连铸连轧卷板边部线状缺陷的形成机制

摘要：边部线状缺陷是热轧卷板最为常见的缺陷之一，与连铸坯缺陷、轧制工艺与装备等因素有关，难以判定产生的关键工序，严重影响了热轧卷板表面质量的有效控制。为了探明其形成机制，采用扫描电镜、金相显微镜及酸洗等手段表征了某公司Q355、Q235钢热轧板的典型线状缺陷及铸坯边角部裂纹特征，结果表明：线状缺陷表面及横截面上均含有Ca、Si及连铸保护渣特有的K、Na成分，横截面上裂纹两侧存在氧化铁/夹杂物层、较厚的氧化圆点层以及脱碳与晶粒长大等高温氧化特征，同时Q235连铸坯存在角部横裂纹，因此，连铸结晶器保护渣卷入形成的夹渣/皮下夹杂是热轧卷板线状缺陷的主要诱因；而线状缺陷的表面宽度、横截面上裂纹延伸的深度及其倾斜角度的差异，主要与轧制成品厚度与角部横裂纹的深度有关。控制线状缺陷的关键在于优化连铸结晶器液面控制、保护渣黏度及二次冷却工艺，降低连铸坯边角部的皮下夹杂与裂纹的发生率。

Formation mechanism of linear defects on edge of continuous casting and rolling coil

Edge linear defects in hot rolled coils are related to defects in continuous casting slab, rolling process and equipment, etc., resulting in difficulties in determining defect origin process and effective control of the edge linear defects. In order to ascertain the formation mechanism of linear defects, scanning electron microscope, optical microscope and pickling were used to characterize the typical linear defects and characteristics of cracks in slab edges and corners of the hot-rolled sheet of a company′s Q355 and Q235 steel hot-rolled plates. The results show that the surface and cross section of the linear defect contain Ca, Si, and K and Na components in mold flux of continuous casting, with iron oxide/inclusion layers, thick oxide dot layers and high temperature oxidation characteristics such as decarburization and grain growth, in addition, Q235 continuous casting slab has corner cracks. Therefore, the mold flux inclusions/subcutaneous inclusions of slabs formed in continuous casting mold are the main causes of linear defects in hot-rolled coils;and the differences of these defect characteristics such as surface width, crack extension depth, and inclination angle of linear defects, mainly relate to the thickness of the rolled product and the depth of the transverse cracks at the corners. The key countermeasures of controlling linear defects in hot-rolled coils is to optimize the liquid steel level control of the continuous casting mold, the viscosity of the mold flux, and the secondary cooling process, in order to reduce the incidence of subcutaneous inclusions and cracks at the corners of the continuous casting slab.