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

Tetsu-to-Hagané Vol. 89 (2003), No. 5

  • Structure and Energy of Interphase Boundaries in Steel

    pp. 497-509

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    Current understanding of the structure and energy of interphase boundaries formed during phase transformations and precipitations in steel was reviewed. First, the classification of boundary structure was revisited. In the following, the methods to measure boundary energy and the typical values measured in ferrous alloys were summarized. Finally, the calculation of boundary energy, in accordance with classical and nonclassical nucleation theories, and by the use of atomic potential was reviewed.
  • Modelling Simultaneous Reactions of Niobium Precipitates in Ferritic Stainless Steels

    pp. 510-517

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    Niobium is used as an important alloying element in the design of heat-resistant ferritic stainless steels for automotive exhaust systems.When in solid solution, the niobium improves both the high temperature strength and the resistance to thermal fatigue. However, it also forms several kinds of precipitates during service at elevated temperatures, making it important to understand the kinetics of its precipitation and coarsening. In the present work, the kinetics of simultaneous reactions of niobium precipitates in ferrite have been modelled, taking into account the multicomponent nature of the diffusion process whilst at the same time allowing for capillarity effects. The model can predict not only volume fraction changes but also particle size. The experimental results on Fe2Nb Laves phase and Fe3Nb3C carbide in a 19Cr-0.8Nb steel have been obtained and shown to be comparable to be the calculations with new simultaneous model.
  • Influence of Sulfur Content and Deformation Temperature on Precipitation Behavior of MnS in 3% Si Steel

    pp. 518-523

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    The precipitation behavior of MnS in 3% Si steel was investigated for understanding the metallurgical sequence of precipitation during hot rolling process in 3% Si steel. Precipitation was strongly accelerated by supercooling prior to aging or deformation even at high temperatures. Relatively large and inhomogeneously dispersed MnS precipitates were observed on subgrain boundaries and dislocations at higher temperatures. Fine and dense dispersed MnS precipitates were observed in the specimen deformed at lower temperatures. These fine precipitates were not always observed on dislocations. The precipitation behavior was strongly affected by S content. An increase in S content promoted fine precipitation. The precipitation rate estimated from classical nucleation model agreed with results at region of fine and dense precipitation.
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    Readers Who Read This Article Also Read

    1. Dissolution Behavior of MnS Inclusions in Low Alloy Steel in High Temperature Water Zairyo-to-Kankyo Vol.42(1993), No.10
    2. Morphology Control of MnS in Steel during Solidification Tetsu-to-Hagané Vol.80(1994), No.8
    3. The Growth of MnS Precipitates in Fe-Si Alloys Tetsu-to-Hagané Vol.88(2002), No.9
  • Precipitation Strengthening at Elevated Temperature in Fe-Cu Alloys

    pp. 524-529

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    The mechanism of strengthening by copper (Cu) particles at 873K was discussed in terms of the interaction between dislocation and Cu particles in aged Fe-Cu alloys. The moving dislocation was pinned by the Cu particles and passed through them when the bowing angle reached some critical value (θc). The value of θc 873K was around 33 degree regardless of the Cu particle size, although the θcat room temperature increased with an increase in the diameter of particle. This indicates that the interaction at 873K is significantly different from that at room temperature. TEM observation for the 3%Cu steel deformed by 1.9% at 873K revealed that the strain around dislocation is released at the particle/matrix interface and the attractive interaction is working between the dislocation and the Cu particles. The precipitation strengthening was evaluated by considering the effect of mean particle spacing (λ) and the θc and it was found that the increment of yield stress was smaller than the Orowan stress.
  • Influence of Second Phase Particles on Recrystallization in Fe-36Ni Alloy

    pp. 530-535

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    The influences of second phase particle in the size range 1 to 10 μm in Fe-36Ni alloys on recrystallization were investigated and the sample was cold-rolled with 50% and 80% cold rolling reductions, and annealed at 873K or 923K. Orientation measurements of grains have been made using electron backscattered diffraction in SEM (SEM-EBSD). The obtained results are as follows. (1) In case of the 50% cold rolling reduction, deformation zones with 5 μm width was formed near the second phase particle. (2) (110) orientation grain as the nuclei from the deformed structure was observed. (3) (123) orientation in the sample included second phase particles was formed around the second phase particle. And (115) orientation in the sample without second phase particles was formed around the second phase particle.
  • Influences of Heating and Coiling Temperatures on Recrystallization during Hot-rolling Process in Ti Added High-purity Ferritic Stainless Steel

    pp. 536-543

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    The influence of heating and coiling temperatures on recrystallization of Ti added high purity ferritic stainless steels (17Cr-0.1Ti-ultra low C, N) was investigated to claryfy the conditions of accelerating recrystallization in the coiling processes. The recrystallization during coiling significantly retarded when the heating temperature before rolling was higher than 1423K or coiling temperature was lower than 1023K. The effect of heating temperature was explained by the precipitation of Ti4C2S2. At heating temperature higher than 1423K, Ti4C2S2 dissolved and they finely re-precipitated during finishing rolling. These fine precipitates significantly retarded the recrystallization. At low coiling temperature, retardation of recrystallization was caused by the precipitation of FeTiP. At the coiling temperature below 1023K, FeTiP finely precipitated during coiling before recrystallization and retarded recrystallization. When P content decreases, precipitation temperature of FeTiP becomes lower and recrystallization at 1023K was accelerated. Two important conditions to accelerate recrystallization were confirmed. One is low heating temperature less than 1423K to prevent the dissolution of Ti4C2S2. The other is coiling temperature higher than 1023K to prevent the precipitation of FeTiP. In the latter case, the decrease of P content is also effective.
  • Effect of Ti Content on Grain Growth of Ultra Low-carbon Steel Sheets

    pp. 544-551

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    The effect of Ti content on the grain growth of Ti-added ultra low-carbon steels during annealing after cold-rolling has been investigated. The experimental result showed that the grain growth rate is slower in steels with lower Ti content than in steels with higher Ti content; the average grain size increased with 1/10th power of time in low-Ti steels and with 1/3rd power of time in high-Ti steels. This difference would be related to the change of precipitates. In low-Ti steels, the change in the average size of precipitates was small. This might be related to the disappearance of TiS and appearance of Ti4C2S2 during annealing. On the other hand, Ti precipitates in high-Ti steels ripened during annealing and this ripening of precipitates would control the grain growth; the Ostwald ripening. The grain growth is dependent on the chemical composition of steels other than Ti and the re-heating condition of slabs; the increases of Mn, the decrease of S and the decrease of re-heating temperature of slabs accelerated the grain growth rate even in low-Ti steel. This experimental result on the grain growth behavior agrees with the calculation result considering the Zener force.
  • Development of Recovery Technology of Valuable Metal Contained in Industrial Sludge by Smelting Reduction Furnace with Two-stage Tuyeres

    pp. 552-558

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    A new technology for recycling stainless pickling sludge has been developed by combination of a smelting reduction process of the sludge and a separation process of fluorine from the pickling.
    The stainless pickling sludge, which is generated by the neutralization of the waste pickling liquor, contains valuable metals such as iron, chromium and nickel. The sludge, however, also contains fluorine and the fact has obstructed the recycle of the sludge.
    We carried out bench-scale and pilot plant tests of the smelting reduction with coke packed bed in order to develop the recovery technology of valuable metal contained in the sludge. The reduction of the metal hydroxides and the behavior of the fluorine in the furnace were studied through the tests. A newly modified index of heat level was installed. By considering decomposition heat of hydroxide and reaction heat with carbon, reduction indices were uniformly described. Complete recovery of the valuable metals in the sludge was achieved at the pilot plant tests. The fluorine distribution ratio into slag was 21% and we expect that the fluorine leaching from the slag is 0.32 mg/l, which is lower than the environmental standard value of 0.8 mg/l. The pH of the cooling water for the off gas is mainly determined by the balance of the ion contents of fluorine and chlorine as the acid elements and that of calcium, sodium and potassium as the alkali elements.
  • Effect of CaO Addition on the Reducibility of FeO-SiO2 Based Slag Powder

    pp. 559-564

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    Recently, it is strongly desired to lower slag content of iron are sinter. In the sintering of the low slag ratio, one of the problems is the decrease of melt, which is needed to agglomerate the iron ores. As one of the methods to solve the problem, the use of silicate melt mainly composed of FexO-SiO2 is considered. However, the reducibility of the sinter tends to become worse by increasing FexO content. In the present study, the method is examined to improve the reducibility by adding CaO in the silicate melt and by precipitating the mineral phases composed of CaO and SiO2. In the examination, it is important to know the reducibility of FexO in CaO-SiO2-FexO slag. The hydrogen reduction rate of FexO in the formed slag and the precipitated phases by the addition of CaO to FexO-SiO2 melt at 1573K was measured at 1173K; the effect of CaO on the reducibility of FexO in CaO-SiO2-FexO slag has been investigated. From the results, it is found that the composition of the equilibrated slag at 1573K largely affects the reducibility of the slag at 1173K. The reducibility is improved by the precipitation of 2CaO SiO2 after CaO addition. The reducibility of both 2FeO·SiO2 and CaO ·FeO ·SiO2 are very low. Accordingly, it is important to prevent the formation of 2FeO SiO2 and CaO· FeO ·SiO2 to attain the higher final fractional reduction. The reducibility of the slag containing 5 mass% Al2O3 is a little better than that of the Al2O3-free slag, and the dependence of the reducibility on slag composition shows the same tendency as that of the Al2O3-free slag.
  • Effects of Rapid Preheating on Coal Structure and Coke Strength

    pp. 565-572

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    At present, a rapid preheating process of coals prior to coke ovens is under investigation in the national project (SCOPE21). The effects of the rapid preheating of coals on the coal chemical structures and the resultant coke strength have been examined in the present study.
    The cracking reactions of the coals appeared to be suppressed in the case of the rapid preheating (104°C/min) of the coals up to their softening temperatures, compared to slow preheating (10°C/min): the ratios of O/C and H/C in the rapidly preheated coals were found to be almost identical to those of the untreated coals. The NMR study revealed the enhanced mobility and structural relaxation of the coal molecules due to the rapid preheating.
    The rapid preheating of the coals improved the resultant coke strength. The enhanced mobility of the coal molecules, observed in the present study, is thought to improve the thermoplasticity of the coals at the melting stage, which lead to the improved strength of the resultant cokes.
    The effect of CO2 and H2O gas in the preheater on the resultant coke strength was studied. The estimated concentrations of CO2 and H2O gas was found to hardly oxidize the coals in the preheater.
    In conclusion, the rapid preheating process of coals is thought to be effective to improve the resultant coke strength.
  • Bottom Shape of Blast Furnace Deadman and its Floating/Sinking Behavior by 3-dimensional Model Experiment

    pp. 573-580

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    A three-dimensional scale model which simulates the motion of a deadman in a blast furnace was used as the experimental apparatus. The floating and sinking movement and the bottom shape of the deadman were investigated. The effects of factors such as furnace bottom shape and liquid level on the deadman behavior were examined.
    When the water level moved up or down, the movement of the deadman bottom center lagged behind the change in water level. When the deadman touched the furnace bottom before floating, floating of the deadman started earlier with air injection from the tuyere than without it.
    In the condition without blast from the tuyere, the initial shape was almost unaffected even if one of the factors such as water level, a load on the deadman, or exhaust of particles was changed as a variable. In contrast, in the condition with blast from the tuyere, when particles were exhausted simultaneously with water supply or drainage, the deadman bottom gradually rose up toward the furnace wall.
    Measures for floating or sinking of the deadman in an actual blast furnace were evaluated from the balance between solid load and buoyancy. In some cases a negative correlation was found between the calculated sinking depth and the center temperature of furnace bottom with some time delay. In other cases, a negative correlation was not found. It was considered that when a deadman behaves in a specific manner, it may affect the temperature of furnace bottom through the liquid flow in the hearth region.
  • Reaction Rate of Coke and Other Carbonaceous Materials with CO2

    pp. 581-586

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    The reaction rates of Bintyo char, Bamboo char, activated carbon, coke, graphite and glassy carbon with pure CO2 have been obtained by thermo gravimetric method. After the reaction, N2 was introduced to the reaction tube to obtain the amount of adsorbed CO on the active reaction sites. The specific surface area after the reaction was measured by BET method. The results are summarized as follows:
    The reaction rate per unit mass were in the order of Bintyo char_??_Bamboo char_??_activated carbon>graphite_??_coke>glassy carbon. The difference between the maximum and minimum were two orders of magnitude. The activation energy was 180 to 220 kJ/mol. The reaction rate per unit area of graphite, however, was an order of magnitude higher than those of Bintyo char and Bamboo char. The estimated amount of adsorbed CO was positively correlated to the reaction rate. The reaction rate constant which is independent from surface area was obtained using the reaction rate and the amount of CO adsorption, respectively per unit area. They are ranged in the order of 10-4 to 10-3 1/s. The differences among Bintyo char, Bamboo char and graphite became smaller. The enthalpy of CO adsorption was estimated as about 230 kJ/mol for Bintyo char and Bamboo char, and about 280 kJ/mol for graphite.
  • Simultaneous Determination of Arsenic, Bismuth and Antimony in Steels by High Power Nitrogen Microwave Induced Plasma Atomic Emission Spectrometry Coupled with Hydride Generation Method

    pp. 587-592

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    The feasibility of an annular-shaped high power nitrogen microwave induced plasma atomic emission spectrometry (N2-MIP-AES) has been studied for the simultaneous determination of arsenic, bismuth and antimony in combination with the hydride generation method. Under the optimized experimental conditions, the best attainable detection limits at As I 228.812, Bi I 223.061 and Sb I 231.147 nm lines were 7.13, 116 and 14.6 ng/ml for arsenic, bismuth and antimony, respectively, with linear dynamic ranges of 30 to 10000 ng/ml for As or Sb and 300 to 10000 ng/ml for Bi in concentrations. The presence of several diverse elements was found to cause more or less a depressing interference with the proposed technique. Of the several pre-reductants examined, thiourea was found to be the most preferable to reduce arsenic and antimony from their pentavalent state to trivalent one prior to hydride generation. Therefore, thiourea was utilized as a pre-reductant for the determination of total arsenic and antimony concentration, i.e., As(III)+ As(V) and Sb(III)+Sb(V). When arsenic, bismuth and antimony in steels were determined simultaneously, a large amount of Fe(III) in the solution caused a severe depressing interference, while the presence of Fe(II) showed little or no significant interference. Of the several interference-releasing agents examined, L-ascorbic acid was found to be the most preferable to reduce Fe(III) to Fe(II). The proposed method, using both thiourea as a pre-reductant and L-ascorbic acid as an interference-releasing agent, was applied to the simultaneous determination of low concentrations of arsenic, bismuth and antimony in steels. The results obtained by this method were in good agreement with their certified values.
  • Wear Resistance and Resistance to Surface Roughening of Newly Developed Material for Piercing Roll with High Carbon and Low Hardness

    pp. 593-600

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    Piercer roll in the pipe manufacturing mills requests the wear resistance and bite efficiency. Usually, low carbon alloyed cast steels are applied to the piercer roll, but have low wear resistance. In order to improve the wear resistance of piecer roll, three tests samples with high carbon content and low hardness are investigated. The most suitable carbon level was determined by using the high temperature wear simulator. The main results obtained are as follows; (1) Microstructure of new piecier roll material consists of ferrite and spheroidal carbide. (2) The carbides except spheroidal carbide cause to heat cracks by thermal shocks to decrease wear resistance and resistance to surface roughness. (3) In case of high temperature wear testing, the micro-structure of matrix consisting much of ferrite structure shows the good wear resistance than the matrix of pearlite structure contained. (4) Ferrite structure is worn by abrasion mechanism and pearlite causes to fatigue cracking and pealing off. (5) The excellent wear resistance of low hardness roll consists of the spheroidal carbide and ferrite, which is gripping carbide and is worn keeping the effective surface roughness.
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  • Effect of Silicon Content and Lubricant on the Machinability of Hot Working Tool Steel

    pp. 601-608

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    Influence of silicon content of hot working tool steel (4mass%Cr-Mo-V-Ni) on machinability at milling process has been investigated. The effect of silicon was compared with that of lubricant, concerning tool life, cutting forces and temperature, chip detachment from a tool, tool damages, surface oxidization, microstructure and shapes of chip.
    The observation of the chip detached from the tool by high-speed video camera showed that increasing the amount of silicon resulted in suppression of adhesion between the tool and the chips. This lubrication effect of silicon coincided with the effect of lubricant sprayed. The suppression of adhesion by increasing silicon content prolonged the tool lives, probably because the scale of Fe-Si-O formed on the surface of the chips of high silicon steel lubricated more effectively than that of Fe-Cr-O formed on the surface of low silicon steel.
  • Effect of Initial Microstructures before Annealing on Ductility of TRIP-Aided Steel Sheets

    pp. 609-615

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    The TRIP-aided steel sheets have been known as the excellent formable high strength steel sheet with high elongation property. However, itsstretch-flangeability, which is another necessary property of products, is inferior. In this research, the stretch-flangeability of the retained austenite steel sheets with the annealed martensitic or bainitic matrix of 0.15C-1.5Si-2.0Mn and 0.20C-1.5Si-1.5Mn steels were investigated.
    Initial microstructures as bainite, martensite, and ferrite-martensite were prepared by hot rolling, and following annealing heat treatment was applied. These steel sheets applied with such process increase the stretch-flangeability also elongation than the conventional retained austenite steel with polygonal-ferrite matrix and block-like retained austenite. Furthermore, the higher stretch-flangeability shows in the case that initial microstructure of the hot rolled sheet is full-bainite or full-martensite rather than ferrite-martensite microstructure. In these steel sheets, lath-type retained austenite morphology and annealed martensitic or bainitic matrix without polygonal-ferrite causing high HV ratio are considered as effective factors for increasing stretch-flangeability.
  • Grain Size Control by Oxide Dispersion in Austenitic Stainless Steel

    pp. 616-622

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    The difference in the deoxidation condition between Al-Ca system and Si-Mn system was discussed in terms of grain growth behavior of anaustenitic stainless steel (Fe-17%Cr-9%Ni-3%Cu-low C, N alloy). In the steel deoxidized by Si-Mn, oxide inclusion exists as MnO-SiO2 particles in the as-cast ingot. However, once this steel is annealed at 1523K for 3.6 ks, a part of MnO-SiO2 particles decomposes and MnO-Cr2O3 particles are newly formed on the annealing. The particle size of MnO-Cr2O3 is about 0.2μm span and this size is much smaller than that of MnO-SiO2 particles (about 1μm). This oxide transition from MnO-SiO2to MnO-Cr2O3 is very useful for suppressing the grain growth of recrystallized austenite grains on annealing at 1373K after 65% cold working because the reprecipitated fine oxide particles pin the austenite grain boundary effectively. The relation between austenite grain size and oxide particles dispersion is not explained by the wellknown Zener's relationship but done by the Doherty's theory in which a half of particles are thought on grain boundary and play a role to pin the grain boundary. On the other hand, in the steel deoxidized by Al-Ca, stable Al2O3-CaO particles are formed in the as-cast ingot. This oxide is so stable that it never causes the oxide transition on annealing like that in the steel deoxidized by Si-Mn. Therefore, the grain refining through recrystallization process is never expected in the steel deoxidized by Al-Ca.

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