WO2018043456A1 - 高強度冷延薄鋼板及びその製造方法 - Google Patents
高強度冷延薄鋼板及びその製造方法 Download PDFInfo
- Publication number
- WO2018043456A1 WO2018043456A1 PCT/JP2017/030849 JP2017030849W WO2018043456A1 WO 2018043456 A1 WO2018043456 A1 WO 2018043456A1 JP 2017030849 W JP2017030849 W JP 2017030849W WO 2018043456 A1 WO2018043456 A1 WO 2018043456A1
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- WIPO (PCT)
- Prior art keywords
- less
- steel sheet
- martensite
- hot
- grain size
- Prior art date
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- C—CHEMISTRY; METALLURGY
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- 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
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- C—CHEMISTRY; METALLURGY
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- 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
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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
- C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/0226—Hot rolling
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- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/0405—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing of ferrous alloys
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
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- C21D8/0436—Cold rolling
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- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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- C21D2211/00—Microstructure comprising significant phases
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/22—Electroplating: Baths therefor from solutions of zinc
Definitions
- the present invention relates to a high-strength cold-rolled thin steel sheet having a tensile strength (TS) of 980 MPa or more and suitable for automobile parts and a method for producing the same.
- TS tensile strength
- a DP steel sheet made of a composite structure of soft ferrite and hard martensite is known.
- C 0.07 to 0.25%
- Si 0.07 to 0.25%
- Mn 1.5 to 3.0%
- Ti 0.005 to 0.09%
- B 0.0001 to 0.01%
- P 0.001 to 0.03%
- S 0.0001 to 0.01%
- Al 2.5% or less
- N 0.00.
- a high-strength steel sheet having a maximum tensile strength of 900 MPa or more with good ductility is disclosed, which is a martensite having a block size of 1 ⁇ m or less and a C concentration in the martensite of 0.3 to 0.9%.
- the DP steel sheet has a defect that since martensite having high hardness exists in the steel sheet structure, voids are generated at the interface with the soft ferrite during the punching process, and the hole expandability is poor.
- a TRIP steel sheet containing retained austenite is known as a steel sheet having both high ductility and high strength. Residual austenite undergoes a processing-induced transformation to martensite during deformation, so that both high strength and high ductility can be achieved.
- the TRIP steel sheet also has a drawback that voids tend to be generated at the interface with soft ferrite due to transformation of retained austenite into martensite during punching, resulting in poor hole expandability.
- Patent Document 2 by mass, C: 0.05 to 0.35%, Si: 0.05 to 2.0%, Mn: 0.8 to 3.0%, P: 0.0010 to A component satisfying 0.1%, S: 0.0005 to 0.05%, N: 0.0010 to 0.010%, Al: 0.01 to 2.0%, the balance being Fe and inevitable impurities
- the steel structure is mainly composed of ferrite, bainite, or tempered martensite, and contains a residual austenite phase of not less than 3% and not more than 30%.
- the central concentration Cgc of the retained austenite phase and the grain boundary concentration of the retained austenite grain By including 50% or more of retained austenite grains in a range where Cgb satisfies Cgb / Cgc> 1.3, the stability of the retained austenite phase interface is improved, and a high-strength thin steel sheet excellent in elongation and hole-expandability is obtained. Shows.
- the high-strength cold-rolled thin steel sheet is required not to crack during welding in order to obtain excellent joint strength during resistance welding.
- zinc on the steel sheet surface melts during resistance welding, and tensile stress is generated in the vicinity of the weld, resulting in liquid metal embrittlement and cracking in the steel sheet. May occur. Therefore, in Patent Document 3, by mass%, C: 0.015 to 0.072%, Si: 1.2% or less, Mn: 0.5 to 3.0% or less, P: 0.020% or less, S: 0.030% or less, sol. Al: 0.002 to 1.20%, Si + sol.
- galvanized steel sheet with a tensile strength of 450 MPa or more, satisfying Al + 0.4 ⁇ Mn ⁇ 1.4%, with the balance being composed of Fe and inevitable impurities An excellent high tensile galvanized steel sheet is disclosed.
- Japanese Patent No. 4925611 JP 2011-195957 A Japanese Patent No. 3758515
- Patent Document 1 In order to have both high ductility and hole expansibility in a high-strength cold-rolled thin steel sheet, it is necessary to add C, Si, Mn, etc., but there is a problem that resistance weldability decreases as the content of these increases.
- the steel sheet of Patent Document 1 actively uses Si to obtain 50% or more ferrite and martensite with a C concentration of 0.3 to 0.9%. Not considered.
- the high-strength thin steel sheet of Patent Document 2 also contains a large amount of C and Si in order to obtain retained austenite, but no consideration is given to weldability.
- the steel plate In resistance welding (spot welding), the steel plate normally contacts the electrode vertically, but when assembling automobile parts, depending on the welding location, the steel plate or electrode tilts and the angle between the steel plate and electrode deviates from the vertical ( Striking angle). In that case, the stress load applied to the steel sheet during welding becomes non-uniform, and cracks are likely to occur at places where the load is large.
- the steel sheet of Patent Document 3 no consideration is given to weld cracking when the hitting angle is applied, and no consideration is given to hole expansibility.
- the present invention advantageously solves the above-mentioned problems of the prior art, has a tensile strength of 980 MPa or more, has high ductility, and has high resistance weldability with high hole expansibility, and production thereof It aims to provide a method.
- the inventors of the present invention are keen to obtain a high-strength cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more, excellent not only in ductility but also in hole expandability, and also in weldability, and a method for producing the same. Repeated examination. As a result, the inventors built fine ferrite, retained austenite, martensite (as-quenched martensite), bainite, and tempered martensite, and further controlled the inter-particle distance of martensite (as-quenched martensite).
- TS tensile strength
- the cooling stop temperature after hot rolling is controlled, and further, cold rolling is performed within an appropriate condition range, and further annealing conditions
- the crystal grains of ferrite, retained austenite, martensite, bainite, and tempered martensite can be made minute in the final steel structure.
- the present invention has been made based on the above knowledge and has the following features.
- B 0.0002% or more and 0.0040% or less
- the component composition comprising the balance Fe and inevitable impurities, Contains 35% or less ferrite, 1% or more and 10% or less retained austenite, 2% or more and 12% or less as-quenched martensite, and 25 to 70% bainite and tempered martensite in total.
- the component composition further includes, by mass%, V: 0.005% to 0.200%, Cr: 0.05% to 0.20%, Mo: 0.01% to 0.20 %: Cu: 0.05% to 0.20%, Ni: 0.01% to 0.20%, Sb: 0.002% to 0.100%, Sn: 0.002% to 0 100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0005% or more and 0.0050% or less.
- a steel slab having the composition described in [1] or [2] is hot-rolled at a hot rolling start temperature of 1100 ° C. or higher and 1300 ° C. or lower, and a finish rolling temperature of 800 ° C. or higher and 1000 ° C. or lower, After the hot rolling, hot rolling is performed after cooling to a cooling stop temperature of 500 ° C. or less under conditions where the average cooling rate in the temperature range from 700 ° C. to the cooling stop temperature is 5 ° C./s or more and 50 ° C./s or less.
- a cold rolling step in which cold rolling is performed, and the cold-rolled steel sheet obtained in the cold rolling step is held at a temperature range of 750 ° C. to 900 ° C. for 10 seconds to 900 seconds, and after the holding, 5 ° C.
- a high-strength cold-rolled thin steel sheet having a tensile strength of 980 MPa or more, having both excellent ductility and hole expandability, and excellent weldability can be obtained.
- the high-strength cold-rolled thin steel sheet of the present invention is, in mass%, C: 0.04% to 0.12%, Si: 0.15% to 0.95%, Mn: 2.00% to 3. 50% or less, P: 0.050% or less, S: 0.0050% or less, N: 0.0100% or less, Al: 0.010% or more and 2.0% or less, Ti: 0.005% or more. 075% or less, Nb: 0.005% or more and 0.075% or less, B: 0.0002% or more and 0.0040% or less, and has a component composition composed of the balance Fe and inevitable impurities.
- the above component composition is further in mass%, V: 0.005% to 0.200%, Cr: 0.05% to 0.20%, Mo: 0.01% to 0.20% Cu: 0.05% to 0.20%, Ni: 0.01% to 0.20%, Sb: 0.002% to 0.100%, Sn: 0.002% to 0.000%. 100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0005% or more and 0.0050% or less These elements may be contained.
- % representing the content of the component means “% by mass”.
- C 0.04% or more and 0.12% or less
- C has a high solid solution strengthening ability and is effective for increasing the strength of the steel sheet, and is also effective for residual austenite, martensite, bainite and tempered martensite in the present invention. Contributes to formation. In order to obtain such an effect, a content of 0.04% or more is required. If C is less than 0.04%, it becomes difficult to obtain desired retained austenite and martensite. On the other hand, when the content exceeds 0.12%, retained austenite, martensite, bainite, and tempered martensite are excessively formed, so that ductility and hole expandability are lowered, and weldability is further lowered. Therefore, the C content is 0.04% or more and 0.12% or less.
- the preferable C content for the lower limit is 0.05% or more. More preferably, it is 0.06% or more, More preferably, it is 0.07% or more.
- the preferable C content for the upper limit is 0.11% or less. More preferably, it is 0.10% or less or less than 0.10%, and further preferably 0.09% or less.
- Mn 2.00% or more and 3.50% or less
- Mn contributes to increasing the strength of the steel sheet by solid solution strengthening or improving hardenability, and is an austenite stabilizing element, so that desired retained austenite and martensite are secured. It is an indispensable element. In order to acquire such an effect, content of 2.00% or more is required. On the other hand, if the content exceeds 3.50%, the weldability is deteriorated, excessive austenite and martensite are generated excessively, and the hole expandability is further deteriorated. For this reason, Mn content shall be the range of 2.00% or more and 3.50% or less.
- the preferable Mn content for the lower limit is 2.20% or more. More preferably it is 2.40% or more, and still more preferably 2.60% or more.
- a preferable Mn content for the upper limit is 3.30% or less. More preferably it is 3.10% or less, and still more preferably 2.90% or less.
- P 0.050% or less
- P is an element contributing to an increase in the strength of the steel sheet by solid solution strengthening.
- the content exceeding 0.050% causes a decrease in weldability and promotes grain boundary fracture due to grain boundary segregation. Therefore, the P content is 0.050% or less.
- the lower limit of the P content is not particularly limited, but excessively reducing the P content leads to an increase in production cost, so the P content is preferably 0.0001% or more.
- S 0.0050% or less
- S is an element that segregates at the grain boundaries and embrittles the steel during hot working, and also exists in the steel as a sulfide such as MnS and lowers local deformability. If the content exceeds 0050%, the hole expandability is lowered. For this reason, S is limited to 0.0050% or less.
- the lower limit of the S content is not particularly limited, but excessively reducing the S content leads to an increase in production cost, so the S content is preferably 0.0001% or more.
- N 0.0100% or less
- N is an element that is present in the steel as a nitride and lowers the local deformability, and if it exceeds 0.0100%, the hole expandability is lowered. For this reason, N content is limited to 0.0100% or less.
- the lower limit of the N content is not particularly limited, but excessively reducing the N content leads to an increase in manufacturing cost, so the N content is preferably 0.0001% or more.
- Al 0.010% or more and 2.0% or less
- Al is a ferrite-forming element, and is an element that suppresses the formation of carbide (cementite) and contributes to the stabilization of retained austenite in the same manner as Si.
- it is necessary to contain 0.010% or more.
- it is 0.015% or more, More preferably, it is 0.020% or more.
- the Al content is set to 2.0% or less.
- it is 1.8% or less, More preferably, it is 1.6% or less. Even if the total of Al and Si is 0.95% or less, the effect of the present invention is obtained.
- Ti not only forms fine carbides and nitrides, but also suppresses coarsening of crystal grains and refines the steel structure after heating, thereby increasing the strength. It is an element that contributes to the rise of. Furthermore, addition of Ti is effective in order not to react B with N. In order to obtain such an effect, it is necessary to contain 0.005% or more of Ti. Preferably it is 0.010% or more, More preferably, it is 0.020% or more. On the other hand, if the Ti content exceeds 0.075%, carbides and nitrides are excessively generated, resulting in a decrease in ductility. For this reason, Ti content is taken as 0.005% or more and 0.075% or less of range. Further, the Ti content is preferably 0.060% or less, more preferably 0.050% or less.
- B 0.0002% or more and 0.0040% or less
- B is an effective element that improves hardenability and contributes to an increase in strength. In order to acquire such an effect, it is necessary to contain 0.0002% or more. Preferably it is 0.0007% or more, More preferably, it is 0.0011% or more. On the other hand, if the content exceeds 0.0040%, martensite is excessively generated, so that ductility and hole expandability are lowered. For this reason, B content is taken as 0.0002% or more and 0.0040% or less of range. Further, the B content is preferably 0.0035% or less, more preferably 0.0030% or less.
- the above-described components are basic components.
- V 0.005% to 0.200%
- Cr 0.05% to 0.20%
- Mo 0.01% to 0.20%
- Cu 0.05% to 0.20%
- Ni 0.01% to 0.20%
- Sb 0.002% to 0.100%
- Sn 0.002% to 0.100%
- Ca 0.0005% to 0.0050%
- Mg 0.0005% to 0.0050%
- REM 0.0005% to 0.0050%
- V contributes to strengthening of the steel sheet by generating V-based precipitates, and also contributes to the refinement and homogenization of the steel structure.
- the V content is set to 0.005% or more. Preferably it is 0.007% or more, More preferably, it is 0.010% or more. On the other hand, if the content exceeds 0.200%, V-based precipitates are excessively generated and ductility is lowered. For this reason, when it contains V, it is preferable to limit V content to the range of 0.005% or more and 0.200% or less. Further, the V content is preferably 0.100% or less, more preferably 0.050% or less.
- Cr contributes to an increase in strength by solid solution strengthening and contributes to an increase in strength by improving hardenability and promoting martensite formation. In order to acquire such an effect, 0.05% or more of content is required. More preferably, it is 0.06% or more, More preferably, it is 0.07% or more. On the other hand, when it contains Cr exceeding 0.20%, a martensite will produce
- ⁇ Mo contributes to increasing the strength of the steel sheet by solid solution strengthening, improves hardenability, and promotes the formation of martensite, thereby contributing to an increase in strength.
- a content of 0.01% or more is required. More preferably, it is 0.02% or more, More preferably, it is 0.04% or more.
- Mo content when it contains exceeding 0.20%, a martensite will produce
- it contains Mo it is preferable to limit Mo content to 0.01% or more and 0.20% or less of range. Further, the Mo content is preferably 0.15% or less, more preferably 0.10% or less.
- Cu contributes to an increase in strength by improving the hardenability and promoting the formation of martensite while contributing to an increase in strength of the steel sheet by solid solution strengthening.
- 0.05% or more of content is required.
- it is 0.06% or more, More preferably, it is 0.07% or more.
- the Cu content is preferably 0.15% or less, more preferably 0.10% or less.
- Ni contributes to an increase in strength by solid solution strengthening and contributes to an increase in strength by improving hardenability and promoting martensite formation.
- 0.01% or more of content is required.
- it is 0.02% or more, More preferably, it is 0.05% or more.
- it is preferable to limit Ni content to the range of 0.01% or more and 0.20% or less.
- the Ni content is preferably 0.15% or less, more preferably 0.10% or less.
- Sb and Sn have an action of suppressing decarburization of the steel sheet surface layer (region of about several tens of ⁇ m from the surface in the thickness direction) caused by oxidation of the steel sheet surface.
- By suppressing such decarburization of the steel sheet surface layer it is possible to prevent a reduction in the amount of martensite produced in the steel sheet surface layer, which is effective in securing a desired steel sheet strength.
- Sb and Sn are respectively contained exceeding 0.100%, the effect is saturated. For this reason, when it contains these, it is preferable to limit Sb and Sn to the range of 0.002% or more and 0.100% or less, respectively.
- Ca, Mg, and REM are all elements used for deoxidation, and are elements that have an action of spheroidizing the shape of the sulfide and improving the adverse effects on the local ductility and hole expansibility of the sulfide.
- the contents of Ca, Mg, and REM need to be 0.0005% or more, respectively.
- it exceeds 0.0050% and contains excessively inclusions and the like are increased, and surface defects and internal defects are generated, resulting in a decrease in ductility and hole expandability. For this reason, when it contains these, it is preferable to limit Ca, Mg, and REM to the range of 0.0005% or more and 0.0050% or less, respectively.
- the balance other than the above components is Fe and inevitable impurities.
- the element when the said arbitrary component is included below the said lower limit, the element shall be included as an unavoidable impurity.
- the steel structure contains 35% or less ferrite, 1% or more and 10% or less retained austenite, and 2% or more and 12% or less as-quenched martensite, with the balance being from bainite and tempered martensite. Become.
- the average crystal grain size of ferrite is 5.0 ⁇ m or less
- the average crystal grain size of retained austenite is 2.0 ⁇ m or less
- the average crystal grain size of martensite is 3.0 ⁇ m or less
- bainite and tempered martensite is 3.0 ⁇ m or less
- the average crystal grain size of the site phase 4.0 ⁇ m or less
- the average inter-particle distance of martensite satisfies 1.0 ⁇ m or more.
- Ferrite Volume ratio of 35% or less and an average crystal grain size of 5.0 ⁇ m or less
- Ferrite is a structure that contributes to improvement of ductility (elongation).
- the ferrite has a volume fraction of 35% or less.
- Range Preferably it is 33% or less, More preferably, it is 30% or less.
- the volume fraction of ferrite is preferably 10% or more from the viewpoint of improving ductility. More preferably, it is 15% or more, More preferably, it is 20% or more.
- the average crystal grain size of ferrite exceeds 5.0 ⁇ m, voids generated at the punched end during the hole expansion are easily connected during the hole expansion, so that a good hole expansion property cannot be obtained.
- the average crystal grain size of ferrite is set to a range of 5.0 ⁇ m or less. Preferably it is 4.5 micrometers or less, More preferably, it is 4.0 micrometers or less.
- the average crystal grain size of ferrite is usually 1.0 ⁇ m or more and 2.0 ⁇ m or more.
- Residual austenite volume ratio of 1% or more and 10% or less and an average crystal grain size of 2.0 ⁇ m or less
- Residual austenite is a ductile phase itself, but it is a structure that contributes to further improving ductility by strain-induced transformation. Yes, it contributes to improvement of ductility and improvement of strength-ductility balance.
- the retained austenite needs to be 1% or more by volume ratio. Preferably it is 2% or more, more preferably 3% or more. On the other hand, if it exceeds 10%, the hole expandability is lowered. For this reason, a retained austenite shall be 1% or more and 10% or less by volume ratio. Further, the retained austenite is preferably 8% or less, more preferably 6% or less.
- the average crystal grain size of retained austenite is set to a range of 2.0 ⁇ m or less. Preferably it is 1.5 micrometers or less, More preferably, it is 1.0 micrometers or less. Moreover, the average crystal grain size of retained austenite is usually 0.1 ⁇ m or more or 0.3 ⁇ m or more.
- Martensite Volume ratio 2% or more and 12% or less and average grain size 3.0 ⁇ m or less Martensite needs 2% or more by volume ratio to obtain a tensile strength of 980 MPa or more. Preferably it is 4% or more, more preferably 6% or more. On the other hand, if it exceeds 12%, voids are likely to occur at the interface with the ferrite during the hole expansion test, leading to a decrease in the hole expansion rate. For this reason, martensite is in a range of 2% to 12% by volume. Further, the volume ratio of martensite is preferably 11% or less, more preferably 10% or less. The martensite here is martensite as it is quenched and is distinguished from tempered martensite described later.
- the average crystal grain size of martensite is set to a range of 3.0 ⁇ m or less. It is preferably 2.5 ⁇ m or less, more preferably 2.0 ⁇ m or less. Moreover, the average crystal grain size of martensite is usually 0.5 ⁇ m or more or 0.7 ⁇ m or more.
- Bainite and tempered martensite Average crystal grain size of 4.0 ⁇ m or less
- tempered martensite means that martensite generated when cooled to a cooling temperature range is heated to a reheating temperature range in an annealing process. , Martensite that has been tempered when held. Bainite and tempered martensite reduce the difference in hardness between soft ferrite, hard martensite and retained austenite, and contribute to the improvement of hole expansibility. For this reason, it is necessary to contain bainite and tempered martensite having an average crystal grain size of 4.0 ⁇ m or less in the structure.
- the average crystal grain size of bainite and tempered martensite is set to a range of 4.0 ⁇ m or less. Preferably it is 3.8 micrometers or less, More preferably, it is 3.4 micrometers or less.
- the average crystal grain size is usually 1.5 ⁇ m or more and 2.0 ⁇ m or more.
- the “average crystal grain size of bainite and tempered martensite” means an average crystal grain size derived without distinguishing between bainite and tempered martensite.
- the total volume ratio of bainite and tempered martensite is 25% or more because it reduces the hardness difference between soft ferrite, hard martensite, and retained austenite and contributes to improvement of hole expansibility. Preferably it is 30% or more, more preferably 35% or more. Further, if the total volume ratio becomes excessively large, the ductility is lowered, so that it is made 70% or less. Preferably it is 65% or less, More preferably, it is less than 60%. The volume ratio of tempered martensite is often 59% or less.
- Martensite average inter-particle distance 1.0 ⁇ m or more Voids are formed at the interface between the soft phase and the hard phase, and grow by joining adjacent voids to form cracks. If the distance between the voids is small, the connection of the voids is easily generated, which causes a decrease in local deformability and hole expansibility. Therefore, in order to ensure good ductility and hole expansibility, it is necessary that the average inter-particle distance of martensite be 1.0 ⁇ m or more. Preferably it is 1.5 micrometers or more, More preferably, it is 2.0 micrometers or more. The average interparticle distance is preferably 9.0 ⁇ m or less, more preferably 7.0 ⁇ m or less.
- the martensite average interparticle distance ⁇ m was calculated using the following equation (1) (Tetsu-to-Hagane, vol. 91, (2005), p. 796-802). Moreover, as mentioned above, martensite means martensite as quenched. Moreover, it exists in the tendency for uniform elongation (uEl) to increase by making the average inter-particle distance of a martensite into the said range.
- ⁇ m ⁇ 0.9 (V m / 100) ⁇ 1/2 ⁇ 0.8 ⁇ ⁇ d m (1)
- V m volume fraction of martensite (%)
- d m the average grain size of martensite ([mu] m).
- non-recrystallized ferrite, pearlite, and cementite may be generated. If the above limitation is satisfied, the object of the present invention can be achieved. However, the volume ratio is preferably 10% or less for non-recrystallized ferrite, 5% or less for pearlite, and 5% or less for cementite.
- the thickness of the high-strength cold-rolled thin steel sheet can be appropriately set according to the application. Generally, it is 0.8 to 2.5 mm.
- the high-strength cold-rolled thin steel sheet having the composition and structure described above may further have a plating layer on the surface in order to improve corrosion resistance.
- the plating layer is preferably any one of a hot dip galvanized layer, an alloyed hot dip galvanized layer, or an electrogalvanized layer.
- a hot dip galvanized layer, the alloyed hot dip galvanized layer, and the electrogalvanized layer a known hot dip galvanized layer, alloyed hot dip galvanized layer, and electrogalvanized layer are all suitable.
- the high-strength cold-rolled thin steel sheet of the present invention has a tensile strength (TS) of 980 MPa or more, a breaking elongation (El) of 12% or more, and a hole expansion ratio ⁇ of 40% or more, as measured in Examples.
- TS tensile strength
- El breaking elongation
- ⁇ hole expansion ratio
- TS is usually 1200 MPa or less
- El is 20% or less
- ⁇ is 80% or less.
- uEl is often 9.5% or more.
- uEl is 10.0% or more.
- a hot rolling process, a pickling process, a cold rolling process, and an annealing process are sequentially performed on the steel material having the above composition to obtain a high-strength cold-rolled thin steel sheet.
- Steel slabs to be used for hot rolling are prepared by casting a slab of a predetermined size by a continuous casting method because the molten steel having the above composition is melted by a conventional melting method such as a converter and segregation of components is less likely to occur. It is preferable to use a piece (steel material). In addition, what was obtained by the ingot-making method or the thin slab casting method may be used.
- the steel material having the above composition is subjected to a hot rolling process to obtain a hot rolled sheet (hot rolled sheet steel).
- the steel material having the above composition is reheated, and in addition to the method of performing hot rolling, the cast steel slab is inserted into a heating furnace without being cooled and reheated.
- a method of rolling the steel slab immediately after it is heated without cooling the steel slab a method of rolling the steel slab immediately after casting, or the like can be applied. Specific conditions in the hot rolling process will be described below.
- Hot rolling start temperature 1100 ° C. or higher and 1300 ° C. or lower If the hot rolling start temperature is lower than 1100 ° C., the rolling load increases and the productivity decreases, whereas if it exceeds 1300 ° C., the heating cost only increases. Therefore, the hot rolling start temperature is set to a range of 1100 ° C. or higher and 1300 ° C. or lower.
- Finish rolling temperature 800 ° C. or more and 1000 ° C. or less If the finish rolling temperature is less than 800 ° C., the steel structure becomes non-uniform, and the ductility and hole expandability after the annealing process are lowered. By setting the finish rolling temperature to 800 ° C. or higher, rolling is completed in the austenite single phase region, and a homogeneous steel sheet structure is obtained. Preferably it is 850 degreeC or more. On the other hand, if the finish rolling temperature exceeds 1000 ° C., the structure of the hot-rolled steel sheet becomes coarse, and a structure having a desired crystal grain size cannot be obtained after the annealing process. Preferably it is 950 degrees C or less. Therefore, finish rolling temperature shall be 800 degreeC or more and 1000 degrees C or less.
- the hot-rolled steel sheet is controlled to a structure mainly composed of bainite. If it is less than 5 ° C./s, ferrite or pearlite is excessively generated in the structure of the hot-rolled steel sheet. Preferably it is 15 degrees C / s or more. On the other hand, if it exceeds 50 ° C./s, the effect of suppressing the formation of ferrite or pearlite is saturated.
- the average cooling rate is set to 5 ° C./s or more and 50 ° C./s or less.
- about 700 degreeC after hot rolling may be allowed to cool or may be cooled by a cooling means, and the cooling conditions are not particularly limited.
- Cooling stop temperature 500 ° C. or less
- the hot-rolled steel sheet is homogenized into a bainite-based structure.
- the steel structure after the annealing step particularly ferrite and martensite, is refined, and an effect of obtaining a desired inter-particle distance of martensite is obtained.
- it exceeds 500 ° C. ferrite or pearlite is excessively generated in the steel structure of the hot-rolled steel sheet, and the steel structure after the annealing process becomes inhomogeneous.
- the lower limit of the cooling stop temperature is not particularly specified, but if it is less than 350 ° C., hard martensite is excessively generated in the structure of the hot-rolled steel sheet, and the rolling load during cold rolling may increase. For this reason, the cooling stop temperature is preferably 350 ° C. or higher.
- the pickling conditions are not particularly limited, and any conventional pickling method using hydrochloric acid, sulfuric acid or the like can be applied.
- the cold rolling step is a step of cold rolling the hot-rolled sheet that has undergone the pickling process to obtain a cold-rolled sheet (cold-rolled steel sheet) having a predetermined thickness.
- Rolling ratio of cold rolling 30% or more and 70% or less
- processing strain is introduced into the steel sheet.
- the annealing process which is the next process
- recrystallization in the annealing temperature region is promoted, and the crystal grain size of the final structure is controlled.
- the rolling reduction is less than 30%, the processing strain applied to the steel sheet is insufficient, and recrystallization cannot be sufficiently achieved in the annealing process. Since the average inter-particle distance of martensite cannot be obtained, ductility and hole expandability deteriorate.
- the rolling reduction exceeds 70%, processing strain is excessively introduced into the steel sheet, and recrystallization in the annealing temperature region is excessively promoted in the annealing process, and ferrite, martensite, bainite, or tempered martensite.
- the average crystal grain size becomes coarse. Therefore, the rolling rate of cold rolling is in the range of 30% to 70%.
- the obtained thin cold-rolled sheet is subjected to an annealing process.
- the annealing process is performed to form desired ferrite, retained austenite, martensite, bainite, and martensite on the steel sheet, thereby obtaining a high-strength cold-rolled thin steel sheet having both high ductility and high hole expansibility.
- Specific conditions for this annealing step are as follows.
- Annealing temperature 750 ° C. or more and 900 ° C. or less If the annealing temperature is less than 750 ° C., the volume fraction of austenite in the annealing temperature region decreases, so not only the ferrite is generated excessively, but also recrystallization does not proceed sufficiently. Unrecrystallized ferrite is also generated excessively, and the hole expandability is reduced. On the other hand, if the annealing temperature exceeds 900 ° C., the austenite grains become excessively coarse and it is difficult to obtain a desired crystal grain size. For this reason, annealing temperature shall be 750 degreeC or more and 900 degrees C or less. A preferable annealing temperature for the lower limit is 770 ° C. or higher. A preferable annealing temperature for the upper limit is 880 ° C. or lower. The heating conditions up to the annealing temperature are not particularly limited.
- Holding time in the annealing temperature range 10 seconds or more and 900 seconds or less If the holding time in the annealing temperature range is less than 10 seconds, not only recrystallization does not proceed sufficiently, but austenite cannot be sufficiently generated in the annealing temperature range, Ultimately, unrecrystallized ferrite and ferrite are excessively formed. Moreover, even if it hold
- “holding” includes not only isothermal holding but also slow cooling and heating in the temperature range.
- Average cooling rate to the cooling stop temperature 5 ° C / s or more If the average cooling rate from the annealing temperature to the cooling stop temperature is less than 5 ° C / s, not only ferrite but also pearlite is generated excessively during cooling. It is difficult to obtain a desired amount of bainite and tempered martensite.
- the cooling is preferably gas cooling, but may be performed by combining furnace cooling, mist cooling, roll cooling, water cooling, and the like.
- the upper limit of an average cooling rate is not specifically limited, Usually, it is 50 degrees C / s or less.
- Cooling stop temperature 100 ° C. or more and 250 ° C. or less
- the cooling stop temperature is less than 100 ° C.
- a large amount of martensite is generated when cooling is stopped, and a large amount of tempered martensite is formed when reheating is performed.
- the cooling stop temperature exceeds 250 ° C., not only the finally obtained martensite becomes excessive, but also the desired average interparticle distance ⁇ m cannot be obtained, and the hole expandability is lowered. Therefore, the cooling stop temperature is limited to a temperature in the cooling stop temperature range of 100 ° C. or more and 250 ° C. or less.
- Reheating temperature 300 ° C. or more and 400 ° C. or less Reheating is performed in order to temper martensite generated during cooling and transform untransformed austenite to bainite to obtain bainite and retained austenite. If the reheating temperature is less than 300 ° C., the martensite is not sufficiently tempered, the resulting martensite phase is excessive, and the desired average interparticle distance ⁇ m cannot be obtained. Spreadability is reduced. On the other hand, when the reheating temperature exceeds 400 ° C., not only ferrite is generated excessively but also a desired amount of martensite cannot be obtained. Therefore, the reheating temperature is limited to 300 ° C. or more and 400 ° C. or less. The heating conditions up to the reheating temperature are not particularly limited.
- Holding time in the reheating temperature region 10 seconds or more and 1800 seconds or less If the holding time in the reheating temperature region is less than 10 seconds, the martensite is not sufficiently tempered, and the final martensite is excessive. In addition, the desired average interparticle distance ⁇ m cannot be obtained, and the hole expandability is reduced. On the other hand, even if it exceeds 1800 seconds, the steel structure is not affected. For this reason, the holding time in the reheating temperature region was set to 10 seconds or more and 1800 seconds or less.
- “holding” includes not only isothermal holding but also slow cooling and heating in the temperature range.
- the cooling after holding in the reheating temperature range does not need to be specified, and can be cooled to a desired temperature such as room temperature by an arbitrary method such as cooling.
- temper rolling may be performed after annealing.
- the elongation rate in this temper rolling is not particularly defined, excessive elongation is preferably from 0.1% to 2.0% because ductility is lowered.
- a plating process may be further performed to form a plating layer on the surface.
- the plating treatment is preferably galvanizing treatment, galvanizing treatment and alloying treatment, or electrogalvanizing treatment.
- hot dip galvanizing treatment hot dip galvanizing treatment, alloying treatment, and electrogalvanizing treatment, all known processing methods are suitable.
- Molten steel having the composition shown in Table 1 was melted in a converter and formed into a slab having a thickness of 230 mm by a continuous casting method.
- the obtained steel material was subjected to a hot rolling process under the conditions shown in Table 2 to obtain a hot-rolled steel sheet.
- the obtained hot-rolled steel sheet was pickled and subjected to a cold rolling process at a cold reduction rate shown in Table 2 to obtain a cold-rolled steel sheet.
- hydrochloric acid was used for pickling. Subsequently, it annealed on the conditions shown in Table 2.
- GI hot dip galvanized thin steel sheet
- the hot dip galvanizing treatment uses a continuous hot dip galvanizing line to reheat the annealed cold-rolled annealed plate to a temperature in the range of 430 to 480 ° C. as necessary, and a hot dip galvanizing bath (bath temperature: 470). C.) and adjusted so that the adhesion amount of the plating layer was 45 g / m 2 per side.
- the bath composition was Zn-0.18 mass% Al.
- the bath composition was Zn-0.14 mass% Al, and after the plating treatment, alloying was performed at 520 ° C. to obtain alloyed hot dip galvanized thin steel sheets (GA).
- the Fe concentration in the plating layer was set to 9% by mass or more and 12% by mass or less.
- some cold-rolled steel sheets are further subjected to electrogalvanizing treatment using an electrogalvanizing line after the annealing process so that the amount of coating is 30 g / m 2 per side.
- a plated thin steel sheet (EG) was used.
- a specimen for microstructural observation is taken from a high-strength cold-rolled thin steel sheet that has been annealed or further plated, and corresponds to 1 ⁇ 4 of the plate thickness in the rolling direction cross section (L cross section). It was polished, corroded (3 vol.% Nital liquid corrosion) so that the position to be observed became an observation surface, and observed at a magnification of 5000 times using an SEM (scanning electron microscope). Using the obtained SEM image, the tissue fraction (area ratio) of each phase was determined by image analysis, and the value was treated as the volume ratio. In the image analysis, “Image-Pro” (trade name) of Media Cybernetics was used as analysis software.
- ferrite is gray
- as-quenched martensite and retained austenite cementite is white
- bainite and tempered martensite are intermediate colors between gray and white, so each phase was judged from the color tone.
- the structure in which retained austenite and cementite are observed in fine lines or spots in ferrite is bainite
- the structure in which cementite is observed in fine lines or spots in martensite is tempered martensite.
- the area of each ferrite grain, bainite and tempered martensite grain is obtained by image analysis, the equivalent circle diameter is calculated from the area, and the average crystal is obtained by arithmetically averaging these values. The particle size was taken.
- the average crystal grain size of the residual austenite grains was observed at a magnification of 15000 times using a TEM (transmission electron microscope), and the area of the residual austenite grains was determined from the obtained TEM image by image analysis. The equivalent circle diameter was calculated, and these values were arithmetically averaged to obtain the average crystal grain size.
- a specimen for X-ray diffraction is taken from a cold-rolled steel sheet that has been annealed or further plated, and is ground and polished so that the position corresponding to 1/4 of the sheet thickness becomes the measurement surface.
- the amount of retained austenite was determined from the diffracted X-ray intensity by the X-ray diffraction method.
- the incident X-ray was a CoK ⁇ ray.
- all of the integrated intensities of the peaks of the ⁇ 111 ⁇ , ⁇ 200 ⁇ , ⁇ 220 ⁇ , ⁇ 311 ⁇ planes of austenite and the ⁇ 110 ⁇ , ⁇ 200 ⁇ , ⁇ 211 ⁇ planes of ferrite are used.
- the strength ratio was calculated for the combination, the average value thereof was determined, and the amount of retained austenite (volume ratio) of the steel sheet was calculated.
- the welding current and welding time were adjusted so that the nugget diameter was 4 ⁇ t mm (t: the thickness of the high-strength cold-rolled steel sheet). After welding, the test piece is cut in half, and the cross section is observed with an optical microscope. If no crack of 0.1 mm or more is observed, the resistance weld cracking resistance is considered good, and is evaluated as “ ⁇ ”. The case where a crack of 1 mm or more was observed was designated as “x”.
- Each of the present invention examples 1 to 13 has a structure containing predetermined ferrite and retained austenite, martensite, bainite and tempered martensite, high tensile strength TS: 980 MPa or more, and total elongation El It is a high-strength cold-rolled thin steel sheet having a high ductility of 12% or more, a high hole-expandability of 40% or more, and an excellent resistance spot weldability.
- No. Nos. 15, 17, and 19 contain C, Si, and Mn in excess, so that cracks are generated during welding. In 15 and 19, a desired structure cannot be obtained, and El and ⁇ are insufficient.
- No. Nos. 14, 16, 18, 20 to 22 are inferior in at least one characteristic of TS, El, and ⁇ because the component composition in the steel is out of the limited range.
- the composition of the components in the steel is within the limited range, but the manufacturing method is out of the scope of the present invention. Therefore, in the final steel sheet structure, the ferrite phase in the proper form, the retained austenite phase, the martensite phase, A structure containing a bainite phase and a tempered martensite phase cannot be obtained, and at least one characteristic of TS, El, and ⁇ is inferior.
- the examples of the present invention are high-strength cold-rolled thin steel sheets having high strength, high ductility and high hole expansibility, and excellent weldability.
Abstract
Description
体積率で、35%以下のフェライトと、1%以上10%以下の残留オーステナイトと、2%以上12%以下の焼入れままマルテンサイトと、合計で25~70%のベイナイトおよび焼戻しマルテンサイトとを含む鋼組織と、を有し、前記フェライトの平均結晶粒径:5.0μm以下であり、前記残留オーステナイトの平均結晶粒径:2.0μm以下であり、前記焼入れままマルテンサイトの平均結晶粒径:3.0μm以下であり、前記ベイナイトおよび焼戻しマルテンサイト相の平均結晶粒径:4.0μm以下であり、前記焼入れままマルテンサイトの平均粒子間距離が1.0μm以上を満たす高強度冷延薄鋼板。
Cは、高い固溶強化能を有し、鋼板強度の増加に有効であるとともに、本発明における残留オーステナイト、マルテンサイト、ベイナイト、および焼戻しマルテンサイトの形成に寄与する。このような効果を得るためには、0.04%以上の含有を必要とする。Cが0.04%未満では、所望の残留オーステナイトおよびマルテンサイトを得ることが困難になる。一方、0.12%を超える多量の含有は、残留オーステナイト、マルテンサイト、ベイナイト、および焼戻しマルテンサイトが過剰に生成するため、延性と穴広げ性が低下し、更に、溶接性の低下を招く。したがって、C含有量は0.04%以上0.12%以下とする。下限について好ましいC含有量は0.05%以上である。より好ましくは0.06%以上、さらに好ましくは0.07%以上である。上限について好ましいC含有量は0.11%以下である。より好ましくは0.10%以下や0.10%未満、さらに好ましくは0.09%以下である。
Siは、フェライト中で高い固溶強化能を有し、鋼板強度の増加に寄与するとともに、炭化物(セメンタイト)の生成を抑制し、残留オーステナイトの安定化に寄与する。また、フェライトに固溶したSiは、加工硬化能を向上させ、フェライト相自身の延性向上に寄与する。このような効果を得るためには、0.15%以上の含有を必要とする。一方、Si含有量が0.95%を超えると、残留オーステナイト安定化の寄与は飽和し、更に、溶接性の低下も招く。このため、Si含有量は0.15%以上0.95%以下の範囲とする。なお、下限について好ましいSi含有量は0.25%以上である。より好ましくは0.30%以上、さらに好ましくは0.35%以上である。上限について好ましいSi含有量は0.85%以下である。より好ましくは0.80%以下、さらに好ましくは0.70%以下である。
Mnは、固溶強化あるいは焼入れ性向上により、鋼板の強度増加に寄与するとともに、オーステナイト安定化元素であるため、所望の残留オーステナイトおよびマルテンサイトの確保に必要不可欠な元素である。このような効果を得るためには2.00%以上の含有を必要とする。一方、3.50%を超える含有は、溶接性を低下させる上、残留オーステナイトおよびマルテンサイトを過剰に生成させ、更に、穴広げ性の低下を招く。このため、Mn含有量は2.00%以上3.50%以下の範囲とする。なお、下限について好ましいMn含有量は2.20%以上である。より好ましくは2.40%以上、さらに好ましくは2.60%以上である。上限について好ましいMn含有量は3.30%以下である。より好ましくは3.10%以下、さらに好ましくは2.90%以下である。
Pは、固溶強化により鋼板の強度増加に寄与する元素である。一方、0.050%を超える含有は、溶接性の低下を招くとともに、粒界偏析による粒界破壊を助長する。このため、P含有量は0.050%以下とする。P含有量の下限は特に限定されないが、過剰にP含有量を低減することは製造コストの増加につながるため、P含有量は0.0001%以上が好ましい。
Sは、粒界に偏析して熱間加工時に鋼を脆化させるとともに、MnSなどの硫化物として鋼中に存在して局部変形能を低下させる元素であり、0.0050%を超える含有は穴広げ性の低下を招く。このため、Sは0.0050%以下に限定する。S含有量の下限は特に限定されないが、過剰にS含有量を低減することは製造コストの増加につながるため、S含有量は0.0001%以上が好ましい。
Nは、窒化物として鋼中に存在して局部変形能を低下させる元素であり、0.0100%を超える含有は穴広げ性の低下を招く。このため、N含有量は0.0100%以下に限定する。N含有量の下限は特に限定されないが、過剰にN含有量を低減することは製造コストの増加につながるため、N含有量は0.0001%以上が好ましい。
Alは、フェライト生成元素であり、Siと同様に炭化物(セメンタイト)の生成を抑制し、残留オーステナイトの安定化に寄与する元素である。このような効果を得るためには、0.010%以上含有する必要がある。好ましくは0.015%以上、より好ましくは0.020%以上である。一方、2.0%を超えると効果が飽和するため、Al含有量は2.0%以下とする。好ましくは1.8%以下、より好ましくは1.6%以下である。なお、AlとSiの合計が、0.95%以下であっても本発明の効果を奏する。
Tiは、微細な炭化物や窒化物を形成するのみならず、結晶粒の粗大化を抑制し、加熱後の鋼組織を微細化することにより、強度の上昇に寄与する元素である。更に、BをNと反応させないために、Tiの添加は有効である。このような効果を得るためには、Tiを0.005%以上含有する必要がある。好ましくは0.010%以上、より好ましくは0.020%以上である。一方、Ti含有量が0.075%を超えると、炭化物や窒化物が過剰に生成し、延性の低下を招く。このため、Ti含有量は0.005%以上0.075%以下の範囲とする。また、Ti含有量は、0.060%以下が好ましく、より好ましくは0.050%以下である。
Nbは、微細な炭化物や窒化物を形成するのみならず、結晶粒の粗大化を抑制し、加熱後の鋼組織を微細化させることにより、強度の上昇に寄与する。このような効果を得るためには、0.005%以上含有する必要がある。0.010%以上が好ましく、より好ましくは0.015%以上である。一方、Nb含有量が0.075%を超えると、炭化物や窒化物が過剰に生成し、延性の低下を招く。このため、Nb含有量は0.005%以上0.075%以下の範囲とする。また、Nb含有量は、0.060%以下であることが好ましく、より好ましくは0.050%以下である。さらに好ましくは0.040%未満である。
Bは、焼入れ性を向上させ、強度の上昇に寄与する有効な元素である。このような効果を得るためには、0.0002%以上含有する必要がある。好ましくは0.0007%以上、より好ましくは0.0011%以上である。一方、0.0040%を超える含有は、マルテンサイトを過剰に生成させるため、延性および穴広げ性を低下させる。このため、B含有量は0.0002%以上0.0040%以下の範囲とする。また、B含有量は0.0035%以下が好ましく、より好ましくは0.0030%以下である。
フェライトは、延性(伸び)の向上に寄与する組織である。しかし、体積率が35%を超えると、所望量のベイナイトおよび焼戻しマルテンサイトを得ることが困難となる等して、穴広げ性が低下してしまうため、フェライトは、体積率で35%以下の範囲とする。好ましくは33%以下、より好ましくは30%以下である。また、延性向上の観点からフェライトの体積率は10%以上が好ましい。より好ましくは15%以上、さらに好ましくは20%以上である。
残留オーステナイトは、それ自体、延性に富む相であるが、歪誘起変態してさらに延性の向上に寄与する組織であり、延性の向上および強度-延性バランスの向上に寄与する。このような効果を得るためには、残留オーステナイトは、体積率で1%以上とする必要がある。好ましくは2%以上、より好ましくは3%以上である。一方、10%を超えて多くなると、穴広げ性の低下を招く。このため、残留オーステナイトは、体積率で1%以上10%以下の範囲とする。また、残留オーステナイトは、8%以下が好ましく、より好ましくは6%以下である。
マルテンサイトは、980MPa以上の引張強さを得るために、体積率で2%以上必要である。好ましくは4%以上、より好ましくは6%以上である。一方、12%を超えると、穴広げ試験時にフェライトとの界面にボイドが生じやすくなり、穴広げ率の低下を招く。このため、マルテンサイトは、体積率で2%以上12%以下の範囲とする。また、マルテンサイトの体積率は11%以下が好ましく、より好ましくは10%以下である。なお、ここでいうマルテンサイトは、焼入れままマルテンサイトであり、後述する焼戻しマルテンサイトとは区別される。
ここでいう「焼戻しマルテンサイト」とは、焼鈍工程において、冷却温度域まで冷却した際に生成したマルテンサイトが、再加熱温度域まで加熱され、保持された際に焼戻されたマルテンサイトのことである。ベイナイトおよび焼戻しマルテンサイトは、軟質なフェライトと、硬質なマルテンサイトおよび残留オーステナイトとの硬度差を小さくし、穴広げ性の向上に寄与する。このため、組織中に平均結晶粒径が4.0μm以下のベイナイトおよび焼戻しマルテンサイトを含有することが必要である。一方、4.0μmを超えると、穴広げ時の打ち抜き破面に生成したボイドが穴広げ中に連結しやすくなるため、良好な穴広げ性が得られない。このため、ベイナイトおよび焼戻しマルテンサイトの平均結晶粒径は4.0μm以下の範囲とする。好ましくは3.8μm以下、より好ましくは3.4μm以下である。また、上記平均結晶粒径は、通常、1.5μm以上や2.0μm以上である。なお、「ベイナイトおよび焼戻しマルテンサイトの平均結晶粒径」とは、ベイナイトと焼戻しマルテンサイトとを区別せずに導出した平均結晶粒径を意味する。
ボイドは軟質相と硬質相の界面に生成し、隣接するボイド同士が連結することで成長し、き裂となる。ボイド同士の距離が小さいとボイドの連結が容易に生じるため、局部変形能や穴広げ性の低下を招く。したがって、良好な延性や穴広げ性を確保するには、マルテンサイトの平均粒子間距離を1.0μm以上にすることが必要である。好ましくは1.5μm以上、より好ましくは2.0μm以上である。また、上記平均粒子間距離は、9.0μm以下が好ましく、より好ましくは7.0μm以下である。なお、マルテンサイトの平均粒子間距離Λmは下記の(1)式を用いて算出した(Tetsu-to-Hagane,vol.91,(2005),p.796-802)。また、上記の通り、マルテンサイトは焼入れままマルテンサイトを意味する。また、マルテンサイトの平均粒子間距離を上記範囲にすることで、均一伸び(uEl)が高まる傾向にある。
Λm={0.9(Vm/100)-1/2-0.8}×dm・・・(1)
ここで、Vm:マルテンサイトの体積率(%)、dm:マルテンサイトの平均結晶粒径(μm)とする。
熱間圧延開始温度が1100℃未満では圧延負荷が増大し、生産性が低下する一方、1300℃超では加熱コストが増大するだけである。したがって、熱間圧延開始温度は、1100℃以上1300℃以下の範囲とする。
仕上げ圧延温度が800℃未満では、鋼組織が不均一となり、焼鈍工程後の延性や穴広げ性が低下する。仕上げ圧延温度を800℃以上とすることで、オーステナイト単相域で圧延が完了し、均質な鋼板組織が得られる。好ましくは850℃以上である。一方、仕上げ圧延温度が1000℃超では熱延鋼板の組織が粗大となり、焼鈍工程後に所望の結晶粒径を有する組織が得られない。好ましくは950℃以下である。したがって、仕上げ圧延温度は800℃以上1000℃以下とする。
熱間圧延後、700℃から冷却停止温度までの平均冷却速度を5℃/s以上50℃/s以下とすることで、熱延鋼板はベイナイトを主体とする組織に制御される。5℃/s未満では、熱延鋼板の組織にフェライトもしくはパーライトが過剰に生成してしまう。好ましくは15℃/s以上である。一方、50℃/sを超えるとフェライトもしくはパーライトの生成を抑制する効果が飽和する。そこで、上記平均冷却速度を5℃/s以上50℃/s以下とする。なお、熱間圧延後から700℃までについては、放冷でも、冷却手段による冷却でもよく、冷却条件は特に限定されない。
上記冷却の冷却停止温度を500℃以下とすることにより、熱延鋼板はベイナイト主体の組織に均質化される。この均質化により、焼鈍工程後の鋼組織、特にフェライトやマルテンサイトが微細化する上、所望のマルテンサイトの粒子間距離が得られる効果が得られる。一方、500℃を超えると熱延鋼板の鋼組織にフェライトもしくはパーライトが過剰に生成し、焼鈍工程後の鋼組織が不均質となる。この不均質により、所望の平均結晶粒径を有するフェライトまたはマルテンサイトが得られない上、所望のマルテンサイトの平均粒子間距離が得られなくなるため、穴広げ性が劣化する。なお、冷却停止温度の下限は特に規定しないが、350℃未満では、熱延鋼板の組織に硬質なマルテンサイトが過剰に生成し、冷間圧延時の圧延負荷が増大する場合がある。このため、冷却停止温度は350℃以上が好ましい。
冷間圧延では、鋼板に加工歪が導入される。これにより、次工程である焼鈍工程で、焼鈍温度域での再結晶を促進し、最終組織の結晶粒径を制御する。圧下率が30%未満では、鋼板に加わる加工歪が不足し、焼鈍工程で再結晶が十分に達成しないため、最終組織の鋼組織には、未再結晶フェライトが過剰に生成する上、所望のマルテンサイトの平均粒子間距離を得られなくなるため、延性と穴広げ性が劣化する。一方、70%を超える圧下率は、鋼板に加工歪が過度に導入されてしまい、焼鈍工程で、焼鈍温度域での再結晶が過度に促進され、フェライト、マルテンサイト、ベイナイトまたは焼戻しマルテンサイトの平均結晶粒径が粗大となる。したがって、冷間圧延の圧延率は30%以上70%以下の範囲である。
焼鈍温度が750℃未満では、焼鈍温度域のオーステナイトの体積分率が少なくなるため、フェライトが過剰に生成するだけでなく、再結晶も十分に進行しないため、未再結晶フェライトも過剰に生成し、穴広げ性が低下する。一方、焼鈍温度が900℃を超えると、オーステナイト粒が過度に粗大化し、所望の結晶粒径を得ることが困難となる。このため、焼鈍温度は750℃以上900℃以下とする。下限について好ましい焼鈍温度は770℃以上である。上限について好ましい焼鈍温度は880℃以下である。なお、焼鈍温度までの加熱条件は特に限定されない。
焼鈍温度域での保持時間が10秒未満では、再結晶が十分に進行しないだけでなく、焼鈍温度域でオーステナイトが十分に生成できず、最終的に未再結晶フェライトおよびフェライトが過剰に生成する。また、900秒を超えて保持しても、最終的に得られる鋼組織や機械的特性に影響は表れない。このため、焼鈍温度域での保持時間は10秒以上900秒以下の範囲とする。なお、ここで「保持」とは、等温保持以外に、当該温度域での徐冷、加熱をも含むものとする。
焼鈍温度から冷却停止温度までの平均冷却速度が5℃/s未満では、冷却中にフェライトだけでなく、パーライトが過剰に生成するだけでなく、所望量のベイナイトおよび焼戻しマルテンサイトが得ることが困難になる。なお、冷却は、ガス冷却が好ましいが、炉冷、ミスト冷却、ロール冷却、水冷などを組み合わせて行うことも可能である。また、平均冷却速度の上限は、特に限定されないが、通常、50℃/s以下である。
冷却停止温度が100℃未満では、冷却停止時に多量のマルテンサイトが生成し、再加熱時に多量の焼戻しマルテンサイトとなるため、延性が低下する。一方、冷却停止温度が250℃を超えると、最終的に得られるマルテンサイトが過剰となるだけでなく、所望の平均粒子間距離Λmが得られなくなり、穴広げ性が低下する。したがって、冷却停止温度は100℃以上250℃以下の冷却停止温度域の温度に限定した。
再加熱は、冷却中に生成したマルテンサイトを焼戻すとともに、未変態オーステナイトをベイナイト変態させ、ベイナイトおよび残留オーステナイトを得るために施される。再加熱温度が300℃未満では、マルテンサイトの焼戻しが十分に施されず、最終的に得られるマルテンサイト相が過剰となる上、所望の平均粒子間距離Λmが得られなくなり、延性および穴広げ性が低下する。一方、再加熱温度が400℃を超えると、フェライトが過剰に生成するだけでなく、所望量のマルテンサイトが得られなくなる。したがって、再加熱温度は、300℃以上400℃以下に限定した。なお、再加熱温度までの加熱条件は特に限定されない。
再加熱温度域での保持時間が10秒未満では、マルテンサイトの焼戻しが十分に施されず、最終的に生成するマルテンサイトが過剰となる上、所望の平均粒子間距離Λmが得られなくなり、穴広げ性が低下する。一方、1800秒を超えても鋼組織に影響しない。このため、再加熱温度域での保持時間は10秒以上1800秒以下とした。なお、ここで「保持」とは、等温保持以外に、当該温度域での徐冷、加熱をも含むものとする。
まず、焼鈍され、あるいはさらにめっき処理を施された高強度冷延薄鋼板から組織観察用試験片を採取し、圧延方向断面(L断面)で板厚の1/4に相当する位置が観察面となるように、研磨し、腐食(3vol.%ナイタール液腐食)し、SEM(走査型電子顕微鏡)を用いて5000倍の倍率で観察した。得られたSEM画像を用いて、画像解析により各相の組織分率(面積率)を求め、その値を体積率として扱った。なお、画像解析では、解析ソフトとしてMedia Cybernetics社の「Image-Pro」(商品名)を使用した。なお、SEM画像では、フェライトは灰色、焼入れままマルテンサイトおよび残留オーステナイト、セメンタイトは白色を呈し、更に、ベイナイトおよび焼戻しマルテンサイトは灰色と白色の中間色を呈するため、その色調から各相を判断した。また、フェライト中に残留オーステナイトやセメンタイトが微細な線状または点状に観察される組織はベイナイトとし、マルテンサイト中にセメンタイトが微細な線状または点状に観察される組織は焼戻しマルテンサイトとした。また、得られたSEM画像を用いて、画像解析により、各フェライト粒、ベイナイトおよび焼戻しマルテンサイト粒の面積を求め、該面積から円相当直径を算出し、それらの値を算術平均して平均結晶粒径とした。
焼鈍され、あるいはさらにめっき処理を施された高強度冷延薄鋼板から、引張方向が圧延方向と垂直な方向(C方向)となるようにJIS 5号引張試験片を採取し、JIS Z 2241(2011)の規定に準拠して、引張試験を実施し、引張特性(引張強さTS、破断伸びEl)を求めた。また、均一伸びuElも求めた。なお、TS:980MPa級では、El:12.0%以上である場合を、良好な強度延性バランスであるとした。また、均一伸び(uEl)が9.5%以上であることが多いことも確認した。
(3)穴広げ試験
焼鈍され、あるいはさらにめっき処理を施された高強度冷延薄鋼板から、100mmW×100mmLサイズの試験片を採取し、JIS Z 2256(2010)の規定に準拠して、クリアランス12±1%にて、10mmφの穴を打ち抜き、60°の円錐ポンチを上昇させ穴を広げた際に、き裂が板厚方向を貫通したところでポンチの上昇を止め、き裂貫通後の穴径と試験前の穴径から穴広げ率λ(%)を測定した。なお、TS:980MPa級では、λ:40%以上である場合を、良好な穴広げ性であるとした。
焼鈍され、あるいはさらにめっき処理された高強度冷延薄鋼板から、150mmW×50mmLサイズの試験片を1枚用い、もう1枚は590MPa級溶融亜鉛めっき鋼板を用いて抵抗溶接(スポット溶接)を実施した。2枚の鋼板を重ねた板組について、溶接ガンに取付けられたサーボモータ加圧式で単相交流(50Hz)の抵抗溶接機を用いて板組を3°傾けた状態で抵抗スポット溶接した。溶接条件は加圧力を4.0kN、ホールドタイムを0.2秒とした。溶接電流と溶接時間はナゲット径が4√t mm(t:高強度冷延薄鋼板の板厚)になるように調整した。溶接後は試験片を半切して、断面を光学顕微鏡で観察し、0.1mm以上のき裂が認められないものを耐抵抗溶接割れ性が良好であるとし、「○」と評価し、0.1mm以上のき裂が認められたものを「×」とした。
Claims (5)
- 質量%で、
C:0.04%以上0.12%以下、
Si:0.15%以上0.95%以下、
Mn:2.00%以上3.50%以下、
P:0.050%以下、
S:0.0050%以下、
N:0.0100%以下、
Al:0.010%以上2.0%以下、
Ti:0.005%以上0.075%以下、
Nb:0.005%以上0.075%以下、
B:0.0002%以上0.0040%以下を含み、残部Feおよび不可避的不純物からなる成分組成と、
体積率で、35%以下のフェライトと、1%以上10%以下の残留オーステナイトと、2%以上12%以下の焼入れままマルテンサイトと、合計で25~70%のベイナイトおよび焼戻しマルテンサイトとを含む鋼組織と、を有し、
前記フェライトの平均結晶粒径:5.0μm以下であり、前記残留オーステナイトの平均結晶粒径:2.0μm以下であり、前記焼入れままマルテンサイトの平均結晶粒径:3.0μm以下であり、前記ベイナイトおよび焼戻しマルテンサイト相の平均結晶粒径:4.0μm以下であり、
前記焼入れままマルテンサイトの平均粒子間距離が1.0μm以上を満たす高強度冷延薄鋼板。 - 前記成分組成は、さらに、質量%で、
V:0.005%以上0.200%以下、
Cr:0.05%以上0.20%以下、
Mo:0.01%以上0.20%以下、
Cu:0.05%以上0.20%以下、
Ni:0.01%以上0.20%以下、
Sb:0.002%以上0.100%以下、
Sn:0.002%以上0.100%以下、
Ca:0.0005%以上0.0050%以下、
Mg:0.0005%以上0.0050%以下、
REM:0.0005%以上0.0050%以下のうちから選ばれる少なくとも1種の元素を含有する請求項1に記載の高強度冷延薄鋼板。 - 表面に、溶融亜鉛めっき層、合金化溶融亜鉛めっき層、あるいは電気亜鉛めっき層のいずれかを有する請求項1または2に記載の高強度冷延薄鋼板。
- 請求項1または2いずれかに記載の成分組成からなる鋼スラブを、熱間圧延開始温度1100℃以上1300℃以下、仕上げ圧延温度800℃以上1000℃以下で熱間圧延し、該熱間圧延後、700℃から冷却停止温度までの温度域の平均冷却速度が5℃/s以上50℃/s以下の条件で500℃以下の冷却停止温度まで冷却した後に巻取る熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板に、酸洗処理を施す酸洗工程と
前記酸洗工程で酸洗された熱延鋼板に、圧延率が30%以上70%以下の冷間圧延を施す冷間圧延工程と、
前記冷間圧延工程で得られた冷延鋼板を750℃以上900℃以下の温度域で10秒以上900秒以下保持し、該保持後、5℃/s以上の平均冷却速度で、100℃以上250℃以下の冷却停止温度まで冷却した後、300℃以上400℃以下の再加熱温度域まで加熱し、再加熱温度域で10秒以上1800秒以下保持する焼鈍工程とを有する高強度冷延薄鋼板の製造方法。 - 前記焼鈍工程後に、溶融亜鉛めっき層、合金化溶融亜鉛めっき層、あるいは電気亜鉛めっき層のいずれかを形成するためのめっき処理を施すめっき工程を有する請求項4に記載の高強度冷延薄鋼板の製造方法。
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Also Published As
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MX2019001794A (es) | 2019-06-13 |
US11136644B2 (en) | 2021-10-05 |
KR20190028758A (ko) | 2019-03-19 |
JPWO2018043456A1 (ja) | 2018-08-30 |
EP3476963B1 (en) | 2020-04-08 |
EP3476963A1 (en) | 2019-05-01 |
CN109642281B (zh) | 2021-02-23 |
US20190203315A1 (en) | 2019-07-04 |
EP3476963A4 (en) | 2019-06-19 |
KR102197431B1 (ko) | 2020-12-31 |
JP6323627B1 (ja) | 2018-05-16 |
CN109642281A (zh) | 2019-04-16 |
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