Steel plate and method of producing same, and steel pipe or tube

A steel plate that has a chemical composition containing, in mass %, C:

• 0.030% to 0.080%, Si:
• 0.01% to 0.50%, Mn:
• 0.80% to 1.80%, P:
• 0.010% or less, S:
• 0.0010% or less, Al:
• 0.030% to 0.080%, Nb:
• 0.080% or less, N:
• 0.0080% or less, Ca:
• 0.0005% to 0.0050%, Mg:
• 0.0005% to 0.0050%, and 0:
• 0.0003% to 0.0030%, optionally at least one selected from the group consisting of Cu:
• 0.30% or less, Ni:
• 0.30% or less, Cr:
• 0.50% or less, Mo:
• 0.50% or less, V:
• 0.100% or less, Ti:
• 0.100% or less, Zr:
• 0.0200% or less, and REM:
• 0.0200% or less, and has an ACR of 1.00 or more as determined by the following Expression (1), with the balance being Fe and inevitable impurity, wherein,
• in a region inward from a depth of ¼ thickness from both surfaces of the steel plate in the thickness direction, inclusions are present that have an average composition of 10 mass % to 40 mass % MgO and 10.0 mass % or less MnS, and an average MnS aspect ratio of 10 or less,
• an average value of the top 10% of the circle equivalent diameters of the inclusions is 2.0 μm or less, and
• a crack area ratio CAR after an HIC test in the region is 1.0% or less,
• ACR
• =
• {
• [
• Ca
• ]
• -
• (
• 1.23
• [
• O
• ]
• -
• 0
• .
• 0
• ⁢
• 0
• ⁢
• 0
• ⁢
• 3
• ⁢
• 6
• ⁢
• 5
• )
• }
• /
• (
• 1.25
• [
• S
• ]
• )
• (
• 1
• )