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Design of dual phase high strength steel sheets for autobody.

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).

[FIGURE 1 OMITTED]

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).

[FIGURE 2 OMITTED]

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 = [[sigma].sub.0.2%] true yield stress--Y[S.sub.true] or true flow stresses-- [[sigma].sub.0.05], [[sigma].sub.0.1]) as well as its geometry. The stiffness can be predicted using the following relationship:

STF = [V.sub.0][YS/x.E] (1)

or by elastic work:

STFW = [V.sub.0][Y[S.sup.2]/[x.sup.2].E] (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):

YS = [[sigma].sub.0] + [k.sub.yx][d.sup.-0.5] + [DELTA][[sigma].sub.PR] + [DELTA][[sigma].sub.D] + [DELTA][[sigma].sub.S] + [DELTA][[sigma].sub.IN] + [DELTA][[sigma].sub.p] + [DELTA][[sigma].sub.f] (3)

where d--the ferritic grain, or diameter of cells of dislocation martensite,

[k.sub.y]--the characteristic of a barrier of grain boundaries against dislocation movement,

[[sigma].sub.0]--stress required for movement of dislocations in crystalographical lattice,

[DELTA][[sigma].sub.PR]--contribution of hardening by perlite,

[DELTA][[sigma].sub.D]--contribution of dislocation hardening,

[DELTA][[sigma].sub.S]--contribution of substitutional hardening,

[DELTA][[sigma].sub.IN]--contribution of interstitial hardening,

[DELTA][[sigma].sub.P]--contribution of precipitation hardening,

[DELTA][[sigma].sub.f]--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:

Y[S.sub.true] = YS + BH + WH (4)

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

To evaluate the true flow stresses of different steel sheets, the following Hollomon equation can be used:

[sigma] = K.[[epsilon].sup.n] (5)

and WH effect

WH = K.[[epsilon].sup.n] - YS (6)

where YS or Y[S.sub.0.2%]--yield stress at static tensile test,

BH--bake hardening effect (interstitial hardening),

WH--work hardening effect.

UTS--ultimate tensile strength,

[[epsilon].sub.r] 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.

ST = YS/x (7)

Or

S[T.sub.true] = Y[S.sub.true]/x (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.

W = [YS + UTS/2].[[epsilon].sub.r] (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):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (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 (FM) 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).

[FIGURE 5 OMITTED]

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 (Hrivnak, A. & Evin, E., 2004):

1. area of tension: [[epsilon].sub.2] < 0,

2. area of plane strain: [[epsilon].sub.2] = 0,

3. area of stretching: [[epsilon].sub.2] > 0.

[FIGURE 6 OMITTED]

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 ([[epsilon].sub.2] > 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 ([[epsilon].sub.2] < 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, Hrivnak, 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.

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.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Stress of material in the area of stretching ([[epsilon].sub.2] > 0) was modelled by Erichsen test. Stretchability is expressed as IE height of cup. Stress of material in the area of deep drawing ([[epsilon].sub.2] < 0) was modelled by cup test. Drawability is expressed as the limiting draw ratio as follows:

LDR = [D.sub.0max] (11)

where [D.sub.0max]--maximum blank diameter by maximum drawing load, [d.sub.0]--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.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

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[degrees] 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.

[FIGURE 12 OMITTED]

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:

EEA = [YS/YSD[C.sub.04]]. 100 [%]. (12)

Effectiveness of energy absorption is calculated as follows:

EEA = [[YSS + UTS]]/2].[[epsilon].sub.r]/[Y[S.sub.DC04] + UT[S.sub.DC04]/2].[[epsilon].sub.r DC04]. 100 [%] (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 ([A.sub.50] = 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).

[FIGURE 13 OMITTED]

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:

EST = [K.[[epsilon].sup.n]/K.[[epsilon].sup.n.sub.DC04]]. 100[%] (14)

[FIGURE 14 OMITTED]

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% ([epsilon] = 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:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (15)

and effectiveness of energy absorption by 5% degree of deformation

EEA = [W/W[D.sub.C04]. 100[%] (16)

[FIGURE 15 OMITTED]

Based on the test results of technological forrnability it is possible to compare the forrnability of dual phase steel sheets from the viewpoint the forrnability of conventional low-carbon steel sheets. The classification of conventional low-carbon steel sheets suitable for deep drawing is given in Table 4.

[FIGURE 16 OMITTED]

The elongation values ([A.sub.50] = 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 (Hrivnak, 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%


[FIGURE 17 OMITTED]

[FIGURE 18 OMITTED]

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      CSQ:   Vm > 25%
  quality


[FIGURE 19 OMITTED]

[FIGURE 20 OMITTED]

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.; Tkacova, J. & Tkac, J. (2012). Aspects of steel sheets selection for car-body components (2nd 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

Hrivnak, 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. ISIJInternational, Vol. 47, No. 7, (2007), pp. 1058-1064, ISSN 0915-1559

Rosenberg, G.; Burikova, K. & Juhar, E. (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

Authors' data: Prof. Ing. CSc. Evin, E[mil] *; Ing. PhD. Tomas, Miroslav] *; 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 Kosice, Letna 9, 040 01, Kosice, Slovakia, ** University of Technology, Karlsplatz 13, 1040, Vienna, Austria, *** University of Security Management in Kosice, Kukucinova 17, 040 01, Kosice, Slovakia, emil.evin@tuke.sk, miroslav.tomas@tuke.sk, katalinic@mail.ift.tuwien.ac.at, wessely@vsbc.sk, jozef.kmec@tuke.sk

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
Tab. 1. Volume fraction of the individual
structural components

Method of production                  Intercritical annealing

Designation of material                    A                   B

Designation of specimen         A1     A2     A3     A4    B1    B2

Martensite volume fraction     19.9   25.4   20.3   27.9   31    31

Ferrite volume fraction [%]    80.1   74.6   79.7   72.1   69    69

Ferrite grain size [[micro]m]  4.3    3.1    4.3    3.1    3.8    4

Method of production                 Controlled rolling

Designation of material                      C

Designation of specimen         C1     C2    C3     C4    C5

Martensite volume fraction      25     52    25     27    29

Ferrite volume fraction [%]     75     48    75     73    71

Ferrite grain size [[micro]m]   4.5    4.2     4    3.6     4

Tab. 2. Mechanical properties of experimental materials

Material      Yield      Tensile       Total         Uniform
            strength    strength    elongation    (homogenous)
               Re          Rm       [A.sub.50]     deformation
              [MPa]       [MPa]         [%]

DC 04          210         350          40            0.251
A1             299         593          31            0.242
A2             361         647          26            0.212
A3             304         596          30            0.238
A4             361         646          24            0.195
B1             443         792          22            0.188
B2             437         791          22            0.180
C1             460         646          24            0.189
C2             492         733          15            0.130
C3             464         624          23            0.185
C4             458         656          27            0.206
C5             495         627          21            0.174

Material     Strain-     Constant    Plastic
            hardening        k        strain
             exponent      [MPa]      ratio
                n                       r

DC 04         0.200         470        1.60
A1            0.229        1076        1.01
A2            0.196        1113        1.03
A3            0.211        1052        1.05
A4            0.180        1073        1.04
B1            0.184        1336        0.71
B2            0.166        1270        0.82
C1            0.166        1070        0.81
C2            0.130        1153        0.63
C3            0.167        1085        0.82
C4            0.172        1070        0.67
C5            0.165        1080        0.78

Tab. 3. Measured and calculated values of
technological characteristics

                         Experiment

Material    IE [mm]     LDR             FLD0

A1           10.0      2.096     0.28 [+ or -] 0.03
A2            9.5      2.083     0.25 [+ or -] 0.03
A3            9.9      2.096     0.28 [+ or -] 0.03
A4            9.3      2.068     0.24 [+ or -] 0.03
B1            9.1      2.033     0.22 [+ or -] 0.02
B2            9.0       2.01     0.22 [+ or -] 0.02
C1            9.0       1.97     0.24 [+ or -] 0.02
C2            8.1       1.93     0.18 [+ or -] 0.02
C3            9.1      1.957     0.23 [+ or -] 0.02
C4            9.4       1.97     0.26 [+ or -] 0.03
C5            8.9       1.97     0.22 [+ or -] 0.03

               Numerical simulation

Material    IE [mm]     LDR      FLD0

A1             --        --       --
A2            9.4      0.482    0.280
A3             --        --       --
A4            9.4      0.486    0.260
B1             --        --       --
B2             --        --       --
C1            9.3      0.496    0.242
C2            9.1      0.527    0.194
C3             --        --       --
C4             --        --       --
C5            9.1      0.503    0.241

Tab. 4. Classification of formability of conventional steel
sheets (Hrivoak, A. & Evin, E., 2004)

Qualitative              Material
classification

                  DIN      EN         STN
                  1623     10130      42 0127

CQ                St 12    Fc PO 1    11 331
DQ                St 13    Fc PO 3    11 321
DDQ               St 14    Fc PO 4    11 305
EDDQ                       Fc PO 5    KOHAL ISO
                           Fc PO 6      IF IS

Qualitative           Mechanical properties
classification

                  Rp min       Rm      [A.sub.80]
                   [MPa]     [MPa]       min [%]

CQ                  280     270-410        28
DQ                  240     270-370        34
DDQ                 210     270-330        38
EDDQ                180     270-340        40
                                           38

Qualitative         Mechanical properties
classification

                  [r.sub.min]    [n.sub.min]

CQ
DQ                    1.3
DDQ                   1.6        0.18
EDDQ                  1.9        0.21
                     1.8 *       0.22 *

* [r.sub.min] and [n.sub.min] 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
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Title Annotation:Chapter 46
Author:Evin, E.; Tomas, M.; Katalinic, B.; Wessely, E.; Kmec, J.
Publication:DAAAM International Scientific Book
Article Type:Report
Geographic Code:4EXSV
Date:Jan 1, 2013
Words:5348
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