JP4591456B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a method for continuous casting of steel.

  In recent years, the demand for improving the quality of steel products, particularly steel sheets for automobiles, has become strict, and the demand for high-quality slabs with excellent cleanliness has increased from the slab stage. Slab defects include those caused by inclusions and bubbles, and those caused by segregation of components in the molten steel, and the flow in the mold is closely related to these, so many studies and inventions have been made. It was. As one of them, a flow control method in a mold using a magnetic field is considered.

  For example, two upper and lower magnetic poles facing each other across the mold long side are arranged on the back side of the mold long side, and a magnetic field in which a DC static magnetic field and an AC moving magnetic field are superimposed on the magnetic pole arranged on the lower side or the upper side A method of controlling molten steel flow in a mold is disclosed in which a DC static magnetic field and an AC moving magnetic field are superimposed on a magnetic pole arranged on the magnetic pole, and a DC static magnetic field is applied to the magnetic pole arranged on the lower side (for example, patent document) 1).

  There is also known an apparatus for controlling molten steel flow in a mold by flowing appropriate linear driving AC current and braking DC current to a plurality of installed electric coils (see, for example, Patent Document 2).

  There is also a flow control technique in the mold in which an AC moving magnetic field whose phase is shifted by 120 degrees and a DC static magnetic field are superimposed (see, for example, Patent Document 3).

  There is an electromagnetic stirring method in which a static magnetic field is applied to a position surrounding the molten steel flow discharged from the immersion nozzle, the flow velocity is lowered, and an electromagnetic stirring device is installed at a position downstream of the static magnetic field (see, for example, Patent Document 4). .

  A magnet (400 to 2000 gauss) that forms a moving magnetic field is installed on the upper part of the mold, and the moving magnetic field is applied to the ascending and reversing molten steel flow that flows through the meniscus surface layer, and a magnet that forms a static magnetic field 500 mm or more below the meniscus (1000 A casting method and an apparatus are shown in which a static magnetic field is applied to a molten steel flow in which the discharged molten steel flow falls upon hitting the short side of the mold (see, for example, Patent Document 5).

  In addition, there is a casting technique in which a moving magnetic field is applied by a magnet having a core center between the molten metal surface and a discharge hole (downward 50 degrees or more) and a static magnetic field is applied by a magnet having a pole core disposed below the immersion nozzle. (For example, refer to Patent Document 6).

  A magnet for electromagnetic stirring is installed above the lower end of the immersion nozzle, a magnet that can apply a moving magnetic field and a static magnetic field is installed below the lower end of the immersion nozzle, and casting uses a static magnetic field and a moving magnetic field according to the steel type and casting speed. There is also a method (see, for example, Patent Document 7).

  When casting steel while blowing Ar gas, a static magnetic field with a magnetic flux density of 0.1 Tesla or more is applied to the molten steel flow immediately after coming out of the immersion nozzle, and continuously stirred or stirred by an electromagnetic stirring device at the top. A technique for periodically changing the direction is known (for example, see Patent Document 8).

  An electromagnetic brake is disclosed in which a static magnetic field is applied by an electromagnet having substantially the same length as the long side that is installed facing the long side of the mold (see, for example, Patent Document 9).

  A magnetic pole that is bent or inclined toward the upper side of the mold from a predetermined position inside the mold width center or the short side of the mold to the vicinity of both ends is installed at the center of the width below the submerged nozzle discharge hole. There is also a method for controlling the flow of molten steel in a mold by applying a magnetic field (see, for example, Patent Document 10).

  A meniscus flow rate is controlled to 0.20 to 0.40 m / s by applying a DC magnetic field having a substantially uniform magnetic flux density distribution in the mold thickness direction over the entire width of the mold and controlling the discharge flow from the immersion nozzle. Techniques are disclosed (for example, see Patent Document 11).

  Disclosed is a casting method for controlling the flow of molten steel by applying a static magnetic field by applying a direct current to a plurality of coils provided below the submerged nozzle discharge hole or by applying a moving magnetic field by applying an alternating current. (For example, refer to Patent Document 12).

  Moreover, when controlling the molten steel flow in a casting_mold | template by electromagnetic induction, there also exists a technique which applies the static alternating current magnetic field of frequency 1-15Hz to molten steel (for example, refer patent document 13).

  In a slab continuous casting machine, a technique (M-EMS) for obtaining a horizontal molten steel swirl flow along a mold wall by electromagnetic stirring is disclosed (for example, see Non-Patent Document 1).

  A technique for braking (EMLS) or accelerating (EMLA) the discharged molten steel flow by applying an AC moving magnetic field to the discharged flow from the immersion nozzle is disclosed (for example, see Non-Patent Document 2).

  It has a static magnetic field arranged to control the molten steel current supplied into the mold on the long side of the mold, and a moving magnetic field generator is arranged on the upper side to flow from the horizontal section center to the short return side And a structure above the mold (see, for example, Patent Document 14).

  In addition, an electromagnetic stirrer is installed at the upper part of the mold and an electromagnetic brake is installed at the lower part of the mold to control the discharge flow from the immersion nozzle (see, for example, Patent Document 15).

  In continuous casting, using a static magnetic field on the molten steel surface in the continuous casting mold, using a straight nozzle as the nozzle for continuous casting, using a traveling magnetic field at the discharge port, and using a static magnetic field below it, control the molten steel flow in the mold A technique is disclosed (for example, refer to Patent Document 16).

  There is a steel casting method in which a static magnetic field and a high-frequency magnetic field are superimposed on the entire width direction by a magnet placed on the submerged nozzle discharge hole and a static magnetic field is applied by a magnet placed below the discharge hole. (For example, refer to Patent Document 17).

  During continuous casting of slabs, a uniform static magnetic field in the mold thickness direction is applied to the entire width of the slab outside the mold, acting on the upper and lower parts of the submerged nozzle discharge hole, giving effective braking force to the molten steel discharge flow, and making the flow uniform There is also a technique to make (see, for example, Patent Document 18).

  In addition, by applying a low-frequency AC static magnetic field that does not move with time and exciting low-frequency electromagnetic vibrations on the solidification front surface, the columnar dendrite on the solidification front surface is broken and suspended in the molten metal. A technique for reducing crystallization and center segregation is described (for example, see Patent Document 19).

In addition, a DC magnetic field and a fixed AC magnetic field are superimposed and applied in the casting thickness direction, and these magnetic fields are applied from a pair of magnetic fields disposed above or further below the submerged nozzle discharge port. The technique whose frequency is 0.01-50 Hz is shown (for example, refer patent document 20).
JP-A-10-305353 (page 3, left column, lines 16 to 32) Japanese Patent No. 3067916 (page 3, left column, lines 40 to 48) JP-A-5-154623 (right column on page 2, line 5 to line 11) JP 61-193755 (page 2, upper left column, line 18 to lower right column, line 43) Japanese Patent No. 2610741 (right column on page 2, line 35 to line 47) JP-A-6-226409 (page 2, left column, line 34 to right column, line 4) JP-A-9-262651 (page 2, right column, lines 19 to 26) JP 2000-271710 A (right column on page 2, line 13 to line 18) JP-A-3-258442 (page 3, upper right column, lines 14 to 19) JP-A-8-19841 (page 3, left column, lines 1 to 5) International Patent Publication No. W095 / 26243 (page 5, column 18 to 21) JP-A-9-262650 (2nd page, right column, lines 24 to 41) JP-A-8-19840 (page 2, right column, lines 37 to 40) Japanese Patent Laid-Open No. 61-140355 (2nd page, lower right column, lines 5 to 19) JP 63-119959 A (page 2, lower left column, line 20 to lower right column, line 6) Japanese Patent No. 2856960 (right column on page 2, line 11 to line 23) JP-A-6-190520 (page 2, right column, lines 36 to 44) Japanese Patent Laid-Open No. 2-284750 (the second page, lower right column, lines 7 to 12) JP-A-2-274350 (page 2, upper right column, line 13 to lower left column, line 13) Japanese Unexamined Patent Publication No. 2000-326054 (page 3, left column, lines 10 to 28) Iron and steel 66 (1980), PS797 (page 1, line 5 to line 6) Materials and Processes vol 3 (1990), P.I. 256 (first page line 25 to line 30)

  As described above, there are various types of flow control in the mold using a magnetic field. However, due to the recent increase in surface quality needs and the demand for cost reduction, better slab surface quality improvement technology is desired. Effective mold flow control is required. An object of the present invention is to provide a continuous casting method obtained by improving the in-mold flow control technique.

  In the method of the present invention, the flow velocity distribution in the thickness direction of the mold is regulated. That is, near the center of the thickness, the flow velocity is reduced and the entrainment of mold flux is suppressed, while the flow velocity at the solidification interface close to the mold wall surface is increased to provide a cleaning effect on the bubbles and inclusions, thereby capturing the solidification nuclei. Suppress.

  As a means for this purpose, it is necessary to devise an AC magnetic field application mode, and model experiments and simulation calculations were performed. As a result, the following conclusions were obtained.

  (1) Macro flow caused by a moving magnetic field suppresses trapping of bubbles and inclusions at the solidification interface, but sometimes increases the entrainment of mold flux, which may deteriorate the quality.

  (2) When the position where the oscillating magnetic field is strongly received is fixed when applying the oscillating magnetic field, there may be a portion where the trapping of bubbles and inclusions cannot be sufficiently suppressed at the position where the electromagnetic force is weak.

  (3) For this reason, it is effective to move the peak position of the Lorentz force by the oscillating magnetic field.

  (4) In order to move the peak position of the Lorentz force, the phases of three adjacent coils or coil groups may be set so that the phase of the middle coil is the last. Here, the oscillating magnetic field refers to a magnetic field in which the direction of the Lorentz force is reversed with time.

The above (4) will be described below. An oscillating magnetic field is applied to each of the comb-like coils as shown in FIG. 1, and the phase is changed for each coil. 2-5 is explanatory drawing of the phase provided for every such coil. The numbers attached to the sides of each of the oscillating magnetic field generating coils 31a and 31b in the figure indicate the phase angle (degree) of the current of the coil at a certain time. 2 to 5 show the case of two-phase alternating current, FIG. 2 shows a moving magnetic field, FIG. 3 shows an oscillating magnetic field, and FIGS. 4 and 5 show examples in which the peak position of the oscillating magnetic field is moved. As shown in FIG. 4 and FIG. 5, three or more electromagnets are arranged in the mold long side width direction of the continuous casting mold, and the phase of the current applied to the adjacent electromagnets increases or decreases in one direction. However, by setting at least the middle phase to be delayed from the phases on both sides, the magnetic field does not move in one direction but moves while vibrating. As described above, vibrations can be obtained by providing an array portion in which the phase of three or more adjacent coils is n , 3n, 2n (where n is 90 ° for two-phase alternating current and 60 ° or 120 ° for three-phase alternating current). The peak position of the magnetic field can be moved.

  Here, when an oscillating magnetic field is simply induced, there are places where the amplitude of the oscillating magnetic field is large and small. By moving this peak position, it is possible to clean the solidification interface at all positions.

  Here, an example in which the number of comb teeth of the coil is 12 is shown, but the number of comb teeth can be selected from 4, 6, 8, 10, 12, 16 and the like, and the alternating current is two-phase, Any of three phases may be used.

  In the magnetic field in the thickness direction as shown in JP-A-6-190520 (Patent Document 17), the Lorentz force is concentrated on the solidification interface or the molten steel surface by utilizing the skin effect of the alternating current. Then, the Lorentz force cannot be concentrated efficiently. For this reason, it is important to concentrate the electromagnetic field on the solidification interface, and it is necessary to control the magnetic field line distribution.

  As a means for this, it has been found that it is effective to arrange and alternate electromagnets whose phases are alternately reversed in the width direction. When vibrating in the thickness direction, the electromagnetic force cannot be concentrated on the mold wall surface, that is, the solidification interface, and thus the magnetic field needs to be vibrated in the width direction. Here, the phase of the current that is passed through the alternating electromagnets needs to be substantially reversed, and for this purpose, the alternating phase differs by 120 degrees or more, and three or more comb-like coils are formed in the width direction, and By inverting the phase of adjacent coils, the magnetic field in the width direction can be oscillated.

  Next, by superimposing a static magnetic field, the B term in the formula F = J × B is increased, so that the Lorentz force can be increased. However, the direction of the Lorentz force does not superimpose a static magnetic field. Unlike the above, the flow state of the molten steel also changes, and the flow in the width direction and the casting direction increases, so that it is possible to expect a cleaning effect of bubbles and inclusions trapped at the solidification interface. Moreover, by superimposing the AC magnetic field and the DC magnetic field, the flow rate of the molten metal at the center of the thickness can be reduced, and the entrainment of mold flux can also be prevented.

  If the frequency of the alternating current at this time is too low, macro flow is excited, and if it is too high, the molten steel does not follow the electromagnetic field, so it was found that the range of 1 Hz to 8 Hz is appropriate. Furthermore, in order to concentrate the electromagnetic field on the solidification interface, the coil structure is important. In particular, it is effective to optimize the distance between the magnetic poles of the coil and the distance to the molten steel. Furthermore, in the technique disclosed in Japanese Patent Application Laid-Open No. 2-274350 (Patent Document 19), the dendrite breaks and changes from a columnar crystal structure to an equiaxed crystal structure. In ultra-low carbon steel and the like, only the columnar crystal structure becomes easier to control as a texture during rolling, so that there is a problem that it is difficult to align the crystal orientation by equiaxed crystallization. Therefore, the upper limit of the intensity | strength of an alternating oscillating magnetic field and a direct current magnetic field is required.

  Therefore, in the method of the present invention, by applying a DC magnetic field in the thickness direction while oscillating the magnetic field in the width direction, a large flow can be induced in the melt in the mold width direction and the casting direction, which is greatly different from the conventional method. Therefore, the above actions can be expected.

  According to the present invention, it is possible to cast slabs with less trapped bubbles, non-metallic inclusions and slab surface segregation, surface defects caused by mold flux and internal inclusions, and high-quality metal products can be manufactured. became.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a plan view of a steel continuous casting mold 21 and an arrangement example of electromagnets 31 and 32.

  The casting mold 22 is suspended from the bottom of the upper tundish and is immersed in the casting mold 21 to supply molten steel 23. An oscillating magnetic field generating coil 31 and a DC magnetic field generating DC coil 32 in which electromagnets (coils) 1a to 12a and 1b to 12b are arranged are arranged outside the long side of the continuous casting mold 21. An oscillating current that generates an oscillating magnetic field is added to each of the electromagnets (coils) 1a to 12a and 1b to 12b, and the peak value of the oscillating current is applied so as to move along the long side width direction of the mold. This movement is applied so that the phases of adjacent coils have an array portion of n, 2n, n or n, 3n, 2n.

  2 to 5 show the distribution of the phase of the oscillating magnetic field of the coils 1a to 12a and 1b to 12b at a certain moment by describing them with numbers (phase angle values). The peak position of the oscillating magnetic field sequentially moves in the direction along the long side of the mold 21. FIG. 2 shows a two-phase alternating moving magnetic field in which the adjacent coils have a phase difference of 90 degrees and differ by 180 degrees between the opposing coils 31a and 31b. In FIG. 3, the phase difference between adjacent coils is 180 degrees, and a two-phase alternating vibration magnetic field having the same phase is applied to the opposing coils 31 a and 31 b. In FIG. 4, a half-wave rectified two-phase alternating current having a phase difference of 90 degrees between adjacent coils and 180 degrees different between the opposing coils 31a and 31b is applied. In FIG. 5, a half-wave rectified two-phase alternating current having a phase difference of 120 degrees between adjacent coils and 60 degrees different between the opposing coils is applied.

  By the method of the present invention, it is possible to effectively vibrate only the solidification interface and suppress the trapping of bubbles and inclusions, so that the surface quality of the slab can be greatly improved.

  About 300 tons of molten steel was melted in a converter and made into an ultra-low carbon steel aluminum killed steel by RH treatment, and a slab was cast by a continuous casting machine. Table 1 shows typical molten steel components. The width of the slab was 1500 to 1700 mm, the thickness of the slab was 220 mm, and the throughput of the molten steel was in the range of 4 to 5 ton / min. As the coil structure, a comb-like iron core divided into 12 parts in the width direction as shown in FIG. 1 was used and arranged so as to generate a magnetic field whose phase was alternately reversed in the width direction of the mold 21. The maximum magnetic flux by the AC magnetic field was 1000 gauss. Table 2 summarizes the experimental conditions and experimental results. The code | symbol of the coil pattern in Table 2 is as follows.

A: n, 2n, n (Example)
B: n, 3n, 2n (Example)
C: 0, n, 2n, 3n (comparative example)
D: 0, 2n, 0, 2n (comparative example)
However, n is a phase angle, n = 90 degrees in a two-phase alternating current, and n = 60 degrees or 120 degrees in a three-phase alternating current.

The surface segregation of the slab was etched after slab grinding, and the number of segregations per 1 m ² was examined by visual observation. Moreover, the surface defect of the coil after cold rolling was visually inspected, and after collecting a defect sample, the number of defects due to the mold flux was investigated by analyzing the defect portion. The amount of inclusions was measured after the inclusions were extracted by the slime extraction method from the 1/4 thickness position of the slab. The surface segregation, mold flux defects, and amount of inclusions were indexed with 10 being the worst of all conditions, and displayed as a linear ratio.

  As can be seen from Table 2, by applying an oscillating magnetic field, surface segregation, defects due to mold flux, bubbles, and non-metallic inclusions can be reduced.

  If the intensity of the oscillating magnetic field is too strong, the flux entrainment on the surface of the molten metal will increase and the surface quality will deteriorate, and if the frequency is too high, the molten metal will not be able to follow the magnetic field and the solidification interface cleaning effect will be reduced. However, bubbles and inclusion defects are increasing.

It is the plane schematic diagram which showed the coil and casting_mold | template by this invention. It is the schematic diagram which showed the phase of the coil. It is the schematic diagram which showed the phase of the coil. It is the schematic diagram which showed the phase of the coil. It is the schematic diagram which showed the phase of the coil.

Explanation of symbols

1a to 12a, 1b to 12b Coil 21 Mold 22 Nozzle 23 Molten steel 31a, 31b Coil for generating oscillating magnetic field (electromagnet)
32a, 32b DC coil (electromagnet)

Claims (2)

The electromagnets having an arrangement portion in which the phase of three or more adjacent coils is n, 3n, 2n are arranged along the mold long side direction of the continuous casting mold, and the peak position of the oscillating magnetic field is generated while generating the oscillating magnetic field. A continuous casting method of steel, wherein the steel is moved along the long side direction.
However, n = 60 ° or 120 ° for three-phase alternating current and n = 90 ° for two-phase alternating current. 2. The steel continuous casting method according to claim 1, wherein a DC magnetic field is superimposed on the oscillating magnetic field .

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