WO2024122037A1 - Tôle d'acier à haute résistance, élément formé à l'aide d'une tôle d'acier à haute résistance, composant de structure d'ossature d'automobile ou composant de renforcement d'automobile composé d'un élément, et procédés de production de tôle d'acier à haute résistance et d'élément - Google Patents

Tôle d'acier à haute résistance, élément formé à l'aide d'une tôle d'acier à haute résistance, composant de structure d'ossature d'automobile ou composant de renforcement d'automobile composé d'un élément, et procédés de production de tôle d'acier à haute résistance et d'élément Download PDF

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WO2024122037A1
WO2024122037A1 PCT/JP2022/045373 JP2022045373W WO2024122037A1 WO 2024122037 A1 WO2024122037 A1 WO 2024122037A1 JP 2022045373 W JP2022045373 W JP 2022045373W WO 2024122037 A1 WO2024122037 A1 WO 2024122037A1
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strength steel
sheet
steel sheet
steel plate
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PCT/JP2022/045373
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English (en)
Japanese (ja)
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悠佑 和田
秀和 南
勇樹 田路
由康 川崎
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Ｊｆｅスチール株式会社
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Priority to JP2023519413A priority Critical patent/JP7367893B1/ja
Priority to PCT/JP2022/045373 priority patent/WO2024122037A1/fr
Publication of WO2024122037A1 publication Critical patent/WO2024122037A1/fr

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  • the present invention relates to high-strength steel plates, components made of high-strength steel plates, automobile structural components or automobile reinforcing parts made of components, and methods for manufacturing high-strength steel plates and components.
  • Patent Document 1 proposes a high-strength steel plate having a specified component composition and microstructure, and in which the grain size of the iron carbide contained in the low-temperature transformation phase is 500 nm or less.
  • parts such as crash boxes have punched end faces and bent parts, so the steel plates used in these parts are required to have good ductility, stretch flangeability, and bendability of the shear end faces.
  • parts using high-strength steel plates with a tensile strength of 780 MPa or more are used in low-temperature environments, there is a risk that their toughness will deteriorate and cracks will occur during a collision. Therefore, automotive steel plates are required to have excellent low-temperature toughness to prevent cracks during a collision when used in low-temperature environments.
  • high strength steel plate refers to a steel plate having a tensile strength (TS) of 780 MPa or more as determined by a tensile test described later.
  • Excellent component strength means that the yield ratio (YR) determined by the tensile test described below is 55% or more.
  • excellent ductility means that the total elongation (El) determined by the tensile test described below is 10% or more.
  • excellent stretch flangeability means that the hole expansion ratio ( ⁇ ) determined by the hole expansion test described below is 20% or more.
  • Excellent bendability at the sheared end surface means that the ratio (Rs/Rg) of the limit bending radius (Rs/t) determined in a bending test of a sample having a sheared end surface (described later) to the limit bending radius (Rg/t) determined in a bending test of a sample having a ground end surface is 1.50 or less.
  • excellent low-temperature toughness means that the low-temperature toughness parameter (P) is 3000 or more in the Charpy impact test described below.
  • the gist of the present invention is as follows. (1) In mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, A composition comprising N: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; At the 1/4 plate thickness position, The area ratio of martensite is 10% or more and 80% or less, The area ratio of bainite is 2% or more and 70% or less, The area ratio of ferrite is 80% or less, a steel structure in which the area ratio of retained austenite is 15% or less and the ratio of the number of martensite blocks in which metastable carbides are present to the number of mar
  • the composition further includes, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less,
  • the high-strength steel plate has a Vickers hardness of 85% or less of the Vickers hardness at a 1/4 position in the plate thickness direction of the high-strength steel plate, and has a surface soft layer which is a region within 200 ⁇ m from the surface of the high-strength steel plate in the plate thickness direction;
  • the ratio of the number of measurements in which the nano hardness of the sheet surface at a position of 1/4 of the sheet thickness direction depth of the soft surface layer from the surface of the high strength steel sheet is 7.0 GPa or more to the total number of measurements is 0.10 or less,
  • the high-strength steel plate according to any one of (1) to (5) above having a metal plating layer containing at least one of zinc and aluminum in a total amount of 50% by mass or more on one or both outermost layers of the high-strength steel plate.
  • a first cooling step in which the first cooling rate in a temperature range from T2 to 750°C is 2.0°C/s or more;
  • a second heating step is performed under conditions of a temperature X (° C.) and a holding time Y (s) that satisfy the following formula 2.
  • Formula 2 7000 ⁇ (273+X)(20+log(Y/3600)) ⁇ 13000 (10)
  • a metal plating containing more than 50 mass% of one or more selected from Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi
  • any of the manufacturing methods according to (9) to (12) above comprising a step of applying metal plating containing 50 mass% or more in total of at least one of zinc and aluminum to the steel sheet subjected to the first heating and the second heating steps.
  • a method for manufacturing a component comprising the step of subjecting any one of the high-strength steel plates according to (1) to (6) above to at least one of forming and joining to form a component.
  • the present invention it is possible to provide a high-strength steel sheet excellent in part strength, ductility, stretch flangeability, bendability of a sheared end surface, and low-temperature toughness. Also, it is possible to provide a member made using the high-strength steel sheet. Furthermore, according to the present invention, there can be provided a method for manufacturing the above-mentioned high-strength steel plate and a method for manufacturing a member using the high-strength steel plate. In addition, according to the present invention, it is possible to provide an automobile frame structural part or an automobile reinforcing part made of the above-mentioned member.
  • FIGS. 2A and 2B are schematic diagrams showing the preparation of samples for V-bending and orthogonal VDA bending tests in the examples, in which Fig. 2A shows V-bending (primary bending) and Fig. 2B shows orthogonal VDA bending (secondary bending).
  • 3(a) is a front view of a test member
  • FIG. 3(b) is a front view of a test member
  • FIG. 3(c) is a schematic diagram showing an axial crush test.
  • the high-strength steel plate of the present invention (hereinafter, for convenience, also referred to as "steel plate”) has a chemical composition and a steel structure described below.
  • composition of the high strength steel plate of the present invention (hereinafter, for convenience, also referred to as the “composition of the present invention") will be described.
  • “%” in the composition of the present invention means “mass%” unless otherwise specified.
  • C is one of the important basic components of steel, and in particular in the present invention, it affects the area ratio of martensite. If the C content is too low, the area ratio of martensite decreases, making it difficult to achieve a TS of 780 MPa or more. For this reason, the C content is 0.030% or more, preferably 0.040% or more, and more preferably 0.050% or more. On the other hand, if the C content is too high, the amount of retained austenite increases excessively, and the hardness of martensite generated from the retained austenite during punching increases significantly. As a result, crack propagation during hole expansion is promoted, the hole expansion ratio decreases, and the stretch flangeability decreases.
  • the residual austenite undergoes stress-induced transformation, which reduces the YR and reduces the strength of the part.
  • the C content is 0.500% or less, preferably 0.400% or less, and more preferably 0.300% or less.
  • Silicon is a component that increases the strength of a steel sheet by suppressing the precipitation of cementite in martensite and by solid solution strengthening.
  • the silicon content is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.
  • the Si content is too high, the carbide precipitation during bainite transformation is significantly suppressed, the residual austenite increases excessively, and the hardness of martensite generated from the residual austenite during punching increases significantly. As a result, crack growth during hole expansion is promoted, the hole expansion ratio decreases, and the stretch flangeability decreases.
  • the residual austenite undergoes stress-induced transformation, which reduces the YR and reduces the part strength.
  • the Si content is 2.50% or less, preferably 2.00% or less, and more preferably 1.50% or less.
  • Mn is one of the important basic components of steel, and particularly in the present invention, it affects the area ratio of martensite. If the Mn content is too low, the area ratio of martensite decreases, making it difficult to achieve a TS of 780 MPa or more. Therefore, the Mn content is 0.10% or more, preferably 0.90% or more, and more preferably 1.80% or more. On the other hand, if the Mn content is too high, the austenite is stabilized, the residual austenite is excessively increased, and the hardness of the martensite generated from the residual austenite during punching is greatly increased.
  • the Mn content is 5.00% or less, preferably 4.20% or less, and more preferably 3.60% or less.
  • P is a component that segregates at prior austenite grain boundaries to embrittle the grain boundaries, thereby reducing the ultimate deformability of the steel sheet, thereby reducing ⁇ and reducing bendability.
  • the P content is 0.100% or less, and preferably 0.070% or less.
  • the lower limit of the P content is not particularly limited, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to set the lower limit to 0.001% or more.
  • ⁇ S 0.0200% or less ⁇ S exists as a sulfide and is a component that reduces the ultimate deformability of the steel sheet, thereby reducing ⁇ and reducing bendability. Therefore, the S content is 0.0200% or less, preferably 0.0050% or less.
  • the lower limit of the S content is not particularly limited, but due to constraints on production technology, it is preferably set to 0.0001% or more.
  • Al is an effective component for sufficient deoxidation and reducing inclusions in steel, but if the Al content is too high, a large amount of ferrite is generated, the hole expansion ratio decreases, and the stretch flangeability may decrease. Therefore, the Al content is 1.000% or less, preferably 0.500% or less, and more preferably 0.100% or less. On the other hand, in order to perform stable deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.
  • N exists as a nitride and is a component that reduces the ultimate deformability of the steel sheet, thereby reducing ⁇ and reducing bendability. Therefore, the N content is 0.0100% or less, preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more.
  • O exists as an oxide and is a component that reduces the ultimate deformability of the steel sheet, thereby reducing ⁇ and reducing bendability. Therefore, the O content is 0.0100% or less, and preferably 0.0050% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more.
  • the high strength steel plate of the present invention further comprises, in addition to the above-mentioned composition, in mass%: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.020% or less, Te: 0.020% or less, At least one element selected from the group consisting of Hf: 0.10% or less and Bi: 0.200% or less may be contained. These elements may be contained alone or in combination of two or more kinds.
  • the Ti, Nb or V content is preferably 0.200% or less, and more preferably 0.100% or less.
  • the Ti, Nb or V content is 0.001% or more.
  • the Ta or W content is preferably 0.10% or less, and more preferably 0.08% or less.
  • the Ta or W content is preferably 0.01% or more.
  • the B content is preferably 0.0100% or less, and more preferably 0.0003% or more.
  • the B content is preferably 0.0003% or more.
  • the Cr, Mo or Ni content is 1.00% or less, and more preferably 0.80% or less.
  • the Cr, Mo or Ni content is 0.01% or more.
  • the Co content is preferably 0.010% or less, and more preferably 0.008% or less.
  • the Co content is 0.001% or more.
  • the Cu content is preferably 1.00% or less, and more preferably 0.80% or less.
  • the Cu content is 0.01% or more.
  • the Sn content is preferably 0.200% or less, and more preferably 0.100% or less. There is no particular lower limit for the Sn content, but since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more.
  • the Sb content is preferably 0.200% or less, and more preferably 0.100% or less.
  • the Sb content is preferably 0.001% or more.
  • the Ca, Mg or REM content is 0.0100% or less, and more preferably 0.0050% or less.
  • the Ca, Mg or REM content is 0.0001% or more.
  • the Zr or Te content is 0.100% or less, and more preferably 0.080% or less.
  • the Zr or Te content is 0.001% or more.
  • the Hf content is preferably 0.10% or less, and more preferably 0.08% or less.
  • the Hf content is preferably 0.01% or more.
  • the Bi content is preferably 0.200% or less, and more preferably 0.100% or less.
  • the Bi content is 0.001% or more.
  • the high-strength steel plate according to one embodiment of the present invention has a composition containing the above essential components and optional components, with the balance being Fe and unavoidable impurities.
  • the unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. These unavoidable impurities are permitted to be contained in an amount of 0.100% or less in total.
  • ⁇ Area ratio of martensite 10% to 80%>
  • the area ratio of martensite is 10% or more, preferably 15% or more, and more preferably 20% or more.
  • the area ratio of martensite is 80% or less, preferably 75% or less, and more preferably 70% or less.
  • martensite includes lower bainite, martensite that has undergone self-tempering during cooling performed in the annealing step described later, and martensite that has been tempered in the second heating step described later. As described later, the martensite was observed at a position corresponding to 1/4 of the thickness of the steel plate.
  • ⁇ Area ratio of bainite 2% to 70%>
  • the hardness difference between the structures is reduced and ⁇ is increased.
  • the crack propagation at the interface is suppressed, and thus low-temperature toughness is improved.
  • the area fraction of bainite is 2% or more, preferably 3% or more, and more preferably 4% or more.
  • the area ratio of bainite is 70% or less, preferably 60% or less, and more preferably 50% or less.
  • bainite is a mixed structure of angular bainitic ferrite, iron-based carbides, and retained austenite that is formed in a temperature range of Ms or higher and 700° C. or lower.
  • the observation position of bainite is a quarter position of the sheet thickness of the steel sheet, as described later.
  • the area ratio of ferrite 80% or less
  • the desired strength can be easily obtained.
  • the effect of the present invention can be obtained even if the area ratio of ferrite is 0%.
  • the area ratio of ferrite is 80% or less, preferably 75% or less, and more preferably 70% or less.
  • the area ratio of ferrite is preferably 10% or more, and more preferably 15% or more.
  • ferrite is soft BCC iron formed at relatively high temperatures, and includes allotriomorph ferrite and idiomorph ferrite.
  • the observation position of ferrite was a quarter position of the sheet thickness of the steel sheet, as described later.
  • the method for measuring the area ratios of martensite, bainite, and ferrite is as follows. First, a sample is cut out from a steel sheet so that a thickness cross section (L cross section at 1/4 of the sheet thickness) parallel to the rolling direction becomes an observation surface. The observation surface of the sample is mirror-polished with diamond paste, then finish-polished with colloidal silica, and further etched with 1 volume % nital to reveal the structure. Next, the observation surface of the sample is observed at a magnification of 3000 times using a scanning electron microscope (SEM) under the condition of an acceleration voltage of 10 kV, and SEM images of three visual fields (one visual field is 40 ⁇ m ⁇ 30 ⁇ m) are obtained.
  • SEM scanning electron microscope
  • the area ratio of each structure is calculated using Adobe Photoshop (manufactured by Adobe Systems). Specifically, the value obtained by dividing the area of each structure by the measured area is regarded as the area ratio of each structure. The area ratio of each structure is calculated for three fields of view, and the average value thereof is regarded as the area ratio of each structure.
  • Bainite is a mixed structure region consisting of gray angular bainitic ferrite, white contrasting iron carbides, and needle-shaped retained austenite. Martensite has a hierarchical structure with minute internal irregularities. These can be distinguished from one another.
  • the area ratio of the retained austenite is 15% or less, and preferably 10% or less. There is no particular lower limit, and this effect can be obtained even if the area ratio of the retained austenite is 0%.
  • the method for measuring the area ratio of retained austenite is as follows. First, the steel plate is ground so that the 1/4 position of the plate thickness (the position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) becomes the measurement surface, and then the plate is further polished by 0.1 mm by chemical polishing to obtain a sample. For the measurement surface of the sample, an X-ray diffractometer is used to measure the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite), and the (200), (211), and (220) planes of bcc iron, using a Co K ⁇ radiation source.
  • the intensity ratio of the integrated reflection intensity of each surface of the fcc iron to the integrated reflection intensity of each surface of the bcc iron is calculated.
  • the average value of the nine intensity ratios is taken as the volume fraction of the retained austenite.
  • This volume fraction of the retained austenite is considered to be three-dimensionally uniform, and is taken as the area fraction of the retained austenite at the 1/4 position of the plate thickness of the steel plate.
  • the steel structure of the present invention may have a structure (remaining structure) other than the above-mentioned martensite, bainite, ferrite, and retained austenite.
  • the remaining structure is a structure other than martensite, bainite, ferrite, and retained austenite, and may be any structure known as a steel sheet structure, such as pearlite and alloy carbonitrides precipitated in ferrite. It should be noted that iron-based carbides present in bainite, metastable carbides precipitated in martensite, and iron-based carbides such as cementite precipitated in martensite are not included in the remaining structure.
  • the area ratio of the remaining structure is preferably 3% or less so as not to impair the effects of the present invention.
  • the metastable carbides precipitated in the martensite block improve low-temperature toughness while maintaining excellent part strength, ductility, shear edge bendability, and stretch flangeability. This is believed to be because the metastable carbides precipitated in the martensite block suppress the initiation and propagation of cracks at low temperatures.
  • ratio p the ratio of the number of martensite blocks in which metastable carbides exist to the number of martensite blocks.
  • ratio p is 2% or more, preferably 5% or more, more preferably 10% or more, even more preferably 20% or more, and particularly preferably 30% or more.
  • the upper limit of ratio p is not particularly limited and may be 100%.
  • metastable carbides are metastable carbides that precipitate during the tempering process of martensite.
  • Metastable carbides are, for example, Fe carbides (iron-based carbides) other than cementite, and include at least one type of carbide selected from the group consisting of epsilon ( ⁇ ) carbides, eta ( ⁇ ) carbides, and chi ( ⁇ ) carbides.
  • the method for measuring the ratio (ratio p) of the number of martensite blocks containing metastable carbides to the number of martensite blocks is as follows. First, a steel sheet is ground so that a 1/4 position of the sheet thickness (a position corresponding to 1/4 of the sheet thickness in the depth direction from the surface of the steel sheet) becomes an observation surface, and then electrolytic polishing is performed to prepare a sample. The observation surface of the prepared sample is observed using a transmission electron microscope (TEM) at an acceleration voltage of 200 kV. When an electron beam is incident on the martensite block from the [100] direction, an electron diffraction pattern of the parent martensite is obtained.
  • TEM transmission electron microscope
  • Adjacent martensite blocks have different crystal orientations across the block boundaries, and therefore can be distinguished from each other by the different contrast in the bright-field image. Martensite can be distinguished from ferrite and bainite by the high density of dislocations observed in martensite and the relatively low dislocation density in ferrite and bainite.
  • FIG. 1 is an example of an electron diffraction pattern of martensite in which carbides are present.
  • the electron diffraction pattern of the carbides is obtained in addition to the electron diffraction pattern of the parent martensite ( ⁇ ), as shown in FIG.
  • black circles indicate electron diffraction spots of the martensite parent phase when the electron beam is incident from the [100] direction
  • white circles indicate electron diffraction spots of carbides.
  • the martensite block is defined as a martensite block containing metastable carbides.
  • the ratio of the number of martensite blocks containing metastable carbides to the number of martensite blocks can be rephrased as "the ratio of the number of martensite blocks with a ratio dc/dm of 1.020 or more and 1.150 or less to the number of martensite blocks.”
  • Metastable carbides may be present inside the martensite blocks or at boundary portions such as block boundaries, but are preferably present inside the martensite blocks.
  • Average number density of metastable carbides in martensite blocks containing metastable carbides 1 x 10 6 /mm 2 or more
  • a high number density of metastable carbides in the martensite blocks is preferred for better low temperature toughness reasons, as it is believed that a high number density of metastable carbides provides greater resistance to crack propagation in the martensite at low temperatures.
  • the average number density of metastable carbides in a martensite block in which metastable carbides are present (hereinafter also referred to as "number density n”) is preferably 1 x 10 pcs/mm 2 or more, more preferably 10 x 10 pcs/mm 2 or more, and even more preferably 100 x 10 pcs/mm 2 or more.
  • the upper limit of the number density n is not particularly limited, and the number density n can be, for example, 10,000,000 ⁇ 10 6 pieces/mm 2 or less, preferably 1,000,000 ⁇ 10 6 pieces/mm 2 or less, more preferably 100,000 ⁇ 10 6 pieces/mm 2 or less, and even more preferably 10,000 ⁇ 10 6 pieces/mm 2 or less.
  • the method for measuring the average number density (number density n) of metastable carbides in a martensite block in which metastable carbides are present is as follows.
  • a selected area electron diffraction pattern is obtained for a single martensite block in which metastable carbides are present, and a dark-field image is obtained using the electron diffraction spots obtained from the metastable carbides.
  • the metastable carbides show white contrast.
  • An area of 300 nm ⁇ 300 nm is photographed within a single martensite block, and the number of metastable carbides is counted. Note that adjacent martensite blocks may exist across a block boundary within the 300 nm ⁇ 300 nm area.
  • the area of a martensite block with metastable carbides is defined as the area of a single martensite block for which a selected-area electron diffraction pattern was obtained. Adjacent martensite blocks are distinguished from each other by their contrast in bright-field images due to different crystal orientations across the block boundaries.
  • the average of these values is regarded as the average number density (number density n) of the metastable carbides in the martensite block in which the metastable carbides are present.
  • Average circle equivalent diameter of metastable carbides 20 nm or less
  • the average circle-equivalent diameter of the metastable carbides in the martensite blocks is preferably 20 nm or less, and more preferably 5 nm or less.
  • the method for measuring the average value of the circle equivalent diameter of metastable carbides in a martensite block is as follows.
  • a selected area electron diffraction pattern is obtained for a single martensite block in which metastable carbides are present, and a dark-field image is obtained using the electron diffraction spots obtained from the metastable carbides.
  • the metastable carbides show white contrast.
  • a dark field image of a 300 nm x 300 nm area within a single martensite block is taken and image processing is performed to obtain a binary image in which metastable carbides can be distinguished.
  • the binary image is subjected to particle analysis to determine the circle equivalent diameter for each metastable carbide particle. If metastable carbides overlap in the dark field image, the binary image is segmented using the Watershed method. The circle equivalent diameter is determined for each of all metastable carbides present in the 300 nm ⁇ 300 nm region (three visual fields). The average of the circle equivalent diameters for the three visual fields is determined and this is set as the average circle equivalent diameter of the metastable carbides in the martensite block.
  • the standard deviation ⁇ n of the nanohardness is 0.60 ⁇ [H n ] ave or less, preferably 0.50 ⁇ [H n ] ave or less, where [H n ] ave is the average value of the nanohardness.
  • the lower limit of the standard deviation ⁇ n of the nanohardness is not particularly limited and may be 0.
  • the average nano-hardness [Hn] ave is preferably 3.0 GPa or more and 9.0 GPa or less, and more preferably 3.5 GPa or more and 8.5 GPa or less.
  • a method for measuring the standard deviation ⁇ n of nanohardness will be described.
  • a nanoindentation device equipped with a Berkovich indenter is used. After cutting out a sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is the observation surface, the observation surface is mirror-polished using diamond paste, and then finish-polished using colloidal silica.
  • the nanohardness of 225 or more points is measured for the sample under load control conditions of a loading rate and unloading rate of 50 ⁇ N/s, a maximum load of 500 ⁇ N, and a data collection pitch of 5 msec.
  • the measurement position is set to 1/4 the plate thickness from the surface of the high-strength steel plate, and the distance between the indentations is set to 2 ⁇ m or more.
  • a histogram is created from the nanohardness measurement results obtained at 225 or more points, and the standard deviation is calculated and the result is defined as the standard deviation of the nanohardness ⁇ n .
  • the average value of the nanohardness measurement results obtained at 225 or more points is defined as [H n ] ave .
  • the high-strength steel sheet has a soft surface layer formed on the surface of the base steel sheet.
  • the soft surface layer contributes to suppressing the propagation of bending cracks during press forming and vehicle body collision, thereby improving bending fracture resistance.
  • the base steel sheet refers to a high-strength steel sheet that is the base (undercoat) for the various platings in the case of a steel sheet that has been subjected to a plating treatment, such as a hot-dip galvanized steel sheet, a galvannealed steel sheet, an electrolytic galvanized steel sheet, or a steel sheet that has been plated with other metals, and refers to a high-strength steel sheet in the case of a steel sheet that has not been plated.
  • the surface layer refers to a region corresponding to a thickness of 200 ⁇ m from the surface of the base steel sheet to a depth of 200 ⁇ m in the sheet thickness direction.
  • the soft layer refers to a region having a Vickers hardness of 85% or less of the Vickers hardness of a cross section (a plane parallel to the steel sheet surface) at 1/4 of the sheet thickness of the base steel sheet.
  • the soft layer includes a decarburized layer in the surface layer of the base steel sheet.
  • the surface soft layer refers to a soft layer included in the surface layer, and may be a soft layer in its entirety or a part of it.
  • the surface soft layer may be a region corresponding to a thickness of 200 ⁇ m or less from the surface of the base steel sheet in the sheet thickness direction.
  • a region having a Vickers hardness of 85% or less on a cross section (plane parallel to the steel plate surface) at 1/4 of the plate thickness of the base steel plate is formed at a predetermined depth from the surface of the base steel plate in the plate thickness direction, when the predetermined depth is within 200 ⁇ m in the plate thickness direction, the region corresponding to the thickness from the surface to the predetermined depth in the plate thickness direction is the surface soft layer, and when the predetermined depth is more than 200 ⁇ m in the plate thickness direction, the region corresponding to a thickness of 200 ⁇ m from the surface of the base steel plate to a depth of 200 ⁇ m in the plate thickness direction is the surface soft layer.
  • the lower limit of the thickness of the soft surface layer is not particularly limited, but is preferably 8 ⁇ m or more, and more preferably more than 17 ⁇ m.
  • the Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.
  • the proportion of nano hardness of 7.0 GPa or more should be 0.10 or less.
  • the proportion of nano hardness of 7.0 GPa or more is 0.10 or less, it means that the proportion of hard structures (martensite, etc.), inclusions, etc. is small, and it is possible to further suppress the generation and connection of voids and crack growth in hard structures (martensite, etc.) and inclusions during press forming and collision, and to obtain excellent R/t and SF max .
  • the standard deviation ⁇ of the nanohardness of the sheet surface at a position 1/4 of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less
  • the standard deviation ⁇ of the nanohardness of the sheet surface at a position 1/2 of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet is 2.2 GPa or less
  • a more preferred range for the standard deviation ⁇ of the nanohardness of the sheet surface at a position 1/4 of the way from the base steel sheet surface to the soft surface layer in the sheet thickness direction is 1.7 GPa or less.
  • a more preferred range for the standard deviation ⁇ of the nanohardness of the sheet surface at a position 1/2 of the way from the base steel sheet surface to the soft surface layer in the sheet thickness direction is 2.1 GPa or less.
  • the nano-hardness of the plate surface at the 1/4 and 1/2 positions in the plate thickness direction depth is a hardness measured by the following method. First, if a plating layer is formed, after peeling off the plating layer, mechanical polishing is performed from the surface of the base steel sheet to a position 1/4 of the depth in the sheet thickness direction of the soft surface layer, buff polishing with diamond and alumina is performed, and further colloidal silica polishing is performed.
  • the nano hardness is measured using a Berkovich-shaped diamond indenter under the following conditions: load: 500 ⁇ N, measurement area: 50 ⁇ m ⁇ 50 ⁇ m, and impact spacing: 2 ⁇ m.
  • the soft surface layer is mechanically polished to a position halfway down the thickness direction, buffed with diamond and alumina, and then polished with colloidal silica.Then, the nano-hardness is measured with a Berkovich diamond indenter under the following conditions: load: 500 ⁇ N, measurement area: 50 ⁇ m ⁇ 50 ⁇ m, and impact spacing: 2 ⁇ m.
  • the thickness of the surface soft layer can be measured by the following method. After smoothing the thickness cross section (L cross section) of the base steel sheet parallel to the rolling direction by wet polishing, measurements were taken at 1 ⁇ m intervals using a Vickers hardness tester with a load of 10 gf from a position 1 ⁇ m from the surface of the base steel sheet in the thickness direction to a position 100 ⁇ m in the thickness direction. After that, measurements were taken at 20 ⁇ m intervals up to the center of the thickness. The area where the hardness has decreased to 85% or less compared to the hardness at the 1/4 position in the thickness direction was defined as the soft layer (surface soft layer), and the thickness in the thickness direction of this area was taken as the thickness of the soft layer.
  • the high-strength steel sheet according to an embodiment of the present invention preferably has a first plating layer, which is a metal plating layer, on one or both surfaces of the base steel sheet.
  • the first plating layer is formed directly on the surface of the base steel sheet and is a metal plating layer containing more than 50 mass% in total of one or more selected from Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb and Bi, and is not a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a zinc plating layer of an electrogalvanized layer, or a hot-dip aluminum plating layer.
  • the first plating layer is preferably a metal electroplated layer, and the following description will be given taking a metal electroplated layer as an example.
  • the outermost metal electroplating layer helps to prevent bending cracks during press forming and vehicle collisions, further improving bending fracture resistance.
  • the metal type of the metal electroplating layer may be any of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, but Fe is more preferable.
  • the coating weight of the Fe-based electroplating layer is more than 0 g/ m2 , and preferably 2.0 g/ m2 or more. There is no particular upper limit to the coating weight of the Fe-based electroplating layer per side, but from the viewpoint of cost, the coating weight of the Fe-based electroplating layer per side is preferably 60 g/ m2 or less.
  • the coating weight of the Fe-based electroplating layer is preferably 50 g/ m2 or less, more preferably 40 g/ m2 or less, and even more preferably 30 g/ m2 or less.
  • the adhesion weight of the Fe-based electroplating layer is measured as follows. A 10 x 15 mm sample is taken from the Fe-based electroplated steel sheet and embedded in resin to create a cross-section embedded sample. Three random locations on the cross section are observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based plating layer, and the average thickness of the three fields of view is multiplied by the specific gravity of iron to convert it into the adhesion weight of the Fe-based plating layer per side.
  • SEM scanning electron microscope
  • the Fe-based electroplating layer in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can be used.
  • the composition of the Fe-based electroplating layer is not particularly limited, but it is preferable that the composition contains one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 10 mass% or less, with the remainder consisting of Fe and unavoidable impurities.
  • the C content is preferably 0.08 mass% or less.
  • the high-strength steel sheet according to one embodiment of the present invention may have a second plating layer, which is a metal plating layer, as an outermost layer on one or both sides of the high-strength steel sheet.
  • the second plating layer contains at least one of zinc and aluminum in a total amount of 50 mass % or more, and may be a hot-dip galvanized layer, a galvannealed layer, an electrolytic galvanized layer, a hot-dip aluminum plating layer, or the like.
  • the second plating layer may be formed directly on one or both surfaces of the base steel sheet surface, or it may be formed on the first plating layer.
  • the hot-dip galvanized layer, alloyed hot-dip galvanized layer, and electrolytic galvanized layer refer to a plating layer containing Zn (zinc) as a main component (Zn content of 50.0 mass % or more).
  • the plating layer of an aluminum-plated steel sheet refers to a plating layer containing Al (aluminum) as a main component (Al content is 50.0 mass % or more).
  • the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less Fe, and 0.001 mass% or more and 1.0 mass% or less Al.
  • the hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of more than 0.0 mass% and 3.5 mass% or less.
  • the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.
  • the galvannealed layer is preferably composed of, for example, 20% by mass or less Fe and 0.001% by mass or more and 1.0% by mass or less Al.
  • the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of more than 0% by mass and 3.5% by mass or less.
  • the Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more.
  • the Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.
  • the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g/m 2 or more and 80 g/m 2 or less.
  • the coating weight of the zinc plating layer is measured as follows.
  • a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution.
  • a sample of a steel sheet having a zinc plating layer is then immersed in the treatment solution to dissolve the zinc plating layer.
  • the mass loss of the sample before and after dissolution is then measured, and the value is divided by the surface area of the base steel sheet (the surface area of the part that was covered with plating) to calculate the coating weight (g/ m2 ).
  • the thickness of the high-strength steel plate is not particularly limited, and can be 0.3 mm or more and 3.0 mm or less.
  • the production method of the present invention is also a method for producing the high-strength steel plate according to the present invention described above.
  • the temperature in the production method is based on the surface temperature of the steel slab or steel plate, unless otherwise specified.
  • ⁇ Hot rolling, pickling and cold rolling> In the manufacturing method of the present invention, first, a steel slab having the above-mentioned component composition of the present invention is subjected to hot rolling, pickling and cold rolling to obtain a cold-rolled sheet.
  • the steel slab may be, for example, a molten steel having the composition of the present invention obtained by melting a steel material and solidifying the molten steel.
  • the method for melting steel is not particularly limited, and known melting methods such as converter molten steel and electric furnace molten steel can be used.
  • the method for producing a steel slab from molten steel is not particularly limited, and known methods such as continuous casting, ingot casting, and thin slab casting can be used. From the viewpoint of preventing macrosegregation, it is preferable to produce the steel slab by the continuous casting method.
  • the produced steel slab is, for example, cooled to room temperature once, then heated again and hot rolled (rough rolling and finish rolling), and then coiled. In this way, a hot-rolled sheet is obtained.
  • the produced steel slab may be charged into a heating furnace as a hot piece without being cooled to room temperature, or may be roughly rolled immediately after being slightly kept at room temperature.
  • the temperature at which the steel slab is heated is preferably 1100° C. or higher from the viewpoint of dissolving carbides and reducing the rolling load.
  • the slab heating temperature is preferably 1300° C. or lower. The steel slab heated to the slab heating temperature is subjected to rough rolling.
  • the standard deviation of nanohardness can be reduced to 0.60 ⁇ [H n ] ave or less. This is thought to be because solute atoms such as Si and Mn rapidly diffuse through dislocations and grain boundaries of recrystallized grains during the plastic deformation and dynamic recrystallization of austenite grains during rough rolling, resulting in an appropriate distribution and uniform plastic deformation resistance in local regions.
  • the average strain rate during rough rolling is defined as the total rolling reduction ⁇ (-) from the first mill to the final mill in rough rolling divided by the time t R (s) required from the start of rolling at the first mill to the completion of rolling at the final mill in rough rolling ( ⁇ /t R ).
  • the average strain rate during rough rolling is less than 1 ⁇ 10 ⁇ 4 /s, the recovery of dislocations in austenite grains is promoted, the driving force for recrystallization is reduced, and dynamic recrystallization is suppressed, resulting in insufficient diffusion of solute atoms such as Si and Mn, and the standard deviation of nanohardness exceeds 0.60 ⁇ [H n ] ave . Therefore, rough rolling is performed under the conditions of an average strain rate of 1 ⁇ 10 ⁇ 4 /s to 1 ⁇ 10 ⁇ 1 /s and a total reduction of 50% or more.
  • the average strain rate during rough rolling is preferably 1 ⁇ 10 ⁇ 3 /s to 1 ⁇ 10 ⁇ 2 /s.
  • the total reduction during rough rolling is preferably 60% or more.
  • the rough rolling end temperature is preferably set to 950° C. or more.
  • the rough rolling end temperature can be set to, for example, 1250° C. or less.
  • the slab heating temperature is set to a low temperature, it is preferable to heat the roughly rolled sheet using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.
  • the temperature when performing the finish rolling is preferably equal to or higher than the Ar3 transformation point. This reduces the rolling load. Furthermore, the rolling reduction in the non-recrystallized state of austenite is reduced, and the development of abnormal structures elongated in the rolling direction is suppressed, resulting in excellent workability.
  • Finish rolling may be performed continuously by joining the rough rolled sheets together.
  • the rough rolled sheets may be wound up once before finishing rolling is performed.
  • Lubricated rolling is also preferred from the viewpoint of making the steel sheet shape and material uniform.
  • the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.
  • the coiling temperature after the hot rolling is preferably 300° C. or more and 700° C. or less from the viewpoint of improving the sheet passing property during the cold rolling and annealing described later.
  • the hot-rolled sheet obtained by hot rolling is pickled.
  • oxides on the surface of the hot-rolled sheet are removed, and excellent chemical conversion treatability and quality of the plating layer can be obtained in the final product, a high-strength steel sheet.
  • Pickling may be performed once or multiple times.
  • the hot-rolled sheet is optionally subjected to a softening heat treatment and then cold-rolled. In this way, a cold-rolled sheet is obtained.
  • the total reduction ratio of the cold rolling is preferably 20% or more and 75% or less.
  • the number of rolling passes and the reduction ratio of each pass are no particular limitations on the number of rolling passes and the reduction ratio of each pass.
  • a first plating step may be included in which a first plating layer, which is a metal plating layer, is formed on one or both sides of the steel sheet after the hot rolling step (after the cold rolling step if cold rolling is performed) and before the annealing step.
  • the first plating step is preferably a metal electroplating step.
  • a metal plating process such as metal electroplating may be applied to the surface of the cold-rolled sheet obtained as described above to produce a pre-annealed metal-plated steel sheet having a pre-annealed metal plating layer formed on at least one side.
  • the metal plating layer referred to here may be the above-mentioned first plating layer.
  • the pre-annealed metal-plated steel sheet is preferably a pre-annealed metal electroplated steel sheet provided with a pre-annealed metal electroplating layer.
  • the metal electroplated steel sheet before annealing means that the metal electroplating layer has not been subjected to an annealing process, and does not exclude the case where the hot-rolled sheet before the metal electroplating process, the pickled sheet after hot rolling, or the cold-rolled sheet has been annealed in advance.
  • the metal species of the electroplating layer can be any of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, but since Fe is more preferable, the manufacturing method of Fe-based electroplating is described below.
  • the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 0.5 mol/L or more in terms of Fe 2+ . If the Fe ion content in the Fe-based electroplating bath is 0.5 mol/L or more in terms of Fe 2+ , a sufficient Fe deposition amount can be obtained. In addition, in order to obtain a sufficient Fe deposition amount, the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 2.0 mol/L or less.
  • the Fe-based electroplating bath may contain at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in addition to Fe ions.
  • the total content of these elements in the Fe-based electroplating bath is preferably such that the total content of these elements in the Fe-based electroplating layer before annealing is 10 mass% or less.
  • the metal elements may be contained as metal ions, and the nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acid, etc.
  • the iron sulfate plating solution may also contain a conductivity aid such as sodium sulfate or potassium sulfate, a chelating agent, or a pH buffer.
  • the temperature of the Fe-based electroplating solution is preferably 30° C. or higher from the viewpoint of constant temperature retention, and is preferably 85° C. or lower.
  • the pH of the Fe-based electroplating bath is also not particularly limited, but is preferably 1.0 or higher from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or lower from the viewpoint of the electrical conductivity of the Fe-based electroplating bath.
  • the current density is preferably 10 A/dm 2 or higher from the viewpoint of productivity, and is preferably 150 A/dm 2 or lower from the viewpoint of facilitating control of the deposition amount of the Fe-based electroplating layer.
  • the sheet passing speed is preferably 5 mpm or higher from the viewpoint of productivity, and is preferably 150 mpm or lower from the viewpoint of stably controlling the deposition amount.
  • a degreasing treatment and water washing for cleaning the surface of the cold-rolled sheet, and further, a pickling treatment and water washing for activating the surface of the cold-rolled sheet can be performed.
  • the Fe-based electroplating treatment is performed following these pretreatments.
  • the method of the degreasing treatment and water washing is not particularly limited, and a conventional method can be used.
  • various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid, and mixtures thereof are preferred.
  • the concentration of the acid is not particularly limited, but is preferably 1% by mass or more and 20% by mass or less from the viewpoints of the ability to remove the oxide film and the prevention of roughness (surface defects) due to over-pickling.
  • the pickling solution may also contain an antifoaming agent, a pickling promoter, a pickling inhibitor, and the like.
  • the obtained cold-rolled sheet is subjected to a first heating at a temperature of 750° C. or more.
  • the cold-rolled sheet may be one that has been subjected to an electroplating treatment, but may not be one that has been subjected to said treatment.
  • the first heating temperature is 750° C. or higher, and preferably 770° C. or higher.
  • the upper limit of the heating temperature is not particularly limited, but is preferably 950° C. or less from the viewpoint of operability and the like.
  • heating time is not particularly limited, but if the time is too short, the reverse transformation to austenite may not proceed sufficiently, so the time is preferably 30 seconds or more, and more preferably 60 seconds or more.
  • the upper limit of the heating time is not particularly limited, and can be, for example, 6000 s or less, and preferably 3000 s or less, where "s" means seconds.
  • the dew point of the annealing atmosphere in the first heating is preferably -30°C or higher.
  • the annealing atmosphere in the annealing process is more preferably -15°C or higher, and even more preferably -5°C or higher.
  • the dew point of the annealing atmosphere in the annealing process is preferably 30°C or lower.
  • the first average cooling rate v1 is 2.0° C./s or more, preferably 3.0° C./s or more, and more preferably 5.0° C./s or more.
  • the upper limit of the first average cooling rate v1 is not particularly limited, but from the viewpoint of reducing the capital investment burden, it is preferably 60.0° C./s or less.
  • the cooling in the temperature region T1 is preferably continuous cooling.
  • the cooling rate from the first heating temperature to 750° C. is not particularly limited.
  • the cold-rolled sheet that has passed through the temperature range T1 is then subjected to a residence time step of holding at a residence temperature T2 of 350°C or more and 550°C or less.
  • the cold-rolled sheet that has passed through the first cooling step is then held at a residence temperature T2 of 350°C or more and 550°C or less for a residence time t2 (s) that satisfies F defined by formula 1 of 0.20 or more and 0.90 or less, thereby causing bainite transformation.
  • this residence time t2 (s) from an expansion curve obtained from a Formaster test, the bainite transformation and the associated precipitation of iron carbides and the distribution of C into untransformed austenite are optimized. Since the expansion curve depends on the steel composition and the thermal history up to the first cooling, it is necessary to determine an expansion curve for each steel composition and thermal history from the first heating temperature to T2 °C, and select an appropriate residence time t2 .
  • the holding time t2 is the time t or more at which F becomes 0.20, and preferably the time t or more at which F becomes 0.30.
  • the bainite transformation proceeds excessively, the amount of martensite decreases, and TS decreases.
  • the holding time t2 is equal to or shorter than the time t at which F becomes 0.90, and preferably equal to or shorter than the time t at which F becomes 0.80.
  • the relationship between F and t in formula 1 is calculated as follows.
  • the steel slab is subjected to a process up to the first cooling, and then is retained at a retention temperature T2 of 350° C. to 550° C.
  • the process up to the first cooling includes a hot rolling process, a pickling and cold rolling process, a first heating process, and a process up to the first cooling.
  • an expansion curve during retention at retention temperature T2 is obtained. Retention at T2 is continued until expansion stops.
  • the expansion amount at the start of retention at retention temperature T2 is set to 0, and the expansion amount at retention is set to 1.
  • the expansion curve is fitted with Equation 1 to calculate constants k and n. This determines the relationship between F and t at retention temperature T2 .
  • Formula 1: F 1-exp(-kt n ) t: residence time (s) k, n: constants obtained from the expansion curve of the Formaster test
  • the cooling stop temperature is Ms-20°C or less. This allows the martensitic transformation to proceed sufficiently. If the cooling stop temperature exceeds Ms-20°C, the untransformed austenite does not transform to martensite, the amount of retained austenite becomes excessive, and good part strength and stretch flangeability cannot be obtained.
  • the cooling stop temperature may be room temperature.
  • Ms is the temperature (Ms point) at which martensitic transformation begins to occur, and a value measured by the test described below is used.
  • the second average cooling rate v2 is 5° C./s or more, and preferably 8° C./s or more.
  • the upper limit of the second average cooling rate v2 is not particularly limited, but from the viewpoint of reducing the capital investment burden, it is preferably 60.0° C./s or less.
  • the cooling rate outside the temperature region T3 is not particularly limited.
  • the Ms point is a value measured by a Formaster test as follows. Using a Formaster testing machine, the steel slab is subjected to a process up to the end of the retention process, and then cooled to room temperature at a second average cooling rate of 5°C/s or more. The temperature at which martensitic transformation occurs and expansion begins during the second cooling is defined as the Ms point.
  • the upper limit of the second average cooling rate is not particularly limited, but can be, for example, 100°C/s or less.
  • the ratio (ratio p) of the number of martensite blocks in which metastable carbides exist to the total number of martensite blocks can be increased, thereby improving low temperature toughness.
  • the temperature X (° C.) is a value higher than room temperature.
  • the temperature X (° C.) of the second heating is higher than the cooling stop temperature of the second cooling step, heating is performed from the cooling stop temperature to the temperature X (° C.) of the second heating.
  • the cooling stop temperature of the second cooling step and the temperature X (°C) of the second heating may be the same, in which case it means that the cooling stop temperature is maintained at the temperature X (°C) of the second heating.
  • the value of the variable part Z in the above formula 2 is too small, that is, if the temperature X is too low and/or the holding time Y is too short, metastable carbides are not sufficiently precipitated, and the proportion p becomes low. Therefore, from the viewpoint of increasing the proportion p, the value of the variable part Z is 7000 or more, and preferably 8000 or more. On the other hand, if the value of the variable part Z is too high, i.e., if the temperature X is too high and/or the holding time Y is too long, the metastable carbides will transition to cementite and the proportion p will decrease. For this reason, from the viewpoint of increasing the proportion p, the value of the variable part Z is 13000 or less, and preferably 12000 or less.
  • the temperature X (unit: ° C.) preferably satisfies the following formula 3. This increases the number density (number density n) of metastable carbides in the martensite block in which the metastable carbides are present.
  • Formula 3 100 ⁇ X ⁇ 400
  • the temperature X is preferably 100° C. or higher, more preferably 120° C. or higher, and even more preferably 150° C. or higher.
  • the temperature X is preferably 400° C. or less, more preferably 380° C. or less, and further preferably 350° C. or less.
  • hot-dip galvanizing it is preferable to immerse the steel sheet in a zinc plating bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like.
  • a plating bath with an Al content of 0.10 mass% to 0.23 mass%, with the balance being Zn and unavoidable impurities.
  • the coating weight of the steel sheet having a hot-dip galvanized layer (hot-dip galvanized steel sheet) (GI) and the steel sheet having a galvannealed layer (galvannealed hot-dip galvanized steel sheet) (GA) is preferably 20 to 80 g/ m2 per side.
  • the coating weight can be adjusted by gas wiping or the like.
  • Skin pass rolling (optional)
  • the obtained high strength steel sheet may be subjected to skin pass rolling.
  • the plating treatment may be performed.
  • the reduction ratio in the skin pass rolling is preferably 0.05% or more from the viewpoint of increasing the yield strength.
  • the upper limit of the reduction ratio is not particularly limited, but is preferably 1.50% from the viewpoint of productivity.
  • Skin pass rolling may be performed either online or offline. The skin pass may be performed at a single time to the desired rolling reduction, or may be performed in several steps.
  • Example 1 the sheet was held at the first heating temperature of 800 ° C. for 200 s, then cooled to 480 ° C. under the condition of an average cooling rate of 18 ° C./s, and then held at 480 ° C. for 1000 s.
  • the expansion curve obtained was fitted with Equation 1 to obtain k and n.
  • ⁇ Low temperature toughness test> Six of the obtained steel plates (1.6 mmt steel plates) were stacked and bonded together to prepare a Charpy impact test piece with a thickness of 9.6 mmt. The notch was a 2 mm U-notch. Using this test piece, a Charpy impact test was performed at -40°C, and the Charpy absorbed energy obtained was measured. In addition, the ductile fracture rate was measured by observing the fracture surface after the test.
  • the low-temperature toughness parameter P was defined as the product of Charpy absorbed energy (unit: J) and ductile fracture surface area ratio (%) (Charpy absorbed energy (J) x ductile fracture surface area ratio (%)) and was calculated. When the low temperature toughness parameter P is 3000 or more, it is determined that the low temperature toughness is excellent.
  • TS is 780 MPa or more, and the parts have excellent strength, ductility, stretch flangeability, bendability of the sheared end surface, and low-temperature toughness.
  • the parts are inferior in one or more of strength, ductility, stretch flangeability, bendability of the sheared end surface, and low-temperature toughness.
  • the composition of the metal electroplating layer is Fe: 95-100% by mass for Fe-based electroplating, Ni: 95-100% by mass for Ni-based electroplating, and the remainder in each case is unavoidable impurities.
  • a punch B2 was pressed into the test piece T1 placed on a support roll A2 so that the bending direction was perpendicular to the rolling direction, and an orthogonal bending (secondary bending) was performed.
  • D1 indicates the width (C) direction
  • D2 indicates the rolling (L) direction.

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Abstract

La présente invention propose une tôle d'acier à haute résistance ayant d'excellentes propriétés de résistance, ductilité, façonnage à des formes complexes, pliabilité en section côté cisaillement, et de ténacité à basse température de composant. La tôle d'acier à haute résistance a une composition de composant prescrite, et une composition d'acier dans laquelle le rapport de surface de martensite est de 10 à 80 %, le rapport de surface de bainite est de 2 à 70 %, le rapport de surface de ferrite est inférieur ou égal à 80 %, le rapport de surface d'austénite résiduelle est inférieur ou égal à 15 %, et le rapport du nombre de blocs de martensite dans lequel du carbure métastable existe au nombre de blocs de martensite est supérieur ou égal à 2 %. Lorsque la nanodureté est mesurée à 225 points ou plus à une épaisseur de position de 1/4 de feuille, l'écart type σnde la nanodureté par rapport à la nanodureté moyenne [Hn]ave est inférieur ou égal à 0,60×[Hn]ave.
PCT/JP2022/045373 2022-12-08 2022-12-08 Tôle d'acier à haute résistance, élément formé à l'aide d'une tôle d'acier à haute résistance, composant de structure d'ossature d'automobile ou composant de renforcement d'automobile composé d'un élément, et procédés de production de tôle d'acier à haute résistance et d'élément WO2024122037A1 (fr)

Priority Applications (2)

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JP2023519413A JP7367893B1 (ja) 2022-12-08 2022-12-08 高強度鋼板、高強度鋼板を用いてなる部材、部材からなる自動車の骨格構造部品用又は自動車の補強部品、ならびに高強度鋼板及び部材の製造方法
PCT/JP2022/045373 WO2024122037A1 (fr) 2022-12-08 2022-12-08 Tôle d'acier à haute résistance, élément formé à l'aide d'une tôle d'acier à haute résistance, composant de structure d'ossature d'automobile ou composant de renforcement d'automobile composé d'un élément, et procédés de production de tôle d'acier à haute résistance et d'élément

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008123267A1 (fr) * 2007-03-22 2008-10-16 Jfe Steel Corporation Tôle d'acier galvanisé à chaud, haute résistance, possédant une excellente aptitude au moulage, et son procédé de production
WO2009119751A1 (fr) * 2008-03-27 2009-10-01 新日本製鐵株式会社 Tôle d'acier galvanisée à haute résistance, tôle galvanisée à chaud alliée à haute résistance et tôle d'acier laminée à froid à haute résistance qui excellent en termes d'aptitude au moulage et au soudage, et procédé de fabrication de toutes ces tôles
WO2017168957A1 (fr) * 2016-03-31 2017-10-05 Ｊｆｅスチール株式会社 Tôle d'acier mince, tôle d'acier plaquée, procédé de production de tôle d'acier laminée à chaud, procédé de production de tôle d'acier très dure laminée à froid, procédé de production de tôle d'acier mince, et procédé de production de tôle d'acier plaquée
WO2018151331A1 (fr) * 2017-02-20 2018-08-23 新日鐵住金株式会社 Tôle d'acier haute résistance
WO2022185805A1 (fr) * 2021-03-02 2022-09-09 Ｊｆｅスチール株式会社 Tôle d'acier, élément, procédé de production de ladite tôle d'acier et procédé de production dudit élément
WO2022185804A1 (fr) * 2021-03-02 2022-09-09 Ｊｆｅスチール株式会社 Tôle d'acier, élément, procédé de production de ladite tôle d'acier et procédé de production dudit élément

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008123267A1 (fr) * 2007-03-22 2008-10-16 Jfe Steel Corporation Tôle d'acier galvanisé à chaud, haute résistance, possédant une excellente aptitude au moulage, et son procédé de production
WO2009119751A1 (fr) * 2008-03-27 2009-10-01 新日本製鐵株式会社 Tôle d'acier galvanisée à haute résistance, tôle galvanisée à chaud alliée à haute résistance et tôle d'acier laminée à froid à haute résistance qui excellent en termes d'aptitude au moulage et au soudage, et procédé de fabrication de toutes ces tôles
WO2017168957A1 (fr) * 2016-03-31 2017-10-05 Ｊｆｅスチール株式会社 Tôle d'acier mince, tôle d'acier plaquée, procédé de production de tôle d'acier laminée à chaud, procédé de production de tôle d'acier très dure laminée à froid, procédé de production de tôle d'acier mince, et procédé de production de tôle d'acier plaquée
WO2018151331A1 (fr) * 2017-02-20 2018-08-23 新日鐵住金株式会社 Tôle d'acier haute résistance
WO2022185805A1 (fr) * 2021-03-02 2022-09-09 Ｊｆｅスチール株式会社 Tôle d'acier, élément, procédé de production de ladite tôle d'acier et procédé de production dudit élément
WO2022185804A1 (fr) * 2021-03-02 2022-09-09 Ｊｆｅスチール株式会社 Tôle d'acier, élément, procédé de production de ladite tôle d'acier et procédé de production dudit élément

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