Classification method of continuous casting machine
At present, continuous casting machines have been widely used in steel mills, with various forms and different uses. The name of the continuous casting machine is also very inconsistent. Now introduce the classification method of continuous casting machine according to general habits:
(1) The appearance classification of continuous casting machine includes: vertical continuous casting machine, vertical bending continuous casting machine, arc continuous casting machine, ultra-low head (elliptical) continuous casting machine, horizontal continuous casting machine, wheel continuous casting machine Wait.
(2) According to the cast slab section classification:
-Billet continuous casting machine: The section is less than or equal to 150×150mm called billet, and larger than 150×150mm is called bloom. In billet continuous casting, 120×120mm is used as a dividing line. Above this value, immersion nozzle and mold slag are used for casting, and below this value, open casting or gas-protected casting is used. If the ratio of the long side to the wide side of the rectangular section is less than 3, it is also called a billet continuous caster.
-Slab continuous caster: the section of the cast slab is rectangular. The width-to-thickness ratio is generally above 3.
-Round billet continuous casting machine: the section of the cast billet is circular, with a diameter of Φ60～Φ400mm.
-Special-shaped billet continuous casting machine: Pouring special-shaped sections such as I-beams.
-Continuous casting machine for both square and slab: On one casting machine, both slab and square billet can be poured.
(3) According to the drawing speed classification, there are: high drawing speed continuous casting machine and low drawing speed continuous casting machine. The main difference between them is that the cast slab is straightened with liquid core at high drawing speed, and the cast slab is fully solidified and straightened at low drawing speed.
(4) According to the classification of the hydrostatic head of the steel: the high-head type continuous casting machine with higher static pressure is called the vertical type continuous casting machine and the vertical bending type continuous casting machine. The lower static pressure is called the low-end continuous casting machine, such as the arc, ellipse, and horizontal continuous casting machine.
The number of continuous casting machines, the number of machines and the number of streams
In continuous casting production, a set of continuous casting equipment that shares the same ladle for pouring first-rate or multiple-strand casting billets at the same time is called a continuous casting machine. A continuous casting machine can be a multi-unit or a single unit. The so-called unit is a set of continuous casting equipment that has an independent transmission system and working system in a continuous casting machine, which can still work as usual when the other machine has an accident is called a unit. For each continuous casting machine, the total number of casting slabs that can be cast at the same time is called the number of continuous casting machine streams. Where a continuous casting machine has only one unit, and only one casting billet can be poured, it is called a first-class machine. If more than two slabs can be poured at the same time, it is called one machine with multiple streams. Where a continuous casting machine has multiple units and casts multiple slabs separately, it is called multiple machines and multiple streams.
Compared with multi-machines and multi-streams, the equipment is lighter and less investment. However, if there is an outflow accident for one machine with multiple flows, the production of the gold machine may be stopped, and the coordination of production operations and flows may be difficult. In recent years, the highest billet has been poured into 8 streams, and most of them use 2 to 4 streams. The slab is poured at most 4 streams, most of which use l~2 streams.
The composition of continuous casting equipment and its layout in the workshop
Continuous steel casting equipment is mainly composed of ladle (steel drum) turntable, ladle, tundish (tank), crystallizer (primary cooling), crystallizer vibration mechanism, secondary cooling device, billet drawing (straightening) device, cutting It consists of 9 parts including the device and the casting billet delivery device.
At present, the continuous steel casting workshop mainly has two types of layout: horizontal and vertical: the center line of the continuous caster is perpendicular to the column line of the factory building is horizontally arranged, and the center line of the continuous caster and the column line of the factory building are parallel to the vertical arrangement. The former is suitable for workshops with large production scale and multiple continuous casting machines, while the latter is mostly used when the number of continuous casting machines is small or the old factory is remodeled.
Continuous steel casting operation process
The molten steel preliminarily made from the steelmaking furnace (converter, electric furnace, open hearth) is injected into the ladle and deoxidized and alloyed at the same time. Then it is transported to the ladle refining station, where the temperature and composition of the molten steel are adjusted, and sent to the continuous casting platform. Since it is difficult to control the flow of molten steel with ladle pouring directly, a tundish is used to temporarily store molten steel before pouring into the mold. The molten steel is rapidly cooled in the water-cooled mold to form a cast slab with a solidified billet shell and unsolidified molten steel inside. The solidified billet shell at the outlet of the lower end of the mold should have enough thickness to ensure that the internal molten steel does not flow out (the flow of molten steel is called pull-out). As the draw roll slowly pulls the slab with liquid core out of the mold, the molten steel in the tundish is also continuously injected into the mold at the same time. In this way, a very long slab with a liquid core can be obtained. The cast slab with liquid core is forced to be cooled by spraying water in the secondary cooling zone. When it is pulled to the specified position, the inside of the cast slab is completely solidified. Then the cast billet is cut into a specified size and sent to the subsequent process by the billet discharging device.
Types of continuous steel casting machine
At present, there are 5 types of continuous casting machines used in various countries in the world: vertical, vertical bending, arc, oval and horizontal.
The pouring operation, crystal solidification, secondary cooling, cutting and other processes of the vertical continuous casting machine are all carried out sequentially on the vertical line. The vertical bending type casting machine is first vertical. After the cast slab is solidified, it is bent 90° into a horizontal shape, and then cut and shipped out. The equipment height is lower than that of the vertical type. The arc continuous caster pours molten steel into the arc mold, and then runs along the arc track, passes through a quarter arc, and then discharges the billet in the horizontal direction. The equipment height is further lower than that of the vertical bending type. Further improvement on the basis of the arc continuous casting machine, the oval continuous casting machine appeared. The horizontal continuous casting machine is currently in the development stage.
According to incomplete statistics, among the continuous casting machines currently built in the world, the vertical type accounts for 17%, the vertical bending type accounts for 21%, the arc type accounts for 55%, and other types account for 7%. At present, the newly built continuous casting machine has the most arc shape.
Features of vertical continuous caster
The main equipment of the vertical continuous casting machine from the tundish to the cutting station are all arranged in a vertical line. The entire fuselage stands on the ground of the workshop or is arranged in a deep pit underground. This type of continuous caster has a small footprint and compact equipment; high-temperature cast slabs have no bending deformation, and there are few cracks on the surface and internal of the cast slabs; inclusions in the unsolidified liquid in the cast slabs are easy to float, and the molten steel is relatively "clean"; secondary cooling device Simple structure and easy maintenance. The shortcomings of the vertical continuous caster are: the machine body is as high as 20-40m, the workshop must be heightened or the pit must be deepened, and the infrastructure cost is expensive; because the height of the caster cannot be increased too high, it can only be poured at a low speed and the productivity is low; The high static pressure easily causes the solidified billet shell to swell (called bulge), which leads to the deterioration of the quality of the cast billet.
Features of vertical bending continuous caster
Compared with the vertical type continuous casting machine, the height of the machine body is lowered, which saves investment; it is easier to discharge the billet in the horizontal direction, and it is easier to lengthen the machine body, which can realize high-speed pouring; the inclusions in the unsolidified molten steel in the cast slab are easy to float up. The inclusions are evenly distributed. The disadvantage is that because the cast slab has to be bent once and straightened once, it is easy to produce internal cracks; although the height of the caster is reduced, the infrastructure cost is still high.
There are two types of vertical-bending continuous casting machines: one is that there is a vertical section under the mold, and the top bend enters the arc section when the billet is fully solidified or nearly solidified, and then straightened. It is mainly suitable for billets. The other is a straight mold with a vertical section (2 to 3 meters), with a liquid core that is bent at multiple points or gradually bent into the arc section and then straightened. The advantages of this continuous casting machine: 1) There is a vertical section to fully float the inclusions in the liquid cavity; 2) The casting billet has multiple points or gradually bends, which avoids the local concentration of bending deformation stress and effectively reduces the formation of cracks; 3 ) Due to the bending with liquid core, the drawing speed can be increased and the productivity of the casting machine can be improved. At present, in order to solve the quality problem of the accumulation of arc inclusions in the cast slab of the arc continuous casting machine, the construction of vertical bending continuous casting machines has an increasing trend.
Features of arc continuous caster
The molds of the vertical and vertical bending continuous casting machines are straight, while the arc continuous casting machine uses an arc mold with a certain radius of curvature. The crystallizer, secondary cooling device, and tension-correcting device are all arranged on a quarter of an arc of a circle with a certain radius. The casting slab is bent when it solidifies in the mold. The casting slab with liquid core is pulled out of the mold and runs along the arc-shaped track, and continues to spray water for cooling. The solidification is completed at the quarter arc, and then straightened. And pull it out and send it to the cutting station.
The height of the arc continuous caster is only one third of that of the vertical type. The construction cost is low, the hydrostatic pressure of the steel is small, the billet bulge between the rolls is small, and the billet quality is good; it is also easier to extend the body, so it can be High-speed pouring, high productivity. The shortcomings of the arc continuous caster are: due to the bending and straightening of the cast slab, it is easy to cause internal cracks; the distribution of inclusions in the cast slab is uneven, and there is a collection of inclusions on the inner arc side; the equipment is more complicated and the maintenance is more difficult.
Although the arc continuous caster has shortcomings, it is still the most widely used model in steel mills in the world due to technological advances in equipment and technology.
Features of ultra-low head continuous caster (elliptical continuous caster)
The so-called ultra-low head continuous casting machine means that the height of the continuous casting machine is lower than that of the general continuous casting machine, and its liquid core pressure head is naturally very low, so it is called the ultra-low head continuous casting machine. At present, there is no unified definition standard in various countries. Comcast uses the ratio of the caster height h to the slab thickness H as an index to define:
h/H>50 high pressure head continuous caster h/H=40～50 normal pressure head continuous caster h/H=25～40 low pressure head continuous caster h/H<25 ultra low head continuous caster
The advantages of the ultra-low-head continuous casting machine are:
(1) Due to the low hydraulic head of the steel, the small hydrostatic pressure of the steel and the small bulge deformation, after calculation, when the radius of the continuous caster decreases from R=10.5m to R=3m, the bulge deforms in the two-phase zone of the slab The strain caused is only less than 10% of the original, which improves the quality of the cast billet. Due to the small bulge deformation, the structure of the nip roller in the secondary cooling zone is simplified;
(2) The main equipment of the continuous casting machine is placed in a horizontal position, which is convenient for maintenance;
(3) The equipment is light in weight and reduces the cost.
However, due to the low pressure head, it is not conducive to the floating of inclusions, so measures should be taken to purify the molten steel and protect the pouring.
This type of model is suitable for the transformation of old factories. my country's Tiangang, Handan Iron and Steel, Maanshan Iron and Steel and other enterprises have built this kind of ultra-low head continuous casting machine, and achieved good production results.
Features of horizontal continuous caster
The tundish, crystallizer, secondary cooling device and billet drawing device of the horizontal continuous casting machine are all placed on the ground plane in a straight line and horizontal arrangement. The molten steel is poured into the tundish, and the tundish is connected with the inlet end of the mold. The molten steel in the tundish enters the water-cooled mold through the separating ring at the junction of the mold and the middle and the ladle, and solidifies into a certain shape of the shell with a liquid core. The cast billet is pulled out from the mold, solidified after secondary cooling, and sent to the cutting station to be cut to length.
The horizontal continuous casting machine has low equipment height, which can be installed in the original workshop, and the capital construction cost is greatly reduced; the equipment is simple, light in weight, and convenient to maintain; the tundish and the mold are sealed and connected to prevent secondary oxidation, and the purity of the steel is high; Convenient operation, no mold level control is required, mold does not need mold slag, nozzle is not easy to be blocked, etc.; the casting billet does not need to be straightened, and alloy steel with strong crack sensitivity can be poured.
The above-mentioned advantages of the horizontal continuous casting machine make it particularly suitable for electric furnace steelmaking plants with many alloy steel varieties and small batches.
The development level of continuous casting has three major technical keys: the first is the material, life and cost of the separating ring; the second is the coating and lubrication of the mold; the third is the billet drawing mechanism that satisfies the drawing-stop-reverse-pushing method. In recent years, breakthroughs have been made in these three key technologies. At present, there are horizontal continuous casting machines used in industrial production in Germany, Japan, and the United States. my country's Maanshan Iron & Steel and other units have carried out the development test of horizontal continuous casting machine. The Hengyang Steel Pipe Plant in my country used a horizontal continuous casting machine to achieve full continuous casting. In the next few years, the horizontal continuous casting technology of small and medium steel plants in my country will have a greater development.
Centrifugal Rotary Continuous Casting Machine
Research on this model began in the 1950s. In 1969, the first centrifugal rotary vertical continuous caster was put into production in France. This type of casting machine can only cast round billets, also known as rotating round billet continuous casters. The characteristics of this model are: (1) The mold rotates at a speed of 30-150r/min during the pouring process, the molten steel produces centrifugal force, and the inclusions produce centripetal force, which promotes the inclusions to float up and remove and improve the cleanliness of the steel. (2) Due to the rotation, the molten steel washes away the dendrites at the solidification front, enlarges the equiaxed crystal zone in the center of the slab, and reduces segregation and porosity. (3) The round billet has accurate geometric dimensions and good surface quality. Therefore, the round billet produced by the rotary continuous casting machine can be directly sent to the tube for rolling without surface cleaning.
However, the disadvantages of this model are: (1) The structure of the casting machine is complex, and the equipment and maintenance costs are high. (2) The height of the casting machine is large, the workshop is required to be high, and the investment is large. (3) Only round billets can be poured, and the production flexibility is small. (4) Since it is a vertical type, increasing the speed is limited, so this type of machine has not been greatly developed.
Wheel belt continuous casting machine
The main body of the wheeled continuous caster is composed of wheels with crystallization grooves and a ring-shaped steel belt. The crystallization tank is made of heat-resistant copper alloy, and the trapezoidal crystallization tank with El is sealed with steel tape to form a crystal cavity. The molten steel starts to solidify after being injected into the crystallization cavity from the tundish, and moves synchronously with the rotating crystallization wheel until near the lower tangent point of the wheel. Cut into a cut-to-length finished blank.
The wheels can be placed vertically or horizontally. Its characteristics are: 1) High speed, drawing speed up to 5-10m/min, easy to realize continuous casting and rolling together with rolling mill. 2) The height of the casting machine is low, the equipment is simplified, and the investment is low. 3) There is no mold vibration, no vibration marks on the surface of the casting slab, and the surface quality is good. Wheel casters are widely used for casting non-ferrous metals, and are still being tested for casting billets.
Features of thin slab caster
The continuous casting machine that can cast slabs with a thickness of 20-50mm and a width of 900-1600mm is called a thin slab continuous caster.
The continuous caster built by Nucor Corporation of the United States has a billet thickness of 50mm, a billet width of 1295-1560mm, and an annual output of 1 million tons/year. The model is vertical-bend type, the mold is straight. In order to insert the nozzle, the upper mouth is made into a funnel shape with an opening of 170mm. Its longitudinal section is composed of two broken lines. The mold is long and the length is 1100mm. The narrow side distance can be adjusted to adapt to thin slabs of different widths.
There is a 700mm long cooling grid under the mold to support the weak shell that has just been pulled out. Three sets of nip rollers are installed under the grid. The first set is a segmented roller with a relatively small roll diameter and roll distance. The total length of the three sets of nip rollers is 4600mm.
The cast slab completely cooled in the secondary cooling zone is topped by a hydraulic top bending roll to form an arc of R=3m, and then the cast slab is pulled out by a four-roller tensioning and leveling machine, and finally cut into a cut-to-length finished billet with mechanical shears.
The mold vibration adopts a four-eccentric vibration mechanism, which vibrates according to a sinusoid, with an amplitude of ±3mm, and the vibration frequency changes with the drawing speed, that is, ƒ=60Vmin-1, V is the drawing speed, and the unit is m·min-1. The highest frequency is 400min-1. For example, when the pulling speed V=4m/min, ƒ=240min-1.
The advantages of thin slab casters are simplified production process, light equipment, reduced capital investment, energy saving and lower production costs.
How many types of steel and slab cross-sections are poured by the continuous caster
According to statistics, there are currently more than 130 steel types that can be poured by continuous casters, and there are more than 500 different steel grades. Including carbon steel, low alloy steel, stainless steel, silicon steel, high speed steel, bearing steel and high temperature alloys. About 63% of them are carbon steel and 37% are alloy steel and stainless steel.
The sections that can be cast in continuous casting are as follows: billet: the section is less than 150×150mm, and the smallest section is 50×50mm; bloom: the section is greater than 150×150mm, and the largest billet is 450×450mm; slab (Rectangular billet): The minimum section is 50×108mm, and the largest is 300～250×2500～3000mm; Round billet: diameter from Φ50mm to Φ450mm; Special-shaped billet: such as I-shaped billet, hollow round billet, etc.
Principles to be considered for matching between continuous caster and steelmaking furnace
The continuous casting machine can be produced in conjunction with open hearth, electric furnace, and converter. The following points must be paid attention to:
(1) Multi-furnace continuous pouring: Due to the long open-hearth smelting time, continuous casters generally can only cast in a single furnace. In conjunction with converters and electric furnaces, in order to give full play to the production capacity of the continuous caster, increase the yield of molten steel and reduce costs, it is necessary to realize continuous casting of multiple furnaces.
(2) The pouring time of the continuous caster and the tapping cycle of the converter should be coordinated well.
(3) The production capacity of the continuous caster should be appropriately greater than the production capacity of the steelmaking furnace to meet the production potential of the smelting equipment. Generally, there should be a surplus of 10-20%.
(4) Choose appropriate refining equipment outside the furnace that meets product quality requirements to ensure the quality of continuous casting molten steel. Principles to be considered for the cooperation of continuous caster and rolling mill
The following points should be considered for the cooperation of continuous casting machine and rolling mill:
(1) Steel grade: At present, there are more than 100 steel grades for continuous casting. The next step of processing should be selected according to product quality.
(2) Cast slab section: The size and shape of the cast slab section are mainly determined by the capacity of the rolling mill and the quality requirements of the rolling mill products. The production section of the continuous caster should be compatible with the section of the billet required by the rolling mill. The billet continuous caster mainly supplies the 650, 500 or 400 rolling mills to produce profiles and wire rods. It is generally an instant success. The typical billet supply section is: 650 rolling mill, 140-220mm square billet, cast slab fixed-length 2.8-3.2m; 500/350 rolling mill, 120-140mm square billet, fixed-length 1.4-1.6m; 400/300 rolling mill, 90～ 120mm square billet, cut-to-length 1.4～1.6m.
The cast billet section is larger than 160mm square billet, which is mainly used to produce section steel such as rails, I-beams, channel steel, etc. Continuous casting slabs are mainly used for thick plate rolling mills or wide strip rolling mills to produce medium and thick plates or hot-rolled coils.
(3) Compression ratio: Generally speaking, a small section of the supplied billet can reduce the number of heating and rolling passes, improve the productivity of the rolling mill and save energy. However, the cross-section should meet a certain compression ratio to obtain qualified product quality as a prerequisite. The so-called compression ratio refers to the ratio of the cross-sectional area of the cast slab to the cross-sectional area of the rolled material. In order to ensure that the internal structure of the rolled material is dense and has good mechanical properties, different products require different compression ratios. When the compression ratio is 3, the mechanical properties required by general products can be achieved; when the compression ratio is 4, the product obtains good physical properties and can homogenize the center structure; when the compression ratio is 5-8, the columnar crystal structure of the rolled material can be destroyed , To homogenize the structure of the steel and obtain good mechanical and physical properties.
(4) Shape tolerance: For billets, the shape is required to be regular, the deviation of the two diagonals of the section is less than 2.5-3%, and the deviation of the section corner is 90 ± 2°. If the cast billet is severely diamond-shaped, it may cause steel stacking when pushing the billet in the heating furnace, or the billet may have difficulty biting into the pass during rolling.
For slabs, severe bulging should be prevented. The maximum warpage of the billet is 10mm/m.
Principles to be considered when choosing a continuous casting machine
When designing a steel plant, the most important thing is the choice of continuous caster. A suitable model should be determined according to the product plan, product quality and reduced construction investment. Choosing the right model will create conditions for future production management and high-yield and high-quality continuous casting machines.
The basic principles for choosing a continuous casting machine are:
(1) The production capacity of the continuous casting machine should match the production capacity of the steelmaking furnace. In order to give full play to the production potential of steelmaking, the production capacity of the continuous caster should be 10-20% rich.
(2) Meet the requirements of steel grade and rolling section specifications. According to the statistical data of production practice, the current vertical, vertical bending, and arc continuous casting machines can meet the requirements of pouring carbon steel, low alloy steel, alloy steel and different specifications of sections, but the arc continuous casting machine The most widely used (approximately more than 60% of the construction of continuous casting machines).
(3) Meet product quality requirements. The quality of the slab mainly includes two aspects, one is the internal cracks and segregation of the slab, and the other is the purity of the slab. The former is determined by the static pressure of the molten steel to cause the billet to bulge and bend and straighten. In the order of vertical→vertical bending→arc→ellipse→horizontal, the height of the casting machine is reduced, and the tendency of cracks generated is also reduced. The latter is related to the floating of inclusions in the liquid cavity of the crystallizer. In the order of vertical→horizontal, as the height of the caster decreases, the accumulation of inclusions in the cast slab gradually increases.
After comprehensively evaluating the above two contradictory influencing factors, the arc continuous casting machine is still a relatively ideal model, coupled with the use of external refining, non-oxidizing pouring and other measures, it can fully meet the requirements of product purity. In order to completely solve the shortcomings of the arc inclusions in the cast slab of the arc continuous casting machine, the construction of the vertical bending type continuous casting machine with a straight mold with a vertical section (2 ~ 3m) has a new development trend.
(4) Reduce construction investment and equipment manufacturing costs.
Regarding the choice of models, there is no uniform standard. It is necessary to combine the factory product plan and comprehensively consider the aspects of productivity, product quality, casting machine height, and investment. The construction of the continuous casting machine should be regarded as a systematic project. At the upstream of the continuous caster, it is necessary to consider that steelmaking should have a guarantee system (such as out-of-furnace refining) that provides qualified quality molten steel for continuous casting. At the downstream of the continuous caster, the possibility of hot delivery, hot charging, and direct rolling of the cast slab must be considered. And the quality assurance system for rolled products. In the continuous casting machine itself, the equipment, technology and control system for producing defect-free billets should be considered.
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