HOT-ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Abstract

A hot-rolled steel sheet having a predetermined chemical composition is adopted, wherein a region, where the angle of rotation between a line normal of a surface and a pole (011) near the line normal becomes 5° or less, is 0.150 or less from the surface in terms of a position of the sheet thickness direction standardized by a sheet thickness, and a region, where the angle of rotation becomes 20° or more, is 0.250 or more from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness and a method of manufacturing the same.

Description

HOT-ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME [Technical field of the invention]

[0001] The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same. Priority is claimed in Japanese Patent Application No. 2020-082656, filed on May 8, 2020, the contents of which are incorporated herein by reference. [Background of the technique]

[0002] In recent years, efforts have focused on reducing the weight of automobiles and every component within them. Optimally designing component shapes ensures rigidity and thus allows for weight reductions. Furthermore, in blank-formed components, such as press-formed parts, weight can be reduced by decreasing the thickness of the component material sheets. However, to ensure static fracture resistance and yield strength while reducing sheet thickness, high-strength materials are necessary. Specifically, for automotive suspension components such as lower control arms, trailing arms, and control arms, studies have been initiated on the application of steel sheets with a yield strength exceeding 780 MPa.Since these automotive suspension components are manufactured by bending and similar forming in steel sheets, the steel sheets applied to these automotive suspension components are required to have excellent formability.

[0003] . 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 ranges, thereby controlling the grain sizes and aspect ratios of the previous austenite and reducing anisotropy.

[0004] 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 ranges at a predetermined final rolling temperature range, thereby improving toughness.

[0005] In order to further reduce the weight of automobiles, each component of the machine or similar, it is also expected to apply steel sheets that have a sheet thickness based ML / a / ZUZZ / UI uouo in a cold-rolled steel sheet to automotive suspension components. The techniques described in Patent Document 1 and Patent Document 2 are effective in the manufacture of automotive suspension components to which a high-strength steel sheet is applied.

[0006] However, the present inventors found that even in steel sheets to which the techniques of Patent Document 1 and Patent Document 2 are applied, there are cases where the fatigue properties (durability and impact resistance) after the steel sheets are formed into component shapes are insufficient. This is thought to be due to the formation of a sharp, hollowed-out portion, like a fine crack, in the cross-section of the inside of a bend (hereafter simply referred to as the inside bend) in a part that forms the bend, even when no load simulating the operating environment is applied after the bend is formed. This hollowed-out portion is thought to cause a notching effect, like a fine crack, and degrade the durability of the components.The inventors discovered that forming a sharp, hollowed-out section, such as a fine crack in the inner curve, becomes easier as the strength of a steel blade increases. [Documents of the prior art] [Patent documents] uouo

[0007] [Patent Document 1] Japanese Patent No. 5068688 [Patent Document 2] Japanese Patent No. 3858146 [Description of the invention] [Problems to be solved by the invention]

[0008] The inventors investigated hollow sections that form on the inside bend to enable the provision of a high-strength steel sheet, which has been improved in terms of a sharp hollow section on an inside bend that begins during bend forming. As a result, the present inventors discovered that the sharp hollow section, like a fine crack on the inside bend (hereafter referred to as the hollow section on the inside bend), is not a fine crack and is attributed to the irregularity formed by the plastic buckling of the surface layer of the steel sheet outward from the plane in a microscopic region during bend forming.Furthermore, the present inventors discovered that, in the event that the depth of a hollowed-out part in the inner curve exceeds a certain value, the fatigue properties of the hot-rolled steel sheets deteriorate significantly.

[0009] ML / a / ZUZZ / UI uouo An object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent formability and allowing the reduction of the depth of a hollowed-out portion in the inner curve formed during bending, and a method of manufacturing the same. [Means to solve the problem]

[0010] As a result of inventive studies, the present inventors found that the depth of a recessed portion in the inner curve formed during bending can be reduced to the point where the component's performance is not degraded by establishing a chemical composition and metallographic structure appropriate for achieving high strength and, furthermore, by particularly controlling the rotation angle of a specific crystal orientation in the direction of the sheet thickness. High strength in the present embodiment means that the tensile strength (maximum) is 880 MPa or more. In addition, excellent formability means that the hole expansion rate is 35% or more.

[0011] The essence of the present invention based on the findings described above is as follows. (1) A hot-rolled steel sheet in accordance with ΜΛ / a / ZUZZ / UI uouo an aspect of the present invention contains, as a chemical composition, in % by mass: ΜΛ / a / ZUZZ / UI white C : 0.060% to 0.170%, Si : : 0.030% to 1.700%, Mn : : 1.20% to 3.00%, Al; : 0.010% to 0.700%, Nb: : 0.005% to 0.050%, P: 0.0800% or less, S : 0.0100% or less, N: 0.0050% or less, Water: 0% to 0.1800%, Mo: 0% to 0.150%, V: 0% to 0.3000%, Cr: 0% to 0.500%, B: 0% to 0.0030%, and a residue consisting of Fe and an impurity, wherein, in metallographic structures at a position of 1 / 4 in the strip thickness direction from a surface and at a position of 1 / 2 in the strip thickness direction from the surface, by % by volume, bainite and martensite are a total of 80.0% or more, ferrite is 20.0% or less, and cementite and residual austenite are a total of 0% to 10.0%, in a metallographic structure of a region from the surface to a position of 100 pm in the strip thickness direction from the surface, a mean grain diameter of the above austenite grains is less than 30.00 pm, a region, where the angle of rotation between a line normal to the surface and a pole (011) near the line normal is 5oo less, is 0.150 or less from the surface in terms of a standardized sheet thickness direction position by a sheet thickness, a region, where the rotation angle between the surface normal line and the pole (011) near the normal line is 20° or more, is 0.250 or more from the surface in terms of the standardized sheet thickness direction position by the sheet thickness, and a tensile strength is 880 MPa or more. (2) Hot-rolled steel sheet according to (1) may further contain, as a chemical composition, by % by mass, one or more selected from the group consisting of Ti: 0.0200% to 0.1800% Mo: 0.030% to 0.150%, V: 0.0500% to 0.3000% Cr: 0.050% to 0.500%, and B: 0.0001% to 0.0030%. (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 casting step of, in the continuous casting of a slab having the chemical composition according to (1), performing the continuous casting such that an average surface temperature gradient in a region from a meniscus to 1.0 m from the meniscus is from 300 to 650 °C / m to obtain the slab, a heating step of heating the slab to 1200°C or more and holding the slab at that temperature for 30 minutes or more, a hot rolling step consisting of rough rolling the slab and finishing rolling such that the total reduction of the roll in a temperature range of 870°C to 980°C is 80% or more, the time elapsed between the rolling stands in the temperature range of 870°C to 980°C becomes 0.3 to 5.0 seconds, and a total reduction of lamination in a temperature range below 870°C becomes less than 10%, a cooling step of cooling for 30.0 seconds or less to cool to a temperature range below 300°C after final lamination, and a winding step such that the winding temperature is below 300°C after cooling. (4) The method of manufacturing hot-rolled steel sheet according to (3) may further include a holding heat treatment step in a temperature range of 200°C or more and less than 450°C for 90 to 80,000 seconds ML / a / ZUZZ / UI uouo after rolling. [Effects of the invention]

[0012] According to aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent formability and allowing the reduction of the depth of a hollowed-out part in the inner curve formed during bending and a method of manufacturing the same. [Brief description of the drawings]

[0013] FIG. 1 is a view showing a relationship between the position of the standardized sheet thickness direction by the sheet thickness of a region where the angle of rotation between a normal line of a surface of a steel sheet and a pole (011) near the normal line becomes 5oo less and a depth of a hollowed-out part in the inner curve in an example. FIG. 2 is a view showing a relationship between a position of the standardized sheet thickness direction by the sheet thickness of a region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 20° or more and the depth of the recessed part in the inner curve in the example. ML / a / ZUZZ / UI uouo FIG. 3 is a view showing a relationship between the position of the standardized sheet thickness direction by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 500 less, the position of the standardized sheet thickness direction by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 20° or more, and the result of the evaluation of the hollowed-out part on the inside curve in the example. [Modalities of the invention]

[0014] A hot-rolled steel sheet according to the present embodiment (hereinafter referred to simply as the steel sheet in some cases) will now be described in detail. However, the present invention is not limited solely to the configuration described herein and may be modified in various ways within the scope of the essence of the present invention. The numerical limit ranges expressed below using 'a' include the lower and upper limits in the ranges. Numerical values ​​expressed with 'more than' and 'less than' are not included in the numerical ranges. % with 'uouo' with respect to chemical compositions indicates % by mass.

[0015] Hot-rolled steel sheet according to the present specification contains, in % by mass, C: 0.060% to 0.170%, Si: 0.030% to 1.700%, Mn: 1.20% to 3.00%, Al: 0.010% to 0.700%, Nb: 0.005% to 0.050%, P: 0.0800% or less, S: 0.0100% or less, N: 0.0050% or less, and a residue of Fe and an impurity. Each element will be described in detail below.

[0016] C: 0.060% to 0.170% Carbon (C) is a key element in determining the strength of hot-rolled steel sheet. When the carbon content is less than 0.060%, a tensile strength of 880 MPa or higher is not achievable. Therefore, the carbon content is typically set at 0.060% or higher. Ideally, the carbon content should be 0.080% or higher. On the other hand, when the carbon content exceeds 0.170%, the hole expansion capacity of the hot-rolled steel sheet deteriorates, and it is not possible to achieve a hole expansion rate of 35% or more. Hot-rolled steel sheets with a hole expansion rate of less than 35% are not suitable for components. Therefore, the carbon content is set at 0.170% or less. The carbon content is preferably 0.150% or less.

[0017] ML / a / ZUZZ / UI uouo Yes: 0.030% to 1.700% Silicon (Si) is an element that improves the strength of hot-rolled steel sheet by strengthening the solid solution. Furthermore, Si also suppresses carbide formation and softening during heat treatment. To achieve these effects, the Si content is set at 0.030% or higher. Preferably, the Si content is 0.050% or higher. On the other hand, since silicon has a high oxide-forming capacity, when the silicon content is excessive, an oxide forms in a weld or the volume percentage of residual austenite exceeds 10%, and the expansion capacity of the hole in the hot-rolled steel sheet is impaired. Therefore, the silicon content is set at 1,700% or less. To further suppress softening during quenching, the silicon content is preferably set at 1,300% or less.

[0018] Mη: 1.20% to 3.00% Manganese (Mn) is a necessary element for improving the strength of hot-rolled steel sheet. When the Mn content is less than 1.20%, it is not possible to obtain a tensile strength of 880 MPa or higher. Therefore, the Mn content is set at 1.20% or higher. The preferred Mn content is 1.50% or higher. ML / a / ZUZZ / UI uouo Furthermore, when the Mn content exceeds 3.00%, the toughness of a cast slab deteriorates, and hot rolling is not possible. Therefore, the Mn content is set at 3.00% or less. The preferred Mn content is 2.70% or less.

[0019] Al: 0.010% to 0.700% Aluminum (Al) is an element that acts as a deoxidizing agent and improves the cleanliness of steel. To achieve this effect, the Al content is set at 0.010% or higher. The Al content is preferably 0.100% or higher. On the other hand, when the Al content exceeds 0.700%, casting becomes difficult. Therefore, the Al content is set at 0.700% or less. Al is an oxidizing element, and the Al content is preferably 0.300% or less to achieve an additional improvement in continuous castability and a cost reduction effect.

[0020] Nb: 0.005% to 0.050% To obtain an average grain diameter of less than 30.00 pm for the preceding austenite grains in a hot rolling pass, the Nb content must be set at 0.005% or higher. When the Nb content is less than 0.005%, it is not possible to obtain an average grain diameter of less than 30.00 pm for the preceding austenite grains in the pass. ML / a / ZUZZ / UI hot rolling, and in the end a desired metallographic structure cannot be obtained. Therefore, the Nb content is set at 0.005% or more. The Nb content is preferably 0.010% or more or 0.020% or more. On the other hand, when the Nb content exceeds 0.050%, the toughness of the cast slab deteriorates, and hot rolling is not possible. Therefore, the Nb content is set at 0.050% or less. The Nb content is preferably 0.040% or less.

[0021] P: 0.0800% 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 brittle the hot-rolled steel sheet becomes. For hot-rolled steel sheets used in automotive suspension components, a P content of up to 0.0800% is acceptable. Therefore, the P content is typically set at 0.0800% or less. Preferably, the P content is 0.0500% 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 set at 0.0005% or higher.

[0022] ML / a / ZUZZ / UI UDUO S: 0.0100% or less If molten steel contains a high amount of sulfur (S), manganese sulfide (MnS) forms, degrading the hole expansion capacity and toughness of the hot-rolled steel sheet. Therefore, the sulfur content is set at 0.0100% or less. Preferably, the sulfur content is 0.0080% or less. When the sulfur content is reduced to less than 0.0001%, the cost of desulfurization increases significantly, and therefore, the sulfur content can be set at 0.0001% or more.

[0023] 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 amount of residual austenite in the hot-rolled steel sheet increases, and in some cases, the hole expansion capacity of the hot-rolled steel sheet deteriorates, as does the toughness of the slab. Therefore, the N content is typically set at 0.0050% or less. Preferably, the N content is 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 set at 0.0001% or higher.

[0024] ML / a / ZUZZ / UI uouo The remainder of the chemical composition of the hot-rolled steel sheet according to this embodiment may be Fe and an impurity. In this embodiment, an impurity means a substance that is incorporated from the raw material ore, scrap, a manufacturing environment, or the like, and is permitted to the extent that the hot-rolled steel sheet according to this embodiment is not adversely affected.

[0025] Hot-rolled steel sheet produced according to this specification may contain one or more of the elements Ti, Mo, V, Cr, and B as an arbitrary element in place of Fe. If the arbitrary element is not present, the lower limit of the content is 0%. Each arbitrary element is described below.

[0026] Ti: 0% to 0.1800%, Titanium (Ti) is an element that increases the strength of hot-rolled steel sheet by precipitating as a fine carbide within the steel and can therefore be present in the steel. To reliably achieve this effect, the Ti content is preferably set at 0.0200% or higher. However, even with a Ti content exceeding 0.1800%, the effect described above is saturated. Therefore, the Ti content is preferably set at 0.1800% or lower. IVIA / a / ZUZZ / UI uouo

[0027] Mo: 0% to 0.150% Mo is an element that improves the hardenability of steel and can be included as an element that adjusts the strength of hot-rolled steel sheet. To reliably achieve the effect described above, the Mo content is preferably set at 0.030% or higher. 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 set at 0.150% or lower.

[0028] V: 0% to 0.3000% Zinc (V) is an element that produces an effect similar to titanium (Ti). In order to reliably achieve precipitation hardening through the formation of a fine carbide, the zinc content is preferably set at 0.0500% or higher. However, when zinc is present in excess, a nitride forms in the steel, which degrades the toughness of the slab and makes threading difficult. Therefore, the zinc content is preferably set at 0.3000% or lower.

[0029] Cr: 0% to 0.500% Cr is an element that develops an effect similar to that of Mn. In order to reliably obtain an effect of ML / a / ZUZZ / UI uouo improvement of the strength of hot-rolled steel sheet, the Cr content is preferably set at 0.050% or more. On the other hand, even when it contains more than 0.500% Cr, the effect described above is saturated. Therefore, the Cr content is preferably set at 0.500% or less.

[0030] B: 0% to 0.0030% Boron (B) is an element that produces an effect similar to that of molybdenum (Mo), improving the hardenability and increasing the strength of hot-rolled steel sheets. To reliably achieve this effect, the B content is preferably set at 0.0001% or higher. However, even with a B content exceeding 0.0030%, the effect described above becomes saturated; therefore, the B content is preferably set at 0.0030% or lower.

[0031] The chemical composition of hot-rolled steel sheet described above can be analyzed using a spark discharge emission spectrophotometer or similar instrument. For carbon (C) and sulfur (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 infrared absorption are adopted. ML / a / ZUZZ / UI uouo Furthermore, for N, an identified value is adopted by melting a test piece collected from the hot-rolled steel sheet in a stream of helium and measuring N using a thermal conductivity method.

[0032] The metallographic structure of the hot-rolled steel sheet, according to the present method, will now be described. The characteristics of the metallographic structure are limited to the point that an effect can be achieved not only on improving the strength and formability of the hot-rolled steel sheet, but also on reducing the depth of the hollows in the inner curve.

[0033] In hot-rolled steel sheet according to the present method, in the metallographic structures at a position 1 / 4 in the sheet thickness direction from the surface and at a position 1 / 2 in the sheet thickness direction from the surface, in % by volume, bainite and martensite are a total of 80.0% or more, ferrite is 20.0% or less, cementite and residual austenite are a total of 0% to 10.0%, in the metallographic structure in a region from the surface to a position 100 pm in the sheet thickness direction from the surface, the average grain diameter of the above austenite grains is less than IVIA / a / ZUZZ / UI uouo 30.00 pm, a region where the rotation angle between the surface normal line and a pole (011) near the normal line becomes 500 or less, is 0.150 or less from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, a region where the rotation angle between the surface normal line and the pole (011) near the normal line becomes 20° or more, is 0.250 or more from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness. Each regulation will be described below.

[0034] Bainite and martensite: Total 80.0% or more If the total volume percentage of bainite and martensite is less than 80%, it is not possible to obtain a tensile strength of 880 MPa or more and / or a hole expansion rate of 35% or more. Therefore, the total volume percentage of bainite and martensite is set at 80.0% or more. The volume percentage of bainite and martensite is preferably 83.0% or more. Martensite may be tempered and may contain residual cementite and austenite. The volume percentage of cementite and residual austenite may be 10.0% or less.

[0035] uouo Ferrite: 20.0% or less When the ferrite volume percentage exceeds 20.0%, the combined volume percentages of bainite and martensite do not reach 80.0% or more, and the desired tensile strength cannot be achieved. Therefore, the ferrite volume percentage is set at 20.0% or less. To further improve strength, the ferrite volume percentage is preferably 17.0% or less, and more preferably 15.0% or less. The ferrite volume percentage can be set at 10.0% or more to ensure the bore's expansion capacity.

[0036] Residual cementite and austenite: 0% to 10.0% As described above, there are cases where martensite contains residual cementite and austenite. When the volume percentage of residual cementite and austenite exceeds 10.0%, the expansion of the hole in the hot-rolled steel sheet deteriorates due to the impairment of local deformability. Therefore, the volume percentage of residual cementite and austenite is set at 10.0% or less. The volume percentage of residual cementite and austenite is preferably 7.0% or less, and more preferably 5.0% or less. The volume percentage of residual cementite and austenite is preferably as small as possible, and therefore the ΜΛ / a / ZUZZ / UI uo lower limit is 0%.

[0037] Method for measuring the percentage of ferrite volume The ferrite volume percentage is determined by the ratio of crystal grain areas where an iron-based carbide does not form, obtained by observing the structure in a metallographic photograph. A sample is taken such that a cross-section of the sheet thickness intersecting the rolling direction of the hot-rolled steel sheet at a right angle is visible. This cross-section is etched using a nital etching solution with a concentration of 3% to 5% to make the ferrite visible. The structure is then observed using metallographic photographs, each taken at 500x to 1000x magnification at the 1 / 4 position along the sheet thickness direction from the surface of the hot-rolled steel sheet and at the 1 / 2 position along the sheet thickness direction from the surface.For a given steel type, metallographic structure photographs are prepared in three or more fields of view at each of the 1 / 4 position relative to the sheet thickness from the surface and the 1 / 2 position relative to the sheet thickness from the surface. The ratio of the ferrite area observed in each metallographic structure photograph is obtained, and the value is calculated. ML / a / ZUZZ / UI uouo average of the same, thus obtaining the percentage by volume of ferrite. The iron-based carbide is recognized as a black granular contrast that has a circle equivalent diameter of 1 pm or less in the photograph of the metallographic structure and is observed in the crystal grain.

[0038] Method for measuring the percentage of volume of bainite and martensite. As the total volume percentages of bainite and martensite in the present form, a value is used obtained by subtracting the volume percentage of ferrite and the total volume percentages of cementite and residual austenite measured by a method described below from 100.0%.

[0039] Method for measuring the percentage of residual austenite volume The volume percentage of residual austenite is measured by EBSP. EBSP analysis is performed using a sample collected from the same location as the sample collection point when measuring the ferrite volume percentage: 1 / 4 of the way down the sheet thickness from the surface of the hot-rolled steel sheet and 1 / 2 of the way down the sheet thickness from the surface. The sample must be polished with silicon carbide paper from #600 to #1500 grit and then finished to a mirror surface with a liquid containing diamond powder with a grain size of 1 to 6 µm dispersed in a dilute solution, such as alcohol or pure water, and then finished by electropolishing to sufficiently relieve stress in the cross-section to be measured.In electrolytic polishing, to eliminate mechanical polishing stress in an observed section, the sample should be polished to a minimum of 20 µm and a maximum of 50 µm. Preferably, the sample is polished to 30 µm or less, taking into account the roll-off at the end. In EBSP measurements, the acceleration voltage is set to 15–25 kV, measurements are taken at intervals of at least 0.25 pm, and crystal orientation information at each measurement point is obtained at 150 pm or more in the sheet thickness direction and 250 pm or more in the rolling direction. From the resulting crystal structures, grains with an fcc crystal structure are identified as 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 residual austenite is then calculated, yielding the residual austenite area ratio. This residual austenite area ratio is considered the percentage of the total area of ​​the austenite. IVIA / a / ZUZZ / UI uouo volume of the residual austenite. Here, the greater the number of measurement points, the better; 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 electron beam's scattering width. Therefore, measurement intervals are set at 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 sheet width direction. The measurement instrument 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 x 10“5Pa or less, the irradiation current level is set at 13, and the electron beam irradiation level is set at 62.

[0040] Method for measuring the volume percentage of cementite: The volume percentage of cementite is measured using a sample collected in the same position as the sample collection position when measuring the volume percentage of ferrite at position 1 / 4 in the direction of the sheet thickness from the sheet surface. IVIA / a / ZUZZ / UI uouo rolled steel and at the 1 / 2 position in the sheet thickness direction from the surface. The cross-section of the sheet thickness is polished with abrasive paper or alumina abrasive grains to a mirror finish, then etched with a 3% nital and picral solution, and observed using a scanning electron microscope (SEM). Subsequently, a plurality of fields of view are captured using a photographic device connected to the SEM at 2000x magnification, so that the total area of ​​the observed field of view is 1.6 * 10⁷ pm² or more, and the cementite area ratio is measured using image analysis software such as particle analysis software. Thus, the cementite area ratio is obtained. The obtained cementite area ratio is considered to be the volume percentage of cementite.

[0041] Average grain diameter of previous austenite grains: Less than 30.00 pm The hollowed-out portion on the inner curve is caused by the plastic buckling of the crystal grains in the surface layer of the hot-rolled steel sheet and is affected by the sizes of the bainite and martensite structures, which have low deformability. For these structures, the size of the preceding austenitic grain becomes the maximum unit (i.e., there is no case in IVIA / a / ZUZZ / UI (where bainite and martensite are larger than the preceding austenitic grain). Characteristically, the bainite and martensite are divided into several structural units called blocks. To ensure that the depths of the hollowed-out portions in the inner bend are less than 30.0 pm, the average grain diameter of the preceding austenite grains, which becomes the maximum size of the bainite and martensite structural units (primary phases, representing 80.0% or more by volume, of the hot-rolled steel sheet produced according to this modality), is set at less than 30.00 pm. To further suppress the deterioration of fatigue properties attributed to the hollowed-out portions in the inner bend, the average grain diameter of the preceding austenite grains is preferably set at less than 20.00 pm.Furthermore, since the deterioration of fatigue properties attributed to the hollowed-out parts in the inner curve is affected by the average grain diameter of the preceding austenite grains in the surface layer region, it is in a surface layer region (a region from the surface of the hot-rolled steel sheet to a position of 100 pm in the sheet thickness direction from the surface) that the average grain diameter of the preceding austenite grains is set at less than 30.00 pm.

[0042] IVIA / a / ZUZZ / UI uouo Method for measuring the average grain diameter of previous austenite grains To 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 with 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 of the hot-rolled steel sheet 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 image of the structure, the sample is irradiated with an electron beam in a vacuum at 9.6 * 10⁻⁵ Pa or less, an accelerating voltage of 15 kV, and an irradiation current level of 13 kV. A secondary electron image of the surface layer region (the region from the surface of the steel sheet) is then captured. IVIA / a / ZUZZ / UI (hot-rolled to the 100 pm position in the sheet thickness direction from the surface). The number of captured fields of view is set to 10 or more fields of view. In the captured secondary electron image, the former austenite grain boundaries are captured as a bright contrast. The equivalent circle diameter is calculated for one of the former austenite grains that is included in the observed field of view. The operation described above is performed on all former austenite grains that are included in the observed field of view, except for former austenite grains that are not completely included in the captured field of view, such as former austenite grains at the end of the captured field of view, and the equivalent circle diameters of all former austenite grains in the captured field of view are obtained.The average grain diameter of the above austenite grains is obtained by calculating the average value of the equivalent circular diameters of the above austenite grains obtained in the captured individual fields of view.

[0043] Region where the rotation angle between the surface normal line and the pole (011) near the normal line becomes 5oo less: 0.150 or less from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, and region where the rotation angle between the surface normal line and the pole (011) near the normal line becomes 20° or more: 0.250 or more from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness. The present inventors found that when a region where the angle of rotation between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line becomes 5° or less, occurs at 0.150 or less of the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, and a region where the angle of rotation becomes 20° or more occurs at 0.250 or more of the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, it is possible to reduce the depths of the hollowed-out portions on the inner curve in an arbitrary direction of the sheet surface. The position of the sheet thickness direction standardized by the sheet thickness is expressed as d / t, where d represents the depth of the sheet thickness direction and t represents the sheet thickness.

[0044] As described above, the hollowed-out parts on the inner curve are attributed to a plastic buckling phenomenon. IVIA / a / ZUZZ / UI microscopic plastic buckling in the surface layer of the hot-rolled steel sheet. The present inventors considered this plastic buckling phenomenon as a microscopic plastic flow and understood that the plastic buckling phenomenon is the result of a basic behavior caused by the rotation of the crystal grains. In the case of bending distortion, the number of rotated crystal grains depends on the distortion gradient from the neutral axis to the surface of the sheet thickness. The present inventors considered that the distribution of orientation groups that have different crystal rotation behaviors in the direction of the sheet thickness causes an imbalance in local distortion and promotes buckling in the surface layer of the hot-rolled steel sheet.

[0045] Therefore, the inventors paid attention to and investigated the relationship between the depths of the recessed portions on the inner curve and the crystal orientations in the direction of the sheet thickness. As a typical crystal orientation, a pole (011) is drawn in the direction of the sheet thickness and divided into a region where the rotation angle is 50° or less and the crystal orientation remains unchanged, and a region where the rotation angle is 20° or less and the crystal orientation remains unchanged. The present inventors considered the thickness within a range IVIA / a / ZUZZ / UI UDUO, in which the crystal orientation does not change, causes irregular distortions in the sheet thickness direction, and they investigated the relationship between the proportions of the depths in the sheet thickness direction in the individual ranges and the depths of the hollowed parts in the inner curve. As a result, as shown in FIG. 1 and FIG. 2, when the region where the rotation angle between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line reaches 500 or less is present in more than 0.150 in terms of the position of the sheet thickness direction (depth of the sheet thickness direction d / sheet thickness t) standardized by the sheet thickness, the depths of the hollowed parts in the inner curve reach 30.0 pm or more.Furthermore, it was found that even when the region where the rotation angle between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line reaches 20° or more, it is present at less than 0.250 in terms of the position of the sheet thickness direction standardized by the sheet thickness. Similarly, the depths of the hollowed-out parts on the inner curve become 30.0 pm or more. Figure 1 is a view obtained from an example that will be described below and shows the relationship between the position of the sheet thickness direction. IVIA / a / ZUZZ / UI uouo normalized by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 5oo less and the depth of the hollowed part in the inside curve. FIG. 2 is a view obtained from an example that will be described below and a view showing the relationship between the position of the direction of the sheet thickness normalized by the sheet thickness of the region where the angle of rotation between the normal line of the surface and the pole (011) near the normal line becomes 20° or more and the depth of the hollowed part in the inside curve.

[0046] Based on the research described above, the present inventors found that, to reduce the depth of the recessed portion on the inner curve, the most favorable range exists for the depth ratios of the region where the angle between the normal line of the hot-rolled steel sheet surface and the pole (011) becomes 500 less and the region where the rotation angle becomes 20° or more. As shown in FIG. 3, when the region where the rotation angle between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line is 500 less, it occurs at 0.150 or less from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, and the region where the rotation angle becomes 20° or more is present at 0.250 or more of the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, it is possible to make the depths of the hollowed parts on the inside curve less than 30.0 pm. FIG. 3 is a view obtained from the example that will be described below and a view showing the relationship between the position of the sheet thickness direction standardized by the sheet thickness of the region where the angle of rotation between the surface normal line and the pole (011) near the normal line becomes 500 less, the position of the sheet thickness direction standardized by the sheet thickness of the region where the angle of rotation between the surface normal line and the pole (011) near the normal line becomes 20° or more, and the result of the evaluation of the hollowed part on the inside curve in the example.

[0047] Next, a method for measuring the region that has a predetermined rotation angle between the normal line of the steel sheet surface and the pole (011) near the normal line will be described. EBSP performs the measurement using a sample that has IVIA / a / ZUZZ / UI uouo A cross-section finished to a mirror surface by the same method as the sample used for the previous measurement of the volume percentage of the austenite grains. The sample must be finished by electropolishing in order to sufficiently relieve stress in the cross-section to be measured. In electropolishing, to eliminate mechanical polishing stress in an observed section, the sample must be polished to a minimum of 20 µm and a maximum of 50 µm. Preferably, the sample is polished to 30 µm or less, taking into account the roll at the end. In EBSP measurement, the acceleration voltage is set to 15 to 25 kV, and the measurement range is set to cover the full thickness of the sheet. The measurement range 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 30 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 the steel sheet surface and the pole (011) near the normal line is measured using the following method. IVIA / a / ZUZZ / UI uouo

[0048] The rotation angle between the normal line of the hot-rolled steel sheet surface and the (011) pole near the normal line is a value measured by plotting the crystal orientation data obtained from 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 (011) orientation poles are displayed such that the normal lines (origin: ND) become the normal lines to the hot-rolled steel sheet surface, the horizontal axis TD becomes the sheet width direction, 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 full thickness range of the sheet. This group of points is divided into 20 sections in the sheet thickness direction, and a pole figure (011) is drawn. On the pole figure (011), at each depth direction position from the surface of the steel sheet drawn as described above, the angle between the origin ND (normal line to the surface of the hot-rolled steel sheet) and the nearest pole (011) is measured. uouo This measurement value is defined as the angle of rotation between the surface normal line and the pole (011) near the normal line. A value obtained by dividing each depth direction position by the sheet thickness is defined as the sheet thickness direction position (depth of sheet thickness direction d / sheet thickness t) standardized by the sheet thickness, and the region where the rotation angle becomes 5° or less and the region where the rotation angle becomes 20° or more are obtained at this sheet thickness direction position standardized by the sheet thickness.

[0049] Tensile strength: 880 MPa or more In hot-rolled steel sheet produced according to this specification, the tensile strength is 880 MPa or higher. When the tensile strength is less than 880 MPa, it is difficult to apply the hot-rolled steel sheet to automotive suspension components. The tensile strength may be 900 MPa or higher. The tensile strength is preferably as high as possible, but it may be 1500 MPa or lower to minimize the weight reduction effect of the high hardening of the hot-rolled steel sheet. Tensile strength is measured by performing a tensile test in accordance with JIS Z 2241: 2011 using a IVIA / a / ZUZZ / UI uouo test piece no. 5 of JIS Z 2241: 2011. The position in which the tensile test piece is picked up is the central position in the direction of the width of the sheet, and a direction perpendicular to the rolling direction is the longitudinal direction.

[0050] Hole expansion rate: 35% or more In hot-rolled steel sheet produced according to this method, the hole expansion rate is 35% or higher. When the hole expansion rate is less than 35%, induced fracture occurs in the flash portion, making it difficult to apply the hot-rolled steel sheet to automotive suspension components. The hole expansion rate can be set at 50% or higher to reduce the flattening rate of the flash portion and decrease the load on the die during a pressing step. If the hole expansion rate is set at 80% or higher, flattening can be eliminated, and the rigidity of the components can be improved by achieving a sufficient flash height. Therefore, the hole expansion rate can be set at 80% or higher. The hole expansion rate is measured by performing a hole expansion test in accordance with JIS Z 2256: 2010. uouo

[0051] A preferred method of manufacturing hot-rolled steel sheet according to the present embodiment will now be described. A casting step and a hot-rolling step, described below, are important steps for controlling the distribution of the crystal orientation in the sheet thickness direction and the average grain diameter of the austenite grains above, which are necessary requirements for reducing the depths of the hollowed-out parts on the inner curve.

[0052] The preferred method of manufacturing hot-rolled steel sheet according to the present modality includes the following steps. A casting step, in the continuous casting of a slab having a predetermined chemical composition, involves performing continuous casting such that the average surface temperature gradient in a region from a meniscus to 1.0 m from the meniscus becomes 300 to 650 °C / m to obtain the slab; a heating step involves heating the slab to 1200°C or more and holding the slab at that temperature for 30 minutes or more; and a hot rolling step consists of rough rolling the slab and then performing a final rolling. IVIA / a / ZUZZ / UI uouo of finishing such that the total reduction of the rolling in a temperature range of 870°C to 980°C is 80% or more, an elapsed time between rolling boxes in the temperature range of 870°C to 980°C becomes 0.3 to 5.0 seconds, and a total reduction of rolling in a temperature range below 870°C becomes less than 10%, a cooling step consisting of, after the final rolling, cooling a hot-rolled steel sheet for 30.0 seconds or less to cool the hot-rolled steel sheet to a temperature range below 300°C, and a coiling step consisting of, after cooling, coiling the hot-rolled steel sheet such that the coiling temperature is below 300°C. The preferred method of manufacturing hot-rolled steel sheet according to the present modality may further include a heat treatment step consisting of, after rolling, holding the hot-rolled steel sheet at a temperature range of 200 °C or higher and lower than 450 °C for 90 to 80000 seconds. Each step will be described below.

[0053] Casting step In the continuous casting of a slab having the chemical composition described above, the surface temperature gradient The average surface temperature gradient in a region from the meniscus to 1.0 m from the meniscus is set at 300 to 650 °C / m. The surface temperature gradient in the early solidification stage affects the angle of rotation between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line. In the present embodiment, the average surface temperature gradient refers to a temperature gradient obtained by dividing the temperature in a mold in contact with a solidified shell by the distance from the meniscus. The temperature is measured with thermocouples embedded in the mold. The thermocouples are embedded at a position 0 mm below the meniscus that is 0.010 mm or less from the outer surface (solidified shell) of the mold and 1.0 mm below the meniscus that is at 0.0.10 mm or less from the outer surface (solidified shell) of the mold in the central part of the long side surface of the slab in the width direction. The thermocouple, which is embedded at the position 0 mm below the meniscus, must be 0.040 mm or less, and preferably 0.005 mm or less, from the meniscus (in one casting direction). A value obtained by dividing each measured temperature by the distance from the section is considered the average surface temperature gradient.

[0054] When the average surface temperature gradient IVIA / a / ZUZZ / UI uouo In the meniscus region, at 1.0 m from the meniscus, the temperature gradient is less than 300 °C / m. The region where the angle of rotation between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line is less than 50° is present at more than 0.150 m from the surface, measured from the standardized sheet thickness direction. On the other hand, when the average temperature gradient in the region described above is greater than 650 °C / m, the region where the angle of rotation between the normal line of the hot-rolled steel sheet surface and the pole (011) near the normal line is 20° or more is present at less than 0.250 m from the surface, measured from the standardized sheet thickness direction. Therefore, the average surface temperature gradient in the region from the meniscus to 1.The average surface temperature gradient is set at 300 to 650 °C / m from the meniscus, and the slab is manufactured. The lower limit of the average surface temperature gradient is preferably 350 °C / m to 400 °C / m, and the upper limit of the average surface temperature gradient is preferably 600 °C / m to 550 °C / m.

[0055] The average casting speed in the pouring pass can be within a normal range, it can be 0.8 m / min or faster, or it can be 1.2 m / min or faster. From the point From a cost reduction perspective, the average pouring speed in the casting step is preferably set at 1.2 m / min or faster. On the other hand, when the average pouring speed exceeds 2.5 m / min, the cooling temperature gradient in the slab thickness direction increases due to the higher pouring speed, and the internal stress of the slab during solidification increases, making it easier for defects to develop. Therefore, the average pouring speed is preferably 2.5 m / min or slower. Furthermore, when the average pouring speed is 0.6 m / min or slower, the cooling temperature gradient in the slab thickness direction decreases, but economic efficiency is significantly affected. Therefore, the average pouring speed is preferably between 0.6 and 2.5 m / min.

[0056] Warm-up step The slab obtained by continuous casting is heated so that the surface temperature reaches 1200°C or higher and is held at this temperature for 30 minutes or more, thus solubilizing the slab. When the heating temperature is below 1200°C, homogenization and dissolution of the carbide by dissolution treatment do not occur, and ferritic transformation takes place, resulting in decreased strength. IVIA / a / ZUZZ / UI uouo of the hot-rolled steel sheet. If the slab contains Ti, the heating temperature is preferably set to 1230°C or higher to more reliably form a solid Ti solution. Furthermore, regarding the roughing temperature before heating, the slab 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. The slab is heated in the heating step by loading it into a controlled furnace at a predetermined temperature, and the time required for the slab surface temperature to reach 1200°C or higher should be set at 30 minutes or more, which is sufficient. When the holding time at the temperature range of 1200°C or higher is less than 30 minutes, it is not possible to obtain the desired amount of bainite and martensite.The holding time is preferably 40 minutes or more, 60 minutes or more, or 100 minutes or more. For example, the heating temperature should be 1400 °C or less, and the heating time should be 300 minutes or less. Furthermore, if the slab contains titanium, the time required for the slab's surface temperature to reach 1230 °C or higher must be set at 60 minutes or more, which is sufficient. In the oven, the slab is placed on a platform of inorganic material, and the slab can dissolve. ML / a / ZUZZ / UI UDUO heating it to a temperature equal to or lower than that at which the heated slab does not dissolve due to a reaction between the inorganic substance and the iron at this time.

[0057] Hot rolling step After heating the slab, rough rolling is performed, followed by finish rolling within a range described below. Finish rolling is carried out such that the total rolling reduction within a temperature range of 870°C to 980°C reaches 80% or more. The total rolling reduction is preferably 85% or greater. 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 austenite grains reaches 30.00 pm or more. The total rolling reduction mentioned here is a value obtained by summing the rolling reduction at each rolling stand where the contact temperature falls between 870°C and 980°C.When the final rolling temperature exceeds 980°C, the average grain diameter of the austenite grains becomes large, regardless of the total rolling reduction in the rolling stand, and it is not possible to control the depth of the hollowed-out sections on the inner bend to less than 30.0 pm. The total rolling reduction within the temperature range of 870°C to 980°C can be established. IVIA / a / ZUZZ / UI uouo at 98% or less. Furthermore, when the total rolling reduction below 870°C is 10% or more, the region where the rotation angle between the normal line of the steel sheet surface and the pole (011) near the normal line becomes less than 500 is present at more than 0.150 from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness. Therefore, the total rolling reduction below 870°C is set at less than 10%. The total rolling reduction below 870°C is preferably less than 7%.

[0058] In the hot rolling process, when the total sheet reduction ratio ((1 - t / to)x100), which is the ratio between the sheet thickness to after rough rolling and the product sheet thickness t after finishing 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 ratio is limited to 80% or more. This total sheet reduction ratio is preferably as high as possible since it increases throughput; however, if the total sheet reduction ratio exceeds 98%, the load on the rolling mill increases. IVIA / a / ZUZZ / UI uouo the costs of roller replacement and the like. Therefore, the overall sheet reduction rate, which is the ratio of the sheet thickness after rough rolling to 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.

[0059] The total number of rolling stands is not particularly limited and can be determined based on the rolling mill's capabilities, such as load capacity or torque. In cases where the contact temperature between stands rises from 870°C to 980°C to two or more, and the time between stands exceeds 5.0 seconds, the austenite grains grow in the affected section, and the average grain diameter reaches 30.00 pm or more, which is undesirable. Therefore, within the temperature range of 870°C to 980°C, the time between stands is set at 5.0 seconds or less. Preferably, this time is 4.0 seconds or less. Furthermore, if the time between individual rolling stations is less than 0.3 seconds, the load on the rolling roller increases.Therefore, the time between individual lamination boxes is set at 0.3. IVIA / a / ZUZZ / UI uouo seconds or more. The elapsed time is preferably 1.0 second or more or 2.0 seconds or more. This contact temperature can be obtained from the surface temperature of the steel sheet measured with a thermometer, such as a radiation thermometer installed in each rolling stand.

[0060] Cooling step After final rolling, the hot-rolled steel sheet is cooled to a temperature below 300°C and then wound at a winding temperature below 300°C to achieve a tensile strength of 880 MPa or higher. The winding temperature is preferably 280°C or lower. The winding temperature may be adjusted to 20°C or higher. As it cools after final rolling, the hot-rolled steel sheet is cooled so that the cooling time after final rolling (the time elapsed from the completion of final rolling to the start of winding) is 30.0 seconds or less to achieve a desired amount of bainite and martensite formation and a tensile strength of 880 MPa or higher. The cooling time is preferably 25.0 seconds or less.For cooling after final lamination, a cooling method can be selected, such as water cooling or air cooling on an output table, so that the cooling time is as desired.

[0061] The average surface temperature of the steel sheet along the entire length of a coil, measured with a thermometer installed along the section from the cooling unit to the 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. However, to reduce material variation within the coil, the winding temperature at any point along the coil is preferably set to a maximum of 450°C or lower. In other words, the surface temperature of the steel sheet is preferably set to 450°C or lower along the entire length of the coil.

[0062] Hot-rolled steel sheet manufactured by the method described above can be cooled to room temperature or water-quenched after being wound into a coil. If cooled to room temperature, the hot-rolled steel sheet can be unwound and pickled or subjected to IVIA / a / ZUZZ / UI UDUO a tempering lamination (skin pass) to adjust residual stress or shape. The tempering lamination reduction should be set to 0.5% or less.

[0063] Heat treatment step In hot-rolled steel sheets manufactured using the steps described above, heat treatment can be performed by holding the sheet at a temperature between 200°C and 450°C for 90,000 to 80,000 seconds to further improve hole expansion. When the heat treatment 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 heat treatment temperature is 450°C or higher, there are instances where the volume percentages of cementite and residual austenite in the hot-rolled steel sheet increase regardless of the holding time, and the hole expansion capacity of the hot-rolled steel sheet deteriorates.The average rate of temperature increase during the heat treatment stage is not particularly limited, but it is preferably 0.01 °C / s faster to avoid a decrease in heat treatment efficiency. Furthermore, the atmosphere during the... The heat treatment environment can be an oxidizing atmosphere or an atmosphere replaced with nitrogen or similar substances. Heat treatment can be performed on hot-rolled steel sheet in coil form; however, in this case, the holding time is preferably set at 120 seconds or more to reduce variation within the coil. When the holding time exceeds 80,000 seconds, the material quality rarely changes, and the economic efficiency of the heat treatment is affected; therefore, the holding time can be set at 80,000 seconds or less. There are no particular restrictions on the heat treatment method; however, when the heat treatment time is 2,000 seconds or less, the heat treatment is preferably performed after the coil has been unwound to allow for soaking.Heat-treated hot-rolled steel sheet can be cooled to room temperature and then pickled to remove scale formed by hot rolling or heat treatment if necessary. [Examples]

[0064] Examples of the present invention will now be described. The conditions in the examples are illustrative of the conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to these illustrative conditions. The present invention is capable of adopting a variety of conditions within the scope of its essence, provided that the object of the present invention is achieved.

[0065] Slabs with the chemical composition shown in Table 1 were manufactured by continuous casting. The casting speed was 0.9 m / min. Additionally, a mold was cooled to change the average surface temperature gradient in a region from the meniscus to 1.0 m from the meniscus, and hot-rolled steel sheets were obtained. The maximum time between stands in Tables 2 and 3 is the maximum value of the elapsed times between individual rolling stands within a temperature range of 870 °C to 980 °C during final rolling. In all examples, the elapsed time between individual rolling stands within the temperature range of 870 °C to 980 °C was 0.3 seconds or more. The ROT cooling time in Tables 2 and 3 indicates the time elapsed from the completion of final rolling to the start of winding.In addition, after final rolling, the slabs were cooled to rolling temperatures after ROT cooling in Table 2 and Table 3 and then rolled.

[0066] In Test No. 24 of Table 2 and Test No. 37 of IVIA / a / ZUZZ / UI uouo Table 3, because cracks were identified, it was not possible to perform the test after casting. Furthermore, in Test No. 30 of Table 3, because nozzle clogging during continuous casting was significant, and there was concern about the incorporation of an oxide deposit or similar, the test was not performed after casting. In Tests Nos. 14 to 18 and 20 to 23 in Table 2 and Tests Nos. 38 and 48 in Table 3, heat treatment was performed after hot rolling. A test piece was taken from the hot-rolled steel sheet and its metallographic structure was measured using the method described above. Additionally, the tensile strength and hole expansion rate were measured using the following methods on the same steel sheet. Furthermore, the hollowed-out sections on the inner curve were evaluated using the following method.

[0067] Method for measuring tensile strength and pass / fail determination criteria Tensile strength was obtained by performing a tensile test in accordance with JIS Z 2241: 2011 using test piece no. 5 of JIS Z 2241: 2011. The position in which the tensile test piece was picked up was the central position in the direction of the sheet width, and a direction perpendicular to a rolling direction was the longitudinal direction. In the case where the tensile strength was 880 MPa or more, the hot-rolled steel sheet was considered approved for having high strength, and in the case where the tensile strength was less than 880 MPa, the hot-rolled steel sheet was determined to be a failure for not having high strength.

[0068] Method for measuring the rate of hole expansion and pass / fail determination criteria The hole expansion rate was obtained by performing a hole expansion test in accordance with JIS Z 2256: 2010. In the case where the hole expansion rate was 35% or more, the hot-rolled steel sheet was considered to have excellent formability, and in the case where the hole expansion rate was less than 35%, the hot-rolled steel sheet was determined to have poor formability.

[0069] Method for evaluating the hollowed-out portion on the inner curve after formation and pass / fail determination criteria The suppression of deterioration of high-strength steel sheets due to the hollowed-out parts in the curve The internal integrity of suspension components at the time of their application can be evaluated using the following method. A recessed portion is generated on the inside of a steel sheet within a bend, in a section that does not come into contact with a die inside a bend during bend forming. Even when attempting to form a vertical wall portion in a pressure-formed component with a complex shape, a non-contact section is generated. Reproducing such a non-contact state inside a bend can be achieved using a regulated V-block method, for example, in JIS Z 2248:2014 or similar; however, with respect to a punch, an opening portion can be provided so that a non-contact section can be created in the central part of the V.

[0070] In cases where the shape of a pressed component is complex, the recessed portion is not a feature in a specific direction on the sheet surface, and it becomes necessary to remove a recessed portion on the inner curve in an arbitrary direction. Therefore, V-bend tests were performed with respect to the displacement direction L of a coil of steel sheet, the L direction, a C direction orthogonal to the L direction, and five additional directions at 15° intervals between the L and C directions. Bending tests were performed in these directions (a total of seven directions), and the maximum depth of the recessed portion on the inner curve was used as an index for evaluation.For pressed components with a complex shape, such as suspension components, the radius of the bent portion (radius of curvature) varies depending on the design. However, when considering the actual application, R / t, the ratio of the radius of curvature R to the sheet thickness t, can be considered the minimum radius of curvature to be 1.5. With radii of curvature greater than this, the curvature distortion gradient in the direction of the sheet thickness becomes small, which is not a reliable indicator. Therefore, in the examples presented, pass or fail was determined based on the maximum depth of the indentation obtained by performing bending tests with a bending radius for which R / t was set at 1.5. When the depth of the indentation on the inside bend is less than 30.0 pm, no deterioration of the component's fatigue properties is recognized.Therefore, in a case where the depth of the hollowed-out portion on the inner bend was less than 30.0 pm, the hot-rolled steel sheet was deemed acceptable, as the depth of the hollowed-out portion formed during bending could be reduced. On the other hand, in a case where the depth of the hollowed-out portion on the inner bend was greater than 30.0 pm, the hot-rolled steel sheet was deemed acceptable. IVIA / a / ZUZZ / UI uouo was 30.0 min or more, the hot-rolled steel sheet was determined to be a failure since the depth of the hollowed-out part on the inside curve that formed during bending could not be reduced.

[0071] In evaluating the hollow portion on the inside bend of a component, the minimum depth detectable by a dye penetration test method, which is typically adopted, is 30.0 pm. The depth of the hollow portion on the inside bend was measured by cutting a location in a bent piece that did not come into contact with a punch along a cross-section orthogonal to the bend axis, grinding the cut to remove burrs, and observing the cross-section. The depth of a crack (the depth of the hollow portion on the inside bend) was obtained by measuring the distance in the depth direction from a tangent line to the inside bend to the center of the sheet thickness at this cross-section.The presence or absence of a hollowed-out part can be determined by the penetrant liquid testing method, which is normally adopted as a non-destructive method; however, normally, the accuracy is approximately 30.0 pm, which is not adequate.

[0072] IVIA / a / ZUZZ / UI uouo The measurement results described above are shown in Tables 4 and 5. In addition, the results obtained in the examples are shown in Figures 1 through 3. Figure 1 is a view showing the relationship between a position in the standardized sheet thickness direction and the sheet thickness in a region where the angle of rotation between the normal line of the steel sheet surface and a pole (011) near the normal line becomes 50° or less, and the depth of a hollowed-out portion on the inside curve. Figure 2 is a view showing the relationship between a position in the standardized sheet thickness direction and the sheet thickness in a region where the angle of rotation between the normal line of the steel sheet surface and a pole (011) near the normal line becomes 20° or more, and the depth of a hollowed-out portion on the inside curve.Figure 3 shows the relationship between the position of the standardized sheet thickness direction by the sheet thickness of the region where the rotation angle between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 500 less, the position of the standardized sheet thickness direction by the sheet thickness of the region where the rotation angle between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 20° or more, and the result of the evaluation of the hollowed-out part in the curve. IVIA / a / ZUZZ / UI uouo interior .

[0073] IVIA / S / ZUZZ / UI uouo [Table 1] Steel Chemical composition (% by mass), the remainder is Fe and impurities Note C Si Mn Al Nb PSN Ti Mo V Cr BA 0.050 0.350 1.29 0.310 0.015 0.0090 0.0030 0.0028 Comparative steel B 0.130 0.720 2.40 0.050 0.018 0.0080 0.0040 0.0032 Steel of the present invention C 0.190 1.210 2.60 0.030 0.008 0.0100 0.0020 0.0033 Comparative steel D 0.080 0.070 1.81 0.512 0.006 0.0090 0.0030 0.0030 Steel of the present invention E 0.075 0.560 2.10 0.030 0.008 0.0070 0.0020 0.0038 Steel of the present invention F 0.143 1.290 2.60 0.270 0.022 0.0080 0.0035 0.0035 Steel of the present invention G 0.125 1.850 2.45 0.020 0.015 0.0090 0.0032 0.0028 Comparative steel H 0.120 1.110 1.13 0.030 0.016 0.0060 0.0021 0.0022 Comparative steel I 0.165 1.010 2.40 0.022 0.008 0.0070 0.0018 0.0031 Steel of the present invention J 0.082 0.620 3.07 0.018 0.007 0.0060 0.0036 0.0028 Comparative steel K. 0.168 1.680 2.85 0.020 0.006 0.0100 0.0030 0.0025 Steel of the present invention L 0.063 0.050 2.72 0.030 0.037 0.0080 0.0026 0.0031 Steel of the present invention M 0.082 0.070 1.86 0.715 0.011 0.0090 0.0026 0.0029 Comparative steel N 0.062 0.060 1.31 0.020 0.021 0.0090 0.0026 0.0029 Steel of the present invention 0 0.067 0.048 2.69 0.029 0.003 0.0080 0.0026 0.0030 Comparative steel P 0.110 0.930 2.54 0.021 0.036 0.0070 0.0019 0.0031 Steel of the present invention Q 0.122 0.720 2.30 0.032 0.072 0.0068 0.0022 0.0027 Comparative steel R 0.151 1.420 2.31 0.032 0.026 0.0080 0.0030 0.0070 Comparative steel S 0.130 0.710 2.18 0.032 0.020 0.0080 0.0020 0.0028 0.1350 0.0014 Steel of the present invention T 0.132 0.730 2.22 0.018 0.021 0.0060 0.0030 0.0034 0.1250 0.1760 0.030 Steel of the present invention u 0.144 1.180 2.57 0.030 0.008 0.0080 0.0022 0.0032 0.130 Steel of the present invention V 0.115 0.960 2.56 0.040 0.032 0.0080 0.0040 0.0025 0.0180 0.068 0.030 0.0017 Steel of the present invention w 0.093 0.440 1.48 0.173 0.018 0.0082 0.0042 0.0032 0.0130 0.0630 0.082 0.0014 Steel of the present invention X 0.060 0.032 1.31 0.020 0.030 0.0060 0.0020 0.0031 0.0450 0.081 0.373 Steel of the present invention Y 0.132 0.730 2.12 0.052 0.013 0.0770 0.0013 0.0033 0.0021 Steel of the present invention z 0.141 0.701 2.11 0.038 0.014 0.0090 0.0082 0.0021 Steel of the present invention. The underlines indicate that the corresponding values ​​are outside the scope of the present invention.

[0074] uouo [Table 2] Test No. Steel Average surface temperature gradient in the region from the meniscus to 1.0 m from the meniscus °C / m Slab heating temperature °C Slab heating residence time min Total rolling reduction in the temperature range of 870°C to 980°C % Total rolling reduction in the temperature range below 870°C % Maximum time between boxes SPO Cooling time ROT sec Winding temperature after cooling ROT °C Tempering rolling reduction % Heat treatment temperature °C Heat treatment time sec. Note 1 A 321 1225 121 83 0 2.9 15.0 25 0.2 No treatment No treatment Comparative Example 2 B 262 1222 131 86 0 2.8 14.3 34 0.2 No treatment No treatment Comparative Example 3 B 315 1234 74 88 6 2.7 13.8 29 0.2 No treatment No treatment Example of the present invention 4 B 638 1243 42 85 0 2.7 12.6 29 0.2 Without treatment Without treatment Example of the present invention 5 B 698 1229 167 85 0 2.7 12.5 29 0.2 Without treatment Without treatment Comparative example 6 C 552 1236 175 82 0 2.8 13.7 225 0.2 Without treatment Without treatment Comparative example 7 D 443 1236 148 95 0 2.7 12.7 26 0.2 Without treatment Without treatment Example of the present invention 8 E 296 1240 238 86 0 3.8 13.0 28 0.2 Without treatment Without treatment Comparative example 9 E 340 1238 233 72 0 4.2 13.7 27 0.2 Without treatment Without treatment Comparative example 10 E 312 1233 154 85 7 3.6 15.2 29 0.2 No treatment No treatment Example of the present invention 11 E 642 1217 88 85 0 3.5 14.4 28 0.2 No treatment No treatment Example of the present invention 12 E 662 1235 67 85 0 3.8 14.1 28 0.2 No treatment No treatment Comparative example 13 E 313 1222 77 85 13 4.5 14.4 28 0.2 No treatment No treatment Comparative example 14 F 325 1220 45 83 0 2.9 14.2 125 0.2 440 98 Example of the present invention 15 F 322 1220 84 85 0 2.5 32.3 20 0.2 437 78480 Comparative Example 16 F 641 1225 69 87 0 2.6 14.5 25 0.2 435 78480 Example of the present. Invention 17 F 294 1219 76 86 0 2.2 14.3 21 0.2 433 103 Comparative Example 18 G 419 1229 77 93 0 2.8 14.6 28 0.2 290 135 Comparative Example 19 H 584 1227 173 89 0 2.4 14.4 25 0.2 No treatment No treatment Comparative Example 20 I 552 1230 108 83 0 4.4 14.1 28 0 458 102 Comparative Example 21 I 541 1238 111 82 0 3.8 14.4 189 0 215 78840 Example of the present invention 22 I 532 1232 106 79 0 4.2 14.3 281 0 436 74160 Comparative Example 23 I 653 1222 82 85 0 3.9 15.2 223 0 436 74160 Comparative Example 24 J 529 Not performed due to slab cracking Comparative Example 25 K 492 1233 109 96 0 3.2 20.2 287 0.2 No treatment No treatment Example of the present invention The underlines indicate that the corresponding values ​​are outside the scope of the present invention. IVIA / S / ZUZZ / UI UDOU

[0075] MA / a / ZUZZ / UI uouo [Table 3] Test No. ^zero | Average surface temperature gradient in the region from the meniscus to 1.0 m from the meniscus °C / m Slab heating temperature °C Slab heating residence time min Total rolling reduction in the temperature range of 870°C to Total rolling reduction in the temperature range below 870°C % Maximum time between boxes seq Cooling time ROT se£j Winding temperature after cooling ROT °C Tempering rolling reduction % Heat treatment temperature °C Heat treatment time sec. Note 26 K 501 1216 221 88 0 3.3 20.5 343 0.2 No treatment No treatment Comparative example 27 L 473 1227 100 89 0 3.3 15.5 282 0 No treatment No treatment Example of the present invention 28 L 483 1229 167 82 0 5.2 15.2 281 0 No treatment No treatment Comparative example 29 L 321 1232 126 77 0 4.9 15.3 266 0 No treatment No treatment Comparative example 30 M Not performed due to nozzle obstruction in continuous casting step Comparative example 31 N 444 1189 46 91 0 1.9 12.6 252 0.2 No treatment No treatment Comparative example 32 N 454 1240 29 85 0 2.9 13.1 191 0.2 No treatment No treatment Comparative example 33 N 455 1230 39 84 0 2.0 13.3 325 0.2 No treatment No treatment Comparative example 34 N 452 1223 38 86 0 1.9 12.9 292 0.2 No treatment No treatment Example of the present invention 35 0 521 1230 154 90 0 4.9 13.1 122 0.2 No treatment No treatment Comparative example 36 P 456 1241 139 81 0 4.2 12.9 25 0.2 No treatment No treatment Example of the present invention 37 Ω 482 Not performed due to slab cracking Comparative example 38 R 491 1320 153 94 0 3.2 14.8 25 0.2 440 103 Comparative example 39 S 309 1272 143 92 0 3.0 14.9 275 0 No treatment No treatment Example of the present invention 40 Γ 310 1269 232 91 0 3.3 15.0 189 0 No treatment No treatment Example of the present invention 41 Γ 298 1224 273 92 0 3.3 15.2 167 0 No No Example. Comparative treatment 42 U 611 1223 218 82 0 4.1 14.2 25 0.2 No treatment No treatment Example of the present invention 43 V 421 1223 221 81 0 3.5 14.9 28 0.2 No treatment No treatment Example of the present invention 44 W 503 1242 228 83 0 3.6 14.1 181 0.2 No treatment No treatment Example of the present invention 45 X 592 1256 269 85 0 4.4 14.7 286 0 No treatment No treatment Example of the present invention 46 Y 467 1251 245 93 0 3.8 14.3 171 0 No treatment No treatment Example of the present invention 47 z 489 1255 249 91 0 3.9 15.1 276 0 No treatment No treatment Example of the present invention 48 F 320 1231 65 87 0 2.4 28.7 20 0 441 78480 Example of the present invention The underlines indicate that the corresponding values ​​are outside the scope of the present invention.

[0076] uouo [Table 4] Test No. Position of the sheet thickness direction from the surface standardized by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 5° or less Position of the sheet thickness direction from the surface standardized by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 20° or more Average grain diameter of previous austenite grains Volume of bainite + martensite % Volume of ferrite % Volume of cementite + residual austenite % Tensile strength MPa Rate of hole expansion % Depth of the hollowed portion in the inner bend m Note 1 0.137 0.263 19.81 72.3 27.4 0.3 855 73 25.4 Comparative Example 2 0.172 0.265 15.49 82.3 16.6 1.1 1160 45 33.4 Comparative Example 3 0.144 0.273 14.82 86.1 13.0 0.9 1182 46 27.4 Example of the present invention 4 0.122 0.254 16.11 87.5 11.3 1.2 1196 51 21.2 Example of the present invention 5 0.128 0.233 18.20 88.3 10.4 1.3 1208 52 35.1 Comparative Example 6 0.131 0.262 23.77 83.5 5.0 11.5 1306 32 27.3 Comparative Example 7 0.128 0.273 20.20 80.2 17.2 2.6 1084 62 24.4 Example of the present invention 8 0.152 0.268 23.13 84.3 15.4 0.3 994 73 30.4 Comparative example 9 0.139 0.271 34.30 83.0 16.8 0.2 953 71 40.3 Comparative example 10 0.143 0.268 23.40 83.1 16.6 0.3 972 64 27.3 Example of the present invention 11 0.132 0.254 21.60 84.3 15.4 0.3 1020 68 23.1 Example of the present invention 12 0.128 0.247 24.30 82.2 17.3 0.5 983 70 31.5 Comparative example 13 0.156 0.262 26.40 80.5 19.1 0.4 910 88 30.9 Comparative example 14 0.141 0.271 10.35 81.5 13.7 4.8 935 46 18.3 Example of the present invention 15 0.143 0.265 12.33 78.3 16.1 5.6 854 51 26.5 Comparative example 16 0.118 0.253 9.88 81.3 15.5 3.2 982 45 21.6 Example of the present. Invention 17 0.155 0.259 11.47 80.6 13.1 6.3 975 51 30.8 Comparative Example 18 0.129 0.272 9.76 83.5 5.4 11.1 1212 21 18.2 Comparative Example 19 0.113 0.262 8.84 70.6 27.3 2.1 863 48 13.6 Comparative Example 20 0.119 0.267 26.30 83.3 6.4 10.3 1254 28 26.1 Comparative Example 21 0.121 0.259 23.16 86.5 9.3 4.2 1435 38 23.3 Example of the present invention 22 0.126 0.263 30.90 88.1 4.3 7.6 1331 42 33.8 Comparative example 23 0.123 0.248 24.68 87.1 5.6 7.3 1321 44 30.6 Comparative example 24 Not carried out due to cracking of the slab Comparative example 25 0.128 0.262 11.30 85.3 10.7 4.0 1013 38 12.6 Example of the present invention The underlines indicate that the corresponding values ​​are outside the scope of the present invention and the feature is not preferable.

[0077] uouo [Table 5] Test No. Position of the direction of the sheet thickness from the surface standardized by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 5° or less Position of the direction of the sheet thickness from the surface standardized by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line becomes 20° or more Average grain diameter of previous austenite grains Volume of bainite + martensite % Volume of ferrite % Volume of cementite + residual austenite % Tensile strength MPa Rate of hole expansion % Depth of the hollowed portion in the inner bend m Note 26 0.119 0.264 19.50 79.3 12.3 8.4 870 33 20.6 Comparative Example 27 0.128 0.266 6.38 81.2 16.2 2.6 932 63 8.31 Example of the present invention 28 0.118 0.268 32.30 80.7 17.0 2.3 907 66 34.4 Comparative example 29 0.139 0.268 31.80 81.0 17.7 1.3 910 66 34.6 Comparative example 30 Not realized due to nozzle obstruction in the continuous casting step Comparative example 31 0.121 0.273 14.32 78.6 19.6 1.8 875 43 18.3 Comparative example 32 0.131 0.267 16.75 70.6 27.3 2.1 821 51 15.8 Comparative example 33 0.127 0.263 17.08 74.6 24.1 1.3 867 55 22.1 Comparative example 34 0.124 0.274 14.23 80.6 18.3 1.1 886 68 17.3 Example of the present invention 35 0.122 0.259 32.50 80.6 17.8 1.6 953 82 36.6 Comparative example 36 0.120 0.268 25.60 86.3 11.5 2.2 1183 43 24.6 Example of the present invention 37 Not realized due to slab cracking Comparative example 38 0.123 0.266 8.30 88.2 1.5 10.3 1240 21 11.3 Comparative example 39 0.145 0.252 5.38 82.3 14.6 3.1 1260 36 26.3 Example of the present invention 40 0.147 0.267 10.20 83.2 12.6 4.2 1220 42 17.3 Example of the present invention 41 0.157 0.275 8.33 82.2 13.7 4.1 1160 46 31.6 Comparative example 42 0.109 0.256 25.60 88.9 8.0 3.1 1316 36 25.3 Example of the present. Invention 43 0.140 0.261 18.80 84.0 12.4 3.6 1203 43 28.6 Example of the present invention 44 0.117 0.275 22.60 82.9 14.8 2.3 1140 55 24.5 Example of the present invention 45 0.126 0.265 25.10 80.3 18.7 1.0 997 46 26.2 Example of the present invention 46 0.123 0.255 18.90 85.3 11.6 3.1 1183 45 17.1 Example of the present invention 47 0.121 0.262 17.67 83.2 14.3 2.5 1163 41 16.8 Example of the present invention 48 0.139 0.268 12.20 80.8 13.3 5.9 912 63 10.9 Example of the present invention The underlines indicate that the corresponding values ​​are outside the scope of the present invention and the feature is not preferable. IVIA / a / ZUZZ / UI uouo

[0078] In Tests No. 2, 8, 13, 17 and 41 where the region where the rotation angle between the normal line of the surface of the hot-rolled steel sheet and the pole 5 (011) near the normal line becomes 5oo less was not present at 0.150 or less from the surface in terms of the position of the direction of the sheet thickness standardized by the sheet thickness, the depth of the hollowed part on the inside curve became 30.0 gm or more. Furthermore, in Tests No. 5, 12 and 23 where the region where the rotation angle between the normal line of the surface of the hot-rolled steel sheet and the pole (011) near the normal line becomes 20° or more was not present at 0.250 or more from the surface in terms of the position of the sheet thickness direction standardized by the sheet thickness, the depth of the hollowed part on the inside curve became 30.0 pm or more.

[0079] In Tests Nos. 9, 22, 29, and 35, where the average grain diameter of the austenite grains was 30.00 pm or more, regardless of the fact that the hot-rolled steel sheet has the characteristic of crystal orientation, the depth of the hollow section on the inner curve became 30.0 pm or more. That is, it was found that controlling the average grain diameter of the austenite grains serves as a prerequisite for achieving the effect of controlling the crystal orientations in the direction of the sheet thickness to obtain a hollow section depth on the inner curve of less than 30.0 pm.

[0080] Therefore, the crystal orientation characteristic can be ordered by the average surface temperature gradient in the region from the meniscus to 1.0 m from the meniscus. In Tests No. 2, 8, 17, and 41, the average surface temperature gradients in the region from the meniscus to 1.0 m from the meniscus were all less than 300 °C / m. On the other hand, in Tests No. 5, 12, and 23, the average surface temperature gradients in the region from the meniscus to 1.0 m from the meniscus were greater than 650 °C / m. IVIA / a / ZUZZ / UI uouo

[0081] It is found that, in Test No. 13 where the average surface temperature gradient in the region from the meniscus to 1.0 m from the meniscus was 313 °C / m and the total rolling reduction in the temperature range of less than 870 °C during final rolling exceeded 10%, the position of the sheet thickness direction standardized by the sheet thickness of the region where the rotation angle between the normal line of the steel sheet surface and the pole (011) near the normal line became 500 less became 0.156 and it was not possible to reduce the depths of the hollowed parts on the inside curve.

[0082] In Tests No. 3 and 10, where the average surface temperature gradients in the region from the meniscus to 1.0 m from the meniscus were close to 313 °C / m and the total rolling reductions in the temperature range below 870 °C during finishing rolling were different, the position of the sheet thickness direction standardized by the sheet thickness of the region where the angle of rotation between the normal line of the steel sheet surface and the pole (011) near the normal line became 0.500 or less. From these examples, it is determined that the total rolling reduction in the temperature range IVIA / a / ZUZZ / UI uouo less than 870°C during finishing lamination less than 10% is an appropriate condition.

[0083] It is found that the metallographic structure fractions of hot-rolled steel sheet depend on the cooling conditions after rolling and the coiling conditions, and excellent tensile strength and hole expansibility can be obtained through appropriate chemical composition along with the conditions described above.

[0084] Based on what has been described above, it was found that, within the scope of the essence of the present invention, the tensile strength is 880 MPa or more, the expansion capacity of the hole is excellent, and a hollowed-out part on the inner curve, which has been a problem in the application of hot-rolled steel sheets to components, can be improved. [Industrial applicability]

[0085] According to aspects of the present invention, it is possible to provide a hot-rolled steel sheet that has high strength and excellent formability and that allows the reduction of the depth of a hollowed-out part in the inner curve that is formed during bending and a method of manufacturing the same.

Claims

1. A hot-rolled steel sheet comprising, as chemical composition, in % by mass: C: 0.060% to 0.170%; Si: 0.030% to 1.700%; Mn: 1.20% to 3.00%; Al: 0.010% to 0.700%; Nb: 0.005% to 0.050%; P: 0.0800% or less; S: 0.0100% or less; N: 0.0050% or less; Ti: 0% to 0.1800%; Mo: 0% to 0.150%; V: 0% to 0.3000%; Cr: 0% to 0.500%; B: 0% to 0.0030%; 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, bainite and martensite are a total of 80.0% or more, ferrite is 20.0% or less, and residual cementite and austenite are a total of 0% to 10.0%, 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 less than 30.00 pm, a region, where the angle of rotation between a surface normal line and a pole (011) near the normal line is 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness, a region, where the angle of rotation between the surface normal line and the pole (011) near the normal line is 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness, and a tensile strength is 880 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: Ti: 0.0200% to 0.1800%; Mo: 0.030% to 0.150%; V: 0.0500% to 0.3000%; Cr: 0.050% to 0.500%; and B: 0.0001% to 0.0030%.

3. A method of manufacturing hot-rolled IVIA / a / ZUZZ / UI uouo 75 steel sheet according to claim 1 or 2, comprising: a casting step of, in the continuous casting of a slab having the chemical composition according to claim 1, performing the continuous casting such that an average surface temperature gradient in a region from a meniscus to 1.0 m from the meniscus is from 300 to 650°C / m to obtain the slab; a heating step of heating the slab to 1200°C or more and holding the slab at that temperature for 30 minutes or more; A hot rolling step consisting of rough rolling on the slab and finishing rolling such that the total rolling reduction in a temperature range of 870°C to 980°C is 80% or more, the time between rolling stands in the temperature range of 870°C to 980°C becomes 0.3 to 5.0 seconds, and a total reduction of lamination in a temperature range below 870°C becomes less than 10%; a cooling step of 30.0 seconds or less to cool to a temperature range below 300°C after final lamination; and a winding step, winding so that the winding temperature is below 300°C after cooling.

4. The method of manufacturing the hot-rolled IVIA / a / ZUZZ / UI steel sheet according to claim 3, further comprising: a holding heat treatment step in a temperature range of 200°C or higher and lower than 450°C 5 for 90 to 80000 seconds after rolling.