Effect Of Aging Treatment On Phosphorus Grian Boundary Segregation In Medium Manganese TRIP Steel

In recent years,medium-manganese TRIP steel has become the most promising automotive steel with excellent mechanical properties and low cost.Therefore,further improving the comprehensive mechanical properties of medium manganese TRIP steel has become the main research direction of scholars.In order to improve the strength of medium-manganese TRIP steel,phosphorus element is often added to the steel for solid solution strengthening.However,phosphorus not only acts as a solid solution strengthening,but also tends to segregate at the grain boundary,thereby increasing the brittleness of the medium-manganese TRIP steel.Therefore,it is necessary to study the segregation behavior of phosphorus in medium-manganese TRIP steel.In this paper,the hot-rolled 0.2C-5Mn TRIP steels with different niobium content were used as experimental materials,The medium-manganese TRIP steel were annealed at 650?for 2h and then constant temperature aging for 20⁷0h at 550?.The segregation and precipitation behavior of phosphorus in medium-manganese TRIP steel were investigated using field emission scanning electron microscopy?FE-SEM?and electron backscatter diffraction?EBSD?.The experimental results show that phosphorus non-equilibrium segregation in medium manganese TRIP steel occurred after aging treatment.During aging,the boundary segregation content of phosphorus increases evidently with increasing aging time until reaching a maximum value in approximately 50h and then decreases gradually with further increasing aging time.The critical time of non-equilibrium segregation is about 50h,which is rather close to the theoretical calculation value of 49h.There is a certain overlap between the phosphorus segregation zone and the carbon segregation zone,that is where the amount of phosphorus segregation is high,the carbon content is relatively high.But conversely,where carbon content is high,phosphorus segregation is not necessarily exist.This experimental phenomenon of carbon and phosphorus is more conform with the theory of co-segregation.In addition,compared with the maximum phosphorus segregation in the segregation zone of steel A and steel B,it is found that Nb microalloying has the characteristics of reducing phosphorus segregation in steel,which can reduce the maximum phosphorus segregation content by 6.57%¹9.5%.According to the analysis results of the EBSD Kikuchi pattern,when the phosphorus content of phosphorus segregation zone is 1.15at.%,the precipitate phase is the matrix phase??-BCC phase?,and phosphorus is in a segregation state.When the phosphorus content in the segregation zone is in the range of 2.28⁶.51at.%,phosphorus is in a precipitation state,the precipitates phase is Fe₄P,FeP and FeP₄,and the number of phosphorus atom increases with increasing phosphorus segregation content.