JPS62199765A - Material for high surface pressure structural parts - Google Patents

Abstract

PURPOSE:To produce a material for high surface pressure structural parts having improved pitting resistance by successively subjecting a steel stock contg. specific ratios of C, Si, Mn, Al, and N to a carburization treatment, chemical vapor deposition treatment, hardening and tempering. CONSTITUTION:The steel contg., by weight %, 0.15-0.30% C, <=1.0% Si, <=1.5 Mn, <=0.1% Al, and <=0.03% Ni, contg., at need 1 or <=2 kinds among <=5.0% Ni, <=5.0% Cr, and <=0.8% Mo, further contg. 1 or >=2 kinds among <=0.5% V, <=0.5% Ti, <=0.5% Nb, and <=0.5% Zr and consisting of the balance Fe and inevitable impurities is used as the stock and is subjected to the carburization treatment. TiC or TiN is then deposited by evaporation thereon to form a hard layer and such steel stock is subjected to hardening and tempering so that the hardness at 0.05mm depth from the surface is >=700Hv and the depth of the effective hardened layer to provide 550Hv is >=0.50mm. The material for the machine structural parts for a largesized gear, etc., which exhibits the excellent pitting resistance under a high surface pressure is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a material for mechanical structural parts that are used while rolling under high surface pressure, such as large gears.

(Prior art and problems) Among mechanical structural parts that are used by rolling under high surface pressure, for example, parts that are used under particularly high surface pressure such as large gears of 200 kgf/mm" or more, Excellent pitting resistance is required.

Pitting occurs when a gear slides on its surface. In other words, when there is a difference in the peripheral speed of the tooth surfaces of meshing gears, a peak of shear stress appears on the surface, and the fatigue phenomenon caused by this shear stress causes pits to form on the surface. This is a phenomenon in which similar defects occur, and the higher the single surface pressure, the better the pitting resistance is desired to extend the life of the gear.

In order to improve pitting resistance, it is essential to increase the surface hardness by subjecting the steel material to surface hardening treatment.

Conventionally, as a surface hardening treatment for this purpose, a gas carburizing treatment based on case hardening steel has been adopted, but this method has had the problem that a surface hardness of only Hv 700 to 8 oO can be obtained at most.

Therefore, Ti is used as a surface treatment to dramatically increase surface hardness.
Attempts have been made to chemically deposit a hard layer of C, TiN, or the like. However, if only vapor deposition is performed on ordinary case-hardened steel, although it is possible to increase the surface hardness with a hardened layer on the surface, sufficient strength just below the surface cannot be obtained, and it does not contribute to improving fatigue strength. do not.

(Objective of the Invention) The present invention solves the drawbacks of the above-mentioned prior art, and in particular
The object of the present invention is to provide a material for mechanical structural parts such as large gears that can exhibit excellent pitting resistance even when used under high surface pressures such as kic f/mm'' or higher.

(Structure of the Invention) In order to achieve the above object, the present inventors first investigated TiN, TiC, Al as a method that can dramatically increase the surface hardness.
When attempts were made to chemically or physically coat the surface of steel materials with hard particles such as No. 202, surface hardness of Hv 2000 to 400° was obtained in all cases by vapor deposition.

However, although the surface hardness is significantly higher than before, in the case of high surface pressure mechanical structural parts such as gears, not only surface hardness but also appropriate hardness and hardening depth of each part are required. When the hardness of the surface layer is dramatically high, as in the case of the hardness of each part, it is necessary to ensure an appropriate hardening depth so that the difference in hardness between each part and the surface layer is as smooth as possible. As a result of research, it was found that the above-mentioned vapor deposition treatment alone is insufficient, and that a combination of the material of the steel material, carburizing treatment before the vapor deposition treatment, and heat treatment after the vapor deposition treatment is essential.

In other words, the material for high surface pressure mechanical structural parts according to the present invention is:
C: 0.15-0.30%, Si≦1.0%, Mn
≦1.5%, Al≦0.1% and N≦0.03%, and if necessary, Ni≦5.0%, Cr≦5.0% and Mo≦1.80%. One type or two or more types, or further V≦1.5%, Ti≦1.5%.

One or two of Nb≦1.5% and Zr≦1.5%
After carburizing the material, it has a hard layer deposited with TiC1TiN, etc., and is 0.05 mm from the surface after quenching and tempering. Hardness at depth is Hv700 or more,
And, the effective hardened layer depth to obtain Hv550 is 0.50
It is characterized by being larger than mm.

The present invention will be explained in detail below based on examples.

First, the reasons for limiting the components of the steel material in the present invention are shown below.

C: C is an element necessary to obtain the specified strength of each part by carburizing, quenching and tempering, and for that purpose 0.1
Contain 5% or more. However, if the content is too large, the strength of each part becomes too high and both fatigue strength and impact resistance deteriorate, so the upper limit is set at 0.30%.

Si: Si is a necessary element as a deoxidizing agent, but if it is too large, it impedes forgeability, so it is added in an amount of 1.0% or less.

Mn: Mn is added to improve hardenability, but since too much Mn inhibits forgeability, it is added in an amount of 1.0% or less.

Al, N: Al and N are effective elements for forming AlN and preventing coarsening of crystal grains, and for this purpose, each of Al≦0.
A range of 1%, N:i; 0.03% is sufficient.

Ni, Cr, Mo: Ni, Cr, and Mo are all elements that are effective in improving hardenability, and can be added in appropriate amounts as necessary. When adding, Ni≦5.0%, Cr
One or more of them are added within the range of Mo≦5.0% and Mo≦1.80%. Even if each element is added in a larger amount, the above effects are saturated. In addition, Ni and Mo
has the effect of improving the hardenability as well as the toughness of the carburized layer.

V, Ti, Nb, Zr: V, Ti, Nb, and Zr are all elements that form carbides or carbonitrides, contribute to grain refinement, and improve impact resistance. Appropriate amounts of one or more of them can be added. When adding each element, the upper limit is 0.5%, and even if added in excess of 0.5%, the above effects will be saturated.

In the present invention, steel having the above-mentioned composition is used as a material, and it is necessary to coat it with hard particles of about 10 μm such as TiN, TiC, Al, ○, etc. by chemical vapor deposition (CVD). As a result, the surface layer of the steel cable has a surface hardness of Hv2.
The hard layer can be made extremely hard with a hardness of about 000 to 4000. Of course.

Physical vapor deposition (PVD) replaces chemical vapor deposition (CVD).
) may be employed, and the processing conditions for either vapor deposition method are not particularly limited.

However, although the surface layer of the steel material becomes extremely hard by the above vapor deposition method, the difference in hardness between each part is extremely large, and the pitting resistance required for high surface pressure structural parts does not improve. Carburizing is performed before the above vapor deposition method, and quenching and tempering are performed after vapor deposition. As a result, the vapor deposited layer (hard layer)
The hardness distribution in the lower part of the layer changes smoothly, making it possible to achieve a stress distribution in which sufficient compressive residual stress remains directly under the first hard layer, thereby improving pitting resistance.

That is, as shown in Fig. 1, in the present invention, in order to obtain the above-mentioned hardness distribution, carburizing treatment is first performed to obtain the hardness distribution shown in Fig. 1.
A carbon distribution as shown in a) is secured, and then a hard layer is formed on the surface layer of the steel material by vapor deposition.

At this stage, the hardness distribution is as shown in Figure (b), where there is a large difference between the hardness of the hard surface layer and the hardness of each part. Based on the obtained C distribution, it is possible to obtain a hardness distribution in which the hardnesses of both the hardnesses are gently curved, as shown in FIG. 3(c). The hardness at a depth of 0.05 mm from the surface of sag 1 is H
It is possible to obtain a hardness distribution in which the effective hardened layer depth is 0.50 mm or more when v700 or more and I (v550) is obtained, and the pitting resistance can be dramatically improved.

At this time, there are no particular restrictions on the treatment conditions for carburizing, quenching, and tempering, and the quenching method may be a normal quenching method, or so-called high-density energy quenching such as induction quenching, laser quenching, or plasma quenching.

Although it is possible to obtain a hardness distribution that gradually decreases by increasing the C content and quenching and tempering without carburizing, the strength of each part will be too high. The aim is to suppress this to a level sufficient to ensure the minimum required strength of each part, and to increase the amount of C only to the required depth in the carburizing process. therefore,
It is also conceivable that carburizing treatment may be performed after vapor deposition before quenching.

The carburizing effect is more effective if it is carried out before forming a hard layer on the surface layer by vapor deposition.

(Example) A steel material having the chemical composition shown in Table 1 was melted, cast to a diameter of 30 mm, and then normalized to form a roller pitting test piece. This was subjected to carburizing treatment under the conditions shown in Table 2, and then chemical vapor deposition treatment to form a hard layer shown in the same table,
Then, after hardening from 800 to 850°C, tempering is performed at 180°C.

A roller pitching test was conducted.

In addition, a Charpy impact test piece (notch shape: 10 mm R, 2 mm depth) was prepared in the same manner, subjected to the above composite treatment, and then subjected to an impact test.

In addition, as a roller pitting test, a test piece having the shape and dimensions (mm) shown in FIG. 2 was brought into contact with a small roller 1 as shown in FIG. The small roller (test piece) was tested under the test conditions of a surface pressure of 300 kgf/ms+2 (thick arrow in the same figure) and a slip ratio of 140% (i.e., the ratio of the circumferential speed to the large roller).
Rotate 1, B,. The lifespan (times) was investigated.

The results of each of the above tests are shown in Table 3. In addition, the hardness of the hardened surface layer, the hardness of each part, the hardness at a depth of 0.05 mm from the surface and the effective hardened layer depth are Hv550 according to JIS.
The results are also listed in Table 2.

[Margin below 1] As can be seen from Tables 2 and 3, all of the comparative examples Nα6.12 that were simply carburized as in the past had poor pitting resistance, and that they were not subjected to chemical vapor deposition treatment. Comparative example Nα7.13, which was only subjected to further treatment, had good toughness, but
Since the hardness distribution is unfavorable, the pitting resistance is rather deteriorated.

On the other hand, the present inventions (1) and (2) have significantly improved pitting resistance, and in particular, the present invention (2) has poor toughness due to grain refinement, even though the material is strengthened. Since the problem has been overcome, the impact value has also improved.

(Effects of the Invention) As described in detail above, according to the present invention, it is possible to increase the surface hardness in an instant, and moreover, it is possible to achieve a preferable hardness distribution and stress distribution while maintaining an appropriate core strength. Because you can get
It is possible to significantly improve pitting resistance.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating changes in the hardness distribution of the hardened layer during the manufacturing process of the material for high surface pressure structural parts according to the present invention.
a) shows the C content distribution after carburizing, (b) shows the hardness distribution after vapor deposition treatment, (c) shows the hardness distribution after quenching and tempering, and Figures 2 and 3 show the roller pitting test, respectively. It is a figure which shows the shape of a piece, a dimension (mm), and a test procedure.

Claims (3)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.15 to 0.3
0%, Si≦1.0%, Mn≦1.5%, Al≦0.1
% and N≦0.03%, with the balance consisting of Fe and unavoidable impurities, and after carburizing the material, TiC
, has a hard layer deposited with TiN, etc., and has a hardness of Hv at a depth of 0.05 mm from the surface after quenching and tempering.
700 or more, and an effective hardened layer depth at which Hv550 is obtained is 0.50 mm or more. (2) C: 0.15-0.30%, Si≦1.0%, M
n≦1.5%, Al≦0.1% and N≦0.03%, furthermore Ni≦5.0%, Cr≦5.0% and Mo≦0.
．． Contains one or more of 80%, and the remainder is Fe.
After carburizing the material, it has a hard layer deposited with TiC, TiN, etc., and after quenching and tempering, the hardness reaches a depth of 0.05 mm from the surface. A material for high surface pressure structural parts, characterized in that the effective hardened layer depth at which Hv 700 or more and Hv 550 is obtained is 0.50 mm or more. (3) C: 0.15-0.30%, Si≦1.0%, M
n≦1.50%, Al≦0.1% and N≦0.03%, furthermore Ni≦5.0%, Cr≦5.0% and Mo≦
One or more of 0.80% and V≦0.5%
, Ti≦0.5%, Nb≦0.5% and Zr≦0.5%
A steel containing one or more of the following, the remainder being Fe and unavoidable impurities, is used as a material, and after carburizing the material, T
It has a hard layer deposited with iC, TiN, etc., and the hardness at a depth of 0.05 mm from the surface after quenching and tempering is Hv700 or more, and the effective hardened layer depth to obtain Hv550 is 0. A material for high surface pressure structural parts, characterized by a thickness of 50 mm or more.