Incompatible effects of B and B plus Nb additions and inclusions' characteristics on the microstructures and mechanical properties of low-carbon steels

The influence of boron as well as boron with niobium additions on the phase transformation behaviour, resultant microstructures, and mechanical properties of thermomechanically controlled hot-rolled and direct-quenched low-carbon bainitic steel plates was investigated. Also, the probable factors that could inhibit their specific merits on hardenability, phase transformation behaviour and mechanical properties, were studied. Continuous cooling transformation diagrams of both deformed and non-deformed austenite were constructed for the investigated steels. Laser scanning confocal microscopy (LSCM) and field emission scanning electron microscopy (FESEM) were employed to examine the microstructures, besides detailed analyses of the non-metallic inclusions using FESEM combined with INCA software. Moreover, the precipitates were investigated qualitatively using both FESEM as well as transmission electron microscopy (TEM). The results showed that the addition of boron or boron with niobium led to an increase in the critical transformation temperatures (AC1 and AC3) covering the intercritical range. The addition of boron with niobium decreased the bainite start transformation temperature (Bs), while the lone addition of boron had a slight or insignificant effect on Bs. While the addition of boron alone had no effect on the hardness, ultimate tensile strength (UTS), yield strength (YS), and elongation to fracture, augmenting it with niobium led to a marginal increase in the UTS and YS. In general, the addition of boron with or without niobium deteriorated the impact toughness of the investigated steel. These were explained in terms of the slight changes in the chemical composition and cleanness of the investigated steels and considering various microstructural features i.e., prior austenite grain size, effective grain size, coarsest grain size and precipitates characteristics, particularly the formation of coarse (Fe,Cr)23(B,C)6.