Strengthening and toughening mechanism of a Cu-bearing high-strength low-alloy steel with refined tempered martensite/bainite (M/B) matrix and minor inter-critical ferrite

The microstructure–mechanical property relationship of a Cu-bearing low-carbon high-strength low-alloy steel, subjected to a novel multistage heat treatment including quenching (Q), lamellarization (L) and tempering (T), is presented. Yield strength of 989.5 MPa and average toughness at − 80 °C of 41 J were obtained in this steel after quenching and tempering (QT) heat treatments. Specimen QLT gained a little lower yield strength (982.5 MPa), but greatly enhanced average toughness at − 80 °C (137 J). To further clarify the strengthening and toughening mechanisms in specimen QLT, parameters of microstructural characteristic and crack propagation process were compared and analyzed for specimens Q, QL, QT and QLT. The microstructure of tempered martensite/bainite (M/B) in specimen QT changed to refined tempered M/B matrix mixed with minor IF (inter-critical ferrite) in specimen QLT. Cu-rich precipitates existed in tempered M/B for both specimens QT and QLT, as well as in IF. Compared with QT, adding a lamellarization step before tempering made the effective grains of specimen QLT refined and also led to coarser Cu-rich precipitates in tempered M/B matrix. The weaker strengthening effect of coarser Cu-rich precipitates should be a key reason for the slightly lower yield strength in specimen QLT than in specimen QT. No austenite was found in all specimens Q, QL, QT and QLT. Specimen QLT showed purely ductile fracture mode at − 80 °C due to the refined effective grains. The greatly improved toughness is mainly attributed to the enhanced energy of crack propagation. The combination of refined microstructure, softened matrix and deformation of minor ‘soft’ IF during crack propagation led to the most superior toughness of specimen QLT among all specimens.