Interrupted and Isothermal Solidification Studies of Low and Medium Carbon Steels
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INTRODUCTION
DURING the past quarter century, the vital importance of the metallurgical condition of austenite in affecting the final properties of plain carbon and low alloy steel has been clearly demonstrated. There exists, for example, an extensive literature on how the size, shape, degree of recrystallization, and composition of hot-rolled austenite grains can have a profound influence on the nature of the transformation products which form upon cooling and, therefore, on the final mechanical properties.[~-5] By comparison, our understanding of the factors which influence the hot workability of plain carbon and low alloy steel is rather limited. Since hot workability is often assessed by hot ductility, problems in hot workability are frequently associated with the lack of adequate hot ductility. There have been several microstructural features found to be responsible for low hot ductility in various temperature regimes in plain carbon and low alloy steels, t6] These features include nonmetallic inclusions, precipitates, large austenite grain sizes, and ferrite f i l m s . [6"71 Of principal interest in the present study are the factors which are responsible for the large austenite grain sizes which are often found in as-cast structures and which are at least partially responsible for low hot ductility. For example, low hot ductility caused by very large austenite grains I6,71has led to slab cracking problems during the unbending o r straightening stage of continuous casting tS] and during the slab rough rolling stage of hot charging, tg] Furthermore, unusually large N.S. POTTORE, formerly with the Basic Metals Processing Research Institute, Department of Materials Science and Engineering, University of Pittsburgh, is Staff Engineer, Research Laboratory, Inland Steel Company, East Chicago, IN 46312. C.I. GARCIA, Research Associate Professor, and A.J. DeARDO, Professor, are with the Basic Metals Processing Research Institute, Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted June 6, 1990. METALLURGICAL TRANSACTIONS A
austenite grains which exist after solidification and cooling may cause large austenite grains to form during slab reheating prior to conventional hot or controlled rolling. Large austenite grains in reheated slabs have been associated with a poor response to subsequent hot rolling and inferior final properties, c'~ Although the austenite grain size which exists at temperatures below 1400 ~ during cooling following solidification is quite important, there is a lack of information in the literature concerning both grain size data of this kind and the factors which control this grain size. In this case, one central question is how do the steel composition, solidification rate, and subsequent cooling rate affect the primary solidification structure and the subsequent phase transformations which eventually lead to an austenite grain size at and below 1400 ~ While there have been numerous studies concerning the s t r u c t u r e D1'
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