This article introduces the development trend of today’s continuous casting, focusing on the key component to achieve high-speed continuous casting of billet – the crystallizer. The behavior of traditional crystallizers and the technical characteristics of several new crystallizers were analyzed, and the design ideas of high-speed continuous casting crystallizers were summarized.
Keywords: high-speed continuous casting; crystallizer; reverse taper
The two basic trends in the development of today’s continuous casting are: (1) Strive to cast a slab that is as close to the final product as possible, that is, “near net shape continuous casting”. (2) Continuously improve continuous casting productivity, and the most effective way to improve continuous casting productivity is to increase the casting speed, that is, “high-speed continuous casting.” Therefore, in recent years, high-speed casting has become one of the major issues in the development of continuous casting technology, and its research and development situation is exciting. In particular, the development of high-speed continuous casting of billet is particularly prominent.
High-speed casting is a systematic project, which includes the design and manufacturing of the crystallizer, process temperature control, formulation of the cooling system, roller arrangement, and development of mold powder, etc. But the key technology is how to make the high-temperature molten steel form a uniform and thick shell in a very short time when passing through the crystallizer, so as to withstand the static pressure of the molten steel and suppress the occurrence of steel breakouts.
Analysis of traditional crystallizer behavior
As an important component of the continuous casting machine, the crystallizer is responsible for the initial solidification of hydraulic molten steel. For traditional crystallizers, when high-temperature molten steel is poured into the crystallizer, the contact between the molten steel and the water-cooled crystallizer wall will quickly form a thin initial shell. As solidification proceeds, the shell gradually thickens. Due to the shrinkage effect, the billet shell will break away from the wall, and inside the billet shell, due to the dual effects of the reheat of high-temperature molten steel and static pressure, the billet shell will be squeezed against the wall. This disengagement and squeezing state occurs alternately and continues until the strength of the billet shell is enough to resist the static pressure of molten steel.
In the traditional crystallizer, the growth and thermal stress of the green shell are extremely uneven, which is one of the shortcomings. Second, in order to compensate for the shrinkage of the billet shell, the crystallizer is designed with an inverted taper (for example, 0.4% to 0.9% for billets). Inverse taper is a very important parameter that varies with steel type and solidification heat transfer. If the inverse taper is too small, the billet shell will break away from the copper wall prematurely, creating an air gap and reducing the cooling effect; if the inverse taper is too large, the drawing speed will be unstable, the billet will vibrate, and in severe cases, a pull-off accident will occur. The design of the traditional mold back taper is single linear and is fixed during the casting process. However, in actual production, the solidification shrinkage of molten steel in the crystallizer is non-linear, so the optimal solidification effect of molten steel cannot be obtained in the crystallizer. Third, regarding the length of the crystallizer, due to the existence of frictional resistance and the formation of the air gap in the lower part of the crystallizer, the heat transfer efficiency drops significantly (equivalent to only about 1/2 of the upper part). In this way, not only the length of the traditional crystallizer is limited, but also under the condition of a single, linear taper, an overly long crystallizer has little practical significance from the perspective of heat transfer.
From the above analysis, changing the design of the traditional crystallizer and developing a high-efficiency heat transfer crystallizer are the main measures to achieve high-speed casting.
Introduction to billet high-speed continuous casting mold
The characteristic technology of this crystallizer is that the upper copper wall of the crystallizer is surrounded by a convex shape, which gradually transitions downward to a flat surface. The angles of the upper wall are 96°, 93°, and 90° from top to bottom. The design idea is that the crystals protrude outward from the upper part of the wall and then gradually become straight downwards. This can not only reduce the air gap between the billet shell and the crystallizer, significantly increase the heat transfer efficiency, but also well balance the tensile stress generated inside the billet shell due to the longitudinal thermal gradient, so that the outlet billet shell can be evenly thickened and the drawing speed can be increased. Amplitude increased. This mold can cast a 150 square billet at a casting speed of up to 3.5 m/min. Suitable for casting plain carbon steel, high carbon hard wire steel, cold pier steel, etc.
The copper tube of the crystallizer is lengthened to 1000 mm, and a parabolic inverted taper is adopted in the length direction of the crystallizer, with zero taper in the corner area 300~400 mm from the top. The design idea is to change the traditional linear taper to a parabolic taper to better adapt to the solidification shrinkage law of molten steel in the mold, minimize the air gap, and uniformly grow the shell. At the same time, by designing the taper of the lower corner of the mold to zero, the increase in friction is effectively suppressed, making it possible to lengthen the length of the mold, and providing the necessary conditions for extending the effective residence time of the shell in the mold. This kind of crystallizer is suitable for casting spring steel, hard wire steel, cold pier steel, and high-quality carbon steel.
Its characteristic technology is that the crystallizer is composed of three parts when viewed from the axial direction.
The upper part has varying tapers in both the axial and transverse directions, and the middle of the transverse direction is convex outward. The middle part has varying tapers in the axial direction, and is square in the transverse direction. The outlet has varying tapers in the axial and transverse directions, and the middle of the transverse direction is concave inward. To compensate for the air gap caused by thermal deformation of the mold and shrinkage of the slab. This can reduce the friction in the exit corner area, make the billet shell uniform and grow quickly, thereby increasing the casting speed. Now Yongxin and Shajing are already in use and the effect is very good.
In summary, although the forms of billet high-speed continuous casting molds are different, their design ideas can be summarized in three points: (1) Reduce the air gap between the billet shell and the mold. (2) Increase or extend the effective cooling length and time of the shell in the crystallizer. (3) Improve the contact between the billet shell and the crystallizer wall.