High-strength hot-rolled steel sheet, member, and method for manufacturing same

Abstract

The purpose of the present invention is to provide a high-strength hot-rolled steel sheet having high strength and high ductility as well as excellent buckling resistance, high stretch flangeability and excellent bendability; a member; and a method for manufacturing the same.　The high-strength hot-rolled steel sheet has a prescribed component composition and has a steel structure with, in area ratio, the area ratio of tempered martensite of more than 0% and 99.8% or less, the area ratio of lower bainite of more than 0% and 99.8% or less, the combined area ratio of the lower bainite and the tempered martensite of 86.0 to 99.8%, fresh martensite of 0 to 7.0%, retained austenite of 0.2 to 7.0%, and an average solid solution C amount in retained austenite of 0.50 to 1.10%, with a number density of hard phases having an area of 50.0 μm2 or greater of 500 / mm2 or less, a 90th percentile of a nano-hardness gradient measured at a 1.0 μm pitch of 0.80 to 3.50 GPa, and a tensile strength of 980 MPa or greater.

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, since these parts undergo burring and bending processes to ensure rigidity, the steel plates used as the material for these parts require excellent bendability. In addition, undercarriage parts and frame members undergo burring and bending processes at the ends of the parts to ensure rigidity, so the steel plates used as the material for these parts require excellent bendability. During the bending process of steel plates, buckling may occur unintentionally. This buckling is difficult to detect because the surface of the bent area is curved, and it becomes even more difficult to detect if the buckling is corrected by processing afterward. Once buckling occurs, the toughness of the area is reduced, and there is a risk that cracks may form and the part may break due to instantaneous loads during actual use or repeated loads over a long period, potentially impairing the vehicle's operation and safety. Therefore, the steel plates used as the material for undercarriage parts require excellent buckling resistance.

[0013] As described above, it is important that steel sheets, which are the raw materials for automotive parts such as suspension components and frame members, possess high strength, high ductility, high elongation flangeability, excellent bendability, and excellent buckling resistance.

[0014] 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 elongation flangeability, excellent buckling resistance, and excellent bendability.

[0015] 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 buckling resistance, and excellent bendability.

[0016] 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 elongation flangeability, excellent buckling resistance, and excellent bendability.

[0017] 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 ≥ 4.2% When 1180 MPa ≤ TS < 1310 MPa, U.El ≥ 4.0% When 1310 MPa ≤ TS, U.El ≥ 3.5% 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 ≥ 50% If 1180 MPa ≤ TS < 1310 MPa, λ ≥ 45% If 1310 MPa ≤ TS, λ ≥ 40% In order to ensure the reliability of automobile parts, such as undercarriage parts and frame members, the steel plates that make up these parts must have excellent buckling resistance. In this invention, excellent buckling resistance means that a 90° V-block bending test in accordance with JIS Z 2248 is performed using the method described below. That is, the G / r value (G / r) of the unloaded test piece satisfies the following formula under the following conditions, where r / t is obtained by dividing the bending radius r (mm) by the thickness t (mm) of the steel plate. Here, the G / r value (G / r) is the value obtained by dividing the distance G (mm) between the bottom of the bend of the test piece and the line segment with length r / 2 (mm) perpendicular to the pressing direction of the press fitting, when the line segment is inscribed inside the bend of a cross section perpendicular to the width direction of the test piece, by the bending radius r (mm). When 980 MPa ≤ TS < 1180 MPa, G / r ≤ 0.100 is satisfied under the condition r / t = 2.0 ± 0.2. When 1180 MPa ≤ TS < 1310 MPa, G / r ≤ 0.100 is satisfied under the condition r / t = 3.5 ± 0.2. When 1310 MPa ≤ TS, G / r ≤ 0.100 is satisfied under the condition r / t = 4.5 ± 0.2.

[0018] Automotive parts, such as suspension components and frame members, undergo 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 90° V-block bending test is performed in accordance with JIS Z 2248 using the method described below, the R / t value satisfies the following equation. 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 R is the thickness t (mm) of the steel plate. If 980 MPa ≤ TS < 1180 MPa, R / t ≤ 2.0 If 1180 MPa ≤ TS < 1310 MPa, R / t ≤ 3.5 If 1310 MPa ≤ TS, 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 99.8%, and the area ratio of lower bainite was greater than 0% and less than or equal to 99.8%. Furthermore, the total area ratio of the lower bainite and tempered martensite was set to 86.0% or more and less than or equal to 99.8%. In addition, the area ratio of fresh martensite was set to 7.0% or less (including 0%), the area ratio of retained austenite was set to 0.2% or more and less than or equal to 7.0%, 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 described above is 500 particles / mm². ２ The following criteria were used: the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch was set to be between 0.80 GPa and 3.50 GPa, and the tensile strength was set to be 980 MPa or higher. This revealed that a high-strength hot-rolled steel sheet could be obtained that maintained high strength while also possessing high ductility, high elongation flangeability, excellent buckling resistance, 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%. Area ratio of lower bainite: 8% or less, area ratio of lower bainite: greater than 0% and 99.8% or less, total area ratio of lower bainite and tempered martensite: 86.0% or more and 99.8% or less, area ratio of fresh martensite: 7.0% or less (including 0%), area ratio of retained austenite: 0.2% or more and 7.0% or less, average solid solution carbon content in retained austenite: 0.50% or more and 1.10% or less, area 50.0 μm ２ Number density of the hard phase described above: 500 particles / mm² ２The following describes a high-strength hot-rolled steel sheet having a 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch of 0.80 GPa or more and 3.50 GPa or less, 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 at a finish rolling completion temperature of 800°C or higher and 980°C or lower to obtain a hot-rolled steel sheet; and a cooling stop temperature of Ms°C or lower and a cooling temperature of Ms°C or lower for the hot-rolled steel sheet after the hot-rolling step. １Accumulated tempering parameter above ℃: Cooling is carried out under the condition of 25.4 or less, and cooling is stopped, the first cooling step; the hot-rolled steel sheet after the first cooling step is reheated under the conditions of time from the stop of cooling to the start of reheating: less than 60 seconds, reheating stop temperature: 320℃ or more and 480℃ or less, the reheating step; the hot-rolled steel sheet after the reheating step is heat-preserved in a temperature range of heat-preserving temperature: 320℃ or more and 480℃ or less, the heat-preserving step; and the hot-rolled steel sheet after the heat-preserving step is cooled to room temperature in the second cooling step, and the accumulated tempering parameter PT2 from the start of reheating in the reheating step to the steel sheet temperature of the hot-rolled steel sheet reaching room temperature in the second cooling step is 24.0 or more and 30.0 or less, and the cooling stop temperature of the first cooling step is Tq, and PT2 satisfies 22.0 ≦ PT2 + 0.00717×(Ms - Tq) - Mn×Si^0.5 ≦ 29.0, a method for manufacturing a high-strength hot-rolled steel sheet. Here, T １ is the higher one of 300℃ and the cooling stop temperature of the first cooling step, and Ms is Ms(℃) = 539 - 423×C - 30.4×Mn + 30.0×Al - 12.1×Cr - 17.7×Ni - 7.5×Mo, and each element symbol in the formula represents the content (mass%) of each element, and in the case of an element not contained, it is taken as 0. [6] The method for manufacturing a high-strength hot-rolled steel sheet according to [5], wherein plating treatment is performed on the hot-rolled steel sheet after the second cooling step. [7] A method for manufacturing a member, which has a step of subjecting at least one of forming processing or joining processing to the high-strength hot-rolled steel sheet manufactured by the method for manufacturing a high-strength hot-rolled steel sheet according to [5] or [6] to form a member.

[0023] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having high strength, high ductility, high elongation flange property, excellent buckling resistance, and excellent bending property.

[0024] FIG. 1 is a schematic view of the buckling resistance evaluation method in the present invention.

[0025] Hereinafter, embodiments of the high-strength hot-rolled steel sheet, member, and their manufacturing methods of the present invention will be described. However, the present invention is not limited to the following embodiments.

[0026] [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 "%".

[0027] 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 fresh martensite and the area of ​​50.0 μm² are affected. 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.

[0028] 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. In addition, Si improves the strength-ductility balance of the lower bainite and tempered martensite after the 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.

[0029] 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-solution carbon content in retained austenite within a 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 is 5.00% or more, 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%.

[0030] P: 0.100% or less. P is an element that dissolves and contributes to an increase in the strength of steel. However, P is also an element that segregates at the austenite grain boundaries during hot rolling, causing slab cracking during hot rolling. Further, it segregates at the grain boundaries and reduces ductility. Therefore, it is preferable to make the P content as low as possible, but the inclusion of P up to 0.100% is acceptable. Accordingly, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content is not particularly limited, but from the viewpoint of productivity, etc., the P content is preferably 0.001% or more.

[0031] S: 0.0200% or less. S combines with Ti or Mn to form coarse sulfides, which accelerate the generation of voids, thereby reducing ductility, elongation flangeability, and bendability. Therefore, it is preferable to make the S content as low as possible, but the inclusion of S up to 0.0200% is acceptable. Accordingly, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. The lower limit of the S content is not particularly limited, but from the viewpoint of productivity, etc., the S content is preferably 0.0001% or more.

[0032] Al: 0.010% or more and 2.000% or less. Al acts as a deoxidizer and is an element effective in improving the cleanliness of steel. If the Al content is less than 0.010%, the effect is not sufficient, so the Al content is 0.010% or more. Also, like Si, Al leaves austenite and contributes to an improvement in ductility. Also, like Si, it improves the strength-ductility balance of bainite and martensite after passing through an appropriate heat retention process. On the other hand, excessive inclusion of Al leads to an increase in oxide-based inclusions and reduces ductility, elongation flangeability, wrinkle suppression ability, and bendability. Therefore, the Al content is set to 2.000% or less. Also, the Al content is preferably 1.000% or less.

[0033] 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, elongation flangeability, 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.

[0034] A high-strength steel sheet according to one embodiment of the present invention has a component 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 component composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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 such effects, when V is included, it is preferable to have a V content of 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 V-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, 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.

[0039] Cr: 1.00% or less. Like Mn, Cr inhibits the formation of ferrite and contributes to the formation of bainite and martensite. To obtain such effects, 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.

[0040] Mo: 0.500% or less Mo increases the tempering softening resistance of steel and contributes to improving the strength of steel sheets. In addition, 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, it is preferable to have a Mo content of 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, which may worsen the stretch flangeability and bendability. Therefore, when Mo is included, the Mo content should be 0.500% or less.

[0041] B: 0.0100% or less. B is an element that contributes to the formation of lower bainite and tempered martensite by segregating at the 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 becomes saturated. Therefore, when B is included, the B content should be 0.0100% or less.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 content is below the preferred lower limit, it will not impair the effects of the present invention, and therefore they will be included as unavoidable impurities. Other unavoidable impurities include, for example, O and H. The content of O and H is preferably 0.01000% or less, each.

[0055] 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.

[0056] Area ratio of tempered martensite: greater than 0% and less than or equal to 99.8%. 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. The promotion of lower bainite formation by martensite results in an area of ​​50.0 μm. ２The number density of the hard phase and the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch can be brought within the desired range. Tempered martensite, together with lower bainite, is a useful phase from the viewpoint of obtaining high ductility, high elongation flangeability, excellent buckling resistance, and excellent bendability while maintaining high strength of 980 MPa or more in tensile strength. 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 99.8%, it becomes impossible to set the area ratio of retained austenite to 0.2% or more. Therefore, the area ratio of tempered martensite is set to 99.8% 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 99.8%, more preferably 99.0% or less, even more preferably 95.0% or less, particularly preferably 90.0% or less, and most preferably less than 85.0%.

[0057] Lower bainite area ratio: greater than 0% and less than or equal to 99.8% 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 buckling resistance, 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 99.8%, it becomes impossible to set the area ratio of retained austenite to 0.2% or more. For this reason, the area ratio of lower bainite is set to 99.8% or less. The lower limit of the area ratio of lower bainite is preferably 0.1% or more, more preferably 3.0% 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 99.8%, more preferably 98.0% or less, even more preferably 90.0% or less, and most preferably less than 85.0%.

[0058] Total area ratio of lower bainite and tempered martensite: 86.0% or more and 99.8% or less From the viewpoint of obtaining high ductility, high elongation flangeability, excellent buckling resistance, 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 86.0% 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 99.8%, the area ratio of retained austenite cannot be set to 0.2% or more. For this reason, the total area ratio of lower bainite and tempered martensite is set to 99.8% or less. The total area ratio of lower bainite and tempered martensite is preferably 99.5% or less, more preferably 99.0% or less, and even more preferably 98.0% or less.

[0059] Area ratio of fresh martensite: 7.0% or less (including 0%) Fresh martensite is hard and becomes a void formation site during molding, thus reducing stretch flangeability, buckling resistance, and bendability. Therefore, the area ratio of fresh martensite should be 7.0% 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%.

[0060] Area ratio of retained austenite: 0.2% or more and 7.0% or less. Retained austenite contributes to high ductility and excellent wrinkle suppression. Therefore, the area ratio of retained austenite should be 0.2% or more. Preferably, the area ratio of retained austenite is 0.5% or more, more preferably 1.0% or more, and even more preferably 2.0% or more. On the other hand, if the area ratio of retained austenite exceeds 7.0%, the stretch flangeability and bendability decrease. Therefore, the area ratio of retained austenite should be 7.0% or less. Preferably, the area ratio of retained austenite is 6.0% or less, more preferably 5.5% or less, and even more preferably 5.0% or less.

[0061] 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).

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] Fresh martensite and retained austenite have the same contrast in SEM and are difficult to distinguish. Therefore, in the observation using SEM, the area ratio is calculated as the hard phase without distinguishing fresh martensite and retained austenite. Further, the area ratio of retained austenite is determined by X-ray diffraction method, and the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite described below from the area ratio of the hard phase calculated from the SEM image.

[0069] The area ratio of retained austenite is measured as follows. That is, after the hot-rolled steel sheet is machined and ground to the 1 / 4 position of the sheet thickness in the sheet thickness direction (depth direction), chemical polishing with oxalic acid is performed on a sheet thickness of 100 μm or more to obtain an observation surface. Then, the observation surface is observed by X-ray diffraction method. CoKα ray is used as the incident X-ray. Then, 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 obtained, and the volume ratio 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 ratio of retained austenite is taken as the area ratio of retained austenite.

[0070] Average carbon content in solid solution in retained austenite: 0.50% or more and 1.10% or less From the viewpoint of obtaining high ductility, the average carbon content in solid solution in retained austenite is 0.50% or more. The average carbon content in solid solution in retained austenite is preferably 0.55% or more, more preferably 0.60% or more. On the other hand, when the average carbon content in solid solution in retained austenite exceeds 1.10%, the ductility rather decreases. Therefore, the average carbon content in solid solution in retained austenite is 1.10% or less. The average carbon content in solid solution in retained austenite is preferably 1.05% or less, more preferably 1.00% or less.

[0071] The average carbon content in solid solution in retained austenite is measured as follows. That is, the observation surface on which the area ratio 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) 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 %).

[0072] Area 50.0μm ２ Number density of the hard phase described above: 500 particles / mm² ２ In the present invention, the hard phase refers to a phase consisting of fresh martensite and retained austenite. The hard phase does not contain Fe-based carbides compared to lower bainite and / or tempered martensite. Furthermore, fresh martensite and retained austenite have brighter contrast in SEM images compared to upper bainite, lower bainite and / or tempered martensite, polygonal ferrite, and acicular ferrite. For this reason, the hard phase can be distinguished from these structures using SEM. From the viewpoint of obtaining high elongation flangeability and excellent bendability, the area is 50.0 μm². 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.

[0073] Here, the area is 50.0 μm². ２ The number density of the hard phase described above 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 around the position at 1 / 4 of the plate thickness of the high-strength hot-rolled steel sheet.

[0074] 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 μm. 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.

[0075] 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch: 0.80 GPa or higher and 3.50 GPa or lower. From the viewpoint of ensuring excellent buckling resistance, the steel structure of steel plates requires hardness irregularities on the order of μm. Generally, steel plates have hardness irregularities on the order of tens of μm or more due to irregularities in the component concentration during casting. Therefore, if the hardness irregularities on the order of μm are insufficient, there will be areas of tens of μm or more that are soft and easily deformable, reducing buckling resistance. In other words, the presence of hardness irregularities on the order of μm disperses strain during deformation of the steel plate, improving buckling resistance. This hardness irregularity on the order of μm can be expressed by the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch. From the viewpoint of ensuring excellent buckling resistance, the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch should be 0.80 GPa or higher. The 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch is preferably 1.00 GPa or higher, more preferably 1.20 GPa or higher, and even more preferably 1.40 GPa or higher. On the other hand, if the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch exceeds 3.50 GPa, voids are more likely to occur during deformation of the steel sheet, and the stretch flangeability and bendability decrease. For this reason, the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch should be 3.50 GPa or lower. The 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch is preferably less than 3.30 GPa, more preferably less than 3.10 GPa, and even more preferably less than 2.90 GPa.

[0076] Here, the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch 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.

[0077] 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 1.0 μm, with 50 points x 50 points. The gradient of nanohardness measured at a 1.0 μm pitch is then determined as the difference in hardness between measurement points with a distance of 1.0 μm between them. The 90th percentile of the gradient of nanohardness measured at a 1.0 μm pitch at a total of 24010 locations (49 locations x 49 locations x 2 directions x 5 measurement ranges) is calculated and set as the 90th percentile of the gradient of nanohardness measured at a 1.0 μm pitch. Here, the 90th percentile is the 21,609th smallest value when all 24,010 values ​​are arranged in ascending order according to JIS Z 8101.

[0078] Next, the mechanical properties of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0079] 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.

[0080] 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 may be any known plating layer, but a zinc plating layer is preferred, for example. 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-Zn-Si-Zn-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.

[0081] 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.

[0082] [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 buckling resistance 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.

[0083] [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. The hot-rolled steel sheet after the hot-rolling step is cooled to a cooling stop temperature of Ms°C or lower and a cooling temperature of Ms°C or lower. １The process consists of: a first cooling step where the hot-rolled steel sheet is cooled under the condition that the cumulative tempering parameter PT is 25.4 or less above ℃, and then the cooling is stopped. A reheating step where the hot-rolled steel sheet after the first cooling step is reheated under the condition 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℃ or higher and 480℃ or lower. A heat retention step where the hot-rolled steel sheet after the reheating step is kept warm in the heat retention temperature range of 320℃ or higher and 480℃ or lower. A second cooling step where the hot-rolled steel sheet after the heat retention step is cooled to room temperature. Furthermore, the cumulative tempering parameter PT2 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, and the cooling stop temperature of the first cooling step is Tq, and PT2 satisfies the relationship 22.0 ≤ PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 ≤ 29.0. Furthermore, the process may optionally include a plating step in which the manufactured high-strength hot-rolled steel sheet is plated.

[0084] Unless otherwise specified, the temperatures mentioned above refer to the surface temperature of the steel material and steel plate.

[0085] [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.

[0086] [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 resistance to punching roughness. 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.

[0087] [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.

[0088] 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.

[0089] [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").

[0090] 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 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 (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo Cooling Ms°C or lower T １ Cumulative tempering parameters above °C: 25.4 or less during cooling, Ms°C or less T １When the cumulative tempering parameter above °C exceeds 25.4, excessive carbon diffusion from lower bainite to untransformed austenite occurs during cooling, in parallel with the formation of lower bainite, 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. Therefore, during cooling, the temperature drops below Ms°C. １ The cumulative tempering parameter above ℃ shall be 25.4 or less. During cooling, Ms ℃ or less T １ The cumulative tempering parameter above °C is preferably 25.2 or less, more preferably 25.0 or less. The Ms temperature is the temperature defined by the above-mentioned Ms formula. １ The temperature 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.

[0091] During cooling, below Ms℃T １ The cumulative tempering parameter for temperatures above °C is calculated as PTC1 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 ptc1 is calculated for each time point. Next, this result is used to further calculate the formula for PTC1. ptc1(t) = -18000 / (T + 273) + 48 where t: time (seconds), T: steel sheet 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 plate 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 plates.

[0092] Time from cooling stop to reheating start: Less than 60 seconds. If the time from cooling stop to reheating start 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. ２The 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.

[0093] 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.

[0094] [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.

[0095] [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.

[0096] The cumulative tempering parameter PT2 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 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 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 reaches room temperature in the second cooling process is greater than 30.0, the area ratio of retained austenite decreases. Therefore, the cumulative tempering parameter from the start of reheating in the reheating process until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling process is set to 30.0 or less. The cumulative tempering parameter from the start of reheating in the reheating process until the steel sheet 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.

[0097] 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 where t: time (seconds), T: steel plate temperature at time t (°C) PT2 = log(10^pt2(0) + 10^pt2(0.01) + 10^pt2(0.02) + ...) 22.0 ≤ PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 ≤ 29.0 If the difference between the cooling stop temperature Tq and Ms is small in the first cooling process, and PT2 is small in the reheating process, heat retention process, and second cooling process, the formation form of martensite and lower bainite is not appropriate. As a result, the hardness of the partially tempered martensite and lower bainite increases. This increases the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch. Therefore, PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 should be 22.0 or greater. That is, 22.0 ≤ PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5. PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 is preferably 22.5 or greater, more preferably 23.0 or greater, and even more preferably 23.5 or greater. On the other hand, if the difference between the cooling stop temperature Tq and Ms is large in the first cooling step, and PT2 is large in the reheating step, heat retention step, and second cooling step, the irregularity of the hardness of the tempered martensite and lower bainite decreases. As a result, the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch decreases. Therefore, PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 should be 29.0 or less. That is, PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 ≤ 29.0. PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 is preferably 28.5 or less, more preferably 28.0 or less, and even more preferably 27.5 or less.

[0098] 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.

[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, it is preferable to apply the plating process at least once to the hot-rolled steel sheet after the second cooling process. The plating process in the plating process is not particularly limited, and examples include known plating processes.

[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. The steel material after the heating process was rough-rolled to obtain a rough-rolled sheet. The surface of the obtained rough-rolled sheet was subjected to high-pressure water descaling with an impact pressure of 10.0 MPa. The rough-rolled sheet that had been subjected to high-pressure water descaling was then subjected to finish rolling at the finish rolling completion temperature [°C] shown in Table 2 to obtain a hot-rolled steel sheet. 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 temperature (Ms°C or less T) during cooling. １ The cumulative tempering parameter PTC1 for temperatures above °C is listed. After the first cooling process, the obtained hot-rolled steel sheet was subjected to a reheating process. Table 2 lists the conditions for the reheating process, including the time [seconds] from the end of cooling to the start of reheating, and the reheating stop temperature [°C]. After the reheating process, the obtained hot-rolled steel sheet was subjected to a heat retention process, followed by a second cooling process. Table 2 lists the conditions for the first cooling process, reheating process, heat retention process, and second cooling process as PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 with Eq. In addition, the cumulative tempering parameter PT2 from the start of reheating until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature is listed as a condition for the heat retention process and the second cooling process.

[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 lower bainite, TM is tempered martensite, FM is fresh martensite, γ is retained austenite, UB is upper bainite, P is pearlite, F is ferrite, and θ is the area percentage of cementite. %Cγ is the average amount of dissolved carbon in retained austenite, and NDHP is the area of ​​50.0 μm².２ The number density of the hard phase described above, DH90, is the 90th percentile of the nanohardness gradient measured at a 1.0 μm pitch.

[0108]

[0109] Furthermore, tensile tests, hole expansion tests, and 90° V-block bending tests were performed according to the following procedure, and the tensile strength (TS), uniform elongation (U.El), critical hole expansion ratio (λ), R / t value (R / t), and G / r value (G / r) 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, 4.2% or more (U.El ≥ 4.2%) was considered to have high ductility. For 1180 MPa ≤ TS < 1310 MPa, 4.0% or more (U.El ≥ 4.0%) was considered to have high ductility. For 1310 MPa ≤ TS, 3.5% or more (U.El ≥ 3.5%) was considered to have high ductility. Anything that did not meet these criteria was considered to lack high ductility.

[0114] (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. λ was considered to have high elongation flange properties if it was 50% or more (λ≧50%) when 980MPa ≤ TS < 1180MPa, 45% or more (λ≧45%) when 1180MPa ≤ TS < 1310MPa, and 40% or more (λ≧40%) when 1310MPa ≤ TS. Any other values ​​were considered to lack high elongation flange properties. 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.

[0115] (3) 90° V-block 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.

[0116] Furthermore, the following conditions were set for r / t, which is obtained by dividing the bending radius r (mm) by the thickness t (mm) of the steel plate: When 980 MPa ≤ TS < 1180 MPa, the condition was r / t = 2.0 ± 0.2. When 1180 MPa ≤ TS < 1310 MPa, the condition was r / t = 3.5 ± 0.2. Also, when 1310 MPa ≤ TS, the condition was r / t = 4.5 ± 0.2. The buckling resistance of the hot-rolled steel plate was evaluated for the unloaded test specimen as follows: As shown in Figure 1, when a line segment with length r / 2 (mm) perpendicular to the pressing direction of the press fitting was inscribed inside the bend of a cross section perpendicular to the width direction of the test specimen, the distance G (mm) between the bending bottom of the test specimen and the line segment was divided by the bending radius r (mm) to obtain the G / r value (G / r). The results are shown in Table 4. A G / r value of 0.100 or less (G / r ≤ 0.100) was considered to indicate excellent buckling resistance, while any other value was considered to indicate poor buckling resistance.

[0117] In the examples of the present invention, high-strength hot-rolled steel sheets are obtained that are high in strength, have high ductility, excellent buckling resistance, 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% 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: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is as follows: Area ratio of tempered martensite: more than 0% and 99.8% or less, Area ratio of lower bainite: more than 0% and 99.8% or less, Total area ratio of lower bainite and tempered martensite: 86.0% or more and 99.8% or less, Area ratio of fresh martensite: 7.0% or less (including 0%), Area ratio of retained austenite: 0.2% or more and 7.0% or less. Average solid-solution carbon content in retained austenite: 0.50% to 1.10%, area 50.0 μm² ２ Number density of the hard phase described above: 500 particles / mm² ２ The following describes a high-strength hot-rolled steel sheet having a nanohardness gradient measured at a 1.0 μm pitch with a 90th percentile of 0.80 GPa to 3.50 GPa, 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 at a finish rolling completion temperature of 800°C or higher and 980°C or lower to obtain a hot-rolled steel sheet; and cooling the hot-rolled steel sheet after the hot-rolling step at a cooling stop temperature of Ms°C or lower and a cooling temperature of Ms°C or lower. １ The process comprises: a first cooling step in which the hot-rolled steel sheet is cooled and stopped at a cumulative tempering parameter of 25.4 or less above °C; a reheating step in which the hot-rolled steel sheet after the first cooling step is reheated under the conditions of a time from the stop of cooling to the start of reheating of less than 60 seconds, 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 PT2 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 or higher and 30.0 or lower, and the cooling stop temperature of the first cooling step is Tq, and the PT2 is A method for manufacturing high-strength hot-rolled steel sheets that satisfies the relationship 22.0 ≤ PT2 + 0.00717 × (Ms - Tq) - Mn × Si^0.5 ≤ 29.

0. Here, T １ is the higher of 300°C and the cooling stop temperature of the first cooling step, and Ms is given by 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 above 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 second cooling 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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