Microstructure Evolution And Mechanical Properties Of N Micro-Alloyed 0.2C-5Mn Steel

Advanced high strength steel has become a hot research topic at home and abroad during recent years.As the third generation of advanced high strength steel,its strong strength and toughness is mainly used to achieve by TRIP effect of austenite.At present,nitrogen alloying has been successfully applied to stainless steel and high manganese steel,and the N in steel could form nitrides and solid solution was found,nitride can play a role of pinning the grain boundaries and refining grain,and the concentration of dissolved nitrogen in steel has a direct impact on the tensile strength of steel.In addition,nitrogen can enhance the austenite in the form of interstitial atoms,effectively improve the austenitic stability.However,there are few reports on the study of nitrogen alloying of carbon structural steels.In this paper,with nitrogen micro-alloyed 0.2C-5Mn steel for the research,the investigation were carried out on the effect of austenite reverse transformation annealing on evolution of microstructure and mechanical properties of hot rolled sheet,one stage cold sheet and two stage cold sheet.The main research contents and results of this paper are as follows:(1)The effect of austenite reverse transformation annealing on evolution of microstructure and mechanical properties of hot rolled sheet was sudied.The hot rolled sheet were annealed at 620?680? for 1?24h,and the lamellar microstructure of austenite and tempered martensite was obtained.With the increase of annealing temperature and holding time,the floating convex structure increased,the width of laths was increased,and the precipitates were dissolved gradually.With the increase of annealing time,the volume of retained austenite increased initially and then decreased;The yield strength of tested steel decreased obviously,the tensile strength increased initially and then decreased,total elongation increased significantly,maximum values of Rm×A(44.8GPa%)was obtained after annealing treatment for 3h.With the increase of annealing temperature,the volume of retained austenite increased gradually,the yield strength of tested steel decreased obviously,the tensile strength increased,and maximum values of RmxA(40.4GPa%)was obtained after annealing treatment at 635?.With the annealing treatment at 680? for 24h,the stability of austenite decreased remarkably,which resulted in a sharp decrease in the volume fraction of retained austenite at room temperature,which was only 16.2%.(2)The effect of austenite reverse transformation annealing on evolution of microstructure and mechanical properties of one stage cold sheet was studied.The cold rolled sheet with the thickness was 0.7 or 1.15mm were annealed at 635? for 10min-15h,and the microstructure was consist of severely deformed martensite and ferrite before the annealing treament,it was obtained the equal axial grains recovery and recrystallization.With the increase of annealing time,the floating convex structure increased,the grain size of both phase was increased,the profile was more clear,and the precipitates were dissolved gradually,the volume of retained austenite increased gradually,The yield strength of tested steel decreased,total elongation increased initially and then decreased,work hardening was more obvious,and the yield elongation became shorter.Maximum values of RmxA(38.9GPa%,36.9GPa%)was obtained after annealing treatment at 635 ? for 1h.(3)The effect of austenite reverse transformation annealing on evolution of microstructure and mechanical properties of two stage cold sheet.The two stage cold rolled sheet were annealed at 635 and 650? for 30min-6h,and it was obtained the equal axial grains after recovery and recrystallization.With the increase of annealing time,the volume of retained austenite increased initially and then decreased,and maximum volume of retained austenite(29.6%)was obtained after annealing treatment for 1h,The yield strength of tested steel decreased significantly,and maximum values of Rm×A(40.9GPa%)was obtained after annealing treatment at 63 5 ? for 2h.Increasing the annealing temperature and prolonging the holding time were beneficial to shorten the yield elongation,and the effect of increasing annealing temperature was more obvious.(4)The Ac3 temperature of tested steel was 696? which is 23 ? lower than that of the 0.2C-5Mn steel,demonstrated that the N effectively stabilize the austenite and expanded austenite phase region;N existed in the form of solid solution to ferrite and austenite,and the formation of precipitates;In the process of austenite reverse transformation annealing,the partitioning rule of N was similar to C.(5)The curve of the work hardening of the hot rolled plate and the cold rolled plate had obvious characteristics.The hot-rolled sample which was annealed at 635 ? for 3h was obtained large amounts of retained austenite,and the clear steps in true stress-strain curve were visible,showed a discontinuous hardening behavior,and this was rarely reported in previous studies;The 0.7mm cold-rolled sample which was annealed at 635? for 1h was obtained a long yield platform nearly to 30%,it is much longer than 0.2C-5Mn steel which was reported in previous documents,and N is conducive to the formation of Cottrell atmosphere which could pin the dislocation and extend the yield platform;In addition,the grain boundaries could be pinned by nitride which may reduce the grain size,then ultimately lead to the extension of the yield platform;The work hardening rate of 1.15mm cold-rolled sample which was annealed at 635? for 1h was showed a cyclical change which is similar to the pulse waveform from 0 to 2000MPa,this is due to the occurrence of work softening and work hardening,and this results have not yet reported in previous research.