| Dynamic secondary cooling and soft reduction are effective technologies for the improvement of strand quality in continuous slab casting.A comprehensive understanding of the roller-layout of caster as well as the heat transfer,solidification and thermo-mechanical behaviors of strand is essential for the modelling and control of dynamic soft reduction.In this dissertation,the key technologies of dynamic soft reduction for an ultrahigh strength steel (UHSS) have been studied in view of experiment,design,modelling and dynamic control. And a dynamic secondary cooling and soft reduction control system has been developed based on the experiment of high temperature mechanical properties of strand and the design of roller-layout of caster.Firstly,thermo-mechanical parameters and plastic curves of an UHSS at elevated temperatures have been obtained using the physical simulation system Gleeble-1500D.The measured results showed that the tensile strength subject to solidified type and reheated type thermal histories approximately decreased linearly with the increasing temperature.The tensile strength with solidified type thermal history was lower than that with reheated type,and the influence of thermal history on the tensile strength could be contributed to the activation energy for deformation.A viscoplastic constitutive equation with variables of strain rate,temperature, stress,solid fraction and activation energy for deformation was constructed,which could be applied on the thermo-mechanical analysis and the process parameters determination of dynamic soft reduction technology.Secondly,two differential roller-layouts of caster with slab cross dimension of 1100mm×180mm have been designed by the multi-point bending and straightening principle as well as the improved Concast continuous bending and straightening curve method.The corresponding mechanical inspection of calculation results indicated that the two design methods were reasonable and reliable.Thirdly,a three-dimensional model of the steady heat transfer and solidification has been developed based on the realistic roller-layout and spray nozzle distribution in continuous slab casting.The contact cooling of individual rolls and the characteristics of spray nozzle were involved in the model.The calculated slab surface temperature and shell thickness accorded with the measured ones derived from pyrophotometer and pin-shooting experiments, respectively.The three-dimensional model was applied to optimize the temperature distribution in continuous slab casting with effective improvement.Fourthly,the aimed slab surface temperatures were determined according to the high temperature plastic curves,the caster roller-layout parameters and the metallurgical principles in the secondary cooling zone.Therefore,a dynamic secondary control model has been developed based on the two-dimensional transient heat transfer module and the incremental PID algorithm module.The testing results showed that the present online model and incremental PID algorithm could effectively control the aimed slab surface temperature with the operational conditions variation in each loop of secondary cooling zone,and the solidification end position could be predicted.Finally,the dynamic soft reduction parameters of low carbon steel and the UHSS with slab cross dimension of 950～1950mm×170mm were determined based on the high temperature mechanical properties and constitutive equation.As a result,a dynamic soft reduction control system in continuous slab casting,Visual Cast-Dynamic,has been developed by multithreading method.The dynamic soft reduction system was implemented by two control modes,i.e.,soft reduction and soft clamping,which were based on critical strain and critical stress of steel in the mushy zone measured by experiment.The process parameters determined by the developed dynamic soft reduction control system were in agreement with those in practice.It demonstrated that the dynamic soft reduction model in this study was correct with the ability of online control.
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