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

Tetsu-to-Hagané Advance Publication

  • Effect of Laminated Structure on Mechanical Properties of Composition-modulated Co-Ni Laminated Plating

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    DOI:10.2355/tetsutohagane.TETSU-2018-140

    A composition-modulated Co-Ni laminated plating has been developed to extend the lifetime of molds used for continuous steel casting. We have investigated the relationship between the laminated structure and the mechanical properties of the plating films. The tensile strength of as-plated film increased with increasing thickness of the constituent layers, while the elongation did not depend on the thickness of the constituent layer and remained almost constant in the range from 3 to 5%. Heat treated at 400°C improved the tensile strength and the elongation. The improvement in the elongation was remarkable, and the elongation reached 13% in the film composed of layers with a thickness of 0.8 μm. The layer with low Ni content had an hcp structure, and that with high Ni content comprised two phases of the hcp and fcc structures in the as-plated state. By the heat treatment, the high Ni-content layer turned into the single fcc phase, while the low Ni-content layer kept the hcp phase, and accordingly, the film structure changed into the one where the lamination of the hcp and fcc layers was distinct. The fact that the fcc layers, which was easily deformed, were formed continuously in the lateral direction, was seemed to contribute to the significant improvement in the elongation after heat treatment.
  • Extraction of Phosphorus and Recovery of Phosphate from Steelmaking Slag by Selective Leaching

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    DOI:10.2355/tetsutohagane.TETSU-2018-128

    In steelmaking slag, phosphorus is distributed between liquid phase and solid solution of 2CaO·SiO2 and 3CaO·P2O5 at refining temperature. Using the difference in water solubility of the solid solution and the other phases, we are developing the process to separate phosphorus by acid leaching. In this paper, the optimum condition of leaching by nitric acid by the control of valency of Fe in slag was investigated and the recovery of phosphorus from leachate was also conducted. To dissolve the solid solution, pH has to be decreased to 3 and the solid solution containing FeO showed lower dissolution ratio. To suppress the dissolution of other phases in slag at this pH, slow cooling to avoid the formation of glassy phase was necessary. The artificially steelmaking slag with the appropriate composition was made and subjected to the leaching experiment. The dissolution ratio of phosphorus has reached about 91% and the phosphorus content in the residue was enough low. After the separation of residue, pH of the leachate increased to precipitate phosphate. At pH=7, 80% or more of the phosphorus in the leachate was precipitated and the phosphate content of the precipitate was about 25% after calcination.
  • Time-resolved and In-situ Observation of δ-γ Transformation during Unidirectional Solidification in Fe-C Alloys

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    DOI:10.2355/tetsutohagane.TETSU-2018-145

    Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that the massive-like transformation, in which the γ phase was produced through the solid – solid transformation and the partition of substitute elements such as Mn and Si at the δ/γ interface could be negligibly small, was selected in the unidirectional solidification of 0.3 mass%C steel at a pulling rate of 50 μm/s. The massive-like transformation produced fine γ grains in the vicinity of the front of δ/γ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δ/γ front and the advancing front of coarse γ grains was as short as 200 μm. Namely, the fine γ grains disappeared within 10 s by the growth of coarsen γ grains along the temperature gradient. In addition, the observation of the δ/γ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like growth of γ phase occurred at a growth rate of 5 μm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during conventional solidification processes.
  • Effect of Prior Structure to Intercritical Annealing on Rapid Formation of Ultrafine Ferrite + Austenite Structure and Mechanical Properties in 0.1%C-2%Si-5%Mn Steels

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    DOI:10.2355/tetsutohagane.TETSU-2018-121

    Ultrafi ne ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation AHSS. This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafi ne grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32000 MPa%) which is superior to conventional TRIP steels is also obtained.
  • Effect of Initial Microstructure on Creep Strength of ASME Grade T91 Steel

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    DOI:10.2355/tetsutohagane.TETSU-2018-066

    In order to understand the cause of heat-to-heat variation in creep strength of Grade T91 steels, influence of initial microstructure on creep strength has been investigated. Distribution of chromium concentration considered to be a remained segregation was observed, corresponding to a lamellar contrast parallel to longitudinal direction of boiler tube. Standard deviation (SD) of ΔCr was employed as an indicator of degree of segregation, and good correlation was recognized between SD of ΔCr and creep rupture life at 650°C. Remained segregation was reduced by renormalizing heat treatment at not 1250°C, but 1200°C. Creep rupture life of the steel subjected to renormalizing heat treatment at 1200°C and tempering at 760°C, followed by normalizing and tempering with a standard heat treatment condition for Grade T91 steel, was prolonged for 2.3 to 2.8 times. Strengthening effect of renormalizing at 1200°C to reduce a remained segregation was confirmed by creep tests for up to about 10,000h at 600 and 650°C. Decreases in number density of M23C6 carbide particles, length of high angle boundaries and average KAM value during creep exposure have been promoted by a presence of remained segregation. Since diffusion is enhanced by concentration gradient of elements, degradation due to microstructural change is promoted by a presence of remained segregation. Segregation should be reduced to obtain high creep strength with homogenized concentration of chemical composition.
  • Effect of Heat Treatment on Peel Strength of 16Cr-Ferritic Stainless Steel / Aluminum Rolled Clad Sheet and Proposed Joint Mechanism

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    DOI:10.2355/tetsutohagane.TETSU-2018-133

    The peel strength of 16Cr-ferritic stainless steel / aluminum rolled clad sheet is improved by heat treatment at 300 to 500 °C after clad rolling. The joint mechanism was studied by analyzing the fracture surface after the peel test, taking into account the characteristics of the peel test method.The detailed investigation into the fracture surface after peel test allows us to classify fracture sites into three categories: 1) ductile fracture in aluminum base material, 2) flake-like fracture in 16Cr-stainless steel, and 3) fracture in the intermediate interface layer between aluminum and 16Cr-stainless steel. It was revealed that the intermediate layer breakage mainly occurs in the as-rolled clad sheet; whereas after heat treatment at 300 to 500°C, the aluminum base material breakage predominantly occurs.Close observation into the state of deformation at the crack tip of the peel tested specimen leads us to conclude that clad rolled and subsequently annealed aluminum undergoes plastic deformation during the peel test because significant softening easily takes place preferentially in aluminum during low temperature heat treatment. This might imply the decrease in peel strength by heat treatment, which is different from the fact. However, the load area responsible for peeling force during the peel test is inferred to substantially increase, which gives rise to the increase in peel strength. This is because the peel strength is simply evaluated by dividing the peel force by the width of the specimen.
  • First-principles Calculations of the Effects of Mn, Cr, and Ni on Hydrogen Diffusion in BCC, FCC, and HCP Fe

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    DOI:10.2355/tetsutohagane.TETSU-2018-070

    The effects of Mn, Cr, and Ni addition on the hydrogen diffusion behavior in BCC, FCC, and HCP Fe was investigated by means of first-principles calculations. Diffusion coefficients were estimated quantitatively from the migration energy calculated by the nudged elastic band method and the vibrational energy at every stable and metastable site. The addition of Mn, Cr, and Ni to a BCC lattice has a blocking effect on hydrogen diffusion and decreases the diffusion coefficient of hydrogen. Crystal orbital Hamilton population (COHP) analysis revealed that the weakened bonding between the added element and hydrogen is the origin of the blocking effect. On the other hand, the addition of Mn, Cr, and Ni to FCC and HCP lattices resulted in the formation of hydrogen trap sites. In the FCC case, the diffusion coefficients of Fe31MnH, Fe31CrH, and Fe32H, showed similar values, while that of Fe31NiH was lower. In the HCP case, the diffusion coefficients of the three additional elements showed a decreasing trend. Based on the results of the COHP analysis, we conclude that the octahedral interstitial sites around the additional elements become trap sites in FCC and HCP Fe due to the strengthened bonding between Fe and H.
  • Crystal Plasticity Analysis on Ductility of Ferrite/Cementite Multilayers: Effect of Dislocation Absorption Ability of the Hetero Interface

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    DOI:10.2355/tetsutohagane.TETSU-2018-082

    Strain hardening behavior of ferrite layers in the microstructure of drawn pearlite wire is studied theoretically and numerically. It is shown that stress field associated to dislocations could diminish quickly if the dislocations enter the phase or grain boundaries and decompose into smaller segments to distribute along the boundary. Some atomistic simulations of single-phase media validate this phenomenon; dislocations show to pass, decompose or accumulate on tilt-type grain boundaries depending on their atomistic configuration. Mechanical responses of nine-layered pearlite models subjected to tensile load are analyzed by a strain gradient crystal plasticity finite element code, where possible passage or absorption of dislocations is expressed in the model of dislocation mean free path. The critical resolved shear stress for slip systems consists of the lattice friction, the Taylor and Orowan terms and the strain hardening is given by the Taylor one. The density evolution of accumulated dislocations is evaluated by the model of Kocks and Mecking where the dislocation mean free path plays a major role. Results show that the smaller the dislocation absorption ability of the phase boundary and thinner the layer thickness, larger the strain hardening becomes. Slip localization in cementite layers is shown to be suppressed when the strain hardening of ferrite layers is higher, and this trend is consistent with results obtained in previous studies by molecular dynamics simulation and classical elasto-plasticity analyses. Scale sensitive phenomena taking place at phase boundaries in layered structure are briefly discussed in views of atomistic process and continuum mechanics.
  • Structural and Mechanical Characterizations of Top Dross in a Molten Zinc Bath

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    DOI:10.2355/tetsutohagane.TETSU-2018-127

    In a molten zinc bath in a continuous galvanizing line (CGL), top dross particles crystallize as Fe-Al-Zn intermetallic compounds. These particles easily adhere to the steel sheets causing surface defects. Therefore, controlling the top dross particles is a key issue. The present study focused on the structural and mechanical characterizations of top dross particles using an electron probe micro analyzer, X-ray diffraction, electron back scattering diffraction, Vickers hardness measurement and nano-indentation measurement. The following results were obtained: (1) The crystal structure of top dross particles Fe2Al5Znx having Fe: 37~38 wt%, Al: 44~45 wt% and Zn: 18~19 wt% belongs to the orthorhombic system with a lattice constant of a=7.61 Å, b=6.48 Å and c=4.23 Å. The a axis of Fe2Al5Znx becomes shorter, while the b and c axes become longer compared to those of binary Fe2Al5. (2) The top dross particles with the faceted interface were postulated to coarsen by the mechanism of the anisotropic interface energy between the top dross particles and molten Zn as a driving force rather than by the aggregation mechanism. (3) The hardness and the elastic modulus of the top dross particles are the lowest in the [001] direction like Fe2Al5, and are lower than those of Fe2Al5. (4) The fracture toughness of top dross particles is approximately 1.1 MPa·m1/2, which is slightly lower than that of Fe2Al5.
  • Effect of Lattice Defects on Tribological Behavior for High Friction Coefficient under TCP added PAO Lubrication in Nanostructured Steels

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    DOI:10.2355/tetsutohagane.TETSU-2018-043

    The effect of lattice defects on the tribological behavior for high friction coefficient under tricresyl phosphate (TCP) added poly-α-olefin (PAO) lubrication was investigated in the nanostructured steels produced by heavy plastic deformation processes. In the surface-nanostructured SUJ2 bearing steel, the tribological behavior with high friction coefficient was observed in the ball-on-disk tests in comparison with the non-deformed steel. In addition, the similar phenomenon was observed in the ultra-low carbon (ULC) steel with high-density of lattice defects (grain boundary, dislocation and so on). By increasing the density of lattice defects, higher friction coefficient was shown. The reason of the tribological behavior with high friction coefficient seems that the compound film of Fe-O-P system formed in the ball-on-disk test was worn.
  • Mechanical Property of Ultrafine Elongated Grain Structure Steel Processed by Warm Tempforming and Its Application to Ultra-High-Strength Bolt

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    DOI:10.2355/tetsutohagane.TETSU-2018-077

    Our strategy to enhance fracture properties of ultra-high-strength low-alloy steel with a yield strength of 1.4 GPa or over is to arrest the propagation of brittle crack in a hierarchical, anisotropic and ultrafine grain structure designed to be fail-safe, in addition to suppressing the crack initiation. The present article reviews strength, ductility, toughness and delayed fracture resistance of ultra-high-strength low-alloy steel with an ultrafine elongated grain structure that was processed by deformation of a tempered martensitic structure at an elevated temperature (warm tempforming). The evolution of heterogenous microstructure during warm tempforming using multi-pass caliber rolling and the microstructural factors controlling the strength and the fracture properties of the warm tempformed steels are discussed. Furthermore, we introduce the application of the warm tempformed steel with an ultrafine elongated grain structure to ultra-high-strength bolt.

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