JPH0693374A - High strength steel for induction hardening - Google Patents

Abstract

PURPOSE:To improve induction hardenability and also to improve torsional strength and rolling contact fatigue strength by incorporating specific amounts of Cr, Ni, and Mo into a steel and further adding N actively. CONSTITUTION:The steel for induction hardening has a composition consisting of, by weight ratio, 0.40-0.65% C, 0.01-0.60% Si, 0.30-1.20% Mn, <=0.018% P, <=0.010% S, 0.005-0.018% Al, 0.0100-0.0250% N, <=0.0020% O, <=0.0030% Ti, one or >=2 kinds among 0.30-0.60% Cr, 0.30-0.60% Ni, and 0.03-0.13% Mo, and the balance Fe with impurity elements. By this method, nonmetallic inclusions in the steel can be decreased, and the occurrence of strain and quenching crack of parts at the time of induction hardening can be inhibited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to induction hardening which is excellent in rolling contact fatigue strength and static strength used in automobile parts such as outer races of constant velocity joints, transmission gears and other induction hardening parts. For steel.

[0002]

2. Description of the Related Art Among carbon steels for machine structures, medium carbon ones are surface hardened by induction hardening and satisfy characteristics such as rolling contact fatigue strength and static strength required for automobile parts, Furthermore, it has excellent forgeability and is inexpensive. Therefore, conventionally, for example, in the outer race of a constant velocity joint, etc., medium carbon carbon steel for mechanical structure (S
45C to S58C) have been used.

[0003]

However, in recent years, the performance of automobile engines has been rapidly improved, and particularly for drive transmission system parts such as the outer race of constant velocity joints, the high output of the engine is high. In order to avoid an increase in weight and an increase in size of parts, demands for higher strength, improved durability, and lighter weight have become extremely strong.

It is well known that induction hardening involves rapidly heating the surface of a part and then rapidly cooling it, so that the residual compressive stress in the hardened part improves fatigue strength. In addition, in order to improve the torsional strength or the bending strength, which is the static strength, it can be dealt with by increasing the hardening depth by induction hardening.

However, in the carbon steel for mechanical structure of medium carbon which has been conventionally used for automobile parts, the mass effect is large and the hardenability is relatively poor, so that a sufficient hardening depth satisfying the design strength is obtained. However, there is a drawback in that it is very difficult to set the induction hardening conditions because the hardening depth varies significantly between the parts that are significantly affected by the shape.

Therefore, in order to improve and stabilize the induction hardenability, SCr440 or SC which is a low alloy steel for machine structure is used.
It is thought that this can be dealt with by changing to steel having high hardenability such as M440. However, these low alloy steels for machine structures need to solve problems such as cracks during forging, cracks due to induction hardening, and distortion. Furthermore, in terms of component forming, there is a tendency to shift from conventional hot forging to warm forging and cold forging, and in terms of workability, in low alloy steel for machine structural use,
There are many problems, the cost will be much higher than that of conventional steel, and it is difficult to deal with.

Further, in addition to static strength such as torsional strength, rolling contact fatigue strength is extremely important for parts such as outer races of constant velocity joints, and improvement of rolling contact fatigue strength is also an important subject. . However, in the conventional steel, the strength of the induction-hardened layer is limited, and further, the rolling contact fatigue strength cannot be expected to improve due to the influence of non-metallic inclusions.
Furthermore, in terms of forming parts, there is a tendency to shift from conventional hot forging to warm forging and cold forging, and it is necessary to improve workability. There is almost no improvement in workability.

The present invention has been made in view of the above problems of conventional steels used for induction hardening parts for automobiles and the like. It is an object of the present invention to provide a high-strength induction hardening steel which has improved mechanical strength, improved rolling contact fatigue strength, and improved workability such as forgeability.

[0009]

In order to solve the above-mentioned problems, the present invention has been conducted by the inventors of the present invention as a result of intensive studies on steel that has been induction hardened, and as a result, the present invention has been completed with the following findings. did.

First, in order to improve the static strength such as torsional strength or bending strength, it is effective to improve the hardenability of steel to increase the hardening depth by induction hardening. Therefore, addition of alloys such as Cr and Ni can be considered. However, the addition of these alloys causes distortion, cracking and the like of the component during induction hardening, and significantly deteriorates the forgeability, leading to a decrease in the productivity of the component. Therefore, the inventors of the present invention have found that in induction hardening, a large hardening depth can be obtained by adding a small amount of alloy, and this also hardly causes deterioration of the manufacturability of parts.

Specifically, Cr: 0.30 to 0.60%, Ni: 0.30
~ 0.60%, and Mo: 0.03 ~ 0.13% of 1 to 2
If more than one type of alloy is added, it is possible to obtain the required hardening depth in induction hardening with almost no loss of manufacturability. Furthermore, by positively adding N,
It has been found that the yield strength and the torsional strength of the induction-hardened structure that has been tempered at a low temperature increase. In particular
It is necessary to contain 0.0100% or more.

Secondly, in order to improve the rolling contact fatigue strength, the relationship between the rolling contact fatigue strength and the non-metallic inclusions and impurity elements in the steel in the induction hardened steel was studied. As a result, it was discovered that the most harmful substances to rolling contact fatigue strength were coarse oxide inclusions and TiN inclusions. Ideally, these inclusions should be eliminated, but it is difficult in the current steelmaking technology, so the optimum method in the current steelmaking technology was examined.

First, in order to reduce coarse oxide inclusions, it is effective to set the O content in the steel to 0.0020% or less, preferably 0.0010% or less, and to set the Al content in the steel to 0.005 to 0.018%. I understood. Reduction of O content in steel reduces oxide inclusions, and when Al content is less than 0.005%, coarse oxide inclusions such as SiO ₂ increase, and when Al content exceeds 0.018%, Al ₂ content increases. Oxide inclusions such as 0 ₃ become coarse. Therefore, the oxide inclusions contained in the steel can be made as small as possible and fine by adjusting the amounts of O and Al.

Although TiN inclusions have not been considered so far in steels for mechanical structures, the Ti content as impurities is 0.0030% or less, preferably 0.
By setting the content to be not more than 15%, the adverse effect on the rolling contact fatigue strength is eliminated. Furthermore, MnS by reducing the amount of S
It was found that strengthening the crystal grain boundaries of steel by reducing inclusions and P content is also effective in improving the rolling contact fatigue strength of induction hardened steel. Further, it has been found that reducing these inclusions and impurity elements has a great effect on improving forgeability and reducing cracks during induction hardening at the same time.

The present invention has been completed based on the above findings, and the first aspect of the present invention is, in terms of weight ratio,
C: 0.40 to 0.65%, Si: 0.01 to 0.60%, Mn: 0.30 to 1.20
%, P: 0.018% or less, S: 0.010% or less, Al: 0.005
~ 0.018%, N: 0.0100 ~ 0.0250%, O: 0.0020% or less, Ti: 0.0030% or less, Cr: 0.30 ~ 0.60%, Ni: 0.30
It is a high-strength induction hardening steel, characterized in that it contains one or more of 0.6 to 0.60% and Mo: 0.03 to 0.13%, and the balance is Fe and impurity elements.

Next, the reasons for limiting the chemical composition of the steel for induction hardening of the present invention will be explained. C: 0.40 to 0.65% C is an element necessary for obtaining induction hardening hardness and internal hardness, and in order to secure the desired induction hardening hardness and internal hardness, 0.40% or more should be contained. is necessary. However, if the content exceeds 0.65%, cracking tends to occur during induction hardening, so the upper limit was made 0.65%.

Si: 0.01 to 0.60% Si is an element necessary for deoxidizing molten steel at the time of steel making, and at least 0.01% or more is necessary to obtain sufficient deoxidation. However, if the content exceeds 0.60%,
The upper limit was set to 0.60% because it strengthens ferrite, impairs workability, and increases oxide inclusions.

Mn: 0.30 to 1.20% Mn is an element necessary for deoxidizing molten steel at the time of steel making, and is an element necessary for strengthening the matrix. To obtain the above effect, at least 0.30% It is necessary to contain the above. However, if the content exceeds 1.20%, the workability and machinability deteriorate, and cracks tend to occur during induction hardening, so the upper limit was made 1.20%.

P: 0.018% or less P segregates at the grain boundaries to reduce the static strength and rolling contact fatigue strength of the induction-hardened part, so P must be 0.018% or less. It is preferably 0.015% or less.

S: 0.010% or less S forms sulfide-based inclusions and impairs rolling contact fatigue strength and forgeability, so the content must be 0.010% or less.

Cr: 0.30 to 0.60% Cr is an element that improves the hardenability of steel and has a great effect on increasing the hardening depth in induction hardening. Therefore, it is necessary to contain at least 0.30% or more. But 0.
If the content exceeds 60%, the forgeability deteriorates, so the upper limit was made 0.60%.

Ni: 0.30 to 0.60% Ni is an element that improves the hardenability and toughness of steel, and is particularly effective in increasing the hardening depth by induction hardening. To obtain this effect, it is necessary to contain at least 0.30% or more. However, if the content exceeds 0.60%, the forgeability deteriorates, so the upper limit was made 0.60%.

Mo: 0.03 to 0.13% Mo is an element that improves the hardenability and temper softening resistance of steel, and is particularly effective in increasing the hardening depth by induction hardening. Therefore, it is necessary to contain at least 0.03% or more. However, if the content exceeds 0.13%, the forgeability deteriorates, so the upper limit was made 0.13%.

Al: 0.005 to 0.018% Al is an element necessary for deoxidizing steel, but if it is less than 0.005%, deoxidation is insufficient and coarse oxide inclusions such as SiO ₂ are produced. Also, if the content exceeds 0.018%, Al
_The content of ₂ O ₃ inclusions was set to 0.005 to 0.018% because coarsening of ₂ O ₃ inclusions reduces rolling contact fatigue strength.

N: 0.0100 to 0.0250% N has the effect of increasing the yield point by fixing mobile dislocations, and it is necessary to contain 0.0100% or more. However, if the content exceeds 0.0250%, the workability deteriorates, so the upper limit was made 0.0250%.

O: 0.0020% or less O is an element which forms oxide inclusions and reduces rolling contact fatigue strength, and it is necessary to reduce it as much as possible, and the content thereof is made 0.0020% or less. It is preferably 0.0010% or less.

Ti: 0.0030% or less Ti is an element that forms TiN inclusions and reduces rolling contact fatigue strength, and it is necessary to reduce the content as much as possible, and the upper limit was made 0.0030%. It is preferably 0.0015% or less.

[0028]

EXAMPLES Next, the characteristics of the steel of the present invention will be described by way of examples in comparison with comparative steel and conventional steel. Table 1 shows the chemical composition of these test steels. In Table 1, steels A to D are steels of the present invention. E to L steels are comparative steels, and E steel is P
And comparative steels with a high S content, F steels with a low Al content, G steels with a high Al content, H steels with a high Ti content, and I steels with a high Mo content. Comparative steel, J steel is Ni, C
Comparative steels with low r and Mo contents, K steel with a high Cr content, and L steel with a low N content.
M to O steels are conventional steels, M steel is JIS S53C equivalent steel, N steel is SCr440 equivalent steel, and O steel is JIS SCM440 equivalent steel.

The test steels were measured for torsional strength, induction hardenability, rolling contact fatigue strength and forgeability, and the results obtained are shown in Table 3. Regarding the torsional strength, a solid test piece of 35φ was induction hardened at 20 KH _Z , then tempered at 180 ° C. for 90 minutes, and the maximum shear stress at the time of breaking was measured using a torsion tester. evaluated. For induction hardenability, 40φ solid test piece 20KH
After induction hardening with _Z , tempering was performed at 180 ° C for 90 minutes, the hardness distribution from the surface was measured, and the effective hardening depth was determined to obtain induction hardening.

Regarding rolling contact fatigue strength, a 60φ thrust life test piece was subjected to induction hardening and tempering treatment, and then tested at maximum Hertz stress of 560 Kgf / mm ² , stress repetition number of 1500 cpm in lubricating oil, until peeling. The life was sorted by the cumulative damage rate, and the B ₁₀ life (life at a cumulative damage rate of 10%) was obtained and evaluated. Regarding forgeability, a cylindrical test piece of height / diameter = 1.5 was subjected to a cold both-end restrained upsetting test, and the upsetting rate at which 50% of the test pieces cracked was evaluated as the critical upsetting rate.

As is clear from Table 3, the comparative steel E
Steel has a B ₁₀ life of 1.40 × 10 ⁷ and a critical upsetting rate of 45%, and is inferior in rolling contact fatigue strength and forgeability. F steel has a B ₁₀ life of 0.85 × 10 ⁷ like E steel. The rolling contact fatigue strength and the forgeability are inferior at the critical upsetting ratio of 40%, and the G and H steels have good induction hardenability, but the B ₁₀ life is 0.90 to 1.20 × 10 ⁷ , the critical upsetting. 40-45% coverage
And rolling contact fatigue strength and forgeability are inferior. I steel is excellent in torsional strength, induction hardenability and rolling contact fatigue strength, but the limit upsetting ratio is 30% and the forgeability is low. Steel has good forgeability, but maximum shear stress is 1500 N / mm ² , effective hardening depth is 5.3 mm, B ₁₀ life is 2.00
× 10 ⁷ is inferior in torsional strength, induction hardenability and rolling contact fatigue strength, and K steel is good in torsional strength, induction hardenability and rolling contact fatigue strength contrary to J steel, but the limit upsetting The forgeability is inferior at 30%, and although L steel is excellent in induction hardenability and forgeability, the maximum shear stress is 17
00N / mm ² , effective hardening depth is 3.80mm, and torsional strength and rolling contact fatigue strength are poor.

Similar to J steel, M steel which is a conventional steel has good forgeability, but is inferior in torsional strength, induction hardenability and rolling contact fatigue strength, and N steel is induction hardenability.
Rolling contact fatigue strength and forgeability are inferior, and O steel is inferior in rolling contact fatigue strength and forgeability. In contrast to these comparative steels and conventional steels, the steels A to D of the present invention have maximum shear stress of 2100 to 2200 N / mm ² , effective hardening depth of 10.3 to 10.8 mm,
B ₁₀ life is 4.00-5.30 × 10 ⁷ and critical upset rate is 55-60%,
It has been proved that the steel of the present invention has excellent properties because it has excellent torsional strength, induction hardenability, rolling contact fatigue strength and forgeability.

[0033]

As described above, the steel of the present invention has the high strength and durability required for induction hardening parts for automobiles such as outer races of constant velocity joints and transmission gears. In response to demands such as weight reduction, by adding Cr, Ni, Mo in appropriate amounts and positively adding N, and by significantly reducing the content of non-metallic inclusions and impurity elements in steel, induction hardening is possible. The high strength that suppresses the occurrence of distortion and quench cracking of parts, and improves the static strength such as torsional strength and the induction hardenability and rolling contact fatigue strength without impairing the manufacturability such as forgeability. It is a steel for induction hardening and is a very useful steel.

[Table 1]

[Table 2]

[Table 3]

Claims (1)

[Claims] 1. A weight ratio of C: 0.40 to 0.65%, Si: 0.
01 to 0.60%, Mn: 0.30 to 1.20%, P: 0.018% or less,
S: 0.010% or less, Al: 0.005 to 0.018%, N: 0.0100
~ 0.0250%, O: 0.0020% or less, Ti: 0.0030% or less,
Cr: 0.30 to 0.60%, Ni: 0.30 to 0.60%, Mo: 0.03 to 0.13
%, A high strength induction hardening steel, characterized in that it contains one or two or more of Fe and the balance Fe and impurity elements.