Segregation effects in a high-strength wind power steel: Microstructure, mechanical properties, and hydrogen embrittlement sensitivity of base metal and simulated HAZ

Despite the application of Thermo-Mechanical Controlled Processing (TMCP) technology, continuous banded segregation structures were still observed in the central region of a 25 mm thick 550 MPa grade wind power steel plate. This segregation results in delamination behavior in the low-temperature impact samples of the base metal and a reduction in low-temperature toughness. Welding simulation experiments revealed an optimal range of welding heat input, specifically 14–20 kJ/cm, which enables both the near-surface and the core of the steel plate to achieve fine lath bainite structures and optimal low-temperature impact toughness. However, it was also found that the segregation in the core of the base metal is inherited by the welding heat-affected zone (HAZ), particularly the segregation of Mn, which is difficult to eliminate during the welding thermal cycling process. This leads to the formation of hard phases such as MnS inclusions, martensite (M), and M/A (martensite/austenite) islands. Additionally, voids were observed at the interface between MnS and the matrix, which significantly deteriorate the low-temperature impact toughness of both the matrix and HAZ. Furthermore, the distribution of large-sized (Nb,Ti)(C,N) precipitates along the centerline of the segregation zone also adversely affects low-temperature toughness and plasticity. Moreover, the study found that segregation and welding processes significantly increase the hydrogen embrittlement sensitivity of the steel plate, with the welding process having a more pronounced effect. The simulated welding sample fracture occurred in the intercritical HAZ (ICHAZ) rather than the coarse-grained HAZ (CGHAZ). This is attributed to the dual effects of two-phase thermal cycling and segregation in ICHAZ, which promote the formation of numerous coarse M/A islands, inducing brittle crack initiation and propagation within the M/A or at the interface with ferrite. Additionally, hydrogen-induced austenite grain boundary cracking behavior was observed in CGHAZ.