Research On Microstructure And Mechanical Properties Of High Manganese Austenitic Steel For LNG Storage Tanks

With the improvement of people’s awareness of environmental protection,the demand for clean energy is rising all over the world.Natural gas,as one of the cleanest resources,has drawn more and more attention from all countries.In recent years,the supply of LNG has been increasing year by year,and LNG storage tanks have also been the focus of research.LNG storage tanks materials usually include aluminum alloy,austenitic stainless steel,9%Ni steel and Invar alloy,with lower strength and higher cost.Therefore,high manganese austenitic steel for cryogenic application is of great research valuable,which can combine high strength and excellent toughness at cryogenic temperature.And it has stable organization and lower cost,without low-temperature ductile-to-brittle transition temperature.In this paper,the alloy composition designing,dynamic recrystallization behavior,static recrystallization behavior,rolling and heat treatment process,and fracture behavior at low temperature of high manganese austenitic steels were studied.The main results are as follows:(1)The alloy composition was designed from the consideration of Cr equivalent and Ni equivalent,and the chemical composition of the experimental steel falls in the austenite phase region of the schaeffler histogram.Through the study of the dynamic recrystallization behavior of the experimental steel,the true stress-true strain curves of the experimental steel under high temperature deformation were obtained.The influences of deformation temperature,deformation and deformation rate on the dynamic recrystallization behavior were also studied.Then the activation energy Q the Z parameter of the experimental steel have been calculated.Also the linear relationship between the critical strain and the Z parameter was obtained by calculating.(2)The static recrystallization of the experimental steel was accelerated with the increasing deformation temperature,prolonging of the interval time and the increasing of strain rate through researching on the static recrystallization behavior of the experimental steel.Also,the static recrystallization activation energy of the experimental steel and its static recrystallization kinetics equation were obtained.(3)The research of laboratory rolling and heat treatment of the experimental steel has been studied.The hot rolling austenite and carbide which distributed along the austenite grain boundaries.It should be quickly quenched after hot rolling.After the heat treatment of solution treatment,the microstructure of the experimental steel is austenite lonely,which has large number of annealing twins in the austenite.Fine and homogeneous austenite organization will be obtained with reducing the solution treatment temperature and shorten the solution treatment time.The experimental steel still has good toughness even at cryogenic temperature.And the best heat treatment process is to solution treat at 900℃ with 30min.The excellent plasticity and toughness is due to the interaction of dislocations and twins in the microstructure.(4)It is found that the impact energy of the experimental steel decreases with the decrease of the test temperature.The impact fractures are all composed of dimples under different temperature,which show the typical ductile fracture morphology.And the experimental steel has a characteristic of stable structure.There exist serious plastic deformation and a lot of deformation twins near the fracture.The micropores generated in this process aggregate into cracks with deformation.And the cracks continue to expand with deformation until the ductile fractures occur,which reveals the fracture is microvoid coalescence fracture.