Microstructure And Mechanical Properties Of The Duplex Stainless Steel 00Cr25Ni7Mo4N After Cold Rolling And Subsequent Annealing

A duplex stainless steel(00Cr25Ni7Mo4N) with approximately equal fractions of ferrite and austenite phases has been cold rolled up to 90%, followed by annealing at temperatures up to 1000°C. The specimens of the as-received, defoemed and after different annealing treatment were measured by microhardness and tensile tests. Optical microscopy, electron back scattered diffraction and transmission electron microscope were used to characterize the microstructure after deformation and subsquence annealing. The main findings are summarized as follows:(1) After cold rolling to 90% thickness reduction a nanoscale laminated ferrite and austenite structure was formed where both the ferrite and austenite phases were further subdivided by lamellar boundaries. The average lamellar boundary spacings measured along the normal direction(ND) are 103 nm in the α phase and 83 nm in the γ phase. The tensile strength is 1.4 GPa cold-rolled sample, which is about twice of the strength for the as-received material(704 MPa), but the tensile elongation is only 4%.(2) The microstructure of the austenite remains in deformed structure and recovered structure was investigated in the ferrite phase after annealing at 800~850℃ for 1min. After annealing at 900~1000℃ for 1min, recrystallization occurred in austenite phase and the deformed structure transferred into ultrafined grains with annealing twins. The subgrains of ferrite grew, forming bamboo-type microstructure surrounding by ultrafined ferrite subgrains.(3) The mechanical properties of the samples were optimized through deformation following subsequence annealing. An excellent combination of high strength(1.0 GPa) and good tensile elongation(32%) is achieved in the deformed sample after annealing at 900℃ for 1 min.(4) Back stress hardening enhanced the strength of the heterogeneous lamella structure. After annealing at 900℃ for 1min,a heterogeneous lamella structure with the hard austenite layers embedded in the soft ferrite layer matrix were obtained. The heterogeneous lamella structure, increasing phase boundary density and heterogeneous microstructure in ferrite layers were more effective for pile-up and accumulation of geometrically necessary dislocations,leading to a high back stress than the as-received materials.(5) The strain hardening behavior of the duplex stainless steel was mathematical analysed based on modified Crussard–Jaoul equation. There was just one stage of strain hardening for deformed microstructure, which may be resulted from the formation of comparable strength between austenite and ferrite after cold rolling. After annealing at 900℃ for 1min,three stages recovered, exhibiting similar strain hardening behavior to the as-received samples, which is associated with mechanisms of ferrite and austenite, i.e., the onset of yielding in austenite phase; deformation twinning in austenite, respectively. The deformation twinning in the third stages improved the uniform elongation more effectively.