Effect of Alloying Elements on Microstructure and Mechanical Properties of Air-Cooled Bainitic Steel
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INTRODUCTION
BAINITIC steel with fine bainitic laths and film type of retained austenite sandwiched between two bainitic laths are proven to provide a good combination of strength and toughness.[1,2] Such steels are developed by two-stage processing with heat treating the steel to the austenite zone and cooling subsequently to a bainitic bay. Further, the steel is isothermally heat treated in a bainitic bay for a period until the formation of bainite and is subsequently cooled to room temperature.[3,4] However, instead of the two-stage cost of intense processing to obtain a desired combination of strength toughness, an air-cooled bainitic steel could provide a cost-effective material with the desired properties.[3,4] In this respect, attempts have been made either by alloy modification or altering the processing route. The generic design principles of such steels were explored by Gomez et al.,[3,4] and they used Mn content of < 0.1 wt pct to avoid banding in bainitic steel during solidification.[3] Nickel is also added in such steel to improve the toughness of steel by stabilizing the retained
MINAL SHAH, S.K. DAS, and S. GHOSH CHOWDHURY are with the Materials Engineering Division, CSIR-National Metallurgical Laboratory, Jamshedpur, 831007, India. Contact e-mail: [email protected] Manuscript submitted August 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite. Lowering of Cr content was done to maximize the amount of bainitic transformation, and Al was added in a small amount for deoxidation.[3] Sourmail et al.[5] reported the possibility of carbide-free bainite formation during air cooling. In terms of process modification, isothermal treatment was considered for obtaining ultrafine high strength steel.[6,7] However, the process is not economically viable because of the long holding time required for isothermal treatment. Adjustment of composition in the steel is one of the preferred routes to obtain bainitic microstructure during air cooling after austenitization.[5,6,8] It is reported that the air-cooled bainitic steel has a higher amount of bainite and better mechanical properties than austempered bainitic steel.[9] Continuously cooled bainitic steel has shown a very good combination of strength and toughness.[5,6] Das and Haldar[8] made an attempt to design bainitic steel that could be produced directly in a hot strip mill by accelerated cooling on the run-out table followed by coiling and reported a tensile strength of ~ 1370 MPa with ductility of 21 pct.[8] Phases, such as allotriomorphic ferrite, Widmansta¨tten ferrite, martensite, and blocky austenite, are to be avoided to improve the toughness of the steel.[10] Other researchers[11–15] reported that carbide-free bainite microstructure with high silicon provides very good wear resistance (rolling/ sliding). The unique combination of strength and toughness of high Si bainitic steel were attributed to the synergistic effect of ultrafine microstructure and absence of coarse carbides.[11–15] Fang et al.[16] reported the development of manganese cont
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