Study On The Continuous Annealing Process Of Cold-rolled SUS430Ferritic Stainless Steel

In this study. the mathematical models of temperature, microstructure and mechanical properties of SUS430cold-rolled stainless steel in a continuous annealing furnace were investigated by experimental and numerical methods. The mathematical correlations of heat flux on the surface of cold-rolled strip inside the heat section, air jet cooling section, air-atomized cooling section were evaluated, based on this, the temperature prediction model of cold-rolled strip was established. The recrystallization kinetics and grain growth kinetics equations of cold-rolled SUS430stainless steel were estimated by isothermal annealing experiment. The evolution of microstructure and mechanical properties of cold-rolled SUS430stainless steel during a continuous annealing process (CAP) were studied by a continuous annealing experiment. Based on the results obtained in continuous annealing experiment, the yield strength, the elongation, and the quantitative relationship between the yield strength and the softening fraction were determined for cold-rolled SUS430stainless steel. Finally, the temperature prediction model, softing kinetics model and mechanical properties prediction model were combined by Sheil superposition principle, and this integrated model was used for the prediction of temperature, microstructure and mechanical properties of cold-rolled SUS430stainless steel during CAP as well as for the development of new annealing processes.Above all, the following innovative research conclusions are listed below:(1) Based on one-dimensional inverse heat conduction method, an industry-scale test facility was built in this study for analyzing the characteristic of heat transfer during air-atomized cooling of high temperature surface. For a widely used industry flat pattern air-atomized spray nozzle, mathematical correlations were proposed, signifying the dependence of surface heat flux in different boiling regimes for water flow rates V≤0.83×10-5m3/s. The results will provide a theoretical basis for predicting the temperature of the cold-rolled strip during the spray-cooling processes and for designing the cooling process.(2) Based on the detailed analysis of heat transfer mechanism inside the heat section, air jet cooling section and air-atomized cooling section, temperature prediction model of the strip during practical continuous annealing process was established. The control function of strip temperature is one-dimensional unsteady equation, heat transfer mechanism in different furnace section are reflected by boundary conditions. By this method, the relative error between the simulation result and experimental result is less than10%.(3) Microstructure and hardness evolution of cold-rolled SUS430stainless steel during isothermal annealing process were studied, and the recrystallization kinetics and grain growth model was established. This models can be used to predict the mean grain size and the softening fraction of cold-rolled SUS430steel during CAP.(4) The effect of the annealing temperature on the microstructure, strength, elongation, and anisotropy properties of cold-rolled SUS430stainless steel was investigated by a continuous annealing experiment. In this study, referring to the Hall-Petch equation, the quantitative relationship between yield strength and the softening fraction of cold-rolled SUS430stainless steel was established. Meanwhile, the correlation between yield strength and elongation of cold-rolled SUS430stainless steel was determined.(5) By using the coupled-model, the evolution of temperature, microstructure, and mechanical properties of cold-rolled SUS430stainless steel during CAP can be predicted. The simulated result was used to evaluate the problem existed in the practical continuous annealing process of cold-rolled SUS430stainless steel. Meanwhile, the coupled-model can be used for the development of new annealing processes.The continuous annealing process of cold-rolled SUS430stainless steel was studied by experimental and numerical methods. The results of our study provide a theoretical basis for the development of CAP.