Patent ID: 8920583
Filing Date: 2014-12-30
Classification: C21D,C22C,Y10T

Abstract:
1. A cold-formed, electric-resistance-welded steel pipe excellent in deformation characteristics consisting of, in mass % C: 0.04 to 0.10% Mn: 1.00 to 2.50%, Si: 0.80% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.10% or less, and N: 0.01% or less, one or more of Mo: 0.60% or less, Cr: 1.00% or less, Cu: 1.00% or less, V: 0.30% or less, B: 0.0003 to 0.003%, Ca: 0.01% or less, or REM: 0.02% or less, one or more of Ni: 1.00% or less, Nb: 0.01 to 0.30%, or Ti: 0.005 to 0.03%, and a balance of iron and unavoidable impurities, and wherein the cold-formed, electric-resistance-welded steel pipe has an outer diameter of 114 to 610 mm, a wall thickness of 5 to 15 mm, and a ratio of wall thickness to outer diameter of 0.03 or greater, wherein a content of Mn and a content of one or more of Cr, Ni, Mo and Cu satisfy wherein the cold-formed, electric-resistance-welded steel pipe has two-phase microstructure in the entire steel pipe consisting of a martensite-austenite phase having an area fraction of 2 to 10%, and a soft phase comprising at least one of a high-temperature tempered martensite and a high-temperature tempered bainite, wherein the martensite-austenite phase, the high-temperature tempered martensite and the high-temperature tempered bainite are obtained by heat treatment at a temperature of Ac wherein the cold-formed, electric-resistance-welded steel pipe has a work-hardening coefficient in the circumferential direction of 0.10 or greater.