184 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

Structure and mechanical properties of newly-developed high-strength TRIPLEX type steels

Liwia Sozańska-Jędrasik1*, Janusz Mazurkiewicz1, Wojciech Borek1, Leszek A. Dobrzański2

1Instytut Materiałów Inżynierskich i Biomedycznych, Politechnika Śląska, Gliwice, Polska, 2Centrum Naukowe Asklepios, Gliwice; *liwia.sozanska-jedrasik@polsl.pl

The paper presents the results of mechanical properties research, fractographic investigations of fractures and microstructure of experimental high-strength high-manganese X98MnAlSiNbTi24–11 and X105MnAlSi24–11 TRIPLEX type steels. In order to determine the mechanical properties of the tested steels, microhardness tests and a static tensile test were performed, and yield stress, tensile strength and elongation of the tested steels were determined. The per-formed microstructure tests of the analysed steels using light microscopy allowed for the identification of austenitic-ferritic structure with the participation of carbides. It was found that the structure of both tested steels, which have undergone hot rolling followed by water cooling, consist of austenite grains with numerous annealing and deformation twins along with ferrite bands. The investigated steels are characterized by the mixed fractures of formed after a static tensile test after forging where there are areas of ductile fracture with small areas of transcrystalline and intergranular brittle fractures. The fractures after a static tensile test and after hot rolling and cooling in water have a dimple morphology characteristic of plastic fractures. The increase in hardness after thermomechanical treatment and after a static tensile test is caused by strain hardening affecting the achieved values of strength, yield point and hardness. The obtained research results allow to assess the impact of both the chemical composition and the applied thermomechanical treatment technology on the properties of newly developed steels

Key words: high manganese steels, microstructure, fractures, microhardness, static tensile test.

Inżynieria Materiałowa 5 (225) (2018) 184÷191DOI 10.15199/28.2018.5.4

MATERIALS ENGINEERING

1. INTRODUCTION

High-manganese steels belong to the high-strength AHSS group steels (Advanced High Strength Steels) of the second generation, are characterised by a good combination of strength and plastic prop-erties. Fe–Mn–Al–C steels (TRIPLEX type) belong to the group of high manganese steels of different participating in the structure of the three phases: austenite, ferrite and carbides, including those responsible for very good mechanical properties of these disper-sion steels of κ-(Fe, Mn)3AlC carbides [1÷15]. Nano-sized carbides type M3C — (Fe, Mn)3AlC (so-called κ carbides) precipitation in this type of steels is affected by the addition of Al greater than 5% [1÷4]. These carbides shape the strength properties of this group of steels. The mechanical properties of these steels are determined to a large extent by the location, dimensions, coherence with the ma-trix as well as the morphology of κ-(Fe, Mn)3AlC carbides. κ car-bides can also cause the appearance of the brittleness of steel during plastic deformation at room temperature when they are formed at grain boundaries as large precipitates [1÷10, 14÷17]. Due to the relatively high addition of Al ~11%, as well as Mn (18÷28%) and C (0.5÷1.2%), metallurgy, processing, microstructure and defor-mation mechanisms of these steels are significantly different from those in traditional structural steels. These steels are characterised by 15% lower density compared to conventional structural steels (Fig. 1), making them very attractive from the point of view of their use in structural elements. Addition of Al to steel with a high con-tent of Mn affects two important effects: increasing the stacking fault energy and the precipitation of κ-(Fe, Mn)3AlC carbide. The dominant mechanisms in Fe–Mn–Al–C steels include: microband induced plasticity (MBIP), dynamic slip band refinement (DSBR), shear band induced plasticity (SIP), transformation induced plas-ticity (effect TRIP) and Twinning-Induced Plasticity (effect TWIP) [1÷7, 12÷16, 18].

Fe–Mn–Al–C steels, depending on the chemical composi-tion and thermomechanical treatment, are characterised by a wide range of strength properties. The key tool for determining strength

properties are stress diagrams as a function of strain [1, 2, 22]. The mechanical properties of high manganese steels can be shaped by appropriately selected thermomechanical treatment. Thermome-chanical treatment allows increasing the strength of the material without reducing its ductility [2÷8, 10÷12].

2. THE AIM OF THE WORK

The aim of the work was to determine the mechanical and plas-tic properties in reference to with the microstructure of the newly manganese X98MnAlSiNbTi24–11 and X105MnAlSi24–11 type TRIPLEX steels after hot rolling with cooling in water on a semi-industrial scale. Obtained results of the research will allow to as-sessing the impact of both the chemical composition and the ap-plied thermomechanical treatment technology on the properties of the tested steels.

Fig. 1. Comparison of the density of steelRys. 1. Porównanie gęstości stali

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3. RESEARCH MATERIAL AND METHODS

The subject of this research was two experimental high-manganese steels developed at the Silesian University of Technology in Gli-wice, X98MnAlSiNbTi24–11 (steel A) and X105MnAlSi24–11 (steel B) TRIPLEX type steels containing, among others, about 1% carbon, 24% manganese and 11% aluminium. Detailed chemical compositions of the tested steels are presented in Table 1.

Steels were melted in a laboratory vacuum induction furnace the batch weight 25 kg and cast under argon to a round hot-topped mould, converging downwardly with internal dimensions: bottom Ø122 mm, top Ø145 mm, h = 200 mm — without a hot head (with a hot head 300 mm). After cooling in the air, a thermomechanical treatment was made using the forging method on a high-speed hy-draulic press for flat bars with a thickness of approx. 20 mm and a width of 210 mm. The forging temperature ranged from 1200 to 900°C with in-process heat so that the material did not cool down below 900°C. Test sections of examined steels with dimensions 5×205×600 mm were hot rolled on the line for semi-industrial roll-ing simulation (LPS) at the Institute of Ferrous Metallurgy in Gli-wice. Earlier on the same processing line pre-4-rolled flat stages after forging of the above-mentioned bands having a thickness of 5 mm. The batch was austenitized at 1150°C for 15 minutes. The material in the form of the aforementioned test sections was sub-jected to a four-stage rolling on a single-stand two-high reversing mill (LPS — maximum pressure force 2.5 MN, maximum roll-ing speed 1.7 m/s) with a roll diameter of 550 mm and a width of 700 mm at a roll speed of 0.74 m/s in the temperature range from 1100°C to 850°C with a 0.23 strain for each pass. A detailed dia-gram of the hot rolling process is shown in Figure 2. After the last stage hot rolling, the strip was cooled in water and flat bars 210 mm wide and about 3.2 mm thick were obtained.

Samples for structural tests were ground and mechanically pol-ished on abrasive papers and discs moistened with diamond suspen-sion. To reveal the structure as a reagent, a 5% solution of HNO3 in ethyl alcohol was used. The etching time was 10÷70 s, depending on the degree of plastic deformation.

Observations of the structure of the investigated steels were carried out using the Axio Observer light microscope from Zeiss. Observations of fractures after a static tensile test of tested steels were made in a scanning electron microscope. Research using the SUPRA 35 scanning electron microscope from Zeiss was carried out at 20 kV acceleration using secondary electron detection (SE).

The hardness tests were carried out on the FM-700 hardness tester from Future-Tech using the Vickers method according to EN ISO 6507–-1. Hardness was measured at a load of 4.905 N (500 gf), the ac-tion of the load time was 15 seconds. Hardness test after static tensile test was performed on the longitudinal metallographic section failure from the fracture resulting from the test with a step of 0.25 mm.

The mechanical properties tests were carried out on the universal Z100 testing machine from Zwick, and the TestXpert II program was used to develop the test results. The static tensile test was per-formed using a Makro extensometer and a 2 mm thick flat sample with a measuring length of 50 mm and a width of 12.5 mm. The static tensile test was carried out in accordance with EN 10002–1 standard. At least three samples were used in the research, which consisted of ones cut in the direction of rolling, hot plastic working and free forged samples, as it is shown in Figure 2.

4. RESULTS AND THE DISCUSSION

The newly developed A and B TRIPLEX type steels, after forging, are characterized by an austenitic-ferritic structure with carbides, what was confirmed by structures observation (Fig. 3 and 4) us-ing light and scanning electron microscopy and also described in other publications [17, 19]. In both studied steels, ferritic areas with diversified morphology are visible at the boundaries of austenite grains. In B steel, the ferritic areas are unevenly distributed (in the form of clusters) over the grain boundaries and considerably wider, while in the case of A steel the ferrite grains are distributed much more evenly along the boundaries of the austenite grains. The aver-age equivalent diameter of austenite grain in A steel is 42 μm, while ferrite is around 6 μm. In B steel average grain equivalent diameter of austenite is higher by 50%, and is 62 μm, the average ferrite grain diameter is 11 μm, compared to A steel gives a result twice as large. Analysing the above data, it can be concluded that Nb and Ti addi-tions inhibit the growth of austenite grains more than ferrite. B steel is characterised by a higher volume fraction of ferrite in relation to steel with micro-additives Nb and Ti, the volume fraction of ferrite in B steel is 50% higher than in A steel, while in A steel the volume fraction of ferrite is 6%. In both A and B steels in austenite grains, there are few and small ferritic areas. In A steel disclosed, among others multicomponent carbides based on niobium and titanium, located primarily on the boundaries of austenite and ferrite grains. The location of the above-mentioned carbides affects the limita-tion of grain growth, and their fragmentation increases the strength properties of steel [17, 19].

The microstructure of A and B steels after the four-step hot-roll-ing with a true strain for each passage of ε = 0.23 and cooling in water are shown in Figures 5 and 6. Structural analysis was carried out on samples cut according to the direction of rolling. On the basis of metallographic examinations, it was found that the structures of both steels are austenite grains with numerous annealing and de-formation twins and a ferrite band. A characteristic feature of the tested austenitic-ferritic structure are elongated ferrite grains, which results from the low tendency of ferrite to recrystallise [1]. The for-mation of ferrite bands parallel to the rolling direction is influenced by the high aluminium content (~11%) in the analysed steels [1]. After hot rolling in A steel, the proportion of ferrite is 5%, while in B steel 10%.

Table 1. Chemical composition of newly developed high manganese steels, wt %Tabela 1. Skład chemiczny nowoopracowanych stali wysokomangano-wych, %mas.

Steel X98MnAlSiNbTi24–11 (steel A)

C Mn Al Si Nb Ti Ce La Nd Pmax Smax0.98 23.83 10.76 0.20 0.048 0.019 0.029 0.006 0.018 0.002 0.002

Steel X105MnAlSi24–11 (steel B)

C Mn Al Si Nb Ti Ce La Nd Pmax Smax1.05 23.83 10.76 0.10 — — 0.037 0.011 0.015 0.005 0.005

Fig. 2 Scheme of hot rolling process and cooling of tested steels of TRI-PLEX typeRys. 2. Schemat procesu walcowania na gorąco i chłodzenia badanych stali typu TRIPLEX

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Fig. 3. Microstructure of steel A after forging (a) and detail from figure a (b)Rys. 3. Mikrostruktura stali A po kuciu swobodnym (a) i szczegół z ry-sunku a (b)

Fig. 4. Microstructure of steel B after forging (a) and detail from figure a (b)Rys. 4. Mikrostruktura stali B po kuciu swobodnym (a) i szczegół z ry-sunku a (b)

Fig. 5. Microstructure of steel A after hot rolling and cooling in water (a) and detail from figure a (b)Rys. 5. Mikrostruktura stali A po walcowaniu na gorąco i chłodzeniu w wodzie (a) i szczegół z rysunku a (b)

Fig. 6. Microstructure of steel B after hot rolling and cooling in water (a) and detail from figure a (b)Rys. 6. Mikrostruktura stali B po walcowaniu na gorąco i chłodzeniu w wodzie (a) i szczegół z rysunku a (b)

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On the basis of structure investigations of A and B steels after forging subjected to a static tensile test (Fig. 7, 8), the grain refin-ing of austenite grain in both steels was observed. The structure of the investigated steels after hot rolling and cooling in water in both tested steels subjected to a static tensile test (Fig. 9, 10) is characterized by elongated grains in the direction of rolling which is strongly visible in the case of ferrite grains. In the austenite grains, numerous twins of deformation were noticed. Small ferrite grains are distributed over the boundaries of austenite grains. Comparing structure after a four-step hot rolling and the static tensile tests, it can be concluded that the structure has not been fragmented, be-cause the average grain diameter after hot rolling and static tensile test in the case of A steel is about 25 μm and in B steel about 45 um. On the other hand, as a result of the mechanisms of cold plastic deformation such as slip and mechanical twins, the resulting grains deformed in accordance with the direction of tensile forces.

The fractures after a static tensile test of investigated steels af-ter the forging (Fig. 11) are characterised by mixed fractures: there are both areas of ductile fracture and small areas of transcrystal-line and intercrystalline brittle fracture. In A steel (Fig. 11a) a larger share of the dimple morphology was observed than in the case of B steel (Fig. 11b), which is characterized by a system of recesses and convexities of various sizes and shape. The dimple morphology of fracture structure is made of micropores, microcracks or microvoids that form during the load [20÷22]. On the surface of the fractures of both steels, distinctive faults were noted. Tongue morphology with arose as a result of deflection of the brittle fracture when encounter-ing twins were identified in A steel. While in B steel, fractures in the form river-like morphology were observed, occurring when the crack face encounters screw dislocations that intersect the face of cleavage [20÷22]. Comparing the fractures after a static tensile test (Fig. 12) of the samples after forging (Fig. 11) in B steel numerous

Fig. 8. Microstructure of steel B after forging and static tensile test (a) and detail from figure a (b)Rys. 8. Mikrostruktura stali B po kuciu swobodnym i rozciąganiu statycz-nym (a) i szczegół z rysunku a (b)

Fig. 9. Microstructure of steel A after hot rolling and cooling in water and static tensile test (a) and detail from figure a (b)Rys. 9. Mikrostruktura stali A po walcowaniu na gorąco i chłodzeniu w wodzie oraz rozciąganiu statycznym (a) i szczegół z rysunku a (b)

Fig. 7. Microstructure of steel A after forging and static tensile test (a) and detail from figure a (b)Rys. 7. Mikrostruktura stali A po kuciu swobodnym i rozciąganiu statycz-nym (a) i szczegół z rysunku a (b)

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Fig. 10. Microstructure of steel B after hot rolling and cooling in water and static tensile test (a) and detail from figure a (b)Rys. 10. Mikrostruktura stali B po walcowaniu na gorąco i chłodzeniu w wodzie oraz rozciąganiu statycznym (a) i szczegół z rysunku a (b)

Fig. 11. Fractures after the static tensile test after forging steel: a) A, b) BRys. 11. Przełomy po statycznej próbie rozciągania po kuciu swobodnym stali: a) A, b) B

Fig. 12. Fractures after the static tensile test after hot rolling and water cooling in steel: a) A, b) BRys. 12. Przełomy po statycznej próbie rozciągania po walcowaniu na gorąco i chłodzeniu w wodzie stali: a) A, b) B

secondary cracks were observed, which were not observed in steel A. Figure 12 shows the fractures after a static tensile test after hot rolling and cooling in water. In the fractures of both examined steel, recesses and convexities forming the so-called dimple morphology characteristic of ductile fractures. At the bottom of dimple has been observed precipitates. In B steel additional areas of intercrystalline fractures were also observed (Fig. 12b) [20÷22].

TRIPLEX type steels tested after forging are characterized by hardness from 381 to 391 HV0.5 (Fig. 13a). The difference in hard-ness for both steels in the input state for thermomechanical treat-ment is small and amounts to 10 HV0.5, which is the difference be-low 2.5%. In the case of hardness of both steels, after hot rolling and cooling in water, the hardness of steel A increased to 421 HV0.5, while steel B to 440 HV0.5. On the basis of the hardness tests of the material after static tensile test demonstrated a gradual increase in hardness together the closer to the place of the fracture, which is caused by the increasing strain hardening (Fig. 13b).

The yield stress Rp0.2, the tensile strength Rm and the elonga-tion A of the tested steels were determined on the basis of static tensile test curves. Based on the analysis of strain–stress curves, it was found (Fig. 14a) that the value of tensile strength after forging amounts for A steel 1049 MPa and 895 MPa for B steel. The values of tensile strength after hot rolling and cooling in water increasing respectively to A steel 1142 MPa and B steel 1141 MPa. B steel after forging is characterized by average yield stress of 875 MPa, whereas after hot rolling and cooling in water 1057 MPa. In the case of A steel, the average yield stress after forging is 929 MPa, and 1086 MPa after hot-rolling and cooling in water. Definitely, much higher plastic properties of tested steels after thermome-chanical treatment are a consequence of changes in the structure and above all break the ferrite bands (Fig. 14b). The high ratio

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– In the structure of the investigated steels, after hot rolling, ther-momechanical treatment and cooled in water after static tensile test, grain ferrite fragmentation and numerous twins deforma-tions in austenite grains were observed. As a result of a static tensile test, the austenite grains have not been fragmented, the size of which, both after hot rolling and after static tensile test, is for A steel ~25 μm, and ~45 μm for B steel. At austenite grain boundaries there were small ferrite grains distributed, also the elongation of the ferrite grains in accordance with the direction of action of the tensile forces were noted.

– Mixed fractures are characterised by static tensile test employed after forging in the areas whereas ductile fracture with small zones of transcrystalline and brittle intercrystalline fractures are present. On the fractures, characteristic steps have been noticed — river-like and tongue morphology. The fractures after a static tensile test, after hot rolling and cooling in water have a dimple morphology characteristic of ductile fractures. At the bottom of the dimples observed minor precipitates.

– The hardness of the steels after hot rolling and cooling in wa-ter, and also after a static tensile test is greater than after forg-ing. Near the resulting fractures after static tensile test hard-ness of examined steels after hot rolling ranks in the range of 500÷503 HV0.5, and after forging 426÷438 HV0.5. The increase

Fig. 13. The hardness of the tested steel after forging, and hot rolling and cooling in the water (a), hardness of tested steels after static tensile tests after forging, hot rolling and cooling in water as a function of distance from the front of the fracture (b)Rys. 13. Twardość badanych stali po kuciu swobodnym oraz walcowa-niu na gorąco i chłodzeniu w wodzie (a), twardość badanych stali po rozciąganiu statycznym po kuciu swobodnym oraz walcowaniu na gorąco i chłodzeniu w wodzie w funkcji odległości od czoła przełomu (b)

Fig. 14. The tensile strength and yield strength (a), the total elongation of the tested steels after forging, hot rolling and cooling in water (b)Rys. 14. Wytrzymałość na rozciąganie i umowna granica plastyczności (a), wydłużenie całkowite badanych stali po kuciu swobodnym oraz wal-cowaniu na gorąco i chłodzeniu w wodzie (b)

R0.2/Rm = 0.95÷0.99 for thermomechanical treated steels proves the low inclination of the tested TRIPLEX steels to gradually stain strengthening during cold deformation. While the high value of Rm·Ag guarantee the stability of plastic deformation conditions dur-ing cold forming during technological development.

5. SUMMARY

– The structure of A and B TRIPLEX steels after forging are aus-tenite, ferrite and carbides. In A steel, ferritic areas are evenly distributed at the boundary of austenite grains, while in B steel ferrite grains are arranged in the form of strongly differentiated by volume (morphologically) microareas along the boundaries of austenite grains. The addition of Nb and Ti affects the grain refinement in A steel. In both steels a few and relatively small ferritic areas appear inside the austenite grains.

– After hot rolling and cooling in water, it was found that the struc-tures of both tested steels consist of austenite grains with numer-ous annealing twins and ferrite bands. In B steel was found high-er ferrite volume fraction of about 50%. The low tendency to recrystallization of ferrite leads to elongation of the grains. The relatively high aluminium content in steels affects the formation of ferrite bands parallel to the direction of rolling [1].

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in hardness is caused by mechanical strain strengthening affect-ing the achieved values of strength, yield point and hardness.

– The average value of tensile strength of the steel tested after forging is from 1049 MPa for A steel to 895 MPa for B steel, whereas after hot rolling and cooling in water from 1142 MPa for A steel up to 1141 MPa for B steel.

– Thermomechanical treatment contributes to a significant in-crease in the elongation value: in the case of steel A with 2% af-ter forging up to 25% after hot rolling and cooling in water, while in steel B with 0.5% after forging up to 27% after hot rolling.

– The high product Rm·Ag guarantees the stability of plastic cold deformation conditions during technological shaping, and the ra-tio Rp0,2/Rm at the level of 0.95÷0.99 indicates a low tendency of the tested TRIPLEX steel to gradually stain strengthening.

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.

Liwia Sozańska-Jędrasik is a scholarship holder of the Visegrad International Scholarship Grant for the period September 2017 to July 2018, so some of the research was conducted in collaboration with Ing. Martin Kraus from VŠB – Technical University of Os-trava in the Czech Republic.

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Struktura a właściwości mechaniczne nowoopracowanych wysokowytrzymałych stali manganowych typu TRIPLEX

Liwia Sozańska-Jędrasik1*, Janusz Mazurkiewicz1, Wojciech Borek1, Leszek A. Dobrzański2

1Instytut Materiałów Inżynierskich i Biomedycznych, Politechnika Śląska, Gliwice, Polska, 2Centrum Naukowe Asklepios, Gliwice; *liwia.sozanska-jedrasik@polsl.pl

Inżynieria Materiałowa 5 (225) (2018) 184÷191DOI 10.15199/28.2018.5.4

MATERIALS ENGINEERING

Słowa kluczowe: stale wysokomanganowe, mikrostruktura, przełomy, mikrotwardość, statyczna próba rozciągania.

1. CEL PRACY

Celem pracy było określenie właściwości mechanicznych oraz plastycznych w powiązaniu z mikrostrukturą nowoopracowanych stali wysokomanganowych X98MnAlSiNbTi24–11 i X105MnAl-Si24–11 typu TRIPLEX po walcowaniu na gorąco w skali półprze-mysłowej i chłodzeniu w wodzie. Uzyskane wyniki badań pozwolą na ocenę wpływu zarówno składu chemicznego, jak i zastosowanej technologii obróbki cieplno-mechanicznej na właściwości bada-nych stali.

2. MATERIAŁ I METODYKA BADAŃ

Przedmiotem badań były eksperymentalne stale wysokomangano-we X98MnAlSiNbTi24–11 (stal A) i X105MnAlSi24–11 (stal B) typu TRIPLEX zawierające m.in. około 1% węgla, 24% manganu i 11% aluminium. Szczegółowe składy chemiczne badanych sta-li przedstawiono w tabeli 1. Obserwacje struktury badanych stali wykonano na mikroskopie świetlnym, natomiast obserwacje prze-łomów powstałych po statycznej próbie rozciągania prowadzono w skaningowym mikroskopie elektronowym. Badania twardości wykonano sposobem Vickersa wg EN ISO 6507–1. Twardość mie-rzono przy obciążeniu 4,905 N (500 gf), czas działania obciążenia wynosił 15 s. Badania właściwości mechanicznych wykonano na uniwersalnej maszynie wytrzymałościowej.

3. WYNIKI I ICH DYSKUSJA

Nowoopracowane stale A i B typu TRIPLEX po kuciu swobodnym charakteryzują się strukturą austenityczno-ferrytyczną z udziałem węglików, co potwierdzono na podstawie obserwacji struktury (rys. 3 i 4) za pomocą mikroskopii świetlnej i elektronowej oraz opisano także w innych publikacjach [17, 21]. W obu badanych stalach na granicach ziaren austenitu są widoczne obszary ferrytyczne o zróż-nicowanej morfologii. Strukturę stali A i B po czteroetapowym walcowaniu na gorąco i chłodzeniu w wodzie przedstawiono na rysunkach 5 i 6. Analizę strukturalną przeprowadzono na próbkach wyciętych zgodnie z kierunkiem walcowania. Na podstawie badań metalograficznych stwierdzono, że strukturę obu stali stanowią ziarna austenitu z licznymi bliźniakami wyżarzania oraz pasma fer-rytu. Cechą charakterystyczną są wydłużone ziarna ferrytu, co wy-nika z małej skłonności ferrytu do rekrystalizacji. Po walcowaniu w stali A udział ferrytu wynosi 5%, natomiast w stali B 10%.

Przełomy powstałe po statycznej próbie rozciągania badanych stali w stanie po kuciu swobodnym (rys. 11) charakteryzują się prze-łomem mieszanym: występują zarówno obszary przełomu ciągli-wego, jak i niewielkie obszary przełomu transkrystalicznego i mię-dzykrystalicznego kruchego. W stali A (rys. 11a) zaobserwowano

większy udział budowy plastrowej niż w przypadku stali B (rys.11b). Na powierzchni przełomów obu stali zauważono charak-terystyczne uskoki. Uskoki o morfologii języków zidentyfikowano w stali A. Natomiast w stali B zauważono uskoki w postaci rzek. Porównując przełomy po statycznej próbie rozciągania (rys. 12) z próbkami po kuciu swobodnym (rys. 11) w stali B zaobserwowa-no liczne pęknięcia wtórne, których nie zauważono w stali A. Na rysunku 12 przedstawiono przełomy po statycznej próbie rozciąga-nia po walcowaniu na gorąco i chłodzeniu w wodzie tworzące tzw. budowę plastrową charakterystyczną dla złomów plastycznych. Na dnie wgłębień zaobserwowano drobne wydzielania.

Badane stale typu TRIPLEX po kuciu swobodnym charaktery-zują się twardością od 381 do 391 HV0,5 (rys. 13a). W przypad-ku mikrotwardości obu stali po walcowaniu na gorąco i chłodze-niu w wodzie zauważono wzrost twardości stali A do 421 HV0,5, natomiast stali B do wartości 440 HV0,5. Umowną granicę pla-styczności Rp0,2, wytrzymałość na rozciąganie Rm oraz wydłużenie A badanych stali określono na podstawie otrzymanych krzywych uzyskanych ze statycznej próby rozciągania. Na podstawie anali-zy krzywych rozciągania stwierdzono (rys. 14a), że wytrzymałość na rozciąganie po kuciu swobodnym stali A wynosi 1049 MPa i 895 MPa stali B. Wytrzymałość na rozciąganie po walcowaniu na gorąco i chłodzeniu w wodzie zwiększyła się i wynosiła odpowied-nio: stali A 1142 MPa, stali B 1141 MPa.

4. PODSUMOWANIE

– Po walcowaniu na gorąco i chłodzeniu w wodzie stwierdzono, że struktura obu badanych stali składa się z ziaren austenitu z licznymi bliźniakami wyżarzania oraz pasm ferrytu. W stali B stwierdzono większy udział ferrytu o około 50%. Skutkiem małej skłonności do rekrystalizacji ferrytu są wydłużone ziarna. W strukturze badanych stali po walcowaniu na gorąco obrabia-nych cieplno-plastycznie i chłodzonych w wodzie po statycz-nym rozciąganiu zaobserwowano rozdrobnienie ziaren ferrytu oraz liczne bliźniaki odkształcenia w ziarnach austenitu.

– Przełomami mieszanymi charakteryzują się przełomy badanych stali powstałe po statycznej próbie rozciągania po kuciu swobod-nym, w których występują obszary przełomu ciągliwego z nie-wielkimi obszarami przełomu transkrystalicznego i międzykry-stalicznego kruchego. Przełomy po statycznej próbie rozciągania i po walcowaniu na gorąco i chłodzeniu w wodzie mają budowę plastrową charakterystyczną dla złomów plastycznych. Na dnie wgłębień zaobserwowano drobne wydzielenia.

– Wzrost twardości jest spowodowany umocnieniem mechanicz-nym wpływającym na osiągane wartości wytrzymałości, granicy plastyczności oraz twardości. Obróbka cieplno-plastyczna wpły-wa na znaczne zwiększenie wydłużenia.