The effect of Nb on microstructure and thermal-shock resistance was investigated for as cast and annealed JIS SCH21 steels. Primary NbC carbide were crystallized together with M₂₃C₆ at grain boundary in as cast steel. Decomposition of primary M₂₃C₆ was detected, while NbC was also hardly decomposed when annealed at elevated temperature. It was found that chromium carbides had low thermal stability but primary NbC carbides played a role in preventing propagation of micro-cracks and restraining shape deformation under condition both heating and quenching. As a result, thermal-shock resistance was improved in the Nb-added steel.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 474–478.