Hereditary Effects of Segregation on Weld Microstructure and Properties of High-Strength Wind Power Steel and Analysis via In Situ Tensile Tests

A systematic comparative study was conducted on the microstructural and performance differences in the heat-affected zone (HAZ) of Q460 (20 mm) and Q500 (25 mm) grade wind power steels dominated by ferrite/pearlite and acicular ferrite/bainite, respectively. In situ tensile tests reveal the HAZ performance degradation mechanism induced by centerline segregation. Compared with Q460, Q500 welded joints show higher tensile strength and fatigue limit, but poorer low-temperature impact toughness and crack tip opening displacement properties. Centerline segregation occurs in both steels and is more severe in the thicker Q500 plate. Segregation bands are enriched in C, Mn, Nb, and Ti, indicating inheritance of base metal segregation in the HAZ. Apart from martensite, strip-shaped MnS inclusions and irregular (Nb, Ti)(C, N) composite phases precipitate along segregation bands. Under stress, these brittle phases preferentially nucleate microvoids, which coalesce and induce macroscopic fracture through void linkage and interface debonding. MnS and (Nb, Ti)(C, N) phases contribute more significantly to HAZ property degradation than segregation-induced martensite, acting as a key factor for the deteriorated mechanical performance of both the plate center and HAZ.