| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a steel which has excellent HAZ toughness even when subjected to large heat input welding of a heat input of 50 kJ/mm or more and to provide a method for producing the same. <P>SOLUTION: In the steel, (a) the composition of the entire oxide inclusions satisfies 5-50% ZrO<SB POS="POST">2</SB>, 5-50% REM oxides, and 50% or less (exclusive 0%) CaO when measured and expressed in terms of mass of a single oxide, (b) among the entire inclusions, the number of inclusions with a circle-equivalent diameter of 0.1-2 μm is 120 pieces/mm<SP POS="POST">2</SP>or more, the number of oxides with circle-equivalent diameters of above 3 μm is 5.0 pieces/mm<SP POS="POST">2</SP>or less, and the number of oxides with circle-equivalent diameters of above 5 μm is 5.0 pieces/mm<SP POS="POST">2</SP>or less. When the composition of the entire inclusions are measured, (c-1) the numerical ratio of the REM and Zr-containing inclusions I satisfying an REM/Zr mole ratio of 0.6-1.4 is 30% or more based on the number of the entire inclusions and/or (c-2) the numerical ratio of the REM, Zr, Al, Ca, and-Ti-containing inclusions II in which the ratio of the total number of moles of REM and Zr to the total number of moles of Al, Ca, and Ti satisfies 0.5-1.2 is 40% or more based on the number of the entire inclusions. <P>COPYRIGHT: (C)2012,JPO&INPIT |
