To provide an untempered rolled steel bar for hot forging excellent in machinability, fatigue strength, and a fracture toughness value, a hot forged roughly shaped material, and a common rail and a method for producing the same.
A rolled steel bar for hot forging has chemical composition 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, 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 in an R/2 part of a longitudinal section of the rolled steel bar is represented 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 number density of a sulfide having an equivalent-circle diameter of 0.3-1.0 μm observed per unit area in an R/2 part of a transverse section of the rolled steel bar is 500 pieces/mm2 or more. Wherein, each chemical symbol for an element in formula represents a content (mass%) of each chemical symbol for an element.
HIGASHIDA SHINJI
MATSUMOTO HITOSHI
NEISHI YUTAKA
MAKINO TAIZO
MORITA KOJI
TANIMURA YOSHIHIRO
SAJIMA NAOYUKI
ITO TOSHIMASA
DENSO CORP
| JPH10195599A | 1998-07-28 |
| WO2009064013A1 | 2009-05-22 | |||
| WO2010103772A1 | 2010-09-16 |
Chihara
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