Effects of Heat Treatment Processes on the Microstructure and Properties of 07MnNiMoDR Steel

This study investigates three different heat treatment processes applied to industrially produced 07MnNiMoDR steel: conventional quenching at 900°C followed by tempering at 610°C (QT), intercritical quenching at 880°C followed by tempering at 610°C (IT), and quenching at 900°C followed by intercritical quenching at 880°C and then tempering at 610°C (QIT). The relationship between the microstructure and mechanical properties of the experimental steels under these different heat treatment processes was analyzed using scanning electron microscopy, electron backscatter diffraction, tensile testing, and Charpy impact testing. The results indicate that the microstructure of the QT steel is primarily bainite. The IT steel mainly consists of bainite and relatively coarse‐grained ferrite. In contrast, the QIT steel exhibits a microstructure of bainite and fine, equiaxed ferrite. The effective grain sizes of the three experimental steels were 4.26 μm, 4.71 μm, and 3.80 μm, respectively, and their tensile strengths were 654.4 MPa, 615.9 MPa, and 646 MPa, respectively. The QIT steel showed a high fraction of Σ3 coincidence site lattice boundaries, reaching 28%, and its impact absorbed energy at −85°C reached 168 J. The low‐temperature toughness of the QIT steel is improved compared to the conventional heat treatment process, with a reduction in the ductile‐brittle transition temperature. Grain refinement is identified as the key factor for improving the low‐temperature toughness.