JP3149668B2 - Continuous casting equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting apparatus capable of improving the surface quality and internal quality of a slab by uniformizing the "flow state of molten metal in a mold" in continuous casting using a mold having a rectangular cross section. It is about.

[0002]

2. Description of the Related Art At present, in continuous casting of steel or the like, powder for continuous casting is poured into a mold to improve the lubrication between the mold and the slab to reduce the mechanical frictional force and to reduce the mechanical friction force. Measures have been taken to suppress heat transfer to the mold and make the slab thickness uniform. However, when using powder for continuous casting, it is known that the charged powder is easily caught in the solidified shell by the flow near the surface of the molten metal and is caught by the solidified shell, which is a major cause of deteriorating the quality of the slab. ing. In addition to the powder for continuous casting, inclusions in the tundish or from the immersion nozzle or inclusions precipitated from the molten metal are also trapped in the solidified shell, which has been a major cause of the deterioration of the slab quality. .

For this reason, in order to suppress product defects due to the above, flow control of molten metal in a mold using electromagnetic stirring is generally performed, but the flow control factors are not sufficiently understood or the control means is inappropriate. Therefore, it has been considered very difficult to obtain a sufficiently satisfactory result.

[0004] However, various proposals have been made so far to solve the above problems. Recently, a three-phase electromagnetic stirring coil has been installed in a continuous casting mold of molten steel having a rectangular cross section, and at the same time, each phase has been provided. A DC current whose current value changes periodically with a phase shift of 120 degrees is applied (it becomes a current obtained by superimposing the DC current and the AC current), and the magnetic field due to the AC component promotes the flow of molten steel near the solidification interface. At the same time, the inclusion of inclusions in the solidified shell is prevented, and the flow of molten steel in the center of the mold on the molten steel surface is suppressed by the magnetic field of the DC component, thereby preventing powder entrainment. A control method ”has been proposed (Japanese Unexamined Patent Publication No.
154623).

[0005] However, according to the study by the present inventors and others, it was found from the proposals described in Japanese Patent Application Laid-Open No. 5-154623 that "AC components and DC components generated using a three-phase electromagnetic stirring coil installed in a continuous casting mold were used. The method of controlling the flow of molten steel by means of a magnetic field does not significantly improve the effect of reducing the surface or internal defects of a slab of rectangular cross-section, and sometimes this method can cause It became clear that ripples and fluctuations were promoted, and conversely, a phenomenon that powder was easily entangled in molten steel was likely to be caused.

Accordingly, an object of the present invention is to solve the above-mentioned problems and to establish a continuous casting means capable of stably producing a slab having excellent surface quality and internal quality. Was cut.

[0007]

Means for Solving the Problems The present inventors have conducted detailed studies from various viewpoints in order to achieve the above object, and as a result, the following has been confirmed. That is, usually, when pouring molten steel into a continuous casting mold having a rectangular cross section, an immersion nozzle having two openings opened toward the short side of the mold is used. The flow velocity of the molten metal flowing out of the opening of the immersion nozzle is extremely large, and the molten metal flow flowing out of the immersion nozzle is divided into a flow going upward in the mold and a flow going downward in the mold when colliding with the short side of the mold. Here, the direction of the molten metal flowing toward the upper part of the mold is changed in the vicinity of the surface of the molten metal in the mold, and is directed toward the immersion nozzle at the center of the mold. Then, near the immersion nozzle in the center of the mold, the molten metal flows coming from the left and right sides of the mold collide, and a wave having nodes and antinodes is formed by superimposition of the waves. In addition, the molten metal flow collides with the short side and the long side of the mold, so that waves are reflected, and the reflected waves also overlap, so that the surface of the molten metal in the mold fluctuates or fluctuates in the vertical direction,
Different drifts with different flow velocities from the left and right sides (both short sides) of the mold toward the center will occur. These undulations in the molten metal surface and fluctuations in the vertical direction, as well as drifts from both short sides of the mold toward the center, promote powder entrainment and are a major cause of quality deterioration of the slab.

Therefore, to improve the quality of the continuous cast slab, A) suppression of undulation of the molten metal surface in the mold and fluctuation of the molten metal surface in the vertical direction, and B) drift of the molten metal on both sides of the immersion nozzle in the mold. It is indispensable to take measures such as suppressing the flow rate and the direction of the flow as uniform as possible, and reducing the fluctuation of the molten metal flow.
On the other hand, it is also important to increase the flow velocity of the molten metal at the solidification interface in order to prevent inclusions from being trapped in the solidified shell, and it is necessary to have a means for controlling the flow of the molten metal that satisfies such conditions.

In the method disclosed in Japanese Patent Application Laid-Open No. HEI 5-154623, as described above, the flow rate of the molten metal on the surface of the molten metal is suppressed by the DC component from the three-phase electromagnetic stirring coil, thereby reducing the undulation of the molten metal. On the other hand, while trying to promote the flow of the molten metal near the solidified shell by the moving magnetic field due to the AC component from the three-phase electromagnetic stirring coil,
Means for suppressing the flow of molten metal on the surface of the molten metal by means of a DC component while the molten metal near the solidified shell is caused to flow by the moving magnetic field due to the alternating current component from the electromagnetic stirring coil of the phase, It is difficult to control the flow of the molten metal in the vicinity of the surface of the molten metal, for example, to eliminate the drift of the molten metal.

Therefore, the present inventors have proposed the technique disclosed in Japanese Unexamined Patent Application Publication No.
As a result of further study to find a means for controlling the flow of molten metal that can obtain a remarkable action and effect far exceeding the method according to No. 54623, "in particular, a moving magnetic field type electromagnetic stirring coil is opposed to the mold wall in a specific arrangement. By simply installing it, it is possible to form a uniform and stable flow of molten metal in a continuous casting mold that is convenient for improving the quality of slabs. "

The present invention has been completed on the basis of the above findings and the like. "A continuous casting apparatus having a mold having a rectangular cross section, as shown in FIG. Opposite to both long side walls, a pair of closed loop-shaped moving magnetic field stir coils 2 such as circular, elliptical or polygonal shapes symmetrical with respect to the center of the mold long side wall 1 is disposed. , Descends to the molten metal 3 in the mold at the center of the mold
By adopting a configuration in which “ two rotational flows symmetrical with respect to the immersion nozzle 5 as a boundary ” are generated, the drift of the molten metal and the fluctuation of the molten metal surface are suppressed, and the entrainment of the continuous casting powder is prevented as much as possible. A sufficient feature is that a sufficient flow velocity at the solidified shell interface is secured to prevent inclusions from being trapped, and that high-quality slabs with sufficiently excellent surface and internal quality can be manufactured stably. are doing. FIG. 2 is a plan view for explaining the arrangement of the electromagnetic stirring coil 2 with respect to the mold. In each figure, reference numeral 4 denotes a long side wall of the mold, and reference numeral 5 denotes an immersion nozzle.

Hereinafter, the operation and effects of the continuous casting apparatus according to the present invention will be described in comparison with those of the comparison apparatus.

As described above, when injecting molten metal into a continuous casting mold, an immersion nozzle having two discharge ports is usually used, but the molten metal poured into the mold from the discharge port of this immersion nozzle is usually used. The flow moves as shown in FIG.

That is, since the immersion nozzle is arranged so that the two discharge ports face each short side 1 of the mold, the molten metal stream a ejected from the discharge ports firstly flows toward the short side 1 of the mold. Go forward. And the molten metal stream which collided with the short side 1 of the mold is
The flow is divided into a flow b toward the upper side of the mold and a flow c toward the lower side of the mold. Further, the molten metal flow b toward the upper part of the mold changes its direction in the vicinity of the molten steel surface and becomes a flow d toward the direction of the immersion nozzle 5 at the central part of the mold. Therefore, the melt flow d from the left and right in the mold toward the center collides at the position of the immersion nozzle 5,
The molten metal flow is in a very turbulent state. For this reason, the surface of the molten metal in the mold is wavy or fluctuated, and the drift of the molten metal is generated, which causes a surface defect or an internal defect in the slab.

[0014] The continuous casting apparatus according to the present invention controls the flow of the molten metal in the mold to suppress the occurrence of the above-mentioned defects. That is, as shown in FIGS. 1 and 2, two sets of closed loop shapes (circular in the figure) are opposed to both long side walls 4, 4 of the continuous casting mold and symmetrically with respect to the center of the long side wall. shape)
A moving magnetic field type electromagnetic coil 2 is installed. In this way, the moving magnetic field type electromagnetic stirring coil 2 having a circular shape (closed loop shape) facing the long side wall of the mold 2
Is provided, a circular flow parallel to the long side wall of the mold can be generated by the electromagnetic force in the molten metal in the mold.

Therefore, the natural flow of the molten metal (flow of the molten metal shown in FIG. 3) generated when no electromagnetic force is applied to the molten metal 3 poured from the immersion nozzle 5 disposed at the center of the continuous casting mold. Without opposition, a smooth flow as shown in FIG. 4 can be provided. That is, a) the jet flow a from the discharge port of the immersion nozzle, a) the jet flow a collides with the short side of the mold by the circular moving magnetic field type electromagnetic stirring coil 2 installed opposite to the long side wall of the mold. The generated ascending flow b, c) can apply an electromagnetic propulsion force or an electromagnetic deterrent to the flow d, toward the center of the mold in the vicinity of the molten metal surface, so that the respective flows can flow uniformly without generating stagnation. Become like

Of course, also in this case, the molten metal flows from the left and right sides (both short sides) of the mold collide at the position of the immersion nozzle at the center of the mold. As a result, a smooth flow toward the lower side of the mold is generated, so that the molten metal flows from the left and right sides (both short sides) of the mold can be joined at the position of the immersion nozzle to form a smooth downward flow.

That is, according to the apparatus of the present invention, two smooth rotational flows symmetrical to the left and right can be given to the molten metal in the mold by the boundary of the immersion nozzle. Since they can be the same, there is almost no deviation in the molten metal flow.

In addition, it is easy to easily control the flow velocity of the "flow d toward the center of the mold near the surface of the molten metal".
In the continuous casting apparatus according to the present invention, the collision of the molten metal stream in the mold can be prevented in this way, and the above-described smooth molten metal stream is formed. Also, it is possible to suppress the reflection and the collision, so that the ripples and fluctuations of the mold surface are significantly reduced. Therefore, the phenomenon that the continuous casting powder is caught in the molten metal is suppressed, and the deterioration of the slab quality due to the phenomenon can be prevented as much as possible.

Further, according to the continuous casting apparatus according to the present invention, the flow of the "spout flow a from the discharge port of the immersion nozzle" or the "upflow b generated by the collision of the spout flow a with the short side of the mold" is reduced. It is also possible to partially promote the flow of the molten metal without disturbing the flow, so that the molten metal can be given a flow that is fast enough that inclusions are not trapped in the solidified shell at the solidification interface, and the slab from this surface The quality improvement is also very remarkable.

As described above, the apparatus according to the present invention enables the molten metal flow in the mold to flow evenly and smoothly without causing stagnation, and furthermore, allows the molten metal to flow smoothly without impairing the uniform and smooth flow. It is also easy to control the flow of the molten metal so as to satisfy the contradictory condition of "reducing the flow velocity at the solidification interface and increasing the flow velocity at the solidification interface", and has a remarkable effect on improving the quality of the continuous cast slab.

Next, the effects of the present invention will be described more specifically with reference to examples.

EXAMPLE A continuous casting test of molten steel was carried out using a continuous casting apparatus shown in FIGS. 1 and 2 in which a circular magnetic stirring coil of a moving magnetic field type was disposed opposite to both long side walls of a mold.
The chemical composition of the molten steel used for the test was Fe-0.15wt% C-
0.27wt% Si-1.35wt% Mn-0.01wt% P-0.02wt% S
A 1600 mm wide by 270 mm thick slab was cast at a casting speed of 1.8 m / min. Further, the casting test using the continuous casting apparatus was performed for two cases, “when electromagnetic stirring was performed” and “when electromagnetic stirring was not performed”. The magnetic field strength is 400
It was set to 0 Gauss.

According to this test, "fluctuation level of molten steel in the mold", "deviation of solidified shell thickness near molten steel level", "state of inclusions in slab", and "situation of slab surface defect" Was examined for “when electromagnetic stirring was performed” and “when electromagnetic stirring was not performed”, and the results are shown in FIG.
~ 8.

FIG. 5 shows the amount of vertical fluctuation of the molten steel surface in the mold between "when electromagnetic stirring is performed" and "when electromagnetic stirring is not performed". The average value of the fluctuation of the molten steel surface when the electromagnetic stirring was not performed was as large as 6 mm, but the average value of the fluctuation of the molten steel surface was reduced to 3 mm when the electromagnetic stirring was performed. Can be confirmed to be stabilized.

FIG. 6 shows the deviation of the thickness of the solidified shell near the molten steel surface (when the electromagnetic stirring is performed) and when the electromagnetic stirring is not performed (the shell in the cross section of the shell at a position 100 mm from the meniscus). The difference between the maximum value and the minimum value of the thickness is compared with each other, but the average value of the deviation of the thickness of the solidified shell when electromagnetic stirring is not performed is 3
The average value of the deviation of the thickness of the solidified shell is 1 mm when the electromagnetic stirring is performed, and it can be confirmed that the thickness of the solidified shell is uniform by performing the electromagnetic stirring. In addition, the measurement of the solidified shell thickness in the vicinity of the molten metal surface is as follows.
Ferrosulfur was added to the molten steel during casting.

FIG. 7 shows the inclusion index (the ratio of the area occupied by inclusions per unit area, which is the ratio of the area occupied by the inclusions in the slab) when the electromagnetic stirring is performed and when the electromagnetic stirring is not performed. Although the average value of the inclusion index when the electromagnetic stirring was not performed was as large as 10, the inclusion index was large when the electromagnetic stirring was performed. Has a small average value of 2, and it can be confirmed that inclusion in the slab is significantly reduced by performing electromagnetic stirring.

FIG. 8 shows the index of occurrence of surface defects of the slab (the ratio of the total length of cracks per unit length of the slab) when “electromagnetic stirring is performed” and “when electromagnetic stirring is not performed”. Although the average value of the index of occurrence of surface defects when electromagnetic stirring was not performed was as large as 8, whereas the index of occurrence of surface defects was determined when electromagnetic stirring was performed. Has a small average value of 3, and it can be confirmed that the surface quality of the slab is improved by performing the electromagnetic stirring.

[0027]

As described above, according to the present invention, the drift of the molten metal flow in the mold is eliminated, the fluctuation of the molten metal level is suppressed, the powder for continuous casting is prevented from being entangled in the molten metal, and the molten metal is removed from the mold. It is possible to provide a continuous casting apparatus that not only can equalize the heat transfer of the jet to the jet but also can easily secure the flow velocity at the solidification interface to prevent the inclusions from being captured. Industrially useful effects such as high-quality cast slabs with excellent internal quality can be stably cast.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a configuration of a continuous casting apparatus according to the present invention.

FIG. 2 is a plan view illustrating an arrangement of an electromagnetic stirring coil with respect to a mold of the apparatus shown in FIG.

FIG. 3 is an explanatory diagram of a molten metal flow poured into a mold when electromagnetic stirring is not performed.

FIG. 4 is an explanatory view of a molten metal flow poured into a mold when electromagnetic stirring is performed by the apparatus according to the present invention.

FIG. 5 is a graph comparing the molten steel level variation in the molten steel in the mold.

FIG. 6 is a graph comparing the deviation of the thickness of the solidified shell.

FIG. 7 is a graph comparing the inclusion index in a slab.

FIG. 8 is a graph comparing occurrence indices of slab surface defects.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Short side wall of a mold 2 Electromagnetic stirring coil of a moving magnetic field type 3 Melt 4 Long side wall of a mold 5 Immersion nozzle

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-50-28430 (JP, A) JP-A-4-319905 (JP, A) JP-A-4-190949 (JP, A) JP-A-2- 235554 (JP, A) JP-A-5-154623 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/04 311

Claims (1)

(57) [Claims] 1. A pair of closed loops symmetrical with respect to the center of a long side wall of a mold, opposed to both long side walls of a continuous casting mold having a rectangular cross section.
"Immersion nozzles" , each of which is equipped with a moving magnetic field type magnetic stirring coil in the shape of a
Continuous casting apparatus so as to generate two rotating streams of the right and left symmetry with a symmetry boundary "to.