The studied Cr-Co-Ni-Mo gear steel (/% : 0.05 ~ 0. 15C

4. 2 ~ 6. 3Ni

14. 5 ~ 16. 2Co

4. 2 ~ 6. 4Ct

3. 8 〜5.3Mo

0. 8 ~ 1. 5W

0.1 ~0.4V

≤ 0.005S

≤ 0. 008P

non-adding Nb or adding 0. 01 ~0. 03Nb) is steelmaking by 200 kg vacuum induction melting + vacuum arc remelting process and rolling to Φ50 mm bar products. The testing specimen is solid-solution-treated at 1 050℃ for 1 h and oil cooling

and cold-treated at -80℃ for 2 h

then aging-treated at 560 °C for 7 h. Results show that in steel bearing Nb

Nb microalloy carbon-nitride exists at grain boundary or sub-grain boundary

it has the fine grain strengthening function

as a result with adding 0. 01% ~ 0

03% Nb the tensile

yield strength and impact energy of steel increase respectively from non-Nb steel 1 850 MPa

1 598 MPa and 22 J to 2 057 MPa

1 964 MPa and 33 J.