Effect on inclusion and microstructure of ultra-low S and low P steels with vacuum treatment

摘要

The simultaneous desulphurization and dephosphorization of molten steel with CaO-based slag containing BaO for vacuum treatment,and inclusion in steel and microstructure of steel are carried out separately.These results show that desulfurization rate exceed 92％ and dephosphorization rate exceed 50％,final [S] content and [P] content in molten steel are less than 0.0024％ and 0.020％ respectively for all heats,and the lowest final [S] content and [P] content are 0.0012％ and 0.010％ respectively,they satisfy the demand of ultra-low S steels and low P steels;Comparing a grade 1.5 of inclusion level of the compared sample without vacuum treatment,inclusion level of these treated samples with vacuum is all less than grade 1.0 and that is less than grade 0.5 for 78％ of treated samples; And micro-structure of the compared sample is ferrite + pearlite in steel base,but micro-structure of these treated samples is all needle-like fenrite + pearlite + netted ferrite in steel base and netted ferrite + pearlite in local area.This kind of fiber structure which comes from netted ferfite + needle-like ferrite and grade 0.5 of inclusion level can make steel possess better mechanical properties.