Advantages of Si Deoxidation of Bearing Steels for Steel Cleanness and for Composition and Morphology of Nonmetallic Inclusions in Rolled Product

摘要

The conventional production of bearing steel requires aluminum deoxidation with regard to lowest contents of oxygen improving the rolling contact fatigue life to a very high level. However, for more than 20 years, Saarstahl AG has used the advantages of silicon deoxidation's ability to avoid the formation of pure high melting alumina inclusions to fulfill the stringent requirements of premium quality for the bearing and the automotive industries. Silicon deoxidized heats have an excellent castability during billet and bloom casting due to the absence of clogging. Saarstahl's steel production is characterized by the use of alloys with very low contents of aluminum and titanium as well as slag formers without any harmful components. Due to very strict ladle metallurgy, the oxygen content in the finished product is in the range of 4 ppm to 8 ppm. The result is an excellent cleanness of the finished product with regard to ultrasonic immersion tank testing and microscopic evaluation according to ASTM E45, Standard Test Methods for Determining the Inclusion Content of Steel, that easily meets the requirements of the ball bearing industry. In the rolled material, harmless low melting inclusions with similar hardness to the steel matrix were observed. Both stringer (C-type) and intercepted (B-type) inclusions were analyzed having the same composition and mineralogical phases. With an increasing reduction ratio, a transition from stringers to intercepted inclusions occurs without changing the chemical composition of these inclusions. Using selective treatment methods in the secondary metallurgy, a very low level of low melting inclusions was achieved in the rolled product.