The Ferrite Transformation in Hot Deformed 0.036% Nb Austenite at Temperatures Above the Ae3

Abstract

A 0.036% Nb microalloyed steel was deformed in torsion over the temperature range 816–896°C in a 2%H2–Ar gas atmosphere. Strains of 0.5–5.0 were applied at strain rates of 0.04 and 0.4 s−1. The experimental parameters were varied in order to study the effects of strain, strain rate and temperature on the formation of ferrite by dynamic transformation (DT) at temperatures above the Ae3. The critical strain for ferrite formation by DT was 0.5 and the volume fraction formed increased with strain and slightly with strain rate. It was also observed that the applied strain has a far greater influence on the transformation than the time. Average ferrite grain sizes of 2 to 3.5 μm were produced, the size increasing with the transformation temperature and decreasing strain rate. By comparison with the behavior of plain C steels, it is evident that the addition of niobium slows the reverse transformation to a considerable degree. Two stages were detected in the reverse transformation: i) in stage I, observed during the initial 200 s of isothermal holding, the deformation-induced ferrite was fairly stable; ii) in stage II, observed after 200 s of holding, the reverse transformation began to take place, going to completion in about 400 s. The results of these experiments support the view that it is the stored energy of the ‘inhomogeneously’ distributed dislocation (i.e. those in shear bands and sub-boundaries) that provides the driving force for such “non-equilibrium” transformation.