Use of CO2 in argon-oxygen refining of molten metal

Abstract

An improved argon-oxygen decarburization process for refining stainless steel comprising injecting a three component gas mixture consisting of oxygen, argon and carbon dioxide into the molten steel during decarburization. .[.The critical upper limit of CO 2 injection into the melt is defined, below which the carbon removal efficiency is improved..]. The preferred flow rate of CO 2 during each of the three principal phases of decarburization is set forth as a function of the flow rate of oxygen and argon, bath temperature and carbon content of the melt.

What is claimed is: 
     
       1. In a process for decarburizing a chromium-containing molten steel characterized by the subsurface injection of oxygen and at least one inert gas selected from the group consisting of helium, neon, krypton, argon, xenon and nitrogen, into a mass of said molten steel, wherein at least a portion of said oxygen reacts with the carbon in said molten steel to form a volatile carbon oxide, comprising a first phase of decarburization wherein the temperature of said molten steel is adjusted to the desired operating range; a second phase of decarburization wherein the carbon content of the molten steel is reduced to a predetermined value corresponding approximately to the carbon content of the melt in equilibrium with CO at a partial pressure of 1 atmosphere and at a temperature within said desired operating range; and a third phase of decarburization wherein the carbon content of the melt is reduced from said predetermined value to approximately the desired carbon content of the molten steel, the improvement comprising: injecting a gas consisting essentially of CO 2  into the molten steel during said third phase of decarburization in an amount less than that defined by the formula:   F.sub.CO.sbsb.2 = [F.sub.I P/(1-P - 2XF.sub.O.sbsb.2 ]     where:     F co .sbsb.2 = flow rate of CO 2 , (cfm)   F i  =  flow rate of said inert gas, (cfm)   P = equilibrium partial pressure of CO for the particular bath temperature and carbon content of the molten steel, (atmospheres), and   X = carbon removal efficiency in the absence of CO 2 .   
     
     
       2. The improvement as in claim 1 wherein the flow rate of CO 2  injected into the melt is defined approximately by the formulae: ##EQU2##   F.sub.O.sbsb.2 = F.sub.T - F.sub.I - F.sub.CO.sbsb.2     where:   F co .sbsb.2 = flow rate of CO 2 , (cfm)   F o .sbsb.2 = flow rate of O 2 , (cfm)   F i  = flow rate of inert gas, (cfm)   F t  = total gas flow rate for the particular system, (cfm)   t = blowing time, (minutes)   C i  = carbon content of the melt at the start of the blow, (percent)   C f  =  carbon content of the melt at the end of the blow, (percent)   T i  = metal bath temperature at the start of the blow, (° F)   T f  = metal bath temperature at the end of the blow, (° F)   W = total weight of molten metal, (tons)   K r  = measured heat loss coefficient of the vessel (° F/min)   X = carbon removal efficiency in the absence of CO 2 , and   Z i  = enthalpy of the inert gas at the operating temperature of the bath.   
     
     
       3. The improvement as in claim 1 further including the step of injecting a gas consisting essentially of CO 2  into the molten steel during said second phase of decarburization approximately as defined by the formulae: ##EQU3##   F.sub.O.sbsb.2 = F.sub.T - F.sub.I - F.sub.CO.sbsb.2     where:   F co .sbsb.2 = flow rate of CO 2 , (cfm)   F o .sbsb.2 = flow rate of O 2 , (cfm)   F i  = flow rate of inert gas, (cfm)   F t  = total gas flow for the particular system, (cfm)   t = blowing time, (minutes)   C i  = carbon content of the melt at the start of the blow, (percent)   C f  = carbon content of the melt at the end of the blow, (percent)   T i  = metal bath temperature at the start of the blow, (° F)   T f  = metal bath temperature at the end of the blow, (° F)   W = total weight of molten metal, (tons)   K r  = measured heat loss coefficient of the vessel (° F/min)   X = carbon removal efficiency in the absence of CO 2 , and   Z i  = enthalpy of the inert gas at the operating temperature of the bath.   
     
     
       4. The improvement as in claim 1 further including the step of injecting a gas consisting essentially of CO 2  into the molten steel during said first phase of decarburization for a period of time defined approximately by the formula:   t = W(T.sub.f - T.sub.i)/[0.40X + 1.35 (1-X)][F.sub.O.sbsb.2 + 1/2 F.sub.CO.sbsb.2 ]-0.62 F.sub.CO.sbsb.2 - KrW-Z.sub.i F.sub.I     where:   F co .sbsb.2 = flow rate of CO 2 , (cfm)   F o .sbsb.2 = flow rate of O 2 , (cfm)   F i  =  flow rate of inert gas, (cfm)   t = blowing time, (minutes)   T i  = metal bath temperature at the end of the blow, (° F)   W = total weight of molten metal, (tons)   K r  = measured heat loss coefficient of the vessel (° F/min)   X = carbon removal efficiency in the absence of CO 2 , and   Z i  = enthalpy of the inert gas at the operating temperature of the bath.