Effects of Si Content on the Growth of Oxide Layers in Carbon Steels during the Heating Process

Abstract

A prevalent metal surface defect is hot-rolled iron oxide; thus, it is critical to regulate the production and growth of oxidized iron during the hot-rolling process. To analyze the influence of Si content on the growth laws of the oxidized layer in carbon steel during heating, three types of carbon steel with significant differences in Si content were selected for research on the growth laws of the oxidized layer at different heating temperatures. The production law and micromorphology of the oxidized layer were analyzed using methods such as scanning electron microscopy and thermodynamic phase diagram calculation, and an oxidation dynamic model was obtained. The predicted control values of the model are highly consistent with the measured values. This study reveals that the heating temperature significantly impacts the thickness of the oxidized layer of carbon steel. At temperatures below 500 °C, the oxidation is not evident, and the layer is thin. Between 500 °C and 900 °C, the steel’s composition affects the thickness. Carbon steels with high Si content form a dense iron olivine layer, which slows down the oxidation rate. However, heating temperatures above 900 °C cause the protective oxidized film to reach the melting point of iron olivine, increasing the oxidation rate. At 1200 °C, the oxidized layers of the three types of carbon steel remain consistent. This paper’s research findings offer theoretical guidance for large-scale industrial production practices and serve as a reference for similar studies on steel oxidation behavior.

1. Introduction

Hot-rolled oxidized iron sheet is a common metal surface defect that not only reduces the mechanical properties of the material but also can cause corrosion and other quality issues. Therefore, it is crucial to control the generation and development of hot-rolled oxidized iron in the production process. According to the different temperature stages and process environments generated, it can be divided into [1,2,3]: primary oxidized iron sheet (heating furnace-generated oxidized iron sheet, generally 1350 °C–1150 °C), secondary oxidized iron sheet (usually between rough rolling and finishing rolling in the two-stage rolling process, generally above 950 °C) [4], and tertiary oxidized iron sheet (between 950 °C–350 °C). According to the chemical composition and Fe-O co-precipitation transformation law, a hot-rolled oxidized iron sheet can be Fe₂O₃ (red, powdery), Fe₃O₄ (blackish-brown, scale-like), or FeO (blackish-brown, granular) [5,6]. At present, the main control technologies for hot-rolled oxidized iron sheets include [7,8,9]: (1) physical method: This method mainly separates the iron in hot-rolled oxidized iron sheets through physical processes. Common physical methods include high-pressure water descaling, brush roll descaling, sandblasting/shot peening descaling, etc. The advantage of this method is its low processing cost, but the disadvantage is that the effect of external forces on treatment results is large, and there is only a small amount of oxidized iron sheet at the interface. (2) chemical reduction method, which is a commonly used hot-rolled oxidized iron sheet treatment method, mainly through chemical reactions to reduce the iron oxide in the oxidized iron sheet back to iron. The advantage of this method is a good treatment effect, but the disadvantage is that it requires a large consumption of energy and chemicals, and common ones include the H₂ reduction method. The disadvantage is that the maintenance of the reduced atmosphere requires additional expensive equipment, with high investment costs, safety hazards, and environmental hazards. (3) Thermal melting method: This is a new hot-rolled oxidized iron sheet treatment method that uses laser, plasma, or an electric arc to heat and melt the surface of the oxidized iron sheet at high temperatures. The advantages of this method are high efficiency and digital precision control, but the disadvantage is that the equipment investment cost is high.

At the same time, alloy elements and their content have a great influence on oxidized iron sheets. Alloy elements react with oxygen in the oxidized iron sheet to form alloy compounds, thus affecting the composition of the oxidized iron sheet structure and the rate of surface oxidation of the steel [10]. During high-temperature oxidation, Cr and Si elements will accumulate at the interface between the iron sheet and the base metal [5], reacting with iron and oxygen ions to generate Fe³⁺ and Cr³⁺, forming (Fe, Cr)₂O₃, and Fe²⁺ and Fe³⁺, Cr³⁺ and Cr²⁺, forming the mixture (Fe, Cr)₃O₄ (Fe₂O₃ + FeO, Cr₂O₃ + CrO) [11,12]. These oxides can slow down the diffusion of iron oxygen ions, improve the anti-oxidation of steel containing Cr and Si, and enhance the adhesion of oxidized iron [1]. Silicon content has a significant impact on the surface quality of hot-rolled steel plates. Si’s influence on hot-rolled steel plate-oxidized iron sheets is mainly manifested in the following aspects: (1) Formation of red oxidized iron sheets: when the Si content in the steel matrix exceeds 0.5%, the outermost layer of oxidized iron sheets formed during hot rolling usually appears red. This is because at high temperatures, Si will react with oxygen to produce SiO₂, which then reacts with Fe₂O₃ to form red Fe₂SiO₄ spinel. This red-oxidized iron sheet will affect the surface quality of the strip, reducing its decorative and coating properties. (2) Influence on peelability of oxidized iron sheets: when Si content is low, large pores will be formed in the oxidized iron sheets. During air cooling, thermal stress will cause cracks to form in the iron sheets, which will extend to the interface between the oxidized layer and the base metal, resulting in overall peeling during descaling. This kind of peeling phenomenon will not only affect the surface quality of the steel plate but also increase the difficulty and cost of subsequent treatment. (3) Influence on oxidation mechanism: as the Si content in steel increases, the steel’s oxidation mechanism will change from internal oxidation to external oxidation. This is because Si can promote Fe’s outward diffusion, thereby accelerating the steel’s oxidation rate. In addition, Si can also change the structure and properties of oxides, further affecting the oxidation process. (4) Influence on structure and adhesion of oxidized iron sheets: during heating, Si will first be oxidized into SiO₂, which then reacts with FeO to further form Fe₂SiO₄ spinel. With increasing Si content, Fe₂SiO₄ in the oxidized iron sheets will also increase, thereby improving the adhesion and structure of the oxidized iron sheets. This improvement in adhesion can reduce the peeling and falling off of oxidized iron sheets, improving the service life and safety of steel plates. Yu et al. [13] found that reducing heating temperature and Si content can improve the surface quality of automotive steel containing Si. Cao et al. [14] studied the thickness and morphology characteristics of silicon steel under high-temperature oxidation with Si content ranging from 0.75% to 2.2%. The results showed that when the oxidation temperature was lower than the melting point of Fe₂SiO₄, increasing Si content could improve the anti-oxidation performance of silicon steel, while when the temperature was higher than the melting point of Fe₂SiO₄, increasing Si content would actually increase the oxidation rate of silicon steel. Yang et al. [15] found that when the heating temperature was 700 °C~900 °C, the constant rate of silicon steel’s oxidation decreased with increasing Si content.

It can be seen that alloy elements in steel and heating temperature will have an important influence on the surface layer of steel plates. Therefore, previous scholars have carried out various studies. Sun et al. [16] studied the high-temperature oxidation rate of silicon steel and found that during the initial stages of oxidation, linear law fitted well with experimental steel grades. However, when the critical thickness was reached by the generated layers, the growth law changed towards a parabolic law, and the silicon element content in steel correlated positively with the critical thickness value for the generated layers. Cao et al. [17] conducted experiments on silicon steel under dry air and steam conditions to study its oxidative layer thickness and morphology. They found that the steel’s oxidation rate increased when the atmosphere was steam, and a silicon-rich layer formed at the interface between the oxidative layer and the base metal. Suarez L. et al. [18] found defects in silicon steel plates during hot rolling due to molten-state ferrihydrite formation, impacting surface quality and subsequent processing effects. To sum up, current work focusing on high-temperature oxidative behaviors for silicon steel plates is relatively abundant; however, there are few reports about how different silicon contents affect growth laws for oxidative layers during the heating process of carbon steel plates. Therefore, this paper aims to select silicon steel plates with significantly different silicon contents to study the influence of the rules of the silicon element on thickness and structural morphology for oxidative layers during the heating process to provide the experimental basis for the establishment of the oxidative layer removal process and improvements to the heating system in actual production.

2. Experimental Materials and Methods

This research materials selected for the experiment are silicon-containing steel plates with silicon contents of 0.04%, 0.9%, and 3.2%, respectively. There is a significant gradient in the silicon content among the three steel plates. The chemical compositions of the three materials are shown in

Table 1. Chemical composition of test steel (wt.%).

Sample	C	Si	Mn	P	S	Fe
0.04Si	0.07	0.04	0.27	0.0091	0.0027	Bar.
0.90Si	0.07	0.90	0.28	0.0214	0.0085	Bar.
3.28Si	0.08	3.28	0.29	0.0106	0.0055	Bar.

. The test steel plates were cut along the rolling direction after removing the surface oxide layer using an acid-free descaling device and then ultrasonically cleaned before being placed in a vacuum drying oven for preservation. Figure 1 shows the scanning electron microscope (JSM-7800F, JEOL) SEM and EDS analysis diagram after the surface oxide layer was removed. In the scanned area, the oxygen atom accounts for about 1.61% and the iron atom accounts for about 97.73%.

(1) Research on the transformation law of iron oxide and its compounds: Using the Phase Diagram module of FactSage Education for thermodynamic calculations and analysis, specifically Firstly, setting up the binary element Fe-O, calculating the phase diagram at 0–1200 °C under one standard atmosphere, and analyzing the transformation laws of FeO, Fe₂O₃, and Fe₃O₄. Secondly, setting up the ternary element Fe-Si-O, calculating the ternary phase diagram at 900 °C and 1200 °C under one standard atmosphere, and analyzing the transformation law of Fe-Si-O compounds to explore the influence of Si and O content on the transformation law of ternary compounds.

(2) Study the oxidation kinetics of iron oxide. The experiment was carried out in a controlled-resistance heating furnace (HT1350 type, Longkou Electric Furnace Factory). Before the experiment, the furnace was preheated to ensure a stable atmosphere inside the furnace. Under the same experimental atmosphere, the effects of 14 different heating temperatures of 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, 950 °C, 1000 °C, 1050 °C, 1100 °C, and 1200 °C on the production of iron oxide were studied. Modern testing methods, such as optical microscopy and SEM, were used to analyze the morphology and microstructure content of iron oxide at different temperatures. The effects of heating temperature, heating time, furnace gas composition, steel chemical composition, and other factors on the increase in oxidation weight were analyzed.

3. Results and Discussion

3.1. Fe-O Transformation Laws

The color and smoothness of the oxidation layer on test steel plates vary significantly at different heating temperatures [19]. When the heating temperature is below 400 °C, the oxides on the surface of all three test steel plates are blue, very dense, and smooth. As the heating temperature rises above 400 °C, the color of the oxidation layer on the test steel plate gradually changes from blue to light red, and the surface remains relatively smooth. However, when the heating temperature exceeds 900 °C, the surface oxidation layer gradually turns gray and black, and the flatness of the surface deteriorates with increasing heating temperature. When the heating temperature is above 1100 °C, there are obvious bumps and bubbles on the surface of the test steel plate, and it tends to flake off upon cooling. Figure 2 shows photos of the color and smoothness of the surface oxidation layer of 0.9Si steel at 400 °C and 1100 °C.

The Fe oxidation layer is composed of FeO, Fe₃O₄, and Fe₂O₃ from the inside out. The coefficient of thermal expansion between Fe and FeO is 0.84, and the coefficient of thermal expansion between FeO and Fe₃O₄ is also 0.84, indicating good bonding. However, the coefficient of thermal expansion between Fe₃O₄ and Fe₂O₃ is greater than that between Fe and FeO and FeO and Fe₃O₄, and the higher the temperature, the greater the difference in expansion rate [13]. When the expansion difference reaches a certain value, the outermost Fe₂O₃ layer begins to deviate partially from the Fe₃O₄ layer, but it does not fall off, resulting in bumps and bubbles. When the expansion difference further increases, large internal stresses will be generated, causing the Fe₂O₃ layer to separate and crack from the Fe₃O₄ layer, leading to rapid deterioration in the surface smoothness of the steel plate.

The transformation of iron oxide scale (Fe-O) during the formation of oxidized scale on high-strength steel surfaces is shown in Figure 3. Calculations and analyses have found that the degree of deformation of the oxidized scale is influenced by the amount of deformation of the base material. When the amount of deformation of the base material is small, the oxidized scale can undergo plastic deformation together with the steel substrate without cracking [13,14]. In addition, changes in oxygen content can also affect the behavior of the iron oxide scale. When the oxygen content is lower than that in air, no eutectic structure will be formed in the produced iron oxide scale. However, when the oxygen content is close to or higher than that in air, a FeO eutectic reaction will occur on the iron oxide scale. Isothermal treatment at different temperatures can cause structural changes in the iron oxide scale. For example, at temperatures of 600 °C and 550 °C, the iron oxide scale has a typical two-layer structure of FeO and Fe₃O₄. As the temperature decreases below 450 °C, large blocks of primary eutectic tissue Fe₃O₄ appear in the FeO layer, and some secondary eutectic tissue appears.

3.2. Fe-Si-O Phase Diagram and Transformation Laws

The transformation law of iron, silicon, and oxygen during hot rolling is shown in Figure 4. The iron-silicon-oxygen phase diagram is a ternary phase diagram, which reflects the interaction and phase transformation rules between iron, silicon, and oxygen under different temperature and pressure conditions. Each point on the phase diagram represents a specific chemical composition, and the position of the point indicates the phase state of that composition under the corresponding conditions. In the phase diagram, the interaction between iron, silicon, and oxygen forms various phases, including iron phases (α-Fe, γ-Fe), iron oxide phases (Fe₃O₄, Fe₂O₃, FeO), iron silicate phases (FeSiO₃, Fe₂SiO₄), and silicon phases (Si) [20]. The transformation law of the iron-silicon-oxygen system mainly includes the oxidation-reduction reaction of iron oxides, the oxidation reaction of iron, and the reaction between silicon and iron oxides. At high temperatures, Fe₃O₄ will decompose into FeO and O₂, which is an oxidation-reduction reaction. As the temperature increases, the equilibrium of this reaction moves to the right, producing more FeO and O₂. On the surface of iron, oxygen can react with iron to form iron oxide, which is an oxidation reaction. As the temperature increases, the rate of this reaction increases, producing more iron oxide. Silicon can react with iron oxide to form iron silicate, which is a reduction reaction. As the temperature increases, the rate of this reaction also increases, producing more iron silicate. When the experimental steel is heated to 900 °C–1200 °C, the total number of chemically combined phases produced is the same, and the temperature has little effect on the type and production conditions of phase transformation. However, due to the continuous diffusion of high-temperature oxidation [O] into the steel matrix caused by high-temperature retention and high-temperature oxidation, the oxygen enrichment section of the phase diagram develops, producing large amounts of Fe₂O₃, Fe₃O₄, Fe₂SiO₄, and other compounds. This is because in the hot rolling process, the interaction transformation laws between Fe-Si-O, directly affect the properties of the scale. These phases have different stable regions at different temperatures and pressures, forming complex phase transformation laws. During hot rolling, the transformation law of scale is mainly affected by temperature, pressure, and silicon content. As the temperature increases, the rate of oxidation-reduction reaction of iron oxide increases, producing more FeO and O₂, while the rate of oxidation reaction of iron also increases, producing more iron oxide. Therefore, high temperatures are conducive to the transformation of scale. Increasing pressure can promote the oxidation-reduction reaction of iron oxide, producing more FeO and O₂, but has little effect on the oxidation reaction of iron. Silicon can react with iron oxide to form iron silicate, so the content of silicon will directly affect the transformation law of scale. With increasing silicon content, the amount of iron silicate produced increases, and the transformation of scale is more thorough.

3.3. Effect of Si Content on Oxidation Behavior

The average thickness of the oxidation layer of 0.04Si steel, 0.9Si steel, and 3.2Si steel at a heating temperature of 400 °C was shown in Figure 5. There were slight differences in the oxidation layer thickness of the tested steels at 400 °C, but the overall change was not significant. The reason for this is that at this temperature, the energy fluctuation of Fe atoms in the steel is not high, making it difficult for them to migrate outward, and the diffusion of the oxidizing atmosphere into the steel is not obvious either. As a result, the reaction rate is slow, and the macroscopic appearance shows a relatively thin oxidation layer.

Further analysis of the oxidation layer morphology of the tested steels when heated to 800 °C showed that the oxidation layer thickness of both 0.04Si steel and 0.9Si steel had a slight increase between 600 °C and 800 °C, while there was almost no change in the thickness of the 3.2Si steel in this temperature range. The oxidation layer of 3.2Si steel was the thinnest, followed by 0.9Si steel, while that of 0.04Si steel was the thickest, as shown in Figure 6. It is believed that when the tested steels are heated above 600 °C due to the high temperature, the outward diffusion rate of Fe atoms in both 0.04Si steel and 0.9Si steel increases, and the chemical energy of the oxidizing atmosphere increases, promoting their reaction and accelerating the oxidation of the steel, resulting in a significant increase in the thickness of the oxidation layer. At 600 °C, a certain thickness of iron olivine and alumina layers formed in 0.9Si steel hindered part of the diffusion of Fe atoms, resulting in a thinner oxidation layer compared to that of 0.04Si steel. In contrast, due to its higher Si content, at 600 °C, more compact iron olivine layers formed on the surface of 3.2Si steel, reducing the diffusion rate of Fe atoms to the FeO, Fe₃O₄, and Fe₂O₃ layers and slowing down the growth of Fe₃O₄ and Fe₂O₃, resulting in almost no increase in the thickness of the oxidation layer.

After analyzing the growth law of oxidation layers in three test steel plates with significantly different Si content, it was found that when the Si content is greater than 0.2%, at temperatures below 1173 °C, iron olivine (Fe₂SiO₄) with anchor-like morphology is formed on the surface of the steel, firmly pinning the oxidation layer to the substrate surface and improving the adhesion of the oxidation layer itself [13]. At temperatures above 1173 °C, due to exceeding the melting point of iron olivine, molten Fe₂SiO₄ and FeO envelop each other to form an eutectic product of FeO/Fe₂SiO₄. There are large pores in FeO and FeO/Fe₂SiO₄ eutectic products, and the conversion of the FeO/Fe₂SiO₄ eutectic product into a molten state destroys the protective oxide film formed by the substrate, making the adhesion between the oxidation layer and the base metal weaker and causing the oxidation layer to peel off easily. For porous, loose, and unevenly distributed 3.2Si steel, at 1200 °C, the melting of Fe₂SiO₄ reduces the adhesion between the substrate and the oxidation layer, resulting in holes, cracks, or even separation between the substrate and the oxidation layer. It should not be overlooked that due to heating exceeding the phase transformation temperature of the test steel, metal will undergo phase transformation, and the stress caused by phase transformation tissue can act on the oxidation layer. During cooling, due to differences in properties between the substrate and the oxidation layer, different volumetric shrinkage occurs in various parts, resulting in stress between the oxidation layer and the substrate [21,22]. Therefore, the stress caused by tissue during heating and cooling processes makes cracks appear at the interface between the substrate and the oxidation layer or even causes it to peel off. At this time, due to improper force or pressing during polishing or inlaying of the sample, cracks may also occur in the oxidation layer or even peel off [18,19].

3.4. Kinetic Analysis of Si Oxidation Behavior

The kinetics of Si oxidation behavior were analyzed through the observation of the oxidized layer on the steel plates’ cross-section using a metallographic microscope. Since the thickness of some oxidized layers was uneven, five points were measured within the analysis area, and their average value was taken. Some samples did not obtain valid data at certain temperature points, and to ensure the consistency of the test, these samples were not subjected to a secondary test. The average thickness of the oxidized layer on the tested steel plates increased with the increase in temperature. When the heating temperature was below 500 °C, the degree of oxidation of the steel was not obvious, and the thickness of the oxidized layer was extremely thin. Between 500 °C and 900 °C, the thickness of the oxidized layer began to increase. When the temperature exceeded 900 °C, the thickness of the oxidized layer increased sharply. When the temperature was higher than 900 °C, steel with a higher Si content had a thinner oxidized layer than steel with a lower Si content. At 1200 °C, the thickness of the oxidized layer on the three types of steel was similar. The average thickening curve of the oxidized layer on the tested steel plates at different temperatures is shown in Figure 7.

High-temperature heating promotes the diffusion reaction of Fe and O atoms, i.e., Fe from the steel plate matrix and O from the air, which generates an oxide layer at high temperatures, with the reaction process described as Fe + O → FeO/Fe₂O₃/Fe₃O₄. This creates conditions for the formation of the oxidized layer [13]. From the steel matrix to the steel surface, the oxygen content gradually increases while the iron content gradually decreases. Based on the Fe/O atomic ratio, the iron oxides formed are FeO, Fe₃O₄, and Fe₂O₃ in order. At the same time, the silica-rich phase Fe₂SiO₄ is formed at the interface between the matrix and the iron oxide. For steel with a higher Si content, after reaching a certain temperature and starting to oxidize, due to the stronger affinity between Si and O (stability of oxide), compared to Fe, Si is more easily oxidized and generates SiO₂ preferentially on the substrate. The subsequent formation of FeO reacts with Si or Si oxide on the surface of the steel matrix to form a dense iron olivine layer, which hinders the outward diffusion of Fe atoms and slows down the growth rate of FeO and Fe₃O₄. Even if the oxidizing atmosphere enters the substrate through the iron olivine layer, it will preferentially react with Si to generate SiO₂, thereby reducing the speed of oxidation of the steel plate. At temperatures above 1173 °C, exceeding the melting point of iron olivine, it causes iron olivine to melt into a liquid state, thus destroying the protective oxide film formed by the substrate, causing an increase in the migration rate of Fe atoms, increasing the oxidation rate, and greatly promoting the oxidation process of the steel plate [9]. Figure 8 shows a comparison between the fitted dynamic model curve of oxidation and the measured values.

The kinetic model of the oxidized layer thickness of the three tested steel plates and the heating temperature was obtained through regression analysis [23].

Table 2. Parameters of the oxidation dynamic model.

Sample	A1	A2	A3	A4
0.04Si	23.26300	2,440,894.75392	2291.66558	0.00347
0.90Si	33.16924	5,229,486.46421	2809.67759	0.00254
3.28Si	30.69415	8,305,158.01963	3344.35849	0.00197

shows the values of the fitting model parameters.

$$y=A1+\frac{A2-A1}{1+10^{A4(A3-x)}}$$

(1)

where y is the thickness of the oxidized iron sheet, in μm; x represents the oxidation temperature, in °C; and A1, A2, A3, and A4 are model coefficients.

After analyzing the measured and fitted values of the average thickness of the oxidized layer, it was found that the thickness growth curves of the oxidized layers of the three tested steel grades all showed a parabolic growth law [14]. The overall trend of 0.04Si steel was more obvious than that of 0.9Si steel and 3.2Si steel, and the fitted values were most consistent with the measured values. The trends of 0.9Si steel and 3.2Si steel were roughly similar, and the consistency between the fitted values and the measured values was relatively good, indicating that the model had high robustness and accuracy and could be used for predicting the oxidation behavior of this type of steel during production.

4. Conclusions

In order to study the effect of Si content on the growth law of the oxidized layer during the heating process of carbon steel, the oxidation behavior of carbon steel with different Si content at different temperatures was studied, and the influence of Si content and heating temperature on the growth law of the oxidized layer of carbon steel was revealed. The main conclusions are as follows:

(1)

By selecting three steel grades with large differences in silicon content, the influence of silicon content on the growth law of the oxidized layer during the heating process of carbon steel was analyzed. When the heating temperature is below 500 °C, the oxidation degree of the steel is not obvious, and the oxidized layer is extremely thin. When the temperature is between 500 °C and 900 °C, the thickness of the oxidized layer begins to increase, and when it exceeds 900 °C, the thickness of the oxidized layer increases sharply. At 1200 °C, the thicknesses of the oxidized layers of the three steels are similar.

(2)

The kinetic model of the thickness of the oxidized layer of the three tested steel plates and the heating temperature were obtained, and the growth curve of the thickness of the oxidized layer showed a parabolic growth law. The fitting value of 0.04Si steel is most consistent with the measured value, followed by 0.9Si steel and 3.2Si steel.

(3)

For steel with a higher silicon content, the iron olivine layer formed on the surface can effectively prevent the reaction between Fe and O, slowing down the increase in thickness of the oxidized layer. When the heating temperature exceeds the melting point of iron olivine, the inhibition effect of iron olivine decreases, and the thickness of the oxidized layer increases rapidly.

(4)

The morphology of iron olivine, phase transformation stress during heating, cooling shrinkage, and preparation methods will cause cracks in the oxidized layer, or even peeling off.