Sliding member steel material and method for manufacturing sliding member steel material

Abstract

Provided is a steel material for a sliding member that is excellent in sliding properties and workability. The steel material for a sliding member is formed of a steel material having a C content of 0.30 to 0.60 mass%, the structure of the steel material for a sliding member includes at least one of tempered martensite and bainite and iron carbide, the volume fraction of the sum of the tempered martensite and the bainite is 80% or more, the volume fraction of the iron carbide is 2.0% or more, the Vickers hardness is 300 or more and 600 or less, and the volume fraction X of the 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.

Description

Technical Field

[0001] This 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 automotive parts, railway vehicle parts, building components, and pipes. In particular, carbon steel and alloy steel for mechanical structures are often used as materials for sliding components, such as gears and shafts, in power transmission systems due to their high mechanical strength.

[0003] The biggest problem with sliding components lies in the friction and wear between the components, which are considered a cause of overall mechanical system inefficiency and poor performance. Looking ahead, we anticipate that as mechanical systems become smaller and lighter, the environment for sliding components will become increasingly demanding. In components such as crankshafts in automotive engines, improving the resistance to adhesion of rotating sliding parts, along with miniaturization and weight reduction, will remain a persistent challenge. To address these issues, it is necessary to develop steels for sliding components with superior sliding properties compared to current methods, in preparation for the overall miniaturization and weight reduction of mechanical systems.

[0004] One of the problems to be solved regarding steel for sliding components, from the perspective of extending component life and improving reliability, is improving wear resistance. Increasing the hardness of the steel is considered effective in improving wear resistance. However, increasing hardness impairs the machinability of the steel, posing a risk during mass production. Therefore, as a method to improve the sliding performance of sliding components, selectively controlling the microstructure of only the surface layer and hardening only that portion is effective.

[0005] For example, Japanese Patent Application Publication No. 1-230746 discloses the following: In a sliding member having a fixed part made of cast iron and a sliding member made of a high-hardness material with a hardness higher than that of cast iron, the surface structure of the fixed part is set as a structure formed of a hardened layer and oxides, wherein the hardened layer is formed of martensite or a mixed structure of martensite, pearlite, ferrite and graphite.

[0006] Besides controlling hardness, there are methods to improve adhesion resistance by controlling precipitates in the steel to suppress adhesion. Japanese Patent Application Publication No. 2013-227674 discloses a gear having a steel microstructure in its surface layer containing 1 to 10% retained austenite in tempered martensite and / or tempered bainite by area, and precipitating more than 5% carbides by area, and a nitrogen concentration of 2.0 to 6.0% at a depth of 20 μm from the surface.

[0007] Japanese Patent Application Publication No. 2010-100881 discloses a sliding component that has undergone carburizing or carburizing and nitriding. In this component, in the surface layer extending 10 μm from the surface of the sliding surface, the Vickers hardness at a depth of 10 μm from the surface of the sliding surface is 700 or higher, the average particle size of the cementite is 0.6 μm or less, and the number density of cementite particles in a cross-section perpendicular to the sliding surface is 1 particle / μm. 2 above.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 1-230746

[0011] Patent Document 2: Japanese Patent Application Publication No. 2013-227674

[0012] Patent Document 3: Japanese Patent Application Publication No. 2010-100881 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] In sliding components such as rotating shafts and crankshafts, it is crucial to suppress friction and wear between the surfaces of the sliding components, and to function in a state that prevents mechanical damage and thermal cracking caused by overload. Increasing hardness is one way to improve wear resistance, but increasing hardness can also become a major cause of impaired machinability. Furthermore, preventing adhesion in sliding components is important.

[0015] The objective of this invention is to provide a steel for sliding components with excellent sliding properties and machinability. Another objective of this invention is to provide a method for manufacturing a steel for sliding components with excellent sliding properties and machinability.

[0016] Solution for solving the problem

[0017] In one embodiment of the present invention, the sliding component steel is formed from steel with a C content of 0.30 to 0.60% by mass. The microstructure of the sliding component steel includes at least one of tempered martensite and bainite, and iron carbides. The volume fraction of the tempered martensite and bainite is 80% or more, and the iron carbides are 2.0% or more. The Vickers hardness is 300 or more and 600 or less. The volume fraction X of the iron carbides and the Vickers hardness Hv satisfy the following relationship (1).

[0018] X≥-0.065×Hv+36.5 (1)

[0019] The unit of X is %, and the unit of Hv is Hv.

[0020] Alternatively, for the steel used in the sliding component according to one embodiment of the present invention, the chemical composition of the steel, in mass %, is: C: 0.30-0.60%, Si: 0.01-2.00%, Mn: 0.10-2.00%, Al: less than 0.060%, N: less than 0.020%, P: less than 0.10%, S: less than 0.20%, Cr: 0-0.50%, balance: Fe and impurities.

[0021] A method for manufacturing steel for sliding parts according to one embodiment of the present invention is a method for manufacturing the aforementioned steel for sliding parts, the method comprising the following steps: a step of quenching a billet by cooling it at a temperature of 830°C or higher and 1100°C or lower, wherein the billet is cooled at a cooling rate of 300°C / second or higher from the holding temperature to 300°C; and a step of tempering the quenched billet by holding it at a temperature of 200°C or higher and 600°C or lower.

[0022] The effects of the invention

[0023] According to the present invention, steel for sliding parts with excellent sliding properties and machinability can be obtained. Attached Figure Description

[0024] Figure 1 It is a concave-convex image of steel obtained using an atomic force microscope.

[0025] Figure 2 It is an image of the adhesive force of steel obtained using an atomic force microscope.

[0026] Figure 3 It is a scatter plot showing the relationship between the Vickers hardness of steel and the volume fraction of iron carbides.

[0027] Figure 4 It is a graph showing the relationship between the Vickers hardness of steel and the width of the wear mark obtained by a sliding test conducted by a ball-and-disc friction and wear testing machine.

[0028] Figure 5 This is an example of using an atomic force microscope to measure the concave-convex image obtained from a test piece whose surface has been machined using Ar ions.

[0029] Figure 6 This is an example of iron carbides detected using image analysis software.

[0030] Figure 7 This is a schematic diagram of a ball-and-disc type friction and wear testing machine. Detailed Implementation

[0031] In order to develop steel with excellent sliding and machinability, the inventors investigated the sliding and machinability of steel. As a result, the following insights were obtained.

[0032] Figure 1 and Figure 2 The image was obtained using an atomic force microscope (AFM). Figure 1 It is a concave-convex image. Figure 2 It is an adhesive image. Figure 1 In the diagram, raised areas are represented by white, and recessed areas by black. Figure 2 In the diagram, the parts with stronger adhesion are represented in white, and the parts with weaker adhesion are represented in black.

[0033] Figure 1 The unevenness pattern was obtained using AFM measurements on samples whose surfaces had been milled with Ar ions. This milling process contrasts this with milling of a soft iron matrix containing iron carbides, where the iron carbides remain as raised areas, thus allowing for AFM searching of the iron carbides. Figure 1 In the image, the white, raised portion is iron carbide. The adhesive force was measured within the same range. Figure 2 ,from Figure 1 and Figure 2 It can be seen that iron carbides have relatively low adhesion.

[0034] Therefore, it is believed that increasing the volume fraction of iron carbides can improve adhesion resistance. On the other hand, increasing the volume fraction of iron carbides also leads to a decrease in the hardness and wear resistance of the steel.

[0035] Figure 3 This is a scatter plot showing the relationship between the Vickers hardness of the steel produced in the embodiments discussed below and the volume fraction of iron carbides. Figure 4 This is a graph showing the relationship between the Vickers hardness of steel and the width of the wear mark obtained through a sliding test conducted using a ball-and-disc friction and wear testing machine. The smaller the wear mark width, the higher the wear resistance.

[0036] exist Figure 3 and Figure 4 In this context, a hollow circle represents the case where the volume fraction of iron carbides X and the Vickers hardness Hv of the steel satisfy the following relationship (1), while a solid circle represents the case where the volume fraction of iron carbides X and the Vickers hardness Hv of the steel do not satisfy relationship (1). Furthermore, Figure 4 The triangle symbol in the diagram represents steel with a structure set to a quenched state.

[0037] X≥-0.065×Hv+36.5 (1)

[0038] The unit of X is %, and the unit of Hv is Hv.

[0039] from Figure 3 and Figure 4 It can be seen that if the volume fraction X of iron carbides and the Vickers hardness Hv of steel satisfy the relationship (1), excellent wear resistance can be obtained.

[0040] Based on the above insights, the present invention has been completed. Hereinafter, a steel material for a sliding component according to an embodiment of the present invention will be discussed in detail.

[0041] [Chemical Composition]

[0042] The sliding component steel of this embodiment is formed from steel with a C content of 0.30 to 0.60% by mass. There is a tendency that the higher the C content, the higher the volume fraction of carbides. Furthermore, there is a tendency that the higher the C content, the higher the Vickers hardness of the sliding component steel. If the C content falls outside the range of 0.30 to 0.60% by mass, it becomes difficult to satisfy the relationship between the volume fraction of iron carbides and Vickers hardness (1), or even if the relationship (1) is satisfied, it is impossible to obtain steel with an excellent balance between sliding properties and workability. The lower limit of the C content of the sliding component steel of this embodiment is preferably 0.32% by mass, more preferably 0.35% by mass, more preferably 0.38% by mass, and more preferably 0.40% by mass. The upper limit of the C content of the sliding component steel of this embodiment is preferably 0.58% by mass, and more preferably 0.55% by mass.

[0043] In this embodiment, the C content of the steel used for the sliding component can be 0.30 to 0.60% by mass, and other properties are not particularly limited. It can also have a chemical composition as described below. In the following description, "%" for element content indicates mass%.

[0044] C: 0.30~0.60%

[0045] Carbon (C) improves the hardenability of steel. As already mentioned, if the C content deviates from an appropriate range, it becomes difficult to satisfy the relationship between the volume fraction of iron carbides and Vickers hardness (1), or even if the relationship (1) is satisfied, it is impossible to obtain steel with an excellent balance between smoothness and machinability. Therefore, the C content is 0.30–0.60%. The lower limit of the C content is preferably 0.32%, more preferably 0.35%, more preferably 0.38%, and even more preferably 0.40%. The upper limit of the C content is preferably 0.58%, and more preferably 0.55%.

[0046] Si: 0.01~2.00%

[0047] 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 can be from 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%, even more preferably 0.80%, even more preferably 0.60%, and even more preferably 0.40%.

[0048] Mn: 0.10~2.00%

[0049] Manganese (Mn) improves the hardenability of steel. On the other hand, if the Mn content is too high, the workability of the steel decreases. Therefore, the Mn content can be from 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%, even more preferably 1.50%, even more preferably 1.00%, and even more preferably 0.90%.

[0050] Al: below 0.060%

[0051] 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 can also 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%. If the deoxidation effect brought about by Al is obtained, the Al content can also be set to 0.020% or more.

[0052] N: below 0.020%

[0053] Nitrogen (N) reduces the hot workability of steel. Therefore, the N content can be 0.020% or less. The upper limit of the N content is preferably 0.018%, more preferably 0.015%, more preferably 0.010%, and even more preferably 0.005%. On the other hand, excessively restricting the N content increases manufacturing costs. Therefore, the lower limit of the N content can also be set at 0.0010%.

[0054] P: below 0.10%

[0055] Phosphorus (P) is an impurity. P segregates at grain boundaries, reducing the hot workability and toughness of steel. Therefore, the P content can be 0.10% or less. Preferably, the P content is 0.03% or less, and more preferably 0.02% or less. The P content is preferably as low as possible.

[0056] S: below 0.20%

[0057] Sulfur (S) is sometimes added to improve the machinability (cutting properties) of steel. However, if the S content is too high, the steel's resistance to quenching cracks decreases. Therefore, the S content can 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%. If the improved machinability resulting from S is achieved, the S content can also be set to 0.020% or more.

[0058] Cr: 0–0.50%

[0059] Chromium (Cr) is an arbitrary element. That is, the steel used for the sliding parts in this embodiment may not contain Cr. Cr improves the hardenability of steel. This effect can be obtained as long as it contains a small amount of Cr. On the other hand, if the Cr content is too high, the workability of the steel will decrease. Therefore, the Cr content can also 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%.

[0060] In this embodiment, the balance of the chemical composition of the steel used for the sliding component may also be Fe and impurities. Here, impurities refer to elements mixed in from the ore or waste used as raw material for steel, or elements mixed in from the environment during the manufacturing process.

[0061] The steel used for the sliding parts in this embodiment can also be made of carbon steel or alloy steel for mechanical structures. Preferably, the steel used for the sliding parts in this embodiment is made of carbon steel for mechanical structures as specified in JIS G 4051:2016, or alloy steel for mechanical structures as specified in JIS G 4053:2016. Particularly preferred are S45C and S50C of JIS G 4051:2016, and SMn438 of JIS G 4053:2016. Furthermore, to improve machinability, steel containing 0.20% by mass or less of sulfur (S) in these steels can also be used.

[0062] [organize]

[0063] The microstructure of the steel used for the sliding component in this embodiment includes at least one of tempered martensite and bainite (including tempered bainite, the same applies below) and iron carbides, with a volume fraction of at least 80% for the sum of tempered martensite and bainite and at least 2.0% for iron carbides.

[0064] In this embodiment, the total volume fraction of tempered martensite and the volume fraction of bainite in the microstructure of the steel used for the sliding component are 80% or more. The microstructure of the steel used for the sliding component in this embodiment may include at least one of tempered martensite and bainite.

[0065] In this embodiment, the machinability of the sliding component steel is ensured by tempering it to produce a microstructure containing a predetermined amount of iron carbides. In contrast, it is difficult to ensure good machinability when the microstructure of the sliding component steel is set to a quenched state microstructure (a microstructure mainly composed of quenched martensite). The sliding component steel in this embodiment preferably contains tempered martensite.

[0066] Excellent wear resistance is difficult to obtain when the sum of the volume fractions of tempered martensite and bainite is less than 80%. 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.

[0067] In this embodiment, iron carbides are considered as a separate microstructure in the calculation of the volume fraction of the microstructure, thus distinguishing them from tempered martensite and bainite. That is, the portion in which iron carbides are precipitated is not included in the volume of tempered martensite or bainite.

[0068] In this embodiment, the volume fraction of iron carbides in the steel for the sliding component is 2.0% or more. Specifically, the iron carbides in the steel for the sliding component in this embodiment are at least one of ε-carbides and cementite. The steel for the sliding component may contain one or more types of iron carbides. When multiple types of iron carbides are contained, the volume fraction of the iron carbides is the sum of the volume fractions of these iron carbides.

[0069] If the volume fraction of iron carbides is less than 2.0%, it is difficult to obtain excellent wear resistance. The lower limit of the volume fraction of iron carbides is preferably 3.0%, more preferably 5.0%, and even more preferably 7.0%. The upper limit of the volume fraction of iron carbides is preferably 18.0%, more preferably 15.0%, even more preferably 12.0%, even more preferably 10.0%, and even more preferably 8.0%.

[0070] The volume fraction of iron carbides can be adjusted by utilizing the carbon content of the steel and the tempering conditions. Specifically, there is a tendency for a higher carbon content to result in a higher volume fraction of iron carbides. In addition, regarding tempering conditions, there is a tendency for higher holding temperatures and longer holding times to result in a higher volume fraction of iron carbides.

[0071] The microstructure of the steel used for the sliding component in this embodiment may also contain a small amount of microstructure other than tempered martensite, bainite, and iron carbides. Microstructure other than tempered martensite, bainite, and iron carbides includes, for example, ferrite, pearlite, retained austenite, and MnS. The total volume fraction of microstructure other than tempered martensite, bainite, and carbides in the microstructure of the steel used for the sliding component in this embodiment is preferably 5.0% or less, more preferably 3.0% or less, more preferably 2.0% or less, and more preferably 1.0% or less.

[0072] Vickers hardness

[0073] The Vickers hardness of the steel used for the sliding parts in this embodiment is 300 or higher and 600 or lower. If the Vickers hardness is less than 300, it is difficult to obtain excellent wear resistance. On the other hand, if the Vickers hardness is higher than 600, the machinability decreases. From the viewpoint of wear resistance, the lower limit of the Vickers hardness is preferably 350, more preferably 400, more preferably 450, more preferably 500, and more preferably 530. From the viewpoint of machinability, the upper limit of the Vickers hardness is preferably 580, more preferably 560, more preferably 550, more preferably 530, and more preferably 520.

[0074] The Vickers hardness of steel used for sliding parts can be adjusted by utilizing the carbon content, quenching conditions, and tempering conditions. Specifically, there is a tendency for higher carbon content to result in higher Vickers hardness. Regarding quenching conditions, there is a tendency for higher cooling rates to result in higher Vickers hardness. Furthermore, regarding tempering conditions, there is a tendency for lower holding temperatures and shorter holding times to result in higher Vickers hardness.

[0075] [Relation (1)]

[0076] In this embodiment, the volume fraction X of iron carbides in the steel used for the sliding component satisfies the following relationship (1) with the Vickers hardness Hv of the steel. By satisfying the relationship (1), excellent wear resistance can be obtained.

[0077] X≥-0.065×Hv+36.5 (1)

[0078] The unit of X is %, and the unit of Hv is Hv.

[0079] [other]

[0080] In this embodiment, the average minor axis length of the iron carbides in the steel used for the sliding component is preferably 0.027 μm or less. By dispersing the iron carbides in this shape, the overall hardness of the steel can be maintained. If the iron carbides are too large, the influence on the softness of the matrix increases, and the wear resistance decreases. The average minor axis length of the iron carbides is preferably 0.025 μm or less.

[0081] In this embodiment, the steel used for the sliding component preferably does not have any of the following on its surface: a nitrided layer, a carburized layer, or a carburized-nitrided layer.

[0082] In this embodiment, the Vickers hardness of the surface of the steel used for the sliding component is 300 or more and 600 or less, and the volume fraction X of iron carbides on the surface and the Vickers hardness Hv preferably satisfy the above-mentioned relationship (1).

[0083] In the above, "surface Vickers hardness" more specifically refers to the Vickers hardness of the region at a depth of 100 μm or less from the surface of the steel for sliding parts. "Volume fraction of iron carbides on the surface" more specifically refers to the volume fraction of iron carbides in the microstructure of the region at a depth of 100 μm or less from the surface of the steel for sliding parts.

[0084] [Manufacturing method of steel for sliding parts]

[0085] The following describes a method for manufacturing the steel for the sliding component according to this embodiment.

[0086] Prepare a billet having the above-described chemical composition. The billet may be, for example, a hot-forged product. For instance, a hot-forged product can be prepared as a billet by melting steel having the above-described chemical composition, performing continuous casting or initial rolling to obtain a steel billet, and then hot-forging the steel billet to machine it into the approximate shape of a sliding part. Machining or other machining processes can also be performed on the hot-forged billet.

[0087] After holding the billet at a temperature above 830°C and below 1100°C, quenching is performed by cooling at a rate of 300°C / second or higher from the holding temperature down to 300°C. If the holding temperature is too low, a uniform microstructure cannot be obtained. On the other hand, if the holding temperature is too high, grain coarsening occurs. If the cooling rate is too low, the desired microstructure cannot be obtained. Furthermore, there is a tendency that the higher the cooling rate in the quenching process, the higher the Vickers hardness of the steel for sliding parts that can be obtained.

[0088] The quenched billet is tempered at a temperature between 200°C and 600°C. There is a tendency that the higher the holding temperature and the longer the holding time, the lower the Vickers hardness of the resulting sliding component steel. Furthermore, there is a tendency that the higher the holding temperature and the longer the holding time, the higher the volume fraction of iron carbides in the microstructure of the resulting sliding component steel. If the holding temperature deviates from this range, it becomes difficult to set the volume fraction of iron carbides and the Vickers hardness within the predetermined range. The quenching and tempering conditions are adjusted according to the chemical composition of the steel to ensure that the volume fraction of iron carbides X and the Vickers hardness Hv satisfy the relationship (1). Thus, the sliding component steel of this embodiment can be obtained.

[0089] The steel for sliding components according to one embodiment of the present invention has been described above. The steel for sliding components of this embodiment possesses excellent sliding properties and machinability. Therefore, the steel for sliding components of this embodiment is suitable as a material for sliding components. A sliding component is, for example, a crankshaft.

[0090] Example

[0091] The present invention will now be described in more detail with reference to specific embodiments. However, the present invention is not limited to these embodiments.

[0092] Steel with the chemical composition shown in Table 1 was melted using a 10kg vacuum induction melting furnace to produce ingots.

[0093] [Table 1]

[0094] Table 1

[0095]

[0096] The ingot was hot-forged at 950–1200°C to a thickness of 30 mm, a width of 100 mm, and a length of 290 mm, and then rolled to a thickness of 7 mm and a width of 110 mm. The rolled material was cut into pieces with a width of 15 mm, a length of 60–120 mm, and a thickness of 7 mm, and then subjected to the heat treatments described in Table 2. Furthermore, the microstructure before heat treatment was ferrite-pearlite (F+P). The values ​​in the "Cooling Rate" column of "Quenching" in Table 2 represent the cooling rate from the holding temperature of quenching up to 300°C.

[0097] [Table 2]

[0098]

[0099] After heat treatment, multiple 20mm square, 2mm thick test pieces were collected from each material. These test pieces were used for microstructure observation, Vickers hardness measurement, and evaluation of lubrication properties.

[0100] The test specimens used for tissue observation were surface-machined using Ar ion milling. The specimen was irradiated with an Ar ion beam at an angle of 80° or greater perpendicular to the specimen, thus milling away the iron matrix, which is softer than iron carbides. The iron carbides remained in a raised form, allowing for the search of iron carbides using atomic force microscopy (AFM). An example of a surface roughness image of the machined specimen obtained using AFM is shown below. Figure 5 In the middle, white indicates convex parts, and black indicates concave parts. Figure 5 The white part is iron carbide.

[0101] Concave-convex images were obtained at three locations on the test piece within a 2μm × 2μm area using AFM (Aspect-Factor Method), and the volume fraction of iron carbides was calculated using image analysis software, ImageJ. After adjusting the image contrast and resolution to enable particle size detection using the image analysis software, the images were binarized, and particle analysis was performed using the software's particle analysis function. An example of iron carbides detected using image analysis software is shown below. Figure 6 In the middle, the area fraction of iron carbides was calculated at the three observed locations, and their average value was obtained. The obtained area fraction was regarded as the volume fraction of iron carbides.

[0102] The volume fraction of ferrite was calculated using secondary electron microscopy (SEM) images (concave-convex images). The sample surface was etched with a nitric acid-ethanol etching solution until only the ferrite was etched, creating a concave appearance. Secondary electron microscopy images were then obtained at 1000x magnification. The images were input into ImageJ image analysis software. Freehand selection and polygon selection functions were used to select the relevant areas, and the selected areas were masked. Binarization was performed in the same manner as for iron carbides. The particle analysis function of the image analysis software was used to detect the corresponding areas. The area fraction of ferrite was calculated at the three observed locations, and their average value was determined. The obtained area fraction was considered as the volume fraction of ferrite.

[0103] The volume fraction of retained austenite was measured using X-ray diffraction. For the volume fraction of MnS, the area ratio of MnS was determined by photographing the surface of the test specimen using an optical microscope (magnification: 210x, field of view: 1218 μm × 1218 μm), and this area ratio was considered as 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 carbides, retained austenite, ferrite, and MnS from 100%.

[0104] The morphology of iron carbides was determined using image analysis from the concave-convex image obtained when calculating the volume fraction of iron carbides. Specifically, the image was binarized, and the particle analysis function of the image analysis software was used to approximate all particles in each observation field with an ellipse. The average minor axis length and average major axis length were calculated at the three observed locations, and their average values ​​were obtained.

[0105] The Vickers hardness was measured at 5 points with a test force of 1 kgf (9.807 N), and the average value was calculated.

[0106] Table 3 shows the microstructure and Vickers hardness of each steel after heat treatment. In Table 3, the volume fraction column for microstructure indicates quenched martensite ("M"), bainite ("B"), tempered martensite ("TM"), and retained austenite ("γ").

[0107] [Table 3]

[0108]

[0109] The surface of the test piece used in the sliding test was mirror-finished. The sliding test was conducted using a ball-and-disc friction and wear testing machine. Figure 7 The diagram shows a schematic of the testing machine. Alumina balls are used, the load is set to 10 N, and the sliding speed is set to 10 mm / s. After the sliding test, the width of the sliding marks is measured. If the average width of the sliding marks is less than 160 μm, the wear resistance is rated as "good"; if it exceeds 160 μm, the wear resistance is rated as "unacceptable".

[0110] Table 4 shows the Vickers hardness, carbide (iron carbide) volume fraction, and sliding test results for each steel. For machinability, a Vickers hardness below 600 is rated "Good," while a value above 600 is rated "Unacceptable." In the comprehensive evaluation, steels with both "Good" machinability and wear resistance are designated "Acceptable," while steels with either "Unacceptable" machinability or wear resistance are designated "Unacceptable."

[0111] [Table 4]

[0112]

[0113] As shown in Table 4, the Vickers hardness of steels No. 4–7, 9, 10, and 14–17 is 300–600, and the volume fraction of iron carbides X and the Vickers hardness Hv satisfy the relationship (1). The wear mark width after sliding tests of these test materials is less than 160 μm, exhibiting excellent wear resistance. In addition, the Vickers hardness Hv of these test materials is less than 600, and their machinability is also excellent.

[0114] The wear marks of steels No. 1 to 3, 8, 12, and 13 after sliding tests exceeded 160 μm. This is believed to be because the volume fraction of iron carbides X and the Vickers hardness Hv do not satisfy the relationship (1).

[0115] No.11 steel has a microstructure that is in a quenched state. No.11 steel has good wear resistance, but its Vickers hardness Hv exceeds 600, resulting in poor machinability.

[0116] Figure 3 It is a scatter plot showing the relationship between the Vickers hardness of steel and the volume fraction of iron carbides. Figure 4 This is a graph showing the relationship between the Vickers hardness of steel and the width of the wear mark obtained from a sliding test using a ball-and-disc friction and wear test. Figure 3 and Figure 4 In the figure, the hollow circle symbol represents the case where the volume fraction X of iron carbides and the Vickers hardness Hv of steel satisfy the relationship (1), and the solid circle symbol represents the case where the relationship (1) is not satisfied. Figure 4 The triangle symbol in the diagram represents steel (No. 11) with a quenched microstructure. From... Figure 3 and Figure 4 It can be seen that if the volume fraction X of iron carbides and the Vickers hardness Hv of steel satisfy the relationship (1), excellent wear resistance can be obtained.

[0117] The present invention has been described above as an embodiment, but the above embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above embodiment, and can be implemented by appropriate modifications without departing from its spirit.

Claims

1. A type of steel for sliding components, formed from steel with a carbon content of 0.30 to 0.60% by mass. The microstructure of the steel used for the sliding component includes at least one of tempered martensite and bainite, as well as iron carbides, with a volume fraction of at least 80% for the sum of the tempered martensite and the bainite, and at least 2.0% for the iron carbides. A Vickers hardness of 300 or higher but below 600. The volume fraction X of the iron carbide and the Vickers hardness Hv satisfy the following relationship (1). X≥-0.065×Hv+36.5 (1) The unit of X is %, and the unit of Hv is Hv.

2. The steel for the sliding component according to claim 1, wherein, The chemical composition of the steel, expressed as C (by mass%), is 0.30–0.60%. Si: 0.01~2.00% Mn: 0.10~2.00% Al: below 0.060% N: below 0.020% P: below 0.10% S: Below 0.20% Cr：0～0.50％， Balance: Fe and impurities.

3. The steel for sliding components according to claim 1 or 2, wherein, The average minor axis length of the iron carbide is less than 0.027 μm.

4. The steel for sliding components according to claim 1 or 2, wherein, The sliding component is made of steel that does not have any of the following layers on its surface: nitrided layer, carburized layer, and carburized-nitrided layer.

5. The steel for sliding components according to claim 1 or 2, wherein, The Vickers hardness of the steel surface of the sliding component is above 300 and below 600, and the volume fraction X of the iron carbides on the surface and the Vickers hardness Hv satisfy the relationship (1).

6. The steel for sliding components according to claim 1 or 2, wherein, The Vickers hardness is above 300 and below 550.

7. A method for manufacturing steel for sliding components, which is the method for manufacturing steel for sliding components according to claim 1 or 2, wherein the method for manufacturing steel for sliding components comprises the following steps: A quenching process is performed by holding the billet at a temperature above 830°C and below 1100°C, and then cooling it at a cooling rate of 300°C / second or higher from the holding temperature down to 300°C; and The quenched billet is tempered at a temperature above 200°C and below 600°C.

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