The Test Material and the Scope of Tests Carried Out

In recent years, there has been a growing interest in steels of multi-phase structures, composed of a ferritic matrix, in which islets occur, that contain bainite-B, martensite-M and the untransformed austenite-A 1-9 . The multi-phase structure of steel with untransformed austenite promotes the increase of plasticity induced by the phase transformation – the TRIP effect 3-4, 6-8 . In steels containing up to 1.6% Mn, the stabilization of austenite was achieved by increasing the content of carbon, aluminium or silicon. In the examined steel 6Mn16 containing up to 4% Mn and up to max. 0.08% C, the effect of carbon on austenite stabilization is less than that in steels with a higher carbon content. Manganese stabilizes austenite and has a higher solubility in austenite than in ferrite. The diffusion coefficient of Mn in ferrite, on the other hand, is higher by a factor from 10 to 10 compared to austenite. In order to achieve the equilibrium partition of Mn between austenite and ferrite in steels containing up to 1.6% Mn, soaking times from approx. 10 to 10 seconds (277 to 27777 hrs) would have to be used in the inter-critical soaking processes 7 . To reduce the soaking time until a significant Mn partition between ferrite and austenite had been achieved, an unconventional heat treatment operation was applied. This consisted in the annealing of steel 6Mn16 below the temperature Ac1 to enrich the cementite with Mn. Then, the carbides enriched with Mn were dissolved in austenite during soaking in the interval ( + ). In order to precisely determine the parameters of this heat treatment, the critical temperatures Ac1 an Ac3 were established and the effect of Mn on the kinetics of austenite decomposition from the complete austenitization and from ( + ) interval was determined. The partition of manganese between ferrite and austenite at the inter-critical soaking temperature and, after cooling, between the ferritic matrix and the bainitic-martensitic islets was assessed.