HIGH-STRENGTH STEEL, PRODUCTION METHOD THEREFOR, STEEL PIPE, AND PRODUCTION METHOD THEREFOR

摘要

Provided is a technology whereby a tensile strength of at least 620 MPa (at least API X80 grade) as required for API X80 or higher grade steel pipes can be obtained even after extended ageing in a medium temperature range. The high-strength steel is characterized by: comprising a specified component composition; having a parameter Peff of at least 0.050%; tensile strength (TS) and tensile strength (TSO) fulfilling the formula (TSO-TS)/TSO ≤ 0.050, said tensile strength (TS) being the tensile strength at 350°C measured after ageing performed under conditions in which the Larson Miller Parameter (LMP) = 15,700 and said tensile strength (TSO) being the tensile strength at 350°C measured before said ageing; and having a toughness of at least 100J at vE-20, in a welding heat-affected zone formed when welded.