High-strength hot-rolled steel sheet, member, and production methods for high-strength hot-rolled steel sheet and member

Abstract

There is provided a high-strength hot-rolled steel sheet which has a high strength of 1180 MPa or more, excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range, and, in addition, has excellent low-temperature toughness. The high-strength hot-rolled steel sheet of the present invention has a predetermined chemical composition, and has a steel microstructure including a predetermined area ratio of upper bainite, a predetermined total area ratio of lower bainite and tempered martensite, a predetermined area ratio of fresh martensite, and a predetermined area ratio of retained austenite. Further, the steel microstructure has the following feature: the average amount of solute C in retained austenite: not less than 0.60 mass % and not more than 0.90 mass %, the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more: 10.0% or less, the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more: 10.0% or less, and the total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in hard-phase regions (retained austenite and fresh martensite): 1.0% or more.

Description

Technical Field

[0001] The present invention relates to a high-strength hot-rolled steel sheet and a member, and a method for producing them.Background Art

[0002] In recent years, improving the fuel efficiency and electric power consumption of automobiles has been an important issue from the viewpoint of global environmental conservation. There has been a growing trend toward a reduction in the weight of automotive bodies by increasing the strength and reducing the thickness of steel sheets, which serve as a material for automotive members or parts. Therefore, high-strength hot-rolled steel sheets have been increasingly used as a material for automotive members. Such high-strength hot-rolled steel sheets are used not only for automotive structural members and frame members, but for automotive chassis members, truck frame members, etc.

[0003] In particular, a high-strength hot-rolled steel sheet having a tensile strength of 1180 MPa or more is expected as a material that can dramatically improve the fuel efficiency and electric power consumption of automobiles through their weight reduction.

[0004] Various studies have been conducted to date on such a steel sheet which can be used as a material for automotive members.

[0005] For example, Patent Literature 1 discloses a high-strength hot-rolled steel sheet having excellent flangeability and low-temperature toughness and having a maximum tensile strength of 980 MPa or more. The steel sheet has a chemical composition comprising, in mass %, C: 0.01 to 0.20%, Si: 2.50% or less (0 not included), Mn: 4.00% or less (0 not included), P: 0.10% or less (0 not included), S: 0.03% or less (0 not included), Al: 0.001 to 2.00%, N: 0.01% or less (0 not included), O: 0.01% or less (0 not included), one or both of Ti and Nb: 0.01 to 0.30% in total, with the balance being iron and incidental impurities. The microstructure of the steel sheet comprises one or both of tempered martensite and lower bainite in a total volume fraction of 90% or more. 1 x 10 6< (number / mm 2< ) or more of iron carbides are present in the one or both of tempered martensite and lower bainite, the effective grain size of the one or both of tempered martensite and lower bainite is 10 µm or less, the aspect ratio of the effective grains of the one or both of tempered martensite and lower bainite is 2 or less, and the standard deviation σ of the distribution of Vickers hardness is 15 or less.

[0006] Patent Literature 2 discloses a hot-rolled steel sheet having a chemical composition comprising, in mass %, C: 0.15 to 0.30%, Si: 0.50 to 4.00%, Mn: 2.00 to 4.00%, P: 0.100% or less, S: 0.005% or less, Al: 0.010 to 0.500%, N: 0.010% or less, V: 0.20 to 1.00%, Nb: 0 to 0.10%, Ti: 0 to 0.10%, B: 0 to 0.0050%, Cr: 0 to 1.000%, Mo: 0 to 0.500%, Cu: 0 to 3.00%, and Ni: 0 to 1.50%, with the balance being Fe and impurities. The Si / V ratio is 10.0 or less and, when the steel contains one or both of Cr and Mo, the contents of Cr, Mo and V satisfy the relationship: (2Cr + Mo) / 2V ≤ 2.0. When the steal contains Ni, it contains Cu. The steel sheet contains tempered martensite at an area ratio of 90% or more. Cementite having a major axis of 400 nm or less and an aspect ratio of 3 to 5 accounts for 70% or more of all cementite contained in the tempered martensite. Further, V-containing carbides having an equivalent circle diameter of 8 to 15 nm have been precipitated in the tempered martensite at a number density of 30 number / µm 2< or more.

[0007] Patent Literature 3 discloses a high-strength hot-rolled steel sheet having a tensile strength of 1180 MPa or more and an arithmetic average surface roughness Ra of 2.00 µm or less, and having a chemical composition comprising, in mass %, C: not less than 0.09% and not more than 0.20%, Si: not less than 0.2% and not more than 2.0%, Mn: not less than 1.0% and not more than 3.0%, P: 0.100% or less, S: 0.0100% or less, Al: not less than 0.01% and not more than 2.00%, N: 0.010% or less, Ti: not less than 0.001% and less than 0.030%, and B: not less than 0.0005% and not more than 0.0200%, and further comprising at least one selected from the group consisting of Cr: not less than 0.10% and not more than 1.50%, Mo: not less than 0.05% and not more than 0.45%, Nb: not less than 0.005% and not more than 0.060%, and V: not less than 0.05% and not more than 0.50%, with the balance being Fe and incidental impurities. The steel sheet has a microstructure comprising an upper bainite phase and a second phase. The upper bainite phase has an area ratio of not less than 50% and less than 90%, and an average grain size of 12.0 µm or less. The second phase is at least one selected from the group consisting of a lower bainite phase and / or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase, and has an area ratio of not less than 10% and less than 50%. The perimeter of the second phase, having an equivalent circle diameter of 0.5 µm or more, is 300,000 µm / mm 2< or more.Citation ListPatent Literature

[0008] PTL 1: Japanese Patent No. 6354268 PTL 2: Japanese Patent No. 6835294 PTL 3: Japanese Patent No. 6819840 Summary of InventionTechnical Problem

[0009] At present, hot-rolled steel sheets of at most 980 MPa grade are practically used for automotive members. Automotive parts, in particular chassis parts such as suspension parts, need to be formed into complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets, which are to be used as materials for members having such complex shapes, are required to have excellent formability.

[0010] On the other hand, in general, increasing the tensile strength of a steel sheet sometimes causes a reduction in properties such as ductility and stretch-flangeability. Thus, when a high-strength hot-rolled steel sheet, particularly one having a tensile strength of 1180 MPa or more, is formed, forming defects such as cracking are likely to occur on localization of strain. To prevent such forming defects, a steel sheet is required to not only have high ductility and stretch flangeability but have high strain dispersibility. In particular, when a steel sheet is to be formed into a complex shape, the steel sheet needs to have excellent strain dispersibility over a wide strain range, ranging from a low strain to a high strain.

[0011] Further, increasing the tensile strength of a steel sheet reduces its low-temperature toughness. When brittle cracking occurs in a suspension part or the like, having a low-temperature toughness, due to an impact load applied to the part in cold climates, the propagation of the brittle cracking cannot be stopped. In other words, a part having a low-temperature toughness may fracture or separate due to brittleness in the event of a collision, etc. Thus, it is possible that the part cannot achieve the strength assumed in design. Therefore, low-temperature toughness is required for materials for automotive parts or the like.

[0012] In fact, the steel sheets disclosed in Patent Literatures 1 to 3 cannot be said to have excellent low-temperature toughness in addition to excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range, while having such a high strength as a tensile strength (TS) of 1180 MPa or more.

[0013] As described above, none of the conventional techniques have established a hot-rolled steel sheet which has excellent low-temperature toughness in addition to excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range, while having such a high strength as a tensile strength (TS) of 1180 MPa or more.

[0014] It is therefore an object of the present invention to provide a high-strength hot-rolled steel sheet which has such a high strength as a tensile strength (TS) of 1180 MPa or more, excellent ductility and stretch flangeability, excellent strain dispersibility over a wide strain range, and, in addition, excellent low-temperature toughness.

[0015] In the present invention, "excellent ductility" means that the uniform elongation (total elongation at maximum test force, hereinafter also referred to as U. El) measured in a tensile test according to JIS Z 2241:2011 satisfies the following formula: U. El ≥ 5.0%. Excellent ductility is required to form automotive parts, in particular suspension parts having a complex shape.

[0016] In the present invention, "excellent stretch flangeability" means that the limiting hole expansion ratio (λ) measured in a hole expansion test according to JIS Z 2256:2020 satisfies the following formula: λ ≥ 30%. Excellent stretch flangeability is required particularly for a steel sheet for a suspension part because such a steel sheet is subjected to stretch flanging after punching.

[0017] A steel sheet is subjected to severe forming to produce an automotive part, in particular a suspension part. It is therefore necessary to reduce localization of strain in the steel sheet. In particular, in order to perform severe forming of a steel sheet while maintaining a high strength of TS of 1180 MPa or more, the steel sheet is required to have excellent strain dispersibility over a wide strain range. The phrase "have excellent strain dispersibility over a wide strain range" means that the steel sheet has both excellent strain dispersibility in a low strain range and excellent strain dispersibility in a high strain range.

[0018] When a steel sheet has excellent strain dispersibility in a low strain range, the steel sheet can avoid cracking and wrinkling that occur from the early stage of forming. In the present invention, "excellent strain dispersibility in a low strain range" means that the yield ratio (YR) = YS / TS which is defined in terms of TS and 0.2% proof stress (hereinafter also referred to as YS), measured in a tensile test according to JIS Z 2241:2011, satisfies the following formula: YR ≤ 0.93.

[0019] When a steel sheet has excellent strain dispersibility in a low strain range and also in a high strain range, the steel sheet can avoid cracking and wrinkling that occur on severe forming. In the present invention, "excellent strain dispersibility in a high strain range" means that the work hardening index (hereinafter also referred to as n-value), measured according to JIS Z 2253:2020 in the strain range of 0.7 × U. El to 0.9 × U. El, satisfies the following formula: n-value ≥ 0.080.

[0020] In the present invention, "excellent low-temperature toughness" means that the brittle-ductile fracture transition temperature (vTrs), measured in a Charpy impact test according to JIS Z 2242:2018 using a sub-size test specimen (V-notch), satisfies the following formula: vTrs ≤ -40°C.Solution to Problem

[0021] The present inventors, through their intensive studies conducted to achieve the above object, found that a high-strength hot-rolled steel sheet, which has excellent low-temperature toughness in addition to high strength, excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range, can be obtained by appropriately adjusting the chemical composition of the hot-rolled steel sheet, and controlling the microstructure of the steel sheet as follows to make its tensile strength 1180 MPa or more: the area ratio of upper bainite: not less than 30.0% and less than 90.0%, the total area ratio of lower bainite and tempered martensite: not less than 6.0% and not more than 60.0%, the area ratio of fresh martensite: not less than 2.0% and not more than 20.0%, the area ratio of retained austenite: not less than 2.0% and not more than 10.0%, the average amount of solute C in retained austenite: not less than 0.60 mass % and not more than 0.90 mass %, the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more: 10.0% or less, the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more: 10.0% or less, and, assuming that the retained austenite and the fresh martensite are hard phases, the total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in the hard-phase regions is 1.0% or more.

[0022] The present invention has been accomplished based on the above finding and on additional studies.

[0023] Thus, the present invention can be summarized as follows. [1] A high-strength hot-rolled steel sheet having a chemical composition comprising, in mass %: C: not less than 0.100% and not more than 0.250%, Si: not less than 0.20% and not more than 2.00%, Mn: not less than 1.00% and less than 3.50%, P: not more than 0.100%, S: not more than 0.0200%, Al: not less than 0.010% and not more than 2.000%, and N: not more than 0.0100%, and at least one selected from Ti: not less than 0.005% and not more than 0.200% and Nb: not less than 0.005% and not more than 0.200%, with the balance being Fe and incidental impurities, wherein the steel sheet has the following microstructure: the area ratio of upper bainite: not less than 30.0% and less than 90.0%, the total area ratio of lower bainite and tempered martensite: not less than 6.0% and not more than 60.0%, the area ratio of fresh martensite: not less than 2.0% and not more than 20.0%, the area ratio of retained austenite: not less than 2.0% and not more than 10.0%, the average amount of solute C in retained austenite: not less than 0.60 mass % and not more than 0.90 mass %, the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more: 10.0% or less, the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more: 10.0% or less, and, assuming that the retained austenite and the fresh martensite are hard phases, the total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in the hard-phase regions is 1.0% or more, and wherein the steel sheet has a tensile strength of 1180 MPa or more. [2] The high-strength hot-rolled steel sheet according to [1], wherein the chemical composition further comprises, in mass %, one or more selected from the following groups a and b: group a: one or more selected from V: not less than 0.001% and not more than 0.100%, Cr: not less than 0.005% and not more than 1.000%, and Mo: not less than 0.005% and not more than 0.500%, and group b: one or more selected from B: 0.0100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. [3] The high-strength hot-rolled steel sheet according to [1] or [2], having a coating layer on its surface. [4] A member formed using the high-strength hot-rolled steel sheet according to any one of [1] to [3]. [5] A method for producing the high-strength hot-rolled steel sheet according to [1] or [2], comprising: a heating step of heating a steel material having the chemical composition to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step under the following conditions: finish rolling start temperature: not less than 1000°C and not more than 1250°C, and finish rolling delivery temperature: not less than (RC1 - 100)°C and not more than (RC1 + 100)°C, thereby forming a hot-rolled steel sheet; a first cooling step of cooling and coiling the hot-rolled steel sheet after the hot-rolling step under the following conditions: time from the end of the hot rolling to the start of cooling: within RC2 seconds, average cooling rate: FF°C / s or more, and cooling stop temperature: not less than 350°C and not more than (Bs + 50) °C; a first holding step of holding the hot-rolled steel sheet after the first cooling step under the following conditions: holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of not less than 350°C and not more than 450°C: not less than 20 minutes and not more than 500 minutes; a second holding step of holding the hot-rolled steel sheet after the first holding step under the following condition: holding time in the temperature range of not less than 250°C and less than 350°C: not less than 30 minutes and not more than 500 minutes; and a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower, wherein RC1, RC2, FF, and Bs are defined by the following formulae: Bs = 830 − 270 × C − 90 × Mn − 70 × Cr − 37 × Ni − 83 × Mo where the element symbols each represent the content of the element in mass %, which is 0 when the element is not present. [6] A method for producing the high-strength hot-rolled steel sheet according to [3], comprising: a heating step of heating a steel material having the chemical composition to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step under the following conditions: finish rolling start temperature: not less than 1000°C and not more than 1250°C, and finish rolling delivery temperature: not less than (RC1 - 100)°C and not more than (RC1 + 100)°C, thereby forming a hot-rolled steel sheet; a first cooling step of cooling and coiling the hot-rolled steel sheet after the hot-rolling step under the following conditions: time from the end of the hot rolling to the start of cooling: within RC2 seconds, average cooling rate: FF°C / s or more, and cooling stop temperature: not less than 350°C and not more than (Bs + 50)°C; a first holding step of holding the hot-rolled steel sheet after the first cooling step under the following conditions: holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of not less than 350°C and not more than 450°C: not less than 20 minutes and not more than 500 minutes; a second holding step of holding the hot-rolled steel sheet after the first holding step under the following condition: holding time in the temperature range of not less than 250°C and less than 350°C: not less than 30 minutes and not more than 500 minutes; a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower; and a coating step of subjecting the hot-rolled steel sheet to a coating treatment, wherein RC1, RC2, FF, and Bs are defined by the following formulae: Bs = 830 − 270 × C − 90 × Mn − 70 × Cr − 37 × Ni − 83 × Mo where the element symbols each represent the content of the element in mass %, which is 0 when the element is not present. [7] A method for producing a member, comprising a step of subjecting the high-strength hot-rolled steel sheet according to any one of [1] to [3] to at least one of forming and joining. Advantageous Effects of Invention

[0024] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet which has such a high strength as a tensile strength (TS) of 1180 MPa or more, excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range, and, in addition, has excellent low-temperature toughness.Description of Embodiments

[0025] Embodiments of a high-strength hot-rolled steel sheet and a method for producing the same according to the present invention will now be described. It should be noted that the present invention is not limited to the following embodiments.[1] High-Strength Hot-Rolled Steel Sheet

[0026] A description will be first given of the chemical composition of a high-strength hot-rolled steel sheet according to an embodiment of the present invention. Units in the chemical composition are "mass %", but will be expressed simply as "%" unless otherwise specified.C: not less than 0.100% and not more than 0.250%

[0027] C is an element that has the effect of improving the strength of steel. C improves hardenability and thereby promotes the formation of bainite, thus contributing to increasing the strength. C contributes to increasing the strength of steel also by increasing the strength of martensite. To achieve a tensile strength of 1180 MPa or more, the C content needs to be 0.100% or more. Therefore, the C content is made 0.100% or more. The C content is preferably 0.105% or more, more preferably 0.110% or more. On the other hand, if the C content exceeds 0.250%, the strength of martensite will increase excessively. This increases the difference in strength between upper bainite as a main phase and fresh martensite / retained austenite, resulting in a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, the C content is made 0.250% or less. The C content is preferably 0.220% or less, more preferably 0.200% or less.Si: not less than 0.20% and not more than 2.00%

[0028] Si has the effect of suppressing the formation of Fe carbides, and is useful to prevent the precipitation of cementite upon upper bainite transformation, thereby obtaining an appropriate amount of upper bainite. Si can therefore distribute C in untransformed austenite. Accordingly, in the below-described second cooling step, the untransformed austenite is transformed into fresh martensite and / or retained austenite. This makes it possible to obtain desired fresh martensite and desired retained austenite. To achieve these effects, the Si content needs to be 0.20% or more. Therefore, the Si content is made 0.20% or more. The Si content is preferably 0.60% 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 2.00%, the subscales will be formed too thick. Therefore, the surface roughness of the steel sheet will be too high even when descaling of the steel sheet surface is performed during hot rolling, resulting in a deterioration in the pre-paint treatability of the high-strength hot-rolled steel sheet when it is painted. Therefore, the Si content is made 2.00% or less. The Si content is preferably 1.60% or less, more preferably 1.30% or less.Mn: not less than 1.00% and less than 3.50%

[0029] Mn stabilizes austenite and contributes to inhibiting the formation of ferrite and to forming fresh martensite and / or retained austenite. To achieve these effects, the Mn content needs to be 1.00% or more. Therefore, the Mn content is made 1.00% or more. The Mn content is preferably 1.50% or more. On the other hand, if the Mn content is 3.50% or more, the area ratio of upper bainite decreases, the area ratios of lower bainite and tempered martensite increase, U. El decreases, and YR increases. Therefore, the Mn content is made less than 3.50%. The Mn content is preferably 3.20% or less, more preferably 3.00% or less.P: 0.100% or less

[0030] P is dissolved in steel and contributes to increasing the strength of steel. On the other hand, P is an element which segregates at austenite grain boundaries during hot rolling, causing slab cracking during hot rolling. In addition, P segregates at grain boundaries and reduces uniform elongation. Therefore, the P content is preferably as low as possible; however, the inclusion of P content in an amount of up to 0.100% is acceptable. Thus, the P content is made 0.100% or less. The P content is preferably 0.030% or less. While the lower limit of the P content is not particularly limited, the P content is preferably 0.001% or more, for example, from the viewpoint of productivity.S: 0.0200% or less

[0031] S combines with Ti and Mn to form coarse sulfides. The coarse sulfides accelerate the formation of voids, resulting in a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, the S content is preferably as low as possible; however, the inclusion of S in an amount of up to 0.0200% is acceptable. Therefore, the S content is made 0.0200% or less. The S content is preferably 0.0080% or less. While the lower limit of the S content is not particularly limited, the S content is preferably 0.0001% or more, for example, from the viewpoint of productivity.Al: not less than 0.010% and not more than 2.000%

[0032] Al is an element which acts as a deoxidizer and which is effective in improving the cleanliness of steel. The effect is insufficient when the Al content is less than 0.010%; therefore, the Al content is made 0.010% or more. As with Si, Al has the effect of suppressing the formation of Fe carbides and inhibiting the precipitation of cementite during upper bainite transformation. This contributes to the formation of fresh martensite and / or retained austenite in the second cooling step. The Al content is preferably 0.015% or more, more preferably 0.020% or more. On the other hand, the inclusion of Al in an amount of more than 2.000% increases oxide inclusions, resulting in a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, the Al content is made 2.000% or less. The Al content is preferably 1.000% or less, more preferably 0.300% or less.N: 0.0100% or less

[0033] N combines with a nitride-forming element and precipitates as a nitride, which generally contributes to grain size refinement. On the other hand, N combines with Ti at a high temperature to form coarse nitrides. Therefore, the inclusion of N in an amount of more than 0.0100% causes a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, the N content is made 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less. While the lower limit of the N content is not particularly limited, the N content is preferably 0.0005% or more, for example, from the viewpoint of productivity.

[0034] The chemical composition of the high-strength hot-rolled steel sheet further comprises at least one selected from Ti: not less than 0.005% and not more than 0.200%, and Nb: not less than 0.005% and not more than 0.200%.Ti: not less than 0.005% and not more than 0.200%

[0035] Ti is an element that has the effect of improving the strength of the steel sheet through precipitation strengthening or solid solution strengthening. Ti is also an element which is effective in improving stretch flangeability by increasing the hardness of relatively soft upper bainite and reducing the difference in hardness between upper bainite and hard fresh martensite. In addition, Ti increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the non-recrystallization region of austenite and thereby contributing to grain size refinement in a BCC phase. These effects can be achieved when the Ti content is 0.005% or more. Therefore, when the steel contains Ti, the Ti content is made 0.005% or more. The Ti content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of Ti precipitates will be formed, which rather deteriorates stretch flangeability. Therefore, when the steel contains Ti, the Ti content is made 0.200% or less. The Ti content is preferably 0.150% or less, more preferably 0.120% or less.Nb: not less than 0.005% and not more than 0.200%

[0036] As with Ti, Nb is an element that has the effect of improving the strength of the steel sheet through precipitation strengthening or solid solution strengthening. Nb is also an element which is effective in improving stretch flangeability by increasing the hardness of relatively soft upper bainite and reducing the difference in hardness between upper bainite and hard fresh martensite. Further, as with Ti, Nb increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the non-recrystallization region of austenite and thereby contributing to grain size refinement in a BCC phase. In addition, Nb is a carbide-forming element and segregates at interfaces between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces a driving force for bainite transformation, thereby stopping upper bainite transformation while leaving untransformed austenite. The untransformed austenite is transformed into fresh martensite and / or retained austenite in the second cooling step. Therefore, when the steel contains Nb, the Nb contributes to the formation of desired area fractions of fresh martensite and retained austenite. These effects can be achieved when the Nb content is 0.005% or more. Therefore, when the steel contains Nb, the Nb content is made 0.005% or more. The Nb content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of Nb precipitates will be formed, which rather deteriorates stretch flangeability. Furthermore, it is possible that fresh martensite may increase excessively, resulting in a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, when the steel contains Nb, the Nb content is made 0.200% or less. The Nb content is preferably 0.150% or less, more preferably 0.120% or less.

[0037] The above-described components are the basic components of the high-strength hot-rolled steel sheet of the present invention. The high-strength hot-rolled steel sheet of the present invention can have a chemical composition containing the above-described components, with the balance being Fe and incidental impurities.

[0038] In addition to the above-described components, the high-strength hot-rolled steel sheet of the present invention may further contain one or more selected from the following groups a and b: (group a) one or more selected from V: not less than 0.001% and not more than 0.100%, Cr: not less than 0.005% and not more than 1.000%, and Mo: not less than 0.005% and not more than 0.500%, and (group b) one or more selected from B: 0.0100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. V: not less than 0.001% and not more than 0.100%

[0039] V is an element that has the effect of improving the strength of the steel sheet through precipitation strengthening and solid solution strengthening. Further, as with Ti, V increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the non-recrystallization region of austenite and thereby contributing to grain size refinement in a BCC phase. In addition, as with Cr, V is a carbide-forming element and segregates at interfaces between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces a driving force for bainite transformation, thereby stopping upper bainite transformation while leaving untransformed austenite. The untransformed austenite is transformed into fresh martensite and / or retained austenite in the second cooling step. Therefore, when the steel contains V, the V contributes to the formation of desired area fractions of fresh martensite and retained austenite. To achieve these effects, the V content is preferably made 0.001% or more. Therefore, when the steel contains V, the V content is preferably 0.001% or more. The V content is more preferably 0.010% or more. On the other hand, if the V content exceeds 0.100%, fresh martensite will increase excessively, resulting in a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, when the steel contains V, the V content is made 0.100% or less. The V content is preferably 0.050% or less.Cr: not less than 0.005% and not more than 1.000%

[0040] Cr is a carbide-forming element and segregates at interfaces between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces a driving force for bainite transformation, thereby stopping upper bainite transformation. The untransformed austenite, which remains due to the stop of its transformation into upper bainite, is transformed into fresh martensite and / or retained austenite in the second cooling step. Therefore, when the steel contains Cr, the Cr contributes to the formation of desired area fractions of fresh martensite and retained austenite. To achieve these effects, the Cr content is preferably made 0.005% or more. Therefore, when the steel contains Cr, the Cr content is preferably 0.005% or more. The Cr content is more preferably 0.010% or more. On the other hand, Cr is an element that deteriorates corrosion resistance and pre-paint treatability. Therefore, when the steel contains Cr, the Cr content is made 1.000% or less. The Cr content is preferably 0.700% or less.Mo: not less than 0.005% and not more than 0.500%

[0041] Mo promotes the formation of bainite through improvement in hardenability, contributing to increasing the strength of the steel sheet. As with Cr, Mo is a carbide-forming element and segregates at interfaces between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces a driving force for bainite transformation, contributing to the formation of fresh martensite and retained austenite in the second cooling step. To achieve these effects, the Mo content is preferably made 0.005% or more. Therefore, when the steel contains Mo, the Mo content is preferably 0.005% or more. The Mo content is more preferably 0.010% or more. On the other hand, if the Mo content exceeds 0.500%, fresh martensite will be formed excessively, resulting in a reduction in uniform elongation and a deterioration in stretch flangeability. Therefore, when the steel contains Mo, the Mo content is made 0.500% or less. The Mo content is preferably 0.300% or less. When the content of any of V, Cr, and Mo is less than the lower limit, the component is regarded as an incidental impurity.B: 0.0100% or less

[0042] B is an element that segregates at prior-austenite grain boundaries and suppresses the formation of ferrite, thereby promoting the formation of upper bainite and contributing to improving the strength of the steel sheet. To achieve such effects, when the steel contains B, the B content is preferably made 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, the above-described effects will saturate. Therefore, when the steel contains B, the B content is made 0.0100% or less. The B content is preferably 0.0060% or less.Cu: 1.000% or less

[0043] Cu is dissolved in steel and contributes to increasing the strength of steel. Further, Cu promotes the formation of bainite through improvement in hardenability, contributing to increasing the strength. To achieve such effects, when the steel contains Cu, the Cu content is preferably made 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the surface quality of the hot-rolled steel sheet will be worsened, and the fatigue properties of the hot-rolled steel sheet will be deteriorated. Therefore, when the steel contains Cu, the Cu content is made 1.000% or less. The Cu content is preferably 0.500% or less.Ni: 1.000% or less

[0044] Ni is dissolved in steel and contributes to increasing the strength of steel. Further, Ni promotes the formation of bainite through improvement in hardenability, contributing to increasing the strength. To achieve such effects, when the steel contains Ni, the Ni content is preferably made 0.005% or more. The Ni content is more preferably 0.010% or more, even more preferably 0.040% or more. On the other hand, if the Ni content exceeds 1.000%, fresh martensite and retained austenite will increase excessively, resulting in a deterioration in the ductility of the hot-rolled steel sheet. Therefore, when the steel contains Ni, the Ni content is made 1.000% or less.Sb: 0.200% or less

[0045] Sb is an element which is effective in inhibiting denitrification, deboronation, etc., thereby preventing a reduction in the strength of steel. When the steel contains Sb, the Sb content is preferably made 0.002% or more to achieve this effect. On the other hand, if the Sb content exceeds 0.200%, the toughness of the steel will be reduced, which may cause slab cracking and hot rolling cracking. Therefore, when the steel contains Sb, the Sb content is made 0.200% or less. The Sb content is preferably 0.100% or less, more preferably 0.050% or less.Sn: 0.200% or less

[0046] As with Sb, Sn is an element which is effective in inhibiting denitrification, deboronation, etc., thereby preventing a reduction in the strength of steel. When the steel contains Sn, the Sn content is preferably made 0.005% or more to achieve this effect. On the other hand, if the Sn content exceeds 0.200%, the toughness of the steel will be reduced, which may cause slab cracking and hot rolling cracking. Therefore, when the steel contains Sn, the Sn content is made 0.200% or less. The Sn content is preferably 0.100% or less, more preferably 0.050% or less.Ta: 0.100% or less

[0047] Ta increases TS through the formation of fine carbides, nitrides or carbonitrides. In addition, Ta is partly dissolved in Nb carbides or Nb carbonitrides to form complex precipitates such as (Nb,Ta) (C,N). This prevents coarsening of precipitates and stabilizes precipitation strengthening, thereby increasing TS. To achieve such effects, when the steel contains Ta, the Ta content is preferably made 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may sometimes be formed. This may result in a deterioration in stretch flangeability. Therefore, when the steel contains Ta, the Ta content is made 0.100% or less. The Ta content is preferably 0.050% or less, more preferably 0.020% or less.W: 0.500% or less

[0048] W is an element which is effective in improving hardenability and adjusting TS to a more suitable range. To achieve such effects, when the steel contains W, the W content is preferably made 0.001% or more. The W content is more preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of hard fresh martensite will increase excessively, which may cause a reduction in ductility and in work hardening ability in a high strain range. Therefore, when the steel contains W, the W content is made 0.500% or less. The W content is preferably 0.100% or less, more preferably 0.050% or less.Mg: 0.0200% or less

[0049] As with Ca, Mg controls the shape of oxide inclusions, sulfide inclusions, etc., thereby contributing to preventing cracking in the sheared edge surface of the steel sheet and to further improving stretch flangeability. To achieve such effects, when the steel contains Mg, the Mg content is preferably made 0.0010% or more. On the other hand, the inclusion of Mg in an amount of more than 0.0200% reduces the cleanliness of the steel, which may rather cause cracking in the sheared edge surface or stretch flange cracking. Therefore, when the steel contains Mg, the Mg content is made 0.0200% or less. The Mg content is preferably 0.0050% or less.Zn: 0.0200% or less

[0050] Zn makes the shape of inclusions spherical, thereby contributing to preventing cracking in the sheared edge surface of the steel sheet and to further improving stretch flangeability. To achieve such effects, when the steel contains Zn, the Zn content is preferably made 0.0010% or more. On the other hand, the inclusion of Zn in an amount of more than 0.0200% reduces the cleanliness of the steel, which may rather cause cracking in the sheared edge surface or stretch flange cracking. Therefore, when the steel contains Zn, the Zn content is made 0.0200% or less. The Zn content is preferably 0.0050% or less.Co: 0.0200% or less

[0051] As with Zn, Co makes the shape of inclusions spherical, thereby contributing to preventing cracking in the sheared edge surface of the steel sheet and to further improving stretch flangeability. To achieve such effects, when the steel contains Co, the Co content is preferably made 0.0010% or more. On the other hand, the inclusion of Co in an amount of more than 0.0200% reduces the cleanliness of the steel, which may rather cause cracking in the sheared edge surface or stretch flange cracking. Therefore, when the steel contains Co, the Co content is made 0.0200% or less. The Co content is preferably 0.0050% or less.Zr: 0.0200% or less

[0052] As with Zn and Co, Zr makes the shape of inclusions spherical, thereby contributing to preventing cracking in the sheared edge surface of the steel sheet and to further improving stretch flangeability. To achieve such effects, when the steel contains Zr, the Zr content is preferably made 0.0010% or more. On the other hand, the inclusion of Zr in an amount of more than 0.0200% reduces the cleanliness of the steel, which may rather cause cracking in the sheared edge surface or stretch flange cracking. Therefore, when the steel contains Zr, the Zr content is made 0.0200% or less. The Zr content is preferably 0.0050% or less.Ca: 0.0200% or less

[0053] Ca controls the shape of oxide inclusions, sulfide inclusions, etc., thereby contributing to preventing cracking in the sheared edge surface of the steel sheet and to further improving stretch flangeability. To achieve such effects, when the steel contains Ca, the Ca content is preferably made 0.0005% or more. On the other hand, the inclusion of Ca in an amount of more than 0.0200% increases Ca inclusions and reduces the cleanliness of the steel, which may rather cause cracking in the sheared edge surface or stretch flange cracking. Therefore, when the steel contains Ca, the Ca content is made 0.0200% or less. The Ca content is preferably 0.0050% 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, REM: 0.0200% or less

[0054] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM (rare earth metal), like Ca, each controls the shape of oxide inclusions, sulfide inclusions, etc., thereby contributing to preventing cracking in the sheared edge surface of the steel sheet and to further improving stretch flangeability. To achieve such effects, when the steel contains any of the above elements, the content of that element is preferably made 0.0005% or more. On the other hand, the inclusion of such an element in an amount of more than 0.0200% reduces the cleanliness of the steel, which may rather cause cracking in the sheared edge surface or stretch flange cracking. Therefore, when the steel contains any of the above elements, the content of that element is made 0.0200% or less. REM is a collective term for Sc, Y, and elements from lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. The steel sheet may contain one or more of these elements as REM, and the REM content herein refers to the total content of such elements.

[0055] A description will now be given of the steel microstructure of the high-strength hot-rolled steel sheet according to an embodiment of the present invention.Area ratio of upper bainite: not less than 30.0% and less than 90.0%

[0056] Upper bainite is a phase useful to ensure appropriate amounts of fresh martensite and retained austenite by utilizing diffusion of C from the upper bainite to untransformed austenite. Therefore, the area ratio of upper bainite is made 30.0% or more. The area ratio of upper bainite is preferably 40.0% or more, more preferably 50.0% or more. On the other hand, if the area ratio of upper bainite is 90.0% or more, it is difficult to achieve a tensile strength of 1180 MPa or more. Therefore, the area ratio of upper bainite is made less than 90.0%. The area ratio of upper bainite is preferably less than 85.0%, more preferably less than 80.0%, and even more preferably less than 70.0%.Total area ratio of lower bainite and tempered martensite: not less than 6.0% and not more than 60.0%

[0057] Lower bainite and tempered martensite each have a hardness which is intermediate between the hardness of soft upper bainite and the hardness of hard fresh martensite, and are phases useful to ensure high strength and excellent stretch flangeability. Therefore, the total area ratio of lower bainite and tempered martensite is made 6.0% or more. The total area ratio of lower bainite and tempered martensite is preferably 8.0% or more, more preferably 10.0% or more. On the other hand, if the total area ratio of lower bainite and tempered martensite exceeds 60.0%, ductility will be reduced. Therefore, the total area ratio of lower bainite and tempered martensite is made 60.0% or less. The total area ratio of lower bainite and tempered martensite is preferably 55.0% or less, more preferably 50.0% or less.Area ratio of fresh martensite: not less than 2.0% and not more than 20.0%

[0058] Fresh martensite, when it is present adjacent to other phase(s), improves strain dispersibility in a low strain range. Therefore, the area ratio of fresh martensite is made 2.0% or more. The area ratio of fresh martensite is preferably 3.0% or more, more preferably 3.5% or more. On the other hand, if the area ratio of fresh martensite exceeds 20.0%, stretch flangeability will deteriorate. Therefore, the area ratio of fresh martensite is made 20.0% or less. The area ratio of fresh martensite is preferably 17.0% or less, more preferably 15.0% or less.Area ratio of retained austenite: not less than 2.0% and not more than 10.0%

[0059] The area ratio of retained austenite is made 2.0% or more from the viewpoint of achieving good ductility and excellent work hardening ability in a high strain range. The area ratio of retained austenite is preferably 2.5% or more, more preferably 2.8% or more, and even more preferably 3.0% or more. On the other hand, if the area ratio of retained austenite exceeds 10.0%, stretch flangeability will deteriorate. Therefore, the area ratio of retained austenite is made 10.0% or less. The area ratio of retained austenite is preferably 9.0% or less, more preferably 8.0% or less.

[0060] The area ratio of the remaining microstructure of the steel sheet is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 5.0% or less. The area ratio of the remaining structure may be 0%.

[0061] The remaining microstructure is not particularly limited, and may be, for example, polygonal ferrite, acicular ferrite, pearlite, or a carbide such as cementite. The type of the remaining microstructure can be determined by observation using, for example, an SEM (Scanning Electron Microscope).

[0062] Upper bainite is an aggregation of ferrite laths having a misorientation of less than 15°, and is a microstructure which includes Fe carbides and / or retained austenite at the ferrite lath boundaries (in some cases, however, neither Fe carbides nor retained austenite is present at the ferrite lath boundaries). The microstructure either has no Fe carbides inside the ferrite laths, or has Fe carbides inside the ferrite laths, with the number of the Fe carbides present inside the ferrite laths being smaller than the number of the Fe carbides present at the ferrite lath boundaries. The ferrite lath boundaries sometimes disappear or become unclear when the upper bainite is held at a high temperature after its formation. Such upper bainite can be distinguished from polygonal ferrite and acicular ferrite, which have a small misorientation in the crystal grains, by measuring the misorientation in the crystal grains using EBSD (Electron Backscatter Diffraction). Unlike polygonal ferrite and acicular ferrite, upper bainite internally has a relatively high dislocation density. Therefore, lath-like ferrite can also be distinguished from polygonal ferrite and acicular ferrite using a TEM (Transmission Electron Microscope). When retained austenite is present between ferrite laths, only the ferrite lath portion is taken as upper bainite as distinct from the retained austenite.

[0063] Lower bainite and / or tempered martensite is an aggregation of ferrite laths having a misorientation of less than 15°, and is a microstructure which includes Fe carbides inside the ferrite laths (in some cases, Fe carbides are present also at the ferrite lath boundaries), with the number of the Fe carbides present inside the ferrite laths being larger than the number of the Fe carbides present at the ferrite lath boundaries.

[0064] Lower bainite and tempered martensite can be distinguished from each other by observing the orientation and crystal structure of Fe carbides in the ferrite laths using a TEM. However, in the present invention, the two are not distinguished because they have substantially the same properties.

[0065] Lower bainite and / or tempered martensite can be distinguished from upper bainite by their brighter contrast under an SEM and their inclusion of a large amount of fine Fe carbides and / or retained austenite.

[0066] In the present invention, assuming that fresh martensite and retained austenite are hard phases, the hard phases (fresh martensite and retained austenite), unlike lower bainite and / or tempered martensite, do not include Fe carbides. Further, fresh martensite and retained austenite have a brighter contrast in SEM images as compared to upper bainite, lower bainite and / or tempered martensite, polygonal ferrite, and acicular ferrite. Therefore, the hard phases can be distinguished from these microstructures using an SEM.

[0067] Fresh martensite and retained austenite have similar shapes and contrasts under an SEM, making them difficult to distinguish. Therefore, the area ratio of each of fresh martensite and retained austenite is determined by the below-described method.

[0068] The area ratios of upper bainite, lower bainite and tempered martensite, fresh martensite, retained austenite, and the remaining microstructure are herein measured at a 1 / 4 thickness position on the high-strength hot-rolled steel sheet in the following manner.

[0069] A sample is cut out from the hot-rolled steel sheet such that a thickness-direction cross section, parallel to the rolling direction of the hot-rolled steel sheet, serves as an observation surface. Subsequently, the observation surface of the sample is mirror-polished using diamond paste. Subsequently, the observation surface of the sample is finish-polished using colloidal silica, and then etched with 3 vol % Nital to expose the microstructure.

[0070] The observation surface of the sample is observed in ten fields of view, each field having a scope of 25.6 µm × 17.6 µm, using an SEM (Scanning Electron Microscope) under the conditions of an acceleration voltage of 15 kV and a magnification of 5000 times.

[0071] Fresh martensite and retained austenite, constituting the hard phases, have similar contrasts in SEM, and therefore it is difficult to discriminate them. Therefore, the area ratio of retained austenite is first determined by X-ray diffractometry. The area ratio of fresh martensite is then determined by subtracting the below-described area ratio of retained austenite from the area ratio of the hard phases, calculated from an SEM image thereof.

[0072] The area ratio of retained austenite is measured as follows. The hot-rolled steel sheet is mechanically ground in the thickness direction (depth direction) to a 1 / 4 thickness position, and then a thickness of 100 µm or more is removed by chemical polishing with oxalic acid to expose an observation surface. The observation surface is then observed by X-ray diffractometry. CoKα rays are used as incident X-rays. The ratios 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 are determined, and the volume fraction of retained austenite is calculated from the diffraction intensity ratios between the planes. Assuming that retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area ratio of retained austenite.Average amount of solute C in retained austenite: not less than 0.60 mass % and not more than 0.90 mass %

[0073] If the average amount of solute C in retained austenite is less than 0.60 mass %, the retained austenite will be transformed in the early stage of forming, resulting in a reduction in the work hardening ability in a high strain range, that is, a decrease in the n-value. Therefore, the average amount of solute C in retained austenite is made 0.60 mass % or more. The average amount of solute C in retained austenite is preferably 0.63 mass % or more, more preferably 0.65 mass % or more. On the other hand, if the average amount of solute C in retained austenite exceeds 0.90 mass %, the retained austenite will not be transformed sufficiently in the early state of forming, resulting in a reduction in the work hardening ability in a low strain range, that is, an increase in YR. Therefore, the average amount of solute C in retained austenite is made 0.90 mass % or less. The average amount of solute C in retained austenite is preferably 0.87 mass % or less, more preferably 0.85 mass % or less.

[0074] The average amount of solute C in retained austenite is measured as follows. The observation surface, on which the area ratio of retained austenite was measured, is observed by X-ray diffractometry using CuKα rays as incident X-rays. The lattice constant (a γ ) of austenite is determined from the position of the diffraction peak of the (220) plane of fcc iron (austenite), and the average amount of solute C in retained austenite is determined by the following formula: a γ (Å) = 3.572 + 0.033 × C - 0.00157 × Si + 0.0012 × Mn.

[0075] In the formula, Si and Mn each represent the content (mass %) of the element in the steel, and C represents the average amount of solute C (mass %) in retained austenite. The element symbols in the formula each represent the content (mass %) of the element in the steel.Total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more: 10.0% or less

[0076] If coarse crystal grains are present in the steel microstructure, the low-temperature toughness and stretch flangeability of the steel sheet will deteriorate, that is, vTrs will increase. Therefore, the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more is made 10.0% or less. The total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more is preferably 7.0% or less, more preferably 5.0% or less. There is no particular limitation on the lower limit of the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more; the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more may be 0%.

[0077] "Crystal grain" herein refers to a region surrounded by a grain boundary having a misorientation of 15° or more, and can be measured by EBSD (Electron Backscatter Diffraction).

[0078] The "equivalent circle diameter" of a crystal grain refers to a value calculated as 2√(A / π) [µm], where A [µm 2< ] represents the area of the grain.

[0079] The total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more is measured at a 1 / 4 thickness position on the high-strength hot-rolled steel sheet in the following manner.

[0080] A sample is cut out from the hot-rolled steel sheet such that a thickness-direction cross section, parallel to the rolling direction of the hot-rolled steel sheet, serves as an observation surface. Subsequently, the observation surface of the sample is mirror-polished using diamond paste. Subsequently, the observation surface of the sample is finish-polished using colloidal silica. The observation surface of the sample is observed in ten fields of view, each field having a scope of 100 µm × 100 µm, using an EBSD method (electron beam acceleration voltage: 20 keV, measurement interval: 0.1 µm step). Using OIM Analysis software manufactured by TSL, grain boundaries having a misorientation of 15° or more are visualized, and the equivalent circle diameter of each crystal grain is determined by image processing, and the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more is determined.Total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more: 10.0% or less

[0081] If crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more are present in the steel microstructure, they will serve as starting points for the formation of voids during forming, leading to a deterioration in stretch flangeability. Therefore, the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more is made 10.0% or less. The total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more is preferably 9.0% or less, more preferably 8.0% or less. There is no particular limitation on the lower limit of the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more; the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more may be 0%.

[0082] The "aspect ratio" of a crystal grain herein refers to a value obtained by dividing the length of the major axis of an ellipse, which approximates the region of the crystal grain, by the length of the minor axis of the ellipse.

[0083] The total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more is measured at a 1 / 4 thickness position on the high-strength hot-rolled steel sheet in the following manner.

[0084] In the measurement of the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more, grain boundaries having a misorientation of 15° or more are visualized. Thereafter, the equivalent circle diameter and aspect ratio of each crystal grain are determined, and the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more is determined.Total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in hard-phase regions which are assumed to be composed of retained austenite and fresh martensite: 1.0% or more

[0085] In order to ensure excellent work hardening ability in a high strain range, it is necessary to ensure a certain area ratio of retained austenite and, in addition, ensure that the distribution of the amount of solute C in the retained austenite has appropriate average and variance. The average amount of solute C in retained austenite has been described above. Through studies on the variance of the distribution of the amount of solute C in retained austenite, it has been found that assuming that retained austenite and fresh martensite are hard phases, the variance of the distribution of the amount of solute C in retained austenite is high when the total area ratio (hereinafter also referred to as "S 3 ") of regions (island regions), having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in the hard-phase regions is 1.0% or more. In other words, the work hardening ability in a high strain range is low when S 3 is less than 1.0%. Therefore, S 3 is made 1.0% or more. S 3 is preferably 1.5% or more, more preferably 2.0% or more. In view of the fact that it is difficult to make S 3 10.0% or more, and that the variance of the distribution of the amount of solute C in retained austenite will not be excessively high, S 3 is preferably made less than 10.0%.

[0086] The total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in the hard-phase regions (retained austenite and fresh martensite) is measured in the following manner.

[0087] For the hard-phase regions, identified in the above-described measurement of the area ratios of the hard phases, their equivalent circle diameters and aspect ratios are determined by image processing. Regions having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more are identified, and the total area ratio of the regions is determined. In the present invention, the hard-phase regions include regions composed solely of retained austenite, regions composed solely of fresh martensite, and regions composed of adjacent retained austenite and fresh martensite. As described above, retained austenite and fresh martensite have similar shapes and contrasts in SEM and are difficult to distinguish. In addition, in the present invention, retained austenite and fresh martensite have similar properties as a hard phase. Therefore, regions composed of adjacent retained austenite and fresh martensite are herein taken as a single hard-phase region.

[0088] A description will now be given of the mechanical properties of the high-strength hot-rolled steel sheet according to an embodiment of the present invention.Tensile strength (TS): 1180 MPa or more

[0089] The tensile strength of the high-strength hot-rolled steel sheet according to an embodiment of the present invention is 1180 MPa or more. The tensile strength is preferably less than 1470 MPa.

[0090] The uniform elongation (U. El), limiting hole expansion ratio (λ), yield ratio (YR), work hardening index (n-value), and brittle-ductile fracture transition temperature (vTrs) of the high-strength hot-rolled steel sheet according to an embodiment of the present invention are as described above.

[0091] The tensile strength (TS), the uniform elongation (U. El), the yield ratio (YR), and the work hardening index (n-value) are measured by a tensile test according to JIS Z 2241:2011, which will be described later in the Examples. The limiting hole expansion ratio (λ) is measured by a hole expansion test according to JIS Z 2256:2020, which will be described later in the Examples. The brittle-ductile fracture transition temperature (vTrs) is measured by a Charpy impact test according to JIS Z 2242:2018, which will be described later in the Examples.

[0092] The high-strength hot-rolled steel sheet of the present invention may have a coating layer on its surface. The coating layer is not particularly limited, and may be a known coating layer.[2] Member and Method for Producing Member

[0093] A description will now be given of a member according to an embodiment of the present invention, and a method for producing the member.

[0094] A member according to an embodiment of the present invention is a member formed using (as a material) the high-strength hot-rolled steel sheet described above. An exemplary method for producing the member comprises subjecting the high-strength hot-rolled steel sheet as a material to at least one of forming and joining.

[0095] The high-strength hot-rolled steel sheet has excellent low-temperature toughness in addition to high strength, excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range. Therefore, the member according to an embodiment of the present invention can advantageously be used particularly as a member having a complex shape for use in the automotive field.[3] Method for Producing High-Strength Hot-Rolled Steel Sheet

[0096] A description will now be given of a high-strength hot-rolled steel sheet production method according to an embodiment of the present invention.

[0097] The high-strength hot-rolled steel sheet production method according to an embodiment of the present invention comprises: a heating step of heating a steel material having the above-described chemical composition to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step under the following conditions: finish rolling start temperature: not less than 1000°C and not more than 1250°C, and finish rolling delivery temperature: not less than (RC1 - 100)°C and not more than (RC1 + 100)°C, thereby forming a hot-rolled steel sheet; a first cooling step of cooling and coiling the hot-rolled steel sheet after the hot-rolling step under the following conditions: time from the end of the hot rolling to the start of cooling: within RC2 seconds, average cooling rate: FF°C / s or more, and cooling stop temperature: not less than 350°C and not more than (Bs + 50)°C; a first holding step of holding the hot-rolled steel sheet after the first cooling step under the following conditions: holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of not less than 350°C and not more than 450°C: not less than 20 minutes and not more than 500 minutes; a second holding step of holding the hot-rolled steel sheet after the first holding step under the following condition: holding time in the temperature range of not less than 250°C and less than 350°C: not less than 30 minutes and not more than 500 minutes; and a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower.

[0098] The symbols RC1, RC2, FF, and Bs are defined by the following formulae: Bs = 830 − 270 × C − 90 × Mn − 70 × Cr − 37 × Ni − 83 × Mo

[0099] In the above formulae, the element symbols each represent the content of the element in mass %, which is 0 when the element is not present.

[0100] The above temperatures each refer to the surface temperatures of the steel material or the steel sheet unless otherwise specified. The above average cooling rate refers to the average cooling rate of the surface of the steel sheet. Further, the average cooling rate refers to [(cooling start temperature - cooling stop temperature) / cooling time from cooling start temperature to cooling stop temperature] unless otherwise specified.

[0101] First, a steel material such as a slab, having the above-described chemical composition, is prepared. There is no particular limitation on a method for producing the steel material such as a slab; any common method can be used. An exemplary method for producing the steel material involves producing molten steel having the above-described chemical composition, for example, in a converter using a known method, and then producing a slab by a casting method such as continuous casting. Other known casting methods, such as ingot casting / blooming, may also be used to produce the steel material. Scrap may be used as a raw material for the steel material.[Heating Step]Heating temperature of steel material: 1150°C or higher

[0102] In a steel material, such as a slab, which has been cooled to a low temperature, most of carbonitride-forming elements, such as Ti, are precipitated non-uniformly as coarse carbonitrides. The presence of the coarse nonuniform precipitates leads to deterioration of various properties (strength, resistance to punching defects, etc.). Therefore, the steel material is heated before hot rolling to dissolve the coarse precipitates. To sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the steel material is made 1150°C or higher. The heating temperature of the steel material is preferably 1180°C or higher, more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel material is too high, it may cause the formation of slab defects and a reduction in yield due to removal of scale. Therefore, 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. The steel material before hot rolling may be subjected to hot rolling (hot direct rolling) directly after casting while keeping the steel material at a high temperature (i.e., while maintaining a temperature within the above-described heating temperature range).[Hot Rolling Step]

[0103] Next, the steel material heated to 1150°C or higher (including one sent directly from a casting process while maintaining a high temperature of 1150°C or higher) is subjected to hot rolling, which consists of rough rolling and finish rolling. Conditions for rough rolling are not particularly limited as long as desired sheet bar dimensions can be ensured.

[0104] The steel material is roughly rolled to obtain a rough-rolled sheet. Before performing finish rolling on the rough-rolled sheet, it is preferably subjected to descaling (high-pressure water descaling) by spraying high-pressure water onto the steel sheet at the entry of a finish rolling mill.

[0105] In order to remove primary scale formed before finish rolling, the rough-rolled sheet is preferably subjected to high-pressure water descaling. The impact pressure of high-pressure water descaling (also referred to simply as "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 a force per unit area at which high-pressure water hits the surface of the rough-rolled sheet. While the upper limit of the descaling impact pressure is not particularly limited, the pressure 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 between stands during finish rolling. If necessary, the rough-rolled sheet may be cooled between stands during finish rolling.Finish rolling start temperature: not less than 1000°C and not more than 1250°C

[0106] The rough-rolled sheet is subjected to finish rolling at a predetermined finish rolling start temperature and a predetermined finish rolling delivery temperature to obtain a hot-rolled steel sheet (finish-rolled sheet). If the finish rolling start temperature is too low, recrystallization of austenite grains is unlikely to occur during finish rolling, leading to an increase in the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more. This results in a deterioration in stretch flangeability. Therefore, the finish rolling start temperature is made 1000°C or higher. The finish rolling start temperature is preferably 1020°C or higher, more preferably 1040°C or higher. On the other hand, if the finish rolling start temperature is too high, there will be a remarkable growth of austenite grains, resulting in coarsening of the austenite grains and an increase in the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more. This increases vTrs. Therefore, the finish rolling start temperature is made 1250°C or lower.Finish rolling delivery temperature: not less than (RC1 - 100)°C and not more than (RC1 + 100)°C.

[0107] If the finish rolling delivery temperature is too low, rolling may be performed at a two-phase region (ferrite + austenite) temperature. This may result in a failure to sufficiently achieve a desired area ratio for the respective phase, and to ensure a tensile strength of 1180 MPa or more. Therefore, the finish rolling delivery temperature is made (RC1 - 100)°C or higher. The finish rolling delivery temperature is preferably (RC1 - 80)°C or higher, more preferably (RC1 - 50)°C or higher. On the other hand, if the finish rolling delivery temperature is too high, there will be a remarkable growth of austenite grains, resulting in coarsening of the austenite grains and an increase in the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more. This increases vTrs. Therefore, the finish rolling delivery temperature is made (RC1 + 100)°C or lower. The finish rolling delivery temperature is preferably (RC1 + 80)°C or lower, more preferably (RC1 + 50)°C or lower.

[0108] RC1 is defined by the following formula (1):

[0109] In formula (1), the element symbols each represent the content of the element in mass % in the above-described chemical composition, which is 0 when the element is not present.[First Cooling Step]

[0110] Next, the hot-rolled steel sheet (finish-rolled sheet) obtained by the finish rolling is subjected to cooling (hereinafter also referred to as "forced cooling") from the above-described finish rolling delivery temperature to the below-described cooling stop temperature at the below-described average cooling rate.Time from the end of hot rolling to the start of cooling: within RC2 seconds

[0111] 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, the growth of austenite grains will occur, leading to an increase in the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more. This increases vTrs. Therefore, the cooling start time is made RC2 seconds or less. The cooling start time is preferably (RC2 - 0.5) seconds or less, more preferably (RC2 - 1.0) seconds or less. The lower limit of the cooling start time is not particularly limited; the cooling start time may be 0 seconds.

[0112] RC2 is defined by the following formula (2):

[0113] In formula (2), the element symbols each represent the content of the element in mass % in the above-described chemical composition, which is 0 when the element is not present.Average cooling rate: FF°C / s or more

[0114] In the forced cooling, if the average cooling rate from the finish rolling delivery temperature to the cooling stop temperature (hereinafter also referred to as the "average cooling rate in forced cooling") is too low, ferrite transformation will occur before upper bainite transformation, resulting in a failure to achieve a desired area ratio of an upper bainite phase. In addition, pearlite and the like will be formed, resulting in a failure to achieve desired area ratios of fresh martensite and retained austenite. This makes it difficult to achieve a TS of 1180 MPa or more. Therefore, the average cooling rate in forced cooling is made FF°C / s or more. The average cooling rate in forced cooling is preferably (FF + 5)°C / s or more, more preferably (FF + 10)°C / s or more. There is no particular limitation on the upper limit of the average cooling rate in forced cooling; however, if the average cooling rate is too high, it is possible that control of the cooling stop temperature may be difficult, leading to a difficulty in achieving a desired steel microstructure. From this viewpoint, the average cooling rate is preferably 500° C / s or less, more preferably 300°C / s or less, even more preferably 200°C / s or less, and particularly preferably 150°C / s or less.

[0115] FF is defined by the following formula (3):

[0116] In formula (3), the element symbols each represent the content of the element in mass % in the above-described chemical composition, which is 0 when the element is not present.Cooling stop temperature: not less than 350°C and not more than (Bs + 50) °C

[0117] If the cooling stop temperature is too low, the area ratios of upper bainite and retained austenite will decrease, and the area ratios of lower bainite and tempered martensite will increase. This reduces U. El and the n-value, and increases YR. Therefore, the cooling stop temperature is made 350°C or higher. The cooling stop temperature is preferably 370°C or higher, more preferably 380°C or higher. On the other hand, if the cooling stop temperature is too high, the area ratios of upper bainite and retained austenite will decrease. This reduces the n-value. In addition, it is possible that pearlite and the like may be formed, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the cooling stop temperature is made (Bs + 50)°C or lower. The cooling stop temperature is preferably (Bs + 20)°C or lower, more preferably Bs°C or lower.

[0118] Bs is defined by the following formula (4): Bs = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo (4)

[0119] In formula (4), the element symbols each represent the content of the element in mass % in the above-described chemical composition, which is 0 when the element is not present.[First Holding Step]

[0120] Next, the hot-rolled steel sheet after the first cooling step is held in the temperature range of not less than 350°C and not more than (Bs + 50) °C. The holding of the hot-rolled steel sheet in this temperature range is preferably performed when the heat retention of the steel sheet is enhanced, for example, after the steel sheet is coiled after the first cooling step. In the first holding step, the hot-rolled steel sheet may be held at a constant temperature, or the temperature of the steel sheet may be raised or lowered during the holding step as long as it is within the temperature range of not less than 350°C and not more than (Bs + 50) °C. The rise or lowering of the temperature may be performed by any method, such as external heating, transfer of heat to the outside, or use of heat generation or heat absorption in the hot-rolled steel sheet.Holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of not less than 350°C and not more than 450°C: not less than 20 minutes and not more than 500 minutes

[0121] If the holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C exceeds 1000 minutes, carbides and pearlite will be formed, leading to a decrease in the area ratios of fresh martensite and retained austenite. This makes it difficult to achieve a TS of 1180 MPa or more. Therefore, the holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C is made 1000 minutes or less. The holding time in this temperature range is preferably 800 minutes or less, more preferably 500 minutes or less. The holding time in this temperature range is preferably 20 minutes or more from the viewpoint of achieving a predetermined area ratio of upper bainite.

[0122] The first holding step is an important step for achieving a predetermined area ratio of upper bainite, diffusing C from the upper bainite to untransformed austenite, and appropriately diffusing C into the untransformed austenite in the below-described second holding step, thereby achieving an appropriate distribution of solute C in retained austenite. If the holding time in the temperature range of not less than 350°C and not more than 450°C is less than 20 minutes in the first holding step, the area ratios of upper bainite and retained austenite will decrease, leading to an increase in YR and a reduction in uniform elongation. Therefore, the holding time in the temperature range of not less than 350°C and not more than 450°C is made 20 minutes or more. Thus, in the first holding step, the holding time in the temperature range of not less than 350°C and not more than (Bs + 50) °C is made 1000 minutes or less, and the holding time in the temperature range of not less than 350°C and not more than 450°C is made 20 minutes or more. The holding time in the temperature range of not less than 350°C and not more than 450°C is preferably 25 minutes or more, more preferably 30 minutes or more. On the other hand, if the holding time in the temperature range of not less than 350°C and not more than 450°C exceeds 500 minutes, carbides will be formed and the area ratio of upper bainite will increase. This makes it difficult to achieve a TS of 1180 MPa or more. Therefore, the holding time in the temperature range of not less than 350°C and not more than 450°C is made 500 minutes or less. Thus, in the first holding step, the holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C is made 1000 minutes or less, and the holding time in the temperature range of not less than 350°C and not more than 450°C is made 500 minutes or less.[Second Holding Step]

[0123] Next, the hot-rolled steel sheet after the first holding step is held in the temperature range of not less than 250°C and less than 350°C. In the second holding step, the hot-rolled steel sheet may be held at a constant temperature, or the temperature of the steel sheet may be raised or lowered during the holding step as long as it is within the temperature range of not less than 250°C and less than 350°C. The rise or lowering of the temperature may be performed by any method, such as external heating, transfer of heat to the outside, or use of heat generation or heat absorption in the hot-rolled steel sheet.Holding time in the temperature range of not less than 250°C and less than 350°C: not less than 30 minutes and not more than 500 minutes

[0124] The second holding step is an important step for appropriately diffusing C into untransformed austenite, and achieving a predetermined average amount of solute C in retained austenite and a predetermined S 3 . If the holding time in the temperature range of not less than 250°C and less than 350°C exceeds 500 minutes, excessive concentration of C in untransformed austenite will occur and the average amount of solute C in retained austenite will increase. This results in a deterioration in strain dispersibility in a low strain range, that is, an increase in YR. In addition, it is possible that carbides may be precipitated from untransformed austenite, leading to a reducing in the amount of retained austenite and a reduction in uniform elongation and the n-value. Therefore, the holding time in the temperature range of not less than 250°C and less than 350°C is made 500 minutes or less. The holding time is preferably 450 minutes or less, more preferably 400 minutes or less. On the other hand, if the holding time in the temperature range of not less than 250°C and less than 350°C is less than 30 minutes, S 3 will decrease, and the variance of the distribution of the amount of solute C in retained austenite will decrease, resulting in a decrease in the n-value. Therefore, the holding time in the temperature range of not less than 250°C and less than 350°C is made 30 minutes or more. The holding time is preferably 40 minutes or more, and more preferably 45 minutes or more.[Second Cooling Step]

[0125] Next, the hot-rolled steel sheet after the second holding step is cooled to 100°C or lower. While conditions for cooling the steel sheet to 100°C or lower after the second holding step are not particularly limited, the average cooling rate during cooling from 250°C to a cooling stop temperature of 100°C or lower is preferably 200°C / s or less. When a coating step, which will be described later, is performed after the second holding step, the steel sheet after the coating step is cooled to 100°C or lower in the second cooling step.

[0126] The high-strength hot-rolled steel sheet of the present invention is produced by the above-described steps.

[0127] Thereafter, temper rolling (skin pass rolling) may be performed on the steel sheet in the usual manner. Further, pickling may be performed to remove scale from the steel sheet. When the high-strength hot-rolled steel sheet of the present invention has a coating layer on its surface, the high-strength hot-rolled steel sheet production method further includes a step of subjecting the hot-rolled steel sheet to a coating treatment. In that case, the coating treatment may be performed on the hot-rolled steel sheet, for example, and not limitingly after the second holding step or the second cooling step. The coating treatment in the coating step is not particularly limited, and may be a known coating treatment.EXAMPLES

[0128] The following examples illustrate the present invention in greater detail. It should be noted that the present invention is not limited to the examples.[Production of High-Strength Hot-Rolled Steel Sheet]

[0129] Molten steel having a chemical composition shown in Table 1 below (the balance consisting of Fe and incidental impurities) was produced in a converter, and the molten steel was subjected to continuous casting to produce a steel material. The steel material produced was subjected to a heating step at a heating temperature [°C] shown in Table 2 below. The steel material after the heating step was rough rolled to obtain a rough-rolled sheet. The surface of the rough-rolled sheet was subjected to high-pressure water descaling at an impact pressure of 10.0 MPa. The rough-rolled sheet that has undergone the high-pressure water descaling was subjected to finish rolling at a finish rolling start temperature [°C] and a finish rolling delivery temperature [°C], shown in Table 2 below, to obtain a hot-rolled steel sheet. After completion of the hot rolling (finish rolling), the hot-rolled steel sheet was subjected to a first cooling step. Table 2 below describes conditions for the first cooling step, including the cooling start time (time from the end of hot rolling to the start of forced cooling) [s], the average cooling rate (average cooling rate during cooling from the finish rolling delivery temperature to the cooling stop temperature) [°C / s], and the cooling stop temperature [°C]. Thereafter, the hot-rolled steel sheet after the first cooling step was coiled at a cooling stop temperature [°C] shown in Table 2 below.

[0130] The coiled hot-rolled steel sheet after the first cooling step was subjected to a first holding step in which the steel sheet was held at a temperature in the range of not less than 350°C and not more than (Bs + 50)°C for a holding time I [min] shown in Table 2 below, and at a temperature in the range of not less than 350°C and not more than 450°C for a holding time II [min] shown in Table 2 below. The holding times I shown in Table 2 are each a holding time (min) in the temperature range of not less than 350°C and mot more than (Bs + 50)°C, and the holding times II are each a holding time (min) in the temperature range of not less than 350°C and not more than 450°C. The hot-rolled steel sheet after the first holding step was subjected to a second holding step in which the steel sheet was held at a temperature in the range of not less than 250°C and less than 350°C for a holding time shown in Table 2 below. The holding times shown in Table 2 are each a holding time (min) in the temperature range of not less than 250°C and less than 350°C. The hot-rolled steel sheet after the second holding step was subjected to a second cooling step in which the steel sheet was cooled to 100°C or lower. In this manner, a high-strength hot-rolled steel sheet was obtained. The high-strength hot-rolled steel sheet was subjected to temper rolling, followed by pickling (hydrochloric acid concentration: 10% by mass, temperature: 85°C) to remove scale. [Table 1]Steel symbolChemical composition (mass %)RemarksCSiMnPSAlNTiNbOtherA0.1021.262.190.0030.00060.0420.00280.188--Suitable steelB0.1441.132.430.0080.00070.0390.00330.085--Suitable steelC0.1510.922.710.0130.00190.0400.00350.039--Suitable steelD0.1501.532.520.0200.00310.0450.00290.007--Suitable steelE0.1191.032.280.0070.00600.0380.0031-0.081-Suitable steelF0.1371.061.720.0100.00181.1200.00340.082--Suitable steelG0.0341.242.850.0090.00080.0370.00400.056--Comp. steelH0.2810.781.690.0120.00140.0320.00420.032--Comp. steelI0.1420.062.810.0070.00080.0390.00360.101--Comp. steelK0.2071.180.680.0170.00260.0360.00300.053--Comp. steelL0.1250.773.630.0120.00190.0340.00320.036--Comp. steelM0.1861.082.080.0090.00100.0300.00380.071-V:0.033Suitable steelN0.2410.861.520.0100.00180.0380.00340.082-B:0.0055Suitable steelO0.1741.012.200.0040.00090.0360.00440.078-Cu:0.20Suitable steelP0.1101.141.960.0150.00070.0300.00340.119-Cr:0.71Suitable steelQ0.1441.022.340.0100.00110.0310.00330.094-Cr:0.12Suitable steelR0.1780.951.160.0060.00100.0350.00280.087-Ni:0.89Suitable steelS0.1680.901.890.0100.00170.0400.00330.097-Mo:0.41Suitable steelT0.2111.012.130.0150.00140.0320.00390.033-Sb:0.005Suitable steelU0.1590.942.360.0050.00200.0430.00400.089-Sn:0.007Suitable steelV0.1711.152.570.0080.00150.0410.00370.048-Ta:0.005Suitable steelW0.1851.162.330.0100.00110.0450.00380.036-W:0.016Suitable steelX0.1490.922.400.0120.00120.0370.00320.107-Mg:0.005Suitable steelY0.2211.022.220.0050.00160.0400.00290.023-Zn:0.003Suitable steelZ0.1810.612.080.0170.00100.0350.00380.101-Co:0.008Suitable steelAA0.1180.942.310.0150.00170.0310.00310.156-Zr:0.003Suitable steelAB0.1901.112.240.0140.00090.0330.00320.029-Ca:0.004Suitable steelAC0.1511.202.680.0100.00150.0440.00290.075-Ce:0.002Suitable steelAD0.1931.142.330.0110.00130.0330.00360.019-Se:0.007Suitable steelAE0.1470.902.650.0060.00120.0370.00390.070-Te:0.019Suitable steelAF0.1191.022.570.0150.00130.0310.00350.127-Ge:0.015Suitable steelAG0.1671.392.300.0050.00110.0450.00380.035-As:0.012Suitable steelAH0.1800.962.030.0150.00070.0340.00420.093-Sr:0.010Suitable steelAI0.1761.082.540.0030.00060.0320.00430.020-Cs:0.007Suitable steelAJ0.1840.912.000.0130.00130.0330.00340.103-Hf:0.004Suitable steelAK0.2111.162.030.0190.00170.0450.00310.034-Pb:0.005Suitable steelAL0.2151.032.240.0120.00190.0430.00420.030-Bi:0.002Suitable steelAM0.1431.082.530.0050.00200.0350.00360.095-REM:0.002Suitable steelAN0.1520.872.660.0280.00440.0520.00530.0110.012Suitable steelAO0.1081.223.470.0060.00070.0320.00330.055-V:0.088, Cr:0.26Suitable steelAP0.1211.033.120.0110.00140.0400.00280.018-Cr:0.33, Mo:0.011Suitable steelAQ0.1851.142.460.0390.00190.0380.00240.037-V:0.008, Cr:0.008, Mo:0.007Suitable steelAR0.1011.282.820.0070.00060.0340.00280.034-Cr:0.40, B:0.0007Suitable steel*The underlines indicate that the values are outside the scope of the present invention. [Table 2] No.Steel symbolHeating stepHot rolling stepFirst cooling stepFirst holding stepSecond holding stepRemarksHeating temp. (°C)Finish rolling start temp. (°C)Finish rolling delivery temp. (°C)Cooling start time (s)Average cooling rate (°C / s)Cooling stop temp. (°C)Holding time I (min)Holding time II (min)Holding time (min)1A127011909600.3110420380180100Inventive Ex.2B127011709400.580430300200130Inventive Ex.3C121011608500.6120440220160120Inventive Ex.4D116010908600.490400200200150Inventive Ex.5E128012109500.880430240160100Inventive Ex.6F121011309000.49044090016080Inventive Ex.7C121012909300.580450300220120Comp. Ex.8C12309608400.4100430240220100Comp. Ex.9C1220118010800.890410280280140Comp. Ex.10C120011208001.060400150150200Comp. Ex.11C123011509405.080440350180200Comp. Ex.12C124011008900.614109080180Comp. Ex.13C123011409300.470660190100150Comp. Ex.14C122011609000.7603000080Comp. Ex.15C124010709100.2405202000400160Comp. Ex.16C121011709000.550450950900360Comp. Ex.17C124011209300.3120500301090Comp. Ex.18C122011809300.6804101501501000Comp. Ex.19C123011109200.69042025018010Comp. Ex.20G120011109400.980440100100180Comp. Ex.21H120010909101.01004008080170Comp. Ex.2222 I126011409201.270500180110200Comp. Ex.24K122011209300.8120420130130150Comp. Ex.25L123011308901.190410140120180Comp. Ex.26M125012009200.580480300180160Inventive Ex.27N129012008801.4150420210200190Inventive Ex.28O126011409100.4100450290240150Inventive Ex.29P125011209002.260460120100110Inventive Ex.30Q125011509300.530400190190450Inventive Ex.31R126012108702.110044011010050Inventive Ex.32S127012009301.6504208080150Inventive Ex.33T121010408300.1190380210210110Inventive Ex.34U126011809501.41204008080120Inventive Ex.35V120010808501.0306002105070Inventive Ex.36W118010008601.810043020018090Inventive Ex.37X128012009201.690410250250280Inventive Ex.38Y121010908501.690360140140200Inventive Ex.39Z130012008801.1120430210180350Inventive Ex.40AA133012409500.36042018018070Inventive Ex.41AB115010709201.4504408080140Inventive Ex.42AC122011709002.360450150120240Inventive Ex.43AD121011809901.71304008080320Inventive Ex.44AE123011708902.080420320320500Inventive Ex.45AF122011108800.750470340280100Inventive Ex.46AG122010709101.560400480480110Inventive Ex.47AH129011509000.78042012012080Inventive Ex.48AI124011508801.3304005050200Inventive Ex.49AJ128011308701.7904603020200Inventive Ex.50AK125011308501.211042016016030Inventive Ex.51AL123011508601.890410140140120Inventive Ex.52AM124011008601.61004306060300Inventive Ex.53AN123011408700.290430808060Inventive Ex.54AO126011809600.110410140140120Inventive Ex.55AP124011409200.4110400110110130Inventive Ex.56AQ122011309000.270400100100100Inventive Ex.57AR123011308800.320420200200140Inventive Ex. *The underlines indicate that the values are outside the scope of the present invention. [Evaluation of High-Strength Hot-Rolled Steel Sheet]

[0131] The steel microstructure of each of the high-strength hot-rolled steel sheets obtained was identified in the above-described manner. The measurement results are shown in Table 3. In Table 3, UB represents upper bainite, LB represents lower bainite, TM represents tempered martensite, FM represents fresh martensite, γ represents retained austenite, F represents ferrite, P represents pearlite, and θ represents carbides (excluding carbides contained in upper bainite, lower bainite, and tempered martensite). In Table 3, S 1 represents the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more. S 2 represents the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more. S 3 represents the total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in hard-phase regions which are assumed to be composed of retained austenite and fresh martensite. [Table 3]No.Steel symbolSteel microstructureRemarksArea ratios of phasesRemaining microstructureAverage amount of solute C in γS 1 S 2 S 3 UBLB + TMFMγ(%)(%)(%)(%)(mass %)(%)(%)(%)1A84.18.03.34.6-0.738.45.62.1Inventive Ex.2B73.313.67.85.3-0.755.25.92.6Inventive Ex.3C55.230.49.35.1-0.700.06.82.5Inventive Ex.4D47.141.77.24.0-0.680.03.22.1Inventive Ex.5E65.822.06.45.8-0.714.22.13.4Inventive Ex.6F69.715.84.79.8-0.660.02.64.5Inventive Ex.7C49.635.810.34.3-0.6815.10.33.1Comp. Ex.8C69.218.17.25.5-0.690.019.83.1Comp. Ex.9C47.339.09.93.8-0.7216.60.02.7Comp. Ex.10C23.134.911.40.9F0.700.025.60.2Comp. Ex.11C50.235.210.14.5-0.6713.83.52.2Comp. Ex.12C17.210.51.30.7F, P0.508.13.80.2Comp. Ex.13C22.553.412.70.8P0.484.22.20.4Comp. Ex.14C0.096.22.41.4-0.732.61.50.3Comp. Ex.15C82.412.61.00.4θ, P0.510.03.10.2Comp. Ex.16C94.23.20.30.2θ0.850.02.90.1Comp. Ex.17C24.858.215.61.4-0.763.82.00.5Comp. Ex.18C53.540.42.73.4-1.027.23.41.6Comp. Ex.19C54.331.79.84.2-0.654.82.60.6Comp. Ex.20G68.32.41.22.2F0.736.24.52.3Comp. Ex.21H49.319.32833.1-0.730.03.12.9Comp. Ex.22I94.25.30.30.2-0.682.13.01.4Comp. Ex.24K14.40.032.10.8F0.703.85.62.2Comp. Ex.25L18.7 61.174.64.32.4-0.650.04.83.2Comp. Ex.26M25.16.27.6-0.654.42.52.9Inventive Ex.27N68.815.37.18.8-0.730.04.32.3Inventive Ex.28O63.526.26.34.0-0.702.51.53.1Inventive Ex.29P76.511.96.55.1-0.730.00.41.2Inventive Ex.30Q55.738.92.23.2-0.744.33.42.4Inventive Ex.31R66.820.29.23.8-0.690.06.91.4Inventive Ex.32S57.532.07.43.1-0.751.72.42.6Inventive Ex.33T60.927.06.65.5-0.650.08.42.9Inventive Ex.34U60.430.95.73.0-0.698.41.51.8Inventive Ex.35V32.951.99.16.1-0.710.06.61.2Inventive Ex.36W55.927.410.56.2-0.700.08.52.7Inventive Ex.37X53.739.74.12.5-0.740.05.01.6Inventive Ex.38Y59.025.88.86.4-0.700.05.72.9Inventive Ex.39Z65.128.93.62.4-0.651.47.33.0Inventive Ex.40AA79.213.34.53.0-0.709.12.41.2Inventive Ex.41AB62.622.78.85.9-0.750.03.01.8Inventive Ex.42AC54.336.64.34.8-0.710.02.53.2Inventive Ex.43AD58.930.83.96.4-0.758.34.02.4Inventive Ex.44AE50.744.03.22.1-0.700.01.62.7Inventive Ex.45AF57.934.05.03.1-0.740.04.71.4Inventive Ex.46AG61.523.58.16.9-0.722.66.13.0Inventive Ex.47AH62.328.55.53.7-0.750.03.31.9Inventive Ex.48AI54.437.04.04.6-0.680.03.72.6Inventive Ex.49AJ69.023.63.53.9-0.710.03.32.6Inventive Ex.50AK63.321.48.17.2-0.730.05.81.2Inventive Ex.51AL57.625.610.36.5-0.650.06.92.4Inventive Ex.52AM50.538.17.44.0-0.740.07.32.2Inventive Ex.53AN64.821.88.64.8-0.710.05.41.8Inventive Ex.54AO82.610.34.22.9-0.650.52.41.4Inventive Ex.55AP65.225.26.33.3-0.630.03.62.2Inventive Ex.56AQ52.532.09.75.8-0.730.02.82.5Inventive Ex.57AR87.46.83.12.7-0.600.23.81.9Inventive Ex.*The underlines indicate that the values are outside the scope of the present invention.

[0132] Further, each of the high-strength hot-rolled steel sheets obtained was subjected to a tensile test, a hole expansion test and a Charpy impact test, which were conducted in the following manner, to evaluate the tensile strength (TS), uniform elongation (U. El), limiting hole expansion ratio (λ), yield ratio (YR), work hardening index (n-value), and brittle-ductile fracture transition temperature (vTrs).(1) Tensile Test

[0133] The tensile test was conducted according to JIS Z 2241:2011. In particular, a JIS No. 5 test specimen was taken from each of the high-strength hot-rolled steel sheets obtained such that the longitudinal direction of the specimen was perpendicular to the rolling direction of the steel sheet. Using the test specimen, a tensile test was conducted at a crosshead speed of 10 mm / min to measure YS, TS, U. El, T. El (total elongation), and n-value. The n-value was measured in a strain range of 0.7 × U. El to 0.9 × U. El, where U. El is the measured value. The YR was calculated from YS / TS. The results are shown in Table 4.

[0134] A TS of 1180 MPa or more (TS ≥ 1180 MPa) was evaluated as acceptable, while a TS value which is outside this range was evaluated as unacceptable.

[0135] A U. El of 5.0% or more (U. El ≥ 5.0%) was evaluated as acceptable (excellent ductility), while a U. El value which is outside this range was evaluated as unacceptable.

[0136] A YR of 0.93 or less (YR ≤ 0.93) was evaluated as acceptable (excellent strain dispersibility in a low strain range), while a YR value which is outside this range was evaluated as unacceptable.

[0137] An n-value of 0.080 or more (n-value ≥ 0.080) was evaluated as acceptable (excellent strain dispersibility in a high strain range), while an n-value which is outside this range was evaluated as unacceptable. A test specimen having both an acceptable YR and an acceptable n-value was evaluated as having excellent strain dispersibility over a wide strain range.(2) Hole Expansion Test

[0138] The hole expansion test was conducted according to JIS Z 2256:2020. In particular, a 100 mm × 100 mm test specimen was taken by shearing from each of the high-strength hot-rolled steel sheets obtained. A 10-mm diameter hole (initial hole of the test specimen) was punched into the test specimen with a clearance of 12% ± 1%. Subsequently, while applying a blank holder force of 9 tons (88.26 kN) to an area around the hole using a die with an inner diameter of 75 mm, a conical punch having an apex angle of 60° was pressed into the hole, and the diameter of the hole of the test specimen at the crack initiation limit (upon the occurrence of cracking) was measured. The limiting hole expansion ratio λ (%) was then determined by the following formula. The λ serves as an index for evaluating stretch flangeability. The results are shown in Table 4.

[0139] A λ of 30% or more (λ ≥ 30%) was evaluated as acceptable (excellent stretch flangeability), while a λ value which is outside this range was evaluated as unacceptable. λ % = D f − D 0 / D 0 × 100 where D f is the diameter (mm) of the hole of the test specimen upon the occurrence of cracking, and D 0 is the initial diameter (mm) of the hole of the test specimen.(3) Charpy Impact Test

[0140] A 2.5-mm thick sub-size test specimen (V-notch) was taken from each of the high-strength hot-rolled steel sheets obtained such that the longitudinal direction of the test specimen was perpendicular to the rolling direction. A Charpy impact test was then performed on the test specimen according to JIS Z 2242:2018 to measure a brittle-ductile fracture transition temperature (vTrs) and evaluate toughness. In the case of a hot-rolled steel sheet having a thickness of more than 2.5 mm, a test specimen was prepared by double-side grinding to a thickness of 2.5 mm, and the specimen was subjected to the Charpy impact test. In the case of a hot-rolled steel sheet having a thickness of 2.5 mm or less, a test specimen was prepared with the original thickness and subjected to the Charpy impact test. The results are shown in Table 4.

[0141] A vTrs of -40°C or lower (vTrs ≤ -40°C) was evaluated as acceptable (excellent low-temperature toughness), while a vTrs value which is outside this range was evaluated as unacceptable.

[0142] As can be seen in Table 4, according to the present invention, it is possible to achieve a high-strength hot-rolled steel sheet which has a tensile strength of 1180 MPa or more, excellent ductility and stretch flangeability, and excellent strain dispersibility over a wide strain range, and, in addition, has excellent low-temperature toughness. [Table 4]No.Steel symbolYSTSU. EIT. EIλYRn-valuevTrsRemarks(MPa)(MPa)(%)(%)(%)(°C)1A91312307.514.7630.740.107-45Inventive Ex.2B92812196.913.1510.760.110-50Inventive Ex.3C90812326.614.2410.740.101-80Inventive Ex.4D89212118.515.3320.740.094-65Inventive Ex.5E87712246.313.6420.720.103-50Inventive Ex.6F102812178.715.1680.840.118-60Inventive Ex.7C88311925.711.2240.740.1040Comp. Ex.8C87012476.111.9210.700.109-70Comp. Ex.9C82812015.611.4180.690.088-10Comp. Ex.10C72810949.212.1220.670.092-40Comp. Ex.11C84411845.511.4310.710.089-10Comp. Ex.12C56283611.519.0460.670.067-40Comp. Ex.13C64494310.416.9320.680.070-50Comp. Ex.14C121212733.710.8890.950.065-55Comp. Ex.15C98910454.413.8520.950.065-55Comp. Ex.16C96110195.314.2570.940.071-60Comp. Ex.17C119612604.210.4430.950.068-50Comp. Ex.18C117212266.010.8580.960.068-45Comp. Ex.19C88212356.413.3400.710.069-50Comp. Ex.20G52286212.116.4780.610.110-40Comp. Ex.21H78312613.84.0110.620.099-55Comp. Ex.22I48279113.117.2710.610.098-50Comp. Ex.24K59811293.43.630.530.101-50Comp. Ex.25L119512563.17.2610.950.105-65Comp. Ex.26M90912128.312.7380.750.103-50Inventive Ex.27N97212348.813.4420.790.088-70Inventive Ex.28O92912257.412.3440.760.096-55Inventive Ex.29P85712227.411.9600.700.096-60Inventive Ex.30Q104612527.010.8460.840.091-50Inventive Ex.31R92611987.512.1530.770.094-75Inventive Ex.32S95512556.711.5430.760.087-55Inventive Ex.33T95412488.212.2390.760.096-90Inventive Ex.34U95912506.611.4480.770.084-45Inventive Ex.35V103012167.413.1380.850.109-70Inventive Ex.36W91312378.412.5360.740.113-75Inventive Ex.37X95312466.510.9520.760.086-60Inventive Ex.38Y100012977.813.0370.770.100-70Inventive Ex.39Z102512267.411.9430.840.083-55Inventive Ex.40AA97512706.611.0690.770.086-45Inventive Ex.41AB92412228.012.7390.760.107-60Inventive Ex.42AC91212597.012.2380.720.105-65Inventive Ex.43AD95812737.712.7340.750.102-45Inventive Ex.44AE108312236.210.3380.890.081-60Inventive Ex.45AF89811996.611.1610.750.093-65Inventive Ex.46AG90012388.513.1440.730.129-55Inventive Ex.47AH94012267.711.9430.770.090-60Inventive Ex.48AI92012417.312.3400.740.095-65Inventive Ex.49AJ93712067.712.0440.780.090-70Inventive Ex.50AK93312429.014.0350.750.113-80Inventive Ex.51AL96412628.313.2370.760.101-75Inventive Ex.52AM92812397.211.0430.750.096-85Inventive Ex.53AN83611817.512.2380.710.099-55Inventive Ex.54AO97012026.610.9520.810.084-45Inventive Ex.55AP83311886.811.7400.700.115-55Inventive Ex.56AQ80711978.313.2330.670.124-55Inventive Ex.57AR102512336.510.3550.830.089-60Inventive Ex.The underlines indicate that the values are outside the scope of the present invention.

Claims

1. A high-strength hot-rolled steel sheet having a chemical composition comprising, in mass %: C: not less than 0.100% and not more than 0.250%, Si: not less than 0.20% and not more than 2.00%, Mn: not less than 1.00% and less than 3.50%, P: not more than 0.100%, S: not more than 0.0200%, Al: not less than 0.010% and not more than 2.000%, and N: not more than 0.0100%, and at least one selected from Ti: not less than 0.005% and not more than 0.200% and Nb: not less than 0.005% and not more than 0.200%, with the balance being Fe and incidental impurities, wherein the steel sheet has the following microstructure: the area ratio of upper bainite: not less than 30.0% and less than 90.0%, the total area ratio of lower bainite and tempered martensite: not less than 6.0% and not more than 60.0%, the area ratio of fresh martensite: not less than 2.0% and not more than 20.0%, the area ratio of retained austenite: not less than 2.0% and not more than 10.0%, the average amount of solute C in retained austenite: not less than 0.60 mass % and not more than 0.90 mass %, the total area ratio of crystal grains having an equivalent circle diameter of 15.0 µm or more: 10.0% or less, the total area ratio of crystal grains having an equivalent circle diameter of 8.0 µm or more and an aspect ratio of 4.0 or more: 10.0% or less, and, assuming that the retained austenite and the fresh martensite are hard phases, the total area ratio of regions, having an equivalent circle diameter of 4.0 µm or less and an aspect ratio of 2.0 or more, in the hard-phase regions is 1.0% or more, and wherein the steel sheet has a tensile strength of 1180 MPa or more.

2. The high-strength hot-rolled steel sheet according to claim 1, wherein the chemical composition further comprises, in mass %, one or more selected from the following groups a and b: group a: one or more selected from V: not less than 0.001% and not more than 0.100%, Cr: not less than 0.005% and not more than 1.000%, and Mo: not less than 0.005% and not more than 0.500%, and group b: one or more selected from B: 0.0100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

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

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

5. A method for producing the high-strength hot-rolled steel sheet according to claim 1 or 2, comprising: a heating step of heating a steel material having the chemical composition to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step under the following conditions: finish rolling start temperature: not less than 1000°C and not more than 1250°C, and finish rolling delivery temperature: not less than (RC1 - 100)°C and not more than (RC1 + 100)°C, thereby forming a hot-rolled steel sheet; a first cooling step of cooling and coiling the hot-rolled steel sheet after the hot-rolling step under the following conditions: time from the end of the hot rolling to the start of cooling: within RC2 seconds, average cooling rate: FF°C / s or more, and cooling stop temperature: not less than 350°C and not more than (Bs + 50)°C; a first holding step of holding the hot-rolled steel sheet after the first cooling step under the following conditions: holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of not less than 350°C and not more than 450°C: not less than 20 minutes and not more than 500 minutes; a second holding step of holding the hot-rolled steel sheet after the first holding step under the following condition: holding time in the temperature range of not less than 250°C and less than 350°C: not less than 30 minutes and not more than 500 minutes; and a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower, wherein RC1, RC2, FF, and Bs are defined by the following formulae: RC 1 = 850 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V RC 2 = 1.2 + C + N + 0.1 × Mn + 10 × Ti + 50 × B + 0.1 × Cr + 0.5 × Mo + 20 × Nb + 1.5 × V FF = 10 ∧ 3.9 − 1.2 × 2.5 × C + Mn + 0.7 × Cr + 0.1 × Mo + 0.5 × Ni Bs = 830 − 270 × C − 90 × Mn − 70 × Cr − 37 × Ni − 83 × Mo where the element symbols each represent the content of the element in mass %, which is 0 when the element is not present.

6. A method for producing the high-strength hot-rolled steel sheet according to claim 3, comprising: a heating step of heating a steel material having the chemical composition to 1150°C or higher; a hot-rolling step of hot-rolling the steel material after the heating step under the following conditions: finish rolling start temperature: not less than 1000°C and not more than 1250°C, and finish rolling delivery temperature: not less than (RC1 - 100)°C and not more than (RC1 + 100)°C, thereby forming a hot-rolled steel sheet; a first cooling step of cooling and coiling the hot-rolled steel sheet after the hot-rolling step under the following conditions: time from the end of the hot rolling to the start of cooling: within RC2 seconds, average cooling rate: FF°C / s or more, and cooling stop temperature: not less than 350°C and not more than (Bs + 50)°C; a first holding step of holding the hot-rolled steel sheet after the first cooling step under the following conditions: holding time in the temperature range of not less than 350°C and not more than (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of not less than 350°C and not more than 450°C: not less than 20 minutes and not more than 500 minutes; a second holding step of holding the hot-rolled steel sheet after the first holding step under the following condition: holding time in the temperature range of not less than 250°C and less than 350°C: not less than 30 minutes and not more than 500 minutes; a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower; and a coating step of subjecting the hot-rolled steel sheet to a coating treatment, wherein RC1, RC2, FF, and Bs are defined by the following formulae: Bs = 830 − 270 × C − 90 × Mn − 70 × Cr − 37 × Ni − 83 × Mo where the element symbols each represent the content of the element in mass %, which is 0 when the element is not present.

7. A method for producing a member, comprising a step of subjecting the high-strength hot-rolled steel sheet according to any one of claims 1 to 3 to at least one of forming and joining.

Images