Steel Part and Method for Manufacturing the Same

A plurality of layers are laminated on at least part of the member under treatment made of steel, the plurality of layers having carbon concentrations higher than that of the member under treatment and 1.0 wt. % or less, the carbon concentration of an outermost layer of the plurality of layers being the highest. A method for manufacturing a steel part, including spraying powder containing carbon on at least part of an member under treatment made of steel so as to form a first layer having a carbon concentration higher than that of the member under treatment and spraying powder containing carbon on at least part of the first layer so as to form a second layer having a carbon concentration higher than that of the first layer. Carbon concentrations of a plurality of layers including the first layer and the second layer are 1.0 wt. % or less.

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Description
TECHNICAL FIELD

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

BACKGROUND ART

Since machine parts such as drive parts, gears, and bearings are always exposed. to heavy loads, they need to have high mechanical strength such as hardness and fatigue resistance. These machine parts are made from steels used for machine structures such as carbon steel, chromium. steel, chromium molybdenum steel, and nickel chromium-molybdenum steel.

The steels used for machine structures may need two opposite properties for an outer portion and an inner par Lion, that is, a surface of the steels needs high fatigue resistance and the material itself needs high fracture resistance to secure shock resistance of the part. The material may have a base material having a comparatively low carbon concentration and a high fracture resistance, for example, a low alloy steel (such as SCM415 defined in JIS-G4104). The surface of the material is often solid-soluted with carbon so as to increase the carbon concentration, and carburizing treatment and carbonitriding treatment are often performed to improve hardness and fatigue resistance.

However, when carbon is simply dispersed on the surface of the material, a surface hard layer has a gradient in carbon concentration distribution. Thus, it is difficult to form a thick composition having a desired carbon concentration. When the surface hard layer is thickened, the surface is excessively carburized (excessive carburization). As a result, the surface hard layer embrittles. For example, PTL 1 discloses a method for forming a hard film of high carbon steel or a high-carbon low-alloy steel on a surface of a base material and thermally diffuse-bonding the base material and the hard film so as to secure desired strength.

CITATION LIST Patent Literature

PTL 1: JP 11-222663 A.

SUMMARY OF INVENTION Technical Problem

However, according to the method disclosed in PTL 1, since there is a large difference between the carbon concentration of the base material as a member under treatment and that of the coating film of the high carbon steel, the member under treatment and the surface hard layer easily peel off between the base material and the coating film, which is a problem of such a method.

An object of the present invention is to prevent the member under treatment and the surface hard layer from peeling off.

Solution to Problem

To accomplish the foregoing object, configurations described for example in the claims are used.

Advantageous Effects of Invention

According to the present invention, the member under treatment and the surface hard layer can be prevented from peeling off.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example of a block diagram of a steel part.

FIG. 2 is an example of a block diagram of the steel part.

FIG. 3 is an example of a process drawing showing a process for forming a first layer and a second layer on a member under treatment.

FIG. 4 is an example of a schematic diagram showing a steel part according to a second example.

FIG. 5 is an example of a schematic diagram showing a cross section of the steel part according to the second example.

FIG. 6 is an example of a process drawing showing a process for forming a steel part according to the second example.

 DESCRIPTION OF EMBODIMENTS

A steel part according to the present invention includes a plurality of layers having high carbon concentrations than that of a member under treatment (high carbon steel layers) formed on the surface of the member under treatment (steel, part). The carbon concentrations of the high carbon steel layers are 1.0 wt. % or less. The outermost layer has the highest carbon concentration.

FIG. 1 shows an embodiment of the present invention. A first layer 102 and a second layer 103 are successively laminated on at least part of a member under treatment 101 so as to form a steel part 100. The first layer 102 and the second layer 103 are high carbon steel layers having carbon concentrations of 1.0 wt. % or less. The carbon concentrations of the first layer 102 and the second layer 103 are higher than that of the member under treatment. The carbon concentration of the second layer is higher than that of the first layer.

FIG. 2 shows another embodiment. According to the present embodiment, a third layer 104 is laminated on the second layer of the steel part shown in FIG. 1. The carbon concentration of the third layer 104 is higher than that of the second laver. The carbon concentration of the third layer 104 is 1.0 wt. % or less.

Examples of the member under treatment on which a surface treatment is performed includes low carbon steel or low carbon alloy steel. The materials having high fracture resistance such as chromium steel, chromium molybdenum steel, chromium molybdenum nickel steel, chromium manganese steel, and chromium nickel, steel (stainless steel) are exemplified, and alloy compositions thereof are defined in domestic and foreign standards such as JIS and ASTM.

The plurality of high carbon steel layers coating the member under treatment is required to have carbon concentrations that are higher than that of the member under treatment and 1.0 wt. % or less. Since the carbon concentrations of these layers are higher than that of the member under treatment, these layers have higher fatigue resistances. In addition, when the carbon concentrations of these layers are 1.0 wt. % or less, mesh-shaped cementite can be prevented from excessively deposited on a grain boundary. Thus, the surface can be prevented from embrittling, and as a result, these high carbon steel layers become long fatigue-life layers. The high carbon steel lavers have at least the first layer and the second layer. When necessary, the high carbon steel layers also have one or more layers formed on the first and second layers. The carbon concentration increases from the member under treatment toward the outermost layer.

To increase the carbon concentration of the steel part by coating the member under treatment with the layer without carburizing and diffusing, a plurality of layers is formed to decrease differences of carbon concentrations between the layers, and therefore, peeling between the member under treatment and the high carbon steel layer and peeling, cracking and the like between the high carbon steel, layers can be reduced. In addition, the desired carbon concentrations and thicknesses of the layers can be freely adjusted. When the layer is a multi-layer composed of three or more layers, even if the difference between the carbon concentration of the outermost layer and the carbon concentration of the member under treatment is large, since the differences of carbon concentrations of individual layers can be decreased, peeling between the member under treatment and the layer and peeling and the like between the layers can be reduced.

Although the drawing clearly separates layers, since carbon slightly diffuses from a layer that has a high carbon concentration to a layer that has a low carbon concentration, there is a gentle gradient of carbon concentrations at boundaries of layers in the film thickness direction. According to the present invention, a portion where there is a gradient of carbon concentrations is permitted as inter-layers (boundaries of layers).

Alloy compositions other than carbon of individual layers are not restricted. Examples of these steel materials are high carbon steel and high carbon alloy steel. In particular, it is preferable that the compositions of alloy elements of the first layer, the second layer, and at least one layer formed thereon if necessary (hereinafter these layers are referred to as the plurality of high carbon steel layers) are nearly the same as that of the member under treatment, so that the high carbon steel layers can be unified with the member under treatment and that a conjugated compound can be prevented from locally being formed. Alternatively, other alloy elements may be adjusted so as to improve properties such as corrosive resistance and heat resistance other than fatigue resistance, at the same time.

Although the plurality of high carbon steel layers may be formed on the entire surface of the member under treatment with equal thicknesses, the plurality of high carbon steel layers may have a thickness distribution on the surface of the member under treatment, and may be formed only on part of the member under treatment. As a particular example from machine parts, each of the layers may be formed only at a portion that needs especially high fatigue resistance such as a contact portion with a bearing of a shaft, tooth of a gear, a contact portion with a member of a press roll. These partial treatments are preferable when a portion that needs fracture resistance and a portion that needs fatigue resistance are individually controlled. Further, each of the layers may be formed on the surface of the member under treatment with distribution such as a portion having no layers, a portion having only the first layer, and a portion having both the first layer and the second layer. Likewise, at least one layer formed on the second layer may be formed on part of the surface of the member under treatment.

The plurality of high carbon steel layers can be formed according to various methods. As methods that are excellent in forming speed and adherence to the member under treatment, a cold spraying method, a warm spraying method, a plasma spraying method, an arc spraying method, a flame spraying method, a building-up welding method, an aerosol deposition method, and so forth are known. FIG. 3 shows a process for successively forming individual layers.

(a) A member under treatment 101 is prepared to form high carbon steel layers.

A first layer 102 is formed on the member under treatment 101. Materials of the individual layers are prepared in the form of powder, wires, rods, and the like, according to the individual methods. According to the method shown in FIG. 3, powder is sprayed to be deposited on the member under treatment. For example, using powder having a low carbon concentration (first powder), the layer is formed according to the foregoing method. The carbon concentration of the first powder is higher than that of the member under treatment and 1.0 wt. % or less. Alternatively, the first powder may be a mixture of carbon and another powder. When the layer is formed, the carbon concentration of the layer contained in the entire layer may be higher than that of the member under treatment and 1.0 wt. % or less.

(c) A second layer 102 is formed on the first layer 102. The second layer is formed on the first layer 102 using powder (second powder) having a higher carbon concentration than that of the first layer. Like the first powder, the second. powder may be a mixture of carbon and another powder. When the layer is formed, the carbon concentration of the entire layer may be higher than that of the first layer and 1.0 wt. % or less.

(d) As a result, a steel part 100 having two high carbon steel layers is formed.

Even if the high carbon steel layers is a multilayer, it is preferable that the mixing ratios of a plurality of types of powders having different carbon concentrations are changed, because the kinds of materials required for forming the layer. Although film forming conditions are appropriately adjusted depending on a method, a member under treatment, and materials of individual layers that are used, it is preferable to form the layer at a temperature of the member under treatment higher than the room temperature, because the film forming efficiency is improved, adherence of each of the layers to the member under treatment is improved, and mutual diffusion on the interface of each of the layers is accelerated. However, it is desirable that the film forming temperature is adjusted depending on various conditions such as heat resistances and oxidizing resistances of the materials of the member under treatment and the individual layers. After forming the layers as described above, thermal treatment such as quenching and annealing and surface treatment such as carburizing and nitriding can be performed.

Hereinbelow, examples will be described with reference to the accompanying drawings.

EXAMPLE 1

In the present example, an example in which the steel part 100 is a plate will be described. FIG. 1 is a block diagram of the steel part 100 in the present example. In the present example, the member under treatment 101 used for the steel part 100 was a stainless steel plate having a length of 50 mm, a width of 50 mm, and a. thickness of 10 mm (JIS standard: SUS 304, NISSHIN STEEL CO., LTD, 0.05 wt. %).

The first layer 102 and the second layer 103 were made from stainless steel powder (DAP304L, DAIDO STEEL LTD). Two types of powder, stainless steel powder A that is additive free and stainless steel powder B that holds graphite powder of 2.0 wt. % (SIGMA-ALDRICH CO. LLC) were mixed at predetermined weight ratios to prepare material powders of the first layer 102 and the second layer 103. In the present example, the material powders of the first layer 102 were mixed so that the carbon concentration became 0.4 wt. % (stainless steel powder A: stainless steel powder B=8:2 (weight ratio) and the material powders of the second layer 103 were mixed so that the carbon concentration became 0.8 wt. % (stainless steel powder A: stainless steel powder B=6:4 (weight ratio), and the mixed material powders were uniformly maxed by a V-shape rotating mixer to prepare the material powders of each of the first layer 102 and the second layer 103. FIG. 1 is a schematic diagram showing individual layers so that they can be easily distinguished. Table 1 lists real thicknesses of individual films that are formed. Theoretically, the carbon concentrations when the powder is adjusted matches the carbon concentrations of the formed layers. However, since carbon is lost while materials are adjusted and layers are formed, the carbon concentrations of the layers are slightly lower than those of the material powders.

FIG. 3 shows a process for forming the first layer and the second layer in the present example. The first layer and the second layer were formed by a cold spraying method under the condition that nitrogen gas was used as carrier gas at a pressure of 4 MPa, the temperature of the member under treatment was 400° C., and the nozzle distance was 20 mm. When the member under treatment is heated, since powder easily adheres to the member under treatment, the adherence between layers further improves. Thereafter, material powders were changed and the second layer was formed by the cold spraying method in the same conditions. Thereafter, the heat treatment was performed at 800° C. for 30 minutes so that graphite powder held on stainless steel powder B was solidified in each of the layers and then quenching is performed at a speed of 100° C. per second to form the steel part 100.

COMPARATIVE EXAMPLE 1

In the configuration of Example 1, a first layer having a carbon concentration of 0.8 wt. % was formed by the cold spraying method, and a second layer was not formed. The other forming conditions were the same as those of Example 1.

In Example 1, also in a Falex test conducted with a test piece of around 1 mm thick and Vickers hardness (Hv) of 920 on the surface of the steel part 100, the surface treatment layer having good adherence without inter-laver peeling can be obtained. However, in Comparative Example 1, there was a problem in adherence due to small peeling between the member under treatment 101 and the first layer 102.

TABLE 1 EXAMPLE COMPARATIVE 1 EXAMPLE 1 FIRST THICKNESS (mm) 0.10 1.03 LAYER AVERAGE CARBON 0.36 0.75 CONCENTRATION (wt. %) VICKERS HARDNESS 710 910 (Hv) SECOND THICKNESS (mm) 1.01 LAYER AVERAGE CARBON 0.77 CONCENTRATION (wt. %) VICKERS HARDNESS 920 (Hv) ADHERENCE BETWEEN BASE NO CRACKING MATERIAL AND LAYER PEELING

EXAMPLE 2

In the present example, an example in which a steel part 100 is a shaft part will be described. FIG. 4 is a schematic diagram of the steel part 100. The steel cart 100 used a member under treatment 101 of chrome molybdenum steel (JIS standard.: SCM 415, DAIDO STEEL CO., LTD., 0.15 wt. %) having a diameter of 30 mm and a length of 300 mm. A first layer 102, a second layer 103, and a third layer 104 were formed at an end portion of the member under treatment.

FIG. 5 is a sectional view taken along a line A-A′ of FIG. 4. FIG. 5 shows a cross section to a line C-C. The first layer 102, the second layer 103, and the third layer 104 were formed at the end portion of the member under treatment 101. All the three layers were formed within 100 mm from the end portion, the first layer 101 and the second layer 102 were formed at the portion between 100 mm and 110 mm from the end portion, and only the first layer 101 was formed at the portion between 110 mm and 120 mm from the end portion of the member under treatment 101. FIG. 4 is a schematic diagram showing individual layers so that they can be easily distinguished. Table 2 lists real thicknesses of individual films that are formed.

Each of these layers was made by mixing two types of powder, chrome molybdenum steel powder A and chrome molybdenum steel powder B at predetermined weight ratios. Chrome molybdenum steel powder A is made of chrome molybdenum steel powder (SCM 415, EPSON ATMIX CORPORATION) that has the same composition as that of the member under treatment 101, and chrome molybdenum steel powder B is made by increasing only the carbon concentration of the chrome molybdenum steel powder A to 2.0 wt. %, to prepare the material powders of these layers. In the present example, the material powders of the first layer 102 were mixed so that the carbon concentration became 0.4 wt .% (chrome molybdenum steel powder A: chrome molybdenum steel powder B=86:14 (weight ratio)), the material powders of the second layer 103 were mixed so that the carbon concentration became 0.6 wt. % (chrome molybdenum steel powder A: a chrome molybdenum steel powder B=76.24 (weight ratio)), and the material powders of the third layer 103 were mixed so that the carbon concentration became 0.8 wt. % (chrome molybdenum steel powder A: chrome molybdenum steel powder B=65:35 (weight ratio)), and the mixed material powders were uniformly mixed by a V-shape rotating mixer to prepare the material powders of these layers

FIG. 6 shows a process for forming the first layer, the second layer, and the third layer in the present example. Each of the layers were formed using a plasma spraying method. The first layer, the second layer, and the third layer formed in this order by changing the material powders in the same conditions. In this configuration, since each of the layers was partly formed, while the member under treatment 101 pre-heated at 400° C. was being rotated, the member under treatment 101 was scanned by a thermal spray nozzle 106 to form the individual layers at desired portions as shown in FIG. 6.

EXAMPLE 3

In the configuration of example 2, the material powders of the third layer were prepared so that their carbon concentration became 1.0 wt. % (chrome molybdenum steel powder A: chrome molybdenum steel powder B=54:46 (weight ratio)) by the plasma spraying method. The other forming conditions were same as those of Example 2.

COMPARATIVE EXAMPLE 2

In the configuration of Example 2, the material powders of the first layer were prepared so that the carbon concentration became 0.8 wt. % (chrome molybdenum steel powder A: chrome molybdenum steel powder B=65:35 (weight ratio)) by the plasma spraying method, and the second layer and the third layer were not formed. The other forming conditions of Comparative Example 2 were the same as those of Example 2.

COMPARATIVE EXAMPLE 3

In the configuration of Example 2, the material powders of the third layer were prepared so that the carbon concentration became 1.1 wt. % (chrome molybdenum steel powder A: chrome molybdenum steel powder B=49:51 (weight ratio)) by the plasma spraying method. The other forming conditions of Comparative Example 3 were the same as those of Example 2.

The steel parts 100 obtained in Examples 2 and 3 and Comparative Examples 2 and 3 were smoothened by a mechanical polishing method or a buffing method so that the surface roughness (Ra) became 1.0 μm or less. Thereafter, a Falex test was conducted in lubrication oil for a film-forming portion of the third layer 103 based on ASTM-D-3233. Table 2 shows the thicknesses of the individual layers measured by cutting the steel parts 100 after the Falex test was confucted, average carbon concentrations measured by an electron beam micro-analyzer (SHIMAZU CORPORATION), presence or absence of inter-layer peeling observed by an optical microscope, surface roughness (Ra) of the outermost layers, and cross-sectional Vickers hardness measured by a micro Vickers hardness meter (SHIMAZU CORPORATION).

When the Falex test was conducted with an approximately 1-mm thick test piece having a Vickers hardness (Hv) of 930 or higher on the surface of steel parts 100 in Examples 2 and 3, surface treatment layers having excellent adherence without inter-layer peeling were obtained. It was confirmed that in each of the surface treatment layers of each sample, mesh-shaped cementite was not deposited on a grain boundary and the layers did not excessively carburize.

In contrast, in Comparative Example 2, there was a problem in adherence due to small peeling between the member under treatment 101 and the first layer 102. Further, it was confirmed that in the conditions of Comparative Example 3, the surface roughness after the Falex test was conducted was larger than that of the other steel parts, and the steel part 100 was damaged by abrasion. As a result of an observation of the composition, it was confirmed that mesh-shaped cementite that was characteristic of hypereutectoid steel of a steel material was deposited on a grain boundary, and an carburized portion was damaged.

From each of the foregoing evaluations, it was confirmed that the steel parts having configurations disclosed in the present invention have a surface treatment layer having excellent surface hardness and excellent adherence to a member under treatment. Although a shaft as a machine part has been described in this section, it is clear that the embodiments of the present invention can be applied to various machine parts such as drive parts, gears, and bearings.

TABLE 2 EXAMPLE EXAMPLE COMPARATIVE COMPARATIVE 2 3 EXAMPLE 2 EXAMPLE 3 FIRST THICKNESS (mm) 0.10 0.10 1.03 0.08 LAYER AVERAGE CARBON 0.32 0.33 0.78 0.32 CONCENTRATION (wt. %) VICKERS HARDNESS 610 630 900 610 (Hv) SECOND THICKNESS (mm) 0.11 0.09 0.13 LAYER AVERAGE CARBON 0.55 0.57 0.53 CONCENTRATION (wt. %) VICKERS HARDNESS 800 820 770 (Hv) THIRD THICKNESS (mm) 0.99 1.05 1.03 LAYER AVERAGE CARBON 0.77 0.99 1.05 CONCENTRATION (wt. %) VICKERS HARDNESS 940 930 960 (Hv) SURFACE ROUGHNESS (Ra, μm) 1.1 0.9 2.1 5.4 ADHERENCE BETWEEN BASE NO NO CRACKING NO MATERIAL AND LAYER PEELING PEELING PEELING

REFERENCE SIGNS LIST

  • 100 steel part
  • 101 member under treatment
  • 102 first layer
  • 103 second layer
  • 104 third layer
  • 105 spray nozzle
  • 106 spray nozzle

Claims

1. A steel part having a member under treatment made of steel, a plurality of layers being laminated on at least part of the member under treatment, the plurality of layers having carbon concentrations higher than that of the member under treatment and 1.0 wt. % or less, the carbon concentration of an outermost layer of the plurality of layers being the highest.

2. The steel part according to claim 1,

wherein the plurality of layers are three layers or more.

3. The steel part according to claim 1,

wherein the member under treatment has an area in which the plurality of layers are laminated and an area in which the plurality of layers are not laminated.

4. The steel part according to claim 1,

wherein the plurality of layers includes a first layer and a second layer laminated successively on the member under treatment, the first layer having an area in which the second layer is laminated and an area in which the second layer is not laminated.

5. A method for manufacturing a steel part, comprising:

spraying powder containing carbon on at least part of an member under treatment made of steel so as to form a first layer having a carbon concentration higher than that of the member under treatment; and
spraying powder containing carbon on at least part of the first layer so as to form a second layer having a carbon concentration higher than that of the first layer,
wherein carbon concentrations of a plurality of layers including the first layer and the second layer are 1.0 wt. % or less.

6. The method for manufacturing the steel part according to claim 5, comprising:

spraying powder containing carbon on at least part of the second layer so as to form a third layer having a carbon concentration higher than that of the second layer.

7. The method for manufacturing the steel part according to claim 5,

wherein the plurality of layers are formed while the member under treatment is heated.
Patent History
Publication number: 20160138151
Type: Application
Filed: Jun 10, 2013
Publication Date: May 19, 2016
Inventors: Kousuke KUWABARA (Tokyo), Minseok PARK (Tokyo)
Application Number: 14/897,113
Classifications
International Classification: C23C 8/66 (20060101); C23C 28/04 (20060101); C23C 4/06 (20060101);