Technological Study And Numerical Simulation On Hot Continuous Rolling Of GH4169 Alloy

As an iron-nickel-chromium base wrought superalloy, GH4169 alloy has a predominant position by terms of consumption in all superalloys, since this alloy exhibits excellent properties and has wide application. Hot continuous rolling has a higher efficiency and the products possess top quality; the production way of this hot-rolled alloy has been changed from the traditional one-rolling in open-train mill to the hot continuous rolling. Nevertheless, the factors, e.g. billet reheating temperature, the rolling speed, cooling mode and the solution way, have much influence on the microstructures and properties of the final products.The hot-rolled round rod of GH4169 alloy in hot continuous rolling process on the production line for high and special alloy steels is selected as the subject investigated in this dissertation. In this work, the mathematical models of grain growth and flow stress, the finite element model for the coupled thermo-mechanical analysis and the processing maps for workability analysis have been well established. And a systemic analysis is made on the technology of reheating and rolling for the billet, cooling and solution for the final product in the rolling process. The main contents are summarized as follows,(1) For the billet reheating prior to the rough rolling, we chose original square billet with a starting grain size as experimental material, the grain sizes of thin specimens reheated under different temperatures for different times were obtained and mathematical models of grain growth under isothermal and non-isothermal conditions established. The validity of these models was further verified by experiments and practical production.(2) By conducting uniaxial hot compression test, the true stress-true strain curves have been obtained in order to simulate the hot rolling process. A flow stress model has been regressed to meet accurate finite element calculation and the accuracy is verified by comparing the calculated values to the experimental ones. On the basis of this model, the flow stress model considering the dynamic recrystallization and meta-dynamic recrystallization for hot continuous rolling was established to realize the coupled thermo-mechanical analysis.(3) Based on large deformation elastic-plastic finite element theory, a coupled thermo-mechanical FE model was constructed on ANSYS10.0/LS-DYNA. This FE model includes temperature field model, flow stress model and micro structural evolution model. Also, all kinds of initial conditions, boundary conditions were taken into account. The validity of this FE model is verified by a practical rolling process including the rough rolling and the hot continuous rolling.(4) The middle cross-section of the billet, the elements and the nodes in that section are regarded as the analytical objects. The variables, e.g. effective strain, effective strain rate, effective stress, temperature and grain size of these elements or nodes have been simulated for hot continuous rolling. At the same time, the influence of initial temperature and initial velocity of the middle billet on those variables was analyzed to find the key to design hot continuous rolling scheme.(5) The processing maps of GH4169 alloy have been developed based on dynamic material model theory and the values in true stress-true strain curves obtained on cylindrical uniaxial hot compression experiments. The stability during deformation process, the occurrence conditions of dynamic recrystallization and the possibility to realize hot continuous rolling were analyzed on those maps.(6) The hot continuous rolling technology of GH4169 alloy is preliminarily worked out by combination of the analytical results of processing maps and numerical simulation. The results are shown as follows. The reheating temperature of tunnel furnace is 1050±5℃and the reheating time is from 2 to 5 min; the initial velocity of the middle billet is controlled between 0.15-0.35m·s-1 as the rolling process is finished from fourth to six pass, 0.15-0.25m·s-1 as the rolling process is finished from seventh to tenth pass, no more than 0.15m·s-1 as the rolling process is finished form eleventh to sixteenth pass.(7) Three cooling and solution ways were simulated on thermal mechanical simulator, and the influence on grain refinement andδphase precipitation was analyzed. The on-line solution is the best cooling and solution way according to the experimental results.