INTRODUCTION

The use of manganese oxide in the direct alloying of steel is of great interest because of the economic benefit of not employing expensive manganese ferroalloys. It is well known that the traditional use of manganese alloys in the reduction and alloying of steel results in manganese extraction no better than 50% [1]. The use of manganese oxide is especially attractive because small regional deposits may be employed.

Direct alloying has been tested for steel productions in converters and electrosmelting furnaces. In the extensive research on direct alloying, the best-known studies of steel alloying by means of oxides are [2–6].

Because of resource scarcity, Japan has been most active in pursuing direct alloying. At present, manganese ore accounts for 50% of the manganese used in steel alloying in Japan [7]. Most research on direct alloying by means of manganese ore focuses on converter production of steel [8–13]. Extensive research on direct alloying in electrosmelting furnaces is summarized in [14].

The simplest and most expedient approach to direct alloying is to add sinter or rich manganese ore to the furnace or ladle, with melting and the introduction of reducing agent. Thus, direct alloying is possible in both oxidative and reductive conditions in the ladle treatment of steel.

OXIDATIVE TECHNOLOGY

To assess the direct alloying of steel by manganese, Astra software is used for thermodynamic modeling of the introduction of manganese ore from the Durnovsk field (Kemerovo region) in the furnace [15]. The initial data are as follows. Before the introduction of manganese ore, the steel contains up to 0.8% C and ~0.2% Mn. The basic components in the Durnovsk ore are as follows: 47% MnO, 14% FeO, 21% SiO2, 2% CaO, 2% MgO, and 4% Al2O3. The quantity of metal in the electrosmelting arc furnace is 100 t. The quantity of initial slag in the furnace is 5 t. The slag composition in the furnace is as follows: 20% FeO, 45% CaO, 16% SiO2, 8% MgO, 3% Al2O3, and 8% MnO. The quantity of ore added is 400–1200 kg. The temperature of the process is 1873 K.

In Fig. 1, we show the modeling results for direct alloying of steel in oxidative conditions, with different quantities of added manganese ore.

Fig. 1.
figure 1

Variation in manganese content in the steel in the course of injection, when 400 kg () and 1200 kg () of manganese ore is introduced and without added ore ().

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The results show that, in the electrosmelting furnace, it is possible to increase the manganese content in the metal. Adding ore in the proposed quantities poses no technical difficulties. It may easily be introduced through the furnace crown. However, in determining the optimal quantity of ore, we need to take account the carbon content of the metal discharged from the furnace. In smelting medium-carbon and especially high-carbon steel, the manganese content in the steel on account of direct alloying may reach practically the requirements for carbon and low-alloy steel without significant manganese (0.5–0.6%).

For low-carbon steel, adding more ore proves ineffective, since most of the reduced manganese will be lost in the final stage of oxygen injection. As we see in Fig. 1, adding about 200–400 kg of manganese ore is sufficient to more or less double the residual manganese content.

Note that, in oxygen injection, carbon-bearing powder is intensively transferred to the slag in the electrosmelting furnaces. That decreases the MnO content in the slag, according to the data in [16], for example.

One important measure of the effectiveness of direct alloying is the extraction of manganese from its oxide. This may be assessed on the basis of the ratio of the manganese entrained by the metal to the manganese entrained by the ore. Theoretically, this ratio may be calculated from data regarding the equilibrium manganese distribution between the metal and the slag.

Assessment of the equilibrium in the metal–slag system within the smelter on the basis of the theory of regular ionic solution is promising [17–19]. However, the results obtained are not sufficiently precise [17]. To assess the capabilities of direct alloying, we conduct industrial trials.

In electrosmelting arc furnaces, the metal melts, the phosphorus and carbon concentrations are decreased, and the steel is heated to specified temperature. The process is intensified by means of burners and tuyeres for the injection of carbon-bearing powder so as to foam the slag. Then the steel is discharged to the ladle and undergoes treatment in the ladle–furnace unit. At discharge, the slag is segregated by means of the furnace’s discharge system. To form active ladle slag at discharge, slag-forming mixture based on lime and fluorspar is added. In parallel, ferroalloys are added for preliminary reduction of the metal. After treatment in the ladle–furnace unit, the steel is sent to a continuous-casting machine.

In the experimental melts, Durnovsk manganese ore (500–900 kg) is added to the ladle together with the slag-forming mixture. In producing carbon steel, the manganese ore is preliminarily dried. The results of 50 melts without disruption of the technological instructions are analyzed. The mean manganese extraction is 0.98. This is a satisfactory result, indicating a stable process.

The assimilation of silicon from silicon-bearing ferroalloys is 63%. In the absence of manganese ore, this figure is 67%.

The increment in phosphorus content during ladle treatment is less than 0.002%. That matches, or is even less than, the figure in the standard technology.

Note that the main problem in using manganese ore and concentrates is that the actual manganese content is fairly low (about 40%) [20].

Thus, together with manganese, large quantities of slag-forming compounds (oxides) are added to the metallurgical system.

REDUCTIVE TECHNOLOGY

To assess the capabilities of manganese ore in direct alloying, the accompanying chemical compounds in the ore must be taken into account.

The initial data are as follows. The basic components in the manganese ore are as follows: 47% MnO, 14% FeO, 21% SiO2, 2% CaO, 2% MgO, and 4% Al2O3. The quantity of metal is 100 t. The quantity of initial slag in the ladle is 2 t. The initial slag composition in the ladle is as follows: 0.79% MnO, 1.34% FeO, 20.2% SiO2, 51.5% CaO, 8% MgO, and 5% Al2O3.

The addition of silicon is responsible for the reduction of manganese and iron oxide from the slag and ore. In the calculations, it is important to estimate the content of manganese oxides in the slag at the end of ladle treatment. To that end, the relationship between the FeO content and MnO content in the ladle slag must be assessed on the basis of industrial data.

The results are presented in Fig. 2.

Fig. 2.
figure 2

Relation between the FeO content and MnO content in the ladle slag on treatment of the steel in the ladle–furnace unit.

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As expected, there is a characteristic relation between the FeO content and the MnO content in the slag. On that basis, if the content of one oxide is known, the content of the other may be predicted. In the calculations, we assume that the FeO content in the slag within the ladle–furnace unit is 1.34% (the mean value for the experimental melts), while the equilibrium MnO content is 0.79% (the value calculated from Fig. 2). All of the FeO and MnO (more than 1.34% FeO and 0.79% MnO) supplied to the slag with the manganese ore will be reduced by the silicon added to the melt for that purpose. The slag basicity must remain unchanged. To that end, the corresponding quantity of lime must be added to the ladle. These features of the technology, which are necessary to ensure desulfurization of the steel, must be taken into account in the analysis.

On the basis of these findings, we write a balance equation from which the basic parameters of direct alloying may be calculated with the addition of different quantities of manganese ore. The results are plotted in Fig. 3.

Fig. 3.
figure 3

Influence of the quantity M of manganese ore added on the change in manganese content in the steel (a), the manganese assimilation Kass (b), the slag mass in the ladle (c), and the silicon consumption in the reduction of the manganese ore (d).

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CONCLUSIONS

Direct alloying is effective with wide variation in the quantity of added manganese ore . That permits the alloying of carbon and low-alloy steels with manganese assimilation no less than 0.97.

The main factor limiting the use of direct alloying in the ladle–furnace unit is that the slag mass in the ladle is increased.

In practice, without any additional measures, direct alloying increases the manganese content in the steel by about 0.3%. To obtain higher values, slag may be drained from the ladle. However, that complicates the technology.

The efficiency is higher when using direct alloying in oxidative conditions to increase the residual manganese content in the steel prior to ladle treatment.