LOW-YIELD-RATIO HIGH-STRENGTH ELECTRIC RESISTANCE WELDED STEEL TUBE AND PRODUCTION METHOD THEREOF

摘要

PROBLEM TO BE SOLVED: To provide an electric resistance welded steel tube which has limited C content so as to increase its weldability and toughness, achieves high strength and low yield ratio without containing any Cr and is excellent in toughness, and a production method thereof. SOLUTION: The electric resistance welded steel tube has a composition comprising≤0.25% C, appropriate amounts of Mn and Ti, limited amounts of Si, Al, P, S and N, 0.05-2.0% Mo and >0.1% but≤1.0% V, has a metal structure composed of tempered martensite and has Ceq of≥0.45, provided that Ceq is defined by the equation: Ceq=C+Mn/6+Ni/15+(Mo+V)/5. B, Ni and Nb may be further added. The yield strength is≥800 MPa, while the yield ratio is≤90%, and the Charpy absorbed energy is≥200 J. The production method includes the steps of heating the electric resistance welded steel tube at a temperature ranging from Ac3to (Ac3+100)°C, quenching the same, and tempering the same at≤550°C. COPYRIGHT: (C)2010,JPO&INPIT