Effect of Nb Microalloying and Hot Rolling on Microstructure and Properties of Ultrathin Cast Strip Steels Produced by t
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NTRODUCTION
CASTRIP* process is a revolutionary new
*CASTRIP is a registered trademark of Castrip LLC, Charlotte, NC.
twin-roll strip casting method of producing steel sheet directly from the liquid. Using this technique, thin (~1 to 1.5 mm) strip steel can be produced with significantly less energy, time, and floor space, while maintaining higher production levels compared to conventional slab casting techniques.[1–3] Initially, only plain carbon steels were produced using this particular strip casting method, but the production of higher strength grades using microalloyed steels has more recently been established following extensive plant trials and fundamental investigations. This study was undertaken as part of the supporting research for the manufacture of microalloyed ultrathin cast strip (UCS) steels. The effect of microalloying additions such as Nb on steel microstructure was hitherto unknown for this UCS steel. A unique
KELVIN Y. XIE and LAN YAO, Postdoctoral Students, CHEN ZHU, Research Assistant, JULIE M. CAIRNEY, Senior Lecturer in Materials Characterization, and SIMON P. RINGER, Director, are with the Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, NSW 2006, Australia. Contact e-mail: [email protected] CHRIS R. KILLMORE, Product Design Manager, FRANK J. BARBARO, Product Applications Manager, and JAMES G. WILLIAMS, Product Technology Manager, are with Metallurgical Technology, BlueScope, Five Islands Rd, Port Kembla, NSW 2500, Australia. Manuscript submitted June 28, 2010. Article published online February 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
feature of UCS steel is that the solidification process occurs within 200 to 300 ms, compared to conventional slab casting in which several minutes are required for complete solidification. The aim of this study is to provide new information about the microstructureproperty relationships in microalloyed grades of these exciting new steels, which are expected to form an entirely new sheet steel product category with the potential to replace both hot-rolled and cold-rolled steel sheet in many applications. To this end, a series of UCS steels produced by the CASTRIP process with varying levels of Nb content were manufactured for study in both as-cast and hot-rolled states. It is most noteworthy that the effects of hot rolling on steel microstructures and mechanical properties were investigated by many researchers, and a very large body of knowledge is available in this area.[4–8] It is generally accepted that hot rolling leads to finer ferrite grains. This is because hot rolling recrystallizes austenite grains, resulting in smaller austenite grains and, subsequently, finer ferrite grains. Furthermore, hot rolling refines ferrite grain size by introducing strain fields within any nonrecrystallized austenite grains, providing extra nucleation sites for ferrite to nucleate and grow intragranularly in addition to along prior austenite grain boundaries. Hot rolling was also found to retard the ferrite grain growth in t
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