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Materials Science and Engineering A 438440 (2006) 296299

Continuous cooling transformation diagrams and propertiesof micro-alloyed TRIP steels

M. Zhang a,b,, L. Li a, R.Y. Fu a, D. Krizan c, B.C. De Cooman ca Department of Materials Science & Engineering, Shanghai University, 149 Yanchang Road, 200072 Shanghai, China

b Shanghai Huizhong Automotive Manufacturing Co. Ltd. 1493 S. Pudong Road, 200122 Shanghai, Chinac Laboratory for Iron and Steelmaking, Ghent University, Technologiepark 903, B-9052, Belgium

Received 8 April 2005; received in revised form 23 November 2005; accepted 12 January 2006

bstract

Continuous cooling transformation (CCT) diagrams and properties of four kinds of low-silicon CMnSiAl transformation-induced plasticityTRIP) steels with different carbon contents, with or without microalloy element Ti/V, as well as a reference TRIP steel containing 1.19 wt.%i were studied. The CCT diagrams exhibited that as the carbon equivalent (CE) increased, it caused a shift of the ferrite forming and pearliteorming temperatures to the right side and the bainite forming and martensite forming to the lower temperatures of the diagram. The microstructuralvolution obtained from the dilatometry samples revealed that the highest cooling rates produced fully martensitic microstructure in all cases except

he reference TRIP steel. As the cooling rate decreased, more ferrite and bainite were formed. The increase of CE caused the increase of the amountf martesite in the microstructure. Tensile test and Erichsen test of the investigated steels showed an excellent mechanical strength and ductilityombination, with tensile strength between 800 and 1000 MPa, total elongation of around 20%, and a quite good formability with a dome heightf about 10 mm in all cases.

2006 Elsevier B.V. All rights reserved.

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eywords: Micro-alloyed; TRIP steels; CCT diagram; Microstructure; Mechan

. Introduction

In general, the increase of strength will result in the decreasef formability or ductility. However, it has been shown that thetrain-induced transformation phenomenon in retained austenite1] results in an excellent combination of strength and duc-ility. Transformation-induced plasticity (TRIP) steels have a

ultiphase microstructure consisting of ferrite, bainite, andetained austenite. Their excellent strength-ductility combina-ion is resulted from the presence of retained austenite, whichransforms to martensite during plastic deformation, and makeshe TRIP steels very attractive for structural parts in the automo-ive industry. The chemical composition of the traditional TRIPteel is 0.10.4 C1.5 Mn1.5 Si. In recent years, content ofi is lowered to reduce the industrial casting and the surface-

uality or galvanising problems. Alternative low Si TRIP steelssing Al have been developed, principally to encounter withigh Si contents. The demand for higher strength TRIP grades

Corresponding author. Tel.: +86 21 58201188x2512; fax: +86 21 58306293.E-mail address: zhangmei3721@hotmail.com (M. Zhang).

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921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.01.128roperty; Formability

8001200 MPa) for automobile applications can be met byncreasing the carbon content to around 0.4%, however theseigher carbon levels introduce serious weldability problems anday cause hot rolling difficulties, particularly for wide formats.hus the micro-alloying concept (Ti, Nb and V) has been incor-orating in the TRIP steel in order to keep the C content low (usu-lly

M. Zhang et al. / Materials Science and Eng

Table 1Chemical composition of the studied steels in wt.%

Code C Mn Al Si P Ti V CE

P 0.11 1.67 0.038 1.19 0.013 0.44O 0.21 1.64 1.37 0.31 0.081 0.073 0.55Z 0.24 1.51 1.12 0.27 0.082 0.54SJ

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0.26 1.64 1.35 0.39 0.049 0.099 0.053 0.610.34 1.75 1.32 0.46 0.055 0.12 0.033 0.71

Steel P is the reference 1.19 Si-contained TRIP steel withoutny addition of micro-alloy. Steel Z is a CMnSiAlP TRIPteel. Steel O is a V-added micro-alloyed CMnSiAlP TRIPteel. Steel S and J are V/Ti-added TRIP steels.

The material used in dilatometry was vacuum remelted andot rolled. The specimen for dilatometry was of a diameter of.5 mm and a length of 5 mm. During testing, the specimensere protected from oxidation with a vacuum of 3 103 mbar.The heating and cooling process in dilatometry for CCT dia-

ram constructing was as follow. The sample was heated atrate of 2 K/s to the austenitizing temperature 1200 C, heldmin; and then cooled at a rate of 5 K/s to the soaking tem-erature 950 C, held 2 min; at last cooled to room temperaturet different cooling rates, 0.3, 1, 2, 5, 10, 30 K/s, respectively.he microstructures on the dilatometry samples were examinedsing light optical microscopy (LOM), and Vickers hardnessas also measured.After cold rolled to a thickness of 1 mm, the specimens

or tensile test and Erichsen test were intercritically annealedt 800 C and austempered at 460 C to obtain TRIP-assisted

icrostructure.Tensile test was carried out using 25 mm gauge length and

mm width specimens. Erichsen test was also carried out using0 mm 90 mm samples. The microstructures of the studied

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Fig. 1. CCT diagrams anineering A 438440 (2006) 296299 297

RIP steels were examined using LOM and scanning electronicroscopy (SEM).

. Results

Fig. 1(a)(e) show the CCT diagrams constructed from theilatometry curves, and Fig. 1(f) is the measured microhard-ess profile of the dilato samples. From the CCT diagrams ofll the studied steels, it can be found that as the carbon equiva-ent increases, it causes a shift of the ferrite forming and pearliteorming temperatures to the right side and the bainite form-ng and martensite forming to the lower temperatures. Fromig. 1(f), the relationship between the hardness HV1 and theooling rate can be seen. The hardness of dilatometer sam-les after different cooling reveals that the hardness increasess the cooling rate increases. The hardness also increases withhe increase of the carbon equivalent. The highest hardness isained by steel J at cooling rate of 30 K/s.

Fig. 2 exhibits the microstructures of the dilatometry sam-les. The label at the upright corner of each image indicates theteel code and the cooling rate. The microstructural evolutionbtained from the dilatometry samples reveals that the highestooling rate couples fully martensitic microstructure (Fig. 2(a),d) and (g)) except the reference steel (steel P, Fig. 2(m)). As theooling rate decreases, more ferrite and bainite is formed. Thencrease of CE causes the increase of the amount of martensiten the microstructure. This can be more clearly shown in steelwith CE of 0.71 where almost fully martensitic microstruc-

ure is achieved even at a slow cooling rate of 5 K/s as shown in

ig. 2(c).

Fig. 3 shows the mechanical properties. Obviously, the ulti-ate tensile strength (UTS) increases with the increase of carbon

ontent, and the highest UTS of over 1 GPa is obtained in steel

d hardness profile.

298 M. Zhang et al. / Materials Science and Engineering A 438440 (2006) 296299

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Fig. 2. Microstructures

. On the other hand, the total elongation (EL) decreases withhe increase of carbon content (except steel J). The investigatedteels have an excellent combination of mechanical propertynd ductility, with the UTS between 800 MPa and 1 GPa, EL ofround 20%. The V-added steel shows a higher yield strengthYS) and a lower uniform elongation (EU).Erichsen test results are shown in Fig. 4 where the formabil-ty of the studied steels decreases slightly with the increase ofarbon content. Steel Z, steel S, steel J show a dome height ofbout 810 mm.

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e dilatometry samples.

. Discussion

As known, a variety of alloying elements is available to con-rol the transformation behavior of steel such that, depending onhe available processing conditions, the desired microstructures obtained.The addition of element Al in TRIP steel can shift the austen-te area of the equilibrium phase diagram to the right side ashown in Fig. 5. The amount of ferrite is increased and the car-on content of austenite is raised as well as the stability of the

M. Zhang et al. / Materials Science and Eng

Fig. 3. Tensile test result of various steels.

Fig. 4. Dome height of three kinds of steels.

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[Aided High Strength Ferrous Alloys Proceedings, GRIPS, Ghent, Belgium,20023, p. 199.

[4] C. Scott, in: M.A. Baker (Ed.), International Conference on Advanced High-Strength Sheet Steels for Automotive Applications Proceedings, WinterPark, CO, USA, 2004, pp. 181193.Fig. 5. Equilibrium phase diagram of Steel J.ineering A 438440 (2006) 296299 299

etained austenite. As a result, the ferrite and pearlite area arehifted to the right side, and martenite and bainite to the loweremperature part, as shown in CCT diagrams.

In addition, the microstructure of the V-added TRIP steelhows extremely fine grain size in steel O.

The left side of the CCT diagrams can be used to deter-ine the microstructural evolution of the most critical zones

n the laser welds, such as fusion zone, fusion boundarynd super critical heat-affected zone. The other part of CCTiagram representing lower cooling rates, can also be usedn higher heat input operations such as arc welding or hotolling.

. Conclusions

The highest cooling rates in this paper cause the formationf full martensite. The hardness increases as the cooling ratencreases. The hardness also increases with an increase of thearbon content.

The studied micro-alloyed TRIP steels have an excellent com-ination of mechanical strength, with tensile strength between00 and 1000 MPa, and total elongation of 20%.

The studied TRIP steels have a fairly good formability, withdome height of about 10 mm.

cknowledgement

The authors are thankful to the Fund of Vanadium Interna-ional Technical Committee for the financial support.

eferences

1] V.F. Zackay, E.R. Parker, D. Fahr, R. Bush, Trans. Am. Soc. Met. 60 (1967)252.

2] D. Krizan, in: M.A. Baker (Ed.), 45th MSS Conference Proceedings, vol.XLI, ISS, Chicago, USA, 912 November, 2003, pp. 437448.

3] J. Ohlert, in: B.C. De Cooman (Ed.), International Conference on TRIP-

Continuous cooling transformation diagrams and properties of micro-alloyed TRIP steelsIntroductionExperimental procedureResultsDiscussionConclusionsAcknowledgementReferences