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 degrees C for I 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. (C) 2009 Published by Elsevier B.V.