High-strength hot-rolled steel plate and member, and manufacturing method thereof

Abstract

We provide high-strength hot-rolled steel sheets that have a high 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. The high-strength hot-rolled steel sheet of the present invention has a predetermined chemical composition, and contains upper bainite, a sum of lower bainite and tempered martensite, fresh martensite, and retained austenite, each at a predetermined area ratio, and further has a steel structure having an average solute C amount 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: 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, and a total area ratio of regions of the hard phase (retained austenite and fresh martensite) having an equivalent circle diameter of 4.0 μm or less and an aspect ratio of 2.0 or more: 1.0% or more.

Description

[Technical field]

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

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

[0003] In particular, high-strength hot-rolled steel sheets with a tensile strength of 1,180 MPa or more are expected to be a material that can dramatically improve the fuel efficiency and electricity consumption of automobiles by reducing their weight.

[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%, of C: 0.01 to 0.20%, Si: 2.50% or less (not including 0), Mn: 4.00% or less (not including 0), P: 0.10% or less (not including 0), S: 0.03% or less (not including 0), Al: 0.001 to 2.00%, N: 0.01% or less (not including 0), O: 0.01% or less (not including 0), 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 contains one or both of tempered martensite and lower bainite in a volume fraction of 90% or more in total, and one or both of the tempered martensite and lower bainite has an iron-based carbide content 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: 0.15-0.30%, Si: 0.50-4.00%, Mn: 2.00-4.00%, P: 0.100% or less, S: 0.005% or less, Al: 0.010-0.500%, N: 0.010% or less, V: 0.20-1.00%, Nb: 0-0.10%, Ti: 0-0.10%, B: 0-0.0050%, Cr: 0-1.000%, Mo: 0-0.500%, Cu: 0-3.00%, and Ni: 0-1.50%, with a Si / V ratio of 10.0 or less, and with a low content of Cr and Mo. When one or two of the above elements 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, with the balance being Fe and impurities. The tempered martensite contains 90% or more in terms of area ratio. The cementite contained in the tempered martensite has a major axis of 400 nm or less and an aspect ratio of 3 to 5 of 70% or more. The tempered martensite contains 30 V-containing carbides having a circular 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 100%."

[0007] Patent Document 3 describes a steel sheet having a tensile strength of 1180 MPa or more, an arithmetic average surface roughness Ra of 2.00 μm or less, and containing, by 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 ratio 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 ratio 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. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6354268 [Patent Document 2] Patent No. 6835294 [Patent Document 3] Patent No. 6819840 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the practical use of hot-rolled steel sheets for automotive parts is currently limited to the 980 MPa class. Automobile parts, especially suspension parts, need to be made into complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets, which are the raw material for parts with such complex shapes, are required to have excellent formability.

[0010] On the other hand, increasing the tensile strength of steel sheets generally leads to a decrease in properties such as ductility and stretch flangeability. Therefore, when high-strength hot-rolled steel sheets, particularly those with a tensile strength of 1180 MPa or more, are formed, forming defects such as cracks tend to occur when strain is localized. To prevent such forming defects, steel sheets are required to have not only ductility and stretch flangeability but also high strain dispersion. In particular, when forming into complex shapes, 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 suspension parts in cold regions due to an impact load, the propagation of the brittle crack cannot be stopped. In other words, if low-temperature toughness is low, the part may break or separate brittlely in the event of a collision, 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 cannot be said to have high strength, such as a tensile strength TS of 1180 MPa or more, while also having excellent ductility and stretch flangeability, excellent strain dispersion ability over a wide strain range, and excellent low-temperature toughness.

[0013] As described above, in the conventional technology, there is no established technology for a hot-rolled steel sheet that has a high strength of tensile strength (TS) of 1180 MPa or more, excellent ductility and stretch flangeability, excellent strain distribution ability 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, as well as 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 having complex shapes such as suspension parts. U.El≧5.0%

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

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

[0018] By having excellent strain dispersion ability 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 dispersion ability in the low strain range means that the yield ratio (YR) = YS / TS, which is defined with respect to 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 dispersion ability in the low strain range and also excellent strain dispersion ability in the high strain range, it is possible to prevent cracks and wrinkles caused by severe processing. In the present invention, excellent strain dispersion ability in the high strain range means that, for the work hardening index (hereinafter also referred to as n value) measured in accordance with JIS Z 2253:2020, the n value measured in the strain range of 0.7×U.El to 0.9×U.El with respect to the 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 by a Charpy impact test in accordance with JIS Z 2242:2018 using a subsize test piece (V notch) satisfies the following formula. vTrs≦-40℃ [Means for solving the problem]

[0021] Now, the present inventors have conducted extensive research in order to achieve the above object. As a result, it has become possible to appropriately adjust the 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, an area ratio of crystal grains having an equivalent circle diameter of 8.0 μm or more: 10.0% or less, and an area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more: 10.0% or less. It has been discovered that by setting the total area ratio of crystal grains having a circular equivalent diameter of 4.0 μm or less and an aspect ratio of 4.0 or more to 10.0% or less, 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 circular 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 distribution ability over a wide strain range, as well as excellent low-temperature toughness, can be obtained.

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

[0023] That is, the gist and configuration of the present invention are as follows. [1] In mass percent, 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% to 2.000%; and N: Contains 0.0100% or less, Further, 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 is contained; The balance is Fe and unavoidable impurities. The steel structure is Area ratio of upper bainite: 30.0% or more and less than 90.0% Total area ratio of lower bainite and tempered martensite: 6.0% to 60.0% Area ratio of fresh martensite: 2.0% to 20.0% Area ratio of retained austenite: 2.0% or more and 10.0% or less, Average amount of solute C in retained austenite: 0.60 mass% or more and 0.90 mass% or less, The total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more: 10.0% or less, The total area ratio of crystal grains having an equivalent circle diameter of 8.0 μm or more and an aspect ratio of 4.0 or more: 10.0% or less, When the retained austenite and the fresh martensite are considered as hard phases, the total area ratio of the regions of the hard phase having a circle equivalent diameter of 4.0 μm or less and an aspect ratio of 2.0 or more is 1.0% or more; High-strength hot-rolled steel plate with a tensile strength of 1180MPa 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: V: 0.001% or more and 0.100% or less, 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% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less, one or more selected from [3] A high-strength hot-rolled steel sheet according to [1] or [2], having a plating layer on its surface. [4] A member made using the high strength hot rolled steel sheet according to any one of [1] to [3] above. [5] A method for producing a high strength hot rolled steel sheet according to the above [1] or [2], A heating step of heating a steel material having the above-mentioned composition to 1150°C or higher; A hot rolling process in which the steel material after the heating process is hot rolled under the conditions of a finish rolling start temperature of 1000°C or more and 1250°C or less and a finish rolling end temperature of (RC1-100)°C or more and (RC1+100)°C or less to obtain a hot rolled steel sheet; A first cooling process in which the hot-rolled steel sheet after the hot rolling process is cooled and coiled under the conditions of a time from the end of the hot rolling to the start of cooling: RC2 seconds or less, an average cooling rate: FF ° C. / second or more, and a cooling stop temperature: 350 ° C. or more (Bs + 50) ° C. or less; 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 in the temperature range of 350 ° C. or more and (Bs + 50) ° C. or less: 1000 minutes or less, and a holding time in the temperature range of 350 ° C. or more and 450 ° C. or less: 20 minutes or more and 500 minutes or less; A second holding step in which the hot-rolled steel sheet after the first holding step is held at a temperature range of 250 ° C. or more and less 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 less; A method for producing a high strength hot rolled steel sheet having the above structure. Here, RC1, RC2, FF, and Bs are defined by the following expressions, respectively. RC1=850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V RC2=1.2+C+N+0.1×Mn+10×Ti+50×B+0.1×Cr+0.5×Mo+20×Nb+1.5×V FF=10^(3.9-1.2×(2.5×C+Mn+0.7×Cr+0.1×Mo+0.5×Ni)) Bs=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo In the above formulas, each element symbol represents the content of each element in mass %, and the content of an element that is not contained is set to 0. [6] A method for producing a high strength hot rolled steel sheet according to the above [3], A heating step of heating a steel material having the above-mentioned composition to 1150°C or higher; A hot rolling process in which the steel material after the heating process is hot rolled under the conditions of a finish rolling start temperature of 1000°C or more and 1250°C or less and a finish rolling end temperature of (RC1-100)°C or more and (RC1+100)°C or less to obtain a hot rolled steel sheet; A first cooling process in which the hot-rolled steel sheet after the hot rolling process is cooled and coiled under the conditions of a time from the end of the hot rolling to the start of cooling: RC2 seconds or less, an average cooling rate: FF ° C. / second or more, and a cooling stop temperature: 350 ° C. or more (Bs + 50) ° C. or less; 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 in the temperature range of 350 ° C. or more and (Bs + 50) ° C. or less: 1000 minutes or less, and a holding time in the temperature range of 350 ° C. or more and 450 ° C. or less: 20 minutes or more and 500 minutes or less; A second holding step in which the hot-rolled steel sheet after the first holding step is held at a temperature range of 250 ° C. or more and less 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 less; A plating process for plating the hot-rolled steel sheet; A method for producing a high strength hot rolled steel sheet having the above structure. Here, RC1, RC2, FF, and Bs are defined by the following expressions, respectively. RC1=850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V RC2=1.2+C+N+0.1×Mn+10×Ti+50×B+0.1×Cr+0.5×Mo+20×Nb+1.5×V FF=10^(3.9-1.2×(2.5×C+Mn+0.7×Cr+0.1×Mo+0.5×Ni)) Bs=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo In the above formulas, each element symbol represents the content of each element in mass %, and the content of an element that is not contained is set to 0. [7] A method for producing a component, comprising a step of subjecting the high-strength hot-rolled steel sheet according to any one of [1] to [3] above to at least one of forming and joining to form a component. Effect of the Invention

[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 dispersion ability over a wide strain range, and excellent low-temperature toughness. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 plate First, the composition of the high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that the unit of the composition is always "mass%", but hereinafter, unless otherwise specified, it will be simply represented as "%".

[0027] C: 0.100% or more and 0.250% or less C is an element that has the effect of improving the strength of steel. C promotes the formation of bainite by improving hardenability, and contributes to high strength. C also contributes to high strength by increasing the strength of martensite. In order to obtain a tensile strength of 1180 MPa or more, the C content needs to be 0.100% or more. Therefore, the C content is 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, and the strength difference between the upper bainite as the main phase and the fresh martensite and the retained austenite increases, resulting in a decrease in uniform elongation and stretch flangeability. Therefore, the C content is 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, and is useful for suppressing the precipitation of cementite during upper bainite transformation and obtaining an appropriate amount of upper bainite. As a result, C is distributed to the untransformed austenite, and in the second cooling step described below, the untransformed austenite becomes fresh martensite and / or retained austenite, and the desired fresh martensite and retained austenite can be obtained. In order to obtain these effects, the Si content needs to 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, and even if descaling is performed in the hot rolling process, the surface roughness of the steel sheet surface becomes excessive, and the paint pretreatment property when painting is applied to the high-strength hot-rolled steel sheet is deteriorated. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less, 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 obtain such 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, when the Mn content is 3.50% or more, the area ratio of upper bainite decreases, the area ratio of lower bainite and tempered martensite increases, U.El decreases, and YR increases. Therefore, the Mn content is set to less than 3.50%. The Mn content is preferably 3.20% or less, more preferably 3.00% or less.

[0030] P:0.100% or less P is an element that dissolves and contributes to increasing the strength of steel. However, P is also an element that causes slab cracking during hot rolling by segregating to the austenite grain boundaries during hot rolling. It also segregates to the grain boundaries and reduces 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 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content is not particularly limited, but from the viewpoint of productivity, the P content is preferably 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. The lower limit of the S content is not particularly limited, but from the viewpoint of productivity, the S content is preferably 0.0001% or more.

[0032] Al: 0.010% or more and 2.000% or less Al acts as a deoxidizer and is an element effective in improving the cleanliness of steel. If the Al content is less than 0.010%, the effect is insufficient, so 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 suppresses the precipitation of cementite during upper bainite transformation. This contributes to the generation 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 of more than 2.000% leads to an increase in oxide-based inclusions, which 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, and generally contributes to grain refinement. However, N combines with Ti at high temperatures to form coarse nitrides, so a content of more than 0.0100% causes a decrease 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, and more preferably 0.0050% or less. The lower limit of the N content is not particularly limited, but 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% or more and 0.200% or less, and Nb: 0.005% or more and 0.200% or less.

[0035] Ti: 0.005% or more and 0.200% or less Ti is an element that has the effect of improving the strength of a steel sheet by precipitation strengthening or solid solution strengthening. Ti is an element that is also effective in improving stretch flangeability by improving the hardness of the relatively soft upper bainite and reducing the hardness difference between the upper bainite and the hard fresh martensite. Ti also increases the recrystallization temperature of austenite during hot rolling, making it possible to roll in the austenite unrecrystallized region, and contributing to refinement of the grain size of the BCC phase. These effects can be obtained 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, when the Ti content exceeds 0.200%, a large amount of Ti-based precipitates are generated, which rather reduces the 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 Nb, like Ti, is an element that has the effect of improving the strength of steel sheets by precipitation strengthening or solid solution strengthening. Nb is an element that is effective in improving stretch flangeability by improving the hardness of the relatively soft upper bainite and reducing the hardness difference between the upper bainite and the hard fresh martensite. In addition, like Ti, Nb increases the recrystallization temperature of austenite during hot rolling, making it possible to roll in the austenite unrecrystallized region, and contributing to refinement of the grain size of the BCC phase. Furthermore, Nb is a carbide-forming element, and segregates at the interface between the upper bainite and untransformed austenite during upper bainite transformation after coiling of the hot-rolled steel sheet. This reduces the transformation driving force of bainite, and is an element that has the effect of stopping 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 a desired area fraction. 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 further preferably 0.030% or more. On the other hand, when the Nb content exceeds 0.200%, a large amount of Nb-based precipitates are generated, which rather reduces the stretch flangeability. In addition, there is a risk that fresh martensite increases excessively, which reduces the uniform elongation and stretch flangeability. Therefore, when Nb is contained, the Nb content is set to 0.200% or less. The Nb content is preferably 0.150% or less, and 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] The high strength hot rolled steel sheet of the present invention may further contain, in addition to the above-mentioned components, one or more selected from the following groups a and b. (Group a) One or more 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 Bottom, 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 the following: 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 has the effect of improving the strength of the steel sheet by precipitation strengthening and solid solution strengthening. Similarly to Ti, V 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. Similarly to Cr, V 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 transformation driving force of bainite, and is an element that has the effect of stopping upper bainite transformation while leaving untransformed austenite. The 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 fresh martensite and retained austenite with a desired area fraction. In order to obtain these effects, it is preferable that the V content is 0.001% or more. Therefore, when V is contained, it is preferable that the V content is 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 decrease. 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, and during the upper bainite transformation after coiling of the hot-rolled steel sheet, it segregates at the interface between the upper bainite and untransformed austenite, reducing the driving force of the bainite transformation and causing the upper bainite transformation to halt. The untransformed austenite remaining due to the halt 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 fresh martensite and retained austenite with a desired area fraction. In order to obtain 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, since Cr is an element that deteriorates corrosion resistance and paint pretreatment properties, when Cr is contained, the Cr content is 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 the improvement of hardenability, and contributes to improving the strength of the steel sheet. In addition, Mo, like Cr, 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 transformation driving force of bainite, and contributes to the formation of fresh martensite and retained austenite in the second cooling process. In order to obtain 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%, fresh martensite is excessively formed, which deteriorates uniform elongation and stretch flangeability. Therefore, when Mo is contained, the Mo content is 0.500% or less. The Mo content is preferably 0.300% or less. Note that when the contents of V, Cr, and Mo are less than the lower limit, the components are considered to be included as unavoidable impurities.

[0042] B: 0.0100% or less B is an element that segregates at prior austenite grain boundaries and inhibits 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 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. Cu also promotes the formation of bainite through improved hardenability, thereby contributing to improved strength. In order to obtain such effects, when Cu is contained, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. However, when the Cu content exceeds 1.000%, it leads to a deterioration in the surface properties of the hot-rolled steel sheet, and deteriorates the fatigue properties of the hot-rolled steel sheet. Therefore, when Cu is contained, the Cu content is 1.000% or less. The Cu content is preferably 0.500% or less.

[0044] Ni: 1.000% or less Ni is an element that dissolves in solid solution and contributes to increasing the strength of steel. Ni also promotes the formation of bainite through improved hardenability, thereby contributing to improved strength. In order to obtain such effects, 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 is effective in suppressing denitrification, deboronization, etc., and suppressing a decrease in the strength of steel. When Sb is contained, the Sb content is preferably 0.002% or more in order to obtain the above effect. On the other hand, if the Sb content exceeds 0.200%, the toughness of the steel decreases, and slab cracks and hot rolling cracks 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 is effective in suppressing denitrification, deboronization, etc., and suppressing the decrease in strength of steel. When Sn is contained, the Sn content is preferably 0.005% or more in order to obtain the above effect. On the other hand, if the Sn content exceeds 0.200%, the toughness of the steel decreases, and slab cracks and hot rolling cracks 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 generate composite precipitates such as (Nb,Ta)(C,N). This inhibits the coarsening of precipitates and stabilizes precipitation strengthening. This improves TS. In order to obtain such effects, when Ta is contained, the Ta content is preferably 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. This may cause a decrease in stretch flangeability. Therefore, when Ta is contained, the Ta content is 0.100% or less. The Ta content is preferably 0.050% or less, 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. In order to obtain such an 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 further preferably 0.030% or more. On the other hand, when the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, which may lead to a decrease in ductility and work hardening ability in the high strain region. Therefore, when W is contained, the W content is 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, and contributes to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to obtain such effects, 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 cause sheared edge cracking or stretch flange cracking. Therefore, when Mg is contained, the Mg content is 0.0200% or less. The Mg content is preferably 0.0050% or less.

[0050] Zn: 0.0200% or less Zn makes the shape of inclusions spherical, which contributes to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to obtain such effects, when Zn is contained, the Zn content is preferably 0.0010% or more. However, when the Zn content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may 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 makes the shape of inclusions spherical, which contributes to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to obtain such effects, 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 cause sheared edge cracking or stretch flange cracking. Therefore, when Co is contained, the Co content is 0.0200% or less. The Co content is preferably 0.0050% or less.

[0052] Zr: 0.0200% or less Like Zn and Co, Zr makes the shape of inclusions spheroidal, which contributes to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to obtain such effects, 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 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 inclusions, and contributes to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to obtain such effects, when Ca is contained, the Ca content is preferably 0.0005% or more. However, when the Ca content exceeds 0.0200%, the Ca-based inclusions increase, the cleanliness of the steel deteriorates, and this may cause sheared edge cracking or stretch flange cracking. Therefore, when Ca is contained, the Ca content is 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) control the shape of oxides and sulfide-based inclusions, as well as Ca, and contribute to suppressing cracking at the sheared edge of the steel sheet and further improving stretch flangeability. In order to obtain 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, when the content of each of the above elements exceeds 0.0200%, the cleanliness of the steel deteriorates, which may cause sheared edge cracking or stretch flange cracking. Therefore, when the above elements are contained, the content of each of the above elements is 0.0200% or less. Note that REM is a general term for Sc, Y, and elements 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 ratio of upper bainite: 30.0% or more and less than 90.0% Upper bainite is a useful phase for securing an appropriate amount of fresh martensite and retained austenite by utilizing the diffusion of C from the upper bainite to the 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 ratio of lower bainite and tempered martensite: 6.0% to 60.0% The lower bainite and tempered martensite have intermediate hardness between the soft upper bainite and the hard fresh martensite, and are useful phases for ensuring high strength and excellent stretch flangeability. Therefore, the total area ratio of the lower bainite and tempered martensite is set to 6.0% or more. The total area ratio of the lower bainite and tempered martensite is preferably 8.0% or more, more preferably 10.0% or more. On the other hand, if the total area ratio of the lower bainite and tempered martensite exceeds 60.0%, the ductility decreases. Therefore, the total area ratio of the lower bainite and tempered martensite is set to 60.0% or less. The total area ratio of the lower bainite and tempered martensite is preferably 55.0% or less, more preferably 50.0% or less.

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

[0059] Area ratio of retained austenite: 2.0% to 10.0% From the viewpoint of obtaining good ductility and excellent work hardening ability in a high strain range, the area fraction of the retained austenite is set to 2.0% or more. The area fraction of the retained austenite is preferably 2.5% or more, more preferably 2.8% or more, and further preferably 3.0% or more. On the other hand, if the area fraction of the retained austenite exceeds 10.0%, the stretch flangeability decreases. Therefore, the area fraction of the retained austenite is set to 10.0% or less. The area fraction of the retained austenite is preferably 9.0% or less, and 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 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, for example, by observation with a SEM (Scanning Electron Microscope).

[0062] Upper bainite is an aggregate of lath-shaped ferrite with an orientation difference of less than 15°, and has Fe-based carbides and / or residual austenite at the lath-shaped ferrite interface (including cases where the lath-shaped ferrite interface does not have Fe-based carbides and / or residual austenite), and means a structure in which the lath-shaped ferrite does not have Fe-based carbides inside, or even if the lath-shaped ferrite has Fe-based carbides inside, the number of Fe-based carbides inside the lath-shaped ferrite is smaller than the number of Fe-based carbides at the lath-shaped ferrite interface. Upper bainite may be held at high temperatures after formation, causing the interface of the lath-shaped ferrite to disappear or become unclear, but it can be distinguished from polygonal ferrite and acicular ferrite, which have small crystal orientation differences within the crystal grains, by measuring the crystal orientation difference within the crystal grains using EBSD (Electron Backscatter Diffraction). Moreover, 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 TEM (Transmission Electron Microscope). When retained austenite is present between the laths, only the lath ferrite is regarded as upper bainite and is distinguished from the retained austenite.

[0063] Lower bainite and / or tempered martensite is an aggregate of lath-shaped ferrite with an orientation difference of less than 15°, and is a structure having Fe-based carbides inside the lath-shaped ferrite (however, this also includes cases where Fe-based carbides are present at the lath-shaped ferrite interfaces), and refers to a structure in which the number of Fe-based carbides present inside 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 crystal structure of Fe-based carbides in the lath ferrite using a TEM, but in the present invention, they have substantially the same properties and therefore are not distinguished from each other. Lower bainite and / or tempered martensite have brighter contrast when viewed under SEM than upper bainite and can be distinguished from them by the fact that they contain 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) do not contain Fe-based carbides, as compared with 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 when viewed with an SEM, making it difficult to distinguish between them. Therefore, the area ratios of fresh martensite and retained austenite are determined by the method described below.

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

[0066] 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. Next, the observation surface of the sample is mirror-polished using diamond paste. Next, the observation surface of the sample is 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 SEM (Scanning Electron Microscope) under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times.

[0068] The fresh martensite and the retained austenite constituting the hard phase have similar contrast in SEM and are difficult to distinguish from each other. Therefore, the area ratio of the retained austenite is obtained by X-ray diffraction, and the area ratio of the fresh martensite is obtained 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 the retained austenite is measured as follows. That is, the hot-rolled steel sheet is mechanically ground in the thickness direction (depth direction) to a position of 1 / 4 of the sheet thickness, and then the sheet thickness of 100 μm or more is chemically polished with oxalic acid to obtain the observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays. Then, the ratio of the diffraction intensity of each of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of the (200) and (211) planes of bcc iron is obtained, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each plane. Then, the retained austenite is considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

[0070] Average amount of dissolved C in retained austenite: 0.60% by mass or more and 0.90% by mass or less If the average amount of solute C in the retained austenite is less than 0.60% by mass, the retained austenite will transform in the early stage of processing, and the work hardening ability in the high strain region will decrease. That is, the n value will decrease. Therefore, the average amount of solute C in the retained austenite is set to 0.60% by mass or more. The average amount of solute C in the retained austenite is preferably 0.63% by mass or more, more preferably 0.65% by mass or more. On the other hand, if the average amount of solute C in the retained austenite exceeds 0.90% by mass, the retained austenite will not transform sufficiently in the early stage of processing, and the work hardening ability in the low strain region will decrease. That is, the YR will increase. Therefore, the average amount of solute C in the retained austenite is set to 0.90% by mass or less. The average amount of solute C in the retained austenite is preferably 0.87% by mass or less, more preferably 0.85% by mass or less.

[0071] The average amount of solute C in the retained austenite is measured as follows. That is, the surface on which the area ratio of the retained austenite was measured is observed by X-ray diffraction. CuKα rays are used as the incident X-rays, and the lattice constant of austenite (a γ ) was obtained, and the average amount of dissolved C in the retained austenite was calculated using the following formula. a γ (Å)=3.572+0.033×C-0.00157×Si+0.0012×Mn In the formula, Si and Mn represent the content (mass%) of each element in the steel, and C 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 with a circle equivalent 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 are reduced. That is, vTrs increases. Therefore, the total area ratio of crystal grains having a circle equivalent diameter of 15.0 μm or more is set to 10.0% or less. The total area ratio of crystal grains having a circle equivalent 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 a circle equivalent diameter of 15.0 μm or more, and the total area ratio of crystal grains having a circle equivalent diameter of 15.0 μm or more may be 0%.

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

[0074] The equivalent circle 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 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. Next, the observation surface of the sample is mirror-polished using diamond paste. Next, the observation surface of the sample is finish-polished 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 OIMAnalysis software manufactured by TSL, grain boundaries with a crystal orientation difference of 15° or more were visualized, the circle equivalent diameter of each crystal grain was obtained by image processing, and the total area ratio of crystal grains with a circle equivalent diameter of 15.0 μm or more was calculated.

[0077] Total area ratio of crystal grains with a circle equivalent diameter of 8.0 μm or more and an aspect ratio of 4.0 or more: 10.0% or less If the steel structure contains crystal grains having an equivalent circle diameter of 8.0 μm or more and an aspect ratio of 4.0 or more, they will become the starting point of void generation during processing, and the stretch flangeability will decrease. 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 sheet thickness of a high strength hot rolled steel sheet 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, the 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 obtained, 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 obtained.

[0080] When retained austenite and fresh martensite are considered as hard phases, the total area ratio of the regions of the hard phase having a circle equivalent 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 that the distribution of the amount of solute C in the retained austenite has an appropriate average and variance while ensuring a predetermined amount of area ratio of the retained austenite. 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, it was found that when the retained austenite and fresh martensite are hard phases, if the total area ratio (hereinafter, the area ratio is also referred to as "S3") of the hard phase regions having a circle equivalent diameter of 4.0 μm or less and an aspect ratio of 2.0 or more (island regions) is 1.0% or more, the variance of the distribution of the amount of solute C in the retained austenite is large. That is, if S3 is less than 1.0%, the work hardening ability in the high strain region decreases. Therefore, S3 is set to 1.0% or more. S3 is preferably set to 1.5% or more, more preferably 2.0% or more. It is difficult to make S3 10.0% or more, and since the variance of the distribution of the amount of dissolved C in the retained austenite does not become excessively large, it is preferably made less than 10.0%.

[0081] Here, the total area ratio of the regions of the hard phase (retained austenite and fresh martensite) 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 region identified in the measurement of the area ratio of the hard phase were obtained by image processing. Each region having a circle equivalent diameter of 4.0 μm or less and an aspect ratio of 2.0 or more was identified, and the total area ratio was obtained. In the present invention, the hard phase region includes a region composed of only retained austenite, a region composed of only fresh martensite, and a region composed of adjacent retained austenite and fresh martensite. As described above, the retained austenite and fresh martensite have similar shapes and contrasts in SEM and are difficult to distinguish, and in the present invention, both the retained austenite and fresh martensite have similar characteristics as hard phases. Therefore, a region composed of adjacent retained austenite and fresh martensite is 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): 1180MPa 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, and preferably less than 1470 MPa.

[0085] In addition, the uniform elongation (U.El), limiting hole expansion ratio (λ), yield ratio (YR), work hardening exponent (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 conforming to JIS Z 2241:2011, which will be described later in the examples. The limiting hole expansion ratio (λ) is measured by a hole expansion test conforming to JIS Z 2256:2020, which will be described later in the examples. The brittle-ductile fracture transition temperature (vTrs) is measured by a Charpy impact test conforming to 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] Components and their manufacturing methods Next, a member according to one embodiment of the present invention and a method for manufacturing the same will be described.

[0089] A member according to an embodiment of the present invention is a member made of the above-mentioned high-strength hot-rolled steel sheet (as a raw material). Also, as a manufacturing method for the member, for example, a manufacturing method can be mentioned 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] Here, the above-mentioned high-strength hot-rolled steel sheet has high strength, excellent ductility and stretch flangeability, excellent strain dispersion ability over a wide strain range, and also excellent low-temperature toughness. Therefore, the member according to one embodiment of the present invention is particularly suitable for application to members having complex shapes used in the automotive field.

[0091] [3] Manufacturing method for high-strength hot-rolled steel sheets Next, a method for producing 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-mentioned 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 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. the hot-rolled steel sheet after the first cooling step is held under 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 less than 450°C or less; the second holding step is held under conditions of a holding time of 30 minutes or more and less than 500 minutes in a temperature range of 250°C or more and less than 350°C or less; and the second cooling step is cooled to 100°C or less. Here, RC1, RC2, FF, and Bs are defined by the following expressions, respectively. RC1=850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V RC2=1.2+C+N+0.1×Mn+10×Ti+50×B+0.1×Cr+0.5×Mo+20×Nb+1.5×V FF=10^(3.9-1.2×(2.5×C+Mn+0.7×Cr+0.1×Mo+0.5×Ni)) Bs=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo In the above formulas, each element symbol represents the content of each element in mass %, and the content of an element that is not contained is set to 0. Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel material and the steel plate. The above average cooling rate refers to the average cooling rate of the steel plate surface. Unless otherwise specified, the average cooling rate is [(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-mentioned composition is prepared. The method for producing the steel material such as a slab is not particularly limited, and a commonly used method can be used. As a method for producing the steel material, for example, a method in which molten steel having the above-mentioned composition is melted using a known method in a converter or the like, and a slab is produced by a casting method such as a continuous casting method can be used. As a method for producing the steel material, a known casting method such as an ingot casting-blooming rolling method can also be used. Scrap can also be used as the raw material for the steel material.

[0094] [Heating process] Heating temperature of steel material: 1150℃ or higher In the steel material such as a slab after being cooled to a low temperature, most of the elements forming 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 lead to 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 directly subjected to hot rolling (direct rolling) after casting while still at a high temperature (i.e., while maintaining the temperature within the above heating temperature range).

[0095] [Hot rolling process] Next, the steel material heated to 1150°C or higher (including steel 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 can be secured.

[0096] A steel material is roughly 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 entry side of the finish rolling mill.

[0097] In order to remove the primary scale generated before the finish rolling, it is preferable to perform high-pressure water descaling on the rough rolled plate. The collision pressure of the 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 plate. The upper limit of the descaling collision pressure is not particularly limited, but is preferably 15.0 MPa or less, more preferably 14.5 MPa or less, and more preferably 12.0 MPa or less. In addition, high-pressure water descaling may be performed during rolling between the stands of the finish rolling. In addition, the rough rolled plate may be cooled between the stands of the finish rolling, if necessary.

[0098] Finish rolling start temperature: 1000℃ to 1250℃ The rough rolled sheet is subjected to finish rolling at a predetermined finish rolling start temperature and finish rolling end temperature to obtain a hot rolled steel sheet (finish rolled sheet). If the finish rolling start temperature is too low, recrystallization of austenite grains during finish rolling is difficult, and 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 increases. This leads to a decrease in stretch flangeability. For this reason, the finish rolling start temperature is set to 1000 ° C or more. The finish rolling start temperature is preferably 1020 ° C or more, and more preferably 1040 ° C or more. On the other hand, if the finish rolling start temperature is too high, the grain growth of austenite grains occurs significantly, the austenite grains become coarse, and the total area ratio of crystal grains having a circle equivalent diameter of 15.0 μm or more increases. This leads to an increase in vTrs. For this reason, the finish rolling start temperature is set to 1250 ° C or less.

[0099] Finish rolling end temperature: (RC1-100)℃ or higher (RC1+100)℃ or lower If the finish rolling end temperature is too low, the 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 obtained sufficiently, and a tensile strength of 1180 MPa or more cannot be secured. 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, the grain growth of austenite grains occurs significantly, the austenite grains become coarse, and the total area ratio of crystal grains with a circle equivalent diameter of 15.0 μm or more 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 lower, and more preferably (RC1+50) ° C or lower. 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) In the formula (1), each element symbol represents the content by mass % of each element in the above-mentioned composition, and the content of an element that is not contained is set to 0.

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

[0101] Time from the end of hot rolling to the start of cooling: RC within 2 seconds The time from the end of finish rolling to the start of forced cooling (cooling start time) is controlled. If the cooling start time is too long, grain growth of austenite grains occurs, and the total area ratio of crystal grains with a circle equivalent diameter of 15.0 μm or more increases. This increases vTrs. For this reason, 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. The lower limit of the cooling start time is not particularly limited, 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 by mass % of each element in the above-mentioned composition, and the content of an element that is not contained is set to 0.

[0102] Average cooling rate: FF℃ / sec or more In the forced cooling, if the average cooling rate from the finish rolling end temperature to the cooling stop temperature (hereinafter also referred to as the "average cooling rate of forced cooling") is too slow, ferrite transformation occurs before upper bainite transformation, and the desired area ratio of upper bainite phase cannot be obtained. In addition, pearlite and the like are generated, and the desired area ratio of fresh martensite and retained austenite cannot be obtained. This makes it difficult to achieve a TS of 1180 MPa or more. For this reason, the average cooling rate of forced cooling is set to FF ° C. / sec or more. The average cooling rate of forced cooling is preferably (FF + 5) ° C. / sec or more, and more preferably (FF + 10) ° C. / sec or more. On the other hand, the upper limit of the average cooling rate of forced cooling is not particularly limited, but if it is too fast, it may be difficult to control the cooling stop temperature and 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) In the formula (3), each element symbol represents the content by mass % of each element in the above-mentioned composition, and the content of an element that is not contained is set to 0.

[0103] Cooling stop temperature: 350℃ or more (Bs+50)℃ or less If the cooling stop temperature is too low, the area ratio of upper bainite and retained austenite decreases, and the area ratio of lower bainite and tempered martensite increases. As a result, U.El and 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 ratio of upper bainite and retained austenite decreases. As a result, n value decreases. In addition, pearlite and the like may be generated, making it difficult to achieve TS of 1180MPa or more. 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×C-90×Mn-70×Cr-37×Ni-83×Mo...(4) In the formula (4), each element symbol represents the content by mass % of each element in the above-mentioned composition, and the content of an element that is not contained is set to 0.

[0104] [First holding process] Next, the hot-rolled steel sheet after the first cooling step is held in a temperature range of 350°C or more and (Bs+50)°C or less. 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, in 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 the temperature is in the range of 350°C or more and (Bs+50)°C or less. Any means for increasing or decreasing the temperature may be used, and heating from the outside or extracting heat to the outside may be used, or heat generation or absorption inside the hot-rolled steel sheet may be utilized.

[0105] Holding time in the temperature range of 350°C or higher (Bs+50)°C or lower: 1000 minutes or less, and holding time in the temperature range of 350°C or higher and 450°C or lower: 20 minutes or more and 500 minutes or less If the holding time in the temperature range of 350°C or more and (Bs+50)°C or less exceeds 1000 minutes, carbides and pearlite are generated, and the area ratio of fresh martensite and retained austenite decreases. This makes it difficult to achieve a TS of 1180 MPa or more. For this reason, the holding time in the temperature range of 350°C or more and (Bs+50)°C or less is set to 1000 minutes or less. The holding time in the above temperature range is preferably 800 minutes or less, and more preferably 500 minutes or less. The lower limit of the holding time in the above temperature range is preferably 20 minutes or more, from the viewpoint of obtaining a predetermined amount of area ratio of upper bainite.

[0106] The first holding step is an important step for obtaining a predetermined area ratio of upper bainite, diffusing C from the upper bainite to the untransformed austenite, and appropriately diffusing C to the untransformed austenite in the second holding step described later, thereby making the distribution of the amount of dissolved C in the retained austenite appropriate. If the holding time in the temperature range of 350°C to 450°C is less than 20 minutes in the first holding step, the area ratio of upper bainite and the retained austenite decreases, the YR increases, and the uniform elongation decreases. For this reason, 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, more preferably 30 minutes or more. On the other hand, if the holding time in the temperature range of 350°C to 450°C 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 more. Therefore, the holding time in the temperature range of 350°C to 450°C is set to 500 minutes or less. 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 500 minutes or less.

[0107] [Second holding process] 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 holding step so long as the temperature is in the range of 250° C. or more and less than 350° C. Any means may be used for increasing or decreasing the temperature, and the temperature may be increased or decreased by heating from the outside or extracting heat to the outside, or by utilizing heat generation or absorption inside the hot-rolled steel sheet.

[0108] Holding time in the temperature range of 250℃ to 350℃: 30 minutes to 500 minutes The second holding step is an important step for obtaining a predetermined average amount of dissolved C in the retained austenite and a predetermined S3 by appropriately diffusing C into the untransformed austenite. If the holding time exceeds 500 minutes in the temperature range of 250°C or more and less than 350°C, excessive C concentration occurs in the untransformed austenite, and the average amount of dissolved C in the retained austenite increases. Therefore, the strain dispersion ability in the low strain region decreases, that is, YR increases. In addition, carbides may precipitate from the untransformed austenite, reducing the amount of dissolved austenite and reducing 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, S3 decreases, and the distribution of the amount of dissolved C in the retained austenite decreases, thereby reducing the 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 30 minutes or more, preferably 40 minutes or more, and more preferably 45 minutes or more.

[0109] [Second cooling process] 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. In addition, as described below, in the case where a plating process is carried out after the second holding step, the steel sheet after the plating process 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. In addition, pickling may be performed to remove scale. In addition, when the high strength hot rolled steel sheet of the present invention has a plating layer on the surface, the manufacturing method of the high strength hot rolled steel sheet further includes a plating process of plating the hot rolled steel sheet. In this case, although there is no particular limitation, for example, the hot rolled steel sheet after the second holding process or the second cooling process may be plated. The plating process in the plating process is not particularly limited, and examples thereof include known plating processes. EXAMPLES

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

[0113] [Manufacturing of high-strength hot-rolled steel sheets] Molten steel having the composition shown in Table 1 below (the balance being Fe and unavoidable impurities) was melted in a converter, and a steel material was produced by a continuous casting method. The produced steel material was subjected to a heating process at the heating temperature [°C] shown in Table 2 below. The steel material after the heating process was rough rolled to obtain a rough rolled plate. The surface of the obtained rough rolled plate was subjected to high-pressure water descaling at a collision pressure of 10.0 MPa. The rough rolled plate subjected to high-pressure water descaling was subjected to finish rolling at the finish rolling start temperature [°C] and finish rolling end temperature [°C] shown in Table 2 below to obtain a hot rolled steel plate. After completion of hot rolling (finish rolling), the obtained hot rolled steel plate was subjected to a first cooling process. The following Table 2 lists the conditions for the first cooling step, such as the cooling start time (the time from the end of hot rolling to the start of forced cooling) [sec], the average cooling rate (the average cooling rate from the finish rolling end temperature to the cooling stop temperature) [°C / sec], 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 more (Bs+50)°C or less for a holding time I [min] shown in Table 2 below, and at a temperature range of 350°C or more and 450°C or less for a holding time II [min] shown in Table 2 below. The holding time I shown in Table 2 is the holding time (min) in the temperature range of 350°C or more and (Bs+50)°C or less, and the holding time II is the holding time (min) in the temperature range of 350°C or more and 450°C or less. 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 more and less than 350°C under the holding time conditions shown in Table 2 below. The holding time shown in Table 2 indicates the holding time (min) in the temperature range of 250°C or more and less than 350°C. The hot-rolled steel sheet after the second holding step was subjected to a second cooling step in which the steel sheet was cooled to 100°C or less. In this manner, a high-strength hot-rolled steel sheet was obtained. The obtained high-strength hot-rolled steel sheet was subjected to temper rolling, and then pickled (hydrochloric acid concentration: 10 mass %, temperature: 85° C.) to remove scale.

[0115] [Table 1]

[0116] [Table 2]

[0117] [Evaluation of high-strength hot-rolled steel sheets] The steel structure of the obtained high-strength hot-rolled steel sheet was identified in the above-mentioned manner. The measurement results are shown in Table 3. In Table 3, UB is upper bainite, LB is lower bainite, TM is tempered martensite, FM is fresh martensite, γ is retained austenite, F is ferrite, P is pearlite, and θ is carbide (excluding carbides contained in upper bainite, lower bainite, and tempered martensite). In addition, in Table 3, S1 is the total area ratio of crystal grains having a circle equivalent diameter of 15.0 μm or more. S2 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. S3 is the total area ratio of the region of the hard phase having a circle equivalent diameter of 4.0 μm or less and an aspect ratio of 2.0 or more when the retained austenite and fresh martensite are hard phases.

[0118] [Table 3]

[0119] In addition, tensile tests, hole expansion tests and Charpy impact tests were conducted as follows to evaluate the tensile strength (TS), uniform elongation (U.El), limiting hole expansion ratio (λ), yield ratio (YR), work hardening index (n value) and brittle-ductile fracture transition temperature (vTrs).

[0120] (1) Tensile test The tensile test was performed in accordance with JIS Z 2241:2011. Specifically, JIS No. 5 test pieces 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 pieces, tensile tests were performed at a crosshead speed of 10 mm / min to measure YS, TS, U.El, T.El (total elongation), and n-value. Here, the n-value was measured with a strain range of 0.7×U.El to 0.9×U.El relative to the U.El. 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 acceptable, and anything other than that was considered to be unacceptable. A U.El of 5.0% or more (U.El ≧5.0%) was considered to be acceptable (having excellent ductility), and anything other than that was considered to be unacceptable. A YR of 0.93 or less (YR≦0.93) was deemed to be acceptable (having excellent strain dispersion ability in the low strain range), and anything other than that was deemed to be unacceptable. An n value of 0.080 or more (n value ≧ 0.080) was deemed to pass (having excellent strain dispersion ability in the high strain range), and anything else was deemed to fail. Materials that passed both YR and n value were evaluated as having excellent strain dispersion ability over a wide strain range.

[0121] (2) Hole expansion test The hole expansion test was performed in accordance with JIS Z 2256:2020. Specifically, a test piece of 100 mm × 100 mm was taken from the obtained high-strength hot-rolled steel sheet by shear processing. A hole of 10 mm in diameter (the hole of the initial test piece) was punched in the test piece with a clearance of 12% ± 1%. Next, a wrinkle holding force of 9 ton (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm, and in that state, a conical punch with an apex angle of 60 ° was pressed into the hole, and the diameter of the hole of the test piece at the crack occurrence limit (when cracks occurred) was measured. Then, the limit hole expansion ratio: λ (%) was calculated by the following formula. In addition, λ is an index for evaluating the stretch flangeability. The results are also shown in Table 4. A λ of 30% or more (λ≧30%) was judged to be acceptable (having excellent stretch flangeability), and any other value was judged to be unacceptable. λ(%)={(D f -D0) / D0}×100 Where: D f : diameter of the hole in the test piece when the crack occurred (mm) D0: Initial diameter of the hole in the test piece (mm) It is.

[0122] (3) Charpy impact test From the obtained high-strength hot-rolled steel sheet, a sub-size test piece (V-notch) with a thickness of 2.5 mm was taken 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 the toughness. Here, for hot-rolled steel sheets with a thickness of more than 2.5 mm, test pieces were prepared with a thickness of 2.5 mm by double-sided grinding, 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 shown in Table 4. A vTrs of -40°C or lower (vTrs≦-40°C) was deemed to be acceptable (having excellent low-temperature toughness), and anything other than that was deemed to be unacceptable.

[0123] As shown in Table 4, according to the present invention, a high-strength hot-rolled steel sheet can be obtained which has a 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.

[0124] [Table 4]

Claims

1. In mass percent, 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; Further, 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 is contained; The balance is Fe and unavoidable impurities, The steel structure is Area ratio of upper bainite: 30.0% or more and less than 90.0%; Total area ratio of lower bainite and tempered martensite: 6.0% or more and 60.0% or less, Area ratio of fresh martensite: 2.0% or more and 20.0% or less, Area ratio of retained austenite: 2.0% or more and 10.0% or less, Average amount of solute C in retained austenite: 0.60 mass% or more and 0.90 mass% or less, The total area ratio of crystal grains having an equivalent circle diameter of 15.0 μm or more: 10.0% or less, The total area ratio of crystal grains having an equivalent circle diameter of 8.0 μm or more and an aspect ratio of 4.0 or more: 10.0% or less, When the retained austenite and the fresh martensite are defined as hard phases, a 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 is 1.0% or more; A high-strength hot-rolled steel plate having a tensile strength of 1180 MPa or more.

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

3. The high strength hot rolled steel sheet according to claim 1, having a plating layer on a surface thereof.

4. A high-strength hot-rolled steel sheet as described in claim 2, having a plating layer on its surface.

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

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

7. A method for producing a high strength hot rolled steel sheet according to claim 3 or 4, A heating step of heating a steel material having the above-mentioned composition to 1150°C or higher; A hot rolling process in which the steel material after the heating process is hot rolled under the conditions of a finish rolling start temperature of 1000 ° C. or more and 1250 ° C. or less and a finish rolling end temperature of (RC1-100) ° C. or more and (RC1+100) ° C. or less to obtain a hot rolled steel sheet; A first cooling process in which the hot-rolled steel sheet after the hot rolling process is cooled and coiled under the conditions of a time from the end of the hot rolling to the start of cooling: RC 2 seconds or less, an average cooling rate: FF ° C. / second or more, and a cooling stop temperature: 350 ° C. or more (Bs + 50) ° C. or less; 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 in a temperature range of 350 ° C. or more and (Bs + 50) ° C. or less: 1000 minutes or less, and a holding time in a temperature range of 350 ° C. or more and 450 ° C. or less: 20 minutes or more and 500 minutes or less; A second holding step in which the hot-rolled steel sheet after the first holding step is held at a temperature range of 250 ° C. or more and less 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 less; A plating process for plating the hot-rolled steel sheet; A method for producing a high strength hot rolled steel sheet having the above structure. Here, RC1, RC2, FF, and Bs are defined by the following expressions, respectively. RC1=850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V RC2=1.2+C+N+0.1×Mn+10×Ti+50×B+0.1×Cr+0.5×Mo+20×Nb+1.5×V FF=10^(3.9-1.2×(2.5×C+Mn+0.7×Cr+0.1×Mo+0.5×Ni)) Bs=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo In the above formulas, each element symbol represents the content of each element in mass %, and the content of an element that is not contained is set to 0.

8. A method for manufacturing a component, comprising a step of subjecting the high strength hot rolled steel sheet according to any one of claims 1 to 4 to at least one of forming and joining to form a component.