The Nb-Ti microalloying undersea linepipe steel HSL450S(0.06C
0.40Cr
0. 15Mo
0.02Ti
0.05Nb) has been developed by 90 t EBT arc funace- 100tLF+VD-wire feeding - Φ220 mm CC flow sheet.It was obtained by analyzing 30 heats that [O
]
was 35.2×10
-6
and [N
]
was78.8×10
-6
.The structure of steel was tempered bainite-pearlite + polygonal ferrite. The mechanical properties of Φ102 mm×14 mm and Φ168.3 mm×8.7 mm tube of steel HSL450S with grain size rating 10~11 met the requirement of standard of for undersea linepipe steel X60 and X65 grade.