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Apr 2, 2021
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What are the reasons for the porosity and shrinkage of continuous casting slabs

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1.Introduction

A steel-making plant of a steel (group) company built a 1800mm slab continuous caster. The designed billet specifications are 200mm, 220mm, 250mm in thickness and 1400mm~1800mm in width. During the trial operation, Q235 and Q345 steel slabs with a cross-section of 250mm×1550mm were produced. The macrostructure inspection results showed that the center segregation and center porosity were serious. We excerpted the low power inspection reports of 13 furnace batch numbers and carried out comparative inspections, and found that the center segregation is generally B2.5 and the center porosity is about 2. After the cast slab is rolled into a product, when the two cross-sections are perpendicular to each other in the welding test, local delamination appears on the oxygen cutting or notch. According to the actual production situation of the plant, in order to reduce the center segregation and center porosity as much as possible and produce high-quality cast slabs, the reasons for the formation of center segregation and center porosity defects were analyzed, and specific preventive measures were put forward.

  1. Causes of center segregation and center porosity

Central segregation means that during the solidification process of molten steel, when the solute elements are distributed in the solid and liquid phases, it is manifested as the uneven distribution of elements in the slab. The content of C, S, P and other elements in the center of the slab is significantly higher than other elements. Location. The solidification end area in the center of the slab thickness often shows “V” segregation. Porosity in the center refers to the formation of small voids between the dendrites in the center of the thickness of the cast slab at the end of solidification of molten steel. There are many reasons for center segregation and center porosity, and these two kinds of defects are often associated with each other.

2.1. The columnar crystals in the solidification structure of the slab are too developed

One of the formation mechanisms of center segregation and center porosity is the “solidified crystal bridge” theory, that is, during the solidification of the slab, the instability of the heat transfer of the slab causes the growth rate of columnar crystals to be different, and the preferential growth of columnar crystals is in the center of the slab When they meet, they form a “bypass”. The molten steel in the liquid cavity is separated by a “solidified crystal bridge”. The molten steel at the lower part of the crystal bridge cannot be supplemented by the upper molten steel during solidification and shrinkage, forming loose or shrinking cavities, accompanied by central segregation. When the columnar crystals in the solidified structure are too developed, the “solidified crystal bridges” are more likely to be formed, and the center segregation and center porosity are more likely to occur in the cast slab.

2.2. The content of solute elements that are easy to segregate in molten steel is too high

The second of the formation mechanism of center segregation and center porosity is the precipitation and enrichment theory of solute elements that are easy to segregate in molten steel, that is, the solute elements in molten steel have a dissolution equilibrium on the solid-liquid phase boundary during the crystallization process of the cast slab from the case to the center. Moving, the easily segregated elements such as C, S, and P are precipitated as columnar grains and discharged into the unsolidified molten metal. As the crystallization continues, these easily segregated elements are enriched in the center of the slab or the solidification end area, thereby Produce center segregation and center looseness.

2.3. Bulging of the blank shell

The third mechanism of the formation of center segregation is the theory of cavity suction, that is, if the billet shell bulges during the solidification process of the cast slab, cavities will be generated in the center of the cast slab. These cavities have a negative pressure suction effect and enrich The molten steel of the solute elements is sucked into the center of the slab to cause center segregation; at the end of solidification, the volume shrinks from ten liquids to solids, resulting in a certain cavity, and the molten steel enriched with solute elements at the solidification end is sucked into the center of the slab. Lead to center segregation. Therefore, the larger the billet bulge, the more serious the center segregation will be.

  1. Countermeasures against center segregation and center looseness

Based on the analysis of the causes of center segregation and center porosity, if measures can be taken to promote the equiaxed crystallization of the solidification structure of the center of the cast slab, reduce the content of easily segregated elements in the molten steel, and control the bulge of the cast slab, the generation of center segregation and center porosity can be slowed down .

3.1. Improve the purity of molten steel

The carbon content in steel is closely related to the solidification structure, which affects the growth ratio of columnar crystals and equiaxed crystals, which will inevitably play a decisive role in the generation of center segregation and center porosity of the cast slab. Studies have shown [1] that under the same other conditions, a steel with a carbon content of 0.3%, 0.1%, and 0.6% was cast, and it was found that the length of the columnar crystals, the width of the center segregation, and the center cavities were The carbon content increases in the order of 0.3%~0.1% and 0.6%. Therefore, it is necessary to increase the carbon hit rate in converter production and accurately control the carbon content in molten steel. S, P, etc. in molten steel are easily segregated elements, and their content and distribution in molten steel affect the center segregation and center porosity of the cast slab. By smelting clean steel, such as hot metal pretreatment or ladle desulfurization, the content of easily segregated elements such as S and P in molten steel is reduced, and the purity of molten steel is improved, which can effectively prevent the occurrence of center segregation and center looseness.

3.2. Control the bulge of the billet

Controlling the billet bulge can effectively slow down the occurrence of center segregation. The size of the billet bulge is mainly related to the roller spacing in the secondary cold zone, the thickness of the billet shell, and the hydrostatic pressure of the steel. The smaller the roller spacing, the thicker the billet shell, the smaller the hydrostatic pressure of the steel, and the smaller the bulge. Therefore, when designing the continuous casting machine, as far as possible, the design should adopt small roll diameter and close-packed roll arrangement to reduce the roll spacing; use rigid multi-section rolls to prevent deformation of the backup roll; the continuous caster should not be too high in order to reduce the liquid cavity Height, reduce the static pressure of the molten steel; in the production, the nip roller in the second cold zone needs to be strictly aligned to the arc.

3.3. Control the pouring temperature and drawing speed

Pouring temperature is an important factor affecting the growth of columnar crystals. The pouring temperature is high and the columnar crystals of the cast slab are developed: the pouring temperature is low, and the equiaxed crystals of the cast slab are developed. Therefore, in the case of not causing the nozzle to freeze, it should be possible to use low superheat pouring. In the production operation, based on the experience of each factory, the corresponding steel ladle and intermediate tank target superheat benchmarks can be formulated for different steel clocks. The experience of a domestic factory is: When producing low-carbon steel ([C]≤0.08%), it is best to control the superheat of the molten steel in the molten steel tank and the tundish within 600℃ and 300℃ respectively; produce peritectic steel and For medium carbon steel (0.08%≤[C]≤0.30%), the target superheat of molten steel in the molten steel tank and the tundish should be controlled within 550℃ and 250℃ respectively. The drawing speed is also an important factor affecting the growth of columnar crystals. The drawing speed is high, the slab stays in the mold for a short time, and the slab liquid core is prolonged. This not only delays the nucleation and growth of equiaxed crystals, enlarges the columnar crystal area, but also has a risk of bulging. Also increase. Therefore, under the premise of not affecting the output, the drawing speed should not be too high. In production practice, different operating modes (such as starting pouring, rapid replacement of intermediate tanks, rapid replacement of immersion nozzles, replacement of mold mold slag, and continuous casting of dissimilar steels in different operating modes (such as starting pouring, rapid replacement of intermediate tanks, rapid replacement of mold powder, and continuous casting of different steel types) Process, termination of pouring, etc.) to formulate corresponding control standards. The specific standards can be set by accumulating experience in production, or by referring to the data of steel mills with successful production experience.

3.4. Optimize secondary cooling technology

The secondary cooling technology has an important influence on the surface quality and internal quality of the cast slab, and the formation of defects such as center segregation and center porosity is closely related to it. Secondary cooling technology includes segmentation of the secondary cooling zone, selection and configuration of nozzles in the secondary cooling zone, and determination of water spray conditions (such as flow and pressure).

The sections of the secondary cooling zone should be arranged according to the continuous caster’s roll row, and divided along the drawing direction from top to bottom according to the principle of increasing the length of each cooling section. Generally, the slab continuous caster has 7-9 cooling sections.

The nozzle structure in the second cooling zone determines the flow density distribution, droplet velocity and droplet diameter of the cooling water. Compared with the pressure water nozzle, the air-water nozzle has the advantages of large spray water flow adjustment range, large cooling intensity, uniform cooling, and not easy to be blocked, 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 number of nozzles in the second cooling section should ensure that the actual working water volume change range of the nozzle is always within its normal adjustment range. .

The secondary cooling water volume, air volume and pressure must be determined according to the billet specifications, steel type, product quality requirements and production experience. The principle of the total distribution of the secondary cooling water is to gradually decrease from top to bottom along the 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 steels that are not sensitive to cracks, the upper part of the second cold zone is strongly cooled and the lower part is slowly cooled; for steels that are sensitive to cracks, the second cold zone is slowly cooled from top to bottom; the steel types that are more sensitive to internal cracks than surface cracks, two 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 caster, the corresponding secondary cooling water meter is generally designed according to the high-temperature mechanical properties of the steel grade in the product outline and its quality requirements and the specifications of the cast slab. The formulation of the second cold water meter should ensure sufficient cooling intensity and reasonable distribution of cooling water. If the strength of the secondary cooling is not enough, the surface temperature of the cast slab will be higher, the liquid core of the cast slab will be lengthened, the equiaxed crystal region will be enlarged, and the ability of the slab shell to resist bulging deformation caused by the hydrostatic pressure of the steel will be weakened, which will promote center segregation and center looseness Formation and expansion. A document [5] studied the secondary cooling system of a 1350mm slab caster in a steel plant, and found that from the exit of the secondary cooling zone IV to the exit of the V zone, the surface temperature of the cast slab rose greatly, which caused the slab shell to resist bulging. The ability to deform is reduced, and due to thermal expansion, the center of the slab has a suction effect, which intensifies 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 applied to the coil, it will produce a directional electromagnetic force that has a strong stirring effect on molten steel. The electromagnetic force pushes the unsolidified molten steel in the slab shell to circulate in a certain direction, destroys the coarse columnar crystals that have been formed in the solidified structure of the molten steel, and refines the crystal grains; hinders the further formation of columnar crystals and increases The equiaxed crystal ratio is improved; the uneven distribution of carbon components and sulfides in the center of the cast slab is improved, and the chances of collision and aggregation of the inclusions are increased, so that the size of the inclusions increases and is easy to float, so as to slow the center segregation. And the center is loose.

The electromagnetic stirring technology is used on the slab continuous casting machine. To make it fully exert the stirring effect and significantly reduce the center segregation and center looseness, it is necessary to accurately calculate the specific installation position of the electromagnetic stirring device and the electromagnetic thrust at the center of the slab. Studies have shown [4] that it is more appropriate for the electromagnetic stirring device to be installed in the range of 25%-40% unsolidified molten steel. At this time, the equiaxed crystal rate is high; the electromagnetic thrust is controlled in the range of 65mmFe~147mmFe, and the stirring effect is ideal.

3.6. Use light reduction technology

The soft reduction technology is to apply a uniform external force near the end of the continuous casting slab liquid core to make the casting slab a certain amount of compression to compensate for the solidification shrinkage of the casting slab [6]. The use of light reduction technology can eliminate or reduce the internal voids formed by the shrinkage of the casting slab, and prevent the molten steel enriched with solute elements between the crystals from flowing laterally to the center of the casting slab; at the same time, the squeezing effect produced by the light reduction can also promote the liquid core The molten steel enriched with solute elements flows in the opposite direction along the billet drawing direction, so that the solute elements are redistributed in the molten steel, so that the solidification structure of the cast slab is more uniform and dense, which can improve center segregation and reduce center porosity.

Continuous casting production is a continuous and dynamic process. Due to the continuous changes of molten steel temperature, slab thickness, steel grade, drawing speed and water spray conditions, the solidification position of the slab liquid core is also constantly changing. The static soft reduction technology requires that the solidification end position of the cast slab remains basically unchanged, and the soft reduction area needs to be set in advance, and the corresponding sector roll gap should be adjusted. In order to give full play to the best effect of soft reduction and pinpoint the freezing point, the application of soft reduction technology has evolved from static to dynamic. Dynamic soft reduction can dynamically control the position and amount of soft reduction according to the change of the drawing speed and the position of the solidification end of the cast slab. The dynamic soft reduction system developed by VAI consists of a core part [7, 8]: a SMART segment with a remote control device and 4 position adjustment hydraulic cylinders, which can be automatically positioned on the support frame, and the lifting of the driving roller is composed of a The transmission hydraulic cylinder is realized, and the clamping is completed by 4 position hydraulic cylinders equipped with a built-in position transmitter. The ASTC system for automatic adjustment of the taper of the SMART sector with computer remote control 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 pouring and tailing states; dynamic calculation model DYNACS system It can accurately control and determine the solidification point of the cast slab according to the actual water flow, the drawing speed, the steel grade and the degree of superheat. VAI’s dynamic soft reduction technology has been successfully used in Meishan Iron and Steel, Wuhan Iron and Steel, Rautaruukki and AvestaPolauit steel mills in Finland, ILVA steel mills in Italy, POSCO in South Korea, Voestalpinestahl Steel in Austria, and Bethlehem Steel in the United States. And the effect is good.

  1. Conclusion

There are many factors that affect the center segregation and center looseness of the cast slab. According to their formation reasons, the following preventive countermeasures can be taken in the 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 bulge of the casting billet with technologies such as small roll diameters and close rows of rolls and rigid multi-section rolls.

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 high.

4) Optimize the secondary cooling technology, select suitable nozzles, and 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 the molten steel unsolidified rate, and the electromagnetic thrust is controlled within the range of 65mmFe to 147mmFe.

6) Using mature dynamic soft reduction technology.

#Continuous casting

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