JP2741222B2 - Method for manufacturing a nitrided steel member - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a steel member subjected to a nitriding treatment, and more particularly to a method for producing a steel member having an excellent fatigue life by subjecting a steel member to a nitriding treatment and a shot peening treatment. .

(Prior art)

Conventionally, as a technology for manufacturing a steel member such as a gear having excellent fatigue strength using a steel member, a technology of carburizing a steel member and a technology of nitriding or nitrocarburizing a steel member have been widely put into practical use. I have.

When the above nitriding or nitrocarburizing treatment is performed, a high hardness nitride coating made of iron nitride (Fe ₂ N, Fe ₃ N, Fe ₄ N) is formed on the surface layer of the steel member, and at the same time, the inside of the steel member is also formed. A nitride (diffusion hardened layer) is formed, and the fatigue life is significantly improved.

From the viewpoint that it is important to form the diffusion hardened layer inside the above-mentioned nitride coating as deeply as possible, it has been proposed to optimize the steel structure during the nitriding treatment and change the metal structure to a ferrite + pearlite structure. I have. (Japanese Patent Publication No. 61-3118
No. 4).

On the other hand, as a technique for increasing the fatigue life of a steel member, a technique of imparting a compressive residual stress to the surface layer portion and the inside of the steel member by shot peening treatment and forming a martensite structure by work hardening has been widely used.

For steel members such as gears, a technique for further improving the fatigue life by performing shot peening treatment on the steel member after the carburizing treatment has been put to practical use. When peening is performed, many cracks are generated in brittle nitride with high hardness and the effect of nitriding or nitrocarburizing is lost, so a technique that combines shot peening with nitriding or nitrocarburizing is used. Has not been put to practical use.

However, for engine valve springs, etc.
A technique of removing the surface nitride film by electrolytic polishing or the like after nitriding the steel member and then performing shot peening has also been put to practical use, but this method removes the nitride film. The effect of the treatment is greatly reduced.

[Problems to be solved by the invention]

The following three points are considered when considering the cause of the generation of many cracks in nitride when a shot peening treatment is applied to a steel member subjected to the nitriding treatment or the nitrocarburizing treatment.

(1) that iron nitride _{(Fe 2 N, Fe 3 N} , Fe 4 N) nitride film composed of the brittle essentially high hardness, (2) the toughness of the base material of the nitride subcapsular steel member is sufficiently Instead, the steel sheet is yielded by the impact force of shot peening, and loses the support function of supporting the nitride film from the inside of the base material. (3) The shot peening condition for the steel member subjected to the nitriding treatment or the nitrocarburizing treatment is appropriate. Is not possible, and other causes.

As described in the above publication, when a steel member having a ferrite + pearlite structure is subjected to nitriding treatment and shot peening, the ferrite + pearlite structure does not have too high hardness. The generation of cracks in the nitride film is unavoidable and is not suitable for practical use.

An object of the present invention is to eliminate the causes of the above (2) and (3), establish a technique for performing shot peening on a steel member subjected to nitriding or nitrocarburizing treatment, and significantly improve the fatigue life. Is to provide a method of manufacturing the same.

[Means for solving the problem]

In the method for producing a nitrided steel member according to claim 1, the steel member is cooled after hot forging or after being heated to an austenitizing temperature or higher, and cooled to obtain a metal structure mainly composed of bainite. This steel member is subjected to a nitriding treatment or a nitrocarburizing treatment. Then, the shot member / nitride compound film thickness is 15 to 60, and the Rockwell hardness of the shot is 45 to 6.
The shot peening process is performed under the condition of 0, shot speed = 50 to 120 m / sec. The method for producing a steel member according to claim 2 is the method according to claim 1, wherein, as a steel material of the steel member, C is 0.15 to 0.35, Si is 0.50 or less, Mn is 0.50 to 1.20, and Cr is 0.80-1.20, V 0.05-0.2
0, Mo is characterized by using a steel material containing 0.05 to 0.50.

[Action]

In the method for manufacturing a nitrided steel member according to the first aspect, first, the steel member is cooled after hot forging or is heated to an austenitizing temperature or higher, and then cooled to obtain a metal structure mainly composed of bainite. . Although the metal structure of bainite is lower than martensite, it is very hard, and it is harder and tougher than ferrite + pearlite structure. Instead, the support function of supporting the surface nitride coating from the inside is maintained. Since the above-mentioned bainite structure easily generates a nitride compound, it is advantageous to use a bainite structure, but the area ratio of the bainite structure is desirably 50% or more. The remainder is ferrite, pearlite, and sorbite (tempered martensite).

Next, when the above steel member is subjected to nitriding treatment or soft nitriding treatment, nitriding of iron nitride (Fe ₂ N, Fe ₃ N, Fe ₄ N) having a thickness of about 10 μm to several tens μm is performed on the surface layer of the steel member. While a film of the compound is formed, nitrogen atoms are diffused inside to form iron nitride.

Then, although subjected to a shot peening treatment to the steel member, the shot peening, shot size / nitride compound film thickness (D / t) = 15~60, shot Rockwell hardness (H _R C) = 45-60, shot speed (V) = 50-120
It is performed under shot peening conditions of m / sec.

Here, when D / t <15, the effect of shot peening does not reach the metal structure under the nitride compound film, the formation of residual stress becomes insufficient, and the fatigue life due to shot peening cannot be improved. On the other hand, when D / t> 60, the impact strength of the shot becomes excessive, and the metal structure under the nitride compound film becomes a yield state, and cracks occur in the film.

In H _R C <45, since poor working force of the shot peening, the formation of residual compressive stress in the subcapsular metal structure is insufficient. If H _R C> 60, the shot is too hard and cracks occur in the coating. When V <50 m / sec, the kinetic energy of the shot is not sufficient, the machining force is insufficient, and V
At> 120 m / sec, the kinetic energy of the shot becomes excessive and cracks occur in the coating.

When manufacturing a steel member that has been nitridated as described above,
The metal structure of the steel member is hard and excellent in toughness. A nitride compound film is formed on the surface layer, and a diffusion nitride layer is formed by diffusion in the internal metal structure, so that the hardness and fatigue life are improved. By peening process,
Residual compressive stress is appropriately formed in the metal structure under the nitride compound film without causing cracks in the nitride compound film, and the fatigue life is improved by the action of the residual compressive stress. In the method of manufacturing a steel member according to claim 2, basically the same operation as in claim 1 is achieved, but as a steel material of the steel member, C is 0.15 to 0.35, Si is 0.50 or less by weight%, Mn is 0.50 to 1.20, Cr is 0.80 to 1.20, V is 0.05 to 0.2
0, Mo uses a steel member containing 0.05 to 0.50, and since this steel member contains an appropriate amount of Mo, Mn, and Cr, which are quenchability improving elements, it may be continuously adjusted and cooled after hot forging or After heating to an austenitizing temperature or higher, continuous adjustment and cooling can be performed to obtain a metal structure mainly composed of bainite.

〔The invention's effect〕

According to the method for producing a nitrided steel member according to claim 1, the metal structure of the steel member is changed to a structure mainly composed of bainite, and the nitriding treatment or the nitrocarburizing treatment is performed. By applying the shot peening process under the above-described predetermined conditions, it is possible to perform the shot peening process on the nitrided or soft-nitrided steel member without destroying the nitride compound film. This makes it possible to manufacture a steel member that has been subjected to nitriding treatment or nitrocarburizing treatment and has a significantly improved fatigue life. According to the method of manufacturing a steel member according to the second aspect, basically the same effects as in the first aspect are achieved, but as a steel material of the steel member, C is 0.15 to 0.35 by weight%,
Is 0.50 or less, Mn is 0.50 to 1.20, Cr is 0.80 to 1.20, and V is 0.
05-0.20, Mo used 0.05-0.50 steel material.Since this steel material contains Mo, Mn and Cr which are hardenability improving elements, it is continuously adjusted and cooled after hot forging. Alternatively, after heating to an austenitizing temperature or higher, continuous adjustment and cooling can be performed to obtain a metal structure mainly composed of bainite.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

A method for manufacturing a steel member subjected to a nitriding treatment according to the present embodiment will be described with reference to FIG.

As a steel material suitable for applying the production method of the present application, C is 0.15 to 0.35, Si is 0.50 or less, and Mn is 0.
50-1.20, Cr 0.80-1.20, V 0.05-0.20, Mo 0.05
Desirably, the steel contains up to 0.50.

First, a steel material is cut into a shape suitable for hot forging, and then the steel material is hot forged to obtain a steel member having a predetermined shape. However, the temperature at the time of the hot forging is higher than the austenitizing temperature (about 730 ° C.).

Next, the hot forged steel member having a temperature higher than the austenitizing temperature is adjusted and cooled at a cooling rate of 0.4 to 4.0 ° C./sec to change the metal structure of the steel member into a structure mainly composed of bainite. Alternatively, the metal structure of the steel member is changed to a structure mainly composed of bainite by cooling after hot forging and then heating again to the austenitizing temperature or higher and then adjusting and cooling. The bainite-based structure may be formed by conventional well-known constant temperature cooling.However, when the above steel material is used, Mo, Mn, and Cr, which are quenchability improving elements, are appropriately Since it has been added, a bainite structure can be obtained by continuous adjustment cooling as described above. The metal structure mainly composed of bainite may be a ferrite + pearlite structure, but preferably contains bainite in an area ratio of 50% or more.

Next, the steel member having a metal structure mainly composed of bainite is machined or, if necessary, subjected to a tempering treatment and then machined into a shape of a machine part. In this case, a metal structure of ferrite + bainite + sorbite (tempered martensite) is obtained, and it is desirable to include bainite in an area ratio of 50% or more.

Next, the steel member is subjected to a nitriding treatment or a nitrocarburizing treatment,
About 10μm iron nitride (Fe ₂ N, Fe ₃ N, Fe
Inside form a diffusion nitrided layer to form a coating of ₄ N). Since the techniques of the nitriding treatment and the nitrocarburizing treatment are known, detailed description is omitted.

Next, a shot diameter (D) / iron nitride coating thickness (t) = 15 to 60 and a Rockwell hardness (H
_R C) = _45~60, subjected to shot peening at a shot speed (V) = 50~120m / sec condition. However, the shot diameter (D) and the film thickness (t) are the same unit (for example, μm).
Use the value at.

The steel member obtained by the above manufacturing method has the advantage of nitriding treatment (high hardness iron nitride coating and diffusion nitrided layer) in addition to the advantage of bainite structure (high hardness and high toughness), and reduces the fatigue life. Excellent) and the advantage of the shot peening treatment (residual compressive stress is formed in the internal metal structure and the fatigue life is excellent).

Next, a description will be given of Experimental Examples I and II actually performed to examine the correlation between the processing conditions of the shot peening processing and the improvement of the fatigue life.

<Experimental Example I> The composition of the steel material used is shown in Table 1 below.

(1) Preparation of test piece After hot forging a steel material with the above composition, it was heated to 900 ° C,
Ferrite + bainite structure (area ratio of bainite 70%) after cooling at a cooling rate of ° C / sec and austempering
Was manufactured.

Next, the test piece was machined to have a diameter of 10 mm and a notch R =
A 1.0 mm fatigue test piece was prepared.

(2) Nitriding Treatment Next, the test piece was subjected to a gas soft nitriding treatment under the conditions of 570 ° C. × 3 H _r , NH ₃ / RX = 50/50. The film thickness of the iron nitride film formed on the surface by this nitriding treatment was 10 μm.
Note that RX is an endothermic modified gas.

(3) Shot peening treatment The above-mentioned fatigue test pieces were subjected to a shot peening treatment using steel shots of various diameters, and then subjected to a fatigue test.

Conditions of the shot peening, shot Rockwell hardness (H _R C) = 48, shot velocity (V) = 80m / sec, was fatigue test stress = 45Kgf / mm ^2.

(4) Fatigue test results The fatigue test results are shown as shot diameter (D) and iron nitride film thickness (t). The D / t parameter is plotted on the horizontal axis, and the fatigue life ratio is plotted on the vertical axis. The result as shown in the figure was obtained. Here, the fatigue life ratio = 1.0 is the case of the test piece which is not subjected to the shot peening treatment while being subjected to the nitriding treatment.

As can be seen from FIG. 1, the fatigue life was improved by a factor of 4 or more when D / t = 15 to 60.

When D / t <15, the shot diameter (D) is small or the film thickness (t) is large, so that Hertz contact stress due to the shot is generated in the surface layer, and the effect of shot peening under the iron nitride film is reduced. Because the formation of the residual compressive stress is insufficient, the improvement of the fatigue life is insufficient.

At D / t> 60, the impact strength of the shot is excessive and the Hertzian contact stress extends to the inside, and the steel material inside yields and the supporting action to support the surface iron nitride coating from the inside becomes insufficient. In addition, cracks are easily generated in the iron nitride coating, and the improvement of the fatigue life is insufficient.

<Experimental Example II> A test piece manufactured in the same manner as in Experimental Example I above was subjected to a shot peening treatment with a shot diameter (D) of 0.4 mm, that is, D / t = 40, and with different shot hardness and shot speed, and a fatigue test. (Fatigue test stress = 45 kgf / mm ² ).

As a result, the results were as shown in Table 2 below, and the results are shown in FIG.

As can be seen from Table 2 and FIG. 3, the shot of Rockwell hardness (H _R C) = 45~60, shot velocity (V) = 50
When it is in the range of ~ 120 m / sec, the fatigue life is improved four times or more.

In H _R C <45, the working force of the shot is small, since the formation of the residual compressive stress is insufficient, is not sufficient improvement in fatigue life.

In H _R C> 60, excessive hardness of the shot, the generation of cracks in the iron nitride coating is not sufficient improvement in fatigue life.

When the shot speed (V) is less than 50 m / sec, the kinetic energy of the shot is insufficient and the working force is insufficient, so that the fatigue life is not sufficiently improved. Shot speed (V)> 120m / s
In the case of ec, the improvement of the fatigue life is not sufficient because the processing power of the shot becomes excessive.

When the results of Experimental Examples I and II are combined, the most desirable conditions of the shot peening process are as follows.

(I) Shot diameter (D) / Film thickness of iron nitride coating (t) = 15
To 60, (ii) available for Rockwell hardness (H _R C) = 45~60, as (iii) Shot velocity (V) = 50~120m / sec above, metal steel member composed mainly of bainite The steel member is subjected to nitriding or soft nitriding,
After that, when the shot peening process is performed under the above conditions,
As described above, it is possible to manufacture a steel member having both the characteristics of bainite, the characteristics obtained by nitriding, and the characteristics obtained by shot peening.

[Brief description of the drawings]

The drawings relate to an embodiment of the present invention. FIG. 1 is a manufacturing process diagram of a method for manufacturing a steel member subjected to nitriding treatment according to the present embodiment, and FIG. 2 is a shot diameter / iron nitride obtained in Experimental Example I. FIG. 3 is a diagram showing the relationship between the film thickness of the coating and the fatigue life ratio, and FIG. 3 is a diagram showing the relationship between the Rockwell hardness of the shot obtained in Experimental Example II and the fatigue life ratio of the shot speed.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI // C22C 38/00 301 C22C 38/00 301Z 38/24 38/24

Claims (2)

(57) [Claims] The steel member is cooled after hot forging or is heated to an austenitizing temperature or higher and then cooled to obtain a metal structure mainly composed of bainite. Then, the shot diameter / nitride compound film thickness = 15 to
60. A method for producing a nitrided steel member, wherein a shot peening treatment is performed under the conditions of a Rockwell hardness of a shot = 45 to 60 and a shot speed = 50 to 120 m / sec. 2. The steel material of the steel member is C in weight%.
Is 0.15 to 0.35, Si is 0.50 or less, Mn is 0.50 to 1.20, and Cr is 0.
The method according to claim 1, wherein a steel material containing 80 to 1.20, V of 0.05 to 0.20, and Mo of 0.05 to 0.50 is used.