JP7410369B2 - Method for reducing internal defects in slabs and slab manufacturing equipment - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for reducing internal defects in slabs cast by a continuous casting machine and slab manufacturing equipment.

Voids and segregation occur in slabs due to uneven solidification timing. If voids or segregation remain in the slab, there is a problem that the quality of the final product will be impaired. In order to improve such internal defects (voids, etc.), it is known that applying pressure to the slab is effective (for example, Patent Document 1).

Internal defects tend to cluster in the final solidification region. The final solidification part is located near the center of the thickness of the slab, but in slabs that are wide relative to the thickness, the final solidification part is located near the width edges rather than the width center. Likely to happen. Therefore, as a method for increasing the improvement effect near the width end of the slab, for example, Patent Document 2 discloses that, for an extra-thick steel plate, both ends in the width direction of the slab are reduced by a width of 150 mm or more. A method is disclosed in which the thickness of the part is increased and then horizontal reduction is applied using a press.

Japanese Unexamined Patent Publication No. 62-33048 Japanese Patent Application Publication No. 10-263614

However, in the technique described in Patent Document 2, horizontal rolling is performed using a press machine, but since the target is extremely thick steel plates, a huge apparatus of, for example, 6000 tons is required. Therefore, productivity is reduced due to intermittent operation using huge equipment.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to reduce internal defects in slabs using compact equipment and without reducing productivity. It is an object of the present invention to provide a new and improved method for reducing internal defects in slabs and slab manufacturing equipment that enable the following.

In order to solve the above-mentioned problems, according to one aspect of the present invention, after the slabs continuously sent out from a continuous casting machine are width-reduced in the machine or on the downstream side of the continuous casting machine, the slabs that have been width-reduced are A method for reducing internal defects in a slab by horizontal rolling is provided.

Here, width reduction and horizontal rolling of the slab may be performed after the solidification state of the slab in the cross section becomes a completely solidified state.

In addition, by controlling at least one of the following: the casting speed by the continuous casting machine, the cooling temperature of the mold, the electromagnetic stirring conditions of the molten steel in the mold by the electromagnetic stirring device, and the cooling conditions by the secondary cooling device, The final solidification point may be located near the width end of the slab. Here, the widthwise position of the final solidification point of the slab refers to the widthwise position of the point farthest from the molten steel surface in the mold in the widthwise distribution of the final solidification point of the slab.

Here, when the plate thickness of the slab is H and the plate width is W, the widthwise position of the final solidification point of the slab may be in the range of 0.5H to 0.25W from the width end of the slab, Alternatively, it may be within a range of 0.5H to 2.0H from the width end of the slab.

Alternatively, the width-reduced slab may be horizontally rolled in a state where the temperature at the center of the cross-section of the width-reduced slab is higher than the temperature of the surface of the slab.

The amount of width reduction of the slab due to width reduction may be 5 mm or more.

Further, in the horizontal rolling, the slab may be rolled down so that the thickness of the slab becomes at least the thickness of the slab before width rolling.

Furthermore, the slab may be horizontally rolled before the slab is width-reduced or after the slab is horizontally rolled.

Moreover, in order to solve the above-mentioned problem, according to another aspect of the present invention, a width reduction device for width reduction of slabs continuously delivered from the continuous casting machine is provided inside or downstream of the continuous casting machine; A slab manufacturing facility is provided, which includes a horizontal rolling device that is provided on the downstream side in the rolling direction of a width reduction device and horizontally rolls the slab.

The slab manufacturing equipment may include a second horizontal rolling device that horizontally rolls the slab, upstream in the rolling direction with respect to the width reduction device or downstream in the rolling direction with respect to the horizontal rolling device.

As explained above, according to the present invention, internal defects in slabs can be reduced with compact equipment and without reducing productivity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing an example of a configuration of continuous casting equipment for carrying out a method for reducing internal defects in slabs according to an embodiment of the present invention. In the continuous casting equipment of FIG. 1, it is a top view which shows the width reduction device which rolls down the width of a slab immediately after continuous casting, and the horizontal rolling device which horizontally rolls a slab. FIG. 2 is an explanatory diagram showing another example of the configuration of continuous casting equipment for implementing the method for reducing internal defects in slabs according to the embodiment. It is a schematic diagram which shows the cross-sectional shape of the slab after continuous casting, after width reduction, and after horizontal rolling. It is an explanatory view showing an example of width reduction amount ΔE and ultrasonic flaw detection test results. FIG. 7 is an explanatory diagram showing another example of the width reduction amount ΔE and the ultrasonic flaw detection test results. It is an explanatory view showing an example of board thickness h after horizontal rolling and an ultrasonic flaw detection test result. It is an explanatory view showing another example of plate thickness h after horizontal rolling and ultrasonic flaw detection test results. FIG. 2 is an explanatory diagram showing an example of a configuration of continuous casting equipment in which a second horizontal rolling device is arranged upstream of a width rolling device. FIG. 2 is an explanatory diagram showing an example of the configuration of a continuous casting facility in which a second horizontal rolling device is arranged downstream of a horizontal rolling device. 2 is a graph showing the results of a numerical analysis of the internal quality improvement effect of the cross section of a slab after width reduction and horizontal rolling using the continuous casting equipment shown in FIG. 1; This is the result when the position in the width direction is expressed non-dimensionally by the thickness of the slab. 2 is a graph showing the results of a numerical analysis of the internal quality improvement effect of the cross section of a slab after width reduction and horizontal rolling using the continuous casting equipment shown in FIG. 1; This is the result when the position in the width direction is expressed dimensionlessly using the slab width. It is an explanatory view showing one example of composition of a secondary cooling device of continuous casting equipment. It is an explanatory view showing an example of arrangement of a spray nozzle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that, in this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals and redundant explanation will be omitted.

<1. Equipment configuration>
In explaining the method for reducing internal defects in a slab according to each embodiment of the present invention, first, the equipment configuration for carrying out the method for reducing internal defects in a slab will be described based on FIGS. 1 to 3. FIG. 1 is an explanatory diagram showing an example of the configuration of continuous casting equipment for implementing the method for reducing internal defects in slabs according to the present embodiment. FIG. 2 is a plan view showing an example of the width reduction device 130 and the horizontal rolling device 140 installed in the continuous casting equipment of FIG. 1. FIG. 3 is an explanatory diagram showing another example of the configuration of continuous casting equipment for implementing the method for reducing internal defects in slabs according to the present embodiment.

In the method for reducing internal defects in a slab according to the present embodiment, a slab immediately after continuous casting is subjected to width reduction and then horizontally rolled. For slabs that have many internal defects near their width edges rather than at the center of their width, width reduction and horizontal rolling are performed under conditions where the internal temperature is higher than the surface side, thereby reducing the voids in the slab. Effectively reduces internal defects such as In particular, it is effective to horizontally roll the width-reduced slab in a state where the temperature at the center of the cross-section of the slab is higher than the temperature at the surface of the slab. Slabs, which are wide slabs relative to their thickness, tend to have many internal defects near their width ends, while billets and blooms, which have approximately the same width and thickness, are used for rod-shaped pipes. , many internal defects occur at the center of the width. Therefore, the slab whose internal defects are to be reduced by the method according to the present invention is a slab whose width is wider than the thickness of the slab or the like.

As equipment for implementing the method for reducing internal defects in slabs according to the present embodiment, for example, as shown in FIG. 1, a continuous casting equipment 100 in which a width reduction device 130 and a horizontal rolling device 140 are installed inside the machine, or, As shown in FIG. 2, there is a hot rolled thin slab continuous casting facility 200, etc., which is equipped with a width reduction device 220 and a horizontal rolling device 230.

Continuous casting equipment 100 shown in FIG. 1 is equipment for manufacturing slab 5 by continuously casting molten steel using continuous casting mold 115. Although the continuous casting equipment 100 shown in FIG. 1 is a vertical bending type continuous casting equipment, the present invention is not limited to this example, and may be a curved type, a vertical type, or the like continuous casting equipment. As shown in FIG. 1, the continuous casting equipment 100 includes a ladle 111, a tundish 113, a mold 115, a secondary cooling device 120, a width reduction device 130, a horizontal rolling device 140, and a slab cutting machine. 150.

The ladle 111, which is a movable container for conveying molten steel, supplies the molten steel in the ladle 111 to the tundish 113 from above the tundish 113. The molten steel supplied to the tundish 113 is continuously supplied into the mold 115 after inclusions in the molten steel are removed within the tundish 113. The molten steel supplied to the mold 115 is cooled at the contact portion with the mold plate, and becomes a slab 5 including an unsolidified portion inside the solidified outer shell. As the slab 5 moves below the mold 115, solidification of the unsolidified portion inside progresses, and the thickness of the solidified outer shell gradually increases. The slab 5 including the solidified shell and unsolidified portion is pulled out from the lower end of the mold 115. The slab 5 pulled out from the mold 115 is moved downstream in the casting direction while the secondary cooling device 120 is supported by the support rolls 121 . While the secondary cooling device 120 is moving, cooling water is sprayed onto the slab 5 by a spray nozzle (not shown).

The slab 5 that has passed through the secondary cooling device 120 is rolled down in the width direction by a width rolling device 130, and then horizontally rolled by a horizontal rolling device 140. The width reduction device 130 may be an edger rolling mill including edger rolls 131, 131 provided at both ends of the slab 5 in the plate width direction, as shown in FIG. However, the present invention is not limited to this example, and the width reduction device 130 may be a rolling mill other than an edger rolling mill, such as a sizing press. The slab 5 rolled by the horizontal rolling device 140 is cut into a predetermined length by a slab cutting machine 150, as shown in FIG. Transported to process equipment.

The hot rolling thin slab continuous casting equipment 200 shown in FIG. 3 is equipment for manufacturing thin plate materials, and endlessly manufactures slabs 5 having a wide width relative to their thickness from continuous casting to hot rolling. . As shown in FIG. 3, the hot rolled thin slab continuous casting equipment 200 includes a thin slab continuous casting machine 210, a width reduction device 220, a horizontal rolling device 230, a rough rolling mill 240, a finishing rolling mill 250, A cooling device 260 and a winder 270 are provided. The continuous thin slab casting machine 210 includes a ladle 211, a tundish 213, a mold 215, and a secondary cooling device 217, and is configured similarly to the continuous casting equipment 100 shown in FIG. The slab 5 cast by the continuous thin slab casting machine 210 is rolled down in the width direction by the width reduction device 220, and then horizontally rolled by the horizontal rolling device 230. The width reduction device 220 and the horizontal rolling device 230 can be configured as shown in FIG. 2, equivalent to the continuous casting equipment 100 in FIG. The slab 5 rolled by the horizontal rolling device 230 passes through a rough rolling mill 240 and a finishing rolling mill 250, is rolled to a predetermined thickness, and is cooled by a cooling device 260. The slab 5 cooled by the cooling device 260 is wound into a coil by the winder 270.

In both the continuous casting equipment 100 shown in FIG. 1 and the hot rolled thin slab continuous casting equipment 200 shown in FIG. In the hot-rolled thin slab continuous casting equipment 200 of FIG. Before being roughly rolled, the slab 5 is subjected to width reduction and horizontal rolling. By performing width reduction and horizontal rolling on the slab 5 immediately after continuous casting, internal defects in the slab 5 are improved. Since a width reduction device and a horizontal rolling mill are added to the equipment, internal defects in the slab 5 can be reduced with a compact equipment. In addition, if the width reduction device is an edger rolling machine, the operation will be continuous rather than intermittent as in the case of using a press, so internal defects in the slab 5 can be eliminated without reducing productivity. can be reduced.

<2. First embodiment>
[2-1. mechanism]
First, based on FIG. 4, a mechanism in which internal defects are reduced by the method for reducing internal defects in a slab according to the first embodiment of the present invention will be described. FIG. 4 is a schematic diagram showing the cross-sectional shape of the slab 5 after continuous casting, width reduction, and horizontal rolling.

Internal defects such as voids that occur inside the slab tend to be concentrated in the final solidification area near the center of the thickness of the slab. This tends to occur near the width edges. In order to improve the internal defects in the slab, it is effective to reduce the slab, and at this time, it is effective to plastically deform the slab while applying a large hydrostatic pressure to the internal defect portion. By subjecting the final solidified part of the slab to rolling that causes large plastic deformation under high hydrostatic stress, the internal quality of the slab can be improved and the quality of the final product can be improved.

Therefore, in order to apply large plastic deformation under high hydrostatic pressure to the vicinity of the width end of the slab, where internal defects are likely to occur, the inventor of the present application has developed a method of horizontal rolling after increasing the thickness by width reduction. I thought it was effective.

Here, when the slab is subjected to width reduction and horizontal rolling, the cross-sectional shape of the slab 5 changes as shown in FIG. 4.

The cross-sectional shape of the slab immediately after continuous casting shown in the upper part of FIG. 4 is the slab 5 after passing through the secondary cooling device 120 of the continuous casting equipment 100 in FIG. This is the cross-sectional shape of the slab 5 after passing through the continuous thin slab casting machine 210 of the equipment 200. At this time, the slab 5 has a substantially rectangular cross-sectional shape with a plate thickness H and a plate width W. A region 5a of the slab 5 indicates a region at the end of the plate width where internal defects such as voids are likely to occur.

When the width of the slab 5 immediately after continuous casting is reduced by the width reduction amount ΔE using the width reduction device, the slab 5 becomes a so-called dogbone shape in which thickening is concentrated at the width end portion as shown in the center of FIG. At this time, the region 5a where internal defects such as voids are likely to occur is located in the thick portion of the slab 5. Thereafter, when the width-reduced slab 5 is horizontally rolled, it becomes a substantially rectangular slab 5 with a thickness h as shown in the lower side of FIG. At this time, since the thick portion on the end side of the slab 5 after width reduction is reduced from the width center, high hydrostatic stress and large plastic deformation are effectively applied to the area 5a, and as a result, Voids inside the piece are reduced. In this way, by horizontally rolling the slab 5 immediately after continuous casting after width reduction, the voids inside the slab can be reduced.

Note that width reduction and horizontal rolling of the slab may be performed after the solidified state in the cross section of the slab becomes a completely solidified state, or may be performed in an unsolidified state.

[2-2. Width reduction amount ΔE]
In the method for reducing internal defects in a slab according to the present embodiment, the width reduction amount ΔE of the slab by the width reduction device may be 5 mm or more. If the width reduction amount ΔE is too small, the difference in thickness between the thickened part at the width end and the width center part will be small, so horizontal rolling performed after width reduction is sufficient to reduce the area 5a where internal defects such as voids are likely to occur. Hydrostatic stress and plastic deformation cannot be applied, and voids remain inside the slab.

In order to investigate the value of the effective width reduction amount ΔE, the following verification was performed. In this verification, a slab of general low carbon steel cast to a plate thickness of 250 mm and a plate width of 1600 mm was subjected to width reduction by a width reduction amount ΔE at the exit side of a continuous casting machine, and then horizontally rolled in one pass. It was rolled to a thickness of 220 mm. An ultrasonic flaw detection test specified in JIS G 0801 was carried out on a range of 100 to 300 mm from the end of the plate width of the slab thus obtained, and the internal quality of the slab was evaluated. The results are shown in FIG. In FIG. 5, the ○ mark indicates a quality level that passed in the ultrasonic flaw detection test for cast slabs specified in JIS G 0801, and the x mark indicates a quality level that failed. From FIG. 5, it was found that if the width reduction amount ΔE was ensured to be 5 mm or more, a slab with excellent internal quality could be obtained.

In addition, similar verification was conducted using slabs of other steel types. Here, a medium carbon steel slab cast to a plate thickness of 200 mm and a plate width of 1200 mm is width-reduced by ΔE at the exit side of a continuous caster, and then rolled to a thickness of 160 mm by one pass of horizontal rolling. did. An ultrasonic flaw detection test specified in JIS G 0801 was carried out on a range of 100 to 300 mm from the end of the plate width of the slab thus obtained, and the internal quality of the slab was evaluated. The results are shown in FIG. In FIG. 6, the ○ mark indicates a quality level that passed in the ultrasonic flaw detection test for cast slabs specified in JIS G 0801, and the x mark indicates a quality level that failed. From FIG. 6, it was found that as in the case of FIG. 5, if the width reduction amount ΔE was ensured to be 5 mm or more, a slab with excellent internal quality could be obtained.

From the above, it can be said that the width reduction amount ΔE for reducing the width of the slab by the width reduction device may be 5 mm or more. Note that the upper limit of the width reduction amount ΔE is not particularly limited from the viewpoint of reducing voids as shown in FIGS. 5 and 6, but if the width reduction amount ΔE is too large, the casting immediately after continuous casting Since the slab has low ductility, cracks may occur in the slab. In the method for reducing internal defects in slabs according to the present embodiment, it is sufficient to perform a width reduction of, for example, about 5 to 10 mm.

[2-3. Reduction amount in horizontal rolling (plate thickness h after horizontal rolling)]
In addition, in the method for reducing internal defects in a slab according to the present embodiment, the slab 5 whose width has been reduced by the width reduction device is horizontally rolled by horizontal rolling. is preferably at least equal to or less than the plate thickness H of the slab 5 before width reduction. This is because if the plate thickness h of the slab 5 after horizontal rolling is larger than the plate thickness H of the slab 5 before width reduction, internal defects such as voids inside the slab cannot be sufficiently reduced.

In order to investigate the effective plate thickness h after horizontal rolling, the following verification was performed. In this verification, a slab of general low carbon steel cast to a thickness of 250 mm and a width of 1600 mm was subjected to a width reduction of 20 mm at the exit side of continuous casting, and then rolled to a thickness h by one pass of horizontal rolling. did. An ultrasonic flaw detection test specified in JIS G 0801 was carried out on a range of 100 to 300 mm from the end of the plate width of the slab thus obtained, and the internal quality of the slab was evaluated. The results are shown in FIG. In FIG. 7, the ○ mark indicates a quality level that passed in the ultrasonic flaw detection test for slabs specified in JIS G 0801, and the x mark indicates a quality level that failed. From FIG. 7, it can be seen that if the plate thickness h after horizontal rolling is 250 mm, that is, equal to or less than the plate thickness H before width reduction, a slab with excellent internal quality can be obtained.

In addition, similar verification was conducted using slabs of other steel types. Here, a medium carbon steel slab cast to a thickness of 220 mm and a width of 1500 mm was subjected to a width reduction of 20 mm at the exit side of continuous casting, and then rolled to a thickness h by one pass of horizontal rolling. An ultrasonic flaw detection test specified in JIS G 0801 was carried out on a range of 100 to 300 mm from the end of the plate width of the slab thus obtained, and the internal quality of the slab was evaluated. The results are shown in FIG. In FIG. 8, the ○ mark indicates a quality level that passed in the ultrasonic flaw detection test for cast slabs specified in JIS G 0801, and the x mark indicates a quality level that failed. From FIG. 8, it can be seen that if the plate thickness h after horizontal rolling is 220 mm, that is, equal to or less than the plate thickness H before width reduction, a slab with excellent internal quality can be obtained.

[2-4. Improving the internal quality of slab near the center of width]
In the method for reducing internal defects in a slab according to the present embodiment, as shown in FIG. 1 or 3, the slab 5 after continuous casting is width-reduced and then horizontally rolled to eliminate internal defects such as voids. High hydrostatic stress and large plastic deformation are effectively applied to areas where defects are likely to occur, thereby reducing internal defects in the slab 5. At this time, in order to further improve the internal quality near the center of the slab 5 in the width direction, a second horizontal rolling mill is installed on the upstream side in the rolling direction with respect to the width reduction device or on the downstream side in the rolling direction with respect to the horizontal rolling device. Further devices may be provided.

For example, by arranging the second horizontal rolling device upstream of the width reduction device in the rolling direction, the internal quality of the slab 5 near the center in the width direction can be improved. In this case, it is also possible to further improve the ductility of the slab 5 and secure the width reduction amount ΔE necessary for improving the internal quality of the slab 5. As a specific equipment configuration, for example, like the continuous casting equipment 100A shown in FIG. 9, a second horizontal rolling device 160 is installed upstream in the rolling direction of the width reduction device 130 of the continuous casting equipment 100 shown in FIG. Just set it up. Similarly, in the case of the hot rolled thin slab continuous casting equipment 200 shown in FIG. 3, a second horizontal rolling device may be provided upstream of the width rolling device 220 in the rolling direction.

Also, by arranging the second horizontal rolling device downstream of the horizontal rolling device in the rolling direction, the internal quality of the slab 5 near the center in the width direction can be improved. In this case, the second horizontal rolling apparatus is installed immediately after the horizontal rolling apparatus, that is, immediately after the horizontal rolling apparatus 140 of the continuous casting equipment 100 shown in FIG. 1, or the hot rolled thin slab continuous casting shown in FIG. It may be installed between the horizontal rolling device 230 and the rough rolling mill 240 of the equipment 200. In addition, in the case of the hot rolled thin slab continuous casting equipment 200, it is also possible to utilize the rolling by the rough rolling mill 240 as rolling by the second horizontal rolling device. Furthermore, when the second horizontal rolling device is placed downstream of the horizontal rolling device in the rolling direction, it is not necessarily necessary to place the second horizontal rolling device in the same equipment as the width reduction device and the horizontal rolling device. . For example, as shown in FIG. 10, a second horizontal rolling device 310 may be installed in a post-process facility of the continuous casting facility 100 shown in FIG. 1, such as a hot rolling facility 300.

<3. Second embodiment>
[3-1. overview]
In the first embodiment, regarding improvement of internal quality by rolling down the slab, when the solidified state in the width cross section of the slab is completely solidified or unsolidified, the slab is width rolled down to form a dogbone shape. After increasing the thickness, it was horizontally rolled. This is based on the fact that the final solidification point (also called crater end), where internal defects tend to be concentrated, is more likely to occur near the width ends than in the width center.

On the other hand, in conventional continuous casting, attempts have been made to suppress and prevent the accumulation of internal defects near the width ends by aiming to make the final solidification point uniform in the width direction. Specifically, by controlling the casting speed of the continuous casting machine, the cooling temperature of the mold, the electromagnetic stirring conditions of the molten steel in the mold (for example, the flow of molten steel), or the cooling conditions of the secondary cooling device, the final solidification point can be controlled. The shape of the distribution in the width direction of the slab (that is, the shape of the crater end) has been oriented to be uniform in the width direction. However, it is difficult to stably obtain a final solidification point that is uniform in the width direction of the slab. This is because the shape of the final solidification point distribution in the slab width direction is strongly influenced by the temperature of the molten steel supplied to the mold and the flow behavior of the molten steel in the mold, both of which change with casting conditions and the passage of time. This is because, in response to such changes, it is essential to carefully and appropriately control the electromagnetic stirring conditions, the cooling conditions of the mold and the slab by secondary cooling water, etc.

Therefore, in this embodiment, the final solidification point of the slab is purposely made non-uniform in the width direction, and the final solidification point of the slab is cast under the condition that it is located near the width end, and then inside the continuous casting machine. Alternatively, after width reduction is performed on the downstream side, the width-reduced slab is horizontally rolled. This is based on the fact that, as explained in the first embodiment above, when horizontal rolling is applied to a slab after width reduction, the internal quality improvement effect is maximized near the width end. The final solidification point of the slab is located downstream of the final solidification point near the width center, and by concentrating internal defects near the final solidification point in the width direction, the quality of the slab can be further effectively improved. By thinking. In addition, especially when performing unsolidified rolling, it is important to control the solid phase ratio at the rolling position to a desired value, but according to the internal defect reduction method according to the present embodiment, Since it is not necessary to reduce the pressure at 30 degrees, some degree of non-uniformity in the final solidification point in the width direction can be tolerated, which also has the advantage of increasing the degree of freedom in operation.

[3-2. Control of final freezing point]
First, the reason why the width direction position of the final solidification point of the slab is located near the width end portion will be explained based on FIGS. 11A and 11B. FIGS. 11A and 11B are graphs showing an example of the width direction distribution of the internal quality improvement effect in horizontal rolling after width reduction of a slab. Figure 11A shows the results when the position in the width direction from the width end is expressed dimensionless by slab thickness, and Figure 11B shows the result when the width direction position from the width edge is rendered dimensionless by slab width. The results are shown in the case. In FIGS. 11A and 11B, the plate thickness H of the slab is 100 mm and the plate width W is 1200 mm (Case 1), the plate thickness H is 250 mm, and the plate width W is 1200 mm (Case 2), and the plate thickness An example is shown in which H is 250 mm and the plate width W is 2000 mm (Case 3). The horizontal axes in FIGS. 11A and 11B indicate that position 0 on the left side indicates the width end, and as it moves toward the right side, it moves toward the center of the width.

The internal quality improvement effect shown on the vertical axis in Figures 11A and 11B was obtained by numerical analysis from the deformation of the slab caused by horizontal rolling after width reduction using the continuous casting equipment shown in Figure 1. It is calculated based on stress and strain behavior, and indicates that the higher the value, the higher the internal quality improvement effect. Looking at FIGS. 11A and 11B, in all of Cases 1 to 3, the internal quality improvement effect is maximum between the width ends and the width center. Therefore, by accumulating the internal quality defects of the slab near the area where the internal quality improvement effect is maximum, and applying width reduction to the slab and then horizontal reduction, the internal quality of the area is improved. can be dramatically improved.

Here, it is known that internal defects are difficult to accumulate in the range of 0.5H from the width end of the slab due to cooling from the side end surface. Furthermore, as shown in Fig. 11A, a high internal quality improvement effect can be obtained in the range from the width end of the slab to 2.0H, and as shown in Fig. 11B, the range from the width end of the slab to 0.25W. It is. Therefore, after accumulating internal defects in the range of 0.5H to 0.25W from the width end of the slab or in the range of 0.5H to 2.0H from the width end of the slab, width reduction is performed. By horizontally rolling the slab after increasing its thickness at the width end, it is possible to obtain even better internal quality over the entire width of the slab. This is because the internal defects of the slab were accumulated near the above-mentioned width ends, so areas other than these areas (for example, near the width center) already had good internal defects before width reduction and horizontal rolling. You can get quality. Therefore, by applying the complementary method according to the present embodiment, it is possible to obtain good internal quality throughout the width direction of the slab, and it is possible to improve the quality of the final product.

Note that the final solidification position can be detected using existing methods, such as a method that is determined by performing a rivet driving test in advance, a non-contact detection method using ultrasonic waves, etc., or a method that is calculated from solidification heat transfer analysis. method etc. may be used.

The method of controlling the widthwise position of the final solidification point of the slab to the desired position is based on the casting speed of the continuous casting machine, the cooling temperature of the mold, the electromagnetic stirring conditions of the molten steel in the mold using an electromagnetic stirring device, or two methods. It is conceivable to control at least one of the cooling conditions by the secondary cooling device. Among these, cooling condition control using a secondary cooling device is preferable because it is possible to control the widthwise position of the final solidification point of the slab without having a large adverse effect on productivity or the surface quality of the slab. Therefore, as an example of controlling the position in the width direction of the final solidification point of the slab according to the present embodiment, a method for controlling the position in the width direction of the final solidification point by controlling the cooling conditions of the secondary cooling device is described based on FIGS. 12 and 13. explain. FIG. 12 is an explanatory diagram showing a configuration example of the secondary cooling device 120 of the continuous casting equipment 100. FIG. 13 is an explanatory diagram showing an example of the arrangement of spray nozzles.

FIG. 12 shows the secondary cooling device 120 of the continuous casting equipment 100 of FIG. 1 in more detail. As shown in FIG. 12, the secondary cooling device 120 cools the slab 5 passing through the secondary cooling zone, and a plurality of support rolls 121 that are arranged opposite to each other so as to sandwich the slab in the secondary cooling zone. A cooling device (not shown) is included. The support rolls 121 include a group of support rolls 121a on the movable surface side (so-called L surface side) of the slab, and a support roll group 121a on the fixed surface side (so-called F surface side) so as to guide the slab 5 in the casting direction A along the slab path. side) support roll group 121b.

The slab 5 pulled out from directly below the mold 115 passes through the curved section 12A and then through the horizontal section 12B. The most downstream side of the curved part 12A is a straightening part 12C that changes the shape of the slab 5 from vertical to horizontal, and when passing through the straightening part 12C, the slab 5 is straightened from its curved shape to horizontal. .

The secondary cooling device 120 is installed between directly below the mold 115 and the width reduction device (see width reduction device 130 in FIG. 1). The secondary cooling device 120 constitutes cooling zones a1 to a12 for cooling the slab 5. A plurality of spray nozzles that spray secondary cooling water toward the long side surfaces of the slab 5 are arranged in each of the cooling zones a1 to a12. For example, as shown in FIG. 13, the plurality of spray nozzles constituting the secondary cooling device 120 include a spray nozzle 125c installed at the width center side and a spray nozzle 125e installed at the width end side. The spray nozzle 125c sprays secondary cooling water onto the width center side of the long side surface of the slab, and the spray nozzle 125e sprays secondary cooling water onto the width end side of the long side surface of the slab. These spray nozzles 125c and 125e are provided on both the L side and the F side of the slab passage, and spray secondary cooling water onto the entire long sides of the slab 5.

The water volume density X (l/min/m ² ) of the secondary cooling water of the spray nozzle 125c installed on the width center side, and the water volume density Y (of the secondary cooling water of the spray nozzle 125e installed on the width end side) l/min/m ² ) is configured to be independently controllable. Thereby, the cooling capacity at different positions in the width direction of the slab 5 can be set differently. In this embodiment, for example, when the final solidification point of the slab is to be located closer to the width end side with respect to a certain standard condition, the cooling capacity at least on the width end side is weakened, or the cooling capacity on the width center side is weakened. All you have to do is strengthen it. Therefore, at least the water volume density Y of the secondary cooling water of the spray nozzle 125e installed on the width end side should be reduced, or the water volume density X of the secondary cooling water of the spray nozzle 125c installed on the width center side. The cooling device is controlled to either increase the

Note that the installation range of the spray nozzle 125e installed on the width end side is within the range of distance L from the width end, but this installation range is such that the internal quality improvement effect shown in FIGS. 11A and 11B, for example, can be achieved. It may be determined according to the position where the maximum value is reached. Further, in FIG. 13, the water density of the secondary cooling water can be controlled independently on the width end side and the width center side, but the present invention is not limited to such an example, and the water density can be controlled more finely in the width direction. The cooling device may be configured to be able to control the water volume density of the secondary cooling water. In this manner, by controlling the cooling conditions of the secondary cooling device 120, for example, the water volume density of the secondary cooling water, the widthwise position of the final solidification point of the slab can be controlled.

In addition to controlling the cooling conditions of the secondary cooling device 120, the casting speed of the continuous casting machine can be changed, the cooling temperature of the mold 115, and the conditions of electromagnetic stirring of molten steel in the mold by the electromagnetic stirring device 117 can be changed. By doing so, the widthwise position of the final solidification point of the slab can be controlled. The widthwise position of the final solidification point of the slab may be controlled by changing at least one of these conditions. As mentioned above, it is preferable for operation to control the cooling conditions of the secondary cooling device 120, but it may be difficult to control the widthwise position of the final solidification point of the slab to a desired position by this alone. Therefore, by changing a plurality of conditions, the widthwise position of the final solidification point of the slab can be controlled more flexibly.

The treatment of the slab 5 after cooling by the secondary cooling device 120 may be performed by horizontal rolling after width reduction, as in the first embodiment. Therefore, detailed explanation will be omitted in this embodiment.

[A. Width reduction and horizontal rolling of slab immediately after continuous casting]
First, we verified the effects of width reduction and horizontal rolling on slabs immediately after continuous casting. In this verification, a general low carbon steel slab cast to a plate thickness of 250 mm and a plate width of 1600 mm was subjected to a width reduction of 30 mm at the exit side of a continuous casting machine (CC) as in Example 1, and then in one pass. The steel plate was horizontally rolled to a thickness of 220 mm, and then hot rolled to produce a thick steel plate with a thickness of 50 mm.

As Comparative Example 1, a slab of general low carbon steel similar to Example 1 was hot rolled without width reduction (that is, only one pass of horizontal rolling was performed to a thickness of 220 mm). A thick steel plate with a thickness of 50 mm was manufactured. In addition, as Comparative Example 2, the slab of general low carbon steel similar to Example 1 was rolled to a thickness of 220 mm by width reduction of 30 mm and one pass of horizontal rolling, instead of immediately after continuous casting. This was carried out after reheating on a rolling line, and a thick steel plate with a plate thickness of 50 mm was manufactured.

For these manufactured thick steel plates, specimens were cut from an area of 100 to 200 mm from the width end at the middle part in the longitudinal direction of the steel plate and an area of ±10 mm from the center of the plate thickness, and the size of the voids (porosity) was determined using a microscope. The number of pieces was observed and investigated. The results are shown in Table 1.

In Example 1, no voids were observed inside the steel plate, and a steel plate with reduced internal defects was obtained by performing width reduction and subsequent horizontal rolling on the outlet side of the continuous caster (CC). This is because horizontal rolling was performed after increasing the thickness at the width end by width reduction, and the temperature distribution in the cross section of the slab is such that the center of the slab is on the surface side (i.e., the plate thickness This is due to the fact that the slab was rolled at the exit side of the continuous casting machine (CC), where the temperature distribution was higher than that at the end (in both the direction and width direction). When the center of the slab has a higher temperature distribution than the surface side of the slab, the center of the slab has a distribution such that the deformation resistance is lower than the surface side of the slab. . Therefore, by performing width reduction and horizontal rolling on such a slab, it is possible to apply high hydrostatic stress and large plastic deformation to the area where internal defects exist in the slab, and improve the internal quality. It is thought that a good slab could be subjected to hot rolling.

On the other hand, in Comparative Example 1, in which only horizontal rolling was performed, horizontal rolling was performed without increasing the thickness at the width end of the slab, so sufficient hydrostatic stress was applied at the center of the thickness of the slab and near the width end. Also, plastic deformation could not be applied and voids remained. In addition, in Comparative Example 2, width reduction and horizontal rolling are performed after the slab (material to be rolled) is reheated in a heating furnace. The temperature distribution will be lower than the above. For this reason, sufficient hydrostatic stress could not be applied to the center of the rolled material during width reduction during hot rolling and subsequent horizontal rolling, and voids remained.

[B. Installation of second horizontal rolling equipment]
Next, we verified the effect of the second horizontal rolling device installed to improve the internal quality near the center of the width of the slab.

In this verification, as Example 2, a general low carbon steel slab cast to a plate thickness of 250 mm and a plate width of 1600 mm was subjected to a width reduction of 20 mm at the exit side of a continuous caster (CC), and then 1 The steel plate was rolled to a thickness of 220 mm by horizontal rolling passes, and then hot rolled to produce a thick steel plate with a thickness of 100 mm.

In addition, as Example 3, a slab of general low carbon steel similar to Example 2 was subjected to a width reduction of 20 mm at the exit side of continuous casting, and then rolled to a thickness of 220 mm by horizontal rolling in the first pass. Then, the steel plate was further rolled to a thickness of 180 mm by a second horizontal rolling pass using a second horizontal rolling device, and then hot rolled to produce a thick steel plate with a thickness of 100 mm.

Furthermore, as Example 4, a slab of general low carbon steel similar to Example 2 was rolled to a thickness of 220 mm by horizontal rolling at the exit side of continuous casting, and then subjected to a width reduction of 20 mm. After rolling to a thickness of 180 mm by horizontal rolling using a horizontal rolling machine, hot rolling was performed to produce a thick steel plate with a thickness of 100 mm.

On the other hand, as Comparative Example 3, the first pass of horizontal rolling was performed to a thickness of 220 mm, the second pass of horizontal rolling was performed to a thickness of 180 mm, and then hot rolling was performed to obtain a thickness of A 100 mm thick steel plate was manufactured.

For these manufactured thick steel plates, specimens were cut from an area of 100 to 200 mm from the width end at the middle part in the longitudinal direction of the steel plate and an area of ±10 mm from the center of the plate thickness, and the size of the voids (porosity) was determined using a microscope. The number of pieces was observed and investigated. The results are shown in Table 2. In Table 2, the size and number of voids (porosity) are shown separately for those within a range of ±400 mm from the center of the board width and those within a range of 400 mm from both width ends.

From Table 2, it can be seen that compared to Comparative Example 3, steel sheets with superior internal quality can be obtained in any of Examples 2 to 4 of the present invention. In particular, in Examples 3 and 4, in which width reduction and horizontal rolling were performed immediately after continuous casting, and one or more passes of horizontal rolling were added, the number of voids was smaller than in Comparative Example 2, and the internal quality was better. It can be seen that it is possible to obtain a steel plate with The slabs used in this verification, which have a thick manufacturing thickness compared to the slab thickness, are likely to have internal defects, but after performing width reduction at the exit side of the continuous caster (CC), two-pass A slab with good internal quality is heated by horizontal rolling, or by horizontal rolling at the exit side of a continuous caster (CC), width reduction, and then one more pass of horizontal rolling. This is thought to be due to the fact that it could be used for inter-rolling.

[C. Control of the widthwise position of the final freezing point]
Next, the effect of locating the final solidification point of the slab in the width direction toward the width end side was verified.

In this verification, a general low carbon steel slab was cast in a continuous casting machine to a plate thickness of 250 mm and a plate width of 1600 mm, and after a width reduction of 30 mm was performed on the exit side of the continuous casting machine, the first pass The steel plate was horizontally rolled to a thickness of 220 mm and then hot rolled to produce a thick steel plate with a plate thickness (H) of 100 mm.

In this verification, slabs were cast by varying the water volume density of the secondary cooling water in the width direction of the slab in multiple ways in the secondary cooling zone in the continuous casting machine shown in FIGS. 12 and 13. That is, the water density of the secondary cooling water in the width direction of the slab was changed to obtain a slab having a shape of distribution of a plurality of final solidification points in the width direction of the slab (that is, a shape of a crater end). The shape of the final solidification point distribution in the slab width direction was determined by solidification heat transfer analysis.

In order to evaluate the internal quality of these manufactured thick steel plates, specimens were cut from an area ±10 mm from the center of the plate thickness in the middle part of the steel plate in the longitudinal direction, and the size and number of voids (porosity) were observed using a microscope. I looked it up. Table 3 shows the relationship between the position of the final solidification point in the slab width direction and the size and number of voids (porosity).

From Table 3, thick steel plates with good internal quality were obtained when the final solidification point in the slab width direction was in the range of 125 mm to 500 mm from the slab width end, that is, in the range of 0.5H to 2.0H. It was a case of being located within the range. This is due to the thickening of the width end of the slab due to width reduction, which results in large plastic deformation under higher hydrostatic stress during horizontal rolling. This is thought to be because by controlling the widthwise position of the solidification point, horizontal reduction was performed with internal defects accumulated in the area, and the internal quality was effectively improved.

Although preferred embodiments of the present invention have been described above in detail with reference to the accompanying drawings, the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea stated in the claims. It is understood that these also naturally fall within the technical scope of the present invention.

In the above explanation, an example was explained in which the present invention is applied to a vertical bending type continuous casting machine, but the present invention is not limited to this, and the present invention can be applied to a vertical type continuous casting machine in which the roll group is arranged vertically or a curved type continuous casting machine. It can also be applied to continuous casting machines. Furthermore, the present invention is applicable not only to slabs but also to casting of slabs such as blooms.

5 Slab 5a Region of plate width end where internal defects are likely to occur 7 Slab (after cutting)
111, 211 ladle 113, 213 tundish 117 electromagnetic stirring device 115, 215 mold 120, 217 secondary cooling device 121 support roll 125c, 125e spray nozzle 130, 220 width reduction device 131 edger roll 140, 230 horizontal rolling device 150 slab Cutting machine 160, 310 Second horizontal rolling device 210 Thin slab continuous casting machine 240 Roughing mill 250 Finishing mill 260 Cooling device 270 Winding machine

Claims (5)

Immediately after continuous casting, the slab continuously sent out from the continuous casting machine is width-reduced, and then the width-reduced slab is horizontally rolled ,
By controlling at least one of the casting speed by the continuous casting machine, the cooling temperature of the mold, the electromagnetic stirring conditions of the molten steel in the mold by the electromagnetic stirring device, and the cooling conditions by the secondary cooling device, The width direction position of the final solidification point is located in the range of 0.5H to 0.25W from the width end of the slab, or in the range of 0.5H to 2.0H from the width end of the slab,
After the solidification state of the slab in the cross section becomes a completely solidified state, width reduction and horizontal rolling of the slab are performed,
A method for reducing internal defects in a slab, comprising horizontally rolling the width-reduced slab in a state where the temperature at the center of the cross section of the width-reduced slab is higher than the temperature of the surface of the slab.
Note that H is the thickness of the slab, and W is the width of the slab. The method for reducing internal defects in a slab according to claim 1 , wherein the amount of width reduction of the slab due to the width reduction is 5 mm or more. Reducing internal defects in a slab according to claim 1 or 2, wherein in the horizontal rolling, the slab is rolled down so that the thickness of the slab becomes at least the thickness of the slab before the width reduction. Method. Between the continuous casting machine and the slab cutting machine, or between the continuous casting machine and the rough rolling mill,
a width reduction device that reduces the width of slabs continuously delivered from the continuous casting machine;
a horizontal rolling device that is provided on the downstream side in the rolling direction of the width reduction device and horizontally rolls the slab;
Equipped with
Immediately after continuous casting, the slab continuously sent out from the continuous casting machine is width-reduced by the width-reducing device, and then the width-reduced slab is horizontally rolled by the horizontal rolling device,
By controlling at least one of the casting speed by the continuous casting machine, the cooling temperature of the mold, the electromagnetic stirring conditions of the molten steel in the mold by the electromagnetic stirring device, and the cooling conditions by the secondary cooling device, The width direction position of the final solidification point is located in a range of 0.5H to 0.25W from the width end of the slab, or in a range of 0.5H to 2.0H from the width end of the slab,
After the solidification state of the slab in the cross section becomes a completely solidified state, width reduction and horizontal rolling of the slab are performed,
A slab manufacturing equipment that horizontally rolls the width-reduced slab in a state where the temperature at the center of the cross section of the width-reduced slab is higher than the temperature of the surface of the slab.
Note that H is the thickness of the slab, and W is the width of the slab. between the continuous casting machine and the slab cutting machine, or between the continuous casting machine and the rough rolling mill, and upstream in the rolling direction with respect to the width reduction device, or the horizontal The slab manufacturing equipment according to claim 4 , further comprising a second horizontal rolling device that horizontally rolls the slab on the downstream side in the rolling direction with respect to the rolling device.

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