KR100353631B1 - Heat treatment method of bearing steel casting with excellent center segregation - Google Patents

Abstract

The present invention improves the heat treatment temperature and cooling conditions of continuous cast Bloom to reduce material center segregation, which reduces the fatigue life of ball bearings, thereby reducing segregation at the center of the bloom slab and at the same time of cementite cementite. The present invention relates to a heat treatment method of a ball bearing steel slab having a reduced thickness. In order to reduce the central segregation of a slab in a bloom state in the manufacture of bearing steel, the slab is heated to a temperature in the range of 1250 to 1280 ° C., and the thickness of cementite cementite is reduced. It is characterized by cooling at a cooling rate of 8 to 15 ℃ / sec in the temperature range of 850 to 400 ℃ before phase transformation.

According to the present invention, the center segregation of the ball bearing steel slab is greatly reduced, the thickness of the cementite cementite is significantly reduced, and the quality of the bearing steel material is improved.

Description

Heat treatment method of bearing steel casting with excellent center segregation

The present invention relates to a heat treatment method of a ball bearing steel having excellent central segregation, and more particularly, heat treatment of a continuously cast bloom to reduce material center segregation which decreases the fatigue life of the ball bearing. The present invention relates to a method for heat treatment of a ball bearing steel slab which reduces the segregation at the center of a bloom slab by improving the temperature and cooling conditions and at the same time reduces the thickness of cementite cementite.

Segregation due to the amount of carbon generated in the center of cast steel during ball bearing steel casting has the greatest adverse effect on the mechanical properties of the ball bearing steel.

Bearing steel containing about 1% by weight of carbon (C) and 1.4% by weight of chromium (Cr) is refined while maintaining a strong reducing atmosphere in the ladle after steelmaking in the converter to reduce the amount of non-metallic inclusions, RH) After the degassing process, the oxygen content is lowered to 10ppm, and then hot scarfing is performed to remove defects on the surface of the material after continuous casting. Soaking Heat Treatment is performed to remove segregation at the center of the cast slab in the bloom state and is rolled to Billet through natural air cooling.

Subsequently, in order to soften the material in the second and third wire rod factories, ultra-cold operation is performed to produce the material. The produced material is processed into ball bearings through drawing and spheroidizing heat treatment, cold rolling and grinding, and then quenching. It is produced as ball bearing, the final product, through quenching, tempering and polishing processes.

The ball bearing produced through the above process is very fragile due to the formation of cementite in the center, so it is cracked at the center of the material which is the center segregation part of the center segregation part and the center part of the wire rod during the continuous casting during the fatigue test or use. This happens frequently.

This is a conventional technique for reducing segregation in the center of cast steel and the formation of cementite cementite, which have the greatest adverse effect on the mechanical properties of the bearing steel.The superheat of molten steel is adjusted to 20 ℃ or lower and the casting speed is about 0.63m / Adjust min to reduce segregation by increasing the ratio of equiaxed crystals formed in cast steel, or adjust the cooling rate in the forced air cooling zone during wire rod manufacture to reduce the amount of cementite cementite formed in the center of wire rod and at the same time There is a method of improving material quality by making the layer thickness of (Pearlite) smaller.

In addition, Japanese Patent Application Laid-Open No. 96-132205 discloses a method of reducing segregation in bearing steel by performing a light pressure of 10 to 100 mm when operating in a vertical player, and in Japanese Patent Application Laid-Open No. 94-248302. Attempts have been made to control segregation by means of equipment, such as the installation of rolls on the solidification part and the application of light pressure to control the pressure.

However, in the case of the bearing steel material produced by the prior art as described above, the center segregation still remains, so that the problem that the quality of the material is deteriorated due to the generation of cornerstone cementite due to such residual segregation has not been solved.

The present invention has been made in order to solve the problems of the prior art described above, heat treatment of the bearing steel slab improved the quality of the bearing steel material produced by significantly reducing the center segregation generated in the center of the material and the thickness of the cementite cementite The purpose is to provide a method.

1 is a view showing a manufacturing process of a bearing steel,

Figure 2 is a view showing a comparison of the cooling conditions of the present invention and the prior art,

3 is a view showing a comparison of the segregation index of the present invention and the prior art,

Figure 4 is a view showing a comparison of the thickness of the cementite cementite of the present invention and the prior art.

In order to achieve the above object, in the present invention, in the method of heat-treating the cast steel in the bloom (Bloom) state in the production of bearing steel, in order to reduce the center segregation, the cast steel is heated to a temperature in the range of 1250 ~ 1280 ℃, the thickness of cementite cementite It provides a heat treatment method of the bearing steel cast for cooling to a cooling rate of 8 ~ 15 ℃ / sec in the temperature range of 850 ~ 400 ℃ before phase transformation.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments.

In the heat treatment method of the present invention, as shown in Fig. 1, the ball-shaped ball bearing steel which is continuously cast in order to effectively reduce the center segregation remaining in the cast steel, and has undergone a hot scarping process to remove surface defects of the material. In the process of soaking heat, the cast is heated at a heating temperature of 1250 to 1280 ° C, but when heated to 1250 ° C or less, diffusion of alloying elements that cause segregation in the center is delayed and thus there is little effect of reducing segregation. If heated above 1280 ℃, the size of austenite becomes larger and thicker cementite cementite is formed around the grain boundary even though the cooling rate of the cast is controlled in the post-process. The effect of reducing the stone is negligible and the heating temperature is limited to 1250 to 1280 ° C.

In addition, as shown in FIG. 2, in the present invention, unlike the conventional natural cooling, in order to reduce the thickness of the cementite cementite during cooling of the cast steel, the cooling rate is adjusted to 8 to 15 ° C / sec in the temperature range of 850 to 400 ° C. Cool down.

At this time, it is preferable that the cooling rate is adjusted to 8 to 15 ° C / sec by a forced cooling method using compressed air.

The cooling rate is adjusted to 8 to 15 ° C./sec because the accelerated cooling effect is small when the casting cooling rate is 8 ° C./sec or less, so that the thickness reduction effect of the cornerstone cementite is small, and the decrease in the layer thickness of the pearlite is small. If the cooling rate is higher than 15 ° C / sec, harmful cold tissue such as bainite or martensite may be formed, causing cracks on the surface.

It adjusts at 8-15 degreeC / sec.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

Table 1 shows the component composition of the target material to which the present invention is applied.

Steel grade Chemical component (wt%) C S O ₂ (ppm) N ₂ (ppm) Si P Mn Cr Ti Cu SUJ2 1.02 0.003 10 34 0.23 0.012 0.37 1.45 0.003 a very small amount

In the case of the present embodiment, the inventive material was heated for 6 hours with the heating temperature of 1250 to 1280 ° C. during the crack heat treatment of the bloom-shaped slab of the above-described continuous casting, and cooled by adjusting the phase transformation cooling rate to 10 ° C./sec.

The comparative material was heated at a heating temperature of 1200 ° C. for 6 hours, and the cooling rate was the same at 10 ° C./sec.

3 and 4 are measured the invention materials while changing the heating temperature to 1250 ℃, 1260 ℃, 1270 ℃, 1280 ℃, and the comparative material was measured by repeated tests four times (Segregation Ratio) and the thickness of the cement cementite In comparison with the present invention, the segregation index is significantly reduced in the case of the inventive material, and the thickness of the cementite cementite is significantly reduced.

Therefore, according to the present invention, the central segregation of the bearing steel slab is significantly reduced, the cementite cementite is reduced and the ferrite layer spacing is reduced, thereby improving the mechanical properties such as improving the surface hardness of the manufactured bearing steel and improving the fatigue life. do.

Claims (1)

In the process of heat-treating the cast steel in the bloom state of bearing steel, in order to reduce the center segregation, the cast steel is heated to a temperature in the range of 1250 to 1280 ° C. and 850 to 400 ° C. before the phase transformation to refine the thickness of the cementite cementite. A heat treatment method for a bearing steel slab, characterized by cooling at a temperature ranging from 8 to 15 ° C / sec.