JPH0790363A - Production of machine structural part excellent in mechanical strength - Google Patents

Abstract

PURPOSE:To produce a machine structural part further improved in mechanical properties such as bearing strength, bending fatigue strength and twisting fatigue strength. CONSTITUTION:A steel contg., by weight, 0.35 to 0.65% C, 0.03 to 1.50% Si, 0.3 to 1.0% Mn and 0.1 to 3.0% Cr, furthermore contg., at need, one or >=two kinds among 0.01 to 1.5% Al, 0.05 to 0.5% V and 0.05 to 0.5% Mo, similarly, 0.5 to 2.0% Ni, similarly, one or two kinds of 0.005 to 0.05% Ti and 0.01 to 0.10% Nb and, similarly, one or >=two kinds among 0.02 to 0.40% S, 0.01 to 0.50% Pb, 0.0003 to 0.010% Ca, 0.005 to 0.10% Te and 0.01 to 0.50% Bi, and the balance Fe with impuritiets is subjected to nitriding treatment at the Ac1 transformation point or above of the steel to <=950 deg.C, is cooled by air cooling or by using a refrigerant and is thereafter subjected to induction hardening.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a mechanical structural part used for obtaining a mechanical structural part having excellent mechanical strength such as surface pressure strength, bending fatigue strength, and torsional fatigue strength. .

[0002]

2. Description of the Related Art Conventionally, in order to improve mechanical strength such as surface pressure strength, bending fatigue strength and torsional fatigue strength of machine structural parts, it is often practiced to subject steel to surface hardening treatment. Surface hardening treatments such as carburizing, nitriding, and induction hardening are often adopted. (For surface hardening treatments for this type of steel, see, for example, "3rd Edition Iron and Steel Handbook, Volume VI Secondary Processing, Surface Treatment, and Heat Treatment."・ Welding ”Showa 5
May 31, 1995 Published by The Iron and Steel Institute of Japan, 56th edition
Pages 2 to 600, “14. Surface hardening ”for a detailed explanation. ).

In such a surface hardening treatment, for example, by using carburizing or carbonitriding treatment using a case-hardening steel having a C content as low as 0.13 to 0.23%, wear resistance and fatigue are improved. It is common to obtain mechanical structural parts with improved strength, but in addition to this, nitriding treatment (for example,
(Tufftride treatment, gas soft nitriding treatment, ion nitriding treatment)
Also induction hardening is often done.

Recently, a composite treatment of induction hardening after the nitriding treatment has been performed.

In this case, not only nitrogen diffused during nitriding but also carbon contained in steel contributes to induction hardening. And the martensite obtained by this is
It exhibits excellent temper softening resistance and cracking resistance, and significantly improves mechanical properties such as rolling life.

[0006]

By the way, in the case of the conventional nitriding treatment, in order to reduce the strain during the heat treatment as much as possible, the nitriding treatment is performed at a temperature lower than the Ac ₁ transformation point, which is lower than 590 ° C. In this case, long-time treatment of 3 to 10 hours was required for low temperature treatment. In the case of 590 ° C. or higher (Ac ₁ transformation point or higher), expansion / contraction occurs due to transformation, so the effect of low strain, which is the greatest advantage of the nitriding treatment, disappears, and it was not adopted.

Further, in the case of only induction hardening, there is a problem that the surface hardness is low, the rolling life is remarkably short, and the tempering softening resistance is also low. The challenge was to make it possible to further utilize the excellent tempering softening resistance and cracking resistance of the site by a short-time treatment.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and has excellent resistance to temper softening and cracking of martensite obtained by rapidly cooling austenite containing nitrogen and carbon. It is possible to make better use of the generated resistance by a short-time treatment, and to obtain mechanical structural parts that are more excellent in mechanical strength such as surface pressure strength (pitting fatigue resistance), bending fatigue strength, and torsional fatigue strength. Has an aim.

[0009]

The method of manufacturing a mechanical structural component according to the present invention is, by weight%, C: 0.35 to 0.65.
%, Si: 0.03 to 1.50%, Mn: 0.3 to 1.
0:%, Cr: 0.1-3.0%, and in some cases, Al: 0.01-1.5%, V: 0.05-
One or more of 0.5% and Mo: 0.05 to 0.5%, Ni: 0.5 to 2.0%, and Ti: 0.005 to 0.05% , Nb: 0.01 to
0.10% of 1 type or 2 types, similarly, S: 0.02
0.40%, Pb: 0.01 to 0.50%, Ca: 0.
0003-0.010%, Te: 0.005-0.10
%, Bi: 0.01 to 0.50%, 1 or 2
A steel containing at least seeds and the balance Fe and impurities is subjected to a nitriding treatment at a temperature not lower than the Ac ₁ transformation point of the steel and not higher than 950 ° C., and after cooling with air or a refrigerant, the nitrided layer becomes austenite. The feature is that the induction hardening is performed under the conditions.

Next, the reasons for limiting the chemical composition (% by weight) of steel applied in the method of manufacturing a mechanical structural part according to the present invention will be described.

C is a basic element that determines the strength of mechanical structural parts, but if the content is too small, the strength cannot be secured sufficiently, so C is set to 0.35% or more. However, if the C content is too large, the toughness may be deteriorated, so the content is made 0.65% or less.

Si is an element which is useful as a deoxidizing agent during the melting of steel, and is also useful for improving the resistance to temper softening and improving the rolling life. And 0.03% or more. But Si
If the content is too large, workability and toughness are deteriorated, so the content is made 1.50% or less.

Mn is an element that acts as a deoxidizing agent and a desulfurizing agent during the melting of steel, and is useful for improving induction hardenability. To obtain such an action, Mn is set to 0.3% or more. However, if the Mn content is too high, the toughness is deteriorated, so the content is made 1.0% or less.

Cr is an element useful for improving the nitriding property, particularly for increasing the nitriding depth, and also for improving the induction hardenability.
% And above. However, even if the Cr content is increased, the effect of improving the nitriding characteristics is saturated, so the content is made 3.0% or less.

All of Al, V and Mo are elements useful for improving nitriding characteristics, Al is an element particularly useful for improving surface hardness, and V is particularly useful for improving nitriding depth. Since it is an element that is also useful for improving the core hardness, and Mo is an element that is particularly useful for improving the nitriding depth and the induction hardenability, 0.01% or more for Al,
0.05% or more for V, 0.05 for Mo
One or two or more of% or more may be optionally contained. However, if the Al content is too large, the effect of improving the nitriding characteristics is saturated, so it is set to 1.5% or less, and if the V content is too large, the effect of improving the nitriding characteristics is saturated, so it is set to 0.5 or less. If the Mo content is too high, the effect of improving the hardenability is saturated and the machinability is deteriorated, so 0.5% or less is preferable.

Since Ni is an element that contributes to the improvement of the induction hardenability and the toughness, 0.5% or more may be contained in some cases. However, Ni
If the content is too large, the effect of improving the hardenability is saturated and the machinability is deteriorated. Therefore, the content is preferably 2.0% or less.

Since Ti and Nb are elements useful for refining the crystal grains and improving the nitriding characteristics, the Ti content is 0.
Depending on the case, 005% or more and 0.01% or more of Nb may be contained. However, Ti, Nb
If the content is too large, the toughness will decrease, so
0.05% or less for Ti, 0.1 for Nb
It is better to be 0% or less.

Since S, Pb, Ca, Te, and Bi are all elements that contribute to the improvement of machinability, in the case of mechanical structural parts which are required to have excellent machinability, these 1 It is also possible to add seeds or two or more kinds. In this case,
0.02% or more for S, 0.01 for Pb
%, Ca may be 0.0003% or more, Te may be 0.005% or more, and Bi may be 0.01% or more. However, if these contents are too high, strength (especially roller pitting strength)
As well as decreasing the toughness in the longitudinal (rolling) direction, S is 0.40% or less, Pb is 0.50% or less, and Ca is 0.010%.
Hereinafter, Te is preferably 0.10% or less and Bi is preferably 0.50% or less.

In the present invention, the steel having the above composition is subjected to a nitriding treatment at a temperature not lower than the Ac ₁ transformation point of the steel and not higher than 950 ° C., cooled by air or a refrigerant, and then induction hardened. The surface pressure fatigue strength (pitting fatigue resistance) is improved by obtaining martensite different from the martensite obtained by induction hardening ordinary carbon steel by nitriding and induction hardening.

In this case, martensite obtained by quenching austenite containing nitrogen and carbon with a refrigerant is superior in temper softening resistance to martensite obtained by quenching austenite consisting of carbon only. In addition to having excellent cracking resistance, it is possible to obtain a mechanical structural component with significantly improved surface pressure fatigue strength, bending fatigue strength, and torsional fatigue strength by performing a combined heat treatment of nitriding and induction hardening.

The nitriding temperature is said to be not lower than the Ac ₁ transformation point of steel and not higher than 950 ° C. This is more nitriding temperature Ac ₁ transformation point (e.g., 590 ° C. or higher) enables rapid nitriding is because the it is possible to greatly shorten the nitriding time, below Ac ₁ transformation point at the time of nitriding treatment N is a nitride of Cr, A ■, V, etc., or FeN ₄ (γ phase),
It exists in steel in the form of FeN ₂ (ε phase) and can dissolve only in the parent phase at a maximum of about 0.1%, but at the Ac ₁ transformation point or higher, it dissolves up to about 2.35%. It is possible to increase the amount of N used as N + C martensite during the induction hardening after the nitriding treatment time and to shorten the nitriding treatment time significantly.

However, if the temperature exceeds 950 ° C., the partial pressure of NH ₃ is increased and carburization becomes the main stream, so the temperature is set to 950 ° C. or less.

And as such soft nitriding treatment,
Gas nitriding treatment can be used, and in addition, nitriding treatment such as tufting treatment and ion nitriding treatment can be used.

Then, after the nitriding treatment at the Ac ₁ transformation point or more and 950 ° C. or less, the induction hardening treatment is performed.
Since induction hardening is a short-time heating, nitrogen diffusion hardly occurs during induction hardening. Therefore, it is necessary to obtain an appropriate pattern of the nitrogen diffusion layer before induction hardening. Moreover, nitriding is performed at a temperature lower than the Ac ₁ transformation point (for example,
When performed at a temperature lower than 590 ° C.), the diffusion depth is very shallow, and long-time treatment is required. However, when the Ac ₁ transformation point or higher (for example, 590 ° C. or higher), it is possible to obtain a deep nitrogen diffusion pattern in a short time, and thus it is easy to control an appropriate nitrogen diffusion layer pattern.

Further, the residual stress on the surface due to induction hardening also contributes to the improvement of surface pressure fatigue strength, bending fatigue strength and torsional fatigue strength.

By the way, the usual nitriding treatment is carried out at a temperature below 590 ° C., which is a temperature below the Ac ₁ transformation point, in order to reduce strain during heat treatment as much as possible. When the Ac ₁ transformation point or higher (590 ° C. or higher), expansion / contraction occurs due to transformation, and the effect of low strain, which is the greatest advantage of the nitriding treatment, disappears. However, in the present invention, since induction hardening is performed after the nitriding treatment to exceed the transformation point, it is not always necessary that the strain be low after the nitriding treatment.
Also, the higher the temperature, the higher the diffusion rate of nitrogen,
There is an advantage that the nitriding time can be significantly shortened.

[0027]

According to the method of manufacturing a mechanical structural component having excellent mechanical strength of the present invention, C: 0.35% by weight.
0.65%, Si: 0.03 to 1.50%, Mn: 0.
3 to 1.0%, Cr: 0.1 to 3.0%, and in some cases, Al: 0.01 to 1.5%, V: 0.
05-0.5%, Mo: 1 of 0.05-0.5%
Kind or two or more kinds, similarly, Ni: 0.5 to 2.0%,
Similarly, Ti: 0.005 to 0.05%, Nb: 0.0
1 to 0.10% of 1 type or 2 types, similarly, S: 0.0
2 to 0.40%, Pb: 0.01 to 0.50%, Ca:
0.0003 to 0.010%, Te: 0.005 to 0.
10%, Bi: 0.01 to 0.50% of one or two or more of the steel, and the balance Fe and impurities with respect to steel, at a temperature of Ac ₁ transformation point or more and 950 ° C. or less of the steel. After nitriding and cooling with air or refrigerant,
Particularly preferably, induction hardening is carried out under the condition that the nitrided layer becomes austenite. Therefore, nitrogen + carbon martensite obtained by rapidly cooling austenite containing nitrogen and carbon is formed in the surface layer mechanical structure. It becomes a part and has even better resistance to temper softening and cracking, and mechanical properties such as surface pressure strength (pitting fatigue resistance), bending fatigue strength and torsional fatigue strength that are even better. It becomes a structural part.

[0028]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were melted, forged to a diameter of 32 mm, and after normalizing,
As shown in FIG. 1, D ₁ = 26 mm, D ₂ = 22 mm,
A roller pitching test piece 1 having L ₁ = 28 mm, L ₂ = 51 mm, L ₃ = 130 mm was prepared, and as shown in FIG. 2, D ₅ = 8 mm, D ₆ = 15 mm, L ₅ = 5.
0 mm, L ₆ = 80 mm, L ₇ = 210 mm, R = 30
An Ono-type rotary bending test piece 2 having a size of mm was produced and used as a test material.

For each of the test materials except Comparative Example d-4, Table 5 and for the roller pitching test piece 1, the conditions and inventions shown in "Gas nitriding temperature" and "Treatment time" in Tables 3 and 4 Regarding the Ono-type rotary bending test piece 2 of Example D and Comparative Example d-3, as shown in FIG.
By performing the gas nitriding treatment under the condition of ° C x 2 hours, the values shown in the columns of "surface hardness after nitriding" and "nitriding layer depth" of Table 3 and Table 4 were obtained for the roller pitching test piece 1. A nitriding layer was obtained.

Subsequently, among the test materials after the nitriding treatment, with respect to the test materials other than Comparative Example d-3, Table 6 (high frequency was used for the roller pitting test piece 1 and the Ono-type rotary bending test piece 2). The induction hardening is performed under the conditions shown in (4), and the roller pitching test piece 1 has a "surface hardness after induction hardening" and a "nitriding depth after induction hardening" in Table 3 and Table 4 by performing induction hardening. "," And "hardened layer depth after induction hardening" are obtained, and the Ono-type rotary bending test piece 2 is hardened with the nitriding depth after induction hardening shown in FIG. Layers were obtained.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

Next, for each roller pitching test piece 1, a roller pitching test was carried out under the conditions of small roller (test piece): diameter 26 mm, large roller (counterpart material): diameter 130 mm, slip ratio: 40%, rotation speed: 1580 rpm. When carried out, similarly, the results shown in the columns of "pitching life" in Tables 3 and 4 were obtained.

As a result, in each of Examples A to L of the present invention, the pitching life was more than 10 ⁷ times, and had excellent surface pressure fatigue strength (pitting fatigue strength).

On the other hand, in the case of Comparative Example d-1 in which the nitriding treatment time is short, in the case of Comparative Example d-2 in which the nitriding treatment temperature is high, in Comparative Example d-3 in which only the nitriding treatment is performed and the induction hardening treatment is not performed. In the case of Comparative Example d-4 in which the nitriding treatment was not performed and only the induction hardening treatment was performed, the pitching life was short and the contact pressure fatigue strength was low.

On the other hand, the invention sample D and the comparative samples d-3 and d
For Ono-type rotary bending test piece 2 made of -4 steel type,
When the Ono-type rotary bending fatigue test was performed under the condition of the number of revolutions: 3500 rpm, the SN diagram shown in FIG. 4 was obtained.

As a result, the invention sample D had excellent bending fatigue strength, while the comparative sample d-
In the case of 3 and d-4, the bending fatigue strength was inferior.

[0042]

INDUSTRIAL APPLICABILITY In the method for manufacturing a mechanical structural component according to the present invention, a steel having a specific composition is subjected to a nitriding treatment at a temperature not lower than the Ac ₁ transformation point of the steel and not higher than 950 ° C. and cooled with air or a refrigerant. After that, since induction hardening is performed, it is possible to further utilize the excellent temper softening resistance and cracking resistance of martensite obtained by rapidly cooling austenite containing nitrogen and carbon, It is possible to provide a mechanical structural component having excellent mechanical properties such as surface pressure strength (pitting fatigue strength), bending fatigue strength, and torsional fatigue strength, which is a remarkably excellent effect.

[Brief description of drawings]

FIG. 1 is an explanatory view showing the shape of a roller pitching test piece.

FIG. 2 is an explanatory view showing the shape of an Ono-type rotary bending test piece.

FIG. 3 is a graph illustrating the influence of gas nitriding temperature on the nitriding depth after induction hardening.

FIG. 4 is a graph illustrating an SN diagram of an Ono-type rotary bending fatigue test piece using Inventive Example D steel and Comparative Examples d-3 and d-4 steel.

Claims (5)

[Claims] 1. C: 0.35 to 0.65% by weight,
Si: 0.03 to 1.50%, Mn: 0.3 to 1.0
%, Cr: 0.1 to 3.0%, balance Fe and impurities with respect to steel, the Ac ₁ transformation point of the steel or higher and 950 ° C.
A method for producing a mechanical structural component having excellent mechanical strength, which comprises performing nitriding treatment at the following temperature, cooling with air or a refrigerant, and then performing induction hardening. 2. In the steel, Al: 0.01 to 1.5%,
The production of a mechanical structural component excellent in mechanical strength according to claim 1, which contains one or more of V: 0.05 to 0.5% and Mo: 0.05 to 0.5%. Method. 3. The method for producing a mechanical structural component excellent in mechanical strength according to claim 1, wherein the steel contains Ni: 0.5 to 2.0%. 4. Ti: 0.005 to 0.05 in steel
%, Nb: 1 to 2 out of 0.01 to 0.10% or 2
The method for producing a mechanical structural component having excellent mechanical strength according to claim 1, further comprising a seed. 5. S: 0.02 to 0.40%, P in steel
b: 0.01 to 0.50%, Ca: 0.0003 to 0.
010%, Te: 0.005 to 0.10%, Bi: 0.
The method for producing a mechanical structural component excellent in mechanical strength according to any one of claims 1 to 4, containing one or more of 01 to 0.50%.