JPH07286257A - Production of nitriding steel member excellent in cold forgeability and fatigue strength - Google Patents

Abstract

PURPOSE:To produce a nitriding steel member excellent in cold forgeability and having high fatigue strength, at the time of subjecting a steel having a specified compsn. to gaseous nitriding treatment, by prescribing the starting temp. and finishing temp. of the nitriding treatment to specified temp. ranges and furthermore continuously raising the temp. therebetween. CONSTITUTION:A steel contg., by weight, 0.10 to 0.30% C, <0.35% Si, <1.0% Mn, <0.05% S, 0.5 to 1.5% Cr, 0.1 to 0.5% Al and 0.1 to 0.5% V or furthermore contg. one or two kinds among <0.1% Ni, <1.0% Cu and <0.2% Mo is charged to a gaseous nitriding furnace having an N2-NH3-CO2 atmosphere, is heated and is subjected to nitriding treatment. The nitriding treatment is executed in such a manner that the starting temp. is regulated to 480 to 550 deg.C and the finishing temp. to 560 to 630 deg.C and its temp. is continuously raised therebetween, by which the nitriding steel member excellent in cold forgeability and having high fatigue strength can be produced.

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 cold forgeability and fatigue characteristics, which is suitable for mechanical structural parts such as gears which are required to have excellent cold forgeability and fatigue strength. .

[0002]

2. Description of the Related Art Gas nitriding is one of the surface hardening treatment methods for improving the fatigue strength of machine structural steel. This gas nitriding treatment is effective as a surface hardening treatment method for machine structural parts that require dimensional accuracy, because the heat treatment strain is smaller than that of the same surface hardening treatment methods such as induction hardening and carburizing hardening.

Conventionally, JIS has been used as a steel material for nitriding treatment.
SACM645, JIS SCM435, etc. are used. However, SACM645 has a surface hardness of Hv1.
Since it is too hard as about 000 and a brittle compound layer is formed on the surface, high fatigue strength cannot be obtained. In addition, SCM4
No. 35 also cannot obtain a sufficient hardened layer depth, so that high fatigue strength cannot be obtained. Further, since both steels have high material hardness, they have problems such as poor cold forgeability and machinability.

Therefore, in order to improve the fatigue strength, Cr-M is disclosed in JP-A-4-45244, JP-A-4-66646, JP-A-5-25538 and the like.
High-fatigue strength nitriding steel or soft nitriding steel based on o-V steel has been proposed. On the other hand, in order to improve the cold workability, JP-A-5-171347 discloses Si or Mn.
There has been proposed soft nitrided steel in which the amount is regulated and Cr and V are added together.

[0005]

However, Japanese Patent Laid-Open Nos. 4-45244 and 4-66646,
The nitriding steel or the soft nitriding steel disclosed in Japanese Patent Laid-Open No. 25538/1993 has an increased fatigue strength due to the combined addition of Cr, Mo, and V, but since it contains a large amount of Mo, the material hardness is high. Is too high, and cold forgeability and machinability are extremely poor.

The steel for soft nitriding disclosed in Japanese Patent Laid-Open No. 5-171347 has a material hardness of Hv 200 or less and thus has good cold forgeability, but has a surface hardness after nitriding treatment. Moreover, even if the conventional nitriding method is adopted, a large hardened layer depth cannot be obtained, and the fatigue strength cannot be said to be sufficient. The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for manufacturing a nitrided steel member having both excellent fatigue properties and cold forgeability.

[0007]

Means and Actions for Solving the Problems The inventors of the present application first studied in detail the effects of chemical components, composition, and nitriding conditions on fatigue strength, and as a result, obtained the following two findings. First, in order to increase the fatigue strength of a component such as a gear that is subjected to stress concentration, it is necessary that the surface hardness after nitriding treatment be Hv 700 or more and that the hardening depth be sufficient. In order to increase the surface hardness, it is essential to add 0.1% or more of Al, and by further adding Cr in combination, sufficient surface hardness can be obtained, and by adding V, a large hardened layer depth can be obtained. it can.

Secondly, the compound layer formed on the surface portion of the component by gas nitriding is very brittle, and when the thickness is too thick, it is easily peeled off, resulting in a decrease in fatigue strength. A two-step nitriding method has been conventionally used as a nitriding method for suppressing the formation of such a compound layer. However, the two-step nitriding method cannot provide a sufficient hardened layer depth and also has an insufficient fatigue strength. It was On the other hand, by defining the treatment start temperature and the treatment end temperature in a certain specific temperature range and applying a method of continuously raising the temperature during that period, even if a compound is formed, the temperature is raised immediately to form a compound layer. Fatigue strength is remarkably improved because it easily disappears and the formation of a compound layer is suppressed, and a large hardening depth is obtained.

Then, as a result of further studies to obtain further excellent cold forgeability, C, Si, Mn and Cr were obtained.
Adjust the amount of addition of Hv22
It was found that by setting it to 0 or less, the required cold forgeability can be secured while maintaining excellent fatigue properties.

The present invention was made on the basis of such findings, and firstly, C: 0.10 to 0.30w.
t%, Si: 0.35 wt% or less, Mn: 1.0 wt%
Hereinafter, S: 0.05 wt% or less, Cr: 0.5 to 1.5
wt%, Al: 0.1 to 0.5 wt%, V: 0.1
For steel containing 0.5 wt%, the treatment start temperature is 48
Excellent cold forgeability and fatigue characteristics, characterized by performing a nitriding treatment in which the temperature is 0 to 550 ° C., the treatment end temperature is 560 to 630 ° C., and the temperature is continuously raised from the treatment start to the treatment end. And a method for manufacturing a nitrided steel member.

Secondly, C: 0.10 to 0.30 wt%,
Si: 0.35 wt% or less, Mn: 1.0 wt% or less,
S: 0.05 wt% or less, Cr: 0.5 to 1.5 wt
%, Al: 0.1 to 0.5 wt%, V: 0.1 to 0.5
wt%, Ni: 1.0 wt% or less, C
u: 1.0 wt% or less, Mo: 0.2 wt% or less steel containing one or more, the treatment start temperature is 48
Excellent cold forgeability and fatigue characteristics, characterized by performing a nitriding treatment in which the temperature is 0 to 550 ° C., the treatment end temperature is 560 to 630 ° C., and the temperature is continuously raised from the treatment start to the treatment end. And a method for manufacturing a nitrided steel member.

The reasons for limitation of the present invention will be described below. First, the reasons for limiting the chemical components will be shown. (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 strength of the core portion 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 wt%, the material strength becomes too high, the toughness deteriorates, and the machinability or cold forgeability also remarkably decreases. Therefore, the C content is 0.10 to 0.30w
The range was t%.

(2) Si: 0.35 wt% or less Si is necessary as a deoxidizing material, but since it forms a solid solution in ferrite and deteriorates cold forgeability, it is set to 0.35 wt% or less.

(3) Mn: 1.0 wt% or less Mn is added as a deoxidizer and is necessary for ensuring strength. However, if it exceeds 1.0 wt%, the cold forgeability deteriorates like Si, so the content was made 1.0 wt% or less.

(4) S: 0.05 wt% or less S is an element that improves machinability, but 0.05 wt%
If it exceeds, the amount of sulfide-based inclusions increases too much, which not only causes cracking during cold forging, but also leads to a decrease in fatigue strength. Therefore, the upper limit is set to 0.05 wt%.

(5) Cr: 0.5 to 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 0.5 wt
If it is less than 0.1%, the effect is small, and if it exceeds 1.5% by weight, the depth of the hardened layer is decreased conversely, and the material hardness becomes too high, which deteriorates cold forgeability. Therefore, the Cr content is 0.5 to
It is set to 1.5 wt%.

(6) Al: 0.1 to 0.5 wt% Al is an element that increases the surface hardness after nitriding treatment.
However, if the amount is less than 0.1 wt%, the required surface hardness cannot be obtained, and if it exceeds 0.5 wt%, the hardened layer depth is adversely affected. Therefore, the Al amount is set to the range of 0.1 to 0.5 wt%.

(7) V: 0.1 to 0.5 wt% V is an element that improves the depth of the hardened layer after nitriding.
However, if the amount is less than 0.1 wt%, the effect is insufficient, and if added in excess of 0.5 wt%, the effect is saturated and the cost becomes disadvantageous. Therefore, the amount of V is set to the range of 0.1 to 0.5 wt%.

Further, one or more of the following Ni, Cu and Mo may be contained for the main purpose of enhancing the strength and toughness of the material. (8) Ni: 1.0 wt% or less Ni is an element that improves the toughness of the material without reducing the cold forgeability. However, if added in excess of 1.0 wt%, not only the hardness increases and the cold forgeability is adversely affected, but also it is a very expensive element, which is disadvantageous in terms of cost. Therefore, the upper limit is set to 1.0 wt%.

(9) Cu: 1.0 wt% or less Cu is an element that precipitates and hardens during the nitriding treatment and improves the strength of the core of the nitriding material. However, if added in excess of 1.0 wt%, the toughness decreases, so the upper limit is 1.0
It was set to wt%.

(10) Mo: 0.2 wt% or less Mo is an element that increases the strength of the material and at the same time increases the depth of the hardened layer after the nitriding treatment. However, 0.2 wt%
If added in excess of 0.1%, the hardness increases and the cold forgeability remarkably deteriorates, so the upper limit was made 0.2 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 less than 480 ° C., an effective hardening depth cannot be obtained because the nitriding reaction is slow, while if it exceeds 550 ° C., the compound layer thickness becomes large and fatigue occurs. It adversely affects strength.
Therefore, the nitriding treatment start temperature is set in the range of 480 ° C to 550 ° C.

(2) Treatment end temperature: 560 to 630 ° C. When the treatment end temperature is less than 560 ° C., effective diffusion depth cannot be obtained because nitrogen diffusion is slow, while when it exceeds 630 ° C., nitrogen is more deep inside. The diffusion reduces the surface hardness and deteriorates the fatigue strength. Therefore, the nitriding treatment end temperature is set to 560 to 6
The range was 30 ° C.

(3) Continuously raising the temperature from the start to the end of the treatment As described above, by continuously raising the temperature from the start to the end of the treatment, even if the compound is formed at a low temperature, the temperature is immediately raised. As a result, the compound layer easily disappears, and as a result, the formation of the compound layer is suppressed, and a large curing depth can be 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 raised from the start to the end of the treatment, but it is preferable to raise the temperature linearly.

[0025]

EXAMPLES Examples 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, hot rolled into a plate having a thickness of 30 mm, and a normalizing treatment at 900 ° C x 1 hour was performed to obtain a material. After measuring the hardness of the material, it was processed into an Ono-type rotary bending fatigue test piece having a notch with a notch coefficient of 1.8. Then, No. 1 in Table 1 was used. The sample materials 1 to 14 were subjected to a nitriding treatment. The nitriding treatment was performed using a gas nitriding furnace in an N ₂ —NH ₃ —CO ₂ atmosphere by a method (gradient nitriding method) of continuously raising the temperature in the temperature pattern shown in FIG. The nitriding treatment is performed at a treatment start temperature of 510 ° C. and a treatment end temperature of 62.
It was carried out under the conditions of 0 ° C. and treatment time: 20 hours.

An Ono-type rotary bending fatigue test was conducted using the test material thus nitrided, and the stress value at the number of repetitions of 10 ⁷ was determined as the fatigue strength. Further, the surface hardness after nitriding (hardness at a position of 0.05 mm from the surface) and the depth of the hardened layer (distance at which Hv becomes 420) were also measured. The results are shown in Table 2. In Tables 1 and 2, the test material No. Nos. 1 to 11 are examples of the present invention. 1
2 to 21 are comparative examples.

[0027]

[Table 1]

[0028]

[Table 2]

As is apparent from Table 2, No. 1 which is an example of the present invention. All of 1 to 11 have a material hardness of Hv220 or less and are excellent in cold forgeability, and the surface hardness after nitriding treatment is Hv.
It was confirmed that high fatigue strength was obtained because v700 or more and the hardened layer depth was 0.4 mm or more.

On the other hand, No. which is a comparative example. In No. 12, the V 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. In addition, No. No. 13 had a lower surface hardness and a lower fatigue strength because Al was lower than the range specified in the present invention. No. In No. 14, the amount of Al was higher than the range specified in the present invention, so the depth of the hardened layer was small and the fatigue strength was low. No. In Nos. 15 and 16, the amounts of Cr and C were lower than the ranges specified in the present invention, so the hardened layer depth was small and the fatigue strength was low. In addition, No. 17, 18, 1
In Nos. 9 and 20, the Cr content, Mn content, Si content, and C content were higher than the ranges specified in the present invention, so the fatigue strength was high, but the material hardness was high and the cold forgeability was poor. Furthermore N
o. In No. 21, the S content was higher than the range specified in the present invention, so the fatigue strength was low even though the surface hardness and the depth of the hardened layer were within the proper ranges.

(Embodiment 2) Next, No. The steel having the composition of No. 1 was processed into an Ono-type rotary bending fatigue test piece having a notch with a notch coefficient of 1.8 as in Example 1, and three temperature patterns shown in FIG. The nitriding treatment was performed for 20 hours under the conditions shown in. Then, a fatigue test was conducted on these test materials. Table 3 also shows the surface hardness after nitriding, 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.
The symbols A to D are examples of the present invention, and the symbols E to L are comparative examples that deviate from the nitriding treatment conditions of the present invention.

[0032]

[Table 3]

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

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

The symbols I and J adopt the two-stage nitriding method, and although the compound layer thickness is small, the hardened layer depth is small, so that the fatigue strength was low. The symbols K and L adopted the most general one-step nitriding method, and the depth of the hardened layer was not sufficient, and the compound layer was too thick, so the fatigue strength was low.

[0036]

As described above, according to the present invention, for a steel material having a specific composition, the processing start temperature and the processing end temperature are regulated within a specific temperature range, and the temperature is continuously raised between them, so that the cooling is performed. It is possible to obtain a nitrided steel member that is excellent in hot forgeability and has high fatigue strength.

[Brief description of drawings]

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

Front Page Continuation (72) Inventor Tetsuo Shirakami 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inventor Moriyuki Ishiguro 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Co., Ltd. (72) Inventor Hitoshi Kabazawa, 1-share company, 2358, Hashido, Hasuda City, Saitama, Japan Techno Co., Ltd. (72) Inventor, Mihiro Kuwahara, 777 Shimojo, Nagaoka, Niigata Nagaoka Electronics Co., Ltd.

Claims (2)

[Claims] 1. C: 0.10 to 0.30 wt%, Si:
0.35 wt% or less, Mn: 1.0 wt% or less, S:
0.05 wt% or less, Cr: 0.5 to 1.5 wt%, A
1: 0.1-0.5 wt%, V: 0.1-0.5 wt%
For steel containing 480 to 550
Nitriding steel having excellent cold forgeability and fatigue properties, characterized by being subjected to a nitriding treatment for continuously raising the temperature from the start of treatment to the end of treatment in the range of 560 to 630 ° C. A method of manufacturing a member. 2. C: 0.10 to 0.30 wt%, Si:
0.35 wt% or less, Mn: 1.0 wt% or less, S:
0.05 wt% or less, Cr: 0.5 to 1.5 wt%, A
1: 0.1-0.5 wt%, V: 0.1-0.5 wt%
In addition, Ni: 1.0 wt% or less, Cu: 1.
The treatment start temperature is 480 to 55 for steel containing one or more of 0 wt% or less and Mo: 0.2 wt or less.
0 ° C, the treatment end temperature is in the range of 560 to 630 ° C,
A method for producing a nitrided steel member excellent in cold forgeability and fatigue characteristics, which comprises performing a nitriding treatment for continuously raising the temperature from the start of treatment to the end of treatment.