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

Abstract

Provided are a high-strength hot-rolled steel sheet having high strength, high ductility, excellent strain dispersibility, high stretch flangeability, and bendability, a member, and methods for manufacturing the same.　The high-strength hot-rolled steel sheet has a prescribed component composition, and has a steel structure in which, in terms of area ratio, the total of bainite and tempered martensite is 80.0% or more, pearlite is 10.0% or less, retained austenite is 15.0% or less, the total of retained austenite and pearlite is 1.0% or more, the total of tempered martensite and bainite having a circle-equivalent diameter of 5.0 μm or more is 10.0% or less, and the proportion of line segments having a hard phase fraction of 20.0% or more among line segments having a length of 200 μm parallel to the rolling direction is 30.0% or less, the tensile strength TS (MPa) and the component composition (mass%) satisfy the expression 0 ≦ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) – TS ≦ 250, where each element symbol in the formula represents the content (mass%) of each element, and the high-strength hot-rolled steel sheet has a tensile strength of 980 MPa or more.

Description

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

[0001] The present invention relates to a high-strength hot-rolled steel sheet suitable as a material for automotive members and a method for manufacturing the same.

[0002] In recent years, from the perspective of global environmental conservation, improving the fuel efficiency and electricity consumption of automobiles has become an important issue. Therefore, there has been an active movement to reduce the weight of the automobile body by increasing the strength and reducing the thickness of the steel sheet used as the material for automotive members. In addition, in hybrid vehicles and electric vehicles, the vehicle weight increases due to the installation of a large-capacity battery as the energy source for driving, so an increase in the strength of automotive members corresponding thereto is required. For this reason, high-strength hot-rolled steel sheets are actively adopted as materials for automotive members. The use of high-strength hot-rolled steel sheets is not only for structural members and skeletal members of automobiles, but also for underbody members, truck frame members, etc.

[0003] In particular, a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more is expected as a material that can dramatically improve the fuel efficiency and electricity consumption of automobiles by coping with weight reduction and an increase in vehicle weight due to battery loading.

[0004] Various studies have been made so far as such a steel sheet used as a material for automotive members.

[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, by mass%, it contains C: 0.12% or more, 0.25% or less, Si: 0.01% or more, 2.0% or less, Mn: 0.5% or more, 3.0% or less, P: 0.020% or less, S: 0.010% or less, Al: 0.001% or more, 0.10% or less, B: 0.0005% or more, 0.0050% or less, Cu: 0% or more, 0.50% or less, Ni: 0% or more, 0.50% or less, Cr: 0% or more, 0.50% or less, Mo: 0% or more, 0.50% or less, V: 0% or more, 0.05% or less, Ca: 0% or more, 0.05% or less, and REM: 0% or more, 0.01% or less, and Nb: 0.001% or more, 0. A hot-rolled steel sheet is disclosed, characterized in that it contains 0.20% or less of Ti, and one or two of the following: 0.001% or more and 0.20% or less, with the remainder being Fe and impurities, and the metal structure at a position 1 / 4 thickness from the surface contains tempered martensite with an area ratio of more than 90%, the average grain size of carbides in the tempered martensite is 10 nm or less, the average grain size of prior austenite grains is less than 40 μm, the aspect ratio of prior austenite grains is 3.5 or less, and the X-ray random intensity ratio in the {112}<110> direction at a position 1 / 2 thickness from the surface is 4.0 or less.

[0007] Patent Document 3 states that, "The composition is such that, by mass%, it contains C: 0.05 to 0.20%, Si: 0.5 to 1.2%, Mn: 1.5 to 4.0%, P: 0.10% or less, S: 0.03% or less, Al: 0.001 to 2.0%, N: 0.01% or less, O: 0.01% or less, and B: 0.0005 to 0.010%, with the remainder being Fe and unavoidable impurities, and the microstructure contains upper bainite with an area ratio of 80% or more in the surface region from the surface of the steel sheet to the 1 / 10 position of the sheet thickness, and fresh martensite and / or retained austenite with a total area ratio of 2% or more, and in the internal region from the 1 / 10 position to the 3 / 10 position of the sheet thickness" A high-strength steel sheet is disclosed, comprising 70% or more upper bainite by area ratio and 3% or more fresh martensite and / or retained austenite by total area ratio, having an average grain size of 6 μm or less in the surface region from the steel sheet surface to the 1 / 10 thickness position, the difference (HV2-HV1) between the hardness of the surface region from the steel sheet surface to the 1 / 10 thickness position (HV1) and the hardness of the internal region from the 1 / 10 thickness position to the 3 / 10 thickness position (HV2) being 5% to 15% of [0.3 × tensile strength (MPa)], having a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio R / t of the limit bending radius R to the sheet thickness t of 1.5 or less.

[0008] Patent No. 6465266 Patent No. 7115628 Patent No. 7168137

[0009] 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 undercarriage components such as suspension parts, need to have complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets used as materials for such complexly shaped components require excellent formability.

[0010] On the other hand, increasing the tensile strength of steel sheets generally reduces their formability. In particular, steel sheets used as components for automobile undercarriages require not only high ductility but also excellent strain distribution, as well as high elongation flangeability and bendability. It is important to ensure these qualities while maintaining high strength.

[0011] In fact, the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to possess high strength, such as a tensile strength (TS) of 980 MPa or more, as well as high ductility, excellent strain distribution ability, and high elongation flangeability and bendability.

[0012] 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, excellent strain distribution ability, and high elongation flangeability and bendability.

[0013] Therefore, the present invention aims to solve the problems of the prior art and 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, excellent strain distribution ability, and high elongation flangeability and bendability.

[0014] 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: U.El ≥ 5.0% Automotive parts, especially undercarriage parts, are subjected to rigorous processing, so it is necessary to suppress the localization of strain. In particular, in order to perform rigorous processing using steel plates with high strength of TS 980 MPa or more, it is necessary to disperse the strain without localizing it and prevent forming defects such as cracks and wrinkles. If the steel plate has excellent strain dispersion ability, cracks and wrinkles can be prevented. In this invention, excellent strain dispersion ability means that the yield ratio YR = YS / TS, defined for TS and 0.2% proof stress (hereinafter also referred to as YS) measured in a tensile test in accordance with JIS Z 2241, satisfies the following equation. YR ≤ 0.95 Since undercarriage parts undergo stretch flange forming after punching, high stretch flange properties are required. In the present invention, high stretch flange properties mean that the average limit hole expansion ratio (λ), measured in a hole expansion test in accordance with JIS Z 2256 using three test pieces, satisfies the following formula. In particular, undercarriage parts undergo stretch flange forming after punching, so excellent stretch flange properties are required. λ ≥ 40% Since undercarriage parts undergo bending forming at burring processes and part ends to ensure rigidity, high bendability is required. In the present invention, high bendability means that a bending test in accordance with JIS Z 2248 is performed, and the minimum bending radius at which no crack occurs in any test piece is defined as the limit bending radius R (mm), and the R / t value obtained by dividing R by the thickness t (mm) of the steel plate satisfies the following formula. If 980 MPa ≤ TS < 1180 MPa, then R / t ≤ 1.50. If 1180 MPa ≤ TS < 1310 MPa, then R / t ≤ 3.50. If 1310 MPa ≤ TS, then R / t ≤ 4.50.

[0015] The inventors diligently conducted research to achieve the above objectives.

[0016] As a result, the composition of the hot-rolled steel sheet was appropriately adjusted, and the tensile strength TS (MPa) and the composition of the sheet were set to satisfy the following equation: 0 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 250 Here, each element symbol represents the content (mass %) of each element, and 0 is used if an element is not present. Furthermore, the structure of the hot-rolled steel sheet was set to contain a total area ratio of bainite and tempered martensite of 80.0% or more, an area ratio of hard phase of less than 20.0%, and an area ratio of pearlite of 10.0% or less. Furthermore, the hard phase included retained austenite, and the area ratio of said retained austenite was set to 15.0% or less. Furthermore, the total area ratio of retained austenite and pearlite was set to 1.0% or more, and the total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more was set to 10.0% or less. In addition, the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more was set to 30.0% or less. By using such a component composition and steel structure, and achieving a tensile strength of 980 MPa or more, it was found that a high-strength hot-rolled steel sheet can be obtained that maintains high strength while also possessing high ductility, excellent strain distribution ability, and high elongation flangeability and bendability.

[0017] This invention was completed based on the above findings and further considerations.

[0018] 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 0.300% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and less than 4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is such that the total area ratio of bainite and tempered martensite is 80.0% or more, the area ratio of the hard phase is less than 20.0%, and the area ratio of pearlite is 0% or less. It contains 10.0% or less of the above, and the hard phase contains retained austenite, with the area ratio of the retained austenite being 15.0% or less, the total area ratio of the retained austenite and the pearlite being 1.0% or more, the total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more being 10.0% or less, and the percentage of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more being 30.0% or less, and the tensile strength TS (MPa) and the component composition (mass%) are, A high-strength hot-rolled steel sheet that satisfies the equation 0 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 250, where each element symbol in the above equation represents the content (mass %) of each element, and 0 is used for elements that are not present, and the tensile strength is 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 start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of 800°C or higher and 980°C or lower to obtain a hot-rolled steel sheet; and a method for hot-rolling the hot-rolled steel sheet after the hot-rolling step, with a time from the end of the hot-rolling to the start of cooling of 2.0 seconds or less and an average cooling rate A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a cooling step of cooling under the conditions of FF°C / s or more and a cooling stop temperature Tq: Ms-450°C or more and Ms+30°C or less; a heating step of raising the temperature of the hot-rolled steel sheet after the cooling step under the conditions of a maximum reach temperature Tm: 300°C or more and 500°C or less and Tm-Tq≧10°C; and a holding step of holding the hot-rolled steel sheet after the heating step in a temperature range of Tq or more and Tm°C or less for a holding time of 5 seconds or more. Here, FF and Ms are defined by the following formulas, respectively.FF (°C / s) = 10^(3.5 - 1.2 × (2.5 × C + Mn + 0.7 × Cr + 0.1 × Mo + 0.5 × Ni)) Ms (°C) = 550 - 361 × C - 39 × Mn + 30 × Al - 35 × V - 20 × Cr - 5 × Mo - 10 × Cu - 37 × Ni - 5 × W + 15 × Co In each of the above formulas, the element symbols represent the mass percentage content of each element, and the content of elements that are not present is set to 0. [6] A method for manufacturing a high-strength hot-rolled steel sheet according to [5], further comprising a plating step of applying a plating treatment to the hot-rolled steel sheet after the holding step. [7] A method for manufacturing a component, comprising a step of applying at least one of forming or joining to 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].

[0019] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet that has high strength, high ductility, excellent strain distribution ability, and high elongation flangeability and bendability.

[0020] This is a schematic diagram for determining the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase accounts for 20.0% or more, according to the present invention.

[0021] The following describes embodiments of the high-strength hot-rolled steel sheet and its manufacturing method according to the present invention. However, the present invention is not limited to the following embodiments. [1] High-strength hot-rolled steel sheet First, the component composition of a high-strength hot-rolled steel sheet that has not undergone plating treatment 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 "%".

[0022] C: 0.040% or more and 0.300% 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 0.300%, the area ratio of the hard phase increases, and the elongation flangeability and bendability decrease. Also, the tensile strength increases excessively, and the ductility decreases. Therefore, the carbon content is set to 0.300% or less. Preferably, the carbon content is 0.250% or less, and more preferably 0.220% or less.

[0023] Si: 0.20% to 2.00% Si has the effect of suppressing the formation of Fe-based carbides and suppressing cementite precipitation during the holding process. As a result, C is distributed to the untransformed austenite, and after cooling to room temperature, a portion of the untransformed austenite becomes retained austenite, contributing to improved ductility. In addition, Si improves the strength-ductility balance of bainite and tempered martensite after an appropriate holding process. In order to obtain these effects, the Si content must be 0.20% or more. Therefore, the Si content is set to 0.20% or more. Preferably, the Si content is 0.40% or more, and more preferably 0.60% or more. On the other hand, Si is an element that forms subscale on the surface of the steel sheet during hot rolling. If the Si content exceeds 2.00%, the subscale becomes too thick, and even if descaling is performed in the hot rolling process, the surface roughness of the steel sheet becomes excessive, and the pretreatment properties for painting high-strength hot-rolled steel sheets deteriorate. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less, and more preferably 1.30% or less.

[0024] Mn: 1.00% or more and less than 4.00% Mn stabilizes austenite, suppresses ferrite formation, and contributes to the formation of bainite and tempered martensite. To obtain these effects, the Mn content must be 1.00% or more. Therefore, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.20% or more, more preferably 1.50% or more. On the other hand, if the Mn content is 4.00% or more, bainite transformation is more likely to occur, the hard phase increases, and the elongation flangeability and bendability decrease. Therefore, the Mn content is set to less than 4.00%. Preferably, the Mn content is less than 3.50%, more preferably less than 3.20%, and even more preferably less than 3.00%.

[0025] P: 0.100% or less. P is an element that contributes to increasing the strength of steel by solid solution. However, P is also an element that causes slab cracking during hot rolling by segregating at the austenite grain boundaries. Furthermore, segregation at grain boundaries reduces ductility. For this reason, it is preferable to keep the P content as low as possible, but a P content of up to 0.100% is acceptable. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.030% or less. There is no particular lower limit to the P content, but from the viewpoint of productivity, etc., it is preferable that the P content be 0.001% or more.

[0026] S: 0.0200% or less. S combines with Ti and Mn to form coarse sulfides, which accelerate void formation and reduce ductility, elongation flangeability, and bendability. Therefore, it is preferable to keep the S content as low as possible, but an S content of 0.0200% or less is acceptable. Accordingly, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. There is no particular lower limit to the S content, but from the viewpoint of productivity, etc., it is preferable that the S content be 0.0001% or more.

[0027] 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 tempered martensite after appropriate holding processes. On the other hand, excessive Al content leads to an increase in oxide inclusions, reducing ductility, elongation flammability, and bendability. Therefore, the Al content should be 2.000% or less. Preferably, the Al content is 1.000% or less.

[0028] N: 0.0100% 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 0.0100% causes a decrease in ductility, elongation, flangeability, and bendability. For this reason, the N content should be 0.0100% 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.

[0029] 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.

[0030] The basic component composition of a high-strength hot-rolled steel sheet that has not undergone plating treatment according to one embodiment of the present invention has been described above, but other elements described below may be further included as needed.

[0031] 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 the refinement of grain size of 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 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.

[0032] 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-preserved region and contributing to the refinement of grain size in bainite and tempered martensite. To obtain such effects, if Nb is included, the Nb content is preferably 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 bendability. In addition, the hard phase may increase excessively, which may reduce ductility, ductility, 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.

[0033] V: 0.400% or less V 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-preserved region and contributing to the refinement of grain size of bainite and tempered martensite. To obtain such effects, when V is included, the V content is preferably 0.005% or more. The V content is more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if the V content exceeds 0.400%, the hard phase may increase excessively, and ductility, elongation flangeability, and bendability may decrease. 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.

[0034] Cr: 1.00% or less. Like Mn, Cr inhibits ferrite formation and contributes to the formation of bainite and tempered 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.

[0035] 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 holding process and contributes to an increase in the amount of 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. However, if the Mo content exceeds 0.500%, a hard phase may be excessively formed, which may worsen ductility, elongation flangeability, and bendability. Therefore, when Mo is included, the Mo content should be 0.500% or less.

[0036] B: 0.0100% or less. B is an element that contributes to the formation of 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.

[0037] Cu: 1.00% or less. Cu is an element that contributes to increasing the strength of steel by solid solution. Furthermore, Cu promotes the formation of bainite and tempered martensite through improved hardenability, thereby contributing to increased strength. To obtain these 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. Preferably, the Cu content is 0.50% or less.

[0038] 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 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%, the hard phase may increase excessively, potentially degrading the ductility, elongation flangeability, and bendability of the hot-rolled steel sheet. Therefore, when Ni is included, the Ni content should be 1.00% or less.

[0039] 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.

[0040] Sn: 0.200% or less. Similar to Sb, Sn is an element effective in suppressing denitrification, deboronization, etc. and suppressing a decrease in the strength of steel. When containing Sn, it is preferable to set the Sn content to 0.005% or more in order to obtain the above effect. On the other hand, when the Sn content exceeds 0.200%, the toughness of the steel decreases, and slab cracking and hot rolling cracking may occur. Therefore, when containing Sn, the Sn content is set to 0.200% or less. The Sn content is preferably 0.050% or less.

[0041] Ta: 0.100% or less. By forming fine carbides, nitrides or carbonitrides, Ta increases the strength of steel. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides to generate composite precipitates such as (Nb, Ta)(C, N). Thereby, coarsening of the precipitates is suppressed, and precipitation strengthening is stabilized. Thereby, the strength of the steel is improved. In order to obtain such an effect, when containing Ta, it is preferable to set the Ta content to 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. Thereby, there is a risk that the elongation flange property and bendability decrease. Therefore, when containing Ta, the Ta content is set to 0.100% or less. The Ta content is preferably 0.050% or less.

[0042] W: 0.500% or less. W is an element effective in enhancing hardenability and adjusting the strength of steel to a more suitable range. In order to obtain such an effect, when containing W, it is preferable to set the W content to 0.001% or more. The W content is more preferably 0.030% or more. On the other hand, when the W content exceeds 0.500%, the area ratio of the hard phase excessively increases, and there is a risk of causing a decrease in the elongation flange property and bendability. Therefore, when containing W, it is preferable to set the W content to 0.500% or less. The W content is preferably 0.100% or less.

[0043] Mg: 0.0200% or less. Mg controls the shape of oxide and sulfide inclusions and contributes to suppressing cracks on the sheared end face of the steel sheet and further improving the stretch flange formability. To obtain such an effect, when Mg is contained, the Mg content is preferably 0.0010% or more. However, when the Mg content exceeds 0.0200%, the cleanliness of the steel deteriorates, and it may conversely cause cracks on the sheared end face or stretch flange cracks. Therefore, when adding Mg, the Mg content is set to 0.0200% or less. The Mg content is preferably 0.0100% or less.

[0044] Zn: 0.0200% or less. Zn spheroidizes the shape of inclusions and contributes to suppressing cracks on the sheared end face of the steel sheet and further improving the stretch flange formability. To obtain such an effect, when Zn is contained, the Zn content is preferably 0.0010% or more. However, when the Zn content exceeds 0.0200%, the cleanliness of the steel deteriorates, and it may conversely cause cracks on the sheared end face or stretch flange cracks. Therefore, when containing Zn, the Zn content is set to 0.0200% or less. The Zn content is preferably 0.0100% or less.

[0045] Co: 0.0200% or less. Co, like Zn, spheroidizes the shape of inclusions and contributes to suppressing cracks on the sheared end face of the steel sheet and further improving the stretch flange formability. To obtain such an effect, when Co is contained, the Co content is preferably 0.0010% or more. However, when the Co content exceeds 0.0200%, the cleanliness of the steel deteriorates, and it may conversely cause cracks on the sheared end face or stretch flange cracks. Therefore, when containing Co, the Co content is set to 0.0200% or less. The Co content is preferably 0.0100% or less.

[0046] Zr: 0.0200% or less. Like Zn and Co, Zr contributes to the suppression of cracking at the shear end face of steel plates and further improvement of tensile flange properties 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 cracking at the shear end face and tensile 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.

[0047] Ca: 0.0200% or less. Ca controls the shape of oxide and sulfide-based inclusions, contributing to the suppression of cracking at the shear end face of steel plates and further improvement of tensile 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 shear end face cracking and tensile flange cracking. Therefore, when Ca is added, the Ca content should be 0.0200% or less. The Ca content is preferably 0.0100% or less.

[0048] 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, contributing to crack suppression at the shear end face of steel sheets and further improvement of elongation 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 shear end face cracking and stretch flange cracking. Therefore, when adding each of the above elements, the content of each element should be 0.0200% or less. The Hf content is preferably 0.0100% or less. The Pb content is preferably 0.0100% or less. The Bi content is preferably 0.0100% or less. The REM content is preferably 0.0100% or less. 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.

[0049] Furthermore, regarding the above-mentioned Ti, Nb, V, Cr, Mo, B, Cu, Ni, Sb, Sn, Ta, W, Mg, Zn, Co, Zr, Ca, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities.

[0050] In addition, unavoidable impurities include, for example, H and O. Preferably, the amounts of H and oxygen are 0.01000% or less, respectively.

[0051] Next, the microstructure of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0052] Total area ratio of bainite and tempered martensite: 80.0% or more From the viewpoint of obtaining high ductility, high elongation flangeability, and high bendability while maintaining high strength, the total area ratio of bainite and tempered martensite is set to 80.0% or more. The total area ratio of bainite and tempered martensite is preferably 85.0% or more, more preferably 88.0% or more, and even more preferably more than 90.0%. The total area ratio of bainite and tempered martensite is preferably 99.0% or less.

[0053] Area ratio of hard phase: Less than 20.0% From the viewpoint of obtaining good elongation flange properties and bendability, the area ratio of the hard phase is set to less than 20.0%. The area ratio of the hard phase is preferably less than 18.0%, more preferably less than 15.0%, and even more preferably less than 10.0%. The lower limit of the area ratio of the hard phase is not particularly limited and may be 0%. Here, the hard phase in the present invention refers to a structure consisting of one or two of fresh martensite and retained austenite.

[0054] Perlite area ratio: 0% to 10.0%. Perlite contributes to excellent work hardening ability and good stretch flange properties. However, if the perlite area ratio exceeds 10.0%, the stretch flange properties actually decrease. Therefore, the perlite area ratio should be 10.0% or less. Preferably, the perlite area ratio is 8.0% or less, more preferably 5.0% or less, and even more preferably 3.0% or less. The lower limit of the perlite area ratio is not particularly limited and may be 0%.

[0055] Furthermore, it is preferable that the area ratio of the remaining tissue other than bainite, tempered martensite, fresh martensite, retained austenite, and pearlite be 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, cementite, and other carbides. The type of remaining tissue can be confirmed, for example, by observation using an SEM (Scanning Electron Microscope).

[0056] Area ratio of retained austenite: 15.0% or less. Retained austenite contributes to good ductility and excellent work hardening ability. However, from the viewpoint of ensuring high elongation flangeability and bendability, the area ratio of retained austenite should be 15.0% or less. The area ratio of retained austenite is preferably 12.0% or less, more preferably less than 10.0%. There is no particular lower limit to the area ratio of retained austenite, and it may be 0%. The area ratio of retained austenite is preferably 1.0% or more, more preferably 1.5% or more, and even more preferably 2.0% or more. Here, retained austenite refers to the area ratio of retained austenite contained in the hard phase, and is the ratio of retained austenite to the total steel structure.

[0057] Total area ratio of retained austenite and pearlite: 1.0% or more From the viewpoint of obtaining excellent work hardening ability, the total area ratio of retained austenite and pearlite is set to 1.0% or more. Preferably, the total area ratio of retained austenite and pearlite is 1.2% or more, more preferably 1.5% or more, and even more preferably 2.0% or more. The total area ratio refers to the ratio of the total retained austenite and pearlite to the total steel structure.

[0058] Here, bainite and tempered martensite are aggregates of lath-like ferrite grains with an orientation difference of less than 15°, and have a structure containing Fe-based carbides and / or retained austenite phases at and / or within the lath-like ferrite interfaces. However, this also includes cases where Fe-based carbides and / or retained austenite phases are not present at and / or within the lath-like ferrite interfaces. After formation, bainite and tempered martensite may undergo grain coarsening due to the loss of some of the lath-like ferrite interfaces when held at high temperatures. However, by measuring the crystal orientation difference within the grains using EBSD (Electron Backscatter Diffraction), they can be distinguished from polygonal ferrites and acicular ferrites, which have small crystal orientation differences within the grains. Furthermore, unlike polygonal ferrite and acicular ferrite, bainite and tempered martensite have a relatively high dislocation density internally. For this reason, bainite and tempered martensite can be distinguished from polygonal ferrite and acicular ferrite using a TEM (Transmission Electron Microscope).

[0059] Furthermore, if bainite and tempered martensite contain retained austenite, only the lath-like ferrite portion is considered bainite and tempered martensite, and is distinguished from the retained austenite. Also, if bainite and tempered martensite contain Fe-based carbides, the contained Fe-based carbides are also considered bainite and tempered martensite.

[0060] Bainite and tempered martensite can be distinguished from each other by observing the orientation and crystal structure of the included Fe-based carbides using TEM. However, in this invention, they are not distinguished because they have substantially the same properties. Furthermore, bainite is generally distinguished in more detail by its morphology and the precipitation position of Fe-based carbides. However, in this invention, fine bainite has substantially the same properties regardless of detailed characteristics. Therefore, as described later, if the total area ratio of the bainite and martensite with an equivalent circle diameter of 5.0 μm or more is 10.0% or less, bainite is not distinguished in detail.

[0061] The hard phases, fresh martensite and retained austenite, exhibit brighter contrast in SEM images compared to bainite and tempered martensite, polygonal ferrite, and acicular ferrite. Therefore, the hard phases can be distinguished from these structures using SEM. Furthermore, the hard phases often lack Fe-based carbides, and even when they do, the amount of Fe-based carbides is smaller and / or finer compared to bainite and tempered martensite, allowing them to be distinguished from bainite and tempered martensite using SEM.

[0062] As described later, the hard phase is unevenly distributed, so to determine its area ratio, a laser microscope is used to easily observe a wide area at lower magnification. In images observed with a laser microscope using Repera etching solution, the hard phase appears bright, while other tissues appear dark, allowing for differentiation.

[0063] Fresh martensite and retained austenite have similar shapes and contrasts under SEM and laser microscopy, making them difficult to distinguish; therefore, the area ratio of retained austenite is determined by the method described below.

[0064] Here, the area ratios of bainite and tempered martensite, hard phase (fresh martensite + retained austenite), pearlite, and the remaining microstructure are measured as follows. The observation range is the thickness cross section of the high-strength hot-rolled steel sheet parallel to the rolling direction, and is the range from the 1 / 8 thickness position to the 3 / 8 thickness position, centered at the 1 / 4 thickness position.

[0065] First, a sample is cut from a hot-rolled steel sheet so that the cross-section parallel to the rolling direction of the sheet becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample is finished with colloidal silica, and finally, the microstructure is revealed by etching with 3 vol. % nital.

[0066] Then, using a Scanning Electron Microscope (SEM), 10 fields of view of 42.7 μm × 32.0 μm on the observation surface of the sample are observed under the conditions of acceleration voltage: 15 kV and magnification: 3000x. Each phase is identified, and the area percentage of bainite, tempered martensite, and pearlite is calculated. If any remaining tissue exists in addition to bainite, tempered martensite, and pearlite, and the hard phase (fresh martensite + retained austenite), the area percentage of the total tissue is calculated together.

[0067] Because the hard phase (fresh martensite + retained austenite) is unevenly distributed, it is measured by observing a wider area at lower magnification. The observation surface of the polished sample is etched with Repera etching solution, and ten 200 μm × 200 μm fields of view of the sample's observation surface are observed using a laser microscope at a magnification of 500x. The hard phase and other structures are identified, and the area percentage of the hard phase is calculated. Here, the pixel size was set to 0.10 to 0.30 μm. In addition, to determine the percentage of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more, the observation field of view is set so that two sides are parallel to the rolling direction and the other two sides are parallel to the plate thickness direction.

[0068] The area fraction of retained austenite is measured as follows: After mechanically grinding a hot-rolled steel sheet in the thickness direction (depth direction) up to 1 / 4 of the sheet thickness, the section from 100 μm to 150 μm is chemically polished with oxalic acid to create the observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the ratio of the diffraction intensity of the (200), (220), and (311) surfaces of fcc iron (austenite) to the diffraction intensity of the (200) and (211) surfaces of bcc iron is determined. Then, the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0069] The total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more is 10.0% or less. Since the presence of coarse crystal grains reduces bendability, from the viewpoint of obtaining excellent bendability, the total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more is set to 10.0% or less. The total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more is preferably 8.0% or less, more preferably 6.0% or less, and even more preferably 5.0% or less. The total area ratio of bainite and tempered martensite is preferably 0% or more.

[0070] Here, the area ratio of the total bainite and tempered martensite with an equivalent circular diameter of 5.0 μm or more is measured in the thickness cross section of the high-strength hot-rolled steel sheet parallel to the rolling direction, in the range from the 1 / 8 thickness position to the 3 / 8 thickness position centered at the 1 / 4 thickness position, as follows.

[0071] First, the crystal orientations of bainite and tempered martensite are measured in 10 fields of view, the same field of view used for the area fraction measurement of bainite and tempered martensite mentioned above, using EBSD (electron backscatter diffraction) attached to the SEM without etching. The step size at this time is set to 0.1 μm. From the obtained crystal orientation map, crystal grains of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or larger are identified and their area fractions are calculated.

[0072] The proportion of line segments with a hard phase fraction of 20.0% or more among line segments with a length of 200 μm parallel to the rolling direction: 30.0% or less. If the steel structure in the thickness direction is non-uniform, that is, if the area ratio of line segments with a hard phase fraction of 20.0% or more among line segments with a length of 200 μm parallel to the rolling direction exceeds 30.0%, high elongation flangeability cannot be obtained. This is because voids generated during deformation of the steel plate become more likely to connect. Therefore, the proportion of line segments with a hard phase fraction of 20.0% or more among line segments with a length of 200 μm parallel to the rolling direction should be 30.0% or less. The proportion of line segments with a hard phase fraction of 20.0% or more among line segments with a length of 200 μm parallel to the rolling direction is preferably 20.0% or less, more preferably 10.0% or less, even more preferably 5.0% or less, and particularly preferably 1.0% or less. Here, the fraction of the hard phase in the line segment is the length fraction, that is, the fraction of the total length occupied by the hard phase relative to the total length of the line segment.

[0073] The method for calculating "the percentage of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more" will be explained below with reference to Figure 1. Figure 1(a) is an example of a microstructure classification image used to calculate the area fraction of the hard phase, in which the hard phase and other parts are color-coded. Figure 1(b) shows the profile of the hard phase fraction with respect to the plate thickness direction in the microstructure classification image of (a). The percentage of the line segments is measured as follows using the 10 fields of view of the observation image used to calculate the area fraction of the hard phase mentioned above, in a plate thickness cross section parallel to the rolling direction of a high-strength hot-rolled steel plate, in the range from the 1 / 8 position to the 3 / 8 position centered at the 1 / 4 position of the plate thickness. First, as shown in Figure 1(a), the percentage of the hard phase in line segments with a length of 200 μm parallel to the rolling direction is calculated at each position in the plate thickness direction of the observation image with a field of view of 200 μm × 200 μm. As a result, as shown in Figure 1(b), a profile of the hard phase fraction with respect to the plate thickness direction is obtained for each of the 10 observation fields. For each observation field, the proportion of plate thickness positions where the hard phase fraction is 20.0% or more is calculated relative to all plate thickness positions, and the average of this proportion is taken over the 10 fields. In this way, the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more was determined.

[0074] The "area ratio of the hard phase" and the "percentage of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more" mentioned above are measured and calculated from a common observation image.

[0075] 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.

[0076] Next, the mechanical properties of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0077] 0 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 250 From the viewpoint of achieving both high strength and excellent ductility, it is important to temper the tempered martensite and bainite appropriately according to the composition of the steel. After repeated studies on the relationship between the optimal degree of tempering and the composition of the steel, the following was found: If 0 > 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS, the tempering is insufficient, and the ductility and elongation flangeability decrease. Therefore, we set 0 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS. Preferably, 20 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS, and more preferably 50 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS. On the other hand, if 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS > 250, the tempered martensite and bainite will be excessively tempered, and conversely, ductility commensurate with the strength cannot be secured. Therefore, 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 250. Preferably, 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 230, and more preferably, 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 200. Here, each element symbol in the above formula represents the content (mass %) of each element, and if an element is not contained, it is set to 0.

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

[0079] Plating Layer The high-strength hot-rolled steel sheet of the present invention has a plating layer on its surface. This plating layer is not particularly limited and may be any known plating layer, but a zinc plating layer is preferred. When a zinc plating layer is used, it is preferable that it contains 0.08% to 0.30% Al. Furthermore, even if 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. Moreover, this zinc plating layer may be an alloyed zinc plating layer that has undergone alloying treatment. In addition to a zinc plating layer, other known plating layers such as Zn-Al plating layers, Zn-Al-Mg plating layers, Al-Zn plating layers, and Al-Si plating layers containing more than 0.30% Al, which are produced by hot-dip plating or various electroplating methods, may also be used as the plating layer.

[0080] [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.

[0081] The above-described high-strength hot-rolled steel sheet possesses high strength, excellent ductility and elongation flangeability, and excellent strain distribution ability over a wide strain range, in addition to excellent low-temperature toughness. Therefore, the component according to one embodiment of the present invention is particularly suitable for application to components with complex shapes used in the automotive field.

[0082] [3] Method for manufacturing high-strength hot-rolled steel sheets Next, a method for manufacturing high-strength hot-rolled steel sheets according to one embodiment of the present invention will be described. The method for manufacturing high-strength hot-rolled steel sheets according to one embodiment of the present invention comprises the following steps: A heating step in which the steel material is heated to 1150°C or higher. A hot rolling step in which the steel material after the heating step is hot-rolled to a hot-rolled steel sheet under the conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of 800°C or higher and 980°C or lower. A cooling step in which the hot-rolled steel sheet after the hot rolling step is cooled under the conditions of a time from the end of hot rolling to the start of cooling of 2.0 seconds or less, an average cooling rate of FF°C / s or higher, and a cooling stop temperature Tq of Ms-450°C or higher and Ms+30°C or lower. Here, FF (°C / s) is defined as 10^(3.5 - 1.2 × (2.5 × C + Mn + 0.7 × Cr + 0.1 × Mo + 0.5 × Ni)), and Ms (°C) is defined as 550 - 361 × C - 39 × Mn + 30 × Al - 35 × V - 20 × Cr - 5 × Mo - 10 × Cu - 37 × Ni - 5 × W + 15 × Co. In the above formulas, each element symbol represents the mass percentage content of each element, and the content of elements that are not present is set to 0. Next, there is a heating step in which the temperature of the hot-rolled steel sheet after the cooling step is raised under the conditions that the maximum temperature Tm is between 300°C and 500°C, and Tm - Tq ≥ 10°C. After the heating step, the hot-rolled steel sheet is held in a temperature range between Tq°C and Tm°C for a holding time of 5 seconds or more. Furthermore, the process may optionally include a plating step in which the manufactured high-strength hot-rolled steel sheet is plated.

[0083] Unless otherwise specified, the above temperatures refer to the surface temperature of the steel material and steel plate. The above average cooling rate refers to the average cooling rate of the steel plate surface. Unless otherwise specified, the average cooling rate is calculated as [(cooling start temperature - cooling stop temperature) / cooling time from cooling start temperature to cooling stop temperature].

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

[0085] [Heating Process] Heating temperature of steel material: 1150°C or higher After cooling to a low temperature, most of the elements that form carbonitrides, such as Ti, precipitate unevenly as coarse carbonitrides in the steel material such as slabs. The presence of these coarse and uneven precipitates leads to deterioration of various properties, such as strength and 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 (i.e., while maintaining a temperature within the above heating temperature range).

[0086] [Hot Rolling Process] Next, the steel material heated to 1150°C or higher (including materials delivered directly at high temperature after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The 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.

[0087] Finish Rolling Start Temperature: 1000°C or higher, 1250°C or lower. A hot-rolled steel sheet (finished rolled sheet) is obtained by performing finish rolling on a rough-rolled sheet at a predetermined finish rolling start temperature and finish temperature. If the finish rolling start temperature is too low, recrystallization of austenite grains becomes difficult during finish rolling, and the austenite becomes flattened, increasing the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more. This reduces the elongation flange property. For this reason, the finish rolling start temperature should be 1000°C or higher. The finish rolling start temperature is preferably 1020°C or higher, and more preferably 1040°C or higher. On the other hand, if the finishing rolling start temperature is too high, significant grain growth occurs in austenite grains, causing them to coarseen. This increases the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction exceeds 20.0%. This reduces the elongation flange properties. Additionally, the area ratio of the total bainite and tempered martensite with an equivalent circular diameter of 5.0 μm or more increases. This reduces the bendability.

[0088] Finish rolling completion temperature: 800°C or higher, 980°C or lower. If the finish rolling completion temperature is below 800°C, the austenite grains elongate, and the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more increases. This reduces the stretch flange properties. 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 the austenite grains occurs, causing the austenite grains to coarseen, and the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more increases. This reduces the stretch flange properties. In addition, the total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more increases. This reduces the flexibility. Therefore, the finish rolling completion temperature should be 980°C or lower. Preferably, the finish rolling completion temperature should be 970°C or lower, and more preferably 950°C or lower.

[0089] [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 below at the average cooling rate described below (hereinafter also referred to as "forced cooling").

[0090] Time from end of hot rolling to start of cooling: within 2.0 seconds. The time from the end of finish rolling to the start of forced cooling (cooling start time) is controlled. If the cooling start time is too long, austenite grain growth occurs, and the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the fraction of the hard phase is 20.0% or more increases. This reduces the elongation flange property. Also, the area ratio of the total bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more increases. This reduces the bendability. For this reason, the cooling start time should be 2.0 seconds or less. The cooling start time is preferably 1.8 seconds or less, more preferably 1.5 seconds or less, and even more preferably 1.0 second or less. Note that there is no particular lower limit to the cooling start time, and as an example, the cooling start time is greater than 0 seconds.

[0091] Average cooling rate: FF°C / s or higher. In forced cooling, if the average cooling rate from the finish rolling end temperature to the cooling stop temperature (hereinafter also referred to as the "average cooling rate of forced cooling") is too slow, ferrite and pearlite will be formed during cooling, and the desired area ratio of bainite and tempered martensite cannot be obtained. This reduces the elongation flange properties and bendability. For this reason, the average cooling rate of forced cooling should be FF°C / second or higher. The average cooling rate of forced cooling is preferably FF + 5°C / second or higher, and more preferably FF + 10°C / second or higher. On the other hand, there is no particular upper limit to the average cooling rate of forced cooling, but if it is too fast, it becomes difficult to control the cooling stop temperature, and the temperature difference between the inside of the steel sheet and the surface of the steel sheet becomes large, making it difficult to obtain the desired steel structure inside the steel sheet. For this reason, 500°C / second or less is preferred, 300°C / second or less is more preferred, 200°C / second or less is even more preferred, and 150°C / second or less is particularly preferred. FF is defined by the following formula (1). FF (°C / s) = 10^(3.5 - 1.2 × (2.5 × C + Mn + 0.7 × Cr + 0.1 × Mo + 0.5 × Ni)) ... (1) However, each element symbol in equation (1) represents the mass percentage content of each element in the above-described component composition, and the content of elements that are not present is considered to be 0.

[0092] Cooling stop temperature Tq: Ms-450°C or higher and Ms+30°C or lower. If the cooling stop temperature Tq is less than Ms-450°C, the total area ratio of retained austenite and pearlite decreases. This reduces the strain distribution capacity. For this reason, the cooling stop temperature Tq should be Ms-450°C or higher. The cooling stop temperature Tq is preferably Ms-400°C or higher, more preferably Ms-300°C or higher, even more preferably Ms-200°C or higher, and particularly preferably Ms-150°C or higher. If the cooling stop temperature Tq exceeds Ms+30°C, the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the fraction of the hard phase is 20.0% or more increases. This reduces the elongation flange property. In addition, the hard phase increases, which may further reduce the elongation flange property and bendability. For this reason, the cooling stop temperature Tq should be Ms+30°C or lower. The cooling stop temperature Tq is preferably Ms + 20°C or lower, more preferably Ms + 10°C or lower, even more preferably less than Ms, and particularly preferably Ms - 5°C or lower. Ms is defined by the following formula (2): Ms (°C) = 550 - 361 × C - 39 × Mn + 30 × Al - 35 × V - 20 × Cr - 5 × Mo - 10 × Cu - 37 × Ni - 5 × W + 15 × Co ... (2) However, each element symbol in formula (2) represents the mass percentage content of each element in the above-described component composition, and the content of elements that are not present is considered to be 0.

[0093] [Heating Process] Next, the hot-rolled steel sheet after the cooling process is heated to a maximum temperature Tm of 300°C or more and 500°C or less, with Tm - Tq ≥ 10°C. The means of heating the steel sheet may be any; external heating may be used, or the heat generated by the hot-rolled steel sheet itself may be utilized.

[0094] Maximum temperature Tm: 300°C or more and 500°C or less. If the maximum temperature exceeds 500°C, the tempering of tempered martensite and bainite becomes excessive, and the condition 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 250 is no longer satisfied. This reduces ductility. In addition, pearlite may increase, potentially further reducing ductility. For this reason, the maximum temperature Tm should be 500°C or less. Preferably, the maximum temperature Tm is 480°C or less, more preferably 450°C or less, and even more preferably less than 430°C. On the other hand, if the maximum temperature reached is less than 300°C, the tempering of the tempered martensite and bainite becomes insufficient, and the condition 0 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS is no longer satisfied. As a result, the ductility and elongation flangeability decrease, and even if a steel structure with the desired area ratio is obtained, the desired ductility and elongation flangeability cannot be obtained. For this reason, the maximum temperature reached Tm should be 300°C or higher. Preferably, the maximum temperature reached Tm is 320°C or higher, more preferably 340°C or higher, and even more preferably over 360°C.

[0095] Tm - Tq ≥ 10°C. If Tm - Tq < 10°C, the proportion of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more increases. This reduces the stretch flange properties. Therefore, Tm - Tq ≥ 10°C is set. Preferably, Tm - Tq ≥ 15°C, more preferably Tm - Tq ≥ 20°C, and even more preferably Tm - Tq ≥ 30°C. The values ​​of Tm and Tq are as described above.

[0096] [Holding Process] Next, the hot-rolled steel sheet after the heating process is held in a temperature range of Tq to Tm°C for a holding time of 5 seconds or more. The conditions of the hot-rolled steel sheet during holding in this temperature range, other than the temperature, are not particularly limited, but it is preferable to wind the hot-rolled steel sheet after the heating process, for example, into a coil, to enhance its heat retention.

[0097] Holding time in the temperature range of Tq to Tm°C: 5 seconds or more. If the holding time in the temperature range of Tq to Tm°C is less than 5 seconds, the proportion of line segments with a hard phase fraction of 20.0% or more among the 200 μm length line segments parallel to the rolling direction increases. This reduces the stretch flange properties. For this reason, the holding time in the temperature range of Tq to Tm°C should be 5 seconds or more. Preferably, the holding time in the temperature range of Tq to Tm°C should be 10 seconds or more, more preferably 30 seconds or more, and most preferably 600 seconds or more. There is no particular upper limit to the holding time in the temperature range of Tq to Tm°C, but from the viewpoint of production efficiency, 20 hours or less is preferred.

[0098] The high-strength hot-rolled steel sheet of the present invention is manufactured through the above process. Temper rolling (skin pass rolling) may be performed. Alternatively, pickling may be performed to remove scale.

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

[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 below. 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 at 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 start temperature [°C] and finish rolling end 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 cooling process. Table 2 below lists the conditions for the cooling process, including the cooling start time (time from the end of hot rolling to the start of forced cooling) [seconds], the average cooling rate (average cooling rate from the end temperature of finish rolling to the cooling stop temperature) [°C / s], and the cooling stop temperature Tq [°C]. After the cooling process was completed, the obtained hot-rolled steel sheet was subjected to a heating process. Table 2 lists the conditions for the heating process, including the maximum temperature reached Tm and Tm-Tq [°C]. Subsequently, the hot-rolled steel sheet after the cooling process was wound up. The wound hot-rolled steel sheet after the heating process was subjected to a holding process. Table 2 below lists the conditions for the holding process, including the holding time [seconds] in a temperature range of Tq or more and Tm°C or less. In this way, a high-strength hot-rolled steel sheet was obtained. The obtained high-strength hot-rolled steel sheet was subjected to temper rolling, and then pickled (hydrochloric acid concentration: 10 mass%, temperature 85°C) to remove scale. The plate thickness was set to 3.0 mm.

[0103]

[0104]

[0105] [Evaluation of High-Strength Hot-Rolled Steel Sheets] Using the obtained high-strength hot-rolled steel sheets, the microstructure of the high-strength hot-rolled steel sheets was identified according to the procedure described above. The measurement results are shown in Table 3. In Table 3, B is the area percentage of bainite, TM is the area percentage of tempered martensite, HP is the area percentage of hard phase, γ is the area percentage of retained austenite, P is the area percentage of pearlite, and F is the area percentage of ferrite. Also, S １ This is the area ratio of the total area of ​​bainite and tempered martensite with an equivalent circular diameter of 5.0 μm or more, S ２This is the ratio of line segments with a length of 200 μm parallel to the rolling direction in which the fraction of the hard phase is 20.0% or more.

[0106]

[0107] Furthermore, tensile tests, hole expansion tests, and bending tests were performed according to the following procedure, and the tensile strength (TS), uniform elongation (U.El), critical hole expansion ratio (λ), yield ratio (YR), and R / t value (R / t) were evaluated according to the following criteria. In addition, the value of 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS (RCS) was calculated. The measurement results are shown in Table 4. The presence or absence of a plating layer is also shown in Table 4. Here, - indicates no plating layer, GA indicates alloyed hot-dip galvanizing, and GI indicates non-alloyed hot-dip galvanizing.

[0108] (1) Tensile Test Tensile tests were conducted in accordance with JIS Z 2241. Specifically, JIS No. 5 test specimens were taken from the obtained high-strength hot-rolled steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. Tensile tests were performed on the taken test specimens under the condition of a crosshead speed of 10 mm / min, and YS, TS, and U. El were measured. YR was calculated from YS / TS. The results are shown in Table 4. A TS of 980 MPa or higher (TS ≥ 980 MPa) was considered a pass, and anything else was considered a fail. A U. El of 5.0% or higher (U. El ≥ 5.0%) was considered a pass (exhibiting excellent ductility), and anything else was considered a fail. A YR of 0.95 or lower (YR ≤ 0.95, i.e., 95% or lower) was considered a pass (exhibiting excellent strain distribution ability), and anything else was considered a fail.

[0109] (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. Note that λ is an index used to evaluate the stretch flange property. The results are shown in Table 4. A λ of 40% or more (λ ≥ 40%) was considered a pass (excellent stretch flange property), and anything else was considered a fail. 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.

[0110] (3) Bending Test The obtained high-strength hot-rolled steel sheets were subjected to shearing, and bending tests were taken on 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. 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 R / t value is an index for evaluating bendability. The results are shown in Table 4. For the R / t value, a value of 1.5 or less (R / t value ≤ 1.5) was considered acceptable (indicating high bendability) when the pressure was 980 MPa ≤ TS < 1180 MPa, 3.5 or less (R / t value ≤ 3.5) when the pressure was 1180 MPa ≤ TS < 1310 MPa, and 4.0 or less (R / t value ≤ 4.0) when the pressure was 1310 MPa ≤ TS. Any other value was considered unacceptable.

[0111] In the examples of the present invention, high-strength hot-rolled steel sheets are obtained that are high in strength, have high ductility, excellent strain distribution ability, and high elongation flangeability and 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 0.300% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and less than 4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is as follows: total area ratio of bainite and tempered martensite: 80.0% or more, area ratio of hard phase: less than 20.0%, and area ratio of pearlite: 0% or more and 10.0% or less, the hard phase includes retained austenite, with an area ratio of retained austenite of 15.0% or less, and a total area ratio of retained austenite and pearlite of 1.0% or more. A high-strength hot-rolled steel sheet having a total area ratio of bainite and tempered martensite with an equivalent circle diameter of 5.0 μm or more: 10.0% or less, a ratio of line segments with a length of 200 μm parallel to the rolling direction in which the hard phase fraction is 20.0% or more: 30.0% or less, a tensile strength TS (MPa) and the component composition (mass%) satisfying the formula 0 ≤ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) - TS ≤ 250, where each element symbol in the formula represents the content (mass%) of each element, and 0 is used for elements that are not contained, and 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. A high-strength hot-rolled steel sheet according to claim 1 or 2, having a plating layer on its surface.

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

5. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 1 or 2, comprising: a heating step of heating a steel material to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step to a hot-rolled steel sheet under the conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of 800°C or higher and 980°C or lower; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step under the conditions of a time from the end of the hot-rolling to the start of cooling of 2.0 seconds or less, an average cooling rate of FF°C / s or higher, and a cooling stop temperature Tq of Ms-450°C or higher and Ms+30°C or lower; and a heating step of raising the steel sheet temperature of the hot-rolled steel sheet after the cooling step under the conditions of a maximum temperature Tm of 300°C or higher and 500°C or lower, and Tm-Tq ≥ 10°C. A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a holding step, in which the hot-rolled steel sheet after the heating step is held in a temperature range of Tq or more and Tm°C or less for a holding time of 5 seconds or more; and a holding step, in which the hot-rolled steel sheet is held. Here, FF and Ms are defined by the following formulas, respectively: FF (°C / s) = 10^(3.5 - 1.2 × (2.5 × C + Mn + 0.7 × Cr + 0.1 × Mo + 0.5 × Ni)) Ms (°C) = 550 - 361 × C - 39 × Mn + 30 × Al - 35 × V - 20 × Cr - 5 × Mo - 10 × Cu - 37 × Ni - 5 × W + 15 × Co In each of the above formulas, the element symbols represent the mass percentage content of each element, and the content of elements that are not present is set to 0.

6. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 5, further comprising a plating step of applying a plating treatment to the hot-rolled steel sheet after the holding step.

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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