| The low-temperature bainitic steels consist of nano-sized bainitic ferrite laths,film-like retained austenite and micro-nano-structured blocky retained austenite,with ultra-high strength and good ductility and toughness.Low temperature bainite steel with 0.79C is the main object in the present thesis.Scanning electron microscope?SEM?,electron backscattered diffraction?EBSD?,X-ray diffractometer?XRD?and other experimental methods were used.Comparative analysis of one-step bainite isothermal transformation process,two-step bainite isothermal transformation process and quenching-partitioning-bainite process?QPB?on bainite transformation and microstructure.Study on erosion resistance of bainitic steels with different properties through 0.55C and 0.79C bainitic steels.The hydrogen permeation curves of different microstructures of 0.79C bainite steel were measured by equipment such as dual-unit electrochemical workstation,and the influence of microstructure on hydrogen diffusion coefficient was studied.Through the experimental analysis and discussion,the following conclusions are obtained:1.After bainite isothermal transformation in different temperature and time,the microstructures and properties of 0.79C steel are different.The tensile strength of 300?for 12 hours bainite is 1525 MPa,and the elongation is 15.1%.The tensile strength of 200?15 days bainite is 1928 MPa.2.Two-step bainite process and QPB process can effectively shorten the heat treatment isothermal time,obtain a well-transformed,better performance structure.The tensile strength reaches 2040 and 1894 MPa respectively,and the elongation both reaches12.5%.3.Compared with two-step bainite of 0.79C steel,the 0.55C steel has lower strength toughness and high elongation.Medium and high carbon low temperature bainite steels exhibit different wear resistance performance due to different mechanisms in erosion wear at different angles.Bainite steel with high toughness has low wear rate in small angle erosion,and bainite with high hardness has low wear rate in large angle erosion.4.After the first hydrogen permeation of the sample,the second hydrogen permeation rate increase due to the filling of the irreversible hydrogen trap and the increase of the hydrogen atom channel,but the second maximum current is lower than the initial hydrogen permeation.The initial hydrogen diffusion coefficients of industrial pure iron and rolling down 10%iron are 1.79 and 1.54×10-6 cm2/s respectively.The hydrogen diffusion coefficients of different microstructure of 0.79C bainitic steel are obviously different.The hydrogen diffusion coefficients of pearlite,200?and 250?bainite are 9.36?1.66?1.09×10-8 cm2/s respectively. |
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