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The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels
Materials Science & Engineering A 556 (2012) 246–252
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels Chan-Woo Yang a, Seung-Mok Cho b, Youn-Hee Kang c, Jong-Sub Lee c, Jin-Woo Park a,n a
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Hyundai Welding Co. Ltd., Pohang, Republic of Korea c Steel Solution Group, POSCO R&D Center, POSCO Co. Ltd., Pohang, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 15 May 2012 Accepted 27 June 2012 Available online 5 July 2012
We present the structure-property relationships of oxide scales and interfaces formed on carbon steels. We select four different steels having the same alloying elements, but different compositions. Using thermodynamic calculations, the phases of the scales formed at a fixed temperature are predicted as a function of oxygen partial pressures (pO2 ). The model steels are oxidized under the same condition to the calculations for a fixed time. The scale microstructures are analyzed by electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The mechanical strength and fracture patterns of the scales are analyzed by tension test. According to the analysis and test results, the adhesion strength of the scales are determined by the contents of reactive elements such as Si and Cr that form continuous oxide layers along the interfaces. The overall structure of the scales and the cohesion strength depend on O concentration in the scales which is controlled by C content. pO2 gradient in the layers of mostly Fe oxides becomes greater with more C for a fixed Si concentration increasing the fracture resistance. The predicted phases agree well with the microstructural analysis results. & 2012 Elsevier B.V. All rights reserved.
Keywords: Oxides scales Si Steel Adhesion strength Thermodynamic calculations
1. Introduction Steel has been the most widely used structural materials in the modern world [1]. For the last a few decades, the consumption of the steel has been steadily increased not only in the traditional applications such as ships and automobiles, but also in the relatively new areas like oxide fuel cells (SOFC) and flexible substrates [1,2]. As the applications have become diverse, there has been significant technology development in alloy design to meet various property requirements [3]. Among fabrication processes of various steel alloys, hot strip rolling using conventional thick slab casting and rolling technology are still one of the most extensively used processes [4]. The hot strip rolling process comprises several major steps, which are slab reheating, hot rolling and coiling. In and between each step, the steel slabs are heated by combustion of natural gas in a reheat furnace to the desired temperatures. Because of the oxidizing nature of the atmosphere, layers of oxide scales (up to several millimeters) form on the slab surfaces [5]. The scales are usually removed by hydraulic descalers using high-pressure water at each pass or at selective passes in the hot
n
Corresponding author. Fax: þ82 2 312 5375. E-mail address: [email protected] (J.-W. Park).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.06.082
rolling process [6]. In practice, however, the complete removal of scales exactly at the scale-substrate interface is impossible. A thin scale layer or some parts of the thick scale remain on the substrate surface [3]. The scale residues may grow further if O is available and will undergo structural changes even during cooling after coiling, which is the end of the process [1,7,8]. These scale residues significantly degrade the quality of the final products in several ways [1]. Due to the oxidation on the surface, the composition of the surface is altered, which modifies the physical and chemical properties of the steel [1] and has numerous consequences on later applications [8]. To maintain the surface properties such as the resistances to corrosion, friction, and wear that are originally designed by alloying, it is thus essential to understand the phase transformation processes [6,9]. The oxide layer formed on diluted steel alloys consists essentially of Fe oxides and the growth mechanism is understood. However, the addition of the alloying elements significantly modifies the oxidation mechanism altering the morphological evolution, crystalline structure, and chemical composition of the interface and the alloys near the surfaces even if the concentrations of the added elements are low [8–10]. The scales remained on the surface and the altered surface compositions degrade the weldability of the alloys most of all [11,12]. The weldability is a critical factor for the applicability of newly developed materials [13]. In welding the plates manufactured
C.-W. Yang et al. / Materials Science & Engineering A 556 (2012) 246–252
by hot strip rolling, the scale residues on the mating surfaces are inserted along the weld interface and become facture initiation sites at strains so much lower than the base metals [11,12]. For this reason, it has been known that the weldability is strongly related to the amount of scale residues on the strip surfaces [12]. Research effort has been made to investigate the factors for the adhesion strength between the scales and the steels [7,14]. However, in reality, there are few parameters to vary in the hot rolling process [8]. In this study, we extend the previous investigation by others to the effects of major alloying elements that have been added for balancing the strength and ductility and high temperature oxidation resistance of the steels on the cohesive and adhesive strengths of the scales [3,15]. We select four steels with different contents of C, Si, Cr, and Mn. The types of oxide phases formed on the model steels are predicted by thermodynamic calculations. The model steels are oxidized at a fixed temperature and the hydraulic descaling is done once. The microstructures of the scales remained on the substrates and the interfaces are analyzed by high resolution X-ray diffraction (HR-XRD), EPMA, and high resolution TEM (HR-TEM). The analysis results are compared with the predicted phases. The strength of the scales on each alloy is evaluated by the uniaxial tension test. Based on our analysis results, the relationship between the microstructures and the mechanical properties of the scales are explained.
2. Calculation and experimental procedures The compositions of model alloys are summarized in Table 1. ULCS in Table 1 is an ultra-low carbon steel and has the largest amount of Si among the four model alloys. MCS and MCS-Si are medium carbon steels ( o0.59 wt%) with a similar level of C and Mn compositions, but MCS-Si has about five times larger Si content than MCS. HCS is a high carbon steel ( 40.6 wt%). It has a similar level of Si content to MCS-Si, but has the highest composition of Cr among the model alloys. Mn concentration is about a half of MCS and MCS-Si. The types and fractions of the scale phases on each alloy as a function of pO2 at 900 1C and under an atmospheric pressure are calculated using Thermo-Calcs software (SSUB database) [16]. The steels in Table 1 are oxidized at 1100–1200 1C for 4–5 h and are cooled down in the air. This oxidation process is done in a pilot plant of POSCO Research Center for simulating the initial reheating process of the hot rolling in which the steel slabs are heated by combustion of natural gas in a reheat furnace [1]. During the reheating process, a few millimeter thick oxide scales are formed on the steels in general. Like the same way as being done in the real hot rolling process, the scales are removed once by the high pressurized water hydraulic descaler at the exit of the furnace. The microstructures of the scales remained on the steel surfaces after the descaling are analyzed using HR-XRD, EPMA, and HR-TEM. The analysis results are compared with the predicted phases by the calculations. The mechanical strength and the adhesion between the scales and the steels are evaluated by the uniaxial tension test. The
Table 1 Compositions and concentration (wt%) of the elements in alloys A–D Sample alloy names ULCS MCS MCS-Si HCS
C
Si
Cr
Mn
Ni
Cu
o 0.003 0.500 0.470 0.800
2.0 0.04 0.195 0.170
0.01 0.06 0.020 0.163
0.15 0.70 0.708 0.397
0.007 0.010 0.010 0.008
0.009 0.029 0.006 0.008
247
Fig. 1. Schematic descriptions of the tension test and a test sample.
uniaxial tension tester and the size of the test specimen are presented in Fig. 1. Increasing tensional strains during the test, compressive stress is also induced in the scales along the direction perpendicular to the tension due to the mismatches in the mechanical properties between the steel and the scale layers, which is called as Poisson’s contraction [17]. The strain rate is 2.5 10 5/s and the moments of the fracture and the delamination of the scales are recorded by a high-speed camera equipped with a magnification lens. After the tests, the fractured and delaminated surfaces of the specimens are examined by EPMA and the phases on the brittle crack paths are analyzed.
3. Results 3.1. Calculation results Considering the environments where the reheating process is done and the rate of cooling in reality, the calculations are done at 900 1C, not 1100–1200 1C. Also, the commercial database provides a very limited data for the oxide phases above 1100 1C. Fig. 2(a)–(d) present the possible equilibrium phases at the fixed T and within a range of pO2 and the fractions of the phases. The phase fractions in Fig. 2 are weight-based excluding the fractions of the steels. The diffusion of elements including O cannot be considered in the thermodynamic calculations. However, in reality, there are compositional gradients from top surfaces through the substrates and the diffusion is one dimensional considering the sample geometry described in Fig. 1. Hence, the oxide phases predicted in Fig. 2(a)–(d) are likely to form as a stack of thin layers. Based on these assumptions, the oxide formed at a lower pO2 in Fig. 2 may exist closer to the interface with the substrate than the phase formed at a higher pO2 as illustrated by the inset diagram in Fig. 2(a). + According to Fig. 2, the scales are mostly Fe1 xO (wustite of iron protoxide) over pO2 of 10 18. A gas phase (CO and CO2) and a FeXaOb type spinel structure, Fe2SiO4 (Fayalite), Mn2SiO4, and FeCr2O4 are also major phases over 10 18. Fe1 xO becomes unstable under 570 1C and is transformed into Fe2O3 (hematite) and Fe3O4 (magnetite) by the diffusions of Fe from the substrate and O from the scale surface [8]. Below pO2 of 10 18, Si-, Cr-, and Mn-oxides coexist.
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100
6 4 1
10-2 2
10-3 10-4
4, 6
5
1
1
10-5 10-6 10-26
3
Mass fraction of phase
Mass fraction of phase
10-1
100
10-22 10-20 Oxygen partial pressure/Pa
7 2
10-3
5
3 8
10-4
10-24
10-22 10-20 Oxygen partial pressure/Pa
10-18
100
6
6
10-1 2
10-2
5
10-3
1
4
3
10-4 10-5
Mass fraction of phase
10-1 Mass fraction of phase
10-2
10-6 10-26
10-18
100
10-6 -26 10
10-1
10-5
Substrate 10-24
6
10-3
5
2
10-2 1
8
4
3
10-4 10-5
10-24
10-22 10-20 Oxygen partial pressure/Pa
10-18
10-6 -26 10
10-22 10-20 10-24 Oxygen partial pressure/Pa
10-18
Fig. 2. The thermodynamic calculation results: phase fractions in the scales as a function of oxygen partial pressure (pO2 ) on (a) ULCS, (b) MCS, (c) MCS-Si and (d) HCS.
Among the three reactive elements, Si is the most reactive with oxygen based on Fig. 2. SiO2 is produced at the lowest pO2 among the oxides. In all the alloys in Table 1, the Si-oxides such as SiO2, Fe2SiO4 (Fayalite), and Mn2SiO4 exist in the greatest fractions over the largest range of pO2 . The reactivity of Mn seems to be the next to Si in the alloys. As presented in Fig. 2(b)–(d), Mn2SiO4 is preferentially formed to MnO with increasing Si. Croxides form at the highest pO2 among the three reactive elements. At the given conditions in this calculation, FeCr2O4 is more stable Cr-oxide than Cr2O3 based on Fig. 2(a)–(d).
3.2. The scale microstructures According to OM analysis, the average thicknesses of the scales remained on the substrates of ULCS, MCS-Si, and HCS after the descaling are about 42, 50, and 65 mm, respectively, while, on MCS, most of the scale layer is delaminated by the descaling. The phase analyses of the scales are done using HR-XRD, EPMA, and HR-TEM and the results are summarized in Figs. 3–5. The HR-XRD results in Fig. 3 agree well with the prediction by the thermodynamic calculations in Fig. 2. Fe2O3 and Fe3O4 are transformed from FeO on cooling and are the major phases with FeO in all the samples. Also, Fe2SiO4 and SiO2 are found in all the scales. As presented in Fig. 3, Mn2SiO4 and FeCr2O4 are identified more than MnO and Cr2O3, which also corresponds well with the calculation results in Fig. 2.
Fig. 3. XRD analysis of the scales before the tension test in ULCS, MCS, MCS-Si and HCS.
Fig. 4(a)–(d) present the EPMA elemental mapping results of the scale cross-sections where the atomic concentration (%) of each element is identified by different colors and degrees of
C.-W. Yang et al. / Materials Science & Engineering A 556 (2012) 246–252
249
Fig. 4. The cross-sectional FE-SEM images and EPMA mappings of the alloying elements in: (a) ULCS, (b) MCS-Si and (c) HCS.
contrast. As demonstrated in Fig. 4(a), ULCS has the largest concentration of O in the scale among the samples. In addition, a large amount of Si-oxide particles are found throughout the scales. A five to six micron-thick band-like layer of Si-oxide is also formed along the interfaces. Based on the results in Fig. 2(a) and Fig. 3, the Si-oxide layer and particles seems to be Fe2SiO4. As O concentration is considered, the scale on MCS-Si consists of three layers as presented in Fig. 4(b). The major phase in the top layer with high O concentration seems to be Fe2O3 transformed from Fe1 xO as mentioned in Section 3.1 [6,8]. The middle layer with a relatively low concentration of O appears to consist of FeO, Fe3O4, and Mn2SiO4 based on Figs. 3 and 4(b). Mn-oxides are known to be soluble in Fe-oxides [8]. Like on ULCS, a layer of Si is found along the interface of the scale on MCS-Si according to the elemental mapping result in Fig. 4(b). The interfacial layer appears to be Fe2SiO4 like on ULCS and is the third, bottom layer in the scale on MCS-Si according to Fig. 4(b). In the scales on HCS, O concentration is the lowest among the three samples as presented in Fig. 4. An oxide layer with the high concentration of Cr is formed along the interface with HCS as well as the layers of Si according to the elemental mappings in Fig. 4(c). The interfacial layers are Fe2SiO4 and FeCr2O4 according to Fig. 2(d) and Fig. 3. Nevertheless, Mn-oxides seem to form throughout the scale like in the middle layer of the scale on MCSSi. Based on Fig. 3, Mn-oxides are Mn2SiO4 and MnO. About 10 mm-thick scales from the substrates of MCS-Si and HCS including interfaces are analyzed by HR-TEM and the results are summarized in Fig. 5(a) and (b). The results in Fig. 5(a) and (b) confirm that the thin interfacial layers with the high Si concentration on MCS-Si and HCS are Fe2SiO4. In the scale above the Fe2SiO4 layer, Fe3O4 particles are found to be embedded in the matrix of FeO in MCS-Si. Near the interface of HCS, Fe2SiO4 exists in two types: a discrete thin layer and particles (Fig. 5(b)). Hence, the total interfacial region with the high Si concentration (i.e., Sioxides) in the scale on HCS is so much thicker than that on MCSSi. Unlike MCS-Si, the scale near the interface is a mixture of FeO, Fe3O4, and Fe2SiO4. 3.3. Tension test results Fig. 5. HR TEM analysis results of the interfaces between the scale and the steel substrate in: (a) MCS-Si (in counterclockwise direction from the left, a STEM image, a transmission image, and SADPs) and (b) HCS (in counterclockwise direction from the left, FE-SEM image of the FIB sample, a STEM image, a transmission image, and SADPs).
All the samples except MCS are tensioned to 5% in engineering strain. Until the end of the test, the scales on ULCS are neither cracked nor delaminated. The tension test results of MCS-Si and
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HCS are summarized with the images captured in the in-situ recording of the tests by the high speed camera in Fig. 6. As appeared in Fig. 6(a), there are already cracks and local delamination on MCS-Si before the tension test, which seems to be caused by the hydraulic descaling. As the engineering strain is increased, the cracking and delamination of the scales are further progressed and most of the scales are delaminated over 1.2% of the strain. The red circles depict flying pieces of the scale detached from the substrate during the test. As presented in Fig. 6(b), HCS shows a completely different fracture pattern from MCS-Si. On the scale surfaces, neither a crack nor a delamination is found before the tension test. As the sample is tensioned, small cracks are initiated perpendicular to the tensional direction and a large number of vertical wrinkles are observed on the scale surfaces. When the strain is up to 1.5%, a
longitudinal cracking occurs across the scale and a layer of the scale is completely delaminated from the substrate at 1.8% strain as presented in the second and third captured images in Fig. 6(b). The interfacial delamination must occur at less than 1.5% of the strain. In the second to fourth captured images in Fig. 6(b), it should be noted that the vertical wrinkles are also found on the substrate surfaces even after the global delamination of the scale. HR-XRD analysis is done on the fractured surfaces and the wrinkled layers are revealed as mostly Fe2SiO4, FeCr2O4, and FeO as shown in Fig. 6(d). In addition, the sample surfaces after the delamination are analyzed by FE-SEM (Fig. 6(c)). Based on Fig. 6(c), the wrinkles seem to be buckling delamination by the compression induced due to the mismatches in Poisson’s ratio between the substrate and the scales as mentioned in Section 2.
Fig. 6. Stress–strain curves and high speed camera captured images of the tension tests of: (a) MCS-Si, (b) HCS, (c) FE-SEM image of the buckling delamination on the top surface of the HCS tension test specimen after global delamination and (d) HR-XRD analysis results of the fractured surfaces of MCS-Si and HCS.
C.-W. Yang et al. / Materials Science & Engineering A 556 (2012) 246–252
251
Fig. 7. The top and cross-sectional FE-SEM images and EPMA mappings of the alloying elements of the scale residues after the tension test of: (a), (b) MCS-Si and (c), (d) HCS.
After tensioned up to 5% engineering strain, the samples are examined by EPMA. Fig. 7(a)–(d) present the elemental mapping results of the top surfaces and cross-sections of MCS-Si and HCS, respectively. According to Fig.7(a) and (b), most of the scales are removed from the MCS-Si substrate by the tension test as expected from Fig. 6(a). According to the HR-XRD results in Fig. 6(d), the scales that are still remained on the substrate seem to be the third layer with the high Si concentration mentioned in Section 3.2. Fig. 7(c)–(d) confirm that the HCS substrate is still covered with a thin layer of the scale. The global delamination in Fig. 6(b) is the layer of low O concentration as presented in Fig. 4(c) and the scale remained on HCS is the thin layer with the high concentrations of Si and Cr along the interfaces.
4. Discussion It is experimentally proved that the interface and overall scale microstructures are determined by the contents of Si, Cr, and C and the cohesion and adhesion strengths of the scales are highly related to the scale microstructures. Particularly, Si and Cr form the continuous interfacial oxide layers (Fig. 4) and the adhesion strength of the scales is increased with the thickness of the layers according to the tension test results. This result contradicts the previous report that scale spallation increases with the Si content since the Si layer plays a diffusion barrier layer and the density of micro-pores at the interface is increased with Si [3].
The scales on ULCS that has the highest Si concentration and the lowest C concentration among the model alloys show the highest fracture and adhesion strengths. As presented in Fig. 2(a), O is mostly consumed by the reactions with Fe and Si and the formation of gas phases with C is minimized in ULCS compared to other alloys. As a result, the thickest Fe2SiO4 layer is formed along the interfaces in ULCS and O concentration in the scale is the highest among the steels. The thick scales above the interfacial Fe2SiO4 layer have a composite like structure of Fe2SiO4 and Feoxides. To the contrary, most of the scales on MCS with the smallest Si concentration among the samples are delaminated in the hydraulic descaling process. MCS-Si and HCS has a similar level of Si concentration, but HCS has higher contents of C and Cr. Both the adhesion and fracture strengths of HCS are greater than MCS-Si. The stronger scale adhesion on HCS than on MCS-Si seems to be because the thin layer of Cr-oxide is also formed along the interfaces in addition to the Fe2SiO4 layers. Also, the Fe2SiO4 exist in particles along the interfacial region as well as in the thin layers as mentioned in Section 3.2. Due to high C concentration, a large fraction of O in the scales is consumed to form CO and CO2 gases in MCS-Si and HCS as presented in Fig. 2(c) and (d). As the interfacial Si-and Croxide layer formation increases, the layers play as diffusion barrier layers of O and Fe between the surface and the substrate [3]. Hence, the O concentration gradient in the scale from the top surface to the interface is increased. In MCS-Si, O depletion zone is observed in the middle of the scales as presented in Fig. 4(a). Due to this, the scale on MCS-Si
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has multiple-layered structures and becomes vulnerable to cracking due to the low interfacial adhesion between the layers. As HCS has almost twice C concentration of MCS-Si and has a thicker interfacial oxide layers, O concentration is the lowest among the samples and is almost uniform throughout the scales as shown in Fig. 4(b). The scale on HCS looks to have a more homogeneous structure than the scale on MCS-Si and is mechanically stronger.
5. Conclusions The structure-mechanical property relationship of the scales is clearly presented in this work. Also, the two major factors for the efficiency of descaling (descaled amount of the oxides) are revealed as the pO2 gradient that is determined by C contents and the contents of reactive elements such as Si and Cr. The former forms a continuous reaction layers with strong adhesion to the steel substrates along the interfaces. As the latter becomes stiffer, a thicker and homogeneous scale is formed and, naturally, the scale is more resistant to external tension due to the increased stiffness. With increasing Si, not only a thicker Si oxide layer is formed along the interfaces, but also the Si oxides are distributed throughout the scale, which forms a composite-like structure with the enhanced fracture strength. Our results reveal that, to improve the descaling efficiency by decreasing interfacial adhesion, the contents of elements that form oxides soluble to the Feoxide base scales and/or are reactive with Si and Cr such as Mn should be increased when others are fixed.
Acknowledgments POSCO R&D center is acknowledged for this research. This work was also supported by a grant (M-2009-01-0014) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, South Korea. References [1] R.Y. Chen, W.Y.D. Yuen, Oxid. Metals 59 (2003) 433–468. [2] S. Megel, E. Girdauskaite, V. Sauchuk, M. Kusnezoff, A. Michaelis, J. Power Sources 196 (2011) 7136–7143. [3] S. Taniguchi, K. Yamamoto, D. Megumi, T. Shibata, Mater. Sci. Eng. A 308 (2001) 250–257. [4] X.G. Shi, B. Han, X. Xu, Y. Dong, R.D. Liu, J. Iron Steel Res. Int. 18 (2011) 999–1003. [5] R.Y. Chen, W.Y.D. Yuen, Oxid. Metals 53 (2000) 539–560. [6] Y.L. Yang, C.H. Yang, S.N. Lin, C.H. Chen, W.T. Tsai, Mater. Chem. Phys. 112 (2008) 566–571. [7] V.V. Basabe, J.A. Szpunar, ISIJ Int. 48 (2008) 467–474. [8] D. Geneve, D. Rouxel, P. Pigeat, B. Weber, M. Confente, Appl. Surf. Sci. 254 (2008) 5348–5358. [9] R.Y. Chen, W.Y.D. Yuen, Mater. Sci. Forum 522–523 (2006) 451–460. [10] R. Krein, M. Palm, Acta Mater. 56 (2008) 2400–2405. [11] B.A. Baker, V.W. Hartmann, L.E. Shoemaker, S.A. McCoy, S. Rajendran, Trans. Indian Inst. Met. 56 (2003) 327–333. [12] Y.-H. Kang, S.-H. Um, J.-S. Lee, J.-B. Lee, Y.-B. Gong, K. Horii, in: Korean Welding Society Korean Welding Society, Busan, Korea, 2006, pp. 117–119. [13] J.W. Park, S.S. Babu, J.M. Vitek, E.A. Kenik, S.A. David, J. Appl. Phys. 94 (2003) 4203–4209. [14] R. Bhattacharya, G. Jha, S. Kundu, R. Shankar, N. Gope, Surf. Coatings Technol. 201 (2006) 526–532. [15] D. Geneve, D. Rouxel, B. Weber, M. Confente, Mater. Sci. Eng. A 435–436 (2006) 1–11. [16] ThermoCalc, Ver. S, Stockholm, Sweden, Thermo-Calc software. [17] C.J. Xie, W. Tong, Acta Mater. 53 (2005) 477–485.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels Chan-Woo Yang a, Seung-Mok Cho b, Youn-Hee Kang c, Jong-Sub Lee c, Jin-Woo Park a,n a
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Hyundai Welding Co. Ltd., Pohang, Republic of Korea c Steel Solution Group, POSCO R&D Center, POSCO Co. Ltd., Pohang, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 15 May 2012 Accepted 27 June 2012 Available online 5 July 2012
We present the structure-property relationships of oxide scales and interfaces formed on carbon steels. We select four different steels having the same alloying elements, but different compositions. Using thermodynamic calculations, the phases of the scales formed at a fixed temperature are predicted as a function of oxygen partial pressures (pO2 ). The model steels are oxidized under the same condition to the calculations for a fixed time. The scale microstructures are analyzed by electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The mechanical strength and fracture patterns of the scales are analyzed by tension test. According to the analysis and test results, the adhesion strength of the scales are determined by the contents of reactive elements such as Si and Cr that form continuous oxide layers along the interfaces. The overall structure of the scales and the cohesion strength depend on O concentration in the scales which is controlled by C content. pO2 gradient in the layers of mostly Fe oxides becomes greater with more C for a fixed Si concentration increasing the fracture resistance. The predicted phases agree well with the microstructural analysis results. & 2012 Elsevier B.V. All rights reserved.
Keywords: Oxides scales Si Steel Adhesion strength Thermodynamic calculations
1. Introduction Steel has been the most widely used structural materials in the modern world [1]. For the last a few decades, the consumption of the steel has been steadily increased not only in the traditional applications such as ships and automobiles, but also in the relatively new areas like oxide fuel cells (SOFC) and flexible substrates [1,2]. As the applications have become diverse, there has been significant technology development in alloy design to meet various property requirements [3]. Among fabrication processes of various steel alloys, hot strip rolling using conventional thick slab casting and rolling technology are still one of the most extensively used processes [4]. The hot strip rolling process comprises several major steps, which are slab reheating, hot rolling and coiling. In and between each step, the steel slabs are heated by combustion of natural gas in a reheat furnace to the desired temperatures. Because of the oxidizing nature of the atmosphere, layers of oxide scales (up to several millimeters) form on the slab surfaces [5]. The scales are usually removed by hydraulic descalers using high-pressure water at each pass or at selective passes in the hot
n
Corresponding author. Fax: þ82 2 312 5375. E-mail address: [email protected] (J.-W. Park).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.06.082
rolling process [6]. In practice, however, the complete removal of scales exactly at the scale-substrate interface is impossible. A thin scale layer or some parts of the thick scale remain on the substrate surface [3]. The scale residues may grow further if O is available and will undergo structural changes even during cooling after coiling, which is the end of the process [1,7,8]. These scale residues significantly degrade the quality of the final products in several ways [1]. Due to the oxidation on the surface, the composition of the surface is altered, which modifies the physical and chemical properties of the steel [1] and has numerous consequences on later applications [8]. To maintain the surface properties such as the resistances to corrosion, friction, and wear that are originally designed by alloying, it is thus essential to understand the phase transformation processes [6,9]. The oxide layer formed on diluted steel alloys consists essentially of Fe oxides and the growth mechanism is understood. However, the addition of the alloying elements significantly modifies the oxidation mechanism altering the morphological evolution, crystalline structure, and chemical composition of the interface and the alloys near the surfaces even if the concentrations of the added elements are low [8–10]. The scales remained on the surface and the altered surface compositions degrade the weldability of the alloys most of all [11,12]. The weldability is a critical factor for the applicability of newly developed materials [13]. In welding the plates manufactured
C.-W. Yang et al. / Materials Science & Engineering A 556 (2012) 246–252
by hot strip rolling, the scale residues on the mating surfaces are inserted along the weld interface and become facture initiation sites at strains so much lower than the base metals [11,12]. For this reason, it has been known that the weldability is strongly related to the amount of scale residues on the strip surfaces [12]. Research effort has been made to investigate the factors for the adhesion strength between the scales and the steels [7,14]. However, in reality, there are few parameters to vary in the hot rolling process [8]. In this study, we extend the previous investigation by others to the effects of major alloying elements that have been added for balancing the strength and ductility and high temperature oxidation resistance of the steels on the cohesive and adhesive strengths of the scales [3,15]. We select four steels with different contents of C, Si, Cr, and Mn. The types of oxide phases formed on the model steels are predicted by thermodynamic calculations. The model steels are oxidized at a fixed temperature and the hydraulic descaling is done once. The microstructures of the scales remained on the substrates and the interfaces are analyzed by high resolution X-ray diffraction (HR-XRD), EPMA, and high resolution TEM (HR-TEM). The analysis results are compared with the predicted phases. The strength of the scales on each alloy is evaluated by the uniaxial tension test. Based on our analysis results, the relationship between the microstructures and the mechanical properties of the scales are explained.
2. Calculation and experimental procedures The compositions of model alloys are summarized in Table 1. ULCS in Table 1 is an ultra-low carbon steel and has the largest amount of Si among the four model alloys. MCS and MCS-Si are medium carbon steels ( o0.59 wt%) with a similar level of C and Mn compositions, but MCS-Si has about five times larger Si content than MCS. HCS is a high carbon steel ( 40.6 wt%). It has a similar level of Si content to MCS-Si, but has the highest composition of Cr among the model alloys. Mn concentration is about a half of MCS and MCS-Si. The types and fractions of the scale phases on each alloy as a function of pO2 at 900 1C and under an atmospheric pressure are calculated using Thermo-Calcs software (SSUB database) [16]. The steels in Table 1 are oxidized at 1100–1200 1C for 4–5 h and are cooled down in the air. This oxidation process is done in a pilot plant of POSCO Research Center for simulating the initial reheating process of the hot rolling in which the steel slabs are heated by combustion of natural gas in a reheat furnace [1]. During the reheating process, a few millimeter thick oxide scales are formed on the steels in general. Like the same way as being done in the real hot rolling process, the scales are removed once by the high pressurized water hydraulic descaler at the exit of the furnace. The microstructures of the scales remained on the steel surfaces after the descaling are analyzed using HR-XRD, EPMA, and HR-TEM. The analysis results are compared with the predicted phases by the calculations. The mechanical strength and the adhesion between the scales and the steels are evaluated by the uniaxial tension test. The
Table 1 Compositions and concentration (wt%) of the elements in alloys A–D Sample alloy names ULCS MCS MCS-Si HCS
C
Si
Cr
Mn
Ni
Cu
o 0.003 0.500 0.470 0.800
2.0 0.04 0.195 0.170
0.01 0.06 0.020 0.163
0.15 0.70 0.708 0.397
0.007 0.010 0.010 0.008
0.009 0.029 0.006 0.008
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Fig. 1. Schematic descriptions of the tension test and a test sample.
uniaxial tension tester and the size of the test specimen are presented in Fig. 1. Increasing tensional strains during the test, compressive stress is also induced in the scales along the direction perpendicular to the tension due to the mismatches in the mechanical properties between the steel and the scale layers, which is called as Poisson’s contraction [17]. The strain rate is 2.5 10 5/s and the moments of the fracture and the delamination of the scales are recorded by a high-speed camera equipped with a magnification lens. After the tests, the fractured and delaminated surfaces of the specimens are examined by EPMA and the phases on the brittle crack paths are analyzed.
3. Results 3.1. Calculation results Considering the environments where the reheating process is done and the rate of cooling in reality, the calculations are done at 900 1C, not 1100–1200 1C. Also, the commercial database provides a very limited data for the oxide phases above 1100 1C. Fig. 2(a)–(d) present the possible equilibrium phases at the fixed T and within a range of pO2 and the fractions of the phases. The phase fractions in Fig. 2 are weight-based excluding the fractions of the steels. The diffusion of elements including O cannot be considered in the thermodynamic calculations. However, in reality, there are compositional gradients from top surfaces through the substrates and the diffusion is one dimensional considering the sample geometry described in Fig. 1. Hence, the oxide phases predicted in Fig. 2(a)–(d) are likely to form as a stack of thin layers. Based on these assumptions, the oxide formed at a lower pO2 in Fig. 2 may exist closer to the interface with the substrate than the phase formed at a higher pO2 as illustrated by the inset diagram in Fig. 2(a). + According to Fig. 2, the scales are mostly Fe1 xO (wustite of iron protoxide) over pO2 of 10 18. A gas phase (CO and CO2) and a FeXaOb type spinel structure, Fe2SiO4 (Fayalite), Mn2SiO4, and FeCr2O4 are also major phases over 10 18. Fe1 xO becomes unstable under 570 1C and is transformed into Fe2O3 (hematite) and Fe3O4 (magnetite) by the diffusions of Fe from the substrate and O from the scale surface [8]. Below pO2 of 10 18, Si-, Cr-, and Mn-oxides coexist.
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100
6 4 1
10-2 2
10-3 10-4
4, 6
5
1
1
10-5 10-6 10-26
3
Mass fraction of phase
Mass fraction of phase
10-1
100
10-22 10-20 Oxygen partial pressure/Pa
7 2
10-3
5
3 8
10-4
10-24
10-22 10-20 Oxygen partial pressure/Pa
10-18
100
6
6
10-1 2
10-2
5
10-3
1
4
3
10-4 10-5
Mass fraction of phase
10-1 Mass fraction of phase
10-2
10-6 10-26
10-18
100
10-6 -26 10
10-1
10-5
Substrate 10-24
6
10-3
5
2
10-2 1
8
4
3
10-4 10-5
10-24
10-22 10-20 Oxygen partial pressure/Pa
10-18
10-6 -26 10
10-22 10-20 10-24 Oxygen partial pressure/Pa
10-18
Fig. 2. The thermodynamic calculation results: phase fractions in the scales as a function of oxygen partial pressure (pO2 ) on (a) ULCS, (b) MCS, (c) MCS-Si and (d) HCS.
Among the three reactive elements, Si is the most reactive with oxygen based on Fig. 2. SiO2 is produced at the lowest pO2 among the oxides. In all the alloys in Table 1, the Si-oxides such as SiO2, Fe2SiO4 (Fayalite), and Mn2SiO4 exist in the greatest fractions over the largest range of pO2 . The reactivity of Mn seems to be the next to Si in the alloys. As presented in Fig. 2(b)–(d), Mn2SiO4 is preferentially formed to MnO with increasing Si. Croxides form at the highest pO2 among the three reactive elements. At the given conditions in this calculation, FeCr2O4 is more stable Cr-oxide than Cr2O3 based on Fig. 2(a)–(d).
3.2. The scale microstructures According to OM analysis, the average thicknesses of the scales remained on the substrates of ULCS, MCS-Si, and HCS after the descaling are about 42, 50, and 65 mm, respectively, while, on MCS, most of the scale layer is delaminated by the descaling. The phase analyses of the scales are done using HR-XRD, EPMA, and HR-TEM and the results are summarized in Figs. 3–5. The HR-XRD results in Fig. 3 agree well with the prediction by the thermodynamic calculations in Fig. 2. Fe2O3 and Fe3O4 are transformed from FeO on cooling and are the major phases with FeO in all the samples. Also, Fe2SiO4 and SiO2 are found in all the scales. As presented in Fig. 3, Mn2SiO4 and FeCr2O4 are identified more than MnO and Cr2O3, which also corresponds well with the calculation results in Fig. 2.
Fig. 3. XRD analysis of the scales before the tension test in ULCS, MCS, MCS-Si and HCS.
Fig. 4(a)–(d) present the EPMA elemental mapping results of the scale cross-sections where the atomic concentration (%) of each element is identified by different colors and degrees of
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Fig. 4. The cross-sectional FE-SEM images and EPMA mappings of the alloying elements in: (a) ULCS, (b) MCS-Si and (c) HCS.
contrast. As demonstrated in Fig. 4(a), ULCS has the largest concentration of O in the scale among the samples. In addition, a large amount of Si-oxide particles are found throughout the scales. A five to six micron-thick band-like layer of Si-oxide is also formed along the interfaces. Based on the results in Fig. 2(a) and Fig. 3, the Si-oxide layer and particles seems to be Fe2SiO4. As O concentration is considered, the scale on MCS-Si consists of three layers as presented in Fig. 4(b). The major phase in the top layer with high O concentration seems to be Fe2O3 transformed from Fe1 xO as mentioned in Section 3.1 [6,8]. The middle layer with a relatively low concentration of O appears to consist of FeO, Fe3O4, and Mn2SiO4 based on Figs. 3 and 4(b). Mn-oxides are known to be soluble in Fe-oxides [8]. Like on ULCS, a layer of Si is found along the interface of the scale on MCS-Si according to the elemental mapping result in Fig. 4(b). The interfacial layer appears to be Fe2SiO4 like on ULCS and is the third, bottom layer in the scale on MCS-Si according to Fig. 4(b). In the scales on HCS, O concentration is the lowest among the three samples as presented in Fig. 4. An oxide layer with the high concentration of Cr is formed along the interface with HCS as well as the layers of Si according to the elemental mappings in Fig. 4(c). The interfacial layers are Fe2SiO4 and FeCr2O4 according to Fig. 2(d) and Fig. 3. Nevertheless, Mn-oxides seem to form throughout the scale like in the middle layer of the scale on MCSSi. Based on Fig. 3, Mn-oxides are Mn2SiO4 and MnO. About 10 mm-thick scales from the substrates of MCS-Si and HCS including interfaces are analyzed by HR-TEM and the results are summarized in Fig. 5(a) and (b). The results in Fig. 5(a) and (b) confirm that the thin interfacial layers with the high Si concentration on MCS-Si and HCS are Fe2SiO4. In the scale above the Fe2SiO4 layer, Fe3O4 particles are found to be embedded in the matrix of FeO in MCS-Si. Near the interface of HCS, Fe2SiO4 exists in two types: a discrete thin layer and particles (Fig. 5(b)). Hence, the total interfacial region with the high Si concentration (i.e., Sioxides) in the scale on HCS is so much thicker than that on MCSSi. Unlike MCS-Si, the scale near the interface is a mixture of FeO, Fe3O4, and Fe2SiO4. 3.3. Tension test results Fig. 5. HR TEM analysis results of the interfaces between the scale and the steel substrate in: (a) MCS-Si (in counterclockwise direction from the left, a STEM image, a transmission image, and SADPs) and (b) HCS (in counterclockwise direction from the left, FE-SEM image of the FIB sample, a STEM image, a transmission image, and SADPs).
All the samples except MCS are tensioned to 5% in engineering strain. Until the end of the test, the scales on ULCS are neither cracked nor delaminated. The tension test results of MCS-Si and
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HCS are summarized with the images captured in the in-situ recording of the tests by the high speed camera in Fig. 6. As appeared in Fig. 6(a), there are already cracks and local delamination on MCS-Si before the tension test, which seems to be caused by the hydraulic descaling. As the engineering strain is increased, the cracking and delamination of the scales are further progressed and most of the scales are delaminated over 1.2% of the strain. The red circles depict flying pieces of the scale detached from the substrate during the test. As presented in Fig. 6(b), HCS shows a completely different fracture pattern from MCS-Si. On the scale surfaces, neither a crack nor a delamination is found before the tension test. As the sample is tensioned, small cracks are initiated perpendicular to the tensional direction and a large number of vertical wrinkles are observed on the scale surfaces. When the strain is up to 1.5%, a
longitudinal cracking occurs across the scale and a layer of the scale is completely delaminated from the substrate at 1.8% strain as presented in the second and third captured images in Fig. 6(b). The interfacial delamination must occur at less than 1.5% of the strain. In the second to fourth captured images in Fig. 6(b), it should be noted that the vertical wrinkles are also found on the substrate surfaces even after the global delamination of the scale. HR-XRD analysis is done on the fractured surfaces and the wrinkled layers are revealed as mostly Fe2SiO4, FeCr2O4, and FeO as shown in Fig. 6(d). In addition, the sample surfaces after the delamination are analyzed by FE-SEM (Fig. 6(c)). Based on Fig. 6(c), the wrinkles seem to be buckling delamination by the compression induced due to the mismatches in Poisson’s ratio between the substrate and the scales as mentioned in Section 2.
Fig. 6. Stress–strain curves and high speed camera captured images of the tension tests of: (a) MCS-Si, (b) HCS, (c) FE-SEM image of the buckling delamination on the top surface of the HCS tension test specimen after global delamination and (d) HR-XRD analysis results of the fractured surfaces of MCS-Si and HCS.
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Fig. 7. The top and cross-sectional FE-SEM images and EPMA mappings of the alloying elements of the scale residues after the tension test of: (a), (b) MCS-Si and (c), (d) HCS.
After tensioned up to 5% engineering strain, the samples are examined by EPMA. Fig. 7(a)–(d) present the elemental mapping results of the top surfaces and cross-sections of MCS-Si and HCS, respectively. According to Fig.7(a) and (b), most of the scales are removed from the MCS-Si substrate by the tension test as expected from Fig. 6(a). According to the HR-XRD results in Fig. 6(d), the scales that are still remained on the substrate seem to be the third layer with the high Si concentration mentioned in Section 3.2. Fig. 7(c)–(d) confirm that the HCS substrate is still covered with a thin layer of the scale. The global delamination in Fig. 6(b) is the layer of low O concentration as presented in Fig. 4(c) and the scale remained on HCS is the thin layer with the high concentrations of Si and Cr along the interfaces.
4. Discussion It is experimentally proved that the interface and overall scale microstructures are determined by the contents of Si, Cr, and C and the cohesion and adhesion strengths of the scales are highly related to the scale microstructures. Particularly, Si and Cr form the continuous interfacial oxide layers (Fig. 4) and the adhesion strength of the scales is increased with the thickness of the layers according to the tension test results. This result contradicts the previous report that scale spallation increases with the Si content since the Si layer plays a diffusion barrier layer and the density of micro-pores at the interface is increased with Si [3].
The scales on ULCS that has the highest Si concentration and the lowest C concentration among the model alloys show the highest fracture and adhesion strengths. As presented in Fig. 2(a), O is mostly consumed by the reactions with Fe and Si and the formation of gas phases with C is minimized in ULCS compared to other alloys. As a result, the thickest Fe2SiO4 layer is formed along the interfaces in ULCS and O concentration in the scale is the highest among the steels. The thick scales above the interfacial Fe2SiO4 layer have a composite like structure of Fe2SiO4 and Feoxides. To the contrary, most of the scales on MCS with the smallest Si concentration among the samples are delaminated in the hydraulic descaling process. MCS-Si and HCS has a similar level of Si concentration, but HCS has higher contents of C and Cr. Both the adhesion and fracture strengths of HCS are greater than MCS-Si. The stronger scale adhesion on HCS than on MCS-Si seems to be because the thin layer of Cr-oxide is also formed along the interfaces in addition to the Fe2SiO4 layers. Also, the Fe2SiO4 exist in particles along the interfacial region as well as in the thin layers as mentioned in Section 3.2. Due to high C concentration, a large fraction of O in the scales is consumed to form CO and CO2 gases in MCS-Si and HCS as presented in Fig. 2(c) and (d). As the interfacial Si-and Croxide layer formation increases, the layers play as diffusion barrier layers of O and Fe between the surface and the substrate [3]. Hence, the O concentration gradient in the scale from the top surface to the interface is increased. In MCS-Si, O depletion zone is observed in the middle of the scales as presented in Fig. 4(a). Due to this, the scale on MCS-Si
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has multiple-layered structures and becomes vulnerable to cracking due to the low interfacial adhesion between the layers. As HCS has almost twice C concentration of MCS-Si and has a thicker interfacial oxide layers, O concentration is the lowest among the samples and is almost uniform throughout the scales as shown in Fig. 4(b). The scale on HCS looks to have a more homogeneous structure than the scale on MCS-Si and is mechanically stronger.
5. Conclusions The structure-mechanical property relationship of the scales is clearly presented in this work. Also, the two major factors for the efficiency of descaling (descaled amount of the oxides) are revealed as the pO2 gradient that is determined by C contents and the contents of reactive elements such as Si and Cr. The former forms a continuous reaction layers with strong adhesion to the steel substrates along the interfaces. As the latter becomes stiffer, a thicker and homogeneous scale is formed and, naturally, the scale is more resistant to external tension due to the increased stiffness. With increasing Si, not only a thicker Si oxide layer is formed along the interfaces, but also the Si oxides are distributed throughout the scale, which forms a composite-like structure with the enhanced fracture strength. Our results reveal that, to improve the descaling efficiency by decreasing interfacial adhesion, the contents of elements that form oxides soluble to the Feoxide base scales and/or are reactive with Si and Cr such as Mn should be increased when others are fixed.
Acknowledgments POSCO R&D center is acknowledged for this research. This work was also supported by a grant (M-2009-01-0014) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, South Korea. References [1] R.Y. Chen, W.Y.D. Yuen, Oxid. Metals 59 (2003) 433–468. [2] S. Megel, E. Girdauskaite, V. Sauchuk, M. Kusnezoff, A. Michaelis, J. Power Sources 196 (2011) 7136–7143. [3] S. Taniguchi, K. Yamamoto, D. Megumi, T. Shibata, Mater. Sci. Eng. A 308 (2001) 250–257. [4] X.G. Shi, B. Han, X. Xu, Y. Dong, R.D. Liu, J. Iron Steel Res. Int. 18 (2011) 999–1003. [5] R.Y. Chen, W.Y.D. Yuen, Oxid. Metals 53 (2000) 539–560. [6] Y.L. Yang, C.H. Yang, S.N. Lin, C.H. Chen, W.T. Tsai, Mater. Chem. Phys. 112 (2008) 566–571. [7] V.V. Basabe, J.A. Szpunar, ISIJ Int. 48 (2008) 467–474. [8] D. Geneve, D. Rouxel, P. Pigeat, B. Weber, M. Confente, Appl. Surf. Sci. 254 (2008) 5348–5358. [9] R.Y. Chen, W.Y.D. Yuen, Mater. Sci. Forum 522–523 (2006) 451–460. [10] R. Krein, M. Palm, Acta Mater. 56 (2008) 2400–2405. [11] B.A. Baker, V.W. Hartmann, L.E. Shoemaker, S.A. McCoy, S. Rajendran, Trans. Indian Inst. Met. 56 (2003) 327–333. [12] Y.-H. Kang, S.-H. Um, J.-S. Lee, J.-B. Lee, Y.-B. Gong, K. Horii, in: Korean Welding Society Korean Welding Society, Busan, Korea, 2006, pp. 117–119. [13] J.W. Park, S.S. Babu, J.M. Vitek, E.A. Kenik, S.A. David, J. Appl. Phys. 94 (2003) 4203–4209. [14] R. Bhattacharya, G. Jha, S. Kundu, R. Shankar, N. Gope, Surf. Coatings Technol. 201 (2006) 526–532. [15] D. Geneve, D. Rouxel, B. Weber, M. Confente, Mater. Sci. Eng. A 435–436 (2006) 1–11. [16] ThermoCalc, Ver. S, Stockholm, Sweden, Thermo-Calc software. [17] C.J. Xie, W. Tong, Acta Mater. 53 (2005) 477–485.