Κ-carbide Precipitation 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.

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.

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

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.

Study on the influence of κ-carbide on the high temperature flow behavior of the medium-Mn lightweight steel: Modeling and characterization

Isothermal compression tests were carried out on the Fe-11Mn-10Al-0.9C medium-Mn lightweight steel in the temperature range from 800 °C to 1100 °C and strain rate range from 0.001 s−1 to 10 s−1 to investigate the influence of κ-carbide on the hot flow behavior. The true stress-true strain curves were analyzed to evaluate the role of κ-carbide on the softening phenomena during straining. It's found flow stress drops more remarkably from the peak point to the end of deformation at low temperature and low strain rate, that condition is favorable for the precipitation of κ-carbide instead of the occurrence of DRX. The activation energy of hot deformation based on the peak stress was calculated as 454.517 kJ mol−1, but this typical strain-compensated Arrhenius-type constitutive equation was found inappropriate to model the flow behavior of the present steel, especially at the low temperature regime. By comparison, a physically-based constitutive model consisting of Bergström's equation (ε<εp) and Kolmogorov-Johnson-Mehl-Avrami's equation (ε>εp) was developed and could predict the flow behavior of the steel accurately. Electron backscattered diffraction (EBSD) analysis revealed the formation of necklace structure at high strain rate and the dynamic recrystallization (DRX) proceeded more adequately with the decrease in strain rate and increase in the deformation temperature. The precipitation of κ-carbides in both austenite grain interior and boundaries (GI and GBs) were enhanced at relatively low deformation temperature (e.g., 800 °C) and greatly improved the peak stress by hindering the dislocation motion. With the increase of deformation, the GB κ-carbide gradually coarsened and induced the decomposition of deformed austenite via the eutectoid reaction (γ→α+κ) at lower strain rate (e.g., 0.001 s−1) due to the sufficient transformation time, consequently leading to the remarkable flow stress softening. The microstructure characterization result was in agreement with variations of parameters (i.e., U, Ω, ρp and nA) with different deformation condition.

Optimizing the microstructure and mechanical properties of Fe-30.5Mn–8Al-1.0C lightweight austenitic steel through thermomechanical treatment

This study presents an application of thermomechanical treatment technique (cold rolling followed by aging treatment) to significantly enhance the mechanical properties of a novel lightweight austenitic steel (Fe-30.5Mn–8Al-1.0C), mainly focusing on the impacts of cold-rolling reduction and subsequent aging temperature on its microstructure, texture, and tensile properties. It has been found that the dislocation density increased with an increase in cold rolling reduction from 25 % to 45 % and reached a saturation at a reduction of 45 %, facilitating the activation of deformation twins and accelerating κ-carbide precipitation. The tensile strength of the steel exhibited significant improvement with increased cold-rolling reduction, owing to the combined effects of dislocation strengthening and the Hall–Petch mechanism. A tensile strength as high as 1.6 GPa was obtained in samples treated by 65 % cold rolling reduction. Aging treatment can generally enhance the ductility in most samples, owing to an improved work hardening capability induced by κ-carbide precipitation. Consequently, an optimum thermomechanical process, involving a 45 % cold rolling reduction followed by aging at 600 °C for 10 min, was established to achieve a better combination of tensile strength (1428 ± 35.8 MPa) and elongation (26.7 ± 1.3 %), showcasing its potential for applications demanding high strength and exceptional ductility.

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.

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.

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.

Effects of C and Al Alloying on Constitutive Model Parameters and Hot Deformation Behavior of Medium-Mn Steels.

Single-pass isothermal hot compression tests on four medium-Mn steels with different C and Al contents were conducted using a Gleeble-3500 thermal simulation machine at varying deformation temperatures (900-1150 °C) and strain rates (0.01-5 s-1). Based on friction correction theory, the friction of the test stress-strain data was corrected. On this basis, the Arrhenius constitutive model of experimental steels considering Al content and strain compensation and hot processing maps of different experimental steels at a strain of 0.9 were established. Moreover, the effects of C and Al contents on constitutive model parameters and hot processing performance were analyzed. The results revealed that the increase in C content changed the trend of the thermal deformation activation energy Q with the true strain. The Q value of 2C7Mn3Al increased by about 50 KJ/mol compared with 7Mn3Al at a true strain greater than 0.4. In contrast, increasing the Al content from 0 to 1.14 wt.% decreased the activation energy of thermal deformation in the true strain range of 0.4-0.9. Continuing to increase to 3.30 wt.% increased the Q of 7Mn3Al over 7Mn by about 65 KJ/mol over the full strain range. In comparison, 7Mn1Al exhibited the best hot processing performance under the deformation temperature of 975-1125 °C and strain rate of 0.2-5 s-1. This is due to the addition of C element reduces the δ-ferrite volume fraction, which leads to the precipitation of κ-carbides and causes the formation of microcracks; an increase in Al content from 0 to 1.14 wt.% reduces the austenite stability and improves the hot workability, but a continued increase in the content up to 3.30 wt.% results in the emergence of δ-ferrite in the microstructure, which slows down the austenite DRX and not conducive to the hot processing performance.

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.

The effect of nano-sized κ-carbides on the mechanical properties of an Fe-Mn-Al-C alloy

The strengthening via precipitation of nano-sized κ-carbides leads to exceptional strength-ductility balance in low-density steels. During aging, such nanocarbides form through spinodal decomposition by fluctuations in the aluminum and carbon content in the austenite, followed by short-range ordering. At lower aging temperatures and short aging times, the κ-carbides are very fine, coherent with the matrix, and homogeneously distributed. When the aging temperature increases, heterogeneous nucleation initiates on the grain boundaries, and the κ-carbides become coarse and lose coherency with the austenite matrix, leading to the deterioration of the mechanical properties. This work studies a fully austenitic hot rolled Fe-29Mn-8.7Al-0.9C alloy after different aging treatments. Two aging treatments were selected for a detailed study of the microstructure based on the exceptional strength-ductility balance demonstrated by these conditions. The samples aged at 550 °C for 8 h exhibited an ultimate tensile strength of 1141 MPa and strain at failure around 49%. The second aging treatment selected was aging at 600 °C for 1 h, and these samples exhibited an ultimate tensile strength of 1084 MPa, with strain at failure 62%. The size and morphology of the austenite grains and the annealing twins were studied through EBSD. Additionally, the size, morphology, and volume fraction of the nano-sized κ-carbides were studied using TEM. Both aging conditions led to microstructures consisting of a matrix formed by equiaxed austenite grains with homogeneously distributed intragranular κ-carbides. The κ-carbides were coherent with the matrix and showed globular morphology with a diameter between 3 and 6 nm and coherent with the austenite matrix. The interaction between gliding dislocations and κ-carbides was analyzed. It was shown that the mechanical behavior of the studied material is characterized by very high sensitivity to the size of κ-carbides and, therefore, can be tailored by appropriate aging treatment.

Effect of alloying content on microstructure and mechanical properties of Fe–Mn–Al–C low-density steels

A series of low-density steels with alloying contents of C from 0.6 to 1.5%, Mn from 20 to 35%, and Al from 6.0 to 12.0% were fabricated in the laboratory. The microstructure, density and mechanical properties of the resulting steels were examined to explore the effects of varying alloy content. The results reveal a significant influence of alloy content on the microstructure of the low-density steels after solution processing in terms of ferrite phase fraction, grain size, and κ-carbide size. It was found that the dependence of alloy density on the C, Mn, and Al content can be formulated ρ = 8.1–0.102C-0.10Al-0.01Mn g/cm3 (with the alloy content given as mass%). Both the yield strength and the work hardening rate varied significantly with alloying content, where the yield strength decreased with increasing Mn content, and increased with increasing Al and C content. The impact toughness of the low-density steels was found also to be strongly affected by the alloying content, showing Charpy impact toughness lower than 10 J for C content above 1.2%, for Mn content below 25%, and for Al content above 10%. The effects of alloy content on the mechanical properties are attributed to differences in the precipitation and coarsening of κ-carbides during the solution treatment process.

Microstructural characterization of welded plates of austenitic low-density Fe-Mn-Al-C steels microalloyed with Ti/B and Ce/La

The excellent combination of tensile strength and elongation of low-density (LD) steels of the Fe-Mn-Al-C system has generated great interest in the metallurgical field. The need to understand the effects produced by welding processes has increased the studies carried out on the weldability of LD steels. Researchers have focused their attention on the weldability issues of LD steels, which occur due to the high Mn, Al, and C contents and their harmful effects in welded joints, such as segregation, hot cracking, and phase transformations, which, in turn, may decrease mechanical resistance. The main objective of this research work is to study the microstructural and mechanical behavior of the fusion zone (FZ) and different heat-affected zones (HAZs) generated by the welding of Fe-31Mn-8Al-1.9 C, Fe-31Mn-8Al-1.9C-Ce/La, and Fe-27Mn-7Al-1.2C-Ti/B austenitic LD steels in 3.5 mm thickness plates through an autogenous GTAW process. The welded plates were mechanically, metallographicaly, and structurally characterized by means of Vickers microhardness (HV), light optical microscopy (LOM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Results exhibited strong variations in the microhardness profile of the welded joint, particularly in the heat-affected zones. The lowest hardness values (≈ 250 HV) were found in the FZ, which was associated with the dendritic structure. The HAZ-1 near to the FZ showed hardness values in the range of 300–375 HV, whereas HAZ-2 furthest from the FZ showed the highest hardness values (up to 430 HV), which could be associated with moderate and high κ-carbide precipitation with modulated structure, respectively. The welded joints of the LD steels under study did not exhibit cracking and showed austenite as the main phase, as well as low presence of α-ferrite, κ-carbides, and β-Mn.

The effects of partial recrystallization annealing and aging treatment on the microstructural evolution and mechanical behavior of austenitic lightweight steel

The introduction of dislocations and nanoprecipitates effectively enhances the mechanical properties of lightweight steel. Herein, cold-rolled lightweight steel was processed through partial-recrystallization annealing and subsequent short aging to achieve a microstructure comprising fine recrystallized grains containing nanosized κ-carbides and non-recrystallized austenite grains containing dense dislocations and nanosized κ-carbides. Thus, the steel exhibited a high yield strength of 1363 MPa, which was primarily attributed to the fine-grain strengthening from the recrystallized austenite grains, dislocation strengthening from the non-recrystallized austenite grains, and precipitation strengthening from the nanosized κ-carbides. Notably, even at this high yield strength, the steel exhibited excellent ductility, with a total elongation exceeding 25%. Moreover, the heterogeneous microstructure of the steel promoted rapid precipitation of κ-carbides during aging. The pre-existing dislocations in the non-recrystallized austenite grains facilitated an accelerated diffusion of the constituent elements of the steel, effectively achieving precipitation strengthening of the κ-carbides. This prevented the precipitation of the intergranular κ-carbides around austenite grain boundaries due to over-aging, which could lead to a detrimental reduction in the ductility of the steel.

Deformation behaviors and microstructure evolution of a novel multiphase medium Mn high Al lightweight steels with excellent strength-ductility combinations

The effects of phase constituents on the deformation behavior and the correlated mechanical properties at different solution temperatures were investigated in medium Mn high Al lightweight steels with different Al contents (Fe-12Mn-(6,8)-Al-1C). This study indicates that the mechanical properties of medium Mn high Al lightweight steels can be significantly enhanced though changing the content of Al and solution treatment. 6Al steel shows lower yield strength (YS) and tensile elongation (TEL) values than those of 8Al steel at the same temperature due to the unitary austenite phase. When Al is added to 8 wt%, both the precipitation of intergranular and intragranular κ-carbide are promoted and the constituents of multiple phases (austenite + ferrite +κ-carbide) result in ultra-high strengthening. The apparent strain partitioning causes by the different characteristics between different phases leads to the high initial strain hardening. During the deformation process, the sufficient formation of slip bands in austenite grains keeps the high strain hardening exponent and postponed the plastic stability, which also contribute to a pronounced ductility. The existence of multiple phases owing to the addition of Al is the key feature of excellent properties in the investigated lightweight steel.

Precipitation behavior of κ-carbides and its relationship with mechanical properties of Fe–Mn–Al–C lightweight austenitic steel

The effects of the aging process on the κ-carbide precipitation behavior and thus on the mechanical properties of an Fe-30.5Mn–8Al-1.0C lightweight austenitic steel were experimentally studied. The morphology of intragranular κ-carbide precipitation evolved from spherical to cuboidal and then to rectangular with increasing aging temperature and time, and intergranular κ-carbides precipitate in the high-temperature-aged steels and exhibit lath-like and lamellar morphologies. For steels with intragranular κ-carbide precipitation, the yield strength becomes significantly improved with the persisting aging owing to the enhanced dislocation shearing strengthening via κ-carbide growth, and the increase in yield strength exceeded the contribution of work hardening. Intergranular precipitation in higher-temperature-aged steel has a negative effect on the strength and ductility; it partly counteracts the strength increase due to intragranular precipitation. In summary, the studied steel aged at 550 °C for 1–2 h exhibited a better strength–ductility match.

Achieving high strength and ductility in Fe–Mn–Al–C austenitic steel via vanadium microalloying and aging

An investigation was performed to evaluate the effect of vanadium (V) addition on the microstructure and the tensile behavior of Fe–27Mn–8Al-1.0C austenite lightweight steel after annealing and subsequently aging. The grain size of austenite in V-added steels was effectively refined and many nano-sized VC particles were observed in the austenite grains, compared to V-free steels, under annealed conditions. Subsequent aging promotes the precipitation of κ-carbide from the austenite matrix, and the dual-nanoprecipitation consisting of nano-sized VC and κ-carbide particles further improves the strength of the steel. Because of this, the current steels have good ductility (up to 41% total elongation) even when aged and high yield and ultimate tensile strengths of up to 1009 MPa and 1243 MPa, respectively. By examining the dislocation structure of the specimens during deformation, it was also possible to determine how precipitates affected the rate of strain hardening. As a result, the strain hardening rate of the V-added steel after aging is effectively improved at the initial deformation by the precipitation of VC particles, which compensates for the decrease in strain hardening rate due to shearable κ-carbide. This study provides a strategy to obtain an excellent combination of strength and ductility in lightweight steel by V microalloying and simple thermomechanical treatment.

Nano-precipitation behavior and mechanical properties of Ti-containing Fe–12Mn–9Al–3Cr-1.4C medium-Mn lightweight steel

The role of Ti in lightweight steel has been widely concerned due to its influence on the precipitation of nano-particle and its inherent low density. In this work, the nano-precipitation behavior and mechanical properties of Ti-containing Fe–12Mn–9Al–3Cr-1.4C-xTi (wt.%, x = 0.02/0.04) medium-Mn lightweight steels were systematically investigated. It was found that the increase of 0.02 wt% Ti content had little influence on the two-phase structure of ferrite and austenite. TEM characterization revealed that Ti could suppress the precipitation and growth of κ-carbide in austenite while promoting the precipitation of TiC particle. Based on thermodynamics and first-principles calculations, the suppression of κ-carbide precipitation by the Ti element resulted from the increased formation energy of κ-carbide and the decreased diffusion coefficient of C in austenite. The reduction of κ-carbide decreased the precipitation strengthening in lightweight steels. However, the precipitation of TiC particle not only resulted in additional strength, but also refined the austenite grain and introduced a large number of dislocations, improving the grain boundary strengthening and dislocation strengthening. For the aged lightweight steels at 450–550 °C for 3 h, the increase of 0.02 wt% Ti content mainly affected the precipitation strengthening of κ-carbide and dislocation strengthening. Especially, when the ageing temperature exceeded 500 °C, the precipitation strengthening of κ-carbides became the primary factor contributing to the reinforcement of lightweight steels.

Microstructure, mechanical properties and deformation behavior of Cr-containing triplex low-density steels with different C content

In this work, Fe–20Mn–9Al–3Cr-xC (wt.%, x = 0.8–1.4) steels were designed to investigate the microstructure, mechanical properties and strain hardening behavior of Cr-containing Triplex low-density steel with different C content. Due to the presence of 3 wt% Cr, all the steel strips with different C content consisted of ferrite and austenite matrix with κ-carbide precipitation. With increasing the C content from 0.8 wt% to 1.4 wt%, the ferrite content decreased from 36 vol% to 4 vol%. Simultaneously, the precipitation and growth of κ-carbides in austenite was accelerated significantly. Quantitative analysis of the yield strength indicated that the strength contribution by κ-carbides increased from 36.3 MPa in 0.8C steel strip to 409.4 MPa in 1.4C steel strip, and became the primary strengthening factor when the C content exceeded 1.0 wt%. Fortunately, due to Cr addition, no intergranular κ-carbide was observed even in the 1.4C steel strip, and the steel strip demonstrated a good elongation of more than 33%. During plastic deformation, the slip band-dominated dislocation substructure made the 0.8C steel strip show a three-stage strain hardening, whereas a four-stage strain hardening occurred in the 1.0C, 1.2C and 1.4C steel strips due to the successive formation of slip band and microband. In addition, it was found that the plastic deformation in ferrite was prior to that in austenite, and the formation of ferrite improved the strain hardening capacity of steel strips at the early deformation stage. The findings provided valuable insights into the design of high-performance Triplex low-density steels by composition optimization.