| The offshore oil and natural gas industry has become the pillar of marine economy in China. The exploit and utilization of offshore oil and gas resource cannot simply be done without offshore engineering equipment in exploration, exploit, storage and transportation or any other aspects. Presently, steel plates with high mechanical properties used for China’s offshore engineering equipment, especially ultra-heavy plates, still depend on imported from overseas. Lacking of cutting edge technology is a severe limitation on the offshore oil developing progress and independence in China, and also slows down the steps of China’s marine economy development. Rolling process is the key point of ultra-heavy plate manufacturing, as for some steel plates with special mechanical properties requirements, the corresponding heat treatment methods are mattered as well. The thickness section effect of ultra-heavy plate (including deformation inhomogeneity and heat distribution inhomogeneity) is a ubiquitous problem in the manufacturing process, and directly deteriorates the microstructure and property homogeneity of steel. Few effective theories could direct the experimentation and manufacturing which are directed by trial and error method more often. Ni-Cr-Mo-B microalloyed steel is a new type of high strength low alloy steel, its plate products such as heavy plate and ultra-heavy plate can be used for manufacturing offshore engineering equipment. By using Ni-Cr-Mo-B steel as the research object, this present work has studied the elevated-temperature flow behavior, microstructure evolution and its modeling, and hot processing map firstly, and then multi-level multi-field coupling simulation (mechanical-thermal-microstructural) is conducted as a further research on the hot compression deformation and rolling process of ultra-heavy plate, in order to provide a reference and evidence for the development and optimization of hot working process of Ni-Cr-Mo-B ultra-heavy plate.Elevated-temperature flow behavior can be characterized by constitutive equation which has its significance on the design and optimization of material forming process and also is the cornerstone of thermo-mechanical coupling analysis. Plenty of studies have focused on the establishing methods of constitutive equation, and due to the complexity and sophistication of flowing deformation behavior itself, there is no unified establishing method unfortunately. In this work, based on the thermal compression experiment, a modified strain-compensated Arrhenius type and a modified ZA type constitutive equation have been established to describe the flow behavior of Ni-Cr-Mo-B steel, which, subsequently, has been estimated by the employment of the BP Artificial Neural Network (BP-ANN). Moreover, in a point view of statistics, the previous stated three methods are comparatively evaluated to verify which is the optimum method, and the result is:BP-ANN has the highest estimation accuracy, the second one is modified strain-compensated Arrhenius type and the one with lowest estimation accuracy is modified ZA type, however, the modified strain-compensated Arrhenius type constitutive equation has a good mathematical expression, and also maintained a satisfied estimation accuracy, therefore, the modified Arrhenius type has been chosen for multi-level multi-field coupling simulation of rolling process of Ni-Cr-Mo-B ultra-heavy plate.This study focuses on the hot deformation microstructure evolution of the Ni-Cr-Mo-B microalloyed steel, and the result indicates that the dynamic recrystallization behavior of the Ni-Cr-Mo-B steel is quite sensitive to the deformation temperature and strain rate, with the increase of deformation temperature and decrease of strain rate, the volume fraction of dynamic recrystallization is increased, dynamic softening effect is enhanced, and also, the size of fully dynamic recrystallization grain is enlarged. Based on the Avrami equation, the modified dynamic recrystallization kinetic equation of the Ni-Cr-Mo-B steel and mathematical model between temperature compensation of strain rate factor (Z parameter) and grain size of dynamic recrystallization are well established, and the estimation results of these models are well matched with experimental results as well. In addition, the characteristic parameters (peak stress, critical strain, etc.) of hot deformation of the Ni-Cr-Mo-B steel have been identified accurately based on the flow curves, as the mathematical relationship of characteristic parameters and Z parameter is established as well.In this paper, thermal processing map of the Ni-Cr-Mo-B steel has been established based on dynamic material model (DMM). The analysis indicates that there exist two flow stability region at higher strain levels (ε= 0.7), and corresponding process parameter range of these are:temperature in the range of 910℃-1030 ℃ with the strain rate in the range of 0.0067 s-1～0.15 s-1 and temperature in the range of 1030℃～l150℃with the strain rate in the range of 0.06 s-1～0.61 s-1. Moreover, the flow instability regions perform characters of low temperature with low strain rate and high temperature with high strain rate. In summary, the recommendation is the hot working processes should be taken at the flow stability regions. While in combination of using thermal processing map and constitutive equation to optimize the hot working process, these two methods are backed up and complemented each other which is well performed in this research.The rolling process of ultra-heavy plate is such a complicated nonlinear process, but with continuous development and improvement on finite element technology, there is a certain available way of researching. Based on the secondary development of finite element technology, the multi-level multi-field coupling simulation (mechanical-thermal-microstructural) has been conducted for the rolling process of the Ni-Cr-Mo-B ultra-heavy plate in this work. The microstructure evolution models used in the coupling simulation are the previous stated dynamic recrystallization kinetics model and grain size model. Moreover, during the secondary development, the unsteady state feature of actual rolling is considered, the relevant mathematical models are processed to fit the feature, and the simulation method is testified by the unsteady axial compression experiment in order to ensure the reliability of coupling simulation method. The simulation results indicate that low speed rolling (less than 1 m·s-1) facilitates the deformation; heavy reduction facilitates the improvement on the deformation inhomogeneity in the thickness direction of ultra-heavy plate; while high temperature rolling facilitates the occurrence of dynamic recrystallization of ultra-heavy plate and grain refinement. Therefore, the recommended ultra-heavy plate rolling process condition is rolling with high temperature, low speed and heavy reduction.Besides, the numerical simulation of roller quenching process was also conducted for some ultra-heavy plate with the requirement of quenching and tempering. First, the simulation methodology of the quenching process for Ni-Cr-Mo-B ultra-heavy plate was stuied based on the physical analysis of quenching process and heat transfer theory. By comparison with the experimental results, the simulation method proposed in this paper is valid. Furthermore, the effect of surface cooling rate on temperature distribution in cross section of ultra-heavy plate was studied. The result shows that improving the cooling rate of the surface in quenching within a certain range could increase the cooling rate of interior of steel plate, improve the temperature section effect and the inhomogeneity of microstructure and mechanical property of the steel; But because the plate is particularly thick, and although a higher cooling rate could make the surface of steel plate cooling down rapidly, there is no help for improving the cooling rate at the interior of ultra-heavy plate. And the recommendation for the manufacturing is selecting an appropriate quenching cooling rate according to the specific working condition.The above research results can provide guidance for the hot processing design and optimization, and microstructure control of Ni-Cr-Mo-B ultra-heavy plate. This paper reflects the close links between computing, experiment and data, and is an attempt and exploration on materials and processes optimization design under the framework of Materials Genome Initiative (MGI). |
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