| abstract |
The steel sheet according to any one of the above items (1) to (4), wherein the steel sheet contains 0.38 to 0.55% of C, 1.0 to 2.0% of Si, 0.20 to 2.0% of C, 0.005 to 0.10% of S, 0.01 to 2.0% of Cr, 0.003 to 0.10% of Al, 0.0005 to 0.0030% %, 0 to 1.0% of Cu, 0 to 3.0% of Ni, 0 to 0.50% of Mo, 0 to 0.10% of Nb, 0 to 0.30% of V, 0 to 0.005% of Ca, 0 to 0.30% , The balance being Fe and impurities, and having a chemical composition satisfying P? 0.030% and N? 0.008% [3.4N? Ti? 3.4N + 0.02] in the impurities and the microstructure is ferrite (F), lamellar pearlite ) And cementite (C), and the average particle diameter of F in the region from the surface to the half radius of the surface is 10 mu m, the area ratio of LP is 20%, and the number of spherical cementites (SC) × 10 5 / mm 2, also the number of the area ratio of ≥20% in the center of LP and SC <4 × 10 is a 5 / mm 2, the average in the vertical cross-sectional view, from the surface of the area to the half position of a radius F Steering rack bars with aspect ratio ≥ 3. This steel material has high base material toughness and good machinability without performing tempering treatment. |