The effect of chemical composition and annealing condition on the microstructure and tensile properties of a resource-saving duplex stainless steel

Abstract

Edge cracking phenomenon occurs in ‘Mn + N′ duplex stainless steels because of inferior thermoplasticity that is mainly induced by the non-uniform deformation between ferrite and austenite phase with different strength during hot rolling, limiting its application. To address this problem, a Fe–19Cr-0.6Al–1Ni–12Mn duplex stainless steel with an optimized alloy composition was trial-prepared, where Ni is substituted by Mn and Cr is partially substituted by Al. This enabled a high content ferrite to be present during hot rolling to reduce the adverse effect of austenite on thermoplasticity. Thus, the edge cracking of hot rolled plates was solved. Furthermore, the formation of austenite was governed by annealing after cold rolling. In view of this, it is significant to study phase transformation and precipitation in duplex stainless steel during annealing. A large number of experimental results indicated that there were two types of transformation during the annealing process: the transformation of ferrite to austenite and recovery and recrystallization of ferrite. After annealing at lower temperature, the austenite was characterized by the banded structure at the grain boundaries of ferrite. Besides, the formation of austenite restricted recrystallization of ferrite. With the increase of annealing temperature, the growth of austenite grain would be enhanced, and the banded austenite evolved into bamboo morphology or island morphology. When the annealing temperature was greater than 900 °C, recrystallization of ferrite was promoted, and transformation of ferrite to austenite was inhibited. However, the precipitation of σ phase increased with the decrease of annealing temperature below 800 °C, reducing plasticity and pitting resistance for the investigated steel.