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

Tetsu-to-Hagané Vol. 101 (2015), No. 1

  • Effect of Flux Addition Method on Hot Metal Desulfurization by Mechanical Stirring Process

    pp. 1-10

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    DOI:10.2355/tetsutohagane.101.1

    Effect of a flux addition method on hot metal desulfurization by a mechanical stirring was investigated in a 1/12-water model and a 70 kg hot metal desulfurization tests. Following flux addition method was employed; top addition on bath surface at the first period of desulfurization, continuous addition from the top and powder blasting with nitrogen gas. Desulfurization rate of powder blasting is larger than continuous addition and top addition. The obtained aggregated slag (desulfurization flux) particle diameters after desulfurization were 0.76 mm (top addition) and 0.38~0.44 mm (powder blasting), respectively. Desulfurization behavior was analyzed assuming the interfacial area between the flux and the hot metal is proportional to the estimated aggregated slag particle size. Based on the analysis, the effect of powder blasting with nitrogen gas on improvement of desulfurization efficiency was interpreted to be the combined results of a) promotion of flux dispersion, avoiding aggregation at the time of flux addition and b) prevention of slag particle aggregation in hot metal during stirring.
  • Influence of Lime Coating Coke on NOx Concentration in Sintering Process

    pp. 11-18

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    DOI:10.2355/tetsutohagane.101.11

    Decreasing NOx emission in sintering process is a key issue in steel industry. NOx emission in sintering process is decreased by coke combustion under high temperature. It has been investigated that coating layer of CaO-Fe2O3 composition on coke surface (CF coating method) is effective for decreasing NOx. It has been considered that CaO-Fe2O3 coating layer promotes high temperature combustion and functions as catalyst for reduction of nitrogen oxide.
    In this study, CaO coating of coke (Lime coating coke: LCC) has been studied as simple technology for decreasing NOx. As the results, LCC has been effective like a CF coating method and it has been understood that CF melt formation on coke surface is important for decreasing NOx. About coating CaO ratio, 10% was preferable. And decrease in mixing time of LCC with iron ores (Post-mixing) was also effective. By LCC post-mixing, 17.6% NOx decreased and sinter productivity increased.
  • Effect of Coke Combustion Rate Equation on Numerical Simulation of Temperature Distribution in Iron Ore Sintering Process

    pp. 19-24

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    DOI:10.2355/tetsutohagane.101.19

    It is thought that numerical simulation model for estimation of temperature distribution in the iron ore sintering process is useful tool for stable operation of this process. The purpose of this study is to develop new numerical simulation model based on a combination of Hottel’s equation and coke combustion rate equation in quasi-particle derived from previous study. The following results were obtained. More practical temperature distribution could be calculated using coke combustion rate from previous work than in case of using only Hottel’s equation. Decrease of temperature distribution in the sintering layer was found with consideration of the liquid phase formation effect on coke combustion rate. This effect will increase with increasing of fine coke amount in the process.
  • Reduction of CO2 Emissions by Use of Pre-reduced Iron Ore as Sinter Raw Material

    pp. 25-32

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    DOI:10.2355/tetsutohagane.101.25

    In order to reduce CO2 emissions at an ironmaking process, it is an effective measure to decrease a bonding agent rate at an iron ore sinter plant. In this study, effect of using a pre-reduced iron ore as a part of a sinter raw material on a sintering process was investigated mainly from a viewpoint of decreasing a bonding agent rate.
    Two brands of pisolitic iron ores were reduced up to wustite at 1173 K with reducing gas of which an oxidation degree was 55%. The pre-reduced iron ore was stable against reoxidation in the atmosphere and through a cyclic wet and dry treatment.
    Two brands of pisolitic iron ores and a Marra Mamba iron ore were pre-reduced and then used in a sinter pot test. A use of the pre-reduced iron ore was effective in decreasing a bonding agent rate at a given productivity. The reoxidation heat of the pre-reduced iron ore was estimated to be less than the combustion heat of the bonding agent being saved by use of the pre-reduced iron ore. The reoxidation heat is more effective in the sintering process than the combustion heat. The decrease of the bonding agent resulted in reduction of NOx emissions.
    A mass and heat balance shows that a use of a pre-reduced iron ore as a sinter raw material enables reduction of CO2 emissions not only at an ironmaking process but also at a whole integrated steel works.
  • Evaluation of Susceptibility to Hydrogen Embrittlement for Vanadium Added Spring Steel with Tensile Strength of 2 GPa Class

    pp. 33-39

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    DOI:10.2355/tetsutohagane.101.33

    The susceptibility to hydrogen embrittlement for vanadium added spring steel (9254 V) with a tensile strength of 2 GPa class was evaluated by conventional strain rate tensile tests (CSRT) and torsion tests, using smooth specimens respectively. In the CSRT evaluation, the maximum tensile stress decreased with an increment in the diffusible hydrogen content, especially over 5.5 massppm the maximum tensile stress tended to fall off to the lower limit of 1.1 GPa and the fracture appearance changed to fully intergranular.
    On the other hand, in the torsion tests, the maximum shear stress hardly exhibited any decrease until the hydrogen content reached 6 massppm, where the cracking trace changed from shear plane (transverse direction of the specimen) to a resolved tensile stress plane (45º against the shear plane); fractgraphically, from micro-void coalescence (MVC) to intergranular fracture, and the torsional strength began to decrease.
    The resistance to hydrogen embrittlement as regards the CSRT properties of 9254 V was superior to that of vanadium-free SAE9254 but with the same tensile strength. Although the superior performance for 9254 V is partially attributable to the reduction of phosphorous and sulfur contents, it should be noted that the addition of vanadium causes refining the prior austenite grains followed by an effective reduction of the intergranular segregation in phosphorus and sulfur, and probably hydrogen trapping at the vanadium carbide interface. However, there was no difference between 9254 V and SAE9254 as regards the torsion properties insusceptible to hydrogen compared with CSRT.
  • Effect of Nb Contents on Size of Ferrite Grain in Ultra-low Carbon Steel for Cans

    pp. 40-45

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    DOI:10.2355/tetsutohagane.101.40

    Fineness of ferrite grain is desirable for good surface quality of film laminated steel for drawn cans. And softness of steel is suitable for drawn cans from the view point of high formability. Generally, ultra low carbon steel (ULC) is soft, but it has coarse ferrite grains. On the other hand, low carbon steel (LC), which has fine ferrite grains, is hard and has poor formability, and therefore it is not suitable for drawing. In order to improve these contradictory properties, the amount of carbon and niobium in ULC steel is varied to control the size of precipitates of niobium carbide, which contribute to reduction of grain size. The study suggested that the newly developed steel has a potential to have an excellent balance of both properties.
  • Effects of Temperature and Strain Rate on Deformation Twinning in Fe-Si Alloy

    pp. 46-50

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    DOI:10.2355/tetsutohagane.101.46

    In this study, the effects of temperature and strain rate on the deformation twinning behavior in Fe-5%Si alloy were investigated. Tensile tests at various temperatures (198-248 K) and strain rates (0.1-0.01 s–1) were carried out up to a strain of 0.8%. The presence of deformation twins was confirmed in all the tensiled specimens. The results of crystal orientation analysis by SEM-EBSP indicated that the {112} plane is the twinning plane for the twins formed in the grains. The area fraction and the width of deformation twin increased with decreasing temperature or with increasing strain rate. This tendency can be explained by the relationship between the resolved shear stress and the dislocation velocity.
  • Effect of Cold-working and Mo Addition on Creep Behavior in High Mn Austenitic Stainless Steels

    pp. 51-58

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    DOI:10.2355/tetsutohagane.101.51

    The effect of cold-working and Mo addition on creep behavior in 15Mn-17Cr-0.09C-0.39N-(0.01, 0.5, 1.0, 1.5, 2.0) Mo steels has been investigated. The steels with solution treatment at 1373 K and ones with 40% cold-rolling are subjected to a creep test at 873 K under 200-400 MPa, respectively. Time to rupture (tr) of both steels increases with increasing the Mo concentration. Since tr in the cold-rolled steels is shorter then that in solution-treated ones in a same creep condition, cold-working is suggested to be harmful for creep strength. Creep rate in a primary creep region of cold-rolled steels with more than 1.0 Mo is smaller than that for solution-treated ones because many dislocations are induced by the cold-working. This nature is advantageous as sealing materials for high temperature. Mo addition causes decreasing of minimum creep rate in the solution-treated steels. A linear relationship exists between logarithm of minimum creep rate and Mo content. In the cold-rolled steels with 0.01 Mo and 0.5 Mo, recrystallization is partially occurred during a prescribed heat treatment before the creep tests. Therefore, the creep rate is promptly accelerated. In the cold-rolled steels with more than 1.0 Mo, the suppression of recrystallization by Mo yields the small minimum creep rate and the extension of tr. Fine precipitates of Laves phase are generated in grain interior in tertiary creep region of cold-rolled steels with 1.5 and 2.0 Mo. It also contributes to the extension of tr due to the retarding effect of creep rate acceleration.
  • Effect of Aging Treatment on Hydrogen Embrittlement of Drawn Pearlitic Steel Wire

    pp. 59-64

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    DOI:10.2355/tetsutohagane.101.59

    Hydrogen embrittlement has become a crucial issue with the promotion of high-strength steel. As-drawn pearlitic steel wire is well known to have superior resistance to hydrogen embrittlement. The resistance to hydrogen embrittlement is clarified as being further improved by aging treatment at 100-ºC and 300-ºC for 10-min. of as-drawn 0.8 mass% C pearlitic steel wire with φ5.0 mm (ε=1.9). The higher the aging temperature is, the better the resistance to hydrogen embrittlement becomes. Simultaneously, the strength even increased slightly by aging treatment. The mechanism is investigated by exploiting thermal desorption analysis (TDA) and the newly developed TEM precession analysis. Aging at 100-ºC led to a decrease in the hydrogen content at peak I around 100-ºC in the TDA curve, which is inferred to be caused by C segregation to dislocations resulting in improvement of hydrogen embrittlement. Aging at 300-ºC further improved the resistance to hydrogen embrittlement, which is presumably brought about by the local recovery of the heterogeneously deformed lamellar ferrite area together with the C segregation to dislocations. Here, the strength increased slightly by aging due to the softening factor of recovery and the hardening factor of strain aging.
  • Flow Stress Variation Due to Pre-strain at Extra Low Temperature in Nickel Added Steel

    pp. 65-72

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    DOI:10.2355/tetsutohagane.101.65

    The stress-strain curves of 12 mass% Nickel added steel are investigated at the two temperatures of room temperature and –196 ºC. The stress-strain curves at room temperature after pre-straining at –196 ºC are also compared. The pre-strain at –196 ºC doesn’t induce the strain hardening effect on the yield strength in reloading at room temperature, but increases the flow stress at room temperature. The retained austenite is also measured using XRD and EBSD in the specimens subjected to the different temperature and the straining histories. The flow stress at room temperature after pre-straining at –196 ºC proportionally goes up with an increase of the stress induced martensite. Stability of the retained austenite and hardness of the stress induced martensite are discussed from the viewpoint of local concentration of the austenite former elements.

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