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

Abstract

The present invention provides a high-strength hot-rolled steel sheet that has a high strength of at least 1180 MPa, has excellent ductility and stretch-flangeability, has excellent strain dispersion capability over a wide range of strain, and also has excellent low-temperature toughness.　This high-strength hot-rolled steel sheet has a steel structure that has a prescribed component composition and a prescribed area percentage of fresh martensite, a prescribed area percentage of retained austenite, and a prescribed total area percentage of upper bainite, lower bainite, and tempered martensite, the average amount of C in solid solution in the retained austenite being 0.60–0.90 mass%, the total area percentage of crystal grains that have a circle-equivalent diameter of at least 15.0 μm being no more than 10.0%, the total area percentage of crystal grains that have a circle-equivalent diameter of at least 8.0 μm and an aspect ratio of at least 4.0 being no more than 10.0%, and the total area percentage of regions that have a circle-equivalent diameter of no more than 4.0 μm and an aspect ratio of at least 2.0 among regions of hard phases (retained austenite and fresh martensite) being at least 1.0%.

Description

High-strength hot-rolled steel sheets and components, and methods for manufacturing the same

[0001] The present invention relates to a high-strength hot-rolled steel sheet and member, and a method for manufacturing the same.

[0002] In recent years, improving the fuel efficiency and electric power consumption of automobiles has become an important issue from the perspective of preserving the global environment. Therefore, there has been a growing trend to reduce the weight of automobile bodies by increasing the strength and thinning of steel sheets, which are used as the raw material for automobile components. For this reason, high-strength hot-rolled steel sheets are being actively adopted as materials for automobile components. High-strength hot-rolled steel sheets are used not only for automobile structural and skeletal components, but also for underbody components, truck frame components, etc.

[0003] In particular, high-strength hot-rolled steel sheets having a tensile strength of 1180 MPa or more are expected to be materials that can dramatically improve the fuel economy and electricity consumption of automobiles through weight reduction.

[0004] Various studies have been conducted so far on steel sheets that can be used as raw materials for such automobile parts.

[0005] For example, Patent Document 1 describes a steel sheet having a chemical composition, in mass %, containing 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 unavoidable impurities, and the microstructure containing one or both of tempered martensite and lower bainite in a volume fraction of 90% or more in total, and iron-based carbides in one or both of the tempered martensite and lower bainite in a volume fraction of 1×10 ６ (pcs / mm ２) or more, the effective grain size of one or both of the tempered martensite and the lower bainite is 10 μm or less, the aspect ratio of the effective grains of one or both of the tempered martensite and the lower bainite is 2 or less, and the standard deviation σ of the Vickers hardness distribution is 15 or less.

[0006] Patent Document 2 describes a steel sheet containing, by 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 a Si / V ratio of 10.0 or less, and When one or two of the above are contained, the contents of Cr, Mo and V satisfy the relationship of (2Cr+Mo) / 2V≦2.0, when Ni is contained, Cu is contained, and the balance is Fe and impurities. The steel has a chemical composition containing 90% or more tempered martensite in terms of area ratio, and the content of cementite contained in the tempered martensite is 70% or more, the major axis of which is 400 nm or less and the aspect ratio is 3 to 5, and the tempered martensite contains 30 V-containing carbides having a circle equivalent diameter of 8 to 15 nm per μm. ２ "A hot-rolled steel sheet characterized in that the precipitates are formed at a number density of at least 10 ...

[0007] Patent Document 3 describes a steel sheet having a tensile strength of 1180 MPa or more, an arithmetic mean roughness Ra of the surface of 2.00 μm or less, and containing, in mass %, C: 0.09% or more and 0.20% or less, Si: 0.2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less, S: 0.0100% or less, Al: 0.01% or more and 2.00% or less, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005% or more and 0.0200% or less, and further containing Cr: 0.10% or more and 1.50% or less, Mo: 0.05% or more and 0.45% or less, Nb: 0.005% or more and 0.060% or less, and V: 0.05% or more and 0.50% or less. and a microstructure including an upper bainite phase and a second phase, wherein the area fraction of the upper bainite phase is 50% or more and less than 90%, the average grain size of the upper bainite phase is 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, the area fraction of the second phase is 10% or more and less than 50%, and the perimeter of the second phase having an equivalent circle diameter of 0.5 μm or more is 300,000 μm / mm ２ The above is a high strength hot rolled steel sheet.

[0008] Patent No. 6354268 Patent No. 6835294 Patent No. 6819840

[0009] However, currently, the practical application of hot-rolled steel sheets for automotive components remains at the 980 MPa level. Automotive components, particularly suspension components, need to be formed into complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets, which are used as raw materials for components with such complex shapes, are required to have excellent formability.

[0010] On the other hand, increasing the tensile strength of a steel sheet generally leads to a decrease in properties such as ductility and stretch-flangeability. Therefore, when forming a high-strength hot-rolled steel sheet, particularly one having a tensile strength of 1180 MPa or more, forming defects such as cracks are likely to occur when strain is localized. To prevent such forming defects, the steel sheet is required to have not only ductility and stretch-flangeability but also high strain dispersion. In particular, when forming into a complex shape, it is necessary to have excellent strain dispersion over a wide strain range, from low to high strain.

[0011] Furthermore, increasing the tensile strength of steel plate reduces its low-temperature toughness. If low-temperature toughness is low, when a brittle crack occurs in a suspension part or the like due to an impact load in a cold region, the propagation of the brittle crack cannot be stopped. In other words, if low-temperature toughness is low, the part may fracture or separate due to brittleness in the event of a collision, etc., and the part strength expected in the design may not be achieved. For this reason, low-temperature toughness is necessary for materials used in automobile parts, etc.

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

[0013] As described above, in the prior art, there is no established technology for a hot-rolled steel sheet that has a high strength of 1180 MPa or more in tensile strength (TS), as well as excellent ductility and stretch flangeability, excellent strain dispersibility over a wide strain range, and excellent low-temperature toughness.

[0014] Therefore, an object of the present invention is to provide a high-strength hot-rolled steel sheet having a high tensile strength of 1180 MPa or more, excellent ductility and stretch flangeability, excellent strain dispersion ability over a wide strain range, and excellent low-temperature toughness, and a manufacturing technology for the same.

[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 in accordance with JIS Z 2241:2011 satisfies the following formula: Excellent ductility is required for molding automobile parts, particularly parts with complex shapes such as suspension parts: U.El≧5.0%

[0016] In the present invention, excellent stretch flangeability means that the limiting hole expansion ratio (λ) measured in a hole expansion test in accordance with JIS Z 2256:2020 satisfies the following formula: λ≧30%. In particular, suspension parts are required to have excellent stretch flangeability because they are stretch flanged after punching.

[0017] Automobile parts, especially suspension parts, are subjected to severe processing, so it is necessary to suppress strain localization. In particular, to perform severe processing while maintaining a high strength of 1180 MPa or more, excellent strain dispersibility over a wide strain range is required. Excellent strain dispersibility over a wide strain range means having both excellent strain dispersibility in the low strain range and excellent strain dispersibility in the high strain range.

[0018] By having excellent strain dispersibility in the low strain range, it is possible to prevent cracks and wrinkles from occurring from the early stage of processing. In the present invention, excellent strain dispersibility in the 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 in accordance with JIS Z 2241:2011, satisfies the following formula: YR≦0.93

[0019] By having excellent strain dispersibility in the low strain range and also having excellent strain dispersibility in the high strain range, cracks and wrinkles caused by severe processing can be prevented. In the present invention, excellent strain dispersibility in the high strain range means that the work hardening index (hereinafter also referred to as n value) measured in accordance with JIS Z 2253:2020, where n value is measured 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 using a sub-size test piece (V-notch) in a Charpy impact test in accordance with JIS Z 2242:2018 satisfies the following formula: vTrs≦−40°C

[0021] The present inventors have conducted extensive research to achieve the above object, and as a result, it has become possible to appropriately adjust the chemical composition of a hot-rolled steel sheet and to modify the steel structure of the hot-rolled steel sheet to have an area ratio of upper bainite: 30.0% or more and less than 90.0%, a total area ratio of lower bainite and tempered martensite: 6.0% or more and 60.0% or less, an area ratio of fresh martensite: 2.0% or more and 20.0% or less, an area ratio of retained austenite: 2.0% or more and 10.0% or less, an average amount of solute C in the retained austenite: 0.60 mass% or more and 0.90 mass% or less, a total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more: 10.0% or less, and an area ratio of crystal grains having an equivalent circle diameter of 8.0 μm or more: 10.0% or less. It has been found that by setting the total area ratio of crystal grains having a circle equivalent diameter of 4.0 μm or less and an aspect ratio of 4.0 or more to 10.0% or less, and when the retained austenite and the fresh martensite are considered to be hard phases, the total area ratio of regions in the hard phase having a circle equivalent diameter of 4.0 μm or less and an aspect ratio of 2.0 or more to 1.0% or more, and setting the tensile strength to 1180 MPa or more, a high-strength hot-rolled steel sheet having high strength, excellent ductility and stretch flangeability, excellent strain dispersibility over a wide strain range, and excellent low-temperature toughness can be obtained.

[0022] The present invention was completed based on the above findings and further investigations.

[0023] That is, the gist of the present invention is as follows: [1] A steel having a chemical composition containing, by mass%, C: 0.100% to 0.250%, Si: 0.20% to 2.00%, Mn: 1.00% to less than 3.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% to 2.000%, and N: 0.0100% or less, and further containing at least one element selected from Ti: 0.005% to 0.200% and Nb: 0.005% to 0.200%, with the balance consisting of Fe and unavoidable impurities, and having a steel structure having an area ratio of upper bainite: 30.0% to less than 90.0%, and a total area ratio of lower bainite and tempered martensite: 6.0% to 60.0%. a high-strength hot-rolled steel sheet having an area ratio of fresh martensite of 2.0% or more and 20.0% or less, an area ratio of retained austenite of 2.0% or more and 10.0% or less, an average amount of dissolved carbon in the retained austenite of 0.60 mass% or more and 0.90 mass% or less, a total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more of 10.0% or less, a 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 of 10.0% or less, a 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 of the hard phase of which the retained austenite and the fresh martensite are hard phases of 1.0% or more, and a tensile strength of 1180 MPa or more. [2] The high-strength hot-rolled steel sheet according to [1], wherein the chemical composition further contains, in mass%, one or more selected from the following groups a and b:Group a: one or more elements selected from V: 0.001% or more and 0.100% or less, Cr: 0.005% or more and 1.000% or less, and Mo: 0.005% or more and 0.500% or less; Group b: 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, [3] A high-strength hot-rolled steel sheet according to [1] or [2], having a plating layer on the surface. [4] A member made using the high-strength hot-rolled steel sheet according to any one of [1] to [3]. [5] A method for producing a 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 conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of (RC1-100)°C or higher and (RC1+100)°C or lower to obtain a hot rolled steel sheet; and a first cooling step of cooling the hot rolled steel sheet after the hot rolling step under conditions of a time from the end of the hot rolling to the start of cooling of RC2 seconds or less, an average cooling rate of FF°C / second or higher, and a cooling end temperature of 350°C or higher and (Bs+50)°C or lower, and coiling the hot rolled steel sheet. a first holding step of holding the hot-rolled steel sheet after the first cooling step in a temperature range of 350°C or higher and (Bs+50)°C or lower for a holding time of 1000 minutes or less, and in a temperature range of 350°C or higher and 450°C or lower for a holding time of 20 minutes or more and 500 minutes or less; a second holding step of holding the hot-rolled steel sheet after the first holding step in a temperature range of 250°C or higher and lower than 350°C for a holding time of 30 minutes or more and 500 minutes or less; and a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower.Here, RC1, RC2, FF, and Bs are defined by the following equations, respectively. RC1 = 850 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V RC2 = 1.2 + C + N + 0.1 x Mn + 10 x Ti + 50 x B + 0.1 x Cr + 0.5 x Mo + 20 x Nb + 1.5 x V FF = 10^(3.9 - 1.2 x (2.5 x C + Mn + 0.7 x Cr + 0.1 x Mo + 0.5 x Ni)) Bs = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo The element symbols in the above formulas represent the content of each element in mass%, and the content of elements that are not contained is 0. [6] A method for producing a high strength hot rolled steel sheet according to [3], comprising: a heating step of heating a steel material having the above-mentioned chemical composition to 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step under conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of (RC1-100)°C or higher and (RC1+100)°C or lower to obtain a hot rolled steel sheet; and a first cooling step of cooling the hot rolled steel sheet after the hot rolling step under conditions of a time from the end of the hot rolling to the start of cooling of RC2 seconds or less, an average cooling rate of FF°C / second or higher, and a cooling end temperature of 350°C or higher and (Bs+50)°C or lower, and coiling the hot rolled steel sheet. a first holding step of holding the hot-rolled steel sheet after the first cooling step in a temperature range of 350°C or higher and (Bs+50)°C or lower for a holding time of 1000 minutes or less and in a temperature range of 350°C or higher and 450°C or lower for a holding time of 20 minutes or more and 500 minutes or less, a second holding step of holding the hot-rolled steel sheet after the first holding step in a temperature range of 250°C or higher and lower than 350°C for a holding time of 30 minutes or more and 500 minutes or less, a second cooling step of cooling the hot-rolled steel sheet after the second holding step to 100°C or lower, and a plating step of plating the hot-rolled steel sheet, wherein RC1, RC2, FF, and Bs are defined by the following formulas, respectively.RC1 = 850 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V RC2 = 1.2 + C + N + 0.1 x Mn + 10 x Ti + 50 x B + 0.1 x Cr + 0.5 x Mo + 20 x Nb + 1.5 x V FF = 10^(3.9 - 1.2 x (2.5 x C + Mn + 0.7 x Cr + 0.1 x Mo + 0.5 x Ni)) Bs = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo The element symbols in the above formulas represent the content of each element in mass%, and the content of elements that are not contained is 0. [7] A method for manufacturing a component, 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 to form a component.

[0024] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having a high tensile strength of 1180 MPa or more, excellent ductility and stretch flangeability, excellent strain dispersibility over a wide strain range, and excellent low-temperature toughness.

[0025] Hereinafter, embodiments of the high-strength hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be described, however, the present invention is not limited to the following embodiments.

[0026] [1] High-strength hot-rolled steel sheet First, the chemical composition of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that although the unit of chemical composition is "mass %", it will be simply expressed as "%" hereinafter unless otherwise specified.

[0027] C: 0.100% or more and 0.250% or less. C is an element that improves the strength of steel. C promotes the formation of bainite by improving hardenability, contributing to high strength. C also contributes to high strength by increasing the strength of martensite. To obtain a tensile strength of 1180 MPa or more, the C content must be 0.100% or more. Therefore, the C content is set to 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 increases excessively, increasing the strength difference between the upper bainite as the main phase and the fresh martensite and retained austenite. As a result, uniform elongation and stretch flangeability deteriorate. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less, more preferably 0.200% or less.

[0028] Si: 0.20% or more and 2.00% or less. Si has the effect of suppressing the formation of Fe-based carbides, suppressing the precipitation of cementite during upper bainite transformation, and is useful for obtaining an appropriate amount of upper bainite. This distributes C to untransformed austenite, which then transforms into fresh martensite and / or retained austenite in the second cooling process described below, thereby obtaining the desired fresh martensite and retained austenite. To achieve these effects, the Si content must be 0.20% or more. Therefore, the Si content is set to 0.20% or more. The Si content is preferably 0.60% or more. On the other hand, Si is an element that forms subscale on the steel sheet surface during hot rolling. If the Si content exceeds 2.00%, the subscale becomes too thick, resulting in excessive surface roughness of the steel sheet surface even after descaling in the hot rolling process, which deteriorates the paint pretreatment properties when painting is applied to high-strength hot-rolled steel sheets. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less, and more preferably 1.30% or less.

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

[0030] P: 0.100% or less P is an element that contributes to increasing the strength of steel by dissolving in solid solution. However, P is also an element that segregates at austenite grain boundaries during hot rolling, causing slab cracking during hot rolling. Furthermore, P segregates at grain boundaries, reducing uniform elongation. For this reason, it is preferable to keep the P content as low as possible, but a P content of up to 0.100% is acceptable. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. While there is no particular lower limit for the P content, from the viewpoint of productivity, etc., the P content is preferably set to 0.001% or more.

[0031] S: 0.0200% or less S combines with Ti and Mn to form coarse sulfides, which accelerate the generation of voids, thereby reducing uniform elongation and stretch flangeability. Therefore, it is preferable to keep the S content as low as possible, but a S content of 0.0200% or less is acceptable. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. Note that there is no particular limit on the lower limit of the S content, but from the viewpoint of productivity, it is preferable that the S content be 0.0001% or more.

[0032] Al: 0.010% or more and 2.000% or less. Al acts as a deoxidizer and is an element effective in improving the cleanliness of steel. Since the effect is insufficient when the Al content is less than 0.010%, the Al content is set to 0.010% or more. Similarly to Si, Al has the effect of suppressing the formation of Fe-based carbides and suppressing 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, an Al content exceeding 2.000% increases oxide-based inclusions and reduces uniform elongation and stretch flangeability. Therefore, the Al content is set to 2.000% or less. The Al content is preferably 1.000% or less, more preferably 0.300% or less.

[0033] N: 0.0100% or less N combines with nitride-forming elements to precipitate as nitrides, generally contributing to grain refinement. However, N combines with Ti at high temperatures to form coarse nitrides, so a content of more than 0.0100% causes deterioration in uniform elongation and stretch flangeability. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less. While there is no particular lower limit for the N content, from the viewpoint of productivity, the N content is preferably 0.0005% or more.

[0034] The chemical composition of the high-strength hot-rolled steel sheet further contains at least one selected from Ti: 0.005% to 0.200% and Nb: 0.005% to 0.200%.

[0035] Ti: 0.005% or more and 0.200% or less Ti is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. Ti is an element that is effective in improving stretch flangeability by increasing the hardness of relatively soft upper bainite and reducing the hardness difference between upper bainite and hard fresh martensite. Ti also increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite unrecrystallized region and contributing to refinement of the grain size of the BCC phase. These effects are achieved when the Ti content is 0.005% or more. Therefore, when Ti is contained, the Ti content is set to 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-based precipitates is formed, which actually reduces stretch flangeability. Therefore, when Ti is contained, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, and more preferably 0.120% or less.

[0036] Nb: 0.005% or more and 0.200% or less. Like Ti, Nb is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. Nb is an element that is effective in improving stretch flangeability by increasing the hardness of relatively soft upper bainite and reducing the hardness difference between upper bainite and hard fresh martensite. Similarly to Ti, Nb increases the recrystallization temperature of austenite during hot rolling, thereby enabling rolling in the austenite unrecrystallized region and contributing to refinement of the grain size of the BCC phase. Furthermore, Nb is a carbide-forming element that segregates at the interface between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces the driving force for bainite transformation and stops the upper bainite transformation while leaving the untransformed austenite. The untransformed austenite becomes fresh martensite and / or retained austenite in the second cooling process. Therefore, when Nb is contained, Nb contributes to the formation of fresh martensite and retained austenite with desired area fractions. These effects are obtained when the Nb content is 0.005% or more. Therefore, when Nb is contained, the Nb content is set to 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, when the Nb content exceeds 0.200%, a large amount of Nb-based precipitates is formed, which actually reduces stretch-flange formability. In addition, fresh martensite may increase excessively, which may reduce uniform elongation and stretch-flange formability. Therefore, when Nb is contained, the Nb content is set to 0.200% or less. The Nb content is preferably 0.150% or less, more preferably 0.120% or less.

[0037] The above-mentioned 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 may have a composition containing the above-mentioned components with the balance being Fe and unavoidable impurities.

[0038] In addition to the above-mentioned components, the high-strength hot-rolled steel sheet of the present invention may further contain one or more elements selected from the following group a and group b: (Group a) one or more elements selected from V: 0.001% to 0.100%, Cr: 0.005% to 1.000%, and Mo: 0.005% to 0.500%; (Group b) 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%. one or more selected from 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

[0039] V: 0.001% or more and 0.100% or less. V is an element that improves the strength of a steel sheet through precipitation strengthening and solid-solution strengthening. Similarly to Ti, V increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite unrecrystallized region and contributing to refinement of the grain size of the BCC phase. Similarly to Cr, V is a carbide-forming element that segregates at the interface between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces the driving force for bainite transformation, thereby terminating upper bainite transformation while leaving untransformed austenite. Untransformed austenite becomes fresh martensite and / or retained austenite in the second cooling process. Therefore, when V is contained, V contributes to the formation of desired area fractions of fresh martensite and retained austenite. To achieve these effects, the V content is preferably 0.001% or more. Therefore, when V is contained, the V content is preferably 0.001% or more. The V content is more preferably 0.010% or more. However, if the V content exceeds 0.100%, fresh martensite increases excessively, and uniform elongation and stretch flangeability deteriorate. Therefore, when V is contained, the V content is set to 0.100% or less. The V content is preferably 0.050% or less.

[0040] Cr: 0.005% or more and 1.000% or less. Cr is a carbide-forming element. During upper bainite transformation after coiling of the hot-rolled steel sheet, Cr segregates at the interface between upper bainite and untransformed austenite, reducing the driving force of the bainite transformation and causing the upper bainite transformation to stall. The remaining untransformed austenite resulting from stalling of the transformation to upper bainite becomes fresh martensite and / or retained austenite in the second cooling process. Therefore, when Cr is contained, Cr contributes to the formation of desired area fractions of fresh martensite and retained austenite. To achieve these effects, the Cr content is preferably 0.005% or more. Therefore, when Cr is contained, the Cr content is preferably 0.005% or more. The Cr content is more preferably 0.010% or more. However, because Cr is an element that deteriorates corrosion resistance and paint pretreatment properties, when Cr is contained, the Cr content is set to 1.000% or less. The Cr content is preferably 0.700% or less.

[0041] Mo: 0.005% or more and 0.500% or less. Mo promotes the formation of bainite through improved hardenability, contributing to improved strength of the steel sheet. Similarly to Cr, Mo is a carbide-forming element and segregates at the interface between upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces the driving force for bainite transformation and contributes to the formation of fresh martensite and retained austenite in the second cooling process. To achieve these effects, the Mo content is preferably 0.005% or more. Therefore, when Mo is contained, the Mo content is preferably 0.005% or more. The Mo content is more preferably 0.010% or more. However, when the Mo content exceeds 0.500%, excessive fresh martensite is formed, deteriorating uniform elongation and stretch flangeability. Therefore, when Mo is contained, the Mo content is set to 0.500% or less. The Mo content is preferably 0.300% or less. When the content of V, Cr, or Mo is less than the lower limit, the component is considered to be contained as an unavoidable impurity.

[0042] B: 0.0100% or less 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. In order to obtain such an effect, when B is contained, the B content is preferably 0.0005% or more. On the other hand, when the B content exceeds 0.0100%, the above-mentioned effect saturates. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0060% or less.

[0043] Cu: 1.000% or less Cu is an element that dissolves and contributes to increasing the strength of steel. Furthermore, Cu promotes the formation of bainite through improved hardenability, thereby contributing to improved strength. To achieve this effect, when Cu is contained, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. However, a Cu content exceeding 1.000% leads to a deterioration in the surface quality of the hot-rolled steel sheet, deteriorating the fatigue properties of the hot-rolled steel sheet. Therefore, when Cu is contained, the Cu content is set to 1.000% or less. The Cu content is preferably 0.500% or less.

[0044] Ni: 1.000% or less Ni is an element that contributes to increasing the strength of steel by dissolving in solid solution. Ni also promotes the formation of bainite through improved hardenability, thereby contributing to improved strength. To achieve this effect, when Ni is contained, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.010% or more, and even more preferably 0.040% or more. However, when the Ni content exceeds 1.000%, fresh martensite and retained austenite increase excessively, deteriorating the ductility of the hot-rolled steel sheet. Therefore, when Ni is contained, the Ni content is set to 1.000% or less.

[0045] Sb: 0.200% or less Sb is an element that suppresses denitrification, deboronation, etc., and is effective in suppressing a decrease in the strength of steel. When Sb is contained, the Sb content is preferably 0.002% or more to obtain the above effect. On the other hand, if the Sb content exceeds 0.200%, the toughness of the steel decreases, and slab cracking and hot rolling cracking may occur. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably 0.100% or less, and more preferably 0.050% or less.

[0046] Sn: 0.200% or less Like Sb, Sn is an element that suppresses denitrification, deboronation, etc., and is effective in suppressing a decrease in the strength of steel. When Sn is contained, the Sn content is preferably 0.005% or more to obtain the above effect. On the other hand, if the Sn content exceeds 0.200%, the toughness of the steel decreases, and slab cracking and hot rolling cracking may occur. Therefore, when Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably 0.100% or less, and more preferably 0.050% or less.

[0047] Ta: 0.100% or less Ta increases TS by forming fine carbides, nitrides, or carbonitrides. In addition, Ta partially dissolves in Nb carbides or Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This improves TS. To achieve this effect, when Ta is contained, the Ta content is preferably 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. This may result in reduced stretch flangeability. Therefore, when Ta is contained, the Ta content is set to 0.100% or less. The Ta content is preferably 0.050% or less, and more preferably 0.020% or less.

[0048] W: 0.500% or less W is an element effective for improving hardenability and adjusting TS to a more suitable range. To achieve this effect, when W is contained, the W content is preferably 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 fraction of hard fresh martensite increases excessively, which may result in a decrease in ductility and work hardening ability in the high strain region. Therefore, when W is contained, the W content is set to 0.500% or less. The W content is preferably 0.100% or less, and more preferably 0.050% or less.

[0049] Mg: 0.0200% or less Like Ca, Mg controls the shape of oxide and sulfide inclusions, contributing to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. To achieve this effect, when Mg is contained, the Mg content is preferably 0.0010% or more. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause sheared edge cracking or stretch flange cracking. Therefore, when Mg is contained, the Mg content is set to 0.0200% or less. The Mg content is preferably 0.0050% or less.

[0050] Zn: 0.0200% or less Zn spheroidizes the shape of inclusions, contributing to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. To achieve this effect, when Zn is contained, the Zn content is preferably 0.0010% or more. However, if the Zn content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may instead cause sheared edge cracking or stretch flange cracking. Therefore, when Zn is contained, the Zn content is set to 0.0200% or less. The Zn content is preferably 0.0050% or less.

[0051] Co: 0.0200% or less Like Zn, Co spheroidizes the shape of inclusions, contributing to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. To achieve this effect, when Co is contained, the Co content is preferably 0.0010% or more. However, if the Co content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause sheared edge cracking or stretch flange cracking. Therefore, when Co is contained, the Co content is set to 0.0200% or less. The Co content is preferably 0.0050% or less.

[0052] Zr: 0.0200% or less Like Zn and Co, Zr spheroidizes the shape of inclusions, contributing to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. To achieve this effect, when Zr is contained, the Zr content is preferably 0.0010% or more. However, if the Zr content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause sheared edge cracking or stretch flange cracking. Therefore, when Zr is contained, the Zr content is set to 0.0200% or less. The Zr content is preferably 0.0050% or less.

[0053] Ca: 0.0200% or less Ca controls the shape of oxide and sulfide-based inclusions, contributing to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. To achieve this effect, when Ca is contained, the Ca content is preferably 0.0005% or more. However, if the Ca content exceeds 0.0200%, the Ca-based inclusions increase, deteriorating the cleanliness of the steel, which may actually cause sheared edge cracking or stretch flange cracking. Therefore, when Ca is contained, the Ca content is set to 0.0200% or less. The Ca content is preferably 0.0050% or less.

[0054] 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. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM (rare earth metals) all control the shape of oxide and sulfide-based inclusions, as with Ca, and contribute to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to achieve such effects, when the above elements are contained, it is preferable that the content of each of the above elements is 0.0005% or more. However, if the content of each of the above elements exceeds 0.0200%, the cleanliness of the steel deteriorates, which may lead to shear edge cracking or stretch flange cracking. Therefore, when each of the above elements is contained, the content of each of the above elements is set to 0.0200% or less. Note that REM is a collective term for Sc, Y, and elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. REM can contain one or more of these elements, and the REM content here refers to the total content of these elements.

[0055] Next, the steel structure of the high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0056] Area fraction of upper bainite: 30.0% or more and less than 90.0% Upper bainite is a useful phase for ensuring an appropriate amount of fresh martensite and retained austenite by utilizing the diffusion of C from upper bainite to untransformed austenite. Therefore, the area fraction of upper bainite is set to 30.0% or more. The area fraction of upper bainite is preferably 40.0% or more, more preferably 50.0% or more. On the other hand, if the area fraction of upper bainite is 90.0% or more, it becomes difficult to achieve a tensile strength of 1180 MPa or more. Therefore, the area fraction of upper bainite is set to less than 90.0%. The area fraction of upper bainite is preferably less than 85.0%, more preferably less than 80.0%, and even more preferably less than 70.0%.

[0057] Total area fraction of lower bainite and tempered martensite: 6.0% or more and 60.0% or less Lower bainite and tempered martensite have intermediate hardness between soft upper bainite and hard fresh martensite, and are useful phases for ensuring high strength and excellent stretch flangeability. Therefore, the total area fraction of lower bainite and tempered martensite is set to 6.0% or more. The total area fraction 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 fraction of lower bainite and tempered martensite exceeds 60.0%, ductility decreases. Therefore, the total area fraction of lower bainite and tempered martensite is set to 60.0% or less. The total area fraction of lower bainite and tempered martensite is preferably 55.0% or less, more preferably 50.0% or less.

[0058] Area fraction of fresh martensite: 2.0% or more and 20.0% or less Fresh martensite improves strain dispersion in the low strain region by being adjacent to other phases. Therefore, the area fraction of fresh martensite is set to 2.0% or more. The area fraction of fresh martensite is preferably 3.0% or more, more preferably 3.5% or more. On the other hand, if the area fraction of fresh martensite exceeds 20.0%, stretch flangeability deteriorates. Therefore, the area fraction of fresh martensite is set to 20.0% or less. The area fraction of fresh martensite is preferably 17.0% or less, more preferably 15.0% or less.

[0059] Area fraction of retained austenite: 2.0% or more and 10.0% or less From the viewpoint of obtaining good ductility and excellent work hardening ability in a high strain range, the area fraction of retained austenite is set to 2.0% or more. The area fraction 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 fraction of retained austenite exceeds 10.0%, stretch flangeability deteriorates. Therefore, the area fraction of retained austenite is set to 10.0% or less. The area fraction of retained austenite is preferably 9.0% or less, more preferably 8.0% or less.

[0060] The area ratio of the remaining structure other than the above 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 also be 0%.

[0061] The remaining structure is not particularly limited, and examples thereof include carbides such as polygonal ferrite, acicular ferrite, pearlite, cementite, etc. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).

[0062] Upper bainite is an aggregate of lath-shaped ferrite with a misorientation of less than 15°, a structure having Fe-based carbides and / or retained austenite at the lath-shaped ferrite interfaces (however, this also includes cases where Fe-based carbides and / or retained austenite are not present at the lath-shaped ferrite interfaces), and a structure in which no Fe-based carbides are present inside the lath-shaped ferrite, or even if Fe-based carbides are present inside the lath-shaped ferrite, the number of Fe-based carbides present inside the lath-shaped ferrite is smaller than the number of Fe-based carbides present at the lath-shaped ferrite interfaces.Upper bainite can be held at high temperatures after its formation, causing the interfaces of the lath-shaped ferrite to disappear or become unclear, but it can be distinguished from polygonal ferrite and acicular ferrite, which have small misorientation within the crystal grains, by measuring the misorientation within the crystal grains using EBSD (Electron Backscatter Diffraction). Furthermore, unlike polygonal ferrite and acicular ferrite, upper bainite has a relatively high dislocation density inside. Therefore, lath ferrite can be distinguished from polygonal ferrite and acicular ferrite using a transmission electron microscope (TEM). When retained austenite is present between the laths, only the lath ferrite portion is considered to be upper bainite and is distinguished from the retained austenite.

[0063] Lower bainite and / or tempered martensite are aggregates of lath-shaped ferrite with an orientation misorientation of less than 15°, and are structures having Fe-based carbides within the lath-shaped ferrite (including cases where Fe-based carbides are also present at the lath-shaped ferrite interfaces), meaning structures in which the number of Fe-based carbides present within the lath-shaped ferrite is greater than the number of Fe-based carbides present at the lath-shaped ferrite interfaces. Lower bainite and tempered martensite can be distinguished from each other by observing the orientation and crystalline structure of the Fe-based carbides within the lath-shaped ferrite using a TEM. However, in the present invention, the two are not distinguished because they have essentially the same properties. Lower bainite and / or tempered martensite can be distinguished from upper bainite by their brighter contrast in SEM observations and the inclusion of fine and large amounts of Fe-based carbides and / or retained austenite.

[0064] In the present invention, when fresh martensite and retained austenite are defined as hard phases, the hard phases (fresh martensite and retained austenite) contain no Fe-based carbides compared to lower bainite and / or tempered martensite. Furthermore, fresh martensite and retained austenite have brighter contrast in SEM images than upper bainite, lower bainite and / or tempered martensite, polygonal ferrite, and acicular ferrite. Therefore, the hard phases can be distinguished from these structures using SEM. Fresh martensite and retained austenite have similar shapes and contrasts in SEM, making them difficult to distinguish. Therefore, the area fractions of fresh martensite and retained austenite are determined using the method described below.

[0065] Here, the area ratios of upper bainite, lower bainite and tempered martensite, fresh martensite, retained austenite, and the remaining structure are measured at a 1 / 4 position in the plate thickness direction of the high-strength hot-rolled steel plate as follows.

[0066] Specifically, a sample is cut out from a hot-rolled steel sheet so that the observation surface is a cross section of the sheet thickness parallel to the rolling direction of the hot-rolled steel sheet. The observation surface of the sample is then mirror-polished using diamond paste. The observation surface of the sample is then finish-polished using colloidal silica, and then etched with 3 vol% nital to reveal the structure.

[0067] Then, ten visual fields of 25.6 μm×17.6 μm on the observation surface of the sample are observed using a scanning electron microscope (SEM) under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times.

[0068] The fresh martensite and the retained austenite that constitute the hard phase have similar contrast in an SEM image and are difficult to distinguish from each other. Therefore, the area ratio of the retained austenite is determined by X-ray diffraction, and the area ratio of the fresh martensite is determined by subtracting the area ratio of the retained austenite (described later) from the area ratio of the hard phase calculated from the SEM image.

[0069] The area fraction of retained austenite is measured as follows. Specifically, a hot-rolled steel sheet is mechanically ground in the thickness direction (depth direction) to a position corresponding to one-quarter of the sheet thickness, and then a portion of the sheet thickness of 100 μm or more is chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays. The ratio of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200) and (211) planes of bcc iron is then calculated, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0070] Average amount of solute C in retained austenite: 0.60 mass% or more and 0.90 mass% or less. If the average amount of solute C in retained austenite is less than 0.60 mass%, the retained austenite transforms at an early stage of working, reducing the work hardening ability in the high strain region. That is, the n value decreases. Therefore, the average amount of solute C in retained austenite is set to 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 does not transform sufficiently at an early stage of working, reducing the work hardening ability in the low strain region. That is, the YR increases. Therefore, the average amount of solute C in retained austenite is set to 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.

[0071] The average amount of solute C in the retained austenite is measured as follows. That is, the observation surface on which the area ratio of the retained austenite has been measured is observed by X-ray diffraction. CuKα rays are used as the incident X-rays, and the lattice constant (a γ ) was obtained, and the average amount of dissolved C in the retained austenite was calculated using the following formula: γ (Å) = 3.572 + 0.033 × C - 0.00157 × Si + 0.0012 × Mn In the above formula, Si and Mn represent the content (mass%) of each element in the steel, and C in the formula represents the average amount (mass%) of dissolved C in the retained austenite. Each element symbol in the formula represents the content (mass%) of each element in the steel.

[0072] Total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more: 10.0% or less If coarse crystal grains are present in the steel structure, the low-temperature toughness and stretch flangeability of the steel sheet will decrease. 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 set to 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 lower limit for the total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more, and the total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more may be 0%.

[0073] Here, the crystal grains are regions surrounded by grain boundaries with a crystal orientation difference of 15° or more, and are measured by EBSD (Electron Backscatter Diffraction) method.

[0074] The circle equivalent diameter is defined as the area A [μm ２ ], the value is calculated as 2√(A / π) [μm].

[0075] The total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more is measured at a 1 / 4 position in the sheet thickness direction of a high-strength hot-rolled steel sheet as follows.

[0076] That is, a sample is cut out from the hot-rolled steel sheet so that the plate thickness cross section parallel to the rolling direction of the hot-rolled steel sheet becomes the observation surface. Then, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample is polished to a finish using colloidal silica, and 10 fields of view of 100 μm × 100 μm of the observation surface of the sample are observed by the EBSD method (electron beam acceleration voltage: 20 keV, measurement interval: 0.1 μm step). Using TSL's OIM Analysis software, grain boundaries with a crystal orientation difference of 15° or more are visualized, and the circle equivalent diameter of each crystal grain is determined by image processing, and the total area ratio of crystal grains with a circle equivalent diameter of 15.0 μm or more is calculated.

[0077] 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 If crystal grains having an equivalent circle diameter of 8.0 μm or more and an aspect ratio of 4.0 or more exist in the steel structure, they become the origin of void generation during processing, thereby reducing 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 set to 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 set to 9.0% or less, more preferably 8.0% or less. 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 is not particularly limited, 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 may be 0%. Here, the aspect ratio is a value obtained by dividing the length of the major axis of an ellipse by the length of the minor axis of the ellipse when the region of a crystal grain is approximated as an ellipse.

[0078] 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 position in the plate thickness direction of a high-strength hot-rolled steel plate as follows.

[0079] That is, 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 crystal orientation difference of 15° or more are visualized. Then, 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.

[0080] When retained austenite and fresh martensite are considered as hard phases, the total area ratio of regions in the hard phase having an equivalent circle diameter of 4.0 μm or less and an aspect ratio of 2.0 or more: 1.0% or more In order to ensure excellent work hardening ability in the high strain region, it is necessary to ensure a predetermined amount of area ratio of retained austenite, and also to ensure that the distribution of the amount of solute C in the retained austenite has an appropriate average and variance. The average amount of solute C in the retained austenite is as described above. As a result of examining the variance of the distribution of the amount of solute C in the retained austenite, when retained austenite and fresh martensite are considered as hard phases, it was found that the total area ratio of regions (island regions) in the hard phase having an equivalent circle diameter of 4.0 μm or less and an aspect ratio of 2.0 or more (hereinafter, the area ratio is referred to as "S ３ It has been found that when the S content is 1.0% or more, the distribution of the amount of dissolved C in the retained austenite is large. ３ If S is less than 1.0%, the work hardening ability in the high strain region decreases. ３ is 1.0% or more. ３ is preferably 1.5% or more, and more preferably 2.0% or more. ３ It is difficult to make the content of C equal to or greater than 10.0%, and the dispersion of the distribution of the amount of dissolved C in the retained austenite does not become excessively large, so it is preferably set to less than 10.0%.

[0081] Here, the total area ratio of the hard phase (retained austenite and fresh martensite) regions having an equivalent circle diameter of 4.0 μm or less and an aspect ratio of 2.0 or more is measured as follows.

[0082] That is, the circle-equivalent diameter and aspect ratio of the hard phase regions identified in the measurement of the hard phase area ratio were determined by image processing. Regions with a circle-equivalent diameter of 4.0 μm or less and an aspect ratio of 2.0 or more were identified, and their total area ratio was 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 and are difficult to distinguish using an SEM. Furthermore, in the present invention, retained austenite and fresh martensite both have similar properties as hard phases. Therefore, regions composed of adjacent retained austenite and fresh martensite are considered to be a single hard phase region.

[0083] Next, the mechanical properties of the high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0084] Tensile strength (TS): 1180 MPa or more The high-strength hot-rolled steel sheet according to one embodiment of the present invention has a tensile strength of 1180 MPa or more. The tensile strength is preferably less than 1470 MPa.

[0085] 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 one embodiment of the present invention are as described above.

[0086] The tensile strength (TS), uniform elongation (U.El), yield ratio (YR), and work hardening index (n value) are measured by a tensile test in accordance with 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 in accordance with 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 in accordance with JIS Z 2242:2018, which will be described later in the examples.

[0087] The high-strength hot-rolled steel sheet of the present invention may have a plating layer on the surface thereof. The plating layer is not particularly limited, and examples thereof include known plating layers.

[0088] [2] Member and manufacturing method of member Next, a member according to one embodiment of the present invention and a manufacturing method thereof will be described.

[0089] A member according to one embodiment of the present invention is a member made using (as a raw material) the above-mentioned high-strength hot-rolled steel sheet. Examples of a method for manufacturing the member include a manufacturing method in which the raw material high-strength hot-rolled steel sheet is subjected to at least one of forming and joining to form the member.

[0090] The high-strength hot-rolled steel sheet has high strength, excellent ductility and stretch flangeability, excellent strain distribution ability over a wide strain range, and excellent low-temperature toughness. Therefore, the member according to one embodiment of the present invention is particularly suitable for use in members with complex shapes used in the automotive field.

[0091] [3] Manufacturing Method of High-Strength Hot-Rolled Steel Sheet Next, a manufacturing method of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0092] A method for producing a high-strength hot-rolled steel sheet according to one embodiment of the present invention includes 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 conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of (RC1-100)°C or higher and (RC1+100)°C or lower to produce a hot-rolled steel sheet, and a hot-rolling step of hot-rolling the hot-rolled steel sheet after the hot-rolling step under conditions of a time from the end of the hot rolling to the start of cooling of RC2 seconds or less, an average cooling rate of FF°C / second or higher, and a cooling end temperature of 350°C or higher and (Bs+50)°C or lower. The hot-rolled steel sheet after the first cooling step is cooled and coiled under the conditions of a first cooling step, a first holding step in which the hot-rolled steel sheet after the first cooling step is held in a temperature range of 350°C or higher and (Bs + 50)°C or lower for a holding time of 1000 minutes or less and a temperature range of 350°C or higher and 450°C or lower for a holding time of 20 minutes or higher and 500 minutes or less, a second holding step in which the hot-rolled steel sheet after the first holding step is held in a temperature range of 250°C or higher and lower than 350°C for a holding time of 30 minutes or higher and 500 minutes or less, and a second cooling step in which the hot-rolled steel sheet after the second holding step is cooled to 100°C or lower. Here, RC1, RC2, FF, and Bs are defined by the following formulas, respectively. RC1 = 850 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V RC2 = 1.2 + C + N + 0.1 x Mn + 10 x Ti + 50 x B + 0.1 x Cr + 0.5 x Mo + 20 x Nb + 1.5 x V FF = 10^(3.9 - 1.2 x (2.5 x C + Mn + 0.7 x Cr + 0.1 x Mo + 0.5 x Ni)) Bs = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo The element symbols in the above formulas represent the content of each element in mass%, and the content of elements that are not contained is 0. Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel material and the steel sheet. The above average cooling rate refers to the average cooling rate of the steel sheet surface. Unless otherwise specified, the average cooling rate is defined as [(cooling start temperature - cooling stop temperature) / cooling time from the cooling start temperature to the cooling stop temperature].

[0093] First, a steel material such as a slab having the above-described composition is prepared. The method for producing a steel material such as a slab is not particularly limited, and a commonly used method can be used. Examples of the method for producing a steel material include a method in which molten steel having the above-described composition is produced using a known method in a converter or the like, and a slab is produced by a casting method such as continuous casting. Known casting methods such as ingot making and blooming may also be used to produce the steel material. Scrap may also be used as the raw material for the steel material.

[0094] [Heating Process] Heating temperature of steel material: 1150°C or higher In steel material such as a slab after being cooled to a low temperature, most of the elements that form carbonitrides, such as Ti, are precipitated non-uniformly as coarse carbonitrides. The presence of these coarse and non-uniform precipitates leads to deterioration of various properties (e.g., strength, punching roughness resistance, etc.). For this reason, the steel material is heated before hot rolling to dissolve the coarse precipitates. In order to sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the steel material is set to 1150°C or higher. The heating temperature of the steel material is preferably 1180°C or higher, 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 occurrence of slab defects or a decrease in yield due to scale-off. For this reason, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. The steel material before hot rolling may be subjected to hot rolling (direct rolling) directly after casting while still at a high temperature (i.e., while maintaining a temperature within the above heating temperature range).

[0095] [Hot Rolling Step] Next, the steel material heated to 1150°C or higher (including steel that is directly shipped at a high temperature of 1150°C or higher after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The conditions of rough rolling are not particularly limited as long as the desired sheet bar dimensions are secured.

[0096] A steel material is rough-rolled to obtain a rough-rolled plate. Before the rough-rolled plate is subjected to finish rolling, it is preferable to perform descaling (high-pressure water descaling) by spraying high-pressure water on the inlet side of a finish rolling mill.

[0097] In order to remove primary scale generated before finish rolling, it is preferable to subject the rough-rolled sheet to high-pressure water descaling. The collision pressure of high-pressure water descaling (also simply referred to as "descaling collision 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 collision pressure is the force per unit area at which high-pressure water collides with the surface of the rough-rolled sheet. There is no particular upper limit for the descaling collision pressure, but it is preferably 15.0 MPa or less, more preferably 14.5 MPa or less, and even more preferably 12.0 MPa or less. Note that high-pressure water descaling may be performed during rolling between stands in the finish rolling. Furthermore, if necessary, the rough-rolled sheet may be cooled between stands in the finish rolling.

[0098] Finish rolling start temperature: 1000°C or higher and 1250°C or lower A hot-rolled steel sheet (finish-rolled sheet) is obtained by finish-rolling a rough-rolled sheet at a predetermined finish-rolling start temperature and finish-rolling end temperature. If the finish-rolling start temperature is too low, austenite grains are less likely to recrystallize during finish rolling, and the total area ratio of crystal grains with a circle-equivalent diameter of 8.0 μm or higher and an aspect ratio of 4.0 or higher increases. This reduces stretch-flange formability. Therefore, the finish-rolling start temperature is set to 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, significant grain growth of austenite grains occurs, the austenite grains become coarse, and the total area ratio of crystal grains with a circle-equivalent diameter of 15.0 μm or higher increases. This increases vTrs. For this reason, the finish rolling start temperature is set to 1250°C or lower.

[0099] Finish rolling end temperature: (RC1-100)°C or higher and (RC1+100)°C or lower. If the finish rolling end temperature is too low, rolling may be performed at the two-phase region temperature of ferrite + austenite. As a result, the desired area ratio for each phase cannot be sufficiently obtained, and a tensile strength of 1180 MPa or higher cannot be ensured. For this reason, the finish rolling end temperature is set to (RC1-100)°C or higher. The finish rolling end temperature is preferably (RC1-80)°C or higher, and more preferably (RC1-50)°C or higher. On the other hand, if the finish rolling end temperature is too high, significant grain growth of austenite grains occurs, the austenite grains become coarse, and the total area ratio of crystal grains with an equivalent circle diameter of 15.0 μm or higher increases. This increases vTrs. For this reason, the finish rolling end temperature is set to (RC1+100)°C or lower. The finish rolling end temperature is preferably (RC1 + 80) ° C. or less, and more preferably (RC1 + 50) ° C. or less. RC1 is defined by the following formula (1): RC1 = 850 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V (1) where each element symbol in formula (1) represents the content of each element in mass% in the above-mentioned composition, and the content of an element that is not contained is set to 0.

[0100] [First Cooling Step] Next, the hot-rolled steel sheet (finish-rolled sheet) obtained by finish rolling is cooled (hereinafter also referred to as "forced cooling") from the finish-rolling end temperature described above to a cooling stop temperature described below at an average cooling rate described below.

[0101] Time from the end of the hot rolling to the start of cooling: RC2 seconds or less 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, grain growth of austenite grains occurs, and the total area ratio of crystal grains with an equivalent circle diameter of 15.0 μm or more increases. This increases vTrs. Therefore, the cooling start time is set to RC2 seconds or less. The cooling start time is preferably (RC2 - 0.5) seconds or less, and more preferably (RC2 - 1.0) seconds or less. Note that there is no particular limitation on the lower limit of the cooling start time, and the cooling start time may be 0 seconds. RC2 is defined by the following formula (2): RC2=1.2+C+N+0.1×Mn+10×Ti+50×B+0.1×Cr+0.5×Mo+20×Nb+1.5×V (2) In the formula (2), each element symbol represents the content of each element in mass % in the above-mentioned composition, and the content of an element that is not contained is set to 0.

[0102] Average Cooling Rate: FF°C / sec or More In forced cooling, if the average cooling rate from the finish rolling end temperature to the cooling stop temperature (hereinafter also referred to as the "average cooling rate in forced cooling") is too slow, ferrite transformation occurs before upper bainite transformation, and the desired area ratio of upper bainite phase cannot be obtained. Furthermore, pearlite and the like are formed, and the desired area ratios of fresh martensite and retained austenite cannot be obtained. This makes it difficult to achieve a TS of 1,180 MPa or more. For this reason, the average cooling rate in forced cooling is set to FF°C / sec or more. The average cooling rate in forced cooling is preferably (FF+5)°C / sec or more, and more preferably (FF+10)°C / sec or more. On the other hand, although there is no particular upper limit for the average cooling rate in forced cooling, if it is too fast, it may become difficult to control the cooling stop temperature, making it difficult to obtain the desired steel structure. From this viewpoint, the average cooling rate is preferably 500°C / sec or less, more preferably 300°C / sec or less, even more preferably 200°C / sec or less, and particularly preferably 150°C / sec or less. FF is defined by the following formula (3): FF=10^(3.9-1.2×(2.5×C+Mn+0.7×Cr+0.1×Mo+0.5×Ni)) (3) where each element symbol in formula (3) represents the content of each element in mass% in the above-mentioned component composition, and the content of an element that is not contained is set to 0.

[0103] Cooling stop temperature: 350°C or higher and (Bs + 50)°C or lower. If the cooling stop temperature is too low, the area fraction of upper bainite and retained austenite decreases, and the area fraction of lower bainite and tempered martensite increases. As a result, U.E.I. and the n value decrease, and YR increases. For this reason, the cooling stop temperature is set to 350°C or higher. The cooling stop temperature is preferably 370°C or higher, and more preferably 380°C or higher. On the other hand, if the cooling stop temperature is too high, the area fraction of upper bainite and retained austenite decreases. As a result, the n value decreases. Furthermore, pearlite and the like may be generated, making it difficult to achieve a TS of 1180 MPa or higher. For this reason, the cooling stop temperature is set to (Bs + 50)°C or lower. The cooling stop temperature is preferably (Bs + 20)°C or lower, and more preferably Bs°C or lower. Bs is defined by the following formula (4): Bs = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo (4) where each element symbol in formula (4) represents the content of each element in mass% in the above-mentioned composition, and the content of an element that is not contained is 0.

[0104] [First Holding Step] Next, the hot-rolled steel sheet after the first cooling step is held in a temperature range of 350°C or higher and (Bs + 50)°C or lower. Holding in this temperature range is preferably performed in a state where the heat retention is enhanced by winding the hot-rolled steel sheet after the first cooling step, for example, into a coil shape. In the first holding step, the hot-rolled steel sheet may be held isothermally, or the temperature may be increased or decreased during the process as long as it is in a temperature range of 350°C or higher and (Bs + 50)°C or lower. Any means for increasing or decreasing the temperature may be used, and may include external heating or heat extraction to the outside, or may utilize heat generation or heat absorption within the hot-rolled steel sheet.

[0105] Holding time in the temperature range of 350°C to (Bs + 50)°C: 1000 minutes or less, and holding time in the temperature range of 350°C to 450°C: 20 minutes to 500 minutes or less. If the holding time in the temperature range of 350°C to (Bs + 50)°C exceeds 1000 minutes, carbides and pearlite are generated, reducing the area fraction 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 350°C to (Bs + 50)°C is set to 1000 minutes or less. The holding time in this temperature range is preferably 800 minutes or less, and more preferably 500 minutes or less. The lower limit of the holding time in this temperature range is preferably 20 minutes or more, from the viewpoint of obtaining a predetermined area fraction of upper bainite.

[0106] The first holding step is an important step for obtaining a predetermined area fraction of upper bainite, diffusing carbon from the upper bainite to untransformed austenite, and appropriately diffusing carbon into the untransformed austenite in the second holding step described below, thereby achieving an appropriate distribution of solute carbon in the retained austenite. In the first holding step, if the holding time in the temperature range of 350°C to 450°C is less than 20 minutes, the area fractions of upper bainite and retained austenite decrease, the YR increases, and the uniform elongation decreases. Therefore, the holding time in the temperature range of 350°C to 450°C is set to 20 minutes or more. That is, in the first holding step, the holding time in the temperature range of 350°C to (Bs + 50)°C is set to 1000 minutes or less, and the holding time in the temperature range of 350°C to 450°C is set to 20 minutes or more. The holding time in the temperature range of 350°C to 450°C is preferably 25 minutes or more, and more preferably 30 minutes or more. On the other hand, if the holding time in the temperature range of 350°C or higher and 450°C or lower exceeds 500 minutes, carbides are generated and the area ratio of upper bainite increases. This makes it difficult to achieve a TS of 1180 MPa or higher. Therefore, the holding time in the temperature range of 350°C or higher and 450°C or lower is set to 500 minutes or less. That is, in the first holding step, the holding time in the temperature range of 350°C or higher and (Bs+50)°C or lower is set to 1000 minutes or less, and the holding time in the temperature range of 350°C or higher and 450°C or lower is set to 500 minutes or less.

[0107] [Second Holding Step] Next, the hot-rolled steel sheet after the first holding step is held in a temperature range of 250° C. or more and less than 350° C. In the second holding step, the hot-rolled steel sheet may be held isothermally, or the temperature may be increased or decreased during the process as long as it is in a temperature range of 250° C. or more and less than 350° C. Any means for increasing or decreasing the temperature may be used, and may include heating from the outside, extracting heat to the outside, or utilizing heat generation or absorption within the hot-rolled steel sheet.

[0108] Holding time in the temperature range of 250°C or higher and lower than 350°C: 30 minutes or longer and 500 minutes or shorter. The second holding step is performed to appropriately cause C diffusion into untransformed austenite, and to obtain a predetermined average amount of dissolved C in the retained austenite and a predetermined amount of S. ３ This is an important process for obtaining the above. If the holding time in the temperature range of 250°C or more and less than 350°C exceeds 500 minutes, excessive C concentration occurs in the untransformed austenite, and the average amount of dissolved C in the retained austenite increases. As a result, the strain dispersibility in the low strain region decreases, that is, the YR increases. In addition, carbides may precipitate from the untransformed austenite, reducing the amount of retained austenite and decreasing the uniform elongation and n value. For this reason, the holding time in the temperature range of 250°C or more and less than 350°C is set to 500 minutes or less. The holding time is preferably 450 minutes or less, and more preferably 400 minutes or less. On the other hand, if the holding time in the temperature range of 250°C or more and less than 350°C is less than 30 minutes, S ３ The n-value decreases as the distribution of the amount of solute C in the retained austenite decreases. Therefore, the holding time in the temperature range of 250°C or higher and lower than 350°C is set to 30 minutes or longer. The holding time is preferably 40 minutes or longer, and more preferably 45 minutes or longer.

[0109] [Second Cooling Step] Next, the hot-rolled steel sheet after the second holding step is cooled to 100°C or less. The conditions for cooling to 100°C or less after the second holding step are not particularly limited, but it is preferable that the average cooling rate from 250°C to a cooling stop temperature of 100°C or less is 200°C / s or less. Note that, as will be described later, when a plating treatment step is performed after the second holding step, the steel sheet after the plating treatment step is cooled to 100°C or less in the second cooling step.

[0110] Through the above steps, the high strength hot rolled steel sheet of the present invention is manufactured.

[0111] Thereafter, temper rolling (skin pass rolling) may be performed according to a conventional method. Furthermore, scale may be removed by pickling. Furthermore, when the high-strength hot-rolled steel sheet of the present invention has a plating layer on its surface, the method for producing the high-strength hot-rolled steel sheet further includes a plating treatment step of plating the hot-rolled steel sheet. In this case, the plating treatment may be performed on the hot-rolled steel sheet after the second holding step or the second cooling step, for example, without being particularly limited thereto. The plating treatment in the plating treatment step is not particularly limited, and examples thereof include known plating treatments.

[0112] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0113] [Production of High-Strength Hot-Rolled Steel Sheet] Molten steel having the chemical composition shown in Table 1 below (the balance consisting of Fe and unavoidable impurities) was melted in a converter, and a steel material was produced by continuous casting. The produced steel material was subjected to a heating step at the 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 obtained rough-rolled sheet was subjected to high-pressure water descaling at an impact pressure of 10.0 MPa. The rough-rolled sheet subjected to high-pressure water descaling was then finish-rolled at the finish-rolling start temperature [°C] and finish-rolling temperature [°C] shown in Table 2 below, to obtain a hot-rolled steel sheet. After completion of hot rolling (finish rolling), the obtained hot-rolled steel sheet was subjected to a first cooling step. The following Table 2 shows the conditions for the first cooling step, including the cooling start time (the time from the end of hot rolling to the start of forced cooling) [seconds], the average cooling rate (the average cooling rate from the finish rolling end temperature to the cooling stop temperature) [°C / second], and the cooling stop temperature [°C]. Thereafter, the hot-rolled steel sheet after the first cooling step was coiled at the cooling stop temperature [°C] shown in Table 2 below.

[0114] 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 range of 350°C or higher (Bs + 50)°C or lower for a holding time I [minutes] shown in Table 2 below, and at a temperature range of 350°C or higher and 450°C or lower for a holding time II [minutes] shown in Table 2 below. The holding time I shown in Table 2 is the holding time (minutes) in the temperature range of 350°C or higher and (Bs + 50)°C or lower, and the holding time II is the holding time (minutes) in the temperature range of 350°C or higher and 450°C or lower. 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 range of 250°C or higher and lower than 350°C under the holding time conditions shown in Table 2 below. The holding times shown in Table 2 indicate the holding times (minutes) in the temperature range of 250°C or higher and lower 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 obtained high-strength hot-rolled steel sheets were subjected to temper rolling, and then to pickling (hydrochloric acid concentration: 10 mass %, temperature: 85° C.) to remove scale.

[0115]

[0116]

[0117] [Evaluation of High-Strength Hot-Rolled Steel Sheets] The steel structure of the obtained high-strength hot-rolled steel sheets was identified in the manner described above. 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 addition, in Table 3, S represents upper bainite, 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). １ is the total area ratio of crystal grains with a circle equivalent diameter of 15.0 μm or more. ２ is the total area ratio of crystal grains having a circle-equivalent diameter of 8.0 μm or more and an aspect ratio of 4.0 or more. ３ is the total area ratio of regions of the hard phase having an equivalent circle diameter of 4.0 μm or less and an aspect ratio of 2.0 or more, when retained austenite and fresh martensite are considered to be hard phases.

[0118]

[0119] In addition, tensile tests, hole expansion tests, and Charpy impact tests were performed according to the following procedures, and 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) were evaluated.

[0120] (1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241:2011. Specifically, JIS No. 5 test specimens were taken from the obtained high-strength hot-rolled steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheets. Using the taken test specimens, a tensile test was conducted at a crosshead speed of 10 mm / min to measure the YS, TS, U.E.I., T.E.I. (total elongation), and n-value. Here, the n-value was measured with a strain range of 0.7 × U.E.I. to 0.9 × U.E.I. The YR was calculated from YS / TS. The results are also shown in Table 4. A TS of 1180 MPa or more (TS≧1180 MPa) was considered to be pass, and any other value was considered to be fail. U.E.I. El of 5.0% or more (U.El≧5.0%) was judged as pass (having excellent ductility), and anything else was judged as fail. YR of 0.93 or less (YR≦0.93) was judged as pass (having excellent strain dispersibility in the low strain range), and anything else was judged as fail. n value of 0.080 or more (n value≧0.080) was judged as pass (having excellent strain dispersibility in the high strain range), and anything else was judged as fail. And, those that passed both YR and n value were evaluated as having excellent strain dispersibility over a wide strain range.

[0121] (2) Hole Expansion Test The hole expansion test was performed in accordance with JIS Z 2256:2020. Specifically, 100 mm × 100 mm test pieces were obtained from the obtained high-strength hot-rolled steel sheet by shearing. A 10 mm diameter hole (the initial test piece hole) was punched into the test piece with a clearance of 12% ± 1%. Next, a blank holding force of 9 ton (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm. In this state, a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when a crack occurred) was measured. The limiting hole expansion ratio: λ (%) was then calculated using the following formula. λ is an index for evaluating stretch flangeability. The results are also shown in Table 4. λ of 30% or more (λ≧30%) was considered to be pass (excellent stretch flangeability), and anything else was considered to be fail. λ (%) = {(D ｆ -D ０ ) / D ０}×100 where, D ｆ D: diameter of the hole in the test piece when a crack occurs (mm) ０ : initial test piece hole diameter (mm)

[0122] (3) Charpy Impact Test Sub-size test pieces (V-notch) with a thickness of 2.5 mm were taken from the obtained high-strength hot-rolled steel sheet so that the longitudinal direction of the test piece was perpendicular to the rolling direction. Then, a Charpy impact test was performed in accordance with the provisions of JIS Z 2242:2018 to measure the brittle-ductile fracture transition temperature (vTrs) and evaluate toughness. For hot-rolled steel sheets with a thickness of more than 2.5 mm, test pieces were prepared by double-side grinding to a thickness of 2.5 mm, and for hot-rolled steel sheets with a thickness of 2.5 mm or less, test pieces were prepared at the original thickness and subjected to the Charpy impact test. The results are also shown in Table 4. A vTrs of -40°C or less (vTrs≦-40°C) was considered to be pass (having excellent low-temperature toughness), and any other value was considered to be fail.

[0123] As shown in Table 4, according to the present invention, a high-strength hot-rolled steel sheet having a tensile strength of 1180 MPa or more, excellent ductility and stretch flangeability, excellent strain distribution ability over a wide strain range, and excellent low-temperature toughness can be obtained.

[0124]

Claims

1. A steel having a composition comprising, by mass%, C: 0.100% or more and 0.250% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, and further comprising at least one selected from Ti: 0.005% or more and 0.200% or less and Nb: 0.005% or more and 0.200% or less, with the balance consisting of Fe and unavoidable impurities, and having a steel structure having an area ratio of upper bainite: 30.0% or more and less than 90.0%, and a combined area ratio of lower bainite and tempered martensite: 6.0% or more and 60.0% or less, a 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; a 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; a 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: 1.0% or more, when the retained austenite and the fresh martensite are considered as hard phases, of regions of the hard phase, 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: 1.0% or more; and a tensile strength of 1,180 MPa or more.

2. The high strength hot rolled steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, one or more elements selected from the following groups a and b: Group a: one or more selected from V: 0.001% or more and 0.100% or less, Cr: 0.005% or more and 1.000% or less, and Mo: 0.005% or more and 0.500% or less; Group b: 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, One or more selected from Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

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

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

5. A method for producing a high strength hot rolled steel sheet according to claim 1 or 2, comprising: a heating step of heating a steel material having the above-mentioned composition to 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step under the conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of (RC1-100)°C or higher and (RC1+100)°C or lower to obtain a hot rolled steel sheet; and a first cooling step of cooling the hot rolled steel sheet after the hot rolling step under the conditions of a time from the end of the hot rolling to the start of cooling of RC2 seconds or less, an average cooling rate of FF°C / second or higher, and a cooling end temperature of 350°C or higher and (Bs+50)°C or lower, and coiling the sheet. A method for producing a high-strength hot-rolled steel sheet, comprising: a first holding step in which the hot-rolled steel sheet after the first cooling step is held under the conditions of a holding time of 1000 minutes or less in a temperature range of 350°C or more and (Bs+50)°C or less, and a holding time of 20 minutes or more and 500 minutes or less in a temperature range of 350°C or more and 450°C or less; a second holding step in which the hot-rolled steel sheet after the first holding step is held under the conditions of a holding time of 30 minutes or more and 500 minutes or less in a temperature range of 250°C or more and less than 350°C; and a second cooling step in which the hot-rolled steel sheet after the second holding step is cooled to 100°C or less. Here, RC1, RC2, FF, and Bs are defined by the following formulas, respectively. RC1 = 850 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V RC2 = 1.2 + C + N + 0.1 x Mn + 10 x Ti + 50 x B + 0.1 x Cr + 0.5 x Mo + 20 x Nb + 1.5 x V FF = 10^ (3.9 - 1.2 x (2.5 x C + Mn + 0.7 x Cr + 0.1 x Mo + 0.5 x Ni)) Bs = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo Each element symbol in the above formulas represents the content of each element in mass%, and the content of elements that are not contained is 0.

6. A method for producing a high strength hot rolled steel sheet according to claim 3, comprising: a heating step of heating a steel material having the above-mentioned composition to 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step under the conditions of a finish rolling start temperature of 1000°C or higher and 1250°C or lower, and a finish rolling end temperature of (RC1-100)°C or higher and (RC1+100)°C or lower to obtain a hot rolled steel sheet; and a first cooling step of cooling the hot rolled steel sheet after the hot rolling step under the conditions of a time from the end of the hot rolling to the start of cooling of RC2 seconds or less, an average cooling rate of FF°C / second or higher, and a cooling end temperature of 350°C or higher and (Bs+50)°C or lower, and coiling the sheet. A method for producing a high-strength hot-rolled steel sheet, comprising: a first holding step in which the hot-rolled steel sheet after the first cooling step is held under the conditions of a holding time of 1000 minutes or less in a temperature range of 350°C or more and (Bs+50)°C or less, and a holding time of 20 minutes or more and 500 minutes or less in a temperature range of 350°C or more and 450°C or less; a second holding step in which the hot-rolled steel sheet after the first holding step is held under the conditions of a holding time of 30 minutes or more and 500 minutes or less in a temperature range of 250°C or more and less than 350°C; a second cooling step in which the hot-rolled steel sheet after the second holding step is cooled to 100°C or less; and a plating step in which the hot-rolled steel sheet is plated. Here, RC1, RC2, FF, and Bs are defined by the following formulas, respectively. RC1 = 850 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V RC2 = 1.2 + C + N + 0.1 x Mn + 10 x Ti + 50 x B + 0.1 x Cr + 0.5 x Mo + 20 x Nb + 1.5 x V FF = 10^ (3.9 - 1.2 x (2.5 x C + Mn + 0.7 x Cr + 0.1 x Mo + 0.5 x Ni)) Bs = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo Each element symbol in the above formulas represents the content of each element in mass%, and the content of elements that are not contained is 0.

7. A method for manufacturing a component, comprising the 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 to form the component.

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