基于流变过程的高温铸坯内部裂纹形成机理及动态行为

The cracks of bloom was one of the key factors affecting the quality of steel. Different levels of cracks were found in bloom during soft-reduction and heavy-reduction process, which was a difficult problem limited uses of the technology. In this project, medium-high carbon steel was used as research object. Internal cracks of bloom would be researched during its rheological process. Several research methods were used in this project such as hot reduction test, properties test of tensile and squeezing, test of fracture, macroscopic and microscopic analysis, and so on. Firstly, forming mechanism of internal cracks with microstructure evolution was explored by research of hot reduction test. Secondly, characters of grain boundary precipitate and grain size distribution were researched in rheological process, and then the mechanism of generation and extending of cracks in grain boundary was deduced. Thirdly, interactive theory of center segregation, center porosity and internal cracks was explored based on segregation elements impacts on the first brittle temperature range and mechanical analysis of porosity. Finally, the model of sensitive zone of crack was formulated through analyzing high temperature critical strength and calculating field of stress in blooms. This proposal would reveal the mechanism of dynamic behavior of generation, welding and extending of internal cracks in rheologic bloom of high temperature. The outcome of this proposal would develop the guideline for optimizing soft-reduction and heavy-reduction and improving the quality of bloom.

连铸坯裂纹一直是影响其质量的关键因素之一，在轻压下及重压下过程中铸坯内部易形成不同程度的裂纹，成为制约该技术的一个难点。本项目以中高碳钢为研究对象，采用高温物理模拟压下试验等手段，结合拉伸、挤压等力学性能测试和材料断裂等实验分析，运用微观及宏观分析方法，开展铸坯流变过程内部裂纹的研究。首先通过高温压下试验研究铸坯流变过程凝固组织的演变，探索组织演变过程内部裂纹的形成机制；其次通过压下及断裂试验研究铸坯流变过程晶界析出物及晶粒分布特征，推导压下过程晶界断裂及扩展机制；再次通过定量分析偏析元素对第I脆性区的影响，结合铸坯缩孔力学分析，探寻中心偏析、中心疏松与内部裂纹的交互作用原理；最后通过铸坯高温临界强度分析和应力场计算，建立内部裂纹敏感区模型。通过本项目研究，将揭示铸坯压下流变过程内部裂纹的产生、焊合及扩展等动态行为机理，为优化轻压下及重压下工艺和提高铸坯质量提供科学依据。

本项目针对连铸坯在轻压下及重压下过程中铸坯内部裂纹的形成机理及动态行为展开了研究，采用了高温物理模拟压下试验等手段，结合拉伸、挤压等力学性能测试和材料断裂等实验分析，从微观和宏观不同角度分析了铸坯流变过程内部裂纹的形成机制。首先通过高温压下试验研究了铸坯流变过程凝固组织的演变，开展了目标钢种热物性参数测定、高温铸坯本构关系模型、铸坯流变过程组织演变规律研究、铸坯凝固组织演变与裂纹形成机制研究等。然后通过实验室铸模模拟压下试验，进行了压下量为1mm-6mm和无压下量压下及断裂试验，结合金属原位分析、EBSD和XCT等检测分析手段，研究了铸坯流变过程凝固组织演变规律，推导了压下过程晶界断裂及扩展机制；同时通过定量分析偏析元素对铸坯凝固过程中脆性区的影响，深入研究了流变过程中心偏析-中心疏松-内部裂纹的交互作用机理；最后开展了铸坯高温临界强度分析和应力场计算，深入分析了铸坯流变过程内部裂纹形成机理及动态行为，揭示了铸坯压下流变过程内部裂纹的产生、焊合及扩展等动态行为机理，为优化连铸生产工艺和提高铸坯质量提供了科学理论支撑。在研究后期，将研究成果应用于连铸生产实践，在工业现场进行了研究结果的应用性验证，取得了较好的成效，在后续工作中将继续为成果的应用开展研究，为实际生产提供有力支撑。