EFFECTS OF SIMULATED ON LINE ACCELERATED COOLING PROCESSING ON TRANSFORMATION TEMPERATURES AND MICROSTRUCTURE IN MICROALLOYED STEELS .1. STRIP PROCESSING

Simulations of industrial thermomechanical processing and on line acce lerated cooling of a low carbon microalloyed strip steel were carried out using a quench deformation dilatometer. Effects of processing para meters, such as accelerated cooling rate over dotT and accelerated coo ling interrupt temperature T-I on the critical transformation temperat ures and final microstructure were determined. The most important on l ine accelerated cooling (OLAC) processing parameter is the accelerated cooling interrupt temperature, which controls whether the transformed microstructure is predominantly ferrite or bainite. A variety of (Ti, Nb,Fe) carbide, nitride, and carbonitride precipitates are present in the OLAC processed samples. The final precipitate distribution is deve loped at three stages of processing, namely: dissolution and coarsenin g of pre-existing precipitates at the reheat temperature, precipitatio n in deformed austenite during the deformation schedule, and precipita tion in ferrite after the interruption of accelerated cooling. Maximum precipitations strengthening occurs for T-I=700-640 degrees C. (C) 19 96 The Institute of Materials.