CONTINUOUS COOLING TRANSFORMATION TEMPERATURES DETERMINED BY COMPRESSION TESTS IN LOW-CARBON BAINITIC GRADES

The transformation behaviors of six steels containing microalloying ad ditions of B, Nb, and Mo were investigated under continuous cooling co nditions. Continuous cooling compression (CCC) tests were employed to study the effects of chemical composition (mainly, Nb, Mo, and B) and deformation parameters (reheat temperature, prestrain, and holding tim e) on the transformation temperatures (A(r3) and B-s). It was found th at for the Mo-Nb-B, Mo-B, and B steels, the transformation temperature s are relatively stable, and vary in a range of about 20 degrees C whe n the reheat temperature is changed from 900 degrees C to 1200 degrees C. Both the stress-temperature curves and the associated microstructu res show that transformation in the Mo-Nb-B steel is basically of the gamma-to-B type; i.e., the resulting microstructure is low carbon bain ite. By contrast, for the Nb-B steels, the transformation temperatures vary significantly when the reheat temperature is changed. The concen tration of boron in solution strongly affects the transformation behav ior of this type of steel. In the Nb-48B steel, the latter is of the g amma-to-B type, while in grades with either higher (Nb-64B) or lower ( Nb-15B) boron concentrations, it is mainly of the gamma-to-alpha type. Large Fe-23(C,B)(6) particles, which were found at low reheat tempera tures and long holding times, are considered to be responsible for rai sing the transformation temperatures.