PURPOSE: To obtain a high yield. ratio and high strength resistance welded steel tube for a vehicle door impact bar by subjecting a steel having a specified compsn. constituted of C, Mn, Si, Nb, Cr, Mo, V, Ti, B and Fe to hot and cold rolling and making a tube.
CONSTITUTION: A steel contg., by weight, 0.05 to 0.3O% C, 0.6 to 2.0% Mn and 0.03 to 0.70% Si, furthermore contg., at need, one or more kinds among ≤0.05% Nb, ≤1.2% Cr, ≤0.5% Mo, ≤0.1% V, ≤0.05% Ti and ≤0.005% B and the balance Fe with inevitable impurities is subjected to hot rolling and cold rolling. This cold rolled steel sheet is heated to about 800 to 900°C and is soaked for about ≥30sec. Next, the steel sheet is subjected to water hardening and is rapidly cooled at about 200 to 1500°C/sec. After that, the steel sheet is subjected to overaging treatment at about 100 to 300°C according to necessary to make a tube by a resistance butt seam welding equipment. In this way, the high yield ratio and high strength resistance welded steel tube for a vehicle door impact bar having ≥85% yield ration and 120 to 170kgf/mm2 tensile strength can be obtd.
TAKAMURA TOSHIHIRO
ISHIZAWA YOSHIKAZU
SUGIMOTO YUJI
MORIMOTO YOSHITOSHI
SUGAMASA TETSUAKI
NISHIMURA FUMIHIRO
