Abstract: On the basis of statistical analysis of qualified ratio for ultrasonic detection, the qualified and unqualified samples of heavy and medium plates by ultrasonic detection were analyzed through macrostructure examination，metallographic examination and SEM analysis etc. Results showed that the main factor affecting the disqualification of heavy and medium plate by ultrasonic detection were the small cracks in the center of steel plates, which resulted from the combined action of center segregation and the hydrogen of steel. The qualified ratio of steel plates 25 mm or above by ultrasonic detection were increased evidently by reducing the content of hydrogen in steel, improving the center segregation of slabs simultaneously and optimizing the production plan and process flow for different steel grades with different thickness, and then the qualified ratio of heavy steel plates（25 mm or above） and sheet steel by ultrasonic detection were increased from 88.61 % to 96.27 % and from 97.08 % to 98.44 %.

Key words: ultrasonic detection, segregation, hydrogen, crack