Microstructure Evolution And Mechanical Properties Of N Or Nb Micro-alloyed Medium Manganese Steels

Advanced high-strength steel(AHSS)is a high-quality steel with high strength and high toughness.After years of research and development,it has been widely used in the automotive industry and other industrial fields.As a typical type of the third generation advanced high strength steel,medium manganese(TRIP)steel both has good strength and good toughness.Alloying of steels has always been an important direction of research and development in many research institutions both at home and abroad,and alloying is one of the most direct and effective methods to control the microstructure and properties of steel,Alloying can effectively improve the comprehensive performance of steel products.The strength and toughness of the medium manganese steel mainly depends on the stability of the TRIP effect in the deformation process.The strength and toughness of the medium manganese steel mainly depends on the stability and sustainability of the trip effect during deformation process,as two typical micro-alloying elements,nitrogen and niobium can effectively refine grains in steel,and the formed precipitates are beneficial to the improvement of steel strength.As an interstitial atom,the nitrogen element can effectively improve the stability of austenite;the solid solution of niobium in austenite can improve the stacking fault energy of the alloy,inhibit the TRIP effect.In this paper,nitrogen microalloyed 8Mn steel and niobium microalloyed 8Mn steel were studied.Effects of nitrogen and niobium on Microstructure and properties of 8Mn steel was studied.And the influence of austenite reverse transformation annealed about the microstructure evolution,element distribution behaviors and mechanical properties changes of the hot rolled plate and cold rolled plate of niobium microalloyed 8Mn steel and the cold rolled plate of nitrogen microalloyed 8Mn steel has been studied separately.The main research contents and results of this paper are as follows:(1)The microstructure evolution,element distribution behavior and mechanical properties change of cold rolled annealed steel 8Mn by nitrogen microalloying was studied.Nitrogen microalloying made the microstructure of the experimental steel refined,and the yield strength and tensile strength increased.The microstructure of cold rolled sheet of the experimental steel after annealed is austenite-ferrite dual phase structure.With the increase of austenite inverse transformation annealed temperature and the prolongation of annealed time,the grain size increases gradually,and the microstructure tends to be equiaxed and homogenized.With the increase of annealed temperature,the tensile strength and elongation at break of the experimental steel increase first and then decrease,and the yield strength gradually decreases.With the increase of annealed holding time,the yield strength of experimental steel decreases gradually,and the tensile strength decreases first and then increases.The best comprehensive performance is obtained when annealed at 620? for 3 hours,and the product of strength and ductility is 39.6 GPa·%.(2)The microstructure evolution,element distribution behavior and mechanical properties change of hot rolled annealed steel 8Mn by niobium microalloying was studied.Niobium microalloying made tensile strength increased,but plastic decreased.The microstructure of hot rolled sheet after annealed is austenite and tempered martensite dual phase structure.With the increase of annealing temperature of austenite inverse transformation,the lath structure is continuously coarsened.With the increase of annealing time,the microstructure tends to homogenize and the precipitates coarser.With the increase of annealing temperature and the prolongation of annealing time,the tensile strength and elongation at break of the experimental steel increases first and then decreases.When the experimental steel annealed at 610? for 6h,the best properties of Rm×A(41.4GPa·%)was obtained,with a tensile strength of 1690 MPa and an elongation at break of 24.5%.(3)The microstructure evolution,element distribution behavior and mechanical properties change of hot rolled annealed steel 8Mn by niobium microalloying was studied.Niobium microalloying made the microstructure of the experimental steel refined,and the yield strength increased,but plastic decreased.The microstructure of cold rolled sheet of the experimental steel after annealed is austenite-ferrite dual phase structure.With the increase of annealing temperature of austenite inverse transformation,austenite grain size increases gradually,ferrite grain size increases first and then decreases,and the microstructure tends to be equiaxed and homogenized.With the increase of annealing time,the grain size of austenite and ferrite increases gradually,and the microstructure tends to be equiaxed and homogenized.After austenite reverse transformation,the tensile curve of the cold rolled sheet after annealed of the experimental steel is dominated by the yield extension,which is characterized by a long yield platform.With the increase of annealing temperatue and the prolongation of annealing time,the yield elongation decreases gradually,and the elongation at break increases first and then decreases,yield elongation gradually decreases.(4)In 8Mn steel,nitrogen and niobium are mainly present in the form of precipitates and solid solution in both austenite and ferrite.Both nitrogen and niobium elements can effectively refine the microstructure of experimental steel and obtain ultrafine grain structure.Both nitrogen and niobium elements lead to the extension of the yield platform and the increase of tensile strength of the experimental steel,but the niobium element will lead to the reduction of the plasticity of the experimental steel.