亚包晶钢Q345B连铸大方坯表面裂纹原因分析

Analysis on the Formation of Surface Crack on Casting Bloom of Hypo-peritectic Steel Q345B

针对国内某钢厂利用电炉-LF-VD流程生产的亚包晶钢Q345B铸坯及轧材表面存在大量缺陷的现状，利用光学显微镜、扫描电镜以及能谱分析仪对铸坯表面裂纹进行了分析，并对析出物进行热力学计算，以此来分析出该钢种的铸坯产生裂纹的原因。研究结果表明：裂纹附近存在脱碳层及铁索体膜；裂纹处发现P、S富集及AI、Mg、Ca、Si等元素的夹杂物破坏了钢基体的连续性；保护渣及析出物对铸坯表面质量影响严重。为了防止缺陷的发生，应将钢水过热度控制在15～30℃；将结晶器中液渣层控制在8．5～9mm并提高析晶相比率。同时，将钢水中氮含量稳定控制在35×10-～45×10-6，并且适当提高Ti处理效率，降低A1N数量，对改善连铸坯表面质量有着很大的作用。

Large amounts of surface cracks were found on the 300 mm 360 mm continuous casting blooms for hypoperitectic steel produced by EAF ＋ LF ＋ VD processes after cooling. The cracks were an alyzed by optical microscopy, SEM and EDS, and thermo dynamical calculation was carried out at the same time in order to find out the reasons for formation of the cracks. The results indicate that decarburi zed layer and ferrite film are found near the cracks and enrichment of P and S found at the cracks; inclu sions containing elements of A1, Mg, Ca and Si wreck continuity of the steel base ; mold flux and precipi tates have significant effects on surface quality of the casting bloom. In order to protect casting bloom from crack forming, it is necessary to control the superheat of steel between 15 and 30 ℃ maintain the liquid powder layer between 8.5 and 9 mm, and increase the crystallization rate of mold powder. Keeping the N content of the steel at 35 x 10 6 to 45 x 10 6 and increasing the Ti treatment effeciency to decrease the amount of AIN in bloom also contributes to improve the quality of the casting bloom.