Microstructure And Properties Of Ultra-high Strength Bainitic Steel

With the continuous development of society,the industrial demand for steel materials is gradually increasing.Ultra-high strength steel is widely used in engineering machinery,bridges,ships,oil pipelines and other fields due to its excellent strength performance.It is the development direction of ultra-high strength steel to ensure high strength,good plastic toughness and weldability of material at one time.In this paper,two kinds of experimental steel with appropriate alloying elements were used as the research object(0.2C-1.5Si-3.0Mn-2.0Ni-0.5Cr-0.1V and 0.4C-1.5Si-2.0Mn-2.0Ni-0.5Cr-0.1V).The kinetic laws of bainite transformation were studied by comparing experimental steel of different C and Mn contents.On this basis,the effects of different heat treatment processes on the microstructure evolution in high strength bainitic steel were studied,and the mechanism of strengthening and toughening in high strength bainitic steel was revealed.The main contents and results are as follows:(1)The martensitic transformation initiation temperature of different austenite grain size was determined.The results showed that the martensitic transformation temperature decreased with the reduction of austenite grain size.(2)The kinetic law of bainite transformation of experimental steel was studied by using phase change test.The bainite transformation in isothermal process was calculated by expansion method and lattice constant method.The results showed that the kinetics of bainite transformation is significantly accelerated during the isothermal process of Ms temperature.The austenite grain size has a remarkable effect on the kinetics of bainite transformation.Specifically in the high temperature region of bainite transformation,the bainitic transformation kinetics accelerated with the increase of the austenite grain size.In the low temperature region of bainite transformation,the bainitic transformation kinetics slowed down with the increase of austenite grain size.(3)The martensite band caused by the segregation of Mn can be eliminated under the reasonable two-stage process condition.The strength and toughness of the experimental steel are simultaneously improved because of uniform bainite structure.(4)The strength and plasticity matching of experimental steel under different heat treatment conditions was studied by heat treatment experiment.The results showed that the strength and plasticity matching was higher under the two-stage heat treatment process when compared with the one-stage heat treatment process,and product of strength and elongation was up to 21888MPa%.