HOT ROLLED STEEL SHEET AS STOCK FOR STEEL TUBE FOR HYDROFORMING, STEEL TUBE FOR HYDROFORMING, AND THEIR MANUFACTURING METHODS

摘要

PPROBLEM TO BE SOLVED: To provide a steel tube for hydroforming having excellent THF (tube hydroforming) characteristics, a hot rolled steel sheet as a stock for the steel tube, and their manufacturing methods. PSOLUTION: The steel sheet and the steel tube are characterized by including: a chemical composition which is composed of, by mass, 0.02 to 0.20% C, 0.05 to 1.50% Si, 0.2 to 3.0% Mn, 0.003 to 0.10% P, 0.01% S, 0.01 to 2.0% Al, 0.01% N, 0.01% O (oxygen), further one or more selected from the group consisting of 0.25% Ti, 0.1% Nb and 0.5% V, and the balance Fe with impurities and satisfies specific inequalities; a metallic structure having 1.0 to 10 m average ferrite grain size and 50% ferrite area ratio; surface characteristics in which the area ratio of insular scale flaws is 10% in total; and mechanical properties of 590 MPa tensile strength (TS). PCOPYRIGHT: (C)2009,JPO&INPIT