Vortex flow measurements have been carried out with PIV (particle image velocimetry) to understand the flow in a bath with an immersion cylinder under mechanical agitation condition. An impeller is placed on the centerline of the bath and the cylinder is immersed slightly into the bath at an offset radical position. A horizontal rotational motion of water prevails in the absence of the immersion cylinder and the axis of the rotational motion overlaps the centerline of the bath. On the other hand, the axis of the rotational motion inclines in the presence of the immersion cylinder and a strong downward motion of liquid is induced in the bath. This downward motion enhances the entrainment of low-density particles initially placed on the bath surface. The inclination angle decreases with an increase in the rotation frequency of the impeller. Accordingly, the effect of the immersion cylinder becomes weak as the rotation frequency increases under the present experimental conditions.
Decrease of carbon dioxide emission is a serious subject in the steel industry. Although low reducing agent operation of blast furnace is a primary method in ironmaking, many different ways should be taken. Utilization of biomass as a carbon-neutral reducing agent is an attractive one for ironmaking. However, since the calorific value is relatively low as an injection material and the oxygen content derived from the functional group is higher than coal, the direct injection of raw biomass into the blast furnace is not favorable. Optimized carbonization of biomass is proposed to keep high carbon yield and to attain selective oxygen removal by controlling the atmosphere, heating time and temperature condition. In carbonization, it was clarified that the carbonization of biomass from 300 to 500°C in the inert gas atmosphere improves the replacement ratio of coke and the crushability by intentionally changing the biomass composition and structure. It was estimated that the combustibility of the carbonized biomass became similar to pulverized coal. The effect of carbonized biomass injection into blast furnace was estimated with Rist diagram. The injection of biomass carbonized in the optimized condition effectively decreases CO2 emission from the ironmaking process.
Anionic polymer dispersant (APD) has been applied to the granulation process for enhancing sintering productivity. It is reported that APD accelerates micro-particle dispersion. In this study, the amount of micro-particles whose diameter was less than 10 μm has been surveyed by means of Andreasen pipette method. And in order to understand granulation mechanism focused on micro-particle, effect of micro-particles on granulation has been analyzed by granulation test with single ore. As results, total amount of micro-particle was in the range from 2 to 10% in all ores. The increase in amount of micro-particles by APD differed according to ores. These differences mostly could be explained with mineral characteristics of micro-particle. From the granulation test, the optimum moisture in wet state did not correspond with one in dry state. In wet state, micro-particles dispersed by APD increased effective volume of water, resulting in lowing water requirement for optimum granulation. Therefore, APD addition is seemed to enable optimum moisture of granulation to move wet optimum moisture closer to dry optimum moisture. In dry state, dispersing micro-particles may result in concentrating in contact point of iron ores and reinforces the strength of granules. This understanding based on this result can make comprehensive interpretation for granulation.
Effects of Ti addition on as-cast austenite grain size in S45C carbon steel have been investigated in range of Ti addition between 0 and 0.5 mol%. The cooling rate was set to be 0.03°C/s. It is shown that the average austenite grain diameter decreases from 1900 to 250 μm as the Ti addition increases from 0 to 0.5 mol%. The microstructural observation revealed that the motion of austenite grain boundary is inhibited at inter-dendritic position, which is ascribable to pinning effect of Ti(C,N) particle. The CALPHAD analysis showed that this particle crystallizes mainly in L+γ−Fe+Ti(C,N) phase field. As increasing Ti addition, the size of Ti(C,N) particle does not substantially change, however, the number of the Ti(C,N) particles increases, leading to further refinement of the austenite grains.
Silicon crystal pulling from the melt of Si–45mass%Ni alloy is performed as a model experiment of a silicon solidification refining process with silicon alloy melts. A needle-like silicon crystal is successfully grown, using a seed crystal with a ‹211› crystallographic orientation to the pulling direction, which is poly-crystalline contains a large number of twins inside. A rod-type crystal with Si–Ni eutectics engulfed inside is also obtained. Appropriate ranges of melt superheating and pulling velocity for crystal growth are estimated to be from one to five degree Celsius and from 0.018 to 0.15 mm/min, respectively. It is necessary to decrease the pulling velocity to less than 0.03 mm/min for obtaining the sufficient epitaxial growth on the seed crystal.
Mold flux is widely used in continuous casting of steel. The heat transfer control in the mold is one of the important roles of mold flux. The mild cooling due to crystallization of mold flux is established for middle carbon steel, which has a high tendency for longitudinal cracking. However, the mechanism of crystallization has not well understood. Therefore, the kinetics of crystallization via glassy state has been analyzed in this study, using a commercial mold flux, which easily crystallizes. Quenched mold flux was prepared and heat treated at some temperatures for various periods. After heat-treatment, the mold flux was analyzed by XRD to confirm the phase, glass or crystalline. The time for crystallization as a function of heat-treatment temperature was experimentally determined. Then the apparent activation energy for crystallization was estimated using Arrhenius plot. The calculated value is 193 kJ/mol and this may correspond to the activation energy for diffusion of some molecules or ions in a glassy state of mold flux.
Cooling curve of sample surface of ultra low carbon steel at initial stage of solidification was measured using a new temperature measurement system that consisted of a two-dimensional optical pyrometer and a chill plate which made of transparent sapphire glass. The presence of recalescence phenomena was observed on the measured cooling curve of ultra low carbon steel sample and the recalescence temperature existed in the range of peritectic transformation temperature as well as low carbon and middle carbon steel samples. The difference of temperature at each measured point of sample surface was very small until the temperature reached the recalescence temperature, but the difference of temperature became larger after recalescence because the thermal deformation of solidifying shell seemed to be generated. From experimental results for tensile strength and density during solidification, these values changed depending on the phase and the change of values between δ phase and γ phase was large. As the unevenness of solidified shell was generated by deformation accompanying with peritectic transformation and the degree of unevenness could be arranged by the both temperature range, difference of tensile strength and difference of density during peritectic transformation.
Quantitative detection of phosphorus in steel by Laser ablation–laser induced fluorescence spectrometry has been studied. Solid steel samples were irradiated by pulsed Nd:YAG laser (ablation laser) to generate atomic vapor, which was then illuminated by the probe laser. Laser induced fluorescence spectrum of phosphorus with the highest selectivity was obtained with excitation at 255.49 nm among transitions of atomic phosphorus in 253–256 nm. Dependences of fluorescence intensity on pulse energies of ablation laser and probe laser were investigated. Fluorescence intensity was found to be logarithmically proportional to the ablation laser pulse energy with the slope of 3.2–3.3. Fluorescence intensities were in a good linear correlation with contents of phosphorus in steel samples in a range of 37–350 μg/g. Relative standard deviation were 5.6 and 12.8% at 72 and 170 μg/g of phosphorus, respectively.
We have developed a numerical model to simulate the hydrogen desorption profiles for pure iron and eutectoid steel, which is obtained in thermal desorption analysis (TDA). Our model incorporates the equation of McNabb and Foster without the hydrogen diffusion term combined with the Oriani's local equilibrium theory. It is found that the present numerical model successfully simulates the hydrogen desorption profile using the concentration of hydrogen trapping sites which is inferred from experiments both for pure iron and for eutectoid steel. We further verify the model by discussing the trapping site concentration and the effect of hydrogen diffusion.
The Spark electric discharge optical emission spectrometry has been widely applied to a rapid composition analysis of molten elements in the field of steel making processes. The advantage of this method is that the simultaneously composition analysis from light elements to heavy ones is possible and that the acid soluble elements and insoluble inclusions can be analyzed separately. In this study, spark phenomena in discharge and sample surface morphology were investigated using high-speed camera, SEM-EDS, and spark-OES methods. On the basis of detailed analyses of spark images between electrode and sample surfaces, it was found that the explosion process of inclusion consists of the following three stages. At first, a breakdown between sample surface and electrode occurs, and then inclusions are transformed to cathode points. Finally, vapor jet spouts out to electrode and collides with electrons, Ar+, and Ar* leading to strong plasma light. In this process plural inclusions are initially destroyed and dispersed finely into the matrix. These findings led us a new model for the selective discharge of inclusions in spark discharge.
Although several studies on the friction stir welding of high temperature materials have recently been reported, their practical use has not yet occurred due to some problems to be solved such as decreases in the corrosion resistance and joint efficiency during a high-speed joining. In this study, the effect of the welding speed on the joint efficiency was investigated in detail. As a result, it has been clarified that at a rotational speed of 600 rpm, the friction stir welding of 304 austenitic stainless steel is possible up to the joining speed of 1200 mm/min, and the tensile strength of the joint exceeds that of the parent material up to the joining speed of 1150 mm/min. In addition, the corrosion resistance is significantly improved at the higher welding speeds. No rust was observed during the salt spray testing of the 1000 mm/min joint. Thus, an increase in the welding speed can improve productivity and the product quality by decreasing corroded region. These results are expected to extend to an actual application to products by further improving the technology.
Hot-dip 55%Al–Zn alloy coated steel sheet has superior corrosion resistance both on flat panel and near shear cut edge in various atmospheric environments. However, the corrosion often occurs near the shear cut edge in continuous wet conditions, for example, NaCl solution spray test (SST, JIS Z2371), and it has been explained by galvanic model. In this paper, it is clarified that the corrosion near shear cut edge is suppressed by artificial sea water and its second ingredient, MgCl2. To make clear the corrosion mechanisms on MgCl2 suppression of the corrosion near shear cut edge, the measurement of cathodic polarization curve on the Fe electrode after the corrosion test using a AZ/Fe/AZ galvanic electrode, the estimation of corrosion products by numerical analysis in consideration of the substances migration and the precipitation reactions, and the analysis of corrosion products by FT-IR spectroscopy were conducted. As a result, in the galvanic condition, it is clarified that Mg(OH)2 precipitates on the Fe exposed on the cut edge. Moreover, the corrosion near shear cut edge is suppressed by the effect of the cathodic reaction suppression of Mg(OH)2.
Cu precipitation hardening behavior and mechanical properties were investigated in Cu added martensitic ultra-high strength steels. In this work, hardness measurement, TEM observation, lattice parameter measurement and tensile testing were conducted for 0.19%C–1.5%Mn steels with addition of up to 4% Cu those were water-quenched followed by aging at 250 through 550°C for 20 s through 360 min. Cu added steels exhibited higher hardness than a Cu free steel in each aging condition. Precipitated Cu content estimated by lattice parameter well corresponded to hardness increment in 4% Cu steels compared with Cu free steels aged at the same aging condition. Regarding the tensile properties of the aged Cu free and 4% Cu steels, 4% Cu steel exhibited superior balance of tensile strength and elongation with tensile strength level of 1300 MPa. Lower activation energy estimated by peak hardness increment than that of Cu diffusion in bcc-Fe matrix suggested that high dislocation density introduced by martensitic transformation accelerated the growth of Cu precipitates. And smaller Cu precipitation hardening in martensite matrix compared with that in ferrite matrix was also discussed taking into consideration nonlinear summation of dislocation hardening and precipitation hardening.
In this paper, a dynamic substance flow model of zinc in Japan was conducted. Currently, 60% of zinc is consumed for galvanized steels (galvanized steel sheets and other galvanized steels), followed by brass in Japan. To analyze the substance flow of zinc in a dynamic way, the linkages of zinc with other substances were considered in this model. Furthermore, the dissipated zinc during the use phase was taken into account using the existing results of atmospheric exposure tests. At the end-of-products, most of zinc is recovered as steel scraps or copper alloy scraps. In the re-melting process in electric arc furnace (EAF) of steel, zinc is vaporized, separated from steel, and generated as EAF dust. So, zinc can come back into the zinc cycle again. On the other hand, when zinc is used in copper alloys, zinc is not recycled as zinc but mixed into copper cycle. The mass balances of zinc in the substance flow analysis were verified by comparing estimated values of the model with statistics and other estimation at the recovering processes of zinc. Using this model, in-use stock of zinc in Japan was estimated as approximately 330 kt in 2005, which was about five times as much as recovered zinc. The amounts of unrecovered zinc in 2005 were estimated as around 190 kt. Of these losses, 70% were unrecovered in the steel cycle, e.g. dissipated into environment as sacrificial materials or with uncollected steels. Finally, complete zinc flows in Japan were determined with the results of this model.
We estimated probable length of continuation of steelmaking slag as an iron fertilizer enhancing marine phytoplankton growth. Bioavailability of the slag was determined by a recovery in growth rate by repeated addition of macronutrients other than iron during a prolonged culture period. Degree of the recovery was gradually reduced by repeated nutrient spikes possibly because concentration of the iron released from the slag decreased with culture age. No recovery can be performed when iron supply from the slag was exhausted. Therefore, we could calculate the final day of the bioavailability of the slag by extrapolating the reduction course of the recovery. We also made SEM observations on the slag particles before and after the culture experiments. Results indicate that the slag particles added to phytoplankton cultures at the concentration of 20 mg L−1 continually release bioavailable iron for up to 50 d. In the slag we used, submicron particles were often found, while predominant fraction was 5–20 μm before experiments. In the 5–20 μm fraction, three types of the particles were recognized by SEM: amorphous, crystalline and cotton-ball types. After the culture, only the crystalline type of the particle was observed insoluble. Surface structure and size of these particles are likely determinative in bioavailability and durability of the iron contained in the steelmaking slag.