Hot-rolled steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide a hot-rolled steel sheet that has a tensile strength of not less than 1,180 MPa and that has excellent bending workability and high fatigue strength, and a method for producing the hot-rolled steel sheet.　The hot-rolled steel sheet has a specific component composition, and the metal structure thereof is such that: at a position at 1 / 4 of the sheet thickness, the martensite area ratio is not less than 90%; in a steel sheet surface layer which is positioned in the range of 50-150 μm from the surface of the steel sheet in the depth direction of the sheet thickness, the upper bainite area ratio is not less than 80%, and the martensite area ratio is 0-20%; in the steel sheet surface layer, the total amount of Ti and Nb which are contained in a precipitate having a particle size of not more than 100 nm is 0.010-0.050 mass% in terms of the equivalent circle diameter; and the dislocation density in the steel sheet surface layer is not more than 3.0×1015 / m2.

Description

Hot-rolled steel sheet and method for manufacturing the same

[0001] The present invention relates to a hot-rolled steel sheet suitable as a material for automotive parts, particularly one with excellent bendability and fatigue strength, and to a method for manufacturing the same.

[0002] In the industrial sector, in order to address climate change, CO2 ２ Technological development aimed at reducing emissions is continuously progressing. In the automotive sector, improving fuel and electricity consumption through vehicle weight reduction is a key development challenge, and steel sheets used in automotive parts require even higher strength. Generally, the workability of steel sheets decreases with increasing strength, so there is a demand for steel sheets that are both high-strength and highly workable. Hot-rolled steel sheets with a tensile strength of over 980 MPa are often used for parts that are primarily bent, such as frames, so particularly excellent bendability is required. In addition, frame parts are subjected to repeated loads while the vehicle is in motion and when loading and unloading luggage, so fatigue strength is also required.

[0003] Various technological developments have been made to improve the bendability of steel plates (Patent Documents 1-3).

[0004] Patent Document 1 discloses a technique for improving the bendability of high-strength steel sheets by promoting decarburization on the surface of the steel sheet and creating a composite structure of ferrite and tempered bainite in the surface region.

[0005] Furthermore, Patent Document 2 discloses a technique for improving the bendability of high-strength steel sheets by forming a soft surface layer consisting of 65% or more ferrite and 5-20% pearlite on one or both sides of the center of a steel sheet containing 85% or more tempered martensite, and setting the average spacing between pearlite particles in the soft surface layer to 3 μm or more, thereby making the Vickers hardness (Hc) of the center of the sheet thickness and the Vickers hardness (Hs) of the soft surface layer 0.50 ≤ Hs / Hc ≤ 0.75. The above technique improves bendability by softening the surface of the steel sheet. However, since fatigue fracture is a phenomenon that originates from the surface of the steel sheet, the fatigue strength decreases significantly when the surface of the steel sheet is softened.

[0006] Patent Document 3 discloses a technique for obtaining a high-strength steel sheet with high fatigue strength by controlling the metallic structure of the steel sheet so that the total volume fraction of tempered martensite and lower bainite is 90% or more, and the average effective grain size in the area within 50 μm from the surface is 6 μm or less.

[0007] Japanese Patent Publication No. 2021-507107, International Publication No. 2020 / 196060, International Publication No. 2014 / 188966

[0008] The inventors applied the conventional techniques described in Patent Documents 1 to 3 to steel sheets with a tensile strength exceeding 1180 MPa and conducted bending tests simulating the bending process of actual parts. They found that the methods in Patent Documents 1 to 3 could not suppress shear end face cracking or ridge cracking. Furthermore, a decrease in fatigue strength was confirmed in the methods in Patent Documents 1 and 2. Thus, conventional techniques have not established a method that can suppress cracking during the bending process of high-strength hot-rolled steel sheets with a tensile strength (TS) exceeding 1180 MPa in actual parts while simultaneously maintaining fatigue strength.

[0009] This invention has been made in view of the above circumstances, and aims to provide a hot-rolled steel sheet and a method for manufacturing the same that has a tensile strength of 1180 MPa or more, as well as excellent bendability and high fatigue strength.

[0010] Here, excellent bendability means that in a bending test using a specimen with a sheared end face, the limit bending radius R, which does not cause cracking such as ridge cracking or end face cracking, divided by the plate thickness t, is 2.5 or less (R / t). Furthermore, high fatigue strength means that in a full double-handed planar bending fatigue test, 1 × 10 ７ This refers to a pulse intensity of 550 MPa or higher.

[0011] The inventors diligently investigated conditions for suppressing end-face cracking during press brake bending tests. As a result, they found that high fatigue strength can be achieved while suppressing cracking during bending by controlling the chemical composition and the metal structure of the steel sheet surface within a specific range. Furthermore, the inventors discovered that precisely controlling the conditions of the finish rolling and heat treatment is effective in achieving the above-mentioned metal structure.

[0012] Based on the above findings, the gist of the present invention is as follows: [1] The composition is such that, in mass%, C: 0.03-0.20%, Si: 0.1-2.0%, Mn: 0.5-3.5%, Ti: 0.005% or more and less than 0.205%, P: 0.100% or less, S: 0.02% or less, Al: less than 1.55%, with the remainder being Fe and unavoidable impurities, and the metal structure is such that at the 1 / 4 position of the plate thickness, martensite accounts for 90% or more by area ratio, and in the surface layer of the steel plate located in the range of 50-150 μm from the surface of the steel plate in the thickness direction, upper bainite accounts for 80% or more by area ratio and martensite accounts for 0-20% by area ratio, and the total amount of Ti and Nb contained in precipitates with a particle size of 100 nm or less in the equivalent circle diameter in the surface layer of the steel plate is 0.010-0.050 mass%. The dislocation density on the surface layer of the steel plate is 3.0 × 10 １５ / m ２ The following hot-rolled steel sheet: [2] The hot-rolled steel sheet according to [1], wherein the component composition further includes one or more selected by mass% from: Cr: 0.05% or more and less than 2.05%, Mo: 0.05% or more and less than 2.05%, V: 0.05% or more and less than 1.05%, Cu: 0.05% or more and less than 4.05%, Ni: 0.005% or more and less than 2.050%, Nb: 0.005% or more and less than 0.205%, B: 0.0003 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010% or more and less than 0.1050%, Sn: 0.0010% or more and less than 0.5050%. [3] A method for manufacturing a hot-rolled steel sheet, comprising: heating a slab having the component composition described in [1] or [2] above, performing rough rolling, and then performing finish rolling, wherein the amount of reduction for each rolling pass is set such that the sum of the reduction strains Sr ΣSr of the rolling passes performed at an entry temperature Tn satisfying formula (1) is 1.0 to 3.0, the time from the end of the finish rolling to the start of cooling is 10.0 s or less, and the temperature range from the start of cooling after the end of the finish rolling to 300°C is cooled under conditions where the average cooling rate is 50°C / s or more, the winding temperature is 300°C or less, and then heat treatment is performed by holding the temperature in the temperature range of 400°C to 750°C for 0 to 3600 s. Tn ≤ 74.35 × t －０．６８５-500{0.1-(Ti+Nb)}+950...(1) Here, t is the time it takes for the slab to pass from the current finishing rolling stand to the next stand (or the time until cooling begins in the case of the final pass). Also, each element symbol in equation (1) represents the mass %) of each element in the slab, and is set to 0 if it is not present. The reduction strain Sr is defined by the following equation (2): Sr = ln(WTe / WTd)...(2) Here, WTe is the thickness of the slab on the entry side of the pass (mm), and WTd is the thickness of the slab on the exit side of the pass (mm).

[0013] According to the present invention, a hot-rolled steel sheet with a tensile strength exceeding 1180 MPa can be obtained, in which cracking such as ridge cracking and end face cracking during bending of actual parts is suppressed. When the hot-rolled steel sheet of the present invention is applied to automobile structural parts, frame parts, and truck frame parts, safety is ensured and the weight of the automobile body can be reduced, thus yielding significant industrial benefits.

[0014] Figure 1 is a schematic diagram illustrating the bending test method according to the present invention. (a) is a schematic diagram observed from the cross-section of the test specimen after the bending test, and (b) is a schematic diagram observed from the side of the bending test specimen from the outer bending surface.

[0015] The hot-rolled steel sheet (hereinafter also referred to as steel sheet) and its manufacturing method according to the present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0016] <Hot-Rolled Steel Sheet> The hot-rolled steel sheet of the present invention has a specific component composition and a specific metal structure. Here, the component composition and metal structure will be described in that order. First, the component composition of the hot-rolled steel sheet of the present invention will be described. Note that "%" representing the content of the component composition means "mass %".

[0017] <Component Composition> The component composition of the hot-rolled steel sheet of the present invention is, in mass%, C: 0.03 to 0.20%, Si: 0.1 to 2.0%, Mn: 0.5 to 3.5%, Ti: 0.005% or more and less than 0.205%, P: 0.100% or less, S: 0.02% or less, Al: less than 1.55%, with the remainder being Fe and unavoidable impurities.

[0018] C: 0.03-0.20% C is an effective element for increasing TS and YS by generating and strengthening bainite and martensite. If the C content is less than 0.03%, these effects are not sufficiently obtained, and a TS of 1180 MPa or higher cannot be obtained. Therefore, the C content should be 0.03% or more. From the viewpoint of obtaining a TS of 1180 MPa or higher more stably, the C content should preferably be 0.04% or more, more preferably 0.05% or more. The C content should still preferably be 0.06% or more. On the other hand, if the C content exceeds 0.20%, the desired metallic structure cannot be obtained in the region 50-150 μm from the surface of the steel sheet, and excellent bendability cannot be obtained. Therefore, the C content should be 0.20% or less. From the viewpoint of bendability, the C content should preferably be 0.18% or less, more preferably 0.15% or less. Even more preferably 0.13% or less.

[0019] Si: 0.1-2.0% Si is an effective element for increasing TS and YS by solid solution strengthening of steel and suppressing tempering softening of martensite. To obtain such effects, the Si content must be 0.1% or more. Preferably, the Si content is 0.2% or more, more preferably 0.4% or more, and even more preferably 0.5% or more. On the other hand, if the Si content exceeds 2.0%, the chemical treatment properties of the steel sheet decrease, and it becomes unusable as a steel sheet for automobile parts. Therefore, the Si content should be 2.0% or less. Preferably, it is 1.8% or less, and more preferably 1.5% or less. Even more preferably, the Si content is 1.4% or less.

[0020] Mn: 0.5-3.5% Mn is an effective element for increasing TS and YS by generating martensite and bainite. If the Mn content is less than 0.5%, diffusion transformation structures such as polygonal ferrites, which provide sufficient effects, are formed, and the metallic structure of the present invention cannot be obtained. Therefore, the Mn content should be 0.5% or more. The Mn content is preferably 1.0% or more, more preferably 1.2% or more. On the other hand, if the Mn content exceeds 3.5%, the desired metallic structure cannot be obtained in the surface layer of the steel sheet located in the range of 50-150 μm in the thickness depth direction from the surface of the steel sheet, and excellent bendability cannot be obtained. Therefore, the Mn content should be 3.5% or less. The Mn content is preferably 3.0% or less, more preferably 2.5% or less.

[0021] Ti: 0.005% or more and less than 0.205% Ti is an effective element for promoting the transformation from processed austenite to upper bainite on the steel sheet surface by forming carbides and suppressing recrystallization during finish rolling. To obtain such an effect, the Ti content must be 0.005% or more. The Ti content is preferably 0.020% or more, more preferably 0.025% or more. The Ti content is even more preferably 0.030% or more. On the other hand, if the Ti content is 0.205% or more, the amount of coarse Ti carbonitrides that become fracture initiation points increases, and the bendability decreases. Therefore, the Ti content should be less than 0.205%. The Ti content is preferably 0.150% or less, more preferably 0.120% or less. Even more preferably 0.114% or less.

[0022] P: 0.100% or less. Since P embrittles steel and promotes bending cracks, it is desirable to reduce its amount as much as possible. In this invention, a P content of up to 0.100% is acceptable. Therefore, the P content should be 0.100% or less. Preferably, the P content should be 0.030% or less. More preferably, it should be 0.025% or less, and even more preferably, 0.020% or less. There is no particular lower limit, but if the P content is less than 0.001%, the effect saturates, while it leads to a significant decrease in production efficiency and an increase in costs, so a P content of 0.001% or more is preferable.

[0023] S: 0.02% or less. Since sulfur (S) embrittles steel and promotes bending cracks, it is preferable to reduce its amount as much as possible. In this invention, an S content of up to 0.02% is acceptable. Therefore, the S content is 0.02% or less. Preferably, the S content is 0.0050% or less, more preferably 0.0040% or less. Even more preferably, it is 0.0034% or less. There is no particular lower limit, but since the effect saturates when the S content is less than 0.0002%, while it leads to a significant decrease in production efficiency and an increase in costs, an S content of 0.0002% or more is preferable.

[0024] Al: Less than 1.55% Al acts as a deoxidizing agent and is preferably added in the deoxidation process. From the viewpoint of using it as a deoxidizing agent, an Al content of 0.01% or more is preferable. On the other hand, if a large amount of Al is included, a large amount of polygonal ferrite will be formed, and the metal structure of the present invention cannot be obtained. In the present invention, an Al content of less than 1.55% is permissible. Therefore, the Al content is less than 1.55%. The Al content is preferably 0.30% or less, more preferably 0.10% or less. Even more preferably 0.08% or less.

[0025] The remainder consists of Fe and unavoidable impurities.

[0026] The above components constitute the basic component composition of the hot-rolled steel sheet of the present invention. In the present invention, the following elements may be optionally and appropriately included: One or more elements selected from the following: Cr: 0.05% or more and less than 2.05%, Mo: 0.05% or more and less than 2.05%, V: 0.05% or more and less than 1.05%, Cu: 0.05% or more and less than 4.05%, Ni: 0.005% or more and less than 2.050%, Nb: 0.005% or more and less than 0.205%, B: 0.0003 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010% or more and less than 0.1050%, Sn: 0.0010% or more and less than 0.5050%.

[0027] Cr, Mo, V, Cu, and Ni are effective elements that promote martensite formation and contribute to increased strength. To obtain this effect, when Cr, Mo, V, Cu, and Ni are included, the content of each element should be above the lower limit values ​​mentioned above. Specifically, when Cr, Mo, V, Cu, and Ni are included, the Cr content should be 0.05% or more, the Mo content 0.05% or more, the V content 0.05% or more, the Cu content 0.05% or more, and the Ni content 0.005% or more. The Cr content is preferably 0.10% or more. The Mo content is preferably 0.10% or more. The V content is preferably 0.10% or more. The Ni content is preferably 0.100% or more. On the other hand, if the content of each element Cr, Mo, V, Cu, and Ni exceeds the upper limit values ​​mentioned above, the desired bendability may not be obtained. Therefore, specifically, when Cr, Mo, V, Cu, and Ni are contained, the Cr content should be less than 2.05%, the Mo content less than 2.05%, the V content less than 1.05%, the Cu content less than 4.05%, and the Ni content less than 2.050%. Furthermore, the Cr content should preferably be 1.00% or less, more preferably 0.80% or less. The Mo content should preferably be 0.50% or less, more preferably 0.40% or less. The V content should preferably be 0.50% or less, more preferably 0.40% or less. The Cu content should preferably be 0.60% or less, more preferably 0.50% or less. The Ni content should preferably be 0.600% or less, more preferably 0.500% or less.

[0028] Nb is an effective element for increasing the strength of steel by forming carbides. It is also an effective element for suppressing recrystallization during finish rolling and promoting the transformation from processed austenite to upper bainite on the surface of the steel sheet. To obtain these effects, if Nb is included, the content should be 0.005% or more. Preferably, the Nb content should be 0.010% or more. More preferably, it should be 0.015% or more. On the other hand, if the Nb content exceeds 0.205%, the desired bendability may not be obtained. Therefore, if Nb is included, the Nb content should be less than 0.205%. Furthermore, preferably, the Nb content should be 0.150% or less. More preferably, it should be 0.100% or less.

[0029] B is an effective element that enhances the hardenability of steel sheets, promotes the formation of martensite, and contributes to increased strength. To obtain these effects, when B is included, the B content should be 0.0003% or more. Preferably, the B content is 0.0005% or more. More preferably, the B content is 0.0006% or more, and even more preferably, 0.0007% or more. On the other hand, if the B content exceeds 0.0050%, the amount of B-based compounds increases, the hardenability decreases, and the metallic structure of the present invention may not be obtained. Therefore, when B is included, the B content should be 0.0050% or less. Preferably, the B content is 0.0040% or less. More preferably, it is 0.0038% or less.

[0030] Ca and REM are elements that are effective in improving processability by controlling the morphology of inclusions. To obtain such effects, when Ca and REM are included, their respective contents should be Ca: 0.0001% or more and REM: 0.0001% or more. The Ca content is preferably 0.0005% or more. The REM content is preferably 0.0005% or more. On the other hand, if the Ca and REM contents exceed the above upper limits, the amount of inclusions may increase and processability may deteriorate. Therefore, when Ca and REM are included, the Ca content should be 0.0050% or less and the REM content should be 0.0050% or less. The Ca content is preferably 0.0030% or less. The Ca content is more preferably 0.0028% or less. The REM content is preferably 0.0030% or less. The REM content is more preferably 0.0028% or less. REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content referred to here is the total content of these elements.

[0031] Sb is an element that is effective in suppressing the decrease in steel strength by inhibiting denitrification, deboronization, etc. To obtain such effects, when Sb is included, the Sb content should be 0.0010% or more. Preferably, the Sb content is 0.0050% or more. More preferably, it is 0.0100% or more. If the Sb content exceeds the above upper limit, it may lead to embrittlement of the steel sheet. Therefore, when Sb is included, the Sb content should be less than 0.1050%. More preferably, the Sb content is 0.0500% or less. More preferably, it is 0.0450% or less.

[0032] Sn is an element that is effective in suppressing pearlite formation and preventing a decrease in steel strength. To obtain this effect, when Sn is included, the Sn content should be 0.0010% or more. Preferably, the Sn content is 0.0050% or more. More preferably, it is 0.0060% or more. If the Sn content exceeds the above upper limit, it may lead to embrittlement of the steel sheet. Therefore, the Sn content should be less than 0.5050%. Preferably, the Sn content is 0.0500% or less. More preferably, it is 0.0450% or less.

[0033] Even if the contents of Cr, Mo, V, Cu, Ni, Nb, B, Ca, REM, Sb, and Sn are less than the above lower limit values, the effects of the present invention are not impaired. Therefore, when the contents of these elements are less than the above lower limit values, these elements are treated as being included as inevitable impurities. In addition, examples of inevitable impurities other than these elements include N, Na, Mg, Zr, Hf, Ta, W, etc., and the total of these is 0.020% or less.

[0034] Next, the metallographic structure of the hot-rolled steel sheet of the present invention will be described. <Metallographic Structure> The metallographic structure of the hot-rolled steel sheet of the present invention contains martensite at an area ratio of 90% or more at the 1 / 4 plate thickness position. Also, in the steel sheet surface layer region located in the range of 50 to 150 μm in the plate thickness depth direction from the steel sheet surface, upper bainite is contained at an area ratio of 80% or more, and martensite is contained at an area ratio of 0 to 20%. The range of 50 to 150 μm in the plate thickness depth direction from the steel sheet surface as described above is the range from the position 50 μm inside in the plate thickness depth direction from the steel sheet surface to the position 150 μm inside in the plate thickness depth direction from the steel sheet surface.

[0035] At the 1 / 4 plate thickness position, martensite is 90% or more in area ratio. In the present invention, in order to obtain high TS and high YS, the metallographic structure is mainly composed of martensite except for the steel sheet surface layer region located in the range of 50 to 150 μm in the plate thickness depth direction from the steel sheet surface. For the metallographic structure of the part excluding the steel sheet surface layer region, the 1 / 4 plate thickness position may be observed. If the area ratio of martensite is less than 90%, the desired TS and YS cannot be obtained. Therefore, the area ratio of martensite is 90% or more. The area ratio of martensite is preferably 95% or more, more preferably 99% or more. The area ratio of martensite may be 100% or less. Examples of the remaining structure include upper bainite, lower bainite, and retained austenite. In addition, if the remaining structure listed above is 10% or less, it does not affect the properties, so the remaining structure is preferably 10% or less. The lower limit is not particularly limited, but the remaining structure may be 0% or more.

[0036] In the present invention, the metal structure in the surface region of the steel sheet, located in the range of 50 to 150 μm from the surface of the steel sheet in the thickness direction, is controlled so that the area ratio of upper bainite is 80% or more and the area ratio of martensite is 0 to 20% in order to suppress cracking in bending tests, especially end face cracking. The metal structure in the surface region of the steel sheet should be observed within the range described above. If the area ratio of upper bainite is less than 80%, end face cracking will occur in bending tests, and excellent bendability cannot be obtained. Therefore, the area ratio of upper bainite should be 80% or more. The area ratio of upper bainite is preferably 82% or more, more preferably 85% or more. Also, the area ratio of upper bainite may be 100% or less. Furthermore, the upper bainite structure on the surface of the steel sheet is a result of bainite transformation being accelerated by the processing strain accumulated during finish rolling, and therefore has an elongated shape in the rolling direction (average length of crystal grains in the rolling direction / average length in the thickness direction is 5 or more and 100 or less). In addition, if the area ratio of martensite in the surface of the steel sheet exceeds 20%, end cracks occur in bending tests, and excellent bendability cannot be obtained. Therefore, the area ratio of martensite should be 20% or less. Preferably, the area ratio of martensite is 18% or less, more preferably 15% or less. Also, the area ratio of martensite may be 0% or more. The remaining structure may include lower bainite and retained austenite.

[0037] The area ratios of martensite, upper bainite, and lower bainite refer to the ratios of the areas of each microstructure in the observed area. The area ratio of each microstructure is determined by cutting out samples from the obtained hot-rolled steel sheet, polishing the cross-section parallel to the rolling direction, corroding with 3% nital, taking three fields of view at a magnification of 1000 times with a SEM (scanning electron microscope) at the 1 / 4 position of the sheet thickness, and obtaining the area ratio of each microstructure from the image data of the secondary electron images using Image-Pro manufactured by Media Cybernetics, and taking the average area ratio of the three fields of view as the area ratio of each microstructure. For the surface layer region of the steel sheet, three fields of view are taken at a magnification of 1000 times with a SEM (scanning electron microscope) so that a region of 150 μm from the steel sheet surface is included, and the field of view from a position 50 μm to 150 μm in the sheet thickness depth direction from the steel sheet surface is selected. In the SEM image data, upper bainite is black or dark gray including carbides or martensite with a linear interface, lower bainite is black or dark gray or gray or light gray including carbides with aligned orientations, martensite is black or dark gray or gray or light gray or white or light gray without carbides including carbides with multiple orientations, and retained austenite is white or light gray without carbides. Since martensite and retained austenite may not be distinguishable, the retained austenite is determined by the method described below, and the area ratio of martensite is obtained by subtracting the area ratio of retained austenite from the total area ratio of martensite and retained austenite obtained from the SEM image.

[0038] The area ratio of retained austenite is determined by grinding the annealed steel sheet to 1 / 4 + 0.1 mm of the sheet thickness, further polishing 0.1 mm by chemical polishing, measuring the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron (ferrite) using the Kα1 line of Mo with an X-ray diffractometer for the polished surface, obtaining the volume ratio from the intensity ratio of the integrated reflection intensities from each plane of fcc iron to the integrated reflection intensities from each plane of bcc iron, and taking this as the area ratio of retained austenite.

[0039] Furthermore, although not generally included in the present invention, ferrite is a structure that is black or dark gray and does not contain carbides internally, or contains only a small amount, or does not have martensite with linear interfaces, while pearlite can be distinguished as a black and white layered or partially interrupted layered structure.

[0040] In the surface region of the steel sheet, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less in equivalent circular diameter is 0.010 to 0.050 mass%. Ti and Nb precipitates with a particle size of 100 nm or less in equivalent circular diameter inhibit dislocation movement and delay the formation of dislocation cell structures and bundle structures, which are precursor processes for fatigue crack initiation. To obtain such an effect, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less must be 0.010 mass% or more. Preferably, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less is 0.011 mass% or more, and more preferably 0.012 mass% or more. The total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less refers to the total amount of Ti and Nb contained in precipitates with a particle size of 100 nm or less. On the other hand, if the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less exceeds 0.050 mass%, the strength of the steel sheet surface layer increases excessively, and the bendability decreases. Therefore, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less should be 0.050 mass% or less. Preferably, it should be 0.045 mass% or less, and more preferably 0.040 mass% or less.

[0041] The total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less is determined by the following method. Specifically, a test piece is prepared by grinding the surface of a steel plate to a thickness of 50 μm and protecting areas other than the ground surface with vinyl tape to prevent dissolution by the electrolyte. Constant current electrolysis is then performed in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol). At this time, the current density is 20 mA / cm². ２As for the electrolysis amount, it is set to about 0.2 g. After the test piece after electrolysis with deposits adhering to the surface is taken out from the electrolytic solution and immersed in an aqueous sodium hexametaphosphate solution (500 mg / L) (hereinafter referred to as SHMP aqueous solution), ultrasonic vibration is applied to peel off the deposits from the test piece and extract them into the SHMP aqueous solution. Then, the SHMP aqueous solution containing the deposits is filtered using a filter with a pore size of 100 nm. Next, after the filtrate is dried, nitric acid, perchloric acid, and sulfuric acid are added for acid decomposition, and the decomposition solution is analyzed with an ICP emission spectroscopic analyzer to measure the absolute values of Ti and Nb in the decomposition solution. The absolute values of the obtained Ti and Nb are divided by the electrolyte mass to obtain the amounts of Ti and Nb (mass %) contained in the deposits with a particle size of 100 nm or less (mass % when the total composition of the test piece is 100 mass %). Next, the sum of the obtained amounts of Ti (mass %) and Nb (mass %) is divided by the sum of the contained amounts of Ti (mass %) and contained amount of Nb (mass %) in the test piece, and the total amount of Ti (mass %) present as deposits containing Ti with a particle size of 100 nm or less and the total amount of Nb (mass %) present as deposits containing Nb with a particle size of 100 nm or less are obtained. The electrolyte mass is determined by measuring the mass of the test piece after deposit peeling and subtracting it from the mass of the test piece before electrolysis.

[0042] The dislocation density in the surface layer region of the steel sheet is 3.0×10 １５ / m ２ Hereinafter, upper bainite, which is the main metal structure in the surface layer region of the steel sheet of the present invention, is a structure having a high dislocation density. In a structure with a high dislocation density, the distance between dislocations is small, so when repeatedly loaded, a dislocation cell structure or a bundle structure is likely to be formed. Therefore, in order to obtain a high fatigue strength, it is effective to reduce the dislocation density. In the present invention, the dislocation density in the surface layer region of the steel sheet is set to 3.0×10 １５ / m ２ or less. Preferably, it is 2.0×10 １５ / m ２ or less. Since the lower the dislocation density, the better, there is no need to define a lower limit. However, in upper bainite, the dislocation density can be reduced within an industrially achievable range of 5.0×10 １４ / m ２To this extent. Here, since it is difficult to measure the dislocation density for each phase, the dislocation density of the present invention is defined as that of all phases located in the range of 50 to 150 μm in the thickness direction from the surface of the steel plate.

[0043] The dislocation density is determined by the following method: A test specimen is prepared by grinding and polishing the surface of a steel plate to 50 μm. The polished surface is irradiated with X-rays (Cu-Kα rays), and the resulting diffraction profile is analyzed using the CMWP method shown in Reference 1 to calculate the dislocation density. [Reference 1] G. Ribarik, J. Gubicza and T. Ungar: Mater. Sci. Eng. A, 387-389(2004), 343

[0044] As described above, the hot-rolled steel sheet of the present invention has excellent bendability, which will be explained in detail below. The bending test will be conducted as described in the examples. Figure 1 shows a schematic diagram illustrating the bending test. Specifically, as will be described later, the bending test involves cutting out a bending test piece 1 by shearing, leaving the end face created by the shearing process, setting the bending test piece 1 so that the fracture surface side of the sheared end face is on the outside of the bend, and then performing the bending process. Since cracks will occur as described later if the bending radius is small, the bending test is performed on the bending test piece 1 using various bending radii and various plate thicknesses t of bending test pieces to select the limit bending radius R at which no cracks occur. In Figure 2, (a) is a schematic diagram observing the cross-section of the bending test piece after the bending test, and (b) is a schematic diagram observing the side of the bending test piece from the outside of the bend. Cracks are mainly classified into two types: end face cracks 10 and ridge cracks 12. The cross-section of the bending test specimen 1 observed in Figure 2(a) is the shear end face 14, and a face crack 10 occurs from the outer side of the bend due to the bending process. Reference numeral 15 indicates the fracture surface side. The face crack 10 occurs in the thickness direction of the plate, or along a direction perpendicular to the cross-section of the bending test specimen 1 in Figure 2(a). The direction perpendicular to the cross-section of the bending test specimen 1 corresponds to the rolling direction 11 of the steel plate in Figure 2(b). In addition, a ridge crack 12 occurs along the rolling direction 11 at a position corresponding to the apex 13 of the bending process in the view of the bending test specimen 1 shown in Figure 2(b). The presence or absence of cracks is determined by the method described later in the examples. The bending radius at which no cracks, including face cracks 10 and ridge cracks 12, occur in the bending test is defined as the limit bending radius R, and a hot-rolled steel plate with R / t (limit bending radius R divided by plate thickness t) of 2.5 or less is judged to have excellent bendability. The lower limit of R / t is not particularly limited, but it may be 0.1 or greater.

[0045] The hot-rolled steel sheet of the present invention may be either a hot-rolled steel sheet with black scale as it is, or a hot-rolled steel sheet with white scale that has been further pickled after hot rolling. Furthermore, the hot-rolled steel sheet of the present invention preferably has a thickness of 10.0 mm or less. In particular, when the hot-rolled steel sheet of the present invention is used as a material for automobile parts, a thickness of 1.0 mm or more is preferred. A thickness of 1.5 mm or more is more preferred, and a thickness of 2.0 mm or more is even more preferred. Furthermore, when the high-strength hot-rolled steel sheet of the present invention is used as a material for automobile parts, a thickness of 6.0 mm or less is even more preferred.

[0046] Furthermore, the width of the hot-rolled steel sheet of the present invention is preferably 500 mm or more, and more preferably 700 mm or more. The width of the high-strength hot-rolled steel sheet of the present invention is preferably 1800 mm or less, and more preferably 1400 mm or less.

[0047] <Method for Manufacturing Hot-Rolled Steel Sheets> The hot-rolled steel sheet of the present invention is manufactured by heating a slab having the above-mentioned component composition, performing rough rolling, and then performing finish rolling, in which the total reduction strain Sr (formula (2) below) of each rolling pass is set to 1.0 to 3.0, in which the entry temperature Tn of each rolling pass satisfies the following formula (1), and the rolling is performed with the reduction amount of each pass set to 1.0 to 3.0, the time from the end of the finish rolling to the start of cooling being 10.0 s or less, and the temperature range from the start of cooling to 300°C being cooled at an average cooling rate of 50°C / s or more, and winding at 300°C or below. A detailed explanation follows below. Note that the above-mentioned temperature is the surface temperature at the center of the width of the steel sheet, and unless otherwise specified, the above-mentioned average cooling rate is [(start of cooling - stop of cooling) / cooling time from start of cooling to stop of cooling].

[0048] Steel having the above-mentioned composition is melted using known methods such as converters, electric furnaces, and vacuum melting furnaces, and cast into steel billets (slabs) using known methods such as continuous cooling or ingot-inflesh casting. The slabs are then heated, either directly or after cooling, and subjected to rough rolling. The conditions for rough rolling do not need to be specifically defined and can be carried out according to conventional methods. After rough rolling, finish rolling is performed under predetermined conditions.

[0049] The total reduction strain Sr (formula (2) below) of a rolling pass performed under conditions where the entry temperature Tn satisfies the following formula (1) is 1.0 to 3.0. Finish rolling under the above conditions is important for obtaining the desired microstructure in the region 50 to 150 μm from the surface of the steel sheet. If ΣSr is less than 1.0, the microstructure in the surface layer of the steel sheet located in the range of 50 to 150 μm in the thickness direction from the surface of the steel sheet becomes martensite or lower bainite, and the desired microstructure cannot be obtained. Therefore, ΣSr should be 1.0 or higher. Preferably, ΣSr should be 1.3 or higher. Furthermore, if ΣSr exceeds 3.0, strain-induced precipitation occurs, and the precipitates grow during post-heat treatment, reducing the amount of precipitates with an equivalent circle diameter of 100 nm or less. As a result, the total amount of Ti and Nb contained in the precipitates with a particle size of 100 nm or less becomes less than 0.010 mass%, and the fatigue strength decreases. Therefore, the sum of Sr ΣSr must be 3.0 or less. Tn ≤ 74.35 × t －０．６８５ -500{0.1-(Ti+Nb)}+950...(1) Here, t is the time it takes for the slab to pass from the current finishing rolling stand to the next stand (or the time until cooling begins in the case of the final pass). Also, each element symbol in equation (1) represents the mass %) of each element in the slab, and is set to 0 if it is not present. The reduction strain Sr is defined by the following equation (2): Sr = ln(WTe / WTd)...(2) Here, WTe is the thickness of the slab on the entry side of the pass (mm), and WTd is the thickness of the slab on the exit side of the pass (mm).

[0050] Time from the end of finish rolling to the start of cooling: 10.0 s or less. If the time from the end of finish rolling to the start of cooling exceeds 10.0 s, the recovery of dislocations on the surface of the steel sheet is accelerated, and the metallic structure of the present invention cannot be obtained in the surface of the steel sheet located in the range of 50 to 150 μm in the thickness direction from the surface of the steel sheet. Therefore, the time from the end of finish rolling to the start of cooling (time from the end of finish rolling to the start of cooling) should be 10.0 s or less. Preferably, the time from the end of finish rolling to the start of cooling should be 8.0 s or less. Furthermore, it is preferable that the lower limit of the time from the end of finish rolling to the start of cooling should be 0.1 s or more. Note that air cooling means exposure to the atmosphere (air cooling) without performing active cooling (accelerated cooling) by pouring water or the like.

[0051] Average cooling rate in the temperature range from the cooling start temperature to 300°C: 50°C / s or more If the average cooling rate in the temperature range from the cooling start temperature to 300°C after the completion of finish rolling is less than 50°C / s, upper bainite, ferrite, and pearlite will form at the 1 / 4 thickness position of the sheet, and the metal structure of the present invention cannot be obtained. Therefore, the average cooling rate in the temperature range from the cooling start temperature (accelerated cooling start temperature) to 300°C should be 50°C / s or more. The average cooling rate should preferably be 80°C / s or more, and more preferably 100°C / s or more. There is no particular upper limit specified for the average cooling rate, but from the viewpoint of the shape stability of the steel sheet, the average cooling rate should preferably be 1000°C / s or less. More preferably 800°C / s or less. The cooling start temperature is, for example, the finish rolling completion temperature (rolling temperature of the final pass).

[0052] Winding temperature: 300°C or less. If the winding temperature exceeds 300°C, a tensile strength of over 1180 MPa cannot be reliably obtained. Therefore, the winding temperature should be 300°C or less. Preferably, the winding temperature is 280°C or less, and more preferably 250°C or less. The lower limit is not particularly limited, but preferably the winding temperature is 0°C or higher, and more preferably 25°C or higher.

[0053] Other than the conditions for the manufacturing method described above, there are no particular limitations, but it is preferable to adjust the conditions as appropriate during manufacturing as follows. For example, the heating temperature of the slab is preferably 1100°C or higher from the viewpoint of segregation removal and precipitate solid solution, and more preferably 1150°C or higher. Also, from the viewpoint of energy efficiency, it is preferable to be 1300°C or lower. Finish rolling is preferably 5 passes or more from the viewpoint of reducing coarse grains that would otherwise lead to a decrease in workability. There is no particular upper limit, but it is preferable to be 7 passes or less. After winding, the steel sheet is subjected to heat treatment. The heat treatment may be performed after winding, for example, after cooling to room temperature, or it may be performed immediately after winding. Room temperature refers to -30 to 60°C, and more preferably 0 to 40°C.

[0054] Heat treatment temperature: In the temperature range of 400°C to 750°C, holding for 0 to 3600 s. If the heat treatment temperature is less than 400°C, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less is less than 0.010 mass%, and the dislocation density is 3.0 × 10⁻⁶. １５ / m ２ If the temperature exceeds a certain limit, the desired surface metallic structure cannot be obtained. Therefore, the heat treatment temperature should be 400°C or higher. A heat treatment temperature of 420°C or higher is preferable, and 450°C or higher is more preferable. If the heat treatment temperature exceeds 750°C, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less exceeds 0.050 mass%, and the desired surface metallic structure cannot be obtained. Therefore, the heat treatment temperature should be 750°C or lower. A heat treatment temperature of 740°C or lower is preferable, and 730°C or lower is more preferable. The desired metallic structure can be obtained even if the holding time at the heat treatment temperature is 0 s, that is, if heating is stopped immediately after reaching the predetermined heat treatment temperature. Therefore, the holding time at the above heat treatment temperature should be 0 s or more. A holding time of 30 s or more is preferable, and 60 s or more is more preferable. On the other hand, if the material is held at the predetermined heat treatment temperature for more than 3600 s, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less becomes less than 0.010 mass%. Therefore, the holding time at the above heat treatment temperature should be 3600 s or less. Preferably, the holding time should be 2700 s or less, and more preferably 1800 s or less. The heat treatment method is not limited, but equipment such as a continuous annealing furnace, induction heating device, or batch furnace can be used. After heat treatment, the material may be allowed to cool to room temperature in the atmosphere, or air cooling or water cooling may be performed as needed.

[0055] Slabs of steel with the component composition shown in Table 1 were melted in a vacuum melting furnace and subjected to finish rolling, cooling, accelerating cooling, coiling, and heat treatment under the conditions shown in Table 2 to produce hot-rolled steel sheets. The total number of passes for finish rolling was seven. Using the obtained hot-rolled steel sheets, microstructure observation, tensile tests, and bending tests (press brake processing) were performed according to the following test methods.

[0056] From the steel sheet obtained for microstructural observation, a specimen for microstructural observation was taken so that the cross-section parallel to the rolling direction was the observation surface. The surface of the obtained specimen was polished, and the surface was further corroded using an etching solution (3% nital solution) to reveal the metallic structure. Targeting the surface layer of the steel sheet located at the 1 / 4 position and in the thickness depth direction from the steel sheet surface to a depth of 50 to 150 μm, an SEM was used to obtain SEM images of the metallic structure by taking three field images at 1000x magnification. The SEM images obtained by the above method were analyzed by image processing, and the area ratios of martensite, upper bainite, and lower bainite were quantified. Here, since it is difficult to distinguish the area ratios of martensite and retained austenite dispersed in the metallic structure using SEM, as described above, the total area ratio of martensite and retained austenite was first determined from the SEM image, and the area ratio of retained austenite was separately determined by X-ray diffraction. The area ratio of martensite was then quantified by subtracting the area ratio of retained austenite from the total area ratio.

[0057] From the hot-rolled steel sheet obtained from the tensile test, a JIS No. 5 tensile test was taken so that the tensile direction was perpendicular to the rolling direction according to JIS Z 2201, and the strain rate was set to 10 according to JIS Z 2201. －３ A tensile test was performed using a setting of / s to determine the tensile strength. In this invention, a tensile strength of 1180 MPa or higher was considered acceptable.

[0058] From the hot-rolled steel sheet obtained from the bending test, a rectangular test piece measuring 200 mm in the rolling direction and 100 mm in the rolling width direction was cut by shearing. Leaving the end face created by the shearing process intact, the test piece was set so that the fracture surface side of the sheared end face was on the outside of the bend, and press brake processing was performed. An Amada hydraulic bending machine was used for press brake processing. Here, in a typical 90° V bending test as described in JIS Z 2248, the effects of shearing or gas cutting are removed from the end face of the test piece as needed, and in most cases the test is conducted in accordance with this. However, in this invention, in order to reproduce the processing conditions of an actual part, the bending test was conducted with the sheared end face intact, i.e., under conditions unfavorable to bending cracks. Other conditions were tested based on the conditions described in JIS Z 2248. Regarding cracks, as described in JIS Z 2248 above, the presence or absence of cracks was determined by visually confirming that no cracks had occurred. A passing grade was defined as a plate with a passing grade of R / t, which is the limit bending radius R at which no cracks such as ridge cracks or end face cracks occur, divided by the plate thickness t, and is 2.5 or less.

[0059] From the hot-rolled steel sheet obtained from the planar bending fatigue test, specimens were taken so that the longitudinal direction of the specimen was perpendicular to the rolling direction, and the planar bending fatigue test was performed in accordance with the provisions of JIS Z 2275. The stress loading mode was set to a stress ratio R = -1 and a frequency f = 25 Hz. The stress-fracture life (S-N) curve was obtained by changing the load stress amplitude, and 1 × 10⁻⁶ ７ The time intensity for each trial was determined.

[0060] The examples of the invention are all hot-rolled steel sheets having a tensile strength of 1180 MPa or more, excellent bendability, and high fatigue strength. On the other hand, the comparative examples that fall outside the scope of the present invention either do not have a tensile strength of 1180 MPa or more, do not have excellent bendability, or do not have high fatigue strength.

[0061]

[0062]

[0063]

[0064] ​1. Bending test specimen 2. Bending radius R 3. Plate thickness 10. End face crack 11. Rolling direction 12. Ridge crack 13. Bending apex 14. Shear end face 15. Fracture surface side

Claims

1. The composition is as follows, by mass%, C: 0.03-0.20%, Si: 0.1-2.0%, Mn: 0.5-3.5%, Ti: 0.005% or more and less than 0.205%, P: 0.100% or less, S: 0.02% or less, Al: less than 1.55%, with the remainder being Fe and unavoidable impurities; the microstructure is such that at the 1 / 4 position of the plate thickness, martensite accounts for 90% or more by area; in the surface layer of the steel plate located in the range of 50-150 μm from the surface of the steel plate in the thickness direction, upper bainite accounts for 80% or more by area and martensite accounts for 0-20% by area; and the total amount of Ti and Nb contained in precipitates with a particle size of 100 nm or less in the equivalent circle diameter in the steel plate surface layer is 0.010-0.050% by mass. The dislocation density on the surface layer of the steel plate is 3.0 × 10 １５ / m ２ The following are hot-rolled steel sheets.

2. The hot-rolled steel sheet according to claim 1, wherein the component composition further includes one or more selected from, in mass%, Cr: 0.05% or more and less than 2.05%, Mo: 0.05% or more and less than 2.05%, V: 0.05% or more and less than 1.05%, Cu: 0.05% or more and less than 4.05%, Ni: 0.005% or more and less than 2.050%, Nb: 0.005% or more and less than 0.205%, B: 0.0003 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010% or more and less than 0.1050%, and Sn: 0.0010% or more and less than 0.5050%.

3. A method for manufacturing a hot-rolled steel sheet, comprising: heating a slab having the component composition described in claim 1 or 2; performing rough rolling; and in subsequent finish rolling, setting the reduction amount for each rolling pass such that the sum of the reduction strains Sr ΣSr of the rolling passes performed at an entry temperature Tn satisfying formula (1) is between 1.0 and 3.0; making the time from the end of the finish rolling to the start of cooling 10.0 s or less; cooling the temperature range from the start of cooling after the end of the finish rolling to 300°C at an average cooling rate of 50°C / s or more; setting the winding temperature to 300°C or less; and then performing a heat treatment in which the temperature range is between 400°C and 750°C for 0 to 3600 s. Tn ≤ 74.35 × t －０．６８５ -500{0.1-(Ti+Nb)}+950...(1) Here, t is the time it takes for the slab to pass from the current finishing rolling stand to the next stand (or the time until cooling begins in the case of the final pass). Also, each element symbol in equation (1) represents the mass %) of each element in the slab, and is set to 0 if it is not present. The reduction strain Sr is defined by the following equation (2): Sr = ln(WTe / WTd)...(2) Here, WTe is the thickness of the slab on the entry side of the pass (mm), and WTd is the thickness of the slab on the exit side of the pass (mm).

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