| With the continuous development of iron and steel industry,thin slab casting-direct rolling technology is widely accepted as an optimal route for producing high valued flat steel products.The Tata Steel's IJmuiden factory introduced a Direct Sheet Plant(DSP)in 2000.To expand the DSP product range and improve the quality of existing thin slab continuously cast products,a computer modeling and simulation technology can be regarded as an effective method.Thus,SteelSim,a finite element model for DSP thin slab continuous casting process,was developed by Institute for Energy Technology(IFE),Tata Steel,Research&Development(R&D)and University of Science and Technology Beijing(USTB).Currently,SteelSim has been able to couple the multi-field in thin slab continuous casting process of steel.However,the current model is not perfect and needs to be further improved.It is necessary to carefully analyze the calculation results of the model and find out the the shortcomings.Secondly,in the process of SteelSim modeling,the accuracy of calculation results depends on the reliability of input parameters.Constitutive parameters are one of the most important input parameters,which need to be measured at the laboratory scale.For the quality control of continuously cast thin slabs,one of the main problems is to minimize the formation of hot cracks,especially for the steel during peritectic solidification.However,an indisputable fact is that so far almost all the mainstream cracking criteria are using two-phase(solid and liquid)models,which can not reflect the characteristics of peritectic solidification.Based on the above situation,the current study takes the DSP thin slab continuous casting of steel as the background and mainly carries out the following research contents:? Multi-field finite element coupling simulation of thin slab continuous casting process.? High temperature mechanical properties of a hyperperitectic steel AHSS(0.22C-1Si-2.1Mn).? Peritectic solidification characteristics and hot cracking susceptibility assessment of AHSS steel.In the part of finite element modeling,the strand temperature distribution,stress distribution and deformation behavior,during thin slab continuous casting with Liquid Core Reduction(LCR),of a Low Carbon Aluminum Killed(LCAK)steel and a high silicon Non-Grain Oriented(NGO)electrical steel were studied by using the three-dimensional Finite Element Method(FEM)model,SteelSim.Special attention is paid to the deformation part of the strand at the liquid core reduction(LCR)section,whereby the main strand thickness reduction occurs.From the calculated results,it is shown that bulging at narrow face can be found in both cases,but the bulging deflections are different.Compared to the LCAK case,a larger displacement can be observed at the narrow face in the NGO case.In addition,the NGO case shows an early unbalanced stress distribution at the LCR section,which leads to strand broadening.On the other hand,the LCAK case has relatively a more stable development of stresses and negligible strand broadening.In the study on high-temperature mechanical properties of a hyperperitectic steel AHSS(0.22C-1Si-2.1Mn),according to the characteristics of thin slab continuous casting,the experimental scheme on hot tensile test is designed and validated by the analysis of fracture surfaces.The mechanical properties of the specimens at 1173 K-1573 K(900 ?-1300 ?)and strain rate of 10-3 s-1-1 s-1 were tested.Experimental data from the hot tensile tests were employed to determine the constitutive parameters,activation energy maps and hot processing maps.The results from calculation of constitutive model show that the original Arrhenius-type constitutive equations can accurately describe the constitutive behavior of the steel.The analysis results of activation energy map show that for a constant strain rate,special attention should be paid to the specimen deformed at 1373 K(1100 ?),where the maximum value of activation energy and the defect-free microstructures can be possibly attained.For a given temperature,the strain rate of 10-2 s-1 should be investigated because the minimum value of the activation energy is obtained at this strain rate and the microstructure and the mechanical properties of the material can be changed obviously.Combining the calculation results of hot processing map with the microstructure analysis of the cross section of the tensile specimen,the results indicated that the void formation is the main unstable factor for the current hot deformation process.In the aspect of assessment of hot cracking susceptibility,the peritectic solidification process of AHSS steel was in-situ locally observed by confocal scanning laser microscope(CSLM).By analyzing the solidification process,it is shown that the variation of solute concentration and the heterogeneous distribution of liquid phase may be the main reasons for the difference in the features of the peritectic reaction among the dendrites.Combining the whole experimental solidification process,it is found that the solidification sequence of the studied steel has been changed within the visible experimental range.According to the characteristics of peritectic solidification,the Feurer's criterion is modified.Based on the results of the local experimental observation and theoretical calculation,crack susceptibility of the hyperperitectic steel under experimental solidification conditions and equilibrium solidification conditions are evaluated using the modified Feurer's criterion.It was found that the local rate of solidification shrinkage under experimental solidification conditions is much higher than that under equilibrium solidification conditions.The results also reflect the shortcomings of the current mainstream cracking criteria in evaluating the cracking susceptibility of steel during peritectic solidification.The comprehensive application of the above studies lays a good foundation for the process optimization and quality control of DSP thin slab continuous casting process,and will play an important role in the following studies. |
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