CC BY
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Logunova O.S., Matsko I.I, Posochov I.A.
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Shatokhin I.M., Bigeev V.A., Shaymardanov K.R., Manashev I.R.
INVESTIGATION OF COMBUSTION IN TITANIUM-FERROSILICON SYSTEM
Abstract. Results of self-sustaining combustion process in the titanium-ferrosilicon system investigations are presented. These data were used for experimental-industrial technology developing of production ferro silico titanium with high titanium content for steel alloying.
Keywords: titanium, titanium containing steel, ferrotitanium, ferrosilicotitanium, titanium ferrosilicide, self-propagating high-temperature synthesis, combustion rate, combustion temperature.
At present, titanium is actively used for alloying of wide assortment of steel thanks to its specific properties. Titanium is used for modern HSLA-steels, pipe-steels, stainless steels and others. Mainly, the mechanism of its influence on steel quality is associated with formation of titanium carbides, nitrides and carbonitrides in steel.
Ferrotitanium is usually used for steel alloying. Depending on the method of production, ferrotitanium can be with high (~70% Ti) and low (<40% Ti) titanium content. High-grade ferrotitanium is usually obtained by melting titanium-containing waste in induction furnaces. Low-grade ferrotitanium is produced by recovery from ilmenite in special melting aggregates. Ilmenite concentrate, iron ore, aluminum powder, ferrosilicon and lime are used as raw materials. Sometimes, when high-purity steel grades are produced, vacuum-melted ferrotitanium is used. Standard ferrotitanium contains considerable amount of impurities (such as non-ferrous metals, nitrogen, oxygen, hydrogen, carbon, sulfur, phosphorus), which come to alloy from raw materials and atmosphere during production. Moreover, assimilation degree of titanium from ferrotitanium remains low.
That is why, full or partial replacement of ferrotitani-um by alternative titanium containing alloys is actual. This problem can be solved by creating of complex master-alloys, which contain titanium as basic element and high-level elements, such as Si, Al, Ca, etc. It is supposed, that these elements will protect titanium against oxidation, because they are strong deoxidizers. In this way, titanium assimilation will be higher.
Thus, to be most effective, new alloy should meet the following requirements:
- high titanium content;
- low impurities content;
- the presence of elements with high affinity to oxygen.
This alloy will give high and stable titanium assimilation, will allow to produce steels with narrow titanium
limits and will reduce impurities content in metal. The most economical alternative to ferrotitanium can be ferro silico titanium.
Usually ferro silico titanium is obtained by melting titanium, metallic silicon and low-carbon steel in induction furnace or by recovery from ilmenite ore.
But it is impossible to produce ferro silico titanium with high titanium content (more than 30% Ti) by furnace methods, because of high melting point of titanium sili-cides and strong liquation during crystallization. Moreover, alloy, obtained by furnace methods, has higher concentration of nitrogen, oxygen and hydrogen, penetrating into the melt from the atmosphere. So, it is important to use fundamentally different method of titanium ferrosili-cide obtaining that enables high product recovery with low impurities content with the lowest electricity costs. Such method is self-propagating high-temperature synthesis (SHS). When SHS is applied, no traditional furnaces are used. The process is carried out in special SHS-reactors at the atmosphere of inert gas or vacuum. At the combustion synthesis, as well as at the traditional metallo-thermic process, energy source is the heat of chemical reactions. But unlike metallothermy, SHS is slagless.
The Ti-Si system has 5 silicides: TiSi, TiSi2, Ti5Si3, Ti3Si4 h Ti3Si. Ti5Si3 has the highest melting point. Physical and chemical properties of titanium silicides are presented in Table 1. The formation of titanium sili-cides occurs with great heat release, so that adiabatic combustion temperature is high. Regularities of interaction of titanium and silicon are studied in detail in works [3, 4]. It was shown, that combustion in powder compound of titanium and silicon can be implemented in a wide range of parameters changing such as the components ratio, powders dispersion, etc. It can be expected, that heat release during chemical interaction in ternary system Ti-Si-Fe will be enough to carry out the process in self-propagating mode.
Table 1
Physical and chemical properties of titanium silicides [5]
Characteristics TÍ5SÍ3 TiSi TiSi2 Ti3Si4 Ti3Si
Heat formation, kJ/mol 581,2 132,8 136,6 - -
Crystallization temperature, °K 2120 1760 1540 1920 1170
Density, g/cm3 4,3 4,03 4,21 - -
Adiabatic combustion temperature, °K 2500 2000 1800 - -
It is known, that high exothermicity of raw materials interaction is the essential condition for all SHS-reactions. To determine the possibility of self-propagating process, calculation of adiabatic temperature in ternary system Ti-Si-Fe with iron content of 0 to 90% was carried out using thermodynamic data and methodology, represented at [5, 6]. During calculation it was assumed, that iron did not take part in the reaction and titanium silicide Ti5Si3 and a-iron are combustion products:
5Ti + 3Si + xFe ^ Ti5 Si3 + xFe
(1)
Combustion temperature was calculated using the following formula:
(1 -M) .AH%Sh =
= (1 • (Q -Vt5S3 ■ Ln¡Sh\
(2)
The results show, that adiabatic temperature remains enough high despite heat effect reduction, when iron is added to (5Ti + 3Si)-compound (more than 2000°K when iron concentration is changed between 0 and 50%). Therefore, we can expect, that process can be carried out in self-propagating mode in wide range of Ti, Si, Fe concentration.
To carry out the researches, a laboratory SHS-reactor with 20 liters volume was constructed. This plant allows to carry out researches of SHS-processes in a wide range of pressure: from 0,01 to 10 MPa. Ti-Si-Fe-system is qualified as SHS-gas-free system, that is, all raw materials and products are in condensed state. There is no relationship between combustion parameters and pressure and this is the distinction of these systems. The relationships between combustion rate, combustion temperature, raw components ratio and particle size of titanium powder in titanium-ferrosilicon system were investigated using a laboratory SHS-reactor (Fig. 2).
Porous titanium powder TPP-4 TU 1791-44905785388-99 produced by JSC «VSMPO-AVISMA Corporation» and ferrosilicon powder FeSi75 GOST 1415-93 produced by JCS «Chelyabinsk Electrometallurgical Integrated Plant» are used as raw materials. The temperature was measured by thermocouple method using tungsten-rhenium thermocouples TR-5/TR-20, an analogue transducer RL-16AIF and a signal multiplier RL-4DA200. The combustion rate was measured by a video camera.
where L
Vt
are the heat
Ti5Si3 ' Ti5Si3
and the degree of Ti5Si3 fusion respectively; AH^ and kHTFe
are enthalpy change of titanium silicide formation and iron when temperature increases from To to Tad; (x is the amount of iron in the combustion products.
The results of calculation are presented in Fig. 1. It is relationship curve between adiabatic temperature of combustion of (5Ti + 3Si)-Fe-compound and concentration of iron in it.
3000
Fig. 2. Laboratory SHS reactor scheme
10
20
30
40 50 60 Iron concentration, %
70
80
90
100
Fig. 1. Influence of iron concentration on adiabatic temperature of combustionof (5Ti + 3Si)-Fe-compound
Combustion rate and temperature are basic parameters of a SHS-method. These parameters determine process productivity and safety, as well as equipment requirements. It is known, that modes and possibility of SHS depend on many factors. Most important factors, except high exothermicity, are raw components ratio and powder particle size.
In general, combustion synthesis in Ti-Si-Fe-system can be implemented using different schemes: Ti + Si + Fe; Ti + FeSi + Fe; Ti + FeSi; FeTi + Si + +Fe; FeTi + Si and others. In this work
0
Shatokhin I.M., Bigeev V.A., Shaymardanov K.R., Manashev I.R.
processing in titanium-ferrosilicon system is investigated.
Combustion synthesis in this system was carried out in a wide range of raw components ratio (Fig. 3 and 4). As a result of combustion product with 60-75% Ti and 18-27% Si is formed. Fig. 3 shows the relationship between combustion temperature and tita-nium/ferrosilicon ration in the mixture.
As we can see, combustion temperature depends weakly on components ratio and remains almost constant in a wide range of raw components ratio (1720 ± 50°K). We think it happens because the changing of raw components ratio in studied ranges affects only on the proportion of the liquid phase in products, but the temperature of forming melted products, that is close to measured temperature, remains constant.
Fig. 4 shows the relationship between combustion rate and component ratio of (5Ti + 3Si)-Fe-compound
We can see from the figure, that combustion rate reduces both when titanium concentration increases and when it reduces. Maximum combustion rate was obtained on the compound with 72% Ti.
Fig. 5 shows the relationship between combustion temperature and titanium powder particle size when component ratio provides formation of Ti5Si3.
The figure shows that temperature remains at the same level (1720 ± 50°K) and changes weakly when titanium powder particle size is varied.
The next figure presents the relationship between combustion rate and titanium powder particle size (Fig. 6). This figure demonstrates, that combustion rate changes weakly (2 mm/s ± 0.25 mm/s) when titanium powder particle size is varied. Fig. 5 and 6 demonstrate that combustion in this system can be carried out in wide range of titanium powder particle size and combustion temperature with almost constant rate.
Q. £
1800
1750
1700
1650
1600
1550
1,5 1,7 1,9 2,1 2,3 2,5
The ratio of titanium to ferrosilicon, %
2,7
2,9
3,1
Fig. 3. Relationship between combustion temperature and components ratio
E £
-Q £ o o
1,5
2,5
3,5
4
3
2
1
0
2
3
The ratio of titanium to ferrosilicon, %
Fig. 4. Relationship between combustion rate and components ratio
1760
1740
1720
V
1700
<u
3
ra 1680
<u
o
F 166U
<u
l-
1640
1620
1600
0 0,5 1 1,5 2
Avarage particle size, mm
Fig. 5. Relationship between combustion temperature compound and titanium powder particle size in titanium-ferrosilicon compound
0,5
1,5
Avarage particle size, mm
Fig. 6. Relationship between combustion rate and titanium powder particle size in titanium-ferrosilicon system
Thus, these investigations have shown that SHS-technology can be applied to obtain ferro silico titanium with high titanium content. It is possible to obtain alloys with various titanium content and various densities by varying of initial mixture and particle size of initial powders. Moreover, lack of waste and electricity consumption will bring low cost and provide high competitiveness of products. At the same time, producing of new alloy in inert gas atmosphere will increase the purity of end product.
Investigations results were used for developing the experimental-industrial technology of ferro silico titanium production by SHS-method. Specifications for optimal SHS-ferro silico titanium composition are developed (Table 2).
Table 2
Chemical composition of SHS-ferro silico titanium
Grade Ti Si C S P O N H
max
FST 70 61-74 18-27 0,15 0,005 0,009 0,1 0,05 0,005
Conclusions
The process of obtaining ferro silico titanium with high titanium content (to 75%) by self-propagating high-temperature synthesis was investigated. The relationships between combustion rate, combustion temperature and initial components ratio, titanium powder particle size were obtained. As it turned out, temperature depends weakly on initial components ratio and titanium powder particle size and remains at the same level (about 1720 ± 50°K). Combustion rate is also weakly depends on titanium powder particle size, but we can see, that combustion rate reduces when titanium content both increases and reduces. Maximum combustion rate was obtained on the compound with 72% titanium. Based on these data, experimental-industrial technology of obtaining ferro silico titanium by SHS-method was developed.
References
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Parsunkin B.N., Andreev S.M., Akhmetov T.U., Mukhina E.Y.
OPTIMAL ENERGY-EFFICIENT COMBUSTION PROCESS CONTROL IN HEATING FURNACES OF ROLLING MILLS
Abstract. Considering continuous energy price rising, energy- efficient combustion process control is of current interest because circa 15% of the consumed firing is expended in rolling production for metal heating.
Effective solution of this problem is possible by using the automated systems of optimal control, based on optimizing control algorithms of search type. Such management systems have the ability to provide search and to maintain the maximum value of the optimized parameters under uncertainty and the lack of accurate quantitative model of the processing. Keywords: temperature, air volume control, objective variable, extremal control, combustion control.
In iron and steel industry circa 15% of the consumed firing is expended in rolling production in metal heating. Therefore, energy-efficient combustion process control is of current interest, especially considering continuous energy price rising.
Optimal energy-efficient combustion process control is a difficult task in continuous furnaces of modern highperformance hot rolling mills, when their operating rate varies from 100 up to 1000 t / h, and the initial temperature of continuous cast billets, feeding to heating, ranges
