Production of Low-P Steel

Production of low phosphorus steel using the basic oxygen steelmaking process is conventionally achieved by maintaining a highly basic slag. This leads to high flux consumption, generates a large volume of slag and restricts recycling of the slag due to presence of free lime. The environmental impact goes further, since the calcined lime involves depletion of a mineral resource, consumption of fossil fuels and release of CO₂. However, recent studies indicate that adequate dephosphorisation is possible in the BOF even if the slag basicity is reduced from the current practice (3.3–3.6) to a level of 2.7–2.8. Phosphorus partitioning deviates considerably from equilibrium and post stirring of bath helps in lowering bath phosphorus. Induction furnace is widely used to produce structural grade steel. Since lining is usually acidic, effective phosphorus removal is not achieved. But using basic lining reasonably low phosphorus steel could be produced.     

Modelling inclusion evolution in Al–Ti-killed steels during ladle mixing process

A reaction model was developed to better understand the evolution of inclusions in Al–Ti-killed steels during the ladle mixing process. The fluctuation of steel chemistry gave rise to the transient evolution of inclusions during the mixing process. The formed Al₂O₃ in the steel can be hardly influenced by the addition of FeTi, while adding Al can effectively modify the TiO_x-containing oxides to solid Al₂O₃. The formation of Al₂O₃–TiO_x inclusions can be suppressed by increasing Al and lowering Ti in the steel. The alloying sequence of adding Ti after the Al addition is beneficial to improve the recovery of Ti. The one-point strong air absorption may cause the formation of the unwanted Al₂O₃–TiO_x inclusions in Al–Ti-killed steels. The critical oxygen contents in the molten steel with varying Al and Ti concentrations were predicted to avoid the formation of Al₂O₃–TiO_x inclusions and Ti loss.     

Improvement of reaction efficiency in hot metal dephosphorisation

Abstract

In Japan, the hot metal pretreatment process has been developed to refine hot metal under conditions where each impurity can be removed most efficiently. At Nippon Steel, three types of hot metal pretreatment process, using torpedo car, hot metal ladle, or LD converter, are employed, that make a great contribution to the reduction of slag volume. Recently, Nippon Steel has developed a new hot metal pretreatment called the multirefining converter (MURC) process, in which dephosphorisation and decarburisation are carried out continuously in the same converter. Nevertheless, since the dephosphorisation efficiency of CaO is less than 30%, CaO is now being used in far greater quantities than that stoichiometrically required to make 3CaO.P₂O₅. As hot metal dephosphorisation is a non-equilibrium reaction, in which hot metal is in contact with slag whose oxygen activity differs greatly from that of hot metal, it is important to increase the interfacial oxygen activity. From the results of a fundamental experiment, the 2CaO.SiO solid phase, in which P₂O₅ can be dissolved, has a great effect on the dephosphorisation reaction.     

Distribution of P2O5 between Phosphorus‐Enrichment Phase and Matrix Phase in Phosphorus‐Containing Slag

In order to make better recovery utilization of the phosphorus in the steel slag, and as P₂O₅ mainly exists in phosphorus‐enrichment phase and matrix phase of steel slag, it is a must to study the distribution of P₂O₅ between those two phases in the phosphorus‐containing slag and its related influencing factors. Heat experiments were conducted in this research, and the final slag was analyzed with the method of scanning electron microscope (SEM) + energy disperse spectroscopy (EDS). Then L_p, r_p, and C_p in both phosphorus‐enrichment phase and matrix phase were calculated, respectively. From this study, is has been found that CaO% in matrix phase, the basicity of slag and mass fraction of P₂O₅ in both phosphorus‐enrichment phase and matrix phase influence the distribution ratio of P₂O₅ in the slag. Adding CaF₂ also has effect on P₂O₅ partition while adding MgO and MnO has no obvious effect. Besides, for the low content of T.Fe in matrix phase, there is no linear relationship between L_p or $\gamma _{{\rm P}_{2} {\rm O}_{5} } $ and T.Fe in matrix phase.     

Investigation on the removal efficiency of inclusions in Al-killed liquid steel in different refining processes

Industry trials were carried out to study the removal efficiency of inclusions in Al-killed liquid steel in the processes of BOF–LF–RH–CC and BOF–RH–CC. It was found that the removal efficiency of inclusions has a high dependence on inclusion types. Solid inclusions are more easily to be removed than liquid inclusions. The removal efficiency of solid Al₂O₃ inclusions is higher than that of solid CaO–Al₂O₃–MgO inclusions. As liquid CaO–Al₂O₃–MgO inclusions coexisted with solid CaO–Al₂O₃–MgO inclusions in the liquid steel, the low removal efficiency of inclusions in RH degassing process was found in BOF–LF–RH–CC process. However, high removal efficiency and ultra-low total oxygen (T.O) content were obtained in BOF–RH–CC process because the inclusions were mainly composed of solid Al₂O₃ although initial T.O content before RH degassing was relatively high. This is due to the fact that solid Al₂O₃ tends to form cluster-shaped inclusions which have both a higher contact angle and a lower work of adhesion with steel than calcium aluminate, resulting in easier removal by RH degassing. Therefore, it is proposed to weaken steel–slag reaction and calcium treatment before RH degassing to retain solid Al₂O₃ inclusions in the steel.     

Soda slag system for hot metal dephosphorization

Phosphorus partition ratios between Na₂O-SiO₂-PO_2.5 slags and carbon saturated iron have been measured as a function of slag composition at 1200 °C. To avoid excessive Na₂O loss by reaction with carbon, the initial slag-metal interfacial area was increased by using a powder mixture of slag and metal to reduce the time for the equilibrium. The results indicate that phosphorus partition ratios, L_p, are relatively high, about 20 for an equimolar slag (X _{Na 2 O}/X _{S 1 O 2} = 1) and are independent of P level in the slag. The phosphate capacity and the activity coefficients of PO_2.5 and FeO for these slags are calculated from the experimental results using available thermodynamic data. The oxygen potential at the slag-metal interface was found not to be controlled by Si in metal under an Ar atmosphere or by CO partial pressures in CO-Ar mixtures. Experimental results and thermodynamic calculations show thatP _{O 2} at the interface between Na₂O-SiO₂ slags and carbon saturated iron is controlled by the C-CO equilibrium where the CO pressure is determined by C-Na₂O equilibrium for which the CO pressure is close to 1 atmosphere in all cases.     

A thermodynamic model for calculating manganese distribution ratio between CaO–SiO2–MgO–FeO–MnO–Al2O3–TiO2–CaF2 ironmaking slags and carbon saturated hot metal based on the IMCT

A thermodynamic model (IMCT-L_Mn) for calculating manganese distribution ratio between CaO–SiO₂–MgO–FeO–MnO–Al₂O₃–TiO₂–CaF₂ slags and carbon saturated liquid iron have been developed based on the ion and molecule coexistence theory. The predicted manganese distribution ratio shows a reliable agreement with the measured ones. With the aid of the IMCT-L_Mn model, the respective manganese distribution ratio of (Mn²⁺?+?O^2?), MnO·SiO₂, 2MnO·SiO₂, MnO·Al₂O₃, MnO·TiO₂, and 2MnO·TiO₂ are investigated. The results indicate that the structural units SiO₂?+?FeO play a key role in CaO–SiO₂–MgO–FeO–MnO–Al₂O₃–TiO₂–CaF₂ slags in demanganisation process in the course of hot metal treatment at 1673?K. The manganese distribution ratio at a given binary basicity range increases with CaF₂ content, while that decreases with TiO₂ content at different binary basicity scopes, which demonstrate that high Mn in the metal is favoured by TiO₂ content. In the present study, various critical experiments are carried out in an effort to clarify the effect of temperature on demanganisation ability, indicating that the lower temperature of molten metal is, the faster the rate of demanganisation reaction is and the shorter the thermodynamic equilibrium time is and the lower end-point Mn content is. It can be deduced from the obtained experimental results that the greater oxygen potential of slags or iron-based melts, lower content of basic oxides in slags, and lower temperature at reaction region is benefit for demanganisation reaction.     

Formation of Inclusion in Ti–Al Killed Low C–Mn Steel

The formation of inclusion in Ti–Al complex deoxidized C–Mn steel was investigated. When Al content in steel is very low ([Al]=0.0005%), for 0.003%<[Ti]<0.007%, the inclusion is the Al₂O₃–SiO₂–MnO–TiO_x composite inclusion; for [Ti]≥0.009%, the inclusion is TiO_x in the steel. When [Ti]=0.005%, [Al]<0.001%, the inclusion is the Al₂O₃–SiO₂–MnO–TiO_x composite inclusion; while [Al]>0.006%, inclusions would be pure Al₂O₃. The experimental results agree with the thermodynamics conclusions.     

Oxygen Activity Calculations of Molten Steel: Comparison With Measured Results

This study sets out to find some model/s that could calculate the closest oxygen activity of molten steel to the measured oxygen activity in an ASEA‐SKF ladle furnace. Ten steel heats grade SAE 52100 were chosen, the oxygen activities of the molten steel after vacuum degassing process were measured, then by means of two different equations and one computer programs the theoretical oxygen activities were calculated and compared to the measured results. In order to calculate the activities of Al₂O₃ in the top slag four different models and two different constants were applied. Both Wagner and Turkdogan's equations were found to be useful. It was seen that increasing the wt% Al from 0 up to 0.05 in molten steel, increasing CaO/Al₂O₃ ratio in top slag, and reducing equilibrium temperature of slag‐steel could contribute to reduction of oxygen activity of molten steel. The scanning electron microscope observations revealed that the main types of observed non‐metallic inclusions in these samples were spinels and calcium aluminates and by increasing the CaO content of the inclusions their equivalent circle diameters grew.     

Dephosphorization equilibria between pure molten iron and CaO-saturated FeOn–CaO–SiO2 and FeOn–CaO–Al2O3 slags

Metal-slag refining reactions have been investigated to determine dephosphorization equilibria in steelmaking using CaO-saturated slags, low in P₂O₅–content, based on the systems FeO_n–CaO–SiO₂ and FeO_n–CaO–Al₂O₃. Slag compositions have been optimized with respect to basicity and oxygen potential to achieve maximum partition ratios wt.%(P₂O₅)/wt.%[P] and minimum phosphorus contents in pure molten iron at 1550, 1600 and 1700°C. Both slag systems prove to be effective dephosphorizers. Optimal slag compositions are around 10 wt.% SiO₂ near the CaO–3CaO · SiO₂ double saturation in the case of FeO_n–CaO–SiO₂ slags and at Al₂O₃ contents tending to zero in the case of FeO_n–CaO–Al₂O₃ slags. Attempts were also made to present phosphate capacities C_PO4^3?, fractions of free oxygen ions x_O^2? and theoretical optical basicities Λ as a function of the FeO_n content of slags.     

Effect of Physicochemical Properties of Slag and Flux on the Removal Rate of Oxide Inclusion from Molten Steel

The slag-metal reaction experiments were carried out using a high-frequency induction furnace to confirm the effect of slag composition on the removal rate of inclusions in molten steel through the CaO-based slags. The apparent rate constant of oxygen removal (k _O) was obtained as a function of slag composition. It increased with increasing basicity, and the content of MgO and CaF₂, whereas it decreased by increasing the content of Al₂O₃ in the slag. The removal rate of inclusions was strongly affected not only by the driving force of the chemical dissolution but also by the viscosity of the slags and fluxes.     

Studies on Dephosphorisation of Steel in Induction Furnace

Steel melters who use recycled steel scrap in small induction furnaces have no control on phosphorus input from scrap. For furnaces of capacity less than 2T, dephosphorisation in external ladle is not possible owing to excessive heat loss. Studies on dephosphorisation were thus carried out in the induction furnace itself using lime based fluxes with and without addition of fluxing agents like CaF₂ and Na₂O. The fluxes were added directly and also added through injection. The effects of individual components of fluxes on dephosphorisation were estimated. Correlation were developed between dephosphorisation and different expressions of basicity values. An operator can easily use these correlation to predict dephosphorisation levels depending on type of flux used and mode of addition.     

Effect of Al2O3 and MgO on Interfacial Tension Between Calcium Silicate‐Based Melts and a Solid Steel Substrate

The effect of Al₂O₃ and MgO on the interfacial tension between the molten CaO–SiO₂‐based slag and solid steel at 1773 K was studied. The interfacial tension of molten slags slightly increased with increasing Al₂O₃, but no significant change of interfacial tension was observed with higher MgO. Fourier transform infrared (FTIR) of as‐quenched slag samples indicated the slag structure to polymerize with Al₂O₃ additions, but depolymerize with MgO additions. Further detailed studies of the slag surface using X‐ray photoelectron spectroscopy (XPS) showed the fraction of free oxygen ions to decrease with higher Al₂O₃ but remained constant at higher MgO. The results suggested that interfacial tension decreases not only with the depolymerization of the melt, but also with an increase in the free oxygen ions at the molten slag/solid steel interface.     

Effect of protected electroslag remelting on cleanliness of G20CrNi2Mo bearing steel

To meet the high cleanliness requirements of bearing steel used in high-speed railway trains, a new production process combining vacuum induction melting and electroslag remelting (ESR) was used to produce G20CrNi2Mo bearing steel. To investigate the effect of remelting on the cleanliness of the steel, two kinds of G20CrNi2Mo steels were prepared using an ESR furnace with and without high-purity argon protection. The results show that the G20CrNi2Mo electrodes smelted using a vacuum induction furnace have very high cleanliness 0.010[P%]–0.004[S%]–0.0012[O%]–0.0041[N%]). Unprotected ESR leads to an increased oxygen content, while protected ESR prevents any increase in oxygen content. Both protected and unprotected ESR results in significant desulphurisation, with desulphurisation rates reaching over 50%. The protected ESR process removes Al₂O₃–SiO₂–MnO inclusions, and the remaining inclusions in the steel can be divided into two categories, Al₂O₃ and Al₂O₃–MnS.     

In Situ ceramic particle-reinforced aluminum matrix composites fabricated by reaction pressing in the TiO2 (Ti)-Al-B (B2O3) systems

Particulate TiB₂ reinforced aluminum-based metal matrix composites (MMCs) were successfully fabricated by means of the reaction processing method. TiB₂ particulates were formed in situ through the reaction of Ti and B in Ti-Al-B, TiO₂ and B in TiO₂-Al-B, and TiO₂ and B₂O₃ in TiO₂-Al-B₂O₃ systems. The results showed that in situ TiB₂ particulates formed in the Ti-Al-B system had a size of 5 μm and they exhibited block and rodlike structures. Moreover, coarse Al₃Ti blocks several tens of micrometers in size were also formed simultaneously. On the other hand, equiaxed Al₂O₃ and TiB₂ particulates with a size of less than 2 μm were formed in situ in the TiO₂-Al-B and TiO₂-Al-B₂O₃ systems. The Al₃Ti phase was completely eliminated in the TiO₂-Al-B system with increasing B content. Tensile tests revealed that the Al₂O₃ · TiB₂/Al composite fabricated from the TiO₂-Al-B system exhibits excellent mechanical properties. The yield strength of the Al₂O₃ · TiB₂/Al composite appeared to increase with increasing TiB₂ content. The yield strength of the Al₂O₃ · TiB₂/Al composite could be further increased by introducing CuO into the TiO₂-Al-B system. Such an increment in mechanical strength arose from the strengthening effect caused by the Al₂Cu precipitates. The incorporation of CuO had no effect on the in situ reaction process of the TiO₂-Al-B system. Finally, the effect of SiC addition on the microstructure and mechanical properties of the composites fabricated from the TiO₂-Al-B and TiO₂-Al-B-CuO systems was also investigated.     

Addition of Dispersoid Titanium Oxide Inclusions in Steel and Their Influence on Grain Refinement

In this article, the addition of dispersoid titanium oxide inclusions into liquid steel, the effect of additions on the inclusions found in the steel and on grain refinement, and acicular ferrite formation were studied. Different TiO₂-containing materials and addition procedures into liquid steel were tested in experimental heats to obtain inclusions that promote grain refinement and acicular ferrite formation in C-Mn-Cr steel. Different additives with metallic Ti and TiO₂ were added into the steel melt just before casting or into the mold during casting to create Ti-containing inclusions. The aluminum content in steel was lowered by an addition of iron oxide. The samples taken from steel melts and ingots were studied with a scanning electron microscope to find inclusions and to analyze them. Thermodynamic calculations showed that the Al content should be low (<50 ppm) to obtain Ti oxide dominating inclusions, whereas Al₂O₃ were formed at higher Al contents. When TiO₂ was added late before casting, the oxide inclusions were Ti oxides and were mixed with Ti, Al, and Mn oxides. Small inclusions around 1 μm were detected in the samples with TiO _x or TiN as the main component. It could be concluded that the additions resulted in a clearly higher number and in a smaller size of TiO _x inclusions than just by adding metallic Ti. Selected samples were brought for subsequent hot rolling and heat-treatment experiments to find out the grain-refining effect and the eventual formation of acicular ferrite. Grain refinement was observed clearly, but the presence of acicular ferrite could not be confirmed definitely.     

Control of MgO·Al2O3 Spinel Inclusions during Protective Gas Electroslag Remelting of Die Steel

The effect of calcium treatment and/or aluminum-based deoxidant addition on the oxygen control and modification of MgO·Al₂O₃ spinel inclusions during protective gas electroslag remelting (P-ESR) of H13 die steel with low oxygen content was experimentally studied. It is found that all the inclusions in the consumable electrode are MgO·Al₂O₃ spinels, besides a few MgO·Al₂O₃ spinels surrounded by an outer (Ti,V)N or MnS layer. After P-ESR refining combined with proper calcium treatment, all the original MgO·Al₂O₃ spinels in the electrode (except for the original MgO·Al₂O₃ spinels having been removed in the P-ESR process) were modified to mainly CaO-MgO-Al₂O₃ and some CaO-Al₂O₃ inclusions, both of which have a low melting point and homogeneous compositions. In the case of only Al-based deoxidant addition, all the oxide inclusions remaining in ESR ingots are MgO·Al₂O₃ spinels. The operation of Al-based deoxidant addition and/or calcium treatment during P-ESR of electrode steel containing low oxygen content is invalid to further reduce the oxygen content and oxide inclusions amount compared with remelting only under protective gas atmosphere. All the original sulfide inclusions were removed after the P-ESR process. Most of the inclusions in ESR ingots are about 2 μm in size. The mechanisms of non-metallic inclusions evolution and modification of MgO·Al₂O₃ spinels by calcium treatment during the P-ESR process were proposed.     

Influence of Al2O3 and TiO2 degradation behaviour of sinter and hematite at low temperatures on reduction

Abstract

It is known that the presence of Al₂O₃ and TiO₂ tends to increase the intensity of the reduction degradation of sinter. To enable a basic understanding of the phenomena taking place, the present work evaluates some aspects of paramount importance, such as: (a) microstructural changes of sinter containing Al₂O₃ and TiO₂ during low temperature reduction; (b) theoretical and experimental volume changes occurring during the hematite to magnetite reduction step as well as the effect of Al₂O₃ and TiO₂ in solid solution with hematite, using absolute dilatometry; and (c) crystal lattice distortions of hematite and magnetite owing to solid solution with Al₂O₃ and TiO₂, determined by line broadening of X-ray diffraction peaks. The results show that hematite containing Al₂O₃ and TiO₂, subjected to a reduction process to the magnetite stage, yields a magnetite phase with a distorted crystal structure. This phenomenon, in association with structural deformation resulting from the volumetric expansion of hematite crystals during reduction, can cause crack generation and propagation in the sinter, leading to disintegration.