Rolled round steel material for steering rack bar and steering rack bar

摘要

The purpose of the present invention is to provide a rolled round steel material for a steering rack bar having a high base material toughness and an excellent machinability for machining a deep hole in the length direction of the radial center part without performing a refinement treatment. A rolled round steel material for a steering rack bar having a chemical composition containing 0.38 to 0.55% of C no more than 1.0% of Si 0.20 to 2.0% of Mn 0.005 to 0.10% of S 0.01 to 2.0% of Cr 0.003 to 0.10% of Al and 0.003 to 0.03% of N with the remainder being made up by Fe and impurities the impurities containing no more than 0.030% of P. The microstructure comprises ferrite (F) lamellar pearlite (LP) and cementite (C). In a cross section perpendicular to the rolling direction the average grain diameter of the F in a region extending from the surface to a position halfway along the radius is no greater than 10 µm the area ratio of the LP is less than 20% and the number of spherical cementite (SC) among the C is greater than or equal to 4 × 10 per mm. In the center part the area ratio of the LP is greater than or equal to 20% and the number of SC is less than 4 × 10 per mm. In a cross section which passes through the center line of the round steel material and is parallel to the rolling direction the average aspect ratio of the F in the region extending from the surface to the position halfway along the radius is equal to or greater than 3.