JPH116048A - Carburized member and carburizing method for steel containing boron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a hardness on a steel surface by uniformly heating a steel containing a specified small quantity of boron in a carburizing atmosphere so as to easily/efficiently carburize carbon in a steel surface. SOLUTION: By increasing a carbon concentration with carburizing a steel surface through a carburizing gas, a hardness of a surface layer part is increased while maintaining toughness at a core part. In this case, a steel uniformly containing boron with a quantity of 10-30 ppm is used, a steel surface is carburized with using a gas atmosphere having a carbon potential of 0.4-0.8 equivalent weight % as a carburizing gas. By using a steel containing boron for a steel to be surface hardened, in the case of a low carbon potential of the carburizing gas, the steel can be carburized at a relatively low temp. and in a short time, thus, carburizing is performed at a low cost and a life of carburizing equipment is extended.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburizing member and a method for carburizing boron-containing steel.

[0002]

2. Description of the Related Art Conventionally, a technique of increasing the hardness of steel or the like by incorporating carbon into steel or the like is well known. In particular, the technique of carburizing that maintains the core toughness while increasing the carbon concentration of only the surface layer of steel to make the surface layer a hard layer is often used.

[0003] Carburizing is a method in which a steel material is heated from about 900 ° C to 1200 ° C together with a carbon feed agent to cause carbon to penetrate its surface. As the carbon feed agent, solid carbon, a molten carbon salt, or a carbon compound gas is used, and gas carburization using a carbon compound gas as the carbon feed agent is most often used.

[0004]

However, in general, gas carburization is carried out at an ambient gas temperature of about 900 ° C to 1200 ° C.
The heating is performed in a high-temperature heating furnace at a temperature of ° C. Under such high-temperature conditions, the heating furnace is severely damaged and the life of the heating furnace is shortened. In order to maintain the inside of the heating furnace at a high temperature, a considerable cost is imposed due to energy consumption.

On the other hand, in order to effectively penetrate and diffuse carbon into the surface layer of steel to form a hard layer having a predetermined carbon concentration, the carbon potential in the atmospheric gas must be increased. For example, in order to make the surface layer of low carbon steel eutectoid steel having a carbon concentration of 0.8% by weight, it is necessary to set the carbon potential in the atmosphere gas to 0.8 equivalent weight% or more, which increases cost. Become. Further, in order to infiltrate and diffuse carbon to a predetermined depth, a long time is required as a carburizing time, and the productivity is poor.

The present invention was conceived in view of the above circumstances, and provides a carburizing member that exhibits high strength even when the surface carbon concentration is low, and reduces the carburizing time,
It is an object of the present invention to provide a carburizing method capable of imparting a desired hardness to the surface layer of steel even when the carbon potential in the carburizing gas is low.

[0007]

DISCLOSURE OF THE INVENTION In order to solve the above-mentioned problems, the present invention takes the following technical means.

That is, according to a first aspect of the present invention,
A carburized member is provided, wherein the surface layer of steel containing boron uniformly is carburized so as to contain 0.4% to 0.8% by weight of carbon. Preferably,
The surface layer of the boron-containing steel is carburized to contain 0.4% to 0.7% by weight of carbon, more preferably 0.4% to 0.6% by weight of carbon. .

Preferably, the boron content is 10 to
30 ppm (on a weight basis).

According to a second aspect of the present invention, there is provided a method for carburizing steel, characterized in that carbon is infiltrated from the surface of steel containing boron uniformly in a high-temperature atmosphere containing a carbon compound gas. Provided.

Specifically, the method is carried out by heat-treating steel containing boron uniformly in advance, for example, in a heating furnace containing a carbon compound gas so that carbon enters and diffuses from the surface of the steel. The steel thus heat-treated is quenched and then tempered to produce steel with high surface strength.

The temperature of the high temperature atmosphere is preferably 84
The temperature is set at 0 ° C. to 870 ° C., and the carbon potential in the high-temperature atmosphere is preferably set at 0.4 equivalent weight% to 0.8 equivalent weight%. The amount of boron contained in the steel may be about 10 to 30 ppm.

The steel may be carbon steel, chromium steel, chromium-molybdenum steel, or the like. Examples of the carbon compound gas include known carburizing gases such as natural gas, carbon monoxide, methane gas, propane gas, and the like. Oil vapor or alcohols are used.

[0014] In a steel containing no boron, the highest ultimate hardness is reached in the case of a eutectoid steel having a carbon concentration of 0.8% by weight. On the other hand, in the steel containing boron, the carbon concentration is set to 0.1.
The highest reached hardness is reached at around 5% by weight. This has been made clear by experiments performed by the present inventors.

The inventor of the present invention carburized carbon steel containing boron uniformly at various carbon concentrations, quenched and tempered to obtain carburized parts (test pieces), and determined the hardness of each carburized member. A measurement was made. The result is shown in FIG. Note that Vickers hardness was adopted as the hardness of the carburized member.

As is apparent from FIG. 1, when the carbon concentration in the surface layer of the carburized part is 0.5 to 0.6% by weight, that is, when the carbon concentration in the surface layer is lower than that of steel containing no boron. Has reached the highest attainable hardness.

Therefore, in the case of boron-containing steel, it is possible to obtain a carburized member having a hardness equivalent to that of eutectoid steel by using a carburizing gas having a lower carbon potential than steel containing no boron. In addition, since the carburized member has a low surface carbon concentration when exhibiting the highest ultimate hardness, a carburized member exhibiting the highest ultimate hardness can be obtained in a short carburizing time. Therefore, by these, the cost of the carburizing operation can be reduced, and the operability is improved by shortening the time.

Further, since the amount of carbon to be carburized to obtain the highest ultimate hardness of the above carburized member is small, it is possible to carry out heat treatment at a lower temperature than conventional carburizing to carry out carburizing.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described together with comparative examples.

[0020]

[Embodiment 1] In this embodiment, 13 pp
A ring gear made of steel containing m boron (module 1.75 mm, mass 0.95 kg)
Was carburized. Specifically, the atmosphere in the heating furnace was adjusted to 920 ° C. and the carbon potential was adjusted to 0.55 equivalent weight%. After the ring gear was carburized in this furnace for 90 minutes, the carbon potential in the atmosphere was further reduced to 0. Diffusion treatment was performed at 920 ° C. for 60 minutes at 45 equivalent weight%. The ring gear thus carburized was subjected to quenching and tempering to measure the hardness and structure of the surface and core of the ring gear.
Table 1 shows the results.

(Measurement of Hardness) The hardness of the surface portion and the core portion of the ring gear was measured using a so-called Vickers hardness meter. That is, the area of the depression formed by pressing the tip of the diamond indenter with a predetermined load (1 kg) was measured, and the hardness was calculated by dividing the applied load by the area of the depression.

(Microstructure Observation) The microstructure of the above-mentioned ring gear was obtained by etching a cut and polished sample with a 5% nital corrosive solution for 15 seconds, washed with water and dried, and observed with a metallographic microscope.

[0023]

Embodiment 2 In this embodiment, the surface of a ring gear (module 1.75 mm, mass 0.95 kg) made of the same steel as the steel used in Embodiment 1 was carburized. Specifically, the atmosphere in the heating furnace is set to 920
C., carbon potential was adjusted to 1.10 equivalent weight%, and after the ring gear was carburized for 62 minutes in this furnace, the carbon potential in the atmosphere was further reduced to 0.80%.
Diffusion treatment was performed at 920 ° C. for 40 minutes with the equivalent weight%. The ring gear thus carburized was quenched and tempered, and the hardness and the structure of the surface and core of the ring gear were measured in the same manner as in Example 1. Table 1 shows the results.

[0024]

[Embodiment 3] In this embodiment, 13 pp is applied to S25C.
A ring gear made of steel containing m boron (module 1.75 mm, mass 0.95 kg)
Was carburized. Specifically, the atmosphere in the heating furnace was adjusted to 855 ° C. and the carbon potential was 1.10 equivalent weight%. After the ring gear was carburized for 90 minutes in the furnace, the carbon potential in the atmosphere was further reduced to 0.80%. Diffusion treatment was performed at 855 ° C. for 60 minutes with the equivalent weight%. The ring gear thus carburized was quenched and tempered, and the hardness and the structure of the surface and core of the ring gear were measured in the same manner as in Example 1. Table 1 shows the results.

[0025]

Embodiment 4 In this embodiment, the surface of a ring gear (module 1.75 mm, mass 0.95 kg) made of the same steel as the steel used in Embodiment 3 was carburized. Specifically, the atmosphere in the heating furnace is set to 855.
° C, the carbon potential was adjusted to 1.10 equivalent weight%, and the ring gear was carburized in this furnace for 70 minutes.
Diffusion treatment was carried out at 855 ° C. for 48 minutes at an equivalent weight%. The ring gear thus carburized was quenched and tempered, and the hardness and the structure of the surface and core of the ring gear were measured in the same manner as in Example 1. Table 1 shows the results.

[0026]

Embodiment 5 In this embodiment, the surface of a ring gear (module 1.75 mm, mass 0.95 kg) made of the same steel as the steel used in Embodiment 3 was carburized. Specifically, the atmosphere in the heating furnace is set to 855.
° C, the carbon potential was adjusted to 0.60 equivalent weight%, and the ring gear was carburized in this furnace for 90 minutes.
Diffusion treatment was performed at 855 ° C. for 60 minutes with the equivalent weight%. The ring gear thus carburized was quenched and tempered, and the hardness and the structure of the surface and core of the ring gear were measured in the same manner as in Example 1. Table 1 shows the results.

[0027]

Embodiment 6 In this embodiment, the surface of a ring gear (module 1.75 mm, mass 0.95 kg) made of the same steel as the steel used in Embodiment 3 was carburized. Specifically, the atmosphere in the heating furnace is set to 855.
° C, the carbon potential was adjusted to 1.10 equivalent weight%, and the ring gear was carburized in this furnace for 70 minutes.
Diffusion treatment was carried out at 855 ° C. for 48 minutes at an equivalent weight%. The ring gear thus carburized was quenched and tempered, and the hardness and the structure of the surface and core of the ring gear were measured in the same manner as in Example 1. Table 1 shows the results.

[0028]

Embodiment 7 In this embodiment, the surface of a ring gear (module 1.75 mm, mass 0.95 kg) made of the same steel as the steel used in Embodiment 3 was subjected to carburizing treatment. Specifically, the atmosphere in the heating furnace is set to 855.
° C, the carbon potential was adjusted to 0.65 equivalent weight%, the ring gear was carburized in this furnace for 74 minutes, and then the carbon potential in the atmosphere was further reduced to 0.55%.
Diffusion treatment was performed at 855 ° C. for 50 minutes with the equivalent weight%. The ring gear thus carburized was quenched and tempered, and the hardness and the structure of the surface and core of the ring gear were measured in the same manner as in Example 1. Table 1 shows the results.

[0029]

Comparative Example 1 In this comparative example, a chromium steel (SCr4
The surface of a ring gear (module 1.75, mass 0.95) made of 20) was carburized.
Specifically, the atmosphere in the heating furnace was adjusted to 920 ° C. and the carbon potential was 1.10 equivalent weight%. After the ring gear was carburized in this furnace for 90 minutes, the carbon potential in the atmosphere was further reduced to 0.80%. 9 equivalent weight%
Diffusion treatment was performed at 20 ° C. for 60 minutes. The carburized ring gear was subjected to quenching and tempering to measure the hardness and structure of the surface and core of the ring gear. Table 1 shows the results.

[0030]

[Table 1]

【The invention's effect】

As is clear from comparison between Example 1 and Comparative Example 1, in the case of steel containing boron, even if the carbon potential in the carburizing atmosphere gas is about half that of steel containing no boron, good results can be obtained. Is given hardness. That is, by using steel containing boron, a predetermined hardness can be imparted even when the carbon potential in the carburizing atmosphere gas is low.

As is clear from the comparison between Example 2 and Comparative Example 1, in the case of steel containing boron, even if the carburization time is about 2/3 that of steel containing no boron, the surface portion of the steel is not affected. Has good hardness. That is, the use of steel containing boron can shorten the carburizing time.

As is apparent from a comparison between Example 3 and Comparative Example 1, the carbon potential in the carburizing atmosphere of steel containing boron is lower than that of steel containing no boron.
Even in the case where the hardness is lower than the comparative example, although the hardness is slightly lower than that of the comparative example 1, the hardness is favorably given to the surface of the steel. That is, by using boron-containing steel, carburization can be performed at a lower temperature than before.

When the carbon concentration is low and the carburizing time is short as in Example 4, when the carbon potential is low and the atmospheric gas temperature is low as in Example 5, the carburizing time is short and the atmospheric gas is low as in Example 6. Even when the temperature is low, or when the carbon potential is low and the ambient gas temperature is low and the carburizing time is short, the steel surface is satisfactorily imparted with hardness.

Therefore, in order to carburize the surface portion of the steel and to give the surface portion the highest ultimate hardness or a hardness close thereto, it is effective to use steel containing boron. In this case, the carburizing time can be shortened, so that productivity is improved, and carburizing can be performed more favorably in a low-temperature atmosphere than before, so that damage to the heating furnace can be reduced and the heating furnace can be reduced. Long life. In addition to these, a substantially predetermined strength can be imparted to the surface portion of the steel under a low carbon potential, so that the cost required for carburizing can be significantly reduced.

[Brief description of the drawings]

FIG. 1 shows Vickers hardness of carburized parts obtained by carburizing a surface layer with various carbon concentrations.

Claims (6)

[Claims] 1. A carburized member characterized in that a surface layer of steel containing boron uniformly is carburized so as to contain 0.4 to 0.8% by weight of carbon. 2. The boron content is 10 to 30 ppm.
The carburized member according to claim 1, wherein 3. A carburizing method for boron-containing steel, characterized in that carbon is infiltrated from the surface of steel containing boron uniformly in a high-temperature atmosphere containing a carbon compound gas. 4. The boron-containing steel according to claim 1, wherein said boron-containing steel is 10-30 pp.
The method for carburizing boron-containing steel according to claim 3, wherein a material containing boron of m is used. 5. The carburizing method for boron-containing steel according to claim 3, wherein the temperature of the high-temperature atmosphere is 840 ° C. to 870 ° C. 6. The carbon potential in the high-temperature atmosphere is from 0.4 equivalent weight% to 0.8 equivalent weight%.
A method for carburizing boron-containing steel according to any one of claims 3 to 5.