The effect of carbon content on solidification of steel in the continuous casting mold
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1. I N T R O D U C T I O N
THE surface quality of continuously cast steels is influenced to a great extent by the solidification conditions in the mold. Beside the avoidance of steel making defects such as pinholes, scums and subsurface inclusions, it is important to ensure sufficient safety against the formation of surface and subsurface solidification cracks and also against breakouts. In order to realize these targets, the basic conditions appear to be maintaining a uniform shell growth at an appropriate growth rate which, in turn, depend on good strand/ mold contact. It was revealed in a systematic investigation of mold heat flux and mold friction as function of steel carbon content (Fig. 1) that concentrations around 0.1 wt pct C lead to a distinct minimum in strand/mold-contact, owing to a very "rough" and "rippled" surface, l Incidentally, such a minimum is also found in the relationship between carbon content and subsurface cracking (Fig. 2) of continuously cast strand sections. 2 Such tendency to nonuniform shell growth of low carbon steels is particularly strong in case of casting with oil lubricant whereas higher uniformity can be obtained with slag as lubricant3,4--provided that mold powder properties and other casting conditions are properly selected to maintain a stable slag film) In order to explain the peculiar effect of carbon content on mold heat flux, the enhanced shrinkage of steels undergoing the delta/gamma-transformation immediately after solidification has been suggested as the controlling mechanism. 6,7 Further studies showed that the effect of shrinkage behavior is most critical at the very beginning of shell formation i.e. in the meniscus region: 8 due to shrinkage, the shell is bending inwards and the resultant gap between strand surface and mold wall causes a reduction in heat flux with subsequently
M. WOLF is Manager Process Technology, CONCAST AG, CH-8027 ZOrich, Switzerland. W. KURZ is Professor Department of Materials Engineering, Swiss Federal Institute of Technology, CH1007 Lausanne, Switzerland. Manuscript submitted July 3 l, 197~9.
retarded shell growth. Recent investigations about the shape of the liquid steel meniscus in molds 9 also point to the importance of mechanical resistance of the first solidified layer against ferrostatic pressure. Hence, the mechanical properties of steels at high temperature seem to be another factor for the intensity of strand/mold-contact. For instance, the observed increase in mold heat flux with higher Por S-content had already been interpreted in terms of reduced high temperature strengthl~ based on corresponding measurements of mechanical properties. ~ Such reasoning would suggest that steels with high carbon content, characteristic for high heat flux and uniform shell growth, should have a much lower strength at high temperature than the low carbon steels since shrinkage behavior being not as much different from low carbon steels as the solidification behavior would indicate: However, according to the data of Fig. 3, no significant effect
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