WO2016190396A1 - 鋼板及びその製造方法 - Google Patents
鋼板及びその製造方法 Download PDFInfo
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- WO2016190396A1 WO2016190396A1 PCT/JP2016/065629 JP2016065629W WO2016190396A1 WO 2016190396 A1 WO2016190396 A1 WO 2016190396A1 JP 2016065629 W JP2016065629 W JP 2016065629W WO 2016190396 A1 WO2016190396 A1 WO 2016190396A1
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- C—CHEMISTRY; METALLURGY
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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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- 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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- C21D6/00—Heat treatment of ferrous alloys
- 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
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- 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
- C21D8/0226—Hot rolling
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
Definitions
- the present invention relates to a steel plate and a manufacturing method thereof.
- a steel sheet containing 0.1 to 0.7% by mass of carbon is formed into a blank material by press forming, hole expanding forming, bending forming, drawing forming, thickening and thinning forming, or a combination thereof. It is used as a material for manufacturing drive system parts such as automobile gears and clutches by forming such as forging. Since the strength of such a component is ensured by quenching and tempering the steel sheet, the steel sheet is required to have high hardenability.
- the steel sheet as the material for the drive system parts is required to have high cold formability.
- Component molding is mainly drawn and / or thickened, and in component molding, the greatest factor that affects the formability of a material is plastic anisotropy. Improvement of plastic anisotropy in a steel sheet is necessary for application to forming a part of a steel sheet.
- Patent Document 1 as steel for mechanical structure whose toughness is improved by suppressing coarsening of crystal grains in carburizing heat treatment, C: 0.10 to 0.30%, Si: 0.05 ⁇ 2.0%, Mn: 0.10 ⁇ 0.50%, P: 0.030% or less, S: 0.030% or less, Cr: 1.80 ⁇ 3.00%, Al: 0.005 ⁇ Containing 0.050%, Nb: 0.02 to 0.10%, N: 0.0300% or less, the balance being Fe and inevitable impurities, the structure before cold working is a ferrite pearlite structure, and ferrite A steel for machine structural use having an average particle size of 15 ⁇ m or more is disclosed.
- Patent Document 2 as steel having excellent cold workability and carburizing hardenability, C: 0.15 to 0.40%, Si: 1.00% or less, Mn: 0.40% or less, sol. Al: 0.02% or less, N: 0.006% or less, B: 0.005 to 0.050%, with the balance being Fe and inevitable impurities, mainly composed of ferrite phase and graphite phase A steel having a structure that achieves this is disclosed.
- Patent Document 3 discloses a steel material for a carburized bevel gear having excellent impact strength, a high toughness carburized bevel gear, and a manufacturing method thereof.
- Patent Document 4 cold forging is performed after spheroidizing annealing, and the parts produced in the carburizing and quenching and tempering process have excellent workability while suppressing the coarsening of crystal grains even in subsequent carburizing.
- Steel for carburized parts having excellent impact resistance and impact fatigue resistance is disclosed.
- Patent Document 5 as cold tool steel for plasma carburizing, C: 0.40 to 0.80%, Si: 0.05 to 1.50%, Mn: 0.05 to 1.50%, and V: 1.8 to 6.0%, Ni: 0.10 to 2.50%, Cr: 0.1 to 2.0%, and Mo: 3.0% or less
- the steel which contains 2 or more types and remainder consists of Fe and an unavoidable impurity is disclosed.
- Patent Document 6 in C: 0.25 to 0.75%, the carbide particle size and spheroidizing rate are specified, the cold rolling rate and box annealing conditions, the hot rolling coiling temperature, and the texture specification It has been proposed to limit the r value and ⁇ r by defining the improvement in the in-plane anisotropy.
- Patent Documents 7 and 8 it is proposed to reduce the ⁇ r value and improve the in-plane anisotropy by defining the heating and annealing conditions of the hot rolled material between the stands of the finish rolling mill. .
- Patent Document 8 proposes a steel sheet with reduced in-plane anisotropy by prescribing finish rolling at a temperature not lower than the Ar3 point and winding at 500-630 ° C. in hot rolling.
- JP 2013-040376 A Japanese Patent Laid-Open No. 06-116679 JP 09-201644 A JP 2006-213951 A Japanese Patent Laid-Open No. 10-158780 JP 2000-328172 A JP 2001-073076 A Japanese Patent Laid-Open No. 2001-073077
- the present invention improves the hardenability and material formability, and in particular, a steel plate suitable for forming parts such as gears by forming by cold forging such as thickening and a method for manufacturing the same.
- the purpose is to provide.
- the ferrite phase has low hardness and high ductility. Therefore, it is possible to improve the material formability by increasing the grain size in a structure mainly composed of ferrite.
- carbides in the steel sheet are strong particles that prevent slipping, and by allowing carbides to exist at the ferrite grain boundaries, it is possible to prevent the propagation of slips across the crystal grain boundaries and suppress the formation of shear bands. It can improve the cold forgeability and at the same time improve the formability of the steel sheet.
- cementite is a hard and brittle structure, and if it exists in the state of pearlite, which is a layered structure with ferrite, the steel becomes hard and brittle, so it must be present in a spherical shape. In consideration of cold forgeability and generation of cracks during forging, the particle size needs to be in an appropriate range.
- the metal structure of the steel sheet after coiling after hot rolling becomes a bainite structure in which cementite is dispersed in fine pearlite or fine ferrite with a small lamellar spacing, so that the temperature is relatively low (400 ° C to 550 ° C). Take up with.
- cementite dispersed in the ferrite is also easily spheroidized.
- the cementite is partially spheroidized by annealing at a temperature just below the Ac1 point as the first stage annealing.
- annealing is performed at a temperature between Ac1 point and Ac3 point (so-called two-phase region of ferrite and austenite), and a part of the ferrite grains is left, and a part thereof is austenite transformed. Thereafter, the ferrite grains left by slow cooling were grown, and austenite was transformed into ferrite by using the ferrite grains as a nucleus, so that cementite was precipitated at the grain boundaries while obtaining a large ferrite phase, and the above structure was realized.
- the present invention has been made on the basis of these findings, and the gist thereof is as follows.
- the hot-rolled steel sheet is heated to an annealing temperature of 725 ° C. or more and 790 ° C. or less at a heating rate of 1 ° C./hour or more and 80 ° C./hour or less, and subjected to the second stage annealing for holding for 3 hours or more and less than 10 hours.
- the annealed hot-rolled steel sheet is 1 ° C./hour or more and 100 ° C./hour or less.
- C is an element that forms carbides and is effective in strengthening steel and refining ferrite grains. In order to suppress the occurrence of satin during cold forming and to ensure the surface appearance of the cold-formed product, it is necessary to suppress the coarsening of the ferrite grain size.
- C is set to 0.10% or more.
- 0.14% or more the volume fraction of carbide increases, and when a load is instantaneously applied, a crack that becomes the starting point of fracture is generated, and there is a concern that the moldability and impact resistance characteristics are deteriorated. .
- C is made 0.40% or less. Preferably it is 0.38% or less.
- C is more than 0.40%. Preferably it is 0.44% or more. If C exceeds 0.70%, a large amount of cracks that are the starting points of fracture are generated, and the fatigue characteristics are deteriorated. Therefore, C is set to 0.70% or less. Preferably it is 0.66% or less.
- Si 0.01-0.30%
- Si is an element that affects the form of carbides and contributes to the improvement of material moldability.
- Si is made 0.01% or more. Preferably, it is 0.07% or more.
- Si When Si exceeds 0.30%, the hardness increases due to the solid solution strengthening of ferrite, the ductility decreases, cracking is likely to occur during cold forging, formability during cold forging and after carburizing and quenching and tempering. Since the impact resistance is deteriorated, Si is made 0.30% or less. Preferably it is 0.28% or less.
- Mn is an element that controls the form of carbide in two-stage annealing. If it is less than 0.30%, it becomes difficult to form carbides at the ferrite grain boundaries in the slow cooling after the second stage annealing, so Mn is 0.30% or more. Preferably it is 0.40% or more.
- Mn exceeds 1.00%, the toughness after carburizing, quenching and tempering is lowered, but the strength is improved.
- Mn is made 1.00% or less. Preferably it is 0.96% or less.
- Mn When increasing the strength, Mn is over 1.00%. Preferably it is 1.10% or more. When Mn exceeds 3.00%, the toughness after carburizing and quenching and tempering is remarkably lowered, so Mn is made 3.00% or less. Preferably it is 2.70% or less.
- Al 0.001 to 0.10%
- Al is an element that acts as a deoxidizer and stabilizes ferrite. If it is less than 0.001%, the effect of addition cannot be sufficiently obtained, so Al is made 0.001% or more. Preferably it is 0.004% or more.
- Mo is made 0.50% or less. Preferably it is 0.40% or less.
- B is an element that enhances hardenability and further increases toughness.
- the required hardenability is required, so 0.0004 to 0.01% is added. If it is less than 0.0004%, the effect of addition cannot be obtained, so B is made 0.0004% or more. Preferably it is 0.0010% or more.
- B is 0.01% or less.
- Ti is made 0.10% or less.
- it is 0.07% or less.
- the following elements are impurities and must be controlled to a certain amount or less.
- P 0.02% or less
- P is an element that segregates at the ferrite grain boundaries and suppresses the formation of carbides at the ferrite grain boundaries. Therefore, the smaller the P, the better.
- the content of P may be 0, but if it is reduced to less than 0.0001%, the refining cost increases significantly, so the practical lower limit is 0.0001 to 0.0013%.
- P is set to 0.02% or less. Preferably it is 0.01% or less.
- S is an impurity element that forms non-metallic inclusions such as MnS.
- Non-metallic inclusions are the starting point of cracking during cold forging, so the smaller the S, the better.
- the content of S may be 0, but if it is reduced to less than 0.0001%, the refining cost increases significantly, so the practical lower limit is 0.0001 to 0.0012%.
- S When S exceeds 0.01%, non-metallic inclusions are generated and the material formability deteriorates, so S is set to 0.01% or less. Preferably it is 0.009% or less.
- N 0.02% or less
- N is an element that embrittles ferrite when present in a large amount. Therefore, the smaller N, the better.
- the N content may be 0, but if it is reduced to less than 0.0001%, the refining cost will increase significantly, so the practical lower limit is 0.0001 to 0.0006%.
- N exceeds 0.02%, the ferrite becomes brittle and the material formability deteriorates, so N is made 0.02% or less. Preferably it is 0.017% or less.
- N is set to 0.01% or less to suppress ferrite embrittlement. Preferably it is 0.007% or less.
- O when present in a large amount, is an element that promotes the formation of coarse oxides. Therefore, the smaller the amount of O, the better. However, if it is reduced to less than 0.0001%, the refining cost will increase significantly, so it is made 0.0001% or more. Preferably it is 0.0011% or more.
- Sn 0.05% or less
- Sn is an element inevitably mixed from the steel raw material. Therefore, the smaller the Sn, the better.
- the S content may be 0, but if the content is reduced to less than 0.001%, the refining cost increases significantly, so the practical lower limit is 0.001 to 0.002%.
- Sn is made 0.05% or less.
- Sn is 0.04% or less.
- Sb 0.05% or less
- Sb is an element that is inevitably mixed from the steel raw material, segregates at the ferrite grain boundary, and reduces the number of carbides at the ferrite grain boundary. Therefore, the smaller the Sb, the better.
- the Sb content may be 0, but if the content is reduced to less than 0.001%, the refining cost increases significantly, so the practical lower limit is 0.001 to 0.002%.
- Sb segregates at the ferrite grain boundary, the number of carbides at the ferrite grain boundary decreases, and the material formability deteriorates, so Sb is made 0.050% or less. Preferably it is 0.04% or less.
- As is an element that is inevitably mixed in from the steel raw material and segregates at the ferrite grain boundaries, like Sn and Sb. Therefore, the smaller As, the better.
- the content of As may be 0, but if it is reduced to less than 0.001%, the refining cost increases significantly, so the practical lower limit is 0.001 to 0.002%.
- As is segregated at the ferrite grain boundary, the number of carbides at the ferrite grain boundary is reduced, and the material formability is lowered, so As is made 0.050% or less. Preferably, it is 0.04% or less.
- the steel sheet of the present invention contains the above elements as basic components, but may further contain the following elements for the purpose of improving the cold forgeability of the steel sheet.
- the following elements are not essential for obtaining the effects of the present invention, so the content may be zero.
- Nb is an element that is effective for controlling the morphology of carbides, and is an element that contributes to improving toughness by refining the structure.
- Nb is preferably 0.001% or more. More preferably, it is 0.002% or more.
- Nb is 0.10. % Or less.
- it is 0.09% or less.
- V is an element that is effective in controlling the morphology of carbides, and is an element that contributes to improving toughness by refining the structure.
- V is preferably 0.001% or more. More preferably, it is 0.004% or more.
- V is 0.10. % Or less.
- V is 0.09% or less.
- Cu is an element that segregates at the ferrite grain boundary, and is an element that contributes to improvement in strength by forming fine precipitates. In order to obtain the strength improvement effect, Cu is preferably 0.001% or more. More preferably, it is 0.008% or more.
- Cu is made 0.10% or less. Preferably it is 0.09% or less.
- W is an element effective for controlling the form of carbide.
- W is preferably 0.001% or more. More preferably, it is 0.003% or more.
- Ta 0.001 to 0.10%
- Nb, V, and W is an element effective for controlling the morphology of carbides.
- Ta is preferably 0.001% or more. More preferably, it is 0.007% or more.
- Ni is an element effective for improving the impact resistance of the molded product.
- Ni is preferably 0.001% or more. More preferably, it is 0.002% or more.
- Ni is made 0.10% or less.
- it is 0.09% or less.
- Mg is an element that can control the form of sulfide by addition of a small amount.
- Mg is preferably 0.0001% or more. More preferably, it is 0.0008% or more.
- Ca is an element that can control the form of sulfide with a small amount of addition.
- Ca is preferably 0.001% or more. More preferably, it is 0.003% or more.
- Ca is 0.05% or less.
- it is 0.04% or less.
- Y is 0.05% or less. To do.
- it is 0.03% or less.
- Zr 0.05% or less
- Zr is an element that can control the form of sulfide by adding a small amount.
- Zr is preferably 0.001% or more. More preferably, it is 0.004% or more.
- La is an element that can control the form of sulfide by adding a small amount, but is also an element that segregates at the ferrite grain boundary and reduces the number of carbides at the ferrite grain boundary.
- La is preferably 0.001% or more. More preferably, it is 0.003% or more.
- La is segregated at the flight grain boundary, the number of carbides at the ferrite grain boundary is reduced, and the material formability is lowered, so La is made 0.05% or less. Preferably it is 0.04% or less.
- Ce is segregated at the ferrite grain boundary, the number of carbides at the ferrite grain boundary is reduced, and the material formability is lowered, so Ce is made 050% or less. Preferably it is 0.04% or less.
- the balance of the component composition is Fe and inevitable impurities.
- the structure of the steel sheet of the present invention is substantially a structure composed of ferrite and carbide.
- carbides include compounds in which Fe atoms in cementite are substituted with alloy elements such as Mn and Cr, and alloy carbides (M 23 C 6 , M 6 C MC, etc. [M: Fe and other metal elements added as alloys]).
- a shear band is formed in the macro structure of the steel sheet, and slip deformation is concentrated near the shear band. Slip deformation is accompanied by dislocation growth, and a region having a high dislocation density is formed in the vicinity of the shear band. As the amount of strain applied to the steel sheet increases, slip deformation is promoted and the dislocation density increases.
- the formation of a shear band is understood as a phenomenon in which a slip generated in one crystal grain overcomes the grain boundary and continuously propagates to adjacent crystal grains. Therefore, in order to suppress the formation of shear bands, it is necessary to prevent the propagation of slip across the grain boundary.
- Carbides in the steel sheet are strong particles that prevent slipping, and by allowing the carbides to exist at the ferrite grain boundaries, it is possible to prevent the propagation of slips across the crystal grain boundaries and suppress the formation of shear bands. It becomes possible to improve cold forgeability. At the same time, the formability of the steel sheet is improved.
- the formability of a steel sheet is largely due to the accumulation of strain (accumulation of dislocations) in the crystal grains. If the propagation of strain to adjacent crystal grains is prevented at the grain boundaries, the amount of strain in the crystal grains is reduced. Increase. As a result, the work hardening rate is increased and the moldability is improved.
- the carbide In order to obtain such an effect, the carbide needs to be dispersed in an appropriate size in the metal structure. Therefore, the average particle diameter of the carbide is set to 0.4 ⁇ m or more and 2.0 ⁇ m or less. When the average particle size of the carbide is less than 0.4 ⁇ m, the hardness of the steel sheet is remarkably increased and the cold forgeability is lowered. More preferably, it is 0.6 ⁇ m or more.
- the average particle diameter of the carbide exceeds 2.0 ⁇ m, the carbide becomes a starting point of cracking during cold forming. More preferably, it is 1.95 ⁇ m or less.
- cementite which is a carbide of iron
- the area ratio is set to 6% or less.
- perlite Since perlite has a unique lamellar structure, it can be distinguished by SEM and optical microscope observation.
- the area ratio of pearlite can be obtained by calculating the region of the lamellar structure in an arbitrary cross section.
- the present inventors did not adopt the above observation method as a general analysis method, and searched for a simpler and more accurate evaluation index.
- the number of carbides in ferrite grains the number of carbides at the ferrite grain boundary with respect to A: the ratio of B: B / A can be used as an index, and the cold forgeability and formability can be quantitatively evaluated, and It was found that when the ratio: B / A exceeds 1, the cold forgeability and the drawability / thickening formability are remarkably improved.
- Any of buckling, folding, and folding that occurs during cold forging of a steel sheet is caused by the localization of strain associated with the formation of a shear band. Formation and strain localization are alleviated and the occurrence of buckling, folding, and folding is suppressed.
- Carbide is observed with a scanning electron microscope. Prior to observation, a sample for tissue observation was wet-polished with emery paper and polished with diamond abrasive grains having an average particle size of 1 ⁇ m, and the observation surface was finished to a mirror surface, and the tissue was then washed with a 3% nitric acid-alcohol solution. Etch. The observation magnification is 3000 times, and 8 fields of view of 30 ⁇ m ⁇ 40 ⁇ m in a 1/4 layer thickness are taken at random.
- region is measured in detail with image analysis software (Win ROOF by Mitani Corporation).
- carbides having an area of 0.01 ⁇ m 2 or less are excluded from evaluation targets.
- the ferrite grain size is preferably 3 ⁇ m or more and 50 ⁇ m or less in terms of improving cold forgeability. If the ferrite particle size is less than 3 ⁇ m, the hardness increases and cracks and cracks are likely to occur during cold forging, so the ferrite particle size is preferably 3 ⁇ m or more. More preferably, it is 5 ⁇ m or more.
- the ferrite grain size exceeds 50 ⁇ m, the number of carbides at the grain boundaries that suppress the propagation of slip is reduced and the cold forgeability is lowered. Therefore, the ferrite grain size is preferably 50 ⁇ m or less. More preferably, it is 40 ⁇ m or less.
- the ferrite grain size is the image taken by observing the structure etched with a 3% nitric acid-alcohol solution with an optical microscope or a scanning electron microscope after the observation surface of the sample surface is polished to a mirror surface by the above-described procedure. Can be measured by applying the line segment method.
- Vickers hardness When the Vickers hardness exceeds 170 HV, the ductility is reduced, buckling out of the surface is likely to occur during compression deformation such as thickening, and internal cracking is likely to occur during cold forging, and the impact resistance is Since it deteriorates, Vickers hardness shall be 170HV or less. In order to ensure the ductility and impact resistance, the Vickers hardness is preferably 150 HV or less. More preferably, it is 140HV or less.
- the manufacturing method of the present invention is based on the basic idea that the steel strip having the above-described composition is used to consistently manage the hot rolling conditions and the annealing conditions and to control the structure of the steel sheet.
- a steel slab in which molten steel having a required composition is continuously cast is subjected to hot rolling.
- the slab after continuous casting may be directly subjected to hot rolling, or may be subjected to hot rolling after being once cooled and heated.
- the heating temperature is preferably 1000 ° C. or more and 1250 ° C. or less, and the heating time is preferably 0.5 hours or more and 3 hours or less.
- the temperature of the steel slab subjected to hot rolling is preferably 1000 ° C. or more and 1250 ° C.
- the steel slab temperature or the steel slab heating temperature is preferably 1000 ° C. or more, and the heating time is preferably 0.5 hours or more. More preferably, it is 1050 ° C. or more and 1 hour or more.
- Finish rolling in hot rolling is completed at a temperature range of 820 ° C or higher, preferably 900 ° C or higher and 950 ° C or lower.
- the finish rolling temperature is set to 820 ° C. or higher. In terms of promoting recrystallization, the temperature is preferably 900 ° C. or higher.
- finish rolling temperature exceeds 950 ° C, a thick scale is generated in the run-out table (ROT) through the plate.
- ROT run-out table
- finish rolling temperature shall be 950 degrees C or less. Preferably it is 920 degrees C or less.
- the winding temperature is 400 ° C or higher and 550 ° C or lower. This is a temperature lower than a general winding temperature, and is a condition that is not normally performed particularly when the C content is high.
- the structure of the steel sheet can be a bainite structure in which carbides are dispersed in fine ferrite.
- the winding temperature is 400 ° C. or higher. Preferably it is 430 degreeC or more.
- the coiling temperature is 550 ° C. or less. Preferably it is 520 degrees C or less.
- the steel plate after pickling is cold-rolled before the annealing treatment, the ferrite grains become finer, so that the steel plate becomes difficult to soften. Therefore, in the present invention, it is not preferable to perform cold rolling before annealing, and it is preferable to perform annealing treatment without pickling after pickling.
- the first stage annealing is performed in a temperature range of 650 to 720 ° C., preferably the A c1 point or less.
- the carbide is coarsened and partially spheroidized, and the alloy elements are concentrated in the carbide, thereby improving the thermal stability of the carbide.
- the first stage heating rate exceeds 150 ° C./hour, the temperature difference between the outer peripheral portion and the inside of the hot-rolled steel sheet coil increases, and slag and seizure due to the difference in thermal expansion occurs. Unevenness is formed on the surface.
- cracks are generated as a starting point, and cold forgeability is deteriorated, and impact resistance after carburizing and quenching and tempering is reduced. It shall be below °C / hour. Preferably it is 130 degrees C / hour or less.
- the annealing temperature in the first stage annealing (hereinafter referred to as “first stage annealing temperature”) is 650 ° C. or more and 720 ° C. or less. If the first stage annealing temperature is less than 650 ° C., the carbide is not sufficiently stabilized, and it becomes difficult to leave the carbide in the austenite during the second stage annealing. For this reason, the first stage annealing temperature is set to 650 ° C. or higher. Preferably it is 670 degreeC or more.
- the first-stage annealing temperature is set to 720 ° C. or less. . Preferably it is 700 degrees C or less.
- the annealing time in the first stage annealing (hereinafter referred to as “first stage annealing time”) is 3 hours or more and 60 hours or less. If the first stage annealing time is less than 3 hours, the carbide is not sufficiently stabilized, and it becomes difficult to leave the carbide in the austenite during the second stage annealing. For this reason, the first stage annealing time is set to 3 hours or more. Preferably it is 5 hours or more.
- the first stage annealing time is set to 60 hours or less. Preferably it is 55 hours or less.
- the temperature is raised to 725 to 790 ° C., preferably in the temperature range from A c1 to A 3 , and austenite is generated in the structure.
- the carbides in the fine ferrite grains are dissolved in the austenite, but the carbides coarsened by the first stage annealing remain in the austenite.
- the ferrite grain size When cooled without performing the second stage annealing, the ferrite grain size does not increase and an ideal structure cannot be obtained.
- the heating rate of the second stage annealing to the annealing temperature (hereinafter referred to as “second stage heating rate”) is 1 ° C./hour or more and 80 ° C./hour or less.
- austenite is generated and grows from the ferrite grain boundary.
- by slowing the heating rate up to the annealing temperature it becomes possible to suppress austenite nucleation and increase the grain boundary coverage of the carbide in the structure formed by annealing after annealing.
- the second stage heating rate exceeds 80 ° C./hour, in the hot-rolled steel sheet coil, the temperature difference between the outer peripheral portion and the inside increases, and scouring and seizure due to a large difference in thermal expansion due to transformation occurs. Unevenness is formed on the surface of the steel plate. At the time of cold forging, cracks are generated starting from this unevenness, cold forgeability and formability are reduced, and impact resistance after carburizing and quenching and tempering is also reduced, so the second stage heating rate is 80 ° C / Less than hours. Preferably it is 70 degrees C / hour or less.
- the annealing temperature in the second stage annealing (hereinafter referred to as “second stage annealing temperature”) is 725 ° C. or higher and 790 ° C. or lower.
- second stage annealing temperature is set to 725 ° C. or higher. Preferably it is 735 ° C or more.
- the second stage annealing temperature is set to 790 ° C. or less. Preferably it is 770 degrees C or less.
- the annealing time in the second stage annealing is 3 hours or more and less than 10 hours. If the second stage annealing time is less than 3 hours, the amount of austenite produced is small, and the dissolution of carbides in the ferrite grains does not proceed sufficiently, making it difficult to increase the number of carbides at the ferrite grain boundaries, In addition, the ferrite grain size is reduced. For this reason, the second stage annealing time is set to 3 hours or more. Preferably it is 5 hours or more.
- the second stage annealing time exceeds 10 hours, it becomes difficult to leave the carbide in the austenite and the manufacturing cost increases, so the second stage annealing time is set to less than 10 hours. Preferably it is 8 hours or less.
- the steel sheet is cooled to 650 ° C. at a cooling rate of 1 ° C./hour or more and 100 ° C./hour or less.
- the cooling rate is low, but if it is less than 1 ° C./hour, the time required for cooling increases and the productivity decreases, so the cooling rate is 1 ° C./hour or more. Preferably, it is 10 ° C./hour or more.
- the cooling rate exceeds 100 ° C./hour, austenite is transformed into pearlite, the hardness of the steel sheet is increased, cold forgeability is lowered, and impact resistance after carburizing, quenching and tempering is lowered. Therefore, the cooling rate is set to 100 ° C./hour or less. Preferably it is 80 degrees C / hour or less.
- the steel sheet cooled to 650 ° C. is cooled to room temperature.
- the cooling rate at this time is not limited.
- the atmosphere in the two-stage annealing is not particularly limited to a specific atmosphere.
- any atmosphere of 95% or more nitrogen atmosphere, 95% or more hydrogen atmosphere, or air atmosphere may be used.
- the manufacturing method that consistently manages the hot rolling conditions and annealing conditions of the present invention and performs the structure control of the steel sheet, the formability during cold forging combined with drawing and thickening is achieved. Further, it is possible to produce a steel sheet that is excellent and further has excellent hardenability necessary for improving impact resistance after carburizing, quenching, and tempering.
- the conditions in the examples are examples of conditions adopted for confirming the feasibility and effects of the present invention. It is not limited.
- the present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
- Example 1 Continuous cast slabs (steel slabs) having the composition shown in Table 1 and Table 2 (continuation of Table 1) were hot-rolled after heating for 1.8 hours at 1240 ° C, and after hot rolling at 920 ° C, on the ROT was cooled to 530 ° C. at a cooling rate of 45 ° C./second and wound at 520 ° C. to produce a hot-rolled steel sheet coil having a thickness of 5.2 mm.
- Discharge the hot-rolled steel sheet coil pickle it, insert it into a box-type annealing furnace, control the annealing atmosphere to 95% hydrogen-5% nitrogen, and then heat from room temperature to 705 ° C at 100 ° C / hour. And kept at 710 ° C. for 24 hours to make the temperature distribution in the hot-rolled steel sheet coil uniform.
- Table 3 shows the ferrite particle size ( ⁇ m), average carbide particle size ( ⁇ m), pearlite area ratio (%), Vickers hardness (HV), number of grain boundary carbides / intragranularity of the steel sheets shown in Table 1 and Table 2.
- the critical cooling rate was determined by creating a CCT diagram. If the hot-rolled steel sheet is cooled at a cooling rate slower than the determined critical cooling rate, the hardenability at the time of quenching after being formed into a part is deteriorated, a pearlite structure is formed, and sufficient strength cannot be obtained. Therefore, a small critical cooling rate is necessary to obtain a high quenching strength. If the critical cooling rate is 280 ° C./second, it can be determined that the hardenability is improved.
- the average carbide particle size is 0.4 to 2.0 ⁇ m
- the pearlite area ratio is 6% or less
- the number of grain boundary carbides / intragranular carbides is more than 1
- I1 / I0 is less than 1. Therefore, the Vickers hardness is in the range of 100 HV to 170 HV, and
- the Vickers hardness exceeds 150
- the number of field carbides / intragranular carbides is less than 1.
- the critical cooling rate exceeds 280 ° C./second, and the hardenability is lowered.
- Example 2 No. of invention steel plate. 1-5, no. 16-19, no. 31, no. 33 and no.
- the production method under conditions outside the condition range defined in the present invention was applied to 35 steel types.
- Table 4 shows the production conditions
- Table 5 shows the ferrite grain size ( ⁇ m), Vickers hardness (HV), number of grain boundary carbides / number of intragranular carbides of steel sheets produced under the production conditions shown in Table 4.
- X-ray intensity ratio I1 / I0, r-value anisotropy index
- the present invention it is possible to provide a steel plate excellent in hardenability and material formability and a method for manufacturing the steel plate.
- the steel sheet of the present invention is suitable for obtaining parts such as gears by forming by cold forging such as thickening. Therefore, this invention has a high applicability in steel plate manufacture and utilization industry.
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Abstract
Description
Cは、炭化物を形成し、鋼の強化及びフェライト粒の微細化に有効な元素である。冷間成形時、梨地の発生を抑制し、冷間成形品の表面美観を確保するためには、フェライト粒径の粗大化を抑制する必要がある。
Siは、脱酸剤として作用する他、炭化物の形態に影響を及ぼし、素材成形性の向上に寄与する元素である。脱酸効果を得るために、Siは0.01%以上とする。好ましくは、0.07%以上である。
Mnは、2段焼鈍において、炭化物の形態を制御する元素である。0.30%未満では、2段目焼鈍後の徐冷において、フェライト粒界に、炭化物を生成させることが困難となるので、Mnは0.30%以上とする。好ましくは0.40%以上である。
Alは、脱酸剤として作用するとともに、フェライトを安定化する元素である。0.001%未満では、添加効果が十分に得られないので、Alは0.001%以上とする。好ましくは0.004%以上である。
Crは、熱処理時の炭化物の安定化に有効な元素である。0.010%未満では、浸炭時に炭化物を残存させることが困難となり、表層におけるオーステナイト粒径が粗大化し、強度が低下するので、Crは0.010%以上とする。好ましくは0.050%以上である。
Moは、Mn、Crと同様に、炭化物の形態制御に有効な元素である。0.001%未満では、添加効果が得られないので、Moは0.001%以上とする。好ましくは0.005%以上である。
Bは、焼入れ性を高め、さらに靭性を高める元素である。本発明鋼板においては、所要の焼入れ性が必要であるので、0.0004~0.01%添加する。0.0004%未満では、添加効果が得られないので、Bは0.0004%以上とする。好ましくは0.0010%以上である。
Tiは、窒化物を形成し、結晶粒の微細化に寄与するとともに、Bの添加効果を有効に発揮させる作用をなす元素である。0.001%未満では、添加効果が得られないので、Tiは0.001%以上とする。好ましくは0.010%以上である。
Pは、フェライト粒界に偏析し、フェライト粒界における炭化物の生成を抑制する作用をなす元素である。それ故、Pは、少ないほど好ましい。Pの含有量は0でもよいが、0.0001%未満に低減すると、精錬コストが大幅に増加するので、実質的な下限は0.0001~0.0013%である。
Sは、MnSなどの非金属介在物を形成する不純物元素である。非金属介在物は、冷間鍛造時に割れの起点となるので、Sは、少ないほど好ましい。Sの含有量は0でもよいが、0.0001%未満に低減すると、精錬コストが大幅に増加するので、実質的な下限は0.0001~0.0012%である。
Nは、多量に存在すると、フェライトを脆化させる元素である。それ故、Nは、少ないほど好ましい。Nの含有量は0でもよいが、0.0001%未満に低減すると、精錬コストが大幅に増加するので、実質的な下限は0.0001~0.0006%である。
Oは、多量に存在すると、粗大な酸化物の形成を促す元素である。それ故、Oは、少ないほど好ましいが、0.0001%未満に低減すると、精錬コストが大幅に増加するので、0.0001%以上とする。好ましくは0.0011%以上である。
Snは、鋼原料から不可避的に混入する元素である。それ故、Snは、少ないほど好ましい。Sの含有量は0でもよいが、0.001%未満に低減すると、精錬コストが大幅に増加するので、実質的な下限は0.001~0.002%である。
Sbは、Snと同様に、鋼原料から不可避的に混入して、フェライト粒界に偏析し、フェライト粒界の炭化物の個数を低減する元素である。それ故、Sbは、少ないほど好ましい。Sbの含有量は0でもよいが、0.001%未満に低減すると、精錬コストが大幅に増加するので、実質的な下限は0.001~0.002%である。
Asは、Sn、Sbと同様に、鋼原料から不可避的に混入し、フェライト粒界に偏析する元素である。それ故、Asは、少ないほど好ましい。Asの含有量は0でもよいが、0.001%未満に低減すると、精錬コストが大幅に増加するので、実質的な下限は0.001~0.002%である。
Nbは、炭化物の形態制御に有効な元素であり、また、組織を微細化して靭性の向上に寄与する元素である。添加効果を得るためには、Nbは0.001%以上とするのが好ましい。より好ましくは0.002%以上である。
Vも、Nbと同様に、炭化物の形態制御に有効な元素であり、また、組織を微細化して靭性の向上に寄与する元素である。添加効果を得るためには、Vは0.001%以上とするのが好ましい。より好ましくは0.004%以上である。
Cuは、フェライト粒界に偏析する元素であり、また、微細な析出物を形成して強度の向上に寄与する元素である。強度向上効果を得るためには、Cuは0.001%以上とするのが好ましい。より好ましくは0.008%以上である。
Wも、Nb、Vと同様に、炭化物の形態制御に有効な元素である。添加効果を得るためには、Wは0.001%以上とするのが好ましい。より好ましくは0.003%以上である。
Taも、Nb、V、Wと同様に、炭化物の形態制御に有効な元素である。添加効果を得るためには、Taは0.001%以上とするのが好ましい。より好ましくは0.007%以上である。
Niは、成形品の耐衝撃特性の向上に有効な元素である。添加効果を得るためには、Niは0.001%以上とするのが好ましい。より好ましくは0.002%以上である。
Mgは、微量の添加で硫化物の形態を制御できる元素である。添加効果を得るためには、Mgは0.0001%以上とするのが好ましい。より好ましくは0.0008%以上である。
Caは、Mgと同様に、微量の添加で硫化物の形態を制御できる元素である。添加効果を得るためには、Caは0.001%以上とするのが好ましい。より好ましくは0.003%以上である。
Yは、Mg、Caと同様に、微量の添加で硫化物の形態を制御できる元素である。添加効果を得るためには、Yは0.001%以上とするのが好ましい。より好ましくは0.003%以上である。
Zrは、Mg、Ca、Yと同様に、微量の添加で硫化物の形態を制御できる元素である。添加効果を得るためには、Zrは0.001%以上とするのが好ましい。より好ましくは0.004%以上である。
Laは、微量の添加で硫化物の形態を制御できる元素であるが、フェライト粒界に偏析し、フェライト粒界の炭化物の個数を低減する元素でもある。硫化物の形態制御効果を得るためには、Laは0.001%以上とするのが好ましい。より好ましくは0.003%以上である。
Ceは、Laと同様に、微量の添加で硫化物の形態を制御できる元素であるが、フェライト粒界に偏析し、フェライト粒界の炭化物の個数を低減する元素でもある。硫化物の形態制御効果を得るためには、Ceは0.001%以上とするのが好ましい。より好ましくは0.003%以上である。
そのためには、冷却速度は遅い方が好ましいが、1℃/時間未満であると、冷却に要する時間が増大し、生産性が低下するので、冷却速度は1℃/時間以上とする。好ましくは10℃/時間以上である。
表1及び表2(表1の続き)に示す成分組成の連続鋳造鋳片(鋼片)を、1240℃で1.8時間加熱後に熱間圧延し、920℃で仕上げ熱延後、ROT上で45℃/秒の冷却速度で530℃まで冷却し、520℃で巻き取り、板厚5.2mmの熱延鋼板コイルを製造した。
発明鋼板のNo.1~5、No.16~19、No.31、No.33、及び、No.35の12の鋼種に対し、本発明で規定する条件範囲外の条件の製造方法を適用した。表4に、その製造条件を示し、表5に、表4で示す製造条件で製造した鋼板の、フェライト粒径(μm)、ビッカース硬さ(HV)、粒界炭化物数/粒内炭化物数、X線強度比:I1/I0、r値の異方性指数|Δr|、及び、臨界冷却速度(℃/秒)を示す。
Claims (2)
- 質量%で、
C :0.10~0.70%、
Si:0.01~0.30%、
Mn:0.30~3.00%、
Al:0.001~0.10%、
Cr:0.010~0.50%、
Mo:0.0010~0.50%、
B :0.0004~0.01%、
Ti:0.001~0.10%、
P :0.02%以下、
S :0.01%以下、
N :0.0200%以下、
O :0.0200%以下、
Sn:0.05%以下、
Sb:0.05%以下、
As:0.05%以下、
Nb:0.10%以下、
V :0.10%以下、
Cu:0.10%以下、
W :0.10%以下、
Ta:0.10%以下、
Ni:0.10%以下、
Mg:0.05%以下、
Ca:0.05%以下、
Y :0.05%以下、
Zr:0.05%以下、
La:0.05%以下、及び
Ce:0.05%以下
を含有し、残部がFe及び不可避不純物である鋼板であって、
上記鋼板の金属組織が、
炭化物の平均粒径が0.4μm以上、2.0μm以下、
パーライトの面積率が6%以下、
フェライト粒内の炭化物の個数に対するフェライト粒界の炭化物の個数の比率が1超、及び
上記鋼板の1/2板厚部分の板面における{211}<011>のX線回折強度をI1、{100}<011>のX線回折強度をI0としたときI1/I0<1
を満たし、
上記鋼板のビッカース硬さが100HV以上150HV以下である
ことを特徴とする鋼板。 - 請求項1に記載の鋼板を製造する製造方法であって、
請求項1に記載の成分組成の鋼片を、820℃以上950℃以下の温度域で仕上げ圧延を完了する熱間圧延を施し熱延鋼板とし、
上記熱延鋼板を400℃以上550℃以下で巻き取り、
巻き取った熱延鋼板に酸洗を施し、
酸洗した熱延鋼板を30℃/時間以上150℃/時間以下の加熱速度で、650℃以上720℃以下の焼鈍温度に加熱して、3時間以上60時間以下保持する1段目の焼鈍を施し、次いで、
熱延鋼板を1℃/時間以上80℃/時間以下の加熱速度で、725℃以上790℃以下の焼鈍温度に加熱して、3時間以上10時間未満保持する2段目の焼鈍を施し、
焼鈍後の熱延鋼板を、1℃/時間以上100℃/時間以下の冷却速度で650℃まで冷却する
ことを特徴とする鋼板の製造方法。
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