EXPERIMENTAL PROCESSING OF ULTRA-LOW CARBON STEEL USING VACUUM TREATMENT

Abstract

This paper presents experimental process of ultra-low carbon (ULC) steel using vacuum heat treatment. After adjusting the chemical compositions as desired, the ULC steel was casted into plate, hot-forged and cold-rolled to sheet of 1 mm thickness, finally annealed at 800oC. Microstructure, crystalline phase, non-metallic inclusions and mechanical properties of the ULC steels were characterized by optical microscopy, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and tensile test. Under argon vacuum atmosphere, decarburization occurred and C contents of the treated steels were reduced to 36 and 40 ppm corresponding to the decarburizing rate of 84.2 and 82.4%, respectively. The vacuum induction melting is thought to accelerate the rate of carbon removal from liquid steel. Electromagnetic force was attributed to promote the decarburization due to increasing the mass transfer coefficient during vacuum treatment. The annealed steels obtained a good combination of the strength and ductility; the total elongations were 45.2 and 42.9 %, while the yield strengths were 199 and 285 MPa, respectively. The results indicated that the ULC steels have only ferrite phase, of which grains size were 30 μm in average. The relative volume of non-metallic inclusions in the ULC steels was calculated as 0.23 vol. %, resulting positive contribution in the mechanical properties.