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High-strength bainitic steels have created a lot of interest in recent times because of their excellent combination of strength, ductility, toughness, and high ballistic mass efficiency. Bainitic steels have great potential in the fabrication of steel armor plates. Although various approaches and methods have been conducted to utilize the retained austenite (RA) in the bainitic matrix to control mechanical properties, very few attempts have been conducted to improve ballistic performance utilizing transformation-induced plasticity (TRIP) mechanism. In this study, high-strength bainitic steels were designed by controlling the time of austempering process to have various volume fractions and stability of RA while maintaining high hardness. The dynamic compressive and ballistic impact tests were conducted, and the relation between the effects of TRIP on ballistic performance and the adiabatic shear band (ASB) formation was analyzed. Our results show for the first time that an active TRIP mechanism achieved from a large quantity of metastable RA can significantly enhance the ballistic performance of high-strength bainitic steels because of the improved resistance to ASB formation. Thus, the ballistic performance can be effectively improved by a very short austempering time, which suggests that the utilization of active TRIP behavior via tuning RA acts as a primary mechanism for significantly enhancing the ballistic performance of high-strength bainitic steels.

The bending angle at the peak load is regarded as the most important parameter for evaluating bending properties of hot-press-forming (HPF) steels. However, it is not a mechanics-based parameter for the bending criterion, and the data interpretation is difficult because bending criteria in relation with microstructures and associated bending mechanisms have not been verified yet. In this study, effects of coating and baking treatments on bending angles at the peak load of three kinds of 1470 MPa-grade HPF steels were investigated by interrupted three-point bending tests coupled with direct microstructural observation. According to direct observations of sequential cracking processes of V-shaped crack (V-crack), bending procedures were classified into four stages: (1) formation of small V-crack, (2) increase in number and size of V-cracks, (3) initiation of shear-crack propagation from the V-crack tip, and (4) further propagation and opening of the shear crack. The minimum bending angle required for initiating the shear-crack propagation from the V-crack tip was defined as a critical angle, which meant the boundary between the 2nd and 3rd stages. The present bending behavior related with critical bending angle and V-cracking could be interpreted similarly by the fracture-mechanics concept, i.e., the initiation of shear-crack propagation.

Effects of Cr addition (0, 3, and 6 wt%) on Charpy impact properties of Fe-C-Mn-Cr-based steels were studied by conducting dynamic compression tests at room and cryogenic temperatures. At room temperature, deformation mechanisms of Charpy impacted specimens were observed as twinning induced plasticity (TWIP) without any transformation induced plasticity (TRIP) in all the steels. At cryogenic temperature, many twins were populated in the Cr-added steels, but, interestingly, fine e-martensite was found in the 0Cr steel, satisfying the Shoji-Nishiyama (S N) orientation relationship, { 111 }γ//{ 0002 }e and 101 >γ// 11 2 ¯ 0 >e. Even though the cryogenic-temperature staking fault energies (SFEs) of the three steel were situated in the TWIP regime, the martensitic transformation was induced by Mn- and Cr-segregated bands. In the 0Cr steel, SFEs of low-(Mn,Cr) bands lay between the TWIP and TRIP regimes which were sensitively affected by a small change of SFE. The dynamic compressive test results well showed the relation between segregation bands and the SFEs. Effects of Cr were known as not only increasing the SFE but also promoting the carbide precipitation. In order to identify the possibility of carbide formation, a precipitation kinetics simulation was conducted, and the predicted fractions of precipitated M23C6 were negligible, 0.4–1.1 × 10−5, even at the low cooling rate of 10 °C/s.

Formation possibilities of adiabatic shear bands (ASBs) or cracks in two commercial ultra-high-strength armor steels composed of martensite were evaluated by a split Hopkinson pressure bar so that a plausible forecasting method of ASB formation and cracking during the dynamic compression or ballistic impact might be suggested. The present dynamic compressive test effectively investigated the ASB formation behavior and provided a good idea on how many ASBs or cracks form during the ballistic impact.

The formability of austenitic high-Mn steels is a critical issue in automotive applications under non-uniformly-deformed environments caused by dynamic strain aging. Among austenite stabilizing alloying elements in those steels, Cu has been known as an effective element to enhance tensile properties via controlling the stacking fault energy and stability of austenite. The effects of Cu addition on formability, however, have not been sufficiently reported yet. In this study, the Cu addition effects on formability and surface characteristics in the austenitic high-Mn TRIP steels were analyzed in consideration of inhomogeneous microstructures containing the segregation of Mn and Cu. To reveal determining factors, various mechanical parameters such as total elongation, post elongation, strain hardening rate, normal anisotropy, and planar anisotropy were correlated to the hole-expansion and cup-drawing test results. With respect to microstructural parameters, roles of (Mn,Cu)-segregation bands and resultant Cu-rich FCC precipitates on the formability and surface delamination were also discussed.

An austenitic Hadfield steel shows highly sustained strain hardening as well as excellent tensile strength and ductility, but its yield strength is relatively low (400 to 500 MPa). In order to overcome this shortcoming, two multi-layer steel (MLS) sheets were fabricated by the hot-roll-bonding of the Hadfield steel with two kinds of martensitic hot-press-forming (HPF) steels. Carburized and decarburized layers were formed near Hadfield/HPF interfaces by the C diffusion from the high-C Hadfield (1.2 pct) to low-C HPF (0.23 and 0.35 pct) layers. The tensile test results indicated that the two MLS sheets were fractured right after the yielding with almost no plastic deformation because the intergranular fracture appeared in the carburized and HPF layers. This intergranular fracture was caused by Cr7C3 carbide precipitation and C segregation at prior austenite grain boundaries as well as considerably high residual stresses generated in both Hadfield and HPF layers. The tempering at 473 K (200 °C) was adopted for relieving residual stresses, while most of grain-boundary carbides remained. The 473 K (200 °C)-tempered MLS sheets showed improved tensile properties over the non-tempered MLS sheets, and well satisfied a rule of mixtures.

An austenitic Hadfield steel sheet shows a relatively low yield strength of 0.4–0.5 GPa and serrated flows in spite of excellent tensile strength and ductility along with highly-sustained strain hardening. In order to overcome the shortcomings, a multi-layer steel (MLS) sheet was fabricated by a roll-bonding with an ultra-high-strength martensitic hot-press-forming (HPF) steel sheet. Near the Hadfield/HPF interface, the carburized and decarburized layers were formed by the carbon diffusion from the Hadfield (1.2% of C) to HPF (0.23% of C) layers, and could generate a kind of very thin multi-layers of 35 μm in thickness. All tensile properties of the Hadfield/HPF MLS sheet (yield strength; 946 MPa, tensile strength; 1291 MPa, elongation; 44.5%) were superior to those of the Hadfield sheet. Interestingly, the persistent elongation up to 44.5%, which is higher than that of the Hadfield steel, in the present MLS sheet is a quite unique and interesting characteristic. The simultaneous enhancement of strength and ductility of the MLS sheet was explained by the contributions of 1) populated twin formation, 2) generation of geometrically necessary dislocations (GNDs), and 3) increase of back stress inside thin interfacial layers.

The metastable high- and medium entropy alloys (HEAs or MEAs) have drawn many attentions regarding deformation mechanisms and mechanical properties. Most of their studies have conducted under quasi–static or uniaxial tensile/compressive loading conditions. For cryogenic applications, however, the fracture or impact toughness should be carefully evaluated because it is one of the most important indices for the low–temperature performance. In this study, quasi–static and dynamic tensile properties of a metastable VCrFeCoNi HEA were investigated at room and cryogenic temperatures, and they were systematically correlated with the Charpy impact toughness. Under the quasi–static tensile loading, the Twinning Induced Plasticity (TWIP) mechanism occurred at room temperature, while the Transformation Induced Plasticity (TRIP) from FCC to BCC phases via an intermediate HCP phase occurred at cryogenic temperature. Under the dynamic loading, more deformation twins were formed at room temperature, and the amount of martensite reduced at cryogenic temperature. These variations of twinning and martensitic transformation were elucidated by the raised flow stress and by the adiabatic heating effect, respectively. They were confirmed by combining with ab–initio calculations, leading to the strong dependency of the energetic stability of BCC and HCP phases relative to the FCC phase. As a result, a plenty of deformation twins under the dynamic loading resulted in the high impact toughness of 112.6 J at room temperature. The martensitic transformation and consequently refined network structure played key roles in sustaining the remarkable toughness and in preventing the DBT phenomenon as the test temperature decreased.

The hot-press-forming (HPF) steels might contain a few MnS inclusions, and their brittle characteristics and stringer shape can deteriorate the impact toughness and formability. In this study, effects of shape and orientation of MnS particles on Charpy impact and bending properties of 1470-MPa-grade HPF steel sheets were investigated by conducting ex situ three-point bending or tension tests. According to direct observations of sequential cracking processes of MnS particles in the longitudinal-oriented specimen, largely elongated (high-aspect-ratio-valued) MnS particles were prone to form cracks at low strains. In the transverse-oriented specimen, MnS/matrix interfaces were debonded at the lower strains, and were liable to develop into cracks along the longitudinal direction. These observed results in this study indicated that the MnS cracking and MnS/matrix interfacial debonding were major fracture-initiation sites in the longitudinal- and transverse-oriented specimens, respectively, thereby leading to the decreases in both Charpy impact and bending properties in the MnS-containing HPF steel sheet.

A boron-containing 1.8–2.0-GPa-grade hot-stamping steel, which has been attracting great attentions as reinforcing automotive parts, often leads to the deteriorated resistance to hydrogen embrittlement due to its high strength level. The micro-alloying elements such as Nb and Mo have been known to improve the resistance to hydrogen embrittlement by refining grains and providing interfaces of precipitates. Ti is also utilized for the mentioned purposes; however, the effect of Ti on hydrogen embrittlement, particularly in (Nb + Mo) multi-alloyed system, has not been revealed clearly yet. In this study, therefore, the alloying effects of Ti on resistance to hydrogen embrittlement were investigated via controlling Ti content and conducting slow-strain-rate tensile (SSRT) tests and thermal desorption analyses (TDA) after the hydrogen charging. The complex addition of Nb, Mo, and Ti promotes the formation of nanoscale (Nb,Ti)C and (Nb,Mo,Ti)C complex precipitates along with coarse Ti(C,N) particles. The increased Ti content to 0.03 wt% increases the volume fraction of nanoscale precipitates, which effectively refines the prior austenite grain size and interfacial incoherency, thereby providing stable hydrogen trapping sites with the higher activation energy for hydrogen desorption. Although the increased Ti also promotes the formation of brittle coarse Ti(C,N) particles, this negative effect of the particles can be minimized or prevented by the decrease in particle size due to the interaction with Nb and Mo.

Hydrogen embrittlement of multi-layered steel (MLS) is significantly affected by a degree of hydrogen permeation from surface layers to interfaces. Here, the surface microstructure of multi-layered steel was modified for the first time using ultrasonic nanocrystal surface modification (UNSM), and the roles of UNSM-treated surface layers in strength and hydrogen permeation behavior were investigated by examining microstructural evolutions of the surface layer. Since the UNSM induced the compressive residual stress, grain refinement, and deformation twin formation at the specimen surface, the yield strength greatly improved via synergetic contributions of the grain refinement effect and dislocation strengthening. In addition, the UNSM-affected zone of 150–210 µm along depth direction effectively suppressed the hydrogen permeation by supplying compressive residual stresses and hydrogen trapping sites including grain boundaries, dislocations, and twin boundaries.

Hot press forming (HPF) steel sheets are austenitized, press-formed, and rapidly cooled to obtain a martensitic microstructure with an ultra-high strength. When they are insufficiently austenitized, their microstructures might contain a small amount of untransformed ferrite, which can deteriorate impact toughness as well as strength, but its causes and relevant fracture mechanisms have not been clearly verified yet. In this study, thus, 1.8-GPa-grade HPF sheets were austenitized at various temperature and time, and their tensile and Charpy impact test results were analyzed in relation with untransformed ferrite and its effect on fracture mechanisms. In the HPF sheets containing the untransformed ferrite, voids were formed mostly at ferrite/martensite interfaces, and were grown and propagated linearly to form a cleavage crack, whereas deformation bands were well developed without voids or cracks in the non-ferrite-containing sheets. The highly localized strains accommodated in the soft ferrite made ferrite/martensite interfaces or ferrite itself work as fracture initiation sites, which led to the brittle fracture and consequently to the deterioration of impact energy. This result can provide an important idea for optimization of austenitization conditions demanded for ultra-high strength and excellent impact toughness in HPF applications.

Quasi-static and dynamic compressive properties of an FCC-based metastable HEA (composition; V10Cr10Fe45Co35 (at.%)) showing both Transformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) were investigated at room and cryogenic temperatures. During the quasi-static and dynamic compression at room temperature, the FCC to BCC TRIP occurred inside FCC grains, and resulted in very high strain-hardening rate and consequently maximum compressive strength over 1.6 GPa. The dynamic compressive strength was higher by 240 MPa than the quasi-static strength because of strain-rate-hardening effect, and kept increasing with a high strain-hardening rate as the twinning became activated. The cryogenic-temperature strength was higher than the room-temperature strength as the FCC to BCC TRIP amount increased by the decrease in stability of FCC phase with decreasing temperature. Under dynamic loading at cryogenic temperature, twins were not formed because the increase in SFE due to adiabatic heating might not be enough to reach the TWIP regime. However, the dynamically compressed specimen showed the higher strength than the quasi-statically compressed specimen as the strain-rate-hardening effect was added with the TRIP.

In this study, a high-strength bainitic steel was fabricated and austempered at 350 °C for 15 min, 90 min, and 24 h to control the volume fraction of retained austenite (RA) while keeping an appropriate hardness level. Their dynamic compressive properties were investigated in relation to the ASB formation and cracking by the SHPB. The specimen austempered for 90 min showed the highest volume fraction of RA along with the highest compressive properties. This specimen also possessed the high resistance to form a transformation-ASB (tASB), which was attributed to the active deformation-induced martensitic transformation of RA. Since the applied dynamic energy was consumed by the martensitic transformation as well as the ASB formation, the active transformation effectively delayed the critical strain for initiating formation. Therefore, this critical strain concept, which was well correlated with dynamic compressive properties, would be an important key factor for suppressing the ASB formation and for improving the ballistic performance.

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.

Austenitic high-Mn steels are regarded as a promising candidate for high-strength cold-rolled steels because their combination of strength and ductility improves greatly by combined effects of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Although it is well known that Al plays key roles in tensile properties of austenitic TWIP steels, its effects in austenitic TRIP or TRIP/TWIP steel are still unclear yet. In this study, three austenitic steels (composition; 0.4C–15Mn–1Si-(0,0.5,1)Al-0.3Mo-0.5V (wt.%)) were fabricated, and the effect of Al alloying on microstructures and tensile properties were investigated in relation to the deformation behavior with TRIP and TWIP mechanisms. A partial recrystallization was conducted for enhancing the yield strength, which was characterized with electron backscatter diffraction (EBSD) grain orientation spread maps. The present steels showed 1 GPa of yield strength achieved by partial recrystallization with the precipitation of (V + Mo) complex carbides. Particularly in the non-Al-alloyed steel, the e-martensite formed in the early deformation stage, and the martensitic transformation continued until the failure, thereby resulting in the highest tensile strength (1.5 GPa) along with the highest strain hardening.

In this study, laboratory-scale Split Hopkinson pressure bar (SHPB) tests were conducted on commercial armor plates composed of 2139 and 7056 Al alloys to investigate their dynamic compressive properties in relation to adiabatic shear band (ASB) formation. The results were then correlated to those of the ballistic performance as measured from V50 ballistic impact tests. In the SHPB tests, the 2139 alloy was compressed and flattened with traces of cracking, whereas cracks initiated in a shear mode in the 7056 alloy and propagated to produce several pieces. The cracking behavior was further analyzed by interrupted SHPB tests, which confirmed that shear cracking occurred through the formation of deformed ASBs and subsequent locally transformed ASBs. These processes started much earlier in the 7056 alloy; therefore, it showed a lower critical strain for initiating the ASBs and lower resistance to ASB formation. This result corresponded well with the actual V50 ballistic impact test results, which confirmed the better ballistic performance of the 2139 alloy. The resistance to dynamic and ballistic deformation was quantitatively discussed using the susceptibility to ASB formation.

Room- and cryogenic-temperature compressive properties for the three V10Cr10Co30FexNi50-x (x = 40, 45, 50) high entropy alloys are investigated under quasi-static and dynamic loading conditions. The face-centered cubic (FCC) stability, which is estimated by the calculation of Gibbs free energy difference between FCC and body-centered cubic (BCC) phases, decreases in increasing order of Fe content. The 40Fe and 45Fe alloys consist of FCC single phase, whereas the 50Fe alloy is composed of a duplex structure with ~95 vol% of body-centered cubic (BCC) lath martensite. Under the quasi-static loading condition at room temperature, the dislocation slip prevails in the 40-at.%-Fe-containing alloy, while the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) are activated in the other alloys. The decrease in testing temperature lowers the FCC stability, activating both TWIP and TRIP mode. Under the dynamic loading, the magnitude of TWIP effect enhances, but that of TRIP effect reduces in comparison to the quasi-static loading. The adiabatic heating induced from the dynamic loading increases the FCC stability, thereby resulting in the reduced TRIP effect. Despite the increased stability, the TWIP occurs more actively because the dynamic loading enables the increased flow stress to readily exceed the critical stress required for TWIP. These variations of deformation mechanisms according to the FCC stability, testing temperature, loading rate, adiabatic heating effect, and increased flow stress effect are systematically correlated by the shifting of stability on the slip-TWIP-TRIP mechanism band.

Layered architectures have been applied to twinning-induced plasticity (TWIP) steel in terms of configuring multi-layered TWIP and martensitic steel (MLS) sheets to overcome the limitation of low yield strength. However, hydrogen embrittlement (HE) inevitably appears in materials used for high-strength purposes, with the interfacial layers presenting localization sites of hydrogen, thereby limiting the materials’ broad structural applications. Here, we present a novel design for MLS sheets that exhibit both strong resistance to HE and a good strength–ductility balance via surface shielding. Our hydrogen penetration data demonstrates that the proposed austenitic shielding structure effectively acts as a barrier to hydrogen due to the reduced hydrogen diffusivity, compared with existing MLS sheets. We propose the optimal surface thickness required to play the role of shielding layer based on the quantified diffusivity and the rule of mixtures of the layer fractions, which presents a plausible novel design for layered architectures that exhibit strong resistance to HE and possess tunable mechanical properties.

The microstructural evolution and formation mechanism of adiabatic shear band (ASB) in a high-strength armor steel were investigated using a laboratory-scale split Hopkinson pressure bar, and the results were correlated with the actual ballistic impact behavior. The interrupted dynamic compressive test results reveal that a deformed ASB (dASB) starts to form right after the stress collapse and it develops into a transformed ASB (tASB). In the ballistic impact, wide tASBs form mostly at the perforated surface, and narrower tASBs are branched from the tASB. Very fine equiaxed grains of ∼190 nm in the tASB developed during the dynamic compression indicates that the dynamic recrystallization occurs even in 86.5 μs, and then the grains grow up to 260 nm in 9.5 μs. Rotational dynamic recrystallization mechanism and grain-growth rate model were proposed based on the calculation of temperature rise from a thermo-elasto-plastic finite element method, which provide a reasonable explanation for the formation and growth of fine equiaxed grains during both the dynamic compression and ballistic impact. A linkage of equiaxed subgrains and elongated parent subgrains demonstrates that the equiaxed subgrains did not evolve from nucleation and growth processes but from the sub-boundary rotation. Based on the underlying formation mechanisms and kinetics of ASBs, this study would suggest a reliable method to interpret the ASB formation and associated fracture mechanism during the ballistic impact.

In this study, roles of strain-hardening rate on susceptibility of adiabatic shear band (ASB) formation and subsequent cracking were investigated in two ultra-high-strength armor steel plates heat-treated differently. The quenched and tempered steel contained ~2% of retained austenite in the tempered martensitic matrix, while the quenched and austempered steel contained ~4% retained austenite in the bainitic matrix partly with the tempered martensite. The actual ballistic impact test results revealed the lower sensitivity of ASB formation in the austempered steel than in the tempered steel, which corresponded well to the higher critical strain for ASB formation in the dynamic compressive test using a laboratory-scale split Hopkinson pressure bar (SHPB). The austempered steel caused the higher internal stress among various constituents, and all the retained austenite transformed into martensite during the deformation, thereby leading to transformation-induced plasticity (TRIP) effect. The higher strain-hardening rate induced by these higher internal stress and TRIP effect increased resistance to ASB formation, which was confirmed by a calculation of ASB susceptibility. Thus, the austempered steel was much less susceptible to the ASB formation during the ultra-high-speed deformation. Consequently, the increased resistance to ASB formation retarded the initiation and propagation of ASBs and cracks.

Adiabatic shear band (ASB) is a well-known structure formed during high strain-rate deformation in various metallic materials, and significantly affects dynamic properties as well as ballistic performance. Retained austenite contained in recently developed high-strength martensitic or bainitic armor steels usually triggers a TRansformation-Induced Plasticity (TRIP), which enhances a strain hardening capability. However, this TRIP effect on ASB formation has hardly been investigated yet. In this study, a laboratory-scale split Hopkinson pressure bar was utilized to describe the ASB formation behavior in four high-strength martensitic or bainitic armor steels. Their formation possibilities were then correlated with hardness, strength, and dynamic compressive properties coupled with the TRIP. This TRIP occurring under adequate austenite stability improved the strain hardening, sufficiently absorbed the dynamically applied energy, and effectively suppressed the ASB formation. These behaviors suggest the favorable utilization of retained austenite in high-strength armor steels for wide-range ballistic applications.

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.