Materials Science and Engineering A 497 (2008) 451–461

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Materials Science and Engineering A

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Splitting in Dual-Phase 590 high strength steel platesPart I. Mechanisms

Min Yanga,b, Yuh J. Chaob,∗, Xiaodong Lib, Jinzhu Tanc,b

a School of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, PR Chinab Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USAc College of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, PR China

a r t i c l e i n f o

Article history:Received 13 February 2008Received in revised form 21 July 2008Accepted 22 July 2008

Keywords:SplittingDP590 steelHot-rolled steelHDGI steelCharpy V-notch impactInclusion

a b s t r a c t

Charpy V-notch impact tests on 5.5 mm thick, hot-rolled Dual-Phase 590 (DP590) steel plate were evalu-ated at temperatures ranging from 90 ◦C to −120 ◦C. Similar tests on 2.0 mm thick DP590 HDGI steel platewere also conducted at room temperature. Splitting or secondary cracks was observed on the fracturedsurfaces. The mechanisms of the splitting were then investigated. Fracture surfaces were analyzed byoptical microscope (OM) and scanning electron microscope (SEM). Composition of the steel plates wasdetermined by electron probe microanalysis (EPMA). Micro Vickers hardness of the steel plates was alsosurveyed. Results show that splitting occurred on the main fractured surfaces of hot-rolled steel speci-mens at various testing temperatures. At temperatures above the ductile–brittle-transition-temperature(DBTT), −95 ◦C, where the fracture is predominantly ductile, the length and amount of splitting decreasedwith increasing temperature. At temperatures lower than the DBTT, where the fracture is predominantlybrittle, both the length and width of the splitting are insignificant. Splitting in HDGI steel plates onlyappeared in specimens of T-L direction. The analysis revealed that splitting in hot-rolled plate is caused

by silicate and carbide inclusions while splitting in HDGI plate results from strip microstructure due to itshigh content of manganese and low content of silicon. The micro Vickers hardness of either the inclusions

es is h

1

inpbosv

cstmAts

[Vn[i

dfso

apv

0d

or the strip microstructur

. Introduction

Dual-phase steel is a mixture of ferrite matrix and martensiteslands decorating grain boundaries. Some have additions of bai-ite. The soft phase ferrite provides the ductility while the hardhase martensite offers the strength. The steel having the com-ined phases appears to possess superior mechanical propertiesver conventional mild steels and high strength low alloy (HSLA)teels. It therefore has quickly become one of the most popular andersatile materials in today’s automotive industry [1–4].

To meet different design requirements of individual auto-bodyomponents, a wide variety of DP grades exhibiting differenttrength and ductility levels are currently produced by steel indus-ry. Numerous investigations have been performed to study the

echanical characteristics and microstructures of DP steels [5–14].mong these studies, Chao et al. [14] have noticed the splitting in

he fracture surface of Charpy V-notch impact specimens of DP590teel. Similar studies on pipeline steels, such as X60, X70 and X80

∗ Corresponding author. Tel.: +1 803 777 5869; fax: +1 803 777 0106.E-mail address: chao@sc.edu (Y.J. Chao).

ts(bTfep

921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2008.07.067

igher than that of the respective base steel.© 2008 Elsevier B.V. All rights reserved.

15–20], suggest that splitting on the fracture surface in Charpy-notch impact specimens affects the measured fracture tough-ess and therefore safety evaluation of pipeline steels. Ray et al.21] further found that the inclusions and microstructure of HSLA’snfluence their mechanical properties and fracture behavior.

Splitting is a phenomenon in which secondary cracks perpen-icular to the main crack and parallel to the plate surface appear inracture testing, as shown in Fig. 1. It most occurs in high strengthteels and can show up in either dynamic or static tests. The sec-ndary cracks can be one or multiple.

Charpy impact tests can reveal a material’s capacity in energybsorption under impact conditions, which is a very importantroperty for safety assessment in automotive industry. However,ery little has been reported on the mechanisms of splitting andhe potential effect of splitting on the Charpy impact energy of DPteels. Since splitting was observed in DP590 in Charpy impact testsChao et al. [14]), we further investigated the splitting mechanisms

y studying the inclusions and microstructures of the steel plates.he results are reported here as Part I. Quantitative analysis of theractured surfaces (e.g. the areas of the secondary cracks) and itsffect on the Charpy impact energy is presented in a follow-upaper, Part II.

452 M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461

Fig. 1. Schematics of splitting on the broken Charpy V-notch (CVN) impact speci-m(

2

ads[

33di

Ft

1tta

CstitumFwas placed into liquid nitrogen and ethanol mixed liquid or boil-ing water, respectively, for a period of time to achieve a uniformtemperature in both the inside and the surface of the specimen.The specimen was then placed in the testing anvil and allowed to

TC

M

HH

en: (a) CVN impact specimen before Charpy test, (b) broken impact specimen, andc) splitting at the fracture surface of one piece of the broken specimen.

. Experimental procedure

Two types of DP590 steel plates were used in this study. One is5 mm thick, hot-rolled plate and the other is a 2 mm thick, hotip galvanized (HDGI) plate. The chemical composition and ten-ile properties of these two steel plates are listed in Tables 1 and 222–24].

The Charpy V-notch impact test samples were cut from a

04.8 mm × 304.8 mm × 5.5 mm hot-rolled DP590 plate and a30 mm × 150 mm × 2 mm HDGI DP590 plate in both L-T and T-Lirections, as shown in Fig. 2. The specimens were prepared accord-

ng to ASTM Standard E-23, i.e., the in-plane dimensions were

able 1hemical composition of DP590 (wt%)

aterial C Mn P S Si

ot-Rolled 0.08 1.24 0.018 0.005 1.14DGI 0.098 1.58 0.015 0.003 0.198

Fig. 4. Optical micrographs of DP 590 steel in the middle of the plate: (a) loca

ig. 2. Schematic diagram of the Charpy V-notch Impact test specimens showinghe T-L and L-T direction of the plate.

0 mm × 55 mm with a 2 mm deep 45◦ V-notch having a 0.25 mmip radius in the middle of the specimen. Five samples cut fromhe HDGI DP590 steel plate were bond together using QM-50Adhesives to achieve 10 mm thickness for impact testing.

Impact tests were performed using a Tinius Olson pendulumharpy impact tester with a maximum capacity of 339 J. All HDGIteel samples were tested at room temperature. The test tempera-ure of hot-rolled plate samples ranged from −120 ◦C to 90 ◦C at annterval of 10 ◦C. To control the specimen temperature at impact, ahermocouple was welded onto each specimen near the notch facesing a Hughes 110-V thermocouple capacitor-discharge welder toonitor the temperature of the testing specimen while in the anvil.

or temperatures below or over room temperature, the specimen

Fig. 3. Schematic diagram of metallographic sample location.

Cr Ni Mo + Ti + V Al Fe

0.87 0.01 – 0.043 96.5840.281 0.022 0.068 0.057 97.678

tion of micrographs in sample, (b) hot-rolled steel, and (c) HDGI steel.

M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461 453

Table 2Tensile properties of DP590 steels

M

HH

wtpws

Fd

aterial Yield strength (MPa)

ot-rolled 437DGI 370

arm up or cool down to the desired testing temperature. Oncehe desired testing temperature was reached from the thermocou-le readout, the impact hammer was released and the specimenas hit and broken. The impact energy was then recorded from the

cale on the impact tester in ft-lb.

tdge

ig. 5. Macro fractographs of hot-rolled steel impact specimens at different temperatureirection, and (c) L-T direction.

Ultimate tensile strength (MPa) Total elongation (%)

621 29.9620 25

After the Charpy V-notch impact tests, the fracture surface andhe cross-section of fracture surface of the broken samples in T-Lirection were examined by optical microscope (OM) to investi-ate the splitting. A scanning electron microscope (SEM) was alsomployed to examine the associated fracture modes. The fracture

s and in different directions (a) schematic diagram of sample orientation, (b) T-L

454 M. Yang et al. / Materials Science and En

F(

simmenCF

3

oarmcioi

mb

3

sAiBi

aitaLdidi

to

idrthQC

ig. 6. Macro fractographs of HDGI steel impact specimens in (a) T-L direction, andb) L-T direction.

urfaces of selected T-L specimens were grinded and polished tonvestigate the mechanisms of splitting. Selected broken T-L speci-

ens were sectioned parallel to the fracture surface to examine theicrostructure, composition and micro Vickers hardness by OM,

lectron probe microanalysis (EPMA) and the micro Vickers hard-ess tester. All polished specimens were sectioned from the brokenharpy specimens at locations away from the broken surface (seeig. 3) and etched with 4% picral reagent.

. Experimental results

The optical micrographs of DP590 steel plates in the middlef the plate at a plane perpendicular to the transverse directionre shown in Fig. 4. Fig. 4(b) shows the microstructure of hot-olled steel consisting of banded ferrite (the bright phase) and

artensite (the dark phase) along the rolling direction. Some pre-

ipitates were also distributed in the ferrite. There are also somenclusions in the middle of the thickness. The strips composedf bright particle phases, black dots and dark belt are observedn the middle of the HDGI steel shown in Fig. 4(c). The strip

3

Tc

Fig. 7. Effect of splitting on the CVN impact energy of hot-r

gineering A 497 (2008) 451–461

icrostructures are apparently ferrite, martensite and some fineainite.

.1. Macroscale analysis of fracture surface

Fig. 5 shows the macroscopic fractographs of hot-rolled steelpecimens at different temperatures in both T-L and L-T directions.pparent splitting can be observed in the fracture surface of spec-

mens in the T-L direction as the test temperature is below 80 ◦C.oth the length and the number of splitting decreased with increas-

ng test temperature.As for the specimens in the L-T direction, splitting gets longer

nd deeper as the test temperatures vary from 80 ◦C to−80 ◦C. Theres only short and discontinuous splitting when the test tempera-ure is below −80 ◦C. More shorter and smaller secondary splitsppear at positions away from the notch tip in both the T-L and-T specimens when the test temperature is below −60 ◦C. Due touctile deformation, global shrinkage or necking of the specimens

n the thickness direction occurred and this shrinkage graduallyiminished as the test temperature reduces indicating migration

nto brittle regime.The fracture surface of HDGI steel specimen in the T-L (L-T) direc-

ion is relatively rough (smooth), as shown in Fig. 6. No splitting wasbserved from the macro fractographs of the HDGI steel.

Fig. 7 shows the ductile–brittle-transition curve of the CVNmpact energy of hot-rolled steel. According to ASTM-E3, theuctile–brittle-transition-temperature (DBTT) of this DP590 hot-olled steel is determined as −95 ◦C. It appears that, aside fromhe difference in rolling direction, specimens with splitting exhibitigher CVN impact energy in both the upper shelf and lower shelf.uantitative analysis of the splitting areas and its relation with theVN energy are reported in Part II of the paper.

.2. Microscale analysis of fracture surface

The SEM micrographs of the fracture surface of hot-rolled steel-L impact specimens at 90 ◦C are shown in Fig. 8. Scattered smallracks can be found, which are not obvious in the macro fractograph

olled steel; splitting tends to shift the curve upward.

M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461 455

Fig. 8. SEM fractographs of hot-rolled steel T-L impact specimens at 90 ◦C; (a) surface profile of the specimen, (b) enlarged region of zone A in (a), and (c) enlarged region ofzone B in (b).

Fig. 9. SEM fractographs of hot-rolled steel T-L impact specimens at 70 ◦C; (a) fracture surface profile of the specimen (arrow indicating the cracking direction of the maincrack), (b) enlarged region of the zone marked by C in (a) which is inside of the splitting, and (c) enlarged region of zone D in (a) showing dimples and the cracking direction.

Fig. 10. SEM fractographs of hot-rolled steel T-L impact specimens at −80 ◦C; (a) surface profile of the specimen, (b) enlarged region of the fracture propagation zone markedby E in (a), and (c) enlarged region of zone F in (a).

Fig. 11. SEM fractographs of hot-rolled steel T-L impact specimens at −110 ◦C; (a) surface profile of the specimen, (b) enlarged region of the fracture propagation zone markedby G in (a), and (c) enlarged region of zone H in (a) which is near the tip of the splitting.

456 M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461

F ofile os age of

impr

Fp

ig. 12. Scanning electron fractographs of the HDGI steel specimens: (a) fracture prmall splittings, (c) fracture profile of the specimen in L-T direction, and (d) SEM im

n Fig. 5(b). It appears that the small splittings are due to the stripicrostructure as pointed out by the white arrow in Fig. 8(c). The

resence of small splitting on the fracture surface of DP590 hot-olled steel Charpy V-notch impact specimens appears to be at all

tl

s

ig. 13. Microstructure near the splitting of polished fracture surface of hot-rolled steelropagating zone of the splitting showing the inclusions, and (c) one side of the splitting

f specimen in T-L direction, (b) SEM image of the zone marked by A in (a) showingthe zone marked by B in (c).

esting temperatures. The splitting tends to be small at upper andower shelf, but more obvious in the transition-temperature range.

Fig. 9 shows a portion of the fracture surface containing oneplitting from the hot-rolled steel T-L impact specimens at 70 ◦C.

specimen at 70 ◦C; (a) the tip of the splitting and its location in specimens, (b) theshowing the pores.

M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461 457

Fig. 14. Microstructure around the splitting of polished fracture surface of the specimen at −70 ◦C; (a) propagating zone of splitting, (b) merge of pores, and (c) the side ofthe splitting.

F harpyt (cut plm

TaFAtTtct

sawiaowg

st

mib

opotto[

3o

TC

E

NMCFSCA

ig. 15. Macro and micro characteristic of cross-section of hot-rolled DP590 steel Che broken specimen and the cut plane, (b) macro-morphology of the cross-section

arked by A in (b).

he fracture surface appearance inside the splitting, see Fig. 9(b),ppears to be cleavage while on the surface of the main crack, seeig. 9(c), to be typical parabolic tearing dimple type ductile fracture.

very interesting observation from the shape of the dimples ishe tearing direction as pointed by the white arrow in Fig. 9(c).his tearing or “fracture” direction suggests that the splitting orhe secondary cracks occurred prior to the propagation of the mainrack and the presence of the secondary cracks possibly reducedhe stress concentration at the main crack tip.

Figs. 10(b) and 11(b) are from the hot-rolled steel T-L testpecimens at −80 ◦C and −110 ◦C, respectively. The fracture appear-nce appears to be quasi-cleavage and cleavage, respectively,hich reflects the gradual reduction of the toughness and ductil-

ty with decreasing temperature. Similar to Fig. 9(b), the fractureppearance inside the splitting clearly shows the characteristicsf cleavage, as shown in Fig. 10(c). It suggests that the splitting

as resulted from brittle phases as contrast to ductile dimple, void

rowth fracture.Fig. 12 shows the micrographs of the fracture surface of HDGI

teel specimens. Relatively small splitting marks are apparent inhe T-L samples, as shown in Fig. 12(a) and (b). There are strip

tastt

able 3omposition of analyzed points in hot-rolled steel (wt%)

lement 1 2 3 4 5

i 0 0.0170 0.0259 0 0n 1.4469 1.4849 1.4156 1.6157 1.5985

r 0.0711 0.0749 0.0755 0.1182 0.0773e 97.113 96.742 96.864 96.911 95.344i 1.2056 1.2399 1.2264 1.2869 1.3700

0.1228 0.4050 0.3491 0 1.5554l 0.0334 0.0363 0.0428 0.0433 0.0331

V-notch impact specimen tested at −70 ◦C: (a) schematic diagram of one-half ofane) showing several splittings, and (c) enlarged view from the root of the splitting

icrostructures at the bottom of the splitting. The secondary cracksn the strip microstructure imply that the strip microstructure isrittle.

Fig. 12(c) and (d) show the existence of holes around inclusionsn the fracture surface of HDGI L-T samples. No distinct splitting isresent on the fracture surface of the L-T samples. It appears that thebvious strip microstructure is only along the rolling direction ofhe plate, and is narrow and discontinuous in the transverse direc-ion. It is therefore concluded that the strip microstructure, perhapsriginated from the fabrication process, contributed to the splitting25] in this steel plate.

.3. Microstructure of polished fracture surface and cross-sectionf fracture surface

Fracture surfaces from some broken specimens were polished

o examine the microstructures. Fig. 13 shows the microstructureround the splitting of a polished fracture surface of the hot-rolledteel specimen at 70 ◦C. The microstructure near the splitting con-ains typical textured characteristics. As 4% picral reagent can revealhe presence of carbides, the bright gray phases in Fig. 13 are ferrite

6 7 8 9 10

0.0466 0 0.0503 0.0313 01.5486 1.5970 1.4928 2.2914 1.43680.0932 0.1353 0.1264 0.1578 0.0872

96.577 94.025 96.915 92.186 97.2291.2724 1.5800 1.3284 2.0417 1.20940.4226 2.5954 0.0492 3.2577 00.0266 0.0607 0.0335 0.0324 0.0287

4 and Engineering A 497 (2008) 451–461

adtFbdss

iacfas

dsrttdfw

3

ataa

taPrP9itwctctcso(aot

Fig. 16. Location of hot-rolled steel EPMA analyzed points.

Tt

t

TC

E

VNFMCSAC

58 M. Yang et al. / Materials Science

nd the dark gray phases are martensite. Both ferrite and martensiteistribute as strips parallel to the splitting. The tip of the split-ing propagated along the martensite, as shown in Fig. 13(a). Inig. 13(b), one can see the inclusions inside the splitting as pointedy the arrows in the figure. Additionally, in Fig. 13(c) there areeformed pores along the side of the splitting. Evidently, the inclu-ions fell off from the matrix metal during the impact causing theplitting to occur.

Fig. 14 shows the microstructure around the splitting of pol-shed fracture surface of hot-rolled steel specimen at −70 ◦C. Therere many pores along the side of splitting. At lower temperature, theohesion at inclusion and matrix metal interface is weak and there-ore is easy to break upon impact. When the pores were deformednd enlarged by stress, the close-by pores were merged to formplitting, as shown in Fig. 14(b).

The cross-section view of the splitting in the direction perpen-icular to the splitting in hot-rolled steel specimen at −70 ◦C ishown in Fig. 15. It can be seen that there are many pores around theoots of the splitting, especially near the tip of the root, indicatinghe propagation of the splitting in the depth direction is attributedo the linkage of pores. As the pores are formed due to the inclusionsuring the impact, the postulation that splitting on the fracture sur-

ace in hot-rolled steel DP590 Charpy V-notch impact specimensas caused by inclusion is further verified.

.4. Concentration of alloy elements of DP590 steel

The concentrations of alloy elements of the DP590 hot-rollednd HDGI steel in the middle of the thickness along the rolling direc-ion (as shown in Fig. 4(a)) were surveyed by electron microprobenalysis. The alloy element concentrations of DP590 hot-rolled steelt points shown in Fig. 16 are listed in Table 3.

In Fig. 16, there is an inclusion (i.e. white dots) filled band inhe middle running from left to right. The points 1, 2, 3, 5 and 6re located in the martensite and are outside of the band region.oint 4 is located at the ferrite and is outside of the inclusion bandegion. Point 7 is in the martensite inside the inclusion band region.oint 8 is in the ferrite and is inside the inclusion band region. Pointis located inside the inclusion. Point 10 is at the location where

nclusion fell off. By analyzing the data in Table 3, one can concludehat the white region (i.e. point 9 in Fig. 16) is an inclusion mixedith silicate and carbide. Obviously, the average concentrations of

arbon and silicon inside the inclusion band region are higher thanhose outside of the band region, but other element concentrationshanged very little. Inclusion in this case (see point 9 in Table 3) con-ains the highest concentration of carbon, silicon, manganese andhromium. The carbon concentration of martensite inside the inclu-ion band zone (see point 7 in Table 3) is much higher than those

utside the inclusion band region. The ferrite inside the band zonei.e. point 8 in Table 3) contains a bit more carbon, silicon, nickelnd chromium, but less manganese and aluminum than the ferriteutside of the inclusion band region. The martensite at point 5 con-ains high content of carbon, and therefore it is the plate martensite.

ibtPc

able 4omposition of analyzed points in DP590 HDGI steel (wt%)

lement 1 2 3 4

0 0 0 0.0029i 0.02633 0.01857 0 0.0192e 97.888 97.641 97.137 94.754n 1.846 2.08 2.364 2.133

r 0.193 0.2 0.2176 0.198i 0 0.0215 0.0136 0.0057l 0.0468 0.0381 0.0379 0.049

0 0 0.229 2.8374

Fig. 17. Location of HDGI steel EPMA analyzed points.

he concentrations of alloying elements at point 10 are similar tohose of ferrite outside of the inclusion band region.

The concentrations of alloying elements of DP590 HDGI steel athose points shown in Fig. 17 are also listed in Table 4. The points

nside the ferrite grain, such as points 1, 2 and 7, do not contain car-on and alloying element concentrations at these points are lowerhan those of other points (i.e. points 3, 4, 5, 6, 8, and 9 in Fig. 17).oint 8 is located at the grain boundary of ferrite. It contains a littlearbon and no silicon, nickel and vanadium. The concentrations of

5 6 7 8 9

0.02775 0.00385 0.0359 0 00.0258 0.0308 0.0078 0 0.0214

95.447 97.486 97.872 97.823 96.9512.142 2.12 1.804 1.8776 2.29850.197 0.1723 0.2088 0.2029 0.19370.0115 0 0.0029 0 00.0335 0.0481 0.068 0.05145 0.03892.115 0.1386 0 0.0446 0.4965

M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461 459

Fig. 18. Micro hardness of DP590 hot-rolled steel; (a) optical micrograph showing the locations of indentation for hardness measurement; (b) Vickers hardness value atvarious points in (a).

F cations

oicmcPtcpmotctoch

3

tF

hnh

sih1

4

amwH

s

TC

S

CP

ig. 19. Micro hardness of DP590 HDGI steel: (a) optical micrograph showing the lohown in (a).

ther elements at point 8 are similar to those at point 7. Point 3s located inside the strip microstructure region. It has the highestontent of manganese. Its concentrations of carbon, silicon, alu-inum and chromium are higher than those of point 8, while the

oncentrations of other elements are the same as those of point 8.oint 9 is also located inside the strip microstructure region, buthe color of its microstructure is darker than that of point 3’s. Onlyarbon and nickel concentrations at point 9 are higher than that ofoints 3. Point 6 is inside the particle microstructure of the stripicrostructure region. No silicon exists at point 6. Compared with

ther points inside the strip microstructure region, point 6 con-ains lowest content of carbon and lower content of manganese andhromium. Points 4 and 5 are located in the gray microstructure ofhe strip microstructure region. They have much higher contentf carbon than all other points. In general, the concentrations ofarbon and manganese inside the strip microstructure are muchigher than those outside of the strip microstructure.

.5. Micro hardness of DP590 steel

Micro Vickers hardness values across the thickness of thewo steel plates were measured and the results are shown inigs. 18 and 19, respectively. As shown in Fig. 18(b), the highest

ituaF

able 5eq and Pcm of hot-rolled steel at points shown in Fig. 16

equence Original 1 2 3 4

eq 0.5088 0.4284 0.7202 0.6530 0.3465cm 0.2237 0.2389 0.5246 0.4650 0.1296

of the indentation for hardness measurement; (b) Vickers hardness value at points

ardness value is from the inclusion in the middle of the thick-ess, whereas the lowest value is from the ferrite. The difference ofardness value between the two is 52 MHV.

The highest hardness value in DP590 HDGI steel is from thetrip microstructure, and the lowest hardness appears to be locatednside the grain of ferrite, as shown Fig. 19. The difference inardness value between strip microstructure and ferrite grain is56 MHV, a value much higher than that in hot-rolled DP590 steel.

. Discussion

Alloying elements affect the characteristics of microstructurend consequently the mechanical properties of steels. Silicon andanganese are main alloying elements in DP590 hot-rolled steel,hile manganese is the predominant alloying element in DP590DGI steel.

Manganese is soluble in both austenite and �-ferrite, andtrengthens the ferrite in carbon steels by solid solution strengthen-

ng. Manganese in the amount of 1–1.5% is added in dual-phase steelo ensure sufficient harden-ability so that martensite is formedpon rapid cooling [26]. However, according to the Fe–C–Mn trinarylloy phase diagram [27], manganese makes the eutectoid point ofe–C phase diagram move to the lower left. Thereby, manganese

5 6 7 8 9 10

1.8944 0.7555 2.9544 0.3820 3.7583 0.30731.6849 0.5479 2.7346 0.1753 3.4487 0.1165

460 M. Yang et al. / Materials Science and Engineering A 497 (2008) 451–461

Table 6Ceq and Pcm of HDGI steel at points shown in Fig. 17

Sequence Original 1 2 3 4 5 6 7 8 9

C 3.234P 2.954

ittne

iiohmtaa

sTh

t

C

b

P

sT

(nAp

mTatdvbdEestPmToi

sisfipf

shott

oetHhswsio

5

irf

(

(

(

Acknowledgements

eq 0.4273 0.348 0.3888 0.667cm 0.204 0.1024 0.115 0.3585

ncreases the stability of austenite and promotes the precipita-ion of metallic carbide (Me3C). Moreover, manganese enhancedhe strip microstructure in steel [28], and hard phases such as bai-ite and martensite aggregated in the strip microstructure tend tombrittle the steel.

Silicon that is an element to promote the formation of ferrites added into dual-phase steel to provide solid solution harden-ng and balance the action of manganese to restrain the presencef strip microstructure. As stated above that HDGI steel containsigh content of manganese and low content of silicon. Its stripicrostructure is composed of bainite and martensite. On the con-

rary, the hot-rolled steel contains high content of both manganesend silicon. So, there is no strip microstructure in hot-rolled steels in HDGI steel.

Carbon is an important alloying element in steel. While marten-ite in steel contains high content of carbon, it gets hard and brittle.herefore, high concentration of carbon results in high strength,igh hardness, and low ductility of steel.

The combined effect of alloying elements can be evaluated byhe carbon equivalent (Ceq) as calculated by [26]

eq = C + Mn/6 + Si/24 + Ni/15 + Cr/5 + Mo/4

+Cu/13 + P/2 (1)

The effect of alloying elements on the propensity of cracking cane assessed with “crack sensitive factor” (Pcm) using [26]

cm = C + (Mn + Cu + Cr)/20 + Mo/15 + V/10 + Si/30

+Ni/60 + 5B (2)

Using the values in Tables 1, 3 and 4, the Ceq and Pcm of the twoteels at the points measured by EPMA are calculated and listed inables 5 and 6.

As shown in Table 5, both the Ceq and the Pcm of the inclusioni.e. point 9 in Fig. 16) are the highest in hot-rolled steel. The hard-ess is the highest as well in the material as shown in Fig. 18(b).ll these indicate that the inclusion in the steel is the most brittlehase and prone to cracking in the DP590 hot-rolled steel.

However, the Ceq and Pcm of ferrite around the inclusions areuch lower than those at the inclusions, see points 8 and 9 in

able 5 and Fig. 16. It is relatively soft and could contribute to largemount of plastic deformation. During impact at high temperature,he ferrite can absorb most of the impact energy through plasticeformation, but the inclusions, which are hard, cannot deformery much. Consequently, de-cohesion happens at the interfaceetween the inclusion and ferrite (see Fig. 13(c)). As the plasticeformation of ferrite increases, de-cohesion expands into pores.ventually, splitting is formed when the adjacent pores combineach other becoming crack, as shown in Fig. 8(b) and (c). The cohe-ion at the ferrite and inclusion interface becomes weaker as theemperature decreases. It can also be observed in Table 5 that the

cm of martensite inside the band region of inclusion (i.e. point 7) isuch higher than that of the ferrite in the same region (i.e. point 8).

he microstructure around the splitting of polished fracture surfacef impact specimen at 70 ◦C (see Fig. 13) indicates that part of thenclusion fell off from the ferrite to form pores that initiated the

Gsa

6 2.5191 0.5292 0.3502 0.3981 0.91988 2.2355 0.2541 0.1044 0.1486 0.6215

plitting and the splitting then propagated along the martensitenside the band of the inclusion. The pores at the side of splitting ofpecimens at −70 ◦C shown in Figs. 14 and 15 indicate that the inter-acial bond between inclusion and ferrite gets further weaker. And,nclusions completely fell off the ferrite to produce pores. Theseores were then extended and combined by the impact stress toorm splitting (or secondary cracks).

Because the Ceq value of the strip microstructure zone in HDGIteel is highest as listed in Table 6, the hardness of this area is alsoighest as shown in Fig. 19(b). Furthermore, because of the amountf silicon in the material, the lowest hardness of HDGI steel is lowerhan that of hot-rolled steel, and the difference in hardness acrosshe thickness direction in HDGI steel is bigger.

The Pcm of the strip microstructure zone is higher than thatf the base (HDGI or hot-rolled) steel. Consequently, splitting isasier to initiate and propagate in this zone. This is evidenced fromhe experimental results, i.e. both the strip microstructure in theDGI steel and the splitting are parallel to the rolling direction. Theard phases such as bainite and martensite appeared inside theplitting. Because the thickness of the HDGI steel studied in thisork is relatively thin, the out of plane constraint is small. Then,

plitting is therefore short and shallow. In summary, the splittingn DP590 HDGI steel is due to the strip microstructure, and its effectn the impact energy of steel may be ignored.

. Conclusion

The splitting or secondary cracking phenomenon in Charpympact specimens made of DP590 steels in the form of 5.5 mm hot-olled and 2.0 mm HDGI was studied. Conclusions can be drawn asollows:

1) Splitting phenomenon exits in hot-rolled steel impact speci-mens at various testing temperature. In the region when thetemperature is higher than the DBTT, longer, narrower and moresplitting (or secondary cracks) appear with decreasing temper-ature. As the temperature is lower than the DBTT, sub-splittingbecomes shorter with decreasing temperature.

2) Splitting in hot-rolled steel occurred due to silicate and carbideinclusions. During impact, pores firstly formed by de-cohesionat the interface between ferrite and inclusion, then wereenlarged by the applied impact stress, and finally combinedwith each other to initiate the secondary cracks or the splitting.Subsequently, these splitting propagate along the martensite orthe interface between the inclusion and ferrite.

3) Splitting in HDGI steel only appears in T-L impact specimen andis caused by strip microstructure. High content of manganeseand low content of silicon lead to the strip microstructure withaggregated bainite and martensite in it.

The authors would like to thank Profs. Zuocheng Wang, Jinqiangao and Shitong Li at Shandong University for their helpful discus-ion. Financial support to Min Yang from China Scholarship Councilnd Shandong University is acknowledged.

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