JPH0947853A - Method for stirring molten steel in continuos casting mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove inclusion in a solidified shell and to prevent the entrapment of powder on molten metal surface by controlling a ratio of Lorenz's force in the accelerating steps at the initial period and the latter period to a specific value and securing the flow speed of molten metal to a specific value. SOLUTION: A mold 1 is formed as a rectangular shape and electromagnetic force is applied to mutually facing direction along the faced long sides to stir the molten steel. Then, accelerating cores 2a at the initial period and accelerating cores 2b at the latter period are arranged around the mold 1, respectively. The ratio F2 /F1 of the Lorenz's force F2 in the accelerating step at the latter period, in which the flow directs from the inside to the short sides, with the accelerating cores 2b to the Lorenz's force F1 in the accelerating step at the initial period with the accelerating cores 2a, is controlled in the range of 0.15-0.5. Further, the flowing speed of molten steel is secured to 20-60cm/sec. By this method, the product quality can be improved.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method of electromagnetically stirring molten steel in a continuous casting mold.

[0002]

2. Description of the Related Art Conventionally, stirring is performed by applying electromagnetic force to molten steel in a continuous casting mold in order to prevent non-metallic inclusions causing surface defects from being trapped on the surface layer of the slab. Is being done. FIG. 6 is an explanatory view of the stirring flow on the molten metal level in the mold when the core of the electromagnetic stirring device is installed around the mold for continuous casting to electromagnetically stir the molten steel, and FIG. 7 is shown in FIG. A Lorentz force distribution diagram in the core height direction of the electromagnetic stirrer, and FIG. 8 is a three-dimensional explanatory view of the stirring flow of molten steel in the continuous casting mold shown in FIG.
In FIG. 6, the core 2 of the electromagnetic stirrer is installed along the long side of the mold 1, and an electromagnetic force is applied to the core 2 in the opposite direction along the facing long side. Molten steel is supplied from the nozzle 3.

On the molten steel surface in the mold 1, the stirring flows 5 and 6 are generated by the electromagnetic force in the direction of the arrow 4 in the core 1. Among the agitated flows, the agitated flow 5 is the agitated flow due to the electromagnetic force of the molten metal surface, and the agitated flow 6 is the reverse flow 6 generated by the collision rising flow in the center of the core.

Normally, as shown in FIG. 7, due to the structure of the core 2, the largest Lorentz force is generated at the center in the height direction. Therefore, as shown in FIG. 8, the flow velocity at the center in the height direction becomes faster than the flow velocity at the molten metal surface, collides with the short side to become the upflow 6a, and the upflow 6a becomes the reverse flow 6. However, the reverse flow 6 interferes with the stirring flow 5 on the molten metal surface, and as a result, flow stagnant areas A and B are generated.

However, since the inclusions are captured in the stagnation areas A and B, there is a problem that defects are concentrated in this area.

In order to solve this problem, Japanese Patent Laid-Open No. 61-14052 proposes a core structure shown in FIG. In this core structure, the length of core 2 is
It is intended to suppress the collision flow on the short side by making it shorter than the long side and applying no electromagnetic force at the end of the core 2.

Further, the core structure shown in FIG. 10 is also proposed in Japanese Patent Application Laid-Open No. 58-100955. In this core structure, an accelerating core 2c and a flow velocity maintaining core 2d having the same electromagnetic force direction and a decelerating core 2e for applying an electromagnetic force in the opposite direction are used to accelerate the flow of molten steel into an accelerating region and a maintaining region. 11 is a graph showing the flow velocity of the molten steel flow in the longitudinal direction of the mold, divided into three regions of the deceleration region. FIG. 11 is a graph showing the flow velocity of the molten steel flow in the longitudinal direction of the mold. It is said to be decelerated by an electromagnetic force in the opposite direction nearby.

[0008]

However, in the core structure disclosed in Japanese Patent Laid-Open No. 61-14052, it is necessary to move the core in the long side direction according to the mold width. Therefore, a space for moving the core is secured. Also, there is a problem that not only investment for installing the mobile device is required, but also control is difficult.

According to the method described in Japanese Patent Laid-Open No. 58-100955, the short side collision upflow at the center in the height direction becomes weak, but the flow velocity is rapidly increased by applying a brake force to the molten steel flow near a part of the corner. Is reduced. Therefore, as in the flow pattern shown in FIG. 10, stirring is performed only in the flow velocity maintaining region, and the flow velocity decreases in the acceleration region and the deceleration region. In particular, on the surface of the molten metal with a small Lorentz force, there is almost no flow at some corners of the mold under the influence of the upward flow of the pouring discharge flow, and conversely, defects are increased. Further, the power of each core needs to be changed according to the width of the mold, so that its control is complicated. Therefore, the present invention provides an electromagnetic stirring method for uniformly stirring the molten steel in the continuous casting mold.

[0010]

SUMMARY OF THE INVENTION The present invention provides a method for electromagnetic stirring of molten steel in which electromagnetic force is applied in opposite directions along opposed long sides in a rectangular continuous casting mold. The ratio of Lorenzka in the initial acceleration stage, which is directed from the short side to the inner side along the long side: F _l, and the Lorenzka in the latter acceleration stage, which directs the flow of molten steel from the inside to the other short side: F ₂ . It is an electromagnetic stirring method for molten steel, characterized in that F ₂ / F ₁ is controlled within a range of 0.15 to 0.5 to secure a flow rate of molten steel at 20 to 60 cm / s.

The present invention does not use the conventional pattern of acceleration, maintenance, and deceleration shown in FIGS. 10 and 11, but the acceleration stage is divided into two stages, and the acceleration force in the latter half is compared with the acceleration force in the first half. It weakens the force to a certain range and electromagnetically stirs,
The upper limit of the accelerating force in the latter half is the value at which the short side collision upflow of the flow in the center of the height direction of the core weakens and the stagnant area of the molten metal is eliminated, and the lower limit is near the corner due to the decrease of the stirring force of the molten metal It is a necessary condition that the flow velocity of is not decreased.

[0012]

In the core structure shown in FIG. 5, when the Lorentz force of the core 2a and the Lorentz force of the core 2b on the molten metal surface are equal, the product of the Lorentz force F ₁ of the core ₁ and the mold width L is 1 It shows that the flow velocity is proportional to the squared power.

In this state, the short side collision upflow at the center in the height direction of the core becomes an inward flow on the surface of the molten metal, which interferes with the stirring flow on the surface of the molten metal to produce a stagnant region (see FIG. 8). In other words, when considering the flow on the molten metal surface, it is necessary to consider not only the action of the horizontal section of the mold but also the three-dimensional flow caused by the action of the electromagnetic force distribution in the height direction. On the surface of the bath, a and b in FIG.
The flow velocity at each of the points c and c is the lowest at the point c, as shown by the lines a, b and c in FIG. 20 cm / s to remove inclusions by applying a flow velocity to the solidified shell surface
The above flow rate is required. On the other hand, in order to prevent powder entrapment on the molten metal surface, it must be 60 cm / s or less. However, the Lorentz force F ₁ is 0.1 ≦
When F ₁ · L ≦ 0.6 (N ^1/2 / m),
The flow velocity does not exceed the powder entrainment flow velocity of 60 cm / s at points a, b, and c, and the flow velocity of 20 cm / s required to sufficiently remove inclusions can be secured at points a and b. At the point, a stagnant area is formed and the flow velocity is 20c.
Since it is less than m / s, there is a problem that inclusions cannot be sufficiently removed at this portion.

[0014] To solve this problem, by weakening the Lorentz force F ₂ of the core 2b against the Lorentz mosquitoes F _l of the core 2a, the molten steel flow velocity impinging on the short side in the core height direction center It becomes smaller and the upward flow due to this is suppressed, and the reverse flow is weakened accordingly. As a result, a stagnant region is not generated due to the interference between the reverse flow and the stirring flow on the molten metal surface, and the flow velocity required for removing inclusions can be secured over the entire mold width. At this time, in order to eliminate the stagnation area of the molten metal surface, it is necessary to set the ratio of F _{2 /} F _l to a certain threshold value or less. In addition, in order to maintain the stirring power on the surface of the molten metal, it is necessary to set it to another threshold value or more. In this study, 0.15 ≦ F
_It is the most desirable flow to control in the range of _{2 /} F _l ≦ 0.5.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to FIGS.

FIG. 1 is a diagram showing the core structure and flow pattern of the present invention.

In FIG. 1, an initial acceleration core 2a and a late acceleration core 2b are installed around the mold 1. Lorentz force in the initial acceleration stage by the initial acceleration core 2a: F ₁ and the ratio of the Lorentz force in the latter acceleration stage: F ₂ of the latter acceleration core 2b in which the flow goes from the inner side to the short side: F ₂ / F ₂ / F
₁ is controlled within the range of 0.15 to 0.5.

[0018] FIG. 2 is a graph of the results of testing for the effect of removing inclusions in the case of weak F ₂ against F _1.
The horizontal axis represents the ratio of F ₂ to F _l , and the vertical axis represents the amount of inclusions at point c shown in FIG. 5, F _l = 0.16 N / m ³ , L = lm
This is for the case of (F ₁ · L = 0.16N ^1/2 / m). The amount of inclusions was set to 1 when no electromagnetic stirring was performed. As shown in this graph, F
_{When 2} exceeds 50% of F _l , a sufficient effect of removing inclusions cannot be obtained due to the stagnant region of the flow at point c. Also,
If it is less than 15%, the effect of removing inclusions is also reduced due to insufficient stirring force at some corners.

Therefore, it is necessary to set F ₂ within the range of 15 to 50% of F ₁ in order to eliminate the stagnation region at the point c and obtain a sufficient stirring force. As is clear from the graph, by operating in the above range, the amount of inclusions at point c can be reduced to about half or less.

[0020]

INDUSTRIAL APPLICABILITY According to the present invention, in electromagnetic stirring of molten steel, the Lorentz force in the latter acceleration stage: F ₂ is set within the range of 15-50% of the Lorentz force in the initial acceleration stage: F _l. It is possible to eliminate the stagnant zone of the molten steel flow and to obtain a sufficient stirring force. As a result, it is possible to prevent the inclusions on the solidified shell from being removed and to prevent the powder surface from being caught up in the molten metal surface. Has the effect of being peeled off.

[Brief description of drawings]

FIG. 1 is an explanatory view of a core structure and molten steel flow for carrying out the present invention.

FIG. 2 is a graph showing the relationship between F ₂ / F ₁ and the inclusion amount index.

FIG. 3 is a diagram showing changes in the flow rate of molten steel according to the present invention.

FIG. 4 is a relationship diagram between a flow velocity and (F _l·L ) ^1/2 .

5 is an explanatory diagram of flow velocity measurement points in FIG. 4. FIG.

FIG. 6 is an explanatory diagram of a stirring flow on a molten metal surface in a continuous casting mold having a conventional electromagnetic stirring device.

7 is a Lorentz force distribution diagram in the core height direction of the electromagnetic stirring device of FIG.

8 is an explanatory view of a stirring flow of molten steel in the continuous casting mold shown in FIG.

FIG. 9 is an explanatory diagram of a core structure of a conventional electromagnetic stirrer.

FIG. 10 is an explanatory view of another core structure of a conventional electromagnetic stirrer.

11 is a change diagram of the flow velocity of molten steel by the electromagnetic stirrer shown in FIG.

[Explanation of symbols]

1 mold, 2 cores, 2a initial acceleration core, 2b
Late acceleration core, 2c acceleration core, 2d flow velocity maintaining core, 2e deceleration core, 3 immersion nozzle,
4 electromagnetic force direction, 5,6 stirring flow, 6a reverse flow

Claims (1)

[Claims] 1. A method for electromagnetic stirring of molten steel in which electromagnetic force is applied in opposite directions along opposite long sides in a rectangular continuous casting mold, in which molten steel flows from one short side to the long side. initial acceleration phase of the Lorentz mosquito directing inward Te: Lorentz force F _l and late acceleration phase for directing molten steel flow from the inside to the other short side ratio of F ₂ F ₂
/ F ₁ is controlled in the range of 0.15 to 0.5, and the flow rate of the molten steel is secured at 20 to 60 cm / sec.