The warm V-bendabilities and hydrogen embrittlement properties of ultrahigh-strength Quenching and Partitioning (QP)-Transformation-Induced Plasticity (TRIP) steel sheets were investigated to apply the QP-TRIP steel sheets for automotive structural parts manufactured by cold or warm press forming. V-bending tests were carried out at a crosshead speed of 1 mm/min at V-bending temperatures of 25, 100 and 150 degrees C using a hydraulic servo testing machine with a 88-deg. V-punch (punch tip radius R = 2 mm, R/t0 = 1.7) and a V-die (die groove size l = 12 mm, die shoulder diameter 0.8 mm) using V-bending specimens with dimensions of 5-mm width, 50-mm length and 1.2-mm thickness without and with hydrogen charging. Hydrogen charging was conducted by means of cathodic charging using a 3 wt% NaCl + 3 g/L NH4SCN solution at a current density of 10 A/m2 for 48 h before V-bending. The main results were obtained as follows. (1) QP-A steel enabled to conduct V-bending at a V-bending temperature T = 25 degrees C although the bending angle after unloading (theta 2) was less than 90-deg. (2) When V-bending tests were carried out at T = 100 degrees C, QP-B, C, and E steels without hydrogen and QP-B steel with hydrogen charging enabled to conduct V-bending. In addition, QP-B steel was also possible to carry out the V-bending at T = 150 degrees C. These results implied that the V-bending at warm temperatures can improve the V-bendabilities of the QP-TRIP steels.

The damage initiation and evolution behaviors of ferrite-martensite dual phase (DP), transformation-induced plasticity (TRIP)-aided dual-phase (TDP), quenched and tempered (QT), and TRIP-aided martensitic (TM) steels during tensile deformation were investigated. Voids were initiated at the phase boundaries and inside the martensite in the DP and TDP steels, whereas fine voids were observed at the prior austenite, packet, and block boundaries in the QT and TM steels. In the DP and TDP steels, the size of the voids remarkably increased with the plastic strain, even though the number of voids increased slightly. By contrast, the QT and TM steels exhibited a drastic increase in the number of voids, whereas a slight increase in the size of the voids was observed. The voids in the TM steel hardly extended as the plastic strain increased unlike those in the QT steel. The extent of voids in the DP and TDP steels might be attributed to stress and plastic strain partitioning between the different phases during tensile deformation. In addition, the promotion of void initiation and suppression of void growth might be attributed to the fine and uniform martensite matrix in the QT and TM steels. The suppression of void initiation in the TDP steel and void growth in the TM steel might be attributed to the stress and plastic strain relaxations at the void initiation site and the vicinity of voids owing to the effective martensitic transformation of retained austenite.

High-manganese cast steel is widely used in railway crossings due to its high toughness, wear resistance, and exceptional work-hardening capability. However, the mechanisms of work hardening and crack initiation under low slip ratios have not been fully clarified, as most previous studies focused on higher slip ratios that do not reflect actual railway operations. This study aims to elucidate the fundamental mechanisms of work hardening and damage formation in high-manganese cast steel under low slip conditions by employing a twin-disc rolling contact fatigue tester. Experiments were conducted using SCMnH3 rail specimens and SSW-QS wheel materials under a contact pressure of 900 MPa with slip ratios of 0% and 0.22%. Rolling cycles were varied up to 500,000. Results revealed pronounced wear and microcrack formation under low slip, whereas no-slip conditions produced negligible wear. Microstructural observations showed deformation bands in both conditions; however, low slip produced dense deformation twins extending deeper beneath the surface, increasing in density and depth with rolling cycles. EBSD IQ and IPF maps confirmed significant strain accumulation and crystallographic reorientation near the surface, accompanied by plastic-flow-like features that governed crack initiation and propagation. Hardness measurements indicated substantial work hardening under low slip, reaching approximately 590 HV with saturation beyond 300,000 cycles. XRD analysis revealed broadened diffraction peaks and localized α’-martensite formation within the extreme surface layer, suggesting transformation-induced hardening. These findings demonstrate that work hardening under low slip is primarily driven by the accumulation of deformation twins and dislocations, complemented by localized martensitic transformation. Crack initiation occurs along regions of concentrated plastic flow, consistent with damage patterns observed in actual manganese crossings. The study provides essential insights into the deformation and damage mechanisms of high-manganese cast steel under realistic wheel–rail contact conditions.

The influence of cold rolling on hydrogen desorption behavior in Al-Cu-Mg and Al-Zn-Mg alloy plates was investigated. Solution-treated plates were cold-rolled with total reductions of 0, 50, 80 and 90%, and kept at room temperature for 168 h. Vickers hardness of the specimens increased with the rolling reduction. Thermal desorption analysis revealed that a desorption peak at about 350 degrees C in the specimen without cold rolling (0% reduction), attributable to hydrogen trapped by dislocations, changed markedly with rolling reduction: the desorption peak split into two peaks and the desorption amount of the peak at lower temperature increased notably with increasing cold reduction. To investigate the cause of this phenomenon, hardness measurement and electron backscattering diffraction analysis were carried out for the specimen 90% cold-rolled and then heated to 350 degrees C. The hardness was decreased by about 50% by the heating, and partially recrystallized microstructure containing some recrystallized grains in the recovered matrix was observed in the heated specimen. From these results, recovery and partial recrystallization were attributable to the appearance of the low-temperature peak. The change in hydrogen desorption behavior with holding time for up to 144 h at room-temperature was also examined in the 90% cold-rolled specimens. The desorption amount corresponding to the two peaks described above stemming from hydrogen atoms trapped by dislocations decreased with the holding time at room temperature, indicating that the hydrogen atoms trapped by dislocations can be released during room-temperature holding. The whole results obtained in this study have clarified the role of deformation microstructures on desorption behavior of the hydrogen atoms trapped by dislocations.[doi:10.2320/jinstmet.J202601]

It has been reported that severely cold worked Fe-24.6Ni-5.8Al-0.4C ( mass% ) had a yield strength of 2 GPa and afracture elongation of 20 %, in which huge amount of Luders-type deformation was observed. In the present article, we summarize the reports for high-strength Fe-Ni-Al-C, Fe-Mn, Fe-Cr-Ni and Fe-Ni-Mn base steels with the Luders-type deformation so far, and provide our latest data on the effects of alloying elements and the cold-rolling reduction on the microstructure and mechanical properties of cold-rolled Fe-Ni-Al-C alloys. Previous reports imply that the phase stability of gamma phase affects the size of Luders elongation, while the strategies to control the microstructure to achieve high strength and high ductility are currently unknown. Our latest study also shows that the gamma- phase stability affects the Luders strain. In addition, it is confirmed that severe cold rolling by 80 % enables the prolonged Luders strain as much as 25 % in nominal strain. This prolonged Luders strain is achieved by multiple propagation of Luders-type bands

The effect of nanostructures on the hardness of Al-3.9Cu-1.5Mg alloys with and without cold rolling (CR) during natural and isothermal aging at 190 degrees C for up to 48 h has been investigated by in situ high energy laboratory small-angle X-ray scattering combined with micro-Vickers and calorimetry measurements. In the naturally aged samples, the formation of Cu-Mg co-clusters of 1 nm in diameter was observed. In the specimens with CR, the formation of clusters due to CR was observed at the beginning, while little change were observed during aging. Consequently, the total amount of clusters was less than in the specimens without CR in the later stage of natural aging. The change in the amount of age hardening corresponded to the change in the amount of Cu-Mg co-cluster formation, indicating that the Cu-Mg co-cluster was responsible for the precipitation strengthening at room temperature. At 190 degrees C for the artificial aging process, precipitate scattering of three different sizes and shapes was observed, i.e., the intermediate and S phases were precipitated in addition to the Cu-Mg co-cluster. The continuous change in volume fraction and size with time suggests that the Cu-Mg co-cluster has grown into an intermediate phase. The time evolution of hardness at 190 degrees C artificial aging without CR was simply explained by Orowan's equation. These results show that the dispersion state of precipitates mainly affects the hardness in AlCu-Mg alloys with low Cu/Mg ratios rather than the phases themselves.

The effects of mean normal stress on the deformation properties such as the strain-hardening, strain-induced martensite transformation, and micro-void initiation behaviors of low-carbon ultrahigh-strength TRIP-aided bainitic ferrite (TBF), bainitic ferrite/martensite (TBM), and martensite (TM) steels were investigated to evaluate the various cold formabilities. In addition, the deformation properties were related to the microstructural properties such as the matrix structure, retained austenite characteristics, and second-phase properties. Positive mean normal stress considerably promoted strain-induced martensite transformation and micro-void initiation, with an increased strain-hardening rate in an early strain range in all steels. In TM steel, the primary martensite matrix structure suppressed the micro-void initiation through high uniformity of a primary martensite matrix structure and a low strength ratio, although the strain-induced transformation was promoted, and a large amount of martensite/austenite constituent or phase was contained. A mixed matrix structure of bainitic ferrite/primary martensite in TBM steel also suppressed the micro-void initiation because of the refined microstructure and relatively stable retained austenite. Promoted micro-void initiation of TBF steel was mainly promoted by a high strength ratio.

The damage to the shear-punched surface layers such as strain-hardening, strain-induced martensite transformation, and micro-void initiation behaviors was evaluated in the third-generation low-carbon advanced ultrahigh-strength TRIP-aided bainitic ferrite (TBF), bainitic ferrite–martensite (TBM), and martensite (TM) steels. In addition, the surface layer damage was related to (1) the mean normal stress generated during shear-punching and (2) microstructural properties such as the matrix structure, retained austenite characteristics, and second-phase properties. The shear-punched surface layer damage was produced under the mean normal stress between zero and negative in all the steels. The TBM and TM steels achieved relatively small surface layer damage. The small surface layer damage resulted in excellent cold stretch-flangeability, with a high crack-propagation/void-connection resistance on hole expansion.

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance.

To analyze various types of cold formability in TRIP-aided polygonal ferrite (TPF), annealed martensite (TAM), and bainitic ferrite (TBF) steels, the effects of the mean normal stress on the strain-hardening, strain-induced martensite transformation, and void-formation behaviors were investigated. The strain-hardening behavior was influenced by positive mean normal stress and was hardly influenced by zero and negative mean normal stresses in all steels. Positive mean normal stress promoted the strain-induced martensitic transformation behavior, especially in TBF steel due to the high mechanical stability of the retained austenite. The void-formation behavior was also promoted by positive mean normal stress, especially in TPF steel. These behaviors were also related to the microstructural properties, such as the matrix structure, retained austenite characteristics, and second phase.

The effects of hydrogen on the tensile properties, fatigue life, and tensile and fatigue fracture morphologies of nitrogen-added ultrahigh-strength transformation-induced plasticity (TRIP)-aided martensitic (TM) steels were investigated. The total elongation and number of cycles to failure (Nf) of the hydrogen-charged TM steels decreased with the addition of nitrogen; in particular, adding 100 ppm of nitrogen decreased the total elongation and Nf of the TM steels. The quasi-cleavage cracking around the AlN occurred near the sample surface, which is the crack propagation region, although dimples appeared at the center of the fracture surface in the tensile samples. The initial fatigue crack initiated at the AlN precipitate or matrix/AlN interface, located at the notch root. During crack propagation, new cracks were initiated at the AlN precipitates or matrix/AlN interfaces, while quasi-cleavage crack regions were observed around the AlN precipitates. The decrease in the total elongation and Nf of the hydrogen-charged TM steel with 100 ppm of added nitrogen might be attributable to the crack initiation around the AlN precipitates formed by a large amount of hydrogen trapped at the AlN precipitates and matrix/AlN interfaces, and to the dense distribution of AlN, which promoted crack linkage.

To understand the process and mechanism for hydrogen embrittlement in steels, visualization of the location of hydrogen is essential. In the present study, two visualization techniques, hydrogen microprint technique (HMT) and tritium autoradiography (TAR), were applied to a pure iron sheet 20% tensile-deformed with cathodic hydrogen charging. When the specimen was covered with photographic emulsion shortly (40 min) after the deformation, HMT showed that the charged hydrogen atoms diffused out at majorly grain boundaries and minorly in the grain interiors. The TAR, conducted on the same sample but completely de-hydrogenated and then charged with tritium, revealed that hydrogen enhances the formation of vacancies or vacancy clusters with plastic deformation, which are located along grain boundaries and deformation bands and act as relatively stable trapping sites for tritium.

To investigate the effect of the microstructure adjacent to the grain boundaries on the mechanical properties and hydrogen embrittlement susceptibilities in the Al-Cu base 2219 alloy, alloy specimens were solution -treated and then aged at 100 degrees C, 130 degrees C, and the usual aging temperature of 190 degrees C, to control the alloy microstructure in the vicinity of grain boundaries. The slow strain rate technique was conducted on the specimens in humid air and dry nitrogen gas environments to evaluate the effect of environmental hydrogen on them. Transmission electron microscopy was used to measure the grain boundary precipitate size and precipitate -free zone width of the specimens. Thermal desorption analysis was conducted on the gauge sections of the fractured specimens to evaluate the trapping sites and amount of hydrogen desorbed. The specimens aged below 190 degrees C had finer grain boundary precipitates than those aged at 190 degrees C. The test environment did not affect the specimen strength under any of the aging conditions 100 degrees C, 130 degrees C, and 190 degrees C. Some samples had intergranular fractures on their entire fracture surfaces, irrespective of the test environment. The thermal desorption analysis results showed no significant difference between the hydrogen emission spectrum and the amount of hydrogen released within each temperature range. Thus, hydrogen embrittlement does not occur in the 2219 alloy, irrespective of the characteristics of its microstructure adjacent to the grain boundaries. [doi:10.2320/matertrans.MT-L2023012]

To attain the aim of weight reduction and safety improvement of vehicles, some high strength steel sheets have been developed and investigated. TRIP-aided steel sheets with transformation-induced plasticity (TRIP) of the retained austenite have high strength and ductility, and excellent hydrogen embrittlement resistance. In previous study, as high strength TRIP-aided steel for forging parts, the volume fraction of retained austenite in the TRIP-aided steel could be increased by hot forging with austempering. Similarly, our research group reported that the thermomechanical process of hot rolling following by austempering could also increase the amount of retained austenite in the TRIP-aided steel sheet. The tensile properties and formabilities of TRIP-aided steel sheet subjected to the thermomechanical rolling just before austempering possess obvious advantages compared with those of TRIP-aided steel sheet without thermomechanical rolling process (with only austempering). These excellent mechanical properties may be caused by the finely dispersed retained austenite and refined bainitic ferrite and/or martensite brock by thermomechanical rolling process.

Nanoscale microstructural analysis and evaluation of mechanical properties were conducted on severely cold-rolled aluminum alloys. In order to examine the effect of alloying elements on microstructure and mechanical properties, Al-Cu -Mg, Al-Mg -Si and Al-Zn-Mg-Cu alloy sheets were prepared for systematic investigation. The SAXS and SANS were used to analyze the nanoscale microstructures in solution-treated and rolled samples of Al-Cu -Mg and Al-Zn-Mg-Cu alloys, and the results quantitatively revealed that nanoscale clusters are formed regardless of with or without rolling. HR-TEM, HAADF-STEM, thermal analysis, hardness and electrical conductivity measurements also suggested that the clusters are formed in cold-rolled samples. Mechanical property evaluations showed that strength generally increased, and ductility decreased with increasing cold-rolling reduction. The strength tended to increase with increasing solute content regardless of the alloy system.

The effects of partial replacement of Si by Al on the microstructure, tensile properties, and Charpy impact toughness were investigated using 0.2%C-Si/Al-Mn-Cr-B TRIP-aided martensitic steels to promote the application of galvanized third-generation ultrahigh- and high-strength steels. The impact toughness was related to the microstructural and mechanical properties. The partial replacement decreased the volume fraction of retained austenite and increased the mechanical stability, accompanied by softening and an increase in the volume fraction of the primary martensite. Resultantly, the partial replacement decreased strength and ductility. The impact absorbed energy (value) at 25 °C was slightly increased by the partial replacement. The increased impact absorbed energy was mainly caused by high crack/void propagation energy due to the softened primary martensite and a small contribution of the stabilized retained austenite. The 50% shear fracture ductile-to-brittle transition temperature was marginally raised by the partial replacement. The raised transition temperature was mainly associated with an increase in a unit crack path of quasi-cleavage/cleavage fracture.

The effect of stability of austenite (gamma) phase on Luders-like deformation behavior of cold-rolled Fe-Ni-Al-C alloys was investigated by digital image electron backscatter diffraction (EBSD) analysis. In this study, cold-rolled Fe-23Ni-5.0Al-0.5C (mass%) alloy (23Ni alloy) and Fe-25Ni-5.0Al-0.5C (mass%) alloy (25Ni alloy) were used. The tensile strength increased and the total elongation decreased with decreasing Ni content. Luders-like deformation after yielding was observed up to a nominal strain of approximately 0.04 for the 23Ni alloy and approximately 1.0 for the 25Ni alloy. The transformation from gamma to martensite (alpha') with a specific crystal orientation relationship was observed during tensile deformation, and it was found that the volume fraction of a' during tensile deformation increased more easily in 23Ni alloy than in 25Ni alloy. In addition, the increment of a' phase in 25Ni alloy was higher during Luders-band propagation than after the band propagation. These results suggested that change of the Luders-like deformation behavior was due to the change in gamma stability.

Severe plastic deformation processing and subsequent aging treatment have been known to be effective for achieving higher strength than the conventional aging treatment in aluminum alloys. This study prepared the Al-Cu-Mg-based alloy sample, Al-5.3Cu-2.8Mg (mass%). The alloys were solution treated at 480, 495 and 505°C, and cold-rolled by 90%. The effect of process condition and test environment on tensile properties in cold-rolled Al-Cu-Mg alloys was investigated. Results confirm that strength and ductility were improved with increasing the solution heat treatment temperature regardless of test environment. 0.2% proof stress and ultimate tensile strength were higher than aging treatment specimens, but elongation to failure was lower than aged one. Hydrogen embrittlement susceptibility increased with increasing solution treatment temperature. Ductile fracture with many dimples is observed in both cold-rolled and aged specimens. Second-phase particles were observed at the bottom of the dimples. There was no significant difference in fracture surface between the different test environments.

The warm V-bending and hydrogen embrittlement properties of the ultrahigh-strength transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel sheets were investigated to apply for the automotive structural parts manufactured by cold- or warm-press forming. The V-bending tests were carried out at a forming speed of 1 mm/min and a forming temperature (T, °C) of 25 and 100 ℃ using a hydraulic servo type universal testing machine with a 88-degree V-punch and a V-die using V-bend specimens with dimensions of 5 mm width, 50 mm length and 1.2 mm thickness without and with hydrogen. Hydrogen charging was conducted by means of cathodic charging using a 3 wt

Tensile properties have been evaluated for four kinds of Al-Mg-Si base alloys which were cold rolled by 90% and subsequently aged for 1 min, 30 min, 18 h at 170°C. To study the effect of alloy compositions and aging conditions on deformation behavior, fracture surface observations and strain distribution analysis during tensile deformation were performed. Strain distribution in the tensile specimens was evaluated by digital image correlation method. The ductility in the specimens aged for 1 min and 30 min after cold rolling was improved due to increase in both uniform and local elongation. All the specimens showed ductile fracture, while the specimen added with 0.71%Fe showed slightly lower ductility. The addition of copper improved strength without deteriorating ductility. The relation between mechanical properties and microstructure is discussed to obtain basic information required to improve strength and ductility of Al-Mg-Si alloys.