50/ Manufacturing method of 50kg/ grade steels for pressure vessels with excellent toughness and weldability

摘要

PURPOSE: A method for manufacturing 50 kg/mm¬2 grade steels for pressure vessels with excellent toughness and weldability is provided. CONSTITUTION: The method for manufacturing 50 kg/mm¬2 grade steels for pressure vessels includes the steps of (a) normalizing a steel comprising 0.18 wt.% or less of C, 0.50 wt.% or less of Si, 1.5 wt.% or less of Mn, Al 0.01 to 0.08 wt.%, a balance of Fe and incidental impurities at Ac3 to 930 deg.C wherein carbon equivalent (Ceq) is less than 0.40 wt.%; and (b) cooling the steel at a cooling rate of 0.5 to 5 deg.C/sec after normalizing. The steel further comprises at least one element selected from the group consisting of 0.5 wt.% or less of Cu or Ni, 0.10 wt.% or less of Nb, 0.10 wt.% or less of V, Ti 0.005 to 0.030 wt.%, B 0.0005 to 0.003 wt.%.