Low-density Steel Research Articles

Strain hardening and stored energy in high-Mn austenitic based low-density steel

In the present study an attempt has been made to understand the high strain hardening in austenitic low-density steel using two full field measurements of temperature and strain on the sample during tensile deformation. Based on these measurements’ evolution of energy storage during deformation was calculated. High energy storage has been attributed to the interaction between dislocation groups belonging to different slip bands and destruction of short-range order (SRO) clusters. This in turn, dictates the range of the elastic stress field of slip bands. Furthermore, weak deformation heterogeneity ensued just after yield and continued with a fixed strain pattern up to facture. This did not result in macroscopic instability but manifested as microstructural heterogeneity in the form of micro-band and nano twins at later stages of deformation. A mathematical model of energy storage based on dislocation entrapment is discussed.

Effect of cold rolling and subsequent heat treatment on microstructural evolution and mechanical properties of Fe-Mn-Al-C-(Ni) based austenitic low-density steels

Thermo-mechanical processing in a two-phase domain of low-density steel (LDS) has the potential of enhancing strength-ductility combination. In the present study, the effects of alloy composition, cold rolling and heat treatment temperature (973–1173 K) on concurrent precipitation of κ-carbide/B2 particles and recrystallization and subsequently, mechanical properties include hardness variation and tensile response are evaluated. In order to understand the synergistic effect of Mn/Ni ratio towards selecting the two-phase processing window, three LDS alloy compositions, namely Fe–28Mn–9Al-0.9C, Fe–28Mn–9Al-0.9C–5Ni and Fe–15Mn–9Al-0.9C–5Ni (in weight percent) are vacuum induction melted, hot rolled and then cold rolled to 50% reduction in thickness at room temperature. The LDS alloys containing Ni showed much delayed recrystallization as compared to Ni-free LDS. The Mn/Ni ratio played a vital role towards controlling precipitation and recrystallization and accordingly, the mechanical properties. All the three alloys exhibited similar hardness, strength and ductility levels after 50% cold reduction. Upon heat treatment, there was precipitation of intergranular and intragranular B2 phase particles along with κ-carbides, whose fraction decreased with increasing the temperature. The tensile response of LDS alloys was influenced primarily by the volume fraction of these precipitates as well as the recrystallized austenite grains. Planar slip activity, dynamic slip band refinement in conjunction with dislocation-twin interaction mechanisms were noticed in fully recrystallized austenite and thereby leading to enhanced strain hardening and ductility. Furthermore, lowering Mn in Ni containing LDS reduced the processing temperature window of the alloy in two-phase region.

Achieving ultra-high yield strength in austenitic low-density steel via drastic VC precipitation

In this study, the precipitation behavior of V in austenitic low-density steel (LDS) and its influence on microstructure and mechanical properties were investigated by X-ray diffraction, scanning electron microscope, transmission electron microscope, electron backscatter diffraction, physicochemical phase analysis and tensile testing in an attempt of enhancing the yield strength. Forging samples of novel Fe–25Mn–10Al-1.1C-(0,0.5)V (wt.%) austenitic LDSs were annealed at 900 °C followed by water quenching. Results showed that a drastic VC precipitation could be achieved in the austenitic matrix, which is mainly attributed to the decreased solid solubility products of VC and the reduction of the nucleation barrier due to the 10 wt% Al addition as well as the high C content. Thus, an extremely high yield strength of 1120 MPa, an increase of 260 MPa compared with that of the V-free steel, was obtained due to the precipitation strengthening and dislocation strengthening effect. Moreover, the addition of V inhibited the precipitation of κ-carbide and increased the ferrite content. Dispersedly distributed VC precipitates enhanced the initial work hardening rate and delayed the recrystallization of the V-containing steel. The V-containing steel exhibited primary planar slip but deformation twins were also observed in some austenite grains due to its decreased stacking fault energy. This study thus offers a potential strategy to significantly improve the yield strength of low-density steels through a suitable addition of V and corresponding simple annealing treatment.

Work hardening behavior and substructure evolution of a low-density steel during compressive deformation

In the present work, the room temperature work hardening behavior and substructure evolution of Fe–17.5Mn–8.3Al–0.74C-0.14Si lightweight steel were investigated during compressive deformation. A series of compression tests were conducted under strain rates of 0.001s−1, 0.01s−1, and 0.1s−1. In addition, the compression tests were interrupted at strain levels of 0.25, 0.75, and 1.25 under the strain rate of 0.001s−1 to study the evolution of the substructure under compression. The starting and deformed microstructures were characterized through high-resolution electron back scatter diffraction (EBSD) technique. The obtained flow curves indicate the high capability of the experimented material for strain hardening. According to the microstructural observation, the outstanding mechanical properties are attributed to the capability of the alloy in cell structure formation and its progressive evolution to subgrains. Such extended dynamic recovery was followed by continuous dynamic recrystallization (CDRX) and deformation-induced ferrite transformation (DIFT) to accommodate the high amount of compressive strain (∼2.5). These two mechanisms result in a final refined microstructure with ultrafine equiaxed grains which provides a great combination of strength and formability.

Studies on Hot Deformation Behavior and Dynamic Recrystallization in a High Al Ferritic Low-Density Steel

Studies on Hot Deformation Behavior and Dynamic Recrystallization in a High Al Ferritic Low-Density Steel

Effect of early stage of κ-carbides precipitation on tensile properties and deformation mechanism in high Mn–Al–C austenitic low-density steel

High Mn–Al–C austenitic low-density steels possess the great combination of strength and ductility due to their excellent strain hardening capability. At present, there is no consensus on the controlling factors of the deformation mechanism, of which κ-carbides may play an important role. Abundant literatures have investigated the influence of long-term or peak aging heat-treatment so far, while few reports have focused on the early stage of κ-carbides precipitation. In this study, samples characterized by short-range order (SRO) and early precipitation states of κ-carbides were processed by rapidly quenched (RQ) and water quenched (WQ) treatment, respectively. When the ordering process from SRO to ultrafine nano κ-carbides was stopped by the increase of cooling rate, the ultimate tensile strength of the alloy was significantly increased without a decrease in yield strength, which indicates a dramatic increase in the work hardening rate. TEM investigation clearly revealed that the precipitation of κ-carbides slowed down the evolution of microstructure, which is an essential factor for the deterioration of work hardening capability. Another interesting result was that although the stacking fault energy (SFE) of the alloy was high (∼108 mJ/m2), twinning phenomena was observed in the RQ sample, implying that SRO should assist the formation of deformation twins, which provided a novel example for the development of twins in high SFE alloys.

A novel method for preparing diffusion couple to investigate the interaction between AlN and low-density steel at 1373 K

• A novel method for preparing the diffusion couple was performed. • The particle precipitation zone consisting of AlN, Al 2 O 3, and MnAl 2 O 4 was detected. • Fine AlN particles precipitated on the grain boundary during heat treatment. • The formation of oxide particles at the AlN/steel interface was clarified. A novel liquid–metal-suction (LMS) method was proposed to prepare the diffusion couple. The interaction between AlN and low-density steel (LDS) after heating at 1373 K for 10 h was observed. The AlN and LDS were tightly combined, and the interface between AlN and LDS was visible. After heat treatment, the diffusion of nitrogen from the AlN substrate to the LDS matrix resulted in the precipitation of fine AlN particles on the grain boundary. Due to traces of Y 2 O 3 in the AlN substrate, the MnAl 2 O 4 and Al 2 O 3 particles were detected on the steel side near the interface. This LMS method can also be used to investigate the interaction between other types of NMIs and steels during heat treatment.

The Microstructure and Mechanical Properties of Welded Joints of Hot-Rolled and Post-Weld Solution-Treated Fe-12.5Mn-9.8Al-1.0C Low-Density Steel

The Microstructure and Mechanical Properties of Welded Joints of Hot-Rolled and Post-Weld Solution-Treated Fe-12.5Mn-9.8Al-1.0C Low-Density Steel

Microstructure evolution and microhardness of thermal-simulated HAZ in Fe–Mn–Al–C steel

The microstructure evolution and microhardness of Fe–Mn–Al–C steel heat-affected zone (HAZ) were investigated systematically using the weld thermal simulation technique. The results illustrated that the microstructure in HAZ with different peak temperatures (600, 800, 1000 and 1300°C) and heat inputs (10, 20 and 30 kJ cm–1) mainly consisted of ferrite and austenite. When the peak temperature was 1300°C, there are many high-angle grain boundaries which may inhibit crack propagation. With the increase in heat input, κappa carbide precipitation increased, the grain size became finer and microhardness increased. The microhardness was 438 HV when the peak temperature was 1300°C at 30 kJ cm–1. Highlights The welding of high carbon, high aluminium and low-density steel was simulated. The HAZ was composed of austenite and a small amount of high-temperature δ-ferrite. The proportion of high- and low-angle grain boundaries has a great influence on the mechanical properties. The welding heat input was 30 kJ cm–1, the grain size of the HAZ decreased and microhardness was the largest.

Microstructure and mechanical response of dissimilar joint of ferritic interstitial-free steel to austenitic low-density steel produced by diffusion bonding

Lightweight Fe–Mn–Al–C steels, also referred as low-density steels (LDS), have gained significant importance in the recent past owing to their reduced density, high strength and ductility, along with good oxidation resistance at elevated temperatures. The present study is aimed at exploring the feasibility of solid-state joining of LDS to the other prominent ferrous alloys, for example, interstitial-free steel (IFS), in order to extend their applications. In the present work, diffusion bonding has been attempted to produce IFS-to-LDS joints with and without the application of nickel (Ni) interlayer. Diffusion bonding is performed at two different temperatures of 850 °C and 950 °C for 30 min at the bonding pressures of 10, 20 and 30 MPa under high vacuum conditions. The interfacial microstructures of the joints are examined using an optical microscope and scanning electron microscope in conjunction with energy dispersive spectroscopy and electron backscattered diffraction. Microhardness and nanoindentation measurements across the interface and shear tests are performed to evaluate the interfacial bond strength of the joints. The joint produced with the application of Ni interlayer at 950 °C under 30 MPa bond pressure exhibited superior shear properties with a maximum shear strength of ∼340 MPa. The formation of stable oxides resulting, especially by diffusion of aluminum from LDS to the faying surfaces is found to be detrimental to establish a good interface at lower bonding parameters. The application of higher bonding pressure and Ni interlayer along with enhanced diffusional activities from LDS to IFS within the austenite phase are found to be effective in achieving a sound joint. The metallurgical aspects involved in the formation of the reaction products and subsequently, the influence of the evolved interface microstructure on the mechanical response of the joints are discussed in-detail.

The Effect of Strain Rate on the Deformation Behavior of Fe-30Mn-8Al-1.0C Austenitic Low-Density Steel

Automotive steels suffer different strain rates during their processing and service. In this study, the effect of strain rates on the tensile properties of fully austenitic Fe-30Mn-8Al-1.0C (wt.%) steel was investigated, and the dominant deformation mechanism was clarified. Conventional and interrupted tension tests and various microscopic characterization methods were carried out in this study. The results indicate that the yield strength increases with the increasing strain rate in the range of 10−4–10−1 s−1, and a good strength–ductility combination was achieved in the sample deformed at 10−3 s−1. In the process of straining at 10−3 s−1, microbands and deformation twins were observed. Thus, the combination of microband induced plasticity (MBIP) together with twinning induced plasticity (TWIP) leads to a continuous strain hardening behavior, and consequently to superior mechanical properties. However, adiabatic heating that leads to the increase in stacking fault energy (SFE) and inhibits the TWIP effect, as well as thermal softening jointly induces an anomalous decrease in tensile strength at the high strain rate of 10−1 s−1.

Microstructure and Continuous Cooling Transformation of an Fe-7.1Al-0.7Mn-0.4C-0.3Nb Alloy

Reducing pollutant emissions and improving safety standards are primary targets for modern mobility improvement. To meet these needs, the development of low-density steels containing aluminum is a new frontier of research for automotive applications. Low-density Fe-Mn-Al-C alloys are promising. In this regard, an alloy with high aluminum content and niobium addition belonging to the Fe-Mn-Al-C system was evaluated to understand the possible phase transformations and thus obtain a transformation diagram by continuous cooling to help future processing. Dilatometry tests were performed in a Gleeble thermomechanical simulator with different cooling rates (1, 3, 5, 10, 15, 20, 30, and 50 °C/s). Chemical analyses carried out simultaneously with dilatometry tests showed the presence of proeutectoid ferrite (αp), δ-ferrite, retained austenite, and niobium carbide (NbC). In the case of low cooling rates (1 and 3 °C/s), lamellar colonies of the eutectoid microconstituents were observed with a combination of α-ferrite and k-carbide. For higher cooling rates (5 to 50 °C/s), martensite was observed with body-centered cubic (BCC) and body-centered tetragonal (BCT) structures.

Effect of κ Carbides on Deformation Behavior of Fe-27Mn-10Al-1C Low Density Steel

Fe-Mn-Al-C steel, which is a potential lightweight material for automobiles, has a variety of microstructures and good mechanical properties. The effect of κ carbides on the mechanical properties and strain hardening rate of Fe-27Mn-10Al-1C (wt.%) low density steel was studied by short-time heat treatment to control the precipitation behavior of κ carbides. Quenched specimens have an excellent combination of strength and plasticity and continuous high strain hardening rate, which is due to the uniform distribution of κ carbides with an average size of 1.6 nm in an austenite matrix. The fracture mode of the sample changed from ductile fracture to cleavage fracture, which was because the aging treatment promoted the precipitation of B2 phases and κ carbides at grain boundaries. The size and volume fraction of nanoscale κ carbides in austenite grains increase with the increase of aging temperature, and the yield strength increases but the density of slip bands decreases, resulting in the gradual decrease of strain hardening rate.

Enhancement of strength and ductility of Fe–13Al–16Mn–5Ni −0.8C steel by annealing at high temperature

The mechanical properties and microstructural evolution of low-density (6.89 g/cm3) steel having composition of Fe–13Al–16Mn–5Ni-0.8C has been investigated after annealing at 720 °C and 1050 °C, respectively. At 720 °C, nanosized B2-particles of different morphologies were formed inside austenite and ferrite grains. After annealing at 1050 °C the selected area diffraction (SAED) and HAADF analysis confirmed the formation of rod-shaped ordered Ni3Al-type particles of FCC crystal structure on the (111) plane of austenite grains. Moreover, inside the ferrite grains, new austenite grains up to 30 μm in size were observed. Ordered Ni3Al-type rod and spherical shape particles contributed to an increase of 7% in the average ultimate tensile strength due to hindering of dislocations movement on the active slip plane of austenite grains. A significant increase of 233% in average elongation to failure was also achieved with high-temperature annealing due to the evolution of new austenite grains and the disappearance of B2 particles. The presence of fibrous dimple fracture in the sample annealed at 1050 °C is consistent with the high ductility observed. This is in stark comparison to the sample annealed at 720 °C which showed failure through ductile tearing and cleavage.

An accelerated aging assisted by electric current in a Fe-Mn-Al-C low-density steel

An aging method assisted by electric current was applied to a Fe-18Mn-9Al-1C (wt.%) low-density steel. It improves the microstructure and therefore significantly increases both the yield strength and ductility of the steel. This current-assisted aging method can increase the yield strength by 178 MPa and elongation by 1.16 times in only 0.5 min at 450 °C. However, the yield strength is increased only 90 MPa by the traditional aging method (heat conduction) at 450 °C for 180 min, and the elongation is even decreased from 42.0% to 31.6%. The obvious improvement in yield strength by the current-assisted aging for a short time is resulted from the fact that the current-assisted aging promotes a rapid precipitation of nano-scale κ-carbides in γ-austenite by reducing the thermodynamic barrier and accelerating the atomic diffusion. This work demonstrates that this current-assisted aging method is significantly time saving and cost-effective for low-density steels, with potential for various industrial applications.

Tensile deformation behavior related with strain-induced martensitic transformation in a duplex Fe-Mn-Al-C low-density steel

The tensile deformation behavior of a duplex Fe-Mn-Al-C low-density steel with a structure of banded ferrite and austenite has been studied in the present work. The steel exhibits a good strength-ductility match and a typical three-stage deformation behavior, under the joint contributions of transformation-induced plasticity (TRIP) effect, twinning-induced plasticity (TWIP) effect and dislocation slipping. Even though the steel has a stacking fault energy of 53 mJ∙m −2 that is suitable for the TWIP effect, it just worked at the final deformation stage. The TRIP effect played an important role during the majority of deformation process except at an initial stage, and the austenite was dramatically consumed at the later stage. This is mainly determined by an unnormal stability of austenite. For estimating the austenite stability, the relationship between the critical size of austenite grain ( D crit ) and critical temperature for strain-induced transformation ( M s σ ) was established. The calculated D crit of 12 μm reflects that the strain-induced martensitic transformation dominates the TRIP effect in the duplex steel. • Joint effect of TRIP and TWIP effects contribute to the excellent ductility. • This duplex low-density steel shows a three-stage deformation behavior. • A quantitative estimation method of austenite stability is proposed.

Effect of Aging Treatment on the Microstructure and Mechanical Properties of Fe-Mn-Al-C Low Density Steel

Fe-Mn-Al-C steel has excellent weight reduction potential. In this study, the microstructure and mechanical properties of Fe-25Mn-11Al-1.0C steel after solution treatment at 1050 °C and aging at 450~650 °C are studied. The results show that with the increase of aging temperature, the strength increases first and then decreases. The plasticity and strength show a trade-off relationship. With the increase of the aging temperature, the plasticity first decreases and then increases. The above-mentioned changes in mechanical properties are closely related to the changes in the morphology and distribution of κ-carbides during the aging process. In general, the precipitation of κ-carbides is beneficial to obtain higher strength, but has a detrimental effect on plasticity.

Revealing effect of aluminum alloying on work hardening and impact behaviors of low-density Fe-18Mn-1.3C-2Cr-(4, 11)Al casting steel

The present study designed two kinds of Fe-18Mn-1.3C-2Cr-(4, 11)Al (wt.%) low-density steels. Tensile and impact tests were carried out to evaluate the work hardening and impact toughness properties via aluminum (Al) alloying control. Meanwhile, microstructure evolution and fracture morphology were investigated by X-ray diffraction (XRD), a scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD), a transmission electron microscope (TEM), and a stereo-optical microscope (OM). It is found that the Al addition obviously promotes the dislocation planar slipping, resulting in cleavage and brittle impact fracture in 11wt.% Al steel. Besides, the microband-induced plasticity (MBIP) mechanism is found in 4wt.% Al containing steel, introducing considerable work hardening capacity and impact toughness of 156.8±17.4 J. The present study provides a direct illustration of the relationship between work hardening and impact toughness behaviors of these two low-density steels for potential application as impact-resistant components.

Tensile deformation-induced twinning in an austenitic low density steel over a wide range of temperature

A Fe30Mn11Al1CNbV low-density steel was processed by solid solution treatment and deformed at different temperatures from -196°C to +200°C by tensile testing. It was found that the deformation microstructure is mainly dominated by thin dislocation slip bands during tensile deformation, and deformation twinning was found near grain boundaries at a strain of 0.5, regardless of deformation temperature. The stacking fault energy is calculated by consideration of both chemical composition and the deformation temperature, which is high and normally should not initiate deformation twinning. However, deformation twinning is occasionally observed in the deformation microstructure at room temperature, and becomes to be one of the dominating features at liquid nitrogen temperature. Based on the analysis of work hardening and microstructure evolution, it is proposed that the deformation behaviour of the austenitic low density steel is controlled by the continuous thinning of dislocation slip bands at temperatures of 25°C and 200°C, but by a synergistic behaviour of slip bands and deformation twinning at -196°C.

Effect of annealing time on the microstructure and SCC behavior of an austenite-based low-density steel in a marine atmosphere

The microstructure and stress corrosion cracking (SCC) behavior of the Fe-Mn-Al-C-Ni low-density steels (LDSs) with different annealing times were studied in a simulated marine atmosphere, and the relation between them was discussed. The results indicated that the SCC mechanism of the LDSs was a combination of anodic dissolution and hydrogen embrittlement. This behavior was greatly facilitated by a micro-galvanic corrosion effect between B2 phase and austenite. The SCC resistance of the LDSs increased gradually as the annealing time extended, attributing to the transformation of B2 band to B2 particle, the decrease in the potential difference, and the homogenization of the strain distribution.