Study On The Characteristics Of Nb And Ti Precipitates Of The Weathering Steel

High strength and high toughness weathering steel can be very easily obtained bycombining micro-alloying technology and the controlled rolling and cooling process.Micro-alloying elements niobium and titanium play an important role in solution strength,grain refinement, and precipitation strength. Therefore, to master the function mechanismof the niobium and the titanium precipitates in the steel is very important. In order toobserve and characterize the constituent, morphology, size, distribution, species of theniobium and titanium precipitates, and finally analysis the precipitation difference causedby the niobium and titanium content, some instruments and testing method such as OM,SEM, TEM and XRD were used. The results show:Most niobium and titanium precipitates are complex niobium and titanium carbide orcarbonitride, while few of them exist as (Ti,Nb)N. The size of most carbides andcarbonitride is less than100nm, only a few are in the range of100-450nm.(Ti,Nb)N isvery large, in the range of1-3μm. Niobium and titanium carbide and carbonitride presentin the grain boundary are relatively large, in the range of100-450nm, while most of themin the dislocations and matrix are less than50nm. When the ratio of atomic concentrationof niobium and titanium below1, the niobium and titanium precipitates are square or closeto square,and when the ratio above1, whey are spherical or close to spherical. SEADshows the precipitates are face-centered cubic structure. XRD shows the main precipitatesin the steel are NbC0.83, TiC, NbN, NbN0.85, and a little TiN.In the niobium-bearing steel with the least amount of niobium(0.021%), the meansize of all precipitates is the smallest, the volume fraction of unit is the biggest, and theprecipitation strength is the biggest. In the niobium and titanium-bearing steel, the meansize of all precipitates is the largest, the precipitate density is the biggest.