ULTRA HIGH STRENGTH COLD-ROLLED STEEL SHEET AND A HOT DIP PLATED STEEL SHEET WITH A LOW YIELD RATIO AND A METHOD FOR MANUFACTURING THE SAME, CAPABLE OF IMPROVING TENSILE STRENGTH BY CONTROLLING ALLOY COMPOSITION AND PROCESSING CONDITIONS

摘要

PURPOSE: An ultra high strength cold-rolled steel sheet and a hot dip plated steel sheet with a low yield ratio and a method for manufacturing the same are provided to have 1180MPa or more of tensile strength and 0.85 or less of low yield ratio by controlling alloy composition and a annealing process.;CONSTITUTION: An ultra high strength steel sheet is made of a cold-rolled steel sheet or hot dip plated steel sheet. The ultra high strength steel sheet contains one or more kinds of 0.1-0.2 weight% of carbon, 0.1-1.0 weight% of silicon, 2.0-2.5 weight% of manganese, 0.03-0.5 weight% of aluminum, 0.01-0.05weight% of titanium, 0.01-0.05weight% of niobium, 0.0005-0.005 weight% of boron, 0.05-0.5 weight% of copper, 0.05-0.5 weight% of nickel, 0.05-1.0 weight% of chrome, remaining amount of iron, and unavoidable impurities. The ultra high strength steel sheet has 0.85 or less of yield ratio 0.85 and 1180MPa or more tensile strength. A strength steel sheet contains 0.15 weight% or less of phosphorus, 0.02 weight% or less of sulfur, and 60pppm or less of nitrogen. The strength steel sheet has 780-980MPa of yield strength and 6% or more of elongation ratio.;COPYRIGHT KIPO 2013;[Reference numerals] (AA) Start; (BB) End; (S110) Hot rolling process; (S120) Cold rolling process; (S130) Annealing process(maintaining heating→primary cooling→secondary cooling); (S140) Autempering process; (S150) Hot-dip coating process