Improvement of the out-of-furnace steel treatment technology for improving its mechanical properties

. The problem of increasing the competitiveness of technology today is largely determined by the quality of manufacturing large-sized cast billets for their supporting systems. Many factors are involved in forming the service properties of a casting. Each factor is important and affects the quality of the resulting casting. The presence of "cold" slag does not allow refining processes to be fully realized during the induction melting of steel. Therefore, attempts to achieve the same results for induction melting steel without additional furnace treatment as for electric arc melting steel do not always lead to success. In recent years, various methods of out-of-furnace steel processing have been developed, which make it possible to eliminate certain disadvantages of induction melting. One of them is the modification of steel in the bucket with rare earth metals (REM). Additional modification of steel in the ladle along with REM also allows for further improvement of steel quality. Currently, REM, with a high affinity for oxygen, sulfur, nitrogen, non-ferrous metals, and other impurities, are widely used in the steel production. Studies have been carried out to improve the technology of out-of-furnace steel processing and its mechanical properties. The effect of alkaline earth and rare earth master alloys on impact strength and the contamination index with non-metallic inclusions of cold-resistant steel is shown.


Introduction
Large cast parts of a particularly responsible purpose, such as a side frame and a spring beam, are used on biaxial bogies of 1520 mm gauge freight cars and are manufactured according to GOST 32400-2013 following technical requirements.These parts are cast from low-carbon, low-alloy manganese steel grade 20GL.Steel grade 20GL refers to coldresistant steels and is smelted in electric furnaces with the main lining.Steel's mass fraction of sulfur and phosphorus should not exceed 0.020% (of each element) [1,2].
The following quality indicators are monitored on such parts: appearance, dimensions, chemical composition, mechanical properties, yield strength, time resistance, elongation and contraction, and impact strength at negative temperatures (-60 °C).
Removal of harmful impurities from metal is based on their transfer to slag and the creation of conditions that prevent their reverse transition from slag to metal.By changing the composition, quantity, and temperature of slag, it is possible to significantly influence *Corresponding author: tv574toirov@mail.ru the processes of refining metal from sulfur and phosphorus.Therefore, obtaining a slag of the necessary composition with the appropriate physical properties is one of the important tasks in metal smelting [1,8,11].
To date, in many foundries that have found themselves in connection with the urbanization of cities near residential areas, electric arc furnaces have been replaced by induction crucible furnaces, as more environmentally friendly in terms of noise, smoke, and dust formation.
Induction furnaces are considered the best option for producing steel castings.Still, the forced replacement of electric arc furnaces with induction furnaces has negatively affected the quality indicators of castings [12,16,20].
Typically, slags in induction furnaces do not perform such functions as oxidation, reduction, desulfurization, and dephosphorylation.In the process of metal melting, slags in the furnace are formed due to oxidation of the charge components and the lining materials of the crucible.These slags are usually removed at the end of melting and "fresh" from the battle of glass and quartz sand for acidic furnaces and from lime and fluorspar for furnaces with the main lining.The main purpose of these slags is to protect the liquid metal from interaction with the atmosphere and to reduce the loss of energy emitted by the surface of the metal mirror.Significant energy losses through the slag surface lead to low fluidity and large temperature differences in height (up to 1200 K) [3,14].
Slags as a refining phase in metal smelting in induction furnaces have not yet found wide application in metallurgical practice.This is primarily due to the low reactivity of the slags, which are heated only in the contact zone with the metal surface, the small size of the metal-slag contact surface, and the cooling of the slags by the crucible lining [4,5,15].
The presence of "cold" slag does not allow refining processes to be fully realized during the induction melting of steel.Therefore, attempts to achieve the same results for induction melting steel without additional furnace treatment as for electric arc melting steel do not always lead to success.Moreover, the stricter the requirements for castings, the more prerequisites for obtaining unsatisfactory results.

Methods
Based on the above, in recent years, various methods of out-of-furnace steel processing have been developed, which make it possible to eliminate certain disadvantages of induction melting [6,7].One of them is the modification of steel in the bucket with rare earth metals (REM).For cast carbon and alloy steels, depending on the purpose of the castings, three variants of ladle treatment of the melt for deoxidation-modification are recommended: a) deoxidation with aluminum -for general purpose castings; b) aluminum deoxidation and silicocalcium modification -for castings of responsible purpose; c) deoxidation with aluminum and modification with silicocalcium, as well as rare earth metals (REM) -for castings of a particularly responsible purpose.
The use of alkaline earth metals (AEM) can significantly increase the impact strength of steel, as well as the purity of steel for non-metallic inclusions.To study the effect of the amount of ligature on the impact strength and the contamination index for non-metallic inclusions of steel grade 12HGFL, laboratory research was conducted using the AEM.The results of the experiments are shown in Figure 1 [17,18].As seen from Figure 1, the impact strength's dependence on the shield's flow rate in the entire temperature range (from plus 20 to minus 80 ° C) has an extreme character.At low costs, the impact strength of steel increases, at the consumption of 0.4-0.5% ligature, it reaches a maximum, and at a higher consumption, it decreases.Due to the use of AEM, the impact strength of steel can be almost doubled.With an increase in impact strength, the contamination index of steel with non-metallic inclusions decreases [8,9].
Additional modification of steel in the ladle along with rare earth metals (REM) also allows for further improvement of steel quality.
Currently, REM, with a high affinity for oxygen, sulfur, nitrogen, non-ferrous metals, and other impurities, are widely used in the production of steel.The effect of REM is manifested both in reducing the content of these impurities in the steel and in transferring them from active forms to passive ones, which helps to clean the grain boundaries and ensures the formation of a finely dispersed dendritic structure.By cleaning steel from harmful impurities, REM improves its casting properties, fluidity, feeding conditions, and crack resistance of castings, as well as reduces the anisotropy of the mechanical characteristics of steel.The high efficiency of the effect of REM on the properties of steel is due to their favorable effect on the composition, type, shape, quantity, and uniformity of the distribution of the formed HB, a significant improvement in the macro-and microstructure of the workpiece, reducing its physical and chemical heterogeneity, providing increased density and dispersion of the crystal structure in all areas of the cast workpiece, including small cross-section [10,[13][14][15][16][17][18]20].
Other researchers believe that the role of REM is to achieve such a total sulfur content in liquid steel, which is significantly lower than the values obtained with conventional desulfurizers, such as manganese, magnesium, and others [11,13].
REM (cerium, yttrium, etc.) chemically binds sulfur and phosphorus dissolved in steel to form sulfides, oxysulfides, and phosphides.In particular, cerium forms Ce 2 S 3 sulfide, Ce 2 O 2 S 3 oxysulfide, and Ce 2 P and CeP phosphides by reactions: The reaction products are isolated in the form of separate rounded non-metallic inclusions.As a result, the grain boundaries become cleaner, the intercrystalline bond is strengthened, and the mechanical properties of steel (relative elongation δ, relative contraction ψ, and impact strength KCU -KCV) increase.Therefore, it is believed that when processing REM steel, not only deep deoxidation of steel is achieved, but also modification of the melt [12,19,20].

Results and Discussion
The following experiments were carried out using AEM and REM. Figure 2 shows the effect of various variants of bucket treatment of 20GML steel on the impact strength.Fig. 2. The effect of AEM and REM on the impact strength of steel 20GL: 1 is 0,1 % Al; 2 is 0,1 % Al + 0,2 % Si-Ca; 3 is 0,3 % Fe-Si-ΣREM; 4 is 0,4 % Fe-Si-Ce Figure 3 shows the effect of the consumption of REM (Fe-30Si-30ΣREM) on the impact strength of steel 20GL.As seen from the figure, the dependence of the impact strength on the consumption of REM has the same extreme character as when processing REM.As the consumption of REM increases, it initially increases, reaches a maximum, and then falls (curve 3).Moreover, at subzero temperatures, the impact strength values are lower, but the nature of the dependence remains (curve 4).During filtration, due to the cleaning of steel from nonmetallic inclusions, the impact strength of steel increases.At the same time, the nature of the dependence remains the same (curves 1 and 2).

Conclusion
The results of theoretical analysis, experimental studies, and the conclusions made in this work allowed us to formulate the following recommendations for improving the technology of out-of-furnace processing of 20GL steel with the use of AEM and REM: a) it is mandatory to purge the steel in the bucket with argon for homogenization by chemical composition and temperature; b) to carry out bucket deoxidation with aluminum and modification with calcium in silicocalcium and rare earth metals in the form of ferrocerium, yttrium, Fe-Si-REM, yttrium or cerium mischmetal.This leads to an increase in the degree of absorption of REM while increasing the mechanical properties of 20GL steel.

Fig 1 .
Fig 1. Influence of Ca-and Va-containing ligature on the impact strength and index of contamination with non-metallic inclusions of steel 12HGFL: 1 is index of contamination of steel with non-metallic inclusions; 2-6 are impact strength of steel at +20, -20, -40, -60 and -80 ºC

Fig 3 .
Fig 3.The effect of REM consumption on the impact strength of 20GL steel at different temperatures: 1 and 2 are filtered and unfiltered steel at 20 º; 3 and 4 are the same at minus 50 ºC