a high strength , high toughness steel alloy is disclosed . the alloy has the following weight percent composition . element c 0 . 30 - 0 . 55 mn 0 . 6 - 1 . 75 si 0 . 9 - 2 . 8 cr 0 . 6 - 2 . 5 ni 2 . 70 - 7 . 0 mo + ½ w 0 . 25 - 1 . 3 cu 0 . 30 - 1 . 25 co 0 . 01 max . v + × nb 0 . 10 - 1 . 0 ti 0 . 01 max . al 0 . 015 max . ca 0 . 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 usual impurities . also disclosed is a hardened and tempered steel article having very high strength and fracture toughness and formed from the alloy 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 .