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

Abstract

Provided is a high-carbon steel sheet that has a greater amount of carbon and still has favorable workability. A high-carbon steel sheet according to the present invention has a chemical composition that includes, by 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, the remainder being Fe and impurities. In an L cross-section taken at a 1 / 4 thickness position from the surface of the high-carbon steel sheet, the average particle size of prior austenite grains is no more than 20 μm, and the flatness of the prior austenite grains as defined by (length of major axis-length of minor axis) / (length of minor axis) is at least 5.0. The metal structure at a 1 / 4 thickness position from the surface of the high-carbon steel sheet includes 25–65 area% of ferrite, 35–75 area% of pearlite, and no more than 5 area% of structures other than ferrite and pearlite.

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] Chain components, gear components, and other parts are manufactured by first forming them into a predetermined shape and then performing heat treatment. The steel used as the material for such components is required to have high strength after heat treatment and to be soft and highly workable before heat treatment. Increasing the carbon content is effective in increasing the strength after heat treatment, but increasing the carbon content reduces the workability before heat treatment.

[0007] The object of the present invention is to provide a high-carbon steel sheet that has good workability even when the carbon content is high, 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. In the L-section at a position 1 / 4 of the thickness from the surface, the average particle size of prior austenite grains is 20 μm or less, the flattening ratio defined as (length of major axis - length of minor axis) / (length of minor axis) is 5.0 or more, and the metallic 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, with the structure other than ferrite and pearlite being 5 area percent or less.

[0009] The above-mentioned high-carbon steel sheet may contain 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] 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%, with the remainder being Fe and impurities. In the L-section at a position 1 / 4 of the thickness from the surface, the average particle size of prior austenite grains is 20 μm or less, and the aspect ratio, defined as (length of major axis - length of minor axis) / (length of minor axis), is 5.0 or more.

[0011] 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%, 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. In the L-section at a position 1 / 4 of the thickness from the surface, the average particle size of prior austenite grains is 20 μm or less, and the flattening ratio, defined as (length of major axis - length of minor axis) / (length of minor axis), is 5.0 or more.

[0012] 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 )

[0013] According to the present invention, a high-carbon steel sheet with good workability can be obtained even with a high carbon content.

[0014] 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.

[0015] To improve the workability of steel sheets, it is effective to increase the proportion of soft ferrite in the ferrite-pearlite structure. On the other hand, it is difficult to increase the ferrite proportion when the carbon content is high. The inventors have found that by controlling the shape of austenite grains after hot rolling, it is possible to promote ferrite precipitation during cooling. Specifically, they found that by making the austenite grains fine and extremely flattened, it is possible to increase the ferrite proportion and ensure workability even with a high carbon content.

[0016] Such a structure 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 structure with a high ferrite fraction can be obtained.

[0017] 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.

[0018] [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.

[0019] C: 0.30-0.70% Carbon (C) improves the hardenability of steel and contributes to improved strength after heat treatment. It also refines and flattens the prior austenite grains in the metal structure of the steel sheet, contributing to improved workability. On the other hand, if the C content is too high, it becomes difficult to ensure formability (workability) 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%.

[0020] 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%.

[0021] 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%.

[0022] 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%.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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%.

[0028] 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%.

[0029] 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%.

[0030] 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%, more preferably 0.0008%, and even more preferably 0.0010%. The upper limit of the B content is preferably 0.0030%, and even more preferably 0.0025%.

[0031] 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.

[0032] [Metal structure] In the high-carbon steel sheet according to this embodiment, the average particle size of prior austenite grains in the L-section at a position 1 / 4 of the thickness from the surface is 20 μm or less, and the aspect ratio, defined as (length of major axis - length of minor axis) / (length of minor axis), is 5.0 or more.

[0033] In this embodiment, the austenite grains (previous austenite grains) immediately before cooling after hot rolling are made fine (average grain size of 20 μm or less) and extremely flattened (flatness ratio of 5.0 or more). The flattening of the austenite grains means that dislocations are accumulating within them. By making the austenite grains fine and flattened, the density of dislocations at grain boundaries, which serve as nucleation sites, and within the grains can be increased, thereby increasing the amount of ferrite precipitated. This softens the metal structure and improves workability.

[0034] The upper limit of the average particle size of the prior austenite grains is preferably 18 μm, and more preferably 16 μm. The lower limit of the average particle size of the prior austenite grains is not particularly limited, but is, for example, 3 μm, preferably 5 μm, and more preferably 8 μm.

[0035] The lower limit of the flattening ratio of the prior austenite grains is preferably 5.5, more preferably 6.0, and still more preferably 6.5. The upper limit of the flattening ratio of the prior austenite grains is not particularly limited, but for example it is 11.0, preferably 10.0, more preferably 9.0, and still more preferably 8.0.

[0036] The average grain size and flatness of the prior austenite grains are measured as follows: A test piece is taken from an arbitrary position at least 50 mm away from the end face of the high-carbon steel sheet, so that the metal structure can be observed 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) on the thickness cross section (L section) parallel to the rolling direction. Next, the thickness cross section is mirror-polished, and the polished surface is etched with 3% nital (3% nitric acid-ethanol solution), and the metal structure is observed with a scanning electron microscope (SEM). Specifically, an area in which 500 or more crystal grains (prior austenite grains) can be observed in one field of view is imaged by SEM observation, and the average grain size and flatness are determined by performing image analysis using the image analysis software "WinROOF" manufactured by Mitani Corporation. The captured image is divided into minute regions of 5 μm × 5 μm, and the number of cementite particles present in each region is determined. In each region, the number of cementite particles is measured per 25 μm area. ２ The value obtained by dividing by this is the local density of cementite particles in that region (unit: particles / μm). ２ ) This local density is calculated for the entire region, and the local density is set to 0.00 to 0.20 particles / μm ２ By color-coding the range and visualizing it as a two-dimensional map, the local density is 0.15 particles / μm ２ The regions where the above areas appear continuously are extracted as high-density regions. Since these high-density regions correspond to prior austenite grain boundaries, the boundary positions are identified using continuous line segments of the extracted regions and considered as prior austenite grain boundaries. The average grain size is determined by the intercept method. The flatness is calculated by approximating each prior austenite grain with an ellipse, determining the length of the major axis and the length of the minor axis, and then determining the flatness of each prior austenite grain ((length of major axis - length of minor axis) / (length of minor axis)). The arithmetic mean of the flatness of all prior austenite grains within the field of view is taken as the flatness of the prior austenite grains in the steel sheet.

[0037] The high-carbon steel sheet according to this embodiment has a microstructure at a position 1 / 4 of the thickness from the surface that contains 25 to 65 area percent of ferrite and 35 to 75 area percent of pearlite, with the remaining microstructure other than ferrite and pearlite accounting for 5 area percent or less. The microstructure shall be measured in an L-shaped cross section (a cross section including the rolling direction and the thickness direction).

[0038] If the area fraction of ferrite is too low, formability may decrease. The lower limit of the area fraction of ferrite is preferably 30%, more preferably 40%, still more preferably 50%, and still more preferably 55%. On the other hand, within the carbon content range of the high-carbon steel sheet according to the present embodiment, it may be difficult to make the area fraction of ferrite greater than 65 area%. The upper limit of the area fraction of ferrite is preferably 60%.

[0039] Within the carbon content range of the high-carbon steel sheet according to the present embodiment, it may be difficult to reduce the area fraction of pearlite to lower than 35 area%. The lower limit of the area fraction of pearlite is preferably 40%. On the other hand, if the area fraction of pearlite is too high, formability may decrease. The upper limit of the area fraction of pearlite is preferably 70%, more preferably 60%, still more preferably 50%, and still more preferably 45%.

[0040] Structures other than ferrite and pearlite include, but are not limited to, martensite, bainite, retained austenite, and the like. The area fraction of structures other than ferrite and pearlite is preferably 3% or less, more preferably 2% or less. If the area fraction of structures other than ferrite and pearlite is too high, formability may decrease.

[0041] The area fraction of the metal structure of the high-carbon steel sheet is measured as follows.

[0042] A test specimen is collected from any position at least 50 mm away from the end face of the high-carbon steel sheet such that the metal structure can be observed at the 1 / 4 thickness position from the surface (the region from 1 / 8 depth to 3 / 8 depth of the thickness from the surface) of a thickness cross section parallel to the rolling direction (L cross section). Next, after the thickness cross section is mirror-polished, the polished surface is etched with 3% nital, and the structure in a region of 300 µm in the thickness direction and 800 µm in the rolling direction is observed at a magnification of 100 times using an optical microscope. There are at least three observation regions. Image analysis is performed on the structure photographs obtained through this structure observation to obtain the area fractions of ferrite and pearlite.

[0043] Since ferrite is not easily corroded by nital etching, it appears pale (white) and has an equiaxed grain morphology. On the other hand, for pearlite, a layered structure consisting of ferrite regions (bright regions) and cementite regions (dark regions) is observed through nital etching. Based on these characteristics, the two metallographic structures can be distinguished from each other.

[0044] The area fraction of retained austenite can be measured by X-ray diffraction. First, a test piece is collected from any position at least 50 mm away from the end face of the high-carbon steel sheet, and chemical polishing is performed from the surface to a position of 1 / 4 of the thickness from the surface (a region from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface). At the position of 1 / 4 of the thickness from the surface, the integrated intensities of a total of 6 peaks, namely α(110), α(200), α(211), γ(111), γ(200), and γ(220), are obtained using Co-Kα radiation, and the volume fraction of retained austenite is calculated by the average intensity method. The volume fraction of retained austenite obtained is regarded as the area fraction of retained austenite.

[0045] [Mechanical Properties, etc.] The high-carbon steel sheet according to the present embodiment preferably has a Vickers hardness, which is used as an index of workability, of 135 HV or less. If the Vickers hardness is too high, workability will decrease. The upper limit of Vickers hardness is more preferably 132 HV, and still more preferably 130 HV. The lower limit of Vickers hardness is not particularly limited, but is, for example, 120 HV, and more preferably 125 HV.

[0046] Vickers hardness is measured using the following procedure. First, a test piece is taken from an arbitrary position at least 50 mm away from the end face of the high-carbon steel sheet, so that the Vickers hardness can be measured at a position 1 / 4 of the way from the surface to the thickness (the region from 1 / 8 of the thickness to 3 / 8 of the thickness) on the sheet thickness cross section (L section) parallel to the rolling direction. Next, the sheet thickness cross section is polished to a mirror finish by mechanical polishing. On this polished surface, the Vickers hardness (HV) is measured at 12 locations on a straight line parallel to the rolling direction, at a position 1 / 4 of the thickness from the surface, with an indentation load of 20 gf. The arithmetic mean of the Vickers hardness of the 10 points obtained by excluding the lowest and highest values ​​from these 12 measured Vickers hardness points is taken as the Vickers hardness of the high-carbon steel sheet. It is preferable that the distance between each measurement point be at least four times the distance of the indentation. The distance of four times or more the indentation mentioned here refers to the distance obtained by multiplying the length of the diagonal of the indentation created by the diamond indenter during Vickers hardness measurement by a value of four times or more.

[0047] The high-carbon steel sheet according to this embodiment preferably has a tensile strength of 435 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 420 MPa, and even more preferably 410 MPa. The lower limit of the tensile strength is not particularly limited, but for example, it is 390 MPa.

[0048] Tensile strength shall be measured in accordance with JIS Z 2241:2011, using a No. 5 test specimen. The sampling location for the tensile test specimen shall be from the edge in the width direction to the 1 / 4 portion of the plate width (the region from the 1 / 8 position from the edge to the 3 / 8 position from the edge to the width direction), and the orientation of the test specimen shall be such that the longitudinal direction is perpendicular to the rolling direction (direction C).

[0049] [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.

[0050] 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.

[0051] 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).

[0052] 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.

[0053] 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 1.2 to 9.0 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.

[0054] 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 metal structure of the steel sheet as long as they are within the range of general conditions. For this reason, 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.

[0055] 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.

[0056] 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 strength and formability of the final 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 (the average grain size of the prior austenite grains cannot be reduced to 20 μm or less), resulting in a decrease in the strength and formability of the final steel sheet. The lower limit of the heating temperature is preferably 1150°C. The upper limit of the heating temperature is preferably 1300°C.

[0057] 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.

[0058] 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.

[0059] (b1) The inlet-side temperature of the first stand 14s among the plurality of stands is 850°C or higher. If the inlet-side temperature of the first stand 14s is too low, it becomes difficult to suppress recrystallization of austenite grains, and prior austenite grains cannot be made fine and flat. Therefore, the inlet-side temperature of the first stand 14s is 850°C or higher. The lower limit of the inlet-side temperature of the first stand 14s is preferably 900°C. The upper limit of the inlet-side temperature of the first stand 14s is not particularly limited, and is, for example, 1200°C.

[0060] (b2) The outlet-side temperature of the final stand 14f1 among the plurality of stands is 800 to 900°C. If the outlet-side temperature of the final stand 14f1 is too low, it becomes difficult to suppress recrystallization of austenite grains during rolling, and prior austenite grains cannot be made fine and flat. On the other hand, if the outlet-side temperature of the final stand 14f1 is too high, the amount of dislocations introduced into austenite grains becomes insufficient, and prior austenite grains cannot be made fine and flat. Therefore, the outlet-side temperature of the final stand 14f1 is 800 to 900°C. The lower limit of the outlet-side temperature of the final stand 14f1 is preferably 820°C. The upper limit of the outlet-side temperature of the final stand 14f1 is preferably 880°C.

[0061] (b3) The cumulative rolling reduction is 90% or more. If the cumulative rolling reduction in finish rolling (the cumulative rolling reduction from stand 14s to stand 14f1) is too small, sufficient shear strain cannot be applied, and prior austenite grains cannot be made fine and flat. Therefore, the cumulative rolling reduction is 90% or more. The lower limit of the cumulative rolling reduction is preferably 92%. The upper limit of the cumulative rolling reduction is not particularly limited, and is, for example, 98%. Note that the cumulative rolling reduction R (%) can be obtained from the following formula when the plate thickness before finish rolling is t0 (mm) and the plate thickness after finish rolling is t1 (mm): R = (t0 - t1) / t0 × 100

[0062] (b4) In each of the final four stands 14f1 to 14f4 among the plurality of stands, σ defined by the following formula (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.

[0063] 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.

[0064] 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 preventing the prior austenite grains from becoming fine and flattened. On the other hand, if σ is too small, the amount of dislocations introduced is insufficient, also preventing the prior austenite grains from becoming fine and flattened. The lower limit of σ is preferably 75, and more preferably 80. The upper limit of σ is preferably 95, and more preferably 90.

[0065] (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.

[0066] 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, making it impossible to make the prior austenite grains fine and flat. 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.

[0067] 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, making it impossible to ultimately make the prior austenite grains fine and flat. 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.

[0068] 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.

[0069] 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 metal structure 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.

[0070] Hot-rolled steel sheets are manufactured through the above process.

[0071] 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.

[0072] 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 good workability can be obtained even with a high carbon content.

[0073] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0074] 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 1110 to 1343°C, rough rolling was performed, and then hot-rolled steel sheets were manufactured under the conditions shown in Table 2.

[0075]

[0076]

[0077] In Table 2, "T0" is the heating temperature of the slab before rough rolling, "T1" is the temperature at the entry side of the first stand for finish rolling, and "T2" is the temperature at the exit side of the final stand for finish rolling. "t0" is the thickness of the slab before finish rolling, "t1" is the thickness of the slab after finish rolling, and "R" is the cumulative reduction ratio in finish rolling. "σ1" to "σ4" are the values ​​of σ at the last four stands of finish rolling. "p1" to "p3" are the inter-pass times 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 end of cooling, and "T3" is the winding start temperature. The time from the start of cooling to the end of cooling was set to 0.3 seconds or more, and the average cooling rate from the end of cooling to the start of winding was set to less than 90°C / second.

[0078] The manufactured hot-rolled steel sheet was pickled and then annealed by holding it at 700°C for 40 seconds to produce the final steel sheet.

[0079] The average grain size and flatness of the prior austenite grains in each steel plate were measured as follows: A test specimen was taken from a position at least 50 mm away from the end face of each steel plate, and the L-section was mirror-polished. The polished surface was then etched with 3% nital (3% nitric acid-ethanol solution), and the metal structure was observed using a scanning electron microscope (SEM) at a position 1 / 4 of the thickness from the surface. Specifically, an area in which more than 500 crystal grains (prior austenite grains) could be observed in one field of view was imaged using SEM observation, and the average grain size and flatness were determined by performing image analysis using the image analysis software "WinROOF" manufactured by Mitani Corporation using the method described above.

[0080] 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.

[0081] The Vickers hardness of each steel plate was measured using the following procedure. First, a test piece was taken from a position at least 50 mm away from the end face of each steel plate, so that the Vickers hardness could be measured at a position 1 / 4 of the way from the surface to the thickness (from 1 / 8 of the way from the surface to 3 / 8 of the way from the surface) on the plate thickness cross section (L section) parallel to the rolling direction. Next, the plate thickness cross section was polished to a mirror finish by mechanical polishing. On this polished surface, the Vickers hardness (HV) was measured at 12 locations on a straight line parallel to the rolling direction, at a position 1 / 4 of the way from the surface to the thickness, with an indentation load of 20 gf. The arithmetic mean of the Vickers hardness of the 10 points obtained by excluding the lowest and highest values ​​from these 12 measured Vickers hardness points was taken as the Vickers hardness of the steel plate. The distance between each measurement point was set to a distance of at least four times the indentation distance.

[0082] The tensile strength of each steel sheet was measured by tensile testing. The tensile strength was measured 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 sheet, 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).

[0083] 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.

[0084]

[0085] As shown in Tables 1 to 3, steel plates No. 1 to 8 had an average particle size of prior austenite grains of 20 μm or less and an aspect ratio of 5.0 or more. These steel plates had a Vickers hardness of 135 HV or less. Furthermore, the tensile strength of all of them was 410 MPa or less.

[0086] Steel plates No. 9 to No. 21 had either the average grain size or flatness of the prior austenite grains (hereinafter collectively referred to as "shape of prior austenite grains") outside the appropriate range. These steel plates had a Vickers hardness greater than 135 HV. Furthermore, their tensile strength was greater than 410 MPa.

[0087] The improper shape of the prior austenite grains in steel sheet 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 improper shape of the prior austenite grains in steel sheet 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 improper shape of the prior austenite grains in steel sheet 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.

[0088] The improper shape of the prior austenite grains in steel sheet No. 12 is thought to be due to a cumulative reduction ratio (R) that was too low during finish rolling. The improper shape of the prior austenite grains in steel sheet No. 13 is thought to be due to a σ that was too low at the final four stands during finish rolling. The improper shape of the prior austenite grains in steel sheet No. 14 is thought to be due to a σ that was too high at the final four stands during finish rolling.

[0089] The improper shape of the prior austenite grains in steel sheet No. 15 is thought to be due to the short inter-pass time between the last four stands during finish rolling. The improper shape of the prior austenite grains in steel sheet No. 16 is thought to be due to the long inter-pass time between the last four stands during finish rolling. The improper shape of the prior austenite grains in steel sheet No. 17 is thought to be due to the long time between the end of finish rolling and the start of cooling.

[0090] The improper shape of the old austenite grains in steel plate No. 18 is thought to be due to an excessively low average cooling rate from the start of cooling to the end of cooling. The improper shape of the old austenite grains in steel plate No. 19 is thought to be due to an excessively low winding start temperature. The improper shape of the old austenite grains in steel plate No. 20 is thought to be due to an excessively high winding start temperature.

[0091] The reason the shape of the old austenite grains in steel plate No. 21 was not appropriate is thought to be because the carbon content of steel plate No. 21 (steel type I) was too low.

[0092] 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

In mass percent, C: 0.30-0.70%, Si: 0.07-1.00%, Mn: 0.2-3.0%, Cr: 0.01-1.50%, Ti: 0 to 0.50%, Nb: 0 to 0.30%, V: 0 to 0.30%, B: 0 to 0.0035%, The remainder has a chemical composition consisting of Fe and impurities. In the L-shaped cross-section at a position 1 / 4 of the thickness from the surface, the average particle size of the prior austenite grains is 20 μm or less, and the flattening ratio, defined as (length of the major axis - length of the minor axis) / (length of the minor axis), is 5.0 or more. A high-carbon steel sheet in which the metallic 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 accounts for 5 area percent or less. 　 A high-carbon steel sheet according to claim 1, The aforementioned chemical composition, in mass%, Ti: 0.01 to 0.50%, Nb: 0.01-0.30%, V: 0.01 to 0.30%, and B: 0.0004-0.0035%, A high-carbon steel sheet containing one or more selected from the group consisting of the following. 　 A method for producing a high-carbon steel sheet according to claim 1 or 2, The process involves hot-rolling a slab to produce a hot-rolled steel sheet, The process of pickling the hot-rolled steel sheet, The process includes annealing the pickled hot-rolled steel sheet, The process of hot rolling to produce hot-rolled steel sheets 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 process in which, after the rough rolling process, the roughly rolled slab is passed through four or more rolling stands in succession and rolled to form a steel strip. (c) A cooling step in which cooling is started within 2.0 seconds after the finish rolling step and the steel strip is cooled at a cooling rate of 100°C / second or more, and (d) After the cooling step, a winding step in which the steel strip is wound at a winding start temperature of 550 to 650°C. The aforementioned finishing rolling process is: The temperature on the input side of the first stand of the aforementioned plurality of stands is 850°C or higher. The temperature at the exit of the final stand of the aforementioned plurality of stands is 800 to 900°C. The cumulative reduction rate is 90% or more. In each of the last four stands of the aforementioned plurality of stands, σ, as defined by the following formula (1), is between 70 and 100. A method for manufacturing high-carbon steel sheets, wherein 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+30000 / (T+273))・e 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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