Steel
A steel consists of, 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, and satisfies Formula (1) to Formula (4) described in the present specification, wherein: its microstructure is composed of ferrite and pearlite having a total area fraction of 5.0 to 100.0%, and a hard phase having a total area fraction of 0 to 95.0%; a proportion of a total area of CaO—CaS—MgO—Al 2 O 3 composite oxides with respect to a total area of oxides in the steel is 30.0% or more; and a number density of oxides having an equivalent circle diameter of 20.0 μm or more is 15.0 pieces/mm 2 or less.
1 . 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—Al 2 O 3 composite oxides containing:
at least one selected from the group consisting of CaO and Cas,
MgO, and
Al 2 O 3 ,
a proportion of a total area of the CaO—CaS—MgO-Al 2 O 3 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/mm 2 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.
2 . The steel according to claim 1 , further comprising one or more elements selected from the group consisting of:
Cu: 0.20% or less,
Ni: 0.20% or less,
B: 0.0050% or less,
Nb: 0.100% or less, and
Ti: 0.100% or less.