Kinetics of the Formation of Nanosize Niobium Carbonitride Precipitates in Low-Alloy Structural Steels

A detailed study is made using original methods of kinetic features of nanosize niobium carbonitride precipitate formation in relation to metal temperature and deformation regimes on the example of an industrial melt of low-alloy structural steel. It is established that in the absence of deformation there are considerable kinetic difficulties for niobium carbonitride precipitation, and for its occurrence to some significant extent a time interval of not less than 20–40 min at 900°C is necessary, even in the presence of considerable supersaturation and solid solution supercooling. With equivalent conditions, preliminary deformation considerably accelerates formation of nanosize niobium carbonitride precipitates that commences approximately after 10 sec and ceases after isothermal soaking at 900°C in 200–300 sec. A fundamental dependence is demonstrated for the amount and mechanism of nanosize niobium carbonitride precipitate formation on thermal prehistory. In addition, prior cooling to room temperature and subsequent heating to an exposure temperature of 700°C leads to a marked increase in number of nanosize carbonitride precipitates formed.