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

Tetsu-to-Hagané Vol. 81 (1995), No. 3

  • Relationships between Microstructure and Mechanical Properties in Steels

    pp. 157-166

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  • Effect of Ar Bubbling on Removal Rate of Alumina Inclusion in Al Deoxidized Steel

    pp. 167-172

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    Effect of Ar bubbling on the deoxidation rate of molten steel with Al has been investigated in 20 kg scale induction furnace. The results are summarized as :
    (1) The deoxidation was completed within 5 to 15 min depending on experimental conditions. The final total oxygen content was 30 to 45 ppm.
    (2) The deoxidation model has been proposed in which deoxidation rate should be determined by the balance of removal and reentrainment rates of alumina inclusion.
    (3) The removal rate constant was obtained at each condition. The apparent activation energy was obtained as 284, 355, 368 kJ/mol, respectively with Ar flow rate of 0, 5, 10 l(STP)/min.
    (4) The removal rate constant was expressed as a function of total stirring energy density for induction stirring(εind)and Ar gas stirring(εB/B), namely, k=k1indB/B)n.The value of n was obtained as 0.50 to 0.65.
    (5) The contribution of entrapment of inclusion in bubbles was estimated quantitatively. The transfer coefficient of inclusion migration to bubble was also estimated.
  • Cold Model Experiments on the Enhancement of Scrap Melting in a Bath Accompanied by Gas Injection

    pp. 173-178

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    A fundamental study based upon an aqueous system was carried out to reveal the effect of gas injection on the melting rate of solid bodies such as scrap. Ice spheres and ice cylinders were used as models of solid metals for the aqueous system. As the first step, the melting behavior of a single ice sphere or cylinder in a water bath was observed in this study using a high-speed video camera and by eye inspection. The melting rate of the ice was highly increased with an increase in gas flow rate. An estimation method of the melting time of the ice was proposed. Measured values of the melting time were found to compare well with estimated ones. The estimation method of the melting time originally proposed for the aqueous system was applicable to Wood's metal systems by adding time required for a solid Wood's metal sphere or cylinder to be heated up to its melting point.
  • Levitation and Heating of Metallic Ball in Cold Crucible Simultaneously Supplied with Two Frequencies

    pp. 179-184

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    The levitation forces working toward the metallic ball in the cold crucible and the temperature rise of the metallic ball were measured under supplying two different frequencies (3kHz and 30kHz) simultaneously with the object of the optimum operation of the cold crucible type levitation melting. The results obtained are as follows.
    (1) The levitation forces supplied by two frequencies take additional values of 3kHz and 30kHz which are supplied independently.
    (2) The behavior of temperature rise of the metallic ball in the case of supplying two frequencies are almost equivalent to additive behavior of 3kHz and 30kHz power supplying independently.
    A new control technique for levitation melting where rather lower frequency is mainly allotted for levitation and higher frequency is mainly allotted for heating has been suggested by the above stated results.
  • Influence of Heating Temperature and Strain on Surface Crack in Carbon Steel Induced by Residual Copper

    pp. 185-190

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    Mechanism of surface crack formation of steel induced by residual copper (Cu) is investigated using a new technique of Greeble test. Two kinds of experiments were carried out, to clarify the effect of temperature and to understand the behavior of crack growth. Crack is caused by liquid Cu, which precipitates at steel-scale interface during oxidation. However, no crack formed at higher temperature. Micro analysis indicates that it is due to the formation of liquid scale above eutectic temperature of FeO-2FeO·SiO2. Liquid Cu-precipitates are trapped in the liquid scale area, and they cannot penetrate into austenite grain boundaries. The fact that silicon addition reduces the crack formation also supports this mechanism.
    Deformation test with various strains reveals that there exist two stages in the behavior of crack growth. At the firststage, crack grows deeper, because liquid Cu penetrates into the boundary. The crack stops growing along the depth direction and opens its width in the second stage, because of the lack of liquid Cu. That means the amount of Cu-precipitates decides the crack depth.
  • Development of Accurate Temperature Control in Hot Strip Mill

    pp. 191-196

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    A temperature control of hot strips on the run-out table due to a proper cooling plays a significant role for controlling their mechanical properties. For high carbon steels, for example, the temperature control is quite difficult because of a remarkable heat development (thermal generation) caused by latent heat of pearlite transformation. To realize required temperature control, a simplified metallurgical online model has been developed which predicts the transformation behavior and the resultant heat development. In this model, history of removed thermal energy during cooling process has been introduced instead of conventional temperature history. As a result, high accuracy has been realized on special steel without any human intervention to cooling control system.
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  • Precipitation Behaviour of Titanium Carbo-sulfide in Extra-low-carbon Titanium-added Steels

    pp. 197-202

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    Precipitation behaviour of titanium carbo-sulfide in extra-low-carbon titanium-added steels was studied in detail by Transmission Electron Microscopy (TEM).
    Co-precipitation of TiS and MnS was observed to form in chain-like groups at austenite grain boundaries during holding the specimen at 950°Cafter reheating at 1250°C. This seemed to result from preferential precipitation at interfaces with the help of preceding grain boundary of S. On the other hand, hot working at 950°C suppressed the co-precipitation of TiS and MnS at austenite grain boundaries through recrystallization, and enhanced the strain-induced Ti4C2S2 homogeneous precipitation within austenite grains. In the case of reheating at 1250°C followed by holding at 950°C, and then keeping at 700°C, co-precipitated TiS and MnS at prior austenite grain boundaries gradually changed to Ti4C2S2 with the keeping time, and also fine precipitation of Ti4C2S2 additionally occurred within ferrite grains.
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    Readers Who Read This Article Also Read

    1. Analysis of Precipitates and Precipitation Behavior in Nb-Ti Bearing Steels Tetsu-to-Hagané Vol.82(1996), No.2
    2. Rate of Dephosphorization of Liquid Iron by Solid Lime Tetsu-to-Hagané Vol.58(1972), No.14
    3. Strengthening Mechanisms of Ti and Nb Bearing Steel Plates Produced by Thermomechanical Control Process Tetsu-to-Hagané Vol.82(1996), No.6
  • Effects of Alloying Elements and Oxygen Potential on Equilibrium Carbon Content in Gas Carburizing

    pp. 203-208

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    In order to control the surface carbon content of gas-carburized steels, the effects of alloying elements on equilibrium carbon content of low alloy steels based on JIS SCr420 grade were investigated. 1.0mm and 0.3mm thick specimens were carburized for 108ks or 288ks. As the carburizing time was long, the carbon content of the specimens deviated from the equilibrium value based only on the carbon potential in the atmosphere while excluding oxygen potential. The deviation was affected by Si, Cr, V content which are oxidized in the surface layer.
    These results show that the equilibrium carbon content in gas carburizing atmosphere is also affected by oxygen potential in the atmosphere ; i.e. surface oxidaiton of alloying elements decrease the amount of the alloying elements in austenite, which changes the activity coefficient of carbon. Furthermore it is indicated that the growth of oxide results in re-carburization or decarburization. Therefore, oxygen potential in gas carburizing atmosphere is one of the important variables affecting the surface carbon content of gas-carburized steels.
  • Fracture Toughness in the Transition Region of a Carbon Steel and a Ferritic Nodular Cast Iron

    pp. 209-213

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    In order to characterize the fracture toughness in the ductile-brittle transition region for thick-walled cylinders of ASME SA350 Gr.LF5 carbon steel and JIS FCD300LT ferritic nodular cast iron, elastic-plastic fracture toughness tests were carried out. The specimens were fatigue precracked compact tension (CT) specimens of 25mm in thickness. The tensile testing machines used were Instron type, electrohydraulic type and drop-weight type ones.
    In the static fracture toughness test on a FCD300LT cast iron. CT specimens were often fractured at somewhat higher loads after the initiation of pop-in cracks. Although the scatter of pop-in fracture toughness was small, the values of critical J-integral at the unstable brittle fracture scattered largely.
    In the transition region of SA350 steel, the initiation of pop-in crack was not observed, and fracture toughness scattered largely.
    At the propagation of the unstable crack near the transition temperature, the Weibull distribution provides good fits for the critical CTOD and the critical J-integral. This distribution can be mainly interpreted by the scatter of the distance between the precrack tip and the origin of unstable brittle fracture.
  • Long-term Creep Strength Property and Microstructural Stability of 12Cr Steel

    pp. 214-219

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    Heat-to-heat variation of long-term creep strength property observed for 12Cr steel was investigated from viewpoints of creep deformation behaviour and stability of microstructure. Heat-to-heat variations of creep deformation behaviour and creep rupture strength properties were described by the differences in the parameter α in a modifiedθprojection method which indicates the acceleration in creep rate at the tertiary stage. It was shown that the magnitude of α and its change with creep depend on the morphology and stability of initial microstructure, and correlate with microstructural changes accompanied by recovery and recrystallization under tertiary creep. It is concluded therefore that the creep strength and rupture life can be controlled by the microstructure and its stability of 12Cr steels and expressed only by the rate constant α.
  • Collective Evaluation of Temperature and Stress Dependence of Creep Rupture Life in Austenitic Stainless Steels

    pp. 220-224

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    Temperature and stress dependence of creep rupture life was examined on 24 heats of type 304, 316, 321 and 347 stainless steels. Collective evaluation of the results indicated the existence of three regions, H, M and L, with different activation energy, Q, and stress exponent, n, for rupture life : region H at a shorter rupture life, region L at a longer rupture life, and region M in between the two regions. The values of Q and n decrease with increasing rupture life as expected from creep fracture theories, suggesting that a longer rupture life than the actual value would be predicted erroneously if it is estimated from the short term data taking the high values of Q.
    Transgranular and intergranular fracture take place respectively in the regions H and M, and the fracture process in the two regions is controlled by creep deformation. The intergranular fracture in the region L starts from cavities nucleated at sigma-phase/matrix interfaces, and this fracture process is controlled by grain boundary diffusion. The changes in Q and n coincide fairly well with the changes in creep mechanism. This fact points out that the changes in Q and n are inherent to the austenitic stainless steels and the same thing will happen in any materials similar to the four types of stainless steels.
  • Resistance against the Stable Crack Growth and Brittle Fracture Initiation Site Controlling the Ductile-Brittle Fracture Transition Behaviors of Low Carbon Steels

    pp. 225-230

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    The fracture process of low carbon steels have been investigated by means of a fracture mechanics test and microscopic observations. Three steels with characteristic differences in the ductile-brittle transition behaviors such as the transition temperature, R-curve and brittle fracture initiation stage are employed. A new microstructural parameter, the constraint factor, is defined to take into account the constraint of plastic deformation by grain boundary carbides. The constraint factor is successfully correlated with the resistance against the stable crack growth. The crack tip field is analyzed in terms of the elastic-plastic analyses in terms of J/σy and COD, and the elastic-plastic analyses are found to be applicable at the ductile-brittle transition region. The brittle fracture initiation is controlled either by the local stress or by the local strain for respective steels. The constraint factor is also correlated with the location of the brittle fracture initiation sites. The estimated local stress at the brittle fracture initiation site shows a unique temperature dependence irrespective of the microstructure of steels. It is suggested that the nucleation of an incipient crack at the brittle initiation site is microstructure dependent through a local deformation behaviors.
  • Quality Evaluation of Line Pipes by the Chevron Notched Drop Weight Tear Test

    pp. 231-236

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    The WG in High Strength Line Pipe Research Subcommittee of Steel Pipes and Tubes Committee of The Joint Research Society of ISIJ investigated the applicability of the Chevron Notched Drop Weight Tear Test to Japanese high toughness line pipes, and developed a new test method, i.e., the Slit Chevron Notched Drop Weight Tear Test. The effects of the slit width and the notch angle of the specimen was investigated in details, and the established method was verified by the burst tests on actual pipes.The fracture appearance transition temperature of the established test is a little safe side estimation to the brittle fracture of line pipes, and the impact energy of the test gives the prediction to the ductile crack propagation and arrest in natural gas transmission pipelines. It is expected that the developed method serves for the quality evaluation of high toughness line pipes against brittle fracture and ductile crack propagation and arrest.
  • Environmental Effect on the Ductile-Brittle Transition of High Purity As-rolled Chromium

    pp. 237-242

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    Effect of environment on ductile-brittle transition of high purity as-rolled chromium were experimentally investigated using a small punch (SP) test in ten different atmospheres. The ductile-brittle transition temperature (DBTT) of pure chromium is sensitive to the test environment. DBTT under the environment of water or air is lower than that in vacuum. Also the ductility of chromium at room temperature in air is higher than that in a dry hydrogen gas atomosphere.
    The values of the SP energy are higher in a wet gas atmosphere than that in a dry gas atmosphere. This result is contrary to the observation in high-strength steels or intermetallic compounds in which the ductility becames poorer when tested in a wet gas atmosphere.
  • 誌上討論

    pp. 243-248

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  • Introduction to Reaction Kinetics for Material Processing (I)

    pp. N84-N94

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  • Capacities of Molten Slag

    pp. N95-N100

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    1. Equilibrium between Fe0-MnO-SiO2 Slags and Molten Iron Tetsu-to-Hagané Vol.51(1965), No.6
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