JPH0790380A - Production of induction-hardened part - Google Patents

Abstract

PURPOSE:To provide a method for producing an induction-hardened part from which quenching cracks are prevented and having excellent torsion strength. CONSTITUTION:Steel stock having a compsn. contg. 0.4 to 0.8% C, 0.3 to 1.70% Mn, 0.005 to 0.15% S, 0.015 to 0.05% Al, 0.01 to 0.3% Nb, 0.005 to 0.05% Ti, 0.0005 to 0.005% B and 0.002 to 0.02% N, and/or specified amounts of one or >=two kinds among Cr, Mo, Ni and V and/or specified amounts of one or >=two kinds of Ca and Pb and in which the contents of Si, P, Cu and O are reduced is heated to a forging temp. at a temp. rising time of <=30min, is forged in an austenitic temp. range of <=1000 deg.C, is thereafter cooled from the forging temp. to 500 deg.C at the average cooling rate of >=0.5 deg.C/sec and is subsequently subjected to induction hardening-tempering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an induction-hardened part, and more particularly to a method for manufacturing an induction-hardened part which has excellent torsional strength as a mechanical part and is less likely to cause quench cracks during manufacturing. It is a thing.

[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 in 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. Regarding the improvement of the strength of the induction hardening shaft, Japanese Unexamined Patent Publication (Kokai) No. 4-2181641 discloses Si: 0.
05% or less, Mn: more than 0.65 and 1.7 or less low Si
By using a steel material for high-strength shaft components of a specific component system characterized by a high Mn and
It has been shown that a torsional strength of 60 kgf / mm ² can be obtained. Thus, the maximum torsional strength that can be realized at present is about 160 kgf / mm ² .

[0003]

However, under the present circumstances, the above-mentioned level of the torsional strength of 160 kgf / mm ² cannot be said to be sufficient as the strength level of the power transmission system parts of the 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 provide a method for manufacturing an induction-hardened component which prevents quenching cracks and has an excellent torsional strength of 160 kgf / mm ² or more.

[0004]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made earnest 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, if the high C content and the hardenability are improved, the risk of quench cracking increases.

(2) Quenching cracks are former austenite grain boundary cracks, and in order to prevent quenching cracks, the following points are important points. 1) Addition of Ti-B 2) Reduction of P, Cu and O contents 3) Si that strengthens ferrite and enhances susceptibility to quench cracking
To reduce. 4) The grain size of the former austenite after induction hardening is refined by the combination of the following methods. -Carbonitride formation by Nb addition. -Forging is performed in the austenite temperature range of 1000 ° C or less before induction hardening to refine the structure before induction hardening.

(3) Mn, Cr, Mo, etc. melt into cementite during forging heating-cooling before induction hardening,
Since there is a risk that the hardenability will decrease, the temperature is raised and cooled rapidly during forging heating. Note that this also promotes the refinement of the structure before induction hardening.

(4) Further, carbonitride-forming elements such as Al simultaneously form oxide-based inclusions to increase the susceptibility to quench cracking, thus reducing the amount of oxygen. The present invention has been made based on the above new findings, and the gist thereof is as a weight ratio, C: 0.4 to 0.8% Mn: 0.3 to 1.70. % S: 0.005-0.15% Al: 0.015-0.05% Nb: 0.01-0.3% Ti: 0.005-0.05% B: 0.0005-0.005 % N: 0.002-0.02% is contained, Si: 0.15% or less, P: 0.020% or less, Cu:
It is limited to 0.05% or less, O: 0.002% or less, or further, Cr: 0.05 to 1.5% Mo: 0.05 to 0.5% Ni: 0.1 to 3.5% V: 0.03 to 0.5% of 1 type or 2 or more types, and further, Ca: 0.0005 to 0.010% Pb: 0.05 to 0.5%, 1 type or 2 types. A steel material containing the above and the balance being iron and unavoidable impurities is heated to a forging temperature within a heating time of 30 minutes or less, and after forging in an austenite temperature range of 1000 ° C. or less, the forging temperature to 500 ° C. is 0. A method for producing an induction-hardened component is characterized by cooling at an average cooling rate of 0.5 ° C./sec or more, and then performing induction hardening-tempering.

[0008]

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 hardenability, but if it is less than 0.3%, this effect is insufficient. On the other hand, if it exceeds 1.7%, 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 0.3 to 1.7%.

Next, S exists as MnS in steel and contributes to improvement of machinability and refinement of structure, but 0.00
If it is less than 5%, the effect is insufficient. On the other hand, 0.15
If it exceeds%, the effect is saturated and rather the toughness deteriorates and the anisotropy increases. For the above reason, the S content is set to 0.005 to 0.15%. Next, Al is added as a deoxidizing element and a grain refinement element.
%, The effect is insufficient, while 0.05%
If it exceeds 1.0, the effect is saturated and rather the toughness is deteriorated, so the content was made 0.015 to 0.05%.

Nb is added for the purpose of refining austenite grains during induction hardening heating by forming carbonitrides in steel. However, if it is less than 0.01%, the effect is insufficient, while if it exceeds Nb: 0.30%, the effect is saturated, and such excessive addition is not preferable from the economical point of view. For the above reasons, the Nb content is set to 0.01% to 0.3%. Ti is also N in steel
TiN is formed by combining with, but is added for the purpose of 1) refining austenite grains during induction hardening heating, and 2) preventing BN precipitation by completely fixing solid solution N, that is, securing solid solution B. However, if less than 0.005%, the effect is insufficient. On the other hand, if it exceeds 0.05%, the effect is saturated and rather the toughness is deteriorated, so the content was made 0.005 to 0.05%.

B is added as a solid solution in order to increase the grain boundary strength by segregating to the austenite grain boundaries at the grain boundaries and expelling grain boundary impurities such as P and Cu from the grain boundaries. However, if less than 0.0005%, the effect is insufficient, while if over 0.05% is added,
Rather, it causes grain boundary embrittlement, so its content should be 0.0005.
Was made 0.005%. Further, N is added for the purpose of refining austenite grains during high frequency heating due to precipitation of carbonitrides such as AlN. However, if it is less than 0.002%, its effect is insufficient, while if it exceeds 0.02%. However, the effect is saturated and rather BN is formed to reduce the amount of solid solution B, so the content was made 0.002 to 0.02%.

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 less. Since these adverse effects become particularly remarkable when the content exceeds 0.15%,
The upper limit was 15%. P causes grain boundary segregation at the austenite grain boundaries, lowers the grain boundary strength, and easily causes brittle fracture under torsional stress, and thus lowers the strength. In particular, when P exceeds 0.02%, the strength is significantly reduced,
The upper limit was 0.02%. In addition, when aiming at further strengthening, it is desirable that the content of P is 0.009% or less.

Cu, like P, also causes grain boundary segregation at the austenite grain boundaries, causing a decrease in strength. Especially Cu
Is more than 0.05%, the strength will be significantly reduced.
The upper limit was 0.05%. Further, O causes grain boundary segregation to cause grain boundary embrittlement, forms hard oxide inclusions in steel, and easily causes brittle fracture under torsional stress.
This will cause a decrease in strength. In particular, when O exceeds 0.0020%, the strength is markedly reduced, so 0.0020% was made the upper limit.

According to the second aspect of the present invention, by adding Cr, Mo, Ni, and V, 1) the induction hardening hardness is increased due to the improvement of the hardenability, the depth of the hardened layer is increased, and 2) the grain boundary segregation occurs in the austenite grain boundaries. It is a steel that is further strengthened and prevented from quenching cracks by increasing the grain boundary strength or preventing the brittle fracture by improving the toughness in the vicinity of the grain boundary. However, C
r: less than 0.05%, Mo: less than 0.05%, Ni:
If it is less than 0.15 and V: less than 0.03%, this effect is insufficient. On the other hand, Cr: more than 1.5%, Mo: 0.5%
If the content is more than Ni, more than 3.5% and V is more than 0.5%, this effect is saturated, and such excessive addition is not preferable from the economical viewpoint. For the above reasons, the content of Cr: 0.
05-1.5%, Mo: 0.05-0.5%, Ni:
0.1 to 3.5% and V: 0.03 to 0.5%. The addition of V also has the effect of increasing the strength by increasing the hardness of the core due to precipitation strengthening.

A third aspect of the present invention is an invention relating to a manufacturing method excellent in workability in a manufacturing process of an induction-hardened part.
In the steel of the present invention, in order to improve machinability, 1 of Ca and Pb is added.
It is possible to contain one kind or two kinds. Note that Ca not only improves the machinability, but also has the effect of combining with P in the steel to form a phosphide, reducing the grain boundary segregation amount of P, and increasing the grain boundary strength. However, the Ca content is 0.00
If the content is less than 05% and the Pb content is less than 0.05%, these effects are insufficient, while Ca: more than 0.01%, Pb:
If it exceeds 0.50%, these effects are saturated, and such excessive addition is not preferable from the economical point of view. For the above reasons, the content of Ca is 0.0005 to 0.0
10%, Pb: 0.05 to 0.5%.

Next, in the present invention, the above steel material is heated to a forging temperature at a temperature rising time of 30 minutes or less, and then 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, during forging heating-cooling before induction hardening, Mn, Cr, Mo, etc. melt into cementite, and it becomes difficult to secure sufficient hardenability.
This phenomenon is caused by a temperature rise time of more than 30 minutes and a forging temperature of ~ 500.
The average cooling rate between 0.5 ° C. and 0.5 ° C./sec is particularly remarkable, so the temperature rising time to the forging temperature is within 30 minutes, and the average cooling rate between the forging temperature and 500 ° C. is 0.5 ° C./sec. That's it. Hereinafter, the effects of the present invention will be described more specifically by way of examples.

[0017]

EXAMPLE A steel bar having a diameter of 50 mm and a composition shown in Tables 1 and 2 was forged with a surface reduction rate of 50% under the conditions shown in Table 3 and machined into a twisted test piece having a parallel portion of 20 mm. Then, induction hardening was performed under the conditions of a frequency of 30 KHz and heating conditions of 1050 ° C. for 4 seconds, and tempering was performed under the conditions of 170 ° C. for 1 hour. These samples were examined for the presence of quench cracks, and a torsion test was conducted for those that did not cause quench cracks. 16m parallel part as a test piece for torsional strength evaluation
A notched torsion test piece having a notch having a tip R of 0.25 mm and a depth of 2 mm at the center of mφ was used.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

Tables 4 and 5 show the results of evaluation of the torsional strength of each steel material together with the presence or absence of quench cracks during high frequency heating. As is clear from Tables 4 and 5, it is understood that each of the samples prepared by the method of the present invention has an excellent torsional strength of 160 kgf / mm ² or more and has a low susceptibility to quench cracking. On the other hand, Comparative Example 4 is a case where the temperature rise time to the forging temperature exceeds the range of the present invention, and Comparative Examples 9 and 20 are the forging temperature to 5
This is a case where the average cooling rate between 00 ° C. is below the range of the present invention, and none of them achieves the torsional strength of 160 kgf / mm ² or more. Further, Comparative Example 22 is a case where the content of C is below the range of the present invention, and 160 kgf / mm ²
The above torsional strength is not achieved.

Next, Comparative Examples 24, 25 and 27 are Nb,
This is a case where the contents of B and Ti are below the range of the present invention, respectively, and Comparative Examples 23, 26, 28, 29, 30, 31.
Is the case where the contents of C, B, Si, P, Cu and O are above the range of the present invention, and Comparative Examples 3 and 8 are the cases where the forging temperature is above the range of the present invention. Burning cracks have also occurred.

[0023]

[Table 4]

[0024]

[Table 5]

[0025]

As described above, by using the method of the present invention, it is possible to manufacture an induction-hardened component having an excellent torsional strength of 160 kgf / mm ² or more and having few quench cracks. Since it becomes possible to manufacture power transmission system parts that can allow higher engine output, industrial effects are extremely remarkable.

Claims (3)

[Claims] 1. As a weight ratio, C: 0.4 to 0.8% Mn: 0.3 to 1.70% S: 0.005 to 0.15% Al: 0.015 to 0.05% Nb : 0.01 to 0.3% Ti: 0.005 to 0.05% B: 0.0005 to 0.005% N: 0.002 to 0.02% Si: 0.15% or less , P: 0.020% or less, Cu:
It is limited to 0.05% or less and O: 0.002% or less, and the steel material consisting of the balance iron and unavoidable impurities is heated to the forging temperature within a heating time of 30 minutes or less, and the austenite temperature range of 1000 ° C or less After the forging, the forging temperature to 500 ° C. is cooled at an average cooling rate of 0.5 ° C./sec or more, and then induction hardening-tempering is performed. 2. The steel further comprises Cr: 0.05 to 1.5% Mo: 0.05 to 0.5% Ni: 0.1 to 3.5% V: 0.03 to 0.5%. The method for producing an induction-hardened component according to claim 1, which contains one or more types. 3. The induction-hardened component according to claim 1 or 2, wherein the steel further contains one or two of Ca: 0.0005 to 0.010% Pb: 0.05 to 0.5%. Manufacturing method.