Ca-treated Steel Research Articles

Practical Implications of Using an Online Data-Driven Optimizer for Calcium-Treated Steels

Calcium (Ca) additions during secondary steelmaking are a well-adopted practice to transform solid oxide non-metallic inclusions (NMIs) into globular-shaped liquid oxides. The claimed hypothesis that liquid NMIs reduce SEN clogging has been proven in the past by researchers. However, the exact quantity of Ca needed to transform the physical state of NMIs during steelmaking remains uncertain. Operators in the steel plant use a consistent quantity of Ca additions for specific steel grades, but this approach does not account for the varying physical states and evolving dynamics of NMIs characteristics in each ‘heat’. To overcome this, a study was conducted to explore the impact of varying Ca additions on the transformation and behavior of NMIs in low-alloyed Ca-treated steel grades. The aim was to establish a more reliable and responsive approach to Ca treatment, potentially leading to more effective control in preventing submerged entry nozzle (SEN) clogging. The proposed methodology involved online monitoring of NMIs state coupled with controlled variations in Ca addition, deviating from fixed quantity, to observe its effects on NMIs state transformations. Through careful analysis of collected data and the implementation of a data-driven optimizer, this study reports the practical implications of using optimal amounts of Ca during secondary steelmaking. The resulting change due to dynamic calcium silicide (CaSi)-cored wire additions and their impact on SEN clogging were evaluated. The findings reveal the significant role of optimal CaSi wire additions, leading to improved steel castability and a notable 30 pct reduction in SEN clogging tendencies. The results obtained after the implementation of the data-driven optimizer ‘ClogCalc’ have significant implications for steel manufacturers, offering new insights into enhancing Ca treatment efficiency.

Online Supervisory System for In-Process Optimization of Calcium Additions by Continuously Monitoring the State of Non-metallic Inclusions Inside Low-Alloyed Liquid Steels

A decision support system was developed using supervised machine learning (ML) approach for optimization of calcium (Ca) additions by continuously monitoring the physical state of non-metallic inclusions (NMIs) inside low-alloyed liquid steels. In this work, two instances were considered to design the base algorithm for the proposed supervisory system: (1) Clogging of submerged entry nozzle (SEN) during continuous casting of steels due to accumulation of solid oxide non-metallic inclusions (NMIs) and (2) Ca treatment during secondary steelmaking for modification of oxide NMIs from solid to liquid state to avoid SEN clogging. At first, experimental investigations were carried out on liquid steel samples from three low-alloyed Ca-treated steel grades from the same steel family to evaluate the characteristics of solid oxide NMIs that cause SEN clogging. In the next step, data-driven models were developed using an in-house ML algorithm trained primarily with process data for calculating the value of the newly proposed dummy parameter ‘Clog.’ These models, after testing, were architected to develop a supervisory system based on experimental investigations and data-driven models. The objective of this proposed supervisory system was to predict the optimum quantity of Ca needed for successful modification of NMIs from solid to liquid state to avoid SEN clogging based on the forecasted ‘Clog’ value. Finally, industrial data from ~ 3000 heats were tested to verify the results obtained from the developed supervisory system. The results confirmed that this novel supervisory system could predict the optimum class of Ca for all studied steel grades with 95 to 98 pct accuracy. The integration of this online supervisory system in steel production is expected to minimize operators’ corrective actions in achieving realistic control of Ca additions.

Coarsening Mechanisms of CaS Inclusions in Ca-Treated Steels

In this work, the coarsening mechanisms of CaS inclusions in liquid steel were investigated by analyzing inclusions in experimental and industrial samples. A detailed particle size distribution evolution was reported. The observed CaS coarsening rate was compared with the theoretical coarsening rate calculated by using the models proposed in the literature. For both experimental and industrial data, it was observed that the coarsening mechanisms varied during different stages of Ca treatment. It was found that in the early stage (after Ca addition) of experiments and during Ca addition under industrial conditions, the coarsening of CaS was governed by diffusion-controlled growth. As the Ca dissolved in steel diminished, the coarsening was governed by collision-dependent mechanisms. For experimental conditions, the growth of CaS was controlled by the Brownian collisions, while the coarsening by turbulent collisions was the dominant mechanism under industrial conditions.

Inclusion Evolution in Al-Killed Ca-Treated Steels at Heat Treatment Temperature In Situ Observed Using Confocal Scanning Laser Microscope

Inclusion Evolution in Al-Killed Ca-Treated Steels at Heat Treatment Temperature In Situ Observed Using Confocal Scanning Laser Microscope

The Effect of Aluminum Addition on the Evolution of Inclusions in an Aluminum-Killed Calcium-Treated Steel

In the current study, the reduction of CaO-Al2O3 inclusions in steel by the subsequent addition of aluminum is revealed through industrial trials, laboratory experiments, and thermodynamic calculations. During the industrial production of the Al-killed Ca-treated steel, the Al2O3 content in CaO-Al2O3 inclusions increased from 72.05% to 79.83%, after the aluminum addition. In laboratory experiments, the addition of aluminum increased the Al2O3 content, and meanwhile lowered the contents of CaS and CaO in the inclusions. The average diameter of the inclusions decreased and the number density increased after the Al addition in steel, due to the formation of tiny inclusions. Based on the thermodynamic consideration, the transformation of the inclusions was attributed to the re-equilibrium of the reaction between steel and the inclusions after aluminizing. The Al2O3-riched calcium aluminate with a high melting point increased the possibility of the nozzle clogging during the continuous casting process, reducing the effect of the calcium treatment. Therefore, the addition of aluminum after calcium treatment should be avoided as far as possible.

Wear mechanisms of PcBN tools when machining AISI 316L

This paper investigates the machinability of AISI 316L stainless steel without and with NMI treatment, when machined with three different PcBN tool grades: low cBN content (50% vol.), medium (65% vol.) and high (90% vol.). The NMI treatment consisted of resulfurized and Ca-treated steels with two different Si/Al ratio. The tool materials were evaluated in terms of tool life, showing the best performance for the medium cBN content grade. Results showed that the crater wear has a more significant effect on the tool performance than the standard wear criteria of maximum flank wear. The worn tools were examined using scanning electron microscopy (SEM), focused ion beam (FIB) and transmission electron microscopy (TEM). In most cases, the crater exhibited three distinct regions from the edge to the end of the contact zone: the plateau, middle crater, and the upper region. Plateau and crater region displayed tool protective layer (TPL), consisting mostly of Al, Si, Mg and Ca-oxides; and some dimples in the tool from preferential wear of cBN grains. In the upper region of the best performing material, a ~1.5 μm thick TPL of mostly MnCr2O4 spinel was found, a result of the transfer of Mn- and Cr-rich oxide NMIs found in this material or oxidation of workpiece material, during the machining process.

Onset of the degradation of CVD α-Al2O3 coating during turning of Ca-treated steels

The ability to control the shape, distribution and composition of non-metallic inclusions has had an important impact on many aspects of steel making. One such impact is on the machinability. Ca-treatments have shown to be able to reduce the abrasiveness of oxide inclusions, improve chip-breaking and lead to formation of deposits that reduce tool wear. However, machining Ca-treated steels with Al2O3 coated cemented carbide tools has not been as advantageous as expected. This study investigates the mechanisms behind the anomalous wear of Al2O3 coatings when turning soft Ca-treated steels.Longitudinal turning tests at a range of speeds (vc = 100–600 m/min) show rapid localized degradation of the Al2O3 coating limited to the sliding zone. Detailed analysis of the degradation mechanisms was performed using scanning and transmission electron microscopy. The results demonstrate a presence of chemical interactions between the alumina coating and non-metallic inclusions. The interaction resulted in the formation of mainly calcium aluminates and partly alumina-magnesia spinel. In-operando infrared thermography measurements indicate cutting temperatures of 850–1000 °C. Thermodynamic calculations give that CaO and MgO readily reacts with Al2O3, while the reaction with CaS requires presence of additional oxygen at these cutting conditions. Additional turning experiments investigate the influence of oxygen by controlling the cutting environment by adding oxygen (compressed air) or removing oxygen (supply argon). These additional tests show that the presence of additional oxygen has a limited impact on the possible Ca–Al2O3 interaction. This demonstrat a potential for further machinability improvements by controlling the chemical interaction between Ca and Mg based non-metallic inclusions and alumina coatings.

Dependence of the Clogging Possibility of the Submerged Entry Nozzle during Steel Continuous Casting Process on the Liquid Fraction of Non-Metallic Inclusions in the Molten Al-Killed Ca-Treated Steel

In the current study, the nozzle clogging behavior and inclusion composition in Al-killed Ca-treated steels were observed to investigate the relationship between the liquid fraction of non-metallic inclusions and the clogging possibility of the submerged entry nozzle. Clogging materials were mainly MgO-Al2O3 with less than 20% liquid phases, while most of the inclusions were full liquid CaO-Al2O3-MgO in tundish at the casting temperature. Thus, it was proposed that the nozzle clogging can be effectively avoided by modification of solid inclusions to partial liquid ones rather than full liquid ones. There was a critical value of liquid fraction of inclusions causing the nozzle clogging. A critical condition of the inclusion attachment on the nozzle wall was a function of cosθN−S+cosθI−S<0. With the increase of T.Ca content in steel, the evolution route of inclusions was solid MgO-Al2O3→liquid CaO-Al2O3-MgO→solid CaS and CaO. To avoid the clogging of the submerged entry nozzle (SEN) under the current casting condition, the appropriate T.Ca concentration range in Al-killed Ca-treated steels can be enlarged from the 100% liquid inclusion zone of 10–14 ppm to the 20% liquid inclusion zone of 4–38 ppm.

Preparation and application of ZrB2-SiCw composite powder for corrosion resistance improvement in Al2O3–ZrO2–C slide plate materials

ZrB2-SiCw (w-whisker) composite powders were synthesized by a microwave-assisted carbo/borothermal reduction method under argon atmosphere using zircon, boric acid, and activated carbon as starting materials. The optimized ZrB2-SiCw composite powder was obtained by adjusting heating temperature and amounts of boric acid and activated carbon. Further, the effect of the optimized ZrB2-SiCw composite powder addition on corrosion resistance of the Al2O3–ZrO2–C slide plate material against O2 and Ca-treated steel was investigated. The results showed that the relative content of ZrB2-SiCw increased with increasing temperature. The excess of boric acid favored the formation of the ZrB2-SiCw, and the optimum amount of boric acid was overdosed to 30–45 wt%. The corrosion resistance and thermo-mechanical properties of Al2O3–ZrO2–C slide plate material could be improved effectively with 6 wt% ZrB2-SiCw composite powder addition, which was attributed to the excellent oxidation and corrosion resistance of ZrB2-SiCw composite powder. In addition, ZrB2-SiCw could confer excellent thermal shock resistance and reduce the generation of cracks, ultimately preventing corrosive media from infiltrating into slide plates, further increasing corrosion resistance of the plates.

Formation and Deformation Mechanism of Al2O3-CaS Inclusions in Ca-Treated Non-Oriented Electrical Steels

Industrial trials were performed to study the effect of calcium treatment on inclusions in non-oriented electrical steels. The evolution and characterization of inclusions in both molten steel and rolled steel were investigated, including a thermodynamic analysis using FactSage 7.1. In the Ca-treated steel, alumina inclusions were transformed into Al2O3-CaO-CaS, with a mass fraction of CaO that increased with increasing the Ca/S ratio. Inclusions of Al2O3-CaO-CaS were classified into wrapping and adhesion type according to their morphologies. Adhesion-type Al2O3-CaO-CaS inclusions were observed only in the steel with Ca/S > 0.84. The two types of Al2O3-CaO-CaS inclusions were transformed into Al2O3-CaS with distinctive morphologies. The mass fraction of Al2O3 and CaS in the inclusions was experimentally found to depend on the Ca/S ratio of the steel and confirmed by thermodynamic analysis. The two types of Al2O3-CaS inclusions could hardly be deformed during the hot-rolling process of the steel but showed different deformation behavior during the cold-rolling process of the steel. The component of CaS in the adhesion-type Al2O3-CaS inclusions was more easily separated from Al2O3 and formed a tail along the rolling direction of the steel, while only a little part of the CaS component broke off from the wrapping-type Al2O3-CaS inclusions.

Modification of Non-Metallic Inclusions in Oil-Pipeline Steels by Ca-Treatment

Corrosion rate in different steel grades (including oilfield pipeline steels) is determined by the presence of non-metallic inclusions (NMI) in steels. Specifically, the effect of different inclusions on the quality of steels depends on their characteristics such as size, number, morphology, composition, and physical properties, as well as their location in the steel matrix. Therefore, the optimization and control of NMI in steels are very important today to obtain an improvement of the material properties of the final steel products. It is well known that a Ca-treatment of liquid steels in ladle before casting is an effective method for modification of non-metallic inclusions for improvement of the steel properties. Therefore, the NMI characteristics were evaluated in industrial steel samples of low carbon Ca-treated steel used for production of oil-pipelines. An electrolytic extraction technique was used for extraction of NMI from the steel samples followed by three-dimensional investigations of different inclusions and clusters by using SEM in combination with EDS. Moreover, the number and compositions of corrosion active non-metallic inclusions were estimated in hot rolled steel samples from two different heats. Finally, the corrosion resistance of these steels can be discussed depending on the characteristics of non-metallic inclusions present in the steel.

Transient Behavior and Thermodynamics of Inclusions in Al-Ti-Deoxidized and Ca-Treated Steel

Ti-alloyed steels have severe nozzle clogging problems, which are detrimental to the steel production process. However, little research on the Al-Ti-Ca-O complex inclusion in molten steel has been carried out. Based on that, experiments with different calcium additions were carried out in an Al2O3 crucible at 1873 K to investigate the variation of inclusions in Ti-bearing Al-killed steel in the current article. It was found that calcium treatment significantly influenced the morphology, composition distribution, and size of oxides in Ti-bearing Al-killed steel. Inclusions with a higher CaO or TiOx content were liquid at 1873 K (1600 °C), indicating that the inclusions in Ti-contained steel can be modified to liquid ones through the proper calcium treatment process. The appropriate amount of calcium should be added to the steel depending upon the aluminum and titanium contents. Thermodynamics of the inclusions in the Al-Ti-Ca-O molten steel system at high temperature and during the solidification process were comprehensively calculated, including all types of inclusions such as calcium oxide, aluminum oxide, titanium oxide, calcium aluminate, calcium titanate, and liquid inclusion. The thermodynamic calculations agree well with the experimental results and can precisely predict the formation of the inclusions in Al-Ti-Ca deoxidized steel.

Tracing the Origin of Non-ferrous Oxides in Lamination Defects on Hot-Rolled Coils: Mold Slag Entrainment vs Submerged Entry Nozzle Reaction Products

In this work, lamination defects (slivers) on hot-rolled coils of Ca-treated steel were investigated for microstructure and composition using optical and scanning electron microscopy combined with microanalysis (SEM/EDS). The goal was to identify possible origins for the observed defects which contain a complex assemblage of phases, such as different types of calcium aluminates (CA, CA2, CA6), melilite (C2AS), spinel (MA), and a newly identified phase, CNA2. Mold slag similar to that employed during the cast was absent. Analysis of the bulk composition of some of the defects indicated these to be too rich in alumina to be derived from mold slag through steel-slag redox exchange. In contrast, microstructural observation of the inner side of the submerged entry nozzles (SEN) used during casting showed deposits with compositions comparable to those of the defect material. Based on an estimation of the chemical evolution of mold slag interacting with steel, it is found that the defects are not likely to be entrained mold slag but remobilized SEN deposits, as supported by several microstructural and trace phase criteria. However, it should be noted that extensive reduction of mold slag by steel can lead to compositions rich in sodic-calcic aluminates (CNA2). Therefore, differentiation between specific locations of the defect materials within a casting system requires detailed analysis from the potential sources of origin as well as from the materials found in the defects.

Effects of the composition of Ca-rich inclusions on tool wear mechanisms during the hard-turning of steels for transmission components

This study describes the influence of the steel characteristics of Ca-treated carburising steel grades during hard part turning of synchronising rings in gearbox production. The main focus was on the chemical composition of the non-metallic inclusions in the evaluated workpieces and their effect on the PCBN tool wear. In addition, a Ca-treated carburising steel grade was compared to a standard steel grade.Machining tests were performed at the transmission machining site at Scania in order to evaluate the PCBN cutting tool life as defined by the generated surface roughness during actual production. The progression of flank and crater wear was evaluated by using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a secondary electron (SE) detector.The Ca-treated steel showed a more than doubled tool life than that of the standard steel grade. The superior machinability was linked to the formation of a Ca-enriched slag barrier composed of (Mn,Ca)S and (Ca,Al)(O,S). It is believed that the stability of the protective deposits is essential to minimise diffusion-induced chemical wear of the PCBN tool.

A kinetic model for Ca treatment of Al-killed steels using FactSage macro processing

A kinetic model was developed to investigate the evolution of inclusions during the Ca treatment of Al-killed steels. Evolutions of steel chemistry and inclusion composition in Al-killed Ca-treated steels from previous experimental works were accurately predicted using the current kinetic model. The model was widely used to predict the evolution of inclusions during the Ca treatment of Al-killed steels with varying concentrations of Al, S, O and Ca. During the Ca injection of Al-killed steels, a large number of CaS and CaO inclusions are transiently generated in the Ca-rich zone due to the excessive Ca. Al2O3 inclusions are gradually modified to calcium aluminates in the bulk steel with the addition of Ca. The superfluous Ca or S may lead to the formation of solid CaS in the liquid steel.

In-situ observation of the behaviour of non-metallic inclusions at different interfaces in the system steel-slag-refractory

The system steel-slag-refractory and reactions of non-metallic inclusions at its interfaces are decisive in numerous metallurgical processes. In order to obtain the desired steel cleanness level, a detailed understanding of the occurring interactions is needed. This study focuses on the in-situ observation of different reactions of non-metallic inclusions in steel and slag. For this purpose a Confocal Scanning Laser Microscope attached to a high temperature furnace is applied which enables the visualization of metallurgical phenomena at steelmaking temperatures up to 1700 °C. Two different examples are discussed: First, the agglomeration of different inclusion types in a Ca-treated steel is described and corresponding attraction forces are calculated. Secondly, the dissolution of Al2O3 and MgOAl2O3 in two slags of the system CaO-Al2O3-MgO-SiO2 is examined, including the evaluation of the governing dissolution mechanism. Finally, an outlook on further experimental possibilities is given.

The effect of inclusion composition on tool wear in hard part turning using PCBN cutting tools

This work reports on hard part turning of carburizing steels using a PCBN cutting tool in fine machining. Emphasis is on the link between composition of the inclusions in work material and wear mechanisms of the cutting tool. A Ca-treated machinability improved 20NiCrMo steel was included together with three other carburizing steels with different inclusion characteristics.Machining tests were conducted to examine cutting tool life and its balance between excessive flank wear and crater wear. The wear mechanisms were examined using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a secondary electron (SE) detector.The longest tool life was obtained when cutting the Ca-treated steel. The improved machinability is linked to the deposition of complex (Mn,Ca)S and (Ca,Al)(O,S) protective slag layers that form on the rake face of the cutting tool during machining. Cutting in this steel also resulted in a typical ridge formation in the tool edge crater. Transfer of workpiece material to the rake face crater is characteristic in hard part turning of steel with high cleanliness. This is suggested to be related to the lack of the sulfides that lubricate conventional machinability treated steels, and that the crater wear of low-sulfur steel is more pronounced than for steels with higher sulfur content.

Study of Inclusions in High Sulfur, Al-Killed Ca-Treated Steel via Experiments and Thermodynamic Calculations

In the production of special steels, inclusions have a significant influence on final product quality. Moreover, there is a growing need for constant improvement of mechanical and machinability properties, such as in the production of complex pieces for the automotive applications. The main objective of this study was to conduct a thermodynamic study aimed at preventing clogging problems during continuous casting (CC) and the formation of undesirable inclusions. Alumina (Al2O3), spinels, and calcium sulfide (CaS) are potential sources of defects during rolling and steel forging. The specific goals of this study were: (1) To determine the phases and compounds formed in the inclusions as a function of composition and casting temperature adopted for high sulfur Al-killed Ca-treated steel, and (2) To establish chemical composition conditions resulting in the formation of less harmful inclusions for steel castability. Based on the literature and previous studies by the authors, calculations were carried out using FactSage commercial software and related databases. The calculations were performed using the steel temperature and chemical composition in the CC tundish. The results of these calculations were used to estimate the composition of steel and non-metallic inclusions (oxides and sulfides) at the given steel temperature. Based on these calculations, it was predicted a calcium content range where inclusions contained a high proportion of liquid phase (i.e., “liquid window”). The results of thermodynamic simulation were compared to industrial outcomes in terms of inclusion composition and casting index. The thermodynamic predictions were validated and confirmed by industrial results.

Evaluation of Inclusion Characteristics in Low-Alloyed Steels by Mainly Using PDA/OES Method

The characteristics of non-metallic inclusions (such as number, size and volume fraction) in liquid steel samples taken during ladle treatment and casting of industrial heats of two low-alloyed Ca-treated steel grades were evaluated by using the Pulse Distribution Analysis with Optical Emission Spectroscopy (PDA/OES) method. These results were compared to data obtained by Scanning Electron Microscope observations of inclusions after electrolytic extraction from steel samples (the EE method). It was found that the PDA/OES method can be used for a relative estimation of the homogeneity of the distribution of non-metallic inclusions in steel samples. Bottom and middle parts of the steel samples showed more homogeneous results with respect to the characteristics of the investigated Al2O3, CaO–Al2O3 and CaO–Al2O3–CaS inclusions. The numbers of inclusions in the size ranges 2.0–5.7 µm and 1.4–5.7 µm in samples taken before and after a Ca addition, respectively, showed a relatively good agreement between both methods. Furthermore, the calculated volume fractions for inclusions in the size range 2–13 µm obtained by the PDA/OES method agreed satisfactorily well with those obtained from the EE method. Finally, the minimum sizes of inclusions in steel samples, which can reliably be detected by the PDA/OES method, were estimated for steels with different concentrations of Al in steel and Al2O3 in inclusions.

Dynamic Fracture Toughness of High Strength Cast Steels

The dynamic fracture toughness of Cr and Mo steels with nickel contents of 0, 1.56, and 5.5 wt.% was evaluated and compared to a lightweight steel of composition Fe-30.40%Mn-8.83%Al-1.07%Si-0.90%C-0.53%Mo. Each steel was heat treated to a Rockwell C-scale hardness range of 36 to 38. The 4130, 4325, and HY130 steels were quench-hardened and tempered. The lightweight steel was solution treated, water quenched, and age hardened. In the hardness range of Rockwell C36 to C38, the lightweight steel, the 4325 steel, and the Al-killed and Ca-treated HY130 steel had similar dynamic fracture toughness values of 153, 153, and 165 kJ/m2, respectively. The 4130 steel had a much lower toughness of 94 kJ/m2. The lightweight Fe-Mn-Al-C alloy performed better at Rockwell C32, producing the highest dynamic fracture toughness of 376 kJ/m2. Toughness of the Cr and Mo steels was strongly dependent on deoxidation practice. Alloys treated with ferro-titanium showed a reduction in toughness, which was attributed to TiN particles and in one case eutectic Type II sulfides. Addition of misch metal to an aluminum and ferro-titanium treated HY130 steel eliminated the Type II sulfides and increased the dynamic fracture toughness from 58 to 88 kJ/m2. HY130 obtained the highest toughness (165 kJ/m2) when aluminum deoxidation was followed by calcium treatment.