The impacts of M/A constituents decomposition and complex precipitation on mechanical properties of high-strength weathering steel subjected to tempering treatment
Liyang Zhao, Qiuming Wang, Qiuming Wang, Genhao Shi, Bing Hu, Shibiao Wang, Mingliang Qiao, Qingfeng Wang, Qingfeng Wang, Riping Liu
Abstract
The impact of microstructure evolution on the mechanical properties of a typical 500 MPa-grade weathering steel produced by an identical thermo-mechanical control process and different tempering treatments at 450–650 °C were thoroughly investigated mainly using scanning electron microscopy (SEM) equip with an electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and X-ray diffractometer (XRD). Results indicated that the as-rolled steel consists of granular bainitic ferrite (GBF) and a considerable amount of martensite/austenite (M/A) constituents, exhibiting unsatisfied mechanical properties. As the tempering temperature increased from 450 to 550 and 650 °C, the massive twin-type M/A constituents decomposed preferentially into carbides following the sequence of Fe3C→(Cr, Mn, Fe)3C→(Cr, Mn, Fe)3C + (Cr, Mn)23C6, with a small amount of fine lath-type M/A constituents remained. The matrix recovered with the bainitic ferrite laths merged, the dislocation density decreased and the proportion of high-angle grain boundary (HAGB) increased. Nanoscale (Ti, Nb)C and ε-Cu particles also precipitated simultaneously, leading to an increase of yield strength. The strain hardening capacity and hence the tensile strength reduced resulting from the decomposition of M/A constituents. Moreover, the impact toughness was first enhanced by tempering at 450–600 °C with the decreasing twin-type M/A constituents content and increasing HAGBs proportion, and then deteriorated by tempering at 650 °C due to the precipitation of necklace-like M23C6 carbides at the grain boundaries. An optimum mechanical property combination was achieved via tempering at 550–600 °C.