The Effect of Annealing Temperature and Temper-Rolling on Microstructure,Mechanical Properties,and Lüders Strain of Fe–0.25C–5.9Mn–1.0Al–1.57Si Medium Mn Steel

This study investigates the effect of austenite reverted transformation (ART) annealing temperature and temper-rolling on the microstructure, mechanical properties, and deformation behaviors of cold-rolled Fe–0.25C–5.9Mn–1.0Al–1.57Si transformation-induced plasticity (TRIP) steel. The cold-rolled steel annealed at 700 °C demonstrates excellent mechanical properties. The ultimate tensile strength, total elongation, and product of strength and elongation are observed as 1212 MPa, 31.8%, and 38.6 GPa%, respectively. The excellent combination of strength and ductility is related to the discontinuous TRIP effect; still, an inhomogeneous deformation is observed during tensile deformation, known as the Lüders strain. Temper-rolling is used for the ART-annealed specimens at 700 and 720 °C, and yield point elongation decreases when temper-rolling reduction increases. When the temper-rolling reduction increases by 8%, the yield point elongation of the specimen annealed at 700 °C is noted at 1%, while the specimen annealed at 720 °C exhibits continuous yielding. The strain-induced martensite transformation and increased dislocation density in the ferritic matrix improve the early-stage strain hardening rate, thus suppressing the Lüders band's formation.     

Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part II. Jerky Flow and Instantaneous Strain Rate

The jerky and smooth flow curves in high-manganese twinning induced plasticity (TWIP) steels were investigated by comparing Fe-Mn-C and Fe-Mn-Al-C systems. The pronounced serrations on the flow curves of Fe-Mn-C TWIP steel, produced during tensile testing at 300 K (27 °C) and 373 K (100 °C), were shown to be the result of localized high-temperature Portevin Le-Chatelier (PLC) bands moving across the gage length throughout the deformation. The speed of the PLC bands and their temperature effects were found to be strongly dependent on the applied strain rate, which was controlled by adjusting the cross-head speed of the tensile testing machine. The localized temperature-dependent stacking fault energy (SFE) variations resulting from the PLC effect and adiabatic heating were analyzed and compared for both slow and fast deformation rates. The instabilities in the measured logarithmic strain values caused by jerky flow could cause the local strain rate to deviate systematically from the targeted (applied) strain rate. These instabilities are better observed by calculating the instantaneous strain rate (ISR) values for each instant of deformation along the entire gage length. Finally, a new type of diagram was developed by plotting the true stress against the ISR values. From the diagram, the onset of different mechanisms, such as deformation twinning, nonpronounced, and pronounced serrations, could be marked precisely.     

Hydrogen Effects in Prestrained Transformation Induced Plasticity Steel

Thermal desorption analysis (TDA) was performed on laboratory heat-treated transformation induced plasticity (TRIP) steel with 14.5?pct retained austenite (RA), ultimate tensile strength (UTS) of 880?MPa, and elongation to failure of 33?pct. Samples were tensile prestrained 5?pct at 253?K (?C20?°C), 296?K (23?°C), and 375?K (102?°C) to generate different amounts of deformation-induced martensite, 10.5, 5.5, and 0.5?pct, respectively, prior to cathodically charging to a hydrogen content of 1 to 2 ppm. TDA was performed on charged samples to determine the location and strength of hydrogen trapping sites. TDA results suggest that dislocations were the main trapping sites in prestrained TRIP steel. The TDA peak intensity increased with prestrain, suggesting that the quantity of hydrogen trap sites increased with deformation. Tensile tests were performed on the four hydrogen-charged TRIP steel conditions. As confirmed with transmission electron microscope images, samples with more homogeneous dislocation distributions (i.e., prestrained at 375?K (102?°C)) exhibited greater resistance to hydrogen embrittlement than samples that included a high dislocation density adjacent to the formations of strain-induced martensite (i.e., samples prestrained at 253?K (?C20?°C) and 296?K (23?°C)).     

Study on the Strain Hardening Behaviors of TWIP/TRIP Steels

Due to the complex coupling of twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP), and dislocation glide in TWIP/TRIP steels, it is difficult as well as essential to build a comprehensive strain hardening model to describe the interactions between different deformation mechanisms (i.e., deformation twinning, martensitic transformation, and dislocation glide) and the resulted strain hardening behaviors. To address this issue, a micromechanical model is established in this paper to predict the deformation process of TWIP/TRIP steels considering both TWIP and TRIP effects. In the proposed model, the generation of deformation twinning and martensitic transformation is controlled by the stacking fault energy (SFE) of the material. In the thermodynamic calculation of SFE, deformation temperature, chemical compositions, microstrain, and temperature rise during deformation are taken into account. Varied by experimental results, the developed model can predict the stress–strain response and strain hardening behaviors of TWIP/TRIP steels precisely. In addition, the improved strength and enhanced strain hardening in Fe-Mn-C TWIP/TRIP steels due to the increased carbon content is also analyzed, which consists with literature.     

Processing of Ultrafine-Grained Twinning-Induced Plasticity Steel Using Martensite Treatment

For the first time, martensite treatment was used to fabricate an ultrafine-grained (UFG) twinning-induced plasticity (TWIP) steel. The effects of cold rolling with 70 pct reduction at the liquid nitrogen temperature and subsequently annealing at 973 K (700 °C) for 5 to 20 minutes on the microstructure and mechanical properties of Fe-22Mn-0.4C-1.5Al-1Si TWIP steel were investigated. The results showed that a fully recrystallized UFG TWIP steel with a mean grain size of about 400 to 600 nm can be produced by the designed martensite treatment. The UFG TWIP steel exhibited high yield and tensile strengths and relatively high ductility.     

Microstructure Evolution and Mechanical Behavior of a CMnSiAl TRIP Steel Subjected to Partial Austenitization Along with Quenching and Partitioning Treatment

The present study investigated the microstructure evolution and mechanical behavior in a low carbon CMnSiAl transformation-induced plasticity (TRIP) steel, which was subjected to a partial austenitization at 1183 K (910 °C) followed by one-step quenching and partitioning (Q&P) treatment at different isothermal holding temperatures of [533 K to 593 K (260 °C to 320 °C)]. This thermal treatment led to the formation of a multi-phase microstructure consisting of ferrite, tempered martensite, bainitic ferrite, fresh martensite, and retained austenite, offering a superior work-hardening behavior compared with the dual-phase microstructure (i.e., ferrite and martensite) formed after partial austenitization followed by water quenching. The carbon enrichment in retained austenite was related to not only the carbon partitioning during the isothermal holding process, but also the carbon enrichment during the partial austenitization and rapid cooling processes, which has broadened our knowledge of carbon partitioning mechanism in conventional Q&P process.     

Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part I. Mechanism Maps and Work-Hardening Behavior

Thermodynamic stacking fault energy (SFE) maps were developed using the subregular solution model for the Fe-Mn-Al-C system. These maps were used to explain the variations in the work-hardening behavior of high-manganese steels, both through experiments and by comparison with the published data. The suppression of the transformation induced plasticity (TRIP) mechanism, the similarity between the shape of the work-hardening rate diagrams for the produced iso-SFE materials, and an earlier onset of stage C of work hardening by decreasing SFE were shown to be efficiently predictable by the given mechanism maps. To overcome the limitations arising from studying the deformation response of high-manganese steels by SFE values alone, for example, the different work-hardening rate of iso-SFE materials, an empirical criterion for the occurrence of short-range ordering (SRO) and the consequently enhanced work-hardening, was proposed. The calculated values based on this criterion were superimposed on the thermodynamics-based mechanism maps to establish a more accurate basis for material design in high-manganese iron-based systems. Finally, the given methodology is able to clarify the work-hardening behavior of high-manganese twinning induced plasticity (TWIP) steels across an extensive range of chemical compositions.     

Solubility of Nitrogen in High Manganese Steel (HMnS) Melts: Interaction Parameter between Mn and N

Nitrogen solubility in Fe-Mn melts was measured using a N₂ bubbling and sampling method at temperatures from 1823 K to 1923 K (1550 °C to 1650 °C) for manganese content to about 25 mass pct. The effect of temperature on the nitrogen solubility was well described based on the thermodynamic behavior of Fe-Mn system. Furthermore, the interaction parameter between Mn and N was evaluated as a function of temperature. The present results can be used in thermodynamic analyses of the formation of nitride compounds such as AlN or TiN in high manganese steel melts for example, transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) aided steels as well as high Mn-N alloyed stainless steels.     

<Emphasis Type="Italic">In Situ</Emphasis> Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced High-Strength Steels

Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third-generation advanced high-strength steels (3GAHSS) under quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt pct) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The medium Mn (10 wt pct) TRIP steel deforms inhomogeneously via propagative instabilities (Lüders and Portevin Le Châtelier-like bands), while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development of a new in situ experimental technique that couples volumetric synchrotron X-ray diffraction measurement of RAVF with surface strain measurement using stereo digital image correlation over the beam impingement area. Measurement results with the new technique are compared to those from a more conventional approach wherein strains are measured over the entire gage region, while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.     

Tailoring the Mechanical Properties of a Twinning-Induced Plasticity Steel by Retention of Deformation Twins During Heat Treatment

The relation between microstructure and mechanical properties of a 30 pct cold-rolled, recovery-annealed, and recrystallization-annealed Fe-23Mn-1.5Al-0.3C twinning-induced plasticity (TWIP) steel was studied. The thermal stability of deformation-induced twin boundaries along with a reduced dislocation density due to annihilation during recovery annealing at 903 K (630 °C) was found to be a simple, promising processing route to overcome the shortcoming of low yield strength usually associated with TWIP steels.     

On the Tensile Behavior of High-Manganese Twinning-Induced Plasticity Steel

High-manganese FeMnC and FeMnAlC austenitic twinning-induced plasticity (TWIP) steel exhibits excellent strain-hardening properties due to the gradual reduction of the mean free path for dislocations glide resulting from deformation twinning. Serrated stress-strain curves are often obtained when this type of steel is tested in a uniaxial tensile test. This phenomenon is due to dynamic strain aging (DSA). It is related to the occurrence of localized Portevin–LeChatelier (PLC) deformation bands. The properties of the PLC bands were accurately determined for a FeMnAlC TWIP steel using a combination of high-sensitivity infrared (IR) thermographic imaging and optical strain analysis carried out in situ during tensile deformation. Strain rate jump tests were conducted at room temperature to measure the instantaneous and steady-state strain rate sensitivity as a function of true stress and true strain. Negative values of the steady-state strain rate sensitivity were measured in both upward and downward jump tests. These measurements explain why FeMnC and FeMnAlC TWIP steels have a limited postuniform elongation. A model for the room-temperature DSA of high-Mn austenitic TWIP steel containing C in solid solution is proposed.     

Environment-Assisted Cracking of Twinning Induced Plasticity (TWIP) Steel: Role of pH and Twinning

This article presents the study of the environment-assisted cracking (EAC) of twinning induced plasticity (TWIP) steels that possess remarkable combination of strength and ductility. EAC of a high-manganese TWIP steel was investigated, using aqueous solutions of different pH, which provided a mechanistic insight into the combined role of the localized deformation due to twinning and the electrochemical characteristic of the steel. Slow strain rate testing in inert environment as well as in acidic, neutral and alkaline solutions, and the fractography of the failed specimens have suggested a profound role of twinning in EAC crack propagation.     

On the Austenite Stability of a New Quality of Twinning Induced Plasticity Steel, Exploring New Ranges of Mn and C

A new high-manganese, low-silicon TWIP steel was studied to evaluate austenite stability after different heat treatment conditions. To determine the phase transformations, dilatometric experiments were performed, and the microstructure was characterized by light optical microscopy, X-ray diffraction, and transmission electron microscopy. Precipitation of lamellar cementite was observed in the microstructure for extended treatment times at 823 K (550 °C). Long isothermal holding at this temperature also caused epsilon martensite formation during cooling, resulting from a decrease in austenite stability due to carbon depletion in the matrix when a quantifiable amount of cementite is formed.     

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Temperature Dependent Deformation Mechanisms of a High Nitrogen‐Manganese Austenitic Stainless Steel

The influence of temperature on the deformation behaviour of a Fe‐16.5Cr‐8Mn‐3Ni‐2Si‐1Cu‐0.25N (wt%) austenitic stainless steel alloy was investigated using transmission electron microscopy and X‐ray diffraction measurements. Recrystallized samples were deformed under tension at ?75°C, 20°C, and 200°C and the microstructures were characterized after 5% strain and after testing to failure. Deformation to failure at ?75°C resulted in extensive transformation induced plasticity (TRIP) with over 90% α′‐martensite. The sample deformed to 5% strain at ?75°C shows that the austenite transformed first to ?‐martensite which served to nucleate the α′‐martensite. Transformation induced martensite prohibits localized necking providing total elongation to failure of over 70%. At room temperature, in addition to some TRIP behaviour, the majority of the deformation is accommodated by dislocation slip in the austenite. Some deformation induced twinning (TWIP) was also observed, although mechanical twinning provides only a small contribution to the total deformation at room temperature. Finally, dislocation slip is the dominant deformation mechanism at 200°C with a corresponding decrease in total elongation to failure. These changes in deformation behaviour are related to the temperature dependence on the relative stability of austenite and martensite as well as the changes in stacking fault energy (SFE) as a function of temperature.     

Production of Ultrahigh-Strength and High-Ductility TWIP Steel by Precipitation of β-Mn during Large Deformation during Warm-Rolling

Herein, twinning-induced plasticity (TWIP) steel having large deformation is rolled at different rolling temperatures to improve the tensile strength and retain a certain plastic deformation capacity. Based on X-ray diffraction and transmission electron microscope analysis, β-Mn is found as the precipitate at the grain boundary during the warm-rolling process (500–650 °C). To investigate the impact of β-Mn on the tensile properties, the microstructure of the TWIP steel rolled at the temperature value of 600 °C is observed by carrying out electron backscatter diffraction and scanning electron microscope measurements. The intergranular β-Mn phase can help the material to accumulate geometric necessary dislocation (GND) density, inhibit crack propagation, as well as improve the strength and plasticity of the material. Once TWIP steel is warm-rolled above the temperature value of 600 °C, and serrated flow appears in the tensile process, which is also conducive to improving the material properties.     

Microstructures and Mechanical Properties of Fe-Mn-（Al, Si） TRIP/TWIP Steels

The mechanical properties and microstructure of two low carbon high manganese steels with 23.8% （No. 1） and 33% （No. 2） （mass percent） of manganese were investigated. The results showed that No. 1 steel possesses high strength and high plasticity, and No. 2 steel has a relatively high strength and extraordinary plasticity. The No. 1 steel exhibits both TRIP （transformation induced plasticity） and TWIP （twin induced plasticity） effects during the deformation; while only TWIP effect appeared under the same deformation condition for No. 2 steel. The comparison between the microstructures and mechanical properties of two steels was made, and the strengthening mechanisms were also analyzed.     

Microstructure and mechanical properties of the hot-rolled Fe16Mn0.6C steel plate

Results presented in this study contribute to investigation of the microstructure and mechanical properties of the hot-rolled Fe16Mn0.6C steel plates.The steel plates have been produced by being hot-rolled at temperatures ranging from 1100℃ to 850℃ in seven passes to 97.5% reduction in thickness and then cooled in a furnace of 650℃.Some plates have been annealed at temperatures ranging from 300℃ to 1100℃ for 5min to 60min,and then followed by water quenching.There are annealing twins in the hot-rolled Fe16Mn0.6C steel.Fe16Mn0.6C steel presents similar ductile behavior as X-IPTM steel,but much higher elongation than commercial martensitic steel (MP) 1000,dual phase (DP) 980,and transformation induced plasticity (TRIP) 980 steels.Fe16Mn0.6C steel experiences γ→ε (-α) transformation in some local regions,but remains mostly austenite during the entire deformation process.Fe16Mn0.6C steel with special mechanical properties can be produced by using the appropriate anneal technology.Twinning induced plasticity(TWIP) effect only occurs in the Fe16Mn0.6C steel annealed at temperature higher than 900℃.     

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N₂+10 pct H₂ gas atmosphere with low DPs [213 K and 243 K (?60 °C and ?30 °C)], the coating layer consisted of δ (FeZn₁₀) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (?10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe₃Zn₁₀) phase crystals, and a thin layer of Г ₁ (Fe₁₁Zn₄₀) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.