High-strength hot-rolled steel sheet, member, and methods for producing same

Abstract

The purpose of the present invention is to provide: a high-strength hot-rolled steel sheet which, in addition to having high strength and high ductility, has excellent wrinkle-suppressing capability, high stretch flangeability, and excellent bendability; a member; and methods for producing the high-strength hot-rolled steel sheet and the member.　Provided is a high-strength hot-rolled steel sheet that has a prescribed component composition, that has a tensile strength of 980 MPa or more, and that has a steel structure, in terms of area ratios, in which the area ratio of tempered martensite is more than 0% and 99.5% or less, the area ratio of lower bainite is more than 0% and 99.5% or less, the total area ratio of lower bainite and tempered martensite is 82.0-99.5%, fresh martensite is 0-8.0%, retained austenite is 0.5-10.0%, the average solid solution C amount in retained austenite is 0.50-1.10%, the median value of the circle equivalent diameter of carbides is 100 nm or less, the number density of carbides having a circle equivalent diameter of 40 nm or more is 30 carbides / μm2 or less, and the area ratio of a region having a hardness of 9.0 GPa or more is 8.0% or less.

Description

High-strength hot-rolled steel sheets, components, and methods for manufacturing them.

[0001] The present invention relates to a high-strength hot-rolled steel sheet suitable as a material for automotive components, components made using the high-strength hot-rolled steel sheet, and methods for manufacturing the same.

[0002] In recent years, improving the fuel efficiency and energy consumption of automobiles has become a crucial issue from the standpoint of protecting the global environment. Therefore, there has been a surge in efforts to lighten automobile bodies by increasing the strength and thinning the steel sheets used as materials for automobile components. Furthermore, hybrid and electric vehicles, with their large-capacity batteries, increase vehicle weight, requiring correspondingly higher strength in automobile components. Consequently, there is a growing need for higher strength in hot-rolled steel sheets, which are primarily used in automobile components such as suspension parts and frame members.

[0003] In particular, high-strength hot-rolled steel sheets with a tensile strength of 980 MPa or more are expected to be a material that can dramatically improve the fuel efficiency and electric power consumption of automobiles by reducing weight and accommodating the increase in vehicle weight due to the installation of batteries.

[0004] Various studies have been conducted on steel sheets used as materials for such automotive components.

[0005] For example, Patent Document 1 states that, in mass percent, C: 0.02 to 0.20%, Si: 0.005 to 2.00%, Mn: 1.30 to 2.40%, P: 0.100% or less, S: 0.0100% or less, sol. Al: 0.001 to 1.00%, Ti: 0.030 to 0.200%, N: 0.0010 to 0.0100%, Nb: 0 to 0.100% The chemical composition consists of V: ​​0-0.50%, Mo: 0-0.50%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-2.00%, B: 0-0.0100%, Ca: 0-0.0100%, Mg: 0-0.0100%, and REM: 0-0.0100%, with the remainder being Fe and impurities, and is found to be 1 / 4 of the plate thickness from the surface. A hot-rolled steel sheet is disclosed, characterized in that, at a given position, the area ratio of ferrite is 10 to 55%, the total area ratio of bainite and martensite is 45 to 90%, the total area ratio of ferrite, bainite and martensite is 90% or more, the average grain size is 12.0 μm or less, and in the texture measured at the center of the sheet thickness, the maximum polar density of the {100}<011>, {211}<011>, {311}<011>, {110}<011> and {332}<113> orientation groups is 8.0 or less, the sum of the polar densities of {211}<011> and {332}<113> is 10.0 or less, and the tensile strength is 950 MPa or more.

[0006] Patent Document 2 states that the chemical composition is, in mass%, C: 0.040 to 0.150%, Si: 0.50 to 1.50%, Mn: 1.00 to 2.50%, P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Ti: 0.005 to 0.150%, B: 0.0005% It contains ~0.0050%, Cr: 0.10~1.00%, Nb: 0~0.06%, V: 0~0.50%, Mo: 0~0.50%, Cu: 0~0.50%, Ni: 0~0.50%, Sb: 0~0.020%, Ca: 0~0.010%, REM: 0~0.010%, and Mg: 0~0.010%, with the remainder being iron. A hot-rolled steel sheet is disclosed, characterized in that, in the metal structure at a position 1 / 4 of the thickness from the surface in the thickness direction, the main phase is 95.00 to 98.00% bainite phase and the second phase is 2.00 to 5.00% tempered martensite phase, the average particle size of the second phase is 1.5 μm or less, the polar density in the (110)<112> direction is 3.0 or less, the average particle size of iron-based carbides is 0.100 μm or less, and in the metal structure from the surface to a position 1 / 16 of the thickness from the surface in the thickness direction, the polar density in the (110)<1-11> direction is 3.0 or less, and the tensile strength TS is 980 MPa or more.

[0007] Patent Document 3 discloses "a high-strength hot-rolled steel sheet having a composition in mass percent of C: 0.02 to 0.23%, Si: 0.10 to 3.00%, Mn: 0.5 to 3.5%, P: 0.100% or less, S: 0.02% or less, Al: 1.5% or less, with the remainder being Fe and unavoidable impurities, a total area ratio of martensite and bainite of 80 to 100%, a maximum grain orientation density of less than 2.5 in the region 5 to 10 μm from the surface in the thickness direction, and a maximum grain orientation density of 2.5 or more in the region 50 to 100 μm from the surface in the thickness direction."

[0008] Patent Document 4 describes a hot-rolled flat steel product made of steel having the following composition (in weight percent): C: 0.1–0.3%, Mn: 1.5–3.0%, Si: 0.5–1.8%, Al: up to 1.5%, P: up to 0.1%, S: up to 0.03%, N: up to 0.008%, with the remainder being iron and unavoidable impurities related to production, provided that if the composition contains at least 1.0 wt% Si, the Al content is a maximum of 0.03 wt%, or if the composition contains 0.5 wt% to 1.0 wt% Si, the Al content is at least 0.5 wt%, wherein the flat steel product has a tensile strength Rm of 800–1500 MPa, a yield strength Rp of over 700 MPa, and an elongation A at break of 7–25%. A hot-rolled flat steel product is disclosed, which has a hole expansion λ of more than 20%, and the structure of the flat steel product consists of at least 85 area percent of martensite, at least half of which is tempered martensite, the remainder of the structure of the flat steel product consists of up to 15 volume percent of retained austenite, up to 15 area percent of bainite, up to 15 area percent of polygonal ferrite, up to 5 area percent of cementite, and / or up to 5 area percent of non-polygonal ferrite, and the structure of the flat steel product has an average kernel average misorientation KAM of at least 1.50° in a measurement range of at least 75 μm x 75 μm.

[0009] Patent No. 6465266 Patent No. 7188618 Patent No. 7207615 Patent No. 7193454

[0010] Incidentally, the practical application of hot-rolled steel sheets used in automotive components is currently limited to those with a tensile strength of around 780 MPa, with the application of 980 MPa class materials being very limited. Automotive parts, especially suspension components and other undercarriage parts and frame members, need to have complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets used as materials for such complexly shaped components require excellent formability.

[0011] On the other hand, increasing the tensile strength of steel sheets generally reduces their formability. Steel sheets used as materials for components with complex shapes require not only high ductility but also high elongation flangeability, and it is important to ensure these properties while maintaining high strength.

[0012] Furthermore, undercarriage components and frame members are critical safety parts whose failure can impair the vehicle's operation and safety, and therefore require high reliability. If these parts have wrinkles, instantaneous loads during actual use or repeated loads over a long period may cause cracks to form in the wrinkled recesses, potentially leading to component failure. Therefore, the steel plates used as the material must have excellent wrinkle-suppressing capabilities. In particular, high-strength steel plates are prone to wrinkle formation even with small distortions during forming, making superior wrinkle-suppressing capabilities crucial for high-strength steel plates.

[0013] Furthermore, since the component undergoes processes such as burring and bending to ensure rigidity, the steel plate used as the material for the component requires excellent bendability.

[0014] As described above, it is important that steel sheets, which are the raw materials for automotive parts such as undercarriage components and frame members, possess high strength, high ductility, high stretchability, excellent wrinkle suppression, and excellent bendability.

[0015] In fact, the steel sheets disclosed in Patent Documents 1 to 4 cannot be said to possess high strength, such as a tensile strength (TS) of 980 MPa or more, as well as high ductility, high stretch flangeability, excellent wrinkle suppression, and excellent bendability.

[0016] As described above, conventional technology has not yet established a method for producing hot-rolled steel sheets that maintain high strength of 980 MPa or more in tensile strength, while also possessing high ductility, high elongation flangeability, excellent wrinkle suppression, and excellent bendability.

[0017] Therefore, the present invention aims to solve the problems of the prior art and provide the following: that is, to provide high-strength hot-rolled steel sheets, components, and manufacturing technologies thereof that maintain high strength with a tensile strength of 980 MPa or more, and further possess high ductility, high stretch flangeability, excellent wrinkle suppression ability, and excellent bendability.

[0018] Here, high ductility is required to form the undercarriage parts. In this invention, high ductility means that the uniform elongation (total elongation at maximum test force; hereinafter also referred to as U.El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following equation: When 980 MPa ≤ TS < 1180 MPa, U.El ≥ 4.7% When 1180 MPa ≤ TS < 1310 MPa, U.El ≥ 4.5% When 1310 MPa ≤ TS, U.El ≥ 4.0% Since the undercarriage parts are stretched flanged after punching, high stretch flange properties are required. In this invention, high stretch flange properties mean that the average limiting hole expansion ratio (λ) measured in a hole expansion test in accordance with JIS Z 2256 using three test pieces satisfies the following equation. In particular, since the undercarriage parts are stretched flanged after punching, high stretch flange properties are required. If 980 MPa ≤ TS < 1180 MPa, λ ≥ 45% If 1180 MPa ≤ TS < 1310 MPa, λ ≥ 40% If 1310 MPa ≤ TS, λ ≥ 35% Steel sheets used as materials for automobile parts, such as undercarriage components and frame members, require excellent wrinkle suppression to ensure the reliability of the parts. In the present invention, excellent wrinkle suppression means that the work hardening index (hereinafter also referred to as the n-value), measured in accordance with JIS Z 2253:2020, satisfies the following formula. For 980 MPa ≤ TS < 1180 MPa, the strain range is 3.5% to 4.5%, and the n value ≥ 0.040. For 1180 MPa ≤ TS < 1310 MPa, the strain range is 3.5% to 4.5%, and the n value ≥ 0.035. For 1310 MPa ≤ TS, the strain range is 3.5% to 4.0%, and the n value ≥ 0.030. Automotive parts, such as suspension components and frame members, are subjected to burring and bending processes to ensure rigidity, and therefore, the steel plates used as the material for suspension components require excellent bendability. In this invention, excellent bendability means that when a bending test is performed in accordance with JIS Z 2248, the R / t value satisfies the following formula. Here, the minimum bending radius at which no crack occurs in any of the test pieces is defined as the limit bending radius R (mm), and t is the thickness of the steel plate (mm).If 980 MPa ≤ TS < 1180 MPa, then R / t ≤ 2.0. If 1180 MPa ≤ TS < 1310 MPa, then R / t ≤ 3.5. If 1310 MPa ≤ TS, then R / t ≤ 4.5.

[0019] The inventors diligently conducted research to achieve the above objectives.

[0020] As a result, the component composition of the hot-rolled steel sheet was appropriately adjusted, and the steel structure of the hot-rolled steel sheet was set so that the area ratio of tempered martensite was greater than 0% and less than or equal to 99.5%, and the area ratio of lower bainite was greater than 0% and less than or equal to 99.5%. Furthermore, the total area ratio of lower bainite and tempered martensite was set to 82.0% or more and less than or equal to 99.5%. In addition, the area ratio of fresh martensite was set to 8.0% or less (including 0%), the area ratio of retained austenite was set to 0.5% or more and less than or equal to 10.0%, and the average solid solution carbon content in retained austenite was set to 0.50% or more and less than or equal to 1.10%. Furthermore, the median equivalent circle diameter of carbides was 100 nm or less, and the number density of carbides with an equivalent circle diameter of 40 nm or more was 30 particles / μm ２ The following was determined. Furthermore, the area ratio of the region with a hardness of 9.0 GPa or higher was set to 8.0% or less, and the tensile strength was set to 980 MPa or higher. As a result, it was found that a high-strength hot-rolled steel sheet could be obtained that maintained high strength while also possessing high ductility, high elongation flangeability, excellent wrinkle suppression ability, and excellent bendability.

[0021] This invention was completed based on the above findings and further considerations.

[0022] In other words, the gist of the present invention is as follows: [1] The component composition is, in mass%, C: 0.040% or more and 0.350%, Si: 0.50% or more and 2.50%, Mn: 1.50% or more and less than 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is, tempered martensite area ratio: greater than 0% and 99.5% or less, lower bainite area ratio: greater than 0% Area ratio of the lower bainite and tempered martensite combined: 99.5% or less; Area ratio of fresh martensite: 82.0% to 99.5%; Area ratio of retained austenite: 8.0% or less (including 0%); Area ratio of retained austenite: 0.5% to 10.0%; Average solid solution carbon content in retained austenite: 0.50% to 1.10%; Median equivalent circle diameter of carbides: 100 nm or less; Number density of carbides with an equivalent circle diameter of 40 nm or more: 30 particles / μm ２The following describes a high-strength hot-rolled steel sheet having an area ratio of 8.0% or less in the region with a hardness of 9.0 GPa or more, and a tensile strength of 980 MPa or more. [2] In addition to the above component composition, the following components are further, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.400% or less, Cr: 1.00% or less, Mo: 0.500% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.02 [1] A high-strength hot-rolled steel sheet according to [1], comprising at least one selected from 0% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. [3] A high-strength hot-rolled steel sheet according to [1] or [2], having a plating layer on its surface. [4] A component made using a high-strength hot-rolled steel sheet according to any one of [1] to [3].A method for manufacturing a high-strength hot-rolled steel sheet as described in [5] [1] or [2], comprising: a heating step of heating a steel material to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step to a hot-rolled steel sheet under the conditions of a finish rolling completion temperature of 800°C or higher and 980°C or lower; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step under the conditions of a cooling stop temperature of Ms°C or lower and a cooling parameter of Ms°C or higher during cooling of 36.0 or lower; and keeping the hot-rolled steel sheet after the first cooling step warm for a heat retention time of 60 seconds or more and an integrated tempering parameter of 8.0 or higher during heat retention of 29.0 or lower. A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a first heat retention step; a reheating step in which the hot-rolled steel sheet after the first heat retention step is reheated at a reheating stop temperature of 320°C or higher and 480°C or lower; a second heat retention step in which the hot-rolled steel sheet after the reheating step is kept hot at a heat retention temperature of 320°C or higher and 480°C or lower, and a heat retention time of 60 seconds or more; and a second cooling step in which the hot-rolled steel sheet after the second heat retention step is cooled to room temperature, wherein the cumulative tempering parameter from the start of reheating in the reheating step until the steel sheet temperature of the hot-rolled steel sheet in the second cooling step reaches room temperature is 24.0 or higher and 30.0 or lower. Here, Ms is defined by the formula Ms(°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo, where each element symbol in the formula represents the content (mass%) of each element, and 0 is used for elements that are not contained. [6] A method for manufacturing a high-strength hot-rolled steel sheet as described in [5], wherein the hot-rolled steel sheet after the first heat retention process is plated. [7] A method for manufacturing a component, comprising the step of forming or joining a high-strength hot-rolled steel sheet manufactured by the method for manufacturing a high-strength hot-rolled steel sheet as described in [5] or [6] to make a component.

[0023] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet that has high strength, high ductility, and high elongation flangeability, in addition to excellent wrinkle suppression ability and excellent bendability.

[0024] The following describes embodiments of the high-strength hot-rolled steel sheets, components, and methods for manufacturing them according to the present invention. However, the present invention is not limited to the following embodiments.

[0025] [1] High-strength hot-rolled steel sheet First, the component composition of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that all units in the component composition are "mass%", but unless otherwise specified, they will be simply referred to as "%".

[0026] C: 0.040% or more and 0.350% or less. Carbon (C) is an element that improves the strength of steel. By improving hardenability, carbon promotes the formation of bainite, contributing to increased strength. Carbon also contributes to increased strength by increasing the strength of martensite. To obtain a tensile strength of 980 MPa or more, the carbon content must be 0.040% or more. Therefore, the carbon content is set to 0.040% or more. Preferably, the carbon content is 0.050% or more, and more preferably 0.060% or more. On the other hand, if the carbon content exceeds 0.350%, the area ratio of the region with a hardness of 9.0 GPa or more, and the number density of carbides increase, and the elongation flangeability and bendability decrease. Also, the tensile strength increases excessively and the ductility decreases. Therefore, the carbon content is set to 0.350% or less. Preferably, the carbon content is 0.250% or less, and more preferably 0.220% or less.

[0027] Si: 0.50% to 2.50% Si has the effect of suppressing the formation of Fe-based carbides and suppresses the precipitation of cementite in processes from the first cooling process, including the first cooling process. As a result, C is distributed to the untransformed austenite, and after cooling to room temperature in the second cooling process, a portion of the untransformed austenite becomes retained austenite, contributing to improved ductility and wrinkle suppression. In addition, Si improves the strength-ductility balance of the lower bainite and tempered martensite after the second heat retention process. In order to obtain these effects, the Si content must be 0.50% or more. Therefore, the Si content is set to 0.50% or more. Preferably, the Si content is 0.60% or more, more preferably 0.70% or more, and even more preferably 0.80% or more. On the other hand, Si is an element that forms subscales on the surface of the steel sheet during hot rolling. If the Si content exceeds 2.50%, the subscale becomes too thick, and even if descaling is performed during the hot rolling process, the surface roughness of the steel sheet becomes excessive, worsening the pre-treatment properties when painting high-strength hot-rolled steel sheets. Therefore, the Si content should be 2.50% or less. Preferably, the Si content is 2.00% or less, and more preferably 1.60% or less.

[0028] Mn: 1.50% or more and less than 5.00% Mn stabilizes austenite, suppresses ferrite formation, and contributes to the formation of lower bainite, tempered martensite, and retained austenite. Mn also stabilizes austenite. This suppresses the transformation of untransformed austenite into lower bainite, carbides, and pearlite in the first and second heat retention processes, and contributes to keeping the average solid-solution carbon content in the retained austenite within the desired range. To obtain such effects, the Mn content must be 1.50% or more. Therefore, the Mn content is set to 1.50% or more. Preferably, the Mn content is 1.70% or more, more preferably 2.00% or more. On the other hand, if the Mn content exceeds 5.00%, bainite transformation is more likely to occur, fresh martensite increases, and elongation flangeability and bendability decrease. Therefore, the Mn content is set to less than 5.00%. The Mn content is preferably less than 4.00%, more preferably less than 3.50%, and even more preferably less than 3.20%.

[0029] P: 0.100% or less. P is an element that contributes to increasing the strength of steel by solid solution. However, P is also an element that causes slab cracking during hot rolling by segregating at the austenite grain boundaries. Furthermore, segregation at grain boundaries reduces ductility. For this reason, it is preferable to keep the P content as low as possible, but a P content of up to 0.100% is acceptable. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.030% or less. There is no particular lower limit to the P content, but from the viewpoint of productivity, etc., it is preferable that the P content be 0.001% or more.

[0030] S: 0.0200% or less. S combines with Ti and Mn to form coarse sulfides, which accelerate void formation, reducing ductility, stretch flangeability, wrinkle suppression, and bendability. Therefore, it is preferable to keep the S content as low as possible, but an S content of 0.0200% or less is acceptable. Accordingly, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. There is no particular lower limit to the S content, but from the viewpoint of productivity, etc., it is preferable that the S content be 0.0001% or more.

[0031] Al: 0.010% to 2.000% Al acts as a deoxidizing agent and is an effective element for improving the cleanliness of steel. Since the effect is insufficient if the Al content is less than 0.010%, the Al content should be 0.010% or more. In addition, like Si, Al retains austenite and contributes to improved ductility. Also, like Si, it improves the strength-ductility balance of bainite and martensite after an appropriate second heat retention process. On the other hand, excessive Al content leads to an increase in oxide inclusions, reducing ductility, stretch flangeability, wrinkle suppression ability, and bendability. Therefore, the Al content should be 2.000% or less. Preferably, the Al content is 1.000% or less.

[0032] N: 0.0200% or less. N precipitates as nitrides by bonding with nitride-forming elements and generally contributes to grain refinement. However, N bonds with Ti at high temperatures to form coarse nitrides, so a content exceeding 0.0200% causes a decrease in ductility, stretchability, wrinkle suppression, and bendability. For this reason, the N content should be 0.0200% or less. Furthermore, the N content is preferably 0.0050% or less. There is no particular lower limit to the N content, but from the viewpoint of productivity, etc., an N content of 0.0005% or more is preferred.

[0033] A high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. Preferably, a high-strength hot-rolled steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities.

[0034] The basic component composition of the base steel sheet for a galvanized steel sheet according to one embodiment of the present invention has been described above, but other elements described below may be further included as needed.

[0035] Ti: 0.200% or less. Ti is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. In addition, Ti increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite-unrecrystallized region and contributing to an improvement in the strength-ductility balance by refining the grain size of lower bainite and tempered martensite. To obtain such effects, if Ti is included, the Ti content is preferably 0.005% or more. More preferably, the Ti content is 0.007% or more, and even more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of Ti-based precipitates will be generated, which may actually reduce the elongation flangeability and bendability. Therefore, if Ti is included, the Ti content should be 0.200% or less. The Ti content is preferably 0.150% or less, and more preferably 0.120% or less.

[0036] Nb: 0.200% or less. Nb, like Ti, is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. Also, like Ti, Nb increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite-unrecrystallized region and contributing to an improvement in the strength-ductility balance by refining the grain size of lower bainite and tempered martensite. To obtain such effects, if Nb is included, the Nb content is preferably 0.005% or more. The Nb content is more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of Nb-based precipitates will be generated, which may actually reduce the elongation flangeability and bendability. In addition, fresh martensite may increase excessively, which may reduce ductility, elongation flangeability, and bendability. Therefore, if Nb is included, the Nb content should be 0.200% or less. The Nb content is preferably 0.150% or less, and more preferably 0.120% or less.

[0037] V: 0.400% or less. Similar to Ti, V is an element that has the effect of improving the strength of the steel sheet through precipitation strengthening and solid solution strengthening. Also, similar to Ti, V raises the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite non-recrystallized region, and contributes to the improvement of the strength-ductility balance by refining the crystal grain size of lower bainite and tempered martensite. To obtain such effects, when V is contained, the V content is preferably 0.005% or more. The V content is more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, when the V content exceeds 0.400%, a large amount of V-based precipitates are generated, which may conversely reduce the stretch flangeability and bendability. Also, the fresh martensite may increase excessively, and there is a risk of reducing ductility, stretch flangeability, and bendability. Therefore, when adding V, the V content is set to 0.400% or less. The V content is preferably 0.200% or less, and more preferably 0.100% or less.

[0038] Cr: 1.00% or less. Similar to Mn, Cr suppresses the formation of ferrite and contributes to the formation of bainite and martensite. To obtain such effects, when Cr is contained, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.10% or more, and even more preferably 0.20% or more. However, since Cr is an element that deteriorates corrosion resistance and pretreatment properties before painting, when adding Cr, the Cr content is preferably 1.00% or less. The Cr content is more preferably 0.80% or less, and even more preferably 0.70% or less.

[0039] Mo: 0.500% or less. Mo increases the tempering softening resistance of steel and contributes to the improvement of the strength of the steel sheet. Also, Mo suppresses the transformation of untransformed austenite to pearlite in the first heat preservation process and the second heat preservation process, and contributes to obtaining an increase in the area ratio of retained austenite and an appropriate average amount of solid solution C in the retained austenite. In order to obtain such an effect, when Mo is contained, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.050% or more, and still more preferably 0.100% or more. On the other hand, when the Mo content exceeds 0.500%, the total area ratio of lower bainite and tempered martensite decreases, deteriorating the elongation flange property and bending property. Therefore, when Mo is contained, the Mo content is set to 0.500% or less.

[0040] B: 0.0100% or less. B is an element that segregates at the prior austenite grain boundary and suppresses the formation of ferrite, thereby contributing to the formation of lower bainite and tempered martensite. In order to obtain such an effect, when B is contained, the B content is preferably 0.0005% or more. On the other hand, when the B content exceeds 0.0100%, the above-mentioned effect saturates. Therefore, when B is contained, the B content is set to 0.0100% or less.

[0041] Cu: 1.00% or less. Cu is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening and solid solution strengthening. In order to obtain such an effect, when Cu is contained, the Cu content is preferably 0.005% or more. However, when the Cu content exceeds 1.00%, it causes a deterioration in the surface properties of the hot-rolled steel sheet. Therefore, when Cu is contained, the Cu content is set to 1.00% or less.

[0042] Ni: 1.00% or less. Ni is an element that contributes to increasing the strength of steel by solid solution. Furthermore, Ni promotes the formation of lower bainite and tempered martensite through improved hardenability, thereby contributing to increased strength. To obtain these effects, it is preferable that the Ni content be 0.01% or more when Ni is included. However, if the Ni content exceeds 1.00%, fresh martensite may increase excessively, potentially degrading the ductility, elongation flangeability, and bendability of the hot-rolled steel sheet. Therefore, when Ni is included, the Ni content should be 1.00% or less.

[0043] Sb: 0.200% or less. Sb is an element that is effective in suppressing the decrease in steel strength by inhibiting denitrification, deboration, etc. When Sb is included, it is preferable to have an Sb content of 0.005% or more to obtain the above effect. On the other hand, if the Sb content exceeds 0.200%, the toughness of the steel will decrease, which may cause slab cracking and hot rolling cracking. Therefore, when adding Sb, the Sb content should be 0.200% or less.

[0044] Sn: 0.200% or less. Like Sb, Sn is an effective element in suppressing the reduction in steel strength by inhibiting denitrification, deboration, etc. When Sn is included, it is preferable to have a Sn content of 0.005% or more to obtain the above effect. On the other hand, if the Sn content exceeds 0.200%, the toughness of the steel will decrease, which may cause slab cracking and hot rolling cracking. Therefore, when Sn is included, the Sn content should be 0.200% or less.

[0045] Ta: 0.100% or less. Ta increases the strength of steel by forming fine carbides, nitrides, or carbonitrides. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides, creating composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates, stabilizes precipitation strengthening, and improves the strength of the steel. To obtain these effects, it is preferable that the Ta content be 0.001% or more when Ta is included. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. This may reduce the elongation flangeability and bendability. Therefore, when Ta is included, the Ta content should be 0.100% or less.

[0046] W: 0.500% or less. W is an effective element for improving hardenability and adjusting the strength of steel to a more suitable range. To obtain such effects, it is preferable that the W content be 0.001% or more when W is included. More preferably, the W content is 0.030% or more. On the other hand, if the W content exceeds 0.500%, the total area ratio of lower bainite and tempered martensite decreases, which may lead to a decrease in tensile flangeability and bendability. Therefore, when W is included, it is preferable that the W content be 0.500% or less.

[0047] Mg: 0.0200% or less. Mg controls the shape of oxide and sulfide inclusions, contributing to further improvements in stretch flange properties and bendability. To obtain these effects, it is preferable that the Mg content be 0.0010% or more when Mg is included. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause stretch flange cracking or bending cracking. Therefore, when Mg is added, the Mg content should be 0.0200% or less.

[0048] Zn: 0.0200% or less. Zn contributes to further improvement of stretch flange properties and bendability by spheroidizing the shape of inclusions. To obtain such effects, it is preferable that the Zn content be 0.0010% or more when Zn is included. However, if the Zn content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause stretch flange cracking or bending cracking. Therefore, when Zn is included, the Zn content should be 0.0200% or less.

[0049] Co: 0.0200% or less. Like Zn, Co contributes to further improvement of stretch flange properties and bendability by spheroidizing the shape of inclusions. To obtain such effects, it is preferable that the Co content be 0.0010% or more when Co is included. However, if the Co content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may conversely cause stretch flange cracking or bending cracking. Therefore, when Co is included, the Co content should be 0.0200% or less.

[0050] Zr: 0.0200% or less. Like Zn and Co, Zr contributes to further improvement of stretch flange properties and bendability by spheroidizing the shape of inclusions. To obtain such effects, it is preferable that the Zr content be 0.0010% or more when Zr is included. However, if the Zr content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause stretch flange cracking or bending cracking. Therefore, when Zr is added, the Zr content should be 0.0200% or less.

[0051] Ca: 0.0200% or less. Ca controls the shape of oxide and sulfide-based inclusions, contributing to further improvements in stretch flange properties and bendability. To obtain such effects, it is preferable to have a Ca content of 0.0010% or more when Ca is included. However, if the Ca content exceeds 0.0200%, the amount of Ca-based inclusions increases, worsening the cleanliness of the steel and potentially causing stretch flange cracking or bending cracking. Therefore, when Ca is added, the Ca content should be 0.0200% or less.

[0052] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM (rare earth metals), like Ca, control the shape of oxide and sulfide-based inclusions, contributing to further improvements in stretchability and bendability. To obtain such effects, when each of the above elements is included, it is preferable that the content of each element be 0.0010% or more. However, if the content of each of the above elements exceeds 0.0200%, the cleanliness of the steel deteriorates, which can actually cause stretching flange cracks or bending cracks. Therefore, when adding the above elements, the content of each element should be 0.0200% or less. Note that REM is a collective term for Sc, Y, and 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content referred to here is the total content of these elements.

[0053] Furthermore, regarding the above-mentioned Ti, Nb, V, Cr, Mo, B, Cu, Ni, Sb, Sn, Ta, W, Mg, Zn, Co, Zr, Ca, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities. Other unavoidable impurities include, for example, O and H. The contents of O and H are preferably 0.01000% or less, respectively.

[0054] Next, the microstructure of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that the area ratio of each microstructure is set to 100% of the total microstructure.

[0055] Area ratio of tempered martensite: greater than 0% and less than or equal to 99.5%. Even if only a very small amount of martensite is produced, it promotes the formation of lower bainite that is formed later, and when tempered, it becomes tempered martensite. By promoting the formation of lower bainite by martensite, the area ratio of the region with a hardness of 9.0 GPa or higher can be brought within the desired range. Tempered martensite, together with lower bainite, is a useful phase from the viewpoint of obtaining high strength of 980 MPa or higher, as well as high ductility, high elongation flangeability, excellent wrinkle suppression, and excellent bendability. In order to secure the desired total area ratio of lower bainite and tempered martensite, the area ratio of tempered martensite is set to greater than 0%. If the area ratio of tempered martensite exceeds 99.5%, it becomes impossible to set the area ratio of retained austenite to 0.5% or higher. Therefore, the area ratio of tempered martensite is set to 99.5% or less. The lower limit of the area ratio of tempered martensite is preferably 0.1% or more, more preferably 1.0% or more, even more preferably 5.0% or more, and particularly preferably more than 15.0%. The upper limit of the area ratio of tempered martensite is preferably less than 99.5%, more preferably 98.0% or less, even more preferably 95.0% or less, particularly preferably 90.0% or less, and most preferably less than 85.0%.

[0056] Lower bainite area ratio: greater than 0% and less than or equal to 99.5% Lower bainite, together with tempered martensite, is a useful phase from the viewpoint of obtaining high strength of 980 MPa or more, as well as high ductility, high elongation flangeability, excellent wrinkle suppression, and excellent bendability, while maintaining high strength. Lower bainite is also a useful phase from the viewpoint of concentrating solid solution carbon in untransformed austenite to secure the desired area ratio of retained austenite. For this reason, the area ratio of lower bainite is set to greater than 0%. If the area ratio of lower bainite exceeds 99.5%, it becomes impossible to set the area ratio of retained austenite to 0.5% or more. For this reason, the area ratio of lower bainite is set to 99.5% or less. The lower limit of the area ratio of lower bainite is preferably 0.1% or more, more preferably 1.0% or more, even more preferably 5.0% or more, and particularly preferably more than 15.0%. The upper limit of the area ratio of the lower bainite is preferably less than 99.5%, more preferably 98.0% or less, even more preferably 95.0% or less, particularly preferably 90.0% or less, and most preferably less than 85.0%.

[0057] Total area ratio of lower bainite and tempered martensite: 82.0% or more and 99.5% or less From the viewpoint of obtaining high ductility, high stretch flangeability, excellent wrinkle suppression ability, and excellent bendability while maintaining high strength with a tensile strength of 980 MPa or more, the total area ratio of lower bainite and tempered martensite is set to 82.0% or more. The total area ratio of lower bainite and tempered martensite is preferably 90.0% or more, more preferably 92.0% or more, and even more preferably 95.0% or more. On the other hand, if the total area ratio of lower bainite and tempered martensite exceeds 99.5%, the area ratio of retained austenite cannot be set to 0.5% or more. For this reason, the total area ratio of lower bainite and tempered martensite is set to 99.5% or less. The total area ratio of lower bainite and tempered martensite is preferably 99.0% or less, more preferably 98.0% or less, and even more preferably 97.5% or less.

[0058] Area ratio of fresh martensite: 8.0% or less (including 0%) Fresh martensite is hard and becomes a void formation site during molding, thus reducing stretch flangeability, wrinkle suppression ability, and bendability. For this reason, the area ratio of fresh martensite should be 8.0% or less. Preferably, the area ratio of fresh martensite should be 6.0% or less, more preferably 4.0% or less, and even more preferably 2.0% or less. The lower limit of the area ratio of fresh martensite is not particularly limited and may be 0%.

[0059] Area ratio of retained austenite: 0.5% or more and 10.0% or less. Retained austenite contributes to high ductility and excellent wrinkle suppression. Therefore, the area ratio of retained austenite should be 0.5% or more. Preferably, the area ratio of retained austenite is 1.0% or more, more preferably 2.0% or more, and even more preferably 2.5% or more. On the other hand, if the area ratio of retained austenite exceeds 10.0%, the stretch flangeability and bendability decrease. Therefore, the area ratio of retained austenite should be 10.0% or less. Preferably, the area ratio of retained austenite is 8.0% or less, more preferably 6.0% or less, and even more preferably 5.0% or less.

[0060] Furthermore, it is preferable that the area ratio of the remaining tissue other than the lower bainite, tempered martensite, fresh martensite, and retained austenite is 10.0% or less. More preferably, the area ratio of the remaining tissue is 5.0% or less. Alternatively, the area ratio of the remaining tissue may be 0%. The remaining tissue is not particularly limited and includes known tissues such as polygonal ferrite, acicular ferrite, upper bainite, pearlite, cementite, and other carbides. The type of remaining tissue can be confirmed, for example, by observation using an SEM (Scanning Electron Microscope).

[0061] Lower bainite and tempered martensite are aggregates of lath-like ferrites with orientation differences of less than 15°, and have a structure containing Fe-based carbides and / or retained austenite phases at and / or within the interfaces of the lath-like ferrites. However, this also includes cases where there are no Fe-based carbides and / or retained austenite phases at and / or within the interfaces of the lath-like ferrites.

[0062] Furthermore, if lower bainite and tempered martensite contain retained austenite, only the lath-like ferrite portion is considered as lower bainite and tempered martensite, and is distinguished from the retained austenite. Also, if lower bainite and tempered martensite contain Fe-based carbides, the contained Fe-based carbides are also considered as lower bainite and tempered martensite.

[0063] Lower bainite and tempered martensite can be distinguished from each other by observing the orientation and crystal structure of the included Fe-based carbides. Fe-based carbides precipitated within tempered martensite exhibit multiple elongation directions within the range of tempered martensite with the same crystal orientation. On the other hand, Fe-based carbides precipitated within lower bainite contain only Fe-based carbides that have elongated in the same direction within the range of lower bainite with the same crystal orientation. Here, Fe-based carbides that have elongated in the same direction refer to Fe-based carbides whose elongation direction difference is 10° or less.

[0064] Fresh martensite and retained austenite do not contain Fe-based carbides compared to lower bainite and / or tempered martensite. Furthermore, fresh martensite and retained austenite exhibit brighter contrast in SEM images compared to upper bainite, lower bainite and / or tempered martensite, polygonal ferrite, and acicular ferrite. Therefore, fresh martensite and retained austenite can be distinguished from these tissues using SEM.

[0065] Here, the area ratios of lower bainite, tempered martensite, fresh martensite, retained austenite, and the remaining microstructure are measured as follows, from the 1 / 8 thickness position to the 3 / 8 thickness position, centered on the 1 / 4 thickness position of the high-strength hot-rolled steel sheet.

[0066] First, a sample is cut from a hot-rolled steel sheet so that the cross-section parallel to the rolling direction of the sheet becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample is finished polished with colloidal silica, and the microstructure is revealed by etching with 3 vol% nital. Then, using a Scanning Electron Microscope (SEM) with an acceleration voltage of 15 kV and a magnification of 3000x, 10 fields of view of the 42.7 μm × 32.0 μm area of ​​the observation surface of the sample are observed, and each phase is identified and the area ratio is calculated.

[0067] Fresh martensite and retained austenite exhibit similar contrast in SEM images, making them difficult to distinguish. Therefore, in SEM observations, the total area fraction is calculated without distinguishing between fresh martensite and retained austenite. Furthermore, the area fraction of retained austenite is determined by X-ray diffraction, and the area fraction of fresh martensite is obtained by subtracting the area fraction of retained austenite (described later) from the total area fraction calculated from the SEM image.

[0068] The area fraction of retained austenite is measured as follows: A hot-rolled steel sheet is mechanically ground in the thickness direction (depth direction) up to 1 / 4 of the sheet thickness. Then, 100 μm of the mechanically ground surface is removed by chemical polishing with oxalic acid to create the observation surface. Next, the observation surface is observed by X-ray diffraction. CoKα rays are used as the incident X-rays. The ratio of the diffraction intensity of the (200), (220), and (311) surfaces of fcc iron (austenite) to the diffraction intensity of the (200) and (211) surfaces of bcc iron is determined, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0069] Average solid-solution carbon content in retained austenite: 0.50% or more and 1.10% or less From the viewpoint of obtaining high ductility and excellent wrinkle suppression ability, the average solid-solution carbon content in retained austenite should be 0.50% or more. Preferably, the average solid-solution carbon content in retained austenite is 0.55% or more, more preferably 0.60% or more. On the other hand, if the average solid-solution carbon content in retained austenite exceeds 1.10%, the ductility and wrinkle suppression ability will decrease. Therefore, the average solid-solution carbon content in retained austenite should be 1.10% or less. Preferably, the average solid-solution carbon content in retained austenite is 1.05% or less, more preferably 1.00% or less.

[0070] The average amount of dissolved carbon in retained austenite is measured as follows: The observation surface on which the area fraction of retained austenite was measured is observed by X-ray diffraction. CuKα rays are used as the incident X-rays, and the lattice constant (a) of the austenite is determined from the position of the diffraction peak of the (220) plane of fcc iron (austenite). γ ) is determined, and the average amount of solid-solution carbon in the retained austenite is calculated using the following formula. a γ (Å) = 3.572 + 0.033 × C - 0.00157 × Si + 0.0012 × Mn In the above formula, Si and Mn represent the mass %) content of each element in the steel, and C represents the average solid solution amount of C in retained austenite (mass %).

[0071] Median equivalent circle diameter of carbide: 100 nm or less Tempered martensite and lower bainite are phases that contain carbides. The formation of carbides contributes to the decrease in the area fractions of fresh martensite and retained austenite. When fresh martensite and retained austenite exist as coarse masses on the order of μm, they become void generation sites during forming, resulting in a decrease in elongation flangeability, wrinkle suppression ability, and bendability. That is, the formation of carbides is important for obtaining desired elongation flangeability, wrinkle suppression ability, and bendability. On the other hand, since carbides can also become microvoid generation sites during forming, if the carbides are coarse, they promote the propagation of cracks that occur between voids caused by coarse masses of fresh martensite and retained austenite, resulting in a decrease in elongation flangeability, wrinkle suppression ability, and bendability. Therefore, the median equivalent circle diameter of carbides is set to 100 nm or less. The median equivalent circle diameter of carbides is preferably 80 nm or less, more preferably 60 nm or less, and even more preferably 40 nm or less. The lower limit of the median equivalent circle diameter of carbides is not particularly limited, but it is preferably 5 nm or more so as not to increase the number density of carbides. Here, the carbides include carbides other than those contained in tempered martensite and lower bainite, for example, carbides in pearlite and carbides precipitated at the boundaries of multiple phases, etc.

[0072] Number density of carbides with an equivalent circle diameter of 40 nm or more: 30 pieces / μm ２ When the number density of the following carbides is large and the distance between carbides is short, when voids are generated due to coarse masses of fresh martensite and retained austenite, it promotes the propagation of cracks that occur between voids, resulting in a decrease in elongation flangeability, wrinkle suppression ability, and bendability. Therefore, the number density of carbides is 30 pieces / μm ２ or less. The number density of carbides is preferably 25 pieces / μm ２ or less, more preferably 20 pieces / μm ２ or less. The lower limit of the number density of carbides is not particularly limited, but it is preferably 0.2 pieces / μm ２ or more in order to keep the median equivalent circle diameter of carbides within a desired range.

[0073] Here, the median of the equivalent circle diameter of the carbides and the number density of carbides with an equivalent circle diameter of 40 nm or more are measured as follows, from the position at 1 / 8 of the plate thickness to the position at 3 / 8 of the plate thickness, centered on the position at 1 / 4 of the plate thickness of the high-strength hot-rolled steel sheet.

[0074] In other words, when calculating the area percentage of the lower bainite, tempered martensite, fresh martensite, retained austenite, and the remaining structure as described above, carbides are also identified separately. Here, carbides embedded in other phases such as lower bainite and tempered martensite are also identified. The median of the equivalent circle diameter of the carbides is calculated by determining the equivalent circle diameter for each carbide particle and then finding the median. The number density of carbides with an equivalent circle diameter of 40 nm or more is calculated by dividing the number of carbides with an equivalent circle diameter of 40 nm or more by the total area of ​​the observation field. Here, the equivalent circle diameter Rc is expressed by the following formula with respect to the area Sc: Rc = (4 × Sc / π)^0.5 Area percentage of the region with hardness of 9.0 GPa or more: 8.0% or less From the viewpoint of ensuring elongation flangeability and bendability, the area percentage of the region with hardness of 9.0 GPa or more is set to 8.0% or less. The area percentage of the region with hardness of 9.0 GPa or more is preferably 6.0% or less, more preferably 4.0% or less. There is no particular lower limit to the area ratio of the region with a hardness of 9.0 GPa or higher, but from the viewpoint of ensuring high ductility, it is preferably 0.2% or more, more preferably 0.4% or more. The region with a hardness of 9.0 GPa or higher consists of fresh martensite, retained austenite, hard tempered martensite, and carbides. Hard tempered martensite refers to the hard region of tempered martensite with a hardness of 9.0 GPa or higher.

[0075] Here, the area ratio of the region with a hardness of 9.0 GPa or higher is measured as follows, from the position at 1 / 8 of the plate thickness to the position at 3 / 8 of the plate thickness, centered on the position at 1 / 4 of the plate thickness of the high-strength hot-rolled steel sheet.

[0076] First, a sample is cut from a hot-rolled steel sheet so that the cross-section parallel to the rolling direction of the sheet becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample is finished with colloidal silica. The amount of polishing for the finish polishing is greater than or equal to the particle size of the diamond paste used in the preceding mirror polishing. After the finish polishing, hardness measurements are performed at five locations using a Berkovich indenter nanoindenter, with a measurement pitch of 1 μm, in a 50x50 point configuration. The total number of measurement points with a hardness of 9.0 GPa or higher is divided by the total number of measurement points (12,500), and the area ratio of the region with a hardness of 9.0 GPa or higher is calculated by multiplying by 100.

[0077] Next, the mechanical properties of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0078] Tensile strength (TS): 980 MPa or more The tensile strength of the high-strength hot-rolled steel sheet according to one embodiment of the present invention shall be 980 MPa or more. Preferably, the tensile strength is less than 1470 MPa. Here, the tensile strength (TS) is measured by a tensile test in accordance with JIS Z 2241, which will be described later in the examples.

[0079] Plating Layer The high-strength hot-rolled steel sheet of the present invention may have a plating layer on its surface. The plating layer is not particularly limited, and for example, known plating layers can be mentioned, but a zinc plating layer is preferred. When a zinc plating layer is used, it is preferable that it contains 0.08% to 0.30% Al. Furthermore, even if elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn are mixed into this zinc plating layer in addition to Zn and Al, and Mg and Si, the effects of the present invention remain unchanged. Furthermore, this zinc plating layer may be an alloyed zinc plating layer that has undergone alloying treatment. In addition to a zinc plating layer, other known plating layers such as Zn-Al-Mg-Zn-Si-Zn-Zin plating layers containing more than 0.30% Al, which are produced by hot-dip plating or various electroplating methods, may also be used as the plating layer.

[0080] Furthermore, the thickness of the high-strength hot-rolled steel sheet of the present invention is not particularly limited, but is preferably 1.0 mm or more and 10.0 mm or less.

[0081] [2] Members Next, a member according to one embodiment of the present invention will be described. A member according to one embodiment of the present invention is a member made using (as a material for) the above-mentioned high-strength hot-rolled steel sheet. Therefore, it has the same steel structure and mechanical properties as the above-mentioned high-strength hot-rolled steel sheet. The above-mentioned high-strength hot-rolled steel sheet maintains high strength with a tensile strength of 980 MPa or more, and further possesses high ductility and high elongation flangeability, as well as excellent wrinkle suppression ability and excellent bendability. For this reason, a member according to one embodiment of the present invention is particularly suitable for application to automobile parts such as undercarriage parts and frame members.

[0082] [3] Method for Manufacturing High-Strength Hot-Rolled Steel Sheets Next, a method for manufacturing high-strength hot-rolled steel sheets according to one embodiment of the present invention will be described. The method for manufacturing high-strength hot-rolled steel sheets according to one embodiment of the present invention comprises the following steps: A heating step in which the steel material is heated to 1150°C or higher. A hot-rolling step in which the steel material after the heating step is hot-rolled to a hot-rolled steel sheet under the conditions of a finish rolling completion temperature of 800°C or higher and 980°C or lower. A first cooling step in which the hot-rolled steel sheet after the hot-rolling step is cooled under the conditions of a cooling stop temperature of Ms°C or lower and a cooling parameter of Ms°C or higher during cooling of 36.0 or lower. A first heat retention step in which the hot-rolled steel sheet after the first cooling step is kept hot for a heat retention time of 60 seconds or more and an integrated tempering parameter of 8.0 or higher and 29.0 or lower during heat retention. A reheating step in which the hot-rolled steel sheet after the first heat retention step is reheated in the temperature range of a reheating stop temperature of 320°C or higher and 480°C or lower. The process includes a second heat retention step in which the hot-rolled steel sheet, after the reheating step, is kept at a heat retention temperature of 320°C to 480°C. A second cooling step in which the hot-rolled steel sheet, after the second heat retention step, is cooled to room temperature. Furthermore, the cumulative tempering parameter from the start of reheating in the reheating step until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling step is set to 24.0 to 30.0. In addition, a plating step may be optionally included in which the manufactured high-strength hot-rolled steel sheet is plated.

[0083] Unless otherwise specified, the temperatures mentioned above refer to the surface temperature of the steel material and steel plate.

[0084] [Manufacturing Process for Steel Slabs (Steel Materials)] First, a steel material such as a slab having the above-mentioned component composition is prepared. The method for manufacturing the steel material such as a slab is not particularly limited, and commonly used methods can be used. As an example of a method for manufacturing the steel material, one can produce a slab by melting molten steel having the above-mentioned component composition using a known method in a converter or the like, and then casting it using a casting method such as continuous casting. As a method for manufacturing the steel material, a known casting method such as ingot-breakdown rolling may also be used. Scrap may also be used as a raw material for the steel material.

[0085] [Heating Process] Heating temperature of steel material: 1150°C or higher After cooling to a low temperature, most of the elements that form carbonitrides, such as Ti, precipitate unevenly as coarse carbonitrides in the steel material such as slabs. The presence of these coarse and uneven precipitates leads to deterioration of various properties, such as strength and elongation flangeability. For this reason, the steel material is heated before hot rolling to solidify the coarse precipitates. In order to sufficiently solidify the coarse precipitates before hot rolling, the heating temperature of the steel material is set to 1150°C or higher. The heating temperature of the steel material is preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel material is too high, it may lead to the occurrence of slab defects and a decrease in yield due to scale-off. For this reason, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. Furthermore, the steel material before hot rolling may be subjected to direct hot rolling (direct rolling) after casting while still at a high temperature, that is, while maintaining a temperature within the above heating temperature range.

[0086] [Hot Rolling Process] Next, the steel material heated to 1150°C or higher (including materials delivered directly at high temperature after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions can be secured. Then, the steel material is roughly rolled to obtain a rough-rolled plate. Before performing finish rolling on the obtained rough-rolled plate, it is preferable to perform descaling (high-pressure water descaling) by spraying high-pressure water at the entrance of the finish rolling mill in order to remove the primary scale that has been generated up to the time of finish rolling. The impact pressure of high-pressure water descaling (also simply called "descaling impact pressure") is preferably 2.5 MPa or higher, more preferably 3.0 MPa or higher, and even more preferably 3.5 MPa or higher. The impact pressure is the force per unit area at which high-pressure water impacts the surface of the rough-rolled plate. The descaling impact pressure is not particularly limited to an upper limit, but is preferably 15.0 MPa or less, more preferably 14.5 MPa or less, and even more preferably 12.0 MPa or less. High-pressure water descaling may be performed during the rolling process between the finish rolling stands. In addition, if necessary, the rough-rolled plates may be cooled between the finish rolling stands.

[0087] Finish rolling completion temperature: 800°C or higher, and 980°C or lower. If the finish rolling completion temperature is less than 800°C, ferrite will form before the end of the finish rolling process, making it impossible to obtain the desired total area ratio of lower bainite and tempered martensite. For this reason, the finish rolling completion temperature should be 800°C or higher. Preferably, the finish rolling completion temperature is 820°C or higher, more preferably 840°C or higher, and even more preferably 860°C or higher. On the other hand, if the finish rolling completion temperature exceeds 980°C, significant grain growth of austenite grains occurs, causing the austenite grains to coarseen, and the untransformed austenite after the formation of lower bainite also coarses. This later becomes fresh martensite, increasing the area ratio of fresh martensite. For this reason, the finish rolling completion temperature should be 980°C or lower. Preferably, the finish rolling completion temperature is 970°C or lower, and more preferably 950°C or lower.

[0088] [First Cooling Process] Next, the hot-rolled steel sheet (finished rolled sheet) obtained by finish rolling is cooled from the finish rolling completion temperature described above to the cooling stop temperature described later (hereinafter also referred to as "forced cooling").

[0089] Cooling stop temperature: Ms°C or lower. If the cooling stop temperature is greater than Ms°C, tempered martensite cannot be obtained. For this reason, the cooling stop temperature should be Ms°C or lower. Preferably, the cooling stop temperature is Ms-5°C or lower, more preferably Ms-10°C or lower, and even more preferably Ms-20°C or lower. The lower limit of the cooling stop temperature is not particularly limited, but in order to obtain the desired area ratio of retained austenite, the cooling stop temperature is preferably -30°C or higher, more preferably 0°C or higher, even more preferably 100°C or higher, and especially preferably greater than 200°C.

[0090] The method for determining the cooling stop temperature is described below. After the finished rolled steel sheet has cooled and its temperature has dropped to 480°C, and before the reheating process is performed, if the temperature drop in the 5 seconds following that point is less than 5°C, and the temperature drop in the 60 seconds following that point is less than 60°C, then that point is considered to be the cooling stop temperature. The steel sheet temperature at that point is then defined as the cooling stop temperature. Note that if the Ms temperature exceeds 480°C, it is outside the scope of this invention.

[0091] Furthermore, Ms is the martensitic transformation initiation temperature defined by the following formula, where each element symbol in the formula represents the content (mass%) of each element, and 0 is used for elements that are not present. Ms (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo Cooling parameter above Ms°C during cooling: 36.0 or less If the cooling parameter above Ms°C during cooling exceeds 36.0, the amount of lower bainite formed above Ms°C during cooling increases, and the amount of dissolved carbon in the untransformed austenite increases as carbon diffuses from the lower bainite to the untransformed austenite. In addition, ferrite and upper bainite may be formed above Ms°C during cooling, which may further increase the amount of dissolved carbon in the untransformed austenite. As a result, the area ratio of fresh martensite increases, and the hardness of tempered martensite increases, increasing the area ratio of the region with a hardness of 9.0 GPa or higher. Therefore, the cooling parameter above Ms°C during cooling should be 36.0 or less. Preferably, the cooling parameter above Ms°C during cooling should be 35.8 or less, more preferably 35.6 or less, and even more preferably 35.4 or less. There is no particular lower limit to the cooling parameter above Ms°C during cooling, but in order to prevent deterioration of the steel plate shape, it is preferable that the cooling parameter above Ms°C during cooling be 32.0 or more. Here, the cooling parameter is calculated as PC as follows. First, the steel plate temperature is measured every 0.1 seconds, interpolated to obtain temperature data every 0.01 seconds, and the following formula for pc is calculated for each time point. Next, this result is used to further calculate the formula for PC. pc(t) = 8000 / (T + 273) where t is the time (seconds) and T is the steel plate temperature (°C) at time t. Then calculate PC = log(10^pc(0) + 10^pc(0.01) + 10^pc(0.02) + ...) + (Ms + 273) / 32.

[0092] [First Heat Retention Process] Next, the hot-rolled steel sheet after the first cooling process is kept warm under the conditions described below. The shape of the hot-rolled steel sheet during heat retention is not particularly limited; it may be wound into a coil or not. The method of heat retention is not particularly limited; the hot-rolled steel sheet may be inserted into a heat retention furnace, or it may simply be allowed to cool. Furthermore, provided that the conditions described below are met, the steel sheet temperature may decrease due to heat removal, or it may increase due to external heating by a heat retention furnace, etc., and internal heat generation such as transformation heat.

[0093] Heat retention time: 60 seconds or more. If the time from the end of cooling in the first cooling step to the start of reheating in the reheating step, i.e., the heat retention time, is less than 60 seconds, the following disadvantages may occur. Specifically, the number density of transition carbides such as ε-carbides generated between the first heat retention step and the subsequent reheating step increases. As a result, the number density of carbides with an equivalent circle diameter of 40 nm or more may increase through the redissolution of transition carbides and the precipitation of carbides such as cementite in the subsequent reheating step and second heat retention step. For this reason, the heat retention time should be 60 seconds or more. Preferably, the heat retention time is 300 seconds or more, more preferably 3600 seconds or more, and even more preferably 10000 seconds or more. The upper limit of the heat retention time is not particularly limited, but from the viewpoint of productivity, 10 ７ It is preferable to have a time of less than a second. Here, the heat retention time refers to the time from the cessation of cooling in the first cooling step to the start of reheating in the reheating step following the first heat retention step.

[0094] Cumulative tempering parameter during heat retention: 8.0 or more and 29.0 or less If the cumulative tempering parameter during heat retention, i.e., from cooling stop to reheating, is less than 8.0, the number density of transition carbides such as ε-carbides generated in the first heat retention step increases. As a result, the number density of carbides with an equivalent circle diameter of 40 nm or more increases through the redissolution of transition carbides and precipitation of carbides such as cementite in the subsequent reheating step and second heat retention step. For this reason, the cumulative tempering parameter during heat retention should be 8.0 or more. Preferably, the cumulative tempering parameter during heat retention is 12.0 or more, more preferably 16.0 or more, and even more preferably 20.0 or more. If the cumulative tempering parameter during heat retention is greater than 29.0, carbides such as cementite coarseen in the first heat retention step and coarseen further in the subsequent second heat retention step, ultimately increasing the median value of the equivalent circle diameter of the carbides. For this reason, the cumulative tempering parameter during heat retention should be 29.0 or less. The cumulative tempering parameter during heat retention is preferably 28.0 or less, more preferably 27.5 or less, and even more preferably 27.0 or less. Here, the cumulative tempering parameter during heat retention is calculated as PT1 as follows. First, the steel sheet temperature is measured every 0.1 seconds and interpolated to obtain temperature data every 0.01 seconds, and the following formula for pt1 is calculated for each time. Next, this result is used to further calculate the formula for PT1. pt1(t) = -18000 / (T + 273) + 48 t: time (seconds), T: steel sheet temperature at time t (°C) PT1 = log(10^pt1(0) + 10^pt1(0.01) + 10^pt1(0.02) + ...) [Reheating process] Next, the hot-rolled steel sheet after the first heat retention process is reheated under the conditions described below. The means for reheating the steel sheet are not particularly limited, but examples include exposing it to a high-temperature atmosphere such as a heat treatment furnace, heating it with a burner, induction heating, or heating it by radiation from other steel sheets. The reheating start temperature is also not particularly limited and may be 320°C to 480°C, which is the heat retention temperature range of the first heat retention process, or it may be less than 320°C. That is, the hot-rolled steel sheet after the first cooling process may be reheated immediately, or it may be reheated after forced cooling or air cooling. The reheating start temperature is preferably 0°C or higher, more preferably more than 100°C, even more preferably more than 150°C, and particularly preferably more than 200°C.

[0095] Reheating stop temperature: 320°C or higher, 480°C or lower. If the reheating stop temperature is below 320°C, the average amount of dissolved carbon in the retained austenite increases. For this reason, the reheating stop temperature should be 320°C or higher. Preferably, the reheating stop temperature is 340°C or higher, more preferably 360°C or higher, and even more preferably 380°C or higher. On the other hand, if the reheating stop temperature is above 480°C, the average amount of dissolved carbon in the retained austenite decreases. Also, the tensile strength may decrease. For this reason, the reheating stop temperature should be 480°C or lower. Preferably, the reheating stop temperature is 460°C or lower, more preferably 440°C or lower, and even more preferably 420°C or lower.

[0096] [Second Heat Retention Process] Next, the hot-rolled steel sheet after the reheating process is kept at a temperature range of 320°C to 480°C. The shape of the hot-rolled steel sheet during heat retention in this temperature range is not particularly limited, but it is preferable to enhance the heat retention of the hot-rolled steel sheet after the reheating process by, for example, placing it in a heat retention furnace or winding it into a coil. The heat retention temperature may be the same as or different from the reheating stop temperature described above, as long as it is within the range of 320°C to 480°C.

[0097] [Second Cooling Process] Next, the hot-rolled steel sheet after the second heat retention process is cooled to room temperature. Room temperature means 50°C or below.

[0098] The cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet reaches room temperature in the second cooling process is 24.0 or more and 30.0 or less. If the cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet reaches room temperature in the second cooling process is less than 24.0, the tempering of the tempered martensite does not progress, and the area ratio of the region with a hardness of 9.0 GPa or more increases. For this reason, the cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet reaches room temperature is set to 24.0 or more. The cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet reaches room temperature in the second cooling process is preferably 25.0 or more, more preferably 26.0 or more, and even more preferably 26.5 or more. On the other hand, if the cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet temperature reaches room temperature in the second cooling process exceeds 30.0, the tempering of the lower bainite and tempered martensite will proceed excessively, reducing the tensile strength. In addition, there is a risk that the area ratio of retained austenite will decrease. For this reason, the cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet temperature reaches room temperature in the second cooling process should be 30.0 or less. The cumulative tempering parameter from the start of reheating in the reheating process until the hot-rolled steel sheet temperature reaches room temperature in the second cooling process is preferably 29.0 or less, more preferably 28.0 or less, and even more preferably 27.5 or less. Here, the cumulative tempering parameter from the start of reheating until the hot-rolled steel sheet temperature reaches room temperature is calculated as PT2 as follows. First, the steel sheet temperature is measured every 0.1 seconds, interpolated to obtain temperature data every 0.01 seconds, and the following formula for pt2 is calculated for each time point. Next, this result is used to calculate the formula for PT2. pt2(t) = -18000 / (T + 273) + 48 where t: time (seconds), T: steel sheet temperature at time t (°C) PT2 = log(10^pt2(0) + 10^pt2(0.01) + 10^pt2(0.02) + ...) Through the above process, the high-strength hot-rolled steel sheet of the present invention is manufactured. In the manufacture of the high-strength hot-rolled steel sheet of the present invention, the hot-rolled steel sheet may be subjected to temper rolling (skin pass rolling).When temper rolling is performed on hot-rolled steel sheets, there are no particular limitations, but for example, temper rolling can be performed on the hot-rolled steel sheets during or between each process after the hot-rolling process. In this case, it is preferable to perform temper rolling at least once, selected from during the first cooling process, between the first cooling process and the first heat retention process, during the first heat retention process, between the first heat retention process and the reheating process, during the second heat retention process, during the second cooling process, and after the second cooling process. Pickling may also be performed to remove scale. When pickling is performed on hot-rolled steel sheets, there are no particular limitations, but for example, pickling can be performed on the hot-rolled steel sheets during or between each process after the first heat retention process. In this case, it is preferable to perform pickling at least once, selected from between the first heat retention process and the reheating process, during the second cooling process, and after the second cooling process.

[0099] [Plating Process] Furthermore, if the high-strength hot-rolled steel sheet of the present invention has a plating layer on its surface, the method for manufacturing the high-strength hot-rolled steel sheet further includes a plating process in which the hot-rolled steel sheet is plated. In this case, although not particularly limited, the plating process can be applied to the hot-rolled steel sheet after the first heat retention process. In this case, it is preferable to apply the plating process at least once, selected from between the first heat retention process and the reheating process, during the reheating process, between the reheating process and the second heat retention process, during the second heat retention process, between the second heat retention process and the second cooling process, during the second cooling process, and after the second cooling process. The plating process in the plating process is not particularly limited, and known plating processes can be cited as examples.

[0100] In the temper rolling, pickling, and plating processes, "between one process and another" means after the completion of one process and before the start of the other. Furthermore, "in the middle of a process" means the entire process from the start to the end of that process.

[0101] [4] Method for manufacturing members As a method for manufacturing members, there is a method of forming a member by subjecting the high-strength hot-rolled steel sheet obtained as described above to at least one of, for example, forming or joining. Examples of forming processes include press forming and roll forming. Examples of joining processes include arc welding and spot welding.

[0102] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0103] [Manufacturing of High-Strength Hot-Rolled Steel Sheets] Molten steel having the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter, and steel material was manufactured by continuous casting. The manufactured steel material was subjected to a heating process at the heating temperatures [°C] shown in Table 2 below. A rough-rolled sheet was obtained by rough-rolling the steel material after the heating process. The surface of the obtained rough-rolled sheet was subjected to high-pressure water descaling with an impact pressure of 10.0 MPa. A hot-rolled steel sheet was obtained by finish-rolling the rough-rolled sheet that had undergone high-pressure water descaling at the finish-rolling completion temperature [°C] shown in Table 2. After the completion of hot rolling (finish rolling), the obtained hot-rolled steel sheet was subjected to a first cooling process. Table 2 shows the conditions for the first cooling process, including the cooling stop temperature [°C] and the cooling parameter PC of Ms°C or higher during cooling. After the completion of the first cooling process, the obtained hot-rolled steel sheet was subjected to a first heat retention process. Table 2 lists the conditions for the first heat retention process, including the heat retention time and the cumulative tempering parameter PT1 during heat retention. After the completion of the first heat retention process, the obtained hot-rolled steel sheet was subjected to a reheating process. Table 2 lists the conditions for the reheating process, including the reheating stop temperature [°C]. After the completion of the reheating process, the obtained hot-rolled steel sheet was subjected to a second heat retention process, followed by a second cooling process. Table 2 lists the conditions for the reheating process, the second heat retention process, and the second cooling process, including the cumulative tempering parameter PT2 from the start of reheating until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature.

[0104] In this way, a high-strength hot-rolled steel sheet was obtained. The sheet thickness was set to 3.0 mm.

[0105]

[0106]

[0107] [Evaluation of High-Strength Hot-Rolled Steel Sheets] Using the obtained high-strength hot-rolled steel sheets, the microstructure of the high-strength hot-rolled steel sheets was identified according to the procedure described above. The measurement results are shown in Table 3. In Table 3, LB is the area percentage of lower bainite, TM is the area percentage of tempered martensite, FM is the area percentage of fresh martensite, γ is the area percentage of retained austenite, UB is the area percentage of upper bainite, P is the area percentage of pearlite, and F is the area percentage of ferrite. %Cγ is the average amount of dissolved carbon in retained austenite, MDC is the median value of the equivalent circle diameter of carbides, NDC is the number density of carbides with an equivalent circle diameter of 40 nm or more, and AFH is the area percentage of the region with a hardness of 9.0 GPa or more.

[0108]

[0109] Furthermore, tensile tests, hole expansion tests, and bending tests were conducted according to the following procedure, and the tensile strength (TS), uniform elongation (U.El), critical hole expansion ratio (λ), n value (n), and R / t value (R / t) were evaluated according to the following criteria. The measurement results are shown in Table 4.

[0110]

[0111] (1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241. Specifically, a JIS No. 5 test specimen was taken from the obtained high-strength hot-rolled steel sheet so that its longitudinal direction was perpendicular to the rolling direction of the steel sheet. Using the taken test specimen, a tensile test was performed under the condition of a crosshead speed of 10 mm / min, and the TS, U. El, and n values ​​were measured. The results are shown in Table 4.

[0112] A total test (TS) of 980 MPa or higher (TS ≥ 980 MPa) was considered a pass, while anything else was considered a fail.

[0113] U.El was defined as follows: For 980 MPa ≤ TS < 1180 MPa, a value of 4.7% or more (U.El ≥ 4.7%) was considered to indicate high ductility. For 1180 MPa ≤ TS < 1310 MPa, a value of 4.5% or more (U.El ≥ 4.5%) was considered to indicate high ductility. For 1310 MPa ≤ TS, a value of 4.0% or more (U.El ≥ 4.0%) was considered to indicate high ductility. Anything that did not meet these criteria was considered to lack high ductility.

[0114] The n-values ​​were defined as follows: For 980 MPa ≤ TS < 1180 MPa, a value of 0.040 or higher (n-value ≥ 0.040) was considered to indicate excellent wrinkle suppression ability. For 1180 MPa ≤ TS < 1310 MPa, a value of 0.035 or higher (n-value ≥ 0.035) was considered to indicate excellent wrinkle suppression ability. For 1310 MPa ≤ TS, a value of 0.030 or higher (n-value ≥ 0.030) was considered to indicate excellent wrinkle suppression ability. Anything that did not meet these criteria was considered to lack excellent wrinkle suppression ability.

[0115] (2) Hole Expansion Test The hole expansion test was conducted in accordance with JIS Z 2256. Specifically, a 100 mm x 100 mm test piece was taken from the obtained high-strength hot-rolled steel sheet by shearing. A hole with a diameter of 10 mm (the initial hole in the test piece) was punched into the test piece with a clearance of 12% ± 1%. Next, a wrinkle-holding force of 9 tons (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm, and a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when cracks occurred) was measured. The limit hole expansion ratio λ (%) was then calculated using the following formula. The results are shown in Table 4.

[0116] λ was defined as follows: For 980 MPa ≤ TS < 1180 MPa, 45% or more (λ ≥ 45%) was considered acceptable (high elongation flange properties); for 1180 MPa ≤ TS < 1310 MPa, 40% or more (λ ≥ 40%) was considered acceptable; and for 1310 MPa ≤ TS, 35% or more (λ ≥ 35%) was considered acceptable. Anything else was considered unacceptable. λ (%) = {(D ｆ -D ０ ) / D ０} × 100 Here, D ｆ : Diameter of the hole in the test specimen at the time of crack initiation (mm), D ０ : This is the diameter (mm) of the hole in the initial test specimen.

[0117] (3) Bending Test The obtained high-strength hot-rolled steel sheets were subjected to shearing, and bending tests were taken of 35 mm (width) x 100 mm (length) specimens so that the longitudinal direction of the specimen was perpendicular to the rolling direction. These specimens, which had sheared end faces, were used. Next, a V-block 90° bending test was performed in accordance with the push bending method specified in JIS Z 2248. At this time, three specimens were used for each steel sheet, and the minimum bending radius at which no crack occurred in any of the specimens was defined as the limit bending radius R (mm). The R / t value was calculated by dividing R by the thickness t (mm) of the hot-rolled steel sheet, and the bendability of the hot-rolled steel sheet was evaluated. The results are shown in Table 4. The R / t values ​​were as follows: For 980 MPa ≤ TS < 1180 MPa, a value of 2.0 or less (R / t value ≤ 2.0) was considered to have excellent bendability. For pressures of 1180 MPa ≤ TS < 1310 MPa, a R / t value of 3.5 or less (R / t value ≤ 3.5) was considered to indicate excellent bendability. For pressures of 1310 MPa ≤ TS, a R / t value of 4.5 or less (R / t value ≤ 4.5) was considered to indicate excellent bendability. Materials that did not meet these criteria were considered to lack excellent bendability.

[0118] In the examples of the present invention, high-strength hot-rolled steel sheets are obtained that are high in strength, have high ductility, excellent wrinkle suppression, and possess high elongation flangeability and excellent bendability. On the other hand, in the comparative examples, at least one of these properties is inferior.

Claims

1. The composition is as follows, in mass%, C: 0.040% or more and 0.350%, Si: 0.50% or more and 2.50%, Mn: 1.50% or more and less than 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is as follows: Area ratio of tempered martensite: greater than 0% and 99.5% or less, Area ratio of lower bainite: greater than 0% and 99.5% or less, Total area ratio of lower bainite and tempered martensite: 82.0% or more and 99.5% or less, Area ratio of fresh martensite: 8.0% or less (including 0%), Area ratio of retained austenite: 0.5% or more and 10.0% or less. Average solid-solution carbon content in retained austenite: 0.50% to 1.10%, Median equivalent circle diameter of carbides: 100 nm or less, Number density of carbides with an equivalent circle diameter of 40 nm or more: 30 particles / μm ２ The following describes a high-strength hot-rolled steel sheet having an area ratio of 8.0% or less of the region with a hardness of 9.0 GPa or higher, and a tensile strength of 980 MPa or higher.

2. In addition to the above component composition, the following may be added by mass: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.400% or less, Cr: 1.00% or less, Mo: 0.500% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less The high-strength hot-rolled steel sheet according to claim 1, comprising at least one selected from Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

3. A high-strength hot-rolled steel sheet according to claim 1 or 2, having a plating layer on its surface.

4. A member made using a high-strength hot-rolled steel sheet according to any one of claims 1 to 3.

5. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 1 or 2, comprising: a heating step of heating a steel material to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step to a hot-rolled steel sheet under the conditions of a finish rolling completion temperature of 800°C or higher and 980°C or lower; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step under the conditions of a cooling stop temperature of Ms°C or lower and a cooling parameter of Ms°C or higher during cooling of 36.0 or lower; a first heat retention step of heat retention the hot-rolled steel sheet after the first cooling step under the conditions of a heat retention time of 60 seconds or higher and an integrated tempering parameter of 8.0 or higher and 29.0 or lower during heat retention; and a reheating step of reheating the hot-rolled steel sheet after the first heat retention step under the conditions of a reheating stop temperature of 320°C or higher and 480°C or lower. A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a second heat retention step of maintaining the heat of the hot-rolled steel sheet after the reheating step at a heat retention temperature of 320°C to 480°C; and a second cooling step of cooling the hot-rolled steel sheet after the second heat retention step to room temperature, wherein the cumulative tempering parameter from the start of reheating in the reheating step until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling step is 24.0 to 30.

0. Here, Ms is defined by the formula Ms(°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo, where each element symbol in the formula represents the content (mass%) of each element, and 0 is used for elements that are not contained.

6. The method for manufacturing a high-strength hot-rolled steel sheet according to claim 5, wherein the hot-rolled steel sheet after the first heat retention step is subjected to a plating treatment.

7. A method for manufacturing a component, comprising the step of forming or joining a high-strength hot-rolled steel sheet manufactured by the method for manufacturing a high-strength hot-rolled steel sheet according to claim 5 or 6 to form a component.

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