WO2022244706A1 - 高強度熱延鋼板及び高強度熱延鋼板の製造方法 - Google Patents
高強度熱延鋼板及び高強度熱延鋼板の製造方法 Download PDFInfo
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- WO2022244706A1 WO2022244706A1 PCT/JP2022/020291 JP2022020291W WO2022244706A1 WO 2022244706 A1 WO2022244706 A1 WO 2022244706A1 JP 2022020291 W JP2022020291 W JP 2022020291W WO 2022244706 A1 WO2022244706 A1 WO 2022244706A1
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- martensite
- bainite
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- steel sheet
- rolled steel
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 88
- 239000010959 steel Substances 0.000 title claims abstract description 88
- 238000004519 manufacturing process Methods 0.000 title claims description 16
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 125
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 86
- 239000013078 crystal Substances 0.000 claims abstract description 41
- 239000000203 mixture Substances 0.000 claims abstract description 19
- 238000005096 rolling process Methods 0.000 claims description 43
- 238000001816 cooling Methods 0.000 claims description 27
- 230000009467 reduction Effects 0.000 claims description 20
- 238000005098 hot rolling Methods 0.000 claims description 11
- 238000000034 method Methods 0.000 claims description 11
- 239000012535 impurity Substances 0.000 claims description 7
- 229910052750 molybdenum Inorganic materials 0.000 claims description 7
- 229910052758 niobium Inorganic materials 0.000 claims description 7
- 229910052719 titanium Inorganic materials 0.000 claims description 7
- 229910052720 vanadium Inorganic materials 0.000 claims description 7
- 238000004804 winding Methods 0.000 claims description 7
- 238000010438 heat treatment Methods 0.000 claims description 4
- 238000012360 testing method Methods 0.000 description 15
- 230000000694 effects Effects 0.000 description 14
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 13
- 229910001566 austenite Inorganic materials 0.000 description 12
- 238000005336 cracking Methods 0.000 description 10
- 239000000126 substance Substances 0.000 description 10
- 150000001247 metal acetylides Chemical class 0.000 description 9
- 229910052761 rare earth metal Inorganic materials 0.000 description 8
- 150000002910 rare earth metals Chemical class 0.000 description 8
- 230000000717 retained effect Effects 0.000 description 8
- 238000004080 punching Methods 0.000 description 7
- 229910000859 α-Fe Inorganic materials 0.000 description 7
- 239000000463 material Substances 0.000 description 6
- 238000009864 tensile test Methods 0.000 description 6
- 230000001276 controlling effect Effects 0.000 description 5
- 230000006872 improvement Effects 0.000 description 5
- 229910052742 iron Inorganic materials 0.000 description 5
- 229910052759 nickel Inorganic materials 0.000 description 5
- 230000009466 transformation Effects 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- 229910001562 pearlite Inorganic materials 0.000 description 4
- 229910001567 cementite Inorganic materials 0.000 description 3
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 3
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 229910052787 antimony Inorganic materials 0.000 description 2
- 229910052796 boron Inorganic materials 0.000 description 2
- 239000003795 chemical substances by application Substances 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
- 239000000446 fuel Substances 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 239000004615 ingredient Substances 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 230000008520 organization Effects 0.000 description 2
- 238000003825 pressing Methods 0.000 description 2
- 238000010008 shearing Methods 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 229910052718 tin Inorganic materials 0.000 description 2
- 239000011800 void material Substances 0.000 description 2
- 241000669298 Pseudaulacaspis pentagona Species 0.000 description 1
- 241000316887 Saissetia oleae Species 0.000 description 1
- 230000001133 acceleration Effects 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000002050 diffraction method Methods 0.000 description 1
- 238000001803 electron scattering Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 235000019362 perlite Nutrition 0.000 description 1
- 239000010451 perlite Substances 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 238000001878 scanning electron micrograph Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 238000010998 test method Methods 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
Classifications
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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
- 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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- 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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
- C21D1/20—Isothermal quenching, e.g. bainitic hardening
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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
- 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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- 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
- C21D8/0221—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
- C22C—ALLOYS
- 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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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- 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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- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- 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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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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- 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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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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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
- C21D2201/00—Treatment for obtaining particular effects
- C21D2201/05—Grain orientation
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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
- C21D2211/002—Bainite
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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
Definitions
- the present invention relates to a high-strength hot-rolled steel sheet and a method for producing a high-strength hot-rolled steel sheet suitable as a material for automobile parts.
- Patent Document 1 it has a specific composition and a bainite phase with an area ratio of more than 95% throughout the plate thickness direction, and bainite in the region from the surface to the 1/4 position of the plate thickness in the plate thickness direction.
- the average grain size of the phase is 5 ⁇ m or less in the thickness cross section parallel to the rolling direction, 4 ⁇ m or less in the thickness cross section perpendicular to the rolling direction, and the width in the thickness direction centering on the thickness center position In a region that is 1/10 of the thickness, by having a structure in which the number of crystal grains extended in the rolling direction with an aspect ratio of 5 or more is 7 or less, punching workability is improved.
- Tensile strength (TS) is 780 MPa or more.
- No. 2003/0000005 discloses a technology related to hot-rolled steel sheets.
- Patent Document 2 it has a specific chemical composition, the grain boundary number density of solid solution C is 1/nm 2 or more and 4.5/nm 2 or less, and is precipitated at the grain boundary in the steel plate.
- a hot-rolled steel sheet having a cementite grain size of 1 ⁇ m or less is described.
- Patent Literature 2 discloses a technique related to a hot-rolled steel sheet having a TS of 540 MPa or more without fracture surface cracks by controlling solute C and grain boundary cementite.
- Patent Document 3 discloses crystal grains having a specific chemical composition and a grain boundary misorientation of 15° or more between adjacent grains, and the average misorientation in the grains is 0 to 0.5°. It contains a certain crystal grain at an area fraction of 50% or more, and the total area fraction of martensite, tempered martensite, and retained austenite is 2% or more and 10% or less, and 40% of Tief represented by a specific formula
- Patent Literature 3 discloses a technique related to a hot-rolled steel sheet in which ductility is improved by controlling the misorientation in crystal grains.
- Patent Document 1 improves the Ra of the punched fracture end surface of the hot rolled steel sheet (improves the punching workability), there is no disclosure of knowledge on suppressing the occurrence of cracks, and the hole expandability is not disclosed. It has not been specifically evaluated and there is room for improvement.
- the technique of Patent Document 2 only confirms the presence or absence of cracks on the member end faces under specific conditions, and cannot be said to stably improve the cracks on the member end faces against variations in clearance. , there is room for improvement.
- Patent Document 3 can improve ductility, there is room for improvement because no consideration has been given to end face crack resistance.
- the present invention is intended to solve the above problems, and provides a high-strength hot-rolled steel sheet and a high-strength steel sheet having excellent ductility, excellent resistance to edge cracking, and excellent hole expandability, which are suitable as materials for automobile parts.
- An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet.
- high strength in the present invention means that TS is 980 MPa or more.
- excellent ductility refers to a uniform elongation of 5.0% or more in a tensile test.
- excellent end face crack resistance refers to a clearance range that does not cause cracks parallel to the plate surface of the sample end face in a sample punched at intervals of 5% from 5 to 30% in the punching test described later. 10% or more can be secured.
- excellent hole expandability refers to a hole expansion rate of 40% or more in a hole expansion test described later.
- the above-described tensile test for measuring TS and uniform elongation, the above-described punching test, and the above-described hole expanding test can be performed by the methods described in the examples below.
- the present inventors focused on a hard phase that increases ductility but reduces end face crack resistance and hole expandability, and improved end face crack resistance by controlling its fraction and crystal orientation. Thought to improve.
- the main phases are martensite and bainite, and a certain amount of martensite is dispersed in the bainite, and the crystal orientation of the martensite in the bainite is is close to the crystal orientation of the bainite around the martensite (the bainite adjacent to the martensite), the end face crack resistance is less likely to decrease and high hole expandability is obtained.
- the gist of the present invention is as follows. [1] in % by mass, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5-3.5%, P: more than 0% and 0.100% or less, S: more than 0% and 0.020% or less, Al: including more than 0% and 1.5% or less, Further, Cr: 0.005-2.0%, Ti: 0.005-0.20%, Nb: 0.005-0.20%, Mo: 0.005-2.0%, V: 0.005-2.0%.
- the steel structure has a main phase of martensite and bainite with a total area ratio of 80 to 100%, The total area ratio of martensite in bainite is 2 to 20%, Among martensite in bainite, the area ratio of martensite having an orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite is A high-strength hot-rolled steel sheet having a ratio of 50% or more to martensite.
- [3] A method for producing a high-strength hot-rolled steel sheet according to [1] or [2] above, heating a slab having the component composition; Then, when performing hot rolling, Rough rolling at 1100°C or higher with 3 passes or more and a rolling reduction of 15% or more per pass, the total rolling reduction at 1000°C or lower being 50% or more, and the total number of passes at 1000°C or lower being 3 or more. After finishing rolling at , allow to cool for 1.0 s or more, then cool under conditions where the average cooling rate from the cooling start temperature to 550 ° C. is 50 ° C./s or more, and then (Ms point -50) ° C.
- the present invention it is possible to provide a high-strength hot-rolled steel sheet excellent in ductility, end face crack resistance, and hole expandability suitable as a material for automotive parts, and a method for producing a high-strength hot-rolled steel sheet. If the high-strength hot-rolled steel sheet of the present invention is used as a raw material for automobile parts, products such as high-strength automobile parts can be obtained without cracking during work.
- the high-strength hot-rolled steel sheet and the method for producing the high-strength hot-rolled steel sheet of the present invention are described in detail below. In addition, this invention is not limited to the following embodiment.
- the high-strength hot-rolled steel sheet of the present invention is a hot-rolled steel sheet called black scale as hot rolled or white scale further pickled after hot rolling.
- the high-strength hot-rolled steel sheet targeted by the present invention preferably has a thickness of 0.6 mm or more and 10.0 mm or less, and when used as a material for automobile parts, it is 1.0 mm or more and 6.0 mm or less. is more preferable.
- the plate width is preferably 500 mm or more and 1800 mm or less, more preferably 700 mm or more and 1400 mm or less.
- the high-strength hot-rolled steel sheet of the present invention has a specific chemical composition and a specific steel structure.
- the chemical composition and the steel structure will be explained in order.
- the chemical composition of the high-strength hot-rolled steel sheet of the present invention is, in mass%, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.5%, P: more than 0% and 0.100% or less, S: more than 0% and 0.020% or less, Al: more than 0% and 1.5% or less, Cr: 0.005 to 2.0%, Ti: One or two selected from 0.005 to 0.20%, Nb: 0.005 to 0.20%, Mo: 0.005 to 2.0%, V: 0.005 to 1.0% It contains more than seeds, and the balance consists of Fe and unavoidable impurities.
- C 0.04-0.18%
- C is an effective element for generating and strengthening bainite and martensite to raise TS. If the C content is less than 0.04%, such an effect cannot be sufficiently obtained, and a TS of 980 MPa or more cannot be obtained. On the other hand, if the C content exceeds 0.18%, hardening of martensite becomes remarkable, and end face crack resistance and hole expansion property of the present invention cannot be obtained. Therefore, the C content should be 0.04 to 0.18%. From the viewpoint of stably obtaining a TS of 980 MPa or more, the C content is preferably 0.05% or more. The C content is preferably 0.16% or less, more preferably 0.10% or less, from the viewpoint of improving end face crack resistance and hole expanding property.
- Si 0.1-3.0%
- Si is an element effective in increasing TS by solid-solution strengthening of steel and suppressing temper softening of martensite.
- it is an element effective in suppressing cementite and obtaining a structure in which martensite is dispersed in bainite.
- the Si content must be 0.1% or more.
- the Si content should be 0.1 to 3.0%.
- the Si content is preferably 0.2% or more.
- the Si content is preferably 2.0% or less, more preferably 1.5% or less.
- Mn 0.5-3.5%
- Mn is an element effective in generating martensite and bainite to raise TS. If the Mn content is less than 0.5%, such an effect cannot be sufficiently obtained, polygonal ferrite or the like is generated, and the steel structure of the present invention cannot be obtained. On the other hand, if the Mn content exceeds 3.5%, bainite is suppressed and the steel structure of the present invention cannot be obtained. Therefore, the Mn content should be 0.5 to 3.5%.
- the Mn content is preferably 1.0% or more from the viewpoint of more stably obtaining a TS of 980 MPa or more. From the viewpoint of stably obtaining bainite, the Mn content is preferably 3.0% or less, more preferably 2.3% or less.
- P more than 0% and 0.100% or less P lowers the end face crack resistance, so it is desirable to reduce the amount as much as possible.
- the P content can be allowed up to 0.100%. Therefore, the P content should be 0.100% or less, preferably 0.030% or less.
- the P content is more than 0%, and if the P content is less than 0.001%, the production efficiency is lowered, so 0.001% or more is preferable.
- the S content should be 0.020% or less, preferably 0.0050% or less, more preferably 0.0020% or less.
- the S content is more than 0%, and if the S content is less than 0.0002%, the production efficiency is lowered, so 0.0002% or more is preferable.
- Al more than 0% and 1.5% or less Al acts as a deoxidizing agent and is preferably added in the deoxidizing step.
- the lower limit of the Al content is more than 0%, and from the viewpoint of use as a deoxidizing agent, the Al content is preferably 0.01% or more. If a large amount of Al is contained, a large amount of polygonal ferrite is generated and the steel structure of the present invention cannot be obtained.
- the present invention allows an Al content of up to 1.5%. Therefore, the Al content is set to 1.5% or less. It is preferably 0.50% or less.
- Cr 0.005-2.0%
- Ti 0.005-0.20%
- Nb 0.005-0.20%
- Mo 0.005-2.0%
- V 0.005- One or Two or More Selected from 1.0% Cr, Ti, Nb, Mo and V are elements effective for obtaining a structure in which martensite is dispersed in bainite.
- the content of one or more elements selected from the above elements should be equal to or higher than the respective lower limits.
- the content of one or more elements selected from the above elements exceeds their respective upper limits, such effects cannot be obtained, and the steel structure of the present invention cannot be obtained.
- Cr 0.005-2.0%, Ti: 0.005-0.20%, Nb: 0.005-0.20%, Mo: 0.005-2.0%, V: 0.005-2.0%. 005 to 1.0%.
- Cr 0.1% or more, Ti: 0.010% or more, Nb: 0.010% or more, Mo: 0.10% or more, V: 0.10% That's it.
- the upper limits are preferably Cr: 1.0% or less, Ti: 0.15% or less, Nb: 0.10% or less, Mo: 1.0% or less, V: 0.1% or less, respectively. 5% or less.
- the balance is Fe and unavoidable impurities.
- unavoidable impurity elements include N, and the allowable upper limit of this element is preferably 0.010%.
- the above ingredients are the basic ingredient composition of the high-strength hot-rolled steel sheet of the present invention.
- the following elements can be further contained as necessary.
- Cu 0.05-4.0%, Ni: 0.005-2.0%, B: 0.0002-0.0050%, Ca: 0.0001-0.0050%, REM: 0.0001- 0.0050%, Sb: 0.0010 to 0.10%, Sn: 1 or 2 or more selected from 0.0010 to 0.50% Cu and Ni generate martensite and have high strength It is an effective element that contributes to In order to obtain such an effect, when Cu and Ni are contained, it is preferable to make each content equal to or higher than the above lower limit. When the respective contents of Cu and Ni exceed the above upper limits, bainite is suppressed and the steel structure of the present invention may not be obtained.
- the Cu content is more preferably 0.10% or more and more preferably 0.6% or less.
- the Ni content is more preferably 0.1% or more, and more preferably 0.6% or less.
- B is an effective element that enhances the hardenability of steel sheets, generates martensite, and contributes to high strength.
- the B content is preferably 0.0002% or more.
- the content is preferably 0.0002 to 0.0050%.
- the B content is more preferably 0.0005% or more, and more preferably 0.0040% or less.
- Ca and REM are elements that are effective in improving workability by controlling the morphology of inclusions.
- the content of Ca is 0.0001 to 0.0050% and the content of REM is 0.0001 to 0.0050%. If the contents of Ca and REM exceed the above upper limits, the amount of inclusions may increase and workability may deteriorate.
- the Ca content is more preferably 0.0005% or more and more preferably 0.0030% or less.
- the REM content is more preferably 0.0005% or more and more preferably 0.0030% or less.
- Sb is an element that suppresses denitrification, deboronization, etc., and is effective in suppressing a decrease in strength of steel.
- the Sb content is preferably 0.0010 to 0.10%. If the Sb content exceeds the above upper limit, the steel sheet may become embrittled.
- the Sb content is more preferably 0.0050% or more, and more preferably 0.050% or less.
- Sn is an element that suppresses the formation of pearlite and is effective in suppressing the strength reduction of steel.
- the Sn content is preferably 0.0010 to 0.50%. If the Sn content exceeds the above upper limit, the steel sheet may become embrittled.
- the Sn content is more preferably 0.0050% or more, and more preferably 0.050% or less.
- the steel structure of the high-strength hot-rolled steel sheet of the present invention has a total area ratio of 80 to 100% martensite and bainite as the main phase, and the total area ratio of martensite in bainite is 2 to 20%.
- the area ratio of the martensite having a crystal orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite is the total martensite is 50% or more for
- Total area ratio of martensite and bainite 80-100%
- a steel structure having mainly martensite and bainite (martensite and bainite being the main phases) is used in order to provide high TS, excellent resistance to edge cracking and hole expansion. If the total area ratio of martensite and bainite is less than 80% with respect to the entire steel sheet structure, at least one of high TS, resistance to edge cracking, and resistance to hole expansion cannot be obtained. Therefore, the total area ratio of martensite and bainite is 80-100%, preferably 83-100%, more preferably 88-100%.
- Total area ratio of martensite in bainite 2-20% Martensite is a steel structure effective for increasing TS, and is a steel structure effective for increasing uniform elongation by being dispersed in bainite. To obtain such an effect, the total area ratio of martensite in bainite must be 2% or more. On the other hand, if the total area ratio of martensite exceeds 20%, at least one of uniform elongation, end face crack resistance and hole expansion property cannot be obtained. Therefore, the total area ratio of martensite is set to 2 to 20%.
- the total area ratio of martensite is preferably 3% or more, more preferably 4% or more.
- the total area ratio of martensite is preferably 15% or less, more preferably 12% or less.
- the area ratio of martensite having an orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite total martensite 50% or more to the site Martensite in which the crystal orientation of the martensite in the bainite has an orientation difference of less than 15° with the crystal orientation of at least one bainite among the bainite adjacent to the martensite (Hereinafter, sometimes referred to as "martensite dispersed phase".)
- the above-mentioned "martensite having a crystal orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite” is, for example, a plurality of It means that when there is martensite surrounded by bainite with a crystal orientation, the orientation difference between at least one bainite among the bainite with a plurality of crystal orientations and the martensite is less than 15°. .
- the martensite dispersed phase has an area ratio of 50% or more.
- the ratio of martensite with a small orientation difference that can suppress void formation is 50% or more, the effect of suppressing connection of voids is increased, and cracking is significantly suppressed.
- the area ratio of the martensite dispersed phase is set to 50% or more with respect to all martensite. It is preferably 60% or more, more preferably 70% or more.
- the upper limit of the area ratio is not particularly defined. It is preferably 99% or less, more preferably 98% or less.
- the martensite dispersed phase can be determined by the method described in Examples below.
- the crystal orientations of bainite and martensite are determined by electron backscattering diffraction (EBSD), and the boundaries of the misorientation of 15° or more are displayed to identify the above martensite and the bainite adjacent to the martensite (adjacent bainite), the area ratio of martensite having a crystal orientation difference of less than 15° from at least one bainite is determined.
- EBSD electron backscattering diffraction
- the structures other than martensite and bainite mentioned above are ferrite, pearlite, and retained austenite.
- the total area ratio of structures other than martensite and bainite shall be less than 20%. If the total area ratio is less than 20%, the characteristics of the present invention can be achieved.
- the area ratio of each structure and the crystal orientation of martensite and bainite can be measured by the methods described in the examples described later.
- the high-strength hot-rolled steel sheet of the present invention is produced by heating a slab having the chemical composition described above and then subjecting it to hot rolling.
- the heated slab is roughly rolled at 1100 ° C. or higher with 3 passes or more and a rolling reduction of 15% or more per pass, and the total rolling reduction at 1000 ° C. or less is 50% or more and 1000 ° C. or less.
- the manufacturing method will be explained in detail below.
- the above temperature is the temperature (surface temperature) of the width center of the slab or steel plate, and the above average cooling rate is the average cooling speed of the width center of the steel plate. These temperatures can be measured with a radiation thermometer or the like.
- Number of passes at 1100°C or higher 3 or more
- the austenite grains are regulated and non-uniformity is eliminated, and the bainite Among the martensites, the area ratio of martensites having a crystal orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite is stably 50% or more of all martensite. If the number of passes at 1100° C. or higher is less than 3, such an effect cannot be sufficiently obtained. Therefore, the number of passes at 1100° C. or higher is set to 3 or more. The number of passes at 1100° C.
- the upper limit of the number of passes at 1100° C. or higher is not particularly specified, but if it exceeds 15, it may lead to hindrance to productivity such as an increase in scale loss, so it is preferably 15 or less.
- Rolling reduction per pass at 1100°C or higher 15% or more In rough rolling of hot rolling, if the rolling reduction per pass at 1100°C or higher is less than 15%, not only does the nonuniformity of the austenite grains disappear, On the contrary, it becomes worse, and martensite having characteristics of this crystal orientation cannot be obtained sufficiently. Therefore, the draft per pass at 1100° C. or higher is set to 15% or higher.
- the rolling reduction per pass at 1100° C. or higher is preferably 18% or higher, more preferably 20% or higher.
- the upper limit of the rolling reduction per pass at 1100° C. or higher is not particularly specified, but if it exceeds 60%, the plate shape may be deteriorated or manufacturing troubles may occur, so 60% or less is preferable.
- Total rolling reduction at 1000° C. or less 50% or more
- the crystal orientation of the present invention that is, martensite and at least one crystal orientation of the bainite adjacent to the martensite
- the crystal orientation of the present invention can account for 50% or more of the total martensite phase.
- the total rolling reduction at 1000° C. or lower in finish rolling of hot rolling is set to 50% or higher. It is preferably 60% or more.
- the upper limit of the total rolling reduction is not particularly defined. If the total rolling reduction is too large, texture may develop and workability such as hole expandability may be impaired, so it is preferably 90% or less.
- the total rolling reduction is the difference between the inlet strip thickness before the first pass in the above temperature range and the outlet strip thickness after the final pass in this temperature range, divided by the inlet strip thickness before the first pass.
- Total number of passes at 1000°C or less 3 times or more In the finish rolling of hot rolling, the reduction at 1000°C or less is distributed multiple times and the reduction rate per pass is reduced, so that the crystal orientation of bainite is improved. Closely oriented martensite (ie, martensite with an orientation difference of less than 15° from at least one crystallographic orientation of adjacent bainite) is more likely to form.
- the total number of passes is 3 or more, and the steel structure of the present invention (i.e., martensite having an orientation difference of less than 15° with at least one crystal orientation of adjacent bainite) has an area ratio of 50% to all martensite. above) can be obtained. It is preferably 4 times or more.
- the upper limit of the total number of passes is not specified. From the viewpoint of production efficiency, etc., it is preferable to set the number to 10 times or less.
- the finish rolling finish temperature is preferably 750 to 1000°C. By controlling the temperature at 750 to 1000° C., stable surface properties can be easily obtained. It is more preferably 780° C. or higher, and more preferably 950° C. or lower.
- Cooling time after finish rolling 1.0 s or more
- the cooling time after finish rolling is set to 1.0 s or longer. It is preferably 1.5s or more.
- the upper limit of the cooling time is not particularly defined. Cooling for 10 s or more may cause formation of structures such as ferrite, which is not desired in the present invention, so the cooling time is preferably 10 s or less.
- Average cooling rate from the cooling start temperature to 550°C 50°C/s or more
- the average cooling rate from the cooling start temperature to 550°C should be 50° C./s or more. It is preferably 80° C./s or more.
- the average cooling rate is preferably 1000° C./s or less from the viewpoint of the shape stability of the steel sheet.
- the cooling start temperature is preferably 700°C or higher. More preferably, the temperature is 720° C. or higher. Moreover, since it is technically difficult to make the cooling start temperature higher than the finish rolling end temperature, the cooling start temperature is preferably equal to or lower than the finish rolling end temperature.
- Winding temperature (Ms point -50) ° C ⁇ 550 ° C If the coiling temperature is less than (Ms point -50)°C, martensite increases and the steel structure of the present invention cannot be obtained. On the other hand, when the temperature exceeds 550°C, ferrite and pearlite are generated, and the steel structure of the present invention cannot be obtained. Therefore, the winding temperature is (Ms point-50)°C to 550°C. It is preferably (Ms point -30)°C or higher, preferably 520°C.
- the Ms point is the temperature at which martensite transformation starts, and can be determined by actually measuring thermal expansion during cooling by a Formaster test or by measuring electrical resistance.
- the slab heating temperature is preferably 1100° C. or higher from the viewpoint of segregation removal, precipitate solid solution, etc., and is preferably 1300° C. or lower from the viewpoint of energy efficiency.
- the finish rolling is performed by 4 or more passes from the viewpoint of reducing coarse grains that cause deterioration of workability.
- the number of passes in finish rolling refers to the total number of passes in finish rolling, and includes the above-mentioned "total number of passes at 1000°C or lower".
- the area ratio of martensite and bainite means the ratio of the area of each structure to the observed area.
- the area ratio of martensite is determined as follows. A sample was cut out from the obtained hot-rolled steel sheet, the thickness cross section parallel to the rolling direction was polished, then corroded with 3% nital, and the 1/4 thickness position was examined by SEM (scanning electron microscope) at a magnification of 1500 times. Three fields of view were photographed. The area ratio of each tissue was obtained from the obtained image data of the secondary electron image using Image-Pro manufactured by Media Cybernetics, and the average area ratio of the visual field was defined as the area ratio of each tissue.
- upper bainite is distinguished as black or dark gray with carbides or martensite with straight interfaces.
- Lower bainite is distinguished as black, dark gray, gray, or light gray containing oriented carbides.
- Martensite is distinguished as black, dark gray, gray, or light gray with multiple orientations of carbides, or white or light gray with no carbides.
- Retained austenite is distinguished as white or light gray with no carbides.
- the total area ratio of martensite and retained austenite obtained from the SEM image is divided by the area ratio of retained austenite obtained by the method described later, and the area ratio of martensite asked for
- martensite may be any martensite such as fresh martensite, autotempered martensite, and tempered martensite.
- the bainite may be any bainite such as upper bainite, lower bainite, and tempered bainite.
- ferrite has a black structure, a dark gray structure that does not have or has a slight amount of carbide inside, or a dark gray structure that does not have a linear interface with martensite Distinguishable as an organization.
- Perlite can be distinguished as a black and white lamellar or partially interrupted lamellar structure.
- the steel plate after annealing was ground to a position of 1/4 + 0.1 mm of the plate thickness, and then the surface was further polished by 0.1 mm by chemical polishing.
- Measurement of integrated reflection intensity of fcc iron (austenite) (200), (220) and (311) planes and bcc iron (ferrite) (200), (211) and (220) planes Did.
- the volume ratio was determined from the intensity ratio of the integrated reflection intensity from each surface of fcc iron to the integrated reflection intensity from each surface of bcc iron, and this was defined as the area ratio of retained austenite.
- crystal orientation The crystal orientations of bainite and martensite are determined by the back electron scattering diffraction method (EBSD) in the same field of view of the same sample used for the above structure observation, and the boundaries of the misorientation of 15° or more are indicated.
- EBSD back electron scattering diffraction method
- a JIS No. 5 tensile test piece (JIS Z 2201) was taken from the obtained hot-rolled steel sheet in a direction parallel to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241 at a strain rate of 10 -3 /s. was performed to obtain TS and uniform elongation.
- a TS of 980 MPa or more and a uniform elongation of 5.0% or more were evaluated as acceptable.
- the end face crack resistance was evaluated by a punching test.
- a test piece having a width of 150 mm and a length of 150 mm was taken from the obtained hot-rolled steel sheet.
- the test piece is punched three times using a ⁇ 10 mm punch under conditions where the clearance is 5%, 10%, 15%, 20%, 25%, and 30%, and the plate surface of the punched end surface (plate surface ) was investigated for the presence or absence of cracks parallel to ), and the end face crack resistance was evaluated.
- the clearance range in which cracks did not occur was 10% or more
- the end face crack resistance was evaluated as acceptable. For example, if the punch test performed in the manner described above yields 10%, 15%, 20%, and 25% crack-free clearance, the crack-free clearance range is the maximum crack-free clearance range. The minimum clearance of 10% is subtracted from the clearance of 25%, resulting in 15%.
- ⁇ Hole expansion test> The hole expansibility was evaluated by a hole expansive test. Using three test pieces punched out under the condition that the clearance is 10% in the punching test, a hole expansion test was performed three times using a 60 ° conical punch according to JFST 1001 (Japan Iron and Steel Federation Standard, 2008). The average hole expansion ratio (%) was obtained by using the hole expansion ratio. A hole expansion ratio of 40% or more was evaluated as acceptable.
- Table 3 shows various evaluation results.
- the invention examples are all high-strength hot-rolled steel sheets having excellent uniform elongation, excellent end face crack resistance, and excellent hole expansibility.
- the comparative examples outside the scope of the present invention did not achieve desired strength, uniform elongation, edge crack resistance, and hole expansibility.
- the present invention it is possible to obtain a high-strength hot-rolled steel sheet having a TS of 980 MPa or more, excellent ductility, excellent end face crack resistance, and excellent hole expandability.
- the use of the high-strength hot-rolled steel sheet of the present invention for automobile parts can greatly contribute to the improvement of collision safety and fuel efficiency of automobiles.
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Abstract
Description
なお、本発明では、上記したTSと一様伸びを測定する引張試験、上記した打ち抜き試験、および上記した穴広げ試験は、後述する実施例に記載の方法で行うことができる。
[1] 質量%で、
C:0.04~0.18%、
Si:0.1~3.0%、
Mn:0.5~3.5%、
P:0%超0.100%以下、
S:0%超0.020%以下、
Al:0%超1.5%以下を含み、
さらに、Cr:0.005~2.0%、Ti:0.005~0.20%、Nb:0.005~0.20%、Mo:0.005~2.0%、V:0.005~1.0%のうちから選ばれる1種または2種以上を含み、
残部がFeおよび不可避的不純物からなる成分組成を有し、
鋼組織は、合計面積率で80~100%のマルテンサイトおよびベイナイトを主相とし、
ベイナイト中のマルテンサイトの全面積率が2~20%であり、
ベイナイト中のマルテンサイトのうち、該マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトの結晶方位との方位差が15°未満であるマルテンサイトの面積率が、全マルテンサイトに対して50%以上である高強度熱延鋼板。
[2] 前記成分組成に加えて、質量%で、
Cu:0.05~4.0%、
Ni:0.005~2.0%、
B:0.0002~0.0050%、
Ca:0.0001~0.0050%、
REM:0.0001~0.0050%、
Sb:0.0010~0.10%、
Sn:0.0010~0.50%
のうちから選ばれる1種または2種以上を含む上記[1]に記載の高強度熱延鋼板。
[3] 上記[1]または[2]に記載の高強度熱延鋼板の製造方法であって、
前記成分組成を有するスラブを加熱し、
次いで、熱間圧延を施すに際し、
1100℃以上で3パス以上かつ1パスあたり15%以上の圧下率で粗圧延し、1000℃以下での合計圧下率が50%以上、1000℃以下での合計パス数が3回以上となる条件で仕上げ圧延した後、1.0s以上放冷し、その後、冷却開始温度から550℃までの平均冷却速度が50℃/s以上となる条件で冷却し、その後、(Ms点-50)℃~550℃の巻取り温度で巻き取る高強度熱延鋼板の製造方法。
本発明の高強度熱延鋼板は、熱間圧延ままの黒皮または、熱間圧延後さらに酸洗する白皮と称される熱延鋼板である。また、本発明が目的とする高強度熱延鋼板は、板厚が0.6mm以上10.0mm以下であることが好ましく、自動車用部品の素材として用いる場合には1.0mm以上6.0mm以下であることがより好ましい。また、板幅は、500mm以上1800mm以下であることが好ましく、700mm以上1400mm以下であることがより好ましい。
Cは、ベイナイトやマルテンサイトを生成および強化させてTSを上昇させるのに有効な元素である。C含有量が0.04%未満では、このような効果が十分得られず、980MPa以上のTSが得られない。一方、C含有量が0.18%を超えると、マルテンサイトの硬化が顕著になって本発明の耐端面割れ性や穴広げ性が得られなくなる。したがって、C含有量は0.04~0.18%とする。C含有量は、980MPa以上のTSをより安定的に得る観点から、好ましくは0.05%以上とする。C含有量は、耐端面割れ性および穴広げ性の向上の観点から、好ましくは0.16%以下とし、より好ましくは0.10%以下とする。
Siは、鋼を固溶強化したり、マルテンサイトの焼き戻し軟化を抑制することでTSを上昇させるのに有効な元素である。また、セメンタイトを抑制して、ベイナイト中にマルテンサイトを分散させた組織を得るのに有効な元素である。このような効果を得るには、Si含有量を0.1%以上とする必要がある。一方、Si含有量が3.0%を超えると、ポリゴナルフェライトが過剰に生成して本発明の鋼組織が得られなくなる。したがって、Si含有量は0.1~3.0%とする。Si含有量は、好ましくは0.2%以上とする。また、Si含有量は、好ましくは2.0%以下とし、より好ましくは1.5%以下とする。
Mnは、マルテンサイトやベイナイトを生成させてTSを上昇させるのに有効な元素である。Mn含有量が0.5%未満では、こうした効果が十分得られず、ポリゴナルフェライト等が生成し、本発明の鋼組織が得られなくなる。一方、Mn含有量が3.5%を超えると、ベイナイトが抑制されて本発明の鋼組織が得られなくなる。したがって、Mn含有量は0.5~3.5%とする。Mn含有量は、980MPa以上のTSをより安定的に得る観点から、好ましくは1.0%以上とする。Mn含有量は、ベイナイトを安定的に得る観点から、好ましくは3.0%以下とし、より好ましくは2.3%以下とする。
Pは、耐端面割れ性を低下させるため、その量は極力低減することが望ましい。本発明では、P含有量が0.100%まで許容できる。したがって、P含有量は0.100%以下とし、好ましくは0.030%以下とする。P含有量は0%超とし、P含有量が0.001%未満では生産能率の低下を招くため、0.001%以上が好ましい。
Sは、耐端面割れ性を低下させるため、その量は極力低減することが好ましいが、本発明ではS含有量が0.020%まで許容できる。したがって、S含有量は0.020%以下とし、好ましくは0.0050%以下、より好ましくは0.0020%以下とする。S含有量は0%超とし、S含有量が0.0002%未満では生産能率の低下を招くため、0.0002%以上が好ましい。
Alは、脱酸剤として作用し、脱酸工程で添加することが好ましい。Al含有量の下限値は0%超とし、脱酸剤として用いる観点からは、Al含有量は0.01%以上が好ましい。多量にAlを含有するとポリゴナルフェライトが多量に生成して本発明の鋼組織が得られなくなる。本発明では、Al含有量が1.5%まで許容される。したがって、Al含有量は1.5%以下とする。好ましくは0.50%以下とする。
Cr、Ti、Nb、MoおよびVは、ベイナイト中にマルテンサイトが分散した組織を得るのに有効な元素である。こうした効果を得るには、上記元素のうちから選ばれる1種または2種以上の元素の含有量が、それぞれの下限値以上である必要がある。一方、上記元素のうちから選ばれる1種または2種以上の元素の含有量が、それぞれの上限値を超えるとこのような効果が得られなくなり、本発明の鋼組織が得られない。したがって、Cr:0.005~2.0%、Ti:0.005~0.20%、Nb:0.005~0.20%、Mo:0.005~2.0%、V:0.005~1.0%のうちから選ばれる1種または2種以上を含有することとする。上記元素を含有する場合は、好ましくは、それぞれCr:0.1%以上、Ti:0.010%以上、Nb:0.010%以上、Mo:0.10%以上、V:0.10%以上とする。上記元素を含有する場合の上限は、好ましくは、それぞれCr:1.0%以下、Ti:0.15%以下、Nb:0.10%以下、Mo:1.0%以下、V:0.5%以下とする。
Cu、Niは、マルテンサイトを生成させ、高強度化に寄与する有効な元素である。このような効果を得るため、Cu、Niを含有する場合には、それぞれの含有量を上記下限値以上とすることが好ましい。Cu、Niのそれぞれの含有量が上記上限値を超えると、ベイナイトが抑制されて本発明の鋼組織が得られなくなる場合がある。Cu含有量は、より好ましくは0.10%以上とし、より好ましくは0.6%以下とする。Ni含有量は、より好ましくは0.1%以上とし、より好ましくは0.6%以下とする。
本発明の高強度熱延鋼板の鋼組織は、合計面積率で80~100%のマルテンサイトおよびベイナイトを主相とし、ベイナイト中のマルテンサイトの全面積率が2~20%であり、ベイナイト中のマルテンサイトのうち、該マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトの結晶方位との方位差が15°未満であるマルテンサイトの面積率が、全マルテンサイトに対して50%以上である。
本発明では、高TSと優れた耐端面割れ性および穴広げ性を備えるため、主にマルテンサイトおよびベイナイトを有する(マルテンサイトおよびベイナイトを主相とする)鋼組織とする。
鋼板組織全体に対し、マルテンサイトおよびベイナイトの合計面積率が80%未満では、高TS、耐端面割れ性および穴広げ性の少なくともいずれか一つが得られなくなる。したがって、マルテンサイトおよびベイナイトの合計面積率は、80~100%とし、好ましくは83~100%とし、より好ましくは88~100%とする。
マルテンサイトは、TSを高めるのに有効な鋼組織であり、さらにベイナイト中に分散することで一様伸びを高めるのに有効な鋼組織である。このような効果を得るには、ベイナイト中のマルテンサイトの全面積率を、2%以上とする必要がある。一方、上記マルテンサイトの全面積率が20%を超えると、一様伸び、耐端面割れ性および穴広げ性の少なくともいずれか一つが得られなくなる。したがって、上記マルテンサイトの全面積率は、2~20%とする。上記マルテンサイトの全面積率は、好ましくは3%以上とし、より好ましくは4%以上とする。上記マルテンサイトの全面積率は、好ましくは15%以下とし、より好ましくは12%以下とする。
ベイナイト中のマルテンサイトにおいて、該マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトにおける結晶方位との方位差が15°未満であるマルテンサイト(以下、「マルテンサイト分散相」と称する場合もある。)の面積率を、全マルテンサイトの面積に対して50%以上とすることで、耐端面割れ性が高められる。その結果、本発明の穴広げ率が得られる。
ここで、上記の「該マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトにおける結晶方位との方位差が15°未満であるマルテンサイト」とは、例えば、複数の結晶方位のベイナイトに囲まれたマルテンサイトがあったときに、複数の結晶方位のベイナイトのうち一つ以上のベイナイトと、該マルテンサイトとの方位差が15゜未満であればよいことを意味する。
本発明の高強度熱延鋼板は、上記成分組成を有するスラブを加熱し、次いで、熱間圧延を施すことにより製造される。上記熱間圧延では、加熱したスラブを、1100℃以上で3パス以上かつ1パスあたり15%以上の圧下率で粗圧延し、1000℃以下での合計圧下率が50%以上、1000℃以下での合計パス数が3回以上となる条件で仕上げ圧延した後、1.0s以上放冷し、その後、冷却開始温度から550℃までの平均冷却速度が50℃/s以上となる条件で冷却し、その後、(Ms点-50)℃~550℃の巻取り温度で巻き取り、室温まで冷却する。
熱間圧延の粗圧延において、1100℃以上でのパス回数を3回以上とすることで、オーステナイト粒が整粒化して不均一が解消し、ベイナイト中のマルテンサイトのうち、該マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトの結晶方位との方位差が15°未満であるマルテンサイトの面積率が、安定して全マルテンサイトに対して50%以上となる。1100℃以上でのパス数が3回未満ではこのような効果が十分得られない。したがって、1100℃以上でのパス数は3回以上とする。1100℃以上でのパス数は、好ましくは、4回以上とし、より好ましくは5回以上とする。1100℃以上でのパス回数の上限は特に規定しないが、15回を超えると、スケールロスの増加等の製造性の阻害を招く場合があるため、15回以下とすることが好ましい。
熱間圧延の粗圧延において、1100℃以上での1パスあたりの圧下率が15%未満ではオーステナイト粒の不均一が解消されないばかりか、返って悪化し、本結晶方位の特徴を有するマルテンサイトが十分に得られなくなる。したがって、1100℃以上での1パスあたりの圧下率は15%以上とする。1100℃以上での1パスあたりの圧下率は、好ましくは18%以上とし、より好ましくは20%以上とする。1100℃以上での1パスあたりの圧下率の上限は特に規定しないが60%を超えると、板形状の悪化や製造トラブルを招く場合があるため60%以下が好ましい。
熱間圧延の仕上げ圧延における、1000℃以下での合計圧下率を50%以上とすることによって、上述した、本発明の結晶方位(すなわち、マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトの少なくとも1つの結晶方位との方位差が15°未満)のマルテンサイト分散相を、全マルテンサイト相に対して50%以上とすることができる。
ここで、合計圧下率とは、上記温度域における最初のパス前の入口板厚と、この温度域における最終パス後の出口板厚との差を、該最初のパス前の入口板厚で除した値の百分率である。
すなわち、(上記温度域における最初のパス前の入口板厚-上記温度域における最終パス後の出口板厚)/(上記温度域における最初のパス前の入口板厚)×100(%)により求められる。
熱間圧延の仕上げ圧延における、1000℃以下での圧下を複数回に分散し、1パスあたりの圧下率を低減させることで、ベイナイトの結晶方位に近い方位のマルテンサイト(すなわち、隣接するベイナイトの少なくとも1つの結晶方位との方位差が15°未満であるマルテンサイト)が生成されやすくなる。合計パス数が3回以上で、本発明の鋼組織(すなわち、隣接するベイナイトの少なくとも1つの結晶方位との方位差が15°未満であるマルテンサイトの、全マルテンサイトに対する面積率を、50%以上とすること)を得ることができる。好ましくは4回以上とする。上記合計パス数の上限は、特に規定しない。生産能率等の観点からは、10回以下とすることが好ましい。
仕上げ圧延終了温度は、750~1000℃とすることが好ましい。750~1000℃に制御することで、安定した表面性状が得やすくなる。より好ましくは780℃以上であり、より好ましくは950℃以下である。
仕上げ圧延後の放冷時間が1.0s(秒)未満では、本発明の結晶方位のマルテンサイト分散相の全マルテンサイト相に対する面積率を50%以上とすることができない。この理由は明らかではないが、放冷することによって、仕上げ圧延で導入された転位が一部回復し、続くベイナイト変態やマルテンサイト変態時の方位選択に影響しているものと考えられる。したがって、仕上げ圧延後の放冷時間は、1.0s以上とする。好ましくは1.5s以上とする。上記放冷時間の上限は特に規定しない。10s以上の放冷は、本発明では望まないフェライトなどの組織の生成を招く場合があるため、上記放冷時間は10s以下とすることが好ましい。
冷却開始温度から550℃までの平均冷却速度が50℃/s未満では、フェライトやパーライトが生成して本発明の鋼組織が得られない。したがって、冷却開始温度から550℃までの平均冷却速度は、50℃/s以上とする。好ましくは80℃/s以上とする。上記平均冷却速度の上限は特に規定しないが、鋼板の形状安定性等の観点からは、上記平均冷却速度は1000℃/s以下とすることが好ましい。
巻取り温度が(Ms点-50)℃未満では、マルテンサイトが増加して、本発明の鋼組織が得られない。一方、550℃を超えると、フェライトやパーライトが生成して、本発明の鋼組織が得られない。したがって、巻取り温度は、(Ms点-50)℃~550℃とする。好ましくは(Ms点-30)℃以上であり、好ましくは520℃である。
また、仕上げ圧延は、加工性の低下を招く粗粒低減等の観点から、4パス以上とすることが好ましい。なお、この仕上げ圧延のパス数とは、仕上げ圧延における全パス数を指し、上記の「1000℃以下での合計パス数」を含めるものとする。
なお、表1の空欄は、意図的に元素を添加しないことを表しており、含有しない(0%)場合だけでなく、不可避的に含有する場合も含む。また、Nは不可避的不純物である。
(各組織の面積率)
マルテンサイト、ベイナイトの面積率とは、観察面積に占める各組織の面積の割合のことである。
得られた熱延鋼板よりサンプルを切り出し、圧延方向に平行な板厚断面を研磨後、3%ナイタールで腐食し、板厚1/4位置をSEM(走査型電子顕微鏡)により1500倍の倍率でそれぞれ3視野撮影した。得られた2次電子像の画像データからMedia Cybernetics社製のImage-Proを用いて各組織の面積率を求め、視野の平均面積率を各組織の面積率とした。
上記組織観察に用いた同サンプルの同視野について、後方電子散乱回折法(EBSD)によりベイナイトおよびマルテンサイトの結晶方位を求め、15゜以上の方位差の境界を表示する。これにより、ベイナイト中に分散されたマルテンサイトのうち、該マルテンサイトと、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトとの結晶方位差が15゜未満のマルテンサイトの面積率を求めた。該マルテンサイトの面積率が全マルテンサイトの面積率に占める割合を求めた。なお、EBSDの測定は、加速電圧30kV、ステップサイズ0.05μmで100μm×100μmの領域について行った。
得られた上記割合を表3に示す。なお、表3中の「隣接Bとの方位差が15°未満のMの割合」は上記割合(%)を意味する。
引張特性の評価は、引張試験により行った。得られた熱延鋼板より、圧延方向に対して平行方向にJIS5号引張試験片(JIS Z 2201)を採取し、歪速度が10-3/sとするJIS Z 2241の規定に準拠した引張試験を行い、TSおよび一様伸びを求めた。
なお、本発明では、TSは980MPa以上を、一様伸びは5.0%以上を、それぞれ合格と評価した。
耐端面割れ特性の評価は、打ち抜き試験により行った。得られた熱延鋼板より、幅が150mm、長さが150mmの試験片を採取した。試験片に対して、クリアランスが5%、10%、15%、20%、25%、30%となる条件で、Φ10mmのポンチを用いて打ち抜きを3回行い、打ち抜き端面の板面(板表面)と平行な亀裂の有無を調査し、耐端面割れ性を評価した。亀裂が生じないクリアランス範囲が10%以上となった場合を、耐端面割れ性が合格と評価した。
例えば、上述の方法で行った打ち抜き試験において、亀裂を生じなかったクリアランスが10%、15%、20%、25%となった場合、亀裂を生じないクリアランス範囲は、亀裂が生じなかった最大のクリアランスである25%から最小のクリアランスである10%の差をとり、15%となる。
穴広げ性は穴広げ試験により評価した。上記打ち抜き試験においてクリアランスが10%となる条件で打ち抜いた試験片3枚を用いて、JFST 1001(日本鉄鋼連盟規格、2008年)に準じて60゜円錐ポンチを用いて穴広げ試験を3回行って平均の穴拡げ率(%)を求め、穴広げ率とした。穴広げ率が40%以上を合格と評価した。
Claims (3)
- 質量%で、
C:0.04~0.18%、
Si:0.1~3.0%、
Mn:0.5~3.5%、
P:0%超0.100%以下、
S:0%超0.020%以下、
Al:0%超1.5%以下を含み、
さらに、Cr:0.005~2.0%、Ti:0.005~0.20%、Nb:0.005~0.20%、Mo:0.005~2.0%、V:0.005~1.0%のうちから選ばれる1種または2種以上を含み、
残部がFeおよび不可避的不純物からなる成分組成を有し、
鋼組織は、合計面積率で80~100%のマルテンサイトおよびベイナイトを主相とし、
ベイナイト中のマルテンサイトの全面積率が2~20%であり、
ベイナイト中のマルテンサイトのうち、該マルテンサイトの結晶方位と、該マルテンサイトに隣接するベイナイトのうち少なくとも1つのベイナイトの結晶方位との方位差が15°未満であるマルテンサイトの面積率が、全マルテンサイトに対して50%以上である高強度熱延鋼板。 - 前記成分組成に加えて、質量%で、
Cu:0.05~4.0%、
Ni:0.005~2.0%、
B:0.0002~0.0050%、
Ca:0.0001~0.0050%、
REM:0.0001~0.0050%、
Sb:0.0010~0.10%、
Sn:0.0010~0.50%
のうちから選ばれる1種または2種以上を含む請求項1に記載の高強度熱延鋼板。 - 請求項1または2に記載の高強度熱延鋼板の製造方法であって、
前記成分組成を有するスラブを加熱し、
次いで、熱間圧延を施すに際し、
1100℃以上で3パス以上かつ1パスあたり15%以上の圧下率で粗圧延し、1000℃以下での合計圧下率が50%以上、1000℃以下での合計パス数が3回以上となる条件で仕上げ圧延した後、1.0s以上放冷し、その後、冷却開始温度から550℃までの平均冷却速度が50℃/s以上となる条件で冷却し、その後、(Ms点-50)℃~550℃の巻取り温度で巻き取る高強度熱延鋼板の製造方法。
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WO2021090642A1 (ja) * | 2019-11-06 | 2021-05-14 | 日本製鉄株式会社 | 熱延鋼板およびその製造方法 |
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Patent Citations (6)
Publication number | Priority date | Publication date | Assignee | Title |
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WO2008123366A1 (ja) | 2007-03-27 | 2008-10-16 | Nippon Steel Corporation | はがれの発生が無く表面性状及びバーリング性に優れる高強度熱延鋼板及びその製造方法 |
JP2012062562A (ja) | 2010-09-17 | 2012-03-29 | Jfe Steel Corp | 打抜き加工性に優れた高強度熱延鋼板およびその製造方法 |
JP2014205890A (ja) * | 2013-04-15 | 2014-10-30 | Jfeスチール株式会社 | 穴拡げ加工性に優れた高強度熱延鋼板およびその製造方法 |
JP2016204690A (ja) | 2015-04-17 | 2016-12-08 | 新日鐵住金株式会社 | 延性と疲労特性と耐食性に優れた高強度熱延鋼板とその製造方法 |
WO2017017933A1 (ja) * | 2015-07-27 | 2017-02-02 | Jfeスチール株式会社 | 高強度熱延鋼板およびその製造方法 |
WO2021090642A1 (ja) * | 2019-11-06 | 2021-05-14 | 日本製鉄株式会社 | 熱延鋼板およびその製造方法 |
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CN117295836A (zh) | 2023-12-26 |
MX2023013343A (es) | 2023-11-27 |
KR20230167426A (ko) | 2023-12-08 |
EP4321645A1 (en) | 2024-02-14 |
JP7239072B1 (ja) | 2023-03-14 |
JPWO2022244706A1 (ja) | 2022-11-24 |
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