During the period of time, the engineer systematically observed the materials, smelting, spheroidizing, gestating and pouring, and found the problem in the 14th smelting. The plant collects the slag from the molten iron after the spheroidization with magnesium addition to a certain amount (this period is about one month), and it is once put into the cupola for melting. The slag which has been treated by spheroidizing on the surface of the molten iron contains a large amount of magnesium sulfide and iron sulfide. After the magnesium sulphide is smelted, the magnesium is reduced by oxidation, and the sulphur remains in the molten iron, so that the S content in the molten iron is increased. When spheroidizing with normal magnesium addition, the high sulfur content in the molten iron consumes more Mg, which lowers the Mg content in the molten iron, resulting in poor spheroidization or spheroidization. To this end, chemical analysis of iron scraps from the roll body of the molten iron is carried out, w(S) is as high as 0.06% to 0.08%, and w(Mg) is only 0.02% to 0.03%. . According to the national standard for cast iron rolls, w(Mg) > 0.04% and w(S) < 0.03% in ductile iron rolls. The analysis data shows that the use of the slag from the surface of the molten iron after spheroidizing treatment is used as the returning charge, which results in the casted rolls being high in S content, low in Mg content and poorly spheroidized. Since then, the use of such slag has ceased, and there has been no problem of inadvertent spheroidization of the infinite cold chill roll.