TWI599661B - Steel plate - Google Patents

Description

Steel plate

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet suitable for a mechanical structural part such as a body structural part of an automobile.

BACKGROUND OF THE INVENTION In order to suppress the amount of carbon dioxide emissions from automobiles, the weight reduction of automobile bodies using high-strength steel sheets is continuously developed. In addition, in order to ensure the safety of the rider, the vehicle body has also begun to use a large number of high-strength steel sheets. In order to promote further weight reduction of the vehicle body, it is important to further increase the strength. On the other hand, depending on the body parts, excellent formability is required. For example, high strength steel sheets for skeletal parts require excellent elongation and hole expandability. In particular, a high-strength steel sheet used for a structure (auxiliary frame) and a reinforcing (reinforcing member) belonging to an automobile frame member requires excellent hole expandability, and is not only good ductility.

However, it is difficult to balance the improvement in strength and the improvement in formability. Although there has been disclosed a technique for achieving both improvement in strength and improvement in formability, it is not possible to obtain sufficient characteristics even by such. In particular, in order to improve productivity, it is desirable to have excellent formability under conditions of high processing speed. However, in conventional steel sheets, the formability at the time of high-speed processing cannot be said to be sufficient. CITATION LIST Patent Literature Patent Literature 1: Japanese Laid-Open Patent Publication No. JP-A No. Hei. No. Hei. No. Hei. No. Hei. Japanese Patent Laid-Open No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. Patent Document 9: Japanese Patent Laid-Open No. 56-6704

SUMMARY OF THE INVENTION

An object of the present invention is to provide a steel sheet which can obtain excellent strength and formability, and is particularly excellent in formability at the time of high speed processing. Means to solve the problem

The inventors carefully studied in order to solve the above problems. As a result, it is understood that in the conventional steel sheet, there is a band-like structure in which a hard tissue composed of a tough body, a worm field body, or a residual Worth field body or any combination thereof is connected in a band shape; and a band structure It constitutes a stress concentration and promotes the formation of pores. It is further understood that, due to the band structure, the formation of pores is densely present, and therefore, the connection of pores is promoted. That is, it is understood that the band structure affects the hole expandability. Further, the inventors have found that in order to enhance the hole expandability, it is important to suppress the band structure.

The band-like structure is formed by segregating alloy elements such as Mn in the smelting stage, and in the hot rolling and cold rolling, the region where the alloying element is segregated is elongated in the rolling direction. Therefore, in order to suppress the band structure, it is important to suppress segregation of alloying elements. Furthermore, the inventors have found that in order to suppress the band-like structure, it is extremely effective to introduce a lattice defect at a high temperature to obtain recrystallization of the Worth field. That is, by recrystallization, the alloy element is promoted to diffuse along the grain boundary of the recrystallized Worthfield body particles, and the alloy elements are distributed in a mesh shape, and segregation of the alloy elements is suppressed.

Based on this knowledge, the inventors further elaborated on the results of the invention, and thought of the following aspects of the invention.

(1) A steel sheet characterized by having the chemical composition shown below: C: 0.05% to 0.40%, Si: 0.05% to 2.00%, Mn: 1.50% to 4.00%, acid-soluble Al, by mass% : 0.01% to 1.00%, P: 0.10% or less, S: 0.01% or less, N: 0.01% or less, Ti: 0.0% to 0.2%, Nb: 0.0% to 0.2%, V: 0.0% to 0.2%, Cr : 0.0%~1.0%, Mo: 0.0%~1.0%, Cu: 0.0%~1.0%, Ni: 0.0%~1.0%, Ca: 0.00%~0.01%, Mg: 0.00%~0.01%, REM: 0.00 %~0.01%, Zr: 0.00%~0.01%, and the remainder: Fe and impurities; in addition, it has the steel structure shown below: in terms of area ratio, fertilizer granules: 5%~80%, by the toughening body , the hard tissue composed of the Matian bulk or the residual Worth field or any combination of the above: 20% to 95%, and the standard deviation of the line rate of the aforementioned hard structure on the line in the plane perpendicular to the thickness direction: When the thickness of the steel sheet is t, it is 0.050 or less in the depth range from 3 t/8 to t/2 from the surface.

(2) The steel sheet according to (1), wherein the steel structure satisfies the residual Worth field body by 5.0% or more in terms of area ratio.

(3) The steel sheet according to (1) or (2), wherein the chemical composition satisfies the following, or any combination of the following: in mass%, Ti: 0.003% to 0.2%, and Nb: 0.003% to 0.2% , or V: 0.003%~0.2%.

(4) The steel sheet according to any one of (1) to (3), wherein the chemical composition satisfies the following or any combination of the above: in mass%, Cr: 0.005% to 1.0%, Mo: 0.005 %~1.0%, Cu: 0.005%~1.0%, or Ni: 0.005%~1.0%.

(5) The steel sheet according to any one of (1) to (4), wherein the chemical composition satisfies the following or any combination of the above: in terms of % by mass, Ca: 0.0003% to 0.01%, Mg: 0.0003 %~0.01%, REM: 0.0003%~0.01%, or Zr: 0.0003%~0.01%. Effect of the invention

According to the present invention, since the steel structure is suitable, excellent strength and moldability can be obtained, and excellent moldability at the time of high-speed processing can be obtained.

Embodiments for Carrying Out the Invention Hereinafter, embodiments of the present invention will be described.

First, the chemical composition of the steel sheet according to the embodiment of the present invention and the flat embryo used for the production thereof will be described. As described above, the steel sheet according to the embodiment of the present invention is produced by multiaxial compression processing, hot rolling, cold rolling, annealing, and the like of the flat embryo. Therefore, the chemical composition of the steel sheet and the flat embryo will take into account these treatments and not only the characteristics of the steel sheet. In the following description, "%" which is a unit of content of each element contained in a steel plate and a flat embryo means "% by mass" unless otherwise stated. The steel sheet according to the present embodiment has the chemical composition shown below: C: 0.05% to 0.40%, Si: 0.05% to 2.00%, Mn: 1.50% to 4.00%, and acid-soluble Al: 0.01% by mass% ~1.00%, P: 0.10% or less, S: 0.01% or less, N: 0.01% or less, Ti: 0.0% to 0.2%, Nb: 0.0% to 0.2%, V: 0.0% to 0.2%, Cr: 0.0% ~1.0%, Mo: 0.0%~1.0%, Cu: 0.0%~1.0%, Ni: 0.0%~1.0%, Ca: 0.00%~0.01%, Mg: 0.00%~0.01%, REM (rare earth metal: Rare earth metal): 0.00%~0.01%, Zr: 0.00%~0.01%, and the remainder: Fe and impurities. The impurities may be exemplified by those contained in raw materials such as ore or scrap, and included in the production steps.

(C: 0.05%~0.40%) C contributes to the improvement of tensile strength. If the C content is less than 0.05%, sufficient tensile strength, for example, a tensile strength of 780 MPa or more cannot be obtained. Therefore, the C content is 0.05% or more, and preferably 0.07% or more. On the other hand, when the C content is more than 0.40%, the matrix of the mai field is hard and the weldability is deteriorated. Therefore, the C content is made 0.40% or less, preferably 0.30% or less, more preferably 0.20% or less.

(Si: 0.05% to 2.00%) Si is strengthened by solid solution, and the tensile strength is improved without deteriorating the hole expandability. If the Si content is less than 0.05%, sufficient tensile strength, for example, a tensile strength of 780 MPa or more cannot be obtained. Therefore, the Si content is made 0.05% or more. Si enhances the ductility of steel and promotes the formation of fat granules and inhibits the band-like distribution of hard tissues. When the Si content is less than 0.20%, a sufficient effect may not be obtained. Therefore, the Si content is preferably made 0.20% or more, and more desirably 0.50% or more. On the other hand, when the Si content is more than 2.00%, the surface properties are deteriorated, or the effect of addition is saturated and the cost is increased. Therefore, the Si content is made 2.00% or less, and preferably 1.50% or less, more preferably 1.20% or less.

(Mn: 1.50% to 4.00%) Mn contributes to an improvement in tensile strength. When the Mn content is less than 1.50%, sufficient tensile strength, for example, a tensile strength of 780 MPa or more cannot be obtained. Therefore, the Mn content is made 1.50% or more, and preferably 1.70% or more. On the other hand, when the Mn content is more than 4.00%, the precipitation amount of MnS increases, and the low temperature toughness deteriorates. Therefore, the Mn content is made 4.00% or less. From the viewpoint of productivity in hot rolling and cold rolling, the Mn content is preferably made 3.00% or less.

(Acid-soluble Al: 0.01% to 1.00%) Acid-soluble Al has a function of deoxidizing steel to improve the steel sheet. If the acid-soluble Al content is less than 0.01%, the effect of utilizing the action cannot be sufficiently obtained. Therefore, the acid-soluble Al content is made 0.01% or more, and preferably 0.02% or more. On the other hand, when the acid-soluble Al content is more than 1.00%, the weldability is lowered, or the oxide-based inclusions are increased to deteriorate the surface properties. Therefore, the acid-soluble Al content is 1.00% or less, and preferably 0.80% or less. Further, the acid-soluble Al does not constitute an acid-insoluble compound such as Al ₂ O ₃ and is soluble in an acid.

(P: 0.10% or less) P is not an essential element and is contained as an impurity, for example, in steel. From the viewpoint of weldability, the lower the P content, the better. In particular, if the P content is more than 0.10%, the weldability is significantly lowered. Therefore, the P content is made 0.10% or less, and preferably 0.03% or less. The reduction in the P content is costly, and the cost is significantly increased when it is to be reduced to less than 0.0001%. Therefore, the P content can also be made 0.0001% or more. P contributes to an increase in strength, and therefore, the P content can be made 0.01% or more.

(S: 0.01% or less) S is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the S content, the better. The higher the S content, the more the precipitation of MnS increases and the low temperature toughness decreases. In particular, if the S content is more than 0.01%, the decrease in weldability and the decrease in low-temperature toughness are remarkable. Therefore, the S content is made 0.01% or less, and preferably 0.003% or less, more preferably 0.0015% or less. The reduction in the S content is costly. When it is desired to reduce it to less than 0.001%, the cost will increase significantly. If it is to be reduced to less than 0.0001%, the cost will increase more significantly. Therefore, the S content can be made 0.0001% or more, and can be made 0.001% or more.

(N: 0.01% or less) N is not an essential element and is contained as an impurity, for example, in steel. From the viewpoint of weldability, the lower the N content, the better. In particular, if the N content is more than 0.01%, the decrease in weldability is remarkable. Therefore, the N content is made 0.01% or less, and preferably 0.006% or less. The reduction in the N content is costly, and the cost is significantly increased when it is to be reduced to less than 0.0001%. Therefore, the N content can also be made 0.0001% or more.

Ti, Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, and Zr are not essential elements, but may appropriately contain a predetermined amount of any element in the steel sheet and the steel.

(Ti: 0.0%~0.2%, Nb: 0.0%~0.2%, V: 0.0%~0.2%) Ti, Nb and V contribute to the improvement of strength. Therefore, it may contain Ti, Nb or V, or any combination of these. In order to sufficiently obtain this effect, the Ti content, the Nb content or the V content or any combination of these may be made 0.003% or more. On the other hand, if the Ti content, the Nb content, or the V content or any combination of these is more than 0.2%, hot rolling and cold rolling may become difficult. Therefore, the Ti content, the Nb content, or the V content, or any combination of these, is made 0.2% or less. That is, it is preferable to satisfy Ti: 0.003% to 0.2%, Nb: 0.003% to 0.2%, or V: 0.003% to 0.2%, or any combination of these.

(Cr: 0.0% to 1.0%, Mo: 0.0% to 1.0%, Cu: 0.0% to 1.0%, Ni: 0.0% to 1.0%) Cr, Mo, Cu, and Ni contribute to the improvement of strength. Therefore, it may contain Cr, Mo, Cu or Ni, or any combination of these. In order to sufficiently obtain this effect, the Cr content, the Mo content, the Cu content, or the Ni content, or any combination of these, is preferably made 0.005% or more. On the other hand, if the Cr content, the Mo content, the Cu content, or the Ni content, or any combination of these, is more than 1.0%, the effect of the above action is saturated and the cost is increased. Therefore, the Cr content, the Mo content, the Cu content, or the Ni content, or any combination of these, is made 1.0% or less. That is, it is preferable to satisfy Cr: 0.005% to 1.0%, Mo: 0.005% to 1.0%, Cu: 0.005% to 1.0%, or Ni: 0.005% to 1.0%, or any combination of these.

(Ca: 0.00% to 0.01%, Mg: 0.00% to 0.01%, REM: 0.00% to 0.01%, Zr: 0.00% to 0.01%) Ca, Mg, REM and Zr contribute to fine dispersion of inclusions. And improve toughness. Therefore, it may also contain Ca, Mg, REM or Zr, or any combination of these. In order to sufficiently obtain the effect, the Ca content, the Mg content, the REM content or the Zr content or any combination of these may be made 0.0003% or more. On the other hand, if the Ca content, the Mg content, the REM content, or the Zr content or any combination of these is more than 0.01%, the surface properties are deteriorated. Therefore, the Ca content, the Mg content, the REM content, or the Zr content or any combination of these may be made 0.01% or less. That is, it is preferable to satisfy Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, REM: 0.0003% to 0.01%, or Zr: 0.0003% to 0.01%, or any combination of these.

REM (rare earth metal) refers to a total of 17 elements of Sc, Y and lanthanoid elements, and "REM content" means the total content of these 17 elements. Lanthanides are industrially added, for example, in the form of rare earth metal alloys.

Next, the steel structure of the steel sheet according to the embodiment of the present invention will be described. The steel sheet according to the present embodiment has a steel structure as follows: in terms of area ratio, the granules are 5% to 80%, and are composed of a tough body, a granulated body or a residual Worth field, or any combination thereof. The hard tissue formed: 20% to 95%, and the standard deviation of the line rate of the hard structure on the line in the plane perpendicular to the thickness direction: the depth from the surface is 3t/8 when the thickness of the steel plate is set to t It is less than 0.050 in the depth range of t/2.

(Fat granules: 5% to 80%) If the area ratio of the granules is less than 5%, it is difficult to ensure elongation at break (EL) of 10% or more. Therefore, the area ratio of the granules is 5% or more, and more preferably 10% or more, and more desirably 20% or more. On the other hand, if the area ratio of the fat granules is more than 80%, sufficient tensile strength, for example, a tensile strength of 780 MPa or more cannot be obtained. Therefore, the area ratio of the fat granules is 80% or less, and preferably 70% or less.

(Hard tissue: 20% to 95%) If the area ratio of the hard structure is less than 20%, it is difficult to obtain sufficient tensile strength, for example, a tensile strength of 780 MPa or more. Therefore, the area ratio of the hard structure is 20% or more, and it is preferable to make 30% or more. On the other hand, if the area ratio of the hard structure is more than 95%, sufficient ductility cannot be obtained. Therefore, the area ratio of the hard structure is 95% or less, and it is preferably 90% or less, and more preferably 80% or less.

(Residual Worth Field (Residual γ): 5.0% or more) If the area ratio of the remaining Worth field is 5.0% or more, it is easy to obtain a fracture elongation of 12% or more. Therefore, the area ratio of the residual Worth field should be made 5.0% or more, and more desirably 10.0% or more. There is no limit to the upper limit of the area ratio of the residual Worth field. However, in the current technical level, it is not easy to manufacture a steel sheet having an area ratio of more than 30.0% of the residual Worth field.

The area ratio of the fat granules and the area ratio of the hard tissues can be measured as follows. First, the sample was taken, and a section perpendicular to the width direction of the 1/4 position of the width of the steel sheet was exposed, and the cross section was etched by a Le Pera etching solution. Next, an optical microscope photograph of a region from the surface of the steel sheet having a depth of 3 t/8 to t/2 was taken. At this time, for example, the magnification is made 200 times. By using the etching of the Le Pera etching solution, the viewing surface can be roughly divided into a black portion and a white portion. Also, the black portion may contain fat bodies, metamorphic bodies, carbides, and corrugated bodies. In the black portion, the portion of the granule containing the layered structure is equivalent to the wave body. In the black portion, the portion containing no layered structure in the granule and containing the lower portion of the tissue corresponds to the granule. In the black portion, a spherical portion having a particularly low luminance and a diameter of 1 μm to 5 μm is equivalent to a carbide. In the black portion, the portion of the granule containing the lower portion of the tissue is equivalent to the toughened body. Therefore, by measuring the area ratio of the portion containing the lower structure in the black portion which does not contain the layered structure in the black portion, the area ratio of the fertilizer body can be obtained, and the area of the portion containing the lower portion of the grain in the black portion can be determined. Rate, the area ratio of the toughened body can be obtained. Further, the area ratio of the white portion is the total area ratio of the Matian bulk or the residual Worth field. Therefore, the area ratio of the self-transformed body and the total area ratio of the Matian bulk and the residual Worth field body can obtain the area ratio of the hard structure. From the optical microscope photograph, the circle-equivalent average diameter r of the hard tissue used for the measurement of the standard deviation of the line rate of the hard tissue described below can be measured.

The area fraction of the residual Worth field can be specified, for example, by X-ray measurement. In this method, for example, mechanical polishing and chemical polishing are used to remove the surface of the steel sheet to a portion of the thickness of the steel sheet, and the characteristic X-rays are MoKα rays. Further, using the following formula, the diffraction peaks of (200), (220), and (311) of the self-centered cubic lattice (bcc) phase (200) and (211) and the face-centered cubic lattice (fcc) phase The integral intensity ratio is used to calculate the area fraction of the residual Worth field. Sγ=(I _200f +I _220f +I _311f )/(I _200b +I _211b )×100 (Sγ represents the area fraction of the residual Worth field, and I _200f , I _220f , and I _311f represent the fcc phase (200, respectively). The diffraction peak intensities of (220) and (311), I _200b and I _211b represent the diffraction peak intensities of (b) and (211) of the bcc phase, respectively.

(Standard deviation of the line segment rate of the hard structure on the line in the plane perpendicular to the thickness direction: when the thickness of the steel sheet is t, the depth from the surface is 3t/8 to t/2, and the depth is 0.050 or less. The steel sheet is fractured by the necking or the generation and joining of the pores in the steel structure during the processing of applying local large deformation such as reaming. In the tensile deformation of the steel sheet during collapse, the central portion of the steel sheet constitutes a stress concentration. Generally, the pores are mainly generated at a position t/2 from the surface of the steel sheet. Further, when the pores are connected and the pores are coarsened to a size larger than t/8, the coarsened pores are used as a starting point to cause fracture. The pore-generating portion constituting the fracture starting point as described above exists in a hard structure having a depth from the surface of 3t/8 to t/2. Therefore, the distribution of the hard structure in the depth range from 3t/8 to t/2 from the surface greatly affects the hole expansibility.

Further, the large standard deviation of the line segment rate of the hard structure in the above-described depth range means that the ratio of the hard structure in the thickness direction fluctuates greatly, that is, the steel structure constitutes a band structure. In particular, when the standard deviation of the line segment rate of the hard structure is more than 0.050, the band structure is remarkable, and the density at the stress concentration is locally increased, and sufficient hole expandability cannot be obtained. Therefore, the standard deviation of the line segment rate of the hard structure is set to be 0.050 or less in a depth range from 3t/8 to t/2 from the surface, and preferably 0.040 or less.

Here, a method of measuring the standard deviation of the line segment rate of the hard structure will be described.

First, in the same manner as the measurement of the area ratio, the photographed optical micrograph was subjected to image processing and binarized into a black portion and a white portion. Fig. 1 is a view showing an example of a binarized image. Next, at the portion of the image depth 3t/8 of the observation object to the depth t/2, the starting point of the line segment is set every r/30 (r is the equivalent average diameter of the circle of the hard tissue). Since the depth range of the observation object is an area of thickness t/8 from 3t/8 to t/2, the number of starting points constitutes 15t/4r. Then, a line segment having a length of 50 r extending from the respective starting points in a direction perpendicular to the thickness direction, for example, in the rolling direction, is set, and the line segment rate of the hard structure on the line segment is measured. Further, the standard deviation of the line segment rate between the 15t/4r line segments is calculated.

There is no limit to the equivalent average diameter r of the circle and the thickness t of the steel sheet. For example, the circle equivalent average diameter r is 5 μm to 15 μm, and the thickness t of the steel sheet is 1 mm to 2 mm (1000 μm to 2000 μm). The interval between the start points of the line segments is not limited, and can be changed in accordance with the resolution of the target image, the number of pixels, and the measurement work time. For example, even if the interval is made to r/10, the same result as when r/30 is created can be obtained.

The depth range from the surface to the depth of 3t/8 to t/2 can theoretically be infinitely subdivided, and the surface perpendicular to the thickness direction is also infinite. However, the line rate cannot be determined for all of them. On the other hand, according to the above-described measuring method, the depth range can be subdivided at a very small interval, and the same result as in the case of infinite subdivision can be obtained. For example, in Figure 1, on the X-X line, the line rate of the hard tissue is high, and on the Y-Y line, the line rate of the hard tissue is low.

According to the present embodiment, for example, a tensile strength of 780 MPa or more can be obtained, and in the method specified in JIS Z 2256, when the test is performed at a hole expansion test speed of 1 mm/sec, 30% or more is obtained. Hole expansion ratio (HER). Moreover, the JIS No. 5 tensile test piece was collected from the steel sheet, and the direction of the stretching direction was orthogonal to the rolling direction, and when measured by the method specified in JIS Z 2241, an elongation at break of 10% or more was obtained. degree.

Next, a method of manufacturing a steel sheet according to an embodiment of the present invention will be described. In the method for producing a steel sheet according to an embodiment of the present invention, multiaxial compression processing, hot rolling, cold rolling, and annealing of the flat embryo having the above chemical composition are performed in this order.

(Multiaxial compression processing) For example, the flat embryo can be melted by using a converter or an electric furnace or the like and melted by the continuous casting method. Instead of the continuous casting method, a block forming method, a thin flat blank casting method, or the like can be used.

The flat embryo is heated to 950 ° C ~ 1300 ° C before being supplied to the multiaxial compression process. The holding time after heating is not limited. However, from the viewpoint of hole expandability, it is preferably 30 minutes or longer, and from the viewpoint of suppressing excessive scale loss, it is preferably made 10 hours or shorter. It is desirable to make it for less than 5 hours. In the case of straight-feed calendering or direct pressure-delay, the flattened embryos may be directly supplied to the multi-axial compression process without heating.

If the temperature of the slab supplied to the multiaxial compression processing is less than 950 ° C, the diffusion of the alloying elements is significantly delayed, and the formation of the band structure cannot be suppressed. Therefore, the temperature of the spheroid is made 950 ° C or higher, and preferably 1020 ° C or higher. On the other hand, if the temperature of the slab supplied to the multiaxial compression processing is more than 1300 ° C, the manufacturing cost is increased, or the scale loss is increased and the yield is lowered. Therefore, the temperature of the slab is 1300 ° C or lower, and preferably 1250 ° C or less.

In the multi-axis compression processing, the flat embryos of 950 ° C to 1300 ° C are subjected to compression processing in the width direction and compression processing in the thickness direction. By multiaxial compression processing, a portion of the alloy element such as Mn concentrated in the flat embryo is subdivided or introduced into a lattice defect. Therefore, in the multiaxial compression processing, the alloying elements are uniformly diffused, and the formation of the banded structure in the subsequent step can be suppressed, and an extremely homogeneous structure can be obtained. In particular, compression processing in the width direction is effective. In other words, by the multiaxial compression processing, the concentrated portion of the alloy element that is connected in the width direction is finely blocked, and the alloy elements are uniformly dispersed. As a result, it is possible to realize the homogenization of the structure which cannot be achieved in the diffusion of the alloy element which is simply heated for a long period of time in a short time.

When the deformation rate per one time in the compression processing in the width direction is less than 3%, the amount of lattice defects introduced by plastic deformation is insufficient, and the diffusion of the alloy elements cannot be promoted, and the formation of the band structure cannot be suppressed. Therefore, the deformation rate per compression processing in the width direction is 3% or more, and preferably 10% or more. On the other hand, if the deformation rate of the compression processing in the width direction is more than 50% each time, the flat embryo is broken, or the shape of the flat embryo becomes uneven, and the size of the hot rolled steel sheet obtained by hot calendering is obtained. Reduced accuracy. Therefore, the deformation rate per unit of the compression processing in the width direction is 50% or less, and preferably 40% or less.

When the deformation rate per unit of the compression processing in the thickness direction is less than 3%, the amount of lattice defects introduced by the plastic deformation is insufficient, and the diffusion of the alloy elements cannot be promoted, and the formation of the band structure cannot be suppressed. Moreover, due to the poor shape, the hot pressing delay may cause a bite of the flattened blank to the calendering roll. Therefore, the deformation rate per unit of the compression processing in the thickness direction is 3% or more, and preferably 10% or more. On the other hand, if the deformation rate in the thickness direction is greater than 50% each time, the flat embryo is broken, or the shape of the flat embryo becomes uneven, and the size of the hot rolled steel sheet obtained by hot rolling is obtained. Reduced accuracy. Therefore, the deformation rate per unit of the compression processing in the thickness direction is 50% or less, and preferably 40% or less.

When the difference between the amount of rolling in the width direction and the amount of rolling in the thickness direction is too large, alloy elements such as Mn cannot be sufficiently diffused in a direction perpendicular to the direction in which the amount of rolling is small, and formation of a band-like structure may not be sufficiently suppressed. In particular, when the difference in the amount of rolling is more than 20%, the band structure is easily formed. Therefore, the difference in the amount of rolling between the width direction and the thickness direction is made 20% or less.

When the multiaxial compression processing is performed at least once, the formation of the band structure can be suppressed. The effect of suppressing the formation of the banded structure is remarkable by the repeated multiaxial compression processing. Therefore, the number of times of multiaxial compression processing is made one or more times, and it is preferable to make it twice or more. When performing multi-axis compression processing for two or more times, the flat embryo can be reheated between the multi-axis compression processes. On the other hand, if the number of times of multiaxial compression processing is more than 5 times, the manufacturing cost is increased, or the scale loss is increased and the yield is lowered. Moreover, the thickness of the flat embryo becomes uneven, and hot rolling may become difficult. Therefore, the number of times of multiaxial compression processing should be 5 or less, and more preferably 4 or less.

(Hot calendering) In the hot calendering, the compact rolling of the flat embryo after the multiaxial compression processing is performed. The temperature of the flat embryo supplied to the finish calendering is set to 1050 ° C to 1150 ° C, and the first rolling is performed by the finish rolling, and then the second rolling is performed, and the winding is performed at 650 ° C or lower. In the first rolling, the rolling reduction ratio (first rolling reduction ratio) in the temperature range of 1050 ° C to 1150 ° C is 70% or more, and in the second rolling, the rolling reduction ratio in the temperature range of 850 ° C to 950 ° C is performed. (The second rolling reduction ratio) is 50% or less.

When the temperature of the slab supplied to the first rolling is less than 1050 ° C, the deformation resistance in the finish rolling is high, and handling becomes difficult. Therefore, the temperature of the flat embryo supplied to the first rolling is 1050 ° C or higher, and preferably 1070 ° C or higher. On the other hand, if the temperature of the flat embryo supplied to the finish calendering is more than 1150 ° C, the scale loss increases and the yield decreases. Therefore, the temperature of the flat embryo supplied to the first rolling is 1150 ° C or lower, and preferably 1130 ° C or lower.

In the first rolling, recrystallization occurs in a temperature region of 1050 ° C to 1150 ° C (Worth field single phase region). When the rolling reduction ratio (first rolling reduction ratio) in the temperature range is less than 70%, a single-phase structure of a Worstian body having a fine crystal grain and a spherical shape cannot be stably obtained, and then a band-like structure is easily formed. Therefore, the first rolling reduction ratio is 70% or more, and preferably 75% or more. The first rolling can be carried out in a single base or in a plurality of bases.

When the rolling reduction ratio (second rolling reduction ratio) of the temperature range of 850 ° C to 950 ° C of the second rolling is more than 50%, the winding is caused by the non-recrystallized Worth body, and a flat band structure cannot be formed. Obtain the desired standard deviation. Therefore, the second reduction ratio is 50% or less. The second rolling can be carried out in a single stand or in a plurality of bases.

When the end temperature of the second rolling is less than 850 ° C, recrystallization cannot be sufficiently caused to easily form a band structure. Therefore, the end temperature is 850 ° C or higher, and preferably 870 ° C or higher. On the other hand, when the end temperature is more than 1000 ° C, crystal grains tend to grow, and it is difficult to obtain a fine structure. Therefore, the end temperature is made 1000 ° C or less, and preferably 950 ° C or less.

When the winding temperature is more than 650 ° C, the surface properties are deteriorated by internal oxidation. Therefore, the winding temperature is 650 ° C or lower, and preferably 450 ° C or lower, more preferably 50 ° C or lower. If the cooling rate from the temperature of the self-refining to the winding temperature is less than 5 ° C / s, it is difficult to obtain a homogeneous structure, and it is difficult to obtain a homogeneous steel structure in the subsequent annealing. Therefore, the cooling rate from the self-finishing to the winding is 5 ° C / s or more, and preferably 30 ° C / s or more. The cooling rate of 5 ° C / s or more can be achieved, for example, by water cooling.

(Cold rolling) Cold rolling is carried out, for example, after pickling of a hot rolled steel sheet. From the viewpoint of homogenizing and miniaturizing the structure of the cold-rolled steel sheet, the rolling reduction ratio of the cold rolling is preferably 50% or more.

(Annealing) For example, annealing is performed by continuous annealing. If the annealing temperature is less than 740 ° C, the retrograde metamorphosis process cannot be sufficiently caused, and a hard structure having an area ratio of 20% or more cannot be obtained. Therefore, the annealing temperature is 740 ° C or higher, and preferably 770 ° C or higher. On the other hand, when the annealing temperature is more than 950 ° C, the productivity is lowered, or the Worth field body is composed of coarse particles, and the fertilizer particles having an area ratio of 10% or more cannot be obtained. Therefore, the annealing temperature is 950 ° C or lower, and preferably 920 ° C or lower.

The annealing time is not limited, however, it should be made for more than 60 seconds. This is to significantly reduce the unrecrystallized structure and to ensure a homogeneous steel structure. After the annealing, the steel sheet is preferably cooled to a temperature region of 600 ° C or more and 720 ° C or less by an average cooling rate of 2 ° C /sec or more and 10 ° C / sec or less. This is to ensure the full area ratio of the fat granules. Preferably, the temperature is cooled from a temperature range of 600 ° C or more to 720 ° C or less to a temperature range of 200 ° C or more and 350 ° C or less by an average cooling rate of 35 ° C /sec or more, and is 200 ° C or more and 350 ° C or less. The temperature zone is maintained for more than 200 seconds. This is to improve the hole expandability by ensuring the ductility of the hard structure.

According to this, a steel sheet according to an embodiment of the present invention can be produced.

The above-described embodiments are merely examples of the specific embodiments of the present invention, and the technical scope of the present invention is not limited by the terms. That is, the present invention can be implemented in various forms without departing from the technical idea or its main features. Example

Next, an embodiment of the present invention will be described. The conditions in the examples are a conditional example used to confirm the workability and effects of the present invention, and the present invention is not limited to the one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

A flat embryo having the chemical composition shown in Table 1 was produced, and after heating the flat embryo at 1250 ° C for 1 hour, multiaxial compression processing was carried out under the conditions shown in Table 2. Next, the slab was reheated to 1,250 ° C, and hot rolled steel sheets were produced by hot rolling under the conditions shown in Table 2. In the hot rolling, the first rolling was performed in four stages, and the second rolling was performed in two stages, and after winding, the temperature was maintained at the winding temperature for one hour. Then, pickling was performed on the hot-rolled steel sheet, and cold rolling was performed by the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. Next, continuous annealing was performed by the temperature shown in Table 2. In the continuous annealing, the temperature increase rate was set to 2 ° C / sec, and the annealing time was made to 200 sec. After being held for 200 seconds, it was cooled to a temperature range of 720 ° C to 600 ° C by an average cooling rate of 2.3 ° C / sec, and further cooled to 300 ° C by an average cooling rate of 40 ° C / sec, and at 300 °C was held for 60 seconds and cooled to a room temperature of about 30 ° C by an average cooling rate of 0.75 ° C / sec. The remainder of the chemical composition shown in Table 1 is Fe and impurities. The bottom line in Table 1 indicates that the value is outside the scope of the present invention.

[Table 1]         <TABLE border="1" borderColor="#000000" width="_0001"><TBODY><tr><td> Table 1 </td></tr><tr><td> Steel </td> <td> Chemical composition (% by mass) </td></tr><tr><td> Mark </td><td> C </td><td> Si </td><td> Mn </ Td><td> P </td><td> S </td><td> Al </td><td> N </td><td> Other </td></tr><tr>< Td> A </td><td> 0.03 </td><td> 1.0 </td><td> 2.6 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> </td></tr><tr><td> B </td><td> 0.07 </td><td> 1.0 </td ><td> 2.6 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Ti:0.03 </td ></tr><tr><td> C </td><td> 0.12 </td><td> 1.0 </td><td> 2.2 </td><td> 0.01 </td><td > 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Ti:0.03 </td></tr><tr><td> D </td><td> 0.21 </td><td> 1.0 </td><td> 1.7 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 < /td><td> </td></tr><tr><td> E </td><td> 0.07 </td><td> 0.02 </td><td> 1.7 </td>< Td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Cr:0.07 </td></tr><tr><td > F </td><td> 0.09 </td><td> 0.2 </td><td> 2.3 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </ Td><td> 0.003 </td><td> </td></tr><tr><td> G </td><td> 0.07 </td><td> 0.2 </td><td > 1.2 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> </td></tr>< Tr><td> H </td><td> 0.09 </td><td> 1.0 </td><td> 2.3 </td><td> 0.01 </td><td> 0.002 </td> <td> 0.03 </td><td> 0.003 </td><td> Nb:0.03 </td></tr><tr><td> I </td><td> 0.11 </td>< Td> 1.1 </td><td> 2.0 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> V: 0.02 </td></tr><tr><td> J </td><td> 0.12 </td><td> 1.0 </td><td> 1.8 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Cr:0.5 </td></tr><tr><td> K < /td><td> 0.12 </td><td> 0.8 </td><td> 1.8 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td ><td> 0.003 </td><td> Mo:0.1 </td></tr><tr><td> L </td><td> 0.09 </td><td> 0.7 </td> <td> 2.1 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Cu:0.12 </td> </tr><tr>< Td> M </td><td> 0.10 </td><td> 1.2 </td><td> 2.0 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Ni:0.1 </td></tr><tr><td> N </td><td> 0.12 </td><td> 1.0 </td><td> 2.2 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Ca: 0.002 </td></tr><tr><td> O </td><td> 0.13 </td><td> 1.0 </td><td> 2.0 </td><td> 0.01 </td ><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Mg:0.002 </td></tr><tr><td> P </td> <td> 0.10 </td><td> 0.5 </td><td> 2.0 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td > 0.003 </td><td> REM:0.002 </td></tr><tr><td> Q </td><td> 0.09 </td><td> 1.0 </td><td> 2.0 </td><td> 0.01 </td><td> 0.002 </td><td> 0.03 </td><td> 0.003 </td><td> Zr:0.002 </td></tr ><tr><td> R </td><td> 0.10 </td><td> 1.1 </td><td> 2.2 </td><td> 0.01 </td><td> 0.002 </ Td><td> 0.03 </td><td> 0.003 </td><td> Ti:0.03 </td></tr></TBODY></TABLE>

[Table 2]

Further, the steel structure of the obtained cold rolled steel sheet was observed. In the observation of the steel structure, the area ratio of the fat granules, the area ratio of the hard tissue (the total area ratio of the tough body, the granulated bulk and the residual Worth field), the corrugated body and the carbide are determined by the above method. The total area ratio and the standard deviation of the line rate of the hard tissue. Table 3 shows these results. The bottom line in Table 3 indicates that the value is outside the scope of the present invention.

Further, the tensile strength TS, the elongation at break EL, and the hole expansion ratio HER of the obtained cold-rolled steel sheet were measured. In the measurement of the tensile strength TS and the elongation at break EL, a JIS No. 5 tensile test piece was prepared, and the test piece was oriented in a direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z 2241. In the measurement of the hole expansion ratio HER, a 90 mm square test piece was taken from the cold rolled steel sheet, and a hole expansion test according to JIS Z 2256 (or JIST 1001) was carried out. At this time, the reaming test speed was made 1 mm/sec. Table 3 also shows these results. The bottom line in Table 3 indicates that the value is out of the desired range. The ideal range here is that the tensile strength TS is 780 MPa or more, the elongation at break EL is 10% or more, and the hole expansion ratio HER is 30% or more.

[table 3]

As shown in Table 3, excellent tensile strength can be obtained in samples No. 2 to No. 4, No. 16, No. 19, No. 21 to No. 30, and No. 33 which are within the scope of the present invention. , elongation at break and hole expandability. Among these, sample No. 23 and the like have an area ratio of the remaining Worth field (residue γ) of 5.0% or more. Therefore, the elongation at break which is superior to the sample No. 16 can be obtained.

On the other hand, in sample No. 1, the C content was too low, the area ratio of the fertilizer granules was too high, and the area ratio of the hard structure was too low, so that the tensile strength was low. In sample No. 18, the Si content was too low, and the area ratio of the fertilizer granules was too low, so the tensile strength was low. In sample No. 20, the Mn content was too low, and the area ratio of the fertilizer granules was too low, so that the tensile strength was low.

In samples No. 5 to No. 8, No. 10 to No. 14, No. 31, and No. 35, since the standard deviation of the line segment rate of the hard structure was too large, the hole expansion ratio was low. Sample No. 9 is that the area ratio of the fat and granules is too high, and the area ratio of the hard structure is too low, so the tensile strength and the hole expansion ratio are low. In the sample No. 15, the deformation rate in the width direction in the multiaxial compression processing was too low, and therefore hot rolling could not be performed thereafter. Sample No. 17 is that the area ratio of the fertilizer granules is too low, and therefore, the elongation at break is low. Sample No. 32 was such that the area ratio of the hard structure was too low, and therefore the tensile strength was low. Sample No. 33 was that the area ratio of the hard structure was too high, and therefore the elongation at break was low. Industrial availability

The present invention can utilize industries associated with, for example, steel sheets suitable for automotive parts.

Fig. 1 is a view showing a method of obtaining a line segment rate of a hard tissue.

Claims (9)

A steel sheet having the following characteristics: having the chemical composition shown below: C: 0.05% to 0.40%, Si: 0.05% to 2.00%, Mn: 1.50% to 4.00%, acid-soluble Al: 0.01% by mass% ~1.00%, P: 0.10% or less, S: 0.01% or less, N: 0.01% or less, Ti: 0.0% to 0.2%, Nb: 0.0% to 0.2%, V: 0.0% to 0.2%, Cr: 0.0% ~1.0%, Mo: 0.0%~1.0%, Cu: 0.0%~1.0%, Ni: 0.0%~1.0%, Ca: 0.00%~0.01%, Mg: 0.00%~0.01%, REM: 0.00%~0.01 %, Zr: 0.00% to 0.01%, and the remainder: Fe and impurities; and, with the steel structure shown below: in terms of area ratio, fertilizer granules: 5% to 80%, from tough body, 麻田散体Or a hard tissue composed of any of the remaining Worstian bodies or any combination of the above: 20% to 95%, and the standard deviation of the line rate of the aforementioned hard structure on the line in the plane perpendicular to the thickness direction: When the thickness is set to t, the depth from the surface is from 3t/8 to t/2, and the depth is 0.050 or less. The steel sheet according to claim 1, wherein the steel structure in the above-mentioned steel structure satisfies the above-mentioned residual Worth field body: 5.0% or more. The steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition satisfies the following or any combination of the above: Ti: 0.003% to 0.2%, Nb: 0.003% to 0.2%, or V: 0.003 by mass% %~0.2%. The steel sheet according to claim 1 or 2, wherein the chemical composition satisfies the following or any combination of the above: Cr: 0.005% to 1.0%, Mo: 0.005% to 1.0%, Cu: 0.005% by mass% ~1.0%, or Ni: 0.005%~1.0%. The steel sheet according to claim 3, wherein the chemical composition satisfies the following or any combination of the above: Cr: 0.005% to 1.0%, Mo: 0.005% to 1.0%, Cu: 0.005% to 1.0, by mass% %, or Ni: 0.005% to 1.0%. The steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition satisfies the following or any combination of the above: Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, REM: 0.0003% by mass% ~0.01%, or Zr: 0.0003%~0.01%. The steel sheet according to claim 3, wherein the chemical composition satisfies the following or any combination of the above: Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, and REM: 0.0003% to 0.01% by mass %, or Zr: 0.0003%~0.01%. The steel sheet according to claim 4, wherein the chemical composition satisfies the following or any combination of the above: Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, and REM: 0.0003% to 0.01% by mass %, or Zr: 0.0003%~0.01%. The steel sheet according to claim 5, wherein the chemical composition satisfies the following or any combination of the above: Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, and REM: 0.0003% to 0.01% by mass %, or Zr: 0.0003%~0.01%.