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

Abstract

The present invention provides high-strength hot-rolled steel sheets, components, and methods for manufacturing them, which possess high strength and ductility, as well as excellent wrinkle suppression, high elongation flangeability, and excellent bendability. [Solution] Having a predetermined component composition, the steel structure has the following area ratios: tempered martensite area ratio: greater than 0% and 99.5% or less, lower bainite area ratio: greater than 0% and 99.5% or less, total area ratio of lower bainite and tempered martensite: 82.0 to 99.5% or less, fresh martensite: 0 to 8.0%, retained austenite: 0.5 to 10.0%, average solid solution carbon content in retained austenite: 0.50 to 1.10%, area: 50.0 μm 2 Number density of the above hard phase: 500 particles / mm² 2 The following describes a high-strength hot-rolled steel sheet characterized by having a surface area ratio of 8.0% or less in the region with a hardness of 9.0 GPa or higher, and a tensile strength of 980 MPa or higher.

Description

[Technical Field]

[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. [Background technology]

[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 higher 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, "In mass%, C: 0.02~0.20%, Si: 0.005~2.00%, Mn: 1.30~2.40%, P: 0.100% or less, S: 0.0100% or less, sol.Al: 0.001~1.00%, Ti: 0.030~0.200%, N: 0.0010~0.0100%, Nb: 0~0.100%, 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~ The chemical composition consists of 0.0100% ferrite, 0-0.0100% Mg, and 0-0.0100% REM, with the remainder being Fe and impurities. At a depth of 1 / 4 of the plate thickness from the surface, the area ratio of ferrite is 10-55%, the total area ratio of bainite and martensite is 45-90%, the total area ratio of ferrite, bainite and martensite is 90% or more, the average grain size is 12.0 μm or less, and the texture measured at the center of the plate thickness is {100}. <011> , {211} <011> , {311} <011> , {110} <011> and {332} <113> The maximum polar density of the azimuthal group is 8.0 or less, and {211} <011> and {332} <113> A hot-rolled steel sheet is disclosed, characterized in that the sum of its extreme densities is 10.0 or less and its tensile strength is 950 MPa or more.

[0006] Patent Document 2 states that the chemical composition is as follows, in mass%, C: 0.040~0.150%, Si: 0.50~1.50%, Mn: 1.00~2.50%, P: 0.100% or less, S: 0.010% or less, Al: 0.010~0.100%, N: 0.0100% or less, Ti: 0.005~0.150%, B: 0.0005~0.0050%, Cr: 0.10~1.00%, Nb: 0~0.06%, V: 0~0.50%, Mo: 0~0.50%, Cu: 0~0.50% The material contains %, 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 and impurities. In the microstructure at a position 1 / 4 of the plate thickness from the surface in the thickness direction, the main phase is 95.00~98.00% bainite phase by area percentage, and the second phase is 2.00~5.00% tempered martensite phase, with the average grain size of the second phase being 1.5 μm or less. (110) <112> A hot-rolled steel sheet is disclosed, characterized in that the polar density in the orientation 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> orientation 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% of C: 0.02-0.23%, Si: 0.10-3.00%, Mn: 0.5-3.5%, 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 80-100%, a maximum grain orientation density of less than 2.5 in the region 5-10 μm from the surface in the thickness direction, and a maximum grain orientation density of 2.5 or more in the region 50-100 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 exceeding 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 % 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 % retained austenite, up to 15 area % bainite, up to 15 area % polygonal ferrite, up to 5 area % cementite, and / or up to 5 area % 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. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 6465266 [Patent Document 2] Patent No. 7188618 [Patent Document 3] Patent No. 7207615 [Patent Document 4] Patent No. 7193454 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] By the way, the practical application of hot-rolled steel sheets used for automotive components mainly remains at a tensile strength of around 780 MPa, and the application of 980 MPa grade is currently limited to only a very small part. Automotive parts, especially underbody parts such as suspension parts and frame members, need to have complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets used as materials for such members with complex shapes are required to have excellent formability.

[0011] On the other hand, when the tensile strength of the steel sheet is increased, generally the formability decreases. Steel sheets used as materials for members with complex shapes are required to have high ductility and high stretch flangeability in addition to high strength, and it is important to ensure both of these while maintaining high strength.

[0012] In addition, underbody parts and frame members are important safety components that affect the running and safety of automobiles if they break, and high reliability is required. If there are wrinkles in these parts, there is a risk that cracks will occur from the concave parts of the wrinkles due to instantaneous loads or long-term repeated loads during actual use, leading to component failure. Therefore, the steel sheets used as materials for these parts need to have excellent wrinkle suppression ability. In particular, for high-strength steel sheets, even when the strain due to forming is small, wrinkles are likely to occur, and it is important for high-strength steel sheets to have excellent wrinkle suppression ability.

[0013] Furthermore, since these parts are subjected to burring and bending operations for reasons such as ensuring rigidity, the steel sheets used as materials for these parts need to have excellent bending properties.

[0014] As described above, it is important for steel sheets used as materials for automotive parts represented by underbody parts and frame members to have high strength while also having high ductility, high stretch flangeability, excellent wrinkle suppression ability, and excellent bending properties.

[0015] In fact, the steel sheets disclosed in Patent Documents 1 to 4 cannot be said to have excellent wrinkle suppression ability and excellent bending properties in addition to maintaining a high strength with a tensile strength (TS) of 980 MPa or more, as well as having higher ductility and higher stretch flangeability.

[0016] Therefore, an object of the present invention is to solve such problems of the prior art and provide the following. That is, while maintaining a high strength with a tensile strength of 980 MPa or more, in addition to high ductility and high stretch flangeability, it is an object to provide a high-strength hot-rolled steel sheet having excellent wrinkle suppression ability and excellent bendability, and a manufacturing technology thereof.

[0017] Here, high ductility is required to form the parts around the feet. In the present invention, high ductility means that the uniform elongation (the total elongation at the maximum test force, hereinafter also referred to as U.El) measured in a tensile test conforming to JIS Z 2241 satisfies the following formula. 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 parts around the feet are subjected to stretch flange forming after punching, high stretch flangeability is required. In the present invention, high stretch flangeability means that the average limiting hole expansion ratio (λ) measured in a hole expansion test conforming to JIS Z 2256 using three test pieces satisfies the following formula. In particular, since the parts around the feet are subjected to stretch flange forming after punching, high stretch flangeability is required. When 980 MPa ≤ TS < 1180 MPa, λ ≥ 45% When 1180 MPa ≤ TS < 1310 MPa, λ ≥ 40% When 1310 MPa ≤ TS, λ ≥ 35% For steel sheets used as materials for automotive parts typified by parts around the feet and frame members, excellent wrinkle suppression ability is required to ensure the reliability of the parts. In the present invention, excellent wrinkle suppression ability means that for the work hardening index (hereinafter also referred to as the n value) measured in accordance with JIS Z 2253:2020, the n value satisfies the following formula. When 980 MPa ≤ TS < 1180 MPa, with a strain range of 3.5% to 4.5%, n value ≥ 0.040 For 1180MPa ≤ TS < 1310MPa, the strain range is 3.5% to 4.5%, and the n value is ≥ 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, undergo burring and bending processes to ensure rigidity, and therefore, the steel plates used as materials for suspension components require excellent bendability. In this invention, excellent bendability means that, in a bending test in accordance with JIS Z 2248, the minimum bending radius at which no crack occurs in any test piece is defined as the limit bending radius R (mm), and the R / t value obtained by dividing R by the thickness t (mm) of the steel plate satisfies the following formula. If 980MPa ≤ TS < 1180MPa, then R / t ≤ 2.0 If 1180MPa ≤ TS < 1310MPa, then R / t ≤ 3.5 If 1310 MPa ≤ TS, then R / t ≤ 4.5 [Means for solving the problem]

[0018] The inventors diligently conducted research to achieve the above objectives.

[0019] 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 the 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 area was 50.0 μm². 2 The number density of the hard phase is 500 particles / mm³. 2Below, 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. This resulted in the discovery 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, and excellent bendability.

[0020] This invention was completed based on the above findings and further considerations.

[0021] In other words, the gist of the present invention is as follows: [1] The composition is as follows, by mass%, C: 0.040% to 0.350%, Si: 0.50% to 2.50%, Mn: 1.50% to less than 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% to 2.000%, N: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is such that the area ratio of tempered martensite is 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 the 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 soluble C content in retained austenite: 0.50% or more and 1.10% or less, area 50.0 μm 2 Number density of the above hard phase: 500 particles / mm² 2 The following describes a high-strength hot-rolled steel sheet characterized by having an area ratio of 8.0% or less in 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 amounts are further included 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. The high-strength hot-rolled steel sheet according to [1], characterized by containing at least one selected from 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], characterized by having a plating layer on its surface. A component characterized by being made of a high-strength hot-rolled steel sheet as described in any of [4][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 at a finish rolling completion temperature of 800°C or higher and 980°C or lower to obtain a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step at a cooling stop temperature of Ms°C or lower, a cooling parameter of Ms°C or higher during cooling of 36.0 or lower, and an integrated tempering parameter of Ms°C or lower T1°C or higher during cooling of 25.4 or lower, and stopping the cooling; and the hot-rolled steel sheet after the first cooling step, A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a reheating step in which the hot-rolled steel sheet is reheated under the conditions of a time of less than 60 seconds from the end of cooling to the start of reheating, and a reheating stop temperature of 320°C or higher and 480°C or lower; a heat retention step in which the hot-rolled steel sheet after the reheating step is kept at a heat retention temperature of 320°C or higher and 480°C or lower; and a second cooling step in which the hot-rolled steel sheet after the heat retention step is cooled to room temperature, wherein the cumulative tempering parameter from the start of reheating in the reheating step to the time when the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling step 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, T1 is the greater of 300°C and the aforementioned cooling stop temperature, and each element symbol in the above formula represents the content (mass%) of each element, with 0 being used for elements that are not present. [6] A method for manufacturing a high-strength hot-rolled steel sheet according to [5], characterized in that the hot-rolled steel sheet after the second cooling step is subjected to a plating treatment. A method for manufacturing a component, characterized by comprising the step of applying at least one of a forming process or a joining process to a high-strength hot-rolled steel sheet manufactured by the method for manufacturing a high-strength hot-rolled steel sheet described in [7], [5], or [6] to form a component. [Effects of the Invention]

[0022] 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. [Modes for carrying out the invention]

[0023] 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.

[0024] [1] High strength hot rolled steel plate 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 in "mass%", but unless otherwise specified, they will be simply referred to as "%".

[0025] C: 0.040 or more and 0.350% or less Carbon (C) is an element that improves the strength of steel. C promotes the formation of bainite by improving hardenability, thus contributing to increased strength. Furthermore, C also contributes to increased strength by enhancing the strength of martensite. To obtain a tensile strength of 980 MPa or higher, the C content must be 0.040% or higher. Therefore, the C content is set to 0.040% or higher. Preferably, the C content is 0.050% or higher, and more preferably 0.060% or higher. On the other hand, if the C content exceeds 0.350%, the area ratio and area of ​​the region with a hardness of 9.0 GPa or higher increase. 2 The number density of the hard phase increases, reducing tensile flangeability and bendability. Furthermore, the tensile strength increases excessively, reducing ductility. Therefore, the carbon content should be 0.350% or less. Preferably, the carbon content is 0.250% or less, and more preferably 0.220% or less.

[0026] Si: 0.50 or more and 2.50% or less Si has the effect of suppressing the formation of Fe-based carbides and inhibits cementite precipitation in processes from the first cooling process onward, including the first cooling process. As a result, carbon 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 heat retention process. 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 subscale 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 in the hot rolling process, the surface roughness of the steel sheet becomes excessive, worsening the pretreatment properties for 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.

[0027] 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. Furthermore, by stabilizing austenite, Mn suppresses the transformation of untransformed austenite into lower bainite, carbides, and pearlite during the heat retention process, and contributes to keeping the average solid soluble C content in retained austenite within the desired range of 1. To obtain these effects, the Mn content needs to 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 is 5.00% or more, bainite transformation is more likely to occur, the hard phase increases, and the elongation flangeability and bendability decrease. Therefore, the Mn content is set to less than 5.00%. Preferably, the Mn content is less than 4.00%, more preferably less than 3.50%, and even more preferably less than 3.20%.

[0028] P:0.100% or less P is an element that contributes to increasing the strength of steel by forming a solid solution. However, P also 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.

[0029] 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 should be 0.0200% or less. Preferably, the S content is 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.

[0030] Al: 0.010% or more and 2.000% or less Al acts as a deoxidizing agent and is an effective element for improving the cleanliness of steel. Since its effect is insufficient at Al content below 0.010%, the Al content should be 0.010% or higher. Furthermore, like Si, Al helps retain austenite, contributing to improved ductility. Also, like Si, it improves the strength-ductility balance of bainite and martensite after appropriate heat retention processes. On the other hand, excessive Al content leads to an increase in oxide inclusions, reducing ductility, elongation flangeability, wrinkle resistance, and bendability. Therefore, the Al content should be 2.000% or less. Preferably, the Al content is 1.000% or less.

[0031] N: 0.0200% or less N precipitates as nitrides by bonding with nitride-forming elements, generally contributing 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. While there is no particular lower limit to the N content, from the viewpoint of productivity, an N content of 0.0005% or more is preferred.

[0032] 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.

[0033] 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.

[0034] Ti: 0.200% or less Ti is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. Furthermore, Ti increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite-unrecrystallized region and contributing to an improved strength-ductility balance through refinement of the grain size of lower bainite and tempered martensite. To obtain these effects, it is preferable that the Ti content be 0.005% or higher when Ti is included. More preferably, the Ti content is 0.007% or higher, and even more preferably 0.010% or higher. On the other hand, if the Ti content exceeds 0.200%, a large amount of Ti-based precipitates are formed, which may actually reduce the ductility and bendability. Therefore, when Ti is included, the Ti content should be 0.200% or lower. Preferably, the Ti content is 0.150% or lower, and more preferably 0.120% or lower.

[0035] 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 non-recrystallized austenite region and contributing to an improved strength-ductility balance through refinement of the grain size of lower bainite and tempered martensite. To obtain these effects, it is preferable that the Nb content be 0.005% or higher. More preferably, the Nb content is 0.010% or higher, and even more preferably 0.020% or higher. On the other hand, if the Nb content exceeds 0.200%, a large amount of Nb-based precipitates may be formed, which may actually reduce ductility, ductility, and bendability. Furthermore, the hard phase may increase excessively, potentially reducing ductility, ductility, and bendability. Therefore, when 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.

[0036] V:0.400% or less V, like Ti, is an element that improves the strength of steel sheets through precipitation strengthening and solid solution strengthening. Also, like Ti, V 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 these effects, when V is included, 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, if the V content exceeds 0.400%, a large amount of Nb-based precipitates may be generated, which may actually reduce the ductility, flammability and bendability. In addition, the hard phase may increase excessively, which may reduce ductility, flammability, and bendability. Therefore, when V is added, the V content should be 0.400% or less. The V content is preferably 0.200% or less, and more preferably 0.100% or less.

[0037] Cr:1.00% or less Like Mn, Cr inhibits ferrite formation and contributes to the formation of bainite and martensite. To obtain this effect, if Cr is included, it is preferable that the Cr content be 0.01% or more. More preferably, the Cr content is 0.10% or more, and even more preferably, 0.20% or more. However, since Cr is an element that worsens corrosion resistance and paint pretreatment properties, if Cr is added, it is preferable that the Cr content be 1.00% or less. More preferably, the Cr content is 0.80% or less, and even more preferably, 0.70% or less.

[0038] Mo: 0.500% or less Mo increases the tempering softening resistance of steel, contributing to improved steel sheet strength. Furthermore, Mo suppresses the transformation of untransformed austenite to pearlite during the heat retention process, contributing to an increase in the area ratio of retained austenite and obtaining an appropriate average solid-solution carbon content in the retained austenite. To obtain these effects, when Mo is included, the Mo content is preferably 0.010% or more. More preferably, the Mo content is 0.050% or more, and even more preferably 0.100% or more. On the other hand, if the Mo content exceeds 0.500%, the total area ratio of lower bainite and tempered martensite decreases, potentially worsening tensile flangeability and bendability. Therefore, when Mo is included, the Mo content should be 0.500% or less.

[0039] B: 0.0100% or less B is an element that contributes to the formation of lower bainite and tempered martensite by segregating at prior austenite grain boundaries and suppressing ferrite formation. To obtain this effect, it is preferable that the B content be 0.0005% or more when B is included. On the other hand, if the B content exceeds 0.0100%, the above effect saturates. Therefore, when B is included, the B content should be 0.0100% or less.

[0040] Cu:1.00% or less Cu is an element that improves the strength of steel sheets through precipitation strengthening and solid solution strengthening. To obtain such effects, it is preferable that the Cu content be 0.005% or more when Cu is included. However, if the Cu content exceeds 1.00%, it leads to a deterioration of the surface properties of the hot-rolled steel sheet. Therefore, when Cu is included, the Cu content should be 1.00% or less.

[0041] Ni: 1.00% or less Ni is an element that contributes to increasing the strength of steel by forming a 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 higher when Ni is included. However, if the Ni content exceeds 1.00%, the hard phase 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.

[0042] Sb: 0.200% or less Sb is an effective element for suppressing the reduction 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.

[0043] 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.

[0044] 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 and stabilizes precipitation strengthening, thereby improving 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 formed. This may reduce the elongation flangeability and bendability. Therefore, when Ta is included, the Ta content should be 0.100% or less.

[0045] W: 0.500% or less W is an effective element for improving hardenability and adjusting the strength of steel to a more favorable 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, it is preferable that the W content be 0.500% or less when W is included.

[0046] Mg: 0.0200% or less Mg controls the shape of oxide and sulfide inclusions, contributing to further improvements in tensile flange properties and bendability. To obtain these effects, it is preferable that the Mg content be 0.0010% or higher when Mg is included. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause tensile flange cracking or bending cracking. Therefore, when adding Mg, the Mg content should be 0.0200% or less.

[0047] 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.

[0048] Co:0.0200% or less Co, like 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 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 actually cause stretch flange cracking or bending cracking. Therefore, when Co is included, the Co content should be 0.0200% or less.

[0049] Zr: 0.0200% or less Zr, like Zn and 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 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 adding Zr, the Zr content should be 0.0200% or less.

[0050] 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 these effects, it is preferable that the Ca content be 0.0010% or higher when Ca is included. However, if the Ca content exceeds 0.0200%, the number 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.

[0051] Se: ≤0.0200%, Te: ≤0.0200%, Ge: ≤0.0200%, As: ≤0.0200%, Sr: ≤0.0200%, Cs: ≤0.0200%, Hf: ≤0.0200%, Pb: ≤0.0200%, Bi: ≤0.0200%, and REM: ≤0.0200% Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM (rare earth metals), like Ca, control the shape of oxide and sulfide inclusions, contributing to further improvements in ductility, flangeability, and bendability. To obtain such effects, it is preferable that the content of each of the above elements be 0.0010% or more when the above elements are included. However, if the content of each of the above elements exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause stretching flange cracks or bending cracks. Therefore, when the above elements are added, the content of each of the above elements 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.

[0052] 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, each.

[0053] 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% for the total microstructure.

[0054] Area percentage 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 forms later, and when tempered, it becomes tempered martensite. The promotion of lower bainite formation by martensite results in an area of ​​50.0 μm 2 The number density of the hard phase and 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 ductility and high elongation flangeability, as well as excellent wrinkle suppression and excellent bendability, while maintaining high strength with a tensile strength of 980 MPa or higher. In order to ensure the desired total area ratio of lower bainite and tempered martensite, the area ratio of tempered martensite is set to be 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 lower. The lower limit of the area ratio of tempered martensite is preferably 0.1% or higher, more preferably 1.0% or higher, and even more preferably 5.0% or higher. The upper limit of the area ratio of tempered martensite is preferably 98.0% or lower, more preferably 90.0% or lower, and even more preferably 85.0% or lower.

[0055] Percentage of lower bainite area: 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 ductility and high elongation flangeability, as well as excellent wrinkle suppression and excellent bendability, while maintaining high strength with a tensile strength of 980 MPa or more. In order to ensure the desired total area ratio of lower bainite and tempered martensite, the area ratio of tempered martensite is set to be 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 more. 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, and even more preferably 5.0% or more. The upper limit of the area ratio of tempered martensite is preferably 98.0% or less, more preferably 90.0% or less, and even more preferably 85.0% or less.

[0056] Total area ratio of lower bainite and tempered martensite: 82.0% to 99.5% From the viewpoint of ensuring high strength, the total area ratio of lower bainite and tempered martensite should be 82.0% or more. Preferably, the total area ratio of lower bainite and tempered martensite is 85.0% or more, more preferably 90.0% or more, and even more preferably 92.0% 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 kept at 0.5% or more. Therefore, the total area ratio of lower bainite and tempered martensite should be 99.5% or less. Preferably, the total area ratio of lower bainite and tempered martensite is 99.0% or less, more preferably 98.0% or less, and even more preferably 97.5% or less.

[0057] Area percentage 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, and bendability. Therefore, 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%.

[0058] Area percentage of retained austenite: 0.5% to 10.0% 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.

[0059] 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).

[0060] 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.

[0061] 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.

[0062] 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 same crystal orientation range of the tempered martensite. 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 same crystal orientation range of the lower bainite. Here, Fe-based carbides that have elongated in the same direction refer to Fe-based carbides whose elongation direction difference is 10° or less.

[0063] Fresh martensite and retained austenite have similar shapes and contrasts in SEM, making them difficult to distinguish; therefore, the area ratios of fresh martensite and retained austenite are determined by the method described below.

[0064] 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.

[0065] 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), 10 fields of view of 42.7 μm × 32.0 μm on the observation surface of the sample are observed under conditions of acceleration voltage: 15 kV and magnification: 3000x, and each phase is identified and the area ratio is calculated.

[0066] Fresh martensite and retained austenite exhibit similar contrast in SEM images, making them difficult to distinguish. Therefore, in SEM observations, fresh martensite and retained austenite are not distinguished; their area fractions are calculated as a single hard phase. Furthermore, the area fraction of retained austenite is determined by X-ray diffraction, and the area fraction of fresh martensite is calculated by subtracting the area fraction of retained austenite (described later) from the area fraction of the hard phase calculated from the SEM image.

[0067] The area ratio of retained austenite is measured as follows: After mechanically grinding the hot-rolled steel sheet in the thickness direction (depth direction) up to 1 / 4 of the sheet thickness, the sheet thickness of 100 μm or more is chemically polished with oxalic acid to create the observation surface. Then, the observation surface is observed by X-ray diffraction. CoKα rays are used as the incident X-rays. The ratio of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200) and (211) planes of bcc iron is determined, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. 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.

[0068] Average solid soluble carbon content in retained austenite: 0.50% to 1.10% From the viewpoint of obtaining high ductility and excellent wrinkle suppression ability, the average amount of C dissolved in retained austenite should be 0.50% or more. The average amount of C dissolved in retained austenite is preferably 0.55% or more, more preferably 0.60% or more. On the other hand, when the average amount of C dissolved in retained austenite exceeds 1.10%, the ductility and wrinkle suppression ability will rather decrease. Therefore, the average amount of C dissolved in retained austenite should be 1.10% or less. The average amount of C dissolved in retained austenite is preferably 1.05% or less, more preferably 1.00% or less.

[0069] The average amount of C dissolved in retained austenite is measured as follows. That is, the observation surface on which the area fraction of retained austenite is measured is observed by X-ray diffraction method. CuKα ray is used as the incident X-ray, and from the position of the diffraction peak of the (220) plane of fcc iron (austenite), the lattice constant (a γ ) of austenite is obtained, and the average amount of C dissolved in retained austenite is obtained 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 content (mass%) of each element in the steel, and C represents the average amount of C dissolved in retained austenite (mass%).

[0070] Number density of hard phases with an area of 50.0 μm 2 or more: 500 pieces / mm 2 or less In the present invention, the hard phase refers to a phase composed of fresh martensite and retained austenite. The hard phase does not have Fe-based carbides compared with lower bainite and / or tempered martensite. Also, fresh martensite and retained austenite have a brighter contrast in the SEM image compared with upper bainite, lower bainite and / or tempered martensite, polygonal ferrite, and acicular ferrite. Therefore, the hard phase can be distinguished from these structures using SEM.

[0071] From the viewpoint of obtaining high elongation flange properties and excellent bendability, an area of ​​50.0 μm² is used. 2 The number density of the hard phase described above is 500 particles / mm³. 2 The following applies: Area 50.0 μm² 2 The number density of the hard phase is preferably 400 particles / mm². 2 More preferably 300 pieces / mm 2 The area is 50.0 μm². 2 There is no particular lower limit to the number density of the hard phase described above, which is 0 particles / mm². 2 That's fine.

[0072] Here, the area is 50.0 μm². 2 The number density of the hard phase described above is measured as follows, centered on the 1 / 4 thickness position of the high-strength hot-rolled steel sheet, from the 1 / 8 thickness position to the 3 / 8 thickness position.

[0073] First, in measuring the area ratio of the hard phase (fresh martensite and retained austenite) as described above, the area of ​​each region of the hard phase is measured. Then, the area is 50.0 μm². 2 By dividing the total number of hard phases by the total area of ​​the observation field, we obtain an area of ​​50.0 μm². 2 The number density of the hard phases described above will be calculated.

[0074] Area ratio of regions with a hardness of 9.0 GPa or higher: 8.0% or less From the viewpoint of ensuring elongation flangeability and bendability, the area ratio of the region with a hardness of 9.0 GPa or higher is set to 8.0% or less. Preferably, the area ratio of the region with a hardness of 9.0 GPa or higher is 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. Here, the region with a hardness of 9.0 GPa or higher is composed of fresh martensite and retained austenite that constitute the hard phase mentioned above, as well as 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 polished using 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, with 50 points × 50 points. The area ratio of the region with a hardness of 9.0 GPa or higher is calculated by dividing the total number of measurement points with a hardness of 9.0 GPa or higher by the total number of measurement points (12,500) and 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 higher The tensile strength of the high-strength hot-rolled steel sheet according to one embodiment of the present invention shall be 980 MPa or higher. Preferably, the tensile strength shall be less than 1470 MPa. Here, the tensile strength (TS) shall be 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. This plating layer is not particularly limited and may be any known plating layer, 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 this zinc plating layer contains elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn in addition to Zn and Al, as well as Mg and Si, the effects of the present invention remain unchanged. Moreover, 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 plating layers containing more than 0.30% Al, Zn-Al-Mg plating layers, Al-Zn plating layers, and Al-Si plating layers, which are produced by hot-dip plating or various electroplating methods, may also be used as the plating layer.

[0080] The thickness of the high-strength hot-rolled steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably about 1.0 mm to 10.0 mm.

[0081] [2] Member Next, a component according to one embodiment of the present invention will be described. A component according to one embodiment of the present invention is a component 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 component 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 sheet Next, a method for manufacturing a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. The method for manufacturing a high-strength hot-rolled steel sheet according to one embodiment of the present invention comprises the following steps: A heating step in which a 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 to a cooling stop temperature of Ms°C or lower, a cooling parameter of Ms°C or higher during cooling of 36.0 or lower, and an integrated tempering parameter of Ms°C or lower T1°C or higher during cooling of 25.4 or lower, and then the cooling is stopped. A reheating step in which the hot-rolled steel sheet after the first cooling step is reheated under the conditions of a time from cooling stop to reheating start of less than 60 seconds and a reheating stop temperature of 320°C or higher and 480°C or lower. The process involves a heat retention step in which the hot-rolled steel sheet, after the reheating step, is kept at a temperature range of 320°C to 480°C. A second cooling step is in which the hot-rolled steel sheet, after the heat retention step, is cooled to room temperature. The cumulative tempering parameter from the start of reheating in the reheating step until the steel sheet temperature reaches room temperature in the second cooling step is set to 24.0 to 30.0. Furthermore, 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-described 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-described 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 for steel materials: 1150℃ or higher In steel materials such as slabs that have been cooled to low temperatures, most of the elements that form carbonitrides, such as Ti, precipitate unevenly as coarse carbonitrides. The presence of these coarse and uneven precipitates leads to deterioration of various properties, such as strength and resistance to punching roughness. Therefore, 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 should be 1150°C or higher. Preferably, the heating temperature of the steel material should be 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. Therefore, preferably, the heating temperature of the steel material should be 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 shipped directly at high temperature after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The rough rolling is only required to ensure the desired sheet bar dimensions, and the conditions are not particularly limited. 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. It is preferable to perform high-pressure water descaling on the rough-rolled plate in order to remove the primary scale that has been generated before 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, 980°C or lower If the finish rolling completion temperature is below 800°C, ferrite will form before the finish rolling is completed, 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 lower bainite is formed 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 obtained by finish rolling (finished rolled sheet) 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: below Ms℃ 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 particularly preferably greater than 200°C. Here, Ms is the martensitic transformation start 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 contained. Ms(℃)=539-423×C-30.4×Mn+30.0×Al-12.1×Cr-17.7×Ni-7.5×Mo Cooling parameters 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. This increases the area ratio of fresh martensite, and the hardness of the tempered martensite and lower bainite increases, resulting in an increase in the area ratio of the region with a hardness of 9.0 GPa or higher. For this reason, the cooling parameter above Ms°C during cooling should be 36.0 or less. Preferably, the cooling parameter above Ms°C during cooling is 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 to prevent deterioration of the steel sheet shape, it is preferable that the cooling parameter above Ms°C during cooling is 32.0 or higher. Here, the cooling parameter PC is calculated 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) Here, t: time (seconds), T: steel plate temperature at time t (°C), PC=log(10^pc(0)+10^pc(0.01)+10^pc(0.02)+...)+(Ms+273) / 32 Cumulative tempering parameter during cooling (below Ms°C and above T1°C): 25.4 or less If the cumulative tempering parameter (Ms°C or T1°C) during cooling exceeds 25.4, excessive carbon diffusion from lower bainite to untransformed austenite occurs in parallel with the formation of lower bainite during cooling, resulting in the creation of coarse regions with high carbon concentration within the untransformed austenite. This results in an area of ​​50.0 μm². 2 The number density of the hard phase increases. For this reason, the cumulative tempering parameter between Ms°C and T1°C during cooling should be 25.4 or less. Preferably, the cumulative tempering parameter between Ms°C and T1°C during cooling should be 25.2 or less, more preferably 25.0 or less. The Ms temperature is the temperature defined by the above-mentioned formula for Ms. T1 is the greater of 300°C and the above-mentioned cooling stop temperature, and if the Ms temperature is less than 300°C, it is outside the scope of the present invention.

[0090] The cumulative tempering parameter for cooling, where Ms℃ is below and T1℃ is above, is calculated as PTC1 as follows. First, the steel plate temperature is measured every 0.1 seconds and interpolated to obtain temperature data every 0.01 seconds. Next, the following formula for PTC1 is calculated for each time point, and this result is used to further calculate the formula for PTC1. ptc1(t) = -18000 / (T + 273) + 48 Here, t: time (seconds), T: steel plate temperature at time t (°C) PTC1=log(10^ptc1(0)+10^ptc1(0.01)+10^ptc1(0.02)+...) [Reheating process] Next, the hot-rolled steel sheet after the first cooling process is reheated under the conditions described below. The means of 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.

[0091] Time from cooling stop to reheating start: Less than 60 seconds If the time between the cessation of cooling and the start of reheating is 60 seconds or more, excessive carbon diffusion occurs from the martensite and lower bainite formed in the first cooling step to the untransformed austenite, resulting in the formation of coarse regions with high carbon concentration within the untransformed austenite. This results in an area of ​​50.0 μm². 2 The number density of the hard phase increases. For this reason, the time from the cessation of cooling to the start of reheating should be less than 60 seconds. Preferably, the time from the cessation of cooling to the start of reheating is less than 30 seconds, more preferably less than 10 seconds, and even more preferably less than 2 seconds. The lower limit of the time from the cessation of cooling to the start of reheating is not particularly limited and may be 0 seconds.

[0092] Reheating stop temperature: 320℃ or higher, 480℃ 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 exceeds 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.

[0093] [Heat retention process] Next, the hot-rolled steel sheet after the reheating process is kept warm in a temperature range of 320°C to 480°C. The shape of the hot-rolled steel sheet during this warming period 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 warming furnace or winding it into a coil. The warming temperature may be the same as or different from the reheating stop temperature mentioned above, as long as it is within the range of 320°C to 480°C.

[0094] [Second cooling process] Next, the hot-rolled steel sheet, after the heat retention process, is cooled to room temperature. Room temperature means 50°C or below.

[0095] 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: 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 total area ratio of lower bainite and tempered martensite decreases. 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 in the second cooling process should be 24.0 or higher. Preferably, 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 25.0 or higher, more preferably 26.0 or higher, and even more preferably 26.5 or higher. On the other hand, 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 greater than 30.0, the area ratio of retained austenite decreases. 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 in the second cooling process should be 30.0 or lower. The cumulative tempering parameter from the start of reheating in the reheating process until the temperature of the hot-rolled steel sheet 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.

[0096] Here, the cumulative tempering parameter PT2, calculated as follows, is obtained from the start of reheating in the reheating process until the hot-rolled steel sheet reaches room temperature in the second cooling process. 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 further calculate the formula for PT2. pt2(t) = -18000 / (T + 273) + 48 Here, t: time (seconds), T: steel plate temperature at time t (°C) PT2=log(10^pt2(0)+10^pt2(0.01)+10^pt2(0.02)+...) The high-strength hot-rolled steel sheet of the present invention is manufactured through the above process. 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 the hot-rolled steel sheet, there are no particular limitations, but for example, temper rolling can be performed on the hot-rolled steel sheet 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 reheating process, during the heat retention process, during the second cooling process, and after the second cooling process. In addition, pickling may be performed to remove scale. When pickling is performed on the hot-rolled steel sheet, there are no particular limitations, but for example, it is preferable to perform pickling at least once on the hot-rolled steel sheet after the second cooling process.

[0097] [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 step of applying a plating treatment to the hot-rolled steel sheet. In this case, although not particularly limited, for example, the plating treatment can be applied to the hot-rolled steel sheet after the second cooling step. The plating treatment in the plating treatment step is not particularly limited, and for example, known plating treatments can be mentioned.

[0098] 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.

[0099] [4] Method of manufacturing the component A method for manufacturing the component involves applying at least one of the following processes to the high-strength hot-rolled steel sheet obtained as described above: forming or joining. Examples of forming processes include press forming and roll forming. Examples of joining processes include arc welding and spot welding. [Examples]

[0100] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0101] [Manufacturing of high-strength hot-rolled steel sheets] A 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 at 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 lists the conditions for the first cooling process, including the cooling stop temperature [°C], the cooling parameter PC of Ms°C or higher during cooling, and the cumulative tempering parameter PTC1 of Ms°C or lower but T1°C or higher during cooling. After the completion of the first cooling process, the obtained hot-rolled steel sheet was subjected to a reheating process. Table 2 shows the conditions for the reheating process, specifically the time t1 [seconds] from the end of cooling to the start of reheating, and the reheating stop temperature Th [°C]. After the reheating process was completed, the resulting hot-rolled steel sheet was subjected to a heat retention process, followed by a second cooling process. Table 2 also shows the conditions for the heat retention process and the second cooling process, specifically the cumulative tempering parameter PT2 from the start of reheating until the steel sheet temperature of the hot-rolled steel sheet reached room temperature.

[0102] In this way, a high-strength hot-rolled steel sheet was obtained. The sheet thickness was set to 3.0 mm.

[0103] [Table 1]

[0104] [Table 2]

[0105] [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, F is the area percentage of ferrite, and θ is the area percentage of cementite. %Cγ is the average amount of solid-soluble carbon in the retained austenite, and NDHP is the area of ​​50.0 μm. 2 The number density of the hard phases described above, AFH, represents the area ratio of the region with a hardness of 9.0 GPa or higher.

[0106] [Table 3]

[0107] Furthermore, tensile tests, hole expansion tests, and bending tests were performed 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.

[0108] [Table 4]

[0109] (1) Tensile test Tensile tests were conducted in accordance with JIS Z 2241. Specifically, JIS No. 5 test specimens were taken from the obtained high-strength hot-rolled steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. Tensile tests were performed on the taken test specimens 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.

[0110] A TS (Total Test) reading of 980 MPa or higher (TS ≥ 980 MPa) was considered a pass, while any other reading was considered a fail.

[0111] 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. Materials that did not meet these criteria were considered to lack high ductility.

[0112] The n-value was defined as follows: For 980MPa ≤ TS < 1180MPa, a value of 0.040 or higher (n-value ≥ 0.040) was considered to indicate excellent wrinkle suppression ability. For 1180MPa ≤ TS < 1310MPa, a value of 0.035 or higher (n-value ≥ 0.035) was considered to indicate excellent wrinkle suppression ability. For 1310MPa ≤ TS, a value of 4.0% 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.

[0113] (2) Hole expansion test The hole expansion test was conducted in accordance with JIS Z 2256. Specifically, a 100mm x 100mm test specimen was taken from the obtained high-strength hot-rolled steel sheet by shearing. A hole with a diameter of 10mm (the initial hole in the test specimen) was punched into the test specimen with a clearance of 12% ± 1%. Next, a wrinkle-holding force of 9 tons (88.26kN) was applied around the hole using a die with an inner diameter of 75mm, and a conical punch with a vertex angle of 60° was pressed into the hole. The diameter of the hole in the test specimen 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. For λ, a value of 45% or more (λ≧45%) was considered acceptable (indicating high elongation flange properties) when 980MPa≦TS<1180MPa, 40% or more (λ≧40%) when 1180MPa≦TS<1310MPa, and 35% or more (λ≧35%) when 1310MPa≦TS. Anything else was considered unacceptable. λ(%)={(D f -D0) / D0} × 100 Here, D f:Diameter of the hole in the specimen at the time of crack initiation (mm), D0:Diameter of the hole in the initial specimen (mm).

[0114] (3) Bending test The obtained high-strength hot-rolled steel sheets were subjected to shearing, and bending tests were taken from 35 mm (width) x 100 mm (length) specimens with sheared end faces, so that the longitudinal direction of the specimen was perpendicular to the rolling direction. These specimens were then 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 1180 MPa ≤ TS < 1310 MPa, a value of 3.5 or less (R / t value ≤ 3.5) was considered to have excellent bendability. For 1310 MPa ≤ TS, a value of 4.5 or less (R / t value ≤ 4.5) was considered to indicate excellent bendability. Materials that did not meet this condition were considered to lack excellent bendability.

[0115] 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 component composition is expressed in mass percent. C: 0.040% or more and 0.350% or less, Si: 0.50% or more and 2.50% or less, 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: Contains 0.0200% or less, The remainder consists of Fe and unavoidable impurities. Steel structure, Area ratio of tempered martensite: greater than 0% and less than or equal to 99.5%. Area percentage of lower bainite: greater than 0% and less than or equal to 99.5% The total area ratio of the lower bainite and the tempered martensite: 82.0% or more and 99.5% or less. Area ratio of fresh martensite: 8.0% or less (including 0%) Area percentage of retained austenite: 0.5% to 10.0% Average solid-solution carbon content in retained austenite: 0.50% to 1.10% Area 50.0μm ２ Number density of the hard phase described above: 500 particles / mm² ２ below, Area ratio of regions with a hardness of 9.0 GPa or higher: 8.0% or less. A high-strength hot-rolled steel sheet characterized by having a tensile strength of 980 MPa or more.

2. In addition to the above component composition, 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: 0.0200% 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 The high-strength hot-rolled steel sheet according to claim 1, characterized by including at least one selected from the following.

3. A high-strength hot-rolled steel sheet according to claim 1 or 2, characterized in that it has a plating layer on its surface.

4. A member characterized by being made of a high-strength hot-rolled steel sheet as described in claim 1 or 2.

5. A member characterized by being made of a high-strength hot-rolled steel sheet as described in claim 3.

6. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 1 or 2, A heating process in which the steel material is heated to over 1150°C, The steel material after the heating process is hot-rolled to produce a hot-rolled steel sheet under the condition that the finishing rolling temperature is 800°C or higher and 980°C or lower. The hot-rolled steel sheet after the hot rolling step is cooled at a cooling stop temperature of Ms°C or lower, a cooling parameter of Ms°C or higher during cooling: 36.0 or lower, and a temperature of Ms°C or lower during cooling T. １ The first cooling step involves cooling under conditions where the cumulative tempering parameter above °C is 25.4 or less, and then stopping the cooling. A reheating step is performed in which the hot-rolled steel sheet after the first cooling step is reheated under the conditions that the time from the end of cooling to the start of reheating is less than 60 seconds, and the reheating stop temperature is 320°C or higher and 480°C or lower. The hot-rolled steel sheet after the reheating process is kept at a temperature range of 320°C to 480°C in a heat retention process, The process includes a second cooling step in which the hot-rolled steel sheet after the heat retention step is cooled to room temperature, A method for manufacturing a high-strength hot-rolled steel sheet, characterized in that the cumulative tempering parameter from the start of reheating in the reheating step to the time when the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling step is 24.0 or more and 30.0 or less. Here, Ms said, Ms(℃)=539-423×C-30.4×Mn+30.0×Al-12.1×Cr-17.7×Ni-7.5×Mo It is defined by the formula, T １ is the greater of 300°C and the aforementioned cooling stop temperature, and each element symbol in the above formula represents the content (mass%) of each element, with 0 being used for elements that are not present.

7. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 6, characterized in that the hot-rolled steel sheet after the second cooling step is subjected to a plating treatment.

8. A method for manufacturing a component, characterized by 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 described in claim 6 to form a component.

9. A method for manufacturing a component, characterized by 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 described in claim 7 to form a component.

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