Bi-sulfide existence in 0Cr18Ni9 steel: correlation with machinability and mechanical properties

摘要

This work investigates the effect of Bi addition on the formation of sulfide inclusions, and also aims to improve the machinability and mechanical properties of type 0Cr18Ni9 stainless steel. Smelting experiments as well as machining and tensile tests were carried out in various low and high Bi content steels. Results show that the forms of Bi-MnS inclusions were: isolated particle, cavity-bridge link, semi- and complete encapsulation. The forms of MnS inclusions were found as: mushroom or rod-like, long strip, spherical, and spindle shape. The normalized frequency of 0.065, corresponding to the inclusion area above 50 μm2, was the greatest at 2300 ppm Bi, reflecting an increase in the number of larger-sized inclusions. The averge hardness of steel matrix and the inclusions at 0 ppm Bi was 179 and 166 kgf/mm2, while that after Bi addition was 154～168 and 155～166 kgf/mm2, respectively. The Bi addition to steel improved the surface finish of the steels after machining and increased the ductility of steel, but had little effect on the yield strength of steel.