Determination of solid solubility products of [Nb][C] in the case and the core of high-temperature carburizing steel by extraction phase analysis method

•The extraction phase analysis method was used to analyze the solid solubility products of [Nb][C] in the case and the core of high-temperature carburizing steel.•The 18CrNiMo7-6 steels with different carbon content of 0.20 wt% and 1.0 wt% were prepared to represent respectively the carburized case and the core.•The extraction phase analysis’s results showed the solid solution products of [Nb][C] in the steel with the carbon content of 1.0 wt% were much lower than those calculated from reported equation.•It was also found that the amounts of NbC precipitates in the steels did not decrease evidently when the austenitizing temperature was below 1323 K.