Chemical Composition and Manufacturing Process Optimization of D36 Offshore Platform Steel

The chemical composition of D36 offshore platform steel was optimized and the three kinds of rolling processes and two kinds of heat-treatments were designed. The microstructures and mechanical properties of the steel in different rolling states and heat-treatment states were studied. The optimal rolling process and subsequent heat-treatment process were determined, realizing the optimization of the production process. The results show that the optimal chemical composition of D36 steel was as follows: 0.11-0.12C, 0.30-0.40Si, 1.55-1.60Mn, ≤0.020P, 0.005S, 0.03-0.06Al, 0.045-0.048Nb, 0.012-0.018Ti, 0.13-0.20Cu, 0.18-0.25Ni, 0.055-0.060V. The rolling and heat-treatment process was that rolling with the controlled rolling and accelerating controlled cooling process followed by the normalization at 890 ℃ for 140 min then fast cooling to the re-reddening temperature of 650 ℃. With this process, the surface microstructure of D36 steel with 60 mm thickness was mainly consisted of ferrite, martensite and bainite and the core microstructure was of fine ferrite and pearlite; the yield strength and tensile strength of the steel were 375, 530 MPa respectively, and the impact energy at -20 ℃ was around 200 J.