Microstructural evolution at the initial stages of continuous annealing of cold rolled dual-phase steel

The performance of cold rolled dual-phase (DP) steels depends on their microstructure, which results from the thermomechanical processing conditions, involving hot rolling, cold rolling, and continuous annealing. The knowledge on the influence of each annealing stage on the microstructure formation is essential for manufacturing high-quality DP steels. In the present work, the effects of some intercritical annealing parameters (heating rate, soaking temperature, soaking time, and quench temperature) on the microstructure and mechanical properties of a cold rolled DP steel (0.08% C-1.91% Mn) were studied. The microstructure of specimens quenched after each annealing stage, simulated on a Gleeble, was analyzed using optical, scanning, and transmission electron microscopy. The tensile properties, determined for specimens submitted to complete annealing cycles, are influenced by the volume fractions of martensite, bainite, martensite/austenite (MA) constituent, and carbides, which depend on annealing processing parameters. The results obtained showed that the yield strength (YS) increase and the ultimate tensile strength (UTS) decrease with the increasing intercritical temperature. This can be explained by the increased formation of granular bainite associated with the increased volume fraction of austenite formed at the higher temperatures. The experimental data also showed that, for the annealing cycles carried out, UTS values in excess of 600 MPa could be obtained with the steel investigated. (C) 2004 Elsevier B.V. All rights reserved.