Phase Transformation Behavior,Microstructure And Mechanical Properties Of Aluminum-containing Medium Manganese Steel With Low Manganse Content

The third generation of high strength steel needs to be designed from the angle of low alloy.The multi-phase,metastable and multi-scale complex structure can be obtained by simple production process,so as to obtain the advanced steel materials with low cost,easy preparation,high strength and toughness.In this paper,from the point of light weight,low alloying and easy production,the medium manganese steel was prepared by adding aluminum and reducing the content of manganese.By adopting austenite reverted transformation(ART)annealing heat treatment,the structure of micron level lamellar ferritic and austenitic dual phase were obtained.The transformation behavior and transformation parameters of the experimental steel were obtained by equilibrium phase diagram calculation and thermal simulation.The microstructure of the experimental steel was quantitatively characterized and analyzed.The relationship between the evolution of annealing structure and the change of properties in the intercritical two-phase region was studied.The quantitative characterization and analysis of the microstructure show that the annealing temperature and time in intercritical two-phase region have a significant effect on the formation of reverted austenite.After oil quenching,the duplex structure of ferrite,austenite and a small amount of martensite are obtained.There are lots of carbides in the ferrite matrix annealed at low temperature and/or in short time.The content of reverted austenite is less and the size is smaller which mainly presents the shape of bamboo-leaf.The content of twin martensite obtained after quenching is less.When the temperature is increased or the annealing time is prolonged,the amount of reverted austenite is increased,and it grows wider along the martensite lath boundary.Finally,they are connected together and transform into a long strip structure.The size difference is gradually increased,and the content of twin martensite after quenching is gradually increased.In the early stage,the formation rate of austenite is faster and then gradually slows down.The content of austenite no longer increases until it reaches saturation,and then the holiding process will make it coarsen.When the content of reversed austenite increases to a certain extent,the content of retained austenite will decrease.The tensile test results show that with the extension of annealing time in the twophase region,the elongation of the samples increases first and then decreases,the yield strength decreases first and then remains unchanged,the tensile strength increases all the time,and the product of strength and elongation increases first and then decreases.There is no Lüders band in the tensile curves of short-time annealed specimens,and this phenomenon disappears with the extension of annealing time.The samples annealed at low temperature and/or in short time have better plasticity,the maximum elongation reaches 32.6%,but the strength is low.The samples annealed at high temperature and/or in long time have higher strength,the maximum strength reaches 1460 MPa,but the plasticity is poor.The samples annealed at 745? for 50 min and 785? for 8 min have good comprehensive mechanical properties,and the product of strength and elongation reaches 30 GPa·%,which is up to the level of conventional medium manganese steel.In conclusion,it is feasible and effective to prepare light medium manganese steel with low alloy and easy preparation by adding aluminum and reducing the content of manganese.Through the analysis of the relationship between microstructure and mechanical properties,it is found that when the ferrite content is high,the test steel shows low strength,high elongation and low work hardening rate.When the content of martensite is high,it shows high strength,low plasticity and high work hardening rate but lasts for a short time.Austenite provides continuous and stable work hardening effect owing to the TRIP effect,which improves the strength.It also improves the plasticity at the same time and then enhance the product of strength and elongation.