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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Advance Publication

  • Microstructural Ductile Fracture Analysis of 1180-MPa Class Martensite-Matrix Dual-phase Steel via in-situ Tensile Test

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    DOI:10.2355/tetsutohagane.TETSU-2022-100

    Martensite-matrix dual-phase (DP) steel is increasingly used for high-strength automobile parts owing to its excellent compatibility, ductility, and tensile strength. However, its higher fracture strain, reflected by the hole expansion ratio, remains an issue hindering further adoption of this material. Therefore, this study conducted a microscale investigation of the ductile fracture behavior of 1180-MPa class martensite-matrix DP steel to obtain a guideline for microstructural design realizing improved fracture strain. In this investigation, in-situ tensile testing was conducted simultaneously with scanning electron microscope observations and crystal plasticity finite-element analysis (CP-FEA). The in-situ tensile test results indicated that microcracks initiated at particular martensite packets and did not propagate into other packets; the CP-FEA results revealed that the martensite crystal orientation caused this behavior to induce remarkable stress and strain localization at interfaces in the vicinity of ferrite islands, relaxing the stress and strain localization at distant martensite packets. Although the cracks observed around the ferrite–martensite interfaces were similar to those observed in conventional ferrite-matrix DP steel, such matrix-phase cracks have rarely been reported except immediately prior to final fracture. Thus, the optimization of ferrite island distribution to suppress the formation of stress and strain localization sites was identified as the key aspect of martensite-matrix DP steel microstructure design. This design aspect can be achieved using a combination of data science and CP-FEA.
  • Effect of Oxygen Enrichment on Melting Behavior in Sintering Process

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    DOI:10.2355/tetsutohagane.TETSU-2022-095

    In the sintering process, productivity and strength are essential indicators, and it is known that they are closely related to the sintering time and temperature. Since the sintering time and high temperature holding time vary greatly depending on the combustion behavior of coke breeze, various studies have been conducted on the influence of oxygen which is critical element in the sintering phenomena. However, the influence of oxygen enrichment on sinter strength and yield has not yet been unified. In this study, the sintering productivity, strength, and yield were investigated when the oxygen concentration of the inlet gas was increased to 40 vol%. In addition, the heat profile on pot tests and the effect of oxygen partial pressure on the melting property were analyzed from the viewpoint of thermodynamics. As a result, it was found that the yield showed the maximum value at 30 vol% oxygen concentration, and the strength increased with oxygen enrichment. As for the yield, it was found that the effect of the heat profile was significant, and the deviation between the heat transfer speed and the combustion speed started when the oxygen concentration exceeded 30 vol%. On the other hand, it was suggested that the strength improvement was caused by the increase in the amount of melt produced by oxygen enrichment in addition to the effect of high temperature holding time.
  • Effect of Alloy Elements on Eutectic Carbide Morphology for High-speed Tool Steel

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    DOI:10.2355/tetsutohagane.TETSU-2022-096

    Effect of alloying elements on the composition distribution, morphology and volume fraction of eutectic carbides were investigated for high-speed tool steels with uniform hardness from 68 to 70HRC after hot working and quenching and tempering. The stability of eutectic carbides at high temperature was also evaluated. M2C, M6C and MC eutectic carbide are observed in as-cast samples similar to the general high-speed tool steels. M2C type carbide increases in volume with increasing Si and V contents, and M6C and MC carbides appear with increasing W and V contents. The amount of M2C eutectic carbide varies with the composition in liquid phase just prior to eutectic solidification. The morphology of the eutectic carbide changes from fine fiber or lamellar type to coarse lamellar or feather type, and the interlayer spacing of eutectic carbide tends to increase with increasing the area fraction of M2C eutectic carbide. Moreover, after heat treatment at 1140℃ for 16 hours, some M2C carbides remain stabled but MC and M6C carbide appears.
  • Effects of Size of Micro Texture Regions on the Dwell Fatigue Properties of Ti-6Al-4V

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    DOI:10.2355/tetsutohagane.TETSU-2022-088

    The cyclic fatigue, dwell fatigue and room temperature creep properties were evaluated in three types of Ti-6Al-4V forged bar samples having different micro-texture-regions (MTR) and tensile properties in the loading direction. In the S-N curve where the stress(σnor) was normalized by 0.2%-proof-stress, the fatigue lives of all samples were almost the same , whereas the dwell fatigue lives were not the same. So the ratio of the cyclic fatigue life to dwell fatigue life (dwell debit) changed to 2–60. In cyclic fatigue the initiation site was a facet of 1–2 α grains, and the fracture surface was typical. In dwell fatigue and creep, on the other hand, facet and dimple regions were confirmed. In addition, the facet region consisted of initiation facets of 1–2 α grains and the propagation facets which were the majority of the facet region. Initiation facets in dwell fatigue occurred earlier than 25 % of the life ratio, and the angle between the c-axis of the α grains with the initiation facets and loading direction was 15–55°. The propagation facets were the MTR in which the angle between the c-axis of the α grains and loading direction was 30° or less. The lengths of the facet regions were proportional to the MTR size. In dwell fatigue, the larger the σnor or MTR size, the larger was the dwell debit. Therefore, the MTR size was considered the dominant factor determining the dwell fatigue life.
  • High Temperature Mechanical Properties and Microstructure in 9Cr or 12Cr Oxide Dispersion Strengthened Steels

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    DOI:10.2355/tetsutohagane.TETSU-2022-080

    Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700°C or 750°C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900°C to 1350°C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900°C to 1100°C, but increased at 1200°C, decreased drastically at 1250°C, and increased again at 1300°C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the δ-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300°C.
  • Effect of Alloying Elements on Grain Boundary Segregation of Boron and Carbon in α-Iron

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    DOI:10.2355/tetsutohagane.TETSU-2022-081

    To clarify the effect of alloying elements (M) on the grain boundary segregation behavior of boron (B) and carbon (C) in a-iron, the grain boundary segregation of B, C and alloying elements was evaluated thermodynamically for the Fe–B–1.0 at.%M and the Fe–C–1.0 at.%M ternary systems (M: Al, Ti, V, Cr, Mn, Nb, Mo) using the parallel tangent law proposed by Hillert. In this calculation, the Gibbs energies of the liquid phase in the Fe–B–M and Fe–C–M ternary systems were applied to those of the grain boundaries. According to the calculated results, in the Fe–B–M ternary systems, co-segregation of Ti, V, Mn or Nb with B was predicted, while no co-segregation behavior was confirmed in the case of Al, Cr or Mo addition; in the Fe–C–M ternary systems, co-segregation of Ti, V, Nb or Mo with C was predicted, while no co-segregation behavior was confirmed in the case of Al, Cr or Mn addition. These co-segregation tendencies correspond well with the formation tendencies of metal borides or metal carbides. Although the present calculated results were based on the assumption that substitutional elements can diffuse sufficiently in addition to interstitial elements B and C, we proposed an equation for the parallel tangent law under paraequilibrium condition in which no partitioning of substitutional elements occurs.
  • Development of Cell Structure and Crack Initiation during Fatigue of an Fe-3 mass%Si Alloy

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    DOI:10.2355/tetsutohagane.TETSU-2022-078

    A cell structure development and a crack initiation during a fatigue of an Fe-3 mass%Si alloy was investigated through electron channeling contrast imaging in a scanning electron microscope and electron back-scatter diffraction analysis. The crystal rotation regions (CRRs), deformation bands (DBs), and cell bands (CBs) together formed a hierarchy in the dislocation structures. In the early stage of fatigue, deformation is constrained near grain boundaries; this impedes further dislocation propagation. This restriction is attributed the formation of CRRs with a width of several hundred micrometers. Further, DBs that were several microns wide were developed inside the CRRs, and CBs with a width of several hundred nanometers were formed inside the DBs. Meanwhile, a crack was initiated from a CRR near a grain boundary. At the crack tip, a DB penetrating the CRR was formed parallel to the crack-propagation direction. It was elucidated that the cell boundary in the DB had a high misorientation angle of approximately 10 degrees, which greatly affected to the crack initiation. In addition, the penetrating DB was composed of elongated cells and cell bands. Prior to crack initiation, the boundaries of the cell bands evolved in proportion to the increasing dislocation density during fatigue. The elongated direction of the cell boundary, which was almost parallel to the {110} plane with a tilt boundary feature, dominated the crack-propagation direction. The formation plane and the cell boundary development process can be explained by analyzing the geometrical relationship of the activated slip systems between adjacent cells.
  • Influence of TiO2% in Iron Sand on Cast Iron Production by Tatara Iron Making

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    DOI:10.2355/tetsutohagane.TETSU-2022-056

    The aim of this project was to learn roles of titanium oxide (TiO2), an impurity contained in iron sand, in the products resulting from traditional iron making processes, tatara operations. For this purpose, iron sand was collected using two different mineral processing methods from four different locations in the Chugoku area of Japan, and these samples were used to run small-scale tatara experiments. Iron sand collected with traditional gravity separation method contained 8 to 12% TiO2, while iron sand collected with modern magnetic separation method contained less than 5% TiO2. When gravity-separated iron sand was used in a tatara under strong reducing conditions, zuku (cast iron) flowed out of the tatara. In contrast, magnetically collected iron sand failed to produce zuku, but did produce raw steel at the bottom of the furnace. Further, even magnetically isolated iron sand could produce zuku when it was supplemented with ilmenite, a titanium-iron oxide containing mineral. The results show that TiO2 plays a key role in producing cast iron in tatara operations, and the fact that Akome iron sand is known to produce cast iron as it contains higher levels of TiO2. In contrast, Masa iron sand which is known to produce steel (tamahagane) contains much less TiO2 and hence is not suitable to produce cast iron. These observations agree with historical descriptions stating that pre-modern tatara operators knew to add iron sand from a specific locality (which is rich in TiO2) to Masa-type iron sand to produce cast iron.
  • Austenite Reversion Behavior of Maraging Steel Additive-manufactured by Laser Powder Bed Fusion

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    DOI:10.2355/tetsutohagane.TETSU-2022-066

    This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550°C for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (γ) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the γ-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing γ phase) localized at cell boundaries would play a role in the nucleation site for the formation of γ phase at 550°C, resulting in the enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.
  • Creep Deformation Behavior and Microstructure of Laves-strengthened Ferritic Heat-resistant Steels Containing Nitrogen or Carbon

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    DOI:10.2355/tetsutohagane.TETSU-2022-065

    The creep deformation behavior and microstructure of a N-containing steel expected to exhibit high creep strength and excellent oxidation resistance were investigated. Even for steel with a high W content, it was possible to form a martensitic microstructure by adding a sufficient amount of N. Comparison of the microstructures of the N-containing steel and a C-containing steel confirmed that the two steels have the same crystal orientation relationship. The N-containing steel precipitated with the Laves phase as a strengthening phase displayed a higher creep strength than conventional steel under relatively high stress. However, the superiority of the creep strength of the N-containing steel relative to the conventional steel decreased under low stress. The stress exponent of the N-containing steel was different from those of the C-containing steel and the conventional steel. This deference considered to be ascribed to the difference of variation behavior of dislocation density during creep deformation.
  • Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

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    DOI:10.2355/tetsutohagane.TETSU-2022-049

    One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

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