JPH0416538B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a steel for carburizing that can stably make the grain size fine even when carburizing is performed at high temperatures. BACKGROUND ART Various gears, bearings, propeller shafts, camshafts, and other parts are used in power transmission mechanisms in automobiles and other transportation machines, industrial machines, agricultural machines, and the like. These parts are generally molded from mechanical structural steel, and the molded products are then subjected to surface hardening treatments such as gas carburizing and carbonitriding. In this case, conventional surface hardening treatments are all about
The process is carried out at a temperature of less than 950°C, requiring a long treatment time to obtain the required depth of carburization or carbonitriding, which has been one of the major factors hindering improvements in parts productivity. Therefore, against this background, 950℃
A vacuum carburizing process has been developed in which carburizing is carried out at higher temperatures to shorten the processing time. However, if conventional case hardened structural steel is used as a raw material and then subjected to the above-mentioned high-temperature vacuum carburizing treatment, the austenite crystal grains will become coarse, resulting in large heat treatment distortions and a decrease in component strength. This problem arises. Therefore, after vacuum carburizing, a so-called grain refining process is sometimes adopted, in which the temperature is rapidly cooled to a temperature below the transformation point, and then heated again to the austenite temperature range and quenched. However, the processing time was not significantly reduced compared to the conventional gas carburizing process, and this was a major impediment to the spread of vacuum carburizing process, which has excellent carburizing performance. The present invention has been made by focusing on the above-mentioned conventional problems, and in particular, the austenite crystal grains do not become coarse even when carburized at a high temperature of 950°C or higher, and the crystal grain size is stabilized. The purpose is to provide a carburizing steel that can be made fine. The carburizing steel according to the present invention has C:
0.1~0.4%, Si: 0.05~1.5%, Mn: 0.05~2.0%,
SolAl: 0.015-0.07%, Ti: 0.007-0.07%, N:
Contains 0.013 to 0.03%, B: regulated to less than 0.0005%, and if necessary, contains one or more of Ni: 4.5% or less, Cr: 2% or less, Mo: 0.6% or less, The remainder is essentially made of Fe, and when a part formed from this carburizing steel is subjected to high-temperature carburizing treatment at 950°C or higher, the austenite crystal grains do not coarsen, and the austenite grains do not coarsen. It is characterized by being able to obtain fine grains with an average grain size of 7.5 or higher. Next, the reason for limiting the composition range (wt%) of the carburizing steel according to the present invention will be explained. C: 0.1 to 0.4% 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 0.4%, toughness and cold forgeability will deteriorate, so it should be in the range of 0.1 to 0.4%. Si: 0.05-1.5% Si is effective as a deoxidizing element, but if it is less than 0.05%, the above deoxidizing effect cannot be obtained, and if it exceeds 1.5%, it deteriorates toughness and cold forgeability.
It should be in the range of 0.05-1.5%. Mn: 0.05-2.0% Mn is effective as a deoxidizing and desulfurizing element, but if it is less than 0.05%, the above deoxidizing and desulfurizing effects cannot be obtained, and sufficient surface hardness after carburizing treatment cannot be obtained. I can't do it. On the other hand, if it exceeds 2.0%, workability and machinability deteriorate. Therefore,
Mn content shall be 0.05-2.0%. Al: 0.015 to 0.07% Al is an element effective in preventing coarsening of austenite crystal grains during carburizing treatment at high temperatures, and for this purpose it is necessary to contain it in an amount of 0.015% or more. However, if the SolAl content exceeds 0.07%, the effect of preventing crystal grain coarsening will be reduced, so the SolAl content should be in the range of 0.015 to 0.07%. Ti: 0.007 to 0.07% Ti is an effective element for preventing coarsening of austenite crystal grains during carburizing treatment at high temperatures, and for this purpose it is necessary to contain it in an amount of 0.007% or more. However, if the content exceeds 0.07%, the effect of preventing crystal grain coarsening is reduced, so it is set in the range of 0.007 to 0.07%. N: 0.013-0.03% N is an element effective in preventing coarsening of austenite crystal grains during carburizing treatment at high temperatures.
For this purpose, it is necessary to contain 0.013% or more. However, if the content exceeds 0.03%, the integrity of the steel ingot or slab will be damaged by N blowholes, so the N content is set in the range of 0.013 to 0.03%. B: less than 0.0005% If the B content exceeds 0.0005%, coarsening of austenite crystal grains tends to occur during carburizing treatment at high temperatures, so the upper limit is regulated to less than 0.0005%. One or more of the following: Ni: 4.5% or less, Cr: 2% or less, Mo: 0.6% or less Ni, Cr, and Mo are effective elements to further improve the hardenability of steel. , Ni content exceeds 4.5%, Cr content exceeds 2%, Mo content 0.6%
If the toughness of the steel is exceeded, the toughness of the steel will deteriorate, so if you want to further improve the hardenability of the steel, Ni: 4.5% or less,
It is preferable to contain one or more of these within the ranges of Cr: 2% or less and Mo: 0.6% or less. Using the carburized steel according to the present invention whose composition has been adjusted in this way as a raw material, structural parts such as gears, ball joints, drive shafts, camshafts, steering parts, bearings, and bearing races are formed and then processed in a vacuum at 950°C or higher. By performing carburizing treatment, structural parts with fine austenite grain size of 7.5 or higher on average, excellent fatigue strength, etc., and good dimensional accuracy can be obtained in a short processing time. Next, an example will be described. After melting steel with the chemical composition shown in Table 1, it is ingot-formed, rolled into a round bar with a diameter of 30 mm, and part of it is cut into a round bar with a diameter of 25 mm, and then vacuum carburized. I processed it. The rest of the round bar was normalized under the conditions of heating at 925° C. for 1 hour and air cooling, then cold forging, and then vacuum carburizing. Then, the prior austenite grain size of the carburized layer was measured for each, as well as the fatigue strength and dimensional accuracy. Further, Table 2 shows the vacuum carburizing treatment conditions at this time. Among these, the prior austenite grain size was measured in accordance with the "Austenite grain size test method for steel" specified in JIS G0551. In addition, the fatigue strength was measured by conducting the Ono rotary bending fatigue test. Furthermore, dimensional accuracy was measured by using a circular test piece with an outer diameter of 90 mm, an inner diameter of 45 mm, and a thickness of 30 mm and examining the amount of dimensional change (μm) in the thickness after the vacuum carburizing treatment. . These results are also shown in Table 1.

【table】

【table】

[Table] As shown in Table 1, in all comparative steels whose Al, Ti, N, and B contents are outside the range of the present invention, the average grain size of prior austenite is smaller than No. 7.5, and austenite crystals are formed during vacuum carburizing. It was clear that the grains had become coarser, and the fatigue strength was low and the amount of dimensional change was large. On the other hand, in the steels of the present invention whose Al, Ti, N, and B contents are within the range of the present invention, it is clear that the average grain size of prior austenite is finer than 7.5, and the fatigue resistance is The strength has been improved and the amount of dimensional change has been reduced, and it has been recognized that it is possible to improve the fatigue strength and dimensional accuracy of structural parts such as gears. As explained above, in the carburizing steel according to the present invention, structural parts such as gears and camshafts are formed using this carburizing steel as a raw material, and then 950
Even when carburizing is performed at high temperatures above ℃, it is possible to prevent austenite crystal grains from becoming coarse during vacuum carburizing, allowing structural parts with excellent fatigue resistance and high dimensional accuracy to be carburized in a short time. It has a very excellent effect of being able to be manufactured through processing.

[Brief explanation of drawings]

The drawings are explanatory diagrams of steps adopted in the embodiments of the present invention.

Claims (1)

[Claims] 1% by weight, C: 0.1 to 0.4%, Si: 0.05 to 1.5%,
Mn: 0.05~2.0%, Al: 0.015~0.07%, Ti:
Contains 0.007-0.07%, N: 0.013-0.03%, B:
A steel for carburizing that is regulated to less than 0.0005%, with the remainder essentially consisting of Fe. 2 In weight%, C: 0.1-0.4%, Si: 0.05-1.5%,
Mn: 0.05~2.0%, Al: 0.015~0.07%, Ti:
0.007-0.07%, N: 0.013-0.03%, and Ni:
For carburizing, containing one or more of the following: 4.5% or less, Cr: 2% or less, Mo: 0.6% or less, B: regulated to less than 0.0005%, and the remainder substantially consisting of Fe. steel.