Evolution of NbN and Z-phase transformation during thermo-mechanical process of Nb-containing AISI 301LN stainless steel

Abstract

The precipitation behavior of Nb-containing precipitates was systematically investigated in Nb-added AISI 301LN stainless steel during hot rolling, intermediate annealing, cold rolling, and final annealing treatment. During hot rolling, Z-phase (NbCrN) and NbN precipitates were preferentially formed along the rolling direction. The formation of these precipitates was promoted by increasing Nb content, while they largely remained undissolved during intermediate annealing treatment owing to their high thermal stability. Cold rolling fragmented coarse precipitates and introduced a high density of defects, thereby increasing the number of favorable nucleation sites for subsequent precipitation. After final annealing treatment, numerous nanoscale precipitates were observed, identified as Z-phase and NbN by chemical and crystallographic indexing. These precipitates exhibited various morphologies including rectangular and spherical shapes, influenced by previous thermo-mechanical process. Quantitative analysis revealed a modest change in the weight fraction of Nb-containing precipitates between hot rolling and cold rolling, followed by a substantial increase after final annealing. Z-phase was consistently identified as the dominant precipitate throughout all processing steps, and its formation appeared to proceed via transformation from pre-existing NbN.