Abstract: The effects of superheat on center segregation in continuously cast slab of X65 pipeline steel were investigated. The results of
macrostructure etching showed that center segregation was dispersed across the equiaxed zone and the negative segregation was thick, which
results in the thick incidence of center segregation in the thickness direction of the slab. However, in highsuperheat cast slab, the center
segregation was compressed in a narrow range in the vicinity of slab center and formed the obvious centerline segregation which was thinner
than that of lowsuperheat cast slab. It is revealed by the chemical analysis results that the fluctuation of element content across the
thickness with lowsuperheat casting was larger than that of high-superheat casting. Compared to the lowsuperheat cast slabs, the
average maximum segregation index of carbon and manganese was decreased from above 1.20 and 1.04to below 1.10 and 1.02 respectively in high-superheat cast slabs. The above mentioned results indicated that high-superheat with soft reduction was more effective to improve and control the center segregation in X65 pipeline steel. The application results of high-superheat (>25  ℃) with soft reduction on X65 acid resistant pipeline steel showed that the average maximum segregation index of carbon and manganese was decreased from 1.18and 1.04 to 1.06and 0.99, while the yield ratio and impact toughness were increased from 0.83 and  370 J  to 0.92 and 430 J respectively.

Key words: X65 pipeline steel, center segregation, low-superheat, high-superheat, maximum segregation ratio, soft reduction, mechanical property