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.
The behaviour of iron ore pellets in a blast furnace must be considered to improve ironmaking operations, especially when a large amount of the pellets is used. This study presents the reduction degree, mineralogical composition, and morphology of the pellet reduced in a gas mixture of 60% CO and 40% Ar at temperatures between 900 and 1,100oC. The pellet was prepared from iron ore from the Cao Bang province, Vietnam, by rotary drum. The obtained results showed that the reduction degree of the pellet increased with increasing reduction time and temperature. The activation energy of the reducing reaction was calculated to be 63.2 kJ/mol, which indicated that reduction occurred more easily in the present condition. X-ray diffraction (XRD) results revealed mineralogical phases such as hematite (Fe2O3), magnetite (Fe3O4), wüstite (FeO), metallic iron (Fe), and fayalite (Fe2SiO4) existing in the pellets when reduced for different times and temperatures. Fe and Fe2SiO4 were found to be the majority in the pellet that was reduced for 90 min at 1,100oC. Scanning electron microscopy (SEM) observations suggested the formation of a liquid phase, e.g., Fe2SiO4, which retarded the reducing reaction because it hindered the diffusion of gas flow inside the pellet. This phenomenon is essential to blast furnace ironmaking because pellets must be completely reduced before they move down to the liquid zone.
This paper investigated change of the mechanical properties of the superheater pipe steel grade P22 (ASTM A335) under constant stress of 9.68 MPa and various temperatures of 500-700 oC. The steel specimens were prepared from the new steel pipe taken from a coal-fired power plant in Vietnam, then heated to the given temperature and held for the predetermined time as 24, 48 and 72 hours in the atmosphere. The results showed that the mechanical properties of the steel P22 decreased with the increased temperature and time. This deterioration of the steel strength was caused by the redistribution of the carbide in the steel subjected under the elevated temperature and the constant stress. Although change in the microstructure including phase and grain size was not observed, the SEM micrographs showed that the carbide of the heated steel accumulated on the grain boundaries and the triple regions when the temperature and holding time increased. This phenomenon was attributed to reduce number of the carbide in the steel, resulting in decrease of the mechanical properties. The obtained results indicated that decrease of the mechanical properties of the steel pipe P22 can occur in the early period of working time in the practice.
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