Efficiency of continuous-caster-mold walls with cylindrical and slit-like channels

One way of improving the durability of the working walls of continuous-caster molds is to increase the rate at which heat is removed from the melt. New designs of mold walls have been developed for this specific purpose. This article compares the thermal state and temperatures of molds having drilled and slit-type channels for coolant water. An algorithm is developed to design bolts for securing the working slabs of the mold. The algorithm was developed with allowance for the nonuniformity of temperature over the surface of the mold walls and with the assumption that the joints between the walls remains intact during service. __________ Translated from Metallurg, No. 6, pp. 40–42, June, 2007.     

Operation of molds and mandrels during electroslag casting of hollow billets

One method of electroslag casting is the remelting of a metallic consumable electrode in a short movable mold as they move toward each other. The main advantages of this method are the saving of copper required to make the mold, a decrease in the electric-furnace height, and a decrease in energy consumption. The most important disadvantage of this method is the erosion of the mold and mandrel copper walls. The wear of the mold’s copper walls depends on the electric current through the mold and the specific surface power released in the slag bath-mold wall contact zone. The erosion-induced fracture of the movable mold and mandrel cannot be fully avoided during electroslag casting; however, the wear (fracture) rate can be reduced using several measures. A procedure is proposed for calculation of a rational geometry of the broadened portion of the movable mold in order to estimate the electrical parameters and the wear resistance of the equipment designed for the electroslag casting of hollow steel billets.     

Simulation of slab formation in a continuous-casting machine

A discrete analog of the differential heat-conduction equation permits the use of nonuniform calculation grids in the simulation of continuous casting. That allows the distribution of the temperature gradients in the model to be taken into account, with corresponding increase in the accuracy of the approximation and the results. A mathematical model is developed for the solidification and shrinkage of continuous-cast slab in the mold. The adoption of a nonuniform grid permits the use of elements measuring 1–2 mm in the simulation. This model is used to study the distortion of the slab cross section at the mold walls. Calculation of the geometric profile permits refinement of the thermal and mechanical interaction of the solidifying shell and the mold walls and determination of the optimal mold taper so as to reduce the risk of surface and subsurface cracking in the slabs.     

钢的连铸过程结晶器与铸坯之间的传热分析

针对连铸过程中的结晶器传热模型，分析总结了结晶器与坯壳之间的热阻构成，并研究了结晶器壁和坯壳界面热流的影响因素，为传热模型的求解和结晶器锥度的优化设计提供了理论依据。     

Molds with narrow walls of variable taper in continuous-casting machines

New designs are proposed for the narrow mold walls in continuous-casting machines at OAO Magnitogorskii Metallurgicheskii Kombinat. The taper of the new walls varies over the height so as allow for shrinkage of the cast billet.     

Eliminating Gaps between the Walls of Sectional Continuous-Caster Molds by the Application of Sprayed Metal Coatings

One reason that continuous-caster molds need to be replaced is that their service life is shortened by the formation and growth of gaps in the corners between the wide and narrow walls of the mold and wear of the walls' lower scn. A technology has been developed to eliminate the gaps and involves the application of a coating to the long side of the narrow walls. A coating of copper of grade MNZhKT is applied in the form of a strip 5–10 mm wide, the width depending on the size of the gap. The coating is adjacent to the working surface of the mold and is lapped after it has been applied. Alternatively, the coating can be formed with a machining allowance of at least 0.3 mm per side and subsequently milled. Deviations of wall width from the nominal dimensions and instances of non-orthogonality are also eliminated by spray-coating the long sides of the narrow walls and leaving an allowance for subsequent milling. One variant of the technology is the application of an MNZhKT coating to the butt joint between the walls jt region of gap formation while the mold is being held in reserve. This coating is then lapped.     

Thermal analysis of mold in continuous slab-casting machine

The factors affecting the thermal state of mold walls in continuous slab-casting machines are analyzed. On the basis of the proposed method and model, the design parameters of a radial mold with slot channels and the parameters of the coolant (water) flow may be calculated.     

On continuous casting of steels using pulse-periodic cooling in a mold

The causes of the difficulties that appear during continuous casting of steels with a wide solidification range are analyzed. It is shown that these difficulties can be overcome using a mold design that provides a pulse-periodic change in the intensity of heat removal from the mold tube walls.     

Wear-resistant aluminum and chromonickel coatings at the narrow mold walls in continuous-casting machines

To extend the working life of molds in continuous-casting machines, a heat- and wear-resistant layer is formed on the narrow M1 copper walls prior to final repair, by spraying on aluminum and chromonickel coatings. The first step is to determine the coating structure, phase composition, and the hardness and microhardness of the surface layers. A chromonickel coating (thickness 0.5–0.6 mm) is sprayed on the surface of the pair of narrow walls of the thick-walled mold. To obtain the required surface purity, the walls must be ground. Efforts are required to increase the thickness of the chromonickel coating. With a coating thickness greater than 0.8 mm, peeling is possible. Therefore, to increase the strength with which the chromonickel coating is bound to the copper base, we consider the possibility of using an aluminum substrate. Thus, to extend the life of the narrow mold walls in a continuous-casting machine, a chromonickel coating with an aluminum substrate may be applied if bonding of the porous layer with the α phase is ensured or the porosity of the layer is reduced.     

Modeling the thin-slab continuous-casting mold

A three-dimensional mathematical model has been developed to compute the thermomechanical state in the mold of thin-slab continuous casters. The thin-slab mold differs from those used in conventional slab casters in that the upper portion of the broad side walls defines a funnel-shaped chamber which allows the nozzle to be submerged into the liquid metal. The chamber converges with distance down the mold, reducing to the rectangular cross section of the finished casting near the mold exit. The new mold, along with casting speeds up to 6 m/min, allows slabs to be cast 50–60 mm thick, compared with 150 to 350 mm in conventional continuous slab casting. However, the mold shape and high casting speed lead to higher mold temperatures and shorter mold life than are found in conventional slab casters. In this article, we develop mathematical models of the process to determine the role of various process parameters in determining the mold life. Finite-element analysis is used to determine the temperatures in the mold and cast slab, and these data are then used in an elastic-viscoplastic analysis to investigate the deformation of the mold wall in service. Cyclic inelastic strains up to 1.75 Pct are found in a region below the meniscus along the funnel edge. These large strains result from the combination of locally high temperatures coupled with geometric restraint of the mold. The deformation leads to short mold life because of thermal fatigue cracking of the mold. The computed locations and time to failure of the mold in fatigue agree very well with observations of the appearance of mold surface cracks in an operating caster. The models are also used to develop an improved mold design. Formerly Research Assistant, Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL     

Optimizing the cross section of continuous-cast slab

New designs are proposed for the narrow mold walls in continuous slab-casting machines at OAO MMK, so as to reduce the incidence of defects at the corner of the slabs and enhance the quality of hot-rolled sheet.     

A water model study of the flow asymmetry inside a continuous slab casting mold

The work studies the extent of asymmetric flow in water models of continuous casting molds of two different configurations. In the molds where fluid is discharged through multiple holes at the bottom, the flow pattern in the lower portion depends on the size of the lower two recirculating domains. If they reach the mold bottom, the flow pattern in the lower portion is symmetrical about the central plane; otherwise, it is asymmetrical. On the other hand, in the molds where the fluid is discharged through the entire mold cross section, the flow pattern is always asymmetrical if the aspect ratio is 1:6.25 or more. The fluid jet swirls while emerging through the nozzle. The interaction of the swirling jets with the wide sidewalls of the mold gives rise to asymmetrical flow inside the mold. In the molds with lower aspect ratios, where the jets do not touch the wide side walls, the flow pattern is symmetrical about the central plane.     

轮带式连铸机结晶器的流场

利用k-ε紊流模型和水模型对结晶器流场进行了数值和物理模拟。结果表明:钢液对坯壳冲刷速度的最大值发生在结晶器入口附近回流区,其值达拉坯速度的2.52倍,并且流动模式不随拉坯速度变化。     

Variables affecting the nature of the chill zone

Structures and substructures in the chill zone have been studied in Al-Cu alloys as a function of the following solidification conditions at the substrate chill: a) heat sink capacity; b) surface microprofile; c) nature of the liquid metal fluid flow as it makes substrate contact. The parameters taken in account both experimentally and analytically are: heat transfer coefficient of the metal/mold interface, surface rugosity of the mold walls, and the Reynolds number of the liquid metal fluid flow. The results obtained show definite correlations between the structural characteristics of the chill zone and the values of the studied parameters.     

New narrow-wall design for the mold of continuous slab-casting machines

A mathematical model of ingot solidification is developed for a continuous slab-casting machine. This provides the basis for a new triplanar design of the narrow copper walls in the mold, with variable taper over the height. The ingot profile required to minimize copper wear in the lower part and ensure sufficient thickness in the upper part is calculated.     

Heat-transfer and solidification model of continuous slab casting: CON1D

A simple, but comprehensive model of heat transfer and solidification of the continuous casting of steel slabs is described, including phenomena in the mold and spray regions. The model includes a one-dimensional (1-D) transient finite-difference calculation of heat conduction within the solidifying steel shell coupled with two-dimensional (2-D) steady-state heat conduction within the mold wall. The model features a detailed treatment of the interfacial gap between the shell and mold, including mass and momentum balances on the solid and liquid interfacial slag layers, and the effect of oscillation marks. The model predicts the shell thickness, temperature distributions in the mold and shell, thickness of the resolidified and liquid powder layers, heat-flux profiles down the wide and narrow faces, mold water temperature rise, ideal taper of the mold walls, and other related phenomena. The important effect of the nonuniform distribution of superheat is incorporated using the results from previous three-dimensional (3-D) turbulent fluid-flow calculations within the liquid pool. The FORTRAN program CONID has a user-friendly interface and executes in less than 1 minute on a personal computer. Calibration of the model with several different experimental measurements on operating slab casters is presented along with several example applications. In particular, the model demonstrates that the increase in heat flux throughout the mold at higher casting speeds is caused by two combined effects: a thinner interfacial gap near the top of the mold and a thinner shell toward the bottom. This modeling tool can be applied to a wide range of practical problems in continuous casters.     

Numerical simulation of the infiltration of fibrous preforms by a pure metal

The numerical simulation of the metal-matrix composites (MMCs) elaboration by an injection process is presented. The equations governing the heat and mass transfers through a porous medium are applied to the metal injection process. The bidimensional numerical model is described based on a finite volume formulation. It is shown that two types of metal solidification appear during the injection: (1) a frontal solidification related to the heat balance between the fibrous preform and the metal and (2) a lateral solidification related to the heat losses toward the mold walls. Numerical tests validate the numerical model in the case of theoretical mono-dimensional geometries when analytical solutions exist. The model is then applied to bidimensional geometries. The preform is progressively closed until the channel is obstructed so that injection is achieved to the “impregnation depth.≓ A systematic study determines the influence on the impregnation depth of different parameters, such as thermal properties of fibers and mold walls, injection flow rate, and preform geometry. Finally, the results of the experimental injection of biphenyl resin through the SAFFIL* preform are discussed and compared with the numerical results.     

Numerical simulation of the infiltration of fibrous preforms by a pure metal

The numerical simulation of the metal-matrix composites (MMCs) elaboration by an injection process is presented. The equations governing the heat and mass transfers through a porous medium are applied to the metal injection process. The bidimensional numerical model is described based on a finite volume formulation. It is shown that two types of metal solidification appear during the injection: (1) a frontal solidification related to the heat balance between the fibrous preform and the metal and (2) a lateral solidification related to the heat losses toward the mold walls. Numerical tests validate the numerical model in the case of theoretical mono-dimensional geometries when analytical solutions exist. The model is then applied to bidimensional geometries. The preform is progressively closed until the channel is obstructed so that injection is achieved to the “impregnation depth.” A systematic study determines the influence on the impregnation depth of different parameters, such as thermal properties of fibers and mold walls, injection flow rate, and preform geometry. Finally, the results of the experimental injection of biphenyl resin through the SAFFIL* preform are discussed and compared with the numerical results.     

Some electrical characteristics of a DC electroslag unit

The various current paths possible in an electroslag unit are proposed and compared with operational results measured on a dc unit. The current found to pass through the mold walls under “mold-isolated” circuitry is small (less than 0.05 × the operating current). The metallurgical consequences of the possible operating modes are outlined and demonstrated.     

Mastery of the Production of Slag-Forming Mixtures at the Nizhniy Tagil Metallurgical Combine

A shop has been organized at the Nizhniy Tagil Metallurgical Combine to make slag-forming mixtures and a factory standard has been developed for its products. The quality of the mixtures is fully consistent with the requirements established for mixtures used in the continuous casting of steel. The mixtures reliably insulate the meniscus of the metal in the mold and provide the necessary degree of lubrication between the skin of the solidifying ingot and the walls of the mold. The technology used to prepare the mixtures makes it possible to change over from one type of slag-forming mixture to another without stopping production. __________ Translated from Metallurg, No. 4, pp. 42–43, May, 2005.