Development of 780 MPa steel plate by direct quenching and tempering processes

Steel with low carbon and micro-alloying was developed and its dynamic continuous-cooling-temperature(CCT) curve was measured.Bainite was formed when the steel plates were produced by direct quenching with a cooling rate between 1 ℃/s and 30 ℃/s after rolling.With the cooling rate increasing,Bs decreases and bainite ferrite become finer,therefore,Vickers hardness of the steel increases.Steel plates with tensile strength of 780 MPa,yield strength of 700 MPa and-40 ℃ Charpy impact energy of 150 J were produced by direct quenching with cooling rate higher than 20 ℃/s and tempering at 610-650 ℃.Steel plate with optimized combination of strength and toughness was produced by tempering at 630 ℃ after direct quenching process.Microstructure of the steel plate was composed of lathy bainite ferrite with 0.5-1.5 μm thickness and martensite-austenite constituents at boundaries of bainite ferrite lath.Bainite ferrite lath was composed of sub units.Spherical cementite was precipitated at bainite ferrite boundaries degenerated after tempering process.It was found that the steel plates exhibited good ability of arresting cracks at-40 ℃ by instrumented impact experiment.The bainitic transformation of the experimental steel can be interpreted by diffusional mechanism.