EFFECT OF CARBON CONTENT ON THE PHASE TRANSFORMATION CHARACTERISTICS, MICROSTRUCTURE AND PROPERTIES OF 500 MPa GRADE MICROALLOYED STEELS WITH NONPOLYGONAL FERRITE MICROSTRUCTURES

The influence of C in the range of 0.011-0.043 wt-% on the phase transformation characteristics, mechanical properties and microstructure of Fe-2.0Mn-0.25Mo-0.8Ni-0.05Nb-0.03Ti steel was investigated. In the dilatometric experiments, it was found that a reduction in the C content increased the phase transformation temperatures, decreased the hardness and promoted quasi-polygonal ferrite (QF) formation over granular bainitic ferrite (GBF) and bainitic ferrite (BF), but at the same time the sensitivity of the phase transformation temperatures and hardness to cooling rates was reduced. Mechanical testing of laboratory hot rolled plates revealed that the targeted yield strength of 500 MPa was reached even in the steel with the lowest C content (0.011wt-%). An increase in C content did not considerably increase the yield strength, although the tensile strength was more significantly increased. Impact toughness properties, in turn, were markedly deteriorated due to this C content increment. Microstructural analysis of the hot rolled plates showed that an increase in C content decreased the fraction of QF and consequently increased the fraction of GBF and BF, as well as the size and fraction of C-enriched secondary microconstituents. In addition, the size of the coarsest crystallographic packets seemed to be finer in the low C steel with QF dominated microstructure than in its higher C counterparts with higher fractions of GBF-BF, even thought the average crystallographic packet size was slightly finer in these higher C steels. Mechanical testing of the simulated CGHAZ’s showed that their toughness properties are not strongly dependent on C content, although there exists a general trend for toughness to slightly weaken with increasing C content. It could be concluded that HAZ toughness properties of these types of steels are acceptable. On the basis of dilatometric experiments, mechanical testing and microstructural analysis it can be stated that a good combination of strength, toughness and weldability as well as microstructural stability can be reached in very low C steels with QF dominated microstructures. Finally, an example of this type of microstuctural concept, which has been successfully