JP2022059872A - Steel sliding part and method for manufacturing the same - Google Patents

Abstract

To provide a steel sliding part excellent in sliding properties.SOLUTION: A steel sliding part has a sliding surface on a surface of the steel sliding part. Integrated intensity of a diffraction line of a (200) plane of ferrite on the sliding surface is 3.0 times or more that of a sample of non-oriented ferrite in X-ray diffraction with Co-K-alpha-ray.SELECTED DRAWING: Figure 2

Description

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

The steel material may slide with other parts and the like not only in the situation where the product is used but also in the manufacturing process. Therefore, understanding, controlling, and improving the frictional characteristics and wear characteristics of steel materials is an important issue.

A typical product in which sliding characteristics are a problem is the crankshaft of an automobile engine. In order to improve the fuel efficiency of automobiles, the crankshaft is required to be lighter and have better seizure resistance. The factors that require improvement in seizure resistance are the decrease in oil film thickness due to the use of low-viscosity engine lubricating oil, and the increase in concerns about seizure due to starting from a state where no hydraulic pressure is applied due to engine stoppage. It can be roughly divided into what is being done and expectations for the benefits of weight reduction due to the reduction in engine lubricating oil usage and pump capacity.

In response to concerns about seizure, engine lubricant manufacturers are developing additives for oils, and bearing manufacturers are developing layers that are responsible for seizure resistance, such as material alloy design and overlays. However, the demand for seizure resistance is increasing, and the development of steel materials with better sliding characteristics is required.

In the case of crankshafts, the controlling factor for seizure is the contact problem between the shaft and the bearing. It is considered that the poor supply of lubricating oil and the temperature rise at the sliding interface between the shaft and the bearing cause the lubricating oil film to break, leading to contact between the shaft and the bearing and seizure. In order to prevent contact between the shaft and the bearing, collision between the convex parts is prevented by reducing the surface unevenness, and the amount of wear is reduced by increasing the hardness of the shaft to avoid contact by wear debris. Measures have been proposed.

Japanese Unexamined Patent Publication No. 7-18379 discloses that steel is subjected to soft nitriding treatment to uniformly form a compound layer having an average of 12 μm or more on the surface, thereby ensuring surface hardness and obtaining seizure resistance. Has been done. Japanese Patent No. 4598885 discloses that an oil film on a sliding surface is secured by defining the relationship between surface hardness and thermal conductivity.

Japanese Unexamined Patent Publication No. 60-1424 discloses a sliding member in which irregularities are formed on the sliding surface and a solid lubricant is held on the uneven surface. Japanese Patent Application Laid-Open No. 2003-253395 has a hardenability index of 1.1 to 1.5, which is the sum of the hardenability conversion parameters f (C), f (Si), f (Mn) and f (Cr). High frequency hardened parts are disclosed.

Japanese Patent Application Laid-Open No. 2020-32455 describes the manufacture of metal parts in which the orientation strength of the crystal plane having an adsorption tendency that the lubricating oil is more easily chemisorbed than other crystal planes is increased by strongly processing the metal parts. The method is disclosed. The publication describes that in the case of steel materials, the crystal plane of {101} is an adsorption surface that is more likely to chemically adsorb lubricating oil than the crystal planes in other directions.

Japanese Unexamined Patent Publication No. 2014-205588 discloses a method for searching for a crystal orientation, a crystal structure and a composition of a material in order to optimize the friction coefficient of the material.

Although it is not related to sliding parts, Patent No. 2535963, Japanese Patent No. 3480072, and Toshiro Tomita, Takashi Tanaka "Formation of {100} aggregate structure and its mechanism by isothermal γ → α transformation in silicon steel sheet", iron And Steel, Vol. 79, No. 12 (1993) discloses a silicon steel sheet having excellent magnetic properties in which <100> axes are densely integrated in the direction perpendicular to the plate surface and a method for manufacturing the same. For example, in Japanese Patent No. 2535963, a steel sheet is heated to the α + γ region or the γ region under a weak decarburization atmosphere to transform the surface layer portion from γ to α, and then heated to the α + γ region or the γ region under a strong decarburization atmosphere. It is described that by transforming the inside from γ to α, a steel sheet having an axial density of the <100> axis in the direction perpendicular to the plate surface is 8 times or more that of the one having no crystal orientation orientation can be obtained.

Japanese Unexamined Patent Publication No. 7-18379 Japanese Patent No. 4598885 Japanese Unexamined Patent Publication No. 60-1424 Japanese Patent Application Laid-Open No. 2003-253395 Japanese Unexamined Patent Publication No. 2020-32455 Japanese Unexamined Patent Publication No. 2014-205588 Japanese Patent No. 2535963 Japanese Patent No. 3480072

Toshiro Tomita, Takashi Tanaka, "Formation of {100} texture by isothermal γ → α transformation in silicon steel plate and its mechanism", Iron and Steel, Vol. 79, No. 12 (1993)

Steel surface hardening processes such as soft nitriding or carburizing are expensive. Forming an oil pool on the surface by shot peening or the like is effective in securing an oil film, but this method is also expensive, and cost reduction is indispensable for use as a general-purpose technology. In addition, it is inevitable that the oil film thickness will decrease due to the temperature rise.

The method of JP-A-2020-32455 has a problem of cost increase due to strong processing and a problem that strong processing cannot be performed depending on the shape of a part. Further, the functions of the lubricating oil are various, and it is not always clear whether controlling the chemisorbency of the lubricating oil contributes to the improvement of sliding characteristics such as seizure resistance.

An object of the present invention is to provide a steel sliding component having excellent sliding characteristics and a method for manufacturing the steel sliding component.

The steel sliding component according to the embodiment of the present invention is a steel sliding component having a sliding surface on the surface of the component, and the sliding surface is a ferrite {200} in X-ray diffraction by Co-Kα rays. The integrated intensity of the diffraction lines on the surface is 3.0 times or more that of the non-oriented ferrite sample.

The method for manufacturing a steel sliding component according to an embodiment of the present invention is the method for manufacturing the steel sliding component described above, wherein the material is vacuumed at a temperature in a two-phase region of ferrite and austenite or a single-phase region of austenite. A step of keeping the temperature at 10 Pa or less is provided.

According to the present invention, a steel sliding component having excellent sliding characteristics can be obtained.

FIG. 1 is an example of the measurement result of electron backscatter diffraction. FIG. 2 is an example of the measurement result of the frictional force by the atomic force microscope. It is a schematic diagram of the ball-on-disc type friction / wear tester.

The present inventors have investigated the sliding characteristics of steel materials by focusing on the relationship between crystal orientation and sliding characteristics.

Seizure is ultimately a contact problem on the surface of the member. The present inventors investigated the frictional properties on the surface of steel materials by an atomic force microscope or the like. As a result, it was found that the {100} plane of ferrite has a smaller coefficient of friction than the other planes. The small coefficient of friction is thought to be due to the small cohesive force that is the main cause of friction. That is, it is considered that the {100} plane of ferrite has a smaller adhesion force than the other planes. Therefore, it is considered that the friction can be reduced and the wear due to the adhesive force can be reduced by increasing the degree of orientation of the {100} surface of the ferrite on the sliding surface of the sliding component.

The present inventors further prepared test pieces in which the degree of orientation of the {100} surface of ferrite was increased, and conducted a friction test in a lubricated and non-lubricated state, and these test pieces had sliding characteristics such as seizure resistance. Was confirmed to be improving.

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

[Steel sliding parts]
The steel sliding component according to the embodiment of the present invention is a steel sliding component having a sliding surface on the surface of the component. This sliding surface has a structure mainly composed of ferrite on the surface layer and has a structure having a high degree of orientation of the {100} surface. Specifically, in this sliding surface, the integrated intensity of the diffraction line of the {200} plane of ferrite is 3.0 times or more that of the non-oriented ferrite sample in the X-ray diffraction by Co-Kα ray. be.

The steel sliding component is, for example, a crankshaft.

The {100} plane of ferrite has a smaller coefficient of friction than the other planes. Therefore, by increasing the degree of orientation of the {100} surface of the ferrite on the sliding surface, the seizure resistance of the sliding component can be improved.

The degree of orientation of the {100} plane of ferrite on the sliding surface is evaluated as follows by X-ray diffraction with Co-Kα rays. A sample is taken from the steel sliding parts so as to include the sliding surface. Since the analysis depth based on the penetration depth of X-rays is about several μm, it is sufficient to collect a sample having a thickness of about 1 mm from the surface. If the sliding surface is a curved surface, the curved surface may be left as it is, but the detection time is adjusted so that the detection intensity is sufficient by examining the X-ray irradiation area. X-ray diffraction is performed on the sliding surface, and the integrated intensity of the diffraction line on the {200} surface is measured. The magnification of this integrated intensity with respect to the integrated intensity of the diffraction line of the {200} plane of the non-oriented ferrite sample measured under the same conditions is defined as the “{100} degree of orientation” of the sliding surface. If measurement cannot be performed under the same conditions, a value standardized using the signal strength of the strongest intensity line may be used. The integrated intensity of the diffraction line of the {200} plane of the non-oriented ferrite sample may be measured by preparing the non-oriented ferrite sample by the powder method, or may be obtained from the database of X-ray diffraction.

When the {100} degree of orientation of the sliding surface is 3.0 or more, excellent seizure resistance can be obtained. The degree of {100} orientation of the sliding surface is preferably 3.5 or more, and more preferably 4.5 or more.

The steel sliding component may have a {100} degree of orientation of 3.0 or more on at least a part of the surface of the part, and does not need to have a {100} degree of orientation of 3.0 or more on the entire surface of the part. For example, in the case of a crankshaft, the degree of orientation of {100} may be 3.0 or more only on the surface of the pin or the journal. However, there is no problem even if the degree of orientation of {100} is 3.0 or more on the entire surface of the component.

In order to make the {100} degree of orientation of the sliding surface 3.0 or more, as described above, the surface layer of the sliding surface has a structure mainly composed of ferrite, and the degree of orientation of the {100} surface is set. It suffices if a high structure (hereinafter referred to as “{100} oriented structure”) is present. The volume fraction of ferrite on the outermost layer of the sliding surface is not limited as long as it satisfies the above-mentioned X-ray diffraction conditions, but is preferably 90% or more, more preferably 95% or more, and further. It is preferably 98% or more.

The {100} oriented structure need only be present in the surface layer portion of the sliding surface. The thickness of the {100} oriented structure is not limited as long as it satisfies the above-mentioned X-ray diffraction conditions. The thickness of the {100} oriented structure may be a thickness corresponding to the life of the member if it is a sliding member that wears, and a member such as a crankshaft that is required to have seizure resistance without being worn. If so, it may be several μm. The lower limit of the thickness of the {100} oriented structure is not limited to this, but is preferably 2 μm, more preferably 20 μm, and even more preferably 200 μm. The upper limit of the thickness of the {100} oriented structure is not limited to this, but is preferably 10 mm, more preferably 5 mm, and even more preferably 1 mm.

The structure of the portion other than the surface layer portion of the sliding surface is not particularly limited, but is preferably ferrite pearlite (including full pearlite), bainite, martensite, or a mixed structure thereof, and particularly preferably ferrite pearlite. be.

The chemical composition of the sliding parts of the steel material is not particularly limited, and a general chemical composition of the steel material can be used as the steel material for the sliding parts. The steel sliding component may be sub-eutectic steel or hyper-eutectoid steel.

The chemical composition of the sliding parts of steel is, for example, C: 0.05 to 1.20%, Si: 6.5% or less, Mn: 5.0% or less, Cr: 1.00% or less, S in mass%. : Includes 0.20% or less. The chemical composition of the sliding parts made of steel may contain elements other than the above (for example, V, Nb, etc.). The chemical composition of the sliding steel parts is preferably C: 0.05 to 1.20%, Si: 6.5% or less, Mn: 5.0% or less, Cr: 1.00% or less, S: 0. .20% or less, balance: Fe and impurities. The lower limit of the C content is preferably 0.20%, more preferably 0.30%. The lower limit of the Si content is preferably 0.05%. The lower limit of the Mn content is preferably 0.05%. The lower limit of the Cr content is preferably 0.05%.

Here, the "chemical composition of the sliding parts made of steel" means the chemical composition of the bulk of the sliding parts made of steel, and means the chemical composition of the portion other than the portion where the {100} oriented structure described above is present. The chemical composition of the portion where the {100} oriented structure is present may differ from the chemical composition of the bulk. For example, when a steel sliding part is manufactured by a method of forming a {100} oriented structure by vacuum annealing as described later, the chemical composition of the portion where the {100} oriented structure is present contains C rather than the bulk chemical composition. The amount etc. will be low. The bulk chemical composition of the steel sliding component can be evaluated by a sample taken from the center position of the wall thickness or the 1/4 position of the outer diameter when the steel sliding component has a shaft shape such as a crankshaft.

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

Steel material Prepare the material for sliding parts. The material is, for example, a hot forged product. For example, steel can be melted, continuously cast or slab-rolled to form steel pieces, and then hot forged to form a rough shape of sliding parts. The material after hot forging may be machined.

The material is kept at a vacuum degree of 10 Pa or less at a temperature in a two-phase region of ferrite and austenite or a single-phase region of austenite. By holding under vacuum, decarburization occurs from the surface of the material, and the {100} surface, which is considered to have a relatively low surface energy, is preferentially generated during the transformation from austenite to ferrite at an isothermal temperature. As a result, the {100} oriented structure described above is formed on the surface layer of the material.

The lower limit of the holding temperature is preferably 800 ° C, more preferably 850 ° C. The upper limit of the holding temperature is preferably 1200 ° C, more preferably 1150 ° C. The degree of vacuum is preferably 1.0 Pa or less, and more preferably 0.01 Pa or less. The holding time depends on the holding temperature and the degree of vacuum, but is preferably 1 hour or more, more preferably 2 hours or more, still more preferably 4 hours or more.

After the vacuum treatment, machining such as grinding or polishing may be performed as long as the {100} oriented structure formed on the surface layer is not completely removed.

The steel sliding parts and the manufacturing method thereof according to the embodiment of the present invention have been described above. The steel sliding parts according to the present embodiment have excellent sliding characteristics.

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

[Evaluation of {100} plane by AFM]
A steel having a component of the machine structural steel S45C was melted, forged and hot-rolled, held at 1250 ° C. for 30 minutes, and then cooled in a furnace to obtain a steel material having a ferrite pearlite structure. A 10 mm square, 2 mm thick test piece was prepared from this steel material, the surface was mirror-polished, and then the crystal orientation of the sample surface was grasped by the electron backscatter diffraction (EBSD) method. An example of the observation result of EBSD is shown in FIG. In FIG. 1, the equivalent planes are shown in the same color.

Then, after removing the deposited carbon on the surface with a spatter cleaner, a horizontal force microscope (AFM) device is used to measure the frictional force under a constant load (2.2 μN) using a cantilever. The frictional force was measured in LFM) mode. EBSD and LFM were performed on the same crystal grains by hitting Vickers marks in advance. An example of the measurement result of LFM is shown in FIG. The measurement area of FIG. 2 corresponds to the area A of FIG.

From this result, it was found that the {100} plane of ferrite has a friction coefficient as low as 10% to 20% as compared with other planes.

[Verification using actual steel materials]
Steels having the chemical compositions shown in Table 1 were melted in a 150 kg vacuum induction melting furnace (VIM) to prepare an ingot.

This ingot was hot forged at 950 to 1200 ° C. to obtain a shaft material having a diameter of 40 mm. A test material having a diameter of 20 mm and a thickness of 3 mm was prepared from this shaft material, and after the heat treatment shown in Table 2, the surface was mirror-finished.

The above-mentioned {100} degree of orientation was measured by the X-ray diffraction method. The results are also shown in Table 2 above.

The sliding characteristics of the test material were evaluated using a ball-on-disc type friction / wear tester. FIG. 3 is a schematic view of a ball-on-disc type friction / wear tester.

As dry slidability, the friction characteristics with unlubricated oil were measured. The test conditions were that the mating ball was alumina, the load was 10 N, and the sliding speed was 10 mm / s. A friction coefficient of 0.30 or less was regarded as low friction, those having low friction when the sliding rotation speed was 15 times were evaluated as "◯", and those having a friction coefficient of 0.30 or less were evaluated as "x".

As an evaluation of seizure resistance in a lubricating oil environment, 2 mL of commercially available engine oil was dropped on the surface of the test material, the ball of the mating material was SUJ2, which is a high Cr steel, the load was 10 N, and the sliding speed was 47 cm / s. A test of sliding for a minute was performed. When the friction coefficient was 0.6 or more during the test time, it was evaluated as “x” as if seizure had occurred, and as “○” if it did not become 0.6 or more.

These results are also shown in Table 2 above. As shown in Table 2, No. 1 having a {100} degree of orientation of 3.0 or more. The test materials 1 to 4 had good dry sliding property and seizure resistance. On the other hand, No. 1 having a {100} degree of orientation less than 3.0. The test materials 5 to 8 had insufficient dry sliding property or seizure resistance.

From this result, it was confirmed that the sliding characteristics can be improved by increasing the degree of orientation of the {100} plane of ferrite on the sliding surface of the sliding component.

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 (4)

It is a steel sliding component having a sliding surface on the surface of the component, and the sliding surface is a ferrite in which the integral strength of the diffraction line of the {200} plane of ferrite is non-oriented in X-ray diffraction by Co-Kα rays. Steel sliding parts, which is 3.0 times more than the sample of. The steel sliding component according to claim 1.
The chemical composition is C: 0.05 to 1.20%, Si: 6.5% or less, Mn: 5.0% or less, Cr: 1.00% or less, S: 0.20% or less in mass%. Including steel sliding parts. The steel sliding component according to claim 1 or 2.
The steel sliding component is a crankshaft, which is a steel sliding component. The method for manufacturing a steel sliding component according to any one of claims 1 to 3.
A method for manufacturing a sliding steel component, comprising a step of keeping the material at a vacuum degree of 10 Pa or less at a temperature in a two-phase region of ferrite and austenite or a single-phase region of austenite.

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