Lauri Holappa

Addition of Titanium Oxide Inclusions into Liquid Steel to Control Nonmetallic Inclusions

Titanium oxide inclusions in steel are well known to inhibit grain growth and act as nucleation sites for acicular ferrite because of absorbing manganese from the surrounding steel resulting in a manganese depleted zone around the inclusion. In this article, the inclusions resulting from TiO2 additions to low-alloyed C-Mn-Cr steel were studied. Different types of TiO2 containing materials were added to liquid steel before or during casting to get small titanium-oxide–rich inclusions in steel. The main goals were to find out what happens to TiO2 in liquid steel after addition and during cooling and to study further what type of inclusions are formed in the steel as a result of the TiO2 addition. Based on the thermodynamic calculations and the results of scanning electron microscope (SEM)-energy dispersive spectroscope (EDS) and SEM-electron backscatter diffraction (EBSD) analysis, TiO2 is first reduced to Ti3O5 in liquid steel at high temperatures and then to Ti2O3 during cooling at around 1573xa0K (1300xa0°C). Both reactions liberate oxygen, which reacts with Ti, Mn, and Al forming complex Ti2O3-rich inclusions. The results also show that TiO2 additions result in more TiOxxa0+xa0MnO inclusions compared with experiments with Ti addition and that the absolute amount of manganese present in the inclusions is much higher in experiments with TiO2 addition than in experiments with Ti additions.

Thermal behaviour of hydrous nickel–magnesium silicates when heating up to 750°C

Abstract The thermal decomposition sequence of hydrous nickel–magnesium silicates from Colombia and Brazil was studied under air/Ar atmosphere from room temperature up to 750°C by differential scanning calorimetry–thermogravimetry followed by X-ray diffraction and scanning electron microscopy analyses. Differential scanning calorimetry curves of the samples obtained showed three endothermic peaks at 100, 250 and 600°C due to the release of free water, the dissociation of goethite and the release of crystalline water respectively. To determine the mineral species and microtexture, the ores were studied by scanning electron microscopy. Scanning electron microscopy–energy dispersive spectroscopy analyses showed that the ores are rich in Mg and Mg–Fe silicates, Cr spinel, Mn oxide, goethite and silica and exhibit complex alteration texture. X-ray diffraction analyses of Colombia-2 and Mirabela (Brazil) after the experiments showed that the dehydroxylation produces an amorphous intermediate phase, which is supposed to be due to the exsolution of silica. However, Colombia-1 sample, which was confirmed to contain antigorite mineral, was observed to undergo dehydration and recrystallisation simultaneously.

Chapter 1.1 – Ironmaking

This chapter describes the ironmaking technology and outlines the process essentials. The main focus was given to the blast furnace process, which is still the dominating technology for ironmaking. The emerging alternative ironmaking technologies—direct reduction and smelting reduction—are only briefly introduced, and will be described in a separate chapter in this book series. n nIn the beginning, there is a short introduction to the ironmaking history followed by a general description of the blast furnace ironmaking process, and its role in the global steelmaking industry. The readers are guided from the equipment aspects of the blast furnace and its auxiliary facilities, to the internal structure and materials interactions within the furnace. The types of raw materials (iron ore, coke and coal, and other fuels), their preparation and properties for the blast furnace process are also described. Furthermore, the aspects of process control, the process performance, and energy consumption are evaluated. In the end, the development trend of various ironmaking technologies is discussed.

Chapter 1.4 – Converter Steelmaking

Converter steelmaking is the main step in ore-based steel production using blast furnace hot metal and steel scrap as basic raw materials. About 70% of steel is nowadays produced via different variants of basic oxygen converters. n nThe converter process was developed in the middle of the nineteenth century using bottom-blown air for oxidation. Oxygen converting was developed about one century later, in the 1950s, first by applying oxygen top-blowing through lance and little later as oxygen bottom-blowing through nozzles. Toward the end of the past century, several new converter technologies were developed by combining top- and bottom-blowing of oxygen and inert gas, argon, or nitrogen. The converter-blowing technique, process control methodology, furnace construction, and long-life lining play key roles in optimal production of crude steel in various converter processes including the optimal achievement of desired end composition and temperature of the steel. n nIn this chapter, basic phenomena occurring in the converter and controlling the process are presented and discussed. These phenomena comprise chemical reactions in liquid metal, slag, and gas phases and at their interfaces, heat effects, and fluid flow and mass transfer phenomena in different reaction zones as well as in the entire system. Oxidation of carbon and other impurities in the iron melt is controlled mainly by mass transport from the oxygen jet to the iron melt in the converter. Other constraints are the thermodynamic affinities of individual oxidation reactions controlling oxygen distribution between the competing oxidation reactions in the oxidation environments, iron melt surface, slag, and surface layers of the melt. n nConverting stainless steel differs from that of carbon grades as the crude steel melt is rich in chromium. To minimize the oxidation of chromium, the oxidation potential must be controlled and decreased along the lowering carbon content. This can be done by diluting oxygen with inert gas, Ar/N2 as is done in AOD process or by decreasing the total pressure by vacuum. Fundamentals of stainless converting will be shortly discussed too.

Interfacial Phenomena in Fe-TiC Systems and the Effect of Cr and Ni

Abstract Titanium carbide is used as reinforcement component in composites due to its stability, hardness and wear resistance. A potential application is local reinforcement of stainless steel castings in which TiC is added only to the surfaces, which acquire wear resistance. This work is part of the study in which Ti and C are added in the mould during casting of stainless steel to form TiC at a specific surface. Wettability, stability and dissolution of the reinforcement particles are key phenomena in producing these composites in liquid state. Wetting experiments between TiC and Fe as well as FeNi and FeCr alloys were performed in order to get information about wettability, dissolution and formation of TiC and other phases related to the systems. Contact angles for the systems were attained without any reactions between the substrate and the alloy during heating. Wetting was not significantly improved by adding Ni or Cr. Dissolution of TiC was very strong into all the alloys. No new phases at the interface were formed in the experiments except for Cr-carbides in Cr-containing alloys. These results gave important information about the interfacial phenomena needed in developing the production methods.

Energy Efficiency and Sustainability in Steel Production

Iron and steel making plays a significant role in global energy consumption and carbon dioxide emissions. The target of limiting the global warming by 2050 is extremely challenging for this energy-intensive branch of industry. In order to be responsible for its own share in cutting CO2 emissions, great advancements must be done. A brief history, present situation and different scenarios are discussed. Possibilities to decrease CO2 emissions in current processes via improved energy efficiency, alternative process routes, energy sources and recycling are examined. On-going and planned efforts and programs of steel producers and institutions, as well as trends of energy generation in the long run are reviewed and evaluated.

Effect of Rice Husk Ash Insulation Powder on the Reoxidation Behavior of Molten Steel in Continuous Casting Tundish

Rice husk ash (RHA) has been widely used as an insulation powder in steel casting tundish. Its effect on the reoxidation of molten steel in tundish as well as on the corrosion of magnesia refractory was investigated. The reoxidation of the steel, indicated by an oxygen pickup, was progressed by increasing the ratio of RHA to the sum of RHA and carryover ladle slag (R ratio) greater than about 0.2. The increase of the silica activity in the slag layer promoted the self-dissociation of SiO2 from the slag layer into the molten steel, resulting in the silicon and oxygen pickup as the R ratio increased. The total number of reoxidation inclusions dramatically increased and the relative fraction of Al2O3-rich inclusions increased by increasing the R ratio. Hence, the reoxidation of molten steel in tundish might become more serious due to the formation of alumina-rich inclusions as the casting sequence increases. MgO in the refractory directly dissolved into the molten slag layer without forming any intermediate compound layer (e.g., spinel), which is a completely different situation from the general slag-refractory interfacial reaction. A flow was possibly induced by the bursting of gas bubbles at the ash-slag (-refractory) interface, since the silica in the RHA powder continuously dissolved into the molten slag pool. Thus, the RHA insulation powder has a negative effect on the corrosion of MgO refractory.

Studies on Interfacial Phenomena in Titanium Carbide/Liquid Steel Systems for Development of Functionally Graded Material

In modern materials applications, versatile, often contradictory requirements are set for properties like high strength, hardness, and toughness. However, e.g., in steel castings, typically only certain surfaces should be hard and wear resistant, whereas the other “bulk” might have only standard properties. Then the critical parts of the surface should be “locally reinforced” to get functionally graded material. Expensive alloying elements are saved, and manufacturing stages are minimized. Titanium carbide is an extremely hard material widely applied in carbide tools. It could be used to reinforce steel castings. When TiC particles are added to liquid steel, wettability, stability, and dissolution are key phenomena that should be understood to better design and control manufacturing processes. In this work, the interfacial phenomena and reactions between TiC and iron/steel melts were examined by wetting experiments with special emphasis on the influence of Cr, Ni, and Mo. No significant effect on wettability was observed by Ni or Mo. High Cr melts showed somewhat higher contact angles. Partial penetration of liquid metal took place in the substrate along the grain boundaries. Ni seemed to promote penetration. During longer experiments, re-precipitation of carbides occurred on the liquid droplet influencing the apparent wetting angle. Cr and Mo promoted carbide formation.

Chapter 1.7 – Inclusion Engineering

The aim of inclusion engineering is to generate such inclusions in steel which are beneficial or at least harmless during the process and in the final steel end product. Thus, it means control of the amount, size distribution, and composition as well as properties of nonmetallic inclusions in steel. In this review, the fundamentals of inclusions formation and control are discussed. As examples, production of steels with deformable silicate inclusions, modification of oxides, and sulfides with calcium, and inclusions for grain refinement are introduced. As a practical steelmaking case, the last half-century progress of ball-bearing steel is reviewed. Electro slag remelting is shortly examined as an example of unconventional method for steel refining, inclusion control, and structure improvement. Finally, some future trends and ideas are considered.

Chapter 1.6 – Secondary Steelmaking

Secondary metallurgy is a very central part of modern steelmaking process. It means a variety of different unit processes via which the final composition and even properties of steels are determined and adjusted. Typical unit processes in secondary steelmaking are deoxidation, desulfurization, degassing, decarburization, alloying and trimming additions as well as heating and temperature adjustment. n nIn this chapter, the fundamentals of thermodynamics and kinetics of unit processes are described. Further, the development and principles of most common technological methods to carry out processes are discussed including gas stirring, vacuum facilities, ladle furnace and chemical heating, and techniques for alloying and trimming additions.