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

摘要

A rolled round steel material for a steering rack bar contains C in an amount of 0.38 to 0.55%, Si in an amount of 1.0% or less, Mn in an amount of 0.20 to 2.0%, S in an amount of 0.005 to 0.10%, Cr in an amount of 0.01 to 2.0%, Al in an amount of 0.003 to 0.10%, B in an amount of 0.0005 to 0.0030%, Ti in an amount of 0.047% or less, Cu in an amount of 0 to 1.0%, Ni in an amount of 0 to 3.0%, Mo in an amount of 0 to 0.50%, Nb in an amount of 0 to 0.10%, V in an amount of 0 to 0.30%, Ca in an amount of 0 to 0.005%, and Pb in an amount of 0 to 0.30%, a remaining portion being constituted by Fe and an impurities, the impurities containing P in an amount of 0.030% or less and N in an amount of 0.008% or less, and has a chemical composition satisfying [3.4N≦Ti≦3.4N+0.02]. The microstructure is constituted by ferrite (F), lamellar pearlite (LP), and cementite (C), and in the latitudinal cross-section, in a region extending from a surface to a position located a distance of ½ the radius therefrom, the average particle diameter of F is 10 μm or less, the area percentage of LP is less than 20%, and the number of pieces of spheroidal cementite (SC) in C is 4×105 pieces/mm2 or more, and, in a central portion, the area percentage of LP is 20% or more and the number of pieces of SC is less than 4×105 pieces/mm2, and in the longitudinal cross-section, the average aspect ratio of F in a region extending from a surface to a position located a distance of ½ the radius therefrom is 3 or more. This steel material has a high base material toughness and good machinability without performing thermal refining.

主权项

1. A rolled round steel material for a steering rack bar, comprising: in mass %,
C in an amount of 0.38 to 0.55%; Si in an amount of 1.0% or less; Mn in an amount of 0.20 to 2.0%; S in an amount of 0.005 to 0.10%; Cr in an amount of 0.01 to 2.0%; Al in an amount of 0.003 to 0.10%; B in an amount of 0.0005 to 0.0030%; Ti in an amount of 0.047% or less; Cu in an amount of 0 to 1.0%; Ni in an amount of 0 to 3.0%; Mo in an amount of 0 to 0.50%; Nb in an amount of 0 to 0.10%; V in an amount of 0 to 0.30%; Ca in an amount of 0 to 0.005%, and Pb in an amount of 0 to 0.30%, a remaining portion being constituted by Fe and impurities, the impurities containing P in an amount of 0.030% or less and N in an amount of 0.008% or less, and the rolled round steel material having a chemical composition satisfying Equation (1) below,
3.4N≦Ti≦3.4N+0.02  (1) where a symbol of an element indicates a content of the element in mass %, wherein a microstructure is constituted by ferrite, lamellar pearlite, and cementite, in a cross-section perpendicular to a rolling direction, in a region extending from a surface to a position located a distance of ½ the radius therefrom, an average particle diameter of the ferrite is 10 μm or less, an area percentage of the lamellar pearlite is less than 20%, a number of pieces of spheroidal cementite of the cementite is 4×105 pieces/mm2 or more, and, in a central portion, an area percentage of the lamellar pearlite is 20% or more and a number of pieces of spheroidal cementite of the cementite is less than 4×105 pieces/mm2, and in a cross-section that passes through a centerline of the round steel material and is parallel to the rolling direction, an average aspect ratio of the ferrite in a region extending from a surface to a position located a distance of ½ the radius therefrom is 3 or more.