Abstract: A rolled steel bar for machine structural use includes a predetermined chemical composition comprising, by mass %, C: 0.45% to 0.65%, Si: higher than 1.00% to 1.50%, Mn: higher than 0.40% to 1.00%, P: 0.005% to 0.050%, S: 0.020% to 0.100%, V: 0.08% to 0.20%, Ti: 0% to 0.050%; Ca: 0% to 0.0030%, Zr: 0% to 0.0030%, Te: 0% to 0.0030%, and a remainder including Fe and impurities, wherein the impurities include: Cr: 0.10% or lower, Al: lower than 0.01%, and N: 0.0060% or lower. In the rolled steel bar for machine structural use, K1 obtained from “K1=C+Si/7+Mn/5+1.54×V” is 0.95 to 1.05, K2 obtained from “K2=139?28.6×Si+105×Mn?833×S?13420×N” is more than 35, a Mn content and a S content satisfy Mn/S?8.0, and a total decarburized depth of a surface layer is 500 ?m or less.

Abstract: A rolled steel bar for machine structural use includes a predetermined chemical composition comprising, by mass %, C: 0.45% to 0.65%, Si: higher than 1.00% to 1.50%, Mn: higher than 0.40% to 1.00%, P: 0.005% to 0.050%, S: 0.020% to 0.100%, V: 0.08% to 0.20%, Ti: 0% to 0.050%; Ca: 0% to 0.0030%, Zr: 0% to 0.0030%, Te: 0% to 0.0030%, and a remainder including Fe and impurities, wherein the impurities include: Cr: 0.10% or lower, Al: lower than 0.01%, and N: 0.0060% or lower. In the rolled steel bar for machine structural use, K1 obtained from “K1=C+Si/7+Mn/5+1.54×V” is 0.95 to 1.05, K2 obtained from “K2=139?28.6×Si+105×Mn?833×S?13420×N” is more than 35, K3 obtained from “K3=137×C?44.0×Si” is 10.7 or more, a Mn content and a S content satisfy Mn/S?8.0, and a total decarburized depth in a surface layer is 500 ?m or less.

Abstract: A rolled steel bar for machine structural use includes a predetermined chemical composition. In the rolled steel bar for machine structural use, K1 obtained from “K1=C+Si/7+Mn/5+1.54×V” is 0.95 to 1.05, K2 obtained from “K2=139?28.6×Si+105×Mn?833×S?13420×N” is more than 35, a Mn content and a S content satisfy Mn/S?8.0, and a total decarburized depth of a surface layer is 500 ?m or less.

Abstract: A rolled steel bar for machine structural use includes a predetermined chemical composition. In the rolled steel bar for machine structural use, K1 obtained from “K1=C+Si/7+Mn/5+1.54×V” is 0.95 to 1.05, K2 obtained from “K2=139?28.6×Si+105×Mn?833×S?13420×N” is more than 35, K3 obtained from “K3=137×C?44.0×Si” is 10.7 or more, a Mn content and a S content satisfy Mn/S?8.0, and a total decarburized depth in a surface layer is 500 ?m or less.

Abstract: A steel for machine structural use, essentially having the following chemical composition:C: 0.45-0.60 wt %,Si: 0.50-2.00 wt %,Mn: 0.10-0.30 (0.30 not inclusive) wt %,P: 0.01-0.10 wt %,S: 0.01-0.20 wt %,V: 0.08-0.15 wt %, andN: 0.0020-0.0050 (0.0050 not inclusive) wt %. The remainder is Fe and impurities inevitably included. The inner structure of the steel is a ferrite-pearlite structure.