Abstract: To solve the problem that the strength and toughness of Cu-bearing medium carbon low alloy steel are difficult to coordinate during high temperature tempering， a novel multi-step heat treatment scheme was designed. The effects of the heat treatment on the microstructure and mechanical properties of the experimental steel were investigated by mechanical properties test， microstructure characterization and thermodynamic calculation. The results show that after quenching-subcritical annealing-inter-critical tempering-tempering， the mixed microstructure with tempered martensite and inter-critical ferrite is obtained. The martensite lath becomes narrower， the microstructure is significantly refined， high angle grain boundaries increase， the lamellar cementite distributed at the prior austenite grain boundaries and lath boundaries is spheroidized， partial Cu-rich phases in the lath are coarsened. Excellent comprehensive mechanical properties can be obtained by the multi-step process， with yield strength of 901MPa， tensile strength of 1003MPa， elongation of 22% and impact energy of 35J. Compared to that after traditional quenching-tempering process， the strength of experimental steel is reduced， but the impact toughness and elongation are increased by around 3 times and 50% respectively.

Key words: Cu-bearing medium carbon steel; multi-step heat treatment; precipitation; thermodynamic calculation; mechanical properties