冷却条件对EH40钢板坯表层奥氏体晶粒长大的影响

Effect of cooling conditions on surface austenite grain growth of EH40 steel slab

铸坯表层异常长大的奥氏体晶粒是产生横裂纹的重要原因之一,研究冷却过程对其生长行为的影响对科学制定连铸工艺、降低铸坯裂纹敏感性有重要意义。采用原创连铸坯凝固过程热模拟方法,再现了EH40低碳船板钢板坯的凝固过程,观察在传统板坯连铸条件下,2种结晶器冷却强度对铸坯表层奥氏体晶粒长大行为的影响。结果表明,在结晶器冷却阶段,热模拟坯表层5 mm的绝大多数奥氏体晶粒短轴尺寸均不超过0.5 mm,但已出现粗大晶粒,且强冷条件下奥氏体晶粒尺寸平均值和极大值均更大,分别为弱冷条件下的2.5倍和2.0倍。在足辊区到矫直点区间,表层奥氏体晶粒生长非常缓慢,平均尺寸仍未超过0.5 mm。矫直点处,结晶器强冷热模拟坯表层20 mm的晶粒短轴最大尺寸为2.2 mm,为弱冷条件下的1.7倍。综上,奥氏体晶粒在连铸不同阶段表现为不同的生长行为,且采用结晶器弱冷更有利于EH40钢板坯获得相对细小的表层奥氏体晶粒。

Abnormal growth of austenite grains on the surface of casting slab is one of leading cause for transverse cracks. Studying the influences of cooling process on austenite growth behavior is meaningful to the scientific formulation of continuous casting process and the reduction of crack sensitivity. Innovative thermal simulator was designed for continuous casting slab solidification process studying, which had been applied to reproduce the cooling and solidification of EH40 low carbon ship plate steel with two kinds of mold cooling intensity, and the grain growth behaviors of surface austenite were observed under the condition of traditional slab continuous casting. The results show that at the mold cooling stage, most of minor axis size of austenite grain within 5 mm to the surface of thermal simulation slab are not more than 0.5 mm, but coarse grains are already appeared, and the average and maximum austenite grain sizes are larger under the condition of strong cooling, which are 2.5 times and 2.0 times that of weak cooling, respectively. From the foot roller to the straightening point, the surface austenite grains grow very slowly, and the average size still does not exceed 0.5 mm. At the straightening point, the maximum grain minor axis size at 20 mm of surface on thermal simulation slab with strong cooling in the mold is 2.2 mm, which is 1.7 times that of weak cooling. In conclusion, the growth behaviors of austenite grains are different in each stage of continuous casting, and weak cooling in mold is more conducive to obtain relatively fine surface austenite grains in EH40 steel slab.