The causes of ultrasonic inspection disqualification for ultra heavy plate were analyzed by means of macroscopic test, metallographic inspection, SEM and EDAX. The results showed that main reasons of ultrasonic test defects are big size of calcium-aluminates inclusions, chain-shaped sulfides inclusion and the crack caused by Mn segregation in the plate. In order to increase the purity of molten liquid steel and improving quality of slab, LF refining slag system was selected, the blowing argon processing of LF and VD was adjusted and the process parameters of casting were optimized. After optimizing,[P]<0.015%,[S]<0.003%,[O]<4×10-6,[H]<1.5×10-6 and[N]<35×10-6. The center segregation and central poriness of slab were less than 1.0 level of C grade. The average qualified ratio of ultrasonic flaw detection is up to 98.8%.% 采用低倍热酸洗、金相、扫描电镜和能谱等方法对特厚板探伤不合格原因进行分析,结果表明,主要原因是钢板中存在大尺寸(20~50μm)钙铝酸盐夹杂物、链状的硫化物夹杂物以及Mn偏析导致的裂纹.通过优化LF炉渣系、VD和LF炉的吹氩制度以及连铸工艺参数,提高了钢水纯净度和板坯质量,钢水中的[P]≤0.015%、[S]≤0.003%、[O]≤4×10-6、[H]≤1.5×10-6、[N]≤35×10-6,板坯的中心偏析和疏松均降为C类1.0级以下,钢板平均探伤合格率提高到了98.8%.
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