Formation mechanism and control of transverse corner cracks in fine blanking steel

Abstract

Owing to the extreme difficulty in their prevention, transverse corner cracks have been frequently observed in fine blanking steel produced on straight arc caster. In this study, the transient thermal module of Ansys was used to calculate the corner temperature distribution of a continuous casting slab. According to the actual cooling rate, a Gleeble 3500 thermal simulator was used to simulate the bending process of continuous casting, and the thermoplastic curves of test steel grades were drawn. Through the observation of fracture morphology and metallographic examination, typical precipitation detection and phase transformation temperature detection, the mechanism of corner cracks in experimental steel grades was determined and the corresponding control measures were formulated. The results showed that under the original process conditions, the corner temperature of the continuous casting slab was in the brittle temperature range of the steel grade when it entered the bending section. During the cooling process of the continuous casting slab, all Ti elements dissolved into the matrix, and the main precipitates were (Cr, Fe)XCY, while the grain boundary pinning was weak. In the high temperature brittle region, deformation-induced ferrite was produced at grain boundaries, which formed a stress concentration during the bending process which led to plastic reduction. The mixed control mode of strong cooling in the cooling stage and weak cooling in the reheating stage could not only ensure the strength of the slab shell in the vertical section, but also promoted the growth of the ferrite film induced by intergranular deformation in the strong cooling stage. It increased the maximum allowable stress of ferrite at the grain boundary. By controlling the temperature and cooling rate in the reheating stage, the growth of ferrite at the grain boundaries and the nucleation of ferrite in grains were accelerated, the stress distribution at grain boundaries and grains was uniform, and the corner cracks were eliminated.