Study on the Transformation Behavior of Q550D Steel at Continuous Cooling Conditions

Taking industrial 30 mm-thick Q550D steel as the research object, JMatPro software was used to simulate and calculate its key parameters, including thermodynamic phase diagram, TTT/CCT curves and mechanical properties. Meanwhile, static thermal simulation tests were conducted to simulate the heat treatment process under different cooling rates, and corresponding samples were prepared. Microstructure characterization and property tests were carried out using metallographic microscope, Vickers hardness tester and other equipment. Finally, the measured CCT curves were plotted and compared with the simulation results. The results show that the microstructure changes regularly with the cooling rate, and the final microstructure is lath martensite. The microhardness increases sharply at first and then tends to be stable with the increase of cooling rate. When the cooling rate exceeds 50℃/s, the microhardness stabilizes at 288-297HV, which is consistent with the variation law of the simulation results.