JP7056237B2 - Manufacturing method of steel material for sliding parts and steel material for sliding parts - Google Patents

Description

The present invention relates to a steel material for sliding parts and a method for manufacturing a steel material for sliding parts.

Against the background of the need for weight reduction of automobiles, there is an increasing demand for narrower and narrower crankshafts. On the other hand, from the viewpoint of improving fuel efficiency, the output of the engine is increasing. Here, the problems are the emergence of seizure risk due to the high surface pressure of the sliding surface and the thinning of the lubricating oil film, and the concern about friction loss due to this. Prevention of seizure is an important issue required for next-generation engine members, and is being studied from various aspects such as the composition and amount of lubricating oil, and the materials of crankshafts and bearings.

Japanese Patent No. 4598885 discloses a crankshaft characterized in that the thermal conductivity κ is 40 W / mK or more and the surface hardness Hv after induction hardening is larger than (2.7 × κ + 420). Has been done. The document states that the controlling factor for seizure is the temperature rise of the sliding surface. It is described that it is effective to increase the thermal conductivity and reduce the coefficient of friction in order to suppress the temperature rise.

Japanese Patent No. 4140283 is characterized in that the structure in the region from the surface layer to 200 nm is a mixed structure of proeutectoid ferrite having an area ratio of 5% or less and pearlite having a lamellar spacing of 30 nm or less. Steel crankshafts are disclosed. According to the document, it is the hardness in the region from the surface layer to 200 nm that affects the wear resistance, and if the hardness measured in this region using a nanoindentation device is 10 GPa or more, the wear resistance is excellent. It is stated that sex can be obtained.

Japanese Patent No. 4598885 Japanese Patent No. 4140283

The crankshaft and the bearing are usually slid in a fluid-lubricated state facing each other with an oil film in between. However, during long-time sliding, there are not a few aspects from fluid lubrication to boundary lubrication. In order to improve the seizure resistance in the boundary lubrication state, it is effective to increase the hardness (nano hardness) within several hundred nm from the surface (sliding surface). If the nano-hardness is increased, excellent seizure resistance can be obtained even if the inside is soft.

On the other hand, when the hardness of steel is increased, there is a problem that plastic deformation is less likely to occur and the steel becomes brittle. Therefore, if the hardness of the steel is increased, wear debris may be generated or cracks may be generated, so that seizure may be more likely to occur.

An object of the present invention is to provide a steel material for sliding parts having improved seizure resistance, and to provide a method for manufacturing a steel material for sliding parts having improved seizure resistance.

The steel material for impulse parts according to one embodiment of the present invention has a chemical composition of% by mass, C: 0.3 to 0.55%, Si: 0.7% or less, Mn: 0.4 to 1.8%. , Cr: 0.3% or less, Ni: 0.5% or less, Cu: 0.5% or less, Al: 0.005 to 0.06%, N: 0.001 to 0.04%, P: 0 .03% or less, S: 0.035% or less, balance: Fe and impurities, and at least a part of the steel material for sliding parts has a martensite fraction as hardened, with a structure up to a depth of at least 200 μm from the surface. The structure is 50 to 80%, the retained austenite fraction is 5 to 10%, and the balance is a ferrite phase.

The method for producing a steel material for sliding parts according to an embodiment of the present invention has a chemical composition of C: 0.3 to 0.55%, Si: 0.7% or less, Mn: 0.4 to% in mass%. 1.8%, Cr: 0.3% or less, Ni: 0.5% or less, Cu: 0.5% or less, Al: 0.005 to 0.06%, N: 0.001 to 0.04% , P: 0.03% or less, S: 0.035% or less, balance: Fe and the step of preparing a material which is an impurity, and at least a part of the material is heated to a heating temperature of 720 to 770 ° C. It comprises a step of holding for 5 to 60 minutes and a step of cooling the heated portion of the material so that the average cooling rate in the temperature range from the heating temperature to 200 ° C. is 10 ° C./sec or more.

According to the present invention, a steel material for sliding parts having improved seizure resistance can be obtained.

FIG. 1 is a flow chart showing an example of a method for manufacturing a steel material for sliding parts according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a pin-on disc type friction test. FIG. 3 is a schematic diagram of nano-hardness measurement by the nano-indentation method. FIG. 4 is a diagram for explaining Rms. FIG. 5 shows the surface observation results of the test material of test number 3 measured by an atomic force microscope after the wear test. FIG. 6 shows the surface observation results of the test material of test number 4 measured by an atomic force microscope after the wear test. FIG. 7 is a surface observation result after the seizure test of the test material of test number 3 measured by an atomic force microscope. FIG. 8 shows the surface observation results of the test material of test number 4 measured by an atomic force microscope after the seizure test.

The present inventors prepared various steel materials and conducted a seizure resistance test assuming a crankshaft for automobiles to evaluate the seizure surface pressure. As a result, the following findings were obtained.

(A) Controlling factors of seizure resistance The seizure surface pressure does not have much correlation with the hardness (Hv hardness) of steel materials, and the hardness within several hundred nm from the surface (sliding surface) (hereinafter referred to as "nano hardness"). ".) Depends on. The higher the nanohardness of the steel material, the higher the seizure surface pressure (the higher the seizure resistance).

However, although the seizure surface pressure can be estimated to some extent from the nanohardness among steel materials having the same structure, the seizure surface pressure cannot be correctly estimated from the nanohardness alone between steel materials having different structures. The seizure surface pressure between steel materials with different structures shows a good correlation with the value obtained by dividing the nano-hardness by the surface roughness of the steel material after sliding. That is, the seizure surface pressure increases as the nanohardness of the steel material is high and the surface roughness of the steel material after sliding is small.

(B) Tissue control for improving seizure resistance The surface of sliding parts is subjected to strong machining due to sliding. In order to improve the nano-hardness of steel materials, it is effective to improve work hardening. In this regard, when nanocrystals are formed by sliding, the movement of dislocations is hindered and work hardening does not occur. In order to maintain work hardening against long-term sliding, it is preferable to increase the density of movable dislocations.

On the other hand, when a sliding component is slid for a long time, repeated shear stress is applied, so that cracks are generated and propagated by the same mechanism as fatigue fracture. When the crack reaches the surface, wear particles are generated, and the wear particles increase the surface roughness. In order to improve seizure resistance, it is necessary to design materials and control the structure to suppress the generation of wear particles.

In order to suppress the generation of wear particles, it is effective to make the surface layer structure a structure with a high density of movable dislocations with respect to plastic deformation. Structures with a high density of mobile dislocations are retained austenite, as-quenched martensite (unquenched martensite, hereinafter sometimes referred to as "fresh martensite"), and a ferrite phase with a low solid solution carbon content. ..

Since the austenite and ferrite phases are soft phases, the required nanohardness cannot be obtained if the surface has only these structures. On the other hand, if the surface is made of only fresh martensite, delayed fracture may occur. Further, when tempering is applied to martensite or a ferrite phase, carbides are precipitated, and the carbides inhibit (immobilize) the movement of dislocations. Therefore, in order to suppress the generation of wear particles, it is preferable to have a structure that is not tempered.

Therefore, a desirable structure for improving seizure resistance is a multiphase structure consisting of fresh martensite, retained austenite, and a ferrite phase. By coexisting these three structures, it is possible to suppress the generation and propagation of cracks while maintaining high strength and high hardness.

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

[Chemical composition]
The steel material for sliding parts according to this embodiment has the chemical composition described below. In the following description, "%" of the element content means mass%.

C: 0.3 to 055%
Carbon (C) enhances the hardenability of steel and contributes to the improvement of hardness. If the C content is less than 0.3%, the hardenability of steel is insufficient. On the other hand, if the C content exceeds 0.55%, the required machinability cannot be obtained. Therefore, the C content is 0.3 to 0.55%. The lower limit of the C content is preferably 0.35%, more preferably 0.38%. The upper limit of the C content is preferably 0.5%, more preferably 0.45%.

Si: 0.7% or less Silicon (Si) suppresses the formation of coarse cementite. This effect can be obtained if even a small amount of Si is contained. On the other hand, if the Si content exceeds 0.7%, the thermal conductivity of the steel is lowered and sufficient seizure resistance cannot be obtained. Therefore, the Si content is 0.7% or less. The lower limit of the Si content is preferably 0.05%, more preferably 0.1%. The upper limit of the Si content is preferably 0.65%, more preferably 0.6%.

Mn: 0.4 to 1.8%
Manganese (Mn) enhances the hardenability of steel and contributes to the improvement of hardness. On the other hand, if the Mn content is too low, the retained austenite will be low, and it will be difficult to obtain a desired structure. On the other hand, when the Mn content exceeds 1.8%, the hot workability deteriorates. Therefore, the Mn content is 0.4 to 1.8%. The lower limit of the Mn content is preferably 0.42%, more preferably 0.45%. The upper limit of the Mn content is preferably 1.6%, more preferably 1.5%.

Cr: 0.3% or less Chromium (Cr) enhances the hardenability of steel and contributes to the improvement of hardness. This effect can be obtained if even a small amount of Cr is contained. On the other hand, when the Cr content exceeds 0.3%, the two-phase region becomes narrow and it becomes difficult to obtain a desired structure. In addition, carbides are likely to precipitate. Therefore, the Cr content is 0.3% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.03%. The upper limit of the Cr content is preferably 0.2%, more preferably 0.1%.

Ni: 0.5% or less Nickel (Ni) enhances the hardenability of steel and contributes to the improvement of hardness. This effect can be obtained if even a small amount of Ni is contained. On the other hand, when the Ni content exceeds 0.5%, retained austenite increases, making it difficult to obtain a desired structure. Therefore, the Ni content is 0.5% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%. The upper limit of the Ni content is preferably 0.4%, more preferably 0.3%.

Cu: 0.5% or less Copper (Cu) enhances the hardenability of steel and contributes to the improvement of hardness. This effect can be obtained if even a small amount of Cu is contained. On the other hand, when the Cu content exceeds 0.5%, the two-phase region becomes narrow and it becomes difficult to obtain a desired structure. Therefore, the Cu content is 0.5% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%. The upper limit of the Cu content is preferably 0.4%, more preferably 0.3%.

Al: 0.005 to 0.06%
Aluminum (Al) forms a nitride and contributes to the miniaturization of austenite grains by the pinning effect of the nitride. If the Al content is less than 0.005%, this effect cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.06%, the machinability of the steel is lowered. Therefore, the Al content is 0.005 to 0.06%. The lower limit of the Al content is preferably 0.01%, more preferably 0.02%. The upper limit of the Al content is preferably 0.05%, more preferably 0.04%.

N: 0.001 to 0.04%
Nitrogen (N) forms nitrides and carbonitrides, and contributes to the miniaturization of austenite grains by the pinning effect of the nitrides and carbonitrides. If the N content is less than 0.001%, this effect cannot be sufficiently obtained. On the other hand, when the N content exceeds 0.04%, the toughness of the steel decreases. Therefore, the N content is 0.001 to 0.04%. The lower limit of the N content is preferably 0.005%, more preferably 0.01%. The upper limit of the N content is preferably 0.035%, more preferably 0.03%.

P: 0.03% or less Phosphorus (P) is an impurity. When the P content exceeds 0.03%, the fatigue strength of the steel decreases. Therefore, the P content is 0.03% or less. The P content is preferably 0.025% or less, more preferably 0.02% or less.

S: 0.035% or less Sulfur (S) is an impurity. When the S content exceeds 0.035%, the hot workability of the steel deteriorates. Therefore, the S content is 0.035% or less. The S content is preferably 0.030% or less, more preferably 0.025% or less.

The balance of the chemical composition of the steel material for sliding parts according to this embodiment is Fe and impurities. Impurities here refer to elements mixed from ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.

[Surface structure]
At least a part of the steel material for sliding parts according to the present embodiment is surface-hardened. The surface hardening treatment is, for example, induction hardening. For example, the steel material for sliding parts according to the present embodiment may be subjected to surface hardening treatment only on a portion to be slid when used as a sliding part, or the entire steel material is subjected to surface hardening treatment. May be.

In the steel material for sliding parts according to the present embodiment, the structure (hereinafter referred to as "surface structure") from the surface of the surface-hardened portion to a depth of at least 500 μm or more remains as martensite (fresh martensite) as it is quenched. It is a multiphase structure composed of austenite and ferrite phase.

As-quenched martensite (fresh martensite) is necessary for high strength and high ductility. If there is too little fresh martensite, sufficient nanohardness cannot be obtained. On the other hand, if there are too many fresh martensites, delayed fracture may occur. The fraction of fresh martensite in the surface structure (hereinafter referred to as "fresh martensite fraction") is 50 to 80%. The lower limit of the fresh martensite ratio is preferably 55%, more preferably 60%. The upper limit of the fresh martensite rate is preferably 75%, more preferably 70%.

When a high load is locally applied to the retained austenite during the sliding process, it transforms into martensite and improves the load bearing capacity. If there is too little retained austenite, this effect will not be sufficient. On the other hand, if there is too much retained austenite, the nanohardness will be insufficient. The retained austenite fraction (hereinafter referred to as "retained austenite ratio") in the surface layer structure is 5 to 10%. The lower limit of the retained austenite rate is preferably 6%. The upper limit of the retained austenite rate is preferably 9%, more preferably 8%.

The rest of the surface structure is the ferrite phase. The ferrite phase relieves strain and residual stresses generated in other structures. The lower limit of the ferrite phase fraction (hereinafter referred to as "ferrite phase fraction") in the surface layer structure is preferably 10%, more preferably 15%. The upper limit of the ferrite phase ratio is preferably 40%, more preferably 30%.

The thickness of the surface layer structure is 200 μm or more. If the thickness of the surface layer structure is less than 200 μm, the effect of the above-mentioned double phase structure cannot be sufficiently obtained. On the other hand, if the structure up to 200 μm from the surface is the above-mentioned double-phase structure, the structure in the region deeper than 200 μm may be a structure other than this. However, the above-mentioned double-phase structure may be used over the entire depth of the steel material. Structures deeper than 200 μm from the surface do not significantly affect sliding properties.

On the other hand, from the viewpoint of workability, it is preferable that only the surface of the steel material is hardened and the inside remains a soft structure. Therefore, from the viewpoint of processability, it is preferable that the thickness of the surface layer structure is as thin as possible with 200 μm as the lower limit. The upper limit of the thickness of the surface layer structure is preferably 1000 μm, more preferably 500 μm.

As described above, in at least a part of the steel material for sliding parts according to the present embodiment, the structure from the surface to a depth of at least 200 μm has a martensite fraction of 50 to 80% and a retained austenite fraction of 5 to 10 as hardened. %, The structure where the balance is a ferrite phase.

The fraction of each structure in the surface structure of the steel material for sliding parts is determined as follows.

From the surface-hardened portion, a test piece containing a position deeper than 200 μm from the surface is collected. Using this test piece, the retained austenite ratio is determined by X-ray diffraction method. Specifically, the integrated intensities of the (200) plane and the (211) plane of the α phase and the (200) plane, the (220) plane, and the (311) plane of the γ phase are measured. The volume fraction Vγ is calculated using the following formula (A) for each combination (2 × 3 = 6 sets) of each surface of the α phase and each surface of the γ phase. The average value of the six sets of volume fraction Vγ is defined as the retained austenite ratio.
Vγ = 100 / (1+ (Iα × Rγ) / (Iγ × Rα)) (A)

Here, "Iα" is the integrated intensity of the α phase, "Rα" is the crystallographic theoretically calculated value of the α phase, "Iγ" is the integrated crystallographic intensity of the γ phase, and "Rγ" is the crystallographic theoretically calculated value of the γ phase. Is.

From the surface-hardened portion, a test piece containing a position deeper than 200 μm from the surface is collected. The cross section parallel to the depth direction of this test piece is polished. The polished surface is etched with a mixed solution of aqua regia and glycerin. Using an optical microscope (observation magnification 100 times), the area ratio of the ferrite phase on the etched surface is measured by a point calculation method based on ASTM E562-11. The measured area ratio of the ferrite phase is defined as the ferrite phase ratio.

The fresh martensite phase ratio is obtained by subtracting the retained austenite ratio and the ferrite phase ratio from 100%. In addition, fresh martensite and tempered martensite can be discriminated by electron backscatter diffraction.

[carbide]
Carbides inhibit (immobilize) the motion of dislocations and reduce the plastic deformability of the tissue. In order to suppress the generation of wear particles, it is preferable to reduce the amount of carbides as much as possible. Therefore, it is preferable that the above-mentioned surface layer structure is an as-quenched structure that is not tempered.

However, in the chemical composition of the steel material for sliding parts according to the present embodiment, carbides such as cementite may be deposited by the autotemper even if tempering is not performed. Even in that case, the effect on the sliding characteristics is small as long as the needle-shaped carbides are deposited at a low density. Specifically, if the average minor axis of the carbides in the fresh martensite is 10 nm or less and the average spacing between the carbides is 100 nm or more, the influence on the sliding characteristics is small.

It should be noted that the precipitation of carbides works disadvantageously on the sliding characteristics, but also has an aspect of improving the strength. Therefore, it may be preferable to precipitate carbides within the above-mentioned range, such as when it is desired to improve the strength at the expense of sliding characteristics.

The average minor axis of carbides is measured as follows.

A test piece is collected from the surface tissue so that the cross section parallel to the depth direction is the observation surface. The observation surface is observed with a transmission electron microscope (TEM) at 40,000 times and 100,000 times in 5 fields so as to include the surface of the steel material for sliding parts. The incident direction of the electron beam is the ferrite <001> direction. The size of the visual field is 2.0 μm × 1.2 μm in the case of 40,000 times, and 0.8 × 0.5 μm in the case of 100,000 times.

The minor axis of each carbide is measured from the electron diffraction image. The minor axis of the carbide is averaged in each field of view, and the average minor axis of the carbide is defined in 10 visual fields. However, when calculating the average, those with a minor axis of less than 1 nm are not included. If the minor axis is less than 1 nm, it is difficult to determine whether it is a carbide.

Even when there is almost no precipitation of carbides, for example, when there is almost no carbide having a minor axis of 1 nm or more, the requirement that "the average minor axis of carbides is 10 nm or less" is satisfied.

The average spacing between carbides is measured as follows.

Collect test pieces from the surface tissue. With a field emission electron probe microanalyzer (FE-EPMA), 500 × 500 points are measured at intervals of 0.06 μm in a region of 30 μm × 30 μm at an acceleration voltage of 15 kV. A measurement point having a C concentration of 1.5% by mass or more is set to 1, and a point having a C concentration of less than 1.5% by mass is set to 0 to binarize to obtain a two-dimensional distribution image of carbides. The average spacing between carbides is derived by the following procedure.
(1) The number of particles PA per unit area is measured from the two _- dimensional distribution image.
(2) The average particle spacing Δ2 on the two-dimensional plane is derived by the following equation.
Δ2 = ^0.500P _A -1 / 2
(Reference) Sakuma, Nishizawa: Bulletin of the Japan Institute of Metals, 10, (1971), P.M. 279-289

Even when there is almost no precipitation of carbides, for example, when there are almost no carbides having a minor axis of 1 nm or more, the requirement that "the average spacing between carbides is 100 nm or more" is satisfied.

[Production method]
Hereinafter, an example of a method for manufacturing a steel material for sliding parts according to the present embodiment will be described. The method for manufacturing a steel material for sliding parts according to the present embodiment is not limited to this.

FIG. 1 is a flow chart showing an example of a method for manufacturing a steel material for sliding parts according to the present embodiment. This manufacturing method includes a step of preparing a material (step S1), a step of heating the material (step S2), and a step of cooling the heated material (step S3).

First, a material having the above-mentioned chemical composition is prepared (step S1). The material is, for example, a hot forged product. In the following, a case where the material is a hot forged product will be described as an example. Steel having the above-mentioned chemical composition is melted and continuously cast or lump-rolled to make steel pieces. Steel pieces are hot forged and processed into a rough shape for sliding parts. The conditions for hot forging are not particularly limited, but for example, the heating temperature is 1000 to 1250 ° C., the holding time at the heating temperature is 10 to 300 minutes, and the finishing temperature is 900 ° C. or higher. The material after hot forging may be machined.

At least a part of the material is subjected to the surface hardening treatment described below. When the steel material for sliding parts according to the present embodiment is used as a sliding part, for example, the surface hardening treatment may be applied only to the portion to be slid, or the entire material may be subjected to the surface hardening treatment. good.

Specifically, first, at least a part of the material is heated to a heating temperature of 720 to 770 ° C. and held for 5 to 60 minutes (step S2).

This heating temperature (720 to 770 ° C.) is a so-called two-phase region. That is, the heated portion of the material has a two-phase structure of austenite and ferrite. If the heating temperature is too low or too high, the desired texture will not be obtained. The lower limit of the heating temperature is preferably 730 ° C. The upper limit of the heating temperature is preferably 760 ° C.

If the retention time is less than 5 minutes, the desired tissue cannot be obtained stably. On the other hand, if the holding time is longer than 60 minutes, the crystal grains become coarse. The lower limit of the holding time is preferably 10 minutes, more preferably 15 minutes. The upper limit of the holding time is preferably 50 minutes, more preferably 45 minutes.

Subsequently, the heated portion of the material is cooled so that the average cooling rate in the temperature range from the heating temperature to 200 ° C. is 10 ° C./sec or more (step S3). As a result, a part of the tissue that was austenite at the heating temperature is transformed into fresh martensite, and the rest becomes retained austenite. This gives a multiphase structure consisting of fresh martensite, retained austenite, and a ferrite phase.

If the average cooling rate in the temperature range from the heating temperature to 200 ° C. is less than 10 ° C./sec, a part of austenite is transformed into pearlite or the like, and a desired structure cannot be obtained. The average cooling rate in the temperature range from the heating temperature to 200 ° C. is preferably 20 ° C./sec or more.

The cooling rate in the temperature range below 200 ° C. is arbitrary. However, if the temperature region lower than 200 ° C. is strongly cooled, residual austenite may not remain or cracks may occur due to transformation expansion. Therefore, it is preferable to air-cool the temperature region as low as 200 ° C.

It is preferable not to perform tempering because carbides do not precipitate after cooling. Since the surface structure of the steel material for sliding parts according to the present embodiment is a multiphase structure composed of fresh martensite, retained austenite, and a ferrite phase, delayed fracture is unlikely to occur even if tempering is not performed.

The steel material for sliding parts and the manufacturing method thereof according to the embodiment of the present invention have been described above. The steel material for sliding parts according to this embodiment has excellent seizure resistance. Therefore, the steel material for sliding parts according to the present embodiment is suitable as a material for sliding parts. The sliding component is, for example, a crankshaft.

Hereinafter, the present invention will be described in more detail by way of examples. The present invention is not limited to these examples.

Steel having the chemical composition shown in Table 1 was melted by a 150 kg vacuum induction melting furnace (VIM), heated to 1200 to 1250 ° C., and hot forged to a 50 mm square and 150 mm length to obtain a material.

The structure of the material after hot forging was a ferrite pearlite structure. Heat treatment equivalent to high-frequency heating and quenching was applied to these materials to prepare test materials of test numbers 1 to 14 shown in Table 2. Table 2 shows the heat treatment conditions after hot forging and the structure after heat treatment.

A plurality of blocks having dimensions: 2.7 mm × 2.0 mm × 2.0 mm were collected from each test material. At that time, the blocks were collected so that the surface of the test material had a surface of 2.7 mm × 2.0 mm (hereinafter referred to as “test surface”). In order to remove the unevenness of the test surface, paper polishing and polishing with colloidal silica were carried out. Further, electrolytic polishing was carried out in order to remove the processed layer generated by paper polishing and polishing with colloidal silica. The total surface thickness removed by paper polishing, polishing with colloidal silica, and electrolytic polishing was about 0.5 μm.

[Identification of surface tissue]
Using the surface perpendicular to the test surface of the block after polishing as the observation surface, the surface structure was specified, the average minor axis of the carbides, and the average spacing between the carbides were measured by the method described in the embodiment. Table 3 shows the surface structure, the average minor axis of carbides, and the average spacing between carbides.

"M", "A", and "F" in the "Structure" column of Table 3 indicate fresh martensite, retained austenite, and ferrite phase, respectively, except as noted in the margin. A "-" in the "Organization" column indicates that the fraction of the relevant organization was below the lower limit of analysis. “-” In the column of “Average minor axis” of “Carbide” in Table 3 indicates that the average minor axis of carbide was less than the lower limit of analysis. A "-" in the "Average interval" column indicates that the measurement of the average interval of carbides has not been performed.

[Measurement of Vickers hardness]
The Vickers hardness was measured using the same block used to identify the surface structure.

[Measurement of nano-hardness and surface roughness]
In order to evaluate the nano-hardness and surface roughness after sliding for a long time, a wear test was performed on each block, and the nano-hardness and surface roughness after the wear test were measured.

First, the pin-on-disk type wear test shown in FIG. 2 was carried out using the above-mentioned block. Specifically, the 800th emery paper 20 is attached to the surface of the rotating disk 10 of the pin-on disc tester, and the test surface of the block 30 is pressed against the emery paper 20 at a surface pressure of 0.3 MPa at room temperature. The rotating disc 10 was rotated until the sliding distance reached 2000 m.

After the wear test, the nano-hardness and surface roughness of the test surface were measured by the nano-indentation method.

For the measurement of nano-hardness, an XP / DCM type nano-indenter manufactured by Agilent Technologies was used. As shown in FIG. 3, the probe 40 of the nanoindenter was brought into contact with the test surface 30a of the test piece 30 for measurement. More specifically, a diamond-made Berkovich-type needle was pushed into the test surface 30a by a continuous rigidity method (CSM method). The conditions of the continuous rigidity method were a frequency of 45 Hz, an amplitude of 2 nm, and a pushing depth of 250 nm.

The surface roughness was evaluated using a Keyence laser microscope VK-X200 and using the element length Rms (μm) defined by the following formula. As shown in FIG. 4, Xsi is a length corresponding to one contour line element.

Further, a seizure resistance test was carried out on each test material at a surface pressure of 100 MPa. Specifically, a columnar sample (shaft diameter 40 mm) imitating a crankshaft was connected to a motor and rotated while being supported by a slide bearing (material is the same alloy). The rotation speed was 6000 rpm, and after applying a commercially available engine oil to a surface pressure of 100 MPa in 60 minutes, the presence or absence of seizure on the sample surface was evaluated.

The evaluation results of the Vickers hardness, nano-hardness, surface roughness (Rms), and seizure test of the test material of each test number are shown.

The test materials of test numbers 4, 7 and 9 had a nano-hardness / Rms value of 7.0 or more. These test materials did not seize in the seizure test. It is considered that these test materials were suppressed from cracking and generation of wear debris even when the surface layer was work-hardened.

The test materials of test numbers 1 to 3, 5, 6, 8, 10 to 14 had a nanohardness / Rms value of less than 7.0, and seizure occurred in the seizure test. It is considered that this is because the surface structure of these test materials was not appropriate.

The surface structure of the test material of Test No. 1 was ferrite pearlite. It is considered that this is because the cooling rate after heating was too low.

The test materials of test numbers 2 and 6 were quenched from the austenite temperature range and tempered. The surface structure of these test materials was a single structure of tempered martensite.

The test materials of test numbers 3, 8 and 10 were quenched from the austenite temperature range. The surface structure of these test materials was a single structure of fresh martensite.

The surface structure of the test material of test number 5 had a low fresh martensite fraction. This is probably because the heating temperature was too low.

The surface structure of the test material of Test No. 11 had a low retained austenite fraction. It is considered that this is because the Mn content of the steel grade D was too low.

The surface structure of the test material of test number 12 had a high retained austenite fraction. It is considered that this is because the Cr content of the steel grade E was too high.

The surface structure of the test material of test number 13 had a high retained austenite fraction. It is considered that this is because the Ni content of the steel type F was too high.

The surface structure of the test material of test number 14 had a low retained austenite fraction. It is considered that this is because the Cu content of the steel grade G was too high.

5 and 6 are surface observation results after the wear test of the test materials of test numbers 3 and 4, respectively, measured by an atomic force microscope (AFM). 7 and 8 are surface observation results after the seizure test of the test materials of test numbers 3 and 4, respectively, measured by AFM.

From FIGS. 5 to 8, the test material of test number 4 maintains a smooth surface as compared with the test material of test number 3 after both the wear test and the seizure test. I understand.

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-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.

Claims (3)

Steel material for sliding parts
The chemical composition is by mass%,
C: 0.3-0.55%,
Si: 0.7% or less,
Mn: 0.4 to 1.8%,
Cr: 0.01-0.3 %,
Ni: 0.01-0.5 %,
Cu: 0.01-0.5 %,
Al: 0.005 to 0.06%,
N: 0.001 to 0.04%,
P: 0.03% or less,
S: 0.035% or less,
Remaining: Fe and impurities,
At least a part of the steel material for sliding parts has a structure from the surface to a depth of at least 200 μm, with a martensite fraction of 50 to 80%, a residual austenite fraction of 5 to 10%, and a ferrite phase remaining as hardened. Steel material for sliding parts, which is a certain structure. The steel material for sliding parts according to claim 1.
The average minor axis of carbides in martensite as hardened is 10 nm or less.
A steel material for sliding parts having an average spacing of 100 nm or more between the carbides. The method for manufacturing a steel material for sliding parts according to claim 1 or 2.
The chemical composition is by mass%, C: 0.3 to 0.55%, Si: 0.7% or less, Mn: 0.4 to 1.8%, Cr: 0.01 to 0.3 %, Ni. : 0.01 to 0.5 %, Cu: 0.01 to 0.5 %, Al: 0.005 to 0.06%, N: 0.001 to 0.04%, P: 0.03% or less , S: 0.035% or less, balance: Fe and the process of preparing materials that are impurities,
A step of heating at least a part of the material to a heating temperature of 720 to 770 ° C. and holding the material for 5 to 60 minutes.
A method for manufacturing a steel material for sliding parts, comprising a step of cooling the heated portion of the material so that the average cooling rate in the temperature range from the heating temperature to 200 ° C. is 10 ° C./sec or more.

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