New T-Bend testing method to examine formability of color coated
steel sheet in different temperatures
Mikko Långvik
Häme University of Applied Sciences
Kauko Jyrkäs
Häme University of Applied Sciences
Antti Markkula
SSAB Europe Oy
Meri Rosenberg
SSAB Europe Oy
ABSTRACT
New testing procedure to measure formability of paint coated steel sheet was
developed. Modified T-bend tests were done with two different paint coating systems
(polyester and polyurethane). Bending was made in two stages in different
temperatures by using tensile test machine and temperature chamber. Pre-bending
was made inside the temperature chamber and actual T-bend was made rapidly
outside the chamber by using Erichsen impact tester Model 471. Forming was
performed in temperatures of -20 oC, -10 oC, 0 oC, +10 oC, +20 oC, +30 oC, +40 oC
and +50 oC. After forming specimen were exposed for 1000h in condensation
humidity test at +60 oC.
Testing procedure was found to be suitable in evaluating formability of different paint
coating systems. Target was to find a method which is able to create 0T tight bends,
but in because of testing procedure research was limited to 2T bends. Method can
still be used to find optimal forming temperature and to find limiting forming
temperature range for demanding bending operations. With small adjustments as
tight as 0T tight bends can also be created.
It was found that forming at elevated temperatures of +40 oC or +50 oC gave much
better results compared to forming in colder environment. It was also found that
forming under +20 oC leads easily to cracking for tight bends (2T-3T). As expected,
polyurethane-coating was verified to have better formability properties than polyestercoating. Difference between coatings is the most significant between forming
temperatures +10 oC to +30 oC.
INTRODUCTION
Color coated steel sheet is a complex multilayer structure which consist of steel, zinc
pre-treatment, primer and one or multiple top paint layers. Because there exist many
different material types in the coating structure, there can be found a variety of
different kind of failure types during forming. This is due to different layers and layer
boundaries have different forming properties. Formability of thin sheet is determined
by the weakest coating layer. When observing forming of paint coated thin sheets,
failure can be defined as visually seen cracking which alters appearance and
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corrosion resistance of the complete product. The best situation in forming would be
that the coating layers have at least the same forming properties as the steel itself.
Unfortunately this is seldom the case [1] – [10].
Paint coatings are polymer based. There exist two important temperature dependent
features in polymeric materials which have an effect on coating flexibility. This means
that paint coating may have a strong dependence between forming temperature and
its behavior in forming. Polymeric materials have normally specific glass transition
temperature (Tg) or temperature range. In many cases T g correlates to the material’s
performance in forming. Other important temperature is brittle-ductile transition
temperature (Tb) which can be found in lower temperatures than T g. When
temperature is below Tb coating is brittle and cannot be formed. Above T b the coating
is hard and ductile and above Tg coating becomes soft. In other words forming should
be performed above certain temperature range to ensure faultless results.
Temperature dependence can be quite steep. Just a few degrees difference in
forming temperature can therefore separate faultless and faulty product. [1, 11, 12,
15]
Glass transition temperature is polymer material dependent feature. Each coating’s
glass transition temperature (Tg) is a net result of the effects of its constituents and
the compositions and ratios of its polymeric building blocks. The binder resin in the
coating determines this behavior. [11, 15, 16]
Colour coated steel sheets are normally zinc coated also. Paint is corrosion resistant
barrier only when it’s intact. Zinc coating ensures that small flaws like scratches in
paint do not give corrosion straight from the beginning. Zinc is more noble metal than
steel and therefore zinc protects exposed steel surface from corrosion. Scratches are
quite rapidly filled by corrosion products of zinc i.e. white rust (zinc oxides), which
reduces corrosion rates to low levels again. [1, 2, 3, 5, 9, 13, 14]
Zinc coating formability is affected by grain size, crystallographic orientation,
thickness, phase composition of intermetallic layer and temperature. Zinc coatings
have strong dependence in their ductile-brittle behavior according to temperature.
Pure zinc becomes brittle at temperatures around +10 oC because low temperature
prohibits some deformation mechanisms in coating. [1, 4, 5, 9, 10] Because zinc
layer is between paint and steel, it is crucial that zinc can withstand forming
operations. If zinc fails in between steel and paint, there occur high stress peaks
locally which can cause cracking of the whole coating system. Still, it is common that
zinc is the first layer to crack during forming. Cracking of zinc can lead to cracking of
paint coating which leads to white rust formation in these areas. It is a known fact
that zinc coating has tendency to crack underneath the paint in tight bends. Those
cracks do not appear visually if paint coating has high enough formability that it can
cover these cracks. [1, 2, 4, 5, 7, 9, 10, 13, 14]
Pre-painted steels are normally quite low-strength and have good formability.
Thickness is also limited to somewhat low values for both steel sheet and zinc
coating. Normally, it is stated that formability of pre-painted steel sheet depends on
the severity of bend radius, paint system and ductility of zinc coating. Other factors
which affect bendability are sheet and coating thickness and strength level of the
steel. Tension-bend cracking has traditionally been minimized by careful material and
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paint-system selection. Besides these traditional bendability criteria, there have been
claims that forming temperature could have a serious effect on forming results [1, 2,
3, 5, 9, 13].
Usage of pre-heating up to +50 oC to +75 oC has been used by a few companies in
roll-forming lines to ensure paint and metallic coatings to stay intact. There have
been investigations which conclude that pre-heating above +50 oC reduces
significantly cracking in both zinc and paint coating by improving ductility of coatings.
Undamaged coating increases the service life of the pre-painted steel sheet
substantially. Normally manufacturers have given handling instructions for customers
from minimum allowed forming temperature of pre-painted sheets. It can be
suggested that they should also give their clients optimal forming temperatures [1].
OBJECTIVE
In this work the formability of pre-painted and zinc coated steel sheets have been
investigated at different temperatures. Formability has been tested by modified Tbend test. Idea has been to examine, how certain forming temperature affects
formability of coating layers. Focus has been on the paint coating cracking behavior.
Aim was to find lowest possible forming temperature for two different paint coating
systems. Other goal was to find optimal forming temperature range for coating
systems.
MATERIALS
Test specimen was pre-painted DX53D+Z275 steel at nominal thickness 0.57 mm.
Paint coatings were polyester and polyurethane based. Polyester-coating has
considerably weaker forming capability than polyurethane-coating. These coating
types were chosen to use common paint coating alternatives with clear difference in
forming properties. Both of these coatings are used e.g. for corrosion protection in
outdoor applications. Nominal thickness including primer for polyurethane-coating
was 40 µm and for polyester-coating 25 µm. Primer for polyurethane-coating was
thickness of 10 µm and for polyester-coating 6 µm.
EXPERIMENTS
Testing was made using Zwick Z050 tensile test machine. Test procedure is basically
modified from standard EN 13523-7 T-bend test. Specimens were pre-formed inside
heat chamber for loose V-shape in certain test temperature. Pre-forming tools were
designed and made in Sheet Metal Centre. Test tooling and assembly can be seen in
Figure 1. After pre-forming, specimens were pulled out from the chamber and desired
T-bend was made rapidly with Erichsen bend and impact tester Model 471. Impact
machines hammer had mass of 2300±200 g while the drop height was 650 mm. To
ensure sufficient closure of T-bends, weight was dropped five times for each
specimen. Certain T-bends were formed by using necessary amount of steel strips of
the test material in between pre-formed test specimen during closing via impacts.
These steel strips were also placed in heat chamber so that they did not have effect
on the temperature of the specimen. Required forming temperature was secured by
using 4 mm thick plates heated up or cooled down in test temperature on top of test
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specimen and underneath it. Test temperature was confirmed by two thermometers,
with one integrated to the heat chamber and with one external.
Figure 1. Pre-bending tooling assembly and the temperature chamber.
Test specimens were steel sheets and their dimensions were approximately 40 mm x
300 mm. Test temperatures were -20 oC, -10 oC, 0 oC, +10 oC, +20 oC, +30 oC, +40
o
C and +50 oC. Target T-bends for polyester-coating were 0T, 0.5T, 1T, 2T and 3T.
Target T-bends for polyurethane-coating were 0T, 0.5T, 1T, 1.5T and 2T. Because of
the thick steel plate above the specimen during impact bend radii were actually larger
(about 2T) than they are in standard T-bend testing. In other words, 0T bend made
by impact machine equals to 2T bend made by standard T-bending machine. So, the
actual sizes of the bends for polyester-coating were measured to be 2T, 2.5T, 3T, 4T
and 5T and for polyurethane coating 2T, 2.5T, 3T, 3.5T and 4T. Three parallel
samples were tested with certain T-bend in each test temperature. In graphs and
tables the actual T-bend values were used.
Bended specimens were checked for cracking after the bend was made. Then
specimens were exposed to +60 oC condensation humidity test (QCT) for 1000 h.
Test machine was made according to standard SFS-EN ISO 6270-1 Paints and
varnishes. Determination of resistance to humidity. Part 1: Continuous condensation.
Climate inside the chamber was held constant during testing. Specimens were
placed in the center of the chamber so that bends pointed upwards. Specimens were
examined and photographed after 24 h, 50 h, 100 h, 150 h, 250 h, 500 h, 750 h and
1000 h.
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Specimens were evaluated by a rating scale of 0-3 points corresponding to the
cracks visually seen on the bent area. 3 points means no cracks at all and 0 points
indicate that the bent area has been cracked entirely. 1 point was given for a
specimen that had a few cracks and 2 points respectively for specimen with slight
cracking within all the specimen area. In this paper the results are given as an
average rating of three specimens.
RESULTS
It was concluded that most of the specimens cracked already in bending if they
cracked at all. These cracks were easily seen in visual inspection. When specimens
were exposed to condensation humidity test only minor changes could be found in
cracking. Polyester-coated samples formed in the range of +20 oC to +30 oC showed
some increase in seriousness of cracking in 2T, 2.5T and 3T bends during
condensation humidity test but overall changes were small. Condensation humidity
test was still very noted to be very useful in evaluation of degradation interface,
because it showed clearly which cracks penetrated through whole paint layer. In
cracks penetrating through paint layer moisture reacts with zinc coating and there
existed a clear formation of white rust which was easily detected in visual inspection.
Polyester-coated specimens tend to suffer cracking much more than polyurethanecoated specimens. In bends of small radius cracking was much more evident than in
bends of higher radius. Polyester-coated specimen with 2T bends cracked before
they were exposed to condensation humidity test if they were formed in +30 oC or
under. Forming temperature had a clear connection to cracking tendency. Forming in
+20 oC and +30 oC seemed to lead in cracking for polyester-coated specimen for 2T,
2.5T and 3T bends. Forming in +40 oC and +50 oC showed that polyester-coating
was able to avoid cracking behavior even for 2T bended specimens. 4T and 5T
bended specimen did not show any tendency for cracking in any forming temperature
above +20 oC degrees. 5T bends avoided all cracking as low temperatures as 0 oC. It
is notable that polyester-coated specimen which had very loose 5T bend cracked
when formed in -20 oC before condensation humidity test. Results are shown in
Tables 1 and 2. Example photos for polyester-coated samples formed in different
temperatures with 2T-bend are showed in Figure 2 and for polyurethane-coated
samples in Figure 3.
Table 1. Rating according to cracking of polyester-coated specimens formed in
different temperatures before and after 1000 h in QCT-chamber. First value is the
mean rating of parallel samples before QCT and latter value is rating after 1000h
QCT.
PES
2T
2,5T
3T
4T
5T
-20oC
-10oC
0oC
+10oC
+20oC
+30oC
+40oC
+50oC
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
3/2
0/0
0/0
0/0
1/0
3/3
0/0
0/0
0/0
1/0
3/3
0/0
0/0
2/1
3
3
1/0
1/0
2/1
3/3
3/3
3/2
3/3
3/3
3/3
3/3
3/2
3/3
3/3
3/3
3/3
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Table 2. Rating according to cracking of polyurethane-coated specimens formed in
different temperatures before and after 1000 h in QCT-chamber. First value is the
mean rating of parallel samples before QCT and latter value is rating after 1000h
QCT.
PUR
2T
2,5T
3T
3,5T
4T
o
-20 C
-20oC
-10oC
0oC
+10oC
+20oC
+30oC
+40oC
+50oC
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
2/1
2/1
3/2
3/2
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
QCT 24h
QCT
o
-10 C
QCT 1000h
o
o
0C
+10 C
o
+20 C
o
+30 C
o
+50 C
o
+40 C
Figure 2. Polyester-coated samples after QCT with 2T bend radius.
QCT 24h
QCT 1000h
o
o
o
-20 C
o
-10 C
0C
o
+20 C
o
+30 C
o
+40 C
Figure 3. Polyurethane-coated samples after QCT with 2T bend radius.
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o
+50 C
+10 C
Polyurethane-coated specimen did not suffer cracking even for 2T bends in forming
temperatures of +20 oC and over. Only minor changes were exposed during
condensation humidity test for 2T bends formed in temperature of +20 oC. On the
other hand polyurethane-coating loses its formability properties in temperatures
below +20 oC. This change is very rapid and clear compared to polyester-coating
which had more step-by-step decrease in formability properties depending on
temperature. When temperature dropped to +10 oC polyurethane-coating was
cracked at 2T, 2.5T and 3T bends clearly even before condensation humidity test
started. At 0 oC and under every sample was cracked in all bend radii before
condensation humidity test started. Level of cracking depended still on temperature.
Worst cracking took place below zero temperatures for both polyester- and
polyurethane-coatings.
CONCLUSIONS
Testing procedure was confirmed to be a suitable way to test formability of different
kinds of color coated steel sheets. Adjustment is needed so that tighter bends than
2T can be produced. Forming temperature has a clear connection to colour coated
steel sheet formability. Specimens which were formed in temperature of +40 oC and
+50 oC showed only minor or no tendency at all for cracking before and during
condensation humidity test at +60 oC treatment.
It is evident that higher forming temperature is much more important when coating
material has lower formability properties as for polyester type of coil paint compared
to polyurethane type. It was shown that formability properties of polyester-coated
specimens were improved dramatically when forming temperature was raised up at
+40 oC. At the same time polyurethane-coated samples showed no tendency to
cracking when formed in +20 oC.
1000 h time in condensation humidity test was too long and basically only 24 h test is
needed to show how cracking of the paint coat is increasing in specimen due to the
effect of elevated heat and humidity.
FURTHER WORK
From the temperature control view, thick plate above formed specimen was a good
solution when preparing certain T-bend. Problem was that part of the impact energy
was “lost” and it made T-bends more “loose” compared to traditionally prepared
T-bends according to EN 13523-7. By discarding the thick plate from testing
procedure it is possible to achieve even 0T bends which enable more accurate
results.
Formability dependence on temperature of different kind of metallic coatings should
be examined from samples without paint. Aim would be to find out optimal forming
temperature for different metallic coatings and to find temperature where cracking
limits formability.
Optimal forming temperature for different kind of paint coatings should be studied.
The other important issue is to find lowest possible forming temperature for each
coating.
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Metallographic analyses of cracked samples are needed to see what caused the
cracking. Another interesting issue is to see how cross-sections of non-cracked,
higher temperature formed specimens differ from cross-sections of cracked
specimens. It should be also examined, that how zinc coating cracking has affected
on paint coating.
ACKNOWLEDGMENTS
The Authors wish to thank FIMECC HYBRIDS for funding of this work and
management of SSAB Europe Oy, R&D facility, Hämeenlinna, Finland for helping us
to publish this article.
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