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

Tetsu-to-Hagané Vol. 101 (2015), No. 6

  • High-temperature in-situ TEM Straining of the Interaction with Dislocations and Particles for Cu-added Ferritic Stainless Steel

    pp. 315-318

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    DOI:10.2355/tetsutohagane.101.315

    Cu is always present in the matrix when ferritic steels were prepared from ferrous scrap. When the ferritic steels are aged thermally, Cu precipitates start appearing and dispersing finely and homogeneously, which may result the steels strengthened by precipitation hardening. In this study, the interactions between Cu precipitates and dislocations were examined via high-temperature in-situ TEM straining. Cu-added ferritic stainless steel (Fe-18.4%Cr-1.5%Cu) was used in the present study. Specimen was aged at 1073 K for 360 ks. Microstructure of specimen was analyzed by JEM-3200FSK and high-temperature in-situ TEM straining was conducted using JEM-1300NEF. Progressing dislocations in matrix contacted with the Cu precipitate at right angle. This result implies that there is an attractive interaction between dislocations and the Cu precipitate. Furthermore, dislocations pass through the particle after contacting it, so that the interaction with dislocations and particles should be explained by Srolovitz mechanism.
  • Strip Warpage Behavior and Mechanism in Single Roll Driven Rolling

    pp. 319-328

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    DOI:10.2355/tetsutohagane.101.319

    Model rolling experiment has been conducted to investigate strip warpage behavior during single roll driven rolling. It became clear that direction of strip warpage changes with change in so-called rolling shape factor which is defined as ratio of contact arc length to strip thickness. In case of the shape factor being small, rolled strip tends to warp toward the idle roll side, which is the consequence of larger exit velocity of the rolled strip for the driven roll side due to higher peripheral velocity of the driven roll. On the other hand, in case of larger shape factor, rolled strip tends to warp toward the driven roll side owing to larger forward slip ratio caused by larger thickness reduction for the idle roll side. In addition, two dimensional steady-state rolling analyses by a rigid plastic finite element method have been conducted to investigate mechanism of the strip warpage behavior. Utilizing fine FE mesh and precise boundary conditions, the results of analyses have shown good agreement both qualitatively and quantitatively in strip warpage behavior with the experimental results mentioned-above. Moreover elaborate mechanical investigation based on the FE analyses have revealed the fact that rolling deformation is realized by a set of macroscopic shear bands which penetrate strip thickness and are inclined with respect to the strip thickness direction, and it is concluded that intensity and configuration of the shear bands determine strip warpage behavior.
  • Characteristics of Heat Transfer at Elastic Contact between Die and Material in Hot Working

    pp. 329-335

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    DOI:10.2355/tetsutohagane.101.329

    In the hot rolling and the forging processes, the temperature of the material being processed is decreased by contact with the work rolls or dies. In addition, the scale layer, which was thermally insulated, was generated on surface of the material. The temperature in the working process can be crucial to the mechanical property of the product. It is important to evaluate contact heat transfer quantitatively. In this study, the contact heat transfer coefficient was investigated in two materials with different surface roughness and scale layer. The materials used were copper and SUS304 stainless steel. Thermocouples were inserted inside the test pieces to estimate the heat flux through the contacting surface. The contact surfaces of the test pieces were roughened to various Ra from 0.15 µm to 2.0 µm by lapping and lathing. The oxide layer was made of FeO and Al2O3 by thermal spray. In the experiment, the copper test piece was first heated to 500 K, after which it was pressed onto the stainless steel test piece, which was at room temperature at various contact pressures from 0.2 MPa to 80 MPa. As the result of the experiment, the heat transfer coefficient became higher as the contact pressure was increased. Applying a parameter based on a theory of total thermal resistance in a composite plane wall, a proportional relationship was shown to exist between surface roughness over thermal conductivity and the heat transfer coefficient.
  • Phase Field Simulation Analysis of Nitrogen Absorption-Phase Transformation Behavior in the High Temperature Stable Ferritic Stainless Steel

    pp. 336-342

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    DOI:10.2355/tetsutohagane.101.336

    The behavior of diffusion and phase transformation in a high temperature stable ferritic stainless steel (SUS444) nitrogen-absorbed at 1450 K for 4 hr under 0.4 MPa N2 gas was investigated with EPMA mapping. While Cr content in the nitrogen absorption layer (NA layer: γ phase) increased, Mo discharged from NA layer was enriched in the grain boundary of α/γ. Using a 1D-phase field simulation (PFS), the phenomenon during the NA treatment in the Fe-19Cr-2Mo-0.5Mn-N alloy was discussed. In the case of PFS with impurity diffusion (lattice diffusion) coefficients in Fe, since the diffusion velocity in γ phase was smaller than that of α phase, concentration distribution between NA layer and matrix α phase is caused by local phase equilibrium in depending on the composition of NA layer. Additionally, diffusion of substitutional atoms is slower than the growing velocity of NA layer that is depended on nitrogen diffusivity. This is the reason why Cr content in the NA layer fluctuates and why the average compositions in α phase and NA layer has the opposite trend with thermodynamic calculated and experimental results. On the other hand, in the case of the simulation under Dγ > Dα in consideration of the grain boundary diffusion of substitutional elements, the segregation behavior of solute elements was in good agreement with the results of EPMA mapping. Thus, it was confirmed that not only the diffusion of N atoms but also the grain boundary diffusion contributes greatly to the growing phenomenon of NA layer in a high stable ferritic stainless steel.
  • Synthesis of Calcium Ferrite from Waste Gypsum Board

    pp. 343-350

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    DOI:10.2355/tetsutohagane.101.343

    Chemical recycle process of waste gypsum board obtaining synthetic calcium ferrite for steelmaking have been investigated. Thermodynamic estimation and results of small-scale heating tests revealed that decomposition of gypsum promoted in proper range of partial pressure of O2 and in lower partial pressure of SO2 through preventing formation of CaS. Adding Fe2O3 to gypsum contributed a lowering of decomposition temperature of gypsum resulting in formation of calcium ferrite in lower temperature. Decomposition of gypsum was incomplete after heating in air at 1180 °C. In contrast, calcium ferrite was successfully obtained by 2-step heat treatment of gypsum ; firstly forming CaS thorough reduction by composite carbon and subsequently heating mixture fine between Fe2O3 and CaS at 1180 °C. Furthermore a possibility of forming calcium ferrite was found even in a single heat treatment if we adapt proper gas and material conditions. After trial tests with using large-scale rotary kiln, it was confirmed that heating of sample consisting with gypsum, coke fine and Fe2O3 at temperature less than 1200 °C resulted in the formation of calcium ferrite accompanying with high desulfurization degree of gypsum.

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