Duplex stainless steel and manufacturing method thereof

A duplex stainless steel containing, by mass %, C:

• not more than 0.03%, Si:
• 0.01 to 2%, Mn:
• 0.1 to 2%, P:
• not more than 0.05%, S:
• not more than 0.001%, Al:
• 0.003 to 0.05%, Ni:
• 4 to 12%, Cr:
• 18 to 32%, Mo:
• 0.2 to 5%, N (nitrogen):
• 0.05 to 0.4%, O (oxygen):
• not more than 0.01%, Ca:
• 0.0005 to 0.005%, Mg:
• 0.0001 to 0.005%, Cu:
• 0 to 2%, B:
• 0 to 0.01%, and W:
• 0 to 4%, and the balance of Fe and impurities, where a number of oxide-based inclusions, which have a total content of Ca and Mg of 20 to 40% by mass and also have a long diameter of not less than 7 μm, is not more than a 10 per 1 mm2 of the cross section perpendicular to the working direction, and a number of oxide-based inclusions, which have a content of S of not less than 15% by mass and also have a long diameter of not less than 1 μm, is not more than 10 per 0.1 mm2 of the cross section perpendicular to the working direction,
• wherein a pitting resistance index PREW represented by the followina equation (1) is not less than 40;
• PREW=Cr+3.3(Mo+0.5W)+16N (1)
• wherein each chemical symbol represents the content of each element (% by mass).