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

Tetsu-to-Hagané Vol. 79 (1993), No. 6

  • Technical Innovation of Continuous Annealing Line for Tin Mill Black Plates

    pp. 619-627

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  • Isolation and Determination of Precipitates in High Alloys and Super Alloys (I)

    pp. 628-637

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    [in Japanese]
  • Effects of Plasma Injection on Stability and Nitrogen Oxide Formation in Combustion of Pulverized Coal

    pp. 638-644

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    The feasibility of the use of a plasma jet was studied for the ignition of pulverized coal combustion and suppression of nitrogen oxide formation. The analyses of the product gases under various experimental conditions led to the following results:
    (1) By the assistance of plasma injection, pulverized coal can be ignited without any addition of auxiliary oil or gas. The proportion of plasma energy necessary to maintain stable combustion does not exceed several percentages of heat input of the coal.
    (2) Suppression of nitrogen oxide formation can be expected even in the range of high combustion efficiency, if the plasma assisted burner is appropriately designed. Nitrogen plasma plays an important roll to reduce the nitrogen oxide.
  • Influence of Stirring on Smelting Reduction Rate of Chrome Ore

    pp. 645-651

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    For the examination of stirring effect in the in-bath smelting reduction process, influence of stirring on the reduction rate of chrome ore was studied. The experiments were mainly carried out using a ten-ton test converter with oxygen top-blowing, nitrogen side-blowing and nitrogen bottom-blowing. After five tons of chrome ore were fed with coke and lime on five tons of molten iron, the stirring effect was examined by changing the gas blowing conditions.
    Main conclusions are as follows:
    (1) The change in the reduction rate caused by the change in the blowing conditions corresponds with the change in the stirring power for the slag layer.
    (2) The nitrogen side-blowing has nearly the same effect on the slag stirring as the nitrogen bottom-blowing has, when the blowing rate is the same.
    (3) The nitrogen side-blowing is suitable for the intensive stirring of slag, which contributes to enhancing the reduction rate of chrome ore.
  • Smelting Reduction Behavior of Prereduced Chromite Ore

    pp. 652-658

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    Chromite ores prereduced to various stages of reduction were reduced by carbon at 1550 and 1600°C in the presence of molten slag of CaO-MgO-Al2O3-SiO2 system.
    Chromium became liable to dissolve in the form of Cr2+ rather than Cr3+ in slag during the smelting reduction of chromite ores prereduced at 1200°C or above, even when the ores had higher iron content. This seemed to be mainly attributed to the fact that the dissociation oxygen pressure of chromite was decreased by preferential reduction of iron oxide in prereduction stage.
    Metal produced in prereduction stage melted and flowed out to liquid droplets in the vicinity of chromite grain during the smelting reduction. As a result, indentations and pores were formed on the surface and inside of chromite grain, respectively.
    The rate of smelting reduction increased with increasing the degree of reduction in prereduction stage. This seemed to be attributed to the increases in number and size of the indentations and pores as well as the change of oxidation state of chromium from Cr3+ to Cr2+.
  • Calculation of Heat Transfer Coefficient at Roll-Solidified Shell Interface of Twin-Roll Type Caster

    pp. 659-664

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    Strip casting experiments on type 304 stainless steel have been carried out in a steel twin-roll caster with rolls 300 mm in diameter and 80 mm wide. The casting speed was 6.68.0 m/min. Thickness of the strips was about 2.4 mm.
    Roll temperature was measured at 1mm deep from outer surface of the roll during casting. Abnormal acceleration behavior of temperature change was detected. The annomalous acceleration of the thermo-electromotive force is caused by reduction in solidified shell thickness during squeezing. Calculation by finite difference method agreed with the result of temperature measurements. Simulation revealed that the heat transfer coefficients are, respectively, 2.54.2 and 1016.7 kW/m2K before and after squeezing.
  • Deformation and Transformation Processes of a Multi-phase Sheet Steel Containing Retained Austenite

    pp. 665-670

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    The deformation and transformation processes of an Fe-0.28%C-1. 41%Si-1.50%Mn steel subjected to various annealing conditions were investigated using in-situ TEM and SEM straining techniques. The retained austenite(γR) was observed in three distinctly different morphological forms, i. e., a film type located between bainite laths, an island type isolated in relatively large ferrite grains, and a granular type located at grain boundaries especially triple junction points. When stress was applied on the steel, the ferrite matrix was found to deform first piling-up dislocations at ferrite-γR boundaries. The internal stress fields generated by these piled-up dislocations seem to act as the driving force for the deformation induced transformation of γR. As the plastic deformation proceeded, fracture began with void formation at ferrite-second phase interfaces leading into subsequent void growth and coalescence until a final failure. The in-situ TEM observation showed that the transformation of γR is accomplished in the two different paths, i. e., γR→twin→α' and γR→α'. The orientation change resulting from a γR→α' transformation can be described by the Kurdjumov-Sachs relationship.
  • Effects of Prestrain on Precipitation Hardening of Cu added Ultra Low Carbon Cold Rolled Steel Sheet

    pp. 671-677

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    Effect of prestrain on the precipitation hardening in 1.4%Cu-added ultra-low carbon Ti-added steel has been studied. The specimens were aged at temperatures below 550°C after annealing or subsequent prestrain up to 10%.
    The as-annealed specimens showed considerable precipitation hardening only around 550°C. While the prestrained specimens hardened at a temperature as low as 350°C. The electrical resistivity and hardness measurements revealed that there existed a new nose for Cu aging around 400°C in addition to normal one around 600°C. The former nose can be related to formation of metastable phase, and was largely accelerated by the prestrain. This acceleration effect can be explained in terms of higher diffusivity of Cu atoms due to excess vacancies formed by the prestrain.
  • Effects of Chemical Composition of Nonmetallic Inclusions on Fatigue Strength of High Strength Steels

    pp. 678-684

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    It is well known that the fatigue strength of high strength steels are strongly influenced by the existence of nonmetallic inclusions. It is said that the effects of nonmetallic inclusions on the fatigue strength of high strength steels depend on their size, shape, chemical composition and locations where they exist. Therefore, nonmetallic inclusions are likely to cause a large scatter in fatigue strength.
    A firm opinion that hard inclusions are more detrimental than soft inclusions for fatigue strength, has prevailed for long years.
    In the present paper, this opinion is proved incorrect by detailed investigations of the chemical composition and size of inclusions observed at fracture origin, i. e., the chemical composition of inclusions is not crucial factor controlling fatigue limit, even if the chemical composition influences the rigidity of inclusions and the residual stresses around inclusions.
    On the contrary, it is verified that two crucial factors which control the fatigue strength are the Vickers hardness of matrix (HV) and the square root of projection area of inclusion (√area). It is shown that the lower bound of large scatter in fatigue strength can be successfully predicted by the statistics of extreme values (√area max) of inclusions contained in many specimens.
  • Effect of Si and Ca Addition on Delayed Fracture of High-strength Steels

    pp. 685-691

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    Effects of Si and Ca addition on delayed fracture of medium carbon steels with 1400N/mm2 strength were investigated. Silicon and calcium were added at concentrations of 02.0% and 3070ppm, respectively. The delayed fracture characteristics were evaluated by FIP (Fédération Internationale de la Précontrainte) test which is a constant tensile load test in 20% NH4SCN solution at 323 K. In order to make clear the effect of adding Ca and Si, the fracture surfaces were examined, and the hydrogen evolution behavior, the diffusion coefficient of hydrogen, and the hydrogen content were measured. It was found that 0.5%Si steels have no effect on the time to fracture regardless of Ca content, while 1.5%Si-30ppm Ca steel has the longest time to fracture. Fractography showed that adding Ca to 0.5%Si steels did not change the intergranular fracture area fraction. However, adding Ca to 1.5%Si steels changed the fracture from intergranular fracture to microvoid coalescence fracture. As for hydrogen behavior after three months from FIP test, 0.5%Si steel released hydrogen at the peak of 500 K, while for the 1.5%Si-30ppm Ca steel the peak was at 700 K. It was suggested that hydrogen released at around 500 K was crucial for delayed fracture characteristics.
  • Effects of O2 and H2O Content in Heating Atmosphere on Scale Properties of Steel Plate at High Temperature

    pp. 692-698

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    The effects of heating atmosphere on oxidation behaviour at high temperature of steels with various Si content(0.060.4%Si), but constant in other elements, have been investigated. The results obtained are summarized as follows.
    Fayalite, Fe2SiO4, formed at the interface of scale/matrix affected the oxidation behaviour. Therefore, solid Fe2SiO4 formed at oxidation temperatures up to 1170°C, which was the eutectic point of Fe2SiO4 and FeO phase, raised heat resistance, but liquid phase of Fe2SiO4 at oxidation temperatures exceeding 1170°C accelerated the oxidation. The change of oxidation behaviour occurred at the Si content between 0.1% and 0.2%. Increasing Si content in steel, Fe2SiO4 formed at the interface of scale/matrix increased gradually. When the whole interface was covered with Fe2SiO4, the oxidation behaviour should be affected by the change of diffusion rate on Fe++ ion from iron matrix to scale. H2O and O2 content in heating atmosphere promoted the oxidation of iron, but the effect of H2O was much more than O2. The mean diameter of pores in scale was in proportion to oxidation weight gain, and therefore Si content and oxidation condition affected on mean diameter of pores in scale through the oxidation weight gain.
  • High Temperature Deformation Behavior of SKD11 Die Steel

    pp. 699-705

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    SKD11 tool steel is one of the steels which contain high compositions of carbon and alloying elements forming a lots of carbides. Major part of these carbides are coarse eutectic carbides which are difficult to be refined through the conventional heat treatment. In order to obtain the refinement of the carbides and the improvement of deformability of SKD11 steel, powder metallurgy method (P/M) of HIP treatment of atomized powder followed by hot forging was utilized and the characteristics of high temperature deformation behaviors of the P/M SKD11 materials and also the effects of the powder particle size and HIP treatment condition were investigated. Maximum elongation of 196% was obtained at the lowest strain rate of 5.6×10-5s-1 in this work and at the test temperature of 1O53K just below Ac1 transformation point. HIP temperature and particle size had generally very small effects on such high temperature deformation behavior. Microstructural observation showed the grain growth and directionality along tensile direction during the deformation at lower strain rates and the test temperature just below Ac1 point. This is cosidered to indicate that the so called "superplasticity due to grain boundary sliding" is not the principal deformation mechanism for the large elongation of the P/M SKD11 steel of the present result.
  • The Composition Dependent Intergranular Corrosion Mechanism in Stainless Steels

    pp. 706-712

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    Intergranular corrosion (IGC) in stainless steels can be well explained by following two mechanisms; "Composition dependent IGC" and "Compound dependent IGC". In this paper, the composition dependent IGC mechanism was discussed. In this mechanism, it has been confirmed that IGC was induced only by the difference of corrosion rates at grainboundaries (GB) and the matrix, due to the change of the chemical composition of GB from that of the matrix.
    The high concentration of P due to the GB segregation causes very severe IGC at active state corrosion potentials as in H2SO4 solution, where the Cr depleted zone never induces IGC. The Cr depleted zone is responsible for IGC in the passive state corrosion potential region as in most of standardized IGC test solutions, where only the high concentration of P at GB is not responsible for IGC.
    In the next paper, the compound dependent IGC mechanism will be presented.
    x

    Readers Who Read This Article Also Read

    1. The Compound Dependent Intergranular Corrosion Mechanism in Stainless Steels Tetsu-to-Hagané Vol.79(1993), No.6
    2. HOT-DUCTILITY OF THE AUSTENITIC STAINLESS STEELS Tetsu-to-Hagané Vol.44(1958), No.6
  • The Compound Dependent Intergranular Corrosion Mechanism in Stainless Steels

    pp. 713-720

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    Following the first paper on the study of the composition dependent intergranular corrosion (IGC) mechanism of stainless steels (SS), the compound dependent IGC mechanism has been studied in this paper. In this mechanism, IGC is induced by the dissolution of grain boundary (GB) precipitates of themselves.
    It was confirmed that the IGC mechanism played an important role mainly in the transpassive state corrosion potential region above about 1.2V(SHE), where neither the Cr depleted zone nor the segregation of P at GB induced IGC. The film-like and amorphous Ni-P compound, [Ni (Fe, Cr)]3P2 was found to precipitate at GB in Type 304 SS and to cause very thin but very deep IGC. Also, intermetallic compounds of Laves and χ phases were identified to precipitate at GB and to induce severe IGC due to the dissolution of themselves in Type 316 SS.
    x

    Readers Who Read This Article Also Read

    1. The Composition Dependent Intergranular Corrosion Mechanism in Stainless Steels Tetsu-to-Hagané Vol.79(1993), No.6
    2. HOT-DUCTILITY OF THE AUSTENITIC STAINLESS STEELS Tetsu-to-Hagané Vol.44(1958), No.6
  • Initiation and Growth of Intergranular Stress Corrosion Cracks for Sensitized 304 Stainless Steel Depending on NaF Concentration of Aqueous Solution

    pp. 721-725

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    Using a slow strain rate technique, SSRT, with CCD camera system, initiation and growth behavior of intergranular stress corrosion cracking, IGSCC, for sensitized 304 stainless steel in aqueous solution containing various concentrations of NaF has been investigated. The largest decrease in maximum stress indicating the highest IGSCC susceptibility was found at 103 ppm NaF. And at the same concentration of 103 ppm NaF, the highest density of crack number and the shortest crack generation time was found. On the other hand, threshold stress intensity factor for IGSCC was independent of concentration of NaF. Crack growth rate showed a little dependence on NaF concentration, exhibiting a decrease at 103 ppm. It was obvious from the results that the highest IGSCC susceptibility at 103 ppm NaF was caused by increasing the number of cracks and decreasing the crack generation time, not by a change in threshold stress intensity factor for SCC or crack growth rate.
  • Generation and Propagation of Intergranular Stress Corrosion Cracks of Sensitized SUS304 Stainless Steel in NaF Aqueous Solution

    pp. 726-731

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    The effects of temperature and applied potential on the intergranular stress corrosion cracking, IGSCC, susceptibility of sensitized SUS304 stainless steel in NaF aqueous solution have been investigated by using slow strain rate test, SSRT, based on the direct observation of crack generation and propagation during the test. It was found that the decrease in the maximum stress, which is usually used as a conventional SCC susceptibility index by SSRT, with increasing temperature was caused not only by the decrease in K'ISCC and crack generation time but also by the increase in crack growth rate and the number of cracks. The result of potential-controlled SSRT proved that the lowest maximum stress at 100 mVAg/AgCl was induced by the decrease in K'ISCC and crack generation time. In conclusion, crack generation is more important compared with crack propagation for deciding the IGSCC susceptibility of sensitized SUS304 stainless steel in NaF solution.
  • Evaluation of Creep Crack Growth Behavior of NCF800H Alloy Based on Creep Fracture Mechanism

    pp. 732-738

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    In order to investigate the relationship between microscopical creep fracture mechanism and creep crack growth rate, creep crack growth tests were conducted on CT specimens of NCF800H alloy. Wedge-type intergranular fracture, transgranular one and cavity-type intergranular one were observed at 873K, 923K and 973K, and 1073K, respectively. For an equal value of C* parameter, creep crack growth rate was the fastest under wedge-type fracture condition, and secondary under cavity-type fracture condition and the slowest under transgranular fracture. Similar dependence of crack growth rate on fracture mode had been observed in creep crack propagation of a SUS316 steel. This corresponds with the difference of creep fracture ductility. Under the cavity-type fracture condition, creep crack growth rate increased as the damaged zone ahead of the crack tip increased. This effect could be explained by Riedel's crack growth model based on the grain boundary cavitation. Therefore, it is necessary to take the effect of creep damaged zone into account, when we evaluate the creep crack growth rate of structural components used for long time at high temperature.
  • Development of Steel Making Refractories

    pp. N397-N404

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  • Cooling Techniques for Hot Steel Products

    pp. N405-N416

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