Variations in microstructure and mechanical properties along thickness direction in a heavy high strength low alloy steel plate

Abstract

A thermal-mechanical control process (TMCP) followed by tempering at 600 °C was applied to produce 420 MPa grade high strength low alloy (HSLA) steel heavy plates. The microstructure and mechanical properties through the thickness were analyzed using various techniques including optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD), tensile and impact tests. Microscopy analyses showed that a fine-grained structure with an average grain size of 6–10 μm could be obtained by TMCP. After subsequent tempering, the microstructure near the surface of the plate was dominated by bainite, while the microstructure of the core was mainly characterized by aggregated carbides and ferrite. In addition, the density of high-angle grain boundaries (HAGB) on the surface was significantly higher than that in the core. Tensile tests showed a decrease in both yield and tensile strengths from 530.1 and 626.9 MPa on the surface to 384.3 and 496.7 MPa in the core, respectively. The contribution of different strengthening mechanisms to the variations in strengths along the thickness was clarified. In addition, the deformation of ferrite and the initiation of micro-voids during tensile tests were also analyzed.