Effect of deformation and cooling rate on the microstructures of low carbon Nb-B steels

The transformation behaviours and microstructural characteristics of three B-containing steels were investigated. In particular, the effects of deformation in the no-recrystallization temperature range and cooling rate were studied by means of compression tests. It was found that over a large cooling rate range (from 1 to 50 degrees C/s), Mo-Nb-B steel exhibits microstructures consisting of a mixture of plate-like or lath-like ferrite with retained austenite or martensite (i.e. M/A) islands. This is basically a low carbon bainitic microstructure, and can be identified as B-3 in the Bramfitt and Speer classification system. The lengths of the ferrite laths increase and the widths decrease as the cooling rate is increased. The shapes and distributions of the M/A islands change from being blocky and randomly distributed to fine and more aligned, as the cooling rate is increased. Also, the lengths of the bainitic ferrite laths are shortened by heavy deformation in the no-recrystallization temperature range. The microstructures of the Nb-15B and B-only steels are basically polygonal ferrite at low cooling rates; however, the fractions of bainite in these two grades increase with cooling rate. The minimum cooling rate required for avoiding polygonal ferrite formation during continuous cooling are much higher in these two grades than in the Mo-Nb-B steel.