HOT-ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Abstract

This hot-rolled steel sheet has a predetermined chemical composition and a predetermined metallographic structure, a ratio between a maximum depth of a region where, on one surface, a rotation angle between a line normal of one surface and a pole (011) near the line normal of one surface is 5° or less and a maximum depth of a region where, on the other surface, a rotation angle between a line normal of the other surface and a pole (011) near the line normal of the other surface is 5° or less is 1.00 to 1.20, and a tensile strength of 1150 MPa or more.

Description

HOT-ROLLED STEEL SHEET AND MANUFACTURING METHOD OF THE SAME Technical field of the invention [1] The present invention relates to a hot-rolled steel sheet and to a method of manufacturing the same. Priority is claimed in Japanese patent application No. 2020-082655, filed on May 8, 2020, the contents of which are incorporated herein by reference. Precedent technique [2] In recent years, efforts have been made to reduce the weight of automobiles and every machine component. Designing an optimal shape for the component ensures rigidity and thus allows for weight reductions in automobiles and every machine component. Furthermore, in components formed from blanks, such as those formed by pressing, weight can be reduced by decreasing the thickness of the component material sheets. However, if the strength characteristics of the components, such as static fracture resistance and yield strength, are to be maintained while reducing sheet thicknesses, it becomes necessary to use high-strength materials. Specifically, for automotive suspension components such as lower control arms, trailing arms, and knuckles, studies have been initiated on the application of steel sheets with a tensile strength exceeding 780 MPa. Since these automotive suspension components are manufactured using processes such as flanging, draw flanging, bending, and similar methods on steel sheets, the steel sheets used for these components must possess excellent formability. [3] For example, Patent Document 1 describes a hot-rolled steel sheet in which, in a hot-rolling step, the final rolling temperature and rolling reduction are set within predetermined intervals, thereby controlling the grain sizes and aspect ratios of the previous austenite and reducing anisotropy. [4] Patent Document 2 describes a cold-rolled steel sheet in which, in a hot-rolling step, the rolling reduction and average strain rate are set within appropriate intervals in a predetermined final rolling temperature range, thereby improving toughness. [5] In order to further reduce the weight of automobiles, each machine component, and the like, it is also expected that steel sheets with a thickness based on a cold-rolled steel sheet will be applied to automotive suspension components. The techniques described in Patent Document 1 and Patent Document 2 are effective in manufacturing automotive suspension components to which a high-strength steel sheet is applied. In particular, these techniques are important discoveries for achieving an effect related to the formability and impact properties of automotive suspension components with a complex shape. [6] However, automotive suspension components are always subjected to cyclic loads attributed to weight-induced vibrations, twisting, buckling, and the like. Therefore, adequate component durability is an important characteristic. As described above, automotive suspension components undergo various forms. In a flat portion near the inside of a portion R that has been bent or bent and bent back, there are many locations where contact with a die is weak. That flat portion near the inside of the portion R has surface properties in which relatively sharp, recessed areas periodically form due to the development of irregularities in the surface layer from forming and contact with a die under a weak load (hereafter, a change in such surface properties will be referred to as forming damage). [7] For example, in Non-Patent Document 1, the development of surface layer irregularities by such formation near the inner bend is simulated using uniaxial distortion, and the fatigue properties of a steel sheet that has been brought into contact with a die are investigated. These fatigue properties of the steel sheet are degraded by the machined parts, but it has been found that the changes differ according to the metallographic structure. In steel sheets of a class higher than 780 MPa that are applied to automotive suspension components, the volume percentage of the complete hard structure increases to develop strength, but there are no techniques that sufficiently improve the fatigue properties of steel sheets that have been damaged by the formation in this strength region. ui ou Documents of the previous technique Patent documents [8] Patent Document 1: Japanese Patent No. 5068688 Patent Document 2: Japanese Patent No. 3858146 Non-patent document Non-patent document 1: Sosei-To-Kakou (Journal of the Japanese Society for Technology of the Plasticity) vol. 57 no. 666 (2016-7) pp. 660 to 666 Description of the invention Problems that must be solved by the invention

[10] The inventors hereof have made a technical development to reduce forming damage and improve fatigue properties. The inventors hereof have recently discovered that if the depths of the recessed parts exceed a certain value, the fatigue properties of hot-rolled steel sheets deteriorate significantly.

[11] Notch sensitivity increases as the strength of a steel sheet increases. Therefore, when applying high-strength steel sheets of a class greater than 780 MPa to automotive suspension components, it is necessary to improve the fatigue properties of the formed parts. One method for reducing notch depths is to increase the contact pressure of a die. However, the contact pressure of a die is a forming factor that controls the amount of viscous plastic flow during forming. Furthermore, when pressing a high-strength steel sheet into a complex shape, it is difficult to increase the contact pressure under a predetermined press load.

[12] In view of the circumstances described above, an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent formability and having excellent fatigue properties in a portion damaged by forming and a method of manufacturing the same. Means to solve the problem

[13] As a result of inventive studies, the inventors hereof paid attention to the fact that the depths of the recessed parts in a formed-damaged portion are derived from the uneven distortion of the front and back surfaces of a hot-rolled steel sheet during forming and found that there is a relationship between the depths of the recessed parts after contact with a die (after forming) and the characteristics of the macroscopic crystal orientation distributions on the front and back surfaces of the steel sheet.The inventors of the present found that, when the appropriate chemical composition and metallographic structure are provided to obtain high strength and excellent formability, and the specific crystal orientations in the direction of the sheet thickness on the front and back surfaces are controlled, it is possible to reduce the depths of the recessed parts in the formation-damaged portion and thus suppress the deterioration of the fatigue properties of the formation-damaged portion.

[14] The essence of the present invention based on the findings described above is as follows. (1) A hot-rolled steel sheet according to an aspect of the present invention containing, as a chemical composition, in % by mass, C: 0.085% to 0.190%, Yes: 0.40% to 1.40%, Mn: 1.70% to 2.75%, Al: 0.01% to 0.55%, Nb: 0.006% to 0.050%, P: 0.080% or less, S: 0.010% or less, N: 0.0050% or less, Ti: 0.004% to 0.180%, B: 0.0004% to 0.0030%, Mo: 0% to 0.150%, V: 0% to 0.300%, Cr: 0% to 0.500%, Ca: 0% to 0.0020%, and a residue consisting of Fe and an impurity, wherein, in metallographic structures at a position of 1 / 4 in the sheet thickness direction from a surface and at a position of 1 / 2 in the sheet thickness direction from the surface, in % by volume, the residual austenite is 3.0% to 12.0%, the bainite is 75.0% or more and less than 97.0%, the ferrite is 10.0% or less, the martensite is 10.0% or less, and the pearlite is 3.0% or less, in a metallographic structure of a region from the surface to a position of 100 pm in the sheet thickness direction from the surface, an average grain diameter of the above austenite grains is 25.0 pm or less, a ratio between a maximum depth of a region where, on one surface, a rotation angle between a normal line of one surface and a pole (011) near the normal line of one surface is 5ooo less and a maximum depth of a region where, on the other surface, a rotation angle between a normal line of the other surface and a pole (011) near the normal line of the other surface is 5ooo less is 1.00 to 1.20. and a tensile strength is 1150 MPa or more. (2) Hot-rolled steel sheet conforming to (1) may further contain, as a chemical composition, in % by mass, one or more selected from the group consisting of, Mo: 0.030% to 0.150%, V: 0.050% to 0.300%, Cr: 0.050% to 0.500%, and Ca: 0.0006% to 0.0020%. (3) A method of manufacturing a hot-rolled steel sheet according to another aspect of the present invention is a method of manufacturing the hot-rolled steel sheet according to (1) or (2), comprising a continuous casting step of, in the continuous casting of a plate having the chemical composition according to (1), performing the continuous casting such that an average cooling rate gradient of a surface temperature in a region from a meniscus to 1.0 m from the meniscus is 0.20 to 15.00°C / s 2 to obtain the plate, a heating step of heating the plate to 1200°C or more, a hot rolling step of rough rolling the plate after heating, and finishing rolling such that the total rolling reduction in a temperature range of 870°C to 980°C is 80% or more and the time between rolling boxes becomes 4.00 seconds or less in the temperature range of 870°C to 980°C, a cooling step of cooling to a temperature range of 300°C to 550°C, and a winding step of winding so that the winding temperature is in the temperature range of 300°C to 550°C after cooling. (4) The method of manufacturing hot-rolled steel sheet in accordance with (3) may further include a tempering step of holding at a temperature range of 200°C or higher and lower than 450°C for 90 to 80,000 seconds after the rolling step. (5) The method of manufacturing hot-rolled steel sheet in accordance with (3) or (4) may further include a plating step to perform a hot-dip galvanizing treatment on the hot-rolled steel sheet after the coiling step or the hot-rolled steel sheet after the tempering step with a thermal history where the residence time within a temperature range of 450°C to 495°C becomes 75 seconds or less. Effects of the invention

[15] In accordance with the aspects of the present invention described above, it is possible to provide a hot-rolled steel sheet having high strength and excellent formability, and having excellent fatigue properties in a forming-damaged portion, and a method for manufacturing the same. In accordance with the aspects of the present invention described above, since the fatigue properties in a forming-damaged portion are excellent, it is possible to reduce the depths of the recessed areas in a flat portion near a portion R that is formed during the forming of portion R. ui ou Brief description of the drawings

[16] Figure 1 is a view showing a relationship between fatigue strength and maximum depth ratio in a damaged portion of the formation in an example. Figure 2 is a view showing a relationship between fatigue strength and average grain diameter of the previous austenite grains in the formation-damaged portion in the example. Figure 3 is a view showing a relationship between an average cooling rate gradient from a surface temperature and a maximum depth ratio in a region from a meniscus to 1.0 m from the meniscus in the example. Modalities of the invention

[17] A hot-rolled steel sheet in accordance with the present embodiment will now be described in detail. However, the present invention is not limited solely to a configuration described herein and may be modified in various ways within the scope of the essence of the present invention. The numerical limit intervals expressed below using 'a' include the lower and upper limits within the intervals. Numerical values ​​expressed with 'greater than' and 'less than' are not included in the numerical intervals. All percentages with respect to chemical compositions indicate mass percentages.

[18] Hot-rolled steel sheet in accordance with the present specification contains, in % by mass, C: 0.085% to 0.190%, Si: 0.40% to 1.40%, Mn: 1.70% to 2.75%, Al: 0.01% to 0.55%, Nb: 0.006% to 0.050%, P: 0.080% or less, S: 0.010% or less, N: 0.0050% or less, Ti: 0.004% to 0.180%, B: 0.0004% to 0.0030%, Mo: 0% to 0.150%, V: 0% to 0.300%, Cr: 0% to 0.500%, Ca: 0% to 0.0020%, and a residue consisting of Fe and an impurity. Each element will then be described in detail.

[19] C: 0.085% to 0.190% Carbon (C) is one of the elements that determines the strength of hot-rolled steel sheet and also affects the amount of residual austenite. When the carbon content is less than 0.085%, it is not possible to adjust the volume percentage of residual austenite to 3.0% or more. Therefore, the carbon content is adjusted to 0.085% or more. The carbon content is preferably 0.115% or more. On the other hand, when the carbon content exceeds 0.190%, the volume percentage of residual austenite increases, and the hole expansion capacity of the hot-rolled steel sheet deteriorates. Therefore, the carbon content is adjusted to 0.190% or less. Ideally, the carbon content should be 0.170% or less.

[20] Yes: 0.40% to 1.40% Silicon (Si) is an element that improves the strength of hot-rolled steel sheet by strengthening the solid solution. Furthermore, Si also suppresses the formation of carbides such as pearlite. To achieve these effects, the Si content is adjusted to 0.40% or higher. Preferably, the Si content is 0.90% or higher. If the Si content is less than 0.40%, the volume percentage of residual austenite will be less than 3.0%, and the volume percentage of pearlite will exceed 3.0%. On the other hand, as the Si content increases, the volume percentage of residual austenite also increases; however, when the Si content exceeds 1.40%, the volume percentage of residual austenite exceeds 12.0%, which degrades the hole expansion capacity of the hot-rolled steel sheet. Furthermore, since Si has a high oxide-forming capacity, when the Si content is excessive, oxides form at welds, or the chemical convertibility of the hot-rolled steel sheet is degraded during component manufacturing. Therefore, the Si content is adjusted to 1.40% or less. The preferred Si content is 1.30% or less.

[21] Mη: 1.70% to 2.75% Manganese (Mn) is a necessary element for improving the strength of hot-rolled steel sheet. When the Mn content is less than 1.70%, the ferrite volume percentage exceeds 10.0%, making it impossible to achieve a tensile strength of 1150 MPa or higher. Therefore, the Mn content is adjusted to 1.70% or higher. Preferably, the Mn content is 1.80% or higher. Furthermore, when the Mn content exceeds 2.75%, the toughness of a cast plate deteriorates, and hot rolling is not possible. Therefore, the Mn content is adjusted to 2.75% or less. The preferred Mn content is 2.70% or less.

[22] Al: 0.01% to 0.55% Aluminum (Al) acts as a deoxidizing agent and improves the cleanliness of steel. To achieve this effect, the Al content is adjusted to 0.01% or more. Ideally, the Al content should be 0.10% or more. On the other hand, when the Al content exceeds 0.55%, casting becomes difficult. Therefore, the Al content is adjusted to 0.55% or less. Al is an oxidizing element, and the Al content is preferably 0.30% or less to achieve an additional improvement in continuous casting capacity and a cost reduction effect.

[23] Nb: 0.006% to 0.050% Nitrogen (Nb) suppresses the abnormal growth of austenite grains in a hot rolling pass and thus reduces the depth of the roughened sections in a formation-damaged area. To achieve this effect, the Nb content is adjusted to 0.006% or higher. When the Nb content is adjusted to 0.025% or higher, the effect described above becomes saturated. On the other hand, when the Nb content exceeds 0.050%, the toughness of the cast plate deteriorates, and hot rolling is not possible. Therefore, the Nb content is adjusted to 0.050% or less. Ideally, the Nb content should be 0.025% or less.

[24] P: 0.080% or less Phosphorus (P) is an impurity that is inevitably incorporated into hot-rolled steel sheet during the manufacturing process. The higher the P content, the more embrittled the hot-rolled steel sheet becomes. For hot-rolled steel sheet used in automotive suspension components, a P content of up to 0.080% is acceptable. Therefore, the P content is adjusted to 0.080% or less. The P content is preferably 0.010% or less. When the P content is reduced to less than 0.0005%, the cost of dephosphorization increases significantly, and therefore the P content can be adjusted to 0.0005% or more.

[25] S: 0.010% or less If molten steel contains a high amount of sulfur (S), manganese sulfide (MnS) forms, degrading the ductility and toughness of the hot-rolled steel sheet. Therefore, the S content is adjusted to 0.010% or less. Preferably, the S content is 0.008% or less. When the S content is reduced to less than 0.0001%, the cost of desulfurization increases significantly, and therefore, the S content can be adjusted to 0.0001% or more.

[26] N: 0.0050% or less Nitrogen (N) is an impurity that is inevitably incorporated into hot-rolled steel sheet during the manufacturing process. When the N content exceeds 0.0050%, the hole expansion capacity of the hot-rolled steel sheet deteriorates. Therefore, the N content is adjusted to 0.0050% or less. Ideally, the N content should be 0.0040% or less. When the N content is reduced to less than 0.0001%, the manufacturing cost of the steel increases significantly, and therefore, the N content can be adjusted to 0.0001% or more.

[27] Ti: 0.004% to 0.180% Titanium (Ti) enhances the effect of the boron (B) content, which will be described below, by forming a nitride. To achieve this effect, the Ti content is adjusted to 0.004% or higher. Preferably, the Ti content is 0.006% or higher. To further enhance the effect of the boron content by containing Ti, the Ti content should be adjusted to 0.013% or higher. On the other hand, titanium (Ti) is an element that degrades the toughness of a plate. When the Ti content exceeds 0.180%, plate cracking frequently occurs, and the solutionization cost increases. Therefore, Ti is kept to 0.180% or less. The preferred Ti content is 0.140% or less, or 0.100% or less.

[28] B: 0.0004% to 0.0030% Boron (B) is an element that suppresses ferrite formation during a cooling step. To achieve this effect, the B content is adjusted to 0.0004% or more. The B content is preferably 0.0011% or more. On the other hand, even when it contains more than 0.0030% B, the effect described above becomes saturated, and therefore the B content is adjusted to 0.0030% or less. The B content is preferably 0.0020% or less.

[29] The remainder of the chemical composition of the hot-rolled steel sheet conforming to this modality may be Fe and an impurity. In this modality, impurity means a substance that is incorporated from the ore as raw material, scrap, manufacturing environment, or the like and is permitted to the extent that the hot-rolled steel sheet conforming to this modality is not adversely affected.

[30] Hot-rolled steel sheet conforming to the present modality may contain one or more of the group consisting of Mo, V, Cr, and Ca as an arbitrary element in place of some Fe. If the arbitrary element is not contained, the lower limit of the content is 0%. Each arbitrary element is described below.

[31] Mo: 0% to 0.150% Molybdenum (Mo) is an element that improves the hardenability of steel and can be included as a strength-adjusting element in hot-rolled steel sheets. To reliably achieve the effect described above, the Mo content is preferably adjusted to 0.030% or more. On the other hand, even when it contains more than 0.150% Mo, the effect described above is saturated. Therefore, the Mo content is preferably adjusted to 0.150% or less.

[32] V: 0% to 0.300% Zinc (V) has an effect on improving strength by forming a fine carbide. To reliably achieve this effect, the zinc content is preferably adjusted to 0.050% or more. However, when zinc is present in excess, a nitride forms in the steel, which degrades the plate's toughness and makes threading difficult. Therefore, the zinc content is preferably adjusted to 0.300% or less.

[33] Cr: 0% to 0.500% Chromium (Cr) is an element that produces an effect similar to that of manganese (Mn). To reliably achieve a strength-enhancing effect in hot-rolled steel sheets, the Cr content is preferably set at 0.050% or higher. However, even with a Cr content exceeding 0.500%, the aforementioned effect is saturated. Therefore, the Cr content is preferably set at 0.500% or lower.

[34] Ca: 0% to 0.0020% Calcium forms fine CaS, thereby improving local ductility and enhancing hole expansion capacity. However, when the calcium content exceeds 0.0020%, manufacturability degrades due to oxide formation in the nozzle during continuous casting, and formability deteriorates due to the carryover of these oxides. Therefore, the calcium content is preferably adjusted to 0.0020% or less. To achieve the effect described above, the calcium content is preferably adjusted to 0.0006% or more.

[35] The above-described chemical composition of the hot-rolled steel sheet can be analyzed using a spark-discharge emission spectrophotometer or similar instrument. For C and S, the values ​​identified by burning the hot-rolled steel sheet in an oxygen stream using a gas component analyzer or similar instrument and measuring C and S by an infrared absorption method are adopted. In addition, for N, a value identified by melting a test piece taken from the hot-rolled steel sheet in a helium stream and measuring N by a thermal conductivity method is adopted.

[36] The metallographic structure of the hot-rolled steel sheet in accordance with this modality will now be described. The characteristics of the metallographic structure are limited to an effect on improving the strength and formability of the hot-rolled steel sheet and also to the extent that fatigue properties can be improved in a forming-damaged portion.

[37] In hot-rolled steel sheet in accordance with the present modality, in the metallographic structures at a position of 1 / 4 in the sheet thickness direction from the surface and at a position of 1 / 2 in the sheet thickness direction from the surface, in % by volume, the residual austenite is 3.0% to 12.0%, the bainite is 75.0% or more and less than 97.0%, the ferrite is 10.0% or less, the martensite is 10.0% or less, and the pearlite is 3.0% or less, in the metallographic structure in a region from the surface to a position of 100 pm in the sheet thickness direction from the surface, the average grain diameter of the above austenite grains is 25.0 pm or less, and the ratio between the maximum depth of a region where, on one surface, the angle of rotation between the line normal of a surface and a pole (011) near the line normal of a surface is 5ooo less and the maximum depth of a region where, on the other surface, the angle of rotation between the line normal of the other surface and a pole (011) near the line normal of the other surface is 5ooo less is 1.00 to 1.20. Each regulation will be described below.

[38] Residual Austenite: 3.0% to 12.0% The volume percentage of residual austenite should be 3.0% or higher to improve the ductility of hot-rolled steel sheet. To improve the fatigue properties of hot-rolled steel sheet, the volume percentage of residual austenite is preferably adjusted to 6.0% or higher. Furthermore, if the volume percentage of residual austenite exceeds 12.0%, the hole expansion capacity of the hot-rolled steel sheet deteriorates. Therefore, the volume percentage of residual austenite is adjusted to 12.0% or less. The volume percentage of residual austenite is preferably 10.0% or less, or 9.0% or less.

[39] Bainite: 75.0% or more and less than 97.0% Bainite is a structure that offers an excellent balance between ductility and hole expansion capacity, while also possessing a predetermined strength. To achieve a total elongation of 13.0% or more, the volume percentage of residual austenite must be reduced to 3.0% or more, and the volume percentage of bainite must be reduced to less than 97.0%. Therefore, the ideal volume percentage of bainite is 95.0% or less. On the other hand, if the bainite volume percentage is less than 75.0%, the homogeneity of the structure is lost and the hole expansion capacity deteriorates. Therefore, the bainite volume percentage is adjusted to 75.0% or higher. Ideally, the bainite volume percentage is 80.0% or higher.

[40] Ferrite: 10.0% or less Ferrite is a structure with a high distortion capacity and is effective in improving the ductility of hot-rolled steel sheet; however, when its volume percentage is too high, the strength of the hot-rolled steel sheet decreases. When the ferrite volume percentage exceeds 10.0%, the strength of the hot-rolled steel sheet decreases, and the tensile strength falls below 1150 MPa. Therefore, the ferrite volume percentage is typically set at 10.0% or less. A lower limit of 6.0% or less is preferred. The lower limit for the ferrite volume percentage is not particularly restricted and can be 0%.

[41] Martensite: 10.0% or less Martensite increases strength but has a low capacity for local distortion, and an increase in its volume percentage degrades the local elongation and hole expansion capacity of hot-rolled steel sheets. When the volume percentage of martensite exceeds 10.0%, the hole expansion rate of hot-rolled steel sheets falls below 35.0%. Therefore, the volume percentage of martensite is typically set at 10.0% or less. A minimum of 6.0% or less is preferred. The lower limit for the volume percentage of martensite is not particularly restricted and can be 0%.

[42] Perlite: 3.0% or less Pearlite is a structure that degrades the hole expansion capacity of hot-rolled steel sheets. When the pearlite volume percentage exceeds 3.0%, the hole expansion rate of the hot-rolled steel sheet falls below 35.0%. Therefore, the pearlite volume percentage is adjusted to 3.0% or less. Ideally, the pearlite volume percentage is 1.5% or less. The lower limit for the pearlite volume percentage is not particularly restricted and can be 0%.

[43] Method for measuring the percentage by volume of residual austenite The volume percentage of residual austenite is measured by EBSP. EBSP analysis is performed at a 1 / 4-inch thickness depth from the surface of the hot-rolled steel sheet (a region extending from a depth of 1 / 8 inch thickness from the surface to a depth of 3 / 8 inch thickness from the surface) and at a 1 / 2-inch thickness depth from the surface (a region extending from a depth of 3 / 8 inch thickness from the surface to a depth of 5 / 8 inch thickness from the surface). A sample should be polished with silicon carbide paper from No. 600 to No. 1.1000 and then finish it to a mirror surface with a liquid containing diamond powder with grain sizes of 1 to 6 µm dispersed in a dilute solution, such as alcohol or pure water, and then finish it by electropolishing in order to sufficiently eliminate stress in a cross-section to be measured. Electropolishing is performed using a liquid mixture of ethanol perchlorate at a liquid temperature of -80°C. Here, the voltage during electropolishing must be adjusted so that the thickness of a polished layer on the surface layer becomes constant and no polishing defect, such as pitting, occurs.

[44] In EBSP measurements, the acceleration voltage is set to 15 to 25 kV, measurements are taken at intervals of at least 0.25 pm, and crystal orientation information is obtained at each measurement point at intervals of 150 pm or more in the sheet thickness direction and 250 pm or more in the roll direction. From the resulting crystal structures, grains with an fcc crystal structure are identified as the residual austenite using a Phase Map function installed in the OIM Analysis software (registered trademark) included with an EBSP analyzer. The ratio of measurement points identified as the residual austenite is then used to obtain the residual austenite area ratio (UI / OU). This residual austenite area ratio is considered the volume percentage of the residual austenite.

[45] Here, the greater the number of measurement points, the more desirable it is; therefore, narrow measurement intervals and a wide measurement range are preferable. However, if the measurement intervals are less than 0.01 pm, adjacent points interfere with the scattering width of an electron beam. Therefore, the measurement intervals are set to 0.01 pm or greater. Furthermore, the measurement range should be set to a maximum of 200 pm in the sheet thickness direction and 400 pm in the lamination direction. Additionally, the measurement is performed using an instrument that includes a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC 5 type detector manufactured by TSL). The vacuum level in the instrument is set to 9.6 * 10-5Pa or less, the irradiation current level is set to 13 and the electron beam irradiation level is set to 62.

[46] Method for measuring the percentage by volume of ferrite The volume percentage of ferrite is determined by the area ratio of crystal grains where an iron-based carbide does not form, as observed in a metallographic photograph. Ferrite is characterized by the absence of subgrain boundaries or transformational interfaces between crystal grains. Crystal grains lacking both iron-based carbide and these interfaces are defined as ferrite grains. A sample is taken such that a cross-section of the sheet thickness intersecting the rolling direction of the hot-rolled steel sheet is visible. This cross-section is then etched for 10 to 15 seconds using ethanol and a nital etching solution adjusted to a concentration of 3% to 5% to make the ferrite visible.The structure is observed using photographs of metallographic structures, each captured at 500x to 1000x magnification at either the 1 / 4 thickness position from the surface of the hot-rolled steel sheet (the region from a depth of 1 / 8 thickness from the surface to a depth of 3 / 8 thickness from the surface) or the 1 / 2 thickness position from the surface (the region from a depth of 3 / 8 thickness from the surface to a depth of 5 / 8 thickness from the surface). An optical microscope is used to capture the photographs of the structure.Metallographic structure photographs are prepared in three or more fields of view, one at a quarter of the way down the thickness of the sheet from the surface and the other at a half of the thickness of the sheet from the surface. The area ratios of the ferrite grains observed in each metallographic structure photograph are obtained, and their average is calculated, thus yielding the average ferrite area ratio. This average value is considered the ferrite volume percentage. Iron-based carbide is recognized as a black granular contrast that has a circle equivalent diameter of 1 pm or less and is observed in the crystal grain. ui ou

[47] Method for measuring the percentage by volume of martensite. The percentage by volume of martensite is the area ratio of martensite identified from photographs of metallographic structures.A sample is collected in such a way that a cross-section of the sheet thickness can be observed intersecting the rolling direction of the hot-rolled steel sheet at right angles, and the structure is observed using photographs of metallographic structures, each captured at a magnification of 500 to 1000 times at the position of 1 / 4 in the sheet thickness direction from the surface of the hot-rolled steel sheet (the region from a depth of 1 / 8 in the sheet thickness direction from the surface to a depth of 3 / 8 in the sheet thickness direction from the surface) or at the position of 1 / 2 in the sheet thickness direction from the surface (the region from a depth of 3 / 8 in the sheet thickness direction from the surface to a depth of 5 / 8 in the sheet thickness direction from the surface).The metallographic structure is etched using a LePera etching solution containing picric acid, sodium sulfite, and ethanol mixed with it at a liquid temperature of 60°C to 80°C for 30 to 60 seconds, thus making it visible. The massive structures observed as a white contrast in the captured structure photographs are the mixed structures of martensite and residual austenite. The area ratio of the mixed martensite and residual austenite structures is considered the total volume percentage of martensite and residual austenite. A value obtained by subtracting the volume percentage of residual austenite, measured by the method described above, from the total volume percentage of martensite and residual austenite is considered the volume percentage of martensite.

[48] ​​Method for measuring the percentage by volume of perlite The volume percentage of pearlite is calculated using the area ratio of the lamellar structures obtained by observing the structure in a metallographic photograph. The same photograph used to measure the volume percentage of ferrite can be used for the metallographic photograph. The pearlite area ratio identified from this metallographic photograph is considered the volume percentage of pearlite.

[49] Method for measuring the percentage by volume of bainite The volume percentage of bainite is a value obtained by subtracting from 100% the total volume percentages of residual austenite, ferrite, martensite, and pearlite measured by the methods described above.

[50] Average grain diameter of previous austenite grains: 25.0 pm or less As described previously, the depths of the recessed areas in the formation-damaged portion are formed by the development of irregularities due to plastic heave on the surface of the steel sheet and contact with a die under bent or bent-back-bent distortion. Among these, the degree of plastic heave on the surface of the steel sheet depends on the magnitude of the effective grain size in the surface layer of the steel sheet. In the metallographic structure, the effective grain size corresponds to the average grain diameter of the preceding austenite grains, and a preceding austenite grain boundary acts as the largest distortion unit. If the average grain diameter of the preceding austenite grains exceeds 25 mm...At 0 pm, the depths of the recessed areas in the formation-damaged portion become deep, and the fatigue properties of the hot-rolled steel sheet in the formation-damaged portion deteriorate. Therefore, the average grain diameter of the fore-austenite grains in a region of the surface layer (a region from the surface to a position 100 pm in the sheet thickness direction from the surface) is adjusted to 25.0 pm or less. The grain diameter of the fore-austenite grains is preferably 20.0 pm or less, or 15.0 pm or less. The average grain diameter of the above austenite grains is preferably as small as possible, but may be 3.0 pm or more, since an extremely high rolling load is needed to adjust the average grain diameter to less than 3.0 pm.

[51] Method of measuring the average grain diameter of previous austenite grainsTo measure the average grain diameter of the austenite grains above, a sample is collected such that a cross-section of the sheet thickness intersecting the rolling direction of the hot-rolled steel sheet at a right angle can be observed. The sample is used after the structure in the sheet thickness cross-section is made visible using a saturated aqueous solution of picric acid and an etching solution of sodium dodecylbenzenesulfonate. In a region of the surface layer (a region from the surface to a position 100 pm in the sheet thickness direction from the surface) of this sample, the equivalent circle diameters of the austenite grains above are measured using a photograph of the structure captured at 500x magnification using a scanning electron microscope. The scanning electron microscope must be equipped with a two-electron detector.For capturing the structural image, the sample is irradiated with an electron beam in vacuum at 9.6 × 10⁻⁵ Pa or less, an accelerating voltage of 15 kV, and an irradiation current level of 13. A secondary electron image of the surface layer region (the region from the surface of the hot-rolled steel sheet to the 100 pm position in the sheet thickness direction from the surface) is captured. The number of captured fields of view is set to 10 or more. In the captured secondary electron image, the foregrain boundaries of austenite are captured as a bright contrast. The equivalent circle diameter is calculated for one of the foregrain grains included in the observed field of view.The operation described above is performed on all anterior austenite grains included in the observed field of view, except for those not fully included in the captured field of view, such as grains at the end of the captured field of view. The equivalent circle diameters of all anterior austenite grains in the captured field of view are then obtained. The average grain diameter of the anterior austenite grains is calculated by averaging the equivalent circle diameters of the anterior austenite grains obtained in the individual captured fields of view.

[52] Ratio between the maximum depth of the region where, on one surface, the angle of rotation between the normal line of one surface and the pole (011) near the normal line of one surface is less than 500 and the maximum depth of the region where, on the other surface, the angle of rotation between the normal line of the other surface and the pole (011) near the normal line of the other surface is less than 500: 1.00 to 1.20 The recessed areas in the forming-damaged portion are created by the development of irregularities due to the plastic deformation of the steel sheet's surface layer and its contact with a die during forming. Based on this, the inventors found that the depth of the recessed areas depends on the degree of distortion of the steel sheet's surface layer and can be reduced by the austenite grain size in high-strength steel. However, the desired fatigue properties cannot be achieved in the forming-damaged portion by controlling the austenite grain size alone. Fatigue damage to a component progresses further in a standing wall and in the flat portion of a section R, as high stress is initiated in the section with the highest stiffness. This section R undergoes bending or folding-back distortion, such as hat formation.As a result of inventive studies, the inventors found that, even in a portion damaged by forming, the desired fatigue properties can be obtained by the fact that the maximum depths of the regions where the angle of rotation between the surface normal line and the pole (011) near the surface normal line is less than 500 differ on the front and back surfaces of the steel sheet and the depths of the recessed parts are obtained by the ratio between the maximum depths of the regions on the front and back surfaces of the steel sheet and the fact that the ratio is 1.00 to 1.20.Therefore, the ratio between the maximum depth of the region where the angle of rotation between the normal line of one surface of the steel sheet and the pole (011) near the normal line of the surface is 500 less and the maximum depth of the region where, on the other surface, the angle of rotation between the normal line of the other surface and the pole (011) near the normal line of the other surface is 500 less (hereafter simply referred to as the ratio between the maximum depths in some cases) is adjusted to 1.00 to 1.20. The ratio between the maximum depths is preferably 1.15 or less or 1.10 or less.

[53] Hereafter, a method for measuring the maximum depth of a region having a predetermined rotation angle between the normal line of a surface and the pole (011) near the normal line of a surface will be described. The measurement is performed by EBSP using a sample with a cross-section finished to a mirror surface by the same method as the sample used for the previous measurement of the volume percent of austenite grains. The sample must be finished by electrolytic polishing to sufficiently relieve stress in the cross-section to be measured. In EBSP measurements, the acceleration voltage is set to 15 to 25 kV, and the measurement interval is set to cover the full thickness of the sheet. The measurement interval should be 1000 pm or more in the rolling direction. Furthermore, since the purpose is to measure the average characteristics of the crystal orientations, the measurement intervals can be 5 pm or more. The measurement intervals are set to 25 pm or less to avoid an increase in the number of crystal grains that are not measured due to error. The crystal orientation data should be recorded along with the measurement coordinate system. From the obtained crystal orientation data, the rotation angle between the normal line of a steel sheet surface and the (011) pole near the normal line is measured using the following method.

[54] The rotation angle between the normal line of a surface (front or back surface) of the hot-rolled steel sheet and the pole (011) near this normal line is a value that is measured by plotting the crystal orientation data obtained by the EBSP measurement on a positive-pole figure. When plotting the crystal orientations on the positive-pole figure, in the positive-pole figure coordinate system, the poles of the (011) orientation are shown such that the normal lines (origin: ND) become the normal lines to the sheet surface of the hot-rolled steel sheet, the horizontal axis TD becomes the direction of the sheet width, and the axis RD orthogonal to the horizontal axis becomes the rolling direction.As described above, the crystal orientation is a group of points measured at predetermined intervals within a measurement range of 1000 pm or more in the rolling direction, covering the entire sheet thickness range. This group of points is divided into 20 sections in the sheet thickness direction, and a pole figure (011) is plotted. On the pole figure (011), at each depth direction position from a surface of the hot-rolled steel sheet plotted as described above, the angle between the origin ND (the normal line of a hot-rolled steel sheet surface) and the nearest pole (011) angle (011) is measured. This measurement value is defined as the rotation angle between the normal line of a surface and the pole (011) near the normal line. At each depth direction position, the maximum depth of a region is obtained where the rotation angle becomes less than 500.

[55] The above operation is performed on the front and back surfaces of the hot-rolled steel sheet, thereby obtaining the maximum depths of the regions having a predetermined angle of rotation between the surface normal line and the pole (011) near the surface normal line on both surfaces of the hot-rolled steel sheet.A value is calculated by dividing the value of, between the front and back surfaces, a surface that has a greater maximum depth by the value of the other surface, thus obtaining the ratio between the maximum depth of the region where, on one surface, the angle of rotation between the normal line of a surface and the pole (011) near the normal line of a surface is 5oo less and the maximum depth of the region where, on the other surface, the angle of rotation between the normal line of the other surface and the pole (011) near the normal line of the other surface is 5oo less.

[56] Tensile strength: 1150 MPa or more In hot-rolled steel sheet according to the present embodiment, the tensile strength is 1150 MPa or higher. When the tensile strength is less than 1150 MPa, the automotive suspension components to which the present invention can be applied are limited. If the tensile strength is inherently less than 1150 MPa, the improvement of fatigue properties in the forming-damaged portion is not considered an issue. The tensile strength may be 1200 MPa or higher, or 1300 MPa or higher. The tensile strength is preferably as high as possible, but it may be 1500 MPa or lower from the standpoint of reducing the weight of the components through the high strengthening of the hot-rolled steel sheet.

[57] Tensile strength is measured by performing a tensile test in accordance with JIS Z 2241: 2011 using a test piece No. 5 of JIS Z 2241: 2011. The position in which the tensile test piece is picked up is the center position in the width direction of the sheet, and a direction perpendicular to the rolling direction is the longitudinal direction.

[58] Total elongation: 13.0% or more The total elongation must be 13.0% or more to prevent narrowing or fracture during the forming of flange portions, protruding portions, and similar automotive suspension components. Therefore, the total elongation may be 13.0% or more. The total elongation is preferably 14.0% or more. Total elongation refers to the elongation at fracture in the tensile test to measure tensile strength.

[59] Hole expansion rate: 35.0% or more Hot-rolled steel sheet produced according to the present embodiment may have a hole expansion rate of 35.0% or more. When the hole expansion rate is less than 35.0%, since the forming-induced fracture occurs at the extreme portion of a cylindrical flake, there is a case where the present invention cannot be applied to automotive suspension components. To further increase the forming height of the cylindrical flake, the hole expansion rate may be 50.0% or more. The hole expansion rate is measured by performing a hole expansion test in accordance with JIS Z 2256: 2010.

[60] Fatigue strength of the portion damaged by the formation: 350 MPa or more For 780 MPa class steel sheets currently in use, the fatigue strength attributed to the recessed portions in the forming-damaged area is not considered a problem, and the fatigue limit ratio becomes 0.45 or higher. In hot-rolled steel sheets produced in accordance with this modality, even when there is a recessed portion in the forming-damaged area, it is necessary to obtain the same fatigue strength as 780 MPa class steel sheets, and therefore the fatigue strength of the forming-damaged portion is preferably 350 MPa or higher. When the fatigue strength of the forming-damaged portion is 350 MPa or higher, the hot-rolled steel sheet can be considered excellent in terms of fatigue strength in the forming-damaged portion. The fatigue limit ratio is a value obtained by dividing the fatigue strength by the tensile strength (fatigue strength / tensile strength).

[61] The fatigue properties of the recessed portion in the damaged area are evaluated by hat forming a hot-rolled steel strip and measuring the fatigue strength using a flat bend fatigue test piece produced from the formed hot-rolled steel strip. In hat forming, as a vertical wall is formed, the hot-rolled steel strip comes into contact with a punch while bending and back, distorting. This allows for the reproduction of recessed portions formed in a flat R-section near the permanent wall portion of a suspension component. For the hot-rolled steel strip used for hat forming, the dimensions are set to a width of 35 mm and a length of 400 mm, with the longitudinal direction as the L-direction.The cap is formed from this hot-rolled steel strip using a square-head punch of approximately R6. In a forming test, a model 145-100 manufactured by Erichsen, Inc. is used. From the vertical wall of a formed cap test piece, a test piece is produced with a shape according to JIS Z 2275:1978 and a fatigue test is performed. As fatigue test conditions, at room temperature, the stress ratio R is set to -1, the frequency is set to 25 Hz, the load is applied repeatedly up to 10⁶ times, and the number of repetitions to fracture is measured. The stress at which the test piece does not fracture up to the 10⁶ application of the load is considered the fatigue strength.

[62] Hot-rolled steel sheet conforming to the present modality, having the chemical composition and metallographic structure described above, may be provided with a surface coating to improve corrosion and other resistance and thereby become surface-treated steel sheet. The coating may be an electrolytic coating or a hot-dip coating. Electrogalvanizing, electroplating with Zn-Ni alloys, and the like are illustrative examples of electrolytic coatings.As a hot-dip plating layer, hot-dip galvanizing, hot-dip galvanizing-annealing, hot-dip aluminizing, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, and similar processes are illustrative examples. The degree of adhesion of the plating is not particularly limited and can be the same as before. Furthermore, it is also possible to further improve corrosion resistance by performing an appropriate chemical conversion treatment (e.g., applying and drying a silicate-based, chromium-free chemical conversion treatment liquid) after plating.

[63] A preferred method of manufacturing hot-rolled steel sheet in accordance with the present modality will now be described. A casting step and a hot-rolling step, which will be described below, are necessary requirements to reduce the depths of the recessed parts in the forming-damaged portion and are important steps for controlling the crystal orientations in the sheet thickness direction and the average grain diameter of the fore-austenite grains.

[64] The preferred method of manufacturing hot-rolled steel sheet in accordance with this modality includes the following steps. A continuous casting step of, in the continuous casting of a plate having a predetermined chemical composition, performing the continuous casting such that the average cooling rate gradient of the surface temperature in a region from the meniscus to 1.0 m from the meniscus is 0.20 to 15.00°C / s2 to obtain the plate, a heating step of heating the plate to 1200°C or more, a hot rolling step of performing rough rolling of the plate after heating and then performing finish rolling such that the total reduction of the roll in a temperature range of 870°C to 980°C is 80% or more and the time elapsed between rolling boxes becomes 4.00 seconds or less in the temperature range of 870°C to 980°C, a cooling step of cooling a hot-rolled steel sheet to a temperature range of 300°C to 550°C, and a coiling step of, after cooling, coiling the hot-rolled steel sheet so that the coiling temperature is in the temperature range of 300°C to 550°C. Each step will be described below.

[65] Continuous casting step In the continuous casting of a plate with the chemical composition described above, the average surface temperature cooling rate gradient in a region from the meniscus to 1.0 m from the meniscus is adjusted to 0.20 to 15.00°C / s². In this embodiment, the average surface temperature cooling rate gradient refers to the change in cooling rate over time within a 1.0 m interval from the meniscus. This average cooling rate gradient can be calculated based on temperature data obtained at positions 0.1 m, 0.5 m, and 1.0 m from the meniscus using a thermometer embedded in a molded copper sheet. At a given time, the temperature measurement values ​​at the positions of 0.1 m, 0.5 m, and 1.0 m from the meniscus are represented by T0.i, T0.5, and T1.0.In a case where the time a solidified layer is present at the position 0.1 m from the meniscus is represented by to.i, the time the solidified layer passes through the position 0.5 m from the meniscus becomes to.5 = (to.i + 0.4 / V), where the casting speed is represented by V (m / s). Similarly, the time the solidified shell passes through the position 1.0 m from the meniscus becomes ti.o = (to.i + 0.9 / V). The cooling rate gradient within the 1.0 m interval from the meniscus expressed using to.i, to.5 and ti.o having the relationship described above and the temperature measurement values ​​To.i, T0.5 and Ti.o at the individual positions becomes (4 / 9) χ v2χ Ti.o + (5 / 9) χ V2χ T0.1 - (1.62 / 1.25) χ V2χ T0.5.

[66] Cooling rate gradients are obtained on the front and back surfaces of the plate at a certain arbitrary time between the beginning and end of the continuous casting of the target steel, and their average value is taken as the cooling rate gradient at that time. The cooling rate gradients at this given time are measured at at least 20 points, and their average value is taken as the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus. Cooling rate gradients can be measured at up to 100 points.

[67] The cooling rate of the surface temperature affects the growth of columnar crystals in the early stage of solidification, and its gradient affects the frequency of columnar crystal colony formation in the surface layer. When the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus is faster than 15.00°C / s², the ratio of maximum depths exceeds 1.20. The average cooling rate gradient in the region is preferably as slow as possible; however, when the average cooling rate gradient is less than 0.20°C / s², cooling control becomes extremely difficult, and therefore the average cooling rate gradient is preferably 0.20°C / s² or faster.

[68] The average pouring speed in the continuous casting step can be within a normal range, such as 0.8 m / min or faster, or 1.2 m / min or faster. From a cost-reduction standpoint, the average pouring speed is preferably set to 1.2 m / min or faster. On the other hand, when the average pouring speed exceeds 2.5 m / min, a defect is likely to initiate in the plate during the solidification process. Therefore, the average pouring speed is preferably 2.5 m / min or slower. Furthermore, when the average pouring speed is less than 0.6 m / min, the cooling temperature gradient in the direction of plate thickness decreases, but economic efficiency is significantly affected. Therefore, the average pouring speed is preferably between 0.6 and 2.5 m / min. The cooling temperature gradient mentioned here is different from the average cooling rate gradient described earlier.

[69] Warm-up step The plate obtained by continuous casting is heated to a surface temperature of 1200°C or higher, at which point it dissolves. If the plate contains titanium (Ti), the heating temperature is preferably set to 1230°C or higher to more reliably form a solid Ti solution. Furthermore, compared to the plate's pre-heating temperature, it may be cooled to room temperature or held at a higher temperature after continuous casting if there is concern about cracking due to thermal stress or similar factors. In the heating step, the plate is placed in a temperature-controlled furnace at a predetermined temperature, and the holding time for the plate's surface temperature to reach 1200°C or higher (holding time) should be set to 30 minutes or more, which is sufficient.Furthermore, if the plate contains titanium, the time required for the heating temperature to reach 1230°C or higher (holding time) should be set to 30 minutes or more, which is sufficient. The upper limit of the holding time should be set to 300 minutes or less. In the furnace, the plate is placed on a platform of inorganic material, and the plate can be dissolved by heating it to a temperature at or below the point at which the plate, heated by a reaction between the inorganic material and the iron, will not dissolve. For example, the heating temperature should be set to 1400°C or less.

[70] Hot rolling pass After heating the plate, rough rolling is performed, followed by finish rolling within a range described below. Finish rolling is carried out such that the total reduction of the rolled material within a temperature range of 870°C to 980°C reaches 80% or more. When the final rolling temperature exceeds 980°C, the average grain diameter of the austenite grains becomes large. Regardless of the total reduction of the rolled material in the rolling stand, it is not possible to reduce the depths of the recessed sections in the formation-damaged portion, and excellent fatigue properties cannot be achieved in the formation-damaged portion.

[71] If the total rolling reduction within the temperature range of 870°C to 980°C is less than 80%, the average grain diameter of the previous austenite grains exceeds 25.0 pm. The total rolling reduction mentioned here is a value obtained by summing the rolling reduction in each roll box where the bite temperature is between 870°C and 980°C. The upper limit of the total rolling reduction within the temperature range of 870°C to 980°C may be set at 95% or less.

[72] Furthermore, in the hot rolling step, when the total sheet reduction rate ((1 - t / to)x100), which is the ratio of the sheet thickness to after rough rolling to the product sheet thickness t after finished rolling, is less than 80%, it is not possible to achieve a total rolling reduction of 80% or more within the temperature range of 870°C to 980°C, regardless of rolling temperature control. Therefore, the total sheet reduction rate is limited to 80% or more. This total sheet reduction rate is preferably as high as possible since it increases throughput; however, if the total sheet reduction rate exceeds 98%, the load on a rolling mill increases, as do the costs of replacing rolls and similar components. Considering the load on the roll, a total sheet reduction rate of 95% or less is more desirable.Therefore, the overall sheet reduction rate, which is the ratio between the sheet thickness after rough rolling and the product sheet thickness after final rolling, is limited to 80% or more. Furthermore, the overall sheet reduction rate is desirable to be 98% or less.

[73] The total number of rolling stands is not particularly limited and can be determined based on the capabilities, such as load capacity or torque, of the rolling machine. Typically, the number of rolling stands where the bite temperature is between 870°C and 980°C is two or more. In the final rolling within the 870°C to 980°C temperature range, if the time between rolling stands exceeds 4.00 seconds, the austenite grains grow in the relevant section, and the average grain diameter of the previous austenite grains becomes greater than 25.0 pm, which is undesirable. Therefore, if the number of rolling stands where the bite temperature is between 870°C and 980°C exceeds two, the time between rolling stands is adjusted to 4.00 seconds or less. In case the time elapsed between the laminate boxes is less than 0.After 30 seconds, the load on the laminating roller increases, and therefore the elapsed time can be adjusted to 0.30 seconds or more. The bite temperature can be obtained from the surface temperature of the steel sheet measured with a thermometer, such as a radiation thermometer installed on each support.

[74] Cooling step After final rolling, the hot-rolled steel sheet is cooled to a temperature range of 350°C to 550°C. When the cooling stop temperature after final rolling is outside the temperature range of 330°C to 550°C, it is not possible to perform the coiling, which will be described below, at a desired temperature range.

[75] Winding pitch After cooling, the hot-rolled steel sheet is coiled so that the coiling temperature falls within the range of 350°C to 550°C to obtain a tensile strength of 1150 MPa or higher. When the coiling temperature is below 350°C, the volume percentage of martensite increases. Therefore, the coiling temperature is set to 350°C or higher. The coiling temperature is preferably 380°C or higher. On the other hand, when the coiling temperature is above 550°C, the volume percentage of bainite decreases, and furthermore, when the coiling temperature is 570°C or higher, the volume percentage of ferrite increases. Therefore, the coiling temperature is set to 550°C or lower. The winding temperature is preferably 480°C or lower.

[76] The average surface temperature of the steel sheet along the entire length of a coil, measured with a thermometer installed in a section from the cooling apparatus to a winding machine after cooling, can be used as the winding temperature. This is because the average surface temperature of the steel sheet along the entire length of the coil is equivalent to the coil temperature after the hot-rolled steel sheet is wound into a coil. To reduce material variation within the coil, the winding temperature at any point along the coil is preferably set to a maximum of 480°C or lower. That is, the surface temperature of the steel sheet is preferably set to 480°C or lower along the entire length of the coil.

[77] In the present embodiment, the time elapsed from the start of cooling in the cooling step to the start of winding in the winding step is preferably set to 30 seconds or less. The elapsed time referred to here is the time taken from the completion of the final rolling to the start of winding. In the chemical composition of the hot-rolled steel sheet according to the present embodiment, the cooling time is not particularly limited; however, when the cooling time is long, the length of the air-cooling zone in a cooling area becomes long, and the thickness of the fouling on the surface layer becomes thick, which increases the cost in a pickling step. Therefore, the cooling time is preferably 30 seconds or less.In the cooling step, the time elapsed from the start of cooling to the start of winding can be adjusted by adjusting the average cooling rate in that step. As a cooling method after final rolling, a method such as water cooling or air cooling on an exit table can be selected to achieve the desired cooling time.

[78] Hot-rolled steel sheet manufactured by the method described above may be allowed to cool to ambient temperature or may be water-cooled after being wound into coils. If cooled to ambient temperature, the hot-rolled steel sheet may be re-coiled and pickled or may be subjected to surface-hardening rolling to adjust residual stress or shape.

[79] Tempering step The preferred method of manufacturing a hot-rolled steel sheet in accordance with the present modality may further include a tempering step to temper the hot-rolled steel sheet manufactured by the steps described above to further improve ductility. If tempering is required, it is preferably carried out under conditions where the hot-rolled steel sheet is held at a temperature between 200°C or higher and 450°C or lower for 90 to 80,000 seconds. When the rolling temperature is below 200°C, a change in material quality is rarely noticeable, and the manufacturing cost increases due to the increased number of passes, which is not desirable. Furthermore, when the tempering temperature is 450°C or higher, the pearlite fraction exceeds 3.0%, thus impairing hole expansion capacity. The average rate of temperature rise during the tempering pass is not particularly limited, but it is preferably 0.01°C / s faster to avoid a decrease in the effectiveness of the heat treatment. Additionally, the atmosphere during tempering can be an oxidizing atmosphere or an atmosphere replaced with nitrogen or a similar substance.Tempering can be performed on hot-rolled steel sheet in coil form; however, in this case, the holding time is preferably set to 1000 seconds or more to reduce variation in the coil. The tempered hot-rolled steel sheet can be cooled to room temperature and then pickled to remove any scale formed by hot rolling or heat treatment if necessary.

[80] Veneering step The preferred method of manufacturing a hot-rolled steel sheet in accordance with this modality may further include a plating step to perform a hot-dip galvanizing treatment on the hot-rolled steel sheet manufactured by the method described above or the hot-rolled steel sheet after the tempering step. In the case of hot-dip galvanizing, it is preferable to set the highest temperature within a range of 450°C to 495°C and to set the residence time within this temperature range to 75 seconds or less. The residence time below 450°C is preferably set, similarly to the tempering step, to a residence time of 90 to 80,000 seconds within the temperature range of 200°C or above and below 450°C. When the highest temperature exceeds 495°C, the volume percentage of residual austenite falls below 3.0%, regardless of the residence time, and the ductility of the hot-rolled steel sheet after plating deteriorates. When the highest temperature falls below 450°C, a defect begins to form in a plating layer, which is undesirable.In cases where other conditions fall within the temperature range described above, the plating method is not particularly limited. The plating adhesion is also not limited and can be the same as before. Furthermore, corrosion resistance can be further enhanced by performing a suitable chemical conversion treatment (e.g., applying and drying a silicate-based, chromium-free chemical conversion treatment liquid) after plating. ui ou Examples

[81] Plates having a chemical composition shown in Table 1 were manufactured by continuous casting. The conditions for continuous casting were those shown in Table 2-1 and Table 2-2. In continuous casting, the average gradient of the surface temperature cooling rate in a region from the meniscus to 1.0 m from the meniscus exceeded 15.00°C / sec 2 in Tests Nos. 4, 5, 10, 13, and 19.

[82] From the obtained plates, hot-rolled steel sheets with a sheet thickness of 2.6 mm were manufactured under the conditions shown in Table 2-1 and Table 2-2. If necessary, annealing and plating treatment was performed under the conditions shown in Table 2-1 and Table 2-2. In the cooling after hot rolling, the temperatures were cooled to the coiling temperatures shown in Table 2-1 and Table 2-2. Furthermore, the times elapsed from the start of cooling in a cooling pass to the start of coiling in a coiling pass were 30 seconds or less.

[83] Table 1 Steel Chemical composition (% by mass), the remainder is Fe and impurities Note C Si Mn Al Nb PSN Ti B Mo V Cr Ca A 0.081 0.82 1.88 0.33 0.009 0.007 0.002 0.0032 0.021 0.0012 Comparative steel B 0.113 0.83 1.83 0.32 0.006 0.008 0.002 0.0028 0.018 0.0011 Steel of the present invention C 0.196 1.41 1.87 0.35 0.007 0.008 0.003 0.0027 0.017 0.0013 Comparative steel D 0.156 1.34 2.40 0.21 0.011 0.007 0.002 0.0028 0.019 0.0016 0.005 0.006 Steel of the present invention E 0.142 1.55 2.38 0.03 0.008 0.007 0.003 0.0028 0.019 0.0017 0.008 0.0002 Comparative steel F 0.178 1.22 1.81 0.32 0.015 0.006 0.004 0.003 0.022 0.0014 0.003 0.006 Steel of the present invention G 0.132 0.92 1.65 0.02 0.034 0.008 0.002 0.003 0.021 0.0006 0.005 0.0002 Comparative steel H 0.134 0.93 3.15 0.03 0.035 0.006 0.003 0.0031 0.022 0.0007 Comparative steel 1 0.138 0.94 2.15 0.03 0.035 0.008 0.002 0.0033 0.007 0.0008 0.006 Steel of the present invention J 0.139 0.92 2.11 0.85 0.032 0.007 0.003 0.0031 0.008 0.0007 0.004 Comparative steel K 0.095 0.73 1.78 0.31 0.009 0.007 0.003 0.0029 0.006 0.001 0.003 Steel of the present invention L 0.161 0.64 2.61 0.51 0.025 0.008 0.002 0.0031 0.047 0.0011 0.004 0.0002 Steel of the present invention M 0.176 1.23 1.80 0.33 0.053 0.008 0.003 0.0026 0.043 0.0019 0.003 Comparative Steel N 0.143 1.18 1.86 0.38 0.015 0.007 0.002 0.0031 0.021 0.0003 0.003 0.004 0.006 Comparative Steel O 0 0.172 0.34 2.33 0.03 0.022 0.006 0.002 0.0033 0.021 0.0022 Comparative Steel P 0.105 1.09 2.65 0.04 0.038 0.008 0.003 0.0035 0.021 0.0014 Steel of the present invention Q 0.167 0.71 2.65 0.52 0.001 0.006 0.002 0.0024 0.031 0.0012 0.003 0.0002 Comparative steel R 0.173 1.26 2.37 0.31 0.019 0.006 0.002 0.0031 0.018 0.0014 0.089 0.004 0.005 Steel of the present invention S 0.176 0.96 2.18 0.09 0.042 0.007 0.003 0.0021 0.132 0.0009 0.153 0.006 Steel of the present invention T 0.181 1.33 2.45 0.06 0.013 0.008 0.003 0.0036 0.009 0.0013 0.003 0.004 0.410 Steel of the present invention U 0.108 1.13 2.63 0.03 0.035 0.006 0.002 0.0031 0.018 0.0011 0.005 0.004 0.006 0.0006 Steel of the present invention V 0.114 0.78 2.29 0.02 0.022 0.073 0.004 0.0028 0.119 0.0022 Steel of the present invention W 0.118 0.76 2.31 0.03 0.018 0.008 0.008 0.0019 0.134 0.0019 Steel of the present invention X 0.162 0.72 1.97 0.31 0.006 0.008 0.003 0.0023 0.175 0.0008 0.210 Steel of the present invention. The underlined values ​​indicate that the corresponding values ​​are outside the scope of the present invention. The blank cells indicate that the corresponding elements are intentionally omitted. ινΐΛ / a / zuzz / uiu / ou

[84] Table 2-1 Test No. Steel Continuous Casting Heating Hot Rolling Coiling Tempering Plating Note Average cooling rate gradient of surface temperature in the region from the meniscus to 1.0 m from the meniscus C / sec2 Heating temperature °C Holding time min Total rolling reduction in the temperature range of 870°C to 980°C % Maximum elapsed time between rolling boxes sec Minimum elapsed time between rolling boxes sec Coiling temperature C Tempering treatment temperature °C Tempering treatment time sec Residence time at the highest temperature and in the temperature range of 450°C to 495°C 1 B 0.38 1246 243 84 3.03 0.45 489 N / A / AN / A Example of the present invention 2 B 3.66 1253 246 86 3.07 0.49 489 N / A / AN / A Example of the present invention 3 B 13.60 1248 250 84 3.04 0.48 498 N / AN / AN / A Example of the present invention 4 B 15.80 1244 256 85 3.05 0.48 494 N / AN / AN / A Comparative example 5 B 20.90 1248 251 84 3.09 0.48 491 N / AN / AN / A Comparative example 6 B 8.90 1251 248 84 3.04 0.49 489 N / AN / AN / A Example of the present invention 7 D 0.62 1276 221 84 2.76 0.37 341 N / AN / AN / A Comparative example 8 D 4.83 1268 226 83 2.60 0.37 362 N / AN / AN / A Example of the present. Invention 9 D 10.95 1270 227 76 2.63 0.33 371 N / A / A / A ​​Comparative Example 10 D 16.30 1271 229 81 2.76 0.33 381 N / A / A / A ​​Comparative Example 11 F 0.83 1231 253 83 3.92 0.49 512 N / A / A / A ​​Example of the present invention 12 F 13.90 1230 245 81 4.67 0.43 516 N / A / A / A ​​Comparative Example 13 F 17.60 1236 229 87 3.82 0.45 510 N / A / A / A ​​Comparative Example 14 I 8.90 1213 235 84 3.15 0.51 541 N / A / N / A Example of the present invention 15 I 8.60 1208 250 86 3.21 0.52 570 N / A / N / A Comparative example 16 I 8.80 1209 256 85 3.24 0.50 553 N / A / N / A Comparative example 17 L 1.90 1278 228 82 3.33 0.37 528 430 96 N / A Example of the present invention 18 L 2.90 1276 232 81 3.22 0.45 531 456 96 N / A Comparative example 19 L 15.30 1251 248 81 3.26 0.32 522 441 96 N / A Comparative example 20 L 14.40 1253 243 81 3.82 0.31 526 426 96 N / A Example of the present invention The underlines indicate that the manufacturing conditions are not preferable. iniΛ / a / zuzz / uiu / ou

[85] Table 2-2 Test No. Steel Continuous Casting Heating Hot Rolling Coiling Tempering Plating Note Average cooling rate gradient of surface temperature in the region from the meniscus to 1.0 m from the meniscus °C / sec2 Heating temperature °C Holding time min Total rolling reduction in the temperature range of 870°C to 980°C % Maximum elapsed time between rolling boxes sec Minimum elapsed time between rolling boxes sec Coiling temperature °C Tempering treatment temperature °C Tempering treatment time sec Residence time at the highest temperature and in the temperature range of 450°C to 495°C 21 K 1.90 1235 260 89 3.02 0.49 388 N / A / A / A ​​Example of the present invention 22 P 8.81 1218 249 81 2.85 0.56 384 N / AN / AN / A Example of the present invention 23 R 9.50 1214 240 82 3.55 0.51 396 216 119 N / A Example of the present invention 24 S 5.70 1276 273 85 3.55 0.51 473 N / AN / AN / A Example of the present invention 25 T 6.80 1221 257 82 3.47 0.53 421 373 42480 N / A Example of the present invention 26 u 6.20 1218 249 91 3.50 0.48 389 N / AN / AN / A Example of the present invention 27 T 6.80 1221 257 82 3.47 0.53 421 N / AN / A 475°C-71 sec Example of the present invention 28 T 6.80 1221 257 82 3.47 0.53 421 N / AN / A 497°C-70 sec Comparative example 29 A 0.98 1228 239 93 3.47 0.50 403 N / A NIA N / A Comparative example 30 c 9.56 1212 244 81 3.82 0.50 382 N / AN / AN / A Comparative example 31 E 7.62 1203 255 86 2.86 0.57 412 N / AN / AN / A Comparative example 32 G 7.40 1222 237 82 3.28 0.53 386 N / AN / AN / A Comparative example. 33 H Corner cracking and plate cracking N / A Comparative example 34 J Corner cracking and plate cracking N / A Comparative example 35 M Corner cracking and plate cracking N / A Comparative example 36 N 14.30 N / A 231 83 3.53 0.53 529 N / A / N / A Comparative example 37 O 5.10 N / A 230 83 3.54 0.50 530 N / A / N / A Comparative example 38 Q 7.80 N / A 249 84 3.61 0.50 491 N / A / N / A Comparative example 39 V 5.56 N / A 270 86 3.50 0.43 481 N / A / N / A Example of the present invention 40 W 5.83 N / A 288 83 3.61 0.46 511 N / A / N / A Example of the present invention 41 X 7.27 N / A 291 81 3.87 0.55 488 N / A / N / A Example of the present invention The underlines indicate that the manufacturing conditions are not preferable. ινΐΛ / a / zuzz / uiu / ou

[86] Table 3-1 Test No. 2 or 1 / 4 Position and 1 / 2 Position Region from the surface to the 100 pm position Mechanical characteristics Note Residual γ volume % Bainite volume % Ferrite volume % Martensite volume % Pearlite volume 0 / / 0 Grain diameter of and previous pm Ratio between the surfaces prior to and behind of maximum depths of regions where the angle of rotation between the surface normal line and the pole (011) near the normal line is 5° or less Tensile strength MPa Total elongation % Hole expansion rate % Fatigue strength of the formation-damaged portion MPa 1 B 6.9 86.8 3.0 2.6 0.7 18.9 1.13 1162 13.4 40.6 386 Example of the present invention 2 B 8.0 85.5 3.4 2.3 0.8 12.3 1.08 1173 13.6 43.2 413 Example of the present invention 3 B 6.2 87.9 2.6 2.5 0.8 17.8 1.14 1154 13.2 45.6 372 Example of the present invention 4 B 8.3 86.9 1.8 2.4 0.6 13.1 1.26 1159 13.1 40.2 330 Comparative example 5 B 6.6 86.6 3.4 2.7 0.7 19.3 1.39 1156 13.6 38.9 304 Comparative example 6 B 7.1 86.9 3.2 2.2 0.6 18.2 1.04 1181 13.7 44.1 422 Example of the present invention 7 D 6.8 74.9 0.0 18.3 0.0 16.9 1.03 1321 13.2 20.3 396 Comparative example 8 D 9.9 82.3 0.2 7.6 0.0 17.4 1.08 1221 14.3 36.8 388 Example of the present invention 9 D 8.1 85.1 0.0 6.8 0.0 26.8 1.02 1204 14.1 41.3 342 Comparative Example 10 D 9.3 82.8 0.8 7.1 0.0 18.6 1.31 1208 15.2 38.8 333 Comparative Example 11 F 7.2 84.4 1.3 6.3 0.8 19.8 1.13 1175 13.6 36.3 366 Example of the present invention 12 F 6.2 86.8 0.0 6.4 0.6 27.6 1.06 1162 14.2 38.3 332 Comparative Example 13 F 8.1 83.3 1.2 6.8 0.6 14.5 1.41 1189 14.6 37.6 274 Comparative example. 14 1 7.2 75.6 8.1 7.9 1.2 8.4 1.01 1163 13.7 37.0 443 Example of the present invention 15 1 5.4 74.2 10.9 8.1 1.4 7.2 1.03 1132 14.8 34.3 412 Comparative example 16 1 6.6 73.6 9.2 8.5 2.1 6.6 1.08 1152 14.3 32.7 400 Comparative example 17 L 8.3 81.3 2.0 6.8 1.6 11.3 1.12 1183 14.6 48.3 376 Example of the present invention 18 L 4.2 85.1 0.0 6.4 4.4 12.9 1.13 1154 13.2 34.8 382 Comparative example 19 L 7.6 84.3 1.1 5.9 1.1 12.8 1.21 1162 14.0 36.5 344 Comparative example 20 L 7.0 82.9 2.1 6.7 1.3 17.9 1.19 1189 13.9 38.5 356 Example of the present invention ινΐΛ / a / zuzz / uiui ou The underlines indicate that the corresponding values ​​are outside the scope of the present Invention, the manufacturing conditions are not preferred, and the characteristics are not preferred.

[87] Table 3-2 Test No. Steel 1 / 4 Position and 1 / 2 Position Region from the surface to the 100 pm position Mechanical characteristics Note Volume of and residual % Bainite Volume % Ferrite Volume % Martensite Volume Pearlite Volume Grain diameter of γ anterior pm Ratio between the surfaces anterior and posterior of maximum depths of regions where the angle of rotation between the surface normal line and the pole (011) near the normal line is 5° or less Tensile strength MPa Total elongation % Hole expansion rate % Fatigue strength of the formation-damaged portion MPa 21 K 4.2 85.8 6.9 1.4 1.7 10.5 1.04 1192 13.3 50.2 451 Example of the present invention 22 P 8.5 83.7 0.0 7.8 0.0 9.7 1.02 1210 15.6 40.8 423 Example of the present invention 23 R 9.8 86.3 0.0 3.9 0.0 11.3 1.04 1208 16.2 55.6 403 Example of the present invention 24 s 11.3 80.1 0.9 6.9 0.8 6.2 1.03 1176 14.3 45.3 396 Example of the present invention 25 T 10.3 87.5 0.0 2.2 0.0 12.9 1.11 1220 16.1 57.3 371 Example of the present invention 26 u 7.7 86.9 1.3 3.3 0.8 5.3 1.10 1163 13.5 48.1 388 Example of the present invention 27 T 11.6 86.8 0.0 1.6 0.0 12.9 1.11 1201 17.3 49.8 400 Example of the present invention 28 T 7.2 87.0 0.0 1.3 4.5 12.9 1.11 1164 14.2 30.1 389 Comparative example 29 A 2.5 95.1 0.3 1.0 1.1 8.8 1.13 1153 10.6 62.3 369 Comparative Example 30 C 13.4 77.5 0.0 9.1 0.0 23.2 1.07 1209 17.3 31.3 366 Comparative Example 31 E 12.6 78.5 0.8 7.8 0.3 16.8 1.05 1182 14.9 33.4 381 Comparative Example 32 G 4.6 77.9 15.6 0.8 1.1 7.6 1.05 1103 16.9 36.3 446 Comparative Example 33 H Corner Cracking and Plate Cracking Example Comparative 34 J Shear cracking and plate cracking Comparative example. 35 M Corner cracking and plate cracking Comparative example 36 N 4.6 71.6 20.6 1.8 1.4 10.6 1.15 1083 13.3 38.3 365 Comparative example 37 0 2.7 88.7 0.3 1.2 7.1 8.9 1.11 1164 11.1 26.5 399 Comparative example 38 Q 6.1 90.3 0.0 2.5 1.1 28.9 1.07 1184 13.9 37.9 328 Comparative example 39 V 3.6 87.6 1.4 4.5 0.0 6.5 1.03 1192 13.4 37.0 403 Example of the present invention 40 w 3.2 89.2 1.2 2.3 0.0 7.3 1.02 1176 14.2 38.9 405 Example of the present invention 41 X 5.1 83.6 3.5 6.8 0.0 6.1 1.10 1247 15.2 40.1 391 Example of the present invention The underlines indicate that the corresponding values ​​are outside the scope of the present invention, the manufacturing conditions are not preferable, and the characteristics are not preferable.

[88] In Tests Nos. 17 to 20, 23, and 25, after hot rolling, the coils were unwound, the steel sheets were cut to a size at which a predetermined characteristic evaluation was possible, and heat treatments (quenching) were performed in a box furnace. In the Tests Nos. 27 and 28, a layer of hot-dip zinc plating was imparted by performing a plating treatment under the conditions shown in Table 2-2.

[89] A tensile test was performed in accordance with JIS Z 2241: 2011 using a JIS Test Piece No. 5 Z 2241: 2011. Tensile strength was obtained from the point of maximum load, and total elongation was obtained from a displacement at fracture. The tensile test piece 15 was collected at its center along the width of the sheet, and perpendicular to the rolling direction was the longitudinal direction.

[90] When the tensile strength was 1150 MPa or more, the hot-rolled steel sheet was determined to be acceptable for having excellent strength, and when the tensile strength was less than 1150 MPa, the hot-rolled steel sheet was determined to be unacceptable for not having excellent strength.

[91] The hole expansion rate was measured by performing a hole expansion test in accordance with JIS Z 2256: 2010. In cases where the total elongation was 13.0% or more and the hole expansion rate was 35.0% or more, the hot-rolled steel sheet was determined to be acceptable due to excellent formability. Conversely, if either of these conditions was not met, the hot-rolled steel sheet was determined to be unacceptable due to insufficient formability.

[92] The fatigue properties of the portion damaged by the forming process were evaluated from a fatigue strength obtained by hat forming on the resulting hot-rolled steel sheet and performing a fatigue test on the formed hot-rolled steel sheet. The conditions for the fatigue test were as described above.

[93] In the case where the fatigue strength was 350 MPa or more, the hot-rolled steel sheet was considered acceptable as being excellent in terms of fatigue properties in the forming-damaged portion, and in the case where the fatigue strength was less than At 350 MPa, the hot-rolled steel sheet was determined to be unacceptable for not being excellent in terms of fatigue properties in the portion damaged by forming.

[94] In Test No. 29, where the C content was low, the amount of residual austenite was small and the total elongation was less than 13.0%. In Test No. 30, where the C and Si content were high, and in Test No. 31, where the Si content was high, the volume percentage of residual austenite was high and the void expansion rate was low.

[95] In Test No. 32, where the Mn content was low, and in Test No. 36, where the B content was low, the tensile strength was less than 1150 MPa. In Test No. 37, where the Si content was low, the volume percentage of residual austenite was low and the total elongation was low. In Test No. 38, where the Nb content was low, the previous austenite grains became coarse and the fatigue strength of the formation-damaged portion was low. Furthermore, in Tests Nos. 33 to 35, hot rolling could not be carried out due to nozzle obstruction during casting and fine cracking in a corner portion, so hot-rolled steel sheets could not be manufactured.

[96] In Test No. 9, where the chemical composition was within the scope of the present invention, but the total reduction of rolling in a temperature range of 870°C to 980°C was less than 80%, and in Test No. 12 where the maximum time elapsed between rolling boxes in the temperature range of 870°C to 980°C exceeded 4.00 seconds, the previous austenite grains became coarse and the fatigue strength of the portion damaged by forming was low.

[97] In Test No. 7, where the winding temperature was low, the hole expansion rate was low. In Test No. 16, where the winding temperature was high, the volume percentage of bainite was low and the hole expansion rate was low. In Test No. 15, where the winding temperature was high, the ferrite volume percentage was high, the tensile strength was less than 1150 MPa, and the hole expansion rate was low.

[98] Among Tests No. 17 to 20, 23 and 25, where tempering was performed after hot rolling, in Test No. 18 where the tempering temperature exceeded 450°C, the volume percentage of pearlite was high and the hole expansion rate decreased. Between Tests No. 27 and 28, where a plating treatment was performed, in Test No. 28, since the highest temperature exceeded 495°C, the percentage by volume of perlite increased and the hole expansion rate decreased.

[99] It was found that, unlike the structural factors governing tensile strength, total elongation, and hole expansion rate, as shown in Figure 1 and Figure 2, fatigue strength in the formation-damaged portion is governed by the ratio of average grain diameter to maximum depth of fore-austenite grains (the ratio of the maximum depth of a region where, on one surface, the angle of rotation between the normal line of one surface and a pole (011) near the normal line of one surface is less than 5oo and the maximum depth of a region where, on the other surface, the angle of rotation between the normal line of the other surface and a pole (011) near the normal line of the other surface is less than 5oo).Furthermore, as shown in Figure 3, the ratio of maximum depths is found to be particularly governed by the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus. As shown in Figure 3, it was found that when the average cooling rate gradient is within the range of 0.20 to 15.00°C / s², the ratio of maximum depths becomes 1.20 or less, and the fatigue strength in the formation-damaged portion becomes 350 MPa or more. Industrial applicability

[100] In accordance with the aspects described above of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent formability and having excellent fatigue properties in a forming-damaged portion, and a method for manufacturing the same. In accordance with the aspects described above of the present invention, given that the fatigue properties in a forming-damaged portion are excellent, it is possible to provide a hot-rolled steel sheet capable of reducing the depths of the recessed areas in a flat portion near a portion R that is formed at the time of forming portion R, and a method for manufacturing the same.

Claims

1. A hot-rolled steel sheet comprising, as chemical composition, in % by mass: C: 0.085% to 0.190%; Si: 0.40% to 1.40%; Mn: 1.70% to 2.75%; Al: 0.01% to 0.55%; Nb: 0.006% to 0.050%; P: 0.080% or less; S: 0.010% or less; N: 0.0050% or less; Ti: 0.004% to 0.180%; B: 0.0004% to 0.0030%; Mo: 0% to 0.150%; V: 0% to 0.300%; Cr: 0% to 0.500%; Ca: 0% to 0.0020%; and a residue consisting of Fe and an impurity, wherein, in metallographic structures at a 1 / 4 position in the strip thickness direction from a surface and at a 1 / 2 position in the strip thickness direction from the surface, in % by volume, the residual austenite is 3.0% to 12.0%, the bainite is 75.0% or more and less than 97.0%, the ferrite is 10.0% or less, the martensite is 10.0% or less, and the pearlite is 3.0% or less, in a metallographic structure of a region from the surface to a position of 100 pm in the sheet thickness direction from the surface, an average grain diameter of the preceding austenite grains is 25.0 pm or less, a ratio between a maximum depth of a region where, on one surface, a rotation angle between a line normal of one surface and a pole (011) near the line normal of one surface is 5° or less and a maximum depth of a region where, on the other surface, a rotation angle between a line normal of the other surface and a pole (011) near the line normal of the other surface is 5° or less is 1.00 to 1.20, and a tensile strength is 1150 MPa or more.

2. The hot-rolled steel sheet according to claim 1, further comprising, as a chemical composition, in % by mass, one or more selected from the group consisting of: Mo: 0.030% to 0.150%; V: 0.050% to 0.300%; Cr: 0.050% to 0.500%; and Ca: 0.0006% to 0.0020%.

3. A method of manufacturing hot-rolled steel sheet according to claim 1 or 2, comprising: a continuous casting step of, in the continuous casting of a plate having the chemical composition according to claim 1, performing the continuous casting such that an average cooling rate gradient of a surface temperature in a region from a meniscus to 1.0 m from the meniscus is 0.20 to 15.00°C / s2 to obtain the plate; a heating step of heating the plate to 12000°C or more; a hot rolling step of performing rough rolling on the plate after heating, and performing finish rolling such that the total rolling reduction in a temperature range of 870°C to 980°C is 80% or more and the time elapsed between rolling boxes becomes 4.00 seconds or less in the temperature range of 870°C to 980°C; a cooling step of cooling to a temperature range of 300°C to 550°C; and a winding step of winding so that the winding temperature is in the temperature range of 300°C to 550°C after cooling.

4. The method of manufacturing the hot-rolled steel sheet according to claim 3, further comprising: a tempering step of holding at a temperature range of 200°C or more and less than 450°C for 90 to 80000 seconds after the rolling step.

5. The method of manufacturing hot-rolled steel sheet according to claim 3 or 4, further comprising: a plating step of performing a hot-dip galvanizing treatment on the hot-rolled steel sheet after the coiling step or the hot-rolled steel sheet after the tempering step with a thermal history wherein the residence time within a temperature range of 450°C to 495°C is reduced to 75 seconds or less.