STUDY OF STRENGTHENING MECHANISMS IN V-N STEELS

The transformation and strengthening behavior of a series of V -N steels during Hot Strip Mill (HSM) simulation was investigated. Of particular interest in this study was the examination of the y to a transformation and the precipitation that takes place during cooling from the finishing rolling temperature to the temperatures reached at the end of the run -out-table (ROT) and during co iling. To clearly distinguish, quan tify, and separate the transformation and precipitation behavior that takes place on the ROT and during coiling, two distinct thermal paths were used; continuous and discontinuous. The continuous cooling thermal path is characterized by the complete proces s, i. e., hot rolling followed by ROT cooling followed by cooling in the coil to room temperature (RT) at 30 degrees C/h. In the discontinuous path, the samples were water quenched (WQ) to RT after reaching the coiling temperature. The transformation and the precip itation behavior were evaluated with standard electron optic techniques, Orientation Imaging Microscopy (OIM) and Image Quality (IQ) analysis. In addition, transmission electron microscopy (TEM)/scanning TEM (STEM) and atom probe tomography (APT) were used to evaluate the precipitation behavior. The results of this study clearly revealed that a) the kinetics and transformation temperature of the y to a transformation behavior are strongly influenced by the N content of the steel; b) the majority of the y to a transformation takes place on the ROT, while the rest takes place during coiling; c) the volume fraction and the nature of the ferrit e (polygonal versus non -polygonal) depends on the steel composition and the coiling temperature; d) the type and extent of precipitation behavior was related to the coiling temperature and the nature of the ferrite formed prior to coiling. The results of this study indicated that the major strengthening mechanisms observed and quantified were related primarily to solid solution strengthening, the average polygonal ferrite grain size, the dislocation density associated with the nature of the non -polygonal fe rrite, and to a lesser extent precipitation.