Hot-rolled steel sheet

Abstract

Provided is a hot rolled steel sheet that has a prescribed chemical composition and contains 80 to 98% of at least one of a ferrite and a bainite in total and 2 to10% of a martensite in terms of areal percentage and that has a metallic structure, wherein: given that a boundary having an orientation difference of 15º or more is defined as a grain boundary and a region that is surrounded by the grain boundary and has a circle-equivalent diameter of 0. 3μm or more is defined as a crystal grain, the proportion of crystal grains having an in-grain orientation difference of 5-14º is 10 to 60% in terms of areal percentage; the average pole density in the {110}<111> and {112}<111> orientations in a region from the surface to the 1 / 6-sheet thickness position is 2.50 or more; and the average pole density in the {100}<011>, {211}<011> , and {332}<113> orientations in a region from the 2 / 5-sheet thickness position to the 3 / 5-sheet thickness position is 7.00 or less.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet.

Background Art

[0002] In recent years, in the automotive industry, weight reduction of vehicle bodies has been demanded from the perspective of improving fuel efficiency. In order to achieve both weight reduction and collision safety of vehicle bodies, increasing the strength of steel sheets used is one effective method, and development of high-strength steel sheets has been promoted under such a background. On the other hand, the workability of steel sheets generally decreases as the strength increases. Therefore, in the development of high-strength steel sheets, it is important to increase the strength while ensuring a certain level of workability.

[0003] In relation to this, in Patent Document 1, a hot-rolled steel sheet having a predetermined chemical composition, a structure including ferrite and bainite with a total area ratio of 80 to 98% and martensite with an area ratio of 2 to 10%, in the structure, when a boundary with an orientation difference of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, the ratio of the crystal grains with an orientation difference of 5 to 14° in the grains is 10 to 60% in terms of area ratio is described. Further, in Patent Document 1, by setting the ratio of the crystal grains with an orientation difference of 5 to 14° in the grains to 10 to 60% in terms of area ratio, it is possible to improve the elongation flangeability and ductility while having high strength, and it is taught that notch fatigue characteristics can be improved by controlling the total area ratio of ferrite and bainite and the area ratio of martensite in the structure within a predetermined range.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] High-strength steel sheets are manufactured by hot rolling a cast slab. It is known that this hot rolling process can result in anisotropy in strength between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular to it. A large anisotropy in strength generally reduces the workability of the steel sheet, which is problematic. Therefore, in order to improve the workability of steel sheets, there is a high demand for high-strength steel sheets with reduced anisotropy in strength, in addition to the stretch flange properties, ductility, and notch fatigue properties described in Patent Document 1.

[0006] Therefore, the present invention aims to provide a hot-rolled steel sheet that, despite having high strength, has improved tensile flange properties, ductility, and notch fatigue characteristics, and reduced strength anisotropy. [Means for solving the problem]

[0007] To achieve the above objective, the inventors focused particularly on the microstructure of hot-rolled steel sheets. As a result, the inventors found that by configuring the microstructure of a hot-rolled steel sheet having a predetermined chemical composition to contain at least one of ferrite and bainite, as well as martensite, in specific proportions, and further controlling the proportion of crystal grains within a predetermined range, the tensile flange properties, ductility, and notch fatigue properties can be improved. In addition, by appropriately controlling the texture between the surface and center of the steel sheet thickness, the anisotropy of strength can be reduced, thus completing the present invention.

[0008] The present invention, which has achieved the above objectives, is as follows. (1) In mass%, C: 0.020~0.070%, Si: 0.010~2.000%, Mn: 0.60~2.00%, Ti: 0.015~0.200%, sol.Al: 0.010~1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0~0.050%, V: 0~0.300%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.000%, B: 0~0.0100%, Sb: 0~1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, Hf: 0~0.0100%, REM: 0~0.1000%, Bi: 0~0.0100%, As: 0~0.0100%, Zr: 0~1.00%, Co: 0~1.00%, Zn: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, and The remainder consists of Fe and impurities. The chemical composition satisfies 0.100 ≤ [Si] + [sol.Al] ≤ 2.500, where [Si] and [sol.Al] are the mass percentages of each element. In area percentage, At least one of ferrite and bainite: 80-98% in total, Martensite: Contains 2-10%, When a grain boundary is defined as a boundary with an orientation difference of 15° or more, and a region enclosed by such grain boundaries with an equivalent circular diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain is 10 to 60% in area percentage. {110} in the region from the surface to a position 1 / 6 of the plate thickness <111> and {112} <111> The average value of the extreme density of azimuth is 2.50 or higher. The hot-rolled steel sheet is characterized by having a metal structure in which the average value of the pole densities in the <011> direction of {100}, the <011> direction of {211}, and the <113> direction of {332} in the region from the 2 / 5 position to the 3 / 5 position of the plate thickness is 7.00 or less. (2) The chemical composition is in mass %, Nb: 0.001 to 0.050%, V: 0.001 to 0.300%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sb: 0.01 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, REM: 0.0001 to 0.1000%, <000第00094>Bi: 0.0001 to 0.0100%, As: 0.0001 to 0.0100%, Zr: 0.01 to 1.00%, Co: 0.01 to 1.00%, Zn: 0.01 to 1.00%, W: 0.01 to 1.00%, and Sn: 0.01 to 1.00% The hot-rolled steel sheet according to (1) above, characterized by containing at least one of them.

Advantages of the Invention

[0009] According to the present invention, it is possible to provide a hot-rolled steel sheet with improved elongation flangeability, ductility, and notch fatigue characteristics and reduced strength anisotropy, despite being high-strength.

Brief Description of the Drawings

[0010] [Figure 1]This figure shows the shape of a saddle-shaped molded product used in the saddle-shaped elongation flange test method. [Figure 2] This figure shows the shape of the fatigue test specimen used to evaluate the fatigue characteristics of the notch. [Modes for carrying out the invention]

[0011] <Hot-rolled steel sheet> The hot-rolled steel sheet according to the embodiment of the present invention is, by mass%, C: 0.020~0.070%, Si: 0.01~2.00%, Mn: 0.600~2.00%, Ti: 0.015~0.200%, sol.Al: 0.100~1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0~0.050%, V: 0~0.300%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.000%, B: 0~0.0100%, Sb: 0~1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, Hf: 0~0.0100%, REM: 0~0.1000%, Bi: 0~0.0100%, As: 0~0.0100%, Zr: 0~1.00%, Co: 0~1.00%, Zn: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, and The remainder consists of Fe and impurities. The chemical composition satisfies 0.100 ≤ [Si] + [sol.Al] ≤ 2.500, where [Si] and [sol.Al] are the mass percentages of each element. In area percentage, At least one of ferrite and bainite: 80-98% in total, Martensite: Contains 2-10%, When a grain boundary is defined as a boundary with an orientation difference of 15° or more, and a region enclosed by such grain boundaries with an equivalent circular diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain is 10 to 60% in area percentage. {110} in the region from the surface to a position 1 / 6 of the plate thickness <111> and {112} <111> The average value of the extreme density of azimuth is 2.50 or higher. {100} in the region from the position of plate thickness 2 / 5 to the position of plate thickness 3 / 5 <011> , {211} <011> and {332} <113> It is characterized by having a metallic structure in which the average value of the polar density in each direction is 7.00 or less.

[0012] As mentioned earlier, it is known that as the strength of steel sheets increases, properties such as tensile flangeability decrease, and that due to hot rolling during steel sheet manufacturing, there may be anisotropy in strength between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular to it. First, in embodiments of the present invention, the metal structure of a hot-rolled steel sheet having a predetermined chemical composition is configured to contain at least one of ferrite and bainite and martensite in specific proportions, more specifically, by configuring it to contain at least one of ferrite and bainite in total (80-98%) and martensite (2-10%) in area percent, it is possible to improve strength, tensile flangeability, ductility, and notch fatigue properties in a balanced manner. In addition, when a crystal grain is defined as a region surrounded by a boundary with an orientation difference of 15° or more and with an equivalent circular diameter of 0.3 μm or more, crystal grains with an orientation difference of 5-14° within the grain are effective in improving strength, tensile flangeability, and ductility. Therefore, by appropriately controlling the proportion of these crystal grains, more specifically by controlling it within the range of 10-60% in area percentage, it becomes possible to further improve the balance between strength, ductility, and ductility.

[0013] On the other hand, to explain the anisotropy of strength in more detail, due to the anisotropic metal structure obtained by hot rolling during steel sheet manufacturing, the tensile strength tends to differ between the rolling direction (L direction) and the width direction perpendicular to it (C direction). Generally, hot-rolled steel sheets tend to exhibit anisotropy of strength such that the tensile strength in the L direction is lower than the tensile strength in the C direction. By improving the stretch flange properties, ductility, and notch fatigue properties mentioned above, as well as reducing such anisotropy of strength, it is possible to greatly improve the workability of high-strength steel sheets used in applications such as automobiles. However, it is generally very difficult to achieve both increased strength of steel sheets and improvements in these properties. Therefore, the inventors focused particularly on the texture of hot-rolled steel sheets in order to achieve both increased strength of steel sheets and improved stretch flange properties, ductility, and notch fatigue properties by reducing anisotropy of strength. As a result, the inventors found that {110} in the region from the surface of the hot-rolled steel sheet to the 1 / 6 position of the sheet thickness <111> and {112} <111> The average value of the polar density of orientation is controlled to be 2.50 or higher, and {100} in the region from the position of 2 / 5 plate thickness to the position of 3 / 5 plate thickness. <011> , {211} <011> and {332} <113> We found that by controlling the average value of the polar density in each direction to 7.00 or less, the anisotropy of strength in the tensile strength of hot-rolled steel sheets in the L and C directions can be significantly reduced.

[0014] To explain in more detail, when analyzing the cross-section of a hot-rolled steel sheet, the crystal orientation differs between the surface layer of the sheet (i.e., the region from the surface of the hot-rolled steel sheet to the 1 / 6 mark of the thickness) and the center layer of the sheet (i.e., the region from the 2 / 5 mark to the 3 / 5 mark of the thickness). More specifically, in the surface layer of the sheet, {110} <111> and {112} <111> The texture in the orientation is well-developed, and it is thought that the strength in the L direction is increased due to the development of this texture. On the other hand, in the center of the plate thickness, {100} <011> , {211} <011> and {332} <113> The texture in the orientation is well-developed, and it is thought that the strength in the C direction is increased due to the development of this texture. However, the influence of the center of the plate is stronger than that of the surface, and therefore it is thought that the strength anisotropy is such that the tensile strength in the L direction is lower than the tensile strength in the C direction. Here, the crystal orientation of a rolled plate is usually indicated by {hkl} or (hkl) for the crystal orientation perpendicular to the rolling plane, and by the crystal orientation parallel to the rolling direction. <uvw>Alternatively, display with [uvw]. {hkl} and <uvw>{110} is a general term for equivalent planes and orientations, while (hkl) and [uvw] refer to individual crystal planes. In the hot-rolled steel sheets according to the embodiments of the present invention, a body-centered cubic structure (bcc structure) is mainly targeted, so for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1), and (0-1-1) are equivalent and indistinguishable. In the embodiments of the present invention, these orientations are collectively represented as {110}.

[0015] Therefore, the present inventors have found that {110} in the surface layer of the plate thickness <111> and {112} <111> While increasing the average value of the polar density in the orientation to a predetermined value or higher to increase the strength in the L direction, {100} in the center of the plate thickness <011> , {211} <011> and {332} <113> To reduce the average value of the polar density in a given direction to below a predetermined value, thereby lowering the strength in the C direction, more specifically, in the region from the surface of the hot-rolled steel sheet to the position 1 / 6 of the sheet thickness {110} <111> and {112} <111> The average value of the polar density of orientation is controlled to be 2.50 or higher, and {100} in the region from the position of 2 / 5 plate thickness to the position of 3 / 5 plate thickness. <011> , {211} <011> and {332} <113> We have found that by controlling the average value of the polar density of orientations to 7.00 or less, the anisotropy of strength in the tensile strength of hot-rolled steel sheets in the L and C directions can be significantly reduced. Here, polar density refers to the ratio of the degree of accumulation in a specific orientation of the test material to that of a standard sample that does not have accumulation in a specific orientation. In the hot-rolled steel sheet according to the embodiment of the present invention, as described above, in order to improve the elongation flangeability, ductility and notch fatigue characteristics, the metal structure is configured to contain at least one of ferrite and bainite and martensite in specific proportions, and the specific crystal grains with an orientation difference of 5 to 14° within the area percentage range of 10 to 60%. Therefore, it is extremely difficult to control the polar density of a specific texture in the surface layer of the sheet thickness and the polar density of a specific texture in the center of the sheet thickness to be within the desired range while maintaining the configuration of the metal structure in this controlled manner. In contrast, as will be explained in detail later in relation to the manufacturing method of hot-rolled steel sheets, in embodiments of the present invention, by making the rolling conditions in the hot-rolling process appropriate, the structure of the metal structure for improving elongation flange properties, ductility and notch fatigue properties is maintained, while the {110} in the surface layer of the sheet thickness <111> and {112} <111> The average value of the polar density of orientation is 2.50 or higher, and {100} is at the center of the plate thickness. <011> , {211} <011> and {332} <113> This makes it possible to create a metallic microstructure in which the average value of the polar density in each direction is 7.00 or less.As a result, according to the embodiments of the present invention, it is possible to achieve high strength, for example, a tensile strength of 540 MPa or more, while simultaneously improving elongation flangeability, ductility, and notch fatigue characteristics, and reducing strength anisotropy. Therefore, the hot-rolled steel sheet according to the embodiments of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is therefore particularly useful in the automotive sector where both of these properties are required.

[0016] The hot-rolled steel sheet according to an embodiment of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0017] [C:0.020~0.070%] Carbon (C) is an effective element for increasing the strength of steel plates. Furthermore, C forms carbides and / or carbonitrides with Ti and Nb in the steel, contributing to precipitation strengthening based on these precipitates and to microstructural refinement due to the pinning effect of these precipitates. To fully obtain these effects, the C content should be 0.020% or higher. The C content may also be 0.022% or higher, 0.025% or higher, 0.028% or higher, or 0.030% or higher. On the other hand, excessive C content may reduce ductility, flangeability, and weldability. Therefore, the C content should be 0.070% or lower. The C content may also be 0.065% or lower, 0.060% or lower, 0.055% or lower, or 0.050% or lower.

[0018] [Si: 0.010~2.000%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect fully, the Si content should be 0.010% or more. The Si content may also be 0.100% or more, over 0.100%, 0.110% or more, 0.120% or more, 0.150% or more, 0.180% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.800% or more, or 1.000% or more. On the other hand, excessive Si content may cause a surface quality defect called Si scale. Therefore, the Si content should be 2.000% or less. The Si content may also be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.

[0019] [Mn: 0.60~2.00%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To obtain these effects to the fullest, the Mn content should be 0.60% or more. The Mn content may be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if the Mn content is excessive, the stretch flange properties may decrease. Therefore, the Mn content should be 2.00% or less. The Mn content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.

[0020] [Ti: 0.015~0.200%] Ti is an element that improves the strength of steel by precipitating as carbides (TiC) in the steel. Furthermore, Ti also fixes carbon (C) by forming carbides, suppressing the formation of cementite, which is detrimental to ductility and flangeability. To fully obtain these effects, the Ti content should be 0.015% or higher. The Ti content may also be 0.020% or higher, 0.030% or higher, 0.040% or higher, or 0.050% or higher. On the other hand, excessive Ti content can lead to coarser carbides and reduced ductility. Therefore, the Ti content should be 0.200% or lower. The Ti content may also be 0.180% or lower, 0.170% or lower, 0.150% or lower, or 0.120% or lower.

[0021] [sol.Al:0.010~1.000%] sol.Al is an element that acts as a deoxidizing agent for molten steel. To obtain this effect sufficiently, the sol.Al content should be 0.010% or more. The sol.Al content may be 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if the sol.Al content is excessive, coarse oxides may form, reducing toughness and ductility, which may lead to fracture during rolling. Therefore, the sol.Al content should be 1.000% or less. The sol.Al content may be 0.800% or less, 0.600% or less, or 0.400% or less. Note that sol.Al refers to acid-soluble Al, which is solid-solution Al present in the steel in a solid-solution state.

[0022] [P:0.100% or less] Excessive phosphorus (P) content can negatively affect weldability and other properties. Therefore, the P content should be 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.

[0023] [S:0.030% or less] Excessive sulfur content can lead to the formation of large amounts of manganese sulfur (MnS), which can reduce toughness. Therefore, the Si content should be 0.030% or less. The S content may be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the S content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0024] [N:0.0060% or less] N preferentially forms precipitates with Ti over C, and may reduce the amount of Ti that is effective in fixing C. Therefore, the N content should be 0.0060% or less. The N content may also be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0025] [O:0.0100% or less] O is an element that is introduced during the manufacturing process. Excessive O content can lead to the formation of coarse inclusions, which can reduce the toughness of the steel sheet. Therefore, the O content should be 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

[0026] The basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may, if necessary, contain at least one of the following optional elements in place of a portion of the remaining Fe.

[0027] [Nb:0~0.050%] Nb is an element that contributes to the refinement of the microstructure and, consequently, the increased strength of steel sheets by forming carbides, nitrides, and / or carbonitrides in steel through a pinning effect. Furthermore, Nb also fixes carbon by forming carbides and / or carbonitrides, suppressing the formation of cementite, which is detrimental to tensile flangeability. While the Nb content may be 0%, it is preferable that the Nb content be 0.001% or higher to obtain these effects. The Nb content may also be 0.005% or higher, 0.010% or higher, or 0.015% or higher. On the other hand, excessive Nb content can lead to the formation of coarse carbides and other particles in the steel, reducing the ductility of the steel sheet. Therefore, the Nb content should be 0.050% or lower. The Nb content may also be 0.040% or lower, 0.030% or lower, or 0.020% or lower.

[0028] [V: 0~0.300%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the V content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 0.300% or less. The V content may also be 0.200% or less, 0.100% or less, or 0.080% or less.

[0029] [Cr: 0~2.00%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. While the Cr content may be 0%, it is preferable that the Cr content be 0.01% or more to obtain such effects. The Cr content may also be 0.03% or more, or 0.05% or more. On the other hand, excessive Cr content may lead to saturation of the effect and an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 2.00% or less. The Cr content may also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0030] [Ni: 0~2.00%] [Cu: 0~2.00%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. The Ni and Cu content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 0.01% or more, and may be 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ni and Cu content be 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0031] [Mo: 0~1.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. While the Mo content may be 0%, it is preferable that the Mo content be 0.001% or more to obtain these effects. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the equipment load may become larger. Therefore, it is preferable that the Mo content be 1.000% or less. The Mo content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0032] [B: 0~0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0033] [Sb: 0~1.00%] Sb is an effective element for improving corrosion resistance. While the Sb content may be 0%, it is preferable that the Sb content be 0.01% or more to obtain this effect. The Sb content may also be 0.02% or more, or 0.05% or more. On the other hand, excessive Sb content may lead to a decrease in toughness. Therefore, it is preferable that the Sb content be 1.00% or less. The Sb content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0034] [Ca: 0~0.0100%] [Mg: 0~0.0100%] [Hf: 0~0.0100%] Ca, Mg, and Hf are elements that can control the morphology of nonmetallic inclusions. The Ca, Mg, and Hf content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the Ca, Mg, and Hf content be 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.002 each is preferable.

[0035] [REM:0~0.1000%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to obtain such an effect, it is preferable that the REM content be 0.0001% or more. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the REM content is excessive, the effect will saturate, and including more REM in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the REM content be 0.1000% or less. The REM content may also be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0036] [Bi: 0~0.0100%] [As:0~0.0100%] Bi and As are elements that are effective in improving corrosion resistance. The content of Bi and As may be 0%, but in order to obtain such an effect, it is preferable that the content of these elements be 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the content of Bi and As be 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.00220% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less, and

[0037] [Zr: 0~1.00%] Zr is an element that can control the morphology of nonmetallic inclusions. While the Zr content may be 0%, it is preferable that the Zr content be 0.01% or higher to obtain such an effect. The Zr content may also be 0.05% or higher, or 0.10% or higher. On the other hand, if the Zr content is excessive, the effect will saturate, and including more Zr than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the Zr content be 1.00% or lower. The Zr content may also be 0.80% or lower, 0.50% or lower, 0.30% or lower, or 0.20% or lower.

[0038] [Co: 0~1.00%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, it is preferable that the Co content be 0.01% or more. The Co content may also be 0.05% or more or 0.10% or more. On the other hand, if the Co content is excessive, the hot workability may decrease and lead to an increase in raw material costs. Therefore, it is preferable that the Co content be 1.00% or less. The Co content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0039] [Zn: 0~1.00%] Zn is an effective element for controlling the shape of inclusions. To obtain such an effect, the Zn content is preferably 0.01% or more. The Zn content may be 0.05% or more or 0.10% or more. On the other hand, if the Zn content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, the Zn content is preferably 1.00% or less. The Zn content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0040] [W: 0~1.00%] W is an element that enhances the hardenability of steel and contributes to improving its strength. While the W content may be 0%, it is preferable that the W content be 0.01% or more to obtain such effects. The W content may also be 0.05% or more, or 0.10% or more. On the other hand, excessive W content may reduce weldability. Therefore, it is preferable that the W content be 1.00% or less. The W content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0041] [Sn: 0~1.00%] Sn is an effective element for improving corrosion resistance. While the Sn content may be 0%, it is preferable that the Sn content be 0.01% or more to obtain this effect. The Sn content may also be 0.02% or more, or 0.05% or more. On the other hand, excessive Sn content may lead to a decrease in toughness. Therefore, it is preferable that the Sn content be 1.00% or less. The Sn content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0042] In the hot-rolled steel sheet according to an embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of hot-rolled steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0043] [0.100≦[Si]+[sol.Al]≦2.500] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention must satisfy the following formula. 0.100 ≤ [Si] + [sol.Al] ≤ 2.500 In the formula, [Si] and [sol.Al] represent the content (mass%) of each element. As mentioned earlier, when a crystal grain is defined as a region enclosed by a boundary with an orientation difference of 15° or more and with an equivalent circular diameter of 0.3 μm or more, crystal grains with an orientation difference of 5 to 14° within the grain are effective in improving strength and ductility. For this reason, in the hot-rolled steel sheet according to the embodiment of the present invention, the balance between strength and ductility is improved by controlling the proportion of such crystal grains to within the range of 10 to 60% in area%, as will be explained in detail later. In addition to the effects described for each element, Si and sol.Al are also effective elements in controlling the proportion of crystal grains with an orientation difference of 5 to 14° within the range of 10 to 60%. This is thought to be due to the fact that the temperature of the Ar3 point rises when Si and sol.Al are included, reducing the transformation strain introduced into the grain. To fully obtain these effects, the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is controlled so that the total content of Si and sol.Al is 0.100% or more, i.e., [Si] + [sol.Al] ≥ 0.100, while controlling the content of each element within the range described above. The total content of Si and sol.Al may be 0.120% or more, 0.150% or more, 0.200% or more, or 0.300% or more. On the other hand, if the total content of Si and sol.Al is too high, ferrite formation may be promoted and the strength may decrease. Therefore, the total content of Si and sol.Al is set to 2.500% or less, i.e., [Si] + [sol.Al] ≤ 2.500. The total content of Si and sol.Al may be 2.000% or less, 1.500% or less, 1.000% or less, or 0.000% or less.

[0044] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the hot-rolled steel sheet can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0045] [Metal structure] [At least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%] The microstructure of the hot-rolled steel sheet according to the embodiment of the present invention includes, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%. By composing the microstructure of the hot-rolled steel sheet with these structures, it is possible to improve the balance between strength, tensile flangeability, ductility, and notch fatigue properties. If the total area percentage of at least one of ferrite and bainite is low or the area percentage of martensite is high, the balance between strength and tensile flangeability will decrease, and the desired properties may not be obtained. Therefore, the total area percentage of at least one of ferrite and bainite should be 80% or more, for example, 82% or more, 85% or more, 88% or more, or 90% or more. Similarly, the area percentage of martensite should be 10% or less, for example, 9% or less, 8% or less, 7% or less, or 6% or less. On the other hand, if the total area percentage of at least one of ferrite and bainite is high or the area percentage of martensite is low, the balance between strength and notch fatigue properties will decrease, and the desired properties may not be obtained. Therefore, the total area ratio of at least one of ferrite and bainite shall be 98% or less, and may be, for example, 96% or less, 94% or less, or 92% or less. Similarly, the area ratio of martensite shall be 2% or more, and may be, for example, 3% or more, 4% or more, or 5% or more.

[0046] The microstructure of the hot-rolled steel sheet may contain either ferrite or bainite, preferably both. Therefore, the area ratio of ferrite and bainite may be 0% for either one, or for example, 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, respectively. Similarly, the area ratio of ferrite and bainite may be 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less, respectively. From the viewpoint of improving the ductility of the hot-rolled steel sheet, the area ratio of bainite is preferably 80% or less, and more preferably 70% or less.

[0047] [Remaining tissue] The remaining microstructure other than ferrite, bainite, and martensite may be 0% in area percentage, but if the remaining microstructure is present, it may be at least one of retained austenite and pearlite. The area percentage of the remaining microstructure is not particularly limited, but may be, for example, 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flange properties, the area percentage of the remaining microstructure is preferably, for example, 10% or less, and may be 8% or less, 6% or less, or 5% or less.

[0048] [Identification of metallographic structure and calculation of area ratio] The identification of the metallic structure and calculation of area ratios in hot-rolled steel sheets are performed by optical microscopy observation and X-ray diffraction after etching with Nital reagent or Repera solution. Microscopy observation is performed on the thickness cross section parallel to the rolling direction and perpendicular to the sheet surface. Specifically, first, a sample is taken from the hot-rolled steel sheet, and the observation surface of the sample is etched with Nital. Next, by performing image analysis on the micrograph obtained using an optical microscope at a depth of 1 / 4 of the sheet thickness in a 300 μm × 300 μm field of view, the area ratios of ferrite and pearlite, as well as the combined area ratio of bainite and martensite, are calculated. Then, using a sample whose observation surface has been Repera-etched, the combined area ratio of retained austenite and martensite is calculated by performing image analysis on the micrograph obtained using an optical microscope at a depth of 1 / 4 of the sheet thickness in a 300 μm × 300 μm field of view. Next, using a sample that has been surface-machined to a depth of 1 / 4 of the plate thickness from the direction normal to the rolling surface, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite. The area fraction of martensite is calculated by subtracting the obtained area fraction of retained austenite from the total area fraction of retained austenite and martensite calculated earlier. Finally, the area fraction of bainite is calculated by similarly subtracting the obtained area fraction of martensite from the total area fraction of bainite and martensite calculated earlier.

[0049] [Percentage of crystal grains with an intra-grain orientation difference of 5-14°: 10-60% by area] In the microstructure of a hot-rolled steel sheet according to an embodiment of the present invention, when a boundary with an orientation difference of 15° or more is defined as a grain boundary, and a region enclosed by such a grain boundary with an equivalent circular diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains with an intra-grain orientation difference of 5 to 14° is controlled to be within the range of 10 to 60% in area percent. Crystal grains with such intra-grain orientation differences are effective in improving strength and ductility. Although not intended to be bound by any particular theory, it is thought that the intra-grain crystal orientation difference correlates with the dislocation density contained in the crystal grain. Generally, an increase in intra-grain dislocation density leads to improved strength but reduces workability. However, in crystal grains with an intra-grain orientation difference controlled to 5 to 14°, it is thought that strength can be improved without reducing workability. In contrast, crystal grains with an intra-grain orientation difference of less than 5° have excellent workability but it is difficult to increase their strength. On the other hand, crystal grains with an orientation difference of more than 14° within the grain have different deformability within the grain and therefore do not necessarily contribute to improving tensile flangeability. Therefore, in the hot-rolled steel sheet according to the embodiment of the present invention, by appropriately controlling the proportion of crystal grains with an orientation difference of 5 to 14° within the grain, more specifically by controlling it to a range of 10 to 60% in area%, it is possible to improve tensile flangeability while achieving the desired steel sheet strength, and to further improve the balance between strength and tensile flangeability. If the proportion of crystal grains with an orientation difference of 5 to 14° within the grain is small, tensile flangeability may decrease. Therefore, from the viewpoint of improving tensile flangeability, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain may be 15% or more, 18% or more, or 20% or more. On the other hand, if the proportion of crystal grains with an orientation difference of 5 to 14° within the grain is large, ductility may decrease. Therefore, from the viewpoint of improving ductility, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain may be 55% or less, 50% or less, 45% or less, or 40% or less.

[0050] [Measurement of the proportion of crystal grains with an orientation difference of 5-14° within the grain] The proportion of crystal grains with an orientation difference of 5-14° within the grain is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from a steel plate so that the cross-section of the plate thickness parallel to the rolling direction and perpendicular to the plate surface becomes the observation surface. Next, at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, an EBSD analysis is performed at a measurement interval of 0.2 μm in a region of 200 μm in the rolling direction of the steel plate and 100 μm in the direction normal to the rolling surface to obtain crystal orientation information. Here, the EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50-300 points / second. Next, for the obtained crystal orientation information, regions with an orientation difference of 15° or more and an equivalent circle diameter of 0.3 μm or more are defined as crystal grains, the average orientation difference within each crystal grain is calculated, and the proportion of crystal grains with an orientation difference of 5 to 14° is determined. The crystal grains and the average orientation difference within each grain defined as described above can be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. In this invention, "intragrain orientation difference" refers to "Grain Orientation Spread (GOS)," which is the orientation dispersion within a crystal grain. The value of the intragrain orientation difference is obtained as the average value of the misorientation between the reference crystal orientation and all measurement points within the same crystal grain, as described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-ray Diffraction Method," Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, pp. 1722-1728. In embodiments of the present invention, the reference crystal orientation is the orientation obtained by averaging all measurement points within the same crystal grain. The GOS value can be calculated using the software "OIM Analysis® Version 7.0.1" included with the EBSD analyzer.

[0051] [Average value of extreme density in a specific orientation at the surface of the plate thickness: 2.50 or higher, and average value of extreme density in a specific orientation at the center of the plate thickness: 7.00 or lower] In the metallographic structure of the hot-rolled steel sheet according to the embodiment of the present invention, {110} is present in the region from the surface of the hot-rolled steel sheet to the position 1 / 6 of the sheet thickness (i.e., the surface layer of the sheet thickness). <111> and {112} <111> The average value of the polar density of orientation is controlled to be 2.50 or higher, and {100} is present in the region from the 2 / 5 position of the plate thickness to the 3 / 5 position of the plate thickness (i.e., the center of the plate thickness). <011> , {211} <011> and {332} <113> The average value of the polar density of orientation is controlled to be 7.00 or less. {110} in the surface layer of the plate thickness. <111> and {112} <111> While controlling the average value of the polar density in the orientation to 2.50 or higher to increase the strength in the L direction, {100} in the center of the plate thickness <011> , {211} <011> and {332} <113> By controlling the average value of the polar density in each direction to 7.00 or less and lowering the strength in the C direction, the difference in tensile strength between the L direction and the C direction of the resulting hot-rolled steel sheet can be reduced, and as a result, the anisotropy of strength in the tensile strength of the L direction and the C direction can be significantly reduced. From the viewpoint of further reducing the anisotropy of strength, {110} in the surface layer of the sheet thickness <111> and {112} <111> A higher average value of the polar density of orientations is preferable, for example, it may be 2.80 or higher, 3.00 or higher, 3.20 or higher, or 3.50 or higher. There is no particular upper limit, but for example, {110} in the surface layer of the plate thickness. <111> and {112} <111> The average value of the polar density in orientation may be 5.00 or less, 4.80 or less, 4.70 or less, 4.50 or less, 4.20 or less, 4.00 or less, or 3.80 or less. Similarly, from the viewpoint of further reducing the anisotropy of strength, {100} at the center of the plate thickness. <011> , {211} <011> and {332} <113> The average value of the polar density of orientations is preferably small, for example, 6.80 or less, 6.50 or less, 6.20 or less, or 6.00 or less. The lower limit is not particularly limited, but for example, {100} at the center of the plate thickness. <011> , {211} <011> and {332} <113> The average value of the polar density of the azimuthal region may be 3.50 or higher, 4.00 or higher, 4.20 or higher, 4.40 or higher, 4.50 or higher, or 5.00 or higher.

[0052] [Measurement of the average value of extreme density in a specific orientation at the surface and center of the plate thickness] {110} in the surface layer of the plate thickness <111> and {112} <111> The average value of the polar density in the orientation and {100} at the center of the plate thickness. <011> , {211} <011> and {332} <113> The average value of the polar density of orientations is measured by EBSD. More specifically, for the measurement of the surface layer of the plate thickness, first, a sample is taken from the steel plate so that the cross-section of the plate thickness parallel to the rolling direction and perpendicular to the plate surface becomes the observation surface. Then, EBSD analysis is performed at measurement intervals of 1 μm on a rectangular region of the steel plate centered at a depth of 1 / 12 of the plate thickness from the surface, with a length of 1000 μm in the rolling direction and a length of 100 μm in the direction normal to the rolling surface, to obtain the crystal orientation information of this rectangular region. The EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points / second. Next, the ODF (Orientation Distribution Function) of this rectangular region is calculated from the crystal orientation information of this rectangular region using the software "OIM Analysis®" attached to the EBSD analysis device. As a method for calculating ODF, the Harmonic Series Expansion (spherical harmonic method) was used with an expansion order of 16. Furthermore, calculations considering orthotropic symmetry were performed. This resulted in {110} <111> and {112} <111> The extreme density for each crystal orientation can be determined, and their arithmetic mean is {110} in the region from the surface of the hot-rolled steel sheet to a position 1 / 6 of the sheet thickness (the surface layer of the sheet thickness). <111> and {112} <111> It is determined as the "average value of the polar density of the orientation". For the measurement of the center of the plate thickness, EBSD analysis was performed on a rectangular area of ​​1000 μm in the rolling direction of the steel plate and 100 μm in the direction normal to the rolling surface, centered at a depth of 1 / 2 the plate thickness from the surface of the steel plate, except that the measurement was performed in the same way as for the measurement of the surface layer of the plate thickness. <011> , {211} <011> and {332} <113> The extreme density for each crystal orientation can be determined, and their arithmetic mean is {100} in the region from the 2 / 5 position to the 3 / 5 position of the plate thickness (center of the plate thickness). <011> , {211} <011> and {332} <113> It is determined as the "average value of the extreme density of direction."Note that the crystal orientation here represents the crystal orientation perpendicular to the surface of the steel plate. Therefore, when performing analysis, it is necessary to consider the orientation of the sample set used for measurement and align the measurement coordinate system of the crystal orientation data with the sample coordinate system.

[0053] [plate thickness] The hot-rolled steel sheet according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 6.0 mm. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 5.0 mm or less, or 4.0 mm or less.

[0054] [Mechanical properties] [Tensile strength: TS] According to the hot-rolled steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 540 MPa or more, can be achieved. The tensile strength is preferably 600 MPa or more, 700 MPa or more, 780 MPa or more, or 850 MPa or more. According to the embodiment of the present invention, despite having such a very high tensile strength, the specific combination of chemical composition and metal structure described above can improve tensile flangeability, ductility and notch fatigue characteristics, as well as reduce strength anisotropy. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, 1180 MPa or less, 1080 MPa or less, or 980 MPa or less. The tensile strength is measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011. The tensile strength obtained in this manner is also referred to as C-direction TS (TSC) in this specification.

[0055] [Full growth: El] According to the hot-rolled steel sheet having the above chemical composition and metallic structure, in addition to high tensile strength, it is also possible to improve total elongation, more specifically achieving a total elongation of 15.0% or more. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more. There is no particular upper limit, but for example, the total elongation may be 40.0% or less or 35.0% or less. The total elongation is measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.

[0056] <Manufacturing method for hot-rolled steel sheets> Next, preferred manufacturing methods for hot-rolled steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing hot-rolled steel sheets according to embodiments of the present invention, and is not intended to limit the hot-rolled steel sheets to those manufactured by the manufacturing methods described below.

[0057] A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention is as follows: (A) A hot rolling process that includes heating a slab having the chemical composition described above in relation to a hot-rolled steel sheet, and then finish rolling it, and satisfying the following conditions (A1) to (A5), and (A1) The heating temperature of the slab shall be between the solution temperature (SRTmin) °C and 1260 °C, as expressed in the following formula 1. (A2) The cumulative strain (εeff.) of the last three stages of finish rolling, as expressed in the following formula 2, is 0.50 to 0.60. (A3) The end temperature of the finish rolling must be Ar3 + 30°C or higher. (A4) In finish rolling, two or more rolling passes are performed at 1100°C or below, in which the shape ratio (X) represented by the following formula 3 is 2.3 or greater. (A5) The rolling temperature of the three stages preceding the finish rolling must be 50°C or higher than the entry temperature (FT0) of the finish rolling. SRTmin=7000 / {2.75-log([Ti]×[C])}-273 ...Formula 1 Here, [Ti] and [C] represent the mass percentage content of each element in the steel. εeff = Σεi(t,T) ···Equation 2 Here, εi(t,T)=εi0 / exp{(t / τR) 2 / 3 } τR = τ0·exp(Q / RT) τ0 = 8.46 × 10 -6 Q=183200J R = 8.314 J / K·mol εi0 represents the logarithmic strain during reduction, t represents the cumulative time (seconds) up to just before cooling in that pass, and T represents the rolling temperature (°C) in that pass. X=2√(R(h0-h1)) / (h0+h1)...Equation 3 Here, R: Rolling mill roll radius (mm) h0: Inlet side plate thickness (mm) h1: Exit side plate thickness (mm) (B) A cooling process comprising: primary cooling of the finish-rolled steel sheet to a temperature range of 650-750°C at an average cooling rate of 10°C / s or more, holding it in the said temperature range for 3.0-10.0 seconds, and then secondary cooling to 100°C or below at an average cooling rate of 30°C / s or more. It is characterized by including [specific features]. The following describes each process in detail.

[0058] [(A) Hot rolling process] [(A1) Slab heating temperature] First, a slab having the chemical composition described above in relation to hot-rolled steel sheets is heated. From a productivity standpoint, the slab is preferably cast using a continuous casting method, but it may also be manufactured by ingot casting or thin slab casting. The heating temperature of the slab must be between the solution temperature (SRTmin) °C and 1260 °C, as represented by Formula 1 below. SRTmin=7000 / {2.75-log([Ti]×[C])}-273 ...Formula 1 Here, [Ti] and [C] represent the mass percentage content of each element in the steel. The hot-rolled steel sheet according to the embodiment of the present invention contains Ti, and if the heating temperature of the slab is below the solution temperature (SRTmin) °C, the Ti will not be sufficiently dissolved. If the Ti is not sufficiently dissolved during slab heating, it becomes difficult to improve the strength of the steel by precipitation strengthening by finely precipitating the Ti as carbides (TiC) in the steel during the cooling process after the hot rolling process. In addition, it becomes difficult to fix carbon by forming carbides (TiC) and suppress the formation of cementite, which is detrimental to tensile flange properties. On the other hand, if the heating temperature of the slab exceeds 1260 °C, the yield decreases due to scale-off.

[0059] [Rough rolling] In this manufacturing method, for example, a heated slab may be subjected to rough rolling before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are ensured.

[0060] [(A2) Cumulative strain (εeff.) in the last three stages of finish rolling: 0.50~0.60] The heated slab, or a slab that has been roughly rolled as needed, is then subjected to finish rolling. In this manufacturing method, it is preferable to perform finish rolling using a tandem rolling mill consisting of five or more rolling stands. When using a tandem rolling mill consisting of five rolling stands, the rolling passes will partially overlap between the last three stages of finish rolling in (A2) and the first three stages of finish rolling in (A5), which will be described later. However, as long as the conditions of (A2) and (A5) are satisfied, the rolling passes under condition (A2) and the rolling passes under condition (A5) may partially overlap. In this manufacturing method, in order to control the proportion of crystal grains with an orientation difference of 5 to 14° within the range of 10 to 60% in area percent, in the finish rolling performed on the heated slab, it is necessary to set the cumulative strain (εeff.) of the last three stages (final three passes) to 0.50 to 0.60 and then perform the cooling process described later. This is for the following reasons. Crystal grains with an intra-grain orientation difference of 5-14° are formed by transformation in a para-equilibrium state at relatively low temperatures. Therefore, by limiting the dislocation density of austenite before transformation within a certain range during the hot rolling process, and by limiting the cooling rate within a certain range during the subsequent cooling process, it is possible to control the formation of crystal grains with an intra-grain orientation difference of 5-14°. In other words, by controlling the cumulative strain in the last three stages of finish rolling and the subsequent cooling, it is possible to control the nucleation frequency and subsequent growth rate of crystal grains with an intra-grain orientation difference of 5-14°. As a result, it is possible to control the area ratio of crystal grains with an intra-grain orientation difference of 5-14° in the hot-rolled steel sheet obtained after cooling. More specifically, the dislocation density of austenite introduced by finish rolling mainly affects the nucleation frequency, and the cooling rate after finish rolling mainly affects the growth rate.

[0061] If the cumulative strain in the last three stages of finish rolling is less than 0.50, the dislocation density of the introduced austenite is insufficient, and the proportion of grains with an orientation difference of 5 to 14° within the grain is less than 10%. On the other hand, if the cumulative strain in the last three stages of finish rolling is greater than 0.60, austenite recrystallization occurs during hot rolling, and the accumulated dislocation density at the time of transformation decreases. As a result, the proportion of grains with an orientation difference of 5 to 14° within the grain is similarly less than 10%. In this manufacturing method, the cumulative strain (εeff.) in the last three stages of finish rolling is calculated by the following equation 2. εeff = Σεi(t,T) ···Equation 2 Here, εi(t,T)=εi0 / exp{(t / τR) 2 / 3 } τR = τ0·exp(Q / RT) τ0 = 8.46 × 10 -6 Q=183200J R = 8.314 J / K·mol εi0 represents the logarithmic strain during reduction, t represents the cumulative time (seconds) up to just before cooling in that pass, and T represents the rolling temperature (°C) in that pass.

[0062] [(A3) Finishing rolling end temperature: Ar3 + 30℃ or higher] In this manufacturing method, the end temperature of the finish rolling must be Ar3 + 30°C or higher. If the end temperature of the finish rolling is below Ar3 + 30°C, there is a risk that processing will be applied to the ferrite, which may be formed in some parts of the microstructure due to variations in the composition of the steel sheet and the rolling temperature. Processed ferrite may cause a decrease in ductility. In addition, if the end temperature of the finish rolling is below Ar3 + 30°C, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain may exceed 60% and become excessively high. In this manufacturing method, Ar3 (°C) is determined by the following formula 4 based on the chemical composition of the hot-rolled steel sheet. Ar3=901-325×[C]+33×[Si]+287×[P]+40×[sol.Al]-92×([Mn]+[Mo]+[Cu])-46×([Cr]+[Ni]) ...Formula 4 Here, [C], [Si], [P], [sol.Al], [Mn], [Mo], [Cu], [Cr], and [Ni] represent the mass %) content of each element in the steel, and 0 indicates that the element is not present.

[0063] [(A4) Two or more rolling passes with a shape ratio (X) of 2.3 or more at 1100°C or below] To reduce the anisotropy of strength, as explained earlier, {110} in the surface layer of the plate thickness <111> and {112} <111> While increasing the average value of the polar density in the orientation to a predetermined value or higher to increase the strength in the L direction, {100} in the center of the plate thickness <011> , {211} <011> and {332} <113> It is necessary to reduce the average value of the polar density in each direction to below a predetermined value in order to lower the strength in the C direction. Therefore, the inventors increased the shear strain introduced into the surface layer of the steel plate thickness during finish rolling, thereby reducing the {110} in the surface layer of the plate thickness. <111> and {112} <111> We found that by increasing the concentration in each orientation, we could control the average value of the extreme densities in these orientations within a desired range. More specifically, by performing two or more rolling passes at 1100°C or below in the finish rolling process, where the shape ratio (X) represented by the following formula 3 is 2.3 or higher, the {110} in the surface layer of the plate thickness was increased. <111> and {112} <111> It becomes possible to increase the average value of the extreme density of azimuth to 2.50 or higher. X=2√(R(h0-h1)) / (h0+h1)...Equation 3 Here, R: Rolling mill roll radius (mm) h0: Inlet side plate thickness (mm) h1: Exit side plate thickness (mm)

[0064] The shape ratio (X) is defined as the roll contact arc length (√(R(h0-h1))) divided by the average plate thickness ((h0+h1) / 2). In this manufacturing method, by using rolls with appropriate roll radii in the rolling mill and rolling at an appropriate reduction ratio, a shape ratio (X) of 2.3 or higher can be achieved, thereby increasing the shear strain introduced into the surface layer of the steel plate. Furthermore, by limiting the rolling temperature to 1100°C or lower, the recovery of the introduced shear strain can be suppressed. Therefore, by performing two or more such rolling passes, sufficient shear strain can be introduced into the surface layer of the plate, and {110} in the surface layer of the plate. <111> and {112} <111> It becomes possible to reliably raise the average value of the polar density of orientation to 2.50 or higher. From the viewpoint of further reducing the anisotropy of strength, it is preferable to perform three or more rolling passes where X is 2.3 or higher and the temperature is 1100°C or lower. There is no particular upper limit to the number of such rolling passes; for example, the number of such rolling passes may be 5 or less. On the other hand, if X is less than 2.3, or the rolling temperature is greater than 1100°C, or if there is one or fewer rolling passes where X is 2.3 or higher and the temperature is 1100°C or lower, it is not possible to introduce sufficient shear strain into the surface layer of the plate thickness. As a result, {110} in the surface layer of the plate thickness <111> and {112} <111> It becomes impossible to increase the average value of the polar density of the orientation to 2.50 or higher. The roll radius of the rolls used in the rolling mill can be selected to an appropriate value within the range where X is 2.3 or higher. Although not particularly limited, for example, the roll radius can be selected from the range of 150 to 400 mm.

[0065] [(A5) Rolling temperature for the three stages prior to finish rolling: FT0-50℃ or higher] In the center of the plate thickness, unlike the surface layer, the strain introduced by finish rolling is reduced, {100} <011> , {211} <011> and {332} <113> It is necessary to limit the average value of the polar density in each orientation to 7.00 or less. Therefore, in this manufacturing method, the rolling temperatures of the three stages preceding the finish rolling, namely the rolling temperature of the first finish rolling stage (FT1), the rolling temperature of the second finish rolling stage (FT2), and the rolling temperature of the third finish rolling stage (FT3), are controlled to be 50°C or higher than the entry temperature of the finish rolling stage (FT0), thereby reducing the shear strain introduced into the center of the plate thickness, and thereby {100} <011> , {211} <011> and {332} <113> The goal is to achieve an average value of 7.00 or less for the extreme density of orientation. The shear strain introduced in the center of the plate thickness is smaller compared to the surface layer of the plate thickness that is in direct contact with the rolls. Therefore, by controlling the rolling temperature of the three stages preceding the finish rolling to a relatively high temperature as described above, it is possible to sufficiently reduce the introduced shear strain. From the viewpoint of further reducing the anisotropy of strength, it is preferable to control the rolling temperature of the three stages preceding the finish rolling to FT0-45°C or higher. On the other hand, if the rolling temperature of even one of the three stages preceding the rolling falls below FT0-50°C, a sufficient reduction in shear strain cannot be obtained, and {100} in the center of the plate thickness... <011> , {211} <011> and {332} <113> It becomes impossible to reduce the average value of the polar density of the orientation to 7.00 or less. The upper limit of the rolling temperature for the three stages preceding the finish rolling is not particularly limited, but for example, the rolling temperature for the three stages preceding the finish rolling may be 1100°C or less or 1000°C or less.

[0066] In this manufacturing method, one or more rolling passes under condition (A4) and one or more rolling passes under condition (A5) may overlap with each other, as long as conditions (A4) and (A5) are satisfied.

[0067] [(B) Cooling process] In this manufacturing method, the finish-rolled steel sheet undergoes a two-stage cooling process in the following cooling step. Specifically, the finish-rolled steel sheet is first cooled to a temperature range of 650-750°C at an average cooling rate of 10°C / s or more, held at this temperature range for 3.0-10.0 seconds, and then secondarily cooled to below 100°C at an average cooling rate of 30°C / s or more. By performing this two-stage cooling in combination with conditions such as (A2) and (A3) in the hot-rolling process, para-equilibrium transformation occurs in the desired relatively low temperature range, thereby reliably controlling the proportion of crystal grains with an orientation difference of 5-14° within the range of 10-60% in area percent. In contrast, if the average cooling rate of the primary cooling is less than 10°C / s or the cooling stop temperature of the primary cooling exceeds 750°C, para-equilibrium transformation occurs at a relatively high temperature, and the proportion of crystal grains with an orientation difference of 5-14° within the area falls below 10%. Furthermore, if the cooling stop temperature for primary cooling is below 650°C, para-equilibrium transformation occurs at a lower temperature than the desired temperature range, and similarly, the proportion of crystal grains with an orientation difference of 5-14° within the grain becomes less than 10%. Moreover, even if the holding time at 650-750°C is less than 3.0 seconds, similarly, the proportion of crystal grains with an orientation difference of 5-14° within the grain becomes less than 10%. On the other hand, if the holding time at 650-750°C exceeds 10.0 seconds or the average cooling rate of secondary cooling is less than 30°C / s, cementite, which is detrimental to scalability, is more likely to form. Also, if the cooling stop temperature for secondary cooling exceeds 100°C, the area fraction of martensite becomes less than 2%. There is no particular upper limit to the average cooling rate of primary and secondary cooling, but for example, the average cooling rate of primary and secondary cooling may be set to 200°C / s or less, taking into account the capacity of the cooling equipment.

[0068] According to the hot-rolled steel sheet manufactured by the above manufacturing method, a microstructure can be obtained in which, by area %, at least one of ferrite and bainite accounts for 80-98% in total, and martensite accounts for 2-10%, and when the boundary with an orientation difference of 15° or more is defined as a grain boundary, and the region enclosed by the grain boundary and with an equivalent circular diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 5-14° within the grain accounts for 10-60% by area %. As a result, despite high strength, it is possible to significantly improve tensile flangeability, ductility, and notch fatigue properties. In addition, in this microstructure, {110} is present in the region from the surface to 1 / 6 of the sheet thickness. <111> and {112} <111> The average value of the polar density of orientation is controlled to be 2.50 or higher, and {100} in the region from the 2 / 5 plate thickness position to the 3 / 5 plate thickness position. <011> , {211} <011> and {332} <113> Since the average value of the polar density in each direction is controlled to 7.00 or less, the anisotropy of strength in the tensile strength of the hot-rolled steel sheet in the L-direction and C-direction can be significantly reduced. Therefore, the hot-rolled steel sheet manufactured by the above manufacturing method can reliably achieve both high strength and excellent workability, which are conflicting properties, making it particularly useful in the automotive sector where both properties are required.

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples. [Examples]

[0070] In the following examples, hot-rolled steel sheets according to the embodiment of the present invention were manufactured under various conditions, and the tensile strength, elongation flange properties, ductility, notch fatigue properties, and strength anisotropy of the obtained hot-rolled steel sheets were investigated.

[0071] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Tables 1 and 2. These slabs were then heated under the conditions shown in Table 3 and subsequently hot-rolled. Hot rolling was carried out by rough rolling and finish rolling. More specifically, rough rolling was performed under the same conditions for all examples and comparative examples, and finish rolling was performed using a tandem rolling mill consisting of seven rolling stands. The entry temperature (F0) of the finish rolling, the rolling temperature of the first stage of finish rolling (FT1), the rolling temperature of the second stage of finish rolling (FT2), the rolling temperature of the third stage of finish rolling (FT3), the end temperature of the finish rolling, and the cumulative strain (εeff.) of the last three stages of finish rolling are shown in Table 2. Furthermore, for finish rolling, a rolling pass with a shape ratio (X) of 2.3 or more was performed at 1100°C or below for the number of times shown in Table 3, using rolls with the roll radii shown in Table 3. Next, the finish-rolled steel sheet was subjected to primary and secondary cooling under the conditions shown in Table 3 to obtain hot-rolled steel sheets with the thicknesses shown in Table 2.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] The properties of the obtained hot-rolled steel sheets were measured and evaluated by the following method.

[0076] [Tensile strength (TSC) and total elongation (El)] Tensile strength (TSC) and total elongation (El) were measured by taking a JIS No. 5 test specimen from the orientation where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.

[0077] [Evaluation of stretch flange properties] The stretch flange properties were evaluated using the saddle-type stretch flange test method with a saddle-type molded product. Specifically, a saddle-shaped molded product simulating a stretch flange shape consisting of a straight section and an arc section, as shown in Figure 1, was press-formed, and the stretch flange properties were evaluated at the limit forming height. In the saddle-type stretch flange test method, a saddle-type molded product with a corner radius of curvature R of 50-60 mm and an opening angle θ of 120° was used, and the limit forming height H (mm) was measured when the clearance when punching out the corner section was 11%. Here, clearance refers to the ratio of the gap between the punching die and the punch to the thickness of the test piece. Since the clearance is actually determined by the combination of the punching tool and the plate thickness, 11% means satisfying the range of 10.5-11.5%. The limit forming height H was determined by visually observing the presence or absence of cracks with a length of 1 / 3 or more of the plate thickness after forming, and the limit forming height at which no cracks were present was defined as the limit forming height. The product of the tensile strength TSC (MPa) and the limit forming height H (mm) (TSC × H) was used as an indicator of elongation flange performance, and the elongation flange performance was evaluated as improved when TSC × H ≥ 19500 MPa·mm.

[0078] [Evaluation of ductility] Ductility was evaluated as improved when the product of TSC (MPa) and El (%) (TSC × El) satisfies TSC × El ≥ 13500 MPa·%.

[0079] [Evaluation of fatigue properties of notches] The notched fatigue characteristics were evaluated as follows. Specifically, fatigue test specimens were taken from the same location as the tensile test specimens, with the shape shown in Figure 2, such that the longer side was parallel to the direction perpendicular to the rolling direction (direction C), and fatigue tests were performed. The fatigue test specimens were ground to a depth of approximately 0.05 mm from the outermost layer. A stress-controlled axial fatigue test was performed with a stress ratio R=0.1 and a frequency of 5 Hz. The stress at which fracture does not occur after 10 million cycles was defined as the notched fatigue limit (FL), and the notched fatigue characteristics were evaluated. As a result of the test, if FL / TSC ≥ 0.25 was satisfied, the notched fatigue characteristics were evaluated as having improved.

[0080] [Evaluation of anisotropy of intensity] To assess the anisotropy of strength, a JIS No. 5 test specimen was first taken from the direction in which the longitudinal direction of the test specimen is parallel to the rolling direction of the hot-rolled steel sheet (L direction), and a tensile test was performed in accordance with JIS Z 2241:2011 to measure the tensile strength in the L direction, i.e., L-direction TS (TSL). Next, if the obtained L-direction TS and the previously determined C-direction TS (TSC) satisfy TSL / TSC ≥ 0.95, the anisotropy of strength was evaluated as having been reduced.

[0081] Hot-rolled steel sheets with a tensile strength (TSC) of 540 MPa or higher, and with TSC × H ≥ 19500 MPa·mm, TSC × El ≥ 13500 MPa·%, FL / TSC ≥ 0.25, and TSL / TSC ≥ 0.95, were evaluated as having high strength while exhibiting improved tensile flangeability, ductility, and notch fatigue characteristics, as well as reduced strength anisotropy. The results are shown in Tables 4 and 5.

[0082] [Table 4]

[0083] [Table 5]

[0084] Referring to Tables 1-5, in Comparative Example 4, the cumulative strain (εeff.) in the last three stages of finish rolling was high, which is thought to have caused austenite recrystallization during hot rolling and a decrease in the accumulated dislocation density during transformation. As a result, the proportion of grains with an orientation difference of 5-14° within the grain was less than 10%, resulting in decreased ductility. In Comparative Example 5, the εeff. was low, which is thought to have resulted in insufficient dislocation density of the introduced austenite. As a result, similarly, the proportion of grains with an orientation difference of 5-14° within the grain was less than 10%, resulting in decreased ductility. In Comparative Example 6, the finishing temperature of the finish rolling was low, causing the proportion of grains with an orientation difference of 5-14° within the grain to exceed 60%, resulting in decreased ductility. In Comparative Examples 7 and 8, the number of rolling passes with a shape ratio (X) of 2.3 or higher at temperatures below 1100°C was insufficient, which is thought to have prevented the introduction of sufficient shear strain into the surface layer of the plate thickness. As a result, {110} in the surface layer of the plate thickness <111> and {112} <111> The average value of the polar density in each direction was less than 2.50, making it impossible to increase the strength in the L direction, resulting in significant anisotropy of strength. In particular, in Comparative Example 7, although finish rolling was performed using rolls with the same roll radius as the other examples, a sufficient shape ratio could not be secured due to the relatively thick plate thickness. On the other hand, in Comparative Example 8, although the plate thickness was the same as the other examples, a sufficient shape ratio could not be secured because the roll radius was relatively small at 150 mm. In Comparative Example 9, it is thought that the rolling temperature (FT3) in the third stage of finish rolling was too low, which prevented the reduction of shear strain introduced into the center of the plate thickness. As a result, {100} <011> , {211} <011> and {332} <113> The average value of the polar density in each orientation exceeded 7.00, making it impossible to reduce the strength in the C direction, resulting in significant anisotropy of strength. In Comparative Example 10, it is thought that the transformation due to para-equilibrium occurred at a relatively high temperature because the average cooling rate of the primary cooling in the cooling process was low. As a result, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain was less than 10%, and the stretchable flange property decreased. In Comparative Example 11, it is similarly thought that the transformation due to para-equilibrium occurred at a relatively high temperature because the cooling stop temperature of the primary cooling was high.As a result, the proportion of grains with an orientation difference of 5-14° within the grain was less than 10%, resulting in reduced tensile flange properties. In Comparative Example 12, the cooling stop temperature of the primary cooling was low, which is thought to have caused the transformation due to para-equilibrium to occur at a lower temperature than the desired temperature range. As a result, similarly, the proportion of grains with an orientation difference of 5-14° within the grain was less than 10%, resulting in reduced tensile flange properties. In Comparative Example 13, the holding time at 650-750°C during the primary cooling was short, similarly resulting in less than 10% of grains with an orientation difference of 5-14° within the grain, resulting in reduced tensile flange properties. In Comparative Example 14, the cooling stop temperature of the secondary cooling in the cooling process was high, resulting in a martensite area fraction of less than 2%. As a result, TSC and notch fatigue properties were reduced.

[0085] Comparative Examples 31 and 33 exhibited reduced ductility due to high C and Mn content, respectively. Comparative Examples 32 and 34 failed to achieve sufficient strength due to low C and Mn content, respectively. Comparative Example 35 experienced cracking during rolling due to high Al content, preventing further testing. Comparative Example 36 saw accelerated ferrite formation and reduced TSC due to high combined Si and sol.Al content. Comparative Example 37 had reduced ductility because the proportion of grains with an orientation difference of 5-14° within the grain was less than 10% due to low combined Si and sol.Al content. Comparative Example 38 had reduced ductility due to coarse carbides (TiC) caused by high Ti content. Comparative Example 39 was thought to have failed to adequately suppress cementite formation due to low Ti content, resulting in reduced ductility.

[0086] In contrast, in all the examples of the invention, the hot-rolled steel sheet has a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, it contains, by area %, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and the proportion of crystal grains with an intra-grain orientation difference of 5-14° is 10-60% by area %, and in the region from the surface to 1 / 6 of the sheet thickness {110} <111> and {112} <111> The average value of the polar density of orientation is 2.50 or higher, and {100} is present in the region from the position of 2 / 5 plate thickness to the position of 3 / 5 plate thickness. <011> , {211} <011> and {332} <113> We were able to obtain a hot-rolled steel sheet having a metallic structure in which the average value of the polar density in each direction was 7.00 or less. As a result, despite having high strength of tensile strength of 540 MPa or more, we were able to improve elongation flangeability, ductility, and notch fatigue characteristics, and significantly reduce the anisotropy of strength in the tensile strength in the L and C directions.< / uvw> < / uvw>

Claims

1. In mass percent, C: 0.020-0.070%, Si: 0.010-2.000%, Mn: 0.60-2.00%, Ti: 0.015-0.200%, Sol. Al: 0.010–1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0-0.300%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.000%, B: 0 to 0.0100%, Sb: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, REM: 0-0.1000%, Bi: 0 to 0.0100%, As: 0 to 0.0100%, Zr: 0 to 1.00%, Co: 0-1.00%, Zn: 0 to 1.00%, W: 0-1.00%, Sn: 0-1.00%, and The remainder consists of Fe and impurities. The chemical composition satisfies 0.100 ≤ [Si] + [sol. Al] ≤ 2.500, where [Si] and [sol. Al] are the content (mass %) of each element. In area percentage, At least one of ferrite and bainite: 80-98% in total, Martensite: Contains 2-10%, When a grain boundary is defined as a boundary with an orientation difference of 15° or more, and a region enclosed by such grain boundaries with an equivalent circular diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains with an orientation difference of 5 to 14° within the grain is 10 to 60% in area percentage. The average value of the extreme density in the {110}<111> and {112}<111> directions in the region from the surface to the position 1 / 6 of the plate thickness is 2.50 or higher. A hot-rolled steel sheet characterized by having a microstructure in which the average value of the extreme densities in the {100}<011>, {211}<011>, and {332}<113> directions in the region from the 2 / 5 thickness position to the 3 / 5 thickness position is 7.00 or less.

2. The aforementioned chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001-0.300%, Cr: 0.01-2.00%, Ni: 0.01-2.00%, Cu: 0.01-2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sb: 0.01 to 1.00%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Hf: 0.0001 to 0.0100%, REM: 0.0001-0.1000%, Bi: 0.0001-0.0100%, As: 0.0001 to 0.0100%, Zr: 0.01-1.00%, Co: 0.01 to 1.00%, Zn: 0.01-1.00%, W: 0.01 to 1.00%, and Sn: 0.01-1.00% The hot-rolled steel sheet according to claim 1, characterized in that it includes at least one of the following.

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