JP7758944B2 - Dummy sheet for twin-drum continuous casting and method for manufacturing thin-walled cast slabs - Google Patents

Description

The present invention relates to a twin-drum continuous casting dummy sheet used at the start of casting in a twin-drum continuous casting machine that produces thin-walled slabs by supplying molten metal to a molten metal reservoir formed by a pair of cooling drums and a pair of side weirs, and a method for producing thin-walled slabs.

A method for producing thin-walled steel slabs is provided using a twin-drum continuous casting machine equipped with cooling drums with internal water-cooling structures. Molten steel is supplied to a molten steel reservoir formed between a pair of rotating cooling drums, solidified shells are formed and grown on the circumferential surfaces of the cooling drums, the solidified shells formed on the circumferential surfaces of the pair of cooling drums are joined at the drum kiss points, and the casting is reduced to produce thin-walled slabs of a predetermined thickness.

When starting casting in this type of twin-drum continuous casting method, as shown in Patent Documents 1 and 2, for example, a dummy sheet is sandwiched between the cooling drums, and the cooling drums are rotated while molten steel is supplied to a molten steel reservoir formed by a pair of cooling drums and a pair of side weirs, forming a thin-walled cast piece connected to the dummy sheet, and then the dummy sheet and the thin-walled cast piece connected to this dummy sheet are pulled out from between the cooling drums.

At the start of casting, casting conditions such as the amount of molten steel supplied to the molten steel reservoir and the temperature of the cooling drum are unstable, which can result in the thin billet connected to the dummy sheet being weak and causing problems such as the thin billet breaking when the dummy sheet is pulled out, making it impossible to start casting. For this reason, the above-mentioned Patent Documents 1 and 2 propose techniques for securing the strength of the thin billet at the start of casting by arranging a reinforcing member connected to the dummy sheet and casting this reinforcing member in molten metal. For example, Patent Document 2 proposes a twin-drum continuous casting dummy sheet 30, as shown in Figure 8, in which a thin or thick reinforcing member 38 made of a high-melting-point material is attached to the tip of the sheet body.

Here, the solidified shell is assumed to form on the circumferential surface of the cooling drum. However, in actual operation, molten steel may also solidify on the surface of the side weir, forming ingots. As this ingot grows and fuses with the solidified shell on the circumferential surface of the rotating cooling drum at the end of the cooling drum, the ingots are peeled off from the surface of the side weirs and, together with the solidified shell, are caught in the drum kissing points and sent downward in the casting machine. At this time, the drum gap temporarily expands beyond the actual thickness of the slab in an attempt to pass the thick ingots, and areas not involving the ingots (such as the center in the width direction) contain a large amount of unsolidified, high-temperature molten steel. This is called a hot band. Because this hot band is hotter and more fragile than the sound areas before and after it in the casting direction, it can break under the weight of the thin-walled slab.

To prevent breakage caused by hot bands and ensure stable casting, it is important to prevent the formation and coarsening of ingots on the side dams.
Hot bands are often caused by the separation of ingots that form on the side gates when the molten metal level is unstable. They also tend to form during the initial stage of casting, when the molten metal level rises. This is because the molten metal is first brought into contact with the side gate surface, which had not previously been in contact with molten metal, and is then cooled, making it easy for ingots to form.

As a method for preventing breakage of thin-walled cast slabs due to hot bands, for example, Patent Document 3 proposes a method of sufficiently preheating the side dams to suppress the generation of ingots on the surface of the side dams at the start of casting.

Japanese Unexamined Patent Publication No. 57-058957 Japanese Unexamined Patent Publication No. 63-224847 Japanese Unexamined Patent Publication No. 62-124051

However, even when the side weirs are preheated as in Patent Document 3, the side weirs are pressed against the end surface of the cooling drum to form a molten steel reservoir, which inevitably results in a drop in the temperature of the side weirs. In particular, when the molten steel level rises to a steady level in the early stages of casting, the molten steel first comes into contact with the side weirs, whose temperature has been reduced by being pressed against the end surface of the cooling drum, making it more likely that ingots will form and increasing the risk of breakage due to hot bands. Furthermore, the higher the melting point of the steel, the more likely it is that the side weirs need to be preheated to a high temperature. Therefore, in the case of carbon steels, which have a relatively higher melting point than stainless steels, the side weirs are not preheated sufficiently, making them more likely to develop hot bands.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a twin-drum continuous casting dummy sheet that, even if metal is formed and grows on the surface of the side dam at the start of casting, removes the metal at an early stage before it coarsens, thereby suppressing breakage of thin-walled slabs caused by hot bands and enabling a stable start to casting, as well as a method for manufacturing thin-walled slabs using this twin-drum continuous casting dummy sheet.

[1] A twin-drum continuous casting dummy seat used in a twin-drum continuous casting apparatus that supplies molten steel to a molten steel reservoir formed by a pair of rotating cooling drums and a pair of side weirs, and produces thin-walled cast strips by forming and growing a solidified shell on the circumferential surface of the cooling drums, the dummy seat comprising: a seat body sandwiched between the pair of cooling drums; and a reinforcing member extending from one end of the seat body in the longitudinal direction on the cast strip connection side and inserted into the molten steel, the reinforcing member comprising:
(a) one or two plate-shaped members disposed along the circumferential surface of one or both of the pair of cooling drums;
(b) at each end on both sides in the width direction,
(b1) Within a range of 10 mm or less from a plane contacting the end surface of the cooling drum,
(b2) a front end surface in the casting direction, which is composed of either or both of an inclined surface inclined in a direction in which the distance d from the plane tangent to the end surface of the cooling drum decreases as casting progresses, and a vertical surface that is perpendicular to the casting direction and faces forward in the casting direction;
(b3) A twin-drum continuous casting dummy seat, characterized in that the casting direction front end surface is located at one or more locations within any 200 mm range in the casting direction.

[2] The twin-drum continuous casting dummy seat according to [1], characterized in that the shape of the inclined surface satisfies formula (1).
Δd/ΔL≦-5/100...(1)
Here, Δd is the change in the distance d of the inclined surface from the plane that contacts the end surface of the cooling drum, and a decrease in d is considered negative, and ΔL is the movement distance of the reinforcing member in the casting direction corresponding to Δd.

[3] The reinforcing member is
(a') Instead of the one or two plate shapes, a plurality of strip-shaped plates extending in the casting direction are arranged without any gaps in the width direction or with a predetermined gap, so that the sum of the widths of the plurality of strip-shaped plates, including the width of the gaps, is the same as the width of the one or two plate shapes, forming an aggregate of strip-shaped reinforcing members;
(b') The dummy seat for twin-drum continuous casting according to [1] or [2], characterized in that, of the band-shaped reinforcing members, the band-shaped reinforcing members located at both ends in the width direction of the sheet body have a front end face in the casting direction at their outer widthwise ends.

[4] The reinforcing member is
(a'') Instead of the two plate shapes, a plurality of strip-shaped plates extending in the casting direction are arranged without any gaps in the width direction or at predetermined gaps, to form an assembly of strip-shaped reinforcing members such that the sum of the widths of the plurality of strip-shaped plates, including the width of the gaps, is the same as the width of the two plate shapes, and further, excluding the strip-shaped reinforcing members located at both ends in the width direction of the sheet body or including the strip-shaped reinforcing members located at both ends in the width direction of the sheet body, the strip-shaped reinforcing members arranged in the width direction are arranged alternately, with one strip-shaped reinforcing member for one cooling drum and one strip-shaped reinforcing member for the other cooling drum being omitted from each other,
(b') The dummy seat for twin-drum continuous casting according to [1] or [2], characterized in that, of the band-shaped reinforcing members, the band-shaped reinforcing members located at both ends in the width direction of the sheet body have a front end face in the casting direction at their outer widthwise ends.

[5] A method for producing thin slabs by supplying molten steel to a molten steel reservoir formed by a pair of rotating cooling drums and a pair of side weirs, and forming and growing a solidified shell on the circumferential surface of the cooling drums, characterized in that a twin-drum continuous casting dummy sheet described in any one of [1] to [4] is used at the start of casting.

As described above, the present invention provides a twin-drum continuous casting dummy sheet that, even if metal is generated and grows on the surface of the side dam at the start of casting, can remove the metal at an early stage before it coarsens, thereby preventing breakage of thin-walled slabs caused by hot bands and enabling a stable start to casting. It also provides a method for manufacturing thin-walled slabs using this twin-drum continuous casting dummy sheet.

FIG. 1 is an explanatory diagram illustrating an example of a twin-drum continuous casting apparatus according to an embodiment of the present invention. 1A and 1B are explanatory views of a dummy seat for twin-drum continuous casting according to one embodiment of the present invention, in which FIG. 1A is a side view and FIG. 1B is a view taken along the line XX. 1 is an explanatory view of a twin-drum continuous casting dummy seat provided with a plate-shaped reinforcing member according to a first embodiment of the present invention. [0023]FIG. FIG. 10 is an explanatory view of a twin-drum continuous casting dummy seat provided with a strip-shaped reinforcing member according to a second embodiment of the present invention. FIG. 10 is a plan view illustrating a twin-drum continuous casting dummy sheet having strip-shaped reinforcing members alternately arranged in the width direction, which is a third embodiment of the present invention, attached to a twin-drum continuous casting apparatus before the start of casting. 1 is an explanatory diagram of the dimensional relationship between different examples of end boundary shapes at the width direction end of a reinforcing member provided in a twin-drum continuous casting dummy seat according to one embodiment of the present invention. FIG. 1 is an explanatory diagram of the dimensional relationship of an example in which the end boundary shape at the width direction end of a reinforcing member provided in a twin-drum continuous casting dummy seat according to one embodiment of the present invention includes a partial circle. FIG. FIG. 10 is an explanatory view of a linear reinforcing member provided in a twin-drum continuous casting dummy seat according to the prior art.

Below, a twin-drum continuous casting dummy sheet (hereinafter simply referred to as a dummy sheet) and a method for manufacturing a thin-walled cast slab using the dummy sheet, which are embodiments of the present invention, will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments.

The thin-walled cast slab 1 produced in this embodiment is made of steel of various compositions, and has a width in the range of 100 mm to 2000 mm, and a thickness in the range of 1 mm to 6 mm.

As shown in FIG. 1, the twin-drum continuous casting apparatus 10 of this embodiment comprises a pair of cooling drums 11, 11, bender rolls 12, 12 that bend the thin cast slab 1, pinch rolls 13, 13 that support the thin cast slab 1, side weirs 15 arranged at the widthwise ends of the pair of cooling drums 11, 11, a tundish 18 that holds molten steel 3 to be supplied to a molten steel reservoir 16 defined by the pair of cooling drums 11, 11 and the side weirs 15, and an immersion nozzle 20 that supplies molten steel 3 from the tundish 18 to the molten steel reservoir 16.

In this twin-drum continuous casting apparatus 10, the molten steel 3 comes into contact with the rotating cooling drums 11, 11 and is cooled, causing solidified shells 5, 5 to grow on the circumferential surfaces of the cooling drums 11, 11. The solidified shells 5, 5 formed on each of the pair of cooling drums 11, 11 are then pressed together at the drum kiss point K (see Figure 2), thereby casting a thin-walled cast strip 1 of a predetermined thickness.

In the twin-drum continuous casting apparatus 10 of this embodiment, at the start of casting, as shown in FIG. 2, a twin-drum continuous casting dummy seat 30 of this embodiment is disposed between the pair of cooling drums 11, 11.
As shown in FIG. 2 , this twin-drum continuous casting dummy seat 30 includes a seat body 31 disposed below the pair of cooling drums 11, 11 to the drum kiss point K, and a reinforcing member 32 disposed to extend from one longitudinal end of the seat body 31 on the slab connection side into the molten steel reservoir 16.

In this state, molten steel 3 is poured from the tundish 18 through the submerged nozzle 20 toward the molten steel reservoir 16. When the surface of the molten steel 3 in the molten steel reservoir 16 reaches a predetermined position, the cooling drums 11, 11 are rotated in the R direction (see Figure 1). The molten steel 3 then moves between the cooling drums 11, 11 along with the reinforcing member 32. As the molten steel 3 solidifies, the reinforcing member 32 and the thin billet 1 are joined together, and the thin billet 1 is then pulled out following the dummy sheet 30.

The reinforcing member 32 is formed from a plate of a high-melting point metal, such as Mo, and the plate thickness can be 0.2 mm or more, preferably 0.5 mm or more and 2 mm or less, taking into account strength and workability.

Here, when molten steel 3 is poured into the molten steel reservoir 16, base metal may form and grow on the surface of the side weir 15. In particular, at the triple junction where the side weir 15, cooling drum 11, and molten steel 3 come into contact, or more precisely, at the area where such triple junctions are connected along the circumferential surface of the cooling drum (also simply referred to as the triple junction area), the molten steel 3 is cooled from two directions, by the cooling drum 11 and the side weir 15, and base metal is likely to form on the surface of the side weir 15 in the vicinity. This base metal coarsens and becomes caught in the drum kiss point K, causing severe hot bands and resulting in the fracture of the thin-walled cast slab 1.

In order to remove the base metal that is likely to form at the triple junction area before it becomes coarse, in the dummy sheet 30 of this embodiment, after it is formed, the attached base metal in the early growth stage is removed by applying an external force from the thick portion of the solidified shell 5, which is formed by the boundary shape of each end on both sides of the width of the reinforcing member 32 arranged nearby.

Here, we will explain the thickened portion of the solidified shell 5 near the reinforcing member 32 on the cooling drum 11, which is created by the boundary shape of both widthwise ends of the reinforcing member 32. When a reinforcing member 32 is present nearby, the reinforcing member 32, made of a high-melting-point metal, acts as a coolant itself. This means that the thickness of the solidified shell 5 is the total thickness resulting from the cooling action of the reinforcing member 32, in addition to the thickness resulting from the cooling action of the cooling drum 11. By utilizing this phenomenon, for example, near a reinforcing member 32 with an end boundary shape that slopes so that the distance d to the plane tangent to the end face of the cooling drum (i.e., the surface of the side weir 15 at the beginning of casting) decreases as casting progresses, a thickened portion can be formed in which the thickness of the solidified shell 5 is thicker than before and after the casting direction due to the increased cooling action of the reinforcing member 32 as the distance d decreases. By applying an external force from the thickened portion of the solidified shell 5 to the deposited metal, the deposited metal can be removed from the surface of the side weir 15.

However, given the relationship between the immobile base metal adhering to the surface of the side weir 15 and the solidified shell 5 that forms on the circumferential surface of the rotating cooling drum 11 and moves in the casting direction, the boundary shape of the reinforcing member end must have a portion where the distance d to the surface of the side weir 15 decreases in the casting direction. If the distance d remains constant in the casting direction, the increase in thickness of the solidified shell 5 due to the cooling action of the reinforcing member 32 also remains constant in the casting direction. Since the distance between the reinforcing member and the base metal adhering to the surface of the side weir 15 remains constant, new external forces cannot be applied to the adhering base metal. Furthermore, if the boundary shape of the reinforcing member end is sloped so that the distance d to the surface of the side weir 15 increases as casting progresses, the cooling effect of the reinforcing member decreases near the end of the reinforcing member, preventing the formation of a thick portion where the solidified shell 5 is thicker than before and after the casting direction. Consequently, the distance between the base metal adhering to the surface of the side weir 15 and the solidified shell 5 increases, preventing the application of new external forces to the adhering base metal.

Therefore, in the twin-drum continuous casting dummy seat 30 according to the first embodiment of the present invention, as shown in FIGS. 3 and 6, the reinforcing member 32 is
(a) One or two plate-shaped reinforcing members 32 a arranged along the circumferential surface of one or both of the pair of cooling drums 11;
(b) At each end of both sides in the width direction of the reinforcing member 32 (32a),
(b1) Within a range of 10 mm or less from a plane that contacts the end surface of the cooling drum 11,
(b2) a casting direction front end surface 35 including either or both of an inclined surface 35a inclined in a direction in which the distance d from a plane tangent to the end surface of the cooling drum 11 decreases as casting proceeds, and a vertical surface 35b perpendicular to the casting direction and facing forward in the casting direction;
(b3) The casting direction front end surface 35 is provided at one or more locations within any range of 200 mm in the casting direction.

The thick-walled portions at the widthwise ends of the solidified shell 5 formed in this manner are relatively thicker than the portion of the solidified shell 5 ahead in the casting direction. Therefore, after contacting the metal adhering to the surface of the side weir 15 near the triple junction, they slip between the cooling drum 11 and the adhering metal, pushing the adhering metal radially outward from the cooling drum 11. As a result, the adhering metal separates from the side weir surface, moving parallel to the surface of the side weir 15 and being pushed radially outward from the cooling drum 11. Before this separation, the pressing force generated between the thick-walled portions and the adhering metal fuses the metal to the solidified shell, and the metal eventually moves along with the solidified shell 5 and becomes entrained at the drum kiss point K. Because the metal entrained at the drum kiss point K is not yet coarsened, it is ultimately discharged from the molten metal pool without generating a severe hot band.

Here, the plane of the dummy sheet 30 that contacts the end face of the cooling drum 11 is assumed to be the plane that contacts the end face of the cooling drum 11 at the start of casting after the dummy sheet 30 is attached to the twin-drum continuous casting apparatus 10. Therefore, for the dummy sheet 30 before attachment to the twin-drum continuous casting apparatus 10, the plane that contacts the end face of the cooling drum 11 corresponds to the plane with which the widthwise end of the cast strand contacts, and the distance d is determined based on this plane. In practice, the twin-drum continuous casting dummy sheet 30 is attached to the twin-drum continuous casting apparatus 10 before the start of casting so that the plane that contacts the end face of the cooling drum 11, as defined by the dummy sheet 30, coincides with the plane that contacts the end face of the cooling drum 11 before the start of casting. Furthermore, for the dummy sheet 30 before attachment to the twin-drum continuous casting apparatus 10, the reinforcing member 32 may be attached to the sheet body 31 so as to ensure the distance d from the plane that contacts the end face of the cooling drum 11 after aligning the widthwise center line of the sheet body 31 with the center line of the cooling drum 11. The reinforcing member 32 can be attached to the sheet body 31, for example, by passing a thin metal wire through holes 36 provided in the reinforcing member 32 and the sheet body 31.

Furthermore, the end faces of the reinforcing member 32 (32a) at both ends in the width direction are not limited to flat surfaces perpendicular to the plate surface of the reinforcing member 32 (32a), but also include inclined surfaces and rounded curved surfaces. If this end face is inclined or curved, the distance d is defined as the shortest distance to the plane tangent to the end face of the cooling drum 11.

In this embodiment, the reinforcing member 32 (32a) has, at each end on both sides in the width direction, a casting direction front end face 35, which comprises either or both of: (b1) an inclined surface 35a within a range of 10 mm or less from the plane tangent to the end face of the cooling drum 11, (b2) which slopes in a direction in which the distance d from the plane tangent to the end face of the cooling drum 11 decreases as casting progresses, and (b3) a vertical surface 35b which is perpendicular to the casting direction and faces forward in the casting direction. The casting direction front end face 35 is the front end face in the casting direction of the width protruding portion 34 which is disposed to protrude outward in the width direction as the end boundary shape of both width direction ends of the reinforcing member 32 (32a). (b1) is a condition for the range in the width direction of the formation of a thick portion of the solidified shell 5 for applying an external force, since the metal to which an external force is applied in this embodiment is located near the triple junction. It is also a condition for the range in the width direction of the front end face 35 of the width-protruding portion 34 in the casting direction, for achieving a thickness change in the solidified shell 5 in the casting direction that fully exhibits the effects of this embodiment. (b2) is a condition for forming a thick portion in which the solidified shell is relatively thicker than those before and after the casting direction. Even if the distance d is within a range of 10 mm or less, if the distance d is constant, the thickness of the solidified shell 5 at the width direction end will be constant except for a limited region in the casting direction behind the vertical surface 35 b (front end face 35 in the casting direction) of the width-protruding portion 34, which has a vertical surface perpendicular to the casting direction (see "Converted Line Fraction Rate (%) of Region (A)" described later). Therefore, even if the deposited metal and the solidified shell come into contact with each other, no pressing force will be generated, and the deposited metal removal effect of this embodiment will not be achieved. Furthermore, if the distance d exceeds 10 mm, the thickness of the solidified shell 5 at the widthwise end will decrease, so no pressing force will be generated and the effect of removing the adhering metal of this embodiment will not be achieved.

The above (b1) and (b2) must be satisfied simultaneously in order to remove the base metal. In the following explanation, the region where both (b1) and (b2) are satisfied will be referred to as region (A). In region (A), the thickness of the solidified shell 5 at the width direction end varies depending on the distance d. The thick portion of the solidified shell 5 at region (A) directly removes the base metal. On the other hand, in regions where either (b1) or (b2) is not satisfied, a solidified shell 5 that has the effect of removing the base metal cannot be formed. In the following explanation, this region will be referred to as region (B). Regions where the distance d exceeds 10 mm do not satisfy (b1). Regions where the distance d is constant or increases do not satisfy (b2).

The preferred range for the distance d of the casting direction front end surface 35, which forms portion (A) of the reinforcing member 32 (32a), is 5 mm or less, because the shorter the distance d, the stronger the pressing force when the solidified shell 5 at the widthwise end contacts the adhering metal, improving the metal removal effect. However, if the distance d is too short, the widthwise protruding portion 34 forming the boundary shape of the widthwise end of the reinforcing member 32 (32a) may scrape the surface of the side dam 15, potentially destroying the side dam 15. Therefore, it is sufficient for the end of the reinforcing member 32 (32a) to be close enough to the side dam 15 so as not to come into contact with it. On the other hand, the distance d of portion (B) of the reinforcing member 32 (32a) may exceed 10 mm.

The shape of the inclined surface 35a of (b2) is preferably such that it satisfies the formula (1).
Δd/ΔL≦-5/100...(1)
Here, Δd is the change in the distance d from the plane that contacts the end surface of the cooling drum 11 to the inclined surface 35a (front end surface 35 in the casting direction) of the widthwise protrusion 34, and is considered negative when d decreases, and ΔL is the distance moved in the casting direction by the reinforcing member 32 (32a) corresponding to Δd (ΔL is an absolute value and takes a positive value). Note that when the front end surface 35 in the casting direction of the widthwise protrusion 34 is a vertical surface 35b that is perpendicular to the casting direction, ΔL can be considered to be 0, and the value of the left side of equation (1) becomes negative infinity, so equation (1) is also valid in this case.

An effective means of changing the distance d is to change the shape of the outer edge boundary of the reinforcing member 32 (32a) in the width direction to a shape other than a straight line parallel to the plane tangent to the end face of the cooling drum 11. For example, any shape, such as a rectangle, triangle, or curve, is acceptable. Even if the shape is straight, the distance d can be changed by tilting it relative to the plane tangent to the end face of the cooling drum 11. The shape of the outer edge boundary of the reinforcing member 32 (32a) in the width direction can be determined based on factors such as ease of processing. Furthermore, as long as the distance d changes in the casting direction, there are no particular limitations on the amount of change in the distance d in the casting direction or the length or spacing in the casting direction corresponding to that change. Furthermore, this edge boundary shape may be periodic or non-periodic.

In this embodiment, the reinforcing member 32 (32a) has at least one casting direction front end surface 35 of the width-protruding portion 34 of portion (A) that satisfies (b1) and (b2) within any 200 mm range in the casting direction (b3). If the casting direction front end surface 35 of the reinforcing member 32 (32a) has a portion (B) that does not satisfy either (b1) or (b2) for a continuous period of 200 mm or more in the casting direction, metal deposits will form and grow coarsely in the area. If this coarse metal deposit is then removed in the subsequent portion (A), severe hot bands will occur, increasing the risk of slab fracture. Therefore, the continuous length of portion (B) in the casting longitudinal direction must be kept less than 200 mm. As long as the continuous length per portion (B) is less than 200 mm, multiple portions (B) may exist along the entire length of the reinforcing member 32 (32a). In other words, no matter where you select a continuous 200 mm stretch along the casting longitudinal direction, it must include at least one section (A).

The proportion of the length of the reinforcing member 32 (32a) occupied by the portion (A) (hereinafter also referred to as the linear fraction (%) of the portion (A)) in the casting direction is preferably 20% or more, and more preferably 50% or more, taking into account the processing load of the end boundary shape of the reinforcing member 32 (32a) and the hot band suppression effect. When the reinforcing member 32 (32a) has portions (A) and (B), the proportion of the portion (A) in the casting direction is preferably 20% or more, and more preferably 50% or more. Furthermore, when the widthwise end boundary shape of the reinforcing member 32 (32a) has a rectangular width-protruding portion 34 that protrudes outward in the width direction and has a vertical surface 35b (casting direction front end face 35) perpendicular to the casting direction, if the casting direction length of the casting direction front end face 35 of this width-protruding portion 34 is extremely short, for example, considering the case where ΔL in Equation (1) is 0, the proportion of the portion (A) in the casting direction may be calculated as 0. However, as described in the explanation of Equation (1) above, if the surface perpendicular to the casting direction rises instantaneously, the left side of Equation (1) becomes negative infinity, and this also indicates that the effect of removing metal adhering to the side dam is very large. Therefore, the effect of removing metal adhering to the side dam is maintained within a certain range in the casting direction behind the vertical surface 35b, which is the front end surface 35 of the widthwise protrusion 34 in the casting direction. Furthermore, because the front end surface 35 of the widthwise protrusion 34 in the casting direction is not an inclined surface whose distance d changes as casting progresses, the removed metal accumulates in front of the vertical surface 35b, which is the front end surface 35 of the widthwise protrusion 34 in the casting direction, thereby providing the effect of artificially increasing the thickness of the solidified shell 5 in the casting direction. Based on estimates from tests conducted by the inventors, it was determined that a 1 mm change in the height of the rectangle (in the width direction of the reinforcing member), i.e., a 1 mm change in the distance d (limited to a range of d≦10 mm) of the vertical surface 35b (front end surface 35 in the casting direction) perpendicular to the casting direction, results in an effect of approximately 2.5 mm in the casting direction. In other words, a 1 mm change in the height of the rectangle is considered to have an effect equivalent to ΔL = 2.5 mm in equation (1). Thus, when the widthwise end boundary shape includes a width-protruding portion 34 having a front end surface 35 in the casting direction that forms a vertical surface 35b perpendicular to the casting direction, ΔL of this vertical surface 35b is zero. Therefore, the "converted line segment ratio (%) of portion (A)" may be used, which converts the proportion of portion (A) in the length of the reinforcing member 32 (32a) into a value of ΔL = 2.5 mm per 1 mm change in the distance d (limited to a range of d≦10 mm) of the vertical surface 35b (front end surface 35 in the casting direction).

Furthermore, in this embodiment, the above conditions (b1) to (b3) must be satisfied at both widthwise ends of the reinforcing member 32 (32a). If the above conditions (b1) to (b3) are not satisfied at either widthwise end of the reinforcing member 32 (32a), the coarsened metal formed on the surface of the side weir 15 will be caught in the drum kiss point K, causing severe hot bands and resulting in fracture of the thin-walled cast slab 1. However, the portion (A) does not need to be in the same position in the casting direction at both widthwise ends of the reinforcing member 32 (32a), and the distance d may vary independently on the left and right. Therefore, the same shape pattern may be repeated on the left and right with a shifted period, or the shape itself may be different on the left and right.

Next, we will explain the preferred number of reinforcing members 32 (32a) provided on the dummy sheet 30 according to this embodiment. The side weirs 15 contact both ends of the pair of cooling drums 11, and the area in contact with the molten metal pool 16 expands to the left and right of the surface of the side weirs 15 as it moves upward from the drum kiss point K, forming a shape similar to an inverted triangle or wedge. The triple junction where the side weirs 15, cooling drum 11, and molten steel 3 meet - or more precisely, the area where the triple junctions are connected along the circumferential surface of the cooling drum (triple junction area) - is located at two positions at each end in the casting width direction, corresponding to the front and back of the slab in the thickness direction, and approaches the thickness of the slab at the drum kiss point K.

As mentioned above, the molten steel 3 is cooled from two directions, the cooling drum 11 and the side weir 15, at the triple junction, making it easy for ingots to form. Removing the ingots formed near each of the two triple junctions at one widthwise end of a pair of cooling drums (in other words, one side weir 15) is effective in preventing hot bands. Therefore, in this embodiment, it is preferable that two reinforcing members 32 (32a), whose end boundary shapes at both ends in the casting width direction vary in the casting direction, be attached, one on each side, along the circumferential surface of each of the pair of cooling drums 11. These two reinforcing members are attached to the sheet body 31 in a stacked manner, resulting in a structure in which one sheet body 31 and two reinforcing members 32 (32a) overlap at the connection between the sheet body 31 and the reinforcing member 32 (32a). When these overlapping joints pass through the drum kiss point K, they are discharged overlapping each other, sandwiching the solidified shells 5 formed around the reinforcing members 32 (32a) of the two cooling drums 11.

Meanwhile, the area where bare metal is particularly likely to form is the narrow region near the drum kiss point K. This is because the gap between the two cooling drums 11 narrows, bringing their triple junctions closer together, making it easier for the molten steel 3 to cool. Using just one reinforcing member 32 (32a) for each of the two cooling drums 11 can adequately remove any bare metal that has formed in this narrow region. Therefore, the benefits of the reinforcing member 32 (32a) of this embodiment can be fully achieved even with just one member. The number of reinforcing members 32 (32a) can be selected by comparing the manufacturing cost and the effort required to deform and shape the two overlapping reinforcing members 32 (32a) to fit the circumferential surface of the cooling drum 11 before casting begins, and the hot band suppression effect.

Next, we will explain the preferred length of the reinforcing member 32 (32a) provided on the dummy sheet 30 according to this embodiment. In order to reinforce the area including the joint between the sheet body 31 and the leading edge of the slab, where strength is unstable due to high temperatures in the early stages of casting, the length is preferably about 1/4 of the circumference of the cooling drum 11 or more. Since the diameter of the cooling drum 11 is generally several hundred mm, the length of the reinforcing member 32 may also be several hundred mm or more.

As described above, Figure 3 shows an example of a dummy sheet 30 of this embodiment, in which plate-shaped reinforcing members 32 (32a) are attached to the sheet body 31. The reinforcing members 32 (32a) have width-protruding portions 34 at both widthwise ends, each having a front end face 35 in the casting direction where the distance d changes. The reinforcing members 32 (32a) in this example have width-protruding portions 34 whose end boundaries facing the surface of the side dam 15 change into a rectangular shape. The distance d becomes smaller at the front end face 35 in the casting direction of the width-protruding portions 34, and the thickness of the solidified shell 5 at the widthwise end is relatively thicker than the portions before and after in the casting direction. Therefore, even if metal is generated and grows on the surface of the side dam at the start of casting, removing the metal at an early stage before it coarsens can suppress fracture of the thin-walled cast strip due to hot bands and enable a stable start of casting. The example of reinforcing member 32 (32a) shown in Figure 3 has the same rectangular pattern at both ends in the width direction, but the pattern period is shifted in the casting direction.

4 is a diagram illustrating a twin-drum continuous casting dummy seat 30 including strip-shaped reinforcing members 32 (32b, 32bc, 32be) according to a second embodiment of the present invention. Up to this point, the first embodiment has been described in which the reinforcing members 32 (32a) are plate-shaped. In the second embodiment of the present invention, the reinforcing members 32 (32b, 32bc, 32be) are
(a') A plurality of strip-shaped plates extending in the casting direction are arranged without any gaps in the width direction or with a predetermined gap between them, so that the sum of the widths of the plurality of strip-shaped plates, including the width of the gaps, is the same as the width of one or two plate shapes, forming an assembly of strip-shaped reinforcing members 32b (32bc, 32be);
(b') A twin-drum continuous casting dummy seat 30, in which the outer end faces of the belt-shaped reinforcing members 32b (32bc, 32be) located at both ends of the sheet body 31 in the width direction have the casting direction front end faces 35 on the outer end faces in the width direction.

In this second embodiment, it is preferable to arrange multiple reinforcing members 32b (32bc, 32be) at predetermined intervals in the width direction. This is because the solidified shell 5 is formed to envelop each of the divided strip-shaped reinforcing members 32b (32bc, 32be), thereby enhancing the reinforcing effect of the solidified shell 5.

Furthermore, of the band-shaped reinforcing members 32b (32bc, 32be) in the second embodiment, the outer widthwise ends of the reinforcing members 32be at both widthwise ends have width-protruding portions 34 with casting direction front end faces 35 of the same configuration as in the first embodiment, so by changing the distance d in the casting direction, the same effect as in the first embodiment can be achieved. Of the multiple reinforcing members 32b (32bc, 32be) in the width direction, the band-shaped reinforcing members 32bc positioned on the inside other than the both ends may be straight band-shaped reinforcing members with a constant width and widthwise side surfaces parallel to the casting direction.

In the example of the reinforcing member 32b (32bc, 32be) shown in FIG. 4, all the strip-shaped plates have the same length, but the lengths of the respective plates may be changed.
In addition, the example of the reinforcing member 32b (32bc, 32be) shown in Figure 4 is an example in which the same rectangular pattern is formed on the outside of each of the widthwise end band-shaped reinforcing members 32be at both ends in the width direction, but the periodicity of the pattern is shifted in the casting direction.

5 is a plan view illustrating a twin-drum continuous casting dummy sheet 30 according to a third embodiment of the present invention, which includes strip-shaped reinforcing members 32 (32b, 32bc, 32be) arranged alternately in the width direction, and which is attached to the twin-drum continuous casting apparatus 10 before the start of casting. The third embodiment of the present invention is different from the second embodiment of the present invention in that the reinforcing members 32b (32bc, 32be) are
(a'') The difference is that, excluding the band-shaped reinforcing members 32be located at both ends of the width direction of the sheet main body 31, or including the band-shaped reinforcing members 32be located at both ends of the width direction of the sheet main body 31, the band-shaped reinforcing members 32bc, 32be arranged in the width direction are arranged alternately, with one band-shaped reinforcing member 32bc, 32be for one cooling drum 11 and one band-shaped reinforcing member 32bc, 32be for the other cooling drum 11 being omitted from each other.

In this third embodiment, the strip-shaped reinforcing members 32bc, 32be are arranged alternately in the width direction, with one reinforcing member removed from each other. This allows the solidified shell 5 to be formed so as to envelop each strip-shaped reinforcing member 32bc, 32be, thereby further enhancing the reinforcing effect of the solidified shell 5.

Furthermore, of the band-shaped reinforcing members 32b (32bc, 32be) of the third embodiment, the outer widthwise ends of the reinforcing members 32be at both widthwise ends have width-protruding portions 34 with casting direction front end faces 35 of the same configuration as in the first and second embodiments. Therefore, by changing the distance d in the casting direction, the same effects as in the first and second embodiments can be achieved. Of the multiple reinforcing members 32b (32bc, 32be) in the width direction, the band-shaped reinforcing members 32bc positioned on the inside other than the both ends may be straight band-shaped reinforcing members with a constant width and widthwise side surfaces parallel to the casting direction.

In the example of the reinforcing member 32b (32bc, 32be) shown in FIG. 5, all the strip-shaped plates have the same length, but the lengths of the respective plates may be changed.
In addition, the example of the reinforcing member 32b (32bc, 32be) shown in Figure 5 is an example in which the same rectangular pattern is formed on the outside of each of the widthwise end band-shaped reinforcing members 32be at both ends in the width direction, but the period of the pattern is shifted in the casting direction.

Below, we will explain the results of experiments conducted in Examples 1 to 3 to confirm the effects of the present invention. Note that the present invention includes the various forms described as embodiments above, and is not limited to the forms of the examples described below.

The common experimental conditions for the thin-walled cast slab manufacturing methods of Examples 1 to 3 are as follows.
Cooling drum diameter: 1200 mm
Casting width: 1300mm
Casting thickness: average 2.0 mm
Casting speed: average 60 m/min
Casting atmosphere: Ar
Casting volume: 10 tons Casting steel type: low carbon steel

The following dummy seats were used:
Seat body: Mild steel sheet material, width 1290 mm
Shape of reinforcing member: Strip-shaped (multiple pieces arranged in the casting width direction)
Reinforcement material: Mo Reinforcement thickness: 0.5 mm
Reinforcement member length: 400 to 800 mm
Reinforcing member width: The reinforcing member width is constant at 15 mm except at both ends (the boundary shape of the width end is a straight line)
The reinforcing members at both ends give a predetermined boundary shape to the outer width surface of the 15 mm wide strip material.

The ends of the sheet body and reinforcing member in the casting direction were overlapped, and holes were drilled in the overlapping area and wire was passed through to join them.

Inventive examples No. 1-6 and No. 1-7 feature two overlapping reinforcing members 32b (32be) at both ends (see width end band-shaped reinforcing member 32be in Figure 5). In other examples, the reinforcing members 32b (32be) at both ends are attached individually without overlapping (see Figure 4). Furthermore, in examples No. 1-6 and No. 1-7 and other than at both ends, the reinforcing members 32b (32bc) are attached individually without overlapping.

The multiple reinforcing members 32b (32bc, 32be) located in the casting width direction were each deformed to fit the circumferential surface of the cooling drum before being attached. More specifically, since there was a pair of cooling drums, the direction of deformation of the reinforcing members 32b (32bc, 32be) was alternated, and the reinforcing members 32b (32bc, 32be) were attached to each cooling drum with every other reinforcing member 32b (32bc, 32be) spaced apart and in a shape that fits the circumferential surface (as in Figure 5, both ends in the width direction were also modified to have one reinforcing member 32be attached at each end). However, since two reinforcing members 32be with modified end boundary shapes were attached to the dummy sheet body at both ends of No. 1-6 and No. 1-7 as shown in Figure 5, one was attached to each cooling drum so that it fit the circumferential surface.

Figure 6 shows the boundary shapes applied to the outer width portions of the reinforcing members 32be at the widthwise ends. Figure 6(a) shows the boundary shape of Example 1, which is rectangular and includes a vertical surface 35b on the front end surface 35 in the casting direction. Figure 6(b) shows the boundary shape of Example 2, which is triangular and includes an inclined surface 35a on the front end surface 35 in the casting direction. Figure 6(c) shows the boundary shape of Example 3, which is curved and includes an inclined surface 35a on the front end surface 35 in the casting direction.

Casting was carried out under the above conditions, and the number of hot bands that occurred within one minute from the start of casting and the presence or absence of fractures in the thin-walled cast slab were visually observed and evaluated. The number of hot bands was evaluated in descending order of quality: ○: 1 or less, △: 2-3, ×: 4 or more. The overall evaluation was ○ for no fractures and 1 or fewer hot bands, △ for no fractures and 2-3 hot bands, and × for fractures. The evaluation results for Examples 1-3 are shown in Tables 1-3, respectively.

Example 1 is an example in which the end boundary shape includes a rectangle, and a schematic diagram of the end boundary shape is shown in Figure 6(a). Casting was performed by repeating one cycle shown in the figure. The circle marks indicate the start and end points of the cycle. In other words, the end point of the previous cycle overlaps with the start point of the next cycle.
When the end boundary shape includes a rectangle, as in Example 1, the end boundary shape is easy to process. As shown in (a) and (b) of Figure 6(a) at the front end surface 35 in the casting direction, the distance d decreases vertically, so the gradient defined by equation (1) is minus infinity, which is very large. The line fraction of the portion (A) relative to the total length of the reinforcing member 32 is expressed as the converted line fraction defined above. The evaluation results are summarized in Table 1.

Inventive Examples No. 1-1 to No. 1-7 met all of the requirements of the present invention, and no slab fracture occurred, earning an overall rating of ○ or △. Among these, when comparing No. 1-1 and No. 1-2, which have a higher converted line fraction ratio of portion (A), with No. 1-3 to No. 1-5, which have a lower converted line fraction ratio of portion (A), it can be seen that No. 1-1 and No. 1-2, which have a higher converted line fraction ratio of portion (A), are able to limit the number of hot bands to one or less. Furthermore, No. 1-1 to No. 1-5 are examples in which there is a single reinforcing member 32be at both ends, while No. 1-6 and No. No. 1-7 is an example in which two reinforcing members 32be are stacked on each end, with one reinforcing member 32be attached to the circumferential edge of each of the two cooling drums. Because the reinforcing members 32be of the present invention are attached to the circumferential edge of each cooling drum, hot bands were further reduced compared to No. 1-2 and No. 1-5.

On the other hand, in Comparative Example No. 1-11, the distance d3 was constant and did not change, L3 exceeding 200 mm, and in Comparative Example No. 1-12, the distances d1 to d4 at each part all exceeded 10 mm. As a result, the metal removal effect was not achieved and the slab fractured, resulting in an overall rating of ×.

Example 2 is an example in which the end boundary shape includes a triangle, and a schematic diagram of the end boundary shape is shown in Figure 6(b). In the figure, the section between L1 and L4 is the portion (A) where the bare metal removal effect is achieved. The evaluation results are shown in Table 2. The line segment ratio (%) of portion (A) to the total length can be defined as (L1 + L4) / (ΣLi) × 100 (i = 1 to 6).
Also, for example, in the section L1 in the figure, the distance d from the plane tangent to the end surface of the cooling drum decreases from d1 to d2, so that Δd=d2−d1 and ΔL=L1.

Inventive examples No. 2-1 and No. 2-2 have a shape that changes into a mountain shape, and No. 2-3 to No. 2-5 have a sawtooth shape in which the distance d of the inclined surface changes vertically.
Inventive Examples No. 2-1 to No. 2-5 satisfied all of the requirements of the present invention, and no slab fracture occurred. The overall evaluation was ○ or △. Among these, comparing No. 2-1 with No. 2-2, No. 2-2, which had a relatively small absolute value of the negative gradient of the region (A), had an increased number of hot bands, although within the allowable range. Furthermore, comparing No. 2-2, No. 2-3, and No. 2-5, the absolute value of the negative gradient of the region (A) was low and comparable, and the linear fraction of the region (A) was also high and comparable. However, No. 2-3, in particular, had a relatively short repetition period of the end boundary shape, which increased the frequency of occurrence of the region (A) and reduced the number of hot bands. Furthermore, No. In No. 2-4, the line segment ratio (%) of the portion (A) to the entire length was 0%, but since the end boundary shape had a casting direction front end face 35 that was a vertical surface 35b perpendicular to the casting direction, the converted line segment ratio (%) of the portion (A) used for evaluation in Example 1 was equivalent to the line segment ratio (%) of the portion (A) in No. 2-1. Furthermore, in No. 2-1 and No. 2-4, the absolute value of the negative gradient of the portion (A) was relatively larger than in the other examples, and therefore no hot bands occurred.

On the other hand, in Comparative Example No. 2-11, the length of section L5, where distance d increases, exceeded 200 mm, and in Comparative Example No. 2-12, the total length of sections L5 and L6 exceeded 200 mm. Therefore, the metal removal effect was not achieved and slab fracture occurred, resulting in an overall rating of ×.

Example 3 is an example where the end boundary shape includes a curve, such as a semicircle or part of a circle, and schematic diagrams of the end boundary shape are shown in Figures 6(c) and 7. In the figure, the section between L2 and L4 is the area (A) where the bare metal removal effect is achieved. The line segment ratio (%) that area (A) accounts for in the total length can be defined as (L2 + L4) / (ΣLi) × 100 (i = 1 to 6). The evaluation results are summarized in Table 3.

Inventive example No. 3-1 is an example in which semicircles are arranged in contact with each other without any gaps (see FIG. 7(a)), while No. 3-2 and No. 3-3 are examples in which parts of the circles are arranged with gaps (see FIGS. 7(b) and 7(c)). No. 3-3 has an opening that is larger than the semicircle (see FIG. 7(c)).
Inventive Examples No. 3-1 to No. 3-3 satisfied all of the requirements of the present invention, and no slab fracture occurred, resulting in an overall rating of ○ or △. Among these, a comparison of No. 3-1, which had a high linear fraction of the portion (A), with No. 3-2 and No. 3-3, which had lower linear fractions of the portion (A), reveals that No. 3-1, which had a high linear fraction of the portion (A), was able to limit the number of hot bands to one or less.

On the other hand, in Comparative Example No. 3-11, the length of section L5, where distance d remained constant and did not change, exceeded 200 mm, so the metal removal effect was not achieved and the slab fractured, resulting in an overall rating of ×.

In Example 3, we explained typical examples of curves that include semicircles and portions of circles, but the present invention is not limited to portions of circles and can use any curve, such as an ellipse or a sine curve.

1 Thin-walled cast slab 3 Molten steel (molten metal)
5 Solidified shell 10 Twin drum continuous casting machine 11 Cooling drum 12 Bender roll 13 Pinch roll 15 Side weir 16 Molten steel reservoir (molten metal pool)
18 Tundish 20 Submerged entry nozzle 30 Dummy seat for twin-drum continuous casting 31 Seat body 32 Reinforcement member 32a Plate-shaped reinforcing member 32b Belt-shaped reinforcing member 32bc Width center belt-shaped reinforcing member 32be Width end belt-shaped reinforcing member 34 Width protruding portion 35 Front end surface in casting direction 35a Inclined surface of front end surface in casting direction 35b Vertical surface of front end surface in casting direction 36 Hole portion 38 Linear reinforcing member

Claims (4)

A twin-drum continuous casting dummy seat used in a twin-drum continuous casting apparatus that supplies molten steel to a molten steel reservoir formed by a pair of rotating cooling drums and a pair of side weirs, and produces a thin-walled cast strip by forming and growing a solidified shell on the circumferential surface of the cooling drums,
a sheet body sandwiched between the pair of cooling drums; and a reinforcing member extending from one end of the sheet body in a longitudinal direction on a side where the slab is connected and inserted into the molten steel,
The reinforcing member is
(a) one or two plate-shaped members disposed along the circumferential surface of one or both of the pair of cooling drums;
(b) at each end on both sides in the width direction,
(b1) Within a range of 10 mm or less from a plane contacting the end surface of the cooling drum,
(b2) A front end surface in the casting direction is formed of either or both of an inclined surface that is inclined in a direction in which the distance d from the plane tangent to the end surface of the cooling drum decreases as casting progresses and whose shape satisfies Δd/ΔL≦−5/100 , and a vertical surface that is perpendicular to the casting direction and faces forward in the casting direction;
(b3) The portion not satisfying either (b1) or (b2) extends continuously for less than 200 mm in the casting direction.
A dummy seat for twin-drum continuous casting, characterized in that
where:
Δd is a change in the distance d of the inclined surface from the plane that is in contact with the end surface of the cooling drum, and a decrease in d is considered negative,
ΔL is the moving distance of the reinforcing member in the casting direction corresponding to Δd. The reinforcing member is
(a') Instead of the one or two plate shapes, a plurality of strip-shaped plates extending in the casting direction are arranged without any gaps in the width direction or at predetermined intervals, so that the sum of the widths of the plurality of strip-shaped plates, including the width of the predetermined intervals , is the same as the width of the one or two plate shapes, forming an aggregate of strip-shaped reinforcing members;
(b') The dummy seat for twin-drum continuous casting according to claim 1, characterized in that, of the band-shaped reinforcing members, the band-shaped reinforcing members located at both ends in the width direction of the sheet body have a front end face in the casting direction at their outer widthwise ends. The reinforcing member is
(a'') Instead of the two plate shapes, a plurality of strip-shaped plates extending in the casting direction are arranged without any gaps in the width direction or at predetermined intervals, so that the sum of the widths of the plurality of strip-shaped plates, including the width of the predetermined intervals , is the same as the width of the two plate shapes, forming an aggregate of strip-shaped reinforcing members;
Furthermore, the assembly of the belt-shaped reinforcing members excluding the belt-shaped reinforcing members located at both ends in the width direction of the sheet main body, or the assembly of the belt-shaped reinforcing members including the belt-shaped reinforcing members located at both ends in the width direction of the sheet main body, are arranged so that the belt-shaped reinforcing members for one cooling drum and the belt-shaped reinforcing members for the other cooling drum are arranged alternately,
(b') The dummy seat for twin-drum continuous casting according to claim 1, characterized in that, of the band-shaped reinforcing members, the band-shaped reinforcing members located at both ends in the width direction of the sheet body have a front end face in the casting direction at their outer widthwise ends. A method for producing a thin cast slab by supplying molten steel to a molten steel reservoir formed by a pair of rotating cooling drums and a pair of side weirs, and forming and growing a solidified shell on a peripheral surface of the cooling drums, comprising:
A method for producing a thin-walled cast strip, characterized in that the twin-drum continuous casting dummy seat according to any one of claims 1 to 3 is used at the start of casting.