Steel material for sliding parts and manufacturing method for steel material for sliding parts

Abstract

Provided is a steel material for sliding components that has excellent sliding properties and workability. The steel material for sliding components: comprises a steel material having 0.30%–0.60% by mass C content; has a structure that includes iron carbide and at least either tempered martensite or bainite; has a total, by volume fraction, of at least 80% tempered martensite and bainite and at least 2.0% iron carbide; and has a Vickers hardness Hv of 300–600. The volume fraction X of the iron carbide and the Vickers hardness Hv fulfill the relational expression (1).　X ≥ –0.065 × Hv + 36.5 (1). The unit for X is % and the unit for Hv is Hv.

Description

[Technical Field]

[0001] The present invention relates to a steel material for sliding parts and a method for manufacturing the steel material for sliding parts. [Background technology]

[0002] Steel materials are widely used in industrial products such as automobile parts, railway vehicle parts, building materials, pipes, etc. In particular, carbon steel materials for machine structures and alloy steel materials for machine structures are often used as materials for sliding parts, such as gears and shafts, which are typified by power transmission system parts, due to their high mechanical strength.

[0003] The biggest issue with sliding parts is friction and wear between parts, which are thought to be the cause of malfunctions and reduced efficiency in the entire mechanical system. As mechanical systems become smaller and lighter in the future, the environment for sliding parts is expected to become even more severe. For example, in the case of crankshafts, which are engine parts for automobiles, improving the seizure resistance of rotating sliding parts is a permanent challenge, along with the need for smaller and lighter parts. To solve these problems, it is necessary to develop steel materials for sliding parts that have better sliding properties than currently available, in preparation for the reduction in size and weight of the entire mechanical system.

[0004] One of the issues that must be resolved for steel materials for sliding parts is the improvement of wear resistance from the viewpoint of extending the lifespan of parts and improving reliability. Increasing the hardness of steel is considered to be an effective way to improve wear resistance. However, increasing hardness impairs the workability of the steel, which poses risks when mass-producing parts. Therefore, an effective method for improving the sliding properties of sliding parts is to selectively control the structure of only the surface layer and harden only that area.

[0005] For example, Japanese Patent Laid-Open Publication No. 1-230746 discloses a sliding component including a fixed member made of cast iron and a sliding member made of a material harder than cast iron, in which the surface layer structure of the fixed member is made to be a hardened layer made of martensite or a mixed phase structure of martensite, pearlite, ferrite, and graphite, and a structure made of oxides.

[0006] As a method other than controlling hardness, there is a method of improving seizure resistance by controlling precipitates in the steel material to suppress adhesion. JP 2013-227674 A describes a gear having a steel material structure in which, in the surface layer, retained austenite is present in an area ratio of 1 to 10% in tempered martensite and / or tempered bainite, and carbides are precipitated in an area ratio of 5% or more, and the nitrogen concentration at a depth of 20 μm from the surface is 2.0 to 6.0%.

[0007] JP 2010-100881 A describes a carburized or carbonitrided sliding part, in which the surface layer from the surface of the sliding surface to a depth of 10 μm has a Vickers hardness of 700 or more, an average particle diameter of cementite particles of 0.6 μm or less, and a number density of cementite particles in a cross section perpendicular to the sliding surface of 1 particle / μm. 2 Thus, the sliding component has been described. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 1-230746 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-227674 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-100881 Summary of the Invention [Problem to be solved by the invention]

[0009] For sliding parts such as rotating shafts and crankshafts, it is important to suppress friction and wear between the surfaces of the sliding parts and to allow them to function in a state where damage such as mechanical damage due to overload and thermal cracking is suppressed. While increasing hardness is one way to improve wear resistance, it can also be a factor that impairs processability. Furthermore, adhesion resistance is important for sliding parts to prevent seizure.

[0010] An object of the present invention is to provide a steel material for sliding parts having excellent sliding properties and workability. Another object of the present invention is to provide a method for manufacturing a steel material for sliding parts having excellent sliding properties and workability. [Means for solving the problem]

[0011] A steel material for sliding parts according to one embodiment of the present invention is a steel material for sliding parts made of a steel material having a C content of 0.30 to 0.60 mass %, wherein the steel material has a structure containing at least one of tempered martensite and bainite and iron carbide, the volume fraction of the sum of the tempered martensite and the bainite being 80% or more and the volume fraction of the iron carbide being 2.0% or more, the steel material has a Vickers hardness of 300 or more and 600 or less, and the volume fraction X of the iron carbide and the Vickers hardness Hv satisfy the following relational expression (1): X≧-0.065×Hv+36.5 (1) The unit of X is %, and the unit of Hv is Hv.

[0012] A steel material for sliding parts according to one embodiment of the present invention may have a chemical composition, in mass%, of C: 0.30 to 0.60%, Si: 0.01 to 2.00%, Mn: 0.10 to 2.00%, Al: 0.060% or less, N: 0.020% or less, P: 0.10% or less, S: 0.20% or less, Cr: 0 to 0.50%, and the balance: Fe and impurities.

[0013] A method for producing a steel material for sliding parts according to one embodiment of the present invention is a method for producing the above-mentioned steel material for sliding parts, and includes the steps of: quenching a material by holding the material at a temperature of 830°C or higher and 1100°C or lower, and then cooling the material from the holding temperature to 300°C at a cooling rate of 300°C / sec or higher; and tempering the quenched material by holding it at a temperature of 200°C or higher and 600°C or lower. [Effects of the Invention]

[0014] According to the present invention, a steel material for sliding parts having excellent sliding properties and workability can be obtained. [Brief explanation of the drawings]

[0015] [Figure 1] Figure 1 shows an image of the surface roughness of a steel material obtained using an atomic force microscope. [Figure 2] Figure 2 shows an image of the adhesion force of steel material obtained using an atomic force microscope. [Figure 3] FIG. 3 is a scatter diagram showing the relationship between the Vickers hardness of steel materials and the volume fraction of iron carbides. [Figure 4] Figure 4 is a graph showing the relationship between the Vickers hardness of steel and the wear scar width obtained by a sliding test using a ball-on-disk friction and wear tester. [Figure 5] Figure 5 shows an example of a topographic image obtained by measuring a test piece whose surface had been processed by Ar ion milling with an atomic force microscope. [Figure 6] Figure 6 shows an example of iron carbide detected using image analysis software. [Figure 7] Figure 7 is a schematic diagram of a ball-on-disk friction and wear tester. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present inventors investigated the sliding properties and workability of steel materials in order to develop a steel material that is excellent in sliding properties and workability, and as a result, obtained the following findings.

[0017] Figures 1 and 2 are images obtained using an atomic force microscope (AFM), with Figure 1 being a topography image and Figure 2 being an adhesive force image. In Figure 1, convex parts are shown in white and concave parts in black. In Figure 2, parts with high adhesive force are shown in white and parts with low adhesive force are shown in black.

[0018] The topography image in Figure 1 was obtained by AFM measurement of a sample whose surface had been processed by Ar ion milling. This processing removes the iron matrix, which is softer than the iron carbide, leaving the iron carbide as a convex portion, making it possible to search for the iron carbide using AFM. In Figure 1, the white areas, i.e., the convex portions, are iron carbide. Figure 2 shows the results of measuring adhesive force over the same area, and it can be seen from Figures 1 and 2 that the adhesive force of iron carbide is weak.

[0019] From this, it is thought that increasing the volume fraction of iron carbide can improve seizure resistance, but on the other hand, increasing the volume fraction of iron carbide is also thought to decrease the hardness of the steel material and reduce its wear resistance.

[0020] Figure 3 is a scatter diagram showing the relationship between the Vickers hardness and the volume fraction of iron carbide of the steel materials prepared in the examples described below. Figure 4 is a graph showing the relationship between the Vickers hardness of the steel materials and the wear scar width obtained in a sliding test using a ball-on-disk friction and wear tester. The smaller the wear scar width, the higher the wear resistance.

[0021] 3 and 4, hollow circles indicate steel materials whose volume fraction X of iron carbides and Vickers hardness Hv satisfy the following relational expression (1), while solid circles indicate steel materials whose structure does not satisfy the relational expression (1). Note that the triangular symbols in Fig. 4 represent steel materials whose structure is as quenched. X≧-0.065×Hv+36.5 (1) The unit of X is %, and the unit of Hv is Hv.

[0022] 3 and 4, it can be seen that excellent wear resistance can be obtained if the volume fraction X of iron carbide and the Vickers hardness Hv of the steel material satisfy the relational expression (1).

[0023] Based on the above findings, the present invention has been completed. A steel material for sliding parts according to one embodiment of the present invention will now be described in detail.

[0024] [Chemical composition] The steel for sliding parts according to this embodiment is made of a steel material having a C content of 0.30 to 0.60 mass%. The higher the C content, the higher the volume fraction of carbides tends to be. Furthermore, the higher the C content, the higher the Vickers hardness of the steel for sliding parts tends to be. If the C content is outside the range of 0.30 to 0.60 mass%, it may be difficult to satisfy the relationship between the volume fraction of iron carbides and the Vickers hardness (1), or even if the relationship (1) is satisfied, it may be impossible to obtain a steel material with an excellent balance between sliding properties and workability. The lower limit of the C content of the steel for sliding parts according to this embodiment is preferably 0.32 mass%, more preferably 0.35 mass%, even more preferably 0.38 mass%, and still more preferably 0.40 mass%. The upper limit of the C content of the steel for sliding parts according to this embodiment is preferably 0.58 mass%, even more preferably 0.55 mass%.

[0025] The chemical composition of the steel material for sliding parts according to this embodiment is not particularly limited as long as it has a C content of 0.30 to 0.60 mass %, but may have, for example, the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0026] C: 0.30-0.60% Carbon (C) improves the hardenability of steel. As mentioned above, if the C content is outside the appropriate range, it becomes difficult to satisfy the relationship (1) between the volume fraction of iron carbides and Vickers hardness, or even if the relationship (1) is satisfied, it may be impossible to obtain a steel material with an excellent balance between sliding properties and workability. Therefore, the C content is 0.30 to 0.60%. The lower limit of the C content is preferably 0.32%, more preferably 0.35%, even more preferably 0.38%, and still more preferably 0.40%. The upper limit of the C content is preferably 0.58%, even more preferably 0.55%.

[0027] Si: 0.01 to 2.00% Silicon (Si) deoxidizes steel. On the other hand, if the Si content is too high, the workability of the steel decreases. Therefore, the Si content may be 0.01 to 2.00%. The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Si content is preferably 1.50%, more preferably 1.20%, more preferably 0.80%, more preferably 0.60%, and even more preferably 0.40%.

[0028] Mn: 0.10 to 2.00% Manganese (Mn) improves the hardenability of steel. On the other hand, if the Mn content is too high, the workability of steel decreases. Therefore, the Mn content may be 0.10 to 2.00%. The lower limit of the Mn content is preferably 0.20%, more preferably 0.40%, and even more preferably 0.60%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.60%, more preferably 1.50%, more preferably 1.00%, and even more preferably 0.90%.

[0029] Al: 0.060% or less Aluminum (Al) deoxidizes steel. On the other hand, if the Al content is too high, the workability of the steel decreases. Therefore, the Al content may be 0.060% or less. The upper limit of the Al content is preferably 0.050%, more preferably 0.040%, and even more preferably 0.030%. To obtain the deoxidizing effect of Al, the Al content may be 0.020% or more.

[0030] N: 0.020% or less Nitrogen (N) reduces the hot workability of steel. Therefore, the N content may be 0.020% or less. The upper limit of the N content is preferably 0.018%, more preferably 0.015%, even more preferably 0.010%, and still more preferably 0.005%. On the other hand, excessive restriction of the N content increases the manufacturing cost. Therefore, the lower limit of the N content may be set to 0.0010%.

[0031] P:0.10% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces the hot workability and toughness of steel. Therefore, the P content may be 0.10% or less. The P content is preferably 0.03% or less, and more preferably 0.02% or less. The P content should preferably be as low as possible.

[0032] S: 0.20% or less Sulfur (S) is sometimes added to improve the workability (machinability) of steel. On the other hand, if the S content is too high, the quench cracking resistance of the steel decreases. Therefore, the S content may be 0.20% or less. The upper limit of the S content is preferably 0.12%, more preferably 0.08%, and even more preferably 0.06%. To obtain the effect of improving workability due to S, the S content may be 0.020% or more.

[0033] Cr: 0 to 0.50% Chromium (Cr) is an optional element. That is, the steel material for sliding parts according to this embodiment does not need to contain Cr. Cr improves the hardenability of steel. This effect can be obtained even if even a small amount of Cr is contained. On the other hand, if the Cr content is too high, the workability of the steel decreases. Therefore, the Cr content may be 0 to 0.50%. The lower limit of the Cr content is preferably 0.01%, and more preferably 0.05%. The upper limit of the Cr content is preferably 0.20%.

[0034] The balance of the chemical composition of the steel material for sliding parts according to this embodiment may be Fe and impurities, which refer to elements that are mixed in from ores or scraps used as raw materials for the steel, or elements that are mixed in from the environment during the manufacturing process, etc.

[0035] The sliding part steel according to this embodiment may be made of carbon steel for machine construction or alloy steel for machine construction. The sliding part steel according to this embodiment is preferably made of carbon steel for machine construction specified in JIS G 4051:2016 or alloy steel for machine construction specified in JIS G 4053:2016. Among these, S45C and S50C specified in JIS G 4051:2016 and SMn438 specified in JIS G 4053:2016 are particularly preferred. Furthermore, these steels may contain 0.20 mass% or less of S to improve workability (machinability).

[0036] [Organization] The structure of the steel material for sliding parts according to this embodiment includes at least one of tempered martensite and bainite (including tempered bainite; the same applies below), and iron carbide, with the volume fraction of the total of tempered martensite and bainite being 80% or more, and iron carbide being 2.0% or more.

[0037] The structure of the steel material for sliding parts according to this embodiment has a total volume fraction of tempered martensite and bainite of 80% or more. The structure of the steel material for sliding parts according to this embodiment may contain at least one of tempered martensite and bainite.

[0038] In this embodiment, the steel material for sliding parts is tempered to have a structure containing a predetermined amount of iron carbide, thereby ensuring the workability of the steel material for sliding parts. In contrast, if the structure of the steel material for sliding parts is an as-quenched structure (a structure mainly composed of as-quenched martensite), it becomes difficult to ensure good workability. The steel material for sliding parts according to this embodiment preferably contains tempered martensite.

[0039] If the sum of the volume fractions of tempered martensite and bainite is less than 80%, it becomes difficult to obtain excellent wear resistance. The sum of the volume fractions of tempered martensite and bainite is preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more.

[0040] In this embodiment, in the calculation of the volume fraction of the structure, iron carbide is treated as an independent structure and is distinguished from tempered martensite and bainite. That is, the portion where iron carbide precipitates is not included in the volume of tempered martensite or bainite.

[0041] The structure of the steel material for sliding parts according to this embodiment has an iron carbide volume fraction of 2.0% or more. Specifically, the iron carbide of the steel material for sliding parts according to this embodiment is at least one of ε carbide and cementite. The iron carbide contained in the steel material for sliding parts may be one type or multiple types. When multiple types of iron carbide are contained, the volume fraction of iron carbide is the sum of the volume fractions of those iron carbides.

[0042] If the volume fraction of iron carbide is less than 2.0%, it becomes difficult to obtain excellent wear resistance. The lower limit of the volume fraction of iron carbide is preferably 3.0%, more preferably 5.0%, and even more preferably 7.0%. The upper limit of the volume fraction of iron carbide is preferably 18.0%, more preferably 15.0%, even more preferably 12.0%, even more preferably 10.0%, and even more preferably 8.0%.

[0043] The volume fraction of iron carbide can be adjusted by the C content of the steel and the tempering conditions. Specifically, the higher the C content, the higher the volume fraction of iron carbide tends to be. Regarding the tempering conditions, the higher the holding temperature and the longer the holding time, the higher the volume fraction of iron carbide tends to be.

[0044] The structure of the steel material for sliding parts according to this embodiment may contain a small amount of structures other than tempered martensite, bainite, and iron carbide. Examples of structures other than tempered martensite, bainite, and iron carbide include ferrite, pearlite, retained austenite, MnS, etc. The volume fraction of structures other than tempered martensite, bainite, and carbides in the structure of the steel material for sliding parts according to this embodiment is preferably 5.0% or less in total, more preferably 3.0% or less, even more preferably 2.0% or less, and even more preferably 1.0% or less.

[0045] [Vickers hardness] The steel material for sliding parts according to this embodiment has a Vickers hardness of 300 or more and 600 or less. If the Vickers hardness is less than 300, it becomes difficult to obtain excellent wear resistance. On the other hand, if the Vickers hardness is higher than 600, workability decreases. From the viewpoint of wear resistance, the lower limit of the Vickers hardness is preferably 350, more preferably 400, even more preferably 450, even more preferably 500, and even more preferably 530. From the viewpoint of workability, the upper limit of the Vickers hardness is preferably 580, more preferably 560, even more preferably 550, even more preferably 530, and even more preferably 520.

[0046] The Vickers hardness of steel for sliding parts can be adjusted by the C content of the steel, the quenching conditions, and the tempering conditions. Specifically, the higher the C content, the higher the Vickers hardness tends to be. Regarding quenching conditions, the faster the cooling rate, the higher the Vickers hardness tends to be. Regarding tempering conditions, the lower the holding temperature and the shorter the holding time, the higher the Vickers hardness tends to be.

[0047] [Equation (1)] In the steel material for sliding parts according to this embodiment, the volume fraction X of iron carbide and the Vickers hardness Hv of the steel material satisfy the following relational expression (1): By satisfying the relational expression (1), excellent wear resistance can be obtained. X≧-0.065×Hv+36.5 (1) The unit of X is %, and the unit of Hv is Hv.

[0048] [others] In the steel material for sliding parts according to this embodiment, the average minor axis length of the iron carbides is preferably 0.027 μm or less. By dispersing iron carbides of this shape, the hardness of the steel material as a whole can be maintained. If the iron carbides are too large, the softness of the matrix will have a large effect, and wear resistance may decrease. The average minor axis length of the iron carbides is preferably 0.025 μm or less.

[0049] The steel material for sliding parts according to this embodiment preferably has no nitride layer, carburized layer or carbonitrided layer on the surface.

[0050] The steel material for sliding parts according to this embodiment preferably has a surface Vickers hardness of 300 or more and 600 or less, and the volume fraction X of iron carbide on the surface and the Vickers hardness Hv preferably satisfy the above-mentioned relational expression (1).

[0051] In the above, "Vickers hardness of the surface" more specifically means the Vickers hardness in a region of the steel material for sliding parts at a depth of 100 μm or less from the surface, and "volume fraction of iron carbide in the surface" more specifically means the volume fraction of iron carbide in the structure in a region of the steel material for sliding parts at a depth of 100 μm or less from the surface.

[0052] [Manufacturing method for steel materials for sliding parts] Hereinafter, a method for manufacturing a steel material for sliding parts according to this embodiment will be described.

[0053] A material having the above-described chemical composition is prepared. The material may be, for example, a hot forged product. For example, steel having the above-described chemical composition is melted and continuously cast or bloomed into a billet, which is then hot forged into a rough shape for the sliding part, which may then be used as the material. The material after hot forging may be subjected to cutting or other processing.

[0054] The material is held at a temperature between 830°C and 1100°C, and then quenched by cooling from the holding temperature to 300°C at a cooling rate of 300°C / second or faster. If the holding temperature is too low, a uniform structure may not be obtained. On the other hand, if the holding temperature is too high, the crystal grains may become coarse. If the cooling rate is too slow, the desired structure may not be obtained. Note that the higher the cooling rate in the quenching process, the higher the Vickers hardness of the final steel material for sliding parts.

[0055] The quenched material is tempered by holding it at a temperature of 200°C or higher and 600°C or lower. The higher the tempering holding temperature and the longer the holding time, the lower the Vickers hardness of the steel material for sliding parts that is finally obtained. Furthermore, the higher the tempering holding temperature and the longer the holding time, the higher the volume fraction of iron carbide in the structure of the steel material for sliding parts that is finally obtained. If the tempering holding temperature is outside this range, it becomes difficult to set the volume fraction of iron carbide and Vickers hardness within the specified range. The quenching and tempering conditions are adjusted depending on the chemical composition of the steel material, etc., so that the volume fraction X of iron carbide and the Vickers hardness Hv satisfy the relational expression (1). In this way, the steel material for sliding parts according to this embodiment is obtained.

[0056] The above describes a steel material for sliding parts according to one embodiment of the present invention. The steel material for sliding parts according to this embodiment has excellent sliding properties and workability. Therefore, the steel material for sliding parts according to this embodiment is suitable as a material for sliding parts. An example of a sliding part is a crankshaft. [Example]

[0057] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0058] Steels having the chemical compositions shown in Table 1 were melted in a 10 kg vacuum induction melting furnace to produce ingots.

[0059] [Table 1]

[0060] This ingot was hot forged at 950-1200°C to a thickness of 30 mm, width of 100 mm, and length of 290 mm, and then rolled to a thickness of 7 mm and width of 110 mm. The rolled material was cut into a width of 15 mm, length of 60-120 mm, and thickness of 7 mm, and subjected to the heat treatment described in Table 2. The structure before heat treatment was ferrite-pearlite (F+P) in all cases. The values ​​in the "Cooling Rate" column under "Quenching" in Table 2 are the cooling rates from the quenching holding temperature to 300°C.

[0061] [Table 2]

[0062] After the heat treatment, several test pieces measuring 20 mm square and 2 mm thick were taken from each material. These test pieces were used to observe the structure, measure Vickers hardness, and evaluate the sliding properties.

[0063] The surface of the test specimen for microstructural observation was processed by Ar ion milling. By irradiating the sample with an Ar ion beam at an angle of 80° or more from the perpendicular direction, the iron matrix, which is softer than the iron carbide, is milled away, leaving the iron carbide as convex portions, making it possible to search for the iron carbide using an atomic force microscope (AFM). Figure 5 shows an example of a surface topography image of a processed test specimen taken with an AFM. White indicates convex portions, and black indicates concave portions. The white areas in Figure 5 are iron carbide.

[0064] The volume fraction of iron carbide was calculated using image analysis software. ImageJ was used to obtain topographical images of a 2 μm x 2 μm area at three locations on the test piece. The image contrast and resolution were adjusted so that particle size could be detected using the image analysis software, and then the images were binarized. The particles were then detected using the particle analysis function of the image analysis software. Figure 6 shows an example of iron carbide detected using the image analysis software. The area fraction of iron carbide was calculated at each of the three observed locations, and their average was calculated. The resulting area fraction was considered to be the volume fraction of iron carbide.

[0065] The ferrite volume fraction was calculated using SEM secondary electron images (topographic images). The sample surface was etched with nital to etch only the ferrite, creating depressions, and then a secondary electron image was acquired at 1000x magnification. The acquired image was imported into the image analysis software ImageJ, and the relevant area was selected using the freehand selection or polygon selection functions. The selected area was masked and binarized in the same way as for iron carbide, and the relevant area was detected using the particle analysis function of the image analysis software. The ferrite area fraction was calculated at each of the three observed locations and their average was calculated. The resulting area fraction was considered to be the ferrite volume fraction.

[0066] The volume fraction of retained austenite was measured by X-ray diffraction. The surface of the test specimen was photographed with an optical microscope (magnification: 210x, field of view: 1218 μm × 1218 μm) to determine the area fraction of MnS, and this area fraction was considered to be the volume fraction. The sum of the volume fractions of tempered martensite and bainite (or the sum of the volume fractions of martensite and bainite) was calculated by subtracting the sum of the volume fractions of iron carbide, retained austenite, ferrite, and MnS from 100%.

[0067] The morphology of the iron carbide was determined by image analysis of the topographical images acquired when determining the volume fraction of iron carbide. Specifically, the images were binarized, and all particles within each observation field were approximated as ellipses using the particle analysis function of the image analysis software. The average minor axis length and average major axis length were determined at each of the three observation points, and then their average was calculated.

[0068] The Vickers hardness was measured at five points with a test force of 1 kgf (9.807 N) and the average was calculated.

[0069] The microstructure and Vickers hardness of each steel after heat treatment are shown in Table 3. In the volume fraction column of the microstructure in Table 3, "M" represents as-quenched martensite, "B" represents bainite, "TM" represents tempered martensite, and "residual γ" represents retained austenite.

[0070] [Table 3]

[0071] The surfaces of the test specimens for the sliding test were mirror-finished. The sliding test was performed using a ball-on-disk friction and wear tester. Figure 7 shows a schematic diagram of the tester. Alumina balls were used, and the load was 10 N and the sliding speed was 10 mm / s. After the sliding test, the width of the sliding marks was measured, and if the average value of the sliding mark width was 160 μm or less, the wear resistance was evaluated as "good," and if it exceeded 160 μm, the wear resistance was evaluated as "poor."

[0072] The Vickers hardness, carbide (iron carbide) volume fraction, and sliding test results for each steel are shown in Table 4. Workability was evaluated as "good" if the Vickers hardness was 600 or less, and "poor" if it exceeded 600. The overall evaluation was "pass" if both workability and wear resistance were "good," and "poor" if either workability or wear resistance was "poor."

[0073] [Table 4]

[0074] As shown in Table 4, the steel materials Nos. 4 to 7, 9, 10, and 14 to 17 had Vickers hardnesses of 300 to 600, and the volume fraction X of iron carbide and the Vickers hardness Hv satisfied the relational expression (1). These test materials had wear scar widths of 160 μm or less after the sliding test, indicating excellent wear resistance. Furthermore, these test materials also had Vickers hardnesses Hv of 600 or less, indicating excellent processability.

[0075] The wear scar width after the sliding test exceeded 160 μm for steel materials Nos. 1 to 3, 8, 12, and 13. This is thought to be because the volume fraction X of iron carbide and the Vickers hardness Hv did not satisfy the relational expression (1).

[0076] Steel No. 11 had an as-quenched structure. Steel No. 11 had good wear resistance, but its Vickers hardness (Hv) exceeded 600, and its workability was poor.

[0077] Figure 3 is a scatter plot showing the relationship between the Vickers hardness of steel and the volume fraction of iron carbide. Figure 4 is a graph showing the relationship between the Vickers hardness of steel and the wear scar width obtained by a sliding test using a ball-on-disk friction and wear test. In Figures 3 and 4, open circles indicate steels whose iron carbide volume fraction X and Vickers hardness Hv satisfy the relationship (1), while solid circles indicate steels whose relationship (1) does not. The triangular symbols in Figure 4 represent steel (No. 11) whose structure is as-quenched. Figures 3 and 4 indicate that excellent wear resistance can be obtained when the iron carbide volume fraction X and Vickers hardness Hv of steel satisfy the relationship (1).

[0078] Although one embodiment of the present invention has been described above, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. A steel material for sliding parts made of steel, The chemical composition of the steel material is, in mass%, C: 0.30-0.60%, Si: 0.01-2.00%, Mn: 0.10-2.00%, Al: 0.060% or less, N: 0.020% or less, P: 0.10% or less, S: 0.20% or less, Cr: 0 to 0.50%, The balance is Fe and impurities. the surface structure contains at least one of tempered martensite and bainite and iron carbide, and the volume fraction of the total of the tempered martensite and the bainite is 80% or more and the volume fraction of the iron carbide is 2.0% or more; The Vickers hardness of the surface is 300 or more and 600 or less, The volume fraction X of iron carbide in the surface structure and the Vickers hardness Hv of the surface satisfy the following relational expression (1), The steel material for sliding parts, wherein the iron carbide has an average minor axis length of 0.027 μm or less. X≧−0.065×Hv+36.5 (1) The unit of X is % and the unit of Hv is Hv.

2. A steel material for sliding parts according to claim 1, A steel material for sliding parts that does not have any nitride layer, carburized layer or carbonitrided layer on its surface.

3. The steel material for sliding parts according to claim 1 or 2, The steel material for sliding parts has a Vickers hardness of the surface of 300 or more and 550 or less.

4. A method for producing a steel material for sliding parts according to claim 1 or 2, a step of holding the material at a temperature of 830°C or higher and 1100°C or lower, and then quenching the material by cooling it from the holding temperature to 300°C at a cooling rate of 300°C / second or higher; and tempering the quenched material by holding it at a temperature of 200°C or higher and 600°C or lower.

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