Effect of Nb content on abnormal grain growth in 444-type ferritic stainless steel during simulated brazing

The significant effect of Nb content on abnormal grain growth in 444-type ferritic stainless steel during simulated brazing (heat treated at 1100 °C and held for 15 min) was studied, which was used in automotive exhaust systems. By slightly increasing of Nb from 0.26 to 0.59 wt%, the starting temperature of abnormal grain growth will decrease from 1100 to 1050 °C, indicating a decrease in brazing performance. Finer ferrite grains can be obtained in the steel plate with high Nb content, but the tendency of retaining sub-grain boundaries will also strengthen. The deformation stored energy associated with high-density dislocations obtained in sub-grains should be the main reason for decrease of the starting temperature of abnormal grain growth. In addition, an increase in Nb content can also improve the dissolution rate of coarse (Nb,Mo)(C,N) particles within the grain, especially the dissolution of Nb-rich precipitates at grain boundaries, which can weaken the grain boundary pinning force and easily cause abnormal grain growth. It seems to be related to the raising of the precipitation temperature of Laves phase and (Nb,Mo)(C,N) precipitates induced by the increase in Nb content.