Low-density Steel Research Articles

Effects of heating temperature and atmosphere on element distribution and microstructure in high-Mn/Al austenitic low-density steel

Effects of heating temperature and atmosphere on element distribution and microstructure in high-Mn/Al austenitic low-density steel

Microstructure and mechanical properties of Fe–30Mn–9Al–C–3Ni low-density steel manufactured by selective laser melting

Low-density Fe–Mn–Al–C steels are widely used in the automotive and aerospace industries as advanced lightweight materials. In this study, highly spherical Fe–30Mn–9Al–C–3Ni powder with a narrow particle-size distribution, low oxygen content, and good flowability was prepared using the plasma rotating electrode process (PREP). Furthermore, Fe–30Mn–9Al–C–3Ni low-density alloy steel was manufactured for the first time using the selective laser melting (SLM) molding technique. Laser power was found to significantly impact the microstructure and mechanical properties of SLM-printed Fe–30Mn–9Al–C–3Ni alloys. The Fe–30Mn–9Al–C–3Ni alloy prepared at a laser power of 110 W (denoted as P-110) exhibited the best mechanical properties, including a yield strength of 887.1 MPa, tensile strength of 1076.8 MPa, elongation of 40.80%, and a maximum hardness value of 294 HV. This study highlights the potential of SLM molding for fabricating low-density Fe–Mn–Al–C steels with complex geometrical shapes and outstanding comprehensive mechanical properties.

Temperature dependence of the deformation behavior and mechanical properties of a Fe–Mn–Al–C low-density steel for cryogenic application

The temperature dependence of the deformation behavior and mechanical properties of a Fe–20Mn–6Al-0.6C-0.15Si austenitic low-density steel was studied by tensile test and Charpy impact test at −196 °C, −77 °C and room temperature (RT), followed by microstructure analyses. The results indicate that the decreased tensile temperature, which is corresponding to reduced stacking fault energy (SFE), promotes the planar glide of dislocations. This led to larger fraction of low-angle grain boundaries (LAGBs) at lower tensile temperature. Consequently, both the tensile strength and elongation were improved as temperature decreased. At lower impact test temperature, the load and energy of crack initiation were larger, indicating a higher resistance to crack initiation. However, the transition from stable to unstable crack propagation occurred more rapidly at lower temperature, resulting in lower crack propagation energy and reduced resistance to crack propagation. This is because less region experienced planar glide. Due to the rapid unstable crack propagation, the Charpy absorbed energy at −196 °C was the lowest. The smaller fracture surface area, larger fibrous zone and more obvious ductile fracture characteristics in radial zone suggest that the toughness is better at higher temperature. Additionally, the presence of secondary cracks at RT delayed fracture, therefore benefitting toughness.

Enhancement of Elastic Modulus by TiC Reinforcement in Low-Density Steel

Enhancement of Elastic Modulus by TiC Reinforcement in Low-Density Steel

Effect of temperature and time on the precipitation of κ-carbides in Fe–28Mn–10Al–0.8C low-density steels: Aging mechanism and its impact on material properties

Effect of temperature and time on the precipitation of κ-carbides in Fe–28Mn–10Al–0.8C low-density steels: Aging mechanism and its impact on material properties

Effect of dislocation density on microstructure and mechanical properties of cold-rolled medium Mn steel

This study investigated the effect of dislocation density on microstructural transformation and mechanical properties of Fe-7.23Mn-4.42Al-0.09C-1.99Si (wt.%) steel. By the tempering treatment, the dislocation density was reduced. The effect of of dislocation density on the plasticity of the steel was analyzed. The dislocation density was reduced from 2.3 × 10−16 m−2 to 1.6 10−16m−2 by tempering. Reducing dislocation density in the metastable austenite resulted in a higher metastable austenite transformation rate. And the low dislocation density steels exhibited a longer strain length during tensile deformation. It was obvserved that the dislocation density of the test steel decreased by 30% after tempering, the austenite transformation rate increased to 53.3%, and tensile strength and elongation increased to 1010 MPa and 44% respectively.

Hydrogen-enhanced microbanding in an austenitic FeMnAlC low-density steel: Effect on hydrogen embrittlement resistance

We have investigated the influence of 101 mass ppm hydrogen content on the room temperature deformation structure and mechanical behavior of an austenitic Fe30Mn6.5Al0.3C (wt.%) low-density steel by several electron microscopy techniques, such as electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The steel exhibits a high hydrogen embrittlement resistance associated with a moderated increase in strength (yield stress increase of 10%) and ductility (increase in the elongation to fracture of ∼ 8%). Analysis of the deformation structure reveals that hydrogen influences the deformation behavior by promoting deformation mechanisms associated with inhomogeneous plasticity (hydrogen-enhanced deformation banding (HEDB)) and strain localization (hydrogen-enhanced microbanding (HEMB)). These deformation mechanisms are ascribed to hydrogen-induced effects on dislocation plasticity, resulting in macroscopic kink bands, sub-micron localized strain gradients, and localized shear at cell blocks. We find that HEMB plays a relevant role in the deformation behavior of sub-micron localized strain gradients by promoting plastic relaxation and the enhanced storage of geometrically necessary dislocations within them. These effects mitigate the activation of damage mechanisms and enhance the strain-hardening capacity, contributing to the high HE resistance of the steel, comparable to that of high HE-resistant fcc alloys and steels.

Enhancing strength and ductility in Fe–20Mn–9Al-1.5C austenitic low-density steels through Ni and Cr alloying synergy adding

In this investigation, a Fe–20Mn–9Al-1.5C lightweight austenitic steel has been developed through strategic alloying with Ni and Cr elements coupled with the employment of a thermo-mechanical processing technique featuring ultra-fast cooling. This composition and treatment synergistically transcend the conventional trade-off between strength and ductility, positioning the mechanical performance of this steel notably above that of comparable low-density austenitic steels previously reported. The coordinated incorporation of Ni and Cr is key to modulating the precipitation threshold for nanoscale κ-carbides and LRO domains, achieving an optimal size for precipitate-induced strengthening. The incorporation of Cr specifically contributes to a dual enhancement mechanism: it fosters the generation of high-density dislocation arrays (HDDA) and refines the dynamic slip band (DSBR) during deformation, which in turn, elevates the strain hardenability and ductility. The remarkable synergy in strength and ductility is attributed to a combination of mechanisms including grain boundary reinforcement, solid solution hardening, the precipitation strengthening effect of nanoscale precipitates, and the activation of secondary slip system dislocations. This synergy is quantitatively reflected in the material's yield strength (YS) exceeding 1100 MPa, ultimate tensile strength (UTS) surpassing 1200 MPa, and a total elongation (TEL) exceeding 45%. The insights gained from this research provide critical guidance for the design and creation of austenitic low-density steels that do not compromise between strength and ductility, thereby offering superior overall performance.

Correlation between grain size, mechanical properties and deformed microstructure of Fe–20Mn–6Al–0.6C–0.15Si low-density steel

Correlation between grain size, mechanical properties and deformed microstructure of Fe–20Mn–6Al–0.6C–0.15Si low-density steel

Deciphering the composition-microstructure correlation in low-density FeMnAlC steels with machine learning

FeMnAlC steels have been regarded as prospective candidates for the advanced high-strength steel with reduced density. However, the correlation between the complex steel composition and the resulting microstructure remains unclear. This study presents a robust and efficient approach for establishing the correlation between the composition and microstructure of low-density FeMnAlC steel using a machine learning algorithm. By employing classification and regression models, we successfully predicted four significant characteristics: the existence of the κ phase, a dual-phase region (γ + κ) at the aging temperature, a solitary γ phase region, and the volume fraction of the κ phase. Notably, the elements Al and C, with a particular emphasis on Al, exert significant influence on the phase structure. For example, when the Mn element content is 30 wt%, an estimated minimum Al content of around 4 wt% is required to guarantee the existence of the κ phase. Furthermore, the anticipated content range of Al and C required for the existence of the (γ + κ) duplex region at the aging temperature spans from 4.5 to 9.3 wt% and 0.4 to 1.1 wt%, respectively. Ultimately, the Fe-30Mn-8Al-1C steel was chosen for experimental validation, and the predicted characteristics exhibited excellent agreement with the experimental results. Our research offers a roadmap for the development of low-density steel, and the approach we devised holds promise for application in other metallic materials as well.

Effect of B2 Precipitation on Hot Ductility of Fe–22Mn–9Al–0.6C Low-Density Steel

Effect of B2 Precipitation on Hot Ductility of Fe–22Mn–9Al–0.6C Low-Density Steel

Impact of Size and Distribution of k-Carbides on the Hydrogen Embrittlement and Trapping Behaviors of a Fe-Mn-Al-C Low-Density Steel.

This study compares the hydrogen embrittlement susceptibility of a Fe-30Mn-8Al-1.2C austenitic low-density steel aged at 600 °C for 0 (RX), 1 min (A1) and 60 min (A60), each exhibiting varying sizes and distributions of nano-sized κ-carbides. Slow strain rate tests were conducted to assess hydrogen embrittlement susceptibility, while thermal desorption analysis was applied to investigate hydrogen trapping behaviors. Fracture surface analysis was employed to discuss the associated failure mechanisms. The results suggest that nano-sized κ-carbides with sizes ranging from 2-4 nm play a crucial role in mitigating hydrogen embrittlement, contrasting with the exacerbating effect of coarse grain boundary κ-carbides. This highlights the significance of controlling the sizes and morphology of precipitates in designing hydrogen-resistant materials.

Electropulsing of low-density duplex steel for strengthening and ductilization by microstructural refinement

The present investigation finds the effect of electropulsing treatment on the mechanisms of refining and reprecipitation of ordered L20 structure (B2) and its impact on tensile properties of low-density duplex steel of Fe-18Mn-10.5Al-1 C-6Ni composition. The selected composition without manganese is melted at 1600°C by induction heating in a vacuum but Mn is added into the melt at 1560°C, in the argon atmosphere to reduce the loss. The melt is cast into a plate form in a copper mold. The cast steel is homogenized, hot rolled, and annealed (PD1-A sample) to get elongated bands, spheroids, and platelets of B2 phase with an austenitic matrix which results in a yield strength of 1015 MPa, and tensile strength of 1285 MPa with a plastic elongation of 16.3%. Electropulsing partially dissolves the B2 phase at a temperature much lower than the equilibrium solvus by reducing barrier energy because of electron wind. As a result, the sizes of bands and spheroids are reduced, and platelets are disintegrated. Electron wind force deforms coarse precipitates, and fragments as well as spheroidized it. Electropulsing of annealed steel (PD1-AE) mobilizes dislocations, and recovers it in B2, but induces localized recrystallization of austenite. At a later time of the signal, the temperature rapidly falls and the matrix becomes supersaturated but electropulsing accelerates low-temperature precipitation of the B2 phase. Therefore, electropulsing can be adopted as a fast manufacturing process to dissolve, deform, fragment, spheroidize and reprecipitate high temperature phase at much lower temperature by dominant athermal effect of electron wind energy over thermal effect. Electropulsing also induces recovery and recrystallization at a relatively low temperature and reduces defect density. Reduction in the overall size of B2 and its distribution towards better uniformity provided an increased yield strength of 1077 MPa, tensile strength of 1355 MPa, and a plastic elongation of 21.6%. The selected duplex low-density steel follows two-stage work hardening of Ludwigson model with an additional stage of dynamic recovery. Fractography analysis confirms that both samples show mixed modes of ductile and brittle fracture, with an increase in size and area percentage of dimples.

Synergistic Effect of Al and Ni on Microstructure Evolutions and Mechanical Properties of Fe-Mn-Al-C Low-Density Steels

In this study, the synergistic behavior of Ni and Al in two low-density steels (Fe-26Mn-10.2Al-0.98C-0.15V (wt. %) and Fe-29Mn-5Al-1C-12Ni (wt. %)) and their influence on microstructures and mechanical properties were investigated. The chemical composition of κ-carbides and B2 precipitated particles as a function of annealing and aging temperature and the matrix within which they formed were elucidated. The microstructures and deformation mechanisms of both steels were studied based on their strengthening contribution. The Fe-26Mn-10.2Al-0.98C-0.15V steel mainly realized precipitation strengthening through κ-carbides and grain boundary strengthening due to full recrystallization. The strengthening caused by Fe-29Mn-5Al-1C-12Ni steel was mainly due to the presence of the B2 phase in the matrix, which was non-coherent with FCC. This led to the Orowan bypass mechanism, which made precipitation strengthening the main strengthening contribution. The synergistic effect led to the shear or bypass mechanism of both steels when plane dislocation slip occurred. In addition, it also had an influence on the work-hardening capability during plastic deformation. This study provides a promising way to further enhance the yield strength of low-density austenitic steels through the synergistic effect of Ni and Al.

Co-precipitation mechanism of Cu-rich phase and κ-carbide precipitates in Fe-28Mn-10Al-1C-3Cu austenitic low-density steel

The excellent comprehensive properties of Fe-28Mn-10Al-1C-3Cu austenitic low-density steel are related to its main precipitates of Cu-rich phase and κ-carbide particles. The co-precipitation mechanism of Cu-rich phase and κ-carbide particles in Fe-28Mn-10Al-1C-3Cu austenitic low-density steel are investigated. Adding Cu to Fe-28Mn-10Al-1C austenitic low-density steel can promote the precipitation of κ-carbide. The κ-carbide can also promote the precipitation of Cu-rich phase with FCC structure. The κ-carbide preferentially precipitate compared to Cu-rich phase. The κ-carbide and Cu-rich phase have a cube-on-cube orientation relationship with the austenitic matrix: [110]γ//[110]κ//[110]Cu and (111)γ//(111)κ//(111)Cu.

Effects of annealing temperature on microstructure and mechanical properties of a low density δ-TRIP steel plate via intercritical quenching-annealing process

In this study, the effects of annealing temperature on microstructure and mechanical properties of Fe–9Mn–5Al-0.3C (wt.%) low density steel plates via intercritical quenching-annealing process were investigated through field-emission scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, and tensile tests. The results showed that the experimental steel maintained micro-laminated microstructure after intercritical quenching-annealing treatment, consisting of δ-ferrite band and tempered martensite lamellar. After annealing at the range of 600–800 °C following quenching of 1050 °C, tempered martensite lamellar mainly changed, and affected the mechanical properties of the steel. The annealed experimental steels exhibited wide yield strength grades (460–620 MPa) with excellent ductility (40–63 %) and low yield-tensile ratio (0.65–0.75). The proportion of austenite, the density of geometrically necessary dislocations, and high-angle grain boundary density of tempered martensite region, as well as the presence or absence of carbides, were considered to contributed to the mechanical properties. Austenitic mechanical ability was analyzed. Various annealed temperature led to the different levels of austenitic stability through changing the enrichment of alloyed elements in austenite, further causing different degrees of TRIP effect. TWIP effect existed during deformation according to estimation of SFE. The steel annealed at 740 °C had maximum uniform elongation (62.4 %) and the highest product of strength and elongation (45.2 GPa%), depending on the maximum proportion of austenite transformed into martensite during deformation. However, the 680 °C annealed steel with the highest austenitic stability exhibited deficient TRIP effect, leading to the occurrence of necking. Moreover, work hardening behavior and fracture morphology of the experimental steel were analyzed.

Uncovering Microstructure–Property Relationship in Ni-Alloyed Fe–Mn–Al–C Low-Density Steel Treated by Hot-Rolling and Air-Cooling Process

Uncovering Microstructure–Property Relationship in Ni-Alloyed Fe–Mn–Al–C Low-Density Steel Treated by Hot-Rolling and Air-Cooling Process

Unexpected strengthening of the Mo-containing low-density steel achieved through a novel nano-ordered phase

Mo element has been considered as an effective element to regulate the nano-precipitation behavior in low-density steel. In this work, a novel nano-ordered phase was found in austenite of the Fe–28Mn–10Al-1.2C (wt.%) low-density steels with 3 wt% and 5 wt% Mo addition. HRTEM characterization indicated that this nano-ordered phase had a chemical composition similar to austenite matrix and a L′12 structure similar to κ-carbides. Although the Mo addition suppressed the precipitation and growth of κ-carbides in austenite, the nano-hardness testing showed that the formation of the novel nano-ordered phase caused by Mo addition resulted in a higher hardness of austenite. Based on the tensile property analysis of the Mo-containing steels, although the Mo addition decreased the precipitation strengthening of intragranular κ-carbides, the novel nano-ordered phase resulted in an additional strength contribution of 309 MPa. In addition, the formation of this novel nano-ordered phase had significant improvement on the strain hardening rate. Consequently, the Mo addition simultaneously increased the strength and plasticity of the low-density steels. The novel nano-ordered phase caused by Mo has rarely been reported in literatures, its contribution on the strength and strain hardening ability indicated its potential application in the regulation of mechanical properties.

A modified Johnson-Cook model to determine plastic flow behavior of Fe-30Mn-9Al-0.8 C low-density steel during warm multiaxial forging

The microstructural changes in Fe-30Mn-9Al-0.8 C low-density steel upto equivalent strain 2.3 during multiaxial forging (MAF) and their impact on tensile strengthening have been thoroughly investigated. The processed sample at 0 pass, 1 pass, 3 pass, and 5 pass with 33% normal strain in each pass are characterized by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). The analysis of XRD confirms a noticeable increase in dislocation density progressing from 0.79 × 1015 m−2 to 8.98 × 1015 m−2, attributed to a decrease in crystallite size and an increase in microstrain from 3.66 × 10−3 to 6.8 × 10−3. The grain size reduces from 50 μm to 13 μm. The yield strength increases from 380 MPa to 1528 MPa, while the ultimate strength increases from 762 MPa to 1548 MPa after 5 passes. It is observed here that both dislocation density and grain boundaries contribute to strengthening the material. However, up to three passes, dislocation strengthening is significant. Additionally, a numerical framework is developed to model the evolution of dislocation density, grain size and yield strength at each stage of MAF. To emphasize the role of strengthening parameters in predicting the flow stresses in every MAF pass accurately, a material VUMAT subroutine in ABAQUS has been created for a modified Johnson-Cook (J-C) constitutive model that accounts for the role of increased dislocation density and grain size. The developed model demonstrates its capability to accurately capture essential material features, including equivalent strain, dislocation density, and grain size. Furthermore, it exhibits the ability to predict the yield strength approximately following each MAF pass.

Secondary Phase Precipitation in Fe-22Mn-9Al-0.6C Low-Density Steel during Continuous Cooling Process.

Secondary phase precipitation in Fe-22Mn-9Al-0.6C low-density steel was investigated during a continuous cooling process with different cooling rates through a DIL805A thermal expansion dilatometer, and the changes in microstructures and hardness by different cooling rates were discussed. The results showed that the matrix of the Fe-22Mn-9Al-0.6C was composed of austenite and δ-ferrite; moreover, the secondary phases included κ-carbide, β-Mn and DO3 at room temperature. The precipitation temperatures of 858 °C, 709 °C and 495 °C corresponded to the secondary phases B2, κ-carbide and β-Mn, respectively, which were obtained from the thermal expansion curve by the tangent method. When the cooling rate was slow, it had enough time to accommodate C-poor and Al-rich regions in the austenite due to amplitude modulation decomposition. Furthermore, the Al enrichment promoted δ-ferrite formation. Meanwhile, the subsequent formation of κ-carbide and β-Mn occurred through the continuous diffusion of C and Mn into austenite. In addition, the hardness of austenite was high at 0.03 °C/s due to the κ-carbide and β-Mn production and C enrichment, and it was inversely proportional to the cooling rate. It can be concluded that the presence of κ-carbide, DO3 and β-Mn produced at the austenitic/ferrite interface when the cooling rate was below 0.1 °C/s resulted in κ-carbide and β-Mn precipitating hardly at cooling rates exceeding 0.1 °C/s, which provides a guideline for the industrial production of Fe-Mn-Al-C low-density steel in the design of the hot working process.