HIGH-STRENGTH STEEL TUBE FOR USE AT LOW TEMPERATURES VERSATILE BY EXCELLENT STRENGTH WITH LONGITUDINAL BEND AND BY THE IMPACT STRENGTH OF HEAT-AFFECTED ZONE WITH WELDING

摘要

1. The tube of high strength steel for low temperature use, comprising: a portion of the base material, which comprises, in weight.%: C: more than 0.03% and 0.08% or less, Si: 0,01-0,5%, Mn : 1.5-3.0%, P: 0.015% or less, S: 0,003% or less, Al: 0,01-0,08%, Nb: 0,005-0,025%, Ti: 0,005-0,025% N : 0,001-0,010%, O: 0.005% or less and B: 0,0003-0,0020% and further contains one or more elements selected izSi: 0,01-1%, Ni: 0,01-1%; Cr: 0,01-1%, Mo: 0,01-1% and V: 0.01-0.1%, the balance Fe and unavoidable impurities, wherein the value of P (in%) calculated by formula (1) below, satisfies 0,19≤R≤0,25, wherein each element represents the content (wt.%) Propert va tensile base material constitute a tensile strength of 760 MPa or more and 930 MPa or less, the uniform elongation of 5% or more, the ratio of the yield strength of 85% or less, and the absorbed energy in the Charpy test at -40 ° C 210 J or more in the case where the sheet thickness is less than 25 mm and 150 J or more in the case where the plate thickness is 25 mm or more; and a portion of the weld metal, in which the composition of the weld metal for welding the longitudinal seam comprises: wt%. C: 0,03-0,10%, Si: 0,5% or less, Mn: 1,5-3,0%, P: 0,015% or less, S: 0,005% or less, Al: 0,05% or less, Nb: 0,005-0,05%, Ti: 0,005-0,03%, N: 0.010% or less, O: 0,015-0,045% uB: 0,0003-0,0050% and further contains one or more elements selected izSu: 0,01-1%, Ni: 0,01-2,5%, Cr: 0,01-1% Mo : 0.01-1.5% and V: 0.1% or less, and the balance Fe and unavoidable impurities, wherein the microstructure of the heat affected zones during welding, wherein the grain size of prior austenite is 50 m or more near to the melting line weld zone of the steel pipe, is a lower bainite structure or a polyphase structure comprising a lower bainite fraction