连铸中(Ti,Nb,V)(C,N)多相异质析出动力学机理及其控制机制研究

The formation of nano-sized (Ti,Nb,V)(C,N) in the microalloyed steel mainly depends on the steel composition and temperature history, and greatly affects its properties during the continuous casting process. However, most of these carbonitrides prefer to precipitate out on the grain boundaries, dislocations or surfaces of existing precipitated particles heterogeneously, which causes very complex kinetic mechanisms. Moreover, the complicated plant conditions often make it impossible to observe and quantify the evolution and effect of carbonitrides inside the casting slabs. In order to solve these problems, based on studying the kinetic mechanisms of heterogeneous nucleation, growth and coarsening of carbonitrides at the different crystal defects in the isothermal and approximate continuous casting process by a combination of physical experiment and numerical simulation, the governing equations and a comprehensive set of numerical models will be developed to describe the multiphase heterogeneous precipitation behaviors of (Ti,Nb,V)(C,N). They includes a heat transfer model, a steel-phase transition model, a segregation model, a thermodynamic model, a non-equilibrium precipitation kinetic model, a grain growth model and a stress analysis model. Computational models will be combined together to calculate the evolution of (Ti,Nb,V)(C,N) composition，amount and size and their influence on the strand properties at the different locations within the casting slabs for the different steel compositions and casting conditions. The results from this project will not only establish a theoretical basis for accurately describing the evolution of the carbonitrides and the related slab properties, but also provide a scientific insight into designing steel grades and technical conditions to produce high-performance steel and realize the intelligent integration in the continuous casting.

微合金钢连铸中纳米级(Ti,Nb,V)(C,N)的形成依赖于钢的成分和温度历程，并对铸坯性能影响显著。然而，这些碳氮化物多在晶界、位错或其它析出表面处异质析出，其动力学机理复杂，且由于连铸现场的复杂性使得其演化和作用过程难以被直接观测和量化。针对上述现状，本项目以连铸中的(Ti,Nb,V)(C,N)作为研究对象，通过物理实验结合数值模拟研究等温和近似连铸中碳氮化物在各晶体缺陷处的多相异质形核、长大和粗化的动力学机理，构建相应的动力学控制方程和数值模型。在此基础上，组合传热、钢相变、偏析、析出热力学和动力学、晶粒生长以及应力分析等多关联模型研究钢材成分和连铸条件等对于铸坯内不同位置碳氮化物析出成分、数量和尺寸演化以及对铸坯性能的影响。本研究不仅能为精确描述连铸中碳氮化物析出和铸坯性能的演化等奠定理论基础，而且可以通过设计钢种和连铸工艺控制析出以制备高性能钢和实现连铸一体化智能等提供科学依据。

微合金钢连铸中纳米级(Ti,Nb,V)(C,N)的形成依赖于钢的成分和温度历程，并对铸坯性能影响显著。这些碳氮化物多在晶界、位错或其它析出表面处异质析出，其动力学机理复杂。针对上述现状，本项目以连铸中的(Ti,Nb,V)(C,N)作为研究对象，通过物理实验结合数值模拟研究等温和近似连铸中碳氮化物在各晶体缺陷处的多相异质形核、长大和粗化的动力学机理，构建相应的动力学控制方程和数值模型。首先，通过在生铁中添加微量钛铁、铌铁和钒铁，在真空感应炉中成功制备出含有不同钛、铌和钒含量的微合金钢。然后，对冶炼出的不同钢样进行了差示扫描量热法分析，并与析出热力学计算进行了对比。对各种钢样在不同保温温度和时间的等温析出进行了激光共聚焦显微镜的观测。再根据传热模型计算了工业用板坯连铸机不同连铸速度下的典型铸坯表面和温度曲线，合理化近似后输入系统用激光共聚焦显微镜对样品进行了实时观测。用扫描电镜和透射电镜技术对所有测试样品中不同的析出颗粒进行了成分测试和尺寸统计。实验结果证明了析出热力学平衡态计算的准确性，通过激光共聚焦显微镜观测到的样品表面浮凸，间接表征了(Ti,Nb,V)(C,N)析出逐步出现在晶粒内部、位错或晶界处，其测量尺寸为纳米级别，远小于样品中原存在的微米级氧化铝夹杂。在钢样的模拟连铸过程中，铸坯表面的(Ti,Nb,V)(C,N)被证实均能在二冷区早期就出现，而中心处的(Ti,Nb,V)(C,N)几乎只会在加热或冷却过程中形成。同时，我们对现有的团簇动力学析出模型进行了改进，提出的组内团簇密度对数线性分布的假设提高了分组法的准确性。最终，我们通过组合传热、钢相变、析出热力学和动力学等多关联模型，建立了针对给定钢材成分和连铸条件模拟铸坯内不同位置(Ti,Nb,V)(C,N)析出成分、数量和尺寸演化的程序化方法。本研究的结果不仅为精确描述连铸中碳氮化物析出和铸坯性能的演化等奠定理论基础，而且为设计钢种和连铸工艺控制析出以实现高效连铸和制备高性能钢等提供了科学依据。