Study On The Heat Treatment, Microstructure And Mechanical Properties Of New High Strength Steels

High strength steel have been widely used in the impacting environment with high strain rates. In this paper, three new high strength steels have been invented by optimizing alloy compositions, namely Z steel, new-1# steel and new-2# steel. Reasonable heat treatment processes were applied, OM, SEM and TEM were used to observe and characterize the microstructures of the three new steels. The quasi-static mechanical properties of the steels were tested by quasi-static tensile, quasi-static compression and impact test. To study the mechanical properties of the three steels at high strain rates, Split Hopkinson Pressure Bar(SHPB) and Taylor bar were employed, while Taylor bar impacting test was numerically simulated by LS-DYNA software. The surfaces of high strength steel were treated by Laser shock processes. The results are as follows.Cryogenic heat treatment procedure and isothermal heat treatment procedure were designed based on the optimized quenching and tempering heat treatment which have been studied before. It can be found that Z steel obtains the best combination of strength and toughness through cryogenic heat treatment, the yield strength of Z steel can reach1446 MPa, while the tensile strength reaches 1800 MPa, the impact toughness is 61J·cm-2.Isothermal heat treatments decrease the strength of Z steel, but increase the toughness. After treated by optimized heat treatment, the yield strengthes of new-1#steel and new-2#steel can reach 1590 MPa and 1577 MPa, the tensile strengthes are 1962 MPa and 1963 MPa, and the impact toughness are 73J·cm-2and 82J·cm-2 respectively. All the steels suffer ductile fracture. The microstructures of the three new steels which exhibit the best mechanical properties are composed of martensite, lower bainite, precipitation carbide and residual austenite.All three new steels show strain rate hardening effect under SHPB dynamic compression test, and new-1# steel is the most sensitive to the changes of strain rates. When impacted under the same strain rate(3200s-1), the dynamic yield stress of new-1# steel is2300 MPa, which is the highest, and the dynamic yield stress of Z steel is 1600 MPa, which is the lowest. Z steel exhibit the best resistance to the nucleation of adiabatic shear bands.In Taylor bar impacting test, each steel suffers the deformation process from slight upset, mushroom-shaped upset, crack to head fracture, where cracks nucleated from ASBs.Z steel show the best resistant to impacting. It can be found from the results of numericalsimulation that with the increasement of impacting velocity, the changing rates of velocity,acceleration and kinetic energy improve greatly.After treated by Laser shock process, there exists a plastic deformation layer on the surface of the high strength steel, which improves the strength of the steel, however decreases the ductility.