Research On The Influence Of Q&P Process On The Mechanical Properties And Microstructure Evolution Of Medium Manganese Steel

In recent years,the total quantity of vehicles in our country has increased,energy consumption has increased,and the problems of environmental pollution caused by automobile manufacturing and automobile exhaust emissions have been aggravating.Under this social background,it is particularly important to realize automobile lightweight.One of the important directions to realize automobile lightweight is to realize material lightweight.By comparing the three generations of advanced high strength steel,it is found that the strength and toughness of the third generation of advanced high strength steel are higher than that of the first generation,and the alloy content and production cost of the second generation of advanced high strength steel are lower.As a typical representative,manganese steel is a hot spot in the research of lightweight steel materials.American scholar Speer proposed the new heat-treatment concept of "Quenching and Partitioning"(Q&P)for the first time in 2003.He found and proved that effective control of retained austenite in steel can be achieved without significantly reducing the strength by effectively controlling it.A kind of cold rolled medium manganese steel(0.12C-7.69Mn-1.45Si-2.76Al)is designed in this thesis.Combined with the research results of other literatures,this thesis uses IA-Q&P(Intercritical annealing-Quenching and Partitioning)to heat treat medium manganese steel,in order to obtain the retained austenite with higher volume fraction and appropriate mechanical stability.Based on previous experiments and theoretical analysis,the process parameters of Q&P process were optimized.The effects of annealing and partition parameters on microstructure evolution and mechanical properties of cold rolled manganese steel and work hardening behavior were studied.The main research contents and results are as follows:(1)Effect of annealing temperature and annealing time on microstructure evolution and mechanical properties of medium manganese steel.The tensile property of the test steel decreases with the increase of annealing temperature.When the intercritical annealing temperature is 680℃,there are 25.3% retained austenite in the test steel,showing excellent comprehensive mechanical properties.With the increase of interannealing temperature,the grain size increases,the fine plate strip retained austenite gradually transforms into massive or equiaxed crystals,and the average Mn concentration in retained austenite also decreases,which makes the stability of retained austenite decrease continuously.Both the volume fraction and stability of the retained austenite affect the tensile properties of the sample.The high volume fraction of the retained austenite and the stability gradient of the retained austenite caused by the fine plate and strip structure lead to the discontinuous TRIP effect of the sample A680,which makes it show excellent tensile properties.With the increase of annealing time,the grain size increases to a certain extent,but not significantly,and the microstructure of each sample is very fine.The volume fraction of retained austenite firstly decreased,then increased and then decreased with the increase of annealing time.The yield platform and sawtooth characteristics can be observed in the tensile curves of the four samples.When the annealing time is 20 min,the test steel shows the best mechanical properties and the product of strength and elongation is 43.5GPa·%.(2)Effects of different partitioning temperature and partitioning time on microstructure evolution and mechanical properties of medium manganese steel.Compared with only annealed samples,Q&P heat treatment can improve the toughness of steel to a certain extent,and the partitioning process significantly affects the morphology of retained austenite.The retained austenite in annealed samples is mainly fine particles and massive,and the stability of retained austenite is poor.When the partitioning temperature is 150℃,heterogeneous austenite structure is formed,and the morphology of retained austenite is granular,massive and lamellar.Retained austenite with higher volume fraction and different stability can have continuous TRIP effect at different stress stages,so as to obtain higher product of strength and elongation.With the increase of partitioning temperature,the content of retained austenite fluctuated greatly in the range of 19.11～25.16%,while the content of carbon in retained austenite only changed slightly in the range of 1.1 ～1.24%.With the increase of partitioning time,the retained austenite ranges from 15.88～25.16%,and the carbon content in the retained austenite ranges from 1.19% to 1.32%,which also changes little.This means that the carbon content is not the key factor controlling the mechanical stability of the heat-treated steel studied in this study.Compared with the sample without the partitioning process,the strength and toughness of medium manganese steel will not be improved if the partitioning time is too long or too short,but will decrease if the partitioning time is too short.The results show that the Q&P heat treatment process with appropriate partitioning time has a significant effect on the improvement of mechanical properties of medium manganese steel.