WO2023053909A1 - 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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WO2023053909A1
WO2023053909A1 PCT/JP2022/033946 JP2022033946W WO2023053909A1 WO 2023053909 A1 WO2023053909 A1 WO 2023053909A1 JP 2022033946 W JP2022033946 W JP 2022033946W WO 2023053909 A1 WO2023053909 A1 WO 2023053909A1
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less
cooling
steel sheet
temperature
particles
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English (en)
French (fr)
Japanese (ja)
Inventor
三周 知場
芳怡 王
洋一郎 松井
真次郎 金子
毅 横田
秀斗 尾園
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JFE Steel Corp
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JFE Steel Corp
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Priority to MX2024003586A priority Critical patent/MX2024003586A/es
Priority to JP2022575216A priority patent/JP7332062B1/ja
Priority to US18/692,925 priority patent/US20240384379A1/en
Priority to KR1020247008877A priority patent/KR20240051976A/ko
Priority to EP22875781.1A priority patent/EP4372119A4/en
Priority to CN202280064002.XA priority patent/CN117980520A/zh
Publication of WO2023053909A1 publication Critical patent/WO2023053909A1/ja
Anticipated expiration legal-status Critical
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a steel sheet suitable for press-formed products having a complicated shape used in automobiles, home appliances, etc. through a press-forming process and having excellent chemical convertibility, a member using the steel sheet, and a method for producing the same. Regarding.
  • 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.
  • austemper treatment (cooling from the single-phase region annealing temperature or the two-phase region annealing temperature to the bainite transformation temperature and isothermally holding is performed to utilize the bainite transformation during isothermal holding or cooling to retain A steel sheet containing C: 0.10 to 0.45%, Si: 0.5 to 1.8%, and Mn: 0.5 to 3.0%.
  • the residual ⁇ is formed by aging treatment in the temperature range of 350 to 500 ° C. for 1 to 30 minutes, and the steel plate has high ductility of TS: 80 kgf / mm 2 or more and TS x EL: 2500 kgf / mm 2 % or more. is obtained.
  • Patent Document 2 a steel sheet containing C: 0.10 to 0.25%, Si: 1.0 to 2.0%, and Mn: 1.5 to 3.0% is annealed at 10 ° C./s or more and hold for 180 to 600 seconds to obtain a residual ⁇ of 5% or more in volume ratio, bainitic ferrite of 60% or more in area ratio, and polygonal ferrite of 20% or less in microstructure. It is disclosed that a steel sheet excellent in both ductility: EL and stretch flangeability: ⁇ can be obtained by controlling .
  • Patent Document 3 Q&P treatment (cooling from the single-phase region annealing temperature or the two-phase region annealing temperature to the martensite start temperature: Ms to the martensite finish temperature: Mf, forming a martensite structure, and then regenerating A process of forming residual ⁇ by causing carbon distribution from the martensitic structure to untransformed ⁇ by heating), and a temperature range of 150 to 350 ° C after annealing a steel plate having a characteristic chemical composition. and then reheating and holding at 350 to 600° C., a structure containing ferrite, tempered martensite, and retained ⁇ is obtained, and a steel plate having both excellent ductility and stretch flangeability is obtained. ing.
  • Both the austempering treatment described in Patent Documents 1 and 2 and the Q&P treatment described in Patent Document 3 are heat treatments for producing TRIP steel sheets, but since tempered martensite that contributes to higher strength can be obtained, more Q&P treatment is suitable for manufacturing high-strength steel sheets.
  • Patent Document 4 by improving the above Q&P treatment and holding the temperature at 470 to 405 ° C. for 14 to 200 seconds during cooling after annealing, carbon is concentrated in untransformed ⁇ by utilizing the upper bainite transformation. and then cooling to Ms-90 to Ms-180 (° C.) for martensitic transformation, and then reheating to distribute carbon from the martensitic structure to untransformed ⁇ , thereby obtaining residual ⁇ .
  • a steel sheet having both high ductility and excellent stretch flangeability is obtained.
  • the amount of Si contained in the steel sheet is large in order to promote efficient carbon enrichment to untransformed ⁇ .
  • steel sheets used for press-formed members are then painted and incorporated into automobiles and the like, so they are subjected to chemical conversion treatment for the purpose of imparting good paintability to the steel sheets.
  • the chemical conversion treatment causes unevenness in the adhered crystal grains due to the chemical conversion treatment, which is a factor in the deterioration of paintability.
  • pickling treatment is usually performed as a pretreatment to improve the chemical conversion treatability.
  • the Si-containing surface oxidized layer causes a problem that the chemical conversion treatability is remarkably deteriorated.
  • Patent Document 5 After continuously immersing and pickling in a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid, a non-oxidizing acid is used. Discloses that the process of continuously immersing the steel sheet in an acid solution containing an oxidizing third acid for re-pickling can provide excellent chemical conversion treatability even to steel sheets with a high Si content. ing.
  • Patent Document 1 Although the conventional TRIP steel described in Patent Document 1 has excellent El, 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.
  • Patent Document 3 achieves relatively high ductility and high stretch flanging formability compared to conventional TRIP steel and steel using bainitic ferrite. It was not sufficient in forming with molded parts, and further improvement in ductility was required. Therefore, considering application to difficult-to-form parts, further improvement of ductility, especially uniform elongation and local elongation at the same time is required.
  • This uniform elongation is U.S., which represents the amount of elongation until necking begins to occur even in El, which is an index of ductility.
  • El the local elongation is the total elongation: T.E. Elongation is the amount of elongation obtained by subtracting uniform elongation from El. Represented by U.E.I. While maintaining El, L.E. El needs to be increased.
  • Patent Document 4 a steel sheet having high ductility and excellent stretch-flange formability can be obtained by holding utilizing the upper bainite transformation during cooling after annealing, and by the subsequent Q&P treatment and bainite transformation after reheating.
  • it promotes carbon distribution from martensite formed in the Q & P treatment to untransformed ⁇ , and contains a large amount of Si, so a specific pickling technique as described in Patent Document 5 is required.
  • Patent Document 5 The pickling described in Patent Document 5 is a technique for imparting excellent chemical conversion treatability to steel sheets, but the running cost is high, and it is costly to pickle all the various steel sheets in the same continuous annealing furnace with this technique. However, the establishment of other technologies has been sought after.
  • the conventional technology is still not sufficient as a technology for steel sheets that ensure high ductility and excellent stretch flanging formability while at the same time possessing excellent chemical conversion treatability.
  • the present invention has been made to solve such problems, and has a tensile strength of 980 MPa or more, and achieves high ductility, excellent stretch flanging formability, and excellent chemical conversion treatability. It aims at providing the manufacturing method of.
  • the tensile strength of 980 MPa or more means that a JIS No. 5 tensile test piece having a tensile direction perpendicular to the rolling direction and a crosshead speed of 10 mm/min are specified in JIS Z 2241 (2011). It means that the tensile strength is 980 MPa or more by a tensile test according to.
  • high ductility is determined by a tensile test conforming to the provisions of JIS Z 2241 (2011) with a crosshead speed of 10 mm / min on a JIS No. 5 tensile test piece having a tensile direction perpendicular to the rolling direction.
  • excellent stretch flanging formability means a hole expansion ratio ⁇ of 45% or more in a hole expansion test conforming to JFST 1001 (Iron Federation Standard).
  • good chemical conversion treatability means that the steel plate is subjected to sulfuric acid electrolytic pickling for 2 seconds at a current density of 20 to 35 A/dm 2 and degreased (treatment temperature 40 ° C., treatment time 120 seconds, spray degreasing). , surface conditioning (pH 9.5, treatment temperature room temperature, treatment time 20 seconds), and then chemical conversion treatment using a zinc phosphate chemical conversion treatment solution (chemical conversion treatment solution temperature 35 ° C., treatment time 120 seconds), It means that there is no surface on which the chemical conversion coating structure is not formed.
  • bainite transformation at around 400° C. causes carbon partitioning into untransformed austenite up to the T0 composition where the free energies of the fcc and bcc phases are equal, after which the bainite transformation stops. Therefore, the coarse and thermally unstable untransformed austenite becomes a hard martensitic structure or mechanically unstable retained ⁇ at the final cooling, which deteriorates the stretch flangeability. Thus, it is difficult to achieve both ductility and stretch flangeability in austempering.
  • the thermally unstable untransformed ⁇ undergoes martensite transformation at the cooling stop temperature of Ms to Mf, and during subsequent reheating Since the steel is tempered, the hardness difference between the hard phase and the soft phase is reduced, the stretch flangeability is excellent, and the ductility can be improved at the same time. Therefore, it can be seen that Q&P treatment (for example, Q&P treatment in which holding is performed during cooling after annealing) is desirable.
  • the Q&P treatment for example, the Q&P treatment that is held during cooling after annealing
  • an alloy design that particularly reduces the Si content is required.
  • the chemical conversion treatability described here is defined as a property that can satisfy the coatability in terms of adhesion amount and unevenness in a general pickling process. Sulfuric acid pickling, etc., but the pickling method is not limited.
  • the present inventors found that a soft ferrite structure is formed and adjacent needle-like ⁇ is formed by heating at a characteristic component and a characteristic temperature rising rate.
  • the present inventors have found that this acicular ⁇ contributes to the distribution of carbon and the formation of residual ⁇ in the formation of the structure during the cooling process, and that even a steel sheet with a low Si content exhibits excellent ductility. It is based on the following outline.
  • the low Si content is not particularly limited, but refers to the case where the Si content is less than 1.60% by mass.
  • the preferred rolling orientation (texture) and rotated cube orientation are obtained by cold rolling that suppresses the development of the shear texture with a cold reduction (reduction) of 5% or more and less than 25% in the first pass.
  • developed cold-rolled steel sheet ( ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011> orientation of the structure
  • a cold-rolled steel sheet having an area ratio of 35% or more and 75% or less with respect to the total structure of the bcc phase is manufactured.
  • the temperature rising rate (average heating rate) from 500 ° C. to Ac1 is set to 15 ° C./sec or less, so that 30% or more of the cold rolled steel sheet Fully recrystallize the cold-rolled structure at a rolling reduction of
  • the austenite ( ⁇ ) that subsequently transforms at temperatures above Ac1 nucleates from the grain boundaries of the recrystallized bcc phase or from residual carbides, but has a specific crystallographic orientation relationship to the surrounding bcc phase; .
  • the degree of interfacial matching is high, and grain growth accompanied by interfacial migration is delayed, but a portion of the interface preferentially migrates in order to approach an equilibrium state, forming needle-like austenite (needle-like ⁇ ).
  • the annealing temperature is two-phase annealing, and from the viewpoint of chemical conversion treatability, 0.6 ⁇ (T-Ac1) / (Ac3-Ac1) ⁇ 1.0 and 840 ° C. Annealing is performed at an annealing temperature T which is as follows.
  • the present invention has been made based on the above findings, and specifically provides the following. [1] % by mass, C: 0.10 to 0.24%, Si: 0.4% or more and less than 1.60%, Mn: 2.0-3.6%, P: 0.02% or less, S: 0.01% or less, sol.Al: less than 1.0%, N: contains less than 0.015%, and satisfies the following formula (1), Having a component composition in which the balance is Fe and unavoidable impurities, Area ratio of polygonal ferrite: 5% or more and 25% or less, Area ratio of upper bainite: 5% or more and 50% or less, Volume fraction of retained austenite: 3% or more and 20% or less, Area ratio of fresh martensite: 12% or less (including 0%), The sum of the area ratios of tempered martensite and lower bainite: 10% or more and 50% or less, and the area ratio of the remaining structure: 5% or less, The ratio of the total number of fresh martensite particles and retained austenite particles having
  • Si and Mn represent Si content (% by mass) and Mn content (% by mass), respectively.
  • Nb 0.2% or less
  • Ti 0.2% or less
  • V 0.2% or less
  • B 0.01% or less
  • Cu 0.2% or less
  • Ni 0.2% or less
  • Cr 0.4% or less
  • the component composition further includes, in % by mass, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.10% or less, Sb: 0.10% or less, REM: The steel sheet according to [1] or [2] above, containing one or more selected from 0.0050% or less.
  • Mg 0.0050% or less
  • Ca 0.0050% or less
  • Sn 0.10% or less
  • Sb 0.10% or less
  • REM The steel sheet according to [1] or [2] above, containing one or more selected from 0.0050% or less.
  • [5] After performing hot rolling and pickling on the steel slab having the chemical composition according to any one of [1] to [3] above, the resulting hot-rolled steel sheet is subjected to cold rolling.
  • a cold rolling process for obtaining a cold rolled steel sheet by applying An annealing step of obtaining a steel plate by subjecting the cold-rolled steel plate to an annealing treatment is Cumulative cold rolling rate: 30 to 85%, By setting the reduction rate of the first pass to 5% or more and less than 25%, ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011>
  • the cold rolling process for obtaining the cold-rolled steel sheet in which the total area ratio of the structure having the ⁇ 011> orientation is 35% or more and 75% or less with respect to the entire structure of the bcc phase includes For the cold-rolled steel sheet, the temperature range of 500 ° C.
  • the first cooling is performed to cool to the first cooling stop temperature Tc1 of 550 ° C. or less and 400 ° C. or more, After the first cooling, performing a first holding for 25 seconds or more at the first cooling stop temperature Tc1, After the first holding, the temperature range of 350 ° C. or lower and 200 ° C. or higher is cooled at an average cooling rate of 3.0 to 80 ° C./s, and cooled to a second cooling stop temperature Tc2 of 320 ° C. or lower and 150 ° C. or higher.
  • a method for producing a steel sheet comprising the step of subjecting the steel plate according to any one of [1] to [3] to at least one of forming and joining to form a member.
  • the steel plate which has the tensile strength of 980 MPa or more, and realizes high ductility, the excellent stretch flanging formability, and favorable chemical convertibility, a member, and its manufacturing method are provided.
  • the steel sheet of the present invention is suitable for complex-shaped press-formed products that are used in automobiles, home appliances, etc. through a press-forming process.
  • FIG. 3 is a diagram showing SEM photographs of a structure of acicular austenite (acicular ⁇ ) observed in a structure after final cooling (after third cooling in the annealing step) and a structure water-cooled after being held at temperature T in the present invention. It is a schematic diagram of acicular austenite (needle-like ⁇ ) and is a diagram for explaining the definition of the aspect ratio of acicular ⁇ .
  • the steel sheet of the present invention is, in mass%, C: 0.10 to 0.24%, Si: 0.4% or more and less than 1.60%, Mn: 2.0 to 3.6%, P: 0.02 % or less, S: 0.01% or less, sol.Al: less than 1.0%, N: less than 0.015%, and satisfying the following formula (1), the balance being Fe and unavoidable impurities
  • the area ratio of polygonal ferrite is 5% or more and 25% or less
  • the area ratio of upper bainite is 5% or more and 50% or less
  • the volume ratio of retained austenite is 3% or more and 20% or less.
  • the ratio of the total number of fresh martensite grains and retained austenite grains having a structure of 5% or less and an equivalent circle diameter of less than 0.8 ⁇ m to the total number of fresh martensite grains and retained austenite grains is 50 % or more and having an aspect ratio of 2.0 or more and an equivalent circle diameter of 0.8 ⁇ m or more, fresh martensite particles and retained austenite particles having an equivalent circle diameter of 0.8 ⁇ m or more.
  • the steel sheet has a ratio of particles to the number of particles of 30% or more.
  • Si and Mn represent Si content (% by mass) and Mn content (% by mass), respectively.
  • the steel sheet of the present invention will be described below in the order of chemical composition and steel structure.
  • the steel sheet of the present invention contains the following components.
  • % which is a unit of content of a component, means “% by mass”.
  • C 0.10-0.24% C is contained from the viewpoint of setting the hardenability of the steel sheet, the strength of martensite, and the volume fraction of retained ⁇ within desired ranges. If the C content is less than 0.10%, the strength and ductility of the steel sheet cannot be sufficiently ensured, so the C content is made 0.10% or more.
  • the C content is preferably 0.12% or more, more preferably 0.14% or more, still more preferably 0.16% or more. If the C content exceeds 0.24%, the toughness of the weld deteriorates. Therefore, the C content is made 0.24% or less. From the viewpoint of improving ductility and toughness of spot welds, the C content is preferably 0.22% or less. From the viewpoint of further improving the toughness of spot welds, the C content is more preferably 0.20% or less.
  • Si 0.4% or more and less than 1.60% Si is contained from the viewpoint of improving ferrite strength, suppressing the formation of carbides in martensite and bainite, and stabilizing residual ⁇ to improve ductility. do. 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 0.5% or more. The Si content is more preferably 0.6% or more. If the Si content is 1.60% or more, the chemical conversion treatability is significantly deteriorated. Therefore, the Si content should be less than 1.60%. Preferably, the Si content is 1.30% or less, more preferably 1.20% or less. More preferably, the Si content is less than 1.0%.
  • Mn 2.0-3.6% Mn secures a predetermined hardenability, suppresses ferrite transformation, and secures a desired area ratio of tempered martensite and/or bainite to secure strength.
  • Mn concentrates in ⁇ during two-phase region annealing of ferrite/ ⁇ , and lowers the Ms point of the retained ⁇ , thereby stabilizing the retained ⁇ and improving the ductility.
  • Mn like Si, suppresses the formation of carbides in bainite and improves ductility.
  • Mn increases the volume fraction of retained ⁇ to improve ductility. From these points, Mn is an important element in the present invention. In order to obtain these effects, the Mn content should be 2.0% or more.
  • the Mn content is preferably 2.1% or more. More preferably, the Mn content is 2.2% or more.
  • the Mn content exceeds 3.6%, the bainite transformation is significantly retarded, making it difficult to ensure high ductility.
  • the Mn content exceeds 3.6%, it becomes difficult to suppress the formation of massive coarse ⁇ and massive coarse martensite, and the stretch flanging formability deteriorates. Therefore, the Mn content should be 3.6% or less. From the viewpoint of promoting bainite transformation and ensuring high ductility, the Mn content is preferably 2.8% or less.
  • Si/Mn ⁇ 0.50 Formula (1) The surface oxides of the steel sheet that significantly deteriorate the chemical conversion treatability are Si-based oxides. Therefore, Si/Mn is set to less than 0.50 for the purpose of forming a Mn-containing oxide that is readily soluble in an acid solution. That is, in the present invention, Si/Mn ⁇ 0.50 as the formula (1).
  • Si and Mn represent Si content (% by mass) and Mn content (% by mass), respectively.
  • chemical conversion treatability can be provided in the dew point range of -50°C or higher and -30°C or lower.
  • Si/Mn is 0.40 or less, more preferably 0.35 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, P 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 peelability in hot rolling and the effect of suppressing nitriding during annealing, but it is an element that deteriorates local elongation in addition to spot weldability.
  • the S content is made 0.01% or less.
  • the S content is preferably 0.0020% or less, and 0.0010%. % is more preferable.
  • the S content is preferably 0.0001% or more from the viewpoint of manufacturing cost.
  • sol. Al less than 1.0% Al is contained for the purpose of deoxidizing or stabilizing residual ⁇ as an alternative to Si. sol. Although the lower limit of Al is not specified, it is preferably 0.01% or more for stable deoxidation. 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 deteriorated. Therefore, sol. Al content is less than 1.0%. In order to obtain high strength, sol. The Al content is preferably less than 0.20%, more preferably 0.10% 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%. Although N may not be included, the N content is preferably 0.0001% or more from the viewpoint of manufacturing cost.
  • 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 Fe and unavoidable impurities.
  • one or two selected from the following (A) and (B) are added as optional elements. It can be contained as appropriate.
  • Nb 0.2% or less
  • Nb is preferably added from the viewpoint of refining the microstructure and improving the defect resistance of spot welds.
  • Nb can be contained because of 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.
  • the Nb content is preferably 0.002% or more, although the lower limit is not particularly specified.
  • 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 deteriorates.
  • it causes an increase in rolling load and deterioration of castability. Therefore, when Nb is contained, the Nb content is made 0.2% or less.
  • the Nb content is preferably 0.1% or less, more preferably 0.05% or less, still more preferably 0.03% or less.
  • Ti 0.2% or less Ti is preferably added from the viewpoint of refining the microstructure and improving the defect resistance of spot welds.
  • N in the steel is fixed as TiN, and it has the effect of improving the hot ductility and the effect of improving the hardenability of B.
  • the Ti content is preferably 0.002% or more, although the lower limit is not specified. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. More preferably, it is 0.010% or more.
  • the Ti content exceeds 0.2%, the rolling load increases and the amount of precipitation strengthening increases, resulting in a decrease in ductility.
  • the Ti content is preferably 0.1% or less, more preferably 0.05% or less. In order to ensure high ductility, the Ti content is more preferably 0.03% or less.
  • V 0.2% 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 effect of precipitating carbides to resist delayed fracture. can be contained from the effect of improving the In order to obtain these effects, although the lower limit is not specified, 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. On the other hand, if a large amount of V is contained, the castability is remarkably deteriorated. The V content is preferably 0.1% or less. The V content is more preferably 0.05% or less.
  • B 0.01% or less B has the advantage of easily generating tempered martensite and/or bainite with a predetermined area ratio.
  • the residual solid solution B improves the delayed fracture resistance.
  • the B content is preferably 0.0002% or more.
  • the B content is more preferably 0.0005% or more.
  • the B content is more preferably 0.0010% or more.
  • the B content shall be 0.01% or less.
  • the B content is preferably 0.0050% or less.
  • the B content is more preferably 0.0030% or less.
  • Cu 0.2% or less Cu improves corrosion resistance in the use environment of automobiles.
  • corrosion products of Cu coat the surface of the steel sheet, which has the effect of suppressing penetration of hydrogen 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. Therefore, although the lower limit is not specified, it is preferable to contain 0.05% or more of Cu from the viewpoint of improving the delayed fracture resistance.
  • Cu content is more preferably 0.10% or more. On the other hand, if the Cu content is too high, surface defects are caused.
  • Ni 0.2% or less
  • Ni is an element that acts to improve corrosion resistance.
  • Ni has the effect of suppressing the occurrence of surface defects that tend to occur when Cu is contained.
  • the Ni content is preferably 0.01% or more, although the lower limit is not specified.
  • the Ni content is more preferably 0.04% or more, still more preferably 0.06% or more.
  • the Ni content is made 0.2% or less.
  • Cr 0.4% 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, although the lower limit is not particularly specified.
  • the Cr content is more preferably 0.03% or more, still more preferably 0.06% or more.
  • the Cr content is made 0.4% or less.
  • Mo 0.15% 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 the cold-rolled steel sheet, so when Mo is contained, the Mo content is made 0.15% or less.
  • Mg 0.0050% 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 are deteriorated. The Mg content is preferably 0.0025% or less, more preferably 0.0010% or less.
  • Ca 0.0050% 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, if a large amount of Ca is added, the surface quality and bendability are deteriorated. The Ca content is preferably 0.0035% or less, more preferably 0.0020% or less.
  • Sn 0.10% 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.
  • the Sn content is preferably 0.002% or more.
  • the Sn content is more preferably 0.004% or more, still more preferably 0.006% or more.
  • the Sn content is more preferably 0.008% or more.
  • the Sn content exceeds 0.10%, castability deteriorates.
  • the Sn content is made 0.10% or less.
  • the Sn content is preferably 0.04% or less, more preferably 0.03% or less.
  • Sb 0.10% 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 fatigue 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 0.10%, the castability deteriorates, and segregation occurs at prior ⁇ grain boundaries, resulting in deterioration of delayed fracture resistance. Therefore, when Sb is contained, the Sb content is made 0.10% or less. The Sb content is preferably 0.04% or less, more preferably 0.03% or less.
  • REM 0.0050% or less REM is an element that suppresses the adverse effects of sulfides on stretch flange formability and improves stretch flange formability by spheroidizing the shape of sulfides.
  • the REM content is preferably 0.0005% or more.
  • the REM content is more preferably 0.0010% or more, still more preferably 0.0020% or more.
  • the REM content exceeds 0.0050%, the effect of improving the stretch flanging formability is saturated.
  • REM includes scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
  • Sc scandium
  • Y yttrium
  • La lanthanum
  • Lu lutetium
  • REM concentration in the present invention is the total content of one or more elements selected from the above REMs.
  • 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 lower limit, the arbitrary element is included as an unavoidable impurity.
  • Area ratio of polygonal ferrite 5% or more and 25% or less
  • the area ratio of polygonal ferrite is set to 5% or more.
  • Polygonal ferrite is preferably 8% or more, more preferably 11% or more.
  • the area ratio of polygonal ferrite is set to 25% or less.
  • Polygonal ferrite is more preferably 23% or less.
  • Upper bainite 5% or more and 50% or less Upper bainite is bainite with little carbide precipitation, and since C is distributed in the surrounding untransformed ⁇ , it can be utilized to form retained ⁇ with high processing stability. .
  • the upper bainite has a hardness intermediate between that of ferrite and martensite, and local elongation is improved by forming a structure with intermediate hardness.
  • the formation of upper bainite in acicular ⁇ formed by annealing promotes the formation of retained ⁇ with a high aspect ratio. Therefore, 5% or more of upper bainite is required at a strength level where the tensile strength (TS) is 980 MPa or more. Therefore, the area ratio of upper bainite is set to 5% or more.
  • the area ratio of upper bainite is 6.0% or more, more preferably 7.0% or more.
  • the area ratio of upper bainite is 45% or less, more preferably 40% or less.
  • volume fraction of retained austenite (retained ⁇ ) 3% or more and 20% or less
  • the volume fraction of residual ⁇ (amount of residual ⁇ ) is preferably 3.0% or more, more preferably 5% or more, and still more preferably 7% or more.
  • This amount of retained ⁇ includes both retained ⁇ generated adjacent to upper bainite and retained ⁇ generated adjacent to martensite and lower bainite. If the amount of retained ⁇ is too large, the strength and stretch flanging formability of the steel are lowered. Therefore, the volume fraction of residual ⁇ is set to 20% or less.
  • the volume fraction of residual ⁇ is preferably 15% or less, more preferably 13% or less.
  • "volume ratio" can be regarded as "area ratio”.
  • Area ratio of fresh martensite 12% or less (including 0%)
  • Fresh martensite is a structure that reduces local elongation, but it is possible to improve strength by forming it within a range that does not deteriorate bendability and hole expansibility. From this point of view, the area ratio of fresh martensite has an upper limit of 12% and includes 0%. The area ratio of fresh martensite may be 12.0% or less.
  • tempered martensite Sum of area ratios of tempered martensite and lower bainite: 10% or more and 50% or less
  • the lower bainite generated by overaging holding at 500°C or less and 350°C or more, the second cooling stop temperature Tc2 of 320°C or less and 150°C or more
  • tempered martensite which is formed by cooling to 350° C. to 550° C. and then tempering it by overaging for 20 to 3000 seconds. Carbon distribution to untransformed ⁇ is suppressed in tempered martensite and lower bainite, in which carbides precipitate in the structure, compared to upper bainite, in which carbides are less precipitated.
  • tempered martensite and lower bainite bring carbon enrichment to untransformed ⁇ by expanding the T0 composition at low temperature, or furthermore, to reduce the amount of fresh martensite during final cooling, these structures are controlled. It is necessary to obtain a retained ⁇ with high processing stability. If the sum of the area ratios of tempered martensite and lower bainite exceeds 50%, precipitation of carbides is promoted, the required amount of retained ⁇ cannot be obtained, and the desired ductility cannot be obtained. Therefore, in the present invention, the total area ratio of tempered martensite and lower bainite is set to 50% or less. Preferably, the sum of these area ratios is 45% or less, more preferably 40% or less.
  • the total area ratio of tempered martensite and lower bainite is set to 10% or more.
  • the sum of these area ratios is 13% or more, more preferably 16% or more.
  • the residual structure is a structure other than polygonal ferrite, upper bainite, retained austenite, fresh martensite, tempered martensite, and lower bainite, such as pearlite.
  • the formation of a pearlite structure inhibits efficient carbon distribution and suppresses the formation of retained ⁇ , thus reducing ductility.
  • the area ratio of the residual structure is 5% or less, the effect on the material quality can be ignored, so the upper limit of the area ratio of the residual structure is set to 5%.
  • the area ratio of the residual tissue may be 0%.
  • the fresh martensite grains and retained austenite grains are structures that do not cause stress concentration during local deformation and do not contribute to the formation of voids, and thus do not deteriorate local ductility and hole expansibility. If the total number of fresh martensite particles and retained austenite particles having an equivalent circle diameter of less than 0.8 ⁇ m is 50% or more of the total number of fresh martensite particles and retained austenite particles, the present invention is excellent. local elongation and hole expansibility.
  • the ratio of the total number of fresh martensite grains and retained austenite grains having an equivalent circle diameter of less than 0.8 ⁇ m to the total number of fresh martensite grains and retained austenite grains is 50% or more. . That is, the following formula (A) is satisfied. 100 ⁇ (equivalent circle diameter: total number of fresh martensite grains and residual ⁇ grains less than 0.8 ⁇ m)/(total number of fresh martensite grains and residual ⁇ grains) ⁇ 50(%) Formula (A) Preferably, the percentage of the left side defined by the above formula (A) is 55% or more.
  • one or two of tempered martensite and lower bainite may be formed in the structure. A sufficient amount of lower bainite can be obtained by cooling to the temperature Tc2, and the lower bainite can be sufficiently obtained by overaging and holding for 20 to 3000 seconds in a temperature range of 350 to 550°C.
  • Fresh martensite particles and residual ⁇ particles having an equivalent circle diameter of 0.8 ⁇ m or more which is the total number of fresh martensite particles and residual ⁇ particles having an aspect ratio of 2.0 or more and an equivalent circle diameter of 0.8 ⁇ m or more. Ratio to the number of: 30% or more In the fresh martensite particles and/or retained austenite particles having an equivalent circle diameter of 0.8 ⁇ m or more, by increasing the aspect ratio of the fresh martensite particles and/or retained austenite particles, local It can reduce stress concentration during deformation, suppress void formation, and improve local ductility and hole expansibility.
  • acicular austenite surrounded by a soft ferrite structure formed in the heating process is transformed into bainite in the subsequent cooling process to increase the area ratio.
  • particles having an equivalent circle diameter of 0.8 ⁇ m or more and an aspect ratio of 2.0 or more are relative to the total number of fresh martensite particles and retained austenite particles having an equivalent circle diameter of 0.8 ⁇ m or more. Desired moldability can be provided by setting the total amount to 30% or more.
  • the total number of fresh martensite particles having an aspect ratio of 2.0 or more and an equivalent circle diameter of 0.8 ⁇ m or more and retained austenite particles having an equivalent circle diameter of 0.8 ⁇ m or more is set to 30% or more. That is, the following formula (B) is further satisfied in addition to the formula (A) described above. Preferably, the ratio of the left side defined by the following formula (B) is 35% or more.
  • the structure of the obtained steel sheet is measured by the following method.
  • Measurement of area ratio of steel structure Cut out an observation sample from a steel plate so that the cross section perpendicular to the steel plate surface and parallel to the rolling direction is the observation surface, and the plate thickness cross section is corroded with 1% by volume nital, and then subjected to a scanning electron microscope.
  • a structure photograph is taken in a region of 3000 ⁇ m 2 or more at a plate thickness of t/4 at a magnification of 2000 times with (SEM). Then, the following items (i) to (iv) are measured respectively. Note that t indicates the plate thickness and w indicates the plate width.
  • Polygonal ferrite and upper bainite Polygonal ferrite (recrystallized F) and upper bainite (UB) both show gray in SEM photographs, but can be distinguished by their shapes.
  • An example of a SEM photograph is shown in FIG. 1 together with an SEM photograph of a structure that has been water-cooled after being held at temperature T.
  • the region indicated by the dashed line in FIG. 1(a) is the needle-like ⁇ structure formed by the annealing process up to the soak holding at the annealing temperature T within the range of the present invention, and the upper bainite (UB) is inside it. is formed around which residual ⁇ or fresh martensite (M) with a high aspect ratio is formed.
  • a similar structure is also observed in the massive ⁇ structure formed by the annealing temperature T up to soaking.
  • the area ratios of polygonal ferrite and upper bainite were measured by point counting according to ASTM E562-11 (2014). Each of the area ratio of polygonal ferrite and the area ratio of upper bainite is an average value of measured values at five locations.
  • Tempered Martensite and/or Lower Bainite Tempered martensite and lower bainite are structures containing carbides that are observed as fine white structures in SEM photographs. It is possible to distinguish between the two by more microscopic observation, but it is difficult to distinguish them by SEM photography. Therefore, in the present invention, tempered martensite and lower bainite are defined as the same structure, and the total area ratio of tempered martensite and lower bainite is measured by the point counting method according to ASTM E562-11 (2014). Let the value which averaged the measured value in five places be the total area ratio of a tempered martensite and a lower bainite.
  • the volume ratio of the residual ⁇ After polishing the steel plate to the position of 1/4 of the plate thickness, the surface was further polished by 0.1 mm by chemical polishing, using the K ⁇ ray of Mo with an X-ray diffractometer, and the FCC iron ( ⁇ ) ( 200) plane, (220) plane, (311) plane, and the (200) plane, (211) plane, and (220) plane of BCC iron (ferrite) are measured, and each plane of BCC iron (ferrite) is measured.
  • the volume ratio of the residual ⁇ obtained from the intensity ratio of the integrated reflection intensity from each surface of the FCC iron ( ⁇ ) to the integrated reflection intensity from the FCC iron ( ⁇ ) is measured.
  • the volume ratio of the residual ⁇ can be the area ratio of the residual ⁇ .
  • Equivalent circle diameter and aspect ratio of fresh martensite grains and/or residual ⁇ grains A sample was cut from a steel plate so that the cross section parallel to the rolling direction was the observation surface, and the structure of the thickness cross section was corroded with a repeller corrosive solution. , Take a picture of the structure in a region of 10000 ⁇ m 2 or more at a plate thickness of t/4 part at 1000 times magnification with a laser microscope (LM). Repeller corrosion is color etching. Fresh martensite particles and/or residual ⁇ particles are extracted by showing fresh martensite and/or residual ⁇ in white contrast. Alternatively, the equivalent circle diameter and aspect ratio of the residual ⁇ particles are measured.
  • particles having an equivalent circle diameter of less than 0.8 ⁇ m are subjected to measurement of the number of particles, and the ratio of the total number of particles to the total number of particles is calculated.
  • the number of particles having an equivalent circle diameter of 0.8 ⁇ m or more was measured, and the number of particles having an aspect ratio of 2.0 or more was measured.
  • a ratio of particles having an aspect ratio of 2.0 or more and an equivalent circle diameter of 0.8 ⁇ m or more to all particles is calculated.
  • the temperature used to heat or cool a steel slab (steel material), steel plate, or the like shown below means the surface temperature of the steel slab (steel material), steel plate, or the like.
  • a steel slab having the chemical composition described above is subjected to hot rolling and pickling, and then the obtained hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • a cold-rolling process for obtaining a cold-rolled steel sheet whose area ratio is 35% or more and 75% or less with respect to the total structure of the bcc phase in the total structure having the ⁇ 011> orientation, and the annealing step includes cold-rolling
  • the temperature range of 500 ° C. or higher and Ac1 or lower is set to an average heating rate of 0.5 to 15 ° C./sec or lower, and 840 ° C.
  • First cooling is performed to cool to the stop temperature Tc1, after the first cooling, first holding is performed to hold the first cooling stop temperature Tc1 for 25 seconds or longer, and after the first holding, a temperature of 350 ° C. or lower and 200 ° C. or higher
  • the range is cooled at an average cooling rate of 3.0 to 80 ° C./s, and subjected to second cooling to a second cooling stop temperature Tc2 of 320 ° C. or lower and 150 ° C. or higher, and the second cooling stop temperature Tc2 is 2 to 20 Second holding for seconds, after the second holding, overaging holding for 20 to 3000 seconds in a temperature range of 350 to 500 ° C., and after the overaging holding, third cooling for cooling. .
  • hot rolling in the hot rolling process includes a method of reheating and then rolling a steel slab having the above-described chemical composition, and a method of directly rolling a steel slab after continuous casting without heating. , a method in which a steel slab after continuous casting is subjected to heat treatment for a short time and then rolled.
  • Hot rolling may be performed according to a conventional method, for example, the slab heating temperature is 1100 ° C. or higher and 1300 ° C. or lower, the soaking time is 20 to 30 minutes, the finish rolling temperature is the Ar3 transformation point (° C.) or higher, and the Ar3 transformation The point (°C) should be +200°C or less, and the winding temperature 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 smelting method for manufacturing the steel slab (steel material) is not particularly limited, and a known smelting method such as a converter or an electric furnace can be adopted. Secondary refining may also be performed in a vacuum degassing furnace.
  • the pickling treatment process is a process of pickling the hot-rolled steel sheet after the hot rolling process.
  • Pickling treatment conditions are not particularly limited, and pickling treatment conditions in known production methods may be adopted.
  • Cold rolling process Cumulative cold rolling rate: 30 to 85% If the rolling reduction (accumulated cold rolling reduction) in the cold rolling treatment is less than 30%, recrystallization is not sufficiently promoted, and the formation of needle-like ⁇ referred to in the present invention is not sufficiently performed. In addition, the desired cold rolling texture does not develop, and ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, as described later, and ⁇ 100 ⁇ 011> orientations do not account for more than 35% of the total bcc phase texture. Therefore, the draft of cold rolling is set to 30% or more.
  • the rolling reduction (accumulated cold rolling reduction) is preferably 40% or more, more preferably 50% or more.
  • the rolling reduction (cumulative cold rolling reduction) is 85% or less.
  • the number of passes is not particularly limited, but may be, for example, 5 passes.
  • the cumulative cold rolling rate (thickness reduction rate) is (1 ⁇ (thickness after cold rolling (after final pass)/thickness before cold rolling) ⁇ 100.
  • Rolling reduction of first pass 5% or more and less than 25%
  • the rolling reduction of the first pass is set to 5% or more from the viewpoint of operability.
  • the rolling reduction in the first pass is 25% or more, the plate temperature during the cold rolling in the first pass is low, so strain of the shear component is imparted to the cold-rolled material, and the desired texture develops. No acicular ⁇ is formed. Therefore, the rolling reduction of the first pass is set to 5% or more and less than 25%.
  • the rolling reduction (thickness reduction rate) of the first pass refers to (1 ⁇ (thickness after first pass cold rolling)/(thickness before cold rolling)) ⁇ 100.
  • the rolling temperature (sheet temperature) in the first pass is preferably 20°C or higher and 40°C or lower.
  • the rolling temperature of the first pass is determined by measuring a portion of the steel plate surface to which lubricating oil has not adhered after the first pass with a radiation thermometer. If the rolling temperature in the first pass is less than 20°C or if the rolling temperature in the first pass exceeds 40°C, the desired texture described above may not develop and acicular ⁇ may not be formed. Therefore, the rolling temperature in the first pass is preferably 20°C or higher and 40°C or lower.
  • Structure of cold-rolled steel sheet after cold rolling ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011> orientation 35% or more and 75% or less in terms of area ratio with respect to the total structure of the bcc phase. have.
  • ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011> orientation to form the desired amount of acicular ⁇ is required to be 35% or more in terms of area ratio with respect to the entire bcc phase structure. Preferably, it is 40% or more.
  • the sum of the textures with ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011> orientation is If the area ratio exceeds 75% with respect to the entire structure, material anisotropy occurs in the steel sheet. Therefore, the total area ratio of the structure having the above specified orientation is set to 75% or less with respect to the total structure of the bcc phase. It is preferably 68% or less, more preferably 65% or less.
  • the hot-rolled steel sheet having the chemical composition described above is subjected to cold rolling treatment at a cold rolling reduction of 30 to 85%, and the rolling reduction in the first pass is 5% or more and less than 25%.
  • the ratio of the area ratio of the bcc phase to the entire structure of the total area ratio of the structure having the above specified orientation can be adjusted to a desired range.
  • a measurement sample whose cross section parallel to the rolling direction is a measurement surface is cut out from the cold-rolled steel sheet after the cold rolling process, and after mechanically or electrolytically polishing the measurement surface, SEM-EBSD (measurement conditions: WD: 20 mm, acceleration voltage: 20 kV).
  • ⁇ ND plane ⁇ ⁇ RD direction> orientation of rolling is ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011> orientation
  • the texture of the cold-rolled steel sheet is evaluated by quantifying the texture area ratio of the bcc phase and calculating the ratio with the area ratio of the bcc phase in all orientations.
  • the temperature range of 500 ° C. or higher and Ac1 or lower is set to an average heating rate (HR1) of 0.5 to 15 ° C./sec for the cold-rolled steel sheet after the cold rolling step, and 840 ° C. or less and 0.6 ⁇ (T - Ac1) / (Ac3 - Ac1) ⁇ 1.0, and after the heating, the dew point Td is -50 ° C. or higher and -30 ° C. or lower.
  • HR1 average heating rate
  • the cold-rolled sheet having the structure after the cold-rolling process described above is heated at an appropriate heating rate to sufficiently promote recrystallization, and then heated to temperature T, or at temperature T. to form acicular austenite. Therefore, the average heating rate is set to 15° C./sec or less in the temperature range of Ac1 or less where austenite transformation does not occur at 500° C. or more.
  • the average heating rate is preferably 10° C./sec or less.
  • the lower limit of the average heating rate is set to 0.5° C./sec or more from the viewpoint of operation.
  • the average heating rate is preferably 1.0° C./sec or higher, more preferably 1.5° C./sec or higher.
  • the average heating rate (° C./s) is calculated from ((Ac1 (° C.) ⁇ 500° C.)/(heating time from 500° C. to Ac1 (° C.) (sec)).
  • Heating to an annealing temperature T that is 840 ° C. or less and 0.6 ⁇ (T - Ac1) / (Ac3 - Ac1) ⁇ 1.0 After the heating, in a furnace with a dew point Td of -50 ° C. or higher and -30 ° C. or lower Annealing at Annealing Temperature T in an Atmosphere
  • a temperature T annealing temperature T
  • acicular austenite as described later can be formed.
  • the annealing temperature T 0.6 ⁇ (T ⁇ Ac1)/(Ac3 ⁇ Ac1) ⁇ 1.
  • the temperature T is set to 840° C. or less.
  • the dew point Td is less than -50°C, good chemical conversion treatability cannot be obtained.
  • the dew point Td exceeds ⁇ 30° C., good chemical conversion treatability cannot be obtained. Therefore, the dew point Td is set to ⁇ 50° C. or higher and ⁇ 30° C. or lower.
  • the dew point Td is preferably -48°C or higher, more preferably -45°C or higher.
  • the dew point Td is preferably ⁇ 32° C. or lower, more preferably ⁇ 34° C. or lower.
  • the soaking time at the annealing temperature T is not particularly limited, it is preferably 25 to 350 seconds, more preferably 50 to 300 seconds, from the viewpoint of element distribution during the two-phase annealing.
  • Ac1 (°C) may be calculated by the following formula based on empirical rules.
  • Ac1 (°C) 723 + 22 x [Si%] - 18 x [Mn%] + 17 x [Cr%] + 4.5 [Mo%] + 16 x [V%]
  • Ac3 (°C) may be calculated by the following formula based on empirical rules.
  • Ac3 (° C.) 910 ⁇ 203 ⁇ [C%] 1/2 +44.7 ⁇ [Si%] ⁇ 30 ⁇ [Mn%]+700 ⁇ [P%]+400 ⁇ [sol.
  • Al%] ⁇ 20 ⁇ [Cu%]+31.5 ⁇ [Mo%]+104 ⁇ [V%]+400 ⁇ [Ti%] [X%] in the above formula is the content (% by mass) of component element X in the steel sheet, and is set to "0" when not contained.
  • Needle-like ⁇ structure formed by the above soaking and holding treatment has a number density of 5/1000 ⁇ m 2 or more
  • needle-like ⁇ is utilized to provide desired formability.
  • a large amount of acicular austenite (needle-like ⁇ ) is formed, a large amount of retained ⁇ having a high aspect ratio tends to be formed.
  • the number density of the needle-like ⁇ structure formed by heating up to the annealing temperature T and holding the soaking is 5/1000 ⁇ m 2 or more. Become. There is no upper limit due to the nature of acicular ⁇ , and the number of acicular ⁇ particles is preferably as large as possible.
  • the cold-rolled steel sheet having the chemical composition and structure described above is heated to the annealing temperature T at an average heating rate of 0.5 to 15 ° C./sec or less in the temperature range of 500 ° C. or higher and Ac1 or lower.
  • the number density of the acicular ⁇ structure can be adjusted within a desired range.
  • the needle-like ⁇ formed in the treatment up to the soaking at the annealing temperature T contributes to the formation of the residual ⁇ that has a high aspect ratio and high processing stability in the subsequent cooling process. is the important point, and the number density of this acicular ⁇ structure is measured.
  • FIG. 1(b) is a photograph of a structure that was water-cooled after being held at a temperature T within the range of the present invention in the annealing process, and needle-like ⁇ , block-like ⁇ , and a ferrite structure are formed.
  • FIG. 2 shows a schematic diagram of a method for measuring the aspect ratio of acicular ⁇ .
  • austenite surrounded by recrystallized ferrite having the same orientation and having an aspect ratio of 3.0 or more is defined as acicular ⁇ .
  • the tip of the acicular austenite may be in contact with other austenite grains, but in that case, it is sufficient to confirm that the adjacent ferrite grains have the same orientation by electron beam backscatter diffraction (EBSD).
  • EBSD electron beam backscatter diffraction
  • First cooling The average cooling rate in the temperature range of 750 to 550 ° C. is set to 6.0 ° C./sec or more, and cooling to the first cooling stop temperature Tc1 of 550 ° C. or less and 400 ° C. or more
  • First holding After the first cooling, the second Hold for 25 seconds or longer at one cooling stop temperature Tc1
  • ferrite transformation predominantly occurs in the temperature range of 750 to 550°C. If ferrite transformation occurs excessively, needle-like ⁇ undergoes ferrite transformation. Therefore, the temperature range of 750 to 550° C. is set to an average cooling rate of 6.0° C./sec or more to suppress ferrite transformation.
  • the average cooling rate is preferably 8.0° C./sec or higher, more preferably 10.0° C./sec or higher.
  • the average cooling rate (° C./sec) is calculated from (750° C. (cooling start temperature) ⁇ 550° C. (cooling stop temperature))/(cooling time from cooling start temperature to cooling stop temperature (sec)). be.
  • the first cooling stop temperature Tc1 in the first cooling is the temperature at which upper bainite transformation occurs. If the first cooling stop temperature Tc1 is higher than 550°C, untransformed austenite undergoes ferrite and/or pearlite transformation, suppressing the formation of retained austenite and making it impossible to ensure desired ductility. On the other hand, when the first cooling stop temperature Tc1 is less than 400°C, untransformed austenite transforms into martensite, C (carbon) cannot be efficiently distributed to the untransformed austenite, and ductility decreases. Therefore, the first cooling stop temperature Tc1 is set to 550° C. or lower and 400° C. or higher. The first cooling stop temperature Tc1 is preferably 500° C. or lower.
  • the first cooling stop temperature Tc1 is preferably 420° C. or higher.
  • the holding time at the first cooling stop temperature Tc1 is set to 25 seconds or more in order to sufficiently perform the bainite transformation.
  • the retention time in the first retention is preferably 30 seconds or longer, more preferably 35 seconds or longer.
  • the holding time in the first holding is preferably 60 seconds or less, more preferably 55 seconds or less.
  • the temperature modulation at the first cooling stop temperature Tc1 is allowed within the range of 550° C. or lower and 400° C. or higher.
  • Second cooling The temperature range of 350 ° C. or lower and 200 ° C. or higher is set to an average cooling rate of 3.0 to 80 ° C./s, and cooled to a second cooling stop temperature Tc2 of 320 ° C. or lower and 150 ° C. or higher After the first holding, As the second cooling, first, the average cooling rate in the temperature range from 350°C to 200°C is set to 3.0 to 80°C/s. If the average cooling rate in the temperature range from 350°C to 200°C is more than 80°C/s, the plate shape deteriorates due to excessively rapid cooling.
  • the average cooling rate in the temperature range from 350° C. to 200° C. is set at 3.0 to 80° C./s.
  • This average cooling rate is preferably 60° C./sec or less, more preferably 50° C./sec or less.
  • this average cooling rate is preferably 5.0° C./sec or more, more preferably 10.0° C./sec or more.
  • the average cooling rate (° C./sec) is calculated from (350° C. (cooling start temperature) ⁇ 200° C. (cooling stop temperature))/(cooling time from cooling start temperature to cooling stop temperature (sec)). be.
  • the steel sheet is cooled to the second cooling stop temperature Tc2 of 320°C or lower and 150°C or higher, martensite transformation occurs. If the second cooling stop temperature Tc2 exceeds 320°C, martensite transformation does not occur, and coarse fresh martensite particles and/or retained austenite are formed during final cooling, so that local elongation and hole expandability deteriorate, and the desired The formability of the product cannot be ensured. Therefore, the upper limit of the second cooling stop temperature Tc2 is set to 320°C.
  • the second cooling stop temperature Tc2 is preferably 300°C or lower, more preferably 280°C or lower.
  • the lower limit of the second cooling stop temperature Tc2 is set to 150°C.
  • the second cooling stop temperature Tc2 is preferably 170°C or higher, more preferably 190°C or higher.
  • Second hold Hold for 2 to 20 seconds or less at the second cooling stop temperature Tc2
  • the holding time at the second cooling stop temperature Tc2 is set to 2 seconds or more, so that the second cooling stop Cooling down to the temperature Tc2 sufficiently causes martensite transformation, and a uniform martensite structure is obtained in the sheet width direction and the sheet thickness direction, thereby reducing variations in material quality.
  • the holding time in the second holding is 20 seconds or less from the viewpoint of operation.
  • the retention time in the second retention is preferably 4 seconds or longer, more preferably 6 seconds or longer.
  • the holding time in the second holding is preferably 17 seconds or less, more preferably 14 seconds or less.
  • the temperature modulation at the second cooling stop temperature Tc2 is allowed within the range of 320° C. or lower and 150° C. or higher.
  • Overaging retention Holding for 20 to 3000 seconds in the temperature range of 350 to 500 ° C. Overaging retention in the temperature range of 350 to 500 ° C. transforms untransformed austenite to bainite, and in addition, the second cooling stop temperature Tc2. This is done for the purpose of promoting carbon distribution to untransformed austenite by tempering the martensite structure generated by cooling. If the overaging temperature exceeds 500° C., decomposition of retained austenite occurs, precipitation of cementite occurs, or pearlite transformation occurs in a part of the structure, and ductility decreases.
  • the overaging temperature is lower than 350 ° C.
  • the untransformed austenite does not transform, and in addition, carbon partitioning from the martensite formed at the second cooling stop temperature Tc2 does not occur, and mechanical stability is improved.
  • a low residual austenite is formed during final cooling. Therefore, the temperature range in which overaging is held is 350 to 500°C.
  • the holding time in overaging holding is 20 seconds or more, carbon distribution occurs from bainite transformation of untransformed austenite and martensite formed at the second cooling stop temperature Tc2, and desired formability can be secured.
  • the holding time in the overaging holding is set to 3000 seconds or less in view of the workability.
  • Third cooling cooling after holding overaging After holding overaging, the steel sheet is cooled to room temperature (10 to 30°C) to obtain the steel sheet of the present invention.
  • temper rolling with an elongation rate of 0.05 to 0.5% can be applied.
  • the steel sheet of the present invention obtained by the method of manufacturing the steel sheet of the present invention preferably has a 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 590 MPa or more, high ductility, excellent stretch flanging formability, and good chemical conversion treatability. Therefore, members obtained using the steel sheet of the present invention also have high strength, and have superior high ductility, excellent stretch flanging formability, and good chemical conversion treatability compared to 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.
  • a steel slab having a thickness of 250 mm having the chemical composition shown in Table 1 is hot rolled (slab heating temperature: 1250 ° C., soaking time: 30 minutes, finish rolling temperature: Ar3 + 50 ° C., coiling temperature: 550 ° C.)
  • the resulting hot-rolled steel sheets were cold-rolled under the conditions shown in Table 2 to produce cold-rolled steel sheets.
  • the cold-rolled steel sheets were annealed in a continuous annealing line under the conditions shown in Table 2 and then temper-rolled at an elongation of 0.2 to 0.4% to produce steel sheets for evaluation.
  • after overaging holding (isothermal holding) it cooled to room temperature (20 degreeC) as 3rd cooling.
  • the obtained steel sheets were evaluated by the following methods.
  • Polygonal ferrite and upper bainite Polygonal ferrite (recrystallized F) and upper bainite (UB) both show gray in SEM photographs, but can be distinguished by their shapes.
  • An example of a SEM photograph is shown in FIG. 1 together with an SEM photograph of a structure that has been water-cooled after being held at temperature T.
  • the region indicated by the dashed line in FIG. 1(a) is the needle-like ⁇ structure formed by the annealing process up to the soak holding at the annealing temperature T within the range of the present invention, and the upper bainite (UB) is inside it. is formed around which residual ⁇ or fresh martensite (M) with a high aspect ratio is formed.
  • a similar structure is also observed in the massive ⁇ structure formed by the annealing temperature T up to soaking.
  • the area ratios of polygonal ferrite and upper bainite were measured by point counting according to ASTM E562-11 (2014). Each of the area ratio of polygonal ferrite and the area ratio of upper bainite was taken as an average value of measured values at five locations.
  • Tempered Martensite and/or Lower Bainite Tempered martensite and lower bainite are structures containing carbides that are observed as fine white structures in SEM photographs. It is possible to distinguish between the two by more microscopic observation, but it is difficult to distinguish them by SEM photography. Therefore, in the present invention, tempered martensite and lower bainite are defined as the same structure, and the total area ratio of tempered martensite and lower bainite was measured by the point counting method according to ASTM E562-11 (2014). A value obtained by averaging the measured values at five locations was taken as the total area ratio of tempered martensite and lower bainite.
  • FIG. 1(b) The SEM photograph shown in FIG. 1(b) is a photograph of a structure that was water-cooled after being held at a temperature T within the range of the present invention in the annealing process, and needle-like ⁇ , block-like ⁇ , and a ferrite structure are formed.
  • FIG. 2 shows a schematic diagram of a method for measuring the aspect ratio of acicular ⁇ .
  • austenite surrounded by recrystallized ferrite having the same orientation and having an aspect ratio of 3.0 or more is defined as acicular austenite.
  • the tip of the acicular austenite may be in contact with other austenite grains, but in that case, it is sufficient to confirm that the adjacent ferrite grains have the same orientation by electron beam backscatter diffraction (EBSD).
  • EBSD electron beam backscatter diffraction
  • the number of needle-like ⁇ in a steel sheet that has been subjected to soaking and holding at the annealing temperature T is determined in five fields of view, and the number of needle-like ⁇ is divided by the total area observed. The number density (pieces/1000 ⁇ m 2 ) was measured. Table 3 shows the results.
  • ⁇ ND plane ⁇ ⁇ RD direction> orientation of rolling is ⁇ 111 ⁇ ⁇ 0-11> orientation, ⁇ 111 ⁇ ⁇ 11-2> orientation, ⁇ 211 ⁇ ⁇ 0-11> orientation, and ⁇ 100 ⁇ ⁇ 011> orientation
  • the texture of the cold-rolled steel sheet was evaluated by quantifying the texture area ratio of the bcc phase and calculating the ratio with the area ratio of the bcc phase in all orientations.
  • Tensile test A JIS No. 5 tensile test piece having a tensile direction perpendicular to the rolling direction was produced from the obtained steel plate. Each test piece was subjected to a tensile test in accordance with JIS Z 2241 (2011). The crosshead speed of the tensile test was 10 mm/min. In addition, the measurement was performed twice, and the measured value was obtained by averaging the measured value, which was defined as the tensile strength (TS) of each steel plate.
  • TS tensile strength
  • Hole expanding test A test piece of 100 mm ⁇ 100 mm was collected, and a hole expanding test based on JFST 1001 (Iron Federation Standard) was performed three times at each sampling position, and the average value of three times (total value of three times ( %)/3) was defined as the hole expansion ratio ⁇ (%).
  • the treatment agents are, in order, degreasing agent FC-E2011, surface conditioning agent PL-X, and chemical conversion treatment liquid Palbond PB-L3065 manufactured by Nihon Parkerizing Co., Ltd. was used.
  • Magnification SEM observation in an area of 10000 ⁇ m 2 or more at 2000 times to observe the surface chemical conversion structure. was evaluated as ⁇ . Table 3 shows the results.
  • the steel sheet of the present invention has a tensile strength of 980 MPa or more, high ductility and excellent stretch flanging formability, and excellent chemical conversion treatability.
  • 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. It has high strength, high ductility, excellent stretch flanging formability, and good chemical conversion treatability. It was found to have good chemical conversion treatability.

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PCT/JP2022/033946 2021-09-30 2022-09-09 鋼板、部材およびそれらの製造方法 Ceased WO2023053909A1 (ja)

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPWO2024203779A1 (https=) * 2023-03-31 2024-10-03
JPWO2024203777A1 (https=) * 2023-03-31 2024-10-03
WO2024203778A1 (ja) * 2023-03-31 2024-10-03 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
JP7817659B1 (ja) * 2024-06-04 2026-02-19 日本製鉄株式会社 鋼板及びそれを含む部品
JP7817660B1 (ja) * 2024-06-04 2026-02-19 日本製鉄株式会社 鋼板及びそれを含む部品
WO2026042396A1 (ja) * 2024-08-20 2026-02-26 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117836459A (zh) * 2021-08-31 2024-04-05 杰富意钢铁株式会社 钢板、构件和它们的制造方法

Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0635619B2 (ja) 1986-02-05 1994-05-11 新日本製鐵株式会社 延性の良い高強度鋼板の製造方法
WO2009099251A1 (ja) * 2008-02-08 2009-08-13 Jfe Steel Corporation 加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
JP4411221B2 (ja) 2004-01-28 2010-02-10 株式会社神戸製鋼所 伸び及び伸びフランジ性に優れた低降伏比高強度冷延鋼板およびめっき鋼板並びにその製造方法
JP5463685B2 (ja) 2009-02-25 2014-04-09 Ｊｆｅスチール株式会社 加工性および耐衝撃性に優れた高強度冷延鋼板およびその製造方法
JP6041079B1 (ja) 2015-07-08 2016-12-07 Ｊｆｅスチール株式会社 冷延鋼帯の製造方法及び製造設備
JP6439903B1 (ja) 2017-04-14 2018-12-19 Ｊｆｅスチール株式会社 鋼板およびその製造方法
JP2019502819A (ja) * 2015-12-15 2019-01-31 ポスコＰｏｓｃｏ 化成処理性及び穴拡げ性に優れた超高強度鋼板及びその製造方法
WO2020080337A1 (ja) * 2018-10-17 2020-04-23 Ｊｆｅスチール株式会社 薄鋼板およびその製造方法
WO2021079756A1 (ja) * 2019-10-23 2021-04-29 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS601492Y2 (ja) 1977-10-07 1985-01-16 日本エツクス線株式会社 螢光ｘ線分析装置等用試料交換装置
JPS6041079U (ja) 1983-08-29 1985-03-23 ソニー株式会社 プリント回路基板
JPH0529523Y2 (https=) 1987-09-04 1993-07-28
JPH0635619A (ja) 1992-07-15 1994-02-10 Nippon Telegr & Teleph Corp <Ntt> 情報多重読取り装置
CN108779536B (zh) * 2016-04-19 2020-06-30 杰富意钢铁株式会社 钢板、镀覆钢板和它们的制造方法
CN111902554B (zh) * 2018-03-30 2022-03-29 日本制铁株式会社 钢板及其制造方法
JP6897874B2 (ja) * 2019-01-09 2021-07-07 Ｊｆｅスチール株式会社 高強度冷延鋼板及びその製造方法

Patent Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0635619B2 (ja) 1986-02-05 1994-05-11 新日本製鐵株式会社 延性の良い高強度鋼板の製造方法
JP4411221B2 (ja) 2004-01-28 2010-02-10 株式会社神戸製鋼所 伸び及び伸びフランジ性に優れた低降伏比高強度冷延鋼板およびめっき鋼板並びにその製造方法
WO2009099251A1 (ja) * 2008-02-08 2009-08-13 Jfe Steel Corporation 加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
JP5463685B2 (ja) 2009-02-25 2014-04-09 Ｊｆｅスチール株式会社 加工性および耐衝撃性に優れた高強度冷延鋼板およびその製造方法
JP6041079B1 (ja) 2015-07-08 2016-12-07 Ｊｆｅスチール株式会社 冷延鋼帯の製造方法及び製造設備
JP2019502819A (ja) * 2015-12-15 2019-01-31 ポスコＰｏｓｃｏ 化成処理性及び穴拡げ性に優れた超高強度鋼板及びその製造方法
JP6439903B1 (ja) 2017-04-14 2018-12-19 Ｊｆｅスチール株式会社 鋼板およびその製造方法
WO2020080337A1 (ja) * 2018-10-17 2020-04-23 Ｊｆｅスチール株式会社 薄鋼板およびその製造方法
WO2021079756A1 (ja) * 2019-10-23 2021-04-29 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of EP4372119A4

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPWO2024203779A1 (https=) * 2023-03-31 2024-10-03
JPWO2024203777A1 (https=) * 2023-03-31 2024-10-03
WO2024203777A1 (ja) * 2023-03-31 2024-10-03 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
WO2024203778A1 (ja) * 2023-03-31 2024-10-03 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
WO2024203779A1 (ja) * 2023-03-31 2024-10-03 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
JP7619534B1 (ja) * 2023-03-31 2025-01-22 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
JP7704309B2 (ja) 2023-03-31 2025-07-08 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
JP7704308B2 (ja) 2023-03-31 2025-07-08 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
JP7817659B1 (ja) * 2024-06-04 2026-02-19 日本製鉄株式会社 鋼板及びそれを含む部品
JP7817660B1 (ja) * 2024-06-04 2026-02-19 日本製鉄株式会社 鋼板及びそれを含む部品
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