Ocean engineering steel has been rapidly developed with the exploration of ocean resourses. A 690 MPa grade low carbon bainite steel was designed for ocean engineering, to upgrade performance by microstructure control and the refinement and dispersion control of precipitates. This steel was tempered on-line with rapid heating rate after control rolling and accelerated cooling process. The results show that the mechanical properties, especially the strength-toughness balance, are strongly influenced by the transformation of untransformed austenite and the condition of precipitates. When fast tempering at 550 £, microstructure recovered and few precipitates appeared, bringing strength down seriously, untransformed austenite turned into martensite/austenite (M/A) islands on cooling.