实现低轧制压缩比制备特厚板的厚板坯连铸重压下应用基础研究

Heavy reduction (HR) is an effective method to improve the homogeneity and compactness of the thick continuous casting slab, and therefore ensure the quality of the extra-thick steel plate with lower rolling ratio. With considering the specific characteristics of HR process such as the lager temperature gradient, higher strain rate and nonuniform solidification structure of the thick slab, the mechanism and new phenomena, such as the healing of solidification shrinkage cavity, the initiation and propagation of cracks, improvement of macrosegregation, the austenite dynamic recrystallization, the strain induced precipitation of microalloy carbonitride, and the microstructure evolution, would be investigated and revealed systematically under the comprehensive effects of HR, electromagnetic stirring (EMS), secondary cooling (SC), and hot charging (HC) process. Combining with the industrial tests, the effects of HR process on the microstructure, performance, and the lowest rolling ratio of the final rolled extra-thick steel plate would be investigated, and the new collaborative process and key equipment of HR-EMS-SC-HC would be developed to achieve the extra-thick steel plate manufacturing with lower rolling ratio. The corresponding assessment standard of homogeneity and compactness for the thick continuously cast slab would be established to estimate the rolling ratio for the extra-thick steel plate. Consequently, the present research could settle the theoretical and the technological foundations of highly efficient and low-cost production of extra-thick steel plate, and the continuous casting theory could be expand developed also.

厚板坯凝固末端重压下是全面提升其致密度与均质度的连铸新技术，也是突破低压缩比直接轧制特厚板的重要手段和关键环节。本项目针对厚板坯重压下大变形过程温度跨度大、应变速率高、组织差异明显的特点，深入系统研究并阐明压下、电搅、冷却、装送全流程多外场协同作用下的厚板坯缩孔焊合与疏松改善机制，裂纹萌生与演变机制，溶质偏析与均质度提升机制，奥氏体动态再结晶与微合金碳氮化物形变诱导析出下的组织演变机制等一系列新现象、新规律；结合工业试验，揭示厚板坯重压下工艺对其终轧组织细化、性能提升的影响规律及其对实现保性能、保探伤特厚板低压缩比轧制的作用机制，研发形成可实现低压缩比轧制特厚板的“压下-电搅-冷却-装送”协同调控新工艺及关键装备控制技术，并建立相应的连铸厚板坯综合质量评价体系，奠定我国微合金特厚板高效、低成本制造的技术基础，丰富发展现代连铸理论。

特厚板广泛于海洋工程、交通运输、国防军工等重大工程与重大装备的关键承压、承重、耐磨部件制造，是实现中国制造、海洋强国、能源安全等国家战略的重要保障。然而，连铸坯中心偏析与疏松等凝固缺陷是制约特厚板的成材率、生产效率及服役稳定性的共性技术难题。在铸坯凝固末端施加大变形压下(即重压下)是解决此问题的创新手段，但因一直未能准确阐明压下过程冶金学行为规律，严重制约了其可靠性与稳定性。鉴于此，本项目系统研究了重压下过程传热变形、溶质传输、缩孔闭合、裂纹萌生扩展风险等冶金学行为规律，阐明了重压下提升连铸坯致密度与均质度的工艺本质，为重压下技术的研发与推广应用奠定了重要理论研究基础。主要研究内容与进展如下：.1. 采用压缩法替代常规拉伸法，系统研究了高温两相、高应变速率等重压下特殊条件下连铸坯金属流变行为规律，建立了基于晶体学参数修正的重压下宽尺度本构模型。.2. 系统研究揭示了连铸坯在辊压力、拉坯力、热应力等综合作用下的变形规律，建立了重压下过程铸坯中心缩孔闭合度理论计算模型，确立了最佳压下区域为铸坯完全凝固后≤5.0m、压下率≥4mm/m。.3. 结合重压下过程铸坯变形特征，研究提出了表层裂纹扩展与中间裂纹萌生临界应变测定方法，确立了中低固相区内压下率≤3mm/m、整个压下过程单道次压下量≤15mm的裂纹风险防制准则。.4. 建立了耦合电磁搅拌、压下、溶质传输、凝固组织生长的连铸多相凝固模型，准确描述了非对称坯壳鼓肚下溶质传输的新特征，阐明了凝固末端大变形压下量(中心固相率fs≥0.9，压下率≥3mm/m)可抑制溶质加速富集的新机制。.5. 建立了连铸重压下过程中奥氏体再结晶动力学模型，阐明了“变形温度-应变速率-初始晶粒尺寸-过渡时间”多因素下的奥氏体再结晶软化行为、碳氮化物析出特征及组织演变规律。.6. 开展了重压下铸坯在加热、轧制过程组织性能演变规律研究及工业试验研究，建立了不同装送工艺下奥氏体晶粒长大模型，揭示了初始组织与织构对轧材力学性能影响规律，形成了“压下-电搅-冷却-装送”协同调控新工艺及关键装备技术，并在鞍钢、河钢等企业推广应用。