Title of the invention: Cold-rolled steel sheet and its manufacturing method
Technical field
[0001]
　The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same.
　This application claims priority under Japanese Patent Application No. 2019-186743 filed in Japan on October 10, 2019 and Japanese Patent Application No. 2019-186957 filed in Japan on October 10, 2019. , The contents are used here.
Background technology
[0002]
　Today, when the industrial technology field is highly divided, 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 demand for high-tensile cold-rolled steel sheets having a thin plate thickness and excellent formability in order to reduce the weight of the vehicle body and improve fuel efficiency. Among steel sheets for automobiles, cold-rolled steel sheets used for car body frame parts are required to have high strength, and further, high formability for expanding the application is required. Examples of the characteristics required for a steel sheet for automobiles are a tensile strength (TS) of 1310 MPa or more, a uniform elongation of 5.0% or more, and a TS × λ (hole expansion) of 35,000 MPa ·% or more. Alternatively, depending on the processing method and the applied parts, the ratio (R / t) of the limit bending R and the plate thickness t at 90 ° V bending is 5.0 or less, and further, hydrogen embrittlement resistance characteristics. It is also required to be excellent.
[0003]
　Although it is effective to have a structure containing ferrite in order to secure ductility such as uniform elongation, it is necessary to harden the second phase in order to obtain a strength of 1310 MPa or more in the structure containing ferrite. However, the hard second phase deteriorates the hole expandability and bendability.
[0004]
　On the other hand, as a technique for improving the hole expandability, bendability, and hydrogen embrittlement resistance of high-strength steel sheets, steel sheets containing tempered martensite as the main phase have been proposed (see, for example, Patent Documents 1 and 2). .. Patent Documents 1 and 2 show that the microstructure is a tempered martensite single-phase structure, which is excellent in hole expandability, bendability, and hydrogen embrittlement resistance.
　However, in the invention of Patent Document 1, the tensile strength is as low as less than 1310 MPa. Therefore, when aiming for higher strength, it is necessary to further improve the workability, bendability, and hydrogen embrittlement resistance that deteriorate with it. Further, in the invention of Patent Document 2, although a high strength of 1310 MPa or more can be achieved, since the material is cooled to near room temperature during quenching, the volume fraction of retained austenite is small and high uniform elongation cannot be obtained. There is.
[0005]
　Further, Patent Document 3 proposes a steel sheet utilizing the TRIP effect of retained austenite as a technique for achieving both high strength and high formability.
　However, since the steel sheet of Patent Document 3 has a ferrite phase, it is difficult to obtain a high strength of 1310 MPa or more, and since there is a difference in strength in the structure, it is inferior in drilling formability and bendability.
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 high tensile strength (TS) of 1310 MPa or more and high formability, preferably a steel sheet having bendability and hydrogen embrittlement resistance, has not been proposed.
　The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a high-strength steel sheet having excellent formability, which is a problem with high-strength steel sheets, and a method for manufacturing the same. In the present invention, high strength means that the tensile strength (TS) is 1310 MPa or more, and excellent formability means that the uniform elongation is 5.0% or more and TS × λ (hole expansion) is 35,000 MPa ·%. It means that it is the above.
　A preferred object of the present invention is to provide a high-strength steel plate having excellent formability, which is a problem with a high-strength steel plate, and having sufficient bendability and hydrogen embrittlement resistance, and a method for producing the same. In the present invention, excellent bendability means that the ratio (R / t) of the limit bending R to the plate thickness at 90 ° V bending is 5.0 or less.
　In the present invention, the cold-rolled steel sheet includes a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface and an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface.
Means to solve problems
[0008]
　The present inventors 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 strength is controlled by controlling the texture inside the steel sheet (for example, at a position of 1/4 of the plate thickness from the surface) after making the metal structure a tempered martensite-based structure containing a predetermined amount or more of retained austenite. It was found that both moldability and formability can be achieved at a high level. Further, after controlling the hot rolling conditions, cold rolling with a cumulative rolling reduction of 60% or less is performed, and then the average heating rate from 550 ° C to 750 ° C is 1.0 ° C / s or more and 50 ° C / s or less. By performing annealing by heating in the γ single phase region and soaking in the γ single phase region, recrystallization during annealing can be suppressed and reverse transformation can be promoted, resulting in a random texture that is advantageous for moldability. I found that I could do it.
　Furthermore, as a result of studies by the present inventors, in addition to the above control, by controlling the texture of the surface layer portion, in addition to strength and formability, bendability and hydrogen embrittlement resistance can be obtained. Also found to be obtained at a high level.
　The present invention has been made based on the above findings. The gist of the present invention is as follows.
[0009]
　The present invention has been made based on the above findings. The gist of the present invention is as follows.
[1] The cold-rolled steel sheet according to one aspect of the present invention has a chemical composition of C: more than 0.140%, less than 0.40%, Si: more than 0.35%, less than 1.50% in mass%. , Mn: more than 1.30%, less than 3.50%, P: 0.10% 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, Cu: 0% or more, 1.00% or less, Ni: 0% or more , 1.00% 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 It contains .0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, and Bi: 0% or more, 0.050% or less, and the balance is Fe and It is composed of impurities, and the structure at the position of 1/4 of the plate thickness from the surface is, by volume ratio, tempered martensite of 80.0% or more, residual austenite of more than 2.5% and less than 10.0%, total 0. % Or more and 15.0% or less ferrite and bainite, 0% or more and 3.0% or less martensite, and a residual structure, and the maximum random specific intensity Iq in the structure is 4.0 or less. The average diameter of the region Rq having an orientation within 10 ° from the crystal orientation at which the random specific strength Iq is maximum is 10.0 μm or less, the surface density of the region Rq is 1000 pieces / mm 2 or more, and the tensile strength. Is 1310 MPa or more, uniform elongation is 5.0% or more, and TS × λ is 35000 MPa ·% or more.
[2] The cold-rolled steel sheet according to the above [1] has a maximum random specific strength Is of 4.0 or less in a structure in a range from the surface to 100 μm in the plate thickness direction, and the random specific strength Is is. The average diameter of the regions Rs having an orientation within 10 ° from the maximum crystal orientation is 10.0 μm or less, the areal density of the regions Rs is 1000 pieces / mm 2 or more, and the limit bending at 90 ° V bending. R / t, which is the ratio of R to the plate thickness t, may be 5.0 or less.
[3] The cold-rolled steel sheet according to the above [1] or [2] may have a tensile strength of 1400 MPa or more.
[4] The cold-rolled steel sheet according to any one of the above [1] to [3] has the chemical composition of Ti: 0.001% or more, less than 0.050%, Nb: 0.001 in mass%. % Or more, less than 0.050%, V: 0.01% or more, 0.50% or less, Cr: 0.01% or more, 1.00% or less, Ni: 0.01% or more, 1.00% or less , Cu: 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: 0.0005% or more, 0.0500% or less, and Bi: 0.0005% or more, 0.050%
It may contain one kind or two or more kinds selected from the following .
[5] The cold-rolled steel sheet according to any one of the above [1] to [4] may be provided with a hot-dip galvanized layer on the surface.
[6] In the cold-rolled steel sheet according to the above [5], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.
[7] The method for producing a cold-rolled steel sheet according to another aspect of the present invention is C: more than 0.140%, less than 0.400%, Si: more than 0.35%, less than 1.50% in mass%. , Mn: more than 1.30%, less than 3.50%, P: 0.10% 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, Ni: 0% or more , 1.00% or less, Cu: 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 It contains 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, and Bi: 0% or more, 0.050% or less, and the balance is Fe and A cast slab having a chemical composition composed of impurities is directly or once cooled and then heated to 1100 ° C. or higher, and hot rolling is performed under the conditions that the rolling temperature FT in the final stage is 920 ° C. or higher and the rolling reduction is 15% or lower. The hot-rolled step of obtaining the hot-rolled steel sheet, the cooling step of cooling the hot-rolled steel sheet so as to pass through the temperature range of 750 ° C. to 650 ° C. within 10 seconds, and the hot-rolled steel sheet after the cooling step. , A winding step of winding at 650 ° C. or lower, and a cold rolling step of pickling the hot-rolled steel sheet after the winding step and cold-rolling at a cumulative reduction rate of 60% or less to obtain a cold-rolled steel sheet. The baking step of heating the cold-rolled steel sheet from 550 ° C to 750 ° C at an average heating rate of 1.0 ° C / s or higher and 50.0 ° C / s or lower and soaking at 820 ° C or higher. The temperature of the cold-rolled steel sheet after the baking step is 50 ° C or higher and 250 ° C or lower so that the average cooling rate in the temperature range of 700 ° C to 600 ° C and the temperature range of 450 ° C to 350 ° C is 5 ° C / s or higher. It is provided with a quenching cooling step of cooling to the above, and a tempering step of tempering the cold-rolled steel sheet after the shrinking cooling step in a temperature range of 200 ° C. or higher and lower than 350 ° C. for 1 second or longer
.
[8] In the method for manufacturing a cold-rolled steel sheet according to the above [7], in the hot rolling step, the rolling temperature FT in the final stage is 920 ° C. or higher and 960 ° C. or lower, and the reduction rate is 10% or higher and 15% or lower. The hot rolling is performed under the condition that the friction coefficient μ is 0.15 or more, and in the baking step, the cold-rolled steel sheet is heated from 550 ° C to 750 ° C at 3.0 ° C / s or more, 50.0. It may be heated at an average heating rate of ° C./s or lower and evenly heated at 820 ° C. or higher.
[9] In the method for manufacturing a cold-rolled steel sheet according to the above [7] or [8], in the annealing cooling step, the cold-rolled steel sheet after the annealing step is subjected to a temperature range of 700 ° C. to 600 ° C. and 450 ° C. You may cool to a temperature of 50 ° C. or higher and lower than 220 ° C. so that the average cooling rate in the temperature range from to 350 ° C. is 5 ° C./s or higher.
[10] In the method for producing a cold-rolled steel sheet according to any one of the above [7] to [9], the chemical composition is, in mass%, 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, Cr: 0.01% or more, 1.00% or less, Ni: 0.01% or more, 1. 00% or less, Cu: 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: 0.0005% or more, 0.0500% or less and Bi: 0.0005% or more, 0. It may contain one or more selected from 050% or less.
[11] In the method for manufacturing a cold-rolled steel sheet according to any one of [7] to [10] above, hot-dip galvanizing is further performed in the temperature range of more than 450 ° C and less than 600 ° C in the annealing cooling step. , May be optionally alloyed.
Effect of the invention
[0010]
　According to the above aspect of the present invention, a cold-rolled steel sheet having high strength and excellent formability, having a tensile strength (TS) of 1310 MPa or more, a uniform elongation of 5.0% or more, and a TS × λ of 35,000 MPa ·% or more, and its production. The method is obtained. Such a steel sheet has sufficient formability that can be applied to processing such as press forming. Therefore, the present invention contributes to the development of industry, such as being able to contribute to solving global environmental problems by reducing the weight of the vehicle body of an automobile.
　Further, according to a preferred embodiment of the present invention, the tensile strength (TS) is 1310 MPa or more, the uniform elongation is 5.0% or more, TS × λ is 35000 MPa ·% or more, and the limit bending R at 90 ° V bending is achieved. The ratio (R / t) of the thickness to the plate thickness t is 5.0 or less, and a 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 forming, and is excellent in hydrogen embrittlement resistance. Therefore, the present invention contributes to the development of industry, such as being able to contribute to solving global environmental problems by reducing the weight of the vehicle body of an automobile.
A brief description of the drawing
[0011]
FIG. 1 shows the average diameter of a region having an orientation within 10 ° from the crystal orientation at which the maximum random specific strength of the texture and the maximum random specific strength are located at a position 1/4 of the plate thickness from the surface, and TS ×. It is a figure which shows the relationship with λ. In FIG. 1, ◯ is a steel sheet of TS × λ ≧ 35,000 MPa ·%, and × is a steel sheet of TS × λ <35,000 MPa ·%.
FIG. 2 shows the areal density of a region having an orientation within 10 ° from the crystal orientation at which the maximum random specific strength of the texture and the maximum random specific strength are located at a position 1/4 of the plate thickness from the surface, and TS ×. It is a figure which shows the relationship with λ. In FIG. 2, ◯ is a steel sheet of TS × λ ≧ 35,000 MPa ·%, and × is a steel sheet of TS × λ <35,000 MPa ·%.
FIG. 3 is a diagram showing the relationship between the finishing temperature and winding temperature of hot rolling and the random specific strength of the texture at a position of 1/4 of the plate thickness from the surface. In FIG. 3, 〇 is a steel sheet having a random specific strength of the texture ≦ 4.0, and × is a steel sheet having a random specific strength of the texture> 4.0.
FIG. 4 is the diameter and surface of a region having an orientation within 10 ° from the crystal orientation at which the soaking temperature and winding temperature at the time of annealing and the crystal orientation at which the random specific strength is maximum at the position of 1/4 of the plate thickness from the surface. It is a figure which shows the relationship with density. In FIG. 4, 〇 is a steel plate having a diameter ≤ 10 μm and an area density ≥ 1000 pieces / mm 2 in a region having an orientation within 10 ° from the crystal orientation at which the random specific strength is maximum, and × is a steel plate having a random specific strength. It is a steel plate having a diameter of a region having an orientation within 10 ° from the maximum crystal orientation> 10 μm and an area density <1000 pieces / mm 2 .
FIG. 5 shows the relationship between the average diameter of the region of the surface layer having the maximum random specific strength of the texture and the orientation within 10 ° from the crystal orientation at which the random specific strength is maximum, and the hydrogen embrittlement resistance. It is a figure which shows. In FIG. 5, ◯ is a steel sheet having good hydrogen embrittlement resistance, and × is a steel sheet having poor hydrogen embrittlement resistance.
FIG. 6 shows the relationship between the surface density of the surface layer portion having the maximum random specific strength of the texture and the region having an orientation within 10 ° from the crystal orientation at which the random specific strength is maximum, and the hydrogen embrittlement resistance. It is a figure which shows. In FIG. 6, ◯ is a steel sheet having good hydrogen embrittlement resistance, and × is a steel sheet having poor hydrogen embrittlement resistance.
FIG. 7 is a diagram showing the relationship between the finishing temperature and winding temperature of hot rolling and the random specific strength of the texture of the surface layer portion. In FIG. 7, ◯ is a steel sheet having a random specific strength of the texture ≦ 4.0, and × is a steel sheet having a random specific strength of the texture> 4.0.
FIG. 8 is a diagram showing the relationship between the soaking temperature and winding temperature during annealing and the diameter and areal density of a region of the surface layer having an orientation within 10 ° from the crystal orientation at which the random specific strength is maximum. Is. In FIG. 8, ◯ is a steel plate having a diameter ≤ 10 μm and an area density ≥ 1000 pieces / mm 2 in a region having an orientation within 10 ° from the crystal orientation at which the random specific strength is maximum, and × indicates a steel plate having a random specific strength. It is a steel plate having a diameter of a region having an orientation within 10 ° from the maximum crystal orientation> 10 μm and an area density <1000 pieces / mm 2 .
Embodiment for carrying out the invention
[0012]
　A 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 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 zinc-plated steel sheet having a hot-dip zinc-plated layer on the surface, or an alloyed hot-dip zinc-plated steel sheet having an alloyed hot-dip zinc plating on the surface. These main conditions are common to hot-dip zinc-plated steel sheets and alloyed hot-dip zinc-plated steel sheets, including steel sheets.
[0013]
　1. 1. Metallographic structure (microstructure)
　First, the metallographic structure of the steel sheet according to the present embodiment will be described.
　In the description of the metal structure of the steel sheet according to the present embodiment, the structure fraction is expressed by the volume fraction. Therefore, unless otherwise specified, "%" represents "volume%".
[0014]
　The steel sheet (including cold-rolled steel sheet, hot-dip zinc-plated steel sheet, and alloyed hot-dip zinc-plated steel sheet) according to the present embodiment has a structure at a position of 1/4 (1/4 thickness) of the plate thickness from the surface in terms of volume ratio. , 80.0% or more tempered martensite, more than 2.5% and less than 10.0% retained austenite, total 0% or more and 15.0% or less ferrite and bainite, 0% or more, 3 Includes .0% or less of martensite.
[0015]

　Tempering martensite is a collection of lath-shaped crystal grains like martensite (so-called fresh martensite). On the other hand, unlike martensite, it is a hard structure containing fine iron-based carbides inside due to tempering. Tempering martensite is obtained by tempering martensite produced by cooling after annealing by heat treatment or the like.
　Tempering martensite is a tissue that is less brittle and more ductile than martensite. In the steel sheet according to the present embodiment, the volume ratio of tempered martensite is set to 80.0% or more in order to improve the strength, hole expandability, bendability and hydrogen embrittlement resistance. The volume fraction is preferably 85.0% or more. The volume fraction of tempered martensite is less than 97.5%.
[0016]
The
　retained austenite improves the ductility of the steel sheet by the TRIP effect and contributes to the improvement of uniform elongation. Therefore, the volume fraction of retained austenite is set to more than 2.5%. The volume fraction of retained austenite is preferably more than 3.5%, more preferably more than 4.5%.
　On the other hand, when the volume fraction of retained austenite becomes excessive, the particle size of retained austenite becomes large. Retained austenite having such a large particle size becomes coarse and hard martensite after deformation. In this case, the starting point of cracking is likely to occur, and the hole-spreading property and bendability are deteriorated. 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%.
[0017]

　Ferrite is a soft phase obtained by annealing in a two-phase region or slow cooling after annealing. When ferrite is mixed with a hard phase such as martensite, it improves the ductility of the steel sheet, 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 the bainite at 350 ° C. or higher and 450 ° 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 as in the case of ferrite described above.
　Therefore, the volume fractions of ferrite and bainite shall be 15.0% or less in total. It is preferably 10.0% or less. Since ferrite and bainite do not have to be included, the lower limit of each is 0%.
　Further, since ferrite is soft with respect to bainite, when the total volume fraction of ferrite and bainite is 15.0% or less, the volume fraction of ferrite is 10.0 in order to achieve high strength of 1310 MPa or more. It is preferably less than%.
[0018]

　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 and easily becomes a cracking starting point at the time of deformation, if the volume fraction of martensite is large, the hole expandability and bendability deteriorate. Therefore, the volume fraction of martensite is set to 3.0% or less. The volume fraction of martensite is preferably 2.0% or less, more preferably 1.0% or less. The lower limit is 0% because martensite does not have to be included.
[0019]
　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 (carbon) in the steel which contributes to the improvement of strength. Therefore, if the pearlite volume fraction exceeds 5.0%, the strength of the steel sheet decreases. Therefore, the volume fraction of pearlite is preferably 5.0% or less. The volume fraction of pearlite is preferably 3.0% or less, more preferably 1.0% or less.
[0020]
　The volume fraction in the structure at the position of 1/4 of the plate thickness from the surface of the steel plate according to the present embodiment is measured as follows.
　That is, for the volume ratios of ferrite, bainite, martensite, tempered martensite, and pearlite, test pieces are collected from any 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 to the surface. The metallographic structure exposed by bainite etching is observed using SEM at a position of 1/4 (1/4 thickness) of the plate thickness. In 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. 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. And.
[0021]
　When measuring the area ratio of each structure, the region where the lower structure 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.
[0022]
　Bainite and tempered martensite can be further distinguished by careful observation of the carbides in the grain.
　Specifically, tempered martensite is composed of martensite truss and cementite formed inside the truss. 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 bainite 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 the cementite variant.
　On the other hand, martensite and retained austenite cannot be clearly distinguished 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 volume fraction of the tissue determined to be martensite or retained austenite.
[0023]
　For the surface integral 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 of 1/4 of the plate thickness, and ferrite (200), (210) by MoKα ray is used. It is quantified from the surface integral strength and the (200), (220), and (311) surface integral strength of austenite.
[0024]
2. 2. Aggregation structure
2.1 Aggregation structure at a position 1/4 of the plate thickness from the surface The
　steel plate according to this embodiment (the steel plate includes cold-rolled steel plate, hot-dip zinc-plated steel plate, and alloyed hot-dip zinc-plated steel plate) is from the surface. The texture at the position of 1/4 (1/4 thickness) of the plate thickness has a maximum random specific strength (Iq) of 4.0 or less and a maximum random specific strength (Iq) measured by the method described later. The average diameter (average region diameter) of the region (Rq) having an orientation within 10 ° from the crystal orientation is 10.0 μm or less, and the region has an orientation within 10 ° from the crystal orientation having the maximum random specific strength. The surface density of the region: Rq) is 1000 pieces / mm 2 or more.
[0025]

　The steel sheet according to the present embodiment has a metal structure mainly composed of tempered martensite, and as described above, control of the texture is effective in improving the hole expandability.
　According to the investigation by the present inventors, the randomization of the texture improves the hole-spreading property. The mechanism by which the texture affects the drilling property is not clear, but if the texture is strong or if there is a coarse region of the crystal orientation in the same orientation in the steel sheet, during machining such as a drilling test. It is presumed that strain is likely to be concentrated and breakage is likely to occur. That is, it is important that the aggregate organization is close to random. In the steel sheet according to the present embodiment, the maximum random specific strength (Iq) of the texture is 4.0 or less. More preferably, the maximum random specific strength is 3.5 or less.
[0026]
In
　the steel plate according to the present embodiment. Not only the maximum random specific strength (Iq), but also the average region diameter of the region (Rq) within 10 ° from the crystal orientation where the random specific strength is maximum is 10.0 μm or less, and the random specific strength (Iq) is The surface density of the region (Rq) having an orientation within 10 ° from the maximum crystal orientation is 1000 pieces / mm 2 or more.
　If the average diameter of the region (Rq) having an orientation within 10 ° from the crystal orientation at which the random specific strength is maximum is more than 10.0 μm, strain concentration during the drilling test tends to occur in that region. The drilling property deteriorates. Further, even in the case of a structure in which the crystal orientation is not randomized such that
　the areal density of such a region is less than 1000 pieces / mm 2 , strain concentration is likely to occur during the hole expansion test, and the hole expansion is also likely to occur. The sex deteriorates.
　Preferably, the average region diameter of the region (Rq) having an orientation within 10 ° from the crystal orientation at which the random specific intensity (Iq) is maximum is 8.0 μm or less, and the surface density of such a region is 1200 pieces / mm. 2 or more, more preferably, the average region diameter of the region (Rq) having an orientation within 10 ° from the crystal orientation at which the random specific intensity (Iq) is maximum is 6.0 μm or less, and the surface of such a region. The density is 1500 pieces / mm 2 or more.
[0027]
　In the texture at a position 1/4 of the plate thickness from the surface of the steel plate according to the present embodiment, the average diameter of the region having the maximum value of the random specific strength and the orientation within 10 ° from the crystal orientation at which the random specific strength is the maximum. And the surface density is measured as follows. That is, the range of 100 μm in the thickness direction and 1000 μm in the longitudinal direction centered on the position of 1/4 of the plate thickness from the surface of the vertical cross section parallel to the rolling direction is measured by EBSD (Electron Back Scattering Diffraction) and converted into EBSD. ODF is calculated using the attached software, TSL OIM Analysis, and the maximum intensity in the Φ2 = 45 ° cross section in the ODF space is evaluated. From this maximum intensity, the maximum random specific intensity and the crystal orientation at which the random specific intensity is maximum. The average diameter and surface density of the region having an orientation within 10 ° are obtained.
[0028]
2.2 Aggregation of surface layer The
　steel plate according to this embodiment has the above-mentioned aggregate structure at a position of 1/4 of the plate thickness from the surface, and will be described later in a range of 100 μm from the surface (surface layer). The average diameter (average) of the region (Rs) having an orientation within 10 ° from the crystal orientation at which the maximum random specific strength (Is) is 4.0 or less and the maximum random specific strength (Is) is measured. Aggregate structure in which the area diameter) is 10.0 μm or less and the surface density of the area (region having an orientation within 10 ° from the crystal orientation where the random specific strength is maximum: Rs) is 1000 pieces / mm 2 or more. It is preferable to have.
[0029]

　As a result of diligent studies by the present inventors, embrittlement and hydrogen resistance are achieved by randomizing the texture in the surface layer portion in the range of 100 μm from the surface. It was found that the embrittlement characteristics were improved. The mechanism by which the bendability and hydrogen embrittlement resistance are improved is not clear, but if the texture is strong or if there is a coarse region with the same crystal orientation in the steel sheet, processing such as a bending test is performed. It is presumed that strain tends to be concentrated at times and breakage in the bending test is likely to occur. In addition, 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 cracking origin is reduced by making the particles finely uniform.
　That is, it is important that the surface texture is random, and in the steel sheet according to the present embodiment, it is preferable that the maximum random specific strength (Is) of the texture is 4.0 or less in the surface layer portion. More preferably, the maximum random specific strength (Is) is 3.5 or less.
[0030]
In
　the steel plate according to the present embodiment. Not only the maximum value of the random specific intensity (Is), but also the average diameter (average area diameter) of the region (Rs) having an orientation within 10 ° from the crystal orientation at which the random specific intensity (Is) is maximum is 10.0 μm or less. Moreover , it is preferable that the surface density of such a region is 1000 pieces / mm 2 or more.
　If the average diameter of the region (Rs) having an orientation within 10 ° from the crystal orientation at which the random specific strength is maximum is more than 10.0 μm, strain concentration during the bending test is likely to occur in that region, and bending is likely to occur. Properties and hydrogen embrittlement resistance are not sufficiently improved. Further, even in the case of a structure in which the crystal orientation is not randomized such that
　the areal density of such a region is less than 1000 pieces / mm 2 , strain concentration is likely to occur during the bending test, and the bendability is improved. Hydrogen embrittlement resistance is not sufficiently improved.
　More preferably, the average region diameter of the region (Rs) having an orientation within 10 ° from the crystal orientation at which the random specific strength (Is) is maximum is 8.0 μm or less, and the surface density of such a region is 1200 pieces / mm. 2 or more. More preferably, the average region diameter of the region (Rs) having an orientation within 10 ° from the crystal orientation at which the random specific strength (Is) is maximum is 6.0 μm or less, and the areal density of such a region is 1500 pieces /. It is mm 2 or more.
[0031]
　The texture up to 100 μm from the surface of the steel sheet according to this embodiment is measured as follows. That is, the range of the vertical cross section parallel to the rolling direction, 100 μm from the surface and 1000 μm in the longitudinal direction, is measured by EBSD (Electron Back Scattering Diffraction), ODF is calculated using TSL OIM Analysis, which is the software attached to EBSD, and ODF. The maximum intensity in the Φ2 = 45 ° cross section in space is evaluated, and from this maximum intensity, the average diameter and surface density of the region having an orientation within 10 ° from the crystal orientation at which the maximum random specific intensity and the maximum random specific intensity are obtained are obtained. Ask.
[0032]
3. 3. 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% unless otherwise specified.
[0033]
　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 desired tensile strength cannot be achieved. In addition, the hole widening property (λ) and bendability are reduced. 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 is deteriorated, and the hole widening property (λ) and the bendability are deteriorated. 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%.
[0034]
　Si: More than 0.35% and less than 1.50%
　Si is a useful element for increasing the strength of steel sheet by solid solution strengthening. Further, since Si suppresses the formation of cementite, it is an effective element for promoting the concentration of C in austenite and 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 the target of 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 during heating in the annealing step is delayed, and the ferrite to austenite transformation may not sufficiently occur. In this case, after annealing, ferrite remains excessively in the structure, the target tensile strength cannot be achieved, and the hole expandability (λ) and bendability deteriorate. Further, when the Si content is 1.50% or more, the surface texture of the steel sheet deteriorates. Further, the chemical conversion treatment property 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.
[0035]
　Mn: More than 1.30% and less than 3.50%
　Mn has an action of improving the hardenability of steel and is an effective element for obtaining the above-mentioned metallographic structure. If the Mn content is 1.30% or less, it becomes difficult to obtain the above metal structure. In this case, sufficient tensile strength cannot be obtained. Therefore, the Mn content is set to more than 1.30%. The Mn content is preferably more than 1.50%, more preferably more than 2.00%.
　On the other hand, when the Mn content is 3.50% or more, the effect of improving the hardenability is diminished due to the segregation of Mn, and the material cost is increased. Therefore, the Mn content is set to less than 3.50%. The Mn content is preferably less than 3.25%, more preferably less than 3.00%.
[0036]
　P: 0.100% or less
　P is an element contained in the steel as an impurity, and is an element that segregates at the grain boundaries and embrittles the steel. Therefore, the smaller the P content is, the more preferable it is, and 0% may be used, but the P content is set to 0.100% or less in consideration of the removal time and cost of P. The P content is preferably 0.020% or less, more preferably 0.015% or less.
[0037]
　S: 0.010% or less
　S is an element contained in steel as an impurity and is an element that forms sulfide-based inclusions and deteriorates bendability. Therefore, the smaller the S content is, the more preferable it is, and it may be 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.
[0038]
　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, the Al content is preferably 0.005% or more, more preferably 0.010% or more in order to surely deoxidize. Further, Al has an effect 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 sufficient tensile strength cannot be obtained. 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 the Al content may be 0%.
[0039]
　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 hole expandability and 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%.
[0040]
　The steel sheet according to the present embodiment contains the above elements, and the balance may be Fe and impurities, but one kind of elements listed below that affect the strength, hole expandability, and bendability are optional elements. Alternatively, two or more kinds may be further contained. However, since these elements do not necessarily have to be contained, the lower limit thereof is 0%.
[0041]
　Ti: 0% or more, less than 0.050%
　Nb: 0% or more, less than 0.050%
　V: 0% or more, 0.50% or less
　Cu: 0% or more, 1.00% or less
　Ti, Nb, V, Cu is an element having an action of improving the strength of a steel sheet by precipitation hardening. Therefore, these elements may be contained. In order to sufficiently obtain the above effects, the Ti and Nb contents are preferably 0.001% or more, and the V and Cu contents are preferably 0.01% or more, respectively. The more preferable Ti and Nb contents are 0.005% or more, respectively, and the more preferable V and Cu contents are 0.05% or more, respectively. 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, V, and Cu, and the lower limit thereof is 0%.
　On the other hand, if these elements are excessively contained, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the hole-spreading property and bendability are impaired. Therefore, when it is contained, the Ti content is less than 0.050%, the Nb content is less than 0.050%, the V content is 0.50% or less, and the Cu content is 1.00% 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. The Cu content is preferably 0.50% or less.
[0042]
　Ni: 0% or more, 1.00% 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
　Ni, Cr, Mo and B is an element that improves hardenability and contributes to increasing the strength of the steel plate, and is an element that is effective in obtaining the above-mentioned metal structure. Therefore, these elements may be contained. In order to sufficiently obtain the above effects, it is preferable that the Ni, Cr, and Mo contents are 0.01% or more, respectively, and / or the B content is 0.0001% or more. More preferably, the contents of Ni, Cr and Mo are 0.05% or more, respectively, and the B content is 0.0010% or more. It is not essential to obtain the above effects. Therefore, it is not necessary to particularly limit the lower limit of the contents of Ni, Cr, Mo, and B, and the lower limit thereof is 0%.
　On the other hand, even if these elements are excessively contained, the effect of the above action is saturated and it becomes uneconomical. Therefore, when it is contained, the Ni content and Cr content are 1.00% or less, the Mo content is 0.50% or less, and the B content is 0.0100% or less. The Ni content and Cr content are preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less.
[0043]
　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
　Ca, Mg and REM , An element that has the effect of improving strength and bendability by adjusting the shape of inclusions. Bi is an element having an action of improving strength and bendability by refining the solidified structure. Therefore, these elements may be contained. In order to sufficiently obtain the above effects, the Ca and Mg contents are preferably 0.0001% or more, and the REM and Bi contents are preferably 0.005% or more, respectively. More preferably, the contents of Ca and Mg are 0.0008% or more, respectively, and the contents of REM and Bi are 0.007% or more, respectively. 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, Bi and REM, and the lower limit thereof is 0%.
　On the other hand, even if these elements are excessively contained, the effect of the above action is saturated and it becomes uneconomical. Therefore, 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.
[0044]
　4. Mechanical characteristics
[Tensile strength is 1310 MPa or more, uniform elongation is 5.0% or more]
[TS × λ is 35000 MPa ·% or more]
[Ratio of limit bending R and plate thickness t in 90 ° V bending (R / t) ) Is 5.0 or less]
　In the steel sheet according to the present embodiment, the tensile strength (TS) is 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 1350 MPa or more, more preferably 1400 MPa or more, and further preferably 1470 MPa or more.
　Further, from the viewpoint of moldability, the uniform elongation (uEl) is 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, TS × λ, which is the product of tensile strength and hole expandability, is 35,000 MPa ·% or more. In order to improve the moldability, TS × λ is preferably 45,000 MPa ·% or more, and more preferably 50,000 MPa ·% or more.
　When the tensile strength of TS × λ is low (for example, less than 1310 MPa), TS × λ ≧ 35000 MPa ·% is relatively easy to be satisfied, but as the tensile strength increases, it becomes difficult to satisfy. In particular, when the tensile strength is 1400 MPa or more or 1470 MPa or more, it is not easy to satisfy TS × λ ≧ 35,000 MPa ·%.
　From the viewpoint of bendability, the ratio (R / t) of the limit bending R and the plate thickness t at 90 ° V bending is preferably 5.0 or less. (R / t) is more preferably 4.0 or less, still more preferably 3.0 or less, in order to improve the bendability.
[0045]
　Tensile strength (TS) and uniform elongation (uEl) are determined by collecting JIS No. 5 tensile test pieces from a steel sheet in the direction perpendicular to the rolling direction and performing a tensile test along JIS Z 2241: 2011.
　Further, the hole expanding property (λ) is evaluated according to the hole expanding test method described in JIS Z 2256: 2010.
　Regarding the limit bending radius (R / t), the radius R is changed at a pitch of 0.5 mm using a 90 ° V bending die to obtain the minimum bending radius that does not cause cracking, and the bending radius is divided by the plate thickness t. Ask for it.
[0046]
　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 that a steel sheet for automobiles will be perforated due to corrosion, it may not be possible to thin the steel sheet to a certain thickness or less even if the strength is increased. One of the purposes of increasing the strength of steel sheets is to reduce the weight by making them thinner, so even if high-strength steel sheets are developed, the application sites are 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.
[0047]
　5. Production conditions
　As a result of studies by the present inventors, by controlling the hot-rolling conditions and making the structure after hot-rolling into coarse crystal grains, the grain boundary area that becomes the recrystallized nuclei is reduced and charcoal is produced. By finely and uniformly dispersing, the reverse transformation nuclei are dispersed to promote the reverse transformation, recrystallization during annealing is suppressed, and the reverse transformation is promoted and annealed in the austenite single phase region to develop the texture. It was found that a near-random aggregate organization can be achieved. Hereinafter, it will be described in detail.
[0048]
　Specifically, the steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (VII).
(I) A cast slab having a predetermined chemical composition is directly or once cooled and then heated to 1100 ° C. or higher, and hot-rolled in the final stage so that the rolling temperature is 920 ° C. or higher and the rolling reduction is 15% or lower. Hot- rolling step
(II) to cool the hot-rolled steel sheet so that it passes through the temperature range of 750 ° C to 650 ° C within 10 seconds
(III) Hot-rolled steel sheet to 650 ° C or less Winding step
(IV) Cold-rolled steel sheet is pickled and cold-rolled at a cumulative reduction rate of 60% or less to obtain a cold-
rolled steel sheet. A rolling step
(VI) in which the temperature from ° C. to 750 ° C. is heated at an average heating rate of 1.0 ° C./s or higher and 50.0 ° C./s or lower and the temperature is equalized at 820 ° C. or higher .
Cooling after the annealing cooling step (VII) that cools the temperature range and the temperature range from 450 ° C to 350 ° C to a temperature of 50 ° C or higher and 250 ° C or lower so that the average cooling rate is 5 ° C / s or higher. Rewinding step of baking a steel plate in a temperature range of 200 ° C. or higher and lower than 350 ° C. for 1 second or longer
　Each step will be described below.
[0049]
[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 slab heating conditions in hot rolling are not limited, but heating to 1100 ° C. or higher is preferable. If the heating temperature is less than 1100 ° C., the homogenization of the material tends to be insufficient.
　In order to control the texture at the position of 1/4 of the plate thickness from the surface, the rolling temperature (FT) in the final stage (final path) of hot rolling is 920 ° C or higher, and the rolling reduction in the final stage is 15% or less. And. When the rolling temperature of the final stage is low or the rolling reduction of the final stage is high, the hot-rolled plate structure after rolling in the final stage becomes fine. When the hot-rolled plate structure with a fine structure is cold-rolled and annealed, recrystallization during heating proceeds, so that the texture is not random and the hole-expandability deteriorates. Therefore, the rolling temperature of the final stage is set to 920 ° C. or higher. The rolling temperature of the final stage is preferably 930 ° C. or higher. The reduction rate of the final stage shall be 15% or less. The reduction rate of the final stage is preferably 13% or less. In principle, the lower limit is zero in order to make the texture random, but in consideration of manufacturability, the reduction rate of the final stage is preferably 5% or more, more preferably 8% or more, and 10%. The above is more preferable.
　On the other hand, when controlling the texture of the surface layer, the rolling temperature (FT) in the final stage of hot rolling is 960 ° C or lower, the rolling reduction in the final stage is 10% or more, and the coefficient of friction in the final stage of rolling is μ. Is preferably 0.15 or more. If the rolling temperature in the final stage of finishing during hot rolling is high, the effect of shear deformation is reduced, the surface texture cannot be formed, and the bendability and hydrogen embrittlement resistance are not sufficiently improved. Therefore, the rolling temperature of the final stage is preferably 960 ° C. or lower. The rolling temperature of the final stage is more preferably 940 ° C. or lower. Further, if the rolling reduction of the final stage is low and the coefficient of friction during the final stage rolling is low, the surface layer portion is not subjected to shear deformation and the texture of the surface layer portion cannot be formed, so that the bendability and hydrogen embrittlement resistance are improved. Not enough. Therefore, it is preferable that the friction coefficient μ in the final finishing stage during hot rolling is 0.15 or more. More preferably, the coefficient of friction μ is 0.20 or more. Further, the reduction rate of the final stage is preferably 10% or more, more preferably 12% or more.
　That is, when the position of 1/4 of the plate thickness from the surface and the texture of the surface layer portion are controlled at the same time as described above, in the hot rolling step, the rolling temperature at the final finishing stage (final pass) during hot rolling ( The FT) is preferably 920 ° C. or higher and 960 ° C. or lower, the rolling reduction in the final stage is preferably 10% or more and 15% or less, and the friction coefficient μ during rolling in the final stage is preferably 0.15 or more.
[0050]
[Cooling step]
　After hot rolling, the material is cooled to the winding temperature so that the time for passing through the temperature range of 750 ° C to 650 ° C (the time for the hot-rolled steel sheet to stay in the temperature range) is 10 seconds or less. .. If the cooling rate in this temperature range is slow, coarse ferrite is generated, so that the structure in which carbides are finely and uniformly dispersed is not obtained. In this case, the texture after cold-rolled annealing is not sufficiently random, and the hole-spreading property deteriorates.
[0051]
[Rewinding process]
　After cooling to the winding temperature as described above, winding is performed. The winding temperature shall be 650 ° C or lower. When the winding temperature exceeds 650 ° C., the structure of the hot-rolled steel sheet becomes a coarse ferrite pearlite structure, and the structure in which carbides are finely and uniformly dispersed is not formed. In this case, the texture after cold rolling and annealing is not sufficiently random, the hole expanding property is deteriorated, and the metal structure of the steel sheet after annealing becomes non-uniform, and the bendability is deteriorated. The winding temperature is preferably 630 ° C or lower, more preferably 620 ° C or lower, and even more preferably 600 ° C or lower.
　On the other hand, when the winding 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 winding temperature is preferably 500 ° C. or higher. If the strength of the hot-rolled steel sheet is high, a softening heat treatment such as BAF may be applied before cold rolling.
[0052]
[Cold rolling process]
　In the cold rolling process, hot-rolled hot-rolled steel sheets are descaled by pickling, etc., and then cold-rolled under the condition that the reduction rate (cumulative reduction rate) is 60% or less. Rolled steel plate. When the rolling reduction in cold rolling is high, recrystallization during annealing is promoted, the texture after annealing does not become random, and the hole expandability, bendability, and hydrogen embrittlement resistance deteriorate. Therefore, the rolling reduction in cold rolling is set to 60% or less. The reduction rate is preferably 55% or less, more preferably 50% or less.
[0053]
[Annealing step]
　The cold-rolled steel sheet after the cold rolling step is subjected to a treatment such as degreasing according to a known method as necessary, and then heated from 550 ° C to 750 ° C at 1.0 ° C / s or higher, 50. Annealing is performed by heating at an average heating rate of 0 ° C./s or less and soaking at 820 ° C. or higher.
　In the annealing step, the heating rate from 550 ° C to 750 ° C is 1.0 ° C / s or more and 50 ° C / s or less. If the heating rate is slow, recrystallization proceeds, the texture after annealing is not sufficiently random, and the hole-spreading property deteriorates. Therefore, the heating rate is set to 1.0 ° C./s or higher. The heating rate is preferably 1.5 ° C./s or higher, more preferably 2.0 ° C./s or higher. When the texture is sufficiently randomized to the surface layer after annealing, the average heating rate up to 750 ° C. is preferably 3.0 ° C./s or more. From the viewpoint of randomizing the texture of the surface layer, the average heating rate up to 750 ° C. is more preferably 5.0 ° C./s or higher.
　On the other hand, if the average heating rate is excessively high, austenite is atomized and the ferrite transformation is excessively promoted. In this case, in the finally obtained steel sheet, the desired structure cannot be obtained, and the strength, hole expandability, and bendability are lowered. Therefore, the heating rate is set to 50.0 ° C./s or less. The heating rate is preferably 30.0 ° C./s or less, more preferably 10.0 ° C./s or less.
　The soaking temperature (annealing temperature) in the annealing step is 820 ° C. or higher. When the soaking temperature is low, austenite single-phase annealing does not occur and the texture is not sufficiently random, and the volume fraction of ferrite increases, resulting in deterioration of hole expandability and bendability. Therefore, the soaking temperature is set to 820 ° C. or higher. The soaking temperature is preferably 830 ° C. or higher or 835 ° C. or higher. The higher the soaking temperature, the easier it is to secure the hole expandability and bendability, but if the soaking temperature is too high, the manufacturing cost increases, so the soaking temperature is preferably 900 ° C. or lower. The soaking temperature is more preferably 880 ° C. or lower, and even more preferably 870 ° C. or lower.
　The soaking time is not limited, but is preferably 30 to 450 seconds. If the soaking time is less than 30 seconds, austenitization may not proceed sufficiently. Therefore, the soaking time is preferably 30 seconds or more. On the other hand, if the soaking time exceeds 450 seconds, the productivity decreases, so that the soaking time is preferably 450 seconds or less.
[0054]
　[Annealed cooling step]
　In order to obtain the above-mentioned metal structure of the annealed cold-rolled steel plate, the average cooling rate in the ferrite transformation temperature range of 700 ° C to 600 ° C and the average of the bainite transformation temperature range of 450 ° C to 350 ° C. Cool to a temperature of 50 ° C. or higher and 250 ° C. or lower so that the cooling rate is 5 ° C./s or higher. When the cooling rate in the above temperature range is slow, the volume ratio of ferrite and bainite at a position 1/4 of the plate thickness from the surface increases, and the volume ratio of tempered martensite decreases. As a result, the tensile strength is lowered and the hole expandability, bendability, and hydrogen embrittlement resistance are deteriorated. Therefore, the average cooling rate from 700 ° C. to 600 ° C. and 450 ° C. to 350 ° C. is 5 ° C./s or higher. The average cooling rate is preferably 10 ° C./s or higher, more preferably 20 ° C./s or higher.
　The cooling shutdown temperature shall be 50 ° C or higher and 250 ° C or lower. If the cooling stop temperature is high, martensite (not tempered) increases in the subsequent cooling after the tempering process, and the hole expandability, bendability, and hydrogen embrittlement resistance deteriorate. Therefore, the cooling shutdown temperature is set to 250 ° C. or lower. On the other hand, if the cooling shutdown temperature is low, the retained austenite fraction decreases, and the desired uniform elongation cannot be obtained. Therefore, the cooling shutdown temperature is set to 50 ° C. or higher. The cooling shutdown temperature is preferably 75 ° C. or higher, more preferably 100 ° C. or higher.
[0055]
[Hot-dip galvanizing step]
[Alloying step]
　In the case of manufacturing a cold-rolled steel sheet (hot-dip galvanized steel sheet) having a hot-dip galvanized layer on the surface, the temperature is further over 450 ° C and below 600 ° C in the hot-dip cooling step. In the region, the cold-rolled steel plate may be immersed in a hot-dip galvanizing bath for hot-dip galvanizing. Further, in the case of manufacturing a cold-rolled steel plate (alloyed hot-dip galvanized steel plate) having alloyed hot-dip galvanizing on the surface, the alloying treatment is performed following the hot-dip galvanizing step, and the plating is used as alloyed hot-dip galvanizing. May be good.
[0056]
[Tempering step]
　The cold-rolled steel sheet after the annealing cooling step is cooled to a temperature of 50 ° C. or higher and 250 ° C. or lower, so that untransformed austenite is transformed into martensite. After that, the cold-rolled steel sheet is tempered at a temperature of 200 ° C. or higher and lower than 350 ° C. 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, bake at a temperature of 200 ° C. or higher and lower than 350 ° C. for 1 second or longer. When the tempering temperature is 350 ° C. or higher, the strength of the steel sheet decreases. Therefore, the tempering temperature is set to less than 350 ° C. The tempering temperature is preferably 325 ° C or lower, more preferably 300 ° C or lower. When it is desired to further increase the tensile strength, it is preferable to lower the tempering temperature. For example, when the tensile strength is 1400 MPa or more, the tempering temperature is preferably 275 ° C. or less, and when the tensile strength is 1470 MPa or more. The tempering temperature is preferably 250 ° C. or lower.
　On the other hand, if the tempering temperature is less than 200 ° C., the tempering becomes insufficient, and the hole-spreading property, bendability, and hydrogen embrittlement resistance deteriorate. Therefore, the tempering temperature is set to 200 ° C. or higher. From the viewpoint of drilling property, bendability, and hydrogen embrittlement resistance, the tempering temperature is preferably 220 ° C. or higher, more preferably 250 ° 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 process. On the other hand, the strength of the steel sheet may decrease after long-term tempering. Therefore, the tempering time is preferably 750 seconds or less, more preferably 500 seconds or less.
[0057]
[Skinpass step]
　The cold-rolled steel sheet after the tempering step may be subjected to skinpass rolling after being cooled to a temperature at which skinpass rolling is possible. When cooling after annealing is water spray cooling using water, dip cooling, air-water cooling, etc., skin pass rolling is performed to remove the oxide film formed by contact with water at high temperature and improve the chemical conversion process of the steel sheet. It is preferable to perform pickling and then plating one or more of a small amount of Ni, Fe, Co, Sn and Cu. Here, the trace amount means a plating amount of about 3 to 30 mg / m 2 on the surface of the steel sheet.
[0058]
　The shape of the steel sheet can be adjusted by skin pass rolling. The elongation rate of skin pass rolling is preferably 0.10% or more. More preferably, it is 0.15% 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 1.00% or less. The elongation rate is more preferably 0.75% or less, further preferably 0.50% or less.
Example
[0059]
　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 cast slab was heated to 1100 ° C. or higher, hot rolled to 2.8 mm, wound and cooled to room temperature. The hot rolling conditions, cooling conditions, and winding temperature were as shown in Tables 2-1 to 2-2.
　Then, the scale was removed by pickling, cold-rolled to 1.4 mm, and then annealed at the soaking temperature shown in Tables 2-1 to 2-2 for 120 seconds. The heating rates at 550 to 750 ° C. during annealing heating are as shown in Tables 2-1 to 2-2.
　After annealing, after cooling to a cooling stop temperature of 50 ° C or higher and 250 ° C or lower so that the temperature range of 700 ° C to 600 ° C and the temperature range of 450 ° C to 350 ° C have an average cooling rate of 20 ° C / s or higher, the table is shown. Heat treatment was performed for 1 to 500 seconds at the tempering temperatures shown in 2-1 to Table 2-2.
　For some examples, hot dip galvanizing and alloying were performed during annealing cooling. “CR” shown in Tables 5-1 to 5-2 is a cold-rolled steel sheet not 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 a temperature of more than 450 ° C. and less than 600 ° C. at about 35 to 65 g / m 2 , and then further alloyed at a temperature of more than 450 ° C. and less than 600 ° C.
[0060]
[table 1]

[0061]
[Table 2-1]

[0062]
[Table 2-2]

[0063]
　From the obtained annealed steel sheet, a test piece for SEM observation is collected as described above, the vertical cross section parallel to the rolling direction is polished, and then the metal structure at the position (t / 4) of 1/4 of the plate thickness is observed. Then, the volume ratio of each tissue 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, as described above, the position of 1/4 of the plate thickness from the surface and the texture of the surface layer (maximum value of random specific strength, orientation within 10 ° from the crystal orientation where the random specific strength is maximum) are determined by EBSD measurement. The average diameter and surface density of the region to be held) were measured respectively.
　The results are shown in Tables 3-1 to 3-2 and Tables 4-1 to 4-2.
[0064]
　Further, the tensile strength (TS), uniform elongation (uEl), hole expansion ratio (λ), critical bending radius (R / t), and hydrogen embrittlement resistance were evaluated as shown below.
[0065]
　Tensile strength (TS) and uniform elongation (uEl) were determined by collecting JIS No. 5 tensile test pieces from annealed steel sheets in the direction perpendicular to the rolling direction and conducting a tensile test along JIS Z 2241: 2011. ..
　The results are shown in Tables 5-1 to 5-2.
[0066]
　The hole expansion ratio (λ) was evaluated by the method described in JIS Z 2256: 2010.
　The results are shown in Tables 5-1 to 5-2.
[0067]
　For the limit bending radius (R / t), which is an index of bendability, the radius R is changed at a pitch of 0.5 mm using a 90 ° V bending die to obtain the minimum bending radius that does not cause cracking. It was calculated by dividing by the thickness (1.4 mm).
　The results are shown in Tables 5-1 to 5-2.
[0068]
　The following tests were conducted to evaluate the hydrogen embrittlement resistance.
　That is, a test piece whose end face is machine-ground is bent into a U-shape by a push-bending method to prepare a U-bending test piece having a radius of 5R, and the non-bent portion is bolted so as to be parallel and elastically deformed. A delayed fracture promotion 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 that did not crack even when the immersion time was 100 hours was evaluated as a steel sheet having good (OK) delayed fracture resistance, and a steel sheet that had cracks 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.
　The results are shown in Tables 5-1 to 5-2.
[0069]
[Table 3-1]

[0070]
[Table 3-2]

[0071]
[Table 4-1]

[0072]
[Table 4-2]

[0073]
[Table 5-1]

[0074]
[Table 5-2]

[0075]
　All of the steels of the present invention had TS of 1310 MPa or more, uEl of 5.0% or more, and TS × λ of 35,000 MPa ·% or more, and were excellent in high strength and formability.
　Further, among the steels of the present invention, when the texture of the surface layer is preferably controlled, TS is 1310 MPa or more, uEl is 5.0% or more, TS × λ is 35000 MPa ·% or more, and further, there are limits. The bending radius (R / t) was 5.0 or less, and the hydrogen embrittlement resistance was also good.
　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 metal structure or the texture was outside the scope of the present invention, the tensile strength, uniform elongation, TS × One of λ did not reach the goal.
Industrial applicability
[0076]
　According to the present invention, a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1310 MPa or more, a uniform elongation of 5.0% or more, and a TS × λ of 35000 MPa ·% or more and excellent formability and a method for manufacturing the same can be obtained. Be done. Since such a steel sheet has sufficient formability that can be applied to processing such as press forming, the present invention can contribute to the development of industry by contributing to solving global environmental problems by reducing the weight of the automobile body. ..
The scope of the claims
[Claim 1]
　Chemical composition by mass%,
　　C: more than 0.140%, less than 0.400%,
　　Si: more than 0.35%, less than 1.50%,
　　Mn: more than 1.30%, less than 3.50%,
　　P: 0.10% 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,
　　Cu: 0% or more, 1.00% or less,
　　Ni: 0% or more, 1.00% 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% Below,
　　REM: 0% or more, 0.0500% or less, and
　　Bi: 0% or more, 0.050% or less, the
　　balance consists of Fe and impurities,
　at the position of 1/4 of the plate thickness from the surface. The structure is by volume
　　80.0% or more tempered martensite, more than
　　2.5% and less than 10.0% retained austenite,
　　total 0% or more and 15.0% or less ferrite and bainite,
　　0% or more, 3.0% or less In the region Rq
　　containing the martensite and the residual structure of the
　above,
　　the maximum random specific intensity Iq is 4.0 or less, and
　　the orientation is within 10 ° from the crystal orientation at which the maximum random specific intensity Iq is maximum. When the average diameter is 10.0 μm or less,
　　the surface density of the region Rq is 1000 pieces / mm 2 or more, the
　tensile strength is 1310 MPa or more, the uniform elongation is 5.0% or more, and TS × λ is 35000 MPa ·% or more. There is a
cold-rolled steel plate.
[Claim 2]
　In the structure in the range from the surface to the plate thickness direction of 100 μm,
　　the maximum random specific strength Is is 4.0 or less, and the
　　region Rs having an orientation within 10 ° from the crystal orientation at which the random specific strength Is is maximum. The average diameter of is 10.0 μm or less,
　　the surface density of the region Rs is 1000 pieces / mm 2 or more, and
　R / t, which is the ratio of the limit bending R at 90 ° V bending to the plate thickness t, is 5. The cold-rolled steel sheet according to claim 1, which is 0.0 or less.
[Claim 3]

The cold-rolled steel sheet according to claim 1 or 2, 　wherein the tensile strength is 1400 MPa or more .
[Claim 4]
　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,
　　Ni: 0.01% or more, 1.00% or less,
　　Cu: 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 : 0.0005% or more, 0.0500% or less, and
　　Bi: 0.0005% or more, 0.050% or less,
one or more selected from the above, according to
claims 1 to 3. The cold-rolled steel plate according to any one item.
[Claim 5]
　The cold-rolled steel sheet according to any one of claims 1 to 4, further comprising a hot-dip galvanized layer on the surface.
[Claim 6]
　The cold-rolled steel sheet according to claim 5, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.
[Claim 7]
　By mass%, C: more than 0.140%, less than 0.400%, Si: more than 0.35%, less than 1.50%, Mn: more than 1.30%, less than 3.50%, 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, Ni: 0% or more, 1.00% or less, Cu: 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: A cast slab containing 0% or more, 0.0500% or less, and Bi: 0% or more, 0.050% or less, and having a chemical composition in which the balance consists of Fe and impurities is directly or once cooled to 1100 ° C. or higher. The hot rolling step of hot rolling to obtain a hot-rolled steel sheet under the conditions that the rolling temperature FT in the final stage is 920 ° C. or higher and the rolling reduction is 15% or less, and the hot-rolled
　steel sheet is 750 ° C. A cooling step of cooling so as to pass through a temperature range of 650 ° C. within 10 seconds,
　a winding step of winding the hot-rolled steel sheet after the cooling step at 650 ° C. or lower, and the
　winding step after the winding step. A cold rolling process in which a hot-rolled steel sheet is pickled and cold-rolled at a cumulative rolling reduction of 60% or less to obtain a cold-rolled steel sheet, and the cold-rolled
　steel sheet is heated from 550 ° C to 750 ° C to 1.0 ° C. A rolling step of heating at an average heating rate of / s or more and 50.0 ° C./s or less and soaking at 820 ° C. or more.
　The cold-rolled steel sheet after the annealing step is 50 ° C. or higher and 250 ° C. or lower so that the average cooling rate in the temperature range of 700 ° C. to 600 ° C. and the temperature range of 450 ° C. to 350 ° C. is 5 ° C./s or higher. A method for manufacturing a cold-rolled steel plate , comprising a annealing cooling step of cooling to
　a temperature and a tempering step of tempering the cold-rolled steel sheet after the annealing-cooling step in a temperature range of 200 ° C. or higher and lower than 350 ° C. for 1 second or longer.
..
[Claim 8]
　In the hot rolling step, the hot rolling under the conditions that the rolling temperature FT in the final stage is 920 ° C. or higher and 960 ° C. or lower, the rolling reduction ratio is 10% or higher and 15% or lower, and the friction coefficient μ is 0.15 or higher. In the
　baking step, the cold-rolled steel sheet is heated from 550 ° C to 750 ° C at an average heating rate of 3.0 ° C / s or higher and 50.0 ° C / s or lower, and leveled at 820 ° C or higher.
The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein the cold-rolled steel sheet is heated .
[Claim 9]
　In the annealing cooling step, the cold-rolled steel sheet after the annealing step is 50.
The method for producing a cold-rolled steel sheet according to claim 7 or 8, wherein the cold-rolled steel sheet is cooled to a temperature of ° C. or higher and lower than 220 ° C.
[Claim 10]
　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, Ni: 0.01% or more, 1.00% or less, Cu: 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 : Any of claims 7 to 9 containing one or more selected from: 0.0005% or more, 0.0500% or less and Bi: 0.0005% or more, 0.050% or less. The method for manufacturing a cold-rolled steel plate according to item 1.
[Claim 11]
　The cold-rolled steel sheet according to any one of claims 7 to 10, further subjected to hot-dip galvanizing in a temperature range of more than 450 ° C. and lower than 600 ° C. and optionally alloyed in the annealing cooling step. Manufacturing method.