metals Article
Effect of Oxidation Temperature on the Oxidation Process of Silicon-Containing Steel Bei He, Guang Xu *, Mingxing Zhou and Qing Yuan The State Key Laboratory of Refractories and Metallurgy, Hubei Collaborative Innovation Center for Advanced Steels, Wuhan University of Science and Technology, 947 Heping Avenue, Qingshan District, Wuhan 430081, China;
[email protected] (B.H.);
[email protected] (M.Z.);
[email protected] (Q.Y.) * Correspondence:
[email protected]; Tel.: +86-15697180996 Academic Editor: Hugo F. Lopez Received: 21 April 2016; Accepted: 27 May 2016; Published: 7 June 2016
Abstract: The oxidation behavior of silicon-containing steel was studied by applying segmented heating routes similar to the atmosphere and heating process in an industrial reheating furnace. The oxidation tests were carried out on a simultaneous thermal analyzer at heating temperatures of 1150 ˝ C–1300 ˝ C. The morphologies of Fe2 SiO4 were observed by SEM, and the penetration depths of the Fe2 SiO4 layer at different oxidation temperatures were determined by using the Image-Pro Plus 6.0 software. The results show that at heating temperatures ě1235 ˝ C, the oxidation rate and total oxidation mass gain have no relation with the heating temperature; the mass gain versus time follows a linear law after about 1164 ˝ C (lower than the eutectic temperature of fayalite). In addition, the oxidation rate first decreases slowly and then drops from 1190 ˝ C to 1210 ˝ C during the isothermal holding stage. With the increase in temperature, the oxidation rate and mass gain also increase gradually; the relationship between the mass gain and time is close to a parabolic law. Moreover, at a heating temperature of 1150 ˝ C, the oxidation rate decreases rapidly during the isothermal holding stage, and the mass gain versus time follows a parabolic law. Keywords: oxidation temperature; oxide scale; fayalite; oxidation kinetics
1. Introduction Silicon (Si) is an important alloying element for improving the corrosion resistance and mechanical performance of conventional alloys [1–3]. However, the addition of Si often leads to red scale, a surface defect of hot rolled steels [4]. In Si-containing steels, red scale is related to the presence of fayalite (Fe2 SiO4 ) which forms by the combination of SiO2 and FeO, firmly bonding steel substrate and iron scale [5,6]. The melting point of Fe2 SiO4 is about 1173 ˝ C and liquid Fe2 SiO4 irregularly penetrates into FeO and the matrix. It is difficult to completely wipe off the FeO layer after descaling due to the very high strength of the eutectic compound Fe2 SiO4 /FeO. The remaining FeO scale is oxidized into red Fe2 O3 during following cooling process. The oxidation temperature influences the oxidation process and Fe2 SiO4 morphology during the reheating process [7–9]. So far, some studies on the effects of the oxidation temperature on the oxidation mass gain and Fe2 SiO4 have been reported. Suarez et al. [10] found that oxidation treatments in air at temperatures higher than 570 ˝ C lead to three different iron oxide layers: wustite (FeO), magnetite (Fe3 O4 ) and hematite (Fe2 O3 ), in increasing order of oxygen content, going from substrate to free surface. Below 570 ˝ C, only the last two layers are thermodynamically stable. Cao et al. [11] investigated the isothermal oxidation kinetics of low carbon steel at temperatures ranging between 500 ˝ C and 900 ˝ C. They reported that the oxidation mass gain per unit area with time has a parabolic relation and that the oxidation rate decreases with time. Mouayd et al. [12] studied the oxidation of silicon-containing steels in the temperature range of 900 ˝ C–1200 ˝ C. They found that a passivation period occurs during the oxidation of silicon steels Metals 2016, 6, 137; doi:10.3390/met6060137
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and that a high silicon concentration leads to a long passivation of oxidation at temperatures lower than 1173 ˝ C. The passivation period is due to the formation of a thin silica layer as Si is more prone to oxidation than iron. This layer dramatically slows down the diffusion of Fe2+ towards the surface in contact with the oxidizing atmosphere. Fukagawa et al. [13] and Okada et al. [5] reported that when the temperature reaches 1173 ˝ C (the eutectic temperature of fayalite), the liquid FeO/Fe2 SiO4 flows into the external scale and develops a net-like distribution, considerably increasing the scale thickness of Fe-Si alloy. In previous studies, the oxidation atmosphere was usually introduced into the chamber of the simultaneous thermal analyzer (STA) during the isothermal oxidation stage. Few studies have adopted heating routes similar to those in the reheating technology used in the industrial production of hot strips; i.e., steel slabs are kept in an oxidation atmosphere at temperatures ranging from low to high temperature during the whole reheating process. In addition, the heating temperatures applied in most studies were below 1200 ˝ C, whereas the heating temperature in industrial production is higher than 1200 ˝ C in most cases. Therefore, it is necessary to investigate the oxidation process of silicon-containing steels under an oxidation atmosphere and temperature similar to those used in industrial production. In the present study, the oxidation kinetics of silicon-containing steel was investigated by using oxidation atmosphere and temperatures similar to those applied in the industrial reheating process. The results provide theoretical reference for controlling oxide scale defects and preventing red scale on the surface of silicon-containing steels. 2. Materials and Methods Silicon-containing steel composed of Fe-0.07C-1.11Si-1.2Mn-0.035Al-0.016Cr (wt. %) was obtained from a hot strip plant. The oxidation tests were carried out on a Setaram Setsys Evo STA (Setaram, Lyon, France). The dimensions of the samples were 15 mm ˆ 10 mm ˆ 3 mm. A hole with a diameter of 4 mm was drilled near the edge center in each sample for suspension in the oxidation chamber. The surfaces of all samples were polished on a series of SiC polishing paper up to 2000 to remove the scale before the tests. The experimental procedure was designed according to the industrial reheating process, as shown in Figure 1. The samples were first heated to 850 ˝ C at a rate of 20 ˝ C/min and then heated to predetermined temperatures (1150, 1190, 1210, 1235, 1260 and 1300 ˝ C) at a slower heating rate of 6.6 ˝ C/min; they were kept at these temperatures for different periods to ensure the same total heating time for all samples. After the isothermal holding, the samples were cooled to room temperature at a rate of 50 ˝ C/min. The accuracy of temperature measurement is ˘0.5 ˝ C, so that the effect of thermal drifts of thermobalance setup on mass gain could be ignored. The atmosphere for slabs in industrial reheating furnace contains 4.0 vol. % oxygen. A binary gas mixture of oxygen and nitrogen with an oxygen concentration of 4.0 vol. % was introduced into the STA chamber during the heating and holding stages to simulate the oxidizing atmosphere of reheating furnace in the industrial production process; the oxygen concentration was changed to 21.0 vol. % during the cooling stage to simulate the oxygen concentration in the air. The mass gain of the samples and the temperature were digitally recorded during the oxidation processes. After the oxidation tests, the samples were molded in resins at room temperature to protect the integrity of the oxide scale. The cross-sections of mounted samples were ground and polished. The microstructures of the oxide scale were observed on a Nova 400 Nano scanning electron microscope (SEM) (FEI, Hillsboro, OR, USA) operated at an accelerating voltage of 20 kV. In addition, the penetration depths of the Fe2 SiO4 layer at different oxidation temperatures were determined by using the Image-Pro Plus 6.0 software (Media Cybernetics, Rockville, MD, USA).
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Figure 1. Oxidation experiment route. Figure 1. Oxidation experiment route. Figure 1. Oxidation experiment route.
3. Results and Discussions 3. Results and Discussions 3. Results and Discussions 3.1. Morphology of Fe 3.1. Morphology of Fe22SiO SiO4 4 3.1. Morphology of Fe2SiO4 Figure 2 shows the morphologic images of Fe SiO44 at different oxidation temperatures. Table 1 Figure 2 shows the morphologic images of Fe22SiO at different oxidation temperatures. Table 1 Figure 2 shows the morphologic images of Fe 2SiO4 at different oxidation temperatures. Table 1 ˝ C (Figure presents the main atomic percentages of the inner layer at temperature °C (Figure 2e); the presents the main atomic percentages of the inner layer at temperature 1260 1260 2e); the inner presents the main atomic percentages of the inner layer at temperature 1260 °C (Figure 2e); the inner layer is a composite of eutectic compounds Fe 2SiO4/FeO. The bright area (Spectrum 1) is FeO, layer is a composite of eutectic compounds Fe2 SiO4 /FeO. The bright area (Spectrum 1) is FeO, while inner layer is a composite of eutectic compounds Fe 2SiO4/FeO. The bright area (Spectrum 1) is FeO, while the dark area (Spectrum 2) is mainly Fe 2 SiO 4 . In addition, Mn is in detected in the and thewhile dark the areadark (Spectrum 2) is mainly In2SiO addition, Mn is detected the bright andbright dark area. 2 SiO4 . Fe area (Spectrum 2) is Fe mainly 4. In addition, Mn is detected in the bright and dark area. The constitution of the inner layer is consistent with the results in the other studies The constitution the inner layer is inner consistent the results in the studies Figure 2 dark area. The ofconstitution of the layer with is consistent with the other results in the [14,15]. other studies ˝ C. However, a [14,15]. Figure 2 shows that the granular FeO + Fe 2SiO4close layer is dispersed close to the substrate at shows that the granular FeO + Fe SiO layer is dispersed to the substrate at 1150 2 4 [14,15]. Figure 2 shows that the granular FeO + Fe 2SiO4 layer is dispersed close to the substrate at ˝ C. At temperatures 1150 °C. However, a continuous net‐like distribution layer of FeO + Fe 2to SiO 4 is observed at 1190 to continuous net-like distribution layer of FeO + Fe2 SiO4 is observed at 1190 1150 °C. However, a continuous net‐like distribution layer of FeO + Fe 2SiO 41300 is observed at 1190 to ˝ 1300 °C. At temperatures higher than the melting point of Fe 2 SiO 4 (1173 °C), the liquid 2SiO higher thetemperatures melting pointhigher of Fe2 SiO C), the liquid is forced penetrate at2Fe the 1300 than °C. At than the melting point of Fe Fe22SiO44 (1173 °C), tothe liquid Fe SiO 4grain is 4 is 4 (1173 forced to to penetrate of the steel steel substrate and and oxide scale [16–18]. The boundaries ofpenetrate the steelat substrate andboundaries oxide scale of [16–18]. The substrate penetration depths ofscale Fe forced at the the grain grain boundaries the oxide [16–18]. The 2 SiO 4 at different penetration depths of Fe 2 SiO 4 at different oxidation temperatures were measured by using oxidation temperatures measured by using the Image-Pro Plus were 6.0 software (Figure 3). Several penetration depths of were Fe2SiO 4 at different oxidation temperatures measured by using the the Image‐Pro Plus 6.0 software (Figure 3). Several images were used to improve the accuracy Image‐Pro Plus to 6.0 software 3). of Several were used to improve the in accuracy of of images were used improve the(Figure accuracy Fe2 SiOimages With the increase temperature 4 measurement. FeFe 2SiO 4 measurement. With the increase in temperature from 1150 to 1190 °C, the depth of Fe 2SiO 21150 SiO 4 measurement. With the increase in temperature from 1150 to 1190 °C, the depth of Fe 2SiO 4 4 from to 1190 ˝ C, the depth of Fe2 SiO4 increases significantly. However, at temperatures ranging ˝ increases significantly. However, at temperatures ranging from 1190 to 1235 °C, the depth of increases significantly. However, at temperatures ranging from 1190 to 1235 °C, the depth of from 1190 to 1235 C, the depth of Fe2 SiO4 increases slowly. As the temperature increases further, the FeFe 2SiO 4 increases slowly. increases further, further, the the depth depth of of 2SiO 4 shows 2SiO 4Fe increases slowly. As As the the temperature temperature increases FeFe 2SiO 4 shows no no depth of 2 SiO4 shows no significant change. significant change. significant change.
Figure 2. SEM cross‐section micrographs showing the distribution of Fe 4 at different oxidation Figure 2. SEM cross-section micrographs showing the distribution of Fe2SiO 2 SiO 4 at different oxidation ˝ C. temperatures: (a) 1150 °C; (b) 1190 °C; (c) 1210 °C; (d) 1235 °C; (e) 1260 °C; (f) 1300 °C. Figure 2. SEM cross‐section micrographs showing the distribution of Fe 2SiO at different oxidation temperatures: (a) 1150 ˝ C; (b) 1190 ˝ C; (c) 1210 ˝ C; (d) 1235 ˝ C; (e) 1260 ˝ C; (f)41300 temperatures: (a) 1150 °C; (b) 1190 °C; (c) 1210 °C; (d) 1235 °C; (e) 1260 °C; (f) 1300 °C.
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Table 1. Main atomic percentages of different layers (at. %). Table 1. Main atomic percentages of different layers (at. %). Chemical Elements O Si Fe
Chemical Elements
O
Si
Bright area (Spectrum 1) 53.71 Bright area (Spectrum 1) 53.71 Dark area (Spectrum 2) 59.66 12.16 ‐
Dark area (Spectrum 2)
Fe
45.62
Mn
45.62 27.29 0.67 59.66 12.16 27.29 0.89
Mn 0.67 0.89
Figure 3. Penetration depths of Fe22SiO Figure 3. Penetration depths of Fe SiO44. .
3.2. Oxidation Analysis 3.2. Oxidation Analysis Figure 4 shows the mass gain per surface square centimeter of samples and the oxidation rate Figure 4 shows the mass gain per surface square centimeter of samples and the oxidation rate as functions of time at different oxidation temperatures. The whole oxidation process includes three as functions of time at different oxidation temperatures. The whole oxidation process includes three stages; slow oxidation, intense oxidation, and final stop oxidation. At a heating temperature ě1235 ˝ C, stages; slow oxidation, intense oxidation, and final stop oxidation. At a heating temperature ≥1235 during the intense oxidation stage, the oxidation rate remains constant, and the mass gain per surface °C, during the intense oxidation stage, the oxidation rate remains constant, and the mass gain per unit with time follows a linear law. At temperatures ranging from 1190 to 1235 ˝ C, during the intense surface unit with time follows a linear law. At temperatures ranging from 1190 to 1235 °C, during oxidation stage, the oxidation rate first decreases slowly and then drops obviously. The mass gain per the intense oxidation stage, the oxidation rate first decreases slowly and then drops obviously. The surface square withcentimeter time is close to atime parabolic law. Atparabolic 1150 ˝ C (lower than the mass gain per centimeter surface square with is close to a law. At 1150 °C eutectic (lower temperature of fayalite-wustite), during the intense oxidation stage, the mass gain versus time shows a than the eutectic temperature of fayalite‐wustite), during the intense oxidation stage, the mass gain parabolic relationship. are three critical temperatures: the starting temperature (SOT, versus time shows a There parabolic relationship. There are three critical oxidation temperatures: the starting point A), the intense oxidation temperature (IOT, point B), and the finishing oxidation temperature oxidation temperature (SOT, point A), the intense oxidation temperature (IOT, point B), and the (FOT, pointoxidation C). Beforetemperature the SOT, the rates of point diffusion combination among ions are due toand the finishing (FOT, C). and Before the SOT, the rates of slow diffusion lower temperature; thus, theare sample almost not lower oxidized. Between the SOTthe andsample the IOT,is the adhesion combination among ions slow isdue to the temperature; thus, almost not of Si and O2 is stronger than that of Fe and O2 at low temperatures2 is stronger than that of Fe and O [19,20], leading to the appearance2 oxidized. Between the SOT and the IOT, the adhesion of Si and O of before the formation the FeO. SiO2 retards the diffusion iron ions and oxygen at SiO low 2 temperatures [19,20], of leading to Because the appearance of SiO 2 before the offormation of the FeO. atoms, the oxidation reaction is slow as the temperature increases. The jump in oxidation rate is due to Because SiO 2 retards the diffusion of iron ions and oxygen atoms, the oxidation reaction is slow as athe buoyancy effect, increases. caused by turbulent at about 43is min in Figure 4b. At theeffect, intensecaused oxidation temperature The jump gas in flows oxidation rate due to a buoyancy by temperature, oxidation rate43 increases This due to the considerable increasethe in turbulent gas the flows at about min in dramatically. Figure 4b. At the isintense oxidation temperature, the diffusion rate at higher temperatures [21–24], resulting in a sharp increase in the oxidation rate. oxidation rate increases dramatically. This is due to the considerable increase in the diffusion rate at After the FOT, the oxidation reaction stops rapidly. According to the mass grain curves, the three higher temperatures [21–24], resulting in a sharp increase in the oxidation rate. After the FOT, the critical temperatures at different oxidation temperatures are determined (Table 2). Because the oxygen oxidation reaction stops rapidly. According to the mass grain curves, the three critical temperatures concentration increases temperatures to 21.0 vol. %, are the oxidation rate(Table increases dramatically the beginning of the at different oxidation determined 2). Because the at oxygen concentration cooling process. increases to 21.0 vol. %, the oxidation rate increases dramatically at the beginning of the cooling process.
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Figure 4. (a) Mass gain versus time and (b) oxidation rate versus time at 1150 to 1300 ˝ C. Figure 4. (a) Mass gain versus time and (b) oxidation rate versus time at 1150 to 1300 °C. Table 2. Transition temperatures of the oxidation reaction. Table 2. Transition temperatures of the oxidation reaction.
Oxidation Starting Oxidation Oxidation Starting Oxidation ˝C Temperature/ Temperature (A)/˝ C Temperature/°C Temperature (A)/°C
Intense Oxidation Finishing Oxidation Intense Oxidation Finishing Oxidation ˝C Temperature (B)/ Temperature (C)/˝ C Temperature (B)/°C Temperature (C)/°C
480 1150 803 803 1150 1150 480 1150 1190 483 1161 1158 1190 483 1161 1158 1210 506 1167 1160 1210 1235 506 1167 491 1164 11621160 1260 487 1162 11611162 1235 491 1164 1300 494 1167 1166 1260 487 1162 1161 1300 494 1167 1166 Regarding to the oxidation kinetics at constant heating temperature, it is often reported that the to the oxidation kinetics at constant heating temperature, is often study reported that massRegarding gain and heating time follows parabolic rule [12,15]. The results in theit present indicate ˝ C. However, the gain and heating time follows parabolic law rule The results in the study that mass the mass gain versus time follows the parabolic at[12,15]. the temperature below 1210present ˝ indicate that the mass gain versus time follows the parabolic law at the temperature below 1210 °C. mass gain changes linearly with time at the heating temperature higher than 1235 C. This point is However, mass gain changes at the heating temperature higher than 1235 °C. useful for the determination oflinearly heating with routestime of silicon-containing steels in industrial production of ˝ This point is useful for the determination of heating routes of silicon‐containing steels in industrial hot strip because the heating temperature is often higher than 1235 C. The results demonstrate that ˝ C.1235 °C. The results production of gains hot strip because are the the heating is often higher than the total mass of oxidation sametemperature at the temperature above 1235 demonstrate that the total mass gains of oxidation are the same at the temperature above 1235 °C. The oxidation kinetics during the isothermal holding period at temperatures below 1235 ˝ C can The oxidation kinetics formula during the isothermal holding period be described by a parabolic proposed by Kofstad et al. [25]: at temperatures below 1235 °C can be described by a parabolic formula proposed by Kofstad et al. [25]: n ∆W Kpp∙t (n > 1) ¨ t pn ą 1q (1) ΔWn“ = K (1)
The oxidation kinetics during the isothermal holding period at temperatures higher than 1235 The oxidation kinetics during the isothermal holding period at temperatures higher than 1235 ˝ C °C can be represented by a linear equation: can be represented by a linear equation: ∆W “ Kl ¨ t (2) ΔW = Kl∙t (2) where, ∆W is the mass gain of the oxide scale in mg/cm2 ;2; K Kl and Kp are the oxidation rate constants at where, ΔW is the mass gain of the oxide scale in mg/cm l and Kp are the oxidation rate constants a constant temperature T; and t is the oxidation time. The value of n in Equation (1) varies from 1 to 2 at a constant temperature T; and t is the oxidation time. The value of n in Equation (1) varies from 1 from a linear relationship to a parabolic law. Therefore, Equation (2) can be integrated into Equationinto (1). to 2 from a linear relationship to a parabolic law. Therefore, Equation (2) can be integrated The oxidation rate constants for different heating temperatures can be calculated based on the Equation (1). experimental data, as shown in Table 3. The kinetic constants increase at 1150 to 1210 ˝ C but remain The oxidation rate constants for different heating temperatures can be calculated based on the similar at temperatures higher than 1235 ˝ C. experimental data, as shown in Table 3. The kinetic constants increase at 1150 to 1210 °C but remain As shown in Figure 4 and Table 2, there is no significant change in the intense oxidation similar at temperatures higher than 1235 °C. temperatures at a heating temperature ě1190 ˝ C, and the average IOT is 1164 ˝ C. The IOT is a temperature range of about 1060 ˝ C–1164 ˝ C, where the end of temperature of the interval is selected as IOT in this paper. It is interesting to note that the intense oxidation temperatures are lower than
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the eutectic temperature of fayalite (1173 ˝ C), indicating that the intense oxidation has no relationship with the melting of fayalite. Table 3. Oxidation kinetics parameters. Temperature (˝ C)
Kl (mg¨cm´2 ¨min´1 )
Kp (mg2 ¨cm´4 ¨min´1 )
1150 1190 1210 1235 1260 1300
0.452 0.459 0.454
0.075 0.141 0.318 -
At temperatures higher than 1164 ˝ C, during the heating stage, the oxidation rate shows no significant change (Figure 4b). Regarding to the oxidation of steels, five main factors should be considered. Firstly, the radius of the oxygen ion is much larger than that of the iron ion; thus, it is difficult for the oxygen ion to diffuse into the reaction interface [26]. As a result, the oxidation rate depending on the diffusion of iron ions increases with the temperature. The liquid Fe2 SiO4 provides diffusion passages for diffusion of iron ions. Thus, the mass gain increases with the temperature. Secondly, manganese (Mn) in the steel substrate can be oxidized [6]. Manganese oxides which are detected in Table 1 increase the oxidation mass gain. Thirdly, the oxidation reaction of carbon (C) causes decarburization in the steel. Simultaneous scaling and decarburization has been studied [27,28], and the general consensus is that during steel oxidation, carbon reacts with the scale via the following reaction [28]: rCs ` FeO “ Fe ` CO (3) where [C] denotes carbon in solution in steel. Further reaction between CO and the scale may produce CO2 via the following reaction: CO ` FeO “ Fe ` CO2 (4) From the reactions, mass loss due to decarburization overlaps with the mass gain from the oxidation. Fourthly, wustite occurs with a broad composition range that can be described as Fe1´x O, with x varying from 0.04 to 0.17 [11]. The ion transport through oxides is dominated by the stoichiometry of the oxide (wustite) that is p-type [26,29]. The fact that x is always positive implies that wustite is cation-deficient and that the interaction between cations and vacancies is the predominant mechanism of mass transport. The amount of iron ions increases and the vacancies of iron ions decrease with the increasing of temperature, leading to a reduction in diffusion passages. Therefore, the oxidation rate decreases with the increase of temperature. Fifthly, the thickness of oxide scale hinders the diffusion of iron ion, decreasing the oxidation rate. The fact that oxidation rate at temperatures higher than 1164 ˝ C has no significant change in Figure 4b indicates the dynamic equilibrium of above five factors affecting oxidation rate. In addition, linear mass gain could be caused by a constant gradient in chemical potential that drives oxygen/iron diffusion. In addition, as shown in Figure 4, at heating temperatures ranging from 1235 to 1300 ˝ C, the oxidation rates of the samples are basically the same, and the mass gain versus time follows an almost linear relationship, indicating that the oxidation mass gain is not sensitive to temperature. The liquid Fe2 SiO4 is forced to penetrate at the grain boundaries of the steel substrate and oxide scale, which plays a role as passage for the ions transfer process and provides fast diffusion passages to accelerate the oxidation rate [14]. In the isothermal holding process at 1235 ˝ C, the oxidation rates of Fe, Mn and C can reach dynamic equilibrium during the isothermal holding period. Therefore, the oxidation rate remains constant. At temperatures higher than 1235 ˝ C, the oxidation rates of Fe, Mn and C can reach dynamic equilibrium, resulting in the same oxidation rate. Therefore, the oxidation rate is not sensitive to temperature.
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Fe, Mn and C can reach dynamic equilibrium during the isothermal holding period. Therefore, the oxidation rate remains constant. At temperatures higher than 1235 °C, the oxidation rates of Fe, Mn Metals 2016, 6, 137 7 of 9 and C can reach dynamic equilibrium, resulting in the same oxidation rate. Therefore, the oxidation rate is not sensitive to temperature. Figure 5a shows the oxidation rate as a function of time in the isothermal holding period at 1150 ˝ C. Figure 5a shows the oxidation rate as a function of time in the isothermal holding period at 1150 As indicated in the figure, the oxidation rate decreases rapidly; Figure 4a shows that the mass gain °C. As indicated in the figure, the oxidation rate decreases rapidly; Figure 4a shows that the mass versus time has a parabolic relationship. BecauseBecause the oxygen is constant, oxidation gain versus time has a parabolic relationship. the concentration oxygen concentration is the constant, the reaction is mainly determined by the iron ion concentration in the interface reaction during the oxidation reaction is mainly determined by the iron ion concentration in the interface reaction ˝ C. The thickness of the oxide scale, the amount of solid-phase isothermal period at 1150 during the holding isothermal holding period at 1150 °C. The thickness of the oxide scale, the amount of particles of SiO and Fe SiO increase the isothermal holding period at 1150 ˝ C, which is 2 2 4 solid‐phase particles of SiO2 and Fe2SiOduring 4 increase during the isothermal holding period at 1150 °C, unfavorable for ion diffusion. Therefore, the iron ion concentration in the reaction interface decreases which is unfavorable for ion diffusion. Therefore, the iron ion concentration in the reaction interface quickly, resulting in a rapid decrease in the oxidation rate. decreases quickly, resulting in a rapid decrease in the oxidation rate. Moreover, Moreover, as as shown shown in in Figure Figure 5b, 5b, the the oxidation oxidation rate rate first first decreases decreases slowly slowly and and then then reduces reduces ˝ C and 1210 ˝ C. In the isothermal holding obviously during the isothermal holding period at 1190 obviously during the isothermal holding period at 1190 °C and 1210 °C. In the isothermal holding period, the amount of liquid Fe22SiO SiO44 increases with the oxidation time, promoting the diffusion of increases with the oxidation time, promoting the diffusion of period, the amount of liquid Fe ions. However, at the same time, the thickness of the oxide scale increases, which is unfavorable for ion ions. However, at the same time, the thickness of the oxide scale increases, which is unfavorable for diffusion. At the beginning of the isothermal holding period, the promotion and inhibition effect on the ion diffusion. At the beginning of the isothermal holding period, the promotion and inhibition effect ion the diffusion of the oxidation reaction reaction basically basically reach dynamic Therefore,Therefore, the oxidation on ion diffusion of the oxidation reach equilibrium. dynamic equilibrium. the rate decreases slowly. With the progress of the isothermal holding time, the thickness of the oxide oxidation rate decreases slowly. With the progress of the isothermal holding time, the thickness of scale increases such that the inhibition effect on the ion diffusion becomes greater than the promotion the oxide scale increases such that the inhibition effect on the ion diffusion becomes greater than the effect. Therefore, the oxidation the rate oxidation decreases obviously with time. In addition, the promotion effectthe of promotion effect. Therefore, rate decreases obviously with time. In addition, temperature on the ion diffusion increases with the temperature, resulting in a slightly higher oxidation promotion effect of temperature on the ion diffusion increases with the temperature, resulting in a rate at high heating temperatures. Because Fe2 SiO4 is in solid phase at 1150 ˝ C, which hinders the slightly higher oxidation rate at high heating temperatures. Because Fe 2SiO4 is in solid phase at 1150 ˝ diffusion ions, thethe decrease in oxidation ratedecrease at 1150 in C isoxidation significantly that at 1190 ˝ C °C, which ofhinders diffusion of ions, the rate greater at 1150 than °C is significantly and 1210 ˝ C during the isothermal holding period. greater than that at 1190 °C and 1210 °C during the isothermal holding period.
Figure 5. Oxidation rate versus time during the isothermal holding period: (a) 1150 ˝ C; (b) 1190 ˝ C Figure 5. Oxidation rate versus time during the isothermal holding period: (a) 1150 °C; (b) 1190 °C and 1210 ˝ C. and 1210 °C.
Figure 6 presents the total mass gain as a function of the oxidation temperature for the same total Figure 6 presents the total mass gain as a function of the oxidation temperature for the same heating time (about 111 min). total mass gain increases significantly at temperatures ranging total heating time (about 111 The min). The total mass gain increases significantly at temperatures from 1150 to 1190 ˝ C. At 1190 ˝ C, the liquid Fe2 SiO4 that provides fast diffusionfast passages promotes the ranging from 1150 to 1190 °C. At 1190 °C, the liquid Fe 2SiO4 that provides diffusion passages promotes the oxidation process, leading to a significant increase in mass gain compared with that at oxidation process, leading to a significant increase in mass gain compared with that at 1150 ˝ C where 1150 where the Fe 2SiO4 The is a total solid mass phase. total mass gain increases slowly at temperatures the Fe°C phase. gainThe increases slowly at temperatures ranging from 1190 to 2 SiO 4 is a solid ˝ C due ranging from to 1235 °C due into the gradual rate, increase in the oxidation rate, previously 1235 to 1190 the gradual increase the oxidation as previously mentioned. Atas temperatures mentioned. At temperatures ≥1235 °C, the total mass gain shows no significant change relative to ě1235 ˝ C, the total mass gain shows no significant change relative to 1235 ˝ C. At temperatures above ˝ 1235 °C. At temperatures above 1235 °C, the oxidation rate is higher, as shown in Figure 4b, resulting 1235 C, the oxidation rate is higher, as shown in Figure 4b, resulting in a larger total mass gain in a larger total mass gain than that at low temperatures. The mass gains are basically the same at than that at low temperatures. The mass gains are basically the same at 1235 to 1300 ˝ C because the 1235 to 1300 °C because the oxidation rates are similar, indicating that the total mass no oxidation rates are similar, indicating that the total mass gain has no relationship with gain has the heating relationship with the heating temperatures higher than 1235 °C. ˝ temperatures higher than 1235 C.
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Figure 6. Total mass gain at different oxidation temperatures. Figure 6. Total mass gain at different oxidation temperatures. 4. Conclusions 4. Conclusions The oxidation behavior of silicon-containing steel was studied by using an atmosphere and The oxidation behavior of silicon‐containing steel was studied by using an atmosphere and heating route similar to those applied in the industrial reheating process. The oxidation experiments heating route similar to those applied in the industrial reheating process. The oxidation experiments were carried out on STA at different oxidation temperatures. The effect of oxidation temperature on the were carried out on STA at different oxidation temperatures. The effect of oxidation temperature on oxidation process of silicon-containing steels was investigated. The results show that at temperatures the oxidation process silicon‐containing steels was a investigated. results show that at higher than 1235 ˝of C, the mass gain versus time follows linear law after The the intense oxidation temperature (lower than the eutectic temperature of fayalite). The oxidation rate and oxidation total temperatures higher than 1235 °C, the mass gain versus time follows a linear law after the intense mass gain have no relationship with the heating temperature. However, the oxidation rate first oxidation temperature (lower than the eutectic temperature of fayalite). The oxidation rate and decreases slowly and then drops markedly during the isothermal holding stage at 1190 ˝ C and 1210 ˝ C. oxidation total mass gain have no relationship with the heating temperature. However, the With the increase in the heating temperature, the oxidation rate and mass gain increase gradually, and oxidation rate first decreases slowly and then drops markedly during the isothermal holding stage at the mass gain with time is close to a parabolic law. Only at the temperatures below 1210 ˝ C does the 1190 °C and 1210 °C. With the increase in the heating temperature, the oxidation rate and mass gain mass gain versus time follow a parabolic law. Moreover, the penetration depth of Fe2 SiO4 and the total mass gain increase quickly and then go up slowly before finally becoming basically unchanged at increase gradually, and the mass gain with time is close to a parabolic law. Only at the temperatures temperatures ranging from 1150 to 1300 ˝ C. below 1210 °C does the mass gain versus time follow a parabolic law. Moreover, the penetration depth of Acknowledgments: Fe2SiO4 and the gain increase the quickly then up slowly before finally The total authorsmass gratefully acknowledge financial and supports fromgo National Natural Science Foundation of China (NSFC) (No. 51274154), the National High Technology Research and Development Program becoming basically unchanged at temperatures ranging from 1150 to 1300 °C. of China (No. 2012AA03A504). State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel Group).
Acknowledgments: The authors gratefully acknowledge the financial supports from National Natural Science Author Contributions: Guang Xu, supervisor, conceived and designed the experiments; Bei He, conducted Foundation of China (NSFC) (No. 51274154), the National High Technology Research and Development experiments, analyzed the data and wrote the paper; Mingxing Zhou, conducted experiments; Qing Yuan, Program of China (No. 2012AA03A504). State Key Laboratory of Development and Application Technology of conducted experiments. Automotive Steels (Baosteel Group). Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
Author Contributions: Xu, supervisor, conceived and designed the experiments; Bei He, conducted decision to publishGuang the results. experiments, analyzed the data and wrote the paper; Mingxing Zhou, conducted experiments; Qing Yuan, References conducted experiments. 1.
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Song, L.; Guo, E.; Wang, L.; Liu, D. Effects of silicon on mechanical properties and fracture toughness of References heavy-section ductile cast iron. Metals 2005, 5, 150–161. [CrossRef] 4. Takeda, M.; Onishi, T. Oxidation behavior and scale properties on the Si containing steels. Mater. Sci. Forum. 1. Nishimura, formation 2006,T. 522,Rust 477–488. [CrossRef]and corrosion performance of Si‐ and Al‐bearing ultrafine
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