With the aim to decrease the reducing agent rate in blast furnace, the use of ferro coke containing metallic iron as a catalyst was proposed. This new burden material is expected to lower thermal reserve zone temperature. For the production of ferro coke, the use of a vertical coke oven is examined to realize the optimum carbonizing process. In this type of the oven, the burden materials have a distribution in residence time due to the friction effect on the wall and the interaction between the burdens. It has a great influence on the properties and quality of the ferro coke. Increasing the size of the vertical coke oven is essential for producing a large amount of ferro coke efficiency with stable quality. In this study, the burden descending behavior in an enlarged vertical coke oven was calculated by Discrete Element Method (DEM). The effects of the shape of the discharging part and the discharging method on the residence time of the burden were analyzed based on the calculated results. In addition, considering the reinforcement of the enlarged coke oven, the burden descending behavior in the vertical coke oven with reinforcing horizontal beam inside was also examined. The calculation results showed that distribution variation in the residence time could be decreased by increase in the size of the vertical coke oven. Furthermore, the results showed the possibility of controlling the residence time by controlling the discharge method.
Increasing coke gasification rate can lowers the temperature of the thermal reserve zone, resulting in a decrease of carbon consumption and a reduction of the reducing agent rate of blast furnaces. To achieve this increase, the enhancement of coke reactivity itself or the close arrangement of iron ore and carbonaceous materials has been investigated in Japan. Against this, RCA, “Reactive Coke Agglomerate,” having a high carbon content, has been developed, and it was found that the agglomerate mixed-in sinter layer had two functions: one having high reducibility itself and the other enhancing the reduction of the surrounding sinter. As a result of the two functions, a significant decrease of the temperature of the thermal reserve zone and an increase of gas utilization by using the agglomerate mixed-in sinter layer in a BIS test was achieved. As for the strength after reaction, disintegration was fairly small in comparison with that of the sinter both in the laboratory scale test and in a basket test using a plant’s vertical probe. Long-term plant trials have been conducted at the Oita Works No.2 Blast Furnace with a maximum use of 54 kg/tHM. It was found that RCA could lower the temperature of the thermal reserve zone and carbon consumption in a commercial blast furnace. Carbon consumption was decreased along the relationship of 0.36 kgC/tHM per 1 kgC/tHM of input carbon from RCA.
In order to clarify the causing mechanism of center cavity in the cast of high Cr steel, the casting experiment of cylindrical ingot in 1 ton scale and the analysis of thermal elastic-plastic stress and strain were carried out. Moreover in relation to these results, the influence of Cr and C concentration on the generation of center cavity was discussed with tensile strength just below the solidus temperature and solidification contraction of the specimens. As the result, though both center line cracks and shrinkage holes were observed in the center cavity of high Cr steel, difference of solidification contraction between specimens was not much. The primary factor of center cavity was proved to be cracking caused by thermal stress under the lower tensile strength just below the solidus temperature associated with the increase of the ferrite amount which should be influenced by Cr and C concentration. Therefore, it was indicated that laying compressive stress on the final solidification part was effective to reduce center cavity in the high Cr and low C steel.
To produce high tensile strength steel sheets in cold rolling with good thickness accuracy, it is important to supply material hot bands without harmful fluctuations in their deformation resistance. For the purpose, it is effective to set a high coiling temperature in the hot rolling stage. On the other hand, it is well known that the surfaces of steel sheets easily form oxide at a high temperature in the case of steel grade which contains Si and Mn in its chemical composition. To avoid the formation of oxide, it is necessary to set a low coiling temperature in the hot rolling stage. By the conventional hot rolling conditions, it is difficult to satisfy conditions both to get uniform formability in cold rolling and to prevent the surface of hot strips from forming internal oxidation zone. This report describes the improvement on the mechanical property without harmful internal oxidation zone of hot strips by the proper heat histories in manufacturing the hot rolled 0.17C-1.3Si-2.0Mn high tensile steel sheet.
If surface defects on hot rolled wire rod subsequently remained, it might become the starting point of cracks in the forging process. Therefore, the surface quality for hot rolled wire rod is strongly required. In order to avoid surface defects, it is important to establish methods for predicting the positions and the criteria of the defects as a function of rolling conditions. In this paper, we assume that the defect is a kind of plastic instability on the surface. We analyze the criteria of the defects by using a geometrical velocity field. By using FEM, we calculate the velocity vector and the geometry of the surface during rolling. The angle between the normal vector and the velocity vector on the surface is constantly changing during rolling. In order to analyze the phenomena, we introduce the new parameters η = v→·n→ and ψ = dη/dθ. Comparing with the experimental results of the position of defects, the velocity gradient (ψ) is a good index for prediction of the defect position. In that case, ψ is locally changing near the inlet on the roll bite. A plastic instability might occur on the surface, as a result of this condition.
Displacement and strain during welding are important parameters for evaluation of weld joint. But in-situ measurement of these values needs non-conductive method because weld joint reaches to high temperature. So we applied electronic speckle pattern interferrometry (ESPI) system to measurement of displacement and strain during gas-tungsten-arc (GTA) welding of 9%Ni steel. This system detects interference of reflected laser illumination from weld joint surface. The change of interference depends on displacement of weld joint surface. ESPI measure-ment starts simultaneously with GTA heating, and continues during cooling after heating finished. ESPI system reveals behavior of two dimensional displacement distribution during welding and martensitic transformation in weld joint. Quantitated displacement record of certain point in GTA heating area shows detail of martensitic transformation. Martensite start temperature (Ms) analyzed by ESPI result shows higher value than conventional value measured by uniform heating and cooling process. This result shows stress-assisted martensitic transformation occurred in this area, because tensile stress is forced after solidification of weld metal at GTA heating area. Quantitated value of Ms raising corresponds to additional chemical driving force for martensitic transformation. This driving force is estimated using free energy calculation. In the case of this study, this driving force is added as tensile stress at GTA heating area and ESPI result is useful to estimate the value of stress.
Reverse roll coating in which a thin single layer of liquid is applied onto a substrate has been used in industry for decades and has been extensively analyzed in the literature. Modern coatings, however, are often composed of more than one layer to improve product performance and to reduce manufacturing cost. Pre-metered methods such as slot, slide and curtain coating are typically used to produce such multilayer coatings. If the caliper of the substrate to be coated is not constant, the coating gap and consequently the final film thickness deposited on the web will also be non-uniform. In this study we focused on the use of reverse roll technique with slot die liquid delivery system to produce a uniform thin two-layer coating. The use of this coating technique to produce such a coating has not been previously explored. The liquid film surface as it is transferred from a rigid steel roll to a deformable urethane covered roll was visualized in order to find out how the uniformity of two layer coating is affected by the speed ratio between two rolls, layers wet thickness and liquid viscosities. The effect of these parameters on the ribbing frequency and amplitude was also investigated. The results show that in two layer coating, as in the single layer reverse transfer, there is a critical web speed above which ribbing occurs. The critical speed is determined by the bottom layer viscosity.
Argon ion sputter etching was applied to three kinds of martensitic stainless steels (SUS410, SUS420J2 and SUS440C) and a ferritic stainless steel (SUS430) at a radio frequency power of 250 W for 0.6 ks to 21.6 ks. When the sputter etching time is 0.6 or 0.9 ks, the pillar-shaped protrusions with diameters smaller than 1 μm are formed on the surface of the SUS420J2 steel. With increasing sputter etching time, the cone-shaped protrusions are formed around the base of the pillars, and the size of the conical protrusions increases to more than 10 μm with further increase in the sputter etching time. When the sputter etching time is 21.6 ks, the size of the protrusion becomes more than 20 μm and the surface of the protrusions is heavily damaged. According to an EDX analysis, the Cr content of the surface of a cone is larger than that of the inside and the matrix surrounding the cone. Other steels show a similar protrusion formation process to the SUS420J2 steel, but the formation speed and the density of the cones are smaller for the SUS410 or SUS430 steels with smaller carbon content than the SUS420J2 steel, whereas they are a little larger for the SUS440C steel with larger carbon content. For the martensitic stainless steels, the quenching increases the hardness of protrusions, which is convenient for a traction roll and a transcription roll to imprint many holes to sheets.
Carbon steels with and without dispersed fine voids, the void diameter was ranging about 3~5 μm, were deformed in compression at elevated temperatures and at some strain rates. The deformation behavior depended on the testing conditions and the steel whether containing voids or not. The peak stress and the peak strain of the carbon steel with voids became higher compared with those of without voids. Occurrence of dynamic recrystallization (DRX) was, therefore, a little delayed by the presence of voids. Furthermore, the yield stress and steady state stress in the steel with voids appeared higher than those without voids. The shape and the distribution of voids drastically changed depending testing condition, i.e., the void size and the aspect ratio become smaller with decreasing strain rate and with increasing temperature. These observed phenomena were discussed in relation with DRX, grain boundary migration, grain boundary sliding and relaxation, etc.
Hydrogen embrittlement of a Fe-18Mn-0.6C-1.5Al steel was observed in tensile deformation during cathodic hydrogen charging. The fracture mode was quasi-cleavage fracture. The relationship between diffusible hydrogen content and fracture stress was arranged by the power law like that for ferritic and Al-free TWIP steels. The Al addition did not affect the magnitude of the degradation of hydrogen embrittlement property at the same current density in TWIP steels. However, the Al-added steel showed a suppression of hydrogen entry and a larger total elongation in comparison to those of the Al-free TWIP steel in the same environment, although the Al addition decreased fracture stress. The larger elongation is one of the reasons for why the Al addition improves the hydrogen embrittlement property of cup specimens.
The purpose of this study is to develop a simulation method to predict the effect of micro-structural characteristics of two-phase steel on two types of ductile properties related to ductile crack growth resistance of a structural component. One of the ductile properties is critical local strain related to shear-slip mode of ductile crack initiation, and another is stress triaxiality dependent ductility related to equi-axed dimple mode of ductile crack growth. A 3D mesoscopic damage simulation method that has been previously proposed by authors are utilized, where a 3D micro-structural FE-model having heterogeneous two phases along with a numerical damage model is applied. On the basis of the method, a small three-point bend specimen model with a deep notch is proposed to predict critical local strain. On the other hand, a method for predicting stress triaxiality dependent ductility is also proposed, where a representative volume element (RVE) loaded under constant stress triaxiality is applied. The two types of ductile properties for ferrite-pearlite two-phase steel predicted by the proposed methods present a good agreement with the experimental results. The developed simulation method can address the effect of micro-structural characteristics of two-phase steel to improve the ductile properties. Layered type morphology of a harder second phase is predicted to provide higher ductile properties than random type morphology as long as volume fraction of a second phase is the same.
Tensile behaviors of cryogenic steels, which contain 0 to 12wt% nickel, were examined to obtain the fundamental knowledge related to the role of retained austenite in the improvement of fracture toughness. Although retained austenite of 8% and 12% nickel steel, which were manufactured by the special heat treatment composed of quenching, intermediate heat treatment, and tempering, was stable under the deep cooling at –196 ºC, it transformed to martensite by straining around 10% at –196 ºC. The strain hardening behavior was divided into two categories. One is that the maximum strain hardening appears just after yielding, then the strain hardening gradually decreases with increasing strain. It was related to 0% to 2% nickel steels, which include less retained austenite. Another is that the strain hardening increases just after yielding with the increase of strain, then it gradually decreases. In this case, higher strain hardening was derived. It was related to 5% to 12% nickel steels which included a certain amount of retained austenite. The increase of strain hardening under low temperature was attributed to the solute nickel and the formation of martensite, which act as a hard second phase, by the work induced martensitic transformation.
We measured a creep deformation in Grade T91 steel under low stress condition using a helicoid spring creep test method. By applying this method, we measured a very low strain rate creep deformation in a short time as there is no need to consider the contribution of the microstructural change to the creep deformation behavior. We distinguished magnitude of several types of time-dependent deformation, including viscoelastic deformation, which occurs during the creep test in order to evaluate a time-dependent plastic deformation in strain-time curves. This consideration derives a conclusion that the time-dependent plastic deformation occurs even in a short time and low stress condition. Moreover, we confirmed a strain rate of whole creep deformation is controlled by the strain rate of the time-dependent plastic deformation at the time of 270 ks in the creep test. We confirmed that though there are almost no changes in the microstructure of Grade T91 steel during creep tests, the susceptibility on strain rate to applied stress has changed depending on the applied stress region. In addition, activation energy of the creep deformation was also different in each stress region. From these results, we concluded that the dominant creep deformation mechanism is different in the low stress region and high stress region.
Tensile properties of 12% nickel steel plates, which were tempered at various temperatures, were examined at 0 ºC, –90 ºC, and –196 ºC. This research was intended to compare the strain hardening behavior of these steels, which reveals good fracture toughness and inferior fracture toughness, and to clarify the influence of work induced martensitic transformation on strain hardening behavior of nickel bearing cryogenic steel. The strain hardening behavior was divided into two categories. One is that the maximum strain hardening appears just after yielding, then the strain hardening gradually decreases with increasing strain. The other is that the strain hardening increases just after yielding with the increase of strain, then it gradually decreases. In the case of 12% nickel steel, which is tempered at optimum temperature and possesses superior fracture toughness, retained austenite is transformed to martensite in the early stage of plastic deformation, then the gradual increase of strain hardening is derived. It leads to the higher strain hardening in total. On the other hand, in the case of 12% nickel steel, which is tempered at higher temperature and possesses inferior fracture toughness, a part of retained austenite was transformed to martensite in elastic region, then the high strain hardening appears just after yielding, but strain hardening quickly decreases with increasing strain.
Toughness, especially ductile-to-brittle transition temperature (DBTT) such as fracture appearance transition temperature (FATT) obtained from Charpy impact test, is one of the most important properties of steels to assure material reliability. One of determining factors of DBTT in ferrite-pearlitic and martensitic steels is well known as the effective grain size (dEFF) on cleavage fracture surfaces. However, in the steels with intermediate stage transformation microstructures (Zw), relationship between DBTT and dEFF has not been clarified because of the complicated microstructures. Meanwhile, absorbed energies of Charpy impact test has been standardised for steel applications. The present study aims to determine the relationship between DBTT and dEFF, and the relationship between upper shelf energy (USE) and plastic properties in a quenched and tempered low carbon high strength steel which has a Zw microstructure consisting of granular bainitic ferrite and quasi-polygonal ferrite. Size distribution of dEFF was measured on the cleavage fracture surfaces revealed by Charpy impact testing at 77 K, and correlation between dEFF and the microstructure was examined. It was found that FATT is inversely proportional to ln (dEFF−1/2) with a slope close to that approximated for ferrite-pearlitic steels. Investigation of the correspondence between cracks and EBSD grain boundaries revealed that dEFF apparently agrees with the grain size of bainitic ferrite enclosed by large angle grain boundaries with misorientation over 15 degrees. Moreover, USE has been proportional to total plastic works until fracture of tensile test. These results indicated that the absorbed energies at any temperatures could be computable.