An Investigation of producability of dual-phase steel at the continuous annealing line of Ereğli Iron and Steel Plant

Ill AN INVESTIGETION OF PRODUCABILITY OF DUAL-PHASE STEEL AT THE CONTINUOUS ANNEALING LINE OF EREGLI IRON AND STEEL PLANT (Ph. D. Thesis) BİLGE DEMİR GAZI UNIVERSITY INSTUTE OF SCIENCE AND TECNOLOGY May 2003 ABSTRACT In this study, two different chemical composition of steels having 0,0640 C%, 1,72 Mn%, 0,46Si%, 0,67 Ni%, and 0,0617C%, l,89Mn%, 0,363Si%, 0.85Ni% were used. The volume fraction of transformed phases from austenite were determined by intercritical annealing of steels having fine and coarse austenite dispersions at three different intercritical annealing temperatures (ICAT) and then cooling at various rates. The results showed that pearlite or bainite were absent in all samples cooled from ICAT even at 0,01°C/s (furnace cooling) cooling rate. It was found out that martensite volume fraction (MVF) transformed from austenite present during ICAT increased on approaching Ai temperature. This increment was higher in samples with fine austenite dispersion during ICAT. By increasing MVF yield and tensile strength increased while uniform and total elongation decreased. At constant MVF (15- 25%) specimen with fine structure exhibited higher yield and tensile strength than coarse ones. Generally, strain values increased with increasing volume friction epitaxial (new) ferrite. Fine structures showed higher yield and tensile strength than coarse ones which exhibited slightly higher ductility. Microvoids were found to be nucleated on the martensite particles, inclusions or martesnsite-ferrite interface in the neck. The density of microvoids increased near to fracture surface. They were smaller in fine structure than that of coarseIV ones The coalescence of microvoids were dominant form of fracture in both structure. Science Code Key word Page number Adviser 710 : Dual-Phase Steels, Ostenite Dispersion, Hardenability, Martensite Volume Fraction, New Ferrit 159 : Assoc. Prof. Mehmet ERDO?AN Assist. Prof. Elmas SALAMCI