DUAL PHASE HIGH STRENGTH STEEL SHEETS FOR AUTOBODY€¦ · strength dual phase steels for automotive applications, including the design of chemical composition, microstructure and

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,


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


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:


(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


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


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.


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


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


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


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.


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


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:


(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


(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)


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


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


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


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


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.

Authors also express their thanks for project APVV-0273-12 “Supporting innovations of autobody components from the steel sheet blanks oriented to the safety, the ecology and the car weight reduction “ supported by Slovak Research and Development Agency. 6. References Aksoy, M.; Karamis, M. B. & Evin, E. (1996). An evaluation of the wear behaviour of a dual-phase low-carbon steel. Wear, Vol. 193, No. 2 (1996), pp. 248-252, ISSN 0043-1648 Dzupon, M. et al. (2007). Dual Phase Ferrite-Martensite Steel Micro-Alloyed with V-Nb. Metalurgija, Vol. 46, No. 1 (2007), pp. 15-20, ISSN 0543-5846 Evin, E. (2011). Desing of dual phase steel sheets for auto body. Acta Mechanica Slovaca, Vol. 15, No. 2 (2011), pp. 42-48, ISSN 1335-2393 Evin, E.; Tkáčová, J. & Tkáč, J. (2012). Aspects of steel sheets selection for car-body components (2

nd part). Ai magazine - automotive industry, Vol. 5, No. 3 (2012), pp.

96-98, ISSN 1337-7612 Hofmann, H.; Mattisen, D. & Schauman, T. W. (2008). Advanced cold rolled steel sheets for automotive. Materialwissenschaft und Werkstofftechnik, Vol. 37, No. 9, pp. 716-723, ISSN 1521-4052 Hrivňák, A. & Evin, E. (2004). Formability of steel sheets: Prediction of higher strength steel sheets formability. Elfa (2004), Kosice, ISBN 80-89066-93-3 Kleiner, M.; Geiger, M. & Klaus, A. (2003). Manufacturing of Lightweight Components by Metal Forming. CIRP Annals - Manufacturing Technology, Vol. 52, No. 2, pp. 521-542, ISSN 0007-8506 Kuziak, R. (2008). Advanced high strength steels for automotive industry. Archives of Civil and Mechanical Engineering, Vol. 8, No. 2 (2008), pp. 104-117, ISSN 1644-9665 Podder, A. S; Bhattacharjed, E. & Raz, R. K. (2007). Effect of Martensite on Mechanical Behavior of Feritte/Bainite Dual phase Steel. ISIJ International, Vol. 47, No. 7, (2007), pp. 1058-1064, ISSN 0915-1559 Rosenberg, G.; Buríková, K. & Juhár, Ľ. (2009). Modification of Strength - Plasticity Properties of Microalloyed Steels by Means of heat Treatment. Manufacturing and industrial engineering, Vol. 8, No. 3 (2009), pp. 49-52, ISSN 1335-7972 Takahashihi, M. (2003). Development of High Strength Steel for Automobiles. Nippon steel report, 88, 2003, pp.1-6 Uthaisangsuk, V.; Prahl, U & Bleck, W. (2008). Micromechanical modelling of damage behaviour of multiphase steels. Computational Materials Science, Vol. 43, 2008, pp. 27-35, ISSN 09270256

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DUAL PHASE HIGH STRENGTH STEEL SHEETS FOR AUTOBODY€¦ · strength dual phase steels for automotive applications, including the design of chemical composition, microstructure and