JP2808801B2 - Method of carburizing Cr-containing steel - Google Patents

Description

The present invention relates to a method for carburizing a Cr-containing steel, and more particularly to a plasma carburizing method for depositing a large amount of fine and spherical carbides.

[Prior art] Conventionally, in order to diffuse and infiltrate carbon into the surface of a steel material, and then harden the surface by quenching to produce a component having a highly wear-resistant surface and a tough core. A method of subjecting Cr-containing steel to gas carburization is known. It is known that Cr acts as a carbide-forming element, and increasing the content of Cr in a steel material promotes carbide formation by carburization.

[Problems to be Solved by the Invention] However, since gas carburizing is performed in an oxygen atmosphere such as CO gas or CO ₂ gas, an oxide layer is formed at a grain boundary of a steel material during the carburizing process. There was a problem.

The problem of the formation of this grain boundary oxide layer is that Cr contained in steel
The situation differs depending on the amount of Cr. In a steel material having a Cr content of 1 to 2%, a grain boundary oxide layer is formed although it is slight, and the Cr content is 2%.
It is known that a grain boundary oxide layer is remarkably formed in a steel material of up to 6%, and an oxide film is formed on the surface in a steel material having a Cr content of 6 to 12%.

In the above case, although the oxide layer is slight, there is a problem that a sufficient amount of carbide is not formed to improve the fatigue characteristics and the roller pitting characteristics. Further, in the above case, there is a problem that the fatigue characteristics and the roller pitting characteristics are rather deteriorated due to the influence of the grain boundary oxide layer. Further, in the above case, there is a problem that carburization becomes impossible because an oxide film is formed on the surface of the steel material.

As described above, carburization with a high carbon concentration cannot be performed by gas carburization generally used conventionally, and it has been difficult to improve fatigue characteristics, roller pitting characteristics, and the like.

On the other hand, it has been proposed in recent years that plasma carburizing is effective for such Cr-containing steel because carburizing can be performed without any problem on high Cr-containing steel and there is no influence of oxidation. Became.

However, in the case of a steel material having a Cr content of 1 to 2%, carburization with sufficient surface carbon concentration could not be performed in the first place. Also, Cr
In a steel material having a content of 2 to 6%, the formed carbide precipitates elongated at the crystal grain boundary to form a so-called reticulated carbide layer, which is not preferable in terms of fatigue characteristics and roller pitting characteristics. Further, in the case of a steel material having a Cr content of 6% or more, there is no problem in the above-described properties, but there is a problem that the cost of the steel material itself is high and generally not applicable.

Then, the present invention solves the above-mentioned problems, and is relatively inexpensive.
Using a steel material with a Cr content of 2 to 6%, a surface carbon concentration of 2% or more, a spherical homogeneous carbide layer, and a Cr-containing steel suitable for manufacturing products having excellent fatigue characteristics and roller pitting characteristics. It aims to provide a carburizing method.

[Means for Solving the Problems] A method for carburizing a Cr-containing steel of the present invention, which is made for the above purpose, is a method for carburizing a Cr-containing steel containing 2 to 6% by weight of Cr, First, the plasma carburizing treatment is performed at a temperature of 900 ° C. or less as a first step, and once the temperature is lowered to the A1 transformation point or less, the temperature is raised to 900 ° C. or more as a second step, and the plasma carburizing treatment is performed again. Features.

[Operation] According to the method for carburizing a Cr-containing steel of the present invention, the first-stage plasma carburizing treatment is performed in a low-temperature region, so that the formed carbide is fine and spherical. This spherical carbide is concentrated in the grain boundary portion of the steel material and exists in a slightly heterogeneous state. In contrast, as the next step, once by lowering the temperature below the A ₁ transformation point, it changes the position of the grain boundary, the shape carbides of the aforementioned granular incorporated in the grains of the metal structure. Next, by performing the plasma carburizing treatment in the second stage, the spherical carbides taken into the structure as described above further grow to become large spherical carbides, and the carbides also occur in the new grain boundary portions. It is formed.
Therefore, a carbide layer having a uniform distribution of carbides is formed.

As a result, according to the method for carburizing Cr-containing steel of the present invention, it is possible to perform carburization with a good distribution and shape of carbide.
The carbide layer at this time is made of a relatively round and large carbide having an area ratio of 20% or more and a surface carbon concentration of 2% or more and an aspect ratio of 2.0 or less. Here, the area ratio is an area ratio of carbide within 0.1 mm from the surface of the steel material.

Here, although mean that efforts to homogenized by treatment to lower the temperature below the A ₁ transformation point, when the first stage is more than 900 ° C., even if the movement of the grain boundaries is found, the final The carbide formed in the steel has a large aspect ratio, which is not preferable in terms of fatigue characteristics.

That is, the action of the present invention is as follows.
The following conduct plasma carburization treatment at a temperature, once after cooling below the A ₁ transformation point can be realized by which comprises carrying out the re-plasma carburization temperature was raised to 900 ° C. or higher in the second stage It is a thing.

EXAMPLES Next, in order to further clarify the present invention, preferred examples to which the present invention is applied will be described.

 FIG. 1 shows a pattern of the carburizing treatment in the embodiment.

In the embodiment, after the steel material is put into the plasma carburizing furnace and evacuated (10 ^-2 Torr to 10 ^-3 Torr), first, the processing temperature as the first stage (first stage processing temperature) T1 [° C.] Raise the temperature. At the first stage treatment temperature T1, prior to the plasma carburizing treatment, Ar gas or H ₂ gas is introduced, plasma discharge is performed, and sputter cleaning using Ar ions or H ₂ ions is performed for 30 minutes. Thereafter, a hydrocarbon gas as a working gas is introduced into the furnace, and the first stage processing time t
Perform plasma carburizing for ₁ minute.

Then, plasma discharge is stopped, the temperature was lowered to a temperature 650 ° C. below the A ₁ transformation point by furnace cooling and held for one hour in the at that temperature time.

Subsequently, the temperature was raised again, and the inside of the furnace was subjected to the second stage processing temperature T ₂ [° C.].
The plasma discharge is performed while introducing the working gas, and the plasma carburizing process as the second stage is performed for the second stage processing time t ₂ [minutes].

After that, the plasma discharge was stopped, and the temperature was lowered to 830 by furnace cooling.
℃, hold at that temperature for 30 minutes, then oil-cooled,
Thereafter, the temperature was maintained at 160 ° C. for 2 hours, and then quenching and tempering were performed by air cooling to room temperature.

Table 1 shows steel materials subjected to carburizing and quenching / tempering by this method.

Further, using this steel material, carburizing was performed under various temperature conditions and processing times according to the above-described processing procedure, and for each, the carbon concentration within 0.1 mm from the surface, the aspect ratio of the formed carbide, and 0.1% from the surface. The surface ratio of carbide at the position of mm was measured.

 Table 2 shows the results.

As shown in Table 2, when the Cr content is 2 to 6%, the first stage processing temperature is set to 900 ° C. or less, and the second stage processing temperature is set to 900 ° C.
In the above examples (No. 1 to No. 4), the surface carbon concentration was 2.1 to 2.6%, which was 2.0% or more, and the aspect ratio was 1.6 to 2.0. Sufficient carbide layers having a spherical shape as shown below and an area ratio of 22 to 29% and 20% or more in all cases were confirmed.

On the other hand, in a comparative example (No. 5) in which a steel material having a Cr content of less than 2% was processed under the condition that the first-stage processing temperature was 900 ° C or higher and the second-stage processing temperature was 900 ° C or lower. Although the carbon concentration was 2.1%, the aspect ratio was 3.4 and the area ratio was 18
%, The amount of carbide formed was smaller than in the examples, and the formed carbide was elongated.

In addition, the steel material C was used as in Example No. 2, but in both the first stage and the second stage, the processing temperature was 900 ° C. or higher.
In o.6), the surface carbon concentration was 2.4% and the area ratio was 26%, which was the same as the result of Examples Nos. 1 to 4, but the aspect ratio was 3.2% and elongated carbides were formed. It was confirmed that.

These differences appear as differences in fatigue limits, as described in Table 2. That is, Examples Nos. 1 to 4 all exhibited satisfactory fatigue characteristics, whereas Comparative Examples Nos. 5 and 6 were poorer in fatigue characteristics compared to these. The same tendency was also confirmed in the roller pitching characteristics.

As a test of the fatigue characteristics, an Ono-type rotary bending test was performed. For the test, a smooth test piece having a parallel portion of 8 mmφ was used. Moreover, none of the test temperature was room temperature, was evaluated by 10 ⁷ times fatigue limit as fatigue characteristics. The roller pitching test was conducted at room temperature under the conditions of a rotation speed of 1500 rpm, a surface pressure of 350 kg / mm ² , and a slip ratio of −40%. ₅₀ life). The example that is described as in the table 10 is B ₅₀ life was more than 10 ⁷ times, which means that as the testing, that where aborted.

On the other hand, in the case of the conventional gas carburization, even if the processing patterns as shown in FIG. 1 are employed using steel materials B to E, there is a problem of an oxide film and grain boundary oxidation. Therefore, it takes too much time to realize the above surface carbon concentration and the like. In addition, fatigue characteristics become poor due to grain boundary oxidation.

In addition, although not described in Table 2, both the first stage treatment temperature and the second stage treatment temperature using steel B were 900
° C or less, but in this case, the surface carbon concentration was 2.0% or more, the area ratio was 20% or more, and the aspect ratio was 2.0% or more.
Although the following carbide layer could be formed, the treatment required a long time and the surface of the steel material was sooted. Therefore, it is unsuitable to adopt it as an actual process from the viewpoint of soot and man-hour.

As described above, according to Examples No. 1 to No. 4, a sufficient carbon layer having a surface carbon concentration of 2.0% or more, an aspect ratio of 2.0 or less, and an area ratio of 20% or more was formed. Products with good fatigue characteristics and roller pitting characteristics could be manufactured.

Further, when the Cr content is about 2 to 6%, such a good product can be manufactured with a relatively inexpensive steel material, which has a large cost effect.

Although the embodiments of the present invention have been described above, the present invention is not limited to these, and it goes without saying that the present invention can be carried out in various modes without departing from the gist thereof.

For example, between the first and second stages, and than it is only necessary to drop the temperature below the A ₁ transformation point, not limited to 650 ° C..

It is also useful to apply shot peening to a part carburized under the conditions of the present invention.

According to the method for carburizing Cr-containing steel of the present invention, a relatively inexpensive
Using a steel material with a Cr content of 2 to 6%, it has a surface carbon concentration of 2% or more, has a uniform carbide layer with a spherical shape, and is suitable for manufacturing products with excellent fatigue characteristics and roller pitting characteristics. It works.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a procedure of a plasma carburizing process in an embodiment. T ₁ … First stage processing temperature T ₂ … Second stage processing temperature t ₁ … First stage processing time t ₂ …… Second stage processing time

Claims (1)

(57) [Claims] 1. A method for carburizing a Cr-containing steel containing 2% by weight to 6% by weight of Cr, wherein the Cr-containing steel is subjected to a plasma carburizing treatment at a temperature of 900 ° C. or less as a first step. after cooling below the a ₁ transformation point, carburizing method of Cr-containing steel which comprises carrying out the re-plasma carburization temperature was raised to 900 ° C. or higher in the second stage.