JP3109146B2 - Manufacturing method of low strain high strength member - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing mechanical structural parts such as gears and shafts, which are required to have excellent wear resistance and fatigue strength, with low distortion and high strength. The present invention relates to a method for producing a low-strain and high-strength member used in the above.

[0002]

2. Description of the Related Art It is desired that components for various mechanical structures such as those for automobiles and machine tools have excellent wear resistance and fatigue strength, especially for gears and shafts.

[0003] In such mechanical structural parts, various surface hardening treatments are often performed as means for improving the wear resistance and fatigue strength.

[0004] Carburizing and quenching are the most widely used in such surface hardening treatments. For such carburizing and quenching, for example, JI
Steel suitable for sintering such as SSCr420, SCM420, SNCM420 is used as a material.

However, in such carburizing and quenching, the carburizing and diffusion treatment is performed in the austenite region at or above the A ₁ transformation point, and thereafter the martensitic transformation is caused by oil quenching. There was a problem that it was easy to occur.

Therefore, in order to prevent distortion of the mechanical structure component due to such heat treatment deformation, a surface hardening treatment at a low temperature without passing through a transformation point, for example, nitriding or nitrocarburizing treatment is performed. Sometimes.

[0007]

However, such a nitriding or nitrocarburizing treatment is usually performed at a temperature of 500 to 600 ° C. below the A ₁ transformation point, so that the strength is improved by passing through the transformation point. However, there is a problem that the strength of the core tends to be reduced, and it is difficult to apply to a component for a machine structure having a sufficient fatigue strength, and it has been a problem to solve such a problem. Was.

[0008]

The present invention is an object of the invention, there is also one has been made in view of the conventional problems described above, the wear resistance without the occurrence of distortion by performing a surface hardening treatment of the low temperature which does not pass through the A ₁ transformation point Can be improved,
It is an object of the present invention to provide a method for manufacturing a low-strain high-strength member capable of obtaining a mechanical structural part having excellent fatigue strength by improving the strength of a core.

[0009]

According to the present invention, there is provided a method for producing a low-strain high-strength member according to the present invention.
25%, Si: 0.50% or less, Mn: 0.50-1.
50%, Cr: 0.5-2.0%, Mo: 0.1-1.
0%, V: 0.05 to 0.50%, Al: 0.50% or less, B: 0.0040 to 0.0200%, and N: 0 when further improvement in machinability is desired. .005-
Steel containing 0.025%, the balance being Fe and impurities, 900 to 1 lower than the normal hot forging temperature
After forging at a temperature of 050 ° C., machining is performed on the member shape, then nitrocarburizing treatment is performed at 550 to 600 ° C., and during the nitrocarburizing treatment, carbides of Mo and V are precipitated and secondarily cured, It is characterized in that the core hardness is Hv304 or more, and as a means for solving the above-mentioned conventional problems with the configuration of the invention relating to the method for manufacturing a low-strain high-strength member as described above. I have.

Next, the chemical composition of the steel material applied in the method for manufacturing a low-strain and high-strength member according to the present invention and the reason for limiting the manufacturing process of the member using the steel material will be described.

C: 0.10 to 0.25% C is an element effective for securing the core hardness of the member for mechanical structure.
% Or more. However, as the C content increases and the core hardness increases, the machinability decreases.
It was as follows.

Si: 0.50% or less Si is an element that is desirably added as appropriate in order to improve the hardenability during cooling after forging, but if it is too much, the soft nitriding property is deteriorated. Therefore, it was set to 0.50% or less.

Mn: 0.50 to 1.50% Mn is preferably contained in an amount of 0.50% or more in order to improve the hardenability during cooling after forging. Therefore, the content is reduced to 1.50% or less.

Cr: 0.5-2.0%, Cr is an element effective for improving hardenability and securing core hardness and effective for improving soft nitriding property. Therefore, in order to obtain these effects, the content is set to 0.5% or more. However, if a large amount is contained, the core hardness becomes too high, and the machinability is reduced.
It was as follows.

Mo: 0.1 to 1.0% Mo improves the hardenability and improves the nitrocarburizing property. Further, Mo precipitates as a carbide Mo ₂ C during the nitrocarburizing treatment, so that it is secondarily hardened. Therefore, the content is 0.1% or more because it is an element effective for increasing the core hardness and improving the fatigue strength of the machine structural member. However, even if the content increases, the effect saturates, rather, the hardness increases too much and the machinability decreases, and the primary carbides precipitate during melting and the toughness in the final product deteriorates. Therefore, 1.
0% or less.

V: 0.05 to 0.50% V improves nitrocarburizing property and carbides V ₄ C ₃ during nitrocarburizing treatment.
The element is effective for increasing the core hardness and improving the fatigue strength of the machine structural member by secondary hardening by precipitating as an element.
However, even if the content is increased, it does not sufficiently form a solid solution at a temperature of 900 to 1050 ° C. and does not contribute to precipitation hardening.

Al: 0.50% or less Al is an element effective for securing soft nitriding properties, and thus can be added as appropriate.
Desirably, it is 0% or more. However, if the content is too large, the toughness deteriorates.

B: 0.0040 to 0.0200% N: 0.005 to 0.025% B and N form compound BN to improve the machinability of the steel material, and are formed into a member shape by machining. In the case where it is further desired to have excellent machinability, the content of B is 0.0040% or more, and the content of N is 0.005% or more. Can be done. However, the upper limit of the N content is 0.025% from the viewpoint of productivity of steel, and the upper limit of the B content corresponding to the upper limit of the N content is 0.0200%, and the B content is higher than this. In some cases, B alone is present, making the hardenability unstable, which is not preferable.

Temperature of steel material before forging: 900 to 1050 ° C. The temperature before forging of a steel material having the above-mentioned chemical composition is 900 to 1050 ° C. lower than a normal hot forging temperature.
However, in this case, if the temperature is lower than 900 ° C., the subsequent forging workability is reduced, and the life of the forging die is shortened.
In order to make o and V form a solid solution, the temperature was set to 900 ° C. or higher. However, when the temperature increases, it becomes impossible to suppress the coarsening of the crystal grains, and it is necessary to perform a solution treatment after the forging, so that the coarsening of the crystal grains is suppressed and the fine grain structure is reduced after the forging. 105 to improve toughness.
0 ° C. or less.

The nitrocarburizing temperature: 550 to 600 ° C. The nitrocarburizing treatment to be performed after forging and machining to the shape of the member is performed by the nitrocarburizing property and the M
5 when considering secondary hardening due to precipitation of o, V carbides
Since it is suitable to carry out in the range of 50 to 600 ° C., such a soft nitriding temperature was used.

[0021]

In the method for producing a low-strain high-strength member according to the present invention, the C content is determined in consideration of the core hardness, and Si, Mn, Cr, and C are determined in consideration of the hardenability after forging. The content of Mo is determined, the content of Cr, Mo, V, and Al is determined in consideration of the nitrocarburizing property, and the content of Mo,
The content of V is determined, and when a further improvement in machinability is required, a steel to which a machinability improving element is added is used as a material, and after a temperature of 900 to 1050 ° C., for example, a member shape is formed. The forging process is performed to a close shape, and after performing machining such as cutting to the member shape, the nitrocarburizing process is performed at 550 to 600 ° C. As a result, the solution treatment after forging can be omitted, and the wear resistance is improved by increasing the surface hardness by performing nitrocarburizing treatment after machining the member shape. It will be.

Further, Mo ₂ C or V ₄ C
₃ such as carbide precipitates eccentric part hardness by curing secondarily is increased become sufficient fatigue strength can be obtained, the surface hardening treatment is a heat treatment since it is a temperature lower than the A ₁ transformation point It becomes a component for a mechanical structure made of a low-distortion high-strength member having little wear, excellent in wear resistance and having stable fatigue strength with little distortion.

[0023]

Embodiment No. 1 in Table 1 A steel having a chemical composition satisfying the present invention shown in 1 to 6 is melted, then rolled to a diameter of 70 mm, heated to a pre-forging heating temperature shown in Table 2, and then forged to form a shape close to a gear. Into a gear shape by performing machining (cutting),
Next, a gas soft nitriding treatment at 580 ° C. × 5 h was performed to obtain a gear having a module of 1.75, a number of teeth of 34, and an inner diameter of 20 mm.

For comparison, No. 1 in Table 1 was used. Using a normalizing material of SCM420 steel having the chemical composition shown in Fig. 7, carburizing at 910 ° C × 3.5h → diffusion of 2h → 830 ° C × quenching by oil cooling after heating for 30 minutes → 160 → A gear having the same specifications as in the above example was obtained through a tempering step at 2 ° C. × 2 h.

The surface hardness, core hardness, and hardened layer depth at which Hv550 was obtained for each of the gears obtained in the above steps were measured. The results are shown in Table 2.

In addition, gas soft nitriding (No.
1 to 6) and carburizing (in the case of Comparative Example No. 7)
In order to investigate the dimensional change before and after the treatment, the dimensional change of the inner diameter (average of n = 25) and the dimensional change of the tooth thickness (the tooth thickness of five teeth) (average of n = 25) were measured. The results are also shown in Table 2.

Further, when the fatigue strength was measured by performing a power circulation type fatigue test, the results are also shown in Table 2.

[0028]

[Table 1]

[0029]

[Table 2]

As is clear from the results shown in Tables 1 and 2, Example No. 1 of the present invention. The dimensional change of the gears according to Comparative Examples Nos. 7, which is considerably smaller than the dimensional change of the carburized and quenched gear due to the fact that the distortion during surface hardening treatment is extremely small and low distortion. It has good fatigue strength comparable to carburized and quenched gears, has significantly lower strain than conventional carburized and quenched gears, and also has higher fatigue strength than conventional nitrocarburized gears Excellent gears were obtained.

Further, Example N of the present invention shown in Table 1
o. No. 1 to 6 forged material having a diameter of 90 mm (heated at 1050 ° C.) and Comparative Example No. A normalizing material having a diameter of 90 mm having a component composition of No. 7 was used as a test material, and the machinability (tool life; tool abrasion was 0.
Table 4 shows the results of evaluation of the cutting length when it becomes 3 mm.
The results are shown in FIG.

[0032]

[Table 3]

[0033]

[Table 4]

As is clear from the results shown in Tables 3 and 4, Example No. 1 of the present invention. In the steels Nos. 1 to 4, the tool life was good in spite of the large material hardness, and in Example No. 4 in which B and N were contained in appropriate amounts in the steel. 5,6
In the steel of Comparative Example No. Although the hardness of the forged material was considerably larger than that of the normalized material of No. 7, the tool life when the forged material was processed was compared with that of Comparative Example No. 7. It was confirmed that the sample had good machinability that exceeded the tool life when the normalized material No. 7 was processed.

[0035]

According to the manufacturing method of low distortion and high strength member according to the present invention, since it is obtained by the configuration described above, A ₁
By applying a low-temperature surface hardening treatment that does not pass through the transformation point, it is possible to improve the wear resistance without generating distortion, and also to improve the core strength during the surface hardening treatment, resulting in excellent fatigue strength A remarkably excellent effect that it becomes possible to manufacture a machine structural part having

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C23C 8/00-8/54

Claims (2)

(57) [Claims] (1) C: 0.10 to 0.25% by weight,
Si: 0.50% or less, Mn: 0.50 to 1.50%,
Cr: 0.5 to 2.0%, Mo: 0.1 to 1.0%,
V: 0.05 to 0.50%, Al: 0.50% or less, steel containing the balance of Fe and impurities is used as a material, and forging is performed at a temperature of 900 to 1050 ° C. lower than a normal hot forging temperature. After performing machining to the member shape, 5
A nitrocarburizing treatment is performed at 50 to 600 ° C.
A method for producing a low-strain, high-strength member, wherein carbides of o and V are precipitated and secondarily hardened to have a core hardness of Hv304 or more. 2. C: 0.10 to 0.25% by weight,
Si: 0.50% or less, Mn: 0.50 to 1.50%,
Cr: 0.5 to 2.0%, Mo: 0.1 to 1.0%,
V: 0.05 to 0.50%, Al: 0.50% or less,
B: 0.0040 to 0.0200%, N: 0.005 to
Steel made of 0.025%, balance Fe and impurities, 900 to 1050 lower than normal hot forging temperature
Temperature, then forging, machining into the shape of the member, nitrocarburizing at 550-600 ° C, and precipitating Mo and V carbides during nitrocarburizing and secondary hardening. A method for producing a low-strain, high-strength member, wherein the part hardness is Hv304 or more.