Abstract
During the heat treatment of 0.3mass%Si-1mass%Mn-0.07mass%P ultra low carbon hot-rolled steel, readily oxidizable elements such as Si, Mn and P were internally oxidized and a depletion layer of these elements was formed at the steel surface region. The amounts of Si and Mn selective surface oxidation of the cold-rolled steel were remarkably suppressed after recrystallization annealing.By this heat treatment of the hot-rolled steel, the galvannealing rate of the galvanized steel was accelerated to a rate equivalent to that of mild steel. As a result, the galvannealing temperature of the galvannealed steel could be lowered and the anti-powdering properties of the galvannealed steel were remarkably improved.The reasons for acceleration of the galvannealing behavior and the improvement of the anti-powdering properties, were considered as follows :1) The amount of Si and Mn selective surface oxidation, which acted as a barrier for the Fe-Zn galvannealing reaction, was suppressed.2) The amount of solute P, which delays the Fe-Zn galvannealing reaction, was reduced.3) The reduction of galvannealing temperature brings about the suppression of the Γ and Γ1 phases formed at the higher temperature, those unfavorable for anti-powdering properties.
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