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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.

We present a new class of metastable high-entropy alloys (HEAs), triggering deformation-induced martensitic transformation (DIMT) from face-centered-cubic (FCC) to body-centered-cubic (BCC), i.e., BCC-DIMT. Through the ab-initio calculation based on 1st order axial interaction model and combined with the Gibbs free energy calculation, the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy (ISFE) in SixV(9-x)Cr10Mn5Fe46Co30 (x = 2, 4, and 7 at.%) alloy system. The ISFE decreases from -30.4 to -35.5 mJ/m2 as the Si content increases from 2 to 7 at.%, which well corresponds to the reduced phase stability of FCC against HCP. The BCC-DIMT occurs in all the alloys via intermediate HCP martensite, and the HCP martensite provides nucleation sites of BCC martensite. Therefore, the transformation rate enhances as the Si content increases in an earlier deformation range. However, the BCC-DIMT is also affected by the phase stability of FCC against BCC, and the stability is the highest at the Si content of 7 at.%. Thus, the 7Si alloy presents the moderate transformation rate in the later deformation range. Due to the well-controlled transformation rate and consequent strain-hardening rate, the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature. Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.

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.

An equiatomic VCoNi medium-entropy alloy possesses high sensitivity to grain-boundary strengthening, achieved by severe lattice distortions. Its ultrafine-grain structure enables 1.5 Gigapascal yield strength even for the fully recrystallized alloy with a single face-centered cubic structure. The high density of grain boundaries also generates high back stresses via piling up of massive dislocations, and the low cross-slip probabilities produce not only robust dislocation-mediated plasticity but also high back stress contribution to flow stress, which affords high strain-hardening capability to ultrafine-grain alloys, with 1.7 Gigapascal ultimate tensile strength with remarkable ductility. Our approach provides a new method for developing ultrastrong metallic materials.

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.

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.

Heterogeneous structured materials achieve a combination of high strength and extreme ductility due to synergetic strengthening driven by conditions in the interfacial region. Although the origin of synergetic strengthening has been revealed to be strain incompatibility in the interfacial region, the effect of the strength difference between hard and soft phases on strengthening has not been investigated well. In the work reported in the present paper, the effect of the difference in strength of the hard and soft phases on synergetic strengthening was investigated by conducting in situ neutron diffraction tensile tests. As a result, it was determined that the dislocation density in a layered sheet of high Mn (HMn) steel/interstitial free (IF) steel is higher than that in a layered sheet of HMn/low carbon steel. The big difference in mechanical properties between HMn steel and IF steel induces a high stress gradient and results in additional dislocations. Because of the high dislocation density and large differences in mechanical property and anisotropy in the HMn/IF layered steel sheet, upper-bound rule-of-mixtures behavior occurs. Therefore, a great difference in the mechanical properties of hard and soft components increases the synergetic strengthening of heterogeneously structured materials.

In order to increase the exhaust-gas temperature of austenitic stainless cast steels used for turbo-charger-housings up to 1050 °C, the more excellent high-temperature strength should be obtained because it works for keeping a sustainability of the housing shape. Three austenitic stainless cast steels were fabricated in this study by adding Nb into the Ni/Mn-replaced conventional ASTM HK40 steel, and their high-temperature strength was investigated in relation with the carbide formation and matrix strengthening. The 1-wt%-Nb-added steel showed the enhancement in high-temperature strength over the non-Nb-added steel. This indicated that the Nb addition was effective on the strength improvement because it raised both the fraction of thermally-stable hard MC-type carbides and the austenite-matrix hardness. However, the 1.5-wt%-Nb-added steel showed the lower high-temperature strength than the 1-wt%-Nb-added steel, although it contained the more MC carbides. The more MC carbides reduced the matrix hardness, enlarged the hardness difference between carbides and matrix, and deteriorated the high-temperature strength. Our results demonstrate that the strengthened matrix as well as populated carbides due to the Nb addition are desirable for further improving the high-temperature strength.

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.

Fracture toughness of intercritically-reheated coarse-grained HAZ (ICRCGHAZ), i.e., local brittle zone, is severely reduced at low temperatures by the presence of undesirable microstructural features such as coarsened prior austenite grains, bainitic microstructures including bainitic ferrite (BF), granular bainite (GB), and acicular ferrite (AF), and martensite-austenite constituent (MA). In this study, effects of the microstructural features on critical CTOD and crack initiation and propagation process were investigated in the ICRCGHAZ of three Mo-added high-strength low-alloy (HSLA) steels. The Mo addition caused the increased volume fractions of GB, BF, and MA and the resultant deterioration of critical CTOD. According to the observation of cracking process, most of hard MAs, particularly MAs located along prior austenite grain boundaries (PAGBs), provided initiation sites of {100} cleavage cracks. In addition, a large hardness difference between PAGB-MA and matrix led to an easy MA/matrix interfacial separation and an increase in crack initiation sites, thereby resulting in very low critical CTOD in the high-Mo-containing steel having the highest fraction of MA. The detailed microscopic observation of cracking process at MAs and macroscopic MA/matrix hardness difference provide plausible ideas for reliably evaluating critical CTOD and for designing HSLA steels in consideration of ICRCGHAZ microstructures.

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.

High-Ni austenitic cast steels were fabricated by replacing the expensive Ni content by inexpensive Mn in a commercial DIN 1.4849 steel (0.4C-2.0Mn-1.5Si-20Cr-38Ni-1.5Nb (wt pct)), and their high-temperature tensile properties were enhanced by controlling the volume fraction and distribution of carbides. The 14-pct-Ni-containing steel showed the highest total fraction and most homogeneous distribution of carbides due to the smallest cell size, which played a role in improving high-temperature properties far better than those of the DIN 1.4849 steel.

Austenitic high-Mn steels have been nominated as desirable ultra-high-strength cold-rolled steels whose mechanical properties are greatly improved by powerful deformation mechanisms of transformation- and twinning-induced plasticity (TRIP and TWIP). In this study, an austenitic high-Mn TRIP steel was suggested to achieve a good strength-ductility balance, and 1–2 wt.% Cu was added as an element for increasing stacking fault energy (SFE) as well as an austenite stabilizer to exploit a transition from TRIP to TWIP. The non-Cu-added steel showed the highest yield and tensile strengths (502 MPa and 1137 MPa, respectively) and the lowest elongation (34.6%) with a serrated flow. Yield and tensile strengths decreased with increasing Cu content, while the elongation was the highest in the 1%-Cu-added steel. TRIP and TWIP mechanisms showed good agreements with calculated SFEs in consideration of (Mn,Cu)-segregated bands. In the non-Cu-added steel, the TRIP occurred step by step as localized deformation bands passed through the specimen gage section to activate the serrated flow, which were reduced (or improved) by the transition from TRIP to TWIP with increasing Cu content. In the 1%-Cu-added steel, overall tensile properties were improved (yield strength; 461 MPa, tensile strength; 1093 MPa, elongation; 65.1%) as both TRIP and TWIP were well homogenized to produce synergic effects.

Although back-stress contributes to the mechanical properties of materials, the degree of strength enhancement from back-stress is not easy to estimate. In this research, back-stress hardening of twinning-induced plasticity (TWIP) + interstitial free (IF)-layered steel sheets were estimated by implementing a non-linear combined isotropic and kinematic hardening model. High back-stress evolution occurs due to plastic strain incompatibility between a TWIP-steel core and IF-steel sheath, and the strength of TWIP + IF layered steel sheath is greater than the strength estimated by the rule-of-mixtures. A non-linear combined isotropic and kinematic hardening model was used to estimate the strength enhancement from back-stress hardening, and the simulation results were correlated with the experimental results. This result shows that the back-stress evolution in heterogeneous materials contributes to their strength, and that the non-linear combined isotropic and kinematic hardening should be included to estimate the degree of back-stress hardening.

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.

The weldability is one of the key factors governing applications of TWinning-Induced Plasticity (TWIP) steels to automotive industries demanding high economy, environmental friendliness, and high performance. During spot welding of Zn-coated Al-containing TWIP steel sheets, liquid metal embrittlement (LME) frequently occurs by Zn infiltration into grain boundaries to form cracks, but the direct observation and detailed analysis of LME cracking are quite difficult because it occurs instantaneously within a second. Here in the present study, the LME was investigated by detailed microstructural evolutions of small Zn infiltrations or cracks as well as formation behavior of various intermetallic phases. In the heat-affected zone, the applied tensile stress and spot-welding heat tore down a diffusion-inhibiting Fe2Al5 layer formed between the Zn-coated layer and the TWIP steel substrate, and formed Zn-containing ferrite (α-Fe(Zn)) particles on the steel surface which provided paths for liquid Zn infiltration. α-Fe(Zn) particles played critical roles in accelerating the LME by reducing the ductility because they were brittle due to high contents of Zn and Al. In the present Al-containing TWIP steels, the increase in welding current generally aggravates the LME.

Austenitic high-Mn TWinning- and Transformation-Induced Plasticity (TWIP and TRIP) steels are strong candidates for GPa-grade cold-rolled steel sheets. The reduction in C or Mn content from high-Mn TWIP steels help generate a TRIP mechanism and prevent serration. However, these high-Mn TRIP steels show low yield strength because of the inherent characteristics of austenite, and often contain a band-shaped segregation of solutes, making the steels acts as hetero-structural materials. Therefore, in this study, we investigate the effects of compositionally-segregated microstructures on tensile properties and serration behavior in precipitation-hardened high-Mn TRIP steels. The present TRIP steels showed high yield strength (778–824 MPa) and an excellent strength-ductility balance, along with serration in their stress-strain curves which could not be explained by existing theories of dynamic strain aging. A considerable amount of martensite was formed step by step as localized deformation bands passed through the specimen gage section, which implied that the serration occurred only when the transformation rate increased substantially. In microstructural aspects, the martensitic transformation occurred sequentially along Mn-segregated bands due to differences in austenite stability and Mn content between high- and low-Mn bands, thereby leading to discontinuous transformation and consequently the serrated flow.

Effects of surface grain refinement and residual stress on the local and global properties of pure Cu processed using ultrasonic nanocrystalline surface modification (UNSM) was investigated. To distinguish each contribution to the local hardness and global tensile properties of the UNSM treated Cu, a stress-relief specimen of the same microstructure as the UNSM treated one was produced by low-temperature annealing after the UNSM treatment. The distinct contributions of residual stress and surface grain refinement to the tensile property of UNSM treated Cu were determined and discussed.

Flattened plates often show the lower or higher yield strength than initial leveled plates because tensile and compressive strains are repeatedly experienced at outer and inner walls during the pipe-forming and flattening, but reasons for the yield-strength variation after the pipe-forming are not sufficiently verified yet. In this study, ten line-pipe steels were fabricated by controlling alloying elements and finish rolling temperatures (FRTs), and the yield strength of pipe-flattened steel plates was predicted by using cyclic simulation tests, based on competing contributions of Bauschinger effect (BE) and strain hardening (SH) effect quantified from yield drop (YD) and yield rise (YR) parameters, respectively. High-FRT-treated steels (H steels) showed the lower BE and the higher SH than low-FRT-treated steels (L steels), thereby resulting in the smaller yield-strength reduction. This lower BE in the H steels was caused by the lower total boundary density, while the higher SH was caused by the higher fraction of granular bainite. According to the SH analyses between the YR parameters obtained from cyclic simulation tests and the yield ratios obtained from ordinary tensile tests, the decrease in yield-strength reduction with decreasing yield ratio was not attributed to the increase in ordinary tensile SH but to the increase in YR parameter.

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.

Austenitic high-Mn steels have been nominated as potential alloys for ultra-high strength and excellent ductility, but the high-Mn content inevitably generates band-shaped Mn-segregation due to dendritic solidification from the steel melt. This segregation band often causes unintended deformation mechanisms and anisotropic mechanical properties, and deteriorates formability. Here, we effectively utilize the Mn-segregated bands for the improvement of tensile properties by actively producing TWinning- and TRansformation-induced plasticity (TWIP and TRIP) mechanisms in high- and low-Mn-segregated bands. We also adopt a mixed microstructure of non-recrystallized and recrystallized austenite mainly formed in high- and low-Mn-segregated bands, respectively. The TWIP+TRIP mechanisms generating highly-sustained strain hardening and high strain hardening, coupled with partial recrystallization and precipitation hardening, are working successfully for overcoming low-yield-strength characteristics of austenite to reach 1 GPa and for achieving the excellent tensile strength of 1.5 GPa and ductility of 44%. Our results demonstrate how the Mn-segregation-induced TWIP+TRIP mechanisms can be a novel idea in ultra-high-strength steel design.