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The Challenge between Energy Saving and Steel Quality: Effect of Scrap / Total Charge Ratio Authors:
Iman El-Mahallawi, Faculty of Engineering Cairo University, Giza, Egypt, E-Mail: [email protected], Tel.: +20-1006044661 Ahmed Shash, Faculty of Engineering Cairo University, Giza, Egypt, E-Mail: [email protected], Tel.: +20-1222144128 Ahmed Ramadan, Beshay Steel, Industrial Zone, Sadaat City, Egypt, E-Mail: [email protected], Tel.: +20-1227261460 Taha Mattar, Centeral Metallurgical Research and Development Institute, Helwan, Egypt, E-Mail: [email protected], Tel.: +20-1001301230 Abstract
With an increasing demand for steel world-wide, scrap metal has become a critical resource, where scrap consumption has been growing at 12% per annum in steel industry. The major environmental benefits of using recycled steel or scrap is shown in the fact that the production of one tone of steel through the EAF routes consumes only 9 - 12.5 Gj/tcs, whereas the BOF steel consumes 28 - 31 Gj/tcs, this significant saving in energy means in addition to saving in energy costs, minimizing CO2 emissions, reducing the solid waste of steel and reducing the consumption of stockpile. But the increased use of scrap metal in the steelmaking process results in rising the content of some residual elements in the recycled materials. It is also agreed that the behaviour of copper and tin in the reheating step during the hot working of steel is the main reason for the appearance of hot shortness, in which during continuous casting or hot rolling, the liquid copper-rich phase weakens grain boundaries and leads to surface cracking. These cracks are generally not eliminated during subsequent processing and cause surface defects in the final product. The increased content of tramp elements means affecting the steel quality by rising problems and deviations from currently used specifications associated with the accumulation of residual elements from the steel scrap. The existence of these tramp elements (Cu, Pb, Ni, Sn, Sb) will affect the mechanical properties of steel as reduction of hot ductility, failure at bending test and hot shortness of steel during reheating as a result of forming of low melting compounds. On the other side, evaluating the current potential and technological strategies aiming at mitigating the accumulation of residual elements in steelmaking industry is becoming a hot spot for recycling of steel. Therefore, the aim of this work is to study the energy saving and gases emissions as well as the problem of residual elements appearing in Egyptian steel industry.
Keywords: Recycled Steel, Energy Saving, Steel Quality, Crack Susceptibility, Tramp Elements.
1. Introduction:
The issue of sustainability should be addressed from a view of life-cycle of materials, in which the life-cycle of manufactured products and their interaction with and impact on the environment are considered simultaneously. To save the world resources "resource consumption" which would mean reducing the use of raw materials since it leads to: increased costs of raw materials, increased energy needs, stockpiles of materials waste, and increased Green House Gas (CO2) emissions, the use of recycled steel is expanding all over the world.
The EAF share of steel production has been increasing at 66% per annum rate since
the 1950s according to Yellishetty et. al. 1 . It has been reported by Janke et. al. 2 that on the average 70% of the amount of steel end products is returned to the materials cycle after 20 years of its manufacture, whilst the remaining 30% is lost by rusting of steel. During the last 60 years the world steel production has increased dramatically,
according to Yellishetty et. al. 1 the world steel production increased from 187 MT to 1299 MT between 1950 and 2006. It has been shown that since 1950, scrap consumption has been growing at 12% per annum in steel industry, with an estimated doubling in consumption between 2009 and 2019, based on estimated EAF steel
production 1 . The major environmental benefits of using recycled steel or scrap is shown from the fact that the production of one ton of steel through the EAF routes consumes only 9 - 12.5 Gj/tcs, whereas the BOF steel consumes 28-31 Gj/tcs, this significant saving in energy means in addition to saving in energy costs, minimizing CO2 emissions and
reducing the solid waste of steel and reducing the consumption of stockpile 1 . Energy is a major challenge for heavy metal industries in Egypt nowadays. The depletion and price increase of energy in Egypt as well as worldwide is imposing several threats to the metal industries. The culture and understanding of Energy auditing and energy efficiency is an important activity that should be implemented by all steel producers. This needs establishing a common benchmarking platform for all steel producers.
Increasing energy efficiency cuts costs and helps to achieve sustainable economic growth, which should be supported by established goals to boost energy productivity. This can only be done through proper technologies and policies. The Egyptian steelmakers should develop and implement practices that will dramatically reduce greenhouse emissions and improve energy efficiency in the industrial sector to facilitate full commercial deployment of energy efficient and low-carbon technologies and practices in industry. The iron and steel sector is not only one of the largest energy using and CO2-emitting industrial sectors but also an area with significant potential for reducing energy use and CO2 emissions. However, decision-makers in industry, analysts and policymakers are challenged to find relevant, usable information that can facilitate the uptake of best practices. By compiling a comprehensive collection of relevant information from around Egypt and making it easily accessible to decision-makers to facilitate increased implementation of best practices.
Scrap is not a waste material but a valuable raw material with high energy content.
Every ton of recycled steel scrap saves 1134 kg iron ore, 635 kg coal and 54 kg lime 3 . The production of new steel from scrap in EAF (electric arc furnace) consumes about half of the energy necessary for producing steel from iron ore. Production of steel from scrap compared to the production based on reduction of iron ore with coke
reduces the emission of CO2 and other harmful gases and consumes less water too 4,3 .
Scrap purification has also economical aspects regarding the value of residual elements in steel.
Unfortunately, the increased use of scrap metal in the steel making process results in rising the amounts of some residual elements in the recycled materials. The behaviour of copper and tin during the reheating of the hot working of steel is the main reason for the appearance of hot shortness in which, during continuous casting or hot rolling, the liquid copper-rich phase weakens grain boundaries and leads to surface cracking. These cracks are generally not eliminated during subsequent processing and cause surface defects in the final product. The increase of some residual elements in the final steel analysis means affecting the steel quality by rising problems and deviations from currently used specifications associated with the accumulation of residual elements from the steel scrap. The existence of these tramp elements (Cu, Pb, Ni, Sn, Sb) will affect the mechanical properties of steel as reduction of hot ductility, failure at bending test and hot shortness of steel during reheating as a result of forming of low melting temperature compounds.
Therefore, the aim of this work is to study the energy saving and the problem of residual elements appearing in Egyptian steel industry. On the other side, evaluating the current potential or technological strategies aiming at mitigating the accumulation of residual elements in steelmaking industry is becoming a hot spot for recycling of steel.
Based on the current state of art, the research group started reviewing the status in
some Egyptian steelmaking companies. A study 5 was carried out at Alexandria National Company of steel at Dekhaila (ANSDK). This study has indicated that the melts produced from charges containing more than 50% scrap contained copper and nickel contents more than (0.1%) compared to (0.02%) for the melts produced from charges containing more than 50% DRI in the charge. The mechanical properties were also determined by the tensile test, the tensile strength and ductility values were correlated to the carbon equivalents. This study has also indicated that higher strengths and lower ductility values were obtained for the reinforced bars (RC bars) made from higher contents of recycled scrap.
According to Egyptian ministry of industry the annual steel yearly production in Egypt for the year 2011 was 11 Million Tons of steel as shown in Table 1. The total consumption of steel in Egypt is around 15 million tons per year in 2011, meaning that Egypt imported about 4 million tons of steel, showing that 90% of these quantities were from Turkey. Some of these steel companies listed in Table 1 are DRI-Integrated plants producing around 3 Million Tons, using more than 25% of scrap. The scrap mix/percentage DRI for each steel plant is ranging from 25% to 70% and in some cases it exceeds these limits to reach 100% scrap. It is expected that, the production will be increasing to reach 17 million tons at 2017 according to the statistics of the Egyptian ministry of industry, where many licenses were given to steel investment companies especially DRI-integrated plants of steel, as Ezz Company DRI, Suez Steel DRI, Beshay Steel DRI and two integrated Mills for rolling and Meltshop (Teba company Integrated Rolling Mill -and Hadeed Al Masreen Integrated Rolling Mills).
Table 1 the annual steel yearly production in Egypt for 2011
The production in one of the steel companies (Beshay Steel) was analysed for the last 5 years and the following experimental work based on the preliminary results presented hereafter.
2. Experimental work:
An experimental programme was carried out on an industrial scale for 140 tons EAF at Beshay steel. Data from a large number of heats conducted at the Egyptian steelmaking factory was collected and studied to analyse energy consumption during the melting stage. Samples with increased tramp elements content were isolated and their mechanical properties were studied.
In the frame of this work, all the manufacturing operational data, the scrap percentage starting from 72 to 100% as well as the Oxygen blowing system data were collected, such as the oxygen line, camera system for controlling and view the surface of melt in the furnace with a monitor as well as the controllable caring oxygen blowing lance, a flow meter and controlling flow systems with display and control panel.
Oxygen in steel industry is important, because it has a considerable effect on the quality of cast steel parts. Solubility of oxygen in steel as well as the quantity of oxygen will be in a relation with the carbon and slag’s lime content, respectively.
Also, as iron and steel sector is one of the largest CO2-emitting industrial sectors, therefore, in this work the relation between charge constituents and CO2 emissions was carried out.
3. Results and Discussion:
3.1 Mechanical Properties Challenge
3.1.1 Failures associated with increased copper
The preliminary analytical study in this work has shown increased copper content in the production. The deviation in the chemical analysis was accompanied with reported failures in the mechanical testing. Table 2 shows the chemical analysis of two rebar sizes that failed during the bending test. Tensile tests were carried out for small specimens taken from the broken bending samples. The displacement was measured with a strain gauge 5 D and the results were recorded in Table 3. Macroscopic images were taken at magnification 2X for fracture surface for size 32 mm samples as illustrated in Figure 1. Microscopic images were taken at
�o. Company �ame Production (M Tons) Market Share %
1 Ezz Group 4.5 40.9
2 Beshay Steel 2 18.6
3 Helwan (Al Massryia) 1.9 17.2
4 Suez steel 1.5 13.6
5 Hadded Almasreen 0.95 8.6
6 Others 0.15 1.3
Total 11 100%
Magnifications 200X from the surface, and core of cross section samples, as shown in Figures 2.a, b and c.
Table 2 Chemical analysis for rebar sizes 16 and 32 mm that failed during bending tests
Size No C Si S P Mn Ni Cr Cu
1 0.43 0.26 0.05 0.03 1.33 0.10 0.04 0.42 16
mm 2 0.4 0.29 0.03 0.03 1.35 0.12 0.14 0.47
3 0.21 0.16 0.04 0.032 0.93 0.142 0.074 0.652 32
mm 4 0.20 0.162 0.044 0.032 0.92 0.142 0.074 0.56
Table 3 Tensile test results
�o. Size Y.S
(MPa)
U.T.S
(MPa)
Elongation % T.S/Y.S
1 16 497 782 11.6 1.57
2 16 488 782 10 1.61
3 32 579 674 15.8 1.16
4 32 560 653 16 1.16
.
Fig. 1 Macroscopic fracture surface for 32 mm size sample.
A B c
Fig. 2 a. Microstructure of Surface 200X, b. Middle 200X and c. Core 200X.
Figures 3.a and b show SEM and XRD analysis of areas where copper segregation was observed. The microstructure shows the appearance of equiaxed grains with
normal grains. Copper has a maximum solubility in ferrite of 0.35%, thus the increased copper contents will cause it to precipitate at the grain boundaries of ferrite forming £ phase which is a rich phase of copper. It is observed from the microstructure and evidently concluded from the SEM and XRD that a thin film of copper with a concentration in the range of 0.5 to 0.55 wt.%, remains in between the grain boundaries, making the inter-crystalline grains so week, reducing the ductility and causing material embitterment, leading to the reported failures as shown in Figure 4. Figure 4 shows the top of billet split during rolling caused by inter granular fracture. It is also noticed that the effect of tramp elements appears more clearly in the bending test rather than reduction in elongation.
a. b.
Fig. 3 SEM and XRD with a. 0.50 wt.% Cu, b. 0.55 wt.% Cu.
Fig. 4 Top of billet spilt during rolling
3.1.2 Relation between copper content and the mechanical properties of steel
This study is based on a series of heats consisting of 100% scrap between the period 2010 and 2014. The used scrap is locally available grade unclassified scrap. Table 4 shows typical plant analysis for two batches from 16 and 32 mm diameter bars.
Effect of increasing Cu% on Ductility
7
10
13
16
19
22
0.3 0.33 0.36 0.39 0.42 0.45 0.48
Cu wt.%
Elo
ng
ati
on
%
Effect of increasing Cu% on Tensile Strength
640
660
680
700
720
740
0.20 0.25 0.30 0.35 0.40 0.45 0.50
Cu wt.%
U.T
.S i
n M
Pa
Table 4 Typical chemical analyses from two rebar sizes
Size No C Si S P Mn Ni Cr Cu
1 0.43 0.26 0.05 0.03 1.33 0.10 0.04 0.42 16
mm 2 0.4 0.29 0.03 0.03 1.35 0.12 0.14 0.47
3 0.21 0.16 0.04 0.032 0.93 0.142 0.074 0.652 32
mm 4 0.20 0.162 0.044 0.032 0.92 0.142 0.074 0.56
Samples from the 16 mm rebars with high percentage of tramp elements as (Cu, Pb, Sn) in the billet, were examined to find the effect of the tramp elements on the mechanical properties of the rebars which were not subjected to tempcore process. Figures 5.a and b show that increasing the Cu content decreases the ductility, in consequence. The copper precipitates causes reduction in ductility and failure during the bending test. On the other side, it was also observed from the results that increasing the Cu content leads to an increase in ultimate tensile strength. Figure 6.b shows the interpolated results of the effect of Cu on the tensile strength.
Fig.5 Effect of increasing Cu% on: a. the ductility, b. the tensile strength for size 16mm
samples without Tempcore process.
It is worth mentioning that the samples from the 16 mm rebars (with high percentage of tramp elements as (Cu, Pb, Sn) in the billet), the chemical composition of which is shown in Table 3, which were subjected to tempcore process were examined to find the effect of applying tempcore process on the bending test. The samples subjected to tempcore process with high percentage of tramp elements didn’t show a remarkable effect on elongation and bending test, however, increasing the Cu content more than 0.5% has shown a slight increase on the ductility and a decrease in the U.T.S.
Since the manganese content is on the high range Mn% (1.33 - 1.35%) with manganese to sulphur ratio (26-45), the Manganese will combine with the total sulphur and form (MnS). The rich copper phase will precipitate at the grain boundaries around the MnS increasing the yield strength and the tensile strength causing grain embrittlement while on the other hand the elongation, toughness, and bendability will be reduced where the presence of copper precipitates will reduce the adhesion of grain and help to cause fracture.
The increased content of tramp copper reduces the elongation to (10- 11.6%), compared to expected elongation for chemical composition free from copper and
other tramp elements (17%), as a result of precipitation of copper at grain boundaries which causes grain boundaries embrittlement and grain boundaries decohesion which causes reduction in elongation and fracture at final.
It is also expected that the brittle copper rich phase (forming as result of selective oxidation of iron during reheating process) will migrate toward grain boundaries and precipitate around the sulphide inclusions, which will weaken the cohesion of the grain boundaries leading to hot shortness. The size and volume fraction of these precipitates depend on the cooling rate. Slow cooling rate results larger sizes of precipitates and less volume fraction which is considered more dangerous, on the other hand the high cooling rate produces fine precipitates or super saturated ferrite of copper.
3.2 Energy Challenge
3.2.1 Relation between Scrap/Charge Ratio and Electrode Consumption
Figure 6 shows the relation between scrap% and electrode consumption. The Figure shows that by increasing the percentage of scrap, the electrode consumption will be reduced from 2 to 1.2 kg/Ton of scrap at 100% Scrap, this means that the amount of energy consumption per ton of scrap will be reduced. Figure 6 also shows that electrode consumption reduces by 0.3 to 0.36 kg/Ton as well as refractory consumption decreases by 0.9 to 1.4 kg/Ton and Tap to Tap time reduces by 5 to 8 minutes.
Effect of Scrap % to Electrode Cosumption
1
1.3
1.6
1.9
2.2
70 75 80 85 90 95 100 105
Scrap / Total Charge Ratio %
Ele
ctro
de
Co
nsu
mp
tio
n (
kg
/ T
on
)
Fig.6 Effect of increasing scrap % on electrode consumption
3.2.2 Relation between Scrap/Charge Ratio and Energy Consumption
Figure 7 shows the relation between the scrap/total charge ratio and electrical energy consumption. It is shown from Figure 7 that increasing the amount of scrap to the total charge ratio reduces the electrical power consumption in the EAF. Preheating of the scrap brings thermal energy into the furnace, as preheating of the scrap to 540°C brings 81 kwh/ton of additional energy. Scrap preheating gives a reduction in energy consumption by 40-60 kwh/ton depending on the scrap preheat temperature.
EAF Power Consumption / Scrap Ratio
400
410
420
430
440
450
460
470
480
490
500
60 70 80 90 100 110
Scrap / Total Charge Ratio %
EA
F P
ow
er
Co
nsu
mp
tio
n
kW
h/t
Fig. 7 Effect of scrap to total charge ratio on the electric power consumption in the EAF.
3.2.3 Relation between Scrap/Charge Ratio and CO2 Consumption
Figure 8 shows the relation between scrap% and CO2 consumption. Blowing oxygen into the furnace to form foam slag alleviates the problem that creates bubbles of CO2, which percolates up into foam. The arc is down in the foamy slag, therefore, the heat is absorbed by the slag instead of moving out to the walls of the vessel and the slag heat is transferred to the molten metal. The effect of increasing the ratio of Lime to the burned O2 on the electrical energy consumption in the EAF for 90% scrap is shown in Figure 9. It is clear from Figure 9 that increasing the ratio of Lime kg / O2 Burning m³/Ton decreases the electrical energy consumed in the EAF. The reaction of carbon with oxygen in the bath to produce CO is important as it supplies a less expensive form of energy to the bath, and performs several important refining reactions. In modern EAF operations, the combination of oxygen with carbon can supply between 30 and 40 % of the net heat input to the furnace. Evolution of carbon monoxide is very important for slag foaming. Coupled with a basic slag, CO bubbles are tapped in the slag causing it to "foam" and helping to bury the arc. This gives greatly improved thermal efficiency and allows the furnace to operate at high arc voltages even after a flat bath has been achieved. One of the reasons of oxygen blowing into EAF is producing heat for heating or melting scrap, decarburizing of steel foaming slag and burning of carbon monoxide (CO). Other important reasons are producing chemical energy to decrease the time of melting and the electrical consumption. Control of the chemical reactions and the metallic constituents in the bath is important as it determines the chemical energy generated and the properties of the final product. Oxygen reacts with aluminium, silicon and manganese to form metallic oxides, which are slag components. These metallic ions tend to react with oxygen before the carbon generating heat energy. They will also react with FeO resulting in a recovery of iron units to the bath. In the refining stage the metallurgical reactions in the steel bath with slag provide the highest amount of the chemical energy for the process.
It is reported 6 that for the production of oxygen the energy of 0.5-1 kWh/Nm³ has to be used so the electrical energy consumption of the furnace can be decreased by higher oxygen injection with gaining up to 5 kWh/Nm³ O2 in the furnace in average
for all reactions. The main exothermic reactions are provided in Table 5 6 . Silicon has the highest energy input with 11.2 kWh/m³ O2 and the free energy is coming down to the carbon combustion to CO with 2.73 kWh/m³ O2. In the refining stage Si, P, and Al
are burned completely due to their high oxygen affinity, Fe, Mn, Cr, and Mo are oxidized with higher oxygen partial pressure. The total energy input by chemical reactions varies from 50 kWh/t to 300 kWh/t depending on the input materials and the
scrap percentage 7 . The effect of increasing the scrap amount to the total charge ratio on the chemical energy generated is shown in Figure 10. The need to develop steelmaking technologies to adopt current trends in using total scrap charges has been
emphasized by one of the authors elsewhere 8 .
Effect of Scrap % to CO2 Consumption
30
35
40
45
50
55
60
60 65 70 75 80 85 90 95 100
Scrap / Total Charge Ratio %
CO2
Co
nsu
mp
tio
n (
m3
/ T
on
)
Fig. 8 The effect of scrap% on CO2 consumption
Electric Energy Consumption in EAF with Lime / O2 Burning
400
410
420
430
440
450
460
470
480
490
500
3.5 4 4.5 5 5.5
Lime kg / O2 Burning in m3/t
Ele
ctr
ic E
nerg
y in
EA
F k
Wh
/t
Fig. 9 The effect of Lime / burned O2 on the electrical energy consumption in the EAF
Table 5 Exothermic chemical reactions in the steel melt 6 .
Chemical Energy / Scrap Ratio
150
200
250
300
350
400
60 65 70 75 80 85 90 95 100 105
Scrap / Total Charge Ratio %
Ch
em
ical
En
erg
y k
Wh
/t
Fig. 10 The effect of scrap to total charge ratio on the chemical energy generated in the EAF
The total power consumption depends on the input material and mainly on the scrap amount. The effect of increasing the scrap to the total charge ratio on the total power consumption is shown in Figure 11, from which it can be seen that the total power consumption decreases as the scrap to the total charge ratio increases.
Total Power Consumption / Scrap Ratio
550
600
650
700
750
800
850
900
60 65 70 75 80 85 90 95 100 105
Scrap / Total Charge Ratio %
To
tal
Po
wer
Co
nsu
mp
tio
n
kW
h/t
Fig. 11 The effect of scrap to total charge ratio on the total power consumption in the EAF
3.3 Environmental Challenge
3.3.1 Relation between scrap% and gas emissions
Figure 12 shows the relation between scrap% and gas emissions. The figure shows that the amount of waste gases increase by increasing the scrap ratio. This may be explained by that at operation with 100% scrap, where there is no enough carbon to produce CO2, which causes, alternatively, the production of carbon mono oxide gas (CO), accordingly for the submergence of the arc in the foam extra amount of carbon is being injected and more oxygen is injected to form carbon mono oxide and slag foams. This means that at higher ratios of scrap, the gas foaming reaction is not sufficient and it is necessary to inject carbon in the bath so the amount of coke added and CO evolving will be higher by increasing the percentage of scrap ratio.
Effect of Scrap % to Waste Gases
10.00
15.00
20.00
25.00
30.00
35.00
40.00
70 75 80 85 90 95 100 105
Scrap / Total Charge Ratio %
Wa
ste
Ga
ses
(m3
/ T
on
)
Fig. 12 The effect of scrap% on gas emissions
4. Conclusion:
1. Increasing the ratio of scrap causes accumulation of undesired elements, especially Cu where Cu% up to 0.8% may be reached.
2. Increasing the scrap ratio in the charge results in a decrease in power consumption.
3. The proper integration of chemical energy during the melting process adds to energy saving in steelmaking.
4. Increasing the scrap to 100% causes an increase in gas emissions.
Acknowledgement
The authors would like also to thank Dr. Yousif Beshay the General Manager of
Beshay Steel & Mr Mohamed Ibrahim plant Manager of ISRM for their generosity for
offering the materials and supporting the experimental tests. The authors, also, would
like to thank Beshay Steel for their support in compiling this work.
References
[1] M. Yellishetty, G. M. Mudd, P.G. Ranjith, A. Tharumarajah, Environmental Science &
Policy, Volume 14, Issue 6, October 2011, p. 650-663.
[2] Janke, L., Czaderski, C., Motavalli, M. & Ruth, J. (2005), Mater. Struct., 38, p. 578-592.
[3] Gartz R, Nylén M, “Miljöaspekter vid produktion, användning och recirkulering av metaller, IM report 3424, 1996. [4] Marique C.,”Tramp elements and steel properties: a progress state of the European project on scrap recycling”, La revue de Métallurgie-CIT pp.433-441, 1998. [5] S. A. Attia, (1999) P.hD Thesis, Alexandria National Company of steel at dekhaila (ANSDK). [6] Pfeifer, H.: Einfluss der Sauerstofftechnologie auf die Leistungssteigerung von Lichtbogenöfen; Jubiläumssitzung des Fachausschusses Hochtemperaturtechnik; Duesseldorf, 2008. [7] Pfeifer, H; Kirschen, M; Simoes, J.P.: Thermodynamic analysis of EF electrical demand, Proceedings EEC 2005 Birmingham. [8] I El-Mahallawi and S. El-Raghy; Reuse of Materials and Byproducts in Construction Waste Minimization and Recycling, Edited by Alan Richardson, Chapter 3, Springer 2013.
