In the press forming of martensitic steel sheets into automotive parts, larger plastic deformation is introduced in the martensite matrix compared to that of ultra-high-strength steel bolts or tool materials. Basic study on the press formability of martensitic steel sheets revealed that tempering below 240-degree C was effective to minimize coarse precipitation of cementite at which cracks often initiated during heavy deformation. At the same time, advanced press forming technique of automotive parts without defects such as cracking and insufficient shape precision has been necessary to be developed together with the material development to apply ultra-high-strength martensitic steel sheets with less formability compared to lower tensile strength grade high-strength steel sheets. In this paper, effect of tempering treatment on the mechanical properties and hydrogen embrittlement from the viewpoint of applicability of martensitic steel sheets for automotive parts will be reviewed. And a unique study of copper (Cu) precipitation in the martensite matrix during tempering aiming for improved ductility of martensitic steel sheets will also be shown. After these reviews with some typical examples of related press forming technologies, key R & D issues for expanded application of martensitic steel sheets for automotive parts will be discussed.

Current understanding of crystallography and interfaces of martensite and bainite in steels are summarized for discussion of uncertain problems. HRTEM observation has given good insights on transformation pathways of lath martensite, and also indicates the same atomic motion in lattice change as an analogous displacive transformation. Recent analyses of lengthening kinetics suggest that carbon diffusion accompanies the growth of bainitic ferrite. A major energetic barrier in bainite transformation comes from accumulation of strain energy. High resulution in-situ observation and advanced atomistic simulation is more important in deeper understanding of the natures of martensite and bainite.

Atomic mechanisms of phase change are not just a curiosity, they help create innovative steels. The goal here is to consider what remains to be usefully explored in the context of bainite, and to set a formidable challenge.

We have formulated a target of reducing total CO₂ emissions by 30% by 2030, compared to the 2013 baseline and of achieving carbon neutrality in 2050. We are working to develop and actually implement breakthrough technologies in steelmaking process and eco-friendly products. Our eco-friendly steel products have advanced functions and reliability and are used in diverse areas including energy, transportation and construction equipment, and household products. They typically help our customers become more efficient while making their products lighter or lengthening product life. That translates into the saving of resources and energy, and into a reduction in CO₂ emissions at the point of use at our customers, thereby contributing to lessening the environmental impact. These steels that support carbon neutralization must have various environmentally friendly properties, such as high strength, high toughness, and resistance to hydrogen embrittlement. Here, we provide an overview of the metallurgy required for the development of these advanced steel materials, with a particular focus on their suitability for a hydrogen society.

The fatigue performance of case hardened carburized steels is related to the resistance of the underlying microstructure to fatigue crack nucleation and growth processes. As carburized steels are often employed in transmission gears, the most relevant loading modes are bending and rolling-sliding contact fatigue (RSCF) where the failure mechanisms and influence of microstructure are unique. This paper highlights the role of the martensitic and retained austenite microstructure and alloying on fatigue crack nucleation and growth during bending fatigue and pitting resistance during RSCF. Fatigue performance of carburized steel is strongly linked to the prior austenite grain size; mechanisms of grain refinement through microalloy precipitates are presented, with particular focus on niobium and molybdenum effects on precipitate coarsening. Carburized microstructures contain significant amounts of metastable retained austenite, the transformation of which during fatigue loading can influence fatigue crack propagation. The role of retained austenite and nickel on pitting fatigue resistance is presented using results from carburized alloys with variations in nickel additions. These alloys were also subjected to cryotreatments to reduce the austenite content and isolate nickel alloying effects. The role of martensitic microstructure and retained austenite, heat treated to have different levels of austenite stability, on fatigue crack growth resistance is also presented in a high carbon 52100 steel. A step quenching heat treatment, which refines the martensitic microstructure and enhances retained austenite stability, exhibits improved fatigue crack growth resistance relative to a condition direct quenched from the austenitizing temperature.

Rigid body rotation is required for paired martensite variants to maintain continuity of deformation at their junction plane. The magnitude of this rigid body rotation, θ, serves as an indicator of the energy barrier associated with variant coupling, and to some extent explains the variant selection rule in the variant pairing. In this paper, an analytical solution for θ is derived. We assume that the deformation gradient of the martensitic plate is given by U=I+gd⊗p as the phenomenological theory of martensite, and the two variants are related by 180° rotations about some axis ê. The variant pairs related by a 180° rotation account for nearly all the major variant pairs observed in steel. Then, θ is analytically determined using d, p, and ê. Notably, θ is zero when d and ê are perpendicular, or when p and ê are parallel.

Changes in dislocation density and internal stress during martensitic transformation (MT) in low-carbon steel are investigated using phase-field simulations, where the elastic energy of the microstructure is formulated by considering both the fcc→bct transformation strain and dislocation evolution in martensite (α') and austenite (γ) phases. The simulation results demonstrate that MT progresses through the formation and growth of clusters composed of three Bain variants of the α' phase. The average dislocation density in the α' phase increases at the onset of MT and remains high throughout the transformation. By contrast, the average dislocation density in the γ phase gradually increases during MT. Furthermore, the average hydrostatic stress in the α' phase fluctuates slightly around zero, while the average hydrostatic stress in the γ phase is negative (compressive stress), with its magnitude increasing as MT progresses.

The mechanical properties of martensite have been extensively studied due to its technological significance. It is well-established that quenched martensite exhibits high mechanical strength but low total elongation and is often deemed as a "brittle" microstructure. However, its mechanical behaviour, including low microplasticity yield and strong initial hardening, resembles paradoxically that of multiphase steels. Consequently, recent research has led to a new perspective, proposing that lath martensite should no longer be viewed as a "uniform and homogeneous" phase but rather as a multiphase aggregate. This perspective arises from the recognition of the sequential nature of the martensitic phase transformation, progressing from the Ms temperature to ambient temperature. This progressive transformation results in the initial martensite laths transforming just below Ms in coarse, undeformed austenite, while subsequent laths transform in a highly constrained environment in terms of size, relaxation possibility, or defect densities. This progressive transformation serves as the primary explanation for the observed microstructural heterogeneities in martensitic steels.

In this paper, we will demonstrate how the dispersion of these microstructural characteristics (microstructure sizes, density of dislocations) and of internal stresses resulting from the phase transformation explain the peculiar behaviour of martensitic steels through a micromechanical approach. We will detail how these characteristics were measured, determined, or estimated. Special attention will be paid to the description of spatial distributions and their stochastic couplings, which we term "soft-soft" correlation. As a discussion, the adaptation of the framework to address martensite tempering, Dual-Phase steels as well as mechanically induced martensite will be presented.

This paper summarizes the hydrogen-related crack propagation behaviour and the deformation microstructure evolution accompanying crack propagation in high-strength martensitic steel investigated by FIB-SEM serial sectioning and TEM. We found that even very small low-angle PAGB segments (sub-micrometre in size) impeded hydrogen-related intergranular crack propagation. Additionally, localized plastic deformation sufficient to produce an ultrafine-grained structure was sometimes observed at the tip of the arrested intergranular cracks. The crystal orientation changed abruptly within 1 μm of the hydrogen-related quasi-cleavage crack tip. At the hydrogen-related quasi-cleavage crack tip, a high dislocation density and deformation microstructures, including the formation of low-energy dislocation structures, were noted. However, the microstructure approximately 1.58 μm away from the crack tip was similar to that in the non-deformed state. Therefore, intense localized plastic deformation played a key role in the hydrogen-related quasi-cleavage crack propagation. The intense localized plastic deformations accompanying the arrest of intergranular cracks and the propagation of quasi-cleavage cracks contributed to intrinsic crack growth resistance in the hydrogen-related fractures.

To understand the influence of composite structures on the brittle-to-ductile transition (BDT) in low-carbon martensite-bainite steel, this study examines the temperature dependence of the impact absorbed energy in five types of steel with identical chemical compositions: fully martensitic steel, fully bainitic steel, and martensite-bainite steels containing 4%, 15%, and 55% bainite fractions, respectively. The BDT temperature was found to be highest for fully martensitic steel, followed by martensite–4% bainite, martensite–15% bainite, martensite–55% bainite, and fully bainitic steel. Despite the significant differences in bainite volume fractions, the BDT temperatures of martensite–15% bainite, martensite–55% bainite, and fully bainitic steel were similar, indicating that the BDT temperature trend cannot be solely explained by the composite rule. To elucidate the trend in BDT temperature based on the shielding theory, the temperature dependence of the 0.2% proof stress was measured for each type of steel. Optical micrographs and the temperature dependence of effective stress showed that dislocations in the bainite phase were preferentially activated, playing a dominant role in determining the yielding behaviour and BDT in the martensite-bainite steels. This behaviour was also associated with the network structure of bainite surrounding the martensite, where the bainite phase underwent plastic deformation immediately after yielding in the steels under investigation.

As-quenched martensite in carbon steels needs to be tempered to restore its ductility and toughness for practical applications. During tempering, a series of reactions causing changes in microstructure and properties are known to occur. In this study, in-situ neutron diffraction was combined with traditional calorimetric and dilatometric analyses to investigate the tempering behaviors and the effects of common alloying elements, i.e., Mn, Cr, Si, Al, on the kinetics in high-carbon martensite during continuous heating. The experimental results revealed that carbon clustering/segregation (0th stage) and metastable carbide precipitation (1st stage) were retarded by Al and Cr additions. Moreover, austenite decomposition (2nd stage) was retarded by Mn and Cr additions in the early step, whereas Al and Si additions also contribute to delaying the kinetics in the late step.

The structures of carbon clusters and carbides in a low-carbon ferritic steel were investigated at atomic-scale by annular dark-field scanning transmission electron microscopy. In the low-carbon ferritic steel aged at 473 K for 1 h, some homogeneously dispersed ε-carbides were formed within the matrix as closely spaced granules aligned to <001> of the ferrite matrix, and others were heterogeneously formed on AlN precipitates. The ε-carbides formed on AlN precipitates were coexisted with carbon clusters with c/a ratio of 1.1, possibly formed via Zener-ordering. In-situ heating experiments showed that ε-carbides acted as precursors for θ-carbides via dissolution of ε-carbides. The microstructural evidence showed that the precipitation sequence of the low-carbon ferritic steels aged at 473 K is proposed as: supersaturated solid-solution → carbon clusters → ε-carbides → θ-carbides, suggesting that Zener-ordering should be the most fundamental step for the carbide precipitation sequence.

Martensite is an important phase in steel, but evaluating its free energy is challenging both experimentally and theoretically. In this study, we explored a method to assess the free energy of martensite using cluster expansion and variational methods (CE-CVM), with the additional application of intermittent lattice deformation. Our analysis of the formation energy of dilute solid solution Fe₆₄C showed that in regions where the c/a ratio exceeds 1.00, carbon dissolution in the c-site is energetically favorable. The dependence of the energy difference on the c/a ratio showed that as c/a increases, the occupancy probability of the c-site approaches 1, while that of the ab-site approaches 0. In previous studies using cluster expansion, energy calculations were performed without considering local strain relaxation due to issues with carbon position deviations affecting precision. This study addresses this by incorporating local strain relaxation into the calculations. Specifically, the initial structure was adjusted to increase the distance between the nearest Fe and C atoms by approximately 0.5 Å, followed by relaxation calculations using first-principles methods. Free energy calculations at T = 400 K indicated that higher axial ratios become more stable with increasing carbon concentration. Furthermore, while previous research overestimated changes in the axial ratio, this study's consideration of local atomic position relaxation improved alignment with experimental data.

Carbon ordering in supersaturated iron is known to induce the tetragonality of the martensite lattice. According to Zener and Khachaturyan, the ordering is determined by strain-induced carbon-carbon interactions. Carbon ordering is probably responsible for the nanostructuring of martensitic microstructures and the resulting high mechanical strength of Fe-C martensite. Understanding the role of carbon content, temperature and applied stress on carbon ordering necessitates the investigation of various phenomena related to carbon migration in the lattice, down to the atomic scale. In this article, we present a unified theoretical treatment of the thermodynamics and kinetics of carbon ordering applicable to ferrite and martensite phases. Our results are based on a mean-field thermo-kinetic model integrating long-range elastic carbon-carbon interactions. All parameters of the model originate from ab initio calculations. Various phenomena such as the effect of stress on the ordering, the thermo-elastic behavior, the anisotropy of carbon diffusion, the internal friction response and the carbon clustering in supersaturated carbon alloys are discussed.

As an example of applying chemical heterogeneity for new microstructure design concept, this study investigates the role of chemical heterogeneity in the microstructure evolution of low carbon steel during quenching and partitioning (Q&P) treatment and its subsequent impact on mechanical properties. The research specifically examines the characteristics of chemical heterogeneity before the Q&P process, focusing on the extent of Mn enrichment and the scale of Mn-enriched domains. The results demonstrate that Mn chemical heterogeneity is pivotal in enhancing both strength and ductility post-Q&P treatment. However, the distinct characteristics of this heterogeneity influence strength and ductility through different underlying mechanisms, highlighting the need for precise control of these heterogeneities to maximize their potential in advanced microstructure design strategies.

Deformation-induced martensitic transformation (DIMT) during various plastic deformations of the bainitic steels with three different carbon contents was studied. The initial microstructures were prepared by the austempering at the same holding temperature (400°C), and they were identified as bainite consisting of bainitic ferrite and the retained austenite whose volume fraction was larger with the larger carbon content. In all of three steels, the compressive deformation showed lower work hardening than the tension. The measurement of both electron back scattering diffraction (EBSD) and in situ neutron diffraction confirmed the less reduction of retained austenite at the compression, indicating the primary reason for the change in work hardening behaviors. In addition, the change in the reduction of retained austenite was observed along the thickness of the bent steels with the different carbon contents because of the change in hoop stress polarity along the thickness. The mechanism of the DIMT dependence on the carbon content and the stress polarity is discussed with deformation texture and the crystallographic orientation dependence on DIMT.

The phase transformation from fcc-to-bcc in quasi-two-dimensional (2D) and three-dimensional (3D) polycrystals of carbon steel was investigated using molecular dynamics (MD) simulations. In particular, taking advantage of recent advancements in interatomic potentials for iron-based alloy systems, the detailed dependence of carbon concentration on the transformation in polycrystalline materials was examined. The atomic structure obtained from MD simulations was analyzed using common neighbor analysis (CNA) to observe the transition in the distribution of fcc and bcc phases during the phase transformation from an atomistic viewpoint. CNA results confirmed that the transformation rate decreased with increasing carbon concentration in both the quasi-2D and 3D models. Moreover, analyses of mean square displacement (MSD) revealed that carbon atoms are constrained by surrounding iron atoms, preventing carbon diffusion during the phase transformation of carbon steels. It is significant that this study has directly revealed the atomic dynamics of the fcc-to-bcc phase transformation in carbon steel.

In ferritic steels, the grain boundary segregation of carbon at ferrite grain boundaries increases the Hall–Petch coefficient and enhances grain refinement strengthening. It is also possible to further enhance the grain refinement strengthening by promoting carbon segregation through low-temperature aging or other methods. On the other hand, carbon content has a significant effect on the contribution of grain refinement strengthening in martensitic steels. By considering martensitic steels in the same manner as ferritic steels, it was speculated that a "hard block boundary" could be designed if carbon could be used more effectively. In this report, we discuss the effect of carbon on grain refinement strengthening and its ability to control the mechanical properties of martensitic steels based on a review of previous studies and newly conducted experiments.

This study evaluates the phase transformation of Fe-0.1 mass% C-2.0 mass% Mn martensitic steel under ultrafast heating and cooling conditions using femtosecond X-ray diffraction to measure the dislocation densities. An X-ray free-electron laser was used with ultrafast heating and cooling to identify the reverse transformation mechanism from martensite (α′) to austenite (γ) and the transformation mechanism from γ to α′. A maximum heating rate of 1.2 × 10⁴ °C s^–1, which is sufficient to avoid diffusive reversion, was achieved, and a reverse transformation during ultrafast heating was observed. The results indicated the formation of a fine microstructure owing to a phase transformation with a high dislocation density and carbon concentration caused by ultrafast heating. Furthermore, a maximum cooling rate of 4.0 × 10³ °C s^–1 was achieved using a gas spraying technique, which was applied immediately after ultrafast heating of the sample to 1200 °C. This cooling rate was sufficient to prevent bainitic transformation, and a transformation during ultrafast cooling was observed. The cooling rate affected the dislocation density of the γ-phase at high temperatures, resulting in the formation of a retained γ-phase owing to ultrafast cooling. An intermediate phase might be formed during the phase transformation from fcc-γ to bcc-α′ during ultrafast cooling, and lattice softening might occur in the carbon steel immediately above the martensitic-transformation starting temperature. These findings will be beneficial for the investigation, development, and industrial applications of functional steels.

The martensitic transformation from the α (BCC) to γ (FCC) in Fe-Mn-Al and Fe-Mn-Al-Ni is thermodynamically analyzed based on the experiments of the specific heat measurements and by the CALPHAD method. In particular, we discuss the phase transformation in terms of the magnetic contribution to the phase stability. In addition, the role of the coherent nano-precipitates that bring about thermoelastic martensitic transformation is also discussed.

Medium-carbon steel martensite is a microstructure present in high-strength steel. Compared with low-carbon steel martensite, medium-carbon steel martensite possesses a more complex and intricate structure, which is assumed to contribute towards ensuring material toughness. Although the microstructure affects mechanical properties, studies on the microstructure formation of medium-carbon steel martensite remain limited. This study investigated the microstructural development during martensitic transformation in medium-carbon steel using a multi-step quenching method. In this study, Fe–0.49C–2.04Mn (mass%) was used and the specimens were observed using scanning electron microscopy and analysed using electron backscatter diffraction. Plate-like martensite regions were observed at the prior austenite grain boundaries in the specimens quenched at 300 °C, suggesting that the plate-like martensite regions form from the austenite grain boundaries during the early stages of martensitic transformation. In the specimens quenched below 300 °C after austenitisation, lath martensite was observed to grow within the grain. However, coarse plate-like martensite was little grown. The observance of lath martensite as the primary microstructure is suggested to be because of its higher nucleation/growth ability than plate-like martensite.

A method to enhance coherency of prior austenite grain boundary (PAGB) was suggested by exploring the relationship between prior austenite misorientation (θ_γ) and martensite misorientation (θ_α′) across PAGB. It was found that a double KS OR (where the variant holds near Kurdjumov-Sachs orientation relationship not only with austenite into which it grows but also with opposite austenite sharing the grain boundary) is a necessary condition for improving PAGB coherency. To achieve a double KS OR, the PAGB should be either an L-PAGB (θ_γ < 10^o) or an H-PAGB (θ_γ > 45^o). L-PAGB always holds double KS OR, whereas H-PAGB requires an initial nucleation event to achieve a double KS OR. The reason why initially nucleated H-PAGB is more likely to hold a double KS OR is related to variant selection to minimize interfacial energy. To make full use of variant selection to enhance PAGB coherency, partial bainite was introduced before martensitic transformation through austempering. As a result, PAGB coherency can be significantly improved while achieving an acceptable hardness loss.

The strain distribution within a final fracture region, i.e., necking deformation region, in the tensile test was visualized by using a combination of replica and digital image correlation methods, and then the relationship between inhomogeneous strain distribution and fracture behavior in martensitic steel was discussed. Strain was inhomogeneously distributed at an initial stage of tensile deformation (before necking deformation). The positions of high- and low-strain regions were maintained from the initial stage of deformation to just before fracture, indicating that more strain concentration occurs at high-strain regions with progressing tensile deformation. In several regions, the values of cumulative ε_Mises were considerably high exceeding 0.8 at just before fracture. It suggests that a portion of the martensitic structure has a large ductility. Cracks nucleated during necking deformation, and the crack positions tended to be high-strain regions. Strain concentration owing to the crack is one of the reasons for this. However, the histogram of the ratio of the local ε_Mises in crack nucleation regions to average ε_Mises calculated from the ε_Mises strain distribution map at a uniform strain revealed that the crack positions are located at high-strain regions even before crack nucleation. Thus, it can be concluded that crack nucleation is easy in high-strain regions and high strain is one of the factors for crack nucleation.

To control the strength-ductility balance of type 304 metastable austenitic stainless steel, it is effective to perform annealing after partially deformation-induced transformation to change the properties of deformation-induced martensite and retained austenite. However, the changes in each microstructure and partitioning of alloying elements after annealing are not necessarily clear. In this study, the dual-phase microstructures of deformation-induced martensite and retained austenite in partially transformed type 304 stainless steel were annealed under different conditions, and the mechanical properties of each phase were investigated. The partially transformed specimen, which was subjected to 30% cold rolling after solution treatment at 1373 K-1.8 ks, has a dual-phase microstructure with approximately 70% deformation-induced martensite and 30% retained austenite. When annealing at 673 and 773 K, the austenite fraction did not change significantly; however, when annealing at 873 K, a reverse transformation occurred, and the austenite fraction increased to approximately 80%. Tensile tests revealed that the cold-rolled specimen exhibited a high yield stress of 1200 MPa and excellent ductility, with a total elongation of 15% owing to the TRIP effect. The yield stress increased at 673 K annealing but decreased at higher annealing temperatures. However, as a result of the nanoindentation tests, the hardness of both martensite and austenite hardly increased, even in the 673 K-annealed specimen, although its yield strength was increased. Therefore, it is presumed that the strength of the interface increased, and the interface became a strong obstacle to dislocation motion.

In situ neutron diffraction experiments during tensile deformation were conducted by using an austenitic stainless steel to investigate the deformation-induced martensitic transformation (DIMT) behavior at cryogenic temperatures. The strength increased, and elongation decreased with a decrease in the deformation temperature. The reduction of area decreased significantly when the temperature decreased from 77 to 20 K. The effect of temperature on the DIMT behavior became smaller with decreasing temperature, and no significant change in the DIMT kinetics was observed at 20 and 77 K.

The effect of loading rate on fracture toughness in hydrogen gas, K_IH, of low alloy martensitic steels, JIS-SNCM439 and JIS-SNB16, was investigated using rising load tests in 85 MPa hydrogen gas at room temperature. The tests were performed at various loading rates ranging from 0.0005 to 3 MPa√m/s, and two methods based on KHKS 0220 and ASTM E399 were used to calculate K_IH. In the evaluation based on KHKS 0220, no difference was recognized in the effect of the loading rate on K_IH for both steels. However, in the evaluation based on ASTM E399, K_IH for JIS-SNCM439 increased with increasing loading rates higher than 0.5 MPa√m/s. In contrast, K_IH for JIS-SNB16 increased at loading rates higher than 0.05 MPa√m/s. This difference is probably attributed to hydrogen diffusion at the initial stage of crack propagation. Accordingly, in rising load tests, lower loading rates are required for JIS-SNB16 than those for JIS-SNCM439, and K_IH for JIS-SNB16 was higher than that for JIS-SNCM439 at similar strength levels. However, no significant difference was observed in fracture surfaces of both steels after the rising load tests, and the relationship between K_IH and the characteristics of their fracture surfaces was unclear.

In this study, we attempted to elucidate the hydrogen embrittlement mechanism in lath martensite steels by multiscale approach across fracture test, microbeam analysis and atomic simulation. It was confirmed that the microcracks initiated not only along prior austenite grain boundaries but also partially along the boundaries existing within the prior austenite grain. To compare hydrogen embrittlement risk on various boundaries, the modelling method was developed to reflect the crystallography on hierarchical lath martensite microstructure. Hydrogen trapping on the boundaries was analysed using molecular dynamics calculations using neural network interatomic potential. Consequently, it was clarified that the hydrogen trapping energy of prior austenite grain boundaries is higher than that of the block and sub-block boundaries. Thus, the initiation risk due to hydrogen was lower in the block and sub-block boundaries which are the boundaries within prior austenite grain. It was suggested that elementary processes of hydrogen-induced isolated microcracks with length of approximately 30~50 μm are the crack initiation on intergranular boundaries with high risk and crack growth along intragranular boundaries with middle risk due to weaker hydrogen trapping.

It is crucial for casing steel pipes for CCS injection wells to have excellent low-temperature toughness. In this study, the effects of impurity elements P and S on the low-temperature toughness of API 5CT L80, which has a strength of 552 MPa class, were evaluated. Fracture surfaces after Charpy impact testing were subjected to fractography using machine learning to quantify the classification and fracture ratio of each fracture surface. The results revealed that the intergranular fracture surface fraction decreased with decreasing amounts of P and S.

V microalloying has been reported to reduce hydrogen embrittlement (HE) susceptibility by trapping hydrogen at V carbides to remove mobile hydrogen that would otherwise accumulate and lead to embrittlement. However, it is unclear whether alloy carbides can offer any benefit to HE resistance once traps saturate under high hydrogen concentrations. In this study, the HE susceptibilities of quenched and tempered 0.4C-1.0Mn-1.0Cr alloys of 1100 MPa tensile strength with and without a 0.15 wt pct V addition were compared for notched specimens hydrogen charged in situ or pre-charged. Cathodic charging was conducted in a 0.5 M H₂SO₄ + 100 ppm As₂O₃ solution with a 0.125 mA∙cm^-2 applied current density. The bulk hydrogen concentration was estimated using melt extraction measurements. HE susceptibility was reduced with the addition of V for 0.5-24 h pre-charging times, demonstrating that, despite increasing hydrogen absorption, V carbide traps effectively mitigated hydrogen accumulation at the notch. In situ cathodic charging also showed that V carbides deterred embrittlement despite higher hydrogen concentration measured in the notched region of the V-added alloy following the test. These results show that, although V carbide precipitation promotes hydrogen absorption, alloy carbides effectively deter hydrogen accumulation and HE, even at the high hydrogen concentrations employed in this study. However, hydrogen can detrap from V carbides and thus reduce mechanical performance when charged specimens are held in ambient conditions for 96 h before testing.

The fatigue limit of steel generally increases with tensile strength; however, when tensile strength exceeds ~1.4 GPa, further increases in tensile strength fail to elevate the fatigue limit or may even decrease it (fatigue limit ceiling). This ceiling poses a challenge for the widespread application of ultrahigh-strength steels. In this study, we successfully achieved a significant improvement in the fatigue limit through pre-fatigue deformation, thereby breaking through the fatigue limit ceiling in 1.6 GPa-grade as-quenched martensitic steel (Fe-3Mn-0.2C (wt.%)). The maximum stress corresponding to the fatigue limit increased from 675 MPa to 1300 MPa (stress ratio: 0.1, 10⁷ cycles). In contrast, a specimen subjected to pre-constant-loading at the same maximum stress as the pre-fatigue training showed only minimal improvement in the fatigue limit. These findings reveal that fatigue deformation, traditionally viewed as detrimental, can be beneficial for fatigue fracture resistance when properly applied. Moreover, the absence of surface cracks in all non-fractured specimens indicates that the fatigue limit of as-quenched martensitic steels aligns with the crack initiation limit, rather than the crack non-propagation limit. Therefore, the significant improvement in the fatigue limit is attributed to the improvement in the crack initiation limit achieved through the pre-fatigue deformation.

Ultrafine-grained (UFG) metastable austenitic steels, produced through plastic deformation and annealing, retain good ductility due to deformation-induced martensitic transformation (DIMT), unlike most UFG metals. However, the effects of grain size on DIMT and deformation behavior are not fully understood. This study addresses this gap by conducting in situ tensile tests with neutron diffraction on coarse-grained (CG: 35 μm) and UFG (0.5 μm) Fe-24Ni-0.3C steels to explore how grain refinement affects deformation and DIMT. Results showed that grain size significantly affects dislocation density and lattice strain partitioning among austenite grains during deformation. The UFG specimen showed a rapid increase in dislocation density within austenite grains and complete grain DIMT, but in a smaller fraction of grains compared to the CG specimen. The dislocation arrangement, represented by the M* parameter, evolved differently between the two specimens, suggesting distinct nucleation mechanisms. In the CG specimen, DIMT occurred in many austenite grains, resulting in significant stress relaxation. In contrast, the UFG specimen experienced DIMT in fewer austenite grains, resulting in reduced stress relaxation. These findings underscore the impact of grain refinement on martensite formation and the mechanical properties of metastable austenitic steels.

The deformation behavior of lath martensite has been extensively explored, with this study focusing on its characteristics through the lens of dislocation theory. We conducted an in-situ neutron diffraction study to investigate the tensile deformation behavior of ultra-low carbon Fe–18Ni alloy under different heat treatment conditions. These alloys were initially quenched and then tempered at 573 K and 773 K. This study aimed to quantitatively evaluate dislocation characteristics and understand their role in the resulting stress–strain curves. To achieve this, we applied the convolutional multiple whole profile (CMWP) method, which determines the microstructural parameters from diffraction line profiles, linking macroscopic deformation behavior with quantitative information on dislocation characteristics without relying solely on local information. The CMWP method evaluates the dislocation characteristics, such as density, arrangement, and character. Our findings revealed that a high dislocation density in the quenched state, which gradually decreased with increasing tempering temperature, affected the mechanical properties such as the tensile strength. Additionally, we closely monitored also changes in the arrangement and character of dislocations as tempering temperature increased and during deformation. The parameters played a crucial role in revealing the evolution of flow stresses.

Some investigations on bent sheet metals have reported that the development of shear bands plays an indispensable role in cracking. On the cross-section perpendicular to the bend axis, cracks are observed along the shear bands, which previously developed in the directions of approximately 45° to the sheet surface. In this study, we investigated the microscale surface changes and internal shear band development of coarse-grained martensitic steel sheets using three-point bending tests with a scanning electron microscope (SEM). Shear band formation was confirmed at the small bending strain stage, causing microscopic grooves with localized strain concentration on the surface. The surface grooves subsequently develop with progressive bending deformation, becoming cracks and leading to fracture. Furthermore, cross-sectional microstructure observation indicated that the initiation and development process of shear bands in martensitic steel sheets is largely influenced by finer structures such as packets and blocks rather than by prior austenite grains.

The present study investigated the effect of cyclic transformation strengthening on the tensile properties and deformation-induced martensitic transformation (DIMT) behavior in the fine-grained Fe-24Ni-0.3C (wt.%) metastable austenitic steel using in-situ synchrotron XRD measurements. The cyclic transformation treatment, which involves forward and reverse phase transformations between martensite and austenite, induced a significantly improved mechanical properties to the material. In-situ XRD measurements revealed that the DIMT during tensile deformation was significantly enhanced by the cyclic transformation treatment, contributing the observed improvement in the mechanical properties.

High-strength, low-to-medium-carbon, low-alloy martensitic steel is increasingly used in automobile structural components. Understanding the controlling factors and kinetics of low-temperature aging is crucial for ensuring quality control in sheet steel and automobile part manufacturing. This study investigates the room-temperature aging behavior of as-quenched autotempered Fe-C lath martensitic steels (C: 0.07–0.77 mass%) via hardness change kinetics and interrupted atom probe (AP) analyses to identify the dominant hardening mechanism. Age-hardening at 23 °C was confirmed in the autotempered lath martensitic steels, including low-carbon steel with a carbon content below 0.25 mass%. Although the maximum hardness increment in lath martensite increased with the initial excess solute carbon (C_sol) within the matrix, the increment per unit C_sol was below that in carbon-supersaturated ferrite. AP and kinetic analyses of the hardness evolution indicated that carbon cluster growth at dislocations dominated the hardening of the martensite.

Influence of austenitizing temperature on acicular ferrite (AF) formation on Ti-Rare Earth Metal-Zr (TRZ) oxide is clarified. The AF formation fraction, PAF, which is a number fraction of oxide particles acting as nucleation sites for AF crystals, and orientation relationships (ORs) formed between γFe, TRZ oxide and AF crystal were investigated. With an increase in austenitizing temperature, P_AF becomes higher. TRZ oxide liquefies at high temperature, and then crystalizes having low index ORs (γFe-TRZ OR) with the surrounding γFe during cooling in austenitizing process. Subsequently, AF crystal nucleates satisfying both near Kurdjumov-Sachs (K-S) OR with γFe and low index ORs (TRZ-AF OR) with TRZ oxide. Therefore, it is considered that increase in austenitizing temperature improves the coherency between γFe, TRZ and AF, resulting in the promotion of AF nucleation at TRZ oxide.

The effect of P on the kinetics of bainite transformation was investigated in Fe–0.08% C–3.0% Mn and Fe–0.08% C–3.0% Mn–0.05% P (mass%) alloys. Alloys transformed isothermally at 803 K for 3–1000 s were analysed using dilatation measurements, scanning electron microscopy, and field-emission electron probe microanalysis. The initiation of transformation in the 0.05% P alloy occurred later than in the P-free alloy, suppressing bainite transformation from the austenite grain boundaries. In addition, the transformation ratio during stasis in the 0.05% P alloy was lower than in the P-free alloy, and the C concentration in the untransformed austenite decreased. The driving force could not explain the suppression of bainite transformation because the presence of 0.05% P had a negligible effect on the thermodynamic stability of the bcc and fcc phases. These results indicate that the segregation of P at the grain boundaries affects the nucleation and growth of bainitic ferrite.

3D printed die-casting molds with conformal cooling channels are spreading out to perform rapid uniform cooling. There is high volume demand of large 3D-printed molds, but conventional alloy powder grades for 3D printing, H13 steel and 18%Ni maraging steel, has issues regarding printability and mold life. Especially, H13 steel powder is subject to frequent cracking due to residual stress during 3D-printing. The estimated cause of cracking is the accumulated strain generated at hardening with each printing layer because of its high carbon content and the martensitic transformation start temperature being higher than the preheating temperature. Otherwise, 18%Ni maraging steel powder is used for large molds because of its printability. However, its poor thermal conductivity compared to H13 steel has a high risk of cracking from cooling channels because it needs to be installed near the cavity surface. In addition, the contained cobalt can be harmful to human bodies.

The novel Co-free and high thermal conductivity powder is designed for large-sized molds modified from the chemical composition of H13 steel. The developed powder is designed to have a lower carbon content than H13 steel, resulting in reduced hardness after martensitic transformation. Furthermore, the transformation start temperature was lowered to near the preheating temperature of conventional 3D-printers by Ni adding, which suppress cracking during printing. The mechanical properties and thermal conductivity are comparable with H13 steel.

Hydrogen embrittlement has long been an obstacle to the development of safe infrastructure. However, in contrast to hydrogen's embrittling effect, recent research has revealed that the addition of hydrogen improves both the strength and uniform elongation of AISI Type 310S austenitic stainless steel. A detailed understanding of how hydrogen affects the deformation mechanism of this steel could pave the way for the development of more advanced materials with superior properties. In the present study, in situ neutron diffraction experiments were conducted on Type 310S steel with and without hydrogen-charged to investigate the effect of hydrogen on the deformation mechanism. In addition to the effect of solid-solution strengthening by hydrogen, the q-value, a parameter representing the proportion of edge and screw dislocations in the accumulated dislocations, was quantitatively evaluated using CMWP analysis on neutron diffraction patterns. The comparison of q-values between the hydrogen-charged and non-charged samples reveals that hydrogen has minimal effect on dislocation character in Type 310S steel.

Low-density ultrahigh-strength steels (LD-UHSS) have garnered significant interest for lightweight design of automotive and aerospace components. However, their application has been limited by poor formability and weldability resulting from the inclusion of aluminum (Al). In this study, we present a successful approach for designing and fabricating a series of LD-UHSS using laser powder bed fusion with in-situ alloying. The addition of Al not only reduces the density of the newly designed LD-UHSS but also significantly strengthens the steels through the precipitation of the B2 phase. Compared to conventional low-density steels, the strength of the δ-ferrite phase in LD-UHSS is notably higher due to the smaller grain size and the presence of massive precipitates, enhancing the work hardening capacity. Additionally, the B2 phase in the developed LD-UHSS exhibits a finer structure, further reinforcing the matrix. However, it should be noted that excessive Al addition can result in steel brittleness due to the extensive precipitation of the B2 phase and increased fraction of δ-ferrite. Additive manufacturing offers a viable pathway for producing LD-UHSS, and the mechanical performance can be effectively optimized by tailoring key phases such as the B2 phase, metastable austenite, and δ-ferrite. This research opens new avenues for the development of lightweight, high-strength steels for various industrial applications.

We propose a novel approach to correlate microstructure to hole expansion ratio (HER). HER is influenced not only by the material's intrinsic edge formability but also by damage accumulated during hole-punching. Therefore, we evaluate HER by the separation into two indicators. Firstly, by making a hole through electrical discharge machining, we can exclude the damage effect induced by the hole-punching process. The HER in this case is named intrinsic HER or HER₀. Next, the punching damage effect is quantified through the difference between HER obtained by the punched hole and HER₀. The thing to note is that microstructural characteristics influence both HER₀ and punching damage effect and by understanding the effects of microstructure on each case, we are able to accurately understand HER from a microstructure perspective. In particular, through an artificial neural network technique, it is confirmed that there is a strong correlation between the uniaxial tensile properties and HER₀. In addition, we could obtain the relationship between the properties obtained by shear test and the punching damage effect.

This study aimed to elucidate the effects of N and Ti contents on the sizes of TiN particles and prior austenite grains in low-alloy steels. Specimens were hot-rolled at a heating temperature of 1250 °C and quenched from 950 °C. The sizes of TiN particles and prior austenite grains were analyzed, revealing that both decreased with increasing N content. The prior austenite grain size was determined based on the ratio of TiN particle size to its volume fraction. As indicated by the Ostwald ripening equation, the reduction in TiN particle size with increasing N content can be attributed to the decreased rate of Ostwald ripening during heating.

Medium Mn steels have an attractive balance of the strength and ductility due to the TRansformation Induced Plasticity (TRIP) effect by the martensitic transformation of the retained austenite. However, the ferrite + austenite intercritical annealing for a long time is required for obtaining more fraction of retained austenite by enrichment of C and Mn because of the slow partitioning of Mn. To obtaining more retained austenite by short intercritical annealing, the effect of the pre-existed austenite fraction (6%~35%) before intercritical annealing on the fraction of the martensite and retained austenite phases was investigated in the 0.16C-0.5Si-3.5Mn (mass%) steel annealed at 690 °C for 180 s . The fraction of the austenite during annealing increased with increasing the fraction of pre-existed austenite, however the most retained austenite of 21% was obtained in the sample with 6% pre-existed austenite annealed at 690 °C for 180 s, not in the sample with 35% pre-existed austenite. The significant partitioning of the Mn in the retained austenite with thin acicular shape was confirmed in the samples with 6% pre-existed austenite by STEM-EDS, while the gradual gradient of Mn content from the ferrite/austenite interface to inside the austenite in the retained austenite with thick lath-shape could be recognized in the sample with 35% pre-existed austenite. This result indicates that a small amount of thin pre-existed austenite is effective to shorten the diffusion distance in austenite for Mn and enrich Mn into austenite during intercritical annealing.

In this study, the microstructural changes during tempering of two types of high strength steel plates with different hardenability were investigated. Steel plates of 40 mm thickness with a bainite or martensite microstructure were prepared by thermo-mechanical controlled rolling and direct quenching. These steel plates were tempered under various conditions, and their hardness and microstructure were investigated. It was found that the near-surface hardness of the steel plate with low hardenability decreased significantly as the tempering parameter increased, while the hardness of both the surface and the interior of the steel plate with high hardenability tended to decrease significantly with an increase in the tempering parameter. Microstructural observations indicated that these significant decreases in hardness corresponded to ferrite nucleation. The results of an EBSD analysis suggested that recrystallized ferrite had formed. Recrystallization occurred when the microstructure before tempering was martensite, but did not occur when it was upper bainite. Therefore, it was suggested that the microstructure before tempering affects the occurrence of recrystallization.