Feasibility of Solid-State Steelmaking from Cast Iron -Decarburization of Rapidly Solidified Cast Iron

Home , Cast iron, Decarburization, Steelmaking

ISIJ International, Vol. 52 (2012), No. 1, pp. 26–34

Feasibility of Solid-state Steelmaking from Cast Iron -Decarburization of Rapidly Solidified Cast Iron-

Ji-Ook PARK, Tran Van LONG and Yasushi SASAKI

Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Hyoja-dong, Pohang, 790-784 South Korea. (Received on September 6, 2011; accepted on September 29, 2011)

To meet the unprecedented demand of environmental issues and tightened production cost, steel industry must develop the disruptively innovative process. In the present study, totally new steelmaking process of ‘Solid State Steelmaking’ (or S3 process) without BOF process or liquid state oxidation process is proposed. The overview of the new process is as follows: (1) High carbon liquid iron from the ironmaking processes is directly solidified by using a strip casting process to produce high carbon thin sheets. (2) Then, the produced cast iron sheet is decarburized by introducing oxidizing gas of H2O or CO2 in a continuous annealing line to produce low carbon steel sheets. The most beneficial aspect of the S3 process is the elimination of several steps such as BOF, and secondary refinement processes and no formation of inclusions. To investigate the feasibility of S3 process, the cast iron strips with various high carbon content produced by a centrifugal slip casting method are decarburized at 1 248 K and 1 373 K by using H2O–H2 gas mixture and its kinetics of the decarburization is investigated. In the decarburization process, the carbon diffusion through the decarburized austenite phase but not the decomposition of cementite is the rate controlling step of the decarburizing process. It is found that 0.5 mass% C sheets can be produced from 3.89 mass% C sheets with the thickness of 1.0 mm within 30 min at 1 373 K. Based on these results, S3 process is confirmed to be feasible as an alternative low cost steelmaking process although the further improvement of the process will be necessary.

KEY WORDS: solid-state steelmaking; strip casting; gaseous decarburization; white cast iron; carbon diffusion.

conventional steelmaking and the solid state steelmaking 1. Introduction process are shown in Fig. 2. The pO2 changes of the con- In the steelmaking process, many efforts have been paid ventional steelmaking process and S3 process are shown in on how to eliminate inclusions or bubbles from liquid steels. Fig. 3. It is noted that there’s no need to struggle for the oxy- The existence of inclusions and bubbles is simply due to the gen control before the casting in S3 process since there’s no liquid phase oxidation process of pig iron melts. The solu- oxygen blowing for decarburization. Since the oxygen con- bility of oxygen in liquid Fe is quite high so that oxygen is tent in molten pig iron is extremely low of about less than easily dissolved into the liquid iron during the liquid state 5 ppm, the oxygen content in strip cast sheet can be less than de-carburization process. The solubility of oxygen in solid 5 ppm. Thus, inclusion formation is practically negligible. iron, however, is extremely small. The intake of oxygen during the decarburization process of the solid phase Fe–C can be negligible. Based on this consideration, as an alternative approach to avoid the inclusion formation, totally new process which is called ‘Solid State Steelmaking’ (or S3 process) has been investigated. Differed from the conventional steel sheet production process, pig iron (high carbon of about 5 mass%) from the ironmaking process is directly solidified into thin sheets by a strip caster and served for continuous solid state decarburization process that removes carbon from cast iron by gas-solid decarburization reaction. Namely, the new process eliminates the liquid state oxidation of conventional BOF process. Hot metal treatment before the strip casting will be preferably carried out to adjust final composition of 3 the sheet product. The process image of S process is sche- Fig. 1. Schematic process flow of Solid state steelmaking(S3) pro- matically shown in Fig. 1. The compositional paths for the cess.

Fig. 2. Compositional paths in the conventional process and S3 process.

Therefore, in this study, the decarburization behaviors of the solid Fe–C alloy with high carbon have been investigated. A large number of studies on the solid state decarburization of Fe–C steel has been carried out until now.7–11) For example, the decarburization process of the austenite of 0.6 mass% C was studied by Nomura7) and he calculated decarburizing ferrite depth considering chemical composition of steel and heating condition. Marder8) studied the effect of carbon content on the kinetics of decarburization in Fe–C alloys of 0.8 mass% C at 1 089 K. In the previous studies, however, the carbon contents were generally less than about 1.0 mass% and they mainly focused on the phase transformation of austenite to ferrite phase by decarburization reaction. In the case of the decarburization of cast iron, however, the decarburization behaviors of the mixed phases of 3 Fig. 3. The pO2 changes in the conventional steelmaking and S cementite and austenite must be investigated. Unfortunately, process. few studies on the decarburization of cast iron have been carried out10,11) and the details of its mechanism have not yet been clearly understood. Another significant benefit of S3 process is the elimination of many steps such as BOF, CC, secondary refinement and reheating or hot rolling process. By eliminating those 2. Experimental steps, the energy, cost, CO2 emission and time in steel pro- 2.1. Apparatus duction process can be drastically reduced. In this study, a centrifugal casting method was employed 3 For the successful implementation and development of S to cast molten iron for high solidification rate. A horizontal process, the clear understanding and verification of the fol- furnace was used for decarburizing the cast iron specimen. lowing two processes are important. The centrifugal casting machine and the decarburizing reac- (1) Strip casting of cast iron. tion system are shown in Figs. 4 and 5, respectively. The gas (2) High speed solid state decarburization process to flow was controlled by the mass flow controller (MFC). The meet the mass production. gases were purified by passing through a drying unit filled Strip casting of cast iron was already investigated by sev- with CaSO4 and then passed through the deoxidizing unit 1–5) eral researchers and cast iron sheets with the thickness of filled with magnesium chips at 723 K. For the setting of the 0.5 mm to 3 mm were successfully produced. Their main suitable ratio of H2O/H2, Ar and H2 mixture was passed aims were the production of ductile cast iron sheet. Namely, through a water bath with a particular temperature. The the produced cast iron sheet was annealed to make precipi- method was the same as that described earlier.12) The vapor tations of fine spheroidal graphite in a ferrite matrix. pressure of water is determined by using the reported mea- Recently, the high carbon steel sheet with about 0.5 mass% sured values.13) To avoid the condensation of the water C has been commercially produced by using strip casting vapor, heating coils were used to heat the gas delivery line 6) process by Nucor. Although the carbon content is relative- after the water bath. The pH2/(pH2O+pH2) was fixed to 0.78. ly low compared with that of cast iron, it supports the fea- An alumina tube inside the resistance furnace was sealed by sibility of the commercial production of cast iron sheet by water-cooling copper end cap. The decarburizing gas was strip casting. Based on these results, it can be said that there injected through a small alumina tube. Samples were easily will be no essential problems for the production of the cast taken out of the furnace or moved to end side of the furnace iron sheet by strip casting. Thus, by using the current strip after decarburization by pulling rod for the water quenching casting technology, the cast iron sheets can be possibly pro- or air cooling. duced.

2.2. Experimental Procedure 2.3. Analysis Methods Cast iron specimen was prepared by the centrifugal cast- After decarburization, the sample was cut into small piec- ing machine. At first, electrolytic iron (high purity) was es by a shearing machine. The average carbon content of the melted with graphite flakes by induction heating at 1 923 K sample was measured by the combustion infrared detection for 1 hr and sampled by a quartz sampling tube and then method. Standard sample calibration was done before mea- water quenched. This master alloy was put into the centrif- suring for accurate results. For the microstructure investiga- ugal casting machine to form a 1 mm thick cast iron strip. tion, a mounting sample was polished and etched by 2% 70 g of this rod type master alloy was mixed with pure elec- Nital solution. An optical microscope was used for micro- trolytic iron to control the carbon concentration, and this structure analysis. EBSD was used to identify phases alloy was put into an alumina crucible with ejecting hole through cross section of the samples. and put into the casting chamber together with a copper mold. Then cast iron strips with three different carbon con- 3. Results tent of 3.89, 4.35 and 4.89 mass% C were prepared. The casting chamber was evacuated by a rotary pump for 3 min- 3.1. Microstructure of Decarburized Cast Iron utes and Ar flushed for 1 minute. After 1 additional cycle The microstructure of the as cast sample (4.35 mass% C, of evacuation and Ar flushing, the master alloy was heated 3.89 mass% C) is shown in Fig. 6, and is consisted of up to 1 896 K by induction heating. After fully melted, the cementite and ferrite phases. The ferrite can be formed by molten alloy was cast. After 1.5 minutes, the mold was put the phase transformation during the cooling process. The into the water for fast cooling. After casting, the strip was average size of cementite may decrease with the solidifica- cut into pieces with 10 mm × 20 mm × 1 mm size for decar- tion cooling rate. It is expected that the decarburization rate burization experiments. of the cast iron with finer cementite will be faster. The H2O/H2 gas mixture was used as a decarburizing gas. The microstructure may have a strong effect on the decarburiza- ratio of pH2O/pH2 was set to avoid the oxidation of the iron tion rate of cast iron. In this study, the sample was prepared strip. A cast iron strip with 10 mm × 20 mm × 1 mm in an by using slip casting. To investigate the supposed decarbur- alumina boat was put into the horizontal furnace and heated ization process in S3 process, the sample should have almost up to 1 248 K or 1 327 K at the 2%H2/Ar atmosphere. 1 248 the same microstructure produced by strip casting. The cool- K and 1 327 K were selected for decarburizing temperature ing rate of the cast iron is the most important factor which because at those temperatures, no ferrite phase and no liquid determines microstructure. Average cooling rate of the cen- phase are formed during decarburization according to the trifugal casting machine is calculated from the secondary arm phase diagram. Heating rate was fixed to 15 K per minutes spacing of austenite in the matrix. From Yoshida’s results,5) in all experiments. When temperature reached to the target the average cooling rate can be obtained by equation temperature, H2O/H2 gas was introduced instead of 2%H2/ Ar for decarburization. Gas flow rate was fixed to 300 cc/ min in all cases. Decarburizing time was 5, 15, 30, 60 and 120 minutes for 1 248 K decarburization and 5, 15, 30 and 60 minutes for 1 373 K decarburization. After decarburization, sample was water quenched or air cooled. Initially, it was supposed that the carbon diffusion may occur around the interface during air cooling. To examine the effect of cooling rate on the carbon diffusion, two different cooling processes were applied. From the preliminary experiment, it was found that the carbon diffusion around the interface during the air cooling is negligible. The difference of the cooling rate is only the formed phase, and the interface position does not changed by the cooling arte. Fig. 4. Centrifugal casting machine.

Fig. 5. Experimental gas flow system.

The observed structures at 1 373 K are almost the same to =−11035.. 028 d2 340( )/v ...... (1) that at 1 248 K, but the moving speed of the interface is %C 43. much faster than that at 1 248 K. Within 30 minutes, the where d2 is secondary dendrite arm spacing(um) and v is interface disappeared by meeting together. The thin white average cooling rate(K/min). By measuring the secondary layer observed just near the surface at 1 248 K as well as arm spacing of 3.89 wt% C as cast strip as shown in Fig. 1 373 K is found to be ferrite phase without cementite based 6(b), the average cooling rate of the centrifugal casting was on EBSD analysis. It means that the carbon concentration at calculated to be 103~104 K/s. From Table 1, the centrifugal the surface is so small that the cementite formation is neg- casting’s cooling rate is comparable with that of the strip ligible even after 5 min decarburization. casting. From these values, it can be expected that the pro- EBSD analysis shows a large fraction of ferrite phase and duced microstructures by the centrifugal casting will not be existence of small amount of cementite at the decarburized so different from those of the strip casting. layer. These phases were formed during cooling after the The cross sectional images of 3.89 mass% C strip as a decarburization. On the other hand, the center area is mainly function of the decarburization time at 1 248 K are shown composed of cementite. This cementite was formed during in Fig. 7. Samples were air cooled and etched with 2% Nital solidification. No notable graphite segregation or agglomer- for 3 seconds. All specimens show clear interface with flat ates were observed. plane and this interface moves from the surface to the center The cross sectional images of 4.35 mass% C strip as a of the strip with the decarburization time. After the decar- function of decarburization time at 1 248 K are shown in burization of 120 min, the interface from the top surface and Fig. 8. The samples were water quenched and etched with that from the bottom surface meet together and disappear. 2% Nital for 3 seconds. Similar with that of the previous hypo-eutectic strip, the clear interface with flat plane was observed. The coarse cementite needles at the center region maintained their shapes until the interface merged. Due to the fast cooling rate by water quenching, the phase of decarburized zone is mainly martensite rather than ferrite or pearlite. The microstructures of 4.89 mass% C strips decarburized at 1 248 K were found to be nearly the same with that of 4.35 mass% shown in Fig. 8. The microstructure images of 4.89 mass% C strips decarburized at 1 373 K with varying time are shown in Fig. 9. Differed from the previous Fig. 6. As cast structure of sample. (a) 4.35 mass% C, (b) 3.89 mass% C results, the position of the interface was not clear at all and the depth of decarburization layer varies with the position. Table 1. Comparison between strip casting and other continuous Furthermore, voids and big agglomerates were observed. It casting process. seems that voids are originated from coarse graphite gran- Total ules and its detachment during polishing. Also this coarse Average Casting Thickness solidification graphite may cause the irregular shape of the interface cooling rate time observed in 4.89 mass% C strip. This result indicates that Conventional the large amount of the initial carbon concentration of the A 150~300 mm ~12 K/s 600~1 100 sec continuous casting strip is not suitable condition for both kinetic analysis and B Thin slab casting 20~50 mm ~50 K/s 40~60 sec practical application. C Strip casting 1~4 mm ~2 000 K/s ~0.15 sec

Fig. 7. Microstructure of 3.89 mass% C strip decarburized at 1 248 K for (a) 5 min., (b) 15 min., (c) 30 min., (d) 60 min. and (e) 120 min.

Fig. 8. Microstructure of 4.35 mass% C strip decarburized at 1 248 K for (a) 5 min., (b) 15 min., (c) 30 min., (d) 60 min. and (e) 120 min.

these. The depth of the interface position was also plotted as a function of the square root of the reaction time, and the results are shown in Fig. 11. They show reasonably good linear relation. Namely, the depth of the interface position can be expressed by the following equation; y = Ct0.5 ...... (3) where, C is a constant and t is reaction time, y is the depth of decarburization or amount of carbon decreased.

4. Discussion 4.1. Kinetics of the Cast Iron Decarburization The decarburized layer thickness follows the parabolic relationship as shown in Fig. 10. It means that the gas film diffusion step is not the rate-limiting process for the decarburization process. As already mentioned, the surface of the Fig. 9. Microstructure of 4.89 mass% C strip decarburized at 1 373 specimen was covered with ferritic phase that was formed dur- K for (a) 5 min., (b) 15 min., (c) 30 min. and (d) 60 min. ing cooling to a room temperature after the decarburization process. It means that the removal rate of carbon at the surface is much faster than that of the carbon supply from the inside 3.2. Interface Moving Rates to maintain the austenitic phase without cementite in the sur- The depth of the interface for 3.89 mass% C and 4.35 face zone. Thus, the surface chemical reaction step is also not mas% C strips with time were plotted respectively in Figs. the rate controlling step in the decarburization process. 10(a) and 10(b). The depth of the interface position was If the dissolution of carbon from the cementite is the rate defined as a distance between the reacting surface and controlling step, the decarburization rate around cementite decarburization interface. The decrease of carbon content can be slower than the austenitic matrix. Typically shown in (Creduced) during the decarburization was calculated from the Fig. 8(c), however, the decarburization interface plane is measured average carbon content (Cav) after the decarbur- essentially flat in all the cases regardless of cementite posi- ization reaction in mass percent unit. tion and size. From this result, the dissolution of carbon from cementite to austenite is also not the rate controlling C = C – C ...... (2) reduced 0 av step. Therefore, the carbon diffusion in the austenite phase where C0 is the initial carbon content in the strip and Cav is from the decarburizing interface to the reacting surface can the average carbon content in the strip after the decarburiza- be confirmed to be the rate controlling step of the gaseous tion. decarburization reaction of the cast iron. It’s hard to determine exact value of depth of decarbur- The apparent decarburization reaction at the interface ization in case of 4.89 mass% C strip due to its irregular pat- with the gas mixture of H2O–H2 is described by the expres- tern as shown before, only 3.89 mass% C and 4.35 mass% sion: C strip’s decarburization results are shown in the figure. H O + C → CO + H ...... (4) From Fig. 10, the reaction temperature has a strong effect 2 2 on the change of depth of decarburized layer, but the initial The decarburization reaction (4) can be further broken down carbon content seems to have negligibly small effects on to the elementary steps:

Fig. 10. (a) The depth of the decarburized layer as a function of the decarburization time for the samples with 3.89 mass% C, and (b) that for the samples with 4.35 mass% C.

Fig. 11. (a) The depth of the decarburized layer as a function of the square root of the decarburization time for the samples with 3.89 mass% C, and (b) that for the samples with 4.35 mass% C.

H2O → Oad + H2 ...... (5)

Oad + H2 → H2O ...... (5)’

Oad + Cad → CO...... (6)

C → Cad ...... (7) where Oad and Cad are the adsorbed oxygen and carbon respectively. Under the chemical equilibrium condition of the reaction (4), the concentration of Oad is determined by the balance between the oxygen supply rate by the reaction (5) and the oxygen removal rate by the reaction (5)’. Name- Fig. 12. Schematic cross sectional carbon concentration profile of the sample during decarburization. ly, the Oad is determined by pH2O/pH2 ratio, or by the oxygen potential of the gas mixture. Due to the fast reaction rates of the reactions of (5) and (5)’, the equilibrium con- region is the same as the initial carbon content of the cast centration of Oad is very rapidly established at the surface. iron. The carbon concentration at the reacting surface was The equilibrium carbon activity at the surface under the fixed to be zero from the experimental results. H2O–H2 gas mixture is negligibly small. In other words, the Figure 12 shows the carbon concentration profile model equilibrium concentration of Cad at the surface will be esti- predicted base on the mentioned assumptions and the exper- mated as zero. The reaction rate of (6) is also known to be imental results. M(x) moves from 0 to L/2 with time and 15–17) extremely rapid. Thus, the Cad is immediately removed phase changes to phase II (austenite phase) as interface by the reaction (6) as soon as the carbon is diffused to the moves. The equilibrium condition at M is simply described surface, or Cad should be essentially zero at all times during by the decarburization reaction. This is the reason that the fer- C (M,t) = C *...... (8) ritic phase without cementite is formed at the surface layer II II even at the very early stage of the decarburization process. From the material balance at the interface, the following relation can be satisfied. 4.2. Diffusion Coefficient of the Carbon Diffusion in the ∂C dM Austenite Phase −D ()II =− (CC* ) ...... (9) II ∂ xM= II 0 Since the carbon diffusion in the austenite phase controls x dt overall reaction, it is important to know the carbon diffusion where the moving phase boundary is at x = M. C0 and CII* coefficient in austenite layer. The diffusion coefficient is denote the average carbon content of as cast strip and solu- generally a function of temperature and average carbon con- bility of carbon at target temperature in the phase II (auste- tent of the matrix. In this estimation, however, the diffusion nite) respectively. Also, DII and Cs represent average diffu- coefficient was assumed to be constant as a first approxima- sion coefficient of carbon in the phase II and surface carbon tion. It was also assumed that carbon content of center concentration respectively. Under these conditions, the fol-

31 © 2012 ISIJ ISIJ International, Vol. 52 (2012), No. 1 lowing relation is obtained. The details can be found else- Many models which predict carbon diffusion coefficient in where.18) Fe–C system as a function of temperature and average car- − * bon content of the matrix have been investigated by several β 2 ()CC ββeerf()= sII ...... (10) researchers. Several reported carbon diffusion coefficients π ()CC* − II 0 as a function of carbon concentration19–21) are plotted in Fig. where β is a dimensionless parameter to be determined once 13. The reported diffusion coefficient formulae19–21) are also 18) the values of (Cs – CII*) and (CII* – C0) are known. The shown in Table 3. In the present carbon diffusion model, position of interface M (x,t) can be described by, however, the diffusion coefficients were assumed to be constant. As a first approximation, this constant value may be MDt = 2β ...... (11) II assumed to the value at the average carbon concentration in Equation (11) simply suggests that the thickness of the the surface layer. The average carbon concentration was decarburized layer is proportional to the square root of the defined by (Cs + CII*)/2 in this study. They are 0.7 mass% reaction time, t1/2. It is well agreed with the experimental at 1 248 K and 0.95 mass% at 1 373 K, respectively. The dif- results shown in Fig. 11. fusion coefficients at the average carbon concentrations are This reasonable agreement means that the described mod- also shown as closed circles in Fig. 13. The evaluated values el in the present study can be practically used to evaluate the are reasonably close to the reported values. interface moving rate during the decarburization process of For the successful development of S3 process, the infor- cast iron. In the Eq. (10), the right hand term can be calcu- mation of the removal rate of carbon from the solidified cast 2 lated from the experimental measurements. Once ββeerf β () iron strip as well as the interface moving rate is essentially value is known, we can evaluate the value of β from the lit- important. The amount of removed carbon from 3.89 mass% erature.18) In case of 3.89 mass% C, value of β is calculated C and 4.35 mas% C strips with time were plotted respec- as 0.50 at 1 248 K and 0.63 at 1 378 K respectively. In case of 4.35 mass% C, value of β is calculated as 0.44 at 1 248 K and 0.60 at 1 378 K respectively. Then, from Eq. (11) with Table 3. Reported models of the carbon diffusion coefficient as a function of temperature and carbon concentration. evaluated β value, we can find out relation between M, DII and t. By curve fitting with the results shown in Fig. 11, DII ⎛ 32 000 ⎞ 2 DC− =+⋅()007.. 006 ⋅−exp⎜ ⎟,/ cm s can be calculated easily by using Eq. (11). The results of cal- CFe()γ ⎝ RT ⎠ Equation culation are shown in Table 2. A119) As already mentioned, the diffusion coefficient generally R=⋅199 ./ cal mol K ,,.% C wt depends on temperature and carbon content of the matrix. =−() ⋅ ⋅ DCCFe()γ − 1023.

⎛ ⋅−15. ⎞ Table 2. Evaluated carbon diffusion coefficients at austenite phase. 4 300C 18 900 −⋅−15. 2 Equation exp⎜ 263..,/C cms 038⎟ 20) ⎝ T ⎠ A2 1248 K 1373 K (C * ~1.45 mass% C) (C * ~1.93 mass% C) II II R=⋅ 831./ J mol K ,,.% C wt 3.89 mass% C –7 2 –7 2 DII = 1.89 × 10 (cm /s) DII = 8.66 × 10 (cm /s) ⎡ ⎤ 18 900⎛ 4 300 ⎞ 15. 2 (Co = 3.89 mass% C) De− =⋅−078..xp⎢ +⎜ −263⎟C cms⎥,/ CFe()γ ⎣ TT⎝ ⎠ ⎦ Equation 4.35 mass% C A321) × –7 2 × –7 2 DII = 1.81 10 (cm /s) DII = 7.76 10 (cm /s) =⋅ (Co = 4.35 mass% C) R 831./ J mol K ,,.% C wt

Fig. 13. The evaluated carbon diffusion coefficients with the previously reported results.

Fig. 14. (a) The amount of the reduced carbon as a function of the reaction time for the samples with 3.89 mass% and (b) that for the samples with 4.35 mass%.

Fig. 15. (a) The amount of the reduced carbon as a function of the square root of the reaction time for the samples with 3.89 mass% and (b) that for the samples with 4.35 mass%. tively in Fig. 14. The amount of removed carbon was also produced by using the strip caster at Nucor.6) This result is plotted as a function of the square root of the reaction time, used as a benchmark to investigate the feasibility of S3 pro- and results are shown in Fig. 15. It shows good linearity. cess. Namely, the production of 0.5 mass% C sheets by S3 Thus, amount of removed carbon is also likely to follow the process is discussed. As shown in Fig. 14, it takes about 30 parabolic law. min to obtain the strip with the average carbon content of In the present study, the diffusion of carbon in the γ phase 0.5 mass% C from the 3.89 mass% C strip at 1 373 K. The process is found to be the rate controlling step. Then the car- commercial casting speed of high carbon strip casting is 6,14) bon removal molar flux at the surface, JC can be equal to the about 60 to 100 m/min. Then, the required length for the carbon diffusion flux at the surface; continuous decarburization treatment line at around 1 373 K must be about 1.8 to 3 km at least. This is a quite demanding ∂C J =−D ()II ...... (12) condition to make S3 process in practice. Thus, to make S3 C II ∂ x=0 x process in practice, the decarburization treatment time Since the interface moving rate is relatively small, we may should be much shortened. The enhancement of the decar- assume that the carbon removal process is carried out under burization rate will be the main target of the future study. the pseudo steady state as a first approximation; ∂C ∂C dM 5. Concluding Remarks −D ()II D≈− ()II =− (CC* ) ...... (13) II ∂ xII==00∂ xM II x x dt In this study, white cast iron strips with three different Then, carbon content (3.89, 4.35 and 4.89 mass% C) are prepared by centrifugal casting method and the gaseous decarburiza- ≈−* dM JC ()CC0 II ...... (14) tion behaviors of these cast iron strips by H2O–H2 gas mix- dt tures have been investigated at 1 248 K and 1 373 K to By integrated Eq. (14) from t0 to t, investigate the feasibility of “direct steel sheet production from solid cast iron”. Based on the microstructures and St()()()≈−CCMCC* =−* 2β Dt ...... (15) 00II II II average carbon content of the decarburized specimen, it has where S(t) is the total amount of the removed carbon for the been found that the rate of decarburization of cast iron strips decarburization time of t. Namely, the amount of the carbon of 1 mm thickness is controlled primarily by slow diffusion removal is proportional to the square root of the reaction process in the austenite phase. The dissociation rate of the time under the assumption of pseudo steady state. The par- cementite into the austenite and carbon is fast enough to abolic dependency shown in Fig. 15 can be explained based maintain the equilibrium concentration distribution between on the Eq. (15). In other words, Eq. (15) can be used to eval- the cementite and the austenite phases at the interface. uate the approximate amount of the carbon reduction from Based on the kinetic of carbon diffusion process, the appar- the cast iron strip by the decarburization reaction. ent carbon diffusion coefficient in the austenite phase is cal- As already mentioned, 0.5 mass% C steel is commercially culated and it shows reasonable agreement with the reported

33 © 2012 ISIJ ISIJ International, Vol. 52 (2012), No. 1 results. The amount of the reduced carbon from the cast iron 27 (1987), 819. 5) C. Yoshida, K. Taniguchi, T. Nakagawa, M. Sudo and T. Nozaki: strip approximately increases with the square root of the Testu-to-Hagané, 72 (1986), 2240. decarburization time. The required decarburization time to 6) D. G. Edelman, P. C. Campbell, C. R. Killmore, K. R. Carprnter, H. produce 0.5 mass% C steel from 3.89 mass% C cast iron R. Kau, J. G. Williams and W. N. Blejde: Iron Steel Technol., 6 (2009), 47. strips with 1.0 mm thickness at 1 373 K is found to be about 7) M. Nomura, H. Morimoto and M. Toyama: ISIJ Int., 40 (2000), 619. 30 min. and this reaction time might be a little bit demand- 8) A. R. Marder, S. M. Perpetua, J. A. Kowalik and E. T. Stephenson: Metall. Trans. A, 16A (1985), 1160. ing for the industrial applications. Although the further 9) R. J. Fruehan and L. J. Martonik: High Temp. Sci., 3 (1971), 244. enhancement of the apparent decarburization rate will be 10) J. Burke, J. Bell and D. R. Bourne: Acta Metall., 8 (1960), 864. certainly required, it can be said the S3 process is feasible, 11) K. Nagasaki and N. Komuro: Tetsu-to-Hagané, 49 (1963), 14. 12) I.-H. Jeong, J.-S. Lee and Y. Sasaki: ISIJ Int., 51 (2011), 805. rather promising as an alternative steelmaking process to 13) J. M. Coulson and F. D. Richardson: Chemical Engineering, vol.1, meet the demands from environmental issues. Despite of its Fluid flow, heat transfer and mass transfer, Butterworth-Henemann 3 Ltd., Oxford, (1966), 691. innovative feature, S can be employed as the actual indus- 14) M. Ferry: Direct strip casting of metals and alloys, Woodhead Pub. trial process with some modifications of the conventional Ltd., Canbridge, (2006), 87. 15) G. R. Belton: Metall. Tans. B, 24B (1993), 241. processes. 16) H-S. Kim, J.-G. Kim and Y. Sasaki: ISIJ Int., 50 (2010), 678. 17) Y. Sasaki and K. Ishii: ISIJ Int., 44 (2004), 439. REFERENCES 18) G. H. Geiger and D. R. Poireir: Transport Phenomena in Metallurgy, Addison-Wesley, London, (1973), 495. 1) J. M. Song, T. S. Lui and L. H. Chen: Metall. Mater. Trans. A, 33A 19) J. I. Goldstein and A. E. Moren: Metall. Trans. A, 9 (1978), 1515. (2002), 1263. 20) A. Ruck, D. Monceau and H. J. Grabke: Steel Res., 67 (1996), 240. 2) G. R. Lee, H. Y. Ra and J. Kor: Foundrymen’s Soc., 20 (2000), 122. 21) E. T. Georgeand and A. H. Maurice: Steel Heat Treatment Handbook, 3) J. Y. Park, K. T. Choi, Jerzy A. Szpunar, K. H. Oh and H. Y. Ra: Marcell Dekker, NY, (1997), 678. Scr. Mater., 46 (2002), 199. 4) C. Yoshida, H. Yasunaka and T. Nozaki: Trans. Iron Steel Inst. Jpn.,