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1

Processing routes, resulting microstructures, and strain rate dependent deformation behaviour of advanced high strength steels for automotive applications

Nanda Tarun, Singh Vishal, Singh Gurpreet, Singh Manpreet, Kumar B. Ravi

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 1

112--135

EN

Automobile industry is continuously striving to obtain light body-in-white structures to meet tightened regulations on flue-gas emissions/crash-testing parameters. ‘Advanced high strength steels (AHSS)’ find increased applications in the automotive industry because of improved crashworthiness/formability at reasonably low costs. AHSS category mainly includes transformation induced plasticity (TRIP) steels, twinning induced plasticity (TWIP) steels, dual phase (DP) steels, complex-phase (CP) steels, and quenching-partitioning (Q&P) steels. AHSSs provide superior strength-ductility combination than conventional high-strength steels by virtue of their multi-phase microstructures. Mechanical properties of AHSSs are greatly influenced by processing routes/derived microstructures. Furthermore, mechanical properties/tensile deformation behavior are also strain rate dependent. During an automobile crash, deformation occurs at strain rates which are exceedingly higher than quasi-static conditions. So, investigation of AHSS properties under both quasi-static as well as high strain rates conditions is important to check applicability for superior crash-resistance. The present work critically reviews details of processing routes, room temperature microstructures, mechanical properties, and finally strain rate dependence of tensile deformation behaviour of AHSSs. Finally, main gaps in existing literature/scope for future research with regards to high strain rate deformation dependent properties of this steel category are presented.

2

The effect of chemical composition on microstructure and properties of TRIP steels

Kučerová L., Bystrianský M.

Journal of Achievements in Materials and Manufacturing Engineering

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2016

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Vol. 77, nr 1

5--12

EN

Purpose: Various alloying strategies can be used to produce advanced high strength steels and this work offers comparison of results achieved for four different low alloyed steels with 0.2-0.4 %C, 0.5-2 %Si, 0.6-1.5 %Mn, 0.03-0.06 %Nb and with 0.8-1.33 %Cr. Microstructures obtained by two methods of thermo-mechanical treatment were analysed for each steel and compared with theoretical predictions of TTT (time temperature transformation) diagrams calculated by JMatPro. Design/methodology/approach: Thermo-mechanical treatment of all steels was carried out at thermo-mechanical simulator. Resulting microstructures were analyses by the means of scanning electron microscopy, mechanical properties were measured by tensile test. Findings: It was found out that microstructures typical for TRIP (transformation induced plasticity) steels can be obtained easily for low carbon steels alloyed by silicon or aluminium-silicon and micro-alloyed by niobium. Chromium addition influenced austenite decomposition causing intensive pearlite formation in low carbon steel and predominantly martensitic microstructure in middle carbon steel. These microstructures were not in agreement with calculated TTT diagrams. Research limitations/implications: To obtain ferritic-bainitic microstructure with retained austenite typical for TRIP steels, chromium alloyed steels require substantial optimisation of processing parameters. This issue should be addressed in future work. Practical implications: JMatPro software is well equipped to calculate TTT diagrams for steels alloyed by manganese, silicon and niobium, however further chromium addition changed behaviour of the steel in a way that the software was not able to predict. Originality/value: Obtained results could be useful for consideration of chemical composition of low alloyed steels with respect to resulting microstructures and properties.

3

Kinetyka przemian przechłodzonego austenitu stali 13MnSi6-5 typu TRIP chłodzonej z zakresu temperatur krytycznych

Kokosza A., Pacyna J., Dziurka R.

Inżynieria Materiałowa

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2015

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Vol. 36, nr 5

324--328

PL

W pracy przedstawiono wyniki badań kinetyki przemian fazowych przechłodzonego austenitu podczas ciągłego chłodzenia stali 13MnSi6-5 typu TRIP. W badaniach tych zastosowano niekonwencjonalną temperaturę wyżarzania w zakresie temperatur krytycznych, od której oziębiano badaną stal. Była ona tylko o 10°C wyższa od temperatury, przy której w badanej stali rozpoczyna się przemiana perlitu w austenit (Ac1s), ale niższa o ok. 60°C od powszechnie zalecanej dla stali typu TRIP temperatury wyżarzania w zakresie dwufazowym, w którym współistnieje austenit i ferryt (Ac1f÷Ac3). Na podstawie analizy dylatometrycznej i metalograficznej dla badanej stali opracowano wykres czas–temperatura–przemiana przy chłodzeniu ciągłym (CTPc), który ilustruje kinetykę przemian fazowych przechłodzonego austenitu w badanej stali. Stwierdzono, że stal 13MnSi6-5 po oziębianiu od 750°C (Ac1s + 10°C) cechuje się niewielką hartownością. Hartowność ta okazała się jednak większa niż po oziębianiu od zalecanej temperatury (810°C). Zaobserwowano również, że przemiana martenzytyczna podczas chłodzenia od tak niskiej temperatury przebiega w dwóch etapach. Obserwacje te mogą wskazywać na tworzenie się w trakcie wygrzewania w 750°C dwóch rodzajów austenitu, różniących się zawartością węgla. Pierwszy z nich (wysokowęglowy) tworzy się z perlitu, natomiast austenit niskowęglowy może powstawać w rezultacie przebiegającej równocześnie przemiany ferrytu w austenit. Zasugerowano, że zastosowanie przedstawionego w pracy sposobu wyżarzania badanej stali typu TRIP może przyczynić się do zwiększenia udziału austenitu szczątkowego w jej mikrostrukturze, a przez to do poprawy jej właściwości mechanicznych.

EN

Comprehensive studies of the phase transformations kinetics of overcooled austenite in the 3MnSi6-5 TRIP steel during continuous cooling were carried out in order to draw the continuous cooling Time–Temperature–Transformation (CCT) diagram. The unconventional annealing temperature within the intercritical temperatures range was applied in these studies. It was only 10°C higher than the temperature at which the pearlite to austenite transformation — in the investigated steel — starts (Ac1s), but lower by approximately 60°C than the commonly recommended for TRIP steels annealing temperature at which austenite and ferrite coexist (Ac1f÷Ac3). It was found that the 13MnSi6-5 steel after cooling from 750°C (Ac1s + 10°C) was characterised by a low hardenability. However, this hardenability was higher than the one after cooling from the recommended temperature (810°C). It was also observed that the martensitic transformation during cooling from such a low temperature proceeds in two stages. These observations may indicate that two types of austenite, with a different carbon content, are formed during annealing at 750°C. The first of them (high carbon) is formed from pearlite, while the low carbon austenite may be formed as a result of the transformation of ferrite to austenite occurring simultaneously. It seems that the application of the annealing type presented in the paper, can contribute to increasing the volume fraction of the retained austenite in the microstructure of the investigated steel and hence to improve its mechanical properties.

4

Kształtowanie mikrostruktury niskowęglowej stali typu TRIP podczas wyżarzania w zakresie temperatur krytycznych

Kokosza A.

Inżynieria Materiałowa

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2014

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Vol. 35, nr 1

19--24

PL

W pracy przedstawiono wyniki badań nad tworzeniem się austenitu w mikrostrukturze stali 13MnSi6-5 (tab. 1) podczas wyżarzania w zakresie temperatur krytycznych. Określono również wpływ temperatury takiego wyżarzania na udział austenitu szczątkowego, jaki pozostaje w mikrostrukturze badanej stali po zahartowaniu. Na podstawie wyników badań dylatometrycznych (rys. 2) stwierdzono, że przemiana perlit → austenit w badanej stali przebiega nie w stałej temperaturze określanej jako Ac1, lecz w pewnym, możliwym do określenia jej zakresie, którego granice wyznaczają wartości Ac1s i Ac1f (tab. 2). Badania metalograficzne potwierdziły, że mikrostruktura stali 13MnSi6-5 po wyżarzaniu w tym zakresie temperatury składa się z ferrytu struktury wyjściowej oraz perlitu i austenitu (rys. 3). Metodą rentgenowskiej ilościowej analizy fazowej wykazano, że najwięcej austenitu szczątkowego pozostało w tych próbkach z badanej stali, które przed zahartowaniem były wyżarzane w najniższej temperaturze z zakresu Ac1s÷Ac1f. Podwyższenie temperatury wyżarzania do zakresu dwufazowego α + γ było przyczyną zmniejszania się udziału austenitu szczątkowego w próbkach badanej stali aż do ok.2,3% obj. Mogło to oznaczać, że podczas wyżarzania w temperaturze 740 lub 750°C najprawdopodobniej miało miejsce silne wzbogacanie w węgiel tworzącego się austenitu. Podczas oziębiania austenit taki cechował się zwiększoną stabilnością i niską temperaturą Ms, dzięki czemu po zahartowaniu w mikrostrukturze stali 13MnSi6-5 było możliwe zachowanie nawet ok. 8,5% obj. austenitu szczątkowego. Określone na podstawie wykonanych badań zależności (rys. 4, 5) potwierdziły, że największe zmiany w mikrostrukturze badanej stali miały miejsce podczas jej wyżarzania w najniższej temperaturze z zakresu Ac1s÷Ac1f. Stwierdzono, że podczas wyżarzania w tym zakresie temperatury austenit tworzy się również z ferrytu. Mogło to być przyczyną zmniejszenia zawartości węgla w tworzącym się austenicie i spadku udziału austenitu szczątkowego w mikrostrukturze stali 13MnSi6-5 po jej zahartowaniu od temperatury wyższej od 750°C. Na podstawie uzyskanych wyników zasugerowano możliwość modyfikacji technologii obróbki cieplnej stali o podobnym składzie chemicznym.

EN

The paper presents the results of research on the austenite formation in the microstructure of 13MnSi6-5 steel (Tab. 1) during annealing in the critical temperature range. The effect of annealing temperature on the volume fraction of the retained austenite remaining in the microstructure of the investigated steel after water quenching was also determined. Based on the results of dilatometric analysis (Fig. 2) it was shown that the austenite → pearlite transformation in the investigated steel does not occur at a constant temperature, which is referred to as Ac1, but in a certain, possible to determine, range which is bounded by Ac1s and Ac1f values (Tab. 2). The metallographic investigation confirmed that the microstructure of the 13MnSi6-5 steel, after annealing in such a temperature range consists of untransformed ferrite, pearlite and austenite (Fig. 3). Quantitative X-ray phase analysis demonstrated that most of the retained austenite remained in the investigated steel samples which were annealed at the lowest temperature in the Ac1s÷Ac1f range before quenching. Surprisingly, it was found that an annealing temperature increasing into the two-phase (α + γ) range, resulted in a reduction of the volume fraction of retained austenite to about 2.3% vol. This could mean that during annealing at 740 or 750°C, there most likely was significant enrichment in carbon of the formed austenite. Such austenite had an increased stability and low Ms temperature and therefore after water quenching, it was possible to maintain about 8.5% vol. retained austenite. The relationships, determined on the basis of the conducted studies (Fig. 4, 5), confirmed that the biggest changes in the microstructure of the 13MnSi6- 5 steel occurred during annealing at the lowest temperature in the Acs1÷Ac1f range. It was also found that during annealing in such a temperature range, the austenite is formed of ferrite simultaneously. This could be the reason for the decrease the carbon content in the formed austenite and consequently the decrease in the volume fraction of retained austenite in the microstructure of the investigated steel after quenching from temperatures higher than 750°C. Based on the obtained results, the possibility of modifying heat treatment technology for TRIP steels with a similar chemical composition was suggested.

5

Formation of Medium Carbon TRIP Steel Microstructure During Annealing in the Intercritical Temperature Range

Kokosza A., Pacyna J.

Archives of Metallurgy and Materials

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2014

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Vol. 59, iss. 3

1017--1022

EN

The paper presents the results of research conducted on austenite formation in the microstructure of 41MnSi6-5 TRIP steel during annealing in the intercritical temperature range. The influence of the annealing temperature on the volume fraction of retained austenite in the microstructure of the investigated steel after water quenching was also determined. Based on the results of a dilatometric analysis and metallographic investigation it was noted that the pearlite-to-austenite transformation does not occur at a constant temperature, which is referred to as Ac1, but rather within some, possible to determine, temperature range which is bounded by the values Ac1s and Ac1f. Moreover, through X-ray analysis, it was stated that the largest amount of retained austenite remained in the samples which were annealed at the lowest temperatures in the Ac1s and Ac1f range prior to quenching. Increasing the annealing temperature to a two-phase a+g (ferrite + austenite) range, resulted in a decrease of the volume fraction of retained austenite. It was also found that during annealing in Ac1s and Ac1f temperature range, austenite is also formed from ferrite simultaneously. This could be the reason for the decrease the carbon content in the formed austenite and consequently the decrease in the volume fraction of retained austenite in the microstructure of the investigated steel, which was quenched after having reached temperatures higher than Ac1s + 30°C.

PL

W pracy przedstawiono wyniki badań nad tworzeniem się austenitu w mikrostrukturze stali 41MnSi6-5 typu TRIP podczas wyżarzania w zakresie temperatur krytycznych. Określono również wpływ temperatury takiego wyżarzania na udział objętościowy austenitu szczątkowego, jaki pozostaje w mikrostrukturze badanej stali po zahartowaniu od takich temperatur. Na podstawie wyników analizy dylatometrycznej oraz badań metalograficznych stwierdzono, że w badanej stali przemiana perlit – austenit nie przebiega w stałej temperaturze określanej jako Ac1s lecz w pewnym, możliwym do określenia jej zakresie, którego granice wyznaczają wartości Ac1s and Ac1f. Ponadto, metodą analizy rentgenowskiej wykazano, że najwięcej austenitu szczątkowego pozostawało w tych próbkach z badanej stali, które przed zahartowaniem były wyżarzane przy najniższej temperaturze z zakresu Ac1s and Ac1f. Podwyższenie temperatury wyżarzania do zakresu dwufazowego a+g (ferryt+ austenit) było przyczyną zmniejszenia udziału austenitu szczątkowego. Stwierdzono, że podczas wyżarzania w zakresie temperatur Ac1s and Ac1f tworzy się również z ferrytu. Mogło to być przyczyną zmniejszenia zawartości węgla w tworzącym się austenicie i spadku udziału austenitu szczątkowego w mikrostrukturze badanej stali po jej zahartowaniu od temperatur wyższych od Ac1s + 30°C.

6

The influence of continuous heating rate on the austenite formation in the medium carbon TRIP steel

Kokosza A., Pacyna J., Bała P., Dziurka R.

Archives of Materials Science and Engineering

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2013

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Vol. 62, nr 1

22--27

EN

Purpose: The purpose of the hereby work was to determine the influence of heating rate on the austenite formation range and to draw the time-temperature-austenitizing diagram at continuous heating for TRIP 41MnSi6-5 steel. Design/methodology/approach: The dilatometric analysis was applied as the basic investigation method. Samples of the tested steel were heated to 1100°C with various heating rates. Changes in the relative elongation (ΔL) were recorded as a temperature function (T), during heating. On the basis of analysing such dependencies, for each heating rate the critical temperatures were determined. Findings: It was found, that during heating of the 41MnSi6-5 steel the austenite formation starts at the higher temperature the faster is the heating. It was observed, that directly before the start of the austenite formation, an unidentified (in the presented here investigations) transformation occurs in the investigated steel, causing its volume increase. Research limitations/implications: The performed investigations indicate that during heating of elements of small thickness or cross-sections - within the critical temperature range - the method of their heating to the required temperature becomes very important. At short heating times incorrectly selected the heating conditions can be the reason of significant errors of the heat treatment. Practical implications: The developed diagram: time-temperature-austenitizing, at a continuous heating (CHT), can be a useful tool supporting the proper selection of heating parameters within the critical temperature range. Originality/value: The dependence of the heating rate and the temperature range, in which austenite is formed in the tested 41MnSi6-5 steel, was found. It was observed that heating of the investigated steel with rates lower than 1°C/s has an insignificant influence on the temperature range within which the austenite formation occurs.

7

Kinetyka przemian fazowych przechłodzonego austenitu stali Mn-Si chłodzonych z zakresu temperatur krytycznych

Pacyna J., Kokosza A.

Inżynieria Materiałowa

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2012

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Vol. 33, nr 5

386--391

PL

W pracy zamieszczono wyniki badań kinetyki przemian przechłodzonego austenitu trzech stopów typu TRIP ( Transformation Induced Plasticity ), o różnym stężeniu węgla (0,076% C, 0,126% C i 0,407% C). Wykonano dla tych stopów wykresy CTPc (Czas-Temperatura-Przemiana przy chłodzeniu ciągłym) (rys. 3÷5). Znajomość takich wykresów, a w szczególności znajomość temperatur krytycznych ( Ac1s, Ac1f, Ac3), temperatury Ms oraz krytycznych szybkości chłodzenia, umożliwiła opracowanie dostosowanej do składu chemicznego badanych stopów technologii obróbki cieplnej – optymalnej dla uzyskania w nich efektu TRIP. Oczekiwano, że po obróbce cieplnej według proponowanych wariantów badane stopy powinny charakteryzować się dużą plastycznością, a po ich końcowym ukształtowaniu, również bardzo dobrą wytrzymałością. Dla zweryfikowania tych oczekiwań na obrobionych cieplnie próbkach z badanych stopów wykonano statyczną próbę rozciągania i wyznaczono własności mechaniczne. Wyniki tych badań (tab. 3) potwierdziły możliwość uzyskania bardzo korzystnej kombinacji własności plastycznych i wytrzymałościowych.

EN

Comprehensive studies of the transformation kinetics of the undercooled austenite in three TRIP (Transformation the Induced the Plasticity) alloys, with different carbon concentration (0.076, 0.126 and 0.407% C) were carried out in order to draw the continuous cooling Time-Temperature-Transformation (CCT) diagrams (Fig. 3÷5). The data obtained from these diagrams, especially concerning critical temperatures (Ac1s, Ac1f, Ac3), Ms temperature as well as critical cooling rates is essential in optimizing the heat treatment procedures of the investigated alloys to obtain the TRIP effect. It was expected, that proposed variants of heat treatment would yield a high plasticity characteristic of the investigated alloys, as well as a high mechanical strength in the final cold-forming condition. These expectations have positively been verified in the static tensile test (Tab. 3).

8

Modyfikacja wtrąceń niemetalicznych pierwiastkami ziem rzadkich w niskostopowych stalach typu C-Mn-Si-Al

Grajcar A.

Rudy i Metale Nieżelazne

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2010

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R. 55, nr 3

143-152

PL

W artykule omówiono modyfikację składu chemicznego wtrąceń niemetalicznych pierwiastkami ziem rzadkich. W części badawczej opracowano trzy stale typu C-Mn-Si-Al z mikrododatkami Nb i Ti, przeznaczone do wytworzenia struktur ferrytyczno-bainitycznych z metastabilnym austenitem szczątkowym. Wytopy wykonano w indukcyjnym piecu próżniowym, a modyfikację wtrąceń niemetalicznych przeprowadzono miszmetalem w ilości 0,77 g na 1 kg stali. Stwierdzono, że stale cechuje duża czystość metalurgiczna związana z małą zawartością siarki, fosforu oraz gazów. Stale zawierają drobne wtrącenia siarczkowe lub siarczkowo-tlenkowe o średniej wielkości od 20 do 25 microm2, a ich udział wynosi od 0,07 do 0,13 %, zależnie od zawartości w stali siarki. Skład chemiczny zdecydowanej większości wtrąceń został zmodyfikowany całkowicie lub częściowo przez Ce, La oraz Nd, co skutkuje małą odkształcalnością wtrąceń niemetalicznych podczas walcowania na gorąco.

EN

The modification of the chemical composition of non-metallic inclusions by rare-earth elements was discussed in the paper. Three C-Mn-Si-Al steels with Nb and Ti microadditions assigned to form ferritic-bainitic structures with metastable retained austenite were developed. The steels were melted in a vacuum induction furnace and a modification of non-metallic inclusions was carried out by the mischmetal in the amount of 0.77 g per 1 kg of steel. It was found that using material charge of high purity and a realization of metallurgical process in vacuous conditions result in a low concentration of sulphur from 0.004 to 0.008 % and oxygen in a range from 6 to 12 ppm. The high metallurgical purity is confirmed by a small fraction of non-metallic inclusions averaging 0.07 % for the steels containing 0.004 % S and 0.13 % for the steel with twice higher sulphur content. A large majority of non-metallic inclusions are fine, globular sulfide or sulfide-oxide particles with a mean size from 20 to 25 microm2. In case of the steels with a lower sulphur content, a modification of the chemical composition of inclusions by rare-earth elements was almost total. As a result they show small ability to elongate in a rolling direction. In the steel containing the higher content of impurities, the mischmetal amount of 0.77 g per 1 kg of steel is sufficient just to a partial chemical composition modification of inclusions, connected with a partial substitution of Mn in sulfide inclusions and Al in oxide inclusions by Ce, La and Nd, forming with oxygen and sulphur compounds with a higher temperature stability compared to manganese and aluminum. A partial propensity of non-metallic inclusions to elongation along hotworking direction is characterized by the aspect ratio of about 1.67. The occurance of dispersive, complex, partiallymodified sulfide-oxide inclusions with a diameter from 30 to 100 nm in the new-developed steels can have a positive influence on the limitation of austenite grain growth during hot-working.

9

Hot-working of advanced high-manganese austenitic steels

Dobrzański L. A., Borek W.

Journal of Achievements in Materials and Manufacturing Engineering

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2010

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Vol. 43, nr 2

507--526

EN

Purpose: The work consisted in investigation of newly elaborated high-manganese austenitic steels with Nb and Ti microadditions in variable conditions of hot-working. Design/methodology/approach: The force-energetic parameters of hot-working were determined in continuous and multi-stage compression test performed in temperature range of 850 to 1100°C using the Gleeble 3800 thermomechanical simulator. Evaluation of processes controlling work-hardening were identified by microstructure observations of the specimens compresses to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light, scanning and electron microscopy as well as X-ray diffraction. Findings: The investigated steels are characterized by high values of flow stresses from 230 to 450 MPa. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25)Mn alloys. Increase of flow stress along with decrease of compression temperature is accompanied by translation of εmax strain in the direction of higher deformation. Results of the multi-stage compression proved that applying the true strain 4x0.29 gives the possibility to refine the austenite microstructure as a result of dynamic recrystallization. In case of applying the lower deformations 4x0.23 and 4x0.19, the process controlling work hardening is dynamic recovery and a deciding influence on a gradual microstructure refinement has statical recrystallization. The steel 27Mn-4Si-2Al-Nb-Ti has austenite microstructure with annealing twins and some fraction of ε martensite plates in the initial state. After the grain refinement due to recrystallization, the steel is characterized by uniform structure of γ phase without ε martensite plates. Research limitations/implications: To determine in detail the microstructure evolution during industrial rolling, the hot-working schedule should take into account real number of passes and higher strain rates. Practical implications: The obtained microstructure - hot-working relationships can be useful in the determination of power-force parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures. Originality/value: The hot-deformation resistance and microstructure evolution in various conditions of hot-working for the new-developed high-manganese austenitic steels were investigated.