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

• Published in: Acta materialia 2018-03, Vol.146, p.126-141
• Main Authors: Li, Yu, Li, Wei, Liu, Wenqing, Wang, Xiaodong, Hua, Xueming, Liu, Huibin, Jin, Xuejun
• Format: Article
• Language: English
• Subjects: Austenite reversion; Co-precipitation hardening; Mechanical stability; Microstructural morphology; Quenching-partitioning-tempering
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2017.12.035

The multiphase microstructure evolution and mechanical properties of an ultra-low carbon medium manganese quenching-partitioning-tempering (QPT) steel have been investigated based on the nanoscale austenite reversion correlated with the co-precipitation behavior. The blocky 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 Cu-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 martensite and elongated Cu-rich particles 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. [Display omitted]