Hot-rolled thin steel sheets

Abstract

DEPCT68 The invention provides for hot-rolled thin sheets of steel with a predetermined chemical composition. Pre-fabricated and integrated steel structure (in percentage by area) total. Residual martensite and austenite: 2 to 30 percent, the sum of perlite and... Cementite: 0 to 3 percent, and the remainder: ferrite and bainite, where the ratio of the area which... Crystalline grains larger than 12 micrometers occupied 50% of the observed area. A percentage or less in texture, which is measured at the midpoint of the sheet thickness. The highest polar density of the group in the alignment direction {100},{211}, {311}, {110}, and {332} are at 8.0 or less, and when divided... Observation site for steel structure in the thickness direction of the plate at 2 micrometer intervals and measurements were taken. The sum of the martensite and austenite remaining at each divided region; the maximum value of this sum will be... It is 3.0 times or less of the average of this sum;

Description

hot rolled steel plate

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

[0002] In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel sheets used is one effective way to achieve both lightweight vehicle bodies and crashworthiness, and against this background, the development of high-strength steel sheets has been progressing. However, as strength increases, the formability of steel sheets generally decreases. For this reason, in the development of high-strength steel sheets, it is important to increase strength while maintaining a certain level of formability.

[0003] In this regard, for example, Patent Document 1 discloses a steel sheet having a predetermined chemical composition, a structure containing polygonal ferrite or the like at an area ratio of 80% or more, and when the standard deviation of microhardness of 50 arbitrary polygonal ferrite particles present within a range of ±100 μm from the center plane in the sheet thickness direction is defined as σHV, the σHV is 30 or less, and Ti-containing carbides are present in the grains of the polygonal ferrite at a ratio of 5 × 10 7 pieces / mm 2 The present invention describes a high-strength hot-rolled steel sheet characterized in that 50% or more of the Ti-containing carbides have an aspect ratio, which is the ratio of the long side length to the short side length, of less than 3, and a tensile strength of 540 MPa or more. Patent Document 1 also teaches that in a polygonal ferrite-based structure having excellent hole expandability, in order to improve strength, Ti precipitation strengthening can be utilized, and hole expandability can be significantly improved by reducing the variation in hardness of individual ferrite grains and reducing the proportion of anisoaxed Ti-containing carbides having an aspect ratio of 3 or more.

[0004] Patent Document 2 describes a hot-rolled steel sheet having a predetermined chemical composition, wherein, at a depth position of ¼ of the sheet thickness from the surface, the area fraction of ferrite is 10 to 55%, the total area fraction of bainite and martensite is 45 to 90%, and the total area fraction of the ferrite, bainite, and martensite is 90% or more, the average grain size is 12.0 μm or less, and in a texture measured at the center of the sheet thickness, the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group is 8.0 or less and the sum of the pole densities of {211}<011> and {332}<113> is 10.0 or less, and the tensile strength is 950 MPa or more. Furthermore, Patent Document 2 teaches that the above-mentioned configuration makes it possible to provide a hot-rolled steel sheet that has high strength and is excellent in elongation, stretch flangeability, and low-temperature toughness.

[0005] International Publication No. 2017 / 022025 International Publication No. 2019 / 009410

[0006] Hot-rolled steel sheets are produced by hot-rolling cast slabs. However, in connection with the hot rolling, the structure and various mechanical properties may exhibit anisotropy between the rolling direction (L direction) and the width direction (C direction) perpendicular thereto. Furthermore, if the degree of such anisotropy increases, the hole expandability (stretch flangeability) corresponding to deformation operations in the entire circumferential direction may decrease. Meanwhile, due to further improvements in workability and customer demands, there is a high demand for high-strength hot-rolled steel sheets that not only have improved hole expandability but also have reduced differences between the L direction and C direction in other mechanical properties, such as uniform elongation and ultimate deformability.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having a novel configuration, which has high strength, but also has improved hole expandability and reduced differences in uniform elongation and ultimate deformability between the L direction and the C direction.

[0008] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the steel structure of hot-rolled steel sheets. As a result, the present inventors have discovered that high strength can be achieved by configuring the steel structure of a hot-rolled steel sheet having a predetermined chemical composition to contain a specific total proportion of martensite and retained austenite, while limiting the proportion of crystal grains having relatively large grain sizes in the steel structure to a predetermined range and suppressing the development of texture in the steel structure, thereby improving hole expandability and reducing the difference in uniform elongation between the L direction and the C direction. Furthermore, by reducing the through-thickness variation of martensite and retained austenite contained in the steel structure, the difference in ultimate deformability between the L direction and the C direction can be reduced, and have completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.025 to 0.080%, Si: 0.001 to 2.000%, Mn: 1.000 to 2.000%, Ti: 0.080 to 0.200%, Al: 0.200 to 1.000%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, Nb: 0 to 0.200%, V: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, the steel has a chemical composition consisting of Sb: 0 to 0.50%, Zn: 0 to 1.00%, B: 0 to 0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Bi: 0 to 0.0100%, Hf: 0 to 0.0100%, As: 0 to 0.1000%, REM: 0 to 0.0100%, and the balance: Fe and impurities; the steel structure in a cross section in the rolling direction in an observation range centered at 1 / 4 thickness from the surface is, in area %, a total of martensite and retained austenite: 2 to 30%, a total of pearlite and cementite: 0 to 3%, and the balance: ferrite and bainite; and the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observation area is 50% or less. (2) A hot-rolled steel sheet according to (1), characterized in that in a texture measured at the center of the sheet thickness, the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group is 8.0 or less, and when an observation field of the steel structure is divided into 2 μm intervals in the sheet thickness direction and the sum of martensite and retained austenite in each divided region is measured, the maximum value of the sum is 3.0 times or less than the average value of the sum.(3) The chemical composition is, in mass%, Cr: 0.001 to 1.00%, Mo: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00%, Co: 0.001 to 1.00%, Nb: 0.001 to 0.200%, V: 0.001 to 1.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Zn: 0.001 to 1.00%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, The hot-rolled steel sheet according to (1) or (2) above, characterized in that it contains at least one of Zr: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, As: 0.0001 to 0.1000%, and REM: 0 to 0.0100%. (4) The hot-rolled steel sheet according to any one of (1) to (3) above, characterized in that it is a plated steel sheet having a plating layer on at least one surface. (5) A part, characterized in that it contains the hot-rolled steel sheet according to any one of (1) to (4) above.

[0010] According to the present invention, it is possible to provide a hot-rolled steel sheet that has high strength, but also has improved hole expandability and reduced differences between the L direction and the C direction in uniform elongation and ultimate deformability.

[0011] <Hot-rolled steel sheet> A hot-rolled steel sheet according to an embodiment of the present invention has, in mass%, C: 0.025 to 0.080%, Si: 0.001 to 2.000%, Mn: 1.000 to 2.000%, Ti: 0.080 to 0.200%, Al: 0.200 to 1.000%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, Nb: 0 to 0.200%, V: 0 to 1.00%, W: 0 to 1.00%, The steel has a chemical composition consisting of Sn: 0 to 1.00%, Sb: 0 to 0.50%, Zn: 0 to 1.00%, B: 0 to 0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Bi: 0 to 0.0100%, Hf: 0 to 0.0100%, As: 0 to 0.1000%, REM: 0 to 0.0100%, and the balance: Fe and impurities, and the steel structure in a cross section in the rolling direction in an observation range centered at ¼ thickness from the surface is, in area %, as follows: total of martensite and retained austenite: 2 to 30%, total of pearlite and cementite: 0 to 3%, and the balance: ferrite and bainite, The ratio of the area occupied by crystal grains having a grain size of more than 12 μm to the observation area is 50% or less; in the texture measured at the center of the sheet thickness, the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group is 8.0 or less; and when the observation field of the steel structure is divided into 2 μm intervals in the sheet thickness direction and the sum of martensite and retained austenite in each divided region is measured, the maximum value of the sum is 3.0 times or less the average value of the sum.

[0012] As mentioned above, it is known that the microstructure and various mechanical properties may exhibit anisotropy between the rolling direction (L direction) and the width direction (C direction) perpendicular to the rolling direction during hot rolling during steel sheet production. As the degree of such anisotropy increases, the hole expandability corresponding to deformation operations in the entire circumferential direction decreases, and the difference between the L direction and the C direction in uniform elongation and ultimate deformability (hereinafter simply referred to as the LC difference) also increases. By improving the hole expandability and reducing the LC difference in uniform elongation and ultimate deformability, it is possible to further improve the workability of high-strength steel sheets used in applications such as automobiles. However, it is generally very difficult to achieve both high strength and improved properties in steel sheets. Therefore, the present inventors conducted research, focusing particularly on the steel microstructure of hot-rolled steel sheets, in order to achieve both high strength and improved properties in steel sheets.

[0013] First, the inventors controlled the C content in the steel to a relatively low range of 0.080 mass% or less in order to reduce the LC difference in ultimate deformability, as will be described in detail later in relation to the chemical composition and manufacturing method of the hot-rolled steel sheet. Meanwhile, the inventors discovered that, despite the relatively low C content, high strength, for example, a tensile strength of 780 MPa or more, can be achieved by utilizing precipitation strengthening by Ti carbide and configuring the steel structure of the hot-rolled steel sheet to contain a specific total proportion of martensite and retained austenite, more specifically, by configuring the steel structure to contain a total of 2 to 30% by area of ​​martensite and retained austenite.

[0014] Next, the inventors conducted further studies focusing on the refinement of the steel structure and texture in order to improve hole expandability and reduce the LC difference in other mechanical properties. As a result, the inventors discovered that by limiting the proportion of crystal grains having a relatively large grain size in the steel structure to a predetermined range and suppressing the development of texture in the steel structure, more specifically, by limiting the proportion of the area occupied by crystal grains with a grain size of more than 12 μm to the observed area in the steel structure to 50% or less, and controlling the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group to 8.0 or less in the texture measured at the center of the thickness of the hot-rolled steel sheet, it is possible to improve hole expandability and reduce the LC difference in uniform elongation. Although it is not intended to be bound by any particular theory, it is believed that by limiting the proportion of the area occupied by crystal grains having a grain size exceeding 12 μm to the observed area of ​​the steel structure to 50% or less, thereby making the steel structure finer and more uniform, the hardness difference in the steel structure can be reduced compared to when the proportion of the area occupied by crystal grains having a grain size exceeding 12 μm to the observed area exceeds 50%, and that hole expandability can be improved due to such a reduction in hardness difference.

[0015] On the other hand, to explain the characteristics of the texture in more detail, first, when analyzing the thickness cross section of a hot-rolled steel sheet, the crystal orientation differs between the center of the thickness and the surface layer of the thickness, which is directly affected by rolling. However, in hot-rolled steel sheets, friction between the rolls and the steel sheet during hot rolling causes shear deformation in opposite directions on the front and back of the steel sheet, so the texture also develops with symmetrical orientations on the front and back of the steel sheet. Therefore, the influence of the texture on the front and back of the steel sheet cancels out, and as a result, it is thought that the relationship between the texture and the mechanical properties is such that the texture in the center of the thickness corresponds more closely to the mechanical properties than the texture in the surface layer of the thickness. For these reasons, in the hot-rolled steel sheet according to the embodiment of the present invention, by controlling the texture in the center of the thickness, improved hole expandability and reduced LC difference in uniform elongation are achieved. Here, the crystal orientation of a rolled sheet is usually represented by {hkl} or (hkl) for the crystal orientation perpendicular to the rolling surface, and by <uvw> or [uvw] for the crystal orientation parallel to the rolling direction. {hkl} and <uvw> are generic terms for equivalent planes and orientations, while (hkl) and [uvw] refer to individual crystal planes. The hot-rolled steel sheet according to an embodiment of the present invention is primarily intended for a body-centered cubic (bcc) structure, and therefore, 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 an embodiment of the present invention, these orientations are collectively represented as {110}.

[0016] The main orientations that develop in the texture at the center of the thickness of a hot-rolled steel sheet include {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113>. The development of any one of these orientations increases the in-plane anisotropy of the mechanical properties of the hot-rolled steel sheet, significantly reducing the hole expandability, which corresponds to deformation operations in the circumferential direction. Therefore, in order to improve hole expandability and reduce the in-plane anisotropy of other mechanical properties, it is important to suppress the development of all of these orientations and randomize the texture. Therefore, in the hot-rolled steel sheet according to an embodiment of the present invention, the pole density of each of these orientations is calculated, and the maximum pole density is controlled to 8.0 or less, thereby sufficiently suppressing the development of the texture and further randomizing the texture. As a result, hole expandability can be further improved in combination with refinement of the steel structure, and the LC difference in uniform elongation can be significantly reduced. Here, the pole density refers to the ratio of the degree of accumulation in a specific direction of the test material to that of a standard sample that does not have accumulation in a specific direction.

[0017] However, while refining the steel structure and suppressing the development of texture can improve hole expandability and reduce the LC difference in uniform elongation, the LC difference in ultimate deformability could not necessarily be sufficiently reduced. Therefore, the inventors conducted further studies to reduce the LC difference in ultimate deformability. As a result, the inventors discovered that the LC difference in ultimate deformability can be reduced by reducing the variation in the martensite and retained austenite contained in the steel structure in the sheet thickness direction, more specifically, by dividing the observation field of the steel structure at 2 μm intervals in the sheet thickness direction and measuring the sum of the martensite and retained austenite in each divided region, and controlling the maximum value of the sum to 3.0 times or less the average value of the sum.

[0018] As described above, the hot-rolled steel sheet according to the embodiment of the present invention is configured to contain 2 to 30% by area of ​​martensite and retained austenite in total to achieve high strength. In relation to this, a predetermined amount of Mn must be contained in the steel to improve the hardenability of the steel. In the hot-rolled steel sheet according to the embodiment of the present invention, the Mn content is set to 1.000 mass% or more. However, if regions where Mn segregate in the steel during the manufacturing process exist, martensite and / or retained austenite are likely to form in these regions during cooling after hot rolling. As a result, it is believed that regions containing large amounts of martensite and / or retained austenite are formed locally in the final hot-rolled steel sheet, causing an LC difference in ultimate deformability. Therefore, the inventors have discovered that, as will be described in detail later in connection with the manufacturing method of the hot-rolled steel sheet, Mn segregation in the steel can be sufficiently suppressed by performing mainly the casting and hot-rolling processes under appropriate conditions, thereby reducing the through-thickness variation of martensite and retained austenite formed during the subsequent cooling process to within the above-mentioned predetermined range. As a result, according to the hot-rolled steel sheet according to the embodiment of the present invention, martensite and retained austenite can be uniformly present in the steel, so that the LC difference in ultimate deformability can be reliably and sufficiently reduced.

[0019] In the hot-rolled steel sheet according to the embodiment of the present invention, as described above, in order to achieve high strength, improved hole expandability, and reduced LC difference in uniform elongation, the structure is configured to contain 2 to 30% martensite and retained austenite in total, the proportion of the area occupied by crystal grains with a grain size of more than 12 μm in the steel structure is limited to a predetermined range, and the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group in the center of the sheet thickness is controlled to 8.0 or less. Therefore, it is extremely difficult to maintain the structure of the steel structure controlled in this way while further controlling the through-thickness variation of martensite and retained austenite contained in the steel structure within a desired range. In contrast, as will be described in detail later in connection with the method for producing a hot-rolled steel sheet, in an embodiment of the present invention, the C content in the steel is limited to 0.080% or less, thereby forming a δ phase during solidification in the casting process and promoting the diffusion of Mn. Furthermore, the subsequent rough rolling is performed by repeatedly introducing strain and recrystallizing the steel, thereby further promoting the diffusion of Mn into the steel, thereby enabling the realization of a steel structure in which the through-thickness variation of martensite and retained austenite is reduced to within the above-mentioned predetermined range. As a result, the hot-rolled steel sheet according to an embodiment of the present invention can improve hole expandability and reduce the difference between the L-direction and the C-direction in uniform elongation and ultimate deformability, despite its high strength, for example, a tensile strength of 780 MPa or more. Therefore, the hot-rolled steel sheet according to an embodiment of the present invention can reliably achieve the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field, where both of these properties are required.

[0020] Hereinafter, the hot-rolled steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0021] [C: 0.025 to 0.080%] C (carbon) is an essential element for ensuring the strength of the steel sheet. To fully obtain this effect, the C content is set to 0.025% or more. The C content may be 0.030% or more, 0.035% or more, 0.040% or more, or 0.045% or more. On the other hand, excessive C content may not sufficiently suppress Mn segregation during the continuous casting process, and as a result, the through-thickness variation of martensite and retained austenite may not be reduced to within the desired range. Therefore, the C content is set to 0.080% or less. The C content may be 0.075% or less, 0.070% or less, 0.065% or less, or 0.060% or less.

[0022] [Si: 0.001 to 2.000%] Si (silicon) is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To achieve these effects, the Si content is set to 0.001% or more. The Si content may be 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, since Si is a ferrite-forming element, it increases the ferrite transformation start temperature (Ar3 point). Therefore, if excessive Si is added, the temperature at which ferrite forms may become excessively high, leading to the formation of coarse ferrite grains and reduced hole expandability. Therefore, the Si content is set to 2.000% or less. The Si content may be 1.500% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.500% or less. In particular, when the Si content is limited to 0.500% or less, the formation of Si scale can be suppressed. As will be described in detail later in relation to the manufacturing method of a hot-rolled steel sheet, by limiting the Si content to 0.500% or less and performing descaling under appropriate conditions before finish rolling, a hot-rolled steel sheet with excellent surface quality can be obtained, and the LC difference in ultimate deformability can be more significantly reduced.

[0023] [Mn: 1.000 to 2.000%] Mn (manganese) is a strong austenite-stabilizing element and is effective in increasing the strength of steel sheets. To fully obtain this effect, the Mn content is set to 1.000% or more. The Mn content may be 1.100% or more, 1.200% or more, 1.300% or more, or 1.500% or more. On the other hand, excessive Mn content may lead to excessive formation of martensite, which may reduce hole expandability. Therefore, the Mn content is set to 2.000% or less. The Mn content may be 1.900% or less, 1.800% or less, 1.700% or less, or 1.600% or less.

[0024] [Ti: 0.080 to 0.200%] Ti (titanium) precipitates in steel as Ti carbides such as TiC, contributing to improved strength through precipitation strengthening. It also contributes to grain refinement through its pinning effect. To fully achieve these effects, the Ti content is set to 0.080% or more. The Ti content may be 0.090% or more, 0.100% or more, 0.110% or more, or 0.120% or more. On the other hand, excessive Ti content may cause the Ti carbides to become coarse, which not only prevents the strength improvement effect of precipitation strengthening from being fully exerted, but may also lead to reduced hole expandability and a reduced LC difference in uniform elongation. Therefore, the Ti content is set to 0.200% or less. The Ti content may be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[0025] [Al: 0.200 to 1.000%] Al (aluminum) acts as a deoxidizer and stabilizes ferrite. If the Al content is low, this effect may not be fully achieved and / or the desired steel structure may not be obtained. Therefore, the Al content is set to 0.200% or more. The Al content may be 0.250% or more, 0.300% or more, 0.350% or more, or 0.400% or more. On the other hand, since Al is a ferrite-forming element, it increases the ferrite transformation start temperature (Ar3 point). Therefore, if excessive Al is added, the temperature at which ferrite forms may become excessively high, resulting in the formation of coarse ferrite grains and reduced hole expandability. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, or 0.500% or less.

[0026] [P: 0.050% or less] P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel sheets, but excessive addition deteriorates weldability and toughness. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but excessively reducing the P content increases the cost of dephosphorization, so from an economical standpoint, the lower limit is preferably set to 0.0001%.

[0027] [S: 0.0100% or less] S (sulfur) is an element contained as an impurity, and forms MnS in steel, which deteriorates toughness and hole expandability. Therefore, the S content is set to 0.0100% or less as a range in which deterioration of toughness and hole expandability is not significant. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. The S content may be 0%, but if the S content is reduced too much, the desulfurization cost will increase, so from an economical viewpoint, the lower limit is preferably 0.0001%.

[0028] [N: 0.0100% or less] N (nitrogen) is an element contained as an impurity, and if the N content is high, coarse nitrides may form in the steel, which may deteriorate the bendability and hole expandability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but if the N content is reduced too much, the cost of denitrification will increase, so from an economical standpoint, the lower limit is preferably set to 0.0001%.

[0029] [O: 0.0100% or less] O (oxygen) is an element contained as an impurity, and if the O content is high, coarse oxides may form in the steel, which may deteriorate bendability and hole expandability. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but from the viewpoint of manufacturing costs, the lower limit is preferably set to 0.0001%.

[0030] 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 contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0031] [Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, Nb: 0-0.200%, V: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, and B: 0-0.0050%] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), Nb (niobium), V (vanadium), W (tungsten), Sn (tin), Sb (antimony), and B (boron) are all elements effective in increasing the strength of steel sheets. The content of these elements may be 0%, but in order to obtain such effects, at least one of these elements may be contained in the hot-rolled steel sheet as necessary. However, excessive inclusion of these elements may saturate the effects and increase manufacturing costs. Therefore, the Cr, Mo, Cu, Ni, Co, V, W, and Sn contents may each be 1.00% or less, 0.60% or less, 0.50% or less, or 0.30% or less. Similarly, the Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less. Similarly, the Sb content may be 0.50% or less, 0.30% or less, or 0.10% or less. Similarly, the B content may be 0.0050% or less, or 0.0020%. Regarding the lower limits of these elements, for example, the Cr, Mo, Cu, Ni, Co, V, W, Sn, and Sb contents may be 0.001% or more, or 0.01% or more. Similarly, the Nb content may be 0.001% or more, or 0.005% or more. Similarly, the B content may be 0.0001% or more, or 0.0005% or more.

[0032] [Zn: 0-1.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, Hf: 0-0.0100%, and REM: 0-0.0100%] Zn (zinc) is an element effective for controlling the shape of inclusions in steel, Ca (calcium), Mg (magnesium), Zr (zirconium), Hf (hafnium), and REM (rare earth metals) are elements that contribute to the fine dispersion of inclusions in steel, and Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. The content of these elements may be 0%, but since each contributes to improving the workability of the steel sheet, at least one of these elements may be contained in the hot-rolled steel sheet as necessary. However, excessive inclusion of these elements may saturate the effects and increase manufacturing costs. Therefore, the Zn content is set to 1.00% or less, and may be set to 0.50% or less, or 0.20% or less. Similarly, the Ca, Mg, Zr, Bi, Hf, and REM contents are set to 0.0100% or less, and may be set to 0.0080% or less, 0.0060% or less, or 0.0030% or less. Regarding the lower limits of these elements, for example, the Zn content may be 0.001% or more, or 0.01% or more. Similarly, the Ca, Mg, Zr, Bi, Hf, and REM contents may be set to 0.0001% or more, or 0.0005% or more.

[0033] [As: 0 to 0.1000%] As (arsenic) is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.0001% or more, and may be 0.0010% or more, 0.0050% or more, or 0.0100% or more. On the other hand, even if excessive As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases manufacturing costs. Therefore, the As content is set to 0.1000% or less, and may be 0.0500% or less, 0.0300% or less, or 0.0200% or less.

[0034] In the hot-rolled steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities, which are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the hot-rolled steel sheet.

[0035] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the hot-rolled steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0036] [Steel structure] [Total of martensite and retained austenite: 2 to 30%, total of pearlite and cementite: 0 to 3%, and balance: ferrite and bainite] In the hot-rolled steel sheet according to an embodiment of the present invention, the steel structure in a cross section in the rolling direction in an observation range centered at ¼ thickness from the surface of the hot-rolled steel sheet is, in area %, total of martensite and retained austenite: 2 to 30%, total of pearlite and cementite: 0 to 3%, and balance: ferrite and bainite.

[0037] In an embodiment of the present invention, martensite includes as-quenched martensite (fresh martensite) and tempered martensite. Furthermore, retained austenite transforms into martensite by strain-induced transformation during deformation of the steel sheet. Therefore, the inclusion of martensite and retained austenite can increase the strength of the hot-rolled steel sheet. To achieve the desired high strength, the total area ratio of martensite and retained austenite is 2% or more, and may be 5% or more, 10% or more, or 15% or more. On the other hand, if martensite and retained austenite are excessively contained, the strength will be high but the hole expandability will be reduced. Therefore, the total area ratio of martensite and retained austenite is 30% or less, and may be 28% or less, 25% or less, 22% or less, or 20% or less.

[0038] Pearlite contains hard and coarse cementite, which can be the origin of fracture during plastic deformation and may reduce the hole expandability of the steel sheet. Therefore, its content, including cementite, is 0 to 3% by area, and may be 0 to 2% or 0 to 1%.

[0039] The remainder of the steel structure in the hot-rolled steel sheet according to the embodiment of the present invention is composed of ferrite and bainite. Ferrite has excellent ductility and contributes to improving elongation. Bainite may be any of upper bainite having carbides between laths, lower bainite having carbides within the laths, bainitic ferrite having no carbides, granular bainitic ferrite in which the lath boundaries of bainite have recovered and become unclear, or a mixed structure thereof.

[0040] [Identification of Steel Structure and Calculation of Area Fraction] The identification of the steel structure and calculation of the area fraction are performed using secondary electron images taken using an FE-SEM. First, a sample is taken from a thickness cross section of the steel sheet parallel to the rolling direction and at the center position in the width direction as the observation surface. The observation surface is mechanically polished to a mirror finish, and then etched using a nital solution. Next, a total of 2.0 × 10 -9 m 2 Secondary electron images are taken of the above areas. From the obtained secondary electron images, the area ratios of the total of martensite and retained austenite, pearlite (total of pearlite and cementite), and the remainder, i.e., ferrite and bainite, are measured. First, regions with high brightness and where the substructure is not revealed by etching are judged to be martensite and retained austenite. Next, regions where cementite is precipitated in a lamellar form are judged to be pearlite (total of pearlite and cementite). The remainder other than the above structures is judged to be ferrite and bainite. For reference, regions with low brightness and where no substructure is observed can be judged to be ferrite, and regions that do not fall into either of the above categories can be judged to be bainite. The area ratios of each structure identified in this way are calculated using the point counting method.

[0041] [Proportion of the area occupied by crystal grains exceeding 12 μm in diameter relative to the observed area: 50% or less] In an embodiment of the present invention, the proportion of the area occupied by crystal grains exceeding 12 μm in diameter relative to the observed area of ​​the steel structure is 50% or less. In the present invention, the grain size of a crystal grain refers to the circle-equivalent diameter of the crystal grain when a crystal grain is defined as a region surrounded by boundaries with an orientation difference of 15° or more. By limiting the proportion of the area occupied by crystal grains exceeding 12 μm in diameter relative to the observed area of ​​the steel structure to 50% or less, the steel structure becomes finer and more uniform, and the hardness difference in the steel structure can be reduced compared to when this requirement is not met. This reduction in hardness difference makes it possible to improve hole expandability. From the perspective of improving hole expandability, a lower area proportion is preferable, and may be, for example, 40% or less, 35% or less, or 30% or less. The lower limit may be 0%, but is not particularly limited. For example, the area proportion may be 5% or more, 10% or more, or 15% or more.

[0042] [Measurement of the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area] The ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the observation surface is a cross section of the sheet thickness parallel to the rolling direction and perpendicular to the sheet surface. Next, at a depth of ¼ of the sheet thickness from the steel sheet surface, a region 200 μm in the rolling direction of the steel sheet and 100 μm in the normal direction to the rolled surface is subjected to EBSD analysis at a measurement interval of 0.2 μm to obtain crystal orientation information. Here, the EBSD analysis is performed at an analysis speed of 50 to 300 points per second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, for the obtained crystal orientation information, regions surrounded by boundaries with an orientation difference of 15° or more are defined as crystal grains, and the proportion of crystal grains with a circle equivalent diameter exceeding 12 μm relative to the observed area is determined. The crystal grains defined as above can be calculated using the software "OIM Analysis (registered trademark)" that comes with the EBSD analysis apparatus.

[0043] [Maximum pole density of {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientations in the texture measured at the center of the thickness of the hot-rolled steel sheet: 8.0 or less] In an embodiment of the present invention, in the texture measured at the center of the thickness of the hot-rolled steel sheet, the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientations is 8.0 or less. By controlling the maximum pole density of these orientations to 8.0 or less in the texture measured at the center of the thickness of the hot-rolled steel sheet, it is possible to further randomize the texture. As a result, in combination with the refinement of the steel structure described above, it is possible to further improve the hole expandability and significantly reduce the LC difference in uniform elongation. From the viewpoint of improving hole expandability and reducing the LC difference in uniform elongation, the more randomized the texture, the more preferable, i.e., the lower the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation groups, the more preferable, for example, 7.0 or less, 6.5 or less, or 6.0 or less. Although the lower limit is not particularly limited, for example, the maximum pole density of these orientation groups may be 2.0 or more, 2.5 or more, 3.0 or more, 3.5 or more, 4.0 or more, or 4.5 or more.

[0044] [Measurement of Maximum Pole Density of {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> Orientation Groups] The maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group in the texture at the center of the sheet thickness is measured by EBSD. More specifically, a sample is first taken from the steel sheet so that the sheet thickness cross section parallel to the rolling direction and perpendicular to the sheet surface serves as the observation surface. EBSD analysis is then performed at measurement intervals of 1 μm on a rectangular region of the steel sheet, 1000 μm in the rolling direction and 100 μm in the normal direction to the rolling surface, centered at a depth position of 1 / 2 the sheet thickness from the surface of the steel sheet, to obtain crystal orientation information of this rectangular region. The EBSD analysis is performed at an analysis speed of 50 to 300 points per second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, from the crystal orientation information of this rectangular region, the ODF (Orientation Distribution Function) of this rectangular region is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis apparatus. As a method for calculating the ODF, the Harmonic Series Expansion (spherical harmonic function method) was used, and the expansion order was set to 16. In addition, calculations were performed taking into account symmetry (orthotropic). This makes it possible to determine the pole density of each of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> crystal orientations, and the maximum value of these pole densities is determined as "the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group in the texture measured at the center of the sheet thickness." Note that, since the crystal orientation here represents the crystal orientation in the direction perpendicular to the steel sheet surface, it is necessary to align the measurement coordinate system of the crystal orientation data with the sample coordinate system during analysis, taking into account the direction of the sample set for measurement.

[0045] [The maximum sum of martensite and retained austenite in the thickness direction is 3.0 times or less the average value of the sum] In an embodiment of the present invention, when the observation field of the steel structure is divided into 2 μm intervals in the thickness direction and the sum of martensite and retained austenite in each divided region is measured, the maximum sum is 3.0 times or less the average value of the sum. By controlling the maximum sum of martensite and retained austenite in each divided region in the thickness direction to 3.0 times or less the average value of the sum, the variation in the thickness direction of martensite and retained austenite is suppressed, i.e., the number of regions where these structures are locally abundant is reduced. As a result, it is possible to reliably and sufficiently reduce the LC difference in ultimate deformability. From the viewpoint of reducing the LC difference in ultimate deformability, the smaller the variation in the thickness direction of martensite and retained austenite, the more preferable. Therefore, for example, the maximum sum of martensite and retained austenite in each divided region in the sheet thickness direction may be 2.8 times or less, 2.5 times or less, 2.2 times or less, 2.0 times or less, or 1.8 times or less of the average value of the sum. Although there is no particular limitation on the lower limit, for example, the maximum sum of martensite and retained austenite in each divided region in the sheet thickness direction may be 1.0 times or more or 1.1 times or more of the average value of the sum.

[0046] [Measurement of the ratio between the maximum value of the sum of martensite and retained austenite in the thickness direction and the average value of the sum] The ratio between the maximum value of the sum of martensite and retained austenite in the thickness direction and the average value of the sum is determined as follows. First, the ratio of the total of 2.0 × 10 -9 m 2 The above area (observation field) is divided into 2 μm intervals in the thickness direction, and the total area ratio of martensite and retained austenite in each divided region is calculated. Next, the highest value of the calculated total area ratios of martensite and retained austenite in each divided region is divided by the average value of all the calculated total area ratios of martensite and retained austenite, and the result is determined as the ratio of the maximum total area ratio of martensite and retained austenite in the thickness direction to the average total area ratio.

[0047] [Preferred Embodiment] In a preferred embodiment of the present invention, the Si content of the hot-rolled steel sheet is 0.001 to 0.500%, and the number density of recesses present on the surface of the hot-rolled steel sheet with a depth exceeding 5 μm is controlled to 1.0 / mm or less per surface length. By limiting the Si content to 0.500% or less, the formation of Si scale can be suppressed. In addition, as will be described in detail later in connection with the manufacturing method of the hot-rolled steel sheet, by appropriately controlling the descaling before finish rolling, it is possible to obtain a hot-rolled steel sheet with excellent surface quality, more specifically, a hot-rolled steel sheet in which the number density of recesses present on the surface with a depth exceeding 5 μm is controlled to 1.0 / mm or less per surface length. Since recesses with a depth exceeding 5 μm can act as stress concentration portions, controlling the number density of such recesses to 1.0 / mm or less to achieve a flatter surface quality makes it possible to more significantly reduce the LC difference in ultimate deformability. From the viewpoint of reducing the LC difference of the ultimate deformability, it is preferable that the number density of recesses having a depth exceeding 5 μm present on the surface of the hot-rolled steel sheet is as small as possible, and may be, for example, 0.8 / mm or less, 0.5 / mm or less, or 0.3 / mm or less. The lower limit is not particularly limited and may be 0 / mm, but for example, the number density of recesses having a depth exceeding 5 μm present on the surface of the hot-rolled steel sheet may be 0.1 / mm or more.

[0048] [Measurement of the Number Density of Depressions Exceeding 5 μm in Depth on the Surface of a Hot-Rolled Steel Sheet] The number density of depressions exceeding 5 μm in depth on the surface of a hot-rolled steel sheet is measured as follows. First, a sample is taken from a thickness cross-section of the hot-rolled steel sheet parallel to the rolling direction and at the center position in the width direction as the observation surface. The observation surface is mechanically polished to a mirror finish, and then a backscattered electron image of the steel sheet surface (or the coating / steel sheet interface if the hot-rolled steel sheet has a coating layer) is taken using an FE-SEM at a magnification of 500x. The obtained backscattered electron image is binarized to clarify the surface of the steel sheet. The converted binarized image is converted into numerical data to obtain a surface height profile. Image analysis software capable of such operations includes, for example, Image J. The center line of the height profile is determined using the least squares method, and regions where the surface height deviates from the center line by more than 5 μm on the negative side are defined as "depressions exceeding 5 μm in depth." A similar analysis is performed so that the total measurement range in the X direction (rolling direction) exceeds 1 mm. For example, if the size in the X direction (rolling direction) in one field of view is 200 μm, the above analysis is performed at least five times, changing the field of view. The number of "depressions with a depth exceeding 5 μm" obtained in each field of view is summed, and this is converted to a number density per 100 μm of surface length. This is determined as the "number density of depressions with a depth exceeding 5 μm present on the surface of the hot-rolled steel sheet." Here, the "surface length" refers to the length along the surface height profile described above, and can be measured using image analysis software. Contact or laser roughness meters are commonly used to measure the height profile of steel sheets. However, if the hot-rolled steel sheet has a coating layer, the coating layer must first be dissolved and stripped with acid. However, this method raises concerns that not only the coating layer but also the base steel interface will be corroded during acid dissolution, resulting in changes to the original unevenness. For this reason, these methods are not recommended.

[0049] [Thickness] The hot-rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although not particularly limited thereto. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0050] [Plated Layer] The hot-rolled steel sheet according to the embodiment of the present invention may be a plated steel sheet having a plated layer on at least one surface, preferably both surfaces. The plated layer is not particularly limited, and may be, for example, an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed hot-dip galvanized layer. These galvanized layers may have any composition known to those skilled in the art, and may contain additive elements other than Zn, such as Al and Mg. The coating weight of the plated layer is not particularly limited, and may be a general coating weight.

[0051] [Mechanical Properties] [Tensile Strength (TS) and Total Elongation (EL)] A hot-rolled steel sheet having the above-described chemical composition and metallographic structure can achieve high tensile strength, specifically a tensile strength (TS) of 780 MPa or more. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. Despite having such extremely high tensile strength, a hot-rolled steel sheet according to an embodiment of the present invention can achieve improved hole expandability and reduced uniform elongation and LC difference in ultimate deformability due to the specific combination of the chemical composition and metallographic structure described above. The upper limit of the tensile strength is not particularly limited, and the tensile strength of the hot-rolled steel sheet may be, for example, 1180 MPa or less, 1080 MPa or less, or 1000 MPa or less. Furthermore, a hot-rolled steel sheet according to an embodiment of the present invention can achieve high ductility, more specifically, a total elongation (EL) of 10.0% or more, preferably 12.0% or more, and more preferably 14.0% or more or 16.0% or more. The upper limit of the total elongation is not particularly limited, but for example, the total elongation of the hot-rolled steel sheet may be 40.0% or less or 30.0% or less. The tensile strength and total elongation are determined by taking JIS No. 5 test pieces from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and a direction in which the longitudinal direction is parallel to the direction perpendicular to the rolling direction (C direction), and conducting a tensile test in accordance with JIS Z 2241:2011. In the present invention, the values ​​of TS and EL when a tensile test is conducted in the C direction are adopted. The reason for conducting a tensile test in the L direction is to determine the LC difference of uniform elongation (uEL C / uEL L ) and the LC difference of ultimate deformability (ε C / ε L) to evaluate the

[0052] [Hole expansion ratio (λ)] A hot-rolled steel sheet having the above-described chemical composition and metallographic structure can achieve high hole expansion properties, specifically a hole expansion ratio (λ) of 50% or more. The hole expansion ratio may be preferably 55% or more, more preferably 60% or more or 70% or more. The upper limit of the hole expansion ratio is not particularly limited, but may be, for example, 130% or less, 120% or less, or 100% or less. The hole expansion ratio is measured by performing the "JFS T 1001 Hole Expansion Test Method" of the Japan Iron and Steel Federation standard.

[0053] [LC difference of uniform elongation (uEL C / uEL L ) )] According to the hot-rolled steel sheet having the above chemical composition and metal structure, the LC difference in uniform elongation (uEL C / uEL L ) can be achieved, for example, a uEL of 0.80 to 1.20, preferably 0.90 to 1.10. C / uEL L can be achieved. C / uEL L is determined by taking JIS No. 5 test pieces from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and a direction in which the longitudinal direction is parallel to the direction perpendicular to the rolling direction (C direction), and conducting a tensile test in accordance with JIS Z 2241:2011. More specifically, the uniform elongation (uEL C ) is the uniform elongation in the L direction (uEL L ) is the LC difference of uniform elongation (uEL C / uEL L ) is determined as

[0054] [LC difference of ultimate deformability (ε C / ε L ) )] According to the hot-rolled steel sheet having the above chemical composition and metallographic structure, the LC difference in ultimate deformability (ε C / ε L ) can be achieved, for example, an ε of 0.70 to 1.30, preferably 0.80 to 1.20, more preferably 0.90 to 1.10. C / ε Lcan be achieved. C / ε L is determined by taking JIS No. 5 test pieces from the direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and the direction in which the longitudinal direction of the test piece is parallel to the rolling direction (C direction), and conducting a tensile test in accordance with JIS Z 2241:2011. More specifically, the ultimate deformability is determined by calculating the difference between the initial thickness (t0) of the parallel part before the tensile test and the thickness (t) of the fracture part after the tensile test for each of the L-direction test piece and the C-direction test piece. f ) is measured and calculated according to the following formula: Next, the obtained ultimate deformability in the C direction (ε C ) is the ultimate deformability in the L direction (ε L ) is the LC difference of ultimate deformability (ε C / ε L ) is determined as

[0055] As described above, the hot-rolled steel sheet according to the embodiment of the present invention can improve hole expandability and achieve a reduced LC difference in uniform elongation and ultimate deformability, despite its high strength, for example, a tensile strength of 780 MPa or more. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent formability, and is therefore particularly useful for use in parts in technical fields where both properties are required. In a preferred embodiment, an automotive part, particularly a part selected from the undercarriage, chassis, wheels, etc. of an automobile, is provided that includes the hot-rolled steel sheet according to the embodiment of the present invention. These automotive parts may at least partially include the hot-rolled steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the above-described chemical composition, metallographic structure, and / or surface texture characteristics. In the portion of the hot-rolled steel sheet that is not in direct contact with a mold during forming, such as press forming, and that is subjected to a relatively small degree of processing, the metallographic structure and / or surface texture characteristics do not change significantly before and after forming.

[0056] <Method for manufacturing hot-rolled steel sheet> Next, a preferred method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention, but is not intended to limit the hot-rolled steel sheet to one manufactured by the manufacturing method described below.

[0057] A method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes: (A) a continuous casting step of casting a slab having the chemical composition described above in relation to the hot-rolled steel sheet so as to satisfy the following formulas (1) to (3): τ: Elapsed time from the start of casting [seconds] τ1: T C (τ) is T δ Time to reach [seconds] τ2:T C Time when (τ) reaches 1673K [sec] T S (τ): Slab surface temperature at time τ [K] T C (τ): Estimated slab internal temperature at time τ [K] T L : Solidification start temperature [K] T δ : Solidification completion temperature [K] f L :Temperature T C (τ) Equilibrium volume fraction of the liquid phase at L :Temperature T C Equilibrium Mn concentration in the liquid phase at (τ) M δ :Temperature T C (τ) Equilibrium Mn concentration of δ at D δ Mn :Temperature T C Mn diffusion coefficient in the δ phase at (τ) [m 2 / second] (B) a hot rolling step which includes heating a cast slab and then rough rolling and finish rolling the slab, and which satisfies the following conditions (B1) to (B3): (B1) the heating temperature of the slab is 1200°C or higher; (B2) the rough rolling includes three or more passes of rolling at a steel plate temperature of 1000°C or higher and a reduction ratio of more than 20%; and (B3) the finish rolling is carried out using a tandem rolling mill consisting of at least five rolling stands, the inlet temperature before the final two passes is 950°C or higher, the outlet temperature of the final pass is 900°C or higher, the reduction ratio of each rolling pass in the final three passes is 25% or less, the time between each rolling pass in the final three passes is 0.2 to 1.0 second, the cumulative reduction ratio of the final three passes is 35 to 55%, and the time from the final pass to the start of cooling is 1.0 to 3.0 seconds. (C) a cooling step in which the finish-rolled steel sheet is primarily cooled to a cooling stop temperature of 600 to 750°C at an average cooling rate of 20°C / sec or more, air-cooled for 3 to 20 seconds, and then secondarily cooled to a coiling temperature of 200°C or less at an average cooling rate of 20°C / sec or more, and then coiled. Each step will be described in detail below.

[0058] [(A) Continuous Casting Step] First, a slab having the chemical composition described above in relation to the hot-rolled steel sheet is cast in a continuous casting step so as to satisfy the following formulas (1) to (3). τ: Elapsed time from the start of casting [seconds] τ1: T C (τ) is T δ Time to reach [seconds] τ2:T C Time when (τ) reaches 1673K [sec] T S (τ): Slab surface temperature at time τ [K] T C (τ): Estimated slab internal temperature at time τ [K] T L : Solidification start temperature [K] T δ : Solidification completion temperature [K] f L :Temperature T C (τ) Equilibrium volume fraction of the liquid phase at L :Temperature T C Equilibrium Mn concentration in the liquid phase at (τ) Mδ :Temperature T C (τ) Equilibrium Mn concentration of δ at D δ Mn :Temperature T C Mn diffusion coefficient in the δ phase at (τ) [m 2 / second]

[0059] When continuously casting a slab from molten steel, the molten steel is initially liquid but gradually solidifies from the surface until it completely solidifies at a certain temperature (i.e., the solidification completion temperature). In this regard, Equation (1) defines the temperature range (partially solidified temperature range) in which the liquid and solid phases coexist before the molten steel completely solidifies. In this temperature range, as solidification progresses from the surface of the molten steel, alloy elements such as Mn are distributed (extracted) from the solid phase to the liquid phase, resulting in Mn enrichment in the liquid phase. This enrichment can cause Mn segregation, which can lead to through-thickness variations in the martensite and retained austenite formed during the cooling process. More specifically, if a large amount of Mn is distributed from the solid phase to the liquid phase, a localized region with a high Mn concentration is formed. Since martensite and retained austenite are likely to form in such a region, this leads to the localized formation of martensite and retained austenite. Therefore, in the temperature range during continuous casting where the liquid phase and the solid phase coexist, it is extremely important to sufficiently suppress the partitioning of Mn from the solid phase to the liquid phase. In relation to this, the inventors have found that satisfying formulas (1) and (2), particularly controlling the value obtained by the left side of formula (1) to be less than 1.00, suppresses such partitioning of Mn, and furthermore, by combining this with formula (3) described later, sufficiently suppresses Mn segregation, thereby reducing the through-thickness variation of martensite and retained austenite formed in the subsequent cooling step.

[0060] Here, τ = 0 in equation (1) means the start of casting, and τ is the internal temperature of the slab when it reaches the solidification completion temperature T δf in the formula (1) means the time (seconds) at which the temperature reaches 0°C. Therefore, formula (1) can be understood as the time integral of the distribution of Mn from the solid phase to the liquid phase. Therefore, the smaller the value of the left side of formula (1), the more the distribution of Mn from the solid phase to the liquid phase is suppressed. From the viewpoint of suppressing the variation in the thickness direction of martensite and retained austenite, the smaller the value of the left side of formula (1), the more preferable it is, and specifically, it is preferably 0.80 or less. f in formulas (1) and (2) L , M L , M δ and D δ Mn is a value determined by the temperature and the chemical composition of the molten steel, and T C (τ) is the internal temperature of the slab at time τ, which is estimated from the surface temperature of the slab using equation (2). Therefore, a person skilled in the art can control the value calculated by the left side of equation (1) within a desired range by appropriately selecting the chemical composition of the molten steel and the temperature history during continuous casting. The step interval of τ on the left side of equation (1) is 1 second. T L , T δ , f L , M L , M δ can be obtained by performing a phase diagram calculation for the target steel composition using, for example, commercially available thermodynamic calculation software "Thermo-Calc 2022b" (Thermo-Calc Software Inc.). The phases to be calculated are LIQUID and BCC_A2, and equilibrium calculations are performed in one axis mode with temperature as a variable. The calculation temperature range only needs to include the temperature range in which the number of moles of the liquid phase is 0 to 1, for example, a range of 1400 to 1600°C. The number of step divisions is 100, and the step method is Normal. The pressure is 100,000 Pa, the system size is 1 Mol, and global minimization is enabled. After calculation, the Mn concentration in the LIQUID phase and the Mn concentration in the BCC_A2 phase are output as variables of temperature, and the Mn concentration at each temperature is calculated. L , M δThe volumes of the LQUID phase and the BCC_A2 phase are output as variables of temperature, and the value obtained by dividing the volume of the LQUID phase by the sum of the volumes of the LQUID phase and the BCC_A2 phase is used as f at each temperature. L The number of moles of the LQUID phase is output, and the lowest temperature at which the number of moles becomes 1 is called T Ｌ The highest temperature at which the number of moles becomes 0 is T δ Also, D δ Mn is calculated by the following formula (4).

[0061] On the other hand, Equation (3) defines the diffusion of Mn after the molten steel is completely solidified. After the molten steel is completely solidified, promoting the diffusion of Mn in the solid phase and uniformly distributing Mn throughout the slab is important for suppressing Mn segregation. Therefore, the inventors focused on the δ phase, which has a fast Mn diffusion rate. More specifically, the inventors found that satisfying Equation (3) while limiting the C content in the steel to 0.080% or less to achieve a single δ phase upon solidification (δ solidification) can sufficiently suppress Mn segregation. If the C content in the steel exceeds 0.080%, a peritectic reaction occurs during solidification from the liquid phase, in which the δ phase and austenite (γ) phase are simultaneously formed without passing through a single δ phase state. As a result, Mn diffusion cannot be sufficiently promoted. This is because when the δ phase and the γ phase are simultaneously generated, Mn is distributed between these two phases, which promotes localization of Mn rather than uniform distribution of Mn. Therefore, in the continuous casting process, it is important to satisfy the formulas (1) to (3) and also to control the C content in the steel to 0.080% or less. Here, formula (3) is the value obtained when the internal temperature of the slab reaches the solidification completion temperature T δThis can be understood as the time integral of the Mn diffusion from the time τ1 when the temperature reaches 1673 K to the time τ2 when the temperature reaches 1673 K. Therefore, the larger the value of the right-hand side of equation (3), the more promoted the Mn diffusion. In this manufacturing method, by controlling the value of the right-hand side of equation (3) to be greater than 3.0, preferably 4.0 or greater, the diffusion of Mn in the δ phase is sufficiently promoted and Mn segregation is suppressed, thereby reliably reducing the through-thickness variation of martensite and retained austenite formed in the subsequent cooling process. As with equation (1), the value calculated by the right-hand side of equation (3) can be controlled within a desired range by appropriately selecting the chemical composition of the molten steel and the temperature history during continuous casting.

[0062] [(B) Hot Rolling Step] The cast slab is heated and subjected to rough rolling and finish rolling in the subsequent hot rolling step, and the heating, rough rolling, and finish rolling of the slab must satisfy the following conditions (B1) to (B3). (B1) The heating temperature of the slab is 1200°C or higher, (B2) the rough rolling includes three or more passes of rolling at a steel plate temperature of 1000°C or higher and a reduction ratio of more than 20%, and (B3) the finish rolling is carried out using a tandem rolling mill consisting of at least five rolling stands, the inlet temperature before the final two passes is 950°C or higher, the outlet temperature of the final pass is 900°C or higher, the reduction ratio of each rolling pass in the final three passes is 25% or less, the time between each rolling pass in the final three passes is 0.2 to 1.0 second, the cumulative reduction ratio of the final three passes is 35 to 55%, and the time from the final pass to the start of cooling is 1.0 to 3.0 seconds.

[0063] [(B1) Slab Heating Temperature: 1200°C or Higher] The slab used contains a relatively large amount of alloying elements, and the slab after continuous casting in particular contains coarse Ti carbides. Therefore, it is necessary to dissolve the alloying elements in the slab, and in particular, it is necessary to sufficiently solubilize Ti. If Ti is not sufficiently solubilized during slab heating, it becomes difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbides in the steel during a cooling process after the hot rolling process. Therefore, in order to sufficiently solubilize Ti, the slab heating temperature must be 1200°C or higher. Although there is no particular upper limit, if the slab heating temperature is too high, the yield will decrease due to scale-off. For this reason, it is preferable to set the slab heating temperature to 1300°C or lower.

[0064] [(B2) Rough Rolling: Three or More Times of Rolling at a Steel Sheet Temperature of 1000°C or Higher and a Reduction Ratio of More than 20%] In rough rolling, rolling is performed three or more times at a relatively high steel sheet temperature of 1000°C or higher and a relatively high rolling reduction ratio of more than 20%. This allows for repeated rolling and recrystallization, and this operation can further promote the diffusion of Mn. As a result, Mn segregation is sufficiently suppressed, and the variation in the thickness direction of the martensite and retained austenite contained in the steel structure of the finally obtained hot-rolled steel sheet can be reduced. More specifically, when the observation field of the steel structure is divided into 2 μm intervals in the thickness direction and the sum of the martensite and retained austenite in each divided region is measured, it becomes possible to control the maximum value of the total to 3.0 times or less the average value of the total. When the rolling is performed two or less times at a steel sheet temperature of 1000°C or higher and a reduction ratio of more than 20%, the maximum total value of martensite and retained austenite in each divided region in the sheet thickness direction cannot be controlled to 3.0 times or less the average value of the total, and as a result, the LC difference in ultimate deformability cannot be sufficiently reduced. The upper limit of the number of rolling times is not particularly limited, but for example, the number of rolling times at a steel sheet temperature of 1000°C or higher and a reduction ratio of more than 20% may be 10 or less, or 8 or less.

[0065] [Descaling] In a preferred embodiment of the present manufacturing method, descaling is performed under predetermined conditions after rough rolling and immediately before finish rolling. More specifically, in addition to limiting the Si content in the steel to 0.500% or less, descaling is performed after rough rolling and within 5.0 seconds before the start of finish rolling under conditions of a steel sheet temperature of 1000°C or higher and a water pressure of 8 MPa or higher. By performing descaling under these conditions, it is possible to obtain a hot-rolled steel sheet with excellent surface quality, more specifically, a hot-rolled steel sheet in which the number density of recesses present on the surface with a depth exceeding 5 μm is controlled to 1.0 / mm or less per surface length. Since recesses with a depth exceeding 5 μm can act as stress concentration portions, controlling the number density of such recesses to 1.0 / mm or less to achieve a flatter surface quality makes it possible to more significantly reduce the LC difference in ultimate deformability.

[0066] More specifically, in steel sheets containing a relatively large amount of Si, Si oxides are formed at the interface between the steel sheet and Fe oxides (also referred to as scale) that form on the steel sheet surface during the hot rolling process. Because the Si oxides firmly adhere to the steel sheet, the scales may not be sufficiently removed even by subsequent descaling using high-pressure water or the like. If insufficiently removed scale is pressed into the steel sheet surface during subsequent finish rolling, unevenness occurs on the steel sheet surface, making it impossible to control the number density of recesses with a depth exceeding 5 μm to 1.0 / mm or less per surface length. Therefore, in a preferred embodiment of the present manufacturing method, the Si content in the steel is first limited to 0.500% or less, thereby suppressing the formation of Si oxides at the interface between the scale and the steel sheet and preventing strong adhesion between the scale and the steel sheet. Next, descaling is performed at a high steel sheet temperature of 1000°C or higher and a relatively high water pressure of 8 MPa or higher, thereby enabling sufficient removal of scale from the steel sheet surface. By performing such descaling immediately before finish rolling, i.e., within 5.0 seconds before the start of finish rolling, it is possible to suppress scale growth between descaling and finish rolling, thereby reliably preventing scale from being pressed into the steel sheet surface during the subsequent finish rolling. If any one of the requirements of a steel sheet temperature of 1000°C or higher, a water pressure of 8 MPa or higher, and descaling within 5.0 seconds before the start of finish rolling is not met, scale removal may be insufficient, making it impossible to achieve the desired surface properties. The upper limit of the steel sheet temperature is not particularly limited, but for example, the steel sheet temperature may be 1150°C or lower. Similarly, the upper limit of the water pressure for descaling is not particularly limited, but for example, the water pressure may be 20 MPa or lower.

[0067] [(B3) Finish Rolling] After rough rolling or after rough rolling and the above-mentioned predetermined descaling, the steel sheet is subjected to finish rolling. Specifically, the finish rolling is carried out using a tandem rolling mill consisting of at least five rolling stands under the following conditions: an inlet temperature before the final two passes of 950°C or higher, an outlet temperature of the final pass of 900°C or higher, a rolling reduction of each rolling pass in the final three passes of 25% or less, a time between each rolling pass in the final three passes of 0.2 to 1.0 second, a cumulative rolling reduction of the final three passes of 35 to 55%, and a time from the final pass to the start of cooling of 1.0 to 3.0 seconds. By performing finish rolling under these conditions, recrystallization can be moderately promoted to prevent the accumulation of strain, thereby suppressing the accumulation of crystal orientations in a specific direction. In other words, the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group measured at the center of the thickness of the hot-rolled steel sheet can be reliably controlled to 8.0 or less. In addition, by performing such finish rolling, excessive growth of recrystallized grains can be suppressed, making it possible to reliably control the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area of ​​the steel structure to 50% or less. As a result, it is possible to improve hole expandability and reduce the LC difference in uniform elongation.

[0068] For example, if the inlet temperature two passes before the final one is less than 950°C or the outlet temperature of the final pass is less than 900°C, recrystallization does not proceed sufficiently, resulting in accumulation of strain, and it may be impossible to control the maximum pole density of the orientation group to 8.0 or less. Furthermore, if the rolling reduction rate of any one of the rolling passes in the final three passes exceeds 25%, excessive strain may be introduced, resulting in the maximum pole density of the orientation group being unable to be controlled to 8.0 or less. Furthermore, if the time between each rolling pass in the final three passes is less than 0.2 seconds or the time from the final pass to the start of cooling is less than 1.0 seconds, recrystallization does not proceed sufficiently, resulting in accumulation of strain, and it may be impossible to control the maximum pole density of the orientation group to 8.0 or less. On the other hand, if the time between each rolling pass in the final three passes exceeds 1.0 second or the time from the final pass to the start of cooling exceeds 3.0 seconds, the recrystallized grains grow excessively, and it may be impossible to control the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area of ​​the steel structure to 50% or less. Furthermore, if the cumulative reduction rate for the final three passes is less than 35%, recrystallization does not proceed, resulting in insufficient grain refinement, and it may be impossible to control the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area of ​​the steel structure to 50% or less. Furthermore, if the cumulative reduction rate for the final three passes exceeds 55%, excessive strain may be introduced, and it may similarly be impossible to control the maximum pole density of the orientation group to 8.0 or less. The cumulative reduction rate for the final three passes is preferably 50% or less. Here, the cumulative reduction rate for the final three passes is calculated using the following formula: Cumulative reduction rate of the final 3 passes (%) = (thickness before the final 3 passes - thickness after the final 3 passes) / thickness before the final 3 passes x 100

[0069] [(C) Cooling Step] Finally, in the next cooling step, the finish-rolled steel sheet is primarily cooled to a cooling stop temperature of 600 to 750°C at an average cooling rate of 20°C / s or more, air-cooled for 3 to 20 seconds, and then secondarily cooled to a coiling temperature of 200°C or less at an average cooling rate of 20°C / s or more, and then coiled. By performing the cooling step so as to satisfy these conditions, it is possible to produce a hot-rolled steel sheet having a steel structure consisting of, in area %, 2 to 30% total martensite and retained austenite, 0 to 3% total pearlite and cementite, and the remainder ferrite and bainite. In particular, the precipitation of Ti carbides can be promoted by primarily cooling to a cooling stop temperature of 600 to 750°C, followed by air-cooling for 3 to 20 seconds. As a result, the strength-improving effect of precipitation strengthening can be fully exerted.

[0070] On the other hand, if the average cooling rate in the primary cooling is less than 20°C / second or the average cooling rate in the secondary cooling is less than 20°C / second, structures such as pearlite are excessively formed, resulting in a decrease in properties such as hole expandability. Furthermore, if the air-cooling time exceeds 20 seconds, structures such as martensite are not sufficiently formed and / or structures such as pearlite are excessively formed, resulting in a failure to obtain the desired strength and / or a decrease in hole expandability. Furthermore, if air-cooling is not performed or the air-cooling time is less than 3 seconds, the steel sheet will be cooled to a coiling temperature of 200°C or less without substantially undergoing intermediate air cooling. As a result, martensite and / or retained austenite are excessively formed, resulting in a similar decrease in properties such as hole expandability. Furthermore, if the coiling temperature exceeds 200°C, structures such as martensite cannot be sufficiently formed, resulting in a failure to achieve the desired strength.

[0071] The obtained hot-rolled steel sheet may be subjected to a post-process such as a plating process for forming a plating layer on one or both sides of the hot-rolled steel sheet. The post-process such as the plating process can be carried out by a conventional method.

[0072] The hot-rolled steel sheet manufactured by the above-described manufacturing method has a predetermined chemical composition in which the C content is controlled to a relatively low range of 0.080 mass% or less, but utilizes precipitation strengthening by Ti carbide, and the steel structure is configured to contain a total of 2 to 30% by area of ​​martensite and retained austenite, thereby achieving high strength, for example, a tensile strength of 780 MPa or more. Furthermore, the ratio of the area occupied by crystal grains with a grain size of more than 12 μm to the observed area in the steel structure is limited to 50% or less, and the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group in the texture measured at the center of the thickness of the hot-rolled steel sheet is controlled to 8.0 or less, thereby improving hole expandability and reducing the LC difference in uniform elongation. In addition, when the observation field of the steel structure is divided into 2 μm intervals in the sheet thickness direction and the sum of martensite and retained austenite in each divided region is measured, the maximum value of the sum is controlled to be 3.0 times or less the average value of the sum, so it is possible to reduce the LC difference in ultimate deformability. Therefore, the hot-rolled steel sheet manufactured by the above manufacturing method can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field where both properties are required to be achieved.

[0073] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0074] In the following examples, the hot-rolled steel sheets according to the embodiments of the present invention were produced under various conditions, and the tensile strength (TS), total elongation (EL), hole expansion ratio (λ), and LC difference of uniform elongation (uEL) of the obtained hot-rolled steel sheets were measured. C / uEL L ) and the LC difference of ultimate deformability (ε C / ε L ) was investigated.

[0075] First, molten steel was cast by continuous casting under the conditions shown in Table 2 to form slabs having various chemical compositions shown in Table 1. These slabs were heated under the conditions shown in Table 2 and then subjected to rough rolling, descaling, and finish rolling. The number of rough rolling passes shown in Table 2 indicates the number of passes at a steel sheet temperature of 1000°C or higher and a reduction ratio of more than 20%. Finish rolling was carried out using a tandem rolling mill consisting of five rolling stands. Finally, the finish-rolled steel sheet was cooled and coiled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 3.0 mm.

[0076]

[0077]

[0078]

[0079] The properties of the obtained hot-rolled steel sheets were measured and evaluated by the following methods.

[0080] [Tensile strength (TS) and total elongation (EL)] The tensile strength (TS) and total elongation (EL) were determined by taking JIS No. 5 test pieces from the direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the hot-rolled steel sheet (L direction) and the direction in which the longitudinal direction was parallel to the rolling direction (C direction), and performing a tensile test in accordance with JIS Z 2241:2011. More specifically, the lower of the obtained L-direction TS and C-direction TS values ​​was determined as the tensile strength (TS) of the hot-rolled steel sheet. Similarly, the lower of the obtained L-direction EL and C-direction EL values ​​was determined as the total elongation (EL) of the hot-rolled steel sheet.

[0081] [Hole Expansion Ratio (λ)] The hole expansion ratio (λ) was measured according to the Japan Iron and Steel Federation standard "JFS T 1001 Hole Expansion Test Method."

[0082] [LC difference of uniform elongation (uEL C / uEL L ) )] LC difference of uniform elongation (uEL C / uEL L) was determined by taking JIS No. 5 test pieces from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the hot-rolled steel sheet (L direction) and a direction in which the longitudinal direction was parallel to the direction perpendicular to the rolling direction (C direction), and conducting a tensile test in accordance with JIS Z 2241:2011. More specifically, the uniform elongation (uEL C ) is the uniform elongation in the L direction (uEL L ) is divided by the LC difference of uniform elongation (uEL C / uEL L ) was decided.

[0083] [LC difference of ultimate deformability (ε C / ε L ) ] LC difference of ultimate deformability (ε C / ε L ) was determined by taking JIS No. 5 test pieces from the direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and the direction in which the longitudinal direction of the test piece is parallel to the rolling direction (C direction), and conducting a tensile test in accordance with JIS Z 2241:2011. More specifically, the ultimate deformability was determined by calculating the difference between the initial thickness (t0) of the parallel part before the tensile test and the thickness (t) of the fracture part after the tensile test for each of the L direction test piece and the C direction test piece. f ) was measured and calculated using the following formula: Next, the obtained ultimate deformability in the C direction (ε C ) is the ultimate deformability in the L direction (ε L ) is the LC difference of ultimate deformability (ε C / ε L ) was decided.

[0084] The TS of the hot-rolled steel sheet is 780 MPa or more, λ is 50% or more, and uEL C / uEL L is 0.80 to 1.20, and ε C / ε L When the tensile strength of the hot-rolled steel sheet was 0.70 to 1.30, the hot-rolled steel sheet was evaluated as having improved hole expandability and reduced differences in uniform elongation and ultimate deformability between the L direction and the C direction, despite its high strength. The results are shown in Table 3.

[0085] In Table 3, "M + retained γ" means the sum of martensite and retained austenite, and "P + θ" means the sum of pearlite and cementite. Furthermore, "M + retained γ max / avg" means the ratio of the maximum value of the sum to the average value of the sum when the observation field of the steel structure is divided into 2 μm intervals in the sheet thickness direction and the sum of martensite and retained austenite is measured in each divided region. Furthermore, "area ratio of grains with a grain size of over 12 μm" means the ratio of the area occupied by crystal grains with a grain size of over 12 μm to the observation area. Furthermore, "maximum pole density" means the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation groups in the texture measured at the center of the sheet thickness of the hot-rolled steel sheet.

[0086]

[0087] With reference to Tables 1 to 3, it is believed that in Comparative Example 2, formula (1) was not satisfied in the continuous casting process, and therefore Mn segregation could not be sufficiently suppressed. As a result, the variation in martensite and retained austenite in the sheet thickness direction increased. In other words, when the observation field of the steel structure was divided into 2 μm intervals in the sheet thickness direction and the sum of martensite and retained austenite in each divided region was measured, the maximum value of the total was greater than 3.0 times the average value of the total, and the LC difference in ultimate deformability increased. It is believed that in Comparative Example 3, formula (3) was not satisfied in the continuous casting process, and therefore Mn segregation could not be sufficiently suppressed. As a result, the variation in martensite and retained austenite in the sheet thickness direction increased, and the LC difference in ultimate deformability increased. It is believed that in Comparative Example 7, the number of rolling passes at a steel sheet temperature of 1000°C or higher and a reduction ratio of more than 20% during rough rolling was small, and therefore Mn diffusion could not be promoted, and Mn segregation could not be sufficiently suppressed. As a result, the variation in martensite and retained austenite in the thickness direction increased, resulting in a large LC difference in ultimate deformability. In Comparative Example 8, the long air-cooling time in the cooling process resulted in insufficient martensite formation and excessive pearlite formation, resulting in reduced TS and λ. In Comparative Example 9, the reduction rate of each rolling pass in the final three passes of finish rolling did not meet the requirement of 25% or less, and the cumulative reduction rate of the final three passes was high, resulting in the introduction of excessive strain and making it impossible to control the maximum pole density to 8.0 or less. As a result, λ decreased and the LC difference in uniform elongation increased. In Comparative Example 10, the time t1 between the rolling pass two passes before the final and the rolling pass one pass before the final, and the time t2 between the rolling pass one pass before the final and the final pass in finish rolling were short, resulting in insufficient recrystallization and the accumulation of strain, making it impossible to control the maximum pole density to 8.0 or less. As a result, λ decreased and the LC difference in uniform elongation increased. In Comparative Example 11, the time t3 from the final pass in finish rolling to the start of cooling was short, so recrystallization did not proceed sufficiently, resulting in accumulation of strain, and the maximum pole density could not be controlled to 8.0 or less.As a result, λ decreased and the LC difference in uniform elongation increased. In Comparative Example 12, the cumulative reduction rate of the final three passes in the finish rolling was low, so recrystallization did not proceed and the crystal grains could not be sufficiently refined. As a result, the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area of ​​the steel structure exceeded 50%, and λ decreased.

[0088] In Comparative Example 13, the inlet temperature two passes before the final pass was low, and the outlet temperature of the final pass was also low, so recrystallization did not proceed sufficiently, resulting in strain accumulation and making it impossible to control the maximum pole density to 8.0 or less. As a result, λ decreased and the LC difference in uniform elongation increased. In Comparative Examples 14 and 15, the average cooling rates for the primary and secondary cooling steps were low, respectively, resulting in excessive pearlite formation and a decrease in λ. In Comparative Example 16, the coiling temperature in the cooling step was high, so martensite could not be sufficiently formed and TS decreased. In Comparative Examples 21 to 23, the air-cooling time in the cooling step was short or air-cooling was not performed, so martensite and / or retained austenite were excessively formed and λ decreased. In Comparative Example 33, the C content was low, so martensite was not formed and TS decreased. In Comparative Example 34, the high C content prevented the formation of a single δ-phase during solidification in the continuous casting process, and Mn segregation could not be sufficiently suppressed. As a result, the variation in martensite and retained austenite in the thickness direction increased, resulting in a large LC difference in ultimate deformability. In Comparative Example 35, the low Ti content prevented sufficient precipitation strengthening by Ti carbide, resulting in a decrease in TS. In Comparative Example 36, the high Ti content is thought to have resulted in the formation of coarse Ti carbides. As a result, λ decreased and the LC difference in uniform elongation increased. In Comparative Example 37, the high Si content resulted in the formation of many coarse crystal grains. As a result, the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area of ​​the steel structure exceeded 50%, resulting in a decrease in λ. In Comparative Example 38, the low Mn content prevented martensite from forming, resulting in a decrease in TS. In Comparative Example 39, the high Mn content resulted in excessive martensite formation, resulting in a decrease in λ. In Comparative Example 40, the Al content was low, so the desired structure was not obtained, and λ decreased. In Comparative Example 41, the Al content was high, so many coarse crystal grains were formed. As a result, the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area of ​​the steel structure exceeded 50%, and λ decreased.

[0089] In contrast, all of the hot-rolled steel sheets according to the examples of the present invention had a predetermined chemical composition, and by appropriately controlling the conditions in the manufacturing method, the steel structure was configured to contain 2 to 30% of martensite and retained austenite in terms of area percentage, thereby achieving a high tensile strength of 780 MPa or more. Furthermore, by limiting the ratio of the area occupied by crystal grains with a grain size of more than 12 μm to the observed area in the steel structure to 50% or less and controlling the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation group to 8.0 or less in the texture measured at the center of the thickness of the hot-rolled steel sheet, it was possible to improve λ and reduce the LC difference in uniform elongation. In addition, when the observation field of the steel structure was divided into 2 μm intervals in the plate thickness direction and the sum of martensite and retained austenite in each divided region was measured, the LC difference in ultimate deformability could be reduced by controlling the maximum value of the sum to 3.0 times or less the average value of the sum.

[0090] In particular, in Examples 1, 17 to 20, 24 to 31, and 42 of the present invention, in which the Si content in the steel was limited to 0.500% or less and descaling before finish rolling was carried out under appropriate conditions, the number density of recesses having a depth of more than 5 μm present on the surface of the hot-rolled steel sheet was 1.0 or less per surface length. As a result, ε C / ε L was controlled within the range of 0.80 to 1.30, and the LC difference in ultimate deformability could be further significantly reduced.

Claims

DEPCT681. Hot-rolled thin steel sheets with the following chemical composition (in percentage by mass): C: 0.025 to 0.080 percent, Si: 0.001 to 2.000 percent, Mn: 1.000 to 2.000 percent, Ti: 0.080 to 0.200 percent, Al: 0.200 to 1.000 percent, P: 0.050 percent or less, S: 0.0100 percent or less, N: 0.0100 percent or less, O: 0.0100 percent or less, Cr: 0 to 1.00 percent, Mo: 0 to 1.00 percent. Concentration of phosphorus: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, Nb: 0 to 0.200%, V: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Zn: 0 to 1.00%, B: 0 to 0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Bi: 0 to 0.0100%, Hf: 0 to 0.0100%, As: 0 to 0.1000%, REM: 0 to 0.0100%.0-100 percent, and the remainder: Fe and impurities, and the steel structure in the cross-section of the rolling direction of the observed range, centered at 1 / 4 of the thickness from the surface, which consists of (in percentage by area) the sum of residual martensite and austenite: 2 to 30 percent, the sum of perlite and cementite: 0 to 3 percent, and the remainder: ferrite and bainite, where the ratio of the area occupied by crystalline grains larger than 12 micrometers compared to the observed area is 50 percent or less, in the texture measured at the center of the sheet thickness, the maximum polar density of the group of the orientation direction {100}. <011> {211} <011> {311} <011> {110} <011> and {332} <113> It is at 8.0 or less, and when the observation area of ​​the steel structure in the sheet thickness direction is divided at 2 micrometer intervals and the total remaining martensite and austenite values ​​at each divided area are measured, the maximum value of this total will be 3.0 times or less of the average of this total.Hot-rolled thin steel sheets according to claim 1, in which the chemical composition includes (in percentage by mass) Si: 0.001 to 0.500 percent and the density by number of the recesses with a depth greater than 5 micrometers present at the surface is 1.0 / mm or less per surface length.

3. Hot-rolled thin steel sheets according to claim 1 or claim 2, in which the chemical composition includes (in percentage by mass) at least one element which is Cr: 0.001 to 1.00 percent, Mo: 0.001 to 1.00 percent, Cu: 0.00 percent. O: 1 to 1.00 percent, Ni: 0.001 to 1.00 percent, Co: 0.001 to 1.00 percent, Nb: 0.001 to 0.200 percent, V: 0.001 to 1.00 percent, W: 0.001 to 1.00 percent, Sn: 0.001 to 1.00 percent, Sb: 0.001 to 0.50 percent, Zn: 0.001 to 1.00 percent, B: 0.001 to 1.00 percent, B: 0.001 to 1.00 percent, Concentrates of magnesium: 0.0001 to 0.0050 percent, Ca: 0.0001 to 0.0100 percent, Mg: 0.0001 to 0.0100 percent, Zr: 0.0001 to 0.0100 percent, Bi: 0.0001 to 0.0100 percent, Hf: 0.0001 to 0.0100 percent, As: 0.0001 to 0.1000 percent, and REM: 0 to 0.0100 percent.Hot-rolled thin steel sheets under Reputation 1 or 2, where hot-rolled thin steel sheets are surface-treated thin steel sheets which are provided with a surface-treated layer on at least one side; 5. Parts which include hot-rolled thin steel sheets under Reputation 1 or 2;