KR20030052755A - Operating method of molten steel flow in mold - Google Patents

Abstract

PURPOSE: A molten steel flow managing method is provided to improve a surface quality of a slab by easily separating argon bubbles and intermediate material through inducing a proper molten steel flow pattern in a mold. CONSTITUTION: At least two factors are selected from the group consisting of an exhaust angle of a submerged nozzle, which determines a molten steel flow pattern, a flow rate of argon provided to prevent a nozzle from being clogged, a submerging depth of the submerged nozzle and a casting speed with respect to width of a mold. Then, remaining factors except for selected two factors are fixed. After that, a flow pattern diagram representing a single rotating current and a dual rotating current is made while varying values of the selected two factors. Then, a working condition is adjusted by selecting a proper area in the flow pattern diagram.

Description

Operating method of molten steel flow in mold

1 is a schematic diagram of a continuous casting machine. The continuous casting process is a process of supplying molten steel from the tundish 102 to the mold 104 via the immersion nozzle 103 in the ladle 101. The molten steel injected into the mold is first cooled in the mold, and solidification proceeds through a second cooling process in which water or water and air are injected together from the bottom of the mold to become a slab. At this time, the surface quality of the produced slab is determined by whether the solidification layer formed in the mold is good, in particular, it is critically affected by the formation process of the initial solidification layer on the upper part of the mold.

The molten steel injected into the mold from the immersion nozzle 103 has various factors such as the immersion depth of the immersion nozzle, the discharge angle of the immersion nozzle, the shape of the immersion nozzle, the amount of argon blown, the casting speed, the casting width, and the mold shape. Thereby forming various flow patterns in the mold. A typical pattern of the flow patterns is shown in FIG. 2.

FIG. 2 (a) shows a single roll flow in which the molten steel flow in the mold is largely causing one vortex, and (b) shows a double roll flow causing two vortices below and below. Flow).

Typical flow patterns are represented by single- and double-rotation flows. The lower the casting speed, the smaller the immersion depth of the immersion nozzle, the larger the blowing amount of argon, the larger the casting width, the easier the single rotational flow is formed. On the contrary, the double rotational flow is more likely to be formed. Therefore, the target quality can be secured by adjusting the flow pattern determination condition during continuous casting.

The flow pattern formed in the mold has a great influence on the formation of the initial solidification layer on the mold in two aspects described below.

First, argon gas and non-metallic inclusions blown to prevent clogging of the nozzle are injected into the mold together with the molten steel discharged from the immersion nozzle, and move along the flow pattern in the mold and float with the molten surface if not removed. It is collected in the coagulation layer. If argon gas is trapped in the coagulation layer, it will cause bubble defects, and if a floating non-metallic substance is trapped in the coagulation layer, it will cause slab line defects. Therefore, it is necessary to induce a flow pattern in which the blown argon gas and non-metallic inclusions are easily floated. The flow pattern is determined by the shape and operation conditions of the immersion nozzle, the shape and operation conditions of the mold, and the discharge amount.

Second, the temperature of the water surface and the flow rate of the surface of the mold are determined according to the flow pattern in the mold. The slowing of the initial solidification layer is an important factor for quality improvement, which is determined by the speed of the surface of the mold and the temperature of the surface of the mold. If the melt flow rate in the mold is extremely small, the molten steel temperature of the stagnant melt surface is lowered, resulting in a dekel, etc., in which molten steel of the melt surface is hardened, or the melt of the mold flux is poor. In addition, when the flow velocity is too high, the quality of the molten mold flux present in the molten surface may be mixed into the molten metal or collected in the initial solidification layer due to the fluctuation of the molten steel. Therefore, inducing the proper flow pattern in the mold to maintain the proper surface water flow rate and temperature is an essential element for securing the slab quality in continuous casting. The flow pattern in the mold is determined by casting width, casting speed, immersion nozzle discharge angle, immersion nozzle immersion depth, immersion nozzle shape, mold shape, argon injection amount, and so on. The proper flow pattern in the mold can be derived.

However, it is difficult to actually evaluate the effects of these various operating factors, and it takes much time in addition to economic costs. In addition, in the prior art, the focus has been mainly focused on argon injection amount, immersion nozzle operation, for example, to secure a water surface speed, and to reduce the molten steel collision speed to the short side. Accordingly, the flow pattern in the mold was controlled by improving only the shape of the immersion nozzle or applying an electromagnetic braking device. That is, by using the operating factors such as the immersion nozzle discharge angle, immersion depth of the immersion nozzle, the strength of the electromagnetic field, as a result, the water surface speed or the water surface temperature is obtained, and the operating conditions have been changed through numerous trials and errors. However, this method is trial and error due to too many manufacturing variables (discharge angle of immersion nozzle, immersion depth of immersion nozzle, immersion nozzle shape, mold shape, argon blowing amount, casting width, casting speed) that determine flow in the mold. It is almost impossible to secure the speed of the water or the temperature of the water surface, and the economic burden is large and there is a problem that takes a lot of time.

In order to solve the above problems, the present invention derives a proper flow pattern in a mold with good quality, and sets up a flow pattern diagram which presents operating conditions (values, criteria) of each operation variable to maintain the pattern. Suggest how to perform operations. By setting the value of the operating variable for the flow pattern diagram area according to the present invention to induce the appropriate molten steel flow pattern in the mold to ensure the surface speed and temperature of the mold in the mold, and to facilitate the separation of argon bubbles and inclusions, slab It provides a way to improve the surface quality.

1 is a schematic diagram of a general continuous casting machine,

2 illustrates a typical in-mold flow pattern,

Figure 3 (a) shows a flow pattern diagram according to the casting speed or the discharge amount and the argon flow rate, (b) shows a flow pattern diagram according to the immersion depth and the argon flow rate by the water model.

According to the present invention for achieving the above object, the discharge angle of the immersion nozzle, which is an element that determines the flow pattern of the molten steel, the argon flow rate injected to suppress the nozzle clogging, the immersion depth of the immersion nozzle, and the discharge amount (to the casting width Casting speed), etc., and after fixing the values of the other elements except the selected two elements, changing the selected two element values and the area in which a single rotational flow, which is the flow pattern of molten steel, appears. Creating a flow pattern diagram showing a region in which rotational flow appears, selecting a suitable region according to the casting speed on the created flow pattern diagram, and adjusting operating conditions with values of the selected element corresponding to the region. Provided is a molten steel flow pattern operating method.

The quality can be improved by setting the flow conditions in the mold having good slab quality, setting an appropriate operating area in the area of this condition, and performing the operation in the operating area.

3 shows a flow pattern diagram according to the discharge amount or the casting speed and the argon flow rate according to the present invention. The flow pattern diagram is divided into a single flow region, a double rotation region, and a transition region that transitions from the single rotation region to the dual rotation region or vice versa. The smaller the argon blowing amount, the higher the casting speed, the easier it is to form a double rotary flow, and under the opposite conditions, it is easy to form a single rotary flow. When the flow pattern in the mold forms a double rotational flow during high-speed casting, it is easy to secure the flow rate of the surface, and it is easy to separate the floating of argon bubbles and inclusions in the mold. On the other hand, induction of a single rotational flow in the mold during high-speed casting, the fluctuation of the hot water surface can cause the mold flux of the hot water surface to be mixed into the molten steel. do. Therefore, it is possible to improve the quality of the slab by operating in the work area 2 shown in Figure 3 during high-speed casting. For example, if the casting speed is located at point A, the quality is improved by selecting the most suitable argon flow for the operation.

On the other hand, in order to induce double rotation during low speed casting, the amount of argon blow should be reduced to a very low level or the depth of the immersion nozzle should be kept deep. However, this causes the blockage of the nozzles or deepens the penetration depth of the inclusions and argon bubbles, thus degrading the quality of the slab. Therefore, it is advantageous to work in the work area 1 which generates a single rotational flow during low speed casting. For example, if the casting speed is located at point B, the work may be performed by selecting the argon flow most suitable for the operation. Although the diagram of FIG. 3 shows a flow pattern diagram of the discharge amount and the argon flow rate or the casting speed and the argon flow rate, it is also possible to prepare a flow pattern diagram using other factors for determining the flow pattern.

Next, we will explain how to create a flow pattern diagram. It is practically impossible to create a flow pattern diagram by experimenting in all cases by continuously changing the values of two flow decision conditions. Therefore, the flow pattern diagram is created by the following method.

First, a flow pattern diagram is created as shown in FIG. 4 through computer simulation. In FIG. 4, the depth of the immersion nozzle and the argon flow rate were selected as flow determination conditions. The mold flow pattern is 0 for single rotational flow, 1 for double rotational flow, and 0.5 for transition region, and quantifies the flow pattern by implementing a number model. Since the goal in this experiment was to induce double rotations, the working area is the area where the double rotations begin to form.

Through the molten steel test in the similar mold, the validity of the created number model is examined. It is a material that can see the inside like acrylic and makes a real mold shape, and replaces molten steel as a liquid such as water. The regions in which the single and double rotation flows appear in the numerical model are extracted to check whether the molten steel flow pattern is valid.

With the flow pattern diagram thus created, a subsequent region can be selected according to the casting speed on the flow pattern diagram, and the operating conditions can be adjusted to the value of the selected element corresponding to the region.

After selecting two elements from the elements that generate double rotation and single rotation flow, such as immersion depth, argon flow rate, immersion nozzle discharge angle and immersion nozzle immersion depth, fix the values of the remaining elements, and then select the selected elements. Improve the surface quality of the slab by creating a flow pattern diagram showing the area representing a single rotational flow and the area representing a double rotational flow while changing the values of these fields and then selecting the optimal conditions for each operation. You can.

The technical spirit of the present invention has been described above with reference to the accompanying drawings, but this is by way of example only for describing the best embodiment of the present invention and not for limiting the present invention. In addition, it is obvious that any person skilled in the art may make various modifications and imitations without departing from the scope of the technical idea of the present invention.

Claims (2)

In the method of controlling the flow pattern of the molten steel in the mold during the continuous casting process, The discharge angle of the immersion nozzle, which determines the flow pattern of the molten steel, the argon flow rate injected to suppress the nozzle clogging, the immersion depth of the immersion nozzle, and the discharge amount (casting speed with respect to the casting width). Selecting the first step; After fixing the values of the other elements except the selected two elements, the flow pattern diagram showing the region in which the single rotational flow, which is the flow pattern of the molten steel, and the region in which the double rotational flow appears, are changed by changing the values of the two selected elements. Creating a second step; Selecting a suitable region according to a casting speed on the created flow pattern diagram, and adjusting operating conditions to a value of the selected element corresponding to the region; In-mold molten steel flow pattern operating method comprising a. The method of claim 1, wherein the preparing of the second stage flow pattern diagram comprises: A first substep of creating a flow pattern diagram through computer simulation using a number model; A second substep of validating the flow pattern diagram created in the first substep through a molten steel test in a similar mold; In-mold molten steel flow pattern operating method comprising a.