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

摘要

A rolled rod steel for hot forging which contains, in terms of mass%, 0.25-0.50% C, 0.40-1.0% Si, 1.0-1.6% Mn, 0.005-0.035% S, 0.005-0.050% Al, 0.10-0.30% V, and 0.005-0.030% N, with the remainder comprising Fe and impurities, the contents of P and O, among the impurities, being 0.035% or less and 0.0030% or less, respectively, and which has a value of Fn1, represented by Fn1=C+Si/10+Mn/5+5Cr/22+1.65V-5S/7, of 0.90-1.20. When the width of nonmetallic inclusions present in an R/2 part of a longitudinal section of the rolled rod steel is expressed by W (µm) and the cumulative distribution function obtained by statistical processing of extreme values is 99.99%, then the estimated maximum width of the nonmetallic inclusions is 100 µm or less. The population density of sulfide grains having an equivalent-circle diameter of 0.3-1.0 µm which are observed per unit area of the longitudinal section is 500 grains/mm² or more.