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.

提出了一种含铜的低碳高强度低合金铜钢，经过淬火（Q），层化（L）和回火（T）等新型多阶段热处理后，其微观结构与力学性能之间的关系。经过淬火和回火（QT）热处理后，该钢获得了989.5 MPa的屈服强度和-80°C的平均韧度41J。试样QLT的屈服强度（982.5 MPa）稍低，但在− 80°C（137 J）时的平均韧性大大提高。为了进一步阐明QLT试样的强化和增韧机理，对Q，QL，QT和QLT试样的显微组织特征和裂纹扩展过程参数进行了比较和分析。QT试样中回火马氏体/贝氏体（M / B）的微观结构变为混有QLT试样中的少量IF（临界铁素体）的细化回火M / B基体。QT和QLT以及IF中，回火M / B中均存在富铜沉淀。与QT相比，在回火之前增加层状化步骤可以使QLT试样的有效晶粒细化，并且在回火后的M / B基体中会导致粗大的富Cu沉淀。较粗的富铜沉淀物的增强作用较弱，这可能是导致试样QLT的屈服强度比试样QT的屈服强度稍低的主要原因。在所有Q，QL，QT和QLT标本中均未发现奥氏体。由于精炼的有效晶粒，试样QLT在− 80°C时表现出纯延性断裂模式。韧性的大大提高主要归因于裂纹扩展能量的增加。细化的组织，软化的基体和裂纹扩展过程中轻微的“软” IF变形共同导致了所有样品中QLT的韧性最高。