Precipitates in two Zr-bearing HSLA steel plates

A comparative study was made of the effect of Zr/N ratio on precipitates and resultant mechanical properties in two Zr-bearing HSLA steel plates by means of transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS). It is shown that the strength of steel plate with hypo-Zr/N ratio is slightly lower than that of hyper-Zr/N ratio steel plate, whereas the impact toughness of steel plate is greatly improved by reducing the Zr/N ratio below stoichiometric ratio. The precipitates in both Zr-containing steel plates are identified to be mainly of nano-sized cuboidal ZrN nitrides, being approximately 25 and 100 nm in hypo- and hyper-Zr/N ratio steel plates, respectively. Prolonged heat treatment at high temperature of 1350 °C for 1 min indicates that the ZrN nanoparticles have a strong resistance to coarsening. Also, the precipitation kinetics of ZrN nanoparticles in both steel plates are calculated and comparatively studied on the basis of Dutta's model.