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EN
Materials scientists are seeking to produce metals with reduced weight and dimensions while maintaining the appropriate mechanical properties. There are several ways to improve the internal structure of metals, such as the ultrasound used to solidify liquid metal. The homogeneity of the grains and the uniformity of the metal structure affects its mechanical strength. This paper presents the results of investigations into the effects of hot deformation parameters in compression on the austenite grain size in the HSLA (High Strength Low Alloy) steel (0.16% C, 0.037% Nb, 0.004% Ti, 0.0098% N). The axisymmetric compression investigations were performed on cylindrical investigation specimens using a Gleeble 3800 thermomechanical simulator with the strain rate of 1÷15.9 s-1 and strain degree ε = 1.2. Before deformation, the research specimens were austenitized at TA = 1100÷1250 °C. The metallographic observations of the primary austenite grains were conducted with an optical microscope, while the structure of dynamically recrystallized austenite, inherited by martensite, was examined by using a scanning electron microscope.
EN
In this study, we present a new method for obtaining the parameters of the Johnson-Mehl-Avrami-Kolmogorov equation for dynamic recrystallization grain size. The method consists of finite-element analysis and optimization techniques. An optimization tool iteratively minimizes the error between experimental values and corresponding finite-element solutions. Isothermal backward extrusion of the AA6060 aluminum alloy was used to acquire the main parameters of the equation for predicting DRX grain size. We compared grain sizes predicted using optimized and reference parameters with experimental values from the literature and found better agreement when the optimized parameters were applied.
EN
In the current study, the hot deformation of medium carbon V-Ti micro-alloyed steel was surveyed in the temperature range of 950 to 1150°C and strain rate range of 0.001 to 1 s-1 after preheating up to 1200°C with a compression test. In all cases of hot deformation, dynamic recrystallization took place. The influence of strain rate and deformation temperature on flow stress was analyzed. An increase in the strain rate and decrease in the deformation temperature postponed the dynamic recrystallization and increased the flow stress. The material constants of micro-alloyed steel were calculated based on the constitutive equations and Zener-Hollomon parameters. The activation energy of hot deformation was determined to be 458.75 kJ/mol, which is higher than austenite lattice self-diffusion activation energy. To study the influence of precipitation on dynamic recrystallization, the stress relaxation test was carried out in a temperature range of 950 to 1150°C after preheating up to 1200°C. The results showed no a stress drop while representing the interaction of particles with dynamic recrystallization.
EN
In this work, the adiabatic shear band of TA1 titanium alloy subjected to electromagnetic impact loading was investigated. The formation of adiabatic shear band and microstructure evolution within it were revealed by microstructure characterizations. Deformation results showed an adiabatic shear band with the width of 10 mm located in shear deformation zone, and most deformations mainly concentrated in the narrow band. The compressive insta-bility and the hardness difference contributed to the formation of adiabatic shear band. Severe shear deformations led to high location density within the adiabatic shear band. A large amount of dislocations distributed in the form of dislocation cells and random dislocations. The rotational dynamic recrystallization mechanism caused that many dy-namic recrystallization grains with the size of 100–200 nm were found inside the adiabatic shear band. Adiabatic temperature rise and distortion energies stored by high dislocation densities provided sub-grain rotations with the driving forces.
EN
The main aim of the presented research is the analysis of damage evolution in 16MnCrS5 steel during hot forming based on results obtained from finite element modelling. Particular attention is put on the interaction between dynamic recrystallization (DRX) and damage initiation at the matrix-inclusion interface. Moreover, a modified Gurson-Tvergaard- Needleman (GTN) model is proposed with the nucleation criterion taken from an extended Horstemeyer model, which predicts damage nucleation based on material softening due to the DRX and stress state in the material.
EN
Isothermal hot compression experiments were carried out using the Gleeble-1500D thermal mechanical simulator. The flow stress of the Cu-1%Zr and Cu-1%Zr-0.15%Y alloys was studied at hot deformation temperature of 550°C, 650°C, 750°C, 850°C, 900°C and the strain rate of 0.001 s-1, 0.01 s-1, 0.1 s-1, 1 s-1, 10 s-1. Hot deformation activation energy and constitutive equations for two kinds of alloys with and without yttrium addition were obtained by correlating the flow stress, strain rate and deformation temperature. The reasons for the change of hot deformation activation energy of the two alloys were analyzed. Dynamic recrystallization microstructure evolution for the two kinds of alloys during hot compression deformation was analyzed by optical and transmission electron microscopy. Cu-1%Zr and Cu-1%Zr-0.15%Y alloys exhibit similar behavior of hot compression deformation. Typical dynamic recovery occurs during the 550-750°C deformation temperature, while dynamic recrystallization (DRX) occurs during the 850-900°C deformation temperature. High Zr content and the addition of Y significantly improved Cu-1%Zr alloy hot deformation activation energy. Compared with hot deformation activation energy of pure copper, hot deformation activation energy of the Cu-1%Zr and Cu-1%Zr-0.15%Y alloys is increased by 54% and 81%, respectively. Compared with hot deformation activation energy of the Cu-1%Zr alloy, it increased by 18% with the addition of Y. The addition of yttrium refines grain, advances the dynamic recrystallization critical strain point and improves dynamic recrystallization.
EN
The hot deformation behaviors of vanadium and titanium microalloyed steels containing different nitrogen contents were studied by performing hot compression tests at various temperatures ranging from 900 to 1050°C and strain rates ranging from 0.1 to 10 s-1. The flow stress curves of the experimental steels were analyzed and the effect of nitrogen on the hot deformation behaviors of the vanadium titanium and nitrogen microalloyed steels was discussed. The results reveal that the flow stress increases with increasing nitrogen addition and the critical strain for the onset of dynamic recrystallization(DRX) also increases by adding nitrogen. Therefore, larger strain should be applied to start DRX in the experimental steel containing higher nitrogen content. The material constants and activation energies for hot deformation were determined by regression method and the effect of nitrogen on the activation energy was also discussed referring to the activation energies from the previous researches. It is found that higher nitrogen content contributes to higher activation energy for hot deformation. Furthermore, the DRX kinetics models for the experimental steels were constructed by regression method and the effect of nitrogen on the DRX rate under various deformation conditions was analyzed. And the inhibition of DRX by increasing nitrogen content is confirmed.
PL
W artykule przedstawiono wyniki badań wpływu grubości materiału wsadowego w postaci taśmy odlewanej metodą Huntera oraz schematu gniotów jednostkowych w procesie walcowania na strukturę i własności mechaniczne blach ze stopu Zn-Cu-Ti, w szczególności na ich podatność do zginania. W tym celu przeprowadzono eksperyment w warunkach przemysłowych, w którym procesowi walcowania poddano trzy kręgi taśmy o zbliżonym składzie chemicznym o trzech różnych grubościach, wytworzone w procesie odlewania ciągłego metodą Huntera, przy zastosowaniu stałych parametrów ciekłego metalu i stałej prędkości odlewania. Proces walcowania ze względu na różnice w grubości materiału wsadowego, realizowany był z zastosowaniem zróżnicowanych schematów gniotów jednostkowych, i przy ustalonych pozostałych parametrach walcowania, takich jak: temperatura wsadu i prędkość walcowania w poszczególnych przepustach. Próbki wytworzonych w ten sposób blach poddano obserwacjom strukturalnym, na podstawie których stwierdzono zróżnicowany stopień ich rekrystalizacji. Na podstawie analizy otrzymanych wyników badań własności mechanicznych stwierdzono, że stopień dynamicznej rekrystalizacji w procesie walcowania jest ściśle związany z własnościami mechanicznymi gotowych blach, jak i ich podatnością do zginania. Przeprowadzony eksperyment pozwolił wyznaczyć optymalną grubość taśmy odlewanej oraz optymalną ilość gniotów w procesie walcownia, która pozwala na uzyskanie pożądanych własności użytkowych gotowych blach.
EN
The paper presents the results of investigations of the influence of thickness of the input material in the form of the Hunter-cast strip and deformation scheme during rolling process on the structure and mechanical properties of Zn-Cu-Ti alloy sheets, in particular their bendability. For this purpose, an industrial experiment was carried out in which three strips with similar chemical compositions and different thickness, manufactured within Hunter method, were subjected to the rolling process. The rolling process due to the difference in the thickness of the input material was carried out at different deformation schemes and at the constant other rolling parameters, ie rolling temperature and rolling speeds in the following rolling passes. Sheet samples produced in this way were subjected to structural observations, on the basis of which their varying degree of recrystallization was found. Based on the analysis of the obtained results of mechanical tests, it was found that the degree of dynamic recrystallization in the rolling process is closely related to the mechanical properties as well as bendability of the finished sheets. The experiment allowed to determine the optimum thickness of the cast strip and the optimum amount of rolling in the reversing rolling mill process, which allows to achieve the desired properties of the finished sheets.
EN
Hot forging is an important process for shaping and property control of lightweight titanium aluminide parts. Dynamic recrystallization and phase transformations play an essential role for the resulting grain size and accordingly the mechanical properties. Due to the fact that titanium aluminides require forging under isothermal conditions, reliable process modeling is needed to predict the microstructure evolution, to optimize the process time and to avoid excessive die loads. In the present study an isothermal forging process of a compressor blade made of TNB-V4 (Ti–44.5Al–6.25Nb–0.8Mo–0.1B, at. %) is modeled using the Finite Element (FE) – Software Q-Form. A microstructure model describing the microstructure evolution during forging is presented. To calibrate the model, the high-temperature deformation behavior was investigated using isothermal compression tests. The tests were carried out at temperatures from 1150°C to 1300°C, applying strain rates ranging from 0.001s-1 to 0.5s-1, up to a true strain of 0.9. The experimentally determined flow stress data were described with model equations determined form the course of the strain hardening rate in Kocks-Mecking plots. An isothermal forging process of a compressor blade was carried out and used to validate the results from the FE simulations.
PL
Plastyczna przeróbka na gorąco jest ważnym procesem po-zwalającym nadawać kształt i kontrolować własności wyrobów z glinków tytanu. Dynamiczna rekrystalizacja i przemiany fazowe odgrywają kluczową rolę w kształtowaniu końcowej wielkości ziarna i, w konsekwencji, własności mechanicznych wyrobu. Ponieważ glinki tytanu wymagają kucia w warunkach izotermicznych, potrzebny jest dokładny model rozwoju mikrostruktury aby umożliwić optymalizację czasu trwania procesu i aby uniknąć przeciążenia matryc. W niniejszej pracy proces kucia łopatki kompresora został zamodelowany metodą elementów skończonych (MES) z wykorzystaniem programu Q-Form. Badanym materiałem był stop TNB-V4 (Ti—44.5Al-6.25Nb-0.8Mo-0.IB, at. %). W pracy przedstawiono zastosowany model rozwoju mikrostruktury. Model został skalibrowany na podstawie wyników prób ściskania na gorąco w warunkach izotermicznych. Badania przeprowadzono w temperaturach w zakresie 1150°C - 1300°C i dla prędkości odkształcenia w zakresie 0.001 s"1 d 0.5 s' . Całkowite odkształcenie w tych próbach wynosiło 0.9. Wyznaczone doświadczalnie naprężenie uplastyczniające zostało opisane za pomocą prędkości umocnienia zgodnie z krzywymi Kocksa-Meckinga. Fizyczny proces kucia łopatki kompresora został wykorzystany do walidacji modelu MES.
EN
Dynamic softening behaviors of a promising biomedical Ti-13Nb-13Zr alloy under hot deformation conditions across dual phase α + β and single phase β regions were quantitatively characterized by establishing corresponding dynamic recovery (DRV) and dynamic recrystallization (DRX) kinetic models. A series of wide range hot compression tests on a Gleeble-3500 thermo-mechanical physical simulator were implemented under the strain rate range of 0.01-10 s−1 and the temperature range of 923-1173 K. The apparent differences of flow stress curves obtained in dual phase α + β and single phase β regions were analyzed in term of different dependence of flow stress to temperature and strain rate and different microstructural evolutions. Two typical softening mechanisms about DRV and DRX were identified through the variations of a series of stress-strain curves acquired from these compression tests. DRX is the dominant softening mechanism in dual phase α + β range, while DRV is the main softening mechanism in single phase β range. The DRV kinetic model for single phase β region and the DRX kinetic model for dual phase α + β region were established respectively. In addition, the microstructures of the compressed specimens were observed validating the softening mechanisms accordingly.
11
Content available remote Perceptive comparison of mean and full field dynamic recrystallization models
EN
Review of dynamic recrystallization models is the subject of the present work. Development of both mean field and full field approaches during last three decades is presented and discussed. Conventional mean field models based on closed form equations as well as differential equations are presented first. Then full field models are elaborated focusing on the cellular automata approach as an example. Capabilities as well as limitations and drawbacks of these approaches are highlighted based on the set of case studies. Experimental data for validation of models were obtained from uniaxial compression tests at Gleeble 3800 thermo-mechanical simulator.
PL
W artykule przedstawiono wyniki badań wpływu temperatury nagrzewania wsadu do procesu walcowania taśmy ze stopu Zn-Cu-Ti w walcarce kwarto-nawrotnej na strukturę i własności przetwarzanych blach, w szczególności na ich podatność do zginania. W tym celu przeprowadzono eksperyment w warunkach przemysłowych, w którym procesowi walcowania poddano pięć kręgów taśmy o zbliżonym składzie chemicznym i przy ustalonych podstawowych parametrach walcowania, tj. schemacie gniotów i prędkościach walcowania w poszczególnych przepustach. Próbki wytworzonych w ten sposób blach poddano obserwacjom strukturalnym, na podstawie, których stwierdzono ich różny stopień rekrystalizacji. Na podstawie analizy otrzymanych wyników badań własności mechanicznych stwierdzono, że stopień dynamicznej rekrystalizacji w procesie walcowania jest ściśle związany z własnościami mechanicznymi gotowych blach, jak i ich podatnością do zginania. Przeprowadzony eksperyment pozwolił wyznaczyć minimalną temperaturę nagrzewania odlanego wsadu do procesu walcowania, która pozwala na uzyskanie pożądanych własności użytkowych gotowych blach.
EN
The paper presents the research results on influence of the charge’s heating temperature intended for rolling process of the sheet made from Zn-Cu-Ti alloy in a four-high reversing mill on the structure and properties of the processed sheets, and its susceptibility to bending in particular. To this end, an experiment in industrial conditions was conducted, in which five coils of similar chemical compositions and established basic rolling parameters, which are draft schemes and rolling velocities in particular roll passes, were put to the rolling process. The sheet samples made according to this method were subjected to structural observations, basing on which various fractions of recrystallization were determined. On the grounds of the analysis concerning mechanical properties it was found that the fraction of dynamic recrystallization during the rolling process is tightly connected with mechanical properties and susceptibility to bending of the ready sheets. The performed test allowed to determine the minimal heating temperature of the casted charge designed for rolling process which permits to obtain desirable utilitarian properties of the ready made sheets.
EN
Samples of selected alloys were compressed in a Gleeble thermo-mechanical simulator at selected temperatures and strain rates to a total true strain of 1. The samples were made of twelve alloys: Monel K-500, Inconel 625, Inconel 718, Permalloy, Waspaloy, Ti-6Al-4V, Ti-6Al-2Sn-4Zr-6Mo, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-10V-2Fe-3Al, Ti-48Al-2Cr-2Nb, 4130 steel and 4340 steel. Exemplary microstructures of the alloys examined after the deformation under the conditions conducive to the occurrence of a partial dynamic recrystallization are shown. The process of dynamic recrystallization has a significant effect on the alloys’ final hardness: it frequently reduces the hardness. The grain size decrease during the dynamic recrystallization process may sometimes increase the alloys’ final hardness.
PL
Próbki wybranych stopów były spęczane w symulatorze termo-mechanicznym Gleeble w wybranych temperaturach oraz z wybranymi prędkościami odkształcenia do wielkości odkształcenia rzeczywistego 1. Próbki do badań wykonano z dwunastu stopów: M onel K -500, Inconel 625, Inconel 718, Permalloy, Waspaloy, Ti-6Al-4V, Ti-6Al-2Sn-4Zr-6Mo, Ti-3Al-8V-6Cr- -4Mo-4Zr, Ti-10V-2Fe-3Al, Ti-48Al-2Cr-2Nb, stal 4130 oraz stal 4340. Przedstawiono przykładowe mikrostruktury badanych stopów po odkształceniu plastycznym. Wyboru mikrostruktur dokonano opierając się na stopniu zaawansowania procesu rekrystalizacji dla zmiennej prędkości odkształcenia. Proces rekrystalizacji dynamicznej ma widoczny wpływ na twardość badanych stopów. Najczęściej proces rekrystalizacji dynamicznej (lub rekrystalizacji zwłocznej) zmniejsza twardość materiału. Jednakże, rozdrobnienie ziarna w wyniku początkowego etapu rekrystalizacji, wpływ odbudowy mikrostruktury oraz rozrostu ziarna na hartowność może skutkować wzrostem twardości w wyniku procesu rekrystalizacji dynamicznej.
EN
The goal of the work was to describe the forging conditions of thermomechanical treatment for Ti-V and Ti-Nb-V microalloyed steels. Conditions of hot-working allowing to obtain both the desired microstructure and mechanical properties of forgings were selected taking into consideration: precipitation analysis of MX-type (M – Nb, Ti, V; X – N, C) interstitial phases in austenite; research on the influence of the austenitizing temperature on the g-phase grain size; investigation of the continuous compression of specimens; and examination of the kinetics of recrystallization of plastically deformed austenite. The precipitation analysis of MX-type interstitial phases in austenite was conducted on the basis of a simplified thermodynamic model for equilibrium conditions as proposed by Adrian, assuming that individual MX phases are soluble in austenite. The effect of the austenitizing temperature in a range from 900 to 1200°C on the prior austenite grain size was investigated to verify the precipitation analysis of MX-type phases. The work also presents the results of the effect of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure. Plastometric tests were carried out using the Gleeble 3800 thermomechanical test simulator. The studies provide the basis for a proper design of the manufacturing process for thermomechanical treatment of forged machine parts obtained from high-strength microalloyed steels.
PL
Celem pracy było opracowanie warunków kucia metodą obróbki cieplno-plastycznej stali mikrostopowych typu Ti-V i Ti-Nb-V. Warunkiobróbki plastycznej na gorąco, zapewniające pożądaną mikrostrukturę i własności mechaniczne odkuwek, dobrano z uwzględnieniem: analizy wydzielania się w austenicie faz międzywęzłowych typu MX (M – Nb, Ti, V; X – N, C), badań wpływu temperatury austenityzowania na wielkość ziarn fazy g, badań ciągłego ściskania próbek oraz badań kinetyki rekrystalizacji austenitu odkształconego plastycznie. Analizę wydzielania faz międzywęzłowych typu MX w austenicie przeprowadzono na podstawie modelu zaproponowanego przez Adriana, zakładającego rozpuszczalność w warunkach równowagi termodynamicznej, indywidualnych faz MX w austenicie. Badania wpływu temperatury austenityzowania w zakresie od 900 do 1200°C na wielkość ziarn austenitu pierwotnego przeprowadzono w celu weryfikacji analizy wydzielania faz typu MX. Wpracyprzedstawiono także wyniki badań wpływu mikrododatków Nb, Ti i V na krzywe płynięcia, kinetykę rekrystalizacji i mikrostrukturę. Badania plastometryczne przeprowadzono przy użyciu symulatora termomechanicznego Gleeble 3800. Przeprowadzone badania stanowią podstawę prawidłowego projektowania procesu technologicznego obróbki cieplno-plastycznej kutych elementów maszyn o wysokiej wytrzymałości ze stali mikrostopowych.
EN
The paper presents analysis of plasticity characteristics and microstructure of magnesium alloys for hot plastic treatment with different aluminium content (3÷8%). Tests were conducted for assessment of susceptibility of tested alloys to hot plastic deformation. A tensile test was run in temperature from 250 to 450°C. Based on the results, ultimate tensile strength (UTS) and reduction of area (Z) were determined for samples. Conducted compression tests allowed to specify the flow stress and microstructure changes after deformation. The activation energy in hot forming was determined for investigated alloys. The parameters of the process where flow is significantly influenced by twin formation in microstructure were determined. A varied plasticity of tested alloys was found depending on aluminium content. Test results will be useful in development of forging technology of selected construction elements which serve as light substitutes for currently used materials.
PL
W pracy analizowano charakterystyki plastyczności i mikrostrukturę stopów magnezu przeznaczonych do przeróbki plastycznej na gorąco ze zróżnicowana zawartością aluminium (3-8%). Badania prowadzono dla oceny podatności badanych stopów do kształtowania plastycznego w warunkach przeróbki plastycznej na gorąco. Przeprowadzono próbę rozciągania w temperaturze od 250 do 450 st.C, na podstawie której określono wytrzymałość na rozciąganie i przewężenie badanych próbek. Wykonane próby ściskania pozwoliły na wyznaczenie naprężenia uplastyczniajacego i zmian mikrostruktury po odkształceniu. Dla badanych stopów wyznaczono energię aktywacji odkształcenia plastycznego na gorąco. Określono parametry procesu, dla których na przebieg plastycznego płynięcia wpływa istotnie bliźniakowanie w mikrostrukturze. Wykazano zróżnicowaną plastyczność badanych stopów w zależności od zawartości aluminium. Wyniki badań będą pomocne do opracowania technologii kucia wybranych elementów konstrukcyjnych stanowiących lekkie zamienniki stosowanych obecnie materiałów.
EN
Purpose: The aim of the paper is to determine the influence of hot-working conditions on microstructure evolution and phase composition of new-developed high-manganese austenitic TRIP-type steels. Design/methodology/approach: The hot-working behaviour was determined in continuous and multistage compression tests performed in a temperature range of 850 to 1100°C by the use of the Gleeble 3800 thermo-mechanical simulator. The processes controlling work hardening and removing it were identified by microstructure evolution observations in different stages of compression with the amount of true strain 4x0.23. Phase composition of steels was confirmed by X-ray diffraction analysis. Findings: It was found that they have austenite microstructure with numerous annealing twins in the initial state. Continuous compression tests realized in the temperature range from 850 to 1050°C with the strain rate of 0.1, 1 and 10 s-1 enabled determination of yield stress values and values of εmax deformations - corresponding to maximum flow stress. The investigated steels are characterized by high values of flow stress from 120 to 380 MPa. Results of the multi-stage compression proved that applying the true strain 4x0.23 gives the possibility to refine the austenite microstructure. 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 conditions relationships and stress-strain curves can be useful in determination of power-force parameters of hot-rolling for sheets with fine-grained austenitic structures. Originality/value: The hot-working behaviour and microstructure evolution in various conditions of plastic deformation for new-developed high-manganese austenitic TRIP-type steels with Nb and Ti microadditions were investigated.
EN
Purpose: The work presents research results of impact of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure of newly elaborated steels assigned for production of forged machine parts, using the method of thermo-mechanical treatment. Design/methodology/approach: The study was performed with the use of Gleeble 3800 simulator. Stress-strain curves were determined during continuous compression test in a temperature range from 900 to 1100°C and at a strain rate of 1, 10 and 50 s-1. In order to determine recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of specimens were done with a given strain at the rate of 10 s-1., in a temperature range from 900 to 1100°C, with isothermal holding of samples between successive stages of deformation for 2 to 100 s. Recrystallization kinetics of plastically deformed austenite was described using the Johnson-Mehl-Avrami equation. The observations of microstructures of thin foils were done using JEOL JEM 3010 transmission electron microscope. Findings: Basing on the analysis of the form and the course of curves obtained in the compression test, it was found that in the studied range of parameters of hot plastic deformation, the decrease of strain hardening of studied steels is caused by the process of continuous dynamic recrystallization. This is also confirmed by calculation results of activation energy of plastic deformation process. Performed two-stages compression tests revealed that microadditions introduced into steel considerably influence the kinetics of static recrystallization. Research limitations/implications: It was found that the time necessary for a total course of recrystallization of austenite is too long to be accepted in the production process of forgings. Practical implications: Executed hot compression tests will contribute to establishing conditions of forging with the method of thermo-mechanical treatment. Originality/value: Strain-stress curves and recrystallization kinetics curves of newly elaborated microalloyed steels have been determined.
EN
In this research work, the double differentiation mathematical method was used to identify more accurately the critical stress (σc) and critical strain (εc) associated with the onset of dynamic recrystallization (DRX), which is based on changes of the strain hardening rate (θ=δσ/δε) as a function of the flow stress (Poliak and Jonas method, simplified by Najafizadeh and Jonas). For this purpose, a low carbon advanced ultra-high strength steel (A-UHSS) microalloyed with different amounts of boron (0, 14, 33, 82, 126 and 214 ppm) was deformed by uniaxial hot-compression tests at high temperatures (950, 1000, 1050 and 1100°C) and constant true strain rates (10-3, 10-2 and 10-1s-1). Results indicate that both σc and εc increase with decreasing deformation temperature and increasing strain rate. On the other hand, these critical parameters tend to decrease as boron content increases. Such a behavior is attributed to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect.
PL
Matematyczna metoda podwójnego różniczkowania została zastosowana dla dokładniejszej identyfikacji krytycznego naprężenia σc i krytycznego odkształcenia εc. związanego z rozpoczęciem rekrystalizacji dynamicznej (DRX). Te dwa parametry są związane z prędkością umocnienia θ=δσ/δε przedstawianą jako funkcja naprężenia uplastyczniającego (metoda Poliaka i Jonasa, uproszczona przez Najafizadeha i Jonasa). W tym celu niskowęglową stal o wysokiej wytrzymałości (ang. advanced ultra-high strength steel -AUHSS) wzbogacono różnymi zawartościami boru (0, 14, 33, 82, 126 i 214 ppm). Tą stal poddano odkształceniom plastycznym w wysokich temperaturach (950, 1000, 1050 i 1100°C przy stałych prędkościach odkształcenia (10-3, 10-2 and 10-1s-1). Otrzymane wyniki wykazały, że zarówno σc jak i εc rośnie wraz z obniżeniem się temperatury odkształcenia i wzrostem prędkości odkształcenia. Z drugiej strony, te krytyczne parametry zmniejszają się gdy wzrasta zawartość boru w stali. Jako przyczynę takiego zachowania uznano wpływ atomów boru w roztworze na granice ziaren austenitu a także wpływ mięknięcia roztworowego.
EN
Purpose: The aim of the paper is to determine the influence of hot-rolling conditions on structure of new-developed high-manganese austenitic steels. Design/methodology/approach: Flow stresses during continuous and multi-stage compression tests were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steels were compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in different stages of hot-rolling was determined in metallographic investigations using light microscopy as well as X-ray diffraction. Findings: The steels are characterized by different microstructure in the initial state. Steel with higher Al concentration has stable microstructure of austenite with annealing twins, while steel with higher Si concentration consists of certain portion of e martensite in form of plates. The flow stresses are in the range of 200-430 MPa for the applied conditions of hot-working and are up to 40 MPa lower compared to continuous compressions. 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. On the basis of analysis of thermo-mechanical treatment carried out in continuous axisymetrical compression test and multi-stage compression test using the Gleeble 3800 simulator allowed to work out a schedule of three different variants of hot-rolling for each of investigated steels 26Mn-3Si-3Al-Nb-Ti and 27Mn-4Si-2Al-Nb-Ti. Research limitations/implications: To fully describe the hot-rolling behaviour of the new-developed steels, further investigations in wider temperature and strain rate ranges are required. Practical implications: Various conditions of hot-rolling for advanced high-manganese austenitic steels can be useful to determine influence of microstructure on mechanical properties obtained in static and dynamic tensile test. Originality/value: Microstructure evolution in various conditions of hot-rolling for advanced high-manganese austenitic steels were investigated.
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Purpose: The aim of the paper is to characterise the microstructure evolution of new-developed 27Mn-4Si-2Al-Nb-Ti high-manganese steel in various conditions of hot-working. Design/methodology/approach: Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed 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 steel has austenite microstructure with annealing twins and some fraction of ĺ martensite plates in the initial state. 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. The flow stresses are in the range of 200-400 MPa for the applied conditions of hot-working. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress to 350 MPa during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. After the grain refinement due to recrystallization, the steel is characterised by uniform structure of ă phase without ĺ martensite plates. Research limitations/implications: To fully describe the hot-working behaviour of the new-developed steel, further investigations in wider temperature and strain rate ranges are required. Originality/value: The hot-deformation resistance and microstructure evolution in various conditions of hot-working for the new-developed high-manganese 27Mn-4Si-2Al-Nb-Ti austenitic steel were investigated.
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