Research On Microstructure, Properties And Microalloying Of Martensitic Low-density Stee

The rapid development of the automotive industry has led to fuel consumption and environmental pollution.Lightweighting can achieve energy saving and emission reduction.Using advanced high-strength steels as body materials is the most effective method to achieve lightweighting.Fe-Al-Mn-C low-density steel with low density and high strength has been the focus of research in the field of automotive steel.Currently,the research is concentrated on low-density steel sheets,and there is a lack of research on low density bar.45Mn5Al4 steel is a martensitic low-density steel,which has the potential to become the bar.In this Paper,the basic characteristics and the organizational properties of 45Mn5Al4steel after heat treatment and the effect of Nb and V microalloying are studied.The phase transformation of 45Mn5Al4 steel was investigated.The austenitic transformation temperature A_c1 and A_c3 are 725℃and 925℃.Only martlistenite transition occured from air cooling to room temperature after austenization.The room temperature organization at 0.01℃/s cooling rate was mainly martensite with a small amount of pearlite and ferrite.Nb and V microalloying had no effect on the phase transformation.The grain growth Pattern of 45Mn5Al4 steel was studied,and the mechanism of the effect of Nb microalloying on grain growth was revealed.The results showed that the grains of 45Mn5Al4 steel grew rapidly with the increase of heating temperature,and the grain coarsening temperature was 1000℃.The grain coarsening temperature was increased to1250℃after Nb microalloying.After adding Nb to the experimental steel,the precipitation of NbC precipitates in the tissue would peg the austenite grain boundaries,resulting in slow grain growth.At a heating temperature of 1250℃,a large amount of NbC precipitates dissolved leading to grain coarsening.The Arrhenius model of grain growth was developed.The thermoplastic behavior of 45Mn5Al4 steel was studied and the mechanism of the effect of Nb and V microalloying on the thermoplasticity was revealed.The results show that the experimental steel has good thermoplasticity and the plasticity increases gradually with the increase of deformation temperature.The Nb and V microalloying inhibited the recrystallization of the experimental steel and raised the recrystallization temperature.The experimental steel recrystallized sufficiently at 1000℃and above tensile.The original grains and recrystallized grains of experimental steel are finer after Nb and V microalloying,which has a greater hindering effect on crack ex Pansion.higher plasticity of Nb and V microalloyed steel.At low-temperature stretching below 1000℃,significant recrystallization still occurs in unmicroalloyed steel,and almost no longer occurs in Nb and V microalloyed.In addition,the large size of NbC generated after Nb microalloying also promotes the generation of microcracks.Therefore,the plasticity of Nb and V microalloyed steels is lower than that of unmicroalloyed steels at lower temperature deformation.The tissue properties of 45Mn5Al4 steel after heat treatment were investigated.The results show that the experimental steel has good strength and plasticity after low temperature tempering,and the organization is tempered martensite.After tempering at350℃,the tensile strength was 1796 MPa,the elongation was 12.48%,and the section shrinkage was 47.58%.The precipitates precipitated along the grain boundaries during tempering at 400℃and above lead to severe tempering brittleness of the experimental steel.The strength and plasticity of the experimental steel were poor after sub-temperature quenching and heat treatment.In addition,Nb and V microalloying during high-temperature tempering improves strength and hardness.However,the large size of NbC produced after Nb microalloying destroys the plasticity and toughness of the experimental steel.