Microstructure and texture evolution of duplex stainless steels with different molybdenum contents

Abstract

The microstructure and texture evolution of duplex stainless steels with different molybdenum contents were studied. For this purpose, hot-rolled 2205 and 2304 samples were solution-annealed at 1100 °C for 1800 s and then cold-rolled to a 75% thickness reduction. This was followed by an isothermal annealing at 900 and 1100 °C for 180 s. The EBSD technique was used to determine the phase ratio, crystallographic texture, and microstructural characteristics. The hot-rolled steel had a work hardened microstructure and the samples submitted to intermediate annealing presented mainly deformation-free grains. Cold rolling resulted in a substantial reduction in the phase spacing with the formation of the strain-induced α′-martensite (SIM) from the metastable austenite. After annealing at 900 °C, the ferrite grains retained the elongated shape of the cold-rolled condition and the primary recrystallization progressed in the austenite phase. Increasing the Mo content increased the resistance to strain-induced martensitic transformation and inhibited the grain growth of austenite phase during annealing at 900 °C. Increasing annealing temperature to 1100 °C promoted the coarsening of both bcc and fcc structures with the formation of annealing twins in the austenitic grains. The recrystallization kinetics and subsequent grain growth occur earlier in ferrite than in austenite phase. Differences in Mo content did not significantly alter the evolution of austenite texture, whereas in the ferrite phase the increase in Mo content suppressed the γ-fiber development. Oriented nucleation was the predominant mechanism observed during the recrystallization of ferritic grains in the 2205 samples. The development of ∑19a boundaries was associated with the selected growth of {554}〈225〉α grains in the annealed 2304 samples. Annealing twins were associated with the formation of Σ3 boundaries after the heat treatments. Grain coarsening resulted in a larger fraction of the special boundaries in ferrite phase but inhibited the CSL formation in the austenite phase.