Effect Of Alloying Elements And Annealing Process On The Microstructure And Properties Of Cold Rolled Medium Mn Steel

With the rapid development and increase of the vehicle population in automotive industry,three major problems emerge: energy consumption,emissions and pollution.It is imperative to improve the performance of automobiles and vehicle safety.The major directions of modern automobile development are weight loss,energy saving and safety,respectively.High plasticity materials are often chosen as body materials because of their high absorption capacity of impact energy to improve the safety of cars.High strength,high plasticity,low cost and easy process are the basic directions of future automotive steel.This essay used the middle manganese alloy composition system.The carbon content was controlled between 0.1% and 0.3%,the content of Mn was controlled from about 4% to 8%,Si and Nb were also added for alloying.This paper took a two-phase zone annealing for four test steels with different alloy compositions.When the annealing temperature was from 570°C to 670°C and annealing time was 1h and 10 h respectively,the effects of annealing temperature,annealing time and alloying elements on the microstructure and mechanical properties of the test steels were investigated.The test results show that as the annealing temperature increases,the tensile strength basically shows an upward trend while the elongation rate goes down.Austenite volume fraction shows a trend of rising first and then decreasing.However,if the annealing temperature is too high,the stability of austenite will decrease.The strength combined with plastic value first increases and then decreases as the annealing temperature increases.When the annealing temperature is 600°C and the microstructure is mainly composed of fine ferrite and austenite,the comprehensive mechanical properties are optimized.When No.3 test steel is annealed at 600°C,the strength combined with plastic under the A80 gauge length reaches 21GPa·%.When annealing at 570 °C for a short time,there are some unrecrystallized structures,and as the annealing time extends to 10 h,the recrystallization process finishes and the elongation rate is higher than the that of the test steel which annealing for 1 h only.The long annealing time could not increase the volume fraction of austenite but in fact reduce the stability of austenite.When the annealing temperature reaches up to 600°C,its elongation will decrease,lower than the elongation in the sample annealing for 1h.The content of undissolved carbides in the four test steel is consistent with the carbon content.When the Mn content is more than 5%,retained austenite volume fraction and product of strength and plasticity are relatively high,the comprehensive mechanical properties of the test steel are excellent.Through the analysis of microstructure and mechanical properties,it is found that cold rolled manganese steel undergoes reverse austenite phase transformation during the two-phase annealing process.A large amount of metastable austenite is obtained,and the microstructure is mainly composed of ultra-fine grained ferrite and a large amount of metastable phases.The work hardening ability of austenite were greatly improved through the TRIP effect,by which not only optimizes the plasticity but also improves the tensile strength of the experimental steel.Its ultra-high tensile strength is mainly provided by the TRIP effect and ultrafine grained ferrite matrix.Mn in the test steel can expand the two-phase region and increase the stability of metastable austenite.At the same tiome,high Mn content increases the retained austenite volume and enhances the TRIP effect.The high strength and plasticity of cold rolled Mn steel are mainly provided by the TRIP effect from the retained austenite phase and the slip of ultrafine grained ferrite and dislocations.