| 摘要 |
A steel product, particularly a heavy-duty gear, having a case of martensite and a core of lower bainite with a Vickers hardness of not less than Hv 600 is obtained by subjecting a medium carbon alloy steel comprising, by weight, 0.45 to 0.60% C, up to 0.50 Si, 0.40 to 1.30% Mn, up to 4.00 Ni, 0.35 to 0.55% Cr, up to 0.70 Mo, balance Fe and incidental impurities to a carbonitriding treatment at a temperature of 800 DEG to 900 DEG C., austempering the carboritrided steel by quenching it in a hot bath of 230 DEG to 300 DEG C. and retaining the steel in the hot bath of the aforementioned temperature for a suitable duration enough to transform the core of the steel into lower bainite and not enough to cause any transformation of the case of the steel, and subsequently cooling the steel.
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