A steel workpiece maintained at a specified quenching temperature is
rapidly cooled to a temperature just above the martensite transformation
start point (Ms point) by being immersed in a high-temperature quenching
oil. Thereafter, the steel workpiece is taken out of the high-temperature
quenching oil so as to be soaked by the heat possessed by the steel
workpiece and subsequently cooled by being immersed in the
high-temperature quenching oil. Through these processes, a temperature
difference between steel workpieces or the portions of a steel workpiece
in the martensite transformation stage is reduced, and a cooling speed in
a high-temperature region (not lower than about 550.degree. C.) is made to
be a slow speed sufficient for restraining a thermal distortion, by which
the quenching distortion and quenching variation can be reduced.
