EFFECT OF S IN STEEL ON HYDRAULIC-DESCALI NG-ABILITY IN SI-ADDED HOT-ROLLED STEEL SHEETS

Effect of S on red scale defect, which is often observed as abnormal r ed scale stripes within normal black scale matrix especially in hot-ro iled Si-added steel sheets, has been studied by means of laboratory ho t rolling test. It was found that the area of red scale defect on the sheet surface decreases with increasing the S content. This can be exp lained by the formation of the eutectic compound of FeO/FeS with the l ow melting point of 940 degrees C between steel substrate and upper eu tectic compound of FeO/Fe2SiO4 with the melting point of 1173 degrees C, i.e., the descaling ability of the primary scale is largely improve d by the existence of the liquid phase in the process. The cause of fo rmation of only FeS without MnS at the scale/steel interface in spite of significantly high amount of 1.5%Mn, is also discussed on the basis of chemical equilibrium diagrams calculated by the thermodynamic data base ''MALT2''.