针对用户反映的莱钢生产的S355NL钢制作的风电法兰个别批次探伤合格率偏低的问题，通过对该批次探伤不合格法兰缺陷部位运用超声波进行定位，对缺陷部位取样进行金相和扫描电镜分析，最终确认了钢水浇注温度过高和铸坯的二次冷却不均匀所导致的中心疏松和中心偏析是造成该批次S355NL钢风电法兰探伤不合格的主要原因，并提出了相应的改进措施。

To the problem of the percent of pass in flaw detection was relatively low for some batches of wind power flange made by S355NL steel from Laiwu Steel Group，the defect positions of unqualified flanges were positioned using ultrasonic wave. Then， the defect position was sampled and analyzed by metallographic and scanning electron microscope. Finally， it was confirmed that the central porosity and segregation， which was caused by over high pouring temperature of liquid steel and the non-uniform secondary cooling of casting blank， was the main cause of the defects of S355NL steel for wind power flange. Moreover， the corresponding improvement measures were proposed.