JPH07238341A - Steel for induction hardening, excellent in high bearing fatigue strength - Google Patents

Abstract

PURPOSE:To produce a steel for induction hardening, excellent in high bearing fatigue strength. CONSTITUTION:A steel, having a composition containing, as alloying elements, 0.40-0.80%, by mass, C, 0.35-3.0% Si, 0.35-1.5% Mn, 0.10-3.0% Cr, and 0.05-0.50% V and having the balance Fe with inevitable impurities, is used after induction hardening. This steel can further contain one or >=2 kinds among <=3.0% Ni, <=1.0% Mo, <=0.10% Nb, 0.0005-0.0050% B, <=0.20% S, <=0.20% Te, and <=0.0050% Ca.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel which is excellent in wear resistance and surface fatigue strength and used by induction hardening.

[0002]

2. Description of the Related Art Up to now, bearing steel represented by SUJ2 or carburized bearing steel represented by SCr420 or SCM420 has been used for parts or parts which are required to have wear resistance and surface fatigue strength. Carbon steel used for induction hardening generally has a shorter rolling life than bearing steel and carburized bearing steel, and was unsuitable for parts that require high levels of wear resistance and surface fatigue strength. .

However, the number of parts that use the induction-hardened portion as it is as a bearing has recently increased. Also, due to the need for energy saving and in-line construction, parts that were conventionally manufactured by carburizing and quenching materials are now manufactured by induction hardening. Along with this, there has been an increasing demand for induction hardening steels having excellent wear resistance, surface fatigue strength and machinability.

In order to improve the wear resistance and the surface fatigue strength of induction hardening steel, it is most effective to increase the amount of C which increases the surface hardness after induction hardening, but the amount of C is Addition of 0.80%, which is the eutectoid point of steel, is the limit, and even if 0.80% or more is added, the surface hardness rather lowers, leading to lower strength improvement. Further, if the amount of C is high, the hardness in the raw material state is increased, and the machinability is impaired. Also, reduction of O content and addition of Si and Cr are being studied,
The wear resistance and surface fatigue strength were not significantly improved.

[0005]

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel for induction hardening which is excellent in wear resistance and surface fatigue strength. Is in the place of providing.

[0006]

As a result of studying various combinations of alloying elements, the present inventor has found that the combined addition of Si and V is extremely effective in order to improve wear resistance and surface fatigue strength. I found that.

That is, in the induction hardened steel of the present invention which has excellent wear resistance and drawing fatigue strength, the content of alloying elements is% by mass.
, C: 0.40 to 0.80%, Si: 0.35 to 3.0%, Mn: 0.35 to 1.5%, Cr: 0.10 to 3.0%, V: 0. The content is 05 to 0.5%, and the balance is Fe and inevitable impurities. Further, in addition to the above alloy elements, in mass%, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.10% or less B: 0.0005 to 0.0050% of 1 It may contain one species or two or more species.

In addition to the above alloy elements, mass%
Then, S: 0.20% or less, Te: 0.20% or less, Ca: 0.0050% or less, and one or more kinds of machinability improving elements can be included.

The reasons for limiting the alloy components will be described below. C: 0.40 to 0.80% C is an essential element for maintaining the strength of steel after induction hardening, and is added by 0.40% or more to maintain the surface hardness after induction hardening. There is a need. However, when the content is added beyond the eutectoid point of 0.80%, the surface hardness is rather lowered, leading to deterioration in strength improvement. In addition, not only the proeutectoid cementite is generated to impair the toughness, but also the material hardness in the raw material state is increased and the machinability is impaired. Therefore, the upper limit of the C content is 0.80%.

Si: 0.35 to 3.0% Si is an element which plays an important role in the present invention.
It is added to impart tempering softening resistance to steel, but its effect can be further enhanced by coexisting with V. In order to exert the effect, the content rate of 0.35% or more is required. However, even if added excessively, the effect is not only saturated, but also causes inconveniences such as impairing forgeability and machinability, so the upper limit of the Si content is set to 3.0.
%.

Mn: 0.35-1.5% Mn is added in an amount of 0.35% or more in order to enhance the hot workability of steel and to secure hardenability. However, if added excessively, the machinability of the material will be impaired, so the upper limit of the Mn content is 1.5%.
And

Cr: 0.10 to 3.0% Since Cr is an element that increases the hardenability of steel and softening resistance, it is added in an amount of 0.10% or more, but even if added in excess, the effect is saturated, Since the cost is unnecessarily increased, the upper limit of the content rate is set to 3.0%.

V: 0.05 to 0.50% V is an element that plays an important role in the present invention together with Si, and has the effect of refining the crystal grain boundaries of steel.
When coexisting with, it has the effect of synergistically increasing the temper softening resistance. In particular, it is an effective element for preventing the phenomenon of breaking in a short life due to rolling fatigue. In order to exert these effects, the V content is 0.0
5% or more is required. However, even if added excessively, the effect is saturated, so the upper limit is made 0.50%.

Ni: 3.0% or less Mo: 1.0% or less Nb: 0.10% or less B: 0.0005 to 0.0050% Ni, Mo and Nb increase the toughness of the hardened layer portion and harden it. Since it is an element that deepens the layer depth, each 3.0% or less,
1.0% or less and 0.10% or less may be added alone or in combination. B is the only element that deepens the depth of the hardened layer without increasing the hardness of steel in the material.
In the range of 0.0005 to 0.0050% where the B effect is most stable, it may be added alone or in combination with other elements.

S: 0.20% or less, Te: 0.20% or less, Ca: 0.0050% or less, which is an element added when it is desired to particularly improve machinability,
S, Ding e and Ca 0.20% or less, 0.2
They may be added individually or in combination within the range of 0% or less and 0.0050% or less. However, an upper limit was set because mechanical properties deteriorate if added more than this.

Induction hardening Not only the surface layer portion is martensitic and hardened by induction hardening, but also a large compressive residual stress is introduced.
Since this compressive residual stress delays the initiation and propagation of fatigue cracks, it is possible to obtain parts having high high surface fatigue strength. Also S
When i is added in an amount of 0.35 to 3.0%, the AC ₃ transformation point rises. Therefore, it is necessary to make the temperature higher than usual in order to completely transform it into austenite. In this case, in quenching and tempering, etc., due to high pressure, oxide scale is attached, resulting in a change in product size and a decrease in yield. In addition, the heat treatment cost increases and the life of the heating furnace decreases. However, in the case of induction hardening, since heating is performed for a short time, the degree of oxidation is remarkably small, so that good surface properties can be secured, and there is an advantage that a high temperature can be easily obtained by adjusting the power easily.

[0017]

EXAMPLE A radial rolling fatigue test piece having a test portion diameter of 12.3 mm was cut out from a hot rolled steel material having the chemical composition shown in Table 1 and subjected to induction hardening and tempering treatment under the conditions shown in Table 2. Further, as a comparison of induction hardening, only Comparative Example 4 was subjected to quenching and tempering treatment under the conditions shown in Table 2. After the treatment, the surface was ground and subjected to a rolling fatigue test. The rolling test was a radial type rolling fatigue test using SUJ2 balls at a surface pressure of 5880 MPa. The test results are shown in Table 3.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

As is apparent from Table 3, Examples 1 to 12 of the present invention in which Si and V are added in combination show a remarkable improvement in the cumulative damage probability 10% (L ₁₀ ) in the rolling fatigue test.

On the other hand, Comparative Example 1 is a conventional steel SAE1.
070, and the L ₁₀ life is short because the Si and V contents are low.

In Comparative Examples 2 and 3, S is different from the Examples.
This is an example in which the i and V contents are low. L ₁₀ life is lower than that of Si, V composite additive. Particularly, in Comparative Example 2 in which only V is added, the life improving effect is small. That is, it is understood that V has the effect of improving the L ₁₀ life only when it coexists with Si.

Examples 1 to 12 in Table 3 are examples satisfying all the composition of components and the conditions for induction hardening according to the present invention, and show rolling fatigue characteristics, drill machinability and induction hardening. Excellent in everything.

On the other hand, Comparative Example 1 is a conventional steel SAE1.
070, and the L ₁₀ life is short because the Si and V contents are low. Further, since the amount of Mn is high, the material hardness is high and the machinability is lower than those of Examples 1-12.

Comparative Examples 2 and 3 are examples having lower Si and V contents than Example 4, respectively. L ₁₀ life is lower than that of Si, V composite additive. Comparative Example 2 in which only V was added
Then, the life improving effect is small. That is, it is understood that the effect of improving the L ₁₀ life is exhibited only when V coexists with Si.

Comparative Example 4 has the same component material as that of Example 1, but is a quenched and tempered material. It is considered that the L ₁₀ life becomes short because the crack compressive residual stress that suppresses the occurrence and propagation of fatigue cracks is small in the quenched and tempered material.

[0028]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide an induction hardening steel excellent in high surface pressure fatigue strength by selecting a unique alloy element.

Claims (3)

[Claims] 1. The content of alloying elements is% by mass, C: 0.40 to 0.80%, Si: 0.35 to 3.0%, Mn: 0.35 to 1.5%, Cr: 0.10 to 3.0%, V: 0.05 to 0.50%, which is excellent in high surface pressure fatigue strength, which is characterized by using steel consisting of the balance Fe and inevitable impurities after induction hardening. Induction hardening steel. 2. In addition to the alloy element according to claim 1, in mass%, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.10% or less B: 0.0005 to 0 A steel for induction hardening excellent in high surface pressure fatigue strength, which is characterized by using a steel containing one or more of 0.0050% and the balance Fe and unavoidable impurities after induction hardening. 3. In addition to the alloy element according to claim 1 or 2, in mass%, one of S: 0.20% or less, Te: 0.20% or less, Ca: 0.0050% or less, or A steel for induction hardening excellent in high surface pressure fatigue strength, characterized by using a steel containing two or more kinds of machinability improving elements and the balance being Fe and unavoidable impurities after induction hardening.