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What are the reasons for the looseness and shrinkage of the continuous casting billet
1 Introduction
A steelmaking plant of an iron and steel (group) company has built a new 1800mm slab continuous casting machine. The designed slab specifications are 200mm, 220mm, 250mm thick and 1400mm~1800mm wide. During the trial operation, Q235 and Q345 steel billets with a cross-section of 250mm × 1550mm were produced, and the low-magnification microstructure inspection results showed that the center segregation and center porosity were serious. We extracted the low magnification inspection reports of 13 batches of furnaces and conducted comparative inspections. We found that the central segregation was generally B2.5, and the central looseness was about 2. When the billet is rolled into a product, when two cross-sections are perpendicular to each other in a welding test, local delamination occurs on the oxygen cut or notch. According to the actual production situation of the plant, in order to reduce center segregation and center porosity as much as possible and produce high-quality slabs, the causes of center segregation and center porosity defects were analyzed, and specific preventive measures were put forward.
2. Causes of central segregation and central porosity
Central segregation refers to the fact that during the solidification process of molten steel, when the solute elements are distributed in the solid and liquid phases, the distribution of elements in the billet is uneven, and the content of C, S, P and other elements in the center of the billet is significantly higher than that of other elements. part. The end region of solidification in the center of the slab thickness often exhibits "V" segregation. Central porosity refers to the formation of tiny voids between dendrites in the center of the slab thickness at the end of solidification of molten steel. There are many reasons for central segregation and central porosity, and these two defects are often accompanied.
2.1. The columnar crystals in the solidification structure of the slab are too developed
One of the formation mechanisms of central segregation and central porosity is the "solidified crystal bridge" theory, that is, during the solidification process of the slab, the instability of the heat transfer of the slab leads to different growth rates of columnar crystals, and the columnar crystals that grow preferentially are in the center of the slab. When they meet, a "bridge" is formed, and the molten steel in the liquid phase cavity is separated by a "solidified crystal bridge". The molten steel at the lower part of the crystal bridge cannot be replenished by the upper molten steel when it solidifies and shrinks, resulting in looseness or shrinkage cavity, which is accompanied by central segregation. When the columnar crystals in the solidified structure are too developed, it is easier to form "solidified crystal bridges", and it is easier to produce central segregation and central looseness in the slab.
2.2. Too high content of segregated solute elements in molten steel
The second formation mechanism of central segregation and central porosity is the theory of precipitation and enrichment of easily segregated solute elements in molten steel, that is, in the process of crystallization of the slab from the shell to the center, the solute elements in the molten steel have a dissolution equilibrium at the solid-liquid phase boundary. Moving, C, S, P and other easy-segregation elements are precipitated in columnar grains and discharged into the molten metal that has not yet solidified. Produce center segregation and center loose.
2.3. The shell is bulging
The third formation mechanism of central segregation is the cavity suction theory, that is, if the billet bulges during the solidification process, cavities will be generated in the center of the billet. The molten steel of solute elements is sucked into the center of the billet, resulting in center segregation; at the end of solidification, the volume shrinks from the ten-liquid to solid transformation, resulting in a certain cavity, which also causes the molten steel enriched with solute elements at the end of solidification to be sucked into the center of the billet. lead to central segregation. Therefore, the larger the bulging amount of the billet, the more serious the center segregation will be.
3. Preventive measures for central segregation and central porosity
From the analysis of the causes of central segregation and central porosity, if measures can be taken to promote the equiaxed crystallization of the solidification structure of the center of the billet, reduce the content of segregation-prone elements in the molten steel, and control the bulging amount of the billet, the generation of central segregation and central porosity can be slowed down. .
3.1. Improve the purity of molten steel
The carbon content in the steel is closely related to the solidification structure, which affects the growth ratio of columnar crystals and equiaxed crystals, and must play a decisive role in the generation of central segregation and central porosity in the slab. Studies have shown that [1], when other conditions are the same, a steel with a carbon content of 0.3%, 0.1% and 0.6% is poured, and it is found that the length of columnar crystal, the width of central segregation and the central loose cavity are determined by The order of carbon content of 0.3%~0.1% and 0.6% increases in turn. Therefore, it is necessary to improve the carbon hit rate in converter production and accurately control the carbon content in molten steel. S, P, etc. in molten steel are easy-to-segregate elements, and their content and distribution in molten steel affect the center segregation and center porosity of the slab. By smelting clean steel, such as molten iron pretreatment or ladle desulfurization and other technologies, the content of easily segregated elements such as S and P in molten steel can be reduced, and the purity of molten steel can be improved, which can effectively prevent central segregation and central porosity.
3.2. Control the bulging amount of the billet
Controlling the bulging amount of the billet can effectively slow down the generation of center segregation. The size of the billet bulging is mainly related to the roller spacing in the second cooling zone, the thickness of the billet shell, and the static pressure of the molten steel. The smaller the roll spacing, the thicker the shell, the smaller the hydrostatic pressure of the molten steel, and the smaller the bulging amount. Therefore, when designing the continuous casting machine, the small roll diameter should be used as closely as possible to reduce the roll spacing; the rigid multi-section rolls should be used to prevent the deformation of the backup rolls; the continuous casting machine should not be too high, so as to reduce the liquid phase cavity height, reduce the static pressure of molten steel; in the production, the clamping rollers in the second cooling zone need to be strictly aligned with the arc.
3.3. Control the pouring temperature and the drawing speed
Pouring temperature is an important factor affecting the growth of columnar crystals. The pouring temperature is high, and the columnar crystal of the slab is developed: the pouring temperature is low, and the equiaxed crystal of the slab is developed. Therefore, in the case of not causing the nozzle to freeze, pouring with low superheat should be used as much as possible. In the production operation, according to the experience of each factory, the corresponding target superheat benchmarks for molten steel tanks and tundish tanks can be formulated for different steel bells. The experience of a domestic factory is: in the production of low carbon steel ([C]≤0.08%), the superheat degree of molten steel in the molten steel tank and the intermediate tank is controlled within 600℃ and 300℃ respectively; in the production of peritectic steel and medium carbon When steel (0.08%≤[C]≤0.30%), the target superheat degree of molten steel in the molten steel tank and the intermediate tank is controlled within 550℃ and 250℃ respectively.
The pulling speed is also an important factor affecting the growth of columnar crystals. The drawing speed is large, the residence time of the billet in the mold is short, and the liquid core of the billet is prolonged, which not only delays the nucleation and growth of the equiaxed crystal, but also enlarges the columnar crystal area, and also causes the risk factor of billet bulging. also increased. Therefore, under the premise of not affecting the output, the drawing speed should not be too large. In production practice, according to different steel grades, different operation modes (such as start of pouring, quick change of tundish, quick change of immersion nozzle, change of mold slag, and continuous casting of different steel grades are required from the end of drawing to the beginning of drawing). process, termination of pouring, etc.) to formulate corresponding control standards. The setting of specific standards can be gradually accumulated experience in production, or can be set with reference to the data of steel mills with successful production experience.
3.4. Optimizing secondary cooling technology
The secondary cooling technology has an important influence on the surface quality and internal quality of the slab, and the formation of defects such as center segregation and center porosity is closely related to it. The secondary cooling technology includes the second cooling zone section, the second cooling zone nozzle selection and configuration, the determination of water spray conditions (such as flow, pressure), etc.
The second cooling zone sections should be arranged according to the roll row of the continuous casting machine, and the length of each cooling section should be gradually increased from top to bottom along the drawing direction. Generally, the slab continuous casting machine has 7~9 cooling sections.
The nozzle structure of the second cooling zone determines the water flow density distribution, water droplet velocity and water droplet diameter of the cooling water. Compared with the pressure water nozzle, the air-water nozzle has the advantages of wide adjustment range of water spray flow, high cooling intensity, uniform cooling, and not easy to block, but it consumes more power during use. Various nozzles have a water volume adjustment range that can maintain their good atomization performance. Therefore, the selection and quantity of nozzles in the second cooling section should ensure that the actual working water volume change range of the nozzle is often within its normal adjustment range. .
The secondary cooling water volume, gas volume and pressure must be determined according to the specifications of the casting billet, steel grades, product quality requirements and production experience. The general distribution principle of the secondary cooling water is to gradually decrease from top to bottom along the billet drawing direction, and the total water distribution ratio between the inner arc and the outer arc of the casting machine is about 2:3. For steel grades that are not sensitive to cracks, the upper part of the secondary cooling zone is strongly cooled and the lower part is slowly cooled; for steel grades sensitive to cracks, the secondary cooling zone is cooled slowly from top to bottom; for steel grades that are more sensitive to internal cracks than surface cracks, the secondary The upper part of the cold zone is slowly cooled and the lower part is strongly cooled [2]. In the design process of the continuous casting machine, the corresponding secondary water meter is generally designed according to the high temperature mechanical properties and quality requirements of the steel grades in the product outline and the specifications of the cast slab. The formulation of the secondary cold water meter should ensure sufficient cooling intensity and reasonable distribution of cooling water. If the secondary cooling strength is not enough, the surface temperature of the billet will be high, the liquid core of the billet will be lengthened, the equiaxed crystal area will be enlarged, and the ability of the billet to resist the bulging deformation caused by the static pressure of the molten steel will be weakened, which will promote center segregation and center looseness. form and expand. There is a literature [5] when studying the secondary cooling system of a 1350mm slab continuous casting machine in a steel plant, it is found that from the exit of zone IV of the second cooling section to the exit of zone V, the surface temperature of the slab rises greatly, which causes the slab shell to resist bulging. The ability to deform is reduced, and the center of the billet has a suction effect due to thermal expansion, which aggravates the severity of center segregation.
3.5, using electromagnetic stirring technology
The electromagnetic stirring technology in continuous casting production is to install the coils arranged according to a certain rule in a certain part of the continuous casting machine. When a directional current is passed through the coil, a directional electromagnetic force that has a strong stirring effect on the molten steel will be generated. The electromagnetic force pushes the unsolidified molten steel in the billet shell to circulate in a certain direction, destroys the coarse columnar crystals formed in the solidified structure of the molten steel, and refines the grains; hinders the further formation of columnar crystals, increasing the It improves the equiaxed grain rate; improves the uneven distribution of carbon components and sulfides and other inclusions in the center of the slab, increases the chance of the inclusions colliding with each other and aggregates, and makes the inclusions increase in size and easily float to slow down the center segregation. and central porosity.
Electromagnetic stirring technology is used in slab continuous casting machine, in order to make full use of stirring effect and reduce center segregation and center looseness, it is necessary to accurately calculate the specific installation position of electromagnetic stirring device and the magnitude of electromagnetic thrust at the center of the slab. Some studies have shown [4] that it is more suitable to install the electromagnetic stirring device in the range of 25%~40% of molten steel unsolidified rate, and the equiaxed crystal rate is high at this time; the electromagnetic thrust is controlled in the range of 65mmFe~147mmFe, and the stirring effect is ideal.
3.6, using light pressing technology
The light reduction technology is to apply a uniform external force near the end of the liquid core of the continuous casting billet to generate a certain amount of compression to compensate for the solidification shrinkage of the billet [6]. The use of light reduction technology can eliminate or reduce the internal voids formed by the shrinkage of the casting billet, and prevent the molten steel enriched with solute elements from flowing laterally to the center of the billet. The molten steel enriched with solute elements flows in the reverse direction along the billet drawing direction, so that the solute elements are redistributed in the molten steel, so that the solidified structure of the billet is uniform and dense, which can improve the center segregation and reduce the center porosity.
Continuous casting production is a continuous and dynamic process. Due to the constant changes of molten steel temperature, slab thickness, steel grade, drawing speed and water spray conditions, the solidification position of the liquid core of the slab also changes constantly. The static light reduction technology requires that the position of the solidification end point of the billet remains basically unchanged. In order to exert the good effect of light pressing and find the freezing point, the application of light pressing technology has developed from static to dynamic. The dynamic light reduction can dynamically control the light reduction position and reduction amount according to the change of the billet drawing speed and the position change of the solidification end point of the casting billet. The dynamic light reduction system developed by VAI consists of a core part [7, 8]: a SMART sector with remote control, 4 position-adjusting hydraulic cylinders, which can be automatically positioned on the support frame, and the lifting and lowering of the drive roller by a The transmission hydraulic cylinder is realized, and the clamping is done by a 4-position hydraulic cylinder equipped with a built-in position transmitter. The automatic taper adjustment ASTC system for computer remote control of the SMART sector can automatically select the target roll gap according to different steel grades, and automatically adjust the roll gap setting value of each sector in the state of pouring and tail billeting; dynamic calculation model DYNACS system , can accurately control and determine the solidification point of the casting billet according to the actual water flow, billet drawing speed, steel grade and superheat degree. VAI's dynamic light reduction technology has been successfully put into use in Meishan Iron and Steel, Wuhan Iron and Steel, Rautaruukki and AvestaPolauit Steel Plants in Finland, ILVA Steel Plant in Italy, POSCO in South Korea, Voestalpinestahl Steel in Austria and Bethlehem Steel in the United States. And it works well.
4. Conclusion
There are many factors affecting the center segregation and center porosity of the billet. According to their formation reasons, the following preventive measures can be taken in design and production:
1) Improve the purity of molten steel, control the carbon content in molten steel, and reduce the content of easily segregated elements such as S and P.
2) The continuous casting machine is designed to control the bulging amount of the casting billet by the technology of small roll diameter and densely arranged rolls, rigid multi-section rolls and other technologies.
3) Control the pouring temperature and drawing speed during production, the pouring temperature should not be too high, and the drawing speed should not be too large.
4) Optimize the secondary cooling technology and select the appropriate nozzles to ensure sufficient cooling intensity and reasonable distribution of cooling water.
5) Using electromagnetic stirring technology, the electromagnetic stirring device is installed at a suitable position within the range of 25%~40% of molten steel unsolidified rate, and the electromagnetic thrust is controlled within the range of 65mmFe~147mmFe.
6) Adopt mature dynamic light pressing technology.
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