Oxide inclusion evolution and factors that influence their size and morphology

The evolution of oxide inclusion size distribution and the shape of the distribution during steelmaking and casting and the process variables that influence the inclusion characteristics at different stages were investigated and documented. A statistical method for transforming the 2D size distribution to their actual 3D distributions and the application of a kinetic model to determine nucleation and growth mechanisms were tested. Finally, laboratory experiments were performed to study the effects of preexisting inclusions, steel active oxygen content, and supersaturation on the size and morphology of Al2O3 inclusions. The inclusion size, composition, and morphology following steel deoxidation were found to depend on the steel conditions during deoxidation, and the method/sequence of deoxidant addition. The oxide size distribution evolved from lognormal to fractal and the distribution shape was quadratic or linear on a log-log plot. The distribution shape was preserved on both 2D and 3D analysis and used to identify new and aged inclusion populations. The Schwartz-Saltykov method for converting 2D data to 3D was found to be inadequate and the applied kinetic model could not explain certain observed trends. Finally, results from the laboratory study showed the oxide inclusion transformations from preexisting FeO and MnO\u27SiO2 inclusions to Al2O3 following Al-deoxidation. The reaction of Al with FeO was relatively fast compared to the sluggish reaction with MnO\u27SiO2. Al2O3 dendrites and clusters were observed after Al-deoxidation. The clusters consisted of spheres and dendrites and three possible sources of cluster formation were identified. The size of the spherical Al2O3 were larger with increased FeO size and results showed that increasing supersaturation had the strongest influence on the length of the dendrite --Abstract, page iv

Grain refinement of high alloy stainless steels in sand and directionally solidified castings

The goal of this research project is to develop an industrially viable melting process that will control the crystallization macrostructure of austenitic grades of cast steels. Titanium nitride (TiN) has proven to be an effective grain refiner of austenite. Theoretical simulation and experimental application has led to the development of a repeatable grain refining melt process for austenitic stainless steel alloys. Grain refinement of the as-cast structure of Cr-Ni stainless steel alloys solidified with primary FCC, BCC and dual FCC/BCC phases was studied experimentally. Refinement was achieved in both cast ferritic and austenitic grades. Dual solidification of FCC/BCC phases resulted in an unrefined macrostructure. It is proposed that solidification sequence can limit the grain refining capability of heterogeneous nuclei. Two inoculation-based melt practices were developed to study grain refinement in cast austenitic stainless steels. The first includes in-situ formation of TiN on to Mg-Al spinel oxides, and the second involves master alloy additions containing preformed TiN. The master alloy method extended the equiaxed zone and improved the distribution of TiN in the casting. The in-situ method showed more effective grain size refinement. The effect of the developed grain refining melt practice on the properties of cast superaustenitic stainless steel (similar to CK3MCuN) was examined. Heat treatment had no effect on the as-cast grain size. The grain refined alloy exhibited a reduction in segregation after heat treatment; an increase in ultimate tensile strength (+11%), yield strength (+13%), ductility (+8%), hardness (+2%), pitting corrosion; a decrease in impact strength and intergranular corrosion rate in comparison to the unmodified, base alloy --Abstract, page iv

Inclusion engineering in FeMnAl steels

Low density high Mn and Al steels, or FeMnAl steels, show great promise for military vehicles and automotive applications in which high strength and toughness is a requirement. However, these steels are subject to processing challenges including development of oxide and nitride inclusions during melting and casting as well as a large as-cast grain size and heavy interdendritic segregation. This can lead to non-uniform heat treatment response and cracking during subsequent hot rolling. Adding up to 10%Al lowers the density of these steels by as much as 15%, unfortunately, this also results in large amounts of hard and faceted AlN inclusions that are known to reduce toughness. Inclusion engineering techniques in other cast alloys can mitigate the effect of harmful inclusions or decrease grain size, segregation, and microporosity and improve ductility and toughness. Unfortunately, there is limited understanding of inclusion evolution in FeMnAl steels and the inclusion engineering strategies to improve mechanical properties. The goal of this research is to explore potential nonmetallic inclusions as inoculants to refine the as-cast grain size as well as potential mitigation of detrimental AlN by soft and globular MnS co-precipitation. MnS was effective at coating most of the AlN inclusions. However, this produced a large overall inclusion population that reduced dynamic fracture toughness. The potential of Ti(C,N), Nb(C,N), and complex Ce-oxides to refine the as-cast grain size was investigated. A decrease in the columnar zone was observed with addition of FeSiMg+FeTi and Ce addition, however, the equiaxed grain size did not decrease. A low N melt practice with Ti additions was effective at eliminating AlN”--Abstract, page iv

Simultaneous modification and grain refinement of the aluminum-silicon-copper alloy using a Magnesium Matrix Alumina Composite as the Master Alloy

The main properties that make aluminum alloys a valuable material are their light weight, strength, corrosion resistance, durability, ductility, formability and thermal conductivity. Due to these unique combinations of properties, the variety of applications for aluminum continues to increase. In addition, to increase the reliability of cast aluminum components the microstructure must be modified and refined. The castings being modified by currently used master alloy have many problems (segregation, porosity). This thesis was designed to investigated the Magnesium Matrix Alumina Composite (MMAC) Master Alloy would be able to solve some of these problems. The MMAC Master Alloy contains nano Alumina particles and Magnesium which have the unique abilities to simultaneously refine and modify the microstructure of the cast aluminum structure. The experimental work performed showed that the MMAC Master Alloy reduces the Secondary Dendrite Arm Spacing and improves the Silicon Modification Level, Porosity Area Fraction and Mechanical Properties of the casting

Investigation of the reaction between mould slag inclusions and high aluminium steels in the continuous caster : innovation report

Mould slag entrainment during the continuous casting process presents a late-stage source of non-metallic inclusions (NMI) with a high likelihood of ending up in the final steel product. The reaction between the entrained slag phase and surrounding liquid steel in the continuous casting mould affects the inclusion morphology and properties. The changes in composition can have adverse effects with the viscosity and heat transfer properties of the mould flux. However, there is a lack of information on the kinetics of the NMI-steel reaction. Using a controlled synthetic inclusion and metal samples, the interaction between inclusion-slag droplets and steel can be investigated to study the reaction and any change in morphology of the inclusion. Using a correlative approach combining High-Temperature Confocal Scanning Laser Microscopy (HT-CSLM), X-ray Computed Tomography (XCT) and advanced electron microscopy techniques offers the ability to observe the size, shape and composition of an unconstrained reacting inclusion within the steel matrix without dissolving the steel. To investigate this, a low aluminium steel (0.04 wt. %) and a high aluminium steel (1 wt. %) in contact with an inclusion-slag phase with a starting composition aligned to a typical silica-rich mould slag. It was found that the reaction between silica and aluminium across the interface of the two phases provided a driving force for spontaneous emulsification of the inclusion to occur. Further tests were carried out with an inclusion particle positioned in a dimple on the surface of the steel heated to different temperatures (representing conditions of solid inclusion – solid steel; liquid inclusion – solid steel; and liquid inclusion – semi-solid steel to calculate reaction times that could be present in the continuous caster as the steel slab is cooling. A mathematical model has also been developed to calculate the kinetics of the reaction by using Gibbs Free energy to calculate the equilibrium constant and the compositions of the components at equilibrium. It has been found that the emulsification of inclusions will have an effect on the floatation rate and potential inclusion removal can be up to 11% longer if spontaneous emulsification were to occur in the continuous caster. Battery cans with inclusions were also investigated and characterised to define a potential pathway for these defects to occur. The knowledge created has a great impact as inclusions affect all grades of steel, which results in huge losses of yield. Improving any aspect of the yield will have an enormous effect on sustainability due to steel manufacturing being extremely energy intrusive and having a large carbon footprint. The steels used in this study AHSS (Advanced High Strength Steel) including TRIP (Trans- formation Induced Plasticity) steel and DP (Dual Phase) steel are becoming prevalent for their low cost and high strength characteristics