| 摘要 |
A steel material for hot forging which has a chemical composition that contains, in mass%, 0.1 to 0.3% of C, 0.16 to 0.50% of Si, 0.3 to 1.0% of Mn, at most 0.030% of S, 0.8 to 1.8% of Cr, 0.02 to 0.06% of Al, 0.010 to 0.025% of N, 0 to 0.50% of Cu, and 0 to 0.50% of Ni with the balance being Fe and impurities, the amounts of P, Ti and O contained in the impurities being 0.020% or less, 0.005% or less, and 0.0020% or less respectively, and that further has an E value of 0.012 to 0.022%, wherein: in the microstructure of a cross section perpendicular to the rolling direction, the matrix is composed of 0 to 10% of bainite in terms of area fraction, ferrite and pearlite; and in an area of 1μm2, AlN particles having equivalent circle diameters of 10 to 100nm accounts for at least 85% of AlN particles having equivalent circle diameters of 10nm or larger. When the Al/N ratio is equal to or higher than 1.93, E = 0.965×N, while when the Al/N ratio is lower than 1.93, E = 0.5×Al. When a product obtained by hot-forging the steel material is heated in a carburizing step, the coarsening of austenite grains can be stably inhibited, even when the hot forging has been conducted in any temperature range selected from a wide variety of temperature ranges. |
