JP4760345B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method capable of improving the solidification structure of a slab when performing continuous casting of a molten metal (hereinafter referred to as molten metal) obtained by melting various metals such as steel.

  Continuous casting is a slab formed in a continuous casting mold by injecting molten metal from above into a mold composed of four sides (hereinafter referred to as a continuous casting mold) and cooling (so-called primary cooling). This is a technique for continuously producing a slab by pulling out the steel sheet downward. On the exit side of the continuous casting mold, the surface layer portion of the slab is solidified (so-called solidified shell). However, since unsolidified molten metal remains inside the solidified portion located on the outer peripheral portion of the slab, cooling is performed by blowing cooling water onto the slab drawn from the continuous casting mold (so-called secondary cooling), Solidify the unsolidified molten metal. Here, the solidified part on the outer periphery and the unsolidified part on the inner side are collectively referred to as a cast piece.

  In the manufacturing process of various metals such as steel, the solidification structure of the slab obtained by continuous casting of the molten metal has a great influence on the properties and materials of the final product. Therefore, in recent years, as the quality required for metal products increases, the need to control the solidification structure of the slab by continuous casting has increased. Particularly in stainless steel plates and silicon steel plates, the prevention of surface defects called ridging is an important issue, and various studies have been conducted to improve the operating conditions in continuous casting to achieve ridging prevention.

A defect called ridging is caused by unevenness in the rolling direction of a steel sheet by subjecting the steel sheet obtained by solidification to the inside of the slab by continuous casting and then cold working (for example, pulling, deep drawing, etc.). This is a phenomenon in which a certain striped pattern occurs. When ridging occurs, not only the appearance of the steel sheet is impaired, but also the electromagnetic characteristics are deteriorated.
It is already known that the solidification structure of the slab has a great influence on the generation of ridging, and it has been found that ridging is likely to occur when a columnar structure develops on the slab. Therefore, in continuous casting, it is necessary to solidify the slab so that a columnar structure does not occur. Therefore, an electromagnetic stirrer is installed in the area where the secondary cooling is performed below the continuous casting mold (hereinafter referred to as a cooling zone), and the unsolidified portion (that is, the unsolidified molten metal) inside the slab is electromagnetically stirred. Techniques for generating equiaxed crystals are widely adopted.

Furthermore, it is known that the formation of equiaxed crystals is promoted by setting the superheat degree of the molten metal injected into the continuous casting mold to 20 ° C. or less.
In addition, various techniques for generating equiaxed crystals in the solidified structure of a slab obtained by continuous casting have been studied.
For example, Patent Document 1 discloses a technique for generating equiaxed crystals at the center of a slab by adding a coolant such as a steel wire to molten steel in a continuous casting mold and performing electromagnetic stirring of the molten steel. Has been.

Patent Documents 2 and 3 disclose techniques in which an ultrasonic wave is applied to a solidified portion on the outer periphery of a slab and vibrated to generate equiaxed crystals in an unsolidified portion inside the slab.
Patent Document 4 discloses a technique for generating equiaxed crystals in an unsolidified portion inside a slab by adjusting the composition and temperature of molten steel.
However, in continuous casting operations, the operating conditions such as casting speed, molten steel composition, molten steel superheat degree, etc. are likely to fluctuate, so all the techniques disclosed in Patent Documents 1 to 4 generate only equiaxed crystals in the slab. This is difficult, and it is inevitable that columnar crystals are partially formed.
JP-A-57-75275 JP 52-62130 A JP-A-64-83350 JP 2002-30395 A

  The present invention solves the above-described problems, and a continuous casting method in which equiaxed crystals are stably generated in a slab when performing continuous casting of various metal melts, in particular, a stainless steel plate or an electromagnetic steel plate by rolling the slab. Furthermore, an object of the present invention is to provide a continuous casting method of a slab that can prevent ridging when cold working is performed.

The present inventors investigated the process of forming equiaxed crystals in order to refine the crystal grains of the slab obtained by continuous casting and suppress the formation of columnar crystals. as a result,
(a) In order to produce a large amount of equiaxed crystals, a large number of solidification nuclei are required.
(b) When vibration is applied to the interface between the solidified part and the unsolidified part of the slab, the fine particles in the solidified part peel off and float in the unsolidified part, which becomes a solidified nucleus.
(c) It has been found that the formation of equiaxed crystals can be promoted by appropriately maintaining the temperature gradient in the vicinity of the interface between the solidified part and the unsolidified part of the slab. Therefore, the present inventors have developed a new technique for vibrating the slab using water vapor generated from the surface of the slab in a cooling zone that performs secondary cooling below the continuous casting mold. A method for stably producing equiaxed crystals has been found.

That is, the present invention relates to a nozzle hole and a slab for injecting high-pressure water in a region where the surface temperature of the slab becomes 400 to 800 ° C. in a cooling zone in which the slab drawn downward from the continuous casting mold is cooled. Is a continuous casting method in which the high-pressure water is sprayed on the slab for 30 seconds or more, with a distance of 350 mm or less and a flow velocity in the nozzle hole of the high-pressure water being 80 m / sec or more.
In the continuous casting method of the present invention, electromagnetic stirring is preferably performed on the unsolidified molten metal in a region where the thickness of the unsolidified portion of the slab is 60% or more of the slab thickness. Further, it is preferable that the degree of superheat ΔT of the molten metal when cast into a continuous casting mold is 30 ° C. or less.

  According to the present invention, high-speed high-pressure water is sprayed on the slab at the cooling zone below the continuous casting mold to destroy the water vapor generated from the surface of the slab, thereby giving vibration to the slab, Formation of crystals can be promoted. As a result, equiaxed crystals can be stably generated without being affected by fluctuations in the casting speed of continuous casting and the degree of superheated molten steel.

  FIG. 1 is a cross-sectional view schematically showing a continuous casting facility to which the present invention is applied. The slab comprising the solidified portion 5 (ie, solidified shell) and the unsolidified portion 7 (ie, unsolidified molten metal) is drawn downward from the continuous casting mold 1 by the driving force of the pinch roll 2 and installed in the cooling zone. The solidified portion 5 is increased in thickness by being cooled by spray water sprayed from a large number of sprays 3.

In the present invention, high-pressure water is sprayed onto the slab in a region 4 where the surface temperature of the slab is 400 to 800 ° C. (hereinafter referred to as a high-pressure water injection region). The difference between this high-pressure water and other normal spray water will be described later together with the reason that the surface temperature of the slab when jetting high-pressure water is 400 to 800 ° C. Are collectively referred to as cooling water.
Water vapor is generated on the surface of the slab as the cooling water evaporates. However, since the high-pressure water destroys the water vapor in the high-pressure water injection region 4, the water vapor is not dissipated. As a result, the slab is vibrated, and at the interface between the solidified part 5 and the non-solidified part 7, a large number of fine particles are separated from the solidified part 5 and float in the non-solidified part 7. These fine particles serve as solidification nuclei and promote the formation of equiaxed crystals.

When the orifice diameter of the nozzle used for injecting the cooling water is d _n (mm) and the flow rate of the cooling water supplied to the nozzle is Q (liter / min), an injection hole provided in the nozzle (hereinafter, nozzle) The flow velocity V (m / sec) of the cooling water in the hole is calculated by the following equation (1). In a nozzle used in general continuous casting, the normal flow rate of cooling water in the nozzle hole is less than 80 m / sec, and the larger the fluid flow rate, the larger the orifice diameter. Here, the cooling water having a flow velocity in the nozzle hole of less than 80 m / sec is referred to as spray water.

V = 400Q / [6π (d _n ) ² ] (1)
In order to stably generate equiaxed crystals in the slab, it is necessary to float a large number of solidified nuclei in the unsolidified portion 7. Therefore, various techniques have been studied to float a large number of solidification nuclei, but these techniques are as follows:
(A) Stirring the unsolidified molten metal to cut the columnar crystals, and floating the broken pieces in the unsolidified portion.
(B) It is roughly classified into two types, in which vibration is given to the interface between the solidified part and the non-solidified part, and the fine particles separated from the solidified part are suspended in the non-solidified part.

As a technique corresponding to the above (A), electromagnetic stirring is known. However, in continuous casting operations, operating conditions such as casting speed and molten metal superheat are likely to fluctuate, so the stirring intensity of the molten metal by electromagnetic stirring also changes. When the stirring strength is excessively strong, negative segregation of components (so-called white band) occurs, and the material properties of the slab are locally degraded.
As a technique corresponding to the above (B), ultrasonic vibration is known. However, a large-scale oscillator is required to apply ultrasonic vibration to a heavy object such as a continuously cast slab. Moreover, considering that the slab is hot, heat resistance must be imparted to the oscillator, which increases the manufacturing cost of the oscillator.

The present inventors conducted experiments using a 30 kg steel ingot and developed a technique for floating a large number of solidification nuclei in the molten metal by a new means completely different from electromagnetic stirring and ultrasonic vibration. The configuration of the small continuous casting machine used in the experiment is the same as in FIG. The experiment will be described below.
In the experiment, a vibration meter was attached to the side surface of the continuous casting mold 1 and vibrations when molten steel (30 kg steel ingot was melted) was measured. The vibration measured by this vibrometer is the vibration of the continuous casting mold 1, but the vibration of the continuous casting mold 1 indicates that the slab is vibrating. It is possible to evaluate vibration. In the cooling zone below the continuous casting mold 1, a plurality of thermometers were installed along the direction of slab travel to measure the surface temperature of the slab.

In the experiment, the flow rate of the cooling water in the nozzle holes in the high-pressure water injection region 4 where the surface temperature of the slab becomes 400 to 800 ° C. was varied and sprayed on the slab.
FIG. 2 is a graph showing the transition of the surface temperature and vibration acceleration of the slab. Although the flow rate of the cooling water was variously changed, in FIG. 2, the flow rate of the cooling water in the nozzle hole is divided into 80 m / sec or more and less than 80 m / sec. Here, the cooling water having a flow velocity of 80 m / sec or more in the nozzle hole is referred to as high-pressure water in order to distinguish it from conventional spray water (flow velocity of less than 80 m / sec).

As is clear from FIG. 2, vibration was generated in the continuous casting mold (ie, slab) slab by spraying high pressure water having a flow velocity in the nozzle hole of 80 m / sec or more onto the slab.
In order to elucidate the mechanism of this phenomenon, the present inventors conducted further studies. As a result, it was found that the behavior of water vapor generated by the evaporation of cooling water on the surface of the slab is involved as shown in FIG. That is, when the water vapor generated from the surface of the slab is directly diffused as shown in FIG. However, as shown in FIG. 3B, when the water vapor generated from the surface of the slab is destroyed by the pressing force of the cooling water, the slab is vibrated.

  The pressing force of the cooling water increases as the flow rate of the cooling water in the nozzle hole increases. Since the flow rate of cooling water in a general continuous casting operation is less than 80 m / sec (that is, spray water) at the nozzle hole as described above, a strong pressing force cannot be obtained. Microscopically, water droplets (about 100 μm in diameter) sprayed from the nozzle holes individually collide with the slab. Therefore, the dispersion of the kinetic energy of the water droplets is a cause of insufficient pressing force.

On the other hand, when the flow velocity in the nozzle hole is 80 m / sec or more (that is, high-pressure water), fine water droplets continuously collide with the slab, so that the growth of the generated water vapor can be suppressed. That is, in order to destroy the water vapor generated from the surface of the slab, a flow velocity of 80 m / sec or more is required at the nozzle hole.
The water vapor generated from the surface of the slab tries to expand rapidly, but the growth is suppressed and destroyed by the pressing force of the high-pressure water. Originally, the energy of water vapor consumed by the expansion of the volume is confined in the vicinity of the surface of the slab by high-pressure water, and is consumed by vibrating the slab.

  However, if the distance between the surface of the slab and the nozzle hole for injecting high-pressure water is excessively wide, it takes time for the fine water droplets to reach the slab, so that water vapor grows during that time. Moreover, since the water drops lose the kinetic energy, the pressing force of the high-pressure water decreases. Therefore, it becomes difficult to destroy water vapor, and vibration of the slab does not occur. According to the study by the present inventors, when the distance between the nozzle hole and the slab exceeds 350 mm, it becomes difficult to destroy the water vapor. Therefore, the distance between the nozzle hole and the slab needs to be within 350 mm.

If the distance between the nozzle hole and the slab is less than 50 mm, the nozzle hole is deformed by radiant heat radiated from the slab, and the nozzle hole diameter and the injection direction are likely to change. Therefore, it is preferable that the distance between the nozzle hole and the slab is 50 mm or more.
In the region where high-pressure water is sprayed on the slab in the cooling zone, the most remarkable vibration is obtained when the surface temperature of the slab is 400 to 800 ° C. When the surface temperature of the slab exceeds 800 ° C., the water vapor forms a film on the surface of the slab and it becomes difficult to destroy the water vapor with high-pressure water. On the other hand, when the surface temperature is less than 400 ° C., the amount of water vapor generated is small, so that sufficient vibration of the slab cannot be obtained even if the water vapor is destroyed.

  Also, the time for spraying high-pressure water on the slab is 30 seconds or more. If the time for spraying high-pressure water is less than 30 seconds, the slab vibrates for a short time, so that a sufficient number of solidification nuclei cannot be suspended in the unsolidified portion. Therefore, the formation of equiaxed crystals cannot be sufficiently promoted, and it is inevitable that columnar crystals are formed. However, if the high-pressure water spraying time exceeds 300 seconds, the cooling rate of the slab increases and it becomes difficult to properly maintain the temperature gradient at the interface between the solidified part and the unsolidified part. Therefore, the high pressure water spraying time is preferably 300 seconds or less.

Furthermore, it is preferable to perform electromagnetic stirring of the unsolidified portion in a region where the thickness of the unsolidified portion is 60% or more of the slab thickness. The reason for this is that in order to achieve the production of 60% or more equiaxed crystals, it is effective to prevent the formation of columnar crystals in the region where the thickness of the unsolidified part is 60% or more. This is because stirring is effective.
It is preferable to set the superheat degree of the molten metal in the tundish to 30 ° C. or less because the formation of equiaxed crystals is more stably promoted.

  Using a slab continuous casting machine with two strands, general steel SUS430 steel and silicon steel slab (width 1500 mm, thickness 215 mm) were cast. In one strand, a high flow rate nozzle is attached to a high-pressure water injection region where the surface temperature of the slab is 400 to 800 ° C. (that is, 3 to 4 m from the meniscus of the continuous casting mold). A nozzle was installed. This high flow rate type nozzle has a flow rate in the nozzle hole of 80 m / sec or more. All the other strands were fitted with normal nozzles.

  The superheat degree of the molten steel in the tundish was 25 to 35 ° C., and the casting speed was maintained at 1.1 m / min. The time for injecting high-pressure water in the high-pressure water injection region was 50 seconds. Other conditions are as shown in Table 1.

  The conventional example is an example in which the flow rate of high-pressure water in the nozzle hole is 80 m / sec or less, and the comparative example is an example in which the flow rate of high-pressure water is 80 m / sec or less, but the distance between the nozzle hole and the slab is 350 mm or more. An example of the invention is an example in which the flow rate of high-pressure water is 80 m / sec or less, and the distance between the nozzle hole and the slab is 350 mm or less. In the case of electromagnetic stirring, it was applied at a constant intensity of 750 A and 2 Hz at a position 3 to 4 m from the meniscus (that is, a region where the thickness of the unsolidified portion is 60 to 80% of the slab thickness).

A sample was collected from the steel slab thus obtained, and the sample was macroetched using nitric acid, and the thickness of the structure in which equiaxed crystals were formed was measured. Table 1 shows the ratio (%) of the thickness of the equiaxed crystal structure to the steel slab thickness as the equiaxed crystal ratio.
Further, the obtained steel slab was rolled into a steel plate, subjected to deep drawing, and then visually observed to investigate the presence or absence of ridging. The results are shown in Table 1 with the steel plate in which ridging was not recognized as good (◯), the one with mild ridging as acceptable (Δ), and the one with severe ridging as impossible (×). Show. A steel plate evaluated as “impossible (×)” corresponds to a product that cannot be shipped as a product.

  As is apparent from Table 1, the equiaxed crystal ratio was less than 60% in the conventional example and the comparative example, whereas the equiaxed crystal ratio was 60% or more in the invention example. Among the examples of the invention, the equiaxed crystal ratio exceeds 75% in an example in which electromagnetic stirring is performed and an example in which the degree of superheat of molten steel is 30 ° C. or less. Therefore, it was confirmed that the formation of equiaxed crystals was promoted by applying the present invention. In the inventive examples, no ridging was generated.

It is sectional drawing which shows typically the continuous casting installation to which this invention is applied. It is a graph which shows transition of the surface temperature and vibration acceleration of a slab. It is sectional drawing which shows the behavior of water vapor | steam typically.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mold for continuous casting 2 Pinch roll 3 Spray 4 High pressure water injection area 5 Solidified part 6 Electromagnetic stirrer 7 Unsolidified part 8 Immersion nozzle

Claims (3)

  The distance between the nozzle hole for injecting high-pressure water and the slab in a region where the surface temperature of the slab becomes 400 to 800 ° C. in the cooling zone where the slab drawn downward from the continuous casting mold is cooled. The continuous casting method is characterized in that the high-pressure water is sprayed on the slab for 30 seconds or longer, with a flow rate of 80 m / sec or higher in the nozzle hole of the high-pressure water.   2. The continuous casting method according to claim 1, wherein electromagnetic stirring is performed on the unsolidified molten metal in a region where the thickness of the unsolidified portion of the slab is 60% or more of the slab thickness.   The continuous casting method according to claim 1 or 2, wherein the degree of superheat of the molten metal when cast into the continuous casting mold is 30 ° C or less.

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