Stainless steel and stainless steel product for oil well

A stainless steel having a chemical composition including, in mass %:

• C:
• 0.001 to 0.06%;
• Si:
• 0.05 to 0.5%;
• Mn:
• 0.01 to 2.0%;
• P:
• up to 0.03%;
• S:
• less than 0.005%;
• Cr:
• 15.5 to 18.0%;
• Ni:
• 2.5 to 6.0%;
• V:
• 0.005 to 0.25%;
• Al:
• up to 0.05%;
• N:
• up to 0.06%;
• O:
• up to 0.01%;
• Cu:
• 0 to 3.5%;
• Co:
• 0 to 1.5%;
• Nb:
• 0 to 0.25%;
• Ti:
• 0 to 0.25%;
• Zr:
• 0 to 0.25%;
• Ta:
• 0 to 0.25%;
• B:
• 0 to 0.005%;
• Ca:
• 0 to 0.01%;
• Mg:
• 0 to 0.01%;
• and
• REM:
• 0 to 0.05%,
• further including one or two selected from the group consisting of:
• Mo:
• 0 to 3.5%;
• and
• W:
• 0 to 3.5%
• in an amount that satisfies Equation (1),
• the balance being Fe and impurities,
• wherein the stainless steel has a matrix structure having, by volume ratio, 40 to 80% tempered martensite, 10 to 50% ferrite and 1 to 15% austenite,
• when a microstructure image with dimensions of 1 mm×1 mm obtained by photographing the matrix structure at a magnification of 100 times is positioned in an x-y coordinate system with an x-axis extending in a wall-thickness direction and a y-axis extending in a length direction and each of 1024×1024 pixels is represented by a gray scale level, β defined by Equation (2) is not smaller than 1.55:
• 1.0≤Mo+0.5W≤3.5 (1),
• where Mo and W are the Mo and W contents in mass %,
• [
• Formula
• ⁢
• ⁢
• 1
• ]
• β
• =
• Su
• Sv
• (
• 2
• )
• in Equation (2), Su is defined by Equation (3), and Sv is defined by Equation (4):
• [
• Formula
• ⁢
• ⁢
• 2
• ]
• Su
• =
• ∑
• u
• =
• 1
• 1023
• ⁢
• 
• F
• ⁡
• (
• u
• ,
• 0
• )
• 
• (
• 3
• )
• Sv
• =
• ∑
• v
• =
• 1
• 1023
• ⁢
• 
• F
• ⁡
• (
• 0
• ,
• v
• )
• 
• (
• 4
• )
• in Equations (3) and (4), F(u,v) is defined by Equation (5):
• [
• Formula
• ⁢
• ⁢
• 3
• ]
• F
• ⁡
• (
• u
• ,
• v
• )
• =
• ∑
• x
• =
• 0
• 1023
• ⁢
• ∑
• y
• =
• 0
• 1023
• ⁢
• f
• ⁡
• (
• x
• ,
• y
• )
• ⁢
• exp
• ⁢
• {
• -
• 2
• ⁢
• π
• ⁢
• ⁢
• i
• ⁡
• (
• ux
• 1024
• +
• vy
• 1024
• )
• }
• (
• 5
• )
• in Equation (5), f(x,y) represents the gray level of the pixel at coordinates (x,y).