Effect Of Intercritical Annealing Process On Microstructure And Mechanical Properties Of Mn-Si Type Medium-Mn Steel

In order to cope with the great pressure of energy saving and emission reduction,green environment protection and safety enhancement,it is an urgent task for the steel and automobile industry to develop high strength,high toughness,high plasticity,light weight and low-density automotive steel.Mn-Si-Aided steel is developed to be studied,thermodynamic calculation and ART(Austenite Reversed Transformation)heat treatment and TEM,EBSD,XRD are used to study the test steel.Through adjusting and controlling the annealing time and temperature,microstructure composed of martensite after hot rolling and UFG ferrite and Austerity,high product of strength and elongation are obtained.Based on the research in this work,several conclusions are obtained as follows.(1)Based on the volume of reatined austenite and the concentration of C、Mn、Si、A1 in austenite,under different annealing temperature,the thermodynamic calculated optimal intercritical annealing temperature is about 640℃,the volume fraction of the remained Austenite under this temperature is about 57%,calculated by the Thermo-calc.(2)The result of the dilatometer is as follow:Acl is 560℃,Ac3 is 730℃.The microstructure of is totally martensite when the cooling rate is over 0.1℃/s,through the tested static CCT curve.The microstructure of hot rolled steel after air cooling experiment is full martensite.(3)As the strain-stress curve of the tested steel shows,it is more advantageous for austenite dynamic recrystallization at low strain rate and high deformation temperature,static recrystalization softening fraction increases with temperature and time increasing.When the strain rate is constant and improved the deformation temperature or the deformation temperature is constant and improved the strain rate,the start time of the recrystallization decreases from the RRT curve.(4)As the annealing temperature increased of the hot rolled medium manganese steel,the width of the lath RA is increasing,the fraction of the retained austenite is increasing,it is proved that the stability of the retained austenite decrease.At the same time the tensile strength increase with increasing the annealing temperature,while the elongation and PSE followed by decrease.The best ART process is:the annealing temperature is 620℃,and holding four hour,the final microstructure of the test steel is UFG ferrite and remained austensite(45%),the PSE(High Product of Strength and Enlongation)of the steel is 43GPa·%,and the tensile strength is 1060MPa,the yield strength is 783MPa,the elongation is 46%.(5)The mocrostructure of the cold rolled steel is elongated martensite.The mocrostructure of the annealed cold rolled steel is UFG ferrite and remained austenite.With the temperature of the ART-I increasing,the grain size of the retained austenite is decreasing,the stability of the retained austenite is enhanced,the tensile strength is increasing and yeild strength is decreasing;the grain size is decreases,the Lüder strain is increasing.As the ART-Ⅱ temperature increase,the tensile strength id enhanced and yield strength is decreasing.The best ART process is:the temperature of ART-I is 650℃,the holding time is three hour,the ART-Ⅱ annealing temperature is 620℃ holding one hour,the elongation is reached 41.78%,the yield strength is 1130MPa,the tensile strength is 1240MPa and the PSE is 52 GPa·%.