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DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
DESIGN OF DUAL PHASE HIGH STRENGTH
STEEL SHEETS FOR AUTOBODY
EVIN, E.; TOMAS, M.; KATALINIC, B.; WESSELY, E. & KMEC, J.
Abstract: The aim of the paper is to present the basic concepts of advanced high strength dual phase steels for automotive applications, including the design of chemical composition, microstructure and mechanical properties development during thermo-mechanical processing as well as characterization production technology
and the potential applications of in-service performance. Dual-phase steel sheets
have very good ability of absorption of kinetic energy on impact and higher strength properties. A good combination of strength and ductile properties of dual phase steels can reduce weight and improve safety (strength, stiffness, absorption energy) of an auto body. Key words: dual phase, weight reduction, auto body, strength, absorption energy
Authors´ data: Prof. Ing. CSc. Evin, E[mil]*; Ing. PhD. Tomas, M[iroslav]*; Univ.
Prof. Dipl.-Ing. Dr.h.c.mult. Dr.techn. Katalinic, B[ranko]**; doc. Ing. CSc.
Wessely, E[mil]***; RNDr. PhD. Kmec, J[ozef]*, * Technical University of Košice,
Letna 9, 040 01, Kosice, Slovakia, ** University of Technology, Karlsplatz 13, 1040,
Vienna, Austria, *** University of Security Management in Kosice, Kukučínova 17,
040 01, Kosice, Slovakia, [email protected], [email protected],
[email protected], [email protected], [email protected]
This Publication has to be referred as: Evin, E[mil]; Tomas, M[iroslav]; Katalinic,
B[ranko]; Wessely, E[mil] & Kmec, J[ozef] (2013) Design of Dual Phase High
Strength Steel Sheets for Autobody, Chapter 46 in DAAAM International Scientific
Book 2013, pp. 767-786, B. Katalinic & Z. Tekic (Eds.), Published by DAAAM
International, ISBN 978-3-901509-94-0, ISSN 1726-9687, Vienna, Austria
DOI: 10.2507/daaam.scibook.2013.46
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
1. Introduction
Research, development, design, construction, manufacture, marketing and
customer support will be increasingly integrated so that they would work together as
a single component virtually joining the clients, designers and manufacturers of
automotive components. In this respect, the European Commission, in collaboration
with other important consortia of steel companies implemented a number of projects
in recent years: Ultra-Light Steel Auto Body - ULSAB, Ultra-Light Steel Auto
Closures - ULSAC, ULSAB-AVC, FSV BEV and SuperLIGHT-CAR, The key
objective was to reduce CO2 emissions and mitigate the climate changes.
Requirements relating to reducing emissions and mitigating climate changes in the
production and operation of the vehicles are required to reconcile with the
requirements of passengers and pedestrians safety as well as power, legislative as
well as designer ones (Evin et al., 2012).
Fig. 1. Key areas of the auto body on impact
The surviving of passengers (passenger safety) in an accident is determined by
the size of the human body congestion and the occupant's survival space - Fig. 1.
Deformation work for plastic deformation of deformation zone components in the
engine compartment and trunk must be consumed during crash for absorption of the
impact kinetic energy. Thus, the larger the deformation work of components in the
area of trunk and engine is, the less overloading of passengers occurs from the
moment of contact of stronger and stiffer components in the front and the rear auto
body part with a fixed barrier (Evin, 2011). Stronger and stiffer components in the
area of cab must prevent the penetration of auto body components into passenger
compartment (cab) during a crash. When designing the SuperLIGHT-CAR concepts,
the components of deformation zones in the area of engine and trunk were made
mostly of DP steels - Dual Phase, TRIP steels - Transformation Induced Plasticity,
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
TWIP - Twinning Induced Plasticity, ASS - austenitic steels. Components in cabin
space (in the passengers zone) were made of ultra-high strength steels (UHSS) with
yield strength higher than 550 MPa (MART martensitic, FB ferritic-bainitic steels,
TWIP steel - Twinning Induced Plasticity, CP-Complex Phase steel, hot-formed
boron steels - formed hot, bored, steel heat-treated after forming - post forming heat
treated) as well as TRIP, TWIP and austenitic steels with a certain degree of
predeformation (e.g. hydromechanical forming). There were also used HSS steels
with yield strength from 210 to 550 MPa and an tensile strength Rm from 270 to 700
MPa (HSIF – High-Strength Interstitial Free, HSLA - High Strength Low-Alloy,
micro-alloyed with BH effect, carbon-manganese sheets), stampings and castings
made of aluminium and magnesium alloys as well as composites (Evin et al., 2012;
Hofmann, 2008; Rosenberg et al., 2009; Kleiner et al., 2003; Aksoy et al., 1996;
Takahashi, 2003). Material composition of the SuperLIGHT-CAR auto body
components allowed reaching the body weight reduction of 74 kg (27%) and 115 kg
(38%).
2. Application Aspects of AHSS
The combination of high strength and ductility that provide modern AHSS can
allow thinner components to be used in the cars construction and also to improve the
safeness due to their high energy-absorption capabilities. The better formability of
AHSS, compared to conventional high strength steels of comparable strength give the
automobile designer a high degree of flexibility to optimize the component geometry.
Other component performance criteria comprise stiffness, durability, crash energy
management (Evin, 2011).
Fig. 2. The primary types of loading of the auto body components
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
The primary types of loading (longitudinal loading tension and compression,
bending, torsion, combined bending and torsion, shear loading,) components of the
body at impact are shown in Fig. 2. For the longitudinal tensile or compressive force
strength and deformation work criteria given in (Rosenberg et al., 2009) can be used
to predict the stiffness.
The stiffness of a component is affected by material properties (module of
elasticity - E, yield stress - YS = σ0.2%, true yield stress - YStrue or true flow stresses -
σ0.05, σ0.1) as well as its geometry. The stiffness can be predicted using the following
relationship:
(1)
or by elastic work:
(2)
The module of elasticity is constant for steel; considering eq. (2) it means
change the steel grade does affect the stiffness due to the yield stress change.
Therefore, to improve stiffness for constant component geometry the material with
higher yield stress must be changed. The yield stress can be predicted by Hall Petch
relationship as the additive effect of the various mechanisms of hardening (Kuziak,
2008, Dzupon et al., 2007):
(3)
where d - the ferritic grain, or diameter of cells of dislocation martensite,
ky - the characteristic of a barrier of grain boundaries against dislocation
movement,
σ0 - stress required for movement of dislocations in crystalographical lattice,
∆σPR - contribution of hardening by perlite,
∆σD - contribution of dislocation hardening,
∆σS - contribution of substitutional hardening,
∆σIN - contribution of interstitial hardening,
∆σP - contribution of precipitation hardening,
∆σf - contribution of phase hardening.
The yield strength increases in two ways: about BH effect (approx. 40 ÷ 60
MPa) due to thermo-mechanical processing when the paint is baked and about WH
effect as a result of deformations – see Fig. 3. AHSS also have good bake hardening
ability (BH effect) and work hardening ability (WH effect) – Fig. 4, then the true
value of the yield stress can be:
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
(4)
Fig. 3. Schematic illustration of BH and WH effects
Fig. 4. BH and WH effect to true stress for AHSS
To evaluate the true flow stresses of different steel sheets, the following
Hollomon equation can be used:
(5)
and WH effect
(6)
where YS or YS0.2% – yield stress at static tensile test,
BH – bake hardening effect (interstitial hardening),
WH – work hardening effect.
UTS –ultimate tensile strength,
εr or UE – uniform (homogenous) deformation,
n – strain hardening exponent,
K – strength coefficient
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
X – degree of safety (x=1.6 ÷ 2).
The strength of a component depends on its geometry and yield and/or tensile
strength - Fig. 5.
(7)
Or
(8)
AHSS provide an advantage in the design flexibility over conventional high
strength steels due to their higher formability and work hardening characteristics.
These grades also have good bake hardening ability - BH. Therefore, it is important
to account for this strength increase during the design process of car components in
order to avoid the over design that may occurs when the design process is based upon
as rolled mechanical properties specification. Both these features enable achieving
high strength of as-manufactured components.
The crashworthiness is an important characteristic that is currently becoming
increasingly important. Recent trends require for a material to absorb more energy in
crash scenario. The potential absorption energy can be assessed based upon the area
under the stress-strain curves.
(9)
Better performance in crash of AHSS compared to classical high strength steels
is associated with higher work hardening rate and high flow stress. This feature
accounts for a more uniform strain distribution in components in the crash event.
Both, work hardening (WH) and bake hardening (BH) significantly improve the
energy absorption characteristics due to the flow stress increase. Then the strain work
(Fig. 5) can be calculated according to equation (10):
(10)
The fatigue properties of structural components depend on geometry, thickness,
applied loads and material endurance limit. Thus, high strength combined with
superior work hardening and bake hardening, resulting in a significant increase in the
as manufactured strength of AHSS components, also results in a better fatigue
resistance.
AHSS which fulfil these requirements include dual-phase ferritic-martensitic (F-
M) steels. Microstructure of dual phase steels is composed of soft ferrite matrix and
10-20% of hard martensite or martensite-austenite (M-A) particles. This type of
microstructure allows achieving the yield strength Re in the range of 300 ÷ 500 MPa
and the ultimate tensile strength in the range of 500 ÷1200 MPa. When the volume
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
fraction of martensite exceeds 20%, DP steels are often called partial martensitic. For
some applications, also baintic constituent may be desirable in the DP steel
microstructure (Uthaisangsuk, 2008; Podder, 2007).
Fig. 5. Schematic illustration of true stress-true strain curve
The contributions of hardening mechanisms in the martensitic structure include
the solid solution substitution element hardening, the precipitation hardening, the
primary austenitic grain size hardening and the martensite morphology hardening.
The dominant hardening effect of martensite in dual phase steels is the carbon
concentration in martensite. It is relatively difficult to formulate regression equations
for the contributions of individual hardening mechanisms in martensite as it is
possible for polygonal ferrite, since it is impossible to separate individual hardening
mechanisms in martensite (Kuziak, 2008).
3. Methods for Prediction of Safety and Technological Formability
Characteristics of Body Components from Steel Sheets
When analyse safety and formability characteristics of auto body components
from steel sheets, it is necessary to define the location and type of failure on stamped
part. Tears occur in consequence of tensile stress in the area of curve - Fig. 6.
Area of failure may be divided on three parts (Hrivňák, A. & Evin, E., 2004):
1. area of tension: ε2 < 0,
2. area of plane strain: ε2 = 0,
3. area of stretching: ε2 > 0.
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
Fig. 6. The primary types of failure of the auto body components
When a car crashes as well as at the production of body components the failures
by pure uniaxial tension or biaxial tension occurs only in rare cases. In the practise it
is ineffective to develop the test method for each shape of car’s components from
steel sheet blanks. The more effective way shows us to compare deformation
properties of steel sheets and components made of steel sheets, based on results of
standard tests that model schemes of its loading at production and its application.
Stress of material in the area of stretching (ε2 > 0) can be modelled by tensile
test, cross tensile test, Erichsen test, bulge test, Marciniak test, Nakazima test, etc.
Stress of material in the area of deep drawing (ε2 < 0) can be modelled by tensile test
with the notch radius on the samples r = 2 mm test, Fukui test, Engelhardt test, etc.
3.1 Experimental Procedure
Experimental research for evaluating the strength and energy absorption and
formability of sheets with higher strength properties was carried out on steel sheets of
F-M produced by intercritical annealing (specimens designated A1, A2, A3, A4, B1,
B2) and specimens produced by the method of controlled rolling (specimens denoted
as C1, C2,C3,C4,C5). The volume proportion of the individual structural components
and the ferrite grain size are shown in Table 1. Metallographic analysis of the
materials A and B show that they have a fine-grained ferrite-martensite structure with
martensite dispersion excluded in the form of small islands which form mainly in the
area of the ferrite grain boundaries (Fig. 7). In the material C martensite formed large
islands and ferrite and martensite grains 'alternated' (Fig. 8) (Evin, 2011,Hrivňák, A.
&Evin, E., 2004).
The materials C had a dual-phase structure. In many cases the second phase
showed a morphological feature of martensite or a mixed nonpolyhedral structure.
Based on the brief analysis of the metallographic structure it may be concluded that a
large difference was detected in the morphology on distribution of martensite in the
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
materials A and B produced by intercritical annealing in comparison with the
materials C produced by controlled rolling.
Method of production Intercritical annealing Controlled rolling
Designation of material A B C
Designation of specimen A1 A2 A3 A4 B1 B2 C1 C2 C3 C4 C5
Martensite volume fraction
[%]
19.9 25.4 20.3 27.9 31 31 25 52 25 27 29
Ferrite volume fraction [%] 80.1 74.6 79.7 72.1 69 69 75 48 75 73 71
Ferrite grain size [µm] 4.3 3.1 4.3 3.1 3.8 4 4.5 4.2 4 3.6 4
Tab. 1. Volume fraction of the individual structural components
To obtain the material properties the tensile machines TiraTEST 2300 and
INSTRON were used. Curves of true stress on strain dependence, normal anisotropy
coefficient, yield strength, tensile strength and total elongation were evaluated in the
terms of requirements of standards STN EN ISO 6892-1, STN EN 42 0435, STN
10130:1991. Values of mechanical properties are shown in Table 2.
Fig. 7. Structure of material A1 Fig. 8. Structure of material C
Material Yield
strength
Re
[MPa]
Tensile
strength
Rm
[MPa]
Total
elongation
A50
[%]
Uniform
(homogenous)
deformation
Strain-
hardening
exponent
n
Constant
K
[MPa]
Plastic
strain
ratio
r
DC 04 210 350 40 0.251 0.200 470 1.60
A1 299 593 31 0.242 0.229 1076 1.01
A2 361 647 26 0.212 0.196 1113 1.03
A3 304 596 30 0.238 0.211 1052 1.05
A4 361 646 24 0.195 0.180 1073 1.04
B1 443 792 22 0.188 0.184 1336 0.71
B2 437 791 22 0.180 0.166 1270 0.82
C1 460 646 24 0.189 0.166 1070 0.81
C2 492 733 15 0.130 0.130 1153 0.63
C3 464 624 23 0.185 0.167 1085 0.82
C4 458 656 27 0.206 0.172 1070 0.67
C5 495 627 21 0.174 0.165 1080 0.78
Tab. 2. Mechanical properties of experimental materials
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
Experiment Numerical simulation Material IE [mm] LDR FLD0 IE [mm] LDR FLD0
A1 10.0 2.096 0.28 ± 0.03 - - -
A2 9.5 2.083 0.25 ± 0.03 9.4 0.482 0.280
A3 9.9 2.096 0.28 ± 0.03 - - -
A4 9.3 2.068 0.24 ± 0.03 9.4 0.486 0.260
B1 9.1 2.033 0.22 ± 0.02 - - -
B2 9.0 2.01 0.22 ± 0.02 - - -
C1 9.0 1.97 0.24 ± 0.02 9.3 0.496 0.242
C2 8.1 1.93 0.18 ± 0.02 9.1 0.527 0.194
C3 9.1 1.957 0.23 ± 0.02 - - -
C4 9.4 1.97 0.26 ± 0.03 - - -
C5 8.9 1.97 0.22 ± 0.03 9.1 0.503 0.241
Tab. 3. Measured and calculated values of technological characteristics
Stress of material in the area of stretching (ε2 > 0) was modelled by Erichsen
test. Stretchability is expressed as IE height of cup. Stress of material in the area of
deep drawing (ε2 < 0) was modelled by cup test. Drawability is expressed as the
limiting draw ratio as follows:
where D0max - maximum blank diameter by maximum drawing load,
d0 - punch diameter.
Technological characteristics obtained by Erichsen test and cup test as well as
these values calculated for selected materials by numerical simulation are shown in
Table 3.
The numerical simulation of Erichsen test and cup test for selected materials
were realised in order to compare experimental and calculated values. Based on tools
dimensions used in experiments virtual CAD models were created as it is shown in
Fig. 9 for Erichsen test and Fig. 10 for cup test.
Fig. 9. Erichsen test simulation model Fig. 10. Cup test simulation model
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
Fig. 11. Simulated drawing force courses at cup test and maximal drawing force for
different blank diameter - material A2
The numerical simulation of tests was done using Pam Stamp 2G simulation
software. Simulation models were meshed, positioned and set-up in pre-processing
module of the software, based on CAD data. To define material models, yield law
and anisotropy type following input data were defined in Pam Stamp 2G
preprocessor:
- basic material data (density, Young's modulus, Poisson's constant),
- blank thickness,
- strain-hardening curve defined by Hollomon’s law according to data shown in
Tab. 3 – constant K and strain hardening exponent n,
- plastic strain ratio r as definition of sheet normal anisotropy,
- rolling direction 0° in x-axis of blanks,
- Yield law defined by Hill 48 model.
Note, the materials were considered here as isotropic so the planar anisotropy of
plastic strain ratio wasn’t considered.
The results of numerical simulations were evaluated in postprocessing module
of Pam Stamp 2G simulation software. The maximum forces and force dependencies
were filtered by MVA filter with the range of 25 due to its course oscillation given by
numerical simulation – Fig. 11. Based on the finding the maximum drawing force,
the IE height of cup in Erichsen test was measured as well as the LDR in cup test was
calculated. The value of FLD0 was calculated by the software using AutoKeeler
mode because of the FLC curves for these materials weren’t experimentally
measured. The results of height of cup IE, LDR and FLD0 reached by numerical
simulation and compared to experimental ones are shown in Table 3. LDR values
were determined from the drawing forces (F draw) and the breaking force (F break)
required to fracture the wall of drawn part - Fig. 12.
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
Fig. 12. Scheme of determination limit blank diameter D0max for A2 material
4. Discussion of Obtained Results
Based on designers’ experiences it is possible to define the requirements for
materials from the viewpoint of static strength and energy absorption reliability
(Evin, 2011). Effectiveness of static strength (Fig. 14) is calculated as follows:
(12)
Effectiveness of energy absorption is calculated as follows:
(13)
Comparison of the mechanical properties specified in the material of the sheets
of the material DC 04 with the measured values obtained for the examined materials
of the F-M steels (Table 2) show that the yield strength (Re = 299-495 MPa) and the
tensile strength (Rm = 593-792 MPa) of all materials was higher that of a mild steel
DC 04. Approximately the same volume fraction of martensite in the structure the
materials produced by intercritical annealing had lower yield limit values than the
materials produced by controlled rolling. The elongation values (A50 = 15-31 %) of
specimens A, B and C varied in the range of materials suitable for slight drawing or
bending and for other materials in the range of materials unsuitable for deep-drawing.
As in the case of strength, the deformation properties values showed no large
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
difference between the materials produced by intercritical annealing and the materials
produced by controlled rolling.
Calculated values of the effectiveness of static strength and energy absorption
according to equation (12), (13), (14),( 16) for high strength dual phase steels has
been compared to the steel sheets DC 04 – Fig. 13 and Fig. 14. These results indicate
the potential for weight reduction from 42 to 135 % with equivalent energy
absorption. As it was mentioned the most of the inner supporting construction
elements of car body are made of steel sheets. These elements are produced by
operations of bending, stretching and deep drawing. During bending deformation
hardening occurs only in small part of bend (in local deformation) of stamped part, in
non-deformed parts (in straight parts of stamped part) deformation strain hardening
doesn’t occur. Stamped parts produced by bending show non-homogenous
distribution of deformation. During deep-drawing and stretching operations of the
stamped parts deformation as well as deformation strain hardening occurs on whole
area. The deformation distributed at stretching is more homogenously than at deep
drawing operations. It is required to calculate with strain hardening but also with
interstitial hardening (BH effect- increasing the strength about approximately 30 to
60 MPa) to optimize the material selection, according to Eq. (4).
Fig. 13. Strength comparing of tested materials to reference material DC 04
The exponent of strain hardening of the material react very sensitively to the
change in the condition of the structure and substructure of the material and enable
the limit of the loss of plastic stability, reduction area, to be expressed more
accurately. Up to this limit there is a guarantee that plastic deformation doesn’t
localize and there is no subsequent failure of the material. Then effectiveness static
strength by 5 % degree of deformation can be calculated according to equation:
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
(14)
Fig. 14. Energy absorption comparing of tested materials to reference material DC 04
Comparison of the constant K specified in the material of the sheets of the
material DC 04 with the measured values obtained for the examined materials of the
F-M steels (Table 2) show that the constant K (K = 1052 - 1336 MPa) of all materials
was higher that of a steel sheets DC 04 and the values of the strain hardening
exponent of materials produced by intercritical annealing were greater or comparable
with the DC 04. Materials produced by rolling have shown lower values of strain
hardening exponent as DC 04. Approximately the same volume fraction of martensite
in the structure of materials produced by intercritical annealing had higher strain
hardening exponent and constant K values than the materials produced by controlled
rolling. The results confirmed the interaction effect of ferrite and martensite reflected
in an increase of dislocation density in ferrite and at the ferrite-martensite boundary
and in an increase in flow stress. However, at assumption that at production of
certain stamped part 5 % (ε = 0.05) deformation and true stress is expressed by
relation (4), dual phases materials shows approximately from 100 to 200% higher
strength as reference material DC 04 – Fig. 13.
Dual phase-steels exhibit of strain hardening effect, i.e. sustain higher stresses at
increased deformation. This effect corresponds to increase in load car crash to the
reference material. Then the strain work (Fig. 15) can be calculated according to
equation:
(15)
and effectiveness of energy absorption by 5 % degree of deformation
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
(16)
Fig. 15. The true stress versus true strain dependence
Based on the test results of technological formability it is possible to compare
the formability of dual phase steel sheets from the viewpoint the formability of
conventional low-carbon steel sheets. The classification of conventional low-carbon
steel sheets suitable for deep drawing is given in Table 4.
Material Mechanical properties
Qual
itat
ive
clas
sifi
cati
on
DIN
1623
EN
10130
ST
N
42 0
127 Rp min
[MPa]
Rm
[MPa]
A80 min
[%] rmin nmin
St 12 Fc PO 1 11 331 280 270 -410 28 CQ
St 13 Fc PO 3 11 321 240 270-370 34 1.3 DQ
St 14 Fc PO 4 11 305 210 270-330 38 1.6 0.18 DDQ
Fc PO 5 KOHAL
ISO
180 270-340 40 1.9 0.21 EDDQ
Fc PO 6 IF IS 38 1.8* 0.22*
* rmin and nmin are mean values CQ - (commercial-drawing quality) grade suitable for parts with lower demands on deformation degree
DQ - (drawing quality) grade suitable for parts with high demands on deformation degree
DDQ - (deep-drawing quality) grade suitable for parts with very high demands on deformation degree
EDDQ - (extra deep-drawing quality) grade suitable for parts with extra high demands on deformation
degree
Tab. 4. Classification of formability of conventional steel sheets (Hrivňák, A. & Evin,
E., 2004)
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
Fig. 16. The dependence of the deformation properties on the volume fraction of
martensite
The elongation values (A50 = 15 ÷ 31%) of specimens A, B and C varied in the
range of materials suitable for slight drawing or bending and for other materials in the
range of materials unsuitable for deep-drawing. As in the case of strength, the
deformation properties values showed no large difference between the materials
produced by intercritical annealing and the materials produced by controlled rolling.
Deformation properties of dual-phase steels (tensibility, uniform elongation UE,
strain-hardening exponent) depend on the volume fraction of martensite – Fig. 16.
Innovation tendencies in automotive industry (decreasing of mass, saving of
energy, ecology) lead to the use of high-strength steels of new conceptions (micro-
alloyed, bake hardening - BH, interstitial free -IF, dual phase - DP, with
transformation induced plasticity - TRIP). Even though they show higher values of
elongation, normal anisotropy coefficient and exponent of strain hardening indicate
the good formability. High-strength steel sheets with tensile strength in the range
from 400 MPa to 800 MPa cannot be classified according to conventional schemes of
evaluation of formability because these steels despite their higher strength show good
formability (Hrivňák, A. & Evin, E., 2004).
Suitability of dual phase steel sheets for deep drawing was evaluated based on
values recommended for qualitative grades of drawing of classical steel sheets (deep
drawing process - values LDR and stretching – values IE and FLD0) - see Fig. 17 and
Fig. 18. Values of limiting ratio (LDR) for examined material evaluated by method of
intercritical annealing varied in the range from 2.068 to 2.096 and in materials
produced by controlled rolling from 1.93 to 1.97. We measured higher values of the
degrees of the LDR in approximately the same volume fraction of martensite in
structure of materials produced by controlled rolling. The diagram LDR in Fig. 19
indicates that materials produced by intercritical annealing appear to be suitable for
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
deep drawing (DDQ) whereas the materials produced by controlled rolling appear to
be suitable for drawing quality (DQ).
On the basis of value IE the materials A and G are suitable for demanding
operations of stretching, and materials A1 and A3 are suitable for middle demanding
operations of stretching - DSQ, and materials A2 and C4 are suitable for lower
demanding operation of stretching - SQ.
Sheet of DDQ quality should be used when drawing steel will not provide a
sufficient degree of ductility for fabrication of parts with stringent drawing
requirements, or applications that require the sheet to be free from aging. This quality
is produced by special steelmaking and finishing practices. It is suitable for
automotive front panels and rear fenders.
Sheet of DQ quality has a greater degree of ductility and is more consistent in
performance than commercial steel, because of higher standards in production,
selection and melting of the steel. It is suitable for automotive panels, audio-visual
equipment, and heating apparatuses.
Based on specification of LDR for classic deep-drawing steel, it is possible to
specify requirements for the volume fraction of martensite F-M steel sheets as
follows:
Extra deep drawing quality EDDQ: Vm < 15 %
Deep drawing quality DDQ: Vm 15 ÷ 20 %
Drawing quality DQ: Vm > 20%
Fig. 17. Drawability qualitative classification of conventional steel sheets
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
Fig. 18. Strechability qualitative classification of conventional steel sheets
However, for the stress-strain states from uniaxial tension to biaxial tension
(stretching) are preferable to use IE and FLD. In terms of suitability for stretching,
materials with martensite precipitated in the form of small islands are classified
according to Fig. 16 as follows:
Extra stretching quality ESQ: Vm < 15 %
High stretching quality HSQ: Vm 15 ÷ 20 %
Stretching quality SQ: Vm 20 ÷ 35 %
Commercial stretching quality CSQ: Vm > 25%
Fig. 19. Specification of limiting drawing ratio versus volume of martensite
DAAAM INTERNATIONAL SCIENTIFIC BOOK 2013 pp. 767-786 CHAPTER 46
Fig. 20. Specification of stretching versus volume of martensite
4. Conclusion
Dual phase steel sheets represent progressive material, but designers often do not know its advantages in comparison with classical steel sheets. In the article, we described approach of predicting of safety characteristics of auto body and technological formability from dual phase steel sheets is based on the concept of producing steel sheets ”to measure for a specific auto body product” taking into account the microstructure of ferritic-martensitic steel sheets, mechanical properties and requirements of efficient economical processing for a specific product.
Knowledge obtained at evaluation of formability of high-strength micro-alloyed and dual-phase steels may be summarized as follows:
1. From this comparison one can see that dual phase steel have 42 and 135% higher
values of strength and also higher values of deformation work. In case of production of steel sheets by stretching with deformation higher than 5% the increase of stress to 100 - 200 MPa occurs.
2. Formability of high strength dual phase steels was compared to formability of deep-drawn steel DC04. Deep drawing capacity steel DC 04 has better formability than dual phase steel, but differences were small in some cases (material A, B, D).
3. Stretching capacity was compared with stretching capacity of dual phase steel sheets with volume fraction of martensite lower than 25 %. Dual phase steel sheets with fine ferrite-martensitic structure with martensite precipitated in form of small islands in grains ferrite boundaries have higher values of strength and plastic properties as steel with martensite precipitated in form of bigger islands.
4. The measured results indicate that it is appropriate to use the Keeler and Brazier empirical relation for prediction of critical values of deformation.
5. Formability of dual-phase ferritic-martensite steels may not be evaluated only on the basis of comparison of mechanical properties values required at conventional steel sheets - Tab. 1. For comparison, on the basis of limit drawing ratio there were determined conditions for quality deep drawings (CQ, DQ, DDQ, EDDQ) on volume
Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E. & Kmec, J.: Design of Dual Phase ...
fraction of martensite in structure and in the same similarly also for stretching capacity (CSQ, SQ, HSQ, ESQ).
5. Acknowledgements
This work is a part of research project VEGA 1/0824/12 “Study of formability aspects of coated steels sheets and tailored blanks“ supported by Scientific Grant Agency of the Ministry of Education, Science and Research of Slovakia.
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