HOLLOW MEMBER FOR VEHICLE REINFORCEMENT

PROBLEM TO BE SOLVED: To provide a hollow member for vehicle reinforcement, which can be manufactured advantageously in terms of joint reliability, energy cost, surface design and product shape.SOLUTION: The hollow member is manufactured by using an electric resistance welded steel tube, as a mother tube, which has a composition containing, by mass, C: 0.15-0.20%, Si: 0.01-0.5%, Mn: 1.5-3.0%, P: 0.020% or less, S: 0.005% or less, N: 0.005% or less, Al: 0.01-0.05%, Ti: 0.1% or less, Nb: 0.1% or less, B: 0.0005-0.0030%, and the remainder being Fe and inevitable impurities, and has a structure containing a tempered martensitic phase: 95 vol.% or more and a residual austenitic phase: less than 5 vol.%. Heat treatment is applied to the whole area or a part of the tube periphery direction in a part of the tube length direction of the mother tube. The tensile strength of a heat-treated part 3 is lower by 400 MPa or more compared with an un-heat-treated part 4, and further the product of the tensile strength and the elongation of the heat-treated part 3 is 12,000 MPa% or more.