The austenite reversion and co-precipitation behavior of an ultra-low carbon medium manganese quenching-partitioning-tempering steel

The multiphase Microstructure evolution and mechanical properties of an ultra-low carbon medium manganese quenching-Partitioning-tempering(QPT) steel hive been investigated based on the nano scale austenite reversion correlated with the co-precipitation behavior. The blocky austenite (aspect ratio < 3) was found to nucleate in the partitioning process, and growth occurred in the tempering stage, but the film austenite (aspect ratio > 3) always formed during the tempering procedure in the alloy-enriched structures after partitioning. The adjacent co-precipitation of Ni-rich particles shells with Cif-rich precipitates was observed in quenching-tempering (QT) steels. The lower annealing temperature of QPT-1 steels resulted in a multiphase constitution of mostly film austenite nanoscale dispersed Ni-rich precipitates and in the obvious coarsening and anisotropic growth of Cu-rich precipitates With a high annealing temperature, QPT-2 steels showed a large percentage of blocky austenite, an entire core-shell co-precipitation structure of Ni-rich and Cu-rich precipitates in martensiie and elongated par tides in the blocky austenite. The introduction of nanoparticles in dual phase' has rarely been found in ultra-low carbon steels with a low alloying degree. The co-precipitation mechanisms are closely related to the cooperative austenite reversion process, which is governed by Mn diffusion and segregation: thiS leads to a different element enrichment degree in dual phase. In addition, the intergranular precipitates in QT-Steels result in stress concentration in the grain boundaries and a very low ductility, although with strong modulus and Orowan hardening effects. The difference of yield strength (67 MPa) in the two QPT steels mainly originate from the contribution of dispersion strengthening effect (44 MPa) under the consideration of the constituent phases. The lower mechanical stability of the blocky austenite in QPT-2 steels results in a lower uniform elongation and impact toughness. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.