Influence of a Cr2O3 glass coating on enhancing the oxidation resistance of 20MnSiNb structural steel

Surface & Coatings Technology 294 (2016) 8–14

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Influence of a Cr2O3 glass coating on enhancing the oxidation resistance of 20MnSiNb structural steel GuoYan Fu a,b, LianQi Wei a,⁎, Xin Shan a,b, XiaoMeng Zhang a, Jian Ding a,b, CuiCui Lv a,b, Ya Liu a,b, ShuFeng Ye a a b

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, PO Box 353, Beijing 100190, China University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 16 June 2015 Revised 11 March 2016 Accepted in revised form 17 March 2016 Available online 19 March 2016 Keywords: 20MnSiNb structural steel Glass coating Oxidation resistance Chromium compound film Silicate film

a b s t r a c t In this paper, a glass protective coating containing chromium oxide deposited on a high-quality 20MnSiNb structural steel was prepared by a slurry-spraying technique at high temperature. Compared with a bare sample, the effect of the coating on the steel was investigated at a temperature range from 700 °C to 1150 °C in the air. The experimental results showed the coating had significant oxidation resistance and effective descaling ability for 20MnSiNb structure steel. The coating could enhance protective effect by 86% after heating for 60 min at 1050 °C compared with the bare sample. The formation of a dense chromium and silicon compound film on the 20MnSiNb structural steel prevented the diffusion of oxygen and iron ions. Due to the difference of the thermal expansion coefficient between the coating and the steel, the coating spalled during the cooling process, which enhanced the descaling ability significantly. The sample was characterized by XRD, SEM-EDS and TGDTA/DSC to clarify the possible protective mechanism, which was the synergy between the glass coating and the protective film formed by the migration of chromium, preventing the diffusion of oxygen and iron ions. Because of the low cost and easy handling method, the coating has potentially applicable for reheating process of 20MnSiNb structural steel. © 2016 Published by Elsevier B.V.

1. Introduction As the biggest consumer steel products in China, rebar accounts for one-fifth of the total steel production and plays an important role in the national economic development [1]. The consumption of the 20MnSiNb structure steel can reach up to 100 million t per year in China. The steel slab of 20MnSiNb is heated below 1100 °C before hot rolling. However, the oxidation of steel is very serious during the heating operation. Therefore, the high temperature treatment leads to oxidation loss of steel and waste of energy (steelworks' gases, the cost to remove the scale and the recycling of the scale) [2]. Meanwhile, the oxidation loss produced in various heat treatments of steel occupied 4%–6% of the total crude steel production in the world [3–5]. Due to mechanical collision, the scales drop to the furnace bottom and corrode the refractories of the furnace during the heat treatment, which will reduce the service life of the furnace lining and cause serious waste [6–8]. Another key factor for the coating is the descaling ability, which is the ability of the scale to detach from the substrate and influences the eventual surface quality of the steel product. In some case, the oxidation of the steel during heating treatment will cause serious economic losses. Therefore, it is very important to solve the oxidation problem and many different methods have been adopted. The most effective ⁎ Corresponding author. E-mail address: [email protected] (L. Wei).

http://dx.doi.org/10.1016/j.surfcoat.2016.03.053 0257-8972/© 2016 Published by Elsevier B.V.

countermeasure for the reduction of the oxidation is to prevent the access of oxidants to the steel slab surface by controlling the atmosphere on the surface or by applying a protective coating onto the surface. Vacuum or an inert gas atmosphere is a good mean to protect steel from oxidation during a reheating process [9–11]. However, the high cost and complex operations limit their application. Coating is considered to be a convenient way to protect some kinds of steels for a long time. For example, the slurry coating, which is an inorganic high temperature oxidation coating, has a low cost and is convenient to be used in practice [9, 12]. Therefore, a lot of works have been done on the coating protection for carbon steel oxidation in recent years [13–15]. The ceramic coating (ceramic based coating) (Al2O3 [16–18], MnO [19], MgO [12], etc.) had a high melting point which could form a ceramic protective layer on the substrate at a high temperature range of 1100–1300 °C to get a good anti-oxidation ability. However, the temperature of the heat rolling treatment for the 20MnSiNb structural steel was b1100 °C. So the ceramic coating was not suitable for the heat treatment of the 20MnSiNb structural steel. The glass coatings were based on silicate, phosphate and borate, which had relative soft point (b800 °C) [9,20–26]. It was well known that glasses could easily form hermetic seals on the surface of the steel substrates and insulated the steel substrates from oxidation [27,28]. Therefore, the glass coating was more suitable than the ceramic coating for the 20MnSiNb structural steel. In this paper, a new glass coating was developed to prevent the oxidation of the 20MnSiNb structural steel and the anti-oxidation ability

G. Fu et al. / Surface & Coatings Technology 294 (2016) 8–14

was evaluated. Finally, the possible protection mechanism of the coating was also investigated. 2. Experiment procedure

9

Table 2 Chemical composition of the glass coating. Component

SiO2

Al2O3

CaO

Fe2O3

K2O

Na2O

TiO2

Cr2O3

Content, wt.%

68.30

9.46

0.49

3.81

3.70

6.63

0.25

6.41

2.1. Preparation of steel sample 20MnSiNb rebar samples in a cube shape with a dimension of 10 mm × 10 mm × 10 mm were prepared. The samples were polished by abrading with SiC papers from 200# grit to 800# grit. Then, the samples were cleaned and rinsed through an ultrasonic treatment and dried. The chemical composition of the samples was shown in the Table 1.

oxidation without scale. αcoated and αbare were the yields of the coated and bare sample after high temperature treatment. The kinetic of oxidation was also investigated by the correlation between reaction rate and temperature. The samples were heated from room temperature to 1100 °C. The weight loss of the sample, which was heated at certain temperature (900 °C, 950 °C, 1000 °C, 1050 °C and 1100 °C) for 5 min, was adopted. The reaction rate (v) was analyzed as in Eq. (4).

2.2. Preparation and application of coating slurry Table 2 showed the composition of the glass coating used in this study, which mainly consisted of metasilicate, chromium oxides and sodium silicate binder. First, 25 wt.% sodium silicate binder was prepared. Then 10 g ± 0.01 g of the metasilicate, 1 g ± 0.01 g of chromium oxides and 4 g ± 0.01 g of sodium silicate binder were transferred into the 100 ml beaker. Then adding a certain amount of water into the beaker to make sure the solid to liquid ratio was 1:3. The mixtures of the coating were ground at 110 r/min for 6 h by a ball mill until the particle size reached above 400# grit. The base material of ball and jar of the mill was ZrO2, the diameters of the balls were 6 mm and 10 mm. Meanwhile the quantities of the balls were 400 and 200 cases respectively. Because of the addition of chromium oxides, the color of the coating was light green. The coating slurry could be applied on the steel by low pressure spray method. The type of the spraying gun was w-77c, which was connected to an air compressor with 8 bar rated pressure. The thickness of the coating was about 0.2 mm after the spraying technology. 2.3. Evaluation of anti-oxidation ability and characterization The anti-oxidation ability was evaluated by the weight changes of 20MnSiNb samples which were heated in a muffle furnace. The samples were heated from room temperature to different temperatures, such as 800 °C, 900 °C, 1000 °C, 1050 °C, 1100 °C and 1150 °C, and maintained for 60 min. Meanwhile, the constant oxidation tests were taken at 1000 °C, 1050 °C and 1100 °C respectively, with different duration from 10 min to 120 min. Then, the coating and scales of the samples were removed by hot compression treatment [12]. Then, we did some artificial processing by the hydraulic machine and hammer for the above treated sample to ensure the samples were handled well. The steel loss of the sample due to oxidation was weighed by a balance with an accuracy of 0.01 g. The anti-oxidation ability, E, was related with the oxidation loss, as analyzed in Eq. (2). To make the results more accurate, four samples were taken in each test, one was bare and the other three samples were coated with a coating. The average value of the three samples was adopted as the anti-oxidation effect. m2  100% Steel yield α ¼ m1 Anti‐oxidation effect E ¼

ð1Þ

αcoated −αbare  100% 1−αbare

ð2Þ

where, m1 and m2 were the weights of the samples before and after Table 1 Chemical composition of 20MnSiNb rebar sample. Component

C

Si

Mn

S

P

Nb

Fe

Content, wt.%

0.24

0.61

1.43

≤0.030

≤0.030

0.033

Balance

Weight loss unit area Δm ¼ M1 −M2

Reaction rate v ¼

Δm Δt

ð3Þ

ð4Þ

where, M1 and M2 were the weights loss unit area of the samples before and after heating at certain temperature without scale. Δt represented the constant time, which was 5 min. The coating and the steel substrate was characterized by scanning electron microscopy (SEM; JSM-6700F, JEOL, Japan), XRD, TG-DTA, DSC, et. al. X-ray diffraction (XRD; X'Pert Pro, Philips, The Netherlands) was carried out from 10°–90° for 3.5 min. The test samples were derived from the scales which were stripped from the bare and coated samples after heating from room temperature to 1050 °C and maintained 60 min by hot compression. Weight and energy changes with the increasing temperature detected using TG-DTA (TG-DTA; STA449, Netzsch, Germany) and DSC (DSC; STA449, Netzsch, Germany). The TG-DTA test was carried out from room temperature to 1200 °C at a heating rate of 10 °C/min, which was the same as the sample heated in muffle furnace. The atmosphere of the test was flowing air, while the nitrogen (20 ml/min) as a simultaneous inert gas flow was employed to protect the chamber walls from oxidation. The coated sample and the bare sample during the tests were 10.236 and 10.273 mg respectively. The DSC test was carried out from room temperature to 900 °C at a heating rate of 10 °C/min. The inert atmosphere of the test was the same as that in TG-DTA test. The coating sample was 10.970 mg. The thermal expansion coefficient of the coating and the steel substrate was measured by thermal dilatometer (CTE; DIL402C, Netzsch, Germany), which was carried out from room temperature to 1200 °C at a heating rate of 5 °C/min. 3. Results and discussion 3.1. Effect of coating The anti-oxidation ability was evaluated by the oxidation burning loss (Fig. 1). As shown in Fig. 1(a), it could be observed that the antioxidation ability of the glass coating was enhanced with the temperature increasing from 800 °C to 1050 °C. The anti-oxidation effect of the coating reached up to 86% for the 20MnSiNb at 1050 °C. The antioxidation effect of the coating began to decrease above 1050 °C and dropped rapidly when the temperature was higher than 1150 °C. For the 20MnSiNb steel, the temperature of hot rolling was lower than 1100 °C. Therefore, the glass coating could develop the action efficiently for the 20MnSiNb. It could be observed from Fig. 1(b) that the antioxidation effects of the glass coating were generally enhanced when the sample was maintained at certain temperature (1000 °C, 1050 °C, 1100 °C) b60 min. The anti-oxidation effects were all higher than 80%, which might be attributed to the density of the glass coating increased and the Cr film and Si film formed with the extension of time. Although

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Fig. 1. Anti-oxidation effect of coating: (a) heating for 60 min at different temperatures, (b) heating for different times at 1000 °C, 1050 °C and 1100 °C.

the anti-oxidation effects began to slightly decrease with longer test time, the anti-oxidation effects were still above 75%. The kinetic of the oxidation was shown in Fig. 2. The apparent activation energy (Ea) was calculated by relevant experimental results. It was well known that the reaction rate (v) was the function of the temperature (T) and the reactant concentration (c). When c was constant (B), the influence of the temperature on the reaction rate could be obtained by studying the influence of the temperature on the reaction rate constants (k). The equation of the reaction rate and temperature was denoted by Eq. (5). v¼Bk

ð5Þ

According to the Arrhenius equation [29]: k ¼ A  e−Ea =RT

ð6Þ

where, A and k were constants. R was 8.314 J·mol-1·K-1. T was the thermodynamic temperature. Through Eqs. (5) and (6), we could get Eqs. (7) and (8). ln k ¼ −

Ea þ ln A RT

ð7Þ

ln v ¼ −

Ea þ ln A þ ln B RT

ð8Þ

We could get the results of weight loss weight at different temperature constant for 5 min in Fig. 2(a) by the experiments and according calculations. Fig. 2(b) was acquired according to Eqs. (3) and (4). Fig. 2(b) showed that the fit lines (q) of the coated and bare samples. The q was equal with ð ERa Þ according to Eq. (8) and results of Fig. 2(b). Ea was analyzed as in Eq. (9). The values of Ea were 65.28 kJ/mol and 98.81 kJ/mol for the bare and coated samples

Fig. 2. Oxidation kinetics of coated and bare samples: (a) weight loss at different temperature constant for 5 min, (b) the reaction rate and temperature.

respectively. Compared with the bare sample, it was difficult for the coated sample to have the oxidation reaction due to the higher Ea. Ea ¼ −q  R

ð9Þ

Based on the thermodynamic results with kinetic results, it was approved that the glass coating had good anti-oxidation ability for the 20MnSiNb steel during the heating process. The descaling ability was evaluated by the thermal expansion coefficient (CTE). The results of the CTE were shown in Fig. 3 and the main corresponding CTE values were shown in Table 3. It could be observed that the CTE values of the glass coating and the steel substrate were mismatched during all test temperature. It also indicated the coating would not accumulate any stress when the glass became a viscous fluid above the softening point (below 700 °C based on Fig. 3(b)). However, the CTE values for the steel substrate fluctuated obviously during 700 °C ~ 900 °C (based on Fig. 3), which was very easy to generate a certain amount of stress at the interface between the coating and the steel substrate. When the temperature became lower than the softening point, the coating behaved again as solid material. Since, in that temperature range, the coating had lower thermal expansion coefficient than the substrate (8.0 vs 15.8 × 10–6/°C, based on Table 3), the coating shrank less than the substrate during cooling and developed high compressive stresses. These stresses were the reason why the coating spalled off the steel substrate spontaneously during cooling. Based on the results of anti-oxidation ability and the thermal expansion coefficient, it was approved that the glass coating made a contribution to reducing the oxidation rate and enhancing the descaling ability of 20MnSiNb steel during the hot rolling treatment system below 1100 °C.

G. Fu et al. / Surface & Coatings Technology 294 (2016) 8–14

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Fig. 3. CTE patterns of samples: (a) bare sample, (b) coating sample.

3.2. Morphology of coating The microstructures of the surface and cross section of the coating stripped from 20MnSiNb rebar steel after heating at high temperature were shown in Fig. 4(a) and (b). It was showed that the density of the glass coating changed with the increase of temperature from 700 °C to 1100 °C. The morphology of the coating was porous when the sample was heated at 700 °C. When the temperature was about 800 °C, a dense coating was formed. It might be the glass coating melted and formed a dense film between 700 °C and 800 °C, which gave the possibility of a good anti-oxidation ability. In Fig. 4(b), it showed that the density of glass coating increased when the temperature was higher than 1000 °C. Based on the results of anti-oxidation, it was approved that 1000–1100 °C was a suitable temperature range that the coating was dense enough to reduce the diffusion of oxygen ions and iron ions. The XRD results of samples were shown in Fig. 5. All samples were heated at 1050 °C for 60 min. The oxide scale was divided into a loose and dense oxide scale. The dense oxide scale was mainly composed of Fe2O3 and Fe3O4, which had good adhesion to the substrate steel. From Fig. 5(a), it could be approved that the inner side scale of the bare samples was composed of Fe2O3 and Fe3O4. It leaded to a big descaling problem which reduced the surface quality of steel. However, in Fig. 5(b), it indicated that the inner side scale of the coated sample was composed of FeCr2O4, (Fe0.6Cr0.4)2O3, and Cr2(FeO2)6. Based on the iconic glass peak in Fig. 5(c), it was approved that the coating belonged to the glass coating. Therefore, from the XRD results, it indicated that on one hand, the scale of the coated sample consisted of chromium compounds and was relative small; on the other hand, the coating was a glass coating which had the mismatched CTE value with that of

Table 3 The CTE values of the steel and the glass coating. Temperature, °C

30.0, 300.0

30.0, 600.0

30.0, 900.0

30.0, 1200.0

CTEsteel, 10−6/K−1 CTEcoating, 10−6/K−1

17.0 8.6

15.8 8.0

11.2

13.6

Fig. 4. (a) The surface SEM micrograph pictures of coating at different temperatures. (b) The SEM micrograph pictures of the cross sections coating of the coatings at different temperatures.

the steel substrate. With the above reasons, the coated sample had a better descaling ability than that of the bare sample. Fig. 6 showed the TG-DTA results of coated and bare sample. In Fig. 6(a), it showed that there was a big exothermic peak at about 650 °C, which indicated the steel began to be oxidized at this temperature. In Fig. 6(b), there had several peaks from 400 °C to 1100 °C, which showed the reactions and changes between the coating and the substrate steel. The peaks (below 650 °C) in the Fig. 6(b) indicated the coating materials transformed to glass by reactions, such as phase transformations and melting reactions. Compared with the bare sample, the coated sample had a better anti-oxidation ability by a relatively smaller exothermic peak at 650 °C. Fig. 7 showed the DSC result of a Cr2O3 glass coating. At 730 °C, the first endothermic second-order transformation represented the glass transition temperature, which was coherent with the range usually observed for soda-lime silicates [30–32]. As the coating had the characters of a glass, the coating showed a high flexibility when the temperature was higher than Tg of the glass. When the temperature further increased, the coating showed viscous

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G. Fu et al. / Surface & Coatings Technology 294 (2016) 8–14

Fig. 7. DSC curve of glass coating.

was denser at 800 °C than 700 °C, which indicated a melting reaction occurred inner the coating. Then the glass protective film formed, which decreased the diffusion of oxygen. Combined with the XRD results, another important exothermic peak observed at 1050 °C in Fig. 6 showed the reactions of phase change of iron, chromium and oxygen between the glass coating and the structure steel. 3.3. Protection mechanism

Fig. 5. XRD patterns of samples: (a) bare sample, (b) coated sample, (c) glass coating.

flow properties. The properties of the coating at this stage provided a basis for a better high temperature protection at the following stage. Fig. 7 also showed that there were some small exothermic peaks at the temperature from 700 °C to 800 °C. As shown in Fig. 4, the coating

Fig. 6. TG-DTA curves samples (a) bare sample (b) coated sample.

Generally, a high-temperature oxidation process could be controlled by outward cation diffusion, inward anion diffusion, or mixed diffusion. According to the Wagner theory of oxidation, one of the mechanisms of oxidation was that the migration of ions or electrons across the scale and the migration was the rate-controlling process [2]. The formation of a dense, well-adherent and slow growing scale on steels by selective oxidation provided protection for steels against oxidation and corrosion [11,12]. As indicated in the TG-DTA result of the coated sample and Fig. 4, the coating formed a dense coating film with the increasing of temperature, which reduced the diffusion rates of iron and the oxygen ions. As a result, the anti-oxidation ability of the coating was improved. Table 1 showed the main components of the coating which was based on silicates. The silicates in the coating formed amorphous silica, which was responsible for improving the oxidation resistance [33]. In order to get good anti-oxidation ability, a glass coating which contained enough Si or SiO2 compound was selected to form the Si concentration layer for the anti-oxidation. Fig. 8 showed the results of EDS mapping of the cross section of the coated samples maintained at 900 °C, 1000 °C, 1050 °C, 1100 °C and 1150 °C for 60 min. As shown in Fig. 8, a smooth Si layer was formed on the interface of scale and steel structure of the coated sample during the high temperature treatment. The Si layer provided a barrier to decrease the diffusion of irons and oxygen ions and thus improved the anti-oxidation ability. Meanwhile, the thickness of the Si layer increased with the increasing of temperature. The Si layer began to disperse and disappear when the temperature was higher than 1100 °C. However, the eutectic temperature for the reaction: eutectic liquid phase → FeO + Fe2SiO4 was about 1170 °C. The molten eutectic fayalite (Fe2SiO4) was formed when the temperature reached above 1170 °C, which brought about the enhanced diffusion of ions and caused severe oxidation of the steel [12,33]. So, the anti-oxidation effect declined seriously when the temperature was higher than 1150 °C. Meanwhile, during the protection of the Si concentration layer, the silica film only allowed the formation of a chromium compound film. As shown in Fig. 8, the chromium began to gather under the silicate layer when the silicate layer formed at the interface. When the oxidation went on, a chromium scale formed over the silica film

G. Fu et al. / Surface & Coatings Technology 294 (2016) 8–14

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Fig. 8. The EDS mapping micrograph pictures of cross section of coated sample at different temperature for 60 min (a) 900 °C, (b) 1000 °C, (c) 1050 °C, (d) 1100 °C, (e) 1150 °C.

and the probability of developing iron layer was very small, which was due to the thermodynamic effects of silica [34]. Thus the silicate in the coating made a big contribution to the anti-oxidation of the steel below 1170 °C. The diffusion of oxygen and iron ions was lower than that of chromium and the difference decreased with the increasing of temperature [35–37]. As shown in Fig. 8, the thickness of chromium layer increased from 900 °C to 1050 °C. When temperature was higher than 1100 °C, the chromium layer began to scatter and even disappeared. According to the results of the anti-oxidation ability, it could be concluded that the chromium made a contribution to the anti-oxidation of steel. When temperature increased, the diffusion of iron and oxygen ions could be equal to or even higher than that of chromium diffusion. For example, the diffusion coefficient of oxygen was higher than that of the chromium between 1100 °C to 1450 °C [36]. The oxygen and iron ions spread to the chromium layer and interacted with the chromium film by the following reaction: 2Fe+ O2 + 2Cr2O3 = 2FeCr2O4. Since the Fe in steel and oxide scales was much higher than Cr, a Fe-rich external scale formed over

the FeCr2O4 layer. Due to iron and oxygen ions diffusion, the chromium film lost the anti-oxidation ability for 20MnSiNb steel. 4. Conclusion In present work, a glass coating was prepared on the 20MnSiNb steel by a simple, efficient and applicable method. Some results were obtained as follows: (1) The formed chromium compound film and silicate film decreased the diffusion rate of iron and oxygen, and then enhanced the anti-oxidation ability of the 20MnSiNb rebar steel. (2) The mixture formed by the reaction between chromium, iron and oxygen was composed of FeCr2O4, (Fe0.6Cr0.4)2O3 and Cr2(FeO2)6, which decreased the diffusion of ions and enhanced the antioxidation. The hybrid productions contained chromium element had a better descaling ability than the mixture composed of iron and oxygen ions.

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(3) The coating enhanced the protective efficiency by 80% from 1000 °C to 1100 °C. The anti-oxidation ability is promoted by 86% compared with the bare sample by heating the sample at 1050 °C for 60 min.

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