Search Results (28)

The development of lightweight steels with high specific strength is critical for automotive applications and energy savings. This study aimed to develop a high-performance lightweight steel with high specific strength by designing an alloy composition and optimizing thermomechanical processing. A novel Fe-28.6Mn-10.2Al-1.1C-3.2Ni-3.9Cr (wt.%) steel was investigated, focusing on microstructural evolution, mechanical properties, and strengthening mechanisms. The steel was processed through hot-rolling, solution treatment, cold-rolling, and subsequent annealing. Microstructural characterization revealed a dual-phase matrix of austenite and ferrite (6.8 vol.%), with B2 precipitates distributed at the grain boundaries and within the austenite matrix, alongside nanoscale κ-carbides (<10 nm). Short-time annealing resulted in the finer austenite grains (~1.1 μm) and the higher volume fraction (5.0%) of intragranular B2 precipitates with a smaller size (~0.18 μm), while long-time annealing promoted the coarsening of austenite grains (~1.6 μm) and the growth of intergranular B2 particles (~0.9 μm). This steel achieved yield strengths of 1130~1218 MPa and tensile strengths of 1360~1397 MPa through multiple strengthening mechanisms, including solid solution strengthening, grain boundary strengthening, dislocation strengthening, and precipitation strengthening. Full article

Ferritic lightweight steels are an emerging class of low-density steels (LDSs) with promising mechanical properties. The study aimed to develop two ferritic lightweight steels with different Mn concentrations. Al was incorporated to achieve the lightweighting effect due to its relatively low atomic mass of substitutional solutions. The C concentration was kept at a minimum level to avoid the precipitation of carbides and the Mn addition was intended to increase solid solution strengthening. Thermodynamic calculations (Thermo-Calc) were employed to design the composition, analyze the phase constituents, and predict the phase transformation behavior. Microstructural investigation and hardness tests were conducted to experimentally verify the calculations. Both produced alloys exhibited a fully ferritic microstructure. Compared to industrially produced DP980 steel, a density reduction of about 7.2% and 8.3% was attained for the Fe-0.04C-5.5Al-1.6Mn-0.075Nb and Fe-0.04C-5.6Al-5.5Mn-0.08Nb steels, respectively. The steel with the higher Mn content showed increased hardness attributed to its solution strengthening effect. An increase in the hardness values was also measured with the progress in hot-rolling thickness reductions for both alloys. The alloying elements influenced the microstructural characteristics, phase transformation behavior, density, and hardness of the newly designed lightweight steels. Full article

To explore the impact of niobium (Nb) addition on the austenitization behavior of Fe-Mn-Al-C lightweight steels, the effects were examined through Thermo-Calc thermodynamic simulations, optical microscopy, transmission electron microscopy (TEM), and the development of austenite grain growth models. Three distinct Fe-Mn-Al-C steel compositions, each containing different Nb contents (0.38%, and 0.56%), were subjected to various austenitization temperatures and aging conditions, and a kinetic model for austenite grain growth was established. The results demonstrate that for heating temperatures below 950 °C, the austenite grain growth rate of the steels was similar. However, at temperatures above 950 °C, the grain growth rate of the steel without Nb (Steel No. 1) increased significantly compared to the niobium-containing alloys. Austenite grain size increased with higher heating temperatures. At constant heating temperatures, longer holding times resulted in larger grain sizes, though the rate of grain size growth diminished over time. Based on the experimental data and the kinetic theory of austenite grain growth, a grain growth model of No. 2 Steel (which contained 0.38% Nb) was established. The predicted grain size values derived from this model closely matched the experimental measurements, indicating a strong correlation and providing valuable insights for future studies. Full article

This research focuses on Fe-18Mn-10Al-1C-5Ni lightweight steel and deeply explores the influences of three different cooling methods, namely, water quenching (WQ), air cooling (AQ), and furnace cooling (FQ), on the precipitation behavior of the B2 phases and κ-carbides in the lightweight steel. The intrinsic relationship among the precipitated phases, mechanical properties, and fracture behavior is revealed. Compared with the WQ sample, the size of the intragranular B2 phase in the AQ sample did not change significantly (an increment of 9 nm), but nano-sized κ-carbides appeared at the grain boundaries and inside the grains. The yield strength and tensile strength of the AQ sample significantly increased to 1232 MPa and 1347 MPa, respectively, while an elongation of 17.4% was still maintained, which benefitted from the synergistic effect of the grain boundary B2, intragranular B2, and nano-sized κ-carbides. When the cooling rate of the heat treatment was further reduced, the size of the intragranular B2 phase in the FQ sample increased slightly (332 nm), and the κ-carbides at the grain boundaries became obviously coarsened (170 nm), resulting in a severe reduction in the elongation (2.3%) because, during the tensile deformation process, the coarsened κ-carbides at the grain boundaries promoted the nucleation of voids and microcracks. The present work provides new insights into the cooling heat treatment process of lightweight steel. Full article

The development of new materials with low weight for the transport industry is required for the saving of natural resources and protection of the environment from carbon dioxide pollution. The microstructure and mechanical properties of the Fe-30Mn-10Al-3.3Si-1C steel in as-cast, quenched, aged, and hot-deformed states were investigated. Austenite, ferrite, and κ-carbides are present in the steel in an as-cast state. Hot deformation of steels was made using the thermal and mechanical simulation system Gleeble-3800 at temperatures of 900–1050 °C and strain rates of 0.1–10 s⁻¹. Mechanical properties in as-cast, annealed, aged, and hot-deformed states were determined by Vickers hardness and compression tests. A constitutive model of the hot deformation behavior of Fe-30Mn-10Al-3.3Si-1C steel with high accuracy (R² = 0.995) was constructed. The finite element analysis of the deformation behavior of the steel under the plane-strain scheme was performed. Compression tests at room temperature have shown an increase in strength and ductility after hot deformation. The strain hardening of ferrite and austenite grain refinement during dynamic recrystallization are the main reasons for the growth of steel’s plasticity and strength. A specific strength of the investigated material is in the range from 202,000 to 233,000 m²/s² which is higher than high-strength steels previously developed and used in the automotive industry. Full article

In this study, the microstructure, mechanical properties, wear resistance, and wear-hardening mechanism of Fe-28Mn-8.5Al-1.0C lightweight wear-resistant steel after heat treatment at different aging temperatures were examined. The results show that the nano-scale κ-carbides precipitated in the grains after aging treatment increased the strength and hardness of the material through the strengthening effect of the second phase. The yield strength of the material is 697 MPa, the tensile strength is 905 MPa, and the hardness is up to 294 HB after aging at 500 °C for 5 h. However, the large-sized κ-carbides precipitating continuously at the grain boundary are unfavorable to the plasticity and toughness of the material. Compared with the aging treatment at 300 °C for 5 h, the elongation and low-temperature impact energy decreased by 12.0% and 47.1%, respectively. Except for the dominant wear mechanism being plastic deformation after heat treatment at 500 °C for 5 h with a 4J impact energy, the predominant wear mechanisms for different impact energies under all other heat treatment conditions are micro-cutting. The increase in aging temperature increases the number and volume of κ-carbide precipitation, which leads to enhanced second-phase strengthening and dislocation strengthening, and the wear resistance of the material is improved. The hardening mechanism of the material after wear at different impact energy levels under aging treatment conditions is a cross-distributed dislocation wall and high-density dislocation entanglement. The increase in aging temperature reduces the spacing of the dislocation wall, increases the area and density of dislocation entanglement, and enhances the work-hardening effect. Full article

The automotive industry’s rapid expansion has made the development of lightweight, high-strength automotive steels essential for both energy efficiency and emission reduction. Among these materials, Fe-Mn-Al-C steel has drawn considerable interest due to its favorable combination of low density and high strength. This research examines the impact of Nb alloying (with Nb content of 0% and 0.5%) and solution treatment on the microstructure and mechanical properties of cold-rolled Fe-28Mn-10Al-C low-density steel. Various methods were employed, including Thermo-Calc thermodynamic simulations, the Olson–Cohen model, X-ray diffraction (XRD), metallographic microscopy, room-temperature tensile testing, and scanning electron microscopy (SEM). The findings demonstrate that Nb alloying significantly refines the austenite grain structure of the Fe-28Mn-10Al-C steel, improving both strength and ductility in comparison to the 0Nb steel. After solution treatment at 1050 °C for 30 min, the cold-rolling-induced defects are effectively removed, leading to a substantial increase in elongation at fracture (38.14–44.45%) and an ultimate tensile strength exceeding 900 MPa. As the solution treatment temperature increases, the austenite grains coarsen, and the number of twins increases, while yield strength and ultimate tensile strength decrease. However, there is a notable enhancement in ductility, with the material exhibiting a ductile fracture mechanism. These results offer valuable insights and a theoretical foundation for further improving the mechanical properties of Fe-Mn-Al-C low-density steels. Full article

In this study, a novel lightweight Fe-Mn-Al-C steel composition and thermomechanical processing route was developed to produce a fully austenitic microstructure with a uniform intragranular dispersion of B2-NiAl precipitation in order to overcome the significant challenge of strengthening hot-rolled Fe-Mn-Al-C steels while retaining toughness. The new composition and processing methods allow for the processing of ultrahigh-strength Fe-Mn-Al-C steel containing nickel as thicker gauge plate for a multitude of new automotive and structural applications where lightweighting is critical. The composition investigated in this study was a fully austenitic Fe-21Mn-9Al-1C-8Ni wt% steel. Two hot rolling methods were investigated: the first procedure involved lower temperature rolling cycles to precipitate B2-NiAl during hot rolling and reheating. The second method involved higher temperature rolling to precipitate B2-NiAl after thermomechanical processing during a short isothermal treatment. The lower temperature rolling produced plate with an ultimate tensile strength of 1120 MPa and a Charpy V-Notch (CVN) toughness of 24 J at −40 °C. After the high temperature rolling procedure, precipitation of B2-NiAl through a subsequent precipitation hardening step resulted in reduced B2-NiAl size and improved the ultimate tensile strength above 1300 MPa. The two novel processing routes of a single composition can be performed with current manufacturing capabilities to produce hot rolled plate strengthened by B2-NiAl precipitation to various hardness (ranging from 33 to 41 HRC) and strength levels (ranging from 1100 to 1320 MPa ultimate tensile strength) while retaining 22–27% elongation. Full article

The influence of Cr addition on the microstructure and tensile properties of Fe-25Mn-10Al-1.2C lightweight steel was investigated. The characteristics of the microstructures and deformation behavior were carried out through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and room temperature tensile testing. Fe-20Mn-12Al-1.5C steel without Cr exhibited a fully austenitic single phase. With the addition of Cr, the volume fraction of ferrite continuously increased. When the content of Cr exceeded 5 wt%, the precipitation of Cr₇C₃ carbides was observed. In the steel with 5 wt% Cr, the quantity of κ carbides remarkably decreased, indicating that the addition of 5 wt% Cr significantly inhibited the nucleation of κ-carbides. As the Cr content increases from 0 wt% to 5 wt%, the austenite grain sizes were 8.8 μm and 2.5 μm, respectively, demonstrating that Cr alloying is an effective method of grain refinement. Tensile strength increased slightly while elongation decreased with increasing Cr content. As the Cr content exceeded 5 wt%, the yield strength increased but the elongation drastically decreased. The steel with 2.5 wt% Cr achieved a synergistic improvement in strength and ductility, exhibiting the best tensile performance. Full article

Aluminum-incorporated medium-manganese steel (MMnS) has potential for lightweight transport applications owing to its impressive mechanical properties. Increasing the austenite volume fraction and making microstructural changes are key to manufacturing MMnS. However, the grain boundary character and strain distribution of intercritically annealed low-density MMnS have not been extensively scrutinized, and the effects of crystallographic texture orientation on tensile properties remain ambiguous. Therefore, in this study, the microstructure, microtexture, strain distribution, and grain boundary characteristics of a hot-rolled medium-Mn steel (Fe–0.2 C–4.3 Al–9.4 Mn (wt%)) were investigated after intercritical annealing (IA) at 750, 800, or 850 °C for 1 h. The results show that the 800 °C annealed sample exhibited the highest austenite volume fraction among the specimens (60%). The duplex microstructure comprised lath-type γ-austenite, fine α-ferrite, and coarse δ-ferrite. As the IA temperature increased, the body-centered cubic phase orientation shifted from <001> to <111>. At higher temperatures, the face-centered cubic phase was oriented in directions ranging from <101> to <111>, and the sums of the fractions of high-angle grain boundaries and coincidence–site–lattice special boundaries were significantly increased. The 800 °C annealed sample with a high austenite content and strong γ-fiber {111}//RD orientation demonstrated a noteworthy tensile strength (1095 MPa) and tensile elongation (30%). Full article

Fe-Mn-Al-C lightweight steels have been of significant interest due to their excellent mechanical properties and unique microstructures. However, there has been limited focus on the dynamic deformation. Here, we systematically investigate the mechanical responses over various strain rates and corresponding microstructure evolution in quasi-static and dynamic compression to reveal the transition of deformation mechanisms. The present lightweight steel exhibits a significant strain rate effect, with the yield strength increasing from 735.8 to 1149.5 MPa when the strain rate increases from 10⁻³ to 3144 s⁻¹. The deformation in ferrite under high-strain-rate loading is dominated by wave slip, forming a cellular structure (cell block). Meanwhile, the deformation in austenite is dominated by planar slip, forming dislocation substructures such as high-density dislocation walls and microbands. In addition, the deformation twinning (including secondary twinning)- and microband-induced plasticity effects are responsible for the excellent dynamic compression properties. This alloy delays damage location while maintaining high strength, making it ideal for shock loading and high-strain-rate applications. The Johnson–Cook (J–C) constitutive model is used to predict the deformation behavior of lightweight steel under dynamic conditions, and the J–C model agrees well with the experimental results. Full article

In this study, the Fe-16Mn-9Al-0.8C-3Ni (wt.%) lightweight steel was fabricated by novel twin-roll strip casting technology. The microstructure, tensile properties and strain-hardening behavior of the present steel have been investigated and compared to those of conventionally processed steels with similar chemical compositions. After annealing, a unique gradient microstructure of intermetallic compound (B2)-austenite was obtained along the thickness direction, consisting of granular B2 (average: 430 nm) and fine austenite (average: 1.82 μm) at the surface layer, blocky B2 (average: 1.03 μm) and medium austenite (average: 3.98 μm) at the quarter layer and polygonal B2 (average: 1.94 μm) and coarse austenite (average: 6.13 μm) at the center layer. The cooperative action of B2 pinning dislocation, plane slip and back stress led to stronger strain hardening, among which the strong back stress effect originated from the multistage discontinuous austenite deformation and the mechanical incompatibility between austenite and B2 is believed to be the most important reason, thereby achieving an excellent balance of strength (ultimate tensile strength: 1147 MPa) and ductility (total elongation: 43.2%). This work not only developed a new processing way to fabricate Ni-containing Fe-Mn-Al-C lightweight steel with outstanding mechanical properties, but also provided a potential solution for manufacturing some other metallic materials accompanied by brittle B2 intermetallic. Full article

The development of new lightweight materials is required for the automotive industry to reduce the impact of carbon dioxide emissions on the environment. The lightweight, high-manganese steels are the prospective alloys for this purpose. Hot deformation is one of the stages of the production of steel. Hot deformation behavior is mainly determined by chemical composition and thermomechanical parameters. In the paper, an artificial neural network (ANN) model with high accuracy was constructed to describe the high Mn steel deformation behavior in dependence on the concentration of the alloying elements (C, Mn, Si, and Al), the deformation temperature, the strain rate, and the strain. The approval compression tests of the Fe–28Mn–8Al–1C were made at temperatures of 900–1150 °C and strain rates of 0.1–10 s⁻¹ with an application of the Gleeble 3800 thermomechanical simulator. The ANN-based model showed high accuracy, and the low average relative error of calculation for both training (5.4%) and verification (7.5%) datasets supports the high accuracy of the built model. The hot deformation effective activation energy values for predicted (401 ± 5 kJ/mol) and experimental data (385 ± 22 kJ/mol) are in satisfactory accordance, which allows applying the model for the hot deformation analysis of the high-Mn steels with different concentrations of the main alloying elements. Full article

High-Mn lightweight steel, Fe-0.9C-29Mn-8Al, was manufactured using steelmaking, ingot-making, forging, and rolling processes. After the final rolling process, a typical austenite single phase was observed on all sides of the thick plate. The microstructural changes after annealing and aging heat-treatments were observed, using optical and transmission electron microscopy. The annealed coupon exhibited a typical austenite single phase, including annealing twins in several grains; the average grain size was 153 μm. After aging heat treatment, κ-carbide was observed within the grains and on the grain boundaries. Additionally, the effect of aging heat treatment on the mechanical properties was analyzed, using a tensile test. The fine κ-carbide that precipitated within the grains in the aged coupon improved the 0.2% offset yield and the tensile stresses, as compared to the as-annealed coupon. To estimate the applicability of high-Mn lightweight steel for low-pressure (LP) steam turbine blades, a low-cycle fatigue (LCF) test was carried out at room temperature. At a total strain amplitude of 0.5 to 1.2%, the LCF life of high-Mn lightweight steel was approximately three times that of 12% Cr steel, which is used in commercial LP steam turbine blades. The LCF behavior of high-Mn lightweight steel followed the Coffin–Manson equation. The LCF life enhancement in the high-Mn lightweight steel results from the planar dislocation gliding behavior. Full article

Fe-Mn-Al-C lightweight steels have been investigated intensely in the last a few years. There are basically four types of Fe-Mn-Al-C steels, ferritic, ferrite-based duplex/triplex (ferrite + austenite, ferrite + austenite + martensite), austenite-based duplex (ferrite + austenite), and single-austenitic. Among these steels, austenite-based lightweight steels generally exhibit high strength, good ductility, and outstanding weight reduction effects. Due to the addition of Al and high C content, κ’-carbide and κ-carbide are prone to form in the austenite grain interior and at grain boundaries of lightweight steels, respectively, and play critical roles in controlling the microstructures and mechanical properties of the steels. The microstructural evolution, strengthening mechanisms, and deformation behaviors of these lightweight steels are quite different from those of the mild conventional steels and TRIP/TWIP steels due to their high stacking fault energies. The relationship between the microstructures and mechanical properties has been widely investigated, and several deformation mechanisms have also been proposed for austenite-based lightweight steels. In this paper, the current research works are reviewed and the prospectives of the austenite-based Fe-Mn-Al-C lightweight steels are discussed. Full article