Abstract: A low alloy steel having a composition containing columbium, vanadium and silicon, advantageously in specified ranges of content, has demonstrated superior mechanical properties, comprising yield strength above 80 ksi, excellent toughness including good impact strength at low temperatures, and good formability evidenced by a suitably high percent elongation. The steel in its preferred embodiments includes a significant amount of manganese, and is economically producible in as-hot-rolled state to achieve the stated properties, subject to attainment of even higher strength by an aging treatment.