WO2023007876A1 - 熱延鋼板 - Google Patents
熱延鋼板 Download PDFInfo
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- WO2023007876A1 WO2023007876A1 PCT/JP2022/017623 JP2022017623W WO2023007876A1 WO 2023007876 A1 WO2023007876 A1 WO 2023007876A1 JP 2022017623 W JP2022017623 W JP 2022017623W WO 2023007876 A1 WO2023007876 A1 WO 2023007876A1
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- WIPO (PCT)
- Prior art keywords
- hot
- less
- rolled steel
- steel sheet
- ferrite
- Prior art date
Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 133
- 239000010959 steel Substances 0.000 title claims abstract description 133
- 238000005096 rolling process Methods 0.000 claims abstract description 58
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 52
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 37
- 239000000126 substance Substances 0.000 claims abstract description 17
- 239000002344 surface layer Substances 0.000 claims abstract description 15
- 239000000203 mixture Substances 0.000 claims abstract description 14
- 229910000734 martensite Inorganic materials 0.000 claims description 50
- 239000002184 metal Substances 0.000 claims description 9
- 229910052751 metal Inorganic materials 0.000 claims description 9
- 239000012535 impurity Substances 0.000 claims description 8
- 238000001816 cooling Methods 0.000 description 41
- 230000000694 effects Effects 0.000 description 20
- 239000013078 crystal Substances 0.000 description 13
- 238000000034 method Methods 0.000 description 13
- 238000007747 plating Methods 0.000 description 10
- 238000004519 manufacturing process Methods 0.000 description 9
- 238000005259 measurement Methods 0.000 description 9
- 230000009466 transformation Effects 0.000 description 8
- 238000005728 strengthening Methods 0.000 description 7
- 238000004458 analytical method Methods 0.000 description 6
- 238000001556 precipitation Methods 0.000 description 6
- 239000010410 layer Substances 0.000 description 5
- 238000004080 punching Methods 0.000 description 5
- 238000012360 testing method Methods 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000009713 electroplating Methods 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 238000005098 hot rolling Methods 0.000 description 4
- 238000007670 refining Methods 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 3
- 238000005336 cracking Methods 0.000 description 3
- 230000008030 elimination Effects 0.000 description 3
- 238000003379 elimination reaction Methods 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 238000005498 polishing Methods 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
- 238000005204 segregation Methods 0.000 description 3
- 238000010008 shearing Methods 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- 229910052782 aluminium Inorganic materials 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000005315 distribution function Methods 0.000 description 2
- 238000005530 etching Methods 0.000 description 2
- 239000000446 fuel Substances 0.000 description 2
- 230000020169 heat generation Effects 0.000 description 2
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 239000013585 weight reducing agent Substances 0.000 description 2
- 229910000838 Al alloy Inorganic materials 0.000 description 1
- 229910021365 Al-Mg-Si alloy Inorganic materials 0.000 description 1
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- 229910000861 Mg alloy Inorganic materials 0.000 description 1
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000007545 Vickers hardness test Methods 0.000 description 1
- 229910007567 Zn-Ni Inorganic materials 0.000 description 1
- 229910007614 Zn—Ni Inorganic materials 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 230000001133 acceleration Effects 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000012670 alkaline solution Substances 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 239000011247 coating layer Substances 0.000 description 1
- 239000008119 colloidal silica Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 239000000356 contaminant Substances 0.000 description 1
- 238000011109 contamination Methods 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 239000002826 coolant Substances 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000000593 degrading effect Effects 0.000 description 1
- 229910003460 diamond Inorganic materials 0.000 description 1
- 239000010432 diamond Substances 0.000 description 1
- 238000003618 dip coating Methods 0.000 description 1
- 238000007598 dipping method Methods 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 238000001887 electron backscatter diffraction Methods 0.000 description 1
- 238000010894 electron beam technology Methods 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 238000005246 galvanizing Methods 0.000 description 1
- 238000005244 galvannealing Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 230000000977 initiatory effect Effects 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 description 1
- 229910010271 silicon carbide Inorganic materials 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
Classifications
-
- 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
- C21D8/0221—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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- 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
-
- 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
Definitions
- the present invention relates to hot-rolled steel sheets. Specifically, it relates to a hot-rolled steel sheet having high strength and excellent fatigue properties and shear workability.
- This application claims priority based on Japanese Patent Application No. 2021-122173 filed in Japan on July 27, 2021, the content of which is incorporated herein.
- the shape of the part is punched out from the steel plate to create a blank material. At this time, if cracking occurs at the sheared surface of the punch, the fatigue durability of the part may not always be improved even if a high-strength steel plate is used.
- Patent Document 1 proposes a hot-rolled steel sheet with excellent fatigue properties at sheared edges by increasing the volume fraction of martensite and decreasing the volume fraction of pearlite.
- Patent Document 2 proposes a steel sheet that is mainly composed of ferrite and bainite structures and has excellent fatigue properties in the punched shear part by reducing the maximum height of the surface of the steel sheet.
- Patent Document 3 proposes a steel sheet in which ⁇ 011> and ⁇ 111> of ferrite and martensite are ensured and ⁇ 001> is suppressed to extend fatigue crack initiation life.
- Patent Document 4 proposes a method of controlling the crystal orientation of the ferrite or bainite main phase structure by controlling the shape ratio up to the final pass in finish rolling.
- Patent Documents 3 and 4 have room for improvement from the viewpoint of further improving shear workability in addition to fatigue properties.
- the present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear workability.
- hard fresh martensite is also a structure that inhibits plastic deformation, so during strong working such as punching, voids are formed around fresh martensite and cracks are likely to occur on the punched shear surface. Therefore, a hot-rolled steel sheet having a multiphase structure utilizing ferrite and fresh martensite generally deteriorates in shear workability.
- the present inventors analyzed each deformation mechanism in detail. As a result, the present inventors have found that by strictly controlling the crystal orientation of ferrite and bainite in desired regions, it is possible to improve the shear workability while ensuring the fatigue properties of the hot-rolled steel sheet. In other words, by controlling the area ratio of the hard fresh martensite and the tempered martensite that is the main phase, the crystal orientation of ferrite and bainite that undergoes large crystal rotation by punching in the desired region while ensuring the fatigue characteristics It was found that the fatigue properties and shear workability of the hot-rolled steel sheet can be achieved at a high level by properly incorporating the
- the hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%, C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5-3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.10 to 1.00%, N: 0.0100% or less, Ti: 0.06-0.20%, Nb: 0 to 0.10%, Ca: 0 to 0.0060%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, V: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0020%, Cu: 0 to 1.00%, Sn: 0-0.50% and Zr: 0-0.050% with the balance being Fe and impurities,
- the metal structure is the area ratio, sum of ferrite and bainite: 30-47%, Tempered martensite: 50-70%, Fresh marten
- the chemical composition is, in mass%, Nb: 0.01 to 0.10%, Ca: 0.0005 to 0.0060%, Mo: 0.02-1.00%, Cr: 0.02 to 1.00%, V: 0.01 to 0.40%, Ni: 0.01 to 0.40%, B: 0.0001 to 0.0020%, Cu: 0.02-1.00%, Sn: 0.01-0.50% and Zr: 0.001-0.050% You may contain 1 type(s) or 2 or more types out of the group which consists of.
- the hot-rolled steel sheet having high strength and excellent fatigue properties and shear workability. According to the hot-rolled steel sheet according to the present invention, it is possible to integrally form a light-weight part of a vehicle body of an automobile or the like, shorten the processing process, improve fuel efficiency, and reduce the manufacturing cost.
- a hot-rolled steel sheet according to one embodiment of the present invention (sometimes referred to as a hot-rolled steel sheet according to this embodiment) will be described.
- the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.
- the hot-rolled steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%. , P: 0.100% or less, S: 0.010% or less, Al: 0.10 to 1.00%, N: 0.0100% or less, Ti: 0.06 to 0.20%, and the balance : Contains Fe and impurities.
- C is an important element for improving the strength of hot-rolled steel sheets. If the C content is less than 0.02%, the desired strength cannot be obtained. Therefore, the C content is made 0.02% or more. It is preferably 0.04% or more, 0.06% or more, or 0.10% or more. On the other hand, when the C content exceeds 0.30%, the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the C content is made 0.30% or less. Preferably, it is 0.25% or less or 0.20% or less.
- Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the fatigue properties of hot-rolled steel sheets. This effect cannot be obtained if the Si content is less than 0.10%. Therefore, the Si content is set to 0.10% or more. It is preferably 0.20% or more, 0.30% or more, or 0.50% or more. On the other hand, when the Si content exceeds 2.00%, the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 2.00% or less. It is preferably 1.80% or less, 1.60% or less, or 1.50% or less.
- Mn is an element effective in improving the strength of hot-rolled steel sheets by improving hardenability and solid-solution strengthening. This effect cannot be obtained if the Mn content is less than 0.5%. Therefore, the Mn content is set to 0.5% or more. It is preferably 0.7% or more or 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, the fatigue properties of the hot-rolled steel sheet deteriorate due to the formation of MnS. Therefore, the Mn content is set to 3.0% or less. It is preferably 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.
- P is an impurity, and the lower the P content is, the more desirable it is, preferably 0%. If the P content exceeds 0.100%, the workability and weldability of the hot-rolled steel sheet are significantly degraded, and the fatigue properties are also degraded. Therefore, the P content is set to 0.100% or less. It is preferably 0.070% or less, 0.050% or less, or 0.030% or less. From the viewpoint of refining cost, the P content may be 0.001% or more.
- S is an impurity, and the lower the S content, the more desirable it is, preferably 0%. If the S content exceeds 0.010%, a large amount of inclusions such as MnS are formed, and the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the S content should be 0.010% or less. It is preferably 0.008% or less and 0.007% or less. If better shear workability is required, the S content is preferably 0.006% or less. From the viewpoint of refining cost, the S content may be 0.001% or more.
- Al is an important element for controlling ferrite transformation. If the Al content is less than 0.10%, the area ratio of ferrite cannot be controlled favorably. Therefore, the Al content is set to 0.10% or more. It is preferably 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, when the Al content exceeds 1.00%, cluster-like precipitated alumina is generated, and the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the Al content is set to 1.00% or less. It is preferably 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.
- N is an impurity, and the lower the N content is, the more desirable it is, preferably 0%. If the N content exceeds 0.0100%, coarse Ti nitrides are formed at high temperatures, degrading the shear workability of the hot-rolled steel sheet. Therefore, the N content is set to 0.0100% or less. It is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. From the viewpoint of refining cost, the N content may be 0.0001% or more.
- Ti is an element that strengthens ferrite by precipitation and is an important element for controlling ferrite transformation to obtain a desired amount of ferrite. If the Ti content is less than 0.06%, the effects of precipitation strengthening and ferrite transformation control cannot be obtained. Therefore, the Ti content is set to 0.06% or more. It is preferably 0.08% or more and 0.10% or more. On the other hand, when the Ti content exceeds 0.20%, inclusions caused by TiN are formed, and the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.20% or less. It is preferably 0.18% or less or 0.16% or less.
- the hot-rolled steel sheet according to the present embodiment may have the chemical composition described above, with the balance being Fe and impurities.
- impurities refers to those mixed in by raw materials such as ores, scraps, and other factors when industrially producing steel materials, and/or in a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. permissible in
- the chemical composition of the hot-rolled steel sheet according to the present embodiment is not essential for satisfying the required properties, but in order to reduce manufacturing variations and further improve strength, the following optional elements are included. good too. However, since none of the following optional elements are essential for satisfying the required properties, the lower limit of their content is 0%.
- Nb is an element that has the effect of increasing the strength of the hot-rolled steel sheet by refining the grain size and strengthening the precipitation of NbC.
- the Nb content is preferably 0.01% or more.
- the Nb content exceeds 0.10%, the above effect is saturated. Therefore, even when Nb is contained, the Nb content is set to 0.10% or less. Preferably, it is 0.06% or less.
- Ca is an element that fixes S in steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the hole expandability of hot-rolled steel sheets.
- the Ca content is preferably 0.0005% or more.
- the Ca content is made 0.0060% or less. Preferably, it is 0.0040% or less.
- Mo is an element effective in improving the strength of hot-rolled steel sheets by precipitation strengthening of ferrite.
- the Mo content is preferably 0.02% or more. More preferably, it is 0.10% or more.
- the Mo content is set to 1.00% or less. It is preferably 0.60% or less, 0.50% or less, or 0.30% or less.
- Cr 0.02 to 1.00%> Cr is an effective element for improving the strength of the hot-rolled steel sheet.
- the Cr content is preferably 0.02% or more. More preferably, it is 0.10% or more.
- the Cr content is set to 1.00% or less. Preferably, it is 0.80% or less.
- V is an element that improves the strength of a hot-rolled steel sheet by precipitation strengthening and dislocation strengthening by suppressing recrystallization.
- the V content is preferably 0.01% or more.
- the V content is set to 0.40% or less. Preferably, it is 0.20% or less.
- Ni is an element that suppresses phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet.
- the Ni content is preferably 0.01% or more.
- the Ni content is set to 0.40% or less. Preferably, it is 0.20% or less.
- B is an element that suppresses phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet.
- the B content is preferably 0.0001% or more.
- the B content is set to 0.0020% or less. Preferably, it is 0.0005% or less.
- Cu is an element that exists in steel in the form of fine particles and improves the strength of hot-rolled steel sheets.
- the Cu content is preferably 0.02% or more.
- the Cu content is set to 1.00% or less. Preferably, it is 0.80% or less.
- Sn is an element that suppresses the coarsening of crystal grains and improves the strength of the hot-rolled steel sheet.
- the Sn content is preferably 0.01% or more.
- the Sn content is set to 0.50% or less. Preferably, it is 0.30% or less.
- Zr is an element that contributes to improving the formability of hot-rolled steel sheets.
- the Zr content is preferably 0.001% or more.
- the Zr content should be 0.050% or less. Preferably, it is 0.030% or less.
- the chemical composition of the hot-rolled steel sheet mentioned above can be measured by a general analytical method.
- it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
- C and S may be measured using a combustion-infrared absorption method
- N may be measured using an inert gas fusion-thermal conductivity method.
- the hot-rolled steel sheet according to the present embodiment has a metal structure in terms of area ratio, the total of ferrite and bainite: 30 to 47%, tempered martensite: 50 to 70%, and fresh martensite: 3 to 10%.
- the pole density of the ⁇ 001 ⁇ plane in the ferrite and bainite in the central region is P i
- the ⁇ 001 ⁇ plane in the ferrite and bainite in the surface layer region is 1.2 to 2.0, where P s is the pole density of .
- the hot-rolled steel sheet according to the present embodiment preferably has a metal structure consisting only of ferrite, bainite, tempered martensite, and fresh martensite. That is, the hot-rolled steel sheet according to the present embodiment has a metal structure in which the area ratios are the total of ferrite and bainite: 30 to 47%, tempered martensite: 50 to 70%, and fresh martensite: 3 to 10%. % only.
- the area ratios of ferrite and bainite, tempered martensite, and fresh martensite in the region from 1/8 depth to 3/8 depth from the surface are defined. The reason is that the metallographic structure in this region exhibits a typical metallographic structure of hot-rolled steel sheets.
- Total of ferrite and bainite 30-47% Ferrite and bainite improve the shear workability of hot-rolled steel sheets. If the total area ratio of ferrite and bainite is less than 30%, the hot-rolled steel sheet may have poor shear workability or may have poor fatigue strength. Therefore, the total area ratio of ferrite and bainite is set to 30% or more. It is preferably 33% or more, 35% or more, or 37% or more. On the other hand, if the total area ratio of ferrite and bainite exceeds 47%, the strength and fatigue properties of the hot-rolled steel sheet may deteriorate, or the shear workability of the hot-rolled steel sheet may deteriorate. Therefore, the total area ratio of ferrite and bainite is set to 47% or less.
- Tempered martensite 50-70% Incorporating fresh martensite is an effective way to improve the fatigue properties of hot-rolled steel sheets. It is effective to include the tempered martensite generated by being annealed.
- the area ratio of tempered martensite is set to 50% or more. Preferably, it is 53% or more or 55% or more.
- the area ratio of tempered martensite exceeds 70%, the hot-rolled steel sheet may have poor shear workability or may have poor strength. Therefore, the area ratio of tempered martensite is set to 70% or less. Preferably, it is 65% or less or 60% or less.
- Fresh martensite improves the fatigue strength of hot-rolled steel sheets. If the area ratio of fresh martensite is less than 3%, the fatigue strength of the hot-rolled steel sheet may deteriorate and/or the strength of the hot-rolled steel sheet may deteriorate. Therefore, the area ratio of fresh martensite is set to 3% or more. It is preferably 4% or more or 5% or more. On the other hand, when the area ratio of fresh martensite exceeds 10%, the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of fresh martensite is set to 10% or less. Preferably, it is 9% or less or 8% or less.
- the area ratio of each tissue is obtained by the following method. First, from the hot-rolled steel sheet, in the thickness cross section parallel to the rolling direction, 1/4 depth of the plate thickness from the surface (area from 1/8 depth to 3/8 depth from the surface) and in the width direction The specimen is taken so that the metallographic structure at the central position can be observed.
- an EBSD analyzer composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the EBSD analysis apparatus is 9.6 ⁇ 10 ⁇ 5 Pa or less
- the acceleration voltage is 15 kV
- the irradiation current level is 13
- the electron beam irradiation level is 62.
- the maximum value of the "Grain Average IQ" of the ferrite region was taken as I ⁇ under the condition that the 5° grain boundary in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°) is regarded as the grain boundary.
- a region exceeding I ⁇ /2 is extracted as bainite, and a region below I ⁇ /2 is extracted as “fresh martensite and tempered martensite”.
- the area ratio of the extracted bainite is obtained.
- fresh martensite and tempered martensite are distinguished by the following method.
- a Vickers indentation is stamped near the observation position. After that, leaving the structure of the observation surface, contamination on the surface layer is removed by polishing, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed with a SEM at a magnification of 3000 times.
- the region having a substructure in the grain and having cementite precipitated with multiple variants is called tempered martensite. determine the site. A region with high brightness and in which the substructure is not revealed by etching is judged to be fresh martensite. By calculating each area ratio, the area ratio of tempered martensite and the area ratio of fresh martensite are obtained.
- a method such as buffing using alumina particles with a particle size of 0.1 ⁇ m or less, or Ar ion sputtering may be used.
- the shear workability of the hot-rolled steel sheet is improved. deteriorates.
- the present inventors have found that cracks during shearing are likely to occur in the central region of the regions obtained by dividing the plate thickness cross-section parallel to the rolling direction into three regions in the plate thickness direction.
- the shear workability of the hot-rolled steel sheet is improved by preferably controlling the pole density of ⁇ 001 ⁇ planes in ferrite and bainite in the central region and the surface layer region.
- the pole density of the ⁇ 001 ⁇ plane in the ferrite and bainite in the central region is P i
- the ⁇ 001 ⁇ plane in the ferrite and bainite in the surface layer region is P i
- P i /P s is less than 1.2
- the ⁇ 001 ⁇ planes are uniformly distributed from the surface of the hot-rolled steel sheet.
- crystal rotation occurs from the sheared surface during punching, sag during shearing increases, and cracks are likely to occur in the sheared punched surface, resulting in deterioration in the shear workability of the hot-rolled steel sheet.
- P i /P s is set to 1.2 or more. It is preferably 1.3 or more, 1.4 or more, or 1.5 or more.
- P i /P s greater than 2.0 indicates excessive concentration of ⁇ 001 ⁇ planes in the central region. In this case, the ⁇ 001 ⁇ plane, which is a brittle fracture surface, increases in the fracture surface, and cracks are likely to occur in the punched shear surface. As a result, the shear workability of the hot-rolled steel sheet deteriorates. Therefore, P i /P s is set to 2.0 or less. It is preferably 1.9 or less, 1.8 or less, or 1.7 or less.
- the central region is the region from the depth of 1/3 of the plate thickness to the depth of 2/3 of the plate thickness from the surface, among the regions obtained by dividing the plate thickness cross section parallel to the rolling direction into three in the plate thickness direction. It's about.
- the surface layer region is the region from the surface to the depth of 1/3 of the plate thickness from the surface, or 2 of the plate thickness from the surface, among the regions obtained by dividing the plate thickness cross section parallel to the rolling direction into three in the plate thickness direction. It is a region from a depth of /3 to the back surface (another surface different from the above surface), and in the present embodiment, any region is not particularly limited.
- ⁇ hkl ⁇ represents a crystal plane parallel to the rolled plane. That is, ⁇ hkl ⁇ indicates that the rolling direction and the ⁇ hkl ⁇ plane are parallel.
- a device combining a scanning electron microscope and an EBSD analysis device and OIM Analysis (registered trademark) manufactured by TSL are used.
- the crystal orientation distribution function (ODF: Orientation Distribution Function) that displays the three-dimensional texture calculated using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and the spherical harmonic function can be used to obtain the extreme density. can. Note that the measurement pitch is 5 ⁇ m/step.
- the measurement range is the central area (the area obtained by dividing the thickness cross section parallel to the rolling direction into three areas in the thickness direction, from the depth of 1/3 of the thickness from the surface to the depth of 2/3 of the thickness from the surface). ), and the surface layer region (out of the regions obtained by dividing the plate thickness cross section parallel to the rolling direction into three in the plate thickness direction, the region from the surface to the depth of 1/3 of the plate thickness from the surface, or 2 of the plate thickness from the surface /3 depth to the back surface (another surface different from the above surface)). Also, the pole density is measured for the regions regarded as ferrite and bainite by the same method as the EBSD measurement described above.
- the hot-rolled steel sheet according to the present embodiment has a tensile strength of 950 MPa or more. It is preferably 1000 MPa or more. If the tensile strength is less than 950 MPa, the applicable parts are limited and the contribution to vehicle weight reduction is small. Although the upper limit is not particularly limited, it may be 1500 MPa or less or 1300 MPa or less from the viewpoint of mold wear suppression. Moreover, the hot-rolled steel sheet according to the present embodiment may have a fatigue limit ratio (fatigue strength/tensile strength) of 0.35 or more.
- the tensile strength is evaluated by conducting a tensile test in accordance with JIS Z 2241:2011.
- the test piece shall be JIS Z 2241:2011 No. 5 test piece.
- the tensile test piece is taken from the 1/4 part from the edge in the width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
- Fatigue strength conforms to JIS Z 2275:1978, takes a No. 1 test piece from a hot-rolled steel sheet, and measures it using a Schenk plane bending fatigue tester. The stress load during measurement is set at a test speed of 30 Hz in both swings, and the fatigue strength is measured at 10 7 cycles. Then, the fatigue limit ratio (fatigue strength/tensile strength) is calculated by dividing the fatigue strength at 10 7 cycles by the tensile strength measured by the tensile test described above.
- the thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may become difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the metal structure described above after hot rolling.
- the hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallographic structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., and may be used as a surface-treated steel sheet.
- the plating layer may be an electroplating layer or a hot dipping layer.
- the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy.
- hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy plating. be.
- the amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.
- an appropriate chemical conversion treatment for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying
- the hot-rolled steel sheet according to the present embodiment has the above-described chemical composition and metallographic structure, regardless of the manufacturing method. However, according to the manufacturing method described below, the hot-rolled steel sheet according to the present embodiment can be stably obtained, which is preferable.
- the hot rolling conditions and subsequent cooling conditions are strictly controlled. A detailed description will be given below.
- the heating temperature of the slab has a great effect on solutionization and elimination of elemental segregation. If the heating temperature of the slab is less than 1100° C., solutionization and elimination of elemental segregation are insufficient, and precipitation strengthening of the final product cannot be obtained sufficiently, resulting in deterioration of tensile strength. Further, when the slab heating temperature exceeds 1350°C, not only the effect of solutionization and elimination of elemental segregation is saturated, but also the average grain size of austenite becomes coarse, resulting in uneven crystal rotation during rolling. , it becomes difficult to obtain the desired texture. Therefore, it is preferable to set the heating temperature of the slab to 1100 to 1350.degree. More preferably, it is 1150 to 1300°C.
- the temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
- the slab is continuously passed through a rolling stand for finish rolling multiple times.
- the rolling conditions at the final 3 stands (the final rolling stand, the rolling stand one before the final, and the rolling stand two before the final) are expressed by the following formula (1).
- formula (2) are preferably satisfied.
- the average value of the final three stands satisfy the following formulas (1) and (2).
- symbol in said Formula (1) is as follows.
- R Roll radius (mm)
- H1 Thickness of steel plate on entry side (mm)
- H2 Thickness of steel plate on delivery side (mm)
- ⁇ T in the above formula (2) is the difference between the steel plate entry-side temperature and the steel plate delivery-side temperature at each rolling stand.
- the middle side of the above formula (1) is a formula for obtaining the rolling shape ratio.
- By controlling the rolling shape ratio it is possible to control the crystal rotation by rolling and obtain the desired crystal orientation in the desired region.
- the average value of the rolling shape ratio of the final three stands is less than 2.0, the compressive strain inside the steel sheet increases due to rolling, and the formation of a rolling recrystallized texture causes the extreme density of the ⁇ 001 ⁇ plane in the central region to increase. descend. As a result, P i /P s becomes less than 1.2. Further, when the average value of the rolling shape ratios of the final three stands exceeds 10.0, strong shear deformation is applied to the surface of the steel sheet, and the pole density of the ⁇ 001 ⁇ plane in the surface layer region is extremely increased.
- the average value of the rolling shape ratios of the final three stands is preferably 2.0 to 10.0. That is, the average value of the rolling shape ratio at the final rolling stand, the rolling shape ratio at the rolling stand one before the final, and the rolling shape ratio at the rolling stand two before the final is 2.0 to 10.0. is preferred.
- controlling ⁇ T which is the difference between the steel plate entry-side temperature and the steel plate delivery-side temperature of each rolling stand, is effective for controlling the temperature inside the steel plate.
- ⁇ T which is the difference between the steel plate entry-side temperature and the steel plate delivery-side temperature of each rolling stand.
- heat removal due to contact with the rolling rolls and heat generation from inside the steel sheet due to working energy and frictional heat with the rolls are generated at the same time.
- the plate thickness becomes thinner and the rolling speed becomes faster, so that the heat removal becomes smaller and the influence of working heat generation becomes larger. Therefore, it is important to manufacture the strip at an appropriate threading speed according to the diameter and surface condition of the rolling rolls and the thickness of the strip to be manufactured.
- the average value of ⁇ T in the final three stands is preferably 5-35. That is, the average value of ⁇ T at the last rolling stand, ⁇ T at the rolling stand one before the last, and ⁇ T at the rolling stand two before the last is preferably 5-35.
- the time until the start of cooling exceeds 1.6 seconds the strain due to rolling is recovered, and the extreme density of the ⁇ 001 ⁇ planes in the surface layer region may not be preferably controlled.
- P i /P s may be less than 1.2.
- the time until the start of cooling is more preferably within 0.6 seconds.
- the finish rolling it is preferable to cool to a temperature range of 600 to 750°C at an average cooling rate of 50°C/sec or more. After that, it is preferable to apply air cooling for 2.0 to 6.0 seconds in the temperature range. If the air-cooling temperature range is less than 600° C. or more than 750° C., ferrite transformation does not proceed sufficiently, and a desired amount of ferrite may not be obtained. As a result, the sum of the area ratios of ferrite and bainite may not be the desired amount.
- the average cooling rate of the primary cooling may be 250° C./s or less from the viewpoint of suppressing the expansion of cooling equipment. Further, when the air cooling time in the temperature range of 600 to 750° C.
- the air-cooling time in the temperature range is less than 2.0 seconds, the area ratio of tempered martensite increases, and the area ratio of fresh martensite may not reach the desired level.
- the air cooling After the air cooling, it is preferable to cool to a temperature range of 200°C or less at an average cooling rate of 40°C/second or more as secondary cooling. If the average cooling rate of the secondary cooling is less than 40° C./sec, the critical cooling rate required for martensite transformation may not be obtained, and the desired amount of fresh martensite and/or tempered martensite may not be obtained. .
- the average cooling rate of the secondary cooling may be 250° C./s or less from the viewpoint of suppressing the expansion of cooling equipment.
- the average cooling rate is a value obtained by dividing the temperature drop range of the steel sheet from the start of cooling to the end of cooling by the time required from the start of cooling to the end of cooling.
- the start of cooling is defined as the start of injection of a cooling medium to the steel plate by the cooling equipment, and the end of cooling is defined as the time of drawing out the steel plate from the cooling equipment.
- cooling facilities have no air-cooling section on the way, and some have one or more air-cooling sections on the way.
- any cooling equipment may be used.
- the steel sheet After cooling to a temperature range of 200°C or less by secondary cooling, the steel sheet is coiled. Since the steel sheet is coiled immediately after the secondary cooling, the coiling temperature is almost equal to the cooling stop temperature of the secondary cooling. If the coiling temperature exceeds 200°C, a large amount of ferrite or bainite may be generated, making it impossible to obtain a desired metal structure. Therefore, the winding temperature, which is the cooling stop temperature, is preferably 200° C. or lower.
- the hot-rolled steel sheet may be temper-rolled according to a conventional method, or may be pickled to remove scales formed on the surface.
- a plating treatment such as hot dip plating or electroplating, or a chemical conversion treatment may be applied.
- the hot-rolled steel sheet according to the present embodiment can be stably manufactured.
- a steel having the chemical composition shown in Tables 1A and 1B was melted, and a slab with a thickness of 240 to 300 mm was produced by continuous casting.
- hot-rolled steel sheets shown in Table 3 were obtained under the manufacturing conditions shown in Tables 2A and 2B.
- finish rolling was performed using a finishing mill having seven rolling stands, F5 (rolling stand two before the last), F6 (rolling stand one before the last) and F7 ( The rolling shape ratio and ⁇ T in the final rolling stand) are described.
- the metal structure area ratio, P i /P s , tensile strength and fatigue limit ratio of the obtained hot-rolled steel sheets were obtained by the methods described above. Table 3 shows the measurement results obtained.
- the hot-rolled steel sheet had high strength and was determined to be acceptable.
- the hot-rolled steel sheet did not have high strength and was determined to be unacceptable.
- the fatigue limit ratio was 0.35 or more, the hot-rolled steel sheet had excellent fatigue strength and was determined to be acceptable. On the other hand, when the fatigue limit ratio was less than 0.35, the hot-rolled steel sheet did not have excellent fatigue strength and was determined to be unacceptable.
- the shear workability of the hot-rolled steel sheets was evaluated by the following method. Based on JIS Z 2256:2020, three punched holes were produced by punching at a clearance of 15% and a punching speed of 3 m/s using a ⁇ 10 mm punch. For three punched holes, the maximum length of cracks on the punched shear plane (the cross section perpendicular to the plate surface) was measured. When the maximum crack length was 300 ⁇ m or more, the hot-rolled steel sheet did not have excellent shear workability and was judged to be unacceptable. On the other hand, when the maximum crack length was less than 300 ⁇ m, the hot-rolled steel sheet was judged to have excellent shear workability and was judged to be acceptable.
- the hot-rolled steel sheets according to the examples of the present invention have high strength and excellent fatigue properties and shear workability.
- the hot-rolled steel sheets according to the comparative examples are inferior in at least one of the above properties.
- the present invention it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear workability. According to the hot-rolled steel sheet of the present invention, it is possible to reduce the weight of automobile bodies, integrally mold parts, and shorten processing steps, thereby improving fuel efficiency and reducing manufacturing costs.
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Abstract
Description
本願は、2021年7月27日に、日本に出願された特願2021-122173号に基づき優先権を主張し、その内容をここに援用する。
(1)本発明の一態様に係る熱延鋼板は、化学組成が、質量%で、
C :0.02~0.30%、
Si:0.10~2.00%、
Mn:0.5~3.0%、
P :0.100%以下、
S :0.010%以下、
Al:0.10~1.00%、
N :0.0100%以下、
Ti:0.06~0.20%、
Nb:0~0.10%、
Ca:0~0.0060%、
Mo:0~1.00%、
Cr:0~1.00%、
V :0~0.40%、
Ni:0~0.40%、
B :0~0.0020%、
Cu:0~1.00%、
Sn:0~0.50%、および
Zr:0~0.050%
を含有し、残部がFeおよび不純物であり、
金属組織が、面積率で、
フェライトおよびベイナイトの合計:30~47%、
焼き戻しマルテンサイト:50~70%、
フレッシュマルテンサイト:3~10%であり、
圧延方向に平行な板厚断面を板厚方向に3分割した領域のうち、中央領域のフェライトおよびベイナイトにおける{001}面の極密度をPiとし、表層領域のフェライトおよびベイナイトにおける{001}面の極密度をPsとしたとき、Pi/Psが1.2~2.0であり、
引張強さが950MPa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、
Nb:0.01~0.10%、
Ca:0.0005~0.0060%、
Mo:0.02~1.00%、
Cr:0.02~1.00%、
V :0.01~0.40%、
Ni:0.01~0.40%、
B :0.0001~0.0020%、
Cu:0.02~1.00%、
Sn:0.01~0.50%、および
Zr:0.001~0.050%
からなる群のうち1種または2種以上を含有してもよい。
Cは熱延鋼板の強度を向上させるために重要な元素である。C含有量が0.02%未満であると、所望の強度を得ることができない。そのため、C含有量は0.02%以上とする。好ましくは0.04%以上、0.06%以上または0.10%以上である。
一方、C含有量が0.30%超であると熱延鋼板のせん断加工性が劣化する。そのため、C含有量は0.30%以下とする。好ましくは0.25%以下または0.20%以下である。
Siはフェライト変態中の炭化物の生成を抑制し、熱延鋼板の疲労特性を向上させる効果を有する元素である。Si含有量が0.10%未満であると、この効果を得ることができない。そのため、Si含有量は0.10%以上とする。好ましくは0.20%以上、0.30%以上または0.50%以上である。
一方、Si含有量が2.00%超であると、熱延鋼板のせん断加工性が劣化する。そのため、Si含有量は2.00%以下とする。好ましくは1.80%以下、1.60%以下または1.50%以下である。
Mnは焼入れ性の向上及び固溶強化によって熱延鋼板の強度を向上させるのに有効な元素である。Mn含有量が0.5%未満であると、この効果を得ることができない。そのため、Mn含有量は0.5%以上とする。好ましくは0.7%以上または1.0%以上である。
一方、Mn含有量が3.0%超であると、MnSが生成することで、熱延鋼板の疲労特性が劣化する。そのため、Mn含有量は3.0%以下とする。好ましくは2.8%以下、2.5%以下、2.3%以下または2.0%以下である。
Pは不純物であり、P含有量は低いほど望ましく、0%であることが望ましい。P含有量が0.100%超であると、熱延鋼板の加工性および溶接性が著しく劣化する上、疲労特性も劣化する。そのため、P含有量は0.100%以下とする。好ましくは0.070%以下、0.050%以下または0.030%以下である。
P含有量は、精錬コストの観点から、0.001%以上としてもよい。
Sは不純物であり、S含有量は低いほど望ましく、0%であることが望ましい。S含有量が0.010%超であると、MnS等の介在物が多量に生成され、熱延鋼板のせん断加工性が劣化する。そのため、S含有量は0.010%以下とする。好ましくは0.008%以下、0.007%以下である。より優れたせん断加工性が要求される場合には、S含有量は0.006%以下とすることが好ましい。
S含有量は、精錬コストの観点から、0.001%以上としてもよい。
Alはフェライト変態を制御するために重要な元素である。Al含有量が0.10%未満であると、フェライトの面積率を好ましく制御することができない。そのため、Al含有量は0.10%以上とする。好ましくは0.20%以上、0.30%以上または0.40%以上である。
一方、Al含有量が1.00%超であると、クラスタ状に析出したアルミナが生成し、熱延鋼板のせん断加工性が劣化する。そのため、Al含有量は1.00%以下とする。好ましくは0.90%以下、0.80%以下、0.70%以下または0.60%以下である。
Nは不純物であり、N含有量は低い程望ましく、0%であることが望ましい。N含有量が0.0100%超であると、高温にて粗大なTi窒化物が形成され、熱延鋼板のせん断加工性が劣化する。そのため、N含有量は0.0100%以下とする。好ましくは0.0080%以下、0.0060%以下または0.0050%以下である。
N含有量は、精錬コストの観点から、0.0001%以上としてもよい。
Tiはフェライトを析出強化させる元素であるとともに、フェライト変態を制御して所望量のフェライトを得るために重要な元素である。Ti含有量が0.06%未満であると、析出強化及びフェライト変態制御の効果を得ることができない。そのため、Ti含有量は0.06%以上とする。好ましくは0.08%以上、0.10%以上である。
一方、Ti含有量が0.20%超であると、TiNを起因とした介在物が生成し、熱延鋼板のせん断加工性が劣化する。そのため、Ti含有量は0.20%以下とする。好ましくは0.18%以下または0.16%以下である。
Nbは結晶粒径の微細化及びNbCの析出強化により、熱延鋼板の強度を高める効果を有する元素である。この効果を得る場合、Nb含有量は0.01%以上とすることが好ましい。
一方、Nb含有量が0.10%超では上記効果は飽和する。そのため、Nbを含有させる場合でも、Nb含有量は0.10%以下とする。好ましくは0.06%以下である。
Caは鋼中のSを球形のCaSとして固定し、MnSなどの延伸介在物の生成を抑制して熱延鋼板の穴拡げ性を向上させる元素である。これらの効果を得る場合、Ca含有量は0.0005%以上とすることが好ましい。
一方、Ca含有量が0.0060%超では上記効果は飽和する。そのため、Caを含有させる場合でも、Ca含有量は0.0060%以下とする。好ましくは0.0040%以下である。
Moはフェライトの析出強化により、熱延鋼板の強度向上に有効な元素である。この効果を得る場合、Mo含有量は0.02%以上とすることが好ましい。より好ましくは0.10%以上である。
一方、Mo含有量が1.00%超では、スラブの割れ感受性が高まりスラブの取り扱いが困難になる。そのため、Moを含有させる場合でも、Mo含有量は1.00%以下とする。好ましくは0.60%以下、0.50%以下または0.30%以下である。
Crは熱延鋼板の強度を向上させるのに有効な元素である。この効果を得る場合、Cr含有量は0.02%以上とすることが好ましい。より好ましくは0.10%以上である。
一方、Cr含有量が1.00%超では熱延鋼板の延性が劣化する。そのため、Crを含有させる場合でも、Cr含有量は1.00%以下とする。好ましくは0.80%以下である。
Vは、析出強化および再結晶の抑制による転位強化によって、熱延鋼板の強度を向上させる元素である。これらの効果を得る場合、V含有量は0.01%以上とすることが好ましい。
一方、V含有量が0.40%超では、炭窒化物が多量に析出して熱延鋼板の成形性が低下する。そのため、V含有量は0.40%以下とする。好ましくは0.20%以下である。
Niは、高温での相変態を抑制し、熱延鋼板の強度を向上させる元素である。この効果を得る場合、Ni含有量は0.01%以上とすることが好ましい。
一方、Ni含有量が0.40%超では、熱延鋼板の溶接性が低下する。そのため、Ni含有量は0.40%以下とする。好ましくは0.20%以下である。
Bは、高温での相変態を抑制し、熱延鋼板の強度を向上させる元素である。この効果を得る場合、B含有量は0.0001%以上とすることが好ましい。
一方、B含有量が0.0020%超では、B析出物が生成して熱延鋼板の強度が低下する。そのため、B含有量は0.0020%以下とする。好ましくは0.0005%以下である。
Cuは、微細な粒子の形態で鋼中に存在し、熱延鋼板の強度を向上させる元素である。この効果を得る場合、Cu含有量は0.02%以上とすることが好ましい。
一方、Cu含有量が1.00%超では、熱延鋼板の溶接性が劣化する。そのため、Cu含有量は1.00%以下とする。好ましくは0.80%以下である。
Snは、結晶粒の粗大化を抑制し、熱延鋼板の強度を向上させる元素である。この効果を得る場合、Sn含有量は0.01%以上とすることが好ましい。
一方、Sn含有量が0.50%超では、鋼が脆化して圧延時に破断し易くなる。そのため、Sn含有量は0.50%以下とする。好ましくは0.30%以下である。
Zrは、熱延鋼板の成形性の向上に寄与する元素である。この効果を得る場合、Zr含有量は0.001%以上とすることが好ましい。
一方、Zr含有量が0.050%超では、熱延鋼板の延性が劣化する。そのため、Zr含有量は0.050%以下とする。好ましくは0.030%以下である。
本実施形態に係る熱延鋼板は、金属組織が、面積率で、フェライトおよびベイナイトの合計:30~47%、焼き戻しマルテンサイト:50~70%、フレッシュマルテンサイト:3~10%であり、圧延方向に平行な板厚断面を板厚方向に3分割した領域のうち、中央領域のフェライトおよびベイナイトにおける{001}面の極密度をPiとし、表層領域のフェライトおよびベイナイトにおける{001}面の極密度をPsとしたとき、Pi/Psが1.2~2.0である。
本実施形態に係る熱延鋼板は、金属組織が、フェライト、ベイナイト、焼き戻しマルテンサイトおよびフレッシュマルテンサイトのみからなることが好ましい。すなわち、本実施形態に係る熱延鋼板は、金属組織が、面積率で、フェライトおよびベイナイトの合計:30~47%、焼き戻しマルテンサイト:50~70%、並びに、フレッシュマルテンサイト:3~10%のみからなることが好ましい。
フェライトおよびベイナイトは、熱延鋼板のせん断加工性を向上させる。フェライトおよびベイナイトの面積率の合計が30%未満であると、熱延鋼板のせん断加工性が劣化する場合または熱延鋼板の疲労強度が劣化する場合がある。そのため、フェライトおよびベイナイトの面積率の合計は30%以上とする。好ましくは33%以上、35%以上または37%以上である。
一方、フェライトおよびベイナイトの面積率の合計が47%超であると、熱延鋼板の強度および疲労特性が劣化するまたは熱延鋼板のせん断加工性が劣化する場合がある。そのため、フェライトおよびベイナイトの面積率の合計は47%以下とする。好ましくは45%以下または43%以下である。
なお、本実施形態では、必ずしもフェライトおよびベイナイトの両方を含有する必要はなく、フェライトおよびベイナイトのいずれか一方のみを含有し、その面積率が上述の範囲であってもよい。
熱延鋼板の疲労特性を向上させるためにはフレッシュマルテンサイトを含ませることが効果的であるが、せん断加工性および疲労特性の両立のためには、マルテンサイト生成温度が高く、冷却中に焼戻しされることで生成される焼き戻しマルテンサイトを含ませることが効果的である。
一方、焼き戻しマルテンサイトの面積率が70%超であると、熱延鋼板のせん断加工性が劣化する場合または熱延鋼板の強度が劣化する場合がある。そのため、焼き戻しマルテンサイトの面積率は70%以下とする。好ましくは、65%以下または60%以下である。
フレッシュマルテンサイトは、熱延鋼板の疲労強度を向上させる。フレッシュマルテンサイトの面積率が3%未満であると、熱延鋼板の疲労強度が劣化する場合および/または熱延鋼板の強度が劣化する場合がある。そのため、フレッシュマルテンサイトの面積率は3%以上とする。好ましくは4%以上または5%以上である。
一方、フレッシュマルテンサイトの面積率が10%超であると、熱延鋼板のせん断加工性が劣化する。そのため、フレッシュマルテンサイトの面積率は10%以下とする。好ましくは、9%以下または8%以下である。
まず、熱延鋼板から、圧延方向に平行な板厚断面で、表面から板厚の1/4深さ(表面から1/8深さ~表面から3/8深さの領域)且つ板幅方向中央位置における金属組織が観察できるように試験片を採取する。
圧延面と{001}面とが平行であると、転位のすべり系が少なく、せん断加工中に結晶回転が起きずに、打ち抜きせん断面に割れが生じやすくなるため、熱延鋼板のせん断加工性が劣化する。本発明者らは、せん断加工中の割れは、圧延方向に平行な板厚断面を板厚方向に3分割した領域のうち、中央領域で発生しやすいことを見出した。本実施形態では、中央領域および表層領域のフェライトおよびベイナイトにおける{001}面の極密度を好ましく制御することで、熱延鋼板のせん断加工性を向上させる。
一方、Pi/Psが2.0超であることは、中央領域に{001}面が過剰に集中していることを示す。この場合、脆性破面である{001}面が破断面に多くなり、打ち抜きせん断面に割れを引き起こしやすくなる結果、熱延鋼板のせん断加工性が劣化する。そのため、Pi/Psは2.0以下とする。好ましくは1.9以下、1.8以下または1.7以下である。
また、{hkl}は圧延面に平行な結晶面を表す。すなわち、{hkl}とは、圧延方向と{hkl}面とが平行であることを示す。
本実施形態に係る熱延鋼板は、引張強さが950MPa以上である。好ましくは1000MPa以上である。引張強さが950MPa未満であると、適用部品が限定され、車体軽量化の寄与が小さい。上限は特に限定する必要は無いが、金型摩耗抑制の観点から、1500MPa以下または1300MPa以下としてもよい。
また、本実施形態に係る熱延鋼板は、疲労限度比(疲労強度/引張強さ)が0.35以上であってもよい。
ただし、上記式(1)中の各符号は以下の通りである。
R :ロール半径(mm)
H1:入側の鋼板厚さ(mm)
H2:出側の鋼板厚さ(mm)
ただし、上記式(2)中のΔTは、各圧延スタンドでの鋼板入側温度と鋼板出側温度との差である。
また、最終3スタンドの圧延形状比の平均値が10.0超であると、鋼板表面に強いせん断変形が加わり、表層領域の{001}面の極密度が極端に増加する。その結果、Pi/Psが1.2未満となってしまう。
そのため、最終3スタンドの圧延形状比の平均値は2.0~10.0とすることが好ましい。すなわち、最終の圧延スタンドにおける圧延形状比、最終から1つ前の圧延スタンドにおける圧延形状比および最終から2つ前の圧延スタンドにおける圧延形状比の平均値は2.0~10.0とすることが好ましい。
また、最終3スタンドのΔTの平均値が35超であると、鋼板表面からの抜熱が大きくなるため、鋼板表面のせん断変形が大きくなる。その結果、表層領域の{001}面の極密度が極端に低下し、Pi/Psが2.0超となってしまう。
そのため、最終3スタンドのΔTの平均値は5~35とすることが好ましい。すなわち、最終の圧延スタンドにおけるΔT、最終から1つ前の圧延スタンドにおけるΔTおよび最終から2つ前の圧延スタンドにおけるΔTの平均値は5~35とすることが好ましい。
また、600~750℃の温度域での空冷時間が6.0秒超であると、フェライトが多量に生成され、フェライトおよびベイナイトの面積率の合計が所望量とならない場合がある。当該温度域での空冷時間が2.0秒未満であると、焼き戻しマルテンサイトの面積率が高くなり、フレッシュマルテンサイトの面積率が所望量とならない場合がある。
JIS Z 2256:2020に準拠して、φ10mmのポンチを用いて、クリアランス15%、打ち抜き速度3m/sで打ち抜くことで、3個の打ち抜き穴を作製した。3個の打ち抜き穴について、打ち抜きせん断面(板面に垂直な断面)における割れの最大長さを測定した。割れの最大長さが300μm以上であった場合、優れたせん断加工性を有さない熱延鋼板であるとして不合格と判定した。一方、割れの最大長さが300μm未満であった場合、優れたせん断加工性を有する熱延鋼板であるとして合格と判定した。
Claims (2)
- 化学組成が、質量%で、
C :0.02~0.30%、
Si:0.10~2.00%、
Mn:0.5~3.0%、
P :0.100%以下、
S :0.010%以下、
Al:0.10~1.00%、
N :0.0100%以下、
Ti:0.06~0.20%、
Nb:0~0.10%、
Ca:0~0.0060%、
Mo:0~1.00%、
Cr:0~1.00%、
V :0~0.40%、
Ni:0~0.40%、
B :0~0.0020%、
Cu:0~1.00%、
Sn:0~0.50%、および
Zr:0~0.050%
を含有し、残部がFeおよび不純物であり、
金属組織が、面積率で、
フェライトおよびベイナイトの合計:30~47%、
焼き戻しマルテンサイト:50~70%、
フレッシュマルテンサイト:3~10%であり、
圧延方向に平行な板厚断面を板厚方向に3分割した領域のうち、中央領域のフェライトおよびベイナイトにおける{001}面の極密度をPiとし、表層領域のフェライトおよびベイナイトにおける{001}面の極密度をPsとしたとき、Pi/Psが1.2~2.0であり、
引張強さが950MPa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、
Nb:0.01~0.10%、
Ca:0.0005~0.0060%、
Mo:0.02~1.00%、
Cr:0.02~1.00%、
V :0.01~0.40%、
Ni:0.01~0.40%、
B :0.0001~0.0020%、
Cu:0.02~1.00%、
Sn:0.01~0.50%、および
Zr:0.001~0.050%
からなる群のうち1種または2種以上を含有することを特徴とする、請求項1に記載の熱延鋼板。
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JP2001040450A (ja) | 1999-07-28 | 2001-02-13 | Nkk Corp | せん断端面の疲労特性に優れた高張力熱延鋼板およびその製造方法 |
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