Abstract: Equilibrium precipitated phases in R5 offshore mooring chain steel in the temperature range of 400 to 1 600℃ were calculated using thermodynamic software Thermo-Calc. The effects of alloying elements on these equilibrium precipitated phases were studied. The results show that main equilibrium precipitated phases in R5 offshore mooring chain steel at temperatures of 400 to 1 600℃ are M₂₃C₆, M₆C, MX and AIN, where M represents Cr, Fe, Mn, Mo, Nb, Ni and V, and X is C, N and vacancy. On this basis the influences of alloying elements C, Cr, Mo and Nb on the equilibrium mole fraction and initial precipitated temperature of M₂₃C₆, MrC and MX phases were discussed. The composition system and heat treatment condition of new R5 offshore mooring chain steel were optimized. The optimized heat treatment process is quenching at 920 ±30℃ and tempering at 600 ± 30℃, and the optimum content of metal element Mo is 4.3%.