Obtaining ultra-low carbon content in metal processing using a circulation vacuum degasser

Introduction

Modern requirements for steel quality, especially for electrical and special grades, necessitate reducing the carbon content to ultra-low values (<0.002 %). In circulation vacuum degassers, this process is complicated by many interrelated factors, including reaction kinetics, melt hydrodynamics, and lining properties [1 – 5].

Materials and methods

The studies were carried out on industrial RH-process units using:

– microstructural analysis of metal samples;

– X-ray microanalysis of inclusions;

– theoretical calculations of phase interaction areas;

– industrial trials with variation of process parameters [6].

Results and discussion

The present work analyzes the following mechanisms of carbon removal under reduced pressure in a circulation vacuum degasser (shown in Fig. 1) [1 – 3; 7; 8]:

1 – desorption of gases from the melt surface;

2 – formation of {CO}bubbles in the melt depth;

3 – formation of {CO}bubbles on the lining surface;

4 – diffusion of the inert gas supplied through the inlet snorkel into the bubbles;

5 – interaction of metal sprays with the vacuum medium in the vacuum chamber.

Mechanisms 1, 4, and 5 operate in the diffusion mode and depend on the area of interaction between the melt and the gas medium, whereas mechanisms 3 and 4 depend on the number of formed {CO} bubbles [9; 10].

Theoretical calculations showed that the interaction area for mechanisms 4, 1, and 5 is 60, 6, and 1 m², respectively. Thus, the main mechanism of carbon removal in the diffusion mode through {CO}formation is mechanism 4.

The rate of carbon removal from the melt by mechanisms 2 and 3 depends on the concentrations of oxygen and carbon in the metal, on the contact area between the lining and the melt (mechanism 3), and on the volume of treated metal (mechanism 2). Theoretical calculations show that, under identical technological parameters of melt treatment, mechanism 2 is 25 times more efficient than mechanism 3.

However, for an objective assessment of carbon removal through formation of {CO} bubbles (mechanism 2), it is necessary to consider the conditions and possibility of their formation [8; 11]. Formation of gas bubbles in the melt depth is possible in the presence of various microinhomogeneities or on the surface of microparticles in the melt. To determine the presence of microparticles in the melt, a metal sample was taken from the converter heat during temperature measurement, before the deoxidation/alloying operation. For this purpose, an immersion sampler was used, from which the deoxidizing element had been removed in advance. After polishing of the test sample, a microstructural analysis was performed to identify nonmetallic inclusions in it, shown in Fig. 2 [5].

The study results revealed the presence of nonmetallic inclusions of various sizes, from 5 to 140 μm (Fig. 3) [5]. In addition, the elemental composition of the nonmetallic inclusions identified in the sample was determined by X-ray microanalysis using a Quanta Inspect S200 scanning electron microscope equipped with a Trident-XM4 system. The results are shown in Fig. 4 [5].

The data shown in Figs. 3 and 4 indicate that the composition of the nonmetallic inclusions corresponds to the composition of the slag formed in the converter, that is, even in non-deoxidized metal (before treatment in the vacuum chamber), nonmetallic inclusions are present that can serve as the basis for the formation of {CO} bubbles.

Thus, it was established that the main mechanisms of carbon removal in a circulation vacuum degasser are the formation of {CO} bubbles in the melt (mechanism 2) [12 – 14] and the diffusion mode (mechanism 4) [15; 16].

When considering the process of carbon removal from the metal, it is necessary to take into account the input of this element from the lining. To achieve an ultra-low carbon content in steel (less than 0.002 %), it is advisable to intensify the degassing mechanisms and reduce carbon input from refractory materials.

Mechanism 2 of carbon removal (formation of {CO} bubbles in the depth of the melt) can be intensified by increasing the number of inhomogeneities in the melt, which is achieved by preliminary deoxidation of overoxidized metal with aluminum before vacuum treatment. For example, at one plant the deoxidation operation using aluminum is permitted to achieve an oxygen concentration of 400 ppm before decarburization in a circulation vacuum degasser. According to theoretical calculations, the optimal values of carbon and oxygen concentration in the melt before vacuum treatment are: [С] = 0.043 %, i.e., [О] = 576.87 ppm.

The rational gas flow rate in the inlet snorkel of the circulation vacuum degasser depends on the state of the lining. In vacuum chambers (140 – 170 t), a large amount of gas is supplied to intensify decarburization processes, which may lead to an increase in bubble size and a decrease in decarburization efficiency [17 – 19].

For example, at one metallurgical plant, a mode of deep decarburization of the melt in an RH vacuum degasser was developed. To increase the contact surface between the metal and the gas phase, the transport gas flow rate was reduced from 140 to 80 m³/h, which made it possible to achieve a carbon content of 0.003 % and lower in the finished steel [7].

Another factor affecting the carbon content is the holding time of the metal before casting. The optimal holding time before casting is 20 – 30 min, taking into account all related operations such as crane handling, sampling, measurements, and others. An increase in this time may lead to a rise in carbon content, whereas a shorter holding time may result in unstable parameters during continuous casting, as illustrated in Fig. 5.

Carbon content is also influenced by other factors, including the operability of the ejectors and the degree of their wear. Regular inspection of the ejectors and assessment of their condition are necessary, since this directly affects the system’s ability to reach low pressure. In practice, it has been observed that the faster the pressure drops below 100 Pa, the higher the probability of effective carbon removal and of achieving a carbon content below 0.002 % in steel.

An important factor is the cleanliness of the off-gas system. The presence of contamination in the form of slag-metal deposits in the gas duct system, particularly in the gas duct elbow, prevents the system from reaching low pressure and, as a consequence, hinders carbon removal (Fig. 6).

The final carbon content can also be affected by the lining [18; 20]. In this case, the residual carbon content depends on the amount of carbon dissolved from the lining of the steel ladle. Studies have shown that replacing carbon-containing refractory materials with carbon-free or low-carbon materials in the bottom area and lower part of the ladle walls significantly reduces the overall rate of carbon dissolution from the lining (by 60 %) [7].

Conclusions

The main mechanisms of metal decarburization in a circulation vacuum degasser are the formation of {CO} bubbles in the melt volume and purging with inert gas. The key technological parameters are a gas flow rate of 80 m3/h and a holding time of 20 – 30 min.

The use of carbon-free lining significantly reduces carbon input into the melt.