Modified austempering effect on tensile properties of Fe-0.2C-1.5Si-1.0Mn steel

The tensile properties of Fe-0.2C-1.5Si-1.0Mn steel can be dramatically imp roved by the following modified austempering treatments: (1) intercritical austenitizing at 1028 K for 30 s in the two-phase region of ferrite (alpha) and austenite (gamma) and then austempering at 673 K for 300 s followed by subsequent air cooling (Inter Aus); (2) conventional austenitizing at 1193 K for 300 s followed by subcritical annealing at 1028 K for 420 s in the t wo-phase region of alpha and gamma and then austempering at 673 K for 120 s with subsequent air cooling (Sub Aus). Both the Inter Aus and Sub Aus stee ls had a triple structure of 50 vol.% alpha, 6 vol.% retained austenite (ga mma(R)) and carbide-free upper bainite (CFUB). The mixed structure of the C FUB and gamma(R) appearing in the Inter Aus steel was dispersed in the alph a phase and hence, the alpha phase areas were connected to each other, whil e the alpha phase associated in the Sub Aus steel appeared as massive form and was enclosed by the mixed structure of the CFUB and gamma(R). Applicati on of the Inter Aus treatment instead of conventional austemper (Con Aus) d ramatically increased total elongation (TE), and hence, it dramatically inc reased the product with the ultimate tensile stress (sigma(u)) (sigma(u) x TE). Compared to the Con Aus, the Inter Aus also significantly decreased th e yield ratio (sigma(y)/sigma(u)). Compared to the Inter Aus, the Sub Aus i ncreased the strength at similar TE and sigma(y)/sigma(u) levels. The resul ts are described and discussed.