Effect of Cr Content on the TWIP Behavior in Fe-Mn-Cr Steels

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ScienceDirect Materials Today: Proceedings 2S (2015) S673 – S676

International Conference on Martensitic Transformations, ICOMAT-2014

Effect of Cr content on the TWIP behavior in Fe-Mn-Cr Steels V. Mertingera, M. Benkeb,*, E. Nagyb a University of Miskolc, Miskolc-Egyetemvaros H3515, Hungary MTA-ME, Materials Science Research Group, Miskolc-Egyetemvaros H3515, Hungary

b

Abstract The characteristics of the martensitic transformations induced by uniaxial tensile tests performed between room temperature and 200°C in two Fe-Mn-Cr alloys with different Cr contents were examined. Different volume fraction of ’ martensite and martensite were detected in the austenitic matrix as the function of parameters of the thermo-mechanical treatments. Quantitative and qualitative phase analyses by X-ray diffraction were carried out on the samples. The crystallographic orientation (texture) of the phases on macro scale was also determined. The decomposition of martensite was followed by DSC measurements. It was found that increasing Cr content increased the stability of austenite, thus, increased the stacking fault energy. The increased Cr content also suppressed the formation of ’ martensite. Deformation of austenite stabilized the austenite phase and led to the formation of textured ε martensite. © 2014 The Authors. Published by Elsevier Ltd. © 2015 Theand Authors. Published by Elsevier Ltd. This is anchairs open of access article under the CC BY-NC-ND license Transformations Selection Peer-review under responsibility of the the International Conference on Martensitic (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2014. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. Keywords: TRIP/TWIP steel, martensite decomposition, texture

1. Introduction It has been known that fcc austenitic steels can transform into ferromagnetic, bcc α’ and/or paramagnetic, hcp ε martensite phases during cooling and/or plastic deformation. Two transformation mechanisms have been observed. The first one describes the direct formation of α’ martensite from γ austenite, while the second one involves a twostep reaction in which ε martensite forms from γ austenite and α’ phase is formed from the ε martensite [1].

* Corresponding author. Tel.: +3646-565-201; fax: +3646-565-214-1130. E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.373

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The mechanism of the austenite–martensite transformation depends on the composition, deformation rate and temperature. The ratio and quantity of the resulting phases determine the properties of the product. The austenite– martensite transformation can even be reversible in nature depending on the composition. Some Fe based alloys such as Fe-Ni-C, Fe-Pt, Fe-Pd or Fe-Mn-Si based alloys have been developed as one way shape memory alloys due to the reversible fcc/hcp martensitic transformation [2,3]. Cr is a favoured alloying element of steels because it improves the corrosion resistance and also has an effect on the stacking faults energy (SFE) of the fcc phase which should be low in order to allow the easy γ →ε transformation [2,4]. However, the effect of a certain alloying element on SFE in a multi-component system strongly depends on the host composition. Thus, the effect of Cr on SFE in TWIP/TRIP steels is not unequivocal [5,6]. Thermo-mechanical treatments of TWIP/TRIP steels also play a very important role on their mechanical properties. From the technological aspect, thermo-mechanical treatments are applied as the typical technological processes during manufacturing of semi-products, and from the microstructure aspect, the applied stress and strain can induce transformations. The interaction of transformation and deformation texture of austenite and the martensite phases has an effect on the reverse ε→γ transformation and the mechanical behavior [7,8]. The present manuscript focuses on the reversibility of the ε→γ transformation and the effect of Cr content and transformation texture on the transformation characteristics in the Fe-Mn-Cr system. 2. Experimental Alloys with different compositions were produced at TU Bergakademie Freiberg. The compositions of steel A and steel B are shown in table 1. The cast ingots were hot rolled to rods with diameter of 10 mm. Tensile test specimens and samples for transformation temperature examinations were machined from the rods. The specimens were solution treated at 1000°C for 30 minutes under nitrogen atmosphere then quenched in room temperature water. The as-quenched samples were designated as initial samples. The ε↔γ transformation temperatures of the quenched alloys were determined by DSC. The results are summarized in Table 1. The measured Ms and As temperatures are in good agreement with observations of Troiani et al. [9]. Tensile tests were performed to fractures at different temperatures ranging from ambient temperature up to 200°C using an Instron 5982 universal mechanical testing machine equipped with a sample furnace. The sample furnace was heated up to 300°C before the tensile tests to reach pure austenitic state of the alloys. Then, test temperatures were reached by cooling the furnace from 300°C to the test temperature (200, 180, 160, 140, 125, 110, 25°C). After the tensile tests, samples were machined form the uniformly elongated part (gauge section) of the specimens for DSC and XRD examinations. The DSC examinations were performed with a Netzsch DSC 204 device with 10 K/min scanning rate in nitrogen media. The XRD texture examinations were carried out on the cross section samples using a Bruker D8 Discovery diffractometer using Co Kα radiation. The texture of the samples was described by texture index numbers calculated by normalizing the measured intensities to the relative intensities (IR) found in the International Centre for Diffraction Data (ICDD) Powder Diffraction File (PDF2) database. Table 1. Composition and transformation temperatures of the examined alloys. Composition [wt%] C

Mn

Cr

Si

Transformation temperatures [°C] S

P

Ms

Mf

As

Af

Steel A

0.02

17.7

2.26

0.1

0.029

0.0051

142

112

188

218

Steel B

0.08

17.7

6.12

0.06

0.025

<0.003

118

95

175

204

3. Results According to Fig. 1 the initial samples of both alloys contained more than 85% of martensite and a small amount of retained austenite. Due to the thermos-mechanical treatments different volume fractions of and ’ martensites were formed in function of the test parameters. Samples of all tensile test temperature contained ε martensite and austenite and only the samples of 25-140°C contained ’ martensite. Samples of high temperatures contained notably high austenite fraction. It is also seen that the austenite fractions are larger for alloy 3.

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Figure 2 shows the heating DSC curves of the samples obtained from the tensile test specimens fractured at different temperatures of steels A and B. The samples that were ferromagnetic (contained ’ martensite) before the DSC examinations remained ferromagnetic after the DSC tests as well. This suggests that the resident ’ martensite did not decompose during the DSC examinations by heating up to 450°C. Thus, the exothermic peaks in Fig. 2 correspond to the ε→γ transformation (decomposition of ε martensite). It can be seen that for both alloys, that the decomposition of the ε martensite occurs at notably higher temperatures than the measured As temperatures of the initial samples. This is caused by the increased dislocation density caused by the severe deformation during tensile tests. The same effect was observed during the thermal cycling of the same alloy [10]. Samples tensile tested at higher temperatures exhibit one narrow, high peak. On the other hand, samples taken from lower temperature tensile tests have low, wide peaks.

Fig. 1. Volume fractions of samples tensile tested at different temperatures (25,110,125,140,160,180,200°C). a) steel A, b) steel B.

Fig. 2. Decomposition of ε martensite in samples tensile tested at different temperatures (25,110,125,140,160,180,200°C). a) steel A, b) steel B.

Fig. 3. Texture numbers of the ε martensite reflections of samples tensile tested at different temperatures. a) steel A, b) steel B.

Figure 3 shows the variation of the texture numbers of the ε martensite {hkl} reflections of samples tensile tested at different temperatures. A quite notable behavior can be observed for the {112} reflections. The texture number is high for samples tested at higher temperatures and becomes lower for samples tested at lower temperatures. For steel A, a sudden drop can be seen in the texture number, while a continuous decrease is seen for steel B.

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4. Discussion The high austenite content in samples tensile tested at high temperatures was caused by the stabilization of the γ phase achieved by deformation above Ms. The stabilization of the γ phase decreases Ms. This stabilization was observed in Cu based shape memory alloys as well [11]. A similar effect was observed during the thermal cyclic examination of TWIP steels carried out by other authors [12-14] and the present authors [10] as well. The decrease of the Ms was associated with the increased dislocation density resulting from the plastic deformation accompanying the ε↔γ transformations. The increased dislocation density inhibits the following ε↔γ transformations [3]. According to the results increasing Cr content increases the fraction of the retained austenite. This suggests that the Cr addition decreases the stability of the martensite phase and increases the stability of the austenite, thus, increases the stacking fault energy of Fe-Mn-Cr alloys. It was also seen that the Cr content suppresses the formation of ’ martensite. It was seen on the DSC curves of the samples tensile tested at different temperatures that the decomposition of the ε martensite phase occurred with different peak shapes. For high tensile test temperatures, one sharp peak was observed, while low, wide peaks were present for low test temperatures. The shape of the DSC peak is strongly affected by the kinetics of the reverse transformation process. The reason of the high, sharp peak is that if the tensile test temperature is well above Ms, the deformation occurs mainly in the γ phase. Thus, all the ε phase is formed from a deformed γ phase during cooling after the tensile test. Since there is a strong orientation relationship between the γ and the ε phases, the ε phase will have a texture that results from the strong deformation texture of the γ phase. This is confirmed by the texture examinations, where strong {112} texture was observed for samples tensile tested at higher temperatures. Such a uniformly oriented ε structure results in discrete As and Af temperatures when subsequently heated, thus, a high, narrow DSC peak will be observed. On the other hand, if the tensile test temperature is below Ms, thermally induced ε and γ are present during the tensile tests. At the end of the tensile test, the orientation and dislocation density of the thermally induced ε will differ from the orientation and dislocation density of the ε that is formed from the deformed γ (thermally or mechanically). It was indeed observed, that the strong {112} texture decreased for lower testing temperatures. The mixture of the two types of ε results in a diffuse decomposition kinetic, that is, a low and wide DSC peak. The formation mechanism of the strong ε {112} texture and the effect of Cr content on the resulting texture of ε phase in the tensile tested samples require further texture investigations on finer scale (EBSD/TEM). But it was shown that for lower Cr content the {112} texture dropped with decreasing testing temperature, and for higher Cr content the {112} texture continuously decreased with decreasing tensile test temperature. Acknowledgements The research work presented in this paper was supported by the Hungarian Scientific Research Fund - OTKA K84065 project and the TÁMOP-4.2.1.B-10/2/KONV-2010-0001 project. References [1] K.D.H. Bhadeshia, R.W.K. Honeycombe, Steels: Microstructure and Properties, third ed., Elsevier, 2006. [2] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, 1998. [3] S. Kajiwara, Mat. Sci. Eng. A 273–275 (1999) 67–88. [4] M. Sade, A. Baruj, H. E. Troiani, Meeting: New Developments on Metallurgy and Applications of High Strength Steels, Buenos Aires, 2008. [5] L. Vitos, J. –O. Nilsson, B. Johansson, Acta Mater. 54 (2006) 3821–3826. [6] A. Dumay, J.-P. Chateau, S. Allain, S. Migot, O. Bouaziz, Mat. Sci. Eng. A 483–484 (2008) 184–187. [7] D. Barbier, N. Gey, S. Allain, N. Bozzolo, M. Humbert, Mat. Sci. Eng. A 500 (2009) 196–206. [8] L. Bracke, K. Verbeken, L. Kestens, J. Penning, Acta Mater. 57 (2009) 1512–1524. [9] H.E. Troiani, M. Sade, G. Bertolino, A. Baruj, ESOMAT 2009, 06002 (2009). [10] V. Mertinger, M. Benke, E. Nagy, T. Pataki, J. Mater. Eng. Perform. 23 (2014) 2347–2350. [11] E. Hornbogen, V. Mertinger, J. Spielfield, Z. Metallkd. 90 (1999) 318–322. [12] A. Baruj, A. Fernández Guillermet, M. Sade, Mat. Sci. Eng. A 273–275 (1999) 507–511. [13] A. Baruj, H.E. Troiani, M. Sade, A. Fernández Guillermet, Philos. Mag. A 80 (2000) 25372548. [14] M. Sade, A. Baruj, H.E. Troiani, Meeting: New Development on Metallurgy and Applications of High Strength Steels, Buenos Aires, 2008.