DUPLEX STAINLESS STEEL, DUPLEX STAINLESS STEEL SLAB, AND DUPLEX STAINLESS STEEL MATERIAL

1 . A duplex stainless steel comprising, in mass %:

C: 0.03% or less;

Si: 0.05% to 1.0%;

Mn: 0.1% to 7.0%;

P: 0.05% or less;

S: 0.0001% to 0.0010%;

Ni: 0.5% to 5.0%;

Cr: 18.0% to 25.0%;

N: 0.10% to 0.30%;

Al: 0.05% or less;

Ca: 0.0010% to 0.0040%; and

Sn: 0.01% to 0.2%,

with the remainder being Fe and inevitable impurities,

wherein a ratio Ca/O of the amounts of Ca and O is in a range of 0.3 to 1.0, and a pitting index PI shown by formula (1) is in a range of less than 30,

PI=Cr+3.3Mo+16N  (1),

(the chemical symbols in the formula (1) indicate the amounts of the elements).

2 . The duplex stainless steel according to claim 1 further comprising:

one or more selected from

Mo: 1.5% or less,

Cu: 2.0% or less,

W: 1.0% or less, and

Co: 2.0% or less.

3 . The duplex stainless steel according to claim 1 or 2 , further comprising:

one or more selected from

V: 0.05% to 0.5%,

Nb: 0.01% to 0.20%, and

Ti: 0.003% to 0.05%.

4 . The duplex stainless steel according to any one of claims 1 to 3 , further comprising:

one or more selected from

B: 0.0050% or less,

Mg: 0.0030% or less, and

REM: 0.10% or less.

5 . A duplex stainless steel comprising, in mass %:

C: 0.03% or less;

Si: 0.05% to 1.0%;

Mn: 0.1% to 4.0%;

P: 0.05% or less;

S: 0.0001% to 0.0010%;

Cr: 23.0% to 28.0%;

Ni: 2.0% to 6.0%;

Co: 0% to 1.0%;

Cu: 0.2% to 3.0%;

Sn: 0.01% to 0.2%;

N: 0.20% to 0.30%;

Al: 0.05% or less; and

Ca: 0.0010% to 0.0040%,

with the remainder being Fe and inevitable impurities,

wherein Ni+Co is in a range of 2.5% or more and a ratio Ca/O of the amounts of Ca and O is in a range of 0.3 to 1.0, and

PI shown by formula (1) is in a range of 30 or more to less than 40,

PI=Cr+3.3Mo+16N  (1),

(the chemical symbols in the formula (1) indicate the amounts of the elements).

6 . The duplex stainless steel according to claim 5 , further comprising:

either one or both of

Mo: 2.0% or less, and

W: 1.0% or less.

7 . The duplex stainless steel according to claim 5 or 6 , further comprising:

one or more selected from

V: 0.05% to 0.5%,

Nb: 0.01% to 0.15%, and

Ti: 0.003% to 0.05%.

8 . The duplex stainless steel according to any one of claims 5 to 7 , further comprising:

one or more selected from

B: 0.0050% or less,

Mg: 0.0030% or less, and

REM: 0.10% or less.

9 . A cast steel of a duplex stainless steel which has a composition according to any one of claims 1 to 8 ,

wherein a fracture reduction of area at 1000° C. is in a range of 70% or more.

10 . A duplex stainless steel material which is manufactured by hot working the cast steel of the duplex stainless steel according to claim 9 .