Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 98 (2012), No. 5

  • Feasibility of Solid-State Steelmaking from Cast Iron
    -Decarburization of Rapidly Solidified Cast Iron-

    pp. 151-160

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    DOI:10.2355/tetsutohagane.98.151

    To meet the unprecedented demand of environmental issues and tightened production cost, steel industry must develop the disruptively innovative process. In the present study, totally new steelmaking process of ‘Solid State Steelmaking’ (or S3 process) without BOF process, or liquid state oxidation process is proposed. The overview of the new process is as follows: (1) High carbon liquid iron is directly solidified by strip casting process to produce high carbon thin sheets. (2) Then, the produced cast iron sheet is decarburized by introducing oxidizing gas of H2O or CO2 in a continuous annealing line to produce low carbon steel sheets. The most beneficial aspect of the S3 process is the elimination of several steps such as BOF, and secondary refinement processes and no formation of inclusions. To investigate the feasibility of S3 process, the cast iron strips with various high carbon content produced by a centrifugal slip casting method are decarburized at 1248K and 1373K by using H2O-H2 gas mixture and its kinetics of the decarburization is investigated. In the decarburization process, the carbon diffusion through the decarburized austenite phase but not the decomposition of cementite is the rate controlling step of the decarburizing process. It is found that 0.5 mass% C sheets can be produced from 3.89 mass% C sheets with the thickness of 1.0 mm within 30 min at 1373K. Based on these results, S3 process is confirmed to be feasible as an alternative low cost steelmaking process although the further improvement of the process will be necessary.
  • Functional Forms of Nitrogen and Sulfur in Coals and Fate of Heteroatoms during Coal Carbonization

    pp. 161-169

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    DOI:10.2355/tetsutohagane.98.161

    The functionalities of the nitrogen (coal-N) and sulfur (coal-S) present in three caking coals with carbon contents of 76-81 wt%-dry and the fate of these heteroatoms during coal carbonization have mainly been studied by the X-ray photoelectron spectroscopy (XPS) and with a fixed bed quartz reactor, respectively. The XPS measurements exhibit that about 80-90 % of the coal-N exists as pyrrolic-N and pyridinic-N, and the rest can be assigned to quaternary-N (pyridinic-N associated with the hydroxyl groups from phenols or carboxylic acids), whereas the coal-S consists of thiophenic-S, alkyl-S and FeS2, and the proportion of the former is as high as 60-70 %. When the coal sample is carbonized in He at a heating rate of 3°C/min, yield of N-containing species observed at 1000°C increases in the order of HCN ≤ tar-N < NH3 < N2 < oil-N < coke-N, regardless of the type of coal, and the latter two are present predominantly as NH4+ and in polynuclear aromatic structures, respectively. On the other hand, yield of S-containing species at 1000°C increases in the order of COS < CS2 < tar-S < oil-S < H2S < coke-S with all coals, and more than 95 % of the coke-S can be regarded as thiophenic-S. These tendencies are also observed with physical mixtures of each coal investigated. Possible mechanisms for the formation of gaseous N- or S-containing compounds in the carbonization process are discussed on the basis of the results of some thermodynamic calculations.
  • Impact Property of Warm-Worked Middle Carbon Ferrite-Pearlite Steels

    pp. 170-176

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    DOI:10.2355/tetsutohagane.98.170

    A purpose of this study is to clarify the generation behavior of the separation on the Charpy impact test and influence of MnS morphorogy on impact properties of warm-worked middle carbon ferrite-pearlite steel. In the warm-worked steels, the separation occurs on the fractured surfaces of the test pieces. Therefore brittle fracture is suppressed in the room temperature and completely changes to ductile fracture. High temperature working is advantageous to improve impact properties by preventing the outbreak of the separation. MnS elongated in the rolling direction has the same effect as the separation. Therefore it is effective for impact properties improvement to reduce elongated MnS by reduction of S content. Furthermore, it is effective for impact properties improvement to make MnS granular by Ca addition.
  • Effect of Deformation Condition on Texture Formation in Fe-3Mass%Si Alloy during High-Temperature Uniaxial Compression Deformation

    pp. 177-183

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    DOI:10.2355/tetsutohagane.98.177

    High temperature uniaxial compression tests are conducted in order to clarify the effect of test conditions to the evolution of microstructures and textures. Fiber texture with {001} and {111} (compression plane) is formed as major and minor component, respectively. In contrast to the deformation at room temperature, axis density at ‹001› is higher than that at ‹111›. EBSD observation shows that the mean grain size of ‹001› oriented grain is larger than the grain with the other orientation. This suggests the development of {001} fiber texture in Fe-3%Si can be attributed to the preferential growth of ‹001› oriented grain, being similar to the behavior of Al solid solution reported before. The orientations of the preferentially grown grains in the both alloys can be understood on the basis of the same hypothesis proposed by two of the authors. In many cases, high angle boundaries (15°∼) are serrated. It is suggested that the serration corresponds to the existence of small angle boundary (2°∼15°). EBSD measurements suggest that small angle grain boundaries in ‹111› oriented grain give local driving force to grain boundary migration into ‹111› oriented grain, and the boundaries in ‹001› oriented grain retard the migration. Examination of the texture sharpness at a strain of -1.0 shows that axis density at ‹001› develops with an increase in temperature and a decrease in strain rate. This suggests that homogeneous distribution of dislocations contributes to the development of ‹001› orientation.
  • Dependence of Cleavage Facet Size in Ferrite Steel on Temperature

    pp. 184-189

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    DOI:10.2355/tetsutohagane.98.184

    The influence of temperature on cleavage fracture facet is evaluated by fractography as a basic research to investigate the mechanism of cleavage crack propagation in the micro scale. Cleavage fracture surfaces are obtained by three point bending test using specimens with machined notches under several test temperatures less than the transition temperature. Equivalent diameters of fracture facets on the cleavage fracture surfaces are measured by SEM observation and image analysis. The results of the distribution of the fracture facet size show a tendency that the facets become finer by lowering test temperature. Then, the grain orientation under the fracture surface is measured by EBSD and the patterns of cleavage fracture facet formations are investigated by using {100}-plane trace. It is found out that a single fracture facet is formed in each grain in case of high test temperature corresponding to the conventional knowledge. On the other hand, multiple fracture facets are intricately formed in most of the grains in case of low test temperature. This fact supports the dependence of the distribution of the fracture facet size on test temperature.
  • Numerical Simulation of Cleavage Fracture Formation in Grain of Ferrite Steel

    pp. 190-196

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    DOI:10.2355/tetsutohagane.98.190

    Dependence of cleavage fracture facet on test temperature is found out in the authors’ previous experimental study. In the present study, a numerical model is developed in order to simulate the formation of the cleavage fracture facet. In this model, three grains are modeled as cubes and their sizes are independently introduced by using the distribution of the test steel. The condition of the breakthrough at grain boundary is defined based on the local fracture stress criterion. The fracture facets in a grain are evaluated by mechanical and topological patterns of fracture. The numerical results show that the multiple fracture facets are frequently formed in one grain and the size of fracture facets become small in case of low fracture arrest toughness. That is, the influence of temperature on the fracture facet distribution in experimental results is simulated by the microscopic fracture arrest toughness in the developed numerical model.
  • Enhanced Lattice Defect Formation Associated with Hydrogen and Hydrogen Embrittlement under Elastic Stress of a Tempered Martensitic Steel

    pp. 197-206

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    DOI:10.2355/tetsutohagane.98.197

    Hydrogen behavior and hydrogen-enhanced lattice defect formation under elastic stress of tempered martensitic steel were clarified with respect to dislocations and vacancies by thermal desorption analysis (TDA) using hydrogen as a probe of defects and a positron probe microanalyzer (PPMA). The relationship between hydrogen embrittlement and lattice defects associated with hydrogen was also investigated. The amount of lattice defects increased gradually with increasing the time of applied stress during hydrogen charging. The specimen fractured under elastic stress in the presence of hydrogen macroscopically showed brittle fracture without necking. Whereas fracture surface was attributed to localized plastic deformation, since the morphology of the microscopic fracture surface was mostly quasi-cleavage fracture. The increased lattice defects in the near-fracture area were subsequently removed by annealing at 200°C. The mean positron annihilation lifetime measured with the PPMA for a fractured specimen was longer in the near-fracture area than in other areas. Thus, the most probable reason for the increase in the amount of lattice defects can be ascribed to an increase in the amount of vacancies or vacancy clusters. Regarding hydrogen embrittlement involving microscopic plastic deformation, the localized enhanced vacancies due to interactions between dislocations and hydrogen under elastic stress directly caused ductility loss, because ductility loss occurred even though hydrogen was completely removed by degassing before the tensile test. Besides hydrogen content and applied stress, the time of formation and accumulation of vacancies are also concluded to be important factors causing hydrogen embrittlement.
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  • Decrease of Sulfide in Enclosed Coastal Sea by Using Steelmaking Slag

    pp. 207-214

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    DOI:10.2355/tetsutohagane.98.207

    Currently in Japan, 15 million tons of steelmaking slag (SMS) as a by-product of the steelmaking process is produced annually. More than 60 % of the SMS is used in civil construction. SMS has special properties which are presently under-exploited. Therefore, research into the greater utilization of the special characteristics of SMS in coastal environments has been undertaken over the last 20 years. It is known that steelmaking slag can reduce hydrogen sulfide in seawater. Hydrogen sulfide is highly toxic and fatal to benthic organisms. It also depletes oxygen and generates blue tide.
    The purpose of this study is to evaluate and demonstrate the effects of removal of hydrogen sulfide in seawater by steelmaking slag. Both the laboratory and the field experiments showed that steelmaking slag removed the hydrogen sulfide from seawater and reduced the concentration of hydrogen sulfide in sediment. The field experiments also indicated that steelmaking slag changed the anaerobic condition of sediment into an aerobic condition. The results imply that effective utilization of steelmaking slag in coastal area restoration can significantly improve the surrounding marine environment.

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