Effect of Heat Treatment on Microstructure and Mechanical Properties of Low Carbon Microalloyed Steel of 1 000 MPa Grade

The effect of heat treatment on the microstructure and mechanical properties of two type steels is investigated by means of using thermo-mechanical controlled process with later off-line quenching and tempering(TMCP+QT) as well as controlled rolling with later direct quenching and tempering(CR+DQ+T) process.Two type steels are C-Mn-Cr-Mo-B steel and C-Mn-Mo-Nb-Cu-B steel respectively,for developing high strength steel of 1000MPa.The microstructure and mechanical properties is investigated by means of SEM and TEM.The results show that C-Mn-Cr-Mo-B steel has excellent combination of strength and low-temperature toughness after treated by TMCP+QT process with 450 ℃ to 550 ℃ tempering for 1 hour.The yield strength is above 1 000 MPa,elongation is above 15% and impact energy at-40 ℃ is more than 30 J.A large number of ε-Cu particles precipitate in C-Mn-Mo-Nb-Cu-B steel after treated by CR+DQ+T process with 450 ℃ tempering,which leads to yield strength increase.Yield strength of the steel further improves to 1 030 MPa because of carbide precipitation of niobium,vanadium and molybdenum after tempered at 500 ℃ to 600 ℃,The yield strength is still above 1 000 MPa,the elongation is above 20% and the impact energy at-40 ℃ is more than 35 J after treated by 620 ℃ tempering.