Thermomechanical and Microstructural Analysis of the Influence of B- and Ti-Content on the Hot Ductility Behavior of Microalloyed Steels

Abstract

The effects of the combined addition of B and Ti, as well as the influence of different strain rates on the hot ductility behavior of low carbon, continuously cast, microalloyed steels were investigated in this work. Tensile tests, microstructure analyses, and thermokinetic simulations were performed with in situ melted samples. Furthermore, prior austenite grain evaluations were carried out for the two different microalloyed steels. Increasing the strain rate brought improvements to the ductility, which was more significant in the steel with the leanest composition. The steel containing more B and Ti presented a better hot ductility behavior under all conditions tested. The main causes for the improvements rely on the precipitation behavior and the austenite–ferrite phase transformation. The preferential formation of TiN instead of fine BN and AlN was seen to be beneficial to the ductility, as well as the absence of MnS. Grain boundary segregation of free B that did not form BN retarded the ferrite formation, avoiding the brittleness brought by the thin ferrite films at the austenite grain boundaries. Furthermore, it was revealed that for the steels in question, the prior austenite grains have less influence on the hot ductility behavior than the precipitates and ferrite formation.

Hot Ductility, Homogeneity of the Composition, Structure, and Properties of High-Strength Microalloyed Steels: A Critical Review

“…Moreover, the addition of Ti reduces the amount of AlN and NbN [95,96]. It should be noted that, with an increase in the strain rate from 10 -4 to 10 -2 s -1 , a decrease in the amount of BN and TiN precipitates and an increase in their size was observed, which was accompanied by an improvement in hot ductility [96].…”

“…More recent works have focused on studying the effect of co-microalloying with boron and titanium [91][92][93][94][95][96]. In the experiments, a cooling rate of 3 • C/s was used.…”

“…This was due to the inhibition of the formation of intergranular ferrite by the segregation of dissolved boron at the grain boundary, since titanium prevented the binding of boron to nitride due to the formation of TiN. Moreover, the addition of Ti reduces the amount of AlN and NbN [95,96]. It should be noted that, with an increase in the strain rate from 10 -4 to 10 -2 s -1 , a decrease in the amount of BN and TiN precipitates and an increase in their size was observed, which was accompanied by an improvement in hot ductility [96].…”