Simultaneous enhancement of strength and ductility in a medium carbon low-alloy steel induced by secondary martensite and Cu-rich particles

This study aims to simultaneously improve the strength and ductility of a medium carbon low-alloy steel via Cu addition and the partitioning treatment. The obtained microstructure was heterogeneous that included temper martensite, retained austenite, M-A island/secondary martensite, bainitic ferrite, and nanosized Cu-rich particles. The partitioning temperature had significantly influence on the microstructure, leading to the distinct variations of mechanical properties. With the increase of partitioning temperature from 240 °C to 330 °C, yield strength decreased from 1470 MPa to 1110 MPa. However, it increased to 1210 MPa when the partitioning temperature further increased to 380 °C. In contrast, the ultimate tensile strength gradually decreased from 2300 MPa to 1440 MPa with the increase of partitioning temperature from 240 °C to 380 °C, while the ductility significantly increased from 9% to 26%. The concurrent increases of yield strength and ductility is attributed to the numerous nanosized Cu-rich particles formed in the temper martensite. In contrast, the increase of ultimate tensile strength was caused by the increasing secondary martensite which had high carbon concentration. By using the band contrast values obtained by EBSD mapping and Gaussian fitting, the distribution of temper martensite, secondary martensite, and bainitic ferrite were quantified. The results showed that the distribution of temper martensite and bainitic ferrite were correlated, which was apt to improve the coordinated deformation between the neighboring phases. In addition, the Cu-rich particles were found to preferentially nucleate at the edge of the strip carbide because the carbide edge acted as a channel for the rapid diffusion of Cu atoms. Surprisingly, the formation of numerous nanosized Cu-rich particles was always accompanied with the increasing ductility without the loss of strength, which provides a great potential for breaking through the trade-off between strength and ductility in steels.