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

Abstract

Provided is a high-strength hot-rolled steel sheet having excellent flatness in addition to high ductility and high stretch flangeability while maintaining high strength with a tensile strength of 1,180 MPa or greater.　The high-strength hot-rolled steel sheet has a prescribed component composition and a steel structure having an area ratio of tempered martensite being 50.0-99.0%, an area ratio of bainite being 0-49.0%, an area ratio of fresh martensite being 0-5.0%, an area ratio of retained austenite being 1.0-20.0%, the total area ratio of the remaining structure being 0-10.0%, and an average solid solution C amount in the retained austenite being 0.50-1.20%, with a steepness being 2.0% or less, a tensile strength being 1,180 MPa or greater, and high ductility and high stretch flangeability.

Description

High-strength hot-rolled steel sheets, components, and methods for manufacturing these.

[0001] This invention relates to high-strength hot-rolled steel sheets, components, and methods for manufacturing the same.

[0002] In recent years, improving the fuel efficiency and energy consumption of automobiles has become a crucial issue from the standpoint of protecting the global environment. This has led to a demand for even higher strength and thinner materials. Consequently, high-strength hot-rolled steel sheets are being actively applied as materials for automobile parts. The use of these high-strength hot-rolled steel sheets is being applied not only to structural and frame members of automobiles, but also to undercarriage members, truck frame members, and construction machinery members.

[0003] As mentioned above, the demand for high-strength hot-rolled steel sheets as a material for automotive parts is increasing year by year. In particular, high-strength hot-rolled steel sheets with a tensile strength (TS) of 1180 MPa or higher are highly anticipated as a material that can dramatically improve the fuel efficiency and electricity consumption of automobiles.

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

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

[0006] Patent Document 2 discloses "a high-strength hot-rolled steel sheet having a composition in mass percent of C: 0.02 to 0.23%, Si: 0.10 to 3.00%, Mn: 0.5 to 3.5%, P: 0.100% or less, S: 0.02% or less, Al: 1.5% or less, with the remainder being Fe and unavoidable impurities, a total area ratio of martensite and bainite of 80 to 100%, a maximum grain orientation density of less than 2.5 in the region 5 to 10 μm from the surface in the thickness direction, and a maximum grain orientation density of 2.5 or more in the region 50 to 100 μm from the surface in the thickness direction."

[0007] Patent Document 3 describes a hot-rolled flat steel product having the following composition (in weight percent): C: 0.1–0.3%, Mn: 1.5–3.0%, Si: 0.5–1.8%, Al: up to 1.5%, P: up to 0.1%, S: up to 0.03%, N: up to 0.008%, with the remainder being iron and unavoidable impurities related to production, provided that if the composition contains at least 1.0 wt% Si, the Al content is a maximum of 0.03 wt%, or if the composition contains 0.5 wt% to 1.0 wt% Si, the Al content is at least 0.5 wt%, wherein the flat steel product has a tensile strength Rm of 800–1500 MPa, a yield strength Rp of over 700 MPa, and an elongation at break A of 7–25%. A hot-rolled flat steel product is disclosed, which has a hole expansion λ of more than 20%, and whose structure consists of at least 85 area percent of martensite, with at least half of the martensite being tempered martensite, the remainder of the structure of the flat steel product consisting of up to 15 volume percent of retained austenite, up to 15 area percent of bainite, up to 15 area percent of polygonal ferrite, up to 5 area percent of cementite, and / or up to 5 area percent of non-polygonal ferrite, and whose structure has an average kernel average misorientation KAM of at least 1.50° in a measurement area of ​​at least 75 μm x 75 μm.

[0008] Patent No. 6465266 Patent No. 7207615 Patent No. 7193454

[0009] Incidentally, the practical application of hot-rolled steel sheets used as materials for automotive parts is currently limited to the 780 MPa class, with the application of the 980 MPa class being very limited. Automotive parts, especially suspension components and frame components, need to have complex shapes to ensure rigidity. Therefore, hot-rolled steel sheets used as materials for such complexly shaped components require excellent formability.

[0010] On the other hand, increasing the tensile strength of steel sheets generally reduces their formability. Steel sheets used as materials for components with complex shapes require not only high ductility but also high elongation flangeability, and it is important to ensure these properties while maintaining high strength.

[0011] Furthermore, high-strength steel sheets used in automotive structural members, frame members, undercarriage members, truck frame members, and construction machinery members require excellent flatness. To increase the strength of hot-rolled steel sheets, it is necessary to create a microstructure in which low-temperature transformation phases such as bainite and martensite are the main phases. To obtain such a microstructure, hot rolling must be performed in the austenite temperature range, followed by cooling to a low temperature, and the transformation from austenite to bainite or martensite must occur at low temperatures. However, when cooling to the low temperature at which transformation to bainite or martensite occurs, temperature unevenness is likely to occur within the steel sheet surface during the cooling process, and the flatness of the steel sheet after cooling deteriorates. In addition, bainite and martensitic transformations, which are shear-type transformations, involve large expansion during the transformation, which also deteriorates the flatness of the steel sheet.

[0012] In fact, the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to possess excellent flatness in addition to high ductility and high elongation flangeability, while maintaining high strength with a tensile strength (TS) of 1180 MPa or more.

[0013] As described above, conventional technology has not yet established a method for producing hot-rolled steel sheets that maintain high strength (tensile strength of 1180 MPa or more), high ductility and high elongation flangeability, in addition to excellent flatness.

[0014] This invention has been made in view of the above circumstances, and aims to provide a high-strength hot-rolled steel sheet that maintains high strength with a tensile strength of 1180 MPa or more, while also having high ductility and high elongation flangeability, as well as excellent flatness.

[0015] Furthermore, the present invention aims to provide a method for manufacturing the above-mentioned high-strength hot-rolled steel sheet, a component made using the above-mentioned high-strength hot-rolled steel sheet, and a method for manufacturing the same.

[0016] In this invention, high ductility means that the uniform elongation (total elongation at maximum test force; hereinafter also referred to as U.El) measured in a tensile test in accordance with JIS Z 2241:2011 satisfies the following formula. In particular, when forming automobile parts, high ductility is required for the steel sheet material. If 1180 MPa ≤ TS < 1310 MPa, U.El ≥ 4.5% If 1310 MPa ≤ TS < 1470 MPa, U.El ≥ 4.0% If 1470 MPa ≤ TS, U.El ≥ 3.5%

[0017] In this invention, high stretch flange properties mean that the average limiting hole expansion ratio (λ), measured in a hole expansion test in accordance with JIS Z 2256:2010 using three test pieces, satisfies the following equation. In particular, since the undercarriage members undergo stretch flange forming after punching, high stretch flange properties are required for the steel plates that make up the undercarriage members. When 1180 MPa ≤ TS < 1310 MPa, λ ≥ 40% When 1310 MPa ≤ TS < 1470 MPa, λ ≥ 30% When 1470 MPa ≤ TS, λ ≥ 20%

[0018] In this invention, excellent flatness means that the steepness is 2.0% or less. The steepness is determined by the following flatness test. A steel plate extruded from a hot-rolled coil is cut to a length of 2000 mm and placed on a surface plate. If the steel plate has a periodically wavy shape within its 2000 mm length, the steepness (%) is defined as the value obtained by dividing the wave height h (mm) by the wave pitch P (mm) and multiplying by 100 [(h / P) × 100]. The wave height h is the length of the largest gap between the surface plate and the steel plate, and the wave pitch P is the length of the shortest wave pitch on the steel plate. If the wave pitch is 2000 mm or more, P is set to 2000 mm. In particular, high-strength steel plates used in structural members, frame members, undercarriage members of automobiles, truck frame members, and construction machinery members may be subjected to the following processing. Specifically, the steel sheet discharged from the hot-rolled coil is first subjected to processing such as shearing using a shearing machine or blanking using a press die. Then, the sheet is positioned for the next processing step, following the first step, by aligning it with a guide. If the flatness of the steel sheet is poor, it may not fit the shape of the guide, making accurate positioning difficult and potentially preventing the formation of the component. Furthermore, even if the component can be formed, shape defects may occur due to the poor flatness of the steel sheet. Therefore, excellent flatness is required for the high-strength steel sheet used as the material for the component. Note that each of the above tests can be performed in more detail using the methods described in the examples below.

[0019] The inventors then diligently conducted research to achieve the above objectives. As a result, they discovered that by appropriately adjusting the component composition of the hot-rolled steel sheet, and by setting the steel structure of the hot-rolled steel sheet to have the following proportions: tempered martensite area ratio: 50.0% to 99.0%, bainite area ratio: 0% to 49.0%, fresh martensite area ratio: 0% to 5.0%, retained austenite area ratio: 1.0% to 20.0%, total area ratio of the remaining structure: 0% to 10.0%, average solid solution carbon content in retained austenite: 0.50% to 1.20%, a steepness of 2.0% or less, and a tensile strength of 1180 MPa or more, a high-strength hot-rolled steel sheet can be obtained that maintains high strength while possessing high ductility, high elongation flangeability, and excellent flatness.

[0020] This invention was completed based on the above findings and further investigations. Specifically, the gist of this invention is as follows: [1] The composition of the steel is such that, in mass%, it contains C: 0.100% or more and 0.500%, Si: 0.20% or more and 2.50%, Mn: 1.00% or more and 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure has the following area ratios: tempered martensite area ratio: 50.0% or more and 99.0%, bainite area ratio: 0% or more and 49.0%, fresh martensite area ratio: 0% or more and 5.0%, retained austenite area ratio: 1.0% or more and 20.0%, and the total area ratio of the remaining structure is 0% or more and 10.0%. A high-strength hot-rolled steel sheet having an average solid solution carbon content of 0.50% to 1.20% in retained austenite, a slope of 2.0% or less, a tensile strength of 1180 MPa or more, and high ductility and high elongation flangeability. [2] The above component composition is further defined in mass% as follows: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.400% or less, Cr: 1.00% or less, Mo: 0.500% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less [1] A high-strength hot-rolled steel sheet according to [1], comprising one or more selected from Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. [3] A high-strength hot-rolled steel sheet according to [1] or [2], having a plating layer on its surface. [4] A component made using the high-strength hot-rolled steel sheet according to any one of [1] to [3].[5] A method for manufacturing a high-strength hot-rolled steel sheet according to [1] or [2] above, comprising a heating step of heating a steel material to 1100°C or higher, and a finishing rolling completion temperature of the steel material after the heating step: A. ３ A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a hot-rolling step of performing finish rolling at a temperature of ℃ or higher and 1000℃ or lower to produce a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to Ms℃ at an average cooling rate of 20℃ / s or higher; a second cooling step of cooling the hot-rolled steel sheet after the first cooling step in a temperature range from Ms℃ to 150℃ at an average cooling rate of less than 20℃ / s; a reheating step of reheating the hot-rolled steel sheet after the second cooling step to a temperature range of 300℃ or higher and 500℃ or lower; and a shape correction step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 200℃ or higher and 500℃ or lower with an elongation rate of 0.10% or higher. ３ A and Ms are calculated using the following formulas: ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo Ms (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo Here, each element symbol in the above formula indicates the content (mass%) of each element, and if it is not contained, it is set to 0. [6] A method for manufacturing a high-strength hot-rolled steel sheet as described in [3] above, comprising a heating step of heating the steel material to 1100°C or higher, and finishing the steel material after the heating step to a finishing rolling temperature: A ３ A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a hot-rolling step of performing finish rolling at a temperature of ℃ or higher and 1000℃ or lower to produce a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to Ms℃ at an average cooling rate of 20℃ / s or higher; a second cooling step of cooling the hot-rolled steel sheet after the first cooling step in a temperature range from Ms℃ to 150℃ at an average cooling rate of less than 20℃ / s; a reheating step of reheating the hot-rolled steel sheet after the second cooling step to a temperature range of 300℃ or higher and 500℃ or lower; a shape-correcting step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 200℃ or higher and 500℃ or lower with an elongation rate of 0.10% or higher; and a plating step of applying a plating treatment to the hot-rolled steel sheet after the shape-correcting step. Here, A ３A and Ms are calculated using the following formulas: ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo Ms (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo Here, each element symbol in the above formulas indicates the content (mass%) of each element, and if it is not contained, it is set to 0. [7] A method for manufacturing a member, comprising the step of forming and joining a high-strength hot-rolled steel sheet according to any one of [1] to [3] above to make a member.

[0021] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet that maintains high strength with a tensile strength of 1180 MPa or more, while also having high ductility, high elongation flangeability, and excellent flatness.

[0022] The following describes embodiments of the high-strength hot-rolled steel sheet and its manufacturing method according to the present invention. However, the present invention is not limited to the following embodiments.

[0023] [1] High-strength hot-rolled steel sheet First, the component composition of the high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that all units in the component composition are "mass%", but unless otherwise specified, they will be simply referred to as "%".

[0024] C: 0.100% or more and 0.500% or less. Carbon (C) is an element that improves the strength of steel. By improving hardenability, carbon promotes the formation of martensite, contributing to increased strength. Furthermore, carbon also contributes to increased strength by increasing the strength of martensite. In order to obtain a tensile strength of 1180 MPa or more, the carbon content must be 0.100% or more. Therefore, the carbon content is set to 0.100% or more. Preferably, the carbon content is 0.120% or more, and more preferably 0.150% or more. On the other hand, if the carbon content exceeds 0.500%, the tensile strength increases excessively, and the ductility or elongation flangeability decreases. Therefore, the carbon content is set to 0.500% or less. Preferably, the carbon content is 0.400% or less, and more preferably 0.350% or less.

[0025] Si: 0.20% to 2.50% Si has the effect of suppressing cementite formation and inhibits cementite precipitation in the second cooling process, reheating process and shape correction process. As a result, C is distributed to the untransformed austenite, and a portion of the untransformed austenite becomes retained austenite, contributing to improved ductility. In addition, Si improves the strength-ductility balance between bainite and tempered martensite. In order to obtain these effects, the Si content must be 0.20% or more. Therefore, the Si content is set to 0.20% or more. Preferably, the Si content is 0.40% or more, more preferably 0.60% or more, and even more preferably 0.80% or more. On the other hand, Si is an element that forms subscale on the surface of the steel sheet during hot rolling. If the Si content exceeds 2.50%, the subscale becomes too thick, and even if descaling is performed in the hot rolling process, the surface roughness of the steel sheet becomes excessive, and the pretreatment properties for painting high-strength hot-rolled steel sheets deteriorate. Therefore, the Si content should be 2.50% or less. Preferably, the Si content is 2.00% or less, and more preferably 1.60% or less.

[0026] Mn: 1.00% to 5.00% Mn stabilizes austenite, suppresses ferrite formation, and contributes to the formation of bainite, tempered martensite, and retained austenite. Furthermore, by stabilizing austenite, Mn suppresses the transformation of untransformed austenite into bainite, carbides, and pearlite during the reheating and shape correction processes, and contributes to keeping the average solid-solution carbon content in the retained austenite within a desired range. To obtain such effects, the Mn content must be 1.00% or more. Therefore, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.40% or more, more preferably 1.70% or more. On the other hand, if the Mn content exceeds 5.00%, bainite transformation is more likely to occur, retained austenite increases, and elongation flange properties decrease. Therefore, the Mn content is set to 5.00% or less. The Mn content is preferably 4.00% or less, more preferably 3.50% or less, and even more preferably 3.20% or less.

[0027] P: 0.100% or less. P is an element that contributes to increasing the strength of steel by solid solution. However, P is also an element that causes slab cracking during hot rolling by segregating at the austenite grain boundaries during hot rolling. Furthermore, segregation at grain boundaries reduces ductility and elongation flangeability. For this reason, it is preferable to keep the P content as low as possible, but a P content of up to 0.100% is acceptable. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.060% or less, and more preferably 0.030% or less. There is no particular lower limit to the P content, but from the viewpoint of productivity, etc., it is preferable that the P content be 0.001% or more.

[0028] S: 0.0200% or less. S combines with Ti and Mn to form coarse sulfides, which accelerate void formation and reduce ductility and elongation flangeability. Therefore, it is preferable to keep the S content as low as possible, but an S content of 0.0200% or less is acceptable. Accordingly, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. There is no particular lower limit to the S content, but from the viewpoint of productivity, etc., it is preferable that the S content be 0.0001% or more.

[0029] Al: 0.010% to 2.000% Al acts as a deoxidizing agent and is an effective element for improving the cleanliness of steel. Since the effect is not sufficient if the Al content is less than 0.010%, the Al content should be 0.010% or more. In addition, Al, like Si, retains austenite and contributes to improved ductility. On the other hand, excessive Al content leads to an increase in oxide inclusions, which reduces ductility and elongation flangeability. Therefore, the Al content should be 2.000% or less. The Al content is preferably 1.000% or less, more preferably 0.200% or less, and even more preferably 0.100% or less.

[0030] N: 0.0200% or less. N combines with nitride-forming elements and precipitates as nitrides, generally contributing to grain refinement. However, since N combines with Ti at high temperatures to form coarse nitrides, a content exceeding 0.0200% causes a decrease in ductility and elongation flanging properties. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less. Although the lower limit of the N content is not particularly limited, from the perspective of productivity and the like, the N content is preferably 0.0005% or more.

[0031] The balance can be Fe and inevitable impurities. Examples of inevitable impurities include H, O, etc.

[0032] As described above, the basic component composition of the high-strength hot-rolled steel sheet according to one embodiment of the present invention has been explained. The high-strength hot-rolled steel sheet according to one embodiment of the present invention can optionally contain the following components in addition to the above component composition.

[0033] Ti: 0.200% or less. Ti is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. In addition, Ti raises the recrystallization temperature of austenite during hot rolling, enabling rolling in the non-recrystallized austenite region and contributing to the improvement of the strength-ductility balance due to the refinement of the crystal grain size of bainite and tempered martensite. To obtain such an effect, when Ti is contained, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.020% or more, still more preferably 0.040% or more. On the other hand, when the Ti content exceeds 0.200%, a large amount of Ti-based precipitates are generated, and conversely, the elongation flanging property is decreased. Therefore, when Ti is contained, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, more preferably 0.120% or less, still more preferably 0.070% or less.

[0034] Nb: 0.200% or less Nb, like Ti, is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. Also, like Ti, Nb increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite-preserved region and contributing to an improvement in the strength-ductility balance by refining the grain size of bainite and tempered martensite. To obtain such effects, if Nb is included, it is preferable that the Nb content be 0.005% or more. The Nb content is more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of Nb-based precipitates are generated, which actually reduces the elongation flangeability. Therefore, if Nb is included, the Nb content should be 0.200% or less. The Nb content is preferably 0.150% or less, more preferably 0.120% or less, and even more preferably 0.060% or less.

[0035] V: 0.400% or less V, like Ti, is an element that improves the strength of steel sheets through precipitation strengthening and solid solution strengthening. Also, like Ti, V increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite-unrecrystallized region and contributing to an improvement in the strength-ductility balance by refining the grain size of bainite and tempered martensite. To obtain such effects, if V is included, the V content is preferably 0.010% or more. The V content is more preferably 0.020% or more, and even more preferably 0.040% or more. On the other hand, if the V content exceeds 0.400%, a large amount of V-based precipitates are generated, which actually reduces the elongation flangeability. Therefore, if V is included, the V content should be 0.400% or less. The V content is preferably 0.200% or less, and more preferably 0.100% or less.

[0036] Cr: 1.00% or less Cr, like Mn, suppresses the formation of ferrite and contributes to the formation of bainite and martensite. In order to obtain such an effect, when Cr is contained, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.10% or more, and still more preferably 0.20% or more. However, since Cr is an element that deteriorates corrosion resistance and pre-painting treatment properties, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less, and more preferably 0.70% or less.

[0037] Mo: 0.500% or less Mo increases the tempering softening resistance of steel and contributes to the improvement of the strength of the steel sheet. Also, Mo suppresses the transformation of untransformed austenite to pearlite in the reheating process and the shape correction process, and contributes to obtaining an increase in the area ratio of retained austenite and an appropriate average amount of solid solution C in the retained austenite. In order to obtain such an effect, when Mo is contained, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.050% or more, and still more preferably 0.100% or more. On the other hand, when the Mo content exceeds 0.500%, the retained austenite excessively increases and the stretch flange property deteriorates. Therefore, when Mo is contained, the Mo content is set to 0.500% or less. The Mo content is preferably 0.250% or less.

[0038] B: 0.0100% or less B is an element that segregates at the prior austenite grain boundaries and suppresses the formation of ferrite, thereby contributing to the formation of bainite and tempered martensite. In order to obtain such an effect, when B is contained, the B content is preferably 0.0005% or more. On the other hand, when the B content exceeds 0.0100%, the above-mentioned effect saturates. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

[0039] Cu: 1.00% or less. Cu is an element that improves the strength of steel sheets through precipitation strengthening and solid solution strengthening. To obtain such effects, when Cu is included, it is preferable that the Cu content be 0.005% or more. More preferably, the Cu content is 0.10% or more, and even more preferably 0.20% or more. However, if the Cu content exceeds 1.00%, it leads to a decrease in the surface properties of the hot-rolled steel sheet. Therefore, when Cu is included, the Cu content should be 1.00% or less. More preferably, the Cu content is 0.70% or less, and more preferably 0.50% or less.

[0040] Ni: 1.00% or less. Ni is an element that contributes to increasing the strength of steel by solid solution. Ni also contributes to strength improvement by promoting the formation of bainite and tempered martensite through improved hardenability. To obtain such effects, if Ni is included, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.10% or more, and even more preferably 0.14% or more. However, if the Ni content exceeds 1.00%, retained austenite increases excessively, degrading the elongation flangeability of the hot-rolled steel sheet. Therefore, if Ni is included, the Ni content should be 1.00% or less. The Ni content is preferably 0.70% or less, more preferably 0.50% or less, and even more preferably 0.30% or less.

[0041] Sb: 0.200% or less. Sb is an element that is effective in suppressing the decrease in steel strength by inhibiting denitrification, deboration, etc. When Sb is included, it is preferable to have an Sb content of 0.005% or more to obtain the above effect. On the other hand, if the Sb content exceeds 0.200%, the toughness of the steel decreases, which may cause slab cracking and hot rolling cracking. Therefore, when Sb is included, the Sb content should be 0.200% or less. The Sb content is preferably 0.030% or less.

[0042] Sn: 0.200% or less. Like Sb, Sn is an effective element in suppressing the reduction in steel strength by inhibiting denitrification, deboration, etc. When Sn is included, it is preferable to have a Sn content of 0.005% or more to obtain the above effect. On the other hand, if the Sn content exceeds 0.200%, the toughness of the steel will decrease, which may cause slab cracking and hot rolling cracking. Therefore, when Sn is included, the Sn content should be 0.200% or less.

[0043] Ta: 0.100% or less. Ta increases the strength of steel by forming fine carbides, nitrides, or carbonitrides. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides, generating composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This improves the strength of the steel. To obtain such effects, when Ta is included, it is preferable to have a Ta content of 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. This may reduce the elongation flangeability. Therefore, when Ta is included, the Ta content should be 0.100% or less. The Ta content is preferably 0.020% or less.

[0044] W: 0.500% or less. W is an element that increases the strength of steel sheets by forming fine carbides, nitrides, or carbonitrides. To obtain such an effect, it is preferable that the W content be 0.001% or more when W is included. More preferably, the W content is 0.030% or more. On the other hand, if the W content exceeds 0.500%, a large amount of coarse precipitates and inclusions may be generated, which may lead to a decrease in elongation flange properties. Therefore, when W is included, the W content should be 0.500% or less. The W content is preferably 0.300% or less, more preferably 0.200% or less, and even more preferably 0.150% or less.

[0045] Mg: 0.0200% or less. Mg controls the shape of oxide and sulfide inclusions, contributing to further improvement of stretch flange properties. To obtain such effects, it is preferable that the Mg content be 0.0010% or more when Mg is included. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause a decrease in stretch flange properties. Therefore, when Mg is included, the Mg content should be 0.0200% or less. The Mg content is preferably 0.0150% or less.

[0046] Zn: 0.0200% or less. Zn contributes to further improvement of stretch flange properties by spheroidizing the shape of inclusions. To obtain this effect, it is preferable that the Zn content be 0.0010% or more when Zn is included. However, if the Zn content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause a decrease in stretch flange properties. Therefore, when Zn is included, the Zn content should be 0.0200% or less.

[0047] Co: 0.0200% or less. Like Zn, Co contributes to further improvement of ductility by spheroidizing the shape of inclusions. To obtain this effect, it is preferable that the Co content be 0.0010% or more when Co is included. However, if the Co content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause a decrease in ductility. Therefore, when Co is included, the Co content should be 0.0200% or less.

[0048] Zr: 0.0200% or less. Like Zn and Co, Zr contributes to further improvement of stretch flange properties by spheroidizing the shape of inclusions. To obtain such an effect, it is preferable that the Zr content be 0.0010% or more when Zr is included. However, if the Zr content exceeds 0.0200%, the cleanliness of the steel deteriorates, which may actually cause a decrease in stretch flange properties. Therefore, when Zr is included, the Zr content should be 0.0200% or less.

[0049] Ca: 0.0200% or less. Ca controls the shape of oxide and sulfide-based inclusions, contributing to further improvement of stretch flange properties. To obtain such effects, it is preferable that the Ca content be 0.0010% or more when Ca is included. However, if the Ca content exceeds 0.0200%, the number of Ca-based inclusions increases, worsening the cleanliness of the steel and potentially causing a decrease in stretch flange properties. Therefore, when Ca is included, the Ca content should be 0.0200% or less.

[0050] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM (rare earth metals), like Ca, control the shape of oxide and sulfide inclusions and contribute to further improvement of stretch flange properties. To obtain such effects, when each of the above elements is included, it is preferable that the content of each element be 0.0010% or more. However, if the content of each of the above elements exceeds 0.0200%, the cleanliness of the steel deteriorates, which can actually reduce its ductility and flange properties. Therefore, when the above elements are included, the content of each element should be 0.0200% or less. Note that REM is a collective term for 17 elements in total, including Sc, Y, and lanthanide elements. REM may contain one or more of these 17 elements. Also, the REM content referred to here is the total content of these elements.

[0051] Next, the microstructure of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. The microstructure of the high-strength steel sheet of the present invention is as follows: tempered martensite area ratio: 50.0% or more and 99.0% or less, bainite area ratio: 0% or more and 49.0% or less, fresh martensite area ratio: 0% or more and 5.0% or less, retained austenite area ratio: 1.0% or more and 20.0% or less, total area ratio of the remaining microstructure: 0% or more and 10.0% or less, average solid-solution carbon content in retained austenite: 0.50% or more and 1.20% or less.

[0052] Area ratio of tempered martensite: 50.0% or more and 99.0% or less Tempered martensite is a useful phase from the viewpoint of obtaining high ductility and high elongation flanging while maintaining high strength of 1180 MPa or more tensile strength. For this reason, the area ratio of tempered martensite is set to 50.0% or more. If the area ratio of tempered martensite exceeds 99.0%, it becomes impossible to set the area ratio of retained austenite to 1.0% or more. For this reason, the area ratio of tempered martensite is set to 99.0% or less. The lower limit of the area ratio of tempered martensite is preferably 60.0% or more, more preferably 70.0% or more, and even more preferably 85.0% or more. Furthermore, the area ratio of tempered martensite is preferably 98.0% or less, more preferably 95.0% or less, and even more preferably 90.0% or less.

[0053] Area ratio of bainite: 0% or more and 49.0% or less. Bainite, like tempered martensite, is a useful phase from the viewpoint of obtaining high ductility and high elongation flangeability while maintaining high strength with a tensile strength of 1180 MPa or more. However, if the area ratio of bainite exceeds 49.0%, it becomes impossible to make the area ratio of retained austenite 1.0% or more. For this reason, the area ratio of bainite should be 49.0% or less. Preferably it is 45.0% or less, more preferably 30.0% or less, and even more preferably 15.0% or less. There is no particular lower limit to the area ratio of bainite, and the area ratio of bainite may be 0%.

[0054] Area ratio of fresh martensite: 0% or more and 5.0% or less. Fresh martensite is hard and becomes a void formation site during molding, thus reducing stretch flangeability. Therefore, the area ratio of fresh martensite should be 5.0% or less. Preferably, the area ratio of fresh martensite should be 4.0% or less, more preferably 3.0% or less, and even more preferably 2.0% or less. There is no particular lower limit to the area ratio of fresh martensite, and the area ratio of fresh martensite may be 0%.

[0055] Area ratio of retained austenite: 1.0% or more and 20.0% or less. Retained austenite contributes to high ductility. Therefore, the area ratio of retained austenite should be 1.0% or more. Preferably, the area ratio of retained austenite is 2.0% or more, more preferably 3.0% or more, and even more preferably 4.0% or more. On the other hand, if the area ratio of retained austenite exceeds 20.0%, the stretch flange properties decrease. Therefore, the area ratio of retained austenite should be 20.0% or less. Preferably, the area ratio of retained austenite is 15.0% or less, more preferably 10.0% or less, and even more preferably 7.0% or less.

[0056] The area ratio of the remaining microstructure is 0% or more and 10.0% or less. The area ratio of the remaining microstructure other than bainite, tempered martensite, fresh martensite, and retained austenite is 10.0% or less from the viewpoint of ensuring high strength, high ductility, and high elongation flangeability. The area ratio of the remaining microstructure is more preferably 5.0% or less. The area ratio of the remaining microstructure may also be 0%.

[0057] The remaining tissue is not particularly limited and may include one or more types, such as polygonal ferrite, acicular ferrite, and pearlite. The type of tissue can be confirmed, for example, by observation using a scanning electron microscope (SEM).

[0058] Bainite and tempered martensite are aggregates of lath-like ferrite with an orientation difference of less than 15°, and have a structure containing Fe-based carbides and / or retained austenite phases at and / or within the interfaces of the lath-like ferrite (however, this also includes cases where Fe-based carbides and / or retained austenite phases are not present at and / or within the interfaces of the lath-like ferrite). When bainite and tempered martensite contain retained austenite, only the lath-like ferrite portion is considered bainite and tempered martensite, and is distinguished from the retained austenite. Furthermore, when bainite and tempered martensite contain Fe-based carbides, the contained Fe-based carbides are also considered part of the bainite and tempered martensite.

[0059] Bainite and tempered martensite can be distinguished from each other by observing the orientation and crystal structure of the Fe-based carbides they contain. Fe-based carbides precipitated within tempered martensite exhibit multiple elongation directions within the same crystal orientation range of the tempered martensite. On the other hand, Fe-based carbides precipitated within bainite contain only Fe-based carbides that have elongated in the same direction within the same crystal orientation range of the bainite. Here, Fe-based carbides that have elongated in the same direction refer to Fe-based carbides whose elongation direction difference is 10° or less.

[0060] Fresh martensite and retained austenite do not contain Fe-based carbides compared to bainite and tempered martensite. Furthermore, fresh martensite and retained austenite exhibit brighter contrast in SEM images compared to bainite, tempered martensite, polygonal ferrite, and acicular ferrite. Therefore, the hard phases (fresh martensite and retained austenite) can be distinguished from these structures using SEM. Because fresh martensite and retained austenite have similar shapes and contrasts in SEM images, making them difficult to distinguish, the respective area percentages of fresh martensite and retained austenite are determined by the method described later.

[0061] Here, the area ratios of bainite, tempered martensite, fresh martensite, retained austenite, and the remaining microstructure are measured at the 1 / 4 thickness position of the high-strength hot-rolled steel sheet as follows.

[0062] Specifically, a sample is cut from a hot-rolled steel sheet so that the 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. Then, the observation surface of the sample is finished polished with colloidal silica, and the microstructure is revealed by etching with 3 vol% nital. Then, using a SEM, 10 fields of view of 42.7 μm × 32.0 μm on the observation surface of the sample are observed under the conditions of acceleration voltage: 15 kV and magnification: 3000x, and each phase is identified and the area ratio is calculated.

[0063] As described above, fresh martensite and retained austenite have similar contrast in SEM images, making them difficult to distinguish. Therefore, in SEM observations, fresh martensite and retained austenite are not distinguished and their area fractions are calculated as a single hard phase. Furthermore, the area fraction of retained austenite is determined by X-ray diffraction, and the area fraction of fresh martensite is obtained by subtracting the area fraction of retained austenite (described later) from the area fraction of the hard phase calculated from the SEM image.

[0064] The area fraction of retained austenite is measured as follows: After mechanically grinding a hot-rolled steel sheet in the thickness direction (depth direction) up to 1 / 4 of the sheet thickness, the sheet thickness of 100 μm or more is chemically polished with oxalic acid to create the observation surface. Then, the observation surface is observed by X-ray diffraction. CoKα rays are used as the incident X-rays. The ratio of the diffraction intensity of the (200), (220), and (311) surfaces of fcc iron (austenite) to the diffraction intensity of the (200) and (211) surfaces of bcc iron is determined, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0065] Average solid-solution carbon content in retained austenite: 0.50% or more and 1.20% or less From the viewpoint of obtaining high ductility, the average solid-solution carbon content in retained austenite should be 0.50% or more. Preferably, the average solid-solution carbon content in retained austenite is 0.55% or more, more preferably 0.60% or more. On the other hand, if the average solid-solution carbon content in retained austenite exceeds 1.20%, the ductility will actually decrease. Therefore, the average solid-solution carbon content in retained austenite should be 1.20% or less. Preferably, the average solid-solution carbon content in retained austenite is 1.10% or less, more preferably 1.00% or less.

[0066] The average amount of dissolved carbon in retained austenite is measured as follows: The observation surface on which the area fraction of retained austenite was measured is observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the lattice constant of austenite (aγ) is determined from the position of the diffraction peak of the (220) plane of fcc iron (austenite). The average amount of dissolved carbon in retained austenite is then calculated using the following formula: aγ (Å) = 3.572 + 0.033 × C - 0.00157 × Si + 0.0012 × Mn In the above formula, Si and Mn represent the mass %) content of each element in the steel, and C represents the average amount of dissolved carbon in retained austenite (mass %).

[0067] Next, the shape of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0068] Steepness: 2.0% or less. Steepness refers to the flatness of the steel sheet. If the steepness is high, the shape may not match the guide during part forming, making accurate positioning difficult and potentially preventing part forming. Even if the part can be formed, poor flatness may result in shape defects. Therefore, high-strength steel sheets require excellent flatness. From this viewpoint, the steepness of the high-strength steel sheet shall be 2.0% or less. Preferably, the steepness is 1.5% or less, more preferably 1.0% or less, even more preferably 0.5% or less, and even more preferably 0.1% or less. The lower limit is not particularly limited, and the steepness may be 0.0%. The steepness can be measured by the flatness test described in the examples.

[0069] Next, the mechanical properties of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0070] Tensile strength (TS): 1180 MPa or more The tensile strength of the high-strength hot-rolled steel sheet according to one embodiment of the present invention shall be 1180 MPa or more. Preferably, the tensile strength is less than 1800 MPa. Here, the tensile strength (TS) can be measured by a tensile test in accordance with JIS Z 2241:2011 as described in the example.

[0071] The high-strength hot-rolled steel sheet according to one embodiment of the present invention is not particularly limited, but it is preferable that the sheet thickness is 0.6 mm or more and 10.0 mm or less. The sheet thickness may be 1.0 mm or more. The sheet thickness may also be 6.0 mm or less. Furthermore, when the high-strength hot-rolled steel sheet according to one embodiment of the present invention is used as a material for automobile parts, it is more preferable that the sheet thickness is 1.0 mm or more and 6.0 mm or less.

[0072] A high-strength hot-rolled steel sheet according to one embodiment of the present invention may have a plating layer on at least one side of the steel sheet. When the high-strength hot-rolled steel sheet has a plating layer, the type of plating layer is not particularly limited and may be, for example, a hot-dip galvanized layer or an electroplated layer. The plating layer may also be an alloyed plating layer. A zinc plating layer is preferred as the plating layer. The zinc plating layer may contain Al or Mg.

[0073] Furthermore, the composition of the plating layer is not particularly limited and can be, for example, a known composition. In the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, the composition of the plating layer can be, for example, one containing Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further containing one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% by mass or more and 3.5% by mass or less, with the remainder being Zn and unavoidable impurities. Also, the amount of plating adhesion is not particularly limited, but for example, the amount of plating adhesion per side can be 20 to 80 g / m². ２ It can be done this way.

[0074] [2] Members and Methods for Manufacturing Members Next, a member and a method for manufacturing the same according to one embodiment of the present invention will be described. A member according to one embodiment of the present invention is a member made using the above-mentioned high-strength hot-rolled steel sheet (as the material). As for the method for manufacturing the member, for example, a manufacturing method can be described in which at least one of forming and joining processes is applied to the high-strength hot-rolled steel sheet, which is the material, to make a member. The forming and joining processes can be appropriately selected according to the shape of the member to be manufactured. For example, as the forming process, general processing methods such as press working can be used without limitation. Also, as the joining process, general welding methods such as spot welding and arc welding, as well as riveting and crimping can be used without limitation.

[0075] Here, the high-strength hot-rolled steel sheet described above maintains high strength with a tensile strength of 1180 MPa or more, and further possesses high ductility, high elongation flangeability, and excellent flatness. Members made using the high-strength hot-rolled steel sheet described above maintain high strength while reducing shape defects. For this reason, members according to one embodiment of the present invention are particularly suitable for application to automobile parts such as undercarriage members and frame members, and construction machinery members.

[0076] [3] Method for manufacturing high-strength hot-rolled steel sheets Next, a method for manufacturing high-strength hot-rolled steel sheets according to one embodiment of the present invention will be described.

[0077] A method for manufacturing a high-strength hot-rolled steel sheet according to one embodiment of the present invention comprises a heating step of heating a steel material having the above component composition to 1100°C or higher, and a finishing rolling completion temperature of the steel material after the heating step: ３The process comprises: a hot rolling step in which finish rolling is performed at temperatures between ℃ and 1000℃ to produce a hot-rolled steel sheet; a first cooling step in which the hot-rolled steel sheet after the hot rolling step is cooled in the temperature range up to Ms℃ at an average cooling rate of 20℃ / s or more; a second cooling step in which the hot-rolled steel sheet after the first cooling step is cooled in the temperature range from Ms℃ to 150℃ at an average cooling rate of less than 20℃ / s; a reheating step in which the hot-rolled steel sheet after the second cooling step is reheated to a temperature range between 300℃ and 500℃; and a shape correction step in which the hot-rolled steel sheet after the reheating step is stretched at a temperature between 200℃ and 500℃ with an elongation rate of 0.10% or more. Unless otherwise specified, the above temperatures refer to the surface temperature of the steel material and the steel sheet. The hot-rolled steel sheet also includes hot-rolled steel strip. The average cooling rate in the temperature range from X°C to Y°C (X > Y) can be calculated as (X°C - Y°C) / cooling time from X°C to Y°C (s).

[0078] First, a steel material such as a slab having the above-described component composition is prepared. The method for manufacturing the steel material such as a slab is not particularly limited, and commonly used methods can be used. As an example of a method for manufacturing the steel material, one can produce a slab by melting molten steel having the above-described component composition using a known method in a converter or the like, and then casting it using a casting method such as continuous casting. As a method for manufacturing the steel material, a known casting method such as ingot-fraction rolling may also be used. Scrap may also be used as a raw material for the steel material.

[0079] [Heating Process] In the steel material such as a slab after being cooled to a low temperature of 1100°C or higher, most of the elements forming carbonitrides such as Ti precipitate non-uniformly as coarse carbonitrides. The presence of these coarse and non-uniform precipitates leads to deterioration of various properties (for example, strength, elongation flangeability, etc.). Therefore, the steel material before hot rolling is heated to dissolve the coarse precipitates. To sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the steel material is set to 1100°C or higher. The heating temperature of the steel material is preferably 1150°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel material becomes too high, it may cause the occurrence of slab defects and a decrease in yield due to scale-off. Therefore, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. Incidentally, the steel material before hot rolling may be directly subjected to hot rolling (direct rolling) after casting while maintaining a high temperature (that is, while maintaining the temperature within the above heating temperature range).

[0080] [Hot Rolling Process] Next, hot rolling consisting of rough rolling and finish rolling is performed on the steel material heated to 1100°C or higher (including those directly sent while maintaining a high temperature after casting). The rough rolling only needs to ensure the desired sheet bar dimensions, and the conditions are not particularly limited. The steel material is rough rolled to obtain a rough rolled sheet. Before performing finish rolling on the obtained rough rolled sheet, it is preferable to perform descaling (high-pressure water descaling) by injecting high-pressure water on the entrance side of the finish rolling mill.

[0081] To remove the primary scale generated before finish rolling, it is preferable to perform high-pressure water descaling on the rough rolled sheet. The impact pressure of the high-pressure water descaling (also simply referred to as "descaling impact pressure") is preferably 2.5 MPa or higher, more preferably 3.0 MPa or higher, and even more preferably 3.5 MPa or higher. The impact pressure is the unit area (cm ２This is the force per unit area. The descaling impact pressure has no particular upper limit, but is preferably 15.0 MPa or less, more preferably 14.5 MPa or less, and even more preferably 12.0 MPa or less. High-pressure water descaling may be performed during the rolling process between the finish rolling stands. In addition, if necessary, the rough-rolled plates may be cooled between the finish rolling stands.

[0082] Finish rolling completion temperature: A ３ The finishing rolling end temperature is A (between ℃ and 1000℃). ３ If the temperature is below ℃, ferrite will form by the end of finish rolling, causing the area ratio of the remaining microstructure to exceed 10.0%. Therefore, the finish rolling completion temperature is A ３ The temperature shall be above ℃. The finishing rolling completion temperature shall be (A ３ (A) ３ (A) ３ A temperature of +60°C or higher is even more preferable. On the other hand, if the finish rolling completion temperature exceeds 1000°C, significant grain growth of austenite grains occurs, causing the austenite grains to coarse, and the untransformed austenite in the martensitic or bainite transformation also coarses. This coarsened untransformed austenite later becomes fresh martensite, so the area ratio of fresh martensite increases. For this reason, the finish rolling completion temperature should be 1000°C or lower. The finish rolling completion temperature is preferably 980°C or lower, and more preferably 960°C or lower. Here, A ３ A can be calculated using the following formula. ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo In the above formula, each element symbol indicates the content (mass %) of each element, and 0 is used if the element is not present.

[0083] [First Cooling Process] Next, the hot-rolled steel sheet (finished rolled sheet) obtained by finish rolling is cooled in the temperature range from the finish rolling completion temperature to Ms°C at an average cooling rate of 20°C / s or more (hereinafter also referred to as "forced cooling").

[0084] Average cooling rate in the temperature range up to Ms°C: 20°C / s or higher. In forced cooling, if the average cooling rate in the temperature range from the end of finish rolling temperature to Ms°C is less than 20°C / s, ferrite transformation occurs during cooling, and the remaining structure exceeds 10.0%. Therefore, the average cooling rate in the temperature range up to Ms°C should be 20°C / s or higher. Preferably, the average cooling rate should be 25°C / s or higher, more preferably 30°C / s or higher, and even more preferably 50°C / s or higher. There is no particular upper limit specified for the average cooling rate, but if the cooling rate becomes too high, it becomes difficult to secure the cooling rate in the subsequent second cooling process. Therefore, it is preferable that the average cooling rate be 300°C / s or less, more preferably 200°C / s or less, and even more preferably 100°C / s or less. The average cooling rate is the average cooling rate on the surface of the steel sheet. Ms is calculated by the following formula. Ms (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo In the above formula, each element symbol indicates the content (mass %) of each element, and 0 is used if the element is not present.

[0085] [Second Cooling Process] Next, the hot-rolled steel sheet after the first cooling process is cooled in the temperature range from Ms°C to 150°C at an average cooling rate of less than 20°C / s.

[0086] Average cooling rate in the temperature range from Ms°C to 150°C: Less than 20°C / s If the average cooling rate in the temperature range from Ms°C to 150°C during the second cooling is 20°C / s or higher, the distribution of C to the untransformed austenite becomes insufficient, and at least one of the desired area fraction of retained austenite and the average amount of solid-soluble C in the retained austenite cannot be obtained. Therefore, the average cooling rate in the temperature range from Ms°C to 150°C should be less than 20°C / s. The average cooling rate is preferably 15°C / s or less, more preferably 10°C / s or less, and even more preferably 5.0°C / s or less. There is no particular lower limit for the average cooling rate, but if the cooling rate becomes too low, productivity will deteriorate. Therefore, the average cooling rate is preferably 0.002°C / s or higher, more preferably 0.005°C / s or higher, and even more preferably 0.010°C / s or higher.

[0087] The second cooling step may be performed before or after winding the hot-rolled steel sheet. Furthermore, the final cooling temperature in the second cooling step is preferably 150°C or lower, and more preferably 100°C or lower. The lower limit of the final cooling temperature is not particularly limited, but typically it should be cooled to ambient temperature. For example, when the hot-rolled steel sheet is allowed to cool outdoors, it should be cooled to ambient temperature. For example, when the hot-rolled steel sheet is allowed to cool outdoors, the final cooling temperature depends on the ambient temperature. Typically, the final cooling temperature is preferably -10°C or higher, more preferably 0°C or higher, and even more preferably 10°C or higher.

[0088] [Reheating Process] Next, the hot-rolled steel sheet after the second cooling process is reheated to a temperature range of 300°C to 500°C. The means for reheating the steel sheet are not particularly limited, but examples include exposing it to a high-temperature atmosphere such as a heat treatment furnace, heating it with a burner, induction heating, or heating it by radiation from other steel sheets.

[0089] Reheating temperature: 300°C or higher and 500°C or lower. If the reheating temperature is less than 300°C, the average amount of solid-soluble carbon in the retained austenite increases. For this reason, the reheating temperature should be 300°C or higher. Preferably, the reheating temperature is 320°C or higher, more preferably 340°C or higher, and even more preferably 360°C or higher. On the other hand, if the reheating temperature is higher than 500°C, the average amount of solid-soluble carbon in the retained austenite decreases. Also, the tensile strength may decrease. For this reason, the reheating temperature should be 500°C or lower. Preferably, the reheating temperature is 480°C or lower, more preferably 460°C or lower, and even more preferably 440°C or lower.

[0090] Furthermore, the hot-rolled steel sheet after the reheating process may be subjected to the subsequent shape-correcting process while maintaining its temperature range. Alternatively, after reheating, it may be held in a temperature range of 300°C to 500°C for 600 seconds or more, and then cooled to room temperature. Note that room temperature is typically the ambient temperature; for example, when cooling the hot-rolled steel sheet outdoors, it may be cooled to the ambient temperature. The room temperature may be 50°C or lower, or 30°C or lower, as an example. Furthermore, there is no particular lower limit to the room temperature; for example, the room temperature may be -10°C or higher, 0°C or higher, or 10°C or higher.

[0091] [Shape Correction Process] Next, the hot-rolled steel sheet after the reheating process is stretched at a temperature of 200°C to 500°C with an elongation rate of 0.10% or more to correct its shape. This shape correction is performed, for example, using at least one of a skin pass mill and a tension leveler. If the sheet has cooled to below 200°C after the reheating process, it is reheated to a temperature of 200°C to 500°C. The means of reheating are not particularly limited, but examples include exposing it to a high-temperature atmosphere such as a heat treatment furnace, heating it with a burner, induction heating, or heating it by radiation from another steel sheet.

[0092] Elongation temperature: 200°C to 500°C By increasing the elongation temperature (the temperature of the steel sheet when shape correction is applied), retained austenite is stabilized, and the transformation of retained austenite to martensite due to elongation is suppressed. However, if the elongation temperature is less than 200°C, the effect is insufficient, and the desired amount of retained austenite cannot be obtained. Alternatively, since the retained austenite, which has a low carbon content and is unstable, transforms during elongation, the average carbon content in the retained austenite increases, and high ductility cannot be obtained. For this reason, the elongation temperature should be 200°C or higher. The elongation temperature is preferably 220°C or higher, more preferably 250°C or higher, and even more preferably 300°C or higher. On the other hand, if the elongation temperature exceeds 500°C, there is a risk that the tensile strength will decrease. For this reason, the elongation temperature should be 500°C or lower. The elongation temperature is preferably 480°C or lower, more preferably 460°C or lower, and even more preferably 440°C or lower.

[0093] Elongation rate: 0.10% or more. If the elongation rate is less than 0.10%, the desired excellent flatness cannot be obtained. For this reason, the elongation rate should be 0.10% or more. Preferably, the elongation rate is 0.20% or more, more preferably 0.30% or more, and even more preferably 0.50% or more. There is no particular upper limit to the elongation rate, but it should be 10.00% or less, where the effect of improving flatness saturates. Preferably, the elongation rate is 5.00% or less, more preferably 3.00% or less, and even more preferably 2.00% or less.

[0094] The high-strength hot-rolled steel sheet of the present invention is manufactured through the above process. Optionally, pickling may be performed to remove scale. Furthermore, the high-strength hot-rolled steel sheet of the present invention may have a plating layer on its surface. If the high-strength hot-rolled steel sheet of the present invention has a plating layer on its surface, the manufacturing method of the high-strength hot-rolled steel sheet further includes a plating treatment step in which the hot-rolled steel sheet is plated. In this case, although not particularly limited, for example, the hot-rolled steel sheet after the shape correction step can be plated. The plating treatment in the plating treatment step is not particularly limited, and examples include known plating treatments. Furthermore, a plating alloying treatment may be optionally performed after the plating treatment.

[0095] For example, hot-dip galvanizing and electro-galvanizing can be applied as plating treatments. Hot-dip galvanizing can be performed, for example, by immersing the base material, a steel sheet, in a zinc plating bath at 440 to 500°C. The zinc plating bath consists of, for example, Zn, Al, and unavoidable impurities, and its composition is not particularly limited, but as an example, the Al concentration in the bath can be 0.001% by mass or more and 1.0% by mass or less. Furthermore, the plating alloying treatment can be performed, for example, by heating the steel sheet after hot-dip galvanizing to an alloying temperature of 450 to 500°C. Furthermore, the electro-galvanizing treatment is not particularly limited, and for example, known conditions for electro-galvanizing treatment can be applied.

[0096] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0097] [Manufacturing of High-Strength Hot-Rolled Steel Sheets] Molten steel having the component composition shown in Table 1 below (the remainder consisting of Fe and unavoidable impurities) was melted in a converter, and steel material was manufactured by continuous casting. The manufactured steel material was subjected to a heating process at the heating temperature [°C] shown in Table 2 below. A rough-rolled sheet was obtained by rough-rolling the steel material after the heating process. The surface of the obtained rough-rolled sheet was subjected to high-pressure water descaling with an impact pressure of 6.0 MPa. A hot-rolled steel sheet was obtained by finish-rolling the rough-rolled sheet that had been subjected to high-pressure water descaling at the finish-rolling completion temperature [°C] shown in Table 2 below. After the completion of hot rolling (finish rolling), a first cooling process was performed on the obtained hot-rolled steel sheet. Table 2 below shows the average cooling rate [°C / s] in the temperature range from the finish-rolling completion temperature to Ms°C as the conditions for the first cooling process. Immediately after the completion of the first cooling process, a second cooling process was performed. Table 2 below shows the average cooling rate [°C / s] in the temperature range from Ms°C to 150°C as a condition for the second cooling process. A reheating process was performed on the hot-rolled steel sheet after the second cooling process. Table 2 below shows the reheating temperature [°C] as a condition for the reheating process. A shape correction process was performed on the hot-rolled steel sheet after the reheating process. Table 2 below shows the elongation temperature [°C] and elongation rate [%] as conditions for the shape correction process. In the shape correction process, at least one of a skin pass mill or a tension leveler was used to perform the shape correction.

[0098] High-strength hot-rolled steel sheets were obtained as described above. The obtained high-strength hot-rolled steel sheets were subjected to pickling (hydrochloric acid concentration: 10% by mass, temperature: 85°C) to remove scale.

[0099] [Evaluation of High-Strength Hot-Rolled Steel Sheets] Using the obtained high-strength hot-rolled steel sheets, the microstructure of the high-strength hot-rolled steel sheets was identified according to the procedure described above. The measurement results are shown in Table 3. In Table 3, TM is tempered martensite, B is bainite, FM is fresh martensite, γ is retained austenite, and %Cγ is the average amount of dissolved carbon in retained austenite.

[0100] Furthermore, tensile tests, hole expansion tests, and flatness tests were conducted according to the following procedure, and the tensile strength (TS), uniform elongation (U.El), critical hole expansion ratio (λ), and steepness were evaluated according to the following criteria. The measurement results are shown in Table 3.

[0101] (1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241:2011. Specifically, a JIS No. 5 test specimen was taken from the obtained high-strength hot-rolled steel sheet so that its longitudinal direction was perpendicular to the rolling direction of the steel sheet. Using the taken test specimen, a tensile test was performed under the condition of a crosshead speed of 10 mm / min, and the TS and U. El were measured. The results are shown in Table 3. A TS of 1180 MPa or higher (TS ≥ 1180 MPa) was considered a pass, and anything else was considered a fail. U. For El, a value of 4.5% or more (U.El ≥ 4.5%) was considered acceptable (indicating high ductility) when 1180 MPa ≤ TS < 1310 MPa, 4.0% or more (U.El ≥ 4.0%) when 1310 MPa ≤ TS < 1470 MPa, and 3.5% or more (U.El ≥ 3.5%) when 1470 MPa ≤ TS. Any other value was considered unacceptable.

[0102] (2) Hole Expansion Test The hole expansion test was conducted in accordance with JIS Z 2256:2010. Specifically, a 100 mm x 100 mm test piece was taken from the obtained high-strength hot-rolled steel sheet by shearing. A hole with a diameter of 10 mm (the initial hole in the test piece) was punched into the test piece with a clearance of 12% ± 1%. Next, a wrinkle-holding force of 9 ton (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm, and a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when cracks occurred) was measured. The limit hole expansion ratio λ (%) was then calculated using the following formula. The results are shown in Table 3. For λ, a value of 40% or more (λ≧40%) was considered acceptable (indicating high elongation flange properties) when 1180 MPa ≤ TS < 1310 MPa, 30% or more (λ≧30%) when 1310 MPa ≤ TS < 1470 MPa, and 20% or more (λ≧20%) when 1470 MPa ≤ TS. Anything else was considered unacceptable. λ(%) = {(D ｆ -D ０ ) / D ０} × 100 Here, in the above formula, D ｆ : Diameter of the hole in the test specimen at the time of crack initiation (mm), D ０ : This is the diameter (mm) of the hole in the initial test specimen.

[0103] (3) Flatness Test A steel sheet (high-strength hot-rolled steel sheet) discharged from a hot-rolled coil was cut to a length of 2000 mm and placed on a surface plate. If the steel sheet had a periodically wavy shape within its 2000 mm length, the degree of steepness [(h / P) × 100] (%) was measured by dividing the wave height h (mm) by the wave pitch P (mm) and multiplying by 100. The results are shown in Table 3. A degree of steepness of 2.0% or less was considered a pass (excellent flatness), and all others were considered a fail. The wave height h is the length of the largest gap between the surface plate and the steel sheet, and the wave pitch P is the length of the shortest wave pitch on the steel sheet. If the wave pitch was 2000 mm or more, P was set to 2000 mm to calculate the degree of steepness.

[0104] As shown in Tables 1 to 3, the examples of invention that satisfy the requirements of the present invention maintained a tensile strength of 1180 MPa or more, while also possessing high ductility, high elongation flangeability, and excellent flatness. On the other hand, the comparative examples that did not satisfy any of the requirements of the present invention failed to obtain one or more of the desired tensile strength, high ductility, high elongation flangeability, and excellent flatness.

[0105]

[0106]

[0107]

Claims

1. The composition of the steel is such that, by mass%, it contains C: 0.100% to 0.500%, Si: 0.20% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100%, S: 0.0200%, Al: 0.010% to 2.000%, and N: 0.0200%, with the remainder being Fe and unavoidable impurities, and the steel structure has the following area ratios: tempered martensite area ratio: 50.0% to 99.0%, bainite area ratio: 0% to 49.0%, fresh martensite area ratio: 0% to 5.0%, retained austenite area ratio: 1.0% to 20.0%, and the total area ratio of the remaining structure is 0% to 10.0%. A high-strength hot-rolled steel sheet having an average solid solution carbon content of 0.50% to 1.20% in retained austenite, a slope of 2.0% or less, a tensile strength of 1180 MPa or more, and high ductility and high elongation flangeability.

2. The above component composition is further defined in mass percent as follows: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.400% or less, Cr: 1.00% or less, Mo: 0.500% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less. The high-strength hot-rolled steel sheet according to claim 1, comprising one or more selected from Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

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

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

5. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 1 or 2, comprising: a heating step of heating a steel material to 1100°C or higher; and a finishing rolling completion temperature of the steel material after the heating step: A ３ A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a hot-rolling step of performing finish rolling at a temperature of ℃ or higher and 1000℃ or lower to produce a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to Ms℃ at an average cooling rate of 20℃ / s or higher; a second cooling step of cooling the hot-rolled steel sheet after the first cooling step in a temperature range from Ms℃ to 150℃ at an average cooling rate of less than 20℃ / s; a reheating step of reheating the hot-rolled steel sheet after the second cooling step to a temperature range of 300℃ or higher and 500℃ or lower; and a shape correction step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 200℃ or higher and 500℃ or lower with an elongation rate of 0.10% or higher. ３ A and Ms are calculated using the following formulas: ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo Ms (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo Here, each element symbol in the above formulas indicates the content (mass %) of each element, and 0 is used if the element is not contained.

6. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 3, comprising: a heating step of heating a steel material to 1100°C or higher; and a finishing rolling completion temperature of the steel material after the heating step: A ３ A method for manufacturing a high-strength hot-rolled steel sheet, comprising: a hot-rolling step of performing finish rolling at a temperature of ℃ or higher and 1000℃ or lower to produce a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to Ms℃ at an average cooling rate of 20℃ / s or higher; a second cooling step of cooling the hot-rolled steel sheet after the first cooling step in a temperature range from Ms℃ to 150℃ at an average cooling rate of less than 20℃ / s; a reheating step of reheating the hot-rolled steel sheet after the second cooling step to a temperature range of 300℃ or higher and 500℃ or lower; a shape-correcting step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 200℃ or higher and 500℃ or lower with an elongation rate of 0.10% or higher; and a plating step of applying a plating treatment to the hot-rolled steel sheet after the shape-correcting step. Here, A ３ A and Ms are calculated using the following formulas: ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo Ms (°C) = 539 - 423 × C - 30.4 × Mn + 30.0 × Al - 12.1 × Cr - 17.7 × Ni - 7.5 × Mo Here, each element symbol in the above formulas indicates the content (mass %) of each element, and 0 is used if the element is not contained.

7. A method for manufacturing a component, comprising the step of applying at least one of forming and joining processes to a high-strength hot-rolled steel sheet according to any one of claims 1 to 3 to form a component.

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