高品质钢连铸坯凝固组织与碳偏析的形貌本构关系研究

Macro/Semi-Macro carbon segregation play a key role for improving the continuously cast product quality. Based on macrostructure qualitative rating comparison and element macro content analysis, the segregation extent has been controlled at different levels by the existing technologies, but there is an obvious shortcoming on segregation morphology. Nowadays, delicacy control is demanded for higher quality requirement. Therefore, morphologies of solidification structure and carbon segregation in continuously cast billet and slab will be investigated in this project for filling the study of short board. Firstly, fractal theory is introduced for multidimensional and multiscale quantitative describing different locations in the transverse and longitudinal macrostructure in terms of number, length, area, body shape and interface outline, from single solidification structure and segregation zone, surface solidification structure cluster and segregation point set to stereoscopic solidification structure and segregation unit based on transverse, vertical and oblique sections. Secondly, from the "output" macrostructure, the "black box" method is used to analyze the liquid flow influences on the solidification structure and segregation morphology by comparing the actual billet or slab, the hot simulation and the numerical calculation. Finally, the morphology constitutive relation between solidification structure and carbon segregation is studied on the basis of the corresponding dimensionless number equation. On the whole, by this project, the theory on the relationship between solidification structure and segregation will be enriched, and a new theoretical basis will be supplied for delicacy control of segregation like defects in continuous casting process in the future.

宏观/半宏观碳偏析是困扰目前高品质钢连铸坯质量提升的关键方面。虽然已有方法能不同程度的减少其发生，但前人工作普遍对偏析等形貌特征描述不足；然而，目前铸坯质量精细化的控制要求又不断提高。因此，本项目将基于分形理论，以连铸方坯与板坯中凝固组织与碳偏析的形貌特征以及二者本构关系为对象，补齐研究短板。首先，引入分形理论，按照单个凝固组织及偏析区、表面凝固组织簇及偏析点集、不同横纵斜截面构成的立体凝固组织与偏析单元的顺序，从数目、长度、面积、主体形状与界面轮廓五个方面对实际铸坯横纵断面多个位置的形貌实现多维多尺度定量描述；然后，从“输出”铸坯着手，采用“黑箱”方法，交叉对比实际铸坯、热态模拟及数值结果，以此逆向分析液相流动复杂行为对形貌的唯象影响规律；最后，利用无量纲准数方程，综合研究凝固组织与偏析的形貌本构关系。研究成果将丰富并创新凝固组织与偏析关系理论，对精细化控制偏析等缺陷具有重要的科学意义。

为实现铸坯偏析缺陷的精细化控制，弥补传统凝固组织与碳偏析形貌表征方法的不足，本项目基于分形理论，对连铸坯凝固组织与偏析形貌特征进行定量化描述并构建二者的形貌本构关系模型。首先，引入分形维数对不同钢种（82B、70#帘线钢、H13模具钢及GCr15轴承钢等）连铸坯的凝固组织与偏析形貌进行多维多尺度定量描述，确定了分形理论定量描述连铸坯凝固组织与碳偏析形貌特征的计算方法及其有效性和普适性，对凝固组织形貌的分形特征也进行了多维描述，发现了枝晶生长形貌复杂化现象，同时分析了工艺参数对凝固组织分形等形貌特征的影响。然后，对组织生长进行了热态模拟，研究了其形貌的分形特征；同时利用相场法对组织形貌的演变进行了数值模拟，研究了过热度和冷却速率等对凝固组织形貌演变过程中分形维数、固-液界面前沿溶质扩散层厚度及偏析等方面的影响；并通过实际连铸坯与热态模拟及数值模拟组织形貌特征的对比研究了液相流动对组织的影响。最后，通过对组织形貌特征的分析与枝晶结构渗透率的计算分析，提出了偏析控制策略，并构建了凝固组织形貌特征、偏析形貌特征与铸坯偏析情况的本构定量关系。总体而言，本项目实现了利用分形理论对连铸坯中凝固组织与碳偏析形貌多维多尺度特征的有效定量表达且丰富完善组织形貌的定量描述方法，同时从形貌角度研究了凝固参数、液相流动等对组织的影响规律且提出了控制策略，并最终构建了凝固组织与碳偏析形貌本构关系模型，为高品质钢偏析缺陷的稳定控制以及对应新技术的开发提供一个新的理论参考。