KR20090061404A

KR20090061404A - Heat treatment for ultra fine grained steel for transmission shaft

Info

Publication number: KR20090061404A
Application number: KR1020070128408A
Authority: KR
Inventors: 강창원
Original assignee: 현대자동차주식회사
Priority date: 2007-12-11
Filing date: 2007-12-11
Publication date: 2009-06-16

Abstract

The present invention relates to a heat treatment method of ultra-fine grain steel for a transmission shaft, and more particularly, a transmission for optimizing heat treatment conditions performed after hot forging to facilitate cold processing such as spline rolling and external turning, hole machining, etc. The present invention relates to a heat treatment method of an ultra fine grain steel for a shaft.

To this end, the present invention comprises the steps of heating the ultra-fine steel forgings made hot forging for 180 minutes at 740 ~ 760 ℃; Cooling to 610-630 ° C. at a rate of 4-5 ° C./min; 15 minutes at 610 to 630 ° C., followed by air cooling; It provides a heat treatment method of ultra-fine grain steel for a transmission shaft characterized in that proceeds to.

Description

Heat treatment for ultra fine grained steel for transmission shaft

Shafts for automobile transmissions typically use carburized alloy steel, and are manufactured through a process such as material → hot / cold forging → heat treatment before processing → machining → carburizing heat treatment, as shown in the process diagram of FIG. 1.

After the forging is performed to the final shape of the product in the state of the raw material, the outer diameter turning, hole drilling and cold forming process spline part rolling, etc. are processed to match the final shape.

At this time, in order to facilitate such processing and homogenization of the microstructure, heat treatment such as normalizing and isothermal annealing is generally performed before processing.

Next, after processing, carburization heat treatment is performed to improve the strength of the product.

Recently, in order to increase the strength and reduce the manufacturing cost of a transmission shaft, a medium-carbon ultrafine steel having a high frequency heat treatment instead of a carburizing heat treatment has been developed.

The ultrafine steel has a grain size of about 5 μm after high frequency heat treatment, and has very high torsional strength and wear resistance compared to general alloy steel having grains of about 20 μm.

The composition and composition ratio of the ultrafine grained steel are based on iron (Fe), about 0.55% by weight of carbon (C), about 0.70% by weight of silicon (Si), about 0.50% by weight of manganese (Mn), and phosphorus (P). 0.020 wt% or less, sulfur (S) 0.020 wt% or less, chromium (Cr) about 0.50 wt%, molybdenum (Mo) about 0.20 wt%, niobium (Nb) about 0.020 wt%, and boron (B) about 30 ppm Made of steel.

The applicant of the present application is to manufacture the shaft for the transmission using the ultra-fine grain steel, the forging by hot forging after cutting the material, and then the conditions of Figure 2 attached to the normalizing heat treatment before general processing (900 ~ 930 ℃ For 120 minutes).

However, in the subsequent machining process, the outer diameter turning and hole drilling process did not have a big problem, but the hardness value of about 230 ~ 240 Hv level was found to be impossible to roll spline part, which is a cold forming process, this problem is relatively high hardness Because of this, it may be determined that cold forming is difficult.

Substantially, in the corresponding shaft for the transmission shown in FIG. 3, a total of three spline rolls should be performed.

Therefore, in order to apply the ultra-fine grain steel to the shaft for the transmission, it is necessary to check the hardness value of the spline rolling, which is a cold forming process, and to obtain an appropriate pre-processing heat treatment condition to obtain the hardness value.

Accordingly, the present invention is a method for applying ultrafine steel to a shaft for a transmission, and when the spline precursor step, which is a cold forming process, is provided, by providing an optimized pre-processing heat treatment condition for the ultrafine steel, a spline precursor that is a cold forming process It is an object of the present invention to provide a heat treatment method of ultra-fine grain steel for a transmission shaft that can be easily made.

The present invention for achieving the above object comprises the steps of heating the ultra-fine steel forgings hot forging for 180 minutes at 740 ~ 760 ℃; Cooling to 610-630 ° C. at a rate of 4-5 ° C./min; 15 minutes at 610 to 630 ° C., followed by air cooling; It provides a heat treatment method of ultra-fine grain steel for a transmission shaft characterized in that proceeds to.

Through the above problem solving means, the present invention can provide the following effects.

By improving the pre-processing heat treatment process for the ultra-fine forged steel, which is hot forged, it is possible to adjust the hardness of the product to a level that can be easily spline rolled, which is a cold forming process. Rolling can proceed very well, and also offers the advantage that the external turning and hole machining can proceed satisfactorily.

Hereinafter, the present invention will be described in more detail.

As described above, the transmission shaft manufacturing process using ultra-fine grain steel proceeds in the order of material → hot forging → heat treatment before processing → machining → high frequency heat treatment.

After the forging is manufactured to be close to the final shape of the part through hot forging, heat treatment is performed before machining, and then the final transmission shaft shape is manufactured by turning, hole machining, and spline part rolling. Heat treatment ensures strength.

According to the present invention, a spline precursor of a cold forming process may be easily formed by performing a preprocessing heat treatment process on a pre-process heat treatment condition optimized for ultrafine steel so that a spline precursor step of a cold forming process may be easily performed. One point is to make sure.

First, looking at the existing pre-processing heat treatment method associated with the present invention.

After hot forging using ultra-fine grained steel, normalizing heat treatment, which is a general heat treatment method before processing, was performed. After heating at 900 to 930 ° C. for 120 minutes using a heat treatment furnace, air was cooled.

Then, it was confirmed that the microstructure was transformed to bainite, the hardness value was about 230 ~ 240 Hv level.

After such heat treatment, there was no big problem such as external diameter turning, hole drilling, etc., but there was a problem in that the spline part rolling, which is a cold forming process, was impossible due to the relatively high hardness value.

In other words, in the hardness condition that the spline part can be rolled, cold forming is possible when the hardness of the forged product is about 190 Hv or less, but the hardness value of the existing heat treatment method is about 230 ~ 240 Hv level, and thus the spline part which is cold forming process It is a problem that cannot be forged.

On the other hand, when the hardness value is less than 175 Hv, chip curling occurs during turning and hole machining, resulting in deterioration of workability, which may cause problems such as poor surface roughness and reduced tool life. In addition, excessive bending may occur.

Accordingly, in manufacturing a shaft for a transmission of ultrafine steel, an appropriate hardness value of a forged product for spline rolling and outer diameter turning, hole drilling, etc., which are cold forming processes, will be 175 to 190 Hv.

Therefore, the present invention proposes a pre-processing heat treatment condition for ultrafine steels to have a hardness value of the 175 ~ 190 Hv level, the heat treatment process diagram is as shown in FIG.

4 is a process chart showing the heat treatment conditions before processing of the ultrafine steel according to the present invention.

First, the step of heating the ultra-fine steel forgings hot forging for 180 minutes at 740 ~ 760 ℃, after heating up to 740 ~ 760 ℃ using a heat treatment furnace, and then for 180 minutes to homogenize the band structure formed during hot forging Will be maintained.

At this time, the temperature is raised to 740 ~ 760 ℃ is to minimize the grain growth during heat treatment, it is preferable to maintain the ultra-fine grain forgings at 740 ~ 760 ℃.

Next, as a step of cooling to 610 ~ 630 ℃ at a rate of 4 ~ 5 ℃ / min, after the above step is cooled to a furnace at a rate of 4 ~ 5 ℃ / min to 610 ~ 630 ℃ temperature below the A3 transformation point, wherein 610 The reason for the furnace cooling at a rate of 4 to 5 ° C. per minute to ˜630 ° C. is to allow the microstructure of the ultrafine grain steel to be transformed into a ferrite + pearlite structure.

Finally, after maintaining for 15 minutes at 610 ~ 630 ℃, air-cooling step, by maintaining for 15 minutes at 610 ~ 630 ℃ to maintain the entire part in the heat treatment furnace at a uniform temperature and then air cooling.

The air cooling is then performed for 15 minutes at 610 to 630 ° C., since the entire part is brought to a uniform temperature condition, and thus the heat deformation error of each part during air cooling can be minimized.

After the heat treatment of the present invention, the microstructure has a mixed structure of ferrite + pearlite, the hardness value was confirmed to have a target hardness value of about 180 Hv through a conventional hardness measurement equipment, the hardness of the 180 Hv level The value would be most appropriate in line with the 175-190 Hv level for spline rolling, a cold forming process.

After applying the heat treatment method of the present invention as described above, spline rolling, which is a cold forming process, in the processing of a part can be performed very well, and also external diameter turning and hole machining can be performed satisfactorily.

After all, through the heat treatment method of the present invention, it is possible to apply the ultrafine steel to the shaft for the transmission including the cold forming process of the spline headlamp.

Hereinafter, the embodiment of the present invention will be described in more detail with a comparative example, but the present invention is not limited by the following examples.

Examples and Comparative Examples

As an example, the ultra-fine steel forgings with hot forging were heated at 750 ° C. for 180 minutes, then cooled to 620 ° C. at a rate of 5 ° C./min, and then air-cooled for 15 minutes at 620 ° C.

As a comparative example, normalization (120 minutes at 900-930 ° C.), which is a conventional heat treatment method, was performed.

After the heat treatment before processing according to the Examples and Comparative Examples, the microstructure was observed through an optical microscope, as shown in the accompanying Figure 5 (a) in the case of normalizing the conventional heat treatment method before processing, the microstructure It can be seen that it is a bainite structure, as shown in the accompanying Figure 5 (b) it can be seen that the microstructure of the ultra-fine grain steel to which the heat treatment method of the present invention is applied shows a mixed structure of ferrite and pearlite having a lower hardness than bainite there was.

Test Example

After the ultra-fine steel forgings with hot forging were heat-treated before processing according to the Examples and Comparative Examples, the hardness was measured three times or more using a conventional hardness measuring instrument. The results are shown in Table 1 below. .

As shown in Table 1 above, the products subjected to the heat treatment before processing of the present invention were measured at hardness values of 182,187,177 Hv over the first to third orders, respectively, and were found to exhibit an average hardness value of 182 Hv, which is cold. It can be seen that the spline precursor, which is a molding process, is possible.

1 is a process chart for explaining a conventional manufacturing method of a shaft for an automobile transmission;

2 is a view for explaining a pre-processing heat treatment method for a conventional ultrafine steel for a transmission shaft,

3 is a photograph showing a state of a transmission shaft,

4 is a heat treatment process diagram illustrating a heat treatment method of the ultrafine grain steel for a transmission shaft according to the present invention;

Figure 5 is a photograph of observing the microstructure of the ultrafine steel after the conventional heat treatment method and the heat treatment method of the present invention.

Claims

Heating the ultra-fine steel forgings for which hot forging is performed at 740 to 760 ° C. for 180 minutes;

Cooling to 610-630 ° C. at a rate of 4-5 ° C./min;

15 minutes at 610 to 630 ° C., followed by air cooling;

Heat treatment method of ultra-fine grain steel for a transmission shaft, characterized in that proceeded to.