大方坯含硫钢表面纵裂纹成因分析与控制

针对大方坯含硫钢表面纵裂纹问题,对裂纹缺陷进行了金相、能谱观察,发现纵裂纹在铸坯出结晶器后发生;通过高温热塑性测试和组织析出区间的确定,分析了硫加入对钢的高温热塑性和组织析出区间的影响,确认在900 ℃以下第三脆性区间,奥氏体晶界形成的先共析铁素体弱化了晶界强度,在热应力、组织应力、机械矫直应力的作用下发生晶界开裂是导致含硫钢表面纵裂纹的根本原因。在此基础上开展了保护渣优化、二冷区改造、铸坯加热和堆冷过程保温等工艺研究。研究结果表明,提高保护渣碱度、降低保护渣熔点和黏度、延长二冷区长度、规范加热和堆冷工艺,大方坯含硫钢铸坯表面纵裂纹得到了较好控制,因纵裂纹导致的圆钢钢质原因修磨率从20%降低到5%以下。

Analysis and control over formation cause of surface longitudinal crack for continuous casting bloom of sulfur bearing steel

In view of the problem of surface longitudinal crack of sulfur-containing steel in bloom, metallographic and energy spectrum observations were made on the crack defects, and it was found that the longitudinal crack occurred after casting bloom out of mold. Through high temperature thermoplastic test determine the range of precipitation and organization, and analyses the influence of sulfur in steel high temperature thermoplastic and organizational precipitation range, the influence of the identified under 900 ℃ the third brittle interval, the formation of austenite grain boundary proeutectoid ferrite weaken the intensity of grain boundary, the thermal stress and organization stress, mechanical straightening stress under the influence of grain boundary cracking occurs. It is the root cause of longitudinal cracks on the surface of steel containing sulfur. On this basis, the optimization of protective slag, reconstruction of secondary cooling zone, heating of casting slab and heat preservation of pile cooling process were carried out. The results show that the longitudinal cracks on the surface of the bloom containing sulfur steel can be well controlled by increasing the basicity of protective slag, reducing the melting point and viscosity of protective slag, extending the length of secondary cooling zone, standardizing heating and cooling process. The grinding rate of round steel caused by longitudinal cracks was reduced from 20% to less than 5%.