Multi-phase microstructure regulation and the influence on mechanical properties of EH500 grade ultra-heavy plate steel for marine engineering

Because of low rolling reduction and central segregation of ultra-heavy plate steel, the poor low-temperature toughness of the center region is a major challenge that limits the application of high-strength ultra-heavy steels. This work systematically investigates the effects of two-step intercritical heat treatment on the regulation of multiphase microstructure and properties of 100 mm EH500 marine engineering steel with severe central segregation. The results showed that after intercritical annealing in the 740 ℃ two-phase region, the yield strength and tensile strength of the experimental steel were 540 MPa and 869 MPa, respectively. The elongation and low-temperature toughness at -40 ℃ were relatively low, only 5.1% and 14 J, respectively. After further tempering at 600, 660, and 680 ℃, the yield strength of the experimental steel did not change significantly, ranging from 528 MPa to 551 MPa, and the tensile strength decreased to 687~730 MPa. The elongation and low-temperature toughness of the experimental steel firstly increased and then decreased with the increase of tempering temperature. When tempered at 660 ℃, the plasticity and toughness were the best, with an elongation of 30.6% and a Charpy impact energy of 163 J at -40 ℃. The microstructure characterization results show that the experimental steel annealed at 740 ℃ is intercritical ferrite (IF) and martensite (M). After further tempering at 600 ℃, multi-phase microstructure of IF, tempered martensite (TM) with fine carbides were obtained. When the tempering temperature is 660 ℃, microstructure of the experimental steel consisted of IF, TM and fine retained austenite (RA), and the content of RA in the central segregation region was significantly higher than that in the matrix, resulting in significantly improvement of the plastic toughness of the experimental steel. When the tempering temperature further rised to 680 ℃, the experimental steel obtained IF, TM, a small amount of retained austenite and high fraction of martensite/austenite (M/A) constituents in the central segregation zone. The large fraction of M/A constituents could worsen the plasticity and toughness of the experimental steel significantly.