structure and phase composition of newly developed high

NR 2/2017 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 69

Structure and phase composition of newly developed high manganese X98MnAlSiNbTi24–11 steel

of TRIPLEX typeLiwia Sozańska-Jędrasik, Janusz Mazurkiewicz*, Wojciech Borek,

Leszek A. Dobrzański1Instytut Materiałów Inżynierskich i Biomedycznych, Politechnika Śląska, Gliwice, Polska, *[email protected]

The work presents the results of investigations of the structure and phase composition of newly developed high manganese X98MnAlSiNbTi24–11 steel of TRIPLEX type. The average density of such steel is 6.67 g/cm3, which is less than for typical structural steels by even 15%. A preliminary analysis of phase composition and structure allows to find an austenitic γ-Fe(Mn, Al, C) structure in the investigated steel with uniformly distributed ferritic α-Fe(Mn, Al) ar-eas elongated towards the boundaries of austenite grains and numerous carbides with differentiated chemical composition and varied size. Nb- and Ti-based complex carbides are dominant in the steel. The investigations of the chemical composition of the carbides revealed in the matrix allow to identify with high probability dispersive κ-(Fe, Mn)3AlC carbides with the nanometric size of approx. 10÷160 nm, which has to be yet confirmed with electron transmission microscopy methods. Fe, Mn and Al as well as small amounts of Nb, Ti and Si are contained in such carbides. The occurrence of aluminium carbonitrides with a fraction of Nb and Ti was also revealed. The size of the above Nb and Ti carbides revealed in solid specimens in the matrix of the studied steel is between approx. 10 nm to 15 μm. X-ray diffraction examinations of carbide isolates prepared by the method of chemical dissolution in HCl showed the ex-istence of NbTiC2 carbides in the studied steel. The diffraction examinations of solid specimens revealed, apart from austenite and ferrite, also the existence of TiC carbides and such initially classified as Mn3.6C0.4 type.

Key words: high manganese steels, microstructure, phase composition, NbTiC2, TiC, κ-(Fe, Mn)3AlC.

Inżynieria Materiałowa 2 (216) (2017) 69÷76DOI 10.15199/28.2017.2.2© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

High manganese steels have become well established in the area of research over high-strength steels which at the same time main-tain high plastic properties. Multiple new steel grades containing high contents of manganese (≥8 wt. %) have been established in the recent years, which are usually classified into three groups de-pending on the chemical composition and structural mechanisms fundamental for achieving the optimum mechanical properties, and are termed TRIP, TWIP and TRIPLEX. High manganese steels are a promising material in terms of its application wherever, apart from high material strength, high plasticity is also required. As far as numerous elaborations have been prepared for TRIP and TWIP steels by various research institutions, describing their structure, in-cluding phase composition in relation to their reinforcement mecha-nisms, there is still scarce knowledge about multiphase TRIPLEX steels, though, in respect of their structure and correlation between their properties and reinforcement mechanisms. The high manga-nese steels described until now, in which the total content of Al and Si should not be less than 12%, have a structure composed of austenite γ-Fe(Mn, Al, C) grains, ferrite α-Fe(Mn, Al) grains and dispersion carbon κ-(Fe, Mn)3AlC1 – x precipitates. A dislocation slip, twinning, transformation of austenite into martensite ε and/or martensite αʹ occur during their plastic deformation, depending on the chemical composition of steels and deformation conditions (rate and temperature), which ensures indirect properties for such steels in relation to TRIP and TWIP steels with considerably lower density (by even 17%) [1÷17].

The research teams of the Institute of Engineering Materials and Biomaterials of the Silesian University of Technology, headed by Professor L. A. Dobrzański and Professor A. Grajcar, have con-ducted — for more than ten years — research over high manganese TRIP and TWIP steels. The research topic discussed in the work results from the continuation of such works, and expands them to

include investigations into newly developed multiphase TRIPLEX steels [1÷3, 10, 15÷16].

Steel with high content of manganese can be used in parts critical for safety by integrating high strength properties and good plastic-ity, and by ensuring high absorption of energy by structural parts, whose task is to absorb energy during an abrupt deformation, e.g. in cars during a road collision [1÷12].

If more than 5% of Al is added to high manganese steels, this supports the precipitation of nanodimensional ordered M3C — (Fe, Mn)3AlC carbides (so-called κ carbides), which control the strength properties of this group of steels. Considering that such par-ticles have a regular, face-centred cubic lattice (fcc), the phase sepa-ration plane of the matrix/κ transformation is strongly dependent on the matrix structure, i.e. a regular face-centred austenite lattice (fcc) or a regular body-centred ferrite lattice (bcc) [17]. In the group of high manganese steels, TRIPLEX steels have a high yield point (700÷1000 MPa), whereas elongation and tensile strength properties rank between TWIP and TRIP steels. The mechanical properties of such steels, as indicated above, are determined to high extent by the location, size and morphology of κ-(Fe, Mn)3AlC carbides. κ car-bides may also be a reason for the occurrence of steel brittleness dur-ing plastic deformation at room temperature, when they are formed on the grain boundaries as large precipitates [1÷10, 13÷17].

2. THE AIM OF THE WORK

The aim of this work was to analyse the structure, includ-ing phase composition, of newly developed high manganese X98MnAlSiNbTi24–11 steel of TRIPLEX type containing Ti and Nb alloy additives with the high content of carbon, being a starting point for further investigations in correlation to mechanical prop-erties and reinforcement mechanisms of such steel with regard to other steels in this group and to steels with similar chemical compo-sition without the above-mentioned alloy additives.

70 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

3. RESEARCH MATERIAL AND METHODOLOGYThe studied material was newly developed high manganese X98MnAlSiNbTi24–11 steel of TRIPLEX type containing of car-bon, Mn, and Al and additives of Nb and Ti. The detailed chemical composition of the studied steel is presented in Table 1.

The studied steel was melted in a laboratory vacuum induction furnace of VSG-50 type produced by Balzers, then it was poured into a cast-iron ingot mould in the atmosphere of argon. The hot plastic working of the ingot was performed after cooling in the air, with open die forging with a high-speed hydraulic press with the load of 300 tonnes. Forging temperature ranged between 1200 and 900°C with reheating between operations so that the material temperature was not below 900°C. 32 mm thick and 20 cm wide section-shaped specimens for structural tests were cut out from the material after forging.

The density of the studied steel was established by two methods: hydrostatic method, and also by measuring the mass and dimen-sions of the specimen using the formula for density.

The specimens for structural examinations were prepared in two ways. Some specimens for structural examinations in a light microscopy (LM) and scanning electron microscope (SEM) were embedded, then ground and polished mechanically with sandpapers and discs wetted with a diamond suspension with the maximum grain size of 1 µm. 5% of HNO3 solution in ethyl alcohol (nital) was used as a reagent to reveal the structure. The time of etching was about 10÷70 s. The specimens for diffraction examinations by EBSD technique in a scanning electron microscope were ground with a grinder-polisher with sandpapers and then polished electro-chemically in a reagent with the following composition: – 950 ml of 99% hydrochloric acid (CH3COOH), – 50 ml of 60% tetraoxochloric acid (HClO4).

The isolates of carbide phases were made to determine the per-centage volume fraction and type of the released carbides in the examined steel. Carbide isolates were made by dissolving a ma-trix of steel specimens with the mass of 45, 57 and 68 g placed in a solution of concentrated hydrochloric acid (HCl) at an elevated temperature and were assisted with ultrasounds. The dissolution methodology and conditions are shown in Table 2. The suspen-sions produced were decanted six times with distilled water. The final rinsing was performed three times using ethyl alcohol. The produced suspension of molecules was filtrated with Nalgene PS Filter Holder and Receiver 250. The sediment was left for drying. After drying the sediment to the value of below 0.5% of moistness, it was weighed and the average percentage content of carbides in the examined steel was estimated.

The structure of the investigated steel was observed with a light microscope, images of the steels structure have been taken with the magnifications of 100÷1000×, and also in a scanning electron mi-croscope, at the accelerating voltage of 5 to 20 kV using secondary electrons (SE), with the magnifications of 50÷120 000×. An EDS

detector and a camera for diffraction tests connected to the above microscope in Trident XM4 system by Edax were used to examine the chemical and phase composition of the specimen microareas, precipitates and particles. EBSD research was carried out at the ac-celerating voltage of 20 kV, working distance of 15 mm and step size of 0.15 µm. Sample was tilted 70 degree. The austenite and fer-rite grain size was estimated by average equivalent grain diameter.

An X-ray phase analysis was carried out with an X-ray X’Pert PRO diffractometer by PANalytical equipped with a strip detec-tor, PIXcel3D, to identify phases existing in the examined steel and carbides from carbide isolates. The parameters of X-ray diffraction examinations are shown in Table 3.

4. RESULTS AND THE DISCUSSION OF RESULTS

X98MnAlSiNbTi24–11 steel of TRIPLEX type is characterised by an austenitic-ferritic structure with the fraction of carbides, which was confirmed on the basis of structure observations with the EBSD technique in a scanning electron microscope and with examinations with an X-ray diffractometer. The examinations of the studied steel performed by means of EBSD, shown in Figure 1, allow to con-firm the distribution and location of the particular above-mentioned structure components together with their morphology and size.

Examinations with a light microscope show that uniformly dis-tributed 1÷14 µm wide and up to 43 µm long ferritic areas elon-gated towards the grain boundaries of austenite are visible on the boundaries of austenite grains (Fig. 2 and 3). The austenite grain equivalent diameter is within the range of 26 µm to 186 µm.

Scarce ferritic areas in austenite grains can also be noticed.A higher concentration of aluminium (10.76%) has influence on

the presence of ferrite in such steel, and also, acc. to the literature data [3], on austenite stacking fault energy (SFE). A properly pre-pared and performed process of steel melting in a vacuum furnace and a mixture of mischmetal (~50% Ce, ~20% La, ~20% Nd) in-troduced into steel have ensured high metallurgical purity of the studied steel. Owing to the amount of phosphorus and sulphur of below 0.002%, the fraction of non-metallic inclusions is low, which was confirmed in a performed analysis of non-metallic inclusions on specimens which did not undergo etching. A volume fraction of non-metallic inclusions in the studied X98MnAlSiNbTi24–11 steel is 0.4%.

A relatively high fraction of Nb and Ti in the studied steel, and a high fraction of carbon are supportive to the presence of carbides with a different morphological form and strongly diversified size in the structure (Fig. 4).

The density examinations of X98MnAlSiNbTi24–11 steel with two methods have shown that the average density is 6.67 g/cm3, which is much lower than for typical structural steels by even up to 15% [21].

Figures 5 and 6 present the results of EDS analysis in the points and for selected representative areas of a specimen in the initial state of X98MnAlSiNbTi24–11 steel. The results of examinations in the microarea 1 (Fig. 5, 6a) allow to determine the concentration Table 1. Chemical composition newly developed high manganese

X98MnAlNbTi24–11 steel, wt. %Tabela 1. Skład chemiczny nowoopracowanej stali wysokomanganowej X98MnAlNbTi24–11, % mas.

C Mn Al Si Nb Ti Ce La Nd Pmax Smax

0.98 23.83 10.76 0.20 0.048 0.019 0.029 0.006 0.018 0.002 0.002

Table 2. Dissolution conditions of steel in HCl to isolate carbides Tabela 2. Warunki rozpuszczania stali w HCl w celu wyizolowania wę-glików

Time, hour Temperature, °C Other

35 room temperature —

12.5 60 a process aided by ultrasound

Table 3. Parameters of X-Ray diffraction examinationsTablica 3. Parametry badań rentgenowskich

Parameters of examinations Steel X98MnAlSiNbTi24–11 Carbide isolates

Angular range, deg 40÷120 20÷120

Step size, deg 0.026

San step time, s 10.2 (30) 30

Lamp cobalt, λ = 1.78901 Å

Tension, kV 40

Current, mA 30


Fig. 1. Microstructure revealed by EBSD technique in SEM in the se-lected microarea of the investigated X98MnAlSiNbTi24–11 steel: a) IQ map, b) EBSD phase mapRys. 1. Mikrostruktura ujawniona techniką EBSD w SEM w wybranym mikroobszarze badanej stali X98MnAlSiNbTi24–11: a) obraz SEM, b) analiza składu fazowego obszaru z rysunku (a)

Fig. 2. Austenitic-ferritic microstructure of high manganese X98MnAlSiNbTi24–11 steel: a) in the initial state, b) details from Fig-ure (a); LMRys. 2. Mikrostruktura austenityczno-ferrytyczna stali wysokomangano-wej X98MnAlSiNbTi24-11 a) w stanie wyjściowym, b) szczegóły z rysun-ku (a); LM

Fig. 3. Austenitic-ferritic microstructure of high manganese X98MnAlSiNbTi24–11 steel: a) in the initial state, b) details from Figure (a); SEMRys. 3. Mikrostruktura austenityczno-ferrytyczna stali wysokomanganowej X98MnAlSiNbTi24-11: a) w stanie wyjściowym, b) szczegóły z rysunku (a); SEM

of Mn, Al and Si in the quantity consistent with the tests of solid specimens from a ladle chemical analysis.

Analysis of the results of chemical composition (Fig. 5, 6) in-dicates the presence of carbides. It was revealed, when examining the chemical composition of carbides shown in Figure 5, that these carbides have a complex chemical composition. Nb- and Ti-based complex carbides are dominant. The presence of Mn, Fe and Al carbides in chemical composition is connected with the excitation of a signal coming from the matrix, sometimes around the carbide


Fig. 4. Microstructure of the studied steel with presence of carbides; SEMRys. 4. Mikrostruktura badanej stali z ujawnionymi węglikami; SEM

Fig. 5. Microstructure of X98MnAlSiNbTi24–11 steel with presence of carbides and nitrides together with marked places of chemical compo-sition measurement by EDS; SEMRys. 5. Mikrostruktura stali X98MnAlSiNbTi24–11 z ujawnionymi wę-glikami i azotkami wraz z zaznaczonymi miejscami badania składu che-micznego za pomocą EDS; SEM

Fig. 6. EDS spectra from the Figure 5: a) area 1, b) point 2, c) point 3, d) point 4, e) point 5Rys. 6. Widma EDS zarejestrowane dla: a) obszaru 1, b) punktu 2, c) punktu 3, d) punktu 4, e) punktu 5 z rysunku 5


or from areas lying directly underneath. For such examinations of carbides carried out to identify their correlation stability of elements in the chemical composition, examinations have to be continued with a beam of electrons with lower energy to minimise the risk of exciting the signals from a matrix, examinations of carbide isolates by EDS (as presented below) or chemical composition examina-tions of carbides by transmission electron microscopy (TEM), as is planned in the further. A surprising examination result was re-vealed in a point 5 in Figure 5. The presence of probably complex aluminium carbonitrides with a fraction of Nb and Ti was found in this point, as confirmed later in the next examination areas. A ladle chemical analysis did not show the content of nitride in the exam-ined steel, which is indicative for the research team that the origin of nitride should be thoroughly reviewed, as such nitrogen enabled the formation of such phases, even more considering that the melt is from a vacuum furnace. The size of the above carbides present in solid specimens in the matrix of the studied steel is between ap-prox. 10 nm to 15 μm. Dispersive κ-(Fe, Mn)3AlC carbides with the nanometric size of 10÷160 nm were also probably revealed during the examinations, which needs to be confirmed with transmission electron microscopy methods (Fig. 7, 8). Fe, Mn and Al as well as small amounts of Nb, Ti and Si are contained in such carbides. Their size, composition and morphology differ significantly from the above-mentioned Nb- and Ti-based carbides.

The investigations of carbides isolated from the matrix of the studied steel give a more complete picture of their morphology, size and chemical composition, as well as a volume fraction (Fig. 9a, b, 10). As compared to steels with a similar chemical composition without the content of strongly carbide forming elements Nb and Ti, the fraction is higher by several dozens of times, up to 50×. It was noted that the isolation of carbides based on chemical quench annealing of a matrix in HCl is related to the dissolution of less du-rable carbide phases such as TiC, which were clearly identified by an X-ray method in the matrix.

This is a hint for further research over carbides in such types of steels not to trust one method of isolation without verifying it with other methods. In the next stage it is planned to isolate carbides in the studied steel with the electrochemical method. The compari-son of the results obtained will be presented in further works for the examined steel. It will be cognitively interesting to compare the properties of steel with such a rich fraction of carbides to base steel containing no Nb and Ti. Simple and obvious analogies are possible here connected with a high fraction of carbides in the examined steel and their effect on reinforcement and plastic properties, but a full picture will be available after finishing the planned strength and plastometric examinations of the steels being compared. An analysis of the isolated carbides only to some extent correlates with quantitative analyses on microsections (etched and unetched). There are no complex TiC carbides in the isolated carbides with the quantitative prevalence of Ti in its chemical composition. A very high concentration of niobium carbides with titanium, of over 80%, is clearly seen for carbides, with a clearly dominant quantitative concentration of niobium, including carbide.

The morphology and size of such carbides is strongly differenti-ated. A small fraction of oxygen and aluminium represents probably the residues of matrix dissolution processes. Interesting results were obtained for an analysis of the smallest isolated carbides extracted from a homogenised ether solution on platinum sputtered glass and on the polished pure copper (Fig. 11). It is clear that the carbides have irregular cubical shape. The size of those that were managed to be measured was between several nanometres to 160 nm.

The examinations performed with an X-ray diffractometer al-lowed to identify the phase composition of the investigated steel and compatible. the results obtained during structural and EBSD examinations (Fig. 12). Reflexes from austenite and ferrite and TiC carbides were found in a solid specimen on a diffraction pattern.

Additional reflexes were also identified, which were initially interpreted as Mn3.6C0.4 phase [22]. However, as no additional

information was available in the literature allowing for such iden-tification in such type of steels, further diffraction examinations with TEM are required. An X-ray examination of carbides (Fig. 13) isolated with the chemical dissolution method in HCl confirmed the existence NbTiC2 carbides only. This means that some of the carbides revealed when examining solid specimens were dissolved for the chosen method of isolation. NbTiC2 carbides are also crystal-lising as TiC in a regular face-centred lattice (fcc), but a higher frac-tion of carbon in a cell raises their chemical resistance to the activity of HCl. A gravimetric analysis of the obtained NbTiC2 carbides al-lowed to estimate their fraction in the studied steel at approx. 1.3%.

Fig. 7. Microstructure of X98MnAlSiNbTi24–11 steel with presence of κ-(Fe, Mn)3AlC carbides together with marked places of chemical composition measurement by EDS; SEMRys. 7. Mikrostruktura stali X98MnAlSiNbTi24–11 z ujawnionymi wę-glikami κ-(Fe, Mn)3AlC wraz z zaznaczonymi miejscami badania składu chemicznego za pomocą EDS; SEM

Fig. 8. EDS spectra from: a) Figure 7 point 1, b) Figure 7 point 2Rys. 8. Widma EDS dla: a) punktu 1, b) punktu 2 z rysunku 7


Fig. 9. Carbides isolated from X98MnAlSiNbTi24–11 steel matrix with marked places of chemical composition measurement by EDS in the indicated microareas; SEMRys. 9. Węgliki wyizolowane z osnowy stali X98MnAlSiNbTi24–11 wraz z zaznaczonymi miejscami badania składu chemicznego za pomocą EDS; SEM

Fig 10. EDS spectra from: a) Figure 9a, b) Figure 9bRys. 10. Widma EDS dla: a) obszaru 1, b) obszaru 2 z rysunku 9

Fig. 11. Carbides isolated from X98MnAlSiNbTi24–11 steel matrix on glass plate; SEMRys. 11. Węgliki wyizolowane z osnowy stali X98MnAlSiNbTi24–11 na płytce szklanej; SEM

Fig. 12. X-ray diffraction pattern of solid X98MnAlSiNbTi24−11 steel specimenRys. 12. Dyfraktogram rentgenowski z litej próbki stali X98MnAlSiNb-Ti24−11

Fig. 13. X-ray diffraction pattern of NbTiC2 carbides isolated from X98MnAlSiNbTi24–11 steelRys. 13. Dyfraktogram rentgenowski węglików NbTiC2 wyizolowanych ze stali X98MnAlSiNbTi24–11


5. SUMMARY – The structure of X98MnAlSiNbTi24–11 steel of TRIPLEX type

in the input state is represented by austenite, ferrite and carbides. Ferritic areas are uniformly distributed on the boundary of aus-tenite grains. Relatively small ferritic areas also occur sometimes inside austenite grains.

– The achieved X98MnAlSiNbTi24–11 steel of TRIPLEX type is characterised by high metallurgical purity, owing to an appropri-ately prepared and performed steel melting process in a vacuum furnace and by introducing a mischmetal mixture into the steel. It is surprising that also few carbonitrides were revealed during chemical composition examinations, which requires an addition-al thorough analysis on the origin of nitrogen, which allowed to produce such phases.

– The results of diffraction and chemical composition examina-tions in microareas using EDS in a scanning electron microscope have confirmed the above-mentioned phase composition of the studied steel and allowed to reveal complex carbides in the ma-trix of the studied steel, mainly based on Nb and Ti, and also, with high probability, κ-(Fe, Mn)3AlC carbides.

– The density of X98MnAlSiNbTi24–11 steel is even up to 15% lower compared to the density of typical structural steels, and accounts for 6.67 g/cm3 [21].

– Phase composition in the form of austenite (γ), ferrite (α) and TiC carbides was identified in the solid specimen of the exam-ined steel based on the results of examinations with an X-ray dif-fractometer. Additional reflexes were also revealed during inves-tigations, and additional examinations and analyses are needed, notably with TEM, to interpret them, and they have been initially classified as Mn3.6C0.4 carbides [22].

– A methodology of isolation was taken into account as a result of examining carbide isolates and such isolation needs to consider the problems of partial dissolution of selected carbide phases in concentrated HCl, as was the case here. The literature reports [18, 19] provide that an alternative method, decreasing a risk of dissolving carbide phases existing in the examined steels during isolation, is the electrochemical carbides isolation method.

– The NbTiC2 phase with the fcc lattice was only identified on X-ray diffraction patterns from the obtained isolates, which un-like carbides (Nb, Ti)C with the fcc lattice.

ACKNOWLEDGEMENTS

Scientific work was financed in the framework of project funded by the National Science Centre based on the decision number DEC-2012/05/B/ST8/00149.

REFERENCES

[1] Eipper K., Frommeyer G., Fussnegger W., Gerick A., Kleineathoefer W.: High-strength DUPLEX/TRIPLEX steel for lightweight construc-tion and use thereof. United States Patent Application Publication US 2007/0125454 A1 (2007).

[2] Dobrzański L. A., Borek W.: Mechanical properties and microstructure of high-manganese TWIP, TRIP and TRIPLEX type steels. Journal of Achievements in Materials and Manufacturing Engineering 55/2 (2012) 230÷238

[3] Grajcar A.: Nowoczesne stale wysokowytrzymałe dla motoryzacji II gene-racji. Stal Metale & Nowe Technologie 7/8 (2013) 10÷13.

[4] Mazancová E., Jonšta Z., Mazanec K.: Structural metallurgy properties of high manganese Fe–Mn–Al–C alloy. Hutnické listy 2 (2008) 60÷63.

[5] Kim H., Suh D., Kim N. J.: Fe–Al–Mn–C lightweight structural alloys: a review on the microstructures and mechanical properties. Science and Technology of Advanced Materials 14 (2013) 1÷11.

[6] Etienne A., Massardier-Jourdan V., Cazottes S., Garat X., Soler M., Zuazo I., Kleber X.: Ferrite effects in Fe–Mn–Al–C triplex steels. Metallurgical and Materials Transactions 45A (2014) 324÷334.

[7] Niewielski G., Kuc D., Cebulski J., Lalik S.: Microstructure and selected properties of Mn–Al duplex steels. Archives of Materials Science and En-gineering 47/1 (2011) 11÷18.

[8] Bausch M., Frommeyer G., Hofmann H., Balichev E., Soler M., Didier M., Samek L.: Ultra high-strength and ductile FeMnAlC light-weight steels (MnAl-steel). European Commission Research Fund for Coal and Steel; Final report Grant agreement RFSR-CT-2006-00027 (2013) 5÷157.

[9] Kozłowski R. H.: Composite of austenitic-ferritic stainless steel. Journal of Materials Processing Technology 53 (1995) 239÷246.

[10] Dobrzański L. A., Borek W.: Hot-working of advanced high-manganese austenitic steels. Journal of Achievements in Materials and Manufacturing Engineering 43/2 (2010) 507÷526.

[11] Sohn S. S., Song H., Suh B. C., Suh J. C., Kwak J. H., Lee B. J., Kim N. J., Lee S.: Novel ultra-high-strength (ferrite + austenite) duplex lightweight steels achieved by fine dislocation substructures (Taylor lattices), grain re-finement, and partial recrystallization. Acta Materialia 96 (2015) 301÷310.

[12] Takahashi M.: Development of high strength steels for automobiles. Nip-pon Steel Technical Report 88 (2003) 2÷7.

[13] Niewielski G., Hetmańczyk M., Kuc D.: Influence of the initial grain size and deformation parameters on the mechanical properties during hot plastic deformation of austenitic steels. Inżynieria Materiałowa 24 (2003) 795÷798.

[14] Mazancová E., Jonšta Z., Mazanec K.: Properties of high manganese Fe–Mn–Al–C alloys. Archives of Materials Science 28/1-4 (2007) 90÷94.

[15] Dobrzański L. A., Grajcar A., Borek W.: Microstructure evolution and phase composition of high-manganese austenitic steels. Journal of Achievements in Materials and Manufacturing Engineering 31/2 (2008) 218÷225.

[16] Dobrzański L. A., Grajcar A., Borek W.: Influence of hot-working con-ditions on a structure of high-manganese austenitic steels. Journal of Achievements in Materials and Manufacturing Engineering 29/2 (2008) 139÷142.

[17] Gutierrez-Urrutia I., Raabe D.: High strength and ductile low density aus-tenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale or-dered carbides. Materials Science and Technology 30/9 (2014) 1099÷1104.

[18] Pietryka I., Lisiak J.: The identification of carbide phases by XRD analysis as the method of assess the extent of the steel damage after long time in service. Archives of Foundary Engineering 3 (2010) 283÷288.

[19] Richter J., Wyciślik A., Cwajna J.: Preparation of carbide isolates of new high-speed steel for analysis by atomic absorption spectrometry. Mikro-chimica Acta 130 (1998) 117÷122.

[20] Kucharska B., Krzywiecki M., Nowak T.: Structure and TRIP effect in AlSi300 steels after tensile deformation. Inżynieria Materiałowa 6 (2015) 409÷413.

[21] Ashby M. F.: Dobór materiałów w projektowaniu inżynierskim. Wydawnictwo Naukowo-Techniczne, Warszawa (1998).

[22] Tirumalasetty G. K., Fang C. M., Jansen J., Yokosawa T., Boeije M. F. J., Sietsma J., van Huis M. A., Zandbergen H. W.: Structural tale of two novel (Cr, Mn)C carbides in steel. Acta Materialia 78 (2014) 161÷172.


Struktura i skład fazowy nowoopracowanej stali wysokomanganowej X98MnAlSiNbTi24-11

typu TRIPLEXLiwia Sozańska-Jędrasik, Janusz Mazurkiewicz*, Wojciech Borek, Leszek A. Dobrzański

Instytut Materiałów Inżynierskich i Biomedycznych, Politechnika Śląska, Gliwice, *e-mail: [email protected]

Inżynieria Materiałowa 2 (216) (2017) 69÷76DOI 10.15199/28.2017.2.2© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: stale wysokomanganowe, mikrostruktura, skład fazowy, NbTiC2, TiC, κ-(Fe, Mn)3AlC.

1. CEL PRACY

Celem pracy była charakterystyka struktury i składu fazowego nowo-opracowanej stali wysokomanganowej X98MnAlSiNbTi24–11 typu TRIPLEX zawierającej dodatki stopowe Ti i Nb przy dużej zawar-tości węgla.

2. MATERIAŁ I METODYKA BADAŃ

Badano stal wysokomanganową X98MnAlSiNbTi24–11 typu TRI-PLEX o składzie chemicznym przedstawionym w tabeli 1. Gęstość stali określono metodą hydrostatyczną, a także na podstawie pomiaru masy i wymiarów próbki. W celu określenia procentowego udzia-łu masowego oraz rodzaju wydzielonych węglików w badanej sta-li wykonano izolaty faz węglikowych przez rozpuszczenie osnowy stalowych próbek w roztworze stężonego HCl (tab. 2). Obserwacje mikrostruktury wykonano na mikroskopie świetlnym AxioObserver firmy Zeiss oraz w skaningowym mikroskopie elektronowym SU-PRA 35 z wykorzystaniem elektronów wtórnych (SE). Skład che-miczny w mikroobszarach badano za pomocą EDS. Rentgenowska jakościowa analiza fazowa została wykonana na dyfraktometrze rentgenowskim X’Pert PRO firmy PANalytical wyposażonym w de-tektor paskowy PIXcel3D (tab. 3).

3. WYNIKI I ICH DYSKUSJA

Stal X98MnAlSiNbTi24–11 typu TRIPLEX charakteryzuje mikro-struktura austenityczno-ferrytyczną z udziałem węglików, co potwier-dzono na podstawie obserwacji z wykorzystaniem techniki EBSD w SEM oraz badań za pomocą dyfraktometru rentgenowskiego. Na podstawie badań EBSD (rys. 1) określono rozmieszczenie poszcze-gólnych składników struktury wraz z ich morfologią i wielkością.

Badania z wykorzystaniem mikroskopu świetlnego ujawniły, że na granicach ziaren austenitu są widoczne równomiernie roz-mieszczone, wydłużone w kierunku granic ziaren austenitu, obsza-ry ferrytyczne o szerokości 1÷14 μm i długości nawet do 43 µm (rys. 2 i 3). Średnia równoważna średnica ziaren austenitu mieści się w przedziale od 26 μm do 186 μm. Można zauważyć także nie-liczne obszary ferrytyczne w ziarnach austenitu.

Dość duża zawartość w badanej stali Nb i Ti oraz C sprzyjają ujawnieniu w strukturze węglików o różnej postaci morfologicznej i silnie zróżnicowanej wielkości (rys. 4).

Badania gęstości stali X98MnAlSiNbTi24–11 wykazały, że wy-nosi ona 6,67 g/cm3. Co jest wartością znacznie mniejszą niż dla typowych stali konstrukcyjnych nawet o 15%.

Wyniki analizy EDS osnowy w mikroobszarze 1 (rys. 5, 6) pozwa-lają stwierdzić udział Mn, Al i Si w ilości zgodnej z badaniami próbek litych. Badając skład chemiczny węglików na rysunku 5 wykazano, że dominują węgliki złożone na bazie Nb i Ti. Wielkość węglików wynosi od 10 nm do 15 μm. Ujawniono również dyspersyjne węgli-ki κ-(Fe, Mn)3AlC o nanometrycznej wielkości 10÷160 nm (rys. 7),

jednak wymaga to potwierdzenia metodami TEM. W skład tego typu węglików wchodzi Fe, Mn i Al oraz niewielkie ilości Nb, Ti i Si. Ich wielkość, skład i morfologia różnią się zdecydowanie od węglików na bazie Nb i Ti.

Wykonane badania za pomocą SEM węglików wyizolowanych z osnowy badanej stali dają pełniejszy obraz odnośnie do ich mor-fologii, wielkości i składu chemicznego, a także udziału masowego (rys. 9, 10). W porównaniu ze stalą o zbliżonym składzie chemicz-nym bez zawartości silnie węglikotwórczych pierwiastków Nb i Ti udział węglików jest nawet do 50× większy.

Węgliki, które zostały wyekstrahowane z homogenizowanego roztworu eteru na szkle napylonym platyną i na polerowanej czystej miedzi, mają regularny sześcienny kształt (rys. 11). Ich wielkość wynosiła od kilku nanometrów do 160 nm.

Na podstawie badań z wykorzystaniem dyfraktometru rentge-nowskiego (rys. 12) i techniki EBSD stwierdzono obecność austeni-tu i ferrytu oraz węglików TiC. Zidentyfikowano także dodatkowe refleksy, które wstępnie zinterpretowano jako fazę Mn3,6C0,4. Anali-za wagowa uzyskanych węglików typu NbTiC2 pozwoliła oszaco-wać ich udział w badanej stali na poziomie ok. 1,3%.

4. PODSUMOWANIE

– Uzyskana stal X98MnAlSiNbTi24–11 typu TRIPLEX charak-teryzuje się czystością metalurgiczną. Ujawniono obecność nie-licznych weglikoazotków aluminium.

– Strukturę stali X98MnAlSiNbTi24–11 typu TRIPLEX w stanie wyjściowym stanowi austenit, ferryt oraz węgliki, które ziden-tyfikowano za pomocą XRD. Obszary ferrytyczne są równo-miernie rozmieszczone na granicach ziaren austenitu. Jednakże wewnątrz ziaren austenitu pojawiają się niewielkie obszary fer-rytyczne.

– Wyniki badań dyfrakcyjnych pozwoliły na ujawnianie węglików złożonych głównie z Nb i Ti oraz węglików κ-(Fe, Mn)3AlC. Analiza składu chemicznego w mikroobszarach z wykorzysta-niem EDS w SEM potwierdziła występowanie takich węglików w badanej stali.

– Gęstość stali X98MnAlSiNbTi24–11 jest nawet do 15% mniej-sza w porównaniu z gęstością typowych stali konstrukcyjnych i wynosi 6,67 g/cm3.

– Badania izolatów węglikowych zwróciły uwagę na metodykę izolacji, która musi uwzględniać możliwość częściowego roz-puszczenia się wybranych faz węglikowych w stężonym HCl. Alternatywną metodą, która zmniejsza zagrożenie rozpuszcze-nia się faz węglikowych występujących w badanej stali w czasie izolacji jest metoda elektrochemicznego izolowania węglików.

– Na podstawie badań rentgenowskich izolatów węglikowych zidentyfikowano wyłącznie fazę typu NbTiC2 o sieci fcc, która w odróżnieniu od węglików (Nb,Ti)C o sieci fcc zawiera w sieci krystalicznej zdecydowanie więcej węgla.

structure and phase composition of newly developed high

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