Discussion and optimization of roll gap shrinkage control technology in slab continuous caster

Based on the formation mechanism and influencing factors of slab center segregation, the impact of the continuous caster roll gap on center segregation was analyzed, and the roll gap shrinkage control technology and its application based on the solidification end were introduced. The application ideas of static light reduction technology were discussed based on the actual situation of Wugang No. 1 slab continuous caster, and measures to ensure roll gap accuracy were put forward focusing on equipment inspection and maintenance. It can be used as a reference for the practical application of roll gap shrinkage control technology to improve the quality of cast slabs.

Keywords: static light pressing; roll gap shrinkage; center segregation

Preface

During the cooling and solidification process of continuous casting billets, there are generally varying degrees of internal defects such as center segregation and center porosity. With the continuous development of continuous casting technology, researchers have developed a variety of technologies to control center segregation and center porosity of cast slabs based on the generation mechanism of center segregation. Such as low-temperature casting technology, electromagnetic stirring technology, strong cooling technology at the end of solidification, light reduction technology, continuous forging and thermal stress reduction technology, etc., have achieved remarkable results in practical applications. Among them, the solidification end dynamic soft reduction technology has been widely used in new continuous casting machines in recent years, improving the internal quality of the slab. Due to equipment structure limitations, some steel mills are unable to achieve dynamic soft reduction on traditional slab continuous casters. However, by drawing on the technical concept of dynamic soft reduction and reasonably setting the roll gap shrinkage, the effect of soft reduction has also been achieved.

The relationship between the roll gap of the continuous casting machine and the center segregation of the slab

Center segregation and crack formation of continuous casting billet

During the cooling and solidification process, the phenomenon that the content of C, S, Mn, P and other elements in the center of the continuous casting slab is higher than that at the edge of the slab is called central segregation.

The “solidification bridge” theory formed by central segregation believes that the surface layer of the cast slab is chilled to form fine dendrites (quench layer) during the initial solidification. As the solidification thickness of the surface layer increases, the internal outward heat transfer capacity decreases, and the cast slab begins to show directional solidification, forming long dendrites (columnar crystals) from the outside to the inside. Due to selective crystallization, solute elements accumulate in the molten pool (liquid phase region). When columnar crystals grow and cause bridging, the molten steel enriched in solute elements cannot be exchanged with other liquids because it is blocked, and positive segregation of elements such as C, S, Mn, and P is formed there. At the same time, the upper molten steel cannot compensate for the solidification shrinkage here due to the “bridging phenomenon”, which is accompanied by residual shrinkage cavities, forming a loose center, and severely forming center cracks, which reduces the internal density of the cast slab and the mechanical properties of the rolled material.

Another “bulging belly” theory formed by central segregation believes that during the solidification process of the cast slab, the bulging of the billet shell causes the flow of solute-enriched liquid between columnar crystals. Or due to the shrinkage of the cast slab at the end of solidification, the flow of solute-enriched liquid at the end of solidification leads to central segregation. The flow of solute-enriched residual mother liquor between dendrites during solidification is the main cause of central segregation. In order to reduce central segregation and porosity, measures must be taken to inhibit the growth of columnar crystals, expand the central equiaxed crystal area, and inhibit the flow of residual molten steel that enriches solutes at the end of the liquid cavity.

Relevant research results show that the end of the slab liquid phase cavity is “V” shaped in the longitudinal section of the slab and “W” shaped in the central section in the width direction of the slab.

Factors affecting the degree of segregation in the center of the slab

According to the verification of the slab solidification principle and production practice, the cross-sectional size of the slab, the element content of the molten steel, the pouring temperature, the casting speed, the secondary cooling water distribution system, the continuous casting machine roll gap and the arc accuracy are the factors that affect the center segregation of the continuous casting slab. degree is an important factor.

Sectional dimensions of cast slab

For slabs, the cross-sectional dimensions are different, the cooling rates are different, and the solidification end positions are also different.

Element content of molten steel

The higher the content of segregation elements C, S, P, etc. in steel, the degree of segregation will also increase accordingly. For example, when the carbon content is between 0.25% and 0.4%, the degree of segregation is small; when the carbon content is greater than 0.4%, the degree of segregation increases sharply.

Pouring temperature

The superheat of molten steel is an important factor affecting the proportion of equiaxed crystals. The central segregation of the slab is caused by the developed columnar crystals, the degree of superheat is high, and the temperature gradient at the solidification front of the slab is large, which is conducive to the growth of columnar crystals. The development of columnar crystals will aggravate the dendrite segregation during the solidification process, causing the flow of residual molten steel enriched in solutes between the dendrites, and aggravating the central segregation.

Casting speed

The casting speed and rate changes of the continuous casting machine have a great influence on the thickness of the solidification shell of the slab, the position of the solidification end, the composition of the solidification structure and the high-temperature mechanical strength of the slab. Frequent changes in the pulling speed will also cause frequent changes in the position of the solidification end. The probability of “bridging” of the solidification front near the solidification end increases accordingly, which can easily cause segregation in the center of the slab to intensify. Sudden changes in the pulling speed will induce central cracks.

Secondary cooling water distribution system

The internal structure of the cast slab is particularly sensitive to the transverse uniformity and longitudinal gradient of the secondary cooling water cooling intensity. Overcooling of the cast slab will lead to the development of columnar crystals, which will reduce the high-temperature strength of the steel. The liquid core will bridge in advance at the end of solidification, and the liquid core will not be replenished in place, resulting in center looseness or segregation. Insufficient cooling of the billet causes the billet shell to be too thin and prone to bulging. In addition, the temperature rise rate of the slab surface should be less than 100°C/m, otherwise the shell’s ability to resist deformation will be reduced. Moreover, thermal expansion will cause a suction phenomenon in the center of the slab, prompting the flow of molten steel and aggravating center segregation and center cracks.

Casting machine roll gap setting and arc alignment accuracy

Equipment precision is the guarantee of process quality. The “bulging belly” theory believes that the bulging belly is closely related to the roller gap setting, roller rigidity, arc alignment accuracy, etc. There are two reasons for a bulging belly. First, changes in process parameters such as pulling speed, cooling intensity, and superheat cause the solidification end of the billet liquid core to be outside the design area. The actual shrinkage of the cast slab does not match the set roll gap standard, resulting in the flow of residual mother liquor enriched in solutes between dendrites during solidification, aggravating segregation. Second, the quality of equipment maintenance is not up to standard or equipment problems cause the roll gap in the sector section to change, causing the billet to bulge.

Effect of roll gap shrinkage on central segregation

For a slab with a certain cross-section size, the arc of the continuous casting machine and the roll gap are two important parameters related to the internal quality of the slab when the pouring superheat, casting speed, and secondary cooling water cooling are stable and reasonable. The reasonable design of the roll gap shrinkage of the sector-shaped roller rows and the stability of the roll gap value can help control the deformation and cracks of the slab, improve center segregation, center porosity and center line cracks, and avoid problems such as delamination of high-strength hot-rolled plates.

The traditional roll gap shrinkage technology is to pre-set a certain taper in the roll gaps of the continuous casting machine roller rows based on the solidification shrinkage characteristics of the steel type to compensate for the solidification shrinkage of the slab without additional reduction. Optimizing the roll gap shrinkage technology of continuous casting machines relies on the concept of roll gap setting and adjustment under dynamic light pressure. A certain amount of reduction is added to the solidification end area of the cast slab where central segregation is prone to occur, to compensate for the solidification volume shrinkage of the solid-liquid mushy area at the end of the cast slab solidification. By shrinking the roller gap, a uniform external force is applied to the end of the liquid core of the continuous casting slab to form a certain amount of compression to compensate for the solidification shrinkage of the slab. On the one hand, it can eliminate or reduce the internal voids formed by the shrinkage of the slab, destroy the “W”-shaped liquid phase cavity formed at the end of solidification, and prevent the solute-enriched liquid steel between the crystals from flowing laterally to the center of the slab. On the other hand, the squeezing effect caused by the shrinkage of the roll gap can also promote the reverse flow of the solute-enriched molten steel in the liquid core along the drawing direction, so that the solute elements are redistributed in the molten steel. This makes the solidification structure of the slab more uniform and dense, achieving the purpose of improving center segregation and reducing center porosity.

Application of roll gap shrinkage control technology

According to the characteristics of their own equipment, many steel plants apply the principle of dynamic soft reduction to design different roll gap shrinkage schemes and conduct soft reduction experiments at the solidification end front. It is proved that the roll gap shrinkage technology has greatly improved the center segregation and center porosity of the slab, and basically eliminated the cracks in the center of the slab. The opening of the continuous casting machine is stable, the equipment is normal, and the effect is relatively ideal.

According to relevant literature, the roll gap shrinkage control technology based on light pressure at the solidification end requires determining the roll gap shrinkage area and reasonably allocating the shrinkage amount based on the position of the solidification end. If the pressing position and the pressing range are too far forward, it will easily cause the narrow surface of the billet to bulge; if it is too far behind, it will not achieve the effect of light pressing. If the roll gap shrinkage is too small, the solidification shrinkage cannot be fully compensated, and the central segregation and central porosity are not significantly improved. When the amount of shrinkage is too large, the cast slab is over-squeezed, causing the molten steel that has not yet solidified and is enriched in solutes to flow into the adjacent bulge area, forming segregation. It will also cause the strain stress and deformation rate of the cast slab to be too large, which will cause internal cracks after exceeding the high temperature strain limit of the steel. At the same time, it will easily cause damage to the rollers and bearing seats in the pressing area.

The key to implementing roll gap shrinkage control technology based on light pressure at the end of solidification is to determine the position of the end of solidification, determine the reduction interval based on the solid phase fraction (0.5≥f≥0.8), and reasonably allocate the shrinkage of each section. In order to use different roll gap shrinkage schemes to reasonably and lightly press the solidification end. The solidification end position depends on process parameters such as steel type, slab cross-section size, pouring superheat, drawing speed, cooling system, etc. If any process parameter changes, the solidification end position will also change. In order to adapt to production under different process parameters, several shrinkage schemes can be determined according to actual conditions.

The position of the solidification end can be determined by the “nail shooting method”, that is, based on the measured thickness change of the shell, the cooling capacity of the system is evaluated and the position of the solidification point is calibrated. Then based on the square root relationship between the thickness of the solidified shell of the cast slab and the solidification coefficient, the solidification coefficient of the cast slab is obtained. The solidification coefficient is an important parameter in formulating the casting slab production process. When the solidification coefficient is stable, the solidification point position can also be calculated based on the square root relationship between the solidification shell thickness of the cast slab and the solidification coefficient, but the solidification coefficient needs to be corrected according to the actual situation.

Discussion on the setting scheme of roll gap shrinkage in the sectoral section of Wugang slab continuous casting machine

Original designed roll gap situation

Wugang 1# continuous casting machine is a straight-bent slab continuous casting machine, which adopts 6-point bending and 4-point straightening, with a total of 12 sector-shaped sections and 5 pairs of rollers in each section. Below the foot roller of the crystallizer is the support section (also called the zero section of the sector section), with a total of 9 pairs of three-section rollers. The first segment of the sector is a three-section roller, and the rest are integral rollers. Sections 1 to 7 are simple arc areas, sections 8 to 9 are straightening sections, and sections 10 to 12 are horizontal sections. It can produce slabs with thicknesses of 210, 250 and 300mm. For a slab with a thickness of 250 mm, the total shrinkage of the basic roll gap in the original design of the sector section is 4.6 mm, the shrinkage between sections is 0.4mm, and the shrinkage within the section is 0.2 mm.

Analysis of pressing parameters

Press down position

The key to determining the pressing position is to accurately determine the solidification end position. Currently, the 1# slab continuous casting machine mainly produces slabs with a thickness of 250 mm. In May 2004, Wugang Steelmaking Plant cooperated with the National Engineering Research Center for Continuous Casting Technology, and combined with the actual production on site, used the “nail shooting method” to measure the thickness of the solidified billet shell of the 250mm×1800mm cross-section continuous casting billet at different positions in the secondary cooling zone. Find the solidification end point of the continuous casting billet, as shown in Table 1. The comprehensive solidification coefficient in the table is calculated according to the square root law of solidification (δ=K solidification√L effective/v), and the correction does not take into account the characteristics of accelerated solidification at the solidification end.

Table 1 Position of solidification point at the end of continuous casting

VarietySection size/mmSuperheat degree/℃Pulling speed/ (m·min-¹)Specific water volume/ (L·kg-¹)Complete solidification point positionDistance to meniscus/mComprehensive solidification coefficient/ (mm/min-1/2 )
Low-carbon steel1800×250about 220.90.4510th section middle section22~23.125.37~24.75
Carbon steel1800×250about 250.850.48End of paragraph 921.824.77

As can be seen from Table 1, the solidification point of low carbon steel is located in the first horizontal section (10#Seg), and the solidifying point of medium carbon steel is located in the last straightening section (9#Seg). The roll gap shrinkage area will include the straightening section. . At this time, the cast slab is subjected to the straightening force and the pressing force of the roll gap shrinkage. Therefore, special attention must be paid to the setting of the roll gap shrinkage, so that the slab cannot be greatly deformed and cracked, and the rollers cannot be deformed due to force. Especially for medium carbon steel, if the roll gap setting in the area near the solidification end is unreasonable or other process parameters fluctuate, the thickness of the molten steel sealed in the solidification bridge will be less than 6 mm. According to relevant literature, if the thickness of the sealed molten steel is about 3 to 4 mm, discontinuity defects will be formed; if the thickness of the sealed molten steel is about 6 mm, continuity defects will be formed. Therefore, it is possible to consider changing the process parameters so that the solidification end position of medium carbon steel avoids the straightening area. When producing other crack-sensitive steel types, the adaptability of process parameters, roll gap settings, and solidification end positions must also be considered.

Pressure area

A large number of studies have proven that central segregation and porosity occur in the liquid-solid two-phase region at the end of solidification, so the solid phase ratio is often used to characterize the location of the depressed region.

When the solid phase ratio in the center is small, the liquid phase upstream in the casting direction can fill the holes caused by the solidification shrinkage due to the static pressure of the molten steel, and the concentrated molten steel between the dendrites will not flow to the center. The area with a small central solid phase ratio is not a critical location for central segregation, and the pressure in this area does not have a significant effect on improving central segregation.

When the central solid phase ratio reaches a certain value, the dendrites are connected by bridges to each other, blocking the downward filling and shrinkage of the upstream liquid phase (small steel ingot theory). If a cavity is created in the center due to bulging or solidification shrinkage, it will cause the residual liquid phase enriched in solutes between dendrites to be sucked into the center of the slab, resulting in central macrosegregation. Such a region is the key location where central segregation occurs, and the solid phase rate at this time can be called the critical feeding solid phase rate. Providing an appropriate amount of reduction can cause the central cavity to disappear, thereby curbing the flow of residual liquid phase between dendrites and reducing central segregation.

When the solid phase rate in the center is large, the mushy areas near the center of the slab thickness are bridged by the secondary dendrite arms, making it impossible for the residual molten steel between the dendrites to flow. Even if there is a large hole in the center, central macrosegregation will not form due to the outflow of the residual liquid phase enriched in solutes between dendrites. Such a location is not a critical location for the generation of central macrosegregation. The solid phase ratio at this time is called the critical flow solid phase ratio. When pressed at such a position, since the residual liquid phase can no longer flow, the cracks generated cannot be welded by liquid phase replenishment. A large number of literatures point out that the area with solid phase ratio between about 0.8 and 0.99 is the internal crack sensitive area. A reasonable solid phase ratio in the reduction zone should be between 0.5 and 0.8. The actual reduction interval can be calculated based on the change pattern of the shell thickness of the 1# continuous casting machine and the actual roll row distribution.

Reduction amount

The amount of reduction is determined by the volume shrinkage of the molten steel during the process of changing from liquid to solid. The distribution of reduction in each section is crucial. The reduction amount D0 can be expressed as: D₀=(D₁+D₂)/m+D₃ (1)

In the formula: D₁- The amount of reduction required to compensate for the solidification shrinkage of the slab/(mm·m-¹); D₂-The amount of reduction required to compensate for the bulge/(mm·m-¹). D₃-Thermal shrinkage of the cast slab/(mm·m-¹); η-Reduction efficiency.

The literature points out that when reduction is not implemented, a large solidification shrinkage flow occurs in the center direction of the thickness of the slab. After implementing a reduction amount (D₀) of 0.26 mm/m in the reduction interval, the flow of molten steel caused by solidification shrinkage can basically be suppressed. If the secondary cooling water is distributed according to the surface temperature and billet bulge limit control system, the reduction required to compensate for the billet bulge can be ignored. According to the heat transfer and solidification theory of the slab, it has been calculated that the basic roll gap thermal shrinkage (D₃) of the slab is 0.2mm/m, and the reduction efficiency (η=reduction in liquid core thickness/reduction amount on the surface of the slab) is 0.4~0.7. According to formula (1), D₀ is calculated to be 0.8~0.9mm·m-¹.

Research shows that a reduction of 0.30 mm/m has no effect on improving the center segregation of the slab, a reduction of 0.75 mm/m has a certain effect, and a reduction of 1.20 mm/m has a significant effect. Under suitable drawing speed conditions, the number of central segregation points larger than 1mm² is almost zero. According to the actual situation, the reduction amount in the reduction interval is selected between 0.4~1.0mm/m. The shrinkage value of each section of the roll gap can be assigned according to the change pattern of the shell thickness and the actual roll row distribution. However, when allocating the reduction amount, the superposition of the reduction stress and the straightening stress in the straightening section must be considered, and the reduction interval and reduction amount must be appropriately moved forward.

Application measures of roll gap shrinkage technology

Establish a roll gap setting plan that adapts to different process parameters

Since the 1# continuous casting machine was put into operation, Wugang Steelmaking Plant has tackled technical issues such as continuous casting slab length measurement, secondary cooling water cooling system optimization, and sample benchmark system accuracy verification. The quality of the slab has improved, but it is still unstable. The important influence of the roll gap on the quality of the slab and the existing problems must be fully understood. Conduct research on roll gap shrinkage control technology and establish roll gap control schemes suitable for various process parameters. Optimization of roll gap setting can be carried out from two aspects.

(1) Combine the process parameters and equipment parameters to analyze and count the existing arc and roll gap measurement data. Based on the sulfur print record of the cast slab and process technical parameters (section size, steel type, superheat, pulling speed, specific water volume, etc.), statistical analysis is performed to calculate the solidification end position and summarize the roll gap setting rules.

(2) Compare the roll gap shrinkage plan determined through theoretical calculations with statistical data, and apply it to actual production for verification after correction. Then establish a standardized and regular roll gap setting plan based on different steel types, pulling speed, specific water volume and other process parameters. For crack-sensitive steel types, study the possibility of changing process parameters to avoid straightening in the high-temperature brittle zone.

Ensure the stability of the set roll gap

After the roll gap of the continuous casting machine is set, it may change during pouring. It is necessary to ensure the stability of the roll gap accuracy from the perspective of equipment maintenance and repair.

The slab is cooled evenly across the wide surface

Check the nozzle arrangement and spray status to prevent overcooling of the corners of the slab and clogging of the nozzles, resulting in uneven cooling.

Arc alignment accuracy

Check the accuracy of the online and offline arc alignment templates, regularly align the arc and perform roll gap measurement and adjustment. Pay special attention to the roll gap formed by the superposition of the arc deviation and the roll gap value in key parts to prevent the formation of bulges.

The roller works normally

Check and eliminate roller deformation, wear, poor rotation, bearing damage and other faults; for the driving roller where the hydraulic cylinder controls the pressing force, the system pressure stability must be ensured to prevent the roll gap at the driving roller from being unstable.

Offline sector section maintenance quality

The lifting and lowering of the sector-shaped frame of the 1# continuous casting machine is driven by a turbine reducer and a screw mechanism. During maintenance, the accuracy of the assembly of the reducer and the screw transmission must be improved. Especially when the turbine reducer is assembled offline, a large preload force must be applied to eliminate gaps and prevent the frame from moving and dislocating after being stressed. Improve the assembly quality of the roller bearings to prevent the bearings from being damaged during use and causing bulging; the hydraulic cylinder of the driving roller must be well sealed to prevent leakage.

Conclusion

(1) The roll gap shrinkage technology based on the principle of dynamic light reduction has a significant effect on improving defects such as segregation, looseness, and cracks in the center of the slab, and is economically feasible, but the position of the solidification end is difficult to determine.

(2) Apply the roll gap shrinkage control technology based on light pressing at the end of solidification. The key is to determine the pressing position. Determine the reduction interval and reduction amount according to the solid phase fraction (0.5≥fs≥0.8), and reasonably allocate the shrinkage amount of each section to facilitate the selection of different roll gap shrinkage plans.

(3) Conduct theoretical calculations based on process and equipment related parameters, design several roll gap setting schemes suitable for different steel types, pulling speeds, and specific water volumes, compare and correct them with statistical data, and apply them to actual production for verification. For crack-sensitive steel types, the possibility of changing process parameters to avoid straightening in the high-temperature brittle zone was studied. After repeated verification and modification, a stable and standardized roll gap setting scheme suitable for different situations was finally optimized.

(4) After the roll gap is set according to the plan, failures such as roller non-rotation or roll collapse may occur during equipment operation, resulting in changes in the roll gap, resulting in increased segregation of the slab or bulging. Therefore, it is also necessary to ensure the stability of the roll gap accuracy from the perspective of equipment maintenance and repair.

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