JPH0517821A - Production of induction hardened parts minimal in quenching crack - Google Patents

Abstract

PURPOSE:To produce machine parts excellent in torsional strength and minimal in quenching crack by subjecting a billet of a medium carbon steel with a specific composition to heating, forging, and cooling under respectively specified conditions and then performing induction hardening and tempering. CONSTITUTION:A billet of a medium carbon steel having a composition which contains, by weight, 0.4-0.8% C, 0.25-0.70% Mn, 0.01-0.15% S, 0.015-0.05% Al, 0.003-0.020% N, and further 0.3-1.5% Cr and/or 0.05-0.5% Mo or further contains one or >=2 kinds among 0.005-0.04% Ti, 0.005-0.1% Nb, and 0.03-0.3% V and in which the contents of Si, P, B, and O as impurities are limited to <0.1%, <0.020%, <0.0005%, and <0.002%, respectively, is heated within a temp. rise time of <=30min up to forging temp., forged into the desired shape in a temp. region not higher than 1000 deg.C, and cooled through the temp. region between the forging temp. and 500 deg.C at >=0.5 deg.C/sec average cooling rate. Finally, hardening and tempering are done by using a high frequency current of 30kHz frequency. By this method, a forged part having >=180kgf/mm<2> torsional strength can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an induction-hardened part having few quenching cracks, and more specifically, an induction-quenched part having excellent torsional strength as a mechanical part and hard to cause quenching cracks during manufacture. The present invention relates to a manufacturing method of.

[0002]

2. Description of the Related Art A shaft-shaped component forming a power transmission system of an automobile has a strong tendency to have higher strength (improvement of torsional strength) with the recent increase in output of an automobile engine. These machine parts are usually manufactured by forming medium carbon steel into a predetermined part shape and subjecting it to induction hardening-tempering. The torsional strength of the induction hardened material is, for example, as shown in "Isuzu Technical Report, No. 67, page 9,"
The deeper the induction hardening depth, the better it is, but at present, the maximum surface shear stress (hereinafter called τmax) is about 150k.
gf / mm ² is the upper limit.

[0003]

[Problems to be Solved by the Invention] However, the above-mentioned τma
At present, the strength level of x = 150 kgf / mm ² cannot be said to be sufficient as the strength level of power transmission system parts of an automobile. Here, in the induction-hardened material, quenching cracks are more likely to occur as the strength is increased, and the suppression thereof is one of the important issues at present. Therefore, an object of the present invention is to prevent quenching cracks and to have τmax of 180 kgf.
An object of the present invention is to provide a method for manufacturing an induction-hardened part having an excellent torsional strength of not less than / mm ² .

[0004]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made extensive studies in order to realize a mechanical component capable of preventing quenching cracks and achieving excellent torsional strength by induction hardening, and obtained the following findings. It was (1) The torsional strength of the induction hardened material is remarkably improved by increasing the C content and improving the hardenability. However, high C
If the hardening and hardenability are improved, the risk of occurrence of quench cracks increases. (2) Quenching cracks are former austenite grain boundary cracks, and in order to prevent quenching cracks, the following points are important points. Reduction of grain boundary segregation elements (P, B) Hardenability is secured mainly by Cr and Mo which are hard to cause quench cracking. Reduces Si, which strengthens the ferrite base and increases the susceptibility to quench cracking. The grain size of the former austenite after induction hardening is refined by the combination of the following methods.・ Properly control the amount of carbonitride forming elements such as Al and the amount of N. -Forging is performed in the austenite temperature range of 1000 ° C or less before induction hardening to refine the structure before induction hardening. (3) In addition, C during forging heating-cooling before induction hardening
Since there is a risk that r and Mo will melt into the cementite and the hardenability will deteriorate, the temperature rise and cooling during the forging heating are performed rapidly. (4) Further, the carbonitride-forming element such as Al simultaneously forms an oxide-based inclusion to increase the susceptibility to quench cracking, thereby reducing the amount of oxygen.

The present invention has been made based on the above new findings, and the gist of the present invention is that C: 0.4 to 0.8%, Mn :. 0.25 to 0.70%, S: 0.01 to 0.15%, Al: 0.015 to 0.05%, N: 0.003 to 0.020%, and Cr: 0. .3 to 1.5%, Mo: 0.05 to 0.
5% of 1 type or 2 types, Si: 0.1% or less, P: 0.020% or less, B: 0.
Less than 0005%, limited to 0: 0.002% or less, or further containing Mo: 0.5% or less, or further, Ti: 0.005-0.04% Nb: 0.005-0. 1% V: 0.03 to 0.3% of 1 type or 2 types or more, and the balance is made of a steel material consisting of iron and unavoidable impurities and heated to a forging temperature within a heating time of 30 minutes, and 1000 After forging in the austenite temperature range of ℃ or less, cooling between forging temperature and 500 ℃ at an average cooling rate of 0.5 ℃ / s or more, and then induction hardening-tempering is characterized by induction hardening with few quench cracks. It is in the manufacturing method of parts.

[0006]

The present invention will be described in detail below. First, the reason why the component content range of the steel of the present invention is limited as described above will be described. First, C is an element effective in increasing the strength of the final product as a mechanical part,
If it is less than 0.4%, the strength of the final product is insufficient, and it is 0.8
If it exceeds%, the toughness of the final product is rather deteriorated.
The content was 0.4 to 0.8%. Next, Mn is an element effective in increasing the strength of the final product through improvement of the hardenability, but if it is less than 0.25%, this effect is insufficient. On the other hand, if it exceeds 0.7%, the forging load in the austenite temperature range of 1000 ° C. or less becomes significantly large.
For the above reason, the Mn content is set to 0.25 to 0.70%. Next, S exists as MnS in the steel and contributes to improvement of machinability and refinement of the structure, but if it is less than 0.01%, its effect is insufficient. On the other hand, if it exceeds 0.15%, the effect is saturated and rather the toughness deteriorates and the anisotropy increases. For the above reasons, the content of S is 0.01 to
It was set to 0.15%. Next, Al is added as a deoxidizing element and a crystal grain refining element, but if it is less than 0.015%, its effect is insufficient, while if it exceeds 0.05%, its effect is saturated and rather toughness is obtained. Therefore, its content is set to 0.015 to 0.05%. Furthermore, N is A
The precipitation behavior of 1N contributes to the refinement of the austenite grain / ferrite / pearlite structure, but 0.003
%, The effect is insufficient, while 0.020
If it exceeds%, the effect is saturated and rather the toughness is deteriorated, so the content is made N: 0.003 to 0.020%.

Next, in the present invention, Cr: 0.3 to 1.5
%, Mo: 0.05 to 0.5% of 1 type or 2 types are contained. Both Cr and Mo improve hardenability,
Although it is an effective element for increasing the strength of the final product,
If Cr: less than 0.3% and Mo: less than 0.05%, this effect is insufficient. On the other hand, Cr: more than 1.5%, Mo:
If it exceeds 0.5%, the forging load in the austenite temperature range of 1000 ° C. or lower remarkably increases. For the above reasons, the content range of the components is Cr: 0.3 to 1.5%, Mo: 0.05 to
0.50%. On the other hand, Si is an element which has a small effect of increasing the hardenability, and on the contrary, enhances the susceptibility to quenching cracks by strengthening the ferrite base and increases the forging load in the austenite temperature range of 1000 ° C. or lower. These adverse effects become particularly remarkable when the content exceeds 0.1%, so 0.1% was made the upper limit. Further, P and B cause grain boundary segregation or center segregation in the steel, which causes quench cracking and deterioration of toughness. Particularly, when P: more than 0.020% and B: 0.0005% or more, these adverse effects become remarkable, so P: 0.020.
% Or less, B: less than 0.0005%. Further, O forms an oxide such as Al ₂ O ₃ and becomes a stress concentration site, which is one of the causes of quench cracking. O: If this content exceeds 0.002%, this effect becomes significant, so the content was made 0.002% or less.

Further, in the present invention, one or more of Ti, Nb and V can be contained as an essential element for the purpose of adjusting the grain size. However, the Ti content is less than 0.005%, the Nb content is less than 0.005%,
If the V content is less than 0.03%, its effect is insufficient, while Ti: more than 0.040%, Nb: 0.10%.
If V exceeds 0.30%, the effect is saturated and the toughness is rather deteriorated.
05-0.040%, Nb: 0.005% -0.1%,
V: 0.03 to 0.3%.

Next, in the present invention, the above steel material is heated to a forging temperature within a heating time of 30 minutes to obtain 100
After forging in the austenite temperature range of 0 ℃ or less, the forging temperature ~
Cooling between 500 ° C. at an average cooling rate of 0.5 ° C./sec or more and then induction hardening-tempering are performed. The reason for limiting the manufacturing method will be described. Forging in the austenite temperature range of 1000 ° C. or lower before induction hardening utilizes work recrystallization in the austenite area to reduce the structure before induction hardening and suppresses quench cracking during induction hardening. This is because. However, if the forging temperature exceeds 1000 ° C, this effect is small, so the forging temperature is set to 1000 ° C or less. Here, Cr and Mo melt into cementite during forging heating-cooling before induction hardening, and it becomes difficult to secure sufficient hardenability. This phenomenon becomes particularly noticeable when the heating time exceeds 30 minutes and the average cooling rate between the forging temperature and 500 ° C is less than 0.5 ° C / sec. Therefore, the heating time to the forging temperature is within 30 minutes, and the forging temperature is less than 30 minutes. ~ 5
The average cooling rate between 00 ° C was 0.5 ° C / sec or more. Hereinafter, the effects of the present invention will be described more specifically by way of examples.

[0010]

EXAMPLE A steel bar having a diameter of 50 mm and having the composition shown in Table 1 was forged with a surface reduction rate of 50% under the conditions shown in Table 2 and machined into a twisted test piece having a parallel portion of 20 mmφ. Then, induction hardening was performed under the conditions of a frequency of 30 KHz and a feed rate of 12 mm / sec, and tempering was performed under the conditions of 170 ° C. × 1 hour. A torsion test was performed on these samples.
Table 3 shows the results of evaluation of the torsional strength of each steel together with the presence or absence of quench cracks during high frequency heating. As is clear from Table 3, the samples according to the method of the present invention each have a τmax of 180 kg.
It can be seen that it has an excellent torsional strength of f / mm ² or more and has a low susceptibility to quench cracking. On the other hand, Comparative Example 1 is a case where the content of C is below the range of the present invention.
6 is the case where the content of Mn is below the range of the present invention, 17 is the case where both the contents of Cr and Mo are below the range of the present invention, and Comparative Examples 5 and 21 are the rises to the forging temperature. When the warm time exceeds the range of the present invention, Comparative Examples 8 and 22 are cases where the average cooling rate between the forging temperature and 500 ° C. is below the range of the present invention, and τmax is 18 in both cases.
The torsional strength of 0 kgf / mm ² or more is not achieved.
In Comparative Examples 10, 11, 14, 15, and 18, C, P,
The contents of B, Si, and O were higher than the ranges of the present invention, and Comparative Examples 12 and 13 were the contents of Al and N lower than the ranges of the present invention. Nos. 7 and 7 are cases where the forging temperature exceeds the range of the present invention, and in both cases, quench cracking occurs.

[0011]

[Table 1]

[0012]

[Table 2]

[0013]

[Table 3]

[0014]

As described above, by using the method of the present invention, it is possible to manufacture an induction-hardened component having excellent torsional strength of τmax of 180 kgf / mm ² or more and few quench cracks. It becomes possible to manufacture power transmission system parts that can allow higher output of the automobile engine, and the industrial effect is extremely remarkable.

Claims (2)

[Claims] 1. A weight ratio of C: 0.4 to 0.8%, Mn: 0.25 to 0.70%, S: 0.01 to 0.15%, Al: 0.015 to 0. 0.05%, N: 0.003 to 0.020%, Cr: 0.3 to 1.5%, Mo: 0.05 to 0.
5% of 1 type or 2 types, Si: 0.1% or less, P: 0.020% or less, B: 0.
Less than 0005%, O: limited to 0.002% or less, the steel material consisting of the balance of iron and unavoidable impurities is heated to the forging temperature within a heating time of 30 minutes, and forged in the austenite temperature range of 1000 ° C or less. Then, a method for producing an induction-hardened component with few quenching cracks, which comprises cooling between forging temperature and 500 ° C at an average cooling rate of 0.5 ° C / sec or more, and then performing induction hardening-tempering. 2. The steel further contains one or more of Ti: 0.005 to 0.04% Nb: 0.005 to 0.1% V: 0.03 to 0.3%. The method for producing an induction-hardened part with few quench cracks according to claim 1.