ROLLED ROUND STEEL MATERIAL FOR STEERING RACK BAR, AND STEERING RACK BAR

摘要

A rolled round steel material for steering rack bars that has a chemical composition which comprises 0.38-0.55% C, up to 1.0% Si, 0.20-2.0% Mn, 0.005-0.10% S, 0.01-2.0% Cr, 0.003-0.10% Al, 0.0005-0.0030% B, up to 0.047% Ti, 0-1.0% Cu, 0-3.0% Ni, 0-0.50% Mo, 0-0.10% Nb, 0-0.30% V, 0-0.005% Ca, 0-0.30% Pb, and Fe and impurities as the remainder, the contents of P and M among the impurities being P≤0.030% and N≤0.008%, and which satisfies 3.4N≤Ti≤3.4N+0.02, and that has a microstructure which comprises ferrite (F), lamellar pearlite (LP), and cementite (C). In a cross-section thereof, a region ranging from the surface to a depth which is 1/2 the radius has an average F-grain diameter of 10 µm or smaller and an areal LP proportion less than 20% and contains spherical cementite (SC), of the C, in an amount of 4×105 grains/mm2 or larger. In a central part thereof, the areal LP proportion is 20% or higher and the number of SC grains is smaller than 4×105 grains/mm2. In a longitudinal section thereof, the F grains contained in a region ranging from the surface to a depth 1/2 the radius have an average aspect ratio of 3 or higher. This steel material has high base-material toughness and satisfactory cuttability without necessitating a tempering treatment.