High-strength hot-rolled steel sheet, member, and methods for producing same

Abstract

Provided is a high-strength hot-rolled steel sheet having excellent delayed fracture resistance and excellent flatness in addition to high stretch flangeability while maintaining a high strength with a tensile strength of 1310 MPa or above.　The high-strength hot-rolled steel sheet has: a predetermined component composition; a steel structure in which the area ratio of tempered martensite is 80% to 100%, the area ratio of bainite is 0% to 20%, the total area ratio of the remaining structure is 0% to 10.0%, and the number density of metastable carbides having a major-axis length of 10 nm or more is 5 × 1010 / mm3; a steepness of 2.0% or less; a tensile strength of 1310 MPa or more; and high stretch flangeability and excellent delayed fracture resistance.

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 1310 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 high elongation flangeability, and it is important to ensure this while maintaining high strength.

[0011] Furthermore, when the TS exceeds 1310 MPa, it is necessary to prevent delayed fracture (hydrogen embrittlement) caused by hydrogen entering from the operating environment; in other words, excellent resistance to delayed fracture is required.

[0012] 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.

[0013] In fact, the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to possess high strength, such as a tensile strength (TS) of 1310 MPa or more, as well as high elongation flangeability, excellent delayed fracture resistance, and excellent flatness.

[0014] As described above, conventional technology has not established a method for producing hot-rolled steel sheets that maintain high strength of 1310 MPa or more in tensile strength, while also possessing high elongation flangeability, excellent delayed fracture resistance, and superior flatness.

[0015] 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 1310 MPa or more, and has high elongation flangeability, as well as excellent delayed fracture resistance and excellent flatness.

[0016] 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.

[0017] In this invention, high stretch flange properties mean that the average limiting hole expansion ratio (λ), measured in a hole expansion test compliant with JIS Z 2256:2010 using three test pieces, satisfies the following equation. In particular, since undercarriage members undergo stretch flange forming after punching, high stretch flange properties are required for the steel plates that make up the undercarriage members. By satisfying the following equation, the invention can be suitably applied to members that require even higher stretch flange properties. When 1310 MPa ≤ TS < 1470 MPa, λ ≥ 50% When 1470 MPa ≤ TS < 1600 MPa, λ ≥ 40% When 1600 MPa ≤ TS, λ ≥ 30%

[0018] In this invention, excellent delayed fracture resistance means that, using a four-point bending test specimen, four-point bending is performed in accordance with ASTM G39-99 (2016), a bending stress (σ) corresponding to the TS of the hot-rolled steel sheet is applied while immersed in a hydrochloric acid solution (pH = 1.0) at room temperature, and the presence or absence of fracture after 100 hours is evaluated, satisfying the following requirements. Structural members, frame members, undercarriage members of automobiles, truck frame members, and construction machinery members may be subjected to stress during use, and there is a risk of delayed fracture due to the intrusion of hydrogen from the operating environment. Therefore, excellent delayed fracture resistance is particularly required for the steel sheets that make up these members. If 1310 MPa ≤ TS < 1470 MPa, σ = 0.9 × TS and no fracture. If 1470 MPa ≤ TS < 1600 MPa, σ = 0.8 × TS and no fracture. If 1600 MPa ≤ TS, σ = 0.7 × TS and no fracture.

[0019] 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.

[0020] The inventors then diligently conducted research to achieve the above objectives. As a result, they appropriately adjusted the component composition of the hot-rolled steel sheet and the steel structure of the hot-rolled steel sheet to have the following characteristics: tempered martensite area ratio: 80% to 100%, bainite area ratio: 0% to 20%, total area ratio of the remaining structure: 0% to 10.0%, and number density of metastable carbides with a major axis length of 10 nm or more: 5 × 10⁻¹⁶ １０ pieces / mm ３ In summary, we found that by setting the steepness to 2.0% or less and the tensile strength to 1310 MPa or more, a high-strength hot-rolled steel sheet can be obtained that maintains high strength while possessing high elongation flangeability, excellent delayed fracture resistance, and excellent flatness.

[0021] The present invention was completed by further investigation based on the above findings. That is, the gist of the present invention is as follows: [1] The composition of the material is such that, by mass%, it contains C: 0.150% or more and 0.500%, Si: 0.20% or more and 2.00%, Mn: 0.80% or more and 3.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000%, and N: 0.0200% or less, with the remainder being Fe and unavoidable impurities, and the steel structure has the following characteristics: Area ratio of tempered martensite: 80% or more and 100%, Area ratio of bainite: 0% or more and 20%, Total area ratio of the remaining structure: 0% or more and 10.0%, Number density of metastable carbides with a major axis length of 10 nm or more: 5 × 10 １０ pieces / mm ３The above describes a high-strength hot-rolled steel sheet with a steepness of 2.0% or less, a tensile strength of 1310 MPa or more, high elongation flangeability, and excellent delayed fracture resistance. [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 the high-strength hot-rolled steel sheet according to [1] or [2], 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 completion 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 150°C or higher and 1000°C or lower to produce a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to 150°C at an average cooling rate of 20°C / s or higher and 300°C / s or lower; a reheating step of reheating the hot-rolled steel sheet after the cooling step to a temperature range of 150°C or higher and 400°C or lower; and a shape correction step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 150°C or higher and 400°C or lower with an elongation rate of 0.10% or higher and 5.0% or lower. Here, A ３ A is calculated using the following formula. ３(°C) = 881 - 206×C + 53×Si - 15×Mn - 20×Ni - 1×Cr - 27×Cu + 41×Mo Here, each element symbol in the above formula indicates the content (% by mass) of each element, and if not contained, it is taken as 0. [6] A method for manufacturing the high-strength hot-rolled steel sheet according to [3] above, a heating step of heating the steel material to 1100 °C or higher, after the heating step, the steel material is subjected to finish rolling at a finish rolling end temperature: A ３ °C or higher and 1000 °C or lower to obtain a hot-rolled steel sheet in a hot rolling step, after the hot rolling step, the hot-rolled steel sheet is cooled in a temperature range up to 150 °C under the condition that the average cooling rate is 20 °C / s or higher and 300 °C / s or lower in a cooling step, after the cooling step, the hot-rolled steel sheet is reheated in a temperature range of 150 °C or higher and 400 °C or lower in a reheating step, after the reheating step, the hot-rolled steel sheet is stretched at a temperature of 150 °C or higher and 400 °C or lower with an elongation rate of 0.10% or higher and 5.0% or lower in a shape correction step, a plating treatment step of subjecting the hot-rolled steel sheet after the shape correction step to a plating treatment, A method for manufacturing a high-strength hot-rolled steel sheet having the above steps. Here, A ３ is obtained by the following formula. A ３ (°C) = 881 - 206×C + 53×Si - 15×Mn - 20×Ni - 1×Cr - 27×Cu + 41×Mo Here, each element symbol in the above formula indicates the content (% by mass) of each element, and if not contained, it is taken as 0. [7] A method for manufacturing a member having a step of subjecting the high-strength hot-rolled steel sheet according to any one of [1] to [3] to at least one of forming and joining to form a member.

[0022] According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having high elongation flangeability, excellent delayed fracture resistance characteristics, and excellent flatness while maintaining a high strength with a tensile strength of 1310 MPa or higher.

[0023] Hereinafter, embodiments of the high-strength hot-rolled steel sheet and its manufacturing method of the present invention will be described. However, the present invention is not limited to the following embodiments.

[0024] [1] High-strength hot-rolled steel sheet First, the component composition of the high-strength hot-rolled steel sheet according to an embodiment of the present invention will be described. In the component composition, the unit is "mass%" in all cases, but hereinafter, unless otherwise specified, it is simply indicated by "%".

[0025] C: 0.150% or more and 0.500% or less. C is an element that has the effect of improving the strength of steel. C promotes the formation of martensite by improving hardenability and contributes to high strength. Also, C contributes to high strength by increasing the strength of martensite. In order to obtain a tensile strength of 1310 MPa or more, the C content needs to be 0.150% or more. Therefore, the C content is set to 0.150% or more. The C content is preferably 0.170% or more, more preferably 0.190% or more. On the other hand, when the C content exceeds 0.500%, the tensile strength increases excessively, and the elongation flangeability or the stress relaxation resistance deteriorates. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.400% or less, more preferably 0.300% or less.

[0026] Si: 0.20% or more and 2.00% or less. Si has the effect of suppressing the formation of cementite and suppresses the precipitation of cementite in the reheating process and the shape correction process. As a result, metastable carbides with a desired number density can be obtained, which contributes to the improvement of stress relaxation resistance. Also, Si improves the strength by improving the tempering softening resistance of martensite. 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 0.40% or more, more preferably 0.60% or more, still more preferably 0.80% or more. On the other hand, Si is an element that forms a subsurface scale on the steel sheet surface during hot rolling. When the Si content exceeds 2.00%, the subsurface scale 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 pre-painting treatment property when painting the high-strength hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less, more preferably 1.20% or less.

[0027] Mn: 0.80% to 3.00% Mn stabilizes austenite, suppresses ferrite formation, and contributes to the formation of tempered martensite. To obtain these effects, the Mn content must be 0.80% or more. Therefore, the Mn content is set to 0.80% or more. Preferably, the Mn content is 1.00% or more, more preferably 1.50% or more. On the other hand, if the Mn content exceeds 3.00%, Mn segregates at the grain boundaries, reducing the delayed fracture resistance. Therefore, the Mn content is set to 3.00% or less. Preferably, the Mn content is 2.50% or less, more preferably 2.10% or less, and even more preferably 1.80% or less.

[0028] 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 tensile flange properties and delayed fracture resistance. 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.

[0029] S: 0.0200% or less. S combines with Ti and Mn to form coarse sulfides, which accelerate void formation and reduce stretch 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.

[0030] Al: 0.010% to 2.000% Al acts as a deoxidizing agent and is an effective element for improving the cleanliness of steel. In addition, Al has the effect of suppressing cementite formation and inhibits cementite precipitation in the reheating process and shape correction process. This makes it possible to obtain metastable carbides with the desired number density, which contributes to improving delayed fracture resistance. Since these effects are not sufficient when the Al content is less than 0.010%, the Al content should be 0.010% or more. On the other hand, excessive Al content leads to an increase in oxide inclusions and reduces ductility. 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.

[0031] N: 0.0200% or less. N precipitates as nitride by bonding with nitride-forming elements and generally contributes to grain refinement. However, N bonds with Ti at high temperatures to form coarse nitrides, so a content exceeding 0.0200% causes a decrease in elongation flange properties. For this reason, the N content should be 0.0200% or less. The N content is preferably 0.0150% or less, and more preferably 0.0050% or less. There is no particular lower limit to the N content, but from the viewpoint of productivity, etc., an N content of 0.0005% or more is preferred.

[0032] The remainder can consist of Fe and unavoidable impurities. Examples of unavoidable impurities include H and O.

[0033] The basic component composition of a high-strength hot-rolled steel sheet according to one embodiment of the present invention has been described above. The high-strength hot-rolled steel sheet according to one embodiment of the present invention may further optionally contain the following components in addition to the above component composition.

[0034] Ti: 0.200% or less. Ti is an element that improves the strength of steel sheets through precipitation strengthening or solid solution strengthening. In addition, Ti increases the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite-unrecrystallized region and contributing to an improvement in the balance of strength and elongation flangeability by refining the grain size of tempered martensite. To obtain such effects, if Ti is included, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.020% or more, and even more preferably 0.040% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of Ti-based precipitates are generated, which actually reduces elongation flangeability. Therefore, if Ti is included, the Ti content should be 0.200% or less. The Ti content is preferably 0.150% or less, more preferably 0.120% or less, and even more preferably 0.100% or less.

[0035] 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-unrecrystallized region and contributing to an improvement in the balance of strength and elongation flangeability by refining the grain size of 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 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 0.080% or less.

[0036] 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 balance of strength and elongation flangeability by refining the grain size of 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 elongation flangeability. Therefore, if V is included, the V content should be 0.400% or less. The V content is preferably 0.300% or less, and more preferably 0.150% or less.

[0037] Cr: 1.00% or less. Like Mn, Cr suppresses ferrite formation and contributes to the formation of tempered martensite. To obtain such effects, if Cr is included, it is preferable that the Cr content be 0.01% or more. More preferably, the Cr content is 0.10% or more, and even more preferably 0.20% or more. However, since Cr is an element that worsens corrosion resistance and paint pretreatment properties, if Cr is included, the Cr content should be 1.00% or less. More preferably, the Cr content is 0.80% or less, and more preferably 0.70% or less.

[0038] Mo: 0.500% or less Mo increases the tempering softening resistance of steel and contributes to improving the strength of steel sheets. To obtain such an effect, if Mo is included, it is preferable that the Mo content be 0.010% or more. More preferably, the Mo content is 0.050% or more, and even more preferably 0.100% or more. On the other hand, if the Mo content exceeds 0.500%, the retained austenite increases excessively, worsening the elongation flange properties. Therefore, if Mo is included, the Mo content should be 0.500% or less. Preferably, the Mo content is 0.250% or less.

[0039] B: 0.0100% or less. B is an element that contributes to the formation of tempered martensite by segregating at the prior austenite grain boundaries and suppressing the formation of ferrite. To obtain this effect, if B is included, it is preferable that the B content be 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, the above effect becomes saturated. Therefore, if B is included, the B content should be 0.0100% or less. The B content is preferably 0.0050% or less.

[0040] 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, it is preferable that the Cu content be 0.005% or more when Cu is included. 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 even more preferably 0.50% or less.

[0041] 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 tempered martensite through improved hardenability. To obtain such effects, if Ni is included, it is preferable that the Ni content be 0.01% or more. The Ni content is more preferably 0.10% or more, and even more preferably 0.15% 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.

[0042] 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.

[0043] Sn: 0.200% or less. Like Sb, Sn is an element that is effective in suppressing the decrease 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 decreases, which may cause slab cracking and hot rolling cracking. Therefore, when Sn is included, the Sn content should be 0.200% or less. The Sn content is preferably 0.140% or less.

[0044] 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.

[0045] 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, and more preferably 0.150% or less.

[0046] Mg: 0.0200% or less. Mg 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 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 0.0120% or less.

[0047] 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.

[0048] 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.

[0049] 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. The Zr content is preferably 0.0120% or less.

[0050] 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.

[0051] 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.

[0052] 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 hot-rolled steel sheet of the present invention is as follows: Area ratio of tempered martensite: 80% or more and 100% or less; Area ratio of bainite: 0% or more and 20% or less; Total area ratio of the remaining microstructure: 0% or more and 10.0% or less; Number density of metastable carbides with a major axis length of 10 nm or more: 5 × 10 １０ pieces / mm ３ That's all.

[0053] Area ratio of tempered martensite: 80% or more and 100% or less Tempered martensite is a useful phase from the viewpoint of obtaining high elongation flangeability while maintaining high strength with a tensile strength of 1310 MPa or more. For this reason, the area ratio of tempered martensite is set to 80% or more. Preferably, the area ratio of tempered martensite is 85% or more, more preferably 90% or more, even more preferably 95% or more, and more preferably 100%.

[0054] Area ratio of bainite: 0% to 20% Although bainite has lower strength than tempered martensite, it is a useful phase from the viewpoint of obtaining high ductility and flangeability. Therefore, the area ratio of bainite can be limited to 20% or less. The area ratio of bainite is preferably 15% or less, more preferably 10% or less, and even more preferably 5% or less. There is no particular lower limit to the area ratio of bainite, and the area ratio of bainite may be 0%.

[0055] The area ratio of the remaining tissue is 0% or more and 10.0% or less. If the area ratio of the remaining tissue other than the tempered martensite and bainite exceeds 10.0%, at least one of the desired high strength, tensile flangeability, and delayed fracture resistance cannot be obtained. Therefore, the area ratio of the remaining tissue should be 10.0% or less. Preferably, the area ratio of the remaining tissue is 5.0% or less. Alternatively, the area ratio of the remaining tissue may be 0%.

[0056] The remaining tissue is not particularly limited and may include one or more types, such as polygonal ferrite, pearlite, fresh martensite, and retained austenite. The type of tissue can be confirmed, for example, by observation using a scanning electron microscope (SEM).

[0057] Tempered martensite is an aggregate of lath-like ferrite with an orientation difference of less than 15°, and has a structure containing Fe-based carbides at the interfaces and within the lath-like ferrite. It may also contain retained austenite at the interfaces of the lath-like ferrite. When tempered martensite contains retained austenite, only the lath-like ferrite portion is considered tempered martensite and is distinguished from the retained austenite. Furthermore, when tempered martensite contains Fe-based carbides, the contained Fe-based carbides are also considered part of the tempered martensite.

[0058] Bainite, like tempered martensite, is an aggregate of lath-like ferrites, but it can be easily distinguished from bainite because it does not contain Fe-based carbides within the lath-like ferrites.

[0059] Here, the area ratio of tempered martensite, bainite, and the remaining microstructure is measured at the 1 / 4 thickness position of the high-strength hot-rolled steel sheet as follows.

[0060] 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.

[0061] 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, they can be distinguished from these tissues using SEM.

[0062] Fresh martensite and retained austenite have similar shapes and contrasts in SEM, making them difficult to distinguish; therefore, the area ratios of fresh martensite and retained austenite are determined by the method described below.

[0063] Perlite refers to a structure containing lamellar ferrite and Fe-based carbides. Because lamellar ferrite has a lower dislocation density compared to lath-like ferrite, pearlite, bainite, and tempered martensite can be easily distinguished using SEM or TEM.

[0064] Polygonal ferrites form at higher temperatures than bainite and, being massive in form, can be easily distinguished from lath-like ferrites using SEM or TEM.

[0065] The fresh martensite and retained austenite exhibit similar contrast in SEM images, making them difficult to distinguish. Therefore, in SEM observations, fresh martensite and retained austenite are not distinguished and are treated as a single hard phase for calculating their area fractions. 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.

[0066] 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.

[0067] Number density of metastable carbides with a long axis length of 10 nm or more: 5 × 10 １０ pieces / mm ３ The metastable carbides described above are metastable carbides that precipitate during the tempering process of martensite. Metastable carbides are, for example, Fe carbides other than cementite (iron-based carbides), and are at least one carbide selected from the group consisting of epsilon (ε) carbides, eta (η) carbides, and chi (χ) carbides. Because metastable carbides have a higher hydrogen trapping capacity compared to cementite, dispersing them in hot-rolled steel sheets provides excellent delayed fracture resistance. Therefore, the number density of metastable carbides with a long axis length of 10 nm or more is 5 × 10⁻⁶. １０ pieces / mm ３ The above is correct. The number density of metastable carbides with a major axis length of 10 nm or more is preferably 10 × 10 １０ pieces / mm ３ More than 20 x 10 １０ pieces / mm３ More preferably 50 x 10 １０ pieces / mm ３ More preferably, 100 x 10 １０ pieces / mm ３ That concludes the explanation. Furthermore, there is no specific upper limit on the number density of metastable carbides with a major axis length of 10 nm or more, but as an example, the number density of the metastable carbides is 1000 × 10 １０ pieces / mm ３ The following is possible:

[0068] Here, we describe a method for measuring the number density of metastable carbides with a major axis length of 10 nm or more. First, a steel plate is ground so that the observation surface is at a position corresponding to 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate), and then electropolished to prepare a sample. The observation surface of the prepared sample is observed using a transmission electron microscope. An electron beam is incident from the

[100] direction of the steel plate at 800,000 nm. ２ Ten fields of view of the same area were observed, and metastable carbides were identified from the restricted-field diffraction patterns of the observed carbides. Dark-field images of the observation field were obtained using electron diffraction spots obtained from the metastable carbides. In the dark-field images, metastable carbides exhibit white contrast. The number of metastable carbides with a major axis length of 10 nm or more was counted by image analysis, and this was divided by the total volume of the observation field (area × thin film thickness) to obtain the number density of metastable carbides with a major axis length of 10 nm or more. The thickness of the thin film used in the transmission electron microscope was measured by EELS (electron energy loss spectroscopy). The major axis length of the metastable carbides was defined as the maximum length of the metastable carbide obtained in the image analysis described above.

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

[0070] 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.

[0071] Next, the mechanical properties of a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0072] Tensile strength (TS): 1310 MPa or more The tensile strength of the high-strength hot-rolled steel sheet according to one embodiment of the present invention shall be 1310 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.

[0073] 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.

[0074] 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, an electroplated layer is preferred. Furthermore, an electroplated zinc layer is preferred as the electroplated layer.

[0075] The composition of the plating layer is not particularly limited and can be, for example, a known composition. The electroplated zinc layer may be a zinc alloy plating layer in which one or more elements selected from Fe, Cr, Ni, Mn, Co, Sn, Pb, Mo, etc., are added to Zn depending on the purpose. The amount of plating deposited is also not particularly limited, but for example, the amount of plating deposited per side may be 20 to 80 g / m². ２ It can be done this way.

[0076] [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.

[0077] Here, the high-strength hot-rolled steel sheet described above maintains high strength with a tensile strength of 1310 MPa or more, and further possesses high elongation flangeability, excellent delayed fracture resistance, and excellent flatness. Members made using the high-strength hot-rolled steel sheet described above maintain high strength, have excellent delayed fracture resistance, and have reduced 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.

[0078] [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.

[0079] 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 150°C and 1000°C to produce a hot-rolled steel sheet; a cooling step in which the hot-rolled steel sheet after the hot rolling step is cooled in a temperature range up to 150°C at an average cooling rate of 20°C / s to 300°C / s; a reheating step in which the hot-rolled steel sheet after the cooling step is reheated to a temperature range between 150°C and 400°C; and a shape correction step in which the hot-rolled steel sheet after the reheating step is stretched at a temperature between 150°C and 400°C with an elongation rate of 0.10% to 5.0%. Unless otherwise specified, the above temperatures refer to the surface temperature of the steel material and steel sheet. 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) is calculated as (X°C - Y°C) / cooling time from X°C to Y°C (s).

[0080] 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.

[0081] [Heating Process] Heating temperature of steel material: 1100°C or higher. In steel materials such as slabs that have been cooled to a low temperature, most of the elements that form carbonitrides, such as Ti, precipitate unevenly as coarse carbonitrides. The presence of these coarse and uneven precipitates leads to deterioration of various properties (e.g., strength, ductility, flangeability, etc.). For this reason, the steel material is heated before hot rolling to solidify the coarse precipitates. In order to sufficiently solidify the coarse precipitates before hot rolling, the heating temperature of the steel material is set to 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 is too high, it may lead to the occurrence of slab defects and a decrease in yield due to scale-off. For this reason, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. Furthermore, the steel material before hot rolling may be subjected to direct hot rolling (direct rolling) after casting while still at a high temperature (i.e., while maintaining a temperature within the above heating temperature range).

[0082] [Hot Rolling Process] Next, the steel material heated to 1100°C or higher (including materials delivered directly at high temperature after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The rough rolling is only required to ensure the desired sheet bar dimensions, and the conditions are not particularly limited. The steel material is roughly rolled to obtain a rough-rolled plate. Before performing finish rolling on the obtained rough-rolled plate, it is preferable to perform descaling (high-pressure water descaling) by spraying high-pressure water at the entrance of the finish rolling mill.

[0083] To remove primary scale generated before finish rolling, it is preferable to perform high-pressure water descaling on the rough-rolled plate. The impact pressure of high-pressure water descaling (also simply called "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 area (cm²) over which the high-pressure water impacts the surface of the rough-rolled plate. 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.

[0084] 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 represents the content (mass %) of each element, and 0 is used if the element is not present.

[0085] [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 150°C under the condition of an average cooling rate of 20°C / s to 300°C / s (hereinafter also referred to as "forced cooling").

[0086] Average cooling rate in the temperature range up to 150°C: 20°C / s or more and 300°C / s or less. In forced cooling, if the average cooling rate from the end temperature of finish rolling to 150°C is less than 20°C / s, ferrite transformation occurs during forced cooling, and the area ratio of the remaining structure exceeds 10.0%. Therefore, the average cooling rate up to 150°C should be 20°C / s or more. The average cooling rate in the aforementioned temperature range is preferably 25°C / s or more, more preferably 30°C / s or more, and even more preferably 50°C / s or more. On the other hand, if the average cooling rate in the aforementioned temperature range becomes too high, the shape of the plate deteriorates significantly, and the desired excellent flatness may not be obtained. Therefore, the average cooling rate in the aforementioned temperature range should be 300°C / s or less. The average cooling rate in the aforementioned temperature range is preferably 200°C / s or less, and more preferably 100°C / s or less. Note that the average cooling rate is the average cooling rate on the surface of the steel plate.

[0087] Furthermore, the cooling completion temperature for the forced cooling is preferably 150°C or lower, and more preferably 100°C or lower. The lower limit of the cooling completion temperature is not particularly limited, but typically it is sufficient to cool it to ambient temperature. For example, when a hot-rolled steel sheet is cooled outdoors, it is sufficient to cool it to ambient temperature. For example, when a hot-rolled steel sheet is cooled outdoors, the cooling completion temperature depends on the ambient temperature. Typically, the cooling completion 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 cooling process is reheated to a temperature range of 150°C to 400°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: 150°C or higher and 400°C or lower. If the reheating temperature is below 150°C, the desired number density of metastable carbides cannot be obtained. For this reason, the reheating temperature should be 150°C or higher. Preferably, the reheating temperature is 170°C or higher, more preferably 190°C or higher, and even more preferably 200°C or higher. On the other hand, if the reheating temperature is above 400°C, the metastable carbides will transition to cementite, and the desired number density of metastable carbides cannot be obtained. Also, the tensile strength may decrease. For this reason, the reheating temperature should be 400°C or lower. Preferably, the reheating temperature is 370°C or lower, more preferably 340°C or lower, and even more preferably 300°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 at a temperature range of 150°C to 400°C for 600 seconds or more, and then cooled to room temperature. 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. Room temperature may be 50°C or lower, or 30°C or lower, as an example. The lower limit of room temperature is not particularly limited, but for example, 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 150°C to 400°C with an elongation rate of 0.10% to 5.0% 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 is cooled to below 150°C in the reheating process, it is reheated to a temperature of 150°C to 400°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 with a burner, induction heating, or heating by radiation from other steel sheets.

[0092] Elongation temperature: 150°C to 400°C By increasing the elongation temperature (the temperature of the steel sheet when shape correction is applied), the precipitation of metastable carbides is promoted, and the hot-rolled steel sheet softens, making it possible to obtain the desired degree of steepness. If the elongation temperature is less than 150°C, the effect is insufficient, and the desired degree of steepness cannot be obtained. For this reason, the elongation temperature should be 150°C or higher. Preferably, the elongation temperature is 170°C or higher, more preferably 190°C or higher, and even more preferably 200°C or higher. On the other hand, if the elongation temperature exceeds 400°C, the metastable carbides transition to cementite, and the desired number density of metastable carbides cannot be obtained. Also, there is a risk of a decrease in tensile strength. For this reason, the elongation temperature should be 400°C or lower. Preferably, the elongation temperature is 370°C or lower, more preferably 340°C or lower, and even more preferably 300°C or lower.

[0093] Elongation rate: 0.10% or more and 5.0% or less. 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. On the other hand, if the elongation rate exceeds 5.0%, excessive strain is introduced into the steel plate, and the desired excellent delayed fracture resistance cannot be obtained. For this reason, the elongation rate should be 5.0% or less. Preferably, the elongation rate is 4.0% or less, more preferably 3.0% or less, and even more preferably 2.0% 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. Electroplating is preferred as the plating treatment in the plating treatment step. This is because if the sheet is reheated to over 400°C after the shape correction step, the metastable carbides may transition to cementite, and the desired number density of metastable carbides may not be obtained. Furthermore, the conditions for the electroplating treatment are not particularly limited, and for example, known electro-zinc plating conditions can be applied.

[0095] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0096] [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 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 150°C as a condition for the cooling process. A reheating process was performed on the hot-rolled steel sheet obtained in the cooling process. Table 2 below lists the reheating temperature [°C] as a condition for the reheating process. After the reheating process, the hot-rolled steel sheet was subjected to a shape correction process. Table 2 below lists 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.

[0097] 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.

[0098] [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 represents tempered martensite, B represents bainite, and the metastable carbide number density is the number density of metastable carbides with a major axis length of 10 nm or more.

[0099] Furthermore, tensile tests, hole expansion tests, delayed fracture tests, and flatness tests were conducted according to the following procedure, and the tensile strength (TS), critical hole expansion ratio (λ), delayed fracture resistance, and steepness were evaluated according to the following criteria. The measurement results are shown in Table 3.

[0100] (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 that the crosshead speed was 10 mm / min, and the TS was measured. The results are shown in Table 3. A TS of 1310 MPa or higher (TS ≥ 1310 MPa) was considered a pass, and anything else was considered a fail.

[0101] (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 50% or more (λ≧50%) was considered acceptable (indicating high elongation flange properties) when 1310MPa ≤ TS < 1470MPa, 40% or more (λ≧40%) when 1470MPa ≤ TS < 1600MPa, and 30% or more (λ≧30%) when 1600MPa ≤ 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.

[0102] (3) Delayed fracture test Four-point bending test specimens measuring 16 mm x 75 mm were taken from the high-strength hot-rolled steel sheet obtained, with the longitudinal direction perpendicular to the rolling direction of the steel sheet. All end faces of the four-point bending test specimens were machine-ground. Using these specimens, four-point bending was performed in accordance with ASTM G39-99 (2016), and the bending stress (σ) corresponding to the TS of the hot-rolled steel sheet shown below was applied while immersed in a hydrochloric acid solution (pH = 1.0) at room temperature, and the presence or absence of fracture after 100 hours was evaluated. For 1310 MPa ≤ TS < 1470 MPa, σ = 0.9 × TS. For 1470 MPa ≤ TS < 1600 MPa, σ = 0.8 × TS. For 1600 MPa ≤ TS, σ = 0.7 × TS. If the specimen did not fracture in 100 hours, the delayed fracture resistance was considered good; if it fractured in less than 100 hours, the delayed fracture resistance was considered poor. The test was conducted with 2 specimens (n=2). If neither specimen fractured, it was considered good (OK: excellent delayed fracture resistance); if even one specimen fractured, it was considered poor (NG). The results are shown in Table 3. Note that if the TS of the high-strength hot-rolled steel sheet was less than 1310 MPa, the delayed fracture test was not performed because the desired strength was not obtained, and the result was shown as -.

[0103] (4) 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 inventive examples that met the requirements of the present invention maintained a tensile strength of 1310 MPa or higher, and in addition to high elongation flange properties, they also possessed excellent delayed fracture resistance and excellent flatness. On the other hand, the comparative examples that did not meet any of the requirements of the present invention failed to obtain one or more of the desired tensile strength, high elongation flange properties, excellent delayed fracture resistance, and excellent flatness.

[0105]

[0106]

[0107]

Claims

1. The composition of the material is such that, by mass%, it contains C: 0.150% to 0.500%, Si: 0.20% to 2.00%, Mn: 0.80% to 3.00%, P: 0.100%, S: 0.0200%, Al: 0.010% to 2.000%, and N: 0.0200%, with the remainder being Fe and unavoidable impurities. The steel structure has the following characteristics: tempered martensite area ratio: 80% to 100%, bainite area ratio: 0% to 20%, total area ratio of the remaining structure: 0% to 10.0%, and number density of metastable carbides with a major axis length of 10 nm or more: 5 × 10⁻⁶ １０ pieces / mm ３ The above describes a high-strength hot-rolled steel sheet with a steepness of 2.0% or less, a tensile strength of 1310 MPa or more, high elongation flangeability, and excellent delayed fracture resistance.

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 150°C or higher and 1000°C or lower to produce a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to 150°C at an average cooling rate of 20°C / s or higher and 300°C / s or lower; a reheating step of reheating the hot-rolled steel sheet after the cooling step to a temperature range of 150°C or higher and 400°C or lower; and a shape correction step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 150°C or higher and 400°C or lower with an elongation rate of 0.10% or higher and 5.0% or lower. Here, A ３ A is calculated using the following formula. ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo Here, each element symbol in the above formula indicates the content (mass %) of each element, and 0 is used if the element is not present.

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 1000°C or higher to produce a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step in a temperature range up to 150°C at an average cooling rate of 20°C / s or higher to 300°C / s; a reheating step of reheating the hot-rolled steel sheet after the cooling step to a temperature range of 150°C or higher to 400°C; a shape-correcting step of stretching the hot-rolled steel sheet after the reheating step at a temperature of 150°C or higher to 400°C with an elongation rate of 0.10% or higher to 5.0%; and a plating step of applying a plating treatment to the hot-rolled steel sheet after the shape-correcting step. Here, A ３ A is calculated using the following formula. ３ (°C) = 881 - 206 × C + 53 × Si - 15 × Mn - 20 × Ni - 1 × Cr - 27 × Cu + 41 × Mo Here, each element symbol in the above formula indicates the content (mass %) of each element, and 0 is used if the element is not present.

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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