High-strength steel sheet, high-strength plated steel sheet, method for producing same, and member

Abstract

The purpose of the present invention is to provide a high-strength steel sheet having excellent YR and a reduced amount of hydrogen trapped in the steel in an acid immersion environment. This high-strength steel sheet has a component composition containing, in mass%, 0.030% to 0.500% of C, 0.01% to 3.00% of Si, 0.10% to 6.00% of Mn, 0.100% or less of P, 0.0200% or less of S, 1.000% or less of Al, 0.0100% or less of N, 0.0100% or less of O, and 0.002% to 0.300% of Sb, with the remainder being Fe and unavoidable impurities, and a steel structure. In the steel structure, the area ratio of martensite is 30% or more, the area ratio of ferrite is 70% or less, and the volume ratio of retained austenite is 20.0% or less at the position of 1 / 4 of the sheet thickness. In addition, the surface layer enrichment index of Sb is 1.2 or more.

Description

Technical Field

[0001] The present invention relates to high-strength steel plates, high-strength coated steel plates, methods for manufacturing them, and components. Background Art

[0002] In order to balance the reduction of CO2 emissions through vehicle lightweighting and the improvement of anti-collision performance, the high-strength of automotive steel plates is being promoted. In addition, new laws and regulations are being introduced one after another. Therefore, for the purpose of increasing the body strength, the application examples of high-strength steel plates for the main structural components and reinforcing components (hereinafter, also referred to as the skeletal structural components of the vehicle, etc.) that form the automotive cab skeleton, especially high-strength steel plates with a tensile strength (hereinafter, abbreviated as TS) of 980 MPa or more, are gradually increasing.

[0003] In addition, when formed into skeletal structural components of a vehicle, etc., the high-strength steel plates used for the skeletal structural components of the vehicle, etc. are required to have high component strength. For the improvement of component strength, for example, increasing the yield ratio of the steel plate (= YS / TS×100, hereinafter abbreviated as YR) is effective. Thereby, the impact absorption energy during vehicle collision is increased.

[0004] As a technology related to such high-strength steel plates, for example, Patent Document 1 discloses "a cold-rolled steel plate and a high-strength hot-dip galvanized steel plate, the cold-rolled steel plate containing, by weight%, C: 0.1 to 0.3%, Si: 1 to 2.5%, Mn: 2.5 to 8%, sol.Al: 0.001 to 0.5%, P: 0.04% or less, S: 0.015% or less, N: 0.02% or less (except 0%), Cr: 0.1 to 0.7%, Mo: 0.1% or less, Ti: (48 / 14)*[N] to 0.1%, Ni: 0.005 to 0.5%, Sb: 0.01 to 0.07%, Nb: 0.1 or less, B: 0.005% or less, with the balance being Fe and other inevitable impurities; the high-strength hot-dip galvanized steel plate is formed with a galvanized layer on the above cold-rolled steel plate, and the average Sb content from the surface of the cold-rolled steel plate to a depth of 0.1 μm inside the galvanized layer is 1.5 times or more the average Sb content at a depth of 0.5 μm or more from the surface of the cold-rolled steel plate, and the surface quality, plating adhesion, and formability are excellent".

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-505963. Summary of the Invention

[0008] However, for high-strength steel sheets with a TS of 980 MPa or more, there is a concern that the steel sheets may break due to delayed fracture in the use environment of automobiles. Therefore, it is also necessary to reduce the amount of hydrogen trapped in steel in an acid immersion environment that is more severe than the use environment of automobiles.

[0009] However, the amount of hydrogen trapped in the steel in an acid immersion environment is not considered in the high-strength steel sheet described in Patent Document 1. Therefore, there is a need to develop a high-strength steel sheet having excellent YR and reduced amount of hydrogen trapped in the steel in an acid immersion environment.

[0010] The present invention has been made in view of the above-mentioned current situation, and an object of the present invention is to provide a high-strength steel sheet having excellent YR and having a reduced amount of hydrogen trapped in the steel in an acid immersion environment.

[0011] In addition, "high strength" means that the tensile strength (TS) measured in accordance with JIS Z 2241:2021 is 980 MPa or more.

[0012] "High YR (high component strength)" means that the YR measured in accordance with JIS Z 2241: 2021 is 65% or more. The YR is preferably 70% or more. The YR is obtained by the following formula (1).

[0013] YR=YS / TS×100····(1)

[0014] “High bendability” means that when the presence or absence of cracks in the ridgeline portion at the bending apex is evaluated in a bending test conducted in accordance with JIS Z 2248:2022, no cracks are generated or micro cracks smaller than 200 μm are generated.

[0015] “The amount of hydrogen trapped in steel under acid immersion is reduced” means that the amount of hydrogen trapped in steel after acid immersion is 5.00 wt.ppm or less. The amount of hydrogen trapped in steel can be measured by a conventionally known method as described in the Examples below.

[0016] The present inventors have conducted intensive studies to achieve the above-mentioned objects and have obtained the following findings.

[0017] (1) By adding Sb and extending the holding time in the high temperature region in each process, the surface layer enrichment index of Sb can be increased, and the amount of hydrogen trapped in the steel in the acid immersion environment can be reduced.

[0018] (2) A high YR can be achieved by forming a structure mainly composed of martensite (quenched martensite and tempered martensite).

[0019] (3) By setting the volume fraction of retained austenite to 20% or less, it is possible to reduce the amount of hydrogen trapped in the steel in an acid immersion environment.

[0020] The present invention has been completed based on the above insights. That is, the gist of the present invention is constituted as follows.

[0021] [1] A high-strength steel sheet having the following composition and steel structure,

[0022] The composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 3.00%, Mn: 0.10% to 6.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, O: 0.0100% or less, and Sb: 0.002% to 0.300%, and the balance is composed of Fe and inevitable impurities;

[0023] In the steel structure, at the 1 / 4 position of the plate thickness, the area ratio of martensite is 30% or more, the area ratio of ferrite is 70% or less, and the volume ratio of retained austenite is 20.0% or less;

[0024] The surface enrichment index of Sb is 1.2 or more.

[0025] [2] The high-strength steel sheet according to [1], 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, 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.

[0026] [3] The high-strength steel sheet according to [1] or [2], wherein the maximum increase rate of the C concentration in the thickness direction from the steel sheet surface is 0.1000 mass% / μm or less.

[0027] [4] The high-strength plated steel sheet according to any one of [1] to [3], which has a plating layer on at least one side.

[0028] [5] A method for manufacturing a high-strength steel sheet, wherein,

[0029] Prepare a steel billet having the composition described in [1] or [2],

[0030] Cool the above billet with the average cooling rate in the temperature range of 700°C to 1000°C being 1500°C / hr or less.

[0031] Next, heat the above billet to a slab heating temperature of 1150°C or higher, and set the residence time from 1100°C to the above slab heating temperature to 20 minutes or more.

[0032] Perform hot rolling on the above billet with the reduction ratio of the first pass of finish rolling being 20% or more to produce a hot-rolled sheet.

[0033] Next, perform pickling on the above hot-rolled sheet to produce a pickled sheet with the average thickness of the front and back of the internal oxide layer being 0.2 μm or more.

[0034] Next, perform cold rolling on the above pickled sheet with the cumulative reduction ratio being 20% to 90% to produce a cold-rolled sheet.

[0035] Next, perform an annealing process of heating the above cold-rolled sheet to a heating temperature of 780°C or higher. At this time, set the average heating rate in the temperature range of 250°C to 700°C to 100°C / s or less, and set the residence time from 750°C to the above heating temperature to 10 s or more.

[0036] Next, perform a cooling process on the above cold-rolled sheet.

[0037] [6] The method for manufacturing a high-strength steel sheet according to [5], wherein, in the above annealing process, the oxygen concentration in the atmosphere from 750°C to the above heating temperature is 0.5 vol% to 5.0 vol%, and the dew point of the atmosphere is set to -35°C or higher.

[0038] [7] The method for manufacturing a high-strength steel sheet according to [5] or [6], wherein, in the cooling process of the above cold-rolled sheet, the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more.

[0039] [8] The method for manufacturing a high-strength steel sheet according to any one of [5] to [7], wherein, in the cooling process of the above cold-rolled sheet, it is held at a holding temperature of 100°C to 450°C for 5 s or more.

[0040] [9] The method for manufacturing a high-strength steel sheet according to any one of [5] to [7], wherein, in the cooling process of the above cold-rolled sheet, the cooling stop temperature is set to 250°C or lower, and then the above cold-rolled sheet is reheated to a reheating temperature higher than (the cooling stop temperature + 50°C) and lower than 450°C, and held at this reheating temperature for 5 s or more.

[0041]

[10] The manufacturing method of the high-strength plated steel sheet according to any one of [5] to [9], wherein, after the annealing step, a plating step of performing plating treatment on at least one side of the cold-rolled sheet is carried out.

[0042]

[11] A component, at least a part of which is made of the high-strength steel sheet according to any one of [1] to [4].

[0043] According to the present invention, it is possible to provide a high-strength steel sheet with excellent YR and a reduced amount of hydrogen trapped in the steel in an acid immersion environment. Detailed implementation manners

[0044] Hereinafter, the implementation manners of the present invention will be described. It should be noted that the implementation manners described below are an example of embodying the present invention, but the constitution of the present invention is not limited to this specific example.

[0045] [High-strength steel sheet]

[0046] First, the composition of the high-strength steel sheet will be described. It should be noted that in the following description, unless otherwise specified, the content "%" of the component element of the steel refers to "mass%".

[0047] [C: 0.030% to 0.500%]

[0048] C is one of the important basic components of the steel. Especially in the present invention, it is an important element that affects the area ratio of martensite, the area ratio of ferrite, and the volume ratio of retained austenite. When the content of C is less than 0.030%, the area ratio of martensite decreases, so the TS decreases and it is difficult to achieve the desired YR. On the other hand, if the content of C exceeds 0.500%, the volume ratio of retained austenite increases, and the amount of hydrogen trapped in the steel in an acid immersion environment increases. In addition, martensite embrittles and it is difficult to achieve the desired bendability. Therefore, the content of C is set to 0.030% to 0.500%. The content of C is preferably 0.050% or more, more preferably 0.070% or more. In addition, the content of C is preferably 0.400% or less, more preferably 0.300% or less.

[0049] [Si: 0.01% to 3.00%]

[0050] Si is one of the important basic components of steel. Especially in the present invention, it suppresses carbide formation during continuous annealing, promotes the formation of retained austenite, and affects the hardness of martensite and the volume fraction of retained austenite. When the content of Si is less than 0.01%, tempering of martensite is carried out and the TS decreases. On the other hand, if the content of Si is greater than 3.00%, the volume fraction of retained austenite increases and the amount of hydrogen trapped in the steel in an acid pickling environment increases. In addition, due to the excessive increase in the carbon concentration in the retained austenite, the hardness of the martensite transformed from the retained austenite during bending deformation increases significantly, resulting in a decrease in bendability. Therefore, the content of Si is set to 0.01% to 3.00%. The content of Si is preferably 0.05% or more, more preferably 0.10% or more. In addition, the content of Si is preferably 2.50% or less, more preferably 2.00% or less, and further preferably 1.80% or less. If Si is 2.50% or less, the increase in the carbon concentration in the retained austenite is appropriate, so the increase in the hardness of the martensite transformed from the retained austenite during bending deformation is appropriate, and excellent bendability can be obtained.

[0051] [Mn: 0.10% to 6.00%]

[0052] Mn is one of the important basic components of steel. Especially in the present invention, it is an important element affecting the area fraction of martensite. When the content of Mn is less than 0.10%, the area fraction of martensite decreases and it is difficult to achieve the desired YR. Therefore, the content of Mn is set to 0.10% or more. On the other hand, if the content of Mn exceeds 6.00%, the maximum increase rate of the C concentration along the plate thickness direction from the steel plate surface exceeds 0.1000% / μm, and in addition, the bendability decreases. Therefore, the content of Mn is set to 6.00% or less. The content of Mn is preferably 0.80% or more, more preferably 1.00% or more. In addition, the content of Mn is preferably 5.00% or less, more preferably 4.50% or less, and further preferably 4.00% or less.

[0053] [P: 0.100% or less]

[0054] When the content of P is large, segregation occurs at the prior austenite grain boundaries, embrittling the grain boundaries, so the ultimate deformation ability of the steel plate decreases and the bendability decreases. Therefore, the content of P is set to 0.100% or less. It should be noted that the lower limit of the content of P is not particularly specified, but since P is a solid solution strengthening element and can increase the strength of the steel plate, it is preferably 0.001% or more. In addition, the content of P is preferably 0.070% or less.

[0055] [S: 0.0200% or less]

[0056] When the content of S is high, it exists as a sulfide, reducing the ultimate deformation ability of the steel plate and, in addition, reducing the bendability. Therefore, the content of S is set to 0.0200% or less. It should be noted that there is no special regulation for the lower limit of the content of S, and from the perspective of production technology constraints, it is preferably 0.0001% or more. In addition, the content of S is preferably 0.0050% or less.

[0057] [Al: 1.000% or less]

[0058] When the content of Al is high, the A3 phase transformation point rises, and the microstructure contains a large amount of ferrite. Therefore, it is difficult to achieve the desired YR. Therefore, the content of Al is set to 1.000% or less. It should be noted that there is no special regulation for the lower limit of the content of Al, but since it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite, the content of Al is preferably 0.001% or more. In addition, the content of Al is preferably 0.500% or less.

[0059] [N: 0.0100% or less]

[0060] When the content of N is high, it exists as a nitride, reducing the ultimate deformation ability of the steel plate and reducing the bendability. Therefore, the content of N is set to 0.0100% or less. It should be noted that there is no special regulation for the lower limit of the content of N, but from the perspective of production technology constraints, the content of N is preferably 0.0001% or more. In addition, the content of N is preferably 0.0050% or less.

[0061] [O: 0.0100% or less]

[0062] When the content of O is high, it exists as an oxide, reducing the ultimate deformation ability of the steel plate and, in addition, reducing the bendability. Therefore, the content of O is set to 0.0100% or less. It should be noted that there is no special regulation for the lower limit of the content of O, but from the perspective of production technology constraints, the content of O is preferably 0.0001% or more. In addition, the content of O is preferably 0.0050% or less.

[0063] [Sb: 0.002% - 0.300%]

[0064] Sb is an important element that affects the surface enrichment index of Sb and the maximum increase rate of C concentration in the plate thickness direction from the steel plate surface in the present invention. When the content of Sb is less than 0.002%, the surface enrichment index of Sb decreases. Therefore, the content of Sb is set to 0.002% or more. On the other hand, if the content of Sb exceeds 0.300%, the maximum increase rate of C concentration in the plate thickness direction from the steel plate surface exceeds 0.1000% / μm, and the bendability decreases. Therefore, the content of Sb is set to 0.300 or less. The content of Sb is preferably 0.003% or more, more preferably 0.004% or more. In addition, the content of Sb is preferably 0.200% or less, more preferably 0.150% or less, and further preferably 0.100% or less.

[0065] The high-strength steel plate has a composition containing the above components 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. It is allowed that the total content of these impurities is 0.100% or less.

[0066] In the high-strength steel plate, in addition to the above composition, it 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, 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, either singly or in combination.

[0067] If Ti, Nb, and V are each 0.200% or less, a large amount of coarse precipitates and inclusions will not be generated, and the ultimate deformation ability of the steel plate will not be reduced, so the bendability will not be reduced. Therefore, when contained, the contents of Ti, Nb, and V are each preferably 0.200% or less. The contents of Ti, Nb, and V are each more preferably 0.100% or less. It should be noted that the lower limit of the contents of Ti, Nb, and V is not particularly specified, but since fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, the strength of the steel plate is improved, so the contents of Ti, Nb, and V are each preferably 0.001% or more.

[0068] If Ta and W are each 0.10% or less, a large amount of coarse precipitates and inclusions will not be generated, and the ultimate deformation ability of the steel sheet will not be reduced, so the bendability will not be reduced. Therefore, when contained, the contents of Ta and W are each preferably 0.10% or less. The contents of Ta and W are each more preferably 0.08% or less. It should be noted that the lower limit of the contents of Ta and W is not particularly specified, but since fine carbides, nitrides or carbonitrides are formed during hot rolling or continuous annealing to increase the strength of the steel sheet, the contents of Ta and W are each preferably 0.01% or more.

[0069] If B is 0.0100% or less, no cracks will occur inside the steel sheet during casting or hot rolling, and the ultimate deformation ability of the steel sheet will not be reduced, so the bendability will not be reduced. Therefore, when contained, the content of B is preferably 0.0100% or less. The content of B is more preferably 0.0080% or less. It should be noted that the lower limit of the content of B is not particularly specified, but since it is an element that segregates at the austenite grain boundaries during annealing and increases hardenability, the content of B is preferably 0.0003% or more.

[0070] If Cr, Mo and Ni are each 1.00% or less, the coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced, so the bendability will not be reduced. Therefore, the contents of Cr, Mo and Ni are each preferably 1.00% or less. The contents of Cr, Mo and Ni are each more preferably 0.80% or less. It should be noted that the lower limit of the contents of Cr, Mo and Ni is not particularly specified, but since they are elements that increase hardenability, the contents of Cr, Mo and Ni are each preferably 0.01% or more.

[0071] If Co is 0.010% or less, the coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced, so the bendability will not be reduced. Therefore, the content of Co is preferably 0.010% or less. The content of Co is more preferably 0.008% or less. It should be noted that the lower limit of the content of Co is not particularly specified, but since it is an element that increases hardenability, the content of Co is preferably 0.001% or more.

[0072] If Cu is 1.00% or less, the coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced, so the bendability will not be reduced. Therefore, the content of Cu is preferably 1.00% or less. The content of Cu is more preferably 0.80% or less. It should be noted that the lower limit of the content of Cu is not particularly specified, but since it is an element that increases hardenability, the content of Cu is preferably 0.01% or more.

[0073] If Sn is 0.200% or less, cracks will not occur inside the steel plate during casting or hot rolling, and the ultimate deformation ability of the steel plate will not be reduced, so the bendability will not be reduced. Therefore, when contained, the content of Sn is preferably 0.200% or less. The content of Sn is more preferably 0.100% or less. It should be noted that the lower limit of the content of Sn is not particularly specified, but since it is an element that improves the hardenability of Sn, the content of Sn is preferably 0.001% or more.

[0074] If Ca, Mg, and REM (Rare Earth Metal) are each 0.0100% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not be reduced, so the bendability will not be reduced. Therefore, when contained, the contents of Ca, Mg, and REM are preferably 0.0100% or less. The contents of Ca, Mg, and REM are each more preferably 0.0050% or less. It should be noted that the lower limit of the contents of Ca, Mg, and REM is not particularly specified, but since they are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformation ability of the steel plate, the contents of Ca, Mg, and REM are each preferably 0.0005% or more.

[0075] If Zr and Te are each 0.100% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not be reduced, so the bendability will not be reduced. Therefore, when contained, the contents of Zr and Te are preferably 0.100% or less. The contents of Zr and Te are each more preferably 0.080% or less. It should be noted that the lower limit of the contents of Zr and Te is not particularly specified, but since they are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformation ability of the steel plate, the contents of Zr and Te are each preferably 0.001% or more.

[0076] If Hf is 0.10% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not be reduced, so the bendability will not be reduced. Therefore, when contained, the content of Hf is preferably 0.10% or less. The content of Hf is more preferably 0.08% or less. It should be noted that the lower limit of the content of Hf is not particularly specified, but since it is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformation ability of the steel plate, the content of Hf is preferably 0.01% or more.

[0077] If Bi is 0.200% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not be reduced, so the bendability will not be reduced. Therefore, when contained, the content of Bi is preferably 0.200% or less. The content of Bi is more preferably 0.100% or less. It should be noted that the lower limit of the content of Bi is not particularly specified, but since it is an element that reduces segregation, the content of Bi is preferably 0.001% or more.

[0078] It should be noted that regarding the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Ca, Mg, REM, Zr, Te, Hf, and Bi, when each content is less than the preferred lower limit value, the effects of the present invention will not be impaired, so they can also be included as inevitable impurities.

[0079] Next, the steel structure of the high-strength steel plate will be described.

[0080] [Area ratio of martensite: 30% or more]

[0081] If the area ratio of martensite is 30% or more, YR and bendability are excellent. Therefore, the area ratio of martensite is set to 30% or more. The area ratio of martensite is preferably 45% or more, more preferably 60% or more, and further preferably 80% or more. It should be noted that the upper limit of the area ratio of martensite is not particularly limited, and even if it is 100%, the desired properties can be obtained. It should be noted that the so-called martensite here includes tempered martensite and bainite in addition to quenched martensite (fresh martensite). It should be noted that the observation position of the area ratio of martensite is set at the 1 / 4 position of the plate thickness of the steel plate as described later.

[0082] [Area ratio of ferrite: 70% or less]

[0083] By making the area ratio of ferrite 70% or less, YR and bendability can be further improved. Therefore, the area ratio of ferrite is set to 70% or less. The area ratio of ferrite is preferably 50% or less, more preferably 40% or less, and further preferably 30% or less. It should be noted that the lower limit of the area ratio of ferrite is not particularly limited, and even if it is 0%, the desired properties can be obtained. It should be noted that the so-called ferrite here includes bainite ferrite. It should be noted that the observation position of the area ratio of ferrite is set at the 1 / 4 position of the plate thickness of the steel plate as described later.

[0084] Here, the measurement methods of the area ratios of martensite (quenched martensite, tempered martensite, and bainite) and ferrite are as follows.

[0085] A specimen was cut out in such a way that the plate thickness cross-section (L-section) parallel to the rolling direction of the steel plate was used as the observation surface. Next, the observation surface of the specimen was polished using diamond polishing paste, and then fine polishing was performed using alumina. Next, the observation surface of the specimen was etched with 3 vol.% nitric acid ethanol to reveal the microstructure. Next, the 1 / 4 position of the plate thickness of the steel plate (corresponding to the 1 / 4 position of the plate thickness in the depth direction from the steel plate surface) was set as the observation position, and 3 fields of view were observed at a magnification of 3000 times by SEM (Scanning Electron Microscope). From the obtained microstructure images, using Adobe Photoshop of Adobe Systems Incorporated, the area ratio was obtained by dividing the area of each constituent microstructure (ferrite, martensite (quenched martensite, tempered martensite, and bainite)) by the measured area, the area ratios of the 3 fields of view were calculated, and the average value of these values was taken as the area ratio of each microstructure. In addition, in the above-mentioned microstructure images, ferrite is a flat microstructure with concave parts and no carbides, tempered martensite and bainite are microstructures with concave parts and contain fine carbides, and quenched martensite is a convex part and has fine unevenness inside the microstructure, and they can be distinguished from each other. It should be noted that since the total area ratio is obtained as the area ratio of martensite for tempered martensite and bainite, they do not need to be distinguished from each other.

[0086] [Volume fraction of retained austenite: 20.0% or less]

[0087] If the volume fraction of retained austenite is 20.0% or less, the amount of hydrogen trapped in the steel can be reduced. Therefore, the volume fraction of retained austenite is set to 20.0% or less. In addition, if the volume fraction of retained austenite is 15.0% or less, the amount of martensite transformed from retained austenite does not increase, and thus excellent bendability can be obtained. Therefore, the volume fraction of retained austenite is preferably 15.0% or less, more preferably 10.0% or less, and further preferably 5.0% or less. It should be noted that the lower limit of the volume fraction of retained austenite is not particularly limited, and even if it is 0%, the desired properties can be obtained.

[0088] Here, the method for measuring the volume fraction of retained austenite is as follows.

[0089] Grinding is carried out in such a way that the observation surface is at a position of 1 / 4 of the plate thickness from the surface of the steel plate, and then, it is further ground by chemical polishing by 0.1 mm. For this surface, the integrated reflection intensities of the (200), (220), (311) planes of fcc iron (austenite) and the (200), (211), (220) planes of bcc iron are measured by an X-ray diffractometer using a Co Kα ray source. The volume fraction of austenite is obtained from the intensity ratio of the integrated reflection intensity of each plane of fcc iron (austenite) to the integrated reflection intensity of each plane of bcc iron, and this is taken as the volume fraction of retained austenite.

[0090] In addition, at the position of 1 / 4 of the plate thickness of the steel plate, the area fraction of the remaining tissue other than martensite, ferrite, and retained austenite is preferably 5% or less. As the remaining tissue, there are tissues known as the tissues of other steel plates, such as pearlite, cementite, metastable carbides (epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, etc.). Identification of the remaining tissue can be carried out, for example, by observation using SEM.

[0091] In addition, the area fraction of the remaining tissue is calculated by the following formula (2).

[0092] [Area fraction of the remaining tissue (%)] = 100 - [Area fraction of martensite (%)] - [Area fraction of ferrite (%)] - [Volume fraction of retained austenite (%)] ···· (2)

[0093] [Surface enrichment index of Sb: 1.2 or more]

[0094] The surface enrichment index of Sb is an extremely important inventive component. By increasing the surface enrichment index of Sb, that is, enriching Sb on the surface, the amount of hydrogen trapped in the steel in an acid pickling environment can be reduced. In order to obtain such an effect, the surface enrichment index of Sb is set to 1.2 or more. The surface enrichment index of Sb is preferably 1.3 or more, more preferably 1.4 or more, and further preferably 1.5 or more. It should be noted that the upper limit of the surface enrichment index of Sb is not particularly limited, but in order to control the maximum increase rate of the C concentration in the plate thickness direction from the steel plate surface within an appropriate range, it is preferably 10.0 or less, more preferably 8.0 or less, and further preferably 6.0 or less.

[0095] Here, the measurement method of the surface enrichment index of Sb is as follows.

[0096] Samples measuring 20 mm in the rolling direction and 20 mm in the width direction were collected from the high-strength steel plate by shearing. Using the surface of the high-strength steel plate as the measurement surface, the intensity of Sb was analyzed in the thickness direction under the discharge conditions of 4 mm from the back surface, a high-frequency condition of 600 V, and 2.8 hPa by Glow Discharge Optical Emission Spectrometry (hereinafter referred to as GDS). The measurement results were organized in a graph of measurement time (s) vs. the intensity of Sb (a.u.). The maximum and minimum values of Sb were obtained within the range where the measurement time was within 1.5 seconds from the start of the measurement. The surface enrichment index of Sb was calculated by dividing this maximum value by the minimum value ((maximum value of Sb) / (minimum value of Sb)).

[0097] [Maximum rate of increase in C concentration in the thickness direction from the steel plate surface: 0.1000 mass% / μm or less (preferred condition)]

[0098] The maximum rate of increase in C concentration in the thickness direction from the steel plate surface is a preferred inventive component required to achieve good bendability. By reducing the maximum rate of increase in C concentration in the thickness direction from the steel plate surface, that is, in the C distribution from the steel plate surface to the inside of the steel plate, there are no locally high parts but a gradual increase, thus enabling the desired bendability to be achieved. To obtain such an effect, the maximum rate of increase in C concentration in the thickness direction from the steel plate surface is preferably 0.1000 mass% / μm or less. The maximum rate of increase in C concentration in the thickness direction from the steel plate surface is more preferably 0.0500 mass% / μm or less, further preferably 0.0300 mass% / μm or less, and most preferably 0.0100 mass% / μm or less. It should be noted that the lower limit of the maximum rate of increase in C concentration in the thickness direction from the steel plate surface is not particularly limited. To prevent a decrease in TS, it is preferably 0.0001 mass% / μm or more, more preferably 0.0002 mass% / μm or more, and further preferably 0.0003 mass% / μm or more.

[0099] Here, the method for measuring the maximum increase rate of the C concentration in the thickness direction from the steel plate surface is as follows. From the high-strength steel plate, a specimen is cut out in such a way that the cross-section parallel to the rolling direction of the steel plate (L cross-section) is the observation surface. Next, the observation surface of the specimen is polished using diamond polishing paste, and then fine polishing is carried out using alumina. Next, using an electron probe microanalyzer (EPMA; Electron Probe Micro Analyzer) (EPMA - 1720HT: manufactured by Shimadzu Corporation), with the surface layer of the steel plate as the observation position, the C concentration of 3 fields of view is measured under the conditions of acceleration voltage: 15 kV, measurement area: thickness 60 μm × width 60 μm, and electron beam current: 50 nA. It should be noted that the measurement data is converted into the C concentration using the calibration curve method. Then, for the 3 fields of view obtained, the average value of the C concentration obtained is calculated in the area of thickness 5 μm × width 60 μm every 5 μm in the thickness direction from the plate surface to a thickness of 60 μm. Next, the average C concentration curve obtained every 5 μm is plotted, and for the value that becomes the maximum slope, it is determined as the maximum increase rate of the C concentration in the thickness direction from the steel plate surface.

[0100] It should be noted that the steel structure of the steel plate is generally approximately symmetric up and down in the thickness direction. Therefore, in the identification of the structure, the volume fraction of retained austenite, the surface enrichment index of Sb, and the measurement of the maximum increase rate of the C concentration in the thickness direction from the steel plate surface, any one of the surfaces of the steel plate (front and back) can be used as a representative, for example, any one of the surfaces of the steel plate (front and back) can be used as the starting point (plate thickness 0 position) of the plate thickness position such as the 1 / 4 position of the plate thickness. The same applies hereinafter.

[0101] The plate thickness of the high-strength steel plate is not particularly limited, and generally can be 0.3 mm or more, and can also be 2.8 mm or less.

[0102] [High-strength plated steel plate]

[0103] One embodiment of the present invention relates to a high-strength plated steel sheet having a high-strength steel sheet and a plating layer formed on at least one side of the high-strength steel sheet. The type of the plating layer is not particularly limited, and for example, it may be any one of a hot-dip plating layer and an electroplated layer. In addition, the plating layer may also be a plated layer after alloying. The plating layer is preferably a galvanized layer. The galvanized layer may contain Al and Mg. In addition, a hot-dip galvanized-aluminum-magnesium alloy (Zn-Al-Mg plating layer) is also preferred. In this case, it is preferred that the Al content is 1% by mass to 22% by mass and the Mg content is 0.1% by mass to 10% by mass, and the balance is Zn. In addition, in the case of the Zn-Al-Mg plating layer, in addition to Zn, Al, and Mg, it may also contain one or more selected from Si, Ni, Ce, and La in a total amount of 1% by mass or less. It should be noted that the plating metal is not particularly limited, and therefore, in addition to plating Zn as described above, Al or the like may also be plated. In addition, the plating layer may be provided on one side of the steel sheet surface or on both sides.

[0104] In addition, the composition of the plating layer is not particularly limited, and a general composition may be used. For example, in the case of a hot-dip galvanized layer and an alloyed hot-dip galvanized layer, the general composition is as follows: containing Fe: 20% by mass or less, Al: 0.001% by mass to 1.0% by mass, and further containing one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass to 3.5% by mass, and the balance is composed of Zn and inevitable impurities. In addition, in the case of a hot-dip galvanized layer, the Fe content in the plating layer is preferably less than 7% by mass. In the case of an alloyed hot-dip galvanized layer, the Fe content in the plating layer is preferably 7 to 20% by mass.

[0105] Furthermore, the plating adhesion amount per side of the plating layer is not particularly limited. For example, in the case of a hot-dip galvanized layer and an alloyed hot-dip galvanized layer (where the hot-dip galvanized layer is alloyed), it is preferably 20 g / m 2 above, and in addition, it is preferably 80 g / m 2 below.

[0106] [Manufacturing method of high-strength steel sheet]

[0107] Next, a manufacturing method of the high-strength steel sheet according to one embodiment will be described.

[0108] First, a steel billet having the above composition is melted, and then a steel billet is manufactured. In the present invention, the melting method of the steel billet is not particularly limited, and known melting methods such as a converter and an electric furnace are suitable. In addition, in order to prevent macrosegregation, it is preferred that the steel billet (slab) is manufactured by a continuous casting method, but it may also be manufactured by an ingot casting method, a thin slab casting method, or the like.

[0109] Next, in the process of cooling the produced steel billet at a temperature above 1000°C, the average cooling rate in the temperature range of 700°C to 1000°C is set to 1500°C / hr or less for cooling.

[0110] [Average cooling rate in the temperature range of 700°C to 1000°C: 1500°C / hr or less]

[0111] The average cooling rate in the temperature range of 700°C to 1000°C is a very important inventive component. By reducing the average cooling rate in the temperature range of 700°C to 1000°C and prolonging the holding time in this temperature range, the surface enrichment index of Sb can be increased, and the maximum increase rate of C concentration in the plate thickness direction from the steel plate surface can be reduced. To obtain such an effect, the average cooling rate in the temperature range of 700°C to 1000°C is set to 1500°C / hr or less. It should be noted that the lower limit of the average cooling rate in the temperature range of 700°C to 1000°C is not particularly specified. To control the maximum increase rate of C concentration in the plate thickness direction from the steel plate surface within an appropriate range, it is preferably 50°C / hr or more. In addition, the average cooling rate in the temperature range of 700°C to 1000°C is preferably 1000°C / hr or less. It should be noted that the temperature of the steel billet here is the temperature of the steel billet surface.

[0112] It should be noted that after the steel billet is cooled according to the conventional method, it can also be temporarily cooled to room temperature and then reheated, but energy-saving processes such as direct rolling can also be applied. Direct rolling is a process of loading the steel billet into the heating furnace in a warm sheet state without cooling to room temperature.

[0113] Next, the steel billet is heated to a slab heating temperature of 1150°C or more, and the residence time from 1100°C to the slab heating temperature is set to 20 minutes or more.

[0114] [Slab heating temperature: 1150°C or more]

[0115] By setting the slab heating temperature to 1150°C or more, the surface enrichment index of Sb can be increased, and the maximum increase rate of C concentration in the plate thickness direction from the steel plate surface can be reduced. To obtain such an effect, the slab heating temperature is set to 1150°C or more. It should be noted that the upper limit of the slab heating temperature is not particularly specified. To control the maximum increase rate of C concentration in the plate thickness direction from the steel plate surface within an appropriate range, the slab heating temperature is preferably 1300°C or less. In addition, the slab heating temperature is preferably 1180°C or more. It should be noted that the slab heating temperature is the temperature of the slab surface.

[0116] [Residence time from 1100°C to the slab heating temperature: 20 minutes or more]

[0117] By setting the residence time from 1100°C to the slab heating temperature to 20 minutes or more, the surface enrichment index of Sb can be increased, and the maximum increase rate of C concentration in the thickness direction from the steel plate surface can be reduced. To obtain such an effect, the residence time from 1100°C to the slab heating temperature is set to 20 minutes or more. It should be noted that the upper limit of the residence time from 1100°C to the slab heating temperature is not particularly specified, and in order to control the maximum increase rate of C concentration in the thickness direction from the steel plate surface within an appropriate range, it is preferably set to 300 minutes or less. In addition, the residence time from 1100°C to the slab heating temperature is preferably 30 minutes or more.

[0118] Next, hot rolling is performed on the steel billet to produce a hot-rolled sheet (hot rolling process). First, the steel billet is rolled into a thin steel sheet under normal conditions. It should be noted that in the case of reducing the slab heating temperature, from the viewpoint of preventing defects during rolling, it is preferable to heat the thin steel sheet using a bar heater or the like before finish rolling. In addition, the finish rolling temperature is preferably above the Ar3 transformation point. If the finish rolling temperature is excessively reduced, the rolling load increases and the reduction ratio in the non-recrystallized state of austenite increases. As a result, abnormal structures elongated in the rolling direction become developed, and sometimes the workability of the steel plate obtained after annealing is reduced. It should be noted that the Ar3 transformation point is obtained by the following formula (3).

[0119] Ar3 (°C) = 868 - 396 × [%C] + 24.6 × [%Si] - 68.1 × [%Mn] - 36.1 × [%Ni] - 20.7 × [%Cu] - 24.8 × [%Cr] ···· (3)

[0120] It should be noted that [%element symbol] in formula (3) represents the content (mass%) of the element in the above composition.

[0121] It should be noted that the thin steel sheets can also be joined to each other and finish rolling can be performed continuously. In addition, the thin steel sheets can be temporarily coiled. In addition, in order to reduce the rolling load during rolling, part or all of the finish rolling can also be performed as lubricated rolling. From the viewpoints of homogenization of the steel plate shape and homogenization of the material quality, lubricated rolling is also effective. It should be noted that the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0122] Moreover, in hot rolling, control of the reduction ratio in the first pass of the above finish rolling is particularly important.

[0123] [Reduction ratio in the first pass of finish rolling: 20% or more]

[0124] By increasing the reduction ratio in the first pass of finish rolling and introducing a large strain at a high temperature, the surface enrichment index of Sb can be increased, and the maximum increase rate of C concentration in the thickness direction from the steel plate surface can be reduced. To obtain such an effect, the reduction ratio in the first pass of finish rolling is set to 20% or more. It should be noted that the upper limit of the reduction ratio in the first pass of finish rolling is not particularly specified, but from the constraints of production technology, it is preferably 80% or less. In addition, the reduction ratio in the first pass of finish rolling is preferably 30% or more.

[0125] [Average front-back thickness of the internal oxide layer of the pickled plate after pickling: 0.2 μm or more]

[0126] After hot rolling, the hot-rolled plate is pickled (pickling process). By increasing the average front-back thickness of the internal oxide layer of the pickled plate after pickling, Sb enriched on the steel plate surface during hot rolling can be retained, so that the surface enrichment index of Sb in the annealed plate can be increased. Therefore, the average front-back thickness of the internal oxide layer of the pickled plate after pickling is set to 0.2 μm or more. The average front-back thickness of the internal oxide layer of the pickled plate after pickling is preferably 0.3 μm or more, and more preferably 0.4 μm or more. In addition, the upper limit of the average front-back thickness of the internal oxide layer of the pickled plate after pickling is not particularly specified. To make the bendability better, it is preferably 4.0 μm or less, and more preferably 2.5 μm or less. It should be noted that the lower limit of the pickling time is preferably 5 seconds or more, and more preferably 10 seconds or more. The upper limit of the pickling time is preferably 200 seconds or less, and more preferably 150 seconds or less.

[0127] Here, the method for measuring the average front-back thickness of the internal oxide layer of the pickled plate after pickling is as follows. A specimen is cut from the head and tail ends and the central part in the width direction of the pickled steel coil in such a way that the plate thickness section (L section) parallel to the rolling direction of the steel plate is used as the observation surface. Next, the observation surface of the specimen is polished using diamond polishing paste. Next, with the surface and back of the pickled plate as the observation positions, 5 fields of view are observed by SEM at a magnification of 2000 times. Then, the internal oxide layer thickness is measured at any 3 places on each of the measured SEM photos. Such evaluations are respectively performed on the surface and back of the head and tail ends of the pickled steel coil, and the internal oxide layer thicknesses of a total of 20 fields of view and 60 places are measured. The average value of these values is used as the average front-back thickness of the internal oxide layer of the pickled plate after pickling.

[0128] It should be noted that after pickling, any heat treatment (hot-rolled plate annealing) can also be performed on the hot-rolled steel plate. The heat treatment conditions are not particularly limited and can be carried out according to conventional methods.

[0129] Next, the pickled plate is cold-rolled to produce a cold-rolled plate. At this time, it is important to satisfy the following conditions.

[0130] [Accumulated reduction ratio of cold rolling: 20% - 90%]

[0131] By setting the cumulative reduction ratio of cold rolling to 20% or more, the surface enrichment index of Sb during annealing can be increased, and the maximum increase rate of C concentration in the thickness direction from the steel plate surface can be reduced. On the other hand, if the cumulative reduction ratio of cold rolling exceeds 90%, the amount of austenite generated at the annealing temperature increases, and the C detachment from the steel plate surface layer during annealing does not occur. As a result, the maximum increase rate of C concentration in the thickness direction from the steel plate surface increases, and the desired bendability cannot be achieved. In addition, if the cumulative reduction ratio exceeds 90%, there is a possibility that the area ratio of retained austenite exceeds 20%. Therefore, the cumulative reduction ratio of cold rolling is set to 20% to 90%. The cumulative reduction ratio of cold rolling is preferably 25% or more, more preferably 27% or more. The cumulative reduction ratio of cold rolling is preferably 75% or less, more preferably 70% or less, and further preferably 60% or less. It should be noted that since strain can be introduced uniformly and efficiently, a uniform structure can be obtained. Therefore, cold rolling is preferably carried out by multi-pass rolling that requires two or more passes, such as tandem multi-stand rolling or reversing rolling.

[0132] It should be noted that the number of rolling passes and the reduction ratio of each pass are not particularly limited, and conventional methods can be used.

[0133] [Annealing process]

[0134] The cold-rolled sheet obtained as described above is annealed (annealing process). At this time, it is important to satisfy the following conditions.

[0135] [Average heating rate in the temperature range of 250°C to 700°C: 100°C / s or less]

[0136] By reducing the average heating rate in the temperature range of 250°C to 700°C, the surface enrichment index of Sb can be increased, and the maximum increase rate of C concentration in the thickness direction from the steel plate surface can be reduced. Therefore, the average heating rate in the temperature range of 250°C to 700°C is set to 100°C / s or less. The average heating rate in the temperature range of 250°C to 700°C is preferably 80°C / s or less, more preferably 60°C / s or less. In addition, the lower limit of the average heating rate in the temperature range of 250°C to 700°C is not particularly specified, but in order to make TS more suitable, it is preferably 5°C / s or more, more preferably 10°C / s or more.

[0137] [Residence time at a heating temperature of 750°C or higher: 10 s or more]

[0138] By extending the residence time below the heating temperature above 750°C, the surface enrichment index of Sb can be increased, and the maximum increase rate of C concentration in the thickness direction from the steel plate surface can be reduced. To obtain such an effect, the residence time below the heating temperature above 750°C is set to 10 s or more. It should be noted that the upper limit of the residence time below the heating temperature above 750°C is not particularly specified. To make TS more appropriate, it is preferably 500 s or less. The residence time below the heating temperature above 750°C is preferably 20 s or more, more preferably 30 s or more. The residence time below the heating temperature above 750°C is more preferably 300 s or less.

[0139] [Heating temperature: 780°C or above]

[0140] When the heating temperature (annealing temperature) is less than 780°C, it becomes an annealing treatment in the ferrite-austenite dual phase region. Therefore, a large amount of ferrite is contained after annealing, and it is difficult to achieve the desired YR. In addition, the surface enrichment index of Sb cannot be increased. Therefore, the heating temperature is set to 780°C or above. It should be noted that the upper limit of the heating temperature is not particularly specified, but the amount of austenite generated during annealing increases, and it is difficult for C to detach from the steel plate surface during annealing. As a result, the maximum increase rate of C concentration in the thickness direction from the steel plate surface increases, and the bendability decreases. Therefore, it is preferably 980°C or less. The heating temperature is preferably 790°C or above, more preferably 800°C or above. In addition, the heating temperature is more preferably 950°C or less. It should be noted that the heating temperature is measured based on the temperature of the steel plate surface.

[0141] It should be noted that the holding time at the heating temperature is not particularly limited and is preferably 10 s to 600 s.

[0142] [Oxygen concentration in the atmosphere below the heating temperature above 750°C: 0.5 vol% - 5.0 vol% (suitable conditions)]

[0143] During annealing, by increasing the oxygen concentration below the heating temperature of 750 °C and decarburizing with oxygen in the air, the maximum increase rate of the C concentration in the plate thickness direction from the steel plate surface can be further reduced. To obtain such an effect, the oxygen concentration below the heating temperature of 750 °C is preferably 0.5% by volume or more, more preferably 1.0% by volume or more, and further preferably 1.5% by volume or more. On the other hand, as the oxygen concentration below the heating temperature of 750 °C increases, the maximum increase rate of the C concentration in the plate thickness direction from the steel plate surface decreases, and the TS decreases. Therefore, the oxygen concentration below the heating temperature of 750 °C is preferably 5.0% by volume or less, more preferably 4.5% by volume or less, and further preferably 4.0% by volume or less. It should be noted that the temperature below the heating temperature of 750 °C is based on the steel plate surface temperature. That is, when the steel plate surface temperature is below the heating temperature of 750 °C, the oxygen concentration is adjusted to the above range.

[0144] [Dew point of the atmosphere below the heating temperature of 750 °C: -35 °C or higher (preferred condition)]

[0145] During annealing, by increasing the dew point of the atmosphere below the heating temperature of 750 °C and decarburizing with moisture in the air, the maximum increase rate of the C concentration in the plate thickness direction from the steel plate surface can be further reduced. To obtain such an effect, the dew point below the heating temperature of 750 °C is preferably -35 °C or higher, more preferably -30 °C or higher, and further preferably -25 °C or higher. It should be noted that the upper limit of the dew point of the atmosphere below the heating temperature of 750 °C is not particularly specified, but since the maximum increase rate of the C concentration in the plate thickness direction from the steel plate surface decreases and the TS decreases, the dew point of the atmosphere below the heating temperature of 750 °C is preferably 15 °C or lower, more preferably 5 °C or lower. It should be noted that the temperature below the heating temperature of 750 °C is based on the steel plate surface temperature. That is, when the steel plate surface temperature is below the heating temperature of 750 °C, the dew point is adjusted to the above range.

[0146] [Cooling process]

[0147] After the annealing process, the cold-rolled sheet is optionally cooled (cooling process). The average cooling rate below the heating temperature and above 400 °C is not particularly limited, but is preferably 5 °C / s to 30 °C / s. In addition, in the temperature range below the heating temperature and above 400 °C, the high-strength steel plate can also be temporarily cooled and the steel plate temperature can be increased again. It should be noted that in the present invention, the so-called "cooling stop temperature" refers to the temperature at which the management of the cooling rate and the holding temperature is stopped for the first time after dropping below 400 °C in the cooling process.

[0148] [Average cooling rate in the temperature range of 250 °C to 400 °C: 1.0 °C / s or higher (preferred condition)]

[0149] The average cooling rate in the temperature range of 250°C to 400°C is preferably 1.0°C / s or more, more preferably 2.0°C / s or more, and still more preferably 3.0°C / s or more. If the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more, the amount of bainite ferrite contained after annealing can be further reduced, and the YR and bendability can be further improved. It should be noted that the upper limit of the average cooling rate in the temperature range of 250°C to 400°C is not particularly specified. From the constraints of production technology, it is preferably 100.0°C / s or less, more preferably 80.0°C / s or less. When the cooling stop temperature exceeds 250°C, the average cooling rate is the value in the temperature range from the cooling stop temperature to 400°C or less. It should be noted that the average cooling rate is measured based on the temperature of the steel plate surface.

[0150] It should be noted that as the cooling method in the temperature range of 250°C to 400°C, gas jet cooling, water mist cooling, water cooling, air cooling, etc. can be applied.

[0151] [Insulation temperature in the cooling process: 100°C to 450°C (suitable conditions)]

[0152] In the cooling process, it is preferably insulated for 5 s or more at the insulation temperature in the temperature range of 100°C to 450°C. By insulating the high-strength steel plate with the temperature range within the above range, the YR and bendability can be made within a more appropriate range. In addition, the area ratio of bainite ferrite can be further reduced, and the TS can be further improved. The insulation temperature in the cooling process is more preferably 150°C or more, and still more preferably 200°C or more. In addition, the insulation temperature in the cooling process is more preferably 400°C or less, and still more preferably 350°C or less. It should be noted that the temperature in the cooling process is based on the steel plate surface temperature.

[0153] [Insulation time in the cooling process: 5 s or more (suitable conditions)]

[0154] By insulating at the insulation temperature in the cooling process, the YR and bendability can be made within a more appropriate range. To obtain such an effect, the insulation time at the insulation temperature in the cooling process is preferably 5 s or more, more preferably 10 s or more, and still more preferably 15 s or more. It should be noted that the upper limit of the insulation time at the insulation temperature in the cooling process is not particularly specified. To make the TS within a more appropriate range, the insulation time at the insulation temperature in the cooling process is preferably 500 s or less, more preferably 250 s or less.

[0155] [Cooling stop temperature: 250°C or less (suitable conditions)]

[0156] In the above cooling process, the cooling stop temperature is preferably 250°C or lower, more preferably 200°C or lower. If the cooling stop temperature is 250°C or lower, a large amount of retained austenite can be prevented from being generated after annealing, and the bendability can be further improved. It should be noted that the lower limit of the cooling stop temperature is not particularly specified, and from the viewpoint of productivity, it is preferably above room temperature. In addition, the cooling stop speed is measured based on the temperature of the steel plate surface.

[0157] It should be noted that when the cooling stop temperature is less than 250°C, the average cooling speed from 250°C to the cooling stop temperature is not particularly specified, but in order to further improve TS, the average cooling speed from 250°C to the cooling stop temperature is preferably 1°C / s or more, more preferably 2°C / s or more. On the other hand, due to production technology constraints, the average cooling speed from 250°C to the cooling stop temperature is preferably 1000°C / s or less, more preferably 150°C / s or less.

[0158] It should be noted that the cold-rolled sheet can also be cooled from the cooling stop temperature to room temperature. The average cooling speed from the cooling stop temperature to room temperature is not particularly limited, and it can be cooled to room temperature by any method. As the cooling method, gas jet cooling, water mist cooling, water cooling, air cooling, etc. can be applied.

[0159] The high-strength steel plate annealed as described above can be cooled to the cooling stop temperature and then rolled. The elongation of the rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation of the rolling performed after cooling to the cooling stop temperature to 0.05% or more, YR can be controlled within a desired range. In addition, the elongation of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation of the rolling after cooling to the cooling stop temperature to 2.00% or less, the volume fraction of retained austenite can be in a more appropriate range, and the bendability and the damage degree of the sheared end face in a corrosive environment can be in a more appropriate range.

[0160] The rolling after cooling to the cooling stop temperature can be performed on a device continuous with the above continuous annealing device (online), or can be performed on a device discontinuous with the above continuous annealing device (offline). In addition, the target elongation can be achieved by one rolling, or multiple rollings can be performed to achieve a total elongation of 0.05% to 2.00%. It should be noted that the rolling described here generally refers to temper rolling, but as long as an elongation equivalent to temper rolling can be applied, it can also be a method using repeated bending with a tension leveling machine, rolls, etc.

[0161] [Reheating process]

[0162] [Reheating temperature: not less than (cooling stop temperature + 50°C) and not more than 450°C (preferred condition)]

[0163] After cooling to the cooling stop temperature, or after further rolling after cooling to the cooling stop temperature, the high-strength steel sheet can be reheated (reheating process). It should be noted that when heat preservation is carried out during the cooling process, the reheating process is not carried out. By reheating the high-strength steel sheet, YR and bendability can be made to be in a more appropriate range. To obtain such an effect, the reheating temperature is preferably not less than (cooling stop temperature + 50°C), more preferably not less than (cooling stop temperature + 100°C), and further preferably not less than (cooling stop temperature + 150°C). On the other hand, as the reheating temperature increases, tempering of martensite occurs and TS decreases. Therefore, the reheating temperature is preferably not more than 450°C, more preferably not more than 400°C, and further preferably not more than 380°C. It should be noted that the above reheating temperature is based on the surface temperature of the steel sheet.

[0164] [Holding time at the reheating temperature: not less than 5 s (preferred condition)]

[0165] By holding at the reheating temperature, YR and bendability can be made to be in a more appropriate range. To obtain such an effect, the holding time at the reheating temperature is preferably not less than 5 s, more preferably not less than 10 s, and further preferably not less than 15 s. It should be noted that there is no particular limitation on the upper limit of the holding time at the reheating temperature. However, to make TS be in a more appropriate range, the holding time at the reheating temperature is preferably not more than 500 s, more preferably not more than 250 s.

[0166] It should be noted that the cooling rate from the reheating temperature to room temperature is not particularly limited, and it can be cooled to room temperature by any method. As the cooling method, gas jet cooling, water mist cooling, water cooling, air cooling, etc. can be applied.

[0167] It should be noted that when the high-strength steel sheet is used as the treatment object, it is usually used as the treatment object after cooling to room temperature.

[0168] [Manufacturing method of high-strength plated steel sheet]

[0169] Next, a method for manufacturing a high-strength plated steel sheet according to an embodiment of the present invention will be described. The method for manufacturing a high-strength plated steel sheet according to an embodiment of the present invention includes the above-described method for manufacturing a high-strength steel sheet and a step of forming a plating layer (plating treatment) on at least one side of the cold-rolled sheet after the annealing step. For example, as the plating treatment, hot-dip galvanizing treatment, an alloying treatment after hot-dip galvanizing can be exemplified. In addition, annealing and galvanizing can be continuously performed on one production line. Further, a plating layer can be formed by electroplating such as electroplating Zn-Ni alloy, or hot-dip galvanized-aluminum-magnesium alloy can be applied. It should be noted that the above description has been centered on the case of galvanizing, but the type of plating metal such as Zn plating and Al plating is not particularly limited.

[0170] It should be noted that when performing hot-dip galvanizing treatment, it is preferable to immerse the high-strength steel sheet in a galvanizing bath at 440°C to 500°C to perform hot-dip galvanizing treatment, and then adjust the plating adhesion amount by gas wiping or the like. In hot-dip galvanizing, a galvanizing bath with an Al content of 0.10 mass% to 0.23 mass% is preferably used. In addition, after hot-dip galvanizing, when performing an alloying treatment of galvanizing, the temperature range is preferably 470°C to 600°C, more preferably 470°C to 560°C. By performing the alloying treatment at 470°C or higher, the Zn-Fe alloying rate can be made more appropriate and the productivity can be made more appropriate. In addition, by performing the alloying treatment at 600°C or lower, it is possible to prevent the untransformed austenite from transforming into pearlite and the TS is more appropriate. In addition, electrogalvanizing treatment can also be performed. In addition, the plating adhesion amount per side is preferably 20 to 80 g / m 2 (double-sided plating), for alloyed hot-dip galvanized steel sheet (GA), it is preferable to make the Fe concentration in the plating layer 7 to 15 mass% by performing the following alloying treatment.

[0171] It should be noted that regarding the plating treatment, after the above annealing step, the high-strength steel sheet can be treated in a temperature range below the heating temperature and above 400°C without cooling, or the cold-rolled steel sheet can be temporarily cooled to less than 400°C, and then the steel sheet temperature can be raised again to above 400°C, and then the plating treatment can be performed.

[0172] Rolling can be performed on the high-strength steel sheet after the above plating treatment. The elongation rate of rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate of rolling performed after the plating treatment to 0.05% or more, the YR can be controlled within a desired range. In addition, the elongation rate of rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of rolling in the plating treatment to 2.00% or less, the volume fraction of retained austenite can be made within a more appropriate range, and the bendability and the damage degree of the shear end face in a corrosive environment can be made within a more appropriate range.

[0173] The rolling after the plating treatment can be carried out on a device continuous with the above-mentioned continuous annealing device (online), or can be carried out on a device discontinuous with the above-mentioned continuous annealing device (offline). In addition, the target elongation can be achieved by one rolling, or multiple rollings can be carried out to achieve a total elongation of 0.05% to 2.00%. It should be noted that the rolling described here generally refers to temper rolling, but as long as it can apply the same elongation as temper rolling, it can also be a method of processing by repeatedly bending using a tension leveling machine, rolls, etc. In addition, a re-heating treatment can also be carried out after the rolling after the plating treatment.

[0174] The conditions of other manufacturing methods are not particularly limited. From the viewpoint of productivity, the above-mentioned annealing, hot-dip galvanizing, alloying treatment of galvanizing, etc. are preferably carried out in a CGL (Continuous Galvanizing Line) which is a hot-dip galvanizing production line. Wiping can be carried out after hot-dip galvanizing to adjust the plating amount. It should be noted that the conditions of plating, etc. other than the above-mentioned conditions follow the conventional methods of hot-dip galvanizing.

[0175] It should be noted that when using a high-strength plated steel sheet as the processing object, it is usually used as the processing object after cooling to room temperature.

[0176] It should be noted that the manufacturing conditions other than the above-mentioned conditions can follow the conventional methods.

[0177] [Component]

[0178] Next, the component according to an embodiment of the present invention will be described.

[0179] The component according to an embodiment of the present invention is a component made of the high-strength steel sheet or high-strength plated steel sheet according to an embodiment of the present invention described above. The component according to an embodiment of the present invention is, for example, a component formed into a target shape by stamping the high-strength steel sheet according to an embodiment of the present invention described above. The component according to an embodiment of the present invention is preferably a component for a skeleton structure component of an automobile or a reinforcing component of an automobile.

[0180] Here, the high-strength steel sheet according to an embodiment of the present invention is a high-strength steel sheet excellent in YR and bendability and with a reduced amount of hydrogen trapped in the steel in an acid immersion environment. Therefore, the component according to an embodiment of the present invention can contribute to the weight reduction of the vehicle body, and thus can be particularly suitable for all components for a skeleton structure component of an automobile or a reinforcing component of an automobile.

[0181] Examples

[0182] A steel having the composition shown in Table 1 and the balance consisting of Fe and inevitable impurities is melted in a converter and made into a slab by continuous casting. After cooling the steel billet above 1000 °C to room temperature, it is heated and then hot-rolled to obtain a hot-rolled sheet. After subjecting the hot-rolled sheet to pickling treatment, cold rolling is carried out to obtain a cold-rolled sheet. Then, the cold-rolled sheet is annealed. Table 2 shows the cooling conditions, hot-rolling conditions, pickling conditions, cold-rolling conditions, and annealing conditions after casting.

[0183]

[0184]

[0185] Next, the cold-rolled sheet is cooled under the conditions shown in Table 2. In some examples, heat preservation or reheating is further carried out to obtain a high-strength cold-rolled steel sheet (CR, uncoated steel sheet). Furthermore, in some examples, a plating treatment is carried out to obtain any one of a hot-dip galvanized steel sheet (GI), an alloyed hot-dip galvanized steel sheet (GA), or an electro-galvanized steel sheet (EG). In the example of obtaining EG, the electro-galvanizing treatment is carried out after cooling and reheating the cold-rolled sheet (i.e., after obtaining CR). In the examples of obtaining GI and GA, the plating treatment is carried out after annealing, and then cooling and reheating are carried out. It should be noted that in Table 2, an example where the heat preservation condition or reheating condition in the cooling process is "-" means that this treatment is not carried out. In addition, in Table 2, an example where the cooling stop temperature is "-" means that cooling is carried out after heat preservation, but the cooling stop temperature is not controlled. In addition, in an example where the cooling stop temperature is not room temperature, a reheating treatment is carried out immediately after reaching the cooling stop temperature shown in Table 2.

[0186] For the hot-dip galvanizing bath, a zinc bath containing Al: 0.14 to 0.19 mass% is used for GI, and a zinc bath containing Al: 0.14 mass% is used for GA, and the bath temperature is 470 °C. For the plating adhesion amount, for GI, it is about 45 to 72 g / m 2 (double-sided plating) on each side, and for GA, it is about 45 g / m 2 (double-sided plating) on each side. In addition, for GA, the Fe concentration in the coating is set to 9 mass% to 12 mass%. For EG, the coating is a Zn-Ni coating, and the Ni content in the coating is set to 9 mass% to 25 mass%. The plating adhesion amount of EG is set to 15 g / m 2 to 100 g / m 2 (double-sided plating).

[0187] The obtained high-strength cold-rolled steel sheet and high-strength plated steel sheet are used as the test steels. According to the above method, the area ratios of martensite and ferrite, the volume ratio of retained austenite, the surface enrichment index of Sb, and the maximum increase rate of C concentration along the plate thickness direction from the steel sheet surface are obtained. In addition, for the remaining part of the microstructure, it is observed according to the method described below. After cutting out a specimen with the plate thickness cross-section (L-section) parallel to the rolling direction of the steel sheet as the observation surface, the observation surface is mirror-polished using diamond polishing paste, then finely polished using colloidal silica, and further etched using 3 vol.% nitric acid ethanol to reveal the microstructure. Under the condition of an accelerating voltage of 15 kV, using SEM, with the 1 / 4 position of the steel sheet thickness as the observation position, three fields of view are observed at a magnification of 5000 times in a field of view range of 17 μm × 23 μm. Carbides are identified as the remaining part of the microstructure from the obtained microstructure images. The results are shown in Table 3.

[0188] Furthermore, according to the following test methods, the tensile properties, bendability, and the amount of hydrogen trapped in the steel in an acid pickling environment are evaluated, and the results are shown in Table 3.

[0189] [Tensile Test]

[0190] The tensile test is carried out in accordance with JIS Z 2241:2021. JIS No. 5 test pieces are sampled from the obtained steel sheet in a direction perpendicular to the rolling direction of the steel sheet, and a tensile test is carried out under the condition of a crosshead speed of 1.67×10 －1 mm / s to measure YS and TS. It should be noted that in the present invention, a case where the yield ratio (YR) is 65% or more is judged to be good. It should be noted that YR is calculated according to the calculation method described in the above formula (1).

[0191] [Bend Test]

[0192] The bending test was conducted in accordance with JIS Z 2248:2022. Strip specimens with a width of 30 mm and a length of 100 mm were collected 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. Then, a 90° V-bending test was carried out under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. It should be noted that in the present invention, the bendability is evaluated by the pass rate of the bending test. The maximum R value where the value obtained by dividing the bending radius (R) by the plate thickness (t) is 5 or less (for example, when the plate thickness is 1.2 mm, the bending radius is 6.0 mm). The bending tests of 5 samples were carried out. Next, it was evaluated whether cracks occurred in the ridge line part of the bending vertex. The case where no cracks were found in all 5 samples was evaluated as "excellent" bendability. In addition, the case where one or more of the 5 samples had microcracks less than 200 μm was judged as "good" bendability. Furthermore, the case where one or more of the 5 samples had microcracks of 200 μm or more was judged as "poor" bendability. Here, the presence or absence of cracks was evaluated by measuring the ridge line part of the bending vertex at a magnification of 40 times using a digital microscope (RH-2000: manufactured by HiROX Co., Ltd.).

[0193] [Hydrogen capture amount in steel in an acid immersion environment]

[0194] Using a crank press, strip specimens with a width (in the direction perpendicular to rolling of the base steel plate): 75 mm and a length (rolling direction): 18 mm were sheared and collected with the shearing separation surface in the direction perpendicular to the rolling of the base steel plate. It should be noted that the shearing conditions of the crank press were set as a shearing angle: 0.5 degrees and a clearance: 10%. Next, the shedding side during the shearing of the crank press was ground by 2 mm to make strip specimens with a width: 75 mm and a length: 16 mm. Next, 0.1 wt% ammonium thiocyanate aqueous solution and McIlvaine buffer solution (pH 4.0) were respectively immersed in a solution in which each was mixed by 50% by volume for 96 hours, then taken out and immersed in liquid nitrogen. Next, after collecting 2 strip specimens with a width: 30 mm and a length: 8 mm from the center of the steel plate, they were rust-removed with a stirrer while being appropriately immersed in liquid nitrogen so that the temperature of the steel plate was not above room temperature, and the 2 specimens were collectively used to measure the amount of hydrogen released from the specimens when heating the test specimens by the temperature rise desorption analysis method. Specifically, the test specimens were heated from room temperature to a temperature rise arrival temperature: 300 °C at a heating rate: 200 °C / hr, and then cooled to room temperature. At this time, the cumulative hydrogen amount released from the test specimens in the temperature range from room temperature to 200 °C during heating (hereinafter, also referred to as the cumulative release hydrogen amount) was measured. The measurement of the hydrogen amount was carried out using a steel hydrogen measurement system (JTF-20AL, manufactured by J-Science Lab Co., Ltd.). Then, the hydrogen capture amount in steel in an acid immersion environment was calculated according to the following formula (4).

[0195] [Amount of hydrogen trapped in acid pickling environment (ppm by mass)] = [Cumulative hydrogen release amount (g)] ÷ [Mass of test piece (g)] × 10 6 ····(4)

[0196] In addition, if the room temperature is in the range of 10 to 25 °C, there is no particular influence on the measurement of the amount of hydrogen trapped in steel in an acid pickling environment. Among them, when the room temperature is outside the range of 10 to 25 °C, 25 °C is taken as the representative temperature of the room temperature, and the target cumulative hydrogen release amount from the test piece in the temperature range from 25 °C to 200 °C is measured.

[0197] Table 3

[0198]

[0199] Underlined part: Indicates outside the scope of the present invention.

[0200] M: Martensite, α: Ferrite, retained γ: Retained austenite; θ: Cementite and / or metastable carbide

[0201] As shown in Table 3, in the examples of the present invention, the amount of hydrogen trapped in YR and steel in an acid pickling environment is excellent. On the other hand, in the comparative examples, one or more of the amount of hydrogen trapped in YR and steel in an acid pickling environment are poor.

[0202] In addition, in the examples of the present invention, in the examples where the maximum increase rate of the C concentration in the thickness direction from the steel plate surface is 0.1000% by mass / μm or less, the volume fraction of retained austenite is 15.0% or less, and the cumulative reduction ratio in the cold rolling process is 75% or less, in addition to the above characteristics, the bendability is also excellent. In the skeleton structure components of automobiles, etc., for example, the collision box, etc. has a bent processing part. Therefore, for such components, from the viewpoint of formability, it is preferable to apply a steel plate with high bendability.

[0203] The above describes the embodiments of the present invention, but the present invention is not limited to the description that constitutes a part of the present invention of this embodiment. That is, all other embodiments, examples, and application techniques, etc. carried out by those skilled in the art based on this embodiment are included in the scope of the present invention. For example, in a series of heat treatments in the above manufacturing method, as long as the heat history conditions are satisfied, the equipment for heat treating the steel plate is not particularly limited.

[0204] Industrial applicability

[0205] According to the present invention, a high-strength steel sheet with excellent YR and reduced hydrogen trapping amount in steel under an acid pickling environment can be obtained. In particular, since the high-strength steel sheet has a reduced hydrogen trapping amount in steel under an acid pickling environment, it can be applied to frame structure components of automobiles of various sizes and shapes while achieving high component strength. Thus, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, and it has great industrial utility value.

Claims

1. A high-strength steel sheet having the following composition and steel structure, The composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 3.00%, Mn: 0.10% to 6.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, O: 0.0100% or less, and Sb: 0.002% to 0.300%, and the balance is composed of Fe and inevitable impurities. In the steel structure, at the 1 / 4 position of the plate thickness, the area ratio of martensite is 30% or more, the area ratio of ferrite is 70% or less, and the volume ratio of retained austenite is 20.0% or less. And, the surface enrichment index of Sb is 1.2 or more.

2. The high-strength steel sheet according to claim 1, 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, 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.

3. The high-strength steel sheet according to claim 1 or 2, wherein, The maximum increase rate of the C concentration in the direction of the plate thickness from the surface of the steel sheet is 0.1000 mass% / μm or less.

4. A high-strength coated steel sheet having the high-strength steel sheet according to any one of claims 1 to 3 and a coating formed at least on one side of the high-strength steel sheet.

5. A method for manufacturing a high-strength steel sheet, Prepare a steel billet having the composition according to claim 1 or 2, Cool the steel billet with an average cooling rate in the temperature range of 700°C to 1000°C of 1500°C / hr or less. Then, heat the steel billet to a slab heating temperature of 1150°C or more, and set the residence time from 1100°C to the slab heating temperature to 20 minutes or more. Perform hot rolling on the steel billet with the reduction ratio of the first pass of finish rolling set to 20% or more to produce a hot-rolled sheet. Then, perform pickling on the hot-rolled sheet to produce a pickled sheet with an average front and back thickness of the internal oxide layer of 0.2 μm or more. Then, perform cold rolling on the pickled sheet with the cumulative reduction ratio set to 20% to 90% to produce a cold-rolled sheet. Then, perform an annealing process of heating the cold-rolled sheet to a heating temperature of 780°C or more. At this time, set the average heating rate in the temperature range of 250°C to 700°C to 100°C / s or less, and set the residence time from 750°C to the heating temperature to 10 s or more. Then, perform a cooling process of cooling the cold-rolled sheet.

6. The manufacturing method of the high-strength steel sheet according to claim 5, wherein, In the annealing process, the oxygen concentration of the atmosphere at 750°C to the heating temperature is 0.5 vol% to 5.0 vol%, and the dew point of the atmosphere is set to -35°C or higher.

7. The manufacturing method of the high-strength steel sheet according to claim 5 or 6, wherein, In the cooling process of the cold-rolled sheet, the average cooling rate in the temperature range of 250°C to 400°C is set to 1.0°C / s or higher.

8. The manufacturing method of the high-strength steel sheet according to any one of claims 5 to 7, wherein, In the cooling process of the cold-rolled sheet, it is held for 5 s or longer at a holding temperature of 100°C to 450°C.

9. The manufacturing method of the high-strength steel sheet according to any one of claims 5 to 7, wherein, In the cooling process of the cold-rolled sheet, the cooling stop temperature is set to 250°C or lower. Then, the cold-rolled sheet is reheated to a reheating temperature that is higher than (the cooling stop temperature + 50°C) and lower than 450°C, and is held at this reheating temperature for 5 s or longer.

10. A method for manufacturing a high-strength plated steel sheet, comprising the method for manufacturing a high-strength steel sheet according to any one of claims 5 to 9 and a process of forming a plating layer on at least one side of the cold-rolled sheet after the annealing process.

11. A component, at least a part of which is made of the high-strength steel sheet according to any one of claims 1 to 3 or the high-strength plated steel sheet according to claim 4.

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