Acicular ferrite microstructure formation in low and medium carbon steels

摘要

Acicular ferrite microstructures produced in weldments are well known for their good combination of strength and toughness properties. This is the reason why there is an increasing interest to produce thiw type of microstructures in the base steel. Acicular ferrite forms in the same range of temperatures than do bainite and, due to this, the formation of both phases is competitive. It is generally accepted that the oxide particles present in the welding pool are the responsible of the intragranular nucleation of acicular ferrite. It seems clear that, if an acicular ferrite microstructure is to be produced in steel it is of paramount importance to reach an optimal distribution of sutiable second phase particles able to induce heterogeneous nucleation of acicular ferrite. However, there is some debate concerning the type of particles and the exact nucleation mechanism able to induce the formation of acicular ferrite more efficiently than do the austenite grain boundaries for bainite. Inoculated steels with titanium oxide particles have been specially produced for this purpose. But, as the present authord have recently demonstrated, other particles are also able to lead to the formation of fully acicular ferrite microstructures in medium carbon steels, allowing to reach a good combination of mechanical properties. The present work continues in the same line, using medium carbon and low carbon microalloyed steels in which the MnS particles covered by a shell of CuS are the nucleants for acicular ferrite formation, in both cases. It has been observed that, depending on the composition of the steel and on the isothermal treatment conditions, microstructures containing different mixtures of acicular ferrite and other transformation products are obtained. To the mentioned is the "bainite like" microstructure obtained at 400degC in the medium carbon steel. In this case, ferrite packets from from the parimary acicular ferrite plates, intragranularly nucleated, and not at the austenite grain boundaries. as usual in bainite.