ELECTRO-RESISTANCE-WELDED STEEL PIPE AND ITS MANUFACTURING METHOD

To provide an electro-resistance-welded steel pipe for a construction machine frame having high strength and excellent low temperature toughness, in particular, in a steel pipe for a crane lattice or crane boom, a steel pipe having less in a surface decarburized layer, and simultaneously an electro-welded steel pipe excellent in welding quality so as to respond to a thick product.SOLUTION: An electro-resistance-welded steel pipe has a predetermined chemical composition, particularly a composition that satisfies the relationship of Ti/N≥4.0, is provided with decarburized layers on both of an inner surface and an outer surface of the steel pipe, and by setting a decarburized layer depth to 0.20 mm at the maximum depth, the tensile strength TS is 780 to 980 MPa, the elongation is 15% or more, and the absorbed energy obtained by the Charpy impact test is 24 J or more.SELECTED DRAWING: None 【課題】高強度かつ優れた低温靭性を有する建機フレーム用の電縫溶接鋼管、とりわけクレーンラチスあるいはクレーンブーム用の鋼管において、表面脱炭層の少ない鋼管であって、同時に厚肉品にも対応するような溶接品質に優れた電縫溶接鋼管を提供する。【解決手段】所定の成分組成、特にＴｉ／Ｎ≧４．０の関係を満足する組成を有し、鋼管の内表面および外表面の両面に脱炭層を備え、該脱炭層深さを、最大深さで０．２０ｍｍ以下とすることで、引張強度ＴＳ：７８０～９８０ＭＰａ、伸び：１５％以上、シャルピー衝撃試験によって得られる吸収エネルギー：２４Ｊ以上となる。【選択図】なし