Steel pipe for pressure piping and starting material for steel pipe

A starting material for a steel pipe for pressure piping, the starting material for a steel pipe to be used for an application for which the starting material for a steel pipe is to be subjected to autofrettage,

• the starting material for a steel pipe comprising an outer surface and an inner surface, wherein
• an average hardness of the steel pipe in an outer layer region from the outer surface to a ¼ wall-thickness depth position is 1.20 times or more an average hardness of the steel pipe in an inner layer region from the inner surface to a ¼ wall-thickness depth position, and
• when the starting material for a steel pipe is subjected to the autofrettage,
• when an outer diameter of the steel pipe after the autofrettage is denoted by D (mm), and an inner diameter of the steel pipe after the autofrettage is denoted by d (mm), and
• when a measured value of a residual stress at an outer surface of the steel pipe after the autofrettage is denoted by σo1 (MPa), a measured value of a residual stress at the outer surface of the steel pipe after the autofrettage and halving is denoted by σo2 (MPa), and a measured value of a residual stress at an inner surface of the steel pipe after the autofrettage and the halving is denoted by σi2 (MPa),
• an estimated value σi1 (MPa) of a residual stress at the inner surface of the steel pipe after the autofrettage is −150 MPa or less, the estimated value σi1 as determined by Formula (i) to Formula (iv) shown below.
• σi1=(−σi2)/(A×(t/T)2−1) (i)
• t/T=((σo2−σo1)/(A×(σo2−σo1)−C×σi2))1/2 (ii)
• A=3.9829×exp(0.1071(D/d)2) (iii)
• C=−3.3966×exp(0.0452×(D/d)2) (iv)