STUDY OF COMPLEX DUCTILE INCLUSIONS CONTROLLING IN MEDIUM-HIGH CARBON STEELS

Formation of a special kind of complex ductile inclusions (CDI), which were character as oxides covered by sulphides, was investigated in a medium-high carbon steel. It was believed that fatigue and toughness properties of carbon steels were benefited from-CDI formation, for oxides were separated from steel matrix. By means of thermodynamic calculation and inclusion microscopy in medium-high carbon steels with different S content and with Ca-treated, CDI in 0.55C steels has been researched. And thermal-stability of the CDI was researched by high temperature quenching. The results show that the frequency of CDI formation increase with S content changes from 0.001% to 0.01%. However, the fraction of covered oxides is no more than 90% without Ca-treated and can be promoted to about 100% in 0.006% S steel after Ca-treated. The reason of improved CDI formation by increasing S content and Ca-treated is that sulphides precipitation temperature could be promoted to higher than solid line of matrix, which is clearly showed by thermodynamic calculation. Furthermore, thermo-stability of CDI is affected heavily by heating history of matrix steel. Sulphide will dissolute from oxide when heat steel casting to an improper high temperature. Fortunately, the dissolution process could be stunted with redundant S content. It shows that thermo-stability of CDI could be received in Ca-treated steels with more than 0.006%S.