In order to investigate the optimum operating conditions for the Fe3C production from iron ore particles in CH4-H2mixtures, experiments were carried out in a fixed bed or a fluidized bed in a flow of 3.33×10-5m3/s of CH4-H2 mixtures and temperature range from 650 to 950°C. The present study has revealed the most appropriate conditions that ratio of CH4 to H2 is 40/60 and temperature is 750°C. Under these experimental conditions most of the reduced iron was converted into Fe3C for about 30 minutes. But during reaction, a part of the Fe3C was decomposed into Fe and C, although Fe3C formed at 750°C was hard to decompose in comparison with those formed at other temperatures. During reaction, if unreduced FeO remained in iron ore it may accelerate the decomposition of Fe3C. Therefore, porous iron ore is better to produce Fe3C than dense iron ore because of high reducibility of porous iron ore.
The experiments using a 100kg induction furnace were carried out to evaluate the effect of combined blowing on efficiency of decarburization of high carbon stainless steel melt. Temperature of hotspot area was measured by an optical fiber scope and fluid motion was observed by a video. Also, micro analysis of samples taken from hotspot area was done. Results were summarized as follows: 1. Efficiency of decarburization with the combined oxygen blowing became larger than that with top oxygen blowing or bottom oxygen blowing. By micro analysis of hotspot samples and video observations, the bottom blowing oxygen gas in combined blowing promoted the interface stirring between metal and slag and the reduction of (Cr2O3) in hotspot region. 2. In combined oxygen blowing with same oxygen blowing conditions, temperature of hotspot area raised with raising the bath temperature, and efficiency of decarburization increased. This reason was explained by the promotion of (Cr2O3) reduction in hotspot by means of raising the hotspot temperature.
From the results of the experiments using a 100kg induction furnace and a 60ton AOD, it was found that efficiency of decarburization with the combined blowing became larger than that with the top blowing or the bottom blowing. It was suggested that the formation and reduction of (Cr2O3) in the hotspot with high temperature have influenced on decarburization reactions. The decarburization potential by chromium-oxide (CRP) was definded as the sum of the discrepancy (-ΔG) from decarburization reaction equilibrium by (Cr2O3) in hotspot and bulk of metal, and experimental results were evaluated by using CRP. There were good correlations between efficiency of decarburization and the value of CRP in every oxygen blowing types. The decarburization rate was controlled by the balance of oxygen blowing rate and rate of carbon supply and conditions of reaction zone, and the decarburization by combined blowing with hard top blowing was promoted by the reduction of (Cr2O3) in hotspot region.
To experimentally obtain the transition probabilities of Ar atom, spectroscopic measurements were carried out on Ar arc plasma for melting metal by using highly sensitive multichannel detector. Then the Boltzmann plot method was newly adopted for determination of transition probabilities, using the values recommended by Wiese et al as starting values. The Boltzmann plot obtained by using present transition probabilities was linear at high correlation coefficient over 0.9 in spite of the variations of plasma temperature, working gas pressure and metallic vapor. Therefore, it was concluded that new transition probabilities can be used in temperature measurement of Ar arc plasma.
This paper treats numerical analyses of the flow structure of molten steel in a mold, focusing upon the unsteady behaviour of the free surface profile and velocity. These calculations were performed using the MAC-type solution method to solve a finite differencing approximation of the three-dimensional Navier-Stokes equations governing incompressible fluid flow. Here, the nonsteady body-fitted coordinate system was used so that the uppermost surface coordinate of the computational domain fits the free surface boundary in the physical space and therefore the mesh system is renewed at each computational time step. Also, the experimental study was undertaken to measure the surface velocity distribution, using water instead of molten steel. It was found that the numerical time-averaged surface velocity distribution is fairly in agreement with the experimental data. According to the results obtained by the present mathematical model, the flow field in a mold, including the free surface configuration and velocity, has been clarified to locally and temporarily fluctuate in somewhat a periodic manner. The effect of the casting speed and the kind of immersion nozzle on the flow structure of molten steel is estimated and discussed from a practical standpoint.
Thin strip casting of 4.5wt% silicon steel has been carried out with a twin roll caster with cooling rolls 550 mm in diameter and 500 mm in width. The casting speed was of the order of 2.1-5 m/sec and the thickness of the strip varied from 0.2 to 0.5 mm. One of the significant technologies for the casting of tonnage strip is the design of the long-lived cooling rolls. Micro-cracks occured around the roll surface when casting heats of more than one ton. In these experiments, the roll remperatures of the inner and outer surfaces were measured during casting and thermal cycle of roll temperature was calculated from the measured values. Roll deformation behavior and working stress were analyzed by the two dimensional finite element method. Based on these theoretical calculations, this paper reports on (1) surface coating by Ni-Cr plating on the rolls, (2) design for an increased cooling water circulation rate in the rolls, and (3) application of a Cu alloy sleeve having excellent high-temperature strength. By adopting these improvements, the authors succeeded in producing a thin strip on a 3 ton/heat scale by twin roll casting.
The testing method for delayed fracture has not been standardized yet. The quantitative evaluation method is necessary to develop low alloy high strength steels with higher delayed fracture resistance. The sustained load tests under cathodic hydrogen charging condition have been investigated considering the severity of the actual environment. The delayed fracture and hydrogen absorption behavior have also been investigated especially in 0.5C-0.3Mn-lCr-0.7Mo0.03Nb-0.3V-low P-low S steels. The maximum hydrogen permeation coefficient, 0.1μA/cm, has been determined at the lowest pH=3.5 realized in the local environment such as in a crevice. The sustained load tests using notched round bar specimens have been carried out under cathodic hydrogen charging condition corresponding to 0.1μA/cm. The results show that this steel has the enough resistance to delayed fracture as 1.3GPa grade-high strength bolts. Although the apparent diffusible hydrogen content is much higher than 1.1GPa grade-JIS SCM440 steel, this steel absorbs the diffusible hydrogen evolved at more than 200°C in thermal analysis. It has been clarified that this diffusible hydrogen evolved at elevated temperatures has no relation with hydrogen embrittlement. Therefore, the susceptibility for embrittlement would be lower in this steel because of the uniform carbides dispersion and the decrease in the internal strain due to the high temperature tempering.
Fatigue is generally a surface related phenomenon as the fatigue cracks usually initiate at the surface and propagate into the bulk material. The surface layer of residual compressive stress induced by shot peening is of primary practical importance. Roughly speaking, the residual stress acts as an applied mean stress and a compressive residual stress will therefore relate fatigue crack initiation and growth. The work hardening results in an increased dislocation density which hinders dislocation movements due to the fatigue load and suppresses localized plastic deformation which is a starting feature for crack initiation. From Mises-Henckey criterion and our previous investigation, it is expected that one of the effective way of obtaining high hardness and residual compressive stress to hard materials is high energy hard shot peening under applied tensile stress. A systematic study of hard shot peening with water jet under various tensile stressing on surface residual stress of carburized SCM420 specimens has been conducted. As a result, the highest peak value, 1400MPa and distributions of residual compressive stress at the surface region were obtained. Also peak residual stresses increase linearly with applied tensile stress. These increments were caused by enhancing effect which is explained by Mises-Henckey criterion of shot penetration to the hardened surface due to applied tensile stress.
As the hardness of high carbon martensitic stainless steels is very high, they are used for cutlery and bearings, etc. However, as huge Cr carbides larger than 20 μm in diameter are formed in these steels, knife edges are easily chipped and fatigue cracks during rolling are initiated from these huge Cr carbides. In this paper, the effect of nitrogen addition to obtain fine Cr carbides in SUS440A (Fe-16.5%Cr-0.65%C) has been studied, and the effect of nitrogen content on both mechanical properties and the corrosion resistance has been investigated. The main results are summarized as follows : The addition of nitrogen more than 0.25% supresses the crystallization of eutectic Cr carbides during casting, and fine Cr carbides can precipitate after hardening heat treatment. As a result, the impact toughness of hardened and tempered sheets is improved, the maximum hardness shows more than HRC60, and the cold workability, the resistance to temper softening and the corrosion resistance are comparable to conventional low nitrogen steels.
Load-displacement curves of a solution-treated β titanium alloy were investigated in the temperature range between 77 and 355K at the crosshead speed from 0.05 to 50mm/min (initial strain rate from 3.3×10-5 to 3.3×10-2s-1). At higher temperature or lower crosshead speed, the alloy work-hardens gradually as it deforms and reaches the plastic instability after showing some uniform elongation. On the other hand, at lower temperature or higher crosshead speed, the alloy shows the plastic instability at the very initial stage of plastic deformation. The above-mentioned results were discussed, based on the equation of plastic instability, dσ/dε=σ. The flow stress increases remarkably with a decrease in test temperature or with an increase in strain rate. However, the work-hardening rate little depends on strain and strain rate, and increases proportionally to the Young's modulus with a decrease in test temperature. Thus, it is concluded that the plastic instability occurrs at the earlier stage of deformation as the temperature lowers or the strain rate increases.
For evaluating the creep damage of 2.25Cr-1Mo steel, intra-granular distortion (IGD) analysis, a method of average plastic distortion calculation in transmission electron microscope (TEM) was applied. Linear relation between creep strain and average intra-granular distortion at 843K and 108MPa reported in previous report was examined with two 2.25Cr-1Mo steels and at various creep conditions. Relations between creep strain and intra-granular distortion of tensile and stress accelerated creep test specimens were explained by single line, though temperature accelerator creep test conditions such as at 943K reduced the distortion in grains with microstructural coarsening. Two steels showed almost same distortion at stress accelerated conditions. Fine Mo2C precipitation with high dislocation density on the middle portion of 2.25Cr-1Mo steel tube used for 220, 000 hours in fossil power plant indicated the applicability of this technique for long time degradation at service condition.
Internal friction in the hydrogen-charged 18Ni maraging steel was measured as a function of temperature from 140 to 350 K at a frequency of about 600 Hz. A characteristic internal friction peak was observed at about 240 K after hydrogen charging under the as-aged condition. The height of the internal friction peak increased with increasing hydrogen content, but appeared to be saturated at a certain level of hydrogen content. The saturated value of the peak height increased with the degree of cold work, that is, with increasing reduction of area for cold rolling. Thus, this internal friction peak was identified as the cold-work peak based on the hydrogen-dislocation interaction in maraging steel, as has been well known in pure iron. The peak can therefore be interpreted to be caused by the hydrogen atoms trapped at dislocations. The hydrogen cold-work peak in the underaged specimen was found to shift to lower temperature compared with that in the optimum aged specimen. This difference in the peak temperature seems to be attributed to the change in the binding energy between hydrogen atoms and dislocations.
The effect of plastic stability of retained austenite on elongation was investigated using Fe-0.17%C-1.99%Si-1.77%Mn hot-rolled high strength steel sheets produced in the mill-scale test. The results obtained are as follows; (1) Total elongation, uniform elongation and local elongation have a tendency to be improved with the increase of the plastic stability of retained austenite when volume fraction of retained austenite is nearly equal. (2) The plastic stability of retained austenite has a tendency to be improved with the increase of carbon contents in retained austenite. (3) The volume fraction of retained austenite at 15% tensile strain, kp and n-value over very small strain interval have strong relation to elongation. They are considered to be indexes for the effect of the plastic stability of retained austenite on elongation.
Sintering treatment in 1atm-N2 gas atmosphere was applied for 23mass% Cr ferritic stainless steel powder compacts, and structural changes of the powder compacts by nitrogen absorption were investigated by means of optical microscopy, chemical analysis and X-ray diffraction. Mechanical properties of the sintered materials were also examined in relation to nitrogen content and microstructures. The results obtained are as follows : (1) In the sintering of powder compacts in N2 gas atmosphere, nitrogen absorption into powder particles causes phase transformations from ferrite to other phases. The phases formed are dependent on the temperature and nitrogen content (2) The amount of nitrogen absorbed into steel powder particles is determined by the surface equilibrium between N2 gas and nitrogen content of steels, so that the saturation nitrogen content increases with a fall in the sintering temperature. At temperatures below 1473K, nitrogen is concentrated enough to form nitrides such as CrN and Cr2 N. (3) At around 1473K, the steel powder can absorb about 1mass% of nitrogen, and this causes a structural change of the matrix from ferrite to austenite which dissolves all of nitrogen. The austenitic structure obtained is so stable at room temperature that the sintered steel does not undergo martensitic transformation during tensile deformation. (4) A sintered steel with the chemical composition of 23mass% Cr-1mass% N has about three times large 0.2% proof stress in comparison with a sintered SUS304L steel with little nitrogen, and also has a good ductility in spite of containing about 12vol.% of retained pores. (5) Sintering in N2 gas atmosphere makes the use of a large amount of nitrogen possible for stainless steels, therefore, stable austenitic structure is easily obtained without adding expensive alloying elements such as nickel.