The Formation of Deformation Induced Ferrite during Mechanical Testing

Abstract

The effects of austenite grain size, level of undercooling, and strain and strain rate in compression on the austenite-to-ferrite transformation were investigated in a 0.1 % C, 1.4 % Mn steel. The influence of this transformation on the hot ductility was then examined. Straining was found to raise the effective Ar₃ temperature almost to the Ae₃, and to accelerate the transformation significantly in both fine (∼25 μm) and coarse (∼200 μm) grained samples. At relatively low levels of undercooling (∼40°C), deformation induced ferrite only formed in regions that were significantly affected by the applied strain. In tension, it was observed that, if thin bands of grain boundary ferrite were present in the microstructure, the ductility decreased significantly and failure always occurred in this second phase. Increasing the strain rate significantly reduced the depth of the ductility trough across its entire width. It is concluded that, for the steel in question, unbending after continuous casting should be performed at strain rates in excess of 3×10^-2s^-1 to ensure good ductility.