Microstructure Regulation And Austenite Thermal Stability Of Low Carbon Medium Manganese Steel

As a new type of high strength plastic steel,Medium Manganese TRIP steel has been favored by more experts and scholars because of its characteristics of low production cost and high comprehensive performance.In this thesis,0.2C-5Mn-0.5Si-1.5Al cold rolled low carbon medium manganese steel was taken as the research object,and the microstructure regulation and austenite thermal stability of 0.2C-5Mn-0.5Si-1.5Al medium manganese steel were studied by means of thermodynamic calculation,XRD,SEM,TEM and hardness tester.The main research contents are as follows:In order to investigate the effect of incomplete austenitizing on pearlite transformation,thermodynamic simulation was carried out on the test steel.The results show that with the increase of incomplete austenitizing temperature and time,the TTT curve of the test steel begins to shift to the right.The pearlite transition start time is delayed and the time required to complete the transition is gradually prolonged.In order to explore the effect of pretreatment process on the microstructure and mechanical properties of the test steel,pretreatment process was carried out on the test steel.The results showed that after incomplete austenitizing at 800℃×30min,the martensite in the tissue at room temperature was as follows:ferrite=6.5:3.5.Subsequently,pearlite with volume fraction of 5%-21.9%was obtained at 550℃for 3-6h.The pearlite group size increases from 0.87±0.15μm to2.34±0.50μm when the time of pearlite transformation is prolonged.Microhardness decreased from 260.31HV to 188.14HV.When the transformation temperature of pearlite increases to 600℃（6h）,the pearlite mainly exists in granular shape,and the spheroidization of carbide is obvious.In order to study the influence of different intercritical annealing processes on the microstructure evolution of the test steel,the test steel after pretreatment（pretreatment process is 800℃×30min-550℃×6h）was taken as the research object,and the intercritical annealing process was conducted on it.The results show that two kinds of RA can be obtained after annealing for a short time（3～10min）.After annealing for 5min,the mass fraction of Mn and grain size of RA are about 8wt%and 1.10μm respectively.The mass fraction of Mn in the RA from pearlite transformation is about 16wt%and the grain size is about 0.19μm.Beck equation for grain growth of filmy and blocky RA is:D=0.0088t^0.5476、D=0.1843t^0.3073.The effect of cold treatment process on two kinds of RA in annealed tissue was investigated.The intercritical annealed medium manganese steel（intercritical annealed at760℃×5min）was used as the research object to conduct cold treatment process.The results showed that the blocky RA almost all underwent martensitic transformation when the cold treatment temperature was reduced to-60℃,while some filmy RA at-100℃.Therefore,the thermal stability of the two kinds of RA is obviously different.