Hot metal desiliconization is carried out by the addition of iron oxide at the tilting runner in the blast furnace casthouse for better hot metal dephosphorization and subsequent BOF operation. The authors proposed the application of a swirling flow to the hot metal at the tilting runner, aiming at effective silicon removal by mixing of the desiliconizing agent and hot metal, since hot metal desiliconization is determined by mass transfer of FexO in the slag in the low FexO concentration range. A basic investigation of the swirling flow was carried out in a water model experiment. The swirling flow was generated in a funnel-shaped container with various flow rates, outlet diameters, etc. The proper flow condition was characterized by a swirl number. The experiment showed that mass transfer of a tracer in model slag was enhanced under the swirling flow condition. A 5-ton hot metal experiment was carried out with a specially designed vessel to generate a swirling flow of hot metal. The desiliconization experiment with addition of iron ore showed that silicon removal was enhanced by application of the swirling flow of hot metal, especially with slag of low FexO content. An analysis was performed for the desiliconization reaction after the addition of iron ore and showed that the mass transfer of Si was increased by almost two times when the swirling flow was applied, which reflects good mixing of the desiliconizing agent and hot metal.
Steelmaking slags are mainly used as aggregates for road and civil constructions and slag products successfully replace natural sand and macadam. However, the slags may contain appreciable amounts of free lime (CaO) that can cause expansive self-destruction by reactions with water and carbon dioxide in the air. It is thus very important in evaluating the quality of the slags as raw materials to develop an accurate and precise method for determination of free lime in the slags. In the present work we developed a method for determination of free lime in the slags that consists of extraction with ethylene glycol followed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and thermogravimetry (TG). The ethylene glycol extraction method (EG method) has been used as a conventional method for analysis of free lime in steelmaking slags. It has been known, however, that the analytical results obtained by the EG method are not reproducible and ethylene glycol dissolves not only CaO but also Ca(OH)2. We re-examined the EG method and optimized the conditions for complete extraction of both CaO and Ca(OH)2 in the slags. We also found that TG enables one to determine Ca(OH)2 in steelmaking slags. The content of CaO in a slag sample is calculated from the amount of calcium determined by the improved EG method and that of Ca(OH)2 obtained by TG.
In hot rolling, lubrication between the work roll and hot strip plays an important role in reducing rolling force and protecting the work roll surface. However, the tribological behavior in hot rolling has not been clarified sufficiently. In this work, the effects of oil amount on the coefficient of friction in hot rolling were investigated in comparison with the case of cold rolling. The oil amount in hot rolling was measured by the amount of oil remaining on the work roll surface after rolling. The results of rolling tests clarified the following points: The coefficient of friction is reduced adequately with a small amount of oil. If the oil amount is increased, a few small oil-pits will form, but no further decrease in the coefficient of friction will be achieved. It is suggested that boundary lubrication is controlling in hot rolling, which is different from the case of cold rolling.
Blistering is a phenomenon in which oxide scale swells during oxidation at high temperatures. Blistered scale causes surface defects in steel products during hot-rolling processes. The present study investigated the effect of oxygen and the scale layer structure on blister initiation when steel is oxidized at high temperatures, and the mechanism of blister formation is discussed. The following conclusions are drawn. Blisters are not formed when the scale is a wustite mono-layer, but start to nucleate when the scale layer structure changes from the wustite mono-layer to a three-layered scale comprised of hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO). The compressive stress in the oxide scale that is applied due to the oxide scale growth is largely released when scale layer structure changes from the wustite mono-layer to the three-layered scale. This result suggests that the compressive stress is not the main factor for blister initiation. It is deduced that the pressure of the CO, CO2, and N2 gases generated at the scale/steel interface is the main factor for blister formation. The hematite layer on the top surface of scale presumably acts as a barrier of gas permeability.
This study investigated resistance of hydrogen embrittlement on a hot-sheared surface of die-quenched steel sheets. The specimens were sheared at 750 °C and 650 °C after austenitization, and then quenched by water cooling. Additionally, the specimens were cathodically hydrogenized for 48 hours to accelerate cracking by hydrogen embrittlement. This sequence resulted in the residual tensile stress of over 1 GPa on the sheared surface and hydrogen concentration of about 1.5 ppm. Despite these severe conditions, cracking by hydrogen embrittlement was not observed. The state of micro-structure in the vicinity of the sheared surface might cause this high resistance against cracking. Indeed, sub-micron grained ferrite or deformed uncertain soft and hard phases, which might be more ductile than martensite, were observed around the sheared surface.
The electric process has the advantage such as the low amount of CO2 emission. But, the tramp elements such as Cu and Sn contained in steel are retained in the puddling process. These tramp elements cause surface cracking of steel during hot rolling process. The surface hot shortness is suppressed by addition of Ni. The scale/steel interface of Ni addition steel is rugged. The rugged scale/steel interface occludes segregated tramp elements into the scale. The authors have reported the influence of shot peening to suppress surface hot shortness. The effect of shot peening was development of microscopically homogenous and microscopically heterogeneous oxidation due to the increase of defects such as dislocations and lattice distortions. The rugged scale/steel interface was produced by microscopically heterogeneous oxidation. The rugged scale/steel interface occluded the enriched Cu into the scale. The suppression of surface hot shortness like Ni addition was occurred by shot peening. This paper describes the mechanism of shot peening on the suppression of surface hot shortness at the initial stage of oxidation. The grain refining is occurs by shot peening at the surface of steel. The refined grains have various crystal orientations which was observed by EBSP. The interior matrix grains have crystal orientation of 3-degree or more. And, the locally-diffused O signal which diffused along refined grain boundaries and dislocation were observed by EDX. The scale/steel interface is being rugged by the microscopically heterogeneous oxidation which was hastened by the locally-diffused O2– ion.
Electrodeposition of Zn-V oxide composite was examined from an agitated sulfate solution without dispersed particles containing Zn2+ and VO2+ at pH 2 and 40 ∫C under galvanostatic conditions. Although V content in deposits decreased for the moment with increasing current density regardless of flow rate of electrolyte, the further increase in current density brought about the increase in V content in deposits. The curves, which shows the relationship between the V content in deposits and current density, shifted to the higher current density region with increasing flow rate of electrolyte. Agitation of the electrolyte decreased the V content of the deposits but reduced the segregation of V oxide. EDX point analysis of cross section of deposits revealed that the V oxide concentrated at the surface of deposits. It was found from the polarization curves in 3% NaCl solution that the corrosion potential of deposited Zn-V oxide films depended on the V content in deposits regardless of the flow rate of electrolyte and it shifted to more noble direction with codeposition of V oxide at V contents in deposits below 2 mass%. At V contents of < 4 mass%, the corrosion current density of deposits decreased with increasing V contents in deposits. The corrosion current densities of deposits obtained from agitated solutions were smaller than that from un-agitated solutions.
The Fe-Zn intermetallic compounds (IMC) layers composed of δp, δk, and Γ phases were fabricated using two different kinds of Fe/Zn diffusion couple (DC), and the fracture toughness of the constituent phases was estimated from the toughness of the IMC layers. In the DCs with sufficient Zn supply, the IMC layers were mainly composed of δp phase after isothermal holding at 450-600 °C for 60 s, while in the DCs with limited Zn supply, the IMC layers were composed of δp, δk, and Γ phases at the early stage of isothermal holding, which was later dominated by Γ phase with further isothermal holding. The average grain size of the IMC phases was found to increase with increased holding temperature. From the comparisons between the IMC layers composed mainly by either δp or Γ phases, toughness of Γ phase was found inferior to that of δp phase. In addition, it was demonstrated that the toughness of δp and Γ was found to increase with decreased average grain size. Whereas, from the results using the multi-phase IMC layers, the toughness of δk phase was found almost equivalent to that of Γ phase.
The effect of Si additions to the steel on the microstructure and growth kinetics of the Fe-Zn intermetallic compound (IMC) in the Fe/Zn diffusion couple with limited Zn supply was experimentally investigated. Three different steels with Si contents of less than 0.01 wt%, 0.44 wt%, and 1.85 wt% were machined in cylindrical shape of 8 mmφ×6 mmh, and their bottom surfaces were polished chemically and mechanically to remove oxide and Si depletion layer. Pure Zn (99.99%) film was inserted between two steel cylinders, and enclosed in the steels in vacuum. The samples were heated at 450 °C for various holding time. Without Si, an IMC layer composed of δp, δk, and Γ phases were formed first, and it was later replaced by an IMC layer composed only of Γ phase by the growth of δk and Γ phases. The addition of Si in steel suppresses the formation of Γ phase as well as the growth rate of δk phase, and IMC layers composed mainly of either δp or δk phases were successfully fabricated. Si was found to accumulate in α-Fe at the α-Fe/δk interface, which indicates that Si partitioning at the α-Fe/δk interface is responsible for the instability of Γ phase and slow supply of Fe from steel substrates. Finally, the fracture toughness and adhesion of IMC layer composed mainly of δk phase were demonstrated inferior to those of δp phase.
Effect of REM (Ce, La, Nd) addition on the austenite grain refinement of heat affected zone in Fe-0.07/C-0.05/Si-1.5/Mn-0.003/S (mass%) steel has been investigated by microstructural observation of SEM/EDS and thermodynamic analysis. It is observed that the 30 ppm of REM addition is most effective for reducing the austenite grain size annealed at 1450 °C for 10 sec, but the grain size increases with increasing REM contents more than 30 ppm. Since the total numbers of precipitates of oxide, sulfide and oxysulfide increase with increasing REM contents, the austenite grain growth can not be explained by Zener’s pinning model. Thermodynamic calculation shows that REM-oxide and MnS in oxysulfides are formed by the miscibility gap in the 30 ppm REM steel, which results in the formation of the liquid MnS-rich precipitates due to eutectic reaction. On the other hand, the melting temperature of MnS-rich precipitates increases with increasing REM amount in oxysulfide, which is not effective for inhibiting grain growth. It is suggested that the austenite grain growth is suppressed by liquid phase pinning effect of MnS-rich precipitates.
The effect of molybdenum (Mo) on softening behavior of cold-rolled high manganese austenitic stainless steels was investigated. The high temperature hardness of cold-rolled Mo-free steel was drastically decreased by prolonged annealing at 873 K. On the other hand, there is not remarkable degradation of high temperature hardness in the steels containing more than 1 mass% Mo. Fully recrystallized structure is observed in Mo-free steel after annealing at 873 K for 1440 ks. Since deformation structures were not disappeared in the steels containing more than 1 mass% Mo even after annealing, the addition of Mo retarded the recrystallization in the work-hardening steels. After annealing, fine particles of M23C6 type carbide precipitated at the grain boundaries in the steel containing Mo. The amount of Mo in M23C6 carbides was increased with increasing Mo content. On the other hand, the size of these carbides slightly decreased with increasing Mo content. These fine carbides strongly prevented the grain boundary movement (migration), therefore recrystallizaiton in the work-hardening steels was retarded. Moreover, the growth of M23C6 carbides at the grain boundary was consistent with Ostwald ripening equation substituted Mo for Cr. This result suggested that the growth rate of M23C6 was controlled by the diffusion of Mo.
Conventional time-temperature-parameter (TTP) methods often overestimate long-term creep rupture life of high Cr ferritic steels. The cause of the overestimation is examined paying attention to temperature and stress dependence of creep rupture life of Gr.91, 92 and 122 steels. In stress-rupture data of all the three steels there are four regions with different values of stress exponent n for rupture life. Activation energies Q for rupture life in the regions take at least three different values. The values of n and Q decrease in a longer-term region. The decrease in Q value is the cause of the overestimation of long-term rupture life predicted by the conventional TTP methods unable to deal with the change in Q value. Therefore, before applying a TTP analysis to stress-rupture data, the data should be divided into several data sets so that Q value is unique in each divided data set. When this multi-region analysis is adopted, all the data points of the steels can be described with higher accuracy, and their long-term rupture life can be evaluated more correctly.
Environmental conditions suitable for growth of sulfate-reducing bacteria are provided by oxygen depletion due to decomposition of organic matter, and as a result, hydrogen sulfide is generated in enclosed coastal seas. It is highly toxic, depletes oxygen and generates blue tide. The purpose of this study is to evaluate the effects of removal of the sulfide in silty sediments by steelmaking slag. The silty sediments were collected in Fukuyama inner harbor located in the back of the bay part of Fukuyama Port, where odor trouble had caused by hydrogen sulfide generation. The steelmaking slag was placed on or mixed with the sediments in plastic containers with tight caps, and the water, sediment and hydrogen sulfide gas were monitored in a laboratory. The results showed that steelmaking slag reduced the dissolved sulfide in both the overlying water and the interstitial water in the sediment as well as the concentration of hydrogen sulfide gas. The analysis of SEM-EDX was suggested that iron sulfide was created on the slag surface immersed in the silty sediment. The results imply that applying steelmaking slag can effectively improve the water and sediment quality of coastal areas by capping on or mixing with the sediments.