宽大断面连铸坯凝固末端变形过程溶质偏析行为及压下工艺研究

The macro-segregation of carbon and alloying elements during the continuous casting process afflict the manufacture of homogeneous wide-large section slab or bloom. In order to conquer this problem, the solidification end reduction technology should be established according to the accurate description of the shell deformation, the solute segregation and the formation and propagation of the internal cracks in wide-large section slab casting process. In this study, the theory calculation model and predication model for solute micro-segregation will be established, and the phase transfer and the solute micro-segregation could be predicted quickly and accurately, when the cooling rate and the solute concentration is unsteady. The coupled model of mushy zone deformation and solute segregation will be presented. The transport phenomena and their interaction during wide-large section continuous casting process, such as solidification, deformation, solute diffusion and segregation, could be described exactly, and then the formation and control mechanism of the mushy zone macro-segregation could be derived. The occurring and propagation criterion of internal cracks for wide-large section continuous casting process could be established based on the accurate prediction results of the comprehensive stress-strain distribution between the ZDT and ZST area, while the limitation criterion of solidification end reduction could be derived also. After that, according to the characteristics of solidification end of wide-large section slab and bloom, such as the large reduction amount, the uneven shell growth, and the high rates of the internal cracks, the theory calculation model and technological equipment will be developed for reducing macro-segregation, and the industrial application will be carried on in order to verifying its reasonability and reliability.

严重的宏观偏析已成为生产高致密度、均质化宽大断面连铸坯的最大障碍。准确揭示宽大断面连铸坯凝固末端变形、溶质偏析、裂纹产生与扩展规律，开发凝固末端压下工艺，是解决此问题的关键。鉴于此，本研究拟开发溶质微观偏析理论计算模型和预测模型，实现液相溶质浓度和冷速连续变化过程中溶质微观偏析与相变的准确而快速预测；研究开发宽大断面连铸坯两相区变形与溶质偏析相耦合的计算模型，准确描述压下过程中凝固、变形、溶质宏观传输、溶质微观偏析等宏微观现象及其相互作用规律，从而揭示宏观偏析的形成与控制机理；准确描述凝固前沿ZDT-ZST区间综合应力应变，研究并提出宽大断面连铸坯凝固末端压下过程内裂纹形成与扩展判据，及压下限定准则；在上述研究基础上，针对压下变形量大、非均匀凝固、高内裂风险的具体特点，研究提出适用于宽大断面连铸坯的凝固末端压下参数理论计算模型与工艺装备技术，并通过工业应用研究，验证工艺的合理性与可靠性。

宽大断面连铸坯严重的中心偏析及缩孔、疏松等内部质量缺陷制约了其品质化、高效化、稳定化制备。目前，凝固末端机压下是改善上述内部质量缺陷的有效技术手段之一。而系统研究宽大断面连铸坯压下过程中传热、变形、传质等行为规律是工艺及装备开发的关键基础，具有十分重要的现实意义与使用价值。.本项目基于高温热压缩实验，准确测定了不同应变速率及温度条件的铸坯金属流变规律，并据此开发了基于晶体学参数修正的重压下宽尺度本构方程。结果表明，该方程可准确描述铸坯压下过程金属流变规律，平均相对误差小于3.7%；.建立了非均匀冷却条件下宽厚板坯鼓肚变形及热收缩三维热/力耦合模型，揭示了其宽厚板坯坯壳鼓肚及热收缩变形规律。结果表明，其鼓肚及热收缩具有明显非均匀特征，且鼓肚引发的凝固前沿张应变呈集中分布趋势。此外，针对宽厚板坯的热收缩非均匀特征，开发了加权收缩辊缝计算方法；.开发了宽大断面凝固末端压下过程三维热/力耦合模型，揭示了复杂工况条件下的铸坯变形及设备受力特征。结果表明：重压下变形可有效传递至铸坯心部，提升铸坯致密度；渐变曲率凸型辊可有效降低系统压力；差异拉矫力条件下，铸坯处于挤压状态可提高压下效率；.开发了凝固末端压下过程变形-流动-传热-传质多场耦合模型，阐明了凝固末端压下过程钢液流动及溶质偏析迁移规律。结果表明：凝固末端机械压下可以减缓溶质富集液相向凝固终点的流动速度，有效降低中心偏析，最佳压下区间的铸坯中心液相率为0.95~0.01；溶质元素偏析分布将明显影响铸流凝固终点位置。.结合工业试验及高温热模拟实验，准确揭示了宽厚板坯重压下过程“温度-变形率-应变速率-初始晶粒尺寸”交互作用下的奥氏体动态再结晶与微观组织演变规律，并开发了重压下过程奥氏体动态再结晶动力学模型与晶粒预测模型。结果表明，凝固末端重压下能够诱导奥氏体再结晶的发生，从而有效细化铸坯组织，为重压下连铸坯铸轧一体化组织协同调控研究奠定了重要基础。