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Journal of Materials Processing Technology 209 (2009) 44764483
Contents lists available atScienceDirect
Journal of Materials Processing Technology
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c
Surface wrinkle defect of carbon steel in the hot bar rolling process
Hyuck-Cheol Kwon a, Ho-Won Lee b, Hak-Young Kim c, Yong-Taek Im b,,Hae-Doo Park a, Duk-Lak Lee d
a Rolling Technology and Process Control G roup, POSCO Technical Research Laboratories, POSCO, 1, Goedongdong, Namgu,
Pohang, Gyeongbuk 790-785, Republic of Koreab National Research Laboratory for Computer Aided Materials Processing, Department of Mechanical Engineering,
Korea Advanced Institute of Science and Technology, 373-1, Gusongdong, Yusonggu, Daejeon 305-701, Republic of KoreacAdvanced Technology Department, Hyundai Mobis Technical Research Institute, 80-10, Mabukdong, Giheunggu, Yongin, Gyeonggi 449-912, Republic of Koread Wire Rod Research Group, POSCO Technical Research Laboratories, POSCO, 1, Goedongdong, Namgu, Pohang, Gyeongbuk 790-785, Republic of Korea
a r t i c l e i n f o
Article history:
Accepted 6 October 2008
Keywords:
Surface wrinkle defect
Hot bar rolling
Specific deformation energy
Instability
a b s t r a c t
It is well known that surface defect is a common problem encountered in the multi-stage hot bar rolling
process of carbon steel. In this study, the phenomenon was investigated by simulating theprocess by the
finite element technique to identify the location where the surface defect might occur and checking the
surface qualities obtained from the compression tests at various temperatures and strain rates to clarify
the important parameter governing the possible surface defect formation. Also, the surface temperature
was measured by employing pyrometer to support the experimental observation. The levels of temper-
ature and specific deformation energy obtained from finite element simulations depending on the roll
groove geometry were compared with the experimental observation to better understand the formation
of the surface defect in the hot rolled bar. Based on this study, the surface defect might be formed by
dissipating the excessive deformation energy accumulated by generating the new surface at the lower
level of temperature where recrystallization cannot occur. According to this work, the comparison of the
specific deformation energy level for determining the instability of the hot working process might be
interesting for further investigation.
2008 Elsevier B.V. All rights reserved.
1. Introduction
Workability for hot deformation depends on both the material
characteristics such as grain size, billet geometry, and distribu-
tion of secondary phase and the processing characteristics such as
strain, strain rate, stress, and temperature. For cold deformation,
the processingcharacteristics are moreimportant than the material
characteristics to govern the workability.
In steelcompany,surfacedefect in the multi-stage hotbar rolling
is one of critical problems to be solved for quality assurance of the
rolled product. The surface defect, which is frequently encountered
in the hot bar rolling of steel, can easily develop into a fatal man-
ufacturing defect during the secondary cold forging process of bar
stocks as shown in Fig. 1. Thus, it is necessary to minimize such
defect formations on the surface of hot rolled bars. Although many
researchers had paid attention to understand and solve the prob-
lems, the phenomenon is not quite well understood yet because of
many deformation stages of roughing,intermediateroughing, inter-
Corresponding author. Tel.: +82 42 869 3237; fax: +82 42 869 3210.
E-mail address:[email protected](Y.-T. Im).
mediate finishing, and finishing mill to produce hot rolled bars as
shown inFig. 2.
Schey (1980) described that cracking of the billet in rolling,
whether in the form of edge or surface cracking, or damage to the
billet center, invariably resulted in increased crap and production
costs. In this work, it was concluded that through thickness inho-
mogeneity resulted into surface or center cracks, whereas lateral
inhomogeneity into edge cracking. From the comparison of bend
test specimens with edge cracking by the help of metallographic
observations, Fitzsimons and Kuhn (1984)pointed out that edge
cracks were observed in the hot bar rolling at high strain rates.
Crowther and Mintz (1986)had investigated the change of duc-
tility at various temperatures depending on carbon contents of the
material.
Barlow et al. (1984)also discussed main rolling defects observed
both during rolling and on finished rolled bars. They related defect
formations to ingot casting such that if the molten metal was not
poured carefully into the ingot at the proper temperature or not
allowed to cool in a controlled environment, it might result into a
longitudinal crack or seam along the bar.Hassani and Yue (1993)
claimed that surface cracks were oxidized in the air in ingot cast-
ing, resulting in defects that lowered the surface quality of the final
product. For rolling in a blooming mill, Milman et al. (1979)estab-
0924-0136/$ see front matter 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2008.10.032
http://www.sciencedirect.com/science/journal/09240136http://www.elsevier.com/locate/jmatprotecmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.jmatprotec.2008.10.032http://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.jmatprotec.2008.10.032mailto:[email protected]://www.elsevier.com/locate/jmatprotechttp://www.sciencedirect.com/science/journal/092401367/27/2019 Material Process Technology.pdf
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Fig. 1. Surface crack observed and upset specimen.
Fig. 2. Schematic of a typical modern rod mill ( Lee, 2004).
lished that the cause of formation of corner cracks in ingots was
due to a combination of three main factors such as an unfavor-
able stress state pattern with the presence of tensile stresses, local
thermal stresses of the same sign, and mechanical concentrators
for those stresses.
Kawano et al. (1999)used thermal and mechanical finite ele-
ment (FE) simulations for calculating the temperature change in
five different roll pass designs. They discussed that temperature
was the most significant processing parameter to control surface
cracks and recommended roll pass design with least temperature
drop during rolling for reducing surface cracks. Kawanishi et al.
(1999)also added parameter of rolling speed for decreasing sur-
face cracking. By the help of finite element investigation, Mantyla
et al. (1993)concluded that the metallurgical aspect showed that
uneven plastic deformation and residual stresses after each pass
of deformation will lead to inhomogeneous recrystallization and
variations of mechanical properties through the thickness of the
material resulted to defects of cross waves and ski-ends impairing
the quality of hot rolled flat products.Atkins (1996)added that not
onlymetallurgybut alsomechanicsand strain ratios during an oper-
ation could have a profound effect on cracking.Topno et al. (2002)
Table 1
Chemical composition of steel used.
C Si Mn P S Cu Al
wt% 0.090 0.028 0.458 0.014 0.005 0.009 0.062 Fig. 3. Location of surface defects in the cross-section of the crop in the roughing
mill.
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Fig. 4. Location of surface defects in the cross-section of the crop in the roughing mill.
Fig. 5. Micrograph of defects in the cross-section of the crop in the roughing mill.
discussed generation of surface defects and corrective measures
observed in the bar mill. Recently, Kim and Im (2002)developed
and validated non-isothermal shape rolling finite element program
which was named CAMProll.BytheaidofCAMProll, Leeet al.(2007)
reconsidered conventional plastic work, generally used as a ductile
fracture criterion in the cold working process, as an instability cri-
terion to predict the surface wrinkle defect during multi-pass hot
bar rolling.
Fig. 6. Traced location of defect in the first stage of the roughing mill.
In the present study, the defect formation was investigated in
terms of the location and type. The temperature variations were
determined experimentally and numerically to better understand
themechanism of surface cracking phenomenon. The hotcompres-
Fig. 7. Measured surface temperatures in the center and edge of the billet before
and after the 1st stage of the roughing mill.
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Fig. 8. FEM analysis result of the 1st stage of the roughing mill: (a) effective strain, (b) effective strain rate (1/s), (c) effective stress (MPa) and (d) temperature (C).
sion tests of cylindrical specimens were carried out up to 50 and
70% reductions in height at various temperaturesand strainrates to
determine the important processing parameter to understand the
surface defect formation. The effect of roll geometry on the max-
imum specific deformation energy accumulated and temperature
distribution was numerically examined. The numerical result was
compared well with the observed industrial data.
2. Analysis of surface crack encountered
Fig. 1shows the surface defect observed after the roughing mill
of the hot bar rolling process. In the same figure the cracking ofthe billet after upsetting of the cylindrical cut of the same bar is
given. In industry, it is not easy to detect such a surface defect in
the middle of production because it is hidden under the skin of the
surface and very thin like hair. In Table 1, the chemical composition
of the material investigated is given.
In order to investigate the cause of forming such cracks, multi-
stage of the roughing mill was carefully examined to determine
the possible location where these cracks occurred. As shown in
Fig. 3,the most likely places of possible cracking were determined
by the inverse finite element simulations using CAMProll as two
regions, which formed 70 left and right from the eccentric cen-
terline. This eccentric centerline was the same as the normal axis
rotated in counterclockwise to the axis of the roll gap for the last
stage of the roughing mill. The cracking occurred at the skewed
places from the normal axis due to the torsional effect during
rolling.
In Fig.4, the cross-sectionalviews of thebar aregiven. According
to this figure the surface cracks look like a concave shape, which is
different from normal shape of the surface line crack. Thus, it was
understood that such a crack was formed in an earlier stage due to
the instability of the surface deformation during rolling instead of
Fig. 9. Strain and temperature history in the edge and center of the billet.
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Fig. 10. Micrographs of the cross-section of compressed specimens (reduction in height: 50%).
Fig. 11. Micrographs of the cross-section of compressed specimens (reduction in height: 70%).
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Fig. 12. Loadstroke curvesobtained fromcompressiontestswith variousstrainrates
for temperatures of (a) 800 C, (b) 900 C, and (c) 1000 C.
regular cracking. This kind of concave shapes will be compressed in
the subsequent deformation stage to form the line cracks as shown
inFig. 5.Thus, such a defect is called here a wrinkle defect in the
present investigation.
In industry, it was very difficult to find out where this kind of
wrinkle defects is formed because of the continuous rolling line.
Therefore, in the present investigation, the multi-stage bar rolling
process was simulated by employing CAMProll to determine the
stage and location where the wrinkle defect might be formed. The
material data used for simulation was obtained from the previous
work. According to FE simulations, the defect might be initiated at
the first stage of roughing mill and located at near the corner edge
of the billet according to the levels of temperature drop and plastic
deformation as shown inFig. 6.
The temperature drop was measured by pyrometer before and
after rolling and simulated by the FE program during rolling as
well. After the reheating furnace, the billet was passed through the
descaler stand and provided into the first stage of roughing mill.
At the descaler stand, the pressurized water was sprayed onto the
billet in order to take off the scale from the billet surface.
Fig. 7shows the temperature variations at the edge and center
areas as introduced inFig. 6before and after rolling. According to
this figure, the measured temperature at the edge area was lower
than that at the center area. This was obvious because of the heatloss atthe edge area dueto largesurface areasthrough radiation and
convection. The temperature change at the center areawas minimal
compared to around 50100 C at the edge area. Inside the billet,
the temperature drop of around 70 C was higher in the tail part
of the billet compared to the front part due to longer disposure to
the coolant at the descaler stand. According to the observation in
industry, the defect was noticeable at the tail part as well.
The effect of processing parameters during rolling was investi-
gated with the FE simulations in terms of strain, strain rate, stress,
and temperature as shown in Fig. 8. At the first stage of rolling,
the rolling speed was relatively lower than the one at other stages,
resultingin the highertemperature dropdue to relatively longer roll
contact with the billet. At the edge corner area, the specific defor-
mation energy and temperature were at maximum and minimum,respectively, because of the thermal contact with rolls.
The temperaturevariationsobtainedfrom measurementsbefore
and after rolling and from simulations during rolling are summa-
rized in Fig. 9. According to this figure, the temperature at the edge
corner area was drastically decreased during rolling and recovered
after rolling due to redistribution of the heat but the level was low-
ered than the one at the entry level. However, the temperature at
the center area was increased during rolling due to heat genera-
tion of plastic deformation and reduced in small amount compared
to the one at the entry level. According to this analysis, it can be
Fig. 13. Mechanism of the wrinkle defect generation during the multi-pass hot rolling.
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Fig. 14. Specific deformation energy and temperature distribution: (a) Caliber 1, (b) Caliber 2, (c) notification of position (P1 means Position 1), and (d) roll and billet shapes.
construed that instability of the deformation might be likely at the
edge corner area because of higher plastic deformation at lower
processing temperatures.
3. Hot compression tests
Because of the similarity of the stress conditions at the free
surface in rolling and simple compression tests, the instability con-
ditions were looked into by carrying out the hot compression tests.
The tests were carried out using Gleeble-3800 at the POSCO tech-
nical research laboratory with their help. In Table 2, the testingconditions are clearly introduced. The specimen was made of a
cylinder with diameter of 10mm and height of 15 mm. The strain
rate was set to be 1s1 and reductions of height were 50 and
70%, respectively. The temperatures of the specimens were var-
ied from 600 to 1000 C with the increment of 50 C. The heating
was increased 10 C persecond andspecimenwas homogenized for
60 s. The specimens were quenched for 2 or 3 s after the compres-
sion to obtain better microstructure. Depending on compression
conditions, surface qualities were varied.
In order to determine the surface quality, the cross-section
was photographed by the optical microscopy and shown in
Figs. 10 and 11.At 50% reduction, the surface was very smooth at
temperatures of 900 and 1000 C. However, the surface was wavy
at temperatures of 700 and 800
C. This phenomenon was moredominant for 70% reductions.
This can be interpreted from the point of energy dissipation.
At hot deformation, the surface was clean because of consumption
of the deformation energy through recrystallization as confirmed
in the load stroke curves in compression in Fig. 12. However, at
Table 2
Experimental conditions investigated.
Parameters Levels
Reduction in height 50, 70 (%)
Strain rate 1 (1/s)
Temperature 6001000 (C)
Specimen dimension 10 mm15mm (diameterheight)Fig.15. (a)Roll groovesand billet geometry,and (b)maximumspecificdeformation
energy.
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cold deformation, higher deformation energy might be dissipated
through the surface expansion, inducing the instability and caus-
ing the irregular surface quality as demonstrated illustrated in
Fig. 13.
4. Process simulations
Since the major processing parameters investigated so far were
related to the levels of the temperature and specific deforma-
tion energy, the effect of roll groove geometries on these two
terms was investigated. The computational results were qualita-
tively compared with the industrial observation of surface cracking
phenomenon.
In Fig. 14, simulationresults are givenfor therolling of theinitial
billet with the corner radius of 8 mm with two different types of
the rolls (the one with regular width and the other with increased
width). By increasing the width of rolls, it was found out that the
maximum specific deformation energy level was decreased from
90 to 70 MPa and the temperature levels were higher. This data
was reasonable according to the industrial observations that the
surface cracking was reduced by increasing the width of rolls. Thus,
it might be worthwhile to look into more carefully thepossibilityto
use the specific deformation energy as a decision criterion to judge
the formation of surface cracking at the hot bar rolling.
InFig. 15,the effect of magnitude of the roll groove radius and
width of the roll geometry on plastic deformation level is given. In
simulations, the widths were varied as 184, 190, and 196 mm and
therollgrooveradii as16,25,and 35mm.As shownin this figure,the
maximum specific deformation energy was obtained for the case
with the width of 184 mm and roll groove radius of 35 mm. Under
the present investigation, it was found outthe specific deformation
energy level decreased as the width and roll groove radius of the
roll were increased and decreased, respectively.
5. Conclusions
In the present investigation, the surface defect was identi-
fied as wrinkle defect occurred at the earlier stage of roughing
mill in the hot bar rolling process and the forming mechanism
of such defect was investigated with the help of recrystallization
behavior. This was confirmed with the hot compression tests. The
instability was correlated with the level of the specific deforma-
tion energy under the present investigation condition. According
to the simulation results, temperature and specific deformation
energy levels were most important parameters to govern the for-
mation of surface defect. Finally, it was found out that the specific
deformation energy level was dependent with the roll geome-
try. Thus, by modifying the roll geometry, the instability can be
reduced according to the limiting value of specific deformation
energy.
Acknowledgements
The authors wish to thank for the grants from the POSCO and
Korea Science and Engineering Foundation (KOSEF) through the
National Research Laboratory Program funded by the Ministry of
Science and Technology (No. R0A-2006-000-10240-0). The techni-
calsupport of the POSCO to carry outthe hot compression tests was
very much appreciated.
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