JPH0539807Y2 - - Google Patents

Description

[Detailed explanation of the idea]

(Industrial Application Field) The present invention relates to an improvement of a pair of endless metal belts in a casting apparatus that continuously produces thin slabs. (Prior art) Conventionally, thin steel sheets are produced by continuously pouring molten steel into a copper mold, cooling and solidifying it to produce a slab with a thickness of 200 to 300 mm, and then rolling this slab into a hot rolling mill and a cold rolling mill. It was manufactured by rolling. However, in recent years, methods of casting thin slabs with a thickness of 50 mm or less have been studied in order to eliminate the hot rolling process as much as possible. One of these is an endless belt type continuous casting machine. This endless belt type continuous casting machine has endless metal belts that are circulated by drive rollers and are arranged facing each other at a predetermined interval, and a dam block that is sandwiched between both belts near both ends of these metal belts and circulates with the belts. Molten steel is poured into the casting space formed by the casting process, the poured molten steel is cooled in both metal belts and both dam blocks, and sequentially solidified into slabs, which are then pulled out downward. Conventionally, low alloy steel with a carbon content of 0.01 to 1.0% by weight has been used as the metal belt. (Problem to be solved by the invention) By the way, when producing slabs using the above-mentioned continuous casting equipment, so-called medium carbon steel in which the carbon content in the molten steel is within the range of 0.09 to 0.30% by weight, Vertical cracks were likely to occur in the slab.
When such cracks occur on the surface of a slab, it is necessary to remove surface defects before sending it to the rolling process, which is a factor that hinders the application of direct rolling, which is originally excellent in terms of thermoeconomics. Various studies have been conducted on the generation mechanism of such surface cracks, that is, longitudinal cracks, in slabs. In particular, the range of 0.09 to 0.30% by weight of C is the peritectic reaction region, where no solidification reaction occurs. It is known that as a result of uniform progress, the thickness of the solidified shell is highly non-uniform, which is likely to cause vertical cracks, etc. (see Fig. 6). Although the mechanism by which such vertical cracks occur has not yet been completely clarified, the mechanism by which such vertical cracks occur is thought to be as follows. In other words, during continuous casting, molten steel is supplied between the belts and a solidified shell is generated, but in this case, the amount of heat removed by the belt is large, so if there is even a slight unevenness in the amount of heat removed, the thickness of the solidified shell will be unstable. When the thickness of the solidified shell becomes uniform, thermal stress due to shrinkage and impact force due to belt fluctuations act on the thinner part of the solidified shell, resulting in vertical cracking. Therefore, in order to prevent the occurrence of vertical cracks in such ultra-thin continuously cast slabs, it is sufficient to make the thickness of the so-called initial solidification shell formed near the meniscus uniform. In addition, this type of continuous casting machine is operated under conditions where one side of the metal belt is in contact with molten steel and the other side is in contact with cooling water, so there is a sudden change in the thickness direction of the metal belt. A gradient temperature difference occurs, and strain due to thermal expansion occurs, and the seventh
Deformation occurs in the width direction, as shown in Figure A, and in the casting direction, as shown in Figure B, resulting in waving. The amount of deformation is greatly influenced by the coefficient of thermal expansion of the metal belt 1, as shown in FIG. As a result, the cooling rate of the surface layer of the slab on the molten steel A side that is in contact with the metal belt 1 becomes uneven, and as a result, the formation of the solidified shell C of the slab also lacks uniformity, and the lower metal In belt 1, as shown in Fig. 9, as solidification progresses immediately after pouring (Fig. A), the solidified shell C and metal belt 1 locally separate from each other, so uniform cooling is not performed and a dent is formed. A solidified shell C with non-uniform thickness is formed (lower figure). On the other hand, as shown in Fig. 10, the upper metal belt 1 also forms a solidified shell C having uneven thickness with depressions as the solidification progresses from immediately after pouring (Fig. A), as shown in Fig. 10. ). That is, even in steels other than medium carbon steels whose compositions are less likely to cause uneven solidification, the formation of the solidified shell C of the slab becomes uneven, resulting in dents on the metal belt 1 side. Since the solidified shell C in this concave portion E is thin, tensile force due to thermal stress is generated in the solidified shell C.
There was a problem in that cracks occurred from the internal solid-liquid interface or the surface of the solidified shell, making it impossible to obtain ultra-thin slabs with good surface properties. Therefore, in order to prevent such cracks from occurring in continuously cast slabs, it is sufficient to reduce the amount of heat removed from the molten steel by the metal belt, and to suppress thermal deformation of the metal belt due to temperature rise. In order to prevent vertical cracks in continuously cast thin slabs, this invention equalizes the thickness of the so-called initial solidification shell that forms near the meniscus, and reduces the amount of heat removed from the molten steel by the belt. The present invention provides a belt for a continuous casting machine for thin cast slabs, which can suppress thermal deformation due to temperature rise of the belt. (Means for Solving the Problems) The belt for a continuous cast thin slab casting apparatus of the present invention has endless metal belts circulated by a driving roller arranged facing each other at a predetermined interval, and near both ends of these metal belts. The belt of the continuous casting apparatus includes a dam block which is sandwiched between both belts and circulates together with the belt, and which supplies molten steel to a casting space surrounded by these metal belts and the dam block to continuously produce thin slabs. , width is 250
A groove with a height or depth of ~750 μm and a height or depth of 60 to 300 μm and continuous peaks and valleys parallel to the casting direction is formed. In the present invention, the width b of the peaks and valleys forming the grooves parallel to the casting direction is set to 250 to 750 μm for the following reason. That is, since the reason for providing the fine grooves is to reduce the amount of heat removed by reducing the contact area of the molten steel with the metal belt, the molten steel A must not easily flow into the valleys of the fine grooves. First, Figure 2 shows the relationship between the pressure of molten steel near the meniscus position and the width ^b of the valley at the limit of molten steel inflow. It works between the belt and the coagulation shell. From the results shown in FIG. 2, when the molten steel pressure is 10 g/cm ² , the width b of the valley where molten steel does not flow is 750 μm or less. Also,
The reason why the width b of the valley is set to 250 μm or more is that processing becomes difficult if the width is less than this. In addition, in the present invention, the height (depth) a of the peaks and valleys that form the grooves parallel to the casting direction is 60~
The reason for setting it to 300 μm is as follows. First, in this invention, the lower limit of the valley depth a is
The reason for setting the depth to 60 μm is that if the depth a is less than 60 μm, the heat insulating effect of air as intended in the present invention is not sufficient. Here, Figure 3 shows the results of measuring the heat flux at a position 20 mm below the meniscus position.
Normally, in order to uniformize the thickness of the solidified shell by slow cooling, it is necessary to reduce the heat flux by 20% or more. From the results shown in Figure 3, the depth of the valley
It can be seen that a diameter of 60 μm or more is required. on the other hand,
The reason why the depth a of the trough is set to 300 μm or less is because the effect hardly changes even if the depth a is exceeded. (Function) As mentioned above, the present invention has a metal belt with a width
Since we have formed grooves that are 250 to 750 μm in height or 60 to 300 μm in height and have continuous peaks and valleys parallel to the casting direction, the surface of the metal belt that comes into contact with the molten steel is reduced, reducing the amount of heat removed by the metal belt. As a result, the vicinity of the meniscus is slowly cooled and the thickness of the solidified shell grows uniformly, and furthermore, thermal deformation due to temperature rise of the metal belt can be reduced and uneven solidification can be suppressed. The solidified shell in the surface layer can be grown uniformly. (Example) A belt for a continuous cast thin slab casting apparatus according to an example of the present invention will be described below with reference to the drawings. FIG. 1A shows a configuration diagram of the casting apparatus, FIG. 1B shows a schematic perspective view of the belt, and FIG. 1C shows a cross-sectional view of the belt. In the figure, reference numeral 1 denotes a pair of endless metal belts, which are guided by drive rollers 2 and hold molten steel A and slab B on their casting surfaces while maintaining a predetermined interval, and on their back surfaces are constantly cooled by water D. It circulates in the direction of the arrow while being cooled. 3 is a dam block, which is guided by a guide roller 4;
It is arranged at both ends between the pair of upper and lower metal belts 1, and is arranged to circulate in synchronization with these metal belts 1. Further, 5 is a gutter for supplying hot water, which supplies molten steel A to a casting space formed by a pair of upper and lower metal belts 1 and both dam blocks 3. The continuous thin slab casting apparatus is constructed as described above, but in the present invention, the endless metal belt 1 is formed as shown in FIGS. 1B and 1C. That is, the surface of the metal belt 1 is machined into fine grooves in which peaks 7 and troughs 6 that are parallel to the casting direction are continuous. The grooves are preferably provided regularly, and their cross-sectional shape is such that they block the inflow of molten steel A while allowing air to flow in, and the peaks 7 constitute the contact area with molten steel A (and slab B). Heat is removed through the lever. By the way, regarding the width b and height or depth a of the peaks 7 or valleys 6 that form this fine groove, b = 250 to 750 μm, a = 60 to
Form to have a thickness of 300 μm. As described above, in the present invention, the width b of the peak portion 7 or the valley portion 6 is 250 to 750 μm, and the height or depth a is 250 to 750 μm.
By setting the processing range to 60 to 300 μm and using this processing range for the metal belt 1, the effect of slow cooling aimed at by the present invention can be obtained. In addition, as mentioned above, reducing the amount of heat removed is effective in producing a uniform initial solidification shell thickness for medium carbon steel, but as shown in Figure 4, ordinary belts without microgroove processing The effect appears by reducing the amount of heat removed to 70% or less. Therefore, it is necessary to reduce the amount of heat removed by 30% or more, but the first
As shown in Figure B, by forming fine grooves on the metal belt 1 in parallel to the casting direction, the area ratio of the part where the inner surface of the metal belt 1 contacts the molten steel A is reduced, and the entire metal belt is slowly cooled. be able to. In addition, with a regular curved or vertical continuous casting machine that uses a water-cooled copper plate, even if steel other than medium carbon steel is cast, uneven solidification and cracking will not occur, but with an endless belt type continuous casting machine, no cracks will occur. When casting, the amplitude of the waving of the metal belt 1 increases due to thermal deformation of the metal belt 1, which causes uneven thickness of the solidified shell and cracks (see FIG. 5). However, by using the metal belt 1 with fine grooves, the contact area ratio between the slab B and the metal belt 1 is reduced, and the amount of heat removed from the slab B to the metal belt 1 is reduced. The temperature rise on the inner surface of the metal belt 1 is reduced, and the thermal deformation of the metal belt 1 is suppressed due to the increase in deformation resistance due to the fine groove processing, so that cracks are significantly reduced. As a specific example, using the apparatus shown in FIG. 1A, the metal belt 1 is a conventional mild steel belt and the following
An ultra-thin slab was cast from molten steel having the chemical composition shown in Table 2 below using a metal belt machined with the microgrooves shown in the table.

【table】

[Table] The casting conditions at this time were slab dimensions: 1000 mm width x 50 mm thickness, and casting speed: 3.0 to 6.0 m/min. Next, as a result of comparing the surface properties of the slabs obtained in these specific examples, it was found that for medium carbon steel, as shown in FIG. In case of (▲
(marked), compared to the case using a conventional belt (△ mark), there was almost no non-uniformity in the solidified shell thickness, and a thin slab with extremely good surface quality was obtained. Also,
As for low carbon steel, as shown in Figure 5, belt undulation amplitude is lower in belts using the belt 1 of the present invention (● and ▲ marks) than in belts using conventional belts (○ and △ marks). As a result, a good thin slab with extremely few surface defects was obtained. (Effects of the invention) As explained above, the present invention is suitable for metal belts used in endless belt type continuous casting equipment with a width of 250 mm
By forming grooves with continuous peaks and valleys parallel to the casting direction with a height or depth of 750 μm and a height or depth of 60 to 300 μm, low carbon In steel, uneven solidification caused by thermal deformation of the metal belt can be suppressed, so the surface defects of slabs that were caused by these are significantly reduced, making it possible to produce thin metal slabs with extremely good surface quality. Can be manufactured stably.

[Brief explanation of the drawing]

Fig. 1 shows an embodiment of the present invention, Fig. A is a block diagram of its casting device, Fig. B is a schematic perspective view of the belt, Fig. C is a cross-sectional view of the belt, and Fig. 2 is a valley of the belt. Figure 3 is a characteristic diagram showing the relationship between the width of the part and the molten steel pressure, Figure 3 is a characteristic diagram showing the relationship between the depth and heat flux reduction rate, and Figure 4 is the relationship between the slow cooling ratio and solidification shell thickness non-uniformity. Figure 5 is a diagram showing the experimental results showing the relationship between the standard deviation of the ripple amplitude of the belt and the total crack length. Figure 6 is a diagram showing the relationship between the carbon content and the frequency of occurrence of longitudinal cracks. Figures 7A and 7B are explanatory diagrams showing the deformation state of the conventional belt in the width direction and casting direction. Figure 8 is a diagram showing the relationship between the coefficient of thermal expansion and the amplitude of waviness of the same belt. Figure 9A and Figure 7B 10A and 10B are explanatory diagrams showing the influence of deformation in the width direction of the metal belt on the solidified shell growth on the lower surface side of the slab, and FIGS. 10A and 10B are the same explanatory diagrams on the upper surface side of the slab. 1 is a metal belt, 2 is a drive roller, 3 is a dam block, 6 is a valley, 7 is a peak, A is molten steel, and B is a slab.

Claims (1)

[Scope of utility model registration request] Endless metal belts circulated by drive rollers are arranged facing each other at a predetermined interval, and a dam block is provided near both ends of the metal belts and circulates together with the belts, and the belt is surrounded by the metal belts and the dam blocks. In the above-mentioned belt of a continuous casting apparatus that continuously produces thin slabs by supplying molten metal to a casting space, the belt is provided with a belt parallel to the casting direction having a width of 250 to 750 μm and a height or depth of 60 to 300 μm. 1. A belt for continuous thin slab casting equipment, characterized in that a groove is formed with continuous peaks and valleys.