Title of the invention: High-strength cold-rolled steel sheet and its manufacturing method
Technical field
[0001]
　The present invention relates to a high-strength cold-rolled steel sheet and a method for producing the same.
　The present application claims priority based on Japanese Patent Application No. 2018-051020 filed in Japan on March 19, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　Today, the industrial technology field is highly divided, and the materials used in each technology field are required to have special and high performance. In particular, with regard to steel sheets for automobiles, in consideration of the global environment, there is a remarkable increase in demand for high-strength steel sheets having a thin wall and high formability in order to reduce the weight of the vehicle body and improve fuel efficiency. Among steel sheets for automobiles, cold-rolled steel sheets (including galvanized steel sheets such as hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets), which are used especially for body frame parts, are not only high-strength, but also aimed at further expansion of application. , High formability is required. Furthermore, there is an increasing concern about hydrogen embrittlement as the strength increases. Therefore, high-strength steel sheets are required to have high strength and good moldability, as well as hydrogen embrittlement resistance after molding. For example, the tensile strength (TS) is 1310 MPa or more, the uniform elongation in the tensile test is 5.0% or more, and the ratio (R / t) of the limit bending radius R to the plate thickness t at 90 ° V bending is 5.0 or less. Therefore, it is required to have excellent hydrogen embrittlement resistance.
[0003]
　A structure containing ferrite is effective for obtaining excellent molding processability. However, ferrite has a soft structure and contributes little to the improvement of strength. Therefore, in order to obtain a strength of 1310 MPa or more in a steel having a structure containing ferrite, it is necessary to harden the second phase. However, the hard second phase deteriorates bendability.
[0004]
　For example, Patent Documents 1 and 2 propose a steel sheet containing tempered martensite as a main phase as a technique for increasing tensile strength without deteriorating bendability. Patent Documents 1 and 2 disclose that the tempered martensite single-phase structure is excellent in bendability. Further, it is disclosed that this tempered martensite has excellent hydrogen embrittlement resistance because it has a structure in which carbides, which are hydrogen trap sites, are finely dispersed.
　However, the invention of Patent Document 1 has a low strength level of less than 1310 MPa. Therefore, when aiming for higher strength, it is necessary to further improve the hydrogen embrittlement resistance and workability that deteriorate with it. Further, the invention of Patent Document 2 has a problem that a high uniform elongation cannot be obtained (low moldability) because the retained austenite is small because the cooling is performed at once during quenching to near room temperature.
[0005]
　Further, as a technique for achieving both high strength and high moldability, Patent Document 3 proposes a steel sheet utilizing the TRIP effect of retained austenite. However, the invention of Patent Document 3 has a ferrite phase. Therefore, it is difficult to obtain high strength of 1310 MPa or more. Further, since there is a difference in strength in the structure, it is required to further improve the bend formability.
Prior art literature
Patent documents
[0006]
Patent Document 1: Japanese Patent Application Laid-Open No. 2009-30091
Patent Document 2: Japanese Patent Application Laid-Open No. 2010-215958
Patent Document 3: Japanese Patent Application Laid-Open No. 2006-104532
Outline of the invention
Problems to be solved by the invention
[0007]
　As described above, conventionally, a steel sheet having a tensile strength (TS) of 1310 MPa or more, high formability, and high hydrogen embrittlement resistance has not been proposed.
　The present invention has been made to solve the above problems, and the problem is a high-strength steel sheet having both formability and hydrogen embrittlement resistance, which are problems in a high-strength steel sheet, at a high level, that is, rolling. The strength (TS) is 1310 MPa or more, the uniform elongation is 5.0% or more, the ratio (R / t) of the critical bending radius R to the plate thickness t at 90 ° V bending is 5.0 or less, and hydrogen embrittlement resistance is further applied. It is an object of the present invention to provide a high-strength cold-rolled steel sheet having excellent chemical embrittlement characteristics and a method for producing the same.
　In the present invention, the high-strength cold-rolled steel sheet includes a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface, and a high-strength alloyed hot-dip galvanized steel sheet.
Means to solve problems
[0008]
　The present inventors have conducted a detailed investigation on the effects of chemical composition and manufacturing conditions on the mechanical properties of high-strength cold-rolled steel sheets. As a result, the structure (metal structure) at a position 1/4 of the thickness from the surface, which is a typical position of the steel sheet, is made into a structure mainly composed of tempered martensite containing retained austenite, and the dew point is controlled during annealing. It has been found that by softening the surface layer and refining the hard phase of the surface layer, it is possible to achieve both formability and hydrogen embrittlement resistance, which are problems in high-strength steel sheets, at a high level. Further, for the surface layer portion, the above-mentioned structure is obtained by adjusting the holding time in the temperature range of more than 425 ° C. and lower than 600 ° C. in cooling after annealing and performing ferrite transformation and bainite transformation of only the surface layer. I found that I could do it.
　The present invention has been made based on the above findings. The gist of the present invention is as follows.
[0009]
(1) The high-strength cold-rolled steel sheet according to one aspect of the present invention has C: more than 0.140%, less than 0.400%, Si: more than 0.35%, less than 1.50%, Mn in mass%. : More than 1.50%, less than 4.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Ti: 0% or more , Less than 0.050%, Nb: 0% or more, less than 0.050%, V: 0% or more, 0.50% or less, Cr: 0% or more, 1.00% or less, Mo: 0% or more, 0 .50% or less, B: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500 % Or less, Bi: 0% or more, 0.050% or less, the balance has a chemical composition consisting of Fe and impurities, and
　the structure at a position 1/4 of the plate thickness from the surface is by volume ratio. More than 70.0% tempered martensite, more than 3.0% and less than 10.0% retained austenite, a total of 25.0% or less ferrite and bainite, and 5.0% or less martensite. The structure at a position 25 μm from the surface contains, by volume, 70% or more of ferrite and bainite in total, and 30% or less of martensite and tempered martensite in total, 25 μm from the surface. The average particle size of the martensite and the tempered martensite is 5.0 μm or less, the tensile strength is 1310 MPa or more, the uniform elongation is 5.0% or more, and the limit bending radius at 90 ° V bending. A high-strength cold-rolled steel plate having R / t, which is the ratio of R to plate thickness t, of 5.0 or less.
(2) In the high-strength cold-rolled steel sheet according to (1) above, the chemical composition is Ti: 0.001% or more and less than 0.050%, Nb: 0.001% or more, 0. Less than 050%, V: 0.01% or more, 0.50% or less, Cr: 0.01% or more, 1.00% or less, Mo: 0.01% or more, 0.50% or less, B: 0. 0001% or more, 0.0100% or less, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.005% or more, 0.0500% The following, and Bi: 0.005% or more, 0.050% or less, may contain one or more.
(3) The high-strength cold-rolled steel sheet according to (1) or (2) above may be provided with a hot-dip galvanized layer on the surface.
(4) In the high-strength cold-rolled steel sheet according to (3) above, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.
(5) The method for producing a high-strength cold-rolled steel sheet according to another aspect of the present invention is, in mass%, C: more than 0.140%, less than 0.400%, Si: more than 0.35%, 1.50. %, Mn: more than 1.50%, less than 4.00%, P: 0.010% or less, S: 0.010% or less, Al: 0.100% or less and N: 0.0100% or less, Ti : 0% or more, less than 0.050%, Nb: 0% or more, less than 0.050%, V: 0% or more, 0.50% or less, Cr: 0% or more, 1.00% or less, Mo: 0 % Or more, 0.50% or less, B: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more , 0.0500% or less, Bi: 0% or more, 0.050% or less, and a cast slab having a chemical composition in which the balance consists of Fe and impurities is directly or once cooled and then heated and hot. The hot-rolling process of rolling to obtain a hot-rolled steel sheet, the cold-rolling process of pickling the hot-rolled steel sheet and cold-rolling to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet having a dew point of -20. An annealing step of soaking and annealing at a temperature of 800 ° C. or higher in an atmosphere containing nitrogen and 1.0% by volume or more and 20% by volume or less of hydrogen at ° C. or higher and 20 ° C. or lower, and after the annealing step, After the first cooling step of cooling the cold-rolled steel sheet to a temperature range of more than 425 ° C. and less than 600 ° C. and the first cooling step, the cold-rolled steel sheet is placed in the temperature range of more than 425 ° C. and less than 600 ° C. for 250 seconds. A holding step of staying for 750 seconds or less, a second cooling step of cooling the cold-rolled steel sheet to a temperature of 50 ° C. or higher and 250 ° C. or lower after the holding step, and 250 on the cold-rolled steel sheet after the second cooling step. A tempering step of tempering at a temperature of ° C. or higher and 350 ° C. or lower for 1 second or longer, a
　　third cooling step of cooling to a temperature at which skin pass rolling is possible after the tempering step, and a skin pass to the
　　cold-rolled steel sheet after the third cooling step. It includes a skin pass process for rolling
.
(6) In the high-strength cold-rolled steel sheet according to (5) above, the chemical composition is Ti: 0.001% or more and less than 0.050%, Nb: 0.001% or more, 0. Less than 050%, V: 0.01% or more, 0.50% or less, Cr: 0.01% or more, 1.00% or less, Mo: 0.01% or more, 0.50% or less and B: 0. 0001% or more, 0.0100% or less, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.005% or more, 0.0500% The following and Bi: One or more kinds consisting of 0.005% or more and 0.050% or less may be contained.
(7) In the method for producing a high-strength cold-rolled steel sheet according to (5) or (6) above, the holding step may include a hot-dip galvanizing step of hot-dip galvanizing the cold-rolled steel sheet.
(8) The method for producing a high-strength cold-rolled steel sheet according to (7) above may include an alloying step of performing an alloying treatment after the hot-dip galvanizing step of the holding step.
Effect of the invention
[0010]
　According to the above aspect of the present invention, the tensile strength (TS) is 1310 MPa or more, the uniform elongation is 5.0% or more, and the ratio (R / t) of the limit bending radius R to the plate thickness t at 90 ° V bending is 5. A high-strength cold-rolled steel sheet having a value of 0.0 or less and excellent hydrogen embrittlement resistance and a method for producing the same can be obtained. Such a steel sheet has sufficient formability that can be applied to processing such as press molding, and is excellent in hydrogen embrittlement resistance, which is a problem in increasing strength. Therefore, the present invention contributes greatly to the development of industry, such as being able to contribute to solving global environmental problems by reducing the weight of the vehicle body.
A brief description of the drawing
[0011]
FIG. 1A shows the residence time in a temperature range of more than 425 ° C. and less than 600 ° C. after annealing and cooling at a depth of 25 μm from the surface in the plate thickness direction, and the volume of ferrite and bainite. It is a graph which shows the relationship with the rate, and the hydrogen embrittlement resistance property at that time.
FIG. 1B shows the residence time in a temperature range of more than 425 ° C. and less than 600 ° C. after annealing and cooling at a depth of 25 μm from the surface in the plate thickness direction, and martensite and tempered martensite. It is a graph which shows the relationship with the volume ratio of a site, and the hydrogen embrittlement resistance property at that time.
[Fig. 1C] The residence time in the temperature range above 425 ° C and below 600 ° C after annealing and cooling at a depth of 25 μm from the surface in the plate thickness direction, and martensite and tempered martensite. It is a graph which shows the relationship with the site particle diameter, and the hydrogen embrittlement resistance property at that time.
Mode for carrying out the invention
[0012]
　A high-strength cold-rolled steel sheet according to an embodiment of the present invention (hereinafter, may be referred to as a steel sheet according to the present embodiment) will be described.
　The metallographic structure and chemical composition of the steel sheet according to the present embodiment and the rolling and annealing conditions in the manufacturing method capable of efficiently, stably and economically manufacturing the steel sheet will be described in detail below. The steel sheet according to the present embodiment is not only a cold-rolled steel sheet having no plating layer on the surface, but also a hot-dip galvanized steel sheet having hot-dip galvanized on the surface or an alloyed steel sheet having hot-dip galvanized on the surface. These include hot-dip galvanized steel sheets, and these main conditions are common to high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets.
[0013]
　1. 1. Metal structure In
　the description of the metal structure of the steel sheet according to the present embodiment, the structure fraction is expressed as a volume fraction. Therefore, unless otherwise specified, "%" represents "volume%".
[0014]
　The steel plate (high-strength cold-rolled steel plate, high-strength hot-dip zinc-plated steel plate, high-strength alloyed hot-dip zinc-plated steel plate) according to the present embodiment has a structure at a position 1/4 (1/4 thickness) of the plate thickness from the surface. In terms of volume ratio, tempered martensite of 70.0% or more, retained austenite of more than 3.0% and less than 10.0%, ferrite and bainite of 25.0% or less in total, and 5.0% or less. Including martensite.
　In addition, the structure at a position 25 μm from the surface contains ferrite and bainite having a total volume ratio of 70.0% or more, and martensite and tempered martensite having a total volume of 30.0% or less, and martensite and tempered. The average particle size of martensite is 5.0 μm or less.
[0015]
　In the steel sheet according to the present embodiment, the structure at a position 1/4 of the plate thickness in the plate thickness direction from the surface showing the typical structure of the steel plate is the tempered martensite-based structure, which is 25 μm in the plate thickness direction from the surface. The structure of the surface layer at the position of is mainly ferrite and bainite. That is, the steel plate in the present embodiment has an inclined structure in which the fraction of the structure differs between the position of 1/4 of the plate thickness and the surface layer portion. Such an inclined structure can be achieved by performing appropriate decarburization during annealing heating and then securing an appropriate residence time during which only the surface layer is ferrite or bainite-transformed during annealing cooling. Detailed conditions will be described in detail in the description of manufacturing conditions.
[0016]
[Regarding the structure at a position 25 μm from the surface (surface layer 25 μm position)] The
　present inventors have diligently studied in order to improve the hydrogen embrittlement resistance property of the high-strength steel sheet. As a result, it was found that the structure of the surface layer has a great influence on the hydrogen embrittlement resistance. Specifically, at a position 25 μm from the surface of the steel plate in the plate thickness direction, the volume fractions of ferrite and bainite are 70.0% or more in total, and the volume fractions of martensite and tempered martensite are 30.0 in total. It was found that when the ratio is less than% and the average particle size of martensite and tempered martensite is 5.0 μm or less, the hydrogen embrittlement resistance is excellent. Although the detailed mechanism for improving the hydrogen embrittlement resistance by using such a structure is not clear, the following reasons can be considered. That is, the surface of the steel sheet is most biased toward bending deformation due to pre-strain in the evaluation of hydrogen embrittlement resistance, but the surface of the steel sheet is soft and the hard phase of the surface layer, which is the starting point of cracking, is reduced. It is considered that hydrogen embrittlement is suppressed by the fact that the hydrogen embrittlement is made fine and uniform and the crack starting points are reduced.
[0017]
　As shown in FIG. 1A, the volume fractions of ferrite and bainite at a position 25 μm from the surface are 70.0% or more in total, and the hydrogen embrittlement resistance is excellent. The volume fractions of ferrite and bainite are preferably 75.0% or more, and more preferably 80.0% or more. The volume fraction of ferrite and bainite may be 100%.
　Further, hard martensite and tempered martensite in the structure of the surface layer portion not only harden the surface layer portion but also increase the starting point of cracking, so that the hydrogen embrittlement resistance is deteriorated. That is, martensite and tempered martensite in the surface structure need to be small and finer. Therefore, the volume fraction of martensite and tempered martensite is set to 30.0% or less as shown in FIG. 1B at a position 25 μm in the plate thickness direction from the surface. The volume fraction of martensite and tempered martensite is preferably 25.0% or less, more preferably 20.0% or less.
[0018]
　Further, as shown in FIG. 1C, the average particle size of martensite and tempered martensite is 5.0 μm or less. The average particle size of martensite and tempered martensite is preferably 4.5 μm or less, and more preferably 4.0 μm or less.
[0019]
[About the structure at the position of 1/4 (1/4 thickness) of the plate thickness from the surface]
　Tempering martensite is a collection of lath-shaped crystal grains like martensite (so-called fresh martensite), but inside by tempering. It is a hard structure containing fine iron-based carbides. Tempering martensite is obtained by tempering martensite produced by cooling after annealing by heat treatment or the like.
　Tempering martensite is a structure that is less brittle and more ductile than martensite. In the steel sheet according to the present embodiment, the volume fraction of tempered martensite is set to 70.0% or more in order to improve the strength and bendability. The volume fraction is preferably 75.0% or more, more preferably 80.0% or more.
[0020]
　Residual austenite improves ductility by the TRIP effect and contributes to improvement of uniform elongation (uniform elongation of 5.0% or more described later). Therefore, the volume fraction of retained austenite is set to more than 3.0% in the structure at a position of 1/4 of the plate thickness in the plate thickness direction from the surface. The volume fraction of retained austenite is preferably 3.5%, more preferably 4.0% or more.
　On the other hand, when the volume fraction of retained austenite becomes excessive, the particle size of retained austenite becomes large, and martensite becomes coarse and hard after deformation. In this case, the starting point of cracking is likely to occur, and the bendability deteriorates. Therefore, the volume fraction of retained austenite is set to less than 10.0%. The volume fraction of retained austenite is preferably less than 8.0%, more preferably less than 7.0%.
[0021]
　Ferrite is a soft phase obtained by two-phase region annealing or slow cooling after annealing. When ferrite is mixed with a hard phase such as martensite, the ductility of the steel sheet is improved, but in order to achieve a high strength of 1310 MPa or more, it is necessary to limit the volume fraction of ferrite.
　Bainite is a phase obtained by holding bainite at 350 ° C. or higher and 550 ° C. or lower for a certain period of time after annealing. Since bainite is soft to martensite, it has an effect of improving ductility, but in order to achieve a high strength of 1310 MPa or more, it is necessary to limit the volume fraction like the above ferrite.
[0022]
　Therefore, the volume fractions of ferrite and bainite shall be 25.0% or less in total. It is preferably 15.0% or less, more preferably 10.0% or less.
[0023]
　Martensite (fresh martensite) is a collection of lath-shaped crystal grains formed by transformation from austenite during final cooling. Since martensite is hard and brittle, it easily becomes a cracking starting point during deformation and deteriorates bendability. Therefore, the volume fraction of martensite is set to 5.0% or less. The volume fraction of martensite is preferably 3.0 or less, more preferably 2.0% or less.
[0024]
　In addition to the above, pearlite may be contained as the residual structure in the structure at the position of 1/4 of the plate thickness from the surface. However, pearlite is a structure having cementite in the structure and consumes C in steel which contributes to the improvement of strength. Therefore, if the volume fraction of pearlite exceeds 5.0%, the strength of the steel sheet decreases. Therefore, the volume fraction of pearlite is 5.0% or less. The volume fraction of pearlite is preferably 3.0% or less, more preferably 1.0% or less.
[0025]
　The volume fraction of the steel sheet according to the present embodiment in the structure at a position 25 μm from the surface and in the structure at a position 1/4 of the plate thickness from the surface is measured as follows. That is, for the volume ratios of ferrite, bainite, martensite, tempered martensite, and pearlite, test pieces are taken from arbitrary positions with respect to the rolling direction and width direction of the steel sheet, and the vertical cross section parallel to the rolling direction is polished to make the steel sheet At a position 25 μm from the surface and a position 1/4 of the plate thickness, the metallographic structure exposed by bainite etching is observed using SEM. In the SEM observation, five fields of view of 30 μm × 50 μm are observed at a magnification of 3000 times, the area ratio of each tissue is measured from the observed images, and the average value is calculated. Since there is no structural change in the direction perpendicular to the rolling direction (steel plate width direction) and the area ratio of the vertical cross section parallel to the rolling direction is equal to the volume ratio, the area ratio is taken as each volume ratio. When measuring the area ratio of each structure, the region where the substructure does not appear and the brightness is low is defined as ferrite. Further, the region where the substructure does not appear and the brightness is high is designated as martensite or retained austenite. In addition, the area where the substructure appears is tempered martensite or bainite.
[0026]
　Bainite and tempered martensite can be further distinguished by careful observation of the carbides in the grains.
　Specifically, tempered martensite is composed of martensite lath and cementite formed inside the lath. At this time, since there are two or more types of crystal orientation relationships between martensite and cementite, cementite constituting tempered martensite has a plurality of variants. On the other hand, bainite is classified into upper bainite and lower bainite. Since the upper bainite is composed of lath-shaped bainitic ferrite and cementite formed at the lath interface, it can be easily distinguished from tempered martensite. The lower bainite is composed of lath-shaped bainite ferrite and cementite formed inside the lath. At this time, the crystal orientation relationship between bainitic ferrite and cementite is one kind unlike tempered martensite, and cementite constituting the lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished based on a variant of cementite.
　On the other hand, martensite is indistinguishable from retained austenite by SEM observation. Therefore, the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite calculated by the method described later from the area fractions other than ferrite, bainite, tempered martensite, and pearlite.
　However, since the C concentration of the surface layer becomes low due to decarburization, retained austenite is not generated. Therefore, in the surface layer portion, the tissue judged to be martensite or retained austenite by SEM observation is judged to be martensite without distinguishing from retained austenite.
　Further, in the steel sheet according to the embodiment, ferrite and bainite are the main structures at the surface layer 25 μm position, but martensite and tempered martensite have a hard structure with respect to these structures.
　Therefore, the particle size of martensite and tempered martensite at the position of 25 μm on the surface layer does not distinguish between martensite and tempered martensite, and is the equivalent circle size of martensite, tempered martensite, or a mixture of martensite and tempered martensite. Is calculated.
　Specifically, the average particle size of martensite and tempered martensite at a position 25 μm from the surface of the steel sheet according to the present embodiment is determined by the following method.
　A test piece is taken from an arbitrary position with respect to the rolling direction and width direction of the steel sheet, and the vertical cross section parallel to the rolling direction is polished. The metallographic structure exposed by etching is observed using SEM. For this structure, the circle-equivalent average diameter of the structure determined to be martensite or tempered martensite described above was calculated by the cutting method described in JIS G 0551 (2013), and the average particle size of martensite and tempered martensite was calculated. And.
[0027]
　For the volume ratio of retained austenite, a test piece is taken from an arbitrary position on the steel sheet, the rolled surface is chemically polished from the surface of the steel sheet to a position inside 1/4 of the plate thickness, and ferrite (200), (210) by MoKα ray ) Surface integral strength and (200), (220), and (311) surface integral strength of austenite are quantified.
　The volume fraction of retained austenite at a position of 25 μm from the surface can be measured by the same method as described above by chemically polishing the rolled surface from the surface of the steel sheet to the position of 25 μm in thickness. However, as described above, since the C concentration of the surface layer portion is lowered by decarburization, retained austenite is not substantially produced. Therefore, the volume fraction of retained austenite at a position 25 μm from the surface does not have to be measured.
[0028]
[Tensile strength is 1310 MPa or more, uniform elongation is 5.0% or more]
[Ratio (R / t) of limit bending radius R and plate thickness t at 90 ° V bending is 5.0 or less] Steel
　sheet according to this embodiment Then, the tensile strength (TS) is set to 1310 MPa or more as the strength that contributes to the weight reduction of the vehicle body of the automobile. From the viewpoint of shock absorption, the strength of the steel sheet is preferably 1400 MPa or more, more preferably 1470 MPa or more.
　Further, from the viewpoint of moldability, the uniform elongation (uEl) is set to 5.0% or more. In order to improve the moldability, the uniform elongation (uEl) is more preferably 5.5% or more. Further, from the viewpoint of moldability, the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending is 5.0 or less. (R / t) is preferably 4.0 or less, and more preferably 3.0 or less in order to improve the moldability.
[0029]
　Tensile strength (TS) and uniform elongation (uEl) are determined by taking a JIS No. 5 tensile test piece from a steel sheet in the direction perpendicular to the rolling direction and performing a tensile test along JIS Z 2241 (2011).
　Regarding the limit bending radius (R), a 90 ° V bending die is used to change the radius R at a pitch of 0.5 mm to obtain the minimum bending radius that does not cause cracking, and the bending radius is divided by the plate thickness t. Ask.
[0030]
　2. 2. Chemical Composition
　of Steel Sheet Next, the chemical composition of the steel sheet according to the present embodiment will be described. Hereinafter, "%" indicating the content of each element in the chemical composition means mass%.
[0031]
　C: More than
　0.140% and less than 0.400% When the C content is 0.140% or less, it becomes difficult to obtain the above metal structure, and the tensile strength cannot be achieved. Further, the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending is deteriorated. Therefore, the C content is set to more than 0.140%. It is preferably more than 0.160%, more preferably more than 0.180%.
　On the other hand, when the C content is 0.400% or more, the weldability deteriorates and the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending deteriorates. The hydrogen embrittlement resistance also deteriorates. Therefore, the C content is set to less than 0.400%. It is preferably less than 0.350%, more preferably less than 0.300%.
[0032]
　Si: More than 0.35% and less than 1.50%
　Si is an element useful for increasing the strength of steel sheet by solid solution strengthening. Further, since Si suppresses the formation of cementite, it has an effect of promoting the concentration of C in austenite, and is an essential element for forming retained austenite after annealing. If the Si content is 0.35% or less, it becomes difficult to obtain the effect of the above action, it becomes difficult to achieve uniform elongation, and the hydrogen embrittlement resistance deteriorates. Therefore, the Si content is set to more than 0.35%. It is preferably more than 0.40%, more preferably more than 0.45%.
　On the other hand, when the Si content is 1.50% or more, the austenite transformation at the time of annealing heating is delayed, and the transformation from ferrite to austenite may not sufficiently occur. In this case, ferrite remains excessively in the structure after annealing, and the target tensile strength cannot be achieved. In addition, the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending deteriorates. Further, when the Si content is 1.50% or more, the surface texture of the steel sheet deteriorates. Further, the chemical conversion processability and the plating property are significantly deteriorated. Therefore, the Si content is set to less than 1.50%. The Si content is preferably less than 1.25%, more preferably less than 1.00%, still more preferably 0.90% or less or 0.85% or less. In particular, when the Si content is less than 1.00%, the plating adhesion is improved.
[0033]
　Mn: More than 1.50% and less than 4.00%
　Mn has an effect of improving hardenability of steel and is an effective element for obtaining the above metal structure. If the Mn content is 1.50% or less, it becomes difficult to obtain the above metal structure. In this case, the tensile strength cannot be achieved. Therefore, the Mn content is set to more than 1.50%. The Mn content is preferably more than 1.75%, more preferably more than 2.00%, still more preferably more than 2.25%.
　On the other hand, when the Mn content is 4.00% or more, the bendability is impaired due to segregation of Mn. In addition, the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending deteriorates, and the hydrogen embrittlement resistance also deteriorates. Furthermore, it causes an increase in material cost. Therefore, the Mn content is set to less than 4.00%. The Mn content is preferably less than 3.50%, more preferably less than 3.20%, still more preferably less than 3.00%.
[0034]
　P: 0.100% or less
　P is an element contained in steel as an impurity, and is an element that segregates at grain boundaries to embrittle the steel. Therefore, the smaller the P content, the more preferably 0%, but the P content is set to 0.100% or less in consideration of the removal time and cost of P. It is preferably 0.020% or less, and more preferably 0.015% or less.
[0035]
　S: 0.010% or less
　S is an element contained in steel as an impurity and forms sulfide-based inclusions to deteriorate bendability. Therefore, the smaller the S content, the more preferably 0%, but the S content is 0.010% or less in consideration of the removal time and cost of S. The S content is preferably 0.005% or less, more preferably 0.003% or less, still more preferably 0.001% or less.
[0036]
　Al: 0.100% or less
　Al is an element having an action of deoxidizing molten steel. When Al is contained for the purpose of deoxidation, 0.005% or more is preferable, and 0.010% or more is more preferable, in order to surely deoxidize. Further, Al has an action of enhancing the stability of austenite like Si, and is an effective element for obtaining the above-mentioned metal structure, and therefore may be contained.
　On the other hand, if the Al content is too high, not only surface defects due to alumina are likely to occur, but also the transformation point is greatly increased, and the volume fraction of ferrite is increased. In this case, it becomes difficult to obtain the above-mentioned metal structure, and the tensile strength cannot be achieved. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less, still more preferably 0.030% or less. Since the steel sheet according to the present embodiment contains Si having a deoxidizing action like Al, it is not always necessary to contain Al, and it may be 0%.
[0037]
　N: 0.0100% or less
　N is an element contained in steel as an impurity, and is an element that produces coarse precipitates and deteriorates bendability. Therefore, the N content is 0.0100% or less. It is preferably 0.0060% or less, and more preferably 0.0050% or less. The smaller the N content, the more preferably 0%.
[0038]
　The steel sheet according to the present embodiment contains the above elements, and the balance may be Fe and impurities, but one or more of the elements that affect the strength and bendability listed below are optional elements. May be further contained. However, since these elements do not necessarily have to be contained, the lower limit thereof is 0%.
[0039]
　Ti: less than 0.050%, Nb: less than 0.050%, V: 0.50% or less
　Ti, Nb and V have an action of improving the strength of the steel sheet by precipitation hardening. Therefore, these elements may be contained. In order to obtain the above effects sufficiently, the lower limit of the Ti and Nb contents is preferably 0.001%, and the lower limit of the V content is preferably 0.01%. The lower limit of the more preferable Ti and Nb contents is 0.005%, and the lower limit of the V content is 0.05%. It is not essential to obtain the above effects. Therefore, it is not necessary to particularly limit the lower limit of the contents of Ti, Nb, and V, and the lower limit thereof is 0%.
　However, if these elements are excessively contained, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability is impaired.
　Therefore, even when it is contained, the Ti content is less than 0.050%, the Nb content is less than 0.050%, and the V content is 0.50% or less. The Ti content is preferably less than 0.030%, more preferably less than 0.020%. The Nb content is preferably less than 0.030%, more preferably less than 0.020%. The V content is preferably 0.30% or less.
[0040]
　Cr: 1.00% or less, Mo: 0.50% or less, B: 0.0100% or less
　Cr, Mo and B have an action of improving the hardenability of steel and affecting the strength. It is an effective element for obtaining the metallographic structure of. Therefore, these elements may be contained. In order to obtain the above effects sufficiently, it is preferable that the lower limit of the content of Cr and Mo is 0.01% and the lower limit of the content of B is 0.0001%. A more preferable lower limit is 0.05% for Cr and Mo, and 0.0010% for B. It is not essential to obtain the above effects. Therefore, it is not necessary to particularly limit the lower limit of the content of Cr, Mo, and B, and the lower limit thereof is 0%.
　However, even if these elements are excessively contained, the effect of the above action is saturated and it becomes uneconomical. Therefore, even when it is contained, the Cr content is 1.00% or less, the Mo content is 0.50% or less, and the B content is 0.0100% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less.
[0041]
　Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0500% or less and Bi: 0.050% or less
　Ca, Mg and REM are solidified by adjusting the shape of inclusions. Both are elements that have the effect of improving strength and bendability by refining the structure. Therefore, these elements may be contained. In order to sufficiently obtain the above effects, the lower limit of the Ca and Mg contents is preferably 0.0001%, and the lower limit of the REM and Bi contents is 0.005%. A more preferable lower limit is 0.0008% for Ca and Mg, and 0.0007% for REM and Bi. It is not essential to obtain the above effects. Therefore, it is not necessary to particularly limit the lower limit of the contents of Ca, Mg, Sb, Zr and REM, and the lower limit thereof is 0%.
　However, even if it is contained in an excessive amount, the effect of the above action is saturated and it becomes uneconomical. Therefore, even when it is contained, the Ca content is 0.0100% or less, the Mg content is 0.0100% or less, the REM content is 0.0500% or less, and the Bi content is 0.050% or less. Preferably, the Ca content is 0.0020% or less, the Mg content is 0.0020% or less, the REM content is 0.0020% or less, and the Bi content is 0.010% or less. REM means a rare earth element and is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the REM content is the total content of these elements.
[0042]
　The steel sheet according to the present embodiment may be provided with a hot-dip galvanized layer on the surface. Corrosion resistance is improved by providing a plating layer on the surface. If there is a concern about perforation due to corrosion, the steel sheet for automobiles may not be thinned to a certain thickness or less even if the strength is increased. One of the purposes of increasing the strength of a steel sheet is to reduce the weight by making it thinner. Therefore, even if a high-strength steel sheet is developed, the application site is limited if the corrosion resistance is low. As a method for solving these problems, it is conceivable to apply plating such as hot dip galvanizing having high corrosion resistance to the steel sheet. Since the steel sheet component according to the present embodiment is controlled as described above, hot-dip galvanizing is possible.
　The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.
[0043]
　3. 3. Manufacturing conditions
　As a result of studies by the present inventors, after appropriate decarburization under predetermined atmospheric conditions during annealing heating, an appropriate residence time is secured during annealing cooling, which is 1/4 of the surface plate thickness. It was found that the structure at the position of is an annealed martensite-based structure, and the structure is different between the surface layer portion and the position of 1/4 of the surface plate thickness, and an inclined structure having excellent hydrogen embrittlement resistance can be achieved. The details will be described below.
[0044]
　Specifically, the steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (IX).
(I) A hot-rolling step in which a cast slab having a predetermined chemical composition is directly or once cooled and then heated and hot-rolled to obtain a hot-rolled steel sheet.
(II) The hot-rolled steel sheet is pickled. , cold rolling step of cold-rolled steel sheet by performing cold rolling,
and (III) the cold-rolled steel sheet, and a dew point of -20 ° C. or higher 20 ° C. or less, the following 20 vol% 1.0 vol% hydrogen in an atmosphere containing a nitrogen, annealing step of annealing by soaking at 800 ° C. or higher temperatures,
(IV) after said annealing, the cold-rolled steel sheet 425 ° C. greater than the first cooling for cooling to a temperature range below 600 ° C. Steps,
(V) A holding step of allowing the cold-rolled steel sheet to stay in the temperature range above 425 ° C. and below 600 ° C. for 250 seconds or more and 750 seconds or less after the first cooling step,
(VI) After the holding step, the cooling second cooling step of cooling the rolled steel sheet to a temperature below 250 ° C. 50 ° C. or higher,
tempering performing (VII) wherein after the second cooling step, tempering at least 1 second at a temperature of 350 ° C. 250 ° C. or higher in the cold-rolled steel sheet Steps,
(VIII) a third cooling step of cooling to a temperature at which skin pass rolling is possible after the tempering step, and
(IX) a skin pass step of performing skin pass rolling on the cold-rolled steel sheet after the third cooling step.
　Hereinafter, each step will be described.
[0045]
[Hot Rolling Step] In the
　hot rolling step, a cast slab having the above-mentioned chemical composition is heated and hot-rolled to obtain a hot-rolled steel sheet. When the temperature of the cast slab is high, it may be subjected to hot rolling as it is without being cooled to around room temperature.
　The conditions for hot rolling are not limited, but it is preferable to heat to 1100 ° C. or higher and hot roll so that the temperature on the exit side of finish rolling is equal to or higher than the Ar3 transformation point. If the heating temperature is less than 1100 ° C., the homogenization of the material tends to be insufficient. Further, when the temperature on the exit side of finish rolling is less than the Ar3 transformation point, the ferrite processed structure remains, resulting in a non-uniform structure, and the structure after annealing is not uniform, which tends to cause deterioration of bendability.
　The hot-rolled steel sheet after hot rolling may be wound into a coil shape. The winding temperature is not particularly limited, but if it exceeds 650 ° C., the structure of the hot-rolled steel sheet becomes a coarse ferrite pearlite structure, the metal structure of the steel sheet after annealing becomes non-uniform, and the bendability deteriorates. Therefore, the upper limit of the winding temperature is preferably 650 ° C. or lower. The winding temperature is preferably 600 ° C. or lower, more preferably 580 ° C. or lower. On the other hand, if the take-up temperature is less than 500 ° C., the strength of the hot-rolled steel sheet becomes high and the load during cold rolling becomes high. Therefore, the take-up temperature is preferably 500 ° C. or higher. When the strength of the hot-rolled steel sheet is high, softening heat treatment such as BAF may be performed before cold-rolling.
[0046]
[
　Cold- rolled step] In the cold-rolled step, the hot-rolled hot-rolled steel sheet is descaled by pickling or the like and then cold-rolled to obtain a cold-rolled steel sheet. The cold rolling (cold rolling) conditions are not particularly limited, but the bendability is improved by promoting recrystallization and homogenizing the metal structure after cold rolling and annealing. Therefore, it is preferable that the cold pressure rate (cumulative reduction rate) is 40% or more. The cold spreading ratio is preferably 45% or more, more preferably 50% or more.
　If the cold pressure ratio is too high, the rolling load increases and rolling becomes difficult. Therefore, the cold pressure ratio is preferably less than 70%. The cold spreading rate is preferably less than 65%, more preferably less than 60%.
[0047]
[Annealing step]
　The steel sheet after the cold rolling step is annealed after being subjected to a treatment such as degreasing according to a known method, if necessary.
　The steel sheet according to this embodiment contains Si. Therefore, from the viewpoint of chemical conversion treatment property of steel sheet or plating adhesion, the atmosphere in the furnace is controlled at the time of annealing heating in order to internally oxidize Si and Mn. Specifically, the atmosphere inside the furnace (heating zone and tropical zone) contains hydrogen with a dew point of -20 ° C or higher and 20 ° C or lower, 1.0% by volume or more and 20% by volume or less, and the balance is nitrogen and impurities. A certain nitrogen-hydrogen mixed atmosphere is used. These atmospheres are appropriately adjusted within this range according to the composition of the steel sheet and the manufacturing conditions. By annealing in this atmosphere, appropriate decarburization occurs on the surface layer of the steel sheet. Therefore, by adjusting the cooling conditions after annealing, it is possible to obtain an inclined structure in which the volume fraction of the structure at the position 1/4 from the surface and the volume fraction of the structure of the surface layer portion are different as described above. Become. That is, since the surface layer having a low C content undergoes ferrite transformation and bainite transformation prior to the start of transformation of the central portion having a high C content due to decarburization, only the surface layer portion becomes soft. By obtaining this inclined structure and the structure structure at a predetermined position, both moldability and hydrogen embrittlement resistance can be achieved at a high level.
　In general, the higher the Si content, the easier it is for decarburization to occur. However, in the steel sheet according to the present embodiment, the upper limit of the Si content is limited in consideration of plating adhesion and the like. Therefore, in the chemical composition of the steel sheet according to the present embodiment, a preferable surface layer structure cannot be obtained unless the temperature history after annealing is controlled.
[0048]
　The soaking temperature in the annealing step is 800 ° C. or higher. If the soaking temperature is less than 800 ° C, the volume ratio of ferrite at the position 1/4 from the surface increases and the ratio of tempered martensite becomes insufficient, resulting in strength and limit bending radius R and plate thickness at 90 ° V bending. It becomes difficult to secure the ratio of t (R / t). The soaking temperature is preferably 820 ° C. or higher, more preferably 840 ° C. or higher. The higher the soaking temperature, the easier it is to secure the strength. However, if the soaking temperature is too high, the manufacturing cost increases. Therefore, the soaking temperature is preferably 900 ° C. or lower. 880 ° C or lower is more preferable, and 870 ° C or lower is even more preferable.
　The soaking time is preferably 30 to 450 seconds. If the soaking time is less than 30 seconds, austenitization does not proceed sufficiently, so that the soaking time is preferably 30 seconds or more. On the other hand, if the heat equalizing time exceeds 450 seconds, the productivity decreases, so that the heat equalizing time is preferably 450 seconds or less.　
[0049]
　In the heating step performed prior to the soaking step in the annealing step, recrystallization is promoted to homogenize the metal structure after annealing to improve bendability, and decarburization of the surface layer is promoted to soften the surface of the steel sheet. It is preferable that the heating rate from 700 ° C. to the soaking temperature is less than 10.0 ° C./s in order to improve the hydrogen embrittlement resistance. It is more preferably less than 8.0 ° C./s, and even more preferably less than 5.0 ° C./s.
[0050]
[First cooling step]
[Holding step]
　The cold-rolled steel sheet after annealing is cooled to a temperature range of more than 425 ° C and less than 600 ° C in order to obtain the above-mentioned inclined structure (first cooling step). It is held in the range (more than 425 ° C., less than 600 ° C.) so that the staying time is 250 seconds or more and 750 seconds or less (holding step). When the cooling stop temperature and the subsequent holding temperature are 425 ° C. or lower, the volume fraction of bainite at a position 1/4 of the plate thickness from the surface of the steel sheet increases, and the volume fraction of tempered martensite decreases. As a result, the tensile strength is lowered and the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending is deteriorated. In the present embodiment, the strength is ensured by the presence of sufficient tempered martensite at a position 1/4 of the plate thickness from the surface of the steel plate. Bainite cannot provide sufficient strength.
　On the other hand, when the cooling stop temperature and the subsequent holding temperature are 600 ° C. or higher, the ferrite fraction increases in the central portion of the steel sheet and the volume fraction of tempered martensite decreases. As a result, the tensile strength is lowered, and the ratio (R / t) of the limit bending radius R and the plate thickness t at 90 ° V bending is deteriorated. Further, the ferrite transformation and the bainite transformation at the surface layer portion of the steel sheet do not proceed, and the surface layer structure as described above cannot be obtained, so that the hydrogen embrittlement resistance deteriorates.
　Therefore, the cooling stop temperature and the holding temperature are set to more than 425 ° C and less than 600 ° C. The holding temperature is preferably more than 440 ° C and less than 580 ° C, more preferably more than 450 ° C and less than 560 ° C. As long as it is within this temperature range, there is no problem even if the temperature is changed during the staying time.
　In the first cooling step, it is preferable to cool at an average cooling rate of 5 ° C./s or more in order to suppress ferrite transformation during cooling. The average cooling rate is more preferably 10 ° C./s or higher.
　FIG. 1A shows the relationship between the volume fractions of ferrite and bainite at a depth of 25 μm (surface layer portion) from the surface of the steel sheet and the residence time at more than 425 ° C and less than 600 ° C. FIG. 1B shows the relationship between the volume fraction of martensite and tempered martensite at a depth of 25 μm (surface layer portion) from the surface of the steel sheet and the staying time at more than 425 ° C and less than 600 ° C. FIG. 1C shows the relationship between the martensite and tempered martensite particle sizes and the residence time above 425 ° C and below 600 ° C. In addition, FIGS. 1A to 1C also show hydrogen embrittlement resistance at that time.
　As shown in FIGS. 1A to 1C, if the residence time is less than 250 seconds, the ferrite transformation and bainite transformation of the surface layer do not proceed, and the untransformed austenite becomes martensite and tempered martensite after final cooling. Not only does the volume ratio of sites and tempered martensite increase, but the particle size also increases. As a result, the above-mentioned surface layer structure cannot be obtained, and the hydrogen embrittlement resistance deteriorates. Therefore, the lower limit of the staying time above 425 ° C. and below 600 ° C. in the holding step is set to 250 seconds or more. The staying time is preferably 300 seconds or longer, more preferably 350 seconds or longer. In the steel sheet according to the present embodiment, the Si content is limited from the viewpoint of plating adhesion, and it is difficult to obtain a decarburized layer on the surface layer, but the staying time is 250 seconds or more in the temperature range of more than 425 ° C and less than 600 ° C. By holding it so as to be, bainite transformation can be caused only in the surface layer portion.
　On the other hand, if the staying time is long, ferrite transformation and bainite transformation occur even at a position 1/4 of the plate thickness from the surface, the desired structure cannot be obtained, the strength of the steel sheet decreases, and the limit bending radius R at 90 ° V bending. The ratio (R / t) of and the plate thickness t deteriorates. Therefore, the upper limit of the staying time above 425 ° C. and below 600 ° C. is 750 seconds or less. The staying time is preferably 650 seconds or less, and more preferably 550 seconds or less.
　In the holding step, it is preferable to use a reducing atmosphere in the furnace from the viewpoint of chemical conversion treatment of the steel sheet or plating adhesion.
[0051]
[Hot-dip galvanizing process]
[Alloying process] When
　producing a cold-rolled steel sheet (hot-dip galvanized steel sheet) having hot-dip galvanizing on the surface, the cold-rolled steel sheet is immersed in a hot-dip galvanizing bath to melt it during the holding process. It may be galvanized. Further, in the case of manufacturing a cold-rolled steel sheet (alloyed hot-dip galvanized steel sheet) having alloyed hot-dip galvanized surface, alloying treatment is performed following the hot-dip galvanized step, and the plating is used as alloyed hot-dip galvanized steel. May be good.
[0052]
[Second cooling step]
[Tempering step] When
　the cold-rolled steel sheet after the holding step is cooled to a temperature of 50 ° C. or higher and 250 ° C. or lower (second cooling step), untransformed austenite is transformed into martensite. In the second cooling step, it is preferable to cool at an average cooling rate of 5 ° C./s or more in order to suppress bainite transformation during cooling. The average cooling rate is more preferably 10 ° C./s or higher. After that, the cold-rolled steel sheet is tempered at a temperature of 200 ° C. or higher and 350 ° C. or lower for 1 second or longer (tempering step), so that a tempered martensite-based structure can be obtained at a position 1/4 of the plate thickness from the surface. ..
　When the hot-dip galvanizing step and / or the alloying step is performed, the temperature of the cold-rolled steel sheet after the hot-dip galvanizing step or the cold-rolled steel sheet after the hot-dip galvanizing step and the alloying step is 50 ° C. or higher and 250 ° C. or lower. After cooling to the above temperature, bake at a temperature of 200 ° C. or higher and 350 ° C. or lower for 1 second or longer.
[0053]
　If the cooling stop temperature in the second cooling step exceeds 250 ° C., the martensitic transformation becomes insufficient, the volume fraction of untempered martensite increases, and the bendability deteriorates. On the other hand, if the cooling stop temperature in the second cooling step is less than 50 ° C., residual austenite does not remain and the ductility deteriorates. Therefore, the cooling stop temperature is set to 50 ° C. or higher and 250 ° C. or lower. The cooling stop temperature is preferably 75 ° C. or higher and 225 ° C. or lower, and more preferably 100 ° C. or higher and 200 ° C. or lower.
　In the subsequent tempering step, if the tempering temperature exceeds 350 ° C., the strength of the steel sheet decreases. Therefore, the tempering temperature is 350 ° C. or lower. The tempering temperature is preferably 330 ° C. or lower, more preferably 310 ° C. or lower.
　On the other hand, if the tempering temperature is less than 200 ° C., the tempering becomes insufficient and the bendability deteriorates. Therefore, the tempering temperature is set to 200 ° C. or higher. The tempering temperature is preferably 250 ° C. or higher, more preferably 260 ° C. or higher, and even more preferably 270 ° C. or higher.
　The tempering time may be 1 second or longer, but 5 seconds or longer is preferable and 10 seconds or longer is more preferable in order to perform a stable tempering treatment. On the other hand, since the strength of the steel sheet may decrease when tempered for a long time, the tempering time is preferably 90 seconds or less, more preferably 60 seconds or less.
[0054]
[Third cooling step]
[Skin pass step]
　The cold-rolled steel sheet after the tempering step is cooled to a temperature at which skin pass rolling is possible (third cooling step), and then skin pass rolling is performed (skin pass step). When cooling after annealing (first cooling step) is water spray cooling using water, dip cooling, air-water cooling, etc., removal of oxide film formed by contact with water at high temperature and chemical conversion processability of steel sheet For improvement, it is preferable to perform pickling and subsequently plating one or more of a small amount of Ni, Fe, Co, Sn, and Cu before the skin pass rolling. Here, the trace amount means a plating amount of about 3 to 30 mg / m 2 on the surface of the steel sheet .
[0055]
　The shape of the steel sheet can be adjusted by skin pass rolling. The elongation rate of skin pass rolling is preferably 0.1% or more. It is more preferably 0.2% or more, still more preferably 0.3% or more. On the other hand, if the elongation rate of skin pass rolling is high, the volume fraction of retained austenite decreases and the ductility deteriorates. Therefore, the elongation rate is preferably 1.0% or less. The elongation rate is more preferably 0.8% or less, further preferably 0.6% or less, and even more preferably 0.5% or less.
Example
[0056]
　The present invention will be described in more detail with reference to Examples. A slab having the chemical composition shown in Table 1 was cast. The slab after casting was heated to 1100 ° C. or higher, hot-rolled to 2.8 mm so that the finish rolling output side temperature was equal to or higher than the Ar3 transformation point, wound at 500 ° C. or higher and 650 ° C. or lower, and then cooled to room temperature.
　Then, the scale was removed by pickling, cold-rolled to 1.4 mm, and then annealed at the soaking temperature shown in Table 2A for 120 seconds. For annealing, the atmosphere inside the furnace during heating and soaking was a nitrogen-hydrogen mixed atmosphere consisting of hydrogen and nitrogen with a dew point of -20 ° C or higher and 20 ° C or lower and 1.0% by volume or more and 20% by volume or less. .. The heating rate from 700 ° C. to the soaking temperature during annealing was set to less than 5.0 ° C./s.
　After annealing, the mixture was cooled to the temperature shown in Table 2A at 10 ° C./s, and then allowed to stay between 425 ° C. and less than 600 ° C. For some examples, hot dip galvanizing and alloying were performed during retention. In Table 2C, CR is a cold-rolled steel sheet that has not been galvanized, GI is a hot-dip galvanized steel sheet, and GA is an alloyed hot-dip galvanized steel sheet. The alloyed hot-dip galvanized steel sheet was subjected to hot-dip galvanizing at about 35 to 65 g / m 2 and then alloyed at a temperature of less than 600 ° C. In this embodiment, the temperature during the staying time of more than 425 ° C. and less than 600 ° C. is constant, but as described above, if it is within this temperature range, there is no problem even if the temperature is changed during the staying time.
　After holding, it is cooled to 50 ° C. or higher and 250 ° C. or lower at 10 ° C./s or higher, then heat-treated for 1 to 90 seconds, and then cooled to 50 ° C. to achieve a skin pass of 0.1% or higher. It was rolled. The tempering temperature was 250 to 350 ° C. for test numbers 1 to 33 and 200 ° C. for test numbers 34.
　For the cold-rolled steel sheets of test numbers 22, 23, and 30 , Ni plating of about 3 to 30 mg / m 2 was applied to the surface of the steel sheet after pickling before skin pass rolling .
[0057]
　From the obtained annealed steel sheet (cold-rolled steel sheet), a test piece for SEM observation was collected as described above, and after polishing the vertical cross section parallel to the rolling direction, the position 25 μm from the steel sheet surface and 1/4 of the sheet thickness. The metallographic structure at the position was observed, and the volume ratio of each structure was measured by image processing. In addition, a test piece for X-ray diffraction was collected, and the volume fraction of retained austenite was measured by X-ray diffraction on a surface chemically polished to a thickness of 1/4 from the surface layer as described above. In addition, the average particle size of martensite and tempered martensite 25 μm from the surface of the steel sheet was measured.
[0058]
　Tensile strength (TS) and uniform elongation (uEl) are determined by collecting a JIS No. 5 tensile test piece from an annealed steel sheet in the direction perpendicular to the rolling direction and performing a tensile test along JIS Z 2241 (2011). It was.
[0059]
　In addition, the following tests were conducted to evaluate the hydrogen embrittlement resistance. That is, a test piece whose end face is mechanically ground is bent into a U shape by a push-bending method to prepare a U-bending test piece having a radius of 5R, tightened with bolts so that the non-bent portions are parallel, and then elastically deformed. A delayed fracture acceleration test was conducted in which hydrogen was allowed to enter the steel sheet by immersing it in hydrochloric acid having a pH of 1. A steel sheet in which cracks did not occur even when the immersion time was 100 hours was evaluated as a steel sheet having good (OK) delayed fracture resistance, and a steel sheet in which cracks occurred was evaluated as defective (NG). In order to eliminate the influence of plating, the plating material was evaluated for hydrogen embrittlement resistance after removing the plating layer with hydrochloric acid containing an inhibitor before the test.
[0060]
　For the limit bending radius (R / t), use a 90 ° V bending die to change the radius R at a pitch of 0.5 mm to obtain the minimum bending radius that does not cause cracking, and divide by the plate thickness of 1.4 mm. I asked for it.
[0061]
[table 1]

[0062]
[Table 2A]

[0063]
[Table 2B]

[0064]
[Table 2C]

[0065]
　Tables 2B and 2C show the metallographic structure observation results and mechanical property survey results of the annealed steel sheet. All of the steels of the present invention have a TS of 1310 MPa or more, a uEl of 5.0% or more, a critical bending radius (R / t) of 5.0 or less, and have good hydrogen embrittlement resistance.
　On the other hand, in the test number (comparative example) in which either the chemical composition or the production method was outside the scope of the present invention and the structure was outside the scope of the present invention, the tensile strength, uniform elongation, limit bending radius, and hydrogen resistance Any one or more of the embrittlement properties did not meet the target. In particular, in Test No. 5, since the Si content was high, the volume fraction of ferrite was high, and the volume fraction of tempered martensite was reduced. As a result, the tensile strength was low and the critical bending radius (R / t) was inferior. In the example of the present invention, the hydrogen embrittlement resistance and the tensile strength are determined by appropriately controlling the heat treatment conditions at the time of annealing to control the metal structure at the position of 1/4 of the plate thickness from the surface of the steel sheet while suppressing the Si content. Achievement of both.
Industrial applicability
[0066]
　According to the present invention, the tensile strength (TS) is 1310 MPa or more, the uniform elongation is 5.0% or more, and the ratio (R / t) of the limit bending radius R to the plate thickness t at 90 ° V bending is 5.0 or less. Further, a high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and a method for producing the same can be obtained. Such a steel sheet has sufficient formability that can be applied to processing such as press molding, and is excellent in hydrogen embrittlement resistance, which is a problem in increasing strength. Therefore, the present invention contributes greatly to the development of industry, such as being able to contribute to solving global environmental problems by reducing the weight of the vehicle body.
The scope of the claims
[Claim 1]
　By mass%,
　C: more than 0.140%, less than 0.400%,
　Si: more than 0.35%, less than 1.50%,
　Mn: more than 1.50%, less than 4.00%,
　P: 0. 100% or less,
　S: 0.010% or less,
　Al: 0.100% or less,
　N: 0.0100% or less,
　Ti: 0% or more, less than 0.050%,
　Nb: 0% or more, 0.050% Less than,
　V: 0% or more, 0.50% or less,
　Cr: 0% or more, 1.00% or less,
　Mo: 0% or more, 0.50% or less,
　B: 0% or more, 0.0100% or less,
　Ca: 0% or more, 0.0100% or
　less, Mg: 0% or more, 0.0100% or
　less, REM: 0% or more, 0.0500% or
　less, Bi: 0% or more 0.050% or less,
　containing
　However , the balance has a chemical composition of Fe and impurities, and
　the structure at a position 1/4 of the plate thickness from the surface is
　　a tempered martensite with a volume ratio of 70.0% or more.
　　It 　　contains more than 3.0% and less than 10.0% retained austenite, a
　　total of 25.0% or less ferrite and bainite, and
　　5.0% or less martensite, and
has a
　structure at a position 25 μm from the surface. , by volume,
　　the ferrite and bainite 70% or more in total,
　　and martensite and tempered martensite of 30% or less in total,
　　includes,
　at a position 25μm from the surface of the martensite and the tempered martensite The average particle size is 5.0 μm or less, the
　tensile strength is 1310 MPa or more, the uniform elongation is 5.0% or more, and the ratio of the critical bending radius R and the plate thickness t in 90 ° V bending is R / A
high-strength cold-rolled steel plate, characterized in that t is 5.0 or less .
[Claim 2]
　The chemical composition is 　Ti: 0.001% or more and less than 0.050%, 　Nb: 0.001% or more and less than 0.050%, 　V: 0.01% or more and 0.50% or less in
　mass%. , 　Cr: 0.01% or more, 1.00% or less, 　Mo: 0.01% or more, 0.50% or less, 　B: 0.0001% or more, 0.0100% or less, 　Ca: 0.0001% or more , 0.0100% or less, 　Mg: 0.0001% or more, 0.0100% or less, 　REM: 0.005% or more, 0.0500% or less, and 　Bi: 0.005% or more, 0.050% or less, The high-strength cold-rolled steel sheet according to claim 1, further comprising one or more of the above .

[Claim 3]
　The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the surface is provided with a hot-dip galvanized layer.
[Claim 4]
　The high-strength cold-rolled steel sheet according to claim 3, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.
[Claim 5]
　By mass%, C: more than 0.140%, less than 0.400%, Si: more than 0.35%, less than 1.50%, Mn: more than 1.50%, less than 4.00%, P: 0. 100% or less, S: 0.010% or less, Al: 0.100% or less and N: 0.0100% or less, Ti: 0% or more, less than 0.050%, Nb: 0% or more, 0.050% Less than, V: 0% or more, 0.50% or less, Cr: 0% or more, 1.00% or less, Mo: 0% or more, 0.50% or less, B: 0% or more, 0.0100% or less, Contains Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, Bi: 0% or more, 0.050% or less. A hot-rolling step in which a cast slab having a chemical composition in which the balance is composed of Fe and impurities is directly or once cooled and then heated and hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled
　steel sheet is acidified. A cold-rolling step of washing and cold-rolling to obtain a cold-rolled steel sheet, and the cold-rolled
　steel sheet having a dew point of −20 ° C. or higher and 20 ° C. or lower and 1.0% by volume or more and 20% by volume or less of nitrogen. An annealing step of soaking and annealing at a temperature of 800 ° C. or higher in an atmosphere containing hydrogen, and a
　first cooling step of cooling the cold-rolled steel sheet to a temperature range of more than 425 ° C. and less than 600 ° C. after the annealing step. a step,
　after the first cooling step, the cold-rolled steel sheet to 425 ° C. greater than the holding step to be staying in the temperature range below 600 ° C. 250 seconds to 750 seconds or less,
　after the holding step, the cold-rolled steel sheet 50 A second cooling step of cooling to a temperature of ° C. or higher and 250 ° C. or lower,
　and a tempering step of tempering the cold-rolled steel sheet at a temperature of 250 ° C. or higher and 350 ° C. or lower for 1 second or longer after the second cooling step.
　　After the tempering step , it is characterized by
　　comprising a third cooling step of cooling to a temperature at which skin pass rolling is possible, and a skin pass step of performing skin pass rolling on the cold-rolled steel sheet after the third cooling step. Manufacturing method of rolled steel sheet.

[Claim 6]
　The chemical composition is Ti: 0.001% or more, less than 0.050%, Nb: 0.001% or more, less than 0.050%, V: 0.01% or more, 0.50% or less in mass%. , Cr: 0.01% or more, 1.00% or less, Mo: 0.01% or more, 0.50% or less and B: 0.0001% or more, 0.0100% or less, Ca: 0.0001% or more , 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.005% or more, 0.0500% or less, and Bi: 0.005% or more, 0.050% or less. The method for producing a high-strength cold-rolled steel sheet according to claim 5, wherein one or more of them are contained.
[Claim 7]
　The method for producing a high-strength cold-rolled steel sheet according to claim 5 or 6, wherein the holding step includes a hot-dip galvanizing step of applying hot-dip galvanizing to the cold-rolled steel sheet.
[Claim 8]
　The method for producing a high-strength cold-rolled steel sheet according to claim 7, further comprising an alloying step of performing an alloying treatment after the hot-dip galvanizing step of the holding step.