Effect of Quenching Temperature on Mechanical Properties of M6C Reinforced 2 200 MPa Ultra-high Strength Steel

The mechanical properties of 40CrNi2Si2MoV martensitic steel are affected by the large size carbides，the strength and toughness of martensitic steel can be effectively improved by optimizing the composition design and reasonable heat treatment process. The effects of M6C carbide on mechanical properties and microstructure of ultrahigh strength steel were studied at different quenching temperatures （860 ℃-1150 ℃）， and the evolution of M6C particle size， quantity and composition was studied by means of SEM and TEM. The results show that large M6C particles reduce the toughness of ultrahigh strength steel， and the M6C particles in the 880 ℃ quenching sample are large （~400 nm）， which is easy to cause stress concentration around the large M6C carbide ， weaken the binding force between the matrix and M6C carbide， and easy to crack around M6C carbide， resulting in material fracture.With the increase of quenching temperature， the size and quantity of M6C particles in the test steel decrease， and the concentration of Si in M6C particles is increased， and the atoms in M（Fe， Mo， Ni， Co）6C are replaced by Si， which improves the stability of M6C carbide. The size of M6C particle in the 1050 ℃ quenching temperature test steel is 25 nm， and the growth of the original austenite grain （8.5 μm） is inhibited. The strength of the test steel is 2227 MPa and the elongation is 7.0%， achieving a good strength and toughness matching.