JP2885061B2 - Method for producing nitrided steel member excellent in fatigue characteristics - 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 producing a nitrided steel member having excellent fatigue characteristics, which is suitable for mechanical structural parts such as gears requiring excellent fatigue strength.

[0002]

2. Description of the Related Art As one of surface hardening treatment methods for improving the fatigue strength of steel for machine structural use, there is a gas nitriding treatment. This gas nitriding treatment is an effective means as a surface hardening method for mechanical structural parts requiring dimensional accuracy because the heat treatment distortion is smaller than that of induction hardening or carburizing quenching which is the same surface hardening method.

[0003] Conventionally, JIS has been used as a steel material for nitriding.
SACM645, JIS SCM435 and the like are used. However, SACM645 has a surface hardness of Hv1.
In addition to a hardness as high as about 000, a brittle compound layer is formed on the surface, so that high fatigue strength cannot be obtained. Also, SCM4
In the case of No. 35, too, a sufficient hardened layer depth cannot be obtained, so that high fatigue strength cannot be obtained.

In order to improve the fatigue strength, Japanese Patent Application Laid-Open Nos. 4-45244, 4-66646, 5-25538, and the like disclose a Cr-M
High fatigue strength steel for nitriding or soft nitrided steel based on oV steel has been proposed. JP-A-63-93821 discloses a method of performing shot peening after nitriding, and JP-A-2-204463 discloses a method of performing two-stage nitriding by ion nitriding. I have.

[0005]

However, the nitriding steels disclosed in JP-A-4-45244 and JP-A-4-66646 have a hardened layer depth due to the combined addition of Cr, Mo and V. Although it becomes large, the fatigue strength is still insufficient because the Al content is low and the surface hardness after nitriding is low.

[0006] Further, the steel for nitrocarburizing disclosed in Japanese Patent Application Laid-Open No. 5-25538 has a high surface hardness after nitrocarburizing treatment since it contains 0.5% or less of Al and has a high fatigue strength. Is also higher. However, this technology employs the conventional nitrocarburizing treatment at a constant temperature, which does not provide a sufficient hardened layer depth and forms a thick compound layer on the surface layer. The fatigue strength is inferior to the material.

Further, the method disclosed in Japanese Patent Application Laid-Open No. 63-93821 needs to add another step called shot peening, and the method disclosed in Japanese Patent Application Laid-Open No. 2-294463 is expensive. It is necessary to use an appropriate method, and there is a problem that all of these methods lead to an increase in cost.

The present invention has been made in view of the above circumstances, and provides a method of manufacturing a nitrided steel member having excellent fatigue strength comparable to carburizing and quenching without causing a large increase in cost. Aim.

[0009]

Means and Action for Solving the Problems The present inventors have studied in detail the effects of chemical components, compositions and nitriding conditions on the fatigue strength, and as a result have obtained the following two findings. First, in order to increase the fatigue strength of components that are subject to stress concentration, such as gears, the surface hardness after nitriding must be H
It is necessary to have v700 or more and have a sufficient curing depth. 0.1% or more Al to increase surface hardness
In addition, sufficient surface hardness can be obtained by adding Cr in combination, and a large hardened layer depth can be obtained by adding Mo and V in combination.

Second, the compound layer formed on the surface of the component by gas nitriding is very brittle, and when the thickness is too large, it is easily peeled off, resulting in a decrease in fatigue strength. Conventionally, a two-step nitriding method has been adopted as a nitriding method for suppressing the formation of such a compound layer. However, the two-step nitriding method does not provide a sufficient hardened layer depth and does not have sufficient fatigue strength. Was. On the other hand, if the process start temperature and the process end temperature are defined in a specific temperature range and a method of continuously increasing the temperature during the process is applied, the temperature is immediately increased even if the compound is formed, and the compound layer is heated. It easily disappears, the formation of the compound layer is suppressed, and a large hardening depth is obtained, so that the fatigue strength is remarkably improved.

The present invention has been made based on such findings, and has a C content of 0.10 to 0.30 wt%,
r: 0.5 to 1.5 wt%, Mo: 0.2 to 1.0 wt%
%, Al: 0.1-0.5 wt%, V: 0.1-0.5
The treatment start temperature is 480 to 5 for steel containing wt%.
A method for producing a nitrided steel member having excellent fatigue properties, characterized by performing a nitriding treatment at 50 ° C and a treatment end temperature in a range of 560 to 630 ° C, and continuously increasing the temperature from the start to the end of the treatment. Is provided.

Hereinafter, the reasons for limitation of the present invention will be described. First, the reasons for limiting the chemical components will be described. (1) C: 0.10 to 0.30 wt% C is an element necessary for securing strength. However, if the amount is less than 0.1 wt%, the core strength becomes too low, so that a sufficient hardened layer depth cannot be obtained, and the required fatigue strength cannot be obtained. On the other hand, if it exceeds 0.3% by weight, the material strength becomes too high, the toughness is deteriorated, and the machinability or cold forgeability is remarkably reduced. Therefore, the C amount is set to 0.10 to 0.30 w
t% range.

(2) Cr: 0.5-1.5 wt% Cr is an element that increases the surface hardness after nitriding and increases the depth of the hardened layer. However, the amount is 0.5wt
%, The effect is small, and if it exceeds 1.5 wt%, the depth of the hardened layer is reduced conversely. Therefore, when the Cr content is 0.5
-1.5 wt%.

(3) Mo: 0.2 to 1.0 wt% Mo is an element which increases the depth of the hardened layer after the nitriding treatment, thereby significantly improving the fatigue strength. However, if the amount is less than 0.2 wt%, the effect is insufficient.
If it exceeds 0 wt%, not only the effect is saturated, but also the material strength becomes too high, so that the machinability or cold forgeability is remarkably reduced, and the cost is disadvantageous. Therefore, the Mo amount is set in the range of 0.2 to 1.0 wt%.

(4) Al: 0.1 to 0.5 wt% Al is an element which remarkably improves the fatigue strength by increasing the surface hardness after the nitriding treatment. However, the amount is 0.1
If it is less than 0.5 wt%, the required surface hardness cannot be obtained, and 0.5 wt%
%, The depth of the hardened layer is adversely affected. Therefore, Al
The amount was in the range of 0.1-0.5 wt%.

(5) V: 0.1 to 0.5 wt% V is an element for improving the depth of the hardened layer after the nitriding treatment.
However, if the amount is less than 0.1 wt%, the effect is insufficient, and if it is added more than 0.5 wt%, the effect is saturated and the cost is disadvantageous. Therefore, the V amount is set in the range of 0.1 to 0.5 wt%.

Next, the reasons for limiting the nitriding conditions will be described. (1) Treatment start temperature: 480 to 550 ° C. If the treatment start temperature is lower than 480 ° C., an effective hardening depth cannot be obtained due to a slow nitridation reaction, while if it exceeds 550 ° C., the thickness of the compound layer increases, resulting in fatigue. Affects strength.
Therefore, the nitriding treatment starting temperature was set in the range of 480 ° C to 550 ° C.

(2) Processing end temperature: 560-630 ° C. If the processing end temperature is lower than 560 ° C., effective diffusion depth cannot be obtained due to slow diffusion of nitrogen. Due to diffusion, the surface hardness decreases and the fatigue strength deteriorates. Therefore, the end temperature of the nitriding treatment is set to 560 to 6
The range was 30 ° C.

(3) Continuously raising the temperature from the start to the end of the process As described above, by continuously raising the temperature from the start to the end of the process, even if the compound is formed at a low temperature, the temperature is raised immediately. The compound layer easily disappears, and as a result, the formation of the compound layer is suppressed, and a large curing depth is obtained.
This is because the fatigue strength is remarkably improved. In the present invention, the mode is not limited as long as the temperature is continuously increased from the start of the process to the end of the process, but it is preferable that the temperature is increased linearly.

[0020]

Embodiments of the present invention will be described below. (Example 1) 150 kg of steel having the composition shown in Table 1 was melted by vacuum melting, formed into a plate having a thickness of 30 mm by hot rolling, and then subjected to a normalizing treatment at 900 ° C for 1 hour, and a notch coefficient was obtained. The specimen was processed into an Ono-type rotating bending fatigue test piece having a notch of 1.8. Then, in Table 1, No. The nitriding treatment was performed on the test materials 1 to 14. Nitriding treatment N ₂ -NH ₃ -C
Using a gas nitriding furnace in an O ₂ atmosphere, the temperature was continuously increased in the temperature pattern shown in FIG. 1 (gradient nitriding method). Note that the nitriding treatment was performed under the conditions of a treatment start temperature: 510 ° C., a treatment end temperature: 620 ° C., and a treatment time: 20 hours.

[0021] In this manner performs a rotary bending fatigue test Ono equation using the test materials subjected to nitriding treatment, were determined stress value at repeated several 10 ⁷ times as fatigue strength. The surface hardness (hardness at a position of 0.05 mm from the surface) after the nitriding treatment and the depth of the hardened layer (distance at which Hv becomes 420) were also measured. Table 2 shows the results.

In Tables 1 and 2, the test material N
o. Nos. 1 to 7 are examples of the present invention. 8 to 14 are comparative examples. In addition, No. Numeral 15 is a conventional example corresponding to SCM420 conventionally used as case hardening steel.
After carburizing at 0 ° C. for 4 hours, quenching, and carburizing and tempering at 120 ° C. for 2 hours, the surface hardness was measured, and the Ono-type rotary bending fatigue test was performed. And the fatigue strength was measured.

[0023]

[Table 1]

[0024]

[Table 2]

As is evident from Table 2, No. 1 which is an example of the present invention. 1 to 7 have a surface hardness of Hv8 after nitriding.
No. 00 or more, and the hardened layer depth was 0.5 mm or more, so that high fatigue strength was obtained. It was confirmed that the fatigue strength was superior to that of the 15 carburized materials.

On the other hand, in Comparative Example No. In No. 8, since the V content was lower than the range specified in the present invention, the depth of the hardened layer was small and the fatigue strength was low. No. In No. 9, the surface hardness was low and the fatigue strength was low because Al was lower than the range specified in the present invention. No. In No. 10, the hardened layer depth was small and the fatigue strength was low because the Al content was higher than the range specified in the present invention. No. In Nos. 11 and 12, since the Mo content and the Cr content were respectively lower than the ranges specified in the present invention, the hardened layer depth was small and the fatigue strength was low. No. No. 13 has a Cr content higher than the range specified in the present invention, and a high surface hardness can be obtained.
The fatigue strength was low due to the small depth of the hardened layer. Further N
o. In No. 14, the C content was lower than the range specified in the present invention, so that the hardened layer depth was small and the fatigue strength was low.

(Embodiment 2) The steel having the composition of No. 1 was processed into an Ono-type rotating bending fatigue test piece having a notch with a notch coefficient of 1.8 in the same manner as in Example 1, and the three temperature patterns shown in FIG. The nitriding treatment was performed for 20 hours under the conditions shown in FIG. Then, a fatigue test was performed on these test materials. Table 3 shows the surface hardness, the depth of the hardened layer, the thickness of the compound layer (the thickness of the nitride layer formed on the surface) and the fatigue strength after the nitriding treatment.
Symbols A to D are examples of the present invention, and symbols E to L are comparative examples out of the nitriding conditions of the present invention.

[0028]

[Table 3]

As is clear from Table 3, symbols A to D of the present invention all have high fatigue strength due to large surface hardness and surface layer depth and small compound layer thickness. It was confirmed that.

On the other hand, although the symbols E to H adopt the gradient nitriding method, the treatment start temperature or the treatment end temperature is out of the range of the present invention, and the surface hardness, hardened layer depth or One of the compound layer thicknesses was insufficient.

Symbols I and J indicate that the two-step nitriding method was adopted, and the fatigue strength was low because the thickness of the compound layer was small but the depth of the hardened layer was small. Symbols K and L are those employing the most common one-step nitriding method, and the fatigue strength was low because the depth of the hardened layer was not sufficient and the thickness of the compound layer was too thick.

[0032]

As described above, according to the present invention, for a steel material having a specific composition, the treatment start temperature and the treatment end temperature are specified in a specific temperature range, and the temperature is continuously raised between them. Since the formation of the compound layer is extremely small, and a high surface hardness and a large hardened layer depth can be obtained, a nitrided steel member having excellent fatigue strength comparable to carburizing and quenching can be obtained without causing a large increase in cost. be able to.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature pattern of a nitriding treatment in the present invention and a comparative example.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hitoshi Kabasawa 2358, Ohirato, Hatsuda-shi, Saitama 1-share, Japan Techno Co., Ltd. (72) Mihiro Kuwahara 777, Shimojo-cho, Nagaoka-shi, Niigata Nagaoka Electronics Co. In-house (56) References JP-A-5-25538 (JP, A) JP-A-63-14854 (JP, A) JP-B-3-12140 (JP, B2) (58) Fields investigated (Int. . ^6, DB name) C23C 8/24 - 8/26 C22C 38 / 00,38 / 24

Claims (1)

(57) [Claims] 1. C: 0.10 to 0.30 wt%, Cr:
0.5 to 1.5 wt%, Mo: 0.2 to 1.0 wt%,
Al: 0.1-0.5 wt%, V: 0.1-0.5 wt%
% Of steel, the treatment start temperature is 480-550.
A method for producing a nitrided steel member having excellent fatigue characteristics, characterized by performing a nitriding treatment at a temperature of 560 to 630 ° C. and a continuous temperature increase from the start to the end of the treatment.