Effect of Titanium on the Precipitation Behavior of Inclusions in Niobium Microalloyed Steel

Experiments were conducted in a high frequency vacuum induction furnace to smelt titanium and niobium microalloyed steel with different titanium addition at 1 550 ℃ under Ar atmosphere,and then the samples underwent heat treatment.The effect of titanium additive amount on composition,microstructure,typical inclusions and macro mechanical property of the titanium and niobium microalloyed steel were analyzed.The results show oxygen content in the steel microalloyed with titanium and niobium decreases to about 20×10-6 after Al is employed to deoxidize and the utilization rate of titanium exceeds 80%.Inclusions in the steel were mainly sphere shaped or approximate to sphere shaped Al₂O₃,SiO₂,and TiO_x based complex inclusions.Cube shaped(Ti,Nb)(C,N) complex inclusions and NbC,TiC inclusions in the steel precipitate around the spherical oxide inclusions.A lot of fine(Ti,Nb)(C,N) precipitations after heat treatment can be seen with confocal laser scanning microscope.After heat treatment,with the increase of titanium content in the steel,inclusions with the size less than 1 μm increase significantly,while the inclusions with the size larger than 1 μm tend to decrease.Heated and held at 1 200 ℃,the austenite grain of the samples became smaller with the increase of titanium addition in the steel.Thus the microstructure was obviously refined in steel,which led to improvement of macro hardness of the steel.