WO2023032651A1 - 鋼板、部材およびそれらの製造方法 - Google Patents

鋼板、部材およびそれらの製造方法 Download PDF

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Publication number
WO2023032651A1
WO2023032651A1 PCT/JP2022/030898 JP2022030898W WO2023032651A1 WO 2023032651 A1 WO2023032651 A1 WO 2023032651A1 JP 2022030898 W JP2022030898 W JP 2022030898W WO 2023032651 A1 WO2023032651 A1 WO 2023032651A1
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steel sheet
content
steel
temperature
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PCT/JP2022/030898
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English (en)
French (fr)
Japanese (ja)
Inventor
洋一郎 松井
三周 知場
真次郎 金子
章紀 中村
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JFE Steel Corp
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JFE Steel Corp
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Priority to EP22864236.9A priority Critical patent/EP4361304A4/en
Priority to KR1020247005332A priority patent/KR20240036625A/ko
Priority to CN202280056863.3A priority patent/CN117836458A/zh
Priority to JP2022569005A priority patent/JP7294545B1/ja
Priority to US18/683,839 priority patent/US20250122602A1/en
Priority to MX2024002273A priority patent/MX2024002273A/es
Publication of WO2023032651A1 publication Critical patent/WO2023032651A1/ja
Anticipated expiration legal-status Critical
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    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Definitions

  • the present invention relates to steel sheets used in various applications such as automobiles and home appliances, members using the steel sheets, and manufacturing methods thereof.
  • TRIP steel in which retained austenite (retained ⁇ ) is dispersed in the microstructure of the steel sheet, has been developed as a technique for improving the ductility of the steel sheet.
  • Patent Document 1 steel containing C: 0.10 to 0.45%, Si: 0.5 to 1.8%, Mn: 0.5 to 3.0% is annealed at 350 to 500 ° C.
  • Patent Document 2 steel containing C: 0.10 to 0.25%, Si: 1.0 to 2.0%, Mn: 1.5 to 3.0% is annealed at 10 ° C./s or more Cool to 450 to 300 ° C., hold for 180 to 600 seconds, and control the space factor to 5% or more of retained austenite, 60% or more of bainitic ferrite, and 20% or less of polygonal ferrite, thereby improving ductility. It is disclosed that a steel sheet excellent in (El) and stretch flanging formability ( ⁇ ) can be obtained.
  • Patent Document 3 a steel sheet having a specific chemical composition is cooled to a temperature range of 150 to 350 ° C. after annealing, and then reheated to 350 to 600 ° C. and held, so that ferrite, tempered martensite, retained austenite It is disclosed that a structure containing is obtained, and high ductility and high stretch flanging formability can be imparted to the steel sheet.
  • the steel is once cooled to a temperature range between the martensitic transformation start temperature (Ms point) and the martensitic transformation completion temperature (Mf point), and then reheated and held to stabilize the residual ⁇ .
  • Ms point martensitic transformation start temperature
  • Mf point martensitic transformation completion temperature
  • Patent Literature 4 discloses a technique that improves the Q&P process described above. That is, in a steel having a specific chemical composition, annealing is performed at a temperature of Ae 3 point ⁇ 10 ° C. or higher in order to reduce polygonal ferrite to 5% or less, and then a relatively high temperature of Ms ⁇ 10 ° C. to Ms ⁇ 100 ° C. By stopping cooling at , upper bainite is generated when reheating to 350 to 450 ° C., and high ductility and high stretch flanging formability are intended to be obtained.
  • Patent Document 5 discloses a method of obtaining a steel sheet with excellent ductility and low-temperature toughness by utilizing bainite produced at low temperature and bainite produced at high temperature. That is, after annealing a steel containing 0.10 to 0.5% of C, it is cooled to 150 to 400 ° C. at a cooling rate of 10 ° C./s or more, and held in that temperature range for 10 to 200 sec. of bainite is generated, reheated to a temperature range of more than 400° C. and 540° C. or less, and held for 50 seconds or more to generate bainite in a high temperature range and obtain a steel plate excellent in ductility and low temperature toughness.
  • Patent Document 6 C: 0.01 to 0.3%, Si: 0.005 to 2.5%, Mn: 0.01 to 3%, Mo: 0.01 to 0.3%, A steel sheet containing Nb: 0.001 to 0.1% is annealed in a high-temperature region having a structure close to a ⁇ single phase, and then cooled to a temperature range of 200 to 450 ° C. and held, so that the main phase is bainite or It is shown that a steel plate having excellent ductility, hole expandability and weldability can be obtained by containing 50 to 97% of bainitic ferrite and containing 3 to 50% of austenite as the second phase.
  • Patent Document 1 Although the conventional TRIP steel described in Patent Document 1 has excellent ductility, it has a problem of very low stretch flange formability.
  • bainitic ferrite is mainly used as the microstructure, and the amount of ferrite is kept to a minimum. For this reason, further improvement in ductility has been desired in consideration of adaptation to difficult-to-form parts.
  • Patent Document 3 achieves relatively high ductility and high stretch flanging formability compared to conventional TRIP steel and steel using bainitic ferrite.
  • breakage was observed in molding difficult-to-form parts such as center pillars, and further improvement in ductility was required.
  • the steel plate to which this technology is applied does not necessarily have a sufficient amount of uniform deformation, which indicates how difficult it is to break.
  • This amount of uniform deformation is the amount of elongation until necking begins to occur even in El, which is an index of ductility. Represented by U.E.I. El needs to be increased further.
  • the amount of polygonal ferrite produced is reduced in order to reduce the amount of massive martensite, and sufficient ductility cannot be ensured.
  • the cooling stop temperature is set relatively high in order to improve El, and since a large amount of untransformed ⁇ remains when cooling is stopped, martensite in the form of blocks tends to remain.
  • the present invention has been made to solve such problems, and has a tensile strength of 980 MPa or more, high ductility, excellent stretch flanging formability, and excellent laser weldability. It is intended to provide a method for producing them.
  • the steel sheet referred to here also includes a galvanized steel sheet whose surface is galvanized.
  • a tensile strength of 980 MPa or more refers to a tensile strength of 980 MPa or more according to JIS Z2241.
  • high ductility means that the total elongation T-El according to JIS Z2241 is 16.0% or more when TS is less than 1180 MPa, 14.0% or more when TS is 1180 MPa or more and less than 1320 MPa, and 13.0% when TS is 1320 MPa or more. It refers to being 0% or more.
  • excellent stretch flanging formability means that a sample of 100 mm ⁇ 100 mm square size is punched using a punching tool with a punch diameter of 10 mm and a die diameter of 10.3 mm (clearance of 13%), and then a conical punch with a vertical angle of 60 degrees.
  • d0 initial hole diameter (mm)
  • d initial hole diameter (mm)
  • d when crack occurs
  • the hole expansion ratio ⁇ (%) ⁇ (d ⁇ d 0 )/d 0 ⁇ 100 is 30% or more.
  • excellent laser weldability means that the following laser welding, fracture mode determination test, and notch tensile test are performed, and the fracture mode is base metal fracture, and HAZ strength ⁇ base metal TS + 50 MPa.
  • the fracture position is 2.0 mm or more away from the weld line (even if it is more than 2.0 mm in part), the base material is fractured, and if it is less than 2.0 mm, the crack propagates along the weld line. If the weld is fractured (a crack develops in the HAZ or melted portion), it is determined that the weld is fractured. (4) Further, from the welded member, a notched test piece in which the weld line is perpendicular to the tensile axis and is located in the longitudinal center of the test piece and the welded portion is notched (see FIG. 1(b)), A tensile test (notch tensile test) is performed. (5) Thus, the strength of the HAZ itself is evaluated by deforming only the HAZ of the welded portion and a small area around it and forcibly breaking the HAZ.
  • the martensitic transformation start temperature Ms or more is average cooling rate (CR2): 10 ° C./
  • the steel is retained at a temperature of 10 seconds or more and 60 seconds or less to form bainitic ferrite with little carbide and residual ⁇ .
  • the bainitic ferrite thus produced is soft and less susceptible to heat during laser welding.
  • C since C is efficiently distributed from the bainitic ferrite to the retained ⁇ during residence, it is possible to obtain a retained ⁇ with a high carbon concentration that contributes to the improvement of ductility in the final structure.
  • a cooling stop temperature of 200° C. or more and 300° C.
  • the temperature range up to (T2) is rapidly cooled at an average cooling rate (CR3) of 3 to 100°C/s. This secondary cooling divides the remaining untransformed ⁇ region by martensite transformation or lower bainite transformation, disperses the residual ⁇ , and reduces the block structure.
  • the massive structure refers to fresh martensite or retained austenite that looks massive on SEM.
  • the T0 composition refers to a composition in which the free energies of austenite and bainite are equal and bainite transformation stops.
  • the temperature range from the cooling stop temperature (T2) to 380°C is reheated at an average heating rate of 2°C/s or more, and the temperature range of 340°C or more and 590°C or less is retained for 20 seconds or more and 3000 seconds or less.
  • carbon is further distributed to the retained ⁇ generated adjacent to the upper bainite during the retention during cooling, thereby stabilizing the retained ⁇ .
  • CR4 average cooling rate
  • the present invention has been made based on the above findings, and specifically provides the following. [1] % by mass, C: 0.06 to 0.25%, Si: 0.4 to 2.5%, Mn: 1.5-3.5%, P: 0.02% or less, S: 0.01% or less, sol.Al: less than 1.0%, N: having a component composition containing less than 0.015% and the balance consisting of Fe and inevitable impurities, area ratio, Polygonal ferrite: 10% or less (including 0%), Tempered martensite: 40% or more, Fresh martensite: 20% or less (including 0%), Bainitic ferrite with 20 or less internal carbides per 10 ⁇ m 2 : 3 to 40%, Having a steel structure with a volume fraction of retained austenite: 5 to 20%, The ratio of the area SC ⁇ 0.5 of the region having a C concentration of 0.50% or more to the area SC ⁇ 0.3 of the region having a C concentration of 0.30% or more SC ⁇ 0.5 / A steel sheet in which SC ⁇ 0.3 ⁇ 100 is 20% or
  • the number density of retained austenite present adjacent to bainitic ferrite having internal carbides of 20 or less per 10 ⁇ m 2 is 50 or more per 10000 ⁇ m 2 steel plate.
  • the component composition further contains, in % by mass, Ti: 0.1% or less, B: The steel sheet according to [1] or [2], containing one or two selected from 0.01% or less.
  • the component composition further contains, in % by mass, Cu: 1% or less, Ni: 1% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, The steel sheet according to any one of [1] to [3], containing one or more selected from Zr: 0.2% or less and W: 0.2% or less.
  • the component composition further contains, in % by mass, Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0030% or less, The steel sheet according to any one of [1] to [4], containing one or more selected from Sb: 0.1% or less and Sn: 0.1% or less.
  • [6] The steel sheet according to any one of [1] to [5], which has a galvanized layer on its surface.
  • [7] A member using the steel plate according to any one of [1] to [6].
  • [8] A cold-rolled steel sheet obtained after hot-rolling and cold-rolling a steel slab having the chemical composition described in [1], [3], [4], or [5] is annealed, The annealing is Annealing temperature: a step of holding at 810 to 900 ° C.; cooling the temperature range from 810 ° C. to 500 ° C. at an average cooling rate CR1: 5 to 100 ° C./s; A step of staying in the temperature range from 500 ° C.
  • an average cooling rate CR3 3 to 100° C./s
  • a method of manufacturing a steel plate comprising: [9] The steel sheet manufacturing method according to [8], wherein hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment is performed in the step of retaining at the average cooling rate CR4: 0.01 to 5°C/s. [10] The method for manufacturing the steel sheet according to [8], including a step of performing an electrogalvanizing treatment after the step of cooling at the average cooling rate CR5: 0.1°C/s or more. [11] A method for manufacturing a member, comprising the step of subjecting the steel plate according to any one of [1] to [6] to at least one of forming and joining to form a member.
  • steel sheets and members having high ductility, excellent stretch flanging formability, and excellent laser weldability can be obtained. Furthermore, according to the present invention, it is also possible to increase the strength. If the steel sheet of the present invention is applied to automobile parts, weight reduction of the automobile parts can be realized, and improvement of fuel efficiency can be expected.
  • FIG. 1 is a diagram for explaining the method for evaluating the laser weldability of a steel plate according to the present invention.
  • FIG. 2 is an example of a SEM photograph of the steel structure of the steel plate.
  • FIG. 3 is a diagram for explaining the method for measuring the steel structure of the steel plate of the present invention.
  • FIG. 4 is a diagram for explaining the method for manufacturing a steel sheet according to the present invention.
  • the steel sheet of the present invention in mass%, C: 0.06 to 0.25%, Si: 0.4 to 2.5%, Mn: 1.5 to 3.5%, P: 0.02% or less , S: 0.01% or less, sol. Al: less than 1.0%, N: less than 0.015%, the balance has a component composition consisting of Fe and inevitable impurities, and the area ratio is polygonal ferrite: 10% or less (including 0% ), tempered martensite: 40% or more, fresh martensite: 20% or less (including 0%), and bainitic ferrite with internal carbides of 20 or less per 10 ⁇ m 2 : 3 to 40% , and the area SC ⁇ 0.5 of the region having a steel structure with a volume fraction of retained austenite: 5 to 20% and a C concentration of 0.50% or more, and a C concentration of 0.30%
  • the ratio S C ⁇ 0.5 /S C ⁇ 0.3 ⁇ 100 of the area S C ⁇ 0.3 of the above region is 20% or more.
  • the steel sheet of the present invention contains the following components.
  • % which is a unit of content of a component, means “% by mass”.
  • C 0.06-0.25%
  • C secures a predetermined strength by securing the area ratio of tempered martensite, secures the volume ratio of retained ⁇ to improve ductility, and concentrates in retained ⁇ to stabilize retained ⁇ . It is contained from the viewpoint of improving ductility.
  • the inclusion of C can improve the HAZ softening resistance by suppressing deformation in the HAZ through an increase in the strength of the melted portion and the rapidly cooled portion from the ⁇ region in the welded joint. If the C content is less than 0.06%, these effects cannot be sufficiently secured, so the lower limit is made 0.06%.
  • the C content is preferably 0.09% or more, more preferably 0.11% or more.
  • the C content exceeds 0.25%, the transformation of upper bainite during intermediate holding during cooling is delayed, making it difficult to form residual ⁇ that is generated adjacent to a predetermined amount of upper bainite. As a result, ductility is reduced. In addition, the amount of clumpy martensite or clumpy residual ⁇ increases, deteriorating the stretch flanging formability. Furthermore, various properties such as HAZ softening resistance, spot weldability, bendability, and hole expandability during laser welding of the steel sheet are significantly deteriorated. Based on these, the upper limit of the C content is set to 0.25%. From the viewpoint of ductility and HAZ softening resistance, the C content is preferably 0.22% or less. From the viewpoint of further improving ductility and HAZ softening resistance, the C content is more preferably 0.20% or less.
  • Si 0.4-2.5% Si strengthens ferrite to increase strength, suppresses the formation of carbides in martensite and bainite, improves the stability of retained ⁇ and improves ductility, and has a solid-solution strengthening amount that is not easily affected by heat. It is contained from the viewpoint of improving the HAZ softening resistance in the weld zone by increasing the From these points of view, the Si content should be 0.4% or more. From the viewpoint of improving ductility, the Si content is preferably 0.6% or more. More preferably, the Si content is 0.8% or more. If the Si content exceeds 2.5%, the rolling load during hot rolling becomes extremely high, making it difficult to produce thin sheets. In addition, the chemical convertability and the toughness of the weld zone deteriorate.
  • the Si content should be 2.5% or less.
  • the Si content is preferably less than 2.0% from the viewpoint of chemical conversion treatability, raw material, and toughness of the weld zone. From the viewpoint of ensuring the toughness of the weld zone, the Si content is preferably 1.8% or less, more preferably 1.5% or less.
  • Mn 1.5-3.5%
  • Mn secures a predetermined area ratio of tempered martensite and/or bainite to ensure strength, lowers the Ms point of retained ⁇ to stabilize retained ⁇ and improves ductility, and has bainite like Si. It is an important element from the viewpoint of suppressing the formation of carbides in the medium to improve ductility and from the viewpoint of increasing the volume fraction of retained ⁇ to improve ductility. In order to obtain these effects, the Mn content should be 1.5% or more. From the viewpoint of stabilizing residual ⁇ and improving ductility, the Mn content is preferably 2.5% or more. The Mn content is preferably 2.6% or more, more preferably 2.7% or more.
  • the Mn content should be 3.5% or less.
  • the Mn content is preferably 3.2% or less. More preferably, the Mn content is 3.1% or less.
  • P 0.02% or less
  • P is an element that strengthens steel, but if its content is large, it deteriorates spot weldability. Therefore, the P content should be 0.02% or less. From the viewpoint of improving spot weldability, the P content is preferably 0.01% or less. Although P may not be included, the P content is preferably 0.001% or more from the viewpoint of manufacturing cost.
  • S 0.01% or less S has the effect of improving scale exfoliation in hot rolling and suppressing nitriding during annealing, but it is an element that reduces spot weldability, bendability, and hole expandability. . From these points, the S content is made 0.01% or less. In the present invention, since the contents of C, Si, and Mn are high, the spot weldability tends to deteriorate, and from the viewpoint of improving the spot weldability, the S content is preferably 0.0020% or less, and 0.0010%. It is more preferable to make it less than. Although S may not be included, the S content is preferably 0.0001% or more from the viewpoint of manufacturing cost. More preferably, the S content is 0.0005% or more.
  • sol. Al less than 1.0% Al is contained for the purpose of deoxidizing and stabilizing residual ⁇ as a substitute for Si. sol. Although the lower limit of Al is not particularly defined, in order to stably deoxidize, sol.
  • the Al content is preferably 0.005% or more. Also, sol. More preferably, the Al content is 0.01% or more. On the other hand, sol. If the Al content is 1.0% or more, the strength of the material is extremely reduced, and the chemical conversion treatability is also adversely affected. Al content is less than 1.0%. In order to obtain high strength, sol. Al is more preferably less than 0.50%, more preferably 0.20% or less.
  • N Less than 0.015% N is an element that forms nitrides such as BN, AlN, and TiN in steel, and is an element that lowers the hot ductility of steel and lowers the surface quality. Also, in steel containing B, there is a problem that the effect of B disappears through the formation of BN. When the N content is 0.015% or more, the surface quality deteriorates significantly. Therefore, the N content should be less than 0.015%.
  • the N content is preferably 0.010% or less. Although N may not be included, the N content is preferably 0.0001% or more from the viewpoint of manufacturing cost. More preferably, the N content is 0.001% or more.
  • the balance other than the above is Fe and unavoidable impurities.
  • the steel sheet of the present invention preferably has a chemical composition containing the basic components described above, with the balance being iron (Fe) and unavoidable impurities.
  • the chemical composition of the steel sheet of the present invention can appropriately contain one or more selected from the following (A), (B), and (C) as the following optional elements.
  • Ti 0.1% or less Ti fixes N in the steel as TiN, and has the effect of improving the hot ductility and the effect of improving the hardenability of B.
  • TiC precipitation has the effect of refining the structure.
  • the Ti content is preferably 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The Ti content is more preferably 0.010% or more.
  • the Ti content is 0.05% or less. In order to ensure high ductility, the Ti content is more preferably 0.03% or less.
  • B 0.01% or less B is an element that improves the hardenability of steel, and tends to generate a predetermined area ratio of tempered martensite and/or bainite. Further, B improves the hardenability in the vicinity of the weld zone and forms a hard structure in the vicinity of the weld zone, thereby improving the HAZ softening resistance. Furthermore, the residual solid solution B improves the delayed fracture resistance.
  • the B content is preferably 0.0002% or more. Also, the B content is more preferably 0.0005% or more. More preferably, the B content is 0.0010% or more.
  • the B content when B is contained, the B content shall be 0.01% or less.
  • the B content is 0.0050% or less. More preferably, the B content is 0.0030% or less.
  • Cu 1% or less Cu improves corrosion resistance in the use environment of automobiles.
  • corrosion products of Cu can coat the surface of the steel sheet and suppress hydrogen penetration into the steel sheet.
  • Cu is an element that is mixed when scrap is used as a raw material. By allowing Cu to be mixed, recycled materials can be used as raw materials, and manufacturing costs can be reduced. From this point of view, it is preferable to contain 0.005% or more of Cu, and more preferably 0.05% or more of Cu from the viewpoint of improving the delayed fracture resistance. More preferably, the Cu content is 0.10% or more. On the other hand, if the Cu content is too high, surface defects will occur. Therefore, when Cu is contained, the Cu content is set to 1% or less. The Cu content is preferably 0.4% or less, more preferably 0.2% or less.
  • Ni 1% or less Ni, like Cu, can improve corrosion resistance. Ni can also suppress the occurrence of surface defects that tend to occur when Cu is contained. From these, it is preferable to contain 0.01% or more of Ni.
  • the Ni content is more preferably 0.04% or more, still more preferably 0.06% or more.
  • the Ni content is set to 1% or less.
  • the Ni content is preferably 0.4% or less, more preferably 0.2% or less.
  • Cr 1.0% or less Cr can be contained from the effect of improving the hardenability of steel and the effect of suppressing the formation of carbides in martensite and upper/lower bainite.
  • the Cr content is preferably 0.01% or more.
  • the Cr content is more preferably 0.03% or more, still more preferably 0.06% or more.
  • the Cr content is set to 1.0% or less.
  • the Cr content is preferably 0.8% or less, more preferably 0.4% or less.
  • Mo 0.5% or less Mo can be contained from the effect of improving the hardenability of steel and the effect of suppressing the formation of carbides in martensite and upper/lower bainite.
  • the Mo content is preferably 0.01% or more.
  • the Mo content is more preferably 0.03% or more, still more preferably 0.06% or more.
  • Mo significantly degrades the chemical conversion treatability of cold-rolled steel sheets. Therefore, when Mo is contained, the Mo content should be 0.5% or less. From the viewpoint of improving chemical convertibility, the Mo content is preferably 0.15% or less.
  • V 0.5% or less
  • V has the effect of improving the hardenability of steel, the effect of suppressing the formation of carbides in martensite and upper/lower bainite, the effect of refining the structure, and the precipitation of carbides to improve delayed fracture resistance. It can be contained from the effect to improve.
  • the V content is preferably 0.003% or more.
  • the V content is more preferably 0.005% or more, still more preferably 0.010% or more.
  • a large amount of V significantly deteriorates castability. Therefore, when V is contained, the V content should be 0.5% or less.
  • the V content is preferably 0.3% or less, more preferably 0.1% or less.
  • the V content is more preferably 0.05% or less, and even more preferably 0.03% or less.
  • Nb 0.1% or less Nb should be contained because it has the effect of refining the steel structure and increasing the strength, the effect of promoting bainite transformation through grain refinement, the effect of improving bendability, and the effect of improving delayed fracture resistance. can be done.
  • Nb improves the hardenability in the vicinity of the welded portion, thereby forming a hard phase in the vicinity of the welded portion and improving the HAZ softening resistance.
  • the Nb content is preferably 0.002% or more.
  • the Nb content is more preferably 0.004% or more, still more preferably 0.010% or more.
  • the precipitation strengthening becomes too strong and the ductility is lowered.
  • it causes an increase in rolling load and deterioration of castability. Therefore, when Nb is contained, the Nb content is set to 0.1% or less.
  • the Nb content is preferably 0.05% or less, more preferably 0.03% or less.
  • Zr 0.2% or less Zr should be contained because of the effect of improving the hardenability of steel, the effect of suppressing the formation of carbides in bainite, the effect of refining the structure, and the effect of precipitating carbides to improve delayed fracture resistance. can be done.
  • the Zr content is preferably 0.005% or more.
  • the Zr content is more preferably 0.008% or more, still more preferably 0.010% or more.
  • the Zr content should be 0.2% or less.
  • the Zr content is preferably 0.15% or less, more preferably 0.08% or less.
  • the Zr content is more preferably 0.03% or less, and still more preferably 0.02% or less.
  • W 0.2% or less W should be contained because of the effect of improving the hardenability of steel, the effect of suppressing the formation of carbides in bainite, the effect of refining the structure, and the effect of precipitating carbides to improve delayed fracture resistance. can be done.
  • the W content is preferably 0.005% or more.
  • the W content is more preferably 0.008% or more, still more preferably 0.010% or more.
  • the W content should be 0.2% or less.
  • the W content is preferably 0.15% or less, more preferably 0.08% or less.
  • the W content is more preferably 0.03% or less, still more preferably 0.02% or less.
  • Ca 0.0040% or less Ca fixes S as CaS and contributes to improvement of bendability and delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, still more preferably 0.0010% or more. On the other hand, when Ca is added in a large amount, it deteriorates the surface quality and bendability. Therefore, when Ca is contained, the Ca content is set to 0.0040% or less. The Ca content is preferably 0.0035% or less, more preferably 0.0020% or less.
  • Ce 0.0040% or less
  • the Ce content is preferably 0.0002% or more.
  • the Ce content is more preferably 0.0004% or more, and still more preferably 0.0006% or more.
  • the Ce content is set to 0.0040% or less.
  • the Ce content is preferably 0.0035% or less, more preferably 0.0020% or less.
  • La 0.0040% or less
  • the La content is preferably 0.0002% or more.
  • the La content is more preferably 0.0004% or more, still more preferably 0.0006% or more.
  • the La content is set to 0.0040% or less.
  • the La content is preferably 0.0035% or less, more preferably 0.0020% or less.
  • Mg 0.0030% or less Mg fixes O as MgO and contributes to the improvement of delayed fracture resistance. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more, still more preferably 0.0006% or more. On the other hand, if a large amount of Mg is added, the surface quality and bendability deteriorate. Therefore, when Mg is contained, the Mg content should be 0.0030% or less. The Mg content is preferably 0.0025% or less, more preferably 0.0010% or less.
  • Sb 0.1% or less Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, thereby suppressing a decrease in the content of C and B in the surface layer. In addition, by suppressing the reduction in the content of C and B, the formation of ferrite in the surface layer of the steel sheet is suppressed, the strength is increased, and the delayed fracture resistance is improved. From this point of view, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.004% or more, still more preferably 0.006% or more. On the other hand, if the Sb content exceeds 0.1%, the castability deteriorates, segregation occurs at prior ⁇ grain boundaries, and the delayed fracture resistance of sheared edges deteriorates. Therefore, when Sb is contained, the Sb content is made 0.1% or less. The Sb content is preferably 0.04% or less, more preferably 0.03% or less.
  • Sn 0.1% or less Sn suppresses oxidation and nitridation of the surface layer of the steel sheet, thereby suppressing a decrease in the content of C and B in the surface layer. In addition, by suppressing the reduction in the content of C and B, the formation of ferrite in the surface layer of the steel sheet is suppressed, the strength is increased, and the delayed fracture resistance is improved. From this point of view, the Sn content is preferably 0.002% or more. The Sn content is preferably 0.004% or more, more preferably 0.006% or more. On the other hand, when the Sn content exceeds 0.1%, castability deteriorates. In addition, Sn segregates in the prior ⁇ grain boundaries, degrading the delayed fracture resistance of the sheared edges. Therefore, when Sn is contained, the Sn content is set to 0.1% or less. The Sn content is preferably 0.04% or less, more preferably 0.03% or less.
  • the optional element contained below the lower limit does not impair the effects of the present invention. Therefore, when the content of the arbitrary element is less than the preferred lower limit, the arbitrary element is included as an unavoidable impurity.
  • Polygonal ferrite 10% or less (including 0%) Polygonal ferrite formed during annealing or cooling process contributes to the improvement of ductility, but causes a decrease in stretch flanging formability due to a difference in hardness from the surrounding hard phase such as martensite. As long as the area ratio of polygonal ferrite is 10% or less, it does not impair the effects of the present invention, so it may be contained. Therefore, in the present invention, the area ratio of polygonal ferrite is set to 10% or less. Polygonal ferrite is preferably 5% or less, more preferably 2% or less. Further, polygonal ferrite may be 0%.
  • Tempered martensite 40% or more In order to obtain a predetermined strength and stretch flanging formability, the area ratio of tempered martensite is 40% or more. The tempered martensite is preferably 50% or more. On the other hand, if the tempered martensite exceeds 80%, the strength is excessively increased, resulting in a decrease in ductility. Therefore, the tempered martensite is preferably 80% or less. Tempered martensite is more preferably 75% or less.
  • Fresh martensite 20% or less (including 0%)
  • a large amount of bainite transformation is to occur in the final tempering step (a step of retaining steel at an average cooling rate CR4, which will be described later)
  • a large amount of massive martensite or massive residual ⁇ remains. Therefore, conventionally, in order to prevent this, a method of promoting bainite transformation by reducing Mn has been used.
  • the Mn content is reduced, the effect of stabilizing the residual ⁇ and the effect of increasing the volume fraction are lost, thereby impairing the ductility.
  • the present invention in which a steel sheet containing a large amount of Mn is subjected to an appropriate cooling treatment, it is possible to realize both the use of bainite transformation and the reduction of the block structure.
  • the area ratio of fresh martensite is set to 20% or less.
  • the content of fresh martensite is preferably 10% or less.
  • Fresh martensite is more preferably 5% or less.
  • fresh martensite may be 0%.
  • the internal carbides contain 20 or less bainitic ferrites per 10 ⁇ m 2 at an area ratio of 3% or more, C is efficiently concentrated in the surrounding residual ⁇ .
  • bainitic ferrite having an area ratio of 20 or less internal carbides per 10 ⁇ m 2 is set to 3% or more.
  • the bainitic ferrite content is preferably 5% or more, more preferably 7% or more.
  • the area ratio of bainitic ferrite having 20 or less carbides per 10 ⁇ m 2 is set to 40% or less. It is preferably 30% or less, more preferably 25% or less. Further, in the present invention, more than 20 bainitic ferrites may be present per 10 ⁇ m 2 of internal carbides.
  • the total area ratio of tempered martensite, fresh martensite, upper bainite, lower bainite, and retained austenite as the residual structure of the polygonal ferrite is set to 90% or more.
  • the residual structure may be a structure containing one or more of tempered martensite, fresh martensite, upper bainite, lower bainite, and retained austenite, and tempered martensite, fresh martensite, upper bainite, and lower bainite.
  • Upper bainite and lower bainite contain bainitic ferrite with 20 or less internal carbides per 10 ⁇ m 2 .
  • the upper bainite and the lower bainite may also contain bainitic ferrite with more than 20 bainitic ferrites per 10 ⁇ m 2 of internal carbides.
  • Retained austenite 5-20%
  • the volume fraction of retained austenite (retained ⁇ ) is set to 5% or more in the entire steel structure.
  • the retained austenite is preferably 7% or more, more preferably 9% or more.
  • This amount of retained ⁇ includes retained ⁇ generated adjacent to bainite. If the amount of retained ⁇ increases too much, the strength, stretch flanging formability, and delayed fracture resistance deteriorate. Therefore, the volume fraction of residual ⁇ is set to 20% or less.
  • the retained austenite is preferably 15% or less.
  • "volume ratio" can be regarded as "area ratio”.
  • the area S C ⁇ 0.5 of the region having a C concentration of 0.50% (mass%) or more occupies the area S C ⁇ 0.3 of the region having a C concentration of 0.30% (mass%) or more.
  • the ratio S C ⁇ 0.5 / S C ⁇ 0.3 ⁇ 100 is 20% or more In order to ensure high ductility, the area S C ⁇ 0.5 of the region where the C concentration is 0.50% or more is C
  • the ratio S C ⁇ 0.5 / S C ⁇ 0.3 ⁇ 100 of the area S C ⁇ 0.3 of the region having a concentration of 0.30% or more is set to 20% or more.
  • the ratio is preferably 25% or more, more preferably 30% or more.
  • the number density of retained austenite existing adjacent to bainitic ferrite with internal carbides of 20 or less per 10 ⁇ m 2 is 50 or more per 10000 ⁇ m 2 (preferred requirement)
  • C is efficiently distributed to the adjacent untransformed austenite, and a high carbon concentration of retained ⁇ that contributes to the improvement of ductility in the final structure is obtained. be done.
  • the number density of retained austenite existing adjacent to bainitic ferrite having 20 or less internal carbides per 10 ⁇ m 2 is 50 per 10000 ⁇ m 2 or more. is preferred.
  • the number density is 70 or more per 10,000 ⁇ m 2 , still more preferably 100 or more.
  • the number density of retained austenite present adjacent to bainitic ferrite having 20 or less internal carbides per 10 ⁇ m 2 is 10000 ⁇ m 2 It is preferably 400 or less, more preferably 300 or less.
  • FIG. 2 shows an example of a SEM photograph of the steel structure of the steel plate.
  • the polygonal ferrite as shown in FIG. 2 is intended for relatively equiaxed ferrite with almost no carbide inside. This is the region that looks blackest in the SEM.
  • Bainitic ferrite is a ferrite structure with formation of carbides or residual ⁇ that appears white in SEM. If it is difficult to distinguish between bainitic ferrite and polygonal ferrite, a ferrite region with an aspect ratio of ⁇ 2.0 is polygonal ferrite, and a region with an aspect ratio >2.0 is bainitic ferrite. It is classified as ferrite and the area ratio is calculated.
  • FIG. 3 is a diagram for explaining the method for measuring the steel structure of the steel plate of the present invention. As shown in FIG.
  • the aspect ratio is obtained by determining the major axis length a where the particle length is the longest, and the particle length when crossing the particle longest in the direction perpendicular to it, the minor axis length b and a/b is the aspect ratio.
  • each particle is divided at positions where the particles are approximately evenly divided, and the size of each particle is measured.
  • the number of carbides inside bainitic ferrite per 10 ⁇ m 2 is obtained by counting the area ratio of bainitic ferrite and the number of carbides inside in an SEM photograph at a magnification of 5000, and calculating the number of carbides from the area of each bainitic ferrite. It can be obtained by dividing by the ratio and converting to a value per 10 ⁇ m 2 .
  • Tempered martensite is a region with an internal lath-like substructure and carbide precipitation in the SEM.
  • Fresh martensite is a blocky region that looks white with no substructure visible in the SEM.
  • a structure having one or more of tempered martensite, fresh martensite, upper bainite, lower bainite, and retained austenite corresponds to the residual structure other than the above polygonal ferrite, and the total area ratio of this structure is It is the area ratio of the region other than the above polygonal ferrite.
  • the area ratio of carbide is very small, it is included in the above area ratio of the residual structure.
  • the volume fraction of retained austenite (retained ⁇ ) was determined by X-ray diffraction after chemical polishing from the surface of the steel plate to the 1/4 thickness position.
  • a Co-K ⁇ ray source was used for incident X-rays, and the volume of retained austenite was determined from the intensity ratios of the (200), (211), and (220) planes of ferrite and the (200), (220), and (311) planes of austenite. Calculate rate.
  • the volume ratio of the residual ⁇ obtained by X-ray diffraction is equal to the area ratio of the residual ⁇ in the steel structure.
  • the number density of retained austenite present adjacent to bainitic ferrite with internal carbides of 20 or less per 10 ⁇ m 2 was obtained in a SEM by mirror-polishing the sample used for observing bainitic ferrite.
  • An electron backscattering diffraction pattern (EBSD) of the same field of view was analyzed using an EBSD analysis program OIM Data Collection ver. 7, and the obtained data were analyzed using TSL OIM Analysis ver.
  • phase map data measures the number density of fcc structures existing adjacent to bainitic ferrite with 20 or less internal carbides per 10 ⁇ m 2 by doing
  • adjacent means that the bcc structure and the fcc structure, which correspond to bainitic ferrite with internal carbides of 20 or less per 10 ⁇ m 2 in the phase map, are in contact. includes the case where fcc structure is included.
  • the steel sheet of the present invention preferably has a tensile strength of 980 MPa or more. More preferably, it is 1180 MPa or more.
  • the upper limit of the tensile strength is preferably 1450 MPa or less, more preferably 1400 MPa or less from the viewpoint of compatibility with other properties.
  • the total elongation T-El is 16.0% or more at TS: less than 1180 MPa, 14.0% or more at TS: 1180 MPa or more and less than 1320 MPa, and 13.0% or more at TS: 1320 MPa or more. Molding stability is remarkably improved. It is preferable to secure a hole expansion ratio ⁇ of 30% or more. From the viewpoint of compatibility with other properties, the upper limit of ⁇ is preferably 90% or less, more preferably 80% or less, at any strength level.
  • the steel plate of the present invention is subjected to laser welding, a fracture mode determination test, and a notch tensile test, and it is preferable that the fracture mode is base metal fracture and HAZ strength ⁇ base metal TS + 50 MPa.
  • the steel sheet of the present invention described above may be a steel sheet having a galvanized layer on its surface.
  • the plating layer may be either a hot-dip plating layer or an electroplating layer.
  • a steel slab having the chemical composition described above is subjected to hot rolling and cold rolling, and then the obtained cold-rolled steel sheet is annealed.
  • a step of holding at 810 to 900 ° C. a step of cooling the temperature range from 810 ° C. to 500 ° C. at an average cooling rate (CR1): 5 to 100 ° C./s, and a martensitic transformation start temperature Ms from 500 ° C. (° C.) or more and a retention stop temperature (T1) of 320° C.
  • average cooling rate 0.01 to 5 ° C./s and a step of cooling to a temperature of 50° C. or less at an average cooling rate (CR5) of 0.1° C./s or more, in this order.
  • the temperature specified in each step in the present invention refers to the surface temperature of the slab (steel slab) or steel plate.
  • FIG. 4 is a diagram for explaining the method of manufacturing a steel plate according to the present invention, and in particular shows changes over time in the surface temperature of a slab (steel slab) or steel plate. The details of each step, including this temperature change over time, will be described below.
  • Hot rolling Steel slabs can be hot rolled by heating and then rolling, by directly rolling the slab after continuous casting without heating, and by subjecting the slab after continuous casting to heat treatment for a short period of time before rolling. There are ways to do so. Hot rolling may be performed according to a conventional method .
  • the temperature of the sample should be 400 to 720°C.
  • the coiling temperature is preferably 430 to 530° C. from the viewpoint of suppressing plate thickness fluctuations and stably ensuring high strength.
  • the rolling reduction (cumulative rolling reduction) may be 30 to 85%. From the viewpoint of stably ensuring high strength and reducing anisotropy, the rolling reduction is preferably 35 to 85%. If the rolling load is high, it is possible to perform a softening annealing treatment at 450 to 730° C. in a CAL (continuous annealing line) or BAF (box annealing furnace).
  • CAL continuous annealing line
  • BAF box annealing furnace
  • Annealing A steel slab having the chemical composition described above is subjected to hot rolling and cold rolling, and then annealed under the conditions specified below.
  • Annealing equipment is not particularly limited, but a continuous annealing line (CAL) or a continuous hot dip galvanizing line (CGL) is preferable from the viewpoint of ensuring productivity and desired heating and cooling rates.
  • CAL continuous annealing line
  • CGL continuous hot dip galvanizing line
  • Annealing temperature maintained at 810 to 900° C.
  • the annealing temperature is set to 810 to 900° C. in order to secure a predetermined area ratio of tempered martensite and/or bainite and a predetermined volume ratio of retained ⁇ .
  • it is preferable to adjust the annealing temperature so as to achieve ⁇ single-phase region annealing.
  • it is 815°C or higher.
  • the annealing temperature should be 900° C. or lower.
  • the annealing temperature is 880°C or less.
  • Average cooling rate (CR1) for the temperature range from 810°C to 500°C Cooling at 5 to 100°C/s After holding at 810 to 900°C, average cooling rate (CR1) for the temperature range from 810°C to 500°C : Cool at 5 to 100°C/s.
  • the average cooling rate (CR1) is preferably at least 8°C/s.
  • the average cooling rate (CR1) is preferably 50°C/s or less, more preferably less than 30°C/s.
  • the average cooling rate (CR1) is "(810 ° C. (cooling start temperature) - 500 ° C. (cooling stop temperature)) / (cooling time from the cooling start temperature of 810 ° C. to the cooling stop temperature of 500 ° C. (seconds ))”.
  • the temperature range from the martensite transformation start temperature Ms (° C.) to the retention stop temperature (T1) of 320° C. or more is retained (slowly cooled) at an average cooling rate (CR2) of 10° C./s or less for 10 seconds or more and 60 seconds or less. Therefore, it is possible to generate bainite with a lower carbide density, and adjacently generate residual ⁇ with a high C concentration.
  • the temperature range is less than Ms or less than 320° C., martensite is generated first, followed by lower bainite, resulting in a decrease in strength.
  • the temperature range is Ms or higher, 320° C. or higher, and 500° C. or lower.
  • This temperature range is preferably 380° C. or higher, more preferably 420° C. or higher.
  • this temperature range is preferably 480° C. or lower, more preferably 460° C. or lower.
  • the average cooling rate (CR2) exceeds 10°C/s, the amount of bainite transformation decreases.
  • the average cooling rate (CR2) is set to 10° C./s or less. If the residence time is less than 10 s, the desired amount of bainite cannot be obtained, and if it exceeds 60 s, the concentration of C from bainite to massive untransformed ⁇ progresses, resulting in an increase in the residual amount of the massive structure. Therefore, the residence time should be 10 seconds or more and 60 seconds or less. From the viewpoint of ensuring bainitic ferrite and retained austenite and improving ductility and HAZ softening resistance, the residence time is preferably 20 seconds or more. Moreover, from the viewpoint of improving the stretch flanging formability by reducing the massive structure, the residence time is preferably 50 seconds or less.
  • the martensite transformation start temperature Ms is measured using a Formaster tester using a cylindrical test piece (diameter 3 mm x height 10 mm), held at a predetermined annealing temperature, and then quenched with helium gas. It can be determined by measuring the change.
  • the average cooling rate (CR2) is "(500° C. (retention start temperature) ⁇ retention stop temperature (T1))/(retention time from 500° C. to retention stop temperature (T1) (seconds))". is.
  • the temperature range from the retention stop temperature (T1) to the cooling stop temperature (T2) of 200°C or higher and 300°C or lower is cooled at an average cooling rate (CR3) of 3 to 100°C/s. It is necessary to cool quickly so as not to thicken too much.
  • the average cooling rate (CR3) in the temperature range from the retention stop temperature T1 of 320 ° C. or higher to the cooling stop temperature T2 of 200 ° C. or higher and 300 ° C. or lower is less than 3 ° C./s, carbon transforms into a massive untransformed ⁇ It thickens, the amount of fresh martensite at the time of final cooling increases, and the stretch flanging formability deteriorates.
  • the average cooling rate (CR3) in the temperature range from the retention stop temperature T1 to the cooling stop temperature T2 of 200° C. or higher and 300° C. or lower is set to 3° C./s or more.
  • the average cooling rate (CR3) is more preferably 5°C/s or higher, still more preferably 8°C/s or higher. If the average cooling rate in this temperature range is too high, the plate shape deteriorates, so the average cooling rate (CR3) in this temperature range is set to 100° C./s or less.
  • the average cooling rate (CR3) is preferably 50°C/s or less.
  • the cooling stop temperature T2 is set to 200° C. or higher.
  • the cooling stop temperature T2 is preferably 220°C or higher, more preferably 240°C or higher.
  • the cooling stop temperature T2 exceeds 300°C, a large amount of untransformed ⁇ remains in the form of lumps, the amount of fresh martensite at the time of final cooling increases, and the stretch flanging formability deteriorates. Therefore, the cooling stop temperature T2 is set to 300° C. or less.
  • the cooling stop temperature T2 is preferably 280° C. or less.
  • the average cooling rate (CR3) is "(retention stop temperature (T1)) - (cooling stop temperature (T2)) / (cooling time from retention stop temperature (T1) to cooling stop temperature (T2) (seconds ))”.
  • the temperature range from the cooling stop temperature (T2) to 380 ° C. is heated at an average heating rate of 2 ° C./s or more.
  • the temperature range from the cooling stop temperature (T2) to 380 ° C. is heated in a short time to form carbides.
  • High ductility can be secured by suppressing precipitation.
  • upper bainite is generated when reheating to 380° C. or higher with martensite or bainite generated by cooling as nuclei. If the average heating rate up to 380°C is slow, these effects cannot be obtained. As a result, the amount of retained ⁇ decreases and the ductility decreases.
  • the average heating rate in the temperature range from the cooling stop temperature (T2) to 380°C is set to 2°C/s or more.
  • the average heating rate is preferably 5° C./s or higher, more preferably 10° C./s or higher.
  • the upper limit of the average heating rate is not particularly limited, it is preferably 50° C./s or less, more preferably 30° C./s or less.
  • the average heating rate is "380° C. (heating stop temperature) ⁇ (cooling stop temperature (T2))/(heating time from cooling stop temperature T2 to 380° C. (heating stop temperature) (seconds))". .
  • the average cooling rate (CR4) should be 0.01° C./s or higher.
  • the average cooling rate (CR4) exceeds 5° C./s, C distribution to residual ⁇ is suppressed, and a sufficient amount of C-enriched region cannot be obtained.
  • the average cooling rate (CR4) shall be 5°C/s or less.
  • the average cooling rate (CR4) is "(cooling start temperature (T3)) - (cooling stop temperature (T4)) / (cooling time from cooling start temperature (T3) to cooling stop temperature (T4) (seconds ))”.
  • the cooling start temperature (T3) and the cooling stop temperature (T4) are not particularly limited as long as they are in the range of 340°C to 590°C, but the cooling start temperature (T3) is in the range of 360 to 580°C. is preferred, and the cooling stop temperature (T4) is preferably in the range of 350 to 450°C.
  • the holding (retention) in the temperature range of 340 to 590° C. may also serve as hot-dip galvanizing treatment. That is, the steel sheet may be subjected to hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment in the above-described step of retaining at an average cooling rate (CR4) of 0.01 to 5° C./s.
  • hot-dip galvanizing treatment it is preferable to immerse the steel sheet in a galvanizing bath at 440° C. or higher and 500° C. or lower, perform hot-dip galvanizing treatment, and then adjust the coating weight by gas wiping or the like.
  • a zinc plating bath having an Al content of 0.10% or more and 0.22% or less for hot-dip galvanization.
  • an alloying hot-dip galvanizing treatment an alloying treatment for galvanizing can be performed after the hot-dip galvanizing treatment.
  • alloying treatment for galvanizing it is preferable to carry it out in a temperature range of 470° C. or higher and 590° C. or lower. This process is a process of performing cooling (retention, slow cooling), but if the above temperature range, retention time range, and average cooling rate CR4 range are satisfied, hot dip galvanizing can be performed during this process. It can be subjected to alloying treatment and zinc plating.
  • the hot-dip galvanizing treatment and the alloying treatment of galvanizing may be accompanied by a temperature rise.
  • the temperature is 0.1°C to a temperature of 50°C or less. Cooling is performed at an average cooling rate (CR5) of at least /s.
  • a steel sheet can be subjected to skin pass rolling from the viewpoint of stabilizing press formability such as adjustment of surface roughness and flattening of the plate shape, and from the viewpoint of increasing YS.
  • the skin pass elongation rate is preferably 0.1 to 0.5%.
  • the plate shape can be flattened by a leveler.
  • the average cooling rate (CR5) to a temperature of 50° C. or lower is preferably 5° C./s or more, more preferably 100° C./s or less.
  • the average cooling rate (CR5) is "(340°C (cooling start temperature) - cooling stop temperature below 50°C)/(cooling time from cooling start temperature to cooling stop temperature (seconds))".
  • low-temperature heat treatment at 100 to 300° C. for 30 sec to 10 days after the above heat treatment or after skin pass rolling.
  • This treatment causes tempering of martensite generated during final cooling or skin-pass rolling, and release of hydrogen that has entered the steel sheet during annealing.
  • Hydrogen can be reduced to less than 0.1 ppm by low temperature heat treatment.
  • electroplating That is, after the step of cooling at an average cooling rate (CR5) of 0.1° C./s or higher, the steel sheet may be electrogalvanized. After electroplating, it is preferable to perform the above-mentioned low-temperature heat treatment from the viewpoint of reducing hydrogen in the steel.
  • the steel plate of the present invention preferably has a plate thickness of 0.5 mm or more. Also, the plate thickness is preferably 2.0 mm or less.
  • the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Further, the method for manufacturing a member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form a member.
  • the steel sheet of the present invention has a tensile strength of 980 MPa or more, excellent ductility, excellent stretch flanging formability, and excellent laser weldability. Therefore, members obtained using the steel sheet of the present invention also have high strength, and have superior ductility, excellent stretch flanging formability, and excellent laser weldability as compared with conventional high-strength members. Moreover, if the member of the present invention is used, the weight can be reduced. Therefore, the member of the present invention can be suitably used for, for example, vehicle body frame parts. Components of the present invention also include welded joints.
  • General processing methods such as press processing can be used without restrictions for molding.
  • general welding such as spot welding and arc welding, riveting, caulking, and the like can be used without limitation.
  • Cold-rolled steel sheets having a thickness of 1.4 mm and having chemical compositions shown in Table 1 were treated under the annealing conditions shown in Table 2 to produce steel sheets of the present invention and steel sheets of comparative examples.
  • Each cold-rolled steel sheet was hot rolled (slab heating temperature: 1200 ° C., soaking time: 60 min, finish rolling temperature: 900 ° C., coiling temperature: 500 ° C. ) and cold rolling (rolling rate (cumulative rolling rate): 50%).
  • the martensite transformation start temperature Ms is obtained by holding a cylindrical test piece (3 mm in diameter x 10 mm in height) with a Formaster tester at a predetermined annealing temperature and then quenching it with helium gas. It was obtained by measuring the volume change of
  • some steel sheets are hot-dip galvanized in a process in which a temperature range of 340 ° C. to 590 ° C. is retained at an average cooling rate of 0.01 to 5 ° C./s for 20 s to 3000 s. It was processed and made into a hot-dip galvanized steel sheet (GI).
  • GI hot-dip galvanized steel sheet
  • the steel sheet was immersed in a galvanizing bath at a temperature of 440° C. or more and 500° C. or less to perform hot-dip galvanizing treatment, and then the coating weight was adjusted by gas wiping or the like.
  • a galvanizing bath with an Al content of 0.10% or more and 0.22% or less was used for hot-dip galvanizing.
  • the hot dip galvanized steel sheets were subjected to an alloying treatment after the hot dip galvanizing treatment as an alloying hot dip galvanizing treatment to obtain an alloyed hot dip galvanizing steel sheet (GA).
  • the alloying treatment was performed in the temperature range of 460°C or higher and 590°C or lower.
  • some of the steel sheets (cold-rolled steel sheets: CR) were electroplated to form electrogalvanized steel sheets (EG).
  • the steel structure was measured by the following method. Table 3 shows the measurement results. Measurement of the area ratio of polygonal ferrite, bainitic ferrite, tempered martensite, and fresh martensite is performed by cutting out a thickness cross-section parallel to the rolling direction, polishing it to a mirror surface, corroding it with 3 vol% nital, and reducing it to 1/4. The thickness position was observed by SEM at a magnification of 5000 times for 10 fields of view.
  • the polygonal ferrite as shown in FIG. 2 is intended for relatively equiaxed ferrite with almost no carbide inside. This is the region that looks blackest in the SEM.
  • Bainitic ferrite is a ferrite structure with formation of carbides or residual ⁇ that appears white in SEM. If it is difficult to distinguish between bainitic ferrite and polygonal ferrite, a ferrite region with an aspect ratio of ⁇ 2.0 is polygonal ferrite, and a region with an aspect ratio >2.0 is bainitic ferrite. It was classified as ferrite and the area ratio was calculated. As shown in FIG. 3(A), the aspect ratio is obtained by determining the major axis length a where the particle length is the longest, and the particle length when crossing the particle longest in the direction perpendicular to it, the minor axis length b and a/b is the aspect ratio. When the particles were in contact with each other, as shown in FIG. 3(B), the particles were divided at positions where the individual particles were approximately evenly divided, and the size of each particle was measured.
  • the number of carbides inside bainitic ferrite per 10 ⁇ m 2 is obtained by counting the area ratio of bainitic ferrite and the number of carbides inside in an SEM photograph at a magnification of 5000, and calculating the number of carbides from the area of each bainitic ferrite. It was obtained by dividing by the ratio and converting to a value per 10 ⁇ m 2 .
  • Tempered martensite is a region with an internal lath-like substructure and carbide precipitation in the SEM.
  • Fresh martensite is a blocky region that looks white with no substructure visible in the SEM.
  • a structure having one or more of tempered martensite, fresh martensite, upper bainite, lower bainite, and retained austenite corresponds to the residual structure other than the above polygonal ferrite, and the total area ratio of this structure is It is the area ratio of the region other than the above polygonal ferrite.
  • the area ratio of carbide is very small, it is included in the above area ratio of the residual structure.
  • the volume fraction of retained austenite (retained ⁇ ) was determined by X-ray diffraction after chemical polishing from the surface of the steel plate to the 1/4 thickness position.
  • a Co-K ⁇ ray source was used for incident X-rays, and the volume of retained austenite was determined from the intensity ratios of the (200), (211), and (220) planes of ferrite and the (200), (220), and (311) planes of austenite. calculated the rate.
  • the volume ratio of the residual ⁇ obtained by X-ray diffraction is equal to the area ratio of the residual ⁇ in the steel structure.
  • the number density of retained austenite present adjacent to bainitic ferrite with internal carbides of 20 or less per 10 ⁇ m 2 was obtained in a SEM by mirror-polishing the sample used for observing bainitic ferrite.
  • An electron backscattering diffraction pattern (EBSD) of the same field of view was analyzed using an EBSD analysis program OIM Data Collection ver. 7, and the obtained data were analyzed using TSL OIM Analysis ver.
  • phase map data (manufactured by EDAX/TSL) to obtain phase map data, and measure the number density of fcc structures existing adjacent to bainitic ferrite with 20 or less internal carbides per 10 ⁇ m 2 was obtained by
  • adjacent means that the bcc structure and the fcc structure, which correspond to bainitic ferrite with internal carbides of 20 or less per 10 ⁇ m 2 in the phase map, are in contact. includes the case where fcc structure is included.
  • a JIS No. 5 tensile test piece and a hole expansion test piece were taken from the obtained steel plate, and a tensile test (based on JIS Z2241) was performed.
  • Table 3 shows the tensile strength TS and total elongation T-El. Those having a tensile strength of 980 MPa or more were judged to be excellent in strength.
  • the total elongation T-El is 16.0% or more when TS is less than 1180 MPa, 14.0% or more when TS is 1180 MPa or more and less than 1320 MPa, and 13.0% or more when TS is 1320 MPa or more. bottom.
  • the stretch flanging formability was evaluated by a hole expansion test conforming to the provisions of the Japan Iron and Steel Federation standard JFST1001 on a hole expansion test piece taken from the steel plate after heat treatment. That is, after punching a 100 mm ⁇ 100 mm square sample with a punch diameter of 10 mm and a die diameter of 10.3 mm (clearance 13%) using a punching tool, a conical punch with a vertical angle of 60 degrees is used to form a punch hole. The hole was widened until a crack penetrating through the thickness of the plate occurred, with the burr that occurred on the outside.
  • d 0 is the initial hole diameter (mm)
  • d is the hole diameter (mm) at the time of cracking
  • the hole expansion ratio ⁇ (%) ⁇ (d ⁇ d 0 )/d 0 ⁇ 100.
  • Table 3 shows. A steel having a ⁇ of 30% or more was judged to be excellent in hole expandability.
  • the weld line is perpendicular to the tensile axis and is located in the longitudinal center of the test piece, and as shown in FIG. (Notch tensile test) was performed.
  • Notch tensile test it is possible to evaluate the strength of the HAZ portion itself by deforming only the HAZ of the welded portion and a small area around it and forcibly breaking the HAZ portion. It is possible to grasp the In this evaluation, it was judged that the laser weldability (HAZ softening resistance) was excellent if the fracture mode in the fracture mode determination test was base metal fracture and the HAZ strength ⁇ base metal TS + 50 MPa in the notched tensile test.
  • inventive examples shown in Tables 2 and 3 are excellent in strength, ductility, hole expandability, and laser weldability (HAZ softening resistance), whereas the comparative examples are inferior in one of them.
  • the present invention has extremely high ductility, excellent stretch flanging formability, and excellent laser weldability, and can be preferably applied for press forming used in automobiles, home appliances, etc. after the press forming process.
  • the steel sheets of the present invention are used in the members obtained by molding, the members obtained by bonding, and the members obtained by further molding and bonding using the steel sheets of the present invention. Since it has strength, excellent ductility, excellent stretch flange formability and excellent laser weldability, it has high strength, excellent ductility, and excellent stretch flangeability like the steel plate of the example of the present invention. It was found to have formability and excellent laser weldability.

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