High Temperature Deformation And Interphase Precipitation Behavior In Ti-Mo-xNb Low Carbon Microalloyed Steel

High strength steel(HSS)is a key lightweight material,due to its significance to national economy,especially for aerospace and transportation.Traditional HSSs strengthened by microalloying elements have some problems,such as the limited fraction,coarse size and unevenly distribution of precipitates,which not only reduces the ductility of the material,but also affects the service safety.Meanwhile,the high cost limits its practical application,which becomes an issue for the development of high-end iron and steel industry.In view of low cost and high performance steels,two kinds of ultra-low carbon Ti-Mo-xNb microalloyed steels are designed.An excellent combination of strength,ductility/toughness and formability is expected by introducing high density of interphase nano-precipitates.The high-temperature compression and isothermal tests are performed to study the hot deformation,ferritic transformation and interphase precipitation behaviors of Ti-Mo-xNb steels.The main contents and conclusions are as follows:(1)A peak stress model of Ti-Mo-xNb steel reveals that there existed an interaction between Nb and deformation rate,i.e.the role of Nb becomes more obvious at high strain rate.(2)Combining Bergstrom and KJMA models,the physically-based models of two-stage flow stress of Ti-Mo-xNb steel were first constructed,indicating an average relative error of less than 5%.The effects of Nb and deformation conditions on physical parameters prove that Nb accelerates the hardening behavior of Ti-Mo steel and caused a high density of peak dislocation,confirming more significant microalloying effect of Nb under high Z parameters.(3)With the prolongation of isothermal time,the volume fraction and grain size of ferrite increase rapidly at first and then tend to be flat gradually.Nb delays the transformation kinetics of ferrite,promotes the refinement of ferrite grain and improves the strength of matrix.(4)As the hodling time is 1200s,the ferrite hardness reaches its peak value,which corresponds a strong interphase nanoprecipitation behavior in Ti-Mo-xNb steel with an average size of about 2-3nm.and.With increasing the holding time,the amount of precipitates increases,but the size increased;Nb promoted the interphase precipitation of Ti-Mo steels.(5)Deformation greatly accelerates the kinetics of ferrite transformation,effectively refines the grain size of ferrite and shortens the starting time of interphase precipitation.