MATERIALS CHARACTERIZATION 43:259–269 (1999)© Elsevier Science Inc., 1999. All rights reserved. 1044-5803/99/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S1044-5803(99)00044-3

259

E

ffects of Microstructure and Notches on the Mechanical Properties of Dual-Phase Steels

Ali Bayram, Agah U uz, and Murat Ula

Faculty of Engineering and Architecture, Uludag University, 16059, Görükle-Bursa, Turkey

A low-carbon (0.1%) steel has been subjected to three different heat treatments to obtaindual-phase steels with different microstructures. The steel with the intercritically annealedmicrostructure of equiaxed ferrite–martensite exhibited the highest tensile strength, thelowest ductility, and intermediate fracture toughness properties. Step quenching also pro-duced an equiaxed ferrite–martensite structure, but the material had the worst mechanicalproperties of the three different heat-treatment conditions. In contrast to the other two heat-treatment conditions, step annealing produced a fibrous (fine, needle-like) ferrite-plus-mar-tensite structure. This gave rise to a material of intermediate tensile strength but with thehighest ductility, notch strength, and fracture toughness. It is argued that optimum me-chanical properties in a dual-phase steel can best be achieved by obtaining a microstructurecontaining fine, fibrous needle-like, martensite. © Elsevier Science Inc., 1999. All rights re-

served.

g

INTRODUCTION

The microstructure of conventional steelsoften makes it impossible to obtain concur-rently good ductility and high strength.However, some applications, especially inthe transportation industries, require eco-nomical high strength steels with goodformability. To achieve weight reductionsand fuel savings in the vehicles and to ful-fill the energy and resource requirements,materials designers have concentrated onthe low-alloy steels. Consequently, dual-phase steels (DPS), mostly containing fer-rite and martensite phases that can be ob-tained by relatively simple heat treatments,have been developed.

The mechanical properties of the DPS aredependent on the morphology of the twophases present. These, in turn, are deter-mined by the annealing temperature andtime, the annealing procedure (continuousor step annealing), alloying elements, andthe quenching media and rate. Because ofthis, many different types of material condi-tions can be produced with unique proper-

ties for specific applications. The source ofstrengthening in DPS derives from the pres-ence of the load-carrying hard martensite ina soft ferrite matrix, the basis of ductility [1].

Many researchers have been working onDPS to determine relationships betweenmicrostructure and mechanical properties,and to develop new materials for specificapplications [2–22]. Cai et al. [2] obtaineddifferent dual-phase microstructures froma low-carbon, Mn-containing steel, andproceeded to measure the kinetics of for-mation of the two-phase structures. Chenand Cheng [3] developed a formulation toevaluate the effect of martensite strengthon the tensile strength of these materials.

The tensile properties of different DPShave been investigated widely [4–12]. Manyresearchers report an increase in tensilestrength with increasing martensite volumefraction (

V

m

) of DPS [5–8]. Chang and Pre-ban [6] also investigated the effect of ferritegrain size on tensile strength. Sarwar andPriestner [7] found that an increase in ten-sile strength could be obtained through anincrease in the martensite volume fraction,

260

A. Bayram et al.

as well as by an increase in the aspect ratio.Also, Tomita [8] investigated the effect of

V

m

and the morphology of the phases. Caiet al. [4], in a different work, obtained vari-ous microstructures with similar

V

m

, andfound that a fine fibrous structure had abeneficial effect on mechanical properties.

Liu et al. [9] observed the effect of priordeformation, and Shen and Priestner [10]the effect of boron, on the microstructureand the mechanical properties of DPS. Fa-tigue properties have also been of interestto researchers [4,13], as well as the Bausch-inger effect [14–16] and the work-harden-ing characteristics [4,16] of these materials.Chen et al. [17] investigated the develop-ment and the characteristics of shear bandsin relation to the void volume fraction, andSidjan and Miyasato [18] observed the nu-cleation and growth of voids during thedrawing of DPS. Çimeno lu and Kayalı[19] investigated yield effects, while Changand Preban [6] examined the factors affect-ing the yield ratio of these materials. Jianget al. [20] established the existence of a rela-tionship between the flow stress and thegrain size of phases based on work-harden-ing theory. Bhattacharyya et al. [21] devel-oped a model to examine the effect of themartensite morphology on the initial plas-tic state of the ferrite matrix and on thestress–strain behavior of DPS during load-ing. Ball et al. [22] investigated the decom-position of austenite during the annealingof a corrosion-resistant steel.

These examples demonstrate that DPShave been of great interest because there areso many microstructures and properties tobe investigated. The aim of the present workwas to produce, from a 0.1%C steel, threedifferent microstructures (using three differ-ent heat-treatment procedures), and to corre-late these microstructures with the mechani-cal properties of the respective materials.

EXPERIMENTAL MATERIALSAND PROCEDURES

The chemical composition of the low-car-bon steel used in this study is given in Ta-ble 1. This high-purity alloy was chosen to

g

minimize the effects of coarse inclusions,because only the ferrite and martensitemorphology were to be examined. The as-received material came in the form of hot-rolled bars, approximately 12mm in diame-ter, and normalized at 910

8

C for 40 min.Three sets of microstructures were pre-

pared by using three different heat-treat-ment procedures: (1) heat-treatment A (MSA)(intercritical annealing): hold at 820

8

C (fer-rite plus austenite region) for 60 min, fol-lowed by water quenching; (b) heat-treat-ment B (MSB) (intermediate quenching orstep annealing): hold at 910

8

C (Austeniteregion) for 40 min, water quench, reheat to820

8

C, hold for 60 min, and water quench;and (c) heat-treatment C (MSC) (stepquenching or continuous annealing): holdat 910

8

C (Austenite region) for 40 min, cooldown to 820

8

C, hold for 40 min, then waterquench. The procedures are representedschematically in Figure 1.

After heat treatment, cross-sections of thesamples were polished, etched with 2% ni-tal, and observed under the light micro-scope to reveal the morphology of the phases.

Tension test specimens were prepared asround bars of approximately 50.3 mm

2

cross-sectional area with a gage length of45mm. Fracture toughness testing was per-formed on circumferentially notched barspecimens as sketched in Fig. 3. The diame-ter

D

was 10mm. Two notch diameters (

d

),6 and 8mm (equivalent to notch depths of 2and 1mm, respectively) were used so thatthe effect of the notch depth could be inves-tigated. Usually notched tensile specimenshave a notch angle of 60

8

. However, for thecurrent work, notch angles of 45, 60, and 75were employed so that the effect of thenotch angle on the mechanical propertiescould be observed. All the test specimenswere machined slightly oversize prior tothe heat treatments, and were ground down

Table 1

Chemical Composition of theSteel (wt %)

C Si Mn S P Fe

0.097 0.06 0.49 0.04 0.01 Balance

Mechanical Properties of Dual-Phase Steels

261

to the specified size afterwards to eliminatethe effects of surface decarburization at hightemperatures.

Both the tensile tests and the fracturetoughness tests were performed on a ZwickTensile Tester at room temperature using acrosshead speed of 4mm/min.

RESULTS AND DISCUSSION

The microstructure of the original materialand those produced by the different heattreatments are illustrated in Fig. 2, and canbe summarized as follows.

AS-RECEIVED (NORMALIZED) MATERIAL [FIG. 2(a)]

Ferrite plus pearlite microstructure. Theferrite grain size is about 7

m

m. The pearliteis distributed uniformly but as irregularlyshaped volumes embedded in the ferritematrix (i.e., typical of a fine normalizedstructure).

MSA (INTERCRITICAL ANNEALED MATERIAL) [FIG. 2(b)]

Equiaxed martensite phase surroundingferrite grains about 7

m

m in diameter. Theferrite grains appear unchanged from thoseobserved in the as-received normalizedmaterial. The ferrite phase did not experi-ence any structural change after quenchingfrom the austenite-plus-ferrite region. Themartensite volume fraction was estimatedto be about 40%, and the plates approxi-mately 7

m

m in width.

MSB (STEP-ANNEALED MATERIAL)[FIG. 2(c)]

Fibrous (fine needle-like) martensite phasedistributed in the ferrite matrix. The initialmartensite structure, which is transformedto ferrite and austenite during the intercriti-cal annealing stage, provides sufficient het-erogeneous nucleation sites for austenite atprior austenite grain boundaries during thisstage. The ferrite formation was presum-ably a process of recovery, recrystallization,and grain growth, which can lead to a sub-grain distribution in the ferrite. Parallelnarrow laths within a prior austenite graincan bring about a fine dispersion of acicularmartensite in a ferrite matrix upon quench-ing. Moreover, quenching from the two-phase region gives rise to a high density offresh dislocations in the ferrite grains dueto the accommodation strain caused by theaustenite-to-martensite transformation.

MSC (STEP-QUENCHED MATERIAL)[FIG. 2(d)]

Ferrite grains, coarsened to approximately10

m

m in diameter, surrounded by coarsemartensite as large as 30

m

m in width. The

FIG. 1. Schematic heat-treatment diagrams of: (a)MSA (intercritical annealing); (b) MSB (intermediatequenching); (c) MSC (step quenching).

262

A. Bayram et al.

martensite volume fraction is increased toabout 60%.

TENSILE PROPERTIES

The room-temperature tensile properties ofthe steel following different heat treatmentsare given in Table 2. Five specimens weretested for each microstructure. From this itcan be seen that the highest strength wasobtained with the MSA (intercritically an-nealed) condition, the ultimate tensile strengthbecoming almost twice that of the as-received(normalized) material. However, the elon-gation and reduction of area decreased con-currently, to about one-half of the originalvalues. Step-annealed (MSB) material ex-hibited a lower strength than MSA, whilestep quenching (MSC) produced the loweststrength of the three heat-treated materials

(but still over 50% greater than that of theoriginal normalized material). The ductilitywas highest with the MSB and lowest withthe MSC. All three heat treatments pro-duced steel with a lower yield ratio (

s

YS

/

s

UTS

) than that of the normalized material.The lower elongation of the MSC mate-

rial compared with other materials couldbe due to the fact that ferrite grains sur-rounded the martensite fracture in a brittlemanner prior to the martensite fractureduring necking. This is caused by the highlocal internal stress produced in the vicin-ity of the ferrite/martensite interface as aresult of the ferrite grains surrounding themartensite during plastic deformation [8].

These observations are in agreement withthose of others [1,4]. The coarse and themore connected martensite geometry causes

FIG. 2. Light micrographs of the microstructures of: (a) normalized (as-received) material (ferrite plus pearlite); (b)MSA material (equiaxed grains of ferrite and martensite); (c) MSB material (fibrous ferrite-plus-martensite struc-ture); (d) MSC material (coarse ferrite-plus-martensite structure).

Mechanical Properties of Dual-Phase Steels

263

more severe inhomogeneous deformationand restricts initial plastic flow to a smallerfraction of the total volume of the ferritematrix. Also, void growth will occur at afaster rate and with less plastic strain whenthe martensite is more interconnected. Con-sequently, the overall ductility will belower [1]. Thus, the fine fibrous microstruc-ture of the MSB material is the reason for itshigher ductility.

Continuous yielding was observed for allthe heat treatment conditions, which is oneof the characteristics of DPS. Continuous

yielding in these steels can be due to: (1) thetransformation of austenite to the hardmartensite phase, introducing a high den-sity of mobile dislocations in the adjacentferrite matrix; and (2) residual stresses fromthe martensite grains [7]. However, Çime-no lu and Kayalı [19] have observed dis-continuous yielding in DPS followingstress relaxation and the strain rate changetests. This effect was attributed to the strainaging of the material.

The yield ratios of the as-received mate-rial and the three heat-treated materials aregiven in Table 2. A slight decrease in the ra-tio is evident going from the normalizedmaterial through MSA, MSB, to MSC. Thisarises from a faster drop in the yieldstrength than in the ultimate tensile strengthin the heat-treated materials. This behaviorhas been observed as the ferrite grain sizeincreases for constant

V

m

or as the volumefraction of martensite increases while thegrain size remains constant [6]. In thepresent case, MSA and MSC, having simi-lar morphologies, can be compared interms of martensite volume fraction andgrain size. The ferrite grain size in MSA isapproximately 7

m

m (as is the martensitewidth), whereas in MSC it is approximately10

m

m (the martensite width is 15

m

m).Although the

V

m

values are also different(about 0.4 in MSA and 0.6 in MSC), thelower yield ratio of MSC is thought to beattributable to the increase in ferrite grainsize. The higher

V

m

value in MSC isthought probably to be a secondary reasonfor the decrease in yield ratio, although itshould be noted that martensite widths inMSC and MSA are also different.

According to Chen and Cheng [3], thevariation of the ferrite phase tensile strength

g

FIG. 3. Sketch of circumferentially notched bar speci-men used in fracture toughness testing. D 5 10mm; d 56 and 8mm; a 5 458, 608, and 758.

Table 2

Tensile Test Data of the Steel with Different Microstructures

MicrostructureYield strength,

MPaUltimate tensile strength,

MPaYieldratio

Elongation,%

Reduction inArea, %

Normalized 309

6

30.8 441

6

19.1 0.70 29.0 67.0MSA 545

6

31.2 865

6

40.1 0.63 13.3 32.0MSB 445

6

25.4 729

6

24.4 0.61 18.0 43.8MSC 401

6

30.6 703

6

41.5 0.57 11.3 37.0

264

A. Bayram et al.

with the martensite volume fraction in DPScan be obtained from Eq. (1):

(1)

where

s

f

is the ultimate tensile strength ofthe ferrite matrix, and

V

m

is the volumefraction of martensite. This equation sug-gests that the ferrite strength increases withincreases in the martensite volume fraction.This may be due to a higher dislocationdensity in the ferrite grains when the vol-ume fraction of martensite increases.

The tensile strength of martensite is de-pendent on its carbon content, and can beestimated with the help of the followingequation:

(2)

where

s

m

is the ultimate tensile strength ofthe martensite, and

C

m

the carbon contentof the martensite phase.

C

m

is related to thecarbon content of the steel,

C

d

, to that of theferrite phase,

C

f

, and to the volume fractionof the martensite itself [Eq. (3)]:

(3)

Thus, as the martensite volume fractionincreases, the amount of carbon in the mar-tensite decreases, and, through substitutingthis equation into Eq. (2), so does the mar-tensite strength also.

The above equations have been appliedto the MSA and MSC materials of thepresent study, but not to the MSB material,which has a distinctly different morphol-ogy (fibrous martensite in a ferrite matrix).If the carbon content of the ferrite in thepresent case can be taken as constant at0.02%, calculations of the tensile strengthsof the ferrite and martensite phases pro-duce the following estimates:

These values can be used to make theo-retical predictions of the tensile strength,

s

DPS

, of DPS using the mixture rule:

(4)

σ f 497 67Vm+=

σm 541 2289 Cm+=

CmCd C f–( )

Vm----------------------- C f+=

MSA σ f 523.8 MPa;= σm 1596.2 MPa=

MSC σ f 537.2 MPa;= σm 1422.6 MPa=

σDPS σfVf σmVm+=

where Vf is the volume fraction of the fer-rite.

According to Eq. (4), MSA and MSC arepredicted to have tensile strengths of 952.8and 1068.4MPa, respectively. However,these predictions are much higher than theexperimental values (865 and 703MPa, re-spectively). Furthermore, the experimen-tally determined tensile strength of MSC islower than that of MSA, in contradiction ofthe theoretical prediction. This suggeststhat the tensile strengths of DPS with dif-ferent heat treatments cannot be comparedon the basis of martensite volume fractions,even though the ferrite and martensitemorphologies produced by each heat treat-ment are similar.

NOTCH STRENGTHENING ANDNOTCH SENSITIVITY

The tensile strength of the notched bars,sNS, calculated by dividing the maximumload by the cross-section area at the notch,are given in Table 3. With the exception ofthe MSA specimens with a notch depth of1mm, all the values are higher than the ten-sile strengths obtained with the corre-sponding unnotched specimens. This isdue to plastic constraint at the notch,because the material possesses ductility.When the stress at the notched cross-sec-tion reaches the yield strength, the materialin the reduced section attempts to stretchplastically in the direction parallel to theloading axis. At the same time, the notchroot material tries to contract but is con-strained by the rest of the material, which isstill encountering an elastic stress. The de-velopment of tensile stresses in the othertwo principal directions makes it necessaryto increase the axial stress to initiate plasticdeformation [23]. This is the reason for thehigher tensile strengths observed in thenotched specimens. The deeper the notchis, the greater the plastic constraint and thehigher the axial stress necessary to deformthe specimen. This effect can be seen in Ta-ble 3, where the 2mm notch specimenshave consistently higher strength valuesthan the 1mm notch specimens.

Mechanical Properties of Dual-Phase Steels 265

The exceptions noted above were theMSA specimens with the 1mm notchdepth, where the strength values werelower than that of the unnotched specimen.As a result, this heat treatment can be con-sidered to a somewhat “notch-sensitive”material. The notch sensitivity of a materialcan be determined through the notchstrength ratio (NSR) [24], where [Eq. (5)]:

(5)

If a material exhibits a significant amountof plasticity, the value of NSR will begreater than 1. However, if NSR is less thanunity, the material is considered to be notchsensitive, as is the case with the MSA mate-rial with a 1mm notch depth. The NSR val-ues in Table 3 also indicate the relative duc-tility of the materials is in the order MSB .MSC . MSA. This is in line with the reduc-tion-of-area values seen in Table 2.

FRACTURE TOUGHNESS

Fracture toughness, Kc, values of the mate-rials were calculated at the onset of fracture

NSR σNS σUTS⁄=

of the circumferentially notched specimensby using Eq. (6) [25]:

(6)

where Pf is the fracture load, D is the diam-eter of the specimen, and d is the diameterof the notched section. The results are sum-marized in Table 3. The Kc values are theaverage of four specimens and the stan-dard deviations from these average values.The results for notched depths of 1mm aredepicted graphically in Fig. 4, with the frac-ture toughness values being plotted as afunction of notch angle. The lines shownrepresent the upper and lower scatterbands of the data.

In all cases, the fracture toughness valuesare greater for the deeper notches, irrespec-tive of microstructure or notch angle. Also,independent of notch depth, the Kc valuesincrease as the notch angles increase from45 to 758 for each microstructure. With re-spect to the microstructures, the MSB hadconsistently higher fracture toughness val-

KcPF

D3 2⁄----------- x 1.72 Dd----

1.27–=

Table 3 Fracture Toughness and Notch Strength Data of the Material for Different NotchDepths and Angles

MaterialNotch angle

(8)Notch depth

(mm)Notch strength

(MPa) NSR

MSA 45 1 785.5 0.91 34.6 6 0.52 1053.9 1.22 47.6 6 0.3

60 1 829.4 0.96 36.7 6 0.32 1064.7 1.23 48.1 6 0.9

75 1 846.5 0.98 37.4 6 1.22 1079.9 1.25 48.8 6 1.8

MSB 45 1 853.8 1.17 37.8 6 1.12 1097.3 1.50 49.5 6 3.7

60 1 874.9 1.20 38.8 6 1.72 1192.7 1.64 53.8 6 4.0

75 1 917.3 1.26 40.6 6 1.02 1249.0 1.71 56.4 6 3.5

MSC 45 1 754.6 1.07 33.3 6 1.22 1032.2 1.47 46.6 6 2.6

60 1 797.7 1.13 35.3 6 0.42 1038.7 1.48 46.9 6 3.2

75 1 809.9 1.15 35.8 6 1.32 1071.2 1.52 48.4 6 1.0

Kc MPa m( )

266 A. Bayram et al.

ues than the other two materials, with thelowest values being recorded with the MSCmaterial.

The higher Kc values observed with thedeeper notches is due the greater plasticconstraint imposed in the smaller cross-sec-tions. However, the reason for the increasein fracture toughness (and notch strength)as the notch angle increases is not as easy toelucidate. The maximum tensile stress,smax, in a notched bar at general yieldingcan be defined as [26; Eq. (7)]:

(7)

where u is the notch angle in radians. Thissuggests a decrease in maximum stress asthe notch angle increases, ultimately be-coming equal to the yield strength whenthe notch angle equals p (i.e., there is nonotch). Thus, the fracture toughness valuewould be expected to decrease as the notchangle increases. However, the opposite isobserved in the present case. On the otherhand, the smaller the notch angle, thesharper the crack tip, the smaller the plasticzone, and the lower the fracture toughness.Therefore, the increase in fracture tough-ness with increases in the notch angle maybe attributed to increased crack tip blunt-ing in these microstructures.

FRACTOGRAPHY

Scanning electron microscope (SEM) im-ages of the fracture surfaces of the tension

σmax σYS 1 π2--- θ

2---–+

=

test specimens are shown in Fig. 5. It can beseen that the MSA material exhibited duc-tile fracture of the martensite by dimpleformation with some cleavage of the ferritegrains [Fig. 5(a)]. The fracture of MSB is en-tirely ductile [Fig. 5(b)], while the fracture

FIG. 4. Fracture toughness as a function of notch angle.

FIG. 5. SEMs of the fracture surfaces of tension testspecimens. (a) MSA—almost completely ductile frac-ture with some cleavage cracking; (b) MSB—com-pletely ductile fracture by dimple formation; (c)MSC—almost completely brittle cleavage cracking.

Mechanical Properties of Dual-Phase Steels 267

surface of MSC exhibits almost total brittlecleavage [Fig. 5(c)].

The propagation of a crack in the hardphase (martensite) into the adjacent ferriteis controlled by the sharpness of the crackand the hardness of the ferrite. If the ferriteis hard, then the plastic zone ahead of thecrack is small and the crack can propagateby cleavage. In the present case, the ferritein the MSC material is harder than that inMSA as a consequence of the higher volumefraction of martensite. Thus, MSC exhibitsmuch more cleavage fracture than MSA.

SEM images of the fracture surfaces ofthe fracture toughness specimens are shownin Fig. 6. As with the tension test speci-mens, there are clear differences in appear-ance with the different microstructures, al-though there are no distinct differences

related to notch angle and notch depthwithin a single microstructure.

The fracture surface of the MSA speci-men with a 608 notch angle and 1mm notchdepth [Fig. 6(a)] appears completely ductileby dimple formation, suggesting that thecracking follows the martensite phase, as isthe case with the MSB material [Figs. 6(b)and (c)]. However, in the MSC material[Fig. 6(d)], complete cleavage cracking ofthe ferrite grains is evident. These featuresare similar to the observations made withthe fracture surfaces of the unnotched ten-sion test specimens.

The fractographic observations supportthe experimental results. Thus, the MSBmaterial, with the highest fracture values,fractures in a completely ductile mode. MSAhas lower fracture toughness values but

FIG. 6. SEMs of the fracture surfaces of notched specimens. (a) MSA with a 608 notch angle and 1mm notch depth–ductile fracture; (b) MSB with a 608 notch angle and 2mm notch depth—fully ductile fracture; (c) MSB with a 758

notch angle and 1mm notch depth—fully ductile fracture; (d) MSC with a 608 notch angle and 1mm notch depth—entire cleavage cracking of ferrite grains.

268 A. Bayram et al.

still exhibits a mostly ductile fracture of themartensite. MSC, with the lowest fracturetoughness values, displays mostly brittlecleavage cracking. Premature failure in theMSC material is thought to take place bythe initiation of cracks in the ferrite regionwhere localized stress concentrations buildup during deformation. Such localizedstresses are probably less in the MSA mate-rial due to the lower martensite volumefraction, which leads to a dimpled ductilefracture of the martensite grains that is typ-ical of low-carbon lath martensites.

The size and the morphology of the sec-ond phase in a two-phase material deter-mines the material’s strengthening mecha-nism. If the second-phase particles exertconstraints on the matrix (as they usuallydo), then the result is an increase instrength. When deformation is applied, thematerial in the vicinity of the second phaseparticles is prevented from deformingfreely, resulting in an increase in the shearstresses required to produce further defor-mation. This type of composite materialdisplays higher work-hardening rates anda higher flow strength than those of thepure matrix. With regard to the shape ofthe martensite, the load transfer is most ef-ficient when it is in the form of fibers ratherthan spheres. This is mainly because theload transfer takes place by shear actionalong the martensite/ferrite (particle/ma-trix) interface, and more interfacial energyis available when the shape is needle-like,providing the volume fraction and thenumber of martensite grains are the same[11]. Accordingly, the MSB microstructure,with its fibrous texture, has a higher frac-ture toughness than the other two micro-structures.

CONCLUSIONS

1. Intercritical annealing of a low-carbonsteel yielded a material with a ferrite-plus-martensite equiaxed grain struc-ture and relatively high strength (MSA).A similar but coarser microstructurewas obtained by step quenching, but the

mechanical properties of this materialare inferior (MSC). Intermediate quench-ing led to a fine fibrous microstructureand a material with better all-round me-chanical properties than the other twomaterials (MSB). Although the MSB ma-terial had a lower tensile strength thanthe MSA material, it had the highestductility, notch strength, and fracturetoughness of the three materials investi-gated. It is concluded that obtaining afine fibrous microstructure leads to opti-mized mechanical properties in dual-phase steels.

2. The notch strength and fracture tough-ness of specimens with a 2mm notchdepth were found to be higher thanthose with a 1mm notch depth. This be-havior arises from the higher plasticconstraint present in the thinner cross-section.

3. The notch strength and fracture tough-ness of all the materials were found toincrease with increases in the notch an-gle for both notch depths. This could bedue to blunting of the crack tip as thenotch angle increases.

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Mechanical Properties of Dual-Phase Steels 269

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Received December 1998, accepted June 1999.

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