Ultrastrong Low-Alloy Steel with Good Ductility via Multiple Strengthening, Suppressed Carbides and Reversed Austenite in Quenching and Tempering

Two low-alloy quenched-tempered steels (#1 steel and #2 steel) are designed and comparatively studied to achieve ultrahigh yield strength (above 2000 MPa) at relatively low material and process costs. #1 steel and #2 steel have chemical compositions (wt pct) of 0.62 C, 1.62 Si, 0.78 Mn, 1.05 Cr, and 0.18 V and 0.55 C, 1.74 Si, 0.67 Mn, 1.12 Cr, 0.14 V, 0.21 Ni, and 0.20 Mo, respectively. The two steels are heated to 950 °C for 30 minutes, oil quenched, tempered at 240 to 440 °C for 90 minutes and water cooled. Under the same heat treatment parameters, the two steels have basically the same strength (tensile strength 2100 to 2400 MPa; yield strength 1950 to 2100 MPa). Interestingly, as the tempering temperature decreases from 440 °C to 240 °C, the elongation of #1 steel decreases from 10.7 to 1.8 pct, but that of #2 steel is quite stable at ~ 10 pct. #2 Steel has a high tempering resistance, and χ-carbide precipitation of #2 steel is significantly suppressed during tempering. Moreover, when tempering at 240 to 400 °C, the austenite volume fraction of #2 steel is higher than that in the quenched state. The reduction in carbide precipitation contributes to carbon segregation at martensite lath boundaries and other locations, and it may promote the formation of reversed austenite during tempering, which is revealed by 3D atom probe tomography (APT). The excellent mechanical properties of #2 steel are mainly related to the reversed austenite, the fine-grained microstructure of the martensite matrix and the reduced carbides acting as crack initiation sites.