Effects of Cooling Rate on the Microstructure and Properties of Coarse‐Grained Zone in Complex‐Phase Steel 780HE

A thermomechanical simulator Gleeble‐3800 is used to simulate the thermal cycles experienced by coarse‐grained heat‐affected zonecoarse‐grained heat‐affected zone (CGHAZ) of 780HE steel under different cooling rates during welding. The effects of cooling rate on the microstructure, precipitates, and mechanical properties of the CGHAZ are systematically investigated. Results indicate that the cooling rate did not significantly alter the types of microstructure, which consistently comprise of lath bainite and granular bainite. When the cooling time from 800 to 500 °C ( t 8/5 ) is 15 s or 20 s, the precipitates are predominantly TiN. As t 8/5 increases to 30 s, (Ti, Nb)C and (Ti, Nb, Mo)C reprecipitate. With decreasing cooling rate, the yield strength and tensile strength initially decrease slightly and then increase, while the uniform elongation exhibits the opposite trend. The impact energy decreases significantly as the cooling rate decreased.