WO2022244707A1 - Tôle en acier laminée à chaud hautement résistante et procédé de fabrication de tôle en acier laminé à chaud hautement résistante - Google Patents

Tôle en acier laminée à chaud hautement résistante et procédé de fabrication de tôle en acier laminé à chaud hautement résistante Download PDF

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WO2022244707A1
WO2022244707A1 PCT/JP2022/020292 JP2022020292W WO2022244707A1 WO 2022244707 A1 WO2022244707 A1 WO 2022244707A1 JP 2022020292 W JP2022020292 W JP 2022020292W WO 2022244707 A1 WO2022244707 A1 WO 2022244707A1
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martensite
less
bainite
rolled steel
steel sheet
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PCT/JP2022/020292
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English (en)
Japanese (ja)
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寛 長谷川
英之 木村
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Ｊｆｅスチール株式会社
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Priority to CN202280034028.XA priority Critical patent/CN117295837A/zh
Priority to JP2022548635A priority patent/JP7239071B1/ja
Priority to KR1020237038669A priority patent/KR20230170031A/ko
Priority to MX2023013344A priority patent/MX2023013344A/es
Priority to EP22804619.9A priority patent/EP4321646A1/fr
Publication of WO2022244707A1 publication Critical patent/WO2022244707A1/fr

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    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high-strength hot-rolled steel sheet and a method for manufacturing a high-strength hot-rolled steel sheet suitable as a material for automobile parts.
  • Patent Document 1 a coated steel sheet having a coating layer made of Al: 50 to 60% by mass and the balance substantially Zn on the surface of the steel sheet and a coating film above the coating layer, the cross-sectional hardness of the base material HM (HV) and cross-sectional hardness HP (HV) of the coating layer satisfy HM>HP and HP ⁇ 90, thereby improving the resistance to bending back.
  • HM base material
  • HP HP
  • Patent Document 2 discloses a hot-rolled steel sheet having a structure containing ferrite as a main phase and retained austenite as a second phase, containing an average of 5% by volume or more of retained austenite, and having a thickness of 0.1 mm from the surface of the steel sheet.
  • the difference (Vmax-Vmin) between the maximum content Vmax and the minimum content Vmin of retained austenite at each position in the thickness direction between 0.1 mm from the back surface is 3.0% by volume or less, and the total thickness equivalent to 2 mm
  • Patent Literature 2 discloses a technique related to a hot-rolled steel sheet having a structure containing ferrite as a main phase and retained austenite so as to have a high total elongation and an improved bend-back property.
  • Patent Document 3 discloses crystal grains having a specific chemical composition and a grain boundary misorientation of 15° or more between adjacent grains, and the average misorientation in the grains is 0 to 0.5°. It contains a certain crystal grain at an area fraction of 50% or more, and the total area fraction of martensite, tempered martensite, and retained austenite is 2% or more and 10% or less, and 40% of Tief represented by a specific formula
  • Patent Literature 3 discloses a technique related to a hot-rolled steel sheet in which ductility is improved by controlling the misorientation in crystal grains.
  • Patent Document 1 is limited to studying only the bending back cracks originating from the coating, and does not study the bending back cracks that occur in hot-rolled steel sheets that do not have a coating layer.
  • Patent Literature 2 only provides knowledge on strength of 900 MPa or less, and does not provide knowledge or suggestions regarding improvement of ductility and bending reversibility at a class of over 980 MPa, which is a more stringent requirement.
  • Patent Document 3 can improve the ductility, there is room for improvement since no study has been made on the bending reversibility.
  • the present invention is intended to solve the above problems, and is a high-strength hot-rolled steel sheet having excellent ductility and excellent bending rebound properties suitable as a material for automotive parts, and a method for producing a high-strength hot-rolled steel sheet. intended to provide
  • high strength in the present invention means that TS (tensile strength) is 980 MPa or more.
  • excellent ductility refers to a uniform elongation of 5.0% or more in a tensile test.
  • excellent bending and unbending properties means that, in the bending and unbending test described later, after bending 90 ° V with a punch having a bending radius of 5 mm, bending back with a flat bottom punch to a bending angle of 10 ° or less. It means that cracks do not occur on the ridgeline of the test piece when it is pressed.
  • the tensile test for measuring the TS and uniform elongation and the bending and bending back test can be performed by the methods described in the examples below.
  • the present inventors focused on the hard phase and came up with the idea of increasing the uniform elongation by promoting work hardening by controlling the fraction of the hard phase.
  • crystal orientation of the hard phase and the region surrounded by the boundary with the misorientation of the adjacent crystals of 15° or more are defined as crystal grains, by controlling the aspect ratio of the crystal grains in the surface layer of the steel sheet, bending It was conceived to improve the bendability.
  • the main phases are martensite and bainite, and the martensite is dispersed in the bainite.
  • the crystal orientation of martensite in bainite and the crystal orientation of bainite around the martensite are controlled to be different.
  • the inventors have found that even a hot-rolled steel sheet of over 980 MPa can be improved in both ductility and unbendability, and have completed the present invention.
  • the gist of the present invention is as follows. [1] in % by mass, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5-3.5%, P: more than 0% and 0.100% or less, S: more than 0% and 0.020% or less, Al: including more than 0% and 1.5% or less, Further, Cr: 0.005-2.0%, Ti: 0.005-0.20%, Nb: 0.005-0.20%, Mo: 0.005-2.0%, V: 0.005-2.0%.
  • the steel structure has a main phase of martensite and bainite with a total area ratio of 80 to 100%, The total area ratio of martensite in bainite is 2 to 20%, Among martensite in bainite, the area ratio of martensite having an orientation difference of 15° or more between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite is more than 50% relative to martensite, The average aspect ratio of the crystal grains present in a region from the surface of the steel sheet to a depth of 5 ⁇ m is 2.0 or less when the crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals.
  • High-strength hot-rolled steel sheet [2] In addition to the above component composition, in mass%, Cu: 0.05 to 4.0%, Ni: 0.005 to 2.0%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, Sn: 0.0010-0.50%
  • the high-strength hot-rolled steel sheet according to [1] above containing one or more selected from among.
  • the present invention it is possible to provide a high-strength hot-rolled steel sheet with excellent ductility and bending unbendability, which is suitable as a material for automotive parts, and a method for producing a high-strength hot-rolled steel sheet. If the high-strength hot-rolled steel sheet of the present invention is used as a raw material for automobile parts, products such as high-strength automobile parts having complex shapes can be obtained.
  • FIG. 1 is a schematic diagram illustrating the aspect ratio of crystal grains in the present invention.
  • the high-strength hot-rolled steel sheet and the method for producing the high-strength hot-rolled steel sheet of the present invention are described in detail below. In addition, this invention is not limited to the following embodiment.
  • the high-strength hot-rolled steel sheet of the present invention is a hot-rolled steel sheet called black scale as hot rolled or white scale further pickled after hot rolling.
  • the high-strength hot-rolled steel sheet targeted by the present invention preferably has a thickness of 0.6 mm or more and 10.0 mm or less, and when used as a material for automobile parts, it is 1.0 mm or more and 6.0 mm or less. is more preferable.
  • the plate width is preferably 500 mm or more and 1800 mm or less, more preferably 700 mm or more and 1400 mm or less.
  • the high-strength hot-rolled steel sheet of the present invention has a specific chemical composition and a specific steel structure.
  • the chemical composition and the steel structure will be explained in order.
  • the chemical composition of the high-strength hot-rolled steel sheet of the present invention is, in mass%, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.5%, P: more than 0% and 0.100% or less, S: more than 0% and 0.020% or less, Al: more than 0% and 1.5% or less, Cr: 0.005 to 2.0%, Ti: One or two selected from 0.005 to 0.20%, Nb: 0.005 to 0.20%, Mo: 0.005 to 2.0%, V: 0.005 to 1.0% It contains more than seeds, and the balance consists of Fe and unavoidable impurities.
  • C 0.04-0.18%
  • C is an effective element for generating and strengthening bainite and martensite to raise TS. If the C content is less than 0.04%, such an effect cannot be sufficiently obtained, and a TS of 980 MPa or more cannot be obtained. On the other hand, if the C content exceeds 0.18%, hardening of martensite becomes remarkable, and the bend unbendability of the present invention cannot be obtained. Therefore, the C content should be 0.04 to 0.18%. From the viewpoint of stably obtaining a TS of 980 MPa or more, the C content is preferably 0.05% or more. The C content is preferably 0.16% or less, more preferably 0.10% or less, from the viewpoint of improving the bending unbendability.
  • Si 0.1-3.0%
  • Si is an element effective in increasing TS by solid-solution strengthening of steel and suppressing temper softening of martensite.
  • it is an element effective in suppressing cementite and obtaining a structure in which martensite is dispersed in bainite.
  • the Si content must be 0.1% or more.
  • the Si content should be 0.1 to 3.0%.
  • the Si content is preferably 0.2% or more.
  • the Si content is preferably 2.0% or less, more preferably 1.5% or less.
  • Mn 0.5-3.5%
  • Mn is an element effective in generating martensite and bainite to raise TS. If the Mn content is less than 0.5%, such an effect cannot be sufficiently obtained, polygonal ferrite or the like is generated, and the steel structure of the present invention cannot be obtained. On the other hand, if the Mn content exceeds 3.5%, bainite is suppressed and the steel structure of the present invention cannot be obtained. Therefore, the Mn content should be 0.5 to 3.5%.
  • the Mn content is preferably 1.0% or more from the viewpoint of more stably obtaining a TS of 980 MPa or more. From the viewpoint of stably obtaining bainite, the Mn content is preferably 3.0% or less, more preferably 2.3% or less.
  • the P content can be allowed up to 0.100%. Therefore, the P content should be 0.100% or less, preferably 0.030% or less.
  • the P content is more than 0%, and if the P content is less than 0.001%, the production efficiency is lowered, so 0.001% or more is preferable.
  • S more than 0% and 0.020% or less S lowers the bending unbendability, so it is preferable to reduce the amount as much as possible, but the present invention allows an S content of up to 0.020%. Therefore, the S content should be 0.020% or less, preferably 0.0050% or less, more preferably 0.0020% or less. The S content is more than 0%, and if the S content is less than 0.0002%, the production efficiency is lowered, so 0.0002% or more is preferable.
  • Al more than 0% and 1.5% or less Al acts as a deoxidizing agent and is preferably added in the deoxidizing step.
  • the lower limit of the Al content is more than 0%, and from the viewpoint of use as a deoxidizing agent, the Al content is preferably 0.01% or more. If a large amount of Al is contained, a large amount of polygonal ferrite is generated and the steel structure of the present invention cannot be obtained.
  • the present invention allows an Al content of up to 1.5%. Therefore, the Al content is set to 1.5% or less. It is preferably 0.50% or less.
  • Cr 0.005-2.0%
  • Ti 0.005-0.20%
  • Nb 0.005-0.20%
  • Mo 0.005-2.0%
  • V 0.005- One or Two or More Selected from 1.0% Cr, Ti, Nb, Mo and V are elements effective for obtaining a structure in which martensite is dispersed in bainite.
  • the content of one or more elements selected from the above elements should be equal to or higher than the respective lower limits.
  • the content of one or more elements selected from the above elements exceeds their respective upper limits, such effects cannot be obtained, and the steel structure of the present invention cannot be obtained.
  • Cr 0.005-2.0%, Ti: 0.005-0.20%, Nb: 0.005-0.20%, Mo: 0.005-2.0%, V: 0.005-2.0%. 005 to 1.0%.
  • Cr 0.1% or more, Ti: 0.010% or more, Nb: 0.010% or more, Mo: 0.10% or more, V: 0.10% That's it.
  • the upper limits are preferably Cr: 1.0% or less, Ti: 0.15% or less, Nb: 0.10% or less, Mo: 1.0% or less, V: 0.1% or less, respectively. 5% or less.
  • the balance is Fe and unavoidable impurities.
  • unavoidable impurity elements include N, and the allowable upper limit of this element is preferably 0.010%.
  • the above ingredients are the basic ingredient composition of the high-strength hot-rolled steel sheet of the present invention.
  • the following elements can be further contained as necessary.
  • Cu 0.05-4.0%, Ni: 0.005-2.0%, B: 0.0002-0.0050%, Ca: 0.0001-0.0050%, REM: 0.0001- 0.0050%, Sb: 0.0010 to 0.10%, Sn: 1 or 2 or more selected from 0.0010 to 0.50% Cu and Ni generate martensite and have high strength It is an effective element that contributes to In order to obtain such an effect, when Cu and Ni are contained, it is preferable to make each content equal to or higher than the above lower limit. When the respective contents of Cu and Ni exceed the above upper limits, bainite is suppressed and the steel structure of the present invention may not be obtained.
  • the Cu content is more preferably 0.10% or more and more preferably 0.6% or less.
  • the Ni content is more preferably 0.1% or more, and more preferably 0.6% or less.
  • B is an effective element that enhances the hardenability of steel sheets, generates martensite, and contributes to high strength.
  • the B content is preferably 0.0002% or more.
  • the content is preferably 0.0002 to 0.0050%.
  • the B content is more preferably 0.0005% or more and more preferably 0.0040% or less.
  • Ca and REM are elements that are effective in improving workability by controlling the morphology of inclusions.
  • the content of Ca is 0.0001 to 0.0050% and the content of REM is 0.0001 to 0.0050%. If the contents of Ca and REM exceed the above upper limits, the amount of inclusions may increase and workability may deteriorate.
  • the Ca content is more preferably 0.0005% or more and more preferably 0.0030% or less.
  • the REM content is more preferably 0.0005% or more and more preferably 0.0030% or less.
  • Sb is an element that suppresses denitrification, deboronization, etc., and is effective in suppressing a decrease in strength of steel.
  • the Sb content is preferably 0.0010 to 0.10%. If the Sb content exceeds the above upper limit, the steel sheet may become embrittled.
  • the Sb content is more preferably 0.0050% or more, and more preferably 0.050% or less.
  • Sn is an element that suppresses the formation of pearlite and is effective in suppressing the strength reduction of steel.
  • the Sn content is preferably 0.0010 to 0.50%. If the Sn content exceeds the above upper limit, the steel sheet may become embrittled.
  • the Sn content is more preferably 0.0050% or more, and more preferably 0.050% or less.
  • the steel structure of the high-strength hot-rolled steel sheet of the present invention has a total area ratio of 80 to 100% martensite and bainite as the main phase, and the total area ratio of martensite in bainite is 2 to 20%.
  • the area ratio of martensites having an orientation difference of 15° or more between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite is the total martensite is more than 50% relative to the grain, and the crystal grains are present in the region from the surface of the steel sheet to a depth of 5 ⁇ m when the region surrounded by the boundary with an orientation difference of 15 ° or more between adjacent crystals is the average aspect of the crystal grains.
  • the ratio is 2.0 or less.
  • Total area ratio of martensite and bainite 80-100%
  • a steel structure that mainly contains martensite and bainite (martensite and bainite are the main phases) is used in order to provide a high TS and excellent bending and unbending properties. If the total area ratio of martensite and bainite is less than 80% with respect to the entire steel sheet structure, either high TS or bending back bending properties cannot be obtained. Therefore, the total area ratio of martensite and bainite should be 80 to 100%. It is preferably 90 to 100%, more preferably 94 to 100%.
  • Total area ratio of martensite in bainite 2-20% Martensite is a steel structure effective for increasing TS, and is a steel structure effective for increasing uniform elongation by being dispersed in bainite. To obtain such an effect, the total area ratio of martensite in bainite must be 2% or more. On the other hand, if the total area ratio of martensite exceeds 20%, the uniform elongation and bending rebound properties are deteriorated. Therefore, the total area ratio of martensite is set to 2 to 20%.
  • the total area ratio of martensite is preferably 3% or more, more preferably 4% or more.
  • the total area ratio of martensite is preferably 15% or less, more preferably 12% or less.
  • the area ratio of martensite having an orientation difference of 15° or more between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite total martensite Martensite in martensite in more than 50% of the site, in which the orientation difference between the crystal orientation of the martensite and the crystal orientation in at least one bainite of the bainite adjacent to the martensite is 15° or more
  • the above-mentioned "martensite having an orientation difference of 15° or more between the crystal orientation of the martensite and the crystal orientation of at least one bainite among the bainite adjacent to the martensite” is, for example, a plurality of It means that when there is martensite surrounded by bainite having a crystal orientation, the orientation difference between at least one bainite among the bainite having a plurality of crystal orientations and the martensite is 15° or more. .
  • the area ratio of the martensitic dispersed phase is set to exceed 50%.
  • the bending and unbending properties of the present invention can be achieved by setting the area ratio to more than 50%.
  • the area ratio of the martensite dispersed phase is set to more than 50% with respect to all martensite. It is preferably 60% or more, more preferably 70% or more.
  • the upper limit of the area ratio of the martensite dispersed phase is not particularly defined. Since it is difficult to make it substantially 100%, less than 100% is preferable.
  • the above-mentioned "martensite dispersed phase” can be measured by the method described in Examples below. Specifically, the crystal orientations of bainite and martensite are obtained by electron electron scattering diffraction (EBSD), and the boundaries of misorientation of 15° or more are displayed. And, among the martensite dispersed in the bainite, the martensite has a crystal orientation difference of 15° or more between the martensite and at least one bainite among the bainite adjacent to the martensite (adjacent bainite). find the rate.
  • EBSD electron electron scattering diffraction
  • the steel structure of the present invention may have ferrite, pearlite, and retained austenite as structures other than martensite and bainite described above.
  • the total area ratio of structures other than martensite and bainite shall be less than 20% (including 0%). If the total area ratio is less than 20%, the characteristics of the present invention can be achieved.
  • Average aspect ratio of crystal grains present in the region from the surface of the steel sheet to a depth of 5 ⁇ m 2.0 or less Crystal grains in the surface layer of the steel sheet become the starting points of cracks during bending and unbending, and grains with a larger aspect ratio are more likely to crack. .
  • the average aspect ratio of crystal grains present in a region from the surface of the steel sheet to a depth of 5 ⁇ m must be 2.0 or less.
  • the average aspect ratio of crystal grains is preferably 1.7 or less, more preferably 1.5 or less.
  • the above-mentioned “crystal grain” refers to a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals.
  • the above-mentioned “aspect ratio” means that, when the maximum length of the grain in the rolling direction is RL and the maximum length of the grain in the plate thickness direction is TL, the maximum length in the rolling direction RL and the plate thickness It is obtained by the ratio of the maximum length TL in the rolling direction (maximum length RL in the rolling direction/maximum length TL in the plate thickness direction).
  • the above-mentioned "average aspect ratio of crystal grains” is the average of the aspect ratios of the crystal grains present in the region from the surface of the steel sheet to a depth of 5 ⁇ m.
  • the area ratio, crystal orientation, and aspect ratio of each structure described above can be measured by the methods described in Examples described later.
  • the high-strength hot-rolled steel sheet of the present invention is produced by heating a slab having the chemical composition described above and then subjecting it to hot rolling.
  • the heated slab is roughly rolled, the total number of passes at 1000 ° C. or higher is 3 or more, the total rolling reduction at 1000 ° C. or lower is less than 50%, and the final pass rolling temperature to the final pass rolling temperature
  • cooling is started in less than 1.0 s, and the average cooling rate from the cooling start temperature to 550 ° C. is 50 ° C./s or more. and then coiled at a coiling temperature of (Ms point -50)°C to 550°C and cooled to room temperature.
  • the manufacturing method will be explained in detail below.
  • the above temperature is the temperature (surface temperature) of the width center of the slab or steel plate, and the above average cooling rate is the average cooling speed of the width center of the steel plate. These temperatures can be measured with a radiation thermometer or the like.
  • Total number of passes at 1000°C or higher 3 or more
  • reduction at 1000°C or higher is performed 3 or more times to promote recrystallization of austenite and form grains with a small aspect ratio in the surface layer of the steel sheet. can be generated. Therefore, the total number of passes at 1000° C. or higher should be 3 or more. Preferably, 4 times or more.
  • the upper limit of the total number of passes at 1000° C. or higher is not specified. From the viewpoint of production efficiency and the like, it is preferable to set the number to 20 times or less.
  • Total rolling reduction at 1000°C or less less than 50%
  • the total rolling reduction at 1000°C or less in the finish rolling of hot rolling is 50% or more
  • grains with a large aspect ratio are formed in the surface layer of the steel sheet, and adjacent bainite is formed.
  • martensite having a crystal orientation close to that is likely to be generated, and the steel structure of the present invention cannot be obtained. Therefore, the total rolling reduction at 1000° C. or less is set to less than 50%. It is preferably less than 40%, more preferably less than 30%. There is no particular lower limit for the total rolling reduction at 1000°C or lower. Since abnormal grains may occur under light reduction, it is preferably 10% or more.
  • the total rolling reduction is the difference between the inlet strip thickness before the first pass in the above temperature range and the outlet strip thickness after the final pass in this temperature range, divided by the inlet strip thickness before the first pass. is a percentage of the value That is, (inlet plate thickness before the first pass in the above temperature range - outlet plate thickness after the final pass in the above temperature range) / (inlet plate thickness before the first pass in the above temperature range) x 100 (%) be done.
  • Total rolling reduction from the final pass rolling temperature to the final pass rolling temperature + 50 ° C. 35% or less
  • the rolling reduction exceeds 35% near the final pass temperature (hereinafter also referred to as FT)
  • expanded grains occur near the surface layer.
  • the average aspect ratio of the crystal grains present in the region from the surface of the steel sheet of the present invention to a depth of 5 ⁇ m cannot be obtained.
  • the amount of strain introduced into austenite becomes excessive, and martensite having the crystal orientation relationship of the present invention cannot be obtained. Therefore, the total rolling reduction from the final pass rolling temperature to the final pass rolling temperature +50° C. is 35% or less, preferably 30% or less.
  • the lower limit is not specified, if the rolling reduction is too low, surface defects may occur, so it is preferably 5% or more, more preferably 10% or more.
  • Cooling time after finish rolling less than 1.0 s After finish rolling, cooling is started in less than 1.0 s (seconds). If the cooling time after finish rolling is 1.0 s or longer, the martensite dispersed phase having the crystal orientation of the present invention cannot be obtained. The reason for this is not clear, but it is thought that the recovery of dislocations introduced by finish rolling is suppressed by reducing the cooling time, which affects the orientation selection during subsequent bainite and martensite transformations. be done. Therefore, the cooling time after finish rolling is set to less than 1.0 s. It is preferably 0.7 s or less. The lower limit of the cooling time is not particularly defined. Since it is difficult to start cooling immediately after rolling due to restrictions on equipment structure, etc., the cooling time is preferably 0.01 s or more.
  • Average cooling rate from the cooling start temperature to 550°C 50°C/s or more
  • the average cooling rate from the cooling start temperature to 550° C. should be 50° C./s or more. It is preferably 80° C./s or higher.
  • the average cooling rate is preferably 1000° C./s or less from the viewpoint of the shape stability of the steel sheet.
  • Winding temperature (Ms point -50) ° C ⁇ 550 ° C If the coiling temperature is less than (Ms point -50)°C, martensite increases and the steel structure of the present invention cannot be obtained. On the other hand, when the temperature exceeds 550°C, ferrite and pearlite are generated, and the steel structure of the present invention cannot be obtained. Therefore, the winding temperature is (Ms point-50)°C to 550°C. It is preferably (Ms point ⁇ 30)° C. or higher, preferably 520° C. or lower.
  • the Ms point is the temperature at which martensite transformation starts, and can be determined by actually measuring thermal expansion during cooling by a Formaster test or by measuring electrical resistance.
  • the slab heating temperature is preferably 1100° C. or higher from the viewpoint of segregation removal, precipitate solid solution, etc., and is preferably 1300° C. or lower from the viewpoint of energy efficiency.
  • the finish rolling is performed by 4 or more passes from the viewpoint of reducing coarse grains that cause deterioration of workability.
  • the number of passes in finish rolling refers to the total number of passes in finish rolling, and includes the above-mentioned "total number of passes at 1000°C or lower".
  • a slab was produced by melting the steel with the chemical composition shown in Table 1 in a vacuum melting furnace. After that, these slabs were heated to 1200° C. and hot rolled under the conditions shown in Table 2 to obtain hot rolled steel sheets. In hot rolling, the total number of passes of finish rolling was set to 7 passes.
  • a blank in Table 1 indicates that the element is not added intentionally, and includes not only the case of no element (0%) but also the case of unavoidable inclusion. Also, N is an unavoidable impurity.
  • the area ratio of martensite and bainite means the ratio of the area of each structure to the observed area.
  • the area ratio of martensite is determined as follows. A sample was cut out from the obtained hot-rolled steel sheet, the thickness cross section parallel to the rolling direction was polished, then corroded with 3% nital, and the 1/4 thickness position was examined by SEM (scanning electron microscope) at a magnification of 1500 times. Three fields of view are photographed. The area ratio of each tissue is obtained from the obtained image data of the secondary electron image using Image-Pro manufactured by Media Cybernetics, and the average area ratio of the field of view is taken as the area ratio of each tissue.
  • upper bainite is distinguished as black or dark gray containing carbide or martensite with straight interfaces, or retained austenite.
  • Lower bainite is distinguished as black, dark gray, gray, or light gray containing oriented carbides.
  • Martensite is distinguished as black, dark gray, gray, or light gray with multiple orientations of carbides, or white or light gray with no carbides.
  • Retained austenite is distinguished as white or light gray with no carbides.
  • the total area ratio of martensite and retained austenite obtained from the SEM image is divided by the area ratio of retained austenite obtained by the method described later, and the area ratio of martensite asked for
  • martensite may be any martensite such as fresh martensite, autotempered martensite, and tempered martensite.
  • the bainite may be any bainite such as upper bainite, lower bainite, and tempered bainite.
  • ferrite is black or dark gray and can be distinguished as a structure that has no or a slight amount of carbide inside and is mainly surrounded by curved boundaries.
  • Perlite can be distinguished as a black and white lamellar or partially interrupted near-lamellar structure.
  • the steel plate after annealing was ground to a position of 1/4 + 0.1 mm of the plate thickness, and then the surface was further polished by 0.1 mm by chemical polishing.
  • Measurement of integrated reflection intensity of fcc iron (austenite) (200), (220) and (311) planes and bcc iron (ferrite) (200), (211) and (220) planes Did.
  • the volume ratio is obtained from the intensity ratio of the integrated reflection intensity from each surface of fcc iron to the integrated reflection intensity from each surface of bcc iron, and this is defined as the area ratio of retained austenite.
  • the crystal orientation of bainite and martensite was obtained by electron backscattering diffraction (EBSD), and by displaying the boundary of the misorientation of 15° or more, it was dispersed in bainite.
  • the area ratio of martensite having a crystal orientation difference of 15° or more between the martensite and at least one bainite among the bainite adjacent to the martensite (adjacent bainite) was determined. Then, the ratio of the martensite area to the total martensite area was determined.
  • the crystal orientation of the surface layer portion of the same sample used for the observation of the structure is determined by EBSD, the boundaries of adjacent crystals with a misorientation of 15° or more are displayed, and the regions surrounded by these boundaries are defined as crystal grains.
  • the maximum length RL in the rolling direction and the maximum length TL in the plate thickness direction are obtained for each grain existing in a region up to 5 ⁇ m in the depth direction (thickness direction) from the steel plate surface ( See Figure 1).
  • the aspect ratio of each grain is calculated from the ratio of the maximum length RL in the rolling direction to the maximum length TL in the thickness direction of these grains (RL/TL), and the average of the calculated values is the average aspect ratio of the grain. ratio.
  • the ratio between the maximum length RL in the rolling direction and the maximum length TL in the thickness direction is determined so that the minimum value of the aspect ratio is 1.0.
  • crystal grains straddling a position of 5 ⁇ m in the depth direction from the steel plate surface are counted as crystal grains within a region up to 5 ⁇ m in the depth direction from the steel plate surface.
  • the EBSD measurement is performed at an acceleration voltage of 30 kV and a step size of 0.10 ⁇ m for a 100 ⁇ m ⁇ 100 ⁇ m region, and the aspect ratio of the crystal grains is measured for all applicable crystal grains in the region (100 ⁇ m ⁇ 100 ⁇ m region). .
  • a JIS No. 5 tensile test piece (JIS Z 2201) was taken from the obtained hot-rolled steel sheet in a direction parallel to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241 at a strain rate of 10 -3 /s. was performed to obtain TS and uniform elongation.
  • a TS of 980 MPa or more and a uniform elongation of 5.0% or more were evaluated as acceptable.
  • the bending-bending-back property was evaluated by a bending-bending-back test.
  • a test piece having a width of 30 mm and a length of 100 mm was taken from the obtained hot-rolled steel sheet so that the longitudinal direction was parallel to the rolling direction.
  • a test piece is subjected to 90° V bending under conditions of a stroke speed of 10 mm/min, a bending radius of 5 mm, and a maximum pressing load of 10 tons.
  • the test piece is turned over and pressed with a flat-bottomed punch at a stroke speed of 10 mm/min. The stroke is stopped at a bending angle of 10° or less, the load is removed, and the sample is taken out.
  • Table 3 shows various evaluation results.
  • the present invention it is possible to obtain a high-strength hot-rolled steel sheet having a TS of 980 MPa or more, excellent ductility, and excellent unbendability.
  • the use of the high-strength hot-rolled steel sheet of the present invention for automobile parts can greatly contribute to the improvement of collision safety and fuel efficiency of automobiles.

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Abstract

L'invention concerne une tôle en acier laminée à chaud hautement résistante possédant une composition spécifique. La composition de l'acier est telle que: la phase principale est composée de martensite et de bainite dans un rapport de surface total compris entre 80 à 100% ; le rapport de surface total de la martensite à l'intérieur de la bainite est compris entre 2 et 20%; dans la martensite à l'intérieur de la bainite, la différence d'orientation entre les cristaux de martensite et au moins une bainite adjacente à cette martensite est supérieure ou égale à 15°, et le rapport de surface de cette martensite par rapport à la totalité de la martensite est supérieur à 50%; lorsque la région délimitée par une bordure dans laquelle la différence d'orientation de cristaux adjacents est supérieure ou égale à 15° est considérée comme des grains cristalllins, le rapport d'aspect moyen desdits grains cristallins situés dans la région de la surface de la tôle d'acier à une profondeur de 5 μm, est inférieur ou égal à 2.
PCT/JP2022/020292 2021-05-17 2022-05-13 Tôle en acier laminée à chaud hautement résistante et procédé de fabrication de tôle en acier laminé à chaud hautement résistante WO2022244707A1 (fr)

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CN202280034028.XA CN117295837A (zh) 2021-05-17 2022-05-13 高强度热轧钢板和高强度热轧钢板的制造方法
JP2022548635A JP7239071B1 (ja) 2021-05-17 2022-05-13 高強度熱延鋼板及び高強度熱延鋼板の製造方法
KR1020237038669A KR20230170031A (ko) 2021-05-17 2022-05-13 고강도 열연 강판 및 고강도 열연 강판의 제조 방법
MX2023013344A MX2023013344A (es) 2021-05-17 2022-05-13 Lamina de acero laminada en caliente de alta resistencia y metodo para fabricar lamina de acero laminada en caliente de alta resistencia.
EP22804619.9A EP4321646A1 (fr) 2021-05-17 2022-05-13 Tôle en acier laminée à chaud hautement résistante et procédé de fabrication de tôle en acier laminé à chaud hautement résistante

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WO2024117442A1 (fr) * 2022-12-01 2024-06-06 현대제철 주식회사 Tôle d'acier laminée à chaud, pièce pour véhicule et son procédé de fabrication

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JP2001032041A (ja) 1999-07-26 2001-02-06 Kawasaki Steel Corp 加工性に優れた高強度熱延鋼板およびその製造方法
JP2008156729A (ja) 2006-12-26 2008-07-10 Nisshin Steel Co Ltd 耐曲げ戻し性に優れたＺｎ−Ａｌ系めっき塗装鋼板およびその製造法
JP2016204690A (ja) 2015-04-17 2016-12-08 新日鐵住金株式会社 延性と疲労特性と耐食性に優れた高強度熱延鋼板とその製造方法
WO2017017933A1 (fr) * 2015-07-27 2017-02-02 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud à haute résistance et procédé de fabrication pour cette dernière
WO2018150955A1 (fr) * 2017-02-17 2018-08-23 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud de haute résistance et son procédé de fabrication
WO2020080553A1 (fr) * 2018-10-19 2020-04-23 日本製鉄株式会社 Tôle d'acier laminée à chaud et procédé pour la fabrication de celle-ci
WO2021090642A1 (fr) * 2019-11-06 2021-05-14 日本製鉄株式会社 Tôle d'acier laminée à chaud et procédé de production correspondant

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2001032041A (ja) 1999-07-26 2001-02-06 Kawasaki Steel Corp 加工性に優れた高強度熱延鋼板およびその製造方法
JP2008156729A (ja) 2006-12-26 2008-07-10 Nisshin Steel Co Ltd 耐曲げ戻し性に優れたＺｎ−Ａｌ系めっき塗装鋼板およびその製造法
JP2016204690A (ja) 2015-04-17 2016-12-08 新日鐵住金株式会社 延性と疲労特性と耐食性に優れた高強度熱延鋼板とその製造方法
WO2017017933A1 (fr) * 2015-07-27 2017-02-02 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud à haute résistance et procédé de fabrication pour cette dernière
WO2018150955A1 (fr) * 2017-02-17 2018-08-23 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud de haute résistance et son procédé de fabrication
WO2020080553A1 (fr) * 2018-10-19 2020-04-23 日本製鉄株式会社 Tôle d'acier laminée à chaud et procédé pour la fabrication de celle-ci
WO2021090642A1 (fr) * 2019-11-06 2021-05-14 日本製鉄株式会社 Tôle d'acier laminée à chaud et procédé de production correspondant

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2024117442A1 (fr) * 2022-12-01 2024-06-06 현대제철 주식회사 Tôle d'acier laminée à chaud, pièce pour véhicule et son procédé de fabrication

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