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 800<sup>o</sup>C. 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.
Highlights
In production of ultra-low carbon (ULC) steel, efficiently removing carbon is an essential step which much affect quality and price of the final product; many attentions have focused on the carbon removal from liquid steel
It was seen that decarburization of the liquid steel occurred during the vacuum treatment, so the C content was reduced to ultrap-ISSN 1335-1532 e-ISSN 1338-1156
The CO bubbles move out from the liquid steel i.e. decarburization can continue at ultra-low carbon level
Summary
In production of ultra-low carbon (ULC) steel, efficiently removing carbon is an essential step which much affect quality and price of the final product; many attentions have focused on the carbon removal from liquid steel. It is possible to lower C content of less than 0.005% ( 50 ppm) using vacuum heat treatment of liquid steel, so the RH degasser is a potential process and provides a high productivity of ULC steel [1,2].
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