Inversed Phosphorus Segregation in Twin Roll Cast Strips for Improvement of Mechanical Properties and Weathering Resistance

In the present paper, twin roll strip casting was applied to fabricating the strips of low carbon steels containing different P contents. P was spontaneously segregated near strip surfaces by the deformation in mushy zone during strip casting. Uniform surface layers enriched in P can be formed after subsequent processing of cold rolling and annealing, which can be used as surface coatings for the improvement of both weathering resistance and mechanical properties including toughness and elongation. In terms of plasticity, the elongation of conventional strips started drastically decreasing with P content beyond 0.15% by weight. By contrast, the elongation of the cast strips containing 0.26% P remained at the level of about 30%, with the tolerated ability of P being increased by about 0.1% by weight by twin roll strip casting. The ductile–brittle transition temperatures (DBTT) for the cast strips after cold rolling and annealing were measured to be lower than those for the conventional strips after hot rolling, cold rolling and annealing by 10–15°C. For 120 cycles of corrosion exposure, the weight gains of the 0.15 and 0.26P cast strips were reduced by more than 30% as compared with that on the conventional strip containing 0.08% P. It has been demonstrated that phosphate enriched layers could be formed beneath the corrosion scales of the 0.15 and 0.26P cast strips after cold rolling and annealing, which reduced the corrosion rate and improved the corrosion resistance.