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Tetsu-to-Hagané Advance Publication

ISIJ International
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575
Publisher: The Iron and Steel Institute of Japan

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  1. Vol. 111 (2025)

    1. No.1

  2. Vol. 110 (2024)

    1. No.16
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    3. No.14
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  3. Vol. 109 (2023)

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  4. Vol. 108 (2022)

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  5. Vol. 107 (2021)

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  6. Vol. 106 (2020)

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  7. Vol. 105 (2019)

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  8. Vol. 104 (2018)

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  9. Vol. 103 (2017)

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  10. Vol. 102 (2016)

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  11. Vol. 101 (2015)

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  12. Vol. 100 (2014)

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  13. Vol. 99 (2013)

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  14. Vol. 98 (2012)

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  15. Vol. 97 (2011)

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  16. Vol. 96 (2010)

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  17. Vol. 95 (2009)

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  18. Vol. 94 (2008)

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  19. Vol. 93 (2007)

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  20. Vol. 92 (2006)

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  21. Vol. 91 (2005)

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  22. Vol. 90 (2004)

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  23. Vol. 89 (2003)

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  24. Vol. 88 (2002)

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  25. Vol. 87 (2001)

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  26. Vol. 86 (2000)

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  27. Vol. 85 (1999)

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  28. Vol. 84 (1998)

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  29. Vol. 83 (1997)

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  30. Vol. 82 (1996)

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  31. Vol. 81 (1995)

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  32. Vol. 80 (1994)

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  33. Vol. 79 (1993)

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  34. Vol. 78 (1992)

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  35. Vol. 77 (1991)

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  36. Vol. 76 (1990)

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  37. Vol. 75 (1989)

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  38. Vol. 74 (1988)

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  39. Vol. 73 (1987)

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  40. Vol. 72 (1986)

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  41. Vol. 71 (1985)

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  42. Vol. 70 (1984)

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  43. Vol. 69 (1983)

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  44. Vol. 68 (1982)

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  45. Vol. 67 (1981)

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  46. Vol. 66 (1980)

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  47. Vol. 65 (1979)

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  48. Vol. 64 (1978)

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  49. Vol. 63 (1977)

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  50. Vol. 62 (1976)

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  51. Vol. 61 (1975)

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  52. Vol. 60 (1974)

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  53. Vol. 59 (1973)

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  54. Vol. 58 (1972)

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  55. Vol. 57 (1971)

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  56. Vol. 56 (1970)

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  57. Vol. 55 (1969)

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  58. Vol. 54 (1968)

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  59. Vol. 53 (1967)

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  60. Vol. 52 (1966)

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  61. Vol. 51 (1965)

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  62. Vol. 50 (1964)

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  63. Vol. 49 (1963)

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  64. Vol. 48 (1962)

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  65. Vol. 47 (1961)

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  66. Vol. 46 (1960)

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  67. Vol. 45 (1959)

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  68. Vol. 44 (1958)

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  69. Vol. 43 (1957)

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  70. Vol. 42 (1956)

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  71. Vol. 41 (1955)

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21 Jan. (Last 30 Days)

Tetsu-to-Hagané Advance Publication

Development of Open-Air Type Super-Rapid Atmospheric-Pressure Plasma Jet Nitriding Process

Junji Miyamoto, Naoyuki Takahashi, Chengji Jin

DOI:

10.2355/tetsutohagane.TETSU-2024-112

Abstract

Plasma nitriding of JIS SKD61 tool steel was performed by open-air type atmospheric-pressure plasma jet. The results of our experiments show that the surface hardness of tool steel work pieces was increased by more than two times that of the core material after within 30 min of treatment time.

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Development of Open-Air Type Super-Rapid Atmospheric-Pressure Plasma Jet Nitriding Process

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Steam Preheating Granulation Technology for Increasing Sinter Productivity

Shohei Fujiwara, Toshiyuki Hirosawa, Hiroshi Tanaka, Hiroshi Tanaka, Takahide Higuchi

DOI:

10.2355/tetsutohagane.TETSU-2024-128

Abstract

As a countermeasure for deterioration of raw materials’ quality, improvement of sinter productivity is needed. To increase sinter productivity, steam heating granulation technologies have been developed. Heated granules reduce water condensation at wetting zone of sintering bed. It improves permeability of sinter bed and sinter productivity. At first, sintering properties and heat conductive efficiency were investigated at laboratory steam injection apparatus. As a result of the laboratory test, it was confirmed that the moisture condensation at the raw material bed was suppressed by steam injection, permeability was improved, and the sinter productivity was improved. Second, on the actual plant test, the rise of raw material’s temperature by steam heating was verified. On that test, granules were heated by 20°C. In terms of difference of heat conductive efficiency between laboratory and actual plant test, heat and material balance were examined to establish the heat transfer model at actual process. According to the heat analysis of actual plant test, indirect heat transfer via lining of drum mixer also contributed to heat granules.

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Steam Preheating Granulation Technology for Increasing Sinter Productivity

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Effect of Solidification Structure Morphology on Macrosegregation Generated by Solidification Shrinkage Flow Due to Bridging

Muneto Sasaki, Yukinobu Natsume

DOI:

10.2355/tetsutohagane.TETSU-2024-129

Abstract

Casting experiments of Al-10 wt.%Cu alloy were carried out using an impreved Satou mold (iST mold). The mold was a rectangular parallelepiped (inner dimensions 30 mmT × 50 mmW × 140 mmH), with a porous alumina plate on the wide side of the mold and a chill set at a height of 70 to 80 mm from the bottom. Four metal materials (stainless, steel, brass, and copper) with different thermal conductivities were used for the chill. To investigate the effect of bridging on the formation of macrosegregation, X-ray CT analysis of the macrosegregation distribution and morphology, observation of micro- and macro-structures, and analysis of temperature and solid fraction distribution were performed for samples obtained under each condition. Bridging formed near the chill under all conditions, and channels consisting of positive segregation and cavities were formed below it. The volume fraction of positive segregation decreased as the thermal conductivity of the chill material increased. In the samples using stainless and copper as chill materials, the volume fractions of positive segregation were 73.8 % and 11.7 %, respectively. Consequently, we confirmed that the bridging-formed conditions have a significant effect on the formation of macrosegregation.

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Effect of Solidification Structure Morphology on Macrosegregation Generated by Solidification Shrinkage Flow Due to Bridging

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Precipitation Behavior of MnS Inclusion in Unidirectionally-solidified Fe–18Mn–1Al–0.3C Steels

Yuewen Fan, Kouki Kameda, Xiaojun Hu, Hiroyuki Matsuura

DOI:

10.2355/tetsutohagane.TETSU-2024-131

Abstract

In order to reasonably control the precipitation of inclusions during solidification in TWIP steels, the precipitation behavior of typical MnS inclusions in high manganese steel was investigated by unidirectional solidification experiments. Through the combined analyses using ASEM-EDS, optical microscope, and thermodynamic calculation, it was found that Mn concentration in the liquid metal region were higher than those in the solid metal region. Furthermore, closer to the inclusion the liquid phase was, higher its Mn content was. In Fe-18mass%Mn-1Al-0.3C, MnS inclusions can precipitate at the positions located in the junction of dendrites at the end of the solidification (solid fraction fS = 0.96), Mn content reaching 34.88 mass%. Already existing Al2O3 particles could become the core of MnS to form composite inclusions to promote the MnS precipitation during the solidification process. When fS achieved 0.7 leading the Mn segregation in the liquid phase to 25 mass%, MnS starts to precipitate to attach the Al2O3 surface to form composite inclusion.

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Precipitation Behavior of MnS Inclusion in Unidirectionally-solidified Fe–18Mn–1Al–0.3C Steels

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Investigation of the Effects of Mechanical Properties and Carbon Content on Cold Cracking in Laser Welds of High-strength Thin Steel Sheets

Hajime Ashida, Hiroki Fujimoto

DOI:

10.2355/tetsutohagane.TETSU-2024-117

Abstract

This study reports a case of cold cracking along welds, which arises from solidification cracking within the crater during the laser welding of high-strength steel sheets. In this investigation, we aimed to delineate the factors influencing cold cracking that originates from solidification cracking in the crater. This was achieved by using steel sheets whose mechanical properties (tensile strength: 0.6 to 1.5 GPa) and chemical composition (carbon content: 0.20 to 0.55%) were individually adjusted. The evaluation method involved performing laser welding in a stitch pattern on an oiled steel sheet, with variations in welding length. The evaluation focused on the maximum welding length at which cold cracking occurred (LMAX). The results indicated that while a high tensile strength of the steel sheet marginally increased the LMAX, the impact remained limited. Conversely, the carbon content of the steel sheet significantly influenced cold cracking; the LMAX for carbon contents of 0.30% and 0.45% was substantially greater than that for 0.20%. However, an unusual behavior was observed at a carbon content of 0.55%, where the LMAX was smaller than that for 0.45%, despite the significant hardening of the weld metal. This phenomenon was hypothesized to occur because the tensile residual stresses in the welds decreased as martensitic transformation starting temperature lowered and the expansion strain during the transformation increased with higher carbon content.

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Investigation of the Effects of Mechanical Properties and Carbon Content on Cold Cracking in Laser Welds of High-strength Thin Steel Sheets

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Effects of S-content on Gigacycle Fatigue Properties of SCM440 Steel

Yoshiyuki Furuya

DOI:

10.2355/tetsutohagane.TETSU-2024-146

Abstract

Our previous study clarified the effect of MnS on gigacycle fatigue properties of SCM440 steel by conducting gigacycle fatigue tests on High-S steels whose S-content was so high as to be close to an upper limit of the JIS standard. On the other hand, this study discusses methods to avoid fatigue-failure from MnS by conducting the gigacycle fatigue tests on Low-S steel whose S-content is as low as possible in commercially available processes. Three types of materials were prepared with different working ratios, and the fatigue tests were conducted in transverse directions. The Low-S revealed higher fatigue strengths than the High-S. The fatigue strengths of Low-S with high working ratios were as high as those of a hot-rolled round bar. The Low-S with the lowest working ratio was fractured from shrinkages, resulting in low fatigue strength. The Low-S with the highest working ratio was fractured from oxides and matrix. As the results, MnS caused fatigue failure in a few specimens of the Low-S only with the middle working ratio. These results meant that the MnS-induced fatigue-failure was avoidable by minimizing the S-content. The fatigue test results were analyzed by using a previously derived prediction. The analysis results suggested that the MnS of High-S and the shrinkages of Low-S were more harmful than others. This was probably attributed to the large number of the MnS of High-S and of the shrinkage of Low-S, which accelerated crack propagations by interaction between the MnS inclusions and between the shrinkages.

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Effects of S-content on Gigacycle Fatigue Properties of SCM440 Steel

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Fracture Behavior of Low Alloy Steel in Concurrent Cathodic Hydrogen Charging SSRT Test

Yoshihiro Nishihara, Ayaka Nozaki, Hiroshi Okano, Shusaku Takagi

DOI:

10.2355/tetsutohagane.TETSU-2024-109

Abstract

In order to reduce the construction cost of hydrogen infrastructure facilities, application of inexpensive materials such as low alloy steel is required. However, when hydrogen embrittlement is a concern, as in the case of low alloy steels, acquisition of material data in high-pressure hydrogen gas and proof of safety are necessary for application. Since the testing in high-pressure hydrogen gas incurs a high cost as a simple evaluation technique for the mechanical properties of materials in high-pressure hydrogen gas. While the comparison of mechanical properties such as total elongation in these two test environments has been an object of research, the difference in fracture processes is still not clear. In this study, slow strain rate tensile (SSRT) tests in high-pressure hydrogen gas and cathodic charge were conducted with two materials that had been controlled to different strengths by heat treatment, and their fracture behavior was investigated. Regardless the fracture mechanisms attributed to material strength, such as quasi-cleavage and intergranular-like fracture, the cathodic hydrogen charging SSRT test showed fracture displacement and a reduction of area equivalent to those in high-pressure hydrogen gas. However, the nominal stress-displacement curves showed different behavior immediately before final fracture. A detailed examination of the fracture processes of the specimens revealed that the difference in the fracture behavior of the specimens in the high-pressure gas test and in the cathodic hydrogen charging test was caused by the difference in hydrogen-induced crack growth behavior.

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Fracture Behavior of Low Alloy Steel in Concurrent Cathodic Hydrogen Charging SSRT Test

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Crack Formation Process in Galvanized Steel Under Dwell Fatigue

Kayo Hasegawa, Shatumbu Thomas Alweedo, Motoaki Morita

DOI:

10.2355/tetsutohagane.TETSU-2024-121

Abstract

The study investigated the dwell fatigue characteristics of hot-dip galvanized steel. Cyclic and dwell fatigue tests were conducted, their fatigue life was compared, and fracture surfaces were analyzed. When the cyclic maximum stress (σmax) was the upper yield stress (σUYS), there was hardly a difference in fatigue life between cyclic and dwell fatigues. In σmax=0.9 × σUYS, the fatigue life in dwell fatigue was shorter than that in cyclic fatigue. The cracks under dwell fatigue were generated in σmax= σUYS before N=10 cycles. Their cracks did not grow until N=100,000 cycles. On the other hand, no cracks were observed on the specimen surface under cyclic fatigue before N=100,000 cycles. The formation of cracks on the surface of the galvanized layer under cyclic dwell was remarkably delayed compared to that under dwell fatigue, regardless of the applied stresses in this study. Therefore, dwell fatigue mode debases the surface of the hot-dip galvanized steel. The applied stress affected the crack morphology on the specimen surface. In σmax= σUYS, the large cracks were observed at the grain boundary triple junctions. In σmax=0.9 × σUYS, not only the cracks at triple junctions of grain boundary but also some cavities along the grain boundaries were detected. Their defects were often reported under creep deformation. The cavities seemed to adjoin each other and coalesce. In the stress relaxation testing, the hot-dip galvanized steel exhibited creep behavior. The decrease in the fatigue life under dwell fatigue would be due to the creep phenomena.

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Crack Formation Process in Galvanized Steel Under Dwell Fatigue

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Microstructural Analysis of Slip Mechanisms in Friction-type Joints Using Ascoated Hot-dip Galvanized Steel and High-strength Bolts

Norihiko L. Okamoto, Hayato Kobayashi, Tetsu Ichitsubo

DOI:

10.2355/tetsutohagane.TETSU-2024-144

Abstract

The friction-type joints using high-strength bolts are frequently employed for the assembly of structural steel components. The drawback of the combination of the friction-type joints and hot-dip galvanized steel plates for highly corrosive environments is the low slip coefficient at the friction interface in the as-coated condition. To increase the slip coefficient, labor-intensive blast processing or phosphate treatment is applied to the surface of the galvanized steel plates before assembly. In this study, we investigated the slip mechanism at the friction interface between as-galvanized steel plates through slip resistance tests on high-strength bolted friction joints, in hope of determining effective methods for overcoming the low slip coefficient in the as-coated condition. In the as-galvanized material, both the outermost Zn- and ζ(FeZn13)-phase layers exhibit c-axis texture. Since the easiest basal (dislocation) slip plane for the Zn phase with the hexagonal close-packed structure is parallel to the friction interface, the Zn phase is geometrically prone to plastic deformation due to the shear stress applied on the friction interface. The evidence that the coarse-grained Zn phase was refined to small crystal grains upon macroscopic slippage at the friction interface indicated that the low slip coefficient was attributed to the readily deformable nature of the outmost Zn phase. Potential strategies for increasing the slip coefficient without pre-surface treatment include strengthening the soft Zn phase through grain refinement or texture modification, or complete removal of the Zn phase during galvanizing.

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Microstructural Analysis of Slip Mechanisms in Friction-type Joints Using Ascoated Hot-dip Galvanized Steel and High-strength Bolts

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Equiaxed Solidification of Metastable Ferrite in Fe–22mass%Mn–0.7mass%C–0.3mass%Ti Alloy Nucleating on Ti Carbonitride

Taka Narumi, Makoto Ohta, Kengo Fujita, Ryoji Katsube, Hideyuki Yasuda

DOI:

10.2355/tetsutohagane.TETSU-2024-125

Abstract

This study demonstrates the effect of Ti addition on phase selection and subsequent ferrite-austenite transformation in Fe-22mass%Mn-0.7mass%C alloy, where the austenite is the primary phase in equilibrium. X-ray radiography revealed that the metastable ferrite nucleated as equiaxed grains in the completely melted specimen. During subsequent cooling, the metastable ferrite massively transformed into the austenite in the solid state, forming multiple austenite grains in each metastable ferrite grain. The ferrite-austenite transformation immediately followed the coarsening of multiple austenite grains within each former metastable ferrite grain. Typical austenite grain size ranged from 100 to 500 μm. In the specimen after the observation, titanium carbonitride (Ti(C,N)), which acts as heterogeneous nucleation agent for the ferrite, was presented and overlaid manganese spinel (MnAl2O4) or Al-Ti oxide. Because disregistry between such oxides and Ti(C,N) can be relatively low, the oxides facilitated the formation of Ti(C,N) in the melt. Regarding the formation of the oxides, it can be postulated that titanium oxides, as a deoxidation product, first combined with soluble Al, Mn, and O to form liquid Al-Mn-Ti oxides. During cooling, MnAl2O4 or Al-Ti oxide was supersaturated in liquid Al-Mn-Ti oxides, which subsequently crystallized and dispersed in the melt. Thus, titanium oxide serves as a precursor to a multistep reaction leading to the formation of Ti(C,N), and its fine dispersion in the melt allows us to control the austenite grain size in the as-cast microstructure through promoting the metastable ferrite nucleation followed by the ferrite-austenite transformation.

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Equiaxed Solidification of Metastable Ferrite in Fe–22mass%Mn–0.7mass%C–0.3mass%Ti Alloy Nucleating on Ti Carbonitride

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Viscosity Measurement of Foam with High Gas Volume Fraction Using Sphere Pull-up and Dam-break Experiments

Shinya Sugi, Yoshihiko Higuchi

DOI:

10.2355/tetsutohagane.TETSU-2024-108

Abstract

Viscosity measurements of a gas-liquid two-phase fluid (foam) with fine bubbles were conducted using a sphere pull-up method and the flow behavior in dam-break experiments was evaluated. The following results were obtained.

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Viscosity Measurement of Foam with High Gas Volume Fraction Using Sphere Pull-up and Dam-break Experiments

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Phase-field Simulation of the Stability of Fe2Al5 Phase at Fe/molten Zn–Al interface

Shunsuke Shiotani, Yuhki Tsukada, Toshiyuki Koyama

DOI:

10.2355/tetsutohagane.TETSU-2024-110

Abstract

The stability of η-Fe2Al5 phase at α-Fe/molten Zn–0.1Al (wt.%) (L) interface at 723 K in Fe–Zn–Al ternary system was investigated by phase-field simulations. Thin layers of intermetallic compound (IMC) phases (η，Г-Fe3Zn10，Г1-Fe5Zn21 and δ1-FeZn7) were placed between the α and L phases, and the growth of the IMC layers and the atomic diffusion of constituent elements along the direction perpendicular to the α/L interface were calculated by one-dimensional phase-field simulation. The simulation result showed that Г and Г1 phases dissolved, and thin η phase and thick δ1 phase remained stable at the α/L interface. Moreover, several phase-field simulations were performed by varying the values of interdiffusion coefficients in each phase. The simulation results showed that the diffusion and partitioning behaviors of Al have a significant effect on the stability of IMC layers at the α/L interface. It was found that the partitioning of Al to the α phase was suppressed due to the fact that the value of interdiffusion coefficient in the α phase was several orders of magnitude smaller than those in the IMC phases. The resultant Al partitioning to the IMC phases was the direct cause of the stabilization of the η phase and the destabilization of the Г and Г1 phases.

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Phase-field Simulation of the Stability of Fe2Al5 Phase at Fe/molten Zn–Al interface

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Trial for Estimation of One-dimensional Fraction Solid in the Horizontal Cross Section of Unidirectionally Solidified Specimen

Hisao Esaka, Akira Takahashi, Masashi Nakamoto, Takeshi Yoshikawa

DOI:

10.2355/tetsutohagane.TETSU-2024-119

Abstract

In order to clarify the location of secondary inclusions formed during solidification, this study proposed a function to estimate the one-dimensional fraction solid at any position from the cross-section of unidirectionally solidified specimen. The function is based on an n-th order function expressed as a combination of the dimensionless distance (r/R) defined within the dendritic domain and the rotation angle θ measured from a specific secondary branch. The function has a simple form and uses only one parameter but the estimated values of fraction solid are ideal both for one-dimensional and two-dimensional ones. In addition, the predicted shape of the cross-sectional structure of the dendrite that can be derived from the proposed function is reasonable. It was also revealed that it is possible to express a variety of shapes from a cell shape without secondary branches to a dendrite shape with well-developed secondary branches, by changing the value of parameter n.

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Trial for Estimation of One-dimensional Fraction Solid in the Horizontal Cross Section of Unidirectionally Solidified Specimen

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Corrosion Resistance of Ni-coated Steel Sheets in Lithium-ion Battery Electrolyte

Misaki Masatsugu, Shintaro Yamanaka, Takehiro Takahashi, Kiyokazu Ishizuka

DOI:

10.2355/tetsutohagane.TETSU-2024-102

Abstract

In order to improve both performance and safety of lithium-ion batteries, we investigated the use of steel sheets which have a higher melting point than aluminum currently used for cell cases of lithium-ion batteries, for cell cases. First, a coating metal that can suppress Fe dissolution was selected, because corrosion resistance to battery electrolyte is important for battery cell cases. We found that Ni has high corrosion resistance to battery electrolyte, and that Ni-coated steel sheets can reduce the risk of short circuits due to decrease in Fe dissolution and re-deposition compared to non-coated steel sheets.

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Corrosion Resistance of Ni-coated Steel Sheets in Lithium-ion Battery Electrolyte

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Improvement of Nitrification of Coke-oven Wastewater Treatment by Adding Iron Source

Toyoshi Yamaguchi, Mariko Shinoda, Atsuharu Inoue, Tomomichi Nakamura, Ryota Murai, Katsunori Takahashi

DOI:

10.2355/tetsutohagane.TETSU-2024-111

Abstract

The coke-oven wastewater contains high concentrations of ammonia, COD (chemical oxygen demand) and toxic compounds such as phenols, cyanides, and thiocyanate. This wastewater is one of the most toxic industrial wastewater. Although the activated sludge process has been applied to treatment of the coke-oven wastewater, the treatment was occasionally deteriorated by inhibition of toxic compounds. Especially, the nitrification step was sensitive to toxic compounds, nitrite (NO2-) was often accumulated due to incomplete nitrification. Nitrite is toxic and has a negative effect on COD degrading bacteria. The complete nitrification was required for stable treatment.

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Improvement of Nitrification of Coke-oven Wastewater Treatment by Adding Iron Source

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Effects of Interface Anisotropy on the Solidification Morphology of Zinc Alloys and Development of Data Assimilation for Their Estimation

Ayano Yamamura, Hideyuki Yasuda, Tomohiro Takaki

DOI:

10.2355/tetsutohagane.TETSU-2024-126

Abstract

The solid–liquid interface energy anisotropy of Zn alloys remains poorly understood. Recently, characteristic 14-arm dendritic growth has been observed using time-resolved X-ray computed tomography at SPring-8 during the solidification of a Zn-4mass%Al alloy. This study investigates the dependence of the dendrite morphologies of Zn alloys on solid–liquid interface energy anisotropy through systematic phase-field simulations of the growth of an isolated equiaxed dendrite. We also develop a data assimilation system to estimate the anisotropy parameters of solid–liquid interface energy and crystal orientation in Zn alloys and validate the system through twin experiments. This study provides insights into the solidification of Zn alloys and a powerful tool for their investigation.

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Effects of Interface Anisotropy on the Solidification Morphology of Zinc Alloys and Development of Data Assimilation for Their Estimation

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In-situ Observation of Inclusion Formation Behaviors during Solidification Process Using Model Alloy

Sakiko Kawanishi, Yuki Tsukahara, Shingo Terashima, Haruto Nakao, Sohei Sukenaga, Hiroyuki Shibata

DOI:

10.2355/tetsutohagane.TETSU-2024-120

Abstract

The formation of secondary inclusions during the solidification process of molten steel is a complex phenomenon triggered by microsegregation. Controlling the dispersion of secondary inclusions in the solidified steel is an important issue that greatly affects the properties of the steel; however, the distribution of inclusions after solidification does not always coincide with the locations of inclusion formation. Therefore, it is still difficult to estimate when, where, and at what supersaturation level inclusions crystallize in the liquid phase, and it is desirable to clarify their formation behavior to control the dispersion of inclusions. In this study, we investigated the formation process of inclusion using a ternary model material of succinonitrile-water-lumogen yellow by in-situ observation, where the formation of oversaturated lumogen yellow can be regarded as the inclusion formation. It was confirmed that the frequency of inclusion formation increased significantly when the solution was held at lower temperatures, i.e., when a large supersaturation ratio was given. The results of the formation frequency indicated that the formation of inclusions occurred in the liquid phase according to the classical nucleation theory.

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In-situ Observation of Inclusion Formation Behaviors during Solidification Process Using Model Alloy

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Crack Initiation and Propagation Mechanism of Rapidly Tempered High-Si Steel during Hydrogen Embrittlement

Manami Sunako, Masataka Mizumoto, Ren Tanaka, Shintaro Kumai, Yuichiro Yamauchi, Azusa Ooi, Eiji Tada

DOI:

10.2355/tetsutohagane.TETSU-2024-123

Abstract

In this study, we focused on fine dispersion of Fe2-3C (ε) by combining the addition of Si and rapid tempering to improve the hydrogen embrittlement resistance of high-strength spring steels. The aim of this study is to clarify the fracture mechanism of rapidly tempered high-Si steels: the JISSUP7 (2.0Si) and SAE9254 (1.4Si) spring steels were tempered at different tempering rates by induction (IH) and furnace heating (FH) methods. Bending test were carried out during the cathodic hydrogen charging to observe the fracture origin and morphology of the steels. The size and volume of carbides were quantified using small-angle X-ray scattering method (SAXS) and synchrotron radiation XRD. The distribution of carbides was observed with the replica method: facets were observed at the fracture origin of the 2.0Si-IH steel and 1.4Si-IH steel, which contained retained γ in the microstructure. It was considered that the facets formed because retained γ at the grain boundaries transformed into martensite during hydrogen embrittlement, promoting intergranular cracking; the 2.0Si-IH steel contained the largest amount of retained γ, but also contained fine Fe2-3C(ε) in the lath. This suggests that dislocations and hydrogen are less likely to accumulate at the grain boundaries, resulting in the longest fracture life. In other words, in rapidly tempered high Si steels (2.0Si-IH steels), the fine dispersion of Fe2-3C (ε) has more influence on the suppression of crack initiation and propagation than the increase in the amount of retained γ, and this contributes significantly to the fracture life.

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Crack Initiation and Propagation Mechanism of Rapidly Tempered High-Si Steel during Hydrogen Embrittlement

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Friction Stir Welding of Fe-15Mn-10Cr-8Ni-4Si Seismic Damping Alloy

Tomoya Nagira, Terumi Nakamura, Takahiro Sawaguchi, Masakazu Mori, Yoshiaki Morisada, Hidetoshi Fujii

DOI:

10.2355/tetsutohagane.TETSU-2024-127

Abstract

Friction stir welding (FSW) was applied to a 10 mm-thick plate for the Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy. A sound FSW joint was obtained successfully without macro-defects such as groove-like defects and tunnel holes. However, small pores with diameters of 1–5 μm were formed owing to the wear of the FSW tool during the FSW. The decrease in the heat input suppressed the tool wear. Consequently, the distribution of small pores was limited to the border of the stir zone at the advancing side under smaller heat input conditions. The stir zone of the FSW specimen produced at 125 rpm showed a higher tensile strength of 759 MPa owing to the grain refinement and the high elongation of 50% compared with the base metal. In addition, the stir zone exhibited a remarkable fatigue life of 9,723 cycles. This was higher than that of the base metal (8,908 cycles). Grain refinement occurred by discontinuous dynamic recrystallization (DDRX) via high-angle boundary bulging and direct nucleation in the high-dislocation area. The increase in the heat input suppressed the DDRX owing to the promotion of dynamic recovery.

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Friction Stir Welding of Fe-15Mn-10Cr-8Ni-4Si Seismic Damping Alloy

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Stability of Oxidation State of Fe in Organic Acid Complex

Masahito Uchikoshi, Shigeru Suzuki

DOI:

10.2355/tetsutohagane.TETSU-2024-134

Abstract

Oral iron medications, a treatment for iron deficiency anemia, frequently cause side effects such as nausea, vomiting, and diarrhea. The mechanism of toxicity has not been elucidated, and therefore measures to reduce toxicity have not been established. The influence of the oxidation state of iron is considered as one of the factors that may cause side effects, so the oxidation state of iron was confirmed using ultraviolet-visible absorption spectroscopy when oral iron medications dissolve in the stomach. The main components of oral iron medications are inorganic and organic acid iron complexes, with citric acid, fumaric acid, sulfuric acid, etc. used as ligands. The influence of the valence of the ligand on the stability of the oxidation state of iron was also examined.

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Stability of Oxidation State of Fe in Organic Acid Complex

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Formation of Micro- and Macro-segregation and Precipitation of Iron-Phosphide during the Solidification of High Phosphorous and Hyper-peritectic Steel

Hiroshi Harada, Kennya Sugimoto, Takuya Takayama

DOI:

10.2355/tetsutohagane.TETSU-2024-094

Abstract

In the production of high strength steel, it is necessary to improve the inner quality of steel slab. Among of the internal defects, the center segregation deteriorates the ductility and fatigue strength of high strength steel. Therefore, it is necessary to understand the generation mechanism of center segregation and develop the countermeasure technologies. In the center-segregated region of high phosphorous and hyper-peritectic steel slabs, iron-phosphide may be observed. However, the precipitation mechanism of iron-phosphide has not been fully clarified. Therefore, in this study, in order to clarify the formation mechanism of micro- and macro-segregation in the final solidification of CC slab, the melting behavior of the center segregation region of high phosphorous and hyper peritectic steel slab has been investigated by using high temperature observation equipment installed with the image furnace. Additionally, the enrichment of solute in the inter-dendritic region has been analyzed by using micro- segregation model with consideration of delta-gamma transition of steel. Under the assumption that the enriched solute in the inter dendritic region was transferred to the final solidification region because of fluid flow and the center-segregation was formed, the re-solidification of micro-segregated liquid was analyzed by using the modified micro-segregation model. At the same time, the possibility of precipitation of iron-phosphide was examined by using solubility limit of iron-phosphide. Moreover, under the basis of Fe-C-P ternary eutectic solidification system, the precipitation mechanism of iron-phosphide during the formation of center segregation was discussed.

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Formation of Micro- and Macro-segregation and Precipitation of Iron-Phosphide during the Solidification of High Phosphorous and Hyper-peritectic Steel

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Effect of Cyclic Stress Change on Creep Behavior in ASME P91 Steel

Kota Sawada, Yasushi Taniuchi, Takehiro Nojima, Kaoru Sekido, Tomotaka Hatakeyama, Kazuhiro Kimura

DOI:

10.2355/tetsutohagane.TETSU-2024-114

Abstract

Effect of cyclic stress changes on creep rupture strength was investigated at 600oC for MGQ and MGS heat of ASME P91 steel. There was no large difference of creep rupture strength among the heats. However, the creep rupture ductility of MGS was lower than that of MGQ in the long-term. The initial creep stress and stress after stress reduction was 120MPa and 24MPa to 84MPa, respectively. The time interval of stress reduction was 6 days for each test. For MGQ heat, no effect of stress reduction to 84MPa on creep rupture strength was observed. The stress reduction to 60MPa slightly increased time to rupture as compared to creep test under constant stress. A small amount of decrease in time to rupture was confirmed in case of stress reduction to 36MPa.Consequently, there was no tendency of effect of stress reduction on creep rupture strength for both heats. The cyclic stress change did not affect the martensitic lath structure and precipitates distribution after creep rupture.

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Effect of Cyclic Stress Change on Creep Behavior in ASME P91 Steel

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Effect of B on Surface Oxidation Behavior and Phosphatability of Si-Mn-added Cold-Rolled Steel Sheets

Shinichi Furuya, Tadachika Chiba, Daisuke Mizuno

DOI:

10.2355/tetsutohagane.TETSU-2024-106

Abstract

The effect of B on the surface oxidation behavior and phosphatability of cold-rolled steel sheets was investigated using 0.001 wt% B-added and B-free steels containing 0.6 wt% Si and 2.0 wt% Mn. The specimens were annealed at 800 ℃ in a 5 vol% H2-N2 atmosphere with a dew point of -50 ℃. The surface oxides of the annealed samples were analyzed by GD-OES, FT-IR, SEM-EDX and TEM. The annealed steel sheets were then subjected to zinc phosphate treatment, and the effect of the surface oxides on phosphatability was evaluated by SEM-EDX. In the initial stage of annealing, fine granular Mn2SiO4 mainly formed and film-like SiO2 partly formed on both steels. As the soaking time at 800 °C increased, the granular Mn2SiO4 increased in the B-free steel. In contrast, in the B-added steel, the granular Mn2SiO4 coarsened, MnSiO3, MnO and B2O3 formed, and the film-like SiO2 formation area expanded. Addition of B reduced the melting point, causing coarsening of Mn2SiO4, exposing the base steel. This results in a difference in the oxygen potential between the exposed area of the steel and the oxide covered area. This local inhomogeneity of the oxygen potential changes the surface oxide species of the B-added steel. To elucidate the reason for the poor phosphatability of the B-added steel, a SEM-EDS analysis of the steel surface in the initial stage of zinc phosphate treatment was conducted, revealing that the coarse Si-Mn complex oxides and large film-like SiO2 inhibited the zinc phosphate reaction.

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Effect of B on Surface Oxidation Behavior and Phosphatability of Si-Mn-added Cold-Rolled Steel Sheets

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Pore Shape Evolution in Nitrided SCM440H Compound Layer and Its Influence on Pitting Resistance during Rolling-Sliding Contact Fatigue

Takashi Maruyama, Osamu Umezawa

DOI:

10.2355/tetsutohagane.TETSU-2024-105

Abstract

Nitriding is a beneficial surface hardening heat treatment to reduce CO2 emissions and shows a potential partially replace carburized steels adopted for sliding parts such as bearings and gears. Both the bending fatigue strength and pitting fatigue strength under high contact pressure were evaluated for nitrided JIS-SCM440H materials using two types of controlled-nitridings, i.e. γ' and e, and conventional gas nitriding. Not only the thickness of the compound layer but also its phase structure such as γ' and e significantly affected the 107 cycles bending fatigue strength, where the γ'-nitrided material was the highest. On the other hand, almost no effects of the thickness and phase structure on the 107 cycles pitting fatigue strength were detected in the materials. Compared to the carburized material, the γ'-nitrided material exhibited higher 107 cycles fatigue strength in bending and pitting, but poor fatigue strength in the lower cycles range. The shape and size of the pores in the compound layer were altered during roller-pitching tests, although there was almost no reduction in the thickness of the compound layer. The pore size was reduced by pore closure, especially at higher contact pressures. Pore closure was more pronounced in the γ'-nitrided materials than in the ε-nitrided materials at lower contact pressures. The 107 cycles pitting fatigue strength was improved after the pore closure, resulting from fewer defects.

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Pore Shape Evolution in Nitrided SCM440H Compound Layer and Its Influence on Pitting Resistance during Rolling-Sliding Contact Fatigue

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Microstructure Development during Creep Deformation of 9Cr-1Mo-V-Nb Steel with Excess Nitrogen Introduced by Solution Nitriding – Multidimensional Scatter Diagram Analysis of STEM-EDS Maps by Machine Learning –

Tomotaka Hatakeyama, Shuntaro Ida, Kota Sawada, Kyosuke Yoshimi

DOI:

10.2355/tetsutohagane.TETSU-2024-115

Abstract

Creep deformation and precipitation behavior of 9Cr-1Mo-V-Nb steel with excess nitrogen introduced by solution nitriding were investigated. Precipitation of Cr2N phase was confirmed in addition to M23C6 and MX phases in the tempered microstructure. The creep strength of the steel was significantly reduced by solution nitriding, while the creep rupture elongation was increased. To characterize the complex precipitation behavior of the nitrogen-added steel, a machine learning-based clustering method of the multidimensional scatter diagram of the X-ray intensity of the alloying elements in each pixel of a STEM-EDS map was developed. Reduced number density of precipitates and enhanced coarsening kinetics of both Cr2N and MX were proposed as the mechanism of weakening caused by excess nitrogen.

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Microstructure Development during Creep Deformation of 9Cr-1Mo-V-Nb Steel with Excess Nitrogen Introduced by Solution Nitriding – Multidimensional Scatter Diagram Analysis of STEM-EDS Maps by Machine Learning –

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Role of Si Addition in Interfacial Reactions of Steel Sheets Hot-dipped in Zn-55%Al Alloy Melt

Yasuo Omi, Dasom KIM, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Suzue Yoneda

DOI:

10.2355/tetsutohagane.TETSU-2024-097

Abstract

This study was set to fundamentally understand the effect of Si addition on the interfacial reaction between Zn-55%Al alloy liquid (corresponding to a nominal composition of Al-25Zn (at%)) and Fe solid in the production process of GALVALUME steel sheets. The pure Fe sheets were hot-dipped in Al-25Zn and Al-25Zn-2Si (at%) alloy melts at 600, 650, and 700oC for 2~3600 s. Significantly thick coatings were formed on Fe sheets hot-dipped in the Al-25Zn binary alloy melt for a longer time than 10 s. The coating thickness became several millimeters after 30 s, resulting in a delamination of the coating. The significant Fe dissolution occurred in the Al-Zn binary alloy melt, accompanied by a significant growth of η phase (Fe2Al5) toward the solid Fe. The growth could be promoted by the Zn-rich liquid phase with a lower melting temperature. However, in the case of hot-dipping in the Al-25Zn-2Si ternary alloy melt, uniform coatings were formed on the hot-dipped Fe sheets due to the suppressed interfacial reactions. The Fe dissolution slightly occurred, and a continuous layer of Si-rich T5 (Fe2Al7.4Si) phase was formed at the interface of solid Fe with the Al-25Zn-2Si alloy melt. The continuous T5 phase layer would play a role in a diffusion barrier at the interface of solid Fe with liquid Al-Zn alloy, resulting in the suppressed interfacial reaction. These interfacial reaction processes are discussed based on thermodynamic calculations of the Fe-Al-Zn ternary and Fe-Al-Zn-Si quaternary systems.

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Role of Si Addition in Interfacial Reactions of Steel Sheets Hot-dipped in Zn-55%Al Alloy Melt

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Formation Behavior of Fe–Zn Intermetallic Layers at the Interface between Fe–Mn and Pure Zn Melt at 460°C

Suzue Yoneda, Naoki Takata

DOI:

10.2355/tetsutohagane.TETSU-2024-116

Abstract

The effect of Mn on the alloying reaction during hot-dip galvanization was investigated. The microstructure of the Fe–Zn intermetallic layers consisted of ζ, δ, and Γ phases for both pure Fe and Fe–2Mn (wt.%) alloy. The intermetallic layers grew thicker with increasing dipping time, and the growth rate of each layer was similar for both substrates. In the case of Fe–2Mn, the formation of the δ1p phase was observed after dipping for 2 s. However, δ1p formation was delayed for pure Fe, indicating that Mn may promote nucleation of the δ1p phase. It is known that the δ1p phase nucleates in the Fe-saturated ζ phase. The Fe content at the ζ/δ1p interface was found to be lower for the Fe–2Mn alloy by electron probe microanalysis, suggesting that the supersaturation of Fe for the nucleation of δ1p is decreased by Mn addition and Mn may stabilize the δ1p phase. Once δ1p became a continuous layer, the growth rates of the δ1p layer on pure Fe and Fe–2Mn were similar. Mn could affect only the nucleation of δ1p during the initial stage of the alloying reaction.

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Formation Behavior of Fe–Zn Intermetallic Layers at the Interface between Fe–Mn and Pure Zn Melt at 460°C

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Precipitation Behavior of MnS from Molten Iron to Al2O3 during Solidification

Akito Takeda, Takuma Kurokawa, Kengo Kato, Hideki Ono

DOI:

10.2355/tetsutohagane.TETSU-2024-096

Abstract

Forming conditions and compositional changes of primary inclusions in molten steel have been studied due to the demand for high cleanliness of steels. MnS, a common inclusion in steel, does not form in molten steel, although it is observed in steel with oxide inclusions such as MnO, Al2O3 and SiO2. On the other hand, Mn and S are enriched in molten steel due to the segregation phenomenon during the solidification process which suggests that MnS form in molten steel during solidification. However, the precipitation behavior of MnS inclusions in molten steel due to the enrichment of Mn and S and the interaction between the primary inclusion and the molten steel is still unclear. In this work, a new experimental technique was developed and the precipitation behavior of MnS from molten steel onto solid Al2O3 was studied. Solid MnS precipitates were observed on the Al2O3 rod immersed in the sample with adding Al whereas precipitates containing MnO, A2O3 and MnS were observed on the Al2O3 rod in the sample without adding Al. Thermodynamic analysis revealed that Mn enriched in molten steel is oxidized to form MnAl2O4 when Al content is low and the MnAl2O4 reacts with S in molten iron to form molten MnO-Al2O3-MnS. MnS can precipitate from the molten MnO-Al2O3-MnS. On the other hand, Mn enriched in molten steel does not react with Al2O3 when Al content is high. Therefore, MnS can precipitate at the final period of solidification where Mn and S are significantly enriched in molten steel.

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Precipitation Behavior of MnS from Molten Iron to Al2O3 during Solidification

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Development of Evaluation Method for Distribution of Inclusions in Micro Segregation / Structure of Unidirectional Solidified Specimen

Akira Takahashi, Anna Sciazko, Masashi Nakamoto, Masanori Suzuki, Hisao Esaka, Naoki Shikazono, Takeshi Yoshikawa

DOI:

10.2355/tetsutohagane.TETSU-2024-101

Abstract

The formation of inclusions during solidification in steelmaking process is a critical issue for the optimal processing and the quality of steel products. Therefore, it is required to clarify the mechanism on the inclusion formation for its adequate control. In the present work, the evaluation method of inclusion distribution via the combination of inclusion positions analysis and image analysis of dendrite structure with machine learning is proposed. Image analysis using a conditional deep convolutional generative adversarial network enabled the detection of domain boundaries and the directions of secondary dendrite arms in the cross-sectional structure of unidirectionally solidified specimens. In addition, by combining this with the analysis of inclusion position, a correlation was confirmed between micro segregation behavior and the formation behavior of TiN inclusions.

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Development of Evaluation Method for Distribution of Inclusions in Micro Segregation / Structure of Unidirectional Solidified Specimen

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Effect of Al Content on Precipitation Behavior of AIN Inclusions during Unidirectional Solidification Process of Fe-(0.5-2.0)%Al-2.0%Mn Alloys

Kenta Imai, Kengo Kato, Hideki Ono

DOI:

10.2355/tetsutohagane.TETSU-2024-099

Abstract

Mn-TRIP steels of which composition is mainly Fe–(0.5–3mass%)Al–(2–10mass%)Mn are expected to be new advanced high-strength sheet steels. During the solidification process of Fe–Al–Mn alloy, AlN inclusions precipitate at the grain boundary, which leads to the severe deterioration of hot ductility. However, the precipitation behavior of AlN inclusion is not known enough. In this work, a unidirectional solidification experiment of Fe–(0.5–2.0)mass%Al–2.0mass%Mn alloys and numerical analysis on the forming condition of AlN were carried out, and the precipitation behavior of AlN inclusions was studied. Al2O3 inclusions were observed in the alloy with 0.5 mass%Al. On the other hand, AlN inclusions were observed in alloys with 1.0, 1.5, and 2.0 mass%Al. The volume fraction of AlN inclusions increased with increasing Al content of the alloy. The thermodynamic analysis revealed that AlN is thermodynamically unstable at temperatures above the liquidus of the alloy. When Al content of molten steel is increased, AlN becomes thermodynamically stable. Accordingly, the forming amounts of AlN in the alloys during the solidification were analyzed considering the segregation. The results show that the precipitation of AlN inclusions increases significantly during solidification due to the enrichment of Al in the liquid phase. In the Fe–(1.0–2.0)mass%Al–2.0 mass%Mn alloy, Al2O3-AlN inclusions were also observed, where AlN is present around Al2O3. These inclusions are considered to be formed by the precipitation of AlN, which becomes stable as the Al concentration increases due to solidification segregation, on Al2O3, which is stable and precipitated in the early stage of solidification.

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Effect of Al Content on Precipitation Behavior of AIN Inclusions during Unidirectional Solidification Process of Fe-(0.5-2.0)%Al-2.0%Mn Alloys

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Evaluation of Activity Coefficients of Oxygen and Nitrogen in Molten Alloy and Its Dominant Factors Based on Solvation Shell Model

Masanori Suzuki, Yusuke Omi, Masashi Nakamoto, Takeshi Yoshikawa

DOI:

10.2355/tetsutohagane.TETSU-2024-100

Abstract

Activity coefficients of light elements in molten metals and alloys are important thermodynamic properties for refining and inclusion controls of metallic materials. Activity measurements of the activities of light elements in pure metals have been carried out by previous studies. Also activities in molten alloys have been previously investigated and summarized using interaction coefficients. However, it is still difficult to accurately explain the activities of light elements in molten alloys over a wide range of temperatures and concentrations. In this study, with the aim of unified understanding of the thermodynamic behavior and solubility of light elements in various molten alloys, we studied the activity coefficients of oxygen and nitrogen in molten alloys using solvation shell model, and examined the factors governing the activity coefficients of oxygen and nitrogen in molten alloys.

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Evaluation of Activity Coefficients of Oxygen and Nitrogen in Molten Alloy and Its Dominant Factors Based on Solvation Shell Model

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Change in Dislocation Density via Ausforming in Fe-5%Mn-C Alloy with Lath Martensitic Structure

Misa Takanashi, Ryota Hidaka, Kota Ohkubo, Takuro Masumura, Toshihiro Tsuchiyama, Satoshi Morooka, Takuya Maeda, Shuichi Nakamura, Ryuji Uemori

DOI:

10.2355/tetsutohagane.TETSU-2024-092

Abstract

The strengthening mechanism of ausforming in martensitic steels is believed to be due to the inheritance of dislocations in austenite by the subsequently transformed martensite. However, no studies to date have quantified the dislocation density before and after ausforming. In this study, the dislocation densities of Fe-5%Mn-C alloys were analyzed, and the relationship between hardening by ausforming and dislocation accumulation, as well as the effect of carbon on this relationship, were investigated. The hardness of ausformed martensite increased with the ausforming reduction in austenite, and the strengthening effect of ausforming increased with the addition of carbon. Similarly, the dislocation density of ausformed martensite increased with the ausforming reduction in austenite, and the dislocation accumulation by ausforming increased with the addition of carbon. Because the hardness of the ausformed martensite follows the Bailey–Hirsch relationship, the strengthening mechanism owing to ausforming could be explained by dislocation strengthening. To understand the dislocation accumulation process during ausforming, the dislocation density of austenite immediately after ausforming was measured by in-situ heating neutron diffraction. Consequently, the dislocation density of the ausformed austenite was not dependent on the carbon content, indicating that dislocations are not inherited in carbon-free steels. By contrast, in steels with sufficient carbon content, not only are dislocations inherited but additional dislocations are introduced during martensitic transformation.

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Change in Dislocation Density via Ausforming in Fe-5%Mn-C Alloy with Lath Martensitic Structure

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Quantitative Understanding of Solute Concentration Distribution by Microsegregation During Solidification

Sakiko Kawanishi, Shingo Terashima, Yuki Tsukahara, Sohei Sukenaga, Hisao Esaka, Hiroyuki Shibata

DOI:

10.2355/tetsutohagane.TETSU-2024-098

Abstract

Microsegregation of solute components during the solidification process causes solute pile-up in the liquid phase, which strongly affects the formation behavior of inclusions. However, there is no quantitative evaluation of solute concentration distribution during dendritic growth. In this study, we established an in-situ observation method for quantitative evaluation of solute concentration distribution using model materials with fluorescent reagents to clarify how the solute pile-up progresses due to microsegregation. In addition to evaluating the physical properties of the model materials necessary for this study, a quantitative evaluation of solute concentration distribution during dendritic growth was successfully achieved. Numerical analysis, taking into account the equilibrium partition of solute components and solute diffusion in each phase, reproduced the measured solute concentration distribution in the liquid phase. Thus, the solute concentration distribution was evaluated by the actual measurement and numerical analysis, and it was clarified that a relatively simple model can represent the progress of microsegregation.

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Quantitative Understanding of Solute Concentration Distribution by Microsegregation During Solidification

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Direct Observation of Atomic Arrangement in Multicomponent Calcium Ferrite Using Scanning Transmission Electron Microscopy

Kenta Takehara, Kohei Ikeda, Takashi Kawano, Takahide Higuchi

DOI:

10.2355/tetsutohagane.TETSU-2024-080

Abstract

To reduce the reducing agent ratio and CO2 emissions in blast furnace operation, it is important to control the material structure of sintered ore, which affects its metallurgical and mechanical properties. Multicomponent calcium ferrites (also called CF or SFCA (silico-ferrite of calcium and aluminum)), which is a type of melting and solidification structure, has attracted considerable interest recently, and the chemical composition and crystal structure of each CF have been researched. Although the crystal structure of CF has conventionally been analyzed mainly by XRD, the atomic arrangement could not be observed directly. Therefore, in this study, CF was investigated at the atomic level by scanning transmission electron microscopy (STEM). This research revealed that acicular CF, which was previously understood to be SFCA-I, has a SFCA (≠ SFCA-I)structure. It was also found that columnar CF had a non-periodic SFCA structure induced with a magnetite-like structure. Furthermore, a CF in which SFCA and SFCA-I were alternately stacked repeatedly was also discovered. This research clarified the fact that CF has a non-periodic structure at the atomic level.

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Direct Observation of Atomic Arrangement in Multicomponent Calcium Ferrite Using Scanning Transmission Electron Microscopy

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Solidification Characteristics and TiC Formation Behaviour in Alloy 800H

Shigeo Fukumoto, Yuto Sakaizawa, Shigeru Kaneko, Nobuhisa Ebihama

DOI:

10.2355/tetsutohagane.TETSU-2024-083

Abstract

It is known that the size distribution of inclusions in steels have a significant effect on material properties. The solidification characteristics and TiC formation behavior of alloy 800H were evaluated both by experiment and simulation in this work. The relationship between dendrite arm spacing and the cooling rate was estimated. TiC particles were observed at the interdendritic region. The size distribution of TiC particles was affected by solidification cooling rate. Solidification analysis using the MPF (Multi-Phase Field) method revealed that TiC formation begins at a solid fraction of 0.79, and solidification accelerates due to TiC formation. It was thought that TiC particles generated in the latter part of solidification aggregate and coalesce without engulfment by the solidified shell. The size distribution of TiC particles was also affected by heat treatment after solidification.

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Solidification Characteristics and TiC Formation Behaviour in Alloy 800H

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Effect of Alumina on the Phase Equilibria of the Iron-rich Corner of the CaO-SiO2-Fe2O3 System at 1240°C in Air

Amane Takahashi, Yukihiro Uchisawa, Hirokazu Sato, Takashi Watanabe, Rie Endo, Masahiro Susa, Miyuki Hayashi

DOI:

10.2355/tetsutohagane.TETSU-2024-079

Abstract

The effect of Al2O3 on the compositional region of silico-ferrite of calcium and aluminum (SFCA) and the liquid phase and the phase equilibria, including SFCA, was investigated in a CaO-SiO2-Fe2O3-5mass%Al2O3 system at 1240 °C in air. To obtain the desired composition, reagent-grade CaCO3, SiO2, Fe2O3, and Al2O3 powders were weighed, mixed, and equilibrated at 1240 °C in air. Each obtained sample was divided into two parts: one was pulverized into a powder and analyzed by XRD, and the other was subjected to microstructural observation and compositional analysis using EPMA. The results revealed that the compositional region of SFCA lies on the CF3-CA3-C4S3 plane and is C/S = 2.77–7.60 for 5 mass% Al2O3. Compared with the SFC composition region for 0 mass% Al2O3, the compositional range of SFCA extended in the CF3-C4S3 direction, suggesting that the addition of Al2O3 contributes to the stability of SFCA. Furthermore, the liquid-phase region was divided into a ferrite melt with a high Fe2O3 concentration and a silicate melt with a high SiO2 concentration, both of which shifted to the lower Fe2O3 side compared to the liquidus isotherm in the CaO-SiO2-Fe2O3 system. Unlike CaO-SiO2-Fe2O3, SFCA-I (SFC-I) was observed in the CaO-SiO2-Fe2O3-5mass%Al2O3 system, thus indicating that the addition of Al2O3 contributes to the stability of SFCA-I.

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Effect of Alumina on the Phase Equilibria of the Iron-rich Corner of the CaO-SiO2-Fe2O3 System at 1240°C in Air

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Equation of Cleavege Fracture and Grain Boundary Fracture Stress Based on Brechet-Louchet Model

Katsutoshi Hyodo, Yosuke Nonaka, Kazuma Itoh, Tetsuya Namegawa

DOI:

10.2355/tetsutohagane.TETSU-2024-074

Abstract

New fracture process model of cleavage fracture initiated from cementite crack was proposed. In addition, the equation of propagation of cementite crack into the ferrite grain was developed based on the Brechet-Louchet model. This equation can reproduce not only ferrite size dependence of cleavage fracture stress that the Petch model can reproduce but both of test temperature dependence and strain rate dependence of fracture stress. Furthermore, in exchanging surface energy for grain boundary cohesive energy in the equation, grain boundary fracture stress can be also estimated.

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Equation of Cleavege Fracture and Grain Boundary Fracture Stress Based on Brechet-Louchet Model

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Phase Stability and Thermal Expansion Properties of Additive Manufactured Super Invar alloy

Senlin Cai, Ryota Nagashima, Yaw Wang Chai, Naoki Sakaguchi, Nobuo Nakada

DOI:

10.2355/tetsutohagane.TETSU-2024-066

Abstract

Super invar alloy, Fe–32%Ni–5%Co, is widely utilized in precision instruments due to its remarkably low thermal expansion coefficient. Additive manufacturing holds promise for fabricating complex-shaped components with this alloy. This study investigated the phase stability and thermal expansion properties of super invar alloy fabricated via Laser Powder Bed Fusion (AM sample), comparing them to those of conventionally cast material (Re-melt sample). Microstructural analysis indicates that the AM sample has a more stable austenitic structure, attributed to minimal micro-segregation. Furthermore, it was observed that the thermal expansion coefficient decreases consistently with higher cooling rates within the temperature range of 400-300 K. As a result, AM sample exhibits lower expansion coefficient and it maintains at lower temperatures.

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Phase Stability and Thermal Expansion Properties of Additive Manufactured Super Invar alloy

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Effect of BN Surface Segregation on Coatability in Hot-dip Galvanizing of B-added Steel

Daisuke Tahara, Katsuya Hoshino, Shoichiro Taira

DOI:

10.2355/tetsutohagane.TETSU-2024-052

Abstract

Boron (B) is frequently used as additives to improve the hardenability of advanced high strength steel. It has been reported that B in steel reacts with atmospheric N2 during annealing at low oxygen potential (low dew point) to form boron nitride (BN) by the thermodynamical calculation. In this study, the effect of BN formation on the steel surface on the coatability during hot-dip galvanizing was investigated, experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperature and dew point, and then hot-dip galvanized. The annealed specimens were also prepared and analyzed with GD-OES, XPS, SEM-EDX and TEM-EELS to investigate the oxide and nitride formation on the steel surface during annealing. As results, coatability deteriorated as the amount of B in steel and the annealing temperature increase, and as the dew point decrease. These trends were not correlated with the amount of oxide but the amount of BN formation, suggesting that BN formation deteriorated the coatability. The surface and cross-sectional analysis revealed that BN formed around the oxide to cover the steel surface. This would lead the deterioration of the coatability because most of the steel surface was covered with BN as well as oxide, which are known to have low wettability with molten Zn.

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Effect of BN Surface Segregation on Coatability in Hot-dip Galvanizing of B-added Steel

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Effect of Re-ignition Method on Sinter Yield Through Improving Carbon Combustion Ratio at Upper Layer of Sinter Packed Bed

Masaru Matsumura, Ryota Kosugi, Yuichiro Yamamoto, Junji Nagata, Kenichi Higuchi

DOI:

10.2355/tetsutohagane.TETSU-2024-031

Abstract

Conventionally, it has been known that the product yield of the upper part of the sintering layer is extremely low, because of the heat loss caused by transferring heat toward the space above sintering layer, and of the large amount of unburned carbon in upper sintering layer.

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Effect of Re-ignition Method on Sinter Yield Through Improving Carbon Combustion Ratio at Upper Layer of Sinter Packed Bed

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Mechanical Properties and Microstructure of High Strength Steel for Fracture Suppression and High Absorbed Energy in Automobile Collision

Shinsuke Komine, Tatsuya Nakagaito, Shinjiro Kaneko, Yuki Toji, Tomohiro Sakaidani, Kentaro Sato

DOI:

10.2355/tetsutohagane.TETSU-2024-035

Abstract

A fundamental study on the axial crush performances of HSS (High Strength Steel) was carried out to clarify the effects of microstructure and mechanical properties on crashworthiness. Axial crush tests were performed to evaluate the crush performances of the HSS with different microstructures and mechanical properties and identify the fracture origins. The cracks in the press formed area were observed and the cracks led to the fractures. The high λ (Hole expansion ratio) steel showed excellent crush performances by crack suppression. Crash deformation in the press formed area was simulated by the ORB (Orthogonally Reverse Bending) fracture tests and the crack suppression factors were investigated. Through the ORB fracture test, it was clarified that the reduction of the hardness gaps between phases and the refinement of the hard phases (Fresh martensite) were effective for suppressing cracks in the press formed area. These microstructures were occurred by the Q&P (Quenching & Partitioning) process for increasing λ. Therefore, it was found that the microstructural design for increasing λ also contributed to excellent crush performances.

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Mechanical Properties and Microstructure of High Strength Steel for Fracture Suppression and High Absorbed Energy in Automobile Collision

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Formation Mechanism of Secondary Inclusions in Fe-36mass%Ni Alloy Using a Novel Combination Analysis Technique

Hiroshi Fukaya, Jonah Gamutan, Makoto Kubo, Shintaro Yano, Shigeru Suzuki, Takahiro Miki

DOI:

10.2355/tetsutohagane.TETSU-2024-018

Abstract

Controlling the size, number, and composition of secondary inclusions is vital in the production of high-quality steels. In this study, experimental and computational investigation of the relationship between secondary inclusion formation in Fe-36mass%Ni alloy and cooling rate was carried out. Assuming the case of large ingots, solidification experiments using various cooling rates (0.17 to 128 K/min) were employed and the size, number, composition, and distribution of inclusions were analyzed by SEM-EDS automatic inclusion analysis. Like previous studies, inclusion number density increased with increasing cooling rate, while inclusion size decreased with increase of cooling rate. On the contrary, oxide inclusion area fraction was found to have little relationship with the cooling rate and was instead found related with oxygen content of the sample. As a new attempt to investigate the relationship between microsegregation and secondary inclusion formation, a combination of SEM-EDS analysis and EPMA mapping analysis was carried out. By superimposing information of microsegregation and inclusions, it was found that high-Al2O3 inclusions formed during the early stage of solidification, whereas low-Al2O3 inclusions formed during the later stage of solidification. These findings suggest that Al2O3 inclusions formed in the early stage of solidification reacted with the remaining Si-enriched liquid steel and changed into low-Al2O3 inclusions. Experimental results were also confirmed by thermodynamic calculations. Present work made it possible to understand deeper the relationship between microsegregation and secondary inclusion formation.

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Formation Mechanism of Secondary Inclusions in Fe-36mass%Ni Alloy Using a Novel Combination Analysis Technique

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