Structure and performance characteristics of mold flux films for continuous casting of special alloy steels

Flux films and liquid slags for low-expansion alloy Fe-36Ni, austenitic stainless steel 304 and martensitic stainless steel 420J2 were taken from casting mold. Chemical compositions were measured by chemical analysis; apparent morphologies of flux films were photographed; cross-section structure, thickness, crystalline fraction and crystalline phases of flux films were examined by SEM, EDS and XRD; viscosity and break temperature were calculated by models. The results showed flux films and liquid slags have similar chemical compositions, but there is a certain difference from the original powders. Flux films for the three steel grades have obvious layered structure and main crystalline phase cuspidine, viscosity and break temperature keep steady during casting. Heat transfer across flux films characterized by morphology, thickness, viscosity, break temperature and crystalline fraction of flux films showed flux film for austenitic stainless steel 304, martensitic stainless steel 420J2 and low-expansion alloy steel Fe-36Ni have the best, moderate and the lowest ability to control heat transfer. Heat transfer across flux films agrees with solidification characteristic of steel grades generally. Low-expansion alloy Fe-36Ni has good surface quality and there is no occurrence of hot-rolling cracking. Stainless steels 304 and 420J2 have no surface cracking but local depressions.