Defect and Diffusion Forum
ISSN: 1662-9507, Vols. 273-276, pp 655-660
doi:10.4028/www.scientific.net/DDF.273-276.655
© 2008 Trans Tech Publications, Switzerland
Online: 2008-02-11
Effect of Si on High-Temperature Oxidation of Steel during Hot Rolling
L. Suarez1,a, J. Schneider1 and Y. Houbaert1,b
1
Department of Metallurgy and Materials Science, Ghent University, Technologiepark 903,
B-9052 Gent, Belgium.
a
[email protected], [email protected]
Keywords: oxide scales; hot rolling; silicon; fayalite
Abstract. An oxide scale layer always forms at the strip surface during the hot rolling process.
As a consequence, de-scaling and pickling operations must be performed prior or after hot rolling.
Many surface defects caused by hot rolling are related to oxidation in the reheating furnace. One of
these is the melting of eutectic FeO/Fe2SiO4 during reheating over 1170°C giving as a result red
scale defects in Si-added steel. On the other hand, steel strip surface oxidation during hot rolling
causes an industrial and environmental problem: secondary oxide is removed after roughing, but
tertiary oxide scales already start to form before entering the finishing stands. Their properties affect
the final steel surface quality and its response to further processing. Furthermore, the addition of
alloying elements has an important impact on scale properties. In particular the alloying of silicon
effects the region between scale and substrate. It causes peculiar surface properties inherited from its
specific oxidation characteristics.
Conventional oxidation experiments in air of silicon steels are a valuable tool to study the
influence of Si on steel oxidation. After oxidation in air in the temperature range of 900-1250°C it
has been observed that Si enhance markedly scale adhesion, especially above 1177°C (the eutectic
temperature of FeO-Fe2SiO4 ) and also at lower temperatures. Special attention has been paid on the
investigation of the effects of alloying Si on the high-temperature oxidation of steel, for a better
understanding of the behaviour of modern steels during hot rolling.
Introduction
In a hot strip mill, slabs are usually heated to a rolling temperature of approximately 1250°C
while the last hot working operation is often conducted above the upper cooling transformation
temperature Ar3. During hot rolling process, oxide layer (named scale) is always developing at the
strip surface. In order to reduce the thickness of this oxide, the strip goes through two de-scaling
stations where scale is removed by high-pressure water jets. Nevertheless, de-scaling is never 100%
effective; a very thin scale layer is always left on the metal surface. Contradictory, a thin scale is
necessary for rolling. It acts as a lubricant between the hot bar and the cold working roll. The slab
goes firstly from the reheating furnace to be discharged for roughing mill. To break the primary
scale, the slab is passed through the first de-scaler just before the roughing mill. Between the
successive rolling passes a secondary scale is formed, which is further removed by the second descaler before the strip enters the finishing mill. The time that the slab runs between the second descaler and the first rolling stand (5-10 s) is sufficient for a new tertiary scale to form [1].
Oxidation treatments in air at temperatures higher that 570°C leads to three different iron oxide
layers: wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3), in oxygen content increasing order,
going from substrate to free surface. Below 570°C, only the last two are thermodynamically stable
[2, 3]. According to the literature [4], the relative thickness fractions of these layers between 700
and 1200°C are 95% wustite, 4% magnetite and 1% hematite at equilibrium, although this balance
can vary from one case to another. It is worth noticing that wustite has got an extremely broad
composition range, characterised by Fe1-xO, with x comprised between 0.04 and 0.17. Furthermore,
the fact that wustite is thermodynamically unstable below 570°C has direct consequences on low
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-18/05/16,13:34:31)
656
Diffusion in Solids and Liquids III
temperature oxide scale behaviour [2, 3, 6]. At hot rolling temperatures, wustite exhibits a
remarkably high plasticity [5, 6].
Oxidation of steels is more
complex than iron. Their oxide
scales are mainly constituted of
iron oxide but their structures
change with the presence of
addition elements, which can be
oxidized too. These elements
sometimes contained in the steel
become concentrated at the
metal/scale
interface.
An
internal diffusion process into
the metal compensates this local
enrichment.
In
fact,
the
oxidation process provokes a
rapid accumulation of these
(more rapid than their inward
diffusion), as a result, the
interface becomes enriched in
Fig. 1. FeO-SiO2 binary system.
alloying additions. This causes
an enormous impact on the
surface quality of the semiproducts. Most alloying elements with high oxygen affinity accumulate in the scale at the
scale/metal interface (Al, Si, P, B, Cr, Mo, Nb, Ti), forming aluminates, silicates, phosphates,
etcetera. These compounds, which often they have a spinel structure; they usually retard the
oxidation reaction and promote void formation [7-9].
In the present work, special attention has been paid on the investigation of the effects of alloying
Si on the high-temperature oxidation of steel, for a better understanding of the behaviour of modern
steels during hot rolling. Si reacts with oxygen diffusing into the steel and precipitates as SiO2.
Then, SiO2 form a separate phase, fayalite Fe2SiO4 that retards the scaling rate. This phase melts at
1170°C (see Fig. 1) resulting in a liquid phase that increases sharply the scaling rate. The effect of
Si for Si-containing steels is studied in literature in connection with the appearance of red scale
[10]. Red scale is one of the defects, which has to be avoided to reach a high surface quality. It was
found that the formation of red Fe2O3 is accelerated by incomplete descaling of FeO. In Sicontaining steels descaling becomes worse due to the appearance of the eutectic compound FeOFe2SiO4 at the steel interface. Red scale consists in irregularly striped defects formed on hot rolled
high strength steel strips containing more than 0.5 % Si [11]. The cause of red scale is the existence
of the liquid phase of Fe2SiO4 which firmly bonds steel substrate and iron scale [10]. The liquid
phase of FeO-Fe2SiO4 penetrates into the scale and steel. After solidification it leads to a strong
adhesion of a part of the scale to the steel. The eutectic compound FeO-Fe2Si O4, which penetrates
into the steel, appears especially at the grain boundaries. Due to the appearance of fayalite at the
steel interface, descaling in Si-containing steels descaling becomes more difficult[11]. In this work
the structure of scale in electrical steels, which may contain Si up to about 3 wt% have been
investigated. Special attention has been paid to the formation of fayalite during the process of
preheating, hot rolling and coiling. To this purpose the temperature region of 750°C up to 1200°C
has been regarded.
Defect and Diffusion Forum Vols. 273-276
657
Experimental Procedure
In order to analyse the high temperature influence on the oxidation of different steel grades, three
steel compositions have been selected. Table 1 shows the compositions of the experimental alloys.
All specimens were 15mm x 15mm x 6.5 mm in dimensions. The samples were placed on refractory
pieces, which where introduced into the electrical resistance furnace for their treatment. Samples
were heated up at six different temperatures (650, 750, 950, 1050, 1150 and 1200ºC) for times
between 16 sec and 2 hours. The oxidation has been carried out by introducing directly the samples
in the oven (already preheated at the desired constant temperature of oxidation. No protection for
oxidation was made during treatment, as it was carried out on air, and the refractory piece (which
contains the specimens on it) was taken out at the end of the treatment for cooling to room
temperature.
Table 1. Chemical composition of the steels.
C
Si
Mn
P
Cr
Al
Cu
Ti
Alloy 1
0,005
0,078
0,55
0,051
0,027
0,036
0,042
0,063
Alloy 2
0,002
1,88
0,048
0,016
0,019
0,075
0,018
0,02
Alloy 3
0,003
3,06
0,066
0,016
0,025
0,096
0,019
0,003
Scale/steel cross sections. Metallographic preparation of specimens covered by oxide scale is
extremely delicate due to its hardness and brittleness. After each oxidation test, each sample was cut
using carefully a saw not to weaken the oxide scale. Most samples were observed in the crosssection. Metallographic samples were prepared with special care in thermo-setting resins with the
purpose of avoid oxide damage. Coated samples are then grounded using 80 to 1200 grit polishing
SiC paper. Finishing is performed with diamond pastes 3 and 1 µm to obtain a mirror polish. After
grinding and polishing, the cross-section of the samples was characterised and their scale thickness
was measured under the optical microscope (OM).
Two techniques were used in order to obtain the characterisation of the oxides layers: Optical
Microscopy (OM) and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEMEDS). SEM analyses were carried out on a ZEISS DSM962 equipped with an Energy Dispersive
Spectrometer (EDS) of Quantum Kevex Instruments. An accelerating voltage of 20keV was used
and Secondary Electron Images (SE) was taken. EDS2004 1.2 Software was used. The main
purpose of this software is to acquire and analyze x-ray spectra, x-ray maps and linescans, and
electron images, to perform a complete analytical analysis of a sample located in a Scanning
Electron Microscope. X-ray linescans are excellent tools for determining the location of phases,
particles, and other features on a sample that can be discriminated by variations in elemental
composition. An X-Ray linescan is performed by acquiring X-ray data at evenly space points along
a horizontal line on the sample. The X-ray is displayed on the screen as line graphs, one for each
element in the element list for the linescan.
658
Diffusion in Solids and Liquids III
Results and discussion
Fig. 2. Cross section of scales formed on different alloys. (a) Typical structure of classical three
layered oxide scale formed on ULC ( alloy 1) at 950ºC during 1 hour of oxidation; (b) Oxide scale
formed on a Fe-2%Si (alloy 2) at 1200°C during 8 min of oxidation; (c) Oxide scale formed on a Fe3%Si (alloy 3) at 1200°C during 8 min of oxidation; (d) Oxide scale formed on alloy 2 at 950°C
during 2 hours of oxidation; (e) Oxide scale formed on alloy 2 at 1150°C during 8 min of oxidation;
(f) Oxide scale formed on alloy 3 at 1150°C during 8 min of oxidation.
In general, the three different iron oxide phases can be found, namely Fe1-xO (wustite), Fe3O4
(magnetite) and Fe2O3 (hematite). Samples from alloy 1 (Table 1) were heated up and oxidised at
six different temperatures (650, 750, 950, 1050, 1150 and 1200ºC) for different times between 16
sec and 2 hours. They show typical results of a classic three layered oxide scale, which comprised a
thin hematite layer on the surface, and intermediate magnetite layer, and a thick wustite layer
immediately over the steel substrate. Figure 2(a) corresponds to cross-section image of ultra-low
carbon (ULC) samples oxidised at 950°C for long oxidation times of 1 hour. After complete
examination of the experiments with ULC (different times and temperatures (650, 750, 950 and
1050°C)), it was observed that the percentage of wustite is significant high over the temperature
range 900-1200°C. Outside this range, the percentage of wustite decreases, while percentages of
magnetite and hematite increase. Air cooling has obviously led to partial decomposition of the
wustite. Proeutectoid magnetite crystals are clearly visible within the wustite. Relating to this nohomogeneus scale aspect, observations agree with literature: growth stress deform and crack the
oxide layer generating porosity and cracks.
Samples from alloys 2 and 3 (table 3.1) are considered to be high in Si content (2 and 3 wt.%).
Samples were heated up and oxidised at six different temperatures (650, 750, 950, 1050, 1150 and
1200ºC) for different times between 16 sec and 2 hours. Oxidation was carried out in air. Fig. 2(b)
and (c) show the typical structure of scale developed under industrial conditions on steel containing
silicon: on the top of the scale normally there is a magnetite and wustite layer.
On the grain boundaries in the scale, iron-silicate can be found. Below this the so-called mosaic
layer can be observed with wustite and there is also a Si- enrichment at the scale–metal interface.
Figure 2(b) corresponds to cross-section images of Fe-2%Si ally sample oxidised at 1200°C during
8 minutes. In this case, the wustite-fayalite eutectic temperature of 1170ºC (Fig. 1) is surpassed. The
Defect and Diffusion Forum Vols. 273-276
659
oxide layer is made of a mixture of various iron oxides. On the top, it is possible to observe the
classical three layered structure of iron oxides, with hematite on the top, magnetite and finally
decomposed wustite (decomposition of wustite into magnetite and iron ). These layers are followed
by a layer made of FeO and fayalite eutectic which binds the innermost layer of the scale to the steel
surface by penetrating into both the substrate. Fig. 2(c) corresponds to Fe-3%Si ally sample oxidised
at 1200°C during 8 min. They show that the continuous layer of fayalite increases as more silicon is
available. Fig.2(d), (e) and (f) correspond to scale layers formed below the eutectic temperature. It is
evident that decreasing the oxidation temperature, the fayalite strongly retards the oxidation rate.
There is an accumulation of fayalite at the steel-oxide interface, which acts as a barrier obstructing
the diffusion of iron until enough stresses are developed within the crust to spall it.
Fig. 3. EDS interface linescans and concentration profiles. (a) and (b): cross section of a scale
formed on a Fe-2%Si (alloy 2) at 1200°C during 8 min of oxidation. (c) and (d):cross section
of a scale formed on a ULC sample oxidised at 1050ºC during 32min.
Although the scale morphology and phase constitution are clearly visible using optical
microscopy, scanning electron microscopy (SEM) was also used in order to confirm the scale
composition. This technique gives a direct conexion between chemical and morphological
information. Fig.3(a) shows a cross section of a scale formed on a Fe-2%Si (alloy 2) at 1200°C
during 8 min of oxidation with the linescan performed on it and its concentration profile (Fig.3(b)).
Figures 3(c) and (d) shows a cross section of a scale formed on a ULC sample oxidised at 1050ºC
during 32min and concentration profile respectively.
660
Diffusion in Solids and Liquids III
Conclusions
In this work the structure of scale in electrical steels, which may contain Si up to about 3 wt%
have been investigated. Special attention has been paid to the formation of fayalite during the
process of preheating, hot rolling and coiling. To this purpose the temperature region of 750°C up to
1200°C was regarded. The structural investigations indicate that oxidation at high temperature of Sialloys produced the classic three layered oxide scale. On the grain boundaries in the scale, ironsilicate can be found. There is also Si- enrichment at the scale –metal interface. This enrichment is
present in the form a mixed wustite-iron silicate (fayalite) phase, FeO-Fe2SiO4. Above 1177ºC, the
eutectic temperature of FeO-Fe2SiO4, the oxidation is strongly accelerated, due to the presence of
the liquid phase at the interface and in the grain boundaries and increasing the scale adhesion to the
steel. Fayalite forms above 1177°C, but also below 1170°C. As a consequence, during hot rolling
process (T>750°C), fayalite is always forming.
References
[1] J.G. Lenard: Metal Forming Science and Practice (Elsevier, Oxford 2002).
[2] P. Kofstad: High temperature corrosion (Elsevier Applied Science Publishers, London 1988).
[3] M.H. Davies, M.T. Simnad and C.E. Birchenall: J. Met. (1951), p. 889.
[4] J. Païdassi: Acta Metall. Vol. 6 (1958), p. 184.
[5] Y. Hidaka, T. Anraku, N. Otsuka: Oxid. Met. Vol. 59 (2003), p. 97.
[6] L. Suarez, X. Vanden Eynde, M. Lamberigts and Y. Houbaert: AIP 907 Vol.2 (2007), p. 1233.
[7] G. Béranger, G. Henry and G. Sanz: The book of the steel (Tech. & Doc Lavoisier, Paris 1994).
[8] H.J. Grabke, V. Leroy and H. Viefhaus: ISIJ Vol. 35 (1995), p. 95.
[9] K. Sachs and C.W. Tuck: The Iron and Steel Institute (1967), p. 1.
[10] T. Fukawaga, H. Okada and Y. Maehara: ISIJ Vol. 34 (1994), p. 906.
[11] H. Okada, T. Fukagawa, H. Ishihara, A. Okamoto, M. Azuma and Y. Matsuda: ISIJ Vol. 35
(1995), p. 886.
[12] S. Taniguchi, K. Yamamoto, D. Megumi and T. Shibata: Mater. Sci. Eng. Vol. A308 (2001), p.
250.
Diffusion in Solids and Liquids III
10.4028/www.scientific.net/DDF.273-276
Effect of Si on High-Temperature Oxidation of Steel during Hot Rolling
10.4028/www.scientific.net/DDF.273-276.655
DOI References
[5] Y. Hidaka, T. Anraku, N. Otsuka: Oxid. Met. Vol. 59 (2003), p. 97.
10.1023/A:1023070016230
[8] H.J. Grabke, V. Leroy and H. Viefhaus: ISIJ Vol. 35 (1995), p. 95.
10.2355/isijinternational.35.95
[10] T. Fukawaga, H. Okada and Y. Maehara: ISIJ Vol. 34 (1994), p. 906.
10.2355/isijinternational.34.843
[11] H. Okada, T. Fukagawa, H. Ishihara, A. Okamoto, M. Azuma and Y. Matsuda: ISIJ Vol. 35 (1995), p.
886.
10.2355/isijinternational.35.886
[12] S. Taniguchi, K. Yamamoto, D. Megumi and T. Shibata: Mater. Sci. Eng. Vol. A308 (2001), p. 250.
10.1016/S0921-5093(00)01977-8