PURPOSE: To manufacture non-heattreated forged products excellent in impact values at low temp. as well as in strength at ordinary temp., by properly controlling the cooling of a steel having a specific composition consisting of C, Si, Mn, Cr, Al, and Fe after hot-forged into the prescribed shape.
CONSTITUTION: A steel having a composition which consists of, by weight, 0.04W0.15% C, ≤1.0% Si, 0.3W3.0% Mn, 0.7W3.0% Cr, 0.01W0.06% Al, and the balance essentially Fe and further contains, is necessary, at least one kind among 0.02W0.50% Mo, 0.01W0.20% Nb, and 0.03W0.30% V and/or at least one kind among ≤0.15% S, ≤0.30% Pb, ≤0.005% Ca, ≤0.30% Bi, and 0.30% Te and in which the values of (Mn+Cr) and Cr/Mn are regulated to ≤4% and ≥1.0, respectively, is hot-gorged into the prescribed shape. Then, the resulting forging is cooled without delay at 0.7W15°C/sec cooling rate down to a temp. right above the Ar3 transformation point. Subsequently, this forging is further cooled from the above-mentioned initial temp. of Ar3 transformation to 150°C at ≥1°/sec cooling rate. In this way, the non-heattreated forged product combining high strength with high toughness can be obtained.
MIYAZAKI TAKANORI
TAKADA KATSUNORI
