The present invention provides a novel heat exchange medium to replace lead. A carbon-steel wire 1A heated in a heating furnace 11 is passed through a bath 12A filled with a liquid-phase Mg-Al-Ca alloy 20 obtained by melting a Mg-Al-Ca alloy in which the main constituent elements are Mg (magnesium), Al (aluminum) and Ca (calcium). When it passes through the bath 12A, the carbon-steel wire 1A, which has been heated for example to about 950°C in the heating furnace 11, is cooled to about 550°C. The Mg-Al-Ca alloy is non-toxic and has no environmental impact as well.
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