JPS6358069B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides continuous casting equipment equipped with an electromagnetic stirring device in which negative segregation caused by electromagnetic stirring is always maintained at a target value regardless of changes in casting conditions. This relates to a method of controlling the current value so that the

(Prior art) Generally, in continuous casting equipment equipped with an electromagnetic stirring device, for the purpose of increasing equiaxed crystallinity and reducing center segregation,
Electromagnetic stirring is applied to the molten steel in the slab during solidification. In this case, as the stirring current increases, the molten steel thrust at the solidification front increases, resulting in effects such as an increase in equiaxed crystallinity and a reduction in the degree of segregation at the center.

However, on the other hand, by stirring the solidifying molten steel, the solidifying front receives the cleaning effect of the molten steel, and the liquid phase concentration at the solidifying front decreases. When the liquid phase concentration decreases, the solute concentration on the solid phase side at the solidification front decreases according to the equilibrium distribution coefficient of the molten steel, and as a result, the molten steel is stirred by electromagnetic stirring. A negative segregation band called the white band occurs. If the degree of segregation of the white band, that is, the negative segregation zone is large, it may impede the hardenability and mechanical properties of the steel material.

Several proposals have been made to solve this problem. For example, in the technology disclosed in Japanese Patent Publication No. 53-25533, when determining the optimum electromagnetic stirring current value, the thickness of the solidified shell at the stirring position is first estimated,
Based on this, the power supply frequency of the stirring device is determined, and the current value is determined from this value, but this method has the drawback of poor accuracy. Further, as another prior art, there is a technique shown in Japanese Patent Application Laid-Open No. 112643/1983. In this technology, when obtaining the electromagnetic stirring current value, the flow rate of cooling water,
Although it is determined based on the water temperature at the entrance and exit side of the mold, the drawing speed of the slab, and the predetermined stirring speed of the molten zone of the slab, this method is also questionable in its accuracy as described above.

(Object of the Invention) The present invention has been made in view of the above-mentioned situation, and aims to provide a method capable of accurately determining the optimum current value that changes depending on the shell thickness and controlling the current value to the optimum current value. purpose.

(Structure and operation of the invention) The present invention uses a model formula to determine the optimum electromagnetic stirring current value from the solidified shell thickness at the electromagnetic stirring position and the set value of the target white band, and based on that value, the current value of the stirring device is It is for controlling.

That is, in continuous casting using electromagnetic stirring, the solidified shell thickness in front of the electromagnetic stirring device is D, the effective distribution coefficient is K _E , the equilibrium distribution coefficient is K _P , the required current value is A, and the coefficient g is given. This is a method for controlling an electromagnetic stirring current in continuous casting, characterized by controlling the current of an electromagnetic stirring device within a range of ±15% of the current value A calculated in .

This will be explained in more detail below.

The present inventors have made the solidification equation under material flow, which has been known in the past, correspond to the operational elements of continuous casting of molten steel, and have found that good results can be obtained by organizing the solidification shell thickness D. learned. Therefore, assuming that the flow velocity U of molten steel is proportional to the square of the electromagnetic stirring thrust H, the cleaning depth at the solidification front is Sh,
When the solidification rate is f, the following formula (1) can be derived as an empirical formula for white band generation.

On the other hand, the following equation (2) holds true according to the law of conservation of matter, where the effective partition coefficient is K _E and the equilibrium partition coefficient is K _p .

K _E =1-(1-K _p )·Sh (2) Here, the effective partition coefficient K _E and the equilibrium partition coefficient K _p will be explained. Figure 1 shows the change in solute concentration at the front of solidification due to stirring of molten steel. Figure 1 a shows the case without electromagnetic stirring, and Figure 1 b shows the case with electromagnetic stirring. The meaning of each symbol in the figure. is as follows.

C _s : Solid phase concentration without stirring C _L : 〃 Liquid phase concentration C _s ′: Solid phase concentration with stirring C _L ′: 〃 Liquid phase concentration C _p : Average concentration K _p : Equilibrium partition coefficient K _E : Effective partition coefficient The effective partition coefficient K _E is the ratio C _s /C _p between the solid phase concentration C _s (or C _s ′) and the average concentration C _p , and the equilibrium partition coefficient K _p
are the solid phase concentration C _s (or C _s ′) and the liquid phase concentration C _L (or C _L ′)
It is expressed as the ratio C _s /C _L ). In addition, electromagnetic stirring thrust H
is proportional to the square of the current A, and if the current is constant, the thrust H between the electromagnetic stirring cores is determined by the position between the cores. That is, the thrust force H is determined from the shell thickness D and is obtained by the following equation (3).

H=A ²・g...(3) Here, g is the shell thickness D and the shape of the electromagnetic stirring device,
That is, it is a coefficient determined by the pole pitch and the distance between cores, usually takes a value between 1.0×10 ^-8 and 0.1, and defines the distribution of thrust H. As the pole pitch increases, the value increases, and conversely, as the pole pitch decreases, the value decreases. Further, as the distance between the cores becomes larger, the value becomes smaller, and conversely, as the distance between the cores becomes smaller, the value becomes larger. As an example, pole pitch is 722mm, distance between cores is 447mm, and shell thickness is D.
is between 30 mm and 175 mm, and a range between 2.1×10 ⁻⁵ and 1.2×10 ⁻⁶ is appropriate.

Since the white band has a one-to-one correspondence with the effective distribution coefficient K _E , the required current value A when the target white band is given is as follows from equations (1) to (3) above.
It can be found as in equation (4).

The current value A obtained above is the optimal value, but
The effect will not be impaired within a certain range, and the range is ±15%.

In the above equation (4), the shell thickness D is determined as shown in FIG. That is, first, the superheating temperature of the molten steel in the tundish, the amount of heat removed in the mold, and the amount of secondary cooling water are collected as process information. Next 2
Next, the amount of cooling water is reevaluated to the heat transfer coefficient, and the estimated shell thickness is calculated to determine the shell thickness at the electromagnetic stirring position. Since the terms other than the shell thickness D are known, that is, K _E is determined by the target white band, and K _P is determined by the steel type, the target electromagnetic stirring current A can be determined from the above equation (4).

In Fig. 2, 1 is the molten steel ladle, 2 is the tundish, 3 is the mold, 4 is the spray zone, and 5 is the molten steel ladle.
is an electromagnetic stirring device.

In this way, according to the method of the present invention, it is possible to control the electromagnetic stirring current so that the target white band is always achieved even if the operating conditions change, or in other words, the thrust at the front of the molten steel is always constant. Become.

(Example) In the continuous casting equipment shown in Fig. 2, the superheating temperature of the molten steel in the tantestine was 26℃, and the amount of heat extracted in the mold was
A shell thickness D = 71 mm in front of the electromagnetic stirring device was determined by a computer from data of 18,800 Kcal/min and secondary cooling water flow rate of 410/min. g was set to 0.8×10 ⁻⁵ based on the shape of the electromagnetic stirring device of this example. Further, when the effective distribution coefficient (ie, white band target value) is set as K _E =0.93 and the equilibrium distribution coefficient is set as K _P =0.34, the current value A is obtained from equation (4) as A = 450 amperes. Based on this value, the current of the electromagnetic stirring device was controlled. The shell thickness D is calculated at regular intervals (48 seconds), the optimum current value is calculated each time, and the current is controlled based on that value. As a result, the equiaxed crystallinity is lower than that of the conventional method. This was improved by 10%, and center segregation was reduced proportionally.

(Effects of the Invention) As explained above, by applying the present invention, the current value of the electromagnetic stirring device can be controlled to a target value, which is effective in improving quality.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the change in solute concentration at the front of solidification due to stirring of molten steel, where a shows the case without electromagnetic stirring and b shows the case with electromagnetic stirring. FIG. 2 is an explanatory diagram of the control method of the present invention. 1: Molten steel pot, 2: Tandeshi, 3: Mold, 4: Spray zone, 5: Electromagnetic stirring device.

Claims (1)

[Claims] 1. In continuous casting using electromagnetic stirring, the thickness of the solidified shell in front of the electromagnetic stirring device is D, and the effective distribution coefficient is
K _E , the equilibrium distribution coefficient is K _p , the required current value is A,
Given the coefficient g, A method for controlling an electromagnetic stirring current in continuous casting, characterized by controlling the current of an electromagnetic stirring device within a range of ±15% of the current value A calculated in .