To produce a large steel plate with few defects.
An ingot having a three-step taper is casted by an underside pouring ingot making method. The ingot body part SA has each different tapers in an upper part, a middle part and a lower part. The taper Tp3 in the upper part and the taper Tp2 in the lower part are 0.02 or more and 0.07 or less, and the taper Tp4 in the middle part is 0.15 or more and 0.24 or less. The boundary between the upper part and the middle part is set to be a position having an H3/T0 of 0.35 or more and 0.5 or less (second position), and the boundary between the middle part and the lower part is set to be a position having an H2/T0 of 0.7 or more and 1.0 or less (first position). A steel plate is produced by rolling the ingot in a specified reduction ratio X.
Makoto Suhara
Koji Segawa