JPH05285601A - Twin roll type continuous casting apparatus - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a casting apparatus equipped with a cooling roll having dimples formed on the outer peripheral surface thereof to reduce the occurrence rate of center porosity generated in a slab and to reduce the time during processing after cold rolling. An object of the present invention is to provide a casting device capable of casting a slab that does not cause cracks. In a twin-roll type continuous casting apparatus 1 for continuously casting thin plate cast pieces, an outer peripheral surface 2a of cooling rolls 2, 3
For the dimples 4 and 5 formed on 3a, the maximum value of the distance between adjacent dimples is Lmax, and the diameter of the cooling roll is D.
Then, it is formed so as to satisfy the relationship of Lmax ≦ D / a (where a is a constant). Due to this relationship, the pitch of uneven solidification becomes small, and the center porosity generation rate can be suppressed to 1% or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a twin roll type continuous casting apparatus (also referred to as a twin drum type casting machine) for continuously casting a thin metal plate directly from a molten metal, and in particular, it has a large number of dimples on its outer peripheral surface. Regarding casting equipment.

[0002]

2. Description of the Related Art A pair of cooling rolls rotating in opposite directions are arranged in parallel and opposite to each other with an appropriate gap, and two side dams (side dams) are pressure-welded to both end faces of these rolls. The outer peripheral surface of the roll and the side weir define a pool of molten metal in the upper part, and the molten metal supplied to this pool is cooled by the outer peripheral surface of the rotating cooling roll while the thin plate is passed through the gap. Slab (eg stainless steel plate)
A so-called twin roll type continuous casting apparatus (hereinafter, casting apparatus) for continuously casting is known.

By the way, in this casting apparatus, the solidified shells formed on the respective surfaces by heat removal from the roll
The two cooling rolls form a slab by being pressed against each other at the closest kissing point, and the urging force of the rolls against the solidified shell (this is called the roll pushing force) is assumed to be too large if the value becomes too large. There may be many cracks in the slab.

Therefore, this roll pushing force is an important control property for preventing the occurrence of cracks, but as another measure, a large number of dimples are formed on the surface of the cooling roll so that the stress exerted by the roll on the surface of the solidified shell is exerted. A casting device that disperses the concentration and prevents the occurrence of cracks and scratches on the surface of the slab is already known.

[0005]

By the way, in the case of a casting apparatus for forming innumerable dimples on the outer peripheral surface of the cooling roll in this manner, since the slab structure has dimples, the columnar shape from the contact line with the cooling roll Akira (Dendrite)
2, the cross-section thereof has a tendency that, as shown in FIG. 2, the line at the tip of the dendrite structure d then becomes a waveform with a pitch p corresponding to the dimple spacing, resulting in a so-called non-uniform solidification structure. There is.

Due to the nonuniform thickness of the dendrites, an unsolidified portion confined by the dendrites extending from the outer peripheral surfaces of both chill rolls is generated in the solidified shell, and the molten metal in this portion is subsequently solidified and equiaxed. A structure e called a crystal is formed. However, since this equiaxed crystal structure e is contracted in the solidification process,
There may be a void f called center porosity at the center, and depending on the void content, cracks may occur from the center porosity f as a starting point during processing after cold rolling of the slab, and the sheet quality may be significantly impaired. There is.

In view of the above problems, it is an object of the present invention to provide a twin roll type continuous casting apparatus in which center porosity is unlikely to occur inside a slab.

[0008]

According to the present invention for the above object, a pair of cooling rolls having a large number of dimples on their outer peripheral surfaces and rotating in opposite directions are arranged in parallel and face each other, and Side weirs are pressure-welded to both end faces of the roll, molten metal is supplied to the molten metal pool defined by the outer peripheral surface of the cooling roll and the side weirs, and thin sheet cast pieces are continuously passed through the gap between the cooling rolls. A twin roll type continuous casting device for casting, wherein the dimples have a center-to-center distance L between any adjacent dimples and a diameter D of the cooling roll.
Then, there is provided a twin roll type continuous casting apparatus characterized by being formed so as to satisfy at least the relationship of L ≦ D / a (where a: a constant).

[0009]

When the pitch with non-uniform solidification increases, the amount of molten metal contained is increased, so that the degree of shrinkage during solidification increases and the tendency of center porosity to occur increases. And this pitch is almost equal to the distance between the dimple centers of the cooling roll. In addition, even if the solidification is unevenly solidified at the same pitch, if the degree of narrowing of the molten metal at the kissing point is smaller, the penetration of the molten metal between the solidified shells is suppressed, and the molten metal is less likely to be confined by the dendrite columnar crystals, which also tends to cause center porosity. Low. And this narrowing down is related to the diameter of the cooling roll.

The inventor of the present application found out the relationship between the diameter of the cooling roll, the distance between the dimple centers, and the frequency of occurrence of the center porosity as described above by the casting test, and further, any adjacent dimples on the outer peripheral surface of the roll. When the dimples are formed so as to satisfy at least L ≦ D / a (where a is a constant), the center porosity occurrence rate should be kept low, where L is the center-to-center distance and D is the diameter of the cooling roll. I confirmed that you can.

[0011]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows a schematic appearance of a twin-roll type continuous casting apparatus 1 (hereinafter, abbreviated as casting apparatus 1) according to the present invention. A pair of cooling rolls 2 and 3 (oppositely arranged with a gap of a predetermined width therebetween). Hereafter, a large number of dimples 4 and 5 are formed on the outer peripheral surfaces 2a and 3a of the roll (abbreviated as roll), and fan-shaped side dams 6 and 7 are pressed against both end surfaces thereof.

The operation of the casting apparatus thus constructed will be described. At the time of casting, the rolls 2 and 3 are rotationally driven in directions opposite to each other as indicated by arrows in the drawing by a drive mechanism (not shown), so that the roll 2 is rotated. , 3 outer peripheral surfaces 2a, 3a
A molten metal (molten metal) 9 typified by stainless steel (for example, SUS304) is supplied to the molten metal pool portion 8 defined by the side dams 6 and 7.

Then, by the cooling action of the rolls 2 and 3, the molten metal 9 is solidified on the outer peripheral surfaces 2a and 3a of the solidified shell 1.
0a, 10b are generated, they grow as the rolls 2, 3 rotate, and they are pressed against each other when the rolls 2, 3 called the kissing point are closest to each other. In this way, the thin plate slab 11 is cast. is there. In the casting apparatus configured and operated as described above, according to the present embodiment, the dimples 4 and 5 formed on the roll outer peripheral surfaces 2a and 3a have a center-to-center distance of L (mm). , 3 is expressed as D (mm), and the casting object is austenitic stainless steel (for example, SUS3
In the case of 04), it is formed and arranged so as to satisfy the relationship of L ≦ D / 400 at any part on the outer peripheral surface of the roll.

The reason for forming such dimples will be described below. FIG. 2 shows a general microstructure cross-section model and a roll cross-section of a cast piece cast in a casting apparatus having dimples on the outer peripheral surface of the roll. In the figure, a structure d is a columnar crystal that grows first in the cast piece structure. (Dendrite),
The structure e is an equiaxed crystal that precipitates thereafter, and f is this equiaxed structure e
It shows the center porosity that tends to appear in the center of. The arrow direction in the figure corresponds to the width direction of the rolls 2 and 3.

In this way, when the dimples 4 and 5 are provided on the roll outer peripheral surfaces 2a and 3a, the boundary line between the equiaxed crystal structure e and the dendrite structure d is substantially equal to the dimple center interval L as shown in the drawing. It tends to be wavy with a pitch p. This is because heat transfer is poor at the portion where the dimples 4 and 5 are formed on the outer peripheral surface of the roll, as compared with the other portions because gas is present there, and therefore the roll 2 and
This is because the heat removal effect due to No. 3 is small and the growth of the dendrite d is smaller than that in the portion where dimples are not provided.

The above-mentioned center porosity f is
The greater the variation in growth of the dendrite d growing from the outer peripheral surface of the same roll, the more likely it is to occur. FIG. 3 is a diagram showing the relationship between the growth variation of dendrites and the pitch p confirmed by the inventor of the present application. In this figure, as parameters showing the growth variation of the dendrite d, the growth length l ₁ from the dimple portion on the outer peripheral surface of the roll to the dendrite tip and the growth length l ₂ from the portion where no dimple is formed to the dendrite tip are The solidification nonuniformity rate G (%) obtained by the following formula is adopted.

G = [(l ₁ -l ₂ ) / l ₁ ] × 100 As is apparent from this figure, the dendrite growth variation, that is, the solidification nonuniformity ratio G, is in proportion to the solidification nonuniform pitch p. .. That is, this figure suggests that the generation rate of the center porosity, which is negative for the quality of the cast slab, is related to the uneven solidification pitch p, that is, the dimple shape formed on the outer peripheral surface of the roll. ing.

(Casting test) In order to investigate the relationship between the distance D between the dimple centers on the outer peripheral surface of the roll and the center porosity occurrence rate S, the distance between the dimple centers was set to 2.0, 2.5,
A casting test for casting a thin slab of SUS304 (austenitic stainless steel) was conducted in a casting machine having a roll diameter of 1200 mm, which was changed to 3.0 mm. The center porosity occurrence rate S is the area occupancy rate of the center porosity per unit area of the cast slab. The results are shown in Table 1 below.

[0019]

[Table 1]

Table 1 shows that the greater the distance L between the dimple centers, the higher the center porosity occurrence rate S. Therefore, in order to suppress the center porosity to a low level, the dimples provided on the outer peripheral surface of the roll should be reduced. The spacing should be small. By the way, even if the solidification of the same pitch p is non-uniform, if the roll diameter is small, the degree of narrowing of the molten metal with respect to the kissing point is so large that confinement of the molten metal by dendrites is likely to occur.

FIG. 4 is a graph showing test results obtained by similarly changing the distance between the dimple centers to another casting apparatus (D = 300, 500, 800 mm) having different roll diameters in addition to the above casting test. The horizontal axis shows the solidification nonuniform pitch p (mm), and the vertical axis shows the center porosity occurrence rate S (%). This figure is similar to the above-mentioned 1200 mm diameter casting apparatus in other roll diameter casting apparatuses,
It can be said that the pitch p and the center porosity generation rate S are in a substantially proportional relationship, and even with the pitch p of the same value, the larger the roll diameter D, the smaller the narrowing of the solidified shell. S shows that it becomes low.

By the way, the center porosity f generated in this way has already been described as having a higher possibility of cracking during processing after cold rolling of a slab as the generation rate S increases. The occurrence rate S is 1.0%
It has been confirmed by the inventor of the present application that when the content is suppressed below, cracking does not occur during processing. FIG. 5 shows the intersection points (a) to (d) of the boundary line of the occurrence rate S = 1.0% shown by the alternate long and short dash line in FIG. 4 and the line of each roll diameter, and the roll diameter D on the horizontal axis. The vertical axis is plotted as the solidification nonuniform pitch p. In the figure, as a reference, S = 0
The pitch p that achieves% is also plotted (dotted line).

This figure shows that under the same center porosity generation rate S, the roll diameter and the nonuniform solidification pitch p are in a substantially proportional relationship, and the center porosity generation is achieved in order to achieve good workability. In order to suppress the rate S, the roll diameter D and the solidification pitch p are 1.0
It indicates that it must be included in the shaded area below the line (solid line) corresponding to%.

As described above, the nonuniform solidification pitch p is substantially equal to the distance L between the dimples formed on the outer peripheral surface of the roll, so that center porosity is less likely to occur inside the cast slab structure. In order to design such casting equipment, the distance between adjacent dimple centers must be L
(Mm) and the roll diameter is D (mm), the solid line in FIG. 4 passes through the point (c) where the roll diameter D = 800 and p = 2.0, and at least L ≦ D / 400 is satisfied. Must be designed to meet.

By designing the rolls in such a manner in the casting apparatus, the center porosity occurrence rate S does not exceed 1% at the maximum around the dimples satisfying the above relationship, and the dimple spacing is smaller than this. Then, as is clear from FIG. 2, the solidification non-uniformity ratio G
Is further reduced, and the center porosity occurrence rate S can be further reduced.

The above relational expression is a conditional expression when the casting material is austenitic stainless steel and the center porosity generation rate S is 1% or less, and of course the required quality (center porosity generation) Rate) and the coefficient change depending on the casting material such as aluminum-killed steel and silicon-killed steel. Therefore, it is expressed as the following formula as a general formula.

L ≦ D / a (where a is a constant that is to be suppressed and is a constant experimentally set depending on the material) In the case of aluminum-killed steel, the width of the liquidus line and the solidus line is small and solidification does not occur. Since the uniformity is likely to occur even with a heat removal difference smaller than SUS304, the value of a becomes larger than 400.

[0028]

As described above, according to the present invention, in the twin roll type continuous casting apparatus, a large number of dimples formed on the outer peripheral surface of the roll are at least the center-to-center distance L between the dimples is equal to the roll diameter. By forming it so as to be less than or equal to 1 of the constant D (the rate of occurrence of center porosity to be suppressed or the value experimentally set by the material), the pitch of non-uniform solidification is reduced to reduce the non-uniform solidification rate, It is possible to prevent the occurrence of cracks during processing after cold rolling by setting the center porosity generation rate to 1% or less.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a twin roll type continuous casting apparatus according to the present invention.

FIG. 2 is a schematic view showing a structure inside a cast piece cast by a casting apparatus including a roll with dimples.

FIG. 3 is a diagram showing a relationship between a solidification nonuniformity pitch p and a solidification nonuniformity ratio G.

FIG. 4 is a diagram showing a relationship between a pitch p of uneven solidification and a center porosity generation rate S for each roll diameter D.

[Fig. 5] Under the condition that the center porosity occurrence rate is constant,
It is a figure which shows the relationship between the pitch p of solidification nonuniformity, and the roll diameter D, and is the figure which showed the area | region with favorable workability with the diagonal line.

[Explanation of symbols]

 1 ... Twin roll type continuous casting device 2, 3 ... Cooling rolls 4, 5 ... Dimples 6, 7 ... Side weir 8 ... Melt pool 9 ... Molten metal

Claims (1)

[Claims] 1. A pair of cooling rolls, which have a large number of dimples on their outer peripheral surfaces and rotate in mutually opposite directions, are arranged in parallel and face each other, and side weirs are pressed against both end faces of the cooling roll. Is a twin roll type continuous casting apparatus for supplying molten metal to the molten metal pool defined by the outer peripheral surface and the side dam, and continuously casting thin plate cast pieces through the gap between the cooling rolls. The dimples are formed so as to satisfy at least the relationship of L ≦ D / a (where a is a constant), where L is the distance between the centers of adjacent dimples and D is the diameter of the cooling roll. A twin roll type continuous casting device characterized in that