Carbide-free Bainite in a High-strength Medium-carbon Steel

摘要

The microstructures and the mechanical properties of the high strength medium-carbon steel were systematically optimized by a series of heat-treatment processes,which contained single-stage ustempering transformation,two-stage austempering transformation and continuous cooling transformation.The heat treatment process of low-temperature bainite in the high strength medium-carbon steel was explored.The relationships between bainite microstructure and mechanical properties were systematically studied using scanning electron microscope,transmission electron microscopy,X-ray as well as metallographic microscope.Results showed that all of the bainite microstructures contained ferrite phase and austenite phase,and the carbides were not observed which due to the material composition containing 1.40％Si and 0.70％Al in the high strength medium-carbon steel.The martensite start temperature (Ms) of the steel was 282℃.The microstructure of low-temperature bainite in the high strength medium-carbon steel,containing fine plate of carbide-free bainitic ferrite (93nm) and thin film of retained austenite,could be produced by continuous cooling transformation around the Ms temperature from 300℃ to 270℃ at a cooling rate of 0.5℃/min after austenitized.The low-temperature bainite exhibited a higher strength and higher impact toughness compared to any other microstructures for the high strength medium-carbon steel.As the single stage austempering temperature increased from 300℃ to 415℃,impact toughness reduced significantly by 60％.