Kinetic analysis of decarburizing during continuous annealing in extra low carbon steel

Abstract

Mechanism of decarburization in ultra low carbon steel sheet at the continuous annealing process has been investigated. Thermo-Calc and DICTRA programs were used for calculation analysis of decarburization and the calculation results were compared with those observed by chemical analysis. Decarburization was observed in the ultra low carbon steel sheets in a short continuous annealing treatment at 750 °C for 10 s. Decarburization and nitriding were observed in 3%H2+97%N2 gas atmospheres. Precipitation of Nb(C, N) affected decarburization kinetics, and the amount of decarburization decreased with the increase of Nb content because precipitates did not resolve during annealing. The decarburization rate obtained by this experiment is in a good agreement with those reported by Collin et al. The rate was dependent upon gas atmosphere and annealing temperature, but independent of carbon content in steel. Control of decarburization kinetics in continuous annealing process is indispensable for quality control of materials, because Yield Strength of ultra low carbon steel sheet after continuous annealing is closely related with the carbon content of steels.