JPH0881738A - 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: This steel for induction hardening excellent in high bearing fatigue strength is characterized by subjecting a steel in which the contents of alloy elements are regulated to, by mass, 0.40 to 0.80% C, <=3.0% Si, 0.35 to 1.5% Mn, 0.10 to 3.0% Cr and 0.05 to 0.50% V, and the balance Fe with inevitable impurities to induction hardening in the temp. range of 950 to 1150 deg.C surface temp. and thereafter cooling the same in the temp. range from the heating temp. to 200 deg.C at >=50 deg.C/s rate. Moreover, this steel may contain one or >=two kinds among <=3.0% Ni, <=1.0% Mo, <=0.10% Nb, 0.0005 to 0.0050% B, <=0.20% S, <=0.20% Te and <=0.0050% Ca as well.

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 typified by SUJ2 and carburized steel typified by SCr420 and SCM420 have been used for rolling element parts or rolling parts which are required to have wear resistance and surface fatigue strength. Came. Carbon steel used for induction hardening generally has a shorter rolling life than bearing steel and carburized steel, and is unsuitable for parts that require high levels of wear resistance and surface fatigue strength.

However, there has recently been an increase in the number of rolling element parts that use the induction-hardened portion as it is as a bearing.
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 alloy elements, the present inventor has found that in order to improve the wear resistance and the contact pressure fatigue strength of induction hardened materials, a supersaturated solid solution is formed in the induction hardened layer. It was found that the above V worked extremely effectively. The reason for this is that V dissolved in supersaturation has a very large tempering softening resistance. The advantage of induction hardening for effective use of V is that it can be heated at a high temperature in a relatively short time, so that the solid solution of V in the matrix is almost completed without causing coarsening of austenite crystal grains. This is because, after heating, it is rapidly cooled with water or a solvent after heating, so that precipitation of V carbonitride in the cooling process can be suppressed, and V can be supersaturated as a solid solution in the matrix.

As a result of further studying the proper conditions for induction hardening, the surface temperature at which V is almost completely solid-dissolved is 950 ° C to 11
It was discovered that V carbonitride precipitation can be largely suppressed by cooling at a rate of 50 ° C./s or more in a temperature range of 50 ° C. and from a temperature after heating to 200 ° C.

That is, the induction hardened steel excellent in wear resistance and surface fatigue strength of the present invention has an alloying element content of mass%.
C: 0.40 to 0.80%, Mn: 0.35
1.5%, Si: 3.0% or less Cr: 0.10 to 3.0%, V: 0.05 to 0.5%
After high-frequency heating of the steel consisting of the balance Fe and unavoidable impurities in the temperature range of 950 to 1150 ° C., the temperature range from the heating temperature to 200 ° C. is 50 ° C. /
Steel for induction hardening, which has excellent high surface pressure fatigue strength and is characterized by cooling at a speed of s or more.

In addition to the above alloying elements, mass%
Then, Ni: 3.0% or less, Mo: 1.0% or less, Nb:
0.10% or less B: One or more of 0.0005 to 0.0050% can be contained.

In addition to the above alloying elements, mass%
And S: 0.20% or less, Te: 0.20% or less, C
a: One or more of 0.0050% or less may be included in the machinability improving element.

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 exceeds the eutectoid point of 0.80%, the surface hardness rather lowers, leading to deterioration of 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: 3.0% or less Si is added to impart tempering softening resistance to steel.
However, even if added excessively, not only the effect is saturated but also inconveniences such as impairing forgeability and machinability are brought about, so the upper limit of the Si content is made 3.0%.

Mn: 0.35 to 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 which plays an important role in the present invention. V is almost completely dissolved in austenite by high frequency heating and V is supersaturated in the matrix by rapid cooling. When dissolved in steel, it has the effect of significantly increasing the temper softening resistance of steel. 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%. Further, the induction hardening condition for exerting the effect of V is as follows: after the surface temperature of the material to be heated is induction heated in the temperature range of 950 to 1150 ° C., the heating temperature up to 200 ° C. is 50 ° C./s.
It is necessary to cool at the above speed.

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%
Hereinafter, Ca: 0.0050% or less, which is an element added when it is desired to particularly improve machinability,
S, Te and Ca are 0.20% or less, respectively.
Independently in the range of 20% or less and 0.0050% or less,
Alternatively, they may be added in combination. However, an upper limit was set because mechanical properties deteriorate if added more than this.

High-frequency heating in a temperature range of 950 to 1150 ° C. for the surface and cooling at a temperature range from the heating temperature to 200 ° C. at a rate of 50 ° C./s or more. The fact that the surface layer part is martensite-hardened by induction hardening and that a large compressive residual stress is introduced into the surface to obtain a part having excellent surface pressure fatigue strength is an effect of induction hardening that has been conventionally used.

In addition to this effect, it is necessary to pay attention to the following points in order to exert the effect of V in the present invention.
That is, in order to obtain the effect of increasing the tempering softening resistance of V, V carbonitride is almost completely dissolved in austenite by high frequency heating and V is supersaturated in the matrix by rapid cooling. . Therefore, the lower limit of the heating temperature for almost completely dissolving V carbonitride is 95
It was set to 0 ° C. The upper limit was set to 1150 ° C to prevent overheating. Further, in order to increase the supersaturated V, it is necessary to minimize the precipitation of V carbonitride during cooling. Therefore, it is necessary to cool the temperature range from the heating temperature to 200 ° C. at a speed of 50 ° C./s or more.

[0020]

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, surface grinding was performed to produce a rolling test piece having a test portion diameter of 12 mm. The rolling test is a radial type rolling fatigue test, and SUJ
The test was performed using a ball made of No. 2 at a surface pressure of 5880 MPa. The test results are shown in Table 3.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

As is apparent from Table 3, the present invention in which V was added and the surface temperature was raised to 950 to 1150 ° C. by high frequency heating, and then the temperature range up to 200 ° C. was cooled at a speed of 50 ° C./s or more It can be seen that Examples 1 to 12 of No. 1 show a remarkable improvement in the cumulative damage probability 10% (L ₁₀ ) in the rolling fatigue test.

On the other hand, Comparative Example 1 having a chemical component except V as compared with Example 4 has a short L ₁₀ life.

Although Comparative Examples 2 to 5 are the same component materials as in Example 3, Comparative Example 2 is a case where the high frequency heating temperature is 920 ° C., which is lower than the claimed range, and Comparative Example 3 is a case where it is high, 1200 ° C. In Comparative Example 2, the solid solution of V was insufficient, so in Comparative Example 3, L was caused by the coarsening of the structure due to overheating.
₁₀ Life is low.

Comparative Example 4 is a case in which the cooling rate is low due to oil cooling, although high frequency heating is used. V compared to proper cooling rate
Since the amount of carbonitrides deposited is large, the L ₁₀ life is short.

Comparative Example 5 is a material for quenching and tempering by heating in a furnace. It is considered that the L ₁₀ life is shortened because the solid solution of V carbonitride is incomplete and the compressive residual stress that suppresses the initiation and propagation of fatigue cracks is small in the quenched and tempered material.

Comparative Example 6 is an example of SCM420 carburized steel which has been conventionally used for rolling element parts. The L ₁₀ life is 35 × 10 ⁷ , and it can be seen that the invention steels of Examples 1 to 12 have the same or longer rolling life.

[0030]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide an induction hardening steel having a high surface pressure fatigue strength by selecting a proper alloying element and appropriate induction hardening conditions.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical indication C22C 38/60

Claims (3)

[Claims] 1. The content of alloy elements is% by mass, C: 0.40 to 0.80%, Si: 3.0% or less Mn: 0.35 to 1.5%, Cr: 0.10 to 3.0%, V: 0.05 to 0.50%, and a steel having a balance Fe and unavoidable impurities having a surface temperature of 950 to 1150.
After high-frequency heating in the temperature range of ℃, 200 from the heating temperature
Induction hardening steel with excellent high surface pressure fatigue strength, characterized by cooling the temperature range up to ℃ at a speed of 50 ℃ / s or more. 2. In addition to the alloy elements of claim 1, in mass%, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.10% or less B: 0.0005 to 0. The surface temperature of the steel containing one or more elements out of 0050% is 950 to 11
After high frequency heating in the temperature range of 50 ℃, from the heating temperature to 2
A steel for induction hardening exemplifying high surface pressure fatigue strength, which is characterized by cooling a temperature range up to 00 ° C at a rate of 50 ° C / s or more. 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 Surface temperature of steel containing two or more machinability improving elements is 950-11
After high frequency heating in the temperature range of 50 ℃, from the heating temperature to 2
Induction hardening steel with excellent high surface pressure fatigue strength, characterized by cooling the temperature range up to 00 ° C at a rate of 50 ° C / s or more.