Steel material

Abstract

A steel material according to the present invention contains C at a proportion of 0.35-0.65 mass%, Si at a proportion of 1.80-2.80 mass%, and Mn at a proportion of 0.70-1.20 mass%, with the remainder consisting of Fe and unavoidable impurities. The equivalent sphere radius of ε carbides (ε-Fe2-3C) as measured by a small angle X-ray scattering technique in a state of having a hardness of 550-630 HV is less than 5.8 nm.

Description

steel material

[0001] The present invention relates to a steel material.

[0002] Steel materials used in automobiles and various industrial machinery parts are generally hardened by quenching. The hardness of steel materials, which are primarily martensitic due to quenching, is determined by the C content in the components, and the hardness of the steel material can be increased by increasing the C content. However, increasing the hardness of steel materials also reduces their toughness, significantly reducing their delayed fracture resistance and corrosion fatigue resistance, so steel materials must have a balance between hardness and toughness.

[0003] As a steel material having high strength and elongation, a technology has been proposed in which a high-strength steel plate containing C, Si, Mn, P, and S is quenched and tempered by high-frequency induction heating or resistance heating to have a tensile strength of 700 to 1,300 MPa, an elongation of 12% or more, and is obtained from a single-phase metal structure of tempered martensite (for example, Patent Document 1).

[0004] Furthermore, as a high strength spring with excellent resistance to hydrogen embrittlement, the following components are used by mass: C: 0.40 to 0.50%, Si: 1.00 to 3.00%, Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%, Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%, Cu: 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05 to 0.50%, Nb: 0.005 to 0.150%, N: 0. It has been proposed to contain 0.0100 to 0.0200%, with P limited to 0.015% or less and S limited to 0.010% or less, with the balance being Fe and unavoidable impurities, and containing an Nb compound containing at least one of Nb carbide, Nb nitride, and Nb carbonitride, and a V compound containing at least one of V carbide and V carbonitride that precipitates around the Nb compound (see, for example, Patent Document 2).

[0005] JP 2018-524475 A JP 2018-154914 A

[0006] In the technology of Patent Document 1, by performing tempering at a high temperature of 500°C to the A1 transformation point, a steel material having excellent elongation can be obtained, but a steel material having high strength cannot be obtained.

[0007] Furthermore, in the technology of Patent Document 2, although a high strength spring can be obtained by using Nb and V compounds, there is a problem of cost because it is a special material.

[0008] The present invention has been made in view of the above, and has an object to provide a steel material having high hardness and toughness.

[0009] In order to solve the above-mentioned problems and achieve the object, the steel material according to the present invention contains C in a proportion of 0.35 mass% or more and 0.65 mass% or less, Si in a proportion of 1.80 mass% or more and 2.80 mass% or less, Mn in a proportion of 0.70 mass% or more and 1.20 mass% or less, with the balance being Fe and unavoidable impurities, and has a hardness of 550 HV or more and 630 HV or less, and has a low content of ε carbide (ε-Fe ２―３ C) The equivalent sphere radius is less than 5.8 nm.

[0010] Furthermore, the steel material according to the present invention contains C in an amount of 0.35 mass% or more and 0.65 mass% or less, Si in an amount of 1.80 mass% or more and 2.80 mass% or less, Mn in an amount of 0.70 mass% or more and 1.20 mass% or less, with the balance being Fe and unavoidable impurities, and has a hardness of 550 HV or more and 630 HV or less, and a fracture time in a 1500 MPa four-point bending delayed fracture test of 150 hours or more.

[0011] Further, the steel material according to the present invention contains C in an amount of 0.35 mass% or more and 0.65 mass% or less, Si in an amount of 1.80 mass% or more and 2.80 mass% or less, Mn in an amount of 0.70 mass% or more and 1.20 mass% or less, and the balance being Fe and inevitable impurities, and has a hardness of 550 HV or more and 630 HV or less and a Charpy impact value of 75 J / cm ２ That's all.

[0012] Furthermore, in the above invention, the steel material according to the present invention contains Cr in proportions of 0.20 mass% to 0.50 mass%, Cu in proportions of 0.10 mass% to 0.40 mass%, Ni in proportions of 0.10 mass% to 0.40 mass%, Ti in proportions of 0.05 mass% to 0.11 mass%, P in proportions of 0.025 mass% or less, S in proportions of 0.025 mass% or less, and the balance being Fe and unavoidable impurities.

[0013] Further, in the steel material according to the present invention, a test piece of φ12.5×15 mm is subjected to a CCT cycle of JASO M609 for 14 days, and the corrosion weight loss is 2200 g / mm ２ The following is the result.

[0014] The present invention has the effect of providing a steel material having high hardness and toughness.

[0015] Fig. 1 is a diagram showing the Charpy impact value of a steel material according to an embodiment of the present invention. Fig. 2 is a diagram showing the time to fracture in a four-point bending test of a steel material according to an embodiment of the present invention. Fig. 3 is a diagram showing corrosion weight loss after a CCT cycle of a steel material according to an embodiment of the present invention. Fig. 4 is a diagram for explaining the ε carbide sphere equivalent radius of a spring steel material according to an example.

[0016] Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the accompanying drawings. Note that the drawings are schematic, and the relationship between the thickness and width of each part, the thickness ratio of each part, etc. may differ from the actual ones, and the drawings may also include parts with different dimensional relationships and ratios.

[0017] (Embodiment) The steel material according to the present invention contains C in a proportion of 0.35 mass% to 0.65 mass%, Si in a proportion of 1.80 mass% to 2.80 mass%, Mn in a proportion of 0.70 mass% to 1.20 mass%, and the balance being Fe and inevitable impurities, and is characterized by having a hardness of 550 HV to 630 HV and any one of the following properties: - The content of ε carbide (ε-Fe) measured by small-angle X-ray scattering ２―３ C) The equivalent sphere radius is less than 5.8 nm. The time to fracture in a four-point bending test with a bending stress of 1500 MPa applied under the CCT cycle of JASO M609 is 150 hours or more. The Charpy impact value is 75 J / cm ２ The above-mentioned characteristics may be combined.

[0018] <Material Composition> The steel material according to the present invention contains C in a proportion of 0.35% by mass or more and 0.65% by mass or less. C contributes to improving the strength of the steel material. If the C content is less than 0.35% by mass, the effect of improving strength is not sufficiently obtained, resulting in insufficient fatigue resistance and sag resistance. Furthermore, if the C content exceeds 0.65% by mass, toughness decreases and cracking becomes more likely to occur. From the above viewpoints, it is preferable that C be contained in a proportion of 0.50% by mass or more and 0.64% by mass or less.

[0019] The steel material according to the present invention contains Si in a proportion of 1.80% by mass or more and 2.80% by mass or less. Si is effective in deoxidizing the steel material and contributes to improving strength and temper softening resistance. If the Si content is less than 1.80% by mass, the above effects are not sufficiently obtained, and carbides generated during tempering become coarse. If the Si content exceeds 2.80% by mass, toughness decreases, making the steel more susceptible to cracking, and decarburization is promoted, resulting in a decrease in wire surface strength. From the above viewpoints, it is preferable that the Si content be 1.8% by mass or more and 2.5% by mass or less.

[0020] The steel material according to the present invention contains Mn in a proportion of 0.70% by mass or more and 1.20% by mass or less. Mn contributes to improving hardenability. If the Mn content is less than 0.70% by mass, it becomes difficult to ensure sufficient hardenability, and the effect of fixing S (MnS formation), which is harmful to ductility and toughness, becomes poor. Furthermore, if the Mn content exceeds 1.20% by mass, ductility decreases, and cracks and surface scratches tend to occur. From the above viewpoints, it is preferable that the Mn content be 0.70% by mass or more and 1.10% by mass or less.

[0021] The steel material according to the present invention is required to contain C, Si, and Mn in the above-mentioned proportions, but may contain elements other than those mentioned above. In addition to C, Si, and Mn, the steel material according to the present invention may contain Cr in proportions of 0.20 mass% to 0.50 mass%, Cu in proportions of 0.10 mass% to 0.40 mass%, Ni in proportions of 0.10 mass% to 0.40 mass%, Ti in proportions of 0.05 mass% to 0.11 mass%, P in proportions of 0.025 mass% or less, and S in proportions of 0.025 mass% or less.

[0022] Cr is effective in preventing decarburization, and also contributes to improving strength and temper softening resistance, thereby improving fatigue resistance. It is also effective in improving warm sag resistance. If the Cr content is less than 0.20 mass%, the above effects cannot be fully achieved. Furthermore, if the Cr content exceeds 0.50 mass%, toughness decreases, and cracks and surface scratches tend to occur. From the above viewpoints, it is preferable that Cr be contained in a proportion of 0.35 mass% or more and 0.45 mass% or less.

[0023] Cu is effective in improving hardenability and can improve fatigue strength by dissolving in ferrite. If the Cu content is less than 0.10 mass%, the above effect cannot be sufficiently obtained. If the Cu content exceeds 0.40 mass%, cracks may occur during hot working. The steel material according to the present invention preferably contains Cu in a proportion of 0.15 mass% or more and 0.40 mass% or less.

[0024] Ni is effective in improving hardenability, suppressing the formation of carbides, and improving fatigue strength. If the Ni content is less than 0.10 mass%, the effect of improving hardenability is insufficient. If the Ni content exceeds 0.40 mass%, not only will cost be a problem, but the amount of retained austenite will increase, reducing fatigue life. The steel material according to the present invention preferably contains Ni in a proportion of 0.15 mass% or more and 0.40 mass% or less.

[0025] Ti combines with C and N to form carbides and nitrides, which act as hydrogen trapping sites, thereby suppressing hydrogen diffusion into the steel material, improving corrosion resistance and delayed fracture resistance, and improving strength and toughness through grain refinement and precipitation strengthening. If the Ti content is less than 0.05 mass%, the above effects cannot be fully achieved. Furthermore, if it exceeds 0.11 mass%, a large amount of TiN is formed, resulting in a decrease in fatigue strength. The steel material according to the present invention preferably contains Ti in a proportion of 0.05 mass% or more and 0.11 mass% or less.

[0026] S forms MnS in the steel material and improves machinability. If the S content exceeds 0.025% by mass, MnS may segregate at grain boundaries, resulting in a decrease in grain boundary strength. Therefore, the S content is preferably 0.025% by mass or less.

[0027] If the P content is high, it segregates at the austenite grain boundaries, reducing the grain boundary strength. The P content is preferably 0.025 mass% or less.

[0028] An example of the steel material according to the present invention is SUP7, but is not limited to this. The steel material according to the present invention has a grain size number of 9 to 12. The grain size number is an index indicating the size of grains, determined by, for example, Japanese Industrial Standard JIS G0551.

[0029] <Carbide> The steel material according to the present invention has ε carbide (ε-Fe ２―３ C) has a spherical equivalent radius of less than 5.8 nm. When the spherical equivalent radius of the ε carbide is less than 5.8 nm, dislocation pinning and dissolved hydrogen trapping become possible, and the sag resistance and delayed fracture resistance of a coil spring formed using this steel material are improved.

[0030] The spherical equivalent radius of ε carbide can be calculated by small-angle X-ray scattering. Small-angle X-ray scattering is a method for evaluating the structure of a substance by irradiating a sample with X-rays and examining the scattering angle and intensity of the X-rays that have passed through the sample. In the present invention, the spherical equivalent radius of ε carbide (ε-Fe ２－３ The size distribution of C) was evaluated.

[0031] For the small-angle X-ray scattering method, a thin steel plate approximately 50 μm thick was prepared, and a scattering profile was obtained using synchrotron radiation (SPring-8). The scattering vector q (= 4π sin θ / λ) was measured at approximately 0.006 to 3, which includes at least a range of 0.04 to 2.0. Data processing involved converting the measurement data into absolute intensities and combining data from different q ranges. Elements required for absolute intensity conversion include, for example, the type of steel (composition), transmittance, instrument constant (calibration factor), and BG measurement data.

[0032] In addition, as a one-dimensional processing, the scattering angle was integrated from the origin of the integrated scattering angle in the scattering angle 2θ (radius) direction, the origin of the scattering angle was determined, camera length calibration (conversion from detector coordinates to diffraction angle) and streak (noise) processing were performed.

[0033] For background processing, data measured without the sample was subtracted as background, and the intensity derived from the sample was calculated based on the following formula: (I_obs-I_dark) / Tr=I_s+I_bg-I_dark, where I_obs is the measured data, Tr is the transmittance, I_s is the intensity derived from the sample, I_bg, I_dark are the backgrounds derived from the device.

[0034] For normalization, the scattering cross section per unit volume was converted into an absolute value (corrected for exposure time, sample thickness, and instrument coefficient).

[0035] After processing, parameter fitting was performed using parameters such as particle size, shape factor, size distribution width, scaling factor (relative volume fraction), and BG (intercept, slope), and the ε carbide (ε-Fe ２－３ C) The equivalent sphere radius was calculated.

[0036] When fitting the scattering intensity I (q, s1, r1, s2, r2), the following assumptions were made: The sum of the volume fractions (1st + 2nd) was set to 10% or less in the following formula (1).

[0037] The volume-weighted frequency distribution was assumed to be a log-normal distribution of the following formula (2). In addition, the equatorial radius refers to the radius of the axis of rotation of the original ellipse, out of the two diameters (major diameter and minor diameter).

[0038] The shape was assumed to be a sphere (ε=1) or a spheroid of revolution represented by the following formula (3).

[0039] The initial parameter values ​​were as follows: (s1, r1, σ1, ε1) = (0.05, 20, 1, 0.5) (s2, r2, σ2, ε2) = (0.1, 10, 0.25, 0.5) s1, s2, r1, and r2 were calculated (fitted) within the range of qmin≦q≦qmax by the least squares method (iterative solution) until the results converged. Here, qmin is the lower limit of the fitting range and is the maximum value among the plots between 0.065 and 0.075, and qmax is the upper limit of the fitting range and is the minimum value among the plots between 0.9 and 1.1. The fitting accuracy is preferably 0.001 or less in terms of the fitting index MSLE (mean square logarithmic error). It is preferable that the number of plots included in the fitting range be 40 or more. In the present invention, ε carbide (ε-Fe ２－３ C) The spherical equivalent radius is calculated using the median value of the size distribution of the measured particles.

[0040] <Physical Properties> The steel material of the present invention has a ε carbide content (ε-Fe) measured by small angle X-ray scattering in a state of 550 HV or more and 630 HV or less. ２―３ C) The equivalent sphere radius is less than 5.8 nm.

[0041] The steel material of the present invention is cut into a rod-shaped test piece of φ8×100 mm in a state of 550 HV to 630 HV and has a time to fracture of 150 hours or more in a four-point bending test in which a bending stress of 1500 MPa is applied under the JASO M609 CCT cycle. The JASO M609 CCT cycle involves salt spray (5% sodium chloride aqueous solution) for 2 hours at 35°C, followed by drying at 60°C for 4 hours at a relative humidity of 20-30%, and then wetting at 50°C for 2 hours at a relative humidity of 95% or more. Here, the state of steel material of 550 HV to 630 HV means, in the case of hot treatment, steel material that has been heated, quenched, and tempered from a green material to have a hardness of 550 HV to 630 HV.

[0042] The steel material of the present invention exhibits high resistance to delayed fracture by having a time to fracture of 150 hours or more in a four-point bending test in which a φ8×100 mm rod-shaped test piece having a strength of 550 HV to 630 HV is subjected to a bending stress of 1500 MPa under the CCT cycle of JASO M609 and the time to fracture is 150 hours or more in a four-point bending test in which a bending stress of 1500 MPa is applied under the CCT cycle of JASO M609 and the time to fracture is preferably 200 hours or more, and more preferably 230 hours or more, in a four-point bending test in which a bending stress of 1500 MPa is applied under the CCT cycle of JASO M609 and the time to fracture is preferably 200 hours or more, and more preferably 230 hours or more.

[0043] The steel material of the present invention has a Charpy impact value of 75 J / cm when in a state of 550 HV or higher and 630 HV or higher. ２ When the Charpy impact value satisfies the above range, the steel material has high toughness. In addition, the Charpy impact value of the steel material is 80 J / cm ２ It is preferable that the concentration is 100 J / cm or more. ２ The Charpy impact value of the steel material is measured in accordance with the Charpy impact test method for metallic materials specified in JIS Z 2242.

[0044] As described above, the steel material according to the present invention has a hardness of 550 HV or more and 630 HV or less. With the hardness in the above range, the steel material has high hardness. It is more preferable that the hardness of the steel material is 560 HV or more and 620 HV or less. The hardness of the steel material is measured in accordance with the Vickers hardness testing method specified in JIS Z 2244.

[0045] Furthermore, the steel material according to the present invention has a corrosion weight loss of 2,200 g / mm after 14 days of a test piece of φ12.5 × 15 mm under the CCT cycle of JASO M609. ２ The CCT cycle for JASO M609 is similar to the four-point bending test described above, in which the specimen is subjected to a salt spray (5% aqueous sodium chloride solution) at 35°C for 2 hours, followed by drying at 60°C for 4 hours at a relative humidity of 20 to 30%, and then wetting at 50°C for 2 hours at a relative humidity of 95% or higher, with the cycle repeated.

[0046] <Manufacturing Method> The steel material according to the present invention is characterized in that the steel material contains 0.35 to 0.65 mass% C, 1.80 to 2.80 mass% Si, 0.70 to 1.20 mass% Mn, with the balance being Fe and unavoidable impurities, and is tempered using a high-frequency induction heating device at 460°C to 490°C for 5 to 30 seconds. This allows the precipitated fine carbides to trap hydrogen and pin dislocations of ε carbides, thereby improving the toughness of even high-strength steel materials.

[0047] In the quenching process, the material is heated to the austenite region above the Ac3 point. Once heated to the austenite region, it is held for a time sufficient for the structure to transform into austenite. The quenching process is preferably carried out by heating to a temperature of 850°C to 1050°C using a high-frequency induction heating device or the like for 5 to 30 seconds. The material is then rapidly cooled from the austenite region to produce martensite.

[0048] In the quenching process, any cooling method can be used as long as it can transform austenite into martensite. For example, the steel material heated to the austenite region can be immersed in oil or water, or can be rapidly cooled by spraying water mist onto the steel material.

[0049] The tempering process involves heating to 460°C to 490°C for 5 to 30 seconds using a high-frequency heating induction device, followed by rapid cooling. The toughness of the steel material is improved by rapidly heating to 460°C to 490°C for 5 to 30 seconds using a high-frequency heating induction device, followed by subsequent rapid cooling. The tempering process preferably involves rapidly heating to 460°C to 490°C at a rate of 50°C / second to 100°C / second. Rapid cooling can also be achieved by, for example, immersing the steel material in oil or water, or by spraying water mist onto the steel material.

[0050] As described above, the steel material according to the present invention has high hardness and high toughness, and therefore has excellent fatigue strength and delayed fracture resistance in the atmosphere and in corrosive environments. The steel material according to the present invention can be suitably used for spring applications, such as automotive suspension springs, valve springs, clutch damper springs, disc springs, stabilizer springs, and torsion bars.

[0051] Example 1: A steel material (SUP7) containing 0.56% by mass or more and 0.64% by mass or less of C, 1.80% by mass or more and 2.20% by mass or less of Si, and 0.70% by mass or more and 1.00% by mass or less of Mn was used. It was heated for 1.5 seconds at a quenching temperature of 850°C to 1000°C using a high-frequency heating device, and then cooled by spraying cooling water onto the steel material. The steel material was then rapidly heated to 460°C at a rate of 51°C / second using a high-frequency heating device, tempered by heating at 460°C for 10 seconds, and then rapidly cooled by spraying cooling water onto the steel material to produce a 600HV steel material. The heat treatment, composition, etc. of the steel material are shown in Table 1.

[0052] Comparative Example 1 A steel material (SAE9254) containing 0.51% by mass or more and 0.59% by mass or less of C, 1.20% by mass or more and 1.60% by mass or less of Si, 0.60% by mass or more and 0.90% by mass or less of Mn, and 0.60% by mass or more and 0.90% by mass or less of Cr was used, and heated to a quenching temperature of 1000°C for 1.5 seconds using a high-frequency heating device, and then placed in a water tank and cooled. Thereafter, the steel material was rapidly heated to a temperature of 460°C to 490°C at a rate of 51°C / second using a high-frequency heating device, and quenched and tempered under the same conditions as in Example 1 to produce a 600HV steel material.

[0053] Comparative Example 2: The same steel material (SUP7) as in Example 1 was used, and was heated for 1.5 seconds at a quenching temperature of 850°C to 1000°C using a high-frequency heating device, and then cooled by spraying cooling water onto the steel material. Thereafter, the steel material was heated to a temperature of 400°C at a rate of 0.2°C / second in a furnace, and heated at a temperature of 400°C for 1800 seconds, and then allowed to cool at room temperature to produce a 600HV steel material.

[0054]

[0055] (Evaluation Method) - Charpy Impact Value - The Charpy impact values ​​of the steel materials of Example 1 and Comparative Examples 1 and 2 were measured. The Charpy impact values ​​were measured using 10 x 5 x 55 mm, U-notch (2 mm deep) test pieces in accordance with the Charpy impact test method for metallic materials specified in JIS Z 2242. The results are shown in Figure 1. - Delayed Fracture Resistance - The steel materials of Example 1 and Comparative Examples 1 and 2 were cut into φ8 x 100 mm rod-shaped test pieces, and a four-point bending test was performed in which a bending stress of 1500 MPa was applied under the JASO M609 CCT cycle to measure the time to fracture. The test was performed in triplicate. The results are shown in Figure 2. - Corrosion Weight Loss - The steel materials of Example 1 and Comparative Examples 1 and 2 were cut into φ12.5 x 15 mm rod-shaped test pieces, and corrosion weight loss was measured after 14 days under the JASO M609 CCT cycle. The results are shown in Figure 3.

[0056] As shown in Figures 1 to 3, Example 1, which was rapidly heated to 460°C to 490°C using a high-frequency heating device and then tempered, had a high Charpy impact value even at high hardness, and the delayed fracture resistance test lasted for more than 200 hours, confirming high toughness and delayed fracture resistance. It was also confirmed that corrosion weight loss was low. On the other hand, Comparative Example 1, which had a low Si content, and Comparative Example 2, which was tempered in a furnace, showed low Charpy impact and delayed fracture resistance at high hardness, and large corrosion weight loss.

[0057] -ε carbide (ε-Fe ２―３ C) spherical equivalent radius - 60 μm thin plate-shaped samples were prepared from the spring steel materials of Example 1, Comparative Example 1, and Comparative Example 2, and scattering profiles were obtained using synchrotron radiation (SPring-8). The scattering vector q (= 4π sin θ / λ) was measured at 0.006 to 3. Data processing involved converting the measurement data into absolute intensities and combining data from different q ranges. Elements required for absolute intensities include the steel type (composition), transmittance, equipment constant (calibration factor), and BG measurement data. Parameter fitting was performed on the processed data using parameters such as particle size / shape factor (assuming ellipsoidal shape), size distribution width, scaling factor (relative volume fraction), and BG (intercept, slope), and the like, to obtain the ε carbide (ε-Fe ２―３The spherical equivalent radius of ε-carbide (ε-Fe ２―３ The spherical equivalent radius of ε-carbide (ε-Fe) was 4.8 nm in Example 1, 5.8 nm in Comparative Example 1, and 6.0 nm in Comparative Example 2. ２―３ Example 1, in which the equivalent sphere radius of C) is less than 5.8 nm, has high toughness and delayed fracture resistance, while Comparative Examples 1 and 2, in which the equivalent sphere radius is 5.8 nm or more, have low Charpy and delayed fracture resistance at high hardness and large corrosion weight loss. In particular, Example 1 and Comparative Example 2, which use the same SUP7, have different properties due to the difference in equivalent sphere radius.

[0058] As explained above, the steel material according to the present invention is suitable for obtaining a steel material having high hardness and toughness.

Claims

1. In a steel material containing C in a proportion of 0.35 mass% or more and 0.65 mass% or less, Si in a proportion of 1.80 mass% or more and 2.80 mass% or less, Mn in a proportion of 0.70 mass% or more and 1.20 mass% or less, with the balance being Fe and unavoidable impurities, the hardness is 550 HV or more and 630 HV or less, and the content of ε carbide (ε-Fe ２―３ C) A steel material having a sphere-equivalent radius of less than 5.8 nm.

2. Steel material containing 0.35% by mass or more and 0.65% by mass or less of C, 1.80% by mass or more and 2.80% by mass or less of Si, 0.70% by mass or more and 1.20% by mass or less of Mn, with the balance being Fe and unavoidable impurities, having a hardness of 550 HV or more and 630 HV or less, and having a time to fracture of 150 hours or more in a four-point bending test in which a bending stress of 1500 MPa is applied under the CCT cycle of JASO M609.

3. In a steel material containing C in proportions of 0.35 mass% to 0.65 mass%, Si in proportions of 1.80 mass% to 2.80 mass%, Mn in proportions of 0.70 mass% to 1.20 mass%, with the balance being Fe and unavoidable impurities, the hardness is 550 HV to 630 HV, and the Charpy impact value is 75 J / cm ２ This is the steel material.

4. A steel material according to any one of claims 1 to 3, containing Cr in proportions of 0.20 mass% to 0.50 mass%, Cu in proportions of 0.10 mass% to 0.40 mass%, Ni in proportions of 0.10 mass% to 0.40 mass%, Ti in proportions of 0.05 mass% to 0.11 mass%, P in proportions of 0.025 mass% or less, S in proportions of 0.025 mass% or less, the balance being Fe and inevitable impurities.

5. After 14 days of use of a φ12.5 x 15 mm test piece subjected to the JASO M609 CCT cycle, the corrosion weight loss was 2,200 g / mm ２ The steel material according to any one of claims 1 to 3, wherein:

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