HIGH STRENGTH STEEL SHEET HAVING EXCELLENT WELDED ZONE PROPERTY FOR LINEPIPE AND THE METHOD FOR MANUFACTURING THE SAME

摘要

A high strength steel sheet for linepipe is provided to obtain excellent physical properties in a welded zone and a yield strength of at least 690 MPa by especially controlling alloy elements and a relational expression and controlling the cooling rate, and a method for manufacturing the high strength steel sheet for linepipe is provided. A high strength steel sheet having excellent welded zone properties for linepipe comprises, by weight percent, 0.04 to 0.1% of C, 0.1 to 0.4% of Si, 1.0 to 2.2% of Mn, 0.01 to 0.05% of soluble Al, 0.005 to 0.02% of Ti, 0.01 to 0.06% of Nb, 0.01 to 0.1% of V, 0.1 to 0.5% of Mo, 0.015% or less of P, and 0.005% or less of S with the balance being Fe and other inevitable impurities, the Ti, N, Nb, and V satisfy 0.005=(Ti-3.4N)+Nb+V=0.15, and a matrix of the steel sheet has a tensile strength of 760 to 904 MPa. The steel sheet further comprises at least one of 1.0% or less of Cu, 1.0% or less of Ni, and 1.0% or less of Cr. A method for manufacturing a high strength steel sheet having excellent welded zone properties for linepipe comprises: reheating a steel slab at a temperature of 1050 to 1180 deg.C, the steel slab comprising, by weight percent, 0.04 to 0.1% of C, 0.1 to 0.4% of Si, 1.0 to 2.2% of Mn, 0.01 to 0.05% of soluble Al, 0.005 to 0.02% of Ti, 0.01 to 0.06% of Nb, 0.01 to 0.1% of V, 0.1 to 0.5% of Mo, 0.015% or less of P, and 0.005% or less of S with the balance being Fe and other inevitable impurities, the Ti, N, Nb, and V satisfying 0.005=(Ti-3.4N)+Nb+V=0.15; finish rolling the reheated steel slab to a reduction ratio of 70 to 82% at the austenite recrystallization temperature or lower; and cooling the rolled steel sheet at a cooling rate of 15 to 30 deg.C/sec and stopping cooling of the rolled steel sheet at a temperature of 350 deg.C or lower.