Enhancing mechanical properties of a V + Ti microalloyed bainitic forging steel via tailoring microstructure through controlled forging

To obtain a refined and homogenized microstructure as well as enhanced combination of strength and toughness, the influence of controlled forging on the microstructure and mechanical properties of a Mn–Cr-type bainitic forging steel microalloyed with V + Ti elements was explored. Microstructural characterization shows that the final microstructure of the as-forged steel changed from primarily granular bainite (GB) to primarily lower lath-bainite (LLB) through controlled forging of mainly finish-forging temperature and post-forging cooling rate. The result of physical–chemical phase analysis shows that almost all the added Ti is in the (V,Ti)(C,N) precipitates, while only ~ 7% of the added V is in the precipitates, and there is only a slight increase in the amount of the (V,Ti)(C,N) precipitates with changing forging condition, and their precipitation strengthening is ~ 40 MPa. The tensile properties except the total elongation increase significantly from the GB sample to the LLB sample, and the latter exhibits identical or even higher mechanical properties than conventional quenched and tempered 42CrMo steel except the yield strength. Instrumental impact tests revealed that the crack initiation energy, propagation energy and the ability of arresting crack growth of the LLB sample are significantly higher than those of the GB sample. The reduction and/or even elimination of large blocky M/A, the increase in high-angle boundaries fraction and microstructure refinement are primarily responsible for this enhancement of doubled impact toughness. It is thus suggested that suitable controlled forging is a promising strategy to regulate and control the microstructure and mechanical properties of bainitic forging steel.