EFFECTS OF VANADIUM AND PROCESSING PARAMETERS ON THE STRUCTURES AND PROPERTIES OF A DIRECT-QUENCHED LOW-CARBON MO-B STEEL

The structures and mechanical properties of a series of thermomechanically processed, direct-quenched martensitic 0.1C-1.4Mn-0.5Mo-B steels containing from 0 to 0.24 wt pct vanadium have been investigated and compared to those obtained after a conventional austenitizing-and-quenching treatment. For all processing conditions, vanadium additions to the base composition are found to increase hardenability (ideal critical parameter, D(I)); the largest effects (up to a 90 pct increase in D(I)) are noted when samples are hot-rolled prior to direct quenching. Vanadium additions are also observed to provide significant strengthening in the quenched-and-tempered condition as the result of the precipitation of fine V-Mo carbides. The strengthening increment due to these precipitates is approximately 100 MPa/0.1 wt pct V over the range of vanadium additions examined. At the same time, however, these precipitates reduce notch toughness; on the average, the 20 J transition temperature increases by about 4-degrees-C for each 10 MPa increment in yield strength. For the conditions examined, the best balance of strength and toughness is obtained in direct-quenched samples which are control-rolled (i.e., rolling is completed below the austenite recrystallization temperature) prior to quenching.