JPH0454736B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a carburizing steel, and particularly to a carburizing steel that has a high hardness of the carburized layer on the surface after carburizing treatment and is suitable as a material for carburized parts having excellent pitting resistance and fatigue strength. Conventionally, mechanical structural parts such as gears, connecting rods, pinions, racks, bearings, bearing races, and steering parts are often carburized to increase their surface wear resistance and fatigue strength. Conventionally, the materials for parts that are subjected to carburizing treatment include SC material, SCr material, SCM material, SMn material, SNC material, specified by JIS.
Structural steel such as SNCM material is usually used. On the other hand, in recent years, in order to improve the driving performance and fuel economy of automobiles, there has been an increasing trend towards reducing the weight of automobile parts. There is also a growing demand for miniaturization of train-related parts. However, when the above-mentioned conventional structural steel is subjected to normal carburizing treatment, there is a problem that sufficient strength cannot be obtained, especially when the parts are downsized. Therefore, it is desired to increase the strength of miniaturized parts, especially pitting resistance and fatigue strength, and in order to improve pitting resistance, excessive carburizing to increase surface hardness has been considered. However, when attempting to excessively carburize the conventional structural steel mentioned above, it is necessary to ensure that the carbon potential of the atmosphere is approximately 3% or more, which is outside the scope of normal carburizing treatment. As a result, there was a problem in that the workability of carburizing treatment was significantly reduced. Special-purpose steels have been developed that are characterized by excessive carburization with such high carbon potential, but these are intended for large parts such as rolling rolls, and they contain large amounts of Cr and Mo. In addition to the disadvantage of high price, especially when this special purpose steel is applied to small parts such as automobile parts, it is difficult to use Cr, Mo, etc. as mentioned above.
Because of its high content, it has very good hardenability, so during the carburizing process, the center of the part is hardened, and when applied to gears, for example, the residual stress at the root of the tooth decreases, reducing its strength. There was a problem. This invention was made by focusing on the above-mentioned conventional problems, and it is possible to carry out excessive carburization even when performing normal carburizing treatment in which the carbon potential of the carburizing atmosphere is about 2.5% or less, and the surface hardness can be reduced. It is an object of the present invention to provide a carburizing steel that can significantly increase the carburizing properties and yield carburized parts with excellent pitting resistance and fatigue strength. The carburizing steel according to the invention has C:
0.1 to 0.3%, Mn: 0.2 to 1.5%, Cr: 1 to 3%, and contains Mn + Cr≦3%, and furthermore, Si: 0.5 to 1.5
%, Al: Contains 0.2-1.5%, if necessary,
Mo: 0.1-0.5%, V: 0.1-0.5%, Ti: 0.1-0.5
%, Zr: 0.1~0.5%, Nb+Ta (one of them is 0)
): Contains one or more of 0.1 to 0.5%, with the remainder consisting of Fe and impurities, and more preferably has a carbon potential of 1.0.
It is characterized by the precipitation of fine spherical carbides in the carburized layer after carburizing in a ~2.5% carburizing atmosphere. Next, the reason for the limit on the composition range (wt%) of the steel for carburizing according to the present invention will be explained. C: 0.1-0.3% C is an element to be included to obtain the strength required for structural parts and surface hardness after carburizing treatment, but if the content is less than 0.1%, the above-mentioned required strength and surface hardness will be reduced. I can't get the feeling,
If it exceeds 0.3%, toughness and cold forgeability will deteriorate, so it is set in the range of 0.1 to 0.3%. Mn: 0.2-1.5% Mn is effective as a deoxidizing and desulfurizing element during steel making, but if it is less than 0.2%, the above deoxidizing and desulfurizing effects will be small, and the surface hardness after carburizing will not be sufficient. I can't get anything. On the other hand, if it exceeds 1.5%, it becomes difficult to control hardenability and workability and machinability deteriorate. Therefore, the Mn content was set in the range of 0.2 to 1.5%. Cr: 1 to 3% Cr is an effective element for precipitating carbides and increasing the hardness of the carburized layer after carburizing treatment, and is contained in an amount of 1% or more to obtain this effect. However, if the Cr content becomes too large, it becomes difficult to control the hardenability, so it is set in the range of 1 to 3%. Mn+Cr≦3% If the Mn and Cr contents are too high, it becomes difficult to control the hardening of the steel as described above, and when applied to gears, for example, the hardenability is excessive, resulting in residual stress at the root of the tooth. becomes smaller and the strength decreases, so
It is also necessary to set an upper limit for the total amount of Mn and Cr, and the total of Mn+Cr is set to 3% or less. Si: 0.5-1.5% Al: 0.2-1.5% Both Si and Al are necessary to promote the precipitation of Cr carbides and ensure sufficient hardness of the carburized layer. 0.5% or more, Al
It is necessary to contain 0.2% or more. However, if the content is too high, the toughness will deteriorate, so
The content of Si must be 1.5% or less, and the content of Al must also be 1.5% or less. Mo: 0.1~0.5%, V: 0.1~0.5%, Ti: 0.1~
One or more of the following: 0.5%, Zr: 0.1~0.5%, Nb+Ta: 0.1~0.5% Mo, V, Ti, Nb, and Ta all cause coarsening of austenite crystal grains during carburizing treatment at high temperatures. Since these elements are effective in preventing the above, it is preferable to contain each of them in an amount of 0.1% or more, if necessary. However, if the content exceeds 0.5% of each, the effect of preventing crystal grain coarsening will decrease, so each
The range is 0.1 to 0.5% (including cases where either Nb or Ta is 0). Gears,
Structural parts such as ball joints, drive shafts, camshafts, steering parts, bearings, and bearing races are molded and then, more preferably, carbon potential in a carburizing atmosphere is formed.
By performing carburizing treatment in an atmosphere with a concentration of 1.0 to 2.5%, fine spherical carbides are precipitated in the carburized layer on the surface, resulting in high surface hardness and excellent pitting resistance and fatigue strength. Obtain structural parts with good dimensional accuracy. Here, it is more desirable that the carbon potential of the carburizing atmosphere is 1.0 to 2.5% because if the carbon potential is too low, excessive carburization becomes difficult and it becomes impossible to increase the surface hardness by carburizing. On the other hand, if the carbon potential is too high, ordinary and simple carburizing treatment cannot be carried out, and the treatment cost becomes extremely high. Examples of the present invention will be described below along with comparative examples. Steel having the chemical composition shown in Table 1 was melted and then formed into ingots, followed by blooming and rolling to produce test pieces with a diameter of 25 mm and a length of 75 mm. Next, each test piece was heated at 925°C for 5 hours in a carburizing atmosphere with the carbon potential shown in Table 2 → oil-cooled at 830°C (oil temperature 70°C) →
Carburizing and quenching under the conditions of heating at 170℃ for 1 hour and cooling in the air.
After tempering, the structure of the carburized layer of each specimen was observed, and the hardness of the carburized layer at a depth of 0.1 mm from the surface layer was measured. The results are shown in Table 2.

【table】

[Table] As is clear from Tables 1 and 2, when carburized molten steel (Nos. 2 to 13) according to the present invention is carburized in a carburizing atmosphere with a carbon potential of 1.25%, fine particles are formed in the carburized layer. Spheroidal carbides were precipitated, and the hardness of the carburized layer was significantly increased, and the results showed that pitting resistance and fatigue strength could be significantly improved when applied to gears, etc. On the other hand, it was also found that if the carbon potential in the carburizing atmosphere was too low, an increase in surface hardness due to excessive carburization could not be obtained. moreover,
In steel (No. 1) that did not satisfy the composition range of this invention, an excessive carburized layer was not formed even when the carbon potential in the carburizing atmosphere was 1.25%, and sufficient surface hardness could not be obtained. Next, sooting tends to occur when the carbon potential is increased in general gas carburizing, but such sooting does not occur in vacuum carburizing, so it is possible to increase the carbon potential relatively easily. Therefore, from this point of view, it is more desirable to perform vacuum carburization when excessive carburization is performed, and examples in which vacuum carburization is performed will be described below together with comparative examples. First, No. 1, 2, 5, 6, 8, 10 shown in Table 1
Each test piece with a diameter of 25 mm and a length of 75 mm was prepared from steel in the same manner as in the above Examples and Comparative Examples and was subjected to vacuum carburizing treatment in a carburizing atmosphere with a carbon potential of 1.7%. This vacuum carburizing treatment was carried out in the above atmosphere under the following conditions: heating at 1040°C for 1.5 hours, quenching at 830°C with oil cooling, and tempering at 170°C for 1 hour.
Thereafter, the structure of the carburized layer of each test piece was observed, and the hardness of the carburized layer at a depth of 0.1 mm from the surface layer was measured. The results are shown in Table 3.

[Table] As shown in Table 3, in the steel for carburizing according to the present invention, even when vacuum carburizing was performed in a carburizing atmosphere with a carbon potential of 1.7%, fine spherical carbides were precipitated in the carburized layer. It was confirmed that the layer hardness was sufficient, and it was also confirmed that vacuum carburizing can be applied when excessively carburizing the steel for carburizing according to the present invention. On the other hand, in the case of the comparative example, coarse net-like carbides were precipitated by high-temperature carburizing treatment, and the hardness of the carburized layer was also low, which was an unfavorable result. As explained above, the carburizing steel according to the present invention has C: 0.1 to 0.3% by weight. , Mn: 0.2~
1.5%, Cr: 1-3% and contains Mn + Cr≦3%, further Si: 0.5-1.5%, Al: 0.2-1.5%,
Mo: 0.1~0.5%, V: 0.1~0.5 as required
%, Ti: 0.1-0.5%, Zr: 0.1-0.5%, Nb+
Ta: Contains one or more of 0.1 to 0.5%, with the balance consisting of Fe and impurities, and fine spherical carbides precipitate in the carburized layer after carburizing, so the carbon potential of the carburizing atmosphere but
Excessive carburization is possible even when normal carburizing is carried out to a level of 2.5% or less, and this excessive carburization can significantly increase the hardness of the surface.
Carburized parts such as gears, connecting rods, pin-ons, racks, bearings, bearing races, and steering parts with excellent pitting resistance and fatigue strength can be obtained, and these parts are being miniaturized based on the demand for weight reduction. Even in some cases, it is possible to obtain sufficiently excellent strength by applying normal carburizing treatment, and there is a great effect that there is no need to perform special carburizing treatment to perform excessive carburizing as in the past. It has the following.

Claims (1)

[Claims] 1% by weight, C: 0.1-0.3%, Mn: 0.2-1.5
%, Cr: 1 to 3% and includes Mn + Cr≦3%,
Further, a carburizing steel containing 0.5 to 1.5% of Si and 0.2 to 1.5% of Al, with the remainder consisting of Fe and impurities, and fine spherical carbides precipitate in the carburized layer after carburizing. 2 In weight%, C: 0.1-0.3%, Mn: 0.2-1.5
%, Cr: 1 to 3% and includes Mn + Cr≦3%,
Furthermore, it contains Si: 0.5 to 1.5%, Al: 0.2 to 1.5%, Mo: 0.1 to 0.5%, and V: 0.1 to 0.5.
%, Ti: 0.1-0.5%, Zr: 0.1-0.5%, Nb+
A carburizing steel containing one or more of Ta: 0.1 to 0.5%, the remainder consisting of Fe and impurities, and fine spherical carbides precipitate in the carburized layer after carburizing.