Si-killed Steel Research Articles

Crystallization Behavior, Viscosity and Structure of CaO–SiO2–MgO–(15~30 Mass Pct) Al2O3 Melts Representing the Oxide Inclusions in Si-Killed Steel

Crystallization Behavior, Viscosity and Structure of CaO–SiO2–MgO–(15~30 Mass Pct) Al2O3 Melts Representing the Oxide Inclusions in Si-Killed Steel

Effect of tundish flux on reoxidation behavior of Si-killed 316L stainless steel

The slag-metal reactions between Si-killed 316L stainless steel and the fluxes rice hush ash (RHA), RHA + CaO–SiO2 (flux A), and RHA + CaO–Al2O3 (flux B) were conducted in an induction furnace at 1773 K to investigate the effects of the various fluxes on the reoxidation behavior of the liquid steel. The total oxygen (T.O) content in the liquid steel increased when the steel reacted with RHA and RHA + A fluxes, whereas T.O decreased when the liquid steel reacted with RHA + B flux. The number density of non-metallic inclusion (NMI) in the steel reacted with RHA, RHA + A, and RHA + B fluxes increases, stays constant, and decreases, respectively. The increase in the number density of NMI is due to the reoxidation of the molten steel caused by the dissociation reaction of SiO2. However, the number density of NMI in the steel decreased since B flux has a higher value of physicochemical factor, i.e., log(ΔC/η), where η is slag viscosity and ΔC represents concentration difference between initial content and saturation limit of target oxide in slag. The reoxidation reaction caused by the presence of SiO2 in the fluxes and inclusion absorption (dissolution) reactions are competitive or coupled each other. Higher SiO2 activity in the fluxes (such as RHA) resulted in easier formation of SiO2-rich inclusions. However, the SiO2 activity in Flux A and Flux B is lower than that in RHA, and the MnO–SiO2 and SiO2 inclusions in the Si-killed steel were reduced by Al to form MnO–SiO2–Al2O3(-Cr2O3) system inclusions in the steels reacted with Flux A and B, respectively.

Aluminum-Free Steelmaking: Desulfurization and Nonmetallic Inclusion Evolution of Si-Killed Steel in Contact with CaO-SiO2-CaF2-MgO Slag

In some applications, deep desulfurization and deoxidation of steels without the use of aluminum are required, using Si as a deoxidant instead, with double-saturated slags in the CaO-SiO2-CaF2-MgO system. This work studied the desulfurization and nonmetallic inclusion evolution for the system using an induction furnace and compared the results with FactSage kinetic simulations. Steel samples were taken from the steel melt and analyzed with ICP-MS and combustion analysis for chemistry, and SEM/EDS for nonmetallic inclusion quantity, size, and composition. The results indicate that the steel was deeply desulfurized, with a final sulfur partition coefficient of 580; MgO was reduced from the slag, yielding dissolved [Mg] that transformed liquid Mn–silicate inclusions into forsterite and MgO. Intentional reoxidation of the melt with oxidized electrolytic iron demonstrated a significant concentration of dissolved [Mg] in the steel, by the formation of additional forsterite and MgO upon reoxidation.

Behaviour of Sulphide and Non-alumina-Based Oxide Inclusions in Ca-Treated High-Carbon Steel

Although alumina-based inclusions in Al-killed steel have been widely investigated, few studies focus on non-metallic inclusions in Si-killed steel. Most high-carbon steel, which is expected to have higher potential for the formation of alumina, is deoxidised by silicon to avoid alumina-based inclusions. Many types of high-carbon steel such as free-cutting steel and grinding media for the mining industry include sulphur and calcium; therefore, non-alumina-based oxide and CaS-based inclusions co-exist in molten steel. In this study, the behaviour of sulphide and non-alumina-based oxide inclusions in Ca-treated liquid steel was investigated via transition of the composition and morphology in the liquid state. Using a confocal scanning laser microscope (CSLM), sulphide particles being pulled toward oxide inclusions were directly observed and changes in the composition of inclusions investigated using different analytical techniques. We identified two types of sulphide phase after remelting and cooling, polygonal particles and a ring-shaped layer around the oxide phase. When the sulphide capacity Cs multiplied liquid fraction of oxide had a higher value, the sulphide layer tended to become wider.

From waste to wealth: recycling the secondary resource from steel ladle as a flux in Si-killed steelmaking process

ABSTRACTWith the regulations pertaining to the environment and waste disposal becoming stringent day by day, the steelmaking industry is being compelled to adopt a zero-waste-policy for sustainability. As a consequence, 3R (reduce, reuse and recycle) technique of waste handling has become the most favoured option among the industries. This paper focuses on recycling of slag generated after secondary refining process. Trials were conducted in two stages, namely (a) prevention of dusting phenomenon of Al-killed steel ladle slag to generate lump slag by suitably selecting the ladle covering compound and chemically modifying the slag, and (b) utilisation of the lump slag as a 100% replacement of synthetic slag in Si-killed steel ladles, leading to a reduction in its specific consumption from 1.2 kg/TLS to zero. As a process requirement, specific power consumption, arcing time and degree of desulphurisation were considered as the major parameters for comparing the results using hypothesis test statistical tool.

Determination of Three-Dimensional Morphology and Inner Structure of Second-Phase Inclusions in Metals by Non-Aqueous Solution Electrolytic and Room Temperature Organic Methods

The secondary-phase particles in metals, particularly those composed of non-metallic materials, are often detrimental to the mechanical properties of metals; thus, it is crucial to control inclusion formation and growth. One of the challenges is determining the three-dimensional morphology and inner structures of such inclusions. In this study, a non-aqueous solution electrolytic method and a room-temperature organic technique were developed based on the principle of electrochemistry to determine the three-dimensional morphologies and inner structures of non-metallic inclusions in Al-killed steel, Si-killed steel, and ductile cast iron. The inclusions were first extracted without any damage to the inclusions, and then the collected inclusions were wrapped and cut through Cu ion deposition. The results revealed that the inclusions in Al-killed steel had an irregular morphology, that those in the Si-killed steel were mainly spherical, and that almost all the spheroidal graphite in the ductile cast iron featured a uniform globular morphology. In addition, nucleation was not observed in the inner structures of the inclusions in the Al-killed steel, while some dendritic or rod-like MnS phase precipitates appeared on the silicate inclusion surfaces, and some silicate-rich phases were detected in their inner matrix. For spheroidal graphite, rare-earth oxides (one particle or more) were observed as nuclei in the center of almost every graphite particle. The formation and evolution of inclusions in these types of metals can be better understood by means of the two developed methods.

Vanadium microalloying for ultra-high strength steel sheet treated by hot-dip metallising

Ultra-high strength steel sheets have been subjected to heat treatments that simulate the thermal cycles in hot-dip galvanising and galvannealing processes and evaluated with respect to their resulting mechanical properties and microstructures. The steels contained suitable contents of carbon (∼0.2%), manganese (1.2%) and chromium (0.4%) to ensure that they could be fully transformed to martensite after austenitisation followed by rapid cooling in a continuous annealing line, prior to galvanising. Different contents of vanadium (0–0.1%) and nitrogen (0.002–0.012%) were used to investigate the possible role of these microalloying elements on the strength of the tempered martensite. Vanadium, especially when in combination with a raised nitrogen content, helps to resist the effect of tempering so that a larger proportion of the initial strengthening is preserved after the galvanising cycle, giving tensile strength levels exceeding 1000 MPa. Different deoxidation practices using aluminium or silicon have also been included. These showed similar strength levels at corresponding carbon contents but the bendability of the Si-killed steel sheet was considerably superior. Microstructural examinations have been made on the annealed steels but the reason for the beneficial effect of vanadium is still not fully explained. It is concluded that microalloying with vanadium is a very promising approach in the development of corrosion-resistant ultra-high strength steel sheet products.

The enhancement of hydrogen attack in steel by prior deformation

An investigation has been made of the cause of enhancement of hydrogen attack (HA) by prior cold working. Bars of 1020 Si-killed steel deformed in three point bending were found to contain microcracks at ferrite/ferrite grain boundaries containing cementite inclusions and at ferrite/pearlite boundaries. Very long cementite plates crack, but shorter cementite inclusions (⪯ ≃2 μm) suffer decohesion from the matrix at or near the inclusion ends. The observed anisotropy of microcracking with respect to grain boundary orientation and sign of the deformation stress can be understood in terms of the stress state at the inclusion/matrix interface during deformation. The microcracks are not eliminated by an annealing treatment which is known to cancel the effect of deformation on HA. It is concluded that residual stresses arising from the difference in deformability between cementite particles and matrix are responsible for the influence of deformation on HA. Formerly with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60201