Hot-rolled steel sheet and method for manufacturing the same

Abstract

The present invention aims to provide a hot-rolled steel sheet having a tensile strength of 1180 MPa or more, as well as excellent bendability and high fatigue strength, and a method for manufacturing the same. The metal has a specific component composition, and its microstructure is such that, at the 1 / 4 position of the plate thickness, martensite accounts for 90% or more by area, and in the surface layer of the steel plate located in the range of 50 to 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 to 20% by area, the total amount of Ti and Nb contained in precipitates with a particle size of 100 nm or less in equivalent circle diameter in the surface layer of the steel plate is 0.010 to 0.050 mass%, and the dislocation density in the surface layer of the steel plate is 3.0 × 10⁻⁶ 15 / m 2 The following are hot-rolled steel sheets.

Description

[Technical Field]

[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. [Background technology]

[0002] In the industrial sector, technological development aimed at reducing CO2 emissions is continuously progressing to address climate change. 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. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Special Publication No. 2021-507107 [Patent Document 2] International Publication No. 2020 / 196060 [Patent Document 3] International Publication No. 2014 / 188966 [Overview of the Initiative] [Problems that the invention aims to solve]

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

[0009] This invention has been made in view of these 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. [Means for solving the problem]

[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] 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, The composition has less than 1.55% Al, with the remainder being Fe and unavoidable impurities. The metallographic structure is such that at the 1 / 4 plate thickness position, martensite is 90% or more in area ratio, in the surface layer of the steel plate located in the range of 50 to 150 μm in the plate thickness depth direction from the steel plate surface, upper bainite is 80% or more in area ratio and martensite is 0 to 20% in area ratio, in the precipitates with a particle size of 100 nm or less in equivalent circle diameter in the surface layer of the steel plate, the total amount of Ti and Nb contained is 0.010 to 0.050% by mass, the dislocation density in the surface layer of the steel plate is 3.0×10 15 / m 2 or less, and it is a hot-rolled steel plate. [2] The component composition further includes, by 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%, Sn: 0.0010% or more and less than 0.5050%, and includes one or more selected from the above, and it is the hot-rolled steel plate according to [1]. [3] Heating a slab having the component composition according to [1] or [2], performing rough rolling, and then in the finish rolling, among each rolling pass, setting the reduction amount of each pass so that the total ΣSr of the reduction strain Sr of the rolling pass performed at the inlet temperature Tn satisfying the formula (1) is 1.0 to 3.0, and performing rolling, the time from the end of the finish rolling to the start of cooling is 10.0 s or less, and cooling is performed under the condition that the average cooling rate is 50 °C / s or more in the temperature range from the start temperature of cooling after the end of the finish rolling to 300 °C, the coiling temperature is set to 300 °C or less, and then heat treatment is performed by holding in the temperature range of 400 °C or more and 750 °C or less for 0 to 3600 s, which is a manufacturing method of a hot-rolled steel plate. 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 0 is used if the element 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 entry side plate of the path (mm), and WTd is the thickness of the exit side plate of the path (mm). [Effects of the Invention]

[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 providing significant industrial benefits. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a schematic diagram illustrating the bending test method according to the present invention. [Figure 2] (a) is a schematic diagram of the cross-section of the specimen after the bending test, and (b) is a schematic diagram of the side view of the bending test specimen from the outer bending surface. [Modes for carrying out the invention]

[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 plate> 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 "%" used to represent the content of the component composition means "mass percent".

[0017] <Component composition> The component composition of the hot-rolled steel sheet of the present invention is, 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.

[0018] C: 0.03~0.20% Carbon (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 higher. From the viewpoint of obtaining a TS of 1180 MPa or higher more stably, the C content should preferably be 0.04% or higher, more preferably 0.05% or higher. The C content should still preferably be 0.06% or higher. On the other hand, if the C content exceeds 0.20%, the desired metallic structure cannot be obtained in the region 50 to 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 conversion treatment properties of the steel sheet deteriorate, making it unsuitable for use 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 are formed, which do not sufficiently produce these effects, and the metallic structure of the present invention cannot be obtained. For this reason, the Mn content should be 0.5% or more. Preferably, the Mn content is 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 to 150 μm in the thickness depth direction from the surface of the steel sheet, and excellent bendability cannot be obtained. For this reason, the Mn content should be 3.5% or less. Preferably, the Mn content is 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. Preferably, the Ti content is 0.020% or more, more preferably 0.025% or more. Even more preferably, the Ti content is 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%. Preferably, the Ti content is 0.150% or less, more preferably 0.120% or less. Even more preferably, it is 0.114% or less.

[0022] P:0.100% or less Since phosphorus (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 a significant decrease in production efficiency and an increase in costs occur, 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 should be 0.02% or less. Preferably, the S content should be 0.0050% or less, more preferably 0.0040% or less. Even more preferably, it should be 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 this 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 metallic 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 should be less than 1.55%. The Al content should preferably be 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 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% to less than 2.05%, Mo: 0.05% to less than 2.05%, V: 0.05% to less than 1.05%, Cu: 0.05% to less than 4.05%, Ni: 0.005% to less than 2.050%, Nb: 0.005% to 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% to less than 0.1050%, Sn: 0.0010% to 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 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 mentioned above, the desired bendability may not be obtained. Therefore, specifically, when Cr, Mo, V, Cu, and Ni are included, 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 martensite formation, 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 an effect, 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 this effect, 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] Furthermore, even if the content of Cr, Mo, V, Cu, Ni, Nb, B, Ca, REM, Sb, and Sn is below the above lower limit, it will not impair the effects of the present invention. Therefore, if the content of these elements is below the above lower limit, these elements will be treated as unavoidable impurities. Other unavoidable impurities include N, Na, Mg, Zr, Hf, Ta, W, etc., but their total content should be 0.020% or less.

[0034] Next, the microstructure of the hot-rolled steel sheet of the present invention will be described. <Metal structure> The metal structure of the hot-rolled steel sheet of the present invention contains 90% or more martensite by area at the 1 / 4 thickness position. Furthermore, in the surface layer region of the steel sheet located in the range of 50 to 150 μm from the surface in the thickness depth direction, it contains 80% or more upper bainite by area and 0 to 20% martensite by area. The range of 50 to 150 μm from the surface in the thickness depth direction described above is the range from a position 50 μm inward from the surface in the thickness depth direction to a position 150 μm inward from the surface in the thickness depth direction.

[0035] At the 1 / 4 thickness point, martensite accounts for 90% or more of the area. In this invention, in order to obtain high TS and high YS, the metal structure is mainly composed of martensite, except for the surface region of the steel sheet located in the depth direction of the sheet thickness from the surface. The metal structure of the portion excluding the surface region of the steel sheet can be observed at the 1 / 4 position of the sheet thickness. If the area ratio of martensite is less than 90%, the desired TS and YS cannot be obtained. Therefore, the area ratio of martensite should be 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. It is preferable that the remaining structure be 10% or less, as this does not affect the properties. There is no particular lower limit, but the remaining structure may be 0% or more.

[0036] In the surface region of the steel sheet, located 50-150 μm from the surface in the thickness direction, the metallic structure consists of upper bainite accounting for 80% or more of the area, and martensite accounting for 0-20% of the area. In this invention, in order to suppress cracking, particularly end-face cracking, in bending tests, the microstructure in a region is controlled so that the area ratio of upper bainite is 80-100%. The microstructure of the steel sheet surface region should be observed within the range described above. If the area ratio of upper bainite is less than 80%, end-face cracking occurs 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. Note that the upper bainite structure on the surface of the steel sheet is a result of bainite transformation being promoted by processing strain accumulated by 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). Furthermore, if the area ratio of martensite exceeds 20% in the surface layer of the steel sheet, 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. Examples of the remaining structure include lower bainite and retained austenite.

[0037] The area ratios of martensite, upper bainite, and lower bainite refer to the proportion of the area occupied by each tissue within the observed area. To determine the area ratio of each tissue, a sample is cut from the obtained hot-rolled steel sheet, the thickness cross section parallel to the rolling direction is polished, and then etched with 3% nital. Three fields of view are taken at 1 / 4 of the sheet thickness using a scanning electron microscope (SEM) at 1000x magnification. The area ratio of each tissue is then determined from the resulting secondary electron image data using Image-Pro from Media Cybernetics, and the average area ratio of the three fields of view is used as the area ratio of each tissue. For the surface region of the steel sheet, three fields of view are taken at 1000x magnification using a scanning electron microscope (SEM) so that the area from the steel sheet surface up to 150 μm is included, and the field of view is selected from 50 μm to 150 μm in the thickness direction from the steel sheet surface. In SEM image data, upper bainite is distinguished as black or dark gray containing carbides or martensite with linear interfaces, lower bainite as black, dark gray, gray, or light gray containing oriented carbides, martensite as black, dark gray, gray, or light gray containing carbides in multiple orientations, or white or light gray without carbides, and retained austenite as white or light gray without carbides. Since martensite and retained austenite may not be distinguishable, retained austenite was determined by the method described later, and the area ratio of martensite was 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 was determined by grinding annealed steel plates to 1 / 4 of their thickness + 0.1 mm, then chemically polishing the surface to a further 0.1 mm. Using an X-ray diffractometer with Mo Kα1 rays, the integrated reflectance intensities of the (200), (220), and (311) faces of fcc iron (austenite) and the (200), (211), and (220) faces of bcc iron (ferrite) were measured. The volume fraction was calculated from the intensity ratio of the integrated reflectance intensity from each face of fcc iron to the integrated reflectance intensity from each face of bcc iron, and this was defined 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 equivalent to 100 nm or less inhibit dislocation movement and delay the formation of dislocation cell structures and bundle structures, which are precursor processes for fatigue crack initiation. To obtain this 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 the 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. 2 The amount of electrolyte used is approximately 0.2 g. After electrolysis, the test specimen with precipitates attached to its surface is removed from the electrolyte and immersed in a sodium hexametaphosphate aqueous solution (500 mg / L) (hereinafter referred to as SHMP aqueous solution). Ultrasonic vibration is applied to detach the precipitate from the test specimen and extract it into the SHMP aqueous solution. Then, the SHMP aqueous solution containing the precipitate is filtered using a filter with a pore size of 100 nm. Next, after the filtrate is allowed to dry, nitric acid, perchloric acid, and sulfuric acid are added to perform acid decomposition. The decomposition solution is analyzed using an ICP emission spectrometer, and the absolute values ​​of Ti and Nb in the decomposition solution are measured. The obtained absolute values ​​of Ti and Nb are divided by the amount of electrolyte to determine the amount of Ti and Nb contained in the precipitates with a particle size of 100 nm or less (mass %) when the total composition of the test specimen is set to 100 mass %. Next, the sum of the obtained Ti (mass%) and Nb (mass%) amounts is divided by the sum of the Ti (mass%) and Nb (mass%) amounts contained in the test specimen to obtain the sum of the Ti (mass%) amount present as precipitates containing Ti with a particle size of 100 nm or less and the Nb (mass%) amount present as precipitates containing Nb with a particle size of 100 nm or less. The electrolyte amount is determined by measuring the mass of the test specimen after precipitate removal and subtracting it from the mass of the test specimen before electrolysis.

[0042] The dislocation density in the surface region of the steel plate is 3.0 × 10⁻⁶ 15 / m 2 below The 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-like structure is likely to be formed. Therefore, in order to obtain 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 set a lower limit. However, in upper bainite, the dislocation density can be reduced within an industrially feasible range up to about 5.0×10 14 / m 2 . Here, since it is difficult to measure the dislocation density of each phase, the dislocation density of the present invention is that of all phases located in the range of 50 to 150 μm in the plate thickness depth direction from the steel sheet surface.

[0043] The dislocation density is determined by the following method. Prepare a test piece obtained by grinding and polishing the steel sheet surface by 50 μm, irradiate the polished surface with X-rays (Cu-Kα rays), and analyze and calculate the obtained diffraction profile by the CMWP method shown in Reference 1. [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 cracking will occur as described later if the bending radius is small, the bending test will be conducted 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 cracking occurs. In Figure 2, (a) is a schematic diagram showing the cross-section of a bent specimen after a bending test, and (b) is a schematic diagram showing the side view of the bent specimen from the outer bending surface. Cracks are mainly classified into two types: end face cracks 10 and ridge cracks 12. The cross-section of the bent specimen 1 observed in Figure 2(a) is the shear end face 14, and end face cracks 10 occur from the outer bending surface due to the bending process. Reference numeral 15 indicates the fracture surface side. End face cracks 10 occur in the thickness direction of the plate, or along a direction perpendicular to the cross-section of the bent specimen 1 in Figure 2(a). The direction perpendicular to the cross-section of the bent specimen 1 corresponds to the rolling direction 11 of the steel plate in Figure 2(b). Also, in the diagram shown in Figure 2(b) as observed from the outer bending surface of the bent specimen 1, ridge cracks 12 occur along the rolling direction 11 at a position corresponding to the apex 13 of the bending process. The presence or absence of cracks is determined by the method described later in the examples. In the bending test, the bending radius at which no cracks, including end face cracks 10 and ridge cracks 12, occur was defined as the critical bending radius R. Hot-rolled steel sheets with a R / t (critical bending radius R divided by plate thickness t) of 2.5 or less were 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 hot-rolled) or a hot-rolled steel sheet with white scale (as hot-rolled and further pickled). 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] <Manufacturing method for 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 amount of reduction for each rolling pass is set so that the sum of the reduction strain Sr (formula (2) below) of each rolling pass performed under the condition that the entry temperature Tn satisfies the following formula (1) is between 1.0 and 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 to 300°C is cooled at an average cooling rate of 50°C / s or more, and the sheet is wound up at 300°C or below. The following provides a detailed explanation. Note that the temperature mentioned above is the surface temperature at the center of the width of the steel plate, and unless otherwise specified, the average cooling rate mentioned above is [(cooling start temperature - cooling stop temperature) / cooling time from cooling start to cooling stop].

[0048] Steel having the above-mentioned component 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 (Equation (2) below) of a rolling pass performed under conditions where the entry temperature Tn satisfies Equation (1) below is ΣSr, and is between 1.0 and 3.0. Finish rolling under the above conditions is crucial for obtaining the desired microstructure in the region 50-150 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 50-150 μm from the surface in the thickness direction will be 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 during post-heat treatment, the precipitates grow, and the number of precipitates with an equivalent circle diameter of 100 nm or less decreases. As a result, the total amount of Ti and Nb contained in precipitates with a particle size of 100 nm or less becomes less than 0.010 mass%, and the fatigue strength decreases. Therefore, the total Sr ΣSr should 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 0 is used if the element 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 entry side plate of the path (mm), and WTd is the thickness of the exit side plate of the path (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 higher If the average cooling rate in the temperature range from the cooling start temperature after the completion of finish rolling to 300°C is less than 50°C / s, upper bainite, ferrite, and pearlite will form at the 1 / 4 thickness position, 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 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, it should be 800°C / s or less. The cooling start temperature is, for example, the finishing rolling completion temperature (rolling temperature of the final pass).

[0052] Winding temperature: 300℃ 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 lower. The winding temperature is preferably 280°C or lower, and more preferably 250°C or lower. The lower limit is not particularly limited, but the winding temperature is preferably 0°C or higher, and more preferably 25°C or higher.

[0053] Aside from the conditions of the manufacturing method described above, there are no particular limitations, but it is preferable to adjust the conditions as appropriate 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 immediately after winding. Room temperature refers to a range of -30 to 60°C, and more preferably 0 to 40°C.

[0054] Heat treatment temperature: Holding at a temperature range of 400°C to 750°C for 0 to 3600 seconds. At heat treatment temperatures below 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 level, 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 will exceed 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 metal 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. For this reason, the holding time at the above heat treatment temperature should be 0 s or more. Preferably, the holding time should be 30 s or more, and more preferably 60 s or more. On the other hand, if the holding time at the predetermined heat treatment temperature exceeds 3600 s, the total amount of Ti and Nb present as precipitates with a particle size of 100 nm or less will be less than 0.010 mass%. For this reason, 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 continuous annealing furnaces, induction heating devices, and batch furnaces 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. [Examples]

[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 7. 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] Tissue observation From the obtained steel plate, a specimen for microstructural observation was taken so that the cross-section parallel to the rolling direction would serve as the observation surface. The surface of the obtained specimen was polished, and the metallic structure was revealed by further etching the surface with an etching solution (3% nital solution). Using a scanning electron microscope (SEM), three fields of view were captured at 1000x magnification, targeting the surface layer of the steel plate located at the 1 / 4 position and in the thickness direction from the steel plate surface to 50-150 μm, to obtain SEM images of the metallic structure. The SEM images obtained using the method described above 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] Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test was taken so that the tensile direction was perpendicular to the rolling direction, in accordance with JIS Z 2201, and the strain rate was set to 10 in accordance with JIS Z 2201. -3 A tensile test was conducted 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] Bending test From the obtained hot-rolled steel sheet, rectangular test pieces measuring 200 mm in the rolling direction and 100 mm in the rolling width direction were cut by shearing. Leaving the end faces created by the shearing process intact, the test pieces were set so that the fractured surface 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 general 90° V bending tests as described in JIS Z 2248, the effects of shearing or gas cutting are removed from the end faces of test pieces as needed, and tests are often conducted in accordance with this. However, in this invention, in order to reproduce the processing conditions of actual parts, the bending test was conducted with the sheared end faces 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 pass was defined as a product where the limit bending radius R, which prevents cracking such as ridge cracks and end face cracks, divided by the plate thickness t, is 2.5 or less (R / t).

[0059] Planar bending fatigue test From the obtained hot-rolled steel sheet, test specimens were taken so that the longitudinal direction of the test specimen was perpendicular to the rolling direction, and a plane 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 (SN) curve was obtained by varying the load stress amplitude, and 1 × 10⁻⁶ values ​​were obtained. 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 achieve a tensile strength of 1180 MPa or more, do not achieve excellent bendability, or do not achieve high fatigue strength.

[0061] [Table 1]

[0062] [Table 2]

[0063] [Table 3] [Explanation of Symbols]

[0064] 1. Bending test specimen 2. Bending radius R 3. Plate thickness 10 End face cracking 11 Rolling direction 12 Ridge split 13. The apex of the bending process 14 Shear end face 15 Fracture side

Claims

1. In mass percent, 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, The composition has less than 1.55% Al, with the remainder being Fe and unavoidable impurities. The metallic structure shows that at the 1 / 4 thickness position, martensite accounts for 90% or more of the area. In the surface layer of the steel plate located in the range of 50 to 150 μm from the surface of the steel plate in the thickness direction, upper bainite accounts for 80% or more of the area, and martensite accounts for 0 to 20% of the area. In the aforementioned steel plate surface layer, the total amount of Ti and Nb contained in precipitates with a particle size of 100 nm or less in terms of circular diameter is 0.010 to 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 aforementioned component composition is further expressed 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: 0.0001-0.0050%, REM: 0.0001-0.0050%, Sb: 0.0010% or more and less than 0.1050% The hot-rolled steel sheet according to claim 1, comprising one or more types of Sn selected from 0.0010% to less than 0.5050%.

3. A slab having the component composition described in claim 1 or 2 is heated and rough-rolled, and in the subsequent finish-rolling, 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 between 1.0 and 3.

0. A method for manufacturing a hot-rolled steel sheet, wherein the time from the end of finish rolling to the start of cooling is 10.0 s or less, and the temperature range from the cooling start temperature after the end of finish rolling to 300°C is cooled under conditions where the average cooling rate is 50°C / s or more, the coiling temperature is 300°C or less, and then a heat treatment is performed in which the temperature range is 400°C to 750°C for 0 to 3600 s. Tn≦74.35×t －０．６８５ -5500;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 0 is used if the element 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 entry side plate of the path (mm), and WTd is the thickness of the exit side plate of the path (mm).

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