Material Process Technology.pdf

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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/09240136


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H.-C. Kwon et al. / Journal of Materials Processing Technology 209 (2009) 44764483 4477

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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4478 H.-C. Kwon et al. / Journal of Materials Processing Technology 209 (2009) 44764483

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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Material Process Technology.pdf