The tested steel M54 (/%：0.28~0.32C

≤0.10Si

≤0.10Mn

≤0.005S

≤0.008P

9.5~10.5Ni

6.6~7.4Co

1.8~2.2Mo

1.1~1.5W

0.7~1.3Cr

0.04~0.16V) is melted by 5.8t vacuum induction furnace +2.2t vacuum consumable electrode furnace

and forged to Ф170mm bar products. The Ф5mm test specimens cut at 1/2R of forged bar are treated by solid solution treatment at 1075°C for 90min

air cooling and at 1060℃ for 75min

oil cooling + deep-cooling treatment at -73 for 120min + aged treatment at 400~600℃ for 300min

air cooling. The effect of aging temperature on mechanical properties and structure of secondary-hardening

ultra-high strength steel M54 is studied and analyzed by optical and scanning electron microscope etc. Results show that tested steel M54 aged at 520°C for 300min has excellent comprehensive mechanical properties-tensile strength 2040MPa

impact energy 62J and fracture toughness 110MPa·m

1/2

with aging at 560°C the morphology of intergranular fracture occurs in steel and the impact energy K

U2

and fracture toughness K

IC

of the steel decrease respectively to 26J and 80.6MPa·m

1/2

and with increasing aging temperature to 600°C the impact energy K

U2

and fracture toughness K

IC

of tested steel increase respectively to 56J and 131MPa·m

1/2

.