【作者】 郭亮亮；

【导师】 姚曼； 沈厚发；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2009， 博士

【摘要】 动态二冷与动态轻压下是现代连铸工艺提高铸坯质量的一项重要技术。研究动态轻压下模型与控制方法,需要对连铸机辊列结构、铸坯凝固传热以及复杂热—力学问题有深入的认识和理解。本文针对超高强度钢(UHSS)动态轻压下工艺的关键技术,开展实验、设计、建模与控制方面的研究。在铸坯高温力学性能实验与铸机辊列设计的基础上,开发了动态二冷与动态轻压下控制系统。首先,在Gleeble—1500D热/力模拟试验机上进行了实验,获得了超高强度钢高温力学性能参数及塑性曲线;同时分别采用凝固法和加热法获得了高温固相区和两相区的拉伸强度。实验结果表明,拉伸强度随着温度的升高而呈近似线性降低;凝固法的拉伸强度小于加热法的实验值,加热法和凝固法拉伸强度的区别是由不同的变形激活能所致。考虑应变速率、温度、应力、固相分数和变形激活能等参数的粘塑性本构方程为轻压下工艺分析和关键参数确定提供了实验及理论依据。其次,分别采用多点弯曲多点矫直和改进的Concast连续弯曲连续矫直曲线对铸坯断面尺寸为1100mm×180mm的板坯连铸机进行了辊列设计。校核计算结果表明,两种方案均满足设计要求,设计结果合格、可靠,设计方法正确。再次,基于板坯连铸机的辊列和喷嘴布置,建立三维稳态传热数学模型,模型考虑了铸坯与辊子间的接触换热及喷嘴特性。计算的温度和凝固坯壳厚度分别与测温和射钉实验结果相吻合。通过模拟计算为实际生产中的二次冷却工艺提出了有效的控制措施。然后,根据铸坯高温塑性曲线、铸机辊列及二冷区冶金准则,制定了超高强度钢等钢种的表面目标温度。同时,开发了动态二冷瞬态传热数学模型,采用增量型PID算法实现了动态二冷控制。结果表明,动态二冷模型与增量型PID算法能够有效控制冷却回路的目标温度,可以准确及时地预测凝固末端位置。最后,以钢的高温力学性能及本构方程为依据,得出了950～1950mm×170mm板坯超高强度钢及低碳钢的动态轻压下模型参数。采用多线程运行方式开发了板坯动态轻压下控制系统Visual Cast—Dynamic。分别以钢的两相区内临界应变和临界应力作为力学判据,设置了位置控制与压下力控制两种动态轻压下控制方式。本研究制定的轻压下工艺与实际已有同类钢种的轻压下工艺一致,说明本文开发的动态轻压下模型正确,相应的动态轻压下系统已具备在线控制能力。更多还原

【Abstract】 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.更多还原