The test super-high strength weathering steel (/%: 0. 062 〜0. 065C

0. 29 〜0. 30Si

1. 23 ~ 1. 27Mn

0. 49 -0. 52Cr

0. 19 -0. 20Ni

0. 29 ~0. 31 Cu

0 ~0. 20Mo

0. 035 ~0. 060Nb) is melted by a 50 kg vacuum induction furnace

forged to 60 mm x 60 mm billet

then hot rolled to 4 mm plate in laboratory by finishing rolling pass at 880℃ ， water cooling to 600℃ ，furnace cooling. The effect of Nb and Nb-Mo microalloying on structure and mechanical properties of steel are studied by optical

scanning electron and transmission electron microscope

and universal tensile test machine. Test results show that as compared with 0. 035% Nb steel the grain size of 0. 060% Nb steel is finer

the strength increases by ~70 MPa

but the elongation is the same as that of 0. 035Nb steel as compared with non-added Mo steel (0. 035Nb steel and 0. 060% Nb steel) the M/A phases in 0. 035% Nb-O. 20% Mo steel increase obviously and bainite structure occurs in steel led to increase of tensile strength and decrease of yield-strength ratio and elongation of steel.