JP3085820B2 - Cooling drum for continuous casting of thin cast slab, continuous casting method, and continuous cast slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous continuous casting method and apparatus such as a twin drum method, and more particularly, to a peripheral shape of a cooling drum used in the apparatus and a casting method using the apparatus. The present invention relates to a thin slab cast by the apparatus.

[0002]

2. Description of the Related Art As a synchronous continuous casting process for producing a cast piece having a thickness equal to or close to that of a hot strip by continuous casting, for example, a twin-drum type continuous casting apparatus has a shaft as shown in FIG. Side dams 7, 7 (the front side is not shown) are press-fitted to both end surfaces of a pair of cooling drums 2, which are horizontal, are installed in parallel with each other, and rotate in opposite directions. By injecting the molten metal from the tundish 1 into the pool 3 formed by the cooling drum 2 and the side weir 7, the molten metal forms a solidified shell at a contact portion with the cooling drum 2, and the solidified shell is kissing. The thin cast slab 5 is formed by pressure bonding with the point 4. However, since the thin cast slab 5 is as thin as about 1 to 7 mm, its surface properties are significantly affected by the formation state of the solidified shell. Sometimes such a thickness nonuniformity defects such as cracks in the cast piece surface occurs.

In order to solve such a problem, it has been proposed to provide a large number of depressions on the peripheral surface of a cooling drum as disclosed in Japanese Patent Application Laid-Open No. 60-184.
No. 449. This depression forms a gas gap between the cooling drum and the solidified shell, which serves as a heat insulating layer.The amount of heat removed from the cooling drum is reduced to slowly cool the molten metal, and the solidified shell thickness is made uniform in the plate width direction. It is assumed that.

Further, in order to efficiently prevent surface cracks of thin cast slabs, Japanese Patent Laid-Open No. 1-83340 discloses a technique in which the shape, size, and distribution of depressions on the peripheral surface of the cooling drum are specified. And Japanese Patent Application Laid-Open Nos. Hei 1-83342 and Hei 3-110044, and are filed by Japanese Patent Application No. 4-646661.

[0005]

In any of the conventional techniques described above, one or two types of depressions having a predetermined range of diameter and depth are provided on the entire peripheral surface of the cooling drum to prevent the surface crack of the slab. I have. However, even if a depression is provided on the entire peripheral surface of the cooling drum, if the casting atmosphere is air, the surface of the pool portion 3 shown in FIG. 1 is oxidized, and the oxide is involved and cracks occur. When the casting atmosphere is an inert gas (eg, Ar gas) that is insoluble in the molten metal, the cavities of the cooling drum are transferred to the slab by the gas in the thermally expanded cavities. ), So that the solidification starting point of starting solidification from the peripheral edge of the convex transfer cannot be controlled, and cracks cannot be completely prevented.

Conversely, the surface of a slab cast using a cooling drum having a depression deep enough to prevent surface cracking under a gas (eg, N ₂ gas) atmosphere soluble in the molten metal has a depth of depression. When a convex transfer with a close height is formed and the transfer height is increased, it becomes a transfer mark after cold rolling and remains on the surface, or uneven gloss due to uneven tissue corresponding to the presence or absence of transfer, etc. Affects the surface of sheet products. It is an object of the present invention to simultaneously prevent surface cracks and defects such as defects in transfer of pits and uneven gloss caused by unevenness in texture of thin sheet products.

[0007]

In order to achieve the above object, the present invention is characterized in that a small-diameter deep dent and a large-diameter shallow dent are provided on the peripheral surface of a cooling drum. Has a diameter of 100 to 300 μ on the peripheral surface of the cooling drum.
m, a depression having a depth of 100 to 500 μm is formed at an area ratio of 15 to 50%, and a diameter of 400 to 1000 μm.
m, a depth of 10 to 100 μm, and an angle between a line perpendicular to a tangent to a peripheral surface of the cooling drum and a side surface of the depression is 45 ° to
It is characterized in that a 75 ° depression is formed at an area ratio of 40 to 60%.

[0008] In the continuous casting using the cooling drum, casting is performed in an atmosphere of a gas soluble in the molten metal or a mixed gas of a gas soluble in the molten metal and an inert gas insoluble in the molten metal. It is characterized by the following. Further, the thin continuous cast slab is cast by the above-mentioned casting method, and a convex transfer having a thickness of 1 to 7 mm and a height of 100 μm or less is formed on the slab surface at an area ratio of 40 to 60%. It is characterized by the following.

[0009]

The present inventor has conducted various studies on the relationship between the surface properties of thin cast slabs and cold rolled sheet products and the depression of the cooling drum. By causing a gas gap to be formed and slowly cooling the solidified shell, and forming a convex transfer 10 (FIG. 2) of the depression in the cast piece, solidification is started from the peripheral edge portion 11 of the convex transfer and solidification is performed. We have found that it is necessary to make it uniform in the width direction. On the other hand, from the viewpoint of the surface properties of the sheet product after cold rolling, if the convex transfer is too high, the convex transfer remains as a transfer mark on the surface of the sheet product even after cold rolling, or uneven gloss due to the presence or absence of the convex transfer. It has also been found that it is necessary to reduce the height of the convex transfer, since the surface transfer occurs and the surface becomes defective.

[0010] The depth of the depression necessary to obtain the above-described action of forming the gas gap is 100 µm to 500 µm. If it is less than 100 μm, the slow cooling effect becomes insufficient and cracks occur. If it exceeds 500 μm, the slow cooling effect becomes too large.

In order to obtain the effect of initiating the solidification of the slab from the peripheral edge of the convex transfer, the casting atmosphere may be a gas soluble in the molten metal (for example, N ₂ gas) or a gas soluble in the molten metal. Inert gas insoluble in the molten metal (eg, Ar
It is necessary to perform casting in an atmosphere of a mixed gas with the gas). In the case of a mixed gas, the ratio of the gas soluble in the molten metal to the mixed gas needs to be 20% or more. When the casting atmosphere is air, the surface of the pool portion 3 shown in FIG. 1 is oxidized, and the oxide is involved, thereby generating cracks. When the casting atmosphere is an inert gas insoluble in the molten metal, or a mixed gas of a gas soluble in the molten metal and an inert gas insoluble in the molten metal, the gas ratio absorbed by the molten metal is less than 20%. In the case of, since the concave of the cooling drum is not transferred to the cast piece by the thermal expansion of the inert gas in the concave of the cooling drum, the solidification starting point of starting the solidification from the peripheral edge of the convex transfer cannot be controlled, Cracking cannot be completely prevented. As a condition of the depression for obtaining the above-mentioned action, a depression having a diameter of 400 μm to 1000 μm is required. When the diameter is 300 μm or less, due to the surface tension of the molten metal, no convex transfer is formed on the thin cast slab even in a gas (eg, N ₂ gas) atmosphere soluble in the molten metal, and a surface crack occurs. When the diameter is 300 to 400 μm, the convex transfer is formed, but the effect of not being formed stably in the slab width direction cannot be obtained. On the other hand, if the diameter exceeds 1000 μm, the convex transfer for starting the solidification becomes too large, and the effect of making the solidification uniform in the slab width direction decreases, so that cracks are likely to occur.

[0012] In order to obtain the above-mentioned action of leaving no transfer marks on the thin sheet product, the diameter of the recess is set to 300 µm or less so that the convex transfer is not formed on the thin cast slab or the depth of the recess is set to 1.
100 μm or less, the height of the convex transfer formed is 100 μm
m or less.

[0013] It is impossible to obtain the above three effects by one kind of depression. In the present invention, the depressions having the above effects are 100 to 300 μm in diameter and 100 depth.
A recess (A pattern) having a diameter of 400 to 1000 μm
m and a depth of 10 to 100 μm (B pattern). Here, even when the diameter of the depression of the pattern A is less than 100 μm, the above effect can be obtained, but the depth becomes too deep with respect to the diameter, so that it is difficult to process the depression stably. It is possible to obtain the above-mentioned effect even when the depth of the depression of the B pattern is less than 10 μm, but dirt (oxide of molten steel or deposits from molten steel) adhering to the drum surface as the casting proceeds. Due to the wear of the drum surface, it is not practical for long-time casting and is not practical.

Therefore, a gas gap is formed on the peripheral surface of the cooling drum between the cooling drum and the solidified shell to obtain only a slow cooling effect of the solidified shell.
0 μm, depth of 100 to 500 μm, 15 to 50%
(A pattern), and in addition, the transfer height where no convex transfer marks exist on the thin plate product is 100 μm.
In order to form the following convex transfer on the slab surface and control the solidification starting point, the diameter is 400 to 1000 μm and the depth is 1
A dent having an angle θ ₁ = 45 to 75 ° formed between a line s perpendicular to the drum tangent line shown in FIG. 2 and a side surface t of the dent and having a surface area of 40 to 60% (pattern B) was formed. SUS with 1-7mm thickness and 800mm width using drum
When a thin cast slab of 304 stainless steel was manufactured, the surface quality of a thin plate product could be greatly improved.

Here, the shape of the convex transfer formed by the depression of the B pattern is similar to a part of the spherical surface.
Is determined by the angle θ ₁ between the diameter d of the dent, the line s perpendicular to the drum contact line and the side surface t of the dent, and the contact angle θ ₂ between the molten steel and the side surface of the dent, as shown in FIG. angle theta ₃ between the line approximates the _{θ 3 = 90 ° -θ 1 -θ} 2. The contact angle θ between the molten steel and the side surface of the depression
₂ is 5 ° ~ 1 depending on steel type and drum surface material
It is about 0 °.

If the angle θ ₃ formed between the spherical portion of the convex transfer and the drum contact line is larger than 40 °, the convex transfer portion tends to fall down during cold rolling irrespective of the height of the convex transfer. Also, transfer marks easily remain on the surface. Stable to prevent occurrence of the transfer marks and, the angle theta ₃ between the spherical portion and the drum contact line of the convex transfer to 40 °, i.e. theta ₂ is 5 ° to 10
Because it is ° the angle theta ₁ between the side surface recess and a line perpendicular to the drum contact line it is necessary to more than 45 °. Also,
Angle θ between the line perpendicular to the drum contact line and the side of the depression
_{When 1} is 75 ° or more, the peripheral edge of the depression is too smooth, so that solidification from the peripheral edge of the depression does not occur stably, and cracks are likely to occur.

Regarding the area ratio of each dent, the pattern B needs to be 40 to 60% because a large number of dents are provided by narrowing the gap in the width direction of the cooling drum in order to start solidification uniformly in the slab width. Become. On the other hand, since the pattern A is for the purpose of slow cooling, the number of the patterns A is not required. Therefore, an area ratio of 15 to 50% is sufficient. Conversely, when the depressions of the pattern A are provided in the cooling drum at an area ratio greater than this, it is difficult to reliably provide the depressions without connecting two or more depressions. The possibility that a large, deep dent is formed increases.

Laser processing is the most suitable for the pattern A dent processing because it is necessary to form small-diameter, deep dents without connecting them to each other. However, photoetching, electron beam processing, plasma processing, etc. Are preferred. In the processing of the B pattern depression, a shallow depression having a larger diameter and a larger area ratio than the A pattern has a high area ratio and a shoulder angle θ _1.
The shot blasting process, which can process a large amount, is optimal, and is preferably performed by adjusting the projection pressure, the projection time, the shot particle size, the shot grain hardness, and the like.

[0019]

EXAMPLE An austenitic stainless steel having the components shown in Table 1 was cast into a strip-shaped thin cast piece 5 having a thickness of 3 mm by a twin-drum continuous casting machine shown in FIG. Was manufactured.

[Table 1] When casting the slab 5, the width is 800 mm and the diameter is 120 mm.
The peripheral surfaces of the 0 mm cooling drums 2 and 2 were machined under the conditions shown in Table 2. In Table 2, the depression A pattern was processed by laser, and the depression B pattern was processed by shot blast. In the example of the present invention, the B pattern is processed after processing the depression A pattern, but there is no difference in the processing order even if the A pattern is processed after processing the depression B pattern.

[Table 2] FIG. 3 shows a cross section of the cooling drum processed under the conditions shown in Table 2. This is obtained by collecting a replica from the peripheral surface of the cooling drum and measuring the replica using a two-dimensional roughness meter based on the replica. (A) shows an example of No1, (B) shows an example of No2, and (C) shows an example of No9. (C) is obtained by processing the dent of (B) after the processing of dent of (A). Also (D)
Is an enlarged view of one dent of (B), from which θ ₁ can be measured.

The surface quality of the finally obtained sheet product was as follows. In the case of Nos. 1 and 5: There is no dent in the B pattern or the diameter is small, so there is no convex transfer on the slab, and since the solidification starting point cannot be controlled, cracks occur and the surface quality is poor. In the case of Nos. 2 and 6: Since the diameter of the A pattern is large, there is a convex transfer on the slab, and the solidification start point can be controlled. Further, since the depth of the depression is deep and a sufficient cooling effect can be sufficiently obtained, there is no occurrence of cracks. However, since the convex transfer height is too high, convex transfer marks remain even after cold rolling, and the surface quality is poor. In the case of No. 3: The pattern A had no depressions and the diameter of the pattern B was small, so that the solidification starting point could not be controlled, and the cooling effect was insufficient, so that cracks occurred and the surface quality was poor. In the case of No. 4: Since there is only the depression of the B pattern, there is a convex transfer on the slab and the solidification start point can be controlled. However, since the depth of the depression is small, the slow cooling effect is insufficient and cracks occur. , Surface quality is poor. For Nos. 7, 8, 9, and 10: There is a slight convex transfer on the slab, and solidification start point control is possible. Further, since the depth of the depression is deep and a sufficient cooling effect can be sufficiently obtained, there is no occurrence of cracks. In addition, since the convex transfer height is low, no convex transfer marks remain after cold rolling, the unevenness of the structure is very small, and the product after cold rolling does not have uneven gloss, and the surface quality is good. In the case of Nos. 11 and 12: Since the casting atmosphere was Ar, there was no convex transfer on the slab, and it was impossible to control the solidification start point, causing cracks and poor surface quality. In the case of No. 13: There is a slight convex transfer on the slab, and the solidification start point can be controlled. Further, since the depth of the depression is deep and a sufficient cooling effect can be sufficiently obtained, there is no occurrence of cracks. Although the height of the convex transfer is low, the angle θ ₁ between the line perpendicular to the tangent to the drum and the side surface of the recess is as small as 20 °, so that the angle θ ₃ between the side surface of the convex transfer and the drum contact line is about 63 °. °, the convex transfer portion tends to fall down during cold rolling regardless of the height of the convex transfer, and there are portions where transfer marks remain on the surface even after cold rolling, and the surface quality is poor. In the case of No. 14, the angle between the line perpendicular to the tangent to the drum and the side surface of the depression is as large as θ ₁ = 80 °, so the control of the solidification start point is not stable, the slab is cracked, and the surface quality is poor. . In the case of Nos. 15 and 16, since the casting atmosphere was air, the surface of the pool 3 shown in FIG. 1 was oxidized, cracks were generated due to the entrainment of the generated oxide, and the surface quality was poor. The above-mentioned convex transfer marks and gloss unevenness were determined by visual observation of the thin cast slab after cold rolling and pickling annealing, and texture unevenness was determined by microscopic observation after polishing and etching the cast slab surface.

[0021]

According to the present invention, in producing a sheet product by cold-rolling a strip-shaped slab having a thickness close to the thickness of a continuously cast product, the surface defect which is the largest surface defect of the sheet product is produced. Since the cracks are prevented and the convex transfer marks and uneven gloss remaining after the cold rolling, which have conventionally been sacrificed to prevent the cracks, are also improved, the industrial effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a front view showing a twin-drum continuous caster incorporating a cooling drum of the present invention.

FIG. 2 is a cross-sectional view of a dent and a cast slab convex transfer formed on a peripheral surface of a cooling drum.

FIGS. 3A and 3B are representative examples of depressions formed on the peripheral surface of a cooling drum, and FIGS. 3A and 3B are comparative examples;
(C) is a sectional view of Example No. 8 of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Tundish 2 ... Cooling drum 3 ... Pool part 4 ... Kissing point 5 ... Thin cast piece 6 ... Pinch roll 7 ... Side dam 8 ... Drum surface 9 ... Depression 10 ... Convex transfer part of cast piece 11 ... Convex transfer Peripheral edge h: Casting convex transfer height d: Depressed diameter θ ₁ : Angle formed by a line perpendicular to the cooling drum tangent line and the concave side surface θ ₂ : Between molten steel surface (cast slab convex transfer portion) and concave side surface Contact angle θ ₃ … Angle between the side surface of the convex transfer and the drum contact line t… Depressed side surface s… Line perpendicular to the cooling drum tangent

Continued on the front page (72) Inventor Mamoru Yamada 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (72) Inventor Keiichi Yamamoto 4-2-2 Kanon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Mitsubishi Heavy Industry Co., Ltd., Hiroshima Laboratory (72) Inventor Kunimasa Sasaki 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Mitsubishi Heavy Industries, Ltd. Hiroshima Works (56) References JP-A-4-238651 (JP, A) JP-A-3-66458 (JP, A) JP-A-6-297110 (JP, A) JP-A-6-134553 (JP, A) JP-A-5-7997 (JP, A) JP-A-4-158957 (JP JP, A) JP-A-4-52053 (JP, A) JP-A-5-261487 (JP, A) JP-A-3-110044 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) B22D 11/06 330 B22D 11/10 360

Claims (3)

(57) [Claims] Claims: 1. An axis is horizontal and installed parallel to each other;
In addition, molten metal is poured into a pool formed by pressing side weirs on both end surfaces of a pair of cooling drums rotating in opposite directions, and a thin cast piece having a thickness corresponding to a gap between the cooling drums is continuously cast. In the apparatus, the diameter of the cooling drum is 10
0-300 μm, depth of 100-500 μm is 15
Formed with an area ratio of about 50% and a diameter of 400
A dent having a surface area of 40 to 60% is formed in which the angle between the line perpendicular to the tangent to the peripheral surface of the cooling drum and the side surface of the depression is 45 to 75 degrees. A cooling drum for continuous casting of thin cast slabs, characterized in that: 2. In the continuous casting using the cooling drum for continuous casting of thin cast slab according to claim 1, a gas soluble in the molten metal, a gas soluble in the molten metal, and an inert gas insoluble in the molten metal. Casting in a mixed gas atmosphere with a thin cast slab. 3. Casting by the casting method according to claim 2.
A thin slab having a thickness of 1 to 7 mm, wherein a convex transfer having a height of 100 μm or less is formed on the surface of the slab at an area ratio of 40 to 60%. Pieces.