The mechanism of the formation of boron ineffective zone and its effect on the properties of Ultralow carbon bainitic steels

Abstract

In the manufacturing of ultralow carbon bainitic (ULCB) steels, boron is an alloying element which is essential to promote the desired bainitic transformation. In order to obtain this hardenability effect, boron must be in solution and it must segregate to the austenite grain boundary and where it decreases the contribution of the boundary's interfacial energy to ferrite nucleation. During the development of ULCB steels in China Steel Corporation, a small boron ineffective zone was found at the centre of steel plates. From electron-probe X-ray microanalysis (EPMA) and boron autoradiograph analysis, it was found that the formation of the boron ineffective zone was due to the centre-line segregation of inclusions which strongly combined with boron and formed a boron-free zone in its vicinity. The microstructure of the boron ineffective zone was a conventional ferrite with a strength which was much lower than that of the surrounding bainite. This resulted in crack separation in the tensile and impact specimens. It was found from a hydrogen-induced-cracking (HIC) test, that the HICs had a propensity to propagate along the boron ineffective zone. From a welding y-grooved test, a higher cold-cracking sensitivity at this boron ineffective zone was also found.