THE INTERMEDIATE TRANSFORMATION OF MN-MO-NB STEEL DURING CONTINUOUS COOLING

The continuous cooling transformation diagram for low-carbon low-alloy steel containing 0. 05% C, 1. 99% Mn, 0. 31% Mo and 0. 06% Nb was constructed by dilatometry and metallography. The intermediate transformation between martensite and polygonal ferrite involves two typical stages: the formation of ferrite matrix and the formation of microphases. Four intermediate transformation products obtained from various cooling rates and designated B//1, B//2, A//1, and A//2, were studied. The B//1 and B//2 structures are composed of pockets of parallel ferrite laths and interlath microphases, which are films of retained austenite in B//1 and are fragments of retained austenite or martensite or martensite-retained austenite (M-A) constituents in B//2. The B//1 structure is further characterized by the appearance of martensite particles inside the ferrite laths. The A//1 structure is comprised of the randomly arranged ferrite groups. Each group contains several short ferrite laths in the same crystallographic orientation and granular M-A constituents or martensite located at the rim of ferrite laths or groups. The A//2 structure is morphologically analogous to Widmanstaetten ferrite. The formation mechanisms of these products are also discussed.