WO2022180954A1 - 鋼板およびその製造方法 - Google Patents
鋼板およびその製造方法 Download PDFInfo
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- WO2022180954A1 WO2022180954A1 PCT/JP2021/042399 JP2021042399W WO2022180954A1 WO 2022180954 A1 WO2022180954 A1 WO 2022180954A1 JP 2021042399 W JP2021042399 W JP 2021042399W WO 2022180954 A1 WO2022180954 A1 WO 2022180954A1
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 96
- 239000010959 steel Substances 0.000 title claims abstract description 96
- 238000000034 method Methods 0.000 title claims description 28
- 238000004519 manufacturing process Methods 0.000 title claims description 17
- 229910045601 alloy Inorganic materials 0.000 claims abstract description 67
- 239000000956 alloy Substances 0.000 claims abstract description 67
- 150000001247 metal acetylides Chemical class 0.000 claims abstract description 62
- 239000013078 crystal Substances 0.000 claims abstract description 27
- 239000000126 substance Substances 0.000 claims abstract description 16
- 239000000203 mixture Substances 0.000 claims abstract description 14
- 238000005096 rolling process Methods 0.000 claims description 81
- 229910000734 martensite Inorganic materials 0.000 claims description 62
- 238000001816 cooling Methods 0.000 claims description 37
- 229910000859 α-Fe Inorganic materials 0.000 claims description 22
- 229910001563 bainite Inorganic materials 0.000 claims description 16
- 238000004804 winding Methods 0.000 claims description 15
- 229910001566 austenite Inorganic materials 0.000 claims description 12
- 238000010438 heat treatment Methods 0.000 claims description 12
- 229910001562 pearlite Inorganic materials 0.000 claims description 12
- 238000003303 reheating Methods 0.000 claims description 10
- 239000012535 impurity Substances 0.000 claims description 7
- 229910052758 niobium Inorganic materials 0.000 claims description 7
- 229910052719 titanium Inorganic materials 0.000 claims description 7
- 229910052750 molybdenum Inorganic materials 0.000 claims description 6
- 229910052720 vanadium Inorganic materials 0.000 claims description 6
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052751 metal Inorganic materials 0.000 claims description 4
- 239000002184 metal Substances 0.000 claims description 4
- 230000006866 deterioration Effects 0.000 description 18
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- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 7
- 239000002245 particle Substances 0.000 description 7
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- 238000005098 hot rolling Methods 0.000 description 3
- 239000002344 surface layer Substances 0.000 description 3
- 238000013022 venting Methods 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 229910052782 aluminium Inorganic materials 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000010894 electron beam technology Methods 0.000 description 2
- 239000000446 fuel Substances 0.000 description 2
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 2
- 230000014759 maintenance of location Effects 0.000 description 2
- 239000000463 material Substances 0.000 description 2
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- 239000010451 perlite Substances 0.000 description 2
- 235000019362 perlite Nutrition 0.000 description 2
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- 239000000243 solution Substances 0.000 description 2
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- 229910052717 sulfur Inorganic materials 0.000 description 2
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- 229910000952 Be alloy Inorganic materials 0.000 description 1
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- 229910001335 Galvanized steel Inorganic materials 0.000 description 1
- 229910000861 Mg alloy Inorganic materials 0.000 description 1
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
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- 229910007567 Zn-Ni Inorganic materials 0.000 description 1
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- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
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- 239000001307 helium Substances 0.000 description 1
- 229910052734 helium Inorganic materials 0.000 description 1
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 1
- 239000007970 homogeneous dispersion Substances 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 238000000691 measurement method Methods 0.000 description 1
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- 238000011084 recovery Methods 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 description 1
- 229910010271 silicon carbide Inorganic materials 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
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- 239000000725 suspension Substances 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
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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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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
- B21C47/02—Winding-up or coiling
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- 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
- 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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- 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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- 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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/02—Hardening by precipitation
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- 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
- 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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- C21—METALLURGY OF IRON
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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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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- 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
- C21D8/0273—Final recrystallisation annealing
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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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- 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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C—ALLOYS
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
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- 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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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
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- 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
- 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
- C21D2211/001—Austenite
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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
- C21D2211/002—Bainite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- 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
- C21D2211/008—Martensite
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- 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
Definitions
- the present invention relates to a steel sheet and a method for manufacturing the same.
- This application claims priority based on Japanese Patent Application No. 2021-030350 filed in Japan on February 26, 2021, the contents of which are incorporated herein.
- Automobile suspension parts are manufactured by subjecting steel plates to burring, stretch flanging, and bending. Therefore, the steel sheets applied to these automobile chassis parts are required to have not only high strength but also excellent formability, particularly hole expandability. In addition, it is required that deterioration in bendability after processing is small.
- a ferrite phase having an area ratio of 95% or more is the main phase, and the ferrite phase is the ratio dN / dL is 0.5 or more, the average particle size defined by (2 ⁇ dL ⁇ dN)/(dL+dN) is 5 ⁇ m or less, and the precipitation density of precipitates of less than 10 nm is 1.0 ⁇ 10 5 / Disclosed is a high-strength steel sheet having a microstructure of 3 ⁇ m or more and having excellent delayed fracture resistance on sheared surfaces.
- the area ratio is 10% or more and 90% or less of ferrite, and 10% or more of tempered martensite and tempered bainite, and the total of the ferrite, the tempered martensite and the tempered bainite is 90%.
- An alloyed hot-dip galvanized steel sheet having a homogeneous dispersion ratio S of 0.75 or more and 1.30 or less is disclosed.
- Patent Documents 1 and 2 do not consider the deterioration of bendability after working.
- the present inventors have found that the techniques described in Patent Documents 1 and 2 need to be further enhanced in strength and hole expansibility.
- the present inventors have found that by strictly controlling the chemical composition and controlling the number density of alloy carbides existing in the grain boundaries and grains, high It has been found that it has strength and excellent hole expansibility, and that the deterioration of bendability after processing can be reduced.
- the inventors have found that the steel sheet can be produced by strictly controlling the conditions in the rough rolling process and the reheating process.
- the gist of the present invention made based on the above knowledge is as follows.
- the steel sheet according to one aspect of the present invention has a chemical composition in mass% of C: 0.030 to 0.180%, Si: 0.030 to 1.400%, Mn: 1.60-3.00%, Al: 0.010 to 0.700%, P: 0.0800% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020 to 0.180%, Nb: 0.010 to 0.050%, Mo: 0-0.600%, V: 0 to 0.300%, Total of Ti, Nb, Mo and V: 0.100-1.130%, B: 0 to 0.0030% and Cr: 0 to 0.500% and the balance consists of Fe and impurities,
- the ratio of the area ratio of the tempered martensite to the total area ratio of the fresh martensite and the tempered martensite is 80.0% or more. good too.
- a method for manufacturing a steel plate according to another aspect of the present invention is the method for manufacturing a steel plate according to (1) above, A rough rolling step of heating a slab having the chemical composition described in (1) above and performing rough rolling in four passes or more in a temperature range of 1000 to 1300 ° C.; After the rough rolling, a finish rolling step in which finish rolling is performed so that the final rolling reduction is 24 to 60% and the finish rolling temperature is in the temperature range of 960 to 1060 ° C.; A cooling step of cooling after the finish rolling so that the average cooling rate in the temperature range of 900 to 650 ° C.
- a winding step of winding in a temperature range of 400 to 580 ° C. After the winding, it is heated to a temperature range of 600 to 750 ° C. at an average heating rate of 0.2 to 5.0 ° C./sec, held in the temperature range of 600 to 750 ° C. for 60 to 3010 seconds, and then a reheating step of cooling so that the average cooling rate in the temperature range of 700 ° C.
- the temperature difference between the final pass and the pass one pass before the final pass is 50°C or less
- the reduction rate of the first to third passes is set to 10 to 30%
- the reduction ratio after the fourth pass is set to 15 to 50%.
- the manufacturing method of the steel plate which can manufacture the said steel plate can be provided.
- the steel plate according to the present embodiment has C: 0.030 to 0.180%, Si: 0.030 to 1.400%, Mn: 1.60 to 3.00%, Al: 0.010 to 0.700. %, P: 0.0800% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020 to 0.180%, Nb: 0.010 to 0.050%, and the balance : Contains Fe and impurities. Each element will be described in detail below.
- C 0.030-0.180% C is an element necessary for obtaining the desired tensile strength of the steel sheet. Desired tensile strength cannot be obtained as C content is less than 0.030%. Therefore, the C content is made 0.030% or more.
- the C content is preferably 0.060% or more, more preferably 0.080% or more, still more preferably 0.085% or more, 0.090% or more, 0.095% or more, or 0.095% or more. 100% or more.
- the C content exceeds 0.180%, the sum of the area ratios of fresh martensite and tempered martensite becomes excessive, and the hole expansibility of the steel sheet deteriorates. Therefore, the C content is made 0.180% or less.
- the C content is preferably 0.170% or less, more preferably 0.150% or less.
- Si 0.030-1.400%
- Si is an element that improves the tensile strength of steel sheets by solid-solution strengthening. If the Si content is less than 0.030%, desired tensile strength cannot be obtained. Therefore, the Si content is set to 0.030% or more. The Si content is preferably 0.040% or more, more preferably 0.050% or more. On the other hand, if the Si content exceeds 1.400%, the area ratio of retained austenite increases, and the hole expansibility of the steel sheet deteriorates. Therefore, the Si content is set to 1.400% or less. The Si content is preferably 1.100% or less, more preferably 1.000% or less.
- Mn 1.60-3.00%
- Mn is an element necessary for improving the strength of the steel sheet. If the Mn content is less than 1.60%, the area ratio of ferrite becomes too high and the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.60% or more.
- the Mn content is preferably 1.80% or more, more preferably 2.00% or more.
- the Mn content is set to 3.00% or less.
- the Mn content is preferably 2.70% or less, more preferably 2.50% or less.
- Al 0.010-0.700%
- Al is an element that acts as a deoxidizing agent and improves the cleanliness of steel. If the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions deteriorate the hole expansibility of the steel sheet. Therefore, the Al content is set to 0.010% or more.
- the Al content is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, if the Al content exceeds 0.700%, casting becomes difficult. Therefore, the Al content is set to 0.700% or less.
- the Al content is preferably 0.600% or less, more preferably 0.100% or less.
- P 0.0800% or less
- P is an element that segregates in the thickness center of the steel sheet.
- P is also an element that embrittles the weld zone. If the P content exceeds 0.0800%, the hole expandability of the steel sheet deteriorates. Therefore, the P content should be 0.0800% or less.
- the P content is preferably 0.0200% or less, more preferably 0.0100% or less. The lower the P content is, the more preferable it is, and 0% is preferable. Therefore, the P content may be 0.0005% or more.
- S 0.0100% or less
- S is an element that embrittles the slab by existing as a sulfide.
- S is also an element that deteriorates the workability of the steel sheet. If the S content exceeds 0.0100%, the hole expansibility of the steel sheet deteriorates. Therefore, the S content should be 0.0100% or less.
- the S content is preferably 0.0080% or less, more preferably 0.0050% or less. The lower the S content, the better, preferably 0%. Therefore, the S content may be 0.0005% or more.
- N 0.0050% or less
- N is an element that forms coarse nitrides in the steel and deteriorates the workability of the steel sheet. If the N content exceeds 0.0050%, the hole expansibility of the steel sheet deteriorates. Therefore, the N content is set to 0.0050% or less.
- the N content is preferably 0.0040% or less, more preferably 0.0035% or less. The lower the N content is, the more preferable it is, preferably 0%. Therefore, the N content may be 0.0005% or more.
- Ti 0.020-0.180%
- Ti is an element that increases the strength of a steel sheet by forming fine nitrides in the steel. Desired tensile strength cannot be obtained as Ti content is less than 0.020%. Therefore, the Ti content is set to 0.020% or more.
- the Ti content is preferably 0.050% or more, more preferably 0.080% or more.
- the Ti content should be 0.180% or less.
- the Ti content is preferably 0.160% or less, more preferably 0.150% or less.
- Nb 0.010-0.050%
- Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling.
- Nb is also an element that increases the strength of the steel sheet by forming fine alloy carbides. If the Nb content is less than 0.010%, desired tensile strength cannot be obtained. Therefore, the Nb content is made 0.010% or more.
- the Nb content is preferably 0.013% or more, more preferably 0.015% or more.
- the Nb content is set to 0.050% or less.
- the Nb content is preferably 0.040% or less, more preferably 0.035% or less.
- Total of Ti, Nb, Mo and V 0.100-1.130%
- the total content of Ti and Nb described above and Mo and V described later is controlled. If the total content of these elements is less than 0.100%, the effect of forming fine alloy carbides to increase the strength of the steel sheet cannot be sufficiently obtained, and the desired tensile strength cannot be obtained. Therefore, the total content of these elements is made 0.100% or more. It should be noted that it is not necessary to contain all of Ti, Nb, Mo and V, and the above effect can be obtained as long as the content of any one of them is 0.100% or more.
- the total content of these elements is preferably 0.150% or more, more preferably 0.200% or more, and still more preferably 0.230% or more.
- the total content of these elements should be 1.130% or less.
- the total content of these elements is preferably 1.000% or less, more preferably 0.500% or less.
- the remainder of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities.
- impurities refers to ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment or the like, or impurities that are allowed within a range that does not adversely affect the steel sheet according to the present embodiment.
- the steel sheet according to the present embodiment may contain the following arbitrary elements instead of part of Fe.
- the lower limit of the content is 0% when the optional element is not included. Each arbitrary element will be described below.
- Mo 0.001-0.600%
- Mo is an element that increases the strength of the steel sheet by forming fine alloy carbides in the steel.
- the Mo content is preferably 0.001% or more.
- Mo content shall be 0.600% or less.
- V 0.010-0.300%
- V is an element that increases the strength of the steel sheet by forming fine alloy carbides in the steel.
- the V content is preferably 0.010% or more.
- the V content is set to 0.300% or less.
- B 0.0001 to 0.0030%
- B is an element that suppresses the formation of ferrite in the cooling process and increases the strength of the steel sheet.
- the B content is preferably 0.0001% or more.
- the B content is set to 0.0030% or less.
- Cr 0.001-0.500%
- the Cr content is preferably 0.001% or more in order to reliably obtain the effect of increasing the strength of the steel sheet due to the Cr content. On the other hand, even if the Cr content exceeds 0.500%, the above effect is saturated. Therefore, the Cr content is set to 0.500% or less.
- the chemical composition of the steel sheet described above can be analyzed using a spark discharge emission spectrometer or the like.
- C and S values identified by burning in an oxygen stream and measuring by an infrared absorption method using a gas component analyzer or the like are adopted.
- N a value identified by melting a test piece taken from a steel plate in a helium stream and measuring it by a thermal conductivity method is adopted.
- the steel sheet according to the present embodiment has a metal structure in terms of area %, bainite: 80.0% or more, total of fresh martensite and tempered martensite: 20.0% or less, total of pearlite, ferrite and austenite: 20 0% or less, and the number density of alloy carbides having a major axis of 10 to 100 nm existing at the grain boundary is 1.0 ⁇ 10 8 to 1.0 ⁇ 10 10 pieces/cm 2 , and The number density of existing alloy carbides having a major axis of 10 nm or less is 1.0 ⁇ 10 16 to 1.0 ⁇ 10 19 pieces/cm 3 .
- the metallographic structure is defined at the position of 1/4 of the plate thickness from the surface (area from 1/8 plate thickness depth from the surface to 3/8 plate thickness depth from the surface). The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.
- Bainite 80.0% or more Bainite is a structure that has a predetermined strength and is excellent in hole expandability. If the area ratio of bainite is less than 80.0%, desired tensile strength and/or hole expansibility cannot be obtained. Therefore, the area ratio of bainite is set to 80.0% or more.
- the area ratio of bainite is preferably 81.0% or more, more preferably 82.0% or more, and still more preferably 83.0% or more. Although the upper limit of the area ratio of bainite is not particularly limited, it may be 100.0% or less, 95.0% or less, or 90.0% or less.
- Total of fresh martensite and tempered martensite 20.0% or less
- Fresh martensite and tempered martensite have the effect of increasing the strength of the steel sheet. Poor expansibility. If the total area ratio of fresh martensite and tempered martensite exceeds 20.0%, the hole expansibility of the steel sheet deteriorates. Therefore, the total area ratio of fresh martensite and tempered martensite is set to 20.0% or less.
- the total area ratio of fresh martensite and tempered martensite is preferably 15.0% or less, more preferably 10.0% or less, and even more preferably 5.0% or less.
- the lower limit of the total area ratio of fresh martensite and tempered martensite is not particularly limited, it may be 0.0% or more, 0.5% or more, or 1.0% or more.
- Percentage of area ratio of tempered martensite 80.0% or more of the total area ratio of fresh martensite and tempered martensite Among the total area ratio of fresh martensite and tempered martensite, tempered martensite By increasing the area ratio of , the hole expansibility of the steel sheet can be further improved. Therefore, the ratio of the area ratio of tempered martensite to the total area ratio of fresh martensite and tempered martensite may be 80.0% or more. Among the sum of the area ratios of fresh martensite and tempered martensite, the ratio of the area ratio of tempered martensite is preferably as high as possible, more preferably 90.0% or more, and may be 100.0%. The area ratio of tempered martensite can be obtained by ⁇ area ratio of tempered martensite/(sum of area ratios of fresh martensite and tempered martensite) ⁇ 100.
- Total of pearlite, ferrite and austenite 20.0% or less
- Ferrite and austenite are structures that deteriorate the strength of the steel sheet.
- Pearlite is a structure that degrades the expandability of the steel sheet. If the total area ratio of these structures exceeds 20.0%, desired tensile strength and/or hole expansibility cannot be obtained. Therefore, the total area ratio of these structures is set to 20.0% or less.
- the total area ratio of these structures is preferably 17.0% or less, more preferably 15.0% or less.
- the lower limit of the total area ratio of pearlite, ferrite and austenite is not particularly limited, it may be 0.0% or more, 5.0% or more, or 10.0% or more.
- an EBSD apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the EBSD apparatus is 9.6 ⁇ 10 ⁇ 5 Pa or less
- the acceleration voltage is 15 kV
- the irradiation current level is 13
- the electron beam irradiation level is 62.
- the area ratio of austenite is calculated using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Thereby, the area ratio of austenite is obtained.
- Austenite is determined to have a crystal structure of fcc.
- the maximum value of the "Grain Average IQ" of the ferrite region was taken as I ⁇ under the condition that the 5° grain boundary in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°) is regarded as the grain boundary.
- a region exceeding I ⁇ /2 is extracted as bainite, and a region having I ⁇ /2 or less is extracted as “pearlite, fresh martensite and tempered martensite”.
- the area ratio of the extracted bainite is obtained.
- the perlite, fresh martensite and tempered martensite are distinguished by the following method.
- a method such as buffing using alumina particles with a particle size of 0.1 ⁇ m or less, or Ar ion sputtering may be used.
- the number density of alloy carbides having a major axis of 10 to 100 nm existing at grain boundaries is less than 1.0 ⁇ 10 8 pieces/cm 2 , it is not possible to reduce deterioration in bendability after working. Therefore, the number density of the alloy carbides is set to 1.0 ⁇ 10 8 pieces/cm 2 or more. It is preferably 2.0 ⁇ 10 8 /cm 2 or more, 5.0 ⁇ 10 8 /cm 2 or more, or 1.0 ⁇ 10 9 /cm 2 or more. If the number density of the alloy carbides exceeds 1.0 ⁇ 10 11 pieces/cm 2 , the strength of the steel sheet decreases. Therefore, the number density of the alloy carbides is set to 1.0 ⁇ 10 11 pieces/cm 2 or less. It is preferably 5.0 ⁇ 10 10 pieces/cm 2 or less and 1.0 ⁇ 10 10 pieces/cm 2 or less.
- alloy carbide refers to carbide containing one or more of Ti, Nb, Mo and V.
- grain boundary means a boundary having a crystal orientation difference of 1.0° or more in the analysis using EBSD described later.
- the minimum major axis of alloy carbides that can be observed in the measurement method described later is 10 nm, so the number density of alloy carbides having a major axis of 10 nm or more is defined.
- coarse alloy carbides having a major axis of more than 100 nm are present at the grain boundaries, microvoids are formed at an early stage of deformation, causing necking.
- the number density of alloy carbides having a major axis of more than 100 nm is low.
- the alloy carbides having a major axis of 10 to 100 nm present at the grain boundaries is within the above range, the alloy carbides having a major axis of more than 100 nm precipitate to an extent that adversely affects the steel sheet according to the present embodiment. Therefore, there is no need to specify the number density of alloy carbides having a major axis of more than 100 nm.
- the number density of alloy carbides having a major axis of 10 to 100 nm existing at grain boundaries is measured by the following method.
- a test piece is taken so that the thickness cross section parallel to the rolling direction serves as the observation surface. After polishing the observation surface of the test piece, nital etching is performed.
- Field emission scanning electron microscope FE-SEM: Field Emission Scanning with a field of view of 5 or more in a region from 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface on the observation surface
- the crystal orientation is analyzed by the electron beam backscatter diffraction method (EBSD: Electron Backscatter Diffraction) using an Electron Microsope. Each field of view is a continuous area. From the obtained crystal orientation map, the boundary where the crystal orientation difference is 1.0° or more is regarded as the crystal grain boundary.
- Whether or not the observed precipitates are alloy carbides is determined by point analysis by SEM-EDS for particles with lower brightness than the iron matrix phase in the field of view of the secondary electron image obtained by SEM observation. Precipitates in which the sum of the peak intensities of (K ⁇ , K ⁇ ), Nb (K ⁇ ), Mo (L ⁇ ), and V (K ⁇ ) is greater than or equal to the peak intensity of Fe (K ⁇ ) are determined to be alloy carbides.
- the number density of alloy carbides having a major axis of 10 nm or less in crystal grains is set to 1.0 ⁇ 10 16 pieces/cm 3 or more. It is preferably 5.0 ⁇ 10 16 particles/cm 3 or more or 1.0 ⁇ 10 17 particles/cm 3 or more. If the number density of the alloy carbides exceeds 1.0 ⁇ 10 19 pieces/cm 3 , the expansibility deteriorates. Therefore, the number density of the alloy carbides is set to 1.0 ⁇ 10 19 pieces/cm 3 or less. It is preferably 5.0 ⁇ 10 18 pieces/cm 3 or less or 1.0 ⁇ 10 18 pieces/cm 3 or less.
- the number density of alloy carbides present in crystal grains and having a major axis of 10 nm or less is measured by the following method.
- the same area as the field of view observed by EBSD is observed using a TEM (transmission electron microscope) at a magnification of 100,000 to 1,000,000.
- the number of alloy carbides having a major axis of 10 nm or less existing within boundaries determined to be grain boundaries by EBSD is calculated.
- the number density of alloy carbides having a major axis of 10 nm or less in the grains is obtained.
- a thin film sample is taken from the test piece.
- Tensile strength TS 1030 MPa or more
- the steel sheet according to the present embodiment has a tensile strength of 1030 MPa or more. If the tensile strength is less than 1030 MPa, it cannot be suitably applied to various automotive underbody parts.
- the tensile strength is preferably 1050 MPa or higher, or 1150 MPa or higher. The higher the tensile strength, the better, but it may be 1450 MPa or less.
- the tensile strength is measured by performing a tensile test in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011.
- the tensile test piece is taken at the central position in the sheet width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
- Hole expansion ratio ⁇ 30% or more
- the steel plate according to the present embodiment may have a hole expansion ratio of 30% or more.
- the hole expansion ratio may be 35% or more, 40% or more, or 45% or more.
- a hole expansion rate is measured by performing a hole expansion test based on JISZ2256:2020.
- the steel sheet according to the present embodiment may be a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance.
- the plating layer may be an electroplating layer or a hot dipping layer.
- the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy.
- hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy plating. be.
- the amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.
- a method for manufacturing a steel sheet according to the present embodiment includes a rough rolling step of heating a slab having the chemical composition described above and performing four or more passes of rough rolling in a temperature range of 1000 to 1300 ° C.; After the rough rolling, a finish rolling step in which finish rolling is performed so that the final rolling reduction is 24 to 60% and the finish rolling temperature is in the temperature range of 960 to 1060 ° C.; After the finish rolling, a cooling step of cooling so that the average cooling rate in the temperature range of 900 to 650 ° C.
- a winding step of winding in a temperature range of 400 to 580 ° C. After the cooling, it is heated to a temperature range of 600 to 750 ° C. at an average heating rate of 0.2 to 5.0 ° C./sec, held in the temperature range of 600 to 750 ° C. for 60 to 3010 seconds, and and a reheating step of cooling so that the average cooling rate in the temperature range of 700° C. is 10° C./second or more.
- the temperature difference between the final pass and the pass one pass before the final pass is 50°C or less
- the reduction rate of the 1st to 3rd passes is 10 to 30%
- the rolling reduction ratio after the fourth pass is set to 15 to 50%.
- the slab having the chemical composition described above is heated and subjected to four or more passes of rough rolling in a temperature range of 1000 to 1300°C. Further, in the rough rolling step, the temperature difference between the final pass and the pass one pass before the final pass is set to 50° C. or less, the reduction rate in the first to third passes is set to 10 to 30%, and the reduction in the fourth and subsequent passes is set. The ratio is 15-50%.
- the temperature for rough rolling is less than 1000° C., precipitation of alloy carbides proceeds, and after the subsequent reheating step, an excessive amount of alloy carbides precipitates at grain boundaries. As a result, deterioration of bendability after processing cannot be reduced. Therefore, rough rolling is performed in a temperature range of 1000° C. or higher. On the other hand, if the rough rolling is performed at 1300°C or higher, the fuel cost increases, so the rough rolling is performed in the temperature range of 1300°C or lower.
- the rough rolling step if less than 4 passes of rough rolling are performed in the temperature range of 1000 to 1300° C., the rolling reduction in one pass increases and the load on the rough rolling mill increases. Therefore, four or more passes of rough rolling are performed in the temperature range of 1000 to 1300°C. Although the upper limit is not particularly defined, the rough rolling performed in the temperature range of 1000 to 1300° C. may be, for example, 6 passes or less.
- the temperature difference between the final pass and the pass one pass before the final pass is set to 50° C. or less. It is preferably 45° C. or lower or 40° C. or lower.
- the temperature difference here is specifically the difference between the slab surface temperature on the exit side of the final pass and the slab surface temperature on the exit side of the pass one pass before the final pass.
- the rolling reduction in the first to third passes is less than 10%, or the rolling reduction in the fourth and subsequent passes is less than 15%, the crystal grains become coarse, and a sufficient amount of alloy is added to the grain boundaries. Carbide cannot be precipitated, and deterioration of bendability after working cannot be reduced. Therefore, the rolling reduction in the first to third passes is set to 10% or more, and the rolling reduction in the fourth and subsequent passes is set to 15% or more.
- the rolling reduction in the first to third passes exceeds 30%, or the rolling reduction in the fourth and subsequent passes exceeds 50%, alloy carbides precipitate, and these alloy carbides coarsen in the subsequent reheating step. do.
- the rolling reduction in the first to third passes is 30% or less, and the rolling reduction in the fourth and subsequent passes is 50% or less. Note that the rolling reduction mentioned here does not mean the cumulative rolling reduction, but rather the rolling reduction for each pass.
- Finish Rolling Step After rough rolling, finish rolling is performed so that the final rolling reduction (rolling reduction of the final pass) is 24 to 60% and the finish rolling temperature is in the temperature range of 960 to 1060°C. If the rolling reduction in the final pass is less than 24%, recrystallization does not proceed sufficiently, the alloy carbides precipitated at the grain boundaries become coarse, and the desired number density cannot be obtained at the grain boundaries. As a result, the desired hole expansibility cannot be obtained and/or the deterioration of bendability after processing cannot be reduced. Therefore, the rolling reduction in the final pass is set to 24% or more.
- the final reduction in finish rolling is preferably 30% or more.
- the upper limit of the final rolling reduction in finish rolling is set to 60% or less from the viewpoint of suppressing an increase in equipment load.
- the final reduction ratio of finish rolling can be expressed by ( 1 ⁇ t/t 0 ) ⁇ 100 (%), where t is the plate thickness after the final pass of finish rolling and t is the plate thickness before the final pass. can.
- finish rolling temperature (the surface temperature of the steel sheet on the exit side of the final pass of finish rolling) is less than 960°C, recrystallization does not proceed sufficiently, and the alloy carbides precipitated at the grain boundaries become coarse. Desired number density cannot be obtained. As a result, the desired hole expansibility cannot be obtained and/or the deterioration of bendability after processing cannot be reduced.
- the finish rolling temperature is preferably 980°C or higher.
- the upper limit of the finish rolling temperature is set to 1060° C. or lower from the viewpoint of suppressing coarsening of the grain size and suppressing deterioration of the toughness of the steel sheet.
- the steel is cooled so that the average cooling rate in the temperature range of 900 to 650°C is 30°C/sec or more. If the average cooling rate in the temperature range of 900 to 650° C. is less than 30° C./sec, a large amount of ferrite and pearlite will be produced, making it impossible to obtain the desired tensile strength. Therefore, the average cooling rate in the temperature range of 900 to 650° C. should be 30° C./second or more. It is preferably 50° C./second or higher, more preferably 80° C./second or higher. Although the upper limit of the average cooling rate in the temperature range of 900 to 650° C. is not particularly limited, it may be 300° C./second or less or 200° C./second or less.
- the average cooling rate referred to in this embodiment is a value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point. After the temperature range of 900 to 650° C. is cooled at the above-mentioned average cooling rate, the cooling up to winding is not particularly limited.
- the steel sheet is wound in a temperature range of 400 to 580°C. If the coiling temperature is less than 400°C, fresh martensite and tempered martensite are excessively formed, and the hole expansibility of the steel sheet deteriorates. Therefore, the winding temperature is set to 400° C. or higher.
- the coiling temperature is preferably 450° C. or higher. Also, if the coiling temperature is higher than 580° C., the amount of ferrite increases and the desired tensile strength cannot be obtained. Moreover, a desired number density cannot be obtained in the crystal grains. Therefore, the winding temperature should be less than 580°C.
- the coiling temperature is preferably 560° C. or lower.
- the steel sheet manufactured by the above method may be allowed to cool to room temperature, or may be water-cooled after being coiled.
- the coil After winding, the coil may be unwound, pickled, and lightly reduced. If the cumulative rolling reduction of light rolling is too high, the dislocation density increases and the hole expandability of the steel sheet may deteriorate. Therefore, when light reduction is performed, the cumulative reduction rate of light reduction is preferably 15% or less.
- the cumulative reduction ratio under light reduction can be expressed by (1 ⁇ t/t 0 ) ⁇ 100(%), where t is the plate thickness after light reduction and t 0 is the plate thickness before light reduction.
- Reheating step After winding or after light reduction, heat to a temperature range of 600 to 750 ° C. at an average heating rate of 0.2 to 5.0 ° C./sec, hold in this temperature range for 60 to 3010 seconds, Cooling is performed so that the average cooling rate from 500 to 700°C is 10°C/sec or more.
- the holding temperature in the reheating step is less than 600° C., a sufficient amount of alloy carbide cannot be precipitated in the crystal grains, and the desired strength cannot be obtained. Therefore, the holding temperature is set to 600° C. or higher. On the other hand, if the holding temperature exceeds 750° C., the alloy carbides in the crystal grains become coarse, and the number density of the alloy carbides in the crystal grains decreases. As a result, desired strength cannot be obtained. Therefore, the holding temperature is set to 750° C. or lower.
- the retention time is set to 60 seconds or longer.
- the holding time exceeds 3010 seconds, the alloy carbides in the crystal grains coarsen, and the number density of the alloy carbides in the crystal grains decreases. As a result, desired strength cannot be obtained. Therefore, the retention time is set to 3010 seconds or less.
- the average heating rate up to the temperature range of 600 to 750° C. is less than 0.2° C./sec, dislocation recovery occurs, the desired strength cannot be obtained, and productivity is reduced. Therefore, the average heating rate up to the temperature range of 600 to 750° C. should be 0.2° C./second or more. On the other hand, if the average heating rate up to the temperature range of 600 to 750° C. exceeds 5.0° C./sec, the fuel cost required for heating increases. Therefore, the average heating rate up to the temperature range of 600 to 750° C. should be 5.0° C./sec or less.
- the average cooling rate in the temperature range of 500 to 700° C. is 10° C./second or more. If the average cooling rate in the temperature range of 500 to 700° C. is less than 10° C./sec, the alloy carbides in the crystal grains coarsen and the number density of the alloy carbides in the crystal grains decreases. As a result, desired strength cannot be obtained. Therefore, the average cooling rate in the temperature range of 500 to 700°C should be 10°C/sec or more. Although the upper limit of the average cooling rate in the temperature range of 500 to 700° C. is not specified, it may be 200° C./sec or less from the viewpoint of suppressing an increase in cooling equipment.
- Slabs having the chemical compositions shown in Table 1 were produced by continuous casting. Using the obtained slabs, steel sheets with a thickness of 3.0 mm were manufactured under the conditions shown in Tables 2A to 3B. In the rough rolling process, 4 to 6 passes of rough rolling were performed. A blank in Table 1 indicates that the element is not intentionally contained.
- the tensile strength TS was 1030 MPa or more, the strength was judged to be high and it was judged as acceptable. On the other hand, when the tensile strength TS was less than 1030 MPa, it was determined that the strength was low and that it was rejected. When the obtained hole expansion ratio ⁇ was 30% or more, it was determined to be acceptable as having excellent hole expansion properties. On the other hand, when the hole expansion rate ⁇ was less than 30%, it was determined to be unacceptable because the hole expandability was inferior.
- drawbent processing was performed as the processing.
- Draw venting was performed by forming a hat part under the conditions shown in FIG.
- a test piece to be formed had a length of 240 mm and a width of 50 mm with the L direction of the steel sheet being the longitudinal direction.
- a test piece was taken such that the vertical wall portion of the hat component was the bent portion.
- a strip-shaped test piece of 100 mm x 30 mm was cut from the 1/2 position in the width direction of the steel plate.
- a bending test was performed in accordance with JIS Z 2248:2006 V-block method (bending angle ⁇ is 90°) for bending in which the bending ridge is parallel to the rolling direction (L direction) (L-axis bending). By determining the minimum bending radius R at which cracks do not occur and dividing it by the plate thickness t, the limit bending R/t was obtained.
- the presence or absence of cracks is determined by observing the bending surface of the test piece after the bending test at a magnification of 10 times or more with a magnifying glass or an optical microscope, and the crack length observed on the bending surface of the test piece is 0.5 mm. It was judged that there was a crack when exceeding
- the R/t before draw-bent processing and the R/t of the processed portion after draw-bent processing were obtained. If the value obtained by dividing the R / t before draw venting by the R / t of the processed part after draw venting is 0.5 or more, it is judged that the bendability deterioration after processing is small, and it is accepted. It was judged and described as "Good” in the table. On the other hand, when the above value was 0.5 or less, it was judged that the bendability after working was greatly deteriorated, it was judged to be unacceptable, and it was described as "NG" in the table.
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Abstract
Description
本願は、2021年2月26日に、日本に出願された特願2021-030350号に基づき優先権を主張し、その内容をここに援用する。
(1)本発明の一態様に係る鋼板は、化学組成が、質量%で、
C :0.030~0.180%、
Si:0.030~1.400%、
Mn:1.60~3.00%、
Al:0.010~0.700%、
P :0.0800%以下、
S :0.0100%以下、
N :0.0050%以下、
Ti:0.020~0.180%、
Nb:0.010~0.050%、
Mo:0~0.600%、
V :0~0.300%、
Ti、Nb、MoおよびVの合計:0.100~1.130%、
B :0~0.0030%、並びに
Cr:0~0.500%
を含有し、残部がFeおよび不純物からなり、
金属組織が、面積%で、
ベイナイト:80.0%以上、
フレッシュマルテンサイトおよび焼き戻しマルテンサイトの合計:20.0%以下、
パーライト、フェライトおよびオーステナイトの合計:20.0%以下であり、
結晶粒界に存在する長径が10~100nmである合金炭化物の個数密度が1.0×108~1.0×1011個/cm2であり、
結晶粒内に存在する長径が10nm以下である合金炭化物の個数密度が1.0×1016~1.0×1019個/cm3であり、
引張強さが1030MPa以上である。
(2)上記(1)に記載の鋼板は、前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの面積率の合計のうち、前記焼き戻しマルテンサイトの面積率の割合が80.0%以上であってもよい。
(3)上記(1)または(2)に記載の鋼板は、前記化学組成が、質量%で、
Mo:0.001~0.600%、
V :0.010~0.300%、
B :0.0001~0.0030%、および
Cr:0.001~0.500%
からなる群のうち1種または2種以上を含有してもよい。
(4)本発明の別の態様に係る鋼板の製造方法は、上記(1)に記載の鋼板の製造方法であって、
上記(1)に記載の化学組成を有するスラブを加熱し、1000~1300℃の温度域で4パス以上の粗圧延を行う粗圧延工程と、
前記粗圧延後に、最終圧下率が24~60%であり、仕上げ圧延温度が960~1060℃の温度域となるように仕上げ圧延を行う仕上げ圧延工程と、
前記仕上げ圧延後に、900~650℃の温度域における平均冷却速度が30℃/秒以上となるように冷却する冷却工程と、
前記冷却後に、400~580℃の温度域で巻取りを行う巻取り工程と、
前記巻取り後に、0.2~5.0℃/秒の平均加熱速度で600~750℃の温度域まで加熱し、600~750℃の前記温度域で60~3010秒保持した後、500~700℃の温度域における平均冷却速度が10℃/秒以上となるように冷却する再加熱工程と、を備え、
前記粗圧延工程では、
最終パスと最終パスから1パス前のパスとの温度差を50℃以下とし、
1~3パス目の圧下率を10~30%とし、
4パス目以降の圧下率を15~50%とする。
また、本発明に係る上記別の態様によれば、上記鋼板を製造することができる、鋼板の製造方法を提供することができる。
Cは、鋼板の所望の引張強さを得るために必要な元素である。C含有量が0.030%未満であると、所望の引張強さを得ることができない。そのため、C含有量は0.030%以上とする。C含有量は、好ましくは0.060%以上であり、より好ましくは0.080%以上であり、より一層好ましくは0.085%以上、0.090%以上、0.095%以上または0.100%以上である。
一方、C含有量が0.180%超では、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計が過剰となり、鋼板の穴広げ性が劣化する。そのため、C含有量は0.180%以下とする。C含有量は、好ましくは0.170%以下であり、より好ましくは0.150%以下である。
Siは、固溶強化によって鋼板の引張強さを向上する元素である。Si含有量が0.030%未満では、所望の引張強さを得ることができない。そのため、Si含有量は0.030%以上とする。Si含有量は、好ましくは0.040%以上であり、より好ましくは0.050%以上である。
一方、Si含有量が1.400%超であると、残留オーステナイトの面積率が多くなり、鋼板の穴広げ性が劣化する。そのため、Si含有量は1.400%以下とする。Si含有量は、好ましくは1.100%以下であり、より好ましくは1.000%以下である。
Mnは、鋼板の強度を向上させるために必要な元素である。Mn含有量が、1.60%未満であると、フェライトの面積率が高くなりすぎ、所望の引張強さを得ることができない。そのため、Mn含有量は1.60%以上とする。Mn含有量は、好ましくは1.80%以上であり、より好ましくは2.00%以上である。
一方、Mn含有量が3.00%超であると、鋳造スラブの靱性が劣化し、熱間圧延することができない。そのため、Mn含有量は3.00%以下とする。Mn含有量は、好ましくは2.70%以下であり、より好ましくは2.50%以下である。
Alは、脱酸剤として作用し、鋼の清浄度を向上させる元素である。Al含有量が0.010%未満であると、十分な脱酸効果が得られず、鋼板中に多量の介在物(酸化物)が形成される。このような介在物は、鋼板の穴広げ性を劣化させる。そのため、Al含有量は0.010%以上とする。Al含有量は、好ましくは0.020%以上であり、より好ましくは0.030%以上である。
一方、Al含有量が0.700%超では、鋳造が困難となる。そのため、Al含有量は、0.700%以下とする。Al含有量は、好ましくは0.600%以下であり、より好ましくは0.100%以下である。
Pは、鋼板の板厚中央部に偏析する元素である。またPは、溶接部を脆化させる元素でもある。P含有量が0.0800%超であると、鋼板の穴広げ性が劣化する。そのため、P含有量は0.0800%以下とする。P含有量は、好ましくは0.0200%以下であり、より好ましくは0.0100%以下である。
P含有量は低い程好ましく、0%であることが好ましいが、P含有量を過剰に低減すると脱Pコストが著しく増加する。そのため、P含有量は0.0005%以上としてもよい。
Sは、硫化物として存在することで、スラブを脆化させる元素である。またSは、鋼板の加工性を劣化させる元素でもある。S含有量が0.0100%超であると、鋼板の穴広げ性が劣化する。そのため、S含有量は0.0100%以下とする。S含有量は、好ましくは0.0080%以下であり、より好ましくは0.0050%以下である。
S含有量は低い程好ましく、0%であることが好ましいが、S含有量を過剰に低減すると脱Sコストが著しく増加する。そのため、S含有量は0.0005%以上としてもよい。
Nは、鋼中に粗大な窒化物を形成し、鋼板の加工性を劣化させる元素である。N含有量が0.0050%超であると、鋼板の穴広げ性が劣化する。そのため、N含有量は0.0050%以下とする。N含有量は、好ましくは0.0040%以下であり、より好ましくは0.0035%以下である。
N含有量は低い程好ましく、0%であることが好ましいが、N含有量を過剰に低減すると脱Nコストが著しく増加する。そのため、N含有量は0.0005%以上としてもよい。
Tiは、鋼中に微細な窒化物を形成することで、鋼板の強度を高める元素である。Ti含有量が0.020%未満であると、所望の引張強さを得ることができない。そのため、Ti含有量は0.020%以上とする。Ti含有量は、好ましくは0.050%以上であり、より好ましくは0.080%以上である。
一方、Ti含有量が0.180%超であると、鋼板の穴広げ性が劣化する。そのため、Ti含有量は、0.180%以下とする。Ti含有量は、好ましくは0.160%以下であり、より好ましくは0.150%以下である。
Nbは、熱間圧延でのオーステナイト粒の異常粒成長を抑制する元素である。またNbは、微細な合金炭化物を形成することで鋼板の強度を高める元素でもある。Nb含有量が0.010%未満であると、所望の引張強さを得ることができない。そのため、Nb含有量は0.010%以上とする。Nb含有量は、好ましくは0.013%以上であり、より好ましくは0.015%以上である。
一方、Nb含有量が0.050%超であると、鋳造スラブの靱性が劣化し、熱間圧延することができない。そのため、Nb含有量は0.050%以下とする。Nb含有量は、好ましくは0.040%以下であり、より好ましくは0.035%以下である。
本実施形態では、上述したTiおよびNb、並びに後述するMoおよびVの含有量の合計を制御する。これらの元素の含有量の合計が0.100%未満であると、微細な合金炭化物を形成して鋼板の強度を高める効果が十分に得られず、所望の引張強さを得ることができない。そのため、これらの元素の含有量の合計を0.100%以上とする。なお、Ti、Nb、MoおよびVの全てを含む必要は無く、いずれか1種でもその含有量が0.100%以上であれば上記効果を得ることができる。これらの元素の含有量の合計は、好ましくは0.150%以上であり、より好ましくは0.200%以上であり、より一層好ましくは0.230%以上である。
一方、これらの元素の含有量の合計が1.130%超であると、鋼板の穴広げ性が劣化する。そのため、これらの元素の含有量の合計は1.130%以下とする。これらの元素の含有量の合計は、好ましくは1.000%以下であり、より好ましくは0.500%以下である。
Moは、鋼中に微細な合金炭化物を形成することで鋼板の強度を高める元素である。この効果を確実に得るためには、Mo含有量は0.001%以上とすることが好ましい。
一方、Mo含有量が0.600%超であると、鋼板の穴広げ性が劣化する。そのため、Mo含有量は0.600%以下とする。
Vは、鋼中に微細な合金炭化物を形成することで鋼板の強度を高める元素である。この効果を確実に得るためには、V含有量は0.010%以上とすることが好ましい。
一方、V含有量が0.300%超であると、鋼板の穴広げ性が劣化する。そのため、V含有量は0.300%以下とする。
Bは、冷却工程でのフェライトの生成を抑制し、鋼板の強度を高める元素である。この効果を確実に得るためには、B含有量は0.0001%以上とすることが好ましい。
一方、0.0030%を超えてBを含有させても上記効果は飽和する。そのため、B含有量は0.0030%以下とする。
Crは、Mnと類似した効果を発現する元素である。Cr含有による鋼板の強度を高める効果を確実に得るためには、Cr含有量は0.001%以上とすることが好ましい。
一方、0.500%を超えてCrを含有させても、上記効果は飽和する。そのため、Cr含有量は0.500%以下とする。
本実施形態に係る鋼板は、金属組織が、面積%で、ベイナイト:80.0%以上、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの合計:20.0%以下、パーライト、フェライトおよびオーステナイトの合計:20.0%以下であり、結晶粒界に存在する長径が10~100nmである合金炭化物の個数密度が1.0×108~1.0×1010個/cm2であり、結晶粒内に存在する長径が10nm以下である合金炭化物の個数密度が1.0×1016~1.0×1019個/cm3である。
ベイナイトは所定の強度を有しながら、穴広げ性に優れた組織である。ベイナイトの面積率が80.0%未満であると、所望の引張強さおよび/または穴広げ性を得ることができない。そのため、ベイナイトの面積率は80.0%以上とする。ベイナイトの面積率は、好ましくは81.0%以上であり、より好ましくは82.0%以上であり、より一層好ましくは83.0%以上である。
ベイナイトの面積率の上限は特に限定しないが、100.0%以下、95.0%以下または90.0%以下としてもよい。
フレッシュマルテンサイトおよび焼き戻しマルテンサイトは鋼板の強度を高める効果があるが、局部変形能が低いため、面積率が高まることで鋼板の穴広げ性が劣化する。フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計が20.0%を超えると、鋼板の穴広げ性が劣化する。そのため、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計は20.0%以下とする。フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計は、好ましくは15.0%以下であり、より好ましくは10.0%以下であり、より一層好ましくは5.0%以下である。
フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計の下限は特に限定しないが、0.0%以上、0.5%以上または1.0%以上としてもよい。
フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合を高めることで、鋼板の穴広げ性をより高めることができる。そのため、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合を80.0%以上としてもよい。フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合は高い程好ましく、より好ましくは90.0%以上であり、100.0%としてもよい。
なお、焼き戻しマルテンサイトの面積率の割合は、{焼き戻しマルテンサイトの面積率/(フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計)}×100で求めることができる。
フェライトおよびオーステナイトは鋼板の強度を劣化させる組織である。パーライトは鋼板の穴広げ性を劣化させる組織である。これらの組織の面積率の合計が20.0%超であると、所望の引張強さおよび/または穴広げ性を得ることができない。そのため、これらの組織の面積率の合計は20.0%以下とする。これらの組織の面積率の合計は、好ましくは17.0%以下であり、より好ましくは15.0%以下である。
パーライト、フェライトおよびオーステナイトの面積率の合計の下限は特に限定しないが、0.0%以上、5.0%以上または10.0%以上としてもよい。
鋼板から、圧延方向に平行な断面で、表面から板厚の1/4深さ(表面から板厚の1/8深さ~表面から板厚の3/8深さの領域)且つ板幅方向中央位置における金属組織が観察できるように試験片を採取する。
結晶粒界には、球状の合金炭化物が存在する。曲げ加工等の変形を受けたとき、変形により蓄積した転位密度が臨界量に達すると、結晶粒界に存在する合金炭化物と母相との界面(結晶粒界に存在する合金炭化物の周り)において、マイクロボイドが発生する。結晶粒界にマイクロボイドが多量に発生すると、曲げ性が顕著に劣化する。結晶粒界に合金炭化物を多量に微細分散させることで、転位の蓄積サイトを分散させることができる。その結果、マイクロボイドが発生しても応力集中を緩和することができ、加工後の曲げ性の劣化を小さくすることができる。
上記合金炭化物の個数密度が1.0×1011個/cm2超であると、鋼板の強度が低下する。そのため、上記合金炭化物の個数密度は1.0×1011個/cm2以下とする。好ましくは5.0×1010個/cm2以下、1.0×1010個/cm2以下である。
本実施形態では、結晶粒界について、後述の測定方法において観察することができる合金炭化物の最小の長径は10nmであるため、長径が10nm以上である合金炭化物の個数密度を規定する。また、結晶粒界に長径が100nm超の粗大な合金炭化物が存在すると、変形の早期にマイクロボイドを形成し、くびれを発生させる。そのため、長径が100nm超である合金炭化物の個数密度は低い方が好ましい。ただし、結晶粒界に存在する長径が10~100nmである合金炭化物の個数密度が上記範囲内であれば、長径が100nm超である合金炭化物は、本実施形態に係る鋼板に悪影響を与えるほど析出しないため、長径が100nm超である合金炭化物の個数密度を規定する必要は無い。
圧延方向に平行な板厚断面が観察面となるように試験片を採取する。試験片の観察面を研磨した後、ナイタールエッチングする。観察面における表面から板厚の1/8深さ~表面から板厚の3/8深さの領域において、5以上の視野にて、電界放射型走査型電子顕微鏡(FE-SEM:Field Emission Scanning Electron Microsope)を用いた電子線後方散乱回折法(EBSD:Electron BackScatter Diffraction)により、結晶方位の解析を行う。各視野は連続した領域とする。得られた結晶方位マップから、結晶方位差が1.0°以上である境界を結晶粒界とみなす。
結晶粒内には、板状の合金炭化物が存在する。微細な合金炭化物を結晶粒内に多量に分散させることで、フェライト、ベイナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトが析出強化される。
上記合金炭化物の個数密度が1.0×1019個/cm3超であると、穴広げ性が劣化する。そのため、上記合金炭化物の個数密度は1.0×1019個/cm3以下とする。好ましくは5.0×1018個/cm3以下または1.0×1018個/cm3以下である。
上述のEBSDによる観察視野と同じ領域について、TEM(透過型電子顕微鏡)を用いて、倍率10万~100万倍で観察する。各視野について、EBSDにより結晶粒界と判別された境界内に存在する、長径が10nm以下である合金炭化物の個数を算出する。得られた合金炭化物の個数を、結晶粒界を除く総観察体積で除することで、結晶粒内に存在する長径が10nm以下である合金炭化物の個数密度を得る。なお、TEMによる観察を行う際には試験片から薄膜試料を採取する。
本実施形態に係る鋼板は、引張強さが1030MPa以上である。引張強さが1030MPa未満であると、様々な自動車足回り部品に好適に適用することができない。引張強さは、1050MPa以上、または1150MPa以上であることが好ましい。
引張強さは高い程好ましいが、1450MPa以下としてもよい。
本実施形態に係る鋼板は、穴広げ率が30%以上であってもよい。穴広げ率は、35%以上、40%以上または45%以上としてもよい。
穴広げ率は、JIS Z 2256:2020に準拠して穴広げ試験を行うことで、測定する。
本実施形態に係る鋼板の製造方法は、上述の化学組成を有するスラブを加熱し、1000~1300℃の温度域で4パス以上の粗圧延を行う粗圧延工程と、
前記粗圧延後に、最終圧下率が24~60%であり、仕上げ圧延温度が960~1060℃の温度域となるように仕上げ圧延を行う仕上げ圧延工程と、
前記仕上げ圧延後に、900~650℃の温度域における平均冷却速度が30℃/秒以上となるように冷却する冷却工程と、
前記冷却後に、400~580℃の温度域で巻取りを行う巻取り工程と、
前記巻取り後に、0.2~5.0℃/秒の平均加熱速度で600~750℃の温度域まで加熱し、600~750℃の前記温度域で60~3010秒保持した後、500~700℃の温度域における平均冷却速度が10℃/秒以上となるように冷却する再加熱工程と、を備える。
また、前記粗圧延工程では、
最終パスと最終パスから1パス前のパスとの温度差を50℃以下とし、
1~3パス目の圧下率を10~30%とし、
4パス目以降の圧下率を15~50%とする。
以下、各工程について説明する。
粗圧延工程では、上述の化学組成を有するスラブを加熱し、1000~1300℃の温度域で4パス以上の粗圧延を行う。また、前記粗圧延工程では、最終パスと最終パスから1パス前のパスとの温度差を50℃以下とし、1~3パス目の圧下率を10~30%とし、4パス目以降の圧下率を15~50%とする。
一方、粗圧延を1300℃以上で行うと、燃料コストの増大を引き起こすため、粗圧延は1300℃以下の温度域で行う。
上限は特に規定しないが、1000~1300℃の温度域で行う粗圧延は例えば6パス以下とすればよい。
ここでいう温度差とは、具体的には、最終パスの出側のスラブ表面温度と、最終パスから1パス前のパスの出側のスラブ表面温度との差である。
なお、ここでいう圧下率とは累積圧下率ではなく、1パス毎の圧下率のことをいう。
粗圧延後には、最終圧下率(最終パスの圧下率)が24~60%であり、仕上げ圧延温度が960~1060℃の温度域となるように仕上げ圧延を行う。
最終パスの圧下率が24%未満であると、十分に再結晶が進行せず、結晶粒界に析出する合金炭化物が粗大化し、結晶粒界において所望の個数密度を得ることができない。その結果、所望の穴広げ性を得ることができない、および/または加工後の曲げ性の劣化を小さくすることができない。そのため、最終パスの圧下率は24%以上とする。仕上げ圧延の最終圧下率は、好ましくは30%以上である。仕上げ圧延の最終圧下率の上限は、設備負荷増大を抑制する観点から60%以下とする。
仕上げ圧延の最終圧下率は、仕上げ圧延の最終パス後の板厚をt、最終パス前の板厚をt0としたとき、(1-t/t0)×100(%)で表すことができる。
仕上げ圧延後は、900~650℃の温度域における平均冷却速度が30℃/秒以上となるように冷却する。900~650℃の温度域の平均冷却速度が30℃/秒未満であると、フェライトおよびパーライトが多量に生成し、所望の引張強さを得ることができない。そのため、900~650℃の温度域の平均冷却速度は、30℃/秒以上とする。好ましくは50℃/秒以上であり、より好ましくは80℃/秒以上である。
900~650℃の温度域の平均冷却速度の上限は特に限定しないが、300℃/秒以下または200℃/秒以下としてもよい。
900~650℃の温度域を上記平均冷却速度で冷却した後、巻取りまでの冷却については特に限定されない。
上述の冷却を行った後、400~580℃の温度域で鋼板を巻取る。巻取り温度が400℃未満であると、フレッシュマルテンサイトおよび焼き戻しマルテンサイトが過剰に生成し、鋼板の穴広げ性が劣化する。そのため、巻き取り温度は400℃以上とする。巻取り温度は、好ましくは450℃以上である。
また、巻取り温度が580℃超であると、フェライト量が増加して所望の引張強さを得ることができない。また、結晶粒内において所望の個数密度を得ることができない。そのため、巻き取り温度は580℃未満とする。巻取り温度は、好ましくは560℃以下である。
軽圧下の累積圧下率は、軽圧下後の板厚をt、軽圧下前の板厚をt0としたとき、(1-t/t0)×100(%)で表すことができる。
巻取り後あるいは軽圧下後は、0.2~5.0℃/秒の平均加熱速度で600~750℃の温度域まで加熱し、この温度域で60~3010秒保持した後、500~700℃の平均冷却速度が10℃/秒以上となるように冷却する。
一方、保持温度が750℃超であると、結晶粒内の合金炭化物が粗大化し、結晶粒内の合金炭化物の個数密度が低下する。その結果、所望の強度を得ることができない。そのため、保持温度は750℃以下とする。
一方、保持時間が3010秒超であると、結晶粒内の合金炭化物が粗大化し、結晶粒内の合金炭化物の個数密度が低下する。その結果、所望の強度を得ることができない。そのため、保持時間は3010秒以下とする。
一方、600~750℃の温度域までの平均加熱速度が5.0℃/秒超であると、加熱に必要な燃料コストが増加する。そのため、600~750℃の温度域までの平均加熱速度は5.0℃/秒以下とする。
500~700℃の温度域における平均冷却速度の上限は特に規定しないが、冷却設備の増大抑制の観点から200℃/秒以下とすればよい。
なお、表1中の空欄は、当該元素を意図的に含有させていないことを示す。
得られた穴広げ率λが30%以上であった場合、穴広げ性に優れるとして合格と判定した。一方、穴広げ率λが30%未満であった場合、穴広げ性に劣るとして不合格と判定した。
ドローベント加工は、d図1に示す条件によりハット部品を成形することで行った。ハット部品の成形では縦壁が形成される際に、鋼板が曲げ曲げ戻し変形を受けながらポンチに接触するため、自動車足回り部品の縦壁部近傍のフラット-R部に形成する凹部を再現することができる。成形に供する試験片は、長手方向を鋼板のL方向として、長さ240mm×幅50mmのサイズとした。後述の曲げ試験では、ハット部品の縦壁部が曲げ部となるように試験片を採取した。
ただし、亀裂の有無は、上記曲げ試験後の試験片曲げ表面を拡大鏡や光学顕微鏡で10倍以上の倍率で亀裂を観察し、試験片の曲げ表面に観察される亀裂長さが0.5mmを超える場合に亀裂有と判断した。
一方、比較例に係る鋼板は、特性のいずれか一つ以上が劣ることが分かる。
Claims (4)
- 化学組成が、質量%で、
C :0.030~0.180%、
Si:0.030~1.400%、
Mn:1.60~3.00%、
Al:0.010~0.700%、
P :0.0800%以下、
S :0.0100%以下、
N :0.0050%以下、
Ti:0.020~0.180%、
Nb:0.010~0.050%、
Mo:0~0.600%、
V :0~0.300%、
Ti、Nb、MoおよびVの合計:0.100~1.130%、
B :0~0.0030%、並びに
Cr:0~0.500%
を含有し、残部がFeおよび不純物からなり、
金属組織が、面積%で、
ベイナイト:80.0%以上、
フレッシュマルテンサイトおよび焼き戻しマルテンサイトの合計:20.0%以下、
パーライト、フェライトおよびオーステナイトの合計:20.0%以下であり、
結晶粒界に存在する長径が10~100nmである合金炭化物の個数密度が1.0×108~1.0×1011個/cm2であり、
結晶粒内に存在する長径が10nm以下である合金炭化物の個数密度が1.0×1016~1.0×1019個/cm3であり、
引張強さが1030MPa以上であることを特徴とする鋼板。 - 前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの面積率の合計のうち、前記焼き戻しマルテンサイトの面積率の割合が80.0%以上であることを特徴とする請求項1に記載の鋼板。
- 前記化学組成が、質量%で、
Mo:0.001~0.600%、
V :0.010~0.300%、
B :0.0001~0.0030%、および
Cr:0.001~0.500%
からなる群のうち1種または2種以上を含有することを特徴とする請求項1または2に記載の鋼板。 - 請求項1に記載の鋼板の製造方法であって、
請求項1に記載の化学組成を有するスラブを加熱し、1000~1300℃の温度域で4パス以上の粗圧延を行う粗圧延工程と、
前記粗圧延後に、最終圧下率が24~60%であり、仕上げ圧延温度が960~1060℃の温度域となるように仕上げ圧延を行う仕上げ圧延工程と、
前記仕上げ圧延後に、900~650℃の温度域における平均冷却速度が30℃/秒以上となるように冷却する冷却工程と、
前記冷却後に、400~580℃の温度域で巻取りを行う巻取り工程と、
前記巻取り後に、0.2~5.0℃/秒の平均加熱速度で600~750℃の温度域まで加熱し、600~750℃の前記温度域で60~3010秒保持した後、500~700℃の温度域における平均冷却速度が10℃/秒以上となるように冷却する再加熱工程と、を備え、
前記粗圧延工程では、
最終パスと最終パスから1パス前のパスとの温度差を50℃以下とし、
1~3パス目の圧下率を10~30%とし、
4パス目以降の圧下率を15~50%とする
ことを特徴とする鋼板の製造方法。
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