Title of invention: High-strength steel sheet
Technical field
[0001]
　The present invention relates to a high-strength steel sheet, specifically, a high-strength steel sheet having a tensile strength of 1300 MPa or more and having excellent seizure curability, which is suitable mainly for structural members such as automobiles used by being pressed. is there.
　The present application claims priority based on Japanese Patent Application No. 2018-141244 filed in Japan on July 27, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In recent years, in order to protect the global environment, improvement in fuel efficiency of automobiles has been required, and in order to reduce the weight and ensure safety of automobile steel sheets, further increase in strength is required. When the strength of the steel sheet is increased, the ductility is generally lowered, which makes cold press forming difficult. Therefore, there is a demand for a material that is relatively soft during molding and is easy to mold, and has high strength after molding, that is, a material having a high baking hardening amount.
[0003]
　The baking hardening is performed by diffusing penetrating elements (carbon and nitrogen) into dislocations formed by press molding (hereinafter, also referred to as “pre-strain”) during coating baking at 150 ° C. to 200 ° C. to fix the dislocations. It is a strain aging phenomenon that occurs.
[0004]
　As shown in Non-Patent Document 1, the amount of baking hardened depends on the amount of penetrating elements in solid solution, that is, the amount of solid solution carbon. Therefore, the amount of baking hardening is higher in martensite, which has a large amount of carbon that can be solid-solved than ferrite, which has a small amount of carbon that can be solid-solved. In this regard, for example, Patent Document 1 discloses a high-strength steel sheet mainly composed of bainite and martensite. In the high-strength steel sheet disclosed in Patent Document 1 , the seizure curability is improved by heating the steel material to a temperature range of 3 Ac points or more and then performing a predetermined treatment to increase the dislocation density. ..
[0005]
　On the other hand, the amount of strain introduced by press molding generally differs depending on the specific conditions and location of the molding process. Therefore, in order to surely improve the seizure curability of the steel sheet even if there is a difference in the amount of strain, it is necessary to uniformly develop the seizure hardening by the same amount regardless of the amount of strain. For that purpose, it is important to evaluate not only by the amount of baking hardening by one prestrain but also by the amount of baking hardening by a plurality of prestrains to produce a material having a small prestrain dependence of the above amount of baking hardening. Become.
[0006]
　However, in Patent Document 1, since only the amount of baking cure when the prestrain is 1% is disclosed in the examples, the amount of baking hardening when the prestrain is other is unknown. The dislocation density is also important as a control factor for the amount of baking hardening, but as shown in Non-Patent Documents 2 and 3, if the dislocation density is too high, the amount of carbon segregation per unit length of dislocations may decrease. , The movable dislocations may be reduced due to the interaction between dislocations. Therefore, as in Patent Document 1, simply increasing the dislocation density may increase the pre-strain dependence of the amount of baking hardening, and as a result, reduce the amount of baking hardening.
[0007]
　As described above, among the steel sheets having excellent seizure curability, (1) the seizure hardening amount is large and (2) the pre-strain dependence of the seizure hardening amount is small (hereinafter, referred to as "uniform seizure curability is high"). It is difficult to achieve both.
Prior art literature
Patent documents
[0008]
Patent Document 1: Japanese Patent Application Laid-Open No. 2008-144233
Non-patent literature
[0009]
Non-Patent Document 1: K.K. Nakaoka, et al. , "Strength, Ductility and Aging Properties of Continuously-Annealed Dual-Phase High-Strength Sheet Steels", Formable HSLA and D.S. Soc. of AIME, (1977) 126-141
Non-Patent Document 2: C.I. Kuang, et al. , "Effective of Temper Rolling on the Bake-Hardening Behavior of Low Carbon Steel", International Journal of Minerals, Metallurgy Unt
. Impact of ", Proceedings of the Japan Society of Mechanical Engineers, 45 (1979) 983-989
Outline of the invention
Problems to be solved by the invention
[0010]
　In order to meet the demand for higher strength in the future, it is necessary to ensure excellent seizure curability. The excellent seizure curability referred to here is (1) a large amount of seizure curing and (2) high uniform seizure curability. However, it is difficult to achieve both (1) and (2) in an ordinary organization having martensite as the main phase, as in Patent Document 1.
[0011]
　Therefore, an object of the present invention is to provide a high-strength steel sheet having a large amount of baking cure and high uniform baking hardening.
Means to solve problems
[0012]
　The present inventors considered that the amount of solid solution carbon and the dislocation density should not be focused on in order to achieve the above object. This is because the amount of solid solution carbon is sufficiently present in martensite, and uniform seizure curability cannot be guaranteed as in Patent Document 1 if the dislocation density is controlled. Therefore, the present inventors considered that it is important to pay attention to the dislocation formation behavior in which seizure hardening is likely to occur.
[0013]
　Dislocations generally refer to linear crystal defects, but for example, when they are entangled to form dislocation cells, they become immobile by themselves. In such a case, the amount of dislocations that adhere due to carbon or the like diffused during baking hardening is reduced, and as a result, the baking hardening amount is reduced. In general, the ease with which dislocation cells are generated depends on the amount of pre-strain, so the amount of baking hardening varies greatly depending on the amount of pre-strain. Therefore, the present inventors considered that the seizure curability could be improved by suppressing the cell formation of dislocations, and studied diligently.
[0014]
　As a result, the present inventors have found that the cell formation of dislocations can be suppressed by precipitating a large amount of precipitates, for example, iron carbides, which are finer than the size of the cells to be formed. The present inventors thought that this would improve the seizure curability, but the precipitation of precipitates such as iron carbides caused a non-uniform hardness difference in the structure, which rather promoted the cell formation of dislocations. The problem arose.
[0015]
　This non-uniform hardness difference was caused by precipitation strengthening due to non-uniform precipitation of the precipitate. The present inventors have found that such non-uniform precipitation occurs from microsegregation, and more specifically, from microsegregation of Si required for precipitating precipitates. In general, microsegregation is a phenomenon in which the concentration of alloying elements generated from solidification is unevenly distributed, and planes perpendicular to the plate thickness direction are connected in layers.
[0016]
　Therefore, the present inventors control the heat spreading process to suppress the microsegregation of Si by making it into a complicated shape to have a uniform structure (hereinafter referred to as a uniform structure), and to produce a finely large amount of precipitates such as iron carbides. It has been found that the seizure curability is greatly improved by uniformly and uniformly precipitating.
[0017]
　The high-strength steel sheet having excellent seizure curability of the present invention that has achieved the above object in this way is as follows.
(1) In terms of mass%,
　C: 0.13 to 0.40%,
　Si: 0.500 to 3.000%,
　Mn: 2.50 to 5.00%,
　P: 0.100% or less,
　S: It contains 0.010% or less,
　Al: 0.001 to 2.000%,
　N: 0.010% or less
, the balance consists of Fe and impurities,
　and contains martensite with an area ratio of 95% or more, and the balance. The structure is 5% or less in area ratio,
　the ratio C1 / C2 of the upper limit value C1 (mass%) and the lower limit value C2 (mass%) of the Si concentration in the cross section in the thickness direction is 1.25 or less, and the
　major axis is 0. A 　high-strength steel plate having a number density of 30 pieces / μm 2 or more of precipitates having an aspect ratio of 1: 3 or more at 0.05 μm or
more and 1.00 μm or less, and a tensile strength of 1300 MPa or more.
(2) The high-strength steel sheet according to (1), wherein the residual structure is made of retained austenite when the residual structure is present.
(3) Further, in mass%,
　Ti: 0.100% or less,
　Nb: 0.100% or less,
　V:
The high-strength steel sheet according to (1) or (2), which contains 0.100% or less of one type or two or more types of 0.100% or less in total.
(4) Further, in
　terms of mass%, one or more of Cu: 1.000% or less,
　Ni: 1.000% or less,
　Mo: 1.000% or less,
　Cr: 1.000% or less
are added in total. The high-strength steel sheet according to any one of (1) to (3), which contains 0.100% or less.
(5) Further, in mass%,
　W: 0.005% or less,
　Ca: 0.005% or less,
　Mg: 0.005% or less
　Rare earth metal (REM): 0.010% or less
One or more types The high-strength steel sheet according to any one of (1) to (4), which contains 0.010% or less in total.
(6) The high-strength steel sheet according to any one of (1) to (5), further containing B: 0.0030% or less in mass%.
The invention's effect
[0018]
　According to the present invention, the microsegregation of Si is made into a uniform structure, and specific precipitates are made to appear on the entire surface of the lath in martensite by heat treatment at a certain temperature, thereby preventing dislocations from forming cells and efficiently dislocations. By diffusing carbon into the dislocations and causing the dislocations to stick to each other, it is possible to provide a high-strength steel plate having excellent seizure curability. This high-strength steel sheet is further increased in strength by being seized during painting after press forming, and is therefore suitable as a structural field in the field of automobiles and the like.
A brief description of the drawing
[0019]
FIG. 1 is an image diagram showing a precipitation state of precipitates in a high-strength steel sheet according to the present invention.
Mode for carrying out the invention
[0020]

　The high-strength steel plate according to the embodiment of the present invention has a mass% of
　C: 0.13 to 0.40%,
　Si: 0.500 to 3.000%, and
　Mn: 2.50 to 5. It contains .00%,
　P: 0.100% or less,
　S: 0.010% or less,
　Al: 0.001 to 2.000%,
　N: 0.010% or less
, and the balance consists of Fe and impurities.
　It contains martensite of 95% or more in area ratio, the residual structure is 5% or less in area ratio, and the ratio of
　the upper limit value C1 (mass%) and the lower limit value C2 (mass%) of the Si concentration in the cross section in the thickness direction. C1 / C2 is 1.25 or less, the
　major axis is 0.05 μm or more and 1.00 μm or less, and the number of precipitates having an aspect ratio of 1: 3 or more is 30 pieces / μm 2 or more, and the
　tensile strength is 1300 MPa or more. It is characterized by being.
[0021]
　First, the chemical composition of the high-strength steel sheet according to the embodiment of the present invention and the slab used for manufacturing the same will be described. In the following description, "%", which is a unit of the content of each element contained in the high-strength steel sheet and the slab, means "mass%" unless otherwise specified.
[0022]
(C: 0.13% to 0.40%)
　C has an effect of increasing the amount of solid solution carbon and enhancing the baking curability. In addition, it has the effect of enhancing hardenability and increasing strength by containing it in the martensite structure. If the C content is less than 0.13%, a sufficient solid solution carbon amount cannot be secured when carbides such as iron carbides are precipitated, and the baking hardening amount decreases. Therefore, the C content is 0.13% or more, preferably 0.16% or more, and more preferably 0.20% or more. On the other hand, if the C content exceeds 0.40%, incomplete martensitic transformation occurs in cooling after annealing, and the retained austenite fraction becomes high, which deviates from the embodiment of the present invention. In addition, the strength is too high to guarantee moldability. Therefore, the C content is 0.40% or less, preferably 0.35% or less.
[0023]
(Si: 0.500% to 3.000%)
　Si is an element necessary for finely and a large amount of precipitates such as iron carbides for suppressing dislocation cells. If the Si content is less than 0.500%, even if the segregation has a uniform structure, sufficient effects cannot be obtained, coarse precipitates are generated, and the formation of dislocation cells cannot be suppressed. Therefore, the Si content is 0.500% or more, more preferably 1.000% or more. On the other hand, if the Si content exceeds 3.000%, the effect of precipitating a large amount of precipitates in a fine manner is saturated, which unnecessarily increases the cost and deteriorates the surface properties. Therefore, the Si content is 3.000% or less, preferably 2.000% or less.
[0024]
(Mn: 2.50% to 5.00%)
　Mn is an element for improving hardenability, and is an element necessary for forming a martensite structure without limiting the cooling rate. In order to effectively exert this effect, the Mn content is 2.50% or more, preferably 3.00% or more. However, the content of excess Mn is 5.00% or less, preferably 4.50% or less because the low temperature toughness is lowered due to the precipitation of MnS.
[0025]
(P: 0.100% or less)
　P is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the P content, the better. In particular, when the P content exceeds 0.100%, the weldability is significantly reduced. Therefore, the P content is 0.100% or less, preferably 0.030% or less. Reducing the P content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the P content may be 0.0001% or more. Further, since P contributes to the improvement of strength, the P content may be 0.0001% or more from such a viewpoint.
[0026]
(S: 0.010% or less)
　S is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the S content, the better. The higher the S content, the higher the amount of MnS deposited and the lower the low temperature toughness. In particular, when the S content exceeds 0.010%, the weldability and low temperature toughness are significantly reduced. Therefore, the S content is 0.010% or less, preferably 0.003% or less. Reducing the S content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the S content may be 0.0001% or more.
[0027]
(Al: 0.001% to 2.000%)
　Al has an effect on deoxidation. In order to effectively exert the above-mentioned actions, the Al content is 0.001% or more, preferably 0.010% or more. On the other hand, when the Al content exceeds 2.000%, the weldability is lowered, oxide-based inclusions are increased, and the surface texture is deteriorated. Therefore, the Al content is 2.000% or less, preferably 1.000% or less.
[0028]
(N: 0.010% or less)
　N is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the N content, the better. In particular, when the N content exceeds 0.010%, the weldability is significantly reduced. Therefore, the N content is 0.010% or less, preferably 0.006% or less. Reducing the N content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the N content may be 0.0001% or more.
[0029]
　The basic composition of the high-strength steel sheet of the present invention and the slab used for its production is as described above. Further, the high-strength steel sheet of the present invention and the slab used for manufacturing the same may contain the following optional elements, if necessary.
[0030]
(Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less)
　Ti, Nb and V contribute to the improvement of strength. Therefore, Ti, Nb or V or any combination thereof may be contained. In order to sufficiently obtain this effect, the content of Ti, Nb or V, or the total content of any combination of two or more of these, is preferably 0.003% or more. On the other hand, if the content of Ti, Nb or V, or the total content of any combination of two or more of these is more than 0.100%, hot rolling and cold rolling become difficult. Therefore, the total content of Ti content, Nb content or V content, or any combination of two or more of these is set to 0.100% or less. That is, the limiting range in the case of each component alone is Ti: 0.003% to 0.100%, Nb: 0.003% to 0.100%, and V: 0.003% to 0.100%. At the same time, the total content when these are arbitrarily combined is preferably 0.003 to 0.100%.
[0031]
(Cu: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cr: 1.000% or less)
　Cu, Ni, Mo and Cr contribute to the improvement of strength. Therefore, Cu, Ni, Mo, Cr or any combination thereof may be contained. In order to sufficiently obtain this effect, the content of Cu, Ni, Mo and Cr is preferably in the range of 0.005 to 1.000% when each component is used alone, and two or more of these are arbitrarily combined. It is preferable that the total content is 0.005% or more and 1.000% or less. On the other hand, if the content of Cu, Ni, Mo and Cr, or the total content of any combination of two or more of these is more than 1.000%, the effect of the above action is saturated and the cost is unnecessarily high. It gets higher. Therefore, the upper limit of the content of Cu, Ni, Mo and Cr, or the total content when two or more of these are arbitrarily combined is 1.000%. That is, Cu: 0.005% to 1.00%, Ni: 0.005% to 1.000%, Mo: 0.005% to 1.000%, and Cr: 0.005% to 1.000%. In addition, the total content when these are arbitrarily combined is preferably 0.005 to 1.000%.
[0032]
(W: 0.005% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.010% or less)
　W, Ca, Mg and REM contribute to fine dispersion of inclusions. , Increase toughness. Therefore, W, Ca, Mg or REM or any combination thereof may be contained. In order to sufficiently obtain this effect, the total content of W, Ca, Mg and REM, or any combination of two or more thereof is preferably 0.0003% or more. On the other hand, when the total content of W, Ca, Mg and REM exceeds 0.010%, the surface texture deteriorates. Therefore, the total content of W, Ca, Mg and REM is 0.010% or less. That is, W: 0.005% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.010% or less, and the total content of any two or more of these is 0. It is preferably .0003 to 0.010%.
[0033]
　REM (rare earth metal) refers to a total of 17 elements of Sc, Y and lanthanoids, and "REM content" means the total content of these 17 elements. Lanthanoids are industrially added, for example in the form of misch metal.
[0034]
(B: 0.0030% or less)
　B is an element for improving hardenability and is an element useful for forming a martensite structure. B is preferably contained in an amount of 0.0001% (1 ppm) or more. However, if B is contained in excess of 0.0030% (30 ppm), the above effect is saturated and it is economically wasteful. Therefore, the B content is set to 0.0030% or less. It is preferably 0.0025% or less.
[0035]
　In the high-strength steel sheet according to the present embodiment, the balance other than the above components is composed of Fe and impurities. Here, the impurity is a component mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when the high-strength steel sheet is industrially manufactured, and is related to the present embodiment. It means a component that is not intentionally added to a high-strength steel plate.
[0036]
　Next, the structure of the high-strength steel sheet according to the embodiment of the present invention will be described. The organizational requirements will be described below, but% related to the organizational fraction means "area ratio".
[0037]
(Martensite: 95% or more) The
　present embodiment is characterized in that martensite is secured at an area ratio of 95% or more. As a result, sufficient solid solution carbon can be secured, and as a result, seizure curability can be enhanced. In order to further enhance such an effect, it is recommended that martensite be secured at an area ratio of 97% or more, and may be 100%, for example.
[0038]
　In the present invention, the area ratio of martensite is determined as follows. First, a sample is taken with the thickness cross section perpendicular to the rolling direction of the steel plate as the observation surface, the observation surface is polished, and the structure at the position 1/4 of the thickness of the steel plate is SEM-EBSD (electron) at a magnification of 5000 times. Observe with a scanning electron microscope with an electron backscatter diffraction device), analyze the image with a field of 100 μm × 100 μm to measure the area ratio of martensite, and the average of these measured values ​​in any 5 or more fields is the book. It is determined as the area ratio of maltensite in the invention.
[0039]
(Remaining structure: 5% or less)
　According to the present invention, the remaining structure other than martensite has an area ratio of 5% or less. In order to further enhance the seizure curability of the high-strength steel sheet, it is preferably 3% or less, more preferably 0%. When a residual tissue is present, the residual tissue can include any tissue and is not particularly limited, but for example, it preferably contains retained austenite or is composed of retained austenite. Trace amounts of retained austenite may be unavoidable depending on the composition of the steel and the manufacturing method. However, such a small amount of retained austenite not only does not adversely affect the seizure curability, but also contributes to the improvement of ductility by the TRIP (Transformation Induced Plasticity) effect when deformed. Can be done. Therefore, the residual structure may contain retained austenite in an area ratio of 5% or less. However, in order to further enhance the baking curability, the content of retained austenite is preferably 3% or less, more preferably 0%.
[0040]
　In the present invention, the area ratio of retained austenite is determined by X-ray diffraction measurement. Specifically, the portion from the surface of the steel sheet to the 1/4 position of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and MoKα rays are used as characteristic X-rays to 1/4 of the depth from the surface of the steel sheet. The X-ray diffraction intensity at the position is measured. Then, from the integrated intensity ratios of the diffraction peaks of the body-centered cubic lattice (bcc) phases (200) and (211) and the face-centered cubic lattice (fcc) phases (200), (220) and (311), the following The area ratio of retained austenite is calculated using the formula of.
　Sγ = (I 200f + I 220f + I 311f ) / (I 200b + I 211b ) × 100 In the
　above formula, Sγ is the area ratio of retained austenite, and I 200f , I 220f and I 311f are the fcc phase (200) and ( I 311f , respectively). 220) and (311) diffraction peak intensities, I 200b and I 211b indicate the intensity of the bcc phase (200) and (211) diffraction peaks, respectively.
[0041]
(Si concentration ratio C1 / C2 is 1.25 or less) The ratio C1 / C2
　of the upper limit value C1 (mass%) and the lower limit value C2 (mass%) of the Si concentration in the thickness direction cross section of the high-strength steel plate is 1. It shall be 25 or less. More preferably, C1 / C2 is 1.15 or less. When C1 / C2 is 1.25 or less, segregation of Si can be controlled, the structure becomes uniform, and precipitates such as iron carbides shown below can be uniformly precipitated, so that uniform baking is possible. Curability can be enhanced.
[0042]
　The degree of segregation of Si represented by C1 / C2 is measured as follows. After adjusting the surface of the steel sheet whose rolling direction is the normal direction (that is, the cross section in the thickness direction of the steel sheet) so that it can be observed, mirror polishing is performed, and the thickness direction of the steel sheet is subjected to an EPMA (Electron Probe Microanalyzer) device. In the cross section, the Si concentration at 200 points is measured at intervals of 0.5 μm from one side to the other along the thickness direction of the steel sheet in a range of 100 μm × 100 μm at the center of the steel sheet. Similar measurements were made on different 4 lines so as to cover almost the entire area within the same 100 μm × 100 μm range, and the highest value was Si among the total 1000 points of Si concentration measured on all 5 lines. The ratio C1 / C2 is calculated with the upper limit value C1 (mass%) of the concentration and the lower limit value C2 (mass%) of the Si concentration.
[0043]
(30 precipitates having a major axis of 0.05 μm or more and 1.00 μm or less and an aspect ratio of 1: 3 or more / number density of 2 or more) In the
　present embodiment, the aspect ratio is 0.05 μm or more and 1.00 μm or less. 30 or more precipitates of 1: 3 or more / μm 2It has a great feature in that it has the above number density. In the present invention, the aspect ratio refers to the ratio of the longest diameter (major diameter) of the precipitate to the longest diameter (minor diameter) of the precipitates orthogonal to it. The precipitate is not particularly limited as long as it satisfies the above requirements for major axis and aspect ratio, and examples thereof include carbides. In particular, when the high-strength steel sheet according to the present invention is produced according to a preferable production method including a heat treatment step described later, the precipitate contains iron carbide or is composed of iron carbide. According to the present invention, by containing a relatively large amount of such precipitates in the structure, for example, the cell formation of dislocations caused by the entanglement of dislocations is suppressed, and this is caused by carbon diffused during baking hardening. The amount of dislocations to be fixed can be increased, and as a result, the amount of baking hardening can be significantly increased. Such findings have not been known in the past, and this time, they were first clarified by the present inventors, which is extremely surprising and surprising. The size of the dislocation cells generated in martensite is about several tens of nm or more and several hundred nm or less. Therefore, in order to suppress the formation of dislocation cells, the size of the precipitate is required to be the same. If the major axis is less than 0.05 μm, the formation of dislocation cell formation cannot be suppressed. Therefore, the major axis of the precipitate is set to 0.05 μm or more. More preferably, it is 0.10 μm or more. On the other hand, if the major axis is larger than 1.00 μm, the precipitate becomes coarse and the amount of solid solution carbon is greatly reduced, and the amount of baking hardening is reduced. Therefore, the major axis of the precipitate is set to 1.00 μm or less. More preferably, it is 0.80 μm or less.
[0044]
　The shape of the precipitate is preferably needle-shaped rather than spherical, and the aspect ratio is preferably 1: 3 or more. If the aspect ratio is less than 1: 3, the shape of the precipitate is considered to be spherical, and the formation of dislocation cells cannot be suppressed. Therefore, the aspect ratio is set to 1: 3 or more. More preferably, it is 1: 5 or more.
[0045]
　The precipitation location of the precipitate is preferably in the lath. This is because the place where the dislocation cells are most easily formed is in the lath, and the dislocation cells are hardly seen between the laths. Here, the lath refers to a structure formed in the former austenite grain boundaries by martensitic transformation. For ease of understanding, FIG. 1 is provided with an image diagram showing a state of precipitation of precipitates in the high-strength steel sheet according to the present invention. Referring to FIG. 1, in the lath structure 3 (FIG. 1 (b)) formed in the former austenite grain boundaries 2 in the microsegregation of Si having the uniform structure 1 (FIG. 1 (a)), between the lath 4 It can be seen that the needle-shaped precipitates 5 are uniformly deposited on the entire surface of the lath 4 (FIG. 1 (c)).
[0046]
　The number density of precipitates shall be 30 pieces / μm 2 or more. If the number density of the precipitates is less than 30 pieces / μm 2 , when dislocations are introduced and moved by prestrain, the dislocations interact with other dislocations before they meet the precipitates, and dislocation cells are formed. Therefore, the number density of precipitates is set to 30 pieces / μm 2 or more. More preferably, it is 40 pieces / μm 2 or more.
[0047]
　In the present invention, the morphology and number density of the precipitate are determined by observation with an electron microscope, and are measured by, for example, TEM (transmission electron microscope) observation. Specifically, a thin film sample is cut out from the surface of the steel sheet from the region from the 3/8 position to the 1/4 position of the thickness of the steel sheet and observed in a bright field. 1 μm 2 is cut out at an appropriate magnification of 10,000 to 100,000 times, and precipitates having a major axis of 0.05 μm or more and 1 μm or less and an aspect ratio of 1: 3 or more are counted. This work is performed in five or more continuous fields of view, and the average thereof is defined as the number density.
[0048]
　Next, the mechanical properties of the present invention will be described.
[0049]
(Tensile strength: 1300 MPa or more)
　According to the high-strength steel plate of the present invention having the above composition and structure, high tensile strength, specifically, 1300 MPa or more can be achieved. Here, the reason why the tensile strength is set to 1300 MPa or more is to satisfy the demand for weight reduction of the automobile body. The tensile strength is preferably 1400 MPa or more, more preferably 1500 MPa or more.
[0050]
　According to the high-strength steel sheet of the present invention, it is possible to achieve an excellent seizure hardening amount. More specifically, according to the high-strength steel plate of the present invention, the stress when the test piece heat-treated at 170 ° C. for 20 minutes after applying the 2% prestrain is re-tensioned, and the stress when the 2% prestrain is applied. It is possible to achieve the baking hardening amount BH such that the value obtained by subtracting the above value is 180 MPa or more, preferably 200 MPa or more. If the BH value is less than 180 MPa, it is difficult to mold and the strength after molding is low, so that it cannot be said that the baking curability is excellent.
[0051]
　Similarly, according to the high-strength steel sheet of the present invention, it is possible to achieve excellent uniform seizure curability. The uniform seizure curability can be evaluated from the viewpoint of whether or not the difference in seizure hardening amount when different prestrains are applied can be controlled to a predetermined value or less. In the present invention, unless otherwise specified, the baking hardening amount difference ΔBH shall mean the absolute value of the difference between BH when the prestrain is 2% and BH when the prestrain is 1%. According to the present invention, the difference in the amount of baking hardening ΔBH can be controlled to 20 MPa or less, preferably 10 MPa or less. Therefore, even if there is a difference in the amount of strain entered during press molding, baking hardening can be uniformly exhibited. That is, it is possible to provide a high-strength steel sheet having a small pre-strain dependence of the amount of seizure hardening (high uniform seizure curability). On the other hand, when the above ΔBH is larger than 20 MPa, the pre-strain dependence of the amount of baking cure is large, and it cannot be said that the uniform baking hardening is excellent.
[0052]

　Next, a preferable method for manufacturing a high-strength steel sheet according to the present embodiment will be described.
[0053]
　The following description is intended to exemplify a characteristic method for manufacturing the high-strength steel plate of the present invention, and the high-strength steel plate of the present invention is manufactured by a manufacturing method as described below. It is not intended to be limited to.
[0054]
　A preferred method for producing a high-strength steel plate of the present invention is a step of casting molten steel having the chemical composition described above to form a
　slab, and rough rolling in which the slab is roughly rolled in a temperature range of 1050 ° C. or higher and 1250 ° C. or lower. In this step, the rough rolling includes reverse rolling with a reduction rate of 30% or less per pass performed an even number of times in 2 passes or more and 16 passes or less, and the reduction rate difference between the two passes when making one round trip. Is 20% or less, the reduction rate of even times in one round trip is 5% or more higher than the reduction rate of odd times, and the rough rolling process is held for 5 seconds or more after the
　rough rolling. A finish rolling process in which finish rolling is performed in a temperature range of ° C. or higher and 1050 ° C. or lower. A finish rolling process in which the
　resulting steel sheet is wound in a temperature range of 400 ° C. or less, a cold rolling process in which the
　obtained hot-rolled steel sheet is cold-rolled at a reduction rate of 15% or more and 45% or less, and the obtained cold-rolled steel sheet Is heated at an average heating rate of 10 ° C./sec or more, held for 10 to 1000 seconds in a temperature range of Ac 3 or more and 1000 ° C. or less, and then cooled to 70 ° C. or less at an average cooling rate of 10 ° C./sec or more. step, and
　the resulting steel sheet to hold the 200 ° C. or higher 350 ° C. or less at a temperature range above 100 seconds, followed by a heat treatment step of cooling to 100 ° C. or less at 2 ° C. / sec or more average cooling rate
is characterized by containing .. Hereinafter, each step will be described.
[0055]
(Slab Forming Step)
　First, a molten steel having the chemical composition of the high-strength steel sheet according to the present invention described above is cast to form a slab to be subjected to rough rolling. The casting method may be a normal casting method, and a continuous casting method, an ingot forming method, or the like can be adopted, but the continuous casting method is preferable from the viewpoint of productivity.
[0056]
(Rough Rolling Step) It
　is preferable to heat the slab to a solution temperature range of 1000 ° C. or higher and 1300 ° C. or lower before rough rolling. The heating holding time is not particularly specified, but it is preferable to hold the slab at the heating temperature for 30 minutes or more in order to bring the slab to a predetermined temperature. The heating holding time is preferably 10 hours or less, more preferably 5 hours or less, in order to suppress excessive scale loss. If the temperature of the slab after casting is 1050 ° C. or higher and 1250 ° C. or lower, the slab may be subjected to rough rolling as it is without being heated and held in the temperature range, and may be directly fed or rolled directly.
[0057]
　Next, by roughly rolling the slab by reverse rolling, the Si segregated portion in the slab formed during solidification in the slab forming step is made into a uniform structure without forming a plate-shaped segregated portion extending in one direction. be able to. To explain the formation of the Si concentration distribution having such a uniform structure in more detail, first, in the slab before the start of rough rolling, the portion where the alloying element such as Si is concentrated is inside from both surfaces of the slab. In the form of a comb, a plurality of them are lined up almost vertically.
[0058]
　On the other hand, in rough rolling, the surface of the slab is stretched in the rolling traveling direction for each rolling pass. The rolling traveling direction is the direction in which the slab advances with respect to the rolling roll. Then, as the surface of the slab is stretched in the traveling direction of rolling in this way, the Si segregated portion growing inward from the surface of the slab is in a state of being inclined in the traveling direction of the slab for each rolling pass. Be made.
[0059]
　Here, in the case of so-called unidirectional rolling in which the traveling direction of the slab in each pass of rough rolling is always the same direction, the Si segregated portion gradually moves in the same direction for each pass while maintaining a slightly straight state. The slope becomes stronger. Then, at the end of the rough rolling, the Si segregation portion is in a substantially parallel posture with respect to the surface of the slab while maintaining a substantially straight state, and a flat microsegregation is formed.
[0060]
　On the other hand, in the case of reverse rolling in which the traveling directions of the slabs in each rough rolling pass are alternately opposite, the Si segregation portion inclined in the immediately preceding pass is inclined in the opposite direction in the next pass. As a result, the Si segregated portion has a bent shape. For this reason, in reverse rolling, the Si segregated portions are alternately bent in a zigzag shape by repeatedly performing the passes in opposite directions alternately.
[0061]
　When a plurality of zigzag shapes that are alternately bent in this way are lined up, the plate-like microsegregation disappears and the Si concentration distribution becomes uniformly intricate. By adopting such a structure, Si can be more easily diffused by the heat treatment in the subsequent process, and a hot-rolled steel sheet having a more uniform Si concentration can be obtained. In addition, since the above-mentioned reverse rolling results in a uniformly intricate Si concentration distribution over the entire steel sheet, such a uniform structure has not only a sheet thickness cross section parallel to the rolling direction but also a sheet thickness having a normal rolling direction. It is similarly formed in the cross section.
[0062]
　If the rough rolling temperature range is less than 1050 ° C., it becomes difficult to complete rolling at 850 ° C. or higher in the final pass of rough rolling, resulting in poor shape. Therefore, the rough rolling temperature range is preferably 1050 ° C. or higher. More preferably, it is 1100 ° C. or higher. If the rough rolling temperature range exceeds 1250 ° C., scale loss increases and slab cracking may occur. Therefore, the rough rolling temperature range is preferably 1250 ° C. or lower.
[0063]
　If the rolling reduction rate per pass in rough rolling exceeds 30%, the shear stress during rolling becomes large, the Si segregated portion becomes non-uniform, and a uniform structure cannot be obtained. Therefore, the rolling reduction rate per pass in rough rolling is set to 30% or less. The smaller the rolling reduction, the smaller the shear strain during rolling, and a uniform structure can be obtained. Therefore, the lower limit of the rolling ratio is not particularly set, but 10% or more is preferable from the viewpoint of productivity.
[0064]
　In order to make the Si concentration distribution a uniform structure, reverse rolling is preferably 2 passes or more, more preferably 4 passes or more. However, if it is applied in excess of 16 passes, it becomes difficult to secure a sufficient finish rolling temperature, so the number is 16 passes or less. Further, it is desirable that each pass in which the traveling directions are opposite to each other is performed the same number of times, that is, the total number of passes is an even number. However, in a general rough rolling line, the entry side and the exit side of rough rolling are located on opposite sides of the roll. For this reason, the number of passes (rolling) in the direction from the entry side to the exit side of rough rolling increases once. Then, in the final pass (rolling), the Si segregated portion becomes a flat shape, and it becomes difficult to form a uniform structure. In the case of rough rolling on such a hot rolling line, it is preferable to leave a gap between rolls in the final pass and omit rolling.
[0065]
　In reverse rolling, if there is a difference in rolling reduction between two passes included in one reciprocating rolling, shape defects are likely to occur, and the Si segregated portion becomes non-uniform, making it impossible to form a uniform structure. Therefore, during rough rolling, the rolling reduction difference between the two passes included in one round trip of reverse rolling is 20% or less. It is preferably 10% or less.
[0066]
　As will be described later, in order to miniaturize the recrystallized structure, tandem multi-stage rolling in finish rolling is effective, but tandem rolling tends to form flat microsegregation. In order to utilize tandem multi-step rolling, it is necessary to make the even-numbered rolling reduction in reverse rolling larger than the odd-numbered rolling, and control the microsegregation formed in the subsequent tandem rolling. The effect becomes remarkable when the reduction rate of even-numbered times (return route) is 5% or more higher than the reduction rate of odd-numbered times (outward route) in one round trip of reverse rolling. Therefore, it is preferable that the even-numbered rolling reduction rate is 5% or more higher than the odd-numbered rolling reduction rate in one round trip of reverse rolling.
[0067]
　In order to make the complex structure of Si produced by reverse rolling in rough rolling uniform by austenite grain boundary movement, it is preferable to hold it for 5 seconds or more from rough rolling to finish rolling.
[0068]
(Finish rolling process)
　After reverse rolling in rough rolling, in order to narrow the interval of Si segregation zone due to the dendrite secondary arm by increasing the rolling reduction in tandem rolling in finish rolling, finish rolling is performed in 4 steps. It is preferably carried out on one or more continuous rolling stands. If the finish rolling temperature is less than 850 ° C, recrystallization does not occur sufficiently and the structure is stretched in the rolling direction, and a plate-like structure due to the stretched structure is generated in the subsequent process. Therefore, the finish rolling temperature is 850 ° C. The above is preferable. More preferably, it is 900 ° C. or higher. On the other hand, when the finish rolling temperature exceeds 1050 ° C., it becomes difficult to generate fine recrystallized grains of austenite, it becomes difficult to segregate Si at the grain boundaries, and the Si segregation zone tends to become flat. Therefore, the finish rolling temperature is preferably 1050 ° C. or lower. If the temperature is appropriate, the rough-rolled steel sheet may be heated after the rough-rolling step and before the finish-rolling step, if necessary. Further, when the reduction ratio of the first stand for finish rolling is set to 15% or more, a large amount of recrystallized grains are generated, and Si is easily dispersed uniformly by the subsequent grain boundary movement. As described above, by limiting not only the rough rolling process but also the finish rolling process, microsegregation of flat Si can be suppressed. The finish rolling temperature refers to the surface temperature of the steel sheet from the start of finish rolling to the end of finish rolling.
[0069]
　If the take-up temperature exceeds 400 ° C., the surface texture deteriorates due to internal oxidation, so the take-up temperature is preferably 400 ° C. or lower. When the steel sheet structure has a homogeneous structure of martensite or bainite, it is easy to form a homogeneous structure by annealing, so that the winding temperature is more preferably 300 ° C. or lower.
[0070]
(Cold Rolling Step) The
　hot-rolled steel sheet obtained in the finish rolling step is pickled and then subjected to cold rolling to obtain a cold-rolled steel sheet. In order to maintain the martensite lath, the reduction rate is preferably 15% or more and 45% or less. When the reduction ratio exceeds 45%, the uniform structure of Si segregation is disturbed, so that in the martensite lath structure, the amount of carbides precipitated between the laths increases, and the needle-like precipitates deposited in the laths decrease. As a result, precipitation of carbide having an aspect ratio of 1: 3 or more is inhibited, which is not preferable. The pickling may be a normal pickling.
[0071]
(Annealing step)
　The steel sheet obtained through the above cold rolling step is annealed. For heating at the annealing temperature, the temperature is raised at an average heating rate of 10 ° C./sec or more, and heating is held for 10 to 1000 seconds in a temperature range of Ac 3 or more and 1000 ° C. or less. This temperature range and annealing time are for austenite transformation of the entire surface of the steel sheet. When the holding temperature exceeds 1000 ° C. or the annealing time exceeds 1000 seconds, the austenite particle size becomes coarse and martensite having a large lath width is formed, resulting in a decrease in toughness. Therefore, the annealing temperature is Ac 3 or more and 1000 ° C. or less, and the annealing time is 10 to 1000 seconds.
[0072]
　The Ac 3 points are calculated by the following formula. Substitute the mass% of the element for the element symbol in the following formula. Substitute 0% by mass for elements that do not contain it.
　Ac 3 = 881-335 x C + 22 x Si-24 x Mn-17 x Ni-1 x Cr-27 x Cu + 41 x Mo
[0073]
　After maintaining the annealing temperature, cooling is performed at an average cooling rate of 10 ° C./sec or higher. In order to freeze the tissue and efficiently induce martensitic transformation, the cooling rate should be high. However, if the temperature is lower than 10 ° C./sec, martensite is not sufficiently produced and the desired tissue cannot be controlled. Therefore, the temperature is set to 10 ° C./sec or higher. If the above cooling rate can be maintained after annealing and holding, a plating step may be added during cooling.
[0074]
　The cooling stop temperature is 70 ° C. or lower. This is because martensite is generated while being quenched on the entire surface by cooling. If cooling is stopped above 70 ° C, tissues other than martensite may appear. In addition, when martensite appears, precipitates such as spheroidized iron carbides appear due to self-tempering. As a result, precipitates such as needle-shaped iron carbides do not precipitate in the subsequent process, the desired precipitates cannot be obtained, and the baking curability deteriorates. Therefore, the cooling stop temperature is 70 ° C. or lower, preferably 60 ° C. or lower.
[0075]
(Heat treatment step)
　The high-strength steel plate according to the present embodiment has a great feature in the precipitation form of precipitates such as iron carbides. Such a precipitate is formed by converting a slab containing an appropriate amount of Si into martensite and then holding it in a temperature range of 200 ° C. or higher and 350 ° C. or lower by heating. When the holding temperature is less than 200 ° C., the major axis of the precipitate is less than 0.05 μm, and the dislocation cells cannot be suppressed. From this, the holding temperature is set to 250 ° C. or higher. If the holding temperature is more than 350 ° C., the precipitates become coarse, the number density is small, and the major axis becomes more than 1.00 μm. As a result, the dislocation cells cannot be suppressed. Therefore, the holding temperature is set to 350 ° C. or lower. The holding time is 100 seconds or more. If the holding time is less than 100 seconds, iron carbide cannot be stably deposited. From this, the holding time is set to 100 seconds or more. Then, from the viewpoint of productivity, the mixture is cooled to 100 ° C. or lower at an average cooling rate of 2 ° C./sec or more.
[0076]
(Skin pass rolling step) After the
　heat treatment step, skin pass rolling (tempering rolling) may be performed arbitrarily. In the high-strength steel sheet according to the embodiment of the present invention, since the dislocation cells are suppressed by the precipitates, the dislocation cells are not formed no matter how much skin pass rolling is performed, and the seizure curability does not deteriorate. However, since it becomes difficult to control the plate thickness, it is preferable that the reduction rate is 2.0% or less. More preferably, the reduction rate is 1.0% or less.
[0077]
　In this way, the high-strength steel sheet according to the embodiment of the present invention can be manufactured.
[0078]
　It should be noted that all of the above embodiments merely show examples of embodiment in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner by these. That is, the present invention can be implemented in various forms without departing from the technical idea or its main features.
Example 1
[0079]
　Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one condition example. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0080]
　A slab having the chemical composition shown in Table 1 was produced, the slab was heated to 1300 ° C. for 1 hour, and then rough-rolled and finish-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet was pickled and cold-rolled at the reduction ratio shown in Table 2 to obtain a cold-rolled steel sheet. Subsequently, annealing and heat treatment were performed under the conditions shown in Table 2. Each temperature shown in Table 2 is the surface temperature of the steel sheet. Further, in Table 2, the "difference in reduction rate between one round-trip internal pass" means that the reduction rate difference was the same in all round-trip paths.
[0081]
　Ac 3 in Table 2 was calculated by the formula shown below. The mass% of the element was substituted for the element symbol in the following formula. For the elements not contained, 0% by mass was substituted.
　Ac 3 = 881-335 x C + 22 x Si-24 x Mn-17 x Ni-1 x Cr-27 x Cu + 41 x Mo
[0082]
[table 1]

[0083]
[Table 2-1]

[0084]
[Table 2-2]

[0085]
　The area ratios of martensite and retained austenite were determined from the obtained cold-rolled steel sheet by using SEM-EBSD and X-ray diffraction method.
[0086]
　In particular, the area ratio of martensite was determined as follows. First, a sample is taken with the thickness cross section perpendicular to the rolling direction of the steel plate as the observation surface, the observation surface is polished, and the structure at 1/4 of the thickness of the steel plate is observed by SEM-EBSD at a magnification of 5000 times. Then, the area ratio of maltensite was measured by image analysis in a field of 100 μm × 100 μm, and the average of these measured values ​​in any of the five visual fields was determined as the area ratio of martensite.
[0087]
　In addition, the steel structure of the obtained cold-rolled steel sheet was observed by TEM to determine the presence or absence of precipitates, their major axis, aspect ratio, and number density. Specifically, a thin film sample is cut out from the surface of the steel sheet from the region from the 3/8 position to the 1/4 position of the thickness of the steel sheet, observed in a bright field, and at an appropriate magnification of 10,000 to 100,000 times. 1 μm 2 was cut out, and the precipitates having a major axis of 0.05 μm or more and 1 μm or less and an aspect ratio of 1: 3 or more were counted and obtained, and this work was performed in five continuous visual fields, and the average was taken as the number density. These results are shown in Table 3.
[0088]
　Further, the tensile strength TS, the elongation at break EL, the baking hardening amount BH, and the baking hardening amount difference ΔBH of the obtained cold-rolled steel sheet were measured. In the measurement of tensile strength TS, elongation at break EL, seizure hardening amount BH, and seizure hardening amount difference ΔBH, JIS No. 5 tensile test pieces with the direction perpendicular to the rolling direction as the longitudinal direction were collected and conformed to JIS Z 2241. A tensile test was performed. The seizure hardening amount BH is a value obtained by subtracting the stress at the time of applying the 2% prestrain from the stress when the test piece heat-treated at 170 ° C. for 20 minutes after applying the 2% prestrain is re-tensioned. The seizure hardening amount difference ΔBH is an absolute value of the difference between BH when the prestrain is 2% and BH when the prestrain is 1%. In order to satisfy the demand for weight reduction of the automobile body, the tensile strength is 1300 MPa or more, preferably 1400 MPa or more, and more preferably 1500 MPa or more. Further, it is preferable that the elongation is 5% or more because it is easy to mold. Further, for BH, if it is less than 180 MPa, it is difficult to mold and the strength after molding becomes low, so 180 MPa or more is required to have excellent baking curability. More preferably, it is 200 MPa or more. Regarding ΔBH, even if there is a difference in the amount of strain applied during press molding, it is necessary to be 20 MPa or less in order to cause uniform baking hardening. More preferably, it is 10 MPa or less.
[0089]
　The degree of segregation of Si represented by C1 / C2 was measured as follows. After adjusting the manufactured steel sheet so that the surface whose rolling direction is the normal direction (that is, the cross section in the thickness direction of the steel sheet) can be observed, mirror polishing is performed, and the steel sheet is mirror-polished by the EPMA device in the thickness direction cross section of the steel sheet. The Si concentration at 200 points was measured at intervals of 0.5 μm from one side to the other along the thickness direction of the steel sheet in the range of 100 μm × 100 μm in the central portion of the steel sheet. Similar measurements were made on different 4 lines so as to cover almost the entire area within the same 100 μm × 100 μm range, and the highest value was Si among the total 1000 points of Si concentration measured on all 5 lines. The ratio C1 / C2 was calculated with the upper limit value C1 (mass%) of the concentration and the lower limit value C2 (mass%) of the Si concentration.
[0090]
[Table 3]

[0091]
[Evaluation Results] As
　shown in Table 3, in Examples 1, 3 to 5, 7, 10, 15, 18, 20, 23, 25, 28, 31 and 34, excellent tensile strength, BH and ΔBH are obtained. I was able to. In each case, the tensile strength was 1300 MPa or more, the BH was 180 MPa or more, and the ΔBH was 20 MPa or less, indicating that the strength was high and the seizure curability was excellent. In the high-strength steel sheets according to these examples, precipitates, particularly iron carbides, were uniformly precipitated on the entire surface of the lath in martensite.
[0092]
　On the other hand, in Comparative Example 2, since the holding time in the heat treatment step was short, the target iron carbide was not sufficiently precipitated, the BH was low, and the ΔBH was high. In Comparative Example 6, since the holding temperature in the heat treatment step was low, the target iron carbide was not sufficiently precipitated, the BH was low, and the ΔBH was high. In Comparative Example 8, since the annealing temperature was too low, a ferrite structure appeared and a sufficient martensite structure could not be obtained, and as a result, TS and BH were low. In Comparative Example 9, since the annealing time was too short, the entire surface was not martensitic, and TS and BH were also low. In Comparative Example 11, since the average cooling rate in the annealing step was too slow, the entire surface did not have a martensite structure, and TS and BH were low. In Comparative Example 12, since the holding temperature in the heat treatment step was too high, the iron carbide became coarse, TS and BH were low, and ΔBH was high. In Comparative Example 13, the C content was too low, so that the solid solution carbon content was reduced and the TS and BH were low. In Comparative Example 14, since the Si content was too low, the desired iron carbide was not sufficiently produced, the BH was low, and the ΔBH was high.
[0093]
　In Comparative Example 16, since the difference in rolling reduction between the two passes during one round trip in the rough rolling step was large, the Si concentration distribution did not have a uniform structure, and ΔBH was high. In Comparative Example 17, since the even-numbered rolling reduction rate in one round trip in the rough rolling process was smaller than the odd-numbered rolling reduction rate, the Si concentration distribution did not have a uniform structure and ΔBH was high. In Comparative Example 19, TS and BH were low because the Mn content was too low. In Comparative Example 21, since the reduction rate of reverse rolling in the rough rolling step was high, the Si concentration distribution did not have a uniform structure, and ΔBH was high. In Comparative Example 22, since the C content was too high, the area ratio of retained austenite (γ) was high, a sufficient martensite structure could not be obtained, and the BH was low. In Comparative Example 24, the time from rough rolling to finish rolling was too short, the Si concentration distribution did not have a uniform structure, and ΔBH was high. In Comparative Example 26, since the number of stands for finish rolling was small, the Si concentration distribution became flat and ΔBH was high. In Comparative Example 27, the rolling reduction of the first stand for finish rolling was too small, the Si concentration distribution became flat, and ΔBH was high. In Comparative Example 29, the finish rolling temperature (finish rolling start temperature in Table 2) was too high, the Si concentration portion distribution became flat, and ΔBH was high. In Comparative Example 30, since the cold rolling ratio was too high, a carbide having a desired aspect ratio could not be obtained, and BH was low and ΔBH was high. In Comparative Example 32, since the number of reverse rolling passes in the rough rolling step was an odd number, the Si concentration distribution did not have a uniform structure, and ΔBH was high. In Comparative Example 33, since the cooling stop temperature in the annealing step was high, spheroidized coarse iron carbide was precipitated, and TS and BH were low and ΔBH was high.
Industrial applicability
[0094]
　The high-strength steel sheet having excellent seizure curability of the present invention can be used as a raw plate for structural materials of automobiles, particularly in the field of the automobile industry.
Description of the sign
[0095]
　1 Uniform structure
　2 Former austenite grain boundaries
　3 Las structure
　4 Las
　5 Precipitates
The scope of the claims
[Claim 1]
　By mass%,
　C: 0.13 to 0.40%,
　Si: 0.500 to 3.000%,
　Mn: 2.50 to 5.00%,
　P: 0.100% or less,
　S: 0.010 %
　Or less, Al: 0.001 to 2.000%,
　N: 0.010% or less
, the balance is composed of Fe and impurities
　, contains martensite with an area ratio of 95% or more, and the remaining structure is the area. The ratio is 5% or less,
　the ratio C1 / C2 of the upper limit value C1 (mass%) and the lower limit value C2 (mass%) of the Si concentration in the thickness direction cross section is 1.25 or less, and the
　major axis is 0.05 μm or more. A 　high-strength steel plate having a number density of 30 pieces / μm 2 or more of precipitates having an aspect ratio of 1: 3 or more at 1.00 μm or less and a
tensile strength of 1300 MPa or more.
[Claim 2]
　The high-strength steel sheet according to claim 1, wherein the residual structure is composed of retained austenite when the residual structure is present.
[Claim 3]
　Further, claim 1 in which, in mass%, one or more of
　Ti: 0.100% or less,
　Nb: 0.100% or less, and
　V: 0.100% or
less are contained in a total of 0.100% or less. Or the high-strength steel sheet according to 2.
[Claim 4]
　Furthermore, in terms of mass%, one or two or more types of
　Cu: 1.000% or less,
　Ni: 1.000% or less,
　Mo: 1.000% or less,
　Cr: 1.000% or less
are 1.000 in total. The high-strength steel sheet according to any one of claims 1 to 3, which contains% or less.
[Claim 5]
　Furthermore, in mass%,
　W: 0.005% or less,
　Ca: 0.005% or less,
　Mg: 0.005% or less
　Rare earth metal (REM): 0.010% or less
One type or two or more types in total The high-strength steel sheet according to any one of claims 1 to 4, which contains 0.010% or less.
[Claim 6]
　The high-strength steel sheet according to any one of claims 1 to 5, further comprising B: 0.0030% or less in mass%.