Effect of heat treatment on the microstructure and mechanical properties of 1000MPa grade structural steel for construction machinery

A low-carbon Mn-Mo-Nb-Cu-B ultra high strength structural steel for construction machinery was designed and the effect of TMCP (thermo-mechanical control process) + tempering (TMCP+T) technology and controlled rolling + direct quenching and tempering (CR+DQ+T) technology on its microstructure and mechanical properties under the same composition was studied. The changes in mechanical properties and microstructures of the steel plates before and after heat treatment were comparatively analyzed. The results show that after tempering from 500℃ to 620℃ for 1h, there is a substantial increase of the steel plates in yield strength. The performance of the steel plates treated by CR+DQ+T process is obviously superior to that by TMCP+T process, and the yield strength of the former is still more than 1000MPa after tempering at 600℃. Besides, the elongation can reach 18% and the impact energy at-40℃ is more than 30J. However, the plasticity of the latter is better but the strength slightly lower. With the tempering temperature increasing, the evolution speed of microstructures under the condition of CR+DQ+T is more quickly than that under the condition of TMCP+T.