Research and application of thin slab continuous casting mold taper design technology

Abstract: The goal is to improve the service life of the mold and the surface quality of the slab. By establishing a calculation model of the funnel area’s own taper, the total taper of the copper mould tube, and the local taper of the copper mould tube, the effects of the width and depth of the funnel zone, the narrow-side copper plate offset, and wear on the distribution of the copper mould tube taper along the height direction were studied. The design concept is proposed that the distribution of the natural taper and local taper along the height direction of the funnel area should match the solidification shrinkage of the cast slab. The shape of the inner cavity of the wide-faced copper plate and the narrow-side taper control process of the thin slab continuous casting mold were optimized and designed, which provided a theoretical basis for the optimization of the copper mould tube operation process and management system. Based on the above research work, a new copper mould tube suitable for the FTSC process was developed and a large number of industrial production tests were conducted. The results show that when pouring a slab with a width of 1520mm, the additional offset of the narrow side can be reduced from 12mm to 7mm, and the service life of the narrow side copper plate has been greatly improved. After the steel passing amount was 22,500 tons, the maximum wear amount of the narrow-side copper plate was reduced from 5mm to less than 2mm; the new mold effectively controlled the occurrence of narrow surface dents and surface longitudinal cracks of the billet.

Keywords: thin slab; continuous casting; copper mould tube; taper; wear; crack

The contact state between the mold wall and the solidified shell is a key factor affecting the surface quality of the cast slab, which is closely related to the optimal design of the mold taper. There have been many related studies on the taper of square billet, round billet, and slab continuous casting molds. Most results show that the ideal mold cavity shape and taper should be able to fully adapt to the shrinkage of the solidification shell, thereby optimizing the contact state between the copper mould tube wall and the solidification shell. Since the growth of the solidified shell thickness follows the square root law, not only the square and round billet molds use parabolic taper copper tubes, but the shape of the narrow copper plate in the slab continuous casting mold is also developing in the direction of double tapers and multi-tapers. . At present, there is little basic research on the definition, calculation method, and narrow surface shape design of thin slab continuous casting mold taper. The author proposed and established a calculation model for the funnel area’s own taper, the total taper of the mold, and the local taper of the mold. With the goal of improving the service life of the mold and the surface quality of the thin slab, the effects of the width and depth of the funnel area, the offset of the narrow-side copper plate, and wear on the longitudinal distribution of the mold taper were studied. The inner cavity shape of the mold copper plate and the narrow side taper control process were optimized and designed, and a new thin slab continuous casting mold suitable for the FTSC process was successfully developed.

1 Calculation model and application of funnel-shaped copper mould tube taper

1.1 Definition and calculation model of thin slab continuous casting mold taper

In order to quantitatively analyze the structural characteristics of the inner cavity of the thin slab continuous casting mold, the taper of the thin slab continuous casting mold is divided into four important parameters, namely the inherent taper of the funnel area and the additional offset of the narrow-side copper plate (called “according to factory custom” Narrow side additional taper”), total mold taper, mold local taper.

1 . 1 . 1 The funnel area has its own taper

The natural taper of the funnel area is the taper formed by the arc length on the cross section of the funnel area of the wide-faced copper plate continuously decreasing in the height direction and tending to a straight line. After the copper plate is processed and formed, if the wear is ignored, the funnel area has its own taper fixed, which has nothing to do with the crystallizer operation process.

The natural taper of the funnel area refers to the difference between the arc length of the upper mouth of the funnel area and the arc length of the lower mouth of the funnel area (for CSP copper mould tube, the lower mouth is a straight line length) divided by the arc length of the upper mouth of the funnel area, that is:

In the formula: ∇1 is the natural taper of the funnel area, %/m; a1 is the arc length of the upper mouth of the funnel area of the crystallizer, mm. a2 is the arc length or straight line length of the lower mouth of the funnel area, mm; H is the height of the funnel area of the crystallizer, m.

After removing H from the denominator of equation (1), the unit of the inherent taper is %. The numerator part in the formula is the line length difference, which is another important parameter that reflects the matching between the funnel curve and the solidification shell shrinkage change curve in the funnel area (the formula mentioned later has a similar meaning).

1. 1. 2 Narrow side additional taper

The additional taper of the narrow side refers to 1/2 of the difference between the upper and lower openings of the inner cavity of the mold copper plate during the pouring process, and the unit is mm. Another outstanding feature of thin slab continuous casting technology is that the additional taper of each narrow-side copper plate can be individually adjusted online during the casting process. When adjusting the additional taper, the CSP mold keeps the width of the upper billet unchanged, while the FTSC mold keeps the width of the lower billet unchanged. The goal of adjusting the narrow side additional taper is to ensure that the narrow-width heat flow ratio of the crystallizer is stable in the range of 60% to 90%. When the narrow-width heat flow ratio is too low, increase the narrow side additional taper, and vice versa, reduce the narrow side additional taper.

1 . 1 . 3 Total taper of copper mould tube

The total taper of the mold is a parameter that takes into account the additional taper of the narrow side, so it is related to the slab section, steel type, etc. The total taper of the funnel crystallizer refers to:

The curve length of the upper mouth of the funnel-shaped copper mould tube(that is, the sum of the arc length and the straight line length) minus the curve length of the lower mouth of the copper mould tube. (That is, when there is no bulge at the lower mouth of the copper mould tube, it is the straight line length; when there is a bulge, it is the sum of the arc and the straight line length), divided by the curve length of the upper mouth of the copper mould tube. The formula is expressed as follows:

In the formula: ∇ is the total taper of the copper mould tube, %/m; b1 is the curve length of the upper mouth of the copper mould tube, that is, the sum of the arc length of the upper mouth and the straight line length, mm. b2 is the curve length of the lower opening of the copper mould tube, mm; h is the total height of the copper mould tube, m.

It can be seen from the meaning of the total taper of the copper mould tube and the calculation model that for a given wide-face copper plate, the additional taper of the narrow face is determined by the total taper.

1. 1. 4 Copper mould tube local taper

The local taper of the mold refers to the taper at a certain position in the height direction. After taking into account the additional taper of the narrow surface, it is the local taper of the mold when pouring a slab with a specific cross-section. When the additional taper of the narrow surface is not considered, it refers specifically to the local own taper of the funnel area, and its calculation model is as shown in Equation (3):

In the formula: ∇2 is the local taper at a distance of x from the lower mouth of the copper mould tube, %/m; c1 is the curve length at a distance of x+0.5Δh from the lower mouth of the copper mould tube, mm. c2 is the length of the curve at a distance of x – 0.5Δh from the lower mouth of the copper mould tube, mm.

The purpose of calculating the local taper of the mold is to study whether its distribution in the height direction can match the solidification shrinkage of the billet shell.

1.2 Comparison of tapers of different types of thin slab continuous casting molds in China

Calculated using the above formula, the structural parameters and natural taper comparison of typical domestic CSP and FTSC process copper mould tube wide-face copper plates are shown in Table 1. The longitudinal distribution of the local natural taper in the funnel area of the copper mould tube is shown in Figure 1.

Table 1 Comparison of structural parameters of the wide-face copper plate inner cavity of the copper mould tube in CSP and FTSC processes

copper mould tube numberWidth of funnel area/mmMaximum depth of copper plate entrance/mmHeight of funnel area/mmTotal height of copper plate/mmLength difference between upper and lower entrance lines of funnel area/mmSelf-taper of funnel area/% (or %·m-1)
CSP111506085011008.330.72(or0.85)
CSP295060850110010.071.05(or1.23)
CSP38805585011009.141.03(or1.21)
CSP48805085011007.560.85(or1.00)
FTSC8004090012005.110.63(or0.70)

Figure 1 Longitudinal distribution of line length differences and local natural tapers in the funnel area of several typical thin slab continuous casting molds

From a theoretical analysis, when the inherent taper of the funnel area is too large, the billet shell cannot shrink into the predetermined shape at the outlet of the funnel area during the downward movement due to the solidification shrinkage of the steel itself. The billet shell will overlap inward under the extrusion of the mold copper plate, resulting in cracks. When the inherent taper of the funnel area is small, the billet shell will quickly break away from the mold wall due to shrinkage during the downward movement, which will also cause a thicker layer of mold slag to be filled between the billet shell and the copper plate of the mold. It hinders the heat transfer of the shell in the funnel area, resulting in uneven thickness of the shell and prone to stress concentration and cracks. In the case where the wide-face copper plate does not have an inverse taper in the thickness direction, in order to make the funnel area meet the needs of the solidification shrinkage of the shell, the inherent taper of the funnel area should generally be controlled in the range of 0.8% to 1.2%.

It can be seen from Table 1 that the FTSC mold copper plate has the smallest opening and its own taper, which is only 0.63%, while the CSP mold copper plate has a relatively larger opening and its own taper. The CSP thin slab continuous casting machines introduced later in China all use CSP4 copper mould tubes.

It can be concluded from Figure 1:

1) The local taper in the funnel area of several typical thin slab molds currently used changes linearly, and the line length difference changes in a parabolic pattern in the height direction, which is similar to the changing pattern of the shrinkage curve of the solidified shell. Therefore, this taper is conducive to improving the heat transfer effect of the copper mould tube as a whole;

2) The CSP process funnel-shaped copper mould tube has a taper of 1.22~2.34%/m in the area 0~300mm from the upper opening. It is larger than the inherent taper (0. 74~1. 22%/m) of the funnel area of the H2 copper mould tube in the FTSC process. Therefore, while keeping the total taper of the copper mould tube the same, it is necessary to increase the taper of the flat areas on both sides of the upper part of the funnel area of the FTSC copper mould tube. That is to increase the additional taper of the narrow surface, and the undesirable consequence of this is that the contact friction between the lower part of the narrow surface copper plate and the shell will be higher in the early use of the new copper plate. It promotes the formation of transverse cracks on the narrow surface of the billet. In addition, the wear of the narrow-surface copper plate will intensify, which not only reduces the service life of the mold, but also in the later period of use, the narrow surface of the billet cannot be effectively supported by the worn copper plate. This in turn will cause longitudinal cracks on the surface of the cast slab;

3) For the existing FTSC copper mould tube, at a distance of 300mm from the lower edge of the copper plate, the change in local taper at this point is not continuous. That is, there is a sudden change between the first and second funnel areas mentioned above, and the corresponding line length difference is not a smooth transition there, but the changes in the longitudinal slab temperature and solidification shrinkage should theoretically be smooth and continuous. Therefore, this is detrimental to improving the contact state between the copper wall and the condensation shell.

At present, when casting SS400 steel in CSP and FTSC molds, the total taper setting values are very different, which are 1.11%/m and 1.58%/m respectively. From this calculation, the relationship between the lower opening width and the additional taper of the narrow surface of several typical thin slab molds is obtained, as shown in Figure 2.

Figure 2 The relationship between the outlet width of several typical thin slab molds and the additional taper of the narrow surface when casting SS400 steel

It can be seen from Figure 2 that, while keeping the total taper the same, there is a linear relationship between the additional taper of the narrow surface and the width of the slab. The wider the billet, the greater the required additional taper on the narrow surface. In comparison, the additional taper on the narrow surface of the FTSC mold is about 6mm larger.

2 Research on taper and longitudinal distribution of thin slab continuous casting mold

2.1 Effect of funnel zone width and depth on natural taper

According to the construction principle of the funnel area surface, combined with the calculation model of the own taper, it can be obtained that the own taper (unit: %) of the funnel area connected by equal radius arcs and the maximum depth D (unit: mm) and W (in mm) of the upper mouth curve can be obtained (unit: mm), the result is shown in equation (4).

∇1= {arctan[0.25W/(D/4+W2/16D-D/2)]×2(2D/4+W2/16D) -W}/{ arctan[0.25W/(D/4+W2 /16D-D/2)]×2(2D/4+W2 /16D)}×100% (4)

From this, we can get the relationship between the maximum funnel zone depth of the upper mouth and its own taper under different funnel zone widths, as shown in Figure 3.

Figure 3 Effect of funnel zone width and depth on inherent taper

It can be seen from Figure 3 that under the condition of constant funnel zone width, as the depth of the funnel zone increases, the spontaneous taper increases. At a constant funnel zone depth, the spontaneous taper decreases as the funnel zone width increases. Therefore, increasing the depth of the funnel area and reducing the width of the funnel area are both means of increasing the inherent taper of the funnel area. In order to make the funnel area curved surface shape meet the needs of solidification shrinkage, the inherent taper should be controlled at 0. 8% ~ 1.2%.

2.2 The influence of narrow-side copper plate offset on the total taper of FTSC copper mould tube

Combining the above calculation model, it can be obtained that when the width of the lower opening of the mold is 1290mm and 1520mm respectively, the impact of the additional taper on the narrow side on the total taper of the existing FTSC mold is shown in Figure 4.

Figure 4 Effect of additional taper on the narrow side on the total taper of the FTSC copper mould tube

It can be seen that if we refer to the design of the CSP mold taper, in order to ensure that the total taper reaches 1.22%, the additional taper of the narrow side of the FTSC mold with billet widths of 1290mm and 1520mm only needs to be controlled at about 5.5mm and 7.0mm.

2.3 The influence of the surface shape and wear of the narrow-edge copper plate on the longitudinal distribution of the local taper of the FTSC copper mould tube

When the FTSC copper mould tubewas put into production, the narrow-side copper plate adopted a multi-taper structure. Before offset and when the range of 0~300mm from the lower opening was zero taper, the longitudinal distribution of the multi-taper narrow-side additional taper is shown in Figure 5.  Since the wide-faced copper plate of the FTSC copper mould tube has its own taper in the range of 0~300mm from the lower opening, it is also 0. Therefore, in order to ensure that the mold has a certain inverse taper in the lower area, when casting a slab of any width, the additional taper of the narrow side is required to be higher than 6. 4mm. This is the main reason why the total taper of the FTSC copper mould tube is relatively higher in the initial stage of production. The narrow-faced copper plate of the single-taper copper mould tube overcomes the inherent defects in this structure. For cast slabs with lower opening widths of 1290mm and 1520mm respectively, when the additional tapers of the narrow sides are controlled at 9mm and 12mm respectively, the longitudinal distribution of the local taper of the copper mould tube formed by a combination of single-taper and multi-taper narrow sides is shown in Figure 6.

Figure 5 Longitudinal distribution of additional tapers when the multi-taper narrow side offset is 0mm and 6.4mm

Figure 6 The influence of the surface shape of the narrow-side copper plate on the local taper of the thin slab mold

It can be seen from Figure 6 that the single-taper narrow side greatly increases the taper of the lower part of the mold, providing conditions for effective support of the narrow side of the shell. In addition, when a multi-taper narrow-edge copper plate is used to cast a slab with a width of 1290mm, the local taper of the lower opening is too low, only 0.34%/m. The air gap between the solidified shell at the lower part of the copper mould tube and the copper wall will increase, and the shell will not be sufficiently cooled. This is one of the main reasons why the incidence of longitudinal cracks and steel breakouts was relatively high when casting narrow-section low-carbon steel in early FTSC molds. If the additional taper of a single-taper narrow copper plate is too high during use, serious wear will occur on the surface of the copper plate. Production practice shows that after the amount of steel passing reaches 130 furnaces (150t per furnace), the maximum wear amount at the lower edge of the narrow copper plate can reach 7~8mm. Combined with the wear amount detection results, the local taper distribution with or without wear is calculated, as shown in Figure 7.

Figure 7 Effect of copper plate wear on local taper of FTSC copper mould tube

It can be seen that wear causes the taper of the lower part of the mold to be greatly reduced, and the support effect of the copper plate on the billet shell becomes worse, which is detrimental to the cooling of the billet and the control of surface longitudinal cracks. In addition, the wear of the lower part of the narrow copper plate will also have a negative impact on the shape of the narrow surface of the billet. When the additional taper on the narrow surface is high and the lower edge of the copper plate is severely worn, the narrow surface of the billet will be severely dented, which is one of the reasons for edge cracks or warping defects in hot-rolled coils. Therefore, the additional taper of the narrow surface should be reasonably controlled to reduce wear as much as possible.

In fact, due to the smaller additional taper of the narrow side of the CSP process funnel-type copper mould tube in use, which is 2~6mm, there is a small amount of wear on the lower part of the narrow side copper plate. For example, if it is 2~3mm, it will cause the taper of the lower part of the CSP funnel-shaped mold to be 0 or form a positive taper, which will lose the support and cooling effect on the narrow surface of the slab, and thus cause longitudinal cracks or steel breakouts.

3 Industrial application of new thin slab continuous casting mold taper optimization design technology

Taken together, the shortcomings of the original FTSC process H2 copper mould tube are as follows:

1) The funnel area and the plane area of the wide-faced copper plate fail to transition smoothly in a tangential manner, which affects the uniformity of the force in the lateral direction of the billet shell;

2) The inherent taper of the funnel area is low, which cannot meet the solidification shrinkage needs of the shell itself, and causes the additional taper of the narrow side to be too high;

3) The lower part of the copper mould tube maintains a funnel area with a maximum depth of 8mm. At the same time, the surface of the foot roller section at the lower mouth of the copper mould tube remains funnel-shaped, which makes the installation and maintenance of the equipment more difficult.

In response to the above problems, the design points and main parameters of the new thin slab continuous casting mold are as follows:

1) The copper plate and the cast rollers in the foot roller section in the lower 300mm height area of the copper mould tube are all changed to a flat shape;

2) The curves that constitute the funnel area are all smoothly transitioned by arcs of equal radius, and the funnel area and the plane area remain tangent;

3) Increase the depth of the funnel area, increase the self-taper to about 0.85%, and moderately increase the width and height of the funnel area. The width of the funnel area of the designed wide-face copper plate of the new copper mould tube is 880mm, and the maximum depth of the upper opening is 50mm. Height is 1000mm;

4) In the height direction, the funnel area and the lower plane area adopt a circular arc to smoothly transition;

5) The narrow surface copper plate adopts a single taper structure. When determining the additional taper of the narrow surface, the total taper of the copper mould tube is slightly higher than the requirements of the CSP process. The setting scheme of the additional taper on the narrow surface of the new copper mould tube is shown in Table 2. The goal of additional taper control on the narrow surface is to ensure that the narrow-to-width heat flow ratio is controlled at 70% to 90% to prevent the occurrence of longitudinal cracks on the surface. The heat flux density across the wide surface of the copper mould tube is generally controlled to be 1600~2100kW/m2. During normal pouring, the setting value of the additional taper for the narrow surface can be adjusted downwards as the drawing speed increases, but it shall not be lower than the lower limit of the setting value stated in the table. When the narrow side of the copper plate has been cast for 100 heats, considering that wear will cause the lower taper to decrease, it is necessary to increase the narrow side offset by 0.5mm compared to the value set in the table.

Table 2 Design of additional taper on the narrow surface of the new copper mould tube

Cast slab width at the bottom of the moldLow carbon steel (SPHC) Medium carbon steel (SS400, Q345B)Microalloyed steel with large solidification shrinkage (SPAH)
1100±504.5~5.55.0~6.06.0~7.0
1200±505.0~6.05.5~6.56.5~7.5
1300±505.5~6.56.0~7.07.0~8.0
1400±506.0~7.06.5~7.57.5~8.5
1500±506.5~7.57.0~8.08.0~9.0
1600±507.0~8.07.5~8.58.5~9.5

A large number of industrial tests were conducted at Tongshan Iron and Steel Co., Ltd. and Tangshan Iron and Steel Co., Ltd. using the new thin slab continuous casting mold. The results show that when pouring medium carbon steel with a width of 1520mm, the additional taper of the narrow surface is reduced from 12mm to 7~8mm on the premise of ensuring that the narrow-width heat flow ratio of the mold reaches 70%~90%. After the amount of steel passing reaches 117 furnaces (150t steel per furnace), the wear amount of the narrow-sided copper plate is reduced to less than 2mm, which nearly doubles the service life of the narrow-sided copper plate of the mold, and the pass rate of hot-rolled coils can reach 99.25%.

4 Conclusion

1) The calculation model of the thin slab continuous casting mold’s own taper, total taper and local taper better describes the structural characteristics of the copper plate inner cavity surface. It can be used for the structural design of the funnel area of the wide copper plate of the copper mould tube and the setting of additional taper for the narrow surface.

2) In order to make the wide-faced copper plate funnel area meet the needs of the solidification shell’s own shrinkage, the self-taper should be controlled at 0.8% ~1.2%. Increasing the depth of the funnel area and reducing the width of the funnel area are the main means to increase the natural taper and reduce the additional taper of the narrow surface.

3) The wear of the lower edge of the narrow copper plate is one of the causes of longitudinal cracks on the surface of the slab, dents on the narrow surface of the slab, and cracks in the hot-rolled edges. While improving the supporting effect of the copper plate at the bottom of the mold on the billet shell, the additional taper on the narrow surface should be prevented from being too high, so as to reduce the wear of the copper plate as much as possible.

4) The new copper mould tube simplifies the processing and installation process of the funnel-shaped casting roller in the foot roller section, which is beneficial to reducing equipment maintenance costs. When pouring medium carbon steel with a width of 1520mm, the additional taper of the narrow surface is reduced from 12mm to less than 8mm. After the steel passing amount reaches 22,500 tons, the wear amount of the narrow-sided copper plate is reduced to less than 2mm, which nearly doubles the service life of the narrow-sided copper plate of the mold, and the pass rate of hot-rolled coils can reach 99.25%.

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