A high strength, high toughness steel alloy is disclosed. The alloy has the following weight percent composition. Element C0.30-0.55 Mn 0.6-1.75 Si0.9-2.8 Cr0.6-2.5 Ni2.70-7.0  Mo + ½ W0.25-1.3  Cu0.30-1.25 Co 0.01 max. V + ( 5/9) × Nb0.10-1.0  Ti 0.01 max. Al0.015 max. Ca0.005 max. The alloy further includes a grain refining element selected from the group consisting of 0.0001-0.01% Mg, 0.001-0.025% Y, and a combination thereof. The balance of the alloy is iron and the usual impurities found in commercial grades of steel alloys produced for similar use and properties. In this regard phosphorus is limited to not more than about 0.01% and sulfur is limited to not more than about 0.001%. Also disclosed is a hardened and tempered article that has very high strength and fracture toughness. The article is formed from the alloy having the weight percent composition set forth above. The alloy article according to this aspect of the invention is further characterized by being tempered at a temperature of about 500° F. to 600° F.