ULTRASONIC INSPECTION METHOD FOR CAST PIECE OR ROLLED STEEL MATERIAL

PROBLEM TO BE SOLVED: To obtain an ultrasonic inspection method in which a flaw part and its size can be judged by a method wherein a nonmetal inclusion or an air bubble which exists in the flaw part is discriminated by using a specific expression and the circle-converted diameter of the projection area to an inspection of the flaw part is judged. SOLUTION: In a method, a nonmetal inclusion or an air bubble which exists at the inside of a cast piece or a rolled steel material is inspected by using a vertical ultrasonic flaw detection method. At this time, a probe is scanned, at a prescribed scanning pitch, on the surface of a material to be inspected, and the intensity (e) of reflected waves in a flaw part from the nonmetal inclusion or the air bubble is measured. Then, the nonmetal inclusion or the air bubble which exists in the flaw part is discriminated by using a relational expression, and the circle-converted diameter (d) of the projection area to an inspection face of the flaw part is judged. In the expression, (c) represents a sound velocity inside the material to be inspected, (t) represents the delay time of reflected waves on the surface and reflected waves in the flaw part, and (k) represents a constant which is decided by the kind of a substance in the flaw part and by the characteristic of a measuring instrument. In the method, the kind, the size and the position of the nonmetal inclusion and the air bubble can be judged.