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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 



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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Conflicts  of  Interest:  The  authors  declare  no  conflict  of  interest.  The  founding  sponsors  had  no  role  in  the  steel. Corros. Sci. 2008, 50, 1306–1312. [CrossRef] design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in  2. Hu, H.J.; Xu, G.; Wang, L.; Xue, Z.L.; Zhang, Y.L.; Liu, G.H. The effects of Nb and Mo addition on the decision to publish the results.  transformation and properties in low-carbon bainitic steels. Mater. Des. 2015, 84, 95–99. [CrossRef] 3.

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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weathering steel. Corros. Sci. 2008, 50, 1306–1312.  Hu,  H.J.;  Xu,  G.;  Wang,  L.;  Xue,  Z.L.;  Zhang,  Y.L.;  Liu,  G.H.  The  effects  of  Nb  and  Mo  addition  on  transformation and properties in low‐carbon bainitic steels. Mater. Des. 2015, 84, 95–99.  Song, L.; Guo, E.; Wang, L.; Liu, D. Effects of silicon on mechanical properties and fracture toughness of  heavy‐section ductile cast iron. Metals 2005, 5, 150–161.   

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