V, N Micro-alloyed Low Carbon Bainitic Steel Component Design And Organizational Characteristics

In this paper, the composition design and transformation characterstics during continuous cooling of vanadium and nitrogen microalloyed low carbon bainitic steels were researched. Seven experimental steels were prepared and CCT curves were determined on Formastor-F II automatic transformation measuring apparatus. For increasing comprehensive performance and refining microstructures of vanadium and nitrogen microalloyed low carbon bainitic steels, thermal simulation experiments were tested by Gleeble-1500. And microstructures and precipitates were observed by Microscope, SEM and TEM. Finally experimental steels were hot rolled and mechanical properties were tested. Results as follows:Similar to Nb-B steel, the steel containing V and B can get fully bainitic structures at the air cooling rate. Removing B from low carbon bainitic steel containing V leads to lower bainite hardenability. And adding Mo and Cr into these steels can improve bainite hardenability, lower critical cooling rate VC-95%B, decrease the bainite transformation temperature and increase hardness.But compared to Cr, Mo is the more effective element to promote bainite transformation.It is found from the results of thermal simulation that the size of laths is reduced and microstructures are refined with increasing deformation. The holding time after deformation will affect the size of microstructures and the size of lath is better refined when the holding time is 120s. The microstructures were thick in samples with different deformation temperature, but after holding 120s after deformation the size of bainite laths is reduced. In addition, these are V(C,N) precipitates in samples with holding, and the precipitates formed in grain boundries can inhibit grain growing and precipitates formed in the grain can improve strength and promote the nucleation of bainite, refine microstructures.The size of bainite laths is smaller after cooling in low carbon steel contain vanadium and nitrogen, and yield strength is 90MPa higher than the steel without vanadium and nitrogen. Also the size of bainite laths is smaller in low carbon steel contain vanadium and nitrogen with relaxation 25s after hot rolled than the steel without relaxation, and comprehensive performance is superior to the steel without relaxation. There are some V(C,N) precipitates which can promote the formation of acicular ferrite in the steel with relaxation 25s. The effect of recystallization and acicular ferrite partitioning on austenite grain can refine microstructure and improve mechanical properties.