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
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].
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].
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.
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
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).
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
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.
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.
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).
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.
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
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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