Behavior of Spot Segregation in Continuously Cast Blooms and the Resulting Segregated Band in Oil Pipe Steels

Abstract

Microscopic dendritic morphology and spot segregation in continuously cast round blooms produced by different intensities of In‐mold Electromagnetic Stirring (M‐EMS) are investigated to elucidate their effect on the segregated bands in the hot‐rolled steel pipes. It is found that segregation spots mainly exist in the center equiaxed crystal zone of round blooms. The ratio of the equiaxed crystal zone area (E.R.) increases with the increase of the M‐EMS intensity. Moreover, the ratio of the total segregation spots area (S.R.) becomes larger with the increase of E.R. After hot deformation, the segregation spots are plastically deformed to form segregate bands in steel pipes. Both the number and the width of the segregated bands increase with the S.R. in round blooms. The severe segregation of Mo, Mn, and Cr in the segregated bands is in accordance with that in the spot segregation. It is necessary to control the degree of the spot segregation in as‐cast products to suppress the formation of the segregated bands in oil pipe steels. According to the experimental results, columnar structure in round blooms is preferred to effectively reduce the degree of the spot segregation and further decreases the occurrence of the segregated bands in steel pipes.