JP2003236646A - Continuous casting method of thin cast slab with excellent surface properties and partition weir - Google Patents

Abstract

[57] [Problem] To reduce the volume of a molten steel pool cooled by a drum in order to prevent generation of a low-temperature portion due to non-uniform temperature of molten steel in a molten metal pool, and to prevent uneven gloss of a steel plate and roughness of a processed surface. The present invention also provides a continuous casting method of a twin-drum thin cast piece capable of suppressing uneven temperature in a molten steel pool cooled by a drum. SOLUTION: A partition weir which does not come into contact with a drum is provided inside a molten metal pool of a pool, and the molten metal pool of the pool is separated into a part surrounded by the partition weir and a part outside the partition weir, The residence time of the molten metal in the molten metal pool outside the weir is 8 seconds or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of casting a thin cast piece of a metal such as steel, stainless steel, copper or aluminum using a twin-drum continuous casting method in which molten metal is partially surrounded by a pair of cooling drums and side weirs. The present invention relates to a method for supplying the molten metal to the molten metal pool and a device for supplying molten metal.

[0002]

2. Description of the Related Art In recent years, attention has been focused on a method for directly producing a thin slab having a thickness of several mm to several tens of mm, which is close to the final shape, from molten metal such as molten steel. This continuous casting method does not require a multi-step hot rolling process as in the prior art, and the rolling to the final shape may be mild, so that the process and equipment can be simplified. One of such continuous casting methods is a twin-drum type continuous casting method.

In this twin-drum system, as shown in FIG. 5, it is formed by a pair of cooling drums 1a and 1b which rotate in mutually opposite directions and side dams 2a and 2b which are in contact with both ends of this drum. Immersion nozzles 6, 6 ′ mounted on the tundish 4 from above in the hot water pool 8
The molten metal 7 is supplied through the cooling pipe to form a pool of a predetermined level, and the solidified shell is cooled and pressure-contacted and integrated at the gap formed at the closest portion of the pair of cooling drums to form a wide thin slab. 3 is to be cast.

In this twin-drum type continuous casting, in order to form a solidified shell having a uniform thickness and stably cast a thin slab without surface defects such as a wound defect, a hot water provided between cooling drums is used. It is required to supply the molten metal as a smooth flow to the pool and to stabilize the cooling conditions in the meniscus near the cooling drum. For example, JP-A-6
Japanese Unexamined Patent Publication No. 2-282753 discloses that a porous filter is incorporated in an immersion nozzle to obtain a discharge flow without disturbance over the entire width of the nozzle.

Further, in Japanese Unexamined Patent Publication No. 8-164454, a core weir is inserted in a drum to suppress the melt flow from the melt supply nozzle toward the edge weir by the core weir so that the melt flow directly to the short side. A twin roll type thin plate continuous casting machine is disclosed which prevents surface defects due to rising of the molten metal near the edge weir and disturbance of the molten metal surface without being applied to the weir. By these techniques, it is possible to prevent a cast piece surface defect caused by the flow or turbulence on the surface of the molten metal or immediately below the surface.

[0006]

However, the formation of a low temperature portion due to the non-uniform molten steel temperature in the molten metal pool inside the pool, and the length of time that the molten steel stays in the molten pool increase, resulting in coarse equiaxed grains. And a new problem arises in which the coarse equiaxed grains exist in the equiaxed zone at the center of the slab. The coarse equiaxed crystal in the central portion becomes a negative segregation due to the distribution of solute components, for example, Ni during solidification, and has a concentration lower than the concentration of solute in the surroundings, which causes uneven gloss and roughened work surface. There was a problem.

The object of the present invention is to insert a simple and large-capacity core weir separately into the space inside the roll to reduce the volume of the molten steel pool cooled by the drum and to cool it by the drum. Another object of the present invention is to provide a method for continuously casting twin-drum type thin cast pieces capable of suppressing non-uniformity of temperature in the molten steel pool and a partition weir used for the method.

[0008]

The gist of the present invention for achieving the above object is as follows. (1) A molten metal is poured into a molten metal pool formed by a part of the periphery of a pair of cooling drums and side dams, and then the molten metal is cooled and solidified on the rotating peripheral surface of the cooling drum to form a thin cast piece. In the twin-drum casting method for continuous casting, a continuous casting method for a thin cast slab having excellent surface properties, characterized in that the residence time of the molten metal in the molten metal pool in the pool is 8 seconds or less. Here, the residence time is the molten metal volume of the molten metal pool / the casting amount per unit time. (2) A partition weir that does not come into contact with the drum is provided inside the molten metal pool in the hot water pool, and the molten metal pool in the hot water pool is separated into a portion surrounded by the partition weir and the outside of the partition weir, and the outside of the partition weir. The method for continuous casting of thin-walled slabs having excellent surface properties, characterized in that the residence time of the molten metal in the molten metal pool volume is set to 8 seconds or less. (3) A downwardly projecting partition weir composed of a refractory wall is provided in the molten metal pool, molten metal is supplied into the partition weir by an immersion nozzle, and a discharge hole or an overflow port provided on the side wall of the partition weir is provided. A continuous casting method for a thin cast slab having excellent surface properties, characterized in that molten steel is supplied to the outside of the partition weir, and the residence time of the molten steel supplied to the outside is set to 8 seconds or less. (4) A downwardly projecting partition weir made of a refractory material in the molten metal pool, and a discharge hole or an overflow port for supplying the molten metal to the outside of the side wall is provided with the molten metal surface position or the molten metal surface. A partition weir for continuous casting, which is provided at a lower position. (5) A thin-walled slab for continuous molten steel casting using the method for casting a thin-walled slab according to any one of (1) to (3), wherein the thin-walled slab is at both end portions in the plate thickness direction. A thin-walled slab having excellent surface properties, which is composed of a columnar crystal zone and an equiaxed crystal zone at the center of the thickness, and does not have a negative segregation zone having a degree of negative segregation of less than 0.90 at the thickness center. . Here, the degree of negative segregation is the Ni concentration in the negative segregation zone / the average Ni concentration in the columnar crystal zone.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of various experiments in which the temperature of molten steel to be injected, the shape of the nozzle, the molten steel height of the molten metal pool, and the drum diameter were changed, the present inventors found that coarse equiaxed crystals were pooled. It was found that it exists in the slab corresponding to the portion of the molten steel having a low temperature, and that it occurs when the residence time in the molten pool exceeds 8 seconds. Therefore, the residence time was set to 8 seconds or less. The present invention has been made based on this finding. That is, in order to suppress the formation of enlarged equiaxed crystals, it is effective to make the temperature in the molten metal pool uniform and to shorten the residence time of the molten steel. Therefore, the effective pool volume is reduced and a uniform flow in the width direction is achieved. This is a method for making the temperature uniform by applying.

A partition weir that does not come into contact with the drum is provided inside the molten metal pool in the molten metal pool, and the molten metal pool in the molten metal pool is separated into a portion surrounded by the partition weir and the outside of the partition weir. It is possible to reduce the volume of the molten metal pool on the outer side of and the residence time of the molten metal in this pool can be set to 8 seconds or less. As a result, direct contact of molten steel on the drum surface where chill crystals etc. which become equiaxed nuclei are generated is prevented as much as possible, and heat is insulated by the partition weir.
Prevents the temperature drop of the molten steel injected into the pool from the immersion nozzle.

As for the shape of the partition weir, a downwardly projecting partition weir made of refractory is provided in the molten metal pool, and molten metal is supplied into the partition weir by a dipping nozzle, and a discharge hole provided on the side wall. Alternatively, the molten steel is supplied from the overflow port to the outside. As a result, the molten metal can be made to flow out uniformly from the pool weir, and the molten steel temperature in the molten steel pool on the drum side can be made uniform.

The shape of the partition weir may be a triangle having an apex below when viewed from the side weir side by combining flat plates and a rectangle when viewed from the drum side, or the surface facing the drum may be curved along the shape of the drum. May be given. By providing the curvature, it becomes possible to immerse the partition weir to a deeper position, and it is possible to further reduce the volume of the pool outside the partition weir.

The supply port for supplying the molten steel from the partition weir to the molten steel pool on the drum side may be rectangular or round. The supply port may be provided not only on the drum side but also on the side dam side. By providing on the side weir side, it is possible to prevent the molten steel temperature near the side weir from decreasing and to suppress the generation of metal. The position of the supply port should be as high as possible above the molten steel pool, but it should be adjusted so as not to disturb the molten metal surface. By making the supply port an overflow port from the molten steel pool in the partition weir, the height of the overflow port determines the height of the molten metal surface, which facilitates the control of the molten metal surface. Since the partition weir has a simple structure, it can be manufactured by laminating plates of refractory material such as alumina graphite, so that the refractory cost can be significantly reduced.

The thin-walled slab produced by twin-drum type continuous casting begins to solidify from the surface cooled by the drum, and columnar crystals grow from both the front and back surfaces of the thin-walled slab, producing equiaxed crystals in the central part. Since the end portions are normally held by side dams made of refractory, columnar crystals do not form and equiaxed crystals also form on the end faces. Coarse equiaxed grains formed in the equiaxed zone in the central part of the thickness become a negative segregation zone having a lower concentration than the surrounding solute concentration because solute components, such as Ni, are distributed during solidification, which causes the gloss Unevenness and roughened skin are generated. If the degree of negative segregation is less than 0.90, uneven gloss and rough skin occur. The cast product produced according to the present invention does not have a partial negative segregation zone in the central equiaxed zone, or exists in the central equiaxed zone because the equiaxed grains do not grow coarsely. The degree of negative segregation in the negative segregation zone is 0.90 or more. Therefore, uneven glossiness and roughened processed surface do not occur. Here, the degree of negative segregation means the Ni concentration in the negative segregation zone / the average Ni concentration in the columnar crystal zone.

[0015]

EXAMPLES Examples of the present invention will be shown below. Austenitic stainless steel is melted and the diameter is 1200m as shown in Fig. 5.
A thin-walled slab having a plate thickness of 2 to 5 mm was manufactured by casting with a twin drum continuous casting machine having an internal water-cooling type, a copper casting drum made of copper with Ni plating on the surface, having a width of 1330 mm. The cross section of the thin cast piece was cut, and the Ni concentration in the cross section was analyzed by EPMA (X-ray microanalyzer).

This cast piece was pickled by a conventional method and then cold rolled,
Annealing etc. were performed and the 0.4-1.5 mm steel plate was created. Further, the uneven glossiness and surface characteristics of this steel sheet and the material obtained by subjecting the steel sheet to overhanging were investigated. The uneven gloss score is 1:
No uneven glossiness, 2: Uneven glossiness is recognized, 3:
Remarkably severe uneven gloss is observed. The surface property is evaluated as follows: 1: no unevenness is observed, 2: unevenness is observed,
3: Remarkably severe irregularities are recognized.

[Example 1] Casting was performed using the nozzle 6 shown in FIG. The width of the nozzle is 800
mm, the thickness is 90 mm at the molten steel surface position, and 50 mm at the lower end of the nozzle
And the immersion depth was 50 mm. As shown in Table 1,
The molten steel residence time in the molten pool was changed by changing the casting speed and the molten metal level (arc angle = angle formed by the line connecting the drum surface and the drum center with the horizontal line). Nos. 1 to 7 are examples of the present invention (having a retention time of 8 seconds or less), No. 8 is an example in which the retention time is deviated, and No. 9 is a conventional example. The results are shown in Table 1.
The material of the present invention was significantly superior in uneven gloss and surface characteristics as compared with the comparative material. Particularly, in No. 1, no partial negative segregation zone was observed.

[0018]

[Table 1]

Example 2 Casting was carried out using the partition weir 11 shown in FIG. 2 in which the drum-side surface of the partition weir is a flat plate. The partition weir had a width of 1200 mm, a thickness of 200 mm at the molten steel surface position, and a dipping depth of 150 mm. The weir was provided with circular or rectangular discharge holes 5, and casting was performed while controlling the molten metal level (arc angle) to be 40 ° so that the discharge holes were below the molten metal surface. At this time, the pool volume on the drum side was 205 kg. Discharge holes 15 were also provided on the side dam side. The casting speed is 50 m / min, and the slab thickness is 3.
It is 5 mm and the casting amount per unit time at this time is 3
It is 0.3 kg / s, and the molten steel residence time outside the partition weir is 5.
6s. The material of the present invention had a gloss unevenness rating and a surface characteristic rating of 1, and was significantly superior in gloss unevenness and surface characteristics as compared with the conventional material shown in No. 9 of Table 1.

[Example 3] Casting was carried out using the partition weir 11 shown in FIG. 3 in which the surface facing the drum of the partition weir is a flat plate. The partition weir has a width of 1000 mm and a thickness of 200 mm at the molten steel surface position. 15 to the weir from the bottom of the nozzle
An overflow port 14 was provided at a position of 0 mm to supply molten steel from the inside of the partition weir to the outside. The overflow hole 14 was fixed at a position where the molten metal surface level (arc angle) was 40 °. At this time, the pool volume on the drum side was 226 kg. The casting speed was 60 m / min, the cast piece thickness was 3.5 mm, the casting amount per unit time at this time was 36.3 kg / s, and the molten steel residence time outside the partition weir was 6.7 s. The material of the present invention has a gloss unevenness rating and a surface characteristic rating of 1, and
Compared with the conventional material shown in No. 9, the unevenness of gloss and the surface characteristics were remarkably excellent.

Example 4 As shown in FIG. 4, casting was performed using a partition weir 11 having an arc on the drum side surface of the partition weir. The partition weir had a width of 1200 mm, a thickness of 204 mm at the molten steel surface position, and an immersion depth of 180 mm. The radius of the circular arc provided on the drum side surface is 630
mm. A circular discharge hole 5 was provided in the weir, and casting was performed while controlling the molten metal surface level (arc angle) to be 40 ° so that the discharge hole was below the molten metal surface. The discharge hole 15 was also provided on the side dam side. At this time, the pool volume on the drum side is 189 kg
Met. Casting speed is 40m / min, slab thickness is 3.5mm
And the casting amount per unit time at this time was 24.2.
kg / s, and the molten steel residence time outside the partition weir is 7.8 s. The material of the present invention has a gloss unevenness score and a surface property score of 1
In comparison with the conventional material shown in No. 9 of Table 1, gloss unevenness and surface properties were remarkably excellent.

[0022]

INDUSTRIAL APPLICABILITY The present invention can inexpensively produce austenitic stainless steel having excellent surface characteristics. Therefore,
It can be said that the invention has extremely high industrial value.

[Brief description of drawings]

FIG. 1 shows a structural example of a nozzle used in Example 1 of the present invention. (A) is a partial sectional front view, (b) is a sectional view of the width center part.

FIG. 2 shows a structural example of a partition weir used in Example 2 of the present invention. (A) is a partial sectional front view, (b) is a sectional view of the width center part.

FIG. 3 shows a structural example of a partition weir having an overflow port used in Example 3 of the present invention. (A) is a partial sectional front view, (b) is a sectional view of the width center part.

FIG. 4 shows a structural example of a partition weir having a discharge hole used in Example 3 of the present invention. (A) is a partial sectional front view, (b)
Is a cross-sectional view of the width center portion.

FIG. 5 is a schematic diagram showing an example of a twin-drum type casting device.

FIG. 6 is an example of Ni distribution in the central portion of a thin cast piece. The white color indicates a high Ni concentration, and the black color indicates a low Ni concentration portion.

[Explanation of symbols]

1a, 1b Cooling drum 2a, 2b Side weir 3 Thin cast pieces 4 tundish 5,5 'outlet 6, 6'immersion nozzle 7 Molten metal 8 Hot water pool 9 Molten metal surface 10 Drum side pool 11 partition weir 12 Partition weir pool 13 Overflow port 14 Side weir side overflow port 15 Side weir side discharge hole

Continued front page    (72) Inventor Takehiko Fuji             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division (72) Inventor Keiji Tsusunari             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division F-term (reference) 4E004 DA13 NA05 SA03                 4E014 DB01

Claims (5)

[Claims] 1. A molten metal is poured into a molten metal pool formed by a part of the periphery of a pair of cooling drums and a side weir, and then the molten metal is cooled and solidified on the rotating peripheral surface of the cooling drum while thin-wall casting. In a twin-drum casting method for continuously casting pieces, a method for continuously casting thin-walled slabs having excellent surface properties, characterized in that the residence time of the molten metal in the molten metal pool in the pool is 8 seconds or less . Here, the residence time is the molten metal volume of the molten metal pool / the casting amount per unit time. 2. A thin-walled casting while injecting molten metal into a molten metal pool formed by a part of the periphery of a pair of cooling drums and side weirs, and then cooling and solidifying the molten metal on the rotating peripheral surface of the cooling drum. In the twin-drum casting method for continuously casting pieces, a partition weir that does not contact the drum is provided inside the molten metal pool in the pool, and the molten metal pool in the pool is surrounded by the partition weir and the outside of the partition weir. The continuous casting method for a thin cast slab having excellent surface properties, characterized in that the residence time of the molten metal in the molten metal pool outside the partition weir is 8 seconds or less. 3. A thin-walled casting while injecting molten metal into a molten metal pool formed by a part of the periphery of a pair of cooling drums and side weirs, and then cooling and solidifying the molten metal on the rotating peripheral surface of the cooling drum. In the twin-drum type casting method of continuously casting pieces, a downwardly projecting partition weir composed of a refractory wall is provided in the pool and the molten metal is supplied into the partition weir by a dipping nozzle to form the side wall of the partition weir. A continuous casting method for thin cast slabs having excellent surface properties, characterized in that molten steel is supplied to the outside of the partition weir from a discharge hole or an overflow port provided in and the residence time of the molten steel supplied to the outside is 8 seconds or less. . 4. A downwardly projecting partition weir made of refractory in a pool of water formed by side dams and a part of the periphery of a pair of cooling drums used in twin-drum casting. A partition casting weir for continuous casting, characterized in that a discharge hole or an overflow port for supplying molten metal to the outside is provided at a position on the surface of the molten metal or a position below the surface of the molten metal. 5. A thin-walled slab for continuously casting molten steel using the thin-walled slab casting method according to any one of claims 1 to 3, wherein the thin-walled slab is at both end portions in the plate thickness direction. A thin cast slab comprising a columnar crystal zone and an equiaxed crystal zone at the center of thickness, wherein there is no negative segregation zone having a degree of negative segregation of less than 0.90 at the center of thickness. Here, the degree of negative segregation is the concentration of Ni in the negative segregation zone / the average concentration of Ni in the columnar crystal zone.