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

Abstract

Provided are: a high-strength hot-rolled steel sheet having high strength and high ductility, as well as excellent wrinkle-suppressing ability, high stretch-flangeability, and excellent surplus deformability; a member; and methods for producing said steel sheet and member.　The high-strength hot-rolled steel sheet has a prescribed component composition, and has a steel structure comprising, in terms of area ratio: more than 0% but no more than 98.5% of tempered martensite, and more than 0% but no more than 98.5% of lower bainite, wherein the total area ratio of the lower bainite and tempered martensite is 80.0-98.5%; 0-9.0% of fresh martensite; and 1.5-15.0% of retained austenite. In the steel structure, the average solid-solution carbon content in the retained austenite is 0.50-1.10%, the number density of hard phases having an area of 50.0 μm2 or more is 500 / mm2 or less, and the solid-solution carbon content distribution balance of the retained austenite is 1.30 or more. The steel sheet has a tensile strength of 980 MPa or more.

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 increased vehicle weight due to the large-capacity batteries, require correspondingly higher strength in automobile components. Consequently, there is a growing need for higher strength in hot-rolled steel sheets, which are primarily used in automobile components such as suspension parts and frame members.

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

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

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

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

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

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

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

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

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

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

[0013] Furthermore, even if defects such as cracks occur in the component due to instantaneous loads or prolonged repeated loads, it is necessary that the component has the ability to absorb energy from deformation so that fatal failure does not occur instantaneously while the vehicle is in motion. For this reason, the steel plate that forms the component must have excellent deformability after being formed into a component (hereinafter referred to as excess deformation capacity).

[0014] 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 stretchability, excellent wrinkle suppression ability, and excellent excess deformation ability.

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

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

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

[0018] Here, high ductility is required to form the undercarriage parts. In this invention, high ductility means that the uniform elongation (total elongation at maximum test force; hereinafter also referred to as U.El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following equation: When 980 MPa ≤ TS < 1180 MPa, U.El ≥ 6.0% When 1180 MPa ≤ TS < 1310 MPa, U.El ≥ 5.5% When 1310 MPa ≤ TS, U.El ≥ 5.0% Since the undercarriage parts are stretched flanged after punching, high stretch flange properties are required. In this invention, high stretch flange properties mean that the average limiting hole expansion ratio (λ) measured in a hole expansion test in accordance with JIS Z 2256 using three test pieces satisfies the following equation. In particular, since the undercarriage parts are stretched flanged after punching, high stretch flange properties are required. If 980 MPa ≤ TS < 1180 MPa, λ ≥ 35% If 1180 MPa ≤ TS < 1310 MPa, λ ≥ 30% If 1310 MPa ≤ TS, λ ≥ 25% Steel sheets used as materials for automobile parts, such as undercarriage components and frame members, require excellent wrinkle suppression to ensure the reliability of the parts. In the present invention, excellent wrinkle suppression means that the work hardening index (hereinafter also referred to as the n-value), measured in accordance with JIS Z 2253:2020, satisfies the following formula when the n-value is measured within a strain range of 3.5% to 4.5%. If 980 MPa ≤ TS < 1180 MPa, the n value ≥ 0.050 If 1180 MPa ≤ TS < 1310 MPa, the n value ≥ 0.040 If 1310 MPa ≤ TS, the n value ≥ 0.035 Steel plates, which are the material for automobile parts such as suspension components and frame members, require excellent excess deformation capacity. In the present invention, excellent excess deformation capacity means that in a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241, the tensile strength (TS) and the nominal stress S1 at strain U. El + 3.0% satisfy the following formula. If 980 MPa ≤ TS < 1180 MPa, then TS - S1 ≤ 100 MPa. If 1180 MPa ≤ TS < 1310 MPa, then TS - S1 ≤ 120 MPa. If 1310 MPa ≤ TS, then TS - S1 ≤ 150 MPa.

[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 98.5%, and the area ratio of lower bainite was greater than 0% and less than or equal to 98.5%. Furthermore, the total area ratio of the lower bainite and tempered martensite was set to 80.0% or more and less than or equal to 98.5%. In addition, the area ratio of fresh martensite was set to 9.0% or less (including 0%), the area ratio of retained austenite was set to 1.5% or more and less than or equal to 15.0%, 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 conditions were met: The solid solution carbon content distribution balance of the retained austenite obtained from the following BCA formula was set to 1.30 or less, and the tensile strength was set to 980 MPa or more. Here, BCA = (θH - θT)^(1.10 - %Cγ) + (θT - θL)^(%Cγ - 0.50) θT (degrees): The angle showing the maximum intensity for the diffraction peak of the (220) plane of retained austenite. θL (degrees): The low-angle side angle showing half the intensity value of the maximum intensity. θH (degrees): The high-angle side angle showing half the intensity value of the maximum intensity. %Cγ (%): The average solid solution carbon content in the retained austenite.

[0021] This revealed that a high-strength hot-rolled steel sheet can be obtained that maintains high strength while also possessing high ductility, high elongation flangeability, excellent wrinkle suppression ability, and excellent excess deformation ability.

[0022] This invention was completed based on the above findings and further considerations.

[0023] 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 98.5% Area ratio of lower bainite: greater than 0% and less than or equal to 98.5%, total area ratio of the lower bainite and tempered martensite: 80.0% or more and less than or equal to 98.5%, area ratio of fresh martensite: 9.0% or less (including 0%), area ratio of retained austenite: 1.5% or more and less than or equal to 15.0%, average solid solution carbon content in retained austenite: 0.50% or more and less than or equal to 1.10%, 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 solid solution carbon balance of retained austenite obtained from the following BCA formula: BCA = (θH - θT)^(1.10 - %Cγ) + (θT - θL)^(%Cγ - 0.50) θT (degrees): angle showing maximum intensity for the diffraction peak of the (220) plane of retained austenite, θL (degrees): angle on the lower side showing half the intensity value of the maximum intensity, θH (degrees): angle on the higher side showing half the intensity value of the maximum intensity, %Cγ (%): average solid solution carbon content in retained austenite, and 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: 0.02 [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 performed under the condition of 25.4 or less, a first cooling step; after the first cooling step, the hot-rolled steel sheet is reheated under the conditions that the time from the cooling stop to the start of reheating is less than 60 seconds, the reheating stop temperature Th is 320 °C or higher and 480 °C or lower, a reheating step; after the reheating step, the hot-rolled steel sheet is heat-insulated in a temperature range where the heat-insulating temperature is 320 °C or higher and 480 °C or lower, a heat-insulating step; and a second cooling step of cooling the hot-rolled steel sheet after the heat-insulating step to room temperature, and the accumulated tempering parameter from the start of reheating in the reheating step to when 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 accumulated tempering parameter at Th - 20 °C or lower in the heat-insulating step and the second cooling step is 20.0 or higher and 29.0 or lower. A method for manufacturing a high-strength hot-rolled steel sheet. Here, T １ is the larger of 300 °C and the cooling stop temperature of the first cooling step, 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, and each element symbol in the formula represents the content (% by 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 forming at least one of a forming process or a joining process on a 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.

[0024] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having high strength, high ductility, high elongation flangeability, excellent wrinkle suppression ability, and excellent excess deformation ability.

[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 the high-strength hot-rolled steel sheet according to an embodiment of the present invention will be described. In the component composition, the unit is "mass %" in all cases, but hereinafter, unless otherwise specified, it is simply indicated by "%".

[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 of ​​50.0 μm² ２ The number density of the hard phase increases, reducing tensile flangeability and excess deformation capacity. 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 cementite precipitation in processes from the first cooling process, including the first cooling process. As a result, C is distributed to the untransformed austenite, and after cooling to room temperature in the second cooling process, a portion of the untransformed austenite becomes retained austenite, contributing to improved ductility and wrinkle suppression. In addition, Si improves the strength-ductility balance of the lower bainite and tempered martensite after the 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: More than 1.50% and less than 5.00% Mn stabilizes austenite, suppresses the formation of ferrite, and contributes to the formation of lower bainite, tempered martensite, and retained austenite. Also, by stabilizing austenite, Mn suppresses the transformation of untransformed austenite into lower bainite, carbides, and pearlite during the holding process, and contributes to keeping the average amount of dissolved C in retained austenite within a desired range. To obtain such an effect, it is necessary to set the Mn content to 1.50% or more. Therefore, the Mn content is set to 1.50% or more. The Mn content is preferably 1.70% or more, more preferably 2.00% or more. On the other hand, when the Mn content becomes 5.00% or more, the arrest of bainite transformation tends to occur, fresh martensite increases, and the elongation flange property and excess deformation ability decrease. Therefore, the Mn content is less than 5.00%. The Mn content is preferably less than 4.00%, more preferably less than 3.50%, still 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 and causes slab cracking during hot rolling. Also, 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. Therefore, the P content is 0.100% or less. The P content is preferably 0.030% or less. Although the lower limit of the P content is not particularly limited, from the viewpoint of productivity and the like, 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 and reduce ductility, elongation flange property, wrinkle suppression ability, and excess deformation ability. Therefore, it is preferable to make the S content as low as possible, but the inclusion of S up to 0.0200% is acceptable. Therefore, the S content is 0.0200% or less. The S content is preferably 0.0080% or less. Although the lower limit of the S content is not particularly limited, from the viewpoint of productivity and the like, the S content is preferably 0.0001% or more.

[0032] Al: 0.010% to 2.000% Al acts as a deoxidizing agent and is an effective element for improving the cleanliness of steel. Since the effect is insufficient if the Al content is less than 0.010%, the Al content should be 0.010% or more. In addition, like Si, Al retains austenite and contributes to improved ductility. Also, like Si, it improves the strength-ductility balance of bainite and martensite after appropriate heat retention processes. On the other hand, excessive Al content leads to an increase in oxide inclusions, reducing ductility, stretch flangeability, wrinkle suppression ability, and excess deformation ability. Therefore, the Al content should be 2.000% or less. Furthermore, 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, stretch flangeability, wrinkle suppression ability, and excess deformation ability. 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 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.

[0035] The basic component composition of a high-strength hot-rolled 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. The Ti content is more preferably 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 are generated, which may actually reduce the stretch flangeability and excess deformation capacity. 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, it is preferable that the Nb content be 0.005% or more. More preferably, the Nb content is 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 ductility, ductility, and excess deformation capacity. In addition, fresh martensite may increase excessively, which may reduce ductility, ductility, ductility, and excess deformation capacity. 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 that the V content be 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 ductility, ductility, and excess deformation capacity. In addition, fresh martensite may increase excessively, which may reduce ductility, ductility, ductility, and excess deformation capacity. 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 excess deformation capacity. 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 the formation of ferrite. To obtain this effect, if B is included, it is preferable that the B content be 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, the above effect becomes saturated. Therefore, if B is included, the B content should be 0.0100% or less. The B content is preferably 0.0050% 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 decrease in the surface properties of the hot-rolled steel sheet. Therefore, when Cu is included, the Cu content should be 1.00% or less. Preferably, the Cu content is 0.50% 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 excess deformation capacity 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 decreases, which may cause slab cracking and hot rolling cracking. Therefore, when Sb is added, the Sb content should be 0.200% or less. The Sb content is preferably 0.050% 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 decreases, which may cause slab cracking and hot rolling cracking. Therefore, when Sn is included, the Sn content should be 0.200% or less. The Sn content is preferably 0.050% 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, generating composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This improves the strength of the steel. To obtain such effects, when Ta is included, it is preferable to have a Ta content of 0.001% or more. 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 tensile flangeability and excess deformation capacity. Therefore, when Ta is included, the Ta content should be 0.100% or less. The Ta content is preferably 0.050% 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, if W is included, it is preferable that the W content be 0.001% or more. 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 stretch flangeability and excess deformation capacity. Therefore, if W is included, it is preferable that the W content be 0.500% or less. More preferably, the W content is 0.100% or less.

[0048] Mg: 0.0200% or less. Mg controls the shape of oxide and sulfide-based inclusions, contributing to further improvement of stretch flange properties. To obtain such 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. Therefore, when Mg is added, the Mg content should be 0.0200% or less. Preferably, the Mg content is 0.0100% or less.

[0049] Zn: 0.0200% or less. Zn contributes to further improvement of stretch flange properties by spheroidizing the shape of inclusions. To obtain this effect, 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. Therefore, when Zn is included, the Zn content should be 0.0200% or less. The Zn content is preferably 0.0100% or less.

[0050] Co: 0.0200% or less. Like Zn, Co contributes to further improvement of stretch flange properties by spheroidizing the shape of inclusions. To obtain this effect, 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. 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 by spheroidizing the shape of inclusions. To obtain such an effect, 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. Therefore, when Zr is added, the Zr content should be 0.0200% or less. The Zr content is preferably 0.0100% or less.

[0052] Ca: 0.0200% or less. Ca controls the shape of oxide and sulfide-based inclusions, contributing to further improvement of stretch flange properties. To obtain such effects, it is preferable that the Ca content be 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. Therefore, when Ca is added, the Ca content should be 0.0200% or less. Preferably, the Ca content is 0.0100% 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 inclusions and contribute to further improvement of stretch flange properties. 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 expansion and flange cracking. 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 respective contents are below the preferred lower limit, the effects of the present invention will not be impaired, and therefore they shall be included as unavoidable impurities. Examples of unavoidable impurities include O and H. The contents of O and H are 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 98.5%. Even if only a very small amount of martensite is produced, it promotes the formation of lower bainite that is formed later, and when tempered, it becomes tempered martensite. The promotion of lower bainite formation by martensite results in an area of ​​50.0 μm. ２The number density of the hard phases described above can be brought within a desired range. Tempered martensite, together with lower bainite, is a useful phase from the viewpoint of obtaining high ductility, high elongation flangeability, and excellent wrinkle suppression while maintaining high strength with a tensile strength of 980 MPa or more. In order to secure 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 98.5%, it becomes impossible to set the area ratio of retained austenite to 1.5% or more. Therefore, the area ratio of tempered martensite is set to 98.5% or less. 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 greater than 15.0%. The area ratio of tempered martensite is preferably less than 98.5%, more preferably 98.0% or less, even more preferably 95.0% or less, even more preferably 90.0% or less, and particularly preferably less than 85.0%.

[0057] Lower bainite area ratio: greater than 0% and less than or equal to 98.5% Lower bainite, together with tempered martensite, is a useful phase from the viewpoint of obtaining high strength of 980 MPa or more, as well as high ductility, high elongation flangeability, and excellent wrinkle suppression ability, 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 98.5%, it becomes impossible to set the area ratio of retained austenite to 1.5% or more. For this reason, the area ratio of lower bainite is set to 98.5% or less. The area ratio of lower bainite is preferably 0.1% or more, more preferably 1.0% or more, even more preferably 5.0% or more, and particularly preferably more than 15.0%. The area ratio of the lower bainite is preferably less than 98.5%, more preferably 98.0% or less, even more preferably 95.0% or less, even more preferably 90.0% or less, and particularly preferably less than 85.0%.

[0058] Total area ratio of lower bainite and tempered martensite: 80.0% or more and 98.5% or less In order to maintain high strength with a tensile strength of 980 MPa or more, and to obtain even higher ductility, high stretch flangeability, and excellent wrinkle suppression ability, the total area ratio of lower bainite and tempered martensite is set to 80.0% or more. The total area ratio of lower bainite and tempered martensite is preferably 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 98.5%, the area ratio of retained austenite cannot be set to 1.5% or more. For this reason, the total area ratio of lower bainite and tempered martensite is set to 98.5% or less. The total area ratio of lower bainite and tempered martensite is preferably 98.0% or less, more preferably 97.0% or less, and even more preferably 95.0% or less.

[0059] Area ratio of fresh martensite: 9.0% or less (including 0%) Fresh martensite is hard and becomes a void generation site during molding, thus reducing stretch flangeability, wrinkle suppression ability, and excess deformation ability. For this reason, the area ratio of fresh martensite is set to 9.0% or less. Preferably, the area ratio of fresh martensite is 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: 1.5% or more and 15.0% or less. Retained austenite contributes to high ductility and excellent wrinkle suppression. Therefore, the area ratio of retained austenite should be 1.5% or more. Preferably, the area ratio of retained austenite is 2.0% or more, more preferably 2.5% or more, and even more preferably 3.0% or more. On the other hand, if the area ratio of retained austenite exceeds 15.0%, the stretch flange properties decrease. Therefore, the area ratio of retained austenite should be 15.0% or less. Preferably, the area ratio of retained austenite is 10.0% or less, more preferably 9.0% or less, and even more preferably 8.0% 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 exhibit similar contrast in SEM images, making them difficult to distinguish. Therefore, in SEM observations, fresh martensite and retained austenite are not distinguished, and 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.

[0069] The area ratio of retained austenite is measured as follows. That is, after the hot-rolled steel sheet is machined and ground to 1 / 4 of the sheet thickness in the sheet thickness direction (depth direction), 100 μm of the sheet thickness of the machined surface is removed by chemical polishing with oxalic acid 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. And 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. And 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 C content in retained austenite: 0.50% or more and 1.10% or less From the viewpoint of obtaining high ductility and excellent wrinkle suppression ability, the average C content in retained austenite is 0.50% or more. The average C content in retained austenite is preferably 0.55% or more, more preferably 0.60% or more. On the other hand, when the average C content in retained austenite exceeds 1.10%, the ductility and wrinkle suppression ability are rather reduced. Therefore, the average C content in retained austenite is 1.10% or less. The average C content in retained austenite is preferably 1.05% or less, more preferably 1.00% or less.

[0071] The average C content 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 obtained, and the average C content 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 C content (mass%) in retained austenite.

[0072] Area 50.0 μm ２Number 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 excess deformation capacity, the area is 50.0 μm². ２ The number density of the hard phase described above is 500 particles / mm³. ２ The area is 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 phases described above is measured as follows, from the 1 / 8 thickness position to the 3 / 8 thickness position, centered around the 1 / 4 thickness position of the high-strength hot-rolled steel sheet. First, in measuring the area ratio of the hard phases (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] Balance of Solid Solution Carbon Distribution in Retained Austenite: 1.30 or Higher To ensure excellent excess deformation capacity, it is necessary to utilize the TRIP effect of retained austenite over a wide strain range during steel sheet deformation. For this purpose, the steel sheet needs to contain retained austenite with a wide range of stabilities, from relatively unstable to relatively stable. The stability of retained austenite varies depending on the amount of solid solution carbon, size, and morphology of the retained austenite, but the amount of solid solution carbon has a particularly strong influence. In other words, to ensure excellent excess deformation capacity, it is necessary for the distribution of solid solution carbon in the retained austenite to be wide. The balance of solid solution carbon in retained austenite can be evaluated using the BCA formula below. From the viewpoint of ensuring excellent excess deformation capacity, the balance of solid solution carbon in retained austenite BCA should be 1.30 or higher. The balance of solid solution carbon in retained austenite BCA is preferably 1.35 or higher, more preferably 1.40 or higher. There is no particular upper limit to the solid-solution carbon balance BCA of retained austenite, but since it is technically difficult to set it to 1.70 or higher, it is preferable to set it to less than 1.70. Here, for the diffraction peak of the (220) plane of austenite using CuKα X-rays as the incident X-ray, if the angle showing the maximum intensity is θT degrees, then the angle on the lower side showing half the intensity value of the maximum intensity is θL degrees, and the angle on the higher side showing half the intensity value of the maximum intensity is θH degrees. Here, if the solid-solution carbon content in retained austenite is %Cγ, then the solid-solution carbon content distribution balance BCA of retained austenite is given by the following equation: BCA = (θH - θT)^(1.10 - %Cγ) + (θT - θL)^(%Cγ - 0.50)

[0075] Next, the mechanical properties of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0076] 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.

[0077] 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 known plating layers can be cited as examples. As a known plating layer, a zinc plating layer is preferred. When a zinc plating layer is used, it is preferable that it contains 0.08% to 0.30% Al. Furthermore, even if elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn are mixed into this zinc plating layer in addition to Zn and Al, and Mg and Si, the effects of the present invention remain unchanged. Furthermore, this zinc plating layer may be an alloyed zinc plating layer that has undergone alloying treatment. In addition to a zinc plating layer, other known plating layers such as Zn-Al-Mg-Zn-Si-Zn-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.

[0078] 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. The thickness of the high-strength hot-rolled steel sheet of the present invention is not particularly limited, but is preferably 10.0 mm or less.

[0079] [2] Members Next, a member according to one embodiment of the present invention will be described. A member according to one embodiment of the present invention is a member made using (as a material for) the above-mentioned high-strength hot-rolled steel sheet. Therefore, it has the same steel structure and mechanical properties as the above-mentioned high-strength hot-rolled steel sheet. The above-mentioned high-strength hot-rolled steel sheet maintains high strength with a tensile strength of 980 MPa or more, and further possesses high ductility and high elongation flangeability, as well as excellent wrinkle suppression ability and excellent excess deformation ability. 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.

[0080] [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. １ A first cooling step involves cooling the hot-rolled steel sheet at a cumulative tempering parameter of 25.4 or less above 2°C. A reheating step involves reheating the hot-rolled steel sheet after the first cooling step at a reheating stop temperature Th of 320°C or higher and 480°C or lower, with a time from cooling stop to reheating start of less than 60 seconds. A heat retention step involves maintaining the heat of the hot-rolled steel sheet after the reheating step at a heat retention temperature of 320°C or higher and 480°C or lower. A second cooling step involves cooling the hot-rolled steel sheet after the heat retention step to room temperature. The cumulative tempering parameter from the start of reheating in the reheating step until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling step is 24.0 or higher and 30.0 or lower, and the cumulative tempering parameter of Th - 20°C or lower in the heat retention step and the second cooling step is 20.0 or higher and 29.0 or lower. Furthermore, an optional plating step may be included in which the manufactured high-strength hot-rolled steel sheet is plated.

[0081] Unless otherwise specified, the temperatures mentioned above refer to the surface temperature of the steel material and steel plate.

[0082] [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.

[0083] [Heating Process] Heating temperature of steel material: 1150°C or higher After cooling to a low temperature, most of the elements that form carbonitrides, such as Ti, precipitate unevenly as coarse carbonitrides in the steel material such as slabs. The presence of these coarse and uneven precipitates leads to deterioration of various properties, such as strength and elongation flangeability. For this reason, the steel material is heated before hot rolling to solidify the coarse precipitates. In order to sufficiently solidify the coarse precipitates before hot rolling, the heating temperature of the steel material is set to 1150°C or higher. The heating temperature of the steel material is preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel material is too high, it may lead to the occurrence of slab defects and a decrease in yield due to scale-off. For this reason, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. Furthermore, the steel material before hot rolling may be subjected to direct hot rolling (direct rolling) after casting while still at a high temperature, that is, while maintaining a temperature within the above heating temperature range.

[0084] [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 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 more, more preferably 3.0 MPa or more, and even more preferably 3.5 MPa or more. 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.

[0085] 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.

[0086] [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").

[0087] 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

[0088] During cooling, below Ms℃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, during cooling, excessive carbon diffusion occurs from the lower bainite to the untransformed austenite in parallel with the formation of the 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 is 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. During cooling, Ms°C or less T １ The cumulative tempering parameter for temperatures above °C is calculated as PTC1 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 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 plate temperature at time t (°C) PTC1 = log(10^ptc1(0) + 10^ptc1(0.01) + 10^ptc1(0.02) + ...)

[0089] [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.

[0090] 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.

[0091] Reheating stop temperature Th: 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.

[0092] [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 Th mentioned above, as long as it is within the range of 320°C to 480°C.

[0093] [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 lower. Cumulative tempering parameter from the start of reheating in the reheating process until the steel sheet temperature of the hot-rolled steel sheet in the second cooling process reaches room temperature: 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 steel sheet temperature of the hot-rolled steel sheet in the second cooling process reaches room temperature 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 steel sheet temperature of the hot-rolled steel sheet in the second cooling process reaches room temperature is set to 24.0 or higher. The cumulative tempering parameter from the start of reheating in the reheating process until the steel sheet temperature of the hot-rolled steel sheet in the second cooling process reaches room temperature is preferably 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 exceeds 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 less. 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 29.0 or less, more preferably 28.0 or less, and even more preferably 27.5 or less.

[0094] Here, the cumulative tempering parameter PT2, calculated as follows, is used 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 and interpolated to obtain temperature data every 0.01 seconds. Next, the following equation for pt2 is calculated for each time point, and this result is used to further calculate the equation for PT2. pt2(t) = -18000 / (T + 273) + 48 where t: time (seconds), T: steel sheet temperature at time t (°C) PT2 = log(10^pt2(0) + 10^pt2(0.01) + 10^pt2(0.02) + ...)

[0095] The cumulative tempering parameter below Th-20°C in the heat retention process and the second cooling process is 20.0 or more and 29.0 or less. After the reheating process is stopped, carbon diffuses from the surrounding structure, such as martensite and lower bainite, into the untransformed austenite that remains, enriching it. The austenite stabilized by carbon enrichment becomes retained austenite that remains even after cooling to room temperature in the second cooling process. In order to make the solid solution carbon distribution balance BCA of retained austenite 1.30 or more, it is necessary to expose the steel sheet to a temperature lower than the reheating stop temperature Th in the second heat retention process and the second cooling process after reheating. That is, from the viewpoint of making the solid solution carbon distribution balance BCA of retained austenite 1.30 or more, the cumulative tempering parameter below Th-20°C (from Th-20°C until reaching room temperature) should be 20.0 or more. The cumulative tempering parameter at Th-20°C or below (from Th-20°C until room temperature is reached) is preferably 22.0 or higher, more preferably 24.0 or higher. On the other hand, if the cumulative tempering parameter from Th-20°C until room temperature is reached is excessive, the solid solution C distribution balance BCA of retained austenite will decrease. For this reason, the cumulative tempering parameter at Th-20°C or below (from Th-20°C until room temperature is reached) should be 29.0 or lower. The cumulative tempering parameter at Th-20°C or below (from Th-20°C until room temperature is reached) is preferably 27.0 or lower, more preferably 26.0 or lower.

[0096] Here, the cumulative tempering parameter for the heat retention process and the second cooling process below Th-20°C (from Th-20°C until room temperature is reached) is calculated as PT3 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 pt3 is calculated for each time point, and this result is used to further calculate the formula for PT3. pt3(t) = -18000 / (T + 273) + 48 where t: time (seconds), T: steel plate temperature at time t (°C) PT3 = log(10^pt3(0) + 10^pt3(0.01) + 10^pt3(0.02) + ...)

[0097] 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.

[0098] [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, for example, the hot-rolled steel sheet can be plated after the second cooling process. The plating process in the plating process is not particularly limited, and for example, known plating processes can be mentioned.

[0099] 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.

[0100] [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.

[0101] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0102] [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 Table 2 lists the cumulative tempering parameter PTC1 for temperatures above ℃. 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 reheating, and the reheating stop temperature [℃]. 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 reheating process, heat retention process, and second cooling process, including the cumulative tempering parameter PT2 from the end of reheating until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature, and the conditions for the heat retention process and second cooling process, including the cumulative tempering parameter PT3 for temperatures below Th-20℃.

[0103] In this way, a high-strength hot-rolled steel sheet was obtained. The sheet thickness is not particularly limited, but in this embodiment of the present invention, it was set to 3.0 mm.

[0104]

[0105]

[0106] [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 phases described above, BCA, represents the distribution balance of solid-solution carbon in retained austenite.

[0107]

[0108] Furthermore, tensile tests and hole expansion tests were conducted according to the following procedure, and the tensile strength (TS), uniform elongation (U.El), critical hole expansion ratio (λ), n value (n), and TS-S1 were evaluated according to the following criteria. The measurement results are shown in Table 4.

[0109]

[0110] (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.

[0111] A total test (TS) of 980 MPa or higher (TS ≥ 980 MPa) was considered a pass, while anything else was considered a fail.

[0112] U.El was defined as follows: For 980 MPa ≤ TS < 1180 MPa, 6.0% or more (U.El ≥ 6.0%) was considered to have high ductility. For 1180 MPa ≤ TS < 1310 MPa, 5.5% or more (U.El ≥ 5.5%) was considered to have high ductility. For 1310 MPa ≤ TS, 5.0% or more (U.El ≥ 5.0%) was considered to have high ductility. Anything that did not meet these criteria was considered to lack high ductility.

[0113] The n-values ​​were defined as follows: For 980 MPa ≤ TS < 1180 MPa, a n-value of 0.050 or higher (n-value ≥ 0.050) was considered to indicate excellent wrinkle suppression ability. For 1180 MPa ≤ TS < 1310 MPa, a n-value of 0.040 or higher (n-value ≥ 0.040) was considered to indicate excellent wrinkle suppression ability. For 1310 MPa ≤ TS, a n-value of 0.035 or higher (n-value ≥ 0.035) was considered to indicate excellent wrinkle suppression ability. Anything that did not meet these criteria was considered to lack excellent wrinkle suppression ability.

[0114] TS-S1 was defined as follows: When 980 MPa ≤ TS < 1180 MPa, TS-S1 ≤ 100 MPa was considered to have excellent excess deformation capacity. When 1180 MPa ≤ TS < 1310 MPa, TS-S1 ≤ 120 MPa was considered to have excellent excess deformation capacity. When 1310 MPa ≤ TS, TS-S1 ≤ 150 MPa was considered to have excellent excess deformation capacity. All other values ​​were considered to lack excellent excess deformation capacity.

[0115] (2) Hole Expansion Test The hole expansion test was conducted in accordance with JIS Z 2256. Specifically, a 100 mm x 100 mm test piece was taken from the obtained high-strength hot-rolled steel sheet by shearing. A hole with a diameter of 10 mm (the initial hole in the test piece) was punched into the test piece with a clearance of 12% ± 1%. Next, a wrinkle-holding force of 9 tons (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm, and a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when cracks occurred) was measured. The limit hole expansion ratio λ (%) was then calculated using the following formula. The results are shown in Table 4. λ was defined as follows: When 980 MPa ≤ TS < 1180 MPa, λ ≥ 35% was considered to have high elongation flange properties. For 1180 MPa ≤ TS < 1310 MPa, a value of 30% or more (λ ≥ 30%) was considered to have high elongation flange properties. For 1310 MPa ≤ TS, a value of 25% or more (λ ≥ 25%) was considered to have high elongation flange properties. Anything else was 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.

[0116] In the examples of the present invention, high-strength hot-rolled steel sheets are obtained that, in addition to high strength and high ductility, have excellent wrinkle suppression ability, high stretch flangeability and excellent excess deformation ability. On the other hand, in the comparative examples, at least one of these properties is inferior.

Claims

1. The composition is as follows, in mass%, C: 0.040% or more and 0.350%, Si: 0.50% or more and 2.50%, Mn: 1.50% or more and less than 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is as follows: Area ratio of tempered martensite: more than 0% and 98.5% or less, Area ratio of lower bainite: more than 0% and 98.5% or less, Total area ratio of lower bainite and tempered martensite: 80.0% or more and 98.5% or less, Area ratio of fresh martensite: 9.0% or less (including 0%), Area ratio of retained austenite: 1.5% or more and 15.0% 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 solid-solution carbon content distribution balance of retained austenite obtained from the following BCA formula: BCA = (θH - θT)^(1.10 - %Cγ) + (θT - θL)^(%Cγ - 0.50) θT (degrees): angle showing maximum intensity for the diffraction peak of the (220) plane of retained austenite, θL (degrees): angle on the lower side showing half the intensity value of the maximum intensity, θH (degrees): angle on the higher side showing half the intensity value of the maximum intensity, %Cγ (%): average solid-solution carbon content in retained austenite, and having a tensile strength of 980 MPa or more.

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. The 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 A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a first cooling step of cooling under the condition that the cumulative tempering parameter above ℃ is 25.4 or less; a reheating step of reheating the hot-rolled steel sheet after the first cooling step under the condition that the time from the cessation of cooling to the start of reheating is less than 60 seconds, and the reheating cessation temperature Th is 320℃ or higher and 480℃ or lower; a heat retention step of heat retention temperature: 320℃ or higher and 480℃ or lower for heat retention; and a second cooling step of cooling the hot-rolled steel sheet after the heat retention step to room temperature, wherein the cumulative tempering parameter from the start of reheating in the reheating step until the steel sheet temperature of the hot-rolled steel sheet reaches room temperature in the second cooling step is 24.0 or higher and 30.0 or lower, and the cumulative tempering parameter below Th - 20℃ in the heat retention step and the second cooling step is 20.0 or higher and 29.0 or lower. １ is the greater of 300°C and the cooling stop temperature of the first cooling process, and Ms is defined by the formula Ms(°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo, where each element symbol in the above formula represents the content (mass%) of each element, and 0 is used for elements that are not present.

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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