JPH02149616A - Manufacture of nitrided steel member - Google Patents

Abstract

PURPOSE:To manufacture a steel member excellent in fatigue strength by forming the structure of a steel member into a metallic structure composed principally of bainite, subjecting the above steel member to nitriding treatment, and then applying shot peening treatment to the above under specific conditions. CONSTITUTION:A steel member is hot-forged and cooled or is heated up to a temp. of the austenitizing temp. or above and cooled, by which the structure of the above steel member is formed into a metallic structure composed principally of bainite in which area ratio of bainite structure is regulated to >= about 50%. Subsequently, nitriding treatment or soft nitriding treatment is applied to the above steel member, by which a film of iron nitride (Fe2N, Fe3N, Fe4N) is formed to a thickness of about 10 to several tens microns on the surface layer of the steel member and also a diffused nitrogen layer is formed inside. Further, shot peening treatment is applied to the above steel member under the conditions of 15-60 ratio of shot diameter to thickness of nitrogen compound film, 45-60 Rockwell hardness of shot, and 50-120m/sec shot velocity. By this method, the steel member remarkably improved in fatigue strength can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing nitrided steel members, and particularly relates to a method for manufacturing steel members with excellent fatigue life by subjecting steel members to nitriding and shot peening. .

[Prior art]

Conventionally, as a technology for manufacturing steel parts such as gears with excellent fatigue strength using steel parts, the technology of carburizing the steel parts and the technology of applying nitriding or nitrocarburizing to the steel parts have been widely put into practical use. There is.

When the above nitriding treatment or soft nitriding treatment is performed, a highly hard nitride film made of iron nitride (FezN, FeJ, FE14N) is formed on the surface layer of the steel member, and at the same time, a nitride (diffusion hardened layer) is formed inside the steel member. ) is formed, significantly improving fatigue life.

From the viewpoint that it is important to form the diffusion hardened layer inside the surface nitride coating as deep as possible, it has been proposed to optimize the steel structure during nitriding treatment and change the metal structure to a ferrite + pearlite structure. There is (Special public official 1986-311)
(See Publication No. 84).

On the other hand, as a technique for increasing the fatigue life of steel members, a technique of imparting compressive residual stress to the surface and interior of the steel member through shot peening treatment and forming a martensitic structure through work hardening has also been widely put into practical use.

For steel parts such as gears, it has been legalized to further improve the fatigue life of the steel parts by subjecting them to the above-mentioned carburizing treatment followed by Ni-shot heating. If shot peening is applied to the steel, many cranks will occur in the hard and brittle nitride, and the effects of nitriding and soft nitriding will be lost.
A technique that uses shot peening treatment together with nitriding treatment or soft nitriding treatment has not been put to practical use.

However, for engine valve springs, etc., a technique has been put into practical use that involves nitriding the steel member, removing the surface nitride film by electrolytic polishing, etc., and then subjecting it to shot peening. Since the nitride film is removed, the effect of the nitriding treatment is greatly reduced.

[Problems to be Solved by the Invention] When we consider the reasons why many cracks occur in the nitride when shot peening is applied to a steel member that has been subjected to the nitriding or nitrocarburizing treatment, we find the following: Three points can be considered.

(1) The nitride film made of iron nitride (FezN, FeJ, Fe4N) is inherently hard and brittle; (2) The base material under the nitride film of the steel member does not have sufficient toughness, making shot peening difficult. (3) Shot peening treatment conditions for nitrided or soft-nitrided steel members are not appropriate, etc. Possible causes.

As described in the above publication, when a steel member with a ferrite + pearlite structure is subjected to nitriding treatment and shot peening, the hardness of the ferrite + pearlite structure is not very high, so due to the cause of (2) above, The occurrence of cracks in the nitride film is unavoidable, making it unsuitable for practical use.

The purpose of the present invention is to eliminate the causes of (2) and (3) above,
It is an object of the present invention to provide a method for manufacturing steel members whose fatigue life is significantly improved by establishing a technique for applying shot peening to steel members that have been subjected to nitriding or soft nitriding.

[Means to solve the problem]

In the method for manufacturing a nitrided steel member according to the present invention, a steel member is cooled after hot forging or heated to an austenitizing temperature or higher and then cooled to obtain a metal structure mainly composed of bainite. A steel member is subjected to nitriding treatment or soft nitriding treatment, and then this steel member is coated with shot diameter/nitride compound film thickness = 15~6o and Rockwell hardness of shot = 45~
60, shot peening treatment is performed under conditions of shot speed = 50 to 120 m/sec.

[Effect]

In the method for manufacturing a nitrided steel member according to the present invention, first, the steel member is hot-forged and then cooled, or alternatively, the steel member is heated to an austenitizing temperature or higher and then cooled to form a metal structure mainly consisting of bainite.

The metal structure of bainite is lower in hardness than martensite, but it is extremely hard, and it is also harder and tougher than the ferrite + pearlite structure, so even when shot peening is applied, the interior of the steel member remains in a yield state. This results in maintaining the supporting effect of supporting the surface nitride film from the inside.

Since the above-mentioned bainite structure easily generates nitride compounds, it is advantageous to use a bainite structure, but it is desirable that the area ratio of the bainite structure is 50% or more. The remainder is ferrite, pearlite, and sorbite (tempered martensite).

Next, when the steel member is subjected to nitriding treatment or soft nitriding treatment,
A nitride compound film of iron nitride (FezN, Fe:+N, FeJ) with a thickness of about 10 μm to several 10X/11 is formed on the surface layer of the steel member, and nitrogen atoms are also diffused inside. Iron nitride is formed.

Next, the above steel member is subjected to shot peening treatment, and the shot peening treatment has a shot diameter/nitride compound film thickness (D/l) of -15 to 60 and a shot Rockwell hardness (H, C) of - 45-60, shot speed (
V) = 15 to 120 m/see shot peening conditions.

Here, when D/l<15, the action of shot peening does not reach the metal structure under the nitride compound coating, and the formation of residual stress becomes insufficient, making it impossible to improve the fatigue life by shot pinning.

On the other hand, when D/l>60, the impact force of the shot becomes excessive, and the metal structure under the nitride compound film enters a yield state, causing cracks to occur in the film.

When HRC<45, the processing power of shot peening is insufficient, and the formation of residual compressive stress in the metal structure under the coating becomes insufficient. In addition, when HRC>60, the shot is too hard and cranks occur in the coating. V < 50 m/s
With EC, the kinetic energy of the shot is insufficient and the processing force is insufficient, and with V>120 m/sec, the kinetic energy of the shot is excessive and cracks occur in the coating.

When a steel member nitrided as described above is manufactured, the metal structure of the steel member is hard and has excellent toughness, and a nitride compound coating is formed on the surface layer, and a diffused nitride layer is also formed in the internal metal structure due to diffusion. is formed, improving hardness and fatigue life, and through shot peening treatment, residual compressive stress is appropriately formed in the metal structure under the nitride compound film without causing cranking in the nitride compound film, and this residual compressive stress is removed. The fatigue life is improved by this action.

(Effects of the invention) According to the method for manufacturing a nitrided steel member according to the present invention,
As explained in the above [Operation] section, by the process of changing the metal structure of the steel member to a structure mainly consisting of bainite, subjecting it to nitriding treatment or soft nitriding treatment, and subjecting it to shot peening treatment under the predetermined conditions, It becomes possible to perform shot peening on a steel member that has been nitrided or soft-nitrided without destroying the nitride compound coating.

This makes it possible to manufacture a steel member that has undergone nitriding or soft-nitriding treatment and has a dramatically improved fatigue life.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

A method for manufacturing a nitrided steel member according to this example will be explained based on FIG. 1.

Steel materials suitable for use in the manufacturing method of the present application include:
In weight percent, 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 to 1.
20,■ is 0°05~0.20, Mo is 0.05~0
．． Preferably, it is a steel containing 50%.

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

Next, the hot-forged steel member whose temperature is higher than the austenitization temperature is cooled at a cooling rate of 0.4 to 4.0°C/sec, so that the metal structure of the steel member is made into a structure consisting mainly of bainite. . Alternatively, after hot forging, the steel member is cooled, heated again to a temperature equal to or higher than the austenitizing temperature, and then adjusted and cooled to change the metallographic structure of the steel member to a structure mainly composed of bainite. The above-mentioned bainite-based structure may be achieved by conventionally well-known constant temperature cooling, but when using the above-mentioned steel materials, Mo, Mn and Cr, which are elements that improve hardenability, may be appropriately added. Since it is added, a bainite structure can be formed by continuous controlled cooling as described above.

The metal structure mainly composed of bainite may be a ferrite-bainite 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, tempered and then machined into the shape of a mechanical part. In this case, the metal structure will be ferrite + bainite + sorbite (tempered martensite), but it is desirable to include haynite in an area ratio of 50% or more.

Next, the above-mentioned steel member is subjected to nitriding treatment or soft nitriding treatment, and iron nitride (FezN, F
A film of e=N, Fe, N) is formed, and a diffusion nitride layer is formed inside. Since the techniques of the nitriding treatment and soft nitriding treatment described above are well known, detailed explanations thereof will be omitted.

Next, the shot diameter (D)/iron nitride liquid film thickness (t) = 15 to 60 and the Rockwell hardness of the shot (H
, IC)=45-60, shot speed (V)=50-1
Shot peening treatment is performed under the condition of 20 m/sec. However, the shot diameter (D) and film thickness (1) are both expressed in the same unit (for example, μm).

In addition to the advantages of the bainitic structure (high hardness and toughness), the steel parts obtained by the above manufacturing method have the advantages of the nitriding treatment (having a high hardness iron nitride coating and a diffusion nitride layer, resulting in a long fatigue life). It has the advantages of shot peening treatment (residual compressive stress is formed in the internal metal structure, resulting in excellent fatigue life).

Next, an example of an experiment actually conducted to investigate the correlation between shot peening treatment conditions and improvement in fatigue life.
and experimental example ■ will be explained.

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

Table 1 (Chemical composition %) (1) Preparation of test piece A steel material with the above composition was heated to 900°C after hot forging, and then cooled at a cooling rate of 1.0°C/sec to undergo austempering treatment. A test piece with a ferrite + bainite structure (area ratio of haynite: 70%) was manufactured.

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

(2) Nitriding treatment Next, the above test piece was treated with 570° (: X 3Hr, N
Gas nitrocarburizing treatment was performed under the condition of H3/RX=50150. The thickness of the iron nitride film formed on the surface by this nitriding treatment was 10 μm. Note that the above RX is an endothermic modified gas.

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

The shot peening conditions are: Rockwell hardness of shot (H, C) = 48, shot speed (V) = 80 m
/sec, fatigue test stress = 45 Kgf/mm''.

(4) Fatigue test results Fatigue test results are calculated based on the shot diameter (D) and the thickness of the iron nitride coating (
1), and when the D/T parameter is plotted on the horizontal axis and the fatigue life ratio is plotted on the vertical axis, the results shown in FIG. 1 are obtained. Here, the fatigue life ratio=1°0 is for a test piece that has been nitrided but not shot peened.

As can be seen from FIG. 1, the fatigue life was improved by more than four times when D/T was in the range of 15 to 60.

When D/T < 15, the shot diameter (D) is small or the film thickness (1) of the film is large, so Hertzian contact stress due to the shot occurs in the surface layer, and the effect of shot peening occurs under the iron nitride film. This results in insufficient formation of residual compressive stress, resulting in insufficient improvement in fatigue life.

When D/T>60, the impact force of the shot is excessive and the contact stress of the shot is applied to the inside, and the internal copper material enters a yielding state, and the supporting effect to support the surface iron nitride coating from the inside is lost. If the iron nitride coating becomes insufficient, cracks are likely to occur in the iron nitride coating, resulting in insufficient improvement in fatigue life.

Experimental Example ■〉 The shot diameter (D
) 0.4 mm, that is, D/T = 40, and subjected to shot peening treatment by changing the shot hardness and shot speed, and subjected to a fatigue test (fatigue test stress -45 gf/mm")
Served.

As a result, the results were as shown in Table 2 below, which is illustrated in Figure 3.

(Next summer, blank space below) As can be seen from Table 2 and Figure 3, the Rockwell hardness of the shot (H, C) -45 to 60, the shot speed (V)
In the range of -50 to 120 m1sec, the fatigue life is improved by more than 4 times.

11. When Ic<45, the shot processing force is small and the formation of residual compressive stress is insufficient, so that the fatigue life is not sufficiently improved.

)! , C>60, the hardness of the shot is excessive and cracks occur in the iron nitride coating, so the fatigue life is not sufficiently improved.

In addition, when shot speed (V) < 50m/sec,
The kinetic energy of the shot is insufficient and the machining force is insufficient, resulting in insufficient improvement in fatigue life. Shot speed (V)
> 120 m/sec, the shot processing force becomes excessive and the fatigue life is not sufficiently improved.

Combining the results of Experimental Example 1 and Experimental Example (2) above, the conditions for shot peening treatment with a desirable amount are as follows.

(i) Shot diameter (D)/Thickness of iron nitride film (1) −
15-60, (ii) Rockwell hardness of shot (H, IC
) -45~60, (iii) Shot speed (V) =50~120m/
sec As mentioned above, if a steel member has a metal structure mainly composed of bainite, and the steel member is subjected to nitriding or soft nitriding treatment, and then shot peening under the above conditions, the characteristics of bainite and It is possible to manufacture a steel member that has both the properties obtained by nitriding treatment and the properties obtained by shot peening treatment.

[Brief explanation of the drawing]

The drawings relate to an example of the present invention, and FIG. 1 is a manufacturing process diagram of the method for manufacturing a nitrided steel member according to this example, and FIG. 2 is a diagram showing the 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 (2), the shot speed, and the fatigue life ratio. Fig. 1 Fig. 2 Shot diameter (D) Nitii b and ti 1st floor Tajide Fyo J1 town ([) Fig. 3 Rockwell hardness of shot

Claims (1)

[Claims] (1) A steel member is cooled after hot forging or heated to a temperature higher than the austenitizing temperature and then cooled to obtain a metal structure mainly composed of bainite, and then the steel member is subjected to nitriding treatment or soft nitriding treatment. , Next, shot diameter/nitride compound film thickness = 15 was applied to this steel member.
60, a shot Rockwell hardness of 45 to 60, and a shot speed of 50 to 120 m/sec. A method for producing a nitrided steel member.