Lightweight Steels Research Articles

Additive manufacturing of Fe–Mn–Al–C lightweight steel by laser powder bed fusion: The role of laser scanning speed on forming quality, microstructure and properties

The Fe–Mn–Al–C lightweight steels have attracted great attention in the military, aerospace, and automotive industries due to their high strength and lightweight design. Nevertheless, the excessive Mn and Al inclusions have led to poor formability and processability, which brings challenges to the manufacture of large size complex structural parts. In this study, the lightweight steel with a composition of Fe–Mn–Al–C was prepared by laser powder bed fusion for the first time, aimed at investigating the effects of laser scanning speed. The results show that different laser scanning speeds have significant effects on the process sensitivity of lightweight steel, and too low or too high laser scanning speeds may lead to suboptimal sample quality, increased porosity and surface roughness. At a scanning speed of 800 mm/s, the optimal conditions for the lowest porosity (0.03%) and surface roughness (Sa = 3.34 μm) are achieved, resulting in the highest quality of the formed products. Furthermore, an increase in scanning speed results in a reduction in grain size and an enhancement of dislocation strength. When the scanning speed is 800 mm/s, the significant enrichment of Ni–Mn phase helps to enhance the pinning ability of dislocations, while the precipitation of nanoscale small and dense κ-carbide optimizes the Orowan strengthening effect. As a result, dislocation movement is effectively impeded, leading to superior comprehensive mechanical properties of exceptional strength-plasticity matched with a tensile strength, yield strength, and elongation after fracture of 1125.2 MPa, 1019.8 MPa, and 41.9%, respectively.

Effects of TRIP and TWIP mechanisms on improved Charpy impact energy of thick hot-rolled duplex lightweight steel plates

Charpy V-notch impact testing serves as a fundamental method for assessing the toughness of conventional steels, but its applicability becomes challenging for duplex (δ-ferrite + γ-austenite) lightweight steels due to their complex microstructures and varied deformation mechanisms. In this study, Charpy absorbed energies of hot-rolled duplex lightweight steel plates, incorporating both transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) mechanisms, were investigated to understand their fracture behavior comprehensively. The microstructural analysis revealed that the D4 (0.4C-15Mn-6Al (wt.%)) steel exhibited lower δ-ferrite volume fraction and superior Charpy absorbed energy than the D2 (0.2C-15Mn-6Al) steel, attributed to its dominance of TWIP mechanism. Conversely, the D2 steel, with many ferrite/austenite interfaces, showed lower strain hardening and absorbed energy. At lower temperatures, both TWIP and TRIP mechanisms operated, but dominant TRIP mechanism along with higher δ-ferrite volume fraction in the D2 steel accelerated crack propagation, further reducing absorbed energy. The superior toughness of the D4 steel, driven by the dominant TWIP mechanism at lower temperature, highlighted its potential for practical utilization of thick hot-rolled lightweight steel plates in automotive, shipbuilding, military, and construction industries.

Enhancing charpy absorbed energy of aged duplex lightweight steel plates through TRIP and TWIP mechanisms

Ensuring toughness in thick hot-rolled plates remains a challenge for lightweight steels in automotive, shipbuilding, military, and construction industries despite improved tensile properties. This study investigated the Charpy absorbed energy of thick hot-rolled Fe-0.4C-15Mn-6Al duplex lightweight steel plates exhibiting TRIP and TWIP mechanisms, aged at 450–500 °C to precipitate κ-carbides. Fracture initiation and propagation energies measured from instrumented Charpy impact testing were analyzed through microstructural and microfracture analyses. The 500 °C-aged (A500) specimen showed the highest Charpy absorbed energy, composed of the highest fracture initiation and propagation energies across all test temperatures, particularly due to active TWIP and TRIP mechanisms along with significant κ-carbide precipitation strengthening. Despite predominantly ductile fracture modes, regardless of aging temperature and test temperature, deformation mechanisms were influenced by stacking fault energy (SFE). Aging resulted in κ-carbide precipitation, reducing C and Mn contents in austenite and lowering SFE. At 25 °C, the superior energy absorption of the A500 specimen (296 J) was attributed to its high flow stress and extensive roughness in the fracture surface due to crack deflection in the fracture initiation region and zigzag crack propagation. The Charpy absorbed energy decreased significantly at lower temperatures due to limited development of slip line field and less zigzag crack propagation. Despite this, the A500 specimen maintained the highest energy absorption due to its optimized TWIP and TRIP mechanisms and κ-carbide precipitation strengthening.

Enhancing strength-ductility synergy in low-Mn lightweight steel by large-deformed warm rolling and flash annealing

The presence of δ-ferrite makes it challenging to achieve higher strength-ductility in low-Mn lightweight steels. In this study, a large deformation warm rolling process was used to construct a precursor enriched with Mn, nano-precipitations, ultra-fine grains, and high-density dislocations. The subsequent flash annealing process not only promotes the formation of multi-heterogeneous structures but also refines and reduces δ-ferrite while increasing the ultrafine-grained hard phase martensite. Additionally, this process enhances the retention of retained austenite, whose ultrafine grains contribute to improved mechanical stability. The synergistic effect of multiple strengthening mechanisms enables low-Mn lightweight steel to achieve outstanding comprehensive mechanical properties, with a tensile strength of 1423 MPa and a total elongation of 23.0 %.

New insights for composition design: A novel synergistic corrosion-resistant effect of Fe–Mn–Al–Cr–Si–Mo–C lightweight steel in 3.5 wt% NaCl solution

Nowadays, a good corrosion resistance of lightweight steel is demanded for its application. However, due to the high contents of Mn and C, which are always corrosion-susceptible elements in steels, the conventional lightweight steel is not desirable for this mission. Although Cr is widely believed to be a common alloying element to improve the corrosion resistance of most steels, it is considered to be ineligible using in the lightweight steel due to the relatively high content of C. This is because the trade-off between corrosion resistance and mechanical properties by alloying Cr. In recent years, the rapid developments of additive and coating manufacturing have provided new possibilities for both the Cr alloying and microstructural modulation to improve corrosion resistance of lightweight steel. In this case, to design a certain chemical composition using in additive and coating manufacturing, it is interesting to clarify the effects of Cr alloying on the corrosion resistance of lightweight steel. To reveal it, in the present work, the Fe–Mn–Al–Cr–Si–Mo–C lightweight steel alloyed with Cr contents of 4 wt%, 8 wt% and 12 wt% are prepared. The corrosion resistances and mechanisms of Cr-alloyed lightweight steels in 3.5 wt% NaCl solution are investigated by electrochemical and immersion tests. The present work may provide new insights for the composition design of lightweight steel, especially for the directions of additive and coating manufacturing.

Excellent combination of strength and ductility in austenitic lightweight steel achieved by warm rolling process

In this study, warm rolling was applied to Fe–28Mn–11Al–1C–5Ni (wt.%) austenitic lightweight steel, which realized the higher dislocation density and finer dislocation-cell structures, achieving a better mechanical properties with ∼2.0 GPa of yield strength and ∼6.8 % of total elongation than cold rolling in the same conditions. In the subsequent partial recrystallization condition, the numerous dislocations and substructures existed in warm rolled steel provide more heterogeneous nucleation sites for grain refinement, resulting in the ultrafine-sized austenite and B2 with the size of 0.36 μm and 0.24 μm, respectively. Combined with grain refinement strengthening, solid solution strengthening, precipitation strengthening, and dislocation strengthening in the unrecrystallized austenite, the present steel shows an ultimate tensile strength of 1734 MPa, a uniform elongation of 21.7 % and a specific tensile strength of 267 MPa g−1 cm3. Such an excellent combination of strength and ductility induced by simple warm rolling and annealing is much better than previously reported for lightweight steels.

In-situ synthesis and characterization of lightweight steel matrix composites reinforced by TiB2

This study aimed to synthesize lightweight steels in situ reinforced with 5 % and 10 vol% of TiB2 and evaluate the microstructural, physical, and mechanical properties. The Fe-(15;20 wt%)Mn-3 wt%Al-0.1 wt%C with 5 and 10 vol% of TiB2 were produced by spray forming followed by hot rolling and annealing. The concept of varying the manganese content was to assess the influence of the matrix constitution on the composite properties. The findings pointed to a direct correlation between the manganese content and the size and morphology of the TiB2 particles. The presence of the ceramic particles re-established the modulus of elasticity above 200 GPa only for composites with a predominantly ferritic matrix. The composites with a matrix composed mostly of austenite showed no gains in ductility during the tensile tests despite the activation of the TWIP effect. However, their wear behavior stands out positively.

Achieving an ultrahigh yield strength of 1.5 GPa in warm-rolled lightweight steel via synergistic multiplication of precipitation and dislocations

The microstructural evolution and tensile property of a Fe-18Mn-8Al-1C-5Ni (wt%) lightweight steel subjected to warm-rolling and subsequent aging treatment have been investigated. During the warm rolling process, dislocations-induced precipitates further hinders movement of dislocations, resulting in the synergistic multiplication of dislocations and B2 particles. Additionally, shear bands observed within the grains also induce the formation of B2 particles. Subsequent aging facilitates the precipitation of coherent κ-carbides within the matrix, with their its size and volume fraction increasing as dislocation density rises. The newly designed warm-rolled and aged steel exhibits a remarkable ultrahigh strength (yield strength of 1537 MPa and tensile strength of 1632 MPa) and a total elongation of 11.29%. Such superior tensile property not only surpass those of other hot rolled lightweight steels strengthened by B2 and/or κ-carbides but also exceed those of the steels subjected to additional cold rolling. This advancement is expected to expand the applications of B2-strengthened steels in response to an increasing demand for lightweight constructions.

Overcoming the strength-ductility trade-off dilemma in austenitic lightweight steel via stepwise controllable intragranular dual nanoprecipitation

The strength-ductility trade-off is an inevitable scenario in precipitation-hardened austenitic lightweight steels. The reduction in ductility is due to the shearing of κ′-carbides by dislocations, resulting in limited work hardening capability. Conversely, non-shearable B2 particles can effectively improve the work hardening rate. However, these B2 particles tend to precipitate along grain boundaries and coarsen due to their incompatibility with the austenite matrix, resulting in negligible strengthening effect or even brittleness. We propose a stepwise controllable dual nanoprecipitation strategy to overcome the strength-ductility trade-off. To implement this strategy, we develop a manufacturing process that includes a high-temperature annealing treatment and a three-step aging process (700 °C/15min + 900 °C/15min + 450 °C/1 h). The higher annealing temperature increases the supersaturation of solute atoms in the austenite matrix, which improves the chemical driving force for the precipitation of intragranular B2 particles during aging at 900 °C. Additionally, the coarsened κ′-carbides (30–35 nm) formed during aging at 700 °C provide heterogeneous nucleation sites for the formation of intragranular B2 particles. Further aging at 450 °C promotes the precipitation of nano-sized κ′-carbides (5 nm) within the austenite matrix. The dual nanoparticles provide a significant precipitation strengthening contribution of 400 MPa without sacrificing ductility. The multiple deformation mechanisms of nanoscale “planar slip and Orowan bowing” provide an efficient source of work hardening capability. The present work aims to overcome the strength-ductility trade-off in austenitic lightweight steels by tailoring the precipitation process, which may provide insight into producing high-performance lightweight steels.

Synergistic strengthening effect achieved good ductility in ultrahigh strength Fe–22Mn–8Al-0.8C austenitic lightweight steels

We present an ultra-high tensile strength of 1.86 GPa with an elongation of 11 % achieved in a Fe–22Mn–8Al-0.8C austenitic lightweight steels in this study. A partial recrystallized microstructure and κ carbides were obtained in the investigated steels by utilizing a simple thermomechanical processing. Interestingly, the existence of κ carbides not only increased the tensile strength of the investigated specimens but a dramatical increasement of ductility (from 4 % increase to 11 % in this work) was also achieved. The formation of intragranular κ carbides in the matrix can reduce the incompatibility within the material, which can further enhance tensile strength while providing good elongations.

Effect of Cr on the Hot Ductility of Austenitic Fe-Mn-Al-C Lightweight Steel

In this study, a hot ductility test was performed for Fe-30Mn-10.5Al-0.9C-Cr austenitic lightweight steels. The test was carried out through a commercial Gleeble simulator at a heating rate of 350 °C/sec and cooling rate of 50 °C/sec, with a stroke rate of 50 mm/sec. Microstructural analysis for understanding the hot ductility behavior was conducted through optical and scanning electron microscopy. The lightweight steels exhibited similar hot ductility behavior in accordance with temperature despite the addition of Cr. The experimental results indicated that the κ-carbide precipitation had an insignificant influence on the hot ductility test. However, ductility at low temperature was induced by slip mechanism, while dynamic recrystallization had significant influence at high temperatures during the on-heating thermal cycle. In the on-cooling thermal cycle, the melted and re-solidified grain boundaries decreased the overall ductility, exhibiting the same tendency as that observed in the on-heating test.

Tribology, corrosion, and tribocorrosion performance of aged lightweight steels: Effects of oxide film and carbide

The study investigated the tribology, corrosion, and tribocorrosion performance of aged Fe–30Mn–8Al–1.2 C–0.1Ti–(2Cr/Mo) steel, with a focus on oxide film and carbide. The precipitates changed from κ carbide to Cr7C3/MoC with the addition of Cr/Mo. The inhomogeneous precipitation of Cr7C3/MoC created zones of varying hardness, one being hard with strong strengthening and the other soft with weak strengthening. The inhomogeneous hardness distribution deteriorates the tribology performance of the steel. By minimizing galvanic coupling corrosion and improving the protectiveness of the oxide film, the addition of Cr/Mo can improve the corrosion and tribocorrosion resistance of lightweight steel.

A strong and ductile ferrite-based lightweight steel manufactured via high-temperature warm rolling process

A trade-off between strength and ductility is unavoidable in ferrite-based lightweight steels due to the presence of δ-ferrite, and improving the properties of such steels requires precise control of the retained austenite and the complete mobilization of multiple strengthening mechanisms. In this study, we obtain a ferrite-based lightweight steel with the chemical composition Fe-0.3C-2.8Mn-3.5Al-0.5Cr-0.2V by adopting a higher temperature warm rolling process, endowing the best comprehensive properties. The results demonstrate that multi-scale heterogeneous microstructures are built without additional recrystallization annealing under the higher temperature (>700 °C) warm rolling process. Meanwhile, a large amount of retained austenite, higher dislocation density and strain-induced vanadium carbide precipitation were obtained through the warm rolling process. As warm rolling refines austenite grains, Mn and Cr are further diffused and enriched, enhancing the chemical stability of the retained austenite. After low-temperature tempering, this warm-rolled steel shows excellent comprehensive properties, including a tensile strength of 1140 MPa and elongation of 30%. The enhanced strength-ductility is attributed to the higher stability of retained austenite, stable maintenance of dislocation density and the pinning effect of precipitation relative to dislocation lines. Furthermore, a stronger discontinuous yielding behavior of the warm-rolled and tempered steel was obtained, enhancing the overall strain.

Dynamic Compression and Constitutive Model in Fe-27Mn-10Al-1C Duplex Lightweight Steel

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.

Simulation of Solidification, Microsegregation, and Heat Treatment of Cr-Based Fe–xMn–7.5Al–1.0C Lightweight Steels

Simulation of Solidification, Microsegregation, and Heat Treatment of Cr-Based Fe–xMn–7.5Al–1.0C Lightweight Steels

Development of functionally graded austenitic lightweight steel through electrically assisted pressure solid-state joining

Functionally graded materials synergistically combine dissimilar components and can be engineered to exhibit gradual controlled variations in composition, structure, or properties, thus featuring advantageous mechanical properties and finding numerous practical applications. However, lightweight functionally graded materials such as those based on lightweight steels (LWSs) remain underexplored, albeit their advancement has significant merit in reducing the overall weight of a component or structure. To address, this study investigates the effective application of austenitic Fe–Mn–Al–C lightweight steels via the fabrication of a functionally graded material, enabling a synergistic combination of their dissimilar properties. Focusing on the mechanical properties of austenitic LWS that can be controlled through κ-carbide precipitation, we propose a novel functionally graded material developed by joining Mo-doped LWS and Si-doped LWS, which exhibit different κ-carbide precipitation behaviors. Electrically assisted pressure joining, an effective solid-state joining technique capable of enhancing atomic diffusion, was employed to strongly bond two dissimilar LWSs with improved joint integrity while preserving a homogeneous austenite matrix at the joint. Mechanical and microstructural characterization demonstrated that a high-quality and reliable solid-state joint was achieved within a short timeframe of a few minutes without elemental segregations and phase transformations in the metal matrix. The opposing tendencies of Mo to retard the κ-carbide kinetics and Si to enhance it resulted in two divided regions: a Mo-doped low hardness zone and a Si-doped high hardness zone in the joined LWS. Furthermore, by exploiting carbon diffusion driven by the chemical potential gradient, we successfully attained remarkable gradients in the amount of κ-carbide precipitate and hardness, from the joint interface to the Si-doped high hardness region. These findings manifest the applicability of the suggested technique in the meticulous design of functionally graded LWS joint materials.

Study on the role of solution treatment on corrosion behavior of duplex lightweight steel in different environments

Diverse microstructure and complex application environments increases the uncertainty of the corrosion resistance of lightweight steels. The effect of solution treatment on the corrosion behavior of Fe–16Mn–10Al–1.5C–0.1Ti lightweight steel in Cl−, HSO3− and Cl−–HSO3− environments was investigated using electrochemical measurement, scanning electron microscope (SEM) and X–ray photoelectron spectroscopy (XPS). The lightweight steels are dominated by austenite and contain a small amount of ferrite. The increase in solution treatment temperature leads to more ferrite formation, but it has less effect on grain size. The decrease in austenite content results in more corrosion microcells and a porous oxide film; therefore, the corrosion resistance of the steel deteriorates with increasing temperature. In addition, in the Cl−–HSO3− environment, the synergistic effect of Cl− and HSO3− leads to the formation of a weak protective oxide film, resulting in a significant increase in corrosion rate compared to the Cl−and HSO3− environment.

Effect of annealing treatment on microstructure and mechanical properties of lightweight steels micro-alloyed with vanadium and niobium

The microstructure evolution and tensile properties of a newly designed Fe-27Mn-10Al-1.4C-0.1V-0.1Nb (wt.%) lightweight steel were investigated at the annealing temperature of 900 °C for 1min, 3min and 10min. The steel's microstructure revealed non-recrystallized austenite grains containing dislocations and fine-sized recrystallized grains and the dense nano-sized (V,Nb)C particles and κ-carbides observed in austenite matrix after annealing for 3 min. The experimental steel achieved ultra-high tensile strength (yield and ultimate tensile strengths of 1152 MPa and 1305 MPa, respectively) because of precipitation strengthening, grain boundary and dislocation strengthening, and excellent ductility (44% of the total elongation). The deformation microstructure of the experimental steel was observed through tensile interruption experiments and TEM characterization. As a result, the pinning effect of (V,Nb)C particles on dislocations and the refinement of the spacing of planar slip bands increase the material's work hardening rate during the first stage of deformation. By activating various dislocation processes involved in the arrangement of slip bands into double dislocation walls and the subsequent evolution of dislocation walls into microbands, the planar slip bands evolve into a microband structure in the later stages of deformation. At later stages of deformation, microband refinement dominates the development of the deformation microstructure of steel.

Simultaneous improvement of corrosion and wear resistance of Fe–Mn–Al–C lightweight steels: The role of Cr/Mo

Fe–Mn–Al–C lightweight steel has promising applications due to its low density and excellent mechanical properties. This study investigates the influence of Cr/Mo on the electrochemical corrosion and wear behavior of lightweight steel. The results showed that adding Cr/Mo significantly improved the corrosion resistance in NaCl/NaHSO3 solution. This improvement was attributed to the presence of protective Cr/Mo oxides and the reduction of harmful Mn oxides, which increased the compactness and charge transfer resistance of the oxide film. The primary wear types of lightweight steels include abrasive, adhesive, fatigue and oxidative wear. The wear test results showed that adding Cr/Mo improved the wear resistance of lightweight steel, which can be attributed to the increased toughness and hardness gradient.

Comprehending the correlation between microstructural features and wear performance in multiphase lightweight steels

This work investigates the wear behavior of three different Fe-Mn-Al-C steels using ball-on-disc reciprocating wear test and 3D optical surface profilometer. Further, the correlation between microstructural features and wear performance of these steels has been established. The experimental results indicate that the wear resistance of the studied steels is in the order: austenite and martensite (AM) > austenite and ferrite (AF) > ferrite and pearlite (FP) steels. In addition to higher hardness, the combined effects of finer width of martensite, larger proportion of high-angle grain boundaries, and higher dislocation density render excellent wear resistance in AM steel. Importantly, the formation of martensite phase during wear test significantly enhances the wear resistance of AM steel. In contrast, the absence of austenite phase reduces the wear resistance of FP steel.