KR100423436B1 - A Method for Manufacturing Bearing Steels Having Excellent Fatigue Property - Google Patents

Abstract

The present invention relates to a method for manufacturing medium-carbon bearing steel with excellent fatigue properties, and its object is to provide a medium-carbon bearing for excellent fatigue performance by controlling the composition of steel and the manufacturing conditions of steelmaking, casting, and steel sheet processes. To manufacture steel.

The present invention for achieving the above object by weight, C: 0.40-0.70%, Si: 0.10-0.40%, Mn: 0.40-1.00%, P: 0.015% or less, S: 0.010% or less, Cr: 0.10- Molten steel composed of 0.40%, Total [O] (total oxygen content) of 10 ppm or less and the remaining Fe and other unavoidable impurities was subjected to furnace external refining by securing 5-7 basicity of ladle slag in the furnace refining process. Next, during the bloom casting, the casting speed is maintained at a casting speed of 0.80-0.9 M / min, and the bloom is cast, and the thus-formed bloom is heated and cracked, followed by rolling. The point concerning bearing steel manufacturing methods is taken as the summary.

Description

A method for manufacturing bearing steels having excellent fatigue property}

The present invention relates to a method for manufacturing a medium-carbon bearing steel that can be applied for automotive hub unit bearing (HUB) with excellent fatigue performance, and more particularly, adopts an optimum alloy component in steelmaking process, The present invention relates to a method for manufacturing bearing steel with improved fatigue performance by performing low speed casting in.

The bearing steel for HUB is manufactured into BILLET by the following process, and the manufactured billet is made of round bar rolling and HUB outer ring material by the first and second demand.

Charter preliminary treatment → converter blow → LF → RH → performance BLOOM → rolled steel sheet → BILLET (120x120mm)]

[BILLET → Round Rolling (23-40mm∮) → Cutting → Hot Forging → Turning → Grinding → Carburizing HUB Bearing Manufacturing → Fatigue Test (HUB Assembly Durability Test)]

The conventional bearing steel for outer ring of HUB was used as JS-S55C steel, which is a structural steel, and mass production application technology such as converter [C] operation and slag composition control, which is a manufacturing technology of high clean steel, was not established. The level of Total Oxygen (Total Oxygen Content), a measure of the cleanliness and fatigue performance of bearing steels, was 10-15 ppm, making it difficult to meet the demand for quality.

Recently, the total [O] demand level of automobile makers (MAKER) and bearing manufacturers is increasing from 10ppm or less to 15ppm or less.

In addition, in the case of conventional steel has also been continuously raised the problem of the presence of large non-metallic inclusions of more than 25㎛.

In addition, conventionally, high speed casting was performed at a casting speed of 0.95-1.00 M / min to improve surface quality during BLOOM casting.

As described above, the above mentioned quality factors are the main cause of fatigue failure early in vehicle mounting after manufacturing HUB bearing.

The present inventors have conducted research and test production in order to improve the above-mentioned problems, and the present invention has been proposed based on the results. The present invention provides a method of controlling the composition of steel and the manufacturing conditions of the steelmaking, rolling, and slab processes. To manufacture bearing steel for medium carbon-based hubs with excellent fatigue performance, its purpose is to manufacture.

Figure 1 is a tissue photograph showing the non-metallic inclusion tissue of the billet 1 / 4t site according to the present invention

1 is a tissue photograph showing a non-metallic inclusion structure of the billet 1 / 4t region according to the comparative example

The present invention for achieving the above object by weight, C: 0.40-0.70%, Si: 0.10-0.40%, Mn: 0.40-1.00%, P: 0.015% or less, S: 0.010% or less, Cr: 0.10- Molten steel composed of 0.40%, Total [O] (total oxygen content) of 10 ppm or less and the remaining Fe and other unavoidable impurities was subjected to furnace external refining by securing 5-7 basicity of ladle slag in the furnace refining process. Next, during the bloom casting, the casting speed is maintained at a casting speed of 0.80-0.9 M / min, and the bloom is cast, and the resulting bloom is heated and cracked, followed by rolling. A bearing steel manufacturing method.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

Bearing steel of the present invention is C: 0.40-0.70%, Si: 0.10-0.40%, Mn: 0.40-1.00%, P: 0.015% or less, S: 0.010% or less, Cr: 0.10-0.40%, Total [O] (Total Oxygen Content) Manufactured using molten steel controlled to 10 ppm or less and the remaining Fe and other unavoidable impurities.

The carbon (C) is a component contributing to the improvement of hardness, and in the case of 0.40% or less, excessive carburizing time and cost are required to match the heat treatment hardness of the steel (JS-SUJ2, 1.0% C-1.4% Cr steel) used as the inner ring of the HUB. There is a problem, and the material hardness is higher than 0.70%, it is preferable to limit the content to 0.40-0.70% due to processing problems.

The silicon (Si) is an element added for the deoxidation effect, and when excessively added, it causes surface decarburization in the bearing manufacturing process, causing deterioration of workability and carburizing property, and therefore it is preferable to limit the content to 0.10-0.40%. .

The manganese (Mn) is an element that improves the hardenability, and has an effect of increasing the final fatigue life of the bearing. However, the content of the manganese (Mn) is limited to 0.40-1.00% because it causes the toughness of the final bearing product to be reduced. .

The phosphorus (P) and sulfur (S) causes surface cracks, internal cracks, center segregation, etc. during BLOOM casting, so the content is limited to P: 0.015 or less and S: 0.010 or less, respectively.

The chromium (Cr) promotes hardenability and carbide formation to improve abrasion resistance. The effect is insufficient at 0.10% or less, and at 0.40% or more, the content thereof increases with the deterioration of toughness of the final product and the increase in production cost. It is preferable to limit to 0.40%.

The total [O] (total oxygen content) is a measure of the level of non-metallic inclusions in the steel, and combines with Al, Ti, etc. in the steel to form high-strength non-stretched inclusions, which causes premature fatigue failure of the HUB bearing, so the content is less than 10 ppm. It is preferable to limit to.

The steel designed with the above composition was refluxed in hot metal pretreatment (Station) at the hot metal pretreatment (station) for 10 minutes or more by adding refractory materials such as CaCO ₃ , CaF ₂ , and sintered semi-reflective light. It is desirable to lower the temperature to 1,650 ° C. or lower, and to carry out low temperature casting.

The reason for this is that it is difficult to sufficiently reduce the contents of harmful components P and S during the Catch [C] operation, which has shorter drilling time than before. To reduce.

After performing the charter preliminary treatment, it is preferable to perform the CATCH [C] operation in which the blowing is stopped at a carbon content of 0.20% or more in the converter blowing process.

The reason is that in the case of high-clean steel such as bearing steel, oxygen is blown until the carbon in the steel becomes zero (ZERO) in the drilling process as in the prior art, so as to form a large amount of non-metallic inclusions by combining with Al and Ti in the steel. This is because non-metallic inclusions in the steel can be reduced by stopping the blowing at 0.20% C or higher to reduce the oxygen blowing.

In the present invention, using the molten steel as described above, in the furnace refining process to ensure the basicity of the ladle slag (LADLESLAG) to 5-7 to perform the furnace external refining, then the casting speed of the bloom rate of 0.80-0.9M / min It maintains as it is, and casts a bloom, heats and cracks the thus produced bloom, and rolls.

In the molten steel, a slag basic composition is secured at a level of 5 to 7 by adding a raw material such as quicklime in an RH degassing process to secure high cleanliness of the steel.

The reason is that in order to manufacture high-clean bearing steel, the inclusions generated in the steel should have good adsorption capacity of the inclusions of slag formed on the molten steel at the time of reflux of molten steel for RH degassing treatment. It is because the low melting point of liquid coexistence can form slag.

The high-purity steel produced as described above is cast into a playing bloom, but the casting speed is preferably maintained at 0.80-0.85 M / min.

The reason is that if it is too fast, the center segregation increases, and if it is too slow, the surface cracks are concerned and productivity is lowered. That is, to reduce segregation of P, S, Cr, etc. formed at the end of the solidification during bloom casting. In addition to the casting speed, the molten steel superheat can be maintained at 30 ° C. or lower. A better effect can be obtained.

The heating and cracking is preferably maintained by heating the surface temperature of the cast bloom to 300-400 ℃ charged to a steel mill factory furnace, heating and cracking 3-4 hours at 1250-1280 ℃.

This is because segregation such as P, S, C, etc. formed in the center of the bloom during the casting process and Cr carbide, etc. are diffused at a high temperature for a long time, which is advantageous for reducing segregation and carbides.

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

Example

The slab having the chemical composition of Table 1 was prepared by the blast furnace-converter-continuous casting method, the slag composition was prepared as shown in Table 2 below.

turn C Si Mn P S S-Al Cr Invention steel One 0.56 0.27 0.82 0.011 0.004 0.028 0.13 2 0.55 0.23 0.84 0.012 0.003 0.026 0.20 3 0.57 0.26 0.85 0.010 0.004 0.026 0.25 4 0.55 0.25 0.83 0.012 0.005 0.025 0.15 5 0.53 0.24 0.83 0.010 0.004 0.026 0.18 Comparative steel 6 0.53 0.24 0.81 0.017 0.010 0.027 - 7 0.55 0.25 0.83 0.015 0.011 0.025 - 8 0.56 0.26 0.85 0.015 0.012 0.026 - 9 0.55 0.24 0.83 0.018 0.013 0.024 -

The casting process is described in more detail as follows.

That is, the molten iron made from the blast furnace is dephosphorized and deflowed in the molten iron preliminary treatment, charged to the converter, and then the molten steel is subjected to RH degassing under the slag composition shown in Table 2 below for laminating and refining the ladle. The molten steel was prepared, and the slag high base also secured the composition by introducing a flux (FLUX) to improve the absorption of the steel inclusions of the slag produced at this time.

The molten steel was continuously cast at a casting speed of 0.85 m / min and a molten steel superheat condition of 25 ° C. to form a bloom, and then charged into a steel plate heating furnace at a temperature of 350 ° C. and heated at 1250 ° C. for 3 hours. After hot rolling, it was prepared as a bilet, and the cleanliness, Total [O] (total oxygen content) and nonmetallic inclusion levels were investigated and the results are shown in Table 3 below.

division number CaO Al ₂ O ₃ SiO ₂ MgO CaO / SiO ₂ CaO / Al ₂ O ₃ Invention steel One 52.5 30.3 9.1 5.5 5.8 1.7 2 53.2 28.4 8.6 5.6 6.2 1.9 3 53.6 32.8 8.8 5.9 6.1 1.6 4 51.8 29.5 8.9 5.3 5.8 1.8 5 54.2 31.0 8.5 6.0 6.4 1.7 Comparative steel 6 54.1 16.7 19.8 6.1 2.7 3.2 7 54.5 16.0 18.5 6.3 2.9 3.4 8 53.0 18.1 17.5 6.0 3.0 2.9 9 55.2 16.6 17.0 6.1 3.2 3.3

division number cleanliness(%) Total [O] .ppm Non-metallic Inclusions (ASTM E45) Sulfide Alumina Silicate Oxide Invention steel One 0.03 8 1.0 / 0.5 0.5 / 0.0 0.0 / 0.0 0.5 / 0.0 2 0.03 7 1.0 / 0.5 0.5 / 0.0 0.0 / 0.0 0.5 / 0.0 3 0.04 7 0.5 / 0.0 0.5 / 0.5 0.0 / 0.0 0.5 / 0.0 4 0.04 8 1.0 / 0.5 1.0 / 0.5 0.0 / 0.0 0.5 / 0.5 5 0.03 9 1.0 / 0.5 0.5 / 0.0 0.0 / 0.0 0.5 / 0.0 Comparative steel 6 0.06 10 1.5 / 0.5 1.0 / 0.5 0.0 / 0.0 1.0 / 0.5 7 0.07 12 1.0 / 0.5 0.5 / 0.5 0.0 / 0.0 0.5 / 0.5 8 0.05 13 1.5 / 0.5 1.0 / 0.5 0.0 / 0.0 0.5 / 0.0 9 0.05 11 1.5 / 0.5 1.0 / 0.5 0.0 / 0.0 1.0 / 0.5

As can be seen in Table 3, in the case of the invention steel that satisfies the conditions of the present invention, Total [O] (total oxygen content) was less than 10 ppm, and also superior to the comparative steel in cleanliness and non-metallic inclusions. .

Meanwhile, the bearing steel billet for HUB manufactured by the above process is subjected to the fatigue performance test in the state of HUB bearing assembly after the bearing is forged at the first demand and round bar rolling and the second demand, and then assembled. Table 4 shows.

division HUB Bearing Assembly Fatigue Performance (Front / Rear) Quality requirements 33.3Hr or more (= 220,000 km of mileage equivalent when applying a vehicle) Invention steel 43-72Hr / 41-65Hr Comparative steel 12-45Hr / 23-43Hr

As can be seen in Table 4, the invention steel that satisfies the conditions of the present invention was not only satisfies the required quality, it was found that the fatigue life of the steel is superior to the comparative steel.

This can be seen from Figures 1 and 2 comparing the non-metallic inclusion structure of the billet 1 / 4t region of the inventive steel (2) and comparative steel (8).

According to the present invention as described above, the optimum alloy component in the steelmaking process of the steel, adopting the [C] operation at 0.20% C or more during the steelmaking process, the upper part of the RH ladle in the furnace refining process of the molten steel slag It is possible to manufacture a billet of HUB bearing steel with excellent fatigue performance by securing the composition with a high base degree, performing low-speed casting to reduce internal segregation in the casting process, and then performing high-temperature diffusion cracking to reduce bloom segregation in the steel slab process.

Claims (3)

By weight%, C: 0.40-0.70%, Si: 0.10-0.40%, Mn: 0.40-1.00%, P: 0.015% or less, S: 0.010% or less, Cr: 0.10-0.40%, Total [O] Oxygen content): The molten steel with 10ppm or less and the remaining Fe and other unavoidable impurities is subjected to external furnace refining by securing the basicity of ladle slag to 5-7 in the furnace external refining process, and then casting speed is 0.80-0.9M / A method for producing a medium carbon bearing steel having excellent fatigue characteristics, characterized in that the casting is performed at a casting speed of min and the rolling is performed after heating and cracking the cast bloom. The method of claim 1, The bloom casting is a method of manufacturing a medium-carbon bearing steel with excellent fatigue characteristics, characterized in that to maintain the molten steel superheat (ΔT) below 30 ℃ The method of claim 1, The heating and cracking method for producing a medium carbon bearing steel having excellent fatigue properties, characterized in that the bloom is heat treated at 1250-1280 ° C. for 3-4 hours after maintaining the surface temperature of 300-400 ° C.