PURPOSE: To prevent the damage to a piano wire due to diamond abrasive grains in the piano wire used for cutting a metal or the like by forming the skin of the prescribed thickness of iron having a ferrite structure.

CONSTITUTION: A high carbon steel element wire 5 drawn out of a reel is heated for one to ten minutes at 850 to 950°C in a gas atmosphere of H2+H2O in a heating furnace 6. With this, it is decarbonized to the prescribed depth from the skin. Next, it is held for ten to twenty seconds at 700 to 750°C in a holding furnace 8, so that the decarbonized part is transformed from an austenite structure into a soft ferrite structure. Next, it is subjected to lead bath quenching in a lead bath furnace 9 of about 550°C for isothermal transformation. After patenting, a pure iron ferrite structure of 10 to 30μ is formed in the skin of the wire 5. At this time, the inside is of a micro-pearlite structure of high carbon steel.