Study of cast microalloyed steels

Improved grades of cast steels were prepared by microalloying C-Mn-Cr steels (0·15-0·22C, wt-%) with V (0·1)+Ti (0·01) or V (0·11)+Nb (0·034)+Ti (0·01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784-1078 MPa (80-110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0·9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the laths into polygonal ferrite occurred. Fine precipitates were deposited on the dislocation substructure.