Hot-rolled and composite organization (組織 熱 延 高) Strength Steel and its manufacturing method

摘要

The steel sheet comprises 0.03-0.15% C, 0.6-1.8% Mn, 0.040.2% P, up to 0.10% Al as a deoxidising agent, S less than 0.008% and the balance Fe. The steel sheet has a dispersed structure comprising 70% or more by a cross-sectional surface ratio of ferrite and 5% or more of martensite structure. The ratio of its yield strength to its tensile strength is 0.7 or more. The steel may further contain 0.2- 2.0% Si or C. A steel melt having the specified composition is formed into a slab. The slab is heated at 1100-1250 deg.C, hot- rolled at a finish temp. of 780-900 deg.C and coiled at lower than 450 deg.C. During a period from completion of hot rolling to coiling. The steel sheet is cooled at 10-200 deg.C/sec.