Steel pipe for automotive door reinforcing member

PURPOSE: To provide a door impact bar with excellent strength and toughness as a whole by forming a half periphery which is not quenched as the inner plate side of a door and a half periphery which is quenched as the outside of the door. CONSTITUTION: A heater which can heat only the half periphery of a steel pipe 3 is installed on the steel pipe 3 to water-cool the whole steel pipe 3 for quenching after heating only the half periphery. The obtained steel pipe 3 is installed inside a door so as to locate a high-strength side half periphery 4 which is quenched on the outer plate 1 side of the door, and a high-toughness side half periphery 5 which is not quenched on the outer plate 2 side of the door. As a result, when a bending load is applied, a range which corresponds to bending abdomen side becomes a high-strength side half periphery 4, so effective strength distribution against buckling is formed. In breaking, cracking occurs from bending back side, so toughness on the bending back side is important, however, a range which corresponds to the bending back side becomes a high-toughness side half periphery 5, thus it is possible to ensure toughness. COPYRIGHT: (C)1993,JPO&Japio