Effect of Cu and Sn on phosphatability of cold-rolled steel sheets

The promotion of scrap recycling in steel making is effective to reduce the discharge of CO2 and prevent the global warming. But, the scrap recycling has been limited to the production of low grade steel products such as shapes and bars, because of the detrimental effect of tramp elements such as Cu and Sn on the productivity and the quality of steel products. In this study, the effect of Cu and Sn on the zinc phosphatability in dip type phosphating of cold-rolled steel sheets annealed in N-2-H-2 and vacuum atmospheres are investigated to obtain the technical information which is necessary to extend the scrap recycling to sheet products. The following results are obtained. 1) The zinc phosphatability of cold-rolled steel sheets annealed in N-2-H-2 atmosphere deteriorates slightly with increasing Cu content, however, it deteriorates remarkably with increasing Sn content. 2) The zinc phosphatability of cold-rolled steel sheets annealed in vacuum atmosphere deteriorates remarkably with increasing both Cu and So contents. 3) The deterioration of zinc phosphatability of cold-rolled steel sheets annealed in both atmospheres with increasing So content is considered to be caused the decrease in surface reactionability with the segregation of So on surface.