高铝钢连铸坯表面纵裂纹形成机理及控制

A new generation of high-Al ferritic lightweight steel (δ-TRIP) is of great importance to lightweighting of automobile. Because of high Al content and δ phase existing throughout the process of casting, the continuous casting slabs of δ-TRIP have a more complex problem of longitudinal surface cracks compared with traditional high Al steels. However, the studies on continuous casting of δ-TRIP are very limited, which hinders its massive production. In this project, in-depth and systematic studies on the formation mechanism of longitudinal surface cracks of δ-TRIP steel slabs will be carried out through SSCT, IMC-B, multi-field coupled numerical simulation and other methods, based on thermodynamic calculation and experimental determination. Firstly, the peritectic reaction characteristics, high temperature mechanical properties and solidified microstructure of δ-TRIP steel with 3-7% Al will be determined; the critical strain for longitudinal cracks will be analyzed and an evaluation model will be established. Secondly, the composition range will be determined after CaO-Al2O3-based mold flux reacts with δ-TRIP steel, and the effect of the melting, viscous flow and crystallization characteristics on CaO-Al2O3-based mold flux’s heat transfer and lubrication will also be investigated. Thirdly, a molten-slab-slag tandem numerical model and an interface friction model in mold will be established to analyze the solidification shell growth and friction behavior; the critical conditions for longitudinal cracks and their control mechanism will be put forward by the combinative analysis of the cracking sensitivity of δ-TRIP steel, and the parameters of mold flux and casting. As a result, this project will provide theoretical guidance for controlling longitudinal surface cracks of continuous casting slabs of high-Al δ-TRIP steel and enrich the researches on solidification theory for high Al steels.

新一代高铝铁素体轻质钢（δ-TRIP）对汽车轻量化具有重要意义。由于高Al含量与凝固过程中始终存在的δ相，使连铸坯表面纵裂纹的形成问题比传统高铝钢更为复杂，但目前关于δ-TRIP钢连铸方面的研究较为缺乏，难以支撑δ-TRIP的规模化生产。本项目拟在热力学计算、物性测定的基础上，结合SSCT、IMC-B和多场耦合数值模拟等方法，对δ-TRIP钢连铸坯表面纵裂纹形成机理进行系统深入研究。主要包括：阐明3-7%Al含量δ-TRIP钢的包晶反应特性、高温力学性能和凝固组织对裂纹敏感性的影响；探明CaO-Al2O3基保护渣在连铸过程中物化性质的变化对δ-TRIP钢凝固坯壳不均匀生长的影响机制；建立钢液-坯壳-液渣串联模型与界面摩擦力数学模型，明确凝固坯壳生长和摩擦行为，系统表征纵裂纹形成的临界条件并提出调控机制。本项目将为δ-TRIP钢连铸坯表面纵裂纹的控制提供理论基础，也将丰富高铝钢凝固理论研究。

高铝δ-TRIP钢具有低密度和良好的强塑性，是理想的汽车轻量化材料。高铝钢连铸坯表面纵裂纹与钢种的裂纹敏感性、保护渣物性变化及结晶器内初生凝固坯壳的均匀生长紧密相关。本项目采用FactSage、Thermo-Calc计算、LSCM原位观察、DSC分析、Gleeble热模拟等方法研究了不同Al含量对δ-TRIP钢裂纹敏感性的影响。结果表明：Al含量对δ-TRIP钢裂纹敏感性影响最大，随着Al含量的增加，δ-TRIP钢包晶反应区域增大，脆性温度区间增加，在第一脆性区内裂纹敏感性增强，第三脆性区变窄、裂纹敏感性减弱，δ相使第一脆性区的裂纹敏感性增强、第三脆性区的裂纹敏感性减弱；提出裂纹指数判据，当应变量大于临界应变量时铸坯产生裂纹。采用FactSage计算、高温静态试验、双膜理论分析、DSC、XRD、拉曼光谱分析等方法研究了CaO-Al2O3基保护渣与δ-TRIP钢的反应性及其对保护渣物化性质的影响。结果表明：保护渣的主要成分在反应初始10min内变化快；明确了反应性对保护渣熔化、结晶度、结晶相类型、尺寸、分布和形貌、熔渣结构以及粘度变化的影响规律。采用数值模拟方法研究了δ-TRIP钢连铸过程的流动、传热和摩擦力分布行为，建立了钢液-坯壳-保护渣模型，计算了坯壳表面受到的摩擦力分布。结果表明：钢种、保护渣及结晶器工艺等因素会影响到坯壳表面温度和摩擦力分布，可从降低铝含量，使用凝固温度较低、粘度较小的保护渣，减小拉速，调整宽/窄面热流比值等方面控制纵裂纹的产生。上述研究初步揭示了高铝δ-TRIP钢铸坯表面纵裂纹的形成机理与调控机制，为新一代高铝低密度钢铸坯表面纵裂纹的深入研究与有效控制提供了理论基础。