Research And Application Of The Process Of High Strength Ultra-heavy Steel Plate

In recent years, with the rapid development of national economy and the increasing of building super high-rise and large-span steel construction projects, the demand for heavy plate with more heavy thickness and better properties used in construction and machinery has been increasing. The work of this dissertation were carried out integrating with the project of "The Development for High Strength Low Alloy High-quality Ultra-heavy Plate"and the production process of two ultra-heavy steel plates, i.e. Q345E steel with60-120mm thickness and Q420E with110mm thickness, were investigated through basic theoretical research, laboratory simulation experiments, mechanics analysis in deforming and industrial trial. The influence of rolling and heat treatment parameters on the microstructure and mechanical properties of Q345and Q420heavy plates were investigated in laboratory based on which the industrial trial were carried out. The reason for high inferior rate by detection examination in Q345industrial trial was clarified. Finally, Q345ultra-heavy steel plate was successfully realized in industrial production. The main work is shown as follows:(1) Regarding the Q420heavy Plate Steel as the research object, thermal simulated experiments with different parameters was carried out in Gleeble-2000thermal simulator, the effect of deformation temperature, deformation and strain rate on the dynamic recrystallization behavior and static recrystallization behavior in the maintaining isothermal time after hot deformation of austenite was studied. meanwhile, the deformation resistance model and the static recrystallization kinetics model. were established. The temperature field and strain field distribution in the rough rolling procedure was simulated using one dimensional implicit difference method and finite element software ANSYS Combining with the results of high temperature recrystallization behavior, the condition of recrystallization taking place was analyzed which in different positions of the heavy plates along thickness direction.(2) Through continuous cooling phase transformation study it was found that with the increase of cooling rate, the ferrite start transformation temperature decreased, the grain was refined, but the bainite start transformation temperature first rising and then began to lower, at the same time the bainite transformation volume gradually increased. As the deformation reduction increases, the whole CCT curve moved towards the upper left orientation, the ferrite transformation was accelerated which reason was the deformation accelerate the diffusion of carbon atom, transformation temperature change higher. As the deformation temperature decreased, the ferrite phase transition temperature increased and the ferrite area expanded. while the bainitic transformation temperature decreases. After the tested steel was deformed in austenite non-recrystallization, the CCT curve contain wide ferrite region in its cooling rate achieved, which microstructure was pro-eutectoid ferrite and pearlite. therefore, for the development of this steel, can take full advantage of the characteristics of its continuous cooling transformation curve, comprehensively utilized the finer grain strengthening, phase change strengthening to improve the performance of the heavy steel plates.(3) The influence of start rolling temperature, cooling rate to the microstructures and properties of Q345and Q420tested steels and rolling mode to the microstructures and properties of Q420tested steels were researched. The results showed that using TMCP process produced Q420ultra-heavy plate, when the total reduction was invariable, increase the total reduction in austenite non-recrystallization, the yield strength increased, the tensile strength increased slightly and elongation slightly lower, while the impact toughness of steel improved, especially the low temperature impact toughness improved significantlly. Comparing with the UPR rolling process, the strength and toughness in the centre layer of the plate remarkably improved, and the elongation changed slightly, which cause is the different grain refinement mechanism in austenite recrystallization region and austenite non-recrystallization region.(4) Comparing with Q420steel plate with different hot rolling process, after normalized the yield strength decreased, the tensile strength and elongation changed little, and the low temperature impact toughness improved significantly. The plate rolled with same rolling process in the experimental normalizing temperature range, with the normalizing temperature decreased, the strength changed little, but the toughness improved significantly, which reason is the reduced number of second-phase precipitates dissolved, the finer of the precipitates; and the slower austenite grain growth rate, both aspects facilitated the toughness of the steel plate. Rolling mode before heat-treatment has a little effect on the properties of the plates, it may be because the smaller difference of austenite grain size in the experimental normalizing temperature range.(5) Rolling process of60mm-120mm Q345E HSLA ultra-heavy plate suitable for the existing wide and heavy steel plate production line was received through the transformation of laboratory results to the factory, which composition was general C-Mn steel without microalloyed element, and the process was two-stage controlled TMCP rolling process, finally realized large-scale industrial production. Through data Statistic it obtained that the plates rolled with this TMCP process were up to250,000t, which qualified rate was92%, detection examination checking qualified rate was99.21%. The chemical composition, hot-rolled and heat treatment process parameters of Q420steel was determined, optimal production process of Q420heavy plate was received, and the110mm Q420E high-strength low-alloy steel products was successfully produced in the industry.(6) On the basis of original TMCP process for Q345ultra-heavy plate, the UPR production process which reduces micro-alloying elements and simplifies working procedure was realized through developing equipment potentialities.The metallic deformation behavior and rolling force and torque parameters in UPR rolling process was studied using continuous velocity field, the upper bound of power, integral mean value theorem and vector inner product method, based on which the feasibility of this process was discussed. The ultra-heavy plates produced using UPR process had a good combination of strength and toughness, and good interior quality due to the improvement of deformation penetrability and uniformity. Furthermore, the rolling passes were greatly reduced thus the rolling cycle was shortened, and also the lumber recovery was increased.