A rolled round steel material for a steering rack bar, having a chemical composition consisting of C: 0.38 to 0.55%, Si: not more than 1.0%, Mn: 0.20 to 2.0%, S: 0.005 to 0.10%, Cr: 0.01 to 2.0%, Al: 0.003 to 0.10%, and N: 0.003 to 0.03%, with the balance being Fe and impurities, and P being not more than 0.030% in the impurities, and a microstructure consisting of ferrite (F), lamellar pearlite (LP), and cementite (C), wherein in a cross-section perpendicular to the rolling direction: an average grain diameter of F≰10 μm, an area fraction of LP is less than 20%, and the number of particles of spheroidal cementite (SC) among C≧4×105/mm2, in a region from the surface to a position at ½ radius; and an area fraction of LP≧20% and the number of particles of SC<4×105/mm2, in a central part, and wherein in a cross-section including a center line of the round steel material and parallel to the rolling direction: an average aspect ratio of F is ≧3 in a region from a surface to a position at ½ radius.
ROLLED ROUND STEEL MATERIAL FOR STEERING RACK BAR AND STEERING RACK BAR
24.03.2016
Patent
Elektronische Ressource
Englisch
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