Synergistic effects of nanoscale precipitations in dual phases and retained austenite by Mn partition on mechanical properties in Mn-TRIP steelsTuesday (27.09.2016) 15:45 - 16:00 Part of:
Two-step “Quench-Partition-Tempering (QPT)” heat treatment procedure has been applied in low carbon Mn-TRIP steels with designed NiAl and Cu-rich precipitates for good combined strength, plasticity and especially low temperature toughness. The designing mechanisms are based on synergistic effects of nanoscale precipitations in dual phases and enough volume of retained austenite by Mn partition. The combination of the two mechanisms is controlled by competition between the amount of retained austenite based on Mn diffusion and nanoprecipitations promotion by Mn segregation at dislocation cores during heat treatment. The approach leads to effective improvement in both strength and elongation.Using transmission electron microscopy (TEM) correlated 3D atom probe tomography (APT), enough retained austenite obtained in one-step “Quench-Partition (QP)” treatment contributed to a good total elongation with low strength without precipitation strengthening. The Mn partition process included not only the Mn diffused from bcc structure to fcc structure for austenite stabilization, but also the enriched element Mn segregated at crystal defects such as dislocation cores in bcc structures. In consideration of precipitate formation conditions, the two-step “QPT” treatment with added extra low temperature tempering was proved beneficial for comprehensive performance with improved strength as well as plasticity, which exhibit a product of tensile strength and ductility of 25000 MPa.%. Interestingly, NiAl and Cu-rich nanoprecpitates during “QPT” two-step aging were observed to nucleate in martensite and austenite respectively. The following low temperature tempering process aimed for sufficient nanoscale phases in respect of precipitation kinetics. The investigation also indicated that the two precipitation hardening effects that occurred simultaneously in Cu-Added steels were more effective compared with Cu-Free steels.Especially, tensile tests in -196? were carried out to research the influence of nanoprecipitates and retained austenite on low temperature toughness. The yield stress and ultimate tensile strength increased with no elongation loss. The obvious strain hardening effects was in relation with the high stability of retained austenite of low carbon, which also offered transformation induced plasticity in low temperature.