The need for lightweight materials has been an important issue in automotive industries to reduce greenhouse gas emission and to improve fuel efficiency. In addition, automotive steels require an ...excellent combination of strength and ductility to sustain automotive structures and to achieve complex shapes, but the traditional approach to obtain a reduction in weight from down-gauged steels with high strength has many limitations. Here, we present a new ferrite–austenite duplex lightweight steel containing a low-density element, Al; this steel exhibits tensile elongation up to 77% as well as high tensile strength (734MPa). The enhanced properties are attributed to the simultaneous formation of deformation-induced martensites and deformation twins and the additional plasticity due to deformation twinning in austenite grains having optimal mechanical stability. The present work gives a promise for automotive applications requiring excellent properties as well as reduced specific weight.
Effects of Mn (19 and 22wt.%) and Al (0 and 2wt.%) contents on tensile and Charpy impact properties in four austenitic high-Mn steels were investigated at room and cryogenic temperatures. The ...cryogenic-temperature tensile test results indicated that the yield strength was higher in the Al-added steels than non-Al-added steels, which could be explained by a stress-induced martensitic transformation in the non-Al-added steels. The reduction in ductility was largest in the 19Mn steel, where the transformations to ε- and α′-martensites occurred and their fraction was highest. Charpy impact energies of the 19Mn and 22Mn steels rapidly dropped with decreasing temperature, whereas those of the 19Mn2Al and 22Mn2Al steels slowly decreased. According to the EBSD analysis data of the cryogenic-temperature Charpy impact specimen, the transformation to ε- and α′-martensites readily occurred in the 19Mn and 22Mn steels, which resulted in the large reduction in impact energy. In the 19Mn2Al steel composed of highly stable austenite, the time needed for sufficient deformation to trigger the martensitic transformation was very short under the impact testing condition. In the Al-added steels, any martensites were not found, while many deformation twins were formed, thereby leading to high Charpy impact energy.
Systematic investigation of microstructure characterization and mechanical property was performed on metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy (HEA) with varying the grain size. The grain ...coarsening led to the decrease in stability of face-centered-cubic (FCC) phase, and consequently generated the formation of laminate-morphology hexagonal-close-packed (HCP) and butterfly-morphology body-centered-cubic (BCC) martensite. During the tensile test, a transformation-induced plasticity (TRIP) from the FCC to BCC via the intermediate HCP occurred, and the accelerated transformation rate of BCC increased the strain-hardening rate. Our results suggest that microstructural evolutions related to thermally-induced martensitic transformation and strain-hardening mechanisms can be well interpreted by understanding the FCC to HCP to BCC martensitic transformation.
Display omitted
High strength and ductility and vehicle's weight reduction are key issues for improving fuel efficiency in automotive industries. In addition, structural reinforcement components require high yield ...strength because the prevention or minimization of deformation is more important than the absorption of impact energy. In this study, new ultra-high-strength duplex lightweight Fe-0.5C-12Mn-7Al-(0,3)Cu (wt%) steels have been developed by varying annealing temperature. Here, Cu, an austenite stabilizer, not only raises the austenite volume fraction but also delays the recrystallization due to a solute drag effect, while it promotes the formation of Cu-rich B2 particles and Cu-segregated interfacial layers. The steels show the planar slip and fine dislocation substructures (Taylor lattices) as desirable deformation mechanisms. Non-shearable Cu-rich B2 particles, solid solution hardening of Cu, and delayed recrystallization greatly improve the yield strength (∼1 GPa) and strain hardening. Through these unique and excellent tensile properties together with weight saving of 10.4%, the present work provides a desirable possibility for applications to automotive reinforcement components preferentially requiring an excellent yield-to-tensile ratio.
Display omitted
Hydrogen embrittlement (HE) has arisen as one of main issues for developing high-strength lightweight steels. The precipitation of fine particles providing as stable H-trapping sites is preferred to ...overcome the intrinsic HE of high-strength steels. However, studies on HE in high-strength lightweight steels along with roles of B2 particles have not been reported yet. In this study, three Fe–0.8C–15Mn–7Al–(0,1,3)Cu duplex lightweight steels were fabricated, and their resistance to HE was evaluated. Roles of Cu addition were investigated by the loss of elongation measured from slow-strain-rate tensile tests and by the concentration and diffusivity of H measured from electrochemical H permeation tests. The Cu addition results in the decreased elongation loss and the lower concentration and effective diffusivity of reversible H, indicating the higher resistance to HE. The unraveled mechanism is that the Cu addition increases the fraction of austenite, where the diffusivity of H is much lower than ferrite, and decreases the strain localization along ferrite grains to reduce the internal diffusion of H during deformation. In addition, it promotes the formation of complex semi-coherent Cu-rich B2 particles, which provides misfit dislocations at interfaces as stable and irreversible H-trapping sites. The B2 particles preferentially nucleated at reversible sites such as grain boundaries and phase interfaces promote a transition from reversible to irreversible sites, which further reduces the diffusivity of H. The present work, thus, would suggest the Cu addition to enhance both tensile properties and resistance to HE for designing high-strength lightweight steels.
Display omitted
Ultra-high-strength (over 1.8 GPa) of hot-stamping steels can be achieved by increasing C content of conventional hot stamping steel (1.5 GPa) or controlling precipitation behavior, but the enhanced ...strength often leads to the deteriorated resistance to hydrogen embrittlement. The complex alloying of Nb and Mo improves the resistance; however, the underlying microstructural evolutions and synergistic mechanisms are not understood yet. In this study, Nb- or (Nb + Mo)-alloyed 1.9 GPa-grade hot-stamping steels were fabricated in the laboratory, and their resistance to hydrogen embrittlement was evaluated by slow strain-rate tensile (SSRT) tests without and after hydrogen charging. Most of Nb consumed to form Nb carbides, hindering the migration of prior austenite grain boundaries mainly by a common Zener's pinning effect. The reduced grain size resulted in the decreased amount of diffusible hydrogen per unit grain boundary area. In addition, coherent or semi-coherent precipitates provided stable hydrogen trapping sites, leading to the low diffusivity and the consequently high resistance to hydrogen embrittlement. In the (Nb + Mo)-alloyed steel, Mo carbides preferred to nucleate at the surface of the pre-existing Nb carbides or to form needle-shaped isolated Mo2C carbide, but a considerably large amount of Mo remained inside the matrix as a solute. The prior austenite grain size was thus further reduced mainly by an additional solute drag effect. The solute Mo also enhanced the grain boundary cohesion, thereby leading to the absence of intergranular fracture and the sufficient post elongation even after the hydrogen charging.
Dual-phase Al0.5CoCrFeMnNi high-entropy alloy consisted of face-centered cubic (FCC) and body-centered cubic (BCC) phases exhibited enhancement of both strength and strain hardening ability at 77 K. ...It resulted from back stress hardening in high work hardening due to large strength difference of two constituent phases with decreasing temperature.
Display omitted
Hydrogen embrittlement (HE) has become an important issue in ultra-strong automotive steel applications. The addition of Mo to commercial 32MnB5 hot-stamping steel is preferred to enhance the ...strength levels with little ductility loss. However, the effects of solute Mo on HE have been rarely studied for developing 32MnB5 steel, and most studies on the alloying of Mo for interfacial cohesion have been conducted theoretically by calculating the cohesive energies in the Fe lattice. In this study, 0.15 wt.% Mo was added to the 32MnB5 steel and the resistance to HE was evaluated experimentally via electrochemical H-charging. The H-charged reference steel shows a large ductility loss (50–79%) after H-charging, while the addition of Mo significantly reduces the loss (17–26%) with sufficient post-elongation, indicating a higher resistance to HE. This is because the solute Mo decreases the H diffusivity, resulted from the high H affinity and repulsive strain field owing to the large atomic size of Mo. The direct observation of crack propagation reveals that the H-induced crack path changes from the prior austenite grain boundaries (PAGBs) to the grain interiors of H-enhanced slip planes. This is attributed to the reduced H- and strain-localization on the PAGBs by the solute Mo and the enhanced grain-boundary cohesion by Mo segregation. This work thus demonstrates the beneficial effects of the addition of Mo on the tensile properties and the intrinsic resistance to HE for the development of ultra-high-strength steels.
Display omitted
A new metastable high-entropy alloy (HEA) system was suggested by thermodynamic calculations based on the Gibbs free energies of FCC and HCP and the associated stacking fault energy (SFE). The ...Fe45Co30Cr10V10Ni5-xMnx (x = 0, 2.5, and 5 at.%) alloys were fabricated, and their tensile properties were evaluated at room and cryogenic temperatures. The relationship between the deformation mechanism and strain hardening behavior was investigated to reveal the role of deformation-induced martensitic transformation on tensile properties. The difference in Gibbs energy decreases with increasing Mn content, leading to the decreased SFE in sequence. At room temperature, ~60% of BCC martensite in the 5Mn HEA contributes effectively to the steady strain hardening, suppressing the plastic instability. This TRIP effect achieves much eminence in the cryogenic deformation, enabling the tensile strength to reach over 1.6 GPa due to 100% of BCC and HCP martensite. In addition to the fraction of martensite, the increased Mn content reduces a critical strain required to trigger the martensitic transformation and then raises the transformation rate. The present findings may provide a guide for the design of metastable HEAs to enhance tensile properties for cryogenic applications through adjusting SFE and TRIP effect.
Weight reduction from down-gauged high-strength steels has been an important issue in automotive industries to improve fuel efficiency. In addition to lightweight needs, automotive steels require an ...excellent combination of specific strength and ductility for forming complex shapes as well as improving crashworthiness qualities. Here in the present study, new ultra-high-strength (ferrite+austenite) duplex lightweight steels containing a low-density element of Al, which exhibit strength above 1GPa and tensile elongation of 46%, have been developed. Improved tensile properties are associated with typical planar glide configurations and high dislocation density walls, configuring Taylor lattices, developed by very fine dislocation structures spaced with intervals between 50 and 100nm. Deformation twinning having extremely small (about 5nm) thickness and spacing is also activated, thereby leading to additionally enhanced ductility. The present lightweight steels have outstanding properties of strength and ductility, easy manufacturing process, and costs of alloying elements as well as reduced specific weight for automotive applications.
