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

Abstract

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