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Abstract

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Introduction

Under the continuous casting of steel is usually taken to mean a system of technology and operations, which provide a quasi-continuous translation zhidkoystali, located in the ladle, the solid state in the form of billets specific geometric shape. Continuous casting process provides a consistent (non-stop) casting a certain number of buckets supplied from the steel-making units, and the resulting preform with cut-to-length, in accordance with the requirements of the customers and then sent to the appropriate roll rolling mills. The yield of non-defective workpiece is 98,5-99,5% by weight of the poured molten steel. Continuous steel casting process is carried out by special machines, nazyvaemyhmashinami continuous casting machine (CCM), which have a structural architecture consisting of a rotary bench to replace the ladle, tundish, the mold, secondary cooling zone of the workpiece, pulling the right gear, etc. Being the continuous casting process is that the liquid steel from the ladle enters the tundish, and then cooled rapidly in a through a rectangular, square, round or special shaped cross-section - the mold, where there is a partial solidification of the ingot is continuously drained and formed a hard shell filled with liquid steel in form and section, corresponding to the finished workpiece.

1. Scheme of radial CCM

The main function of any process is the transfer of steel continuous casting machine from a liquid state to a solid billet produced with giving a specific geometric shape and the provision of quality indicators of the surface and internal structure, regulated by the relevant technical specifications. In order to achieve solid state workpiece must take a certain amount of heat into the environment over time. For normal process cooling is necessary to provide a certain movement of the workpiece at a regulated rate of heat removal (cooling water). The main functional elements of the CCM include: • teeming booth - is intended for placing thereon a ladle, transferring them from the back position to the pouring position and back, to ensure continuous casting, lifting and lowering of the casting ladle and ladle for continuous weighing of the metal; • intermediate bucket truck - used to hold it in the casting and moving out of the reserve position in the work; • intermediate bucket - ensures the supply of the metal in a mold with a certain well-organized flow jet allows pouring steel in a few molds and exercise at the same time casting a production method of "melting in the melting" in smenestalerazlivochnyh buckets without stopping and reducing the casting speed, tundish is the buffer capacity, so as agreed upon with the help of flow of metal from the ladle into the mold; • the mold - is designed to receive molten metal, the formation of a given section of the ingot and its primary cooling (made of copper and cooled with water during the casting); • mechanism for reciprocating the mold - creates conditions that reduce the likelihood of breakthroughs strand shell to exit, and provides a complete "curing" a rupture that emerged during the motion of the ingot in the mold; • secondary cooling zone (WEST) - allows you to create the optimum conditions for the complete solidification of continuously cast ingot to ensure uniform cooling of the blank (water spray nozzles, maintaining its geometric shape rollers prevent buckling) and the required quality of the metal; • tjanushche - straightening machine (TBM) - is designed to pull the cast section of the mold, straightening her on the radial and curved units and supply to the machine (mechanism) for cutting; SST supplies seed to the mold, holding it in the mold at the time of sealing gaps drawing of a continuous casting mold department heads from harvesting the seed and the like; • Machine (mechanism) for cutting blanks - provides separation of continuously metal to length in accordance with the requirements of customers; • seed - is designed to form a temporary "bottom" in the mold before casting and then pulling the workpiece with the clutch SST; • electromagnetic stirring device - to enhance the quality of the workpiece. At present there is a wide variety of operating the continuous casting of steel. All these varieties of plants are classified according to the following features. By type of billet caster at different slab, bloom and varietal. Blanks slab cast onto machines have a sectional shape of a rectangle with a ratio of long side to short> 3 ... 4. On the bloom and billet caster cast billets as a circle, square or rectangle with a smaller aspect ratio. The blanks with the face size> 200 mm are usually called blooms, with a smaller size - Billets. According to the principle of the continuous casting distinguish and semicontinuous casting. On machines for continuous casting ingot is cut into blanks measuring length, allowing you to pour lots of melting melting method for smelting. When polunepreyvnoy casting billet length is due to the design features - speed drawing mechanism, which is chosen for reasons of simplification and reduction in price of cars in these conditions of production. In terms of composition distinguish single-banded and caster. The increase in plant productivity is achieved by casting the metal from the ladle into several molds. Usually varietal cars produced by four - eight streams, and slab - two. Recently manufactured slab machine with four streams. By the nature of the movement of the mold distinguish the following types of casters: - A reciprocating motion, the mold for a certain period while moving ingots or ahead of it, and then returns to its initial position, this type of machine provides the bulk of continuous casting of steel; - With the mold, moving at a speed of ingot, and this ensures no slip relative to the shell of the ingot mold and therefore the friction between them, which reduces the likelihood of rupture at high casting speeds, this type of CCM is the so-called rotary (roller) caster. By location technological axis continuous casting machine are divided into machine with constant curvature axis until the solidification of the ingot (see Figure 1, a-d) and the axis of the machine with the process of solidification of the ingot in the area of ??variable curvature (see Figure 1, f, g) . The most widely used following types of casters: vertical, curved and radial, curved and horizontal ingot. In place of the vertical continuous casting, with a mid-70s of last century, came caster radial type, which are most widely. It is, first of all, was associated with improvement of the process of smelting and reducing the time for the final fine-tuning of the steel to a given chemical analysis.  Machines required at a higher rate casting many streams and had a more compact and cheaper construction.

2. Design features of molds

The mold is water-cooled continuous casting mold, the inner surface of which is exposed to the abrasive effects of moving the crystallizing ingot steel. The mold is the most important hub of CCM. It provides a rapid formation of a thick enough solid "crust" on the surface of continuously cast billets. A copper or steel plates have channels for the flow of cooling water. The walls of the mold are mounted in a rugged housing. The length of the mold caster is (500 ... 1500) mm for the provision of education, "crust" thick (8 ... 25) mm on the surface of the workpiece at the exit of the mold. Internal planed (or wavy) surface, lubrication and continuous vertical reciprocating - translational movement of the mold during casting of steel protects the surface of the workpiece from the formation of defects (longitudinal cracks). Speed ??ratio mold oscillation: speed of up to 70% higher than the speed down. Step rocking - (10 ... 40) mm, frequency - (10 ... 100) cycles per minute. Figure 3 shows the structure of the mold for the continuous casting slab casting (ingot flat section). Liquid steel, crystallizing welded to the "seed" and the separation of "seed" with the workpiece is carried out in the initial part tjanushche-right stand. Then the "seed" once again prepare to work in the usual way. Cooling mold caster system (primary cooling of the cast metal) carried out with water: bottom inlet, the rate of not less than 5 m / s, the temperature at the discharge of not higher than 500 ° C, flow (100 ... 120) m ? / h per 1 m resolution cavity inner diameter crystallizer. Thick-walled mold resistance - (20 ... 45) thousand tons of steel, or up to 85 trunks. Acceptable wear copper plate thickness - 1 mm. Fuel copper - (0.09 ... 0.15) kg / t steel. Normal production length of the mold until recently was 700-800 mm with a minimum size of 500 mm to a maximum of 1200 mm. The modern concept of molds involves a length of about 900-1000 mm, which increases the thickness of a solid crust of the preform at the outlet of the mold during casting at higher speeds. Working part mold is made of either refined copper, or copper alloy with silver or copper alloys with chromium and zirconium. To improve the operational stability of the inner surface of the mold are put special protective coating based on chromium or nickel. Chrome finish - traditional wear resistant coating the inner surface of the mold. It is applied directly on the copper plates for casting bloom and billet, as well as wear-resistant coating that weakens the friction between nickel and copper in the slab caster. The hardness of chromium is equal to about 900 HV, but the thickness of the chromium coating is limited to 0.12-0.13 mm (slab casters) and about 0.20-0.22 mm (billet caster). To date, the main objectives of its application - reducing friction and reducing sticking in the mold with a sharp change in the level of the metal when it starts. In recent years, as a protective coating has been successfully applied nickel and nickel alloys having certain advantages with regard to regulating the intensity of the heat sink. The hardness of these coatings range from 220 to 1200 HV, and the thermal conductivity - from 90 to 30 W / (mK). In practice slab caster using nickel coatings that have a different mold thickness adjustment. This allows greater control over the process of heat transfer in the mold, which is especially important for crack formation steels. Also leading manufacturers use various molds sandwich type nickel coating (+ phosphorus) - cobalt - chromium which greatly increase the resistance sleeve, but they increase the cost price of a few. In terms of structurally part of the copper mold is carried out either in the form of a sleeve or a team. In CCM radial mating molds typically used - usually used for casting square billets to 220-250 mm, and casting the round billet. Mating the molds are made of seamless copper tubes with a wall thickness of 5 ... 20 mm. Of the various methods billets metal forming prepared item with a predetermined cross-sectional profile, called the liner, which is the inner wall of the mold working. Liner is inserted into the steel shell and secured at the top with a flange. The lower part of the sleeve is fixed to the housing with the seal similar free thermal expansion without causing deformation of the walls. Water moves between the shell and the liner in the gap width of 4 ... 7 mm, providing an intensive and uniform heat removal. Warpage is also prevented sleeve device stiffeners. The big advantage is the ability to mold the barrel to achieve high speeds due to the casting of high intensity heat transfer through the thin walls of the sleeve, the lack of joints in the working surface of the wall, which are often the cause of hang-ingot, low consumption of copper, easy removability of used cartridges, and design simplicity and relatively low cost . However, mating molds suitable for casting large rectangular and square cross-sections due to insufficient stiffness of thin-walled copper sleeves, as well as virtually restored when repairs. Operational resistance of mating mold can be 20-25 tonnes of liquid steel. The intensity of the heat in the cartridges is significantly increased by reducing the air gap between the wall of the liner mold crust and a continuous ingot. The air gap can be eliminated or at least reduced to a minimum, and the heat transfer is optimized if the contour of the mold will closely match the contour of the cover of a continuous ingot. In practice, the correction for the natural shrinkage of the continuous casting ingot is carried out by performing the inner surface of the mold liner or in the form of multi-cone, or in the form of so-called parabolic profile. This concept is generally ensured growth rate of extraction harvesting the average 1.5-2.5 times in comparison with the double-cone odnokonusnoy and sleeves.

3. Hardened steel in the mold

In the crystallizer is shaping configuration harvesting by building a solid crust. The process of formation of the solid crust is accompanied by the release of heat into the environment (through the walls of the mold). It is possible to "tacking" (sticking) of a brown solid surface mold which facilitates the formation breakthroughs hard shell at its outlet. During the stay in the melt crystallizer from the workpiece is given 15-30% of the total heat that is accumulated metal. A feature of the mold is the intense heat removal from the workpiece. Thus, the temperature of the liquid steel in the core of the preform located in the mold, at least a few degrees above its solidification starting temperature (liquidus temperature). A typical temperature distribution in the cross section of the mold and preform in a gas gap between the preform and the mold wall For steels of various grades liquidus temperature may range from 1460 ° C to 1539 ° C. The intensity of the heat provided by the scheme depends on the following processes: - Convective flow movements along the border have become hardened; - Heat transfer through a two-phase zone of the workpiece (and the length of the two-phase zone); - The intensity of heat removal through the hardened crust of the workpiece; - Heat transfer gas through the gap between the solidified shell and the inner surface of the preform mold; - The heat sink through the protective coating and the copper wall of the mold itself; - Heat transfer cooling water. Steel flow that enters from the tundish into the mold, has a considerable kinetic energy required for mixing large volumes of liquid metal. Without analyzing in detail the conditions of mixing of the metal in the mold of the incident stream, we note only that, where these flows are of great speed, is more intense heating of the walls of the mold, and the growth of the solid crust of slowing down. Movement of the molten metal in the mold due to the following processes: - The vertical movement of the metal stream from ladle into the mold. When casting metal open jet continues to move vertically downwards and directly into the molten bath blank, penetrating to a depth of several meters. When using the nozzle jet penetrates into the liquid metal bath and the workpiece vertically or at an angle corresponding to the angle of inclination of the holes in the immersion nozzle; - Convection currents in the molten bath workpiece caused by the introduction of the melt jet (or jets) of the metal flowing out of the ladle (nozzle); - Wave processes on the mirror of the metal in the mold, which adversely affect the surface quality of the workpiece due to the capture of slag-forming mixture; - Seething surface of the metal in the casting of an open jet or by blowing argon through the lock-in-One. Rational selection circuit for supplying a jet of metal into the mold defined by a whole set of technological considerations. However, the greatest impact on the warming of the mold wall and the undermining of the solid crust have flows at the intersections of the trajectory of the jet with the surface of the mold. However, in some cases resulting from the nozzle to direct appropriate metal flows obliquely upward to heat the meniscus in the mold at angles of the side faces. In this case, the most complex hydrodynamic and thermal conditions of formation of a crust of hardened metal observed on the border of return flows from the mold wall and insulate the slag on the meniscus. In general, the process of removing heat from the surface of a solid crust of the preform through the walls of the mold are crucial in terms of the dynamics of growth of the solid phase and as a result, formation of surface and subsurface defects. To ensure high performance of CCM and the desired surface quality of the leading firms (manufacturers CCM) seek to optimize the geometry of the mold, moving away from the classic designs. Apparently, efforts to create the optimal geometry of the mold will continue to allow pouring steel with higher speed. A great influence on heat transfer have roughness and waviness of the surface. The distance between the irregularities in the surface roughness exceeds the height of the dozens of times, and with undulations - hundreds of times. When two surfaces come into contact primarily highest irregularities. By increasing pressure on the crust of solidified metal is some uneven deformation and connected to the new terminal, the lower protrusions. Moreover, the number of contact spots and the relative increase of the actual contact area. Under these conditions, heat is transferred from one body to another, primarily through direct contact spots. Therefore, the line of the heat flow is always pulled to the spots where there is a concentration of heat flow. Typically, for analysis of the body contacting contact heat exchange is divided into a number of parallel channels having the same radius and the average number of channels equal to the number of contact points. The duration of contact heat exchange can be increased through technological activities that enhance the plastic properties of the steel at temperatures of its solidification. Pure crystals of iron, for example, at high temperatures, have high ductility and can be deformed due to creep of steel longer than carbon steel. This property of ductile iron, of course, will increase the duration of the contact heat transfer in the mold through the optimization of its geometric shape. The presence of non-metallic inclusions considerably reduces the adhesion between the crystals, and causes a sharp decrease in ductility properties, facilitating more rapid departure of the preform shell mold wall. Therefore ductility at high temperatures depends mainly on the chemical composition of the steel, its melting temperature, the nature of the location of nonmetallic inclusions mode ladle steel deoxidation treatment. If the chemical composition of the steel after deoxidation promotes rapid solidification inclusions in globular form, the ductility at high temperatures increases. When allocating the process of crystallization of low-melting inclusions along the grain boundaries, which are in a period of intensive development of shrinkage are still in a liquid state, the hardened crust shell blanks can withstand without cracking ferrostatic less pressure. Such properties of the metal make it necessary to reduce the speed of continuous casting and require a reduction of the duration of contact heat exchange. In the practice of casting preference for the molds made from primary copper alloys, refined copper and silver (0.15-0.2%), and other ele-ments that have a high thermal conductivity and increases the strength of the material. At the working surface molds as special wear resistant coating applied on the basis of chromium, nickel, etc. To ensure that the intensity of heat are cooled copper wall running water, which is supplied with a high rate in special channels. As a result of staying in a metal mold to form a solid crust, sufficient to ensure that when leaving it blank had a solid shell without cracking allowing breakthroughs and solid frame. The thickness of the solid crust is possible with sufficient accuracy to estimate the approximate empirical formula (square-root law) When deterioration of contact with any portion of the preform mold wall observed locally thinning brown solid, which leads to deterioration of the strength properties of the solid ingot frame in this area. It is most often seen in the corners of the workpiece. In general, when you exit from the mold to form such a solid crust, which is sufficient to withstand the pressure on her ferrostatic steel and tensile stresses caused by the force generated by pulling the workpiece. With the destruction of the solid crust of the ingot is formed longitudinal crack through which flows the liquid steel. This situation is an emergency and leads at least to stop the stream or the whole caster. Depending on the steel grade, the configuration and the workpiece desired casting rate (in terms of reliability of the casting process) thick solid crust at the outlet of the mold in the range 15-18 mm for the billets, blooms 25-30 mm and 25-35 mm for slabs.

Defects associated with the work of the mold of the CCM

All defects found in continuously cast billets can be divided into the following groups: - Defects of geometric shapes (profiles) of the blank; - Surface and subsurface defects; - The internal defects are located within the body of the preform. The main defects of continuously cast billets include: geometry defects, superficial longitudinal, transverse and arachnids cracks, internal cracks, defects in the ingot core, pores and slag inclusions portions etc. Profile of continuous casting defects - are defects in which a cross or longitudinal section of the workpiece is deformed relative to a given geometric configuration. Such defects may be associated with increased speed or casting temperature, with insufficient increased or inhomogeneous cooling of the ingot. The main cause for the development of misconfiguration workpiece is uneven buildup strand shell in the mold. Crust thickness variation at the output of the ingot mold with further intensive cooling of the ingot will promote the development of high rombichnosti workpiece. Nonuniform crust formation ingot mold may be associated with inaccurate centering of the jet of metal from the tundish, wear or deformation of the profile sleeves, uneven cooling of the ingot because of the variable gap between the sidewall and the sleeve mold. High rombichnost continuous casting can cause problems when forming the profile of a roller rolling stands. In addition, high rombichnost workpiece may cause the appearance of other types of defects (eg, surface and internal longitudinal cracks). Transverse fracture surface located at the corners or edges of continuous casting in the transverse direction, i.e., perpendicular to the casting metal. Transverse fractures are placed in the middle of faces and corners of the workpiece. The main causes of transverse cracks are tapered or excessive deformation of the working surface of the mold, poor lubrication or SCO mold, deviation in the alignment of the mold relative to the axis of the stream processing, the deviations of the reciprocating motion of the working path of the mold, etc. All this causes an increase in frictional forces between the working surface and the ingot mold. One of the main factors determining the possibility of formation of transverse cracks in the surface of the workpiece, is to set up the swing of the mold. Deviations in the tilting mechanism (backlash, heartbeat) may lead to changes in the parameters that influence the formation of a hard crust, cause it hang in the mold or discontinuity of the shell. In the formation of transverse cracks in the mold may form spatter, which significantly affects the quality of the surface of the continuous casting and requires her stripping. Another cause of the formation of transverse cracks may appear in the operation of straightening the workpiece if it is subcooled below the hot plastic deformation. Zalivin result from contact of molten metal into the nip between the crust and the wall of the ingot mold, due to curvature of the meniscus formed in the region of their contact. Education Zalivin most likely when interruptions in the metal mold, insufficient or uneven lubrication of the mold, considerable fluctuations in the metal level therein, with the lower metal casting speed and temperature and the like Zalivin are also formed in the case of breaking the shell of the ingot. In general, to prevent defects such as belts, inversions crust ingot Zalivin necessary to prevent delays metering stations tundish, to minimize the amplitude of the oscillations of the metal level in the mold, to optimize the operating conditions grease its surface, to ensure the stability of the casting speed, etc.

Effect of casting speed on the thickness of the crust and the quality of the workpiece

Rate of formation of a crust of solid metal in the mold, as shown by the study, is well described by the equation: X = K * ? The average rate of solidification round billets with a diameter of 170-190 mm ??is equal for low-carbon steels 25-26 mm / min 0.5, for high-chromium 22-24 mm / 0-5 min for high-chromium-nickel steels and hromonikelmargantsovistyh 18-20 mm / min 0 , 5, finally, for the heat resistant steels based on nickel 11-13 mm / min 0.5. To ensure normal conditions of casting thickness of the crust at the exit of the ingot mold should be at least 2.5 mm. When operating the crystallizer length of 100 cm, the casting speed for billets of different low carbon steel sections varies within the following ranges

References

  1. Затуловский С.С., Демченко В.Ф., Юдович А.А. Непрерывное литье стали. №7. М.: Металлургия. 1981.
  2. Барбаев В.И. Оптимизация технологии ковки крупных слитков с целью снижения энергозатрат./ В.И.Барбаев, Е.П. Большина //Сб. научных трудов по материалам межд. научно-практ. конф. «Современные проблемы и пути их решения в науке, транспорте, производстве и образовании' 2008», т. 4, Технические науки, Одесса,2008. – с.65-71.
  3. Большина Е.П. Изучение оптимизационной модели процесса разливки непрерывного слитка./ Е.П. Большина, В.И Барбаев //сб. Казанская наука, №9, вып.1.2010г. – с.97-102.
  4. Источник: http://emchezgia.ru/razlivka/21_skorost'_kristellizatsii.php