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Adhesion behaviour of scales on hot-rolled steel strips produced from continuous casting slabs
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 9359–9367
www.materialstoday.com/proceedings
The 10th Thailand International Metallurgy Conference (The 10th TIMETC)
Adhesion behaviour of scales on hot-rolled steel strips produced from continuous casting slabs Nisachon Na Kalasin, Sirilak Yenchum, Thanasak Nilsonthi* High Temperature Corrosion Research Centre, Department of Materials and Production Technology Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, 1518 Pracharat 1 Road, Wongsawang, Bangsue, Bangkok 10800, Thailand
Abstract The objective of this research was to study the adhesion behaviour of scales on the as-received hot-rolled steel strips produced from continuous casting slabs, i.e. medium and thin slabs. The adhesion test was conducted by the tensile testing machine equipped with a CCD camera to instantaneously observe the scales failure during the test. The adhesion test was also conducted by pickling test via immersion in a 10 %v/v HCl solution at 80°C for observing the scales removing rate. There were two approaches that can be used to evaluate the scales adhesion on the studies steels. This was due to during the hot rolling process the steel strips covers with oxide scales. This scales must be removed by the acid pickling process before further process, e.g. cold rolling process. The results shown that the as-received hot-rolled steel strips produced from medium slab was exhibited higher scales adhesion more than the steel produced from thin slab for both tests. The scales thicknesses on the as-received hotrolled steel strips produced from medium and thin slabs were 9 and 8 μm respectively. A result from tensile test, the mechanical adhesion energies of the scales actually formed on the hot-rolled steel strips produced from medium and thin slabs were 38 and 11 J.m-2 respectively. A result from pickling test, the scales dissolution rate on the as-received hot-rolled steel strips produced from medium and thin slabs were 0.0025 and 0.008 mg.cm-2.s-1. The result was also shown that the time for the complete pickling of scales on the hot-rolled steel strips produced from medium and thin slabs were 25.5 and 23 seconds respectively. This was due to the presence of high amounts of interfacial silica, which was promoted scales adhesion, indicating that longer time needed for scales removal. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Thermal oxide scales; Mechanical adhesion; Pickling behaviour; Hot-rolled steel strips
* Corresponding author. Tel.: +66-2555-2000 Ext. 8610; fax: +66-2587-4335. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.
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1. Introduction The raw material used in a hot rolling process is a slab which can be produced by different routes [1,2]. A slab can be produced by a blast-furnace route starting from iron ore, coal and limestone. The iron produced by a blast furnace is called pig iron. The pig iron is further refined in a steelmaking process resulting in a slab. A slab can also be produced by an electric-arc-furnace route. In this path, a slab is made of recycled steel, or scraps, using an electric-arc-furnace and secondary refining process. Because the scraps are used as main raw material, some elements exist in the steel with the higher content than the one produced using the blast-furnace route, e.g. Si, Cu, Sn, Pb or As [2]. Slabs produced by those processes might be classified by their thickness. The conventional slab produced from the BF route has a typical thickness of ca. 200 mm. For the EAF route, the recycled slab can be either a medium slab in case that its thickness is ca. 80 to 100 mm, or a thin slab in case that its thickness is ca. 50 mm [2]. In a hot rolling process, a slab is reheated in a reheating furnace temperature around 1250 to 1300°C, a primary scales thickness around 2 mm is formed on the slab surface. The scales can be removed by hydraulic descalesr by high pressure water jet, firstly before the entry of slab into the roughing mill and secondly before the entry of steel bar into the finishing stands. After the first descaling, secondary scales produce rapidly. Its thickness is usually less than 100 μm. This scales are secondly removed by water jet before entering the finishing mill. Afterwards, tertiary oxide scales layer developer in the finishing stands at temperature less than 1000°C. They are formed at the first stand, between stands and after the last stand. The oxide scales continuously grow during the steel strips coiling at the temperature less than 1000°C and also during storage until the temperature cools down to room temperature. After the hot rolling process, mechanical breaking of oxide scales followed by acid pickling is normally used to remove the hot-rolling scales. This is an important factor to surface quality of the steel strips for acceptance of the product. The condition of scales formation in a conventional hot strip processing route is summarised in Table 1. Table 1. Scales formation in a conventional hot strip processing route [3]. Type of scales Primary scales Secondary scales Tertiary scales Scales retained on strip
Thickness 1 – 5 mm 100 – 300 μm 10 – 30 μm 5 – 15 μm
Temperatures 1200 – 1300°C 1100 – 1200°C 850 – 1100°C 500 – 880°C
Atmosphere Furnace gas Air/steam Air/steam Air/steam
Duration Hours Minutes Seconds Days
The oxide scales development on the hot-rolled steel strips is reported [4-16]. A classic three-layered oxide scales is produced, which comprises a thin hematite layer, an intermediate magnetite layer, and a thick wustite layer. Sometimes magnetite is observed in the wustite layer due to eutectoid decomposition at cooling. Sometimes also, the hematite layer may be absent. The structure and properties of the oxide scales are influenced by various effects, e.g. alloying elements [17-20] or the oxidising atmospheres [21-23]. The surface quality of the hot-rolled steel product is related to the pickling ability [24-25] and scales adhesion. Several test methods for assessing the scales adhesion have been developed, e.g. the bending test [26-29], the inverted blister test [30] or the micro-tensile test operated in a chamber of a scanning electron microscope [31-36]. Scales adhesion tested by those methods is mainly used on stainless steel. No extensive work has been applied to assess scales adhesion on hot-rolled steel. Our group has been developed by using the macro-tensile testing machine [37-39]. In the present work, the scales failure during tensile loading was conducted by the tensile test with a CCD camera to assess the adhesion of scales on hot-rolled steel strips produced from medium and thin slabs. The result of this test was reported in this paper. 2. Experimental procedure 2.1. Materials The effect of medium and thin slabs on mechanical adhesion was evaluated via the as-received hot-rolled low carbon steel. The steel was cut from strips produced from medium slab called in the present work medium slab and steel produced from thin slab called in the present work thin slab. There slabs were obtained from the electric-arcfurnace route. The finishing and coiling temperatures of the medium and thin slabs were 880°C and 580°C
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respectively. The thickness of study steel was 2.2 mm for steel produced from medium slab and 2.4 mm for steel produced from thin slab. There chemical compositions were shown in Table 2. Table 2. Chemical compositions of the studied steel (wt.%). Steel Medium slab Thin slab
C 0.067 0.057
Si 0.242 0.173
Cu 0.199 0.195
Mn 0.387 0.376
Al 0.030 0.008
P 0.007 0.020
S 0.008 0.005
Fe bal. bal.
2.2. Physico-chemical characterisation The oxide scales structure was observed by the scanning electron microscope (SEM) (FEI, Model QUANTA 450). The energy dispersive X-ray spectroscopy (EDS) (OXFORD INSTRUMENTS, Model X-Max) was equipped with SEM for elemental analysis. The oxide phases were determined by the X-ray diffraction (XRD) technique (BRUKER, Model D8 Advance) using Cu Kα line (λ = 0.15406 nm) with the step size of 0.02 degree/step and the step time of 0.5 s/step. 2.3. Adhesion test The macro-tensile testing machine (Instron, Model 5566) with the load of 10 kN was used. The test was operated at room temperature with the strain rate of 0.04 s–1. The evolution of scales failure was monitored by a high magnification lens with a CCD camera. The video processing with the resolution of 640 × 480 pixels was used. The image acquisition was employed by image framework programming. The frame rate of the CCD camera was 7.5 frames per second. The tensile testing machine with the scales observation set was shown in Fig. 1.
Fig. 1. The tensile testing machine with the observation set.
2.4. Pickling test An immersion test was performed in this work to evaluate pickling ability (Fig. 2). For sample preparation for the test, the as-received hot-rolled steels were prepared with the width and length of 10 and 20 mm, respectively, cleaned with alcohol and dried in air before immediately immersed in a 10%v/v HCl pickling solution at 80°C during 1 to 60 seconds. Weight loss per unit area was recorded as a function of pickling time. After the immersion test, weight loss is plotted as a function of the pickling time. By this graph, dissolution rate of scales and period of complete pickling can be determined as will see in the following results and discussion.
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Distilled water 2
10% HCl at 80°C
Ethanol
Distilled water 1
Distilled water 3
Fig. 2. Immersion testing pattern.
3. Results and discussion 3.1. Physico-chemical characterisation A side view of the scales after the cross section subjected to the SEM was shown in Fig. 3. The averaged thickness of the scales actually formed on the as-received hot-rolled produced from medium and thin slabs was 8.69 and 7.64 µm respectively. It should be noted that the thicknesses of medium and thin slabs before the hot rolling process were particularly varied, but they do not affect to the thicknesses of final oxide scales on steel strips. However, the thicknesses of oxide scales has extremely affected the mechanical adhesion energy. According to Xray diffraction results shown in Fig. 4, hematite and magnetite peaks were observed on the both steels. a
3000 x
b Resin
3000 x
Resin
Fe2O3
Fe2O3
FeO + Fe3O4
FeO + Fe3O4
Medium slab
Thin slab
10 μm
10 μm
Fig. 3. SEM cross-sections of scales on the as-received hot-rolled steel produced from (a) medium slab, and (b) thin slab.
3.2. Adhesion test The macro-tensile tests were employed to investigate the mechanical adhesion of scales on the as-received hotrolled steel produced from medium and thin slabs. During tensile loading, transverse cracks perpendicular to the tensile loading were first observed, followed by local scales spallation.
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b
a
Fig. 4. X-ray diffraction patterns of the as-received hot-rolled steel produced from (a) medium slab; and (b) thin slab.
Figs. 5 and 6 depicts the scales failure after the tensile test which was observed by SEM. A spectrum from an energy dispersive spectroscopy (EDS) of the substrate zone in Figs. 5 (b) and 6 (b) corresponding to the steel-scales interface was shown in Figs. 5 (c) and 6 (c). On that area, the EDS spectrum includes the peaks of Fe, O, C as well as Si. The amount of their elements analysed by EDS point technique was shown in Table 3. It can be observed that the higher amount of Si and O on a steel produced from medium slab than that the steel produced from thin slab. This is directly related to the scales adhesion on steel substrates. It has been widely reported that the oxide containing Si, i.e. fayalite, existing at the steel-scales interface, promoted mechanical adhesion of scales on steel substrates [15,17,40-42] and worsened the picklability of scales [42]. a
ε = 10.67%
200 x
b
3000 x Scales
Spalled area
Scales Steel substrate
100 μm
c
10 μm
Fe Fe O
C
Si
Fe
Fig. 5. (a) Oxide scales and spalled area on the as-received hot-rolled steel produced from medium slab after the tensile test; (b) internal interface between scales and steel substrate magnified from (a); and (c) EDS pattern on metal substrate observed on the substrate area in the upper micrograph.
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a
ε = 10.11%
200 x
b
3000 x
Scales
Spalled area Steel substrate Scales
10 μm
100 μm
c
Fe Fe O
C
Si
Fe
Fig. 6. (a) Oxide scales and spalled area on the as-received hot-rolled steel produced from thin slab after the tensile test; (b) internal interface between scales and steel substrate magnified from (a); and (c) EDS pattern on metal substrate observed on the substrate area in the upper micrograph. Table 3. Elements analysed by EDS point technique of the studied steel (wt.%). Steel Medium slab Thin slab
C 8.26 6.48
O 4.13 2.79
Si 1.02 0.64
Fe 86.59 90.08
The spallation ratio was quantified as the area where scales was spalled out normalised by the total area taken in the picture, in the following. Fig. 7 show the results of spallation ratio was subjected to the imposed strain for the asreceived hot-rolled steel produced from medium and thin slabs. It was found that the spallation ratio of scales on the steel produced from medium slab was lower than that of the steel produced from thin slab. The two times of tensile testing gave the same tendency. Discussing with the spallation ratio, it was found that scales on the steel produced from medium and thin slabs was first spalled, it is more sensitive to spall out with the increase of the imposed strain.
Fig. 7. Spallation ratio of scales on the as-received hot-rolled steel produced from medium and thin slabs as a function of imposed strain.
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Fig. 8 (a), (b) presents strain initiating the first spallation and mechanical adhesion energy respectively. The result shows that strain inducing the first spallation of scales on the steel produced from medium slab was higher than that of scales on the steel produced from thin slab. These results indicated the higher quantified adhesion energies of the scales actually formed on the hot-rolled steel produced from medium slab than that of the scales actually formed on the steel produced from thin slab. Table 4 lists information of the studied steels concerning the thickness of scales actually formed on the hot-rolled steel, strain initiating the first spallation, and mechanical adhesion energy calculated using our model. b
a
Fig. 8. Strain initiating the first spallation and mechanical adhesion energy of scales on the steel produced from medium and thin slabs. Table 4. Information of the studied steel obtained from tensile test. Parameters Oxide thickness (µm) Strain initiating the first spallation (%) Mechanical adhesion energy (J/m2)
Steel produced from medium slab 8.69 1.17± 0.05 37.88 ± 3.60
Steel produced from thin slab 7.64 0.71 ± 0.12 11.32 ± 3.91
A higher measured strain initiating the first spallation of the hot-rolled steel produced from medium slab gave rise to the higher mechanical adhesion energy of the steel produced from medium slab compared to the steel produced from thin slab. The particular chemical composition of the steel produced from medium slab which contains Si higher than the steel produced from thin slab could be one of the reasons of the observed difference. At the internal interface between scales and the steel produced from medium slab, oxide containing Si promoted mechanical adhesion of scales on the steel substrates. In the present case, interfacial silica precipitates increased adhesion between scales and the substrates, resulting in the higher adhesion on a steel produced from medium slab than that the other steel produced from thin slab. 3.3 Pickling test In the experiment, the as-received hot-rolled steel produced from medium slab and steel produced from thin slab was immersed in a 10%HCl solution at 80°C. Fig. 9 plots weight loss per unit area as a function of the pickling time. The initial slope of the curve corresponds to the dissolution rate of the scales. This value is represented in Fig. 10 (a). The slopes at long pickling times, graphically observed to be identical for the two lines in Fig. 9, correspond to the corrosion rate of the steel substrate. From Fig. 9, the dissolution rate of scales on the steel produced from medium slab was slower than that of scales on the steel produced from thin slab. It was also observed that the period of time for the complete pickling of scales on the steel produced from medium slab was ca. 25.5 seconds. It was a bit larger than that of the steel produced from thin slab which was ca. 23 seconds (Fig. 10 b). It might also relate to the high mechanical adhesion of the scales actually formed on the steel produced from medium slab due to the oxide layer contained higher Si.
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Fig. 9. Weight loss for the steel produced from medium and thin slabs in 10%v/v HCl solution at 80°C as a function of pickling time.
a
b
Fig. 10. (a) dissolution rate; and (b) pickling time of scales on the steel produced from medium and thin slabs in 10%v/v HCl solution at 80°C.
4. Conclusions The adhesion behaviour of scales on the as-received hot-rolled steel strips produced from medium and thin slabs was studied. The following conclusions can be drawn. 1. Mechanical adhesion energy conducted by the tensile test of the scales actually formed on the steel produced from medium and thin slabs were 38 and 11 J.m-2 respectively. These results indicated the higher scales adhesion on the steel produced from a medium slab. 2. The pickling test was also conducted. The dissolution rate of scales on the steel produced from medium slab was slower than that of scales on the steel produced from thin slab. The period of time for the complete pickling on the steel produced from medium slab was longer than that of the steel produced from thin slab. These results indicated the higher scales adhesion on the steel produced from medium slab. It might be due to the steel contained more Si oxide, which is hard to be dissolved. Acknowledgements This project is supported by King Mongkut’s University of Technology North Bangkok (KMUTNB), Science and Technology Research Institute (STRI) (grant contract no. KMUTNB-NEW-59-06) and the faculty of engineering (grant contract no. 57-10-09-217). G Steel Public Company Limited is acknowledged for the provision of the hot-rolled steel strips for the study.
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References [1] R.J. Fruehan, Steelmaking and refining volume, eleven ed., The AISE Foundation, USA, 1998. [2] A. Ghosh, A. Chatterjee, Ironmaking and steelmaking: Theory and practice, Prentice Hall of India, New Delhi, India, 2008. [3] R.Y. Chen, W.Y.D, Oxide scaless on hot-rolled steel strips, in: W. Gao, Z. Li(Eds.) Developments in High-Temperature Corrosion and Protection of Materials, Woodhead Publishing, Cambridge, 2008, pp. 192-252. [4] P. Kofstad, High temperature corrosion, Elsevier Applied Science, London, UK, 1988. [5] R.Y. Chen, W.Y.D, Oxid. Met. 53 (2000) 539-560. [6] R.Y. Chen, W.Y.D, Oxid. Met. 56 (2001) 89-118. [7] R.Y. Chen, W.Y.D, Oxid. Met. 57 (2002) 53-79. [8] R.Y. Chen, W.Y.D, Oxid. Met. 59 (2003) 433-468. [9] R.Y. Chen, W.Y.D, ISIJ Int. 45 (2005) 52-59. [10] P. Sarrazin, A. Galerie, J. Fouletier, Mechanisms of high temperature corrosion: a kinetic approach, Materials Science Foundations, 36-37, Trans Tech Publications, Stafa-Zürich, CH, Switzerland, 2008. [11] M.J.L. Gines Benitez, T. Perez, E. Merli, M.A. Firpo, W. Egli, Latin American App. Res. (2002) 281-288. [12] Z.Y. Jiang A.K. Tieu, W.H. Sun, D.B. Wei, Mater. Sci. Eng. A. 435-436 (2006) 434-438. [13] L. Suarez, R. Petrov, L. Kestens, M. Lamberigts, Y. Houbaert, Mater. Sci. Forum. 550 (2007) 557-562. [14] M. Zhang, G. Shao, Mater. Sci. Eng. A. 452-453 (2007) 189-193. [15] Y.-L Yang, C.-H. Yang, S.-N. Lin, C.-H. Chen, W.-T. Tsai, Mater. Chem. Phys. 112 (2008) 566-571. [16] A. Segawa, Mater. Sci. Forum. 696 (2011) 150-155. [17] S. Taniguchi, K. Yamamoto, D. Megumi, T. Shibata, Mater. Sci. Eng. A. 308 (2001) 250-257. [18] L. Suarez, J. Schneider, Y. Houbaert, Defect Diffus. Forum 273-276 (2008) 655-660. [19] S. Chandra-ambhorn, T. Nilsonthi, Y. Wouters, A. Galerie, Corros. Sci. 87 (2014) 101-110. [20] E. Ahtoy, M. Picard, G. Leprince, A. Galerie, Y. Wouters, X. Wang, A. Atkinson, Mater. Chem. Phys. 148 (2014) 1157-1162. [21] W. Wongpromrat, H. Thaikan, W. Chandra-ambhorn, S. Chandra-ambhorn, Oxid. Met. 79 (2013) 529-540. [22] P. Promdirek, G. Lothongkum, S. Chandra-ambhorn, Y. Wouters, A. Galerie, Oxid. Met. 81 (2014) 315-329. [23] W. Wongpromrat, V. Parry, F. Charlot, A. Crisci, L. Latu-Romain, W. Chandra-ambhorn, S. Chandra-ambhorn, A. Galerie, Y. Wouters, Mater. High Temp. 32 (2015) 22-27. [24] A. Chattopadhyay, N. Bandyopadhyay, A.K. Das, M.K. Panigrahi, Scrip. Mater. 52 (2005)211-215. [25] S. Chandra-ambhorn, T. Somphakdee, W. Chandra-ambhorn, Mater. Sci. Forum. 696 (2011) 156-161. [26] A. Galerie, F. Toscan, E. N’Dah, K. Przybylski, Y. Wouters, M. Dupeux, Mater. Sci. Forum 461-464 (2004) 631-638. [27] M.M. Nagl, W.T. Evans, D.J. Hall, S.R.J. Saunders, J. Phy. IV. 3 (1993) 933-941. [28] M.M. Nagl, S.R.J. Saunders, W.T. Evans, D.J. Hall, Corros. Sci. 35 (1993) 965-969. [29] M.M. Nagl, W.T. Evans, D.J. Hall, S.R.J. Saunders, Oxid. Met. 42 (1994) 431-449. [30] J. Mougin, M. Dupeux, L. Antoni, A. Galerie, Mater. Sci. Eng. A. 359 (2003) 44-51. [31] F. Toscan, L. Antoni, Y. Wouters, M. Dupeux, A. Galerie, Mater. Sci. Forum. 461-464 (2004) 705-712. [32] S. Chandra-ambhorn, F. Roussel-Dherbey, F. Toscan, Y. Wouters, A. Galerie, M. Dupeux, Mater. Sci. Tech. 23 (2007) 497-501. [33] G. Bamba, Y. Wouters, A. Galerie, F. Charlot, A. Dellali. Acta Materia. 54 (2006) 3917-3922. [34] S. Chandra-ambhorn, T. Nilsonthi, Y. Madi, A. Galerie, Key Eng. Mater. 410-411 (2009) 187-193. [35] S. Chandra-ambhorn, N. Klubvihok, Oxid. Met. 85 (2016) 103-125. [36] T. Nilsonthi, Key Eng. Mater. 658 (2015) 106-110. [37] K. Ngamkham, S. Niltawach, S. Chandra-ambhorn, Key Eng. Mater. 462-463 (2011) 407-412. [38] S. Chandra-ambhorn, K. Ngamkham, N. Jiratthanakul, Oxid. Met. 80 (2013) 61-72. [39] T. Nilsonthi, J. Tungtrongpairoj, S. Chandra-ambhorn, Y. Wouters, A. Galerie, Steel Res. Int. (2012) 987-990. [40] J. Tungtrongpairoj, Study of thermal oxide scaless on hot rolled carbon steel strips produced from EAF-type slab and different Si containing BF-type slabs, Master Thesis (in English), KMUTNB, Thailand, 2009. [41] S. Chandra-ambhorn, T. Nilsonthi, Y. Wouters, A. Galerie, Steel Res. Int. 81 (2010) 130-133. [42] A. Chattopadhyay, T. Chanda, Scrip. Mater. 58 (2008) 882-885.
ScienceDirect Materials Today: Proceedings 5 (2018) 9359–9367
www.materialstoday.com/proceedings
The 10th Thailand International Metallurgy Conference (The 10th TIMETC)
Adhesion behaviour of scales on hot-rolled steel strips produced from continuous casting slabs Nisachon Na Kalasin, Sirilak Yenchum, Thanasak Nilsonthi* High Temperature Corrosion Research Centre, Department of Materials and Production Technology Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, 1518 Pracharat 1 Road, Wongsawang, Bangsue, Bangkok 10800, Thailand
Abstract The objective of this research was to study the adhesion behaviour of scales on the as-received hot-rolled steel strips produced from continuous casting slabs, i.e. medium and thin slabs. The adhesion test was conducted by the tensile testing machine equipped with a CCD camera to instantaneously observe the scales failure during the test. The adhesion test was also conducted by pickling test via immersion in a 10 %v/v HCl solution at 80°C for observing the scales removing rate. There were two approaches that can be used to evaluate the scales adhesion on the studies steels. This was due to during the hot rolling process the steel strips covers with oxide scales. This scales must be removed by the acid pickling process before further process, e.g. cold rolling process. The results shown that the as-received hot-rolled steel strips produced from medium slab was exhibited higher scales adhesion more than the steel produced from thin slab for both tests. The scales thicknesses on the as-received hotrolled steel strips produced from medium and thin slabs were 9 and 8 μm respectively. A result from tensile test, the mechanical adhesion energies of the scales actually formed on the hot-rolled steel strips produced from medium and thin slabs were 38 and 11 J.m-2 respectively. A result from pickling test, the scales dissolution rate on the as-received hot-rolled steel strips produced from medium and thin slabs were 0.0025 and 0.008 mg.cm-2.s-1. The result was also shown that the time for the complete pickling of scales on the hot-rolled steel strips produced from medium and thin slabs were 25.5 and 23 seconds respectively. This was due to the presence of high amounts of interfacial silica, which was promoted scales adhesion, indicating that longer time needed for scales removal. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Thermal oxide scales; Mechanical adhesion; Pickling behaviour; Hot-rolled steel strips
* Corresponding author. Tel.: +66-2555-2000 Ext. 8610; fax: +66-2587-4335. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.
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1. Introduction The raw material used in a hot rolling process is a slab which can be produced by different routes [1,2]. A slab can be produced by a blast-furnace route starting from iron ore, coal and limestone. The iron produced by a blast furnace is called pig iron. The pig iron is further refined in a steelmaking process resulting in a slab. A slab can also be produced by an electric-arc-furnace route. In this path, a slab is made of recycled steel, or scraps, using an electric-arc-furnace and secondary refining process. Because the scraps are used as main raw material, some elements exist in the steel with the higher content than the one produced using the blast-furnace route, e.g. Si, Cu, Sn, Pb or As [2]. Slabs produced by those processes might be classified by their thickness. The conventional slab produced from the BF route has a typical thickness of ca. 200 mm. For the EAF route, the recycled slab can be either a medium slab in case that its thickness is ca. 80 to 100 mm, or a thin slab in case that its thickness is ca. 50 mm [2]. In a hot rolling process, a slab is reheated in a reheating furnace temperature around 1250 to 1300°C, a primary scales thickness around 2 mm is formed on the slab surface. The scales can be removed by hydraulic descalesr by high pressure water jet, firstly before the entry of slab into the roughing mill and secondly before the entry of steel bar into the finishing stands. After the first descaling, secondary scales produce rapidly. Its thickness is usually less than 100 μm. This scales are secondly removed by water jet before entering the finishing mill. Afterwards, tertiary oxide scales layer developer in the finishing stands at temperature less than 1000°C. They are formed at the first stand, between stands and after the last stand. The oxide scales continuously grow during the steel strips coiling at the temperature less than 1000°C and also during storage until the temperature cools down to room temperature. After the hot rolling process, mechanical breaking of oxide scales followed by acid pickling is normally used to remove the hot-rolling scales. This is an important factor to surface quality of the steel strips for acceptance of the product. The condition of scales formation in a conventional hot strip processing route is summarised in Table 1. Table 1. Scales formation in a conventional hot strip processing route [3]. Type of scales Primary scales Secondary scales Tertiary scales Scales retained on strip
Thickness 1 – 5 mm 100 – 300 μm 10 – 30 μm 5 – 15 μm
Temperatures 1200 – 1300°C 1100 – 1200°C 850 – 1100°C 500 – 880°C
Atmosphere Furnace gas Air/steam Air/steam Air/steam
Duration Hours Minutes Seconds Days
The oxide scales development on the hot-rolled steel strips is reported [4-16]. A classic three-layered oxide scales is produced, which comprises a thin hematite layer, an intermediate magnetite layer, and a thick wustite layer. Sometimes magnetite is observed in the wustite layer due to eutectoid decomposition at cooling. Sometimes also, the hematite layer may be absent. The structure and properties of the oxide scales are influenced by various effects, e.g. alloying elements [17-20] or the oxidising atmospheres [21-23]. The surface quality of the hot-rolled steel product is related to the pickling ability [24-25] and scales adhesion. Several test methods for assessing the scales adhesion have been developed, e.g. the bending test [26-29], the inverted blister test [30] or the micro-tensile test operated in a chamber of a scanning electron microscope [31-36]. Scales adhesion tested by those methods is mainly used on stainless steel. No extensive work has been applied to assess scales adhesion on hot-rolled steel. Our group has been developed by using the macro-tensile testing machine [37-39]. In the present work, the scales failure during tensile loading was conducted by the tensile test with a CCD camera to assess the adhesion of scales on hot-rolled steel strips produced from medium and thin slabs. The result of this test was reported in this paper. 2. Experimental procedure 2.1. Materials The effect of medium and thin slabs on mechanical adhesion was evaluated via the as-received hot-rolled low carbon steel. The steel was cut from strips produced from medium slab called in the present work medium slab and steel produced from thin slab called in the present work thin slab. There slabs were obtained from the electric-arcfurnace route. The finishing and coiling temperatures of the medium and thin slabs were 880°C and 580°C
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respectively. The thickness of study steel was 2.2 mm for steel produced from medium slab and 2.4 mm for steel produced from thin slab. There chemical compositions were shown in Table 2. Table 2. Chemical compositions of the studied steel (wt.%). Steel Medium slab Thin slab
C 0.067 0.057
Si 0.242 0.173
Cu 0.199 0.195
Mn 0.387 0.376
Al 0.030 0.008
P 0.007 0.020
S 0.008 0.005
Fe bal. bal.
2.2. Physico-chemical characterisation The oxide scales structure was observed by the scanning electron microscope (SEM) (FEI, Model QUANTA 450). The energy dispersive X-ray spectroscopy (EDS) (OXFORD INSTRUMENTS, Model X-Max) was equipped with SEM for elemental analysis. The oxide phases were determined by the X-ray diffraction (XRD) technique (BRUKER, Model D8 Advance) using Cu Kα line (λ = 0.15406 nm) with the step size of 0.02 degree/step and the step time of 0.5 s/step. 2.3. Adhesion test The macro-tensile testing machine (Instron, Model 5566) with the load of 10 kN was used. The test was operated at room temperature with the strain rate of 0.04 s–1. The evolution of scales failure was monitored by a high magnification lens with a CCD camera. The video processing with the resolution of 640 × 480 pixels was used. The image acquisition was employed by image framework programming. The frame rate of the CCD camera was 7.5 frames per second. The tensile testing machine with the scales observation set was shown in Fig. 1.
Fig. 1. The tensile testing machine with the observation set.
2.4. Pickling test An immersion test was performed in this work to evaluate pickling ability (Fig. 2). For sample preparation for the test, the as-received hot-rolled steels were prepared with the width and length of 10 and 20 mm, respectively, cleaned with alcohol and dried in air before immediately immersed in a 10%v/v HCl pickling solution at 80°C during 1 to 60 seconds. Weight loss per unit area was recorded as a function of pickling time. After the immersion test, weight loss is plotted as a function of the pickling time. By this graph, dissolution rate of scales and period of complete pickling can be determined as will see in the following results and discussion.
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Distilled water 2
10% HCl at 80°C
Ethanol
Distilled water 1
Distilled water 3
Fig. 2. Immersion testing pattern.
3. Results and discussion 3.1. Physico-chemical characterisation A side view of the scales after the cross section subjected to the SEM was shown in Fig. 3. The averaged thickness of the scales actually formed on the as-received hot-rolled produced from medium and thin slabs was 8.69 and 7.64 µm respectively. It should be noted that the thicknesses of medium and thin slabs before the hot rolling process were particularly varied, but they do not affect to the thicknesses of final oxide scales on steel strips. However, the thicknesses of oxide scales has extremely affected the mechanical adhesion energy. According to Xray diffraction results shown in Fig. 4, hematite and magnetite peaks were observed on the both steels. a
3000 x
b Resin
3000 x
Resin
Fe2O3
Fe2O3
FeO + Fe3O4
FeO + Fe3O4
Medium slab
Thin slab
10 μm
10 μm
Fig. 3. SEM cross-sections of scales on the as-received hot-rolled steel produced from (a) medium slab, and (b) thin slab.
3.2. Adhesion test The macro-tensile tests were employed to investigate the mechanical adhesion of scales on the as-received hotrolled steel produced from medium and thin slabs. During tensile loading, transverse cracks perpendicular to the tensile loading were first observed, followed by local scales spallation.
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b
a
Fig. 4. X-ray diffraction patterns of the as-received hot-rolled steel produced from (a) medium slab; and (b) thin slab.
Figs. 5 and 6 depicts the scales failure after the tensile test which was observed by SEM. A spectrum from an energy dispersive spectroscopy (EDS) of the substrate zone in Figs. 5 (b) and 6 (b) corresponding to the steel-scales interface was shown in Figs. 5 (c) and 6 (c). On that area, the EDS spectrum includes the peaks of Fe, O, C as well as Si. The amount of their elements analysed by EDS point technique was shown in Table 3. It can be observed that the higher amount of Si and O on a steel produced from medium slab than that the steel produced from thin slab. This is directly related to the scales adhesion on steel substrates. It has been widely reported that the oxide containing Si, i.e. fayalite, existing at the steel-scales interface, promoted mechanical adhesion of scales on steel substrates [15,17,40-42] and worsened the picklability of scales [42]. a
ε = 10.67%
200 x
b
3000 x Scales
Spalled area
Scales Steel substrate
100 μm
c
10 μm
Fe Fe O
C
Si
Fe
Fig. 5. (a) Oxide scales and spalled area on the as-received hot-rolled steel produced from medium slab after the tensile test; (b) internal interface between scales and steel substrate magnified from (a); and (c) EDS pattern on metal substrate observed on the substrate area in the upper micrograph.
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a
ε = 10.11%
200 x
b
3000 x
Scales
Spalled area Steel substrate Scales
10 μm
100 μm
c
Fe Fe O
C
Si
Fe
Fig. 6. (a) Oxide scales and spalled area on the as-received hot-rolled steel produced from thin slab after the tensile test; (b) internal interface between scales and steel substrate magnified from (a); and (c) EDS pattern on metal substrate observed on the substrate area in the upper micrograph. Table 3. Elements analysed by EDS point technique of the studied steel (wt.%). Steel Medium slab Thin slab
C 8.26 6.48
O 4.13 2.79
Si 1.02 0.64
Fe 86.59 90.08
The spallation ratio was quantified as the area where scales was spalled out normalised by the total area taken in the picture, in the following. Fig. 7 show the results of spallation ratio was subjected to the imposed strain for the asreceived hot-rolled steel produced from medium and thin slabs. It was found that the spallation ratio of scales on the steel produced from medium slab was lower than that of the steel produced from thin slab. The two times of tensile testing gave the same tendency. Discussing with the spallation ratio, it was found that scales on the steel produced from medium and thin slabs was first spalled, it is more sensitive to spall out with the increase of the imposed strain.
Fig. 7. Spallation ratio of scales on the as-received hot-rolled steel produced from medium and thin slabs as a function of imposed strain.
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Fig. 8 (a), (b) presents strain initiating the first spallation and mechanical adhesion energy respectively. The result shows that strain inducing the first spallation of scales on the steel produced from medium slab was higher than that of scales on the steel produced from thin slab. These results indicated the higher quantified adhesion energies of the scales actually formed on the hot-rolled steel produced from medium slab than that of the scales actually formed on the steel produced from thin slab. Table 4 lists information of the studied steels concerning the thickness of scales actually formed on the hot-rolled steel, strain initiating the first spallation, and mechanical adhesion energy calculated using our model. b
a
Fig. 8. Strain initiating the first spallation and mechanical adhesion energy of scales on the steel produced from medium and thin slabs. Table 4. Information of the studied steel obtained from tensile test. Parameters Oxide thickness (µm) Strain initiating the first spallation (%) Mechanical adhesion energy (J/m2)
Steel produced from medium slab 8.69 1.17± 0.05 37.88 ± 3.60
Steel produced from thin slab 7.64 0.71 ± 0.12 11.32 ± 3.91
A higher measured strain initiating the first spallation of the hot-rolled steel produced from medium slab gave rise to the higher mechanical adhesion energy of the steel produced from medium slab compared to the steel produced from thin slab. The particular chemical composition of the steel produced from medium slab which contains Si higher than the steel produced from thin slab could be one of the reasons of the observed difference. At the internal interface between scales and the steel produced from medium slab, oxide containing Si promoted mechanical adhesion of scales on the steel substrates. In the present case, interfacial silica precipitates increased adhesion between scales and the substrates, resulting in the higher adhesion on a steel produced from medium slab than that the other steel produced from thin slab. 3.3 Pickling test In the experiment, the as-received hot-rolled steel produced from medium slab and steel produced from thin slab was immersed in a 10%HCl solution at 80°C. Fig. 9 plots weight loss per unit area as a function of the pickling time. The initial slope of the curve corresponds to the dissolution rate of the scales. This value is represented in Fig. 10 (a). The slopes at long pickling times, graphically observed to be identical for the two lines in Fig. 9, correspond to the corrosion rate of the steel substrate. From Fig. 9, the dissolution rate of scales on the steel produced from medium slab was slower than that of scales on the steel produced from thin slab. It was also observed that the period of time for the complete pickling of scales on the steel produced from medium slab was ca. 25.5 seconds. It was a bit larger than that of the steel produced from thin slab which was ca. 23 seconds (Fig. 10 b). It might also relate to the high mechanical adhesion of the scales actually formed on the steel produced from medium slab due to the oxide layer contained higher Si.
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Fig. 9. Weight loss for the steel produced from medium and thin slabs in 10%v/v HCl solution at 80°C as a function of pickling time.
a
b
Fig. 10. (a) dissolution rate; and (b) pickling time of scales on the steel produced from medium and thin slabs in 10%v/v HCl solution at 80°C.
4. Conclusions The adhesion behaviour of scales on the as-received hot-rolled steel strips produced from medium and thin slabs was studied. The following conclusions can be drawn. 1. Mechanical adhesion energy conducted by the tensile test of the scales actually formed on the steel produced from medium and thin slabs were 38 and 11 J.m-2 respectively. These results indicated the higher scales adhesion on the steel produced from a medium slab. 2. The pickling test was also conducted. The dissolution rate of scales on the steel produced from medium slab was slower than that of scales on the steel produced from thin slab. The period of time for the complete pickling on the steel produced from medium slab was longer than that of the steel produced from thin slab. These results indicated the higher scales adhesion on the steel produced from medium slab. It might be due to the steel contained more Si oxide, which is hard to be dissolved. Acknowledgements This project is supported by King Mongkut’s University of Technology North Bangkok (KMUTNB), Science and Technology Research Institute (STRI) (grant contract no. KMUTNB-NEW-59-06) and the faculty of engineering (grant contract no. 57-10-09-217). G Steel Public Company Limited is acknowledged for the provision of the hot-rolled steel strips for the study.
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References [1] R.J. Fruehan, Steelmaking and refining volume, eleven ed., The AISE Foundation, USA, 1998. [2] A. Ghosh, A. Chatterjee, Ironmaking and steelmaking: Theory and practice, Prentice Hall of India, New Delhi, India, 2008. [3] R.Y. Chen, W.Y.D, Oxide scaless on hot-rolled steel strips, in: W. Gao, Z. Li(Eds.) Developments in High-Temperature Corrosion and Protection of Materials, Woodhead Publishing, Cambridge, 2008, pp. 192-252. [4] P. Kofstad, High temperature corrosion, Elsevier Applied Science, London, UK, 1988. [5] R.Y. Chen, W.Y.D, Oxid. Met. 53 (2000) 539-560. [6] R.Y. Chen, W.Y.D, Oxid. Met. 56 (2001) 89-118. [7] R.Y. Chen, W.Y.D, Oxid. Met. 57 (2002) 53-79. [8] R.Y. Chen, W.Y.D, Oxid. Met. 59 (2003) 433-468. [9] R.Y. Chen, W.Y.D, ISIJ Int. 45 (2005) 52-59. [10] P. Sarrazin, A. Galerie, J. Fouletier, Mechanisms of high temperature corrosion: a kinetic approach, Materials Science Foundations, 36-37, Trans Tech Publications, Stafa-Zürich, CH, Switzerland, 2008. [11] M.J.L. Gines Benitez, T. Perez, E. Merli, M.A. Firpo, W. Egli, Latin American App. Res. (2002) 281-288. [12] Z.Y. Jiang A.K. Tieu, W.H. Sun, D.B. Wei, Mater. Sci. Eng. A. 435-436 (2006) 434-438. [13] L. Suarez, R. Petrov, L. Kestens, M. Lamberigts, Y. Houbaert, Mater. Sci. Forum. 550 (2007) 557-562. [14] M. Zhang, G. Shao, Mater. Sci. Eng. A. 452-453 (2007) 189-193. [15] Y.-L Yang, C.-H. Yang, S.-N. Lin, C.-H. Chen, W.-T. Tsai, Mater. Chem. Phys. 112 (2008) 566-571. [16] A. Segawa, Mater. Sci. Forum. 696 (2011) 150-155. [17] S. Taniguchi, K. Yamamoto, D. Megumi, T. Shibata, Mater. Sci. Eng. A. 308 (2001) 250-257. [18] L. Suarez, J. Schneider, Y. Houbaert, Defect Diffus. Forum 273-276 (2008) 655-660. [19] S. Chandra-ambhorn, T. Nilsonthi, Y. Wouters, A. Galerie, Corros. Sci. 87 (2014) 101-110. [20] E. Ahtoy, M. Picard, G. Leprince, A. Galerie, Y. Wouters, X. Wang, A. Atkinson, Mater. Chem. Phys. 148 (2014) 1157-1162. [21] W. Wongpromrat, H. Thaikan, W. Chandra-ambhorn, S. Chandra-ambhorn, Oxid. Met. 79 (2013) 529-540. [22] P. Promdirek, G. Lothongkum, S. Chandra-ambhorn, Y. Wouters, A. Galerie, Oxid. Met. 81 (2014) 315-329. [23] W. Wongpromrat, V. Parry, F. Charlot, A. Crisci, L. Latu-Romain, W. Chandra-ambhorn, S. Chandra-ambhorn, A. Galerie, Y. Wouters, Mater. High Temp. 32 (2015) 22-27. [24] A. Chattopadhyay, N. Bandyopadhyay, A.K. Das, M.K. Panigrahi, Scrip. Mater. 52 (2005)211-215. [25] S. Chandra-ambhorn, T. Somphakdee, W. Chandra-ambhorn, Mater. Sci. Forum. 696 (2011) 156-161. [26] A. Galerie, F. Toscan, E. N’Dah, K. Przybylski, Y. Wouters, M. Dupeux, Mater. Sci. Forum 461-464 (2004) 631-638. [27] M.M. Nagl, W.T. Evans, D.J. Hall, S.R.J. Saunders, J. Phy. IV. 3 (1993) 933-941. [28] M.M. Nagl, S.R.J. Saunders, W.T. Evans, D.J. Hall, Corros. Sci. 35 (1993) 965-969. [29] M.M. Nagl, W.T. Evans, D.J. Hall, S.R.J. Saunders, Oxid. Met. 42 (1994) 431-449. [30] J. Mougin, M. Dupeux, L. Antoni, A. Galerie, Mater. Sci. Eng. A. 359 (2003) 44-51. [31] F. Toscan, L. Antoni, Y. Wouters, M. Dupeux, A. Galerie, Mater. Sci. Forum. 461-464 (2004) 705-712. [32] S. Chandra-ambhorn, F. Roussel-Dherbey, F. Toscan, Y. Wouters, A. Galerie, M. Dupeux, Mater. Sci. Tech. 23 (2007) 497-501. [33] G. Bamba, Y. Wouters, A. Galerie, F. Charlot, A. Dellali. Acta Materia. 54 (2006) 3917-3922. [34] S. Chandra-ambhorn, T. Nilsonthi, Y. Madi, A. Galerie, Key Eng. Mater. 410-411 (2009) 187-193. [35] S. Chandra-ambhorn, N. Klubvihok, Oxid. Met. 85 (2016) 103-125. [36] T. Nilsonthi, Key Eng. Mater. 658 (2015) 106-110. [37] K. Ngamkham, S. Niltawach, S. Chandra-ambhorn, Key Eng. Mater. 462-463 (2011) 407-412. [38] S. Chandra-ambhorn, K. Ngamkham, N. Jiratthanakul, Oxid. Met. 80 (2013) 61-72. [39] T. Nilsonthi, J. Tungtrongpairoj, S. Chandra-ambhorn, Y. Wouters, A. Galerie, Steel Res. Int. (2012) 987-990. [40] J. Tungtrongpairoj, Study of thermal oxide scaless on hot rolled carbon steel strips produced from EAF-type slab and different Si containing BF-type slabs, Master Thesis (in English), KMUTNB, Thailand, 2009. [41] S. Chandra-ambhorn, T. Nilsonthi, Y. Wouters, A. Galerie, Steel Res. Int. 81 (2010) 130-133. [42] A. Chattopadhyay, T. Chanda, Scrip. Mater. 58 (2008) 882-885.