Introduction

The assortment of all types of ferroalloys is traditionally determined by the requirements of their consumers, mainly steelmaking enterprises, the technological capabilities of ferroalloy production, and the quality of raw materials. Over the past half-century, steel production technology has undergone significant changes, including the shift of ferroalloy addition operations for alloying, deoxidation, and refining from melting units to ladles1,2,3,4,5. This requires adjustments to several physicochemical properties of ferroalloys due to the reduced temperature in the ladle, limited interaction time of reagents, and other factors. However, the chemical composition of the main grades of ferrochrome in Kazakhstan has not been revised for more than 50–70 years6,7. At the same time, the raw material base of ferroalloy enterprises has significantly changed over the last 50 years8,9,10. Today, low-grade chromite raw materials account for 67% of the global chromite ore reserves11,12,13. Many ferrochrome producing countries are already facing shortages of high-quality ores, both in terms of chemical and granulometric composition14. The gap between the growing demand for ferrochrome by metallurgists and the declining reserves of high-grade chromite ores are expected to increase annually15. Consequently, the inclusion of low-grade chromite ores in production worldwide has become an inevitable necessity. The use of new low-grade chromite raw materials leads to the production of new compositions of ferroalloys, and thus, changes in their physicochemical and consumer properties16. Additionally, in foreign plants producing alloyed steels, the technology involving high-carbon grades of ferrochrome with reduced chromium content, known as “charge chrome”, has become widely adopted. The primary global producers of “charge chrome” are metallurgical enterprises in South Africa and Fin-land. This type of ferrochrome typically contains the following percentages: > 45% Cr; 3–10% Si; 6–8% C. Apart from its use in producing chromium steels, “charge chrome” is also utilized for producing medium-carbon ferrochrome through converter processes17. Unfortunately, “charge chrome” has not yet gained significant adoption in Kazakhstan’s metallurgical plants. This highlights the necessity of studying the properties of newly developed chromium-containing ferroalloys and determining their optimal compositions to meet the modern requirements of steelmaking.

Development of new effective compositions of ferroalloys of the Cr–C–Si–Fe system with improved characteristics (reduced melting time in steel) and the possibility of obtaining them from poor mineral raw materials with a reduced chromium content and a high concentration of iron.

A methodology for determining the optimal composition of ferroalloys has been developed at the IMET Ural Branch of the Russian Academy of Sciences (UrO RAN)18,19,20,21. This methodology is based on the correlation between the composition and properties of alloys and includes two main factors: the preliminary selection of ferroalloy elements according to the specified composition and properties of the treated metal, and the determination of the optimal ratio of elements based on the study of the physicochemical characteristics of ferroalloys and the patterns of their interaction with the iron-carbon melt22,23,24. The methodology takes into account the key factors influencing both the processes of ferroalloy production and application, as well as the properties of the treated steel.

Materials and methods

This study investigates the most significant physicochemical characteristics of complex chromium-containing ferroalloys: density, melting temperatures, and melting time in an iron-carbon melt (liquid steel). These characteristics are essential for selecting compositions, developing alloy production technologies, and ensuring their subsequent application.

The following samples were selected for the study:

  • A standard ferrochrome sample (used as a reference for physicochemical characteristics), produced using the industrial technology adopted at the plant in a submerged arc furnace (Table 1, Alloy 1)

  • Chromium alloy samples with reduced chromium content (Table 1, Alloys 2-4)

  • Chromium alloy samples with varying silicon content at reduced chromium levels (Table 1, Alloys 2, 5, 6).

Table 1 Chemical composition of chromium-containing ferroalloy samples.
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Alloys 2-6 were produced by melting high-carbon ferrochrome (Table 1, Alloy 1), low-carbon ferrochrome (68.1% Cr; 0.03% C; 1.2% Si), ferrosilicon (75.9% Si), steel, ultra-pure carbonyl iron, and metallic manganese in a Tammann furnace under a stream of inert gas (Fig. 1).

Fig. 1
figure 1

High-Temperature Tamman Furnace. (1) Graphite tube; (2) Water-cooled copper jaws; (3) and (4) Water-cooled body; (5) Graphite crucible; (6) Slag; (7) Metal; (8) Refractory body; (9) Thermocouple.

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The crystallization onset temperature for complex multicomponent alloys, such as the studied ferroalloys, must be determined experimentally. For this purpose, we selected the most accurate and straightforward method-recording the temperature curves during the cooling of the melt25. A Tammann-type resistance furnace with an alumina sheath was used as the melting unit. The experiments were conducted under a stream of inert gas. The furnace was heated 50–100 °C above the complete melting temperature of the alloy, followed by cooling at a rate of 10–15 °C/min, during which plateaus were recorded on the cooling curves.

We studied the density of solid chromium-containing ferroalloys. The density of the alloys was determined experimentally using the pycnometric method26.

To investigate the melting time of chromium-containing alloys in an iron-carbon melt, we used a mathematical model for calculating melting time developed by researchers from Ural State Technical University (USTU-UPI) and IMET UrO RAN27. The model examines the melting process of a solid ferroalloy piece fully submerged in an iron-carbon melt (steel). Under real conditions, the initial temperature of the ferroalloy piece is always lower than the crystallization temperature of steel, resulting in the formation of a solid steel shell on its surface at the beginning28,29. The melting process depends primarily on the relationship between the crystallization onset temperature of the ferroalloy, the crystallization temperature of the processed metal, and the steel temperature in the ladle. The methodology for calculating melting time is described in27,28,29,30. For the calculations, the following conditions were assumed: Crystallization temperature of the iron-carbon melt = 1500 °C, Bath temperature = 1600 °C, Initial ferrochrome temperature = 20 °C, Diameter of the initial ferrochrome piece = 50 mm.

Results and discussion

Crystallization onset temperature of ferroalloys

The crystallization onset temperature is one of the key properties of ferroalloys, influencing both the production technology of the ferroalloy and its performance characteristics31.

Experience in ferroalloy production32 shows that the ferroalloy production process proceeds without significant issues when the crystallization onset temperature does not exceed 1450–1500 °C. For the application of ferroalloys in liquid metal processing, some authors33,34 argue that their crystallization onset temperature should be lower than the crystallization temperature of the processed metal. Others35 believe that the melting temperature of the alloy should be lower than the steel temperature in the ladle.

The melting rate of ferroalloys depends on their crystallization onset temperature and the temperature of the processed metal36, as well as the degree of superheat above the liquidus temperature. Therefore, the optimal crystallization onset temperature of a ferroalloy should align with the temperature of the liquid metal at the moment of alloy introduction27.

As demonstrated, the classification of ferroalloys is not absolute but relative-determined in relation to a specific iron-carbon melt. That is, it depends on the specific crystallization temperature of the processed metal and the steel temperature in the ladle. The rational melting temperature for ferroalloys should be at least 200 °C lower than the iron-carbon melt temperature in the ladle32. Using highly refractory ferroalloys for steel alloying is undesirable, as it significantly increases the melting time of the alloys and, consequently, worsens the technical and economic indicators of the melt.

This study determined the crystallization onset temperatures of a group of ferrochrome with varying silicon-to-chromium ratios37,38. The experimental results are presented in Table 2.

Table 2 Crystallization onset temperature of ferroalloys in the Cr–C–Si–Fe system.
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Examining the crystallization onset temperature of high-carbon ferrochrome samples based on their silicon content, it can be noted that with relatively constant chromium content (Table 1, Alloys 2, 5, 6), an increase in silicon content results in a decrease in the crystallization onset temperature (Fig. 1). An increase in silicon content from 0.6 to 10.3%, accompanied by a slight reduction in carbon content from 8.1 to 6.8% and relatively stable chromium content (~ 53% Cr), leads to a consistent decrease in the crystallization onset temperature from 1600 to 1530 °C39.

The observed dependence of the crystallization onset temperature of ferrochrome on silicon content aligns well with the liquidus lines in the phase diagrams of the binary systems Cr–Si and Fe–Si, which are the main constituent components of ferrochrome. According to the equilibrium phase diagram of the Cr–Si system40, an increase in silicon content up to 9% results in a sharp decrease in the liquidus temperature from 1857 to 1700 °C, due to the formation of a low-melting eutectic in the alloy containing 9% silicon and 91% chromium. Further increases in silicon content up to 15% in this system lead to an increase in the melting temperature to 1770 °C, associated with the formation of a more refractory Cr3Si phase. A similar trend is observed in the equilibrium phase diagram of the Fe–Si system, where the liquidus temperature decreases significantly from 1538 to 1202 °C as silicon content increases to 17%, due to the formation of a low-melting eutectic in alloys containing 17–20% silicon and 80–83% iron (the Fe2Si phase melts with a peak at 1215 °C)41. Ferrochrome containing increased silicon levels (Table 1, Alloys 5 and 6) is characterized by lower crystallization onset temperatures (1560 °C and 1530 °C, respectively) compared to the liquidus temperature in the Cr–Fe system (Fig. 2)40 (1580 °C for the same Cr content). This is explained by the presence of a low-melting component-silicon-in ferrochrome (melting temperature 1415 °C).

Fig. 2
figure 2

Liquidus temperature in the Cr–Fe system.

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When studying the effect of the chromium content, it was shown that a reduction of Cr by 18% (from 63 to 45%) in high-carbon ferrochrome (Table 1, Samples 1-4) leads to a decrease in the crystallization onset temperature of the alloy by 50 °C (Fig. 3). According to literature data42, increasing the chromium content above 63% results in a rise in the crystallization onset temperature of high-carbon ferrochrome. For example, ferrochrome containing, wt%: 70.5 Cr; 8.5 C; 0.4 Si, has a final melting temperature of 1674 °C. This dependence aligns with the liquidus line in the equilibrium phase diagram of the binary Cr–Fe system40, where an increase in the chromium content from 25 to 100% results in a gradual increase in the liquidus temperature from 1507 to 1857 °C. It is worth noting that the crystallization onset temperature of high-carbon, low-silicon ferrochrome closely approaches the liquidus temperature in the phase diagram of the Cr–Fe system at the same chromium content, which is due to the specific influence of carbon. According to the phase equilibrium diagram of the binary Cr–C system43, an increase in carbon content up to 3.5% causes a sharp decrease in the liquidus temperature from 1930 to 1498 °C, while further increases in carbon up to 13.2% are accompanied by an increase in the liquidus temperature to 1925 °C. A similar influence of carbon on the liquidus temperature is observed in the isoconcentration section of the ternary Cr–Fe–C diagram at 70% Cr40. Although quantitative relationships vary across different sources44,45,46,47, the qualitative effect of carbon on the liquidus temperature within the considered concentration range remains consistent. Therefore, the liquidus temperature of the Cr–Fe system approaches the crystallization onset temperature of low-silicon, high-carbon ferrochrome containing approximately 8% C.

Fig. 3
figure 3

Dependence of the crystallization onset temperature of high-carbon ferrochrome samples on chromium content (1) for Samples 1-4 and Silicon Content (2) for Samples 2, 5, and 6 (Table 1).

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Thus, it has been established that reducing chromium content and increasing silicon content lead to a decrease in the crystallization onset temperature of ferroalloys. Moreover, the effect of silicon (in combination with the inevitable change in carbon content) is significantly more pronounced than that of chromium. Lowering the crystallization onset temperature of ferrochrome positively impacts its performance characteristics.

The conducted research allows the studied alloys to be conditionally categorized as follows:

  • Standard ferrochrome with high chromium content (63%) and low silicon content (0.2%) is on the conditional boundary between refractory and highly refractory alloys;

  • Ferrochrome with reduced chromium content (45–53%) and increased silicon content belongs to the category of refractory alloys.

Density of ferroalloys

Density is an important physicochemical and structure-sensitive property of alloys. The degree and stability of the assimilation of the main components of the ferroalloy, its melting rate, and the uniform distribution within the liquid metal volume significantly depend on the alloy’s density. In the absence of melt movement, a piece of lightweight ferroalloy quickly floats to the surface and undergoes intense oxidation. Conversely, if a heavy ferroalloy piece is added to the steel, it sinks to the bottom of the ladle48.

In addition, density has a significant influence on the alloy production process. When the density values of slag and metal are close, their separation is poor, leading to the metal appearing above the slag. This results in significant metal losses and complicates the melting and casting process49. From the perspective of alloy production, the alloy’s density should exceed that of the slag by a sufficient margin to ensure clear slag-metal separation. Practical data suggest this requirement is met when the alloy density is above 3200–3500 kg/m3.

In Zhuchkov50, the optimal density of ferroalloys was assessed based on their movement behavior in the ladle. A hydraulic model established the dependence of particle residence time in the liquid on the diameter of the spheres and the ratio of their density to that of the liquid. Optimal ferroalloy densities were determined for particles of various sizes. Considering melting processes, size reduction, oxidation of less dense ferroalloys, and steel shell formation on the particle surface, practical recommendations suggest densities of 5000–7000 kg/m3 for alloys in the 0.10–0.15 m size range and 6300–7000 kg/m3 for alloys in the 0.05–0.10 m size range. Alloys with optimal densities are most effectively involved in the hydrodynamic flow of steel in the ladle and, as a result, melt faster, integrate more completely, and are fully assimilated by the iron-carbon melt51.

This study examines the density of a group of chromium alloys with varying silicon and chromium content. The chemical composition of the alloys is presented in Table 1, and the densities of the studied samples are provided in Table 3.

Table 3 Density of ferroalloys in the Cr–C–Si–Fe system.
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Considering the density of ferrochrome as a function of silicon content in the alloy, it can be noted that with relatively constant chromium content (Table 1, Alloys 2, 5, 6), both the calculated and experimental densities of ferroalloys decrease as the silicon content increases (Fig. 4). An increase in silicon content from 0.6 to 10.3%, with relatively constant chromium content (~ 53% Cr), results in a consistent decrease in experimental density from 7590 to 6800 kg/m3.

Fig. 4
figure 4

Dependence of the Experimental Density of High-Carbon Ferrochrome Samples on Chromium Content (1) for Samples 1-4 and Silicon Content (2) for Samples 2, 5, and 6 (Table 1).

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This dependence of ferrochrome density on silicon content is explained by the significantly lower density of silicon (2330 kg/m3) compared to the densities of the primary components of ferrochrome–chromium and iron—which are 7190 and 7870 kg/m3, respectively, at 20 °C42,52.

Examining the change in ferrochrome density as a function of chromium content, it is observed that increasing chromium content from 45 to 63%, with relatively constant silicon content (less than 1% Si), results in a slight decrease in the additive density of the alloys from 7630 to 7540 kg/m3 (Table 3, Alloys 1-4). At the same time, experimental results indicate that as chromium content decreases, the density of high-carbon ferrochrome increases from 7540 to 7630 kg/m353, likely due to the formation of denser lattice structures of various chromium carbides. According to literature data, increasing chromium content above 63% leads to a further decrease in the density of high-carbon ferrochrome. For example, ferrochrome containing wt%: 70.5 Cr; 8.3 C has a density of 7460 kg/m3.

Thus, the conducted studies established that in high-carbon ferrochrome, a decrease in chromium content from 63 to 45% Cr leads to a reduction in density. In ferrochrome with reduced chromium content (~ 53% Cr) and in complex chromium-manganese alloys, increasing silicon content to 10.3% and 15.9% Si, respectively, also reduces density. Moreover, an increase in silicon content has a significantly greater impact on ferrochrome density than a decrease in chromium content.

The density of high-carbon ferrochrome containing 10.3% silicon does not exceed 7000 kg/m3, making it optimal for alloys intended for ladle treatment of steel. In contrast, the remaining alloys have a density above 7000 kg/m3 and fall into the category of heavy alloys. These ferroalloys, during ladle treatment of steel, sink to the bottom of the ladle and, after a few rebounds, are carried by the melt flows to the corner of the ladle, where they remain stationary until the end of the melting or dissolution process. In such cases, these processes occur very slowly. When using such ferroalloys for steel alloying and deoxidation in the ladle, it is advisable to employ various methods to reduce the melting time of the ferroalloys (e.g., steel stirring in the ladle, reducing the size of the alloy pieces, etc.).

Melting time of ferroalloys in an iron-carbon melt

In recent years, there has been a significant increase in scientific interest in studying the kinetics and mechanisms of ferroalloy melting and dissolution in metal. Increasing the dissolution rate of alloys during steel deoxidation and alloying enhances the assimilation of their elements and improves the productivity of steelmaking units. Therefore, understanding the factors that increase the melting and dissolution rates of alloys is essential. These factors can be categorized into groups related to the physicochemical properties of the alloy being melted, the liquid steel, and the nature of their movement54. Extensive research has been conducted on the melting rates of ferroalloys55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70.

Fig. 5
figure 5

Effect of chromium content on the melting time of high-carbon ferrochrome (50 mm fraction) in steel at 1600 °C.

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Fig. 6
figure 6

Effect of silicon content on the melting time of high-carbon ferrochrome (53% Cr, 50 mm fraction) in steel at 1600 °C.

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In the scientific literature, methods of mathematical modeling are widely used to study the melting of ferroalloy pieces in liquid steel55,58,62,66,67,71,72.

The results of the melting time calculations for the studied ferrochrome are presented in Figs. 5 and 6. The alloy with increased silicon content (Table 1, Alloy 6) belongs to the category of low-melting ferroalloys. The melting process of low-melting ferroalloys occurs in three stages:

  • First Stage: During this period, the ferroalloy heats up, and once the surface temperature reaches the crystallization onset temperature, melting begins. The ferroalloy remains encased in a shell of solid steel, whose thickness either increases throughout the stage or decreases toward the end. Freezing of the melt on the surface of the ferroalloy occurs due to the relatively high thermal conductivity of ferroalloys in the solid state compared to the thermal conductivity of the liquid iron boundary layer. The thermal conductivity of iron, like other metals, decreases sharply upon transitioning to the liquid state. The mass of the alloy piece increases relatively quickly at the start of the first stage due to the significant temperature gradient at the boundary between the frozen crust and the alloy. This process slows down as the melting point is approached and ceases entirely once it is reached70.

  • Second Stage: During this period, the ferroalloy melts beneath the solid steel shell due to heat transfer from the iron-carbon melt. Depending on the specific conditions, this stage concludes either with the complete melting of the ferroalloy and the formation of a liquid core within the solid steel shell, or with the complete melting of the solid shell and the immersion of the unmelted part of the ferroalloy piece into the liquid steel.

    Theoretically, during stages 1 and 2, there is no direct contact between the ferroalloy and the liquid steel. Therefore, interactions such as chemical dissolution or oxidation reactions (which involve heat release or absorption) occur only during the third stage73.

  • Third Stage: In this final stage, the ferroalloy interacts directly with the liquid steel. These interactions can accelerate the melting process.

All the studied high-carbon ferrochrome samples with reduced chromium content (Table 1, Alloys 2-6), as previously determined, belong to the category of refractory ferroalloys. The melting process for refractory ferroalloys also occurs in three stages:

  • First Stage: A solid steel shell forms and then melts. During this stage, the ferroalloy initially heats up due to heat transfer from the liquid steel. In the second half of this period, the heat from the iron-carbon melt is used to heat the ferroalloy piece and melt the shell. This stage ends when the solid shell is fully melted.

  • Second Stage: The ferroalloy is heated to the crystallization onset temperature and directly contacts the liquid steel, allowing interaction between them.

  • Third Stage: The solid ferroalloy melts, and the liquid alloy dissolves into the melt. Melting is considered complete when the ferroalloy is fully liquefied. Subsequent distribution of the ferroalloy throughout the ladle volume is approximately the same for all alloys.

According to the modeling results, reducing the chromium content from 63 to 45% (Table 1, Alloys 1-4) decreases its melting time in steel from 191 to 61 s74.

The effect of silicon content in high-carbon ferrochrome (Table 1, Alloys 2, 5, and 6) on melting time in an iron–carbon melt was also studied. The modeling results are presented in Fig. 5. It was shown that increasing the Si content in the alloy from 0.6 to 10.3% leads to a consistent reduction in melting time from 107 to 38 s.

Studies have shown that the properties of high-carbon ferrochrome in steel depends on its Cr and Si content. Reducing chromium content from 63 to 45% decreases the melting time by a factor of 3.1, while increasing silicon concentration to 10% reduces it by a factor of 2.8. This undoubtedly enhances on the consumer properties of the ferroalloy.

When considering the properties of high-carbon ferrochrome with increased silicon content, it is important to note that as the silicon content increases from 0.6 to 10.3%, the carbon content in the alloys decreases from 8.1 to 6.8% (Table 1, Alloys 2, 5, 6). This is because increasing the silicon content reduces the solubility of carbon in the metal.

In terms of crystallization onset temperatures, standard ferrochrome with high chromium content and low silicon content (Table 1, Alloy 1) lies on the conditional boundary between refractory and highly refractory ferroalloys. Its classification depends on the temperature of steel processing: it is considered highly refractory at steel temperatures below 1620 °C and refractory at steel temperatures above 1620 °C. Reducing the chromium content and increasing silicon in high-carbon ferrochrome lowers the crystallization onset temperature of the alloy (Table 2, Alloys 2-6), allowing it to be classified as refractory. This transition positively impacts the physicochemical properties and assimilation of the alloy.

Density studies of high-carbon ferrochrome revealed that reducing the chromium content causes a slight increase in density, while increasing silicon content results in a significant decrease in alloy density. Chromium-containing ferroalloys with high silicon content (≥ 10%) have a density not exceeding 7000 kg/m3, which is optimal for alloys intended for ladle steel treatment. Low-silicon ferroalloys, with a density exceeding 7000 kg/m3, belong to the category of heavy alloys.

The melting time calculations for chromium-containing ferroalloys in an iron-carbon melt showed that reducing chromium content and increasing silicon content consistently reduce the melting time. Lowering chromium content in high-carbon ferrochrome from 63 to 45% reduces its melting time in steel by a factor of 3.2. Increasing silicon content to 10.3% reduces the melting time by a factor of 2.8.

Thus, the obtained data demonstrate that ferrochrome with increased silicon content and reduced chromium content has more favorable physicochemical properties compared to traditional high-percentage (63% Cr) ferrochrome, both in terms of alloy production and its application in steel processing. Moreover, standard ferrochrome, with less than 1% silicon, is used exclusively for steel alloying with chromium. In contrast, "charge chrome," which can contain up to 10% silicon, can be used not only for alloying but also for partial steel deoxidation. Reducing chromium content and increasing silicon content in high-carbon ferrochrome (within the studied ranges) primarily improves its physicochemical characteristics, with the effect of silicon being significantly more pronounced than that of chromium.

Conclusion

The dependence of physicochemical properties (density, melting temperature, melting time) of alloys in the Cr–Si–C–Fe system on chromium and silicon content has been established. It has been shown that increasing the silicon content to 10% and reducing the chromium content from 63 to 53% enhance the performance characteristics of ferroalloys. These improvements include: a reduction in the crystallization onset temperature from 1620 to 1530 °C, a decrease in density from 7540 to 6800 kg/m3, and a threefold reduction in the melting time of an iron-carbon melt. The potential for producing and utilizing alloys with reduced chromium content for steel allowing presents new opportunities for improving the technological parameters of production of chromium-containing steel grades and involving previously unused poor chromium ores in processing by obtaining complex Cr–C–Si–Fe alloys with reduced chromium content.