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제43권제2호(2023. 4) 한국주조공학회지
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- Fazlollah Sadeghi:
박사후연구원
, Tahereh Zargar:
연구원
, Dong-Yong Park:
책임연구원
, NamKyu Park:
부장
,
Yoon-Uk Heo, Jae Sang Lee, Dae Geun Hong, Chang Hee Yim:
교수
Received: Feb. 15, 2023 ; Revised: Mar. 16, 2023 ; Accepted: Mar. 30, 2023
†
Corresponding author: Chang Hee Yim, Sadeghi Fazlollah
Tel: +82-54-276-6010
E-mail: chyim@postech.ac.kr, sadeghi@postech.ac.kr
Journal of Korea Foundry Society
2023. Vol. 43 No. 2, pp. 55~63
http://dx.doi.org/10.7777/jkfs.2023.43.2.55
pISSN 1598-706X / eISSN 2288-8381
©
Korea Foundry Society, All rights reserved.
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creative-
commons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the
original work is properly cited.
Mechanism of Surface Corrosion in the Continuous Casting Guide Rolls
Fazlollah Sadeghi
1
*, Tahereh Zargar
1
, Yoon-Uk Heo
1
, Jae Sang Lee
1
, Dong-Yong Park
2
, NamKyu Park
3
,
Dae Geun Hong
1
, and Chang Hee Yim
1
*
1
Graduate Institute of Ferrous and Energy Materials Technology,
Pohang University of Science and Technology, Pohang 37673, Republic of Korea
2
Aprogen Inc., Pohang 37860,Republic of Korea
3
SungWook Co., Ltd., Yeongcheon 38899, Republic of Korea
Abstract
Due to the importance of the surface on the final slab quality, it is essential to maintain a smooth segment roll surface that is in
touch with the thin solid shell during solidification. In this paper, the surface of the used continuous casting guide roll was analyzed
to realize the mechanism of its surface deterioration. Surface analysis has revealed severe corrosion at two distinct areas leading to
deep roughness occurring on the guide roll. Firstly, the severe corrosion follows prior austenite grain boundary due to exposure with
acidic environment. Also, in heat affected zone (HAZ) where two cladding beads overlap, more severe corrosion takes place. The
overheat input results in local ferritization without full melting which increases retained
δ
-ferrite content almost 10 times higher
than surrounding area. Corrosion was observed to happen at the
δ
-
γ
interface where Cr depletion takes place.
Key words; Continuous casting,
δ
-ferrite, Guide roll and Corrosive wear
The 74
th
World Foundry Congress
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1. Introduction
Continuous casting is the most economical mass
production method around the world. The caster machine
requires precise process control on every part beginning
from tundish to the final slab torch cutter. One of the most
sensitive parts is guide rolls that are positioned immediately
below the downstream of the bottomless copper mold to
the end of the metallurgical length. Guide rolls operate in
high-temperature environments where they are in contact
with the thinnest part of the solidified shell. In the
meantime, the mold flux (coming from the copper mold)
accompanied by air mist blow (to cool down the solid shell
and proceed with solidification) result in the formation of
an extremely acidic environment attacking the guide rolls at
high temperature [1,2]. Thus, it is essential to reduce the
corrosive wear damage to the guide rolls by understanding
the corrosion happening on the surface of the guide rolls.
The continuous casting guide rolls are made of plain
Carbon steel with wire cladding. Martensitic stainless steel
having more than 13 wt.% Cr with added C to increase
Martensitic start temperature (MS) is normally used in
several passes over the guide roll. The purpose is to improve
corrosion resistance, thermal fatigue, and wear resistance of
the guide roll’s surface while the material is easily weldable
[3]. Among all the possible welding processes, submerged
arc welding (SAW) has been successively utilized on
cylinder-shaped guide rolls. This process can be easily
automated resulting in well-aligned beads [4-6].
From a microstructural point of view, most of the
martensitic stainless steels pass through peritectic reaction
during solidification meaning that the primary phase formed
from the melt is
δ
-ferrite followed by
γ
-austenite during
cooling. At low-temperature ranges, the austenite phase
transforms into martensite. Although,
δ
-ferrite phase is
beneficial to enhance the weldability of the material due to its
ductility [7-9], it has poor corrosion resistance. The amount of
retained
δ
-ferrite can be estimated by calculation of Cr and Ni
equivalents according to the Schaeffler diagram [10-13].
Hadizadeh et al. observed the formation of fatigue-related
beach marks called zebra lines happening on the surface of
the failed rolls in the shape of parallel lines. They have
stated that the interconnected
δ
-ferrite network at the
cladding overlay in the overlap region (the boundary
between two passes) is responsible for the formation of
zebra lines at the surface of the roll. In addition, secondary
sigma phase has been mentioned to form in the intermediate
temperatures causing hardness increase and leading to
fatigue progression lines [2]. However, they haven’t stated
the relationship between the retained
δ
-ferrite amount and
formation of cracks at the beads overlap area. In addition, the
corrosive wear not only occurs at the overlap region but also
inside each bead at a less severe rate.
Although, Hadizadeh et al. have stated that severe
corrosion occurs at the boundary between two welding
passes (e.g. welding overlays) so called zebra lines [2], a
more in-depth analysis is necessary to find out why the
reason for severe corrosion and the mechanism of crack
growth. Thus in the current work, a sample of guide rolls
after over 3000 casting heats (here, number of heats refers
to the number of ladles carrying molten steel casting into
the tundish of the continuous caster) was analyzed to find
out the mechanism of corrosion. Furthermore, the effect of
retained
δ
-ferrite on the surface roughness profile was
studied in the overlap region of the wire cladding beads
and inside the beads as well.
2. Experimental
The guide roll samples used in this paper were taken out
from a continuous casting machine after over 3000 number
of heats. The surface image of the roll is shown in Fig. 1.
Detailed chemical composition of the overlayer wire clad is
presented in Table 1. To observe the surface roughness
profile of the damaged guide roll was observed with 3D
optical microscopy using Olympus DSX500 (Olympus
Corporations, Tokyo, Japan). Microstructural observations
were carried out by polishing and etching the top and cross-
section of the samples with Vilella’s etchant [14]. To
characterize the crystal structure and grain size of the samples,
electron backscattered diffraction (EBSD) was used by a
scanning electron microscope (FE-SEM (JEOL JSM-7900F,
Tokyo, Japan) equipped with energy dispersive spectroscopy
(EDS). The EBSD data was then extracted by Oxford Aztec
ver. 6.0 and analyzed by AztecCrystal ver. 2.1.
3. Results and discussion
3.1 Surface analysis
An image of the used guide roll is shown in Fig. 1. This
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guide roll has been used in the first section of the
continuous casting machine for about over 3000 heats of
molten steel. It can be seen that there are obvious parallel
marks on the surface. Hadizadeh et al. have reported the
zebra lines parallel to each other and stated that these
parallel zebra lines correspond to the overlap region of two
wire cladding rounds during clad welding process [2].
A closer look at the surface of the sample is shown in
Fig. 2 at a selected zebra line. The 3D image clearly states
that the zebra line possesses a deeper height compared with
the other regions by average of 250 µm. In addition, out of
the zebra line area, there exist different corrosion marks
similar to grain boundary corrosion marks. Despite the
martensite being the dominant phase at room temperature,
Ta b le 1. Chemical composition of the wire-cladded material used for coating the guide roll
wt. % C Mn Si Cr Ni Mo Fe
Wire cladded 0.1-0.2 0.5-1.5 0.1-0.3 12.5-14.0 1.0-2.0 1.0-2.0 Bal.
Fig. 1.
The used guide roll being installed in the first segment of the continuous casting machine while working for more than 3000 number of
molten steel heats.
Fig. 2.
Optical microscopy and 3D surface mapping of
the used guide roll sample at (a), (b), (c) low
magnification with higher magnification (d),
(e) inside the zebra line, and (f), (g) outside the
zebra line.
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another factor might affect the corrosion effects in these
areas since the rolls operate at high temperatures (about
600oC) at the top segments in a continuous casting machine.
3.2 Microstructural characterization
Further analysis was conducted by grinding and polishing
the cross-section and revealing the microstructure by
etching. Fig. 3 shows the macrostructure of the cladding
overlays positioning one zebra line in the center of the
image. The higher magnification OM image of Fig. 3(b)
depicts deep corrosion cracks (in blue arrows) grown into
the cladding overlay. They appeared as deep corrosion
marks on the surface like grain boundary corrosion marks
(Fig. 2(f)). in addition, the overlap of the two different wire
cladding bead overlays can be distinguished on the left side
of Fig. 3(a). it appears that the overlap of the two beads
does not correspond to the zebra line. In contrast, the heat-
affected zone (HAZ) of the second bead belongs to the
position of the zebra line which is contrary to the former
literature [2].
Due to the high-temperature working condition of the
guide rolls, it is expected that at working temperature, is
above the MS temperature of the wire cladded martensitic
stainless steel. Therefore, the corrosion has taken place at
the prior austenite grain boundary during operation [5].
SEM images were taken from the different distinct areas
of the sample after polishing the ND and RD cross-sections
presented in Fig. 4. Inside the zebra line, corrosion has
occurred uniformly within the retained
δ
-ferrite phases (Fig.
4(a)-(b)). Out of the zebra line, the corrosion has intensified
at a specific point and the crack has grown deep into the
microstructure of the cladding overlay perpendicular to the
surface (Fig. 4(c)-(d)). The corrosion product was formed
around the crack.
EBSD was used to observe the microstructure of the
cladding overlay and reconstruct the prior austenite grain.
Fig. 5 shows a representative area of the top surface and
RD cross-section. The grain boundary between two
adjacent austenite grains can be seen in the reconstructed
image (Fig. 5(b)). In EBSD Fig. 5(c), the only way to
distinguish between martensite and
δ
-ferrite is by observing
the orientation relationship and the strain induced in each
phase by means of kernel average misorientation (KAM)
map. In addition, the retained
δ
-ferrite has low levels of
stains (blue color areas in Fig. 5(c)), while martensite
contains high strain due to the displacive transformation
nature of martensite (green color areas in Fig. 5(c)).
In order to construct the prior austenite grains, the well-
known K-S orientation relationship was used to correlate
the parent austenite with its corresponding child martensite
phase according to the overlap of the two phases PFs
[15,16]. The pole figures of martensite and reconstructed
Fig. 3.
Macrostructure of wire cladding overlay on the used guide roll sample at the selected zebra line with higher magnification OM images
at (b) at the cladding overlay and (c) at HAZ between two passes.
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austenite are presented in Fig. 6 taken from a yellow
dashed area in Fig. 5(b). By comparing both parent and
daughter phases, it can be concluded that all of the
martensite phases possess K-S (Fig. 6(h)); however, the
retained
δ
-ferrite phases don’t obey either of them.
Therefore, the reconstruction fails to obtain reliable
austenite from the
δ
-ferrite phase.
To observe the damaged area more closely, a combined
analysis of EBSD and EDS was performed on a selected
crack in Fig. 7. The results of austenite grain reconstruction
show that the crack has grown from a specific prior
austenite grain boundary (Fig. 7(c)). Observation of the
surface has revealed that the patterns of grain boundary
corrosion marks (Fig. 2(f)) show does not correlate with
actual width of prior austenite grain size. The prior
austenite grain size is much smaller than the grain size that
appeared because of corrosion marks. Thus, it can be
inferred that the corrosion crack does not grow into every
austenite grain boundary. Conclusively, another factor
determines corrosion crack formation and growth. EDS
mapping demonstrates the abundance of Cr, Mo, and
oxygen at the corrosion crack zone.
Further analysis was performed at higher magnification at
the selected crack as shown in Fig 8. It is apparent that the
Fig. 4.
The SEM (a), (c) BSE images and (b), (d) SEI of the (a), (b) cross-section at the zebra line; (c), (d) cross-section out of zebra line at a
corrosion crack mark.
Fig. 5.
EBSD (a) IPF map, (b) IPF reconstruct IPF map, and (c) KAM map of the cross-section of the top cladding overlay.
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corrosion has proceeded at the boundary between retained
δ
-ferrite and martensite. The EDS mapping of important
elements reveals the abundance of Cr in corrosion products
(zero solutions) and the bcc retained
δ
-ferrite phases (Fig.
8(h)). The bcc martensite and retained
δ
-ferrite can be
distinguished by the KAM map where martensite contains
strain due to formation by massive transformation whereas
δ
-ferrite is a remnant of solidification process (Fig. 8). The
Fig. 7.
Combined EBSD (a) band contrast, (b) IPF map at Y-axis, (c) prior austenite grain reconstruction and EDS (d) C Ka, (e) Fe Ka, (f) Cr
Ka, (g) O Ka, and (h) Mo La mapping on a corrosion crack growing at a specific prior austenite grain boundary. Note the austenite grain
width marked by white arrows in (c).
Fig. 6.
Pore figure (PF) of the parent fcc austenite at (a) {100}, (b) {110}, and (c) {111} with its daughter bcc martensite phase at (d) {100},
(e) {110}, and (f) {111} accompanied by the overlap of (g) {110}
fcc
and {100}
bcc
according to N-W and (h) {110}
fcc
and {111}
bcc
according to K-S.
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abundance of oxygen in corrosion product only (not in
δ
-
ferrite) shows that the corrosion initiates and proceeds only
into the
δ
-
α
interface. In addition, a slight carbon enrichment
at the
δ
-ferrite interface reveals carbide formation at the
interface during its decomposition (Fig. 8(f)).
It is important to mention that the corrosion occurring at
the zebra line follows the same mechanism as the cracked
areas grown into the prior austenite grain boundaries. The
corrosion marks can be witnessed in Fig. 4(a), (b). However,
the difference that causes more severe corrosion at the
zebra line arises from the difference in retained
δ
-ferrite
content. In order to find out about the difference in retained
δ
-ferrite content of the HAZ (prone to cause zebra line
corrosion marks on the surface of guide rolls) and matrix
(prone to the formation of corrosion crack along the prior
austenite grain boundary), the samples were etched by
Vilella etchant to reveal the
δ
-ferrite within martensite
phase [17]. Fig. 9 shows the microstructure of clad material
revealing residual
δ
-ferrite at the HAZ (Fig. 9(b)) and
inside the adjacent clad overlay (Fig. 9(c)). Several images
were taken and analyzed by image analysis similar to Fig.
9(d) and 9(e).
Quantitative measurement of residual
δ
-ferrite amount at
the HAZ and inside the clad overlay shows almost 10 times
more at the HAZ region. The equilibrium phase diagram of
the clad material obtained from ThermoCalc software
Fig. 8.
SEM (a) BSE and (b) SE images of a
corrosion crack proceeding into the
cladding overlay accompanied by
higher magnification combined EBSD
(c) KAM, (d) IPF at Y-axis, (e) prior
austenite reconstruction and EDS (f)
C Ka, (g) Fe Ka, (h) Cr Ka, (i) O Ka,
and (j) Mo La mappings at the area
close to rack tip.
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Fig. 9.
Cross-section etched (a) microstructure of the wire cladded
at (b) HAZ and (c) inside the clad overlay and the
corresponding (d), (e) image analysis.
Fig. 11.
Schematic illustration of (a) high-temperature ferritization at the HAZ region resulting in (b) higher retained
δ
-ferrite at room
temperature.
Fig. 10.
(a) Image analysis result of
δ
-ferrite content inside the clad
overlay and HAZ with (b) equilibrium pseudo-binary
phase diagram of cladding material.
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shown in Fig. 10(b) reveals that the dominant phase right
before melting is
δ
-ferrite which belongs to the HAZ area
where heat input is not enough to fully melt the material,
instead heating results in fully
δ
-ferrite phase which
decomposes upon cooling [18]. Thus, a higher amount of
δ
-
ferrite will remain compared to the clad overlay region
(Fig. 10(a)).
Thus, it can be concluded that the microstructure of wire
cladded stainless steel on the guide roll consists of room
temperature martensite and
δ
-ferrite bcc phases are
schematically shown in Fig. 11. At intermediate temperature
during operation, prior austenite grains are exposed to a
corrosive environment and the corrosion is intensified at
higher
δ
-ferrite region. Due to the ferritization that occurs
at HAZ of cladding overlays (Fig. 11(a)), higher corrosion
rates result in zebra line appearance on the surface of guide
roll whereas the deep crack accompanied by corrosion can
be witnessed in a general surface that follows a connected
δ
-ferrite network.
4. Conclusions
After analysis of the used guide rolls installed in the
continuous casting machine for over 3000 heats, the following
conclusions were reached on the surface deterioration
mechanism.
1. Surface corrosion can be divided into two distinct
marks. First, grain boundary corrosion crack occurs because
of the elongated network of retained
δ
-ferrite after dendritic
solidification. Second, zebra lines coincide with the HAZ
of the adjacent cladding beads.
2. Corrosion initiates from the
δ
-
γ
interface at high
temperature and proceeds by the formation of (Cr, Mo)
oxide products as a crack opening exposes the new surface.
3. Despite the findings of former literature [2], it appears
that the zebra line corrosion marks are caused by high retained
δ
-ferrite content at the HAZ zone adjacent to the cladding
overlay and not the overlay itself. In HAZ, ferritization results
in approximately 10 times higher retained
δ
-ferrite content
compared with other neighboring cladded areas.
Acknowledgement
This work was supported by the Technology Innovation
Program (200147323, Development of roll coating process
and coating powder for fabrication of giga grade steel
plate) funded By the Ministry of Trade, Industry & Energy
(MOTIE, Korea). This project aims to improve the life long
of continuous casting guide rolls in the Steel industry.
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