JPH11320050A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cast piece in which both of coarse granular crystal near a center part and coase columnar crystal surrounding this coarse granular crystal are finely formed as equi-axed crystal by making the specific content of Mg contain in molten steel by a ladle, etc. and casting it while alternately and periodically supplying currents generating a revolutional flow in the normal direction and a revolutional flow in the reverse direction in the horizontal surface to the molten steel in an induction electromagnetic device. SOLUTION: At the time of making 0.002-0.01% Mg contain in molten steel, many fine oxides of Mg uniformly dispersed in the unsolidified molten steel become nuclei for generating the crystal respectively and coarse granular crystal at the center part of a cast piece is changed into fine equi-axed crystal. However, coarse columnar crystal existing on the outer layer thereof is not changed into the fine crystal nor the equi-axed crystal. Therefore, the columnar crystal is changed into the fine crystal and the equi-axed crystal by using a revolutional flow being peripodically changed in the normal direction and in the reverse direction to the molten steel during solidifying. The period of the revolutional flow in the normal direction and in the reverse direction is desirable to be 0.1-1.0 times/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for continuously casting steel. The cross section of a normal continuous cast slab has a final solidified part with porosity at the center, a coarse grain crystal part near the center arranged to surround the final solidified part, and a coarse solid part surrounding the coarse granular crystal part. Columnar crystal parts are 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 with excellent low-temperature toughness. The present invention more specifically
The present invention relates to a continuous casting method capable of converting the coarse granular crystals and columnar crystals into fine equiaxed crystals.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. 50-23338 discloses a method in which a molten steel near a solidification interface is provided with a swirling flow flowing in one direction in which the flow velocity does not change by using an induction electromagnetic stirrer, and columnar dendrites are divided. It describes a technique for making crystals equiaxed. However, according to the findings of the present inventors, this method has a small force for equiaxed crystallization, and for example, C
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 solidification 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 part 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 solidified portion, the place where the induction electromagnetic stirrer is disposed is near the final solidified portion, and the meniscus in the mold to the lower part of the mold 10.
There is no arrangement between m.

[0004]

SUMMARY OF THE INVENTION The present invention provides a continuous casting method capable of producing a slab in which both coarse granular crystals near the center and coarse columnar crystals surrounding the same are finely equiaxed. Is an issue.

[0005]

According to the present invention, there is provided (1) a continuous casting apparatus having an induction electromagnetic stirrer between a meniscus in a mold and 10 m below the mold. 0.0002 to 0.01%, and the induction electromagnetic stirrer alternately cycles the current for generating a forward swirling flow and the current for generating a reverse swirling flow in a molten steel in a horizontal plane. It is a continuous casting method characterized by casting while flowing in a continuous manner.

(2) One cycle of the current for generating the swirling flow in the forward direction and the current for generating the swirling flow in the reverse direction is 0.1.
The continuous casting method according to the above (1), wherein the casting speed is 1.0 to 1.0 times / second.

In the present invention, Mg is added to molten steel in an amount of 0.0002 to
0.01% is contained. According to the findings of the present inventors,
When the molten steel contains Mg, the coarse granular crystals near the center of the slab become fine equiaxed crystals. The reason for this is not necessarily clear, but is presumed to be as follows. That is, when the slab is solidified, the solidified layer releases oxygen into the unsolidified molten steel. As a result, oxygen is concentrated in the unsolidified molten steel with the progress of solidification. The Mg contained in the molten steel becomes a large number of fine oxides having a size of 0.01 to 10 μm due to the concentrated oxygen, and is uniformly dispersed during the unsolidification. Since there is not a sufficient number of nuclei in the unsolidified molten steel containing no Mg, the vicinity of the center of the slab has a coarse granular structure, but by containing Mg, it can be uniformly distributed in the unsolidified molten steel. Each of the above-mentioned many fine oxides of dispersed Mg serves as a nucleus for crystal formation. As a result, the coarse granular crystals near the center of the slab become fine equiaxed crystals by adding Mg to the molten steel.

If the Mg content is less than 0.0002%, the density of generation of fine deoxidation products is low, and the effect of converting coarse granular crystals into fine equiaxed crystals is small. On the other hand, if it exceeds 0.01%, large inclusions and cluster-like inclusions are likely to be formed. In order to prevent the generation of large inclusions and cluster-like inclusions, it is desirable to reduce the content of Al in the molten steel.

According to the findings of the present inventors, when molten steel is made to contain Mg, coarse granular crystals near the center become fine equiaxed crystals, but coarse columnar crystals existing in the outer layer become extremely fine. Also, there is no equiaxed crystallization. The reason for this is not clear, but is considered to be due to the following reason.
That is, the columnar crystals are generated at an early stage of solidification. In the early stage of solidification, the amount of solidification is small, so that the amount of oxygen released into the unsolidified molten steel is small, and the amount of unsolidified molten steel is large. For this reason, the concentration of oxygen in the unsolidified molten steel is insufficient, so that the power of converting Mg contained in the molten steel into a fine oxide is small, and therefore, it is necessary to generate a large number of oxides that serve as nuclei for crystal formation. I can't.

FIG. 5 is an explanatory view of induction electromagnetic stirring in continuous casting of a slab, (A) is an explanatory view of a vertical section of the apparatus, and (B) and (C) are explanatory views of a cross section taken along line II. It is. In the figure, 1 is a solidified shell, 2 is unsolidified molten steel, and 3, 3 ′ are induction electromagnetic stirring devices. The induction electromagnetic stirrer for the slab is arranged at corresponding positions on both sides of the wide surface of the slab, and forms a moving magnetic field of a linear motor type.
The unsolidified molten steel 2 is moved in the directions indicated by arrows 4 and 4 '. Although the swirling flow is formed by the 4, 4 ', in this specification, the clockwise swirling flow formed by the 4, 4' is abbreviated as a forward swirling flow.

The induction electromagnetic stirrer of the present invention also has a
As shown in (C), the unsolidified molten steel 2 can be moved in the directions of arrows 5 and 5 '. Although 5,5 'leads to the formation of a swirling flow, in the present specification, the counterclockwise swirling flow formed by 5,5' is abbreviated as a reverse swirling flow. In normal induction electromagnetic stirring, the molten steel 2 is stirred by the swirling flow flowing in one direction without changing the direction of the swirling flow during casting. On the other hand, in the present invention, the columnar crystal is made equiaxed by frequently switching the swirling flow in the forward direction and the reverse direction.

In the present invention, columnar crystals are made equiaxed by generating a swirling flow that periodically changes in the forward and reverse directions in the molten steel during solidification. The usual swirling flow of molten steel formed by an induction electromagnetic stirrer is a swirling flow that always flows in one direction at a flow rate of about 20 to 100 cm / sec. In this swirling flow, columnar crystals must be sufficiently equiaxed. Is difficult. On the other hand, when a swirling flow in which the swirling direction periodically changes in the direction opposite to the normal direction is used, the columnar crystals are refined and are equiaxed. According to the knowledge of the present inventors, the cycle of the swirling flow in the forward direction and the reverse direction is 0.1
In the case of １．０1.0 times / sec, the columnar crystals are particularly refined and are equiaxed.

It is said that when a swirling flow is formed in molten steel, dendrite which causes columnar crystals is cut off, and the degree of superheat of the molten steel is reduced. As a result, the columnar crystals are said to be refined and become equiaxed. The present inventors have investigated a cross section of a cast slab manufactured using a conventional one-way swirling flow. At this time, the dendrite is inclined but is often not divided. From this result, it is considered that the effect of cutting the dendrite is small in the conventional one-way swirling flow, and the equiaxed crystallization of the columnar crystals is mainly achieved by lowering the degree of superheat of the molten steel. On the other hand, the cross section of the slab that has generated the swirling flow in which the direction alternately and periodically changes in the forward direction and the opposite direction has no inclined dendrites, and most of them are separated. From these results, when the swirl flow of the present invention, in which the direction is alternately and periodically changed in the forward direction and the reverse direction, is used, both the effect of dividing the dendrite and the effect of reducing the degree of superheat of the molten steel reduce the fineness of columnar crystals. It is supposed that the formation and equiaxed crystallization remarkably proceeded.

Although the above description has been made with reference to a slab as an example, the present invention is not limited to the slab, and similar effects can be obtained with a bloom, a billet and the like.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention melted 50 kg of molten steel having a carbon content of 0.11%, in which an equiaxed crystal was difficult to be formed, in a high-frequency melting furnace, and added Mg in an amount of 0.003 to 0. 0.005%
Molten steel of the present invention was included. In addition, M
The molten steel of Comparative Example which did not contain g was prepared. The molten steel of the present invention and the molten steel of the comparative example were not
It was poured into a water-cooled copper mold of 0 mm, length 100 mm and height 300 mm. Immediately after the pouring, a swirling flow that alternately alternates in a forward direction and a reverse direction was generated in the molten steel using an induction electromagnetic stirrer, and stopped when the solidification thickness reached 30 mm.

FIG. 1 is a schematic illustration of the current of the induction electromagnetic stirrer in this experiment. When the forward and reverse cycles of the current of the induction electromagnetic stirrer are 0.1 times / second, t ₁ =
One second, t ₂ = 4 seconds. If the cycle of one forward direction and one reverse direction of this current is c times / second, t ₁ =
1 second × (0.1 / c), t ₂ = 4 seconds × (0.1 / c)
And The maximum value + A of the forward current is a current that forms a steady positive swirl flow having a forward flow rate of 30 cm / sec in the molten steel, and the reverse current −A is a reverse current flowing in the molten steel. This is a current that forms a steady reverse swirling flow at a flow velocity of 30 cm / sec.

FIG. 2 shows the current cycle (2) of the induction electromagnetic stirrer.
(t ₁ + 2t ₂ ) and the area ratio of the equiaxed crystal. The area ratio of the equiaxed crystal is a measurement result of a portion 25 mm from the casting surface of the cross section of the slab. As shown in FIG. 2, the area ratio of the equiaxed crystal starts to increase at a current cycle of 0.1 times / second, and increases as the current cycle (times / second) increases. However, if the current cycle is 1.
When it exceeds 0 times / sec, it decreases rapidly. Further, there is no significant difference in the area ratio of equiaxed crystals between the steel containing Mg and the steel not containing Mg.

The following is considered from FIG. That is, if the current cycle is 0.1 times / second or less, the effect of dividing the dendrite is insufficient, and the number of equiaxed crystals does not increase significantly. When the current cycle exceeds once / second, the change in the direction of the swirling flow of the molten steel cannot follow the change in the current, and the effect of dividing the dendrite is reduced. Further, the content of Mg has no significant effect on equiaxed crystallization of coarse columnar crystals 25 mm from the casting surface, which are generated by initial solidification.

FIG. 3 shows the current cycle (2) of the induction electromagnetic stirrer.
t ₁ + 2t ₂ ) and the size of the obtained equiaxed crystal (equivalent circle diameter)
In the figure, the circle equivalent diameter of the equiaxed crystal is a measurement result of a portion 25 mm from the casting surface in the cross section of the cast slab. As shown in FIG. 3, the equivalent circle diameter of the equiaxed crystal starts to be reduced from the current cycle of 0.1 times / second, and the refinement further progresses as the current cycle (times / second) increases. However, when the current cycle exceeds 1.0 times / second, the effect of making the equiaxed crystal fine is significantly reduced. Further, there is no significant difference in the degree of refinement of equiaxed crystals between the steel containing Mg and the steel not containing Mg.

The following can be considered from FIG. That is, when the current cycle is 0.1 to 1 / sec, the number of nuclei generated at the time of columnar crystal formation increases due to the frequent division of the dendrite, resulting in equiaxed crystal formation and remarkable miniaturization. However, if the current cycle is less than 0.1 times / second, the frequency of the dendrite breakage is insufficient, and if the current cycle exceeds 1 time / second, the change in the direction of the swirling flow of the molten steel is Since the change cannot be followed, the effect of miniaturization is reduced. Further, the content of Mg has no significant effect on the refinement of coarse columnar crystals 25 mm from the mold, which occurs at the early stage of solidification.

FIG. 4 shows the current cycle (2
t ₁ + 2t ₂ ) and the size of the equiaxed crystal in the vicinity of the center of the slab (equivalent circle diameter). The equivalent circle diameter of the equiaxed crystal is measured at a portion 45 mm from the casting surface in the cross section of the slab. The result. As shown in FIG.
The equiaxed crystal near the center of the slab appears to be slightly fine when the period of the current of the induction electromagnetic stirring is between 0.1 and 1 / sec, but the degree is not remarkable. On the other hand, when Mg is not contained, the vicinity of the center of the slab is a coarse granular crystal, but by containing Mg, the coarse granular crystal becomes a remarkably fine equiaxed crystal.

[0022]

DETAILED DESCRIPTION OF THE INVENTION The present inventors cast molten steel having a carbon content of 0.12% and a temperature of 1550 ° C. in a tundish at a casting speed of 1.8 m / min in a mold having an inner hole of 250 mm × 1500 mm. At this time, an induction electromagnetic stirrer was disposed at a position 2 m below the mold, and a current of 500 A and a frequency of 2 Hz was passed through the induction electromagnetic stirrer to form a unidirectional swirling flow of 40 cm / sec on the solidification interface. Examination of the slab obtained by this conventional method showed that the equiaxed crystal ratio of the portion corresponding to the position of the swirling flow was 14%, and the equivalent circle diameter of the equiaxed crystal was 3.0 mm. The vicinity of the center of the slab had a coarse granular crystal structure with a circle equivalent diameter of 5 to 8 mm.

The inventors have also found that the carbon content is 0.12.
10% Mg-Ni alloy is added to the molten steel in the ladle to contain 0.002% of Mg, and the molten steel is heated in a tundish at a temperature of 1550 ° C. at a casting speed of 1.8 m / min.
Cast into the same mold as above. 2m below the mold as above
Was used, but the cycle in the forward and reverse directions was 0.5 times / sec, and the maximum swirling flow rate was 40 c
A swirling flow of m / s was formed. The slab obtained by the method of the present invention was examined. The portion corresponding to the position of the swirling flow had an equiaxed crystal ratio of 60%, and the equivalent circle diameter of the equiaxed crystal was 1.1 mm.
Met. In addition, there were no coarse granular crystals near the center of the slab, and fine equiaxed crystals having an equivalent circle diameter of 2 mm or less.

[0024]

According to the present invention, it is possible to manufacture a continuous cast slab in which the columnar crystal parts of the slab are finely equiaxed and the vicinity of the center of the slab is also finely equiaxed.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of a current of the present invention used for induction electromagnetic stirring.

FIG. 2 is a diagram showing forward and reverse periods of a current used for induction electromagnetic stirring and an area ratio of an equiaxed crystal.

FIG. 3 is a diagram showing forward and reverse periods of a current used in an induction electromagnetic stirrer and the equivalent circle diameter of an equiaxed crystal.

FIG. 4 is a diagram showing the presence or absence of Mg and the equivalent circle diameter of an equiaxed crystal near the center of the slab.

FIG. 5 is an explanatory diagram of induction electromagnetic stirring in continuous casting of a slab.

[Explanation of symbols]

1: solidified shell, 2: unsolidified molten steel, 3, 3 ': induction electromagnetic stirrer, 4, 4': positive swirling flow, 5,
5 ′: swirling flow in the reverse direction.

Continued on the front page (72) Inventor Hiroshi Harada 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Inside the Technology Development Division (72) Inventor Kazu Hasegawa 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division

Claims (2)

[Claims] 1. A continuous slab casting device having an induction electromagnetic stirring device between a meniscus in a mold and 10 m below the mold,
Add Mg to molten steel with a ladle, tundish or mold.
0002 to 0.01%, and the induction electromagnetic stirrer alternately and periodically generates a current for generating a swirling flow in the horizontal direction and a current for generating a reverse swirling flow in the molten steel. A continuous casting method characterized by casting while flowing. 2. The method according to claim 1, wherein one cycle of the current for generating the swirling flow in the forward direction and the current for generating the swirling flow in the reverse direction is 0.1 to 1.0 times / sec. The continuous casting method as described.