abstract
A moderately high carbon (0.61%) high silicon steel was subjected to a newly designed heat treatment cycle consisting of continuous cooling for different duration after austenitization followed by austempering at 300,350 and 400 °C to form a very high strength and highly ductile multiphase steels with microstructures consisting of varied amounts of ferrite (formed during continuous cooling), bainite (formed during austempering) and retained austenite. Steels with very high strength up to (tensile strength ~1100–2000 MPa) along with excellent ductility (elongation ~10–32%) were obtained. Effect of continuous cooling duration on ferrite content, amount of carbon diffused in the prior austenite grains, variation of carbon content in the retained austenite (cγ) and its volume fraction (Vγ) have been analyzed. Finally, structure property correlation has been established.
Keywords:
Mixed phase steel Retained austenite Bainite.
Introduction
Development of steels having high strength combined with high ductility at low cost has been the most desirable aspect for metallur-gists. In the recent years, high silicon steels with retained austenite (RA) along with bainite and ferrite as the constituent phases have emerged as a best and cheap option for the automotive industry. Strain induced transformation of RA with moderate stability in to plate type martensite has been observed to enhance the work hardening rate,which results in decreasing the yield strength to ultimate tensile strength ratio along with increase in ductility [1,2]. Stability of RA de-pends on its carbon content, morphology, size and properties of the neigh boring phases present [3–7]. RA has been observed to succumb to strain induced transformation in the samples with variation of carbon content in the austenite retained (cγ) and its volume fraction (Vγ) [4]. In another study [3], weak correlation of uniform elongation with cγ and Vγ has been observed. Rather, better trend in the variation of uniform strain with ratio of carbon in RA to its volume fraction (cγ/Vγ) has been noted. True uniform strain and toughness have been observed to decrease with increase in cγ/Vγ ratio. RA has been observed to be highly stable when surrounded with hard phase [5]. Small grain size of austen-ite has been observed to make it more stable towards strain induced transformation by others.
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