Key metallurgical technology for high-quality bearing steel production based on the nonaluminum deoxidation process

Abstract

Bearing steel is subjected to complex alternating stress conditions for a long time which requires excellent service properties such as high hardness, high wear resistance, high elastic limit, and high contact fatigue strength. Therefore, during bearing steel production, it is necessary to strictly control the process and improve the purity of steel to ensure high precision, long service life, and high reliability of bearings. China has made considerable progress in the production technology of high-quality bearing steel, and some enterprises can produce world-class bearing steel. However, the stability of bearing steel still requires improvement. Currently, the aluminum deoxidation process is mainly used to produce bearing steel at home and abroad. Through aluminum deoxidation and the production of high-alkalinity slag, the oxygen content in liquid steel can be rapidly reduced. The total oxygen mass fraction in high-quality bearing steel can be controlled below 5×10−6. However, fatigue failure caused by occasional Ds-type inclusions still occurs. Concurrently, other problems such as blockage of small billet continuous casting nozzle and difficulty in stable control of ultralow total oxygen and titanium content also occur. To circumvent the aforementioned problems, this study proposed a nonaluminum deoxidation process by adding silicon–manganese alloy for pre-deoxidation during converter tapping, adding silicon deoxidizer to the ladle furnace (LF) slag surface for diffusion deoxidation, and Ruhrstahl‒Heraeus (RH) vacuum deep deoxidation to ensure that the total oxygen mass fraction of the molten steel was approximately 8×10−6, to produce bearing steel. While ensuring the low aluminum and low titanium contents of liquid steel, low-alkalinity slag is used to change the type of inclusions and control the plasticity of inclusions to effectively solve the problem of liquid steel fluidity. The fatigue life of bearing steels by two kinds of processes was evaluated using the ultrasonic fatigue testing machine, the effects of different types of inclusions on fatigue performance were verified, the fatigue fracture mechanism of bearing steels by different processes was analyzed, and the critical size of inclusions causing fatigue cracks was predicted. The application of the aforementioned key technologies plays a guiding role in the large-scale production of nonaluminum deoxidized high-quality bearing steel. However, its quality still lags behind the most advanced production level of bearing steel worldwide, including the poor desulfurization effect caused by the use of low-basicity slag in the refining process, which needs to be further investigated.

Keywords

• bearing steel
• silicomanganese deoxidation
• vacuum deep deoxidation
• inclusion
• ultrasonic fatigue