亚包晶微合金钢表面裂纹缺陷形成及调控机制

The purpose of this project is to solve the problem of longitudinal cracks and austenite grain boundary cracks of hypo-peritectic microalloyed steel by investigating the composition and temperature control strategy for preventing the formation of surface cracks defects in the process of continuous casting. The core idea is to explore the formation mechanism of longitudinal cracks and austenite grain boundary cracks by using the surface roughness to characterize the shrinkage degree of peritectic transformation. Therefore, the whole solidification process of continuous casting from molten steel to strand will be simulated by high temperature confocal microscopy. The observation and analysis methods will be carried out by OM (optical microscope), SEM (scanning electron microscope), and TEM (transmission electron microscope). The influence of solute content on the shrinkage of peritectic transformation and prior austenite grain size under the cooling condition of the mold will be studied. The relationship between the initial austenite grain size and the ferrite dendrites’ interface and size at high temperature will be revealed. The relationship between the high temperature microstructure (austenite grain uniformity and the precipitation behavior of micro alloying element) and the cracks occurrence along the austenite grain boundary will be improved, and the cooling technology of the high plasticity casting structure can be obtained. On basis of these, the quantitative relationship among the composition and temperature control strategy of hypo-peritectic microalloyed steel and the shrinkage degree of peritectic transformation, and the high plastic structure of strand will be established.

本项目的目标是探索连铸过程中防止亚包晶微合金钢表面裂纹缺陷形成的成分和温度控制策略，以解决这类钢一直困扰的铸坯表面纵裂纹和奥氏体晶界微裂纹难题。核心思想是利用表面粗糙度表征包晶转变的收缩程度，以探索纵裂纹和沿奥氏体晶界裂纹形成机制。为此，项目利用高温共聚焦显微镜模拟连铸从钢液到铸坯的整个凝固过程，借助OM（光学显微镜）、SEM（扫描电镜）、TEM（透射电镜）等观察和分析手段，研究结晶器内冷却条件下钢中溶质含量对包晶相变收缩和原始奥氏体晶粒尺寸的影响规律；揭示包晶转变时初始奥氏体晶粒尺寸与高温铁素体枝晶界面和尺寸的对应关系；完善铸坯高温组织（奥氏体晶粒均匀度和微合金元素析出行为）与沿奥氏体晶界裂纹发生率的关系，并寻求获得高塑性铸态组织的冷却工艺。在此基础上，建立起亚包晶微合金钢成分和温度与包晶转变收缩程度以及铸坯表层具有高塑性组织的定量关系。

表面裂纹是包晶钢连铸坯的一种常见缺陷，裂纹的产生与钢初始凝固过程中的包晶转变收缩程度密切相关。针对钢的初始凝固收缩，目前研究主要考虑成分变化的影响，但从实际浇铸过程来看，钢水凝固处于非平衡状态，冷却速率同样会对钢的包晶转变收缩及裂纹形成产生影响。为此，本项目从钢的包晶转变收缩表征方法入手，通过对相变收缩褶皱、表面粗糙度及收缩应变指数的分析，提出了表征包晶转变收缩的新方法。研究了包晶钢成分和热历史（冷却速率）对表面粗糙度的影响规律以及与连铸坯裂纹敏感性的关系。建立了铸坯高温组织（原始奥氏体尺寸和混晶指数）与沿奥氏体晶界裂纹发生率的关系。主要结论如下：1）利用高温激光共聚焦显微镜模拟钢的初期凝固过程，发现包晶相变能够明显的增加奥氏体晶粒表面粗糙度，表面粗糙度与包晶相变收缩程度具有相关性。2）包晶转变收缩，即表面粗糙度随冷却速率的增加呈现先增大后减小的规律。这一规律为控制包晶钢连铸坯表面裂纹发生率提供了两种思路：一种是目前传统连铸广泛采用的缓冷策略，即通过减小初始冷却速率降低钢的裂纹敏感性；另一种则是通过大幅提高钢凝固初期的冷却速率来抑制裂纹的产生，这表明薄带连铸工艺具有浇铸包晶钢的潜力。3）建立了表面粗糙度与连铸坯裂纹敏感性的定量关系。表面粗糙度值大小分为三类：表面粗糙度小于22㎛，钢的裂纹敏感性较弱；表面粗糙度介于22㎛和28㎛之间，钢的裂纹敏感性一般；粗糙度大于28㎛，钢的裂纹敏感性较强。4）以铸态奥氏体晶粒尺寸和混晶指数为判据，建立了连铸坯奥氏体晶界裂纹预测模型，模型准确率由现有的64%提高到97%。本项目研究工作和结果为寻求通过成分和温度控制实现解决包晶钢表面裂纹缺陷打下了基础。