Enhancement of elongation by retained austenite in intercritical annealed 0.4C-1.5Si-0.8Mn steel

An excellent combination of elongation over 30% and high strength of about 100 kgf/mm exp 2 is achieved in processing a 0.4C--1.5Si--0.8Mn steel by intercritical annealing and rapid cooling into bainite transformation temperature to soak for several minutes. This combination is caused by the transformation induced plasticity of retained austenite. A sufficient amount of stable austenite is a requisite for good ductility. For the rapid cooling after annealing, the soaking temperature for the best combination of strength and ductility is immediately above Ac sub 1 . On the other hand, a delay before rapid cooling provides good properties if the soaking temperature is near Ac sub 3 and the subsequent cooling is performed at a lower rate before pearlite transformation; in this case the critical cooling rate is reduced. These phenomena are discussed in terms of the growth of ferrite and the diffusion of alloying elements, including Mn, during slow cooling. 22 ref.--AA