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Materials Science Forum Vols. 783-786 (2014) pp 27-32 Online available since 2014/May/23 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.783-786.27

Mechanical properties of high-manganese austenitic TWIP-type steel

DOBRZAŃSKI Leszek Adam^1a, BOREK Wojciech¹*³, MAZURKIEWICZ Janusz^1c

¹Division of Materials Processing Technology, Management and Computer Techniques in Materials Science, Institute of Engineering Materials and Biomaterials,

Silesian University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland ^[e-mail]; ^[e-mail], ^[e-mail]

Keywords: high-manganese steel, TWIP type steel, thermo-mechanical treatment,

recrystallisation, mechanical properties

Abstract. Taking into consideration increased quantity of accessories used in modern cars, decreasing car’s weight can be achieved solely by optimization of sections of sheets used for bearing and reinforcing elements as well as for body panelling parts of a car. Application of sheets with lower thickness requires using sheets with higher mechanical properties, however keeping adequate formability. The goal of structural elements such as frontal frame side members, bumpers and the others is to take over the energy of an impact. Therefore, steels that are used for these parts should be characterized by high value of UTS and UEl, proving the ability of energy absorption. Among the wide variety of recently developed steels, high-manganese austenitic steels with low stacking faulty energy are particularly promising, especially when mechanical twinning occurs. Beneficial combination of high strength and ductile properties of these steels depends on structural processes taking place during cold plastic deformation, which are a derivative of SFE of austenite, dependent, in turn on the chemical composition of steel and deformation temperature. High-manganese austenitic steels in effect of application of proper heat treatment or thermo-mechanical treatment can be characterized by different structure assuring the advantageous connection of strength and plasticity properties. Proper determinant of these properties can be plastic deformation energy supply determined by integral over surface of cold plastic deformation curve. Obtaining of high strength properties with retaining the high plasticity has significant influence for the development of high- manganese steel groups and their significance for the development of materials engineering.

Introduction

TRIP (TRansformation Induced Plasticity) and TWIP (TWinning Induced Plasticity) belong to the new groups of steels developed in the world in the last few years [1-3]. Newly developed steels, in effect of application of proper heat treatment or heat and plastic working can be characterized by different structure assuring the advantageous connection of strength and plasticity properties. Proper determinant of these properties can be the index equal to product of tension strength and maximum elongation, and the plastic deformation energy supply determined by integral over surface of cold plastic deformation curve. Comparison of results of plastic deformation energy supply tests for different group of steel, including the results of preliminary tests performed by the Team of that paper for high-manganese austenitic steels of TWIP type, for which this plasticity supply was on the level of 270 [MJ/m³] [4-9].

Interest in high manganese steels is a new issue, addressed for a limited number of scientific entities in the world [1-3,10-14]. The test results published so far, described below, mainly relate to steel in supersaturated state. In practice, processes occurring in the steel during heat treatment and hot plastic processing must be taken into consideration, as well as their influence on shaping of initial structure of steel subjected to further cold deformation. In high-manganese austenitic steels in result of cold plastic deformation, mechanisms of intensive twinning and martensitic transition are induced. Admittedly the effect of martensitic transition, in effect of plastic deformation can be obtained in austenitic steels Cr-Ni [15], but they are a relatively expensive material. Therefore, within the last years intensive research is conducted [1-14] on high manganese austenitic steels.

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These steels contain from 20 to 30% of manganese, from 0.01 to 0.6% C and 1-3% Al and 1-3% Si, introduced, among other things, in order to lower the thickness, which can even be below 7 g/cm for this group of steels that allows decreasing the produced components mass even down to 10%. Advantageous mixture of mechanical properties obtained by these steels, i.e. R_m = 600-1300 MPa, R_p0,₂ = 250-450 MPa, A_r = 35-80% strongly depends on the chemical composition, and in particular on Mn concentration [16-19].

Chemical composition is the factor controlling the stacking fault energy and in consequence, decides on the phase transitions and plastic deformation mechanisms. If SFE is low (<20mJ/m2), then the conditions are favourable for martensitic transition. Increase of SFE up to 25mJ/m² will suppresses the martensitic transition, but it will be favourable for mechanical twinning. Twinning occurs within the scope between 25 and 60mJ/m², however, its intensity changes together with stacking fault energy, correspondingly at low SFE value (« 25mJ/m ), the thickness of twins is high and they are evenly distributed as result of almost uniform plastic deformation. However, in case of high SFE values (> 60mJ/m ) the dissociation of dislocations into Shockley's dislocations is very difficult and therefore the dominating mechanism of strengthening is the total dislocation glide. Steels with intermediate SFE value have the tendency for simultaneous occurrence of mechanic twinning and dislocation glide [13-14].

Shaping the strength and plasticity properties greatly depends on the size and shapes of grains of the tested steel before cold plastic deformation. Along with grains size increase, steel elongation occurs even up to 80%, and strength properties decrease. Decreasing of grains size causes lowering of the plasticity properties with simultaneous increase of the plasticity and yield point of the tested steels. Also elaboration of the best correlation between the structure and newly developed groups of steel and strength and plasticity properties are significant and it allow obtaining the highest values of assumed plasticity supply as integral over the cold plastic deformation curve, i.e. energy possible to be accumulated in the operating conditions, especially impact loads[16,20-22].

Materials and experimental procedure

Examinations were carried out on high-manganese TWIP - X8MnSiAlNbTi25-1-3 steel. The chemical composition of steel were shown in Table 1. For investigated melt Nb and Ti microadditions were added in order to refine the structure and achieve precipitation hardening. Steel is characterized by high metallurgical purity, associated with low concentrations of S and P contaminants and gases. Melt were modified with rare earth elements.

Table 1. Chemical composition of new-developed high-manganese TWIP type steel, mass fraction

Steel designation	Chemical composition, mass fraction
	C
X8MnSiAlNbTi25-1-3	0.08

Based on the results obtained during thermo-mechanical treatment carried out in continuous axisymetrical compression test and multi-stage compression tests using the Gleeble 3800 thermo­mechanical simulator [4-9] allowed to work out a schedule of three different variants of hot-rolling of high-manganese austenitic steel (Fig. 1), where the processes eliminating the consequences of strain hardening were, respectively, dynamic recovery, dynamic, metadynamic and static recrystallisation.

Metallographic examinations of samples were carried out using the LEICA MEF4A light microscope. In order to reveal austenite structure, the samples etched in a mixture of nitrous and hydrochloric acid in various proportions. The structure of the investigated steel was also characterised using JEOL JEM 3010 transmission electron microscope working at accelerating voltage of 300 kV. TEM observations were carried out on thin foils. The specimens were ground down to foils with a maximum thickness of 80 |im before 3 mm diameter discs were punched from the specimens. The disks were further thinned by ion milling method with the Precision Ion

Polishing System (PIPS™), using the ion milling device (model 691) supplied by Gatan until one or more holes appeared. The ion milling was done with argon ions, accelerated by voltage of 15 kV.

Results and discussion

After hot-rolling according to scheme shown in figure 1, static tensile tests were performed in order to investigate mechanical properties, especially strain energy per unit volume of high- manganese austenitic TWIP type steel, with various structures after their thermo-mechanical treatment. This group of steels can be applied on constructional elements of the car body which should transfer loads during front or side impact collisions. On figures 2 and 3 are presented austenitic structures of high manganese steels with mechanical and micro twins and slip bands obtained after hot-rolling with a true strain 0.23 according to thermo-mechanical treatment variant No. I or II and after static tensile tests.

Fig. 2. Austenitic structures of high manganese X8MnSiAlNbTi25-1-3 steel with mechanical and micro twins and slip bands obtained after hot-rolling with a true strain 0.23 (Variant No. II) and after static tensile tests

Fig. 4. Influence of various parameters of thermo-mechanical treatment on mechanical properties of X8MnSiAlNbTi25-1-3 high manganese austenitic TWIP type steel: yield stress R_p02, tensile strength R_m and uniform elongation s_um

Fig. 5. Representative tensile curve of investigated TWIP, type steel with designated strain energy per unit volume after static tensile test

Figure 4 present results of static tensile tests which were performed in order to investigate mechanical properties of new developed high-manganese austenitic TWIP type steel, with various structures after various variants of thermo-mechanical treatments. Mechanical twinning induced by the cold working of the high-manganese austenitic TWIP steel has a significant effect on forming their structure and mechanical properties. Twinning induced by the cold working of the high-

manganese austenitic steels results in growth of the strain energy per unit volume after the successive cold deformation. On figure 5 is presented representative tensile curve of the TWIP type steel with designated strain energy per unit volume after the cold deformation equal 263.33 MJ/m³. To increase strain energy per unit volume the temperature of plastic deformation should be decrease below ambient temperature it can be achieved also by increasing the strain rate of cold plastic deformation, for instance for TWIP type steel acceleration of deformation to 500 s^-1 causes that strain energy per unit volume increases by approximately 160 MJ/m³ (Fig. 6) in comparison with energy determined in static condition (Fig. 5). The high-manganese austenitic steels with the properly formed structure and properties and especially with the big strain energy per unit volume yield the possibility to be used for the constructional elements of cards affecting advantageously the passive safety of the vehicles' passengers.

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400 200 0

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50

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Fig. 6. Representative tensile curve of investigated TWIP type steel with designated strain energy per unit volume after dynamic tensile test with a strain rate 500s^-1

Conclusions

High-manganese X8MnSiAlNbTi25-1-3 austenitic TWIP type steel provide an extensive potential for automotive industries through exhibiting the twinning induced plasticity (TWIP) mechanisms. Results obtained for high-manganese austenitic steels with the properly formed structure and properties in the thermo-mechanical processes indicate the possibility and purposefulness of their employment for constructional elements of vehicles, especially of the passenger cars to take advantage of the significant growth of their strain energy per unit volume which guarantee reserve of plasticity in the zones of controlled energy absorption during possible collision resulting from activation of twinning for TWIP type steels, induced by cold working which may result in significant growth of the passive safety of these vehicles' passengers.

Acknowledgements

Project was founded by the National Science Centre based on the decision number DEC-2012/05/B/ST8/00149.

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THERMEC 2013

10.4028/www.scientific.net/MSF.783-786

Mechanical Properties of High-Manganese Austenitic TWIP-Type Steel

10.4028/www.scientific.net/MSF.783-786.27

DOI References

2. G. Frommeyer, U. Brüx, P. Neumann, Supra-ductile and high-strength manganese TRIP/TWIP steels for high energy absorption purposes, ISIJ International 43 (2003) 438-446. http://dx.doi.org/10.2355/isijinternational.43.438

3. O. Grâssel, L. Krüger, G. Frommeyer, L.W. Meyer, High strength Fe-Mn-(Al, Si) TRIP/TWIP steels development - properties - application, International Journal of Plasticity 16 (2000) 13911409. http://dx.doi.org/10.1016/S0749-6419(00)00015-2

5. L.A. Dobrzański, W. Borek, Hot-rolling of advanced high-manganese C-Mn-Si-Al steels, Materials Science Forum 706/709 (2012) 2053-(2058). http://dx.doi.org/10.4028/www.scientific.net/MSF.706-709.2053

6. L.A. Dobrzański, W. Borek, Hot-Working Behaviour of Advanced High-Manganese C-Mn-SiAl Steels, Materials Science Forum 654-656 (2010) 266-269. http://dx.doi.org/10.4028/www.scientific.net/MSF.654-656.266

9. L.A. Dobrzański, A. Grajcar, W. Borek, Microstructure evolution of C-Mn-Si-Al-Nb highmanganese steel during the thermomechanical processing, Materials Science Forum 638 (2010) 3224-3229. http://dx.doi.org/10.4028/www.scientific.net/MSF.638-642.3224

12. O. Kwon, K. Lee, G. Kim, K. Chin, New trends in advanced high strength steel developments for automotive application, Materials Science Forum 638-642 (2010) 136-141. http://dx.doi.org/10.4028/www.scientific.net/MSF.638-642.136

15. S. Ganesh Sundara Raman, K.A. Padmanabhan, Tensile deformation-induced martensitic transformation in AISI 304LN austenitic stainless steel, Journal of Materials Science Letters 13 (1994) 389-392. http://dx.doi.org/10.1007/BF00420808

18. D. Barbier, N. Gey, S. Allain, N. Bozzolo, M. Humbert, Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions, Materials Science and Engineering A 500 (2009) 196­206.

http://dx.doi.org/10.1016/j.msea.2008.09.031

18. L. Bracke, K. Verbeken, L. Kestens, J. Penning, Microstructure and texture evolution during cold rolling and annealing of a high Mn TWIP steel, Acta Materialia 57 (2009) 1512-1524. http://dx.doi.org/10.1016/j.actamat.2008.11.036

19. A. Grajcar, R. Kuziak, Softening kinetics in Nb-microalloyed TRIP steels with increased Mn content, Advanced Materials Research 314-316 (2011) 119-122. http://dx.doi.org/10.4028/www.scientific.net/AMR.314-316.119