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Steelmaking technology for a sustainable society
CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
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Steelmaking technology for a sustainable society Tooru Matsumiya ∗ Nippon Steel Corporation, 20-1 Shintomi, Futtsu 293-8511, Japan
article
info
Article history: Available online 25 March 2011 Keywords: CO2 emission reduction Eco-products City waste recycling Less waste emission
abstract For the reduction of CO2 emission, two major developments are being conducted in COURSE50 (‘‘CO2 Ultimate Reduction in the Steelmaking Process by Innovative Technologies for Cool Earth 50’’). The one is separation of CO2 gas from BFG (Blast Furnace Gas) and recharge of the rest of BFG including H2 and CO into blast furnace. Hydrogen iron ore reduction technology is also going to be developed. The other one is amplification of H2 gas from CH4 , for example, in COG (Coke Oven Gas). The produced hydrogen gas will be supplied to the society or the reformed COG will be charged to blast furnace. In addition to these drastic challenging technology developments, a variety of measures for CO2 reduction is under taken. In the frame of Asia–Pacific Partnership on Clean Development and Climate, the best available technology for energy savings are discussed to be transferred within seven member nations, which has the effect of 1.27 million ton reduction of CO2 emission a year. By supplying energy saving steel products to society such as high strength steels for automobiles and ships, which realizes the fuel consumption reduction, high performance electrical steels for motors and transformers, which realize electricity loss reduction, and by recycling waste city plastics and tires in steel processes for hydrogen gas generation, chemical raw material conversion and iron ore reduction, etc., it is expected that equivalent 10% reduction of CO2 gas emission in steel production is counted. In steelmaking process the reduction of refining slags contributes materials use efficiency and less emission of unuseful byproducts. The control and utilization of nonmetallic inclusions, such as deoxidation products, are one of the key technology for obtaining product performance, which is required in the above-mentioned steel products. In order to optimize steelmaking process for these purposes, computational thermodynamics is applied. Optimization of demanganization, and control of chemical composition of nonmetallic inclusions by the use of computational thermodynamics are mentioned. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. CO2 breakthrough program
For the realization of sustainable society, a variety of approaches is considered in steelmaking sector. At the beginning of this paper CO2 Breakthrough Program will be introduced. Then, incremental reduction programs of CO2 emission will be mentioned: best available technology transfer, eco-products supply to the society, city waste recycle in steel plants, steel plant waste utilization in the society and process improvement and development for less emission. The second through the fourth items are the collaboration between steel manufacturers and the society. At the end, two examples of the use of computational thermodynamics for product and process development including the above-mentioned ecoproduct and eco-process are discussed.
For the reduction of CO2 emission, two major developments are being conducted in COURSE50 (Fig. 1), which is the program in iron and steel production sector of Cool Earth 50, the breakthrough program for CO2 emission reduction in Japan [1]. The one is separation of CO2 gas from BFG (Blast Furnace Gas) and recharge of the rest of BFG enriched in CO gas into blast furnace. The separation technology is being developed both by physical adsorption and chemical adsorption. In the case of chemical adsorption, amine based solvent absorbs CO2 gas in BFG and CO enriched gas is obtained for recharge to BF. After that CO2 gas is released from the solvent at 120–140 °C. Since the reaction is endothermic, waste heat in steel plant is to be utilized, such as sensible heats of BF slag and LD slag. The separated CO2 gas is to be stored in oil and gas well and onshore and offshore aquifers. The storage of CO2 gas is under development in other projects than COURSE50. The other one is amplification of H2 gas from CH4 , tar and light oil in COG. Hydrogen gas is contained in COG by about 60%.
∗
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the frame of Asia–Pacific Partnership on Clean Development and Climate, the best available technology for energy savings are discussed to be transferred within seven member nations, which has the effect of 1.27 million ton reduction of CO2 emission in a year. IEA also estimated that 3.4 million ton reduction of annual CO2 emission is possible by transferring best available technologies in the world (Fig. 2) [2]. 3.2. Eco-products supply
Fig. 1. Outline of COURSE50 development plan [1].
In the first stage of the program the proof test of the following hydrogen gas supply system was conducted: hydrogen gas was separated from COG by using some chemical solvent in hydrogen production station in Kimitsu Works and transported to hydrogen supply stations in metropolitan area in Tokyo for fuel cell cars, etc. The proof test was successful. Since Nippon Steel has steel works with coke oven batteries from north to south of Japan, it has advantage in supply of hydrogen gas nationwide. In the second stage, hydrogen gas amplification has been conducted in Yawata works. CH4 gas is converted to H2 and CO gas by reforming with steam or partial oxidation. Tar and light oil are first converted to CH4 gas by cracking with H2 gas and finally to H2 and CO gas by reforming with steam or partial oxidation, again. For this cracking and reforming sensible heat in COG is applied. The produced hydrogen gas will be supplied to the society or the reformed COG will be charged to blast furnace. Reduction rate of iron ore with hydrogen gas is five times faster than reduction rate with CO gas but it is endothermic reaction. Hydrogen iron ore reduction technology is also going to be developed with these aspects. ULCOS program is under taken in Europe as an equivalent program to COURSE50. They collaborate each other and form IISI (International Iron and Steel Institute)—CO2 breakthrough program.
By supplying energy saving steel products to society such as high strength steels for automobiles and ships, which reduces the fuel consumption, high performance electrical steels for motors and transformers, which reduce electricity losses, and high performance boiler tubes, which increases efficiency of thermal engine by increasing its operation temperature, steel sector farther contributes to the reduction of CO2 emission. High corrosion resistant steels and high fatigue resistant steels elongate the lifetime of steel constructions. High strength steel sheets for automobile plates for ships and high strength wires for bridges reduce the amount of steel use for obtaining the unit performance. They contribute to efficient usage of limited materials resource. Lead free steels, chromate free steel products and polyvinyl chloride free steel products satisfy environmental regulation laws and rules. Table 1 summarizes steel products we developed in each decade and in each sector. It is found that above-mentioned eco-products occupy them more in more recent decade. 3.3. Recycle of city waste in steel plants and utilization of steel plant waste in city
3.4. Process improvements and developments
In addition to these drastic challenging technology developments, a variety of measures for CO2 reduction is under taken. In
Recently new coking process started as proper coke production process in Oita Works. It is called SCOPE21 (Super Coke Oven
0.1
0
0.0
COG recovery Switch from OHF to BOF Steel finishing improvements
he Ot
Ja p
e
Ko re
Ru
Ch
Bra
Ind
rai n Uk
Wo rl
CDQ (or advanced wet quenching) Increased BOF gas recovery Efficient power generation from BF gas
r
0.2
50 an
0.3
100
a
150
US
0.4
ss ia So Afr uth ica Ca na da OE Eu CD rop e
0.5
200
ina
0.6
250
zil
0.7
300
ia
0.8
350
d
400
Specific savings potential (t CO2 per tonne of steel)
3.1. Best available technology transfer
Emissions savings (Mt CO2)
3. Incremental reduction programs of CO2 emission
Waste tires of automobiles reach about 1 million ton a year. 60 thousands ton out of them are recycled in Hirohatas Work for scrap melting and fuel gas production. In scrap melting carbon content is used as heat source and hot metal production and steel content are reused as steel resource. City waist plastics are agglomerated, charged into coke oven battery and converted to coke by 20% for iron ore reduction, COG by 40% for hydrogen gas generation and electricity generation and light oil/tar by 40% for raw materials of various chemicals (Fig. 3) [3]. On the other hand BF slag is used as raw materials of cement, which eliminates CO2 emission during the production of cement by burning limestones. Low-grade waste heats of steel plants are used for steam supply to surrounding factories and homes.
Blast furnace improvements Specific savings potential
Fig. 2. CO2 reduction potential in iron and steel in 2005, based on best available technology transfer [2].
Container
Heavy industry and energy
Electrical machinery and appliance
Construction
Automobile
Fields
• DIS tinplates
• Lamellar-tear resistant steel
• Grain-oriented electrical steel sheets
• EXCELITE (2) • ZINKLITETM (2) • WELCOTETM (2)
• Deep-drawing and ironing steel sheets SSPDXTM
• New tin free steel
differential thickness • Plates for nuclear power plant • 9% Ni plates for LNG tanks
• Wave-shaped plate with
• TMCP steels
r • VIEWKOTE⃝
electrical steels (3) • ALSHEETTM (2)
tube for oil well (3) • High-efficiency boiler tube and pipes • HIARESTTM • high corrosion resistance r stainless YUS270⃝ (2) • CANLITETM ∗ • TULCTM ∗
• High-grade electrically seamed
• Lubricated steel
• Vibration dumping steels
• Laser radiation grain-oriented
(2) • High strength steel wire for long bridge (2) • NS column • NM segment • Ti cladding steels (2)
• HIPERBEAMTM • High strength heat treated rail
• High toughness steel for welding
• Ultra-thin tinplate (0.19 mm)
• High fatigue strength steel (2)
• HAZ fine-grained steel
r • New S-TEN⃝ 1 (2)
under ground wall • Chromate free zinc coated steel sheets (1) • Pb-free tin–zinc coated steel sheets (1) • Ferritic stainless steel with high workability • Thin-gauge high-efficiency electric sheets (3) • High-endothermic sheets
• Hyper joint system • Steel—framed house (2) • Steel segment for continuous
r • SUPERDYMA⃝ (2)
r • HTUFF⃝ • PVC free steel sheets (1)
(2)
• Ni-contained weathering steel
hybrid motors (3)
• Fire-resisting steel
manifold (2)
tanks (1)
• Electrical steel sheets for
• Stainless steels for exhaust
sheets and bars (2) (3)
2000s
• TRIP steels (2) (3) • Steel tubes for hydroforming • Pb-free coated sheets for fuel
90s
• Hot rolled BH steel • DP high tensile strength steel • Various high strength steel
• NS–PACTM steel pipe piling
• SILVERALL0Y E (2)
80s
70s
Table 1 Iron and steel products developed to cope with demands in each decade. T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635 629
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Fig. 5. Outline of MURC (Multi-Refining Converter) process [5].
Fig. 3. Recycle of waste plastic by charging it into coke oven [3].
1000 Temperature (°C)
Reheating(CDQ) 800
Carbonizing
600
200 0
Quenching
Quenching(CDQ)
(SCOPE-21) (SCOPE-21)
400
(Conventional process)
Rapid Preheating 0
2
4
6
8
10 12 Time (h)
14
16
18
20
Fig. 6. The change in CaO consumption with slag recycling [5].
Fig. 4. Coking time in SCOPE21 [4].
for Productivity and Environmental Enhancement toward 21st Century) and has been developed in a national project. In this process coal is rapidly preheated before charging in to traditional coke oven battery. By this rapid preheating non and weak coking coal, which has been considered unsuitable for coke production, can be used as coking coal and in addition coke productivity becomes 2.4 times of that in conventional process since the total resident time of coal in coking process is much reduced (Fig. 4) [4]. In steelmaking process the reduction of refining slags contributes materials use efficiency and less emission of unuseful byproducts. Efficiency of dephosphorization process is aimed by the use of 2CaO · SiO2 precipitation in refining slags, which can contain a large amount phosphor by forming solid solution with 3CaO · P2 O5 over complete range of chemical compositions at steelmaking temperature range, on one hand and MURC (MultiRefining Converter) process has been developed on the other hand (Fig. 5) [5]. In this process dephosphorization and desiliconization are conducted in the first blow in converter and after that phosphorous enriched slag is removed by overflowing where the converter is inclined while hot metal stays inside the converter. Next, the converter is returned its up-right position and decarbonization and additional dephosphorization is conducted in the second blow after adding decarbonization slag. After the second blow the refined molten steel is tapped through a tapping hole while decarbonization slag stays inside the converter this time and is hot recycled as dephosphorization and desiliconization slag of the next charge. In this process, counter flow of hot metal and refining slag is realized and the unit usage of CaO is much reduced. Concomitantly, the amount of slag emitted from steelmaking plant is reduced. The effect of the slag removal ratio after the first blow on the unit usage of CaO was case studied under the condition listed in Table 2. Fig. 6 shows the simulated unit usage of CaO as a function of recycling number of decarbonization slag in the case of 60% in the slag removal ratio after the first blow. As the recycling number increases, the unit usage of CaO decreases and it saturates after more than about 5 times of recycling. Fig. 7 shows the unit usage of CaO at the saturated level as a function of the slag removal ratio.
Fig. 7. The effect of deslagging ratio on the total CaO consumption [5]. Table 2 Calculation conditions to estimate the effect of deslagging rate on the lime consumption [5]. Hot metal (%)
After de–Si, P treatment
After de–C treatment
[P] = 0.1 [Si] = 0.3 [Mn] = 0.3
Temp. = 1350 °C [P] = 0.02% [C] = 3.5% [Si] = 0.01% [Mn] = 0.05% (T.Fe) = 15% (all FeO) (MgO) = 8%
Temp. = 1650 °C [P] = 0.02% [C] = 0.05% [Si] = 0.01% [Mn] = 0.05% (T.Fe) = 15% (all FeO) (MgO) = 8%
It indicates that the unit usage decreases as the removal ratio increases and reaches a saturated level over the 60% in the removal ratio, which is about the half of that in the case of 0% slag removal. Based on these case studies MURC process have been applied to proper operation in Oita, Yawata and Muroran Works. Fig. 8 shows the comparison of unit CaO consumption in various proper operations in Oita Works [6]. By the application of MURC process without decarbonization slag recycling, it becomes about 75% of that without application MURC process and by the application of MURC with slag recycling it reaches less than 60% level. As the summary of this section, in Japan 10% reduction of CO2 emission is aimed by eco-process installations and operation improvements in side steel plants, another 10% reduction is aimed by the cooperation between steel plants and society such as ecoproducts supply to society, city waist recycling in plants, utilization
Total line reduction ratio (Without hot metal treatment = 100%)
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
631
120 100 80 60 40 20 0
Without hot metal treatment
ORP-M
MURC Without slag-recycle
MURC
Fig. 8. Comparison of total lime consumption in various proper operations [6].
of waist slags and heat of plants in society, etc. and another 5% reduction is expected by the transfer of energy saving technology to other countries.
Fig. 9. Relation between fraction solid and temperature obtained in solidification path analysis by various microsegregation models [8].
4. Application computational thermodynamics
Clyne–Kurz and Scheil equations, respectively, and Ω i is the parameter which shows the effect of solute back diffusion and defined by Eq. (4).
4.1. Control and utilization of nonmetallic inclusions
Ω i = α{1 − exp(−1/α i )} − 1/2 exp(−1/2α i )
The control and utilization of nonmetallic inclusions, such as deoxidation products, are one of the key technology for obtaining product performance, which is required in steel products listed in Table 1 including eco-products mentioned in Section 3.3. In order to optimize steelmaking process for these purposes, computational thermodynamics is applied.
where α is called as solidification parameter and defined as DiS tS /L2 . DiS is diffusion coefficient of solute i in the solid phase, tS is local solidification time and L is half of secondary arm spacing. By the adjustment of the solute increment between the calculations at adjacent two steps the effect of back diffusion is included. An example of calculated result for Fe–21% Cr–11% Ni alloy is shown in Fig. 9 [8].
(4)
i
4.1.1. Solidification path analysis Solidification path based on Scheil equation can be done discretely as follows: Some solid fraction is obtained by the equilibrium calculation at a little bit decreased temperature from the liquidus temperature in the first step and after discarding the solid fraction out of the system the next equilibrium calculation is done for the remaining liquid part at a little further lowered temperature in the second step. Another some solid fraction is obtained in the second calculation and this fraction is discarded in the third calculation at farther reduced temperature. By the repetition of these with small temperature decrement interval, solidification microsegregation can be obtained with neglecting buck diffusion of solute into the solid phase. The increment of solute content in liquid dCLi (Scheil) during the increment of solid fraction dfS is the difference of the liquid concentration between two adjacent calculations. Next, solidification path based on Clyne–Kurz equation [7] can be realized as follows: By the comparison between Clyne–Kurz equation, Eq. (1), and Scheil equation, Eq. (2), for microsegregation prediction,
(1 − ki )CLi dfS = {1 − (1 − 2Ω i ki )fS }dCLi (C −K )
(1)
(1 − ki )CLi dfS = {1 − fS }dCLi (Scheil)
(2)
the relation between the solute concentration increments is derived based on both equations during the increase of solid fraction by dfS : dCLi (C −K ) = {1 − fS }dCLi (Scheil) /{1 − (1 − 2Ω i ki )fS }
(3)
where ki is solid/liquid partition coefficient of solute i, CLi is the concentration of solute i in liquid, fS is solid fraction, dCLi (C −K ) and dCLi (Scheil) are the increments of solute i concentration based on
4.1.2. Consideration of nonmetallic inclusion during solidification [9] The existence of various nonmetallic inclusions is also included in the thermodynamic equilibrium calculation at every step during solidification path analysis shown above and it is assumed that the inclusions are homogeneously distributed between solid and liquid phases. That is, it is assumed that pushing out of inclusions by the growing solid does not occur. Based on this assumption the solute contents of inclusions in the liquid part are proportionally included in the equilibrium calculation in the succeeding step and only the increments of the dissolved solutes in the residual liquid are adjusted by Eq. (3) as mentioned above. 4.1.3. Sulfide shape control in anti-HIC steel for line pipe When manganese sulfide, MnS, precipitates at spot-like segregation along the centerline in a continuously cast slab for a line pipe, it becomes film like at the thickness center in a line pipe and it acts as crack initiation site of hydrogen induced cracking, HIC. The spot-like segregation is thought to be formed as follows: The solute enriched residual liquid in the inter-dendritic region is squeezed out due to solidification shrinkage or slab bulging and entrapped in a spot-like form along the centerline of slab. In order to prevent from MnS precipitation, Ca is added in anti-HIC steels so that sulfide changed from MnS to CaS (calcium sulfide). That is, sulfide shape control is targeted. However, MnS precipitation cannot be prevented when the size of spot-like segregation exceeds a certain diameter even if the Ca is properly added to the steel. The average chemical compositions of anti-HIC steel for line pipe is listed in the first line in Table 3 and the chemical compositions of a spot-like segregation is listed in the second line which is equivalent to the solute concentration in the inter-dendritic residual liquid at dendrite fraction solid of 0.90, which is calculated and validated by the observation in the slab. The coupled precipitation model was applied to analyze the chemical compositions change
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Table 3 Chemical compositions of anti-HIC steel on average and in the residual liquid of growing dendrite at fraction solid of 0.9 (calculated value) [10]. (%)
C
Si
Mn
P
S
Al
Ca
O
Average CL90
0.081 0.317
0.16 0.218
1.02 1.32
0.0050 0.0203
0.0012 0.0090
0.0290 0.0447
0.0033 0.0270
0.0025 0.0270
CL90 : Salute content in liquid phase at fs = 0.9 obtained by Clyne–Kurz equation (L = 200 µm, ts = 200 s).
Fig. 10. Changes of the amount of dissolved solutes (a) and nonmetallic inclusions (b) in the residual liquid during solidification of a spot-like segregation with a diameter of 100 µm (calculated values) [10].
of nonmetallic inclusions during the solidification of the spot-like segregation. Fig. 10 shows the result when the spot-like segregation is 100 µm in diameter. At the beginning of the solidification, oxygen is trapped as CaO–Al2 O3 almost completely and dissolved oxygen is less than 1 ppm. The rest of Ca is used for capturing S as CaS and dissolved Ca is about 0.01 ppm. Dissolved Al is about 200 ppm and dissolved S is about 70 ppm. As solidification progresses segregating sulfur decomposes CaO in CaO–Al2 O3 forming CaS. Dissolved Al captures oxygen from the decomposed CaO forming Al2 O3 . As the result, CaS concentration and Al2 O3 concentration in CaO–Al2 O3 increase as fraction solid increases. In this case CaO does not decompose completely throughout the solidification and segregating sulfur is trapped in a form of CaS preventing MnS precipitation, that is, sulfide shape control is successful. When the solidification of a spot-like segregation with a diameter of 1000 µm is analyzed, the general behavior of chemical compositions change of nonmetallic inclusions is the same as that for a spot-like segregation with a diameter of 100 µm as shown in Fig. 11. However, CaO decomposes completely at the very end of solidification. This is due to less effectiveness of back diffusion of sulfur because of the larger diameter of the spot-like segregation and segregating sulfur consumes CaO more rapidly. After that there is no source of Ca which captures segregating S and as the result MnS precipitates, that is, sulfide shape control is not successful in this case. These calculations were done by both analytical method [10] and discrete method [9]. Fig. 12 summarizes
the result of calculation for spot-like segregation with various diameters. It indicates that MnS precipitates when the diameter of the spot-like segregation exceeds 300 µm. Fig. 13 shows the relation between the observed Mn peak concentration in a spotlike segregation and its diameter. Mn peak concentration stepwisely increases when the diameter exceeds 260 µm, which is due to the MnS precipitation in the spot-like segregation. The threshold value agrees well with the calculated results with the coupled precipitation model. In order to suppress the MnS precipitation, the diameter of spot-like segregation should be regulated as well as proper addition of Ca, which can be done by minimizing slab bulging and applying soft reduction of slab thickness to compensate solidification and thermal shrinkage. 4.1.4. Oxide inclusion control in austenitic stainless steel for wire drawing The inclusions in an austenitic stainless steel for wire drawing should be deformable during the wire hot rolling. Otherwise wire breaking occurs at the hard crystalline phases in the inclusions during the process. The coupled precipitation model is also applied to control the oxides inclusions in this steel whose chemical compositions are listed in Table 4 [11]. Fig. 14 shows the calculated Al2 O3 and SiO2 concentrations as functions of fraction solid and total oxygen content in the steel by the use of coupled precipitation model. The calculated concentrations at the fraction solid of 0.5 agree well with the experimentally observed ones. The increase of total oxygen content from 40 to 80 ppm sharply decreases
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
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Table 4 Chemical compositions of the austenitic stainless steel in mass% [11]. Cr
Ni
C
Si
Mn
Al
O
Mo
Ca
Mg
18.3
10.35
0.02
0.5
1.1
0.0015–0.0030
0.0040–0.0150
0.01
0.0005
0.0004
Fig. 13. Mn peak composition as a function of the diameter of spot-like segregation (observed) [10].
Fig. 11. Calculated change of amounts of nonmetallic inclusions in the residual liquid during the solidification of a spot-like segregation with diameter of 1000 µm [10].
Al2 O3 concentration and sharply increases SiO2 concentration, which may change the nature of the oxide inclusions. However, further increase of total oxygen does not change Al2 O3 and SiO2 concentrations so much, that is, the nature of inclusions. The increase only increases the amount of inclusions. By the use of thermodynamic equilibrium calculation the liquidus temperature of inclusions and fractions of various crystalline phases appeared in the inclusions at the various temperature can be obtained [12]. Fig. 15 compares the calculated fractions of every phase in oxide inclusions at various temperatures in the stainless steels with total oxygen content of 40 and 80 ppm. In the inclusions in the steel with total oxygen content of 40 ppm, hard crystalline phases such as Al2 O3 and spinel MgO·Al2 O3 occupy large portion even at high temperature of 1600 °C. These inclusions are considered to be undeformable. On the other hand, in the inclusions in the steel with oxygen content of 80 ppm, liquid phase occupies 80% even at the low temperature of 1200 °C. These inclusions are considered to be deformable. According to this finding an austenitic stainless steel for wire drawing was successfully developed. 4.2. Optimization of demanganization process
Fig. 12. Fraction liquid and temperature at MnS precipitation and temperature at fraction liquid of 0.999 as functions of the diameter of spot-like segregation (calculated) [10].
In order to optimize various refining processes including MURC process, computer simulation of the processes are conducted mainly by the combination of fluid dynamics and computational thermodynamics analyses. The refining of steel is conducted mainly by the use of slag/metal reaction and injection metallurgy. In both cases chemical reaction occurs between liquid metal and refining agents that are frequently liquid oxides. The slag/metal refining process can be analyzed by coupled reaction model developed by Robertson et al. [13]. The reaction between injected flux and metal can be analyzed in the same manner. Fig. 16 shows the schematic view of the model. In the model, thermodynamic equilibrium for each component oxide in the slag and solutes
80 60 40
SiO2 concentration (wt%)
0
20
80 60 40 20 0
Al2O3 concentration (wt%)
100
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
100
634
0
50 100 150 oxygen content (ppm)
200
(a) (Al2 O3 ).
0
50 100 150 oxygen content (ppm)
200
(b) (SiO2 ).
100
100
90
90
80
80
70
70
wt% of phases
wt% of phases
Fig. 14. Alumina and silica contents in inclusions as functions of total oxygen content [11].
60 50 40 30
60 50 40 30
20
20
10
10
0 1100
1200
1300
1400 1500 1600 temperature (°C)
1700
1800
(a) [O] = 40 ppm, fs = 0.5.
0 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 temperature (°C) (b) [O] = 80 ppm, fs = 0.5.
Fig. 15. Calculated fractions of various phases in inclusions at various temperatures [11].
in metal is assumed at slag/metal interface and double diffusion layers are considered along the slag/metal interface. Chemical compositions in the bulk metal and the bulk slag outside the boundary layers are treated as uniform with respective concentrations. Concentrations in slag and metal at the slag/metal interface are calculated by the use of flux conservation equations of metal components and oxygen across the double boundary layers and thermodynamic equilibrium equations for component oxides with solute contents in the metal at the interface. In the case of carbon (C), a rate equation of carbon monoxide (CO) evolution is applied at the slag/metal interface instead of the local thermodynamic equilibrium, C + O = CO, and a mass balance equation for C between C flux from the metal and the CO evolution rate is considered. Once the interfacial concentrations are determined, the flux of each metal component and oxygen from the bulk metal to the bulk slag, or vice versa, as well as the CO evolution rate and decarburization rate are obtained and chemical composition changes in both the bulk slag and metal are calculated. This model was applied to optimize demanganization process of hot metal, where iron oxide powder is injected in a hot metal through an immersed nozzle [14]. The chemical composition change of the injected powder is analyzed with the model. The results are shown in Fig. 17. Silica concentration increases and iron oxide concentration decreases monotonically, while manganese oxide concentration reaches its maximum and decreases as time elapses after the powder is injected. This is because oxygen
FeO
CO
M1aOb
C M1
M2cOd
O M2
SLAG
METAL INTERFACE
Fig. 16. Schematic diagram of the coupled reaction model.
potential in the powder is not enough to oxidize manganese in hot metal after the manganese oxide reaches its maximum and manganese oxide is reduced back to hot metal. Therefore, from the viewpoint of efficient demanganization the injected powder is desired to float up to the top slag when the manganese concentration in the powder reaches the maximum. Since the time at the maximum manganese concentration becomes longer as demanganization process proceeds due to less concentration in silicon and manganese in hot metal, the depth of nozzle immersion should be increased accordingly to realize efficient demanganization. According to this result, the demanganization process is operated successfully.
FeO, SiO2, MnO (wt%)
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
(a) Beginning of operation
FeO, SiO2, MnO (wt%)
Time after injection (sec)
(b) Middle of operation
FeO, SiO2, MnO (wt%)
Time after injection (sec)
(c) End of operation
Time after injection (sec) Fig. 17. Chemical composition changes in the refining powder after it is injected to hot metal in demanganization and desiliconization process [14].
5. Concluding remarks In the first half, firstly the activities in COURSE50 program, CO2 Breakthrough Program in Japan, was explained, which mainly consists of CO2 separation from BFG, hydrogen gas generation from COG and iron ore reduction of hydrogen gas. Secondly, incremental reduction program of CO2 emission is explained, which includes the transfer of best available technology of CO2 emission reduction, the supply of various eco-products including high strength steel sheets and plates and high performance electrical steel sheets for energy savings in society, recycle of automobile tire and city waste plastic in steel plants, utilization of BF slags and waste heat in society and improvements and development of steel processes for less emission of CO2 . In the second half examples of utilization of computational thermodynamics for the development of steel products including eco-products and more environment friendly steel processes. They are for the control and utilization of nonmetallic inclusions and optimization of hot metal demanganization process.
635
In addition, as concluding remarks the author would like to point out the followings from other viewpoint: Sectoral approach proposed by IISI (presently, WSA) should be followed instead of cap and trade for the reduction of CO2 emission. When the environmental tax is to be introduced it should be put on the products on sale based on the LCA figures, that is the amount of CO2 emission. When some products are produced with special care for less emission of CO2 gas, which can be verified by putting eco tag, the amount of the environmental tax is reduced accordingly. Other wise the tax amount is determined based of default values of LCA. In this circumstance, the products, which are produced with less emission of CO2 gas under special care, have better competitiveness in the market, which encourages the selection of environmentally friendly products to the consumers and concomitantly the environmentally friendly production to the manufacturers. References [1] K. Yonezawa, Proc. SCANMT III, vol. 1, MEFOS, Lulea, 2008, pp. 49–58. [2] International energy agency: worldwide trends in energy use and efficiency, 2008, p. 32. [3] K. Kato, S. Nomura, H. Uematsu, J. Mater. Cycles Waste Manage. 15 (2003) 98–101. [4] M. Matsuura, M. Sasaki, K. Kato, I. Komaki, Tetsu to Hagane 89 (2003) 565. [5] Y. Ogawa, M. Yano, S. Kitamura, H. Hirata, Tetsu to Hagane 87 (2001) 21–28. [6] K. Kume, K. Yonezawa, M. Yoshimi, H. Hondo, M. Kumakura, CAMP-ISIJ 16 (3) (2003) 116. [7] T.W. Clyne, W. Kurz, Metall. Trans. A 12A (1981) 965. [8] W. Yamada, T. Mastumiya, B. Sundman, Development of a simulator of solidification path and formation of nonmetallic inclusions during solidification of stainless steels, in: M. Doyama, et al. (Eds.), Computer Aided Innovation of New Materials, Elsevier Science Publisher B.V, North-Holland, 1991, pp. 587–590. [9] W. Yamada, T. Matsumiya, Development of simulation model for composition change of nonmetallic inclusions during solidification of steels, in: Proceedings of the 6th International Iron and Steel Congress, Vol. 1, ISIJ, Ed., 21–26 Oct 1990 (Nagoya), ISIJ, 1990, pp. 618–625. [10] T. Matsumiya, Mathematical analyses of segregations and chemical compositional changes of nonmetallic inclusions during solidification of steels, Trans. Japan Inst. Met. 33 (1992) 783–794. [11] W. Yamada, T. Matsumiya, S. Fukumoto, S. Tanaka, H. Takeuchi, Simulation of the compositions of nonmetallic inclusions in cast stainless steels, in: Proceedings of Intern Conf on Computer-assisted Materials Design and Process Simulation. Tokyo, 1993, ISIJ, pp. 123–128. [12] T. Matsumiya, W. Yamada, T. Ohashi, Estimation of liquidus and solidus temperature of oxide mixtures as nonmetallic inclusions in steels, in: Proc. User application of Alloy Phase Diagram, ASM Int., 4–9 Oct 1986, Lake Buena Vista, FA, 8621–002.. [13] D.G.C. Robertson, B. Deo, S. Oguchi, Multicomponent mixed-transport-control theory for kinetics of coupled slag/metal and slag/metal/gas reactions: application to desulphurization of molten iron, Ironmak. Steelmak. 11 (1984) 41–55. [14] T. Kitamura, K. Shibata, I. Sawada, S. Kitamura, Optimization of refining process by computer simulation, Bull. Jpn. Inst. Met. 28 (1989) 310–312 (in Japanese).
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CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry journal homepage: www.elsevier.com/locate/calphad
Steelmaking technology for a sustainable society Tooru Matsumiya ∗ Nippon Steel Corporation, 20-1 Shintomi, Futtsu 293-8511, Japan
article
info
Article history: Available online 25 March 2011 Keywords: CO2 emission reduction Eco-products City waste recycling Less waste emission
abstract For the reduction of CO2 emission, two major developments are being conducted in COURSE50 (‘‘CO2 Ultimate Reduction in the Steelmaking Process by Innovative Technologies for Cool Earth 50’’). The one is separation of CO2 gas from BFG (Blast Furnace Gas) and recharge of the rest of BFG including H2 and CO into blast furnace. Hydrogen iron ore reduction technology is also going to be developed. The other one is amplification of H2 gas from CH4 , for example, in COG (Coke Oven Gas). The produced hydrogen gas will be supplied to the society or the reformed COG will be charged to blast furnace. In addition to these drastic challenging technology developments, a variety of measures for CO2 reduction is under taken. In the frame of Asia–Pacific Partnership on Clean Development and Climate, the best available technology for energy savings are discussed to be transferred within seven member nations, which has the effect of 1.27 million ton reduction of CO2 emission a year. By supplying energy saving steel products to society such as high strength steels for automobiles and ships, which realizes the fuel consumption reduction, high performance electrical steels for motors and transformers, which realize electricity loss reduction, and by recycling waste city plastics and tires in steel processes for hydrogen gas generation, chemical raw material conversion and iron ore reduction, etc., it is expected that equivalent 10% reduction of CO2 gas emission in steel production is counted. In steelmaking process the reduction of refining slags contributes materials use efficiency and less emission of unuseful byproducts. The control and utilization of nonmetallic inclusions, such as deoxidation products, are one of the key technology for obtaining product performance, which is required in the above-mentioned steel products. In order to optimize steelmaking process for these purposes, computational thermodynamics is applied. Optimization of demanganization, and control of chemical composition of nonmetallic inclusions by the use of computational thermodynamics are mentioned. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. CO2 breakthrough program
For the realization of sustainable society, a variety of approaches is considered in steelmaking sector. At the beginning of this paper CO2 Breakthrough Program will be introduced. Then, incremental reduction programs of CO2 emission will be mentioned: best available technology transfer, eco-products supply to the society, city waste recycle in steel plants, steel plant waste utilization in the society and process improvement and development for less emission. The second through the fourth items are the collaboration between steel manufacturers and the society. At the end, two examples of the use of computational thermodynamics for product and process development including the above-mentioned ecoproduct and eco-process are discussed.
For the reduction of CO2 emission, two major developments are being conducted in COURSE50 (Fig. 1), which is the program in iron and steel production sector of Cool Earth 50, the breakthrough program for CO2 emission reduction in Japan [1]. The one is separation of CO2 gas from BFG (Blast Furnace Gas) and recharge of the rest of BFG enriched in CO gas into blast furnace. The separation technology is being developed both by physical adsorption and chemical adsorption. In the case of chemical adsorption, amine based solvent absorbs CO2 gas in BFG and CO enriched gas is obtained for recharge to BF. After that CO2 gas is released from the solvent at 120–140 °C. Since the reaction is endothermic, waste heat in steel plant is to be utilized, such as sensible heats of BF slag and LD slag. The separated CO2 gas is to be stored in oil and gas well and onshore and offshore aquifers. The storage of CO2 gas is under development in other projects than COURSE50. The other one is amplification of H2 gas from CH4 , tar and light oil in COG. Hydrogen gas is contained in COG by about 60%.
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0364-5916/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.calphad.2011.02.009
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the frame of Asia–Pacific Partnership on Clean Development and Climate, the best available technology for energy savings are discussed to be transferred within seven member nations, which has the effect of 1.27 million ton reduction of CO2 emission in a year. IEA also estimated that 3.4 million ton reduction of annual CO2 emission is possible by transferring best available technologies in the world (Fig. 2) [2]. 3.2. Eco-products supply
Fig. 1. Outline of COURSE50 development plan [1].
In the first stage of the program the proof test of the following hydrogen gas supply system was conducted: hydrogen gas was separated from COG by using some chemical solvent in hydrogen production station in Kimitsu Works and transported to hydrogen supply stations in metropolitan area in Tokyo for fuel cell cars, etc. The proof test was successful. Since Nippon Steel has steel works with coke oven batteries from north to south of Japan, it has advantage in supply of hydrogen gas nationwide. In the second stage, hydrogen gas amplification has been conducted in Yawata works. CH4 gas is converted to H2 and CO gas by reforming with steam or partial oxidation. Tar and light oil are first converted to CH4 gas by cracking with H2 gas and finally to H2 and CO gas by reforming with steam or partial oxidation, again. For this cracking and reforming sensible heat in COG is applied. The produced hydrogen gas will be supplied to the society or the reformed COG will be charged to blast furnace. Reduction rate of iron ore with hydrogen gas is five times faster than reduction rate with CO gas but it is endothermic reaction. Hydrogen iron ore reduction technology is also going to be developed with these aspects. ULCOS program is under taken in Europe as an equivalent program to COURSE50. They collaborate each other and form IISI (International Iron and Steel Institute)—CO2 breakthrough program.
By supplying energy saving steel products to society such as high strength steels for automobiles and ships, which reduces the fuel consumption, high performance electrical steels for motors and transformers, which reduce electricity losses, and high performance boiler tubes, which increases efficiency of thermal engine by increasing its operation temperature, steel sector farther contributes to the reduction of CO2 emission. High corrosion resistant steels and high fatigue resistant steels elongate the lifetime of steel constructions. High strength steel sheets for automobile plates for ships and high strength wires for bridges reduce the amount of steel use for obtaining the unit performance. They contribute to efficient usage of limited materials resource. Lead free steels, chromate free steel products and polyvinyl chloride free steel products satisfy environmental regulation laws and rules. Table 1 summarizes steel products we developed in each decade and in each sector. It is found that above-mentioned eco-products occupy them more in more recent decade. 3.3. Recycle of city waste in steel plants and utilization of steel plant waste in city
3.4. Process improvements and developments
In addition to these drastic challenging technology developments, a variety of measures for CO2 reduction is under taken. In
Recently new coking process started as proper coke production process in Oita Works. It is called SCOPE21 (Super Coke Oven
0.1
0
0.0
COG recovery Switch from OHF to BOF Steel finishing improvements
he Ot
Ja p
e
Ko re
Ru
Ch
Bra
Ind
rai n Uk
Wo rl
CDQ (or advanced wet quenching) Increased BOF gas recovery Efficient power generation from BF gas
r
0.2
50 an
0.3
100
a
150
US
0.4
ss ia So Afr uth ica Ca na da OE Eu CD rop e
0.5
200
ina
0.6
250
zil
0.7
300
ia
0.8
350
d
400
Specific savings potential (t CO2 per tonne of steel)
3.1. Best available technology transfer
Emissions savings (Mt CO2)
3. Incremental reduction programs of CO2 emission
Waste tires of automobiles reach about 1 million ton a year. 60 thousands ton out of them are recycled in Hirohatas Work for scrap melting and fuel gas production. In scrap melting carbon content is used as heat source and hot metal production and steel content are reused as steel resource. City waist plastics are agglomerated, charged into coke oven battery and converted to coke by 20% for iron ore reduction, COG by 40% for hydrogen gas generation and electricity generation and light oil/tar by 40% for raw materials of various chemicals (Fig. 3) [3]. On the other hand BF slag is used as raw materials of cement, which eliminates CO2 emission during the production of cement by burning limestones. Low-grade waste heats of steel plants are used for steam supply to surrounding factories and homes.
Blast furnace improvements Specific savings potential
Fig. 2. CO2 reduction potential in iron and steel in 2005, based on best available technology transfer [2].
Container
Heavy industry and energy
Electrical machinery and appliance
Construction
Automobile
Fields
• DIS tinplates
• Lamellar-tear resistant steel
• Grain-oriented electrical steel sheets
• EXCELITE (2) • ZINKLITETM (2) • WELCOTETM (2)
• Deep-drawing and ironing steel sheets SSPDXTM
• New tin free steel
differential thickness • Plates for nuclear power plant • 9% Ni plates for LNG tanks
• Wave-shaped plate with
• TMCP steels
r • VIEWKOTE⃝
electrical steels (3) • ALSHEETTM (2)
tube for oil well (3) • High-efficiency boiler tube and pipes • HIARESTTM • high corrosion resistance r stainless YUS270⃝ (2) • CANLITETM ∗ • TULCTM ∗
• High-grade electrically seamed
• Lubricated steel
• Vibration dumping steels
• Laser radiation grain-oriented
(2) • High strength steel wire for long bridge (2) • NS column • NM segment • Ti cladding steels (2)
• HIPERBEAMTM • High strength heat treated rail
• High toughness steel for welding
• Ultra-thin tinplate (0.19 mm)
• High fatigue strength steel (2)
• HAZ fine-grained steel
r • New S-TEN⃝ 1 (2)
under ground wall • Chromate free zinc coated steel sheets (1) • Pb-free tin–zinc coated steel sheets (1) • Ferritic stainless steel with high workability • Thin-gauge high-efficiency electric sheets (3) • High-endothermic sheets
• Hyper joint system • Steel—framed house (2) • Steel segment for continuous
r • SUPERDYMA⃝ (2)
r • HTUFF⃝ • PVC free steel sheets (1)
(2)
• Ni-contained weathering steel
hybrid motors (3)
• Fire-resisting steel
manifold (2)
tanks (1)
• Electrical steel sheets for
• Stainless steels for exhaust
sheets and bars (2) (3)
2000s
• TRIP steels (2) (3) • Steel tubes for hydroforming • Pb-free coated sheets for fuel
90s
• Hot rolled BH steel • DP high tensile strength steel • Various high strength steel
• NS–PACTM steel pipe piling
• SILVERALL0Y E (2)
80s
70s
Table 1 Iron and steel products developed to cope with demands in each decade. T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635 629
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Fig. 5. Outline of MURC (Multi-Refining Converter) process [5].
Fig. 3. Recycle of waste plastic by charging it into coke oven [3].
1000 Temperature (°C)
Reheating(CDQ) 800
Carbonizing
600
200 0
Quenching
Quenching(CDQ)
(SCOPE-21) (SCOPE-21)
400
(Conventional process)
Rapid Preheating 0
2
4
6
8
10 12 Time (h)
14
16
18
20
Fig. 6. The change in CaO consumption with slag recycling [5].
Fig. 4. Coking time in SCOPE21 [4].
for Productivity and Environmental Enhancement toward 21st Century) and has been developed in a national project. In this process coal is rapidly preheated before charging in to traditional coke oven battery. By this rapid preheating non and weak coking coal, which has been considered unsuitable for coke production, can be used as coking coal and in addition coke productivity becomes 2.4 times of that in conventional process since the total resident time of coal in coking process is much reduced (Fig. 4) [4]. In steelmaking process the reduction of refining slags contributes materials use efficiency and less emission of unuseful byproducts. Efficiency of dephosphorization process is aimed by the use of 2CaO · SiO2 precipitation in refining slags, which can contain a large amount phosphor by forming solid solution with 3CaO · P2 O5 over complete range of chemical compositions at steelmaking temperature range, on one hand and MURC (MultiRefining Converter) process has been developed on the other hand (Fig. 5) [5]. In this process dephosphorization and desiliconization are conducted in the first blow in converter and after that phosphorous enriched slag is removed by overflowing where the converter is inclined while hot metal stays inside the converter. Next, the converter is returned its up-right position and decarbonization and additional dephosphorization is conducted in the second blow after adding decarbonization slag. After the second blow the refined molten steel is tapped through a tapping hole while decarbonization slag stays inside the converter this time and is hot recycled as dephosphorization and desiliconization slag of the next charge. In this process, counter flow of hot metal and refining slag is realized and the unit usage of CaO is much reduced. Concomitantly, the amount of slag emitted from steelmaking plant is reduced. The effect of the slag removal ratio after the first blow on the unit usage of CaO was case studied under the condition listed in Table 2. Fig. 6 shows the simulated unit usage of CaO as a function of recycling number of decarbonization slag in the case of 60% in the slag removal ratio after the first blow. As the recycling number increases, the unit usage of CaO decreases and it saturates after more than about 5 times of recycling. Fig. 7 shows the unit usage of CaO at the saturated level as a function of the slag removal ratio.
Fig. 7. The effect of deslagging ratio on the total CaO consumption [5]. Table 2 Calculation conditions to estimate the effect of deslagging rate on the lime consumption [5]. Hot metal (%)
After de–Si, P treatment
After de–C treatment
[P] = 0.1 [Si] = 0.3 [Mn] = 0.3
Temp. = 1350 °C [P] = 0.02% [C] = 3.5% [Si] = 0.01% [Mn] = 0.05% (T.Fe) = 15% (all FeO) (MgO) = 8%
Temp. = 1650 °C [P] = 0.02% [C] = 0.05% [Si] = 0.01% [Mn] = 0.05% (T.Fe) = 15% (all FeO) (MgO) = 8%
It indicates that the unit usage decreases as the removal ratio increases and reaches a saturated level over the 60% in the removal ratio, which is about the half of that in the case of 0% slag removal. Based on these case studies MURC process have been applied to proper operation in Oita, Yawata and Muroran Works. Fig. 8 shows the comparison of unit CaO consumption in various proper operations in Oita Works [6]. By the application of MURC process without decarbonization slag recycling, it becomes about 75% of that without application MURC process and by the application of MURC with slag recycling it reaches less than 60% level. As the summary of this section, in Japan 10% reduction of CO2 emission is aimed by eco-process installations and operation improvements in side steel plants, another 10% reduction is aimed by the cooperation between steel plants and society such as ecoproducts supply to society, city waist recycling in plants, utilization
Total line reduction ratio (Without hot metal treatment = 100%)
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
631
120 100 80 60 40 20 0
Without hot metal treatment
ORP-M
MURC Without slag-recycle
MURC
Fig. 8. Comparison of total lime consumption in various proper operations [6].
of waist slags and heat of plants in society, etc. and another 5% reduction is expected by the transfer of energy saving technology to other countries.
Fig. 9. Relation between fraction solid and temperature obtained in solidification path analysis by various microsegregation models [8].
4. Application computational thermodynamics
Clyne–Kurz and Scheil equations, respectively, and Ω i is the parameter which shows the effect of solute back diffusion and defined by Eq. (4).
4.1. Control and utilization of nonmetallic inclusions
Ω i = α{1 − exp(−1/α i )} − 1/2 exp(−1/2α i )
The control and utilization of nonmetallic inclusions, such as deoxidation products, are one of the key technology for obtaining product performance, which is required in steel products listed in Table 1 including eco-products mentioned in Section 3.3. In order to optimize steelmaking process for these purposes, computational thermodynamics is applied.
where α is called as solidification parameter and defined as DiS tS /L2 . DiS is diffusion coefficient of solute i in the solid phase, tS is local solidification time and L is half of secondary arm spacing. By the adjustment of the solute increment between the calculations at adjacent two steps the effect of back diffusion is included. An example of calculated result for Fe–21% Cr–11% Ni alloy is shown in Fig. 9 [8].
(4)
i
4.1.1. Solidification path analysis Solidification path based on Scheil equation can be done discretely as follows: Some solid fraction is obtained by the equilibrium calculation at a little bit decreased temperature from the liquidus temperature in the first step and after discarding the solid fraction out of the system the next equilibrium calculation is done for the remaining liquid part at a little further lowered temperature in the second step. Another some solid fraction is obtained in the second calculation and this fraction is discarded in the third calculation at farther reduced temperature. By the repetition of these with small temperature decrement interval, solidification microsegregation can be obtained with neglecting buck diffusion of solute into the solid phase. The increment of solute content in liquid dCLi (Scheil) during the increment of solid fraction dfS is the difference of the liquid concentration between two adjacent calculations. Next, solidification path based on Clyne–Kurz equation [7] can be realized as follows: By the comparison between Clyne–Kurz equation, Eq. (1), and Scheil equation, Eq. (2), for microsegregation prediction,
(1 − ki )CLi dfS = {1 − (1 − 2Ω i ki )fS }dCLi (C −K )
(1)
(1 − ki )CLi dfS = {1 − fS }dCLi (Scheil)
(2)
the relation between the solute concentration increments is derived based on both equations during the increase of solid fraction by dfS : dCLi (C −K ) = {1 − fS }dCLi (Scheil) /{1 − (1 − 2Ω i ki )fS }
(3)
where ki is solid/liquid partition coefficient of solute i, CLi is the concentration of solute i in liquid, fS is solid fraction, dCLi (C −K ) and dCLi (Scheil) are the increments of solute i concentration based on
4.1.2. Consideration of nonmetallic inclusion during solidification [9] The existence of various nonmetallic inclusions is also included in the thermodynamic equilibrium calculation at every step during solidification path analysis shown above and it is assumed that the inclusions are homogeneously distributed between solid and liquid phases. That is, it is assumed that pushing out of inclusions by the growing solid does not occur. Based on this assumption the solute contents of inclusions in the liquid part are proportionally included in the equilibrium calculation in the succeeding step and only the increments of the dissolved solutes in the residual liquid are adjusted by Eq. (3) as mentioned above. 4.1.3. Sulfide shape control in anti-HIC steel for line pipe When manganese sulfide, MnS, precipitates at spot-like segregation along the centerline in a continuously cast slab for a line pipe, it becomes film like at the thickness center in a line pipe and it acts as crack initiation site of hydrogen induced cracking, HIC. The spot-like segregation is thought to be formed as follows: The solute enriched residual liquid in the inter-dendritic region is squeezed out due to solidification shrinkage or slab bulging and entrapped in a spot-like form along the centerline of slab. In order to prevent from MnS precipitation, Ca is added in anti-HIC steels so that sulfide changed from MnS to CaS (calcium sulfide). That is, sulfide shape control is targeted. However, MnS precipitation cannot be prevented when the size of spot-like segregation exceeds a certain diameter even if the Ca is properly added to the steel. The average chemical compositions of anti-HIC steel for line pipe is listed in the first line in Table 3 and the chemical compositions of a spot-like segregation is listed in the second line which is equivalent to the solute concentration in the inter-dendritic residual liquid at dendrite fraction solid of 0.90, which is calculated and validated by the observation in the slab. The coupled precipitation model was applied to analyze the chemical compositions change
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Table 3 Chemical compositions of anti-HIC steel on average and in the residual liquid of growing dendrite at fraction solid of 0.9 (calculated value) [10]. (%)
C
Si
Mn
P
S
Al
Ca
O
Average CL90
0.081 0.317
0.16 0.218
1.02 1.32
0.0050 0.0203
0.0012 0.0090
0.0290 0.0447
0.0033 0.0270
0.0025 0.0270
CL90 : Salute content in liquid phase at fs = 0.9 obtained by Clyne–Kurz equation (L = 200 µm, ts = 200 s).
Fig. 10. Changes of the amount of dissolved solutes (a) and nonmetallic inclusions (b) in the residual liquid during solidification of a spot-like segregation with a diameter of 100 µm (calculated values) [10].
of nonmetallic inclusions during the solidification of the spot-like segregation. Fig. 10 shows the result when the spot-like segregation is 100 µm in diameter. At the beginning of the solidification, oxygen is trapped as CaO–Al2 O3 almost completely and dissolved oxygen is less than 1 ppm. The rest of Ca is used for capturing S as CaS and dissolved Ca is about 0.01 ppm. Dissolved Al is about 200 ppm and dissolved S is about 70 ppm. As solidification progresses segregating sulfur decomposes CaO in CaO–Al2 O3 forming CaS. Dissolved Al captures oxygen from the decomposed CaO forming Al2 O3 . As the result, CaS concentration and Al2 O3 concentration in CaO–Al2 O3 increase as fraction solid increases. In this case CaO does not decompose completely throughout the solidification and segregating sulfur is trapped in a form of CaS preventing MnS precipitation, that is, sulfide shape control is successful. When the solidification of a spot-like segregation with a diameter of 1000 µm is analyzed, the general behavior of chemical compositions change of nonmetallic inclusions is the same as that for a spot-like segregation with a diameter of 100 µm as shown in Fig. 11. However, CaO decomposes completely at the very end of solidification. This is due to less effectiveness of back diffusion of sulfur because of the larger diameter of the spot-like segregation and segregating sulfur consumes CaO more rapidly. After that there is no source of Ca which captures segregating S and as the result MnS precipitates, that is, sulfide shape control is not successful in this case. These calculations were done by both analytical method [10] and discrete method [9]. Fig. 12 summarizes
the result of calculation for spot-like segregation with various diameters. It indicates that MnS precipitates when the diameter of the spot-like segregation exceeds 300 µm. Fig. 13 shows the relation between the observed Mn peak concentration in a spotlike segregation and its diameter. Mn peak concentration stepwisely increases when the diameter exceeds 260 µm, which is due to the MnS precipitation in the spot-like segregation. The threshold value agrees well with the calculated results with the coupled precipitation model. In order to suppress the MnS precipitation, the diameter of spot-like segregation should be regulated as well as proper addition of Ca, which can be done by minimizing slab bulging and applying soft reduction of slab thickness to compensate solidification and thermal shrinkage. 4.1.4. Oxide inclusion control in austenitic stainless steel for wire drawing The inclusions in an austenitic stainless steel for wire drawing should be deformable during the wire hot rolling. Otherwise wire breaking occurs at the hard crystalline phases in the inclusions during the process. The coupled precipitation model is also applied to control the oxides inclusions in this steel whose chemical compositions are listed in Table 4 [11]. Fig. 14 shows the calculated Al2 O3 and SiO2 concentrations as functions of fraction solid and total oxygen content in the steel by the use of coupled precipitation model. The calculated concentrations at the fraction solid of 0.5 agree well with the experimentally observed ones. The increase of total oxygen content from 40 to 80 ppm sharply decreases
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Table 4 Chemical compositions of the austenitic stainless steel in mass% [11]. Cr
Ni
C
Si
Mn
Al
O
Mo
Ca
Mg
18.3
10.35
0.02
0.5
1.1
0.0015–0.0030
0.0040–0.0150
0.01
0.0005
0.0004
Fig. 13. Mn peak composition as a function of the diameter of spot-like segregation (observed) [10].
Fig. 11. Calculated change of amounts of nonmetallic inclusions in the residual liquid during the solidification of a spot-like segregation with diameter of 1000 µm [10].
Al2 O3 concentration and sharply increases SiO2 concentration, which may change the nature of the oxide inclusions. However, further increase of total oxygen does not change Al2 O3 and SiO2 concentrations so much, that is, the nature of inclusions. The increase only increases the amount of inclusions. By the use of thermodynamic equilibrium calculation the liquidus temperature of inclusions and fractions of various crystalline phases appeared in the inclusions at the various temperature can be obtained [12]. Fig. 15 compares the calculated fractions of every phase in oxide inclusions at various temperatures in the stainless steels with total oxygen content of 40 and 80 ppm. In the inclusions in the steel with total oxygen content of 40 ppm, hard crystalline phases such as Al2 O3 and spinel MgO·Al2 O3 occupy large portion even at high temperature of 1600 °C. These inclusions are considered to be undeformable. On the other hand, in the inclusions in the steel with oxygen content of 80 ppm, liquid phase occupies 80% even at the low temperature of 1200 °C. These inclusions are considered to be deformable. According to this finding an austenitic stainless steel for wire drawing was successfully developed. 4.2. Optimization of demanganization process
Fig. 12. Fraction liquid and temperature at MnS precipitation and temperature at fraction liquid of 0.999 as functions of the diameter of spot-like segregation (calculated) [10].
In order to optimize various refining processes including MURC process, computer simulation of the processes are conducted mainly by the combination of fluid dynamics and computational thermodynamics analyses. The refining of steel is conducted mainly by the use of slag/metal reaction and injection metallurgy. In both cases chemical reaction occurs between liquid metal and refining agents that are frequently liquid oxides. The slag/metal refining process can be analyzed by coupled reaction model developed by Robertson et al. [13]. The reaction between injected flux and metal can be analyzed in the same manner. Fig. 16 shows the schematic view of the model. In the model, thermodynamic equilibrium for each component oxide in the slag and solutes
80 60 40
SiO2 concentration (wt%)
0
20
80 60 40 20 0
Al2O3 concentration (wt%)
100
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
100
634
0
50 100 150 oxygen content (ppm)
200
(a) (Al2 O3 ).
0
50 100 150 oxygen content (ppm)
200
(b) (SiO2 ).
100
100
90
90
80
80
70
70
wt% of phases
wt% of phases
Fig. 14. Alumina and silica contents in inclusions as functions of total oxygen content [11].
60 50 40 30
60 50 40 30
20
20
10
10
0 1100
1200
1300
1400 1500 1600 temperature (°C)
1700
1800
(a) [O] = 40 ppm, fs = 0.5.
0 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 temperature (°C) (b) [O] = 80 ppm, fs = 0.5.
Fig. 15. Calculated fractions of various phases in inclusions at various temperatures [11].
in metal is assumed at slag/metal interface and double diffusion layers are considered along the slag/metal interface. Chemical compositions in the bulk metal and the bulk slag outside the boundary layers are treated as uniform with respective concentrations. Concentrations in slag and metal at the slag/metal interface are calculated by the use of flux conservation equations of metal components and oxygen across the double boundary layers and thermodynamic equilibrium equations for component oxides with solute contents in the metal at the interface. In the case of carbon (C), a rate equation of carbon monoxide (CO) evolution is applied at the slag/metal interface instead of the local thermodynamic equilibrium, C + O = CO, and a mass balance equation for C between C flux from the metal and the CO evolution rate is considered. Once the interfacial concentrations are determined, the flux of each metal component and oxygen from the bulk metal to the bulk slag, or vice versa, as well as the CO evolution rate and decarburization rate are obtained and chemical composition changes in both the bulk slag and metal are calculated. This model was applied to optimize demanganization process of hot metal, where iron oxide powder is injected in a hot metal through an immersed nozzle [14]. The chemical composition change of the injected powder is analyzed with the model. The results are shown in Fig. 17. Silica concentration increases and iron oxide concentration decreases monotonically, while manganese oxide concentration reaches its maximum and decreases as time elapses after the powder is injected. This is because oxygen
FeO
CO
M1aOb
C M1
M2cOd
O M2
SLAG
METAL INTERFACE
Fig. 16. Schematic diagram of the coupled reaction model.
potential in the powder is not enough to oxidize manganese in hot metal after the manganese oxide reaches its maximum and manganese oxide is reduced back to hot metal. Therefore, from the viewpoint of efficient demanganization the injected powder is desired to float up to the top slag when the manganese concentration in the powder reaches the maximum. Since the time at the maximum manganese concentration becomes longer as demanganization process proceeds due to less concentration in silicon and manganese in hot metal, the depth of nozzle immersion should be increased accordingly to realize efficient demanganization. According to this result, the demanganization process is operated successfully.
FeO, SiO2, MnO (wt%)
T. Matsumiya / CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 35 (2011) 627–635
(a) Beginning of operation
FeO, SiO2, MnO (wt%)
Time after injection (sec)
(b) Middle of operation
FeO, SiO2, MnO (wt%)
Time after injection (sec)
(c) End of operation
Time after injection (sec) Fig. 17. Chemical composition changes in the refining powder after it is injected to hot metal in demanganization and desiliconization process [14].
5. Concluding remarks In the first half, firstly the activities in COURSE50 program, CO2 Breakthrough Program in Japan, was explained, which mainly consists of CO2 separation from BFG, hydrogen gas generation from COG and iron ore reduction of hydrogen gas. Secondly, incremental reduction program of CO2 emission is explained, which includes the transfer of best available technology of CO2 emission reduction, the supply of various eco-products including high strength steel sheets and plates and high performance electrical steel sheets for energy savings in society, recycle of automobile tire and city waste plastic in steel plants, utilization of BF slags and waste heat in society and improvements and development of steel processes for less emission of CO2 . In the second half examples of utilization of computational thermodynamics for the development of steel products including eco-products and more environment friendly steel processes. They are for the control and utilization of nonmetallic inclusions and optimization of hot metal demanganization process.
635
In addition, as concluding remarks the author would like to point out the followings from other viewpoint: Sectoral approach proposed by IISI (presently, WSA) should be followed instead of cap and trade for the reduction of CO2 emission. When the environmental tax is to be introduced it should be put on the products on sale based on the LCA figures, that is the amount of CO2 emission. When some products are produced with special care for less emission of CO2 gas, which can be verified by putting eco tag, the amount of the environmental tax is reduced accordingly. Other wise the tax amount is determined based of default values of LCA. In this circumstance, the products, which are produced with less emission of CO2 gas under special care, have better competitiveness in the market, which encourages the selection of environmentally friendly products to the consumers and concomitantly the environmentally friendly production to the manufacturers. References [1] K. Yonezawa, Proc. SCANMT III, vol. 1, MEFOS, Lulea, 2008, pp. 49–58. [2] International energy agency: worldwide trends in energy use and efficiency, 2008, p. 32. [3] K. Kato, S. Nomura, H. Uematsu, J. Mater. Cycles Waste Manage. 15 (2003) 98–101. [4] M. Matsuura, M. Sasaki, K. Kato, I. Komaki, Tetsu to Hagane 89 (2003) 565. [5] Y. Ogawa, M. Yano, S. Kitamura, H. Hirata, Tetsu to Hagane 87 (2001) 21–28. [6] K. Kume, K. Yonezawa, M. Yoshimi, H. Hondo, M. Kumakura, CAMP-ISIJ 16 (3) (2003) 116. [7] T.W. Clyne, W. Kurz, Metall. Trans. A 12A (1981) 965. [8] W. Yamada, T. Mastumiya, B. Sundman, Development of a simulator of solidification path and formation of nonmetallic inclusions during solidification of stainless steels, in: M. Doyama, et al. (Eds.), Computer Aided Innovation of New Materials, Elsevier Science Publisher B.V, North-Holland, 1991, pp. 587–590. [9] W. Yamada, T. Matsumiya, Development of simulation model for composition change of nonmetallic inclusions during solidification of steels, in: Proceedings of the 6th International Iron and Steel Congress, Vol. 1, ISIJ, Ed., 21–26 Oct 1990 (Nagoya), ISIJ, 1990, pp. 618–625. [10] T. Matsumiya, Mathematical analyses of segregations and chemical compositional changes of nonmetallic inclusions during solidification of steels, Trans. Japan Inst. Met. 33 (1992) 783–794. [11] W. Yamada, T. Matsumiya, S. Fukumoto, S. Tanaka, H. Takeuchi, Simulation of the compositions of nonmetallic inclusions in cast stainless steels, in: Proceedings of Intern Conf on Computer-assisted Materials Design and Process Simulation. Tokyo, 1993, ISIJ, pp. 123–128. [12] T. Matsumiya, W. Yamada, T. Ohashi, Estimation of liquidus and solidus temperature of oxide mixtures as nonmetallic inclusions in steels, in: Proc. User application of Alloy Phase Diagram, ASM Int., 4–9 Oct 1986, Lake Buena Vista, FA, 8621–002.. [13] D.G.C. Robertson, B. Deo, S. Oguchi, Multicomponent mixed-transport-control theory for kinetics of coupled slag/metal and slag/metal/gas reactions: application to desulphurization of molten iron, Ironmak. Steelmak. 11 (1984) 41–55. [14] T. Kitamura, K. Shibata, I. Sawada, S. Kitamura, Optimization of refining process by computer simulation, Bull. Jpn. Inst. Met. 28 (1989) 310–312 (in Japanese).