Microstructures and Properties of DP 600 Steel Conventionally Treated and Intercritically Annealed from As-Quenched Martensite

In this work a DP 600 Dual Phase steel, conventionally treated in order to obtain 40 to 60% austenite at the intercritical temperatures, called reference sample, was compared to samples from the same steel, initially fully austenitized and quenched to 100% martensitic structure and subsequently intercritically tempered once ( one step) or twice, ( two steps) at intercritical temperatures so as to obtain the same volume fractions of austenite as the conventional DP steel. The single step heat treatment is QL, quench and lamellarization; the two step heat treatment is called QLT, quench and lamellarization and tempering. Heat treatments were conducted on a quenching dilatometer. Samples were characterized by optical, SEM-FEG, EBSD imagining and X Ray Diffraction. Mechanical properties were evaluated by microhardness and tensile tests on sub-size specimens. The results show that QL samples present a complex microstructure composed of ferrite ( carbide free high temperature tempered martensite) and fresh martensite composed of crystallites of the order of 1 to 5 mu m, with volume fractions of ferrite and martensite similar to the reference samples. X-ray diffraction showed the presence of retained austenite in all treatment conditions, larger for the reference samples when compared with the QL; EBSD images show the retained austenite finely dispersed between the martensite laths and within the limits of martensite blocks. The tensile strength of the QL has higher values than reference DP 600 steel for the similar martensite volume, with smaller uniform and total elongations.