Research And Application Of Phase Transformation And Mechanical Properties Models In Hot Strip Rolling

With the further development of the market economy and rapid development of information technology, customers demand more in categories, quality and cost of steel products. Iron and steel enterprises need to transform from mass production to mass customized production. Flexible rolling technology and online optimization and control technology of microstructures and properties are the key technology. The requirement of predicting mechanical properties based on microstructures is put forward. This thesis focused on the austenite transformation. Changes of dislocation density in deformed austenite and interface position with temperature during continuous cooling had been researched. The relationship between microstructure and stress-strain behaviour was built and online application of the forecasting system of microstructure and mechanical properties was realized.Based on dislocation density changes during deformation and deformation interval, a model of calculating dislocation density in deformed austenite was proposed. Double-hit compression tests were performed on a Gleeble-3500 thernomechanical simulator. Parameters of dislocation density model were obtained after the test data regression. The deformation resistance was calculated based on dislocation density model. The calculated results had a good agreement with experimental results. As static recovery was dominant when the interval time is short (?3s). it was necessary to take the static recovery into account. The static recovery could be neglected when the interval was long (> 3s) as static recrystallization was dominant.A diffusion control model was established for the growth of proeutectoid ferrite during the decomposition of supersaturated austenite in continuous cooling process. The interface positon, carbon diffusion length at austenite side of interface and carbon profile as a function of temperature were obtained. Equilibrium carbon concentration on both sides of interface was calculated by combining super element model and LFG model. Considering the carbon concentration at the austenite side of interface could not reach equilibrium carbon concentration with high cooling rate, a expression C(?)=(1-a·?h)·Ceqy was put forward to modify the carbon concentration at austenite side of interface. The soft impingement effect of interface position was discussed. Ferrite fraction was calculated by austenite transformation model during continuous cooling and the calculated values were in good agreement with literature values.Based on stress-strain equation of ferrite and pearlite as well as Iso-W increment assumption, a new stress-strain model for ferrite-pearlite steel was put forward. The physically based stress-strain equation of ferrite was approximated with Swift's equation in order to obtain the analytic solution. Stress-strain curves of ferrite-pearlite steel with different pearlite fractions, ferrite grain sizes and pearlite interlamellar spacings were calculated as well as tensile strength by combining Considere criterion. A tensile strength model of ferrite-pearlite steel was built based on the relationship between microstructure and property. Considering the plastic relaxation mechanisms, the internal stress was modified with pearlite volume fraction, total strain, yield stress of ferrtie and pearlite when the pearlite volume fraction was more than 20%. The predicted values of tensile strength had a good agreement with experimental values.The forecasting system MPP of microstructures and mechanical properties in hot rolling was developed under visual studio 2005 compiling system with C++ propramming language. Calculating of microstructures and properties during rolling, laminar cooling and coiling was achieved with the system. It was concluded that the system MPP could meet the calculation accuracy in online application by comparing the calculated results with results of business software and test data of actual production. Online prediction of microstructures and properties was realized by combining the forecasting system MPP with control system in CSP production line. It could provide guidance for optimizing technological parameters.