Study on the hot ductility of the B bearing steel and B precipitation behavior

Abstract

Microalloyed steels with strong carbonitride-forming elements are particularly susceptible to surface cracking on slab because of poor hot ductility at the straightening operation of the continuous casting process, which usually occurs in temperature range of 700 ≤ T ≤ 1000°C. According to this mechanism, if the morphology and distribution of the precipitates in the microalloyed steel can be controlled by adjusting its chemical composition, surface cracking may be efficiently suppressed in practical operation. Thus, it is necessary to define clearly the contributing role of microalloying precipitation behavior to hot ductility of microalloyed steel in detail.Addition of boron (B) to plain carbon steel improves hot ductility, because solute B atoms that segregate to grain boundaries can occupy vacancy and thus prevent formation and propagation of micro-cracking at grain boundaries. Meanwhile, B bearing steel (B steel) are susceptible to transverse cracking on the slab surface during the continuous casting process because of poor hot ductility at temperature range of 700 ≤ T ≤ 1000°C, possibly formation of boron-nitride (BN) at austenite grain boundaries. These results suggest that the behavior of B precipitates significantly affects hot ductility of B steel. However, few studies have been conducted on the effect of B precipitation behavior on the hot ductility of B steel. Therefore, in present study, regarding the relation between the hot ductility of B steel and the B precipitation behavior, the effect of thermal treatment and addition of microalloying element was investigated. Hot ductility of B steel has been measured in the laboratory by tensile testing specimens reheated to a high temperature, and then cooled down to the desired test temperature before isothermally testing to fracture. Moreover, isothermal tensile testing has been also performed after imposing different thermal history prior to reaching the test temperature corresponding to the straightening stage of the continuous casting. Experimental work involved metallographic and scanning electron microscopy examination of the fracture surface. The morphology and chemical composition of the B precipitates which mounted on carbon replicas film were determined by using a transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectroscope (EDS). The distribution of B in the B steel was analyzed by particle tracking autoradiography (PTA) and secondary ion mass spectroscopy (SIMS). The hot ductility behavior of B steel strongly was affected by the morphology and the number density of BN precipitates, which was determined by cooling rate and thermal history prior to straining. As the cooling rate and the thermal history gradient prior to straining increased, BN precipitates became smaller and more numerous in the interior of the prior austenite and preferentially at austenite grain boundaries, and therefore led to an increase the tendency for intergranular fracture. The hot ductility of B steel was also affected by amount of BN precipitation and precipitation temperature, which is determined by amount of the N content. Amount and precipitation temperature of BN remarkably increased with increase in N content, and lead to reduced hot ductility of B steel. The hot ductility of B steel could be changed by the distribution of BN precipitates, which was determined by addition of microalloying element such as Nb and Ti. Addition of Nb without Ti to B steel had little influence on the hot ductility, because distribution of BN precipitates in the B steel could not be changed by that. Adding a small amount of Nb plus Ti improved the hot ductility of B steel because the presence of TiN particles before precipitation of BN makes the BN precipitates’ distribution more homogeneous than when these particles are absent. Hot ductility behavior of the B steel was affected by the variability in austenite grain size (AGS) when the BN precipitation distribution remained similar. Hot ductility behavior of the B steel having coarse AGS (>250μm) was dominated by precipitation behavior of BN, and AGS had a relatively minor influence on the hot ductility. In contrast, hot ductility of B steel having a relatively finer AGS (<100μm) was dominated by the effect of AGS refinement rather than effect of BN precipitation behavior.An important insight derived at through this study was that controlling BN precipitation behavior by secondary cooling during continuous casting process, or by adjusting of microallying element contents, may be beneficial in suppressing the corner cracks of microalloyed steel slab with strong nitride-forming elements such as B.