Hot-rolled steel plate

Abstract

Provided is a hot-rolled steel sheet which has a prescribed chemical composition, and in which the steel structure has, in terms of area, a total of 2-30% of martensite and residual austenite and a total of 0-3% of pearlite and cementite, with the remainder being composed of ferrite and bainite, the area of crystal grains having a grain diameter of greater than 12 μm accounts for 50% or less of the observed area, in an aggregate structure measured at a sheet thickness central portion, the maximum polarization density of the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation groups is 8.0 or less, and when the field of observation of the steel structure is segmented at intervals of 2 μm in the sheet thickness direction and the total of martensite and residual austenite is measured for each segmented region, the maximum value among the totals is no more than 3.0 times the average value of the totals.

Description

[Technical Field]

[0001] This invention relates to hot-rolled steel sheets. [Background technology]

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel sheets used is one effective method, and for this reason, the development of high-strength steel sheets is progressing. On the other hand, increasing the strength of steel sheets generally reduces their workability. Therefore, in the development of high-strength steel sheets, it is important to achieve high strength while ensuring a certain level of workability.

[0003] In this regard, for example, Patent Document 1 states that when the standard deviation of the microhardness of any 50 polygonal ferrites having a predetermined chemical composition and a structure containing 80% or more polygonal ferrite by area ratio, and the σHV is defined as the standard deviation of the microhardness of any 50 polygonal ferrites located within ±100 μm from the center plane in the thickness direction, the σHV is 30 or less, and the Ti-containing carbides within the polygonal ferrite grains are 5 × 10 7 pieces / mm 2 The invention describes a high-strength hot-rolled steel sheet characterized by the presence of the above-mentioned Ti-containing carbides, the aspect ratio of which is the ratio of the length of the long side to the length of the short side, being less than 3, and a tensile strength of 540 MPa or more. Furthermore, Patent Document 1 teaches that in a polygonal ferrite-dominant structure having excellent hole-expanding properties, the hole-expanding properties can be significantly improved by utilizing Ti precipitation strengthening to improve strength, reducing the variation in hardness of individual ferrite grains, and reducing the proportion of non-equixaxial Ti-containing carbides with an aspect ratio of 3 or more.

[0004] Patent Document 2 describes a material having a predetermined chemical composition, in which, at a depth of 1 / 4 of the plate thickness from the surface, the area ratio of ferrite is 10-55%, the total area ratio of bainite and martensite is 45-90%, the total area ratio of the ferrite, bainite and martensite is 90% or more, the average grain size is 12.0 μm or less, and the texture measured at the center of the plate thickness is {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the azimuthal group is 8.0 or less, and {211} <011> and {332} <113> A hot-rolled steel sheet is described characterized by having a total extreme density of 10.0 or less and a tensile strength of 950 MPa or more. Furthermore, Patent Document 2 teaches that according to the above configuration, a hot-rolled steel sheet can be provided that is high in strength and has excellent elongation, elongation flangeability and low-temperature toughness. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2017 / 022025 [Patent Document 2] International Publication No. 2019 / 009410 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Hot-rolled steel sheets are manufactured by hot-rolling cast slabs. However, in connection with this hot-rolling process, the microstructure and various mechanical properties may exhibit anisotropy between the rolling direction (L direction) and the width direction perpendicular to it (C direction). Furthermore, if the degree of such anisotropy increases, the hole-expanding ability (stretch flange ability) that responds to deformation operations in the entire circumferential direction may decrease. On the other hand, due to further improvements in workability and customer demands, there is a high need for high-strength hot-rolled steel sheets that, in addition to improved hole-expanding ability, reduce the difference between the L direction and the C direction in other mechanical properties such as uniform elongation and ultimate deformation ability.

[0007] This invention has been made in view of the above circumstances, and its objective is to provide a hot-rolled steel sheet that, despite having high strength, has improved hole-expandability and reduced uniform elongation and the difference between the L-direction and C-direction in ultimate deformation capacity, through a novel configuration. [Means for solving the problem]

[0008] To achieve the above objectives, the inventors focused particularly on the microstructure of hot-rolled steel sheets. As a result, the inventors discovered that by configuring the microstructure of a hot-rolled steel sheet having a predetermined chemical composition to contain martensite and retained austenite in a specific proportion, high strength can be achieved. Furthermore, by limiting the proportion of crystal grains with relatively large grain sizes in the microstructure to a predetermined range and suppressing the development of texture in the microstructure, hole expansion properties can be improved and the difference between the L-direction and C-direction in uniform elongation can be reduced. In addition, by reducing the variation in the thickness direction of martensite and retained austenite contained in the microstructure, the difference between the L-direction and C-direction in ultimate deformability can be reduced, thus completing the present invention.

[0009] The present invention, which has achieved the above objectives, is as follows. (1) In mass%, C: 0.025~0.080%, Si: 0.001~2.000%, Mn: 1.000~2.000%, Ti: 0.080~0.200%, Al: 0.200~1.000%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, 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%, Zn: 0~1.00%, B: 0~0.0050%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr: 0~0.0100%, Bi: 0~0.0100%, Hf: 0~0.0100%, As: 0~0.1000%, REM: 0~0.0100%, and The remainder has a chemical composition consisting of Fe and impurities. The microstructure in the rolling direction cross-section within the observation area centered on the 1 / 4 thickness from the surface is, in area %, Total of martensite and retained austenite: 2-30% Total of perlite and cementite: 0-3%, and The remainder consists of ferrite and bainite. The proportion of the area occupied by crystal grains with a particle size of 12 μm or more relative to the observed area is 50% or less. In the texture measured at the center of the plate thickness, {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the azimuthal group is 8.0 or less. A hot-rolled steel sheet characterized in that, 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 value of the sum is 3.0 times or less the average value of the sum. (2) The hot-rolled steel sheet according to (1) above, characterized in that the chemical composition contains Si: 0.001 to 0.500% by mass, and the number density of recesses on the surface with a depth of 5 μm or more is 1.0 recess / mm or less per surface length. (3) The chemical composition is, in mass%, Cr: 0.001~1.00%, Mo: 0.001~1.00%, Cu: 0.001~1.00%, Ni: 0.001~1.00%, Co: 0.001~1.00%, Nb: 0.001~0.200%, V: 0.001~1.00%, W: 0.001~1.00%, Sn: 0.001~1.00%, Sb: 0.001~0.50%, Zn: 0.001~1.00%, B: 0.0001~0.0050%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Zr: 0.0001~0.0100%, Bi: 0.0001~0.0100%, Hf: 0.0001~0.0100%, As: 0.0001~0.1000%, and REM: 0~0.0100% The hot-rolled steel sheet according to (1) or (2) above, characterized in that it includes at least one of the above. (4) A 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 component characterized by including a hot-rolled steel sheet as described in any one of the above items (1) to (4). [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a hot-rolled steel sheet that, despite having high strength, has improved hole-expanding properties and reduced uniform elongation and the difference between the L-direction and C-direction in ultimate deformation capacity. [Modes for carrying out the invention]

[0011] <Hot rolled steel plate> The hot-rolled steel sheet according to an embodiment of the present invention is, by mass%, C: 0.025~0.080%, Si: 0.001~2.000%, Mn: 1.000~2.000%, Ti: 0.080~0.200%, Al: 0.200~1.000%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, 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%, Zn: 0~1.00%, B: 0~0.0050%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr: 0~0.0100%, Bi: 0~0.0100%, Hf: 0~0.0100%, As: 0~0.1000%, REM: 0~0.0100%, and The remainder has a chemical composition consisting of Fe and impurities. The microstructure in the rolling direction cross-section within the observation area centered on the 1 / 4 thickness from the surface is, in area %, Total of martensite and retained austenite: 2-30% Total of perlite and cementite: 0-3%, and The remainder consists of ferrite and bainite. The proportion of the area occupied by crystal grains with a particle size of 12 μm or more relative to the observed area is 50% or less. In the texture measured at the center of the plate thickness, {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the azimuthal group is 8.0 or less. The observation field of the steel microstructure is divided into 2 μm intervals in the plate thickness direction, and when the total amount of martensite and retained austenite in each divided region is measured, the maximum value of the total is 3.0 times or less the average value of the total.

[0012] As mentioned earlier, it is known that during hot rolling in steel sheet manufacturing, the microstructure and various mechanical properties may exhibit anisotropy between the rolling direction (L direction) and the width direction (C direction) perpendicular to it. When the degree of such anisotropy increases, the hole-expanding ability 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 also simply referred to as the LC difference) also increases. By improving hole-expanding ability and reducing this 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 increased strength of steel sheets and improvements in these properties. Therefore, the inventors of this invention focused particularly on the microstructure of hot-rolled steel sheets in order to achieve both increased strength of steel sheets and improvements in these properties.

[0013] First, the inventors controlled the carbon 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 explained in detail later in relation to the chemical composition and manufacturing method of the hot-rolled steel sheet. On the other hand, despite the relatively low carbon content, the inventors found that high strength, such as a tensile strength of 780 MPa or more, can be achieved by utilizing precipitation strengthening by Ti carbides and by configuring the steel structure of the hot-rolled steel sheet to contain martensite and retained austenite in a specific proportion in total, more specifically, by configuring it to contain 2-30% of martensite and retained austenite in total by area percent.

[0014] Next, the inventors further investigated the refinement and texture of the steel microstructure in order to improve hole expansion properties and reduce LC differences in other mechanical properties. As a result, the inventors limited the proportion of crystal grains with relatively large grain sizes in the steel microstructure to a predetermined range and suppressed the development of texture in the steel microstructure. More specifically, they limited the proportion of the area occupied by crystal grains with a grain size of 12 μm or more to 50% or less of the observation area in the steel microstructure, and in the texture measured at the center of the thickness of the hot-rolled steel sheet, {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> We found that controlling the maximum polar density of orientation groups to 8.0 or less improves hole expansion properties and reduces the LC difference in uniform elongation. Although we do not intend to be bound by any particular theory, we believe that by limiting the proportion of the area occupied by grains with a particle size of 12 μm or more to the observation area of ​​the steel structure to 50% or less, the steel structure becomes finer and more uniform, which reduces the hardness difference in the steel structure compared to the case where the proportion of the area occupied by grains with a particle size of 12 μm or more to the observation area exceeds 50%, and that this reduction in hardness difference leads to improved hole expansion properties.

[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 sheet thickness and the surface layer of the sheet thickness, which is directly affected by rolling. However, in hot-rolled steel sheets, due to friction between the rolls and the steel sheet during hot rolling, shear deformation occurs in opposite directions on the front and back of the sheet, so the texture develops with symmetrical orientations on the front and back of the sheet thickness. Therefore, the effects of the texture on the mechanical properties cancel each other out on the front and back, and as a result, it is considered that the relationship between the texture and mechanical properties is better in the center of the sheet thickness than in the surface layer of the sheet thickness. For this reason, in the hot-rolled steel sheet according to the embodiment of the present invention, by controlling the texture in the center of the sheet thickness, improved hole expansion and reduction of the LC difference in uniform elongation are achieved. Here, the crystal orientation of the rolled sheet is usually indicated by {hkl} or (hkl) for the crystal orientation perpendicular to the rolling plane, and by the crystal orientation parallel to the rolling direction. <uvw>Alternatively, display with [uvw]. {hkl} and <uvw>{110} is a general term for equivalent planes and orientations, while (hkl) and [uvw] refer to individual crystal planes. In the hot-rolled steel sheets according to the embodiments of the present invention, since the body-centered cubic structure (bcc structure) is mainly targeted, for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1), and (0-1-1) are equivalent and indistinguishable. In the embodiments of the present invention, these orientations are collectively represented as {110}.

[0016] The dominant orientation that develops in the texture of the center of the thickness of a hot-rolled steel sheet is {100}. <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> There are several orientation groups. If even one of these orientation groups develops, the in-plane anisotropy in the mechanical properties of the hot-rolled steel sheet increases, and the hole-expanding ability corresponding to deformation operations in the circumferential direction decreases particularly significantly. Therefore, in order to improve hole-expanding ability and reduce in-plane anisotropy in other mechanical properties, it is important to suppress the development of all these orientation groups and randomize the texture. Accordingly, in the hot-rolled steel sheet according to the embodiment of the present invention, the polar density of each of these orientation groups is calculated, and the maximum value of these polar densities, i.e., the maximum polar density, is controlled to 8.0 or less, thereby sufficiently suppressing the development of the texture and making the texture more random. As a result, it is possible to further improve hole-expanding ability in combination with refinement of the steel structure, and to significantly reduce the LC difference of uniform elongation. Here, polar density refers to the ratio of the degree of accumulation in a specific orientation of the test material to that of a standard sample that does not have accumulation in a specific orientation.

[0017] However, while refining the steel microstructure and suppressing the development of texture can improve hole expansion properties and reduce the LC difference of uniform elongation, it was not always possible to sufficiently reduce the LC difference of the ultimate deformability. Therefore, the inventors conducted further studies to reduce the LC difference of the ultimate deformability. As a result, the inventors found that by reducing the variation in the thickness direction of martensite and retained austenite contained in the steel microstructure, more specifically by dividing the observation field of the steel microstructure into 2 μm intervals in the thickness direction and measuring the total of martensite and retained austenite in each divided region, the LC difference of the ultimate deformability can be reduced by controlling the maximum value of the total to 3.0 times or less the average value of the total.

[0018] In the hot-rolled steel sheet according to the embodiment of the present invention, as described above, in order to achieve high strength, the total amount of martensite and retained austenite is configured to be 2 to 30% by area percent. In connection with this, in order to improve the hardenability of the steel, it is necessary to include a predetermined amount of Mn in the steel, and in the hot-rolled steel sheet according to the embodiment of the present invention, the Mn content is 1.000 mass% or more. However, in this case, if there are regions in the steel where Mn is segregated during the manufacturing process, martensite and retained austenite are likely to be generated in such regions during cooling after hot rolling. As a result, it is thought that regions with a localized high concentration of martensite and / or retained austenite are formed in the final hot-rolled steel sheet, causing a difference in LC of the ultimate deformability. Therefore, the inventors have found that, as will be explained in detail later in relation to the manufacturing method of the hot-rolled steel sheet, by mainly carrying out the casting process and the hot-rolling process under appropriate conditions, Mn segregation in the steel can be sufficiently suppressed, thereby reducing the variation in the thickness direction of martensite and retained austenite generated in the subsequent cooling process to within the 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, making it possible to reliably and sufficiently reduce the LC difference of the ultimate deformability.

[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 microstructure is configured to contain a total of 2-30% martensite and retained austenite, and the proportion of the area occupied by crystal grains with a particle size of 12 μm or more in the steel microstructure is limited to a predetermined range, and {100} in the center of the sheet thickness <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the orientation group is controlled to 8.0 or less. Therefore, it is extremely difficult to maintain the structure of the steel microstructure in this controlled manner while further controlling the variation in the thickness direction of martensite and retained austenite contained in the steel microstructure to within a desired range. In contrast, as will be explained in detail later in relation to the manufacturing method of hot-rolled steel sheets, in the embodiment of the present invention, the C content in the steel is limited to 0.080% or less to form a δ phase during solidification in the casting process, thereby promoting the diffusion of Mn. Furthermore, the diffusion of Mn into the steel is further promoted by repeatedly introducing strain and recrystallizing during the subsequent rough rolling. This makes it possible to realize a steel microstructure in which the variation in the thickness direction of martensite and retained austenite is reduced to within the above predetermined range. As a result, according to the embodiment of the present invention, despite having high strength, for example, a tensile strength of 780 MPa or more, it is possible to improve hole expansion properties and reduce the difference between the L direction and C direction in uniform elongation and ultimate deformation capacity. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is therefore particularly useful in the automotive sector where both of these properties are required.

[0020] The following describes in more detail the hot-rolled steel sheet according to the embodiment of the present invention. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0021] [C:0.025~0.080%] Carbon (C) is an essential element for ensuring the strength of steel sheets. To fully obtain this effect, the C content should be 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, if the C content is excessive, Mn segregation during the continuous casting process may not be sufficiently suppressed, and as a result, the variation in martensite and retained austenite in the thickness direction may not be reduced to the desired range. Therefore, the C content should be 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: 0.001~2.000%] Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to improved strength and formability. To obtain these effects, the Si content should be 0.001% or more. The Si content may also 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 initiation temperature (Ar3 point). Therefore, if Si is included in excess, the temperature at which ferrite is formed becomes excessively high, which may lead to the formation of coarse ferrite grains and a decrease in hole expansion properties. For this reason, the Si content should be 2.000% or less. The Si content may also 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 explained in more detail later in relation to the manufacturing method of hot-rolled steel sheets, by limiting the Si content to 0.500% or less, and by performing descaling under appropriate conditions before finish rolling, it is possible to obtain hot-rolled steel sheets with excellent surface properties and to more significantly reduce the LC difference in ultimate deformability.

[0023] [Mn: 1.000~2.000%] Manganese (Mn) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheets. To fully obtain this effect, the Mn content should be 1.000% or more. The Mn content may also 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 martensite formation, reducing hole-expanding properties. Therefore, the Mn content should be 2.000% or less. The Mn content may also be 1.900% or less, 1.800% or less, 1.700% or less, or 1.600% or less.

[0024] [Ti: 0.080~0.200%] Titanium (Ti) 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 obtain these effects, the Ti content should be 0.080% or higher. The Ti content may be 0.090% or higher, 0.100% or higher, 0.110% or higher, or 0.120% or higher. On the other hand, excessive Ti content can lead to coarse Ti carbides, which not only prevents the strength-improving effect through precipitation strengthening from being fully realized, but can also result in reduced hole expansion and a decrease in the LC difference in uniform elongation. Therefore, the Ti content should be 0.200% or lower. The Ti content may be 0.180% or lower, 0.160% or lower, 0.150% or lower, or 0.140% or lower.

[0025] [Al:0.200~1.000%] Aluminum (Al) acts as a deoxidizing agent and stabilizes ferrite. If the Al content is too low, this effect may not be fully achieved, and / or the desired steel structure may not be obtained. Therefore, the Al content should be 0.200% or more. The Al content may also 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 raises the ferrite transformation initiation temperature (Ar3 point). Therefore, if Al is included in excess, the temperature at which ferrite is formed becomes excessively high, which may lead to the formation of coarse ferrite grains and a decrease in hole-expanding properties. Therefore, the Al content should be 1.000% or less. The Al content may also be 0.800% or less, 0.600% or less, or 0.500% or less.

[0026] [P:0.050% or less] Phosphorus (P) 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 should be 0.050% or less. Preferably, the P content is 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but since extremely low P content increases the cost of removing P, from an economic standpoint, it is preferable to set the lower limit to 0.0001%.

[0027] [S:0.0100% or less] S (sulfur) is an element present as an impurity, forming MnS in steel and degrading its toughness and hole-expanding properties. Therefore, the sulfur content should be 0.0100% or less to avoid significant degradation of toughness and hole-expanding properties. Preferably, the sulfur content is 0.0050% or less, 0.0040% or less, or 0.0030% or less. The sulfur content may be 0%, but since extremely low sulfur content increases desulfurization costs, it is preferable from an economic standpoint to set the lower limit at 0.0001%.

[0028] [N:0.0100% or less] Nitrogen (N) is an element present as an impurity, and a high N content can lead to the formation of coarse nitrides in the steel, degrading its bendability and hole-expanding properties. Therefore, the N content should be 0.0100% or less. Preferably, the N content is 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but since extremely low N content increases the cost of nitrogen removal, from an economic standpoint, it is preferable to set the lower limit at 0.0001%.

[0029] [O:0.0100% or less] Oxygen (O) is an element present as an impurity, and a high O content can form coarse oxides in the steel, degrading its bendability and hole-expanding properties. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 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, it is preferable that the lower limit be 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 optionally contain at least one of the following elements in place of a portion of the remaining Fe.

[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 to obtain such effects, at least one of these elements may be included in the hot-rolled steel sheet as needed. However, excessive content of these elements may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the content of Cr, Mo, Cu, Ni, Co, V, W, and Sn should be 1.00% or less, and may be 0.60% or less, 0.50% or less, or 0.30% or less, respectively. Similarly, the Nb content should be 0.200% or less, and may be 0.100% or less, or 0.060% or less. Similarly, the Sb content may be 0.50% or less, or 0.30% or less, or 0.10% or less. Similarly, the B content may be 0.0050% or less, or 0.0020%. For the lower limits of these elements, for example, the Cr, Mo, Cu, Ni, Co, V, W, Sn, and Sb content 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 effective element 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 included in the hot-rolled steel sheet as needed. However, excessive inclusion of these elements may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the Zn content should be 1.00% or less, and may be 0.50% or less or 0.20% or less. Similarly, the Ca, Mg, Zr, Bi, Hf, and REM content should be 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0030% or less, respectively. For 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 content may be 0.0001% or more, or 0.0005% or more, respectively.

[0033] [As: 0~0.1000%] Arsenic (As) is an effective element for improving corrosion resistance. While the As content may be 0%, to obtain this effect, it is preferable that the As content be 0.0001% or more, and may be 0.0010% or more, 0.0050% or more, or 0.0100% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, the As content should be 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 an embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of hot-rolled steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

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

[0036] [Steel structure] [Total of martensite and retained austenite: 2-30%, total of pearlite and cementite: 0-3%, and remainder: ferrite and bainite] In the hot-rolled steel sheet according to the embodiment of the present invention, the microstructure in the rolling direction cross section of the hot-rolled steel sheet, within an observation range centered on the 1 / 4 thickness from the surface, is as follows in area percent: martensite and retained austenite total: 2-30%, pearlite and cementite total: 0-3%, and the remainder: ferrite and bainite.

[0037] In embodiments of the present invention, martensite encompasses both as-quenched martensite (fresh martensite) and tempered martensite. Furthermore, retained austenite transforms into martensite through work-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 obtain the desired high strength, the total area ratio of martensite and retained austenite should be 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 included in excess, while the strength increases, the hole-expanding properties decrease. Therefore, the total area ratio of martensite and retained austenite should be 30% or less, and may be 28% or less, 25% or less, 22% or less, or 20% or less.

[0038] Perlite contains hard, coarse cementite, which can act as a fracture initiation point during plastic deformation, potentially reducing the hole-expanding properties of steel sheets. Therefore, its content, combined with cementite, should be 0-3% by area percentage, or it may be 0-2% or 0-1%.

[0039] In the hot-rolled steel sheet according to the embodiment of the present invention, the remaining part of the steel structure is composed of ferrite and bainite. Ferrite has excellent ductility and contributes to improved elongation. The bainite may be upper bainite having carbides between the laths, lower bainite having carbides within the laths, bainite ferrite without carbides, granular bainite ferrite in which the lath boundaries of bainite have been restored and become indistinct, or a mixed structure thereof.

[0040] [Identification of steel structure and calculation of area ratio] The steel structure is identified and its area fraction is calculated using secondary electron images captured with a FE-SEM. First, a sample is taken from the thickness cross section of the steel sheet parallel to the rolling direction, at the center of the width direction, and the observation surface is used. The observation surface is mechanically polished to a mirror finish, and then etched using Nital solution. Next, in one or more observation fields in the range of 1 / 8 to 3 / 8 thickness centered on 1 / 4 thickness from the surface of the steel sheet on the observation surface, a total of 2.0 × 10⁻⁶ images are taken. -9 m 2 Secondary electron images are taken for the above-mentioned areas. From the obtained secondary electron images, the area percentages of martensite and retained austenite combined, pearlite (sum of pearlite and cementite), and the remainder, i.e., ferrite and bainite, are measured. First, areas with high brightness and where the underlying structure is not revealed by etching are identified as martensite and retained austenite. Next, areas where cementite is precipitated in a lamellar pattern are identified as pearlite (sum of pearlite and cementite). The remainder of the structure other than those described above is identified as ferrite and bainite. For reference, areas with low brightness and where no underlying structure is observed can be identified as ferrite, and areas that do not fall into any of the above categories can be identified as bainite. The area percentage of each structure identified in this way is calculated using the point counting method.

[0041] [Percentage of the area occupied by crystal grains with a particle size greater than 12 μm relative to the observed area: 50% or less] In embodiments of the present invention, the ratio of the area occupied by grains with a particle size exceeding 12 μm to the observation area of ​​the steel microstructure is 50% or less. In the present invention, the particle size of a grain refers to the equivalent diameter of a circle of a grain when a region enclosed by a boundary with an orientation difference of 15° or more is defined as a grain. By limiting the ratio of the area occupied by grains with a particle size exceeding 12 μm to the observation area of ​​the steel microstructure to 50% or less, the steel microstructure can be made finer and more uniform, thereby reducing the hardness difference in the steel microstructure compared to cases where these requirements are not met, and improving hole-expandability due to this reduction in hardness difference is possible. From the viewpoint of improving hole-expandability, a lower ratio of the above area is preferable, and may be, for example, 40% or less, 35% or less, or 30% or less. The lower limit may be 0%, and is not particularly limited, but for example, the ratio of the above area 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 particle size of 12 μm or larger to the observed area] The ratio of the area occupied by crystal grains with a particle size of 12 μm or more to the observation area is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from a steel plate so that the cross-section of the plate thickness parallel to the rolling direction and perpendicular to the plate surface becomes the observation surface. Next, at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, an EBSD analysis is performed at a measurement interval of 0.2 μm in a region of 200 μm in the rolling direction of the steel plate and 100 μm in the direction normal to the rolling surface to obtain crystal orientation information. Here, the EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points / second. Next, for the obtained crystal orientation information, regions enclosed by boundaries with an orientation difference of 15° or more are defined as crystal grains, and the ratio of crystal grains with an equivalent circular diameter exceeding 12 μm to the observation area is determined. The crystal grains defined as described above can be calculated using the software "OIM Analysis®" included with the EBSD analyzer.

[0043] [Texture in {100} measured at the center of the plate thickness] <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> Maximum polar density of azimuthal groups: 8.0 or less] In an embodiment of the present invention, the texture measured at the center of the thickness of the hot-rolled steel sheet is {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of orientation groups is 8.0 or less. By controlling the maximum polar density of these orientation groups 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 hole-expanding properties and significantly reduce the LC difference of uniform elongation. From the viewpoint of improving hole-expanding properties and reducing the LC difference of uniform elongation, the more randomized the texture, the better, i.e., {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the azimuthal group is preferably as low as possible, for example, it may be 7.0 or less, 6.5 or less, or 6.0 or less. The lower limit is not particularly limited, but for example, the maximum polar density of these azimuthal 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] [{100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> [Measurement of the maximum polar density of the azimuthal group] {100} in the texture of the center of the plate thickness <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the orientation group is measured by EBSD. More specifically, first, a sample is taken from a steel plate so that the thickness cross section parallel to the rolling direction and perpendicular to the plate surface becomes the observation surface. Then, EBSD analysis is performed at 1 μm measurement intervals on a rectangular region of the steel plate centered at a depth of 1 / 2 the plate thickness from the surface, with a length of 1000 μm in the rolling direction and 100 μm in the direction normal to the rolling surface, to obtain crystal orientation information for this rectangular region. The EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points / second. Next, the Orientation Distribution Function (ODF) of this rectangular region is calculated from the crystal orientation information of this rectangular region using the software "OIM Analysis®" attached to the EBSD analysis device. As a method for calculating the ODF, Harmonic Series Expansion (spherical harmonic function method) was used with an expansion order of 16. Furthermore, calculations were performed that took orthotropic symmetry into account. This resulted in {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The extreme density for each crystal orientation can be determined, and the maximum value of these extreme densities is "{100} in the texture measured at the center of the plate thickness". <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> This is determined as the "maximum polar density of the orientation group." Note that the crystal orientation here represents the crystal orientation perpendicular to the surface of the steel plate, so when performing the analysis, it is necessary to consider the orientation of the sample set used for measurement and align the measurement coordinate system of the crystal orientation data with the sample coordinate system.

[0045] [The maximum value of the sum of martensite and retained austenite in the thickness direction of the plate is 3.0 times or less the average value of that sum.] In embodiments of the present invention, when the observation field of the steel microstructure is divided into 2 μm intervals in the thickness direction and the total amount of martensite and retained austenite in each divided region is measured, the maximum value of the total is 3.0 times or less the average value of the total. By controlling the maximum value of the total amount of martensite and retained austenite in each divided region in the thickness direction to 3.0 times or less the average value of the total, the variation of martensite and retained austenite in the thickness direction is suppressed, that is, the regions in which these structures are locally abundant are reduced. As a result, it becomes possible to reliably and sufficiently reduce the LC difference of the ultimate deformability. From the viewpoint of reducing the LC difference of the ultimate deformability, it is preferable that the variation of martensite and retained austenite in the thickness direction be small. Therefore, for example, the maximum value of the total amount of martensite and retained austenite in each divided region in the 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 the average value of the total. The lower limit is not particularly limited, but for example, the maximum sum of martensite and retained austenite in each divided region in the thickness direction may be 1.0 times or more, or 1.1 times or more, the average value of the sum.

[0046] [Measurement of the ratio between the maximum value of the total amount of martensite and retained austenite in the thickness direction of the plate and the average value of that total] The ratio of the maximum total amount of martensite and retained austenite in the thickness direction to the average value of that total is determined as follows: First, the total 2.0 × 10⁻¹⁰, as explained regarding the identification of the steel structure and the calculation of the area ratio. -9 m 2 Within the above area (observation field), the observation field is divided into 2 μm intervals in the plate thickness direction, and the total area ratio of martensite and retained austenite in each divided region is calculated. Next, the highest value among the calculated total area ratios of martensite and retained austenite in each divided region is divided by the average value of all calculated total area ratios of martensite and retained austenite, and this value is determined as the ratio of the maximum value of the total of martensite and retained austenite in the plate thickness direction to the average value of that total.

[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 with a depth exceeding 5 μm on the surface of the hot-rolled steel sheet is controlled to 1.0 recess / 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 explained in detail later in relation to the manufacturing method of the hot-rolled steel sheet, by appropriately controlling descaling before finish rolling, it is possible to obtain a hot-rolled steel sheet with excellent surface properties, more specifically, a hot-rolled steel sheet in which the number density of recesses with a depth exceeding 5 μm on the surface is controlled to 1.0 recess / mm or less per surface length. Since recesses with a depth exceeding 5 μm can act as stress concentration areas, controlling the number density of such recesses to 1.0 recess / mm or less and achieving a flatter surface makes it possible to more significantly reduce the LC difference of the ultimate deformability. From the viewpoint of reducing the LC difference in ultimate deformation capacity, it is preferable that the number density of recesses with a depth exceeding 5 μm on the surface of the hot-rolled steel sheet be as small as possible, for example, 0.8 recesses / mm or less, 0.5 recesses / mm or less, or 0.3 recesses / mm or less. The lower limit may be 0 recesses / mm and is not particularly limited, but for example, the number density of recesses with a depth exceeding 5 μm on the surface of the hot-rolled steel sheet may be 0.1 recesses / mm or more.

[0048] [Measurement of the number density of recesses on the surface of hot-rolled steel sheets with a depth exceeding 5 μm] The number density of recesses with a depth exceeding 5 μm on the surface of a hot-rolled steel sheet is measured as follows. First, a sample is taken from a thickness cross section parallel to the rolling direction of the hot-rolled steel sheet, with the thickness cross section at the center in the width direction serving as the observation surface. After mechanically polishing the observation surface to a mirror finish, a backscattered electron image of the steel sheet surface (or the plating / steel sheet interface if the hot-rolled steel sheet has a plating layer) is captured 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 is determined from the height profile using the least squares method, and the region where the surface height deviates from the center line by more than 5 μm to the negative side is defined as a "recess with a depth exceeding 5 μm". A similar analysis is performed so that the measurement range in the X direction (rolling direction) exceeds a total of 1 mm. For example, if the size in the X direction (rolling direction) of one field of view is 200 μm, the above analysis should be performed at least five times with a different field of view. The number of "recesses with a depth of more than 5 μm" obtained in each field of view should be totaled and converted to a number density per 100 μm of surface length to determine the "number density of recesses with a depth of more than 5 μm present on the surface of the hot-rolled steel sheet." Here, 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 common methods for measuring the height profile of steel sheets, but if the hot-rolled steel sheet has a plating layer, the plating layer must first be dissolved and removed with acid. However, with such methods, there is a concern that not only the plating layer but also the base metal interface will be corroded simultaneously during acid dissolution, altering the original surface irregularities. For this reason, these methods are not recommended.

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

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

[0051] [Mechanical properties] [Tensile strength (TS) and total elongation (EL)] According to the hot-rolled steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength (TS) of 780 MPa or higher, can be achieved. The tensile strength is preferably 800 MPa or higher, 820 MPa or higher, or 840 MPa or higher. According to the hot-rolled steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the specific combination of chemical composition and metal structure described above can improve hole expansion, uniform elongation, and reduce the LC difference in ultimate deformability. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1180 MPa or less, 1080 MPa or less, or 1000 MPa or less. Furthermore, according to the hot-rolled steel sheet according to the embodiment of the present invention, high ductility can be achieved, more specifically, a total elongation (EL) of 10.0% or more, preferably 12.0% or more, more preferably 14.0% or more, or 16.0% or more can be achieved. 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. Tensile strength and total elongation are determined by taking JIS No. 5 test specimens from both the direction in which the longitudinal direction of the test specimen is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and the direction in which it is parallel to the direction perpendicular to the rolling direction (C direction), and performing tensile tests in accordance with JIS Z 2241:2011. In this invention, TS and EL are the values ​​obtained when the tensile test is performed in the C direction. The reason for performing the tensile test in the L direction is the difference in LC (uEL) of uniform elongation. C / uEL L ) and the LC difference in the ultimate deformation ability (ε C / ε L ) are to be evaluated.

[0052] [Hole expansion ratio (λ)] According to the hot-rolled steel sheet having the above chemical composition and metal structure, high hole expansion property, specifically, a hole expansion ratio (λ) of 50% or more can be achieved. The hole expansion ratio is 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. For example, the hole expansion ratio may be 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 Standards.

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

[0054] [LC difference in ultimate deformation ability (ε C / ε L )] According to the hot-rolled steel sheet having the above chemical composition and metal structure, the LC difference in ultimate deformation ability (ε C / ε L A significant reduction in ε can be achieved, for example, 0.70 to 1.30, preferably 0.80 to 1.20, and more preferably 0.90 to 1.10. C / ε L It can be achieved. ε C / ε L This is determined by taking JIS No. 5 test specimens from the direction in which the longitudinal direction of the test specimen is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and from the direction in which it is parallel to the direction perpendicular to the rolling direction (C direction), and performing tensile tests in accordance with JIS Z 2241:2011. More specifically, the ultimate deformation capacity is determined for each of the L-direction test specimen and the C-direction test specimen by comparing the initial plate thickness of the parallel section before the tensile test (t0) and the plate thickness of the fractured section after the tensile test (t f The following formula is used to measure and calculate the value of ( ).

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[0055] As described above, the hot-rolled steel sheet according to the embodiment of the present invention has high strength, for example, a tensile strength of 780 MPa or more, yet it can improve hole expansion properties and achieve uniform elongation and a reduction in the LC difference in ultimate deformation capacity. For this reason, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is therefore particularly useful for use in parts in technical fields where both of these properties are required. In a preferred embodiment, an automobile part, particularly a part selected from the undercarriage, chassis, and wheels of an automobile, is provided, which includes the hot-rolled steel sheet according to the embodiment of the present invention. These automobile parts only need to include the hot-rolled steel sheet according to the embodiment of the present invention in at least a part, and therefore at least a part of these parts will satisfy the characteristics of chemical composition, metal structure, and / or surface properties described above. In parts of the hot-rolled steel sheet that do not come into direct contact with the mold during forming such as press forming, and where the degree of processing is relatively low, the characteristics of the metal structure and / or surface properties do not change particularly before and after forming.

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

[0057] A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention is: (A) A continuous casting process for casting a slab having the chemical composition described above in relation to a hot-rolled steel sheet, such that it satisfies the following formulas (1) to (3):

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[0058] [(A) Continuous casting process] First, a slab having the chemical composition described above in relation to hot-rolled steel sheets is cast in a continuous casting process to satisfy the following equations (1) to (3).

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number

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[0059] When slabs are continuously cast from molten steel, the molten steel is initially liquid, but gradually solidifies from the surface, reaching a certain temperature (i.e., the solidification completion temperature) at which point it completely solidifies. In this regard, equation (1) defines the temperature range in which the liquid and solid phases coexist before the molten steel completely solidifies (the temperature range in a partially solidified state). In this temperature range, as solidification progresses from the surface of the molten steel, alloying elements such as Mn are distributed (discharged) from the solid phase to the liquid phase, resulting in an increase in Mn concentration in the liquid phase at this stage. Such Mn concentration can cause Mn segregation, which can lead to variations in the thickness direction of the martensite and retained austenite generated during the cooling process. More specifically, if a large amount of Mn is distributed from the solid phase to the liquid phase, regions with locally high Mn concentrations will be created. Since martensite and retained austenite are easily formed in such regions, this results in the localized formation of martensite and retained austenite. Therefore, in the temperature range where the liquid and solid phases coexist during continuous casting, it is extremely important to sufficiently suppress the distribution of Mn from the solid phase to the liquid phase. In this regard, the inventors have found that by satisfying equations (1) and (2), and in particular by controlling the value obtained by the left side of equation (1) to less than 1.00, such distribution of Mn can be suppressed, and furthermore, in combination with equation (3), which will be explained later, Mn segregation can be sufficiently suppressed, thereby reducing the variation in the thickness direction of martensite and retained austenite generated in the subsequent cooling process.

[0060] Here, in equation (1), τ=0 means the start of casting, and τ1 is the internal temperature of the slab when the solidification is complete temperature T. δ This represents the time (seconds) it takes to reach this point. Therefore, equation (1) can be understood as the integral of the distribution of Mn from the solid phase to the liquid phase with respect to time. Consequently, the smaller the value on the left side of equation (1), the more the distribution of Mn from the solid phase to the liquid phase is suppressed. From the viewpoint of suppressing variations in the thickness direction of martensite and retained austenite, a smaller value on the left side of equation (1) is preferable, and specifically, it is preferable to be 0.80 or less. f in equations (1) and (2) L M L M δ and D δ Mn This is a value determined by the temperature and the chemical composition of the molten steel, 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) to 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. L ,T δ ,f L M L M δ This can be determined, for example, by performing a phase diagram calculation for the target steel composition using commercially available thermodynamic calculation software "Thermo-Calc 2022b" (Thermo-Calc Software Inc.). The phases to be calculated are LIQUID and BCC_A2, and the equilibrium calculation is performed in one-axis mode with temperature as the variable. The calculation temperature range only needs to include the temperature range where the number of moles in the liquid phase is 0 to 1, for example, the range of 1400 to 1600°C is sufficient. The number of step divisions is 100, and the step method is set to Normal. The pressure is 100,000 Pa, the system size is 1 Mol, and global minimization is enabled. After the calculation, the values ​​output with the Mn concentration in the LIQUID phase and the Mn concentration in the BCC_A2 phase as temperature variables are used to determine the M at each temperature. L M δ The volumes of the LIQUID phase and the BCC_A2 phase are output as temperature variables, and the value obtained by dividing the volume of the LIQUID phase by the sum of the volumes of the LIQUID phase and the BCC_A2 phase is the f value at each temperature. L The number of moles in the LIQUID phase is output, and the lowest temperature at which the number of moles becomes 1 is set to T. L Let T be the highest temperature at which the number of Mol becomes 0. δ Let's assume that D δ Mn The following formula (4) shall be used to calculate it.

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[0061] On the other hand, equation (3) defines the diffusion of Mn after the molten steel has completely solidified. After the molten steel has completely solidified, in order to suppress Mn segregation, it is important to promote the diffusion of Mn in the solid phase and distribute Mn uniformly in the slab. Therefore, the inventors focused on the δ phase, in which Mn has a fast diffusion rate. More specifically, the inventors found that by limiting the C content in the steel to 0.080% or less so that it becomes a single δ phase during solidification (δ solidification), and then satisfying equation (3), Mn segregation can be sufficiently suppressed. 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 the single δ phase state. As a result, the diffusion of Mn cannot be sufficiently promoted. This is because when the δ and γ phases are formed simultaneously, Mn distribution occurs between these two phases, and instead of homogenizing the Mn distribution, it promotes the localization of Mn. Therefore, in the continuous casting process, in addition to satisfying equations (1) to (3), it is important to control the C content in the steel to 0.080% or less. Here, equation (3) is expressed as the internal temperature of the slab being equal to the solidification completion temperature T δ This can be understood as the integration of Mn diffusion with respect to time from time τ1, when the temperature reached 0, to time τ2, when the temperature reached 1673K. Therefore, the larger the value on the right-hand side of equation (3), the more Mn diffusion is promoted. In this manufacturing method, by controlling the value on the right-hand side of equation (3) to greater than 3.0, preferably 4.0 or greater, the diffusion of Mn in the δ phase is sufficiently promoted to suppress Mn segregation, thereby reliably reducing the variation in the thickness direction of martensite and retained austenite generated in the subsequent cooling process. Similar to the case of equation (1), the value calculated by the right-hand side of equation (3) can be controlled to a desired range by appropriately selecting the chemical composition of the molten steel and the temperature history during continuous casting.

[0062] [(B) Hot rolling process] The cast slabs are heated and then subjected to rough rolling and finish rolling in the subsequent hot rolling process, and the heating, rough rolling, and finish rolling of the slabs 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 rolling processes in which the steel sheet temperature is 1000°C or higher and the reduction ratio exceeds 20%, and (B3) The finish rolling is carried out using a tandem rolling mill consisting of at least five rolling stands, the entry temperature before the last two passes is 950°C or higher, the exit temperature of the final pass is 900°C or higher, the reduction ratio of each rolling pass in the last three passes is 25% or less, the time between each rolling pass in the last three passes is 0.2 to 1.0 seconds, the cumulative reduction ratio of the last 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℃ or higher] The slabs used contain a relatively large amount of alloying elements, and slabs after continuous casting, in particular, contain coarse Ti carbides. Therefore, it is necessary to dissolve the alloying elements in the slab, and in particular, it is necessary to sufficiently dissolve Ti. If Ti is not sufficiently dissolved during slab heating, it becomes difficult to improve the strength of the steel by precipitation strengthening through the fine precipitation of Ti as carbides in the steel during the cooling process after the hot rolling process. Therefore, in order to sufficiently dissolve Ti, the heating temperature of the slab needs to be 1200°C or higher. There is no particular upper limit, but if the heating temperature of the slab is too high, the yield will decrease due to scale-off. For this reason, it is preferable to heat the slab at 1300°C or lower.

[0064] [(B2) Rough rolling: Rolling at a steel plate temperature of 1000°C or higher and a reduction ratio of more than 20% for three or more times] In rough rolling, by performing rolling at a relatively high temperature of 1000°C or higher and a reduction ratio of more than 20% three or more times, rolling and recrystallization can be repeated, 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 martensite and retained austenite contained in the microstructure of the final hot-rolled steel sheet can be reduced. More specifically, when the observation field of the microstructure is divided into 2 μm intervals in the thickness direction and the sum of martensite and retained austenite in each divided region is measured, it is possible to control the maximum value of the sum to be 3.0 times or less the average value of the sum. If rolling at a steel sheet temperature of 1000°C or higher and a reduction ratio of more than 20% is performed two or fewer times, it is not possible to control the maximum value of the sum of martensite and retained austenite in each divided region in the thickness direction to be 3.0 times or less the average value of the sum, and as a result, the LC difference of the ultimate deformability cannot be sufficiently reduced. There is no particular upper limit on the number of rolling cycles, however, for example, rolling at a steel plate temperature of 1000°C or higher and a reduction ratio of more than 20% may be limited to 10 cycles or less, or 8 cycles or less.

[0065] [Descaling] In a preferred embodiment of this manufacturing method, descaling is performed under predetermined conditions immediately after rough rolling and 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 such conditions, it is possible to obtain a hot-rolled steel sheet with excellent surface properties, more specifically, a hot-rolled steel sheet in which the number density of recesses with a depth of 5 μm or more on the surface is controlled to 1.0 recess / mm or less per surface length. Since recesses with a depth of 5 μm or more can act as stress concentration areas, controlling the number density of such recesses to 1.0 recess / mm or less and achieving a flatter surface makes it possible to more significantly reduce the LC difference in ultimate deformability.

[0066] More specifically, in steel sheets containing a relatively high amount of Si, Si oxide forms at the interface between the Fe oxide (also called scale) generated on the surface of the steel sheet during the hot rolling process. Since this Si oxide firmly adheres the Fe oxide to the steel sheet, the scale may not be sufficiently removed even by subsequent descaling using high-pressure water. If the scale that has not been sufficiently removed is pressed into the surface of the steel sheet during subsequent finish rolling, irregularities will occur on the surface of the steel sheet, making it impossible to control the number density of recesses with a depth of 5 μm or more to 1.0 recess / mm or less per surface length. Therefore, in a preferred embodiment of this manufacturing method, first, the Si content in the steel is limited to 0.500% or less to suppress the formation of Si oxide at the interface between the scale and the steel sheet, preventing the scale and the steel sheet from adhering firmly. Next, by performing descaling at a high temperature of 1000°C or higher and with a relatively high water pressure of 8 MPa or higher, it becomes possible to sufficiently remove the scale from the surface of the steel sheet. Performing this type of descaling immediately before finish rolling, i.e., within 5.0 seconds before the start of finish rolling, can suppress scale growth between the time of descaling and the start of finish rolling, thereby reliably preventing scale from being pressed into the steel sheet surface during the subsequent finish rolling. If any one of the following requirements is not met—steel sheet temperature of 1000°C or higher, water pressure of 8 MPa or higher, and descaling within 5.0 seconds before the start of finish rolling—scale removal may be insufficient, and the desired surface properties may not be achieved. 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) Finishing Rolling] After rough rolling, or after rough rolling plus the above-mentioned predetermined descaling, the steel sheet is subjected to finish rolling. Specifically, this finish rolling is carried out using a tandem rolling mill consisting of at least five rolling stands, under the following conditions: the entry temperature before the last two passes is 950°C or higher, the exit temperature of the final pass is 900°C or higher, the reduction ratio of each rolling pass in the last three passes is 25% or less, the time between each rolling pass in the last three passes is 0.2 to 1.0 seconds, the cumulative reduction ratio of the last 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. By carrying out finish rolling under these conditions, recrystallization can be allowed to proceed appropriately to prevent strain accumulation, thereby suppressing the accumulation of crystal orientations in a specific direction, i.e., in the texture measured at the center of the thickness of the hot-rolled steel sheet {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> This makes it possible to reliably control the maximum polar density of the orientation group to 8.0 or less. In addition, by performing this type of finish rolling, excessive growth of recrystallized grains can be suppressed, making it possible to reliably control the proportion of the area occupied by grains with a particle size of 12 μm or more relative to the observation area of ​​the steel microstructure to 50% or less. As a result, it becomes possible to improve hole expansion properties and reduce the LC difference in uniform elongation.

[0068] For example, if the entry temperature before the last two passes is less than 950°C, or if the exit temperature of the final pass is less than 900°C, recrystallization will not proceed sufficiently, leading to strain accumulation, and it may not be possible to control the maximum polar density of the orientation group to 8.0 or less. Also, if the reduction ratio exceeds 25% in any one of the rolling passes in the last three passes, excessive strain may be introduced, similarly making it impossible to control the maximum polar density of the orientation group to 8.0 or less. Furthermore, if the time between each rolling pass in the last three passes is less than 0.2 seconds, or if the time from the final pass to the start of cooling is less than 1.0 seconds, recrystallization will not proceed sufficiently, leading to strain accumulation, and similarly making it impossible to control the maximum polar 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 if the time from the final pass to the start of cooling exceeds 3.0 seconds, the recrystallized grains may grow excessively, making it impossible to control the ratio of the area occupied by grains with a particle size of 12 μm or more to the observation area of ​​the steel structure to 50% or less. Also, if the cumulative reduction ratio of the final three passes is less than 35%, recrystallization does not proceed, and the grains do not refine sufficiently, making it impossible to control the ratio of the area occupied by grains with a particle size of 12 μm or more to the observation area of ​​the steel structure to 50% or less. Furthermore, if the cumulative reduction ratio of the final three passes exceeds 55%, it may be impossible to control the maximum polar density of the above orientation group to 8.0 or less due to the introduction of excessive strain. Preferably, the cumulative reduction ratio of the final three passes is 50% or less. Here, the cumulative reduction ratio of the final three passes is calculated by the following formula. Cumulative reduction ratio (%) for the final 3 passes = (plate thickness before the final 3 passes - plate thickness after the final 3 passes) / plate thickness before the final 3 passes × 100

[0069] [(C) Cooling process] Finally, the finish-rolled steel sheet is first cooled to a cooling stop temperature of 600-750°C at an average cooling rate of 20°C / second or more in the next cooling process, air-cooled for 3-20 seconds, and then secondarily cooled to a winding temperature of 200°C or less at an average cooling rate of 20°C / second or more before being wound. By carrying out the cooling process to satisfy these conditions, it is possible to produce a hot-rolled steel sheet having a microstructure in which, by area %, the total of martensite and retained austenite is 2-30%, the total of pearlite and cementite is 0-3%, and the remainder is ferrite and bainite. In particular, air-cooling for 3-20 seconds after the first cooling to a cooling stop temperature of 600-750°C can promote the precipitation of Ti carbides. As a result, it is possible to fully demonstrate the strength improvement effect due to precipitation strengthening.

[0070] On the other hand, if the average cooling rate of the primary cooling is less than 20°C / second, or if the average cooling rate of the secondary cooling is less than 20°C / second, excessive formation of structures such as pearlite will occur, resulting in a decrease in properties such as hole expansion ability. Also, if the air cooling time exceeds 20 seconds, sufficient formation of structures such as martensite will not occur, or / or excessive formation of structures such as pearlite will occur, resulting in a failure to obtain the desired strength and / or a decrease in hole expansion ability. Furthermore, if air cooling is not performed or the air cooling time is less than 3 seconds, the material will be cooled to a winding temperature of 200°C or less without essentially going through intermediate air cooling. As a result, excessive formation of martensite and / or retained austenite will occur, similarly reducing properties such as hole expansion ability. Also, if the winding temperature exceeds 200°C, sufficient formation of structures such as martensite will not be possible, and in this case, the desired strength cannot be achieved.

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

[0072] According to the hot-rolled steel sheet manufactured by the above manufacturing method, although it has a predetermined chemical composition controlled within a relatively low range with a C content of 0.080 mass% or less, precipitation strengthening by Ti carbides is utilized, and the steel structure is configured to contain a total of 2 to 30% of martensite and retained austenite in area%, so that high strength, for example, a high strength with a tensile strength of 780 MPa or more can be achieved. Furthermore, the ratio of the area occupied by crystal grains with a grain size exceeding 12 μm to the observed area in the steel structure is limited to 50% or less, and in the aggregate structure measured at the center of the plate thickness of the hot-rolled steel sheet, the maximum pole density of the {100}<011>, {211}<011>, {311}<011>, {110}<011> and {332}<113> orientation groups is controlled to 8.0 or less, so that it is possible to improve the hole expansion property and reduce the LC difference in uniform elongation. In addition, when the observation field of the steel structure is divided at intervals of 2 μm in the plate thickness direction and the total of martensite and retained austenite in each divided region is measured, the maximum value of the total is controlled to be 3.0 times or less of the average value of the total, so that it is possible to reduce the LC difference in the ultimate deformation ability. Therefore, the hot-rolled steel sheet manufactured by the above manufacturing method can surely achieve both high strength and excellent workability, which are contradictory properties, and is particularly useful in the use in the automotive field where both of these properties are required.

[0073] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

Examples

[0074] In the following examples, hot-rolled steel sheets according to embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), total elongation (EL), hole expansion rate (λ), LC difference in uniform elongation (uEL C / uEL L ) and LC difference in ultimate deformation ability (ε C / ε L ) of the obtained hot-rolled steel sheets were examined.

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

[0076] [Table 1-1]

[0077] [Table 1-2]

[0078] [Table 2]

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

[0080] [Tensile strength (TS) and total elongation (EL)] The tensile strength (TS) and total elongation (EL) were determined by taking JIS No. 5 test specimens from both the direction parallel to the rolling direction of the hot-rolled steel sheet (L direction) and the direction parallel to the direction perpendicular to the rolling direction (C direction), and performing tensile tests 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 by following the "JFS T 1001 Hole Expansion Test Method" of the Japan Iron and Steel Federation standard.

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

[0083] [LC difference in ultimate deformation capacity (ε C / ε L )] LC difference in ultimate deformation capacity (ε C / ε L ) was determined by collecting JIS No. 5 test pieces from the directions where the longitudinal direction of the test piece is parallel to the rolling direction of the hot-rolled steel sheet (L direction) and the direction parallel to the direction perpendicular to rolling (C direction), respectively, and conducting a tensile test in accordance with JIS Z 2241:2011. More specifically, the ultimate deformation capacity was calculated by measuring the initial plate thickness (t0) of the parallel part before the tensile test and the plate thickness (t f ) of the fracture part after the tensile test for each of the L-direction test piece and the C-direction test piece according to the following formula.

Equation

[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 ratio was between 0.70 and 1.30, it was evaluated as a hot-rolled steel sheet that, despite having high strength, had improved hole-expanding properties and reduced uniform elongation and the difference between the L-direction and C-direction in ultimate deformation capacity. The results are shown in Table 3.

[0085] In Table 3, "M+Retaining γ" refers to the sum of martensite and retained austenite, and "P+θ" refers to the sum of pearlite and cementite. "M+Retaining γ Maximum / Average" refers to the ratio of the maximum value to the average value of the sum of martensite and retained austenite when the observation field of view of the steel microstructure is divided into 2 μm intervals in the thickness direction and the sum of martensite and retained austenite is measured in each divided region. "Area Ratio of Grain Size Exceeding 12 μm" refers to the ratio of the area occupied by grains with a grain size exceeding 12 μm to the observation area. "Maximum Extreme Density" refers to the {100} of the texture measured at the center of the thickness of the hot-rolled steel sheet. <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> This represents the maximum polar density of the azimuthal group.

[0086] [Table 3]

[0087] Referring to Tables 1-3, it is believed that Comparative Example 2 failed to adequately suppress Mn segregation because it did not satisfy equation (1) in the continuous casting process. As a result, the variation between martensite and retained austenite in the thickness direction increased. Specifically, when the observation field of the steel structure was divided into 2 μm intervals in the thickness direction and the sum of martensite and retained austenite in each divided region was measured, the maximum value of the sum was greater than 3.0 times the average value of the sum, resulting in a large LC difference in the ultimate deformability. It is believed that Comparative Example 3 failed to adequately suppress Mn segregation because it did not satisfy equation (3) in the continuous casting process. As a result, the variation between martensite and retained austenite in the thickness direction increased, leading to a large LC difference in the ultimate deformability. It is believed that Comparative Example 7 failed to adequately suppress Mn segregation because the number of rolling cycles at a steel plate temperature of 1000°C or higher and a reduction ratio of more than 20% was insufficient in the rough rolling process, which prevented the diffusion of Mn from being promoted. As a result, the variation in martensite and retained austenite in the thickness direction increased, and the LC difference of the ultimate deformability increased. In Comparative Example 8, the long air cooling time in the cooling process resulted in insufficient martensite formation and excessive pearlite formation, leading to a decrease in TS and λ. In Comparative Example 9, the requirement that the reduction ratio of each rolling pass in the final three passes of finish rolling be 25% or less was not met, and furthermore, the cumulative reduction ratio of the final three passes was high, resulting in the introduction of excessive strain, which prevented the maximum polar density from being controlled to 8.0 or less. As a result, λ decreased, and the LC difference of the uniform elongation increased. In Comparative Example 10, the time t1 between the rolling pass two passes before the final and the rolling pass before the final pass in finish rolling, and the time t2 between the rolling pass one pass before the final and the final pass were short, resulting in insufficient recrystallization and strain accumulation, which prevented the maximum polar density from being controlled to 8.0 or less. As a result, λ decreased, and the LC difference of the uniform elongation increased. In Comparative Example 11, the time t3 from the final pass in finish rolling to the start of cooling was too short, resulting in insufficient recrystallization and strain accumulation, making it impossible to control the maximum polar density to 8.0 or less.As a result, λ decreased, and the LC difference of the uniform elongation increased. In Comparative Example 12, the cumulative reduction ratio in the final three passes of finish rolling was low, so recrystallization did not proceed, and the grains could not be sufficiently refined. As a result, the proportion of the area occupied by grains with a particle size of 12 μm or larger relative to the observation area of ​​the steel structure exceeded 50%, and λ decreased.

[0088] In Comparative Example 13, the entry temperature was low two passes before the final pass, and the exit temperature in the final pass was also low, resulting in insufficient recrystallization and strain accumulation, making it impossible to control the maximum polar density to 8.0 or less. As a result, λ decreased, and the LC difference of uniform elongation increased. In Comparative Examples 14 and 15, the average cooling rates of the primary and secondary cooling processes were low, respectively, leading to excessive pearlite formation and a decrease in λ. In Comparative Example 16, the coiling temperature in the cooling process was high, preventing sufficient martensite formation and resulting in a decrease in TS. In Comparative Examples 21-23, the air cooling time in the cooling process was short or no air cooling was performed, leading to excessive martensite and / or retained austenite formation and a decrease in λ. In Comparative Example 33, the low carbon content prevented martensite formation and resulted in a decrease in TS. In Comparative Example 34, the high carbon content prevented the formation of a single δ phase during solidification in the continuous casting process, and it is believed that Mn segregation could not be sufficiently suppressed. As a result, the variation in martensite and retained austenite in the thickness direction increased, and the LC difference of the ultimate deformability increased. In Comparative Example 35, due to the low Ti content, sufficient precipitation strengthening by Ti carbides was not obtained, resulting in a decrease in TS. In Comparative Example 36, it is thought that coarse Ti carbides were formed due to the high Ti content. As a result, λ decreased, and the LC difference of uniform elongation increased. In Comparative Example 37, due to the high Si content, many coarse crystal grains were formed. As a result, the proportion of the area occupied by crystal grains with a particle size of 12 μm or more relative to the observation area of ​​the steel structure exceeded 50%, resulting in a decrease in λ. In Comparative Example 38, due to the low Mn content, no martensite was formed, resulting in a decrease in TS. In Comparative Example 39, due to the high Mn content, excessive martensite was formed, resulting in a decrease in λ. In Comparative Example 40, due to the low Al content, the desired structure could not be obtained, resulting in a decrease in λ. Comparative Example 41 had a high Al content, which resulted in the formation of many coarse grains. As a result, the proportion of the area occupied by grains with a particle size of 12 μm or larger relative to the observation area of ​​the steel microstructure exceeded 50%, leading to a decrease in λ.

[0089] In contrast, in all the examples of the invention, the hot-rolled steel sheets had a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, it was possible to construct the steel structure so that the total of martensite and retained austenite in area percentage was 2-30%, thereby achieving a high strength of 780 MPa or more in tensile strength. Furthermore, by limiting the ratio of the area occupied by crystal grains with a particle size of 12 μm or more to the observed area in the steel structure, and by measuring the texture at the center of the thickness of the hot-rolled steel sheet, {100} <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> By controlling the maximum extreme density of the orientation group to 8.0 or less, we were able to improve λ and reduce the LC difference of uniform elongation. In addition, by dividing the observation field of the steel structure into 2 μm intervals in the plate thickness direction and measuring the sum of martensite and retained austenite in each divided region, we were able to control the maximum value of this sum to 3.0 times or less the average value of this sum, thereby reducing the LC difference of the ultimate deformability.

[0090] In particular, in Examples 1, 17-20, 24-31, 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 performed under appropriate conditions, the number density of recesses with a depth of 5 μm or more on the surface of the hot-rolled steel sheet was 1.0 recess / mm or less per surface length, and as a result ε C / ε L This was controlled within the range of 0.80 to 1.30, and the LC difference in the extreme deformation capacity was further significantly reduced.< / uvw> < / uvw>

Claims

1. In mass percent, C: 0.025-0.080%, Si: 0.001-2.000%, Mn: 1.000-2.000%, Ti: 0.080-0.200%, Al: 0.200-1.000%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, Nb: 0 to 0.200%, V: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.50%, Zn: 0 to 1.00%, B: 0 to 0.0050%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Bi: 0 to 0.0100%, Hf: 0-0.0100%, As: 0 to 0.1000%, REM: 0-0.0100%, and The remainder has a chemical composition consisting of Fe and impurities. The microstructure in the rolling direction cross-section within the observation area centered on the 1 / 4 thickness from the surface is, in area %, Total of martensite and retained austenite: 2-30% Total of perlite and cementite: 0-3%, and The remainder consists of ferrite and bainite. The proportion of the area occupied by crystal grains with a particle size of 12 μm or more relative to the observed area is 50% or less. In the texture measured at the center of the plate thickness, the maximum polar density for the {100}<011>, {211}<011>, {311}<011>, {110}<011>, and {332}<113> orientation groups is 8.0 or less. A hot-rolled steel sheet characterized in that, 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 value of the sum is 3.0 times or less the average value of the sum.

2. The hot-rolled steel sheet according to claim 1, characterized in that the chemical composition contains, by mass%, Si: 0.001 to 0.500%, and the number density of recesses on the surface with a depth of 5 μm or more is 1.0 recess / mm or less per surface length.

3. The aforementioned chemical composition is, in mass%, Cr: 0.001-1.00%, Mo: 0.001-1.00%, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00%, Co: 0.001 to 1.00%, Nb: 0.001-0.200%, V: 0.001-1.00%, W: 0.001-1.00%, 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: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, Bi: 0.0001-0.0100%, Hf: 0.0001-0.0100%, As: 0.0001 to 0.1000%, and REM: 0~0.0100% The hot-rolled steel sheet according to claim 1 or 2, characterized in that it includes at least one of the following.

4. The hot-rolled steel sheet according to claim 1 or 2, characterized in that it is a plated steel sheet having a plating layer on at least one surface.

5. A component characterized by comprising the hot-rolled steel sheet described in claim 1 or 2.

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