【作者】 尹合壁；

【导师】 姚曼；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2006， 博士

【摘要】 在连铸结晶器内,热—力学行为互相影响相互作用。结晶器传热能力和特征决定铸坯温度场、坯壳凝固厚度及其分布,影响坯壳收缩和结晶器变形等力学行为;反之,铸坯收缩和结晶器变形影响铸坯和结晶器之间的传热。因此,清楚了解结晶器内的热—力学行为必须建立合适的耦合数学模型。 坯壳和结晶器之间的相对运动形成摩擦力,坯壳受摩擦力的影响而发生力学行为的变化。在实际生产过程中,非理想的复杂的结晶器传热状态使摩擦力发生异常变化。方坯、板坯的形状决定了部分坯壳优先生长。圆坯结晶器由于沿各个方向都对称,其生长不均匀性是随机的,由结晶器各处的传热条件决定。因此,理想状态下的模拟计算不能反映实际生产中连铸结晶器内的热—力学状态。 以安装在圆坯结晶器不同横截面和纵截面内的热电偶所获得的温度为基础,建立基于实测温度数据的多维反问题数学模型。通过确定结晶器和铸坯之间的热阻分布,计算出能及时反映实际生产中沿周向不均匀分布的传热。 以结晶器和铸坯的温度场作为热载荷建立三维力学模型。该模型耦合了摩擦力和铸坯/结晶器的接触状态以及它们的应力、变形,并考虑了结晶器锥度的影响。计算出生产过程中圆坯结晶器和铸坯的热—力学行为,并讨论了摩擦力的影响;同时计算出结晶器和铸坯之间的间隙,包括固液渣膜厚度、气隙尺寸和接触状态,并分析它们之间的互相影响关系。由于实测的传热结果不均匀,结晶器和铸坯的热—力学相关计算结果也具有沿周向分布不均匀特征,更加反映了真实的连铸状态。 基于现场实测数据,研究了正常生产过程中圆坯结晶器热流的实时分布特性,从而分析结晶器的传热行为,为达到实时监控提供基础。 监测分析表明:热流沿结晶器周向分布处于频繁变化中。弯月面区传热很低,但在液面下70—110mm区域内存在热流峰值,并且对工艺参数变化较为敏感,比如浇注温度、拉速、钢水碳含量、保护渣类型等等。本文也分析了正常和异常传热条件下局部温度、热流的稳定性和沿周向不均匀性的变化。在稳定拉坯时,一定变化范围内的工艺参数对结晶器传热的波动性和不均匀性的影响很小,而钢水碳含量、保护渣类型以及结晶器安装等很大程度上决定结晶器传热的波动性和沿周向的不均匀性。 研究发现:结晶器热流和坯壳厚度沿周向的分布特征受结晶器安装状态的影响。统计发现,同一次安装其分布相似,不同安装分布规律不同。高热流区热流沿周向分布的更多还原

【Abstract】 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.更多还原