Development Of The High Strength Heavy Plates For Large Oil Tanks

In accordance with the needs of high strength heavy plates (HSHP) for large oil tanks in the building of the national oil reservation bases, development of the steel composition, optimization of manufacturing technique and industrial trial production were carried out. Meanwhile, the welding and corrosion properties of the plates were also investigated.According to the research progress in the world and the performance requirements of HSHP for large oil tanks, models of the relationship between microstructure and properties, including yield strength, tensile strength and extensibility, were set up based on the various related strengthening mechanisms. In combination with the production conditions and the weldability of plates, the models were applied for the first time in the design and optimization of the steel composition. The optimum compositions of the steel was:(0.085～0.110)%C-(1.50～1.60) %Mn-0.25% Si-0.25% Ni-0.10% Mo-0.05% V-(0.010～0.015)%Ti. and it’s named as 08MnNiVR.The role of trace Nb in the steel during welding under various heat input conditions was discussed. In a welding cycle, dissolution of NbC particles, diffusion of Nb in the austenite substrate and the dragging effect of Nb in solution will influence the impact toughness of the heat effected zone (HAZ). Dissolution of NbC particles will be enhanced with increasing of the heat inputs, which will reduce the keying effect on austenite grain boundaries and the impact toughness of HAZ. When the heat inputs was in between 80kJ/cm-100kJ/cm, as the result of completely dissolved Nb was not able to re-precipitate during cooling and the diffusivity of Nb limited, the Nb-riched zone will be formed and the impact toughness reduce further. When the heat inputs was as high as 120kJ/cm, distribution of Nb in the substrate will be more homogeneous, which was beneficial to the drag of grain boundary movement, so the toughness of HAZ should be improved somewhat. In order to meet the requirements of high heat input welding over 100kJ/cm, Nb was discarded for the first time in the design of composition of HSHP for large oil tanks.By comparing the primary characteristics of simulating quenching and industrial production process, the average cooling rate from quenching temperature to 400℃was proposed as the eigenvalue of the quenching process for the first time. A model of hardness and cooling rate was established on the basis of the CCT curve and Jominy test simulating quenching. According to the actual process the model was amended and then applied to the industry process control.By fractographical observation and analysis of HAZ, it was found that, under the condition of high heat input, the zone of fusion line+1mm in HAZ is the weakest part in the weld joint, therefore, the impact energy of fusion line can be considered as key parameters for the assessment of the weldability of the steel.Based on the above work, the industrial production of the 08MnNiVR heavy plates has passed the technical and construction site technics assessment organized by the National Boilers and Pressure Vessels Standardization Committee, and has been applied successfully in the construction of Zhenhai National Oil Reservation Base.