Intercritical Annealing Research Articles

Role of Solidification Behavior and Various Processing Routes on Microstructure–Property Correlation in Medium‐Mn Steels with and without Nickel: Recent Progresses and Perspectives

Variations in alloying contents, thermo‐mechanical processing (TMP), and heat treatment (HT) routes provide a range of excellent mechanical properties in medium‐Mn steels (MMnS). However, solidification defects limit the industrial production of MMnS as it belongs to the peritectic class of alloys. Considering recent progress, it mandates a comprehensive review of the role of solidification behavior and various processing routes on microstructure–property correlation. Hence, the present article addresses microstructure evolution during solidification, TMP, HT, and corresponding mechanical properties. Quenching & partitioning (Q&P) and intercritical annealing (IA) are considered for the present review. It is observed that δ‐ferrite transforms to austenite by a massive transformation mechanism after the peritectic reaction. High strength but low elongation is found in Q&P as compared to IA MMnS. Further, the occurrence of discontinuous yielding is associated with both HT routes. However, it is preventable with process parameters. Ni addition generally enhances the strength–ductility balance in comparison with Ni‐free MMnS. However, it primarily depends on microstructural characteristics such as phase fraction, distribution, morphology, degree of recrystallization, precipitates, and retained austenite stability. Thus, optimum processing routes can be designed considering the aforementioned factors.

Synergistic improvement in the strength and ductility of a medium Mn steel by single-step warm rolling and intercritical annealing

Synergistic improvement in the strength and ductility of a medium Mn steel by single-step warm rolling and intercritical annealing

A simple strategy to overcome strength-ductility trade-off via tailoring ferrite characteristic in medium Mn steel

This study examined the role of ferrite (tempered martensite) characteristics on the austenite mechanical stability and the mechanical properties of medium Mn steel. It was found that the morphology of ferrite in the annealed microstructure has a significant impact on the austenite mechanical stability and mechanical properties of medium Mn steel. The large massive ferrite grains distributed minimal strain during tensile loading, resulting in significant interphase strain incompatibility and consequently low total elongation and austenite mechanical stability of the experimental steel. Inserting a short-duration high-temperature annealing into the traditional intercritical annealing (IA) process can reduce the proportion of blocky ferrite grains by promoting the nucleation of austenite within the defect-free martensite block in the hot-rolled microstructure of the experimental steel. By adjusting the duration of high-temperature annealing, it is possible to simultaneously optimize the ferrite morphology and intrinsic mechanical stability of austenite, drastically enhancing the mechanical properties of medium Mn steel. In this study, the total elongation of the experimental steel increased from 25.1% to 46.3%, and the product of strength and elongation (PSE) increased from 25.75 GPa·% to 52.78 GPa·%. Ferrite regulation provides a new strategy for medium Mn steel to overcome strength-ductility trade-off through simple processing.

Kinetics of austenite formation during continuous heating in as-cast and as-annealed conditions in a low carbon steel

Abstract Dilatometric analysis was used to study the effect of the initial microstructure on the kinetics of austenite formation during continuous heating in low-carbon steel. Two initial microstructures were analyzed: 1) as-cast condition, composed of columnar grains of Widmastätten ferrite and pearlite, and 2) as-annealed condition, formed of equiaxed grains of ferrite and pearlite. For both initial conditions, it was observed that austenite formation occurs in two stages: the first stage involves the decomposition of pearlite into austenite, and the second stage involves the transformation of ferrite into austenite. In the as-cast condition, a delay in austenite formation was observed, and a decrease in the transformation extent was associated with a higher transformation rate. The apparent activation energies for each stage of austenite formation were calculated using the Kissinger method, with the peak temperature at the point of maximum transformation rate as the criterion. The calculated energies were too high to be associated with a diffusive transformation mechanism, especially under as-annealed condition, so an analytical kinetic model was proposed to evaluate the behavior of the kinetic parameters. Additionally, the phases formed during intercritical annealing were analyzed to evidence the austenite formation and possible segregation of alloying elements. The analysis shows that elemental variations are negligible, so the difference lies mainly in the morphology of the initial condition.

Effect of Intercritical Annealing on Microstructure and Toughness of Medium-Mn Steel with Elongated Prior-austenite Grains Formed via Two-step Hot Rolling Process

Effect of Intercritical Annealing on Microstructure and Toughness of Medium-Mn Steel with Elongated Prior-austenite Grains Formed via Two-step Hot Rolling Process

Enhanced mechanical properties and plastic instability regulation of medium Mn steel via cyclic intercritical annealing and cold rolling

Enhanced mechanical properties and plastic instability regulation of medium Mn steel via cyclic intercritical annealing and cold rolling

Correlation between intercritical annealing duration and the tensile deformation behavior of Fe-7.5Mn-3.25Al-1Si-0.2C steels: Role of austenite stability, δ-ferrite, and texture

Correlation between intercritical annealing duration and the tensile deformation behavior of Fe-7.5Mn-3.25Al-1Si-0.2C steels: Role of austenite stability, δ-ferrite, and texture

The effect of high-temperature intercritical annealing and rolling strain on the development of trimodal microstructure in low-carbon steel

The effect of high-temperature intercritical annealing and rolling strain on the development of trimodal microstructure in low-carbon steel

Influence of Aluminum on Microstructure and Mechanical Properties of Air‐Hardened Medium Manganese Steel

The influence of different aluminum concentrations (0 and 0.25 wt%) on the mechanical properties and the microstructure of recently developed air‐hardening ductile forging steels with 4 wt% manganese is presented together with the effects of additional heat treatments after air cooling. Based on the achieved mechanical properties, the necessity of aluminum alloying is evaluated due to the processing challenges associated with the Fe–Mn–Al–Si system. For this matter, tensile properties, hardness, and Charpy impact toughness areevaluated with respect to microstructural features characterized by light optical microscopy and scanning electron microscopy equipped with electron backscatter diffraction (EBSD). It is demonstrated that the intermediate addition is not sufficient to reach the target values for the impact toughness, which demonstrates the importance of the addition to this steel grade. Additionally, insights on the tempering embrittlement are gained, as it can be shown that 0.25 wt% Al is sufficient to shift the tempering embrittlement from 300 to 350 °C. Finally, potential intercritical annealing temperatures can be specified: while high balances of strength and ductility can be reported at 650 and 675 °C, an exceeding of this temperature leads to insufficient austenite stabilization and consecutively martensite formation during cooling , as demonstrated by EBSD and X‐ray diffraction.

Chemical Composition Optimization and Isothermal Transformation of δ‐Transformation‐Induced Plasticity Steel for the Third‐Generation Advanced High‐Strength Steel Grade

A new low‐manganese transformation‐induced plasticity steel is designed with optimized nickel content to achieve superior strength and ductility while minimizing the use of expensive nickel. The steel is optimized using JMatPro software, then cast, and hot rolled. To assess the effect of intercritical annealing on austenite (martensite at room temperature) volume fraction and carbon content, hot‐rolled steel samples quenched from different annealing temperatures (680–1100 °C) are used. Additionally, hot‐rolled steel coupons are intercritically annealed at about 50% austenite formation temperature (740 °C) and then subjected to isothermal treatments at 300–425 °C for varying times (10–90 min). After optimizing these treatments to maximize retained austenite (RA), tensile specimens are heat‐treated first at 740 °C and then isothermally at 325 °C. Thermodynamic calculations suggest that aluminum combined with silicon may lead to the δ ferrite formation, and even minimal nickel content can stabilize a considerable amount of austenite. In the experimental studies, it is shown that lower‐temperature bainitic holding enhances austenite stability by enriching the carbon content. Optimized two‐stage heat treatments yield up to 25.8% RA, with a tensile strength of 867.2 MPa and elongation of 40.6%, achieving a strength‐elongation product of 35.2 GPa×%, surpassing the third‐generation advanced high‐strength steel grades minimum requirement of 30 GPa×%.

The influence of intercritical temperature, ferrite-austenite morphology, and orientation relationship on martensitic transformation in dual phase steels

Two different morphologies of martensite in dual-phase (DP) steel were obtained through two processing routes. In the first method, the steel was water quenched from a fully austenitic state, followed by intercritical annealing (IA). In the second method, the steel was water quenched, cold rolled, and then subjected to intercritical annealing (C-IA). For IA and C-IA steels, the intercritical temperatures varied from 735oC to 765oC to obtain different fractions of ferrite. To understand the effect of ferrite content, morphology, and the orientation of ferrite/austenite on martensitic transformation, a combination of scanning electron microscopy, electron backscattered diffraction, and dilatometric tests were conducted. It was observed that layered martensite presented in IA samples, and blocky martensite appeared only at prior austenite boundaries at 765oC. In C-IA samples, equiaxed martensite grains were present, which become coarser by elevating the temperature to 765oC. In IA samples, martensitic transformation was much faster than in C-IA samples; the difference in martensitic transformation rate is particularly significant at lower annealing temperatures. Carbon enrichment of the partial austenite had a converse effect on the martensitic transformation at 745°C in both samples. The highest transformation rate was recorded in IA samples at 745°C among all IA and C-IA samples. The ferrite/martensite in IA samples maintained Kurdjumov–Sachs orientation relationship (K-S OR) with prior austenite and selected the same variant in the same block. Whereas, in polygonal samples, ferrite/martensite had an irrational OR with the prior austenite, no variant selection appeared in ferrite and martensite grains. Martensite nucleation accelerated on the ferrite surface when it had a K-S OR with the matrix grain, forming a low-energy interface with the ferrite. In contrast, ferrite surfaces irrationally oriented with the matrix grain did not promote martensite nucleation. This was likely due to the large misorientation and higher carbon build-up near the boundary during intercritical holding. The lower activation energy for martensitic nucleation in IA samples compared to C-IA samples resulted in an accelerated rate of martensite transformation in IA samples.

Influence of Intercritical Annealing on Microstructure, Ductility, and Toughness of Medium Mn Steels

Influence of Intercritical Annealing on Microstructure, Ductility, and Toughness of Medium Mn Steels

Exploring the impact of intercritical annealing on microstructural evolution and mechanical performance in low alloy multiphase TRIP-assisted steels

Understanding the role of the constituent phase on mechanical properties is the key strategy for achieving advanced mechanical properties in multiphase TRIP-assisted steels. This work unravels the opposing role of ferrite on tensile ductility and impact toughness in the constituent phase fraction-controlled multiphase TRIP-assisted steel. The specimens were subjected to three different intercritical annealing (IA) temperatures (775 °C, 800 °C, and 825 °C) for 20 min, and then continuously followed the bainitic isothermal transformation (BIT) at 400 °C for 30 min. With an increase in the IA temperature, the bainite fraction and the stability of retained austenite increased, accompanied by a decrease in the ferrite fraction. In-situ EBSD observation revealed the crack initiation at the deformation-induced martensite/ferrite interface. The strain localization in ferrite, assisted by transforming neighboring austenite to martensite, promoted crack initiation during the tensile test. The tensile elongation decreased from about 28 % to 17 % due to the lower strain partitioning in ferrite originating in the reduced ferrite fraction and TRIP amounts as the IA temperature increased. However, the refined grain size and the reduced ferrite fraction by a higher IA treatment improve impact toughness by about 60 % at room temperature. This study on the imbalance between tensile elongation and impact toughness sheds light on the microstructure design for advanced mechanical properties in multiphase TRIP-assisted steels.

Annealing temperature-dependent microstructure, mechanical, and tribological properties of intercritically heat-treated dual-phase steel

Abstract This study investigates the role of intercritical annealing temperature on microstructural characteristics, mechanical performance, and wear resistance of DP1000 steel. As-received DP1000 steel samples in cold-rolled conditions were subjected to intercritical annealing between 730 and 880 °C with an interval of 30 °C, followed by water quenching. After heat treatment, the specimens were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), mechanical tests consisting of hardness, impact, and tensile tests, and dry sliding wear tests. The findings revealed that equiaxed ferrite grains replaced elongated ones, and the martensite ratio increased from 21% up to 64% with increasing annealing temperature. Due to the formation of equiaxed grains, the impact strength was at least doubled for each specimen by heat treatment, reaching a peak value (9.55 J cm−2) at 760 °C. The hardness (311 HB) and tensile strength (1007 MPa) of the as-received sample were higher than that of the annealed steels up to 820 °C. The mechanical strength of the samples improved at 850 and 880 °C by approximately 10%, and accordingly, the lowest wear rate was obtained at the specimen annealed at 850 °C. The increase in temperature up to 880 °C caused a decrease in wear resistance due to excessive brittleness.

Achieving balanced mechanical properties in Fe–0.16C plain low-carbon steel by trimodal microstructure and fine cementite

In the present study, for the first time, a trimodal microstructure (including coarse, fine, and ultrafine ferrite grains) was created in plain low-carbon steel by simple thermomechanical processing including intercritical annealing (at 770 °C, 800 °C, and 830 °C), 75% cold rolling, and finally heat treatment at 550 °C. The results showed that martensite with two different morphologies consisting of lath (low-carbon) and plate (high-carbon) were created in the microstructures of 800-75% and 830-75% sheets due to the non-homogeneous distribution of carbon atoms as a result of not using the homogenization treatment. After the final heat treatment, the samples exhibited trimodal grain size distribution of ferrite including coarse grains (larger than 5 micrometers), fine grains (between 1 and 5 micrometers), and ultrafine grains (UFGs) (smaller than 1 micrometer). The formation of ultrafine grains was ascribed to the presence of martensite with lath morphology in the 800-75% and 830-75% samples. The size of cementite particles in the 770-75%-550 sample was larger than the other two samples (800-75%-550 and 830-75%-550) owing to the presence of martensite phase only with plate morphology in 770-75%-550 steel. Unlike the 800-75%-550 sheet, the entire microstructure of the 770-75%-550 and 830-75%-550 samples had not undergone complete recrystallization and several deformed ferrite grains were still visible in the microstructures. The strength and hardness of the heat-treated sheets was larger than that of the as-received steel due to the presence of fine spherical cementite particles and fine ferrite grains in the microstructure of the heat-treated samples. Based on the obtained results, the 800-75%-550 sample revealed the best combination of strength-ductility-toughness due to trimodal microstructure along with the formation of fine spherical cementite particles. The failure mode of all heat-treated samples was ductile and there were both fine and coarse dimples in the fracture surfaces.

ИСПОЛЬЗОВАНИЕ ВЗРЫВНОГО ПРЕССОВАНИЯ И ТЕРМИЧЕСКОЙ ОБРАБОТКИ ДЛЯ ПОЛУЧЕНИЯ МАТЕРИАЛОВ НА ОСНОВЕ МЕТАСТАБИЛЬНОЙ ФАЗЫ TI2FE

The basic regularities of formation of structure and phase composition of materials of Fe-Ti system at compaction by explosion of powder mixtures of titanium and iron are considered. The influence of the mechanism of plastic deformation of powder particles on the process of formation of metastable intermetallic phase Ti2Fe with increased hydrogen capacity has been found out. It is shown that an effective method of obtaining materials based on Ti2Fe is the combination of explosive pressing of a mixture of Fe and Ti powders with subsequent thermal treatment in the intercritical temperature range (reaction sintering in the presence of a liquid phase).

Effect of Intercritical Austenitization and Starting Matrix on Martensite Start Temperature and Austenite Carbon Concentration in Ductile Iron

AbstractIn this work, the effect of the intercritical austenitization temperature and time, Ni + Cu concentration and starting matrix (fully ferritic or fully pearlitic) on the volume fraction of austenite (fγ), martensite start temperature (Ms) and carbon concentration of the parent austenite (C0) in ductile iron was investigated using standard metallographic techniques and high-resolution dilatometry. The results showed that a fully pearlitic starting matrix gives higher fγ and C0 and lower Ms than a fully ferritic starting matrix at the same austenitization temperature under continuous heating. On the other hand, the austenitization time has an effect on the parameters under study only during the first minutes of intercritical austenitization. Finally, at the same intercritical austenitization temperature, the addition of Ni + Cu increases the volume fraction of austenite.

Microstrain partitioning, transformation induced plasticity, and damage evolution of a third generation medium Mn advanced high strength steel

An experimental Medium Mn (med-Mn) steel (0.15C-5.8Mn-1.8Al-0.71Si) with a martensitic starting microstructure, intercritically annealed at 685 °C for 120s, was discovered to have a large true strain at fracture (ɛf = 0.61) while also meeting established (strength x elongation) targets (28,809 MPa%), sustained monotonic work hardening and prolonged Transformation Induced Plasticity (TRIP) kinetics. This was found by varying the intercritical annealing (IA) temperature within a narrow temperature interval in order to isolate its impact on TRIP kinetics and damage development on such med-Mn steel. A comprehensive understanding of the microstructural damage processes leading to fracture is presented using quasi in-situ Scanning Electron Microscope tensile testing as well as X-ray Computed Microtomography. In addition, we precisely evaluated the TRIP kinetics of this steel using a combined Digital Image Correlation (DIC) and synchrotron-sourced High Energy X-ray Diffraction technique. With these methods, we demonstrate that an abundance of voids nucleate during deformation, but their growth can be suppressed by prolonging TRIP over a large strain range. Moreover, novel post-processing techniques to assess DIC acquired data at the microscopic scale have been used to gauge the severity of strain partitioning amongst phases and strain gradient evolution across dissimilar phase interfaces. By comparing it to another 3G TRIP-assisted steel and an ultrafine grained Dual Phase steel. Overall, it has been found that, in addition to carefully moderating TRIP kinetics, the introduction of polygonal ferrite, as is conventional in med-Mn steels, enhances the local forming properties and damage tolerance in 3G TRIP-assisted microstructures.

Retained austenite in medium manganese steel with dissimilar morphology and composition

The microstructure and mechanical properties of medium manganese steel with a composition of 0.2C-5Mn-0.5Si-1.5Al (wt.%) were investigated, employing a two-stage heat treatment process. The microstructure following pretreatment comprised pearlite, martensite, and ferrite. Subsequent intercritical annealing resulted in the formation of retained austenite with varying morphology and composition. The filmy retained austenite transformed from pearlite exhibited a higher Mn content compared to the blocky retained austenite transformed from martensite, while also displaying a slower growth rate. Moreover, the filmy retained austenite exhibited superior thermal and mechanical stability. Tensile tests showed that the test steels not only obtained high strength (>1000 MPa) and high elongation (>30%), but the stress-strain curves also showed continuous yielding behaviour and a more reasonable ratio of tensile strength to yield strength.

Understanding low-temperature microstructure and mechanical properties of a newly designed ultra-low carbon medium manganese steel

This study examines the relationship between microstructure and low-temperature mechanical properties of newly designed medium manganese steel with ultra-low carbon, low-Ni content. The microstructural features crucial for impact toughness were identified. The study indicated that post-annealing at 650 °C for 30 min endowed the steel with superior low-temperature mechanical properties, namely tensile strength of 914 MPa, total elongation of 37.2 %, and impact energy of 187 J at −60 °C. With longer intercritical annealing durations, reverted austenite transitioned from a film-like to a blocky morphology, with an increase in both volume fraction and size, accompanied by a substantial decrease in stability. High-stability reverted austenite contributed to sustained work hardening by facilitating a continuous Transformation-Induced Plasticity (TRIP) effect during steel deformation, thereby significantly enhancing the low temperature tensile properties of the steel. Furthermore, enhanced stability of reverted austenite was identified as the pivotal microstructural factor influencing impact toughness.