Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a predetermined chemical composition and has a microstructure containing, by volume, at least 70% in total of martensite, tempered martensite, and bainite, and 5-20% of retained austenite. In a surface layer region ranging from the surface of the sheet to a position at 1 / 10 thickness, the sum of the average polar density of orientation groups of {211}<111> to {111}<112> and the polar density of crystal orientation of {110}<001> is 6.0 or less, the concentration of solute carbon in the retained austenite is at least 0.5 mass%, and the tensile strength is at least 980 MPa.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet. This application claims priority based on Japanese Patent Application No. 2020-026996 filed in Japan on February 20, 2020, and incorporates its content herein by reference.

Background Art

[0002] In recent years, from the perspective of regulations on greenhouse gas emissions associated with measures to combat global warming, further improvement in the fuel efficiency of automobiles has been demanded. And in order to reduce the weight of the vehicle body and ensure collision safety, the application of high-strength steel sheets in automotive parts is increasingly expanding. However, for steel sheets used in automotive parts, not only strength but also various workabilities required during part forming, such as press formability and weldability, are required. Specifically, from the viewpoints of press formability and formability, bend formability and elongation are often required for steel sheets. Since the formability of steel sheets tends to decrease as the strength of the material increases, it is difficult to achieve both high strength and good formability. Therefore, for the application of high-strength steel sheets in automotive parts, it has become an important issue to achieve excellent bend formability and elongation together with a high strength of 980 MPa or more in tensile strength.

[0003]

[0004] Patent Document 1 discloses a method of achieving a tensile strength of 590 MPa or more and 750 MPa or less and excellent bend formability by controlling the composition to contain, in mass %, C: 0.010 to 0.055%, Si: 0.2% or less, Mn: 0.7% or less, P: 0.025% or less, S: 0.02% or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.06 to 0.095%, controlling the structure so that 95% or more of the area ratio consists of ferrite, and controlling the structure so that carbide particle diameters containing Ti in ferrite crystal grains and only TiS with an average diameter of 0.5 μm or less as sulfide containing Ti are dispersed and precipitated.

[0005] Patent Document 2 discloses a method of improving bend formability while maintaining a tensile strength of 780 MPa or more by controlling the composition to contain, in mass %, C: 0.05 to 0.15%, Si: 0.2 to 1.2%, Mn: 1.0 to 2.0%, P: 0.04% or less, S: 0.0030% or less, Al: 0.005 to 0.10%, N: 0.01% or less, and Ti: 0.03 to 0.13%, controlling the structure inside the steel sheet to be a bainite single phase or a structure in which the fraction of bainite exceeds 95%, and controlling the structure of the surface layer portion of the steel sheet so that the fraction of the bainite phase is less than 80% and the fraction of ferrite rich in formability is 10% or more.

[0006] Furthermore, Patent Document 3 discloses that, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 1.5%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.1 to 1.0%, Cr: 0.1 to 1.0% are contained, the tempered martensite phase is the main phase with a volume fraction of 90% or more, the average grain diameter of prior austenite grains in a cross section parallel to the rolling direction is 20 μm or less, and the average grain diameter of prior austenite grains in a cross section perpendicular to the rolling direction is 15 μm or less, and by controlling the structure to reduce the anisotropy of the prior γ grains, a high-strength hot-rolled steel sheet having a high strength with a yield strength of 960 MPa or more, excellent bend formability, and excellent low-temperature toughness can be obtained.

[0007] Patent Document 4 discloses that by controlling the pole density of each orientation in a specific crystal orientation group in the central part of the plate thickness, which is in the range of 5 / 8 to 3 / 8 of the plate thickness from the surface of the steel plate, and setting the rC, which is the r-value in the direction perpendicular to the rolling direction, to be 0.70 or more and 1.10 or less, and the r30, which is the r-value in the direction forming an angle of 30° with the rolling direction, to be 0.70 or more and 1.10 or less, a hot-rolled steel plate with excellent local formability and small anisotropy in bendability can be obtained.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0009]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] As described above, in recent years, it has been required to increase the strength of steel sheets and further improve the bendability and elongation. However, the technologies of Patent Documents 1 to 4 described above cannot be said to sufficiently achieve both strength and bendability and elongation.

[0011] The problem to be solved by the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent bendability and elongation. The bending workability mentioned above is an index showing how difficult it is for cracks to occur in the bent portion when bending, or how difficult it is for the cracks to grow. However, unlike the conventional method, the present invention is intended to cover cracks that occur from the inside of the bent portion when bending (inner bending cracks). [Means for solving the problem]

[0012] The present inventors have investigated the above-mentioned problems, and have found that a steel sheet having a tensile strength of 980 MPa or more while ensuring workability can be manufactured by making the microstructure contain, by volume fraction, 70% or more in total of martensite, tempered martensite, and bainite, and contain 5 to 20% of retained austenite.

[0013] The inventors also conducted extensive research into the bending workability of high-strength steel plates. As a result, they found that the higher the strength of the steel plate, the more likely it is that cracks will occur during bending. In addition, while conventionally, cracks during bending of steel plates generally occurred on the surface or near the end face of the steel plate on the outside of the bend, it was found that as the strength of steel plates increases, microcracks may occur on the inside of the bend. Conventional knowledge has not shown a method for suppressing such microcracks that occur on the inside of the bend (hereinafter referred to as inner bend cracks). Research by the present inventors has revealed that internal bending cracks are more likely to occur in steel plates with tensile strengths of 780 MPa or higher, are more pronounced in steel plates with tensile strengths of 980 MPa or higher, and are an even more pronounced problem in steel plates with tensile strengths of 1180 MPa or higher.

[0014] The present inventors presumed that the mechanism by which the above-mentioned internal cracks occur in bending is due to uneven deformation, and focused on the uniformity of the texture and hardness to search for a method of suppressing the internal cracks in bending. As a result, if the texture is relatively random, the deformation resistance is uniform, so deformation is likely to occur uniformly. However, when a specific texture develops, a deformation bias occurs between the crystals with a large deformation resistance orientation and the crystals with other orientations, making it easier to generate shear deformation bands. Conversely, it was found that when the crystals with a large deformation resistance orientation are reduced, the deformation occurs uniformly and the shear deformation bands are less likely to occur. That is, the inventors have found that bending internal cracks can be suppressed by controlling the texture in the surface layer region in the plate thickness direction where cracks occur, in particular.

[0015] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) The hot-rolled steel sheet according to one aspect of the present invention contains, in mass%, C: 0.02 to 0.30%, Si: 0.01 to 2.50%, Mn: 1.00 to 3.00%, P: 0.100% or less, S: 0.0001 to 0.0100%, Al: 0.005 to 1.000%, N: 0.010% or less, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 0.200%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, W: 0 to 0.100%, B: 0 to 0.0100%, REM: 0 to 0.0300%, Ca: 0 to 0.0300%, Mg: 0 to 0.0300%, and has a chemical composition consisting of the balance being Fe and impurities. The chemical composition satisfies Si + Al ≥ 1.00%. The microstructure contains, in volume ratio, a total of 70% or more of martensite, tempered martensite, and bainite, contains 5 to 20% of retained austenite, and in the surface layer region which is the range from the surface to the position of 1 / 10 of the plate thickness, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> is 6.0 or less. The solid solution carbon concentration in the retained austenite is 0.5 mass% or more, and the tensile strength is 980 MPa or more. (2) The hot-rolled steel sheet described in (1) above may contain one or more selected from the following chemical compositions by mass%: Ti: 0.001 to 0.20%, Nb: 0.001 to 0.20%, V: 0.001 to 0.200%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.005 to 0.100%, B: 0.0005 to 0.0100%, REM: 0.0003 to 0.0300%, Ca: 0.0003 to 0.0300%, Mg: 0.0003 to 0.0300%. (3) The hot-rolled steel sheet described in (1) or (2) above may be provided with a hot-dip galvanized layer on the surface. (4) The hot-rolled steel sheet described in (3) above may have an alloyed hot-dip galvanized layer as the hot-dip galvanized layer.

Advantages of the Invention

[0016] According to the above aspect of the present invention, a hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent bend formability and elongation in which the occurrence of bend internal cracking can be suppressed can be obtained.

Brief Description of the Drawings

[0017]

Figure 1

Embodiments for Carrying Out the Invention

[0018] Hereinafter, a hot-rolled steel sheet according to an embodiment of the present invention (the steel sheet according to this embodiment) will be described.

[0019] 1. Microstructure <In terms of volume ratio, it contains a total of 70% or more of martensite, tempered martensite and bainite, and 5 to 20% of retained austenite> First, the reason for limiting the microstructure will be described. In the steel sheet according to this embodiment, the main phase of the microstructure is one or more selected from martensite, tempered martensite, and bainite with a volume ratio of 70% or more. The microstructure further contains 5 to 20% retained austenite. In order to achieve both a tensile strength (TS) of 980 MPa or more and bend formability in the steel sheet according to this embodiment, one or more selected from martensite, tempered martensite, and bainite, which are low-temperature transformation product phases, are used as the main phase. When attempting to increase the strength in a microstructure configuration with a low total volume ratio of martensite, tempered martensite, and / or bainite, a deformation bias occurs between the hard above-mentioned microstructure and the soft microstructure other than the above, and the bend formability deteriorates. If the total volume ratio of martensite, tempered martensite, and / or bainite is less than 70%, sufficient strength cannot be obtained or sufficient bend formability cannot be obtained. Also, in the steel sheet according to this embodiment, in order to obtain excellent elongation, retained austenite is contained at a volume ratio of 5% or more. If the volume ratio of retained austenite is less than 5%, sufficient elongation cannot be obtained. On the other hand, if manufacturing conditions for allowing more than 20% of retained austenite to remain are selected, other desired microstructures and strengths cannot be obtained. Therefore, the upper limit of the volume ratio of substantial retained austenite is 20%. The remainder other than the above may be one or more of ferrite and pearlite.

[0020] In this embodiment, the volume ratios of pearlite, bainite, tempered martensite, and ferrite are measured by taking a sample with a cross-section in the plate thickness direction parallel to the rolling direction of the hot-rolled steel sheet as the observation surface, polishing the observation surface, nitriding etching it, and observing a range from 1 / 8 to 3 / 8 (1 / 8 thickness to 3 / 8 thickness) of the plate thickness from the surface centered on the position at a depth of 1 / 4 of the plate thickness (1 / 4 thickness) from the surface using a field emission scanning electron microscope (FE-SEM) at a magnification of 5000 times to measure the area ratio of each microstructure and using that as the volume ratio. At that time, 10 fields are measured and the average value is taken as the volume ratio.

[0021] Each tissue has the following characteristics. Therefore, in the measurement of the area ratio, each tissue is identified based on the following characteristics, and its area ratio is determined. Ferrite is equiaxed grains that do not contain iron-based carbides, and pearlite is a lamellar structure of ferrite and cementite. Bainite includes upper bainite and lower bainite. Upper bainite is an aggregate of lath-shaped crystal grains and an aggregate of laths containing carbides between the laths. Lower bainite is an aggregate of lath-shaped crystal grains, contains iron-based carbides with a major axis of 5 nm or more inside, and further, the carbides belong to a single variant, that is, an iron-based carbide group extending in the same direction. Here, the iron-based carbide group extending in the same direction means that the difference in the extension direction of the iron-based carbide group is within 5°. Tempered martensite is an aggregate of lath-shaped crystal grains, contains iron-based carbides with a major axis of 5 nm or more inside, and further, the carbides belong to a plurality of variants, that is, an iron-based carbide group extending in two or more directions. Generally, tempered martensite often refers to those containing iron-based carbides such as cementite, but in this embodiment, martensite containing fine precipitates containing Ti is also defined as tempered martensite.

[0022] Martensite (fresh martensite) and retained austenite are not sufficiently corroded by nital etching, so in the observation by FE-SEM, they can be clearly distinguished from the above-mentioned tissues (ferrite, pearlite, bainite, tempered martensite). Therefore, the volume ratio of martensite can be obtained as the difference between the volume ratio obtained as the area ratio of the uncorroded region observed by FE-SEM and the volume ratio of retained austenite measured by X-ray described later.

[0023] The volume fraction of retained austenite is determined by X-ray diffraction. Specifically, at a depth of 1 / 4 of the sheet thickness of the steel sheet, in the cross-section in the sheet thickness direction parallel to the rolling direction, using Co-Kα rays, the integrated intensities of a total of 6 peaks of α(110), α(200), α(211), γ(111), γ(200), and γ(220) are obtained, and the volume fraction of retained austenite is obtained by calculating using the intensity averaging method. However, for the steel sheet according to the present embodiment, it is only necessary to define the total volume fraction of martensite, tempered martensite, and bainite, and it is not essential to distinguish these structures.

[0024] <The solid solution carbon concentration in the retained austenite is 0.5 mass% or more> By setting the solid solution carbon concentration in the retained austenite to 0.5 mass% or more, the retained austenite is moderately stabilized, and transformation-induced plasticity (TRIP) occurs frequently in the high strain region in the latter stage of deformation, improving the elongation and bend formability of the steel sheet. Therefore, the solid solution carbon concentration in the retained austenite is set to 0.5 mass% or more. The solid solution carbon concentration in the retained austenite is preferably 0.7 mass% or more. By setting the solid solution carbon concentration in the retained austenite to 2.0 mass% or less, excessive stabilization of the retained austenite can be suppressed, and transformation-induced plasticity (TRIP) can be more reliably expressed. Therefore, the solid solution carbon concentration in the retained austenite is preferably set to 2.0 mass% or less.

[0025] The solid solution carbon concentration in the retained austenite is determined by X-ray diffraction. Specifically, in the metal structure at a depth of 1 / 4 of the sheet thickness from the steel sheet surface in the cross-section in the sheet thickness direction parallel to the rolling direction at the center position in the sheet width direction, X-ray analysis using Cu-Kα rays is performed, and the lattice constant a (unit: angstrom) is obtained from the reflection angles of the (200), (220), and (311) planes of the retained austenite, and the solid solution carbon concentration (Cγ) in the retained austenite is calculated according to the following formula (A). Cγ=(a - 3.572) / 0.033 ··· (A)

[0026] <In the surface layer region ranging from the surface to a position of 1 / 10 of the plate thickness, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> is 6.0 or less> The inventors of the present invention have intensively investigated the bendability of high-strength steel sheets. As a result, it has been found that minute cracks may occur on the inner side of the bend as the strength of the steel sheet increases. As a result of further investigation, the mechanism of such bend internal cracking is presumed as follows. During bending, a compressive stress is generated on the inner side of the bend. At first, the entire inner side of the bend deforms uniformly while the processing progresses, but as the processing amount increases, it becomes impossible to bear the deformation only by uniform deformation, and a bias in microscopic deformation occurs (generation of a shear deformation band). As this shear deformation band further grows, cracks occur and grow along the shear band from the inner surface of the bend. The reason why bend internal cracking is likely to occur with increasing strength is presumed to be that due to the decrease in work hardening ability accompanying the increase in strength, uniform deformation becomes difficult to progress, and a bias in deformation is likely to occur, so that a shear deformation band occurs at an early stage of processing (or under loose processing conditions). When the steel sheet is bent, the strain increases toward the surface with the center of the plate thickness as the boundary, and the strain is maximum at the outermost surface. Therefore, the cracks of the bend internal cracking are generated on the surface of the steel sheet. Since the structure of the surface layer region, which is the range from the surface of the steel sheet to 1 / 10 of the plate thickness in the plate thickness direction, contributes to the generation of such cracks, the structure of the surface layer region is controlled.

[0027] The inventors of the present invention focused on the aggregate structure in order to suppress the bias in deformation that causes bend internal cracking during bending. Specifically, when deformation is applied to a steel plate, the ease of slip system activation with respect to deformation varies among different crystal orientations (Schmid factor). This is presumably because the deformation resistance differs for each crystal orientation. That is, if the microstructure is relatively random, the deformation resistance is uniform, and thus deformation is likely to occur uniformly. However, when a specific microstructure develops, a deformation bias occurs between crystals with a high deformation resistance orientation and those with other orientations, making it easier to generate shear deformation bands. Conversely, if the number of crystals with a high deformation resistance orientation is reduced, deformation is likely to occur uniformly, and shear deformation bands are less likely to form.

[0028] Based on the above concept, for the steel plate according to this embodiment, in the surface layer region ranging from the surface to a position 1 / 10 of the plate thickness, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> is set to 6.0 or less. Thereby, bend internal cracking can be suppressed. In the case of a steel plate with different microstructure development on the front and back surfaces, even if only the range from one side surface to a position 1 / 10 of the plate thickness satisfies the microstructure defined in this embodiment, in the bending process when that surface is made the inner side of the bend, the effect of suppressing bend internal cracking can be obtained.

[0029] The orientation group consisting of {211}<111> to {111}<112> and the crystal orientation of {110}<001> are orientations that tend to develop in the surface layer region of a high-strength hot-rolled steel plate produced by a conventional method. Also, since these are orientation groups with particularly high deformation resistance on the inner side of the bend during bending, shear deformation bands are likely to form due to the difference in deformation resistance from other orientation groups. Therefore, bend internal cracking can be suppressed by reducing the pole density of these orientation groups. However, the effect of this embodiment cannot be obtained by reducing only either the average pole density of the orientation group consisting of {211}<111> to {111}<112> or the pole density of the crystal orientation of {110}<001>, and it is important to reduce their sum.

[0030] When the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> in the surface layer region, which is the range from the steel plate surface to 1 / 10 of the plate thickness, exceeds 6.0, shear deformation bands are likely to occur significantly, which becomes a factor in the occurrence of bending internal cracks. In this case, the average value of the minimum bending radius of the L-axis and C-axis / plate thickness, R / t, exceeds 1.5. Therefore, the sum of these is set to 6.0 or less. From this perspective, preferably, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> is 5.0 or less, and more preferably 4.0 or less. The smaller the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001>, the better. However, in high-strength hot-rolled steel sheets of 980 MPa or more, it is difficult to make it less than 0.5, so 0.5 is the practical lower limit.

[0031] The pole density can be measured by the EBSP (Electron BackScatter Diffraction Pattern) method. The sample used for analysis by the EBSP method is mechanically polished on a cut surface parallel to the rolling direction and perpendicular to the plate surface, and the strain is removed by chemical polishing or electrolytic polishing after mechanical polishing. Using this sample, in the range from the steel plate surface to the 1 / 10 position of the plate thickness, the measurement interval is set to 4.0 μm, and the analysis by the EBSP method is performed so that the measurement area is 150000 μm 2 or more.

[0032] Figure 1 shows the crystal orientation distribution function (ODF) of the φ2 = 45° cross-section, the orientation group consisting of {211}<111> to {111}<112>, and the {110}<001> orientation. The orientation group consisting of {211}<111> to {111}<112> refers to the range of φ1 = 85 - 90°, Φ = 30 - 60°, and φ2 = 45° in the crystal orientation distribution function (ODF) of the φ2 = 45° cross-section when the texture analysis is displayed in BUNGE notation. The average pole density of this orientation group is calculated within the above range shown in Figure 1. The {211}<111> to {111}<112> orientation group is strictly in the range of φ1 = 90°, Φ = 30 - 60°, and φ2 = 45° on the ODF, but due to measurement errors caused by specimen processing and sample setting, in the steel sheet according to this embodiment, the average pole density is calculated in the range of φ1 = 85 - 90°, Φ = 30 - 60°, and φ2 = 45°. In the following average pole density analysis, similarly, the range of angles for taking the average value is determined considering the measurement errors caused by specimen processing and sample setting. Similarly, the pole density of the crystal orientation of {110}<001> refers to the range of φ1 = 85 - 90°, Φ = 85 - 90°, and φ2 = 45° in the crystal orientation distribution function (ODF) of the φ2 = 45° cross-section. The pole density of this crystal orientation is calculated within the above range shown in Figure 1.

[0033] Here, the crystal orientation of the rolled plate is usually represented by the lattice plane parallel to the plate surface as (hkl) or {hkl}, and the orientation parallel to the rolling direction as [uvw] or <uvw>is displayed. {hkl} and <uvw>is a general term for equivalent lattice planes and directions, and (uvw) and [hkl] refer to individual lattice planes and directions. That is, in the steel sheet according to the present embodiment, since the bcc structure is targeted, for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1), (0-1-1) are equivalent lattice planes and cannot be distinguished. In such a case, these lattice planes are collectively referred to as {110}.

[0034] 2. Chemical Composition Hereinafter, the chemical composition of the steel sheet according to the present embodiment will be described in detail. In the numerical limit range sandwiched by "~" described below, the values at both ends are included in the range as the lower limit value and the upper limit value. However, the numerical values indicated by "exceeding" or "less than" are not included in the numerical range. "% " regarding the content of each element means "mass%" unless otherwise specified.

[0035] (C: 0.02~0.30%) C is an element effective for increasing the strength of the steel sheet. When the C content is less than 0.02%, it is difficult to ensure a strength of 980 MPa or more. Therefore, the C content is set to 0.02% or more. The C content is preferably 0.13% or more, and more preferably 0.15% or more. On the other hand, when the C content exceeds 0.30%, not only does its effect saturate, but pearlite is preferentially generated and the generation of bainite and retained austenite becomes insufficient, making it difficult to obtain the desired volume fraction of bainite and the volume fraction of retained austenite. Therefore, the C content is 0.30% or less. The C content is preferably 0.25% or less.

[0036] (Si: 0.01~2.50%) Si is an important element that can increase the material strength by solid solution strengthening. When the Si content is less than 0.01%, the strength decreases. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.10% or more, and more preferably 0.30% or more. On the other hand, if the Si content exceeds 2.50%, the surface properties deteriorate. Therefore, the Si content should be 2.50% or less. The Si content is preferably 2.00% or less.

[0037] (Mn: 1.00% - 3.00%) Mn is an effective element for increasing the strength of the steel sheet by increasing the volume ratios of bainite and martensite in the microstructure of the steel sheet. To make the total volume ratio of bainite, martensite, and tempered martensite 70% or more, the Mn content should be 1.00% or more. If the Mn content is less than 1.00%, the volume ratios of these microstructures decrease, and sufficient strength cannot be obtained. On the other hand, if the Mn content exceeds 3.00%, its effect saturates and the economy deteriorates. Therefore, the Mn content should be 3.00% or less.

[0038] (P: 0.100% or less) P is an element that segregates to the center of the plate thickness of the steel sheet and also an element that embrittles the welded part. Although a lower P content is preferable, if the P content exceeds 0.100%, the deterioration of properties becomes significant. Therefore, the P content is limited to 0.100% or less. The P content is preferably 0.050% or less. On the other hand, the lower limit does not need to be particularly defined for the effect to be exerted (it may be 0%), but reducing the P content to less than 0.001% is economically disadvantageous. Therefore, the P content may be 0.001% or more.

[0039] (S: 0.0001 - 0.0100%) S is an element that causes embrittlement of the slab by existing as sulfide. Also, S is an element that deteriorates the formability of the steel sheet. Therefore, the S content is restricted. If the S content exceeds 0.010%, the deterioration of properties becomes significant. Therefore, the S content should be 0.010% or less. On the other hand, the lower limit does not need to be particularly defined for the effect to be exerted (it may be 0%), but reducing the S content to less than 0.0001% is economically disadvantageous. Therefore, the S content should be 0.0001% or more.

[0040] (N: less than 0.010%) N is an element that forms coarse nitrides and deteriorates bend formability and elongation. When the N content exceeds 0.010%, the bend formability and elongation deteriorate significantly. Therefore, the N content is set to 0.010% or less. On the other hand, the lower limit of the N content does not necessarily need to be specified (it may be 0%), but when the N content is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, from the perspective of manufacturing cost, the N content may be 0.0001% or more, or 0.0005% or more.

[0041] (Al: 0.005 - 1.000%) Al is an element effective for controlling the structure in hot rolling and deoxidation. To obtain these effects, the Al content is set to 0.005% or more. When the Al content is less than 0.005%, a sufficient deoxidation effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions become the starting points of cracks during bending and stretch flange processing, deteriorating the workability. On the other hand, when the Al content exceeds 1.000%, the slab becomes brittle, which is not preferable. Therefore, the Al content is set to 1.000% or less. In this embodiment, the Al content means the sol.Al (acid-soluble Al) content. Also, to ensure the area ratio of retained austenite, the total content of Si and Al (Si + Al) is set to 1.00% or more.

[0042] The above are the basic chemical components of the steel sheet according to this embodiment. The chemical composition of the steel sheet according to this embodiment contains the above elements, and the balance may consist of Fe and impurities. In this embodiment, impurities mean those mixed in from ore, scrap as raw materials, or the manufacturing environment, etc., and are allowed within a range that does not adversely affect the steel sheet according to this embodiment. The steel sheet according to this embodiment can further contain the following components for the purpose of improving various properties. The following elements do not necessarily need to be contained, so the lower limit of the content is 0%.

[0043] (Ti: 0 to 0.20%) (Nb: 0 to 0.20%) (V: 0 to 0.200%) Ti (titanium), Nb (niobium), and V (vanadium) are elements that contribute to the improvement of strength. Therefore, Ti, Nb, and / or V may be contained. In order to preferably obtain the above effects, the content is preferably 0.001% or more for each. The Ti content is more preferably 0.02% or more, and the Nb content is more preferably 0.01% or more. On the other hand, even if the content of Ti and Nb exceeds 0.20% and the content of V exceeds 0.200%, not only does the effect by the above action saturate, but it may also be economically disadvantageous. Therefore, when containing, the Ti content is 0.20% or less, the Nb content is 0.20% or less, and the V content is 0.200% or less. The Ti content and the Nb content are preferably 0.15% or less, more preferably 0.10% or less, and the V content is preferably 0.150% or less, more preferably 0.100% or less.

[0044] (Ni: 0 to 2.00%) (Cu: 0 to 2.00%) (Cr: 0 to 2.00%) (Mo: 0 to 2.00%) Ni, Cu, Cr, and Mo are elements that contribute to the high-strength improvement of the steel sheet through microstructure control in hot rolling. This effect becomes remarkable when one or more of Ni, Cu, Cr, and Mo are each contained in an amount of 0.01% or more. Therefore, when obtaining the effect, it is preferable that the content is 0.01% or more for each. On the other hand, when the content of each element exceeds 2.00%, the weldability, hot workability, etc. deteriorate. Therefore, when containing, the content of Ni, Cu, Cr, and Mo is each 2.00% or less.

[0045] (W: 0 to 0.100%) W is an element that contributes to the improvement of the strength of the steel sheet through precipitation strengthening. When obtaining this effect, it is preferable that the W content is 0.005% or more. On the one hand, when the W content exceeds 0.100%, not only does the effect saturate, but the hot workability also deteriorates. Therefore, when adding it, the W content should be 0.100% or less.

[0046] (B: 0 to 0.0100%) B is an element effective for controlling transformation in hot rolling and improving the strength of the steel sheet through structure strengthening. When obtaining this effect, it is preferable that the B content is 0.0005% or more. On the other hand, when the B content exceeds 0.0100%, not only does the effect saturate, but iron-based borides precipitate, and the effect of improving hardenability by solid-solution B is lost. Therefore, when adding it, the B content should be 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0050% or less.

[0047] (REM: 0 to 0.0300%) (Ca: 0 to 0.0300%) (Mg: 0 to 0.0300%) REM, Ca, and Mg are elements that contribute to improving the strength of the steel sheet. If the total of one or more of REM, Ca, and Mg is less than 0.0003%, a sufficient effect cannot be obtained. Therefore, when obtaining the effect, it is preferable that the total content of REM, Ca, and Mg is 0.0003% or more. On the one hand, when REM, Ca, and Mg each exceed 0.0300%, the castability and hot workability deteriorate. Therefore, when adding them, the content of each should be 0.0300% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of the above REM refers to the total content of these elements. In the case of lanthanoids, industrially, they are added in the form of mischmetal.

[0048] The above-described steel components may be measured by general steel analysis methods. For example, the steel components may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using combustion-infrared absorption method, N may be measured using inert gas fusion-thermal conductivity method, and O may be measured using inert gas fusion-non-dispersive infrared absorption method.

[0049] In the steel sheet according to this embodiment, a hot-dip galvanized layer may be further provided on the surface. Further, the hot-dip galvanizing may be an alloyed hot-dip galvanized layer subjected to an alloying treatment. Since galvanizing contributes to improving corrosion resistance, in the case of application to uses where corrosion resistance is expected, it is desirable to use a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet on which galvanizing has been performed. Automotive underbody parts may have a concern of perforation due to corrosion, so even if the strength is increased, it may not be possible to make it thinner than a certain plate thickness. Since one of the purposes of increasing the strength of the steel sheet is weight reduction by thinning, even if a high-strength steel sheet is developed, if the corrosion resistance is low, the applicable parts are limited. As a method for solving these problems, it is conceivable to apply plating such as hot-dip galvanizing with high corrosion resistance to the steel sheet. Since the steel sheet according to this embodiment controls the steel sheet components as described above, hot-dip galvanizing is possible. The plating may be electro-galvanizing or a plating containing Si, Al and / or Mg in addition to Zn.

[0050] 4. Mechanical properties The steel sheet according to this embodiment has a tensile strength (TS) of 980 MPa or more as sufficient strength contributing to weight reduction of automobiles. Preferably it is 1180 MPa or more. There is no particular need to define the upper limit of the tensile strength, but in this embodiment, the substantial upper limit of the tensile strength may be set to 1370 MPa. In addition, the steel sheet according to the present embodiment aims to have a limit bending R / t value, which is an index value for bend internal cracking property, of 1.5 or less. The value of R / t can be obtained, for example, by cutting out a strip-shaped test piece from the 1 / 2 position in the width direction of the hot-rolled steel sheet, and performing bending (L-axis bending) where the bending ridge line is parallel to the rolling direction (L direction) and bending (C-axis bending) where the bending ridge line is parallel to the direction perpendicular to the rolling direction (C direction), in accordance with JIS Z2248:2006 (V-block 90° bending test), investigating the cracks generated on the inside of the bend. The minimum bending radius at which cracks do not occur is obtained, and the value obtained by dividing the average value of the minimum bending radii of the L-axis and the C-axis by the sheet thickness can be used as the limit bending R / t, which is an index value for bend formability. In addition, as an index for the steel sheet according to the present embodiment to have high elongation, it is targeted that the product of the tensile strength TS (MPa) and EL (%) is 19000 (MPa·%) or more. The product of TS and EL is preferably 19120 (MPa·%) or more, more preferably 19600 (MPa·%) or more. It is desirable that the total elongation EL of the steel sheet according to the present embodiment is 16.0% or more. The tensile test is carried out in accordance with JIS Z2241:2011 by taking a JIS No. 5 tensile test piece so that the direction perpendicular to the rolling direction is the tensile direction, and measuring the tensile strength (TS) and the total elongation (EL).

[0051] 5. Manufacturing method Next, a preferred manufacturing method of the steel sheet according to the present embodiment will be described. In order to control the microstructure and aggregate structure of the surface layer region of the steel sheet within the above-mentioned ranges, it preferably includes the following hot rolling process (including heating process, rough rolling process, and finish rolling process), cooling process, heat treatment process, and if necessary, a coiling process, pickling process, and light reduction process between the cooling process and the heat treatment process, and if necessary, a plating process after the heat treatment process to manufacture a hot-rolled steel sheet. Hereinafter, the preferred conditions in each process will be described.

[0052] The manufacturing process prior to hot rolling is not particularly limited. That is, following melting by a blast furnace, an electric furnace, or the like, various secondary melting processes may be performed, and then casting may be carried out by methods such as normal continuous casting or casting by the ingot method. In the case of continuous casting, the cast slab may be cooled to a low temperature once and then reheated before hot rolling, or the cast slab may be directly hot rolled after casting without being cooled to a low temperature. Scrap may be used as the raw material.

[0053] <Heating process> In the heating process, the slab having the above-described chemical composition to be subjected to the rough rolling process is heated to 1200°C or higher. Coarse precipitates (such as iron-based carbides and carbonitrides of alloying elements) precipitated in the slab may inhibit the material stability. Therefore, the slab is heated to 1200°C or higher for the purpose of dissolving such precipitates.

[0054] <Rough rolling process> Next, the heated slab is rough rolled to obtain a rough rolled plate. In this rough rolling process, the thickness of the rough rolled plate after rough rolling is controlled to be more than 35 mm and 45 mm or less. The thickness of the rough rolled plate affects the amount of temperature drop from the tip to the tail end of the rolled plate that occurs from the start to the completion of rolling in the finish rolling process. Also, if the thickness of the rough rolled plate is 35 mm or less or more than 45 mm, the amount of strain introduced into the steel plate during finish rolling in the next process changes, and the processed structure formed during finish rolling changes. As a result, the recrystallization behavior changes, and it becomes difficult to obtain the desired grain structure. In particular, it becomes difficult to obtain the above-described grain structure in the surface layer region of the steel plate. Generally, the thickness of the rough rolled plate after rough rolling is appropriately set from the viewpoint of productivity and the like, and is rarely set for controlling the properties of the steel plate. In contrast, the inventors of the present invention strictly control the thickness of the rough rolled plate in order to control the grain structure in the surface layer region of the steel plate.

[0055] <Finish rolling process> Following rough rolling, multi-stage finish rolling is performed. The inventors have found that it is important for controlling the grain structure to control the sheet thickness, roll shape ratio, temperature, and the contents of Nb and Ti in the steel during the final two stages of the finish rolling process of hot rolling, which have not usually been actively controlled, within appropriate ranges derived by a certain calculation formula. Therefore, in this multi-stage finish rolling, the starting temperature of the finish rolling is 1000°C or higher and 1150°C or lower, and the thickness of the steel sheet before the start of the finish rolling (the thickness of the rough rolled sheet) is more than 35 mm and 45 mm or less. Further, in the rolling one stage before the final stage of the multi-stage finish rolling, the rolling temperature is 960°C or higher and 1020°C or lower, and the reduction ratio is more than 11.0% and 23.0% or lower. Also, in the final stage of the multi-stage finish rolling, it is preferable that the rolling temperature is 930°C or higher and 995°C or lower, and the reduction ratio is more than 11.0% and 22.0% or lower. Further, it is preferable to control each condition during the reduction in the final two stages so that the grain structure formation parameter ω calculated by the following formula (1) satisfies 110 or less. Furthermore, it is preferable to perform the finish rolling under the condition that the total reduction ratio of the final three stages of the multi-stage finish rolling is 35% or higher.

[0056]

Number

[0057]

Number

[0058]

Number

[0059]

Number

[0060]

Number

[0061] [Number]

[0062] [Number]

[0063] [Number]

[0064] In these formulas, PE: Conversion value of recrystallization inhibition effect by precipitation-forming element (unit: mass%) Ti: Ti content contained in steel (unit: mass%) Nb: Nb content contained in steel (unit: mass%) F1 * : Reduction ratio conversion one step before the final step (unit: %) F2 * : Reduction ratio conversion at the final step (unit: %) F1: Reduction ratio one step before the final step (unit: %) F2: Reduction ratio at the final step (unit: %) Sr1: Rolling shape ratio one step before the final step (unitless) Sr2: Rolling shape ratio at the final step (unitless) D1: Roll diameter one step before the final step (unit: mm) D2: Roll diameter at the final step (unit: mm) t1: Plate thickness at the start of rolling one step before the final step (unit: mm) t2: Plate thickness at the start of rolling at the final step (unit: mm) t f : Plate thickness after finish rolling (unit: mm) FT1 * : Conversion rolling temperature one step before the final step (unit: °C) FT2 * : Conversion rolling temperature at the final step (unit: °C) FT1: Rolling temperature one step before the final step (unit: °C) FT2: Rolling temperature at the final step (unit: °C) Each indicates

[0065] However, in Formulas 1 to 8, the numbers 1 and 2 appended to variables such as F1 and F2 are obtained by appending 1 to the variable related to the rolling one step before the final step and 2 to the variable related to the final step of the rolling for the final two steps of the multi-stage finish rolling. For example, in a multi-stage finish rolling consisting of a total of 7 stages of rolling, F1 means the reduction ratio of the 6th stage of rolling counted from the rolling inlet side, and F2 means the reduction ratio of the 7th stage of rolling.

[0066] Regarding the conversion value PE of the recrystallization suppression effect by the precipitate-forming element, since the effects of pinning and solute drag become apparent when the value of Ti + 1.3Nb is 0.02 or more, in Formula 2, when Ti + 1.3Nb < 0.02 is satisfied, PE = 0.01, and when Ti + 1.3Nb ≥ 0.02 is satisfied, PE = Ti + 1.3Nb - 0.01.

[0067] The conversion reduction ratio F1 one step before the final step * Regarding this, since the influence of the reduction ratio F1 one step before the final step on the grain structure becomes apparent when the value of F1 is 12 or more, in Formula 3, when F1 < 12 is satisfied, F1 * = 1.0, and when F1 ≥ 12 is satisfied, F1 * = F1 - 11.

[0068] The conversion reduction ratio F2 of the final step * Regarding this, since the influence of the reduction ratio F2 of the final step on the grain structure becomes apparent when the value of F2 is 11.1 or more, in Formula 4, when F2 < 11.1 is satisfied, F2 * = 0.1, and when F2 ≥ 11.1 is satisfied, F2 * = F2 - 11.

[0069] Formula 1 shows the preferable manufacturing conditions in the finish rolling where the rolling temperature FT2 of the final step is 930°C or more. When FT2 is less than 930°C, it has no meaning for the value of the grain structure formation parameter ω. That is, FT2 is 930°C or more and ω is 110 or less.

[0070] (The starting temperature of finish rolling is 1000°C or higher and 1150°C or lower) If the starting temperature of finish rolling is less than 1000°C, recrystallization of the structure processed by rolling in the previous stage does not occur sufficiently except for the final two stages, the grain structure in the surface layer region of the steel sheet develops, and it is impossible to satisfy R / t, which is the average value of the minimum bending radius of the L-axis and C-axis / plate thickness, of 1.5 or less. Therefore, the starting temperature of finish rolling is preferably 1000°C or higher. More preferably, it is 1050°C or higher. On the other hand, if the starting temperature of finish rolling exceeds 1150°C, the austenite grains become excessively coarse and the toughness deteriorates. Therefore, the starting temperature of finish rolling is preferably 1150°C or lower.

[0071] (Control each condition during the reduction in the final two stages of multi-stage finish rolling, and perform finish rolling under the condition that the grain structure formation parameter ω calculated by Equation 1 is 110 or less) In the production of the steel sheet according to the present embodiment, the hot rolling conditions in the final two stages of multi-stage finish rolling are important. The reduction ratios F1 and F2 during the rolling in the final two stages, which are used for the calculation of ω defined by Equation 1, are numerical values expressed as percentages of the value obtained by dividing the difference in plate thickness before and after rolling in each stage by the plate thickness before rolling. The diameters D1 and D2 of the rolling rolls are measured at room temperature, and there is no need to consider flattening during hot rolling. Also, the plate thicknesses t1 and t2 on the rolling inlet side and the plate thickness t f after finish rolling may be measured in situ using radiation or the like, or may be obtained by calculation from the rolling load considering the deformation resistance and the like. The plate thickness t f after finish rolling may be the final plate thickness of the steel sheet after completion of hot rolling. The rolling start temperatures FT1 and FT2 may be values measured by a thermometer such as a radiation thermometer between the finish rolling stands. The texture formation parameter ω is an index that takes into account the rolling strain introduced into the entire steel plate in the final two passes of finish rolling, the shear strain introduced into the surface layer region of the steel plate, and the recrystallization rate after rolling, and represents the ease of texture formation. When finish rolling in the final two passes is performed under the condition that the texture formation parameter ω exceeds 110, in the surface layer region, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> cannot be made 6.0 or less. Therefore, it is preferable to control the texture formation parameter ω to 110 or less. More preferably, the texture formation parameter ω is 98 or less.

[0072] (The rolling temperature FT1 one pass before the final pass is 960°C or higher and 1020°C or lower) If the rolling temperature FT1 one pass before the final pass is less than 960°C, recrystallization of the structure processed by rolling does not occur sufficiently, and the texture in the surface layer region cannot be controlled within the above range. Therefore, the rolling temperature FT1 should be 960°C or higher. On the other hand, if the rolling temperature FT1 exceeds 1020°C, due to coarsening of austenite grains, etc., the formation state of the processed structure and the recrystallization behavior change, and the texture in the surface layer region cannot be controlled within the above range. Therefore, the rolling temperature FT1 should be 1020°C or lower.

[0073] (The reduction ratio F1 one pass before the final pass is more than 11.0% and 23.0% or less) If the reduction ratio F1 one pass before the final pass is 11.0% or less, the amount of strain introduced into the steel plate by rolling becomes insufficient and recrystallization does not occur sufficiently, and the texture in the surface layer region cannot be controlled within the above range. Therefore, the reduction ratio F1 should be more than 11.0%. On the other hand, if the reduction ratio F1 exceeds 23.0%, the lattice defects in the crystal become excessive and the recrystallization behavior changes, and the texture in the surface layer region cannot be controlled within the above range. Therefore, the reduction ratio F1 should be 23.0% or less. The reduction ratio F1 (%) is calculated as follows. F1 = (t1 - t2) / t1 × 100

[0074] (The rolling temperature FT2 in the final pass is 930°C or higher and 995°C or lower) When the final rolling temperature FT2 is less than 930°C, the recrystallization rate of austenite significantly decreases, and in the surface layer region, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> cannot be made 6.0 or less. Therefore, the rolling temperature FT2 should be 930°C or higher. On the other hand, when the rolling temperature FT2 exceeds 995°C, the formation state of the processed structure and the recrystallization behavior change, so the aggregate structure in the surface layer region cannot be controlled within the above range. Therefore, the rolling temperature FT2 should be 995°C or lower.

[0075] (The reduction ratio F2 in the final stage is more than 11.0% and 22.0% or less) When the reduction ratio F2 in the final stage is 11.0% or less, the amount of strain introduced into the steel sheet by rolling is insufficient, and recrystallization does not occur sufficiently, so the aggregate structure in the surface layer region cannot be controlled within the above range. Therefore, the reduction ratio F2 should be more than 11.0%. On the other hand, when the reduction ratio F2 exceeds 22.0%, the lattice defects in the crystal become excessive and the recrystallization behavior changes, so the aggregate structure in the surface layer region cannot be controlled within the above range. Therefore, the reduction ratio F2 should be 22.0% or less. The reduction ratio F2 (%) is calculated as follows. F2 = (t2 - t f ) / t2 × 100

[0076] (The total reduction ratio F in the final three stages t is 35% or more) The total reduction ratio F in the final three stages t should be larger to promote the recrystallization of austenite. When the total reduction ratio F in the final three stages t is less than 35%, the recrystallization rate of austenite significantly decreases, and in the surface layer region, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> cannot be made 6.0 or less. The total reduction ratio F in the final three stages t is calculated by the following formula. F t = (t0 - t f ) / t0 × 100 Here, t0 is the plate thickness (unit: mm) at the start of rolling two stages before the final stage.

[0077] In the finish rolling process, the above-mentioned various conditions are controlled simultaneously and inseparably. The above-mentioned various conditions do not mean that only one of the conditions needs to be satisfied. When all of the above-mentioned conditions are satisfied simultaneously, the aggregate structure in the surface layer region can be controlled within the above range.

[0078] Subsequent to the finish rolling, a cooling process and a coiling process are performed. Controlling the cooling rate after the finish rolling and performing heat treatment under controlled conditions contribute to the control of the hardness uniformity.

[0079] <Cooling Process> (Cool from 800°C to 450°C at an average cooling rate of 60°C / second or more) In the cooling process, the hot-rolled steel sheet after the finish rolling is cooled to the coiling temperature described below so that the average cooling rate from 800°C to 450°C is 60°C / second or more. This is to suppress the excessive generation of ferrite and pearlite in the temperature range of 800°C to 450°C. Since transformation is unlikely to occur in the temperature range of 800°C or higher, the cooling rate is not specified. However, in general hot-rolling equipment, the cooling zone is reached within several seconds after the completion of the finish rolling. Therefore, the actual holding time at 800°C or higher is within 5 seconds after the completion of the finish rolling. On the other hand, since the temperature range between 800°C and 450°C is a temperature range where transformation occurs, it is cooled at an average cooling rate of 60°C / second or more. If the cooling stop temperature exceeds 450°C or the average cooling rate is less than 60°C / second, ferrite, pearlite, etc. are generated during the cooling process, and it may not be possible to ensure a total of 70% or more of martensite, tempered martensite, and bainite, and it may not be possible to achieve both strength and bend formability. There is almost no concern about the occurrence of ferrite or pearlite transformation at temperatures of 450°C or lower, so there is no need to specify the cooling rate.

[0080] <Coiling Process> The hot-rolled steel sheet after hot rolling may be wound into a coil. If the coiling temperature exceeds 450°C, ferrite, pearlite, etc. may be generated, and it may not be possible to ensure a total of 70% by volume or more of martensite, tempered martensite, and bainite. Therefore, the coiling temperature is set to 450°C or lower.

[0081] <Pickling process> Pickling may be performed on the hot-rolled steel sheet after the cooling process or after the coiling process. By performing pickling, the plating property in the subsequent manufacturing process can be improved, or the chemical conversion treatment property in the automobile manufacturing process can be enhanced. Also, when the hot-rolled steel sheet with scale is lightly rolled, the scale may peel off, and it may become scratched when it is pressed in. Therefore, before performing the light rolling described later, pickling is first performed on the hot-rolled steel sheet. The pickling conditions are not particularly limited, but it is common to pickle with hydrochloric acid, sulfuric acid, etc. containing an inhibitor.

[0082] <Light rolling process> The light rolling process is not essential, but for the purpose of increasing strength by introducing dislocations, rolling may be performed with a rolling reduction rate of 20% or less. However, if the rolling reduction rate exceeds 20%, not only does the effect saturate, but the recovery of the introduced dislocations becomes insufficient, leading to a significant deterioration in elongation. From this, when performing rolling, it is preferable that the rolling reduction rate is 20% or less. The rolling may be performed with a rolling reduction of 20% or less in one pass, or it may be performed in multiple passes so that the cumulative rolling reduction rate is 20% or less.

[0083] <Heat treatment process> (Maintained for 10 seconds or more in the temperature range of 200°C or higher and less than 450°C) The hot-rolled steel sheet after the light rolling process is reheated to a temperature range of 200 to less than 450°C, and heat treatment is performed to hold it in this temperature range for 10 seconds or more. By this heat treatment, the volume ratio of retained austenite in the microstructure can be made 5% or more, and the solid solution carbon concentration in the retained austenite can be made 0.5% by mass or more. If the heat treatment temperature is less than 200°C or the holding time is less than 10 seconds, a sufficient austenite volume fraction or solid solution carbon concentration cannot be ensured. Also, when the heat treatment temperature reaches 450°C or higher, the decrease in strength becomes significant, and a tensile strength of 980 MPa or more cannot be achieved. There is no need to specify the upper limit of the holding time, and it may be determined in consideration of the soaking property and economic rationality according to the heating method. For example, when using a heat treatment facility that runs the steel plate, about 1000 seconds is a realistic upper limit for the purpose of shortening the equipment occupancy time. However, in the case of a box-type heating device, heating for several hours to several tens of hours may be performed as a time sufficient for the temperature inside the coil to be uniform. The holding time means the time during which the steel plate is in the temperature range of 200°C or higher and less than 450°C after reheating. Even if there is a temperature change during this period, it is acceptable as long as it stays in this temperature range for a predetermined time. The cooling after the heat treatment (after the temperature drops below 200°C) is not particularly specified.

[0084] <Plating process> The steel plate according to the present embodiment can be obtained by the manufacturing method including the above steps. However, when the steel plate according to the present embodiment is made into a hot-dip galvanized steel plate or an alloyed hot-dip galvanized steel plate for the purpose of improving corrosion resistance, it is preferable to perform hot-dip galvanizing on the hot-rolled steel plate after the heat treatment process. Since galvanizing contributes to the improvement of corrosion resistance, it is desirable to perform galvanizing in the case of application to uses where corrosion resistance is expected. The galvanizing is preferably hot-dip galvanizing. The conditions for hot-dip galvanizing are not particularly limited and may be performed under known conditions. In addition, an alloyed hot-dip galvanized steel plate can be manufactured by alloying the hot-rolled steel plate (hot-dip galvanized steel plate) after hot-dip galvanizing. Since the alloyed hot-dip galvanized steel plate can impart effects such as improvement in spot weldability and improvement in slidability during drawing forming in addition to the improvement in corrosion resistance, alloying may be performed according to the application. The above hot-dip galvanizing treatment and alloyed hot-dip galvanizing treatment may be performed after cooling to room temperature once after the heat treatment at 200°C or higher and less than 450°C, or may be performed without cooling. In addition to zinc plating, the steel sheet according to the present embodiment can be manufactured even if Al plating, plating containing Mg, or electroplating is performed.

Example

[0085] The hot-rolled steel sheet according to the present invention will be described more specifically below with reference to examples. However, the following examples are examples of the hot-rolled steel sheet of the present invention, and the hot-rolled steel sheet of the present invention is not limited to the following embodiments. The conditions in the examples described below are one set of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these one set of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the object of the present invention.

[0086] Steel having the chemical components shown in Table 1 was cast. After casting, it was reheated as it was or once cooled to room temperature and then reheated to the temperature range shown in Table 2. Then, at a temperature of 1100 °C or higher, the slab was rough-rolled to the thickness of the rough-rolled sheet described in Table 2 to produce a rough-rolled sheet. Next, the obtained rough-rolled sheet was subjected to multi-stage finish rolling consisting of a total of 7 stages. In the multi-stage finish rolling process, finish rolling was started from the rolling start temperature described in Table 2, and by rolling a total of 4 stages excluding the final 3 stages of rolling from the start of rolling, it was rolled to the thickness of the 5th stage rolling time sheet thickness: t0 described in Table 3. Thereafter, after performing the final two-stage hot rolling under the conditions described in Tables 2 to 4, cooling and coiling were performed. The final sheet thickness of the steel sheet after hot rolling was set as the sheet thickness t f after finish rolling.

[0087] Regarding the hot-rolled steel sheet obtained as described above, after pickling, for a part thereof, light reduction was performed under the conditions described in Table 4, and heat treatment was performed. Furthermore, thereafter, for a part thereof, hot-dip galvanizing (GI) or alloyed hot-dip galvanizing (GA) was performed as described in Table 4. The plating bath temperature was 445 °C, and during alloying, it was held at 445 °C for 10 seconds.

[0088]

Table 1

[0089] [Table 2]

[0090] [Table 3]

[0091] [Table 4]

[0092] For the obtained hot-rolled steel sheet, by the method described above, the microstructure was observed in the range of 1 / 8 to 3 / 8 of the sheet thickness (1 / 8 thickness to 3 / 8 thickness) from the surface centering on the position at a depth of 1 / 4 of the sheet thickness (1 / 4 thickness) from the surface, and the volume fractions of martensite (FM), tempered martensite (t-M), bainite (B), retained austenite (γ), ferrite (α), and pearlite (P) were determined.

[0093] In addition, for these hot-rolled steel sheets, by the method described above, the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation of {110}<001> in the surface layer region, which is the range from the surface to the position at 1 / 10 of the sheet thickness, was determined.

[0094] In addition, for these hot-rolled steel sheets, the solid solution carbon concentration in the retained austenite was determined.

[0095] Furthermore, for these hot-rolled steel sheets, from the 1 / 2 position in the width direction of the hot-rolled steel sheet, strip-shaped test pieces were cut out, and bending (L-axis bending) with the bending ridge line parallel to the rolling direction (L direction) and bending (C-axis bending) with the bending ridge line parallel to the direction perpendicular to the rolling direction (C direction) were performed in accordance with JIS Z2248:2006 (V-block 90° bending test), the cracks generated on the inner side of the bending were investigated, and the limiting bending R / t was determined. If R / t is 1.5 or less, it was judged that the bend formability is excellent.

[0096] Also, a JIS No. 5 tensile test piece was taken so that the direction perpendicular to the rolling direction was the tensile direction, and a tensile test was conducted in accordance with JIS Z2241:2011 to measure the tensile strength (TS) and the total elongation (EL). When the product of the tensile strength TS (MPa) and EL (%) was 19000 (MPa·%) or more and the total elongation EL was 16.0% or more, it was judged that the elongation was excellent.

[0097] The respective results are shown in Tables 5 to 6.

[0098]

Table 5

[0099]

Table 6

[0100] As can be seen from Tables 1 to 6, in Examples Nos. 1 to 9, 14, 36, and 38 of the present invention, they had a tensile strength of 980 MPa or more and were excellent in bend processability and elongation. On the other hand, in Comparative Examples Nos. 10 to 12, 15 to 35, and 37 in which one or more of the chemical composition, the microstructure, the sum of the pole densities, and the amount of carbon in solid solution in austenite were outside the scope of the present invention, one or more of the tensile strength, the bend processability, and the elongation did not reach the target values.< / uvw> < / uvw>

Claims

1. By mass%, C: 0.02 - 0.30%, Si: 0.01 - 2.50%, Mn: 1.00 - 3.00%, P: 0.100% or less, S: 0.0001 - 0.0100%, Al: 0.005 - 1.000%, N: 0.010% or less, Ti: 0 - 0.20%, Nb: 0 - 0.20%, V: 0 - 0.200%, Ni: 0 - 2.00%, Cu: 0 - 2.00%, Cr: 0 - 2.00%, Mo: 0 - 2.00%, W: 0 - 0.100%, B: 0 - 0.0100%, REM: 0 - 0.0300%, Ca: 0 - 0.0300%, Mg: 0 - 0.0300%, containing, with the balance being Fe and impurities, having a chemical composition of the chemical composition satisfies Si + Al ≥ 1.00% and the microstructure contains, by volume ratio, a total of 70% or more of martensite, tempered martensite and bainite, and contains 5 - 20% of retained austenite, in the surface layer region, which is the range from the surface to a position of 1 / 10 of the plate thickness, the sum of the average pole density of the orientation group consisting of {211}<111> - {111}<112> and the pole density of the crystal orientation of {110}<001> is 6.0 or less, the solid solution carbon concentration in the retained austenite is 0.5 mass% or more, the tensile strength is 980 MPa or more A hot-rolled steel sheet characterized by the above.

2. The chemical composition is, by mass%, Ti: 0.001 - 0.20%, Nb: 0.001 to 0.20%, V: 0.001 to 0.200%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.005 to 0.100%, B: 0.0005 to 0.0100%, REM: 0.0003 to 0.0300%, Ca: 0.0003 to 0.0300%, Mg: 0.0003 to 0.0300%, containing one or more selected from The hot-rolled steel sheet according to claim 1, characterized in that it is as described above.

3. The hot-rolled steel sheet according to claim 1 or 2, characterized in that it is provided with a hot-dip galvanized layer on the surface.

4. The hot-rolled steel sheet according to claim 3, characterized in that the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

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