Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 1, 2020
Article Number 103
Number of page(s) 7
DOI https://doi.org/10.1051/metal/2019069
Published online 13 January 2020

© EDP Sciences, 2020

1 Introduction

A solid-state steelmaking process is proposed, which can simplify production processes and is environment-friendly [12]. Unlike the conventional production process for steel sheets, liquid iron from the blast furnace is directly solidified into thin iron sheets by casting with twin-roll continuous casters, and then the carbon is removed from the cast iron sheets by gas-solid decarburization reaction. Converter steelmaking and secondary refining in conventional steel sheet production process are also eliminated in the proposed solid-state steelmaking process.

The core points of solid-state steelmaking process: the hot metal from blast furnaces, non-blast furnaces and electric arc furnaces is directly (or after pretreatment) solidified by a twin-roll continuous casters to produce thin iron sheets. This iron sheets are decarburized using an oxidizing atmosphere of H2O/H2 at high temperatures to produce high/low carbon steel sheets. This process removes the high intensity of oxygen supply so that the inclusion formation is possibly avoiding. The most important aspect of this process is the elimination of the basic oxygen furnace, secondary steelmaking of molten steel, continuous casting and hot rolling, and the steelmaking process is completely changed.

This process has very wide popularization and application prospects. On the one hand, hot metal is directly solidified by a twin-roll continuous casters and decarburized using an oxidizing atmosphere to produce the thin steel sheets. On the other hand, this process can be applied to traditional steelmaking process. For example, it is difficult to produce ultra-low carbon steel (< 20 ppm) for basic oxygen furnace, The thin steel sheets are decarburized by gas-solid reaction to produce the ultra-low carbon steel. Producing ultra-low carbon steel by solid-state steelmaking process has many advantages. The advantages of using this method to produce ultra-low carbon steel are that the process can relax the carbon content of basic oxygen furnace steel tapping (such as 0.06%C) and carbon content of RH (such as 0.02%C) to reduce inclusions of the steel and produce ultra-low carbon steel sheets.

In recent years, research on casting iron sheets with twin-roll continuous casters has made considerable progress with the successful production of 0.5–3.0 mm iron sheets [3]. The industrial production of iron sheets of 0.5 mass% C gas has been realized with twin-roll continuous casters [48]. Thus, it supports the feasible production of iron sheets. The solid-state steelmaking process is not only the extension or optimization of conventional steelmaking process, but also a possible alternative steelmaking process to provide industry with a sustainable developing way and protect the environment, although further improvements will be necessary.

The average carbon content after decarburization is an important criterion in judging the feasibility of the solid-state steelmaking process. There have been many studies on the gas-solid decarburization of iron sheets [914]. However, in these studies [1519], the carbon contents of iron sheets were usually less than 1.0 mass%, and they mainly focused on phase transformation from austenite to ferrite phase by decarburization reaction. The gas-solid decarburization of 1 mm high-carbon iron sheet was investigated by Park [1]. He found that the production of 1 mm iron sheets using the solid-state steelmaking process was feasible. However, the thickness of the iron sheets was just 1 mm, which does not meet the required thickness for industrial production. Thicker iron sheets are ideal for decarburization [20]. Most steel sheets produced are thicker than 1 mm. To meet the required thickness for industrial steel sheet production, the gas-solid decarburization of high-carbon iron sheets with greater thickness must be investigated. A detailed analysis of the change mechanism of average carbon content during decarburization reaction is also needed.

2 Experimental

2.1 Experimental material and equipment

The steps for preparing the cast iron specimen are as follows. First, mixed powder (0.72 g high-purity graphite powder + 15.28 g high-purity iron powder) was smelted at 1673 K for 1 h in protective atmosphere. Second, the smelted mixed powder was solidified into Fe-C alloy block by natural cooling in protective atmosphere. Third, 65 × 15 × 2 mm iron sheet was casted with a casting machine, as shown in Figure 1.

The decarburizing reaction system, which consists of an atmosphere control system and a heating control system, is shown in Figure 2. The gas flow was controlled by a mass flowmeter. To set the suitable ratio of H2O/H2, mixed gas (Ar + H2) was passed through a water bath at a particular temperature. The gas was humidified by the water bath and the humidity of the gas was controlled by the temperature of the water bath. The higher temperature of water bath increased the humidifying capacity of the gas. The humidity and temperature of the gas were measured by a hygrometer. A heating pipe between the atmosphere control system and heating control system was used to prevent water vapor from condensing. The heating temperature and holding time can be set with the heating control system.

thumbnail Fig. 1

Casting machine.

thumbnail Fig. 2

Experimental equipment for solid-state decarburization.

2.2 Experimental procedure

First, the iron sheet was placed in the heating field of a quartz tube; then, the quartz tube was sealed and its gas tightness was checked. Second, the heating field was filled protective gas (Ar + H2), and the temperature-raising procedure and temperature of the water bath were set and the calefaction circuit was started. Third, when the temperature of the heating field reached the target temperature, the protective gas was extracted from the heating field and the decarburization gas (high-purity Ar + H2 + H2O) was bubbled. Fourth, after the decarburization reaction, the decarburization gas (high-purity Ar + H2 + H2O) was extracted from the heating field and the protective gas was bubbled, and the iron sheet was naturally cooled in the protective gas. Finally, 800–1000 mg of iron filings from the iron sheet were used to test the carbon contents with a PC-controlled carbon sulfur analyzer.

3 Results and discussion

3.1 Decarburization temperature and time

Approximately 4.10 mass% C and 3.20 mass% C 2 mm iron sheets were placed in the heating field and heated to the target temperatures (1293, 1353, and 1413 K). The temperature of the water bath was 333 K. The total flow of the decarburization gas was 300 mL/min and the ratio of H2 and high-purity Ar was 1/4. The decarburization times were 0 min, 10 min, 30 min, 50 min, 60 min, 70 min, and 80 min, respectively. After the decarburization reaction, the average carbon contents of the iron sheets were tested. The results are listed in Table 1.

From Figure 3, the average carbon contents at 1293, 1353, and 1413 K all decreased as decarburization time passed. The changing trends of the three decarburization-fitted curves are consistent. The absolute value of the slope of the decarburization-fitted curve represents the decarburization rate. All the three decarburization-fitted curves became gentler as decarburization time passed; hence, the three absolute values of the slopes and decarburization rates continuously decreased over time.

The decarburization reaction can be divided into two steps. The first is carbon diffusion from the interior of the sheet to the reaction interface, which is near the sheet surface. The second is the interfacial reaction: (1)

Because the decarburization atmosphere and time are constant during the whole decarburization reaction, the decrease in decarburization rates is attributed to the decrease in the amount of carbon, which diffuses from the interior of the sheet to the reaction interface as the decarburization time passes Thus, the carbon diffusion from the interior of the sheet to the reaction interface is the rate-controlling step of the whole decarburization reaction. The carbon contents of the sheets at the three temperatures remained constant at 60–80 min, and the decarburized carbon contents reached the limit. Therefore, the decarburization time for approximately 4.10 mass% iron sheets at 1293–1413 K should be less than 60 min; the lowest carbon content is approximately 1.0 mass%.

Comparison of the three decarburization-fitted curves at 1293, 1353, and 1413 K shows that the higher the decarburization temperature, the steeper the decarburization curve and the faster the decarburization rate Hence, increasing the decarburization temperature can increase the decarburization rate. At decarburization time of 60 min, the carbon content limits are 2.80 mass%, 1.57 mass%, and 1.09 mass% at 1293, 1353, and 1413 K, respectively. Increasing the temperature also decreases the carbon content limit after decarburization. During the decarburization process, increasing the temperature can accelerate the carbon diffusion from the interior of the sheet to the reaction interface, which is beneficial for the whole decarburization reaction.

Table 1

Experimental results.

thumbnail Fig. 3

Carbon contents at different decarburization times.

3.2 Dynamical analysis

In order to prove that the gas-solid reaction of iron sheet is the first-order, the average carbon content of the sheet (w) in Table 1 was converted for lnw. The relationship between lnw and t was shown in Figure 4.

The first-order reaction equation (2): (2) so, (3)

where w = average carbon content of the sheet, t = decarburization time, w0= initial carbon content of the sheet, and wt = average carbon content of the sheet after decarburization for t minutes.

The lnw-t relation curves in Figure 4 show good linearity. The decarburization reaction is similar to a first-order reaction. Experimental data are fitted into the curves and the equations based on Figures 3 and 4. The reaction rate constant k (-x in the table in Fig. 3 and -b in the table in Fig. 4) is obtained from Figures 3 and 4. The k is used for the following calculation, where k = reaction rate constant.

The relationship of lnk-1/T is shown in Figure 5. Equation (4) is obtained in Figure 5: (4)

The activation energy of decarburization reaction:

Ea = 17432.8 × 8.314 = 144936 J/mol = 144.936 kJ/mol, where Ea = activation energy. (5)

As shown in equation (5), improving the decarburization temperature increases the reaction rate constant k, which represents the reaction rate. As the temperature increases, k also increases and speeds up the decarburization reaction rate. The relationship of T-k is shown in Figure 6.

Equation (6) in Figure 6: (6)

Equation (7), which is used to calculate the carbon contents after decarburization at different decarburization time and temperature, is obtained by combining equations (3) and (6). (7)

As seen in equation (7), improving temperature and extending the time both enhance decarburization. A comparison of the fitted results and experimental results is shown in Figure 7, where Exp = experimental result and Fit = fitted result.

As is shown in Figure 7, the fitted curves of approximately 4.10 mass% iron sheets at 1293, 1353, and 1413 K are very close to the experimental curves, respectively. Hence, equation (7) can accurately describe the changes in average carbon contents during the decarburization process. A comparison of the fitted curve with equation (7) and experimental curve of 3.20 mass% iron sheets at 1413 K is shown in Figure 7. The fitted results and experimental results show good agreement, which indicates that equation (7) is also suitable for describing changes in the carbon contents of low-carbon iron sheets during the decarburization process.

thumbnail Fig. 4

The linear relationship between lnw and t.

thumbnail Fig. 5

Determination of activation energy.

thumbnail Fig. 6

k-T relationship.

thumbnail Fig. 7

Comparisons of fitted results and experimental results.

4 Segmented heating method

The carbon diffusion from interior of the sheet to the reaction interface is the rate-controlling step during the whole decarburization reaction. Improving the temperature not only accelerates the carbon diffusion from interior of the sheet to the reaction interface, but also increases the interfacial reaction rate. Thus, improving the temperature enhances the whole decarburization reaction. As shown in Figure 8, as the carbon content of iron decreases, the melting point of iron is higher in the Fe-C diagram. In the traditional (constant temperature) heating method, the heating temperature is constant during the whole decarburization reaction. Segmented heating means that the heating temperature is increased in stages during the whole decarburization reaction. Some decarburizing routes are shown in Figures 810.

The experimental results in Table 2 show that the decarburization effect with the segmented heating method is much better compared with the traditional heating method.

thumbnail Fig. 8

Decarburizing routes of the initial carbon content 4.15 mass%. a: segmented heating method; b: traditional heating method.

thumbnail Fig. 9

Heating route of segmented heating. a: initial carbon content 4.15 mass%; b: initial carbon content 3.20 mass%.

thumbnail Fig. 10

Decarburizing routes of initial carbon content 3.20 mass%. a: segmented heating method; b: traditional heating method.

Table 2

Experimental results.

5 Conclusion

Equation (7) accurately shows the relation between time, temperature, and average carbon content during the decarburization reaction of 2 mm high-carbon iron sheet. Longer time and higher temperature both enhance decarburization, and improving temperature has better effects on decarburization for industrial applications. The decarburization rates slow down over time; hence, the carbon diffusion from the interior of the sheet to the reaction interface is the rate-controlling step during the whole decarburization reaction. The activation energy of the decarburization reaction is Ea = 144.936 kJ/mol. Equation (7) provides data for use as the basis in designing the heating route of the segmented heating method. The decarburization effects with the segmented heating method are much better compared with the traditional heating method. Decarburization with the segmented heating method not only improves the solid-state steelmaking process, but also has important effects on decarburization annealing.

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Cite this article as: Rong Cheng, Liqun Ai, Lu-kuo Hong, Cai-jiao Sun, Average carbon content change during gas-solid decarburization of 2 mm iron sheet with a high carbon content, Metall. Res. Technol. 117, 103 (2020)

All Tables

Table 1

Experimental results.

Table 2

Experimental results.

All Figures

thumbnail Fig. 1

Casting machine.

In the text
thumbnail Fig. 2

Experimental equipment for solid-state decarburization.

In the text
thumbnail Fig. 3

Carbon contents at different decarburization times.

In the text
thumbnail Fig. 4

The linear relationship between lnw and t.

In the text
thumbnail Fig. 5

Determination of activation energy.

In the text
thumbnail Fig. 6

k-T relationship.

In the text
thumbnail Fig. 7

Comparisons of fitted results and experimental results.

In the text
thumbnail Fig. 8

Decarburizing routes of the initial carbon content 4.15 mass%. a: segmented heating method; b: traditional heating method.

In the text
thumbnail Fig. 9

Heating route of segmented heating. a: initial carbon content 4.15 mass%; b: initial carbon content 3.20 mass%.

In the text
thumbnail Fig. 10

Decarburizing routes of initial carbon content 3.20 mass%. a: segmented heating method; b: traditional heating method.

In the text

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