The microstructure and mechanical properties of controlled rolling and controlled cooling 0. 31% Ni and 0. 88% Ni two Nb-Ti microalloyed NiCr steel are investigated. The results show that granular bainite

upper bainite

acicular ferrite

polygonal ferrite and a little of pearlite in marine steel are obtained by controlled rolling-cooling. Controlled rolling and control! cooling result in the refinement of the feirite grain and the increase of M-A islands. Both steels have high tensile strength

yield strength

elongation and hardness

and the mechanical properties of 0. 88Ni- 0.32Cr steel are higher than those of 0. 31Ni- 0.33Cr steel. The impact energy of longitudinal direction all of the marine steels exceeds 200 J

and that of 0.88Ni- 0.32Cr steel exceeds 300 J at _80℃. 0. 88% Ni is optimum of containing nickel of steel. This is attributed to the ferrite grain refinement strengthening

M-A islands compound strengthening

precipitation strengthening and dislocation strengthening due to controlled rolling and controlled cooling. Nickel effectively improves low-temperature impact toughness of marine steel.