连铸坯纵裂诱导机制及其数值模拟和热态模拟

厚板坯连铸生产中常出现纵裂缺陷，原因是坯壳应力集中，坯壳中心薄的地方受到拉应力超过一次晶界的抗拉强度，先在结晶器内部形成微小裂缝，进入二冷区后扩展成纵裂纹。但该机制缺少定量判据，也缺乏钢种、温度、应力等要素诱导裂纹的机制分析。本申请拟用数值模拟和热态模拟相结合的方法研究纵裂诱导机制。主要工作包括：⑴通过实验测量及理论推导建立裂纹敏感钢种的高温力学性能资料库；⑵开发涉及固、液、两相区三态的基于黏弹塑性本构的三维热力耦合数值模型、板坯纵裂诱导数值模型；⑶利用自主研制的"线收缩-热应力联测仪"来热态模拟钢从液态开始凝固的过程，再现铸坯在不同温度范围受力产生变形，进而出现纵裂的热-力历程，获得临界数据，并与轻压下试验结合验证数值模型的准确性及精度。⑷数值仿真典型钢种在连铸机历经的过程，得到铸坯在高温、中温、低温脆性区，在不同外力的诱导下产生纵裂的临界判据，完善纵裂机制的定量认识,避免纵裂产生。

The longitudinal cracks frequently occur on the surface of the slab in continuous casting process. It is generally accepted that the formation of the cracks is due to the stress concentration on the thin solidified shell. When the tension of stress exceeds the tensile strength of primary dendritic interfaces, the fine cracks may appear on the slab surface in the mold and expand in the secondary cooling zone, which finally leads to the formation of the longitudinal cracks. However, the above induction mechanism lacks a quantitative numerical criterion. Very little information is available to quantitatively analyze the effect of these factors such as steel grade, the temperature, the stress on the formation of the longitudinal surface cracks. In the present study, the numerical simulation and the hot-state simulation are used to investigate the induced mechanism of longitudinal cracks. Firstly, the high-temperature mechanical properties database for the cracking sensitive steel grades is established by the experimational test and theoretical deduction. Combined with the experimental data, a 3D thermo-machnical model based on the constitutive theory of viscoelastic-plasticity considering the solid phase, liquid phase and fuzzy zone of the steel are developed to clarify the formation mechanish of longitudinal cracks. Furthermore, the independent developed "linear shrinkage-thermal stress" coupled instruments are used to perform the hot-state testing to simulate the solidification process and to represent the deformation of the slab in the different temperature range and track the thermal-mechanical history which can lead to the occurrence of the cracks. The obtained results together with the data from the soft reduction industrial test are used to verify the accuracy of the mathematical model. Finally after the verification, the numerical model is used to obtain the quantitative criterion of the occurrence of the longitudinal surface cracks under different operating conditions by simulating the thermal-mechanical history of the cast slab in the high-temperature, middle-temperature and low-temperature brittleness zone.