Microstructure and Mechanical Property of Strip in Fe–23Mn

1. Introduction

TWIP (twinning induced plasticity) steels containinghigh content of Mn, with Al and Si in addition, have rela-tively low specific density of 7.3 g/cm3. TWIP steels are notonly supra-ductile steels, but also possess high crash resist-ance with the specific energy absorption of more than twicethat of conventional high strength deep-drawing steels.1)

Thus, it has the potential to be applied in automotive indus-try in the future for weight reduction, energy saving andhigh safety requirements. Unfortunately, low plasticity andhigh deformation resistance during hot rolling due to highcontents of Mn, Si and Al bring in the difficulties in hotstrip rolling of conventional slabs.18) Severe edge crackscan be formed during hot strip rolling, leading to the edgecutting-off of final products.

Twin-roll strip casting, one of the “near-net-shape” form-ing technologies to produce thin cast strips, has the advan-tage in manufacturing the strips of “non-deformable” steelssuch as high alloy steels because it combines casting androlling into a single step and applies rolling deformation inmushy region.2) Such process has other advantages such aslower production costs and energy consumption, and lowercapital investment as compared to conventional processes.2)

Strip casting also has positive effects on microstructurecontrol such as refined inclusion size and dispersed distri-bution.3)

Therefore, the problems occurred in conventional hotstrip rolling of TWIP steels are expected to be solved by“near-net-shape” forming in twin-roll strip casting.4) Caststrip of TWIP steel with a thickness of up to 6 mm had beensuccessfully produced in semi-industrial scale, which

showed that good edge and surface qualities could be ob-tained.5) However, the detailed mechanical properties andmicrostructure evolution of strip cast TWIP steel have notyet been reported so far.

The present paper described a method to produce caststrip of TWIP steel with no edge cracks by using a labora-tory scale twin-roll strip caster, and explored the underlin-ing mechanisms for the prevention of edge cracking. Mi-crostructures of the cast strip at different processing stageswere studied. TEM was employed to get a better under-standing on the deformation mechanism of cast strip ofTWIP steel.

2. Experimental Procedures

2.1. Material Processing

Table 1 shows the chemical composition of the investi-gated steel. These steels were cast into strip or ingot.

2.1.1. Strip Casting

A vertical type twin-roll strip caster, with the diameter ofcasting rolls being 500 mm and roll width being 254 mm,was used to produce the cast strip. The steel was melted inan induction furnace with the capacity of 40 kg under Argas shielding. The casting speed and initial roll gap wereset to be 0.25 m/s and 2.0 mm, respectively. The moltensteel was poured into a preheated tundish to flow through anozzle into the water-cooled casting rolls and was cast to astrip with the thickness of about 2.8 mm and length of 4 m,during which the roll separating force was measured to beabout 60 kN.

Subsequent processing was employed to eliminate coarse

1340© 2009 ISIJ

ISIJ International, Vol. 49 (2009), No. 9, pp. 1340–1346

Microstructure and Mechanical Property of Strip inFe–23Mn–3Si–3Al TWIP Steel by Twin Roll Casting

S. H. WANG,1) Z. Y. LIU,1) W. N. ZHANG,1) G. D. WANG,1) J. L. LIU2) and G. F. LIANG2)

1) The State Key Laboratory of Rolling Technology and Automation, Northeastern University, Shenyang, Liaoning Province110004, P. R. China. E-mail: [email protected] 2) The Research and Development Institute, Baosteel Co.,Shanghai, P. R. China.

(Received on November 11, 2008; accepted on April 28, 2009)

Strip in TWIP steel with the composition of Fe–23Mn–3Si–3Al fabricated by twin-roll strip casting was in-vestigated. Large edge cracks, which were prone to occur in hot rolled TWIP steels, were avoided by usingtwin-roll strip casting. The cold rolled and annealed strip exhibited a reasonably good combination ofstrength and ductility after annealing at 1 373 K for 20 min, with the tensile strength and elongation being655 MPa and 57.4%, respectively, with the elongation of cast strip being 80% that of the conventionalprocessed counterpart. Fine grain structure formed in cast strip increased the critical resolved shear stress(CRSS) of twinning and suppressed the generation of deformation twins during plastic deformation to makeless contribution to the total elongation.

KEY WORDS: twin-roll strip casting; TWIP steel; deformation twins.

columnar grained structure in cast strip to improve theproperties. After having been solution annealed at 1 373 Kfor 20 min and pickled, the strip was cold rolled to the finalthickness of 1.0 mm. The cold rolled strip was annealed at1 373 K for 6 min, and quenched to room temperature.

2.1.2. Hot Strip Rolling

Ingot in the size of 160 mm in length, 125 mm in widthand 120 mm in height was made by standard vacuum melt-ing and casting by using an induction furnace with the ca-pacity of 50 kg. After having been homogenized at 1 473 Kfor 2 h, the ingot was hot rolled to plate with the final thick-ness of 3.0 mm, followed by solution treatment at 1 373 Kfor 20 min and water quenched to room temperature. Theplate was then cold rolled to 1.0 mm thick strip with thetotal reduction of 67%, subsequently annealed at 1 373 Kfor 6 min, and water quenched to room temperature afterannealing.

2.2. Mechanical Properties and Microstructure Obser-vations

Standard tensile tests were carried out at a strain rate of 10�3 s�1 at room temperature by using the specimenswith length of 25 mm and width of 12.5 mm. Quantita-tive analysis on microstructure was conducted by using a QUANTA600 scanning electron microscopy (SEM)equipped with energy dispersive spectroscopy (EDS) andEPMA-1610. The crystal orientation was analyzed by using

an orientation imaging microscopy (OIM), with the sam-ples having been electro-polished by using a solution ofHClO4 (7 vol%), alcohol (83 vol%) and water (10 vol%).Thin foils with diameter of 3 mm were cut from the de-formed specimens, and prepared by a twin jet electro-pol-ished in a reagent of perchloric acid (10 vol%) and alcohols(90 vol%) for TEM studies. A JEM 2010 transmission elec-tron microscope was used to examine the TEM samples.

3. Results

3.1. Edge Cracks in Hot Rolled Strip

Figure 1 shows the surface appearance of the strips fab-ricated by strip casting and conventional hot rolling. Nocrack could be observed in the cast strip, and good surfacequality was also obtained as shown in Fig. 1(a). By con-trast, severe edge cracks were formed in the hot rolled stripin Fig. 1(b).

Figure 2 shows the solidification structures in cast stripand ingot. Columnar grains sandwiching a small portion ofequi-axed grains were formed in the cast strip in Fig. 2(a).The ingot possessed similar solidification structure in Fig.2(b).

Figures 3(a) and 3(c) show the backscattered electronimages of the cast strip and ingot respectively. Figures 3(b)and 3(d) show the EPMA mappings for Al in the abovementioned specimens. Segregation of Al was formed alongthe inter-dendritic regions in the ingot, and the width of the

ISIJ International, Vol. 49 (2009), No. 9

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Table 1. Chemical composition of the investigated steels, in wt%.

Fig. 1. Surface appearance of (a) twin rollcast strip and (b) conventional hotrolled strip.

Fig. 2. Comparison of optical microstructures between (a) twin roll cast stripand (b) conventional cast ingot.

segregation band was about 3.5 mm, as shown in Fig. 3(d).The particles presented in Fig. 3(c) were confirmed to beAlN. By contrast, no Al segregation could be observedalong the columnar grain boundaries in the solidificationstructure of the cast strip in Fig. 3(b).

Figure 4 shows the analysis of inclusions near stripedges of the hot rolled strip by using SEM. Inclusions were

typically formed inside long cracks and at the crack tips inFig. 4(a), indicating that these inclusions might be thecause for the crack propagation during hot strip rolling.These inclusions were confirmed to be aluminum nitridesand/or aluminum oxides by EDS in Fig. 4(b).

3.2. Microstructure and Mechanical Properties ofCold Rolled and Annealed Strips

Figures 5(a) and 5(b) show the microstructures of thecast strip after the solution treatment at 1 373 K for 5 minand 20 min, respectively. It can be seen that equi-axedgrains gradually formed along the columnar grain bound-aries with increasing solution time, and eventually formedall equi-axed grains. Micro-hardness tests were carried outfor the cast strips after 0, 5 and 20 min of solution anneal-ing, which showed that the hardness values were measuredto be HV 219, 199 and 180 for each specimen, respectively,indicating substantial softening during the solution treat-ment and implying the occurrence of re-crystallization.

Figure 6(a) shows the microstructure of hot rolled plateproduced from the cast ingot. It can be seen that the grainswere elongated in the rolling direction, and a small fractionof dynamic or static recrystallized grains had been formedalong grain boundaries. Figure 6(b) shows the microstruc-ture of the hot rolled strip after solution treatment at1 373 K for 20 min. Its grain size was larger than that of thestrip cast counterpart in Fig. 5(b) because the stored defor-mation energy in hot rolled strip could be higher than that


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Fig. 3. EPMA analysis of (a), (b) twin roll cast strip and (c), (d)conventional cast ingot.

Fig. 4. SEM analysis of the inclusion in crack in conventionalhot rolled strip.

in the cast strip to result in greater driving force for re-crys-tallization.

Figure 7 shows EBSD characterization of the micro-structures in the cold rolled and annealed strips fabricated

by twin-roll strip casting and conventional hot rolling. Finegrains were formed in the cast strip after cold rolling andannealing, with the grain size of about 35 mm in Fig. 7(a);while the grain size in the conventional processed strip wasmeasured to be about 95 mm in Fig. 7(b), greater than thatin the processed cast strip by about 160%.

Figure 8 shows the engineering stress–strain curves ofthe cold rolled and annealed strips fabricated by twin rollstrip casting and conventional hot rolling. Table 2 showsthe mechanical properties of TWIP steels manufactured bythe two different processes. Good mechanical propertiescan be obtained through a series of subsequent processingafter twin-roll strip casting, with the tensile strength and


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Fig. 5. Microstructure of twin-roll strip casting after the solutiontreatment at 1 373 K for (a) 5 min and (b) 20 min.

Fig. 6. Microstructure of hot rolled strip produced from the castingot. (a) As rolled, and (b) after the solution treatment at1 373 K for 20 min.

Fig. 7. EBSD characterization on the microstructures in the coldrolled and annealed strips fabricated by (a) twin-roll stripcasting, and (b) conventional hot rolling and annealing.The calculation of grain size is exclusive of annealingtwin boundaries.

Fig. 8. Engineering stress–strain curves of the cold rolled andannealed TWIP steels fabricated by twin roll cast andconventional cast.

total elongation being 655 MPa and 57.4%, respectively.The product of tensile stress and elongation is 3.76�104

MPa%, and the uniform elongation of the processed caststrip is about 80% that of the conventional counterpart.

Figure 9 shows the comparison of the relationships ofstrain hardening rate, ds /de , with true strain between thetwo strips. Three deformation stages were shown in the ten-sile test for the conventional cold rolled and annealed speci-men. In deformation stage I, ds /de decreased and n in-creased with increasing strain; in deformation stage II,ds /de became to be leveled off, indicating the generationof deformation twinning6,7); in deformation stage III, therewas a slow decrease in strain hardening rate with increasingstrain, suggesting the decreased rate of primary deforma-tion twinning.8) In the processed cast strip, however, no ob-vious plateau occurred in the second stage of deformation,and the strain hardening rate was lower than the conven-tional counterpart in the third deformation stage.

3.3. Deformation Mechanism of the Cold Rolled andAnnealed TWIP Strips

Figure 10 shows the comparison of the microstructureevolutions in the two strips during deformation. In the ten-sile test of the cold rolled and annealed strip-cast speci-mens, no deformation twins were generated when strainwas 0.05. A small number of deformation twins were gen-erated when the specimen was strained to 0.10. When thestrain was 0.25, almost all of austenite grains generated pri-mary deformation twins to accommodate deformation. Bycontrast, deformation twins were generated at the beginningof deformation in conventional processed specimens whenstrained to 0.05 in Fig. 10(d). Half of grains contained de-formation twins when the specimen was strained to 0.10 intensile testing, and when the specimen was strained by0.25, deformation twins were generated in almost all thegrains to accommodate the deformation process, with thesecondary system of deformation twins in considerable

number of grains.Figure 11 shows the TEM micrographs of the mi-

crostructure evolutions in different deformation stages forthe two strips. At the strain of 0.05, planar dislocation wasthe main deformation mode and no deformation twins wereobserved in the cold rolled and annealed cast strip, whiledeformation twins generated in the conventional processedspecimen, and twinning became to be a competitive mecha-nism with dislocation gliding in the deformation process inFig. 11(d). With the deformation going on, the number ofdeformation twins increased, and two deformation twinningsystems activated in the conventional processed specimenin Fig. 11(f), (111̄) and (1̄11̄) deformation twins were ob-served to co-exist in the specimen, as shown by the selectarea diffraction pattern at the upper right corner in Fig.11(f) and its analysis in Fig. 11(g), in good agreement withthe OM observations.


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Table 2. Mechanical properties of cold rolled and annealed TWIP steels.

Fig. 9. Relationships between strain hardening rate, ds /de , withtrue strain of cold rolled and annealed TWIP steels.

Fig. 10. Optical microscopy of cold rolled and annealed strips atdifference tensile strains. (a), (b) and (c) are the twinroll cast specimens strained to 0.05, 0.10, and 0.25 re-spectively, and (d), (e) and (f) are the conventional castspecimens strained to 0.05, 0.10, and 0.25, respectively.

4. Discussion

4.1. Edge Cracking Prevention by Strip Casting

Lack of hot workability is generally believed to be amajor cause for edge cracking in hot rolled TWIP steel. Inaddition, the segregation of Al and AlN inclusions can en-courage the formation of cracks. TWIP steel contains alarge amount of aluminum, with the content of about 3% inweight, which are prone to segregate along the grain bound-aries during solidification. Segregation of Al was observedin the conventional cast ingot in Fig. 3(d). Nitrogen reactswith Al during and/or after solidification to form coarsealuminum nitrides of about several microns, causing theweakening of the casting structure.9) Solution elementssuch as manganese, aluminum and silicon can also be eas-

ily oxidized. Therefore, when the ingot is reheated to a hightemperature during hot deformation, oxidation may haveoccurred along grain boundaries to decrease the strength of grain boundaries further. The formation of nitrides andoxides along the inter-dendritic arms weakens the grainboundaries and deteriorates the poor hot workability bypreferentially forming cracks in these regions during hotplate rolling.

TWIP steel is difficult to be deformed due to its high deformation resistance in hot deformation. Strip castingwhich omit the hot deformation process, can avoid the un-safe hot working regime during hot deformation. Therefore,strips with good surface and edge qualities can be obtained.

4.2. Comparison of Deformation Mechanisms betweenthe Conventional and Cast Strips

Microstructure observations by OM in Fig. 10 and TEMin Fig. 11, indicated that the different deformation mecha-nisms had occurred in the two specimens during tensiletesting. When specimens were strained by 0.05, deforma-tion twins were generated in the conventional cold rolledand annealed strip, but in the twin-roll cast counterpart, nodeformation twins were formed with only planar disloca-tions having been at present, indicating that the deforma-tion twinning occurrence having been retarded.

It is generally accepted that deformation twinning obeysa critical resolved shear stress (CRSS) law similar to theSchmid law for crystallographic slip.12) The CRSS for twin-ning increases with increasing dislocation density and de-creasing grain size because any obstacles to dislocationpropagation can increase the twinning CRSS.12) The grainrefinement in twin-roll cast strip could have led to the in-crease of CRSS, resulting in a retarded generation of defor-mation twins in the deformation process.10,11) Fewer defor-mation twins were formed in the twin-roll cast specimensthan those in the conventional processed specimens withthe same amount of strain in Figs. 10(b) and 10(e). Sec-ondary system of deformation twins generated in consider-able number of grains in the third deformation stage for theconventional processed specimen in Fig. 10(f) and Fig.11(f), suggesting that the activation of two systems of twin-ning was sequential, and the fraction of deformation twinsincreased with increasing strain, in good agreement withthe work by Allain.13) By constrast, no much secondary sys-tem of deformation twins could be observed in strip castspecimen in later deformation stage in Fig. 10(c) and Fig.11(c). When deformation twins are generated in the matrix,dislocation can be restricted between twin lamellas in thefollowing deformation process, because deformation twinsare strong barriers like other high-angle grain bound-aries12–17) against the propagation of other deformationtwins or gliding dislocations. The blocking effect of twinboundaries on dislocations’ motion results in local strainhardening, and transports deformation to un-deformed re-gions to suppress necking from taking place, which eventu-ally leads to large, uniform and necking-free elongation, orthe so-called “TWIP” effect.17,19) Therefore, the delayedgeneration of deformation twinning in fine grained twin-roll cast specimens could have led to lower strain hardeningrate to make less contribution to the total elongation thanthat of the conventional processed specimen, as shown in


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Fig. 11. TEM analysis of cold rolled and annealed strips at dif-ference tensile strains. (a), (b) and (c) are the twin rollcast specimens strained to 0.05, 0.10, and 0.25 respec-tively, (d), (e) and (f) are the conventional cast speci-mens strained to 0.05, 0.10, and 0.25 respectively, and(g) is the analysis of diffraction pattern in Fig. (f ).

Fig. 9.

5. Conclusion

Severe cracks on the sides of hot rolled strip are prohib-ited when producing TWIP steel by using twin-roll stripcasting, in addition, a good combination of strength andductility is obtained, with the tensile strength of 655 MPaand elongation of 57.4%.

The lower elongation of the twin-roll cast specimen at-tributes to its fine grain size, leading to the retardation ofdeformation twin generation and lower quantity of defor-mation twins in the deformation process.

Acknowledgement

This work was supported by the Natural Science Founda-tion together with Baosteel (50873141) and “973” projectunder a contract of 2004CB619108.

REFERENCES

1) G. Frommeyer, U. Brüx and P. Neumann: ISIJ Int., 43 (2003), 438.2) C. A. Santos, J. A. Spim, Jr. and A. Garcia: J. Mater. Process. Tech-

nol., 102 (2000), 33.

3) H. Chen, S. B. Kang, H. Yu, H. W. Kim and G. Min: Mater. Sci. Eng.A, 492 (2008), 317.

4) M. Ha, W. Kim, H.-K. Moon, B.-J. Lee and S. Lee: Metall. Mater.Trans. A, 39 (2008), 1087.

5) B. Engl, D. Senk, J. W. Schmitz and A. Offergeld: US patent2004/0074628Al.

6) E. El-Danaf, S. R. Kalidindi and R. D. Doherty: Int. J. Plast., 17(2001), 1245.

7) S. R. Kalidindi: Int. J. Plast., 14 (1998), 1265.8) E. El-Danaf, S. R. Kalidindi and R. D. Doherty: Metall. Mater.

Trans. A, 30 (1999), 1223.9) T. W. Kim, Y. G. Kim and S. H. Park: US Patent 1997/5647922.

10) R. Ueji, N. Tsuchida, D. Terada, N. Tsuji, Y. Tanaka, A. Takemuraand K. Kunishige: Scr. Mater., 59 (2008), 963.

11) M. R. Barnett: Scr. Mater., 59 (2008), 696.12) J. G. Sevillano: Scr. Mater., 59 (2008), 135.13) S. Allain, J.-P. Chateau, K. Kahmoun and O. Bouaziz: Mater. Sci.

Eng. A, 387–389 (2004), 272.14) L. Rémy: Metall. Trans. A, 12 (1981), 387.15) J. W. Christain and S. Mahajan: Prog. Mater. Sci., 39 (1995), 1.16) L. Lu, Y. Shen, X. Chen, L. Qian and K. Lu: Science, 304 (2004),

422.17) O. Grässel, L. Krüger, G. Frommeyer and L. W. Meyer: Int. J. Plast.,

16 (2000), 1391.18) A. S. Hamada, L. P. Karjalainen and M. C. Somani: Can. Metall. Q.,

46 (2007), 47.19) S. H. Hong and Y. S. Han: Scr. Metall. Mater., 32 (1995), 1489.


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Microstructure and Mechanical Property of Strip in Fe–23Mn