Austenitic Stainless Steel

1 . An austenitic stainless steel comprising base metal and a coating film formed on at least part of a surface of the base metal,

a chemical composition of the base metal consisting of, in mass percent:

C: 0.05% or less;

Si: 1.0% or less;

Mn: 2.0% or less;

P: 0.040% or less;

S: 0.010% or less;

O: 0.020% or less;

N: less than 0.050%;

Ni: 12.0 to 27.0%;

Cr: 15.0% or more to less than 20.0%;

Cu: more than 3.5% to 8.0% or less;

Mo: more than 2.0% to 5.0% or less;

Co: 0.05% or less;

Sn: 0.05% or less;

V: 0 to 0.5%;

Nb: 0 to 1.0%;

Ti: 0 to 0.5%;

W: 0 to 5.0%;

Zr: 0 to 1.0%;

Al: 0 to 0.5%;

Ca: 0 to 0.01%;

B: 0 to 0.01%;

REM: 0 to 0.01%, and

the balance: Fe and impurities, wherein

a chemical composition at a maximum-Cr depth where a concentration of Cr in the coating film reaches a maximum satisfies a following formula (i):

(Cr+Ni+Cu+Mo)/Fe≥1.0  (i)

where each symbol of element in the formula denotes a content (at %) of a corresponding element.

2 . The austenitic stainless steel according to claim 1 , wherein

the chemical composition of the base metal containing one or more elements selected from, in mass percent:

V: 0.01 to 0.5%;

Nb: 0.02 to 1.0%;

Ti: 0.01 to 0.5%;

W: 0.1 to 5.0%;

Zr: 0.02 to 1.0%;

Al: 0.01 to 0.5%;

Ca: 0.0005 to 0.01%;

B: 0.0005 to 0.01%; and

REM: 0.0005 to 0.01%.

3 . The austenitic stainless steel according to claim 1 , wherein

a minimum-Cr depth where the concentration of Cr in the coating film falls to a minimum lies closer to the base metal than the maximum-Cr depth, and

a chemical composition at the maximum-Cr depth satisfies a following formula (ii), and a chemical composition at the minimum-Cr depth satisfies a following formula (iii):

Cr/(Ni+Cu+Mo)≥1.0  (ii)

Cr/(Ni+Cu+Mo)<1.0  (iii)

where each symbol of element in the formulas denotes a content (at %) of a corresponding element.

4 . The austenitic stainless steel according to claim 2 , wherein

a minimum-Cr depth where the concentration of Cr in the coating film falls to a minimum lies closer to the base metal than the maximum-Cr depth, and

a chemical composition at the maximum-Cr depth satisfies a following formula (ii), and a chemical composition at the minimum-Cr depth satisfies a following formula (iii):

Cr/(Ni+Cu+Mo)≥1.0  (ii)

Cr/(Ni+Cu+Mo)<1.0  (iii)

where each symbol of element in the formulas denotes a content (at %) of a corresponding element.