誘導式真空溶解での炭素による脱酸平衡についての一考察

Abstract

As pointed out in previous report, (Tetsu-to-Hagane, Vol 47 (1961) pp.715-720), the degassing effect in vacuum induction melting by carbon should be considered with the study of the phenomenon of the kinetics.Standing on this point, series of melts of iron-base alloys containning chromium have been made in magnesia crucibles in vacuum by carbon degassing practice, and some fundamentals such as the carbon-oxygen relationship, influence of the hydrostatic pressure on degassing effect, the process of CO bubble formation, crucible reaction and effect of Si during the procedure were investigated.The results were as follows:(1) The oxygen in the melt was rapidly reduced down with vigorous boiling. After the boiling was completed, the oxygen reached nearly the constant value. In the author's experiments, the critical value of Pco at this stage obtained by substitution of the experimental activities of carbon and oxygen was some 10mmHg regardless of the alloy composition melted.(2) The oxygen level attainable by carbon degassing process was not influenced by changing the hydrostatic pressure in the vacuum tank in range of 1μHg-10mmHg.(3) Comparing the change in the carbon and oxygen contents during the melt, the carbon lost was far in excess of that necessary to account for the observed oxygen drop.(4) The abovementioned results could be explained as follow:The rapid removal rate of oxygen, at the early stage of the melting, was mainly dependent on the high value of the Pco. As the internal pressure of CO bubbles was gradually reduced and attained to the summed value of the factors, the hydrostatic pressure and surface tension of the molten metal, that limits growth of the bubbles, the bath became quiescent. It was presumed that an equilibrium state might be set up at the stage when the rate of removal of oxygen from the melt was equal to the rate of solution from the refractory.(5) In vacuum melting of austenitic stainless steel, the residual oxygen after the carbondegassing process was determined by the dynamic equilibrium relationship. In this case, the presence of silicon was apt to lower the residual oxygen in comparing with silicon free melts. Such an effect seemed to be attributed to the vaporization of silicon as Si suboxide gas in vacuum.