Establishment of technique for improving surface quality for recycling of steels in resource-cycled society

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The suppression of surface cracking in hot rolling is important in recycling of steel. The surface cracking is caused by copper contained in steel scrap. The behavior of the hot surface cracking and the effect of boron on the suppression of the cracking were investigated in air and LNG combustion gas by using the steel containing 0.5%Cu and the steel containing 0.3%Cu and 0.04%Sn.The surface cracking of Sn-bearing steel is more intensive than that of Sn-free steel. Increasing the oxygen and steam contents accelerates promotes the surface cracking. The occurrence of the cracking is associated with the morphology and amount of Cu-enriched phase at the steel/scale interface. Addition of boron (B) to steel is effective in suppressing the surface cracking. The effect is the largest in air for Sn-free steel and is the smallest in LNG combustion gas for Sn-bearing steel. Addition of B does not change the oxidation behavior and has no effect on the morphology and amount of Cu-enriched phase at the steel/scale interface. Boron is segregated at the austenite grain boundaries of B-bearing steels. In the test for evaluating the easiness of the penetration of liquid Cu into grain boundaries of steel, the penetration is restrained with an increase in B content of steel or Cu-enriched phase. The increase in B content of steel surface can suppress the surface cracking in hot rolling. This is attributed to restraint of penetration of Cu-enriched phase into steel grain boundaries because the decrease in grain boundary energy due to the segregation of B and the increase in solid/liquid interface energy due to dissolution of B into Cu-enriched phase.