Showing papers in "Isij International in 1988"

Investigation of decarburization behavior in RH-reactor and its operation improvement.

Abstract: In order to investigate the decarburization behavior in the RH-reactor, the samples of bulk steels and splashed metals were directly taken from the vacuum vessel by the use of a specially designed sampler in addition to the conventional ladle samples.The analyses of samples have revealed that (i) the carbon content of the bulk steel is uniform throughout the vacuum vessel and (ii) the carbon content of the splashed metal is almost the same as that of the bulk steel. On the basis of the results obtained, several decarburization mechanisms were examined and a new decarburization model has been proposed. In this model, it is assumed that the C-O reaction in the bulk steel is affected by the static pressure of the vessel and also by the evolution pressure of CO bubbles, therefore the reaction gradually decreases with the progress of decarburization. This mechanism covers the reason of low reaction rate in the final stage of decarburization.Along with the discussions of the mechanisms, a new method to measure the circulation flow rate of molten steel has been established and also a new equation which is able to make more precise estimation of the flow rate has been derived. A few experiments have been made with the 100t RH-reactor in Hirohata Works, NSC to accelerate the decarburization rate. It has been found that the use of oval shape snorkels is effective in the early stage, and that argon blowing from the bottom of vacuum vessel is effective in the final stage of decarburization.

Deformation and Recrystallization Behavior of Low Carbon Steel in High Speed Hot Rolling

Abstract: The recrystallization behavior of low carbon steel in high speed hot rolling has been investigated in relation to the strain distribution through the thickness. Sheet specimens with the size of 2×20×250mm were rolled between 650 and 850°C at the rolling speed of 20m/s without lubrication. The rolled specimen was quenched into water with a holding time ranging from 3 to 250ms. The redundant shear strain caused by friction forms severely sheared region beneath the surface. The width of the severely sheared region decreases and the maximum shear strain increases with lowering rolling temperature.A band o f extremely fine recrystallized grains (d=2-10μm) is formed in the severely sheared region when effective strain exceeds a critical value which is dependent on rolling temperature. Recrystallized grains appear even in the sheet quenched after a short holding time of 3ms. The recrystallized grain size is strongly dependent on Zener-Hollomon parameter Z0 at the midthickness of the sheet, and is independent of effective strain and initial grain size. It can not be made clear from the present experiment whether this recrystallization occurs dynamically or statically. However, the structural features of these recrystallized grains suggest that they are dynamically recrystallized ones.

A Mathematical Model for Solute Redistribution during Dendritic Solidification

Abstract: To quantify the solute redistribution during solidification, an approximate solution of the Brody-Flemings model of dendritic solidification has been derived on the assumption of parabolic solidification and a constant partition ratio, and compared with an exact solution of the model. It has been shown that the approximate solution has good accuracy. Further, on the basis of the derived equation, an extended mathematical model is proposed, incorporating a thermal model of solidification into the analysis without assumption on the solidification rate. The extended model includes the treatment of multi-component alloy and δ/γ transformation, the temperature dependence of diffusion coefficients and the estimation of the diffusion path. The extended model provides the results consistent with the available data.

Mathematical analysis of solute redistribution during solidification based on a columnar dendrite model.

Abstract: An exact solution and an approximate solution have been derived for the columnar dendrite model proposed by Ohnaka, 8) in which a solidification rate, equilibrium distribution coefficient and diffusion coefficient in solid are assumed to be constant throughout the solidification. By comparing with the exact solution, it has been shown that the approximate solution has good accuracy. Further the model is extended on the basis of the derived approximate solution by incorporating a thermal model of solidification. The extended model includes multi-component alloy accompanying phase transformation in solid and temperature dependence of diffusion coefficients, and excludes the assumption of a given solidification rate. The calculated results of the extended columnar dendrite model agreed well both with the available experimental data and the calculated results of the extended planar dendrite model proposed previously. The assumption of the solidification geometry is unimportant for quantifying the microsegregation effects.

Oxidation Behavior of Austenitic Heat-resisting Steels in a High Temperature and High Pressure Steam Environment

Abstract: Using a steam oxidation testing apparatus, operating under almost the same steam conditions as those in actual boilers, the influences of temperature and pressure (especially above critical pressure) on steam oxidation were investigated with austenitic heat-resisting steels such as SUS 347-HTB, 17-14CuMo, and the newly-developed 20Cr-25Ni and 22Cr-35Ni steels for advanced fossil steam plants.Oxidation loss in weight increased with higher steam pressure and temperature. Increase of Cr and Ni contents improved the oxidation resistance. Cr concentration in inner scale increased with Cr content of steel. This caused the higher resistance to steam oxidation.The existence of a Ni enriched layer at the scale and metal interface contributed to the improvement of steam oxidation resistance. The Ni concentration of the layer was higher in steels containing a higher Ni content.Morphological investigation of the scale cross sections showed that considerably large amounts of voids and cavities were produced in the outer scale of SUS 347HTB and 17-14CuMo steels oxidized in steam at high temperature and pressure.

Numerical analysis of solute redistribution during solidification accompanying .DELTA./.GAMMA. transformation.

Abstract: A mathematical method is presented to quantify solute redistribution during solidification with δ/γ transformation; new methods to solve the equation of diffusion numerically and to estimate the equilibrium phase diagram of multi-component low alloyed steel are described. Thermal analyses of low alloyed steels were made to verify the mathematical method described. It has been shown that the unequilibrium phase diagram of low alloyed steel for a given cooling rate can be predicted by this method.

Fundamental Study on Snaking in Strip Rolling

Abstract: Three-dimensional analysis for strip rolling, in which elastic deformation of rolls and plastic deformation of a strip are treated as one system, is expanded to calculate the nonsymmetric deformation of both rolls and strip and applied to evaluate the off-centering or cambering of a strip and the fundamental characteristics of snaking of a strip. The phenomena of nonsymmetric rolling and off-centering or cambering of a strip can be predicted accurately by the present analysis.

Comparison of alloy element partition behavior and growth kinetics of proeutectoid ferrite in Fe-C-X alloys with diffusion growth theory.

Abstract: Experimental data in the literature on the partition of alloying element during the growth of Proeutectoid ferrite allotriomorphs in 9 Fe-C-X systems (X=Al, Si, V, Cr, Mn, Co, Ni, Cu and Mo) are compared with the theory of difusional growth which assumes that all component species are in local equilibrium (LE) at the advancing interface and also with the theory which assumes that only local equilibrium with respect to carbon is maintained at the interface (paraequilibrium (PE)). The temperature ranges in which the formation of ferrite occurs with or without alloy partition are explained in a qualitative manner by the former theory. The partition behavior in each alloy is discussed in terms of thermodynamic properties of alloy element. A major deviation of partition from the LE theory occurs at lower reaction temperatures in alloys with higher Mn or Ni content. A practical means of estimating the temperature of the cassation of partition in such alloys is considered. The growth kinetics of alloy element partitioned and no partitioned ferrite allotriomorphs are briefly reviewed. In alloys with X which has a strong affinity with carbon in austenite, such as Cr, Mn, Mo, etc., the growth kinetics differ remarkably from both the LE and the PE theories, the reasons for which are yet to be explained adequately.

Texture formation in low carbon Ti bearing steel sheets by high speed hot rolling in ferrite region.

Abstract: The effect of inhomogeneous shear strain on the texture variation through the thickness of hot rolled low carbon (0.006% C) Ti bearing steel sheets has been investigated. The change in texture with the progress of recrystallization has been also investigated.The steel sheets were rolled in ferritic state at a rolling speed of 20m/s to the reduction of 40% with and without lubrication, and quenched into water at an interval of 3-250ms after rolling. In unlubricated rolling, shear deformation produces a severely sheared region beneath the surface. In lubricated rolling, shear strain is negligibly small throughout the thickness. ||ND axis density increases and ||ND and ||ND axis density decrease with increasing shear strain in unlubricated sheets. The texture of lubricated sheet, which is identical with that at the midthickness of the unlubricated sheet, is uniform through the thickness. Recrystallization reduces ||ND axis density at the severely sheared region and ||ND at the midthickness of the unlubricated sheet. ||ND axis density of the lubricated sheet also decreases with recrystallization. The texture inhomogeneity through the thickness of the sheet rolled without lubrication is retained after completion of recrystallization.

Hot Cracking of Low Alloy Steels in Simulated Continuous Casting-Direct Rolling Process

Abstract: Surface cracking during direct rolling of continuously cast low alloy steel slabs has been investigated by means of hot tensile tests simulating the process. The hot ductility of directly cooled specimens is not reduced by high strain rate deformation corresponding to that in rolling. While, in the specimens predeformed at temperatures ranging from γ/α duplex phase to low temperature γ region at slow strain rate corresponding to that in bending and/or straightening operations of CC slabs, ductile intergranular fracture of austenite is easily induced by the subsequent deformation even at high strain rate, resulting in poor ductility. This ductility loss can be explained in terms of dynamic precipitation of carbide, nitride, or carbonitride such as AlN and/or NbC (or NbC(N)) within the matrix as well as on the γ grain boundaries during the prior slow strain rate deformation. The ductility is also influenced by the final deformation conditions because of the morphology change of the precipitates in the process, although the embrittlement mechanism is essentially the same as in the CC process.

Characteristics of stainless steel strip cast by twin rolls.

Abstract: Experiments on the strip casting of a Type 304 stainless steel have been done by the use of a twin-roll caster with the copper alloy rolls of 400mm diameter and 300mm wide. Cast strips of 120kg with good shape and appearance were obtained with high reproducibility. The casting speed was 17-40m/min, and the length and thickness of the strip were about 30m and O.8-2.3mm, respectively. At an early stage of the casting operation the thickness reduces rapidly, and then approaches a constant value. According to the analysis of heat transfer and thermal expansion of rolls, the early decrease in thickness seems to be caused by the thermal expansion of rolls. The cast strip is not free from defects which should be minimized. The microstructure and segregation of the cast strip have also been inves-tigated. The tensile properties of the cold rolled sheets by this process satisfy those as a Type 304 stainless steel.

Direct observation of molten steel meniscus in CC mold during casting.

Abstract: Free surface fluctuations of molten steel in the mold of a continuous caster are considered to have a significant effect on the surface quality of the cast. To find out the relationship between the surface wave motion of molten steel near the mold wall during casting and mold oscillation, the meniscus of molten steel in the mold was directly observed through a quartz glass window mounted on the mold wall. As a result, it was confirmed that the meniscus fluctuates at the same period and phase as those of the mold oscillation.

Representation of Phosphorus and Vanadium Equilibria between Liquid Iron and Complex Steelmaking Type Slags

Abstract: The relation between phosphate capacity and different scales of optical basicity is examined and discussed.The coexistence of monomer and dimer phosphate ions is considered and the amount of phosphorus as monomer phosphate ions is related to the slag phosphorus content. Excellent relationship is found between phosphorus capacity, temperature and theoretical optical basicity.Optical basicities of TiO2 and VOy are derived and a linear relationship is found between the vanadate capacity, CVO3-4=(%VO3-4)/[%V][%O]5/2, and the optical basicity of basic steelmaking type slaps.

Effect of Na2O addition on the partition of vanadium, niobium, manganese and titanium between CaO-CaF2-SiO2 melts and carbon saturated iron.

Abstract: The effect of Na2O additions to the CaO-CaF2-SiO2 system on the partition of vanadium, niobium, manganese and titanium between CaO- CaF2-SiO2 melts doubly saturated with CaO and 3CaO•SiO2 and molten carbon saturated iron was investigated at 1300°C. The partition of vanadium, niobium and titanium increased with increasing content of Na2O. On the other hand the manganese partition decreased, indicating that manganese oxide behaves as a basic oxide in the CaO-Na2O-CaF2-SiO2 system.Since the vanadium, niobium and titanium have various valencies in ionic melts of slag, the ionic species of these three elements were determined. Each element was shown to exist in slag mainly as Nb5+, V4+ and Ti4+, respectively.Addition of a small amount of Na2O to the CaO-CaF2-SiO2 system enhances the capacity of removing niobium, vanadium and titanium from hot metal to slag. It was proved that the yield of manganese increases and the recovery of niobium, vanadium and titanium to slag increases by addition of Na2O to CaO-CaF2-SiO2 flux for hot metal treatment.Vanadium, niobium, manganese and titanium capacities were calculated and the relationships between these capacities and carbonate and phosphate capacities are discussed.

Rate of Dephosphorization of Liquid Iron-Carbon Alloys by Molten Slags

Abstract: The rate of dephosphorization of liquid iron-carbon alloys by FeO-CaO-SiO2 slaps and the effect o f NaCl or CaCl2 added to the slaps on the reaction rate have been investigated on a laboratory scale at temperatures of 1300-1460°C. The results show that the rate and degree of dephosphorization increase by the addition of NaCl or CaCl2 to the slag. But the degree of dephosphorization decreases with increasing temperature.It is found that the rate of dephosphorization of iron-carbon melt can be explained by the modified mass transfer controlled model in which mass transfer of iron and phosphorus across the slag-metal interface is controlled by diffusion while the evolution of CO gas is the chemical reaction controlled.

Kinetic study on nitrogen absorption and desorption of molten iron.

Abstract: Measurements have been done on the rates of nitrogen absorption and desorption of levitated iron droplets. On the assumption that the mass transfer between two phases determines the total reaction rate, the kinetics of this reaction has been studied. The results obtained in the present work are as follows: The rate equation with the same parameter a conformed to both the rates of nitrogen absorption and of desorption. A decrease in influence of oxygen in the melt on the rate constant of the melt phase with increasing temperature was interpreted to be due to the increase infraction of bare surface. In connection with Arrhenius equation, it was suggested that the rate of surface chemical reaction is not possible to be the rate determining factor.

An Improved Tint Etching Method for High Strength Steel Sheets with Mixed Microstructures

Abstract: An improved tint etching method was developed to obtain a quantitative measure of the microstructure of high strength steel sheets with a mixed structure. The procedures of etching are presented with typical examples. The present method reveals ferrite in blue, martensite and retained austenite in light yellow, and bainite in brown. Each constituent is identified by electron microscopic observations of the same area on etched microstructure. The different colors are analyzed to be a result of difference in the thickness of etched films by means of Auger spectroscopy. The new method is combined with image analysis to determine quantitatively the fraction and morphology of microconstituents.

Mathematical Modeling of the Electromagnetic Stirring of Molten Metal-Solid Suspensions

Abstract: A mathematical formulation is developed to represent surface deformation and fluid flow phenomena in an electromagnetically-stirred suspension. The constitutive relationships for representing non-Newtonian behavior at high solid loading are deduced from established experimental data. The results show that significant deformation of the surface occurs at high coil current. There is a marked decrease in the fluid velocity once the solid fraction exceeds about 10%. In addition, the velocity varies non-linearly with the current, showing a rapid increase once a certain shear threshold is exceeded.

The Copper Partition between Na2S Bearing Fluxes and Carbon-saturated Iron Melts

Abstract: The copper partition between Na2S bearing fluxes and carbon-saturated iron melts has been measured by a chemical equilibration technique at 1473K. The results are discussed in terms of the copper capacity CCu which is defined by the following equation.CCu(copper capacity)=(%CuS0.5)/aCu(l)•P1/4S2Of 5 sulfides of alkali and alkali-earth metals examined, the CCu value has been found to be in the order of Na2S>Li2S>BaS>K2S> CaS, With the NaS0.5-FeS system, the higher (%NaS0.5) leads to the higher CCu. The copper partition ratio LCu(=(%Cu)/[%Cu]) was 17 with [%S]-0.01 for the flux composition of 95%NaS0.5-5%FeS having the maximum CCu. Other thermodynamic properties of the sulfides and Fe-Csat.-Cu-S melts are also studied.

Influence of the Original Structure on the Kinetics of Hydrogen Reduction of Hematite Compacts

Abstract: Green compacts of hematite (54% porosity) were sintered in air at 1000°C for 30min and at 1300°C for 180min to produce porous (35% porosity) and dense (8% porosity) compacts, respectively. Hematite compacts were isothermally reduced with hydrogen at 500-1100°C using weight-loss technique as a function of time. The structural changes accompanying sintering and reduction processes were examined with mercury pressure porosimeter and optical and scanning electron microscopes. The reduction of porous and dense samples reveals the presence of reduction rate minimum at 650°C which was attributed to sintering and densification of the freshly reduced iron around the oxide grains. The retardation in the rate at 650°C was increased with the increase in the reduction extents and with decrease in the porosity of the compacts. The computed values of activation energy were correlated with the corresponding reduction mechanisms.

A Method for Determining Steady State Temperature Distribution within Blast Furnace Hearth Lining by Measuring Temperature at Selected Points

Abstract: A method for determining steady state temperature distribution in a blast furnace hearth lining is given. The isotherms can be determined by soloing so called inverse heat conduction problem, taking into account data obtained by measurements of temperature at selected points within the lining. An example is given.

Looper Optimal Multivariable Control for Hot Strip Finishing Mill

Abstract: A looper optimal multivariable control system based on an optimal regulator theory has been developed. Goal of controlling the hot strip finishing mill loopers is to stabilize rolling operation and to maintain product quality by controlling the tension applied to the strip and the looper angle within their reference values.The most difficult problem in looper control is to achieve simultaneously the interfering tension control and looper angle control. In recent years, a non-interactive control employing cross-controllers has been adopted and brought a dramatic increase in control accuracy, although the response time is limited. Thus, a new method with improved response time is desirable.In this paper, a looper optimal multivariable control based on the optimal regulator theory employing a looper drive system with speed control loop is proposed. Other topics discussed includes the results of an investigation of simplifying the control system by the use of a programmable controller. In addition, it is demonstrated that the optimal multivariable control is effective compared with the conventional control and the non-interactive control as employed in the hot strip finishing mill of Hirohata Works of Nippon Steel Corporation.

Dephosphorization and Desulfurization of Hot Metal by CaO Based Fluxes Containing Fe-oxide and Mn-oxide as Oxidant

Abstract: Experiments on hot metal dephosphorization by using the mixtures of Fe-oxide and Mn-oxide as oxidant were achieved by the use of Tammann furnace. The results were analyzed by the coupled reaction model which consisted of mass transfer equations of each component. Among many unknown parameters in the model, the effective equilibrium constants of P, Mn and S and the mass transfer coefficient in slag phase were changed by the experimental condition and determined by fitting to the experimental result. The observed distribution ratio of P, S and Mn was consistent with the calculated value for each experiment and the applicability of the coupled reaction model was proved.(1) When a mixture of Fe-oxide and Mn-oxide was used as oxidant, as the ratio of FeO to the total content of Fe-oxide and Mn-oxide in flux decreased, the effective equilibrium constants of P and S and the mass transfer coefficient in slag decreased and the effective equilibrium constant of Mn increased. This result indicated that the degree of dephosphorization, desulfurization, Mn recovery and the reaction rate of these reactions increased as the increase of Fe-oxide ratio in flux.(2) The addition of CaF2 and CaCl2 did not significantly change the effective equilibrium constants but modified the mass transfer coefficient in slag. Together with the result of temperature dependence, the necessary amount of CaF2 to obtain the equivalent reaction rate for each temperature was clarified.(3) The activity coefficients of FeO, P2O5 and MnO converted from the effective equilibrium constants were compared with those values calculated by the thermodynamic models. However satisfactory agreements were not obtained especially for γMnO.

Thermodynamic Treatment of Solid/Liquid Equilibrium Distribution Coefficients of Solutes in Multi-component Iron Alloys

Abstract: In this work, the thermodynamic treatment of the equilibrium distribution coefficient kXo in iron base ternary alloys described in the previous work1, 2) was developed for iron base multi-component alloys and the effects of solute-interactions on the equilibrium distribution coefficient in multi-component iron alloys were discussed thermodynamically A new paramete for the evaluation of such effects was introduced Furthermore, the equilibrium distribution coefficients of Cr, Ni, Si, Mn and Mo at various compositions in Fe-Cr-Ni base alloys were measured Small dependence of kXo of those elements on the concentrations of Cr and Ni in those alloys was found and those results were explained by the application of new parameter

Analysis of Converter Process Variables from Exhaust Gas

Abstract: A new mathematical model of which the dynamic equations of material and heat balance are combined with the exhaust gas information obtained by mass-spectrometer has been established for the improvement of the end point control of BOF.The outline of the model calculation is as follows:(1) From the amounts of oxygen and submaterials charged into BOF and the composition and flow rate of the gas exhausted from BOF, the amounts of oxygen consumed on the surface of cavity of molten steel and of oxygen consumed by decarburization can be determined.(2) The transition of composition and temperature of steel bath can be estimated from the amount of oxygen calculated by the above way on the basis of model of reaction theory.At No. 1 BOF shop in Wakayama Steel Works, the model is now being used as the guide of the operation and contributes to the reduction of reblow ratio.

A Latent Heat Method for Macro-Micro Modeling of Eutectic Solidification

Abstract: Two different numerical methods were used to simulate the solidification of eutectic alloys. The first one was the classical approach based on an implicit, one dimensional, axisymmetric finite difference method (EUCAST program), while the second one involved a two dimensional control volume implicit scheme with a boundary condition describing the heat flux at the casting-mold interface (BAMACAST program). To account for the heat evolution term in the conductivity equation, an original latent heat approach was introduced, consisting of the calculation of the latent heat evolved by the fraction of solid formed as a function of time. In turn, the fraction of solid has been calculated based on nucleation and growth kinetics. The heat transfer coefficient used for simulation was determined based on an analytical approach for hollow cylinders.Gray cast iron and aluminum-silicon alloys of eutectic composition were used for the validation experiments. Cylindrical bars of different diameters were poured and the cooling curves were recorded at various locations in the metal and the mold. Experimental and computed data were compared, with excellent results for solidification time, undercooling, width of the mushy zone and heat transfer coefficient.