1987 Volume 73 Issue 7 Pages 884-891
The effects of C, N and Nb contents and processing variables on the chemical compositions of Nb compounds formed in controlled rolled HSLA steel plates were systematically investigated. Identification of Nb compounds and determination of X and Y values in NbCxNy were performed by chemical analysis and X-ray diffraction on the precipitates extracted by electro-chemical method. Nb compound formed in commercial HSLA steels was δ phase of NbCx or NbCxNy. NbCx with X value of 0.84 to 0.86 was formed in the steels with the very low N content or with the addition of small amount of Ti. X + Y value in NbCxNy was almost constant in around 0.83 to 0.86, while X or Y value was widely varied depending on N content in steel.
The δ' phase of NbN was formed exceptionally in the high N-Nb steel containing an extreamly low carbon content less than 0.003%. The reduction of slab reheating temperature in controlled rolling or post heat treatment increased the amount of AlN precipitates, resulting in the decrease of N content in NbCxNy. The map describing formation of various Nb compounds and their chemical compositions was drawn based on C/Nb and N*/Nb mole ratio in steel, where N* was free nitrogen not combined with Al or Ti.