Study On Strength-Toughness Mechanism And Heat Treatment Process Of Medium Manganese Low Temperature Steel

With the rapid development of science and technology,the production and use of liquefied gases are gradually increasing.So the research of storage and transport of these will be the focus of attention.The traditional storage of liquefied gases which are at temperatures below 100 degrees or more lower is mainly nickle-based low-temperature steel.Such low-temperature steel has high nickel content and alloy cost,therefore,reducing nickel content has become an important direction in the development of low-temperature steel.In this paper,through the design idea of using substitution of manganese for nickel,the design of C/Mn low-cost alloy composition is used to study the influence of heat treatment process on the evolution of miscrostructure of medium-manganese low-temperature steel,and the mechanism of strengthening and toughening of medium-manganese low-temperature steel is revealed.The main contents of this paper are as follows:(1)The dynamic recrystallization behavior of the experimental steel was studied by simulated rolling experiment.The dynamic recrystallization model and deformation resistance model of medium manganese low temperature steel were established.The dynamic recrystallization activation energy of experimental steel is Q=527.92 kJ/mol,and the obtained regression model has good reliability.(2)In the simulated experimental steel plate furnace,the heating temperature is increased,and the temperature range of the medium-manganese low-temperature steel.bodyaustenite two-phase region is 596～747℃.Measurement of static CCT curve of the mediummanganese low-temperature steel by experiments,and martensitic transformation only occurs in the continuous coolingh process at 0.1～50℃/s.(3)The experimental study on the microstructure and mechanical properties of the quenched state of the experimental steel through different tempering temperatures.The quenched microstructure is mainly lath martensite with high dislocation density.After tempering,submicron-scale tempered martensite(width≤600 nm)-reverse transformation austenite(width ≤ 250 nm)composite layered structure is formed.As the tempering temperature increases,the yield strength of the experimental steel decreases,the elongation gradually increases,and the impact energy first increases and then decreases.After tempering at 620℃,the best combination of toughness is obtained.(4)The study on the change law of metastable austenite content of experimental steel found that with the increase of tempering temperature of experimental steel,the volume fraction of metastable austenite in the structure increased continuously,up to 28%.The content of C and Mn in austenite decreases with the increase of tempering temperature.The stability of metastable austenite in experimental steel is weakened,and it is easy to spontaneously transform into martensite at low temperature or under small load.Mechanism of Mn partitioning,the metastable austenite volume fraction and stability,the high-angle grain boundary toughening mechanism,the work hardening behavior,the TRIP effect of metastable austenite and the "effective grain size" were studied to interpret the mechanism of strengthening and toughening of medium manganese low temperature steel.(5)Studying the evolution of microstructure and mechanical properties of experimental steel in multi-stage heat treatment(QT,QQT,QLT)system,it is found that the low temperature toughness of experimental steel has been improved after multi-stage heat treatment,When the impact energy at-100℃ has reached 13 7J or more,or the impact energy at-120℃ is higher than 118J and the impact energy at-150℃ is higher than 68J.