JP2002059247A - Method for continuously casting molten steel and continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method with which both of coarse granular crystal near the center and coarse columnar crystal surrounding the above crystal can be formed into fine equiaxial crystals, and a continuously cast slab having fine solidified structure, cast by using this method. SOLUTION: The method for continuously casting molten steel has the peculiarity, in which MgO-Al mixture or MgO-Ti mixture is added into the molten steel in a ladle, a tundish or a mold, and successively, a continuous casting apparatus having electromagnetic coils (3, 3') at the interval from a meniscus in the mold 6 to 10 m below the mold 6 is used, and while vibrating non- solidified molten steel 2 surrounded with the solidified shells 1 into the normal directions (4, 5) and the reverse directions (4', 5') with movable magnetic field generated with these electromagnetic coils, the cast is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for continuously casting steel. More specifically, the present invention relates to a continuous casting method of molten steel capable of forming a 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 same. is there.

[0002]

2. Description of the Related Art A cross section of an ordinary continuous cast slab includes a final solidified portion with porosity at the center thereof, a coarse granular crystal portion near the center arranged to surround the final solidified portion, and
A coarse columnar portion surrounding the coarse granular portion is observed.
If 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, the formability becomes remarkably excellent and, for example, when a slab is made into a thick plate Becomes a thick plate having excellent low-temperature toughness.

The Iron and Steel Handbook, 3rd Edition, II Iron and Steelmaking,
p. 653 shows that low-temperature casting is effective for equiaxed crystallization because the equiaxed crystal increases when the degree of superheat of molten steel is low. However, in low-temperature casting, it is necessary to set the degree of superheat of the molten metal to a temperature close to the liquidus line, and to inject it into the mold from the immersion nozzle, so that solidification abnormalities such as clogging of the immersion nozzle and Dickel formation in the mold are caused. May be invited.

Japanese Patent Application Laid-Open No. 50-23338 discloses a method in which, using an induction electromagnetic stirrer, a molten steel near a solidification interface is provided with a swirling flow that flows in one direction in which the flow velocity does not change, and columnar dendrites are divided to form columnar crystals. A technique for making an equiaxed crystal is described. However, according to the findings of the present inventors, this method has a small force to form an equiaxed crystal, and for example, it is difficult to form an equiaxed crystal.
In the case of molten steel having a content of 0.1% or less, it is difficult to make columnar crystals sufficiently equiaxed.

Japanese Patent Application Laid-Open No. 3-44858 discloses a method for improving a final solidified portion with porosity, which is a quality problem in the case of a cylindrical or prismatic billet.
An induction electromagnetic stirrer is arranged in the vicinity of the final solidification portion of m to 27 m, and a swirling flow in which the direction of the half cycle stirring time is reversed for 5 to 30 seconds is used. However, since this method is a method for improving the final solidification portion, the place where the induction electromagnetic stirrer is arranged and the reversal cycle of the swirling flow are different from those of the present invention.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a continuous casting method capable of producing a slab in which a coarse granular crystal near the center and a coarse columnar crystal surrounding it are finely equiaxed, and a casting method using the same. It is an object to provide a continuous cast slab having a fine solidified structure.

[0007]

According to the present invention, there is provided (1) a continuous casting apparatus having an electromagnetic coil between a meniscus in a mold and 10 m below the mold, in a ladle, a tundish or a mold, an MgO-Al mixture or A continuous casting method for molten steel, characterized in that a molten steel is cast while adding a MgO-Ti mixture and vibrating the molten steel in forward and reverse directions by a moving magnetic field generated by the electromagnetic coil. (2) Claim 1
The molten steel according to claim 1, wherein the forward and reverse accelerations of the vibration waves to be 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. Is a continuous casting method. Also,
(3) In the second aspect, during one cycle of the vibration time, the value of 0.1.
A continuous casting method for molten steel, wherein an acceleration stop time or a power stop time of not less than 03 seconds and not more than 0.3 seconds is provided. (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. . Also, (5) MgO-Al
The MgO content in the mixture or the MgO-Ti mixture is 30% by mass or more, the Al content is 20% by mass or more,
5. The continuous casting method for molten steel according to claim 1, wherein the i content is 20% by mass or more. Also,
(6) The amount of MgO is from 0.01% by mass to 1 based on the amount of molten steel.
The method for continuously casting molten steel according to any one of claims 1 to 5, wherein an MgO-Al mixture or an MgO-Ti mixture is added so as to be in a range of mass%. Also,
(7) A continuous cast slab having a solidified structure made fine by the continuous casting method according to any one of claims 1 to 6. (8) In the continuous casting method according to any one of the first to sixth aspects, the average equiaxed crystal grain size of the cross section of the slab is 0.1.
It is a continuous cast slab characterized by having a microstructure of 5 to 3.0 mm.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic idea of the present invention is to disperse fine oxides in molten steel and apply vibration by an electromagnetic coil to reduce the degree of superheat of the molten steel, thereby obtaining finely dispersed oxides. Is efficiently used as the nucleus for the generation of equiaxed crystals,
It is to produce fine equiaxed crystals in a slab. In order to realize this basic idea, a method of generating a fine oxide that can be a core of an equiaxed crystal in a slab and a re-dissolution of the equiaxed crystal nucleus generated from the fine oxide can be prevented. After reducing the degree of superheat of molten steel, it is important to clarify the vibration conditions of the electromagnetic coil in which the finely dispersed oxide does not aggregate or coalesce.

First, the method will be described. Al-deoxidized molten steel has a large number of Al ₂ O ₃ -based inclusions, but these inclusions are very likely to aggregate and coalesce and become coarse oxides, and thus do not effectively act as nuclei of equiaxed crystals. In contrast,
We have found that MgO-Al mixture or M
was added gO-Ti mixture of Al ₂ O ₃ inclusions Mg
By modifying to O or MgO.Al ₂ O ₃ , it has been found that fine oxides can be uniformly dispersed in molten steel, and that these oxides can easily become nuclei for the generation of equiaxed crystals. MgO-Al mixture or MgO-Ti in molten steel
When the mixture is added, MgO is reduced by Al and Ti as shown in the equations (1) and (2) to generate Mg gas. _{3MgO + 2Al = 3Mg + Al 2} O 3 (1) 2MgO + Ti = 2Mg + TiO 2 (2) The Mg gas is reduced by the Al ₂ O ₃ inclusions in the molten steel (3) and (4), MgO or MgO · Al ₂ O ₃
Generate Al ₂ O ₃ + 3Mg = 3MgO + 2Al (3) 4Al ₂ O ₃ + 3Mg = 3 (MgO · Al ₂ O ₃ ) + 2Al (4) MgO or MgO · Al ₂ O ₃ is more easily wetted by molten steel than Al ₂ O _3. These oxides are finely dispersed in the molten steel.
When continuous casting of molten steel in which fine MgO or MgO.Al ₂ O ₃ is dispersed in molten steel, fine equiaxed crystals are generated with these oxides as nuclei, and the solidification structure can be refined.

In the present invention, the Al concentration in molten steel is 0.1%
When the Al concentration exceeds this, even if the MgO—Al mixture or the MgO—Ti mixture is added, the Al ₂
O ₃ -based inclusions cannot be modified into MgO or MgO.Al ₂ O ₃ , and fine oxides cannot be dispersed in molten steel. The lower limit of the Al concentration in the molten steel is not particularly specified, but is not limited to 0.1.
If it is less than 001%, the effect of deoxidation becomes unstable,
0.001% or more is desirable.

Further, a MgO-Al mixture or MgO
It is necessary to add the -Ti mixture so that the MgO content is from 0.01% by mass to 1% by mass with respect to the amount of molten steel. If the MgO content is less than 0.01% by mass, sufficient Mg gas for reforming the Al ₂ O ₃ -based inclusions in the molten steel is not generated, and if the MgO content exceeds 1% by mass, the Mg concentration in the molten steel is increased. This is because the effect of making the solidified structure in the cast slab into a fine equiaxed crystal is lost in any case because the oxides tend to be coarsened.

MgO—Al mixture or MgO—Ti
MgO content in the mixture is 30% by mass or more, Al and T
The i content needs to be 20% by mass or more, and if it is less than that, the amount of generated Mg gas is greatly reduced in any case.
Effect of modifying al ₂ O ₃ inclusions is compromised. In addition, Mg
O-Al mixture or MgO-Ti mixture contains M
Contaminants other than gO, Al and Ti, such as CaO and Al ₂
O ₃ , SiO _2, etc. may be mixed, but relatively MgO
The content, Al content and Ti content are reduced,
Mass%, it is necessary not to be less than 20 mass%.

In addition, MgO-Al mixture or MgO
The addition of the -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.

Next, will be described. In general, in electromagnetic stirring, it is considered that in order to impart a unidirectional swirling flow to molten steel at a solidification interface, the swirling flow separates columnar dendrites and promotes 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 superheat of molten steel by electromagnetic stirring, it is necessary to increase the swirling flow velocity, in which case fine oxides become coarse due to aggregation and coalescence and function effectively as nuclei of equiaxed crystals. Disappears.

On the other hand, the present invention vibrates molten steel in front of a solidified shell by a moving magnetic field based on an electromagnetic coil to promote heat transfer between the solidified shell and the molten steel, thereby effectively reducing the degree of superheat of the molten steel. In addition, it is possible to prevent re-dissolution of equiaxed crystal nuclei generated from a fine oxide as a starting point. Here, the moving magnetic field is a magnetic field generated by moving and applying a relatively low-frequency magnetic field. Further, the vibration caused by the moving magnetic field is applied to the front surface of the solidified shell, and the flow of molten steel inside the shell is unlikely to occur. Therefore, it also has the effect of suppressing aggregation and coalescence of oxides.

When the current of the electromagnetic coil is varied in the pattern shown in FIG. 1, the vibration flow velocity of the molten steel on the front surface of the solidified shell follows with a slight decrease. 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 oscillating flow is similar to that of electromagnetic stirring, but in the forward acceleration region t2 or the reverse acceleration region t4, the solidified shell is accelerated. It has been found that acceleration is generated in the oscillating flow on the front surface, and has a very large heat transfer promoting effect as compared with the electromagnetic stirring flow at a constant speed. Due to the effect of the 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.

Further, the current of the electromagnetic coil may be a pattern in which an acceleration stop time is provided between one cycle of vibration time as shown in the pattern of FIG. 2. In this case, rapid acceleration is performed by the acceleration stop time. As a result, the effect of relaxing the meniscus is obtained. When the acceleration stop time is longer than 0.3 seconds, the acceleration time is shortened and the heat transfer promoting effect is impaired. On the contrary, when the acceleration stop time is shorter than 0.03 seconds, the meniscus stabilizing effect is impaired. In the case where it is provided, it is preferable to set the time to 0.03 seconds or more and 0.3 seconds or less.

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 cross section taken along an arrow II. . 1 in FIG.
Is a solidified shell, 2 is unsolidified molten steel, and 3, 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 cause the unsolidified molten steel 2 to oscillate in the directions of arrows 4 and 4 'as shown in FIG. 3B by a moving magnetic field based on the electromagnetic coils. In the present invention, since the molten steel in front of the solidified shell is vibrated to promote heat transfer between the solidified shell and the molten steel, it is not necessary to rotate the molten steel during the vibration as shown in FIG. Unsolidified molten steel 2)
May be vibrated in the directions of arrows 5 and 5 ′.

Although the above description has been made with reference to a slab as an example, the present invention is not limited to a slab, and similar effects can be obtained with a bloom, a billet and the like. In addition, 10
Further below m, the solidification from the surface layer of the slab to several tens of mm has already been completed, so in order to make the solidification structure of the entire slab as fine as possible, the electromagnetic coil must be 10 m below the mold from the meniscus in the mold where solidification starts. It is effective to install in the position of.

[0020]

The present invention will be described below with reference to examples and comparative examples.

Example 1 50 kg of molten steel having a carbon content of 0.11% was melted in a high-frequency melting furnace, and 60 mass%
50 g of an MgO-40 mass% Al mixture was added to obtain a molten steel of the present invention. This molten steel is 200m wide at 1600 ° C.
m, a length of 100 mm and a height of 300 mm were poured into a water-cooled copper mold. Immediately after the injection, the molten steel in the mold was solidified while vibrating in a predetermined vibration pattern. In 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 set to 100 amperes maximum and -100 amperes minimum in FIG. Coil current increase time t, which is the forward acceleration time
2. The acceleration was adjusted by adjusting the coil current decrease time t4, which is the acceleration time in the reverse direction. The shorter the time t2 and t4, the higher the acceleration, and the longer the time t2 and t4, the lower the acceleration. Further, the oscillation cycle 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.

After casting, the ingot was cut in a cross section to reveal a solidified structure, and then the equiaxed crystal equivalent diameter (average value over the entire surface) was evaluated. In addition, 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 grain, and b is the minor axis of the crystal grain. ). FIG. 4 shows the relationship between the average equiaxed grain size of the cross section of the slab and the oscillation period of the electromagnetic coil. As can be seen from FIG. 4, the average equiaxed crystal grain size of the slab to which the MgO—Al mixture is added has an absolute value of the acceleration of the vibration wave (because of the forward and reverse acceleration) of 10.
It can be seen that it becomes smaller in a region of not less than cm / s ² and an oscillation period of not less than 0.2 seconds and less than 10 seconds. The acceleration of the vibration wave was calculated from the result of measuring the flow velocity when the mold was filled with mercury and vibrating the mercury with an electromagnetic coil using a propeller current meter. This is because by setting the absolute value of the acceleration of the vibration wave to 10 cm / s ² , the superheat of molten steel was reduced, and the re-dissolution of equiaxed nuclei generated from fine oxides was suppressed. If the oscillation period is 10 seconds or more, the swirling flow due to electromagnetic stirring becomes the same flow state as simply changing the swirling direction. It is considered that, since it is difficult to follow the current change of the coil, the effect of reducing the degree of superheat due to vibration is impaired, and the equiaxed crystal grain size does not become fine as a result of the remelting of the equiaxed crystal nuclei. Therefore, in order to make the solidified structure finer, it is necessary to set the acceleration in the forward and reverse directions of the vibration to 10 cm / s ² or more, and then make the period of the vibration 0.2 seconds to less than 10 seconds.

As is 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 crystal grain size is specified to be 0.5 mm to 3 mm.

Example 2 Carbon content is 0.12% by mass
When casting a molten steel having a temperature of 1550 ° C. in a tundish into a 250 mm × 1500 mm mold at a casting speed of 1.8 m / min, an electromagnetic stirrer is disposed at a position 2 m below the mold, and the electromagnetic coil is provided with 500 A, A current having a frequency of 2 Hz was applied to form a unidirectional swirling flow of 30 cm / s at the solidification interface. Examination of the slab obtained by this conventional method revealed that the average equiaxed grain size of the cross section was 3.5 mm and the solidified structure was not refined.

On the other hand, a mixture of 60% by mass of MgO and 40% by mass of Ti was added to molten steel in a ladle having a carbon content of 0.12% by mass, and the molten steel was heated to a temperature of 1550 in a tundish.
C., at a casting speed of 1.8 m / min. At that time, an electromagnetic coil was arranged at a position 2 m below the mold, and the cycle of this electromagnetic coil current (frequency 2 Hz) was 2 seconds (maximum coil current 500A, minimum coil current -500A, coil current increase time 0.7 seconds, Current decrease 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 a forward / reverse acceleration of 80 cm / s ² . When the slab obtained by the method of the present invention was examined, the average equiaxed grain size of the cross section was 1.3 mm, and the solidified structure was refined.

[0026]

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 the formability of a thin plate is improved.
In the case of a thick plate, a material having excellent low-temperature toughness can be manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a current used for 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 view showing the relationship between the average equiaxed grain size of the cross section of the slab and the vibration period of the electromagnetic coil.

[Explanation of symbols]

1: solidified shell 2: unsolidified molten steel 3, 3 ': electromagnetic coil 4, 4', 5, 5 ': vibration direction

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 11/115 B22D 11/115 L B

Claims (8)

[Claims] In a continuous casting apparatus having an electromagnetic coil between a meniscus in a mold and 10 m below a mold, an MgO-Al mixture or a MgO-Ti mixture is added in a ladle, a tundish or a mold to form a molten steel. A continuous casting method for molten steel, wherein the casting is performed while vibrating in forward and reverse directions by a moving magnetic field generated by an electromagnetic coil. 2. The method according to claim 1, wherein the forward and backward accelerations of the vibration wave oscillated in the forward and reverse directions are 10 cm / s.
² or more, and the oscillation time of one cycle by the electromagnetic coil is set to 0.
A continuous casting method for molten steel, which is performed for at least 2 seconds and less than 10 seconds. 3. The continuous casting method for molten steel according to claim 2, wherein an acceleration stop time or a power stop time of 0.03 seconds or more and 0.3 seconds or less is provided between one cycle of vibration time. . 4. The continuous casting method for molten steel according to claim 1, wherein the molten steel is vibrated in a forward / reverse direction and a swirl flow is applied in a forward or reverse direction. 5. An MgO—Al mixture or MgO—T
i The mixture has a MgO content of 30% by mass or more,
The method for continuously casting molten steel according to any one of claims 1 to 4, wherein the content is 20% by mass or more and the Ti content is 20% by mass or more. 6. The method according to claim 1, wherein the MgO-Al mixture or the MgO-Ti mixture is added so that the MgO content is in a range of 0.01% by mass to 1% by mass based on the amount of molten steel. 5. The continuous casting method for molten steel according to any one of the above items 5. 7. A continuous cast slab having a solidified structure made fine by the continuous casting method according to claim 1. 8. An average equiaxed grain size of a cross section of a slab by a continuous casting method according to any one of claims 1 to 6.
A continuous cast slab having a microstructure of 0 mm.