Lightweight Steels Research Articles

Synergistic strengthening effect induced ultrahigh yield strength in lightweight Fe[sbnd]30Mn[sbnd]11Al-1.2C steel

In the past decades, high MnAlC lightweight steels have been extensively studied due to their excellent mechanical properties. However, the yield strength is usually less than 1.2 GPa for relative lower carbon content (≤1.2 wt %) steels after the general strengthening process. This study was intended to enhance the yield strength of the steel. For this purpose, a novel heterogenous lamellar microstructure was introduced in lightweight steel with the nominal component of Fe30Mn11Al-1.2C (wt. %), which could provide extra back stress strengthening. The microstructure consists of the lamellar fully recrystallized austenite matrix and the small volume fraction (<2%) submicron-sized ferrite which merely distributed in the fine grain layer. The formation process and the underlying strengthening mechanism of the lamellar microstructure were analyzed. Due to the synergistic strengthening of back stress, grain refinement and aging strengthening, ultrahigh yield strength (∼1.4 GPa) and greatest specific yield strength (∼220 MPa g−1 cm3) with good elongation (∼27%) were realized in the present alloy.

Effect of B2 morphology on the mechanical properties of B2-strengthened lightweight steels

Tensile properties and formability of B2-strengthened Fe-15Mn-10Al-0.8C-5Ni and Fe-16Mn-8Al-1C-5Ni lightweight steels have been investigated. Our results show that tensile properties along rolling and transverse directions as well as formability are strongly affected by the morphology of B2. The removal of B2 bands and a development of more homogeneous microstructure containing fine B2 particles in Fe-16Mn-8Al-1C-5Ni steel results in isotropic tensile properties and significant improvement of formability compared to B2 bands containing Fe-15Mn-10Al-0.8C-5Ni steel. Moreover, such improvements are achieved without any degradation of tensile properties, despite a much lower fraction of B2 particles in the former than the latter.

Roles of (Fe, Mn)3Al Precipitates and MBIP on the Hot Ductility Behavior of Fe–30Mn–9Al–0.9C Lightweight Steels

In the automotive industry, lightweight steel has received much attention because steel comprises a significant portion of a vehicle’s total weight. Fe–Mn–Al–C steel is a representative lightweight steel due to its high performance and low density. However, there is insufficient research into the welding characteristics of Fe–Mn–Al–C lightweight steels. In this study, hot ductility tests were conducted on austenitic Fe–30Mn–9Al–0.9C steel in order to understand the welding characteristics (cracking resistance) of the heat affected zone. During the on-heating thermal cycle, ductility was altered by a decrease in microband induced plasticity (MBIP) (softening) and an increase in dynamic recrystallization (DRX) (softening) as the temperature increased. Specifically, in the range of 773–1073 K, ductility was fairly degraded because neither MBIP nor DRX took place. During the on-cooling thermal cycle, ductility behavior was changed by both softening and hardening factors, including formation of brittle (Fe, Mn)3Al intermetallic compounds with grain growth and re-solidified grain boundaries. However, the hardening effect of precipitated κ-carbide was insignificant and might not play a significant role in the hot ductility behavior of the lightweight alloy used in this study.

Phase stability investigation of light-weight alloys based on the FeMnAlC system

Due the necessity of further reducing vehicles weight in the automotive industry, the concept of lightweight steels based on the FeMnAlC system has recently gained a great deal of attention, since it offers a combination of outstanding mechanical properties and specific weight reduction. Such lightweight steels are complex materials with multiphase microstructure. The mechanical properties of the final product are greatly influenced by both the chemistry of the material and the thermo-mechanical processing route applied. Despite the existing body of research on literature, there are many challenges related with their processing and characterization preventing their commercialization. Therefore, the objective of the present work is to deepen into the microstructural development and phase stability at high temperatures of alloys based on the FeMnAlC system as the first step to help define proper hot rolling and annealing parameters and eventually facilitate production processes in such steel grades. A comparison of thermodynamic predictions, data from literature and experimental measurements is finally performed.

Effect of Mn and C on Age Hardening of Fe–Mn–Al–C Lightweight Steels

The effects of Mn and C content on the age hardening of Fe–Mn–Al–C lightweight steels, which have austenitic or duplex (austenite and ferrite) microstructures, were investigated. An increase in Mn content induced a delay of the age hardening that is caused by the formation of intra-granular κ-carbides. In order to interpret the effect of Mn content, first-principles calculations were conducted using the supercells of Fe24Al8C8, Fe24Al8C7, Fe24(Al7Mn)C8, and Fe24(Al7Mn)C7. The calculations showed that an increase in Mn content could be the source of the delay of the intra-granular κ-carbide formation by suppressing C atom’ occupation of the vacancy at the body-centered site of L12. An increase in C content accelerated the formation of intra-granular κ-carbides, which induced the intense age hardening, and coarse inter-granular κ-carbides, which resulted in significant decrease in impact absorbed energy due to inter-granular fracture.

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Fe–Mn–Al–C lightweight steels have great potential to replace Cr–Mo quenched and tempered steels where weight reduction is an important design variable. Previous research has centered around a fully austenitic cast composition of Fe–30Mn–9Al–0.9C–1Si–0.5Mo that achieves high strength through homogenous precipitation of κ-carbide during age hardening. However, the effect of varying Mo content and the corresponding effects on age hardening kinetics and resulting mechanical properties are not well documented. The current study investigates Mo additions in the range of 0–0.7% Mo on the microstructure, age hardening response, and toughness of a Fe–30Mn–9Al–0.8C–1Si steel. Molybdenum was shown to decrease the age hardening response at 530 °C. Higher concentrations of Mo resulted in an increase in the Charpy V-notch impact toughness of aged specimens. Optical microscopy revealed that alloys with Mo additions had much less κ-carbide along grain boundaries in the aged condition and this decreased the propensity of intergranular fracture and increased toughness.

Improved cold-rollability of duplex lightweight steels utilizing deformation-induced ferritic transformation

The Fe–Mn–Al–C duplex lightweight steels have received considerable attention from the automotive industry because of their low density and good mechanical performance. However, their poor cold-rollability limits their applications. This study proposes a thermomechanical treatment that enhances the cold-rollability of duplex lightweight steels via deformation-induced ferritic transformation (DIFT). DIFT activated by warm rolling destroys the initial band structure and causes a partial grain refinement, thereby causing mechanical strengthening without the loss of ductility. Subsequent annealing enabled cold rolling, while the microstructure formed by DIFT was observed to remain. The steels fabricated using the proposed method exhibited more cold-rollability compared with that exhibited by the conventionally hot-rolled samples. Such an improvement can be attributed to the increased austenitic stability and the resultant inhibition of strain-induced martensitic transformation via DIFT.

Precipitation behavior and its effect on mechanical properties in weld heat-affected zone in age hardened FeMnAlC lightweight steels

In this study, we investigated the effect of precipitation during the thermal cycles of age hardening and the welding process in hot rolled austenitic FeMnAlC lightweight steel. After solution treatment, samples were prepared under different age hardening conditions, which was conducted at a temperature of 550 °C in a furnace. Samples representing heat affected zones (HAZs) with three different peak temperature conditions were prepared via a Gleeble simulator. For microstructural analyses, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy (TEM) were carried out. The mechanical characteristics were investigated by tensile, hardness, and cryogenic Charpy impact tests. The results showed that precipitation behavior in the lightweight steels had specific influence in mechanical properties in age hardening and welding process. The age hardening effect was shown with increased tensile strength with loss of ductility. In the solution treated specimen, increased hardness appeared in HAZ compared to the base steel, while HAZ softening occurred in every age-hardened specimen. In the cryogenic Charpy impact test, all of the samples except the solution-treated base steel showed negligible energy absorption with inter-granular fracturing. The trends of the experimented HAZ mechanical properties revealed that the precipitation behavior of κ-carbide was strongly related to the overall mechanical properties. Moreover, we found that the welding process removed the hardening effect of aging as a result of drastic κ-carbide dissolution in the high-temperature HAZ regardless of age hardening conditions. The TEM analysis confirmed that all HAZ simulated specimens showed similar precipitation conditions for all cases. Based on our analysis, we confirmed that the precipitation behavior of κ-carbide was the main factor of different HAZ mechanical properties. We also suggested that discontinuous HAZ mechanical properties should be considered during the welding process because of κ-carbide dissolution in FeMnAlC lightweight steels.

Effect of Mo and Cr additions on the microstructure, mechanical properties and pitting corrosion resistance of austenitic Fe-30Mn-10.5Al-1.1C lightweight steels

Five Fe-30 wt%Mn-10.5 wt%Al-1.1 wt%C steels containing different Mo and Cr contents were prepared to investigate the effect of Mo and Cr addition on the microstructure, mechanical properties and pitting corrosion resistance of austenitic lightweight steels. The microstructures of all samples after solution treatment at 1050 °C consisted of austenite and κ-carbide, while DO3 ordered phases were additionally formed in samples containing 3 wt%Mo-3wt%Cr or 5 wt%Cr. The results of Nanoindentation tests indicated that the intrinsic strength of the austenite matrix decreased with the addition of Mo and Cr due to the suppression of κ-carbide precipitation and then the strength of the DO3 phase is equal or higher as compared to the austenite matrix. The tensile tests also showed that the yield strength decreased when 3 wt%Mo or 3 wt%Cr was added due to the suppression of κ-carbide precipitation, whereas it increased with further additions of Mo and Cr in both cases due to the formation of a DO3 ordered phases and grain refinement. Electrochemical tests showed that the resistance to pitting corrosion was improved by the addition of Mo and Cr due to the formation of a protective passive film; however, the excessive additions of Mo or Cr adversely deteriorated the resistance to pitting corrosion as the DO3 ordered phase which acted as pit initiation sites.

Partitioning of C into κ-carbides by Si addition and its effect on the initial deformation mechanism of Fe-Mn-Al-C lightweight steels

The effect of Si addition on the initial deformation mechanism of Fe–30Mn–9Al–0.9C–0.5Mo cast lightweight steel has been investigated. Atomic-scale analysis by three-dimensional atom-probe tomography confirmed that Si addition accelerates the formation kinetics of the κ-carbide, and increases remarkably the partitioning coefficient of carbon by more than 2 times. Promotion of C partitioning into κ-carbides by Si addition leads to deformation localization in an aged state. This is attributed not only to the increase of the κ-carbide size associated with higher coherency strain, but also to higher frequency of Al-C bonding formation. First-principles calculations indicated that the energy required for a/2<110> shearing of κ-carbide in the aged 1%-Si steel is higher (561 mJ/m2) than that in the aged Si-free steel (494 mJ/m2). Therefore, Si addition leads to a significant energy difference for shearing and results in different shear bands formation. The operative deformation mechanism can be calculated for this alloy system as a function of precipitates volume fraction and size. It was determined that deformation proceeds primarily by κ-carbide shearing as long as the radius of κ-carbides is smaller than 13.4 nm.

Interpretation of the abnormal high-temperature flow behavior of Fe-Mn-Al duplex lightweight steel

The hot deformation behaviors of two Fe-Mn-Al lightweight steels were investigated by hot compression tests on a Gleeble-3500 thermal simulation machine over a practical range of temperatures and strain rates. Optical microscope (OM) and electron backscattered diffraction (EBSD) were carried out to observe the microstructural mo rphologies of samples after hot deformation. Different from the single austenite steel, the hot compression tests of the duplex steel were actually carried out on an austenite/δ-ferrite duplex matrix with various phase compositions. At the commencement of deformation, strain was intensively distributed in δ- ferrite due to its higher stacking fault energy (SFE), and the dynamic softening behavior occurred earlier in δ-ferrite than in austenite during the whole hot deformation period. As a consequence, the prior dynamic recovery (DRV) in δ-ferrite caused the yield-like behavior, and the strain partitioning between δ-ferrite and austenite resulted to a visible serration on the flow curves. The dominant continuous dynamic recrystallization (CDRX) in δ-ferrite at high temperature caused a typical dynamic recovery characteristic, and the dominant discontinuous dynamic recrystallization (DDRX) in austenite at low temperature resulted in a typical dynamic recrystallization characteristic of the flow curves.

Light-weight isometric-phase steels with superior strength-hardness-ductility combination

An optimal proportion of austenitic and martensitic phases is achieved in isometric-phase steels through meticulously designed and elaborately calculated surface nanotechnology. Such isometric-phase steels with nanolamellaed structures and nanograins, exhibit surpassing combinations of strength, hardness and ductility (with yield strength of 1215 MPa, surface hardness of 8.6 GPa, and favorable ductility of 31%). A microstructure-based constitutive model is developed to investigate the deformation mechanism and mechanical performance in isometric-phase steels. We believe the light-weight isometric-phase steels in this work can be regarded as a finer counterpart of dual phase steels for applications in automobile, aircraft and nuclear industry.

Nano-mechanical properties of Fe-Mn-Al-C lightweight steels

High Al Low-density steels could have a transformative effect on the light-weighting of steel structures for transportation. They can achieve the desired properties with the minimum amount of Ni, and thus are of great interest from an economic perspective. In this study, the mechanical properties of two duplex low-density steels, Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8 C (wt.%) were investigated through nano-indentation and simulation through utilization of ab-initio formalisms in Density Functional Theory (DFT) in order to establish the hardness resulting from two critical structural features (κ-carbides and B2 intermetallic) as a function of annealing temperature (500–1050 °C) and the addition of Ni. In the Ni-free sample, the calculated elastic properties of κ-carbides were compared with those of the B2 intermetallic Fe3Al−L12 and the role of Mn in the κ structure and its elastic properties were studied. The Ni-containing samples were found to have a higher hardness due to the B2 phase composition being NiAl rather than FeAl, with Ni-Al bonds reported to be stronger than the Fe-Al bonds. In both samples, at temperatures of 900 °C and above, the ferrite phase contained nano-sized discs of B2 phase, wherein the Ni-containing samples exhibited higher hardness, attributed again to the stronger Ni-Al bonds in the B2 phase. At 700 °C and below, the nano-sized B2 discs were replaced by micrometre sized needles of κ in the Ni-free sample resulting in a lowering of the hardness. In the Ni-containing sample, the entire α phase was replaced by B2 stringers, which had a lower hardness than the Ni-Al nano-discs due to a lower Ni content in B2 stringer bands formed at 700 °C and below. In addition, the hardness of needle-like κ-carbides formed in α phase was found to be a function of Mn content. Although it was impossible to measure the hardness of cuboid κ particles in γ phase because of their nano-size, the hardness value of composite phases, e.g. γ + κ was measured and reported. All the hardness values were compared and rationalized by bonding energy between different atoms.

Improvement of tensile properties in (austenite+ferrite+κ-carbide) triplex hot-rolled lightweight steels

In this study, (austenite+ferrite+κ-carbide) triplex microstructures were obtained from the hot-rolling and subsequent 800 °C-annealing of Fe-0.8C-(12,15,20)Mn-7Al (wt%) lightweight steels, and their effects on ultra-high strength along with good ductility were investigated in relation with microstructural evolutions occurring during the tensile deformation. In the 12-wt%-Mn hot-rolled specimen, lamellar κ-carbides were populated along austenite grain boundaries, which seriously deteriorated the ductility. When the Mn content increased to 15 or 20 wt%, the volume fraction of κ-carbide decreased, and slip bands were sufficiently formed inside austenite grains during the deformation, which resulted in excellent ductility as well as ultra-high strengths. Very high yield strength was also achieved by forming triplex microstructures, in which the austenite deformed during the hot rolling greatly contributed to the strength enhancement. In the 800 °C-annealed specimens, the decreased volume fraction and morphological change from lamellar to polygonal-particle type of κ-carbides enhanced the ductility in spite of the slightly reduced strengths. This was because slip bands were sufficiently formed in austenite grains during the deformation, while voids were formed inside well-developed slip bands. The existence of a large amount of austenite was highly valuable for increasing the ductility by developing fine dislocation substructures such as Taylor lattices as a key feature of deformation mechanisms.

Influence of κ-carbide precipitation on the microstructure and mechanical properties in the weld heat-affected zone in various FeMnAlC alloys

This study aims to investigate the effects of κ-carbide precipitation behavior in the heat affected zone (HAZ) in FeMnAlC lightweight steels. Three alloys with different amounts of Al were prepared by vacuum induction melting and hot rolling. After solution treatment, the HAZ samples were simulated by a Gleeble simulator with two heat inputs of 10 and 30 kJ/cm. Microstructural analysis with XRD and TEM were carried out while sub-sized tensile test, hardness test, and V-notched Charpy impact test were performed for investigating the mechanical properties of the base steels and HAZ. The results showed that the mechanical properties and precipitation of κ-carbide within the HAZ were strongly related to the Al content and heat input; the tensile strength and hardness of the HAZ increased as the Al content and heat input increased while elongation decreased. On the other hand, in the Charpy impact test, fracture mode transitions in the HAZ (ranging from ductile fracture to brittle inter-granular fracture) occurred in accordance with the Al content and heat input. The different fracture behavior was explained by TEM results, which showed precipitation behavior of κ-carbide in HAZ. Coherent intra-granular κ-carbide was found to cause a transition from ductile fracture to trans-granular cleavage, and we observed that a severe drop of the impact toughness occurred when partially coherent inter-granular κ-carbide appeared. Therefore, our results illustrate that the HAZ of lightweight steels with the proper Al content can be strengthened with minimal loss of impact toughness due to κ-carbide precipitation during the welding process.

Investigations of the microstructure evolution and tensile deformation behavior of austenitic Fe-Mn-Al-C lightweight steels and the effect of Mo addition

A series of Fe-30 wt%Mn-10.5 wt%Al-1.1 wt%C steels with Mo addition from 0 to 5 wt% were prepared to investigate the effect of Mo on the microstructure and tensile deformation behavior of austenitic lightweight steel. When the Mo content was below 4 wt%, the microstructure of solution-treated samples consisted of austenite and κ–carbide, while Mo-enriched M6C and M23C6 carbides were additionally precipitated in samples containing Mo more than 4 wt% during a solution treatment at 1050 °C. These carbides inhibited austenite grain growth during the solution treatment, resulting in significant grain refinement in the samples containing more that 4 wt% of Mo. Tensile test results showed that the yield strength gradually decreased with an increase in the Mo content up to 3 wt% due to the suppression of κ–carbide precipitation, whereas it significantly increased when the Mo content exceeded 4 wt% due to grain refinement and precipitation strengthening caused by Mo-enriched carbides. During the tensile deformation, the strain hardening rates of all alloys increased and then the deformation mode subsequently changed with an increase in the Mo content from shearband-induced plasticity (SIP) to microband-induced plasticity (MBIP). Finally, the change in the κ-carbide precipitation behavior upon the addition of Mo and its effect on the deformation behavior were carefully analyzed and discussed through nanoindentation experiments, first-principles calculations and atom probe tomography analyses.

Thermodynamic investigation of the Al-Fe-Mn system over the whole composition and wide temperature ranges

The Al-Fe-Mn system is a core system for both aluminum alloys and lightweight steels. In the present work its phase relations were studied over the entire composition range between RT and well above the melting temperature using the CALPHAD method. For the first time the stable γ1 phase and the metastable φ phase were successfully introduced into the description of the Al-Mn binary system. The wide solubility ranges of the third element in the binary intermetallic compounds, such as Al6Mn, Al13Fe4, γ1 and γ2, were reproduced satisfactorily. The ternary phases, i.e. φ, ζ and D3, were reasonably well described using compound energy models. Moreover, the phase equilibria between the bcc, fcc and β-Mn phases in the Fe-Mn rich part were well predicted using the present description.

Effect of Cooling Rate on the Microstructure and Mechanical Properties of Fe-Mn-Al-C Light-Weight Steels

Effect of Cooling Rate on the Microstructure and Mechanical Properties of Fe-Mn-Al-C Light-Weight Steels

Interpretation of dynamic tensile behavior by austenite stability in ferrite-austenite duplex lightweight steels

Phenomena occurring in duplex lightweight steels under dynamic loading are hardly investigated, although its understanding is essentially needed in applications of automotive steels. In this study, quasi-static and dynamic tensile properties of duplex lightweight steels were investigated by focusing on how TRIP and TWIP mechanisms were varied under the quasi-static and dynamic loading conditions. As the annealing temperature increased, the grain size and volume fraction of austenite increased, thereby gradually decreasing austenite stability. The strain-hardening rate curves displayed a multiple-stage strain-hardening behavior, which was closely related with deformation mechanisms. Under the dynamic loading, the temperature rise due to adiabatic heating raised the austenite stability, which resulted in the reduction in the TRIP amount. Though the 950 °C-annealed specimen having the lowest austenite stability showed the very low ductility and strength under the quasi-static loading, it exhibited the tensile elongation up to 54% as well as high strain-hardening rate and tensile strength (1038 MPa) due to appropriate austenite stability under dynamic loading. Since dynamic properties of the present duplex lightweight steels show the excellent strength-ductility combination as well as continuously high strain hardening, they can be sufficiently applied to automotive steel sheets demanded for stronger vehicle bodies and safety enhancement.

Achievement of high yield strength and strain hardening rate by forming fine ferrite and dislocation substructures in duplex lightweight steel

Lightweight steels containing a considerable content of Al show high specific strength and ductility, but there are some drawbacks such as low yield strength and stringer-type bands formed along the rolling direction. Here we design new duplex lightweight steel in order to complement the drawbacks, and achieve ultra-high yield strength (865MPa), good ductility (41%). Submicron ferrite mainly affects high yield-to-tensile ratio, and high strain hardening is attributed to Lomer-Cottrell lock and planar slip, and cell structure by further deformation in austenite. These results are expected to provide a desirable possibility for applications to reinforcement components requiring high yield-to-tensile ratio.