Considerable efforts have been made during the past two decades to investigate gas injection operations in steelmaking ladles. Towards these, numerous physical and mathematical model studies embodying aqueous as well as full scale systems have been reported. On the basis of an extensive literature search, a summary, discussion and analysis of these are now presented. For the sake of convenience and clarity of presentation, studies have been categorised into three major groups: (1) physical modelling studies, (2) combined physical and mathematical modelling studies and (3) mathematical modelling studies. In each of these categories, a great number of publications on various phenomena, such as gas-liquid interactions, turbulent fluid flow, mixing, solid-liquid mass transfer, etc. have been reported. Accordingly, and as discussed in the text, considerable improvements have resulted in our understanding of the various gas injection induced phenomena in ladle metallurgy operations. Coupled with these, extensive mathematical modelling studies have also lead to a reasonably accurate framework for carrying out engineering design and process calculations. Nonetheless, some obscurities and uncertainties still remain and these are pointed out, together with those areas where further work is needed.
In refining processes for steel and some other metals, gas is blown into molten metal from the top, the bottom or simultaneously from the top and bottom of the vessel to promote mixing and chemical reactions. Since the gas is usually injected at a temperature lower than the molten metal temperature, knowledge about the heat transfer between gas and molten metal is required to predict precisely the mixing effect. Present authors previously carried out cold model experiments for bottom injection to investigate the mechanism of convective heat transfer between bubbles and liquid, using cooled gas and water and proposed an empirical correlation for the Nusselt number. In this paper, the mechanism of convective heat transfer between bubbles and molten metal was investigated using molten Wood's metal. The funtional relationship between the Nusselt number and the Peclet number was the same as that for water models although the Prandtl numbers of water and Wood's metal were much different with each other.
A kinetic study has been made on the reaction between a silica-containing alumina graphite refractory and low carbon molten steel by measuring mass loss of the refractory heated at 1100-1600°C and Si content of the molten steel at 1600°C where the refractory was immersed. The results are summarized as follows: (1) The reaction in the refractory is controlled by the diffusion of SiO gas and CO gas through the pores in the refractory. (2) The rate of reaction between the refractory and the molten steel is controlled by diffusion of the SiO gas and CO gas through the pores of oxide film formed at the refractory-molten steel interface. (3) The rate of the reaction between the refractory and the molten steel is faster in the Ti-killed molten steel than in the Al-killed molten steel sand exhibits steel grade dependence. This steel grade dependence can be ascribed to the differences in the gas permeability through the oxide film formed at the refractory-molten steel interface between the Al-killed molten steel and the Ti-killed molten steel.
With the aim of reducing surface cracks of continuously cast low carbon Nb containing steel, effects of morphology and composition of carbonitrides on high temperature ductility has been investigated under the condition which simulates thermal cycle of steel slab surface passing through spray cooling and bending zones of a vertical-bending continuous caster. Fine Nb carbonitrides precipitation at prior γ grain boundaries has been shown to be enhanced by the amount of temperature variation at slab surface caused by repetitive cooling by spray water and subsequent reheating just after contact with delivering rolls. Microalloying of titanium to the steel has been shown to have marked effects on the degree of supercooling during solidification and δ/γ transformation by high temperature differential themal analysis. In addition to titanium, the similar effect of other nitride forming elements such as Zr, Y microalloyed to the steel has been cofirmed by change in high temperature ductility test. From thermochemical calculation on precipitation of carbonitrides of these elements, it has been suggested that these elements precipitates during solidification and act as nucleation site for precipitation of niobium carbonitrides under nitrogen content reduced by preceding nitride formation of these elements. The improvement in high temperature ductility due to the microalloying of nitride forming element is considered to be caused by the change in chemistry and morphology of carbonitrides.
The improvement of accuracy of head-end strip thickness is required strongly these days. But generally speaking, it is very difficult to maintain accurately under the production of small-lot orders for many different types of products. As one of the positive countermeasures to these difficulties, on-line adaptive methods has been developed at Yawata's hot strip mill. The main idea is based on improvement of prediction accuracy of rolling forces, by means of the forward slip measuring system, in order to estimate coefficient of friction and deformation resistance independently. And further correction method has been introduced employing with fuzzy theory which is a dynamic gap control by finding a certain relationship between predicted rolling force and measured one. We obtained satisfactory results and believe these approach is applicable for a gauge control method under increasingly diversified production style.
The collision dynamics of a water droplet impinging on a rigid surface above the Leidenfrost temperature are described from an experimental point of view. Emphasis is placed upon the collision of relatively small droplets in the range from 300 to 600 μm with the surface. The surface temperature is fixed at 500°C. The first purpose of the present study is to examine whether or not the prior empirical rules obtained by using droplets of 2 to 3 mm give an applicability to the case where the droplet size is much smaller. The second is to clarify the relation between the Weber number and the coefficient of restitution, when the droplet rebounds from the hot surface without disintegrating into a number of particles. Aoso, the maximum value of the diameter of droplet which can spread on the surface and the resident time are examined and discussed bu comparing the result obtained here with the prior empirical formulae proposed by a few researchers.
The technology of deep drawing of commercially pure Ti cold-rolled sheet for obtaining drawn cup without defects was developed. Oil hydraulic press of 490 kN punch force and 98 kN blank holding force (BHF) and 300 mm/min drawing speed were used. Ti sheets (TP28C) of 0.8 mm sheet thickness (t) were used. The punch diameter is φ40 mm, and various radii were used. First, blanks lubricated with machine oil were deep drawn. BHF values were calculated by Dr. Fukui's formula. The limiting drawing ratio (LDR) was about 1.8, inferior to steel sheets. Next, two 30 μm polyethylene (PE) films were used on each side of Ti sheet and BHF values were calculated by Dr. Fukui's formula. When LDR increased to e.g. 2.5, large and numerous wrinkles were found. By BHF increasing to 98 kN, only traces of wrinkles were found. Last, punch lubrication was eliminated. The LDR increased to 2.6, but severe earing was formed. By the consideration of characteristics of test piece of each direction, the breaking mechanism was qualitively explained. By adequate blank shape, this earing was nearly eliminated.
Rectangular blocks of "all weld" C-Mn steel deposit were prepared by multipass submerged arc (SAW) process employing different currents (500-750 A), arc voltages (27-40 V) and speeds (40-60 cm/min), using a 4 mm diameter filler wire and a basic agglomerated flux. Varying the welding parameters influenced the macrostructure comprising coaxial dendrites and reheat refined regions whose amounts were determined by quantitative metallography. The blocks were characterized for their mechanical properties and the structure property correlations wer established. Since the macrostructure emerged as a Key-parameter influencing the mechanical properties of the aforesaid products, model expressions were developed for quantitatively estimating the area fraction of dendrites and the reheat refined regions, starting from the first principles, by incorporating the effect of weld thermal cycle, bead geometry and the resultant structural changes. Validity of the model expressions was experimentally verified. The importance of the study is that it may be possible to optimize the process to attain a desired set of end properties without the necessity of having to run through a very rigorous and time consuming schedule of experimentation.
The microstructure of two HY-130 steel weldments prepared by submerged-arc welding (SAW) and gas-metal-arc welding (GMAW) processes using identical filler wires were investigated. Light and electron microscopy along with microhardness traverses were used to characterize and compare the microconstituents present in the base metal, and weld metal, and heat affected zone (HAZ) of both weldments. The base metal consisted of a bainite-martensite microstructure and was uniform through the plate thickness. The weld metal and HAZ structures consisted of a mixture of lath and twinned martensite, bainite, autotempered martensite, and retained austenite. The GMAW weld metal exhibited a finer lath martensite with more interlath retained austenite and transformation microtwins. The SAW weld metal structure was more bainitic and contained higher inclusion densities as a result of the flux used to make the joint. Microhardness traverses exhibited higher hardness values in the GMAW weld metal compared to the SAW. The HAZ microhardness followed a similar trend for both weldments and showed a maximum value in the fine-grained region near the base metal.
Controlled rolling processes are designed to produce a desired microstructure via the control of hot rolling without subsequent heat treatment. Critical to the success of controlled rolling are the stages that occur after hot deformation, when the steel is cooled to room temperature. This can be divided into two parts: the run out table, where the material is allowed to cool relatively rapidly to a pre-determined temperature, and "coiling" at which point the rolled material is coiled, thus showing down the cooling rate considerably. Controlled rolling schedules generally finish by coiling the steel at temperatures below the bainite transformation start temperature (Bs). Any changes in coiling conditions (temperature and time) in this region can result in variations in bainite characteristics (morphology, size, carbide precipitation, etc.). This in turn, may affect the state of the retained austenite and, consequently, the mechanical properties of Si-Mn TRIP steels, which have bainite as the dominant microconstituent. The effect of changes in the bainite transformation conditions were investigated using two grades of Si-Mn TRIP steels, including one a containing Nb as a microalloy addition. The results reveal that the retained austenite volume fraction was strongly influenced by both bainite formation temperature and hold time, The highest values of total elongation (46 and 33%) and formability index (61180 and 40260 MPa·%) were observed for an intermediate hold time (5 min) and temperature (400°C), respectively. These findings are explained by considering the effect of the bainite transformation on the state of the retained austenite.
A dispersion of precipitates introduces a threshold stress which opposes deformation. The effect of precipitate shape on dispersion strengthening at elevated temperature has been examined. Three kinds of Mod. 9Cr-1Mo steels without Nb(C, N), with spherical Nb(C, N) and with complex Nb(C, N)+V(N, C) "V-wing" were designed to have the same microstructures except for the shape of precipitates. The threshold stresses were measured by tensile and creep tests at 600°C. Two sorts of effects have been observed. The first is that complex precipitates restrain dislocation from climbing even in a condition in which dislocation climbs over a spherical precipitate. Mobile dislocations are considerably trapped at the concavity of the V-wings during deformation. The second effect is that the probability of dislocation trapping at V-wing is larger than that at spherical Nb(C, N). These trapping effects play important roles in the strenghening effect of precipitate.