WO2024053701A1 - Tôle d'acier laminée à chaud - Google Patents

Tôle d'acier laminée à chaud Download PDF

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WO2024053701A1
WO2024053701A1 PCT/JP2023/032635 JP2023032635W WO2024053701A1 WO 2024053701 A1 WO2024053701 A1 WO 2024053701A1 JP 2023032635 W JP2023032635 W JP 2023032635W WO 2024053701 A1 WO2024053701 A1 WO 2024053701A1
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less
rolled steel
steel sheet
hot
rolling
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PCT/JP2023/032635
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English (en)
Japanese (ja)
Inventor
洋志 首藤
栄作 桜田
洵 安藤
壽生 杉山
和政 筒井
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日本製鉄株式会社
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • 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
    • C22C38/18—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

Definitions

  • the present invention relates to a hot rolled steel plate. Specifically, hot-rolled steel sheets are used after being formed into various shapes by press working, etc., and in particular, have high strength and critical thickness reduction rate at break, as well as excellent ductility and shear workability.
  • the present invention relates to a hot rolled steel sheet having excellent fatigue properties after press forming.
  • the critical thickness reduction rate at break and ductility are positioned as important indicators of formability.
  • the critical rupture plate thickness reduction rate is a value determined from the plate thickness of the tensile test piece before rupture and the minimum value of the plate thickness of the tensile test piece after rupture. If the critical thickness reduction rate at rupture is low, it is not preferable because it is likely to break early when tensile strain is applied during press forming.
  • Automotive parts are formed by press forming, and blank plates for press forming are often manufactured by shearing, which is highly productive. Blank plates manufactured by shearing must have excellent end face accuracy after shearing.
  • steel sheets used for automobile parts are required to have excellent fatigue properties after press forming.
  • Patent Document 1 discloses a hot-rolled steel sheet that is a material for a cold-rolled steel sheet with excellent surface properties after press working, in which the degree of Mn segregation and the degree of P segregation in the central part of the sheet thickness are controlled.
  • Patent Document 1 does not consider the critical thickness reduction rate at rupture, shear workability, and fatigue properties after press forming of the hot rolled steel sheet.
  • the present invention was made in view of the above-mentioned circumstances, and the present invention has been made in view of the above-mentioned circumstances.
  • the purpose is to provide rolled steel sheets.
  • the gist of the invention is as follows. (1)
  • the hot rolled steel sheet according to one aspect of the present invention has a chemical composition in mass %, C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, sol.
  • each element symbol in the formulas (A) and (B) indicates the content in mass % of the element, and when the element is not contained, 0% is substituted.
  • P (i, j) in the following formulas (1) to (5) is a gray level co-occurrence matrix
  • L in the following formula (2) is a normalization constant of the brightness value that the SEM image can take.
  • i and j in the following formulas (2) and (3) are natural numbers from 1 to the above L
  • ⁇ x and ⁇ y in the following formula (3) are the following formulas (4) and (5), respectively. It is indicated by.
  • the hot-rolled steel sheet according to (1) above has an average grain size of less than 3.0 ⁇ m in the surface layer region, which is a region starting from the surface and ending at a position 20 ⁇ m deep in the sheet thickness direction. It may be.
  • (3) The hot rolled steel sheet according to (1) or (2) above has the chemical composition in mass%, Ti: 0.001 to 0.500%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 0.01-2.00%, Mo: 0.01-1.00%, Ni: 0.02-2.00%, B: 0.0001 to 0.0100%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005-0.1000%, Bi: 0.0005-0.0200%, As: 0.001 to 0.100%, Zr: 0.01-1.00%, Co: 0.01 to 1.00%, Zn: 0.01-1.00%, W: 0.01 to 1.00%, and Sn: 0.01 to 0.05% It may contain one or more selected from the group consist
  • the hot-rolled steel sheet that has high strength, critical thickness reduction rate at rupture, excellent ductility and shear workability, and has excellent fatigue properties after press forming. can. Further, according to the above-described preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above-mentioned properties and further suppresses the occurrence of cracking in bending, that is, has excellent resistance to cracking in bending. can.
  • the hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile parts, mechanical structural parts, and even building parts.
  • FIG. 1 is an example of a sheared end surface of a hot rolled steel plate according to an example of the present invention. It is an example of a sheared end surface of a hot rolled steel plate according to a comparative example.
  • FIG. 3 is a diagram for explaining press molding performed in an example. It is a figure for demonstrating the punch shape used for the said press forming.
  • the chemical composition of the hot rolled steel sheet according to this embodiment is, in mass%, C: 0.050 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00. %, sol. Al: 0.001 to 0.500%, Cr: 0.060 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0. 0100% or less, and the remainder: contains Fe and impurities, and satisfies formula (A) (0.060% ⁇ Ti+Nb+V ⁇ 0.500%).
  • A formula
  • C 0.050-0.250%
  • C increases the area ratio of the hard phase and also increases the strength of ferrite by combining with precipitation-strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more.
  • the C content is preferably 0.055% or more, more preferably 0.060% or more, even more preferably 0.065% or more.
  • the C content exceeds 0.250%, the area ratio of ferrite decreases, and the ductility of the hot rolled steel sheet decreases. Therefore, the C content is set to 0.250% or less.
  • the C content is preferably 0.150% or less.
  • Si 0.05-3.00%
  • Si has the function of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet, and the function of solid solution strengthening of ferrite to increase the strength of the hot-rolled steel sheet. Further, Si has the effect of making the steel sound by deoxidizing (suppressing the occurrence of defects such as blowholes in the steel). If the Si content is less than 0.05%, the above effects cannot be obtained. Therefore, the Si content is set to 0.05% or more.
  • the Si content is preferably 0.40% or more, more preferably 0.60% or more.
  • the Si content is set to 3.00% or less.
  • the Si content is preferably 2.50% or less, more preferably 2.00% or less.
  • Mn 1.00-4.00% Mn has the effect of suppressing ferrite transformation and increasing the strength of the hot rolled steel sheet. If the Mn content is less than 1.00%, desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more.
  • the Mn content is preferably 1.10% or more, more preferably 1.20% or more.
  • the Mn content exceeds 4.00%, the hard phase becomes periodic band-like due to segregation of Mn, making it difficult to obtain desired shearing workability. Therefore, the Mn content is set to 4.00% or less.
  • the Mn content is preferably 3.50% or less, more preferably 3.00% or less, even more preferably 2.50% or less.
  • each element symbol in the formula (A) indicates the content of the element in mass %, and when the element is not contained, 0% is substituted.
  • Ti, Nb, and V are elements that finely precipitate in steel as carbides and nitrides and improve the strength of steel through precipitation strengthening. If the total content of Ti, Nb and V is less than 0.060%, these effects cannot be obtained. Therefore, the total content of Ti, Nb, and V is set to 0.060% or more. That is, the value of the middle side of the formula (A) is set to 0.060% or more.
  • Ti, Nb, and V are not necessary that all of Ti, Nb, and V be contained; it is sufficient that at least one of them is contained, and the total content thereof may be 0.060% or more.
  • the total content of Ti, Nb and V is preferably 0.080% or more, more preferably 0.100% or more.
  • the contents of Ti, Nb and V are each preferably 0.001% or more.
  • the total content of Ti, Nb and V is set to 0.500% or less. That is, the value of the middle side of the formula (A) is set to 0.500% or less. Preferably it is 0.300% or less, more preferably 0.250% or less, even more preferably 0.200% or less.
  • sol. Al 0.001-0.500%
  • Al has the effect of deoxidizing the steel and making it sound, and also has the effect of promoting the formation of ferrite and increasing the ductility of the hot rolled steel sheet.
  • sol. Al content shall be 0.001% or more.
  • sol. Al content is preferably 0.010% or more.
  • the Al content is preferably 0.450% or less, more preferably 0.400% or less, even more preferably 0.350% or less.
  • sol. Al means acid-soluble Al, and indicates solid solution Al that exists in steel in a solid solution state.
  • Cr:0.060 ⁇ 2.000% Cr has the effect of increasing the hardenability of hot rolled steel sheets. Further, when combined with desired manufacturing conditions, Cr is concentrated in the outermost layer region of the hot rolled steel sheet, thereby suppressing scale growth and having the effect of reducing the arithmetic mean roughness Ra after press forming. If the Cr content is less than 0.060%, the above effects cannot be obtained. Therefore, the Cr content is set to 0.060% or more.
  • the Cr content is preferably 0.200% or more, more preferably 0.400% or more, even more preferably 0.600% or more.
  • the Cr content exceeds 2.000%, the chemical conversion treatability of the hot rolled steel sheet will be significantly reduced. Therefore, the Cr content is set to 2.000% or less.
  • the Cr content is preferably 1.800% or less, more preferably 1.600% or less.
  • P 0.100% or less
  • P is also an element that has the effect of increasing the strength of the hot rolled steel sheet through solid solution strengthening. Therefore, P may be actively included.
  • P is an element that tends to segregate, and when the P content exceeds 0.100%, the ductility and critical thickness reduction rate of the hot rolled steel sheet due to grain boundary segregation decrease significantly. Therefore, the P content is set to 0.100% or less.
  • the P content is preferably 0.030% or less.
  • the lower limit of the P content does not need to be particularly specified and may be 0%, but from the viewpoint of refining cost, it is preferably 0.001%.
  • S 0.0300% or less S forms sulfide inclusions in the steel and reduces the ductility and critical thickness reduction rate at break of the hot rolled steel sheet.
  • the S content is set to 0.0300% or less.
  • the S content is preferably 0.0050% or less.
  • the lower limit of the S content does not need to be particularly specified and may be 0%, but from the viewpoint of refining cost, it is preferably 0.0001%.
  • N 0.1000% or less N has the effect of reducing the ductility and critical thickness reduction rate at break of the hot rolled steel sheet.
  • the N content exceeds 0.1000%, the ductility and critical thickness reduction rate at break of the hot rolled steel sheet are significantly reduced. Therefore, the N content is set to 0.1000% or less.
  • the N content is preferably 0.0800% or less, more preferably 0.0700% or less, even more preferably 0.0100% or less.
  • the lower limit of the N content does not need to be particularly specified and may be 0%, but if one or more of Ti, Nb and V is contained to make the metal structure finer, carbonitriding
  • the N content is preferably 0.0010% or more, more preferably 0.0020% or more.
  • O 0.0100% or less
  • O content is set to 0.0100% or less.
  • the O content is preferably 0.0080% or less, more preferably 0.0050% or less.
  • the O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.
  • the remainder of the chemical composition of the hot rolled steel sheet according to this embodiment may be Fe and impurities.
  • impurities refer to things that are mixed in from ore as a raw material, scrap, or the manufacturing environment, and/or things that are allowed within a range that does not adversely affect the hot rolled steel sheet according to this embodiment. do.
  • the hot rolled steel sheet according to this embodiment may contain the following elements as optional elements in place of a part of Fe.
  • the lower limit of the content is 0%. The arbitrary elements will be explained in detail below.
  • Cu, Mo, Ni, and B all have the effect of improving the hardenability of the hot rolled steel sheet. Further, Cu and Mo precipitate as carbides in the steel and have the effect of increasing the strength of the hot rolled steel sheet. Furthermore, when containing Cu, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.
  • the Cu content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less.
  • Mo has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of precipitating as carbides in the steel to increase the strength of the hot-rolled steel sheet.
  • the Mo content is preferably 0.01% or more, more preferably 0.02% or more.
  • the Mo content is set to 1.00% or less.
  • Mo content is preferably 0.50% or less, more preferably 0.20% or less.
  • Ni has the effect of increasing the hardenability of the hot rolled steel sheet. Further, when containing Cu, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to more reliably obtain the effects of the above action, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically undesirable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
  • B has the effect of increasing the hardenability of the hot rolled steel sheet.
  • the B content is preferably 0.0001% or more, more preferably 0.0002% or more.
  • the B content is set to 0.0100% or less.
  • the B content is preferably 0.0050% or less.
  • Ca, Mg, and REM all have the effect of increasing the ductility of the hot rolled steel sheet by adjusting the shape of inclusions in the steel to a preferable shape. Furthermore, Bi has the effect of increasing the ductility of the hot rolled steel sheet by making the solidification structure finer. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effects of the above action, the content of any one or more of Ca, Mg, REM and Bi is preferably 0.0005% or more.
  • the Ca content or Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, inclusions will be excessively generated in the steel, which will actually reduce the ductility of the hot rolled steel sheet.
  • the Bi content exceeds 0.0200%, the effect of the above action will be saturated, which is not economically preferable. Therefore, the Ca content and Mg content should be 0.0200% or less, the REM content should be 0.1000% or less, and the Bi content should be 0.0200% or less. Bi content is preferably 0.0100% or less.
  • REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids
  • the content of REM refers to the total content of these elements.
  • lanthanoids they are added industrially in the form of mischmetal.
  • the As content be 0.001% or more.
  • the As content is set to 0.100% or less.
  • Zr 0.01 ⁇ 1.00% Co:0.01 ⁇ 1.00% Zn: 0.01-1.00% W: 0.01 ⁇ 1.00% Zr+Co+Zn+W ⁇ 1.00%...(B) Sn: 0.01 ⁇ 0.05%
  • each element symbol in the formula (B) indicates the content of the element in mass %, and when the element is not contained, 0% is substituted.
  • Zr, Co, Zn and W the present inventors have confirmed that the effect of the hot rolled steel sheet according to the present embodiment is not impaired even if these elements are contained in a total of 1.00% or less. There is. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total amount of 1.00% or less.
  • the value on the left side of the equation (B) may be 1.00% or less. Since Zr, Co, Zn, and W do not need to be contained, their respective contents may be 0%. In order to solid solution strengthen the steel sheet and improve its strength, the contents of Zr, Co, Zn, and W may each be 0.01% or more. Moreover, the present inventors have confirmed that even if a small amount of Sn is contained, the effect of the hot rolled steel sheet according to the present embodiment is not impaired. However, if a large amount of Sn is contained, defects may occur during hot rolling, so the Sn content is set to 0.05% or less. Since Sn does not need to be contained, the Sn content may be 0%. In order to improve the corrosion resistance of the hot rolled steel sheet, the Sn content may be 0.01% or more.
  • the chemical composition of the hot-rolled steel sheet described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
  • sol. Al may be measured by ICP-AES using a filtrate obtained by thermally decomposing a sample with an acid.
  • C and S may be measured using a combustion-infrared absorption method, N using an inert gas melting-thermal conductivity method, and O using an inert gas melting-non-dispersive infrared absorption method.
  • the chemical composition may be analyzed after removing the plating layer by mechanical grinding or the like, if necessary.
  • the hot-rolled steel sheet according to the present embodiment has a metal structure at a 1/4 depth position from the surface in the thickness direction, in terms of area%, residual austenite is less than 3.0%, ferrite is 15.0% or more, 60.0%, pearlite is less than 5.0%, and the entropy value is obtained by analyzing the SEM image of the metal structure using the gray level co-occurrence matrix method and is expressed by the following formula (1). is 10.7 or more, the Inverse difference normalized value shown by the following formula (2) is 1.020 or more, and the Cluster Shade value shown by the following formula (3) is -8.0 ⁇ 10 5 to 8.
  • the standard deviation of the Mn concentration is 0.60% by mass or less
  • the solid solution in the outermost layer region is a region starting from the surface and ending at a depth of 5 ⁇ m in the plate thickness direction.
  • the Cr concentration is 0.10% by mass or more
  • the number density of Cr oxides having an equivalent sphere radius of 0.1 ⁇ m or more on the surface is 1.0 ⁇ 10 4 pieces/cm 2 or less.
  • the hot rolled steel sheet according to the present embodiment has high strength and critical thickness reduction rate at rupture, as well as excellent ductility and shear workability, and can obtain excellent fatigue properties after press forming.
  • the microstructure fraction of the metallographic structure, the entropy value, the inverse difference normalized value, the Cluster Shade value, and the standard deviation of the Mn concentration in a region at a depth of 1/4 from the surface in the plate thickness direction are defined. . The reason is that the metal structure at this position shows a typical metal structure of a steel plate.
  • the "surface” here refers to the interface between the plating layer and the steel sheet when the hot-rolled steel sheet has a plating layer
  • the "1/4 depth position from the surface” refers to the hot-rolled steel sheet. A position at a depth of 1/4 of the thickness of the plate in the thickness direction from the surface of the plate.
  • Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite has the effect of increasing the ductility of hot rolled steel sheets through transformation induced plasticity (TRIP).
  • TRIP transformation induced plasticity
  • retained austenite transforms into high-carbon martensite during shearing, which becomes the starting point for cracks during deformation and causes a decrease in the rate of thickness reduction at critical rupture.
  • the area ratio of retained austenite is set to be less than 3.0%.
  • the area percentage of retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since it is preferable to have as little retained austenite as possible, the area ratio of retained austenite may be 0%.
  • Methods for measuring the area ratio of retained austenite include methods using X-ray diffraction, EBSP (electron back scattering diffraction pattern) analysis, and magnetic measurement.
  • the area ratio of retained austenite is measured by X-ray diffraction.
  • the volume fraction of retained austenite is calculated from the integrated intensity using an intensity averaging method. The obtained volume fraction of retained austenite is regarded as the area fraction of retained austenite.
  • the area ratio of ferrite is set to 15.0% or more. Preferably it is 20.0% or more, more preferably 25.0% or more, even more preferably 30.0% or more. On the other hand, since ferrite has low strength, if the area ratio is excessive, desired strength cannot be obtained. Therefore, the ferrite area ratio is set to less than 60.0%. Preferably it is 50.0% or less, more preferably 45.0% or less.
  • Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrite particles, and is a soft metal structure compared to bainite and martensite. If the area ratio of pearlite is 5.0% or more, carbon is consumed by cementite contained in pearlite, and the strength of martensite and bainite, which are the remaining structures, decreases, making it impossible to obtain the desired strength. Therefore, the area ratio of pearlite is made less than 5.0%.
  • the area ratio of pearlite is preferably 3.0% or less. In order to improve the stretch flangeability of the hot rolled steel sheet, it is preferable to reduce the area ratio of pearlite as much as possible, and it is even more preferable that the area ratio of pearlite is 0%.
  • the hot rolled steel sheet according to the present embodiment includes bainite, martensite, and tempered marten with a total area ratio of more than 32.0% and 85.0% or less as residual structures other than retained austenite, ferrite, and pearlite.
  • a hard tissue consisting of one or more types of sites is included.
  • the area ratio of ferrite and pearlite is measured by the following method.
  • the sample has a size that allows observation of about 10 mm in the rolling direction.
  • the cross section of the sample is polished to a mirror finish, and then polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 ⁇ m that does not contain an alkaline solution to remove the strain introduced into the surface layer of the sample. .
  • an EBSD analysis device consisting 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 analyzer is 9.6 ⁇ 10 ⁇ 5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. Note that the number of observation fields is 5.
  • backscattered electron images are taken in each of the same observation fields from which the crystal orientation information described above was obtained.
  • the accelerating voltage is 15 kV
  • the irradiation current level is 12 to 13
  • the electron beam irradiation level is 62.
  • the focal length (WD: Working Distance) is 5 mm.
  • the area ratio of ferrite and pearlite is identified from the backscattered electron image and the crystal orientation information.
  • crystal grains in which cementite is precipitated in a lamellar shape are identified in a backscattered electron image. In the backscattered electron image, cementite is observed as a white contrast.
  • the cementite in this pearlite has a lamellar form, and the crystal grains in which the white contrast of the lamellar form is observed at intervals of 1.0 ⁇ m or less are defined as pearlite crystal grains.
  • the area ratio of pearlite is obtained.
  • the obtained crystal orientation information for the crystal grains excluding the crystal grains determined to be pearlite was used using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer.
  • a region where the Grain Average Misorientation value is 1.0° or less is determined to be ferrite.
  • the Grain Tolerance Angle is set to 15 degrees, and the area ratio of ferrite is obtained by determining the area ratio of the region determined to be ferrite.
  • the area ratio of the residual structure is obtained by subtracting the area ratio of retained austenite, ferrite, and pearlite from 100%.
  • the rolling direction of the hot rolled steel sheet is determined by the following method. First, a test piece is taken so that a cross section parallel to the surface of the hot rolled steel sheet can be observed. After finishing the cross section of the test piece taken in the plate pressure direction with mirror polishing, it is observed using an optical microscope.
  • the observation plane is a plane parallel to the surface of the plate at an arbitrary depth in the range of 1/4 to 1/2 in the plate thickness direction, and on the observation plane, the direction parallel to the stretching direction of the crystal grains is determined as the rolling direction. do.
  • Entropy value 10.7 or more
  • Inverse difference normalized value 1.020 or more
  • E value entropy value
  • I value inverse difference normalized value
  • the E value represents the periodicity of the metal structure.
  • the E value represents the periodicity of the metal structure.
  • the E value decreases.
  • the E value is less than 10.7, secondary shear surfaces are likely to occur. Starting from the periodically arranged structure, cracks occur from the cutting edge of the shearing tool very early in the shearing process, forming a fractured surface, and then a sheared surface is formed again. It is estimated that this makes secondary shear planes more likely to occur. Therefore, the E value is set to 10.7 or more.
  • it is 10.8 or more, more preferably 11.0 or more.
  • the I value represents the uniformity of the metal structure, and increases as the area of a region with a certain brightness increases.
  • a high I value means that the uniformity of the metal structure is high.
  • the I value is set to 1.020 or more. Preferably it is 1.025 or more, more preferably 1.030 or more. The higher the I value, the better, and although the upper limit is not particularly specified, it may be 1.200 or less, 1.150 or less, or 1.100 or less.
  • the Cluster Shade value indicates the degree of distortion of the metal structure.
  • the CS value is a positive value if there are many points with brightness above the average value of the average value of brightness in the image obtained by photographing the metal structure, and a positive value if there are many points with brightness below the average value. It becomes a negative value.
  • the brightness increases where the surface of the object to be observed has large irregularities, and the brightness decreases where the irregularities are small.
  • the unevenness of the surface of an object to be observed is greatly affected by the grain size and intensity distribution within the metal structure.
  • the CS value in this embodiment increases when the strength variation of the metal structure is large or the structure unit is small, and becomes small when the strength variation is small or the structure unit is large.
  • the CS value is set to -8.0 ⁇ 10 5 or more. It is preferably -7.5 ⁇ 10 5 or more, and even more preferably -7.0 ⁇ 10 5 or more.
  • the critical thickness reduction rate at break of the hot rolled steel sheet decreases.
  • the CS value is set to 8.0 ⁇ 10 5 or less. It is preferably 7.5 ⁇ 10 5 or less, and even more preferably 7.0 ⁇ 10 5 or less.
  • the E value, I value and CS value can be obtained by the following method.
  • the imaging area of the SEM image taken to calculate the E value, I value, and CS value is a cross section parallel to the rolling direction at the center in the direction perpendicular to the rolling direction and the sheet thickness direction.
  • the area is 160 ⁇ m ⁇ 160 ⁇ m centered at a 1/4 depth position from the surface in the thickness direction, and the number of observation fields is 5.
  • an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, with a tungsten emitter and an accelerating voltage of 1.5 kV. Based on the above settings, a SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.
  • the obtained SEM image was cut out into an area of 880 x 880 pixels (the observation area is 160 ⁇ m x 160 ⁇ m in actual size), and the limit magnification of contrast enhancement described in Non-Patent Document 3 was set to 2.0. , performs smoothing processing with a tile grid size of 8 ⁇ 8. By rotating the smoothed SEM image counterclockwise in 1 degree increments from 0 degrees to 179 degrees, excluding 90 degrees, and creating images in 1 degree increments, a total of 179 images are obtained. . Next, for each of these 179 images, frequency values of brightness between adjacent pixels are collected in the form of a matrix using the GLCM method described in Non-Patent Document 1.
  • P(i,j) in the following equations (1) to (5) is a gray level co-occurrence matrix, and the value at the i-th row and j-th column of the matrix P is expressed as P(i,j).
  • P(i,j) is a gray level co-occurrence matrix
  • P(i,j) is a gray level co-occurrence matrix
  • P(i,j) is a gray level co-occurrence matrix
  • P(i,j) the value at the i-th row and j-th column of the matrix P.
  • it is calculated using the 256 x 256 matrix P, so if you want to emphasize this point, modify the following formulas (1) to (5) to the following formulas (1') to (5'). be able to.
  • L in the following formula (2) is the number of grayscale levels that the SEM image can take (Quantization levels of grayscale), and in this embodiment, as described above, the SEM image is output with a grayscale of 256 gradations. Therefore, L is 256.
  • Standard deviation of Mn concentration 0.60% by mass or less
  • the standard deviation of Mn concentration of the hot rolled steel sheet according to the present embodiment is 0.60% by mass or less.
  • the standard deviation of the Mn concentration is preferably 0.50% by mass or less, more preferably 0.47% by mass or less.
  • the lower limit of the standard deviation of the Mn concentration is preferably as small as possible from the viewpoint of suppressing excessive burrs, but due to manufacturing process constraints, the actual lower limit is 0.10% by mass.
  • the standard deviation of Mn concentration can be obtained by the following method. First, a sample is taken at the center in a direction perpendicular to the rolling direction and the thickness direction so that a region at a depth of 1/4 from the surface in the thickness direction can be observed in a cross section parallel to the rolling direction. The size of the sample is such that it can be observed about 10 mm in the rolling direction, although it depends on the measuring device. Next, after mirror polishing the sample, the standard deviation of the Mn concentration is measured using an electron probe microanalyzer (EPMA).
  • EPMA electron probe microanalyzer
  • the measurement conditions were an accelerating voltage of 15 kV, a magnification of 5000 times, and a distribution image of Mn concentration in a range of 20 ⁇ m in the sample rolling direction and 20 ⁇ m in the sample plate thickness direction centered on a 1/4 depth position from the surface in the plate thickness direction. Measure. More specifically, the Mn concentration is measured at 40,000 or more locations with a measurement interval of 0.1 ⁇ m. Next, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentration obtained from all measurement points.
  • Solid solution Cr concentration in the outermost layer region 0.10% by mass or more
  • the present inventors found that when the solid solution Cr concentration in the outermost layer region (a region 5 ⁇ m deep from the surface in the plate thickness direction) is high, It has been found that by reducing the number density of Cr oxides with a sphere equivalent radius of 0.1 ⁇ m or more, it is possible to suppress deterioration of fatigue properties of hot rolled steel sheets after press forming. If the solid solution Cr concentration in the outermost layer region is less than 0.10% by mass, deterioration of fatigue properties after press forming of the hot rolled steel sheet cannot be suppressed. Therefore, the solid solution Cr concentration in the outermost layer region is set to 0.10% by mass or more.
  • the solid solution Cr concentration in the outermost layer region is preferably 0.20% by mass or more, more preferably 0.40% by mass or more.
  • the solid solution Cr concentration in the outermost layer region may be 5.00% by mass or less.
  • the region 5 ⁇ m deep from the surface in the thickness direction is the depth in the thickness direction of the range starting from the surface of the hot rolled steel sheet and ending at a position 5 ⁇ m deep in the thickness direction.
  • the solid solution Cr concentration in the outermost layer region can be analyzed by GD-MS (Glow Discharge-Mass Spectrometry) analysis.
  • GD-MS analysis is an analysis method that tracks changes in composition from the surface of a hot rolled steel sheet in the depth direction as the discharge time progresses.
  • a region 5 ⁇ m deep from the surface in the sheet thickness direction startsing from the surface
  • the average value of the Cr concentration in mass % in a region whose end point is a position 5 ⁇ m deep in the plate thickness direction is determined.
  • This operation is performed at any three or more locations (preferably five or more locations) and the average value of the obtained values is calculated to obtain the solid solution Cr concentration in the outermost layer region.
  • the depth position where the Fe concentration is 90% by mass when analyzed by GD-MS is the interface between the plating layer and the hot-rolled steel sheet, that is, the surface of the hot-rolled steel sheet. regarded as.
  • the number density of Cr oxides with a sphere equivalent radius of 0.1 ⁇ m or more on the surface 1.0 ⁇ 10 4 pieces/cm 2 or less
  • the number density of Cr oxides with a sphere equivalent radius of 0.1 ⁇ m or more on the surface is 1 When the number exceeds .0 ⁇ 10 4 pieces/cm 2 , the surface roughness becomes large when the hot rolled steel sheet is press-formed. Since this surface roughness deteriorates the fatigue characteristics of the pressed part, it is preferable that the surface roughness is small.
  • the Cr oxide defined here has an equivalent sphere radius of 0.1 ⁇ m or more, and is relatively coarse. It is thought that this coarse Cr oxide inhibits sliding between the hot rolled steel sheet and the mold, causing an increase in surface roughness.
  • the number density of Cr oxides having an equivalent sphere radius of 0.1 ⁇ m or more on the surface is set to be 1.0 ⁇ 10 4 pieces/cm 2 or less.
  • the number density of Cr oxides with a sphere equivalent radius of 0.1 ⁇ m or more on the surface is preferably 0.8 ⁇ 10 4 pieces/cm 2 or less, more preferably 0.6 ⁇ 10 4 pieces/cm 2 or less. It is.
  • the number density of Cr oxides having an equivalent sphere radius of 0.1 ⁇ m or more on the surface may be 0.1 ⁇ 10 4 pieces/cm 2 or more.
  • the number density of Cr oxides on the surface is measured by the following method.
  • a sample is cut out from a hot-rolled steel plate so that the surface in the thickness direction is the observation surface. After degreasing the observation surface at 60° C. for 60 seconds using FC-E6403 manufactured by Nippon Parkerizing Co., Ltd., it is immersed in acetone and subjected to ultrasonic cleaning for 90 seconds. Thereafter, 10 or more fields of view are observed at a magnification of 3000 times.
  • the composition of the precipitate can be measured by EDS (energy dispersive X-ray spectrometer).
  • the number of regions containing Cr and O with a former equivalent radius of 0.1 ⁇ m or more is counted in each field of view and divided by the measurement area to obtain the number density of Cr oxides. Note that if the precipitate is subjected to EDS analysis and 20 atomic percent or more of each of Cr and O is detected, the precipitate is considered to be a Cr oxide.
  • the above-mentioned measurement is performed after removing the plating layer by pickling with fuming nitric acid.
  • Average crystal grain size in the surface layer region less than 3.0 ⁇ m
  • cracking in bending of hot rolled steel sheets can be suppressed.
  • the mechanism of cracking in bending is estimated as follows. During bending, compressive stress is generated on the inside of the bend.
  • the region 20 ⁇ m deep from the surface in the thickness direction is the depth in the thickness direction of the range starting from the surface of the hot rolled steel sheet and ending at a position 20 ⁇ m deep in the thickness direction.
  • the average grain size in the surface layer region of the hot rolled steel sheet is preferably less than 3.0 ⁇ m. Therefore, in this embodiment, the average grain size in the surface layer region may be less than 3.0 ⁇ m.
  • the average crystal grain size in the surface layer region is more preferably 2.5 ⁇ m or less. Although the lower limit of the average crystal grain size in the surface layer region is not particularly specified, it may be 0.5 ⁇ m.
  • the crystal grain size in the surface layer region is measured using the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method.
  • the EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer, and OIM Analysis (registered trademark) manufactured by AMETEK.
  • OIM Analysis registered trademark manufactured by AMETEK.
  • the analyzable area of the EBSP-OIM method is the area that can be observed with SEM. Depending on the resolution of the SEM, the EBSP-OIM method allows analysis with a minimum resolution of 20 nm.
  • retained austenite is not a structure generated by phase transformation at 600° C. or lower and does not have the effect of dislocation accumulation, so retained austenite is not analyzed in this measurement method.
  • retained austenite having an fcc crystal structure can be excluded from the analysis target.
  • Tensile Strength Properties Among the mechanical properties of hot rolled steel sheets, tensile strength properties (tensile strength, total elongation) are evaluated in accordance with JIS Z 2241:2011.
  • the test piece is a JIS Z 2241:2011 No. 5 test piece.
  • the test piece may be collected at a 1/4 position from the end face in a direction perpendicular to the rolling direction and the plate thickness direction, and the plate width direction may be the longitudinal direction of the test piece.
  • the hot rolled steel sheet according to this embodiment has a tensile strength of 980 MPa or more. Preferably it is 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts will be limited and the contribution to reducing the weight of the vehicle body will be small. Although there is no need to specifically limit the upper limit, it may be set to 1780 MPa from the viewpoint of suppressing mold wear.
  • the total elongation of the hot rolled steel sheet according to the present embodiment is preferably 10.0% or more, and the product of tensile strength and total elongation (TS ⁇ El) is preferably 13000 MPa ⁇ % or more.
  • the total elongation is more preferably 11.0% or more, even more preferably 13.0% or more.
  • the product of tensile strength and total elongation is more preferably 14,000 MPa ⁇ % or more, and even more preferably 15,000 MPa ⁇ % MPa or more.
  • the thickness of the hot rolled steel plate according to this embodiment is not particularly limited, but may be 0.5 to 8.0 mm. If the thickness of the hot rolled steel plate is less than 0.5 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot rolled steel plate according to this embodiment may be 0.5 mm or more. Preferably it is 1.2 mm or more or 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to refine the metal structure and obtain the above-mentioned metal structure. Therefore, the plate thickness may be 8.0 mm or less. Preferably it is 6.0 mm or less.
  • the hot rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metallographic structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet.
  • the plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanizing, electrolytic Zn--Ni alloy plating, and the like.
  • hot-dip plating layer examples include hot-dip galvanizing, alloyed hot-dip galvanizing, 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. Ru.
  • the amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is also possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, applying and drying a silicate-based chromium-free chemical conversion treatment liquid) after plating.
  • a preferred method for manufacturing the hot rolled steel sheet according to this embodiment having the above-mentioned chemical composition and metallographic structure is as follows.
  • the following steps (1) to (11) are sequentially performed.
  • 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.
  • stress refers to the tension applied to the steel plate in the rolling direction.
  • a stress of 170 kPa or more is applied to the steel plate from the end of the last stage of hot rolling until the start of the final stage of rolling.
  • the rolling reduction in the final stage of hot rolling is 8% or more, and the hot rolling is completed so that the rolling completion temperature Tf is 900°C or more and less than 1010°C.
  • a stress of less than 200 kPa is applied to the steel plate after the final stage of hot rolling until the steel plate is cooled to 800°C.
  • Within 1 second after completion of hot rolling cool to a temperature range below hot rolling completion temperature Tf - 50°C, and then cool to a temperature range of 600 to 780°C at an average cooling rate of 50°C/s or more. do.
  • a more preferable cooling condition is to cool to a temperature range below the hot rolling completion temperature Tf - 50° C. within 1 second after the completion of hot rolling.
  • (8) Perform slow cooling at a temperature range of 600 to 780°C with an average cooling rate of less than 5°C/s for 2.0 seconds or more.
  • After completion of slow cooling cool so that the average cooling rate in the temperature range of 450 to 600°C is 30°C/s or more and less than 50°C/s.
  • Cooling is performed so that the average cooling rate in the temperature range from the winding temperature to 450°C is 50°C/s or more.
  • (11) Wind up in a temperature range of 350°C or less.
  • the slab to be subjected to hot rolling is preferably held at a temperature range of 700 to 850°C for 900 seconds or more during slab heating, and then further heated and held at a temperature range of 1100°C or higher for 6000 seconds or more.
  • the steel plate temperature may be varied within this temperature range or may be kept constant.
  • the steel plate temperature may be varied in the temperature range of 1100°C or higher, or may be kept constant.
  • Mn is distributed between ferrite and austenite, and by lengthening the transformation time, Mn can diffuse into the ferrite region. This eliminates the Mn micro-segregation that is unevenly distributed in the slab and significantly reduces the standard deviation of the Mn concentration. Further, by holding the temperature in a temperature range of 1100° C. or higher for 6000 seconds or more, the standard deviation of the Mn concentration can be significantly reduced.
  • Hot rolling it is preferable to use a liver mill or a tandem mill as multi-pass rolling. Particularly from the viewpoint of industrial productivity and the stress load on the steel plate during rolling, it is more preferable that at least the last two stages are hot rolled using a tandem mill.
  • Hot rolling includes rough rolling and finish rolling, each of which is performed multiple times (stages). Rough rolling is a process of rolling a slab to a minimum thickness of 25 mm, and finish rolling is a process of rolling a plate after rough rolling to a target thickness.
  • descaling By performing descaling one or more times in a temperature range of 1150° C. or higher before rough rolling, it is possible to remove the primary scale formed in the heating furnace and suppress the occurrence of subsequent descaling defects.
  • the number density of Cr oxides on the surface of the hot rolled steel sheet can be preferably controlled.
  • the upper limit of the number of times of descaling in the temperature range of 1150° C. or higher is not particularly limited, but may be 5 times or less.
  • rolling and descaling are performed multiple times. During rough rolling, descaling is performed between rollings or after multiple rollings. In this embodiment, during rough rolling, descaling is performed twice or more in a temperature range of 1130°C or higher, and the maximum value of the total rolling reduction between each descaling in a temperature range of 1130°C or higher is set to 40 It is preferable to set it as less than %. By performing descaling two or more times in a temperature range of 1130°C or higher, the thickness of the scale formed in the first stage of rough rolling is reduced or the scale is removed, thereby reducing the number density of Cr oxides on the surface of the hot rolled steel sheet. can be preferably controlled.
  • the number density of Cr oxides can be preferably controlled.
  • the total rolling reduction rate between each descaling in the temperature range of 1130°C or higher is t0, where the plate thickness before the nth descaling in the temperature range of 1130°C or higher is t0 , and the n+1th descaling in the temperature range of 1130°C or higher is
  • the subsequent exit plate thickness is t 1 , it can be expressed as ⁇ (t 0 ⁇ t 1 )/t 0 ⁇ 100(%).
  • rolling may be performed only once, or rolling may be performed multiple times.
  • Hot rolling reduction 90% or more in the temperature range of 850 to 1100°C
  • hot rolling includes rough rolling and finish rolling.
  • the total rolling reduction in the temperature range of 850 to 1100°C is defined as the inlet plate thickness before the first rolling in this temperature range, and the outlet plate thickness after the final rolling in this temperature range.
  • t 1 t 1
  • it can be expressed as ⁇ (t 0 ⁇ t 1 )/t 0 ⁇ 100(%).
  • the band-like structure of the hot rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value is increased.
  • the term "rolling one stage before the final stage of hot rolling” as used herein refers to rolling one stage before the final stage of finish rolling. For example, when finish rolling is performed in seven passes of F1, F2...F6, and F7, the sixth pass (F6) is referred to as the pass of the sixth stage (F6).
  • the stress applied to the steel plate is less than 170 kPa, it may not be possible to set the E value to the desired value.
  • the stress applied to the steel plate is more preferably 190 kPa or more.
  • the stress applied to a steel plate refers to the tension applied in the longitudinal direction of the steel plate, and can be controlled by adjusting the roll rotation speed during tandem rolling. It can be calculated by dividing by the cross-sectional area of the steel plate.
  • Reduction ratio in the final stage of hot rolling 8% or more, hot rolling completion temperature Tf: 900°C or more, less than 1010°C
  • the rolling reduction ratio in the final stage of hot rolling is 8% or more, and hot rolling is completed.
  • the temperature Tf is 900°C or higher.
  • the formation of ferrite in the final structure (metal structure of the hot-rolled steel sheet after production) can be suppressed, and a high-strength hot-rolled steel sheet can be obtained.
  • Tf to less than 1010° C.
  • coarsening of the austenite grain size can be suppressed, the periodicity of the metal structure can be reduced, and the E value can be set to a desired value.
  • Stress applied to the steel plate after the final stage of hot rolling until the steel plate is cooled to 800°C Less than 200 kPa After the final stage of hot rolling, the stress applied to the steel plate until the steel plate is cooled to 800°C It is preferable to apply a stress of less than 200 kPa to the steel plate until the steel plate is heated. By applying a stress of less than 200 kPa to the steel sheet, recrystallization of austenite proceeds preferentially in the rolling direction, and an increase in periodicity of the metal structure can be suppressed. As a result, the E value can be set to a desired value.
  • the stress applied to the steel plate is more preferably 180 kPa or less.
  • Cooling In order to suppress the growth of austenite crystal grains refined by hot rolling, cooling should be performed at 50°C or higher within 1 second after the completion of hot rolling, that is, the amount of cooling within 1 second after completing hot rolling should be It is more preferable that the temperature is at least °C.
  • cooling with a high average cooling rate is performed immediately after the completion of hot rolling, for example, cooling water is applied to the surface of the steel sheet. Just spray it on.
  • the average cooling rate refers to the temperature drop range of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the end of accelerated cooling (when the steel plate is taken out from the cooling equipment). This is the value divided by the time required from the start to the completion of accelerated cooling.
  • the temperature is preferably 300° C./s or less.
  • the cooling stop temperature of accelerated cooling is preferably set to 600° C. or higher in order to perform slow cooling, which will be described later.
  • the average cooling rate is the width of the temperature drop of the steel plate from the cooling stop temperature of accelerated cooling to the stop temperature of slow cooling divided by the time required from the time of stopping accelerated cooling to the time of stopping slow cooling. Refers to value.
  • the time for performing slow cooling is preferably 3.0 seconds or more.
  • the upper limit of the time for slow cooling is determined by the equipment layout, but may be approximately less than 10.0 seconds. Further, although there is no particular lower limit to the average cooling rate of slow cooling, since increasing the temperature without cooling involves a large investment in equipment, it may be set to 0° C./s or more.
  • the average cooling rate in the temperature range of 450 to 600 °C is 30 °C/s or more and less than 50 °C/s. It is preferable to perform cooling so that the average cooling rate in the area is 30° C./s or more and less than 50° C./s.
  • the CS value can be set to a desired value. When the average cooling rate is 50° C./s or more, a flat lath-like structure with low brightness is likely to be formed, and the CS value becomes less than ⁇ 8.0 ⁇ 10 5 .
  • the average cooling rate here refers to the range from the cooling stop temperature of slow cooling with an average cooling rate of less than 5°C/s to the cooling of cooling with an average cooling rate of 30°C/s or more and less than 50°C/s.
  • the temperature drop range of the steel plate to the stop temperature is determined from the time when slow cooling is stopped when the average cooling rate is less than 5°C/s to the time when cooling is stopped when the average cooling rate is 30°C/s or more and less than 50°C/s. This is the value divided by the time required to reach the destination.
  • Average cooling rate in the temperature range from coiling temperature to 450°C: 50°C/s or more In order to suppress the area ratio of pearlite and retained austenite and obtain the desired strength and formability, it is preferable that the average cooling rate in the temperature range is 50° C./s or more. Thereby, the matrix structure can be made hard.
  • the average cooling rate here refers to the range of temperature drop of the steel plate from the cooling stop temperature to the coiling temperature when the average cooling rate is 30°C/s or more and less than 50°C/s. It refers to the value divided by the time required from the time of stopping cooling to winding, which is 30° C./s or more and less than 50° C./s.
  • Winding temperature 350°C or less
  • the winding temperature shall be 350°C or less.
  • the conditions in the Examples are examples of conditions adopted to confirm the feasibility and effects of the present invention.
  • the present invention is not limited to this example of one condition.
  • the present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.
  • the obtained hot-rolled steel sheet was subjected to the above-mentioned method to determine the area ratio of the metallographic structure, E value, I value, CS value, standard deviation of Mn concentration, solid solution Cr concentration in the outermost layer region, and equivalent sphere radius at the surface.
  • the number density of Cr oxides of 0.1 ⁇ m or more, the average crystal grain size in the surface layer region, the tensile strength TS, and the total elongation El were determined.
  • the measurement results obtained are shown in Tables 5A to 6B.
  • the remaining structure was one or more of bainite, martensite, and tempered martensite.
  • Limiting rate of thickness reduction at break The limit rate of thickness reduction at break of hot-rolled steel sheets was evaluated by a tensile test. A tensile test was conducted in the same manner as when the tensile properties were evaluated. When the plate thickness before the tensile test is t 1 and the minimum value of the plate thickness at the center in the width direction (short side direction) of the tensile test piece after rupture is t 2 , (t 1 - t 2 ) ⁇ 100/ By calculating the value of t1 , the critical plate thickness reduction rate at rupture was obtained. The tensile test was carried out five times, and the average value of the three tests, excluding the maximum and minimum values of the limit thickness reduction rate at break, was calculated to obtain the limit thickness reduction rate at break.
  • the critical thickness reduction rate at rupture was 60.0% or more, the hot rolled steel sheet was judged to have passed the test as having a high critical rupture thickness reduction rate. On the other hand, when the critical thickness reduction rate at break was less than 60.0%, it was determined that the hot rolled steel sheet did not have a high critical failure plate thickness reduction rate and was rejected.
  • Shearing workability (secondary shear surface evaluation) The shear workability of the hot-rolled steel sheet was evaluated by a punching test. Three punched holes were made for each example using a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m/s. Next, the cross section perpendicular to the rolling direction and the cross section parallel to the rolling direction of the punched hole were respectively embedded in resin, and the cross-sectional shapes were photographed using a scanning electron microscope. In the obtained observation photograph, a sheared end surface as shown in FIG. 1 or 2 can be observed. Note that FIG. 1 is an example of a sheared end surface of a hot rolled steel sheet according to an example of the present invention, and FIG.
  • FIG. 2 is an example of a sheared end surface of a hot rolled steel sheet according to a comparative example.
  • the diagram shows a sag, a sheared surface, a fractured surface, and a sheared end surface of a burr.
  • FIG. 2 shows the sheared end surface of the burr - sheared surface - fractured surface - sheared surface - fractured surface.
  • the sag is the area of the smooth R-shaped surface
  • the sheared surface is the area of the punched end face separated by shear deformation
  • the fractured surface is the area of the punched end face separated by a crack generated near the cutting edge.
  • a burr is a surface having protrusions protruding from the lower surface of a hot-rolled steel sheet.
  • Fatigue properties after press forming were evaluated by the arithmetic mean roughness Ra of the surface of the hot rolled steel sheet after press forming.
  • one surface A of the hot-rolled steel sheet in the thickness direction was ground to give a thickness of 1.6 mm or less to the other surface B of the test piece 10.
  • Press molding was performed by pulling out in one direction D while pressing with a load.
  • the shape of the punch is shown in FIG. 4 (FIG. 9 of Patent No. 5,655,394).
  • the arithmetic mean roughness Ra of the surface was measured by the following method.
  • measurement points were set at 200 mm intervals in the rolling direction and a direction orthogonal to the rolling direction and the plate thickness direction, and the surface roughness was measured at each measurement point.
  • the measurement length at each measurement point was 5 mm.
  • a roughness curve was obtained by sequentially applying contour curve filters with cutoff values ⁇ c and ⁇ s to the measured cross-sectional curve. Specifically, from the obtained measurement results, components with a wavelength ⁇ c of 0.8 mm or less and components with a wavelength ⁇ s of 2.5 mm or more were removed to obtain a roughness curve.
  • the arithmetic mean roughness Ra of each measurement location was calculated in accordance with JIS B 0601:2013. By calculating the average value of the obtained values, the arithmetic mean roughness Ra of the surface of the hot rolled steel sheet after press forming was obtained.
  • the hot-rolled steel sheet after press-forming had excellent fatigue properties after press-forming and was judged to be acceptable.
  • the arithmetic mean roughness Ra of the surface was more than 3.0 ⁇ m, it was determined that the hot rolled steel sheet did not have excellent fatigue properties after press forming and was rejected.
  • a bending test piece was obtained by cutting out a rectangular test piece of 100 mm x 30 mm at a 1/2 position from the end face of the hot rolled steel plate in a direction perpendicular to the rolling direction and the plate thickness direction. Both bending where the bending ridgeline is parallel to the rolling direction (L direction) (L-axis bending) and bending where the bending ridgeline is parallel to the direction perpendicular to the rolling direction and the plate thickness direction (C-direction) (C-axis bending) A test was conducted in accordance with the V-block method (bending angle ⁇ is 90°) of JIS Z 2248:2022.
  • the minimum bending radius without cracking was determined, and the resistance to internal cracking in bending was investigated.
  • the value obtained by dividing the average value (R) of the minimum bending radius of the L axis and the C axis by the plate thickness (t) was taken as the limit bending R/t and used as an index value of resistance to internal cracking in bending.
  • R/t was 2.5 or less, it was determined that the hot rolled steel sheet had excellent resistance to internal cracking in bending.
  • the presence or absence of cracks can be determined by mirror-polishing a cross section of the test piece cut along a plane parallel to the bending direction and perpendicular to the plate surface, and then observing cracks under an optical microscope. It was determined that a crack was present when the length of the crack exceeded 30 ⁇ m.
  • the hot rolled steel sheets according to the examples of the present invention have high strength and critical thickness reduction rate at rupture, as well as excellent ductility and shear workability, as well as excellent fatigue properties after press forming. It can be seen that it has Furthermore, it can be seen that among the examples of the present invention, the hot-rolled steel sheets in which the average grain size in the surface layer region is less than 3.0 ⁇ m have excellent internal bending cracking resistance in addition to having the above-mentioned properties. On the other hand, it can be seen that the hot rolled steel sheet according to the comparative example has deteriorated in one or more properties.
  • the hot-rolled steel sheet that has high strength, critical thickness reduction rate at rupture, excellent ductility and shear workability, and has excellent fatigue properties after press forming. Can be done. Further, according to the above-described preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above-mentioned properties and further suppresses the occurrence of cracking in bending, that is, has excellent resistance to cracking in bending. can.
  • the hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile parts, mechanical structural parts, and even building parts.

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  • Heat Treatment Of Sheet Steel (AREA)

Abstract

Cette tôle d'acier laminée à chaud a une composition chimique spécifique ; la structure métallique de cette tôle d'acier laminée à chaud à une position où la profondeur à partir de la surface est de 1/4 dans la direction de l'épaisseur de la tôle, comprend, en % en surface, moins de 3,0 % d'austénite résiduelle, pas moins de 15,0 % mais moins de 60,0 % de ferrite et moins de 5,0 % de perlite, tout en ayant une valeur E de 10,7 ou plus, une valeur I de 1020 ou plus, une valeur CS de -8,0 × 105 à 8,0 × 105, et un écart type de la concentration en Mn de 0,60 % en masse ou moins ; la concentration moyenne en Cr en solution solide dans la région de couche la plus à l'extérieur est de 0,10 % en masse ou plus ; et la densité moyenne en nombre d'oxydes de Cr ayant un rayon équivalent de sphère de 0,1 µm ou plus dans la surface est de 1,0 × 104 par cm2 ou moins.
PCT/JP2023/032635 2022-09-08 2023-09-07 Tôle d'acier laminée à chaud WO2024053701A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0479722B2 (fr) * 1983-08-23 1992-12-16 Hitachi Ltd
WO2022044493A1 (fr) * 2020-08-27 2022-03-03 日本製鉄株式会社 Tôle d'acier laminée à chaud
WO2023063010A1 (fr) * 2021-10-11 2023-04-20 日本製鉄株式会社 Tôle d'acier laminée à chaud

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0479722B2 (fr) * 1983-08-23 1992-12-16 Hitachi Ltd
WO2022044493A1 (fr) * 2020-08-27 2022-03-03 日本製鉄株式会社 Tôle d'acier laminée à chaud
WO2023063010A1 (fr) * 2021-10-11 2023-04-20 日本製鉄株式会社 Tôle d'acier laminée à chaud

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