WO2021167079A1 - 熱延鋼板 - Google Patents
熱延鋼板 Download PDFInfo
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- WO2021167079A1 WO2021167079A1 PCT/JP2021/006417 JP2021006417W WO2021167079A1 WO 2021167079 A1 WO2021167079 A1 WO 2021167079A1 JP 2021006417 W JP2021006417 W JP 2021006417W WO 2021167079 A1 WO2021167079 A1 WO 2021167079A1
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- steel sheet
- hot
- less
- rolling
- bending
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 128
- 239000010959 steel Substances 0.000 title claims abstract description 128
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 44
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 41
- 239000013078 crystal Substances 0.000 claims abstract description 35
- 230000000717 retained effect Effects 0.000 claims abstract description 34
- 239000002344 surface layer Substances 0.000 claims abstract description 28
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 26
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 19
- 239000000126 substance Substances 0.000 claims abstract description 16
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 15
- 239000000203 mixture Substances 0.000 claims abstract description 14
- 239000010410 layer Substances 0.000 claims description 8
- 239000012535 impurity Substances 0.000 claims description 4
- 238000005096 rolling process Methods 0.000 description 118
- 238000005452 bending Methods 0.000 description 75
- 230000009467 reduction Effects 0.000 description 42
- 238000000034 method Methods 0.000 description 29
- 230000000694 effects Effects 0.000 description 25
- 239000010936 titanium Substances 0.000 description 23
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 22
- 238000001816 cooling Methods 0.000 description 21
- 238000010438 heat treatment Methods 0.000 description 19
- 238000005246 galvanizing Methods 0.000 description 14
- 238000005098 hot rolling Methods 0.000 description 14
- 239000010955 niobium Substances 0.000 description 14
- 238000001953 recrystallisation Methods 0.000 description 14
- 238000007747 plating Methods 0.000 description 13
- 229910000859 α-Fe Inorganic materials 0.000 description 13
- 238000004519 manufacturing process Methods 0.000 description 12
- 230000008569 process Effects 0.000 description 11
- 238000005336 cracking Methods 0.000 description 10
- 229910052742 iron Inorganic materials 0.000 description 10
- 150000001247 metal acetylides Chemical class 0.000 description 10
- 229910001562 pearlite Inorganic materials 0.000 description 10
- 230000015572 biosynthetic process Effects 0.000 description 9
- 230000007797 corrosion Effects 0.000 description 9
- 238000005260 corrosion Methods 0.000 description 9
- 229910052761 rare earth metal Inorganic materials 0.000 description 9
- 229910001335 Galvanized steel Inorganic materials 0.000 description 8
- 239000008397 galvanized steel Substances 0.000 description 8
- 230000006872 improvement Effects 0.000 description 8
- 238000004804 winding Methods 0.000 description 8
- 229910052758 niobium Inorganic materials 0.000 description 7
- 238000012545 processing Methods 0.000 description 7
- 229910052719 titanium Inorganic materials 0.000 description 7
- 229910052804 chromium Inorganic materials 0.000 description 6
- 229910052750 molybdenum Inorganic materials 0.000 description 6
- 238000005554 pickling Methods 0.000 description 6
- 229920006395 saturated elastomer Polymers 0.000 description 6
- 239000006104 solid solution Substances 0.000 description 6
- 230000009466 transformation Effects 0.000 description 6
- 229910052802 copper Inorganic materials 0.000 description 5
- 230000007423 decrease Effects 0.000 description 5
- 238000005259 measurement Methods 0.000 description 5
- 229910052759 nickel Inorganic materials 0.000 description 5
- 238000012360 testing method Methods 0.000 description 5
- 238000005275 alloying Methods 0.000 description 4
- 238000004458 analytical method Methods 0.000 description 4
- 238000005266 casting Methods 0.000 description 4
- 238000005315 distribution function Methods 0.000 description 4
- 229910052748 manganese Inorganic materials 0.000 description 4
- 229910052698 phosphorus Inorganic materials 0.000 description 4
- 238000009864 tensile test Methods 0.000 description 4
- 229910052720 vanadium Inorganic materials 0.000 description 4
- 239000011701 zinc Substances 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 3
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 3
- -1 baynite Inorganic materials 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 230000007547 defect Effects 0.000 description 3
- 230000006866 deterioration Effects 0.000 description 3
- 238000000445 field-emission scanning electron microscopy Methods 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 229910052757 nitrogen Inorganic materials 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 238000005498 polishing Methods 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 239000000047 product Substances 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- 229910052725 zinc Inorganic materials 0.000 description 3
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 238000005530 etching Methods 0.000 description 2
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 2
- 229910052747 lanthanoid Inorganic materials 0.000 description 2
- 150000002602 lanthanoids Chemical class 0.000 description 2
- 230000007246 mechanism Effects 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 230000001629 suppression Effects 0.000 description 2
- 238000012935 Averaging Methods 0.000 description 1
- 229910001122 Mischmetal Inorganic materials 0.000 description 1
- 241000612118 Samolus valerandi Species 0.000 description 1
- 238000003723 Smelting Methods 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000001186 cumulative effect Effects 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000001887 electron backscatter diffraction Methods 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 239000005431 greenhouse gas Substances 0.000 description 1
- 239000003112 inhibitor Substances 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 238000011946 reduction process Methods 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 238000004904 shortening Methods 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 238000005482 strain hardening Methods 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 1
- 238000010792 warming Methods 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
Images
Classifications
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C—ALLOYS
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- 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
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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- 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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- 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
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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- C21D7/00—Modifying the physical properties of iron or steel by deformation
- C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
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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
- C21D8/0242—Flattening; Dressing; Flexing
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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
Definitions
- the present invention relates to a hot-rolled steel sheet.
- the present application claims priority based on Japanese Patent Application No. 2020-026996 filed in Japan on February 20, 2020, the contents of which are incorporated herein by reference.
- Non-Patent Document 1 reports that bending workability is improved by controlling a single structure such as ferrite, bainite, and martensite by structure control.
- Patent Document 1 describes in terms of mass%, C: 0.010 to 0.055%, Si: 0.2% or less, Mn: 0.7% or less, P: 0.025% or less, S: 0.02. % Or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.06 to 0.095%, and the area ratio is controlled to 95% or more of ferrite.
- Patent Document 2 describes in terms of mass%, C: 0.05 to 0.15%, Si: 0.2 to 1.2%, Mn: 1.0 to 2.0%, P: 0.04% or less. , S: 0.0030% or less, Al: 0.005 to 0.10%, N: 0.01% or less and Ti: 0.03 to 0.13%.
- the phase or bainite is controlled to have a fraction of more than 95%, and the structure of the surface layer of the steel plate has a bainite phase fraction of less than 80% and a highly processable ferrite fraction of 10% or more. By doing so, a method of improving the bending workability while maintaining the tensile strength of 780 MPa or more is disclosed.
- Patent Document 3 describes in terms of mass%, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 1.5%, P: 0.025. % Or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.1 to It contains 1.0%, Cr: 0.1 to 1.0%, the tempered martensite phase is the main phase of 90% or more in volume ratio, and the average particle size of the former austenite grains in the cross section parallel to the rolling direction. Is 20 ⁇ m or less and the average particle size of the old austenite grains in the cross section orthogonal to the rolling direction is 15 ⁇ m or less. It is disclosed that a high-strength hot-rolled steel sheet having excellent bending workability and excellent low-temperature toughness can be obtained.
- Patent Document 4 the extreme density of each direction of a specific crystal orientation group in the central portion of the plate thickness, which is in the plate thickness range of 5/8 to 3/8 from the surface of the steel plate, is controlled, and the direction is perpendicular to the rolling direction.
- the rankford value of rC is 0.70 or more and 1.10 or less
- the rankford value of r30 in the direction forming 30 ° with respect to the rolling direction is 0.70 or more and 1.10 or less. It is disclosed that a hot-rolled steel sheet having excellent local deformability and small bending workability anisotropy can be obtained.
- Patent Documents 1 to 4 are sufficiently compatible with strength, bendability and elongation.
- An object to be solved by the present invention is to provide a high-strength hot-rolled steel sheet having excellent bending workability and elongation and a tensile strength of 980 MPa or more.
- the above-mentioned bending workability is an index indicating that cracks are unlikely to occur in the processed portion during bending processing, or an index indicating that the cracks are unlikely to grow.
- the present invention targets cracks (internal bending cracks) generated from the inside of the bent portion during bending.
- the microstructure contains 70% or more of martensite, tempered martensite and bainite in total and 5 to 20% of retained austenite in terms of volume fraction, thereby ensuring workability and tensile strength. It was found that a steel sheet having a volume of 980 MPa or more can be produced.
- the present inventors presumed that the mechanism of internal bending cracking is due to the bias of deformation, focused on the texture and uniformity of hardness, and searched for a method for suppressing internal bending cracking.
- the texture is relatively random, the deformation resistance is also uniform, so that the deformation tends to occur uniformly.
- a specific texture develops, a crystal having a large deformation resistance and a crystal in other directions It was found that the deformation is biased between the two, and the shear band is likely to be generated.
- the present inventors have found that in-bending cracking can be suppressed by controlling the texture in the surface layer region in the plate thickness direction in which cracks occur.
- the hot-rolled steel sheet according to one aspect of the present invention has a mass% of C: 0.02 to 0.30%, Si: 0.01 to 2.50%, and Mn: 1.00 to 3.00. %, P: 0.100% or less, S: 0.0001 to 0.0100%, Al: 0.005 to 1.000%, N: 0.010% or less, Ti: 0 to 0.20%, Nb : 0 to 0.20%, V: 0 to 0.200%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 2.
- the balance has a chemical composition of Fe and impurities, the chemical composition satisfies Si + Al ⁇ 1.00%, and the microstructure is a total of martensite, tempered martensite and bainite by volume. It consists of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> in the surface layer region, which contains 70% or more, contains 5 to 20% of retained austenite, and ranges from the surface to the position of 1/10 of the plate thickness.
- the hot-rolled steel sheet according to (1) above has a chemical composition of Ti: 0.001 to 0.20%, Nb: 0.001 to 0.20%, V: 0.
- the hot-rolled steel sheet according to (1) or (2) above may be provided with a hot-dip galvanized layer on the surface.
- the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.
- Microstructure ⁇ In volume fraction, it contains 70% or more of martensite, tempered martensite and bainite, and 5 to 20% of retained austenite>
- the main phase of the microstructure is one or more selected from martensite, tempered martensite and bainite having a volume fraction of 70% or more.
- the microstructure also contains 5-20% retained austenite.
- the steel sheet according to the present embodiment contains at least one selected from martensite, tempered martensite, and bainite, which are low-temperature transformation-forming phases, in order to achieve both tensile strength (TS) of 980 MPa or more and bending workability. And.
- TS tensile strength
- the steel sheet according to the present embodiment contains 5% or more of retained austenite in terms of volume fraction in order to obtain excellent elongation. If the volume fraction of retained austenite is less than 5%, sufficient elongation cannot be obtained.
- the upper limit of the volume fraction of substantially retained austenite is 20%.
- the balance other than the above may be one or more of ferrite and pearlite.
- a sample is taken with the cross section in the plate thickness direction parallel to the rolling direction of the hot-rolled steel plate as the observation surface, and the observation surface is polished.
- Nital etching 1/8 to 3/8 (1/8 to 3/8 thickness) of the plate thickness from the surface centered on the depth (1/4 thickness) position of 1/4 of the plate thickness from the surface ) Is observed at a magnification of 5000 times using a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), the area ratio of each tissue is measured, and the volume is taken with it. Let it be a rate. At that time, 10 fields of view are measured, and the average value is taken as the volume fraction.
- FE-SEM Field Emission Scanning Electron Microscope
- each tissue has the following characteristics. Therefore, in the measurement of the area ratio, each tissue is identified based on the following characteristics, and the area ratio is obtained.
- Ferrite is an equiaxed grain that does not contain iron-based carbides
- pearlite is a layered structure of ferrite and cementite.
- Bainite contains an upper bainite and a lower bainite, and the upper bainite is an aggregate of lath-shaped crystal grains and is an aggregate of laths containing carbides between laths.
- the lower bainite is a collection of lath-shaped crystal grains, and contains iron-based carbides having a major axis of 5 nm or more inside, and the carbides belong to a single variant, that is, a group of iron-based carbides extended in the same direction.
- Tempering martensite is a collection of lath-shaped crystal grains, and contains iron-based carbides with a major axis of 5 nm or more inside, and the carbides are further transformed into a plurality of variants, that is, iron-based carbides extended in two or more directions. It belongs to.
- tempered martensite often refers to those containing iron-based carbides such as cementite, but in the present embodiment, martensite containing fine precipitates containing Ti is also defined as tempered martensite.
- Martensite fresh martensite
- retained austenite are not sufficiently corroded by nightal etching and should be clearly distinguished from the above-mentioned structures (ferrite, pearlite, bainite, tempered martensite) when observed by FE-SEM. Can be done. Therefore, the volume fraction of martensite is obtained as the difference between the volume fraction obtained as the area fraction of the uncorroded region observed by FE-SEM and the volume fraction of retained austenite measured by X-ray described later. Can be done.
- the volume fraction of retained austenite is determined by X-ray diffraction. Specifically, ⁇ (110), ⁇ (200), ⁇ ( The integrated intensity of a total of 6 peaks of 211), ⁇ (111), ⁇ (200), and ⁇ (220) is obtained, and the volume ratio of retained austenite is obtained by calculating using the intensity averaging method.
- the total volume fraction of martensite, tempered martensite and bainite may be specified, and it is not essential to distinguish these structures.
- ⁇ Concentration of solid solution carbon in retained austenite is 0.5% by mass or more>
- concentration of solute carbon in the retained austenite is set to 0.5% by mass or more.
- concentration of dissolved carbon in the retained austenite is preferably 0.7% by mass or more.
- the concentration of dissolved carbon in the retained austenite is preferably 2.0% by mass or less.
- the concentration of solute carbon in the retained austenite is determined by X-ray diffraction. Specifically, X-ray analysis with Cu—K ⁇ rays is performed on the metal structure at the center position in the plate width direction and in the cross section in the plate thickness direction parallel to the rolling direction at a depth of 1/4 of the plate thickness from the steel plate surface.
- the average polar density of the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> and the crystal of ⁇ 110 ⁇ ⁇ 001>.
- the sum with the extreme density of the orientation is 6.0 or less>.
- the present inventors diligently investigated the bending workability of high-strength steel sheets. As a result, it was found that minute cracks may occur inside the bend as the strength of the steel sheet increases. As a result of further studies, the mechanism of such bending internal cracking is presumed as follows. During bending, compressive stress is generated inside the bend.
- the strain increases toward the surface with the center of the plate thickness as the boundary, and the strain becomes maximum at the outermost surface. Therefore, cracks in bending internal cracks are generated on the surface of the steel sheet. Since it is the structure of the surface layer region that contributes to the formation of such cracks in the range from the surface of the steel plate to 1/10 of the plate thickness in the plate thickness direction, the structure of the surface layer region is controlled.
- the present inventors focused on the texture in order to suppress the bias of deformation that causes internal cracking during bending. Specifically, when the steel sheet is deformed, the easiness of the slip system to work with respect to the deformation differs in each crystal orientation (Schmid factor). It is considered that this is because the deformation resistance differs depending on the crystal orientation. That is, if the texture is relatively random, the deformation resistance is also uniform, so that the deformation tends to occur uniformly. However, when a specific texture develops, the crystal having a large deformation resistance and the crystal having other directions Deformation bias occurs between them, and a shear deformation zone is likely to occur. On the contrary, if the number of crystals in the direction in which the deformation resistance is large is reduced, it is considered that the deformation occurs uniformly and the shear deformation zone is less likely to occur.
- the steel sheet according to the present embodiment has an orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> in the surface layer region in the range from the surface to the position of 1/10 of the plate thickness.
- the sum of the average polar density and the polar density of the crystal orientation of ⁇ 110 ⁇ ⁇ 001> is 6.0 or less.
- the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> and the crystal orientation of ⁇ 110 ⁇ ⁇ 001> are orientations that easily develop in the surface layer region of the high-strength hot-rolled steel sheet produced by a conventional method. Further, since these are orientation groups in which the deformation resistance is particularly large inside the bending during bending, a shear deformation zone is likely to occur due to the difference in deformation resistance from other orientation groups. Therefore, it is possible to suppress internal bending cracking by reducing the extreme density of these orientation groups.
- the sum of the average polar density of the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> and the extreme density of the crystal orientation of ⁇ 110 ⁇ ⁇ 001> is 5.0 or less, and further. It is preferably 4.0 or less.
- the smaller the sum of the average pole density of the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> and the pole density of the crystal orientation of ⁇ 110 ⁇ ⁇ 001> is preferable, but high-intensity hot rolling of 980 MPa or more is preferable. For steel sheets, it is difficult to make it less than 0.5, so 0.5 is a practical lower limit.
- the extreme density can be measured by the EBSP (Electron Backscatter Diffraction Pattern) method.
- the cut surface parallel to the rolling direction and perpendicular to the plate surface is mechanically polished, and after the mechanical polishing, strain is removed by chemical polishing or electrolytic polishing.
- the measurement interval is set to 4.0 ⁇ m in the range from the surface of the steel sheet to the position of 1/10 of the plate thickness, and the analysis is performed by the EBSP method so that the measurement area is 150,000 ⁇ m 2 or more.
- the average polar density of this azimuth group is calculated in the above range shown in FIG.
- ODF crystal orientation distribution function
- the lattice plane parallel to the plate surface is usually indicated by (hkl) or ⁇ hkl ⁇
- the orientation parallel to the rolling direction is indicated by [uvw] or ⁇ uvw>.
- ⁇ Hkl ⁇ and ⁇ uvw> are generic terms for equivalent grid planes and directions, and (uvw) and [hkl] refer to individual grid planes and directions. That is, since the steel sheet according to the present embodiment targets the bcc structure, for example, (110), (-110), (1-10), (-1-10), (101), (-101). , (10-1), (-10-1), (011), (0-11), (01-1), (0-1-1) are equivalent lattice planes and are indistinguishable. .. In such a case, these lattice planes are collectively referred to as ⁇ 110 ⁇ .
- C (C: 0.02 to 0.30%) C is an element effective for increasing the strength of the steel sheet. If the C content is less than 0.02%, it is difficult to secure a strength of 980 MPa or more. Therefore, the C content is set to 0.02% or more.
- the C content is preferably 0.13% or more, and more preferably 0.15% or more.
- the C content exceeds 0.30%, not only the effect is saturated, but also pearlite is preferentially produced to insufficiently produce bainite and retained austenite, so that the desired volume fraction and residual of bainite are produced. It becomes difficult to obtain the volume fraction of austenite. Therefore, the C content is 0.30% or less.
- the C content is preferably 0.25% or less.
- Si is an important element whose material strength can be increased by solid solution strengthening. If the Si content is less than 0.01%, the strength decreases. Therefore, the Si content is set to 0.01% or more.
- the Si content is preferably 0.10% or more, more preferably 0.30% or more. On the other hand, if the Si content exceeds 2.50%, the surface texture deteriorates. Therefore, the Si content is set to 2.50% or less.
- the Si content is preferably 2.00% or less.
- Mn is an element effective for increasing the volume fraction of bainite and martensite in the microstructure of the steel sheet and increasing the strength of the steel sheet.
- the Mn content is set to 1.00% or more. If the Mn content is less than 1.00%, the volume fraction of these tissues decreases, and sufficient strength cannot be obtained. On the other hand, if the Mn content exceeds 3.00%, the effect is saturated and the economic efficiency is lowered. Therefore, the Mn content is set to 3.00% or less.
- P is an element that segregates in the central portion of the thickness of the steel sheet and is also an element that embrittles the welded portion. It is preferable that the P content is low, but if the P content exceeds 0.100%, the deterioration of the characteristics becomes remarkable, so the P content is limited to 0.100% or less.
- the P content is preferably 0.050% or less.
- the lower limit is not particularly set and the effect is exhibited (0% may be used), reducing the P content to less than 0.001% is economically disadvantageous. Therefore, the P content may be 0.001% or more.
- S is an element that causes embrittlement of the slab by being present as a sulfide. Further, S is an element that deteriorates the moldability of the steel sheet. Therefore, the S content is limited. If the S content exceeds 0.010%, the deterioration of the characteristics becomes remarkable, so the S content is set to 0.010% or less. On the other hand, although the lower limit is not particularly set and the effect is exhibited (0% may be used), reducing the S content to less than 0.0001% is economically disadvantageous, so the S content is set to 0. It shall be 0001% or more.
- N is an element that forms a coarse nitride and deteriorates bendability and elongation.
- the N content exceeds 0.010%, the bending workability and elongation are significantly deteriorated. Therefore, the N content is set to 0.010% or less.
- the lower limit of the N content does not need to be set in particular (may be 0%), but if the N content is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, from the viewpoint of manufacturing cost, the N content may be 0.0001% or more, or 0.0005% or more.
- Al 0.005 to 1.000%)
- Al is an element effective for tissue control and deoxidation in hot rolling.
- the Al content is set to 0.005% or more. If the Al content is less than 0.005%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions serve as a starting point for cracking during bending and stretch flange processing, and deteriorate workability.
- the Al content exceeds 1.000%, the slab becomes embrittled, which is not preferable. Therefore, the Al content is set to 1.000% or less.
- the Al content is determined by sol. It means the Al (acid-soluble Al) content. Further, in order to secure the area ratio of retained austenite, the total content (Si + Al) of Si and Al is set to 1.00% or more.
- the chemical composition of the steel sheet according to the present embodiment may contain the above elements and the balance may be Fe and impurities.
- the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., and are allowed as long as they do not adversely affect the steel sheet according to the present embodiment.
- the steel sheet according to the present embodiment can further contain the following components for the purpose of improving various properties. Since the following elements do not necessarily have to be contained, the lower limit of the content is 0%.
- Ti titanium
- Nb niobium
- V vanadium
- Ti, Nb and / or V may be contained.
- the content is preferably 0.001% or more.
- the Ti content is more preferably 0.02% or more, and the Nb content is more preferably 0.01% or more.
- Ti and Nb are contained in an amount of more than 0.20% and V is contained in an amount of more than 0.200%, the effect of the above action may not only be saturated but also economically disadvantageous.
- the Ti content is 0.20% or less
- the Nb content is 0.20% or less
- the V content is 0.200% or less.
- the Ti content and Nb content are preferably 0.15% or less, more preferably 0.10% or less
- the V content is preferably 0.150% or less, more preferably 0.100% or less. be.
- Ni, Cu, Cr, and Mo are elements that contribute to increasing the strength of steel sheets through structure control by hot rolling. This effect becomes remarkable when one or more of Ni, Cu, Cr, and Mo are contained in an amount of 0.01% or more, respectively. Therefore, when the effect is obtained, the content is preferably 0.01% or more. On the other hand, if the content of each element exceeds 2.00%, weldability, hot workability and the like deteriorate. Therefore, when it is contained, the contents of Ni, Cu, Cr, and Mo are each set to 2.00% or less.
- W is an element that contributes to the improvement of the strength of the steel sheet through precipitation strengthening.
- the W content is preferably 0.005% or more.
- the W content is set to 0.100% or less.
- B is an element effective for controlling the transformation by hot rolling and improving the strength of the steel sheet through the strengthening of the structure.
- the B content is preferably 0.0005% or more.
- the B content exceeds 0.0100%, not only the effect is saturated, but also iron-based boride is precipitated, and the effect of improving the hardenability by the solid solution B is lost. Therefore, when it is contained, the B content is set to 0.0100% or less.
- the B content is preferably 0.0080% or less, more preferably 0.0050% or less.
- REM, Ca, and Mg are elements that contribute to improving the strength of the steel sheet. If the total of one or more of REM, Ca, and Mg is less than 0.0003%, a sufficient effect cannot be obtained. Therefore, when the effect is obtained, the total content of REM, Ca, and Mg is set to 0. It is preferably 0003% or more. On the other hand, if REM, Ca and Mg each exceed 0.0300%, castability and hot workability deteriorate. Therefore, when it is contained, the content of each is set to 0.0300% or less.
- the REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of the REM refers to the total content of these elements.
- lanthanoids they are industrially added in the form of misch metal.
- the above steel composition may be measured by a general analysis method for steel.
- the steel component may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
- C and S may be measured using the combustion-infrared absorption method
- N may be measured using the inert gas melting-thermal conductivity method
- O may be measured using the inert gas melting-non-dispersion infrared absorption method.
- the steel sheet according to the present embodiment may further have a hot-dip galvanized layer on its surface.
- the hot-dip galvanizing may be an alloyed hot-dip galvanizing layer that has been alloyed. Since galvanizing contributes to the improvement of corrosion resistance, it is desirable to use hot-dip galvanized steel sheet with zinc plating or alloyed hot-dip galvanized steel sheet when applied to applications where corrosion resistance is expected. Since there is a concern that the undercarriage parts of an automobile may be perforated due to corrosion, it may not be possible to thin the undercarriage parts below a certain thickness even if the strength is increased. One of the purposes of increasing the strength of a steel sheet is to reduce the weight by making it thinner.
- plating such as hot dip galvanizing, which has high corrosion resistance, to the steel sheet. Since the steel sheet component according to the present embodiment is controlled as described above, hot dip galvanizing is possible.
- the plating may be electrozinc plating, or may be plating containing Si, Al and / or Mg in addition to Zn.
- the steel sheet according to this embodiment has a tensile strength (TS) of 980 MPa or more as a sufficient strength that contributes to weight reduction of automobiles. It is preferably 1180 MPa or more.
- the upper limit of the tensile strength does not need to be set in particular, but in the present embodiment, the upper limit of the substantial tensile strength may be 1370 MPa.
- the steel sheet according to the present embodiment aims to have a limit bending R / t value of 1.5 or less, which is an index value of in-bending crackability.
- the R / t values are, for example, bending (L-axis bending) in which a strip-shaped test piece is cut out from the width direction 1/2 position of the hot-rolled steel sheet and the bending ridge line is parallel to the rolling direction (L direction).
- bending is performed in accordance with JIS Z2248: 2006 (V block 90 ° bending test), and the bending inside is performed. It can be found by investigating the cracks that occur in.
- the minimum bending radius at which cracks do not occur can be obtained, and the value obtained by dividing the average value of the minimum bending radii of the L-axis and the C-axis by the plate thickness can be used as the index value of bending workability as the limit bending R / t.
- the steel sheet according to the present embodiment aims to have a product of tensile strength TS (MPa) and EL (%) of 19000 (MPa ⁇ %) or more as an index of having high elongation.
- the product of TS and EL is preferably 19120 (MPa ⁇ %) or more, more preferably 19600 (MPa ⁇ %) or more.
- the steel sheet according to the present embodiment has a total elongation EL of 16.0% or more.
- the manufacturing process prior to hot rolling is not particularly limited. That is, following the melting in a blast furnace or an electric furnace, various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting or casting by an ingot method. In the case of continuous casting, the cast slab may be cooled to a low temperature and then heated again and then hot-rolled, or the cast slab may be hot-rolled as it is after casting without being cooled to a low temperature. .. Scrap may be used as the raw material.
- the slab having the above-mentioned chemical composition to be used in the rough rolling step is heated to 1200 ° C. or higher.
- Coarse precipitates deposited in the slab can impair material stability. Therefore, the slab is heated to 1200 ° C. or higher for the purpose of dissolving such precipitates.
- the heated slab is roughly rolled to obtain a rough-rolled plate.
- the thickness of the rough rolled plate after rough rolling is controlled to be more than 35 mm and 45 mm or less.
- the thickness of the rough-rolled plate affects the amount of temperature decrease from the tip to the tail of the rolled plate that occurs from the start of rolling to the completion of rolling in the finish rolling process.
- the thickness of the rough-rolled plate is 35 mm or less or more than 45 mm, the amount of strain introduced into the steel sheet during the finish rolling, which is the next process, changes, and the processed structure formed during the finish rolling changes. do. As a result, the recrystallization behavior changes, making it difficult to obtain a desired texture.
- the thickness of the rough-rolled plate after rough-rolling is appropriately set from the viewpoint of productivity and the like, and is rarely set for controlling the characteristics of the steel plate.
- the present inventors strictly control the thickness of the rough-rolled plate in order to control the texture of the surface layer region of the steel sheet.
- ⁇ Finish rolling process> Following rough rolling, multi-step finish rolling is performed.
- the present inventors have determined the plate thickness, roll shape ratio, temperature, Nb and Ti in steel in the final two-stage rolling of the finish rolling process of hot rolling, which has not usually been positively controlled. It was found that controlling the content to an appropriate range derived by a certain formula is important for controlling the texture. Therefore, in this multi-step finish rolling, the start temperature of the finish rolling is 1000 ° C. or higher and 1150 ° C. or lower, and the thickness of the steel plate (thickness of the rough-rolled plate) before the start of the finish rolling is more than 35 mm and 45 mm or less.
- the rolling temperature is 960 ° C. or higher and 1020 ° C. or lower, and the rolling reduction ratio is more than 11.0% and 23.0% or lower.
- the rolling temperature is preferably 930 ° C. or higher and 995 ° C. or lower, and the rolling reduction ratio is preferably more than 11.0% and 22.0% or lower.
- each condition at the time of reduction in the final two stages is controlled, and the texture formation parameter ⁇ calculated by the following equation 1 satisfies 110 or less.
- the numbers 1 and 2 added to the variables such as F 1 and F 2 are the rolling one step before the final step in the final two-step rolling in the multi-step finish rolling. 1 is added to the variable related to rolling, and 2 is added to the variable related to rolling in the final stage.
- F 1 means the rolling reduction of the sixth step counting from the rolling inlet side
- F 2 means the rolling reduction of the seventh step.
- Equation 1 indicates preferable manufacturing conditions in finish rolling in which the rolling temperature FT 2 of the final stage is 930 ° C. or higher, and when FT 2 is less than 930 ° C., it means the value of the texture formation parameter ⁇ . Do not do. That is, FT 2 is 930 ° C. or higher, and ⁇ is 110 or lower.
- the starting temperature of finish rolling is 1000 ° C or higher and 1150 ° C or lower. If the start temperature of the finish rolling is less than 1000 ° C., the structure processed by the rolling in the previous stage is not sufficiently recrystallized except for the final two stages, and the texture of the surface layer region of the steel sheet develops, and the L-axis And R / t, which is the average value / plate thickness of the minimum bending radius of the C-axis, cannot satisfy 1.5 or less. Therefore, the start temperature of finish rolling is preferably 1000 ° C. or higher. More preferably, it is 1050 ° C. or higher. On the other hand, when the start temperature of finish rolling exceeds 1150 ° C., the austenite grains become excessively coarse and the toughness deteriorates. Therefore, it is preferable that the start temperature of finish rolling is 1150 ° C. or lower.
- the finish rolling is performed under the condition that each condition at the time of rolling down the final two steps in the multi-step finish rolling is controlled and the texture formation parameter ⁇ calculated by Equation 1 is 110 or less).
- the hot rolling condition of the final two steps in the multi-step finish rolling is important.
- the rolling reduction rates F 1 and F 2 at the time of rolling in the final two stages used in the calculation of ⁇ defined in Equation 1 are the values obtained by dividing the difference in plate thickness before and after rolling in each stage by the plate thickness before rolling. It is a numerical value expressed as a percentage.
- the diameters D 1 and D 2 of the rolling rolls are measured at room temperature, and it is not necessary to consider the flatness during hot rolling.
- the plate thicknesses t 1 and t 2 on the rolling inlet side and the plate thickness t f after finish rolling may be measured on the spot using radiation or the like, and the deformation resistance and the like are taken into consideration from the rolling load. It may be calculated by calculation.
- the plate thickness t f after finish rolling may be the final plate thickness of the steel sheet after the completion of hot rolling.
- the rolling start temperature FT 1 and FT 2 the values measured by a thermometer such as a radiation thermometer between the finishing rolling stands may be used.
- the texture formation parameter ⁇ is an index considering the rolling strain introduced into the entire steel sheet in the final two stages of finish rolling, the shear strain introduced into the surface layer region of the steel sheet, and the recrystallization rate after rolling.
- the texture formation parameter ⁇ exceeds 110, the average polar density of the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> and ⁇ 110 in the surface layer region.
- ⁇ The sum of ⁇ 001> with the extreme density of the crystal orientation cannot be 6.0 or less. Therefore, it is preferable to control the texture formation parameter ⁇ to 110 or less. More preferably, the texture formation parameter ⁇ is 98 or less.
- Rolling temperature FT 1 one step before the final step is 960 ° C or higher and 1020 ° C or lower. If the rolling temperature FT 1 one step before the final step is less than 960 ° C., the recrystallization of the structure processed by rolling does not sufficiently occur, and the texture of the surface layer region cannot be controlled within the above range. Therefore, the rolling temperature FT 1 is set to 960 ° C. or higher. On the other hand, when the rolling temperature FT 1 exceeds 1020 ° C., the formed state of the processed structure and the recrystallization behavior change due to the coarsening of the austenite grains and the like, so that the texture of the surface layer region cannot be controlled within the above range. .. Therefore, the rolling temperature FT 1 is set to 1020 ° C. or lower.
- the reduction rate F 1 one step before the final step is more than 11.0% and 23.0% or less
- the reduction ratio F 1 one step before the final step is 11.0% or less
- the reduction rate F 1 is set to more than 11.0%.
- the reduction factor F 1 is more than 23.0%, the lattice defects in the crystal become excessive and the recrystallization behavior changes, so that the texture of the surface layer region cannot be controlled within the above range. Therefore, the reduction rate F 1 is set to 23.0% or less.
- Rolling temperature FT 2 in the final stage is 930 ° C or higher and 995 ° C or lower
- the rolling temperature FT 2 of the final stage is set to less than 930 ° C.
- the recrystallization rate of austenite is significantly reduced, and the average polar density of the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112> in the surface layer region.
- the sum of ⁇ 110 ⁇ ⁇ 001> and the extreme density of the crystal orientation cannot be 6.0 or less. Therefore, the rolling temperature FT 2 is set to 930 ° C. or higher.
- the rolling temperature FT 2 exceeds 995 ° C., the formed state of the processed structure and the recrystallization behavior change, so that the texture of the surface layer region cannot be controlled within the above range. Therefore, the rolling temperature FT 2 is set to 995 ° C. or lower.
- the reduction rate F 2 of the final stage is more than 11.0% and 22.0% or less
- Total reduction ratio F t of the final three stages (Total reduction ratio F t of the final three stages more than 35%) The total reduction ratio of F t of the final three stages it is larger in order to promote the recrystallization of the austenite.
- the total reduction rate F t of the final three stages is less than 35%, the recrystallization rate of austenite is significantly reduced, and in the surface layer region, the orientation group consisting of ⁇ 211 ⁇ ⁇ 111> to ⁇ 111 ⁇ ⁇ 112>
- the sum of the average pole density and the pole density of the crystal orientation of ⁇ 110 ⁇ ⁇ 001> cannot be 6.0 or less.
- the total rolling reduction F t of the last three stages is calculated by the following equation.
- F t (t 0 ⁇ t f ) / t 0 ⁇ 100
- t 0 is the plate thickness (unit: mm) at the start of rolling two steps before the final step.
- each of the above conditions is controlled simultaneously and inseparably. It is not necessary for each of the above-mentioned conditions to satisfy only one of the above-mentioned conditions, and when all of the above-mentioned conditions are satisfied at the same time, the texture of the surface layer region can be controlled within the above-mentioned range.
- a cooling process and a winding process are performed. Controlling the cooling rate after finish rolling and further performing the heat treatment under controlled conditions contribute to the control of hardness uniformity.
- ⁇ Cooling process> (Cooling from 800 ° C to 450 ° C at an average cooling rate of 60 ° C / sec or more)
- the hot-rolled steel sheet after finish rolling is cooled to the winding temperature described later so that the average cooling rate from 800 ° C. to 450 ° C. is 60 ° C./sec or more. This is to prevent excessive formation of ferrite and pearlite in the temperature range of 800 ° C. to 450 ° C.
- the cooling rate is not specified because transformation is unlikely to occur in the temperature range of 800 ° C or higher, but in general hot rolling equipment, the cooling zone is reached within a few seconds after the completion of finish rolling, so the holding time at 800 ° C or higher is realistic.
- cooling is performed at an average cooling rate of 60 ° C./sec or more. If the cooling stop temperature exceeds 450 ° C or the average cooling rate is less than 60 ° C / sec, ferrite, pearlite, etc. are generated during the cooling process, and martensite, tempered martensite, and bainite cannot be secured in total of 70% or more. Bending workability may not be compatible. Since there is almost no concern that ferrite or pearlite transformation will occur at a temperature of 450 ° C. or lower, it is not necessary to specify the cooling rate.
- the hot-rolled steel sheet after hot rolling may be wound into a coil. If the winding temperature exceeds 450 ° C., ferrite, pearlite, etc. are generated, and martensite, tempered martensite, and bainite may not be secured in total of 70% by volume or more. Therefore, the winding temperature is set to 450 ° C. or lower.
- the hot-rolled steel sheet after the cooling step or the winding step may be pickled.
- pickling it is possible to improve the plating property in the subsequent manufacturing process and improve the chemical conversion treatment property in the automobile manufacturing process.
- the scale peels off and is pushed in, which may cause a defect. Therefore, the hot-rolled steel sheet is first pickled before the light reduction described later.
- the pickling conditions are not particularly limited, but pickling is generally performed with hydrochloric acid or sulfuric acid containing an inhibitor.
- the light reduction step is not essential, reduction may be applied at a reduction rate of 20% or less in order to increase the strength by introducing dislocations.
- the reduction rate exceeds 20%, not only the effect is saturated, but also the recovery of the introduced dislocations becomes insufficient, resulting in a significant deterioration in elongation.
- the reduction ratio is preferably 20% or less.
- the reduction may be performed by 20% or less in one pass, or may be performed in a plurality of times so that the cumulative reduction rate is 20% or less.
- ⁇ Heat treatment process> (Hold for 10 seconds or more in the temperature range of 200 ° C or higher and lower than 450 ° C)
- the hot-rolled steel plate after the light reduction step is reheated to a temperature range of less than 200 to 450 ° C., and heat treatment is performed to keep the hot-rolled steel plate in this temperature range for 10 seconds or longer.
- the volume fraction of retained austenite in the microstructure can be increased to 5% or more, and the solid solution carbon concentration in the retained austenite can be increased to 0.5% by mass or more. If the heat treatment temperature is less than 200 ° C. or the holding time is less than 10 seconds, a sufficient austenite volume fraction or solid solution carbon concentration cannot be secured.
- the heat treatment temperature is 450 ° C. or higher, the strength is significantly reduced, and the tensile strength of 980 MPa or higher cannot be achieved.
- the upper limit of the holding time it may be determined in consideration of heat equalization and economic rationality according to the heating method. For example, when using a heat treatment facility that runs a steel plate, the practical upper limit is about 1000 seconds for the purpose of shortening the equipment occupancy time, but in the case of a box-type heating device, the temperature inside the coil becomes uniform. The heating may be carried out for several hours to several tens of hours as a sufficient time.
- the holding time means the time during which the steel sheet is in the temperature range of 200 ° C. or higher and lower than 450 ° C. after reheating, and if the steel sheet stays in this temperature range for a predetermined time, the temperature may change in the middle. Cooling after the heat treatment (after the temperature drops to less than 200 ° C.) is not particularly specified.
- a steel sheet according to this embodiment can be obtained by a manufacturing method including the above steps.
- the steel sheet according to the present embodiment is hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet for the purpose of improving corrosion resistance
- the zinc plating is preferably hot dip galvanizing.
- the conditions for hot-dip galvanizing are not particularly limited, and known conditions may be used.
- an alloyed hot-dip galvanized steel sheet can be manufactured. Since the alloyed hot-dip galvanized steel sheet can be provided with effects such as improvement of spot weldability and improvement of slidability during draw forming in addition to improvement of corrosion resistance, alloying may be carried out depending on the application.
- the hot-dip galvanizing treatment and the alloying hot-dip galvanizing treatment may be carried out after being cooled to room temperature once after the heat treatment at 200 ° C. or higher and lower than 450 ° C., or may be carried out without cooling. Even if Al plating, plating containing Mg, and electroplating are performed in addition to galvanization, the steel sheet according to this embodiment can be manufactured.
- the hot-rolled steel sheet according to the present invention will be described in more detail below with reference to an example.
- the following examples are examples of the hot-rolled steel sheet of the present invention, and the hot-rolled steel sheet of the present invention is not limited to the following aspects.
- the conditions in the examples described below are one-condition examples adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to these one-condition examples.
- various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
- the steels having the chemical components shown in Table 1 are cast, and after casting, they are heated as they are or once cooled to room temperature and then reheated to the temperature range of Table 2, and then at a temperature of 1100 ° C. or higher, as shown in Table 2.
- a rough-rolled plate was manufactured by rough-rolling a slab to a rough-rolled plate thickness.
- the obtained rough-rolled plate was subjected to multi-step finish rolling having a total of 7 steps.
- finish rolling is started from the rolling start temperature shown in Table 2, and the final three-step rolling is excluded from the start of rolling.
- Thickness Rolled to a thickness of t 0.
- the final two-stage hot rolling was performed, and then cooling and winding were performed.
- the final plate thickness of the steel sheet after the completion of hot rolling was defined as the plate thickness t f after finish rolling.
- hot-dip galvanizing (GI) or alloyed hot-dip galvanizing (GA) was carried out for a part as shown in Table 4.
- the plating bath temperature was 445 ° C., and the alloying was held at 445 ° C. for 10 seconds.
- the sum of the average pole density of the group and the pole density of the crystal orientation of ⁇ 110 ⁇ ⁇ 001> was obtained.
- a strip-shaped test piece is cut out from the width direction 1/2 position of the hot-rolled steel sheet, and bending (L-axis bending) in which the bending ridge line is parallel to the rolling direction (L direction). Bending is performed in accordance with JIS Z2248: 2006 (V block 90 ° bending test) for both bending (C-axis bending) in which the bending ridge is parallel to the direction perpendicular to the rolling direction (C direction). The cracks formed on the inside were investigated, and the limit bending R / t was determined. When R / t was 1.5 or less, it was judged that the bending workability was excellent.
- JIS No. 5 tensile test pieces are collected so that the direction perpendicular to the rolling direction is the tensile direction, and a tensile test is performed in accordance with JIS Z2241: 2011 to determine the tensile strength (TS) and total elongation (EL). It was measured. When the product of the tensile strength TS (MPa) and EL (%) was 19000 (MPa ⁇ %) or more and the total elongation EL was 16.0% or more, it was judged that the elongation was excellent.
- TS tensile strength
- EL total elongation
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Abstract
Description
本願は、2020年02月20日に、日本に出願された特願2020-026996号に基づき優先権を主張し、その内容をここに援用する。
しかしながら、自動車用部品に供される鋼板においては、強度だけでなく、プレス加工性や溶接性等、部品成形時に要求される各種施工性が要求される。具体的には、プレス加工性や成形性の観点から、鋼板には曲げ加工性及び伸びが要求されることが多い。鋼板の成形性は、材料の高強度化とともに低下する傾向があるので、高強度と良好な成形性とを両立することは難しい。
そのため、自動車用部品における高強度鋼板の適用には、引張強度980MPa以上の高強度とともに、優れた曲げ加工性及び伸びを実現することが重要な課題となっている。
上記した曲げ加工性とは、曲げ加工した際に、加工部にて亀裂が生じにくいことを示す指標、またはその亀裂が成長しにくいことを示す指標である。ただし、本発明では、従来とは異なり、曲げ加工した際に、曲げ加工部の内側から発生する亀裂(曲げ内割れ)を対象とする。
本発明者らの研究により、曲げ内割れは、引張強度780MPa級以上の鋼板で発生しやすくなり、980MPa級以上の鋼板で顕著になり、1180MPa級以上の鋼板で更に顕著な課題となることがわかった。
その結果、集合組織が比較的ランダムであれば変形抵抗も均一であるため、変形が均一に生じやすいが、特定の集合組織が発達すると変形抵抗が大きい方位を持つ結晶とそれ以外の方位の結晶の間に変形の偏りが生じ、せん断変形帯を生じやすくなること、逆に、変形抵抗の大きい方位の結晶を減らすと、変形は均一に生じ、せん断変形帯は生じにくくなることを見出した。すなわち、本発明者らは、特に、亀裂の発生する板厚方向の表層領域における集合組織を制御することで、曲げ内割れを抑制できることを見出した。
(1)本発明の一態様に係る熱延鋼板は、質量%で、C:0.02~0.30%、Si:0.01~2.50%、Mn:1.00~3.00%、P:0.100%以下、S:0.0001~0.0100%、Al:0.005~1.000%、N:0.010%以下、Ti:0~0.20%、Nb:0~0.20%、V:0~0.200%、Ni:0~2.00%、Cu:0~2.00%、Cr:0~2.00%、Mo:0~2.00%、W:0~0.100%、B:0~0.0100%、REM:0~0.0300%、Ca:0~0.0300%、Mg:0~0.0300%、を含有し、残部がFe及び不純物からなる化学組成を有し、前記化学組成が、Si+Al≧1.00%を満足し、ミクロ組織が、体積率で、マルテンサイト、焼き戻しマルテンサイト及びベイナイトを合計で70%以上含有し、残留オーステナイトを5~20%含有し、表面から板厚の1/10の位置までの範囲である表層領域において、{211}<111>~{111}<112>からなる方位群の平均極密度と{110}<001>の結晶方位の極密度との和が6.0以下であり、前記残留オーステナイト中の固溶炭素濃度が0.5質量%以上であり、引張強度が980MPa以上である。
(2)上記(1)に記載の熱延鋼板は、前記化学組成が、質量%で、Ti:0.001~0.20%、Nb:0.001~0.20%、V:0.001~0.200%、Ni:0.01~2.00%、Cu:0.01~2.00%、Cr:0.01~2.00%、Mo:0.01~2.00%、W:0.005~0.100%、B:0.0005~0.0100%、REM:0.0003~0.0300%、Ca:0.0003~0.0300%、Mg:0.0003~0.0300%、から選択される1種又は2種以上を含有してもよい。
(3)上記(1)または(2)に記載の熱延鋼板は、前記表面に溶融亜鉛めっき層を備えてもよい。
(4)上記(3)に記載の熱延鋼板は、前記溶融亜鉛めっき層が合金化溶融亜鉛めっき層であってもよい。
<体積率で、マルテンサイト、焼き戻しマルテンサイト及びベイナイトを合計で70%以上含有し、残留オーステナイトを5~20%含有する>
まず、ミクロ組織の限定理由に関して述べる。
本実施形態に係る鋼板では、ミクロ組織の主相は、体積率で70%以上のマルテンサイト、焼き戻しマルテンサイト及びベイナイトから選択される1種以上である。ミクロ組織は、さらに残留オーステナイトを5~20%含有する。
本実施形態に係る鋼板は、980MPa以上の引張強度(TS)と曲げ加工性を両立するため、低温変態生成相であるマルテンサイト、焼き戻しマルテンサイト及びベイナイトから選択される1種以上を主相とする。マルテンサイト、焼き戻しマルテンサイト及び/またはベイナイトの合計体積率が低い組織構成において強度を高めようとすると、硬質な上記の組織と軟質な上記以外の組織との間に変形の偏りが生じ、曲げ加工性が劣化する。マルテンサイト、焼き戻しマルテンサイト及び/またはベイナイトの合計体積率が70%未満では、十分な強度が得られないか、十分な曲げ加工性が得られない。
また、本実施形態に係る鋼板では、優れた伸びを得るため、残留オーステナイトを体積率で5%以上含有する。残留オーステナイトの体積率が5%未満であると十分な伸びが得られない。一方、残留オーステナイトを20%超残存させる製造条件を選択すると、その他の所望の組織や強度が得られなくなる。そのため、実質的な残留オーステナイトの体積率の上限は20%である。
上記以外の残部は、フェライト、パーライトのうちの1種以上であってもよい。
フェライトは鉄系炭化物を含まない等軸形状をした粒であり、パーライトはフェライトおよびセメンタイトの層状組織である。
ベイナイトは、上部ベイナイトと下部ベイナイトとを含むが、上部ベイナイトは、ラス状の結晶粒の集合であり、ラス間に炭化物を含むラスの集合体である。下部ベイナイトは、ラス状の結晶粒の集合であり、内部に長径5nm以上の鉄系炭化物を含み、さらに、その炭化物が、単一のバリアント、即ち、同一方向に伸張した鉄系炭化物群に属するものである。ここで、同一方向に伸長した鉄系炭化物群とは、鉄系炭化物群の伸長方向の差異が5°以内であるものを意味する。
焼き戻しマルテンサイトはラス状の結晶粒の集合であり、内部に長径5nm以上の鉄系炭化物を含み、さらに、その炭化物が、複数のバリアント、即ち、2方向以上に伸張した鉄系炭化物群に属するものである。一般的に、焼き戻しマルテンサイトはセメンタイト等の鉄系炭化物を含むものを指す場合が多いが、本実施形態ではTiを含む微細析出物を含むマルテンサイトも焼き戻しマルテンサイトと定義する。
ただし、本実施形態に係る鋼板では、マルテンサイト、焼き戻しマルテンサイト及びベイナイトの合計体積率を規定すればよく、これらの組織を見分けることは必須ではない。
残留オーステナイト中の固溶炭素濃度を0.5質量%以上とすることにより、残留オーステナイトが適度に安定化し、変形後期の高歪域において変態誘起塑性(TRIP)が多く生じるようになり、鋼板の伸びおよび曲げ加工性が向上する。したがって、残留オーステナイト中の固溶炭素濃度を0.5質量%以上とする。残留オーステナイト中の固溶炭素濃度は、好ましくは0.7質量%以上である。
残留オーステナイト中の固溶炭素濃度を2.0質量%以下とすることにより、残留オーステナイトの過度な安定化を抑制し、変態誘起塑性(TRIP)をより確実に発現させることができる。したがって、残留オーステナイト中の固溶炭素濃度は2.0質量%以下とすることが好ましい。
Cγ=(a-3.572)/0.033・・・(A)
本発明者らは、高強度鋼板の曲げ加工性について鋭意調査を行った。その結果、鋼板の高強度化に伴い曲げ内側に微小な亀裂が生じることがあることが分かった。さらに検討を行った結果、このような曲げ内割れのメカニズムは以下のように推定される。
曲げ加工時には曲げ内側に圧縮の応力が生じる。最初は曲げ内側全体が均一に変形しながら加工が進むが、加工量が大きくなると均一な変形のみで変形を担えなくなり、ミクロな変形の偏りが生じる(せん断変形帯の発生)。このせん断変形帯が更に成長することで曲げ内側表面からせん断帯に沿った亀裂が発生し、成長する。高強度化に伴い曲げ内割れが発生しやすくなる理由は、高強度化に伴う加工硬化能の低下により、均一な変形が進みにくくなり、変形の偏りが生じやすくなることで、加工早期に(または緩い加工条件で)せん断変形帯が生じるためと推定される。
鋼板が曲げ変形する際、板厚中心を境に、表面に向かってひずみが大きくなり、最表面でひずみは最大となる。したがって、曲げ内割れの亀裂は鋼板の表面に生成する。このような、亀裂の生成に寄与するのは、鋼板表面から板厚方向に板厚の1/10までの範囲である表層領域の組織であるため、表層領域の組織を制御する。
具体的には、鋼板に変形を加えた時、各結晶方位において変形に対するすべり系の働きやすさは異なる(Schmid factor)。これは結晶方位ごとに変形抵抗が異なるからであると考えられる。すなわち、集合組織が比較的ランダムであれば変形抵抗も均一であるため、変形が均一に生じやすいが、特定の集合組織が発達すると変形抵抗が大きい方位を持つ結晶とそれ以外の方位の結晶の間に変形の偏りが生じ、せん断変形帯を生じやすくなる。逆に、変形抵抗の大きい方位の結晶を減らすならば、変形は均一に生じ、せん断変形帯は生じにくくなると考えられる。
表裏面において集合組織の発達が異なる鋼板の場合、片側の表面から板厚の1/10の位置までの範囲のみでも本実施形態で規定する集合組織を満たしていれば、その面を曲げ内側にした時の曲げ加工において、曲げ内割れ抑制の効果を得ることができる。
{211}<111>~{111}<112>からなる方位群の平均極密度と{110}<001>の結晶方位の極密度との和は小さい程好ましいが、980MPa以上の高強度熱延鋼板においては、0.5未満とすることは困難であるため、0.5が実質的な下限である。
同様に{110}<001>の結晶方位の極密度は、φ2=45°断面の結晶方位分布関数(ODF)で、φ1=85~90°、Φ=85~90°、φ2=45°の範囲を指す。この結晶方位の極密度を、図1に示す上記範囲で算出する。
以下、本実施形態に係る鋼板の化学組成について詳細に説明する。
下記する「~」を挟む数値限定範囲には、その両端の値が下限値及び上限値としてその範囲に含まれる。ただし、「超」または「未満」と示す数値は、その値が数値範囲に含まれない。各元素の含有量に関する「%」は、断りがない限り「質量%」を意味する。
Cは、鋼板の強度を高めるために有効な元素である。C含有量が0.02%未満であると、980MPa以上の強度を確保することが難しい。そのため、C含有量を0.02%以上とする。C含有量は、好ましくは、0.13%以上であり、より好ましくは、0.15%以上である。
一方、C含有量が0.30%を超えると、その効果が飽和するばかりでなく、パーライトが優先的に生成してベイナイトおよび残留オーステナイトの生成が不十分となり、所望のベイナイトの体積率および残留オーステナイトの体積率を得ることが困難となる。そのため、C含有量は0.30%以下である。C含有量は、好ましくは0.25%以下である。
Siは、固溶強化により材料強度を高めることができる重要な元素である。Si含有量が0.01%未満では、強度が低下する。そのため、Si含有量は0.01%以上とする。Si含有量は、好ましくは0.10%以上、さらに好ましくは0.30%以上である。
一方、Si含有量が2.50%超では、表面性状が劣化する。そのため、Si含有量は2.50%以下とする。Si含有量は、好ましくは2.00%以下である。
Mnは、鋼板のミクロ組織におけるベイナイト、マルテンサイトの体積率を高めて鋼板の強度を高めるために有効な元素である。ベイナイト、マルテンサイト、焼き戻しマルテンサイトの体積率を合計で70%以上にするために、Mn含有量を1.00%以上とする。Mn含有量が1.00%未満では、これらの組織の体積率が低下し、十分な強度が得られない。
一方、Mn含有量が3.00%超では、その効果が飽和するとともに、経済性が低下する。そのため、Mn含有量を3.00%以下とする。
Pは、鋼板の板厚中央部に偏析する元素であり、また、溶接部を脆化させる元素でもある。P含有量は低い方が好ましいが、P含有量が0.100%超となると特性の劣化が顕著となるので、P含有量を0.100%以下に制限する。P含有量は好ましくは0.050%以下である。
一方、下限は特に定めることなく効果が発揮される(0%でもよい)が、P含有量を0.001%未満に低減することは、経済的に不利である。そのため、P含有量を0.001%以上としてもよい。
Sは、硫化物として存在することで、スラブの脆化をもたらす元素である。またSは、鋼板の成形性を劣化させる元素である。そのため、S含有量を制限する。S含有量が0.010%を超えると特性の劣化が顕著になるので、S含有量を0.010%以下とする。
一方、下限は特に定めることなく効果が発揮される(0%でもよい)が、S含有量を0.0001%未満に低減することは、経済的に不利であるので、S含有量を0.0001%以上とする。
Nは、粗大な窒化物を形成し、曲げ加工性や伸びを劣化させる元素である。N含有量が0.010%を超えると、曲げ加工性や伸びが顕著に劣化する。そのため、N含有量を0.010%以下とする。
一方、N含有量の下限は、特に定める必要はない(0%でもよい)が、N含有量を0.0001%未満に低減すると、製造コストが大幅に増加する。そのため、製造コストの観点からN含有量を0.0001%以上としてもよく、0.0005%以上としてもよい。
Alは、熱延での組織制御及び脱酸に有効な元素である。これらの効果を得るため、Al含有量を0.005%以上とする。Al含有量が0.005%未満では十分な脱酸効果を得ることが出来ず、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曲げ加工や伸びフランジ加工時の割れの起点となり、加工性を劣化させる。
一方、Al含有量が1.000%を超えると、スラブが脆化するので好ましくない。そのため、Al含有量を1.000%以下とする。
本実施形態において、Al含有量は、sol.Al(酸可溶性Al)含有量を意味する。
また、残留オーステナイトの面積率を確保するため、SiとAlとの合計含有量(Si+Al)を1.00%以上とする。
本実施形態に係る鋼板は、各種特性の向上を目的として、さらに下記のような成分を含有することができる。以下の元素は、必ずしも含有する必要はないので、含有量の下限は0%である。
(Nb:0~0.20%)
(V:0~0.200%)
Ti(チタン)、Nb(ニオブ)、V(バナジウム)は、強度の向上に寄与する元素である。したがって、Ti、Nbおよび/またはVを含有させてもよい。上記の効果を好ましく得るためには、含有量がそれぞれ0.001%以上であることが好ましい。Ti含有量は、より好ましくは0.02%以上、Nb含有量は、より好ましくは0.01%以上である。
一方、Ti、Nbを0.20%超、Vを0.200%超含有させても、上記作用による効果は飽和するだけでなく、経済的に不利となる場合がある。したがって、含有させる場合Ti含有量は0.20%以下、Nb含有量は0.20%以下、V含有量は0.200%以下とする。Ti含有量、Nb含有量は、好ましくは0.15%以下、より好ましくは0.10%以下であり、V含有量は、好ましくは0.150%以下、より好ましくは0.100%以下である。
(Cu:0~2.00%)
(Cr:0~2.00%)
(Mo:0~2.00%)
Ni、Cu、Cr、Moは、熱延での組織制御を通じて鋼板の高強度化に寄与する元素である。この効果は、Ni、Cu、Cr、Moの1種又は2種以上を、それぞれ、0.01%以上含有させることで顕著になる。そのため、効果を得る場合、含有量をそれぞれ0.01%以上とすることが好ましい。
一方、各元素の含有量が、それぞれ2.00%を超えると、溶接性、熱間加工性などが劣化する。そのため、含有させる場合、Ni、Cu、Cr、Moの含有量はそれぞれ2.00%以下とする。
Wは、析出強化を通じて鋼板の強度の向上に寄与する元素である。この効果を得る場合、W含有量を0.005%以上とすることが好ましい。
一方、W含有量が0.100%を超えると、効果が飽和するばかりでなく、熱間加工性が低下する。そのため、含有させる場合、W含有量を0.100%以下とする。
Bは、熱延での変態を制御し、組織強化を通じて鋼板の強度を向上させるために有効な元素である。この効果を得る場合、B含有量を0.0005%以上とすることが好ましい。
一方、B含有量が0.0100%超となると、効果が飽和するばかりでなく、鉄系の硼化物が析出して、固溶Bによる焼き入れ性向上の効果が失われる。そのため、含有させる場合、B含有量を0.0100%以下とする。B含有量は、好ましくは0.0080%以下、より好ましくは0.0050%以下である。
(Ca:0~0.0300%)
(Mg:0~0.0300%)
REM、Ca、Mgは、鋼板の強度向上に寄与する元素である。REM、Ca、Mgの1種又は2種以上の合計が0.0003%未満であると、充分な効果が得られないので、効果を得る場合、REM、Ca、Mgの合計含有量を0.0003%以上とすることが好ましい。
一方、REM、Ca、Mgがそれぞれ0.0300%を超えると、鋳造性や熱間での加工性が劣化する。そのため、含有させる場合、それぞれの含有量を0.0300%以下とする。
本実施形態において、REMとは、Sc、Yおよびランタノイドからなる合計17元素を指し、上記REMの含有量は、これらの元素の含有量の合計を指す。ランタノイドの場合、工業的にはミッシュメタルの形で添加される。
亜鉛めっきは耐食性向上に寄与することから、耐食性が期待される用途への適用の場合には亜鉛めっきを実施した溶融亜鉛めっき鋼板、または合金化溶融亜鉛めっき鋼板とすることが望ましい。
自動車の足回り部品は、腐食による穴あきの懸念があることから、高強度化してもある一定板厚以下に薄手化できない場合がある。鋼板の高強度化の目的の一つは、薄手化による軽量化であることから、高強度鋼板を開発しても、耐食性が低いと適用部位が限られる。これら課題を解決する手法として、耐食性の高い溶融亜鉛めっき等のめっきを鋼板に施すことが考えられる。本実施形態に係る鋼板は、鋼板成分を上述のように制御しているので、溶融亜鉛めっきが可能である。
めっきは電気亜鉛めっきであってもよく、Znに加えてSi、Al及び/またはMgを含むめっきであってもよい。
本実施形態に係る鋼板は、自動車の軽量化に寄与する十分な強度として、980MPa以上の引張強度(TS)を有する。好ましくは1180MPa以上である。引張強度の上限は特に定める必要はないが、本実施形態において実質的な引張強度の上限を1370MPaとしてもよい。
また、本実施形態に係る鋼板は、曲げ内割れ性の指標値となる限界曲げR/tの値が1.5以下であることを目標とする。R/tの値は、例えば、熱延鋼板の幅方向1/2位置から、短冊形状の試験片を切り出し、曲げ稜線が圧延方向(L方向)に平行である曲げ(L軸曲げ)と、曲げ稜線が圧延方向に垂直な方向(C方向)に平行である曲げ(C軸曲げ)の両者について、JIS Z2248:2006(Vブロック90°曲げ試験)に準拠して曲げ加工を行い、曲げ内側に生じた亀裂を調査して求めることができる。亀裂の発生しない最小曲げ半径を求め、L軸とC軸との最小曲げ半径の平均値を板厚で除した値を限界曲げR/tとして曲げ加工性の指標値とすることができる。
また、本実施形態に係る鋼板は、高い伸びを有することの指標として、引張強度TS(MPa)とEL(%)との積が19000(MPa・%)以上であることを目標とする。TSとELとの積は、好ましくは19120(MPa・%)以上、より好ましくは19600(MPa・%)以上である。本実施形態に係る鋼板は、さらに、全伸びELが16.0%以上であることが望ましい。
引張試験は、JIS Z2241:2011に準拠して、圧延方向に対して直角方向が引張方向となるようにJIS5号引張試験片を採取し、引張強度(TS)および全伸び(EL)を測定する。
次に、本実施形態に係る鋼板の好ましい製造方法について説明する。
鋼板の表層領域のミクロ組織、集合組織を、上述の範囲に制御するため、以下のような、熱間圧延工程(加熱工程、粗圧延工程、仕上げ圧延工程を含む)、冷却工程、熱処理工程を含み、必要に応じて冷却工程と熱処理工程との間に巻取り工程、酸洗工程及び軽圧下工程を含み、必要に応じて熱処理工程後にめっき工程を含む条件で熱延鋼板を製造することが好ましい。
以下、各工程において好ましい条件を説明する。
加熱工程では、粗圧延工程に供する上述した化学組成を有するスラブを、1200℃以上に加熱する。スラブ内に析出した粗大な析出物(鉄系炭化物や合金元素の炭窒化物など)は、材質安定性を阻害する可能性がある。そのため、このような析出物を溶解させることを目的としてスラブを1200℃以上に加熱する。
次に、加熱されたスラブを粗圧延して、粗圧延板とする。
この粗圧延工程では、粗圧延後の粗圧延板の厚さを35mm超45mm以下に制御する。粗圧延板の厚さは、仕上げ圧延工程における圧延開始時から圧延完了時までに生じる圧延板の先端から尾端までの温度低下量に影響を及ぼす。また、粗圧延板の厚さが、35mm以下または45mm超であると、次工程である仕上げ圧延中に鋼板へ導入されるひずみ量が変化して、仕上げ圧延中に形成される加工組織が変化する。その結果、再結晶挙動が変化して、所望の集合組織を得ることが困難になる。特に、鋼板表層領域で上記した集合組織を得ることが困難になる。
一般に、粗圧延後の粗圧延板の厚さは、生産性等の観点で適宜設定され、鋼板の特性の制御のために設定されることは少ない。これに対し、本発明者らは、鋼板表層領域の集合組織を制御するため、粗圧延板の厚さを厳格に制御している。
粗圧延に続き多段仕上げ圧延を施す。本発明者らは、通常積極的に制御されてこなかった、熱間圧延の仕上げ圧延工程の最終2段の圧延における、圧延時の板厚、ロール形状比、温度、鋼中のNb及びTiの含有量を、ある計算式によって導出される適切な範囲に制御することが集合組織を制御する上で重要であることを見出した。
そのため、この多段仕上げ圧延では、仕上げ圧延の開始温度が1000℃以上1150℃以下であり、仕上げ圧延の開始前の鋼板の厚さ(粗圧延板の厚さ)が35mm超45mm以下である。また、多段仕上げ圧延の最終段より1段前の圧延は、圧延温度が960℃以上1020℃以下であり、圧下率が11.0%超23.0%以下である。また、多段仕上げ圧延の最終段は、圧延温度が930℃以上995℃以下であり、圧下率が11.0%超22.0%以下であることが好ましい。また、最終2段の圧下時の各条件を制御し、以下の式1によって計算される集合組織形成パラメータωが110以下を満たすことが好ましい。さらに、多段仕上げ圧延の最終3段の総圧下率が35%以上である条件で仕上げ圧延を施すことが好ましい。
PE:析出物形成元素による再結晶抑制効果の換算値(単位:質量%)
Ti:鋼中に含まれるTi含有量(単位:質量%)
Nb:鋼中に含まれるNb含有量(単位:質量%)
F1 *:最終段より1段前の換算圧下率(単位:%)
F2 *:最終段の換算圧下率(単位:%)
F1:最終段より1段前の圧下率(単位:%)
F2:最終段の圧下率(単位:%)
Sr1:最終段より1段前の圧延形状比(無単位)
Sr2:最終段における圧延形状比(無単位)
D1:最終段より1段前のロール径(単位:mm)
D2:最終段のロール径(単位:mm)
t1:最終段より1段前の圧延開始時における板厚(単位:mm)
t2:最終段の圧延開始時における板厚(単位:mm)
tf:仕上げ圧延後の板厚(単位:mm)
FT1 *:最終段より1段前の換算圧延温度(単位:℃)
FT2 *:最終段の換算圧延温度(単位:℃)
FT1:最終段より1段前の圧延温度(単位:℃)
FT2:最終段の圧延温度(単位:℃)
をそれぞれ示す。
仕上げ圧延の開始温度が1000℃未満であると、最終2段を除く、前段での圧延によって加工された組織の再結晶が十分に起こらず、鋼板表層領域の集合組織が発達して、L軸及びC軸の最小曲げ半径の平均値/板厚であるR/tにおいて1.5以下を満たすことができない。
したがって、仕上げ圧延の開始温度は1000℃以上とすることが好ましい。より好ましくは1050℃以上である。
一方、仕上げ圧延の開始温度を1150℃超とすると、過度にオーステナイト粒が粗大化し、靱性が劣化する。そのため、仕上げ圧延の開始温度を1150℃以下とすることが好ましい。
本実施形態に係る鋼板の製造においては、多段仕上げ圧延における最終2段の熱延条件が重要となる。
式1で定義するωの計算に用いる、最終2段の圧延時の圧下率F1及びF2は、各段での圧延前後の板厚の差を、圧延前の板厚で除した値を百分率で表した数値である。圧延ロールの直径D1及びD2は、室温で測定したものであり、熱延中の扁平を考慮する必要はない。また、圧延入口側の板厚t1及びt2、並びに仕上げ圧延後の板厚tfは、放射線等を用いてその場で測定してもよいし、圧延荷重から、変形抵抗等を考慮して計算で求めても良い。仕上げ圧延後の板厚tfは、熱延完了後の鋼板の最終板厚としても良い。圧延開始温度FT1及びFT2は、仕上げ圧延スタンド間の放射温度計等の温度計によって測定した値を用いればよい。
集合組織形成パラメータωは、仕上げ圧延の最終2段で鋼板全体に導入される圧延ひずみと、鋼板の表層領域に導入されるせん断ひずみと、圧延後の再結晶速度とを考慮した指標であり、集合組織の形成され易さを意味する。集合組織形成パラメータωが110を超える条件で最終2段の仕上げ圧延を行うと、表層領域にて、{211}<111>~{111}<112>からなる方位群の平均極密度と{110}<001>の結晶方位の極密度との和を6.0以下にできない。したがって、集合組織形成パラメータωは110以下に制御することが好ましい。より好ましくは、集合組織形成パラメータωは98以下である。
最終段より1段前の圧延温度FT1が960℃未満であると、圧延によって加工された組織の再結晶が十分に起こらず、表層領域の集合組織を上記範囲に制御できない。したがって、圧延温度FT1は960℃以上とする。一方、圧延温度FT1が1020℃超であると、オーステナイト粒の粗大化などに起因して、加工組織の形成状態や再結晶挙動が変化するため、表層領域の集合組織を上記範囲に制御できない。したがって、圧延温度FT1は1020℃以下とする。
最終段より1段前の圧下率F1が11.0%以下であると、圧延によって鋼板へ導入されるひずみ量が不十分となって再結晶が十分に起こらず、表層領域の集合組織を上記範囲に制御できない。したがって、圧下率F1は11.0%超とする。一方、圧下率F1が23.0%超であると、結晶中の格子欠陥が過剰となって再結晶挙動が変化するため、表層領域の集合組織を上記範囲に制御できない。したがって、圧下率F1は23.0%以下とする。
圧下率F1(%)は以下のように計算される。
F1=(t1-t2)/t1×100
最終段の圧延温度FT2を930℃未満とすると、オーステナイトの再結晶速度が著しく低下して、表層領域にて{211}<111>~{111}<112>からなる方位群の平均極密度と{110}<001>の結晶方位の極密度との和を6.0以下にできない。したがって、圧延温度FT2は930℃以上とする。一方、圧延温度FT2が995℃超であると、加工組織の形成状態や再結晶挙動が変化するため、表層領域の集合組織を上記範囲に制御できない。したがって、圧延温度FT2は995℃以下とする。
最終段の圧下率F2が11.0%以下であると、圧延によって鋼板へ導入されるひずみ量が不十分となって再結晶が十分に起こらず、表層領域の集合組織を上記範囲に制御できない。したがって、圧下率F2は11.0%超とする。一方、圧下率F2が22.0%超であると、結晶中の格子欠陥が過剰となって再結晶挙動が変化するため、表層領域の集合組織を上記範囲に制御できない。したがって、圧下率F2は22.0%以下とする。
圧下率F2(%)は以下のように計算される。
F2=(t2-tf)/t2×100
最終3段の総圧下率Ftはオーステナイトの再結晶を促進するために大きい方がよい。最終3段の総圧下率Ftが35%未満であると、オーステナイトの再結晶速度が著しく低下して、表層領域において、{211}<111>~{111}<112>からなる方位群の平均極密度と{110}<001>の結晶方位の極密度との和を6.0以下にできない。
最終3段の総圧下率Ftは以下の式で計算される。
Ft=(t0-tf)/t0×100
ここで、t0は最終段より2段前の圧延開始時における板厚(単位:mm)である。
(800℃~450℃までを60℃/秒以上の平均冷却速度で冷却)
冷却工程では仕上げ圧延後の熱延鋼板を、800℃~450℃までの平均冷却速度が60℃/秒以上となるように、後述する巻取り温度まで冷却する。これは800℃~450℃の温度域でフェライトやパーライトが過剰に生成するのを抑制するためである。800℃以上の温度域では変態が起きにくいため冷却速度は規定しないが、一般的な熱延設備では仕上げ圧延完了から数秒以内に冷却帯に到達するため、現実的な800℃以上での保持時間は仕上げ圧延完了後5秒以内である。一方、800℃~450℃の間は変態が生じる温度域であるので、60℃/秒以上の平均冷却速度で冷却する。冷却停止温度が450℃超または平均冷却速度が60℃/秒未満では冷却過程でフェライトやパーライト等が生成し、マルテンサイト、焼き戻しマルテンサイト及びベイナイトを合計で70%以上確保できず、強度と曲げ加工性とを両立できない場合がある。450℃以下の温度ではフェライトやパーライト変態が生じる懸念はほぼないので、冷却速度を規定する必要はない。
熱間圧延後の熱延鋼板はコイル状に巻き取ってもよい。巻取り温度が450℃超では、フェライトやパーライト等が生成し、マルテンサイト、焼き戻しマルテンサイト及びベイナイトを合計で70体積%以上確保できない場合がある。そのため、巻取り温度を450℃以下とする。
冷却工程後または巻取り工程後の熱延鋼板に対して酸洗を行ってもよい。酸洗を実施することで、後の製造工程でのめっき性を改善したり、自動車製造工程での化成処理性を高めたりすることができる。
また、スケールのついた熱延鋼板を軽圧下するとスケールが剥離し、それが押し込まれることで疵になる場合もある。そのため後述する軽圧下を行う前には、まず、熱延鋼板に対して酸洗を実施する。酸洗条件は特に限定されないが、インヒビター入りの塩酸、硫酸などで酸洗するのが一般的である。
軽圧下工程は必須ではないが、転位の導入による高強度化のため、20%以下の圧下率で圧下を加えてもよい。
ただし、圧下率が20%を超えると、効果が飽和するばかりでなく、導入された転位の回復が不十分となり、大幅な伸びの劣化を招く。このことから、圧下を行う場合、圧下率は20%以下とすることが好ましい。圧下は、1パスで20%以下の圧下を実施しても良いし、複数回に分けて行って、累積圧下率が20%以下となるように行っても良い。
(200℃以上、450℃未満の温度域にて10秒以上保持)
軽圧下工程後の熱延鋼板を、200~450℃未満の温度域に再加熱して、10秒以上この温度域に留まるように保持する熱処理を行う。
この熱処理により、ミクロ組織における残留オーステナイトの体積率を5%以上、かつ残留オーステナイト中の固溶炭素濃度を0.5質量%以上にすることができる。
熱処理温度が200℃未満、または保持時間が10秒未満では、十分なオーステナイト体積率または固溶炭素濃度を確保できない。
また、熱処理温度が450℃以上になると、強度の低下が顕著となり、引張強度980MPa以上を達成できなくなる。
保持時間の上限は規定する必要がなく、加熱方法に合わせて均熱性、経済合理性を加味して決めればよい。例えば、鋼板を走行させる熱処理設備を用いる場合は設備占有時間を短縮する目的から1000秒程度が現実的な上限であるが、箱型の加熱装置の場合は、コイル内の温度が均一化するのに十分な時間として、数時間から数十時間の加熱を行ってもよい。
保持時間は、再加熱後、鋼板が200℃以上、450℃未満の温度域にある時間を意味し、この温度域に所定の時間留まっていれば、途中で温度変化があってもよい。
熱処理後(200℃未満の温度に下がった後)の冷却は、特に規定しない。
上記工程を含む製造方法によって本実施形態に係る鋼板が得られる。しかしながら、本実施形態に係る鋼板を、耐食性の向上を目的として溶融亜鉛めっき鋼板または合金化溶融亜鉛めっきとする場合には、熱処理工程後の熱延鋼板に溶融亜鉛めっきを施すことが好ましい。亜鉛めっきは耐食性向上に寄与することから、耐食性が期待される用途への適用の場合には亜鉛めっきを実施することが望ましい。亜鉛めっきは溶融亜鉛めっきであることが好ましい。溶融亜鉛めっきの条件は特に限定されず、公知の条件で行えばよい。
また、溶融亜鉛めっき後の熱延鋼板(溶融亜鉛めっき鋼板)を、合金化することで、合金化溶融亜鉛めっき鋼板を製造できる。合金化溶融亜鉛めっき鋼板は、耐食性の向上に加えて、スポット溶接性の向上や絞り成形時の摺動性向上などの効果を付与できることから、用途に応じて合金化を実施しても良い。
上記の溶融亜鉛めっき処理および合金化溶融亜鉛めっき処理は、上記の200℃以上450℃未満での熱処理後に一度室温まで冷却してから行ってもよいし、冷却せずに行ってもよい。
亜鉛めっき以外に、Alめっき、Mgを含むめっき、電気めっきを実施したとしても本実施形態に係る鋼板を製造できる。
次いで、得られた粗圧延板に対し、全7段からなる多段仕上げ圧延を施した。多段仕上げ圧延工程では、表2に記載の圧延開始温度から仕上げ圧延を開始し、圧延開始から最終3段の圧延を除く、計4段の圧延によって、表3に記載の5段目圧延時板厚:t0の厚さまで圧延した。
その後、表2~表4に記載の各条件で、最終2段の熱間圧延を施したあと、冷却、巻取りを行った。熱延完了後の鋼板の最終板厚を、仕上げ圧延後の板厚tfとした。
さらにその後、一部について表4に記載の通り、溶融亜鉛めっき(GI)または合金化溶融亜鉛めっき(GA)を行った。めっき浴温度は、445℃とし、合金化に際しては、445℃で10秒保持した。
R/tが1.5以下であれば、曲げ加工性に優れると判断した。
これに対し、化学組成、ミクロ組織、極密度の和、オーステナイト中の固溶炭素量の1つ以上が本発明の範囲外である比較例No.10~12、15~35、37では、引張強度、曲げ加工性、伸びのいずれか1つ以上が、目標の値に達していなかった。
Claims (4)
- 質量%で、
C:0.02~0.30%、
Si:0.01~2.50%、
Mn:1.00~3.00%、
P:0.100%以下、
S:0.0001~0.0100%、
Al:0.005~1.000%、
N:0.010%以下、
Ti:0~0.20%、
Nb:0~0.20%、
V:0~0.200%、
Ni:0~2.00%、
Cu:0~2.00%、
Cr:0~2.00%、
Mo:0~2.00%、
W:0~0.100%、
B:0~0.0100%、
REM:0~0.0300%、
Ca:0~0.0300%、
Mg:0~0.0300%、
を含有し、残部がFe及び不純物からなる化学組成を有し、
前記化学組成が、Si+Al≧1.00%
を満足し、
ミクロ組織が、体積率で、マルテンサイト、焼き戻しマルテンサイト及びベイナイトを合計で70%以上含有し、残留オーステナイトを5~20%含有し、
表面から板厚の1/10の位置までの範囲である表層領域において、{211}<111>~{111}<112>からなる方位群の平均極密度と{110}<001>の結晶方位の極密度との和が6.0以下であり、
前記残留オーステナイト中の固溶炭素濃度が0.5質量%以上であり、
引張強度が980MPa以上である
ことを特徴とする熱延鋼板。 - 前記化学組成が、質量%で、
Ti:0.001~0.20%、
Nb:0.001~0.20%、
V:0.001~0.200%、
Ni:0.01~2.00%、
Cu:0.01~2.00%、
Cr:0.01~2.00%、
Mo:0.01~2.00%、
W:0.005~0.100%、
B:0.0005~0.0100%、
REM:0.0003~0.0300%、
Ca:0.0003~0.0300%、
Mg:0.0003~0.0300%、
から選択される1種又は2種以上を含有する
ことを特徴とする請求項1に記載の熱延鋼板。 - 前記表面に溶融亜鉛めっき層を備えることを特徴とする請求項1または2に記載の熱延鋼板。
- 前記溶融亜鉛めっき層が合金化溶融亜鉛めっき層であることを特徴とする請求項3に記載の熱延鋼板。
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