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

Tetsu-to-Hagané Vol. 106 (2020), No. 10

  • Recent Developments in Non-Oriented Electrical Steels

    pp. 683-696

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    DOI:10.2355/tetsutohagane.TETSU-2020-005

    This paper gives an overview of developments in the field of non-oriented electrical steels that are widely used in motor cores in recent years, focusing mainly on texture control. ND//<100> texture is recognized as the ideal texture for application to motor cores. To obtain a suitable texture, precise control is required during each manufacturing process. This involves adjusting the chemical components during the steel making, fine temperature and reduction control during hot rolling, temperature control during hot-band annealing, temperature and reduction control during cold rolling and heating rate and temperature control during final annealing. High silicon Fe-Si alloy containing 6.5 mass% Si has been recognized as a promising core material for high frequency applications. When producing high silicon steel by the continuous chemical vapor deposition (CVD) siliconizing process, materials with a gradient Si concentration in the sheet thickness direction have been found to display superior high-frequency iron loss.
  • Microstructure and Mechanical Properties of a Harmonic Structure Designed Fe-0.3 mass%C Steel

    pp. 735-744

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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.
  • Effect of Wettability on Penetration and Flotation Behavior of a Particle in Refining Process

    pp. 697-707

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    DOI:10.2355/tetsutohagane.TETSU-2020-020

    Powder blasting is often operated in refining process in order to achieve lower sulfur content in molten steel with high desulfurization efficiency. In this study, effects of wettability of a particle on penetration and flotation behavior were examined by water model experiment. A polypropylene particle was blasted onto water surface with Ar gas through a single-hole nozzle, and particle behavior during penetration into water to flotation to water surface was recorded by a high-speed camera. Wettability between the particle and water was changed by applying repellent or hydrophilic material on the particle. According to penetration of the particle, an air column was generated and a residual bubble was remained on the particle after rupture of the air column. The repellent particle floated to the water surface in a short time because maximum penetration depth was small and diameter of a residual bubble was large. On the other hand, the detention time of the hydrophilic particle became longer than the repellent particle because maximum penetration depth was relatively large and the residual bubble was separated from the particle. The reason that wettability between the particle and water affects penetration and floatation behavior is that adhering position of the air column on the particle changed. In the case of repellent particle, the position changed to penetrating direction of the particle. Therefore, the force caused by surface tension of water increases, and the residual air column on the particle after rupture of the air column becomes large.
  • Agglomeration and Removal of Alumina Inclusions in Molten Steel with Controlled Concentrations of Interfacial Active Elements

    pp. 708-718

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    DOI:10.2355/tetsutohagane.TETSU-2020-025

    In this study, Al deoxidation experiments have been performed in a mildly stirred steel bath with controlled O and S concentrations, to investigate the effects of interfacial active elements on the agglomeration and removal of Al2O3 inclusions in molten steel. The decrease rate constants of total Al2O3 inclusions, Al2O3 cluster inclusions, and Al2O3 single inclusions as well as the maximum average diameter of Al2O3 cluster inclusions decrease with increasing O and S concentrations in molten steel. However, the effect of O is much greater than that of S. These experimental results have been analyzed based on the kinetics of Al2O3 inclusion removal and the interfacial chemical interaction between Al2O3 inclusions in molten steel. The following findings have been obtained on the agglomeration and removal mechanisms of Al2O3 inclusions in molten steel. The Al2O3 inclusions in molten steel are removed by a mechanism whereby large Al2O3 cluster inclusions, formed by Al deoxidation, float and separate while repeatedly agglomerating and coalescing with fine Al2O3 single inclusions suspended in molten steel. The agglomeration of Al2O3 inclusions during floating and separation can also be explained by a mechanism whereby the agglomeration force due to the cavity bridge force is exerted between the Al2O3 inclusions and the Al2O3 inclusions come in complete contact when the Al2O3 inclusions with thermodynamically agglomerating tendency are approaching each other. The effects of O and S interfacial active elements are considered in both these mechanisms.
  • In-situ Evaluation Method for Crack Generation and Propagation Behaviors of Iron Ore Burden during Low Temperature Reduction by Applying Acoustic Emission Method

    pp. 719-726

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    DOI:10.2355/tetsutohagane.TETSU-2020-028

    In the field of ironmaking, there are few in-situ non-destructive techniques for systematically evaluating the reduction behaviors of blast furnace (BF) burden materials. The standard method for the evaluation of the reduction disintegration index (RDI) includes low temperature reduction with a constant gas composition at a constant temperature followed by a cold strength test after cooling. During the reduction of iron ore burden in the BF, reduction disintegration proceeds in the course of increasing temperature through crack generation and propagation caused by volume expansion due to the reduction from hematite to magnetite. Acoustic emission (AE) method is an in-situ non-destructive technique to evaluate the crack generation and propagation phenomena in various fields. In this study, we first attempted to apply AE method for a detailed in-situ observation of BF burden materials by a combinational experiment of reduction disintegration. In the case of single-particle reduction, it was possible to detect AEs without friction between the sample and the waveguide giving rise to noise. Although a large number of AEs were measured during cooling of both sinter and pellet, the AE energy of the sinter was larger than that of the pellet. Furthermore, a significant number of AEs were also detected during heating of lump ore containing larger amount of combined water indicating possibility to evaluate the decrepitation behavior of such ores.By the packed bed reduction tests, which could evaluate average properties of iron ore burden, it was found that the AE energy attributed to thermal stress observed during cooling was higher than that to reduction degradation. These results suggest that it will be possible to eliminate the influence of crack generation within the iron ore burdens in the cooling stage by applying the in-situ non-destructive evaluation method developed in this study. It is also expected to apply for the quantitative estimation of the reduction disintegration behavior of burdens in the working BF.
  • Analysis of Liquid Phase Mass Transfer at Free Surface in Gas-stirred Vessel

    pp. 727-734

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    DOI:10.2355/tetsutohagane.TETSU-2020-022

    Gas injection has been widely used to enhance mass transfer and homogenization of temperature and composition of molten steel. In this paper, water model experiments for degassing were performed to investigate mass transfer at free surface in a gas-stirred vessel. Mass transfer coefficients in liquid phase were measured and the correlation with gas flow rate and liquid height were analyzed. A mass transfer model was constructed based on the Levich’s model assuming that a part of energy input by gas injection was consumed near the free surface. As a result, the following dimensionless correlation was obtained:Sh=23[d2v ρ(hvϵ/d)3/4σ1/2Sc1/2The present correlation agrees well with the mass transfer coefficients in liquid phase including previous data for water model systems and that of molten steel systems can be also approximately estimated.
  • Quantitative Study of the Hydrogen Entry Behavior of Low Alloy Steels for Various Sour Environments

    pp. 745-750

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    DOI:10.2355/tetsutohagane.TETSU-2020-044

    Hydrogen entry behavior was investigated with different H2S partial pressures over pH 5.0 and was quantified for various sour environments using American Petroleum Institute grade X65 line pipes and line pipe plates.Hydrogen permeability dramatically decreased for H2S partial pressures of 0.1 MPa exceeding pH 5.5 and for 0.01 MPa exceeding pH 6.2. This is caused by the formation of a stable iron sulfide film. On the other hand, hydrogen permeability proportionally decreased with increasing pH for H2S partial pressure of 0.001 MPa up to pH 6.2. The critical pH at which iron sulfide becomes quite stable was predicted from the equation of the relationship among pH, H2S partial pressure, and iron ion activity based on potential vs. pH in Fe-S-H2O. Hydrogen concentration into steel invading from various sour environments was proposed and quantified.
  • Analysis of Three-dimensional Structure and Formation Mechanism of Degenerate Pearlite in Eutectoid Steel

    pp. 751-756

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    DOI:10.2355/tetsutohagane.TETSU-2020-010

    We investigated the three-dimensional morphology and crystallographic characteristics of cementite in degenerate pearlite in eutectoid steel using FIB-SEM. The three-dimensional morphology of cementite in degenerate pearlite was rod-like and/or incomplete plate-like structures. Combining the three-dimensional image with EBSD measurement data revealed that cementite grows in the low index direction. In addition, we also investigated the relationship between the pearlite morphology and the deviation angle from K-S orientation relationship (K-S OR) between austenite and pearlite during phase transformation. The deviation angle from the K-S OR with austenite in the pearlite growth direction side affected the microstructural evolution. Coarse degenerate pearlite was formed when the deviation angle from K-S OR is small, and fine degenerate pearlite and lamella pearlite tended to be formed as the deviation angle increases. From these results, we concluded that the pearlite morphology including degenerate pearlite can be classified by the deviation from K-S OR with austenite.
  • Effect of Block Size on the Reduction Rate of Dislocation Density during Tempering Treatment of Lath Martensite in Low Alloy Steel

    pp. 757-764

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    DOI:10.2355/tetsutohagane.TETSU-2020-023

    The effect of block size on the reduction rate of dislocation density in quenched and tempered martensite steels was investigated. Three different prior austenite grain sizes were chosen by applying different heat treatment patterns using 0.5mass%C-1mass%Cr- 0.7mass%Mo steel. The block size of lath martensite became finer as the prior austenite grain size became finer. The dislocation density decreased with increase in the holding time at 873 K. In the quenched martensite steel, the dislocation density was independent of the prior austenite grain size. On the other hand, in the martensite steel tempered at 873 K for 3.6×105 s, the dislocation density decreased as the prior austenite grain size became finer. The recovery coefficient m was adopted to discuss the reduction rate of dislocation density. It has been found that m increases as the block size becomes finer and that m increases linearly with the inverse of the block size.
  • Effect of Soft-Fine Particle Peening on Rotating Bending Fatigue Properties of Gas Carburized SCM420H Steel

    pp. 765-776

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    DOI:10.2355/tetsutohagane.TETSU-2020-013

    In order to examine the effects of soft-fine particle peening (Soft-FPP) on the fatigue properties of gas carburized SCM420H steel, rotating bending fatigue tests were conducted for carburized SCM420H steels treated with FPP using shot particles having different hardness. The Soft-FPP treated steels pre-treated with carburizing showed higher fatigue life and fatigue strength at 107 cycles compared with the only-carburized steels. This is attributed to the increase of surface hardness and the generation of compressive residual stress induced by FPP. The carburized steels treated with FPP failed in the subsurface fracture mode from the singular microstructure beneath the carburized layer in the long life region. The transition stress for the carburized steels treated with FPP, at which the surface fracture mode changed to the subsurface one, tended to increase with the hardness of shot particles used in FPP. Furthermore, the release behavior of residual stress induced by FPP during fatigue tests was also examined. There were no noticeable differences in compressive residual stress after the fatigue tests between the two types of carburized specimens treated with FPP; therefore, the Soft-FPP treated steels exhibited same fatigue strength as the steels treated with FPP using hard shot particles. These results suggest that the effect of hardness of shot particles used in FPP on the fatigue properties of carburized steels is not observed.

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