JP2005120466A - HIGH STRENGTH Cr-Mo ALLOY STEEL FOR SPEED CHANGE GEAR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength Cr-Mo alloy steel for a speed change gear which can solve the problem that nickel added when producing carburizing steel for a gear is expensive and reduces workability, and the the problem in the reduction of productivity, the reduction of the service life in a working tool or the like caused by the difficulty in the working of a gear using Ni-Cr-Mo alloy steel. <P>SOLUTION: The high strength Cr-Mo alloy steel for a speed change gear has a composition composed essentially of Fe and having alloy based components comprising, by weight, 0.17 to 0.21% C, ≤0.15% Si, 0.60 to 0.85% Mn, ≤0.02% P, ≤0.03% S, 1.25 to 1.45% Cr, 0.55 to 0.65% Mo and 0.015 to 0.035% Nb, and in which the content of dissolved oxygen is controlled to ≤15 ppm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength Cr-Mo alloy steel for transmission gears, which can realize a cost town while having excellent physical properties, and can perform high-temperature carburization by maximizing the effect of grain refinement. This relates to a high-strength Cr—Mo alloy steel for transmission gears that can reduce the heat treatment cycle time.

  In general, as shown in FIG. 1, the manufacturing process of an automobile transmission gear is as follows: material → hot or cold forging → cooling (air cooling or cooling) → pre-processing heat treatment (ISO annealing or normalizing) → processing (shaving & hobbing) ) → Carburizing heat treatment → Post-processing (polishing, Honing) and the like.

Conventionally, materials often used as alloy steel for automobile transmission gears include Cr alloy steel, Cr-Mo alloy steel, Ni-Cr-Mo alloy steel, etc., which are mainly used after carburizing heat treatment, It can be said that the most suitable alloy steel material is suitable for each process, inexpensive and excellent in physical properties after heat treatment.
That is, those excellent in forgeability and workability, cheap in price, and excellent in physical properties (for example, fatigue characteristics, impact characteristics, etc.) are preferable.

Cr alloy steel and Cr—Mo alloy steel are inexpensive, but they are not excellent in fatigue characteristics and impact characteristics and are often used mainly for gears that do not receive a large load (Japanese Patent Laid-Open No. 9-201644, special characteristics). Kaihei 4-88148).
On the other hand, Ni-Cr-Mo alloy steel is expensive because it contains Ni and has a major disadvantage that it is difficult to process, but it has excellent fatigue characteristics and impact characteristics, so it is mainly applied to gears that receive large loads. (Japanese Patent Laid-Open Nos. 9-296250 and 63-235452).
In automobile manufacturers such as North America and Europe, Ni-Cr-Mo alloy steel is often used for automobile gears and shafts because Ni is cheaper than in Asia.

Usually, in the case of carburized steel for gears, in order to increase hardenability and obtain high strength and high toughness, alloy elements Mo and Ni are added, but Ni is expensive and excessively increases the toughness of the steel. Cause various problems to increase. For example, when processing into a gear, there is a problem that the cutting performance is poor, the tooth surface roughness of the gear is deteriorated, and the life of the processing tool is shortened, leading to a decrease in productivity.
Therefore, development of Cr—Mo alloy steel that can replace Ni—Cr—Mo alloy steel, which is excellent not only in terms of hardenability and workability but also in terms of cost reduction, is desired.
Japanese Patent Laid-Open No. 9-296250 Japanese Patent Laid-Open No. 63-235452 Japanese Patent Laid-Open No. 9-201644 JP-A-4-88148

  The present invention has been made to solve the above-described problems. Nickel added at the time of manufacture of carburized steel for gears is expensive and has a problem that workability is lowered. The object is to provide a high-strength Cr—Mo alloy steel for transmission gears that can solve problems such as a decrease in productivity due to difficulty in processing the gear to be applied and a reduction in the life of the processing tool.

  In addition, the disadvantages of conventional Cr-Mo alloy steel and Ni-Cr-Mo alloy steel have been improved, and the advantages such as high strength, high durability, and compatibility with compact design can be maintained as they are, and the effect of grain refinement has been greatly improved. The object is to provide a high-strength Cr-Mo alloy steel for transmission gears that can reduce the cost town by shortening the heat treatment cycle time by high-temperature carburization due to the improvement.

Hereinafter, the present invention will be described in more detail.
The present invention relates to a high-strength Cr-Mo alloy steel for transmission gears, which contains Fe as a main component, 0.17 to 0.21 wt% C, 0.15 wt% or less Si, 0.85 wt% Mn, 0.02 wt% or less P, 0.03% wt or less S, 1.25 to 1.45 wt% Cr, 0.55 to 0.65 wt% %, And an alloy component containing 0.015 to 0.035% by weight of Nb, and the dissolved oxygen content is 15 ppm or less.

The high-strength Cr-Mo alloy steel for transmission gears according to the present invention has the same or better characteristics in spite of being cheaper than Ni-Cr-Mo alloy steel containing expensive Ni. The effect of extending the life is obtained.
In particular, it is widely applicable to gears and shafts (such as Output Final Gear, Differential Drive Gear, and Input and Output Shaft of high-power manual transmission), which are difficult to apply to conventional Cr-Mo alloy steel due to high load. Have the advantages.

In the high-strength Cr-Mo alloy steel for transmission gears according to the present invention, Ni is not added in order to improve workability. Instead, the content of Cr and Mo is changed to the content of conventional Cr-Mo alloy steel. More characteristic complemented.
Moreover, the problem of the surface abnormal layer increase due to the Cr content increase is overcome by lowering the Si content, and the Mo carbide precipitation problem due to the Mo content increase is usually caused by the Nb content having a higher carbide forming affinity than Mo. Overcame with more.
Moreover, the workability fall by the strength improvement which is a result of the Cr and Mo content increase was improved by increasing S content in the harmless range.

Hereinafter, the main constituent elements of the present invention will be described in more detail.
Table 1 shows a comparison of the components of the Cr-Mo alloy steel according to the present invention and the components of the conventional Cr-Mo alloy steel and Ni-Cr-Mo alloy steel.

Ni is not added to the Cr—Mo alloy steel of the present invention from the viewpoint of improving workability and reducing costs, and C is added in an amount of 0.17 to 0.21% by weight in consideration of physical properties and hardenability.
The Si content was set to 0.15% by weight or less with respect to 100% by weight of the alloy steel to reduce the surface abnormal layer generated during carburizing.
The reason for this is that, in the present invention, the content of Cr, which is likely to cause a surface abnormal layer to improve the characteristics, is increased as compared with the conventional Cr-Mo alloy steel. This is because an adverse effect due to excessive formation of the abnormal layer appears.

The Cr content was set to 1.25 to 1.45% by weight with respect to 100% by weight of the alloy steel. When the Cr content is less than 1.25% by weight, the characteristics of the conventional Cr—Mo alloy steel are obtained. This is because the characteristics are not improved.
In addition, when the Cr content exceeds 1.45% by weight, the cost increases, and when carburizing, Cr carbide (or Cr-Mo carbide) is precipitated on the carburized layer on the surface, and the brittleness becomes strong. This is because the thickness of the surface abnormal layer becomes 15 μm or more despite the Si content being limited to 0.15 wt% or less.

Mo contains 0.55-0.65 weight% more than the conventional Cr-Mo alloy steel with respect to 100 weight% of alloy steel. When the content is less than 0.55% by weight, the improvement effect is not so large compared with the characteristics of the conventional Cr-Mo alloy steel, as in the case of Cr. Therefore, the Ni-Cr-Mo alloy steel is substituted. There is no improvement effect.
When the Mo content exceeds 0.65% by weight, the Cr-Mo based carbide is not precipitated at the grain boundaries in the carburized layer, although the content of Nb having a higher carbide forming affinity than Mo is increased compared to the existing content. However, there is a problem in that fine cracks are generated around the grain boundaries and brittleness is increased.

In order to improve the decrease in workability, S increases the upper limit of the content to 0.03% by weight within a harmless range as compared with the conventional Cr—Mo alloy steel.
In addition, Nb was added in an amount of 0.015 to 0.035% by weight in order to prevent coarsening of crystal grains and maximize the effect of crystal grain refinement during high temperature carburization.
Nb is an element that forms fine Nb nitrides and carbides in the alloy steel and suppresses the coarsening of crystal grains. If its content is less than 0.015% by weight, the effect is not so great. If it exceeds 035% by weight, there is a problem that carbides are excessively precipitated at the grain boundaries to increase brittleness.
In the Cr-Mo alloy steel of the present invention, the upper limit of the Nb content is increased compared to the conventional Cr-Mo alloy steel, but this is to prevent the formation of Mo carbides and Cr carbides due to an increase in the Mo and Cr contents. It is.
Further, in order to reduce the content of impurities in the steel and improve the cleanliness, the amount of dissolved oxygen was suppressed to 15 ppm or less.

Hereinafter, the present invention will be described in more detail through examples, but the present invention is not limited thereto.
The alloy components of Examples and Comparative Examples of the present invention are as shown in Table 2 below, and alloy steels were produced by ordinary methods.
In Table 2, Examples are alloy steels of the present invention, Comparative Examples 1 to 4 are alloy steels manufactured to analyze the influence of properties due to alloy content, and Comparative Examples 5 to 7 are mass produced in the past. And alloy steel in application.

Test Example The basic physical properties necessary for a transmission gear were tested and compared for the examples and comparative examples in Table 2.
Tables 3 and 4 and FIG. 2 show the results. Table 3 shows the test results of fatigue characteristics and impact characteristics after carburizing heat treatment.
Table 4 shows the results of a comparison of processability, which is an important factor when manufacturing gears, and shows the number of gears processed per tool when a specific gear for a manual transmission is shaved. Show.
The workability test was performed on the steel examples of the present invention and the conventional steel comparative examples 5-6.

  FIG. 2 is a result of comparing the fatigue characteristics in the gear state by comparing the steel example of the present invention and the comparative example 7 of the conventional steel currently used as a high-strength material by European manufacturers.

  As can be seen from Table 3, the steels of the examples of the present invention exhibited significantly superior characteristics compared to the comparative example 5 which is a conventional Cr-Mo alloy steel, even though Ni was not added. In comparison with Comparative Examples 6 and 7 which are Ni-Cr-Mo alloy steels, the same or better characteristics were shown.

Summarizing the results of adjusting and comparing the contents of Cr and Mo, when the contents of both alloy elements exceed the content range of the present invention (Comparative Example 1 and Comparative Example 3), the hardenability is high. In addition, the carburized hardened layer has a disadvantage that a large amount of Cr-Mo carbide precipitates at the grain boundaries and the brittleness increases (impact value reduction).
On the other hand, when the content is less than the content of the present invention, it has been found that the fatigue-related characteristics are remarkably inferior.

In addition, the size of the crystal grains when Nb is added is superior in all respects compared to the case where Nb is not added (Comparative Example 5) due to the refinement effect due to the addition of Nb. Addition makes it possible to perform high-temperature carburization and shorten the heat treatment cycle time.
The workability was also significantly improved (about 33%) in the example of the present invention as compared with Comparative Example 6 which is a conventional Ni—Cr—Mo alloy steel. This is because the S content is increased. This is because the deterioration of workability due to the strength improvement was improved.

It is a manufacturing process figure which shows the manufacturing process of the gearwheel for transmissions of a motor vehicle. It is a graph which shows the result of the bending fatigue test of a gearwheel.

Claims (1)

  With Fe as the main component, 0.17 to 0.21 wt% C, 0.15 wt% or less Si, 0.60 to 0.85 wt% Mn, and 0.02 wt% or less P 0.03% by weight or less of S, 1.25 to 1.45% by weight of Cr, 0.55 to 0.65% by weight of Mo, 0.015 to 0.035% by weight of Nb, A high-strength Cr-Mo alloy steel for transmission gears, characterized in that the dissolved oxygen content is 15 ppm or less.

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