Physical Simulation-Based Characterization of HAZ Properties in Steels. Part 2. Dual-Phase Steels

The development of high-strength weldable steels has diversified the range of design alternatives subjected to more and more severe operation conditions. An excellent combination of high work-hardening rate and ductility improved the ultimate tensile strength of dual-phase (DP) steels and made them lucrative for application in automotive industry. The paper focuses on HAZ hardening and softening of three automotive DP steels, which affect their mechanical properties and their crack susceptibility, respectively. A Gleeble 3500 thermomechanical simulator was used to simulate the welding thermal cycles for all HAZ subzones that correspond to TIG welding of three DP steels. Steel samples were heated to different peak temperatures (650, 775, 950, and 1350°C) and cooled at two cooling times of 5 and 30 s. The Rykalin 2D model was used for the numerical simulation of this process. The hardness and microstructure of the specimens were then tested and analyzed using Vicker’s hardness test and optical microscope, respectively. In the investigated cooling time range, the HAZ of the examined cold-rolled DP steels was more susceptible to softening than to hardening. The softening occurred in almost all HAZ sub-regions but was the most pronounced in the fine-grained and intercritical HAZ, where we recorded the highest decrease in hardness for all DP steels. With an increase in heat input and longer cooling time, softening occurred in CGHAZ of all three DP steels under study, which can be attributed to the formation of upper bainite.