Effect of Mn addition on deformation behaviour of 23% Cr low nickel duplex stainless steel

Isothermal compression testing of 23% Cr low nickel duplex stainless steels, with different Mn concentrations, was investigated in the temperature range of 1173-1423 K, the strain rate range of 0.01-10 s−1 and with a height reduction of 70%. Increasing the Mn content from 6.3 to 14.1% affected the deformation dynamic softening behaviour of flow curves at lower temperature and higher strain rate. It was also observed that increasing the deformation stain rate contributed to flow softening, and was caused by dynamic recrystallization (DRX) as a function of Mn addition. Deformation microstructure analysis demonstrated that more complete austenite phase DRX occurred with 10.3% Mn addition than that with 6.3% Mn addition at 1 s−1, but an excessive Mn content of 14.1% was unfavourable due to the increase of austenite fraction and stacking fault energy enhancement. Increasing the strain rate to 10 s−1 promoted austenite phase DRX at 1323 K with the Mn addition under large strain. Meanwhile, the ferrite phase DRX occurred at 1173 K/1-10 s−1 with 6.3% Mn, and was weakened with higher Mn addition. Compared to the 6.3% and 14.1% Mn, continuous DRX occurred at 1323 K/1 s−1 with 10.3% Mn, due to the larger fraction of high angle misorientations. The values for the activation energy and Zener-Hollomon parameter decreased with Mn addition from 6.3 to 10.3%, and then increased with a higher Mn addition of 14.1%, indicating that a proper content of Mn addition can reduce the thermal deformation resistance and promote DRX. The constitutive equations for different Mn addition alloys were developed using a hyperbolic sine equation. Processing maps with different Mn addition for the strains of 0.3, 0.6 and 1.2 were established. The 10.3% Mn content sample corresponded to the lowest instability region, implying that the workability was improved with the middle Mn addition.