CN118202078A

CN118202078A - Steel material for sliding member and method for producing steel material for sliding member

Info

Publication number: CN118202078A
Application number: CN202280073772.0A
Authority: CN
Inventors: 菊地菜摘; 西原基成; 安部达彦
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2021-11-08
Filing date: 2022-10-26
Publication date: 2024-06-14
Also published as: WO2023080029A1; JPWO2023080029A1

Abstract

Provided is a steel material for sliding parts, which has excellent sliding properties and workability. A steel for sliding parts, which is formed from a steel material having a C content of 0.30-0.60 mass%, has a structure comprising iron carbide and at least one of tempered martensite and bainite, and has a volume fraction that is the sum of the tempered martensite and the bainite: 80% or more of the iron carbide: 2.0% or more and a vickers hardness of 300 to 600, wherein the volume fraction X of the carbide and the vickers hardness Hv satisfy the following relational expression (1). X is equal to or more than-0.065 XHv+36.5 (1) X is a unit and Hv is a unit of Hv.

Description

Steel material for sliding member and method for producing steel material for sliding member

Technical Field

The present invention relates to a steel material for sliding members and a method for producing the steel material for sliding members.

Background

Steel materials are widely used in industrial products such as automobile parts, railway vehicle parts, building members, and pipes. In particular, carbon steel for machine construction and alloy steel for machine construction are often used as materials for sliding members represented by power transmission system members such as gears and shafts due to high mechanical strength.

The biggest problem of sliding members is friction and wear between the members, and these are considered to be causes of failure and inefficiency of the entire mechanical system. In the future, the following is expected: as the mechanical system is miniaturized and lightened, the environment of the sliding member becomes more severe. In a crankshaft, which is an engine component of an automobile, for example, improvement of adhesion resistance of a rotary sliding part along with downsizing and weight reduction has been a constant problem. In order to solve these problems, it is necessary to develop a steel material for sliding parts which has better sliding properties than the conventional steel materials, in order to provide for downsizing and weight saving of the entire machine system.

One of the problems to be solved in steel materials for sliding parts is improvement of wear resistance from the viewpoints of extension of the life of the parts and improvement of reliability. For improvement of wear resistance, it is considered that it is effective to increase the hardness of the steel material. However, the increase in hardness deteriorates workability of the steel material, and involves a risk in mass production of the components. Therefore, as a method for improving the sliding property of the sliding member, a method of selectively controlling only the surface layer and hardening only the portion is effective.

For example, japanese patent application laid-open No. 1-230746 discloses the following: in a sliding member including a fixed member made of cast iron and a sliding member made of a material having a higher hardness than that of cast iron, a surface layer structure of the fixed member is a structure made of a hardened layer formed of martensite or a mixed phase structure of martensite, pearlite, ferrite, and graphite, and an oxide.

As a method other than the control of hardness, there is a method of controlling precipitates in steel materials to suppress adhesion and to improve adhesion resistance. JP-A2013-227674 discloses a gear having a steel structure in which retained austenite is present in an area ratio of 1 to 10% in tempered martensite and/or tempered bainite, carbide is precipitated in an area ratio of 5% or more in a surface layer portion, and a nitrogen concentration at a depth of 20 μm from the surface is 2.0 to 6.0%.

Japanese patent application laid-open No. 2010-100881 discloses a sliding member which is carburized or carburized, wherein, in a surface layer portion from a surface of a sliding surface to a depth of 10 μm, a vickers hardness of 10 μm from the surface of the sliding surface: average grain size of cementite grains is 700 or more: number density of cementite grains of 0.6 μm or less in a cross section perpendicular to the sliding surface: 1/μm ² or more.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 1-230746

Patent document 2: japanese patent laid-open No. 2013-227674

Patent document 3: japanese patent laid-open No. 2010-100881

Disclosure of Invention

Problems to be solved by the invention

In sliding members such as a rotary shaft and a crankshaft, it is important to suppress friction and abrasion of surfaces of the sliding members, and to function in a state in which damage such as mechanical damage and thermal cracking due to overload is suppressed. As means for improving the wear resistance, there is an increase in hardness, but an increase in hardness is also a major cause of deterioration in workability. In addition, in order to prevent adhesion in the sliding member, it is important that adhesion is difficult.

The present invention addresses the problem of providing a steel material for sliding members that has excellent sliding properties and workability. Another object of the present invention is to provide a method for producing a steel material for sliding parts, which is excellent in sliding properties and workability.

Solution for solving the problem

A steel for a sliding member according to an embodiment of the present invention is a steel having a C content of 0.30 to 0.60 mass%, and a structure of the steel for a sliding member includes iron carbide and at least one of tempered martensite and bainite, and a volume fraction is a sum of the tempered martensite and the bainite: 80% or more of the iron carbide: 2.0% or more and a Vickers hardness of 300 to 600, wherein 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.

In the steel material for sliding parts according to an embodiment of the present invention, the chemical composition of the steel material may be C:0.30 to 0.60 percent of Si:0.01 to 2.00 percent of Mn:0.10 to 2.00 percent of Al:0.060% or less, N: less than 0.020%, P:0.10% or less, S: less than 0.20%, cr: 0-0.50%, the balance: fe and impurities.

The method for producing a steel material for sliding parts according to an embodiment of the present invention is a method for producing the steel material for sliding parts, comprising the steps of: a step of cooling the blank at a temperature of 830 ℃ to 1100 ℃ and then quenching the blank so that the cooling rate from the holding temperature to 300 ℃ is 300 ℃/sec or more; and tempering the quenched blank at a temperature of 200 ℃ to 600 ℃.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a steel material for sliding parts excellent in slidability and workability can be obtained.

Drawings

Fig. 1 is a view showing an uneven image of steel material obtained by an atomic force microscope.

Fig. 2 is an image of the adhesion of steel material obtained by an atomic force microscope.

FIG. 3 is a scatter diagram showing the relationship between the Vickers hardness of steel and the volume fraction of iron carbide.

FIG. 4 is a graph showing the relationship between the Vickers hardness of steel material and the width of wear marks obtained by a sliding test performed by a ball-and-disc type frictional wear tester.

Fig. 5 shows an example of an uneven image obtained by measuring a test piece whose surface is processed by Ar ion milling using an atomic force microscope.

Fig. 6 is an example of iron carbide detected using image analysis software.

Fig. 7 is a schematic view of a ball-and-disc type frictional wear testing machine.

Detailed Description

The present inventors examined the slidability and workability of steel materials in order to develop steel materials excellent in slidability and workability. As a result, the following findings were obtained.

Fig. 1 and 2 are images obtained by an Atomic Force Microscope (AFM), fig. 1 is a concave-convex image, and fig. 2 is an adhesion image. In fig. 1, the raised portions are indicated as white, and the recessed portions are indicated as black. In fig. 2, the portions having a large adhesion are indicated as white, and the portions having a small adhesion are indicated as black.

The relief image of fig. 1 is obtained by measuring a sample whose surface has been processed by Ar ion milling by AFM. With this processing, milling is performed against the iron matrix soft with iron carbide, and the iron carbide remains in the form of projections, so that searching for iron carbide by AFM can be performed. In fig. 1, white portions, i.e., raised portions, are iron carbides. The result of measuring the adhesion within the same range is fig. 2, and it is understood from fig. 1 and 2 that the adhesion of iron carbide is small.

Thus, it is considered that the adhesiveness resistance can be improved by increasing the volume fraction of iron carbide. On the other hand, if the volume fraction of iron carbide is increased, the hardness and wear resistance of the steel material are also expected to be reduced.

FIG. 3 is a scatter diagram showing the relationship between the Vickers hardness and the volume fraction of iron carbide of steel produced in the examples discussed later. Fig. 4 is a graph showing the relationship between the vickers hardness of steel materials and the width of wear marks obtained by a sliding test performed by a ball-and-disc type frictional wear testing machine. The smaller the wear scar width, the higher the wear resistance.

In fig. 3 and 4, the volume fraction X of the iron carbide and the vickers hardness Hv of the steel material satisfy the following relational expression (1) and the solid circles indicate that the volume fraction X of the iron carbide and the vickers hardness Hv of the steel material do not satisfy the relational expression (1). The triangle in fig. 4 is a symbol of a steel material having a quenched structure.

X≥-0.065×Hv+36.5 (1)

The unit of X is% and the unit of Hv is Hv.

As can be seen from fig. 3 and 4: when the volume fraction X of iron carbide and the vickers hardness Hv of the steel material satisfy the relational expression (1), excellent wear resistance can be obtained.

Based on the above findings, the present invention has been completed. Hereinafter, a steel material for sliding parts according to an embodiment of the present invention will be described in detail.

[ Chemical composition ]

The steel material for sliding members according to the present embodiment is formed 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. Further, the higher the C content, the higher the vickers hardness of the steel for sliding members tends to be. When the C content is out of the range of 0.30 to 0.60 mass%, the following may be present: it is difficult to satisfy the relation (1) between the volume ratio of iron carbide and vickers hardness, or even if the relation (1) is satisfied, a steel excellent in balance between slidability and workability cannot be obtained. The lower limit of the C content of the steel material for sliding parts according to the present 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 material for sliding parts according to the present embodiment is preferably 0.58 mass%, and more preferably 0.55 mass%.

The steel material for sliding parts according to the present embodiment may have a chemical composition in which the C content is 0.30 to 0.60 mass%, and the other components are not particularly limited and may have a chemical composition described below, for example. In the following description, "%" of the content of the element represents mass%.

C：0.30～0.60％

Carbon (C) improves the hardenability of the steel. As described above, if the C content is out of the proper range, the following may be present: it is difficult to satisfy the relation (1) between the volume ratio of iron carbide and vickers hardness, or even if the relation (1) is satisfied, a steel excellent in balance between slidability and workability cannot be obtained. Thus, the C content is 0.30 to 0.60%. The lower limit of the C content is preferably 0.32%, more preferably 0.35%, still more preferably 0.38%, and still more preferably 0.40%. The upper limit of the C content is preferably 0.58%, more preferably 0.55%.

Si：0.01～2.00％

Silicon (Si) deoxidizes the steel. On the other hand, if the Si content is too high, the workability of the steel is lowered. Thus, 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%, more preferably 0.40%.

Mn：0.10～2.00％

Manganese (Mn) improves the hardenability of the steel. On the other hand, if the Mn content is too high, the workability of the steel is lowered. Thus, 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%, more preferably 0.90%.

Al: less than 0.060%

Aluminum (Al) deoxidizes the steel. On the other hand, if the Al content is too high, the workability of the steel is lowered. Thus, 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 still more preferably 0.030%. In the case of obtaining the deoxidizing effect by Al, the Al content may be 0.020% or more.

N: less than 0.020%

Nitrogen (N) reduces the hot workability of the 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%, further preferably 0.010%, further preferably 0.005%. On the other hand, if the N content is excessively limited, the manufacturing cost increases. Therefore, the lower limit of the N content may be set to 0.0010%.

P: less than 0.10%

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

S: less than 0.20%

Sulfur (S) may be added to improve workability (machinability) of steel. On the other hand, if the S content is too high, the quench cracking resistance of the steel is lowered. 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 still more preferably 0.06%. In the case of obtaining the effect of improving workability by S, the S content may be set to 0.020% or more.

Cr：0～0.50％

Chromium (Cr) is an arbitrary element. That is, the steel material for sliding parts according to the present embodiment may not contain Cr. Cr improves the hardenability of the steel. This effect can be obtained as long as a small amount of Cr is contained. On the other hand, if the Cr content is too high, the workability of the steel is lowered. Thus, the Cr content may be 0 to 0.50%. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%. The upper limit of the Cr content is preferably 0.20%.

The balance of the chemical composition of the steel material for sliding parts according to the present embodiment may be Fe and impurities. The impurities herein refer to elements mixed from ores and scraps used as raw materials of steel, elements mixed from the environment of a manufacturing process, and the like.

The steel material for a sliding member according to the present embodiment may be a steel material formed of a carbon steel material for machine structural use or an alloy steel material for machine structural use. The steel material for sliding parts of the present embodiment is preferably produced by JIS G4051: 2016, or JIS G4053: 2016 is formed of an alloy steel material for machine construction. Among them, JIS G4051 is particularly preferred: 2016S 45C, and S50C, and JIS G4053: SMn438 of 2016. In order to improve workability (machinability), a steel material obtained by adding 0.20 mass% or less of S to these steel materials may be used.

[ Tissue ]

The structure of the steel material for a sliding member according to the present embodiment includes iron carbide and at least one of tempered martensite and bainite (including tempered bainite: the same applies hereinafter), and the volume fraction is the sum of tempered martensite and bainite: 80% or more of iron carbide: 2.0% or more.

The sum of the volume fraction of tempered martensite and the volume fraction of bainite in the structure of the steel material for sliding parts according to the present embodiment is 80% or more. The structure of the steel material for sliding parts according to the present embodiment may include at least one of tempered martensite and bainite.

In the present embodiment, the steel for sliding parts is tempered to have a structure containing a predetermined amount of iron carbide, thereby ensuring workability of the steel for sliding parts. In contrast, when the structure of the steel material for sliding parts is a quenched structure (a structure mainly composed of quenched martensite), it is difficult to ensure good workability. The steel material for sliding parts according to the present embodiment preferably contains tempered martensite.

In the case where the sum of the volume fraction of tempered martensite and the volume fraction of bainite is less than 80%, it is difficult to obtain excellent wear resistance. The sum of the volume fraction of tempered martensite and the volume fraction of bainite is preferably 85% or more, more preferably 90% or more, and still more preferably 95% or more.

In the present embodiment, in the calculation of the volume fraction of the structure, the iron carbide is regarded as an independent structure, and is distinguished from tempered martensite and bainite. That is, the portion in which the iron carbide is precipitated is not included in the volume of tempered martensite or bainite.

The volume fraction of iron carbide in the structure of the steel material for sliding parts according to the present embodiment is 2.0% or more. Specifically, the iron carbide of the steel material for sliding parts according to the present embodiment is at least one of epsilon carbide and cementite. The iron carbide contained in the steel material for sliding members may be one kind or plural kinds. In the case of containing a plurality of iron carbides, the volume fraction of the iron carbides is taken as the sum of the volume fractions of the iron carbides.

If the volume fraction of iron carbide is less than 2.0%, it is 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%, more preferably 12.0%, more preferably 10.0%, more preferably 8.0%.

The volume fraction of iron carbide can be adjusted by using 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. In addition, the tempering conditions tend to have a higher holding temperature, a longer holding time, and a higher volume fraction of iron carbide.

The structure of the steel material for sliding parts according to the present embodiment may contain a small amount of structures other than tempered martensite, bainite, and iron carbide. The structure other than tempered martensite, bainite, and iron carbide is, for example, ferrite, pearlite, retained austenite, mnS, or the like. The volume fraction of the structure of the steel material for sliding parts according to the present embodiment excluding tempered martensite, bainite and carbide is preferably 5.0% or less, more preferably 3.0% or less, still more preferably 2.0% or less, and still more preferably 1.0% or less in total.

[ Vickers hardness ]

The steel material for sliding parts according to the present embodiment has a vickers hardness of 300 to 600. 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 workability is lowered. From the viewpoint of abrasion resistance, the lower limit of the vickers hardness is preferably 350, more preferably 400, further preferably 450, further preferably 500, further preferably 530. From the viewpoint of workability, the upper limit of vickers hardness is preferably 580, more preferably 560, further preferably 550, further preferably 530, further preferably 520.

The vickers hardness of the steel for sliding members can be adjusted by using 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. The quenching conditions tend to have a higher cooling rate and a higher vickers hardness. In addition, the lower the holding temperature, the shorter the holding time and the higher the vickers hardness tend to be in the tempering condition.

[ Relation (1) ]

The volume fraction X of iron carbide of the steel material for sliding parts according to the present embodiment satisfies the following relational expression (1) with the vickers hardness Hv of the steel material. By satisfying the relation (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.

[ Others ]

The steel material for sliding members according to the present embodiment preferably has an average short-axis length of iron carbide of 0.027 μm or less. By dispersing the iron carbide having such a shape, the hardness of the entire steel material can be maintained. If the iron carbide is too large, the influence of the softness of the matrix may be large, and the wear resistance may be reduced. The average minor axis length of the iron carbide is preferably 0.025 μm or less.

The steel material for sliding parts according to the present embodiment preferably does not have any one of a nitrided layer, a carburized layer and a carbonitrided layer on the surface.

The vickers hardness of the surface of the steel for sliding parts according to the present embodiment is 300 to 600, and the volume fraction X of iron carbide at the surface and the vickers hardness Hv preferably satisfy the above-described relational expression (1).

In the above description, "the vickers hardness of the surface" more specifically means the vickers hardness of the region having a depth of 100 μm or less from the surface of the steel for sliding members. The "volume fraction of surface iron carbide" more specifically means the volume fraction of iron carbide of the structure of a region having a depth of 100 μm or less from the surface of the steel for sliding members.

[ Method for producing Steel for sliding Member ]

The method for producing the steel material for sliding parts according to the present embodiment will be described below.

A blank having the chemical composition described above was prepared. The blank is, for example, a hot forging. For example, the following hot forging can be used as a blank: the steel having the chemical composition described above is melted, continuously cast or bloomed to form a billet, and then the billet is hot forged to be processed into the general shape of the sliding member. The blank after hot forging may be subjected to cutting or the like.

After the blank is held at a temperature of 830 ℃ to 1100 ℃, the blank is cooled and quenched so that the cooling rate from the holding temperature to 300 ℃ is 300 ℃/sec or more. If the holding temperature is too low, a uniform tissue may not be obtained. On the other hand, if the holding temperature is too high, the crystal grains may coarsen. If the cooling rate is too low, a predetermined tissue may not be obtained. Further, the cooling rate in the quenching step tends to be increased, and the vickers hardness of the finally obtainable steel for sliding parts tends to be increased.

Tempering the quenched blank at a temperature of 200 ℃ or more and 600 ℃ or less. The higher the holding temperature of tempering, the longer the holding time, and the lower the vickers hardness of the finally obtainable steel for sliding parts. In addition, the higher the holding temperature and the longer the holding time of tempering, the higher the volume fraction of iron carbide in the structure of the finally obtainable steel for sliding parts tends to be. If the holding temperature of tempering deviates from this range, it is difficult to set the volume fraction of iron carbide and the vickers hardness to predetermined ranges. The quenching and tempering conditions are adjusted according to the chemical composition of the steel material, etc., so that the volume fraction X of iron carbide and the Vickers hardness Hv satisfy the relation (1). Thus, the steel material for sliding parts according to the present embodiment can be obtained.

The steel material for sliding parts according to the embodiment of the present invention is described above. The steel material for sliding parts according to the present embodiment has excellent sliding properties and workability. Therefore, the steel material for sliding members according to the present embodiment is suitable as a material for sliding members. The sliding member is, for example, a crankshaft.

Examples

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

Steel having the chemical composition shown in table 1 was melted in a 10kg vacuum induction melting furnace to prepare an ingot.

TABLE 1

The ingot was hot forged at 950 to 1200 ℃ and then rolled to a thickness of 7mm and a width of 110mm after a thickness of 30mm, a width of 100mm and a length of 290 mm. The rolled material was cut into pieces having a width of 15mm, a length of 60 to 120mm and a thickness of 7mm, and subjected to the heat treatment described in Table 2. The structure before heat treatment was ferrite-pearlite (f+p). The values in the column "cooling rate" of "quenching" in table 2 are cooling rates from the holding temperature of quenching to 300 ℃.

TABLE 2

After the heat treatment, a plurality of test pieces of 20mm square and 2mm in thickness were collected from each material. Using these test pieces, observation of a tissue, measurement of vickers hardness, and evaluation of slidability were performed.

The test piece for tissue observation was subjected to surface treatment by Ar ion milling. The sample is irradiated with an Ar ion beam at an angle of 80 DEG or more from a direction perpendicular to the sample, whereby an iron matrix softer than the iron carbide is milled, and the iron carbide remains in the form of a protrusion, so that searching for the iron carbide by an Atomic Force Microscope (AFM) can be performed. Fig. 5 shows an example of the surface roughness image of the processed test piece obtained by AFM. White represents convex portions and black represents concave portions. The white part of fig. 5 is iron carbide.

The volume fraction of iron carbide was calculated by obtaining an uneven image in a range of 2 μm×2 μm at 3 sites of the test piece by AFM and using image analysis software. Image analysis software used ImageJ. After the contrast and resolution of the image are adjusted so that the particle size can be detected by the image analysis software, the image is binarized, and particles are detected by using the particle analysis function of the image analysis software. An example of iron carbide detected using image analysis software is shown in fig. 6. The area ratios of iron carbides were calculated at the 3 positions observed, and the average value thereof was calculated. The area ratio obtained was regarded as the volume fraction of iron carbide.

The volume fraction of ferrite was calculated using the secondary electron image (concave-convex image) of SEM. The specimen surface was subjected to corrosion with an aqueous nitric acid-ethanol etchant, and after only ferrite was corroded to recess the specimen surface, a secondary electron image was obtained at a magnification of 1000 times. The acquired image was input to image analysis software ImageJ, the corresponding region was selected using the Freehand selection and Polygon selection functions, the selected region was masked, binarized similarly to the case of iron carbide, and the corresponding region was detected using the particle analysis function of the image analysis software. The area ratio of ferrite was calculated at each of the 3 observed portions, and the average value thereof was obtained. The area ratio obtained was regarded as the volume fraction of ferrite.

The volume fraction of retained austenite was measured by X-ray diffraction. For the volume fraction of MnS, the surface of the test piece was photographed by an optical microscope (magnification: 210 times, size of field: 1218 μm×1218 μm) to obtain the area fraction of MnS, and the area fraction was regarded as the volume fraction. The sum of the volume fraction of tempered martensite and the volume fraction of bainite (or the sum of the volume fraction of martensite and the volume fraction of bainite) is obtained by subtracting the sum of the volume fractions of iron carbide, retained austenite, ferrite and MnS from 100%.

The form of the iron carbide is obtained from the uneven image obtained when the volume fraction of the iron carbide is obtained by image analysis. Specifically, the image is binarized, and all particles in each observation field are elliptically approximated using a particle analysis function of image analysis software. The average minor axis length and the average major axis length were obtained at each of the 3 observed positions, and the average value thereof was obtained.

The vickers hardness was measured at 5 points with a test force of 1kgf (9.807N), and the average value was obtained.

Table 3 shows the heat-treated structure and the Vickers hardness of each steel material. The column "M" of the volume fraction of the structure in table 3 represents quenched martensite (as quenched martensite), "B" represents bainite, "TM" represents tempered martensite, and "residual γ" represents residual austenite.

TABLE 3

The surface of the test piece for the sliding test was subjected to mirror finishing. The sliding test was performed by using a ball-and-disc type frictional wear tester. Fig. 7 shows a schematic diagram of the testing machine. The balls used were alumina balls, the load was 10N, and the sliding speed was 10 mm/sec. After the sliding test, the width of the sliding mark was measured, and if the average value of the sliding mark width was 160 μm or less, the abrasion resistance was evaluated as "good", and if it exceeded 160 μm, the abrasion resistance was evaluated as "bad".

The vickers hardness, the volume fraction of carbide (iron carbide), and the sliding test results of the respective steels are shown in table 4. For workability, the vickers hardness was rated as "good" when it was 600 or less, and as "disqualified" when it exceeded 600. In the comprehensive evaluation, a steel material having both of workability and wear resistance "good" was regarded as "acceptable", and a steel material having either one of workability and wear resistance "unacceptable" was regarded as "unacceptable".

TABLE 4

As shown in Table 4, the steels of Nos. 4 to 7, 9, 10 and 14 to 17 have Vickers hardness of 300 to 600, and the volume fraction X of iron carbide and the Vickers hardness Hv satisfy the relation (1). The test materials showed excellent wear resistance, and the width of the wear mark after the sliding test was 160 μm or less. Further, these test materials have a vickers hardness Hv of 600 or less and also have excellent workability.

The steel products of Nos. 1 to 3, 8, 12 and 13 had wear scar widths exceeding 160 μm after the sliding test. The reason for this is considered that the volume fraction X of iron carbide and vickers hardness Hv do not satisfy the relation (1).

The steel material of No.11 has a quenched structure. The steel product of No.11 has good wear resistance, but has a Vickers hardness Hv exceeding 600 and deteriorated workability.

FIG. 3 is a scatter diagram showing the relationship between the Vickers hardness of steel and the volume fraction of iron carbide. FIG. 4 is a graph showing the relationship between the Vickers hardness of steel materials and the width of wear marks obtained by a sliding test using a ball-and-disc type frictional wear test. In fig. 3 and 4, the volume fraction X of iron carbide and the vickers hardness Hv of the steel material satisfy the relation (1) are represented by open circles, and the relation (1) is not satisfied by filled circles. The triangle in fig. 4 is a symbol of a steel material (No. 11) having a quenched structure. As can be seen from fig. 3 and 4: when the volume fraction X of iron carbide and the vickers hardness Hv of the steel material satisfy the relational expression (1), excellent wear resistance can be obtained.

While the above description has been given of an embodiment of the present invention, the above embodiment is merely an example for carrying out the present invention. Accordingly, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within a range not departing from the gist thereof.

Claims

1. A steel material for sliding parts, which is formed from a steel material having a C content of 0.30 to 0.60 mass%,

The structure of the steel for sliding parts includes iron carbide and at least one of tempered martensite and bainite, and the volume fraction is the sum of the tempered martensite and the bainite: 80% or more of the iron carbide: 2.0% or more of the total weight of the composition,

The Vickers hardness is 300 to 600,

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.

2. The steel material for sliding parts according to claim 1, wherein,

The chemical composition of the steel is as follows in mass percent: 0.30 to 0.60 percent,

Si：0.01～2.00％、

Mn：0.10～2.00％、

Al:0.060% or less,

N: less than 0.020%,

P: less than 0.10 percent,

S: less than 0.20 percent,

Cr：0～0.50％，

The balance: fe and impurities.

3. The steel material for sliding parts according to claim 1 or 2, wherein,

The average minor axis length of the iron carbide is 0.027 μm or less.

4. The steel material for sliding parts according to claim 1 or 2, wherein,

The steel material for sliding members does not have any one of a nitrided layer, a carburized layer and a carbonitrided layer on the surface.

5. The steel material for sliding parts according to claim 1 or 2, wherein,

The steel for sliding members has a Vickers hardness of 300-600, and the volume fraction X of the iron carbide at the surface and the Vickers hardness Hv satisfy the relation (1).

6. The steel material for sliding parts according to claim 1 or 2, wherein,

The Vickers hardness is 300 to 550.

7. A method for producing a steel material for sliding parts according to claim 1 or 2, comprising the steps of:

A step of cooling the blank at a temperature of 830 ℃ to 1100 ℃ and then quenching the blank so that the cooling rate from the holding temperature to 300 ℃ is 300 ℃/sec or more; and

And tempering the quenched blank at a temperature of 200 ℃ to 600 ℃.