Formation of Widmanstatten Ferrite in a 0.036% Nb Low Carbon Steel at Temperatures Above the Ae3

The formation of Widmanstatten ferrite was investigated in a 0.036% Nb microalloyed steel at temperatures above the Ae(3) using optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Such strain-induced ferrite appears to form in two consecutives stages: (i) stage I, first observed at strains below 0.5, leads to the presence of Widmanstatten ferrite plates only about 200nm wide, (ii) stage II, observed at strains >0.5, involves the coalescence of the Widmanstatten plates into grains. Thus the microstructures formed by dynamic transformation are composed of fresh Widmanstatten plates (stage 1) and polygonal grains (stage 2). Over the experimental temperature range of 836-896 degrees C, the ultrafine ferrite plates and grains have areas below 2 mu m(2) and are difficult to detect using optical microscopy.