Evolution of microstructure and texture during casting of AISI 304 stainless steel strip
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DIRECT strip casting (DSC) to produce as-cast steel sheet has attracted much attention because this technology has the capability of producing lower cost products compared with conventional continuous casting (CCC) and thermomechanical processing (TMP) routes.[1] Twin roll casting (TRC) is becoming the favored method of casting steel strip.[2] In this process, molten metal is introduced into a tundish, which flows through a nozzle into the gap between two water-cooled copper rolls that rotate at a given speed to produce strip of thickness of 1 to 2 mm. The high rate of heat extraction from the melt through the rolls results in rapid solidification and produces as-cast steel microstructures radically different to those produced by CCC/TMP.[2–5] In light of these differences, it is necessary to develop a better understanding of the influence of casting and other metallurgical variables associated with TRC on microstructural development of commercially significant steel grades. An understanding of the nucleation and growth of phases during TRC of metals such as austenitic stainless steel (␥ SS) is important because it is anticipated that these steels will be produced in large tonnages by this route.[2] A notable example is AISI 304, which contains the principal alloying elements (mass pct) ⬃18Cr and ⬃8Ni, which infers, from the ternary phase diagram, [6] that the preferred nucleation phase is delta-ferrite (␦ ). However, splat quenching experiments have shown that austenite (␥ ) is the dominant nucleation and solidifying phase due to the high initial heattransfer rate, which favors its formation.[7,8,9] The solidification behavior of this alloy during DSC is also expected to be complex,[2] so it is important to understand the effect of A. HUNTER, Postgraduate Student, is with the Institute for Steel Processing and Products, University of Wollongong, Wollongong NSW 2522, Australia. M. FERRY, Senior Lecturer, is with the School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia. Contact e-mail: [email protected] Manuscript submitted February 19, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
various casting conditions on microstructural development. In the present work, AISI 304 coupons were produced using a dip casting apparatus developed by Strezov,[10] which stimulates closely the conditions of TRC.[10,11] The aim is to investigate the nucleation and growth behavior of grains during solidification as an understanding of variables affecting the as-cast microstructure, and the texture of this steel grade is highly significant, because its commercial production by TRC is now being developed.[2] II. EXPERIMENTAL PROCEDURE A. Production of As-Cast Coupons An AISI 304 stainless steel with melt composition (mass pct) 17.65Cr, 8.35Ni, 0.039C, 1.0Mn, 0.36Si, 0.16Cu, 0.009Nb, 0.01S, and 0.003Ti was used in this work. The mechanisms of nucleation and growth of solidifying grains were studied using Strezov’s melt/substrate contacting apparatus.[10] The technique produces as-ca
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