Influence of heat treatment on microstructure and corrosion resistance of a novel ultra-low carbon Cu–P weathering steel

The microstructure not only governs the mechanical properties of weathering steel (WS) but also influences its corrosion resistance. Microstructures optimization in WS is a viable strategy for developing advanced WS. In this study, a novel Cu–P WS was subjected to heat treatment at various cooling rates to obtain three distinct microstructures. The mechanical properties were evaluated using uniaxial tensile test. The corrosion resistance of these microstructures under simulated industrial atmospheric conditions was assessed by cyclic immersion corrosion tests. The matrix and rust layer were characterized using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), optical microscope (OM), white-light interferometry, Electrochemical impedance spectroscopy (EIS), scanning Kelvin probe force microscope (SKPFM), and micro-Raman. The influence of heat treatment on microstructure and corrosion resistance of this novel Cu–P WS was systematically investigated. The results show that as the cooling rate increased, the microstructure of this Cu–P WS transformed from polygonal ferrite (PF) to granular bainite (GB) and then to lath bainite (LB). Both the yield strength (YS) and ultimate tensile strength (UTS) increased, whereas the uniform elongation (UE) and total elongation (TE) decreased with an increasing cooling rate. There were notable differences in the corrosion kinetics between PF and GB/LB microstructures, whereas the corrosion kinetics of GB and LB microstructures were similar. The microstructure directly influenced the initial electrochemical process at the bare steel surface and the early rust layer morphology. However, as the rust layer thickened, the influence by microstructure on the rust layer evolution decreased. Instead, the proportion of α-FeOOH in rust layer became the dominant factor controlling the subsequent corrosion behavior. From the perspective of corrosion resistance, the novel Cu–P WS exhibits high tolerance to variations in cooling rates, which provides a reference for the thermo-mechanically controlled process (TMCP) design of the ultra-low carbon WS.