Method for heat treating an impact beam of automotive vehicle door and a system of the same

A method for heat treating an impact beam of an automotive vehicle door exhibiting high tensile strength, yield strength and elongation, comprising the steps of:

• preparing hot rolled steel sheets consisting essentially of 0.18 % to 0.22% carbon;
• 1.0 to 1.5% manganese;
• 0.15 to 0.4% silicon;
• not more than 0.025% phosphorous;
• not more than 0.008% nitrogen;
• not more than 0.0010% sulfur;
• 0.01 to 0.04% aluminum;
• 0.001 to 0.003% boron;
• 0.1 to 0.3% chromium;
• 0.01 to 0.035% titanium;
• and the balance essentially consisting of iron;
• roll-forming and welding the steel sheets to make a tube-shaped impact beam;
• carrying out a heat treatment including three steps of high frequency induction heating and air cooling the impact beam consecutively;
• and
• carrying out quenching of water cooling said heat treated and air cooled impact beam;
• wherein said heat treatment step includes the steps of:
• first high frequency induction heating said impact beam for 0.83 to 2.5 seconds at 500.degree. to 720.degree. C. and air cooling said beam for 0.27 to 1 second;
• second high frequency induction heating said impact beam first heat treated for 0.83 to 2.5 seconds at 750.degree. to 850.degree. C. and air cooling said beam for 0.27 to 2 seconds;
• and
• third high frequency induction heating said impact beam second heat-treated for 0.83 to 2.5 seconds at 830.degree. to 960.degree. C. and air cooling said beam for 1.5 to 5 seconds.