High carbon steel sheet and method for producing high carbon steel sheet

Abstract

Provided is a high carbon steel sheet which has high rigidity. The high carbon steel sheet has a chemical composition that contains, in mass%, 0.30-0.70% of C, 0.07-1.00% of Si, 0.2-3.0% of Mn, 0.01-1.50% of Cr, 0-0.50% of Ti, 0-0.30% of Nb, 0-0.30% of V, and 0-0.0035% of B, with the balance being made up of Fe and impurities. The high carbon steel sheet has a structure in which the X-ray diffraction intensity ratio to the random sample in the {110}<112> orientation is 5.0 or more at the position of 1 / 4 the sheet thickness from the surface.

Description

High-carbon steel sheet and method for manufacturing high-carbon steel sheet

[0001] This invention relates to high-carbon steel sheets and methods for manufacturing high-carbon steel sheets.

[0002] The steel used for chain and gear components requires excellent impact resistance.

[0003] International Publication No. 2018 / 151273 describes a hot-rolled steel sheet with excellent impact resistance. This hot-rolled steel sheet contains C: 0.10 to 0.50 mass%, etc., and in an L-section, the metal structure at a position 1 / 4 of the thickness from the surface contains prior austenite grains with an aspect ratio (ratio of average grain size in the rolling direction to average grain size in the thickness direction) of 2.0 or less, an average grain size of 0.1 to 3.0 μm, and a coefficient of variation (standard deviation of grain size distribution / average grain size) of 0.40 or more, as well as a texture with an X-ray diffraction intensity ratio in the {001}<110> direction of a random sample of 2.0 or more, and has a tensile strength of 1180 MPa or more.

[0004] The publication also provides a detailed description of the manufacturing method for the hot-rolled steel sheet mentioned above. Specifically, it describes how to improve toughness by optimizing the reduction ratio in the last four of the multiple finish rolling stands and controlling the process so that austenite recrystallization occurs in the final four stands, thereby reducing the anisotropy of the structure.

[0005] International Publication No. 2018 / 151273

[0006] International Publication No. 2018 / 151273 describes how the impact resistance of a component is enhanced by improving the toughness of the material (more specifically, the ductile-brittle transition temperature). On the other hand, it is also conceivable that the impact resistance of a component can be enhanced by increasing the stiffness (Young's modulus) of the material.

[0007] The object of the present invention is to provide a high-carbon steel sheet having high rigidity and a method for manufacturing a high-carbon steel sheet.

[0008] A high-carbon steel sheet according to one embodiment of the present invention has a chemical composition in mass percent of C: 0.30 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.2 to 3.0%, Cr: 0.01 to 1.50%, Ti: 0 to 0.50%, Nb: 0 to 0.30%, V: 0 to 0.30%, B: 0 to 0.0035%, with the remainder being Fe and impurities, and has a microstructure in which the ratio of X-ray diffraction intensity in the {110}<112> direction to that of a random sample is 5.0 or more at a position 1 / 4 of the thickness from the surface.

[0009] The above-mentioned high-carbon steel sheet may have a chemical composition that contains one or more elements selected from the group consisting of Ti: 0.01 to 0.50%, Nb: 0.01 to 0.30%, V: 0.01 to 0.30%, and B: 0.0004 to 0.0035% by mass.

[0010] The above-mentioned high-carbon steel sheet may have a metallic structure at a position 1 / 4 of its thickness from the surface that contains 25 to 65 area percent of ferrite and 35 to 75 area percent of pearlite, with other structures accounting for 5 area percent or less.

[0011] A high-carbon steel sheet according to one embodiment of the present invention has a chemical composition of mass%, C: 0.30-0.70%, Si: 0.07-1.00%, Mn: 0.2-3.0%, Cr: 0.01-1.50%, with the remainder being Fe and impurities, and has a microstructure in which the ratio of X-ray diffraction intensity in the {110}<112> direction to that of a random sample is 5.0 or more at a position 1 / 4 of the thickness from the surface.

[0012] A high-carbon steel sheet according to one embodiment of the present invention has a chemical composition in mass%, C: 0.30 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.2 to 3.0%, Cr: 0.01 to 1.50%, and further contains one or more selected from the group consisting of Ti: 0.50% or less, Nb: 0.30% or less, V: 0.30% or less, and B: 0.0035% or less, with the remainder being Fe and impurities, and has a microstructure in which the ratio of X-ray diffraction intensity in the {110}<112> direction to that of a random sample is 5.0 or more at a position 1 / 4 of the thickness from the surface.

[0013] A method for manufacturing a high-carbon steel sheet according to one embodiment of the present invention is a method for manufacturing the above-mentioned high-carbon steel sheet, comprising the steps of: hot-rolling a slab to make a hot-rolled steel sheet; pickling the hot-rolled steel sheet; and annealing the pickled hot-rolled steel sheet, wherein the hot-rolling to make a hot-rolled steel sheet includes the following steps (a) to (d): (a) a heating step of heating the slab to a temperature of 1100°C or more and less than 1350°C; (b) a finishing rolling step after the rough-rolling step of passing the roughly-rolled slab through four or more rolling stands in succession to roll it into a steel strip; (c) a cooling step after the finishing rolling step of starting cooling within 2.0 seconds and cooling the steel strip at a cooling rate of 100°C / second or more; and (d) a winding step after the cooling step of winding the steel strip at a winding start temperature of 550 to 650°C. The finish rolling process is characterized by the following conditions: the temperature at the entry side of the first stand of the plurality of stands is 850°C or higher; the temperature at the exit side of the last stand of the plurality of stands is 800 to 900°C; the cumulative reduction ratio is 90% or higher; in each of the last four stands of the plurality of stands, σ, as defined by the following formula (1), is 70 to 100; and the inter-pass time between each of the last four stands of the plurality of stands is 0.2 to 10.0 seconds. σ = exp(0.753 + 3000 / (T + 273)) * ε ０．２１ ・v ０．１３ (1) In equation (1), T is the temperature immediately before entering the stand (in °C), ε is the equivalent plastic strain (in dimensionless units), and v is the strain rate (in seconds). －１ )

[0014] According to the present invention, a high-carbon steel sheet with high rigidity can be obtained.

[0015] Figure 1 is a flow diagram of an example of a method for manufacturing high-carbon steel sheets according to one embodiment of the present invention. Figure 2 is a schematic diagram showing the configuration of an example of a hot rolling mill.

[0016] The inventors have discovered that controlling the texture of the surface layer of a steel sheet increases its Young's modulus. Specifically, they found that increasing the concentration of {110}<112> orientations increases the Young's modulus of the steel sheet. Improving Young's modulus increases the rigidity of the component and leads to improved impact resistance. The inventors have further discovered that this texture improves the formability of the steel sheet even with a high carbon content.

[0017] Such a texture can be obtained by controlling the conditions of the finish rolling. Specifically, in addition to increasing the overall reduction amount during finish rolling, rolling is performed at a low temperature, and a large shear strain is applied to the surface of the steel sheet during rolling. Furthermore, by controlling the rolling conditions so that recrystallization does not occur during rolling, extremely flattened unrecrystallized austenite grains can be obtained. By cooling these unrecrystallized austenite grains to precipitate ferrite, a texture oriented preferentially in the {110}<112> direction can be obtained.

[0018] The present invention was completed based on the above findings. Hereinafter, a high-carbon steel sheet and a method for manufacturing a high-carbon steel sheet according to one embodiment of the present invention will be described in detail.

[0019] [High-carbon steel sheet] [Chemical composition] The high-carbon steel sheet according to this embodiment has the chemical composition described below. In the following description, "%" for element content means mass percent.

[0020] C: 0.30-0.70% Carbon (C) improves the hardenability of steel and contributes to improved strength after heat treatment. It also increases the accumulation in the {110}<112> direction in the texture of the surface layer of the steel sheet, contributing to an improvement in Young's modulus. On the other hand, if the C content is too high, it becomes difficult to ensure formability before heat treatment. Therefore, the C content is 0.30-0.70%. The lower limit of the C content is preferably 0.31%, more preferably 0.33%, more preferably 0.35%, more preferably 0.36%, more preferably 0.38%, and more preferably 0.40%. The upper limit of the C content is preferably 0.68%, more preferably 0.65%, and more preferably 0.60%.

[0021] Si: 0.07–1.00% Silicon (Si) is sometimes used as a deoxidizing agent during refining. Si also has the effect of improving the strength of steel. On the other hand, if the Si content is too high, the toughness of the steel decreases. Therefore, the Si content is 0.07–1.00%. The lower limit of the Si content is preferably 0.10%, and more preferably 0.20%. The upper limit of the Si content is preferably 0.90%, and more preferably 0.80%.

[0022] Mn: 0.2-3.0% Manganese (Mn) improves the hardenability of steel and contributes to improved strength after heat treatment. On the other hand, if the Mn content is too high, it becomes difficult to ensure formability before heat treatment. Therefore, the Mn content is 0.2-3.0%. The lower limit of the Mn content is preferably 0.3%, and more preferably 0.4%. The upper limit of the Mn content is preferably 2.8%, and more preferably 2.6%.

[0023] Cr: 0.01 to 1.50% Chromium (Cr) improves the hardenability of steel and contributes to improved strength after heat treatment. On the other hand, if the Cr content is too high, it becomes difficult to ensure formability before heat treatment. Therefore, the Cr content is 0.01 to 1.50%. The lower limit of the Cr content is preferably 0.05%, and more preferably 0.10%. The upper limit of the Cr content is preferably 1.40%, and more preferably 1.30%.

[0024] The remainder of the chemical composition of the high-carbon steel sheet according to this embodiment consists of Fe and impurities. These impurities refer to elements introduced from the ore or scrap used as raw materials for steel, or elements introduced from the environment during the manufacturing process.

[0025] The impurities contained in the chemical composition of the high-carbon steel sheet according to this embodiment are not limited to these, but include P, S, Cu, Ni, Al, N, O, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, Ca, Mo, Hf, Te, Sr, Bi, and Zn.

[0026] The content of these elements is as follows: P: 0.030% or less, S: 0.035% or less, Cu: 1.00% or less (more preferably 0.30% or less), Ni: 1.00% or less (more preferably 0.20% or less), Al: 1.00% or less (more preferably 0.10% or less), N: 0.020% or less (more preferably 0.010% or less), O: 0.020% or less (more preferably 0.010% or less), W: 0.20% or less (more preferably 0.15% or less), Ta: 0.15% or less, Sn: 0.100% or less (more preferably 0.050% or less), Sb: 0 Preferably, the following percentages are obtained: 500% or less (more preferably 0.050% or less), Co: 3.000% or less (more preferably 0.050% or less), As: 0.100% or less (more preferably 0.050% or less), Mg: 0.050% or less, Y: 0.050% or less, Zr: 0.500% or less (more preferably 0.050% or less), La: 0.050% or less, Ce: 0.050% or less, Ca: 0.050% or less, Mo: 0.2% or less, Hf: 0.1% or less, Te: 0.1% or less, Sr: 0.1% or less, Bi: 0.1% or less, Zn: 0.2% or less.

[0027] The chemical composition of the high-carbon steel sheet according to this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti: 0.50%, Nb: 0.30% or less, V: 0.30% or less, and B: 0.0035% or less. Ti, Nb, V, and B are all arbitrary elements, and the chemical composition of the high-carbon steel sheet according to this embodiment may not contain some or all of these elements.

[0028] Ti: 0-0.50% Titanium (Ti) contributes to improving the strength of steel by suppressing austenite grain growth and refining the crystal grains. This effect can be obtained even if only a small amount of Ti is present. On the other hand, if the Ti content is too high, the toughness of the steel decreases. Therefore, the Ti content is 0-0.50%. The lower limit of the Ti content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Ti content is preferably 0.45%, and more preferably 0.40%.

[0029] Nb: 0-0.30% Niobium (Nb) contributes to improving the strength of steel by suppressing austenite grain growth and refining the crystal grains. This effect can be obtained even if only a small amount of Nb is present. On the other hand, if the Nb content is too high, the toughness of the steel decreases. Therefore, the Nb content is 0-0.30%. The lower limit of the Nb content is preferably 0.01%, more preferably 0.02%, and still more preferably 0.10%. The upper limit of the Nb content is preferably 0.25%, and still more preferably 0.20%.

[0030] V: 0-0.30% Vanadium (V) contributes to improving the strength of steel by suppressing austenite grain growth and refining the crystal grains. This effect can be obtained even if only a small amount of V is present. On the other hand, if the V content is too high, the toughness of the steel decreases. Therefore, the V content is 0-0.30%. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.08%. The upper limit of the V content is preferably 0.25%, and even more preferably 0.20%.

[0031] B: 0-0.0035% Boron (B) improves the hardenability of steel and contributes to improved strength after heat treatment. This effect can be obtained even if only a small amount of B is present. On the other hand, if the B content is too high, the hot workability decreases. Therefore, the B content is 0-0.0035%. The lower limit of the B content is preferably 0.0004%, and more preferably 0.0008%. The upper limit of the B content is preferably 0.0030%, and more preferably 0.0025%.

[0032] The chemical composition of the high-carbon steel sheet described above can be measured using general analytical methods. For example, it can be measured using general methods such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using combustion-infrared absorption spectroscopy, N using inert gas fusion-thermal conductivity spectroscopy, and O using inert gas fusion-nondispersive infrared absorption spectroscopy.

[0033] [Texture] The high-carbon steel sheet according to this embodiment has a texture in which the ratio of X-ray diffraction intensity in the {110}<112> direction to that of a random sample (hereinafter referred to as the "{110}<112> direction X-ray random intensity ratio") is 5.0 or more at a position 1 / 4 of the thickness from the surface.

[0034] The larger the X-ray random intensity ratio in the {110}<112> direction, the larger the Young's modulus of the steel sheet. Also, the larger the X-ray random intensity ratio in the {110}<112> direction, the better the elongation at fracture and the better the formability. The lower limit of the X-ray random intensity ratio in the {110}<112> direction is preferably 5.5, and more preferably 6.0. The upper limit of the X-ray random intensity ratio in the {110}<112> direction is not particularly limited, but for example it is 10.0, and preferably 9.0.

[0035] The X-ray random intensity ratio is generally determined by X-ray diffraction, but in this embodiment, the EBSD (Electron Back Scattering Diffraction Pattern) method is used to determine the ODF (Orientation Distribution Function), and the obtained random intensity ratio in the {110}<112> direction is taken as the X-ray random intensity ratio in the {110}<112> direction. Specifically, the X-ray random intensity ratio in the {110}<112> direction is measured as follows.

[0036] A test specimen is taken from an arbitrary position at least 50 mm away from the end face of the high-carbon steel sheet, in a cross-section (L-section) parallel to the rolling direction, so that the texture at a position 1 / 4 of the thickness from the surface (the region from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface) can be observed.

[0037] The cross-section of the above test specimen is polished in stages using silicon carbide sandpaper from #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the specimen is polished at room temperature using colloidal silica that does not contain alkaline solutions to remove the strain introduced into the surface layer of the test specimen.

[0038] With respect to an arbitrary position in the longitudinal direction (rolling direction) of the cross-section of the obtained test piece and the 1 / 4 thickness position from the surface, measurement is performed by the EBSD (Electron Backscatter Diffraction) method, with a measurement interval of 1 μm and a measurement area of 100000 μm ２ or more, to obtain crystal orientation information. For the measurement, an apparatus composed of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (Velocity detector manufactured by AMETEK) is used. In this case, the vacuum degree inside the apparatus is 9.6×10 －５ Pa or less, the acceleration voltage is 25 kV, and the irradiation current level is 16.

[0039] The obtained crystal orientation information is calculated using the spherical harmonic function method to obtain a crystal orientation distribution function (ODF: Orientation Distribution Function) that three-dimensionally displays the texture. The random intensity ratio of the {110}<112> orientation is obtained from the crystal orientation distribution function.

[0040] "OIMAnalysis (registered trademark)" is used for the analysis, "sample symmetry" is set to "orthorhombic", the crystal orientation distribution function is displayed by the BUNGE method, and the random intensity ratio of each orientation in the cross-section at φ2=45° is calculated. Since there are measurement errors caused by test piece processing and test piece setting, the maximum value of the orientation density within "the range of Φ=85 to 90° and φ1=50 to 60°" is adopted as the random intensity ratio of the {110}<112> orientation.

[0041] Analysis of texture using Euler angles (φ1, Φ, φ2) is widely performed. For example, Hiroshi Inoue: "Lecture (Easy Material Analysis Technology) - Three-dimensional Orientation Analysis of Texture", Keikinzoku, Vol. 42, No. 6 (1992), p. 358 describes the definition of Euler angles (φ1, Φ, φ2). If analysis is performed using the aforementioned software, even a person who does not fully understand the definition of Euler angles (φ1, Φ, φ2) can easily calculate the orientation density of an orientation group expressed by Euler angles.

[0042] Preferably, in the high-carbon steel sheet according to the present embodiment, the metal structure at a position 1 / 4 of the thickness from the surface contains 25 to 65 area% of ferrite and 35 to 75 area% of pearlite, and the area percentage of structures other than ferrite and pearlite is 5 area% or less. The metal structure shall be measured on an L-cross section (a cross section including the rolling direction and the sheet thickness direction).

[0043] If the area percentage of ferrite is too low, formability may deteriorate. The lower limit of the area percentage of ferrite is more preferably 30%, still more preferably 40%, still more preferably 50%, and still more preferably 55%. On the other hand, within the range of the C content of the high-carbon steel sheet according to the present embodiment, it may be difficult to make the area percentage of ferrite larger than 65 area%. The upper limit of the area percentage of ferrite is more preferably 60%.

[0044] Within the range of the C content of the high-carbon steel sheet according to the present embodiment, it may be difficult to reduce the area percentage of pearlite to lower than 35 area%. The lower limit of the area percentage of pearlite is more preferably 40%. On the other hand, if the area percentage of pearlite is too high, formability may deteriorate. The upper limit of the area percentage of pearlite is more preferably 70%, still more preferably 60%, still more preferably 50%, and still more preferably 45%.

[0045] Structures other than ferrite and pearlite include, but are not limited to, martensite, bainite, retained austenite, and the like. The area percentage of structures other than ferrite and pearlite is more preferably 3% or less, and still more preferably 2% or less. If the area percentage of structures other than ferrite and pearlite is too high, formability may deteriorate.

[0046] The area percentage of the metal structure of the high-carbon steel sheet is measured as follows.

[0047] A test specimen is taken from an arbitrary position at least 50 mm away from the end face of a high-carbon steel sheet, so that the metallographic structure can be observed at a position 1 / 4 of the thickness from the surface (from 1 / 8 of the thickness to 3 / 8 of the thickness from the surface) on the sheet thickness cross section (L section) parallel to the rolling direction. Next, the sheet thickness cross section is mirror-polished, and the polished surface is etched with nital. Using an optical microscope, the microstructure is observed at 100x magnification in a region of 300 μm in the thickness direction and 800 μm in the rolling direction. At least three observation regions are used. Image analysis is performed on the microstructure images obtained from this observation to obtain the area ratios of ferrite and pearlite.

[0048] Ferrite is resistant to corrosion by nital etching, appearing pale (white), and possesses an equiaxed grain structure. Pearlite, on the other hand, exhibits a layered structure consisting of ferrite (light) and cementite (dark) regions when etched with nital. These characteristics allow for the differentiation of the two metal structures.

[0049] The area fraction of retained austenite can be measured by X-ray diffraction. First, a test specimen is taken from an arbitrary position at least 50 mm away from the edge of a high-carbon steel sheet, and chemically polished from the surface down to a point 1 / 4 of the way from the surface (the region from 1 / 8 of the way from the surface to 3 / 8 of the way from the surface). At the 1 / 4 point from the surface (the region from 1 / 8 of the way from the surface to 3 / 8 of the way from the surface), the integrated intensities of a total of six peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220), are determined using Co-Kα radiation, and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0050] [Mechanical Properties, etc.] The high-carbon steel sheet according to this embodiment preferably has a Young's modulus of 200 GPa or more, which is an indicator of stiffness. The lower limit of the Young's modulus is more preferably 210 GPa. The upper limit of the Young's modulus is not particularly limited, but is for example 240 GPa, and even more preferably 235 GPa.

[0051] The Young's modulus is measured using the free resonance method (free resonance device) in accordance with JIS Z 2280:1993 (Dynamic Young's Modulus Measurement). The test specimen is taken from the edge in the width direction to the 1 / 4 portion of the width (the region from the edge to the 1 / 8 position to the 3 / 8 position), and the orientation of the test specimen should be with the rolling direction as the longitudinal direction.

[0052] The high-carbon steel sheet according to this embodiment preferably has a fracture elongation of 35% or more, which is an indicator of formability. The lower limit of the fracture elongation is more preferably 40%. The upper limit of the fracture elongation is not particularly limited, but is for example 50%, and even more preferably 45%.

[0053] The elongation at break shall be measured using a No. 5 test specimen of JIS Z 2241:2011, in accordance with JIS Z 2241:2011. The sampling position for the tensile test specimen shall be from the edge in the width direction to the 1 / 4 portion of the width (the region from the 1 / 8 position from the edge to the 3 / 8 position from the edge to the width), and the orientation of the test specimen shall be such that the longitudinal direction is perpendicular to the rolling direction (direction C).

[0054] The high-carbon steel sheet according to this embodiment preferably has a tensile strength of 960 MPa or less. If the tensile strength is too high, sufficient formability may not be obtained. The upper limit of the tensile strength is more preferably 920 MPa. The lower limit of the tensile strength is not particularly limited, but for example, it is 390 MPa.

[0055] [Method for Manufacturing High-Carbon Steel Sheets] Next, an example of a method for manufacturing high-carbon steel sheets according to this embodiment will be described. The high-carbon steel sheets according to this embodiment are not limited to those manufactured by the manufacturing method described below.

[0056] Figure 1 is a flowchart of an example of a method for manufacturing high-carbon steel sheets according to this embodiment. This manufacturing method includes a hot rolling step (step S1) in which a slab having the above-described chemical composition is hot-rolled to make a hot-rolled steel sheet, a pickling step (step S2) in which the hot-rolled steel sheet is pickled, and an annealing step (step S3) in which the pickled hot-rolled steel sheet is annealed.

[0057] The hot rolling process (step S1) will be explained in detail. The hot rolling process includes a heating process (step S1-1), a rough rolling process (step S1-2), a reheating process (step S1-3), a finish rolling process (step S1-4), a cooling process (step S1-5), and a winding process (step S1-6).

[0058] Figure 2 is a schematic diagram showing the configuration of a hot strip mill 10, which is an example of a hot rolling mill. The hot strip mill 10 is equipped with a heating furnace 11, a roughing mill 12, a heating device (bar heater) 13, a finishing mill 14, a cooling device (runout table) 15, and a winding machine 16.

[0059] The slab S is heated to a predetermined temperature by a heating furnace 11 and then roughly rolled by a rough rolling mill 12. The thickness of the slab after rough rolling is not limited to this, but is for example 23 to 60 mm. After rough rolling, the slab S is reheated by a heating device 13 as needed, and then finish-rolled by a finish rolling mill 14 to become a steel strip C. The thickness of the steel strip C after finish rolling is not limited to this, but is for example 1.20 to 9.00 mm. The steel strip C is cooled to a predetermined winding temperature by a cooling device 15, and then wound into a coil by a winding machine 16.

[0060] Of the steps shown in Figure 1, the reheating step (step S1-3) can be omitted. Also, the conditions for the rough rolling step (step S1-2) do not significantly affect the final steel sheet structure as long as they are within the range of general conditions. Therefore, in this embodiment, the conditions for the heating step (step S1-1), finish rolling step (step S1-4), cooling step (step S1-5), and winding step (step S1-6) are specifically limited.

[0061] The hot rolling process (step S1) more specifically includes the following steps (a) to (d): (a) a heating step in which the slab S is heated to a temperature of 1100°C or higher and less than 1350°C; (b) a finishing rolling step in which, after the rough rolling step, the roughly rolled slab is passed through four or more rolling stands in succession to form a steel strip C; (c) a cooling step in which, after the finishing rolling step, cooling is started within 2.0 seconds and the steel strip C is cooled at a cooling rate of 100°C / second or higher; and (d) a winding step in which, after the cooling step, the steel strip C is wound at a winding start temperature of 550 to 650°C.

[0062] The heating temperature in the heating step (step S1-1) is 1100°C or higher and less than 1350°C. If the heating temperature is below 1100°C, the homogenization of the slab S will be insufficient, resulting in a decrease in the final strength and formability of the steel sheet. On the other hand, if the heating temperature is 1350°C or higher, the initial austenite grain size will be large, making it impossible to create a fine structure, resulting in a decrease in the final strength and formability of the steel sheet. The lower limit of the heating temperature is preferably 1150°C. The upper limit of the heating temperature is preferably 1300°C.

[0063] In the finish rolling process (step S1-4), the roughly rolled slab is tandem rolled in a finish rolling mill 14 equipped with four or more rolling stands. The finish rolling mill 14 is preferably equipped with six or seven rolling stands, although this is not limited to these.

[0064] In this embodiment, in addition to the temperature and reduction ratio of the entire finish rolling process, the conditions of the final four stands 14f1 to 14f4 are particularly restricted. More specifically, the finish rolling process (step S1-4) satisfies all of the conditions (b1) to (b5) described below.

[0065] (b1) The temperature of the entry side of the first stand 14s of the multiple stands is 850°C or higher. If the temperature of the entry side of the first stand 14s is too low, it becomes difficult to suppress the recrystallization of austenite grains, and the desired texture cannot be obtained. For this reason, the temperature of the entry side of the first stand 14s is 850°C or higher. The lower limit of the temperature of the entry side of the first stand 14s is preferably 900°C. The upper limit of the temperature of the entry side of the first stand 14s is not particularly limited, but for example it is 1200°C.

[0066] (b2) The temperature of the exit side of the final stand 14f1 of the multiple stands is 800 to 900°C. If the temperature of the exit side of the final stand 14f1 is too low, it becomes difficult to suppress the recrystallization of the austenite grains during rolling, and the desired texture cannot be obtained. On the other hand, if the temperature of the exit side of the final stand 14f1 is too high, the amount of dislocations introduced into the austenite grains becomes insufficient, and the desired texture cannot be obtained. For this reason, the temperature of the exit side of the final stand 14f1 is 800 to 900°C. The lower limit of the temperature of the exit side of the final stand 14f1 is preferably 820°C. The upper limit of the temperature of the exit side of the final stand 14f1 is preferably 880°C.

[0067] (b3) Cumulative reduction ratio of 90% or more If the cumulative reduction ratio in finish rolling (cumulative reduction ratio from stand 14s to stand 14f1) is too small, sufficient shear strain cannot be applied, and the desired texture cannot be obtained. For this reason, the cumulative reduction ratio is 90% or more. The lower limit of the cumulative reduction ratio is preferably 92%. The upper limit of the cumulative reduction ratio is not particularly limited, but for example, it is 98%. The cumulative reduction ratio R (%) can be calculated from the following formula, where the plate thickness before finish rolling is t0 (mm) and the plate thickness after finish rolling is t1 (mm): R = (t0 - t1) / t0 × 100

[0068] (b4) In each of the last four stands 14f1 to 14f4 of the multiple stands, σ determined by the following equation (1) is 70 to 100 σ = exp(0.753 + 3000 / (T + 273)) * ε ０．２１ ・v ０．１３ (1) In equation (1), T is the temperature immediately before entering the stand (in °C), ε is the equivalent plastic strain (in dimensionless units), and v is the strain rate (in seconds). －１ The equivalent plastic strain ε can be calculated using the formula ε = (2 / √3) × (h / H), where h is the thickness of the inlet plate and H is the thickness of the outlet plate. The strain rate v can be calculated using the formula v = ε / tr, where tr is the rolling time.

[0069] In this embodiment, σ is 70 to 100 in each of the four final stands 14f1 to 14f4. That is, in any of the four final stands 14f1 to 14f4, σ is 70 to 100.

[0070] σ is an indicator of the amount of dislocations introduced into the austenite grains during rolling at the relevant stand; a larger σ means more dislocations are introduced. σ increases with increasing equivalent plastic strain ε, increasing strain rate v, and decreasing temperature T. If σ is too large, recrystallization occurs during rolling, releasing dislocations within the austenite grains, and the desired texture cannot be obtained. On the other hand, if σ is too small, the amount of dislocations introduced will be insufficient, and the desired texture cannot be obtained. The lower limit of σ is preferably 75, and more preferably 80. The upper limit of σ is preferably 95, and more preferably 90.

[0071] (b5) The inter-path time is 0.2 to 10.0 seconds between each of the last four stands 14f1 to 14f4 of the multiple stands. In this embodiment, the inter-path time is 0.2 to 10.0 seconds between each of the last four stands 14f1 to 14f4. That is, the inter-path time is 0.2 to 10.0 seconds between stand 14f4 and stand 14f3, between stand 14f3 and stand 14f2, and between stand 14f2 and stand 14f1.

[0072] If the inter-pass time between the last four stands 14f1 to 14f4 is too long or too short, recrystallization is likely to occur during rolling or between passes, preventing the desired texture from being obtained. Therefore, the inter-pass time between each of these stands is 0.2 to 10.0 seconds. The lower limit of the inter-pass time is preferably 0.5 seconds. The upper limit of the inter-pass time is preferably 8.0 seconds.

[0073] Cooling is started within 2.0 seconds after the finish rolling process, and the steel strip C is cooled at a cooling rate of 100°C / second or more (step S1-5). If the time from the finish rolling process to the start of cooling is too long, or if the cooling rate is too low, recrystallization and grain growth may occur during the cooling process, and ultimately the desired texture may not be obtained. Here, "cooling" means water cooling, and "time from the finish rolling process to the start of cooling" means the time from the end of the finish rolling process until water cooling is started. The cooling time (time spent water cooling) is preferably 0.3 seconds or more.

[0074] The time until cooling begins is preferably 1.5 seconds or less, and more preferably 1.0 second or less. The lower limit of the time until cooling begins is not particularly limited, but for example, 0.1 seconds. The lower limit of the cooling rate is preferably 120°C / second, and more preferably 200°C / second. The upper limit of the cooling rate is not particularly limited, but for example, 300°C / second, and more preferably 200°C / second. Note that the cooling rate is the average cooling rate from the temperature immediately before cooling begins to the cooling end temperature (the temperature when water cooling is completed). The cooling conditions from the end of cooling to the start of winding are not particularly limited. Air cooling may be performed, or water cooling may be performed after air cooling. It is preferable that the average cooling rate from the cooling end temperature to the winding start temperature be less than 90°C / second.

[0075] After the cooling process, winding of the steel strip C is started at a winding start temperature of 550 to 650°C (step S1-6). If the winding start temperature is too high or too low, the desired microstructure (metallic structure and texture) cannot be obtained. The lower limit of the winding start temperature is preferably 555°C, and more preferably 570°C. The upper limit of the winding start temperature is preferably 630°C.

[0076] Hot-rolled steel sheets are manufactured through the above process.

[0077] The hot-rolled steel sheet is pickled to remove surface scale (Step S2). The pickled hot-rolled steel sheet is annealed (Step S3). Annealing adjusts the mechanical properties of the hot-rolled steel sheet. The annealing temperature is not particularly limited, but it is preferable to set the holding temperature to 680 to 730°C. The annealing holding time is not particularly limited, but for example, it is 30 seconds to 100 hours.

[0078] An example of a high-carbon steel sheet and a method for manufacturing the same according to one embodiment of the present invention has been described above. According to this embodiment, a high-carbon steel sheet with high rigidity can be obtained.

[0079] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0080] Steel having the chemical composition shown in Table 1 was melted in a converter and cast into 230 mm thick slabs by continuous casting. Although not shown in the table, the impurity content was as follows: P: 0.030% or less, S: 0.035% or less, Cu: 0.30% or less, Ni: 0.20% or less, Al: 0.10% or less, N: 0.010% or less, O: 0.010% or less, W: 0.15% or less, Ta: 0.15% or less, Sn: 0.050% or less, Sb: 0.050% or less, Co: 0.0 The composting ratios were 50% or less, As: 0.050% or less, Mg: 0.050% or less, Y: 0.050% or less, Zr: 0.050% or less, La: 0.050% or less, Ce: 0.050% or less, Ca: 0.050% or less, Mo: 0.2% or less, Hf: 0.1% or less, Te: 0.1% or less, Sr: 0.1% or less, Bi: 0.1% or less, and Zn: 0.2% or less. Subsequently, the slabs were heated to a temperature of 1105 to 1350°C, rough rolling was performed, and then hot-rolled steel sheets were manufactured under the conditions shown in Table 2.

[0081]

[0082]

[0083] In Table 2, "T0" represents the heating temperature of the slab before rough rolling, "T1" represents the temperature at the entry side of the first stand in finish rolling, and "T2" represents the temperature at the exit side of the final stand in finish rolling. "t0" represents the plate thickness before finish rolling, "t1" represents the plate thickness after finish rolling, and "R" represents the cumulative reduction ratio in finish rolling. "σ1" to "σ4" represent the values of σ at the last four stands of finish rolling. "p1" to "p3" represent the inter-pass time between the last four stands of finish rolling. "t" is the time from the end of finish rolling to the start of cooling, "CR" is the average cooling rate from the temperature at the start of cooling to the time point at the end of cooling, and "T3" is the coiling start temperature. It should be noted that the time from the start of cooling to the end of cooling is 0.3 seconds or more, and the average cooling rate from the end of cooling to the start of coiling is less than 90°C / sec.

[0084] After pickling the produced hot-rolled steel sheet, annealing by holding at 700°C for 40 seconds was performed to obtain the steel sheet.

[0085] The random intensity ratio of the {110}<112> orientation of each steel sheet was obtained as follows. From a position at least 50 mm away from the end face of each steel sheet, a test specimen was taken such that the texture at the 1 / 4 thickness position from the surface (the region from 1 / 8 depth to 3 / 8 depth of the thickness (sheet thickness) from the surface) on a cross-section parallel to the rolling direction (L-section) can be observed. After polishing the cross-section of the test specimen step-by-step using silicon carbide grit papers from #600 to #1500, it was finished to a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, polishing was performed using colloidal silica that does not contain an alkaline solution at room temperature to remove strain introduced into the surface layer of the test specimen. For the 1 / 4 thickness position from the surface of the cross-section of the obtained test specimen, the measurement interval was set to 1 μm, and the measurement area is 100000 μm ２ or more. Crystal orientation information was obtained by measurement via the EBSD method. For the measurement, an apparatus composed of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (Velocity detector manufactured by AMETEK) was used. At this time, the vacuum degree inside the apparatus is 9.6×10 －５Below Pa, the acceleration voltage was set to 25 kV and the irradiation current level to 16. The obtained crystal orientation information was used to calculate the ODF using the spherical harmonic method. From the ODF, the random intensity ratio of the {110}<112> orientation was determined. For the analysis, "OIMAnalysis®" was used, and "sample symmetry" was set to "orthorhombic". The ODF was displayed using the BUNGE method, and the random intensity ratio of each orientation in the φ2 = 45° cross section was calculated. Due to measurement errors caused by specimen processing and specimen setting, the maximum orientation density within the range of "Φ = 85 to 90°, φ1 = 50 to 60°" was adopted for the random intensity ratio of the {110}<112> orientation.

[0086] The area ratios of ferrite and pearlite in each steel sheet were measured as follows: A test specimen was taken from a position at least 50 mm away from the end face of each steel sheet, in a thickness cross section (L section) parallel to the rolling direction, so that the metallographic structure could be observed at a position 1 / 4 of the thickness from the surface (the region from 1 / 8 of the thickness to 3 / 8 of the thickness from the surface). Next, the thickness cross section was mirror-polished, and the polished surface was etched with nital. Using an optical microscope, the microstructure was observed at 100x magnification in a region of 300 μm in the thickness direction and 800 μm in the rolling direction. Three observation regions were used. The area ratios of ferrite and pearlite were obtained by performing image analysis on the microstructure photographs obtained from this observation.

[0087] The Young's modulus of each steel sheet was measured using the free resonance method (free resonance device) in accordance with JIS Z 2280:1993 (Dynamic Young's Modulus Measurement). The test specimens were taken from the edge in the width direction to the 1 / 4 portion of the sheet width (the region from the edge to the 1 / 8 position to the 3 / 8 position), and the orientation of the test specimens was with the rolling direction as the longitudinal direction.

[0088] The elongation at break and tensile strength of each steel plate were measured by tensile testing. The tensile tests were conducted in accordance with JIS Z 2241:2011, using a No. 5 test specimen. The tensile test specimens were taken from the edge in the width direction of the plate, at a point 1 / 4 of the way across (the region from 1 / 8 of the width from the edge to 3 / 8 of the width), and the longitudinal direction of the test specimen was perpendicular to the rolling direction (direction C).

[0089] The results are shown in Table 3. In the "Area Percentage of Metallic Structure" column of Table 3, "α" represents ferrite, "P" represents pearlite, and "Other" represents structures other than ferrite and pearlite. "Random Intensity Ratio" is the X-ray random intensity ratio in the {110}<112> direction.

[0090]

[0091] As shown in Tables 1 to 3, steel plates No. 1 to 8 had an X-ray random intensity ratio (hereinafter simply referred to as "random intensity ratio") of 5.0 or higher in the {110}<112> direction. These steel plates had a Young's modulus of 200 GPa or higher and an elongation at break of 35% or higher. The tensile strength of all of them was 920 MPa or lower.

[0092] Steel plates No. 9 to No. 21 had a random strength ratio of less than 5.0 in the {110} <112> orientation. These steel plates had a Young's modulus of less than 200 GPa and an elongation at break of less than 35%.

[0093] The low random strength ratio in the {110}<112> orientation of steel plate No. 9 is thought to be due to the temperature (T1) at the entry side of the first stand during finish rolling being too low. The low random strength ratio of steel plate No. 10 is thought to be due to the temperature (T2) at the exit side of the final stand during finish rolling being too low. The low random strength ratio of steel plate No. 11 is thought to be due to the temperature (T2) at the exit side of the final stand during finish rolling being too high.

[0094] The low random strength ratio of steel plate No. 12 is thought to be due to the cumulative reduction ratio (R) being too low during finish rolling. The low random strength ratio of steel plate No. 13 is thought to be due to the σ being too low at the final four stands during finish rolling. The low random strength ratio of steel plate No. 14 is thought to be due to the σ being too high at the final four stands during finish rolling.

[0095] The low random strength ratio of steel plate No. 15 is thought to be due to the short inter-pass time between the last four stands in the finish rolling process. The low random strength ratio of steel plate No. 16 is thought to be due to the long inter-pass time between the last four stands in the finish rolling process. The low random strength ratio of steel plate No. 17 is thought to be due to the long time between the end of finish rolling and the start of cooling.

[0096] The low random strength ratio of steel plate No. 18 is thought to be due to the average cooling rate being too low from the temperature at the start of cooling to the end of cooling. The low random strength ratio of steel plate No. 19 is thought to be due to the winding start temperature being too low. The low random strength ratio of steel plate No. 20 is thought to be due to the winding start temperature being too high. The low random strength ratio of steel plate No. 21 is thought to be due to the carbon content of steel plate No. 21 (steel type I) being too low.

[0097] Steel plate No. 22 had a random strength ratio of 5.0 or higher in the {110}<112> orientation. Steel plate No. 22 also had a Young's modulus of 200 GPa or higher. However, its fracture elongation was slightly inferior compared to steel plates No. 1 to 8. This is thought to be because the proportion of ferrite was low and the proportion of structures other than ferrite and pearlite was high.

[0098] Although embodiments of the present invention have been described above, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and it is possible to carry out the present invention by appropriately modifying the embodiments described above within the scope of the invention.

Claims

1. A high-carbon steel sheet having a chemical composition in mass%, C: 0.30-0.70%, Si: 0.07-1.00%, Mn: 0.2-3.0%, Cr: 0.01-1.50%, Ti: 0-0.50%, Nb: 0-0.30%, V: 0-0.30%, B: 0-0.0035%, the remainder being Fe and impurities, and having a microstructure in which the ratio of X-ray diffraction intensity in the {110}<112> direction to that of a random sample is 5.0 or higher at a position 1 / 4 of the thickness from the surface.

2. A high-carbon steel sheet according to claim 1, wherein the chemical composition contains one or more selected from the group consisting of, by mass%, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.30%, V: 0.01 to 0.30%, and B: 0.0004 to 0.0035%.

3. A high-carbon steel sheet according to claim 1 or 2, wherein the metal structure at a position 1 / 4 of the thickness from the surface contains 25 to 65 area percent of ferrite and 35 to 75 area percent of pearlite, and the structure other than ferrite and pearlite is 5 area percent or less.

4. A method for manufacturing a high-carbon steel sheet according to claim 1 or 2, comprising: a step of hot-rolling a slab to make a hot-rolled steel sheet; a step of pickling the hot-rolled steel sheet; and a step of annealing the pickled hot-rolled steel sheet, wherein the step of hot-rolling to make a hot-rolled steel sheet includes the following steps (a) to (d): (a) a heating step of heating the slab to a temperature of 1100°C or more and less than 1350°C; (b) a finishing rolling step after the rough-rolling step, passing the roughly-rolled slab through four or more rolling stands in succession to roll it into a steel strip; (c) a cooling step after the finishing rolling step, in which cooling is started within 2.0 seconds and the steel strip is cooled at a cooling rate of 100°C / second or more; and (d) a winding step after the cooling step, winding the steel strip at a winding start temperature of 550 to 650°C, the finishing rolling step is A method for manufacturing high-carbon steel sheets, wherein the temperature at the inlet of the first stand of the plurality of stands is 850°C or higher, the temperature at the outlet of the last stand of the plurality of stands is 800 to 900°C, the cumulative reduction ratio is 90% or higher, in each of the last four stands of the plurality of stands, σ, as defined by the following formula (1), is 70 to 100, and the inter-pass time between each of the last four stands of the plurality of stands is 0.2 to 10.0 seconds. σ = exp(0.753 + 3000 / (T + 273)) * ε ０．２１ ・v ０．１３ (1) In equation (1), T is the temperature immediately before entering the stand (in °C), ε is the equivalent plastic strain (in dimensionless units), and v is the strain rate (in seconds). －１ )

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