JPS63235452A - High-strength steel for drive transmission parts excellent in fatigue strength - Google Patents

Abstract

PURPOSE:To develop a high-strength steel for gear having high dedendum fatigue strength, by manufacturing drive transmission parts having a hard carburized layer on the surface of a gear, etc., by using a low-alloy steel containing specific amounts of Mo and Ni. CONSTITUTION:A high-strength steel, as drive transmission parts for toothed wheels, etc., such as gears and pinions, to be used after subjected to surface- hardening treatment by means of carburizing, is manufactured by using a low- alloy steel having a composition which contains, by weight, 0.15-0.30% C, <0.10% Si, 0.30-1.0% Mn, <0.010% P, 0.005-0.015% S, <15ppm O, 0.2-1.5% Cr, 0.5-2.4% Ni, and 0.5-1.0% Mo and in which the total amount of Mo and Ni is regulated to <=2.5%. In this steel, hardenability in the carburized outermost surface-layer part is secured by the addition of Mo, and, by the addition of Ni, notch sensitivity in the carburized surface-layer part is reduced, so that steel material for drive transmission parts such as gears excellent in fatigue strength and combining high strength with high toughness can be manufactured.

Description

[Detailed Description of the Invention] The present invention relates to high-strength steel for drive transmission parts having excellent fatigue strength, and particularly for use in drive transmission parts such as gears, binions, constant velocity joints, etc. The present invention relates to a high-strength, high-toughness steel with excellent fatigue properties that can be suitably used to manufacture carburized products.

Conventional technology Drive transmission parts such as gears and constant velocity joints in transportation machinery, industrial machinery, agricultural machinery, etc. are usually made of Cr steel such as 5Cr420 steel or Cr steel such as SCM 420 steel.
It is manufactured by forming r-Mo steel into a desired shape and then subjecting it to surface hardening treatment by carburizing.

However, as mentioned above, conventional steels for drive transmission parts, including gears, have M
Elements such as n, CrSMo, etc. are added. Among these elements, Mn and Cr are effective in improving the hardenability inside the carburized layer, but in the outermost carburized layer up to about 20 μm from the surface, due to internal oxidation, they do not form oxides. The amount present in a state that can contribute to improving hardenability is extremely low. In other words, grain boundary oxides of these elements are formed in the carburized outermost layer, which reduces hardenability, making it impossible for carburized products to obtain sufficient surface hardness and surface residual stress.As a result, for example, In the case of gears, high root fatigue strength cannot be achieved.

In addition, in the carburized outermost layer, incompletely hardened structures remain due to the above-mentioned decrease in hardenability.
As a result, the notch sensitivity in the outermost carburized layer increases,
Parts become easy to break.

For example, JP-A-59-74262 and JP-A-60-
The gear steel described in Japanese Patent No. 243252 also has too high notch sensitivity in the carburized outermost layer and does not have sufficient toughness, so that, for example, when cracks occur,
The rate of development is high and the parts have low fatigue strength.

In order to solve the above-mentioned problems in conventional steels for drive transmission parts, the present inventors discovered that MO is an element that is difficult to internally oxidize, and by using this, the hardenability of the outermost carburized layer was ensured. As a result of intensive research, we found that by adding MO in a range of more than 0.5% and less than 1.0%, we were able to secure hardenability in the outermost carburized layer and to add a predetermined amount of MO. The present invention was developed based on the discovery that by adding Ni, it is possible to reduce the notch sensitivity in the carburized outermost layer, thereby obtaining a high-strength, high-toughness steel for transmission parts with excellent fatigue strength. .

That is, an object of the present invention is to provide a high-strength, high-toughness steel for transmission parts that has excellent fatigue strength, and in particular, ensures sufficient carburizing and hardenability in the outermost carburized layer to achieve high dentinal fatigue strength. The object of the present invention is to provide high-strength gear steel that can be obtained.

. The high-strength steel for drive transmission parts with excellent fatigue strength according to the present invention has a weight percentage of C of 0.15 to 0.30%, Si of 0.10% or less, and Mn of 0.3 to 1. 0%, po, oio% or less, S 0.0.05 to 0.015%, 0 15pp
Cr 0.2-1.5%, Ni 0.5-2.4%, Mo more than 0.50% and 1.0% or less, the balance consisting of iron and inevitable impurities.

The chemical components of the steel according to the present invention will be explained.

C is an element necessary to obtain the required strength in drive transmission parts, for example, the required dedendum of 1 degree in gears, and must be added in an amount of at least 0.15%.

However, if it is added in excess of 0.30%, the processability of molding, for example, cutting, turning, drilling, etc., will be poor.

As mentioned above, Si is an element that tends to form grain boundary oxides, especially in the outermost carburized layer. Therefore, in the present invention, the amount of Si added is the minimum amount required for deoxidizing the steel. Therefore, it is preferably added within a range of 1.0% or less.

Mn and Cr are also elements that tend to generate grain boundary oxides next to St in the carburized outermost layer, but on the other hand, they generally have the effect of improving hardenability. However, in the present invention, the hardenability of the carburized part and part 6 is secured to a considerable extent by the addition of Mo and Ni, as will be described later. From the perspective of ensuring workability, the hardenability of the carburized part and part 6 was set to M.
It is preferable to reduce it to an extent that can be secured together with O and Ni.

In other words, it is necessary to keep the Mn and Cr contents within appropriate ranges in order to ensure the workability of steel, such as gear cutting workability. 1
It is preferable to add these elements within a range of 20 or less. The amount of Mn and Cr added increases the D1 value to 120m
In order to soften the steel to the extent necessary for gear cutting, for example, a very long softening process is required. Therefore, in the present invention, Mn
and Cr are added in an amount of 0.3% or more and 0.2% or more, respectively. However, when adding too much, it not only makes machining difficult as described above, but also, in the case of gears, for example, increases the dedendum strength too much, resulting in a decrease in fatigue strength. Therefore, the upper limit of the amount added is 1.0% for Mn and 1.5% for Cr.

Since P has a strong tendency to segregate at grain boundaries, in the present invention, grain boundary strength is strengthened by regulating the amount of P to 0.010% or less.

S forms, for example, sulfide-based inclusions such as MnS, and these inclusions reduce the root strength of gears, for example. However, on the other hand, S has the effect of improving workability such as cutting. Therefore, in the present invention, for example, S is added in a range of 0.005 to 0.015% in order to not reduce the root strength as described above and to improve workability. Ru.

As mentioned above, in the present invention, 15
Regulated below ppm.

Next, research into the mechanism of fatigue fracture shows that the fatigue strength of carburized parts is achieved by improving the hardness and compressive residual stress of the outermost carburized layer. has its limits.

This is due to the carburized product having too high notch sensitivity, as mentioned above.

Ni has the effect of increasing hardenability during carburizing, and furthermore,
During carburizing, the activity of C on the steel surface is increased, the surface carbon concentration is lowered, and a large amount of retained austenite is generated, thereby reducing the hardness of the outermost carburized layer.
This has the effect of reducing notch sensitivity and increasing fatigue strength.

In order to effectively obtain this effect, at least 0.5%
It is necessary to add Ni in an amount of 2.4% or less.However, even if added in excess, the above effect will be saturated and it is not preferable from the economical point of view of steel manufacturing. Added within a range. A particularly preferable range is 2.0% or less.

Furthermore, in order to further increase the fatigue strength of carburized parts, in addition to increasing the surface compressive residual stress and surface hardness, an area ratio of 10 to 40%, preferably 30 to 40%, is added to the carburized outermost layer.
It is particularly effective to coexist with retained austenite within a range of %.

As mentioned above, Mo is difficult to form grain boundary oxides, so it is an extremely important element in the steel of the present invention in order to ensure the hardenability of the outermost carburized layer.

In order to effectively obtain this effect, in the present invention, 0.
It is added in an amount exceeding 50%. Addition amount is 0°50%
If it is below, depending on the carburizing conditions, not only will it not be possible to obtain sufficient hardness in the carburized outermost layer, but in some cases, an incompletely quenched structure will be generated, increasing notch sensitivity. However, adding too much leads to the formation of a large amount of carbide, which actually impairs the effect of improving hardenability, so the upper limit of the amount added is set at 1.0%.

In particular, in carburized products, in order to ensure sufficient hardenability in the outermost carburized layer and at the same time obtain high toughness, MO
It is preferable that the total amount of Ni and Ni is within a range of 2.5% or less.

The steel according to the invention can be produced according to conventional methods,
For example, it may be a hot rolled material or a hot forged material. Furthermore, the method for carburizing the steel of the present invention is not particularly limited, and for example, in addition to gas carburizing, vacuum carburizing and ion carburizing may also be used.

As mentioned above in Matariko Shimai, the steel of the present invention ensures hardenability of the carburized outermost layer by adding a predetermined amount of Mo, and also improves the hardenability of the carburized outermost layer by adding a predetermined amount of Ni. Since it reduces sensitivity and thus has excellent fatigue strength, by carburizing it after forming into gears etc., it is possible to manufacture carburized parts for drive transmission, such as gears, which have excellent fatigue properties.

The present invention will be explained below with reference to 11 Examples, but the present invention is not limited to these Examples in any way.

Inventive steel and comparative steel having the chemical composition shown in Table 1 were melted in a 150 kg vacuum furnace, and after hot forging, annealing, machining, and carburizing to a diameter of 25 fins, their properties were investigated. Ta. Note that Table 1 also shows DI values.

Material Properties Table 2 shows the material properties of each of the above-mentioned steels, that is, surface hardness, surface residual stress, and surface residual austenite amount depending on area ratio. The surface retained austenite IT was determined by taking a microstructure photograph at 5000 times magnification using a scanning electron microscope and measuring the area ratio of the retained austenite portion in the photograph. In Table 2, in Comparative Steels 3 and 4, an incompletely quenched structure remains in the carburized outermost layer.

Further, Fig. 1 shows an electron micrograph of the present invention ml with a surface residual austenite content of 30.2%, and Fig. 2 shows an electron micrograph of a comparative steel with a surface residual austenite content of 48.3%. .

Rotating bending fatigue properties Figure 3 shows the results of the Ono rotary bending fatigue test.

The test was conducted at 20° C. and at a rotational speed of 3600 rpm using a test piece with a diameter of 81 irises at the parallel portion.

Turbine oil #18 was used as a lubricant in a thrust type testing machine for transfer fatigue characteristics.
0, contact stress 500 kgf/mi", rotation speed 3
The relationship between the number of stress repetitions and cumulative failure rate was investigated under the condition of 000 rpm. The results are shown in Figure 4.

Fatigue characteristics as gears Invention steel 1 and comparison steel 4 were used, respectively, module 25, pressure angle 20°, number of teeth 29 (number of teeth of mating gear 30), standard pitch circle diameter 72.5 mm, tooth gi Qm A gear was manufactured, carburized at a temperature of 925°C for 3.5 hours, cooled in a furnace, then heated at a temperature of 850°C for 30 minutes, quenched in oil, and then carburized at a temperature of 180°C. At 2
Hold for an hour and air cool.

Repeated stress was applied to the thus treated gear using a power circulation gear fatigue tester, and after 1×10 17 repetitions, the maximum dedendum strength without tooth breakage was defined as the dedendum strength. As a result, the tooth root strength of the gear made of the steel of the present invention was 70.
kgf/nm", but the gear made of comparative steel 4 was 6 kgf/nm".
6 kgf/n+m".

[Brief explanation of the drawing]

Fig. 1 is an electron micrograph showing the Mi weave in the carburized outermost layer after carburizing an example of the steel of the present invention, and Fig. 2 shows the 'IJi weave in the carburized outermost layer after carburizing an example of the comparative steel. FIG. 3 is a graph showing the rotary bending fatigue strength of the steel of the present invention together with that of comparative steel. FIG. 4 is a graph showing the rolling fatigue characteristics in relation to the number of stress repetitions and cumulative failure rate. It is. Patent applicant: Kobe Steel, Ltd. Representative: Patent attorney: Ittsu Makino Department No. 3 - Figure 1234,! r 67 Planning inclination

Claims (2)

[Claims] (1) In weight%, C 0.15-0.30%, Si 0.10% or less, Mn 0.3-1.0%, P 0.010% or less, S 0.005-0.015% , O 15ppm or less, Cr 0.2-1.5%, Ni 0.5-2.4%, Mo more than 0.50% and 1.0% or less, the balance consisting of iron and inevitable impurities. High-strength steel for drive transmission parts with excellent fatigue strength. (2) The high-strength steel for drive transmission parts according to claim 1, wherein the total amount of Mo and Ni is in a range of 2.5% or less.