Casting Plant Research Articles

Reliability Analysis of a Hot Standby Industrial System

Industries/plants, such as Fibre Optic plant and casting plants, often use programmable logic controllers (PLC) as a two-unit/two-PLC hot standby systems. Optimizing profit and avoiding the major losses are the main objectives of using a two-unit hot standby PLC system. In the two-unit system, one unit is operative and the other is a hot standby. The hot standby unit also fails, but with lower failure rate than the operative unit. The paper outlines a reliability analysis of a two-unit hot standby PLC system, and the real data from an industrial system has been used for the purpose. Four types of failure are seen in the PLCs: input module failure, digital relay burnt/power supply failure, complete unit burnt failure and failure due to corrupted software. The concept of inspection is introduced to detect the type of failure. The model developed is embedded by the types of failure actually depicted in the data and the real failure, repair, replacement, reinstallation rates are used for analysis. Measures of system effectiveness in terms of reliability indices are obtained using semi-Markov processes and regenerative point techniques. Profit incurred to the system is evaluated and related graphs pertaining to the case example are also shown.

Pulsating electromagnetic casting technique for slab casting

To improve the surface quality of cast slabs in the steelmaking process, the pulsating electromagnetic casting (EMC) technique was developed and assessed using a slab casting bench scale test. Numerical simulation of the magnetohydrodynamic field and numerical simulation of the toughness of the mould were also conducted for the slab casting plant test. Consequently, the mould and coil for slab casting plant tests with pulsating EMC were manufactured and the basic functions were verified. Finally slab casting plant tests with the pulsating EMC technique were conducted.

Optimization of mixed casting processes considering discrete ingot sizes

Using linear programming (LP), this research devises a simple and comprehensive scheduling methodology for a complicated, yet typical, production situation in real foundries: a combination of expendable-mold casting, permanent-mold casting and automated casting for large-quantity castings. This scheduling technique to determine an optimal casting sequence is successfully applied to the most general case, in which various types of castings with different alloys and masses are simultaneously produced by dissimilar casting processes within a predetermined period. The methodology proves to generate accurate scheduling results that maximize furnace or ingot efficiency. For multivariable and multi-constraint optimization problems per se, it provides an extremely practical solution which is readily implemented in most real-world casting plants. In addition, incorporating ingot adjustment from the reality of discrete ingot size, this LP scheduling can assist the casting industry in strengthening its competence by heightening ingot utilization as well as satisfying due dates.

Scheduling of casting in real foundries using linear programming

This study develops a simple and convenient scheduling methodology using linear programming (LP) for three typical and distinctive production situations in real foundries: sand casting, low-pressure casting, and a combination of sand casting, low-pressure casting, and automated casting. Proposed to determine an optimal casting sequence, this methodology is successfully applied to all casting environments that produce simultaneously various types of castings with different alloys and weights in a number of orders within a predetermined period. It can also generate accurate scheduling results for maximized furnace or ingot efficiency. The presented LP formulation provides a practical solution that can be implemented in general casting plants. Hopefully, the proposed LP scheduling method will assist the casting industry to strengthen its competence by achieving cost minimization and fulfillment of the production requirement within a due date.

Uncertainty analysis and robust control of temperature fields of casting mould as distributed parameter systems

In the paper, an uncertainty analysis and simulation of a robust control of temperature fields of the casting mould in the benchmark casting plant are presented. Temperature fields are supposed as distributed parameter systems with dynamic models in the form of lumped-input/distributed-output system. For this representation, uncertainty of the controlled system was considered and robust control synthesis with internal model control structure was designed. Simulation of the robust control of the temperature field in the casting mould was carried out using the Distributed Parameter Systems Blockset for MATLAB & Simulink – Third-party MathWorks product.

The CASTRIP® process – recent developments at Nucor steel’s commercial strip casting plant

The CASTRIP® facility at Nucor Steel’s plant in Crawfordsville , Indiana, has been in operation since May 2002. Nucor has also commenced construction of its second strip casting plant in Blytheville, Arkansas. The Crawfordsville plant produces low carbon Ultra-Thin Cast Strip (UCS) material in the thickness range of 0.8 to 1.5 mm with a broad range of mechanical properties which are used in applications replacing cold rolled and hot rolled steel sheet. This paper will report on the recent progress made at the Crawfordsville plant, specifically, the effect of dissolved gasses, such as hydrogen and nitrogen, on castability of UCS produced at the CASTRIP® facility.

System design principles for ultrasonic casting plant for a highly refractory oxide ceramic

Methodological principles are formulated for selecting the scheme and the design parameters for the acoustic system for ultrasonic casting of a highly refractory ceramic. Relationships are established for optimizing the position of the waveguide in relation to the parameters of the acoustic processing and the volume of the simultaneously processed material. The results have been used in setting up an ultrasonic extrusion system.

Prevention of PCDD/F Formation and Minimization of Their Emission at the Stack of a Secondary Aluminum Casting Plant

Results of an extensive 5 year study on a full-scale plant with the specific aim to investigate polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzo furan (PCDF) formation and release in a secondary aluminum casting plant are reported. PCDD and PCDF concentrations were evaluated for all the gaseous and solid streams (no liquid stream was present) as well as for the flue gas upstream and downstream of every single unit of the flue gas cleaning system. The study highlights PCDD and PCDF formation particularly in the narrow 320-360 degrees C range. To prevent formation reactions and/or minimize PCDD and PCDF concentration at the stack, effects of the fabric filter substitution, a quenching chamber and a postcombustor installation together with working conditions are investigated. The flue gas cleaning system results in PCDD and PCDF emission at stack of 0.1-0.2 ng I-TEQ/N m3 and in a mass flow of 250-550 nmol/h. The total PCDD and PCDF release into the environment is 0.06 g I-TEQ/yr and the corresponding emission factor, 0.35 microg I-TEQ/ton. It is shown that the global effects of the technological innovation on the reaction mechanisms are the prevention of PCDD/F formation by de novo synthesis and the minimization of their emission.

Development of solidification microstructures in continuously cast billets of boron and resulphurised steel grades

The continuous casting process equipped by electromagnetic stirrer has been simulated through thermal model in order to identify the growing velocity of the solid front, the thermal gradient and the undercooling featuring the solidification process. The results of the model obtained after imposing the measured boundary condition allowed to describe the solidification process identifying the thermal gradient of the solid front and its velocity of growth. The main parameters featuring the solidification structure have been measured: primary dendrite arm spacings (PDAS), secondary dendrite arm spacings (SDAS) and the extension of the columnar zone, the columnar equiaxed zone and the equiaxed one. The results obtained by the computational activity and by the experimental trial have been combined to determine the condition for the columnar equiaxic transition, to estimate the nucleation and the constitutional undercooling interesting solid liquid system, the relation among the PDAS, the thermal gradient and the growth velocity of the solid front and to improve the relation between the SDAS and the solidification time through the inclusion of the effects produced by the main alloying elements contained within the steel. The performed analysis has permitted to characterise the solidification process of the studied steels and effects produced on their microstructure by the particular configuration of the continuous casting plant which has been considered.

Control issues in continuous casting of steel

Continuous casting plants are highly complex plants whose performance requires reliable mold level control system. Several effects greatly influence the surface quality of the final product and if these are not suppressed sufficiently by the mold level control system, than the production speed, hence the productivity has to be lowered. This paper analyses the most important disturbance and their effects on the control system design, and presents a new solution for the most critical case, mold level hunting.

Failure of Nickel Coating on a Copper Mold of a Slab Caster

Copper molds are critical components in the continuous casting of steel. Surface condition of the mold directly affects product quality, and premature failure/low wear life affects the output of the continuous casting plant. This paper presents the failure analysis of a nickel coating on a copper plate during a plant trial using in situ ultrasonic testing for coating thickness evaluation and in situ testing to confirm the loss of coating. The failure analysis indicated maximum coating wear near the bottom corners of the mold. More wear was found on the wide end face compared with that observed on the narrow face. On top portion of the mold, wear was almost negligible. Based on the wear profile obtained during trial, it was recommended that the coating on the bottom portion of the mold have a higher hardness to improve mold life.

Tundish Flow Optimization and Steel Grade Change Prediction

Tundish design is of great importance for the stability of the casting process and for the quality of the cast product. This paper presents some criteria determined for the optimization of the flow pattern in the tundish, based on the casting experience and Computational Fluid Dynamics. Among such criteria can be mentioned a) steel temperature delivery to the strands, b) inclusions behaviour, c) entrapment of the cover tundish powder, d) erosion of the tundish refractory, etc. The determination of such criteria permits to optimize a tundish design very quickly and in the same time to standardize the tundish geometry according to the casting plant layout and productivity. Additionally, it will be presented the work done for the tuning of a new model that predicts the steel grade change in tundish. The transient fluid dynamics simulations for certain tundish geometry permit to investigate the steel mixing in tundish and strands, tuning the above model. These simulations consider the real tundish filling with new steel grade. For each strand this model predicts the temporally concentration of the two steels during the grade change. The model is set up on some dilution interconnecting elementary cells that describe the shroud and each strand. The cells are considered with uniform concentration. Such a kind of model is very useful in the definition of the technological practice for the steel grade change operation.

Improvements & Innovations in the Continuous Casting Process at POSCO

Continuous casting is the essential process converting liquid steel to solid in the form of slabs or billets/blooms in the steel plant. The economy and quality of the steel products are greatly dependent on how successfully the continuous casting is performed. New technologies have been actively developed in the process during the last decades in order to increase the productivity and, therefore, to decrease the operational cost. Since its first commissioning of a slab caster in 1976, POSCO has constructed a number of continuous slab, bloom and billet casters including a thin slab caster not only for plain carbon steels but for stainless steels. Through the operation of various types of continuous casters for more than 30 years so far, POSCO has steadily developed fundamental technologies and operational know-how and achieved the equipment innovations to improve the surface and internal qualities of cast products as well as to extend the productivity of continuous casters. Furthermore, POSCO has deepened the basic understanding on the solidification phenomena of liquid steel and also accumulated the engineering backgrounds to design the most optimal continuous casters. It has also devised the indispensable and auxiliary equipments and the key technologies to control the process precisely and efficiently in order to guarantee the quality and productivity. An innovative technology under development is the POCAST process, where controlled amount of the pre-molten mold flux instead of conventional powder mold flux is continuously fed into free surface of molten steel through the plunger-type feeding system from the flux melting furnace. In order to prevent the molten flux from freezing at the meniscus, a reflective insulation cover is installed, leading to the suppression of thermal radiation from the molten steel and flux. It is generally understood that, as casting speed increases, the occurrence of breakout increases since mold lubrication becomes insufficient due to the lack of mold flux flow from the meniscus into the solid shell/mold boundary. However, by utilizing the especially composition controlled pre-molten flux, it becomes possible to eliminate the formation of slag bear in the mold. Therefore, the mold flux consumption rate is increased even at the reduced oscillation rate & stroke and more importantly, the mold flux infiltration becomes more uniform throughout the boundary between the mold and the solidified shell. This consequently results in drastic reduction of the formation and depth of the oscillation mark and the occurrence of surface hooks without increasing the possibility of breakout, as has been proved in the casting trials carried out with the 10 ton pilot slab caster in Pohang. A key trend in the development of the continuous casting process is to reduce the thickness of cast products. Examples include thin slab casting and strip casting. In the thin slab casting process, a major drawback is the relatively low casting speed and, as a result, the inefficient equipment layout in the plant where two casters are connected to a hot rolling unit. The drawback could be resolved if the casting speed exceeds a certain limit. At the high casting speed, the productivity of casting becomes equivalent to that of hot rolling, and the thin slab casting plant is to be designed so that one strand

NEW CUT OPTIMIZATION METHOD AT CONTINUOUS CASTING

The paper covers main design principles and description of optimization algorithms for cutting steel at continuous casting plant. The aim is to produce maximum amount of customer bars from liquid steel. The static cutting plan from production planning is dynamically updated taking into account unexpected deviations in steel quality. The optimizing problem type is mixed integer non-linear programming (MINLP).

Vanadium Microalloyed Steel for Thin Slab Casting and Direct Rolling

Vanadium microalloyed steels with high yield strength (»600MPa), good toughness and ductility have been successfully produced in commercial thin slab casting plants employing direct rolling after casting. Because of the high solubility of VN and VC, most of the vanadium is likely to remain in solution during casting, equalisation and rolling. While some vanadium is precipitated in austenite as cuboids and pins the grain boundaries, a major fraction is available for dispersion strengthening of ferrite. Despite a coarse as-cast grain size, significant grain refinement can be achieved by repeated recrystallisation during hot rolling. Consequently, a fine and uniform ferrite grain structure is produced in the final strip. Increasing the V and N levels increases dispersion strengthening which occurs together with a finer ferrite grain size. The addition of titanium to a vanadium containing steel, decreases the yield strength due to the formation of V-Ti(N) particles in austenite during both casting and equalisation. These large particles reduced the amount of V and N available for subsequent precipitation of fine (~5nm) V rich dispersion strengthening particles in ferrite.

Secondary cooling in continuous casting and Leidenfrost temperature effects

The present study was motivated by problems at continuous casting plants where a variety of thermally induced defects were observed. The paper outlines an experimental method for the measurement of cooling intensity in the secondary cooling area where nozzles are applied. The precision of a variety of experimental methods is discussed. The nozzles have been investigated in terms of pressure setting, the influence of casting speed, and behaviour in the overlapping areas. The tests have provided information on heat transfer coefficient characteristics and heat flux distribution on the cooled steel surface. The paper presents new experimental findings regarding specification of the Leidenfrost temperature, which is the point between high and low surface temperature regions where a large difference in cooling intensity is observed. The paper also deals with the problems of homogeneity of cooling along the nozzle spray angle and in the overlapping area where thermal 'stripes' occur. The sensitivity of this problem to the pressure of the coolant is discussed.

Precipitation and phase transformation modelling to predict surface cracks and slab quality

Improved numerical simulation tools provide valuable information for equipment designers and operators of continuous casting plants. In comparison to the collection and evaluation of process data, simulation models based on physical principles are able to provide a more detailed understanding of how individual process parameters can influence the production process and final product quality. One major goal of the numerical modelling of metallurgical processes is to predict the material behaviour using a proper description of production processes and their correlation to product properties and quality. Thus, the amount of expensive experimental trials can be reduced in the process of development. Later for standard production a numerical quality assessment is appropriate to reduce inspection efforts. Utilising commercial and in house made simulation software, the continuous casting process is simulated regarding thermal, mechanical, and phase transformation aspects. In a practical example, metallographic observations of strand surface defects, such as transverse corner cracks, are correlated to material history during the production process. As a result, the initiation of material failure can be localised more precisely and countermeasures can be taken with a higher chance of success compared to models that use only linear combinations of steel analysis and process parameters. The applicability of these simulation tools for computer aided quality control (CAQC) systems is discussed.

Development of Fluoride-Free Fluxes for Billet Casting

More than 90% of the world's steel is produced using the continuous casting process, a method that has seen enormous advances over the last forty years. Mould fluxes play an important part in this process. These fluxes contain fluorides, which can volatilize at operational temperatures polluting both the plant air and cooling water. Airborne fluoride could potentially be a health and safety issue. Waterborne fluoride forms hydrofluoric acid (HF), which can cause plant corrosion, and may lead to contamination of watercourses necessitating water treatment schemes. This adds to production costs and may present potential environmental hazards. These concerns could be reduced or eliminated by removing fluoride from mould fluxes.The present study examines the effect of different fluxing agents upon key mould flux properties. When substituting fluorides for alternative fluxing agents the key design properties of the fluoride-containing flux must be replicated; namely, (i) flux viscosity at 1300°C, (ii) break temperature and (iii) percentage of crystallinity in the solid slag layer. This is to ensure ‘optimal casting’ where operational problems, such as sticker breakouts and defects such as longitudinal cracking, are minimized. In addition, the quality of the steel should not be affected by the substitution. Therefore, any substitute/additive or combination of additives would have to possess the capacity to replicate the effects that fluorine has on mould flux behaviour.This study focuses on B2O3 and Na2O as alternative substitutes for CaF2 in billet fluxes. The new flux has been successfully tested in a plant trial on a continuous casting plant.

Exposure and inhalation risk assessment in an aluminium cast-house

To date the exposure, absorption and respiratory health effects of cast-house workers have not been described since most studies performed in the aluminium industry are focused on exposure and health effects of potroom personnel. In the present study, we assessed the external exposure and the absorbed dose of metals in personnel from the aluminium cast house. This was combined with an evaluation of respiratory complaints and the lung function of the personnel. 30 workers from an aluminium casting plant participated and 17 individuals of the packaging and distribution departments were selected as controls. The exposure was assessed by the quantification of total inhalable fume with metal fraction and by the determination of urinary aluminium, chromium, beryllium, manganese and lead concentration. Carbon monoxide (CO), carbon dioxide (CO2), aldehydes and polyaromatic hydrocarbons and man-made mineral fibres concentration were assessed as well. In order to evaluate their respiratory status each participant filled out a questionnaire and their lung function was tested by forced spirometry. Total inhalable fume exposure was maximum 4.37 mg m(-3). Exposure to the combustion gases, man-made mineral fibres and metal fume was well below the exposure limits. Beryllium could not be detected in the urine. The values of aluminium, manganese and lead in the urine were all under the respective reference value. One individual had a urinary chromium excretion above the ACGIH defined biological exposure index (BEI) of 30 microg g(-1) creatinine. There was no significant difference in any of the categories of the respiratory questionnaire and in the results of the spirometry between cast house personnel and referents (Chi-square, all p > 0.05). Exposure in cast houses seem to be acceptable under these conditions. However, peak exposure to fumes cannot be excluded and the potential risk of chromium and beryllium exposure due to the recycling of aluminium requires further attention.

Production scheduling in a steelmaking-continuous casting plant

In this paper we describe an optimization procedure for planning the production of steel ingots in a steelmaking-continuous casting plant. The strict requirements of the production process defeated most of the earlier approaches to steelmaking-continuous casting production scheduling, mainly due to the lack of information in the optimization models. Our formulation of the problem is based on the alternative graph, which is a generalization of the disjunctive graph of Roy and Sussman. The alternative graph formulation allow us to describe in detail all the constraints that are relevant for the scheduling problem. We then solve the problem by using a beam search procedure, and compare our results with a lower bound of the optimal solutions and with the actual performance obtained in the plant. Computational experience shows the effectiveness of this approach.