Method of making high strength, tough alloy steel

The method of forming a high strength, tough alloy carbon steel comprising heating an alloy steel to a temperature above the austenite transformation temperature to form a homogeneous austenite phase with the alloying elements in solution, and cooling the steel to transform the major portion of austenite to martensite at a temperature of at least about 250.degree. C. to form a microstructure of uniformly dispersed martensite crystals, the major portion of which are in dislocated form, and continuous thin boundary films of stabilized austenite essentially free of carbides separating said martensite crystals, said steel being characterized by a yield strength of at least about 180,000 psi, a room temperature Charpy impact energy of at least about 19 ft/lbs. and a plane strain fracture toughness (K.sub.IC) of at least about 80 KSi-in.sup.1/2, said steel consisting essentially of from about 0.20 to about 0.35 weight % carbon, about 3.0 to 4.5 weight % chromium, and at least 1 weight % of at least one other substitutional alloying element selected from the group consisting of nickel, manganese, molybdenum, cobalt, silicon, aluminum, and mixtures thereof, said steel including a maximum alloy content below that which lowers the martensite transformation temperature to below 250.degree. C.