Rolled steel bar or rolled wire rod for cold-forged component
In a rolled steel bar or rolled wire rod for a cold-forged component having a predetermined chemical composition, Y1 represented by Y1=[Mn]×[Cr] and Y2 represented by Y2=0.134×(D/25.4−(0.50×√[C]))/(0.50×√[C]) satisfy Y1>Y2, the tensile strength is 750 MPa or less, an internal structure is a ferrite-pearlite structure, and the ferrite fraction in the internal structure is 40% or greater. AMOUNT IS 0.30%
1. A rolled steel bar or rolled wire rod for a cold-forged component that has a chemical composition comprising, in mass %:
C: 0.24% to 0.36%;
Si: less than 0.40%;
Mn: 0.20% to 0.45%;
S: less than 0.020%;
P: less than 0.020%;
Cr: 0.70% to 1.45%;
Al: 0.005% to 0.060%;
Ti: greater than 0.010% to 0.050%;
Nb: 0.003% to 0.050%;
B: 0.0003% to 0.0040%;
N: 0.0020% to 0.0080%;
Cu: 0% to 0.50%;
Ni: 0% to 0.30%;
Mo: 0% to 0.050%;
V: 0% to 0.050%;
Zr: 0% to 0.050%;
Ca: 0% to 0.0050%; and
Mg: 0% to 0.0050%
with a remainder of Fe and impurities,
wherein Y1 and Y2 represented by the following Formulas <1> and <2>, satisfy a relationship represented by the following Formula <3>,
a tensile strength is 750 MPa or less,
an internal structure defined as a region from a center to a surface layer portion,
wherein the internal structure consists of ferrite, pearlite and impurities,
wherein the surface layer portion ranges up to 100 um from the surface, and
a ferrite fraction is 40% or greater in the internal structure
Y1=[Mn]×[Cr] Formula <1>,
Y2=0.134×(D/25.4−(0.50×√[C]))/(0.50×√[C]) Formula <1>,
Y1>Y2 Formula <3>,
where [C], [Mn], and [Cr] in the formulas represent respective amounts of elements in mass %, and D represents a diameter of the rolled steel bar or rolled wire rod in the unit of mm.
2. The rolled steel bar or rolled wire rod for a cold-forged component according to claim 1 ,
wherein the chemical composition contains, in mass %, one or more selected from the group consisting of
Cu: 0.03% to 0.50%,
Ni: 0.01% to 0.30%,
Mo: 0.005% to 0.050%, and
V: 0.005% to 0.050%.
3. The rolled steel bar or rolled wire rod for a cold-forged component according to claim 1 ,
wherein the chemical composition contains, in mass %, one or more selected from the group consisting of
Zr: 0.003% to 0.050%,
Ca: 0.0005% to 0.0050%, and
Mg: 0.0005% to 0.0050%.
4. The rolled steel bar or rolled wire rod for a cold-forged component according to claim 2 ,
wherein the chemical composition contains, in mass %, one or more selected from the group consisting of
Zr: 0.003% to 0.050%,
Ca: 0.0005% to 0.0050%, and
Mg: 0.0005% to 0.0050%.
5. The rolled steel bar or rolled wire rod for a cold-forged component according to claim 1 ,
wherein the chemical composition contains, in mass %,
Mn: 0.20% to 0.30%.