Effect of chromium–sulphur interactions on decarburisation of levitated Fe–C alloy droplets by CO 2 –Ar gas mixtures

Background

Since the advent of the argon oxygen decarburisation process, many investigations were undertaken to study and improve the various process routes for stainless steelmaking. The processes are often developed on the basis of manipulation of temperature, gas dilution and pressure, in which the latter is the basis for the vacuum oxygen decarburisation process. It has been recently demonstrated ¹ that using CO₂ for steel decarburisation is a promising approach for the retention of valuable alloying elements. In principle, CO₂ decarburisation is similar to both the dilution (argon oxygen decarburisation) and vacuum (vacuum oxygen decarburisation) processes in that it lowers the oxygen potential in the steel bath, thus giving preference to the oxidation of carbon rather than chromium.

One of the earliest investigations studying the kinetics of CO₂ decarburisation was by Baker et al. ² Iron–carbon droplets with 0 to 5.5 wt-% C were levitated in an inert atmosphere, then subjected to constant flow of CO₂–CO/He mixtures containing up to 100% CO₂. The rate of decarburisation at 1933 K was found to be independent of carbon content in the melt, and the rate varied considerably with oxidant partial pressure in the gas phase. It was therefore concluded that decarburisation by CO₂ is controlled wholly by diffusion in the gas phase at high carbon concentrations.

In their investigation involving levitation experiments at a pressure of 4.053 × 10⁶ Pa, El-Kaddah and Robertson ³ demonstrated that both decarburisation and carburisation of Fe–C liquid alloys by CO₂–CO gas mixtures can be achieved. The kinetics of decarburisation, for 1 g droplets containing 5 and 5.5 wt-% C, was found to obey a model developed for convective mass transfer in the gas phase. Their assessment of the carburisation results, on the other hand, revealed that liquid phase resistance is significant at the reaction temperature of 1923 K. For the gas composition studied, 1.1 or 2.15% CO₂, the carburisation rate was controlled by mixed transport of gas and liquid phase diffusion.

Lee and Rao ^4,5 carried out decarburisation of levitated Fe–C–S droplets in a flowing CO–CO₂ gas stream at 1973 K. The rate decreases significantly with increase in sulphur concentration in the melt. Sulphur, being surface active, was responsible for blocking available sites for CO₂ dissociation. However, it was found that for gas streams containing 10% CO₂ and flowing at 1 L min^− 1, the retarding effect of sulphur diminishes as sulphur concentration increases past >0.05 wt-%. They observed that flowrate of the gas mixture has a relatively small effect on the decarburisation rate compared to the oxidant partial pressure in the gas mixture. It was thus concluded that the process is a mixed controlled scenario of both gas phase mass transfer and dissociative chemisorption of carbon dioxide.

Sun and Pehlke ⁶ have studied the kinetics of carbon oxidation in the presence of silicon and manganese in iron. They observed preferential oxidation of carbon when the alloy (3.35 wt-% C, 2 wt-% Si, 0.36 wt-% Mn) was exposed to CO₂–N₂ gas mixtures. Oxidation of Mn and Si only began when carbon levels were sufficiently low and the rate limiting step changed from CO₂ transport in the boundary layer to solute diffusion in the liquid metal. It was found that sulphur, in the range of 0.002 to 0.5 wt-%, had negligible effect on the decarburisation rate.

Work by Simento et al. ⁷ has shown that the rate of decarburisation of Fe–C droplets, with initial carbon concentrations between 3.38 and 4.18 wt-% C, by CO₂–N₂ gas mixtures is jointly controlled by CO₂ diffusion in the gas phase and subsequent dissociative chemisorption at the gas/metal interface. Small amounts of P (0.1 wt-%), Cr (0.5 wt-%) and S (up to 0.27 wt-%) were alloyed with the Fe–C melt to determine their effect on the decarburisation behaviour. From their experimental results, mostly at 1723 K, the effect of sulphur on the rate was found to obey the Langmuir isotherm for sulphur adsorption. For the melt compositions pertaining to their work, both chromium and phosphorus did not exhibit a noticeable effect on the rate.

It is clear from the above that, for high carbon melts, the rates of CO₂ decarburisation were found to be controlled by either diffusion in the gas phase, surface chemical kinetics or a combination of both. However, limited mention was made with regard to the effects of other alloying elements commonly found in high alloy steel grades on the kinetics of decarburisation by CO₂.

It has been indicated ^8–11 that sulphur plays an important role in the kinetics of decarburisation by CO₂. The presence of sulphur, even in low concentrations, was found to significantly decrease the decarburisation rate. It is generally understood that the blocking of available reaction sites by chemisorbed sulphur is responsible for the rate retardation. As shown in Fig. 1, with relevant data summarised in Table 1, decarburisation rate is hampered by the increasing sulphur concentration in the melt. However, the results from a recent study by the authors ¹² suggest that 0.01 wt-% sulphur in the presence of 20 wt-% Cr did not have an adverse effect on the decarburisation rate of Fe–Cr–C droplets. It can be seen from Fig. 1 that chromium containing melts with 0.01 wt-% sulphur (▪) have similar behaviour to that of essentially sulphur free melts (Δ). This trend is consistently observed within a range of gas flowrates between 500and 1500 mL min^− 1.

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Effects of gas flow characteristics and melt sulphur content on rate of carbon removal

The presence of chromium in significant concentrations may have contributed to this observation due to its effect on reducing the sulphur activity at steelmaking temperatures. This would imply that chromium can mitigate the effect of sulphur coverage. Thus, the present investigation is aimed at providing further insight into the effects of sulphur in the presence of chromium on the decarburisation kinetics of Fe–Cr–C droplets involving CO₂ as the oxidant gas.

Methodology

Equipment

Using a similar approach to that by Lee and Rao ⁴ and Sun and Pehlke, ⁶ decarburisation experiments were performed using an electromagnetic levitation facility. A schematic diagram of the equipment is shown in Fig. 2. The levitation chamber consists of a quartz tube, 15 mm in outer diameter, 13 mm in inner diameter and 304 mm in length, sealed at the upper end with O rings and an optical grade quartz window to permit temperature measurement using a Chino two-colour IR pyrometer, which was calibrated against the melting point of copper to an accuracy of ± 15°. An O ring sealed, rotatable, aluminium platform provided a base for the lower chamber. The platform is fitted with a copper mould for droplet quenching and an alumina charging rod for loading solid specimens into the electromagnetic field within the levitation coil. The rotating action of the platform allowed either the copper mould or the charging rod to be aligned directly below the levitation chamber. The water cooled levitation coil was wound from copper tubing of 3.2 mm in outer diameter and consisted of two segments: the lower cone with three turns and the upper inverted cone with two reverse turns. The lower cone is responsible for providing the droplet with lifting force and heat, while the upper inverted cone controls the vertical and lateral stability of the levitated droplet. The power required for levitation was provided by a high frequency induction generator with a rated terminal output of 10 kW and a frequency range from 150 to 400 kHz.

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Schematic representation of electromagnetic levitation equipment

Development of appropriate rate equation

As discussed in previous work, ¹² the rate of carbon loss is linear down to ∼1 wt-% C, which suggests that mass transport in the liquid phase is not the rate limiting step. If decarburisation is limited solely by the transport of CO₂ from the bulk gas to the interface, the rate can be expressed as follows 1 where k _g is the gas phase mass transfer coefficient, is the CO₂ partial pressure in the bulk gas at the edge of the boundary layer and is the CO2 partial pressure at the gas/metal interface. In the gas phase controlled scenario, is expected to approach zero.

The interfacial reaction for dissociative chemisorption of CO₂ can be written as 2 where v represents a vacant reaction site at the droplet surface, and the superscript i and * denote interface and adsorbed species respectively. Ignoring the reverse reaction, the rate of decarburisation is given by 3 where k _c is the chemical rate constant for the forward reaction described in equation (2). The fractional surface coverage by sulphur θ_s, can be described using the Langmuir adsorption model 4 where K _S is the sulphur adsorption coefficient, a _S is the activity of sulphur in the melt, θ_u represents the fraction of surface sites that cannot be occupied by sulphur and is the activity coefficient of sulphur in the adsorbed layer, which can be taken as unity at carbon saturation. ⁹ For carbon saturated alloys, Sain and Belton ⁹ provided an expression for K _S , in wt-%^− 1, which was based on measurements taken between 1553 and 1873 K 5 The θ_u term was introduced to account for the residual rates observed by many investigators ^5,7,9 who noted that increase in sulphur content beyond 0.1 wt-% does not further decrease the decarburisation rate. Lee and Rao ⁵ obtained a value of 0.085 for θ_u from their study involving levitated droplets. For convenience, equation (4) can be rearranged in terms of available reaction sites 6 When the process of decarburisation is jointly controlled by both gas phase mass transport and interfacial reaction, the overall rate may be derived by combining equations (1) and (3). The resulting expression takes the form 7

Effect of chromium–sulphur interactions on decarburisation rate

As shown in Fig. 3, experimental measurements by Lee and Rao ⁴ have demonstrated the effect of sulphur concentration on the overall rate. The result from Wu et al. ¹² , obtained under similar experimental conditions, is also superimposed on the plot. It can be seen that the process of decarburisation of Fe–C alloys containing 20 wt-% chromium (represented by ▪) is 33% faster than chromium free melts containing the equivalent amount of sulphur, based on the experimental results by Lee and Rao. ⁴

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Effect of melt sulphur content on decarburisation rate

According to equation (7), the fractional surface sites available for the CO₂ dissociative reaction (1 − θ_S), is an important parameter in decarburisation kinetics. It is evident from equations (6) and (7) that the rate would become a sole function of a _S if parameters such as droplet weight, temperature, gas composition and flowrate remain constant. Hence, the effect of an alloying element such as chromium on the decarburisation rate is reflected by virtue of the effect on the sulphur activity coefficient and consequently (1 − θ_S). The interaction parameter for effect of Cr on S as a function of temperature is given by the equation ¹³ 8 At 1873 K, has a value of − 0.0103. Using this figure, the effect of chromium on surface coverage by sulphur can be computed using equation (6). Results of the calculations are summarised in Fig. 4. On the basis of these findings, for melts with identical sulphur concentration of 0.01 wt-%, the fraction of available sites is ∼31 % higher with the melts containing 20 wt-% chromium compared to the chromium free melts. With more sites available on the surface of the droplet, the contribution from chemical reaction to the overall rate equation is expected to increase. Since the interfacial reaction takes place much faster than the transfer of gaseous oxidant across the boundary layer, this phenomenon provides an enhanced rate of decarburisation as indicated by Fig. 3. This means that melts containing 0.01 wt-% sulphur and 20 wt-% chromium exhibit decarburisation behaviour that is similar to melts that are essentially free of sulphur. This finding is particularly significant with respect to the decarburisation of stainless steels when the gas used for oxidation of carbon is CO₂.

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Change of available surface sites with melt sulphur and chromium contents

Conclusions

The objective of the present work was to provide some insights into the effects of chromium, at higher concentrations, on the decarburisation kinetics involving CO₂ as the oxidant gas. It was observed that the rate for Fe–20Cr–C alloys containing 0.01 wt-% S was 33% faster than that reported for chromium free melts with the same sulphur content and essentially the same as that for Fe–C alloys containing zero sulphur and zero chromium. The increase in decarburisation rate of iron–chromium–carbon–sulphur droplets can be explained by the attraction between chromium and sulphur solutes in the liquid phase, which decreases the sulphur activity and results in fewer surface sites occupied by surface active sulphur. This in turn has important implications with regard to preferential oxidation of carbon rather than chromium if stainless steel is refined with carbon dioxide rather than oxygen.

Acknowledgements

Appreciation is expressed to the Natural Sciences and Engineering Research Council of Canada, who provided funding for this project through a Discovery Research Grant.