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A New Approach in Solid State Steelmaking from Thin Cast Iron

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A New Approach in Solid State Steelmaking from Thin Cast Iron ISIJ International, Vol. 58 (2018),ISIJ International, No. 10 Vol. 58 (2018), No. 10, pp. 1791–1800 A New Approach in Solid State Steelmaking from Thin Cast Iron Sheets through Decarburization in CaCO3 Pack Ebrahim SHARIF-SANAVI, Mostafa MIRJALILI* and Jalil VAHDATI KHAKI Department of Materials and Metallurgical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, 91775-1111 Iran. (Received on April 6, 2018; accepted on June 4, 2018) In the solid state steelmaking process used in recent researches, pig iron was directly casted and then decarburized in an oxidizing atmosphere in order to eliminate or reduce the amount of carbon. In the pres- ent study, the feasibility of solid state steelmaking from cast iron was investigated using limestone. For this purpose, white cast iron specimens were buried in a chamber containing CaCO3 powder. Calcination of CaCO3 produces CO2 which participates in cast iron decarburization. In this technique, CO2 reacts with the carbon content of sample according to Boudouard reaction which results in carbon consumption. Furthermore, effects of temperature and time on the decarburization process were investigated. White cast iron samples were decarburized at temperatures of 800, 900 and 1 000°C for 10 and 24 h in the CaCO3 powder pack. Very fine oxide layer was also observed to form during the decarburization process. Finally, samples were studied with optical microscope and SEM to measure the depth of the decarburized layer. However, secondary graphitization was occurred during the decarburization. Results showed that decarburization at 1 000°C for 24 h has led to a completely decarburized layer of 420 μm thickness. In agreement, carbon analysis showed the reduction of carbon content from 3.16 wt% to 0.012 wt%. Kinetic studies revealed activation energy of 125 KJ/mol for decarburization of white cast iron using CaCO3 pow- der pack. KEY WORDS: solid state steelmaking; decarburization; white cast iron; calcium carbonate. observed that the rate of decarburization of cast iron strips 1. Introduction is controlled primarily by carbon diffusion in the austenite In the conventional process of steelmaking, oxidation phase. They found that dissolution rate of the cementite is process of molten pig iron makes inclusions and bubbles fast enough to maintain the local equilibrium concentra- in the liquid phase. Many efforts have made to eliminate tion between the cementite and the austenite phases at the the inclusions and bubbles. Moreover, the solubility limit interface.1,2) of oxygen in liquid iron is almost high; so during the pro- Furthermore, Sasaki2) was investigated commercial pro- cess of carbon removing, oxygen is easily dissolved in the duction of 0.5 wt% C steel by using a strip caster at Nucor3) molten iron. The advantages of solid state steelmaking (S3) to evaluate feasibility of the S3 process. According to processes are eliminating multiple steps (including BOF and McDonald4) investigations, solid state steelmaking process secondary refining), and absence of inclusions.1,2) is ideal for improvement an steel plant to reduce operating Considering environmental problems and economic cost and CO2 emissions, or to reduce the capital cost of a considerations in the steel industry, solid state steelmaking greenfield development. It is considered that the process was introduced by Sasaki et al.1,2) Sasaki’s research was needs to further researches to improve the process kinetics based on the decarburization of cast iron samples using a of solid state decarburization and enable solid state steelmak- decarburizing atmosphere. In their process, high-carbon ing process for decarburizing samples with 4 wt% carbon.4) molten iron was casted directly in form of thin sheets by Recently, production of layer integrated steel with dif- a centrifugal casting method. Then cast iron strips with ferent microstructures and properties with high strength 10 mm × 20 mm × 1 mm dimension were situated into and high ductility was investigated.5) Conventionally, layer a horizontal furnace and heated up to 1 248–1 327 K at integrated steel was produced by laminating several steel 6) H2O/H2 atmosphere. The eliminated carbon was calculated layers and using hot rolling or cold rolling. According to according to the equations of carbon diffusion in a homoge- the results obtained by Sasaki,1,2) a sheet with a three layer neous austenite plane sheet. The calculated decarburization structure can be easily produced that the surface layers have thickness then was verified by empirical investigation. They low carbon contents, in contrast with the inner layer which has initial high carbon concentration. * Corresponding author: E-mail: [email protected] The gaseous decarburization process of white cast iron DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2018-250 consists of elimination of solute carbon in austenite by sur- 1791 © 2018 ISIJ ISIJ International, Vol. 58 (2018), No. 10 face reaction, diffusion of carbon through austenite from the and high carbon iron decarburization were performed in center to the surface of solid sample and decomposition of one chamber for simplification. Moreover, white cast iron the carbon-rich phases.7) Decarburization reactions usually samples were used instead of casted pig iron for feasibility include a combination with one of the followings: study of the solid state decarburization process. For this purpose, white cast iron samples were put inside CaCO CCOC2 O ........................ (1) 3 s 2()gg() powder pack for a while and the decarburized sections were characterized. CHs 2 24()ggCH () .........................(2) COs 22()ggCO () ..........................(3) 2. Experimental In order to use an alloy with a composition close to that CHs 22OC()ggOH () 2()g .................. (4) of pig iron, cast iron samples containing 3.6 wt% carbon Many studies have been focused on the solid state decar- were used. Chemical composition of the utilized cast iron is burization of Fe–C steel which show there are many param- reported in Table 1. For this purpose, samples were casted eters affecting the decarburizing rate, such as the chemical in a shape of sequin with diameter of 4 cm and height of 6 composition,8–12) superficial coating,13) the thermal cycles of mm. Casted sequins were then solidified in a water cooling steel production14) and the atmosphere of the furnace.15–17) mold to achieve a white cast iron structure. Deng et al.15) have investigated influence of oxidizing, inert Before decarburization process, surface oxide scales were and reducing heating atmospheres such as O2, CO2, N2 and removed using SiC grinding papers until a desired surface CO on the decarburization depth of steel. According to their quality was achieved. After that, samples were placed into results, oxidizing gases, such as O2 and CO2 have increased the decarburizing chamber. The decarburization process was the decarburization depth, and the inert gas of N2 and reduc- conducted by using CaCO3 powder pack. For this purpose, ing gas of CO have strongly inhibited decarburization of samples were buried in a closed steel container full of 15) samples. CaCO3 powder, and then heated to elevated temperatures. Liu et al.16) have investigated effects of the temperature Figure 1(a) shows the schematics of decarburization cham- and oxygen concentration on the decarburization of 55SiCr ber. The height and diameter of the chamber was about 7 spring steel. They have decarburized samples in a muffle and 4.7 cm, respectively. Note that all samples were placed furnace (in ambient air) and also in a simultaneous ther- on top of the chamber and at the same position. The particle mal analyzer (in the atmosphere of 2% O2 and 98% N2). size of the used CaCO3 powder is shown in Fig. 1(b) which For samples heated in the ambient air, the decarburization indicates the average particle size of 258 nm. behavior was divided into four types: no decarburization, According to CaCO3 calcination reaction, CO2 is released only partial decarburization, only complete decarburiza- during heating: tion, and finally partial and complete decarburization. By CaCO CaOCO .................... (5) decreasing the oxygen concentration to 2%, only partial 32 (ss)()(g) decarburization was found. However, according to the CO2 then attributes in carbon removing from the speci- results of decarburizing in the thermal analysis, it was men surface, based on Boudouard reaction: impracticable to reduce the thickness of decarburized layer ........................ (6) by decreasing oxygen concentration.16) CC()s OC2g 2 O()g 17) According to Baud et al. research, samples were oxi- Consequently, the weight of CaCO3 powder needed for dized at 700°C in the ambient air, dry air and moist air (31% complete decarburization of sample can be calculated using water vapor). In the ambient air, surface decarburization of stoichiometric values of above reactions. Since the weight the metal was measurable only after a period of 32 h. How- of cast iron sample was 7.5 g, the stoichiometric weight ever, in the presence of 31% water vapor, no decarburiza- of CaCO3 which is needed for total decarburization of the tion was observed, even for an oxidation time of 128 h. In sample is 2.25 g. However, CaCO3 content of chamber contrast, oxidation in completely dry air for 64 h revealed a was considered to be 50 g (almost 20 times higher than the huge decarburization; although, no surface decarburization calculated value). This overuse of CaCO3 is because of the 17) of the sample was observed for periods shorter than 8 h. probable CO2 leakage which does not participate in decarbu- 3 In the atmospheric S process, there is a competition rization process. Our measurements of CaCO3 weight decre- between oxidation and decarburization to occur. It is ment during the process and its corresponding mole of CO2 reported that at higher temperatures (above 1 200°C) oxida- indicated that most of the released CO2 has not participated tion rate is more than decarburization rate.
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