| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a steel sheet that exhibits excellent low temperature toughness and fracture toughness of a welded joint, especially a steel sheet suitably used in a marine structure built in a deep water area and a cold area. <P>SOLUTION: The high strength steel sheet satisfies specified component composition and all of the following conditions (A) to (D). (A) The minimum value of a fraction of acicular ferrite (Amin) is 50 area% or more among the fractions of acicular ferrite in the surface part, in the t (sheet thickness)/4 part, and in the t/2 part, and the difference between the maximum value (Amax) of the fraction and the Amin is 20 area% or less; (B) the maximum value of a high angle grain size (Mmax) is 40 μm or less among the high angle grain sizes in the surface part, in the t/4 part, and in the t/2 part, and the difference between the Mmax and the minimum value of the grain size (Mmin) is less than 40 μm; (C) the difference between the maximum value (Hvmax) and the minimum value (Hvmin) of hardness is 50 or less among the hardness in the surface part, in the t/4 part, and in the t/2 part; and (D) when ultrasonic flaw detection test specified in JIS G 0901 is performed on the whole surface with detection sensitivity of +12 dB, the UT echo height of internal defect is 50% or less. <P>COPYRIGHT: (C)2013,JPO&INPIT |
