JP2919654B2 - Rapid carburizing of steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for rapidly carburizing steel, which has an economic effect such as an improvement in productivity by shortening the processing time and a reduction in maintenance costs by preventing staying.

[0002]

2. Description of the Related Art Conventionally, a general carburizing method uses a shift gas obtained in a shift furnace as a carrier gas.
In recent years, raw materials gas (hydrocarbon gas) and oxidizing gas (air, oxygen, carbon dioxide gas, etc.) have been directly injected into the furnace without using a shift furnace due to demands for quality improvement, processing time reduction, running cost reduction, etc. A method has been proposed in which the steel is introduced and denatured and carburized in the furnace (Japanese Patent Application Laid-Open Nos. 55-128577, 1-38870, and Japanese Patent Application No. 2-1759).
No. 55).

[0003]

In the various carburizing methods as described above, if the treatment time is to be further shortened, the carbon potential of the atmosphere which is kept low so as not to cause staining is reduced. The carburizing treatment may be performed at a higher temperature. However, in this case, it is difficult to control the atmosphere thereafter due to, for example, stinging. Further, the carbon deposited by the sting shortens the life of the heat-resistant parts in the furnace, and requires maintenance in a short time. Problems arise. This is particularly problematic in the conventional general method using the metamorphic gas obtained in the above-mentioned metamorphic furnace, because a large amount of enriched gas is required and the amount of undecomposed enriched gas increases.

Further, a large amount of enriched gas causes a reduction in the partial pressure of CO and a local carburization reaction, causing carburization unevenness, and depending on the shape of the article to be treated, coarse carbides are precipitated to lower the fatigue strength. Was caused. The present invention has been made in view of the above circumstances, and enables rapid carburization with a high carbon potential while maintaining the furnace atmosphere in a controlled state, improving productivity by reducing the processing time, and
It is an object of the present invention to provide a carburizing method that is excellent in economic efficiency, such as preventing maintenance and reducing maintenance costs.

[0005]

In order to achieve the above object, the present invention provides a method for heating an object to be processed to an austenite region temperature and attaining a solid solubility limit of carbon at that temperature or a higher carbon potential. And sooting occurs
A carburization performed in a short time and a diffusion performed with a carbon potential having a low storage limit at the above temperature are repeated, and further, a carburization having a low storage limit during the diffusion is performed. -Control of transition to Bon potential is performed by stopping the introduction of carburizing gas into the furnace or by introducing an oxidizing gas.

[0006]

According to the present invention, after the article to be treated is heated to the austenite region temperature, the atmosphere in the furnace is controlled to the solid solubility limit of carbon at that temperature or a higher carbon potential. However, if the state continues, the steering naturally occurs. Therefore, in the present invention, the introduction of the carburizing gas into the furnace is stopped in a timely manner, or the oxidizing gas such as air, oxygen, or carbon dioxide gas is introduced into the furnace, so that the carbon potential is not caused to stay. It is controlled to a low value.

After the above operation is repeated, the surface carbon concentration can be adjusted by further performing diffusion for an appropriate time as in the conventional case. The shortening of the processing time in the present invention is based on carburizing with a higher carbon potential than that of a conventional general carburizing method, and controlling the atmosphere so as not to cause a stain. That is, as shown in FIG. 2, the average carbon potential value line D is considerably higher than the staking limit line E.

[0008]

An embodiment of the present invention will be described below. FIG. 3 shows an example of a heating temperature curve 1 and a carbon potential control curve 2 in the conventional general carburizing method. That is, the inside of the furnace is heated to an austenite region temperature, for example, 930 ° C., and the article to be treated charged in the furnace is subjected to a carburizing process for 7 hours and a half and a diffusion process for 2 hours, and thereafter 840 ° C. Temperature to the quenching temperature of 20-30
After holding for a minute, quenching is performed. The carbon potential in the conventional general carburizing treatment is controlled to about 0.8%, which is a staying limit at which no sticking occurs, and furthermore, there is no particular change in the carburizing treatment and the diffusion treatment process. there were.

FIGS. 1 and 2 show an example of a heating temperature curve 1 and a carbon potential control curve 2 in the present invention. That is, in the present invention, the heating temperature curve 1 is the same as the conventional general carburizing method, but the carbon potential control curve 2 is completely different from the conventional general carburizing method. More specifically, according to the present invention, in the austenitic region, rapid carburization performed at a solid solubility limit of carbon at that temperature or a higher carbon potential, for example, at a value of 1.1 percent or less. A, and a carbon potential having a low staying limit at the aforementioned temperature, for example, 0.8% and diffusion B performed before and after that, is repeated.

This cycle of high and low carbon potentials is carried out in such a manner that the high carbon potential time is a short time in which no staining occurs, and in practice, the raw material gas and oxidizing gas are introduced into the furnace as raw material gas and oxidizing gas. Gas carburizing method by introducing carbon dioxide gas (Japanese Patent Application No.
No. 175,955) as shown in FIG.
Performed at 0 minute intervals.

[0011] Further, the control for shifting to the carbon potential having a low staying limit during the diffusion is performed by temporarily stopping the introduction of the carburizing gas into the furnace or by introducing the oxidizing gas. In practice, the adjustment was performed by adjusting the opening of a valve for introducing carbon dioxide gas into the furnace. Further, diffusion C is performed for an appropriate time thereafter.

As described above, in the present invention, the atmosphere in the furnace is repeatedly controlled between a high carbon potential and a low carbon potential, so that the sensitivity of the atmosphere is important.
From this point of view, a method using less total gas, that is, a method in which a raw material gas and an oxidizing gas are directly introduced into a furnace is more suitable than the conventional carburizing method using a modified gas. Further, the most suitable method is to increase the carbon potential, that is, the gas carburizing method proposed by the present applicant using carbon dioxide gas as the oxidizing gas.

FIG. 3 shows that the piston (ball outer diameter 80 mm, inner diameter 30 mm) made of the SCM420H to be processed is processed under the above conditions by the conventional general carburizing method in a batch type carburizing furnace, and the effective carburizing is performed. 1.00 to 1.4 depth goal
FIG. 1 shows a pattern including the processing time when the distance is set to 0 mm (Hv513 point). FIG. 1 shows the target (Hv513 point) of the same workpiece and the same effective carburizing depth according to the present invention.
Is a pattern representing the processing time in the case of. As is clear from FIGS. 1 and 3, the total processing time can be reduced by 2 hours and 30 minutes according to the method of the present invention.

Further, in a tray pusher type continuous furnace, SCM420H is used by the conventional general carburizing method and the method of the present invention.
Table 1 below shows a comparison when the outer ring (outer diameter 75 mm, inner diameter 57 mm) was carburized.

[0015]

[Table 1] The treatment conditions were carburization, the diffusion temperature was 930 degrees, and the target of the effective carburization depth was 1.45 to 1.90 mm (Hv513).
Point). As is clear from Table 1, in the case of the continuous furnace, it was possible to reduce the total processing time by 205 minutes. As described above, according to the present invention, it was confirmed that the productivity was improved by about 35% in the case of the batch type carburizing furnace and by about 50% in the case of the tray pusher type continuous furnace. is there.

[0016]

According to the present invention, productivity can be improved by shortening the processing time, and economic effects such as maintenance cost reduction by preventing staying can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a processing step of the present invention.

FIG. 2 is an enlarged explanatory view of a main part of a processing step of the present invention.

FIG. 3 is an explanatory view showing a conventional general processing step.

[Explanation of symbols]

 1 Heating temperature curve 2 Carbon potential control curve A Rapid carburization B Diffusion C Diffusion

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masahiko Watanabe 2-6-26 Minowa-cho, Kohoku-ku, Yokohama-shi, Kanagawa Prefecture Dowa Mining Co., Ltd. Yokohama Plant (72) Inventor Hideki Shikata Minowa-cho, Kohoku-ku, Yokohama-shi, Kanagawa 2-6-26 Dowa Mining Co., Ltd. Yokohama Plant (72) Inventor Toshiyuki Kawamura 2-6-26 Minowa-cho, Kohoku-ku, Yokohama-shi, Kanagawa Prefecture Dowa Mining Co., Ltd. Yokohama Plant (56) References JP Showa 54 -54931 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 8/22

Claims (3)

(57) [Claims] An object to be treated is heated to an austenite region temperature, and at that temperature, a carbon solid solubility limit or a higher carbon potential and a sooting are obtained.
A rapid carburizing method for steel, characterized by repeating carburization performed in a short time that does not occur and diffusion performed at a carbon potential having a low sooting limit at the above temperature. 2. A rapid carburizing method for steel according to claim 1, wherein the control for shifting to a carbon potential having a low stainless steel limit during diffusion is performed by stopping the introduction of a carburizing gas into the furnace. . 3. The rapid carburizing method for steel according to claim 1, wherein the control for shifting to a carbon potential having a low stainless steel limit during diffusion is performed by introducing an oxidizing gas into the furnace.