The Effect of the Si Content on the Morphology and Amount of Fe 2 SiO 4 in Low Carbon Steels

In order to study the effect of the Si content on the morphology, amount, and distribution of fayalite (Fe2SiO4), three low-carbon steels with different Si contents were selected, and reheating tests were conducted in an industrial furnace in a hot strip plant. The results show that Si distributes in two forms—first, Fe2SiO4, in the innermost layer of the oxide scale, and, second, granular SiO2, dispersively distributed in the matrix near the scale. In addition, Fe2SiO4 appears in a net-like form in the innermost layer of the oxide scale close to the iron matrix when the Si content is 1.21 wt. %. However, no obvious net-like Fe2SiO4 is observed when the Si content is less than 0.25 wt. %. Moreover, the inhibition effect of the solid Fe2SiO4 on the oxidation reaction plays a more important role than the promotion effect of the liquid Fe2SiO4 during the entire oxidation reaction. Therefore, the total thickness of the scale decreases with the increase in Si content.


Introduction
Silicon is generally added to steels as one of the solid solution-strengthening elements [1].The surface defects, such as rolled-in scale, red scale, and chromatic aberration, appear in hot-rolled low-carbon steels containing silicon [2,3].The existing research shows that the red scale has a close relationship with the silicon element in steel [4][5][6][7].So far, some studies have been conducted on the red scale.Suarez et al. [8] investigated the influence of silicon on the formation of the oxide scale in hot-rolled strips at high temperatures.They pointed out that Si reacts with oxygen diffusing into steel and precipitates as SiO 2 , which combines with FeO and forms a separate phase called fayalite (Fe 2 SiO 4 ).The melting point of Fe 2 SiO 4 is about 1173 ˝C.Fe 2 SiO 4 begins to form at a temperature above 750 ˝C and primarily aggregates on the interface between the iron matrix and the scale.Fukaga et al. [9] and Onoda et al. [10] analyzed the relationship between the Fe 2 SiO 4 phase and the red scale.They claimed that the oxide layer formed on the steel surface mainly consists of Fe 2 O 3 , Fe 3 O 4 , and FeO.The eutectic FeO/Fe 2 SiO 4 primarily forms in the interface between the matrix and the scale, and irregularly penetrates into FeO and the matrix.It is difficult to absolutely wipe off the FeO layer after descaling due to the very high strength of the eutectic compound.The remaining FeO scale is oxidized into red Fe 2 O 3 during the following cooling process.Furthermore, the descaling process becomes more difficult and more red scale remains when the penetrative depth of the Fe 2 SiO 4 phase in the FeO layer is larger.In addition, only a few studies have been conducted about the effect of the Si element on the content of Fe 2 SiO 4 in low-carbon steel.Schneider et al. [8] investigated the oxidation of Fe-Si alloys at high temperatures from 900 to 1250 ˝C, and found that the amount of Fe 2 SiO 4 and the thickness of the scale increase with the silicon content.In addition, the liquid Fe 2 SiO 4 accelerated the oxidation process.Moreover, Mouayd et al. [11] reported that the penetrative depth of the eutectic FeO/Fe 2 SiO 4 in the scale increases with Si content.
It is generally accepted that the formation of the red scale is related not only to the content of Fe 2 SiO 4 , but also to its morphology and distribution.However, almost all existing studies were performed in laboratories, and oxidizing atmosphere was not added until isothermal holding temperature.Moreover, the effect of Si content on the morphology and quantitative studies on the amount of Fe 2 SiO 4 has scarcely been reported.In the present study, three low-carbon steels with different Si contents were selected, and reheating tests were conducted in an industrial furnace in a hot strip plant to quantitatively study the effect of the silicon content on the morphology and amount of Fe 2 SiO 4 .The novelty in the present study is that a new influence rule of silicon on the oxidation behavior in Si-containing steel has been proposed.In addition, oxidation tests were first conducted with the same atmosphere and heating route as industrial heating technology.

Oxidation Experiment and Sample Preparation
Three low-carbon steels were commercially produced in a hot strip plant.All samples were formed in a cube-shaped structure of 133 mm ˆ39 mm ˆ10 mm.The samples were polished to remove the scale before heating in the furnace.The chemical compositions of the three carbon steels with different Si contents are presented in Table 1.The heating procedure is shown in Figure 1.The samples were heated to 1260 ˝C by segment heating route and held for 40 min, followed by air cooling to room temperature.The heating atmosphere in the furnace contained approximately 2% oxygen, 13% carbon dioxide, 11% water vapor, and 74% nitrogen.After oxidation experiment, specimens were cut using a wire-electrode cutting device.Since the oxide scale in these samples is very brittle and easy to peel off, the cold mounting method was used in the preparation of the samples for microscopic observation.The cold mounting material is composed of 60% acrylic powder and 40% liquid hardener.The cross-sections of mounted samples were grinded and polished.The powder was scraped off from samples without cold mounting for phase analysis via X-ray diffraction (XRD, Panalytical, Almelo, The Netherlands).liquid Fe2SiO4 accelerated the oxidation process.Moreover, Mouayd et al. [11] reported that the penetrative depth of the eutectic FeO/Fe2SiO4 in the scale increases with Si content.It is generally accepted that the formation of the red scale is related not only to the content of Fe2SiO4, but also to its morphology and distribution.However, almost all existing studies were performed in laboratories, and oxidizing atmosphere was not added until isothermal holding temperature.Moreover, the effect of Si content on the morphology and quantitative studies on the amount of Fe2SiO4 has scarcely been reported.In the present study, three low-carbon steels with different Si contents were selected, and reheating tests were conducted in an industrial furnace in a hot strip plant to quantitatively study the effect of the silicon content on the morphology and amount of Fe2SiO4.The novelty in the present study is that a new influence rule of silicon on the oxidation behavior in Si-containing steel has been proposed.In addition, oxidation tests were first conducted with the same atmosphere and heating route as industrial heating technology.

Oxidation Experiment and Sample Preparation
Three low-carbon steels were commercially produced in a hot strip plant.All samples were formed in a cube-shaped structure of 133 mm × 39 mm × 10 mm.The samples were polished to remove the scale before heating in the furnace.The chemical compositions of the three carbon steels with different Si contents are presented in Table 1.The heating procedure is shown in Figure 1.The samples were heated to 1260 °C by segment heating route and held for 40 min, followed by air cooling to room temperature.The heating atmosphere in the furnace contained approximately 2% oxygen, 13% carbon dioxide, 11% water vapor, and 74% nitrogen.After oxidation experiment, specimens were cut using a wire-electrode cutting device.Since the oxide scale in these samples is very brittle and easy to peel off, the cold mounting method was used in the preparation of the samples for microscopic observation.The cold mounting material is composed of 60% acrylic powder and 40% liquid hardener.The cross-sections of mounted samples were grinded and polished.The powder was scraped off from samples without cold mounting for phase analysis via X-ray diffraction (XRD, Panalytical, Almelo, The Netherlands).

Oxide Scale Analyses
Three techniques were used to analyze the constitution of the oxide scale, i.e. a backscattered electron detection (BSED, FEI, Hillsboro, OR, USA), energy-dispersive spectroscopy (EDS, OIMS, Oxford, UK), and XRD.The microstructure and compositions of the oxide scale were analyzed via BSED and EDS on a Nova 400 Nano scanning electron microscope (SEM, FEI, Hillsboro, OR, USA) operated at an accelerating voltage of 20 kV.XRD with Cu Kα radiation was also used to analyze the phase of the oxide scale under the following conditions: acceleration voltage, 40 kV; current, 150 mA; step, 0.06 ˝.The powder sample for XRD was scraped from the oxidized sample.Furthermore, the Image-Pro plus 6.0 software (Media Cybernetics, Rockville, MD, USA) was used to determine the total thickness of the scale and the Fe 2 SiO 4 layer.

Morphology and the Composition of the Oxide Scale
The morphological images of the oxide scale in three low-carbon steels are shown in Figure 2. It can be seen that the oxide scale consists of three layers with different thicknesses.According to the latter results of EDS and XRD, the upper layer primarily contains Fe 2 O 3 .The middle layer is thicker compared to the upper layer and consists of FeO and Fe 3 O 4 (Figure 2a,c,e).The inner layer is a mixture of Fe 2 SiO 4 and FeO.The dark gray Fe 2 SiO 4 distributes in the light gray FeO.Fe 2 SiO 4 appears in the net-like form when the silicon content is high (Figure 2b).However, no obvious net-like Fe 2 SiO 4 is observed when the Si content is low (Figure 2f).Three techniques were used to analyze the constitution of the oxide scale, i.e. a backscattered electron detection (BSED, FEI, Hillsboro, OR, USA), energy-dispersive spectroscopy (EDS, OIMS, Oxford, UK), and XRD.The microstructure and compositions of the oxide scale were analyzed via BSED and EDS on a Nova 400 Nano scanning electron microscope (SEM, FEI, Hillsboro, OR, USA) operated at an accelerating voltage of 20 kV.XRD with Cu Kα radiation was also used to analyze the phase of the oxide scale under the following conditions: acceleration voltage, 40 kV; current, 150 mA; step, 0.06°.The powder sample for XRD was scraped from the oxidized sample.Furthermore, the Image-Pro plus 6.0 software (Media Cybernetics, Rockville, MD, USA) was used to determine the total thickness of the scale and the Fe2SiO4 layer.

Morphology and the Composition of the Oxide Scale
The morphological images of the oxide scale in three low-carbon steels are shown in Figure 2. It can be seen that the oxide scale consists of three layers with different thicknesses.According to the latter results of EDS and XRD, the upper layer primarily contains Fe2O3.The middle layer is thicker compared to the upper layer and consists of FeO and Fe3O4 (Figure 2a,c,e).The inner layer is a mixture of Fe2SiO4 and FeO.The dark gray Fe2SiO4 distributes in the light gray FeO.Fe2SiO4 appears in the net-like form when the silicon content is high (Figure 2b).However, no obvious net-like Fe2SiO4 is observed when the Si content is low (Figure 2f).The thickness of the upper layer in all three carbon steels is thinner compared to other layers.Some dispersive dark spots, confirmed to be Fe3O4 as determined by EDS, can be observed in the middle layer (Figure 2a,c,e).In addition, small dark spots are dispersively distributed in the matrix near the scale, and, according to the EDS results, these spots are confirmed to be SiO2 (Figure 2b,d,f).Fe2SiO4 appears in the net-like form in Steel 1 containing higher silicon content (Figure 2b), whereas no obvious net-like Fe2SiO4 is observed in Steels 2 and 3 with low Si content (Figure 2d,f).According to the distribution of Fe2SiO4 in Figure 2, the amount of Fe2SiO4 decreases with the reduction in the silicon content.In Steels 2 and 3 with low silicon content, the penetration of Fe2SiO4 along the grain boundary is less because of a small amount of Fe2SiO4, leading to unnoticeable net-like Fe2SiO4.In The thickness of the upper layer in all three carbon steels is thinner compared to other layers.Some dispersive dark spots, confirmed to be Fe 3 O 4 as determined by EDS, can be observed in the middle layer (Figure 2a,c,e).In addition, small dark spots are dispersively distributed in the matrix near the scale, and, according to the EDS results, these spots are confirmed to be SiO 2 (Figure 2b,d,f).Fe 2 SiO 4 appears in the net-like form in Steel 1 containing higher silicon content (Figure 2b), whereas no obvious net-like Fe 2 SiO 4 is observed in Steels 2 and 3 with low Si content (Figure 2d,f).According to the distribution of Fe 2 SiO 4 in Figure 2, the amount of Fe 2 SiO 4 decreases with the reduction in the silicon content.In Steels 2 and 3 with low silicon content, the penetration of Fe 2 SiO 4 along the grain boundary is less because of a small amount of Fe 2 SiO 4 , leading to unnoticeable net-like Fe 2 SiO 4 .In Steel 1 with 1.21 wt.% silicon, the net-like Fe 2 SiO 4 can be easily observed in the innermost layer of the oxide scale close to the iron matrix.
EDS and XRD were applied to determine the phases in each layer of the oxide scale.Figure 3 shows the EDS results of the scale in Steel 1 and indicates that each layer of the scale not only contains Fe and O elements, but also includes Mn.Moreover, silicon is detected in the innermost layer of the scale (Figure 3c), indicating that a silicon-enriched phase forms in this layer.Table 2 provides the atomic percentage of the main elements of each layer of the scale in Steel 1.It can be seen that the oxygen content gradually decreases from the upper layer to the inner layer.Moreover, the content of the Mn element increases from outside to inside of the scale as a result of Mn diffusion [12].The single oxide of Mn is not detected in the oxide layers.Steel 1 with 1.21 wt.% silicon, the net-like Fe2SiO4 can be easily observed in the innermost layer of the oxide scale close to the iron matrix.EDS and XRD were applied to determine the phases in each layer of the oxide scale.Figure 3 shows the EDS results of the scale in Steel 1 and indicates that each layer of the scale not only contains Fe and O elements, but also includes Mn.Moreover, silicon is detected in the innermost layer of the scale (Figure 3c), indicating that a silicon-enriched phase forms in this layer.Table 2 provides the atomic percentage of the main elements of each layer of the scale in Steel 1.It can be seen that the oxygen content gradually decreases from the upper layer to the inner layer.Moreover, the content of the Mn element increases from outside to inside of the scale as a result of Mn diffusion [12].The single oxide of Mn is not detected in the oxide layers.The atomic ratios of the oxide in each layer can be calculated according to Table 2.The atomic ratios of Fe/O in the outermost and middle layers are approximately 2/3 and 1/1, respectively.The atomic ratio of Fe/Si/O in the inner layer is about 2/1/4.The corresponding XRD results of the scale in Steel 1 are shown in Figure 4, in which no silicon is detected because it is difficult to scrape the inner layer scale containing silicon from the matrix surface.The phase of each layer can be determined by combining the EDS and XRD results.The upper layer contains Fe 2 O 3 , and the middle layer consists of FeO, and a small amount of Fe 3 O 4 .According to the EDS results in Table 2, the inner layer is a composite of eutectic compounds Fe 2 SiO 4 /FeO.The dark area is mainly Fe 2 SiO 4 , while the bright area is FeO.The constitution of oxide layers is consistent with the results in the other studies [2,9].
Metals 2016, 6, 94 5 of 9 The atomic ratios of the oxide in each layer can be calculated according to Table 2.The atomic ratios of Fe/O in the outermost and middle layers are approximately 2/3 and 1/1, respectively.The atomic ratio of Fe/Si/O in the inner layer is about 2/1/4.The corresponding XRD results of the scale in Steel 1 are shown in Figure 4, in which no silicon is detected because it is difficult to scrape the inner layer scale containing silicon from the matrix surface.The phase of each layer can be determined by combining the EDS and XRD results.The upper layer contains Fe2O3, and the middle layer consists of FeO, and a small amount of Fe3O4.According to the EDS results in Table 2, the inner layer is a composite of eutectic compounds Fe2SiO4/FeO.The dark area is mainly Fe2SiO4, while the bright area is FeO.The constitution of oxide layers is consistent with the results in the other studies [2,9].

Penetrative Depth of Fe2SiO4 and the Total Thickness of the Scale
The Image-Pro plus 6.0, an image-processing software, was used to measure the areas of Fe2SiO4 in unit width.First, the total areas of Fe2SiO4 in inner layers were measured by the color aberration with software.Then, the total areas were divided by the width of measured images to obtain the areas of Fe2SiO4 in unit width.Several images were used to improve the accuracy of the Fe2SiO4 measurement.The measured area can represent the amount of Fe2SiO4, and the results are presented in Figure 5.It can be seen that the amount of the silicon-enriched phase Fe2SiO4 decreases with the reduction in the silicon content.The decreased amount of Fe2SiO4 weakens its anchor effect [10] to scale, which helps to prevent the red scale.Figure 6 shows the relationships between the silicon content, the penetrative depth of Fe2SiO4, and the total thickness of the scale.It is indicated from curve "a" that the penetrative depth of Fe2SiO4 increases with the silicon content.The Pilling-Bedworth ratio (PBR) of Fe oxide or Si oxide is more than 1 at a temperature of 1260 °C [13].PBR is the ratio of the oxide volume and the consumed metal volume.The PBR is greater than 1 because the volume of oxide is larger than that of the consumed metal, leading to a compressive stress in the oxide [14].In other words, during the

Penetrative Depth of Fe 2 SiO 4 and the Total Thickness of the Scale
The Image-Pro plus 6.0, an image-processing software, was used to measure the areas of Fe 2 SiO 4 in unit width.First, the total areas of Fe 2 SiO 4 in inner layers were measured by the color aberration with software.Then, the total areas were divided by the width of measured images to obtain the areas of Fe 2 SiO 4 in unit width.Several images were used to improve the accuracy of the Fe 2 SiO 4 measurement.The measured area can represent the amount of Fe 2 SiO 4 , and the results are presented in Figure 5.It can be seen that the amount of the silicon-enriched phase Fe 2 SiO 4 decreases with the reduction in the silicon content.The decreased amount of Fe 2 SiO 4 weakens its anchor effect [10] to scale, which helps to prevent the red scale.The atomic ratios of the oxide in each layer can be calculated according to Table 2.The atomic ratios of Fe/O in the outermost and middle layers are approximately 2/3 and 1/1, respectively.The atomic ratio of Fe/Si/O in the inner layer is about 2/1/4.The corresponding XRD results of the scale in Steel 1 are shown in Figure 4, in which no silicon is detected because it is difficult to scrape the inner layer scale containing silicon from the matrix surface.The phase of each layer can be determined by combining the EDS and XRD results.The upper layer contains Fe2O3, and the middle layer consists of FeO, and a small amount of Fe3O4.According to the EDS results in Table 2, the inner layer is a composite of eutectic compounds Fe2SiO4/FeO.The dark area is mainly Fe2SiO4, while the bright area is FeO.The constitution of oxide layers is consistent with the results in the other studies [2,9].

Penetrative Depth of Fe2SiO4 and the Total Thickness of the Scale
The Image-Pro plus 6.0, an image-processing software, was used to measure the areas of Fe2SiO4 in unit width.First, the total areas of Fe2SiO4 in inner layers were measured by the color aberration with software.Then, the total areas were divided by the width of measured images to obtain the areas of Fe2SiO4 in unit width.Several images were used to improve the accuracy of the Fe2SiO4 measurement.The measured area can represent the amount of Fe2SiO4, and the results are presented in Figure 5.It can be seen that the amount of the silicon-enriched phase Fe2SiO4 decreases with the reduction in the silicon content.The decreased amount of Fe2SiO4 weakens its anchor effect [10] to scale, which helps to prevent the red scale.Figure 6 shows the relationships between the silicon content, the penetrative depth of Fe2SiO4, and the total thickness of the scale.It is indicated from curve "a" that the penetrative depth of Fe2SiO4 increases with the silicon content.The Pilling-Bedworth ratio (PBR) of Fe oxide or Si oxide is more than 1 at a temperature of 1260 °C [13].PBR is the ratio of the oxide volume and the consumed metal volume.The PBR is greater than 1 because the volume of oxide is larger than that of the consumed metal, leading to a compressive stress in the oxide [14].In other words, during the Figure 6 shows the relationships between the silicon content, the penetrative depth of Fe 2 SiO 4, and the total thickness of the scale.It is indicated from curve "a" that the penetrative depth of Fe 2 SiO 4 increases with the silicon content.The Pilling-Bedworth ratio (PBR) of Fe oxide or Si oxide is more than 1 at a temperature of 1260 ˝C [13].PBR is the ratio of the oxide volume and the consumed metal volume.The PBR is greater than 1 because the volume of oxide is larger than that of the consumed Metals 2016, 6, 94 6 of 9 metal, leading to a compressive stress in the oxide [14].In other words, during the oxidation process, the oxidized part of the metal expands compared with the metal and the compressive stress is produced in the oxide scale.Moreover, the compressive stress at the layer/metal interface is larger than that at the outer position, leading to the pressure difference in different places of the scale.The pressure difference in the liquefied Fe 2 SiO 4 phase at a temperature of 1260 ˝C compels a part of Fe 2 SiO 4 to permeate into the inner scale.The liquid Fe 2 SiO 4 phase distributes along the FeO grain boundary and the net-like Fe 2 SiO 4 phase forms after its solidification.A larger compressive stress due to more Fe 2 SiO 4 in steels with a higher silicon content results in a deeper penetration layer.Furthermore, the above theory can also be used to explain the morphological change of Fe 2 SiO 4 with silicon contents.
the silicon content.However, Li et al. [15] claimed that the total thickness of the scale increases with the silicon content.They explained that the liquefied FeO/Fe2SiO4 provides fast diffusion passages for the ions at 1200 °C and it is an important factor for a sharp increase in the thickness of the scale with the silicon content.Similar results were reported by Mouayd et al. [11].The results of this study are different from theirs.This is because Fe2SiO4 has two opposite effects on the oxidation of steels, i.e., the solid Fe2SiO4 hinders oxidation and the liquid Fe2SiO4 promotes oxidation.In their experiments, the oxidizing atmosphere was pumped in at a temperature higher than the melting temperature of Fe2SiO4 (1173 °C).Therefore, only the promotion effect of Fe2SiO4 was presented and the total thickness of the scale increased with the Si content.However, the experimental procedures in their studies were not suitable for the industrial reheating scenario.In the present study, industrial experiments were conducted and the oxidizing atmosphere was pumped in from the beginning of the test.The solid-state Fe2SiO4 hindered the oxidation reaction at a lower temperature, whereas the liquefied Fe2SiO4 accelerated it when the temperature was higher than the melting temperature of Fe2SiO4 (1173 °C).However, the total thickness of the scale decreased with the increase of the silicon content.The decrease in the total thickness of the scale is attributed to the inhibition effects of Fe2SiO4.It indicates that the inhibition effect of Fe2SiO4 on the oxidation reaction plays a more important role during the entire oxidation reaction.Curve "b" in Figure 6 indicates that the total thickness of the scale decreases with the increase in the silicon content during industrial reheating test.The solid-state Fe 2 SiO 4 at the stage of low temperature below the melting temperature of Fe 2 SiO 4 (1173 ˝C) acts as an ion diffusion barrier to prevent further formation of iron oxide.The solid Fe 2 SiO 4 increases with the silicon content, thus the inhibition effect is enhanced and the total thickness of the scale decreases with the increase in the silicon content.However, Li et al. [15] claimed that the total thickness of the scale increases with the silicon content.They explained that the liquefied FeO/Fe 2 SiO 4 provides fast diffusion passages for the ions at 1200 ˝C and it is an important factor for a sharp increase in the thickness of the scale with the silicon content.Similar results were reported by Mouayd et al. [11].The results of this study are different from theirs.This is because Fe 2 SiO 4 has two opposite effects on the oxidation of steels, i.e., the solid Fe 2 SiO 4 hinders oxidation and the liquid Fe 2 SiO 4 promotes oxidation.In their experiments, the oxidizing atmosphere was pumped in at a temperature higher than the melting temperature of Fe 2 SiO 4 (1173 ˝C).Therefore, only the promotion effect of Fe 2 SiO 4 was presented and the total thickness of the scale increased with the Si content.However, the experimental procedures in their studies were not suitable for the industrial reheating scenario.In the present study, industrial experiments were conducted and the oxidizing atmosphere was pumped in from the beginning of the test.The solid-state Fe 2 SiO 4 hindered the oxidation reaction at a lower temperature, whereas the liquefied Fe 2 SiO 4 accelerated it when the temperature was higher than the melting temperature of Fe 2 SiO 4 (1173 ˝C).However, the total thickness of the scale decreased with the increase of the silicon content.The decrease in the total thickness of the scale is attributed to the inhibition effects of Fe 2 SiO 4 .It indicates that the inhibition effect of Fe 2 SiO 4 on the oxidation reaction plays a more important role during the entire oxidation reaction.

Distribution of Silicon
Line scanning and area scanning for Steel 1 with more silicon were applied to observe the distribution of silicon in the scale and the iron matrix near the scale.Figure 7 presents the results of the Si distribution in the scale by line scanning and area scanning.As shown in Figure 7b, silicon primarily concentrates at the inner scale.Meanwhile, as shown in Figure 7c, a sudden increase in the silicon content indicates that a silicon-enriched phase was formed in the inner scale.

Distribution of Silicon
Line scanning and area scanning for Steel 1 with more silicon were applied to observe the distribution of silicon in the scale and the iron matrix near the scale.Figure 7 presents the results of the Si distribution in the scale by line scanning and area scanning.As shown in Figure 7b, silicon primarily concentrates at the inner scale.Meanwhile, as shown in Figure 7c, a sudden increase in the silicon content indicates that a silicon-enriched phase was formed in the inner scale.The results of the Si distribution in the iron matrix near the scale are presented in Figure 8.According to the results of the energy spectrum in Figure 8c, combined with references [4,5], the dark spots may be classified as silicon dioxide.Figure 9 shows the schematic diagram that explains the formation of the granular silicon dioxide in the iron matrix near the interface.As shown in the diagram, for Si-containing steels, an outer iron oxide layer is initially formed under the oxidizing atmosphere.However, many cracks and holes may exist in the iron oxide thus formed, and these cracks and holes turn out to be the passage by which oxygen permeates into the iron matrix.When the concentration of oxygen is very high, the chemical reaction of Si and O2 takes place to form SiO2 in the iron matrix.A part of SiO2 combines with FeO to form Fe2SiO4, whereas others remain in the iron matrix.Therefore, silicon primarily concentrates in Fe2SiO4 in the inner scale and SiO2 particles in the iron matrix near the scale.The results of the Si distribution in the iron matrix near the scale are presented in Figure 8.According to the results of the energy spectrum in Figure 8c, combined with references [4,5], the dark spots may be classified as silicon dioxide.Figure 9 shows the schematic diagram that explains the formation of the granular silicon dioxide in the iron matrix near the interface.As shown in the diagram, for Si-containing steels, an outer iron oxide layer is initially formed under the oxidizing atmosphere.However, many cracks and holes may exist in the iron oxide thus formed, and these cracks and holes turn out to be the passage by which oxygen permeates into the iron matrix.When the concentration of oxygen is very high, the chemical reaction of Si and O 2 takes place to form SiO 2 in the iron matrix.A part of SiO 2 combines with FeO to form Fe 2 SiO 4 , whereas others remain in the iron matrix.Therefore, silicon primarily concentrates in Fe 2 SiO 4 in the inner scale and SiO 2 particles in the iron matrix near the scale.

Distribution of Silicon
Line scanning and area scanning for Steel 1 with more silicon were applied to observe the distribution of silicon in the scale and the iron matrix near the scale.Figure 7 presents the results of the Si distribution in the scale by line scanning and area scanning.As shown in Figure 7b, silicon primarily concentrates at the inner scale.Meanwhile, as shown in Figure 7c, a sudden increase in the silicon content indicates that a silicon-enriched phase was formed in the inner scale.The results of the Si distribution in the iron matrix near the scale are presented in Figure 8.According to the results of the energy spectrum in Figure 8c, combined with references [4,5], the dark spots may be classified as silicon dioxide.Figure 9 shows the schematic diagram that explains the formation of the granular silicon dioxide in the iron matrix near the interface.As shown in the diagram, for Si-containing steels, an outer iron oxide layer is initially formed under the oxidizing atmosphere.However, many cracks and holes may exist in the iron oxide thus formed, and these cracks and holes turn out to be the passage by which oxygen permeates into the iron matrix.When the concentration of oxygen is very high, the chemical reaction of Si and O2 takes place to form SiO2 in the iron matrix.A part of SiO2 combines with FeO to form Fe2SiO4, whereas others remain in the iron matrix.Therefore, silicon primarily concentrates in Fe2SiO4 in the inner scale and SiO2 particles in the iron matrix near the scale.

Conclusions
In this study, three low-carbon steels with different Si contents were selected, and reheating tests were conducted in an industrial furnace in a hot strip plant.The effect of the Si content on the morphology, amount, and distribution of Fe2SiO4 was quantitatively analyzed via backscattered electron detection (BSED), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD).The results show that Si distributes in two forms, i.e.Fe2SiO4, in the innermost layer of the oxide scale, and granular SiO2, dispersively distributed in the matrix near the scale.In addition, Fe2SiO4 appears in the net-like form in the innermost layer of the oxide scale close to the iron matrix when the Si content is 1.21 wt.%.However, no obvious net-like Fe2SiO4 is observed when the Si content is less than 0.25 wt.%.Moreover, the inhibition effect of the solid Fe2SiO4 on the oxidation reaction plays a more important role than the promotion effect of the liquid Fe2SiO4 during the entire oxidation reaction.Therefore, the total thickness of the scale decreases with the increase in Si content.

Conclusions
In this study, three low-carbon steels with different Si contents were selected, and reheating tests were conducted in an industrial furnace in a hot strip plant.The effect of the Si content on the morphology, amount, and distribution of Fe 2 SiO 4 was quantitatively analyzed via backscattered electron detection (BSED), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD).The results show that Si distributes in two forms, i.e.Fe 2 SiO 4 , in the innermost layer of the oxide scale, and granular SiO 2 , dispersively distributed in the matrix near the scale.In addition, Fe 2 SiO 4 appears in the net-like form in the innermost layer of the oxide scale close to the iron matrix when the Si content is 1.21 wt.%.However, no obvious net-like Fe 2 SiO 4 is observed when the Si content is less than 0.25 wt.%.Moreover, the inhibition effect of the solid Fe 2 SiO 4 on the oxidation reaction plays a more important role than the promotion effect of the liquid Fe 2 SiO 4 during the entire oxidation reaction.Therefore, the total thickness of the scale decreases with the increase in Si content.

Figure 2 .
Figure 2. The morphology micrographs of oxide scale on cross-sections of three steels.(a) The upper and middle layers and (b) the inner layer of Steel 1 (Si 1.21%); (c) the upper and middle layers and (d) the inner layer of Steel 2 (Si 0.25%); (e) the upper and middle layers and (f) the inner layer of Steel 3 (Si 0.09%).

Figure 2 .
Figure 2. The morphology micrographs of oxide scale on cross-sections of three steels.(a) The upper and middle layers and (b) the inner layer of Steel 1 (Si 1.21%); (c) the upper and middle layers and (d) the inner layer of Steel 2 (Si 0.25%); (e) the upper and middle layers and (f) the inner layer of Steel 3 (Si 0.09%).

Figure 5 .
Figure 5.The areas of Fe2SiO4 in three tested steels.

Figure 5 .
Figure 5.The areas of Fe2SiO4 in three tested steels.

Figure 5 .
Figure 5.The areas of Fe 2 SiO 4 in three tested steels.

Figure 6 .
Figure 6.The penetrative depth of fayalite (Fe2SiO4) and total scale thickness in three different silicon-content steels.

Figure 6 .
Figure 6.The penetrative depth of fayalite (Fe 2 SiO 4 ) and total scale thickness in three different silicon-content steels.

Figure 7 .
Figure 7.The results of Si distribution in inner scale by line scanning and area scanning.(a) The morphology micrographs of oxide scale; (b) the results of area scanning; (c) the results of line scanning.

Figure 8 .
Figure 8.The results of Si distribution in iron matrix near scale by area scanning and energy spectrum.(a) The distribution of Si-containing scale and particles; (b) the results of area scanning; (c) the results of EDS.

Figure 7 .
Figure 7.The results of Si distribution in inner scale by line scanning and area scanning.(a) The morphology micrographs of oxide scale; (b) the results of area scanning; (c) the results of line scanning.

Figure 7 .
Figure 7.The results of Si distribution in inner scale by line scanning and area scanning.(a) The morphology micrographs of oxide scale; (b) the results of area scanning; (c) the results of line scanning.

Figure 8 .
Figure 8.The results of Si distribution in iron matrix near scale by area scanning and energy spectrum.(a) The distribution of Si-containing scale and particles; (b) the results of area scanning; (c) the results of EDS.

Figure 8 .
Figure 8.The results of Si distribution in iron matrix near scale by area scanning and energy spectrum.(a) The distribution of Si-containing scale and particles; (b) the results of area scanning; (c) the results of EDS.

Figure 9 .
Figure 9.The schematic diagram of the formation of granular silicon dioxide in iron matrix.

Figure 9 .
Figure 9.The schematic diagram of the formation of granular silicon dioxide in iron matrix.

Table 2 .
The main atomic percentage of each layer in sample Steel 1 (atom %).

Table 2 .
The main atomic percentage of each layer in sample Steel 1 (atom %).