High-strength steel sheet, member using high-strength steel sheet, structural member for automobile building member or structural member for

Abstract

Provided is a high-strength steel sheet having excellent member strength, ductility, stretch flangeability, bendability of a shear end surface, and low-temperature toughness. A high-strength steel sheet having a prescribed component composition and a steel structure in which the area ratio of martensite is 10%-80%, the area ratio of bainite is 2%-70%, the area ratio of ferrite is 80% or less, and the area ratio of retained austenite is 15% or less at a position at 1 / 4 of the sheet thickness. The ratio of the number of martensite blocks in which metastable carbides are present relative to the number of martensite blocks is 2% or more, and the standard deviation [sigma] n of the nano-hardness relative to the average value [Hn] ave of the nano-hardness is 0.60 x [Hn] ave or less when the nano-hardness at 225 points or more is measured at a position of 1 / 4 of the sheet thickness.

Description

Technical Field

[0001] The present invention relates to a high-strength steel plate, a member using the high-strength steel plate, a skeletal structural member or a reinforcing member of an automobile formed of the member, and a method for manufacturing the high-strength steel plate and the member. Background Art

[0002] In order to reduce CO2 emissions and improve collision resistance performance by lightening vehicles, the high-strength of steel plates for automobiles is being promoted. In addition, against the background of the continuous introduction of new laws and regulations, in order to increase the strength of the vehicle body, the cases of applying high-strength steel plates to the main structural members and reinforcing members (hereinafter also referred to as "skeletal structural members of automobiles", etc.) that form the skeleton of the automobile cab are increasing. In particular, the cases of applying high-strength steel plates with a tensile strength (hereinafter also referred to as "TS") of 780 MPa or more are increasing significantly.

[0003] For example, Patent Document 1 proposes a high-strength steel plate having a specified composition and microstructure, and the particle size of iron carbide contained in the low-temperature phase transformation is 500 nm or less.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2008-308717 Summary of the Invention

[0007] For high-strength steel plates used in skeletal structural members of automobiles, excellent component strength (large impact absorption energy during collision) is required, high yield strength (YS) of the steel plate is required, and high yield ratio (YR = yield strength (YS) / tensile strength (TS)) is required. In addition, since components such as collision boxes have blanked end faces and bent portions, good ductility, flange stretching property, and bendability of the shear end face portion are required for the steel plates used for these components. In addition, when a component using a high-strength steel plate with a tensile strength of 780 MPa or more is used in a low-temperature environment, the toughness may deteriorate and cracks may occur during collision. Therefore, for steel plates for automobiles, in order to prevent cracks during collision when used in a low-temperature environment, excellent low-temperature toughness is required.

[0008] In order to increase the ratio of application of high-strength steel plates to automobile components, there is still a need for steel plates that comprehensively satisfy the above characteristics.

[0009] The present invention has been made in view of the above aspects, and an object thereof is to provide a high-strength steel plate and a manufacturing method thereof that are excellent in component strength, ductility, flange stretching property, bendability of the shear end face portion, and low-temperature toughness.

[0010] In addition, an object of the present invention is to provide a member using the above high-strength steel plate and a method for manufacturing the same.

[0011] Here, the "high-strength steel plate" refers to a steel plate having a tensile strength (TS) of 780 MPa or more obtained by a tensile test described later.

[0012] "Excellent component strength" means that the yield ratio (YR) obtained by a tensile test described later is 55% or more.

[0013] "Excellent ductility" means that the total elongation (El) obtained by a tensile test described later is 10% or more.

[0014] "Excellent flange stretching property" means that the hole expansion ratio (λ) obtained by a hole expansion test described later is 20% or more.

[0015] "Excellent bendability of the sheared end face" means that the ratio (Rs / Rg) of the limit bending radius (Rs / t) obtained by a bending test of a sample having a sheared end face to the limit bending radius (Rg / t) obtained by a bending test of a sample having a ground end face is 1.50 or less.

[0016] "Excellent low-temperature toughness" means that the low-temperature toughness parameter (P) in a Charpy impact test described later is 3000 or more.

[0017] The inventors of the present invention conducted in-depth research and found that the above object can be achieved by adopting the following configuration, thus completing the present invention.

[0018] That is, the gist of the present invention is as follows.

[0019] (1) A high-strength steel plate having the following composition and steel structure,

[0020] The composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and the balance is composed of Fe and unavoidable impurities.

[0021] The steel structure has a martensite area ratio of 10% to 80%, a bainite area ratio of 2% to 70%, a ferrite area ratio of 80% or less, a retained austenite area ratio of 15% or less, and a ratio of the number of martensite blocks with metastable carbides to the number of martensite blocks of 2% or more at the 1 / 4 plate thickness position.

[0022] When measuring the nano-hardness at more than 225 points at the position of 1 / 4 of the plate thickness, with respect to the average value of the nano-hardness [H n ave , the standard deviation σ of the nano-hardness n is 0.60 × [H n ave or less.

[0023] (2) The high-strength steel plate according to (1) above, wherein the average value of the number density of the metastable carbides in the martensite blocks where the metastable carbides are present is 1 × 10 6 pieces / mm 2 or more.

[0024] (3) The high-strength steel plate according to (1) or (2) above, wherein the above composition further contains, by mass%, at least one element selected from 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, Hf: 0.10% or less, and Bi: 0.200% or less.

[0025] (4) The high-strength steel plate according to any one of (1) to (3) above, wherein it has a surface soft layer, and the surface soft layer is a region where the Vickers hardness is 85% or less with respect to the Vickers hardness at the position of 1 / 4 of the plate thickness of the above high-strength steel plate and is a region within 200 μm in the plate thickness direction from the surface of the above high-strength steel plate,

[0026] When measuring the nano-hardness at more than 300 points in each 50 μm × 50 μm region of the plate surface at the positions of 1 / 4 and 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the above high-strength steel plate,

[0027] The proportion of the number of measurements with a nano-hardness of 7.0 GPa or more on the plate surface at the position of 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the above high-strength steel plate with respect to the total number of measurements is 0.10 or less,

[0028] The standard deviation σ of the nano-hardness on the plate surface at the position of 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the above high-strength steel plate is 1.8 GPa or less, ​​

[0029] In addition, the standard deviation σ of the nano-hardness of the plate surface at a position half the thickness direction depth of the surface soft layer from the surface of the above high-strength steel plate is 2.2 GPa or less.

[0030] (5) The high-strength steel plate according to any one of the above (1) to (4), wherein a metal plating layer containing one or more metals 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 in a total amount greater than 50% by mass is provided on one or both surfaces of the high-strength steel plate.

[0031] (6) The high-strength steel plate according to any one of the above (1) to (5), wherein a metal plating layer containing at least one of zinc and aluminum in a total amount of 50% by mass or more is provided on the outermost layer of one or both surfaces of the high-strength steel plate.

[0032] (7) A member made of the high-strength steel plate according to any one of the above (1) to (6).

[0033] (8) A skeletal structure member or a reinforcing member of an automobile, which is composed of the member of the above (7).

[0034] (9) A method for manufacturing a high-strength steel plate, comprising the following steps:

[0035] A hot rolling step in which a steel slab having the composition described in the above (1) or (3) is rough-rolled under the condition that the average strain rate is 1×10 -4 / s to 1×10 -1 / s and the total reduction ratio is 50% or more, then finish-rolled, and then coiled to obtain a hot-rolled sheet,

[0036] Then, a pickling and cold rolling step of pickling and cold rolling to obtain a cold-rolled sheet,

[0037] Then, a first heating is performed under the condition that the heating temperature is 750°C or higher,

[0038] A first cooling step, and then a first heating step in which cooling is performed under the condition that the first cooling rate in the temperature range of T2 to 750°C is 2.0°C / s or higher,

[0039] Then, a soaking step in which soaking is performed at a soaking temperature T2 of 350°C to 550°C under the condition that the residence time t (s) defined by the following formula 1 satisfies 0.20 to 0.90,

[0040] Next, a second cooling step of cooling to below Ms - 20°C and with a second average cooling rate in the temperature range of Ms - 20°C to Ms of 5°C / s or more as a condition, and

[0041] Next, a second heating step of performing a treatment under the conditions of a temperature X (°C) and a holding time Y (s) that satisfy the following formula 2;

[0042] Formula 1: F = 1 - exp(-kt n )

[0043] t: residence time (s)

[0044] k, n: constants obtained from the expansion curve of the Formaster test, which is a phase change point determination test and is performed by holding a test piece obtained by subjecting the above steel billet to the process up to the end of the first cooling step at a residence temperature T2 of 350°C to 550°C.

[0045] Formula 2: 7000 ≤ (273 + X)(20 + log(Y / 3600)) ≤ 13000

[0046] (10) The method for manufacturing a high-strength steel sheet according to the above (9), wherein in the above second heating step, the temperature X (°C) satisfies the following formula 3.

[0047] Formula 3: 100 ≤ X ≤ 400

[0048] (11) The method for manufacturing a high-strength steel sheet according to the above (9) or (10), wherein the above first heating step is performed in an atmosphere with a dew point of -30°C or higher.

[0049] (12) The method for manufacturing a high-strength steel sheet according to any one of the above (9) to (11), which includes the following step: performing a metal plating containing one or more metals 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 in an amount greater than 50% by mass on one or both sides of the steel sheet after the above cold rolling step and before the above annealing step.

[0050] (13) The manufacturing method according to any one of the above (9) to (12), which includes the following step: performing a metal plating containing at least one of zinc and aluminum in a total amount of 50% by mass or more on the steel sheet from the above first heating step to the second heating step.

[0051] (14) A manufacturing method of a component, comprising the following steps: performing at least one of forming or joining on the high-strength steel sheet described in any one of the above (1) to (6) to form a component.

[0052] According to the present invention, a high-strength steel sheet excellent in component strength, ductility, stretch flangeability, bendability of the sheared end face, and low-temperature toughness can be provided. In addition, a component using the above high-strength steel sheet can be provided.

[0053] In addition, according to the present invention, a manufacturing method of the above high-strength steel sheet and a component using the high-strength steel sheet can be provided.

[0054] Furthermore, according to the present invention, a skeletal structure component for an automobile or a reinforcing component for an automobile composed of the components described above can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is an example of an electron diffraction pattern of martensite in which carbides are present.

[0056] Figure 2 It is a schematic diagram related to the production of a sample for the V-bending + orthogonal VDA bending test of the examples. Figure 2 (a) Related to V-bending (primary bending), Figure 2 (b) Related to orthogonal VDA bending (secondary bending).

[0057] Figure 3 It is a schematic diagram related to the sample and test for the axial crush test of the examples. Figure 3 (a) is a front view of the test component, Figure 3 (b) is a front view of the test component. Figure 3 (c) is a schematic diagram showing the axial crush test. DETAILED DESCRIPTION OF THE INVENTION

[0058] The present invention will be described based on the following embodiments. The present invention is not limited to the following embodiments.

[0059] [High-strength steel sheet]

[0060] The high-strength steel sheet of the present invention (hereinafter, also referred to as "steel sheet" for convenience) has a composition and a steel structure described later.

[0061] <Composition>

[0062] The composition of the high-strength steel sheet of the present invention (hereinafter, also referred to as "the composition of the present invention" for convenience) will be described. "%" in the composition of the present invention represents "mass%" unless otherwise specified.

[0063] 《C: 0.030% - 0.500%》

[0064] C is one of the important basic components of steel and particularly affects the area ratio of martensite in the present invention. If the C content is too low, the area ratio of martensite decreases, and it is difficult to achieve a TS of 780 MPa or more. Therefore, the C content is 0.030% or more, preferably 0.040% or more, and more preferably 0.050% or more.

[0065] On the other hand, if the C content is too high, the retained austenite increases excessively, and the hardness of the martensite formed from the retained austenite during blanking increases significantly. As a result, the progress of cracks during reaming is promoted, the reaming rate decreases, and the flange stretching property decreases. In addition, YR decreases due to stress-induced phase transformation of the retained austenite, and the component strength decreases. Therefore, the C content is 0.500% or less, preferably 0.400% or less, and more preferably 0.300% or less.

[0066] 《Si: 0.01% - 2.50%》

[0067] Si is a component that increases the strength of the steel sheet by suppressing the precipitation of cementite in martensite and solid solution strengthening. To obtain this effect, the Si content is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.

[0068] On the other hand, if the Si content is too high, the precipitation of carbides during bainite transformation is significantly suppressed, the retained austenite increases excessively, and the hardness of the martensite formed from the retained austenite during blanking increases significantly. As a result, the progress of cracks during reaming is promoted, the reaming rate decreases, and the flange stretching property decreases. In addition, YR decreases due to stress-induced phase transformation of the retained austenite, and the component strength decreases. Therefore, the Si content is 2.50% or less, preferably 2.00% or less, and more preferably 1.50% or less.

[0069] 《Mn: 0.10% - 5.00%》

[0070] Mn is one of the important basic components of steel and particularly affects the area ratio of martensite in the present invention.

[0071] If the Mn content is too low, the area ratio of martensite decreases, and it is 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.

[0072] On the other hand, if the Mn content is too high, austenite is stabilized, the amount of retained austenite increases excessively, and the hardness of martensite formed from retained austenite during blanking increases significantly. As a result, the crack propagation during reaming is promoted, the reaming rate decreases, and the flange stretchability decreases. In addition, due to stress-induced phase transformation of retained austenite, YR decreases and the component strength decreases. Therefore, the Mn content is 5.00% or less, preferably 4.20% or less, and more preferably 3.60% or less.

[0073] 《P: 0.100% or less》

[0074] P segregates at the prior austenite grain boundaries, embrittling the grain boundaries, thus reducing the ultimate deformation capacity of the steel sheet. Therefore, it is a component that reduces λ and decreases bendability. Therefore, the content of P is 0.100% or less, preferably 0.070% or less.

[0075] There is no particular limitation on the lower limit of the content of P. Since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more.

[0076] 《S: 0.0200% or less》

[0077] S exists in the form of sulfides and can reduce the ultimate deformation capacity of the steel sheet. Therefore, it is a component that reduces λ and decreases bendability. Therefore, the content of S is 0.0200% or less, preferably 0.0050% or less.

[0078] There is no particular limitation on the lower limit of the content of S. Due to production technology constraints, it is preferably 0.0001% or more.

[0079] 《Al: 1.000% or less》

[0080] Although Al is an effective component for sufficient deoxidation and reduction of inclusions in steel, if the Al content is too high, a large amount of ferrite is formed, the reaming rate decreases, and the flange stretchability decreases. Therefore, the Al content is 1.000% or less, preferably 0.500% or less, and more preferably 0.100% or less.

[0081] On the other hand, for stable deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and further preferably 0.020% or more.

[0082] 《N: 0.0100% or less》

[0083] N exists in the form of nitrides and reduces the ultimate deformation capacity of the steel sheet. Therefore, it is a component that reduces λ and decreases bendability. Therefore, the content of N is 0.0100% or less, preferably 0.0050% or less.

[0084] The lower limit of the content of N is not particularly limited. Due to production technology constraints, the content of N is preferably 0.0001% or more.

[0085] 《O: 0.0100% or less》

[0086] O exists in the form of an oxide, which reduces the ultimate deformation ability of the steel plate. Therefore, it is a component that reduces λ and bending properties. Thus, the content of O is 0.0100% or less, preferably 0.0050% or less.

[0087] The lower limit of the content of O is not particularly limited. Due to production technology constraints, the content of O is preferably 0.0001% or more.

[0088] 《Optional components》

[0089] In addition to the above-mentioned component composition, the high-strength steel plate of the present invention may further contain, by mass%, at least one element selected from 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, Hf: 0.10% or less, and Bi: 0.200% or less. These elements can be single or in combination of two or more.

[0090] When Ti, Nb or V is contained, in order to avoid the formation of a large amount of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate is reduced, and further λ is reduced and the bending property is lowered. The content of Ti, Nb or V is preferably 0.200% or less, more preferably 0.100% or less, respectively. The lower limit of the content of Ti, Nb or V is not particularly limited. During hot rolling or continuous annealing, the strength of the steel plate is increased by forming fine carbides, nitrides or carbonitrides. Therefore, the content of Ti, Nb or V is preferably 0.001% or more, respectively.

[0091] When Ta or W is contained, in order to avoid the formation of a large amount of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate is reduced, and further λ is reduced and the bending property is lowered. The content of Ta or W is preferably 0.10% or less, more preferably 0.08% or less, respectively. The lower limit of the content of Ta or W is not particularly limited. During hot rolling or continuous annealing, the strength of the steel plate is increased by forming fine carbides, nitrides or carbonitrides. Therefore, the content of Ta or W is preferably 0.01% or more, respectively.

[0092] When containing B, in order to avoid the generation of cracks inside the steel plate during casting or hot rolling, the ultimate deformation ability of the steel plate decreases, thereby reducing λ and the bendability. The content of B is preferably 0.0100% or less, more preferably 0.0003% or more. There is no particular limitation on the lower limit of the content of B. Since it segregates at the austenite grain boundaries during annealing and is an element that improves hardenability, the content of B is preferably 0.0003% or more.

[0093] When containing Cr, Mo or Ni, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, thereby reducing λ and the bendability. The content of Cr, Mo or Ni is respectively preferably 1.00% or less, more preferably 0.80% or less. There is no particular limitation on the lower limit of the content of Cr, Mo or Ni. Since they are elements that improve hardenability, the content of Cr, Mo or Ni is respectively preferably 0.01% or more.

[0094] When containing Co, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, thereby reducing λ and the bendability. The content of Co is preferably 0.010% or less, more preferably 0.008% or less. There is no particular limitation on the lower limit of the content of Co. Since it is an element that improves hardenability, the content of Co is preferably 0.001% or more.

[0095] When containing Cu, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, thereby reducing λ and the bendability. The content of Cu is preferably 1.00% or less, more preferably 0.80% or less. There is no particular limitation on the lower limit of the content of Cu. Since it is an element that improves hardenability, the content of Cu is preferably 0.01% or more.

[0096] When containing Sn, in order to avoid the generation of cracks inside the steel plate during casting or hot rolling, the ultimate deformation ability of the steel plate decreases, thereby reducing λ and the bendability. The content of Sn is preferably 0.200% or less, more preferably 0.100% or less. There is no particular limitation on the lower limit of the content of Sn. Since Sn is an element that improves hardenability, the content of Sn is preferably 0.001% or more.

[0097] When containing Sb, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, thereby reducing λ and the bendability. The content of Sb is preferably 0.200% or less, more preferably 0.100% or less. There is no particular limitation on the lower limit of the content of Sb. Since it is an element that controls the thickness of the surface softened layer and can adjust the strength, the content of Sb is preferably 0.001% or more.

[0098] When Ca, Mg, or REM is contained, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation capacity of the steel sheet decreases, thereby reducing λ and lowering the bendability. The content of Ca, Mg, or REM is preferably 0.0100% or less, more preferably 0.0050% or less, respectively. The lower limit of the content of Ca, Mg, or REM is not particularly limited. Since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation capacity of the steel sheet, the content of Ca, Mg, or REM is preferably 0.0001% or more, respectively.

[0099] When Zr or Te is contained, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation capacity of the steel sheet decreases, thereby reducing λ and lowering the bendability. The content of Zr or Te is preferably 0.100% or less, more preferably 0.080% or less, respectively. The lower limit of the content of Zr or Te is not particularly limited. Since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation capacity of the steel sheet, the content of Zr or Te is preferably 0.001% or more, respectively.

[0100] When Hf is contained, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation capacity of the steel sheet decreases, thereby reducing λ and lowering the bendability. The content of Hf is preferably 0.10% or less, more preferably 0.08% or less. The lower limit of the content of Hf is not particularly limited. Since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformation capacity of the steel sheet, the content of Hf is preferably 0.01% or more.

[0101] When Bi is contained, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation capacity of the steel sheet decreases, thereby reducing λ and lowering the bendability. The content of Bi is preferably 0.200% or less, more preferably 0.100% or less. The lower limit of the content of Bi is not particularly limited. Since it is an element that reduces segregation, the content of Bi is preferably 0.001% or more.

[0102] The high-strength steel sheet according to an embodiment of the present invention has the following composition: containing the above essential components and optional components as required, and the remaining part is composed of Fe and inevitable impurities. Here, as inevitable impurities, Zn, Pb, As, Ge, Sr, and Cs can be cited. These inevitable impurities can be allowed to be contained in a total amount of 0.100% or less.

[0103] 〈Steel structure〉

[0104] The steel structure of the high-strength steel sheet of the present invention will be described.

[0105] 《Area ratio of martensite: 10% - 80%》

[0106] By containing martensite, a TS of over 780 MPa can be easily achieved. Therefore, the area ratio of martensite is 10% or more, preferably 15% or more, more preferably 20% or more.

[0107] On the other hand, if there is too much martensite, El decreases and the ductility deteriorates. Therefore, the area ratio of martensite is 80% or less, preferably 75% or less, more preferably 70% or less.

[0108] Here, martensite includes lower bainite, martensite that has undergone self-tempering during the cooling implemented in the annealing described later, martensite that has been tempered through the second heating process described later, and the like.

[0109] The observation position of martensite is, as described later, the 1 / 4 position of the plate thickness of the steel plate.

[0110] 《Area ratio of bainite: 2% - 70%》

[0111] By containing bainite, the hardness difference between the tissues is reduced and λ increases. In addition, since crack propagation is suppressed at the interface, the low-temperature toughness is improved. Therefore, the area ratio of bainite is 2% or more, preferably 3% or more, more preferably 4% or more.

[0112] On the other hand, if there is too much bainite, a sufficient amount of martensite cannot be ensured and the TS decreases. Therefore, the area ratio of bainite is 70% or less, preferably 60% or less, more preferably 50% or less.

[0113] Here, bainite is a mixed structure of angular bainite ferrite, iron carbide, and retained austenite generated in the temperature range of Ms to 700°C.

[0114] The observation position of bainite is, as described later, the 1 / 4 position of the plate thickness of the steel plate.

[0115] 《Area ratio of ferrite: 80% or less》

[0116] By making the area ratio of ferrite 80% or less, the desired strength can be easily obtained. Even if the area ratio of ferrite is 0%, the effects of the present invention can be obtained. On the other hand, if there is too much ferrite, a sufficient amount of martensite cannot be ensured and the desired TS cannot be obtained. Therefore, the area ratio of ferrite is 80% or less, preferably 75% or less, more preferably 70% or less. On the other hand, in order to increase El and further improve the ductility, the area ratio of ferrite is preferably 10% or more, more preferably 15% or more.

[0117] Here, ferrite is the soft BCC iron formed at a relatively high temperature, including allotriomorph ferrite and idiomorph ferrite.

[0118] The observation position of ferrite is at the 1 / 4 position of the plate thickness of the steel plate as described later.

[0119] The measurement methods for the area ratios of martensite, bainite, and ferrite are as described below.

[0120] First, a sample is cut out from the steel plate in such a way that the cross-section parallel to the rolling direction (the L-section at the 1 / 4 position of the plate thickness) is the observation surface. The observation surface of the sample is mirror-polished using diamond polishing paste, then fine polishing is carried out using colloidal silica, and further etching is performed using 1 vol% nitric acid alcohol to reveal the microstructure.

[0121] Next, the observation surface of the sample is observed using a scanning electron microscope (SEM) at a magnification of 3000 times under the condition of an acceleration voltage of 10 kV to obtain SEM images of 3 fields of view (1 field of view is 40 μm × 30 μm).

[0122] Based on the obtained SEM images, the area ratios of each microstructure are calculated using Adobe Photoshop (manufactured by Adobe Systems). Specifically, the value obtained by dividing the area of each microstructure by the measurement area is used as the area ratio of each microstructure. The area ratios of each microstructure in the 3 fields of view are calculated, and their average value is used as the area ratio of each microstructure.

[0123] In the SEM image, ferrite is a flat tissue area that appears gray and does not enclose carbides with white contrast.

[0124] Bainite is a mixed tissue area composed of gray angular bainite ferrite, iron-based carbides with white contrast, and acicular retained austenite.

[0125] Martensite is a tissue with a hierarchical structure having minute unevenness inside.

[0126] These can be identified from each other.

[0127] 《Area ratio of retained austenite: 15% or less》

[0128] By reducing the retained austenite, good component strength and flange stretchability can be obtained. Therefore, the area ratio of retained austenite is 15% or less, preferably 10% or less. The lower limit is not particularly limited, and even if the area ratio of retained austenite is 0%, this effect can still be obtained.

[0129] The method for measuring the area ratio of retained austenite is described as follows.

[0130] First, grind the steel plate with the measurement surface at a position of 1 / 4 of its plate thickness (a position equivalent to 1 / 4 of the plate thickness in the depth direction from the steel plate surface), and then further grind it by 0.1 mm through chemical polishing to obtain a sample.

[0131] For the measurement surface of the sample, using an X-ray diffractometer and a Co Kα ray source, measure the integrated reflection intensities of the (200), (220), and (311) crystal planes of fcc iron (austenite), and the (200), (211), and (220) crystal planes of bcc iron.

[0132] Calculate the intensity ratios of the integrated reflection intensities of each crystal plane of fcc iron to those of each crystal plane of bcc iron. Take the average value of the 9 intensity ratios as the volume ratio of retained austenite. Consider this volume ratio of retained austenite to be uniform in three dimensions and use it as the area ratio of retained austenite at the 1 / 4 position of the steel plate thickness.

[0133] 《Remaining Structure》

[0134] The steel structure of the present invention may have a structure (remaining structure) other than the above-mentioned martensite, bainite, ferrite, and retained austenite.

[0135] Examples of the remaining structure include structures other than martensite, bainite, ferrite, and retained austenite and structures that are well-known as the structure of a steel plate. For example, pearlite, alloy carbonitrides precipitated in ferrite, etc. can be cited.

[0136] It should be noted that iron-based carbides such as iron-based carbides present in bainite, metastable carbides precipitated in martensite, and cementite precipitated in martensite are not included in the remaining structure.

[0137] For the reason of not impairing the effects of the present invention, the area ratio of the remaining structure is preferably 3% or less.

[0138] 《Ratio of the Number of Martensite Blocks with Metastable Carbides to the Number of Martensite Blocks: 2% or More》

[0139] Due to the metastable carbides precipitated in the martensite blocks, excellent component strength, ductility, shear end face bendability, and stretch flangeability are maintained, and the low-temperature toughness is improved. It is considered that this is because the metastable carbides precipitated in the martensite blocks inhibit the generation and propagation of cracks at low temperatures.

[0140] In order to obtain this effect, the ratio of the number of martensite blocks containing metastable carbides to the number of martensite blocks (hereinafter, also referred to as "ratio p") is 2% or more, preferably 5% or more, more preferably 10% or more, and further preferably 20% or more. Particularly preferably, it is 30% or more. The upper limit of the ratio p is not particularly limited and may also be 100%.

[0141] Here, the metastable carbide is a metastable carbide precipitated during the tempering of martensite. The metastable carbide is, for example, an Fe carbide (ferrous carbide) other than cementite, and at least one carbide selected from ε carbide, η carbide, and χ carbide can be cited.

[0142] 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 described below.

[0143] First, the steel plate is ground so that the observation surface is at a position 1 / 4 of its plate thickness (a position in the depth direction equivalent to 1 / 4 of the plate thickness from the surface of the steel plate), and then electrolytic polishing is performed to produce a sample. The observation surface of the produced sample is observed using a transmission electron microscope (TEM) under the condition of an acceleration voltage of 200 kV.

[0144] If an electron beam is incident from the

[100] orientation of the martensite block, an electron diffraction pattern of the parent-phase martensite is obtained. Adjacent martensite blocks have different crystal orientations via the block boundary, so they have different contrasts in the bright-field image and can thus be distinguished from each other. Since a high density of dislocations is observed in martensite and the dislocation density is low in ferrite and bainite, martensite can be distinguished from ferrite and bainite.

[0145] Figure 1 is an example of the electron diffraction pattern of martensite containing carbides.

[0146] When carbides are present in the observed single martensite block, as Figure 1 shown, in addition to the electron diffraction pattern of the parent-phase martensite (α), an electron diffraction pattern of the carbide can also be obtained.

[0147] Figure 1 In, the black circles represent the electron diffraction spots of the parent-phase martensite when the electron beam is incident from the

[100] orientation, and the white circles represent the electron diffraction spots of the carbide.

[0148] According to Figure 1 the distance D1 between the electron diffraction spots of the parent-phase martensite and the distance D2 between the electron diffraction spots of the carbide in, the ratio dc / dm of the interplanar spacing dc of the carbide to the interplanar spacing dm of the parent-phase martensite is calculated using the following formula 3.

[0149] Equation 3: dc / dm = D1 / D2

[0150] For the observed martensite blocks, when the ratio dc / dm of the interplanar spacing dc of the carbide to the interplanar spacing dm of the parent-phase martensite is in the range of 1.020 to 1.150, the martensite block is defined as a martensite block with metastable carbide. That is, "the ratio of the number of martensite blocks with metastable carbide to the number of martensite blocks" can also be expressed as "the ratio of the number of martensite blocks with a dc / dm ratio of 1.020 to 1.150 to the number of martensite blocks".

[0151] Although the distance D1 between the electron diffraction spots of the parent-phase martensite is a constant value, the value of the distance D2 between the electron diffraction spots of the carbide varies depending on the carbide.

[0152] For example, when the carbide is cementite, the distance D2 is equal to the distance D1. Therefore, the value of the ratio dc / dm is 1 (D1 / D2 = dc / dm = 1).

[0153] On the other hand, the distance D2 of metastable carbide (such as ε-carbide) is shorter than that of cementite, so the ratio dc / dm (= D1 / D2) value is greater than 1. Therefore, the lower limit value of the ratio dc / dm in the case of the presence of metastable carbide is denoted as "1.020".

[0154] It should be noted that the upper limit of the ratio dc / dm is specified as "1.150" according to the distance D2 of the ε-carbide.

[0155] Metastable carbide can exist inside the martensite block or at the boundary parts such as the block boundary, and preferably exists inside the martensite block.

[0156] Observe 50 martensite blocks, and calculate the value obtained by dividing the number of martensite blocks with metastable carbide by the number of observed martensite blocks (the number of martensite blocks with metastable carbide / 50). Multiply the obtained value by 100 to obtain the ratio of the number of martensite blocks with metastable carbide to the number of martensite blocks (ratio p (%)).

[0157] 《Average number density of metastable carbide in martensite blocks with metastable carbide: 1×10 6 pieces / mm 2 or more》

[0158] For the reason of more excellent low-temperature toughness, it is preferable that the number density of metastable carbide in the martensite block is higher. It is considered that when the number density of metastable carbide is high, the resistance to crack propagation in martensite at low temperature becomes greater.

[0159] Specifically, the average number density of metastable carbides in a martensite block containing metastable carbides (hereinafter, also referred to as "number density n") is preferably 1×10 6 pieces / mm 2 or more, more preferably 10×10 6 pieces / mm 2 or more, and still more preferably 100×10 6 pieces / mm 2 or more.

[0160] There is no particular limitation on the upper limit of the number density n. For example, the number density n can be 10000000×10 6 pieces / mm 2 or less, preferably 1000000×10 6 pieces / mm 2 or less, more preferably 100000×10 6 pieces / mm 2 or less, and still more preferably 10000×10 6 pieces / mm 2 or less. Still more preferably.

[0161] The method for measuring the average number density (number density n) of metastable carbides in a martensite block containing metastable carbides is as described below.

[0162] When measuring the ratio p using TEM as described above, a selected area electron diffraction pattern is obtained in a single martensite block containing metastable carbides, and a dark field image is obtained using the electron diffraction spots from the metastable carbides. In the dark field image, the metastable carbides exhibit white contrast.

[0163] A 300 nm×300 nm area inside a single martensite block is photographed, and the number of metastable carbides is counted. It should be noted that martensite blocks adjacent across the block boundary may exist in the 300 nm×300 nm area.

[0164] The area of the martensite block containing metastable carbides is defined as the area of the single martensite block from which the selected area electron diffraction pattern is obtained. Adjacent martensite blocks have different crystal orientations across the block boundary, so they have different contrasts in the bright field image and can be distinguished from each other.

[0165] The above measurement is performed in 3 fields of view, and the value obtained by dividing the number of metastable carbides in the 3 fields of view by the area of the martensite block containing metastable carbides (=number of metastable carbides / area of the martensite block containing metastable carbides) is calculated. The average value of these is taken as the average number density (number density n) of metastable carbides in the martensite block containing metastable carbides.

[0166] "Average Circular Equivalent Diameter of Metastable Carbides: Below 20 nm"

[0167] The smaller the average circular equivalent diameter of the metastable carbides in the martensite laths, the less likely it is to generate cracks in the martensite at low temperatures. Therefore, the low-temperature toughness is more excellent. Accordingly, the average circular equivalent diameter of the metastable carbides in the martensite laths is preferably 20 nm or less, more preferably 5 nm or less.

[0168] The method for measuring the average circular equivalent diameter of the metastable carbides in the martensite laths is as described below.

[0169] When measuring the ratio p using TEM as described above, obtain a selected area electron diffraction pattern in a single martensite lath where metastable carbides are present, and obtain a dark-field image using the electron diffraction spots obtained from the metastable carbides. In the dark-field image, the metastable carbides exhibit white contrast.

[0170] Take a dark-field image of a 300 nm × 300 nm area inside a single martensite lath, perform image processing to obtain a binary image in a way that can distinguish the metastable carbides. Calculate the circular equivalent diameter for each metastable carbide particle by performing particle analysis on the binary image. When the metastable carbides overlap each other in the dark-field image, perform segmentation on the binary image using the watershed algorithm (Watershed method).

[0171] Calculate the circular equivalent diameter for all the metastable carbides present in the 300 nm × 300 nm area (3 fields of view). Calculate the average of the circular equivalent diameters of the 3 fields of view, and use it as the average circular equivalent diameter of the metastable carbides in the martensite lath.

[0172] 〈Nano-hardness〉

[0173] By reducing the standard deviation σ of the nano-hardness of the steel sheet n , excellent component strength, ductility, stretch flangeability, and low-temperature toughness are maintained, and the shear end face bendability is improved. It is considered that this is because the plastic deformation resistance in the local area is homogenized within the structure, so the non-uniformity of plastic deformation in the shearing process part is suppressed, and the bending deformation ability of the shearing process part is improved.

[0174] Here, for example, in a hardness test method other than the nano-indentation method such as the Vickers hardness test, the plastic deformation resistance of the sub-micron level local area of the structure cannot be obtained. Therefore, the problem of the present invention can be solved only by using the nano-indentation method.

[0175] To obtain the above effects, the standard deviation σ of the nano-hardness n relative to the average value of the nano-hardness [H n ​ave , is 0.60×[H n ave Hereinafter, it is preferably 0.50×[H n ave Hereinafter. The standard deviation σ of the nano-hardness n The lower limit of is not particularly limited and may be 0.

[0176] The average value [Hn] of the nano-hardness ave is preferably 3.0 GPa to 9.0 GPa, more preferably 3.5 GPa to 8.5 GPa.

[0177] Here, the method for measuring the standard deviation σ of the nano-hardness n will be described. A nano-indentation device equipped with a Berkovich indenter is used to obtain the nano-hardness.

[0178] After cutting out a specimen with the plate thickness cross-section (L-section) parallel to the rolling direction of the steel plate as the observation surface, the observation surface is mirror-polished using diamond grinding paste, and then fine polishing is performed using colloidal silica. Using the nano-indentation device, under the conditions that the loading speed and unloading speed are 50 μN / s by load control, the maximum load is 500 μN, and the data acquisition interval is 5 msec, the nano-hardness of the specimen is measured at 225 points or more. During the measurement, the position at a distance of 1 / 4 of the plate thickness from the surface of the high-strength steel plate is used as the measurement position, and the distance between the indentations is kept more than 2 μm.

[0179] Based on the obtained nano-hardness measurement results of 225 points or more, a histogram is made to obtain the standard deviation, and this value is used as the standard deviation σ of the nano-hardness n . The average value of the obtained nano-hardness measurement results of 225 points or more is used as [H n ave .

[0180] 〈Surface soft layer〉

[0181] In the high-strength steel plate, a surface soft layer is preferably formed on the surface layer of the base steel plate. The surface soft layer helps to suppress the progress of bending cracks during press forming and vehicle body collision, thus improving the bending fracture resistance characteristics.

[0182] Here, when the base steel plate is a steel plate subjected to plating treatment such as a hot-dip galvanized steel plate, an alloyed hot-dip galvanized steel plate, an electro-galvanized steel plate, and other metal-coated steel plates, it is the high-strength steel plate that is the basis (substrate) of the above various platings, and when no plating treatment is carried out, it is a high-strength steel plate.

[0183] The surface layer refers to the region corresponding to a thickness of 200 μm from the surface of the base steel plate to a depth of 200 μm in the plate thickness direction. ​​​

[0184] The soft layer refers to a region with a Vickers hardness of 85% or less at the cross-section (a plane parallel to the steel plate surface) at a position 1 / 4 of the plate thickness relative to the base steel plate. The soft layer includes the decarburized layer on the surface layer of the base steel plate.

[0185] The surface soft layer refers to the soft layer contained in the surface layer. The entire surface layer can be the soft layer, or a part of the surface layer can be the soft layer. The surface soft layer can be a region corresponding to a thickness within 200 μm in the plate thickness direction from the surface of the base steel plate.

[0186] For example, when a region with a Vickers hardness of 85% or less is formed at a cross-section (a plane parallel to the steel plate surface) at a position 1 / 4 of the plate thickness relative to the base steel plate at a specified depth in the plate thickness direction, if the specified depth is within 200 μm in the plate thickness direction, the region corresponding to the thickness from the surface to the specified depth in the plate thickness direction is the surface soft layer; if the specified depth is greater than 200 μm in the plate thickness direction, the region corresponding to a thickness of 200 μm from the surface of the base steel plate to a depth of 200 μm in the plate thickness direction is the surface soft layer.

[0187] When there is a surface soft layer, the lower limit of the thickness of the surface soft layer is not particularly limited, preferably 8 μm or more, more preferably greater than 17 μm.

[0188] The Vickers hardness is measured based on JIS Z 2244-1(2020) with a load of 10 gf.

[0189] In order to obtain excellent bendability during press forming and excellent bend fracture characteristics during collision, when re-measuring the nano-hardness at 300 points or more in a 50 μm × 50 μm region on the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate (at a position 1 / 4 of the thickness of the surface soft layer in the depth direction from the surface of the base steel plate), preferably the proportion of the nano-hardness of 7.0 GPa or more must be 0.10 or less. When the proportion of the nano-hardness of 7.0 GPa or more is 0.10 or less, it means that the proportion of hard tissues (such as martensite) and inclusions is small, and the generation and connection of voids during press forming and collision of hard tissues (such as martensite) and inclusions can be further suppressed, and further the propagation of cracks, and excellent R / t and SF can be obtained. max 。

[0190] In the present invention, in order to obtain excellent bendability during press forming and excellent bend fracture characteristics during collision, it is preferable that the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less. In addition, the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less. When the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less, and in addition, the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less, it means that the tissue hardness difference in the microscopic region is small, and the generation and connection of voids during press forming and collision can be further suppressed, and further the propagation of cracks can be suppressed, and excellent R / t and SF can be obtained. max 。

[0191] In addition, a more preferable range of the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.7 GPa or less. A more preferable range of the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 2.1 GPa or less.

[0192] Here, the nano-hardness of the plate surface at the 1 / 4 position and 1 / 2 position of the plate thickness direction depth refers to the hardness measured by the following method.

[0193] First, in the case where a plating layer is formed, after the plating layer is peeled off, mechanical grinding is performed at a position 1 / 4 of the plate thickness direction depth from the surface of the base steel plate to the surface soft layer, polishing and grinding are performed using diamond and alumina, and then colloidal silica grinding is performed. Using a diamond indenter in the shape of a triangular pyramid (Berkovich), the nano-hardness is measured under the conditions of a load: 500 μN, a measurement area: 50 μm × 50 μm, and a dot interval: 2 μm.

[0194] In addition, mechanical grinding is performed to a position 1 / 2 of the plate thickness direction depth of the surface soft layer, polishing and grinding are performed using diamond and alumina, and then colloidal silica grinding is performed. Then, using a diamond indenter in the shape of a triangular pyramid, the nano-hardness is measured under the conditions of a load: 500 μN, a measurement area: 50 μm × 50 μm, and a dot interval: 2 μm.

[0195] Here, the thickness of the surface soft layer can be measured by the following method. After smoothing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the base steel plate by wet grinding, a Vickers hardness tester is used to measure at intervals of 1 μm from a position 1 μm in the plate thickness direction from the surface of the base steel plate to a position 100 μm in the plate thickness direction. Then, measurements are made at intervals of 20 μm up to the center of the plate thickness. The region where the hardness is reduced to 85% or less compared to the hardness at the 1 / 4 position of the plate thickness is defined as the soft layer (surface soft layer), and the thickness in the plate thickness direction of this region is taken as the thickness of the soft layer.

[0196] 〈First plating layer〉

[0197] The high-strength steel plate according to an embodiment of the present invention preferably has a first plating layer as a metal plating layer on the surface of one or both sides of the base steel plate. The first plating layer is directly formed on the surface of the base steel plate and is a metal plating layer containing a total of more than 50% by mass of one or more metals 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, excluding hot-dip galvanized layers, alloyed hot-dip galvanized layers, electro-galvanized layers, and hot-dip aluminum plating layers. The first plating layer is preferably a metal electroplating layer. Hereinafter, the metal electroplating layer will be described as an example.

[0198] By forming a metal electroplating layer on the steel plate surface, the outermost metal electroplating layer during press forming and vehicle body collision helps to suppress the generation of bending cracks, so the bending fracture resistance property is further improved.

[0199] The metal substance as the metal electroplating layer can be any one 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, and more preferably Fe. Hereinafter, the Fe-based electroplating layer will be described as an example.

[0200] The adhesion amount of the Fe-based electroplating layer is greater than 0 g / m 2 , preferably 2.0 g / m 2 or more. There is no particular limitation on the upper limit of the adhesion amount on one side of the Fe-based electroplating layer. From the viewpoint of cost, it is preferable that the adhesion amount on one side of the Fe-based electroplating layer is 60 g / m 2 or less. The adhesion amount of the Fe-based electroplating layer is preferably 50 g / m 2 or less, more preferably 40 g / m 2 or less, and further preferably 30 g / m 2 or less.

[0201] The adhesion amount of the Fe-based plating layer is measured as follows. A sample with dimensions of 10×15 mm is taken from the Fe-based electroplated steel sheet and embedded in resin to form a cross-section embedded sample. Using a Scanning Electron Microscope (SEM), any three places of this cross-section are observed at an acceleration voltage of 15 kV and a magnification of 2000 to 10000 times corresponding to the thickness of the Fe-based plating layer. The average value of the thicknesses of the three fields of view is multiplied by the specific gravity of iron, and thus the adhesion amount on one side of the Fe-based plating layer is converted.

[0202] As the Fe-based plating 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 also be used. The composition of the Fe-based plating layer is not particularly limited, and the following composition is preferably: containing 1 or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co with a total of 10 mass% or less, and the remaining part is composed of Fe and inevitable impurities. By making the total amount of elements other than Fe 10 mass% or less, the electrolysis efficiency can be prevented from decreasing, and the Fe-based plating layer can be formed at low cost. In the case of an Fe-C alloy, the content of C is preferably 0.08 mass% or less.

[0203] 〈Second plating layer〉

[0204] The high-strength steel sheet according to one embodiment of the present invention may have a second plating layer as a metal plating layer as the 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 with a total of 50 mass% or more, and can be a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electrogalvanized layer, a molten aluminum plating layer, etc.

[0205] The second plating layer can be directly formed on one or both sides of the surface of the base steel sheet, or can be formed on the first plating layer.

[0206] Here, the hot-dip galvanized layer, the alloyed hot-dip galvanized layer, and the electrogalvanized layer refer to plating layers with Zn (zinc) as the main component (Zn content is 50.0 mass% or more).

[0207] In addition, the plating layer of the aluminized steel sheet refers to a plating layer with Al (aluminum) as the main component (Al content is 50.0 mass% or more).

[0208] Here, the hot-dip galvanized layer preferably consists of, for example, Zn and Fe of 20.0 mass% or less, and Al of 0.001 mass% to 1.0 mass%. Additionally, the hot-dip galvanized layer may optionally contain one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with a total content greater than 0.0 mass% and 3.5 mass% or less. Furthermore, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. It should be noted that the balance other than the above elements is inevitable impurities.

[0209] The alloyed hot-dip galvanized layer preferably consists of, for example, Fe of 20 mass% or less, and Al of 0.001 mass% to 1.0 mass%. Additionally, the alloyed hot-dip galvanized layer may optionally contain one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with a total content greater than 0 mass% and 3.5 mass% or less. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0 mass% or more, and further preferably 8.0 mass% or more. Additionally, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0 mass% or less, and further preferably 12.0 mass% or less. It should be noted that the balance other than the above elements is inevitable impurities.

[0210] In addition, the coating adhesion amount on one side of the above galvanized layer is not particularly limited, and is preferably 20 g / m 2 ~80 g / m 2 .

[0211] The coating adhesion amount of the above galvanized layer is measured as follows. Prepare a treatment solution by adding 0.6 g of a corrosion inhibitor for Fe ("IBIT 700BK" (Japanese registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% hydrochloric acid aqueous solution. Then, immerse a sample of a steel sheet with a galvanized layer in this treatment solution to dissolve the galvanized layer. Next, measure the mass reduction of the sample before and after dissolution, and divide this value by the surface area of the base steel sheet (the surface area of the part covered by the coating) to calculate the coating adhesion amount (g / m 2 ).

[0212] <Other>

[0213] The thickness of the high-strength steel sheet is not particularly limited and can be 0.3 mm to 3.0 mm.

[0214] [Manufacturing method of high-strength steel sheet]

[0215] Next, a method for manufacturing the high-strength steel sheet of the present invention (hereinafter, also referred to as "the manufacturing method of the present invention" for convenience) will be described. The manufacturing method of the present invention is also a method for manufacturing the high-strength steel sheet of the present invention described above. Here, unless otherwise specified, the temperature related to the manufacturing method is based on the surface temperature of the steel slab or steel sheet.

[0216] 〈Hot rolling, pickling, and cold rolling〉

[0217] In the manufacturing method of the present invention, first, a cold-rolled sheet is obtained by subjecting the steel slab having the composition of the present invention described above to hot rolling, pickling, and cold rolling.

[0218] 《Manufacture of steel slab》

[0219] As the steel slab, for example, a steel slab obtained by melting a steel sheet slab into molten steel having the composition of the present invention can be used. The steel melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The method for manufacturing 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 aspect of preventing macrosegregation, it is preferably manufactured by continuous casting.

[0220] 《Hot rolling process》

[0221] After the manufactured steel slab is temporarily cooled to room temperature, for example, it is reheated and hot rolled (rough rolling and finish rolling), and then coiled. Thus, a hot-rolled sheet is obtained. However, the manufactured steel slab can also be charged into a heating furnace in a hot sheet state without being cooled to room temperature, or can be slightly kept warm and then immediately subjected to rough rolling.

[0222] (Rough rolling)

[0223] By rough rolling the steel slab under the following conditions, a rough-rolled sheet is obtained.

[0224] From the viewpoints of melting of carbides and reduction of rolling load, the temperature for heating the steel slab (slab heating temperature) is preferably 1100 °C or higher. On the other hand, in order to prevent an increase in scale, the slab heating temperature is preferably 1300 °C or lower. The steel slab heated to the slab heating temperature is rough rolled.

[0225] By rationalizing the average strain rate and total reduction ratio in rough rolling, the standard deviation of nano-hardness can be reduced to 0.60×[H n ave Hereinafter. It is considered that this is because during the plastic deformation and dynamic recrystallization of austenite grains in rough rolling, solute atoms such as Si and Mn diffuse rapidly through dislocations and grain boundaries of recrystallized grains, and thus are appropriately distributed, and the plastic deformation resistance in local regions is homogenized.

[0226] ​Here, the average strain rate in rough rolling is defined as the total rolling rate ε(-) of rough rolling from the first rolling mill to the last rolling mill divided by the time t R (s) required from the start of rolling using the first rolling mill to the completion of rolling using the last rolling mill in rough rolling, and the value obtained (ε / t R ).

[0227] When the average strain rate in rough rolling is greater than 1×10 －1 / s or the total reduction ratio is less than 50%, the diffusion of solute atoms such as Si and Mn in the plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, and the standard deviation of nano-hardness is greater than 0.60×[H n ave .

[0228] On the other hand, when the average strain rate in rough rolling is less than 1×10 －4 / s, the recovery of dislocations in austenite grains is promoted, the driving force for recrystallization decreases, and dynamic recrystallization is inhibited. Therefore, the diffusion of solute atoms such as Si and Mn becomes insufficient, and the standard deviation of nano-hardness is greater than 0.60×[H n ave .

[0229] Therefore, rough rolling is carried out under the conditions that the average strain rate is 1×10 －4 / s to 1×10 －1 / s and the total reduction ratio is 50% or more. The average strain rate in rough rolling is preferably 1×10 －3 / s to 1×10 －2 / s. The total reduction ratio in rough rolling is preferably 60% or more.

[0230] From the viewpoint of completing the recrystallization of austenite grains, the rough rolling end temperature is preferably 950°C or higher. The rough rolling end temperature can be, for example, 1250°C or lower.

[0231] When the slab heating temperature is set relatively low, from the viewpoint of preventing troubles in hot rolling, it is preferable to heat the rough rolled plate using a bar heater or the like before finish rolling.

[0232] (Finish rolling)

[0233] When performing finish rolling, the temperature (finish rolling temperature) is preferably above the Ar3 transformation point. Thereby, the rolling load is reduced. In addition, the reduction ratio in the unrecrystallized state of austenite is reduced, the development of abnormal structures elongated in the rolling direction is suppressed, and the workability is excellent.

[0234] Finish rolling can be continuously performed by joining rough rolled plates to each other. Before performing finish rolling, the rough rolled plates can be temporarily coiled.

[0235] ​​In order to reduce the rolling load, part or all of the finish rolling can be lubricated rolling. Lubricated rolling is also preferred from the viewpoint of making the shape and material of the steel plate uniform. The friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0236] (Coiling)

[0237] After finish rolling, coiling is carried out to obtain a hot-rolled sheet. From the viewpoint of making the sheet passing property during subsequent cold rolling and annealing good, the coiling temperature after hot rolling is preferably 300°C to 700°C.

[0238] 《Pickling and Cold Rolling Processes》

[0239] The hot-rolled sheet obtained by hot rolling is pickled. By pickling, the oxides on the surface of the hot-rolled sheet are removed, and excellent chemical conversion treatment property and the quality of the plating layer, etc. can be obtained in the high-strength steel sheet as the final product. Pickling can be carried out once or divided into multiple times.

[0240] After optionally performing softening heat treatment on the pickled hot-rolled sheet, cold rolling is carried out. Thus, a cold-rolled sheet is obtained. The conditions of cold rolling are not particularly limited, and the total reduction ratio of cold rolling is preferably 20% to 75%. The number of rolling passes and the reduction ratio of each pass are not particularly limited.

[0241] 《First Plating Process (Optional)》

[0242] In one embodiment of the present invention, a first plating process of forming a first plating layer as a metal plating layer on one or both sides of the steel sheet after the hot rolling process (after the cold rolling process in the case of performing cold rolling) and before the annealing process may be included. The first plating process is preferably a metal electroplating process.

[0243] For example, a metal plating treatment (first plating treatment) such as a metal electroplating treatment can be performed on the surface of the cold-rolled sheet obtained as described above to produce a pre-annealing metal plated steel sheet having a pre-annealing metal plating layer formed on at least one side. It should be noted that the metal plating layer mentioned here can be the above-mentioned first plating layer. The pre-annealing metal plated steel sheet is preferably a pre-annealing metal electroplated steel sheet having a pre-annealing metal electroplating layer.

[0244] The method of metal electroplating treatment is not particularly limited. As the metal plating layer formed on the base steel sheet as described above, a metal electroplating layer is preferred, so metal electroplating treatment is preferably performed. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two can be applied in an Fe-based electroplating bath. In addition, the adhesion amount of the pre-annealing metal electroplating layer can be adjusted according to the energization time, etc. It should be noted that the pre-annealing metal electroplated steel sheet means that the metal electroplating layer does not go through the annealing process, and a method of pre-annealing the hot-rolled sheet, the pickled sheet after hot rolling, or the cold-rolled sheet before the metal electroplating treatment is not excluded.

[0245] Here, the metal substance as the electroplated layer can be any one 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, Bi, and more preferably Fe. Therefore, the manufacturing method of Fe-based electroplating will be described below.

[0246] The content of Fe ions 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 content of Fe ions in the Fe-based electroplating bath is 0.5 mol / L or more in terms of Fe, 2+ a sufficient Fe adhesion amount can be obtained. In addition, in order to obtain a sufficient Fe adhesion amount, the content of Fe ions in the Fe-based electroplating bath before the start of energization is preferably 2.0 mol / L or less.

[0247] In addition, in the Fe-based electroplating bath, in addition to Fe ions, at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co may be contained. The total content of these elements in the Fe-based electroplating bath is preferably 10% by mass or less in the Fe-based electroplated layer before annealing. It should be noted that metal elements may be contained in the form of metal ions, and non-metal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. In addition, the ferric sulfate plating solution may contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0248] Other conditions of the Fe-based electroplating bath are not particularly limited. From the viewpoint of temperature maintenance, the temperature of the Fe-based electroplating solution is preferably 30°C or more and preferably 85°C or less. The pH of the Fe-based electroplating bath is not particularly limited. From the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or more. From the viewpoint of the conductivity of the Fe-based electroplating bath, it is preferably 3.0 or less. From the viewpoint of productivity, the current density is preferably 10 A / dm 2 or more, and from the viewpoint of easily controlling the adhesion amount of the Fe-based electroplated layer, it is preferably 150 A / dm 2 or less. From the viewpoint of productivity, the plate passing speed is preferably 5 mpm or more, and from the viewpoint of stably controlling the adhesion amount, it is preferably 150 mpm or less.

[0249] As the treatment before the Fe-based electroplating treatment, degreasing treatment and water washing for cleaning the surface of the cold-rolled plate, and pickling treatment and water washing for activating the surface of the cold-rolled plate can be implemented. The Fe-based electroplating treatment is implemented after these pretreatment steps. The methods of degreasing treatment and water washing are not particularly limited, and usual methods can be used.

[0250] In pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and their mixtures can be used. Among them, sulfuric acid, hydrochloric acid, and their mixtures are preferred. The concentration of the acid is not particularly limited, and from the viewpoints of preventing the removal ability of the oxide film and surface roughness (surface defects) caused by excessive pickling, it is preferably 1% by mass to 20% by mass.

[0251] In addition, the pickling treatment solution may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0252] 《First Heating Process》

[0253] Next, the obtained cold-rolled sheet is subjected to the first heating at 750 °C or higher. The cold-rolled sheet may or may not be subjected to electroplating treatment.

[0254] If the first heating temperature is too low, the reverse phase transformation to austenite does not proceed sufficiently, the area ratio of martensite decreases, and the desired TS cannot be obtained. Therefore, the first heating temperature is 750 °C or higher, preferably 770 °C or higher.

[0255] The upper limit of the heating temperature is not particularly limited, and from the viewpoints of workability, etc., it is preferably 950 °C or lower.

[0256] The time for heating the cold-rolled sheet at the first heating temperature (heating time) is not particularly limited. If it is too short, the reverse phase transformation to austenite may not proceed sufficiently. Therefore, it is preferably 30 s or longer, more preferably 60 s or longer.

[0257] The upper limit of the heating time is not particularly limited. For example, it may be 6000 s or shorter, preferably 3000 s or shorter. Here, "s" represents seconds.

[0258] The dew point of the annealing atmosphere for the first heating is preferably -30 °C or higher. By carrying out the annealing atmosphere in the annealing process at -30 °C or higher, the decarburization reaction is promoted, and a deeper surface soft layer can be formed. Thus, when 300 or more nano-hardness measurements are made in a 50 μm × 50 μm region on the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate, the proportion of the nano-hardness being 7.0 GPa or higher is 0.10 or less. The annealing atmosphere for the annealing process is more preferably -15 °C or higher, and even more preferably -5 °C or higher. The upper limit of the dew point of the annealing atmosphere for the annealing process is not particularly limited, and from the viewpoints of appropriately preventing oxidation of the surface of the Fe-based electroplating layer and making the coating adhesion good when a galvanized layer is provided, the dew point of the annealing atmosphere for the annealing process is preferably 30 °C or lower.

[0259] 《First Cooling Process》

[0260] Next, the heated cold-rolled plate is cooled. At this time, it passes through the temperature range T1 of T2 to 750 °C. The cold-rolled plate is cooled in the temperature range T1 at the first average cooling rate v1 described below, whereby pearlite transformation can be suppressed, and bainite transformation can be utilized in the subsequent holding process.

[0261] If the first average cooling rate v1 is too low, pearlite transformation occurs in the austenite formed by the first heating, the amount of untransformed austenite decreases, bainite transformation cannot be utilized in the subsequent holding process, the area ratio of bainite decreases, and the low-temperature toughness and flange stretchability deteriorate. Therefore, 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.

[0262] There is no particular limitation on the upper limit of the first average cooling rate v1. From the viewpoint of reducing the equipment investment burden, it is preferably 60.0 °C / s or less.

[0263] The cooling in the temperature range T1 is preferably continuous cooling.

[0264] There is no particular limitation on the cooling rate from the first heating temperature to 750 °C.

[0265] 《Holding Process》

[0266] The cold-rolled plate passing through the temperature range T1 then undergoes a holding process at a holding temperature T2 of 350 °C to 550 °C. By holding the cold-rolled plate after the first cooling process at a holding temperature T2 of 350 °C to 550 °C for a holding time t2 (s) under the condition that F defined by Equation 1 satisfies 0.20 to 0.90, bainite transformation occurs. The holding time t2 (s) is obtained based on the expansion curve obtained from the Formaster test and applied, thereby rationalizing bainite transformation and the precipitation of iron carbide and the distribution of C into the untransformed austenite associated therewith. Since the expansion curve depends on the steel composition and the thermal history up to the first cooling, it is necessary to obtain the expansion curve for each steel composition and the thermal history from the first heating temperature to T2 °C and select an appropriate holding time t2.

[0267] Here, Equation 1 is as follows.

[0268] Equation 1: F = 1 - exp(-kt n )

[0269] t: Holding time (s)

[0270] k, n: Constants obtained from the expansion curve of the Formaster test

[0271] If the retention time (retention time t2) of the cold-rolled sheet during retention is too short, the bainite transformation becomes insufficient, the area ratio of bainite decreases, and in addition, the standard deviation of nano-hardness increases, and the low-temperature toughness, flange stretchability, and bendability of the shear end face deteriorate. Therefore, the retention time t2 is at least the time t when F reaches 0.20, preferably at least the time t when F reaches 0.30.

[0272] On the other hand, if the retention time (retention time t2) of the cold-rolled sheet at the retention temperature T2 is too long, the bainite transformation proceeds excessively, the amount of martensite decreases, and the TS decreases. In addition, cementite precipitates in the untransformed austenite, and it becomes difficult for metastable carbides to precipitate in the subsequent second heating process, and the ratio of the number of martensite blocks with metastable carbides to the number of martensite blocks decreases, and the low-temperature toughness deteriorates. Therefore, the retention time t2 is at most the time t when F reaches 0.90, preferably at most the time t when F reaches 0.80.

[0273] The relationship between F and t in Equation 1 is obtained as follows.

[0274] Perform the processes on the above billet until the first cooling, and then retain it at a retention temperature T2 of 350°C to 550°C. The processes until the first cooling are the hot rolling process, pickling and cold rolling processes, the first heating process, and the first cooling process.

[0275] Using a Formaster tester, obtain the expansion curve during retention at the retention temperature T2. The retention at T2 is carried out until the expansion stops. Set the expansion amount at the start of furnace retention at T2 to 0 and the expansion amount at the stop to 1, and fit the expansion curve using Equation 1 to calculate the constants k and n. Thus, the relationship between F and t at the retention temperature T2 is obtained.

[0276] Equation 1: F = 1 - exp(-kt n )

[0277] t: Retention time (s)

[0278] k, n: Constants obtained from the expansion curve of the Formaster test

[0279] 《Second Cooling Process》

[0280] Next, cool the cold-rolled sheet that has undergone the retention process. The cooling stop temperature is below Ms - 20°C. Thus, the martensite transformation is sufficiently carried out. When the cooling stop temperature is greater than Ms - 20°C, the untransformed austenite does not undergo martensite transformation, and the amount of retained austenite becomes too large, and good component strength and flange stretchability cannot be obtained. The cooling stop temperature can be room temperature. Here, Ms is the temperature at which martensite transformation starts (Ms point), and the value measured by the test described later is used.

[0281] During cooling, although the cold-rolled sheet that has undergone the residence process passes through the temperature range T3 of Ms - 20°C to Ms°C, the cold-rolled sheet is cooled at the second average cooling rate v2 described below in the temperature range T3, thereby suppressing the precipitation of cementite in martensite.

[0282] If the second average cooling rate v2 is too low, cementite precipitates in martensite, suppressing the precipitation of metastable carbides during the subsequent second heating. The ratio of the number of martensite blocks with metastable carbides to the total number of martensite blocks decreases, and the low-temperature toughness deteriorates.

[0283] Therefore, the second average cooling rate v2 is 5°C / s or more, preferably 8°C / s or more. There is no particular limitation on the upper limit of the second average cooling rate v2. From the perspective of reducing the equipment investment burden, it is preferably 60.0°C / s or less.

[0284] The cooling rate other than the temperature range T3 is not particularly limited.

[0285] The Ms point uses the value measured by the Formaster test as follows.

[0286] Using a Formaster testing machine, perform the process on the above billet until the end of the residence process, and then cool it to room temperature at a second average cooling rate of 5°C / s or more. In the second cooling, set the temperature at which martensite transformation occurs and expansion begins as the Ms point. There is no particular limitation on the upper limit of the second average cooling rate. For example, it can be 100°C / s or less.

[0287] 《Second Heating Process》

[0288] Next, perform the second heating on the cold-rolled sheet cooled to the cooling stop temperature.

[0289] By performing the second heating, metastable carbides that improve low-temperature toughness precipitate in the martensite blocks formed during the above cooling. Thus, a high-strength steel sheet with good component strength, stretch flange formability, and bendability of the sheared end face, excellent low-temperature toughness, and a TS of 780 MPa or more is obtained.

[0290] The cold-rolled sheet that has been cooled is subjected to the second heating, heated to the second heating temperature X (unit: °C), and held for a holding time Y (unit: s). The temperature during the holding time Y is within the range of X ± 20°C.

[0291] Here, X and Y satisfy the following formula 2.

[0292] Formula 2: 7000 ≤ (273 + X) × (20 + log(Y / 3600)) ≤ 13000

[0293] For convenience, hereinafter, the "(273 + X) × (20 + log(Y / 3600))" in the above formula 2 will be referred to as "variable part Z".

[0294] By performing the second heating that satisfies the above conditions, the ratio (ratio p) of the number of martensite blocks in which metastable carbides are present to the number of martensite blocks can be increased, and the improvement of low-temperature toughness can be achieved.

[0295] Here, the temperature X (°C) is a value higher than room temperature.

[0296] When the temperature X (°C) of the second heating is higher than the cooling stop temperature of the second cooling process, it is heated from the cooling stop temperature to the temperature X (°C) of the second heating.

[0297] The cooling stop temperature of the second cooling process and the temperature X (°C) of the second heating can be the same. In this case, it means holding at the cooling stop temperature which is the temperature X (°C) of the second heating.

[0298] When the value of the variable part Z of the above formula 2 is too small, that is, when the temperature X is too low and / or the holding time Y is too short, metastable carbides do not precipitate sufficiently, so the ratio p decreases. Therefore, from the viewpoint of increasing the ratio p, the value of the variable part Z is 7000 or more, preferably 8000 or more.

[0299] On the other hand, when the value of the variable part Z is too high, that is, when the temperature X is too high and / or the holding time Y is too long, the migration of metastable carbides to cementite occurs, and the ratio p decreases. Therefore, from the viewpoint of increasing the ratio p, the value of the variable part Z is 13000 or less, preferably 12000 or less.

[0300] In the second heating, the temperature X (unit: °C) preferably satisfies the following formula 3. Thereby, the number density (number density n) of metastable carbides in the martensite blocks in which metastable carbides are present is increased.

[0301] Formula 3: 100 ≤ X ≤ 400

[0302] From the viewpoint of increasing the number density n, the temperature X is preferably 100 °C or more, more preferably 120 °C or more, and further preferably 150 °C or more.

[0303] On the other hand, from the same viewpoint, the temperature X is preferably 400 °C or less, more preferably 380 °C or less, and further preferably 350 °C or less.

[0304] Then, the cold-rolled sheet subjected to the second heating is cooled to room temperature, for example. The cooling rate at this time is not particularly limited.

[0305] Thereby, the high-strength steel sheet of the present invention is obtained by the manufacturing method of the present invention.

[0306] In the manufacturing method of the present invention, when the following plating treatment is carried out, the high-strength steel sheet of the present invention obtained is a plated steel sheet having a plating layer.

[0307] As long as a series of heat treatments in the manufacturing method of the present invention satisfy the above heat history, other conditions are not particularly limited, and the equipment for carrying out the heat treatment is also not particularly limited.

[0308] (Second plating process (optional))

[0309] In one embodiment of the present invention, the following second plating process may be included: A plating treatment is carried out on the steel sheet in the above first heating process to the second heating process to form a second plating layer as a metal plating layer.

[0310] The second plating process is preferably a galvanizing process or a hot dip aluminizing treatment. As the galvanizing treatment in the galvanizing process, for example, hot dip galvanizing treatment, alloying galvanizing treatment, electrogalvanizing treatment, and aluminum plating treatment can be cited.

[0311] In the case of hot dip galvanizing treatment, it is preferable to immerse the steel sheet in a galvanizing bath at 440°C to 500°C and then adjust the plating adhesion amount by gas wiping or the like. As the hot dip galvanizing bath, as long as it becomes the composition of the above galvanized layer, it is not particularly limited. For example, a plating bath having a composition in which the Al content is 0.10% by mass to 0.23% by mass and the balance is composed of Zn and inevitable impurities is preferably used.

[0312] In the case of alloying galvanizing treatment, it is preferable to carry out hot dip galvanizing treatment according to the above-mentioned procedure and then heat the steel sheet having a hot dip galvanized layer (hot dip galvanized steel sheet) to an alloying temperature of 450°C to 600°C to carry out alloying treatment. When the alloying temperature is less than 450°C, the Zn-Fe alloying rate becomes slow, and there is a case where alloying becomes difficult. On the other hand, if the alloying temperature is greater than 600°C, the untransformed austenite transforms into pearlite, and it is difficult to make the TS 590 MPa or more. It should be noted that the alloying temperature is more preferably 510°C or higher. In addition, the alloying temperature is more preferably 570°C or lower.

[0313] In addition, the plating adhesion amounts of the steel sheet having a hot dip galvanized layer (hot dip galvanized steel sheet) (GI) and the steel sheet having an alloyed hot dip galvanized layer (alloyed hot dip galvanized steel sheet) (GA) are both preferably 20 to 80 g / m on one side 2 . It should be noted that the plating adhesion amount can be adjusted by gas wiping or the like.

[0314] In addition, when performing hot-dip aluminizing treatment, which is a specific example of other metal plating treatments, the cold-rolled sheet obtained by annealing the above-described cold-rolled sheet is immersed in an aluminum plating bath at 660 to 730 °C to perform hot-dip aluminizing treatment, and then the plating adhesion amount is adjusted by gas wiping or the like.

[0315] In addition, when performing electro-galvanizing treatment, there is no particular limitation, and it is preferable that the film thickness is in the range of 2 μm to 15 μm.

[0316] 《Skin Pass Rolling (Optional)》

[0317] Skin pass rolling can be performed on the obtained high-strength steel sheet. Skin pass rolling can be performed after the plating treatment.

[0318] From the viewpoint of improving the yield strength, the reduction ratio in skin pass rolling is preferably 0.05% or more. The upper limit of the reduction ratio is not particularly limited, and from the viewpoint of productivity, it is preferably 1.50%.

[0319] Skin pass rolling can be performed online or offline.

[0320] Skin pass rolling with the target reduction ratio can be performed at once, or it can be performed in multiple steps.

[0321] From the viewpoint of productivity, the above-described series of treatments such as annealing and plating treatment are preferably performed using a CGL (Continuous Galvanizing Line).

[0322] [Component and Its Manufacturing Method]

[0323] The component of the present invention and its manufacturing method will be described.

[0324] The component of the present invention is a component made of the above-described high-strength steel sheet of the present invention. The component can be manufactured, for example, by forming the high-strength steel sheet of the present invention into a target shape by stamping or the like.

[0325] The high-strength steel sheet of the present invention is a high-strength steel sheet excellent in the strength, ductility, stretch flangeability, bendability of the shear end face, and bendability of the shear end face of a component. Therefore, by applying a component made of the high-strength steel sheet of the present invention or a high-strength steel sheet to, for example, a frame structure component of an automobile or a reinforcing component of an automobile, it is possible to improve fuel efficiency based on vehicle body weight reduction, and the industrial utilization value is extremely large.

[0326] Examples

[0327] Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited to the examples described below.

[0328] [Test Nos. 1 to 51]

[0329] 〈Manufacture of Steel Plate〉

[0330] Molten steel having the composition shown in Table 1 below (the balance being composed of Fe and inevitable impurities) is melted in a converter and a steel slab is obtained by continuous casting method.

[0331] The obtained steel slab is hot-rolled to obtain a hot-rolled sheet. Specifically, the steel slab is heated to 1250 °C and rough-rolled under the conditions shown in Table 2. Then, after finish-rolling at a finish-rolling temperature of 900 °C, coiling is carried out under the condition of 500 °C, and then it is cooled to room temperature to obtain a hot-rolled sheet.

[0332] After pickling the obtained hot-rolled sheet, softening heat treatment is carried out under the condition of 500 °C, and then cold rolling is carried out under the condition of a rolling ratio of 50%. Thus, a cold-rolled sheet with a thickness of 1.6 mm is obtained.

[0333] The obtained cold-rolled sheet is heated to the first heating temperature shown in Table 2 below and held for 200 s.

[0334] Next, cooling is carried out in such a way that the average cooling rate in the temperature range T1 (750 °C to T2) is the first average cooling rate v1 shown in Table 2. Then, cooling is continued at this cooling rate until the retention temperature T2, and the furnace is held at T2 for the retention time t2 shown in Table 2.

[0335] The parameters F and the retention time t2 satisfy the following formula 1.

[0336] Formula 1: F = 1 - exp( -kt n )

[0337] t: Holding time (s)

[0338] k, n: Constants obtained from the expansion curve of the Formaster test

[0339] The k and n in the above formula 1 are obtained as follows.

[0340] The obtained cold-rolled sheet is cut into a width of 10 mm × a length of 3 mm for the Formaster test. In the Formaster test, an FTM-100 manufactured by Fuji Denpa Koki Co., Ltd. is used. In each example, it is heated to the first heating temperature shown in Table 2 and held for 200 s. Next, it is cooled at an average cooling rate of the first average cooling rate v1 shown in Table 2 in the temperature range T1 (750 °C to T2), and then, it is cooled at this cooling rate to the retention temperature T2, and retained at T2 for 1000 s to obtain an expansion curve. For example, in Example 1, it is 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, retained at 480 °C for 1000 s. The expansion curve thus obtained is fitted using Equation 1 to obtain k and n.

[0341] After the furnace retention at the retention temperature T2 (350 °C to 550 °C), it is cooled to the cooling stop temperature shown in Table 2 at an average cooling rate of the second average cooling rate v2 in the temperature range T3 (Ms - 20 °C to Ms °C).

[0342] The Ms point is obtained as follows.

[0343] The obtained cold-rolled sheet is cut into a width of 10 mm × a length of 3 mm for the Formaster test. In the Formaster test, an FTM-100 manufactured by Fuji Denpa Koki Co., Ltd. is used. In each example, it is heated to the first heating temperature shown in Table 2 and held for 200 s. Then, it is cooled at an average cooling rate of the first average cooling rate v1 shown in Table 2 in the temperature range T1 (750 °C to T2), and then, it is cooled at this cooling rate to the retention temperature T2, retained at T2 for t seconds, and then, finally cooled to room temperature at 30 °C / s to produce an expansion curve during the final cooling. The temperature at which expansion is observed is set as the Ms point. For example, in Example 1, it is held at the first heating temperature of 800 °C for 200 s, then cooled to 480 °C under the condition of the first average cooling rate of 18 °C / s, then retained at 480 °C for 30 s, and next, finally cooled to room temperature at 30 °C / s. The Ms point is obtained based on the expansion curve thus obtained.

[0344] 《Plating Treatment》

[0345] After the furnace retention of a part of the cold-rolled sheet at the retention temperature T2, hot-dip galvanizing treatment is performed to form a plating layer (hot-dip galvanized layer) on both sides. That is, a hot-dip galvanized steel sheet (GI) is obtained. Then, it is cooled to the cooling stop temperature at the second average cooling rate v2.

[0346] A hot-dip galvanizing bath containing 0.20% by mass of Al, with the balance being Zn and inevitable impurities, is used in the hot-dip galvanizing treatment (bath temperature: 470 °C).

[0347] The coating weight on one side of the hot-dip galvanized layer is 45 - 72 g / m 2 or so.

[0348] The composition of the formed hot-dip galvanized layer is as follows: containing 0.1 - 1.0% by mass of Fe and 0.2 - 1.0% by mass of Al, with the balance being Fe and inevitable impurities.

[0349] For another part of the cold-rolled sheet, an alloying hot-dip galvanizing treatment is carried out after soaking in the furnace at the residence temperature T2, and a coating layer (alloying hot-dip galvanized layer) is formed on both sides. That is, an alloying hot-dip galvanized steel sheet (GA) is obtained. Then, it is cooled to the cooling stop temperature at the second average cooling rate v2.

[0350] A hot-dip galvanizing bath containing 0.14% by mass of Al, with the balance being Zn and inevitable impurities, is used in the hot-dip galvanizing treatment (bath temperature: 470 °C).

[0351] The alloying treatment temperature is 550 °C.

[0352] The coating weight on one side of the alloying hot-dip galvanized layer is 45 g / m 2 or so.

[0353] The composition of the formed alloying hot-dip galvanized layer is as follows: containing 7 - 15% by mass of Fe and 0.1 - 1.0% by mass of Al, with the balance being Fe and inevitable impurities.

[0354] In the column of "Coating Type" in Table 3 below, when the hot-dip galvanized layer is formed, it is recorded as "GI", when the alloying hot-dip galvanized layer is formed, it is recorded as "GA", and when no coating layer is formed, it is recorded as "CR".

[0355] The steel sheet cooled to the cooling stop point is reheated to the temperature X [°C] shown in Table 2 and held for the holding time Y [s]. The variable part Z is (273 + X) × (20 + log(Y / 3600)).

[0356] 〈Observation of Steel Structure〉

[0357] The area ratios of martensite, bainite, ferrite, and retained austenite, the ratio (ratio p) of the number of martensite blocks with metastable carbides to the number of martensite blocks, the average value of the number density (number density n) of metastable carbides in the martensite blocks with metastable carbides, and the standard deviation of nano-hardness (225 measurement points) are measured for the obtained steel sheet according to the above method. The results are shown in Table 3 below.

[0358] For the remaining tissue, the area ratio is also measured by a generally known method. Regarding the remaining tissue, "θ" in Table 3 below represents cementite precipitated in ferrite.

[0359] <Evaluation>

[0360] The obtained steel sheet is subjected to the tests described below to evaluate various properties. The results are shown in Table 3 below.

[0361] <Tensile Test>

[0362] The tensile test is carried out in accordance with JIS Z 2241.

[0363] Specifically, a JIS No. 5 test piece is taken from the obtained steel sheet with the long side direction perpendicular to the rolling direction of the steel sheet. Using the taken test piece, a tensile test is carried out under the condition that the crosshead speed is 1.67×10 －1 mm / s to measure the yield strength (YS) [MPa], the tensile strength (TS) [MPa], and the total elongation (El) [%]. Furthermore, the yield ratio (YR) (= 100×YS / TS) [%] is calculated.

[0364] When the tensile strength (TS) is 780 MPa or more, it is judged to be high strength.

[0365] When the yield ratio (YR) is 55% or more, it is judged that the component strength is excellent.

[0366] When the total elongation (El) is 10% or more, it is judged that the ductility is excellent.

[0367] <Hole Expansion Test>

[0368] The hole expansion test is carried out in accordance with JIS Z 2256.

[0369] Specifically, the obtained steel sheet is sheared to take a test piece with dimensions of 100 mm × 100 mm. A hole with a diameter of 10 mm is punched out of the taken test piece with a clearance of 12.5%. Then, using a die with an inner diameter of 75 mm, a conical punch with a vertex angle of 60° is pressed into the hole in a state where the blank holding force is 9 tons (88.26 kN), and the hole diameter Df [mm] at the limit of crack generation is measured. Let the initial hole diameter be D0 [mm], and the hole expansion rate λ [%] is obtained according to the following formula.

[0370] λ = {(Df - D0) / D0} × 100

[0371] When the hole expansion rate (λ) is 20% or more, it is judged that the flange stretching property is excellent.

[0372] <Sheared End Face Bending Test>

[0373] The bending test is carried out in accordance with JIS Z 2248.

[0374] Specifically, a rectangular test piece with a width of 30 mm and a length of 100 mm is taken from the obtained steel plate in a direction parallel to the rolling direction of the steel plate as the axial direction of the bending test. It should be noted that in the bending test of the sheared end face sample, the end face in the long side direction is used as the sheared end face, and in the bending test of the ground end face sample, the end face in the long side direction is used as the ground end face.

[0375] Using the taken test piece, a 90° V-bending test is carried out under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds.

[0376] The bending test is carried out on 5 test pieces with an appropriate bending radius R. Then, it is confirmed whether there are cracks in the ridge part of the bending vertex.

[0377] The presence or absence of cracks is confirmed by observing the ridge part of the bending vertex at a magnification of 40 times using a digital microscope (RH-2000, manufactured by HIROX Corporation).

[0378] Find the minimum bending radius R at which no cracks are generated in all 5 test pieces, and take the value (R / t) obtained by dividing by the plate thickness t as the limiting bending radius. Respectively find the limiting bending radius (Rs / t) of the sheared end face sample and the limiting bending radius (Rg / t) of the ground end face sample. When the ratio (Rs / Rg) of the limiting bending radius (Rs / t) of the sheared end face sample to the limiting bending radius (Rg / t) of the ground end face sample is 1.50 or less, it is judged that the bending property of the sheared end face part is excellent.

[0379] 《Low-temperature toughness test》

[0380] Six obtained steel plates (1.6 mm t steel plates) are overlapped and bonded to fabricate a Charpy impact test piece with a thickness of 9.6 mm t. The notch is a 2 mm U-notch. Using this test piece, a Charpy impact test is carried out at -40 °C, and the Charpy absorbed energy obtained therefrom is measured. In addition, the cross section after the test is observed to measure the ductile fracture rate therefrom.

[0381] The low-temperature toughness parameter P is calculated by defining it as the product of the Charpy absorbed energy (unit: J) and the ductile fracture rate (%) (Charpy absorbed energy (J) × ductile fracture rate (%)).

[0382] When the low-temperature toughness parameter P is 3000 or more, it is judged that the low-temperature toughness is excellent.

[0383]

[0384] [Table 2]

[0385]

[0386] Underline: Indicates outside the scope of the present invention

[0387] [Table 3]

[0388]

[0389] *α n Is the standard deviation of nano-hardness

[0390] Underline: Indicates outside the scope of the present invention

[0391] As shown in Table 3, in the inventive examples, the TS is 780 MPa or more, and the strength, ductility, stretch flange formability, bendability of the shear end face portion, and low-temperature toughness of the components are excellent. On the other hand, in the comparative examples, one or more of the strength, ductility, stretch flange formability, bendability of the shear end face portion, and low-temperature toughness of the components are poor.

[0392] [Test Nos. 52 to 75]

[0393] A steel slab (slab material) having the composition shown in Table 1 and the balance consisting of Fe and inevitable impurities is melted in a converter and the steel slab is obtained by continuous casting. The obtained steel slab is heated to 1250 °C and rough rolled to obtain a thin slab. Then, the obtained thin slab is finish rolled at a finish rolling temperature of 900 °C and coiled under the conditions shown in Table 4 to obtain a hot rolled sheet. After pickling the obtained hot rolled sheet, cold rolling is carried out under the conditions shown in Table 4 to obtain a cold rolled sheet with a thickness of 1.2 mm.

[0394] The obtained cold rolled steel is subjected to a first plating process (metal electroplating process), a first heating process, a first cooling process, a soaking furnace process, a second plating process, a second cooling process, a second heating process, and treatments therein to obtain a steel sheet.

[0395] Here, when the column of the presence or absence of metal electroplating treatment (plating type) in Table 4 is "yes" (Fe), it is an example of performing an Fe-based electroplating treatment, and when it is "(Ni)", it is an example of performing an Ni-based electroplating treatment. The composition of the metal electroplating layer contains Fe: 95 to 100 mass% in Fe-based electroplating and Ni: 95 to 100 mass% in Ni-based electroplating, and the balance is inevitable impurities respectively.

[0396] Here, in the second plating process, a part of the hot rolled sheet (white skin) or the cold rolled sheet is subjected to hot dip galvanizing treatment or alloying galvanizing treatment to obtain a hot dip galvanized steel sheet (hereinafter, also referred to as GI) or an alloyed hot dip galvanized steel sheet (hereinafter, also referred to as GA).

[0397] The galvanizing bath temperature is 470 °C in either the case of manufacturing GI or GA.

[0398] The galvanized coating weight is 45 to 75 g / m on one side (double-sided coating) when manufacturing GI, and 40 to 65 g / m on one side (double-sided coating) when manufacturing GA. 2 (double-sided coating), and 40 to 65 g / m on one side (double-sided coating) when manufacturing GA. 2 (double-sided coating).

[0399] It should be noted that the composition of the galvanized layer of the finally obtained steel sheet contains Fe: 0.1 to 1.0 mass%, Al: 0.20 to 0.33 mass% in GI, and the balance is Zn and inevitable impurities. In addition, in GA, it contains Fe: 7.0 to 12.0 mass%, Al: 0.10 to 0.23 mass%, and the balance is Zn and inevitable impurities.

[0400] In addition, the hot-dip galvanized layer and the alloyed hot-dip galvanized layer are both formed on both sides of the base steel sheet.

[0401] [Thickness of the surface soft layer]

[0402] The measurement method of the surface soft layer is as follows. After the plating layer (hot-dip galvanized layer or alloyed galvanized layer, metal electroplating layer as required) is peeled off, the plate thickness cross-section (L cross-section) parallel to the rolling direction of the base steel sheet is smoothed by wet grinding, and then a Vickers hardness tester is used to measure at 1 μm intervals from a position 1 μm in the plate thickness direction from the steel sheet surface to a position 100 μm in the plate thickness direction with a load of 10 gf. Then, the measurement is continued at 20 μm intervals until the center of the plate thickness. The region where the hardness is reduced to 85% or less compared to the hardness at the 1 / 4 position of the plate thickness is defined as the soft layer (surface soft layer), and the thickness in the plate thickness direction of this region is taken as the thickness of the soft layer.

[0403] [Nanohardness of the surface soft layer]

[0404] After the plating layer (hot-dip galvanized layer or alloyed galvanized layer, metal electroplating layer as required) is peeled off, mechanical grinding is carried out, and polishing and grinding with diamond and alumina and colloidal silica grinding are carried out to a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel sheet. Using a nanoindentation device (tribo-950 of Hysitron), a diamond indenter with a triangular pyramid (Berkovich) shape is used to measure the nanohardness of a total of 512 points under the conditions of a loading speed and an unloading speed of 50 μN / s, a maximum load of 500 μN, a measurement area of 50 μm × 50 μm, a data acquisition interval of 5 msec, and a dot interval of 2 μm.

[0405] Next, mechanical grinding, polishing and grinding using diamond and alumina, and colloidal silica grinding are carried out to the 1 / 2 position in the plate thickness direction of the above-mentioned surface soft layer. Using the tribo-950 of Hysitron Company, the nano-hardness of a total of 512 points is measured under the same conditions as above using a diamond indenter with a triangular pyramid (Berkovich) shape.

[0406] For the obtained steel plate, according to the above test method, the strength, ductility, flange stretchability, bendability of the shear end face part, and low-temperature toughness of the component are evaluated. The results are shown in Table 5 together.

[0407] In addition, according to the following test method, the V-bending + orthogonal VDA bending test and the axial crushing fracture test are carried out. The results are recorded in Table 5 together.

[0408] It should be noted that in the V-bending + orthogonal VDA bending test and the axial crushing test of hot-dip galvanized steel plates with a plate thickness greater than 1.2 mm, considering the influence of the plate thickness, all are carried out on steel plates with a plate thickness of 1.2 mm. The steel plates with a plate thickness greater than 1.2 mm are subjected to single-sided grinding to make the plate thickness 1.2 mm. On the other hand, in the V-bending + orthogonal VDA bending test and the axial crushing test of hot-dip galvanized steel plates with a plate thickness less than 1.2, since the influence of the plate thickness is small, the test is carried out without grinding treatment.

[0409] 《V-bending + orthogonal VDA bending test》

[0410] The V-bending + orthogonal VDA bending test is carried out as follows.

[0411] A test piece of 60 mm × 65 mm is taken from the obtained steel plate by shearing and end face grinding. Here, the 60-mm side is parallel to the rolling (L) direction. With a curvature radius / plate thickness of 4.2, taking the width (C) direction as the axis, 90° bending processing (primary bending processing) is carried out along the rolling (L) direction to prepare the test piece. In the 90° bending processing (primary bending processing), as Figure 2 (a) shows, a punch B1 is pressed into the steel plate placed on a die A1 with a V-groove to obtain a test piece T1. Next, as Figure 2 (b) shows, for the test piece T1 placed on a support roll A2, with the bending direction being perpendicular to the rolling direction, a punch B2 is pressed in to carry out orthogonal bending (secondary bending processing). In Figure 2 (a) and Figure 2 (b), D1 represents the width (C) direction, and D2 represents the rolling (L) direction.

[0412] The conditions of the V-bending in the V-bending + orthogonal VDA bending test are as described below.

[0413] Test method: Die support, punch pressing

[0414] Forming load: 10 ton

[0415] Test speed: 30 mm / min

[0416] Holding time: 5 s

[0417] Bending direction: Rolling (L) direction

[0418] The conditions of the VDA bending in the V-bending + orthogonal VDA bending test are as described below.

[0419] Test method: Roller support, punch pressing

[0420] Roll diameter:

[0421] Radius R of the punch tip: 0.4 mm

[0422] Distance between rollers: (plate thickness × 2) + 0.5 mm

[0423] Stroke speed: 20 mm / min

[0424] Test piece size: 60 mm × 60 mm

[0425] Bending direction: Rolling right angle (C) direction

[0426] In the stroke-load curve obtained when performing the above VDA bending, find the stroke at the maximum load. The average value of the strokes at the maximum load when performing the above V-bending + orthogonal VDA bending test three times is set as SF max (mm). The obtained SF max When it satisfies 26.0 mm or more, it is judged that the fracture resistance during collision (resistance to bending fracture) is excellent.

[0427] 《Axial crushing test》

[0428] The axial crushing test is carried out as follows.

[0429] Take a 150 mm × 100 mm test piece from the obtained steel plate by shearing. Here, the 150 mm side is parallel to the rolling (L) direction. Using a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, perform forming processing (bending processing) to a depth of 40 mm to produce a hat-shaped member 10 as shown in Figure 3 (a) and Figure 3 (b).

[0430] In addition, a steel plate used as the material for the hat-shaped member is cut out into a size of 80 mm × 100 mm. Next, spot welding is performed on the cut steel plate 20 and the hat-shaped member 10 to fabricate a test member 30 as shown in Figure 3 (a) and Figure 3 (b). Figure 3 (a) is a front view of the test member 30 fabricated by spot welding the hat-shaped member 10 and the steel plate 20. Figure 3 (b) is a perspective view of the test member 30. As shown in Figure 3 (b), the position of the spot welding portion 40 is such that the end of the steel plate and the welded portion are 10 mm, and the interval between the welded portions is 20 mm. Next, as shown in Figure 3 (c), the test member 30 and the bottom plate 50 are joined by TIG welding to fabricate an axially crush-tested sample. Next, the impactor 60 is used to perform constant-speed collision on the fabricated axially crush-tested sample at a collision speed of 10 mm / min, and the axially crush-tested sample is crushed by 70 mm. As shown in Figure 3 (c), the crushing direction D3 is a direction parallel to the long side direction of the test member 30.

[0431] Observe the appearance of the test body 30 after the test, and confirm the presence or absence of axial crush fracture (appearance crack).

[0432] In Table 5 below, the case where no appearance crack is seen is recorded as "A", the case where 1 or less appearance cracks are seen is recorded as "B", and the case where 2 or more appearance cracks are seen is recorded as "C". In the case of "A" or "B", it is judged that the fracture resistance during collision (resistance to axial crush fracture) is excellent.

[0433]

[0434]

[0435] As shown in Table 5, in the example of the present invention, TS is 780 MPa or more, and the strength, ductility, stretch flange property, bendability of the shear end face portion, low-temperature toughness, and fracture resistance characteristics during collision (bending fracture characteristics and axial crush characteristics) of the component are excellent.

[0436] According to the present invention, a high-strength steel plate excellent in component strength, stretch flange property, bendability of the shear end face portion, low-temperature toughness, and fracture resistance characteristics during collision (bending fracture characteristics and axial crush characteristics) can be provided. In particular, since the high-strength steel plate of the present invention has excellent various characteristics, it can be used for frame structure components, reinforcement components, etc. of automobiles of various sizes and shapes. Thus, improvement in fuel efficiency can be achieved based on vehicle body weight reduction, and the industrial utilization value is extremely large.

[0437] Symbol description

[0438] A1 mold

[0439] A2 support roll

[0440] B1 punch

[0441] B2 punch

[0442] T1 test piece

[0443] D1 width (C) direction

[0444] D2 rolling (L) direction

[0445] D3 crushing direction

[0446] 10 hat-shaped member

[0447] 20 steel plate

[0448] 30 test member

[0449] 40 spot weld part

[0450] 50 bottom plate

[0451] 60 impactor

Claims

1. A high-strength steel plate having the following composition and steel structure, The composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and the balance is composed of Fe and inevitable impurities. The steel structure is at the 1 / 4 position of the plate thickness. The area ratio of martensite is 10% to 80%, the area ratio of bainite is 2% to 70%, the area ratio of ferrite is 80% or less, the area ratio of retained austenite is 15% or less, and the ratio of the number of martensite blocks with metastable carbides to the number of martensite blocks is 2% or more. Also, when measuring the nano-hardness at more than 225 points at the 1 / 4 position of the plate thickness, with respect to the average value of the nano-hardness [H n ave , the standard deviation σ of the nano-hardness n is 0.60 × [H n ave or less.​​ 2. The high-strength steel sheet according to claim 1, wherein, The average number density of the metastable carbides in the martensite packets with the metastable carbides is 1×10 6 pieces / mm 2 or more.

3. The high-strength steel sheet according to claim 1 or 2, wherein The composition further contains, by mass%, at least one element selected from 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, Hf: 0.10% or less, and Bi: 0.200% or less.

4. The high-strength steel plate according to any one of claims 1 to 3, having a surface soft layer, which is a region where the Vickers hardness is 85% or less relative to the Vickers hardness at the 1 / 4 position of the plate thickness of the high-strength steel plate and is a region within 200 μm in the plate thickness direction from the surface of the high-strength steel plate. When measuring the nano-hardness at 300 points or more in a 50 μm × 50 μm region of each surface at the 1 / 4 position and the 1 / 2 position of the plate thickness direction of the surface soft layer from the surface of the high-strength steel plate. The proportion of the number of measurements with a nano-hardness of 7.0 GPa or more at the 1 / 4 position of the plate thickness direction of the surface soft layer from the surface of the high-strength steel plate relative to the total number of measurements is 0.10 or less. The standard deviation σ of the nano-hardness at the 1 / 4 position of the plate thickness direction of the surface soft layer from the surface of the high-strength steel plate is 1.8 GPa or less. Moreover, the standard deviation σ of the nano-hardness at the 1 / 2 position of the plate thickness direction of the surface soft layer from the surface of the high-strength steel plate is 2.2 GPa or less.

5. The high-strength steel sheet according to any one of claims 1 to 4, wherein, On the surface of one or both sides of the high-strength steel sheet, there is a metal plating layer containing one or more metals 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, with a total content exceeding 50% by mass.

6. The high-strength steel sheet according to any one of claims 1 to 5, wherein, On the outermost layer of one or both sides of the high-strength steel sheet, there is a metal plating layer containing at least one of zinc and aluminum with a total content of 50% by mass or more.

7. A member made of the high-strength steel sheet according to any one of claims 1 to 6.

8. A skeletal structure component or a reinforcing component of an automobile, composed of the member according to claim 7.

9. A method for manufacturing a high-strength steel sheet, including the following steps: Hot rolling process, a steel billet having the composition according to claim 1 or 3 is rough rolled under the conditions of an average strain rate of 1×10 -4 / s to 1×10 -1 / s and a total reduction ratio of 50% or more, then finish rolled, and then coiled to obtain a hot rolled sheet, Next, an acid pickling and cold rolling process is carried out to obtain an acid pickled and cold rolled sheet. Next, a first heating step is carried out under the condition that the heating temperature is 750°C or higher. Next, a first cooling step is carried out under the condition that the first cooling rate in the temperature range of T2 to 750°C is 2.0°C / s or higher. Next, a soaking step is carried out under the condition that the residence temperature T2 is in the range of 350°C to 550°C and the residence time t satisfies 0.20 to 0.90 for F defined by the following formula 1. Next, a second cooling step is carried out by cooling to below Ms - 20°C and with the second average cooling rate in the temperature range of Ms - 20°C to Ms being 5°C / s or higher, and Next, a second heating step is performed under the conditions of a temperature X and a holding time Y that satisfy the following formula 2, where The unit of temperature X is °C, and the unit of holding time Y is s; Equation 1: F = 1 - exp(-kt n ) t: Residence time, unit is s k, n: Constants obtained from the expansion curve of the Formaster test, which is a phase transformation point determination test carried out by holding a test piece obtained by performing the steps up to the end of the first cooling step on the steel billet at a residence temperature T2 of 350°C to 550°C. Formula 2: 7000 ≤ (273 + X)(20 + log(Y / 3600)) ≤ 13000.

10. The manufacturing method of the high-strength steel sheet according to claim 9, wherein, In the second heating step, the temperature X satisfies the following formula 3, unit is °C, Formula 3: 100 ≤ X ≤ 400.

11. The manufacturing method of the high-strength steel sheet according to claim 9 or 10, wherein, The first heating step is carried out in an atmosphere with a dew point of -30°C or higher.

12. The manufacturing method of the high-strength steel sheet according to any one of claims 9 to 11, wherein, It includes the following steps: On one or both sides of the steel sheet after the cold rolling step and before the annealing step, a metal plating containing one or more metals 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 with a total content greater than 50% by mass is carried out.

13. The manufacturing method of the high-strength steel sheet according to any one of claims 9 to 12, wherein, It includes the following steps: A metal plating containing at least one of zinc and aluminum with a total content of 50% by mass or more is carried out on the steel sheet from the first heating step to the second heating step.

14. A method for manufacturing a member, having the following steps: Performing at least one of forming processing or joining processing on the high-strength steel sheet according to any one of claims 1 to 6 to manufacture a member.

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