JPH02243737A - Heat resistant case hardening steel having excellent pitching resistance - Google Patents

Abstract

PURPOSE:To improve the pitching resistance and toughness of the heat resistant case hardening steel by specifying C, Si, Mn, S, Ni, Cr, Mo, P, O, N, Al and Fe. CONSTITUTION:The heat resistant case hardening steel has the compsn. constituted of, by weight, 0.15 to 0.35% C, 0.5 to 2% Si, 0.3 to 1% Mn, 0.005 to 0.01% S, 1 to 3.5% Ni, 0.3 to 1% Cr, 0.05 to 0.8% Mo, <=0.015% P, <=0.0015% O, 0.01 to 0.02% N, 0.02 to 0.05% Al and the balance Fe. The steel has excellent pitching resistance, temper softening resistance and toughness.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a heat-resistant case-hardened steel with excellent pitting resistance.
More specifically, the present invention relates to a heat-resistant case-hardened steel that has excellent pitting resistance, temper softening resistance, and toughness. INDUSTRIAL APPLICATION This invention is utilized for a constant velocity joint, a bearing, a gear, etc.

[Conventional technology]

Conventionally, case hardening steel used for automobiles, power transmission parts, etc. is generally made of chrome! such as 5CR420. m (hereinafter referred to as C
chromium molybdenum steel (hereinafter referred to as Cr-Mo steel) such as SCM420.

[Problem to be solved by the invention]

However, with the increase in output and speed of automobiles, the conventional C
With R steel and Cr-Mo steel, peeling is likely to occur on the contact surface due to high surface pressure contact, and the hardness decreases due to the frictional heat caused by the contact.Furthermore, the toughness is not satisfactory, and the pitting resistance and tempering There was a demand for the development of case-hardened steel with excellent softening resistance and toughness.

In view of the above-mentioned viewpoints, the present invention aims to provide a heat-resistant case-hardened steel having excellent pitting resistance, temper softening resistance, and toughness.

[Means to solve the problem]

As a result of extensive studies by the inventor, the present invention has revealed that the presence of sulfide-based inclusions and oxide-based inclusions becomes a starting point for peeling and inhibits pitting resistance, and that the presence of appropriate amounts of Si, Ni, and Mo is It was discovered that the strength of the base material was improved and the pitting resistance was improved, and furthermore, the inclusion of Sl, Ni, and MO improved the temper softening resistance and also improved the toughness.

That is, the first invention has a weight ratio of C0.15 to C0.15.
35%, Si 0.5-2.0%, Mn 0.3
~1.0%S 0.005~0.015%, Ni
1.0~3.5%, ・Oro, 3~1.0%, Mo
0.05-0.8%, P 0.015% or less, O 0.0
015% or less, N01010~0.020% and A A
The second invention contains 0.020 to 0.050%, and the balance consists of Fe and impurity elements. The third invention further improves the pitting resistance and cold workability of the first invention, and the third invention further improves the S of the first invention by 0.01.
The machinability of the first invention is improved by setting V to 5 to 0.030%, and the fourth invention improves the machinability of the first invention by adding V of 0.05 to 0.03% to the third invention.
The content is 30% to improve toughness and temper softening resistance.

Next, the reason for limiting the components in the present invention will be explained.

C is an element necessary to ensure core hardness through quenching and tempering treatment, and the lower limit was set to 0.15% to ensure this effect. However, if the content increases, machinability and cold workability decrease, as well as toughness, so the upper limit was set at 0.35%.

Si improves the toughness of the carburized layer, improves pitting resistance,
Further, it is an element that contributes to improving the temper softening resistance, and the lower limit was set to 0.5% to ensure this effect. However, if its content increases, machinability and cold forgeability will decrease, and ferrite will remain in the core after carburizing, resulting in a decrease in strength, so the upper limit was set at 2.0%.

Mn is an element necessary to ensure hardenability and deoxidizing effect, and to ensure the effect, the lower limit is set to 0.3%.
And so. However, if its content increases, machinability deteriorates and the abnormal carburized layer increases, so the upper limit was set at 1.0%.

S is an element that generates MnS and improves machinability.

However, on the other hand, S is also an element that reduces fatigue strength and cold workability. Since the purpose of the present invention is to improve pitting resistance, it is necessary to avoid containing a large amount of S, and the content was determined in consideration of the balance with machinability. In the first invention, the lower limit was set to 0.005% in order to ensure the minimum necessary machinability. However, if its content increases, the pitting resistance will be impaired, so the upper limit was set at 0.015%. Further, in the second invention, although there is a slight decrease in machinability, the S content is regulated to 0.005% or less in order to obtain excellent pitting resistance. Also, the third
In the fourth invention, in order to obtain excellent machinability although there is a slight decrease in pitting resistance, the lower limit of S is set to 0.015%, and the upper limit is set in consideration of the decrease in fatigue strength and cold forgeability. 0.0
It was set at 30%.

Ni is an element necessary to ensure the toughness of the carburized layer, stabilize residual austenite, suppress structural changes due to heating, ensure temper softening resistance, improve pitting resistance, and improve hardenability. In order to ensure these effects, the lower limit was set to 1.0%. However, if the content increases, cold forging becomes difficult, and residual austenite increases too much, resulting in insufficient hardness and predetermined carburizing quality, so the upper limit was set at 3.5%.

Cr is an element necessary to improve strength and hardenability through the formation of carbides, and to ensure this effect, the lower limit was set to 0.3%. However, if its content increases, machinability deteriorates and the abnormal carburized layer increases, so the upper limit was set at 1.0%.

Mo is an element necessary to ensure temper softening resistance and pitting resistance at high temperatures, and the lower limit was set to 0.05% to ensure these effects. However, if the content increases, the above effects become saturated and the steel becomes expensive, so the upper limit was set at 0.8%.

A1 is an element that has a deoxidizing effect and suppresses coarsening of crystal grain size during carburizing, and in order to ensure this effect, the lower limit is set to 0.
020%. However, when the content increases, oxide inclusions increase, pitting resistance deteriorates, and cold forgeability also decreases, so the upper limit was set at 0.050%.

P is an element that reduces strength due to agglomeration at grain boundaries or microstructural segregation, and since it is necessary to reduce its content, the upper limit was set at 0.015%.

0 is an element that generates oxide inclusions, reduces pitting resistance, and also deteriorates cold forgeability,
Since it is necessary to reduce the content as much as possible, the upper limit is set to 0.
．． 0015%. In addition, in order to further improve the pitting resistance, it is preferable that the upper limit is 0.0010% or less.

N is an element that generates AIN and suppresses coarsening of crystal grain size during carburizing, and to ensure this effect, the lower limit is set to 0.0.
It was set at 10%. However, if the content increases, the effect becomes saturated, so the upper limit was set at 0.020%.

(2) is an element that contributes to improving toughness, fatigue strength, and resistance to temper softening by refining crystal grains, and the lower limit was set to 0.05% to ensure these effects. However, as the content increases, the above effect becomes saturated, so the upper limit is set to 0.
．． It was set at 30%.

〔Example〕

The characteristics of the steel of the present invention are compared with conventional steel and comparative steel, and are clarified by examples below. Table 1 shows the chemical composition of these test steels.

In Table 1, A-J steel is the steel of the present invention, and A-J steel is the steel of the present invention.
C steel is the first invention steel, D-P steel is the second invention steel, G, H steel is the third invention steel, ■, J steel is the fourth invention steel, K-P steel is the comparison steel, Q, R steel are conventional steel, respectively 5CR420 and SCM42
It is 0.

Table 2 shows the pitting resistance,
The test results for temper softening resistance and toughness are shown. The pitting resistance was evaluated by a radial durability test (maximum contact stress 600 kgf/mm2) based on the number of times at a cumulative failure rate of 10% of the initial life and a cumulative failure rate of 50% of the initial life (each referred to as B1O%BSO).

The temper softening resistance was evaluated based on the amount of decrease in hardness (hardness of carburized layer after carburizing - hardness of carburized layer after tempering at 250°C). Toughness was evaluated by carburizing a JISB No. improved 5R Notchi Yarupi test piece and then performing an impact test.

As is clear from Table 2, steel L is superior to comparative steel in terms of toughness, but inferior in terms of pitting resistance and tempering softening resistance, and comparative steel M is superior in pitting resistance and tempering softening resistance. Both resistance and toughness are inferior, and comparative steels N and O have good toughness but are inferior in pitting resistance and tempering softening resistance, and comparative steel P has poor pitting resistance and tempering softening resistance. Although it is good in terms of toughness, it is poor in toughness.

Furthermore, conventional steels Q and R are inferior in pitting resistance, temper softening resistance, and toughness.

In contrast, the A-J steel, which is the steel of the present invention, has excellent pitting resistance of 0.98 to 6.30xlO1B5o and 1.60 to 10.3x108 at B10, and also has excellent pitting resistance with Regarding resistance, the amount of decrease in hardness (H
V) is 87 to 42, showing excellent heat resistance, and also has a high impact value of 2.55 to 3.51 Kgfm/cm2, and is excellent in toughness. It is clear that both have excellent pitting resistance, temper softening resistance, and toughness.

〔Effect of the invention〕

As mentioned above, in order to eliminate the drawbacks of conventional steel, the present invention incorporates an appropriate amount of Ni into conventional case hardening steel, and also incorporates CrXM.
The content of S and O, which produce sulfide-based inclusions and oxide-based inclusions, has been greatly regulated by setting the O content to an optimal range, and achieving pitting resistance and temper softening without impairing the carburizing properties of conventional steel. It is a case-hardened steel that has improved resistance and excellent toughness, and can be used in high-stress and high-speed automobiles, making it a great contribution to industry.

Claims (4)

[Claims] (1) Weight ratio: C0.15-0.35%, Si0
．． 5-2.0%, Mn0.3-1.0%, S0.005
~0.015%, Ni1.0~3.5%, Cr0.3~
1.0%, Mo0.05-0.8%, P0.015% or less, O0.0015% or less, N0.010-0.020
% and Al 0.020-0.050%, the balance F
A heat-resistant case-hardened steel with excellent pitting resistance characterized by comprising e and impurity elements. (2) Weight ratio: C0.15-0.35%, Si0
．． 5-2.0%, Mn0.3-1.0%, S0.005
% or less, Ni1.0-3.5%, Cr0.3-1.0%
, Mo0.05-0.8%, P0.015% or less, O0
．． 0.010% or less, N0.010-0.020% and A
A heat-resistant case-hardened steel with excellent pitting resistance, characterized in that it contains 0.020 to 0.050% of l and the remainder consists of Fe and impurity elements. (3) Weight ratio: C0.15-0.35%, Si0
．． 5-2.0%, Mn0.3-1.0%, S0.015
~0.030%, Ni1.0~3.5%, Cr0.3~
1.0%, Mo0.05-0.8%, P0.015% or less, O0.0015% or less, N0.010-0.020
% and Al 0.020-0.050%, the balance F
A heat-resistant case-hardened steel with excellent pitting resistance characterized by comprising e and impurity elements. (4) Weight ratio: C0.15-0.35%, Si0
．． 5-2.0%, Mn0.3-1.0%, S0.015
-0.030% or less, Ni1.0-3.5%, Cr0.
3-1.0%, Mo0.05-0.8%, P0.015
% or less, O0.0015% or less, N0.010~0.0
Heat-resistant case hardening with excellent pitting resistance, characterized by containing 20% and 0.020-0.050% Al, further containing 0.05-0.30% V, and the remainder consisting of Fe and impurity elements. steel.