Effect of thermomechanical processing on mechanical behavior and microstructure evolution of C-Mn multiphase high strength cold rolled steel

Multiphase (MP) steels have complex microstructures containing polygonal ferrite, martensite, bainite, carbide, and small amounts of retained austenite. This mixture of phases and constituents is responsible for a good combination of strength and ductility in this class of steels. The present work shows how different annealing parameters can be used to create the suitable microstructure to improve mechanical properties of MP steels. Samples were first heated to 740, 760, or 780 degrees C, held for 300 s, and then quickly cooled to 600 or 500 degrees C. They were then soaked for another 300 s and finally accelerated cooled in the range of 10-30 degrees C s(-1). The microstructures were examined at the end of each processing route using optical, scanning, and transmission electron microscopy. Hardness values were determined for all conditions. Analysis of the available data allowed to establish the simple and yet useful quantitative relationship between the microstructural parameters, cooling rates, and hardness of the steel.