Measurement and analysis of square and round billet continuous casting mold taper

This paper briefly introduces the role of mould tube taper in continuous casting, analyzes the design principles of billet and round billet crystallization taper, and based on the measurement data of different billet and round billet crystal cavity dimensions, analyzes the mold taper in Problems in the production process.

Key words: continuous casting; mould tube taper; parabola

The heat transfer of the mold is the key to control the output and quality of the continuous casting slab, and the air gap formed between the slab shell and the inner wall of the mold is small in size, but its thermal resistance can account for 80% of the entire thermal resistance Therefore, in today when both quality and output are equally important, the taper design and manufacturing level of the copper mould tube are getting more and more attention. This paper firstly introduces the design principles of billet and round billet mold taper and several common high-speed molds. On this basis, based on the measurement data of different mold copper tubes, the current domestic mold taper production Manufacturing problems are analyzed. It can provide reference for production practice.

Quality problems caused by mould tube taper

The role of the mold wall is to support the molten steel to form a billet shell in the mold, and the second is to conduct the heat in the molten steel and the billet shell, so that the billet shell is continuously thickened. As the thickness of the shell increases, the section of the shell shrinks gradually. In order to enable the mold to play the above-mentioned role, the section of the mold must change with the continuous shrinkage of the section of the slab, which can be realized by making the mold into a certain reverse taper. The size of the taper must be appropriate. Excessive taper will cause the mold to squeeze the billet shell, resulting in concave corners, increased friction between the billet shell and the mold, aggravating the wear of the mold, and copper on the surface. In the corner area, hot spots can form due to air gaps, which in turn cause shell thinning and cracks. A small taper will increase the air gap, reduce the heat flow, thin the billet shell, and easily cause steel breakout; in addition, if the taper is too small, it will intensify the rotation of the corner, and induce subcutaneous cracks and longitudinal depressions. Due to the uneven thickness of the air gap and the irregular shape of the longitudinal air gap in the mold, the single taper mold cannot eliminate the adverse effects of the air gap well, especially in continuous casting of low carbon steel or high-speed continuous casting. Its deficiency is particularly obvious. For this reason, with the development of high-speed continuous casting, multi-taper copper mould tube such as double-taper, triple-taper, quadruple-taper and parabolic taper have been developed. Adapted to the shrinkage of the billet shell in the longitudinal direction of the mold, the thickness of the air gap in the longitudinal direction of the mold is further reduced, and the adverse effects of the air gap are better eliminated, and the entire mold is increased.The heat transfer effect provides a guarantee for the high efficiency of continuous casting.

Taper representation method

Among them, Mt is the distance between the two opposite inner walls of the upper part of the copper mould tube, Mb is the distance between the two opposite inner walls of the lower part of the copper mould tube, and Ml is the distance between the upper horizontal plane of the copper mould tube and the lower horizontal plane of the copper mould tube. Through this formula, A mold copper tube is represented by multiple tapers.

Problems in the design and manufacture of domestic copper pipe taper

Using the taper instrument produced by Baotou Lianfang High-tech Co., Ltd. as a measuring tool, the single taper and double taper of the copper mould tube.Degrees and tapers of various curves are measured. The number of sensors in the taper meter and the specific installation position can be proposed by the user. During the measurement process, the measurement signal is processed by the computer, and the measurement curve and measurement value are directly displayed. The measuring instruments are highly integrated, easy to carry and maintain, and easy to use. The measurement accuracy of the copper tube section direction is 0.01 mm, and the measurement accuracy of the copper tube length direction is 0.5 mm. Taking the copper pipes of several domestic copper mould tube manufacturers as objects, the measurement and data analysis are carried out to provide guidance for iron and steel enterprises to choose a suitable copper mould tube.

Different copper mould tubes have different tapers on the two opposite faces

Figure 1 and Figure 2 are the measurement data of the distance between the opposite faces of the new copper tube and the old copper tube of the 180mmx220 mm billet respectively. It can be seen from the two figures that the taper of the new copper pipe is approximately a single taper, where the taper value of the two narrow faces is 0.97%/m, and the taper of the two wide faces is 0.54%/m. For a new copper tube, since the side length difference between the wide side and the narrow side is not very large, the taper of the wide side and the narrow side should be equal (of course, whether they are equal or not, there are still different opinions in the academic circle), otherwise in the continuous During the casting process, serious casting quality defects such as off-square, depression and cracks of the continuous casting billet will be caused. It can also be seen from the figure that the old copper pipe has been severely worn in the length direction, especially at the exit of the copper pipe, the wear is the most serious. This is because the billet shell shrinks more seriously when the molten steel solidifies near the liquid surface during the continuous casting process, and the taper should be larger. In the lower part of the copper mould tube, with the formation of the blank shell, the thermal resistance increases, the shrinkage of the bad shell becomes smaller, and the taper should be smaller. If the copper tube uses a single taper, the taper is a bit small for the upper part of the copper mould tube, but relatively large for the lower part of the copper mould tube, resulting in severe wear at the outlet of the copper mould tube. Therefore, with the development of continuous casting technology, when the steel type is single, multi-taper and parabolic taper copper tubes are the development direction of the mold.

Figure 1  Measurement of the distance between the inner walls of the two narrow surfaces of the 180mm×220mm copper pipe

Figure 2  Measurement of the distance between the two wide inner walls of the 180mm×220mm copper pipe

Problems existing in the shape of the inner cavity of the copper mould tube

Fig. 3 is the measurement data of the distance between the opposite faces of the new copper tube of the 300mmx340mm bloom. In order to obtain the measurement data of two points on the same surface, the taper meter is equipped with two sensors, that is, the distance between the center of the wide surface and the edge of the wide surface can be measured on the inner wall of the opposite surface. From the perspective of taper, the taper at the edge of the wide face of the copper tube is 0.998%/m, the taper at the middle of the wide face of the copper tube is 1.00%/m, and the taper of the mold at different positions on the same face is basically the same. From the shape of the inner cavity of the copper mould tube, the measured values at different positions on the two opposite sides of the copper tube are different, and the measured value at the edge is smaller than that at the center. The shape of the inner cavity of the copper tube of the mold is approximately convex, but it is different from the traditional convex mold The difference is that the cavity in the lower part of the copper mould tube is still convex, but does not gradually transition downwards to a plane, so such a copper mould tube cannot play the role of the convex copper mould tube introduced above.

Figure 3  Measurement of distance between two wide faces of 300mm×340mm bloom copper tube

Problems Existing in Double Taper of Mold

Fig. 4 is the test data of the distance between the 150mm2 billet copper tube relative to the inner wall. Three sensors are installed on the taper meter, two on the side and one on the center, which can measure the distance between three points on the opposite surface at the same time. Since the copper tube of the 150mm2 billet is symmetrical, the distance between the other two opposite surfaces can be measured by flipping the taper meter. Judging from the shape of the inner cavity of the copper mould tube, the values of the two sides are consistent, because the two measurement points on the sides are symmetrical with respect to the central point. But it can also be seen that from the top of the copper tube to the 300mm distance from the top of the copper tube, the distance between the center point is larger than that of the edge point, and the shape of the inner cavity of the copper mould tube is convex. From 300mm from the top of the copper mould tube to the outlet of the copper mould tube, the distance between the measuring points at the edge of the copper mould tube is greater than the center point, and the shape of the inner cavity of the copper mould tube is concave. This kind of copper mould tube is different from the traditional convex copper mould tube. The measured values of the lower side and center of the traditional convex copper mould tube should be consistent, that is, the inner wall of the lower copper mould tube should transition from a convex shape to a plane. In terms of the taper of the copper mould tube, after processing the measured data, it can be seen that the copper tube is a double-tapered copper mould tube, a taper from the top of the copper mould tube to 320mm from the top, and from 320mm from the top to the outlet of the copper mould tube for another taper. The taper of the side and the center is different. The taper of the upper part of the crystallizer at the center point is 6.0%/m, and the taper of the lower part is 0.5%/m. The taper of the upper part is too large, far greater than the traditional value. Such a large taper can easily cause the mold to squeeze the bad shell, resulting in breakout and corner cracks in the continuous casting process. The taper of the edge is also double taper, the taper of the upper part of the copper pipe is 3.06%/m, and the taper of the lower part is 0.47%/m, and there are also cases where the taper of the upper part is too large.

Figure 4  The measurement distance between the inner walls of 150mm2 billet copper tubes

Problems Existing in Parabolic Taper Copper Tubes

Figure 5 is the measured value of the 180mmx220mm billet mold. It can be seen from the figure that the taper of the two wide faces of the copper tube is a parabola. Figure 6 is the measured value of the distance between the two narrow faces of the copper tube. It can be seen from the figure that the copper tube has no taper between 0 and 280 mm of the copper tube. From the distance of 280 mm from the top of the copper tube to the copper tube outlet, this The taper of a section of copper pipe is a parabola. In the continuous casting process, the liquid level is generally located at 135 mm below the top of the copper tube, and there is no taper from the top of the copper tube to the liquid level, which has no effect on continuous casting, but below the liquid level, the heat flux density on the outer surface of the molten steel is relatively large. The shrinkage of the billet shell is relatively serious. If the mold has no taper, an air gap will be formed between the mold and the billet shell. The thermal resistance of the air gap is relatively large, which reduces the internal heat transfer of the molten steel, so that the temperature of the billet shell just formed has been improved Picking up can easily lead to steel breakout.

Figure 5  Measurement data of the distance between the wide faces of 180mmX220mm copper pipes

Figure 6  Measurement of the distance between the narrow faces of the 180mmX220mm billet copper tube

Problems Existing in Taper Measurement of Round Billet Copper Tubes

The taper meter for measuring the taper of the round billet mold is based on the principle of measuring the diameter of the inner cavity of the copper tube in the direction of the mold length, so the taper meter is equipped with a sensor. Figure 7 shows the measurement data of the diameter of the inner cavity of the copper tube of the∮180mm round billet copper mould tube. It can be seen from the figure that from the entrance of the mold to the outlet of the mold, the relationship between the diameter of the mold cavity and the length of the mold is approximately a linear function, so the taper of the 180 copper tube is a single taper, and the taper of the copper tube is obtained by calculation 1.2%/m. Since the round billet only shrinks in the radial direction during the solidification process, its taper should be slightly smaller than that of the same size billet.

Fig. 7  Measurement data of inner wall diameter of ∮180mm round billet copper tube

Design principle of billet copper mould tube taper

The key to the accuracy of the mold taper design lies in the determination of the heat flow of the mold and the selection of the physical property parameters of the cast steel at high temperature. The heat flow of the mold is the external condition that controls the solidification of the molten steel and the growth of the shell in the mold. Therefore, it is necessary to conduct extensive and in-depth research on the thermal response of the copper mould tube under various conditions. Test the temperature change law of the mold when various continuous casting parameters change (such as steel type, casting speed, and billet specifications), and based on this, use the inverse problem model to calculate the heat flow distribution on the contact surface between the mold and molten steel and copper mould tube temperature field.

According to the temperature field obtained from the heat transfer model of the copper mould tube, the program of the elastic-plastic deformation of the mold under high temperature is compiled by using the finite element method, and the deformation of the copper plate of the mold is calculated. For continuous casting of square billets and round billets, since copper tube molds are commonly used, the influence of uneven deformation of the mold must be considered.

The new copper mould tube taper design principle is: during casting, the mold wall under high temperature deformation should fit together with the shrinking billet shell surface. For single taper, the taper range is generally 0.7% to 1.5%/m. Generally speaking, the greater the pulling speed, the smaller the taper; for double taper, the upper taper is 1.5% to 2.8%/m, and the lower The taper is 0.4% to 0.9%/m. In addition, for different steel types, the taper of high carbon steel is generally larger than that of low carbon steel under the same conditions.

Conclusion

(1) Judging from the use of copper mould tube from different manufacturers, there are different degrees of problems in the design of the billet and round billet mold taper problems, resulting in a reduction in the service life of the mold and various defects in the quality of the slab.

(2) The manufacturing level of copper mould tube varies greatly, and the suppliers are diverse. In order to ensure the quality of the continuous casting slab, the taper of the copper tube should be measured before the new mold is used.

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