Macrosegregation in a multicomponent low alloy steel
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I. INTRODUCTION T H E quality of large killed-steel ingots has been improved with recent innovations made in vacuum and deoxidation processing. However, there still are problems of macrosegregation in large ingots, especially channel-type or A-type segregation. Therefore, many theoretical and experimental studies have been carried out to determine the mechanism of the formation of this type of segregation. Mechanisms put forth may be classified as follows: I) Traces of solute enriched liquid flow, concentrated in regions of remelting of the dendrite structure in the mushy zone;~-%~9 2) flow of solute enriched liquidto gaps in the mushy zone caused by tears and solidification shrinkage;8'~ and 3) the traces of gas bubbles escaping through the mushy z o n e .10,Ii In some studies, workers determined that the effect of alloying elements on A-type or channel-type segregation is significant. For example, Smelzer 12 found that "freckles" in a stainless steel ingot were prevented by decreasing the silicon content. Similarly, SuzukiIS found that an increase of silicon in steel promotes the incidence of A-type segregates, whereas an increase of molybdenumdecreases the segregation. When these elements are enriched in the interdendritic liquid, silicon decreases the density of the liquidand molybdenumincreases it. Consequently,silicon enhances and molybdenumsuppresses the natural convection of interdendritic liquid in the mushy zone, suggesting that convection of interdendritic liquid T. FUJII, formerly Research Affiliate, Department of Materials Science and Engineering,Massachusetts Institute of Technology, is now Senior Researcher, Research Laboratories, Kawasaki Steel Corporation, Kawasaki, Japan, D. R. POIRIER is Associate Professor, Department of MetallurgicalEngineering, University of Arizona, Tucson, AR, was Visiting Scientist, Department of Materials Science and Engineering, Massachusetts Institute of Technology, and M. C. FLEMINGS is Ford Professor of Engineering, Department of Materials Science and Engineering, Massachusetts Institute of Technology. Manuscript submitted June 2, 1978. METALLURGICALTRANSACTIONS B
in the m u s h y zone is r e s p o n s i b l e for the f o r m a t i o n of A - t y p e s e g r e g a t e s found in l a r g e s t e e l ingots and c a s t ings. In the past decade, a m o d e l for m a c r o s e g r e g a t i o n has b e e n e x a m i n e d a n a l y t i c a l l y and e x p e r i m e n t a l l y in an extended s e r i e s of s t u d i e s at M a s s a c h u s e t t s I n s t i tute of T e c h n o l o g y . %14-17'2~ The work has b e e n b a s e d upon an a n a l y s i s of the convection of i n t e r d e n d r i t i c liquid d u r i n g s o l i d i f i c a t i o n ; the a n a l y t i c a l and e x p e r i m e n t a l e v i d e n c e shows that fluid flow is the cause of many i m p o r t a n t types of s e g r e g a t i o n in c a s t i n g s and ingots i n c l u d i n g the s e v e r e l o c a l i z e d s e g r e g a t e s r e f e r r e d to as A - t y p e s e g r e g a t e s , " f r e c k l
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