High frequency heat treatment method of ultra-high strength parts

A heat treatment method of hot-stamping-treated steel parts comprising:

• a first step of heating the hot-stamping-treated steel parts until the temperature of a heat treatment zone thereof reaches an AC3 transformation point or higher;
• a second step of maintaining the temperature of the heat treatment zone until phase transformation of the heat treatment zone to austenite structure is completed;
• and
• a third step of cooling the heat treatment zone to provide a transformation to a ferrite-pearlite structure,
• wherein the hot-stamping-treated steel parts comprise:
• about 0.15 to 0.30% by weight of carbon (C), about 0.05 to 0.5% by weight of silicon (Si), about 1.0 to 2.0% by weight of manganese (Mn), about 0.0005 to 0.0040% by weight of boron (B), more than 0% and not more than about 0.003% by weight of sulfur (S), more than 0% and not more than about 0.012% by weight of phosphorus (P), a balance of iron (Fe), and inevitable impurities, the % by weight based on the total weight of the hot-stamping-treated steel parts;
• wherein the heat treatment zone is a spot welding zone;
• wherein the third step comprises maintaining the temperature at about 550 to 700° C. for about 80 seconds or more until the heat treatment zone transforms ferrite-pearlite structure;
• wherein the ferrite-pearlite structure has a tensile strength of about 400 to 500 MPa and an elongation ratio of about 25% or more,
• wherein the third step is carried out to provide a cooling rate of about 5° C./second or less.