JPH06285596A - Method for continuously casting steel - Google Patents

Abstract

PURPOSE:To reduce the production cost by casting a steel while executing rolling reduction to unsolidified part in a strand of steel cast before changing the kind of steel in a mold in the thickness direction. CONSTITUTION:The molten steels having mutually different components are continuously cast. At this time, the unsolidified part 7 in the strand of the steel cast before changing the kind of steel in the mold 4 is cast while executing the rolling reduction to the part 7 with the rolling reduction rolls 6 in the thickness direction. Further, the casting is performed in the order of higher density of the molten steel. Further, one pair of magnetic 8 are arranged on faced wide surfaces in the casting mold 4 and DC magnetic flux in the direction crossing the thickness of the cast slab is given over the whole width of the wide surfaces to form the magnetic field zone by the DC magnetic flux in the casting direction in the mold 4. The molten steel is poured into the casting space at the upper side. In this way, the mix range of the component in the connected part can be reduced without damaging the productivity at the time of sequentially and continuously casting the different kinds of steels.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, when continuously casting a plurality of steel grades having different molten steel components, continuously casts steel in which the component mixing region in a slab between the steel grades having different components is minimized. It is about the method.

[0002]

2. Description of the Related Art In continuous casting of steel, in order to improve productivity and reduce manufacturing cost, molten steel of a plurality of molten steel ladles (melting units) is continuously cast in a single mold. Casting is widely performed one after another. Further, by further expanding this, continuous casting of a plurality of steel types having different molten steel components, that is, so-called different casting of different steel types is being carried out.

When continuously casting different steel types, when the molten steel ladle is exchanged, components are mixed in the cast seam, and as a result, a slab that does not meet the purpose of production is formed. It has been a major problem in continuous casting of different steel types that the cast pieces are subjected to the grade-down treatment or the scrap treatment, or that the rolling pieces before and after the joint portion are held for rolling until the composition determination result is obtained.

Therefore, as a method for reducing the mixing of components in the joint portion which occurs when continuously casting different steel types, Japanese Patent Laid-Open No. 61-
Japanese Patent No. 1459 discloses a method of preventing the components from being mixed in the tundish by injecting molten steel in the rear pot after reducing the residual hot water in the front pot in the tundish as much as possible, and when starting casting of molten steel in the rear pot. Temporarily reduce the casting speed of, the method of preventing the mixing in the mold by shallowing the depth of the discharge flow from the pouring nozzle to penetrate into the molten steel in the mold, temporarily stop the casting at the seam, A method is described in which an iron plate is inserted as a shielding plate in the mold to prevent mixing in the mold, and a method in which a static magnetic field is formed immediately below the mold and casting is performed while suppressing the circulating flow at the joint.

[0005]

However, in the prior art as described above, the casting speed is lowered at each joint, which not only impairs the productivity of continuous casting itself, but also causes the temperature of the slab to partially change. As the mechanical load on the continuous casting machine for bending and straightening the slab becomes large, the life of the equipment is shortened, the schedule of hot direct rolling is impaired, and the iron plate is placed inside the mold. There was a problem that it involved extremely dangerous work for inserting into the.

Furthermore, it cannot be said that the component mixing region at the joint portion during continuous casting of different steel types is not necessarily reduced to a satisfactory degree, and the length or the scrap of the disqualified portion in the obtained slab is reduced. It was difficult to determine the length, and in order to compensate for this, it was inevitable to confirm the slabs before and after the joint by component analysis. Therefore, there is a problem that not only the rolling schedule is disturbed, but also the stock amount is temporarily increased.

[0007]

The gist of the present invention is as follows.

When continuously casting steels having different molten steel components, it is characterized in that the unsolidified portion in the strand of the cast steel is cast while being pressed in the thickness direction before the steel type is changed in the mold. Continuous casting method for steel.

When continuously casting steels having different molten steel components, the steels having a high molten steel density are cast in order from a steel type having a high molten steel density to a steel type having a low molten steel density and cast before the steel type is changed in the mold. A continuous casting method for steel, which comprises casting while unpressurizing an inner unsolidified portion in a thickness direction.

In continuously casting steels having different molten steel components, a pair of magnets are arranged in a wide surface facing each other in the mold or at the rear of the wide surfaces, and a direct current magnetic flux is spread over the wide surface in the direction crossing the thickness of the slab. To form a static magnetic field band by the direct current magnetic flux in the casting direction in the mold, inject molten steel into the upper casting space and cast in order from a steel type with a high molten steel density to a steel type with a small molten steel density, and change the steel type in the mold The continuous casting method for steel, characterized in that the unsolidified portion in the strand of the steel having a high molten steel density cast before the casting is cast while pressing in the thickness direction.

[0011]

The function of the present invention will be described in detail below.

In order to solve the above problems in the prior art, it is important to control mainly the following three factors among the factors affecting the mixing of components during continuous casting of different steel types.

First, the factors that have the greatest influence on the mixing of the components in the mold during continuous casting of different steel types are the infiltration of molten steel discharged from the dipping nozzle into the lower part of the strand pool and the accompanying macroscopic convective mixing. It is believed that there is. As a means to prevent this macroscopic convective mixing, conventionally, casting was temporarily stopped and an iron plate was inserted into the mold as a shielding plate, so that the steel was moved downwards in the strand pool of molten steel after the steel type was changed in the mold (post pot). However, the mixing with the molten steel before the steel type change (front ladle) was minimized, but as described above, the problems associated with the iron plate insertion work occur incidentally.

Therefore, in order to solve this problem, FIG.
As shown in FIG. 2, a pair of magnets 8 are arranged in the mold 4 on opposite sides or behind them, and a DC magnetic flux is applied across the width of the wide surface in a direction that traverses the thickness of the slab. The static magnetic field zone 2 is formed by the direct current magnetic flux in the direction, and molten steel is injected into the upper casting space. Table 1 shows the effect of reducing the depth of penetration of molten steel discharged from the dipping nozzle into the strand pool when a static magnetic field is applied by this method. However, the penetration depth is significantly shortened by the application of a static magnetic field. I understand that When operated most efficiently, the penetration depth is reduced to a depth that approximately corresponds to the position of the static magnetic field band. This is because the static magnetic field exerts a braking effect on the molten steel flow discharged from the nozzle by the Lorentz force, which means that the static magnetic field provides an effect equivalent to the shielding effect by the conventional iron plate. Further, with this method, it is not necessary to reduce the casting speed when changing the steel type, and the problems of the conventional iron plate insertion method are solved.

[0015]

[Table 1]

Next, with respect to the effect of convective mixing due to the difference in the density of molten steel in the strand pool, Table 2 shows the results of investigating the effect of shortening the component mixing region by the static magnetic field. showed that. In the case where the molten steel density of the front ladle is higher than that of the rear ladle (c), a large reduction effect of the component mixing region is recognized as compared with (a) where a static magnetic field is not applied, but it is conversely small ( In b), it can be seen that the component mixing region is not shortened so much and the effect of the static magnetic field is not sufficiently maintained. This is because the molten steel is mixed by the density difference convection after the steel type changing position passes through the static magnetic field zone, that is, in the strand pool below the mold, and the effect of the static magnetic field is not fully utilized. In other words, when casting different steel types one after another, it is important to set the casting order in consideration of the physical properties of the molten steel, especially the density.

[0017]

[Table 2]

Furthermore, the component concentration distribution in the thickness direction and the width direction of the cast slab is detailed under the conditions that the casting order is set in consideration of the difference in density, and the static magnetic field is applied at the seam to minimize convective mixing in the strand pool. The results of the investigation are summarized in FIG. In both the thickness direction and width direction of the slab, the rear ladle molten steel penetrates into the front ladle molten steel side in a concave shape in the casting direction. This recess is almost the same as the volume of shrinkage holes (shrinkage cavities) usually found in the final casting part, and when the steel type in the mold is changed, the unsolidified molten steel in the strand pool already in the continuous casting machine, that is, the solidified molten steel in the front ladle It is presumed that the molten steel in the rear pan was drawn downward due to the effect of shrinkage.

In the case of steel, the solidification shrinkage rate (volume change rate when transforming from solid phase to liquid phase) is about 4%, so if the length of the strand pool is L, the depth ΔL of the recess is Is expressed by Equation 1.

[0020]

[Formula 1] ΔL = 0.04L ... [1]

Further, the solidification thickness d (mm) of steel is generally expressed by the equation 2 using the solidification coefficient K (mm / min ^-0.5 ) and the time t (min) from the start of solidification.

[0022]

## EQU2 ## d = K√t ... [2]

Therefore, assuming that the thickness of the cast piece is D (mm) and the casting speed is Vc (m / min), the following equation 3 is established.

[0024]

[Equation 3] 1 / 2D = K√ (L / Vc) ... [3]

From this, Equation 4 is obtained.

[0026]

[Formula 4] L = Vc · (D / 2K) ²・ ・ ・ [4]

That is, the solidification coefficient K takes a value peculiar to the continuous casting machine and is about K = 25 mm / min ^{-0.5. Therefore} , the strand pool length L is uniquely determined by the slab thickness D and the casting speed Vc. . For example, when casting a 245 mm thick slab at a casting speed of 1 m / min, the strand pool length is estimated to be L = 24 m from the equation [4], and the shrinkage length is ΔL = 0.96 m from the equation [1]. Presumed.

As a method for preventing the movement of molten steel due to this solidification shrinkage, it is known that a light reduction in the thickness direction with respect to an unsolidified cast piece is effective, and the light reduction amount corresponds to the solidification shrinkage of the cast piece. The amount is most desirable, and the effect is not fully exerted when extreme excess or deficiency occurs. FIG. 4 is a diagram schematically showing the component distribution of a slab when it is cast while continuously casting different steel types while pressing the solidified portion of the slab corresponding to the front ladle side in the thickness direction. The unsolidified pressure improves the concave concentration distribution in the slab thickness direction and width direction, and shortens the substantial mixing region.

To summarize the above, in order to reduce the component mixing region in continuous casting of different steel types, it is important to suppress convective mixing in the strand pool and prevent molten steel from moving due to solidification shrinkage.

[0030]

EXAMPLE As shown in FIGS. 1 and 2, continuous casting equipment having a mold 4 in which a pair of magnets 8 were arranged behind the opposing wide surfaces was used to continuously cast different steel types.

The mold shape is 245 mm (thickness) x 1500 m
The m (width) and the casting speed were 1.0 m / min. The position of the static magnetic field band 2 is 450 to 700 m from the meniscus in the mold 4.
The range of m was set, and the intensity of the DC magnetic flux was set to 0.35 tesla at maximum. Molten steel was poured into the upper casting space of the static magnetic field zone 2.

As a compensation for solidification shrinkage, as shown in FIG. 5, a reduction mechanism by a reduction roll 6 was provided below the mold 4 of the continuous casting apparatus, and the slab was cast while being pressed at 1 mm / m in the thickness direction.

After casting, the solute concentration distribution in the obtained slab was investigated to determine the length of the component mixing region. Table 3 shows the relationship between the casting conditions and the length of the component mixing region. In the present invention example,
A large reduction effect of the component mixing region was obtained as compared with the comparative example.

[0034]

[Table 3] Note 1) Molten steel density difference = Front ladle molten steel density-Rear ladle molten steel density Note 2) Component mixing length: Mixing zone length on the front ladle side of the seam

[0035]

EFFECTS OF THE INVENTION According to the present invention, it is possible to reduce the component mixing region of the seam during continuous casting of different steel types without impairing the productivity, and it is possible to greatly reduce the manufacturing cost.

[Brief description of drawings]

FIG. 1 is a schematic view of a continuous casting apparatus that applies a static magnetic field band in a casting direction in a mold.

FIG. 2 is a sectional view in the width direction of a slab of a continuous casting device that applies a static magnetic field band in the casting direction in the mold.

FIG. 3 is a diagram schematically showing component mixing due to solidification shrinkage of the front ladle molten steel during continuous casting of different steel types. In addition, rear pan component concentration>
In the case of the concentration of components in the front pot, the solid line is the lower limit guaranteed concentration of the rear pot, and the broken line is the upper limit guaranteed concentration of the front pot. Further, when the concentration of the rear pan component <the concentration of the front pan component, the solid line is the upper guaranteed concentration of the rear pan, and the broken line is the lower guaranteed concentration of the front pan.

FIG. 4 is a diagram schematically showing reduction of component mixing when compensating for solidification shrinkage of molten steel in the front ladle during continuous casting of different steel types. When the concentration of rear pan component> the concentration of front pan component, the solid line is the lower guaranteed concentration of the rear pan, and the broken line is the upper guaranteed concentration of the front pan. Further, when the concentration of the rear pan component <the concentration of the front pan component, the solid line is the upper guaranteed concentration of the rear pan, and the broken line is the lower guaranteed concentration of the front pan.

FIG. 5 is a diagram showing a continuous casting apparatus used in Examples.

[Explanation of symbols]

 1 Immersion Nozzle 2 Static Magnetic Field Band 3 Long Side of Mold 4 Mold 5 Rectification 6 Rolling Roll 7 Unsolidified Part 8 Magnet

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location B22D 27/09 A 7011-4E

Claims (3)

[Claims] 1. When continuously casting steels having different molten steel components, the unsolidified portion in the strand of the cast steel is cast while being pressed in the thickness direction before the steel type is changed in the mold. Continuous steel casting method. 2. When continuously casting steels having different molten steel components, steels having a large molten steel density are cast in order from a steel having a high molten steel density, and steels having a high molten steel density cast before the steel grade is changed in the mold. The continuous casting method for steel, wherein the unsolidified portion in the strand is cast while being pressed in the thickness direction. 3. When continuously casting steels having different molten steel components, a pair of magnets are arranged in the mold opposite to each other or behind the wide surfaces, and a DC magnetic flux is wide across the width of the wide surface in a direction crossing the thickness of the cast piece. Applied to form a static magnetic field band by the DC magnetic flux in the casting direction in the mold, injecting molten steel into the upper casting space and casting in order from a steel type with a high molten steel density to a small steel type, in the mold A continuous casting method for steel, characterized in that the unsolidified portion in a strand of a steel having a high molten steel density cast before the steel type is changed is cast while being pressed in the thickness direction.