Hydrogen Removal in Circulating Vacuum Degasser under Conditions of PJSC “NLMK”

For high-quality steel smelting, stagebystage production is required, which has a complex of metallurgical units capable for producing products with high performance properties and low content of harmful impurities. One of the harmful impurities is hydrogen, so it is important to limit its content in the metal. To ensure the specified hydrogen content, the metal in the steel outoffurnace treatment at Converter Shop No. 2 (CS2) of PJSC “Novolipetsk Metallurgical Plant” (“NLMK”) is subjected to vacuum treatment in a circulating vacuum degasser. Despite the prevalence of circulating vacuum degassers, theoretically, mechanism of hydrogen removal in these metallurgical units has been insufficiently studied. To increase efficiency of hydrogen removal, theoretical calculations were performed to remove it from the metal. There are several mechanisms for hydrogen removing: direct transfer of hydrogen from metal to the surrounding space; formation of gas bubbles in metal and their direct ascent; nucleation of hydrogen bubbles at the border of refractory wall and metal; and removal of hydrogen by metal blowing with neutral gas (argon). It is shown that the main ways of hydrogen removal in a circulating vacuum degasser are direct transfer of hydrogen from metal to the surrounding space and blowing of melt with transporting gas. In the CS2 of PJSC “NLMK”, both ways are implemented at a circulating vacuum degasser. Vacuum pumps provide pressure in a vacuum chamber of less than 101.3 Pa (0.001 atm). It promotes intensive removal of hydrogen from the metal surface. To ensure circulation of metal, transporting gas argon is supplied to the inlet pipe of the RH degasser, which also takes part in removal of dissolved gases by transferring hydrogen to neutral gas bubbles. Additionally, performed calculations have shown that the main way of degassing in conditions of CS2 of PJSC “NLMK” is removal of hydrogen into the bubbles of carrier gas.


INTRODUCTION
Presently modern metallurgic enterprises need to solve problems on releasing high-quality and competitive products. Melting of high-quality steel requires stage-by-stage production-a complex of melting facilities [1,2]. At each step, the problem of metal refinement as well as its preparation for casting is solved.
In order to provide the given hydrogen content a metal on the area of out-of-furnace steel processing of CS-2 of PJSC "NLMK", it is subjected to processing by vacuum in the plant of circulation vacuuming.
As known, gases can be removed by the following mechanisms [12][13][14]: I way-direct transition of hydrogen from a metal into surrounding space; II way-formation of gas bubbles in the metal and their immediate float; III way-generation of hydro-gen bubbles at the boundary of blast-furnace brickwall with the metal; IV way-removal of hydrogen during blowing of the metal by a neutral gas (argon).
Variations of removal of gas from steel are presented in Fig. 1.
It was shown in works [10][11][12] that the main ways of gas (hydrogen) removal are I and IV, i.e., direct transition of hydrogen from the metal into surrounding space and blowing of the melt by a neutral gas.
Vacuum degasser of a circulation type allows realizing both ways. For example, in CS-2 of PJSC "NLMK", vacuum pumps provide a pressure less than 0.001 atm (101.3 Pa) in the vacuum chamber, which promotes intensive removal of hydrogen from the metal surface. Additionally, to provide circulation of the metal, transporting gas (argon), which is also involved in removal of dissolved gases by transition of hydrogen into bubbles of the neutral gas, is supplied into the inlet pipe of RH plant. Hydrogen removal ways are shown in Fig. 2.
Despite the prescription (1959) of the developed circulation vacuum process [12] and the achieved limits of hydrogen removal [13,[15][16][17], it was not fully identified which of the presented mechanisms is predominant.

RESEARCH
The aim of this work is to determine the main ways to remove hydrogen from metal in conditions of CS-2 of PJSC "NLMK".
Hydrogen removal rate in circulation vacuum degasser [6,11] is described by the following equation: (1)

where [H] t -finite content of hydrogen after time t;
[H] P = 0.64 ppm-equilibrium content of hydrogen at Р = 0.07 kPa (residual pressure in vacuum chamber); [H] 0 -initial content of hydrogen before processing; η = Qt/M-circulation rate; Q-amount of metal supplied into vacuum chamber (circulation rate), t/min; and M-mass of metal in steel-pouring ladle, t.
As known, hydrogen removal rate depends on mass transfer of metal in the inlet pipe of the vacuum chamber. Works [18][19][20][21][22][23][24] present various empiric formulas, by which melting rate in the inlet pipe is calculated depending on different technological and constructional parameters of the metallurgic facility. Most satisfactorily, the metal speed in the inlet pipe of circulating vacuum degasser is described by the equation derived by Japanese scientists [25].
The presented equation (1) accounts various technological and constructional parameters of circulation vacuum degasser at the expense of circulation rate coefficient η.
Graphically, the expression (1) is presented in Fig. 3 at the initial content of hydrogen in metal of 6 ppm.
Removal of hydrogen in circulation vacuum degasser [12] is described by equation (2) where and -concentration of gas in metal flowing into the vacuum chamber at the initial moment of time and t; and c P -equilibrium concentration of gas in metal in the vacuum chamber.
Then, the final content of hydrogen equals to where V-volume of metal in steel-pouring ladle; V* and -volume of metal within the vacuum chamber and flowing per a unit of time through the vacuum chamber; κ*-coefficient of mass transfer; and F*degassing surface of metal portions. According to data of work [12], relation V*/κ*F* corresponds to the value 0.1 C -1 .
If we assume that is the volume of metal flowing per unit of time, it will correspond to the volume metal from which hydrogen is removed from the surface melt per unit time. Then, we obtain the equation removal of hydrogen from the metal surface per unit time.  Fig. 1. Ways of hydrogen removal from the metal.
To determine the surface metal volume in vacuum chamber, from which hydrogen is removed, it is necessary to know the nucleation depth of gas bubbles. If the surface area is constant and constitutes 5.2 m 2 for conditions of circulation vacuum degassing facility of PJSC "NLMK", then the depth of nucleation of gas bubbles depends on the concentration of hydrogen in metal and is determined by the equation [1,12,26]: (5) here, P-pressure in vacuum chamber, atm; H-concentration of hydrogen in metal per a unit of time, ppm.
In conditions of industrial vacuum degassing, pressures P less than 0.001 atm are achieved, which provides hydrogen removal from the melt surface at its concentration in steel not less than 3.5 ppm.
Using equations (2), (4) and (5), it is possible to determine the amount of hydrogen removed from the metal surface. Computed data are presented in Fig. 4, curve 1.
Additionally, it is necessary to consider a question about possible limit of hydrogen removal by the first way, i.e., removal from the metal surface. According to the data of [13], the minimum saturation pressure at which hydrogen bubbles can form on the metal surface corresponds to the hydrogen concentration in the melt, determined by the following equation: where = 28.55 cm 3 /100 g-constant of solubility of hydrogen in iron at temperature of 1600°С [1]; -minimal saturation pressure, Pa. Accordingly, at saturation pressures less than , stable gas bubbles capable of growth do not nucleate. According to work [7], this parameter is determined by the formula (8) where σ-surface tension of metal, kJ/m 2 ; ρ-density of steel, g/cm 3 ; g-free fall acceleration, cm/s 2 .
For operation conditions of PJSC "NLMK" and according to data of works [1,27], this indicator will correspond to 1539 Pa.
The minimum hydrogen content in steel, which can be removed by nucleation from the metal surface, will correspond to 3.5 ppm. When the hydrogen content is less than 3.5 ppm, its removal is possible only by interacting with the bubbles of the transporting gas.
Combining the calculated indicators of the total removal of hydrogen from the metal and the indicators of hydrogen removal by the formation of bubbles on the surface of the melt with the removal limit along the considered way, we obtain the values presented in Table 1 and in Fig. 4.

CONCLUSIONS
Removal of hydrogen from the melt surface is not the main mechanism of degassing. At concentration of hydrogen in steel less than 3.5 ppm, formation of hydrogen bubbles from the metal surface stops. The main way of degassing in conditions of CS-2 of PJSC "NLMK" is removal of hydrogen into bubbles of the transporting gas. study on the removal of hydrogen and nitrogen from the melt of medium carbon steel in vacuum tank degasser, min sat P min sat 4 , P P g = + σρ