JP3381738B2 - Manufacturing method of mechanical structural parts with excellent mechanical strength - Google Patents

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]

[0007]

In the case of induction hardening alone, there is a problem that the surface hardness is low, the rolling life is extremely short, and the temper softening resistance is also low. Therefore, austenite containing nitrogen and carbon is rapidly cooled. The problem was to make it possible to further utilize the excellent tempering softening resistance and cracking resistance of the martensite obtained as described above 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 condition that a nitriding layer is austenized after nitriding at a temperature not lower than the Ac1 transformation point of the steel and not higher than 950 ° C. for steel containing the balance of Fe and impurities containing at least one kind and then cooling with air or a refrigerant. It is characterized in that induction hardening is carried out at a pitching life of more than 10 ⁷ times.

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 Ac1 transformation point of the steel and not higher than 950 ° C., cooled by air or a refrigerant, and then induction hardened. Therefore, by obtaining martensite different from martensite obtained by induction hardening ordinary carbon steel by nitriding and induction hardening, the contact pressure fatigue strength (pitching fatigue resistance) is improved and the pitching life is 10 ⁷ times. To be super.

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 Ac1 transformation point of steel and not higher than 950 ° C. This is because when the nitriding temperature is the Ac1 transformation point or higher (for example, 590 ° C. or higher), rapid nitriding can be performed, and the nitriding time can be significantly shortened. Nitride with Cr, Al, V, etc. or FeN4 (γ phase),
It exists in the steel in the form of FeN2 (ε phase) and can dissolve only in the parent phase at a maximum of about 0.1%, but at the Ac1 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 in the induction hardening after the 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.

Further, the higher the temperature is, the higher the diffusion rate of nitrogen becomes, and the nitriding time can be greatly 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 steels, and the balance Fe and impurities with respect to steel, nitriding at a temperature of Ac1 transformation point of the steel or more and 950 ℃ or less After processing and cooling with air or a refrigerant,
Induction hardening was performed under the conditions that the nitrided layer became austenite, and the pitting life was set to more than 10 ⁷ times. Therefore, nitrogen + carbon martensite obtained by quenching austenite containing nitrogen and carbon was formed in the surface layer portion. It becomes a machine structural part that has better tempering softening resistance and cracking resistance, and it has more mechanical properties such as surface pressure strength (pitching fatigue resistance), bending fatigue strength and torsional fatigue strength. It becomes an excellent machine structural component.

[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, after nitriding the steel of the specific component at a temperature not lower than the Ac1 transformation point of the steel and not higher than 950 ° C., and after cooling with air or a refrigerant. Since induction hardening was performed and the pitting life was over 10 ⁷ times, the excellent tempering softening resistance and cracking resistance of martensite obtained by quenching austenite containing nitrogen and carbon were further utilized. 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. .

[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.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/60 C22C 38/60 (56) References JP-A-2-232353 (JP, A) JP-A-59-50158 (JP , A) JP-A-56-84416 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 1 / 06,6 / 00 C23C 8/26 C22C 38/00-38/60

Claims (5)

(57) [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%, the balance Fe and impurities relative to the steel, Ac1 transformation point of the steel or more and 950 ℃
Nitriding is performed at the following temperatures, and after cooling with air or a refrigerant, induction hardening is performed to obtain a pitching life of 10
^A method for manufacturing a mechanical structural component excellent in mechanical strength, characterized by being over ⁷ times. 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%.