The Study of Deformation Behaviour of DC06 Deep Drawing Steel

Abstract

The occurrence of any cracks or defects in car body parts processed by deep drawing technology is not allowed by high quality standards. This kind of defect is considered as the most dangerous for the process quality and stability because it cannot be easily detected during the manufacturing in the steel plant and also in final inspection after pressing, that's why the occurrence of these defects has always to be studied in detail. For the prevention of defects, it is necessary to study the deformation behaviour of the material in the immediate vicinity the crack tip in detail. For the study the controlled scratched samples were tensile deformed and then were studied using UHR-SEM equipped witd EBSD detector. The EBSD technique allowed detailed inspection of the effect of deformation on the grain structure as changes in grain orientation or local crystal lattice missorientation and thus directly observe and evaluate both, elastic and plastic strain. Obtained results showed that the scratch does not affect deformability of the DC06 deep drawing sheet negatively due to too large tip radius with respect to low sheet thickness.

Full text

The study of deformation behaviour of DC06 deep drawing steel
Michal Tregler1, Pavel Kejzlar2, Tomáš Pilvousek3, Zuzana
Andršová2, Lukáš Voleský2
1Faculty of Mechatronics, Informatics and Interdisciplinary
Studies, Technical University of Liberec, Studentska 1402/2,
461 17 Liberec, Czech Republic. Email:
michal.tregler@tul.cz
2Department of the Preparation and Analysis of
Nanostructures; Institute for Nanomaterials, Advanced
Technologies and Innovation, Technical University of
Liberec, Studentska 1402/2, 461 17 Liberec, Czech
Republic. Email: pavel.kejzlar@tul.cz, lukas.volesky@tul.cz,
zuzana.andrsova@email.cz
3Škoda Auto, VFS1, V. Klementa 869, 293 60 Mladá
Boleslav, Czech Republic. Email: tomas.pilvousek@skoda-
auto.cz
The occurrence of any cracks or defects in car body parts processed by
deep drawing technology is not allowed by high quality standards. This
kind of defect is considered as the most dangerous for the process
quality and stability because it cannot be easily detected during the
manufacturing in the steel plant and also in final inspection after
pressing, that’s why the occurrence of these defects has always to be
studied in detail. For the prevention of defects, it is necessary to study
the deformation behaviour of the material in the immediate vicinity the
crack tip in detail. For the study the controlled scratched samples were
tensile deformed and then were studied using UHR-SEM equipped witd
EBSD detector. The EBSD technique allowed detailed inspection of the
effect of deformation on the grain structure as changes in grain
orientation or local crystal lattice missorientation and thus directly
observe and evaluate both, elastic and plastic strain. Obtained results
showed that the scratch does not affect deformability of the DC06 deep
drawing sheet negatively due to too large tip radius with respect to low
sheet thickness.
Keywords: EBSD, Deformation, Stress, Strain, Structure

1 Introduction
[1] Fracture mechanics is an indispensable tool in improving of mechanical
properties of mechanical parts in modern industrially age. This branch of
material research works with the physics of stress and strain, mainly focused
on the theories of elasticity and plasticity. It deals with the microscopic
crystallographic defects, which are found in real materials in order to predict
the macroscopic mechanical failure of components. Fractography is often
used with fracture mechanics to describe the causes of failures and also
verify the theoretical faults predictions with real life failures [e.g. 1-3].
[2] Electron Back-Scatter Diffraction (EBSD) is a useful technique to obtain
electron diffraction patterns from polycrystalic materials. Obtained patterns
can be used to identify crystal lattice and its orientation and thus give
important information as grain size, grain boundary character, local and bulk
texture and individual grain orientation at crack initiation area. These
possibilities provide complete characterization of microstructure cracks and
strong correlation to both properties and performance of materials. EBSD
data provide powerful tool for strain and/or stress analysis by the use of
different methods, which are described below [4-11].
[3] The first possibility is EBSD Quality Map, which uses the fact that the
diffraction patterns lose their sharpness and contrast in regions with high
dislocation density. The second one is called Boundary Distribution Maps. It
means that deformed regions have a higher concentration of low angle
boundaries. The third method uses Local Misorientation Maps. It counts
average disorientation between each measurement and its 8 neighbours.
These maps display local strain variations, independent from the grain size.
Another mode is called Strain Contouring Maps. The orientation range within
each grain is calculated and then compared according to the grain size.
During the last step is constructed map of the whole area with different colour
gradient of strain.
[4] The goal of present research was to evaluate the effect of potential
scratches, which may occur during sheet manipulation or transport, on the
sheet’s deformability. For the detailed explanation of deformation behaviour
was used high accuracy EBSD measurement.
2 Experimental
[5] For the experiment was used DC06 deep drawing steel sheet [12]. For
the tests were prepared three samples according to CSN-EN ISO 6892-1
(Fig. 1) [13] with dimensions L0 = 80 mm; a = 20 mm; b = 0.7 mm. A
scratchtester was used to make scratch lines under the load of 100 N with a
length of 5 mm perpendicularly to the direction of tensile deformation (as
marked by red colour in Fig. 1). The measured depth of the scratch line was
approx. 100 μm (Fig. 2). Two samples were deformed with a strain rate of 10
mm/min, the third sample was undeformed. The deformations (Δl) were 0
mm; 20 mm and up to rupture of the specimen. The studied areas were: the

source undeformed material; immediate vicinity of scratch (at various degrees
of deformation) and the rupture area. Samples for EBSD analysis were taken
longitudinally to the direction of tensile strain, normally to the scratch path.
Because the sample preparation is the crucial point, the sample’s surface
was finally mechanical-chemically polished using colloidal silica and
subsequently was Ar-ion-beam milled to remove Beilby layer.
*
Fig. 1 A schema of the testing sample. L0 = 80 mm; Lc = 120 mm, a = 0.7 mm; b = 20 mm. A scratch line is marked by
red colour.
Fig. 2 The profile of a scratch line obtained by a mechanical profilometer. The depth is approximately 100 μm, length is
5 mm and width is 400 μm (excluding pile up walls).
[6]
[7] For the structural analysis was used FESEM Zeiss Ultra Plus equipped
with EBSD detector Oxford Nordlys Nano. Operating conditions were:
acceleration voltage 20 kV, working distance approx. 10 mm, sample tilt 70°.
Obtained EBSD data were processed in Oxford Channel5 SW.
3 Results and discussion
[8] The structure of undeformed steel sheet has been studied in the
rolling-, transverse- and normal direction (Fig. 3). The grains are slightly
elongated and show strong orientation along the rolling axis. Grains are
preferentially oriented with its <111> axis parallel to sheet’s normal direction
and <101> axis parallel to the rolling direction.

Fig. 3 EBSD Euler maps showing grain structure of undeformed material.
From the left: rolling-, transverse- and normal direction and <100> pole
figures and inverse pole figures corresponding to rolling direction. The
material shows strong anisotropy.
Fig. 4 Different EBSD maps of scratched undeformed sample. A) IPF Y map; B) Strain-contouring map; C)
Grain Boundary distribution map.
[9] In Fig. 4 there are three different EBSD maps showing a cross-section
of the scratched undeformed sample. In IPF Y map there is visible
considerable grain flattening, refinement and change in its orientation caused
by the tip of the scratchtester under the load of 100 N. It should be noted that
IPF Y map is usable as a tool for deformation evaluation due to the fact that

the starting material showed a strong orientation (grains were preferentially
oriented with its <111> axis parallel to sheet’s normal direction as seen in IPF
Y0 figure in Fig. 3). The strain-contouring map identifies the plastically
deformed (strained) zone by light green colour. Interesting results revealed
sub-grain boundary map, where both, plastically and elastically deformed
zones appeared. In this map dark and light blue shows low angle boundaries
(1 to 3°); middle angles (4-6°) are marked by green and yellow colour; 7-9°
sub-grain boundaries are orange and red; higher angles than 10° were
considered as grain boundaries. According to presented EBSD
measurements, the plastic deformation depth is approximately 140-150 μm;
the elastic deformation reaches depth of approx. 280 μm.
Fig. 5 The broken sample after the tensile deformation.
[10] The fracture occurred at Δl = 41 mm which correspond to relative
deformation of 51 %. A surprising fact is that the fracture occurred outside the
scratch (Fig. 5). The fact that the scratch didn’t act as a stress concentrator
could be explained due to a large scratch-tip radius compared to the sheet
thickness.
Fig. 6 Local missorientation EBSD maps in the scratch area cross-sections.
A) undeformed; B) 25 % deformation; C) at rupture (51 % deformation).
[11] Fig. 6 shows local missorientation maps of undeformed-, 25 %
deformed- and deformed up to rupture sample in the scratch line region.
While the scratch-affected zone doesn’t changed significantly due to

deformation strengthening during tensile loading, the gradual deformation of
the material under the scratch line is obvious. Dark blue areas visible in the
deformed samples (Fig. 6B and C) suggest that strengthening due to plastic
deformation during scratching took place and thus reduced deformation
degree in the area below the scratch.
[12] In Fig. 7 there are a detailed EBSD maps of the rupture area. The IPF X
map demonstrates almost complete <101> grain alignment parallel to tensile
direction, while the previous <111> orientation in normal direction was
mildered (IPF Y map). Obtained pole figures indicate the grain rotation around
<101> parallel to the longitudinal direction with slightly preferred <111>
orientation parallel symmetrically to the normal and transverse direction.
Observed grain alignment well corresponds to the fact that in the BCC lattice
plastic deformation occurs preferably in {110} planes, which are the most
densely occupied by atoms and thus requires the lowest shear stress value
[14].
Fig. 7 EBSD IPF X and IPF Y maps, <100> pole figures and inverse pole
figures of the rupture area. Strong grain <101> orientation parallel to
longitudal (strain) direction.

4 Conclusion
[13] Present paper deals with the deformation study of the deep drawing
ferritic steel, which is used for manufacturing of car body parts. The effect of
the surface scratch on the deformation behaviour was studied. It was
expected that the scratch will act as stress concentrator causing decrease in
deformability and rupture before reaching guaranteed deformation degree.
The study showed that the scratching process only led to deformation
strengthening of the scratch line immediate proximity area and had no effect
on the sample deformability. This could be explained through the large
scratch-tip radius compared to the steel sheet thickness. The usage of
different EBSD data processing techniques was demonstrated and enabled
detailed study and description of the material behaviour during tensile
loading. Further work will deal with the deformation study on the different
types of material defects.
Acknowledgement
Authors wish to thank to the institutional support of Technical
University of Liberec, Faculty of Mechanical Engineering, Department of
Materials and to the project LO1201 “Národní program udržitelnosti I”
and to the Škoda Auto a.s. that participated on the presented research.
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