Steel

A steel comprising, in mass %:

• C:
• 0.25 to 0.45%,
• Si:
• 0.10 to 0.50%,
• Mn:
• 0.40 to 0.70%,
• P:
• 0.015% or less,
• S:
• 0.005% or less,
• Cr:
• 0.80 to 1.50%,
• Mo:
• 0.17 to 0.30%,
• V:
• 0.24 to 0.40%,
• Al:
• 0.005 to 0.100%,
• N:
• 0.0300% or less,
• O:
• 0.0015% or less, and
• the balance being Fe and impurities, wherein
• on a precondition that a content of each element in the steel falls within a range described above, Formula (1) to Formula (4) are satisfied, and
• wherein:
• a microstructure of the steel is composed of:
• ferrite and pearlite having a total area fraction of 5.0 to 100.0%, and
• a hard phase composed of bainite or bainite and martensite having a total area fraction of 0 to 95.0%;
• the steel contains CaO—CaS—MgO—Al2O3 composite oxides containing:
• at least one selected from the group consisting of CaO and Cas,
• MgO, and
• Al2O3,
• a proportion of a total area of the CaO—CaS—MgO-Al2O3 composite oxides with respect to a total area of oxides in the steel is 30.0% or more;
• and
• among oxides in the steel, a number density of oxides having an equivalent circle diameter of 20.0 μm or more is 15.0 pieces/mm2 or less:
• 1.50<0.4Cr+0.4Mo+4.5V<2.45 (1)
• 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<3.50 (2)
• Mo/V≥0.58 (3)
• (Mo+V+Cr)/(Mn+20P)≥2.00 (4)
• where each symbol of an element in Formula (1) to Formula (4) is to be substituted by a content of a corresponding element in mass %, and is to be substituted by “0” if the corresponding element is not contained.