Convection heat transfer coefficient between a spherical particle and a liquid was measured for the range 50<Re_p<5000 and the applicabilities of empirical equations proposed previously to the estimation of convection heat transfer coefficients of solid-liquid system were investigated experimentally. An aluminum sphere, 32 mm in diameter, which was kept at about 60°C initially was dipped into a flow of water at about 25°C and the temperature change of the sphere was measured. The heat transfer coefficient was obtained from the temperature drop curve of the solid. The relationship between the Nusselt number, Nu, and the Reynolds number, Re_p, was obtained from the results.
Convection heat transfer coefficients at several flow systems were measured and the applicability of the equation corrected for the liquid velocity in the bed was examined. For packed bed system, it was found that the local velocity around a sphere became comparatively large, and convection heat transfer coefficients for packed beds tended to become large.

View full abstract

The effect of weir, dam, slotted dam and their combinations on the molten steel flow dynamics in a tundish was investigated in a physical model by studying the flow pattern and residence time distribution. The number, position and size of the above flow modifiers, Froude number, tundish width and inlet-exit distance were varied during the course of the study. For all tundish widths, weirs of all sizes were found to produce short circuiting in the tundish fluid flow system. Whereas use of a dam alone or in combination with a weir or a slotted dam eliminates short circuited flow completely. The residence time and variance are found to be influenced by the tundish width, inlet-exit distance and FM configurations. Empirical correlations were obtained to calculate residence time and variance.

View full abstract

Based on a prototype installation, the mixing time and the fluid flow pattern of CAS (Composition Adjustment by Sealed argon bubbling) were studied by a water model analog simulation. The decisive criterion for this simulation was formulated as a dimensionless number Qμ/(D²σ) which can satisfactorily keep the physical similarity between model and prototype. The critical bottom blowing gas flowrate due to the existence of a slag baffle was defined and used to evaluate the reliability of the simulation. It was found from the investigation that the fluid flow behaviour of CAS is significantly different from that of the conventional ladle bottom-blow stirring. Comments on the design and operation of CAS were proposed. The optimization of CAS was recommended to be conducting the operation with both eccentric bottom blowing and eccentric slag baffle positioning by using ladles of large height/diameter ratio.

View full abstract

Reaction between refractory and molten steel causes clogging in the immersion nozzle in continuous casting process. Reaction mechanism between the silica-containing alumina graphite nozzle and the molten steel was studied on the observation of the used immersion nozzle and the basic experments. The results are summarized as follows,
(1) The used immersion nozzle refractory reveals a reacted zone where the SiO₂ and C contents are reduced and there are many pores.
(2) The overall reaction in the refractory can be arranged as SiO₂ (s)+C (s)=SiO (g)+CO (g)
(3) The reactions of the refractory with the Al-killed molten steel and the Ti-killed molten steel are given by the following equations:
(Al-killed steel) 3SiO₂(s)+3C(s)+4Al=2Al₂O₃(s)+3Si+3C (Ti-killed steel)
low C and high Ti region 5SiO₂(s)+5C(s)+6Ti=2Ti₃O₅(s)+5Si+5C high C and low Ti region 5SiO₂(s)+5C(s)+3Ti=Ti₃O₅(s)+5Si+5CO (g) (4) With the SiO₂-C refractory containing no Al₂O₃, 50% of SiO₂ is reduced by the C to the SiO gas, and the remaining 50% is reduced by the C to SiC. On the other hand, with the Al₂O₃-SiO₂-C refractory, all of SiO₂ is reduced to the SiO gas.

View full abstract

The phosphorus, manganese, and chromium distribution ratios between BaO-MnO fluxes and liquid Fe-Cr-Mn-C_satd. alloys have been measured at 1573 K by using a chemical equilibration technique. The distribution ratio of phosphorus between BaO-MnO fluxes and liquid Fe-Cr-Mn-C_satd. alloys decreases and those of chromium and manganese are nearly constant with increasing chromium content of the metal. With increasing the BaO content of flux the phosphorus distribution ratio is found to increase. The chromium distribution slightly increases and that of manganese decreases at the same time. The possibility of dephosphorization of Fe-Cr-Mn-C_satd. melts by using BaO-MnO fluxes is discussed.

View full abstract

Improving plantwide product quality is a universal concern for every manufacturer. A prototype of a real-time expert system for this purpose has been developed. One of the main functions of the system is quality prediction. The quality of a steel product is predicted by the plantwide quality Expert System (PWQES) by utilizing process variables and operating conditions that change over time. An important issue is how to predict the quality of the product using both real-time data and expert knowledge. Because the knowledge of a steel mill expert includes many uncertainties with regard to the exact causes of specific defects, a method to handle this uncertainty is required in the PWQES. Several methods were examined that deal with uncertainty in prediction of defects in the steel products: production rules with certainty factors, artificial neural networks, and nearest neighbor classifiers. First, a rule set module was built for PWQES that includes steel mill expert knowledge about defects and their causes. Each prediction rule has a certainty factor associated with it. Then, an artificial neural network was constructed and trained using actual process data from the same mill. Finally, a nearest neighbor technique was used to classify products as defective or not. Utilizing five different measurements of prediction error, these different methods then were compared for the mill. As a result, a hybrid system was implemented that utilizes the production rule set and nearest neighbor methods for the quality prediction function.

View full abstract

In order to develop protective coatings for sink rolls used in continuous hot-dip galvanizing, the sprayed WC/Co cermet coating was formed on a mild steel by the High Vilocity Oxygen Fuel Spraying process and its durability in the molten zinc bath (753 K) containing 0-3 mass%Al has been investigated on the basis of the constitutional change measured by SEM and EDS. The following results were obtained.
(1) During immersion periods, Al was enriched in the Zn-Al solution close to the sprayed coating surface and Al-rich phase was deposited on the sprayed coating surface.
(2) Under the sprayed coating surface, the diffusion layer, where Zn was rich and Co was poor, was built up. As Al concentration in the molten zinc bath was increased, the thickness of the diffusion layer was decreased and the durability of the sprayed coating could be kept for longer times.
(3) It is presumably a reason for such improvement of the durability that the Al-rich phase acts as a diffusion barrier against Zn and Co at the interface of molten zinc/sprayed coating and consequently suppresses the growth of the diffusion layer.

View full abstract

High-strength steel of the 490-MPa class strength for offshore structures was developed by making the most of a TiN-MnS complex precipitate effective that is over a wide welding heat input range. At the same time, its HAZ toughness improvement mechanism was clarified. The TiN-MnS system provided superior HAZ toughness with a medium heat input in -60°C Charpy test and -50°C CTOD test and with a high heat input in -60°C Charpy test. This is probably attributable to the TiN compound that pins austenite grains and to the TiN-MnS complex precipitate that produces fine intragranular ferrite (IGF) and reduce the fracture facet unit after welding. IGF may be formed either by increase in driving energy for nucleation in the region where manganese is depleted by the MnS precipitation or decrease in interfacial energy with nucleation by the crystal coherency of TiN and ferrite. The presence of manganese-depleted zone is an indispensable mechanism and is complemented by the crystal coherency of TiN and ferrite.

View full abstract

Through the study of a chemical composition system capable of assuring high strength without impairing HAZ toughness, 590-MPa high tensile strength steel for offshore structure was developed, and its HAZ toughness improvement mechanism was clarified. The high-copper and ultra-low niobium steel provides superior HAZ toughness with medium heat input in -60°C Charpy test and -30°C CTOD test and with high input in -60°C Charpy test. Why HAZ toughness comparable to that of 490-MPa high tensile strength steel is obtained may be considered as follows. The precipitation of copper occurs much later than other precipitation-hardening elements like niobium and vanadium. Copper precipitated by tempering for strengthening goes into solid solution in the heating process of the HAZ. Copper precipitates little on cooling (and heating) during the subsequent welding thermal cycle. The embrittlement of HAZ by the precipitation hardening of copper thus does not take place. This allows the 590-MPa steel to be welded without preheating and cracking, just like 490-MPa steel.

View full abstract

Hot ductility of Ti-added ultra low carbon steels has been studied with particular emphasis on the bahavior of TiS. Hot ductility of low Mn low carbon steels initially solution-treated in the high temperature γ region is reduced with intergranular fracture by high strain rate deformation at temperatures ranging from 900 to 1100°C. This can be explained by the fact that hardening due to dynamic precipitation of Fe-rich sulfide within the matrix accelerates the void initiation at interface of grain boundary precipitate/matrix. In Ti-added steels, the ductility loss is considerably smaller to that of Ti-free low Mn steels, since Ti has a role of trapping S atoms as TiS particles in the same way as Mn. However, nitrogenation due to heating in the nitrogen gas atmosphere or in air changes TiS into the more stable TiN especially in the surface layer. As a result, the extra free S atoms formed by this reaction leads to hot embrittlement by the same mechanism.

View full abstract

CaO-CaF₂ system was examined with respect to dephosphorization of steels with high chromium content. The emphasis was on the effect of chromium oxide on slags phosphate capacity.
Phosphorus distribution between stainless steel and CaO-CaF₂ flux was investigated under different oxidizing potential and temperature. It was found that chromium oxide formed in the oxidizing dephosphorization process, decreases phosphate capacity of slag and significantly affects steel dephosphorization.

View full abstract