The Research On Thermomechanical Processes Of A Mn12Ni2MoTi(Al) Steel

For decades, transformation induced plasticity (TRIP) assisted steels with high tensile strength, exceptional ductility and extraordinary strain hardening potential at room temperature have attracted a great deal of attentions, but their applications are often limited due to the low yield strength. The effects of different heat treatment processes on the microstructure and mechanical properties of a Mn12Ni2MoTi(Al) steel and the ultrafine grained duplex Mn12Ni2MoTi(Al) steel obtained by thermomechanical processes (65% cold rolling and annealing) are investigated by means of XRD, SEM, TEM, Vickers hardness and tensile tests. The main conclusions are as follows:(1) After aging at 480?, the Vickers hardness of as-quenched (1100?-lh) and cold-rolled (65%CR) samples reach the first peak quickly and the second peak slowly. Their Vickers hardness reach the second peaks at 28h (397HV) and 32h (480HV), respectively. After the hardness peaks, their Vickers hardness decrease slowly with the aging time prolonging. The yield strength and total elongation of as-quenched and aging sample (480?-28h) could be improved obviously by the formation of nanoparticles and reverted austenite, and the yield strength of peak aging sample is about double that of as-quenched sample (1100?-1h). After secondary aging at 540? for 32h and 48h in peak aged sample (480?-28h), the total elongation are enhanced to 21% with the increasing of austenite content (40vol.%), but the yield strength and ultimate tensile strength decrease. Although tensile strength could be largely enhanced by aging at 480? for 32h in cold-rolled sample, but low austenite content (5vol.%) results in poor total elongation (5.1%).(2) After annealing at 682-710? for 1h, the deformation microstructure of the cold-rolled sample has transformed into a sub-micron ultrafine grained (UFG) duplex microstructure consisting of austenite and ferrite. In addition, dispersed second-phase particles which are enrich in Ti, Mo, Si could also be formed during annealing at 682-850?, and the relative volume fraction of austenite in the UFG duplex steel at room temperature first increases and then decreases with increasing annealing temperature. Especially for the sample annealing at 710? for lh, not only the volume fraction of austenite increases to 63vol.% but also the average size of recrystallized grains and second-phase particles decrease to as small as 300nm and 120nm, respectively. Owing to the formations of ultrafine grains, second-phase particles and austenite, the yield strength and total elongation of the UFG duplex (710?-1h) sample could be reached 703.8MPa and 22.7%, respectively, which are significantly higher than those of the as-quenched martensitic sample (?0.2=457 MPa,?=12.6%).(3) When annealing at 710?, the formation of dispersed second-phase particles is very quick. With increasing annealing time from 0.25h to 24h, the second-phase particles become coarsening slightly. The particles play a critical role in preventing the ultrafine grains from coarsening, hence the average size of recrystallized grains in the 710?-24h sample could still be as small as 500nm. Even after long time annealing treatment, the samples annealing at 710? from 1h to 4h exhibit yield plateaus during the uniaxial tension, and the values of yield plateaus are between 800MPa and 900MPa. When annealing time increases to 24h, the 710?-24h sample shows continuous yield without a yield plateau and yield strength (?0.2) decreases to 595.2MPa.