JP2008156672A - Carburized component having graphite deposited on the surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a component such as a gear to be used after having been carburized, which has a predetermined strength and an abrasion resistance, and further has a graphite lubricant depositing on the surface, which is desirable for the use in a sliding component. <P>SOLUTION: A steel material to be carburized has an alloy composition comprising, by wt.%, 0.10 to 0.30% C, 0.50 to 3.00% Si, 0.30 to 3.00% Mn, 0.030% or less P, 0.030% or less S, 0.01 to 1.00% Cu, 0.01 to 3.00% Ni, 0.30 to 1.00% Cr, 2.00% or less Mo, further, a specific amount of one or more elements among Al, Nb, Ti, N and B, while satisfying the condition of [Si%]+[Ni%]+[Cu%]-[Cr%]>0.30, and the balance Fe with unavoidable impurities. The method for manufacturing the component includes forming the steel material to be carburized having the above composition into the component shape, subjecting the component to a vacuum carburizing operation of placing the component under a vacuum carburization condition and under a diffusion condition, and placing the component under the carburization condition again at least in the final step of the diffusion stage to produce graphite on the surface of the carburized component. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a carburized component having graphite attached to the surface thereof. The graphite on the part surface serves as a lubricant. Therefore, the component of the present invention is useful as a sliding member.

When steel is formed into machine parts, such as gears, and carburized into products, vacuum carburizing has recently been adopted as a carburizing method instead of the conventionally used gas carburizing method. It has become. This is because the vacuum carburizing method has the following advantages over the gas carburizing method. 1) Since the material is not oxidized because the treatment is performed in a vacuum, grain boundary oxidation, which is likely to occur in the gas carburizing method, is avoided, which contributes to improvement in strength.
2) Due to the structure of the carburizing device, it is easy to perform high-temperature carburizing, which enables quick carburizing.
3) The carburizing gas used is small, and the running cost is low.

When carburizing by the vacuum carburizing method, the mechanism of carburizing is that graphite is first generated from the carburizing gas in contact with the part and deposited on the part surface, which diffuses into the steel under high temperature conditions. I knew that I was going. Therefore, an attempt has been made to predict the carbon concentration distribution on the surface of the vacuum carburized material using this mechanism (Non-Patent Document 1).

Molding parts using graphite steel containing a large amount of carbon as a material, and gas carburizing at a carbon potential of 1.5 to 4.0% at a temperature range of 750 to 950 ° C. to produce graphite on the surface layer of the part Then, a technique for manufacturing a sliding part has been proposed, which is quenched and then used as a lubricant for graphite dispersed on the surface (Patent Document 1). This sliding part is made of martensite in the deep part, and has excellent toughness as compared with cast iron, and has seizure resistance and wear resistance equivalent to cast iron. This part has excellent sliding characteristics because the carbon produced by the carburizing gas precipitates and concentrates with spherical graphite interspersed in the surface layer, and large graphite blocks are dispersed. It is understood that this is due to the mechanism that the lubricant acts when sliding the parts, and even if it wears, the inner graphite mass appears on the surface and acts as a lubricant.
JP 10-265906 A Toshiyuki Morita et al. “Iron and Steel” Vol. 92 (2006) No. 4, p. 36

If the inventor has characteristics as a carburized part and can produce a part with graphite attached to the surface, the inventor has the desired strength and wear resistance, and the lubricant for sliding is already on the surface. It was conceived that a product adhered to the steel was obtained, and it was confirmed that this idea was easily realized by changing the vacuum carburizing process.

An object of the present invention is to provide a part in which a graphite lubricant desirable for a sliding part adheres to the surface in addition to the strength and wear resistance expected as a carburized part, making use of the above-described knowledge of the inventors. .

The carburized part having graphite adhered to the surface of the present invention is a carburized part obtained by vacuum carburizing, and is made by placing the carburizing steel formed into a part shape under vacuum carburizing conditions and then under diffusion conditions. In the vacuum carburizing operation, the following conditions [Si%] + [Ni%] + [Cu%] − [Cr%]> 0.30 are used as carburizing steel.
Carburized parts with graphite adhering to the surface of the carburized parts by using a material having an alloy composition that satisfies the above conditions, and by leaving them under carburizing conditions again at least at the end of the diffusion stage. It is. FIG. 1 illustrates this carburizing operation. A shows a temperature pattern of conventional vacuum carburizing, and B shows a temperature pattern of vacuum carburizing-graphite formation according to the present invention.

Since the carburized part of the present invention has undergone carburizing, carbon exists in the surface layer with a desired concentration distribution, and of course has the strength and wear resistance expected of the carburized part. Since the graphite adhering to the surface acts as a lubricant as it is, resistance and wear associated with sliding of the components are minimized. Therefore, even if a lubricant is not applied again prior to use, it can be used immediately as a sliding member.

The graphite adhering to the surface is formed by revealing the carburizing conditions again at least at the final stage in the diffusion stage following the carburizing stage of vacuum carburizing, specifically by introducing a carburizing gas into the carburizing chamber, Since no special graphite formation step is required, the increase in manufacturing costs is negligible.

In the present invention, as the carburizing steel used as a material for forming the carburized component, C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 by weight%. 3.00%, P: 0.030% or less, S: 0.030% or less, Cu: 0.01 to 1.00%, Ni: 0.01 to 3.00%, Cr: 0.30 to 1 0.000% and Mo: 2.00% or less, Al: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, N: 0.05% or less, and B : 0.01% or less of one type or two or more types, the balance being Fe and inevitable impurities, and the following conditions [Si%] + [Ni%] + [Cu%]-[Cr% ]> 0.30
A steel having an alloy composition satisfying the above is appropriate. With this alloy composition, there is little formation of carbides in the steel, and there is no fear that the carbon concentration in the surface layer of the carburized component that has been subjected to carburizing conditions again in the diffusion stage will be excessively high.

As long as the carburized parts of the present invention are manufactured by vacuum carburizing, various hydrocarbon gases such as acetylene, ethylene, and propane can be used as the carburizing gas. The carburization pattern is also arbitrary. Those skilled in the art will be able to easily determine appropriate vacuum carburizing conditions with reference to the examples described below.

If the suitable alloy composition of the steel for carburizing of this invention is demonstrated, C amount (0.10-0.30%) is a suitable range when obtaining intensity | strength required as a machine part. Mn is added as a deoxidizer during the melting of steel, but does not significantly affect the formation of carbides, so the amount can be selected from a wide range (0.30 to 3.00%). Since P and S are impurities and are unfavorable components for the mechanical properties of the product, the amount should be low. The above values (both 0.030%) are acceptable limits.

Si (0.50 to 3.00%), Ni (0.01 to 3.00%) and Cu (0.01 to 1.00%) are components that suppress the formation of carbides as described above. In addition, each of them must be added so that the value is equal to or more than the above lower limit value, and the value obtained by subtracting the amount of Cr from the total of these amounts exceeds 0.3, preferably 0.5. However, the addition of a large amount reduces the hot workability, so the upper limit is set for each.

Cr: 0.30 to 1.00%
Since Cr is a component that promotes the formation of carbides, it cannot be present in large quantities in the carburizing steel of the present invention. 1.00% is the upper limit of the Cr amount that is possible when there are a large amount of components that suppress the formation of carbides. A Cr amount higher than 1.0% cannot be added from the viewpoint of workability, particularly machinability. However, if it is reduced too much, the hardenability becomes low and the mechanical properties of the product become unsatisfactory, so 0.30% was set as the lower limit.

Al: 0.20% or less Al is added as a deoxidizer, but an excessive addition must be avoided because alumina is formed in the steel, resulting in a decrease in strength. Also, the formation of alumina is not preferable from the viewpoint of impairing workability. For this reason, it is preferable to select an Al addition amount of up to 0.20%. On the other hand, Al also has a function of suppressing coarsening of crystal grains, and 0.005% or more is preferably added in order to obtain the effect.

N: 0.050% or less When N is present, there is an effect of preventing the coarsening of crystal grains. Therefore, it is preferable that at least 0.001% be present. Since this effect is saturated at about 0.050%, there is no point in making it exist beyond this limit.

Mo: 2.00% or less It can be added to improve hardenability and increase temper softening resistance. When the amount is too large, the workability of steel deteriorates, so an appropriate amount of addition of 2.00% or less should be selected.

Nb: 0.20% or less, Ti: 0.20% or less These components are effective for the purpose of suppressing the growth of crystal grains generated during carburizing and maintaining a sized structure. Excessive addition has an adverse effect on processability, so the addition amount is limited to the above limit.

B: 0.01% or less B is effective in improving hardenability, so is added as desired. Since the presence of a large amount is harmful to processability, an addition amount of 0.01% or less is selected.

[Si%] + [Ni%] + [Cu%]-[Cr%]> 0.30 (preferably 0.50)
As described above, Si, Ni, and Cu suppress the formation of carbides, while Cr increases, so that the effects of the former three and the latter are balanced to increase the suppression effect. Thus, the formation of carbides in the carburized surface layer is suppressed, and it is possible to prevent the carbon concentration from becoming excessive when it is again subjected to carburizing conditions to generate graphite. The above equation is a relationship derived from the implementation data described below.

Case-hardened steel having the alloy composition shown in Table 1 was melted. Using each material, machine into the following test gear shape,
Module: 2.5 Teeth: 30
Pitch circle diameter: 82.753mm
Teeth width: 20 mm Twist angle: 25 degrees Under the conditions shown in Table 2, vacuum carburization or vacuum carburization-graphite generation was performed. Graphite formation was performed by supplying a carburizing gas in an amount corresponding to 200 L / hr per m ² of the surface area of the part in the last 5 minutes of the diffusion stage of 30 minutes. After analyzing whether or not carbides were generated in the surface layer of the test gear, which was the carburized part, the gear was rotated under a load, and the surface fatigue strength and the root fatigue strength were measured. The results are shown in Table 2.

Table 2

The gears of the examples of the present invention had no surface formation of carbide in the component surface layer, and had a graphite lubricant on the surface, and exhibited high surface fatigue strength and tooth root fatigue strength. On the other hand, Comparative Examples 1 to 5 and 7 in which carbides are generated in the surface layer have low root fatigue strength or both surface fatigue strength and root fatigue strength, while no carbide is observed. However, although Example 6 which does not have a graphite lubricant on the surface has excellent tooth root fatigue strength, surface fatigue strength was inferior.

It is a conceptual diagram explaining the carburizing operation which obtains the carburized part which the graphite adhered to the surface by this invention, A is a temperature pattern of the conventional vacuum carburizing, B is a temperature pattern of the vacuum carburizing-graphite formation according to this invention.

Claims (2)

A carburized part obtained by vacuum carburizing, and after the carburizing steel formed into a part shape is subjected to vacuum carburizing conditions and then placed under diffusion conditions, the carburizing steel is subjected to the following conditions [ Si%] + [Ni%] + [Cu%]-[Cr%]> 0.30
Carburized parts with graphite adhering to the surface of the carburized parts by using a material having an alloy composition that satisfies the above conditions, and by leaving them under carburizing conditions again at least at the end of the diffusion stage. . As carburizing steel, by weight, C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 to 3.00%, P: 0.030% or less, S: 0.030% or less, Cu: 0.01 to 1.00%, Ni: 0.01 to 3.00%, Cr: 0.30 to 1.00% and Mo: 2.00% or less Furthermore, one or more of Al: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, N: 0.050% or less, and B: 0.01% or less The carburized part of claim 1 obtained by using a steel containing an alloy composition, the balance being Fe and inevitable impurities, and satisfying the condition of claim 1.

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