Analysis Of Thermo-mechanical Behaviors In The Mould For Small Round Billet Continuous Casting

The thermal and mechanical behaviors interact on each other over the whole mould process in continuous casting. The ability and characteristics of mould heat transfer are responsible for strand temperature and thickness of solidifying shell. Heat transfer has great influence on the shell shrinkage and the mould distortion. Whereas, the heat transfer from casting steel to mould is also influenced by the shell shrinkage and the mould distortion. It is important to develop thermo-mechanical coupled mathematical model to make a further understanding of mould process.The non-ideal, complicated mould heat transfer may lead to abnormal mould friction, the direct contact between shell and mould makes monitored mould friction an obvious respond in plant trial, so the influence of mould friction on the stress of strand cannot be neglected. The round billet mould is symmetry, the non-uniformity of shell thickness is random and is determined by the local heat transfer state, which is different from that of slabs, where the mould geometry determines the shell to grow first in corners. The simulation calculation in ideal state does not reflect the real mould process in continuous casting.A multi-dimensional, coupled, inverse problem model is developed from the measured data of mould temperature by thermocouples embedded in various transverse and longitudinal sections for a round billet. By identifying the thermal resistance between the mould and strand, the non-uniform mould heat transfer around the perimeter in each transverse section of mould is calculated, which can reflect the real process.The calculated temperature fields of mould and billet, which are taken as thermal load, are put into a three-dimensional mechanical model. The mould friction, interfacial state between the mould and billet, and their stresses are coupled together into the mould, considering the influence of mould taper and ferro-static of steel. The thermal and mechanical behaviors of billet and mould are predicted. The influence of mould friction is discussed. The solid/liquid slag film thickness, the interaction between the solidified strand and mould, and gap size are predicted from the stress model. Owing to the non-uniform heat transfer, the calculation results in respect of the thermo-mechanical behavior are non-uniform around the perimeter in each transverse section of mould. It is a better reflex from the real continuous casting process.The characteristics of mould heat flux for round billet are studied based on the measured data. The researching results can be provided as a basis for real-time monitoring.The measured results show that, the mould heat flux is in transient variation not only along the height but also around the perimeter in each transverse section of mould. There is low heat transfer of mould in the meniscus region. The peak heat flux locates at 70-11 Omm below meniscus and is sensitive to operational parameters, such as pouring temperature, casting speed, carbon content, powder type, and so on.The variation and non-uniformity of local temperature and heat flux are analyzed under normal and abnormal conditions. The variability and non-uniformity around the perimeter of mould heat transfer are influenced very little by the normal operational parameters, but influenced much by the steel grade, powder type, and mould installation etc.The results show that, both the mould heat flux and shell thickness are non-uniform around the mould perimeter in each transverse section of mould influenced by the mould installation in caster. Heat flux has the similar distribution in the same installation of mould and distinct difference in different installations by statistical analysis from amount of measured data. There is relationship between distributions of mould heat flux around the perimeter in the "high heat flux region" and shell thickness at any mould height. This can provide a basis for uniform strand solidification and for improvement on surface quality, by analyzing and controlling mould heat flux profiles, especially in the vicinity of meniscus.The distribution of mould heat flux and its influencing factors are discussed quantitatively on the basis of measurements and calculations. The relationship between the mould heat flux and shell thickness is discussed. This research helps to apprehend of the distribution of the mould heat flux and profile of solidifying shell thickness under normal production. A method combining on-line detection with numerical simulation provides a basis for the visualization of continuous casting mould process and for the real-time monitoring of high efficiency production of no-defect billet.The mould temperature calculated by two-dimensional model neglecting the longitudinal heat transfer is higher than that by three-dimensional model. However, the heat flux is contrarily lower. The heat flux calculated using Fourier's law simply from the temperature difference between two thermocouples located at different distances from the hot face result in a low estimate of heat flux in the vicinity of meniscus with larger longitudinal heat conduction.The calculation results illustrate that the location and the magnitude of peak mould temperature are depressed by the cold region above meniscus and the thick slag rim (up to 1.0mm). The solid slag film is thin to 0.1mm in the region about 80mm below meniscus,coinciding with formation of the high heat flux region. The liquid lubrication has obvious influence on shell stress in the meniscus region, while the solid-solid contact friction dominates in the lower part of mould. The gap through which the liquid slag filtrates is also non-uniform around the perimeter, determining the profile of mould heat flux to some extent. The shell thickness and mould heat flux share with the similar distribution, both of which are determined by the solidified slag film between the solidified shell and mould before the disappearance of liquid slag.A crack criterion is proposed based on the mould heat flux to predict the crack susceptible area. The possibility of crack formation in the meniscus is comparatively high, and the area with higher crack susceptible is determined by mould installation to some extent, and the crack almost located in the range of arc for the same installation.The inverse problem model is applied to the calculation of heat transfer and solidification for slab continuous casting, and the calculation results are good.All the results provide valuable foundation for the on-line diagnosis of defects, adjustment of operational parameters, optimization of monitoring system, and prediction of abnormity of intelligent mould.