Formation mechanism of a white spot defect on the tin plate steel producedby continuous cast slab

In order to get high productivity on continuous casting process, slab must be cast on higher speed. But high speed cast slab has caused defects on col d rolled sheet owing to the powder used in the mold. In case of producing t in plate sheet by slabs cast under the condition of high speed, the defect which contain composition of the mold powder has been found and named as wh ite spot defect. In this paper, formation mechanism of white spot defect is discussed based on an analysis of this defect and investigation of the operational conditio ns. White spot defects reappear in the laboratory experiment and formation mechanism presumed in this paper proves to be right. The results are summarized as follows; (1) White spot defects on tin plate sheet are consisted of inclusions which include the mold powder of continuous casting process. (2) The formation mechanism is corrosion of tin plate sheet caused by NaOH. This NaOH is formed by chemical reaction between Na of mold powder and wat er which invaded into the inclusion during tin plating process. (3) White spot defect can be decreased by using mold powder containing less amount of Na on continuous casting process.