The redox reaction and vacuum treatment are utilized in the refining process. The steel is held in a vessel, of which inner wall is composed of refractories, with Fe_tO containing slag under gas phase in the refining processes. Therefore, the contents of impurities are limited by the reactions among molten steel, gas, slag and refractories. This means that control of oxygen behavior in the system is the most basic key in the refining technology. In the present paper, the limit of oxygen content and composition of deoxidation products are discussed based on the deoxidation equilibrium of plain and high ally steels. And then the limits of impurities such as phosphorus, sulfur, nitrogen and hydrogen in steel refining process are assessed in consideration of the reactions among liquid steel, gas, refractories and slag containing a small amount of Fe_tO in secondary stellmaking process. The lowest contents of impurities in large scale production of steel have recently reached less than 50 ppm of N, 20approx30 ppm of P and O, 10 ppm of C, S and H, respectively. However, it is very hard in practice to decrease the contents less than the current level. In the present paper, the evidence is shown based on our experimental results on the clean steel production of ultra low P, O and S contents.
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