JP2978335B2 - Belt type continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting thin slabs in belt type continuous casting, and more particularly to a method for obtaining slabs having good surface properties. The so-called present invention is based on the assumption that molten steel has a slab thickness of 30 to 150 mm and a width of 500 to 25.
This is a technique related to the field of belt (for example, ordinary steel strip) continuous casting in which a thin slab of about 00 mm is obtained by continuous casting.

[0002]

2. Description of the Related Art Conventionally, as a kind of continuous casting, for example, JP-A-58-107255 and JP-A-58-1661.
No. 45, Japanese Unexamined Patent Application Publication No. 1-293956, a part of the travel route is provided at a predetermined interval (slab thickness).
And a pair of endless metal belts facing each other facing each other, and a predetermined interval (slab width) synchronously moving with the metal belt and the thin slab held by the metal belts and facing each other. A pair of blocks formed as described above forms a cross-sectional shape corresponding to a desired slab, and the metal belt and the blocks are guided by guide rolls and guide rails so as to make a circular movement along a predetermined movement path. While supporting, a jet nozzle and a cooling pad are arranged on the back side of the metal belt between each guide roll, and the metal belt is cooled by a fluid film formed by ejecting a cooling fluid on the back side of the metal belt, while the casting space of the casting space is cooled. Molten steel is injected from above through an injection nozzle to form a solidified shell along the mold wall of the metal belt, block group, etc., and a slab produced by the growth of the solidified shell. Configured to derive from the casting space from the lower end via a guide roll, a so-called belt type continuous casting machine has been proposed.

[0003]

The purpose of casting steel by such a belt-type continuous caster is to use a conventional slab (200 to 3).
(Thickness: 00 mm) is required to obtain a thinner slab. However, when the slab is thinner, the rolling reduction in the subsequent step becomes smaller, so that good slab surface properties are required from the slab stage.
That is, if there is a defect on the surface of the slab, when the rolling reduction is small, the defect does not disappear sufficiently even by rolling, leading to a serious product defect.

Such surface defects include surface irregularities caused by fluctuations in the molten metal surface, so-called wrinkles, and inclusions (inclusions) such as oxides floating on the molten metal surface. These defects are caused by the start of solidification immediately below the surface of the molten metal.
Since solidification starts immediately below the surface of the molten metal, wrinkles are generated under the influence of fluctuations in the surface of the molten metal, and oxides and the like floating on the surface of the molten metal are involved. As one of these surface defect prevention methods, a method of reducing the flow velocity of molten steel on the molten metal surface and minimizing the fluctuation of the molten metal surface as far as possible has conventionally been adopted.

[0005] However, when the flow rate of the molten steel on the molten metal surface is reduced, solidification of the molten metal surface, so-called deckle, occurs, which hinders the operation and may cause surface defects of the cast slab. Therefore, although reducing the surface flow velocity is effective in preventing the above-mentioned surface defects, it cannot be said to be a good method for securing the stability of operation.

As another method of preventing surface defects, a method of heating or keeping a part of a mold near the molten metal surface to suppress solidification of steel near the molten metal surface, that is, a so-called under-solidification technology has been considered. . However, heat-insulating materials such as a mold heating technique and ceramics used in this method have not been sufficiently developed, and practical use at the present stage involves difficulties. In addition, when a mold such as a belt-type continuous casting machine is constantly rotating, it is very difficult to stably heat and maintain a specific portion near the molten metal surface.

Accordingly, an object of the present invention is to establish a belt-type continuous casting method that does not generate a slab surface defect that occurs when casting steel in the above-described belt-type continuous casting machine and that can ensure stable operation. It is assumed that.

[0008]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides the following casting method for a belt type continuous casting machine. In a belt-type continuous casting machine, when continuously casting steel, while measuring the flow velocity of the molten steel at a depth within 50 mm from the surface of the molten steel held by the mold using a flow velocity measuring device. , The temperature of the molten steel in the tundish is measured, and the molten steel flow rate u
A belt-type continuous casting method characterized by maintaining the relationship between (mm / s) and the degree of superheat of molten steel ΔT (° C.) as follows: (8800 / ΔT) ≦ u ^0.8 ≦ (19000 / ΔT). Here, the flow velocity measurement uses a measuring instrument or the like that measures the flow resistance.

Although the present invention can be applied to a continuous casting machine, a powder is generally used in the continuous casting machine to keep the surface of the molten steel warm. The entanglement rather deteriorates the quality of the cast slab, makes the thickness of the powder layer unstable, loses the effect of keeping the surface warm in the thinned portion of the powder layer, and generates deckle. Therefore, it is difficult at present to adopt the present invention in a continuous casting machine.

[0010]

The inventors of the present invention have found that the cause of the above-mentioned slab surface defects is that solidification starts immediately below the molten metal surface, so that the influence of fluctuations in the molten metal surface, oxides floating on the molten metal surface, and the like. In order to prevent surface defects based on the fact that it is involved, we focus on the molten steel flow near the molten metal surface, casting experiments with a belt-type continuous caster and off-line experiments with various molten steel velocities and their analysis and examination And went.

As a result, in the belt type continuous casting machine,
During continuous casting of steel, the flow velocity of molten steel at a depth within 50 mm from the surface of the molten steel held by the mold is measured using a flow rate measuring device, and the molten steel temperature in the tundish is measured. Measure and based on the measurement result, maintain the relationship between the molten steel flow rate u (mm / s) and the superheat degree of molten steel ΔT (° C.) at (8800 / ΔT) ≦ u ^0.8 ≦ (19000 / ΔT). It has been found that, without causing solidification in the vicinity, surface defects can be prevented, good cast pieces can be obtained, and stable operation is possible.
At this time, the flow rate of the molten steel in the vicinity of the molten metal surface is given by a flow caused by an injection flow from a nozzle or a flow caused by an electromagnetic force such as electromagnetic stirring.

Next, the concept of the present invention will be described. Solidification occurs due to the heat removal of the molten steel from the mold, in the case of the present invention, the belt. FIG. 1 is a diagram schematically showing heat transfer between molten steel 3 and solidified shell 2 and between belt 1 and solidified shell 2. As shown in FIG. 1 and FIG. 2, whether or not the solidified shell 2 is formed and grows is determined from the molten steel 3 indicated by reference numeral 6.
Is determined by the relationship between the heat flux (Q ₁ ) and the heat flux (Q ₂ ) from the solidified shell 2 to the belt 1 indicated by reference numeral 7. That is, Q ₁
When <Q _2, the solidified shell grows and solidification proceeds, whereas when Q ₁ > Q ₂ , the solidified shell does not grow and solidification does not progress. Therefore, if a thermal condition of Q ₁ > Q ₂ is realized over a range of a certain depth from the molten metal surface 4, no solidification occurs in that range.

The heat flux Q ₂ from the solidified shell to the belt near the molten metal surface is about 6 million kcal / m ² hr, whereas the heat flux Q ₁ from the molten steel to the solidified shell varies depending on the flow rate of the molten steel. Heat flux Q ₁ according to the molten steel flow speed increases increases.
If a molten steel flow velocity of a certain size or more is given and the condition of Q ₁ > Q ₂ is achieved, solidification near the molten metal surface does not occur. The present inventors have carried out solidification experiments with varying various molten steel flow speed, it revealed the relationship between the molten steel flow velocity and heat flux Q _1. As a result, it has been found that, for example, when the molten steel superheat degree ΔT is 20 ° C. and the molten steel flow rate is 560 mm / s or more, the above condition of Q ₁ > Q ₂ can be achieved.

In order to change the flow speed of molten steel near the molten metal surface, for example, as shown in FIG. 3, molten steel is injected into a mold formed by the metal belt 1 and the short side block group 8 by the immersion nozzle 9. When the injection flow 10 is generated, the injection angle of the nozzle is changed to change the flow speed of the molten steel, or as shown in FIG. 4, an electromagnetic coil 11 is installed near the belt to perform the electromagnetic stirring operation. The molten steel flow velocity may be changed by changing the force.

However, when the flow rate of the molten steel becomes too large and becomes 1470 mm / s or more, there is no solidified shell,
It has also been found that when heat is transferred directly from the molten steel to the belt and the heat flux becomes too large, an obstacle occurs such that the belt is thermally damaged. Therefore, it was found that the required molten steel flow velocity had a certain range.

Further, an experiment was conducted by changing the superheat degree ΔT of the molten steel, and the relationship between the flow rate u (mm / s) of the molten steel and the superheat degree ΔT (° C.) of the molten steel was calculated as follows: (8800 / ΔT) ≦ u ^0.8 ≦ (19000 / ΔT) to obtain good slab surface properties,
It was clarified that stable operation could be achieved. FIG.
Indicates the region in the present invention by the relationship between the degree of superheat of molten steel ΔT and the flow rate u of molten steel, and the hatched portion is the range region shown in the claims of the present invention.

Molten steel flow rate u (mm / s) and molten steel superheat ΔT
(8800 / ΔT) ≦ u ^0.8 ≦ (19000 / ΔT) The relationship from the molten metal surface is considered as follows. In normal continuous casting, when casting at a casting speed of 2 m / min, inclusions deposited on the surface of molten steel are 5 mm from the molten metal surface.
Within. In belt type continuous casting machines,
Since casting is performed at a casting speed of 20 m / min and a speed 10 times that of a normal continuous casting machine, inclusions concentrated on the molten steel surface expand to within 50 mm from the molten metal surface. Therefore, the molten steel flow rate u (mm
/ s) and the degree of superheat ΔT (° C.) of the molten steel are set to be (8800 / ΔT) ≦ u ^0.8 ≦ (19000 / ΔT). It is necessary.

[0018]

EXAMPLE As shown in FIG. 3, by changing the angle of the injection flow using an injection nozzle, the flow rate of molten steel near the molten metal surface was changed, and a casting experiment was performed by a belt type continuous casting machine under the following casting conditions. Was.

(Example 1) Continuous casting conditions Steel type, casting conditions, degree of superheat of molten steel, injection flow angle, and flow velocity at a certain depth from the molten metal surface are as shown in Table 1.

[0020]

[Table 1]

Casting Results As a result of casting under the above conditions, the surface defect index and belt damage at each molten steel flow rate are as shown in Table 2.
The surface defect index shown in Table 2 indicates the number of hot wrinkles and the number of scum involved per unit area of the slab. It can be seen from Table 2 that the present invention had few surface defects and no belt damage was observed.

[0022]

[Table 2]

(Example 2) Continuous casting conditions Casting was carried out at the same steel type, size and casting speed as in Example 1 while changing the degree of superheat of molten steel and the angle of injection flow (see Table 3).

[0024]

[Table 3]

Casting Results Table 4 shows the results of casting under the above conditions. Table 4 shows that the present invention had few surface defects and no belt damage was observed.

[0026]

[Table 4]

[0027]

As described above, the present invention improves the quality of the obtained slab by preventing surface defects occurring in the slab when performing continuous casting using a belt type continuous casting machine.
The fact that it has enabled stable operations has a great effect on the business field.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat flux in the vicinity of a molten metal surface of a belt type continuous casting machine.

FIG. 2 shows the relationship between the heat flux from the molten steel to the solidified shell (Q ₁ ) and the heat flux from the solidified shell to the belt (Q ₂ ), as well as the flow rate of the molten steel and the heat flux from the molten steel to the solidified shell (Q ₁ ). Q ₁ ).

FIG. 3 is a view schematically showing an injection angle operation of an injection nozzle used for changing a flow rate of molten steel near a molten metal surface.

FIG. 4 is a diagram showing a method of applying an electromagnetic force when using electromagnetic stirring to change the flow rate of molten steel near the molten metal surface.

FIG. 5 is a diagram showing an area of the present invention by ΔT and u.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 metal belt 2 solidified shell 3 molten steel 4 molten surface 5 molten surface molten steel flow 6 heat flux from molten steel to solidified shell (Q ₁ ) 7 heat flux from solidified shell to belt (Q ₂ ) 8 short side block group 9 nozzle 10 Injection flow 11 Electromagnetic coil

Continuation of the front page (72) Inventor Akio Kasama 1 Nishinosu, Oita, Oita, Oita Prefecture Nippon Steel Corporation Oita Works (56) References JP-A-4-305344 (JP, A) JP-A-4- 300051 (JP, A) JP-A-4-284953 (JP, A) JP-A-3-207556 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/06 340 B22D 11/18

Claims (1)

(57) [Claims] 1. A belt-type continuous casting machine, wherein, during continuous casting of steel, the flow rate of molten steel held at a depth within 50 mm from the surface of molten steel held by a mold, and the molten steel in a tundish. The temperature is measured, and based on the measurement result, the relation between the molten steel flow rate u (mm / s) and the molten steel superheat degree ΔT (° C.) is maintained at (8800 / ΔT) ≦ u ^0.8 ≦ (19000 / ΔT). Characteristic belt type continuous casting method.