JPH06328204A - Cooling roll for continuously casting thin cast slab and method for continuous casting and continuously cast slab - Google Patents

Abstract

PURPOSE:To prevent the surface crack on a strip product from a thin cast slab with a synchronous type continuous casting apparatus of twin roll system, etc., and the remain of projecting transfer mark from the recessed parts machined on the peripheral surface of cooling roll and the generation of the unevenness of luster at the same time. CONSTITUTION:The recessed parts 9 having 100-300mum diameter and 100-500mum depth are formed on the peripheral surface 8 of the cooling roll at 15-50% area ratio. Further, the recessed parts 9 having 400-1000mum diameter, 10-100mum depth and 45-75 deg. angle theta formed between the perpendicular line S to the tangent line of the cooling roll and the recessed side surface (t) are formed at 40-60% area ratio. In the atmosphere of the gas absorbed into molten metal or the mixed gas of the gas absorbed into the molten metal and inert gas non-absorbed into the molten metal (e.g. argon), the casting is executed.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art As a synchronous continuous casting process for producing a slab 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. The side dams 7 and 7 (the front side is not shown) are crimped to both end faces of a pair of cooling drums 2 and 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 dam 7, the molten metal forms a solidified shell at the contact portion with the cooling drum 2, and the solidified shell is kissed. The thin cast piece 5 is pressed by the pond 4 to form the thin cast piece 5. However, since the thin cast piece 5 has a thin plate thickness of about 1 to 7 mm, the surface property of the thin cast piece 5 is significantly affected by the formation state of the solidified shell, and the solidified shell 5 is solidified. Sometimes such a thickness nonuniformity defects such as cracks in the cast piece surface occurs.

In order to solve such a problem, it is necessary to provide a large number of depressions on the peripheral surface of the cooling drum.
It is disclosed in Japanese Patent Publication No. 449. This depression forms a gas gap between the cooling drum and the solidification shell, which serves as a heat insulating layer, and reduces the amount of heat removed from the cooling drum to allow slow cooling of the molten metal and make the thickness of the solidification shell uniform in the plate width direction. It is what

Further, in order to effectively prevent the surface cracking of the thin cast slab, there is a technique which defines the shape of the depressions on the peripheral surface of the cooling drum, the size of the depressions, the distribution of the depressions, etc., as disclosed in JP-A-1-83340. It is disclosed in Japanese Patent Application Laid-Open No. 1-83342, Japanese Patent Application Laid-Open No. 3-110044, etc., and is applied for in Japanese Patent Application No. 4-646661.

[0005]

In all of the conventional techniques described above, surface cracking of the cast slab is prevented by providing one or two depressions having a diameter and depth within a predetermined range on the entire peripheral surface of the cooling drum. There is. However, even if the cooling drum is provided with a depression on the entire circumferential surface, when the casting atmosphere is the atmosphere, the surface of the basin 3 shown in FIG. 1 is oxidized, and the oxide is caught and cracks occur. When the casting atmosphere is an inert gas (for example, Ar gas) that is insoluble in the molten metal, the depressions of the cooling drum are convexly transferred to the casting by the gas in the depressions that have undergone thermal expansion (convex projections are transferred to the casting surface. Therefore, it is not possible to control the solidification starting point of starting solidification from the peripheral portion of the convex transfer, and it is not possible to completely prevent cracking.

On the contrary, a cooling drum having a recess having a depth that can prevent surface cracking is used, and the surface of the cast product cast in a melt-soluble gas (for example, N ₂ gas) has a recess depth. When a convex transfer with a similar height is formed and the transfer height becomes large, it becomes a transfer mark after cold rolling and remains on the surface, or uneven gloss occurs due to uneven texture corresponding to the presence or absence of transfer, etc. It adversely affects the surface of thin plate products. It is an object of the present invention to simultaneously prevent surface defects and unevenness in gloss due to residual transfer traces of dents and unevenness in structure, which are defects of such thin plate products.

[0007]

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

In 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. Further, the thin-wall continuous casting slab is characterized in that the thickness is 1 to 7 mm, and convex transfer having a height of 100 μm or less is formed on the surface of the slab with an area ratio of 40 to 60%. .

[0009]

The present inventor has conducted various studies on the relationship between the surface properties of thin cast products and thin-plate products after cold rolling and the depressions of the cooling drum. A gas gap is formed in the slab, the solidified shell is slowly cooled, and a convex transfer 10 of a depression (FIG. 2) is formed in the slab to start solidification from the peripheral portion 11 of the convex transfer and to solidify the slab. We have found that it is necessary to make the width uniform. On the other hand, from the viewpoint of the surface properties of the thin plate product after cold rolling, if the convex transfer is too high, this convex transfer remains as a transfer mark on the surface of the thin plate product even after cold rolling, or uneven gloss due to the presence or absence of convex transfer is observed. It has also been found that it is necessary to reduce the height of the convex transfer because the surface is defective due to the occurrence of defects.

The depth of the depression required to obtain the above-mentioned action of forming the gas gap is 100 μm to 500 μm. When the thickness is less than 100 μm, the slow cooling effect is insufficient and cracks occur, and when it exceeds 500 μm, on the contrary, the slow cooling effect becomes too large, so that the thickness of the cast slab becomes small and the productivity deteriorates, which is not practical.

In order to obtain the action of starting the solidification of the slab from the peripheral portion of the convex transfer, a gas soluble in the molten metal (for example, N ₂ gas) or a gas soluble in the molten metal is used as the casting atmosphere. An inert gas that is insoluble in the molten metal (for example, Ar
It is necessary to cast in an atmosphere of mixed gas with (gas). In the case of mixed gas, the proportion of the gas soluble in the molten metal in the mixed gas must be 20% or more. When the casting atmosphere is the atmosphere, the surface of the pool 3 shown in FIG. 1 is oxidized, and the oxide is caught and cracks occur. Further, when the casting atmosphere is an insoluble gas which is insoluble in the molten metal, or when the mixed gas of the gas soluble in the molten metal and the inert gas insoluble in the molten metal has a gas ratio absorbed by the molten metal of less than 20%. In the case of, since the depression of the cooling drum is not convexly transferred to the slab by thermal expansion of the inert gas in the depression of the cooling drum, it is not possible to control the solidification starting point of starting solidification from the peripheral portion of the convex transfer, Cracks cannot be completely prevented. As a condition for the depression to obtain the above 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, convex transfer is not formed on the thin cast piece even in a gas (eg, N ₂ gas) atmosphere soluble in the molten metal, and surface cracking occurs. When the diameter is 300 to 400 μm, the convex transfer is formed, but the effect cannot be obtained because the convex transfer is not stably formed in the width direction. On the other hand, if the diameter exceeds 1000 μm, the convex transfer that initiates solidification becomes too large, and the effect of making solidification uniform in the width direction of the slab decreases, so that cracks are likely to occur.

In order to obtain the effect of leaving no transfer traces on the above-mentioned thin plate product, the diameter of the recess is set to 300 μm or less so that no convex transfer is formed on the thin cast piece, or the depth of the recess is set to 1
The height of the convex transfer formed to be 100 μm or less is 100 μm.
It must be m or less.

It is impossible to obtain the above-mentioned three actions by one type of depression, and in the present invention, as the depression for obtaining the above-mentioned actions and, the diameter is 100 to 300 μm and the depth is 100.
400 to 1000 μm in diameter as a recess for obtaining the above-mentioned action and
m and a depth of 10 to 100 μm (B pattern). Here, although it is possible to obtain the effect of and even if the diameter of the recess of the A pattern is less than 100 μm, it becomes difficult to stably process the recess because the depth becomes too deep with respect to the diameter. Further, even if the depth of the depressions of the B pattern is less than 10 μm, it is possible to obtain the effect of and, but as the casting progresses, stains (oxides of molten steel or deposits from molten steel) attached to the drum surface, It is not practical because it is not stable against long-time casting due to abrasion of the drum surface.

Therefore, in order to form a gas gap between the cooling drum and the solidification shell on the peripheral surface of the cooling drum to obtain only the effect of slowly cooling the solidification shell, the diameter of 100 to 30 is obtained.
0-50 μm, depth of 100-500 μm 15-50%
(A pattern), and in addition to this, a convex transfer mark does not exist in the thin plate product, and the transfer height is 100 μm.
In order to form the following convex transfer on the surface of the slab and control the solidification starting point, the diameter is 400 to 1000 μm and the depth is 1
An indentation having an angle θ ₁ = 45 to 75 ° formed by a line s perpendicular to the drum tangent line shown in FIG. 2 of 0 to 100 μm and an indentation side surface t is provided at an area ratio of 40 to 60% (B pattern), and cooled. SUS with drum thickness of 1 to 7 mm and width of 800 mm
When a thin cast piece of 304 stainless steel was manufactured, the surface quality of the thin plate product could be greatly improved.

Here, the shape of the convex transfer formed by the depressions of the B pattern is close to a part of the spherical surface, as shown in FIG.
As shown in Fig. 4, the diameter d of the depression is defined by the angle θ ₁ formed by the line s perpendicular to the drum contact line and the depression side surface t, and the contact angle θ ₂ between the molten steel and the depression side surface. The angle θ ₃ formed by the line is approximated to θ ₃ = 90 ° −θ ₁ −θ ₂ . The contact angle θ between the molten steel and the side surface of the depression
₂ depends on steel type and drum surface material, but 5 ° to 1
It is about 0 °.

If the angle θ ₃ formed between the spherical surface of the convex transfer and the drum contact line is larger than 40 °, the convex transfer portion is likely to fall during cold rolling regardless of the height of the convex transfer, and after cold rolling. However, transfer marks are likely to remain on the surface. In order to stably prevent the generation of transfer marks, the angle θ ₃ formed by the spherical surface of convex transfer and the drum contact line is 40 ° or less, that is, θ ₂ is 5 ° to 10 °.
Therefore, it is necessary to set the angle θ ₁ formed by the line perpendicular to the drum contact line and the side surface of the depression to be 45 ° or more. 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 recess is too smooth, so that solidification does not start stably from the peripheral edge of the recess and cracks easily occur.

The area ratio of each depression is required to be 40 to 60% because the pattern B has a large number of depressions with a small gap in the cooling drum width direction in order to start solidification uniformly in the width direction of the slab. Become. On the other hand, since the A pattern is for the purpose of gentle cooling, it does not require as many as the B pattern. Therefore, the area ratio of 15 to 50% is sufficient. On the contrary, when the pattern A dents are provided in the cooling drum with an area ratio higher than this, it is difficult to reliably provide the dents without connecting two or more dents, and as a result, two or more dents are connected and the There is a greater risk of creating large, deep depressions.

Laser processing is the most suitable for the recess processing of the pattern A because it is necessary to form recesses of small diameter and deep without connecting each other, but other photo etching processing, electron beam processing, plasma processing. Etc. are preferred. Further, the processing of the B pattern depression is performed by forming the depression having a larger diameter and shallower than the A pattern with a high area ratio and the shoulder angle θ ₁
Shot blasting, which allows large processing, is preferable, and it is preferable to adjust the projection pressure, the projection time, the shot grain size, the shot grain hardness and the like.

[0019]

EXAMPLE Austenitic stainless steel composed of the components shown in Table 1 was cast by a twin-drum type continuous casting machine shown in FIG. 1 into a strip-shaped thin slab 5 having a thickness of 3 mm, and cold-rolled to a thickness of 0.5 mm. Sheet products were manufactured.

[Table 1] When casting the slab 5, a width of 800 mm and a diameter of 120
The peripheral surfaces of the 0 mm cooling drums 2 and 2 were processed under the conditions shown in Table 2. In Table 2, the recess A pattern was processed by laser and the recess B pattern was processed by shot blast. In the example of the present invention, the B pattern was processed after processing the dent A pattern, but the processing order does not show a difference in the effect even if the A pattern is processed after processing the dent B pattern.

[Table 2] Further, FIG. 3 shows a cross section of the cooling drum processed under each condition of Table 2. This is a replica taken from the peripheral surface of the cooling drum, and a two-dimensional roughness meter was used to measure 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 recess of (A) after processing the recess of (A). Also (D)
Is an enlarged view of one depression in (B), which allows θ _{1 to} be measured.

The surface quality of the finally obtained thin plate product was as follows. In the case of No. 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, cracking occurs because the solidification start point control is impossible, and the surface quality is poor. In case of Nos. 2 and 6: Since the diameter of the A pattern is large, there is convex transfer on the slab and the solidification start point can be controlled. Furthermore, since the depth of the depression is deep and a sufficient cooling effect can be obtained, no cracking occurs. However, since the convex transfer height is too high, the convex transfer marks remain after cold rolling and the surface quality is poor. In the case of No. 3: There is no dent in the A pattern and the diameter of the B pattern is small, so it is not possible to control the solidification start point, and the slow cooling effect is insufficient, so cracks occur and the surface quality is poor. In case of No. 4: Only the pattern B depressions are present, so there is convex transfer on the slab and the solidification start point can be controlled, but since the depth of the depressions is small, the slow cooling effect is insufficient and cracking occurs. , The surface quality is poor. In case of No.7,8,9,10: There is a slight convex transfer on the slab, and the solidification start point can be controlled. Furthermore, since the depth of the depression is deep and a sufficient cooling effect can be obtained, no cracking occurs. In addition, since the convex transfer height is low, no convex transfer marks remain after cold rolling, the texture is minimal, and no gloss unevenness occurs in the product after cold rolling, and the surface quality is good. In the case of Nos. 11 and 12: Since the casting atmosphere is Ar, there is no convex transfer on the slab and cracking occurs because the solidification start point control is impossible, resulting in poor surface quality. In case of No. 13: There is a slight convex transfer on the slab, and the solidification start point can be controlled. Furthermore, since the depth of the depression is deep and a sufficient cooling effect can be obtained, no cracking occurs. Although the convex transfer height is low, the angle θ ₁ between the line perpendicular to the drum tangent and the side surface of the depression is as small as 20 °, so the angle θ ₃ formed between the side surface of the convex transfer and the drum contact line is about 63. As a result, the convex transfer part is likely to collapse during cold rolling regardless of the height of the convex transfer, and there are some transfer marks left on the surface after cold rolling, resulting in poor surface quality. In the case of No. 14: The angle between the line perpendicular to the tangent to the drum and the side surface of the recess is as large as θ ₁ = 80 °, so the control of the solidification start point is not stable and cracks occur in the slab, resulting in poor surface quality. . In the case of Nos. 15 and 16, since the casting atmosphere is the air, the surface of the pool 3 shown in FIG. 1 is oxidized and cracks are generated due to the entrainment of the generated oxide, resulting in poor surface quality. In addition, the above-mentioned convex transfer marks / gloss unevenness was determined by visual observation after cold rolling and pickling annealing of a thin cast piece, and texture unevenness was evaluated by microscopic observation after polishing / etching the surface of the cast piece.

[0021]

According to the present invention, when a thin strip product having a thickness close to that of a continuously cast product is cold-rolled to produce a thin plate product, the surface which is the largest surface defect of the thin plate product. Since the cracks are prevented and the convex transfer marks and uneven gloss remaining after cold rolling, which are conventionally sacrificed to prevent cracks, are also improved, the industrial effect thereof is extremely large.

[Brief description of drawings]

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

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

FIG. 3 is a representative example of a depression formed on the peripheral surface of a cooling drum, and (A) and (B) are comparative examples, and Examples No. 2 and No. 3 respectively.
And (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 ... Hot water pool 4 ... Kissing point 5 ... Thin cast piece 6 ... Pinch roll 7 ... Side dam 8 ... Drum surface 9 ... Recess 10 ... Cast piece convex transfer part 11 ... Convex transfer Peripheral part h ... convex transfer height of cast piece d ... hollow diameter θ ₁ ... angle formed by line perpendicular to tangential line of cooling drum and concave side surface θ ₂ ... between molten steel surface (cast piece convex transfer portion) and concave side surface Contact angle θ ₃ … An angle formed by the side surface of the convex transfer and the drum contact line t… Side surface of the depression s… Line perpendicular to the tangent line of the cooling drum

Front page continuation (72) Inventor Yamada Mamoru, Yamaguchi Prefecture, Hikari City 3434 Shimada, Nippon Steel Co., Ltd. Hikari Steel Works (72) Inventor Keiichi Yamamoto 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi (72) Inventor, Kunimasa Sasaki 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries, Ltd. Hiroshima Works

Claims (3)

[Claims] 1. The axes are horizontal and are installed parallel to each other,
In addition, the molten metal is injected into a molten metal pool formed by crimping side dams on both end faces of a pair of cooling drums rotating in opposite directions, and thin cast pieces having a thickness corresponding to the gap of the cooling drum are continuously cast. In the apparatus, a diameter of 10 is provided on the peripheral surface of the cooling drum.
15 depressions with a depth of 0 to 300 μm and a depth of 100 to 500 μm
Formed with an area ratio of ~ 50% and a diameter of 400
˜1000 μm, depth 10 to 100 μm, and a dent having an angle of 45 ° to 75 ° formed by a line perpendicular to the tangent of the peripheral surface of the cooling drum and the side surface of the dent is formed with an area ratio of 40 to 60%. A cooling drum for continuous casting of thin-walled slabs, which is characterized in that 2. In continuous casting using the cooling drum for continuous casting of thin-walled slabs according to claim 1, a gas soluble in the molten metal or a gas soluble in the molten metal and an inert gas insoluble in the molten metal. A method for casting a thin-walled slab, which comprises casting in an atmosphere of a mixed gas with. 3. 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%. Continuous casting slab.