ROLLED ROD STEEL FOR HOT FORGING, HOT-FORGED ROUGHLY SHAPED MATERIAL, AND COMMON RAIL AND PROCESS FOR PRODUCING SAME

摘要

A rolled steel bar for hot forging consisting, by mass percent, of C: 0.25-0.50%, Si: 0.40-1.0%, Mn: 1.0-1.6%, S: 0.005-0.035%, Al: 0.005-0.050%, V: 0.10-0.30%, and N: 0.005-0.030%, and the balance of Fe and impurities, the contents of P and O in the impurities being P: 0.035% or less and O: 0.0030% or less, and value represented by Fn1 = C + Si/10 + Mn/5 + 5Cr/22 + 1.65V - 5S/7 being 0.90 to 1.20, wherein the predicted maximum width of nonmetallic inclusions at the time when a cumulative distribution function obtained by extreme value statistical processing by taking the width of nonmetallic inclusion in an R₁/2 part of a longitudinal cross section of the rolled steel bar as W (µm) is 99.99% is 100 µm or narrower; and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 µm observed per unit area of the R₁/2 part of a transverse cross section of the rolled steel bar is 500 pieces/mm² or higher;where, the symbol of an element in Formula (i) represents the content (mass%) of the element.