WO2024157551A1 - Steel sheet and member, and method for producing said steel sheet and method for producing said member - Google Patents

Steel sheet and member, and method for producing said steel sheet and method for producing said member Download PDF

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WO2024157551A1
WO2024157551A1 PCT/JP2023/037920 JP2023037920W WO2024157551A1 WO 2024157551 A1 WO2024157551 A1 WO 2024157551A1 JP 2023037920 W JP2023037920 W JP 2023037920W WO 2024157551 A1 WO2024157551 A1 WO 2024157551A1
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steel sheet
particles
temperature
hot
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PCT/JP2023/037920
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French (fr)
Japanese (ja)
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克弥 秦
聖太郎 寺嶋
達也 中垣内
斉祐 津田
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Ｊｆｅスチール株式会社
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Priority to JP2024505642A priority Critical patent/JP7541653B1/en
Publication of WO2024157551A1 publication Critical patent/WO2024157551A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a steel plate, a component made from the steel plate, and a method for manufacturing the same.
  • Patent Document 1 describes: "In mass percent, C: 0.05 or more, 0.35% or less, Si: 0.05% or more, 2.5% or less, Mn: 0.6% or more, 3.0% or less, P: 0.001% or more, 0.1% or less, S: 0.0002% or more, 0.05% or less, N: 0.0010% or more, 0.020% or less, Al: 0.001% or more, 2.0% or less, and a steel composition consisting of the remainder iron and unavoidable impurities, the metal structure of the steel sheet containing one or more of ferrite, bainite and tempered martensite, and containing 3% or more retained austenite, the average grain size of the austenite being 1 ⁇ m or more and 8 ⁇ m or less, and at interfaces where the austenite grains contact the ferrite, bainite and tempered martensite, 50% or more austenite grains have a central concentration Cgc of the austenite
  • Patent Document 2 "In mass percent, C: 0.10-0.50%, Mn: 1.0-3.0% Si: 0.005-2.5%, Al: 0.005-2.5%, Contains P: 0.05% or less, S: 0.02% or less, A galvannealed steel sheet having excellent ductility and corrosion resistance, characterized in that N is limited to 0.006% or less, the sum of Si and Al is Si+Al ⁇ 0.8%, the microstructure contains, by area ratio, 10-75% ferrite and 2-30% retained austenite, and the amount of C in the retained austenite is 0.8-1.0%. has been disclosed.
  • Patent Document 3 "In mass percent, C: 0.05% or more, 0.35% or less, Si: 0.05% or more, 2.0% or less, Mn: 0.8% or more, 3.0% or less, P: 0.0010% or more, 0.1% or less, S: 0.0005% or more, 0.05% or less, 1.
  • a high-strength steel sheet excellent in elongation and press forming stability comprising: N: 0.0010% or more and 0.010% or less; Al: 0.01% or more and 2.0% or less; the balance being iron and unavoidable impurities; and a microstructure having, in terms of area ratio, a ferrite phase and a bainite phase of 10% or more and 93% or less in total, an area ratio of a retained austenite phase of 5% or more and 30% or less, and an area ratio of a martensite phase of 5% or more and 20% or less, the retained austenite phase being in a lath-like and island-like form, and an area ratio ⁇ i of the island-like retained austenite phase and an area ratio ⁇ of the total retained austenite phase satisfying the following formula (1): 0.7 ⁇ i/ ⁇ 0.3...Formula (1)'' has been disclosed.
  • steel sheets used in the frame structure of an automobile are particularly required to have high component strength when press-formed.
  • increasing the yield stress (hereinafter simply referred to as YS) of the steel sheets is an effective way to improve the strength of automotive components.
  • steel sheets used for automobile structural components are formed into complex shapes, they require excellent formability, and in particular, excellent bendability.
  • Patent Documents 1 to 3 cannot be said to satisfy all of the required characteristics mentioned above. Furthermore, in the technology of Patent Document 2, the steel sheet needs to be held for a long time after annealing in order to stabilize the retained austenite. This requires a larger annealing facility, which raises concerns about increased facility costs.
  • the present invention has been developed in order to meet the above-mentioned requirements, and has an object to provide a steel plate having high strength, high YS, excellent ductility, and excellent bendability, together with an advantageous manufacturing method thereof. Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.
  • high strength and high YS mean that the tensile strength (hereinafter also referred to as TS) and YS measured in a tensile test in accordance with JIS Z 2241 respectively satisfy the following formulas.
  • TS tensile strength
  • YS measured in a tensile test in accordance with JIS Z 2241 respectively satisfy the following formulas.
  • Excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula: When 780MPa ⁇ TS ⁇ 980MPa, 19% ⁇ El When 980MPa ⁇ TS, 10% ⁇ El
  • the excellent bendability means that R/t measured by a V-bend test in accordance with JIS Z 2248 satisfies the following formula: In the case of 780 MPa ⁇ TS ⁇ 980 MPa, 2.0 ⁇ R / t In the case of 980 MPa ⁇ TS, 4.0 ⁇ R / t here, R: Limit bending radius (mm) t: thickness of steel plate (mm) It is.
  • the inventors have conducted extensive research and have obtained the following findings.
  • (1) After adjusting the composition of the components to fall within a predetermined range, the total area ratio of ferrite and bainite and the area ratio of martensite are controlled to 10% or more and 87% or less, respectively, which makes it possible to achieve both high strength and excellent ductility.
  • (2) The area ratio of the retained austenite is controlled to 3% or more, and the number density of the particles constituting the retained austenite is controlled to 0.05 particles/ ⁇ m2 or more. This improves both ductility and bendability.
  • A The ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio
  • B The average C concentration of particles with an aspect ratio of 3 or less: 0.3 mass% or more
  • C The average value of the shortest distance between particles with an aspect ratio of 3 or less: 5 ⁇ m or less
  • D The maximum value of the shortest distance between particles with an aspect ratio of 3 or less: 15 ⁇ m or less (4)
  • the Mn concentration distribution in the steel is appropriately controlled, that is, [Mn] C /[Mn] is controlled to be 1.05 to 2.00. This further improves both ductility and bendability.
  • [Mn] C is the average Mn concentration (mass%) of the Mn-enriched region in the steel
  • [Mn] is the average Mn concentration (mass%) in the steel.
  • the gist and configuration of the present invention are as follows. 1. In mass percent, C: 0.05% or more and 0.20% or less, Si: 0.1% or more and 1.8% or less, Mn: 1.5% or more and 3.0% or less, P: 0.001% or more and 0.100% or less, S: 0.0500% or less, Al: 0.010% or more and 1.000% or less, N: 0.0100% or less, and one or two of Nb and Ti: 0.005% or more and 0.200% or less in total; The balance being Fe and unavoidable impurities; Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total, Area ratio of martensite: 10% or more and 87% or less; and area ratio of retained austenite: 3% or more; The number density of particles constituting the retained austenite is 0.05 particles/ ⁇ m2 or more, Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the following (A), (B), (A),
  • A The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio;
  • B The average C concentration of particles having an aspect ratio of 3 or less: 0.3 mass% or more;
  • C The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 ⁇ m or less;
  • D The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 ⁇ m or less.
  • the composition further comprises, in mass %, V: 0.45% or less, B: 0.010% or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Sb: 0.1% or less, Sn: 0.1% or less, Cu: 1.0% or less, Ta: 0.1% or less, W: 0.2% or less, Mg: 0.01% or less, Zn: 0.02% or less, Co: 0.02% or less, Zr: 0.2% or less, Ca: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, 2.
  • the steel sheet according to 1 or 2 above having a soft layer having a thickness of 1 ⁇ m or more and 50 ⁇ m or less.
  • the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate.
  • a steel slab having the component composition described in 1 or 2 above A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more; Next, the steel slab is Finish rolling end temperature: 840°C or more and 1000°C or less, Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C.
  • the hot-rolled steel sheet is A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
  • the cold rolled steel sheet is A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C.
  • the cold rolled steel sheet is Annealing temperature: 750°C or higher and 920°C or lower; and Annealing time: annealing under conditions of 1 second to 30 seconds;
  • the cold rolled steel sheet is A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
  • the cold rolled steel sheet is A residence time in a temperature range of 400° C. to 600° C.: a residence step of 1 second to 90 seconds;
  • the method for producing a steel sheet comprising the steps of:
  • the dew points of the atmospheres in the temperature increasing step and the annealing step are both ⁇ 35° C. or higher;
  • a method for manufacturing a component comprising the step of subjecting the steel plate described in any one of 1 to 5 above to at least one of forming and joining processes to produce a component.
  • a steel plate having high strength, high YS, excellent ductility, and excellent bendability can be obtained. Furthermore, since the steel plate of the present invention has high strength, high YS, excellent ductility, and excellent bendability, it can be used extremely advantageously as a material for automobile frame structural members that have complex shapes.
  • 1 is an example of an observation image used for measuring the particle number density, etc., of retained austenite.
  • C 0.05% or more and 0.20% or less C has the effect of increasing the strength of martensite and bainite and generating appropriate amounts of these phases. This effect makes it possible to ensure a predetermined strength.
  • the C content is less than 0.05%, the strength of martensite decreases.
  • the area ratio of ferrite increases excessively, making it difficult to obtain a predetermined strength.
  • the C content exceeds 0.20%, TS becomes excessively high and El decreases.
  • the strength of martensite increases excessively, decreasing bendability. Therefore, the C content is set to 0.05% or more and 0.20% or less.
  • the C content is preferably 0.07% or more, more preferably 0.09% or more.
  • the C content is preferably 0.18% or less, more preferably 0.17% or less.
  • Si 0.1% or more and 1.8% or less
  • Si is an element that improves the strength of the steel sheet by solid solution strengthening.
  • Si is an element that improves ductility while suppressing strength reduction by increasing the strength of ferrite.
  • Si is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Si is an element that affects the area ratio of ferrite.
  • the Si content is less than 0.1%, the area ratio of ferrite decreases and ductility decreases.
  • the Si content is set to 0.1% or more and 1.8% or less.
  • the Si content is preferably 0.3% or more, more preferably 0.5% or more.
  • the Si content is preferably 1.5% or less, more preferably 1.0% or less.
  • Mn 1.5% or more and 3.0% or less Mn is contained to improve the hardenability of the steel and to ensure a predetermined area ratio of martensite and bainite.
  • the Mn content is set to 1.5% or more and 3.0% or less.
  • the Mn content is preferably 1.65% or more, more preferably 1.8% or more.
  • the Mn content is preferably 2.85% or less, more preferably 2.7% or less.
  • P 0.001% or more and 0.100% or less
  • P is an element that has a solid solution strengthening effect and increases the TS of the steel sheet.
  • the P content is set to 0.001% or more.
  • the P content exceeds 0.100%, P segregates to the prior austenite grain boundary and embrittles the grain boundary. Therefore, when bending stress is applied to the steel sheet, voids are generated and cracks grow along the prior austenite grain boundary, and the desired bendability cannot be obtained. Therefore, the P content is set to 0.100% or less. Due to the constraints of production technology, the P content is preferably 0.002% or more. In addition, the P content is preferably 0.050% or less, more preferably 0.030% or less.
  • S 0.0500% or less S forms MnS, etc., and reduces ductility.
  • the S content is set to 0.0500% or less.
  • the S content is preferably 0.0100% or less, more preferably 0.0080% or less, and further preferably 0.0050% or less.
  • the lower limit of the S content is not particularly limited.
  • the S content is preferably 0.0001% or more.
  • the S content is more preferably 0.0005% or more.
  • Al 0.010% or more and 1.000% or less
  • Al is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Al is an element that affects the area ratio of ferrite.
  • the Al content is less than 0.010%, the area ratio of ferrite decreases and ductility decreases.
  • the Al content exceeds 1.000%, the area ratio of ferrite increases excessively, making it difficult to make TS 780 MPa or more. Therefore, the Al content is 0.010% or more and 1.000% or less.
  • the Al content is preferably 0.015% or more, more preferably 0.030% or more.
  • the Al content is preferably 0.500% or less, more preferably 0.100% or less.
  • N 0.0100% or less
  • N is an element that generates nitride-based precipitates such as AlN that pin grain boundaries, and can be contained to improve elongation.
  • the N content is set to 0.0100% or less.
  • the N content is preferably 0.0070% or less, more preferably 0.0050% or less.
  • the lower limit of the N content is not particularly limited. Due to constraints on production technology, the N content is preferably 0.0006% or more.
  • Nb and Ti are elements that contribute to improving TS, ductility, and further bendability through the refinement of prior austenite grains. That is, Nb and Ti contribute to increasing TS through the refinement of the internal structure of martensite and bainite due to the refinement of prior austenite grains. Furthermore, Nb and Ti also contribute to increasing TS through the formation of fine precipitates, such as carbides, nitrides, and carbonitrides, in the hot rolling process and annealing process.
  • Nb and Ti increase the nucleation sites of ferrite and bainite in the cooling process and retention process by the refinement of prior austenite grains and the formation of fine precipitates, and promote ferrite transformation and bainite transformation. Furthermore, with the increase in the nucleation sites of ferrite and bainite, it becomes possible to appropriately control the distribution state of particles constituting retained austenite. In particular, for particles with an aspect ratio of 3 or less, the above-mentioned (C) and (D) can be appropriately controlled. As a result, Nb and Ti contribute to improving ductility and further bending property. In order to obtain these effects, the total content of Nb and Ti is set to 0.005% or more.
  • the total content of Nb and Ti is set to 0.200% or less.
  • the total content of Nb and Ti is preferably 0.008% or more, more preferably 0.010% or more, even more preferably 0.011% or more, and even more preferably 0.015% or more.
  • the total content of Nb and Ti is preferably 0.150% or less, more preferably 0.080% or less.
  • the Nb content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
  • the Nb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.
  • the Ti content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
  • the Ti content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.
  • the basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities.
  • the steel sheet according to one embodiment of the present invention preferably has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities.
  • the steel sheet according to one embodiment of the present invention may contain at least one selected from the following as an optional added element in addition to the above basic components.
  • V 0.45% or less
  • B 0.010% or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Sb: 0.1% or less, Sn: 0.1% or less, Cu: 1.0% or less, Ta: 0.1% or less, W: 0.2% or less, Mg: 0.01% or less, Zn: 0.02% or less, Co: 0.02% or less, Zr: 0.2% or less, Ca: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less and REM: 0.02% or less.
  • V 0.45% or less
  • the V content is preferably 0.001% or more.
  • the V content is more preferably 0.005% or more.
  • the V content is preferably 0.45% or less.
  • the V content is more preferably 0.060% or less.
  • B 0.010% or less
  • B is an element that enhances hardenability by segregating at the austenite grain boundary.
  • B is an element that controls the generation and grain growth of ferrite in the cooling process after the annealing process.
  • the B content is 0.0001% or more.
  • the B content is more preferably 0.0002% or more.
  • the B content exceeds 0.010%, the amount of nitride-based precipitates such as BN becomes excessive, and ductility may decrease. Therefore, when B is contained, the B content is preferably 0.010% or less.
  • the B content is more preferably 0.0050% or less, and further preferably 0.0030% or less.
  • Cr 1.0% or less
  • Cr is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Cr content to 0.0005% or more. Moreover, the Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.0%, the area ratio of martensite increases and ductility may decrease. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. Moreover, the Cr content is more preferably 0.60% or less, and further preferably 0.30% or less.
  • Ni 1.0% or less
  • Ni is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Ni content to 0.005% or more.
  • the Ni content is more preferably 0.020% or more.
  • the Ni content exceeds 1.0%, the area ratio of martensite increases, and ductility may decrease. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less.
  • the Ni content is more preferably 0.5% or less.
  • Mo 1.0% or less
  • Mo is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Mo content to 0.010% or more.
  • the Mo content is more preferably 0.030% or more.
  • the Mo content is preferably 1.0% or less.
  • the Mo content is more preferably 0.5% or less, and further preferably 0.3% or less.
  • Sb 0.1% or less
  • Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to make the TS 780 MPa or more. Therefore, it is preferable to set the Sb content to 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.1%, the castability decreases. Therefore, when Sb is contained, the Sb content is preferably 0.1% or less. The Sb content is more preferably 0.06% or less, and further preferably 0.04% or less.
  • Sn 0.1% or less Sn suppresses oxidation and nitridation near the steel sheet surface, thereby suppressing the decrease in the content of C and B near the steel sheet surface. This suppresses excessive generation of ferrite near the steel sheet surface, contributing to making the TS 780 MPa or more. From this perspective, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.1%, the castability decreases. Therefore, when Sn is contained, the Sn content is preferably 0.1% or less. The Sn content is more preferably 0.04% or less, and even more preferably 0.02% or less.
  • Cu 1.0% or less
  • Cu is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS.
  • the Cu content is more preferably 0.020% or more.
  • the Cu content exceeds 1.0%, the area ratio of martensite increases excessively, which may reduce ductility.
  • a large amount of coarse precipitates and inclusions may be generated, and such coarse precipitates and inclusions may reduce ductility. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less.
  • the Cu content is more preferably 0.2% or less.
  • Ta 0.1% or less Ta, like Ti, Nb and V, increases TS by forming fine precipitates, such as carbides, nitrides and carbonitrides, in the hot rolling process and annealing process.
  • Ta is partially dissolved in Nb carbides and Nb carbonitrides to generate composite precipitates such as (Nb, Ta) (C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS.
  • the Ta content is 0.001% or more.
  • the Ta content exceeds 0.1%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may reduce ductility and even bendability. Therefore, when Ta is contained, the Ta content is preferably 0.1% or less. The Ta content is more preferably 0.05% or less.
  • W 0.2% or less Like Ti, Nb and V, W increases TS by forming fine precipitates, such as carbides, nitrides and carbonitrides, in the hot rolling process and annealing process. In order to obtain such an effect, it is preferable that the W content is 0.001% or more. The W content is more preferably 0.005% or more. On the other hand, if the W content exceeds 0.2%, a large amount of coarse precipitates and inclusions are generated, which leads to a decrease in ductility. Therefore, when W is contained, the W content is preferably 0.2% or less. The W content is more preferably 0.060% or less.
  • Mg 0.01% or less
  • Mg is an effective element for spheroidizing the shape of inclusions such as sulfides and oxides to improve the hole expandability and bendability of steel sheets.
  • the Mg content is preferably 0.01% or less.
  • the Mg content is more preferably 0.005% or less, and even more preferably 0.001% or less.
  • Zn 0.02% or less
  • Zn is an effective element for making the shape of inclusions spherical and improving the bendability of the steel sheet.
  • the Zn content is preferably 0.001% or more.
  • the Zn content is preferably 0.02% or less.
  • Co 0.02% or less Like Zn, Co is an effective element for making the shape of inclusions spherical and improving the bendability of the steel sheet. In order to obtain such an effect, it is preferable that the Co content is 0.001% or more. On the other hand, if the Co content exceeds 0.02%, a large amount of coarse precipitates and inclusions are generated, which may lead to a decrease in bendability. Therefore, when Co is contained, the Co content is preferably 0.02% or less.
  • Zr 0.2% or less Zr contributes to high strength through refinement of prior austenite grains. Zr also contributes to high strength through reduction of block size and vein grain size, which are internal structural units of martensite and bainite, by refinement of prior austenite grains. Furthermore, Zr improves castability. In order to obtain such an effect, it is preferable that the Zr content is 0.001% or more. However, if a large amount of Zr is added, the amount of coarse precipitates of ZrN and ZrS that remain in an undissolved state in the slab heating process increases, and ductility decreases. Therefore, when Zr is contained, the Zr content is preferably 0.2% or less. The Zr content is more preferably 0.05% or less, and further preferably 0.01% or less.
  • Ca 0.02% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.02%, a large amount of coarse inclusions may be generated, which may reduce ductility and bendability. Surface quality may also deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.02% or less.
  • the lower limit of the Ca content is not particularly limited. For example, the Ca content is preferably 0.0005% or more. In addition, due to constraints on production technology, the Ca content is more preferably 0.0010% or more.
  • Se 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less.
  • Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving the bendability of steel sheet. In order to obtain such an effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more.
  • the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceed 0.02% each, or if the content of As exceeds 0.05%, a large amount of coarse precipitates and inclusions are generated, and the bendability may be deteriorated. Therefore, when these elements are contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are each 0.02% or less, and the content of As is 0.05% or less. Note that Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM may be contained alone or in combination.
  • the steel structure of the steel plate according to one embodiment of the present invention is Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total, Area ratio of martensite: 10% or more and 87% or less, and Area ratio of retained austenite: 3% or more,
  • the number density of particles constituting the retained austenite is 0.05 particles/ ⁇ m2 or more, Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the above (A), (B), (C), and (D),
  • the steel structure has a ratio of [Mn] C /[Mn] of 1.05 to 2.00, [Mn] C being the average Mn concentration (mass %) of Mn-enriched regions in the steel, and [Mn] being the average Mn concentration (mass %) in the steel.
  • the reasons for each of the limitations will be explained below.
  • the area ratio of each phase is the ratio of the area that each phase occupies
  • Total area ratio of one or both of ferrite and bainite (hereinafter also referred to as total area ratio of ferrite and bainite): 10% or more and 87% or less Ferrite and bainite are soft, so they are effective in obtaining excellent ductility. In order to obtain the desired ductility, the total area ratio of ferrite and bainite is set to 10% or more. On the other hand, if the area ratio of ferrite and bainite becomes excessive, it becomes difficult to make TS 780 MPa or more. Therefore, the total area ratio of ferrite and bainite is set to 87% or less.
  • the total area ratio of ferrite and bainite is preferably 20% or more, more preferably 30% or more.
  • the total area ratio of ferrite and bainite is preferably 75% or less, more preferably 65% or less.
  • ferrite and bainite may be contained alone, or both of them may be contained.
  • Area ratio of martensite 10% or more and 87% or less Martensite is hard and is a structure necessary for increasing the strength of steel plate. If the area ratio of martensite is less than 10%, the desired TS cannot be obtained. On the other hand, an excessive increase in the area ratio of martensite causes a decrease in ductility. Therefore, the area ratio of martensite is 10% or more and 87% or less.
  • the area ratio of martensite is preferably 20% or more, more preferably 30% or more. In addition, the area ratio of martensite is preferably 75% or less, more preferably 65% or less.
  • Martensite is a hard structure that is generated by transformation from austenite at a martensite transformation point (also simply referred to as Ms point) or lower. Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.
  • Area ratio of retained austenite 3% or more Retained austenite is a structure necessary for achieving both strength and ductility.
  • the area ratio of retained austenite is set to 3% or more.
  • the area ratio of retained austenite is preferably 5% or more, more preferably 7% or more.
  • the amount of retained austenite becomes excessive, for example, when the steel plate is formed into a part, the retained austenite is transformed into martensite, and the number of starting points of bending cracks increases.
  • the area ratio of retained austenite is preferably 20% or less, more preferably 15% or less.
  • the term "retained austenite” refers to austenite that does not transform into ferrite, martensite, bainite, or other metallic phases and remains. The retained austenite is generated, for example, when elements such as C are concentrated in austenite, causing the martensite transformation point to be lower than room temperature (austenite remains without transforming).
  • the area ratio of the remaining structure other than the above is preferably 15% or less.
  • the area ratio of the remaining structure is more preferably 10% or less, and further preferably 5% or less.
  • the area ratio of the remaining structure may be 0%.
  • the remaining structure is not particularly limited, and examples thereof include carbides such as pearlite and cementite.
  • the type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • Pearlite is formed from austenite at a relatively high temperature and is a structure consisting of lamellar ferrite and cementite.
  • the total area ratio of ferrite and bainite and the area ratio of martensite are measured at a 1/4 position of the sheet thickness of the steel sheet as follows. That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste, and then the observation surface of the sample is finish-polished using alumina. Next, the observation surface of the sample is etched with nital to reveal the structure. Then, the observation surface of the sample is observed in five fields of view at a magnification of 1500 times using a SEM (Scanning Electron Microscope).
  • SEM Sccanning Electron Microscope
  • the following regions are color-coded (defined) from the obtained structure image using Adobe Photoshop from Adobe Systems.
  • the total area ratio of ferrite and bainite, and the area ratio of martensite are calculated by the point counting method. Specifically, 16 ⁇ 15 lattice points are set at intervals of 4.8 ⁇ m in an area of 82 ⁇ m ⁇ 57 ⁇ m in actual length of each SEM image. Then, the number of lattice points on ferrite, bainite, and martensite is counted.
  • Ferrite A black region with a blocky shape. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite at relatively high temperatures. Bainite: A region that is black to dark gray in color and has a blocky or amorphous shape.
  • bainite is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite.
  • Bainite is formed from austenite at a relatively low temperature (Ms point or higher). Bainite contains a relatively small number of carbides. Martensite: A region that is white to light gray in color. As described above, martensite is a hard structure that is generated by transformation from austenite at or below the Ms point. Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.
  • the area ratio of retained austenite is measured at a 1/4 position of the sheet thickness of the steel sheet as follows. That is, the steel plate is mechanically ground in the plate thickness direction (depth direction) to a position of 1/4 of the plate thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoK ⁇ rays are used as the incident X-rays, and the ratio of the diffraction intensity of each of the (200), (220) and (311) faces of fcc iron (austenite) to the diffraction intensity of each of the (200), (211) and (220) faces of bcc iron is obtained.
  • the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face. Then, the retained austenite is considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.
  • the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite determined as described above from 100%.
  • [Area ratio of remaining structure (%)] 100 - [Total area ratio of ferrite and bainite (%)] - [Area ratio of martensite (%)] - [Area ratio of retained austenite (%)]
  • Number density of particles constituting the retained austenite (hereinafter also referred to as the particle number density of the retained austenite): 0.05 pieces/ ⁇ m 2 or more
  • the hard martensite (hereinafter also referred to as the processing-induced martensite) generated by processing-induced transformation from the retained austenite during processing (hereinafter also referred to simply as processing) to form the steel sheet into a part is a structure that promotes the generation of voids and the growth of cracks during processing.
  • the particle number density of the retained austenite is set to 0.05 pieces/ ⁇ m 2 or more.
  • the particle number density of the retained austenite is preferably 0.15 pieces/ ⁇ m 2 or more, more preferably 0.25 pieces/ ⁇ m 2 or more.
  • the upper limit of the particle number density of the retained austenite is not particularly limited. However, if the particle number density of the retained austenite becomes excessive, the number of bending crack initiation points increases. Therefore, the particle number density of the retained austenite is preferably 100 particles/ ⁇ m2 or less, more preferably 10 particles/ ⁇ m2 or less.
  • the ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite (hereinafter also referred to as the ratio of particles with an aspect ratio of 3 or less): 60% or more in area ratio.
  • the ratio of particles with an aspect ratio of 3 or less is set to 60% or more in area ratio.
  • the ratio of particles with an aspect ratio of 3 or less is preferably 65% or more, more preferably 70% or more in area ratio.
  • the upper limit of the ratio of particles with an aspect ratio of 3 or less is not particularly limited, and may be 100% in area ratio.
  • the average C concentration of particles with aspect ratio of 3 or less is set to 0.3 mass% or more.
  • the average C concentration of particles with aspect ratio of 3 or less is preferably 0.5 mass% or more, more preferably 0.7 mass% or more.
  • the upper limit of the average C concentration of particles with aspect ratio of 3 or less is not particularly limited. However, if the C concentration of particles having an aspect ratio of 3 or less is excessively high, the progress of the work-induced transformation from retained austenite to martensite may be excessively suppressed, and sufficient work hardening ability may not be obtained. Therefore, the average C concentration of particles having an aspect ratio of 3 or less is preferably 2.0 mass% or less.
  • (C) Average value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the average interparticle distance value): 5 ⁇ m or less
  • the average interparticle distance value it is important to uniformly disperse the particles of retained austenite.
  • the average interparticle distance value is preferably 4 ⁇ m or less, more preferably 3 ⁇ m or less.
  • the lower limit of the average interparticle distance value is not particularly limited. However, from the viewpoint of cost and productivity, the average interparticle distance value is preferably 0.8 ⁇ m or more.
  • the maximum interparticle distance value is set to 15 ⁇ m or less.
  • the maximum interparticle distance value is preferably 10 ⁇ m or less, more preferably 7 ⁇ m or less.
  • the lower limit of the maximum interparticle distance value is not particularly limited.
  • the maximum interparticle distance value is preferably 1 ⁇ m or more.
  • the particle number density of the retained austenite, the ratio of particles having an aspect ratio of 3 or less, the average C concentration of particles having an aspect ratio of 3 or less, the average interparticle distance, and the maximum interparticle distance are each determined as follows using EBSD (electron backscatter diffraction) attached to an FE-SEM. That is, a sample is cut out from the steel plate so that the plate thickness section (L section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina.
  • EBSD electron backscatter diffraction
  • the observation position is set to 1/4 of the plate thickness position of the steel plate, and a region of 50 ⁇ m x 50 ⁇ m is observed by EBSD.
  • image analysis is performed by ImageJ, and the aspect ratio and center of gravity position of each particle of the observed retained austenite are obtained by the particle analysis function.
  • FIG. 1 an example of the observation image is shown in FIG. 1.
  • the white area is the particle of the retained austenite.
  • the particles constituting the retained austenite are those having a circle equivalent diameter of 0.8 ⁇ m or more.
  • the number of particles constituting the retained austenite is counted, and the number of particles is divided by the area of the observation region to obtain the particle number density of the retained austenite.
  • the area occupied by particles having an aspect ratio of 3 or less among all the particles constituting the retained austenite counted above is divided by the area occupied by all the particles constituting the retained austenite, and the result is multiplied by 100 to determine the ratio of particles with an aspect ratio of 3 or less. Furthermore, for each particle having an aspect ratio of 3 or less, the distance to the nearest particle having an aspect ratio of 3 or less (the distance between the centers of gravity of the particles) is determined. The average and maximum of the calculated distances are then taken as the average interparticle distance and the maximum interparticle distance, respectively.
  • the average C concentration of particles having an aspect ratio of 3 or less is determined as follows. That is, for the sample used in the above EBSD observation, the observation position is set to 1 ⁇ 4 of the sheet thickness position of the steel sheet in the same manner as in EBSD, and the C concentration is measured in a grid pattern in a 23 ⁇ m square area with measurement intervals of 0.1 ⁇ m. Next, an area of particles with an aspect ratio of 3 or less is extracted from the EBSD phase map, and the average value of the C concentration at each measurement point in that area is taken as the average C concentration of particles with an aspect ratio of 3 or less.
  • [Mn] C /[Mn]: 1.05 or more and 2.00 or less The fact that [Mn] C /[Mn], which is the ratio of the average Mn concentration of the Mn-enriched region in the steel to the average Mn concentration in the steel, is large means that the enrichment of Mn in the austenite has progressed in the annealing process.
  • the Mn concentration of the austenite in the steel sheet immediately after the annealing process is one of the factors that determine whether the phase transformed from the austenite in the cooling process and the retention process after the annealing process is ferrite and bainite or martensite.
  • Mn is excessively enriched in the austenite in the annealing process, it leads to a delay in the ferrite transformation and bainite transformation in the cooling process. As a result, the desired area ratio of ferrite and bainite cannot be obtained, and ductility and bendability may be reduced. In addition, the delay in the ferrite transformation and the bainite transformation suppresses the enrichment of C in the untransformed austenite. Therefore, the retained austenite that contributes to improving ductility is not obtained sufficiently. Therefore, [Mn] C /[Mn] is set to 2.00 or less. [Mn] C /[Mn] is preferably set to 1.80 or less, more preferably set to 1.60 or less.
  • [Mn] C /[Mn] is set to 1.05 or more, that is, when the distribution of Mn concentration is moderately non-uniform, ferrite transformation and bainite transformation are promoted in austenite with a low Mn concentration. Accordingly, C concentration in untransformed austenite progresses. This results in good ductility and bendability. Therefore, [Mn] C /[Mn] is set to 1.05 or more. [Mn] C /[Mn] is preferably set to 1.10 or more, more preferably set to 1.15 or more.
  • [Mn] C /[Mn] is calculated as follows. That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, the observation position is set to 1/4 of the plate thickness of the steel plate, and the Mn concentration is measured in a grid pattern with a measurement interval of 0.1 ⁇ m in a 23 ⁇ m square area by EPMA. Then, the average value of the Mn concentration of all the measurement points is set as the average Mn concentration [Mn] (mass%) in the steel.
  • the top 10% of the areas with the highest Mn concentration among all the measurement points are set as the Mn-enriched areas.
  • the average value of the Mn concentration measured in the Mn-enriched areas is set as the average Mn concentration [Mn] C (mass%) of the Mn-enriched areas in the steel.
  • [Mn] C is divided by [Mn] to obtain [Mn] C / [Mn].
  • a soft layer having a thickness of 1 ⁇ m or more and 50 ⁇ m or less it is preferable to have a soft layer having a thickness of 1 ⁇ m or more and 50 ⁇ m or less.
  • a soft layer having a thickness of 1 ⁇ m or more and 50 ⁇ m or less from the surface of the steel sheet in the thickness direction, better bendability can be obtained. Therefore, it is preferable to have a soft layer from the surface of the steel sheet in the thickness direction, and it is also preferable that the thickness is 1 ⁇ m or more.
  • the thickness is 50 ⁇ m or less.
  • the thickness of the soft layer is more preferably 40 ⁇ m or less.
  • the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the sheet thickness of the steel sheet.
  • the thickness of the soft layer is measured as follows. That is, the surface of the thickness cross section (L cross section) parallel to the rolling direction of the steel plate is smoothed by wet polishing. Next, using a Vickers hardness tester, hardness measurements are performed at 1 ⁇ m intervals in the thickness (depth) direction from a position at a depth of 1 ⁇ m from the surface of the steel plate to a position at a depth of 100 ⁇ m under a load of 10 gf.
  • hardness measurements are performed at 20 ⁇ m intervals in the thickness (depth) direction from a position at a depth of 100 ⁇ m from the surface of the steel plate to the center position of the thickness. Then, the hardness obtained at the 1/4 position of the thickness of the steel plate is set as the reference hardness, and the depth position at which the hardness is 65% or less of the reference hardness on the surface side of the 1/4 position of the thickness of the steel plate is specified. Then, the distance from the surface of the steel plate to the deepest depth position at which the hardness is 65% or less of the reference hardness (hereinafter also referred to as the depth of the region at which the hardness is 65% or less of the reference hardness) is measured.
  • any one of the surfaces (front and back) of the steel plate is used as a representative in measuring the thickness of the soft layer.
  • any one of the surfaces (front and back) of the steel plate may be used as the starting point of the plate thickness position (plate thickness 0 position) such as the 1/4 plate thickness position.
  • the surface on which the soft layer is present is used as the starting point of the plate thickness position (plate thickness 0 position).
  • the thickness of the soft layer is the thickness per side.
  • the smaller the fluctuation range of the soft phase thickness the better.
  • the fluctuation range of the soft phase thickness exceeds 20 ⁇ m, the bending property may vary in the longitudinal direction of the steel sheet, and the bending property may be locally deteriorated. Therefore, it is preferable that the fluctuation range of the soft phase thickness is 20 ⁇ m or less.
  • the fluctuation range of the soft phase thickness is preferably 15 ⁇ m or less, more preferably 10 ⁇ m or less.
  • the lower limit of the fluctuation range of the soft phase thickness is not particularly limited.
  • the fluctuation range of the soft phase thickness may be 0 ⁇ m.
  • the fluctuation range of the soft phase thickness is the difference between the maximum and minimum values of the depth of the region having a hardness of 65% or less of the reference hardness measured in the above-mentioned measurement of the soft phase thickness (maximum value-minimum value).
  • the tensile strength of the steel plate according to one embodiment of the present invention is 780 MPa or more.
  • the upper limit of the tensile strength of the steel plate according to one embodiment of the present invention is not particularly limited. However, for example, the tensile strength of the steel plate according to one embodiment of the present invention is preferably less than 1180 MPa.
  • the yield stress (YS), total elongation (El) and R/t of the steel plate according to one embodiment of the present invention are as described above.
  • the tensile strength (TS), yield stress (YS), total elongation (El) and R/t are measured as described later in the examples.
  • the steel sheet according to one embodiment of the present invention may have a zinc plating layer on the surface.
  • the zinc plating layer may be provided on only one surface of the steel sheet, or on both surfaces.
  • the zinc plating layer refers to a plating layer containing Zn as the main component (Zn content of 50.0 mass% or more).
  • Examples of the zinc plating layer include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.
  • the steel sheet having a zinc plating layer can also be called a zinc-plated steel sheet.
  • the above-mentioned steel sheet having a hot-dip galvanized layer and alloyed hot-dip galvanized plating can also be called a hot-dip galvanized steel sheet (GI) and an alloyed hot-dip galvanized steel sheet (GA), respectively.
  • GI hot-dip galvanized steel sheet
  • GA alloyed hot-dip galvanized steel sheet
  • the hot-dip galvanized layer is composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al.
  • the hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%.
  • the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.
  • the galvannealed layer is preferably composed of, for example, Zn, 20% or less by mass of Fe, and 0.001% to 1.0% by mass of Al.
  • the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% to 3.5% by mass.
  • the Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more.
  • the Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.
  • the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g/ m2 or more and 80 g/ m2 or less.
  • the plating weight of the zinc plating layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the test material before and after dissolution is then measured, and the value is divided by the surface area of the steel sheet (the surface area of the part that was covered with plating) to calculate the plating coverage (g/ m2 ).
  • a corrosion inhibitor for Fe Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.
  • the thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.5 mm or less.
  • a member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material).
  • the raw material steel plate is subjected to at least one of forming and joining to form a member.
  • the above steel plate has a TS of 780 MPa or more, a high YS, excellent ductility, and excellent bendability. Therefore, the member according to one embodiment of the present invention has high strength and is particularly suitable for application to complex-shaped members used in the automotive field.
  • a method for producing a steel sheet according to one embodiment of the present invention includes the steps of: A steel slab having the above-mentioned composition is A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more; Next, the steel slab is Finish rolling end temperature: 840°C or more and 1000°C or less, Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C.
  • the hot-rolled steel sheet is A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
  • the cold rolled steel sheet is A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C.
  • the cold rolled steel sheet is Annealing temperature: 750°C or higher and 920°C or lower; and Annealing time: annealing under conditions of 1 second to 30 seconds;
  • the cold rolled steel sheet is A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
  • the cold rolled steel sheet is and a residence step of maintaining the mixture in a temperature range of 400° C. to 600° C. for a residence time of 1 second to 90 seconds.
  • the above temperatures refer to the surface temperatures of the steel slab and the steel plate.
  • a steel slab having the above-mentioned composition is prepared.
  • a steel material is melted to obtain molten steel having the above-mentioned composition.
  • the melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used.
  • the obtained molten steel is then solidified to obtain a steel slab.
  • the method for obtaining a steel slab from molten steel is not particularly limited.
  • a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferable.
  • a conventional method in which the steel slab is cooled to room temperature after being produced and then heated again can be applied.
  • direct rolling a method in which the steel slab is not cooled to room temperature, but is charged as a hot piece into a heating furnace and hot rolled
  • direct rolling a method in which the steel slab is immediately rolled after being slightly kept at heat
  • the slab heating temperature is 1220°C or higher.
  • the slab heating temperature is preferably 1230°C or higher, more preferably 1240°C or higher.
  • the upper limit of the slab heating temperature is not particularly limited.
  • the slab heating temperature is preferably 1400°C or lower.
  • the slab heating temperature is the maximum temperature reached by the steel slab in the slab heating process.
  • the slab heating time is 1.0 hour or more.
  • the slab heating time is preferably 1.1 hours or more, more preferably 1.2 hours or more.
  • the slab heating time is preferably 3.0 hours or less.
  • the slab heating time is a holding time in a temperature range of 1220°C or more.
  • Finish rolling end temperature 840°C or more and 1000°C or less
  • the finish rolling end temperature is less than 840°C, the generation of ferrite is promoted, and excessive ferrite is generated before the coiling of the hot-rolled steel sheet.
  • C is concentrated in the untransformed austenite. Excessive concentration of C in the untransformed austenite promotes pearlite transformation. That is, pearlite is excessively generated in the steel structure of the hot-rolled steel sheet obtained after hot rolling.
  • the finish rolling end temperature is 840°C or more.
  • the finish rolling end temperature is preferably 850°C or more.
  • the finish rolling end temperature is set to 1000° C. or less.
  • the finish rolling end temperature is preferably 950° C. or less, and more preferably 920° C. or less.
  • Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C. (hereinafter, also referred to as the first cooling rate): 10 ° C./sec or more
  • the first cooling rate is slow, the amount of ferrite generated during cooling becomes excessive, leading to the concentration of C in untransformed austenite.
  • Excessive concentration of C in untransformed austenite promotes pearlite transformation. That is, in the steel structure of the hot-rolled steel sheet obtained after hot rolling, pearlite is excessively generated.
  • the first cooling rate is set to 10°C/sec or more.
  • the first cooling rate is preferably 15°C/sec or more.
  • the upper limit of the first cooling rate is not particularly limited. However, from the viewpoint of energy saving of cooling equipment, the first cooling rate is preferably set to 1000°C/sec or less.
  • Coiling temperature 620°C or less If the coiling temperature exceeds 620°C, the amount of pearlite produced during coiling is excessively large, and Mn concentration is promoted. The lower the coiling temperature, the less pearlite is produced, so the lower the coiling temperature is, the more preferable it is. Furthermore, from the viewpoint of finely precipitating carbides and nitrides of Nb and Ti, the lower the coiling temperature is, the more preferable it is. Therefore, the coiling temperature is set to 620°C or less. The coiling temperature is preferably 600°C or less, more preferably 580°C or less. On the other hand, if the coiling temperature is less than 400°C, the steel sheet may be excessively hardened and may cause breakage during cold rolling. Therefore, the coiling temperature is preferably 400°C or more. The coiling temperature is more preferably 450°C or more.
  • descaling may be performed as appropriate to remove the primary and secondary scale formed on the surface of the hot-rolled steel sheet. Before cold rolling the hot-rolled steel sheet, it is advisable to thoroughly pickle the sheet to reduce the amount of remaining scale.
  • the hot-rolled steel sheet may be optionally subjected to hot-rolled sheet annealing.
  • Reduction ratio 20% or more and 80% or less
  • the reduction ratio in cold rolling is 20% or more. That is, if the reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and the TS and bendability of the final product are reduced. Therefore, the reduction ratio is 20% or more. On the other hand, if the reduction ratio exceeds 80%, the shape of the steel sheet is likely to be defective. In addition, there is a risk of non-uniformity in the steel structure and non-uniformity in the amount of zinc coating due to temperature unevenness in the annealing process. Therefore, the reduction ratio is 80% or less. The reduction ratio is preferably 30% or more. In addition, the reduction ratio is preferably 70% or less.
  • Heating rate Average heating rate in the temperature range from 600°C to 750°C (hereinafter also referred to as heating rate): 1°C/sec or more and 15°C/sec or less
  • the heating temperature range When the residence time of the cold-rolled steel sheet in the temperature range from 600°C to 750°C (hereinafter also referred to as the heating temperature range) in the heating process is reduced, Mn diffuses and the concentration of Mn in austenite is suppressed. That is, the longer the residence time in the above heating temperature range, the more the concentration of Mn in austenite is promoted. Therefore, it is effective to shorten the residence time in the heating temperature range, in other words, to increase the heating rate.
  • the heating rate is set to 1°C/sec or more.
  • the heating rate is preferably 2° C./sec or more, more preferably 3° C./sec or more.
  • the heating rate is set to 15° C./sec or less.
  • the heating rate is preferably 12° C./sec or less, more preferably 9° C./sec or less.
  • Dew point of atmosphere -35°C or higher
  • the dew point of the atmosphere in the heating step is -35°C or higher. If the dew point of the atmosphere is less than -35°C, it is difficult to form a soft phase of a desired thickness. Therefore, it is preferable that the dew point of the atmosphere in the heating step is -35°C or higher.
  • the dew point of the atmosphere in the heating step is more preferably -20°C or higher, and further preferably -10°C or higher.
  • the upper limit of the dew point of the atmosphere in the heating step is not particularly limited. In order to set the TS within a suitable range, the dew point of the atmosphere in the heating step is preferably 15°C or lower, more preferably 5°C or lower.
  • the cold-rolled steel sheet is annealed under the conditions of an annealing temperature of 750° C. to 920° C. and an annealing time of 1 second to 30 seconds.
  • Annealing temperature 750°C or more and 920°C or less
  • the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and the desired TS cannot be obtained.
  • the annealing temperature exceeds 920°C, the desired area ratio of ferrite and bainite cannot be obtained, and ductility decreases. Therefore, the annealing temperature is 750°C or more and 920°C or less.
  • the annealing temperature is preferably 880°C or less.
  • the annealing temperature is the maximum temperature reached in the annealing process.
  • the annealing time is important for controlling the aspect ratio of the grains constituting the retained austenite. That is, the shorter the annealing time, the better from the following viewpoints. - It suppresses grain growth during annealing and suppresses the concentration of Mn in austenite. - Promotes ferrite transformation and bainite transformation, and reduces the aspect ratio of the particles that make up retained austenite. - Promotes C concentration in particles with an aspect ratio of 3 or less. ⁇ It suppresses the coarsening of austenite (particles) during annealing and disperses the retained austenite uniformly.
  • the annealing time is set to 30 seconds or less, preferably 25 seconds or less, and more preferably 20 seconds or less.
  • the annealing time is set to 1 second or more.
  • the annealing time is preferably 3 seconds or more, and more preferably 5 seconds or more.
  • the annealing time is the holding time at the annealing temperature.
  • Dew point of atmosphere -35°C or higher
  • the dew point of the atmosphere is set to -35°C or higher in the annealing process following the above-mentioned heating process. If the dew point of the atmosphere is less than -35°C, it becomes difficult to form a soft phase of a desired thickness. Therefore, it is preferable to set the dew point of the atmosphere in the annealing process to -35°C or higher.
  • the dew point of the atmosphere in the annealing process is more preferably -20°C or higher, and further preferably -10°C or higher.
  • the upper limit of the dew point of the atmosphere in the annealing process is not particularly limited.
  • the dew point of the atmosphere in the annealing process is preferably 15°C or lower, more preferably 5°C or lower.
  • the second cooling rate particularly the average cooling rate in the temperature range from annealing temperature -30 ° C. to 600 ° C.
  • the second cooling rate is set to 5 ° C./sec or more.
  • the second cooling rate is preferably 9 ° C./sec or more, more preferably 12 ° C./sec or more.
  • the second cooling rate is set to 100 ° C./sec or less.
  • the second cooling rate is preferably 75 ° C./sec or less, more preferably 50 ° C./sec or less.
  • Cooling stop temperature 400°C or more and 600°C or less
  • the cooling stop temperature is set to 400°C or more.
  • the cooling stop temperature is preferably 430°C or more, more preferably 460°C or more.
  • the cooling stop temperature is set to 600°C or less.
  • the cooling stop temperature is preferably 570°C or less, more preferably 540°C or less.
  • Dew point of atmosphere -35°C or less
  • the dew point of the atmosphere in the cooling step is preferably -60°C or more, more preferably -55°C or more.
  • Residence temperature range 400°C or more and 600°C or less
  • the residence temperature range is set to 400°C or more and 600°C or less from the viewpoint of securing an appropriate amount of bainite and residual austenite. If the residence temperature range is less than 400°C, the number of particles with an aspect ratio of more than 3 increases among all particles constituting the residual austenite. On the other hand, if the residence temperature range exceeds 600°C, ferrite and bainite may be excessively generated, and the desired TS may not be obtained. Therefore, the residence temperature range is set to 400°C or more and 600°C or less.
  • the residence temperature range is preferably 420°C or more, more preferably 440°C or more.
  • the residence temperature range is preferably 560°C or less, more preferably 520°C or less.
  • Residence time 1 second or more and 90 seconds or less
  • the residence time is set to 1 second or more.
  • the residence time is preferably 7 seconds or more, more preferably 15 seconds or more.
  • the residence time is set to 90 seconds or less.
  • the residence time is preferably 80 seconds or less, more preferably 70 seconds or less. Note that the residence time here does not include the residence time in the temperature range of 400°C or more and 600°C or less (before cooling is stopped) in the above cooling step.
  • the cold-rolled steel sheet may be subjected to further surface treatment such as chemical conversion treatment or organic coating treatment.
  • the cold rolled steel sheet may be optionally subjected to a galvanizing treatment.
  • the galvanizing treatment include a hot-dip galvanizing treatment and a hot-dip galvannealing treatment.
  • the treatment conditions may be in accordance with conventional methods.
  • the galvanizing bath is not particularly limited as long as it has the composition of the galvanized layer described above, but it is preferable to use a plating bath with an Al content of 0.10 mass% to 0.23 mass%, with the balance consisting of Zn and unavoidable impurities.
  • hot-dip galvanizing and alloyed hot-dip galvanizing which will be described later, are performed, it is preferable to reheat the cold-rolled steel sheet immediately before the process so that the temperature of the sheet entering the plating bath is higher than the plating bath temperature.
  • alloying hot-dip galvanizing treatment it is preferable to carry out alloying treatment at a temperature range of 450°C to 600°C after carrying out hot-dip galvanizing treatment as described above. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate may be excessively slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and ductility. Therefore, the alloying temperature in the alloying treatment is preferably 450°C to 600°C. The alloying temperature in the alloying treatment is more preferably 460°C or higher, and even more preferably 470°C or higher. The alloying temperature in the alloying treatment is more preferably 580°C or lower, and even more preferably 560°C or lower.
  • the plating weight is preferably 20 g/m 2 or more and 80 g/m 2 or less per side.
  • the plating weight can be adjusted by gas wiping or the like.
  • the steel sheet obtained as described above may be further subjected to temper rolling. If the elongation rate of temper rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when forming the steel sheet into a component may decrease. Therefore, the elongation rate of temper rolling is preferably 2.00% or less.
  • the lower limit of the elongation rate of temper rolling is not particularly limited. From the viewpoint of productivity, the elongation rate of temper rolling is preferably 0.05% or more.
  • Temper rolling may be performed on a device connected to the annealing device for performing each of the above-mentioned steps (online), or on a device not connected to the annealing device for performing each of the steps (offline).
  • the number of rolling times of temper rolling may be one or more than two. As long as the same elongation rate as that of temper rolling can be imparted, rolling using a leveler or the like may be used.
  • Conditions other than those mentioned above are not particularly limited and may be made in accordance with standard methods. According to the method for manufacturing a steel sheet according to one embodiment of the present invention described above, a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be obtained, and the steel sheet can be suitably used, for example, for automobile components.
  • a method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component.
  • the molding method is not particularly limited, and for example, a general processing method such as press molding can be used.
  • the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, riveting, crimping, etc. can be used.
  • the molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.
  • a steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting.
  • the steel slab was then heated under the conditions shown in Table 2, and hot rolling consisting of rough rolling and finish rolling was performed on the steel slab to produce a hot-rolled steel sheet.
  • the slab heating time of No. 13 is the holding time at the slab heating temperature.
  • the obtained hot-rolled steel sheet was then pickled and cold-rolled under the conditions shown in Table 2 to produce a cold-rolled steel sheet.
  • the obtained cold-rolled steel sheet was then subjected to a heating process, an annealing process, a cooling process, and a zinc plating process under the conditions shown in Table 2 to obtain the final steel sheet product. Note that conditions not specified were in accordance with conventional methods.
  • hot-dip galvanizing or alloyed hot-dip galvanizing was performed to obtain hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheet
  • GA alloyed hot-dip galvanized steel sheet
  • the plating bath contained 0.20 mass% Al, with the balance consisting of Zn and unavoidable impurities.
  • the plating bath temperature was 470°C.
  • the plating coating weight was about 45 to 72 g/ m2 per side (double-sided plating).
  • the composition of the zinc plating layer of the finally obtained GI was 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, with the balance consisting of Zn and unavoidable impurities.
  • the plating bath contained 0.14 mass% Al, with the balance consisting of Zn and unavoidable impurities.
  • the plating bath temperature was 470°C.
  • the plating coating weight was about 45 g/ m2 per side (double-sided plating).
  • the alloying temperature was 520°C.
  • the composition of the finally obtained zinc plating layer of GA was 7-15 mass% Fe, 0.1-1.0 mass% Al, with the balance consisting of Zn and unavoidable impurities.
  • the steel structure of the steel plate was identified, the particle number density of the retained austenite, (A) the particle ratio (area%) of aspect ratio: 3 or less, (B) the average C concentration (mass%) of aspect ratio: 3 or less, (C) the average interparticle distance ( ⁇ m), (D) the maximum interparticle distance ( ⁇ m), [Mn] C /[Mn], and the thickness of the soft layer were measured in the above-mentioned manner.
  • the measurement results are shown in Table 3.
  • the soft layer was formed on both sides of the steel plate and had the same thickness on both sides.
  • the soft layer was not confirmed (the thickness of the soft layer was less than 1 ⁇ m), so the column for the thickness of the soft layer in Table 2 is marked with "0".
  • TS tensile strength
  • Yield stress YS
  • El total elongation
  • R/t tensile strength
  • the tensile test was conducted in accordance with JIS Z 2241. That is, JIS No. 5 test pieces were taken from the obtained steel plate so that the longitudinal direction was perpendicular to the rolling direction of the steel plate. Using the taken test pieces, a tensile test was conducted at a crosshead speed of 10 mm/min, and TS, YS and El were measured. The results are shown in Table 3.
  • the V (90°) bending test was performed in accordance with JIS Z 2248. That is, a test piece of 100 mm x 35 mm was taken from the steel plate by shearing and end face grinding. Here, the 100 mm side was taken so as to be parallel to the rolling perpendicular (C) direction (width direction). Next, using the taken test piece, a V (90°) bending test was performed under the following conditions.
  • Bending radius R Varies in 0.5 mm increments
  • Test method Die support, punch press Forming load: 10 tons
  • Test speed 30 mm/min Holding time: 5 seconds
  • Bending direction perpendicular to rolling (C) direction
  • the test was performed three times, and the minimum bending radius at which no cracks occurred in any of the three tests was taken as R.
  • R/t was calculated by dividing R by the plate thickness t.
  • the test pieces were observed at a magnification of 25 times using a Leica stereo microscope, and it was determined that cracks had occurred if a crack with a length of 200 ⁇ m or more was confirmed. The results are shown in Table 3.
  • the present invention provides a steel plate that combines high strength, high YS, excellent ductility, and excellent bendability. Furthermore, the steel plate can be used extremely advantageously as a material for automobile frame structural members that have complex shapes. This allows for improved fuel efficiency through reduced vehicle weight, making the steel plate extremely valuable in industry.

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Abstract

The present invention provides a steel sheet which combines high strength, high YS, excellent ductility, and excellent bendability. The steel sheet has a prescribed component composition and steel structure. In particular, in the steel structure, the following are properly controlled: the retained austenite particle number density; the particle ratio of particles having as aspect ratio of 3 or less; the average C concentration of particles having an aspect ratio of 3 or less; the inter-particle distance average value of particles having an aspect ratio of 3 or less; the inter-particle distance maximum value of particles having an aspect ratio of 3 or less; and [Mn]C/[Mn].

Description

鋼板および部材、ならびに、それらの製造方法Steel plates and members, and their manufacturing methods
　本発明は、鋼板、および、該鋼板を素材とする部材、ならびに、それらの製造方法に関する。 The present invention relates to a steel plate, a component made from the steel plate, and a method for manufacturing the same.
　近年、地球環境保全の観点から、自動車産業では、ＣＯ２などの排気ガスを低減しようとする試みが進められている。具体的には、自動車部材の素材となる鋼板を高強度化し、薄くすることによって、車体を軽量化して燃費を向上させる。これにより、排気ガス量を低減しようとする試みが進められている。 In recent years, from the viewpoint of protecting the global environment, the automobile industry has been attempting to reduce exhaust gases such as CO2 . Specifically, the steel sheets used as the raw material for automobile parts are made stronger and thinner to reduce the weight of the car body and improve fuel efficiency. This is an attempt to reduce the amount of exhaust gas.
　このような自動車部材の素材となる鋼板として、例えば、特許文献１には、
「質量％で、
　Ｃ：０．０５以上、０．３５％以下、
　Ｓｉ：０．０５％以上、２．５％以下、
　Ｍｎ：０．６％以上、３．０％以下、
　Ｐ：０．００１％以上、０．１％以下、
　Ｓ：０．０００２％以上、０．０５％以下、
　Ｎ：０．００１０％以上、０．０２０％以下、
　Ａｌ：０．００１％以上、２．０％以下、
を含有して、残部鉄及び不可避的不純物からなる鋼組成をもち、金属組織はフェライト、ベイナイト、焼戻しマルテンサイトの１種または２種以上を含み、かつ残留オーステナイトを３％以上含む鋼板において、前記オーステナイトの平均粒径が１μｍ以上、８μｍ以下であり、前記オーステナイト粒がフェライト、ベイナイト、焼戻しマルテンサイトと接する界面において、オーステナイト粒の中心濃度Ｃｇｃとオーステナイト粒の粒界の濃度Ｃｇｂが（式１）を満たす範囲にあるオーステナイト粒が５０％以上あることを特徴とする伸びと均一塗装焼付硬化性能に優れた高強度薄鋼板。
　　　　　Ｃｇｂ／Ｃｇｃ ≧ １．１　　　（式１）」
が開示されている。 As an example of a steel sheet that can be used as a material for such an automobile component, Patent Document 1 describes:
"In mass percent,
C: 0.05 or more, 0.35% or less,
Si: 0.05% or more, 2.5% or less,
Mn: 0.6% or more, 3.0% or less,
P: 0.001% or more, 0.1% or less,
S: 0.0002% or more, 0.05% or less,
N: 0.0010% or more, 0.020% or less,
Al: 0.001% or more, 2.0% or less,
and a steel composition consisting of the remainder iron and unavoidable impurities, the metal structure of the steel sheet containing one or more of ferrite, bainite and tempered martensite, and containing 3% or more retained austenite, the average grain size of the austenite being 1 μm or more and 8 μm or less, and at interfaces where the austenite grains contact the ferrite, bainite and tempered martensite, 50% or more austenite grains have a central concentration Cgc of the austenite grains and a grain boundary concentration Cgb of the austenite grains in the range that satisfies (Equation 1).
Cgb/Cgc≧1.1 (Equation 1)
has been disclosed.
　特許文献２には、
「質量％で、
　Ｃ：０．１０～０．５０％、
　Ｍｎ：１．０～３．０％
　Ｓｉ：０．００５～２．５％、
　Ａｌ：０．００５～２．５％、
を含有し、
　Ｐ：０．０５％以下、
　Ｓ：０．０２％以下、
　Ｎ：０．００６％以下
に制限し、上記ＳｉとＡｌの総和をＳｉ＋Ａｌ≧０．８％とし、ミクロ組織が、面積率で１０～７５％のフェライト、２～３０％の残留オーステナイトを含有し、当該残留オーステナイト中のＣ量が０．８～１．０％であることを特徴とする延性及び耐食性に優れた合金化溶融亜鉛めっき鋼板。」
が開示されている。 In Patent Document 2,
"In mass percent,
C: 0.10-0.50%,
Mn: 1.0-3.0%
Si: 0.005-2.5%,
Al: 0.005-2.5%,
Contains
P: 0.05% or less,
S: 0.02% or less,
A galvannealed steel sheet having excellent ductility and corrosion resistance, characterized in that N is limited to 0.006% or less, the sum of Si and Al is Si+Al≧0.8%, the microstructure contains, by area ratio, 10-75% ferrite and 2-30% retained austenite, and the amount of C in the retained austenite is 0.8-1.0%.
has been disclosed.
　特許文献３には、
「質量％で、
　Ｃ：０．０５％以上、０．３５％以下、
　Ｓｉ：０．０５％以上、２．０％以下、
　Ｍｎ：０．８％以上、３．０％以下、
　Ｐ：０．００１０％以上、０．１％以下、
　Ｓ：０．０００５％以上、０．０５％以下、
　Ｎ：０．００１０％以上、０．０１０％以下
　Ａｌ：０．０１％以上、２．０％以下
を含有し、残部鉄及び不可避不純物からなる鋼組成をもち、ミクロ組織は面積率で、フェライト相とベイナイト相が合計１０％以上、９３％以下、残留オーステナイト相が面積率で５％以上、３０％以下、マルテンサイト相が面積率で５％以上、２０％以下、上記残留オーステナイト相がラス状、及び島状形態からなり、島状の残留オーステナイト相の面積率γｉ及び全残留オーステナイト相の面積率γが、以下の式（１）を満たすことを特徴とする伸びとプレス成形安定性に優れた高強度鋼板。
　　　　　０．７≧γｉ／γ≧０．３　　　・・・式（１）」
が開示されている。 In Patent Document 3,
"In mass percent,
C: 0.05% or more, 0.35% or less,
Si: 0.05% or more, 2.0% or less,
Mn: 0.8% or more, 3.0% or less,
P: 0.0010% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
1. A high-strength steel sheet excellent in elongation and press forming stability, comprising: N: 0.0010% or more and 0.010% or less; Al: 0.01% or more and 2.0% or less; the balance being iron and unavoidable impurities; and a microstructure having, in terms of area ratio, a ferrite phase and a bainite phase of 10% or more and 93% or less in total, an area ratio of a retained austenite phase of 5% or more and 30% or less, and an area ratio of a martensite phase of 5% or more and 20% or less, the retained austenite phase being in a lath-like and island-like form, and an area ratio γi of the island-like retained austenite phase and an area ratio γ of the total retained austenite phase satisfying the following formula (1):
0.7≧γi/γ≧0.3...Formula (1)''
has been disclosed.
特開２０１２－０３１５０５号公報JP 2012-031505 A 特開２０１１－１６８８１６号公報JP 2011-168816 A 特開２０１２－０４１５７３号公報JP 2012-041573 A
　ところで、鋼板を高強度化すると、一般的に延性が低下する。しかし、自動車部材の素材となる鋼板には、高い強度と、優れた延性、具体的には、引張試験における全伸び（以下、単にＥｌともいう）の向上とを両立することが要求される。 Increasing the strength of steel sheets generally leads to a decrease in ductility. However, steel sheets that are used as the raw material for automotive components are required to have both high strength and excellent ductility, specifically, improved total elongation (hereinafter simply referred to as El) in tensile tests.
　また、自動車部材のうち、特に、自動車の骨格構造部材などに用いられる鋼板には、プレス成形した際に、高い部材強度を有することが要求される。自動車部材の強度の向上については、例えば、鋼板の降伏応力（以下、単にＹＳともいう）を高めることが有効である。 Furthermore, among automotive components, steel sheets used in the frame structure of an automobile are particularly required to have high component strength when press-formed. For example, increasing the yield stress (hereinafter simply referred to as YS) of the steel sheets is an effective way to improve the strength of automotive components.
　さらに、自動車の骨格構造部材などに用いられる鋼板は複雑な形状に成形されるため、優れた成形性、特には、優れた曲げ性が要求される。 Furthermore, because steel sheets used for automobile structural components are formed into complex shapes, they require excellent formability, and in particular, excellent bendability.
　しかしながら、特許文献１～３に開示される鋼板は、上記の要求特性を全て満足するものとは言えない。また、特許文献２の技術では、残留オーステナイトを安定化させるため、焼鈍後に長時間保持する必要がある。そのため、焼鈍設備が大きくなり、設備費の増加が懸念される。 However, the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to satisfy all of the required characteristics mentioned above. Furthermore, in the technology of Patent Document 2, the steel sheet needs to be held for a long time after annealing in order to stabilize the retained austenite. This requires a larger annealing facility, which raises concerns about increased facility costs.
　本発明は、上記の要求に応えるために開発されたものであって、高い強度と、高いＹＳと、優れた延性と、優れた曲げ性と、を兼備する鋼板を、その有利な製造方法とともに、提供することを目的とする。
　また、本発明は、上記の鋼板を素材とする部材およびその製造方法を提供することを目的とする。 The present invention has been developed in order to meet the above-mentioned requirements, and has an object to provide a steel plate having high strength, high YS, excellent ductility, and excellent bendability, together with an advantageous manufacturing method thereof.
Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.
　ここで、高い強度および高いＹＳとは、ＪＩＳ　Ｚ　２２４１に準拠する引張試験で測定される引張強さ（以下、ＴＳともいう）およびＹＳがそれぞれ、以下の式を満足することを意味する。
・ＴＳ
　７８０ＭＰａ≦ＴＳ
・ＹＳ
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、４２０ＭＰａ≦ＹＳ
　９８０ＭＰａ≦ＴＳの場合、５５０ＭＰａ≦ＹＳ Here, high strength and high YS mean that the tensile strength (hereinafter also referred to as TS) and YS measured in a tensile test in accordance with JIS Z 2241 respectively satisfy the following formulas.
・TS
780MPa≦TS
・Y.S.
When 780MPa≦TS<980MPa, 420MPa≦YS
When 980MPa≦TS, 550MPa≦YS
　優れた延性とは、ＪＩＳ　Ｚ　２２４１に準拠する引張試験で測定される全伸び（Ｅｌ）が、以下の式を満足することを意味する。
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、１９％≦Ｅｌ
　９８０ＭＰａ≦ＴＳの場合、１０％≦Ｅｌ Excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula:
When 780MPa≦TS<980MPa, 19%≦El
When 980MPa≦TS, 10%≦El
　優れた曲げ性とは、ＪＩＳ　Ｚ　２２４８に準拠するＶ曲げ試験で測定されるＲ／ｔが、以下の式を満足することを意味する。
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、２．０≧Ｒ／ｔ
　９８０ＭＰａ≦ＴＳの場合、４．０≧Ｒ／ｔ
　ここで、
　Ｒ：限界曲げ半径（ｍｍ）
　ｔ：鋼板の板厚（ｍｍ）
である。 The excellent bendability means that R/t measured by a V-bend test in accordance with JIS Z 2248 satisfies the following formula:
In the case of 780 MPa ≦ TS < 980 MPa, 2.0 ≧ R / t
In the case of 980 MPa ≦ TS, 4.0 ≧ R / t
here,
R: Limit bending radius (mm)
t: thickness of steel plate (mm)
It is.
　さて、発明者らは、上記の目的を達成すべく、鋭意検討を重ね、以下の知見を得た。
（１）成分組成を所定の範囲に調製したうえで、フェライトおよびベイナイトの合計の面積率、ならびに、マルテンサイトの面積率をそれぞれ１０％以上８７％以下に制御する。これにより、高い強度と、優れた延性とを両立することが可能となる。
（２）残留オーステナイトの面積率を３％以上、かつ、残留オーステナイトを構成する粒子の数密度を０．０５個／μｍ２以上に制御する。これにより、延性と曲げ性の両方が向上する。
（３）残留オーステナイトを構成する粒子のうち、アスペクト比：３以下の粒子について、以下の（Ａ）、（Ｂ）、（Ｃ）および（Ｄ）を満足するように制御する。これにより、残留オーステナイトの安定性が向上する。その結果、残留オーステナイトから硬質なフレッシュマルテンサイトへの加工誘起変態が抑制される。ここで、加工誘起変態は、例えば、プレス成形などの一次加工時に生じる。また、残留オーステナイトの分布形態を制御することにより、硬質なフレッシュマルテンサイトが生成したとしても、応力集中を抑制することが可能となる。これにより、延性と曲げ性の両方がさらに向上する。
（Ａ）残留オーステナイトを構成する全ての粒子に対する、アスペクト比：３以下の粒子の比率：面積率で６０％以上
（Ｂ）アスペクト比：３以下の粒子の平均Ｃ濃度：０．３質量％以上
（Ｃ）アスペクト比：３以下の粒子間の最短距離の平均値：５μｍ以下
（Ｄ）アスペクト比：３以下の粒子間の最短距離の最大値：１５μｍ以下
（４）鋼中におけるＭｎの濃度分布を適切に制御する、すなわち、［Ｍｎ］Ｃ／［Ｍｎ］が１．０５以上２．００以下に制御する。これにより、延性と曲げ性の両方がさらに向上する。ここで、［Ｍｎ］Ｃは鋼中のＭｎ濃化領域の平均Ｍｎ濃度（質量％）、［Ｍｎ］は鋼中の平均Ｍｎ濃度（質量％）である。
　本発明は、上記の知見に基づき、さらに検討を加えて完成されたものである。 In order to achieve the above object, the inventors have conducted extensive research and have obtained the following findings.
(1) After adjusting the composition of the components to fall within a predetermined range, the total area ratio of ferrite and bainite and the area ratio of martensite are controlled to 10% or more and 87% or less, respectively, which makes it possible to achieve both high strength and excellent ductility.
(2) The area ratio of the retained austenite is controlled to 3% or more, and the number density of the particles constituting the retained austenite is controlled to 0.05 particles/ μm2 or more. This improves both ductility and bendability.
(3) Among the particles constituting the retained austenite, the particles having an aspect ratio of 3 or less are controlled to satisfy the following (A), (B), (C), and (D). This improves the stability of the retained austenite. As a result, the processing-induced transformation from the retained austenite to hard fresh martensite is suppressed. Here, the processing-induced transformation occurs during primary processing such as press forming. In addition, by controlling the distribution form of the retained austenite, it is possible to suppress stress concentration even if hard fresh martensite is generated. This further improves both ductility and bendability.
(A) The ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio (B) The average C concentration of particles with an aspect ratio of 3 or less: 0.3 mass% or more (C) The average value of the shortest distance between particles with an aspect ratio of 3 or less: 5 μm or less (D) The maximum value of the shortest distance between particles with an aspect ratio of 3 or less: 15 μm or less (4) The Mn concentration distribution in the steel is appropriately controlled, that is, [Mn] C /[Mn] is controlled to be 1.05 to 2.00. This further improves both ductility and bendability. Here, [Mn] C is the average Mn concentration (mass%) of the Mn-enriched region in the steel, and [Mn] is the average Mn concentration (mass%) in the steel.
The present invention was completed based on the above findings and through further investigation.
　すなわち、本発明の要旨構成は次のとおりである。
１．質量％で、
　　Ｃ：０．０５％以上０．２０％以下、
　　Ｓｉ：０．１％以上１．８％以下、
　　Ｍｎ：１．５％以上３．０％以下、
　　Ｐ：０．００１％以上０．１００％以下、
　　Ｓ：０．０５００％以下、
　　Ａｌ：０．０１０％以上１．０００％以下、
　　Ｎ：０．０１００％以下ならびに
　　ＮｂおよびＴｉの１種または２種：合計で０．００５％以上０．２００％以下
であり、
　残部がＦｅおよび不可避的不純物である、成分組成と、
　　フェライトおよびベイナイトの１種または２種の面積率：合計で１０％以上８７％以下、
　　マルテンサイトの面積率：１０％以上８７％以下、ならびに
　　残留オーステナイトの面積率：３％以上
であり、
　　前記残留オーステナイトを構成する粒子の数密度が０．０５個／μｍ２以上であり、
　　前記残留オーステナイトを構成する粒子のうち、アスペクト比：３以下の粒子が以下の（Ａ）、（Ｂ）、（Ｃ）および（Ｄ）を満足し、
　　［Ｍｎ］Ｃ／［Ｍｎ］が１．０５以上２．００以下であり、［Ｍｎ］Ｃは鋼中のＭｎ濃化領域の平均Ｍｎ濃度（質量％）、［Ｍｎ］は鋼中の平均Ｍｎ濃度（質量％）である、鋼組織と、を有し、
　引張強さが７８０ＭＰａ以上である、鋼板。
（Ａ）残留オーステナイトを構成する全ての粒子に対する、アスペクト比：３以下の粒子の比率：面積率で６０％以上
（Ｂ）アスペクト比：３以下の粒子の平均Ｃ濃度：０．３質量％以上
（Ｃ）アスペクト比：３以下の粒子間の最短距離の平均値：５μｍ以下
（Ｄ）アスペクト比：３以下の粒子間の最短距離の最大値：１５μｍ以下 That is, the gist and configuration of the present invention are as follows.
1. In mass percent,
C: 0.05% or more and 0.20% or less,
Si: 0.1% or more and 1.8% or less,
Mn: 1.5% or more and 3.0% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0500% or less,
Al: 0.010% or more and 1.000% or less,
N: 0.0100% or less, and one or two of Nb and Ti: 0.005% or more and 0.200% or less in total;
The balance being Fe and unavoidable impurities;
Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
Area ratio of martensite: 10% or more and 87% or less; and area ratio of retained austenite: 3% or more;
The number density of particles constituting the retained austenite is 0.05 particles/ μm2 or more,
Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the following (A), (B), (C), and (D),
a steel structure in which [ Mn ]/[Mn] is 1.05 or more and 2.00 or less, [ Mn ] is an average Mn concentration (mass%) in Mn-enriched regions in the steel, and [Mn] is an average Mn concentration (mass%) in the steel;
A steel plate having a tensile strength of 780 MPa or more.
(A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio; (B) The average C concentration of particles having an aspect ratio of 3 or less: 0.3 mass% or more; (C) The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 μm or less; (D) The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 μm or less.
２．前記成分組成が、さらに質量％で、
　Ｖ：０．４５％以下、
　Ｂ：０．０１０％以下、
　Ｃｒ：１．０％以下、
　Ｎｉ：１．０％以下、
　Ｍｏ：１．０％以下、
　Ｓｂ：０．１％以下、
　Ｓｎ：０．１％以下、
　Ｃｕ：１．０％以下、
　Ｔａ：０．１％以下、
　Ｗ：０．２％以下、
　Ｍｇ：０．０１％以下、
　Ｚｎ：０．０２％以下、
　Ｃｏ：０．０２％以下、
　Ｚｒ：０．２％以下、
　Ｃａ：０．０２％以下、
　Ｓｅ：０．０２％以下、
　Ｔｅ：０．０２％以下、
　Ｇｅ：０．０２％以下、
　Ａｓ：０．０５％以下、
　Ｓｒ：０．０２％以下、
　Ｃｓ：０．０２％以下、
　Ｈｆ：０．０２％以下、
　Ｐｂ：０．０２％以下、
　Ｂｉ：０．０２％以下および
　ＲＥＭ：０．０２％以下
のうちから選ばれる少なくとも１種を含有する、前記１に記載の鋼板。 2. The composition further comprises, in mass %,
V: 0.45% or less,
B: 0.010% or less,
Cr: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Sb: 0.1% or less,
Sn: 0.1% or less,
Cu: 1.0% or less,
Ta: 0.1% or less,
W: 0.2% or less,
Mg: 0.01% or less,
Zn: 0.02% or less,
Co: 0.02% or less,
Zr: 0.2% or less,
Ca: 0.02% or less,
Se: 0.02% or less,
Te: 0.02% or less,
Ge: 0.02% or less,
As: 0.05% or less,
Sr: 0.02% or less,
Cs: 0.02% or less,
Hf: 0.02% or less,
Pb: 0.02% or less,
2. The steel plate according to 1 above, containing at least one selected from Bi: 0.02% or less and REM: 0.02% or less.
３．厚さ：１μｍ以上５０μｍ以下の軟質層を有する、前記１または２に記載の鋼板。
　ここで、軟質層とは、硬度が鋼板の板厚１／４位置の硬度の６５％以下になる領域である。 3. The steel sheet according to 1 or 2 above, having a soft layer having a thickness of 1 μm or more and 50 μm or less.
Here, the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate.
４．表面に亜鉛めっき層を有する、前記１～３のいずれかに記載の鋼板。 4. The steel sheet described in any one of 1 to 3 above, having a zinc-plated layer on the surface.
５．前記亜鉛めっき層が溶融亜鉛めっき層または合金化溶融亜鉛めっき層である、前記４に記載の鋼板。 5. The steel sheet according to 4, wherein the zinc-plated layer is a hot-dip galvanized layer or a hot-dip galvannealed layer.
６．前記１～５のいずれかに記載の鋼板を用いてなる、部材。 6. A member made using the steel plate described in any one of 1 to 5 above.
７．前記１または２に記載の成分組成を有する鋼スラブを、
　スラブ加熱温度：１２２０℃以上、および
　スラブ加熱時間：１．０時間以上
の条件で加熱する、スラブ加熱工程と、
　ついで、前記鋼スラブに、
　仕上げ圧延終了温度：８４０℃以上１０００℃以下、
　仕上げ圧延終了温度から７００℃までの温度域での平均冷却速度：１０℃／秒以上、および、
　巻取温度：６２０℃以下
の条件で熱間圧延を施し、熱延鋼板を得る、熱間圧延工程と、
　ついで、前記熱延鋼板に、
　圧下率：２０％以上８０％以下
の条件で冷間圧延を施し、冷延鋼板を得る、冷間圧延工程と、
　ついで、前記冷延鋼板を、
　６００℃から７５０℃までの温度域での平均昇温速度：１℃／秒以上１５℃／秒以下
の条件で昇温する、昇温工程と、
　ついで、前記冷延鋼板を、
　焼鈍温度：７５０℃以上９２０℃以下、および、
　焼鈍時間：１秒以上３０秒以下
の条件で焼鈍する、焼鈍工程と、
　ついで、前記冷延鋼板を、
　焼鈍温度－３０℃から６００℃までの温度域での平均冷却速度：５℃／秒以上１００℃／秒以下、および
　冷却停止温度：４００℃以上６００℃以下
の条件で冷却する、冷却工程と、
　ついで、前記冷延鋼板を、
　４００℃以上６００℃以下の温度域での滞留時間：１秒以上９０秒以下
の条件で滞留させる、滞留工程と、
を有する、鋼板の製造方法。 7. A steel slab having the component composition described in 1 or 2 above,
A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more;
Next, the steel slab is
Finish rolling end temperature: 840°C or more and 1000°C or less,
Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C. / sec or more, and
A hot rolling process in which hot rolling is performed under the condition of a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet;
Next, the hot-rolled steel sheet is
A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
Next, the cold rolled steel sheet is
A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C. of 1° C./sec or more and 15° C./sec or less;
Next, the cold rolled steel sheet is
Annealing temperature: 750°C or higher and 920°C or lower; and
Annealing time: annealing under conditions of 1 second to 30 seconds;
Next, the cold rolled steel sheet is
A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
Next, the cold rolled steel sheet is
A residence time in a temperature range of 400° C. to 600° C.: a residence step of 1 second to 90 seconds;
The method for producing a steel sheet comprising the steps of:
８．前記昇温工程および前記焼鈍工程における雰囲気の露点がいずれも－３５℃以上であり、
　前記冷却工程における雰囲気の露点が－３５℃以下である、前記７に記載の鋼板の製造方法。 8. The dew points of the atmospheres in the temperature increasing step and the annealing step are both −35° C. or higher;
The method for producing a steel sheet according to claim 7, wherein the dew point of the atmosphere in the cooling step is −35° C. or lower.
９．前記滞留工程後、さらに前記冷延鋼板に亜鉛めっき処理を施す、亜鉛めっき処理工程を有する、前記７または８に記載の鋼板の製造方法。 9. The method for producing a steel sheet according to claim 7 or 8, further comprising a zinc plating process in which the cold-rolled steel sheet is subjected to a zinc plating process after the retention process.
１０．前記亜鉛めっき処理が、溶融亜鉛めっき処理または合金化溶融亜鉛めっき処理である、前記９に記載の鋼板の製造方法。 10. The method for producing a steel sheet according to 9, wherein the zinc plating treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.
１１．前記１～５のいずれかに記載の鋼板に、成形加工および接合加工の少なくとも一方を施して部材とする、工程を有する、部材の製造方法。 11. A method for manufacturing a component, comprising the step of subjecting the steel plate described in any one of 1 to 5 above to at least one of forming and joining processes to produce a component.
　本発明によれば、高い強度と、高いＹＳと、優れた延性と、優れた曲げ性と、を兼備する鋼板が得られる。また、本発明の鋼板は、高い強度と、高いＹＳと、優れた延性と、優れた曲げ性と、を兼備するので、複雑な形状となる自動車の骨格構造部材などの素材として、極めて有利に適用することができる。 According to the present invention, a steel plate having high strength, high YS, excellent ductility, and excellent bendability can be obtained. Furthermore, since the steel plate of the present invention has high strength, high YS, excellent ductility, and excellent bendability, it can be used extremely advantageously as a material for automobile frame structural members that have complex shapes.
残留オーステナイトの粒子数密度等の測定に使用した観察画像の一例である。1 is an example of an observation image used for measuring the particle number density, etc., of retained austenite.
　本発明を、以下の実施形態に基づき説明する。
［１］鋼板
　まず、本発明の一実施形態に従う鋼板の成分組成について説明する。なお、成分組成における単位はいずれも「質量％」であるが、以下、特に断らない限り、単に「％」で示す。 The present invention will be described based on the following embodiments.
[1] Steel Plate First, the composition of the steel plate according to one embodiment of the present invention will be described. Note that the unit of the composition is "mass%", but hereinafter, unless otherwise specified, it will be simply shown as "%".
Ｃ：０．０５％以上０．２０％以下
　Ｃは、マルテンサイトおよびベイナイトの強度を高め、これらの相を適正量生成させる効果を有する。このような効果によって、所定の強度を確保すること可能となる。ここで、Ｃ含有量が０．０５％未満では、マルテンサイトの強度が低下する。また、フェライトの面積率が過度に増加して所定の強度を得ることが困難となる。一方、Ｃ含有量が０．２０％を超えると、ＴＳが過度に高くなり、Ｅｌが低下する。また、マルテンサイトの強度が過度に増加し、曲げ性を低下させる。そのため、Ｃ含有量は０．０５％以上０．２０％以下とする。Ｃ含有量は、好ましくは０．０７％以上、より好ましくは０．０９％以上である。また、Ｃ含有量は、好ましくは０．１８％以下、より好ましくは０．１７％以下である。 C: 0.05% or more and 0.20% or less C has the effect of increasing the strength of martensite and bainite and generating appropriate amounts of these phases. This effect makes it possible to ensure a predetermined strength. Here, if the C content is less than 0.05%, the strength of martensite decreases. In addition, the area ratio of ferrite increases excessively, making it difficult to obtain a predetermined strength. On the other hand, if the C content exceeds 0.20%, TS becomes excessively high and El decreases. In addition, the strength of martensite increases excessively, decreasing bendability. Therefore, the C content is set to 0.05% or more and 0.20% or less. The C content is preferably 0.07% or more, more preferably 0.09% or more. In addition, the C content is preferably 0.18% or less, more preferably 0.17% or less.
Ｓｉ：０．１％以上１．８％以下
　Ｓｉは、固溶強化により鋼板の強度を向上させる元素である。また、Ｓｉは、フェライトの強度を増加することにより、強度低下を抑制しながら延性を向上させる元素である。さらに、Ｓｉは、焼鈍工程およびその後の冷却工程において、フェライト変態を促進させる元素である。すなわち、Ｓｉは、フェライトの面積率に影響する元素である。ここで、Ｓｉ含有量が０．１％未満では、フェライトの面積率が減少し、延性が低下する。一方、Ｓｉ含有量が過剰になる、特には１．８％を超えると、熱間圧延時および冷間圧延時の圧延荷重の著しい増加を招く。また、靭性の低下を招く。そのため、Ｓｉ含有量は０．１％以上１．８％以下とする。Ｓｉ含有量は、好ましくは０．３％以上、より好ましくは０．５％以上である。また、Ｓｉの含有量は、好ましくは１．５％以下、より好ましくは１．０％以下である。 Si: 0.1% or more and 1.8% or less Si is an element that improves the strength of the steel sheet by solid solution strengthening. In addition, Si is an element that improves ductility while suppressing strength reduction by increasing the strength of ferrite. Furthermore, Si is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Si is an element that affects the area ratio of ferrite. Here, if the Si content is less than 0.1%, the area ratio of ferrite decreases and ductility decreases. On the other hand, if the Si content is excessive, particularly if it exceeds 1.8%, it will lead to a significant increase in the rolling load during hot rolling and cold rolling. It will also lead to a decrease in toughness. Therefore, the Si content is set to 0.1% or more and 1.8% or less. The Si content is preferably 0.3% or more, more preferably 0.5% or more. The Si content is preferably 1.5% or less, more preferably 1.0% or less.
Ｍｎ：１．５％以上３．０％以下
　Ｍｎは、鋼の焼入れ性を向上させ、マルテンサイトおよびベイナイトの面積率を所定量確保にするために含有させる。ここで、Ｍｎ含有量が１．５％未満では焼入れ性が不足し、フェライトが過剰に生成する。これにより、ＴＳを７８０ＭＰａ以上とすることが困難になる。一方、Ｍｎを過剰に含有させると、ベイナイト変態が遅延され、所定量の残留オーステナイトを得ることが困難となる。これにより、延性の低下を招く。そのため、Ｍｎ含有量は１．５％以上３．０％以下とする。Ｍｎ含有量は、好ましくは１．６５％以上、より好ましくは１．８％以上である。また、Ｍｎ含有量は、好ましくは２．８５％以下、より好ましくは２．７％以下である。 Mn: 1.5% or more and 3.0% or less Mn is contained to improve the hardenability of the steel and to ensure a predetermined area ratio of martensite and bainite. Here, if the Mn content is less than 1.5%, the hardenability is insufficient and ferrite is excessively generated. This makes it difficult to achieve a TS of 780 MPa or more. On the other hand, if Mn is excessively contained, the bainite transformation is delayed and it becomes difficult to obtain a predetermined amount of retained austenite. This leads to a decrease in ductility. Therefore, the Mn content is set to 1.5% or more and 3.0% or less. The Mn content is preferably 1.65% or more, more preferably 1.8% or more. The Mn content is preferably 2.85% or less, more preferably 2.7% or less.
Ｐ：０．００１％以上０．１００％以下
　Ｐは、固溶強化の作用を有し、鋼板のＴＳを上昇させる元素である。このような効果を得るため、Ｐ含有量を０．００１％以上にする。一方、Ｐ含有量が０．１００％を超えると、Ｐが、旧オーステナイト粒界に偏析して粒界を脆化させる。そのため、鋼板に曲げ応力が付加される際に、旧オーステナイト粒界に沿ってボイドの生成および亀裂の進展が生じ、所望の曲げ性が得られない。そのため、Ｐ含有量は０．１００％以下とする。Ｐ含有量は、生産技術上の制約から、好ましくは０．００２％以上である。また、Ｐ含有量は、好ましくは０．０５０％以下、より好ましくは０．０３０％以下である。 P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS of the steel sheet. In order to obtain such an effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates to the prior austenite grain boundary and embrittles the grain boundary. Therefore, when bending stress is applied to the steel sheet, voids are generated and cracks grow along the prior austenite grain boundary, and the desired bendability cannot be obtained. Therefore, the P content is set to 0.100% or less. Due to the constraints of production technology, the P content is preferably 0.002% or more. In addition, the P content is preferably 0.050% or less, more preferably 0.030% or less.
Ｓ：０．０５００％以下
　Ｓは、ＭｎＳ等を形成し、延性を低下させる。また、ＳとともにＴｉが含有される場合には、ＴｉＳ、Ｔｉ（Ｃ、Ｓ）等が形成され、曲げ性を低下させるおそれもある。したがって、Ｓ含有量は０．０５００％以下とする。Ｓ含有量は好ましくは０．０１００％以下、より好ましくは０．００８０％以下、さらに好ましくは０．００５０％以下である。なお、Ｓ含有量の下限は特に限定されるものではない。Ｓ含有量は０．０００１％以上が好ましい。Ｓ含有量は、より好ましくは０．０００５％以上である。 S: 0.0500% or less S forms MnS, etc., and reduces ductility. In addition, when Ti is contained together with S, TiS, Ti(C,S), etc. are formed, which may reduce bendability. Therefore, the S content is set to 0.0500% or less. The S content is preferably 0.0100% or less, more preferably 0.0080% or less, and further preferably 0.0050% or less. The lower limit of the S content is not particularly limited. The S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more.
Ａｌ：０．０１０％以上１．０００％以下
　Ａｌは、焼鈍工程およびその後の冷却工程におけるフェライト変態を促進させる元素である。すなわち、Ａｌは、フェライトの面積率に影響する元素である。ここで、Ａｌ含有量が０．０１０％未満では、フェライトの面積率が減少し、延性が低下する。一方、Ａｌ含有量が１．０００％を超えると、フェライトの面積率が過度に増加し、ＴＳを７８０ＭＰａ以上とすることが困難になる。したがって、Ａｌ含有量は、０．０１０％以上１．０００％以下とする。Ａｌ含有量は、好ましくは０．０１５％以上、より好ましくは０．０３０％以上である。また、Ａｌ含有量は、好ましくは０．５００％以下、より好ましくは０．１００％以下である。 Al: 0.010% or more and 1.000% or less Al is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Al is an element that affects the area ratio of ferrite. Here, when the Al content is less than 0.010%, the area ratio of ferrite decreases and ductility decreases. On the other hand, when the Al content exceeds 1.000%, the area ratio of ferrite increases excessively, making it difficult to make TS 780 MPa or more. Therefore, the Al content is 0.010% or more and 1.000% or less. The Al content is preferably 0.015% or more, more preferably 0.030% or more. Moreover, the Al content is preferably 0.500% or less, more preferably 0.100% or less.
Ｎ：０．０１００％以下
　Ｎは、結晶粒界をピン止めするＡｌＮ等の窒化物系析出物を生成させる元素であり、伸びを良好にするために含有させることができる。しかし、Ｎ含有量が０．０１００％を超えると、ＡｌＮ等の窒化物系析出物が粗大化するため、伸びが低下する。したがって、Ｎ含有量は０．０１００％以下とする。Ｎ含有量は、好ましくは０．００７０％以下、より好ましくは０．００５０％以下である。なお、Ｎ含有量の下限は特に限定されるものではない。生産技術上の制約から、Ｎ含有量は０．０００６％以上が好ましい。 N: 0.0100% or less N is an element that generates nitride-based precipitates such as AlN that pin grain boundaries, and can be contained to improve elongation. However, if the N content exceeds 0.0100%, the nitride-based precipitates such as AlN become coarse, and the elongation decreases. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0070% or less, more preferably 0.0050% or less. The lower limit of the N content is not particularly limited. Due to constraints on production technology, the N content is preferably 0.0006% or more.
ＮｂおよびＴｉの１種または２種：合計で０．００５％以上０．２００％以下
　ＮｂおよびＴｉは、旧オーステナイト粒の微細化などを通じてＴＳおよび延性、さらには曲げ性の向上に寄与する元素である。すなわち、ＮｂおよびＴｉは、旧オーステナイト粒の微細化によるマルテンサイトおよびベイナイトの内部構造の微細化を通じて、ＴＳの上昇に寄与する。さらに、ＮｂおよびＴｉは、熱間圧延工程や焼鈍工程での微細な析出物、例えば、炭化物や窒化物、炭窒化物の形成を通じても、ＴＳの上昇に寄与する。加えて、ＮｂおよびＴｉは、上記した旧オーステナイト粒の微細化と微細な析出物の形成により、冷却工程や滞留工程において、フェライトおよびベイナイトの核生成サイトを増加させ、フェライト変態およびベイナイト変態を促進させる。さらに、上記したフェライトおよびベイナイトの核生成サイトの増加に伴って、残留オーステナイトを構成する粒子の分布状態を適切に制御することが可能となる。特に、アスペクト比：３以下の粒子について、上記した（Ｃ）および（Ｄ）を適切に制御することが可能となる。これにより、ＮｂおよびＴｉは、延性、さらには曲げ性の向上にも寄与する。これらの効果を得るため、ＮｂおよびＴｉの合計含有量を０．００５％以上とする。一方、ＮｂおよびＴｉの含有量が過剰になると、粗大な析出物や介在物が多量に生成し、却って延性および曲げ性の低下を招く。特に、ＮｂとＴｉを複合添加する場合、析出物が安定化して粗大な介在物として残存しやすい。そのため、ＮｂおよびＴｉの合計含有量は、０．２００％以下とする。ＮｂおよびＴｉの合計含有量は、好ましくは０．００８％以上、より好ましくは０．０１０％以上、さらに好ましくは０．０１１％以上、よりさらに好ましくは０．０１５％以上である。ＮｂおよびＴｉの合計含有量は、好ましくは０．１５０％以下、より好ましくは０．０８０％以下である。 One or both of Nb and Ti: 0.005% or more and 0.200% or less in total Nb and Ti are elements that contribute to improving TS, ductility, and further bendability through the refinement of prior austenite grains. That is, Nb and Ti contribute to increasing TS through the refinement of the internal structure of martensite and bainite due to the refinement of prior austenite grains. Furthermore, Nb and Ti also contribute to increasing TS through the formation of fine precipitates, such as carbides, nitrides, and carbonitrides, in the hot rolling process and annealing process. In addition, Nb and Ti increase the nucleation sites of ferrite and bainite in the cooling process and retention process by the refinement of prior austenite grains and the formation of fine precipitates, and promote ferrite transformation and bainite transformation. Furthermore, with the increase in the nucleation sites of ferrite and bainite, it becomes possible to appropriately control the distribution state of particles constituting retained austenite. In particular, for particles with an aspect ratio of 3 or less, the above-mentioned (C) and (D) can be appropriately controlled. As a result, Nb and Ti contribute to improving ductility and further bending property. In order to obtain these effects, the total content of Nb and Ti is set to 0.005% or more. On the other hand, if the content of Nb and Ti becomes excessive, a large amount of coarse precipitates and inclusions are generated, which leads to a decrease in ductility and bending property. In particular, when Nb and Ti are added in combination, the precipitates are stabilized and tend to remain as coarse inclusions. Therefore, the total content of Nb and Ti is set to 0.200% or less. The total content of Nb and Ti is preferably 0.008% or more, more preferably 0.010% or more, even more preferably 0.011% or more, and even more preferably 0.015% or more. The total content of Nb and Ti is preferably 0.150% or less, more preferably 0.080% or less.
　なお、ＮｂおよびＴｉの合計含有量が０．００５％以上０．２００％以下であれば、ＮｂおよびＴｉのそれぞれの含有量は特に限定されない。
　例えば、Ｎｂ含有量は、好ましくは０．００２％以上、より好ましくは０．００５％以上、さらに好ましくは０．０１０％以上である。また、Ｎｂ含有量は、好ましくは０．２００％以下、より好ましくは０．１５０％以下、さらに好ましくは０．０８０％以下である。
　Ｔｉ含有量は、好ましくは０．００２％以上、より好ましくは０．００５％以上、さらに好ましくは０．０１０％以上である。また、Ｔｉ含有量は、好ましくは０．２００％以下、より好ましくは０．１５０％以下、さらに好ましくは０．０８０％以下である。 As long as the total content of Nb and Ti is 0.005% or more and 0.200% or less, the respective contents of Nb and Ti are not particularly limited.
For example, the Nb content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Nb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.
The Ti content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Ti content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.
　以上、本発明の一実施形態に従う鋼板の基本成分組成について説明したが、本発明の一実施形態に従う鋼板は、上記基本成分を含有し、上記基本成分以外の残部はＦｅ（鉄）および不可避的不純物を含む成分組成を有する。ここで、本発明の一実施形態に従う鋼板は、上記基本成分を含有し、残部はＦｅおよび不可避的不純物からなる成分組成を有することが好ましい。本発明の一実施形態に従う鋼板には、上記基本成分に加え、任意添加元素として、以下のうちから選ばれる少なくとも１種を含有させてもよい。
　Ｖ：０．４５％以下、
　Ｂ：０．０１０％以下、
　Ｃｒ：１．０％以下、
　Ｎｉ：１．０％以下、
　Ｍｏ：１．０％以下、
　Ｓｂ：０．１％以下、
　Ｓｎ：０．１％以下、
　Ｃｕ：１．０％以下、
　Ｔａ：０．１％以下、
　Ｗ：０．２％以下、
　Ｍｇ：０．０１％以下、
　Ｚｎ：０．０２％以下、
　Ｃｏ：０．０２％以下、
　Ｚｒ：０．２％以下、
　Ｃａ：０．０２％以下、
　Ｓｅ：０．０２％以下、
　Ｔｅ：０．０２％以下、
　Ｇｅ：０．０２％以下、
　Ａｓ：０．０５％以下、
　Ｓｒ：０．０２％以下、
　Ｃｓ：０．０２％以下、
　Ｈｆ：０．０２％以下、
　Ｐｂ：０．０２％以下、
　Ｂｉ：０．０２％以下および
　ＲＥＭ：０．０２％以下
　なお、上記の任意添加元素は、上記の上限量以下で含有していれば、本発明の効果が得られるため、下限は特に設けない。なお、上記の任意添加元素を後述する好適な下限値未満で含む場合、当該元素は不可避的不純物として含まれるものとする。 The basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities. The steel sheet according to one embodiment of the present invention may contain at least one selected from the following as an optional added element in addition to the above basic components.
V: 0.45% or less,
B: 0.010% or less,
Cr: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Sb: 0.1% or less,
Sn: 0.1% or less,
Cu: 1.0% or less,
Ta: 0.1% or less,
W: 0.2% or less,
Mg: 0.01% or less,
Zn: 0.02% or less,
Co: 0.02% or less,
Zr: 0.2% or less,
Ca: 0.02% or less,
Se: 0.02% or less,
Te: 0.02% or less,
Ge: 0.02% or less,
As: 0.05% or less,
Sr: 0.02% or less,
Cs: 0.02% or less,
Hf: 0.02% or less,
Pb: 0.02% or less,
Bi: 0.02% or less and REM: 0.02% or less. Since the effects of the present invention can be obtained as long as the optional elements are contained in amounts not exceeding the upper limits, no lower limit is set. When the optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.
Ｖ：０．４５％以下
　Ｖは、ＮｂやＴｉと同様、熱間圧延工程や焼鈍工程において、微細な析出物、例えば、炭化物や窒化物、炭窒化物を形成することによって、ＴＳを上昇させる。このような効果を得るためには、Ｖ含有量を０．００１％以上とすることが好ましい。Ｖ含有量は、より好ましくは０．００５％以上である。一方、Ｖ含有量が０．４５％を超えると、粗大な析出物や介在物が多量に生成し、延性が低下するおそれがある。したがって、Ｖを含有させる場合、Ｖ含有量は０．４５％以下が好ましい。Ｖ含有量は、より好ましくは０．０６０％以下である。 V: 0.45% or less Like Nb and Ti, V increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, in the hot rolling process and annealing process. In order to obtain such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.45%, a large amount of coarse precipitates and inclusions may be generated, and ductility may decrease. Therefore, when V is contained, the V content is preferably 0.45% or less. The V content is more preferably 0.060% or less.
Ｂ：０．０１０％以下
　Ｂは、オーステナイト粒界に偏析することにより、焼入れ性を高める元素である。また、Ｂは、焼鈍工程の後の冷却工程におけるフェライトの生成および粒成長を制御する元素である。このような効果を得るためには、Ｂ含有量を０．０００１％以上にすることが好ましい。Ｂ含有量は、より好ましくは０．０００２％以上である。一方、Ｂ含有量が０．０１０％を超えると、ＢＮ等の窒化物系析出物の量が過剰となるため、延性が低下するおそれがある。したがって、Ｂを含有させる場合、Ｂ含有量は０．０１０％以下が好ましい。Ｂ含有量は、より好ましくは０．００５０％以下、さらに好ましくは０．００３０％以下である。 B: 0.010% or less B is an element that enhances hardenability by segregating at the austenite grain boundary. In addition, B is an element that controls the generation and grain growth of ferrite in the cooling process after the annealing process. In order to obtain such an effect, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.010%, the amount of nitride-based precipitates such as BN becomes excessive, and ductility may decrease. Therefore, when B is contained, the B content is preferably 0.010% or less. The B content is more preferably 0.0050% or less, and further preferably 0.0030% or less.
Ｃｒ：１．０％以下
　Ｃｒは、焼入れ性を高めてマルテンサイトの生成を促し、これにより、ＴＳを上昇させる元素である。このような効果を得るためには、Ｃｒ含有量を０．０００５％以上にすることが好ましい。また、Ｃｒ含有量は、より好ましくは０．０１０％以上である。一方、Ｃｒ含有量が１．０％を超えると、マルテンサイトの面積率が増加し、延性が低下するおそれがある。したがって、Ｃｒを含有させる場合、Ｃｒ含有量は１．０％以下が好ましい。また、Ｃｒ含有量は、より好ましくは０．６０％以下、さらに好ましくは０．３０％以下である。 Cr: 1.0% or less Cr is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Cr content to 0.0005% or more. Moreover, the Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.0%, the area ratio of martensite increases and ductility may decrease. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. Moreover, the Cr content is more preferably 0.60% or less, and further preferably 0.30% or less.
Ｎｉ：１．０％以下
　Ｎｉは、焼入れ性を高めてマルテンサイトの生成を促し、これにより、ＴＳを上昇させる元素である。このような効果を得るためには、Ｎｉ含有量を０．００５％以上にすることが好ましい。Ｎｉ含有量は、より好ましくは、０．０２０％以上である。一方、Ｎｉの含有量が１．０％を超えると、マルテンサイトの面積率が増加し、延性が低下するおそれがある。したがって、Ｎｉを含有させる場合、Ｎｉ含有量は１．０％以下が好ましい。Ｎｉ含有量は、より好ましくは０．５％以下である。 Ni: 1.0% or less Ni is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Ni content to 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.0%, the area ratio of martensite increases, and ductility may decrease. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less. The Ni content is more preferably 0.5% or less.
Ｍｏ：１．０％以下
　Ｍｏは、焼入れ性を高めてマルテンサイトの生成を促し、これにより、ＴＳを上昇させる元素である。このような効果を得るためには、Ｍｏ含有量を０．０１０％以上にすることが好ましい。Ｍｏ含有量は、より好ましくは、０．０３０％以上である。一方、Ｍｏ含有量が１．０％を超えると、マルテンサイトの面積率が増加し、所望の延性が得られない場合がある。したがって、Ｍｏを含有させる場合、Ｍｏ含有量は１．０％以下が好ましい。Ｍｏ含有量は、より好ましくは０．５％以下、さらに好ましくは０．３％以下である。 Mo: 1.0% or less Mo is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Mo content to 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.0%, the area ratio of martensite increases, and the desired ductility may not be obtained. Therefore, when Mo is contained, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.5% or less, and further preferably 0.3% or less.
Ｓｂ：０．１％以下
　Ｓｂは、焼鈍中の鋼板表面近傍でのＣの拡散を抑制し、鋼板表面近傍における軟質層の形成を制御するために有効な元素である。鋼板表面近傍において軟質層が過度に増加すると、ＴＳを７８０ＭＰａ以上とすることが困難な場合がある。そのため、Ｓｂ含有量を０．００２％以上とすることが好ましい。Ｓｂ含有量は、より好ましくは０．００５％以上である。一方、Ｓｂ含有量が０．１％を超えると、鋳造性が低下する。したがって、Ｓｂを含有させる場合、Ｓｂ含有量は０．１％以下が好ましい。Ｓｂ含有量は、より好ましくは０．０６％以下、さらに好ましくは０．０４％以下である。 Sb: 0.1% or less Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to make the TS 780 MPa or more. Therefore, it is preferable to set the Sb content to 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.1%, the castability decreases. Therefore, when Sb is contained, the Sb content is preferably 0.1% or less. The Sb content is more preferably 0.06% or less, and further preferably 0.04% or less.
Ｓｎ：０．１％以下
　Ｓｎは、鋼板表面近傍での酸化や窒化を抑制し、それによる鋼板表面近傍でのＣやＢの含有量の低下を抑制する。これにより、鋼板表面近傍において過度にフェライトが生成することが抑制され、ＴＳを７８０ＭＰａ以上とすることに寄与する。このような観点から、Ｓｎ含有量は０．００２％以上とすることが好ましい。ただし、Ｓｎ含有量が０．１％を超えると、鋳造性が低下する。したがって、Ｓｎを含有させる場合、Ｓｎ含有量は０．１％以下が好ましい。Ｓｎ含有量は、より好ましくは０．０４％以下、さらに好ましくは０．０２％以下である。 Sn: 0.1% or less Sn suppresses oxidation and nitridation near the steel sheet surface, thereby suppressing the decrease in the content of C and B near the steel sheet surface. This suppresses excessive generation of ferrite near the steel sheet surface, contributing to making the TS 780 MPa or more. From this perspective, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.1%, the castability decreases. Therefore, when Sn is contained, the Sn content is preferably 0.1% or less. The Sn content is more preferably 0.04% or less, and even more preferably 0.02% or less.
Ｃｕ：１．０％以下
　Ｃｕは、焼入れ性を高めてマルテンサイトの生成を促し、これにより、ＴＳを上昇させる元素である。このような効果を得るためには、Ｃｕ含有量を０．００５％以上にすることが好ましい。Ｃｕ含有量は、より好ましくは０．０２０％以上である。一方、Ｃｕ含有量が１．０％を超えると、マルテンサイトの面積率が過度に増加して延性を低下させるおそれがある。また、粗大な析出物や介在物が多量に生成する場合があり、このような粗大な析出物や介在物が、延性を低下させるおそれもある。したがって、Ｃｕを含有させる場合、Ｃｕ含有量は１．０％以下が好ましい。Ｃｕ含有量は、より好ましくは０．２％以下である。 Cu: 1.0% or less Cu is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Cu content to 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.0%, the area ratio of martensite increases excessively, which may reduce ductility. In addition, a large amount of coarse precipitates and inclusions may be generated, and such coarse precipitates and inclusions may reduce ductility. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less. The Cu content is more preferably 0.2% or less.
Ｔａ：０．１％以下
　Ｔａは、Ｔｉ、ＮｂおよびＶと同様に、熱間圧延工程や焼鈍工程において、微細な析出物、例えば、炭化物や窒化物、炭窒化物を形成することによって、ＴＳを上昇させる。加えて、Ｔａは、Ｎｂ炭化物やＮｂ炭窒化物に一部固溶し、（Ｎｂ，Ｔａ）（Ｃ，Ｎ）のような複合析出物を生成する。これにより、析出物の粗大化を抑制し、析出強化を安定化させる。これにより、ＴＳをさらに向上させる。このような効果を得るためには、Ｔａ含有量を０．００１％以上とすることが好ましい。一方、Ｔａ含有量が０．１％を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合、粗大な析出物や介在物が、延性、さらには曲げ性を低下させる場合がある。したがって、Ｔａを含有させる場合、Ｔａ含有量は０．１％以下が好ましい。Ｔａ含有量は、より好ましくは０．０５％以下である。 Ta: 0.1% or less Ta, like Ti, Nb and V, increases TS by forming fine precipitates, such as carbides, nitrides and carbonitrides, in the hot rolling process and annealing process. In addition, Ta is partially dissolved in Nb carbides and Nb carbonitrides to generate composite precipitates such as (Nb, Ta) (C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS. In order to obtain such an effect, it is preferable that the Ta content is 0.001% or more. On the other hand, if the Ta content exceeds 0.1%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may reduce ductility and even bendability. Therefore, when Ta is contained, the Ta content is preferably 0.1% or less. The Ta content is more preferably 0.05% or less.
Ｗ：０．２％以下
　Ｗは、Ｔｉ、ＮｂおよびＶと同様に、熱間圧延工程や焼鈍工程において、微細な析出物、例えば、炭化物や窒化物、炭窒化物を形成することによって、ＴＳを上昇させる。このような効果を得るためには、Ｗ含有量を０．００１％以上とすることが好ましい。Ｗ含有量は、より好ましくは０．００５％以上である。一方、Ｗ含有量が０．２％を超えると、粗大な析出物や介在物が多量に生成し、延性の低下を招く。したがって、Ｗを含有させる場合、Ｗ含有量は０．２％以下が好ましい。Ｗ含有量は、より好ましくは０．０６０％以下である。 W: 0.2% or less Like Ti, Nb and V, W increases TS by forming fine precipitates, such as carbides, nitrides and carbonitrides, in the hot rolling process and annealing process. In order to obtain such an effect, it is preferable that the W content is 0.001% or more. The W content is more preferably 0.005% or more. On the other hand, if the W content exceeds 0.2%, a large amount of coarse precipitates and inclusions are generated, which leads to a decrease in ductility. Therefore, when W is contained, the W content is preferably 0.2% or less. The W content is more preferably 0.060% or less.
Ｍｇ：０．０１％以下
　Ｍｇは、硫化物や酸化物などの介在物の形状を球状化して鋼板の穴広げ性および曲げ性を向上させるために有効な元素である。このような効果を得るためには、Ｍｇ含有量を０．０００１％以上とすることが好ましい。ただし、Ｍｇ含有量が０．０１％を超えると、表面品質が低下する。また、却って曲げ性の低下を招く場合もある。したがって、Ｍｇを含有させる場合、Ｍｇ含有量は０．０１％以下が好ましい。Ｍｇ含有量は、より好ましくは０．００５％以下、さらに好ましくは０．００１％以下である。 Mg: 0.01% or less Mg is an effective element for spheroidizing the shape of inclusions such as sulfides and oxides to improve the hole expandability and bendability of steel sheets. In order to obtain such an effect, it is preferable to set the Mg content to 0.0001% or more. However, if the Mg content exceeds 0.01%, the surface quality is deteriorated. In addition, it may lead to a deterioration in bendability. Therefore, when Mg is contained, the Mg content is preferably 0.01% or less. The Mg content is more preferably 0.005% or less, and even more preferably 0.001% or less.
Ｚｎ：０．０２％以下
　Ｚｎは、介在物の形状を球状化し、鋼板の曲げ性を向上させるために有効な元素である。このような効果を得るためには、Ｚｎ含有量を０．００１％以上とすることが好ましい。一方、Ｚｎ含有量が０．０２％を超えると、粗大な析出物や介在物が多量に生成し、却って曲げ性の低下を招く場合がある。したがって、Ｚｎを含有させる場合、Ｚｎ含有量は０．０２％以下が好ましい。 Zn: 0.02% or less Zn is an effective element for making the shape of inclusions spherical and improving the bendability of the steel sheet. In order to obtain such an effect, the Zn content is preferably 0.001% or more. On the other hand, if the Zn content exceeds 0.02%, a large amount of coarse precipitates and inclusions are generated, which may lead to a decrease in bendability. Therefore, when Zn is contained, the Zn content is preferably 0.02% or less.
Ｃｏ：０．０２％以下
　Ｃｏは、Ｚｎと同様、介在物の形状を球状化し、鋼板の曲げ性を向上させるために有効な元素である。このような効果を得るためには、Ｃｏ含有量を０．００１％以上とすることが好ましい。一方、Ｃｏ含有量が０．０２％を超えると、粗大な析出物や介在物が多量に生成し、却って曲げ性の低下を招く場合がある。したがって、Ｃｏを含有させる場合、Ｃｏ含有量は０．０２％以下が好ましい。 Co: 0.02% or less Like Zn, Co is an effective element for making the shape of inclusions spherical and improving the bendability of the steel sheet. In order to obtain such an effect, it is preferable that the Co content is 0.001% or more. On the other hand, if the Co content exceeds 0.02%, a large amount of coarse precipitates and inclusions are generated, which may lead to a decrease in bendability. Therefore, when Co is contained, the Co content is preferably 0.02% or less.
Ｚｒ：０．２％以下
　Ｚｒは、旧オーステナイト粒の微細化を通じて高強度化に寄与する。また、Ｚｒは、旧オーステナイト粒の微細化によるマルテンサイトやベイナイトの内部構造単位であるブロックサイズ、ベイン粒径等の低減を通じて高強度化に寄与する。さらに、Ｚｒは、鋳造性を改善する。このような効果を得るためには、Ｚｒ含有量を０．００１％以上とすることが好ましい。ただし、Ｚｒを多量に添加すると、スラブ加熱工程において未固溶で残存するＺｒＮ系やＺｒＳ系の粗大な析出物が増加し、延性が低下する。したがって、Ｚｒを含有させる場合、Ｚｒ含有量は０．２％以下が好ましい。Ｚｒ含有量は、より好ましくは０．０５％以下、さらに好ましくは０．０１％以下である。 Zr: 0.2% or less Zr contributes to high strength through refinement of prior austenite grains. Zr also contributes to high strength through reduction of block size and vein grain size, which are internal structural units of martensite and bainite, by refinement of prior austenite grains. Furthermore, Zr improves castability. In order to obtain such an effect, it is preferable that the Zr content is 0.001% or more. However, if a large amount of Zr is added, the amount of coarse precipitates of ZrN and ZrS that remain in an undissolved state in the slab heating process increases, and ductility decreases. Therefore, when Zr is contained, the Zr content is preferably 0.2% or less. The Zr content is more preferably 0.05% or less, and further preferably 0.01% or less.
Ｃａ：０．０２％以下
　Ｃａは、鋼中で介在物として存在する。ここで、Ｃａ含有量が０．０２％を超えると、粗大な介在物が多量に生成して延性および曲げ性が低下するおそれがある。また、表面品質も劣化する。したがって、Ｃａを含有させる場合、Ｃａ含有量は０．０２％以下が好ましい。なお、Ｃａ含有量の下限は特に限定されるものではない。Ｃａ含有量は、例えば、０．０００５％以上が好ましい。また、生産技術上の制約から、Ｃａ含有量は０．００１０％以上がより好ましい。 Ca: 0.02% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.02%, a large amount of coarse inclusions may be generated, which may reduce ductility and bendability. Surface quality may also deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.02% or less. The lower limit of the Ca content is not particularly limited. For example, the Ca content is preferably 0.0005% or more. In addition, due to constraints on production technology, the Ca content is more preferably 0.0010% or more.
Ｓｅ：０．０２％以下、Ｔｅ：０．０２％以下、Ｇｅ：０．０２％以下、Ａｓ：０．０５％以下、Ｓｒ：０．０２％以下、Ｃｓ：０．０２％以下、Ｈｆ：０．０２％以下、Ｐｂ：０．０２％以下、Ｂｉ：０．０２％以下およびＲＥＭ：０．０２％以下
　Ｓｅ、Ｔｅ、Ｇｅ、Ａｓ、Ｓｒ、Ｃｓ、Ｈｆ、Ｐｂ、ＢｉおよびＲＥＭはいずれも、鋼板の曲げ性を向上させるために有効な元素である。このような効果を得るためには、Ｓｅ、Ｔｅ、Ｇｅ、Ａｓ、Ｓｒ、Ｃｓ、Ｈｆ、Ｐｂ、ＢｉおよびＲＥＭの含有量はそれぞれ０．０００１％以上が好ましい。一方、Ｓｅ、Ｔｅ、Ｇｅ、Ｓｒ、Ｃｓ、Ｈｆ、Ｐｂ、ＢｉおよびＲＥＭの含有量がそれぞれ０．０２％を超えると、また、Ａｓの含有量が０．０５％を超えると、粗大な析出物や介在物が多量に生成し、却って曲げ性が低下する場合がある。したがって、これらの元素を含有させる場合、Ｓｅ、Ｔｅ、Ｇｅ、Ｓｒ、Ｃｓ、Ｈｆ、Ｐｂ、ＢｉおよびＲＥＭの含有量はそれぞれ０．０２％以下、Ａｓの含有量は０．０５％以下とすることが好ましい。なお、Ｓｅ、Ｔｅ、Ｇｅ、Ａｓ、Ｓｒ、Ｃｓ、Ｈｆ、Ｐｂ、ＢｉおよびＲＥＭは、それぞれ単独で含有させてもよいし、複合して含有させてもよい。 Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving the bendability of steel sheet. In order to obtain such an effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceed 0.02% each, or if the content of As exceeds 0.05%, a large amount of coarse precipitates and inclusions are generated, and the bendability may be deteriorated. Therefore, when these elements are contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are each 0.02% or less, and the content of As is 0.05% or less. Note that Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM may be contained alone or in combination.
　上記以外の元素は、Ｆｅおよび不可避的不純物である。 Elements other than those mentioned above are Fe and unavoidable impurities.
　つぎに、本発明の一実施形態に従う鋼板の鋼組織について説明する。
　本発明の一実施形態に従う鋼板の鋼組織は、
　フェライトおよびベイナイトの１種または２種の面積率：合計で１０％以上８７％以下、
　マルテンサイトの面積率：１０％以上８７％以下、ならびに、
　残留オーステナイトの面積率：３％以上
であり、
　前記残留オーステナイトを構成する粒子の数密度が０．０５個／μｍ２以上であり、
　前記残留オーステナイトを構成する粒子のうち、アスペクト比：３以下の粒子が上記（Ａ）、（Ｂ）、（Ｃ）および（Ｄ）を満足し、
　［Ｍｎ］Ｃ／［Ｍｎ］が１．０５以上２．００以下であり、［Ｍｎ］Ｃは鋼中のＭｎ濃化領域の平均Ｍｎ濃度（質量％）、［Ｍｎ］は鋼中の平均Ｍｎ濃度（質量％）である、鋼組織である。
　以下、それぞれの限定理由について説明する。なお、各相の面積率は、鋼組織全体の面積に対して各相が占める面積の割合である。 Next, the steel structure of the steel plate according to one embodiment of the present invention will be described.
The steel structure of the steel plate according to one embodiment of the present invention is
Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
Area ratio of martensite: 10% or more and 87% or less, and
Area ratio of retained austenite: 3% or more,
The number density of particles constituting the retained austenite is 0.05 particles/ μm2 or more,
Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the above (A), (B), (C), and (D),
The steel structure has a ratio of [Mn] C /[Mn] of 1.05 to 2.00, [Mn] C being the average Mn concentration (mass %) of Mn-enriched regions in the steel, and [Mn] being the average Mn concentration (mass %) in the steel.
The reasons for each of the limitations will be explained below. The area ratio of each phase is the ratio of the area that each phase occupies to the area of the entire steel structure.
フェライトおよびベイナイトの１種または２種の合計の面積率（以下、フェライトおよびベイナイトの合計面積率ともいう）：１０％以上８７％以下
　フェライトおよびベイナイトは軟質であるため、優れた延性を得るうえで有効である。所望の延性を得るために、フェライトおよびベイナイトの合計面積率を１０％以上とする。一方、フェライトおよびベイナイトの面積率が過剰になると、ＴＳを７８０ＭＰａ以上とすることが困難となる。そのため、フェライトおよびベイナイトの合計面積率を８７％以下とする。フェライトおよびベイナイトの合計面積率は、好ましくは２０％以上、より好ましくは３０％以上である。また、フェライトおよびベイナイトの合計面積率は、好ましくは７５％以下、より好ましくは６５％以下である。なお、フェライトおよびベイナイトはそれぞれ単独で含有されていてもよく、これらの両方が含有されていてもよい。 Total area ratio of one or both of ferrite and bainite (hereinafter also referred to as total area ratio of ferrite and bainite): 10% or more and 87% or less Ferrite and bainite are soft, so they are effective in obtaining excellent ductility. In order to obtain the desired ductility, the total area ratio of ferrite and bainite is set to 10% or more. On the other hand, if the area ratio of ferrite and bainite becomes excessive, it becomes difficult to make TS 780 MPa or more. Therefore, the total area ratio of ferrite and bainite is set to 87% or less. The total area ratio of ferrite and bainite is preferably 20% or more, more preferably 30% or more. In addition, the total area ratio of ferrite and bainite is preferably 75% or less, more preferably 65% or less. In addition, ferrite and bainite may be contained alone, or both of them may be contained.
マルテンサイトの面積率：１０％以上８７％以下
　マルテンサイトは硬質であり、鋼板の高強度化に必要な組織である。ここで、マルテンサイトの面積率が１０％未満になると、所望のＴＳが得られない。一方、マルテンサイトの面積率の過度の増加は、延性の低下の原因となる。したがって、マルテンサイトの面積率は１０％以上８７％以下とする。マルテンサイトの面積率は、好ましくは２０％以上、より好ましくは３０％以上である。また、マルテンサイトの面積率は、好ましくは７５％以下、より好ましくは６５％以下である。
　なお、マルテンサイトとは、マルテンサイト変態点（単にＭｓ点ともいう。）以下でオーステナイトから変態することにより生成する硬質な組織である。また、マルテンサイトは、焼入れままのいわゆるフレッシュマルテンサイトと、該フレッシュマルテンサイトが焼戻されたいわゆる焼戻しマルテンサイトとの両方を含む。 Area ratio of martensite: 10% or more and 87% or less Martensite is hard and is a structure necessary for increasing the strength of steel plate. If the area ratio of martensite is less than 10%, the desired TS cannot be obtained. On the other hand, an excessive increase in the area ratio of martensite causes a decrease in ductility. Therefore, the area ratio of martensite is 10% or more and 87% or less. The area ratio of martensite is preferably 20% or more, more preferably 30% or more. In addition, the area ratio of martensite is preferably 75% or less, more preferably 65% or less.
Martensite is a hard structure that is generated by transformation from austenite at a martensite transformation point (also simply referred to as Ms point) or lower. Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.
残留オーステナイトの面積率：３％以上
　残留オーステナイトは、強度と延性を両立するために必要な組織である。ここで、残留オーステナイトの面積率が３％未満では、強度と延性を両立することができない。したがって、残留オーステナイトの面積率は３％以上とする。残留オーステナイトの面積率は、好ましくは５％以上、より好ましくは７％以上である。残留オーステナイトの面積率の上限は特に規定されない。ただし、残留オーステナイトが過剰になると、例えば、鋼板を部品に成形する際に、残留オーステナイトがマルテンサイト変態し、曲げ割れの起点が増加する。そのため、残留オーステナイトの面積率は、好ましくは２０％以下、より好ましくは１５％以下である。
　なお、残留オーステナイトとは、オーステナイトからフェライト、マルテンサイト、ベイナイトまたはその他の金属相に変態せずに残ったオーステナイトである。また、残留オーステナイトは、例えば、オーステナイト中にＣ等の元素が濃化することによりマルテンサイト変態点が室温以下となることで、生成する（オーステナイトが変態せずに残留する）。 Area ratio of retained austenite: 3% or more Retained austenite is a structure necessary for achieving both strength and ductility. Here, if the area ratio of retained austenite is less than 3%, it is not possible to achieve both strength and ductility. Therefore, the area ratio of retained austenite is set to 3% or more. The area ratio of retained austenite is preferably 5% or more, more preferably 7% or more. There is no particular upper limit for the area ratio of retained austenite. However, if the amount of retained austenite becomes excessive, for example, when the steel plate is formed into a part, the retained austenite is transformed into martensite, and the number of starting points of bending cracks increases. Therefore, the area ratio of retained austenite is preferably 20% or less, more preferably 15% or less.
The term "retained austenite" refers to austenite that does not transform into ferrite, martensite, bainite, or other metallic phases and remains. The retained austenite is generated, for example, when elements such as C are concentrated in austenite, causing the martensite transformation point to be lower than room temperature (austenite remains without transforming).
　上記以外の残部組織の面積率は１５％以下とすることが好ましい。残部組織の面積率は、より好ましくは１０％以下であり、さらに好ましくは５％以下である。また、残部組織の面積率は０％であってもよい。
　なお、残部組織としては、特に限定されず、例えば、パーライト、および、セメンタイトなどの炭化物が挙げられる。なお、残部組織の種類は、例えば、ＳＥＭ（Ｓｃａｎｎｉｎｇ　Ｅｌｅｃｔｒｏｎ　Ｍｉｃｒｏｓｃｏｐｅ；走査電子顕微鏡）による観察で確認することができる。なお、パーライトは、比較的高温でオーステナイトから生成し、層状のフェライトとセメンタイトからなる組織である。 The area ratio of the remaining structure other than the above is preferably 15% or less. The area ratio of the remaining structure is more preferably 10% or less, and further preferably 5% or less. The area ratio of the remaining structure may be 0%.
The remaining structure is not particularly limited, and examples thereof include carbides such as pearlite and cementite. The type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM). Pearlite is formed from austenite at a relatively high temperature and is a structure consisting of lamellar ferrite and cementite.
　ここで、フェライトおよびベイナイトの合計面積率、ならびに、マルテンサイトの面積率は、鋼板の板厚１／４位置において、以下のように測定する。
　すなわち、鋼板の圧延方向に平行な板厚断面（Ｌ断面）が観察面となるように、鋼板から試料を切り出す。ついで、ダイヤモンドペーストを用いて試料の観察面を研磨し、ついで、アルミナを用いて試料の観察面を仕上げ研磨する。ついで、試料の観察面をナイタールでエッチングし、組織を現出させる。そして、ＳＥＭ（Ｓｃａｎｎｉｎｇ　Ｅｌｅｃｔｒｏｎ　Ｍｉｃｒｏｓｃｏｐｅ；走査電子顕微鏡）により、倍率：１５００倍の条件で、試料の観察面を５視野観察する。ついで、得られた組織画像から、Ａｄｏｂｅ　Ｓｙｓｔｅｍｓ社のＡｄｏｂｅ　Ｐｈｏｔｏｓｈｏｐを用いて以下の領域を色分け（画定）する。そして、ポイントカウンティング法により、フェライトおよびベイナイトの合計面積率、ならびに、マルテンサイトの面積率を算出する。具体的には、各ＳＥＭ像の実長さ：８２μｍ×５７μｍの領域において、４．８μｍの間隔で１６×１５の格子点を設置する。ついで、フェライトおよびベイナイト、ならびに、マルテンサイト上の格子点の数をそれぞれ数える。ついで、フェライトおよびベイナイト、ならびに、マルテンサイト上の格子点の数をそれぞれ、全ての格子点の数で除し、１００を乗じることにより、フェライトおよびベイナイトの合計面積率、ならびに、マルテンサイトの面積率を算出する。
フェライト：黒色を呈した領域であり、形態は塊状である。また、フェライトは、ＢＣＣ格子の結晶粒からなる組織である。フェライトは、比較的高温においてオーステナイトからの変態により生成する。
ベイナイト：黒色から濃い灰色を呈した領域であり、形態は塊状や不定形などである。また、ベイナイトは、上述したように、針状又は板状のフェライト中に微細な炭化物が分散した硬質な組織である。ベイナイトは、比較的低温（Ｍｓ点以上）でオーステナイトから生成する。また、ベイナイトは、炭化物を比較的少数内包する。
マルテンサイト：白色から薄い灰色を呈した領域である。また、マルテンサイトは、上述したように、Ｍｓ点以下でオーステナイトから変態することにより生成する硬質な組織である。マルテンサイトは、焼入れままのいわゆるフレッシュマルテンサイトと、該フレッシュマルテンサイトが焼戻されたいわゆる焼戻しマルテンサイトとの両方を含む。 Here, the total area ratio of ferrite and bainite and the area ratio of martensite are measured at a 1/4 position of the sheet thickness of the steel sheet as follows.
That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste, and then the observation surface of the sample is finish-polished using alumina. Next, the observation surface of the sample is etched with nital to reveal the structure. Then, the observation surface of the sample is observed in five fields of view at a magnification of 1500 times using a SEM (Scanning Electron Microscope). Next, the following regions are color-coded (defined) from the obtained structure image using Adobe Photoshop from Adobe Systems. Then, the total area ratio of ferrite and bainite, and the area ratio of martensite are calculated by the point counting method. Specifically, 16×15 lattice points are set at intervals of 4.8 μm in an area of 82 μm×57 μm in actual length of each SEM image. Then, the number of lattice points on ferrite, bainite, and martensite is counted. Then, the number of lattice points on ferrite, bainite, and martensite is divided by the number of all lattice points, and multiplied by 100 to calculate the total area ratio of ferrite and bainite, and the area ratio of martensite.
Ferrite: A black region with a blocky shape. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite at relatively high temperatures.
Bainite: A region that is black to dark gray in color and has a blocky or amorphous shape. As described above, bainite is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite. Bainite is formed from austenite at a relatively low temperature (Ms point or higher). Bainite contains a relatively small number of carbides.
Martensite: A region that is white to light gray in color. As described above, martensite is a hard structure that is generated by transformation from austenite at or below the Ms point. Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.
　また、残留オーステナイトの面積率は、鋼板の板厚１／４位置において、以下のように測定する。
　すなわち、鋼板を板厚方向（深さ方向）に板厚の１／４位置まで機械研削した後、シュウ酸による化学研磨を行い、観察面とする。ついで、観察面を、Ｘ線回折法により観察する。入射Ｘ線にはＣｏＫα線を使用し、ｂｃｃ鉄の（２００）、（２１１）および（２２０）各面の回折強度に対するｆｃｃ鉄（オーステナイト）の（２００）、（２２０）および（３１１）各面の回折強度の比を求める。ついで、各面の回折強度の比から、残留オーステナイトの体積率を算出する。そして、残留オーステナイトが三次元的に均質であるとみなして、残留オーステナイトの体積率を、残留オーステナイトの面積率とする。 The area ratio of retained austenite is measured at a 1/4 position of the sheet thickness of the steel sheet as follows.
That is, the steel plate is mechanically ground in the plate thickness direction (depth direction) to a position of 1/4 of the plate thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the ratio of the diffraction intensity of each of the (200), (220) and (311) faces of fcc iron (austenite) to the diffraction intensity of each of the (200), (211) and (220) faces of bcc iron is obtained. Next, the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face. Then, the retained austenite is considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.
　さらに、残部組織の面積率は、１００％から上記のようにして求めたフェライトおよびベイナイトの合計面積率、マルテンサイトの面積率、ならびに、残留オーステナイトの面積率を減じることにより求める。
　［残部組織の面積率（％）］＝１００－［フェライトおよびベイナイトの合計面積率（％）］－［マルテンサイトの面積率（％）］－［残留オーステナイトの面積率（％）］ Furthermore, the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite determined as described above from 100%.
[Area ratio of remaining structure (%)] = 100 - [Total area ratio of ferrite and bainite (%)] - [Area ratio of martensite (%)] - [Area ratio of retained austenite (%)]
残留オーステナイトを構成する粒子の数密度（以下、残留オーステナイトの粒子数密度ともいう）：０．０５個／μｍ２以上
　本発明の一実施形態に従う鋼板では、残留オーステナイトの粒子数密度を０．０５個／μｍ２以上とすることが重要である。鋼板を部品に成形する加工（以下、単に加工ともいう）の際に残留オーステナイトから加工誘起変態して生じる硬質なマルテンサイト（以下、加工誘起マルテンサイトともいう）は、加工時のボイド生成およびき裂の進展を助長する組織である。しかし、残留オーステナイトの粒子数密度を０．０５個／μｍ２にすることにより、加工誘起マルテンサイトへの応力が分散して応力集中を抑制し、ボイドの生成およびき裂の進展を抑制することができる。したがって、残留オーステナイトの粒子数密度は０．０５個／μｍ２以上とする。残留オーステナイトの粒子数密度は、好ましくは０．１５個／μｍ２以上、より好ましくは０．２５個／μｍ２以上である。残留オーステナイトの粒子数密度の上限は、特に限定されるものではない。ただし、残留オーステナイトの粒子数密度が過剰になると、曲げ割れの起点数の増加につながる。そのため、残留オーステナイトの粒子数密度は、好ましくは１００個／μｍ２以下、より好ましくは１０個／μｍ２以下である。 Number density of particles constituting the retained austenite (hereinafter also referred to as the particle number density of the retained austenite): 0.05 pieces/μm 2 or more In a steel sheet according to one embodiment of the present invention, it is important to set the particle number density of the retained austenite to 0.05 pieces/μm 2 or more. The hard martensite (hereinafter also referred to as the processing-induced martensite) generated by processing-induced transformation from the retained austenite during processing (hereinafter also referred to simply as processing) to form the steel sheet into a part is a structure that promotes the generation of voids and the growth of cracks during processing. However, by setting the particle number density of the retained austenite to 0.05 pieces/μm 2 , the stress on the processing-induced martensite is dispersed to suppress the stress concentration, and the generation of voids and the growth of cracks can be suppressed. Therefore, the particle number density of the retained austenite is set to 0.05 pieces/μm 2 or more. The particle number density of the retained austenite is preferably 0.15 pieces/μm 2 or more, more preferably 0.25 pieces/μm 2 or more. The upper limit of the particle number density of the retained austenite is not particularly limited. However, if the particle number density of the retained austenite becomes excessive, the number of bending crack initiation points increases. Therefore, the particle number density of the retained austenite is preferably 100 particles/ μm2 or less, more preferably 10 particles/ μm2 or less.
（Ａ）残留オーステナイトを構成する全ての粒子に対する、アスペクト比：３以下の粒子の比率（以下、アスペクト比：３以下の粒子比率ともいう）：面積率で６０％以上
　残留オーステナイトのアスペクト比が大きくなると、加工により生じる加工誘起マルテンサイト周辺に応力集中が起こりやすい。これにより、ボイド生成およびき裂の進展を助長し、延性および曲げ性が低下する。そのため、アスペクト比：３以下の粒子比率は、面積率で６０％以上とする。アスペクト比：３以下の粒子比率は、面積率で好ましくは６５％以上、より好ましくは７０％以上である。アスペクト比：３以下の粒子比率の上限は、特に限定されず、面積率で１００％であってもよい。 (A) The ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite (hereinafter also referred to as the ratio of particles with an aspect ratio of 3 or less): 60% or more in area ratio. When the aspect ratio of the retained austenite is large, stress concentration is likely to occur around the processing-induced martensite generated by processing. This promotes the generation of voids and the propagation of cracks, and reduces ductility and bendability. Therefore, the ratio of particles with an aspect ratio of 3 or less is set to 60% or more in area ratio. The ratio of particles with an aspect ratio of 3 or less is preferably 65% or more, more preferably 70% or more in area ratio. The upper limit of the ratio of particles with an aspect ratio of 3 or less is not particularly limited, and may be 100% in area ratio.
（Ｂ）アスペクト比：３以下の粒子の平均Ｃ濃度：０．３質量％以上
　本発明の一実施形態に従う鋼板では、アスペクト比：３以下の粒子の平均Ｃ濃度を０．３質量％以上とすることが重要である。すなわち、アスペクト比：３以下の粒子の平均Ｃ濃度が高いほど、残留オーステナイトの安定性が高く、優れた強度と延性のバランスが得られる。アスペクト比：３以下の粒子の平均Ｃ濃度が０．３質量％未満では、良好な強度と延性のバランスが得られない。さらに、残留オーステナイトの安定性が低いため、例えば、加工により加工誘起マルテンサイトになる残留オーステナイトが増加し、曲げ性が低下する。したがって、アスペクト比：３以下の粒子の平均Ｃ濃度は０．３質量％以上とする。アスペクト比：３以下の粒子の平均Ｃ濃度は、好ましくは０．５質量％以上、より好ましくは０．７質量％以上である。なお、アスペクト比：３以下の粒子の平均Ｃ濃度の上限は特に限定されるものではない。ただし、アスペクト比：３以下の粒子のＣ濃度が過剰に高まると、残留オーステナイトからマルテンサイトへの加工誘起変態の進行が過度に抑制され、十分な加工硬化能を得られないおそれがある。よって、アスペクト比：３以下の粒子の平均Ｃ濃度は２．０質量％以下が好ましい。 (B) Average C concentration of particles with aspect ratio of 3 or less: 0.3 mass% or more In the steel sheet according to one embodiment of the present invention, it is important to set the average C concentration of particles with aspect ratio of 3 or less to 0.3 mass% or more. That is, the higher the average C concentration of particles with aspect ratio of 3 or less, the higher the stability of the retained austenite, and an excellent balance between strength and ductility can be obtained. If the average C concentration of particles with aspect ratio of 3 or less is less than 0.3 mass%, a good balance between strength and ductility cannot be obtained. Furthermore, since the stability of the retained austenite is low, for example, the amount of retained austenite that becomes processing-induced martensite by processing increases, and the bendability decreases. Therefore, the average C concentration of particles with aspect ratio of 3 or less is set to 0.3 mass% or more. The average C concentration of particles with aspect ratio of 3 or less is preferably 0.5 mass% or more, more preferably 0.7 mass% or more. The upper limit of the average C concentration of particles with aspect ratio of 3 or less is not particularly limited. However, if the C concentration of particles having an aspect ratio of 3 or less is excessively high, the progress of the work-induced transformation from retained austenite to martensite may be excessively suppressed, and sufficient work hardening ability may not be obtained. Therefore, the average C concentration of particles having an aspect ratio of 3 or less is preferably 2.0 mass% or less.
（Ｃ）アスペクト比：３以下の粒子間の最短距離の平均値（以下、粒子間距離平均値ともいう）：５μｍ以下
　本発明の一実施形態に従う鋼板では、残留オーステナイトの粒子を均一に分散させることが重要である。残留オーステナイトの粒子、特に、アスペクト比：３以下の粒子を均一に分散させることにより、加工により生じる加工誘起変態マルテンサイトへの応力集中が抑制され、延性や曲げ性が向上する。そのためには、粒子間距離平均値を５μｍ以下とすることが重要である。粒子間距離平均値は、好ましくは４μｍ以下であり、より好ましくは３μｍ以下である。なお、粒子間距離平均値の下限は特に限定されるものではない。ただし、コストや生産性の観点から、粒子間距離平均値は０．８μｍ以上が好ましい。 (C) Average value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the average interparticle distance value): 5 μm or less In a steel sheet according to one embodiment of the present invention, it is important to uniformly disperse the particles of retained austenite. By uniformly dispersing the particles of retained austenite, particularly the particles with an aspect ratio of 3 or less, stress concentration on the processing-induced transformation martensite caused by processing is suppressed, and ductility and bendability are improved. For this purpose, it is important to set the average interparticle distance value to 5 μm or less. The average interparticle distance value is preferably 4 μm or less, more preferably 3 μm or less. The lower limit of the average interparticle distance value is not particularly limited. However, from the viewpoint of cost and productivity, the average interparticle distance value is preferably 0.8 μm or more.
（Ｄ）アスペクト比：３以下の粒子間の最短距離の最大値（以下、粒子間距離最大値ともいう）：１５μｍ以下
　本発明の一実施形態に従う鋼板では、残留オーステナイトを均一に分散させることが重要である。残留オーステナイトの粒子、特に、アスペクト比：３以下の粒子を均一に分散させることにより、加工により生じる加工誘起変態マルテンサイトへの応力集中が抑制され、延性や曲げ性が向上する。ここで、粒子間距離最大値が大きい、特に１５μｍを超えると、残留オーステナイトの粒子が局所的に存在することになり、延性および曲げ性の低下を招く。そのため、粒子間距離最大値は１５μｍ以下とする。粒子間距離最大値は、好ましくは１０μｍ以下、より好ましくは７μｍ以下である。なお、粒子間距離最大値の下限は、特に限定されない。例えば、粒子間距離最大値は１μｍ以上が好ましい。 (D) Maximum value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the maximum interparticle distance value): 15 μm or less In a steel sheet according to one embodiment of the present invention, it is important to uniformly disperse the retained austenite. By uniformly dispersing the particles of the retained austenite, particularly the particles with an aspect ratio of 3 or less, stress concentration on the processing-induced transformation martensite caused by processing is suppressed, and ductility and bendability are improved. Here, if the maximum interparticle distance value is large, particularly exceeding 15 μm, the particles of the retained austenite will be present locally, resulting in a decrease in ductility and bendability. Therefore, the maximum interparticle distance value is set to 15 μm or less. The maximum interparticle distance value is preferably 10 μm or less, more preferably 7 μm or less. The lower limit of the maximum interparticle distance value is not particularly limited. For example, the maximum interparticle distance value is preferably 1 μm or more.
　ここで、残留オーステナイトの粒子数密度、アスペクト比：３以下の粒子比率、アスペクト比：３以下の粒子の平均Ｃ濃度、粒子間距離平均値および粒子間距離最大値は、ＦＥ－ＳＥＭに付属するＥＢＳＤ（電子線後方散乱回折法）により、それぞれ以下のようにして求める。
　すなわち、鋼板の圧延方向に平行な板厚断面（Ｌ断面）が観察面となるように、鋼板から試料を切り出す。ついで、ダイヤモンドペーストを用いて試料の観察面を研磨する。ついで、アルミナを用いて試料の観察面を仕上げ研磨する。ついで、鋼板の板厚１／４位置を観察位置として、５０μｍ×５０μｍの領域をＥＢＳＤで観察する。そして、ｐｈａｓｅ　ｍａｐを用いて、ＩｍａｇｅＪにより画像解析し、粒子解析機能により、観察される残留オーステナイトの各粒子のアスペクト比および重心位置を求める。参考のため、図１に観察画像の一例を示す。観察画像において、白色の領域が残留オーステナイトの粒子である。なお、残留オーステナイトを構成する粒子は、円相当径で０．８μｍ以上のものを対象とする。そして、残留オーステナイトを構成する粒子の数をカウントし、当該粒子の数を観察領域の面積で除することにより、残留オーステナイトの粒子数密度を求める。また、上記でカウントした残留オーステナイトを構成する全ての粒子のうち、アスペクト比：３以下の粒子が占める面積を、残留オーステナイトを構成する全ての粒子が占める面積で除し、１００を乗じることにより、アスペクト比：３以下の粒子比率を求める。さらに、アスペクト比：３以下の粒子について、それぞれ最も近接するアスペクト比：３以下の粒子との距離（粒子の重心間距離）を求める。そして、算出した距離の平均値および最大値をそれぞれ、粒子間距離平均値および粒子間距離最大値とする。 Here, the particle number density of the retained austenite, the ratio of particles having an aspect ratio of 3 or less, the average C concentration of particles having an aspect ratio of 3 or less, the average interparticle distance, and the maximum interparticle distance are each determined as follows using EBSD (electron backscatter diffraction) attached to an FE-SEM.
That is, a sample is cut out from the steel plate so that the plate thickness section (L section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, the observation position is set to 1/4 of the plate thickness position of the steel plate, and a region of 50 μm x 50 μm is observed by EBSD. Then, using phase map, image analysis is performed by ImageJ, and the aspect ratio and center of gravity position of each particle of the observed retained austenite are obtained by the particle analysis function. For reference, an example of the observation image is shown in FIG. 1. In the observation image, the white area is the particle of the retained austenite. Note that the particles constituting the retained austenite are those having a circle equivalent diameter of 0.8 μm or more. Then, the number of particles constituting the retained austenite is counted, and the number of particles is divided by the area of the observation region to obtain the particle number density of the retained austenite. The area occupied by particles having an aspect ratio of 3 or less among all the particles constituting the retained austenite counted above is divided by the area occupied by all the particles constituting the retained austenite, and the result is multiplied by 100 to determine the ratio of particles with an aspect ratio of 3 or less. Furthermore, for each particle having an aspect ratio of 3 or less, the distance to the nearest particle having an aspect ratio of 3 or less (the distance between the centers of gravity of the particles) is determined. The average and maximum of the calculated distances are then taken as the average interparticle distance and the maximum interparticle distance, respectively.
　また、アスペクト比：３以下の粒子の平均Ｃ濃度は、以下のようにして求める。
　すなわち、上記のＥＢＳＤによる観察で使用した試料について、ＥＢＳＤと同様に鋼板の板厚１／４位置を観察位置として、２３μｍ角の領域において測定間隔：０．１μｍで格子状にＣ濃度を測定する。ついで、ＥＢＳＤのｐｈａｓｅ　ｍａｐからアスペクト比：３以下の粒子の領域を抽出し、当該領域にある各測定点のＣ濃度の平均値を、アスペクト比：３以下の粒子の平均Ｃ濃度とする。 The average C concentration of particles having an aspect ratio of 3 or less is determined as follows.
That is, for the sample used in the above EBSD observation, the observation position is set to ¼ of the sheet thickness position of the steel sheet in the same manner as in EBSD, and the C concentration is measured in a grid pattern in a 23 μm square area with measurement intervals of 0.1 μm. Next, an area of particles with an aspect ratio of 3 or less is extracted from the EBSD phase map, and the average value of the C concentration at each measurement point in that area is taken as the average C concentration of particles with an aspect ratio of 3 or less.
［Ｍｎ］Ｃ／［Ｍｎ］：１．０５以上２．００以下
　鋼中の平均Ｍｎ濃度に対する、鋼中のＭｎ濃化領域の平均Ｍｎ濃度の比である［Ｍｎ］Ｃ／［Ｍｎ］が大きいということは、焼鈍工程においてオーステナイトへのＭｎの濃化が進行したことを意味する。そして、焼鈍工程を経た直後の鋼板におけるオーステナイトのＭｎ濃度は、焼鈍工程後の冷却工程および滞留工程においてオーステナイトから変態する相が、フェライトおよびベイナイトかマルテンサイトかを決める要因の１つとなる。ここで、焼鈍工程においてオーステナイトにＭｎが過度に濃化すると、冷却工程におけるフェライト変態およびベイナイト変態の遅延を招く。これにより、所望のフェライトおよびベイナイトの面積率が得られず、延性および曲げ性が低下するおそれがある。また、フェライト変態およびベイナイト変態の遅延により、未変態オーステナイトへのＣ濃化が抑制される。そのため、延性向上に寄与する残留オーステナイトが十分に得られない。したがって、［Ｍｎ］Ｃ／［Ｍｎ］は２．００以下とする。［Ｍｎ］Ｃ／［Ｍｎ］は、好ましくは１．８０以下、より好ましくは１．６０以下である。一方、［Ｍｎ］Ｃ／［Ｍｎ］が１．０５以上、すなわち、Ｍｎ濃度の分布が適度に不均一である場合、Ｍｎ濃度が低いオーステナイトにおいてフェライト変態およびベイナイト変態が促進される。それに伴って、未変態オーステナイトへのＣ濃化が進行する。これにより、良好な延性および曲げ性が得られる。そのため、［Ｍｎ］Ｃ／［Ｍｎ］は１．０５以上とする。［Ｍｎ］Ｃ／［Ｍｎ］は、好ましくは１．１０以上、より好ましくは１．１５以上である。 [Mn] C /[Mn]: 1.05 or more and 2.00 or less The fact that [Mn] C /[Mn], which is the ratio of the average Mn concentration of the Mn-enriched region in the steel to the average Mn concentration in the steel, is large means that the enrichment of Mn in the austenite has progressed in the annealing process. The Mn concentration of the austenite in the steel sheet immediately after the annealing process is one of the factors that determine whether the phase transformed from the austenite in the cooling process and the retention process after the annealing process is ferrite and bainite or martensite. Here, if Mn is excessively enriched in the austenite in the annealing process, it leads to a delay in the ferrite transformation and bainite transformation in the cooling process. As a result, the desired area ratio of ferrite and bainite cannot be obtained, and ductility and bendability may be reduced. In addition, the delay in the ferrite transformation and the bainite transformation suppresses the enrichment of C in the untransformed austenite. Therefore, the retained austenite that contributes to improving ductility is not obtained sufficiently. Therefore, [Mn] C /[Mn] is set to 2.00 or less. [Mn] C /[Mn] is preferably set to 1.80 or less, more preferably set to 1.60 or less. On the other hand, when [Mn] C /[Mn] is set to 1.05 or more, that is, when the distribution of Mn concentration is moderately non-uniform, ferrite transformation and bainite transformation are promoted in austenite with a low Mn concentration. Accordingly, C concentration in untransformed austenite progresses. This results in good ductility and bendability. Therefore, [Mn] C /[Mn] is set to 1.05 or more. [Mn] C /[Mn] is preferably set to 1.10 or more, more preferably set to 1.15 or more.
　ここで、［Ｍｎ］Ｃ／［Ｍｎ］は、以下のようにして求める。
　すなわち、鋼板の圧延方向に平行な板厚断面（Ｌ断面）が観察面となるように、鋼板から試料を切り出す。ついで、ダイヤモンドペーストを用いて試料の観察面を研磨する。ついで、アルミナを用いて試料の観察面を仕上げ研磨する。ついで、鋼板の板厚１／４位置を観察位置とし、ＥＰＭＡにより、２３μｍ角の領域において測定間隔：０．１μｍで格子状にＭｎ濃度を測定する。そして、全測定点のＭｎ濃度の平均値を、鋼中の平均Ｍｎ濃度［Ｍｎ］（質量％）とする。また、全測定点のうち、Ｍｎ濃度の高い上位１０％まで領域をＭｎ濃化領域とする。そして、Ｍｎ濃化領域で測定したＭｎ濃度の平均値を、鋼中のＭｎ濃化領域の平均Ｍｎ濃度［Ｍｎ］Ｃ（質量％）とする。そして、［Ｍｎ］Ｃを［Ｍｎ］で除することにより、［Ｍｎ］Ｃ／［Ｍｎ］を求める。 Here, [Mn] C /[Mn] is calculated as follows.
That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, the observation position is set to 1/4 of the plate thickness of the steel plate, and the Mn concentration is measured in a grid pattern with a measurement interval of 0.1 μm in a 23 μm square area by EPMA. Then, the average value of the Mn concentration of all the measurement points is set as the average Mn concentration [Mn] (mass%) in the steel. In addition, the top 10% of the areas with the highest Mn concentration among all the measurement points are set as the Mn-enriched areas. Then, the average value of the Mn concentration measured in the Mn-enriched areas is set as the average Mn concentration [Mn] C (mass%) of the Mn-enriched areas in the steel. Then, [Mn] C is divided by [Mn] to obtain [Mn] C / [Mn].
　また、本発明の一実施形態に従う鋼板では、厚さ：１μｍ以上５０μｍ以下の軟質層を有することが好ましい。特に、鋼板表面から板厚方向において厚さ：１μｍ以上５０μｍ以下の軟質層を有することにより、より優れた曲げ性を得ることができる。そのため、鋼板表面から板厚方向において軟質層を有することが好ましく、また、その厚さを１μｍ以上とすることが好ましい。ただし、軟質層が過剰に形成されると、所望のＴＳが得ることが困難となる。そのため、軟質層を有する場合には、その厚さを５０μｍ以下とすることが好ましい。軟質層の厚さは、より好ましくは４０μｍ以下である。 Furthermore, in a steel sheet according to one embodiment of the present invention, it is preferable to have a soft layer having a thickness of 1 μm or more and 50 μm or less. In particular, by having a soft layer having a thickness of 1 μm or more and 50 μm or less from the surface of the steel sheet in the thickness direction, better bendability can be obtained. Therefore, it is preferable to have a soft layer from the surface of the steel sheet in the thickness direction, and it is also preferable that the thickness is 1 μm or more. However, if an excessive soft layer is formed, it becomes difficult to obtain the desired TS. Therefore, if a soft layer is present, it is preferable that the thickness is 50 μm or less. The thickness of the soft layer is more preferably 40 μm or less.
　ここで、軟質層とは、硬度が鋼板の板厚１／４位置の硬度の６５％以下になる領域である。また、軟質層の厚さは以下のようにして測定する。
　すなわち、鋼板の圧延方向に平行な板厚断面（Ｌ断面）について、湿式研磨により、表面の平滑化を行う。ついで、ビッカース硬度計を用いて、荷重１０ｇｆの条件で、鋼板表面から深さ１μｍの位置より深さ１００μｍの位置まで、板厚（深さ）方向に１μｍ間隔で硬度測定を行う。また、同じ条件で、鋼板表面から深さ１００μｍの位置より板厚中心位置まで、板厚（深さ）方向に２０μｍ間隔で硬度測定を行う。そして、鋼板の板厚１／４位置で得られた硬度を基準硬度とし、鋼板の板厚１／４位置よりも表面側において硬度が基準硬度の６５％以下になる深さ位置を特定する。そして、鋼板の表面から硬度が基準硬度の６５％以下になる最深部の深さ位置までの距離（以下、基準硬度の６５％以下となる領域の深さともいう）を測定する。この測定を圧延方向に３ｍｍ以上の間隔をとって５か所で行い、測定した基準硬度の６５％以下となる領域の深さの平均値を軟質層の厚さとする。
　なお、鋼板の鋼組織は、通常、板厚方向に概ね上下対称となるので、軟質層の厚さの測定では、鋼板の表面（オモテ面および裏面）のうち、任意の一面を代表とする。例えば、鋼板の表面（オモテ面および裏面）のうちの任意の一面を板厚１／４位置などの板厚位置の起点（板厚０位置）とすればよい。なお、軟質層が鋼板の片面のみに存在している場合には、軟質層が存在する面を板厚位置の起点（板厚０位置）とする。また、軟質層の厚さは、一面当たりの厚さである。 The soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the sheet thickness of the steel sheet. The thickness of the soft layer is measured as follows.
That is, the surface of the thickness cross section (L cross section) parallel to the rolling direction of the steel plate is smoothed by wet polishing. Next, using a Vickers hardness tester, hardness measurements are performed at 1 μm intervals in the thickness (depth) direction from a position at a depth of 1 μm from the surface of the steel plate to a position at a depth of 100 μm under a load of 10 gf. Also, under the same conditions, hardness measurements are performed at 20 μm intervals in the thickness (depth) direction from a position at a depth of 100 μm from the surface of the steel plate to the center position of the thickness. Then, the hardness obtained at the 1/4 position of the thickness of the steel plate is set as the reference hardness, and the depth position at which the hardness is 65% or less of the reference hardness on the surface side of the 1/4 position of the thickness of the steel plate is specified. Then, the distance from the surface of the steel plate to the deepest depth position at which the hardness is 65% or less of the reference hardness (hereinafter also referred to as the depth of the region at which the hardness is 65% or less of the reference hardness) is measured. This measurement is carried out at five points spaced at intervals of 3 mm or more in the rolling direction, and the average value of the depth of the region where the hardness is 65% or less of the measured reference hardness is regarded as the thickness of the soft layer.
In addition, since the steel structure of a steel plate is generally approximately symmetrical up and down in the plate thickness direction, any one of the surfaces (front and back) of the steel plate is used as a representative in measuring the thickness of the soft layer. For example, any one of the surfaces (front and back) of the steel plate may be used as the starting point of the plate thickness position (plate thickness 0 position) such as the 1/4 plate thickness position. In addition, when the soft layer is present on only one side of the steel plate, the surface on which the soft layer is present is used as the starting point of the plate thickness position (plate thickness 0 position). The thickness of the soft layer is the thickness per side.
　また、鋼板の長手方向（圧延方向）において、軟質相厚さの変動幅は小さいほどよい。特に、軟質相厚さの変動幅が２０μｍを超えると、鋼板の長手方向で曲げ性のバラつきが発生し、局所的に曲げ性が低下するおそれがある。そのため、軟質相厚さの変動幅は２０μｍ以下とすることが好ましい。軟質相厚さの変動幅は、好ましくは１５μｍ以下、より好ましくは１０μｍ以下である。軟質相厚さの変動幅の下限は特に限定されない。軟質相厚さの変動幅は０μｍであってもよい。
　ここで、軟質相厚さの変動幅は、上記の軟質相厚さの測定において測定した基準硬度の６５％以下となる領域の深さの最大値と最小値の差（最大値－最小値）である。 Moreover, in the longitudinal direction (rolling direction) of the steel sheet, the smaller the fluctuation range of the soft phase thickness, the better. In particular, if the fluctuation range of the soft phase thickness exceeds 20 μm, the bending property may vary in the longitudinal direction of the steel sheet, and the bending property may be locally deteriorated. Therefore, it is preferable that the fluctuation range of the soft phase thickness is 20 μm or less. The fluctuation range of the soft phase thickness is preferably 15 μm or less, more preferably 10 μm or less. The lower limit of the fluctuation range of the soft phase thickness is not particularly limited. The fluctuation range of the soft phase thickness may be 0 μm.
Here, the fluctuation range of the soft phase thickness is the difference between the maximum and minimum values of the depth of the region having a hardness of 65% or less of the reference hardness measured in the above-mentioned measurement of the soft phase thickness (maximum value-minimum value).
　つぎに、本発明の一実施形態に従う鋼板の機械特性について、説明する。 Next, we will explain the mechanical properties of a steel plate according to one embodiment of the present invention.
引張強さ（ＴＳ）：７８０ＭＰａ以上
　本発明の一実施形態に従う鋼板の引張強さは、７８０ＭＰａ以上である。本発明の一実施形態に従う鋼板の引張強さの上限は、特に限定されない。ただし、例えば、本発明の一実施形態に従う鋼板の引張強さは、１１８０ＭＰａ未満が好ましい。 Tensile strength (TS): 780 MPa or more The tensile strength of the steel plate according to one embodiment of the present invention is 780 MPa or more. The upper limit of the tensile strength of the steel plate according to one embodiment of the present invention is not particularly limited. However, for example, the tensile strength of the steel plate according to one embodiment of the present invention is preferably less than 1180 MPa.
　なお、本発明の一実施形態に従う鋼板の降伏応力（ＹＳ）、全伸び（Ｅｌ）およびＲ／ｔについては上述したとおりである。また、引張強さ（ＴＳ）、降伏応力（ＹＳ）、全伸び（Ｅｌ）およびＲ／ｔは、実施例において後述する要領で測定する。 The yield stress (YS), total elongation (El) and R/t of the steel plate according to one embodiment of the present invention are as described above. The tensile strength (TS), yield stress (YS), total elongation (El) and R/t are measured as described later in the examples.
　また、本発明の一実施形態に従う鋼板は、表面に亜鉛めっき層を有していてもよい。亜鉛めっき層は、鋼板の一方の表面のみに設けてもよく、両面に設けてもよい。なお、亜鉛めっき層は、Ｚｎを主成分（Ｚｎ含有量が５０．０質量％以上）とするめっき層を指す。亜鉛めっき層としては、溶融亜鉛めっき層や合金化溶融亜鉛めっき層を例示できる。なお、亜鉛めっき層を有する鋼板は、亜鉛めっき鋼板ということもできる。また、上述した溶融亜鉛めっき層および合金化溶融亜鉛めっきを有する鋼板は、それぞれ溶融亜鉛めっき鋼板（ＧＩ）および合金化溶融亜鉛めっき鋼板（ＧＡ）ということもできる。 Furthermore, the steel sheet according to one embodiment of the present invention may have a zinc plating layer on the surface. The zinc plating layer may be provided on only one surface of the steel sheet, or on both surfaces. Note that the zinc plating layer refers to a plating layer containing Zn as the main component (Zn content of 50.0 mass% or more). Examples of the zinc plating layer include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer. Note that the steel sheet having a zinc plating layer can also be called a zinc-plated steel sheet. Furthermore, the above-mentioned steel sheet having a hot-dip galvanized layer and alloyed hot-dip galvanized plating can also be called a hot-dip galvanized steel sheet (GI) and an alloyed hot-dip galvanized steel sheet (GA), respectively.
　ここで、溶融亜鉛めっき層は、例えば、Ｚｎと、２０．０質量％以下のＦｅ、０．００１質量％以上１．０質量％以下のＡｌにより構成することが好適である。また、溶融亜鉛めっき層には、任意に、Ｐｂ、Ｓｂ、Ｓｉ、Ｓｎ、Ｍｇ、Ｍｎ、Ｎｉ、Ｃｒ、Ｃｏ、Ｃａ、Ｃｕ、Ｌｉ、Ｔｉ、Ｂｅ、ＢｉおよびＲＥＭからなる群から選ばれる１種または２種以上の元素を合計で０．０質量％以上３．５質量％以下含有させてもよい。また、溶融亜鉛めっき層のＦｅ含有量は、より好ましくは７．０質量％未満である。なお、上記の元素以外の残部は、不可避的不純物である。 Here, it is preferable that the hot-dip galvanized layer is composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.
　また、合金化溶融亜鉛めっき層は、例えば、Ｚｎと、２０質量％以下のＦｅ、０．００１質量％以上１．０質量％以下のＡｌにより構成することが好適である。また、合金化溶融亜鉛めっき層には、任意に、Ｐｂ、Ｓｂ、Ｓｉ、Ｓｎ、Ｍｇ、Ｍｎ、Ｎｉ、Ｃｒ、Ｃｏ、Ｃａ、Ｃｕ、Ｌｉ、Ｔｉ、Ｂｅ、ＢｉおよびＲＥＭからなる群から選ばれる１種または２種以上の元素を合計で０．０質量％以上３．５質量％以下含有させてもよい。合金化溶融亜鉛めっき層のＦｅ含有量は、より好ましくは７．０質量％以上、さらに好ましくは８．０質量％以上である。また、合金化溶融亜鉛めっき層のＦｅ含有量は、より好ましくは１５．０質量％以下、さらに好ましくは１２．０質量％以下である。なお、上記の元素以外の残部は、不可避的不純物である。 The galvannealed layer is preferably composed of, for example, Zn, 20% or less by mass of Fe, and 0.001% to 1.0% by mass of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% to 3.5% by mass. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.
　加えて、亜鉛めっき層の片面あたりのめっき付着量は、特に限定されるものではないが、２０ｇ／ｍ２以上８０ｇ／ｍ２以下とすることが好ましい。 In addition, the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g/ m2 or more and 80 g/ m2 or less.
　なお、亜鉛めっき層のめっき付着量は、以下のようにして測定する。
　すなわち、１０質量％塩酸水溶液１Ｌに対し、Ｆｅに対する腐食抑制剤（朝日化学工業（株）製「イビット７００ＢＫ」（登録商標））を０．６ｇ添加した処理液を調製する。ついで、該処理液に、供試材となる鋼板を浸漬し、亜鉛めっき層を溶解させる。そして、溶解前後での供試材の質量減少量を測定し、その値を、鋼板の表面積（めっきで被覆されていた部分の表面積）で除することにより、めっき付着量（ｇ／ｍ２）を算出する。 The plating weight of the zinc plating layer is measured as follows.
That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the test material before and after dissolution is then measured, and the value is divided by the surface area of the steel sheet (the surface area of the part that was covered with plating) to calculate the plating coverage (g/ m2 ).
　なお、本発明の一実施形態に従う鋼板の板厚は、特に限定されないが、好ましくは０．５ｍｍ以上３．５ｍｍ以下である。 The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.5 mm or less.
［２］部材
　つぎに、本発明の一実施形態に従う部材について、説明する。
　本発明の一実施形態に従う部材は、上記の鋼板を用いてなる（素材とする）部材である。例えば、素材である鋼板に、成形加工および接合加工の少なくとも一方を施して部材とする。
　ここで、上記の鋼板は、ＴＳ：７８０ＭＰａ以上であり、かつ、高いＹＳ、優れた延性および優れた曲げ性を有する。そのため、本発明の一実施形態に従う部材は、高強度であり、自動車分野で使用される複雑形状部材に適用して特に好適である。 [2] Members Next, members according to one embodiment of the present invention will be described.
A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member.
Here, the above steel plate has a TS of 780 MPa or more, a high YS, excellent ductility, and excellent bendability. Therefore, the member according to one embodiment of the present invention has high strength and is particularly suitable for application to complex-shaped members used in the automotive field.
［３］鋼板の製造方法
　つぎに、本発明の一実施形態に従う鋼板の製造方法について、説明する。 [3] Manufacturing Method of Steel Sheet Next, a manufacturing method of a steel sheet according to one embodiment of the present invention will be described.
　本発明の一実施形態に従う鋼板の製造方法は、
　上記の成分組成を有する鋼スラブを、
　スラブ加熱温度：１２２０℃以上、および
　スラブ加熱時間：１．０時間以上
の条件で加熱する、スラブ加熱工程と、
　ついで、前記鋼スラブに、
　仕上げ圧延終了温度：８４０℃以上１０００℃以下、
　仕上げ圧延終了温度から７００℃までの温度域での平均冷却速度：１０℃／秒以上、および、
　巻取温度：６２０℃以下
の条件で熱間圧延を施し、熱延鋼板を得る、熱間圧延工程と、
　ついで、前記熱延鋼板に、
　圧下率：２０％以上８０％以下
の条件で冷間圧延を施し、冷延鋼板を得る、冷間圧延工程と、
　ついで、前記冷延鋼板を、
　６００℃から７５０℃までの温度域での平均昇温速度：１℃／秒以上１５℃／秒以下
の条件で昇温する、昇温工程と、
　ついで、前記冷延鋼板を、
　焼鈍温度：７５０℃以上９２０℃以下、および、
　焼鈍時間：１秒以上３０秒以下
の条件で焼鈍する、焼鈍工程と、
　ついで、前記冷延鋼板を、
　焼鈍温度－３０℃から６００℃までの温度域での平均冷却速度：５℃／秒以上１００℃／秒以下、および
　冷却停止温度：４００℃以上６００℃以下
の条件で冷却する、冷却工程と、
　ついで、前記冷延鋼板を、
　４００℃以上６００℃以下の温度域での滞留時間：１秒以上９０秒以下
の条件で滞留させる、滞留工程と、を有するものである。
　なお、上記の各温度は、特に説明がない限り、鋼スラブおよび鋼板の表面温度を意味する。 A method for producing a steel sheet according to one embodiment of the present invention includes the steps of:
A steel slab having the above-mentioned composition is
A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more;
Next, the steel slab is
Finish rolling end temperature: 840°C or more and 1000°C or less,
Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C. / sec or more, and
A hot rolling process in which hot rolling is performed under the condition of a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet;
Next, the hot-rolled steel sheet is
A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
Next, the cold rolled steel sheet is
A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C. of 1° C./sec or more and 15° C./sec or less;
Next, the cold rolled steel sheet is
Annealing temperature: 750°C or higher and 920°C or lower; and
Annealing time: annealing under conditions of 1 second to 30 seconds;
Next, the cold rolled steel sheet is
A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
Next, the cold rolled steel sheet is
and a residence step of maintaining the mixture in a temperature range of 400° C. to 600° C. for a residence time of 1 second to 90 seconds.
Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and the steel plate.
　まず、上記の成分組成を有する鋼スラブを準備する。例えば、鋼素材を溶製して上記の成分組成を有する溶鋼とする。溶製方法は特に限定されず、転炉溶製や電気炉溶製等、公知の溶製方法を用いることができる。ついで、得られた溶鋼を固めて鋼スラブとする。溶鋼から鋼スラブを得る方法は特に限定されない。例えば、連続鋳造法、造塊法または薄スラブ鋳造法等を用いることができる。マクロ偏析を防止する観点から、連続鋳造法が好ましい。また、鋼スラブを製造した後、一旦室温まで冷却し、その後、再度加熱する従来法を適用できる。さらに、直送圧延（鋼スラブを室温まで冷却せずに、温片のままで加熱炉に装入し、熱間圧延する方法）や直接圧延（鋼スラブにわずかの保熱を行った後に直ちに圧延する方法）の省エネルギープロセスも問題なく適用できる。 First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The obtained molten steel is then solidified to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited. For example, a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferable. In addition, a conventional method in which the steel slab is cooled to room temperature after being produced and then heated again can be applied. Furthermore, energy-saving processes such as direct rolling (a method in which the steel slab is not cooled to room temperature, but is charged as a hot piece into a heating furnace and hot rolled) and direct rolling (a method in which the steel slab is immediately rolled after being slightly kept at heat) can also be applied without any problems.
［スラブ加熱工程］
　ついで、鋼スラブを加熱する。この際、本発明の一実施形態に従う鋼板の製造方法では、以下の条件を満足させることが重要である。 [Slab heating process]
Next, the steel slab is heated. At this time, in the method for producing a steel plate according to an embodiment of the present invention, it is important to satisfy the following conditions.
スラブ加熱温度：１２２０℃以上
　スラブ加熱温度が１２２０℃以上になると、鋳造時に生成したＮｂおよびＴｉ系の粗大な析出物が十分に固溶する。これにより、粗大な析出物を低減することができる。また、残留オーステナイトの粒子を均一に分散させる、特に、粒子間距離平均値および粒子間距離最大値を所定の範囲に制御することが可能となる。これにより、最終製品となる鋼板の延性および曲げ性が向上する。そのため、スラブ加熱温度は１２２０℃以上とする。スラブ加熱温度は、好ましくは１２３０℃以上、より好ましくは１２４０℃以上である。スラブ加熱温度の上限は、特に限定されない。例えば、スラブ加熱温度は１４００℃以下が好ましい。なお、スラブ加熱温度は、スラブ加熱工程での鋼スラブの最高到達温度である。 Slab heating temperature: 1220°C or higher When the slab heating temperature is 1220°C or higher, the coarse precipitates of Nb and Ti generated during casting are sufficiently dissolved. This makes it possible to reduce the coarse precipitates. In addition, it is possible to uniformly disperse the particles of the retained austenite, in particular to control the average interparticle distance and the maximum interparticle distance within a predetermined range. This improves the ductility and bendability of the steel sheet that is the final product. Therefore, the slab heating temperature is 1220°C or higher. The slab heating temperature is preferably 1230°C or higher, more preferably 1240°C or higher. The upper limit of the slab heating temperature is not particularly limited. For example, the slab heating temperature is preferably 1400°C or lower. The slab heating temperature is the maximum temperature reached by the steel slab in the slab heating process.
スラブ加熱時間：１．０時間以上
　スラブ加熱温度を１２２０℃以上としてスラブ加熱時間を１．０時間以上にすると、鋳造時に生成したＮｂおよびＴｉ系の粗大な析出物が十分に固溶する。これにより、粗大な析出物を低減することができる。また、残留オーステナイトの粒子を均一に分散させる、特に、粒子間距離平均値および粒子間距離最大値を所定の範囲に制御することが可能となる。これにより、最終製品となる鋼板の延性および曲げ性が向上する。そのため、スラブ加熱時間は１．０時間以上とする。スラブ加熱時間は、好ましくは１．１時間以上、より好ましくは１．２時間以上である。スラブ加熱時間の上限は、特に限定されない。例えば、スラブ加熱時間は３．０時間以下が好ましい。なお、スラブ加熱時間は、１２２０℃以上の温度域での保持時間である。 Slab heating time: 1.0 hour or more When the slab heating temperature is 1220°C or more and the slab heating time is 1.0 hour or more, the coarse precipitates of Nb and Ti generated during casting are sufficiently dissolved. This makes it possible to reduce the coarse precipitates. In addition, it is possible to uniformly disperse the particles of the retained austenite, in particular to control the average interparticle distance and the maximum interparticle distance within a predetermined range. This improves the ductility and bendability of the steel sheet that is the final product. Therefore, the slab heating time is 1.0 hour or more. The slab heating time is preferably 1.1 hours or more, more preferably 1.2 hours or more. There is no particular limit to the upper limit of the slab heating time. For example, the slab heating time is preferably 3.0 hours or less. The slab heating time is a holding time in a temperature range of 1220°C or more.
［熱間圧延工程］
　ついで、鋼スラブに熱間圧延を施して熱延鋼板を得る。この熱間圧延工程では、以下の条件を満足させることが重要である。 [Hot rolling process]
Next, the steel slab is subjected to hot rolling to obtain a hot-rolled steel sheet. In this hot rolling process, it is important to satisfy the following conditions:
仕上げ圧延終了温度：８４０℃以上１０００℃以下
　最終製品となる鋼板の鋼組織におけるＭｎの濃化を抑制する観点からは、焼鈍工程前の鋼板の組織においてＭｎ濃化（Ｍｎ濃度のバラつき）を抑制することが重要である。ここで、仕上げ圧延終了温度が８４０℃未満では、フェライトの生成が促進され、熱延鋼板の巻取前に過度にフェライトが生成する。これにより、未変態オーステナイトにＣが濃化する。未変態オーステナイトへの過度なＣの濃化は、パーライト変態を促進する。すなわち、熱間圧延後に得られる熱延鋼板の鋼組織において、パーライトが過度に生成する。パーライトはフェライトとセメンタイトの層状組織であり、Ｍｎはセメンタイトに濃化する。その結果、Ｍｎ濃度のバラつきが生じる。そのため、仕上げ圧延終了温度は８４０℃以上とする。仕上げ圧延終了温度は、好ましくは８５０℃以上である。一方、仕上げ圧延終了温度が過度に高くなると、後述する巻取温度までの冷却が困難になる場合がある。そのため、仕上げ圧延終了温度は１０００℃以下とする。仕上げ圧延終了温度は、好ましくは９５０℃以下、より好ましくは９２０℃以下である。 Finish rolling end temperature: 840°C or more and 1000°C or less From the viewpoint of suppressing the concentration of Mn in the steel structure of the steel sheet to be the final product, it is important to suppress the concentration of Mn (variation in Mn concentration) in the structure of the steel sheet before the annealing process. Here, if the finish rolling end temperature is less than 840°C, the generation of ferrite is promoted, and excessive ferrite is generated before the coiling of the hot-rolled steel sheet. As a result, C is concentrated in the untransformed austenite. Excessive concentration of C in the untransformed austenite promotes pearlite transformation. That is, pearlite is excessively generated in the steel structure of the hot-rolled steel sheet obtained after hot rolling. Pearlite is a lamellar structure of ferrite and cementite, and Mn is concentrated in the cementite. As a result, variation in Mn concentration occurs. Therefore, the finish rolling end temperature is 840°C or more. The finish rolling end temperature is preferably 850°C or more. On the other hand, if the finish rolling end temperature is excessively high, it may be difficult to cool the steel sheet to the coiling temperature described below. Therefore, the finish rolling end temperature is set to 1000° C. or less. The finish rolling end temperature is preferably 950° C. or less, and more preferably 920° C. or less.
仕上げ圧延終了温度から７００℃までの温度域での平均冷却速度（以下、第１冷却速度ともいう）：１０℃／秒以上
　上述したように、最終製品の鋼組織におけるＭｎの濃化を抑制する観点からは、焼鈍工程前の鋼板の組織においてＭｎ濃化（Ｍｎ濃度のバラつき）を抑制することが重要である。ここで、第１冷却速度が遅くなると、冷却中におけるフェライトの生成量が過剰となり、未変態オーステナイトへのＣの濃化を招く。未変態オーステナイトへの過度なＣの濃化は、パーライト変態を促進する。すなわち、熱間圧延後に得られる熱延鋼板の鋼組織において、パーライトが過度に生成する。上述したように、パーライトはフェライトとセメンタイトの層状組織であり、Ｍｎはセメンタイトに濃化する。その結果、Ｍｎ濃度のバラつきが生じる。さらに、第１冷却速度が遅くなると、ＮｂおよびＴｉの炭化物および窒化物といった析出物が粗大化する。そのため、旧オーステナイト粒の微細化および微細析出物によるＴＳの上昇および延性の向上の効果が得られない。よって、第１冷却速度は１０℃／秒以上とする。第１冷却速度は、好ましくは１５℃／秒以上である。第１冷却速度の上限は特に限定されるものではない。ただし、冷却設備の省エネルギーの観点から、第１冷却速度は１０００℃／秒以下とすることが好ましい。 Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C. (hereinafter, also referred to as the first cooling rate): 10 ° C./sec or more As described above, from the viewpoint of suppressing the concentration of Mn in the steel structure of the final product, it is important to suppress Mn concentration (variation in Mn concentration) in the structure of the steel sheet before the annealing process. Here, if the first cooling rate is slow, the amount of ferrite generated during cooling becomes excessive, leading to the concentration of C in untransformed austenite. Excessive concentration of C in untransformed austenite promotes pearlite transformation. That is, in the steel structure of the hot-rolled steel sheet obtained after hot rolling, pearlite is excessively generated. As described above, pearlite is a lamellar structure of ferrite and cementite, and Mn is concentrated in cementite. As a result, variation in Mn concentration occurs. Furthermore, if the first cooling rate is slow, precipitates such as carbides and nitrides of Nb and Ti become coarse. Therefore, the effect of increasing TS and improving ductility due to refinement of prior austenite grains and fine precipitates cannot be obtained. Therefore, the first cooling rate is set to 10°C/sec or more. The first cooling rate is preferably 15°C/sec or more. The upper limit of the first cooling rate is not particularly limited. However, from the viewpoint of energy saving of cooling equipment, the first cooling rate is preferably set to 1000°C/sec or less.
巻取温度：６２０℃以下
　巻取温度が６２０℃超では、巻取時にパーライトが過度に多くなり、Ｍｎ濃化が促進される。巻取温度が低いほど、パーライトの生成量は減少するため、巻取温度は低い方が好ましい。さらに、ＮｂおよびＴｉの炭化物および窒化物を微細に析出させる観点からも、巻取温度は低い方が好ましい。したがって、巻取温度は６２０℃以下とする。巻取温度は、好ましくは６００℃以下、より好ましくは５８０℃以下である。一方、巻取温度が４００℃未満になると、鋼板が過度に硬質化して冷間圧延時の破断を引き起こす可能性がある。したがって、巻取温度は、好ましくは４００℃以上である。巻取温度は、より好ましくは４５０℃以上である。 Coiling temperature: 620°C or less If the coiling temperature exceeds 620°C, the amount of pearlite produced during coiling is excessively large, and Mn concentration is promoted. The lower the coiling temperature, the less pearlite is produced, so the lower the coiling temperature is, the more preferable it is. Furthermore, from the viewpoint of finely precipitating carbides and nitrides of Nb and Ti, the lower the coiling temperature is, the more preferable it is. Therefore, the coiling temperature is set to 620°C or less. The coiling temperature is preferably 600°C or less, more preferably 580°C or less. On the other hand, if the coiling temperature is less than 400°C, the steel sheet may be excessively hardened and may cause breakage during cold rolling. Therefore, the coiling temperature is preferably 400°C or more. The coiling temperature is more preferably 450°C or more.
　なお、熱延鋼板の表面に生成した１次スケールおよび２次スケールを除去するために、デスケーリングを適宜行ってよい。熱延鋼板を冷間圧延する前に、十分酸洗してスケールの残存を軽減するのがよい。また、冷間圧延時の荷重低減の観点から、任意に、熱延鋼板に熱延板焼鈍を施してもよい。 In addition, descaling may be performed as appropriate to remove the primary and secondary scale formed on the surface of the hot-rolled steel sheet. Before cold rolling the hot-rolled steel sheet, it is advisable to thoroughly pickle the sheet to reduce the amount of remaining scale. In addition, from the viewpoint of reducing the load during cold rolling, the hot-rolled steel sheet may be optionally subjected to hot-rolled sheet annealing.
［冷間圧延工程］
　ついで、熱延鋼板に冷間圧延を施して冷延鋼板とする。 [Cold rolling process]
Next, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet.
圧下率：２０％以上８０％以下
　冷間圧延における圧下率は２０％以上とする。すなわち、圧下率が２０％未満では、焼鈍工程において鋼組織の粗大化や不均一が生じやすくなり、最終製品においてＴＳや曲げ性が低下する。したがって、圧下率は２０％以上とする。一方、圧下率が８０％を超えると、鋼板の形状不良が生じやすくなる。また、焼鈍工程での温度ムラによる鋼組織の不均一や、亜鉛めっき付着量の不均一が生じるおそれもある。したがって、圧下率は８０％以下とする。圧下率は、好ましくは３０％以上である。また、圧下率は、好ましくは７０％以下である。 Reduction ratio: 20% or more and 80% or less The reduction ratio in cold rolling is 20% or more. That is, if the reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and the TS and bendability of the final product are reduced. Therefore, the reduction ratio is 20% or more. On the other hand, if the reduction ratio exceeds 80%, the shape of the steel sheet is likely to be defective. In addition, there is a risk of non-uniformity in the steel structure and non-uniformity in the amount of zinc coating due to temperature unevenness in the annealing process. Therefore, the reduction ratio is 80% or less. The reduction ratio is preferably 30% or more. In addition, the reduction ratio is preferably 70% or less.
［昇温工程］
　ついで、冷延鋼板を、焼鈍温度まで昇温する。その際、６００℃から７５０℃までの温度域での平均昇温速度を適切に制御することが重要である。 [Temperature increasing process]
The cold-rolled steel sheet is then heated to an annealing temperature. At this time, it is important to appropriately control the average heating rate in the temperature range from 600°C to 750°C.
６００℃から７５０℃までの温度域での平均昇温速度（以下、昇温速度ともいう）：１℃／秒以上１５℃／秒以下
　昇温工程において冷延鋼板が６００℃から７５０℃の温度域（以下、昇温温度域ともいう）に滞留する時間が減少すると、Ｍｎが拡散し、オーステナイトへのＭｎの濃化が抑制される。すなわち、上記の昇温温度域での滞留時間が長くなるほどオーステナイトへのＭｎの濃化が促進される。そのため、当該昇温温度域での滞留時間を短くする、換言すれば、昇温速度を速めることが有効である。また、昇温温度域での滞留時間を短くすることにより、旧オーステナイト粒の粗大化が抑制される。その結果、残留オーステナイトを構成するアスペクト比：３以下の粒子比率を所定量確保することが可能となる。さらに、フェライト変態およびベイナイト変態を促進させて、残留オーステナイトを均一に分散させる観点からも、昇温速度は速いほうがよい。そのため、昇温速度は１℃／秒以上とする。昇温速度は、好ましくは２℃／秒以上、より好ましくは３℃／秒以上である。一方で、昇温速度が１５℃／秒を超えると、昇温工程でのオーステナイトへのＭｎ濃化が過度に抑制されてしまう。したがって、昇温速度は１５℃／秒以下とする。昇温速度は、好ましくは１２℃／秒以下、より好ましくは９℃／秒以下である。 Average heating rate in the temperature range from 600°C to 750°C (hereinafter also referred to as heating rate): 1°C/sec or more and 15°C/sec or less When the residence time of the cold-rolled steel sheet in the temperature range from 600°C to 750°C (hereinafter also referred to as the heating temperature range) in the heating process is reduced, Mn diffuses and the concentration of Mn in austenite is suppressed. That is, the longer the residence time in the above heating temperature range, the more the concentration of Mn in austenite is promoted. Therefore, it is effective to shorten the residence time in the heating temperature range, in other words, to increase the heating rate. In addition, by shortening the residence time in the heating temperature range, the coarsening of prior austenite grains is suppressed. As a result, it is possible to secure a predetermined amount of particle ratios with aspect ratios of 3 or less constituting the retained austenite. Furthermore, from the viewpoint of promoting ferrite transformation and bainite transformation and uniformly dispersing the retained austenite, it is better to have a fast heating rate. Therefore, the heating rate is set to 1°C/sec or more. The heating rate is preferably 2° C./sec or more, more preferably 3° C./sec or more. On the other hand, if the heating rate exceeds 15° C./sec, Mn concentration in austenite during the heating step is excessively suppressed. Therefore, the heating rate is set to 15° C./sec or less. The heating rate is preferably 12° C./sec or less, more preferably 9° C./sec or less.
雰囲気の露点：－３５℃以上
　鋼板表面から板厚方向に所望の厚さの軟質層を形成し、優れた曲げ性を得る観点から、昇温工程における雰囲気の露点を－３５℃以上とすることが好ましい。雰囲気の露点が－３５℃未満では、所望の厚さの軟質相を形成することが困難となる。よって、昇温工程における雰囲気の露点は－３５℃以上とすることが好ましい。昇温工程における雰囲気の露点は、より好ましくは－２０℃以上、さらに好ましくは－１０℃以上である。なお、昇温工程における雰囲気の露点の上限は特に限定されるものではない。ＴＳを好適な範囲内とするためには、昇温工程における雰囲気の露点は、好ましくは１５℃以下、より好ましくは５℃以下である。 Dew point of atmosphere: -35°C or higher From the viewpoint of forming a soft layer of a desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability, it is preferable that the dew point of the atmosphere in the heating step is -35°C or higher. If the dew point of the atmosphere is less than -35°C, it is difficult to form a soft phase of a desired thickness. Therefore, it is preferable that the dew point of the atmosphere in the heating step is -35°C or higher. The dew point of the atmosphere in the heating step is more preferably -20°C or higher, and further preferably -10°C or higher. The upper limit of the dew point of the atmosphere in the heating step is not particularly limited. In order to set the TS within a suitable range, the dew point of the atmosphere in the heating step is preferably 15°C or lower, more preferably 5°C or lower.
［焼鈍工程］
　ついで、冷延鋼板を、焼鈍温度：７５０℃以上９２０℃以下および焼鈍時間：１秒以上３０秒以下の条件で焼鈍する。 [Annealing process]
Next, the cold-rolled steel sheet is annealed under the conditions of an annealing temperature of 750° C. to 920° C. and an annealing time of 1 second to 30 seconds.
焼鈍温度：７５０℃以上９２０℃以下
　焼鈍温度が７５０℃未満の場合、フェライトとオーステナイトの二相域での加熱中におけるオーステナイトの生成割合が不十分になる。そのため、焼鈍後にフェライトの面積率が過度に増加して、所望のＴＳが得られない。一方、焼鈍温度が９２０℃を超えると、所望のフェライトおよびベイナイトの面積率が得られず、延性が低下する。したがって、焼鈍温度は７５０℃以上９２０℃以下とする。焼鈍温度は、好ましくは８８０℃以下である。なお、焼鈍温度は、焼鈍工程での最高到達温度である。 Annealing temperature: 750°C or more and 920°C or less When the annealing temperature is less than 750°C, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and the desired TS cannot be obtained. On the other hand, when the annealing temperature exceeds 920°C, the desired area ratio of ferrite and bainite cannot be obtained, and ductility decreases. Therefore, the annealing temperature is 750°C or more and 920°C or less. The annealing temperature is preferably 880°C or less. The annealing temperature is the maximum temperature reached in the annealing process.
焼鈍時間：１秒以上３０秒以下
　本発明の一実施形態に従う鋼板の製造方法において、焼鈍時間は、残留オーステナイトを構成する粒子のアスペクト比を制御するために重要である。すなわち、焼鈍時間は、以下の観点から短いほどよい。
・焼鈍中の粒成長を抑制し、オーステナイトへのＭｎの濃化を抑制する。
・フェライト変態およびベイナイト変態を促進し、残留オーステナイトを構成する粒子のアスペクト比を小さくする。
・アスペクト比：３以下の粒子へのＣ濃化を促進させる。
・焼鈍時のオーステナイト（粒子）の粗大化を抑制して、残留オーステナイトを均一に分散させる。
　そのため、焼鈍時間は３０秒以下とする。焼鈍時間は、好ましくは２５秒以下、より好ましくは２０秒以下である。
　一方、焼鈍時間が１秒未満になると、粗大なＦｅ系析出物が溶解しないため、伸びが低下する。したがって、焼鈍時間は１秒以上とする。焼鈍時間は、好ましくは３秒以上、より好ましくは５秒以上である。なお、焼鈍時間とは、焼鈍温度での保持時間である。 Annealing time: 1 second or more and 30 seconds or less In the method for producing a steel sheet according to one embodiment of the present invention, the annealing time is important for controlling the aspect ratio of the grains constituting the retained austenite. That is, the shorter the annealing time, the better from the following viewpoints.
- It suppresses grain growth during annealing and suppresses the concentration of Mn in austenite.
- Promotes ferrite transformation and bainite transformation, and reduces the aspect ratio of the particles that make up retained austenite.
- Promotes C concentration in particles with an aspect ratio of 3 or less.
・It suppresses the coarsening of austenite (particles) during annealing and disperses the retained austenite uniformly.
Therefore, the annealing time is set to 30 seconds or less, preferably 25 seconds or less, and more preferably 20 seconds or less.
On the other hand, if the annealing time is less than 1 second, the coarse Fe-based precipitates do not dissolve, and the elongation decreases. Therefore, the annealing time is set to 1 second or more. The annealing time is preferably 3 seconds or more, and more preferably 5 seconds or more. The annealing time is the holding time at the annealing temperature.
雰囲気の露点：－３５℃以上
　鋼板表面から板厚方向に所望の厚さの軟質層を形成し、優れた曲げ性を得る観点から、前述した昇温工程に引き続き、焼鈍工程でも雰囲気の露点を－３５℃以上とすることが好ましい。雰囲気の露点が－３５℃未満では、所望の厚さの軟質相を形成することが困難となる。よって、焼鈍工程における雰囲気の露点は－３５℃以上とすることが好ましい。焼鈍工程における雰囲気の露点は、より好ましくは－２０℃以上、さらに好ましくは－１０℃以上である。なお、焼鈍工程における雰囲気の露点の上限は特に限定されるものではない。ＴＳを好適な範囲内とするためには、焼鈍工程における雰囲気の露点は、好ましくは１５℃以下、より好ましくは５℃以下である。 Dew point of atmosphere: -35°C or higher From the viewpoint of forming a soft layer of a desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability, it is preferable to set the dew point of the atmosphere to -35°C or higher in the annealing process following the above-mentioned heating process. If the dew point of the atmosphere is less than -35°C, it becomes difficult to form a soft phase of a desired thickness. Therefore, it is preferable to set the dew point of the atmosphere in the annealing process to -35°C or higher. The dew point of the atmosphere in the annealing process is more preferably -20°C or higher, and further preferably -10°C or higher. The upper limit of the dew point of the atmosphere in the annealing process is not particularly limited. In order to set the TS within a suitable range, the dew point of the atmosphere in the annealing process is preferably 15°C or lower, more preferably 5°C or lower.
［冷却工程］
　ついで、上記のようにして焼鈍を施した冷延鋼板を、以下の条件で冷却する。 [Cooling process]
Next, the cold-rolled steel sheet annealed as described above is cooled under the following conditions.
焼鈍温度－３０℃から６００℃までの温度域での平均冷却速度：５℃／秒以上１００℃／秒以下
　この冷却工程ではフェライトおよびベイナイトを生成させるため、冷却速度、特に、焼鈍温度－３０℃から６００℃までの温度域での平均冷却速度（以下、第２冷却速度ともいう）を適切に制御する必要がある。第２冷却速度が遅いと、フェライトが過剰に生成する。加えて、パーライトも過剰に生成し、ＴＳが低下する。さらに、適正量の残留オーステナイトが得られない。そのため、第２冷却速度は５℃／秒以上とする。第２冷却速度は、好ましくは９℃／秒以上、より好ましくは１２℃／秒以上である。一方で、第２冷却速度が１００℃／秒を超えると、フェライト変態およびベイナイト変態が過度に抑制され、延性が低下するおそれがある。したがって、第２冷却速度は１００℃／秒以下とする。第２冷却速度は、好ましくは７５℃／秒以下、より好ましくは５０℃／秒以下である。 Average cooling rate in the temperature range from annealing temperature -30 ° C. to 600 ° C.: 5 ° C./sec or more and 100 ° C./sec or less In this cooling step, in order to generate ferrite and bainite, it is necessary to appropriately control the cooling rate, particularly the average cooling rate in the temperature range from annealing temperature -30 ° C. to 600 ° C. (hereinafter also referred to as the second cooling rate). If the second cooling rate is slow, ferrite is generated excessively. In addition, pearlite is also generated excessively, and TS is reduced. Furthermore, an appropriate amount of retained austenite is not obtained. Therefore, the second cooling rate is set to 5 ° C./sec or more. The second cooling rate is preferably 9 ° C./sec or more, more preferably 12 ° C./sec or more. On the other hand, if the second cooling rate exceeds 100 ° C./sec, ferrite transformation and bainite transformation are excessively suppressed, and ductility may be reduced. Therefore, the second cooling rate is set to 100 ° C./sec or less. The second cooling rate is preferably 75 ° C./sec or less, more preferably 50 ° C./sec or less.
冷却停止温度：４００℃以上６００℃以下
　冷却停止温度が４００℃未満では、ベイナイトのアスペクト比が大きくなり、それに伴って、残留オーステナイトを構成する全ての粒子のうち、アスペクト比：３超の粒子が増加する。したがって、冷却停止温度は４００℃以上とする。冷却停止温度は、好ましくは４３０℃以上、より好ましくは４６０℃以上である。一方、冷却停止温度が６００℃を超えると、パーライトが過剰に生成して所望のＴＳを得られないおそれがある。そのため、冷却停止温度は６００℃以下とする。冷却停止温度は、好ましくは５７０℃以下、より好ましくは５４０℃以下である。 Cooling stop temperature: 400°C or more and 600°C or less When the cooling stop temperature is less than 400°C, the aspect ratio of bainite increases, and accordingly, the number of particles with an aspect ratio of more than 3 increases among all particles constituting the retained austenite. Therefore, the cooling stop temperature is set to 400°C or more. The cooling stop temperature is preferably 430°C or more, more preferably 460°C or more. On the other hand, when the cooling stop temperature exceeds 600°C, there is a risk that pearlite is excessively generated and the desired TS cannot be obtained. Therefore, the cooling stop temperature is set to 600°C or less. The cooling stop temperature is preferably 570°C or less, more preferably 540°C or less.
雰囲気の露点：－３５℃以下
　鋼板表面から板厚方向に形成した、軟質相を均質化させる観点から、冷却工程では雰囲気の露点を－３５℃以下にすることが好ましい。すなわち、冷却の際の雰囲気の露点が－３５℃を超えると、軟質相が均質化されずにムラができてしまうおそれがある。したがって、冷却工程における雰囲気の露点は－３５℃以下とすることが好ましい。冷却工程における雰囲気の露点は、より好ましくは－４０℃以下である。なお、冷却工程における雰囲気の露点の下限は限定されるものではない。冷却工程における雰囲気の露点は、制御性の観点から、－６０℃以上が好ましく、－５５℃以上がより好ましい。 Dew point of atmosphere: -35°C or less From the viewpoint of homogenizing the soft phase formed from the steel sheet surface in the sheet thickness direction, it is preferable to set the dew point of the atmosphere in the cooling step to -35°C or less. That is, if the dew point of the atmosphere during cooling exceeds -35°C, the soft phase may not be homogenized and unevenness may occur. Therefore, it is preferable to set the dew point of the atmosphere in the cooling step to -35°C or less. The dew point of the atmosphere in the cooling step is more preferably -40°C or less. Note that the lower limit of the dew point of the atmosphere in the cooling step is not limited. From the viewpoint of controllability, the dew point of the atmosphere in the cooling step is preferably -60°C or more, more preferably -55°C or more.
［滞留工程］
　ついで、上記のようにして冷却した冷延鋼板を、４００℃以上６００℃以下の温度域において１秒以上９０秒以下滞留させる。 [Retention process]
Next, the cold-rolled steel sheet cooled as described above is retained in a temperature range of 400° C. or more and 600° C. or less for 1 second or more and 90 seconds or less.
滞留温度域：４００℃以上６００℃以下
　滞留温度域は、適正量のベイナイトおよび残留オーステナイトを確保する観点から、４００℃以上６００℃以下とする。滞留温度域が４００℃未満では、それに伴って、残留オーステナイトを構成する全ての粒子のうち、アスペクト比：３超の粒子が増加する。一方、滞留温度域が６００℃を超えると、フェライトおよびベイナイトが過剰に生成して所望のＴＳを得られないおそれがある。そのため、滞留温度域は４００℃以上６００℃以下とする。滞留温度域は、好ましくは４２０℃以上、より好ましくは４４０℃以上である。また、滞留温度域は、好ましくは５６０℃以下、より好ましくは５２０℃以下である。なお、後述する亜鉛めっき処理、特に溶融亜鉛めっき処理や合金化溶融亜鉛めっき処理を行う場合、めっき浴への侵入板温がめっき浴温より高くなるように、亜鉛めっき処理工程の直前に、冷延鋼板を再加熱することが好ましい。 Residence temperature range: 400°C or more and 600°C or less The residence temperature range is set to 400°C or more and 600°C or less from the viewpoint of securing an appropriate amount of bainite and residual austenite. If the residence temperature range is less than 400°C, the number of particles with an aspect ratio of more than 3 increases among all particles constituting the residual austenite. On the other hand, if the residence temperature range exceeds 600°C, ferrite and bainite may be excessively generated, and the desired TS may not be obtained. Therefore, the residence temperature range is set to 400°C or more and 600°C or less. The residence temperature range is preferably 420°C or more, more preferably 440°C or more. In addition, the residence temperature range is preferably 560°C or less, more preferably 520°C or less. In addition, when performing a galvanizing process described later, particularly a hot-dip galvanizing process or an alloyed hot-dip galvanizing process, it is preferable to reheat the cold-rolled steel sheet immediately before the galvanizing process so that the temperature of the sheet entering the galvanizing bath is higher than the temperature of the galvanizing bath.
滞留時間：１秒以上９０秒以下
　適正量の残留オーステナイトを確保するため、上記の滞留温度域での滞留時間（以下、単に滞留時間ともいう）を適正に制御する必要がある。ここで、滞留時間が長いほど、残留オーステナイトは増加する。したがって、滞留時間は１秒以上とする。滞留時間は、好ましくは７秒以上、より好ましくは１５秒以上である。一方、滞留時間が過度に長くなると、ベイナイト量が過剰となり、強度確保に必要なマルテンサイトが得られなくなる。したがって、滞留時間は９０秒以下とする。滞留時間は、好ましくは８０秒以下、より好ましくは７０秒以下である。なお、ここでいう滞留時間には、上記冷却工程における（冷却停止前の）４００℃以上６００℃以下の温度域での滞留時間は含まない。 Residence time: 1 second or more and 90 seconds or less In order to ensure an appropriate amount of retained austenite, it is necessary to appropriately control the residence time in the above residence temperature range (hereinafter, also simply referred to as residence time). Here, the longer the residence time, the more the retained austenite increases. Therefore, the residence time is set to 1 second or more. The residence time is preferably 7 seconds or more, more preferably 15 seconds or more. On the other hand, if the residence time is too long, the amount of bainite becomes excessive, and martensite necessary for ensuring strength cannot be obtained. Therefore, the residence time is set to 90 seconds or less. The residence time is preferably 80 seconds or less, more preferably 70 seconds or less. Note that the residence time here does not include the residence time in the temperature range of 400°C or more and 600°C or less (before cooling is stopped) in the above cooling step.
　また、上記の滞留工程後、冷延鋼板に、さらに化成処理や有機系皮膜処理等の表面処理を施してもよい。 Furthermore, after the above-mentioned retention process, the cold-rolled steel sheet may be subjected to further surface treatment such as chemical conversion treatment or organic coating treatment.
［亜鉛めっき処理工程］
　ついで、任意に、冷延鋼板に亜鉛めっき処理を施してもよい。亜鉛めっき処理としては、例えば、溶融亜鉛めっき処理および合金化溶融亜鉛めっき処理が挙げられる。処理条件は常法に従えばよい。 [Zinc plating process]
Next, the cold rolled steel sheet may be optionally subjected to a galvanizing treatment. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a hot-dip galvannealing treatment. The treatment conditions may be in accordance with conventional methods.
　例えば、溶融亜鉛めっき処理の場合、冷延鋼板を４４０℃以上５００℃以下の亜鉛めっき浴中に浸漬させた後、ガスワイピング等によって、めっき付着量を調整することが好ましい。亜鉛めっき浴としては、前記した亜鉛めっき層の組成となれば特に限定されるものではないが、例えば、Ａｌ含有量が０．１０質量％以上０．２３質量％以下であり、残部がＺｎおよび不可避的不純物からなる組成のめっき浴を用いることが好ましい。また、溶融亜鉛めっき処理および後述する合金化溶融亜鉛めっき処理を行う場合、めっき浴への侵入板温がめっき浴温より高くなるように、当該工程の直前に、冷延鋼板を再加熱することが好ましい。 For example, in the case of hot-dip galvanizing, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. The galvanizing bath is not particularly limited as long as it has the composition of the galvanized layer described above, but it is preferable to use a plating bath with an Al content of 0.10 mass% to 0.23 mass%, with the balance consisting of Zn and unavoidable impurities. In addition, when hot-dip galvanizing and alloyed hot-dip galvanizing, which will be described later, are performed, it is preferable to reheat the cold-rolled steel sheet immediately before the process so that the temperature of the sheet entering the plating bath is higher than the plating bath temperature.
　また、合金化溶融亜鉛めっき処理の場合、上記の要領で溶融亜鉛めっき処理を施した後、４５０℃以上６００℃以下の温度域で合金化処理を施すことが好ましい。合金化温度が４５０℃未満では、Ｚｎ－Ｆｅ合金化速度が過度に遅くなってしまい、合金化が困難となる場合がある。一方、合金化温度が６００℃を超えると、未変態オーステナイトがパーライトへ変態し、ＴＳおよび延性が低下する場合がある。したがって、合金化処理における合金化温度は４５０℃以上６００℃以下が好ましい。合金化処理における合金化温度は、より好ましくは４６０℃以上、さらに好ましくは４７０℃以上である。また、合金化処理における合金化温度は、より好ましくは５８０℃以下、さらに好ましくは５６０℃以下である。 In the case of alloying hot-dip galvanizing treatment, it is preferable to carry out alloying treatment at a temperature range of 450°C to 600°C after carrying out hot-dip galvanizing treatment as described above. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate may be excessively slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and ductility. Therefore, the alloying temperature in the alloying treatment is preferably 450°C to 600°C. The alloying temperature in the alloying treatment is more preferably 460°C or higher, and even more preferably 470°C or higher. The alloying temperature in the alloying treatment is more preferably 580°C or lower, and even more preferably 560°C or lower.
　また、めっき付着量は、片面あたり２０ｇ／ｍ２以上８０ｇ／ｍ２以下とすることが好ましい。なお、めっき付着量は、ガスワイピング等により調節することが可能である。 The plating weight is preferably 20 g/m 2 or more and 80 g/m 2 or less per side. The plating weight can be adjusted by gas wiping or the like.
　また、上記のようにして得た鋼板に、さらに、調質圧延を施してもよい。調質圧延の伸長率は２．００％を超えると、降伏応力が上昇し、鋼板を部材に成形する際の寸法精度が低下するおそれがある。そのため、調質圧延の伸長率は２．００％以下が好ましい。なお、調質圧延の伸長率の下限は特に限定されるものではない。調質圧延の伸長率は、生産性の観点から、０．０５％以上が好ましい。また、調質圧延は上述した各工程を行うための焼鈍装置と連続した装置上（オンライン）で行ってもよいし、各工程を行うための焼鈍装置とは不連続な装置上（オフライン）で行ってもよい。また、調質圧延の圧延回数は、１回でもよく、２回以上であってもよい。なお、調質圧延と同等の伸長率を付与できれば、レベラー等による圧延であっても構わない。 The steel sheet obtained as described above may be further subjected to temper rolling. If the elongation rate of temper rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when forming the steel sheet into a component may decrease. Therefore, the elongation rate of temper rolling is preferably 2.00% or less. The lower limit of the elongation rate of temper rolling is not particularly limited. From the viewpoint of productivity, the elongation rate of temper rolling is preferably 0.05% or more. Temper rolling may be performed on a device connected to the annealing device for performing each of the above-mentioned steps (online), or on a device not connected to the annealing device for performing each of the steps (offline). The number of rolling times of temper rolling may be one or more than two. As long as the same elongation rate as that of temper rolling can be imparted, rolling using a leveler or the like may be used.
　なお、生産性の観点から、上記の焼鈍工程および亜鉛めっき処理工程などの一連の処理は、連続焼鈍ラインであるＣＡＬ（Ｃｏｎｔｉｎｕｏｕｓ　Ａｎｎｅａｌｉｎｇ　Ｌｉｎｅ）や溶融亜鉛めっきラインであるＣＧＬ（Ｃｏｎｔｉｎｕｏｕｓ　Ｇａｌｖａｎｉｚｉｎｇ　Ｌｉｎｅ）で行うのが好ましい。溶融亜鉛めっき処理後は、めっきの目付け量を調整するために、ワイピングが可能である。 From the viewpoint of productivity, it is preferable to carry out a series of processes such as the above annealing process and the zinc plating process in a continuous annealing line (CAL) or a hot-dip galvanizing line (CGL). After the hot-dip galvanizing process, wiping is possible to adjust the coating weight.
　上記した以外の条件については特に限定されず、常法に従えばよい。以上説明した本発明の一実施形態に係る鋼板の製造方法によれば、高い強度と、高いＹＳと、優れた延性と、優れた曲げ性と、を兼備する鋼板が得られ、該鋼板は、例えば、自動車部材に好適に用いることができる。 　Conditions other than those mentioned above are not particularly limited and may be made in accordance with standard methods. According to the method for manufacturing a steel sheet according to one embodiment of the present invention described above, a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be obtained, and the steel sheet can be suitably used, for example, for automobile components.
［４］部材の製造方法
　つぎに、本発明の一実施形態に従う部材の製造方法について、説明する。
　本発明の一実施形態に従う部材の製造方法は、上記の鋼板に、成形加工および接合加工の少なくとも一方を施して部材とする、工程を有する。
　ここで、成形加工方法は、特に限定されず、例えば、プレス成形等の一般的な加工方法を用いることができる。また、接合加工方法も、特に限定されず、例えば、スポット溶接、レーザー溶接、アーク溶接等の一般的な溶接や、リベット接合、かしめ接合等を用いることができる。なお、成形条件および接合条件については特に限定されず、常法に従えばよい。 [4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described.
A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component.
Here, the molding method is not particularly limited, and for example, a general processing method such as press molding can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, riveting, crimping, etc. can be used. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.
　表１に示す成分組成（残部はＦｅおよび不可避的不純物）を有する鋼素材を転炉にて溶製し、連続鋳造法にて鋼スラブとした。ついで、表２に示す条件で、鋼スラブを加熱し、ついで、鋼スラブに粗圧延と仕上げ圧延からなる熱間圧延を施し、熱延鋼板とした。なお、Ｎｏ．１３のスラブ加熱時間は、スラブ加熱温度での保持時間である。ついで、得られた熱延鋼板に、酸洗および表２に示す条件の冷間圧延を施し、冷延鋼板とした。ついで、得られた冷延鋼板に、表２に示す条件で、昇温工程、焼鈍工程および冷却工程、ならびに、一部については亜鉛めっき処理工程を行い、最終製品となる鋼板を得た。なお、明記していない条件は、常法に従うものとした。 　A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The steel slab was then heated under the conditions shown in Table 2, and hot rolling consisting of rough rolling and finish rolling was performed on the steel slab to produce a hot-rolled steel sheet. Note that the slab heating time of No. 13 is the holding time at the slab heating temperature. The obtained hot-rolled steel sheet was then pickled and cold-rolled under the conditions shown in Table 2 to produce a cold-rolled steel sheet. The obtained cold-rolled steel sheet was then subjected to a heating process, an annealing process, a cooling process, and a zinc plating process under the conditions shown in Table 2 to obtain the final steel sheet product. Note that conditions not specified were in accordance with conventional methods.
　ここで、亜鉛めっき処理工程では、溶融亜鉛めっき処理または合金化溶融亜鉛めっき処理を行い、溶融亜鉛めっき鋼板（以下、ＧＩともいう）または合金化溶融亜鉛めっき鋼板（以下、ＧＡともいう）を得た。なお、表２では、亜鉛めっき処理工程の種類についても、「ＧＩ」および「ＧＡ」と表示している。また、表２の亜鉛めっき処理工程の種類が「ＣＲ」のものは、亜鉛めっき処理工程を行わず、冷延鋼板ままであることを意味する。 Here, in the galvanizing process, hot-dip galvanizing or alloyed hot-dip galvanizing was performed to obtain hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA). Note that in Table 2, the type of galvanizing process is also indicated as "GI" or "GA". Also, the type of galvanizing process in Table 2 that is "CR" means that the galvanizing process was not performed and the steel remains cold-rolled.
　ここで、溶融亜鉛めっき処理では、めっき浴は、Ａｌ：０．２０質量％を含有し、残部がＺｎおよび不可避的不純物からなる組成とした。めっき浴温は４７０℃とした。めっき付着量は、片面あたり４５～７２ｇ／ｍ２（両面めっき）程度とした。また、最終的に得られたＧＩの亜鉛めっき層の組成は、Ｆｅ：０．１～１．０質量％、Ａｌ：０．２～１．０質量％であり、残部がＺｎおよび不可避的不純物からなるものであった。 Here, in the hot dip galvanizing treatment, the plating bath contained 0.20 mass% Al, with the balance consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating coating weight was about 45 to 72 g/ m2 per side (double-sided plating). The composition of the zinc plating layer of the finally obtained GI was 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, with the balance consisting of Zn and unavoidable impurities.
　また、合金化溶融亜鉛めっき処理では、めっき浴は、Ａｌ：０．１４質量％を含有し、残部がＺｎおよび不可避的不純物からなる組成とした。めっき浴温は４７０℃とした。めっき付着量は、片面あたり４５ｇ／ｍ２（両面めっき）程度とした。合金化温度は５２０℃とした。また、最終的に得られたＧＡの亜鉛めっき層の組成は、Ｆｅ：７～１５質量％、Ａｌ：０．１～１．０質量％であり、残部がＺｎおよび不可避的不純物からなるものであった。 In the galvannealed hot-dip galvanizing treatment, the plating bath contained 0.14 mass% Al, with the balance consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating coating weight was about 45 g/ m2 per side (double-sided plating). The alloying temperature was 520°C. The composition of the finally obtained zinc plating layer of GA was 7-15 mass% Fe, 0.1-1.0 mass% Al, with the balance consisting of Zn and unavoidable impurities.
　かくして得られた鋼板を用いて、上述した要領により、鋼板の鋼組織の同定、残留オーステナイトの粒子数密度、（Ａ）アスペクト比：３以下の粒子比率（面積％）、（Ｂ）アスペクト比：３以下の平均Ｃ濃度（質量％）、（Ｃ）粒子間距離平均値（μｍ）、（Ｄ）粒子間距離最大値（μｍ）、［Ｍｎ］Ｃ／［Ｍｎ］ならびに、軟質層の厚さの測定を行った。測定結果を表３に示す。なお、軟質層を有する鋼板では、軟質層が鋼板の両面に形成されており、両面とも同じ厚さであった。また、Ｎｏ．２５では、軟質層が確認されなかった（軟質層の厚さが１μｍ未満であった）ため、表２中の軟質層の厚さの欄を「０」と表記している。 Using the steel plate thus obtained, the steel structure of the steel plate was identified, the particle number density of the retained austenite, (A) the particle ratio (area%) of aspect ratio: 3 or less, (B) the average C concentration (mass%) of aspect ratio: 3 or less, (C) the average interparticle distance (μm), (D) the maximum interparticle distance (μm), [Mn] C /[Mn], and the thickness of the soft layer were measured in the above-mentioned manner. The measurement results are shown in Table 3. In the steel plate having a soft layer, the soft layer was formed on both sides of the steel plate and had the same thickness on both sides. In addition, in No. 25, the soft layer was not confirmed (the thickness of the soft layer was less than 1 μm), so the column for the thickness of the soft layer in Table 2 is marked with "0".
　また、以下の要領により、引張試験およびＶ曲げ試験を行い、以下の基準により、引張強さ（ＴＳ）、降伏応力（ＹＳ）、全伸び（Ｅｌ）およびＲ／ｔを評価した。
・ＴＳ
　合格：７８０ＭＰａ≦ＴＳ
　不合格：ＴＳ＜７８０ＭＰａ
・ＹＳ
　合格：
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、４２０ＭＰａ≦ＹＳ
　９８０ＭＰａ≦ＴＳの場合、５５０ＭＰａ≦ＹＳ
　不合格：
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、ＹＳ＜４２０ＭＰａ
　９８０ＭＰａ≦ＴＳの場合、ＹＳ＜５５０ＭＰａ
・Ｅｌ
　合格：
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、１９％≦Ｅｌ
　９８０ＭＰａ≦ＴＳの場合、１０％≦Ｅｌ
　不合格：
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、Ｅｌ＜１９％
　９８０ＭＰａ≦ＴＳの場合、Ｅｌ＜１０％
・Ｒ／ｔ
　合格：
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、２．０≧Ｒ／ｔ
　９８０ＭＰａ≦ＴＳの場合、４．０≧Ｒ／ｔ
　不合格：
　７８０ＭＰａ≦ＴＳ＜９８０ＭＰａの場合、Ｒ／ｔ＞２．０
　９８０ＭＰａ≦ＴＳの場合、Ｒ／ｔ＞４．０ Further, a tensile test and a V-bend test were carried out according to the following procedures, and the tensile strength (TS), yield stress (YS), total elongation (El) and R/t were evaluated according to the following criteria.
・TS
Pass: 780MPa≦TS
Fail: TS<780MPa
・Y.S.
Passed:
When 780MPa≦TS<980MPa, 420MPa≦YS
When 980MPa≦TS, 550MPa≦YS
failure:
When 780MPa≦TS<980MPa, YS<420MPa
When 980 MPa ≦ TS, YS < 550 MPa
・El
Passed:
When 780MPa≦TS<980MPa, 19%≦El
When 980MPa≦TS, 10%≦El
failure:
When 780 MPa ≦ TS < 980 MPa, El < 19%
When 980MPa≦TS, El<10%
R/t
Passed:
In the case of 780 MPa ≦ TS < 980 MPa, 2.0 ≧ R / t
In the case of 980 MPa ≦ TS, 4.0 ≧ R / t
failure:
In the case of 780 MPa ≦ TS < 980 MPa, R / t > 2.0
In the case of 980MPa≦TS, R/t>4.0
　引張試験は、ＪＩＳ　Ｚ　２２４１に準拠して行った。すなわち、得られた鋼板から、長手方向が鋼板の圧延方向に対して直角となるようにＪＩＳ５号試験片を採取した。採取した試験片を用いて、クロスヘッド速度が１０ｍｍ／ｍｉｎの条件で引張試験を行い、ＴＳ、ＹＳおよびＥｌを測定した。結果を表３に併記する。 The tensile test was conducted in accordance with JIS Z 2241. That is, JIS No. 5 test pieces were taken from the obtained steel plate so that the longitudinal direction was perpendicular to the rolling direction of the steel plate. Using the taken test pieces, a tensile test was conducted at a crosshead speed of 10 mm/min, and TS, YS and El were measured. The results are shown in Table 3.
　Ｖ（９０°）曲げ試験は、ＪＩＳ Ｚ ２２４８に準拠して行った。すなわち、鋼板から１００ｍｍ×３５ｍｍの試験片を剪断および端面研削加工により採取した。ここで、１００ｍｍの辺は圧延直角（Ｃ）方向（幅方向）に平行となるように採取した。ついで、採取した試験片を用いて、以下の条件で、Ｖ（９０°）曲げ試験を行った。
　曲げ半径Ｒ：０．５ｍｍピッチで変化
　試験方法：ダイ支持、パンチ押し込み
　成型荷重：１０ｔｏｎ
　試験速度：３０ｍｍ／ｍｉｎ
　保持時間：５秒
　曲げ方向：圧延直角（Ｃ）方向
　試験は３回を行い、３回の試験のいずれでも割れが発生しない最小の曲げ半径をＲとした。そして、Ｒを板厚ｔで除することにより、Ｒ／ｔを算出した。なお、ライカ製実体顕微鏡を用いて倍率：２５倍で試験片を観察し、長さ：２００μｍ以上のき裂が確認された場合に、割れ発生と判断した。結果を表３に併記する。 The V (90°) bending test was performed in accordance with JIS Z 2248. That is, a test piece of 100 mm x 35 mm was taken from the steel plate by shearing and end face grinding. Here, the 100 mm side was taken so as to be parallel to the rolling perpendicular (C) direction (width direction). Next, using the taken test piece, a V (90°) bending test was performed under the following conditions.
Bending radius R: Varies in 0.5 mm increments Test method: Die support, punch press Forming load: 10 tons
Test speed: 30 mm/min
Holding time: 5 seconds Bending direction: perpendicular to rolling (C) direction The test was performed three times, and the minimum bending radius at which no cracks occurred in any of the three tests was taken as R. R/t was calculated by dividing R by the plate thickness t. The test pieces were observed at a magnification of 25 times using a Leica stereo microscope, and it was determined that cracks had occurred if a crack with a length of 200 μm or more was confirmed. The results are shown in Table 3.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-I000003
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-I000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　表３に示したように、発明例ではいずれも、ＴＳ、ＹＳ、ＥｌおよびＲ／ｔの全てが合格であった。また、発明例の鋼板を用いて、成形加工を施して得た部材または接合加工を施して得た部材はいずれも、割れの発生なく目標とする形状を有し、ＴＳ、ＹＳ、ＥｌおよびＲ／ｔの合格基準に達していた。
　一方、比較例では、ＴＳ、ＹＳ、ＥｌおよびＲ／ｔのうちの少なくとも１つが不合格ではあった。 As shown in Table 3, all of the inventive examples passed the TS, YS, El and R/t standards. In addition, all of the members obtained by forming or joining using the steel plates of the inventive examples had the desired shapes without cracking, and met the pass criteria for TS, YS, El and R/t.
On the other hand, in the comparative example, at least one of TS, YS, El and R/t was unsatisfactory.
　本発明によれば、高い強度と、高いＹＳと、優れた延性と、優れた曲げ性と、を兼備する鋼板が得られる。また、当該鋼板は、複雑な形状となる自動車の骨格構造部材などの素材として、極めて有利に適用することができる。これにより、車体軽量化による燃費向上を図ることができるので、産業上の利用価値は極めて大きい。 The present invention provides a steel plate that combines high strength, high YS, excellent ductility, and excellent bendability. Furthermore, the steel plate can be used extremely advantageously as a material for automobile frame structural members that have complex shapes. This allows for improved fuel efficiency through reduced vehicle weight, making the steel plate extremely valuable in industry.

Claims (18)

  1. 　質量％で、
    　　Ｃ：０．０５％以上０．２０％以下、
    　　Ｓｉ：０．１％以上１．８％以下、
    　　Ｍｎ：１．５％以上３．０％以下、
    　　Ｐ：０．００１％以上０．１００％以下、
    　　Ｓ：０．０５００％以下、
    　　Ａｌ：０．０１０％以上１．０００％以下、
    　　Ｎ：０．０１００％以下ならびに
    　　ＮｂおよびＴｉの１種または２種：合計で０．００５％以上０．２００％以下
    であり、
    　残部がＦｅおよび不可避的不純物である、成分組成と、
    　　フェライトおよびベイナイトの１種または２種の面積率：合計で１０％以上８７％以下、
    　　マルテンサイトの面積率：１０％以上８７％以下、ならびに
    　　残留オーステナイトの面積率：３％以上
    であり、
    　　前記残留オーステナイトを構成する粒子の数密度が０．０５個／μｍ２以上であり、
    　　前記残留オーステナイトを構成する粒子のうち、アスペクト比：３以下の粒子が以下の（Ａ）、（Ｂ）、（Ｃ）および（Ｄ）を満足し、
    　　［Ｍｎ］Ｃ／［Ｍｎ］が１．０５以上２．００以下であり、［Ｍｎ］Ｃは鋼中のＭｎ濃化領域の平均Ｍｎ濃度（質量％）、［Ｍｎ］は鋼中の平均Ｍｎ濃度（質量％）である、鋼組織と、を有し、
    　引張強さが７８０ＭＰａ以上である、鋼板。
    （Ａ）残留オーステナイトを構成する全ての粒子に対する、アスペクト比：３以下の粒子の比率：面積率で６０％以上
    （Ｂ）アスペクト比：３以下の粒子の平均Ｃ濃度：０．３質量％以上
    （Ｃ）アスペクト比：３以下の粒子間の最短距離の平均値：５μｍ以下
    （Ｄ）アスペクト比：３以下の粒子間の最短距離の最大値：１５μｍ以下     In mass percent,
    C: 0.05% or more and 0.20% or less,
    Si: 0.1% or more and 1.8% or less,
    Mn: 1.5% or more and 3.0% or less,
    P: 0.001% or more and 0.100% or less,
    S: 0.0500% or less,
    Al: 0.010% or more and 1.000% or less,
    N: 0.0100% or less, and one or two of Nb and Ti: 0.005% or more and 0.200% or less in total;
    The balance being Fe and unavoidable impurities;
    Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
    Area ratio of martensite: 10% or more and 87% or less; and area ratio of retained austenite: 3% or more;
    The number density of particles constituting the retained austenite is 0.05 particles/ μm2 or more,
    Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the following (A), (B), (C), and (D),
    a steel structure in which [ Mn ]/[Mn] is 1.05 or more and 2.00 or less, [ Mn ] is an average Mn concentration (mass%) in Mn-enriched regions in the steel, and [Mn] is an average Mn concentration (mass%) in the steel;
    A steel plate having a tensile strength of 780 MPa or more.
    (A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio; (B) The average C concentration of particles having an aspect ratio of 3 or less: 0.3 mass% or more; (C) The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 μm or less; (D) The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 μm or less.
  2. 　前記成分組成が、さらに質量％で、
    　Ｖ：０．４５％以下、
    　Ｂ：０．０１０％以下、
    　Ｃｒ：１．０％以下、
    　Ｎｉ：１．０％以下、
    　Ｍｏ：１．０％以下、
    　Ｓｂ：０．１％以下、
    　Ｓｎ：０．１％以下、
    　Ｃｕ：１．０％以下、
    　Ｔａ：０．１％以下、
    　Ｗ：０．２％以下、
    　Ｍｇ：０．０１％以下、
    　Ｚｎ：０．０２％以下、
    　Ｃｏ：０．０２％以下、
    　Ｚｒ：０．２％以下、
    　Ｃａ：０．０２％以下、
    　Ｓｅ：０．０２％以下、
    　Ｔｅ：０．０２％以下、
    　Ｇｅ：０．０２％以下、
    　Ａｓ：０．０５％以下、
    　Ｓｒ：０．０２％以下、
    　Ｃｓ：０．０２％以下、
    　Ｈｆ：０．０２％以下、
    　Ｐｂ：０．０２％以下、
    　Ｂｉ：０．０２％以下および
    　ＲＥＭ：０．０２％以下
    のうちから選ばれる少なくとも１種を含有する、請求項１に記載の鋼板。     The composition further comprises, in mass%,
    V: 0.45% or less,
    B: 0.010% or less,
    Cr: 1.0% or less,
    Ni: 1.0% or less,
    Mo: 1.0% or less,
    Sb: 0.1% or less,
    Sn: 0.1% or less,
    Cu: 1.0% or less,
    Ta: 0.1% or less,
    W: 0.2% or less,
    Mg: 0.01% or less,
    Zn: 0.02% or less,
    Co: 0.02% or less,
    Zr: 0.2% or less,
    Ca: 0.02% or less,
    Se: 0.02% or less,
    Te: 0.02% or less,
    Ge: 0.02% or less,
    As: 0.05% or less,
    Sr: 0.02% or less,
    Cs: 0.02% or less,
    Hf: 0.02% or less,
    Pb: 0.02% or less,
    The steel plate according to claim 1, containing at least one selected from the group consisting of Bi: 0.02% or less and REM: 0.02% or less.
  3. 　厚さ：１μｍ以上５０μｍ以下の軟質層を有する、請求項１に記載の鋼板。
    　ここで、軟質層とは、硬度が鋼板の板厚１／４位置の硬度の６５％以下になる領域である。     The steel sheet according to claim 1, having a soft layer having a thickness of 1 μm or more and 50 μm or less.
    Here, the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate.
  4. 　厚さ：１μｍ以上５０μｍ以下の軟質層を有する、請求項２に記載の鋼板。
    　ここで、軟質層とは、硬度が鋼板の板厚１／４位置の硬度の６５％以下になる領域である。     The steel sheet according to claim 2, having a soft layer having a thickness of 1 μm or more and 50 μm or less.
    Here, the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate.
  5. 　表面に亜鉛めっき層を有する、請求項１～４のいずれか一項に記載の鋼板。 The steel sheet according to any one of claims 1 to 4, having a zinc-plated layer on its surface.
  6. 　前記亜鉛めっき層が溶融亜鉛めっき層または合金化溶融亜鉛めっき層である、請求項５に記載の鋼板。 The steel sheet according to claim 5, wherein the zinc-plated layer is a hot-dip galvanized layer or a hot-dip galvannealed layer.
  7. 　請求項１～４のいずれか一項に記載の鋼板を用いてなる、部材。 A member made using the steel plate according to any one of claims 1 to 4.
  8. 　請求項５に記載の鋼板を用いてなる、部材。 A member made using the steel plate described in claim 5.
  9. 　請求項６に記載の鋼板を用いてなる、部材。 A member made using the steel plate described in claim 6.
  10. 　請求項１または２に記載の成分組成を有する鋼スラブを、
    　スラブ加熱温度：１２２０℃以上、および
    　スラブ加熱時間：１．０時間以上
    の条件で加熱する、スラブ加熱工程と、
    　ついで、前記鋼スラブに、
    　仕上げ圧延終了温度：８４０℃以上１０００℃以下、
    　仕上げ圧延終了温度から７００℃までの温度域での平均冷却速度：１０℃／秒以上、および、
    　巻取温度：６２０℃以下
    の条件で熱間圧延を施し、熱延鋼板を得る、熱間圧延工程と、
    　ついで、前記熱延鋼板に、
    　圧下率：２０％以上８０％以下
    の条件で冷間圧延を施し、冷延鋼板を得る、冷間圧延工程と、
    　ついで、前記冷延鋼板を、
    　６００℃から７５０℃までの温度域での平均昇温速度：１℃／秒以上１５℃／秒以下
    の条件で昇温する、昇温工程と、
    　ついで、前記冷延鋼板を、
    　焼鈍温度：７５０℃以上９２０℃以下、および、
    　焼鈍時間：１秒以上３０秒以下
    の条件で焼鈍する、焼鈍工程と、
    　ついで、前記冷延鋼板を、
    　焼鈍温度－３０℃から６００℃までの温度域での平均冷却速度：５℃／秒以上１００℃／秒以下、および
    　冷却停止温度：４００℃以上６００℃以下
    の条件で冷却する、冷却工程と、
    　ついで、前記冷延鋼板を、
    　４００℃以上６００℃以下の温度域での滞留時間：１秒以上９０秒以下
    の条件で滞留させる、滞留工程と、
    を有する、鋼板の製造方法。     A steel slab having the composition according to claim 1 or 2,
    A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more;
    Next, the steel slab is
    Finish rolling end temperature: 840°C or more and 1000°C or less,
    Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C. / sec or more, and
    A hot rolling process in which hot rolling is performed under the condition of a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet;
    Next, the hot-rolled steel sheet is
    A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
    Next, the cold rolled steel sheet is
    A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C. of 1° C./sec or more and 15° C./sec or less;
    Next, the cold rolled steel sheet is
    Annealing temperature: 750°C or higher and 920°C or lower; and
    Annealing time: annealing under conditions of 1 second to 30 seconds;
    Next, the cold rolled steel sheet is
    A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
    Next, the cold rolled steel sheet is
    A residence time in a temperature range of 400° C. to 600° C.: a residence step of 1 second to 90 seconds;
    The method for producing a steel sheet comprising the steps of:
  11. 　前記昇温工程および前記焼鈍工程における雰囲気の露点がいずれも－３５℃以上であり、
    　前記冷却工程における雰囲気の露点が－３５℃以下である、請求項１０に記載の鋼板の製造方法。     The dew points of the atmosphere in the temperature increasing step and the annealing step are both −35° C. or higher;
    The method for producing a steel sheet according to claim 10, wherein a dew point of the atmosphere in the cooling step is −35° C. or lower.
  12. 　前記滞留工程後、さらに前記冷延鋼板に亜鉛めっき処理を施す、亜鉛めっき処理工程を有する、請求項１０に記載の鋼板の製造方法。 The method for manufacturing steel sheet according to claim 10 further includes a zinc plating process in which the cold-rolled steel sheet is subjected to zinc plating after the retention process.
  13. 　前記滞留工程後、さらに前記冷延鋼板に亜鉛めっき処理を施す、亜鉛めっき処理工程を有する、請求項１１に記載の鋼板の製造方法。 The method for manufacturing steel sheet according to claim 11 further includes a zinc plating process in which the cold-rolled steel sheet is subjected to zinc plating after the retention process.
  14. 　前記亜鉛めっき処理が、溶融亜鉛めっき処理または合金化溶融亜鉛めっき処理である、請求項１２に記載の鋼板の製造方法。 The method for manufacturing a steel sheet according to claim 12, wherein the zinc plating treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.
  15. 　前記亜鉛めっき処理が、溶融亜鉛めっき処理または合金化溶融亜鉛めっき処理である、請求項１３に記載の鋼板の製造方法。 The method for manufacturing a steel sheet according to claim 13, wherein the zinc plating treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.
  16. 　請求項１～４のいずれか一項に記載の鋼板に、成形加工および接合加工の少なくとも一方を施して部材とする、工程を有する、部材の製造方法。 A method for manufacturing a component, comprising the steps of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining processes to produce a component.
  17. 　請求項５に記載の鋼板に、成形加工および接合加工の少なくとも一方を施して部材とする、工程を有する、部材の製造方法。 A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 5 to at least one of forming and joining processes to produce a component.
  18. 　請求項６に記載の鋼板に、成形加工および接合加工の少なくとも一方を施して部材とする、工程を有する、部材の製造方法。 A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 6 to at least one of forming and joining processes to produce a component.
PCT/JP2023/037920 2023-01-26 2023-10-19 Steel sheet and member, and method for producing said steel sheet and method for producing said member WO2024157551A1 (en)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2013018740A1 (en) * 2011-07-29 2013-02-07 新日鐵住金株式会社 High-strength steel sheet having superior impact resistance, method for producing same, high-strength galvanized steel sheet, and method for producing same
WO2013125400A1 (en) * 2012-02-22 2013-08-29 新日鐵住金株式会社 Cold-rolled steel sheet and manufacturing method for same
WO2016136810A1 (en) * 2015-02-24 2016-09-01 新日鐵住金株式会社 Cold-rolled steel sheet and method for manufacturing same
WO2019131189A1 (en) * 2017-12-26 2019-07-04 Ｊｆｅスチール株式会社 High-strength cold rolled steel sheet and method for manufacturing same
WO2022270053A1 (en) * 2021-06-24 2022-12-29 Ｊｆｅスチール株式会社 Hot-dip galvanized steel sheet and method for producing same, and member

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2013018740A1 (en) * 2011-07-29 2013-02-07 新日鐵住金株式会社 High-strength steel sheet having superior impact resistance, method for producing same, high-strength galvanized steel sheet, and method for producing same
WO2013125400A1 (en) * 2012-02-22 2013-08-29 新日鐵住金株式会社 Cold-rolled steel sheet and manufacturing method for same
WO2016136810A1 (en) * 2015-02-24 2016-09-01 新日鐵住金株式会社 Cold-rolled steel sheet and method for manufacturing same
WO2019131189A1 (en) * 2017-12-26 2019-07-04 Ｊｆｅスチール株式会社 High-strength cold rolled steel sheet and method for manufacturing same
WO2022270053A1 (en) * 2021-06-24 2022-12-29 Ｊｆｅスチール株式会社 Hot-dip galvanized steel sheet and method for producing same, and member

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