Low-alloy high-strength seamless steel pipe for oil country tubular goods

A low-alloy high-strength seamless steel pipe for oil country tubular goods,

• the low-alloy high-strength seamless steel pipe having a yield strength of 862 MPa or more calculated according to a Japanese Industrial Standard (JIS) Z2241 tensile test, and having a composition that comprises, in mass %,
• C:
• 0.25 to 0.50%,
• Si:
• 0.01 to 0.40%,
• Mn:
• 0.3 to 1.5%,
• P:
• 0.010% or less,
• S:
• 0.001% or less,
• O:
• 0.0015% or less,
• Al:
• 0.015 to 0.080%,
• Cu:
• 0.02 to 0.09%,
• Cr:
• 0.5 to 0.8%,
• Mo:
• 0.5 to 1.3%,
• Nb:
• 0.005 to 0.05%,
• B:
• 0.0005 to 0.0040%,
• Ca:
• 0.0010 to 0.0020%,
• Mg:
• 0.001% or less, and
• N:
• 0.005% or less,
• and in which the balance is Fe and incidental impurities,
• the low-alloy high-strength seamless steel pipe having a microstructure in which the a first number of oxide-base nonmetallic inclusions including CaO, Al2O3, and MgO and having a major diameter of 5 μm or more in the low-alloy high-strength seamless steel pipe, and satisfying first composition ratios represented by formulae (1) and (2), is 10 or less per 100 mm2,
• and in which a second number of oxide-base nonmetallic inclusions including CaO, Al2O3, and MgO and having a major diameter of 5 μm or more in the low-alloy high-strength seamless steel pipe, and satisfying second composition ratios represented by formulae (3) and (4), is 30 or less per 100 mm2,
• (CaO)/(Al2O3)≤0.25 (1)
• 1.0≤(Al2O3)/(MgO)≤9.0 (2)
• (CaO)/(Al2O3)≥2.33 (3)
• (CaO)/(MgO)≥1.0 (4)
• wherein (CaO), (Al2O3), and (MgO) represent the contents of CaO, Al2O3, and MgO, respectively, in the oxide-base nonmetallic inclusions in the steel, in mass %.