JP4065099B2 - Method for continuous casting of molten steel and continuous cast slab - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a continuous casting method for steel. More particularly, the present invention relates to a molten steel continuous casting method capable of making the metal structure of a slab cross-section into a fine equiaxed crystal, and a continuous cast slab having a fine solidified structure cast using the molten steel. is there.
[0002]
[Prior art]
The cross section of a normal continuous cast slab has a final solidified part with porosity at its center, a coarse granular crystal part near the center arranged to surround this final solidified part, and a coarse particle surrounding the coarse granular crystal part. Columnar crystal parts are observed. When this coarse granular crystal and coarse columnar crystal can be made into fine equiaxed crystals, for example, when a slab is made into a thin plate, it becomes a thin plate with remarkably excellent moldability, and for example, when it is made into a thick plate Is a thick plate with excellent low-temperature toughness.
[0003]
“Handbook of Iron and Steel” 3rd Edition, II Steelmaking and Steelmaking, p. No. 653 shows that low temperature casting is effective for equiaxed crystallization because equiaxed crystals increase when the degree of superheated molten steel is low. However, in low-temperature casting, it is necessary to set the superheating degree of the molten metal to a temperature close to the liquidus and to inject it into the mold from the immersion nozzle, so solidification abnormalities such as clogging of the immersion nozzle and deckle generation in the mold May be invited.
[0004]
Japanese Patent Laid-Open No. 50-23338 uses an induction magnetic stirrer to give a swirling flow flowing in one direction in which the flow velocity does not change to the molten steel near the solidification interface, and by dividing the columnar dendrite into equiaxed crystals. The technology to make is described. However, according to the knowledge of the present inventors, this method has a small force for equiaxed crystallization. For example, in the case of a molten steel having a C content of 0.1% or less in which equiaxed crystals are difficult to form, columnar crystals are sufficient. It is difficult to equiaxially crystallize.
[0005]
Japanese Patent Application Laid-Open No. 3-44858 is a method for improving the final solidified portion with porosity, which becomes a quality problem in the case of a cylindrical or prismatic billet, for example, induction electromagnetic stirring near the final solidified portion of 16 m to 27 m below the mold. An apparatus is provided and a swirl flow in which the direction of a half cycle stirring time is reversed for 5 to 30 seconds is used. However, since this method is a method for improving the final solidified portion, the place where the induction electromagnetic stirring device is disposed and the reversal cycle of the swirling flow are different from those of the present invention.
[0006]
[Problems to be solved by the invention]
The present invention relates to a continuous casting method capable of producing a slab in which both coarse granular crystals in the vicinity of the center and coarse columnar crystals surrounding them are finely equiaxed, and continuous casting having a fine solidified structure cast using the same. The challenge is to provide slabs.
[0007]
[Means for Solving the Problems]
In the present invention, (1) in a continuous casting apparatus having an electromagnetic coil between a meniscus in a mold and 10 m below the mold, an MgO-Al mixture or an MgO-Ti mixture is added in a ladle, tundish or mold, Is a continuous casting method of molten steel characterized in that casting is performed while vibrating in a forward and reverse direction by a moving magnetic field generated by the electromagnetic coil. (2) In claim 1, the forward and reverse accelerations of the vibration wave oscillated in the forward and reverse directions are 10 cm / s ² or more, and the vibration time of one cycle by the electromagnetic coil is 0.2 seconds or more and less than 10 seconds. This is a continuous casting method for molten steel. In addition, (3) the continuous casting method of molten steel according to claim 2, wherein an acceleration stop time of 0.03 seconds or more and 0.3 seconds or less or a power supply stop time is provided between vibration periods of one cycle. It is. (4) The continuous casting method for molten steel according to any one of claims 1 to 3, wherein the molten steel is vibrated in the forward and reverse directions, and a swirl flow is applied in the forward or reverse direction. . (5) The MgO-Al mixture or the MgO-Ti mixture has an MgO content of 30% by mass or more, an Al content of 20% by mass or more, and a Ti content of 20% by mass or more. Item 5. A method for continuously casting molten steel according to Items 1 to 4. (6) The MgO-Al mixture or the MgO-Ti mixture is added so that the MgO amount is in the range of 0.01% by mass to 1% by mass with respect to the amount of molten steel. 5. A method for continuously casting molten steel according to 5. (7) A continuous cast slab characterized by having a solidified structure made fine by the continuous casting method of any one of claims 1 to 6. (8) The continuous casting characterized in that the microstructure is 0.5 to 3.0 mm in average uniaxial grain size of the cross section of the slab by the continuous casting method according to any one of claims 1 to 6. It is a slab.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The basic idea of the present invention is that fine oxides are dispersed in molten steel, and vibrations by electromagnetic coils are added to this to lower the superheat of the molten steel. In order to produce fine equiaxed crystals in the slab. In order to realize this basic concept, (1) a method of generating a fine oxide that can become an equiaxed crystal nucleus in a slab, and (2) an equiaxed crystal nucleus formed from a fine oxide as a starting point. It is important to clarify the vibration conditions of the electromagnetic coil in which the finely dispersed oxide does not agglomerate and coalesce while reducing the degree of superheated molten steel so that remelting of the steel can be prevented.
[0009]
First, the method (1) will be described. Although many Al ₂ O ₃ inclusions exist in Al deoxidized molten steel, these inclusions are very easy to agglomerate and coalesce and become coarse oxides, and thus do not act effectively as equiaxed crystal nuclei. In contrast, the present inventors have, MgO-Al mixture in the molten steel, or MgO-Ti mixture is added, by modifying the Al ₂ O ₃ inclusions MgO, or MgO · Al ₂ O ₃ The inventors have found that fine oxides can be uniformly dispersed in molten steel, and that these oxides are likely to become nuclei for the formation of equiaxed crystals. When an MgO—Al mixture or an MgO—Ti mixture is added to the molten steel, MgO is reduced by Al and Ti as shown in the formulas (1) and (2) to generate Mg gas.
3MgO + 2Al = 3Mg + Al ₂ O ₃ (1)
2MgO + Ti = 2Mg + TiO ₂ (2)
This Mg gas reduces Al ₂ O ₃ inclusions in the molten steel according to the formulas (3) and (4) to generate MgO or MgO.Al ₂ O ₃ .
Al ₂ O ₃ + 3Mg = 3MgO + 2Al (3)
4Al ₂ O ₃ + 3Mg = 3 (MgO · Al ₂ O ₃ ) + 2Al (4)
Since MgO and MgO.Al ₂ O ₃ are more easily wetted with molten steel than Al ₂ O ₃ , these oxides are finely dispersed in the molten steel. When a molten steel in which fine MgO or MgO.Al ₂ O ₃ is dispersed in the molten steel is continuously cast, fine equiaxed crystals are generated with these oxides as nuclei, and the solidification structure can be refined.
[0010]
In the present invention, the Al concentration in the molten steel is 0.1% or less, and if the Al concentration exceeds this, even if an MgO-Al mixture or an MgO-Ti mixture is added, the Al ₂ O ₃ inclusions are changed to MgO, Or, it cannot be modified to MgO.Al ₂ O ₃ and fine oxides cannot be dispersed in the molten steel. The lower limit of the Al concentration in the molten steel is not particularly specified, but if it is less than 0.001%, the effect of deoxidation becomes unstable, so 0.001% or more is desirable.
[0011]
Moreover, it is necessary to add the MgO-Al mixture or the MgO-Ti mixture so that the MgO content is 0.01% by mass to 1% by mass with respect to the molten steel amount. If the MgO content is less than 0.01% by mass, sufficient Mg gas will not be generated to modify the Al ₂ O ₃ inclusions in the molten steel. If the MgO content exceeds 1% by mass, the Mg concentration in the molten steel This is because the effect of making the solidified structure in the slab a fine equiaxed crystal is lost because the oxide becomes higher and the oxide becomes coarser.
[0012]
The MgO-Al mixture or the MgO-Ti mixture should have a MgO content of 30% by mass or more, and the Al and Ti contents must each be 20% by mass or more. This greatly reduces the effect of modifying the Al ₂ O ₃ inclusions. Further, MgO-Al mixture or MgO-Ti mixture may contain contaminants other than MgO, Al and Ti, such as CaO, Al ₂ O ₃ , SiO _2, etc. The ratio, the Al content, and the Ti content must be reduced so that they do not become less than 30% by mass and less than 20% by mass, respectively.
[0013]
The addition of the MgO—Al mixture or the MgO—Ti mixture is not limited to the ladle, tundish, or mold, but may be added to the molten steel in the process up to the mold.
[0014]
Next, (2) will be described. In general, in electromagnetic stirring, a unidirectional swirl flow is imparted to the molten steel at the solidification interface, and this swirl flow is considered to break up columnar dendrites and promote equiaxed crystallization, thereby reducing the degree of superheat of the molten steel. The effect is relatively small. In order to increase the effect of reducing the degree of superheated molten steel by electromagnetic stirring, it is necessary to increase the swirl flow velocity. In that case, fine oxides become coarse due to agglomeration and coalescence, and function effectively as nuclei of equiaxed crystals. Disappear.
[0015]
On the other hand, the present invention vibrates the molten steel in front of the solidified shell by a moving magnetic field based on an electromagnetic coil, promotes heat transfer between the solidified shell and the molten steel, and effectively reduces the degree of superheat of the molten steel. It is possible to prevent re-dissolution of equiaxed crystal nuclei generated from various oxides. Here, the moving magnetic field is a magnetic field generated by moving and applying a relatively low frequency magnetic field. Furthermore, vibration caused by the moving magnetic field is applied to the front surface of the solidified shell, and the internal molten steel is less likely to flow, so that it has an effect of suppressing oxide aggregation and coalescence.
[0016]
When the current of the electromagnetic coil is varied in the pattern of FIG. 1, the vibration flow velocity of the molten steel on the front surface of the solidified shell follows the current while gradually decreasing. In the region of t1 or t3 where the vibration flow velocity on the front surface of the solidified shell is constant, the effect of promoting heat transfer by the vibration flow is similar to that of electromagnetic stirring, but in the acceleration region t2 in the forward direction or the acceleration region t4 in the reverse direction, the solidified shell. It has been found that acceleration is generated in the vibration flow on the front surface, which has a much larger heat transfer promoting effect than the electromagnetic stirring flow at a constant speed. Due to the effect of acceleration induced by this vibration, heat transfer between the solidified shell and the molten steel is promoted, and the degree of superheat of the molten steel can be efficiently reduced.
[0017]
Further, the current of the electromagnetic coil may be a pattern in which an acceleration stop time is provided between vibration periods of one cycle as in the pattern of FIG. 2, in which case the rapid acceleration is mitigated by the acceleration stop time, An effect of stabilizing the meniscus can be obtained. If the acceleration stop time exceeds 0.3 seconds, the acceleration time is shortened and the heat transfer promotion effect is impaired. Conversely, if the acceleration stop time is less than 0.03 seconds, the meniscus stabilization effect is impaired. When providing, it is preferable to set it as 0.03 second or more and 0.3 second or less.
[0018]
FIG. 3 is an explanatory view of the present invention in continuous casting of a slab, (A) is an explanatory view of a longitudinal section of the apparatus, and (B) and (C) are explanatory views of a transverse section of an arrow II. In FIG. 3, 1 is a solidified shell, 2 is unsolidified molten steel, and 3 and 3 'are electromagnetic coils. The electromagnetic coils of the slab are arranged at corresponding positions on both sides of the wide surface of the slab, and the unsolidified molten steel 2 is swung in the directions of arrows 4 and 4 'as shown in FIG. In the present invention, the molten steel in front of the solidified shell is vibrated to promote heat transfer between the solidified shell and the molten steel. Therefore, it is not necessary to swivel the molten steel during vibration as shown in FIG. The solidified molten steel 2 may be vibrated in the directions of arrows 5 and 5 ′ as shown in FIG.
[0019]
Although the above description has been given by taking a slab as an example, the implementation of the present invention is not limited to a slab, and the same effect can be obtained with a bloom, billet, or the like. Further, solidification from the slab surface layer to several tens of millimeters has already been completed below 10 m below the mold, so the electromagnetic coil in the mold begins to solidify in order to make the solidified structure of the entire slab as fine as possible. It is effective to install it at a position 10 m below the mold.
[0020]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
[0021]
Example 1
50 kg of molten steel having a carbon content of 0.11% was melted in a high-frequency melting furnace, and 50 g of a 60 mass% MgO-40 mass% Al mixture was added thereto to obtain the molten steel of the present invention. This molten steel was poured at a temperature of 1600 ° C. into a water-cooled copper mold having a width of 200 mm, a length of 100 mm, and a height of 300 mm. Immediately after the injection, the molten steel in the mold was solidified while vibrating with a predetermined vibration pattern. As the vibration pattern, a moving magnetic field was formed by an alternating current having a frequency of 10 Hz, and the current of the electromagnetic coil was 100 amperes at the maximum and -100 amperes at the minimum in FIG. The acceleration was adjusted by adjusting the coil current increase time t2 which is the acceleration time in the forward direction and the coil current decrease time t4 which is the acceleration time in the reverse direction. The shorter t2 and t4, the higher the acceleration, and the longer t2 and t4, the lower the acceleration. Furthermore, the vibration period was changed from 0.2 seconds to 10 seconds by setting the minimum coil current holding time t1 and the maximum coil current holding time t3 to predetermined values.
[0022]
The steel ingot after casting was cut in a cross section, and after revealing a solidified structure, the equivalent circle diameter (average value of the entire surface) of equiaxed crystals was evaluated. The equiaxed crystal grain size was defined as 2 (a · b) ^0.5 so that the grain size in the case of columnar crystals could be evaluated simultaneously (a is the major axis of the crystal grains and b is the minor axis of the crystal grains). . FIG. 4 shows the relationship between the average equiaxed grain size of the slab cross section and the vibration period of the electromagnetic coil. As can be seen from FIG. 4, the average equiaxed grain size of the slab added with the MgO-Al mixture has an absolute value of vibration wave acceleration (because there is acceleration in the forward and reverse directions) of 10 cm / s ² or more. In addition, it can be seen that the vibration period becomes smaller in the region of 0.2 seconds or more and less than 10 seconds. The acceleration of the vibration wave is calculated from the result of measuring the flow velocity when the mold is filled with mercury and the mercury is vibrated by the electromagnetic coil with a propeller anemometer. This is because by setting the absolute value of the acceleration of the vibration wave to 10 cm / s ² , the degree of superheated molten steel is reduced, and the remelting of equiaxed nuclei generated from fine oxides is suppressed. In addition, when the vibration period is 10 seconds or more, the swirling flow due to electromagnetic stirring is in the same flow state as when the swirling direction is periodically changed, and when the vibration period is less than 0.2 seconds, the molten steel in front of the solidified shell is electromagnetic. Since it becomes difficult to follow the current change of the coil, the effect of reducing the degree of superheat due to vibration is impaired, and it is considered that the equiaxed crystal grain size was not reduced as a result of remelting of the equiaxed crystal nucleus. Therefore, in order to refine the solidified structure, it is necessary to set the acceleration in the forward / reverse direction of vibration to 10 cm / s ² or more, and further, to set the vibration period to 0.2 seconds or more and less than 10 seconds.
[0023]
Further, as apparent from FIG. 4, in the slab cast under the conditions of the present invention, the average equiaxed grain size of the cross section of the slab is improved to the range of 0.5 mm to 3 mm. For this reason, in the above (8) of the present invention, the average equiaxed grain size is regulated to 0.5 mm to 3 mm.
[0024]
(Example 2)
When casting a molten steel having a carbon content of 0.12% by mass and a temperature in the tundish of 1550 ° C. into a 250 mm × 1500 mm mold at a casting speed of 1.8 m / min, an electromagnetic stirrer was placed at a position 2 m below the mold. Then, a current of 500 A and a frequency of 2 Hz was passed through the electromagnetic coil to form a unidirectional swirling flow of 30 cm / s at the solidification interface. The slab obtained by this conventional method was investigated, but the average equiaxed grain size in the cross section was 3.5 mm, and the solidified structure was not refined.
[0025]
On the other hand, a 60 mass% MgO-40 mass% Ti mixture was added to the molten steel in the ladle having a carbon content of 0.12 mass%, and the molten steel was subjected to a tundish internal temperature of 1550 ° C. and a casting speed of 1.8 m / min. Then, it was cast into the same mold as above. At that time, an electromagnetic coil is arranged at a position 2 m below the mold, and the period of this electromagnetic coil current (frequency 2 Hz) is 2 seconds (maximum coil current 500 A, minimum coil current −500 A, coil current increase time 0.7 seconds, coil (Current reduction time 0.7 seconds, maximum current holding time 0.3 seconds, minimum current holding time 0.3 seconds), the molten steel in front of the solidified shell was vibrated under the conditions of forward and reverse acceleration of 80 cm / s ² . . When the slab obtained by the method of the present invention was investigated, the average equiaxed grain size in the cross section was 1.3 mm, and the solidified structure was refined.
[0026]
【The invention's effect】
As described above, according to the present invention, it is possible to manufacture a continuous cast slab in which the solidified structure of the slab is finely equiaxed, so that it is possible to produce a thin plate with good formability and a thick plate with low temperature toughness. An excellent material can be manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a current used in an electromagnetic coil of the present invention.
FIG. 2 is a schematic diagram of a current used in the electromagnetic coil of the present invention (when there is an acceleration stop time).
FIG. 3 is an explanatory diagram of vibration application by an electromagnetic coil.
FIG. 4 is a diagram showing the relationship between the average equiaxed grain size of the slab cross section and the vibration period of the electromagnetic coil.
[Explanation of symbols]
1: Solidified shell, 2: Unsolidified molten steel, 3, 3 ': Electromagnetic coils 4, 4', 5, 5 ': Vibration direction

Claims (8)

In a continuous casting machine with an electromagnetic coil between the meniscus in the mold and 10 m below the mold, MgO-Al mixture or MgO-Ti mixture is added in the ladle, tundish or mold, and molten steel is generated in the electromagnetic coil. A continuous casting method for molten steel, characterized by casting while vibrating in a forward and reverse direction by a moving magnetic field. 2. The forward and reverse accelerations of the vibration wave oscillated in the forward and reverse directions are set to 10 cm / s ² or more, and the vibration time of one cycle by the electromagnetic coil is set to 0.2 seconds or more and less than 10 seconds. A continuous casting method for molten steel. 3. The molten steel continuous casting method according to claim 2, wherein an acceleration stop time of 0.03 seconds or more and 0.3 seconds or less or a power supply stop time is provided between vibration periods of one cycle. The continuous casting method for molten steel according to any one of claims 1 to 3, wherein the molten steel is vibrated in a forward / reverse direction and a swirl flow is applied in a forward direction or a reverse direction. The MgO-Al mixture or MgO-Ti mixture has an MgO content of 30% by mass or more, an Al content of 20% by mass or more, and a Ti content of 20% by mass or more. The molten steel continuous casting method according to any one of the above. The MgO-Al mixture or the MgO-Ti mixture is added so that the amount of MgO is in the range of 0.01% by mass to 1% by mass with respect to the amount of molten steel. Continuous casting method for molten steel. A continuous cast slab characterized in that the solidified structure is made fine by the continuous casting method according to any one of claims 1 to 6. A continuous cast slab characterized by having a microstructure with an average equiaxed grain size of the cross section of the slab of 0.5 to 3.0 mm by the continuous casting method according to any one of claims 1 to 6.

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