Steel dissolution in quiescent and gas stirred Fe/C melts
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I.
INTRODUCTION
T H E rate of scrap melting in steelmaking processes is an important process variable. In bottom blown oxygen steelmaking processes, such as OBM and KMS, efforts are being made to increase the quantifies of scrap smelted by the injection of additional fuel as coal, and post combusting the off-gases to increase the overall energy utilization of the processes, r~] The newer KS process t2] can use 100 pct scrap charges, and the rate of scrap melting in this process is critical for smooth operation. It is therefore desirable to understand the physical and chemical factors which influence the rate of scrap melting over a range of process conditions in highly agitated gas stirred iron baths. In the present work, the rate of isothermal dissolution of steel rods in quiescent and inert gas stirred iron/carbon 9 melts was investigated. The objective was to provide information on the effect of gas stirring conditions on the dissolution kinetics of steel in iron/carbon melts under conditions similar to those encountered in converter scrap melting processes.
II.
PREVIOUS WORK
In steelmaking practice, it has been found that the melting rate of scrap is dependent on the difference in carbon content between the scrap and the melt, the degree of superheat above the liquidus temperature, and the surface area of the scrap. Typical scrap consists mainly of low carbon iron with melting points in excess of 1500 ~ In converter steelmaking processes, these temperatures are not exceeded until late in the blow, t31 and insufficient time would be available for complete melting if heat transfer was the only rate controlling process. In effect, the scrap dissolves at temperatures well below 1500 ~ due to the lowering of the melting point by dif-
J.K. WRIGHT, Manager, is with Ferrous Smelting, CSIRO, Division of Mineral and Process Engineering, P.O. Box 312, Clayton, Victoria, 3168, Australia. Manuscript submitted March 11, 1988. METALLURGICAL TRANSACTIONS B
fusion of carbon from the bulk of the melt to the solid/ liquid interface.
A. Static and Rotating Rod Systems Most previous work on the kinetics of the dissolution of steel and iron/carbon alloys has been done using static [4-7] or rotating t4,s,7,81 cylindrical rods immersed in F e / C or pig iron melts at temperatures below the melting points of the rods. The rates of dissolution were usually assessed by the rate of change of the rod diameters. In these laboratory investigations, the steel rods were preheated to temperatures close to the bath temperatures so that heat transfer effects could be neglected. The results of these isothermal investigations showed that the rod dissolution rates depended on how qwickly carbon can be transported to the dissolving interface. Mass transport coefficients were derived by assuming that carbon transport was rate controlling such that the concentration at the interface was equal to the liquidus carbon concentration at the appropriate bath temperature. The mass transport coefficients under natural and forced convection conditions w
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