ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 32 (1992), No. 2

  • Current Progress in Advanced High Cr Ferritic Steels for High-temperature Applications

    pp. 175-181

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    DOI:10.2355/isijinternational.32.175

    The article presents recent developments and future trends in high Cr ferritic heat resistant steels. Research programs are underway worldwide to improve the performance of 8 to 13% Cr ferritic steels for high temperature applications of up to 650°C. We developed the super 12% Cr heat resistant steel called TAF steel in 1956. The creep rupture strength of TAF steel is two or three times higher than those of H46 and AISI422 at 600 and 650°C. Our research in this area was well ahead of work being conducted in other countries.
    Recently the author succeeded in developing TR1100, TR1200, TB9 and TB12 with excellent high temperature properties and room temperature toughness through the improvement of TAF steel. In the near future, these new steels will be applied to turbine rotors, blades, and boiler tubes for advanced supercritical power plants, to fuel cladding, wrapper and steam generator tubing for fast breeder reactors, and to first wall materials for conceptual fusion reactors. These steels are a key to making these plants and reactors practical.
  • Stirring Effect in Bath-smelting Furnace with Combined Blowing of Top and Side Blown Oxygen and Bottom Blown Nitrogen

    pp. 182-189

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    DOI:10.2355/isijinternational.32.182

    For the development of an energy efficient and highly productive bath-smelting process, the influence of gas blowing method on the post combustion ratio (PCR), the heat transfer efficiency (HTE) and the iron ore reduction rate was studied. The experiments were carried out using a 10 ton test converter with top blown oxygen, side blown oxygen and bottom blown nitrogen, receiving a subsidy from the Ministry of International Trade and Industry.
    The combination of top and side blown oxygen and bottom blown nitrogen is found to enable individual control over the stirring intensity of metal and slag with each required degree, resulting in the improvement in PCR, HTE and the reduction rate. The bottom blown nitrogen generates metal droplet which controls PCR and the reduction rate; whereas the side blown oxygen stirs the slag without making excessive amount of metal droplet, increasing HTE without decreasing PCR. The appropriate combination of top and side blown oxygen and bottom blown nitrogen contributes to making favorable conditions to attain the energy efficient and highly productive bath-smelting process.
  • Definition and Determination of Mixing Time in Gas Agitated Liquid Baths

    pp. 190-195

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    DOI:10.2355/isijinternational.32.190

    Definition of mixing time has been examined more closely and the reported concentration vs. time traces were analyzed to determine mixing time. The obtained values of mixing time were compared with those estimated theoretically, employing numerical model of Krishna Murthy. The comparisons suggested that the mixing time values do not depend on the locations of the measuring probe or tracer injection point. Only an improper technique and instruments, or the misunderstanding of the definition of the degree of mixing, led some in the literature to believe that the mixing time was a function of location of tracer injection and position of the measuring probe. It was, therefore, concluded that the true characteristic value of mixing time, for a given set of operating conditions, should be independent of location of measuring probe and tracer injection.
  • Mathematical Modelling of Thermal Stratification and Drainage of Steel Ladles

    pp. 196-202

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    DOI:10.2355/isijinternational.32.196

    Transient analysis of the temperature and velocity distributions of steel during ladle standing and draining has been conducted using a mathematical model based on the PHOENICS numerical package. Parameters investigated were stand time, average steel cooling rate, drainage rate and ladle geometry. Stratification was seen to develop due to natural convection, and the rate of stratification was found to be linearly dependent on the average steel cooling rate, independent of ladle geometry. Qualitative relationships were found between the parameters and the teeming temperature during draining. Plant trials showed good agreement between simulated and actual teeming temperatures.
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    1. Physical Model of Slag Foaming ISIJ International Vol.33(1993), No.1
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  • Effect of Controlled Rolling on Texture Development in a Plain Carbon and a Nb Microalloyed Steel

    pp. 203-212

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    DOI:10.2355/isijinternational.32.203

    The effect of finish rolling temperature was investigated on texture formation in a plain C and a 0.034% Nb microalloyed steel. When finish rolled at 1020°C (i.e. within the γ recrystallization range), the textures in both steels contain the {001}<110> and {110}<110> components. The sharpness of the {001}<110> component generally increases with decreasing finish rolling temperature down to 630°C, while the {110}<110> component gradually weakens and finally disappears after ferrite rolling. The microalloyed steel displays much sharper texture than the plain C steel when finish rolled at 870°C (i.e. within the γ pancaking range for the Nb steel) and at 730°C (in the γ+α intercritical range). After finish rolling at 870°C, the major texture components in the microalloyed steel are {113}<110> and {332}<113>, in addition to the above two, while the plain C steel texture only contains some low intensity maxima. When finish rolled at 730°C, weak peaks appear at {223}<110> and {554}<225> in the plain C steel and stronger ones at {4411}<110> and {554}<225> in the microalloyed steel. After warm rolling at 630°C, the major texture components in both steels are {223}<110>, {554}<225> and {001}<110>.
    The {001}<110> and {110}<110> components are obtained, by transformation, from the {100}<001> (cube) and {122}<212> (twinned cube) components of the recrystallized γ. By contrast, the {113}<110> and {332}<113> components originate, respectively, from the {112}<111> (copper) and {110}<112> (brass) components of the unrecrystallized γ. During continued rolling in the γ+α or α range, these transformation texture components are further modified by deformation and ultimately give rise to the stable end orientations which constitute the well-known warm rolling texture in steels.
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    2. Development of Monobloc Type High-carbon High-alloyed Rolls for Hot Rolling Mills ISIJ International Vol.32(1992), No.11
    3. Pseudoelastic Behavior of Fe–Al Polycrystals ISIJ International Vol.50(2010), No.1
  • Influence of Hot Strip Rolling Parameters on Austenite Recrystallization in Interstitial Free Steels

    pp. 213-221

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    DOI:10.2355/isijinternational.32.213

    The influence of total finishing strain (from 2.1 to 3.2) and first finishing pass temperature (from 990 to 930°C) was investigated by means of multi-pass torsion tests on three interstitial free (IF) steels containing Ti and/or Nb. All the tests were carried out at a strain rate of 2 s–1. Under strip rolling conditions, static recrystallization is responsible for the high degree of interpass softening in the early passes of rolling. During the final passes, dynamic recrystallization occurs to a degree that depends on the composition of the steel, the total finishing strain and the temperature. The critical temperatures of the IF steels were defined by using a simulated plate rolling schedule. Subjected to the same strip rolling schedule, the niobium stabilized steel had the finest, whereas the titanium stabilized grade had the largest ferrite grain size. The present IF steels exhibit higher no-recrystallization temperatures (Tnr's) during strip rolling 1-2 sec interpass times) than under plate rolling conditions (30 sec interpass times).
  • Effects of Cold Deformation on the Morphology of α Precipitates in β Titanium Alloys

    pp. 222-231

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    DOI:10.2355/isijinternational.32.222

    The effects of cold deformation on the morphology of α precipitates were examined for three β titanium alloys which were each mainly deformed by slip, twinning, and stress-induced martensitic transformation.
    Although α precipitated preferentially at β grain boundaries in the undeformed specimens, slight deformation (5% rolling) added another preferential nucleation site peculiar to the deformation modes, which was clearly reflected by the microstructure after aging at 873 K. In Ti-15V-3Cr-3Sn-3Al, slip brought about a striated structure of planar slip bands caused by the extremely localized dislocations, and consequently led to directionally elongated α precipitates aligned along the striations. In Ti-16V-10Sn, deformation twins were frequently accompanied by the internal twins, and the α phase preferentially precipitated at the twin boundaries, especially at the cross points with the internal twins, to finally form film-like α with occasional α precipitates within the twins. In Ti-16V-4Sn, stress-induced orthorhombic martensite (α'') tended to be produced in groups of martensite plates aligned in parallel by cold rolling, leading to a banded structure. This martensite substantially reverted to the β phase during the early stages of aging, and the α phase precipitated preferentially at the interface between the β phases which were originally the α'' and the β matrix. During further aging, the α phase coalesced to form film-like α.
    On the other hand, there was no significant difference reflected by the initial deformation modes in the α morphology after heavy deformation (60% rolling) among the three alloys, and the microstructures were considered to be of a microduplex structure.
  • Characteristics of Vanadium and Titanium Microalloyed Steels Forged at Intermediate (Warm) Temperatures through Simulation by Torsion

    pp. 232-240

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    DOI:10.2355/isijinternational.32.232

    This study concerned the response of medium carbon steels microalloyed with vanadium and titanium, to deformation in intermediate ("warm") temperature zone. Titanium varies from 0.003 to 0.039% in weight.
    A critical automotive component (the constant velocity universal joint) warm-forged has been studied by means of a hot torsion simulation technique. Besides its warm-ductility, the influence of the heating temperature before "warm" forging, the "warm" deformation temperature and the weight percentage of Ti on the final steel properties (grain size, microstructure and hardness), was determined.
    It was found that the optimal ductility is reached between 750 and 775°C and that the best characteristics (smallest grain size and highest hardness) are obtained with a Ti weight percentage of 0.019% and at lower deformation temperatures (from 700 to 750°C). Hence, the automotive component might be warm-forged close to 770°C in order to obtain the best mechanical characteristics with enough warm-ductility to avoid warm-forging problems.
  • Influence of Dynamic Recrystallisation on the Tensile Ductility of Steels in the Temperature Range 700 to 1150°C

    pp. 241-249

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    DOI:10.2355/isijinternational.32.241

    The role of dynamic recrystallisation (DRX) in influencing the hot ductility of plain C-Mn and microalloyed steels was examined by comparing the critical strain for Dynamic recrystallisation with the fracture strain in a hot tensile test. The temperature range examined was 700 to 1150°C and the strain rates were varied from 3×10–2 to 3×10–4 s–1.
    For coarse grained plain C-Mn and C-Mn-Al steels solution treated at 1330°C and cooled to the test temperature, the presence at the γ grain boundaries of thin films of deformation induced ferrite at temperatures between the Ae3 and the undeformed Ar3, leads to strain concentrations which give rise to poor ductility. The presence of these thin films prevents the occurrence of DRX. For these steels, the Ae3 temperature, which marks the onset of good ductility is generally high enough to lead to DRX, so that it is not possible to assess its independent contribution to restoring the hot ductility. In coarse grained C-Mn-Nb-Al steels, DRX and the full recovery of ductility are often not observed until the test temperature is higher than 1000°C. This is related to the strain-induced precipitation of NbCN below 1050°C. However, even when recrystallisation is not possible, the ductility can be improved if the amount of strain-induced NbCN is reduced.
    For fine grained plain C-Mn and microalloyed steels heated directly to the test temperature, DRX often occurs in the trough. Grain boundary migration rates have to be sufficiently high to prevent crack linkage from occurring, and this often necessitates the resolution and coarsening of particles so that they are no longer effective in pinning the boundaries. Finally, of interest in this work was the observation that as the initial grain size do is refined, its influence in encouraging DRX becomes more marked than that given by the simple do1/2 relationship in the equation εp=Ado1/2Zn, where εp is the critical strain to the peak stress, Z is the Zener-Hollomon parameter and A and n are constants.
  • Influence of Heat Treatment on the Properties of Wear Resistant Tungsten Carbide Embedded Nickel Base Coating Produced by Gas Thermal Spray Process

    pp. 250-256

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    DOI:10.2355/isijinternational.32.250

    The hard surfacing of mild steel substrate was carried out by thermal spraying of commercially available nickel base tungsten carbide powder under oxy-acetylene flame. The influence of pre and post spray heating on the morphology, hardness and wear characteristics of the coating were studied. The increase in preheating upto 400°C and post spray heating upto 900°C was found to enhance the hardness and wear resistance of the coating. However, the properties of the coating were found to vary across the coating showing a maximum hardness and wear resistance in the region somewhere in between its surface and the interface with the mild steel. The possibilities of occurring of various kinds of transformation during spraying as well as during post spray heat treatment have been analysed as a cause of variation in distribution of property across the coating.
  • Analysis of the Occurrence of Dynamic Recrystallization in Hot Rolling by Modeling of the Stress-Strain Curve

    pp. 257-259

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    DOI:10.2355/isijinternational.32.257

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