JPH0881733A - High strength steel for induction hardening excellent in machinability - Google Patents

Abstract

PURPOSE: To produce a steel for induction hardening excellent in machinability and excellent in static strength, bending fatigue strength and rolling contact fatigue strength by regulating the compsn. of a steel to the specified one in which the content of C is higher than that in a carbon steel of S45C to S55C, the contents of Mn and Si are reduced and B is added. CONSTITUTION: This high strength steel for induction hardening has a compsn. contg., by mass, 0.50 to 0.80% C, <=0.15% Si, <=0.60%. Mn, 0.0005 to 0.0050% B and <=0.05% Ti, furthermore contg., at need, <=2.0% Cr, <=3.0% Ni, <=1.0% Mo, <=0.10% Nb and 0.05 to 0.50% V, and the balance Fe. Moreover, this steel may contain one or more kinds of machinability improving elements among <=0.20% S, <=0.20% Te and <=0.0050% Ca as well. By the same C content, its static strength, bending fatigue strength and rolling contact fatigue strength can be improved, by the reduction of the contents of Mn and Si, its machinability can be improved, and by the addition of B, deterioration in its hardenability caused by the reduction of the Mn and Si contents can be covered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to machine structural parts such as speed change gears, rolling elements for continuously variable transmissions, constant-velocity joint outer races, and other materials used for machine structural parts after induction hardening. The present invention relates to a high-strength induction hardening steel having excellent static properties, static strength, bending fatigue strength, and rolling contact fatigue strength.

[0002]

2. Description of the Related Art Among the carbon steels for machine structure, medium carbon steels such as S45C to S55C have been surface hardened by induction hardening to obtain bending fatigue strength, rolling contact fatigue strength and static strength. It was However, the carbon steel used for induction hardening is generally low in surface hardness due to the low C content compared with bearing steel and carburized bearing steel, and is suitable for parts that require high rolling contact fatigue strength and static strength. Was inappropriate.

In order to improve the rolling contact fatigue strength and static strength of the induction hardened material, it is most effective to increase the amount of C which increases the surface hardness after induction hardening.
If the amount is high, the hardness in the raw material state is increased, and the machinability and workability are impaired.

Further, in order to improve machinability and workability, it is possible to reduce Mn and Si which are solid solution strengthening elements, but in this case, the induction hardenability is lowered and the required hardened layer depth is reduced. There is a problem that bending fatigue strength and static strength deteriorate because it cannot be secured. As described above, machinability and strength are contradictory, and it has been difficult to obtain a steel having both machinability and strength by the conventional techniques.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide excellent machinability, static strength, bending fatigue strength and rolling. The present invention relates to high strength induction hardening steel having excellent contact fatigue strength.

[0006]

Means for Solving the Problems As a result of studying combinations of various alloying elements, the present inventor has found that the C content in the usual S45C to S55C is improved in order to improve static strength, bending fatigue strength and rolling contact fatigue strength. 0.50 higher than carbon steel
% Or more. Further, in order to improve machinability, the amounts of Mn and Si, which are solid solution strengthening elements, were reduced. in this case,
Induction hardenability is reduced by reducing Mn and Si, but the induction hardenability is ensured by adding B, which is a hardenability improving element, to supplement the decrease in hardenability by reducing the amount of Mn and Si. did. B is said to be ineffective in improving the hardenability in the high C region, but in the case of rapid heating such as induction hardening, the effect of improving the hardenability is higher than that in the case of furnace heating.
It has been found that the hardenability of B is high even if the amount of C is at least%.

That is, according to the present invention, the high-strength induction hardening steel excellent in machinability, the content of alloying elements in mass%, C:
0.50 to 0.80%, Si: 0.15% or less, Mn:
0.60% or less, B: 0.0005 to 0.0050
%, Ti: 0.05% or less, and the balance is Fe and inevitable impurities.

In addition to the above alloy elements, mass%
Then, Cr: 2.0% or less, Ni: 3.0% or less, Mo:
1.0% or less, Nb: 0.10% or less, V: 0.05
~ 0.50% Of these, one or more can be included.

In addition to the above alloying elements, S: 0.20% or less, Te: 0.20% or less, Ca:
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: 0.50 to 0.80% C is an essential element for maintaining the strength of the steel after induction hardening. It secures the surface hardness after induction hardening, and the static strength and bending fatigue strength and It is necessary to add 0.50% or more in order to improve the rolling contact fatigue strength. However, if the content exceeds the eutectoid point of 0.80%, the surface hardness rather lowers, leading to deterioration in strength improvement. Further, not only does proeutectoid cementite generate to impair the toughness, but it also causes adverse effects such as increasing the material hardness in the raw material state and impairing machinability. Therefore, the upper limit of the C content is 0.80.
%.

Si: 0.15% or less Si is an element that acts as a deoxidizing agent during melting, but if it is contained as a normal deoxidizing agent, the hardness of the material is increased and machinability is improved. Since it causes deterioration, it is specified to be 0.15% or less.

Mn: 0.60% or less, Mn is an element that acts as a desulfurizing agent during melting and is an element that improves the induction hardenability, but it is necessary to obtain sufficient hardenability. If added in an amount, since it is a solid solution strengthening element, it increases the hardness of the material and deteriorates machinability and workability. Therefore, the content ratio of Mn is set to 0.60% or less.
The lack of induction hardenability due to a decrease in Mn is complemented by the addition of B.

B: 0.0005 to 0.0050% B is an element that deepens the integrity of the hardened layer without increasing the hardness of the material. By adding B, it is possible to secure the induction hardenability that complements the deterioration of the hardenability due to the reduction of Mn.
It is said that the hardenability effect of B usually disappears at the eutectoid point of 0.80%, but in the case of induction hardening, the distribution of C tends to become non-uniform because of rapid heating, and it is partially in the matrix. Since the amount of C is decreased, the B effect is high even in a high C material. The most stable B effect of 0.00
Addition of 05% or more is required. However, even if added excessively, the effect will rather decrease, so the upper limit is 0.005.
It was limited to 0% or less.

Ti: 0.05% or less Ti is an element necessary for fixing N in steel. T
The production of the iN compound suppresses the production of the BN compound,
The effect of improving the hardenability of B can be secured. However, if it is too large, the toughness is lowered, so the content is limited to 0.05% or less. Further, the desirable addition amount of Ti is Ti / N> 3.4.

V: 0.05 to 0.50% V has the effect of refining the crystal grain boundaries of steel, and further has the effect of 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%.

Cr: 2.0% or less, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.10% or less, C
r, Ni, Mo and Nb increase the toughness of the hardened layer portion,
Since it is an element that deepens the depth of the hardened layer, it may be added individually or in combination at 2.0% or less, 3.0% or less, 1.0% or less and 0.10% or less, respectively.

S: 0.20% or less, Te: 0.20%
Hereinafter, Ca: 0.0050% or less is an element added when it is desired to particularly improve machinability, and is 0.20% or less, 0.20% or less, and 0.00, respectively.
They may be added alone or in combination within a range of 50% or less. However, an upper limit was set because mechanical properties deteriorate if added more than this.

[0018]

【Example】

Each steel material having the chemical composition shown in Table 1 was melted for 2 tons and rolled into a steel bar having a diameter of 32 mm and a diameter of 60 mm. While measuring the hardness of the 1 / 2R part of this steel bar,
A rolling test, a machinability test and an induction hardenability test were conducted under the following test conditions.

A radial type rolling contact fatigue test piece having a test portion diameter of 12.3 mm was carved out, frequency: 100 kHz, method:
Stationary quenching, heating time: 2.5 s, power: 50 kW, maximum heating temperature: 980 ° C, cooling water: water, tempering: 160 ° C
Induction hardening and tempering treatment was performed under the condition of × 1 hour. Further, as a comparison of induction hardening, only Comparative Example 12 was carburized, quenched and tempered under the conditions that the C potential was 0.8% and the depth of the hardened layer was 1.0 mm. After the treatment, the surface was ground and subjected to a rolling fatigue test. The rolling test is a radial type rolling fatigue test, and the surface pressure is 5880M using SUJ2 balls.
The test was performed at Pa.

The machinability test was carried out by a turning test. After turning each steel material having a diameter of 70 mm into a diameter of 60 mm, a turning test under the following conditions was performed. Test conditions are tool: P10, shape:
TNP331, speed: 150m / min, depth of cut: 2m
m / rev, lubricating oil: None. The tool life is shown as a relative value when the flank wear reaches 0.2 mm and the tool life of the steel material of Comparative Example 1 is 1.0.

Induction hardenability has a diameter of 25 mm and a length of 100
A mm bar test piece was processed, induction hardened under the conditions of frequency: 10 kHz, power: 55 kW, heating time: 4 s, and the distance from the surface to the hardness of 500 VV in Vickers hardness was measured.

The above test results are shown collectively in Table 2.

[0024]

[Table 1]

[0025]

[Table 2]

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

On the other hand, in Comparative Example 1, the conventional steel JIS-
S55C, which is inferior to Examples 1 to 12 in rolling fatigue characteristics, machinability, and induction hardenability.

Comparative Example 2 is a conventional steel JIS-S70C, which has a high C content but has excellent rolling fatigue characteristics.
High hardness and poor machinability.

Comparative Example 3 is an example in which B is not added,
The induction hardenability is insufficient. Therefore, B ₁₀ life is also shortened.

Comparative Example 4 is a representative case hardening steel JIS-SC.
It is M420. It is understood that Examples 1 to 12 have almost the same rolling contact fatigue characteristics and machinability as case-hardening steel.

Further, a gear fatigue test was carried out. As the test material, Example 1 was used as an induction hardened material and Comparative Example 4 was used as a carburized and hardened material of a comparative material. The test gear was a combination of two types of gears having a reference pitch circle Φ80 mm, the number of teeth 32 and the reference pitch circle Φ70 mm, and the number of teeth 28. Both gears have a module of 2.5 and a pressure angle of 20 °. Induction hardening is frequency: 150kHz, power: 400k
W, heating time: 0.2 s, the hardened layer depth was 1 mm, and various conditions were adjusted so that contour hardening along the tooth profile was achieved. The carburizing and quenching conditions were also set so that the depth of the hardened layer was 1 mm. The results are shown in Fig. 1. It was confirmed that the steel of the present invention, Example 1, has a fatigue strength equal to or higher than that of the carburized material comparative example 4.

[0032]

According to the present invention, it is possible to provide a high-strength induction hardening steel excellent in machinability by selecting a unique alloy element.

[Brief description of drawings]

 FIG. 1 is a relationship diagram showing load stress and fatigue characteristics of gears.

Claims (3)

[Claims] 1. The alloy element content is mass%, C: 0.50 to 0.80%, Si: 0.15% or less, Mn: 0.60% or less, B: 0.0005 to 0. 0050%, Ti: 0.05% or less, high-strength induction hardening steel with excellent machinability, characterized by the balance Fe and unavoidable impurities. 2. In addition to the alloy element according to claim 1, in mass%, Cr: 2.0% or less, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.10% or less. , V: 0.05 to 0.50% of high-strength induction hardening steel with excellent machinability, characterized by containing one or more elements, and the balance being Fe and unavoidable impurities. . 3. In addition to the alloy elements according to claims 1 and 2,
In mass%, S: 0.20% or less, Te: 0.20% or less, Ca: 0.0050% or less, including one or more kinds of machinability improving elements, and the balance Fe and unavoidable impurities. High-strength induction hardening steel with excellent machinability.