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

Tetsu-to-Hagané Advance Publication

  • Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

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    DOI:10.2355/tetsutohagane.TETSU-2020-003

    High-speed steel type cast iron rolls were developed around 1990 and have been widely used in the former stands of hot strip mills. However, in the latter stands of the hot strip mills, the use of high-speed steel type rolls has been limited due to the insufficient crack resistance. Therefore, in order to improve the wear resistance of the latter stands, enhanced type high-nickel grain rolls in which MC type carbides of high hardness are crystallized in a conventional high-nickel grain roll has been developed. However, since the wear resistance of the enhanced type high-nickel grain roll is significantly inferior to that of the high-speed steel type roll. Therefore, The development of a new cast iron roll with superior wear resistance and is applicable to the latter stands of hot strip mills was studied. The present development roll has improved wear resistance by increasing the amount of addition of the high hardness MC type carbide forming elements. In addition, the reduction of the carbon equivalent for less amount of eutectic carbides resulted in the reduction of the residual stress down to the same level as the high-nickel grain roll, which improved the crack resistance. As a result, it was confirmed that the wear resistance was improved about three times compared with the conventional high-nickel grain rolls. In addition, the results suggest that the wear resistance of work rolls for the hot strip mill is largely controlled by the amount of MC carbides among the rolls with the same hardness.
  • In situ Observation of Abnormal Grain Growth of Austenite in Case Hardening Steel

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    DOI:10.2355/tetsutohagane.TETSU-2020-042

    In order to clarify the mechanism of abnormal grain growth of austenite in case hardening steel, in-situ observation by high temperature EBSD was performed using JIS SCM420 containing Nb (0.2C-0.2Si-0.8Mn-1.1Cr-0.2Mo-0.024Nb steel in mass%). After heating at 1143 K, two growing grains several times larger than the surrounding grains were observed. These growing grains grew abnormally by holding at 1193 K and were adjacent to each other. Since the boundary between the two abnormal grains is a twin boundary, the abnormal grains were observed as if they were one larger abnormal grain. The growth rate of abnormal grain is as high as the initial stage of growth and negligibly small at the latter stage of growth. That is, the grain size of abnormal grain growth of austenite is mainly decided by rapid grain growth for a short time after the start of grain growth. The generation of growing grain of abnormal grain growth of austenite is not affected by orientation and strain distribution. When distant grains which have a twin relationship are adjacent to each other in the grain growth process, grain connection occurs. It doubles the area surrounded by high angle grain boundaries without twin boundary. In addition to the encroachment of surrounding grains by larger grains, austenite grain connection through twin boundary also affects abnormal grain growth of austenite.
  • Development Technology for Prevention of Macro-segregation in Casting of Steel Ingot by Insert Casting in Vacuum Atmosphere

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    DOI:10.2355/tetsutohagane.TETSU-2020-034

    Important large components such as rotors for power generation steam turbines, pressure vessels and reaction vessels are manufactured by the ingot casting. However, it is difficult to manufacture the materials with sufficient properties due to macro-segregation of ingots.For the purpose of developing effective and versatile macro-segregation countermeasures in the casting of large steel ingots for manufacturing large parts for power plants, the insert casting in vacuum atmosphere, in which a core material with the same composition as the base steel is placed at the center of the steel ingot, was studied. The effectiveness of the proposed insert casting as a macro-segregation countermeasure was verified in laboratory experiments. In addition, it has been clarified that good bonding between the core material and the base material can be realized even under conditions where bonding by normal insert casting in air atmosphere is difficult.The smelting behavior of the core material and the solidification behavior of the molten steel in the experiments of macro-segregation reproduction casting and the insert casting were analyzed by the direct finite difference method. The mechanism by which this method suppresses the formation of macro-segregation and the solidification conditions for the suppression, the reasons and conditions for good bonding in this insert casting are clarified by the analyses.Furthermore, in the experiments of insert casting, the cause of internal cracks generated in the solidified shell on the core material surface was considered, and guidelines for preventing the internal cracks were presented.
  • Recovery of Zinc by Reaction between Electric Arc Furnace Dust and Calcium Chloride

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    DOI:10.2355/tetsutohagane.TETSU-2020-032

    The chlorination and volatilization behavior of zinc was investigated by the reaction between ZnO, ZnFe2O4 and CaCl2. Experiments were conducted in a nitrogen atmosphere by using horizontal electric furnace. As a result of changing the ratio of zinc oxide to zinc ferrite at 1000ºC, it was expected that ZnO and ZnFe2O4 were chloride volatilized at the same time. In the kinetic study at 950ºC-1030ºC, Jander’s equation showed the experimental result well in the reaction system of this study, and the activation energy was 237 ± 18 kJ / mol. It was also found that carbon influenced the reaction product determination. FeO and CaFe2O4 were formed in the presence of carbon, and Ca2Fe2O5 was formed in the absence of carbon.
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  • Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

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    DOI:10.2355/tetsutohagane.TETSU-2020-041

    In this study, the relation between the change of the magnetic properties and the creep strength with the addition of Co was investigated for ferritic steel containing 15 mass%Cr. Co addition up to 6 mass% hardly contributes to solid solution strengthening and precipitation strengthening at room temperature. However, in the range of 650ºC to 750ºC, it was confirmed that the addition of Co was effective for the creep strengthening because the steel with a larger amount of Co had higher creep strength. This creep strengthening is explained by a reduction in the diffusion rate associated with a change in magnetic properties by Co addition. The increase of the volume magnetization of the steel with increase of Co amount in the temperature range from room temperature to about 800ºC was confirmed. Comparing the difference in volume magnetization and the ratio of creep strain rate for steels with different amounts of Co, a clear correlation was found between the values. That is, at the temperature at which the difference in volume magnetization becomes maximum, the peak of the creep strain rate ratio was observed. This result is explained as follows. In a low temperature region where the magnetization is large or in a high temperature region above the Curie point of both steels, there is no significant difference in the creep strength between them. However, in the temperature where one steel loses the ferromagnetism but the other steel maintains ferromagnetism, a significant difference in the creep strength was observed.
  • Microstructure and Mechanical Properties of a Harmonic Structure Designed Fe-0.3 mass%C Steel

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    DOI:10.2355/tetsutohagane.TETSU-2020-007

    The microstructure and mechanical properties of harmonic structure designed Fe-0.3mass% carbon steel was investigated. The compacts of Fe-0.3 mass% carbon steel with conventional Homogeneous structure (Homo), and Harmonic Structure (HS) consisting of fine grains (Shell) and coarse grains (Core) were fabricated by a powder metallurgy method. The mechanical milling (MM) leads to the formation of nano ferrite grains at the deformed surface of MM powder particles. After sintering, the Homo and HS compacts had ferrite (α) and perlite (P) phases. The Shell had finer α + P phases than Core, and the fraction of the P in the Shell was larger than that in the Core. It was considered that the carbon segregation occurs at the deformed surface of MM powder particles due to nano ferrite formation. As a result, the number of austenite nuclei increases in Shell. Therefore, the HS compact has both the grain size gradient as well as a phase constituent gradient. As-sintered HS indicated superior mechanical properties compared to the Homo counterparts. The mechanical properties were improved by further heat treatments. Those as-sintered and heat-treated HS compacts indicated a large increase of ductility and tensile toughness. Such outstanding and unique mechanical properties of the HS were attributed to the enhancement of the local elongation after necking. These superior mechanical properties are considered to be due to the micro and macro synergy effects.
  • Crack Initiation and Propagation Behavior of Hydrogen-induced Quasi-cleavage Fracturein X80 Pipeline Steel with Stress Concentration

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    DOI:10.2355/tetsutohagane.TETSU-2019-126

    The processes leading to hydrogen-related fracture in X80 pipeline steel with stress concentration have been investigated comprehensively through observations of fracture surfaces and subsidiary cracks, a stress analysis, crack initiation and propagation analyses and a crystallographic analysis of fracture surfaces. Fracture morphology showed quasi-cleavage (QC) fracture under various amounts of hydrogen. It was found that QC cracks initiated in the area ranging from the notch tip to 100 μm inside based on interrupted tensile tests just before fracture strength with hydrogen charging. Moreover, fracture surface topography analysis (FRASTA) revealed that QC cracks initiated at the notch tip. A finite element analysis indicated that the equivalent plastic strain was maximum at the crack initiation site at the notch tip. In addition, a backscattered electron image showed that nanovoids of 50-250 μm in diameter were present near the initiation site. Regarding the crack propagation process, field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD) and FRASTA results indicated that some microcracks in ferrite grains coalesced stepwise and propagated. Trace analyses using EBSD revealed that the QC fracture surface consisted of {011} slip planes, {001} cleavage planes and non-specific index planes. These findings indicate that QC fracture initiates at the notch tip due to the interaction between dislocations and hydrogen associated with local plastic deformation, and propagates stepwise by coalescence through vacancies, nanovoids and microcracks on various planes associated with/without plastic deformation in ferrite grains.
  • Material Modeling of Hot-Rolled Steel Sheet Considering Differential Hardening and Hole Expansion Simulation

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    DOI:10.2355/tetsutohagane.TETSU-2020-002

    The elastic-plastic deformation behavior of a 440 MPa hot-rolled steel sheet subjected to many linear stress paths is precisely measured using biaxial tensile tests with cruciform specimens (ISO 16842: 2014) and multiaxial tube expansion tests (Kuwabara and Sugawara, 2013) to determine appropriate material models for finite element analysis (FEA). It was found that the Yld2000-2d yield function (Barlat et al., 2003) correctly reproduces the contours of plastic work (CPW) and the directions of the plastic strain rates (DPSR). Differential hardening (DH) models are determined by changing the values of exponent and material parameters of the Yld2000-2d yield function as functions of reference plastic strain. Moreover, FEA of the hole expansion forming of the test material is performed. The DH model correctly predicts the minimum thickness position that matches the fracture position of the specimen in experiment.
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