Development of Multiphase Microstructure with Bainite, Martensite, and Retained Austenite in a Co-Containing Steel Throu
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INTRODUCTION
A novel steel heat treatment involving quenching and partitioning (Q&P) has been proposed to achieve high fraction of retained austenite (RA) through carbon partitioning from supersaturated martensite to RA.[1] Martensite offers high strength, while RA provides the TRIP effect (transformation induced plasticity) during straining, which improves the ductility and formability of the steel.[2] Majority of studies involving Q&P treatment produced martensite-RA structure with a maximum of ~20 pct RA.[3] Very few studies obtained mixed microstructures comprising bainite-martensiteand RA[4] with a maximum bainite fraction of 14 pct and RA fraction less than 5 pct.[5] Present study aims to develop such mixed microstructures with a high fraction of bainite and RA through Q&P treatment. Increase in SANTIGOPAL SAMANTA, Senior Research Fellow, DEBALAY CHAKRABARTI, Assistant Professor, and SHIV BRAT SINGH, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India. Contact e-mail: [email protected] SOURAV DAS, formerly Researcher with the Tata Steel Ltd, Jamshedpur, India, and is now Principal Researcher with the Product Research Group, Research and Development (R&D), Tata Steel Ltd., Jamshedpur, 831 001, India. INDRADEV SAMAJDAR, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai, 400 076, India. ARUNANSU HALDAR, Department Manager, is with the Research, Development and Technology Division (RD&T), Tata Steel Europe, Swinden Technology Centre, Moorgate, Rotherham, S60 3AR, U.K. Manuscript submitted April 11, 2013. Article published online August 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
bainite and RA might lower the strength of the steel, but is expected to improve the ductility. The effect of processing parameters involved in Q&P treatment on the development of mixed phase structure has also been investigated in the present work. It is well-known that the addition of Si can eliminate or delay the carbide formation during the bainitic transformation[6,7] or during the tempering of martensite.[8–10] Addition of Al or even P can have the same effect,[11] although Si being more effective than Al in this respect.[12] Therefore, if carbide precipitation is kinetically suppressed by the addition of Si or Al, then carbon remains available to partition from metastable martensite to RA, which is the basis of Q&P practice.[13] The hypothesis on complete suppression of carbide precipitation is not necessarily true as the carbide particles have been observed in Q&P treated steel containing Si and Al.[14] But the reaction is sufficiently delayed to allow significant C-enrichment of RA during the Q&P treatment. Dilatometric studies on low carbon hypoeutectoid steel by Kim et al.[5] showed a positive dilation during isothermal holding at the partitioning temperature [below martensite start-temperature (Ms)] indicating FCC (austenite) to BCC transform
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