6.1.6. Macro Inclusions and Macro Defects DetectionC-scan ultrasonic method to be used for detection. The detection method was carried out according to SEP1927 (2001 1st edition). The detection frequency ranged from 10 MHz to 15 MHz using four-level sensitivity detection. The detection volume of each furnace steel should not be less than 5 dm3, the defect density of UT image is less than 10 mm/dm3, and the single defect of UT image should be less than 3 mm.