JP2020104149A - Steel continuous casting method for steel - Google Patents

Abstract

To provide a continuous casting method for a steel capable of reducing porosities in a slab.SOLUTION: A continuous casting method for a steel has a process where, while injecting a molten steel 9 into a cooled mold 5 for continuous casting, the molten steel 9 is solidified to form a slab 10, and the slab 10 is pulled out from the mold 5. Next, cooling water is sprayed toward the slab 10 to solidify the molten steel 9 in the slab 10, and then, the slab 10 is depressed. At a position in which the depression is started, the surface temperature of the slab 10 is controlled to 600°C or lower, and also, a difference between the temperature of a slab center and the above surface temperature is controlled to 400°C or higher, and, at a solidification completion position 13 in which the solidification of the molten steel 9 in the slab 10 is completed, the solidification structure of the central part in a slab thickness direction is formed of columnar crystals and/or branched columnar crystals. The slab 10 is depressed by 5 mm or higher.SELECTED DRAWING: Figure 1

Description

The present invention relates to a continuous casting method for steel that reduces voids formed in a slab cast by a continuous casting machine.

In continuous casting of steel, while injecting molten steel into a cooled continuous casting mold, the molten steel is solidified to form a slab, the slab is pulled out of the mold, and cooling water is sprayed toward the slab to cast. The molten steel in the piece is further solidified. A small void (referred to as "porosity") is formed in the slab at the center of the slab at a position where the slab completes solidification (referred to as "solidification completion position") due to solidification contraction or thermal contraction. Hydrogen easily accumulates in the porosity, and if hydrogen remains as a solid solution in the steel sheet and remains, cracks due to hydrogen may occur.

In order to reduce porosity, a method is known in which a reduction roll is arranged near the solidification completion position of the slab and the slab is reduced by the reduction roll to reduce the porosity formed at the center of the slab. In Patent Document 1, at a position where the central solid fraction of the cast piece is 0.8 or more and less than 1, that is, before the molten steel in the cast piece is completely solidified, the cast piece is given a predetermined reduction amount (mm). Discloses a continuous casting method for steel in which a slab is rolled in the thickness direction of the steel. Further, Patent Document 2 discloses a continuous casting method for steel in which the amount of reduction (reduction rate) in the thickness direction of the slab is set to 10 to 50% and the slab after the completion of solidification is rolled down.

JP, 2007-296542, A JP, 2016-175104, A

In the continuous casting method for steel of Patent Document 1, since the cast piece is pressed before the solidification of the cast piece is completed, there is a high possibility that the cast piece will crack.

The steel continuous casting method of Patent Document 2 relates to a technique of reducing porosity by rolling down a slab with a very large force, and when the rolling down described in Patent Document 2 is to be performed by a general continuous casting machine. Is too expensive for equipment. In recent years, for example, steel sheets for industrial machines, construction machines, marine structures and pressure vessels have been required to have a larger thickness, and the thickness of a slab for such steel sheets is about 200 mm at the minimum. In order to apply a reduction of 10% to such a slab in the thickness direction of the slab, it is necessary to use equipment having a large reduction force of several thousand tons. However, in the existing continuous casting machine, it is general that a reduction device having a reduction force of several hundred tons is provided. Therefore, in a general continuous casting machine, when a slab having a large thickness is to be rolled by the method described in Patent Document 2, capital investment is required, and the capital cost becomes excessive.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a continuous casting method of steel capable of reducing porosity in a slab.

The present inventors, as a result of diligent study, the temperature of the slab at the position to start the reduction to a predetermined range by the solidification structure of the slab at the solidification completion position to a specific structure, specific slab The inventors have found that it is possible to cast a slab with suppressed porosity by performing the reduction with a reduction amount or more, and have completed the present invention. That is, the present invention is as follows.
(1) Injecting molten steel into a continuous casting mold while solidifying the molten steel to form a slab, pulling out the slab from the mold, blowing cooling water toward the slab, A method for continuous casting of steel, which comprises rolling down the slab after solidifying molten steel, wherein the surface temperature of the slab is 600° C. or less and the center temperature of the slab and the surface temperature at a position where the rolling is started. Is 400° C. or more, and at the solidification completion position where the solidification of the molten steel in the slab is completed, the solidification structure in the slab thickness direction central portion is columnar crystals and/or branched columnar crystals, and the slab is 5 mm or more. Continuous casting method for rolling steel.
(2) The continuous casting method for steel according to (1), wherein the slab is reduced by 10 mm or more.
(3) The steel continuous casting method according to (1) or (2), wherein the rolling is performed with a pair of rolling rolls extending in the width direction of the slab.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to cast a slab in which the porosity in the slab is suppressed under the reduction by a general force of several hundreds of tons, without the reduction of a large force of several thousand tons.

It is explanatory drawing which shows a continuous casting machine. It is a figure which shows the roll segment which comprises the reduction zone of the continuous casting machine shown in FIG. It is a figure which shows the cross section orthogonal to the casting direction of the roll segment shown in FIG. 7 is a graph showing the relationship between the amount of reduction (mm) and the maximum porosity diameter (mm) in Experiment 2. It is a graph which shows the relationship of the maximum porosity diameter (mm) with the temperature difference (degree C) of the center and surface of the cast in Experiment 3.

The present invention, in the continuous casting of steel, the temperature of the slab at the position to start the rolling down to a predetermined range to the solidification structure of the slab at the solidification completion position to a specific structure, then by rolling down the slab The main purpose is to cast slabs with reduced porosity.

First, it demonstrates with reference to FIG. 1 which shows the continuous casting machine which performs the continuous casting process of the steel which reduces a cast piece after completion of solidification. The continuous casting machine 1 has a mold 5, a tundish 2 installed above the mold 5, and a plurality of slab support rolls 6 arranged below the mold 5 side by side. Although not shown, a ladle containing the molten steel 9 is installed above the tundish 2, and the molten steel 9 is poured into the tundish 2 from the bottom of the ladle. At the bottom of the tundish 2, a dipping nozzle 4 with a sliding nozzle 3 attached is installed, and the molten steel 9 is injected into the mold 5 through the dipping nozzle 4 while the molten steel 9 is allowed to stay in the tundish 2 for a predetermined amount. To be done. A cooling water channel is formed in the mold 5, and cooling water is passed through the cooling water channel. As a result, the molten steel 9 is removed from the inner surface of the mold 5 and solidified to form the solidified shell 11, and the solidified shell 11 is pulled out to form the slab 10 having the unsolidified layer 12 of the molten steel 9 therein. ..

In the gap between the slab support rolls 6 adjacent to each other in the casting direction, a plurality of secondary cooling zones 30 in which spray nozzles (not shown) are arranged are installed immediately below the mold 5 along the casting direction. The slab 10 is cooled while being pulled out by the cooling water sprayed from the spray nozzle of the secondary cooling zone 30. While the slab 10 is conveyed by the slab support roll 6 and passes through the plurality of secondary cooling zones 30, the solidified shell 11 is appropriately cooled, and the solidification of the unsolidified layer 12 proceeds, The solidification of 10 is complete. Although three secondary cooling zones 30 are installed in FIG. 1, the number of the secondary cooling zones 30 is not particularly limited.

At the solidification completion position 13 or at the downstream of the casting direction thereof, there is provided a reduction band 14 composed of a slab support roll group in which a plurality of pairs of slab support rolls 6 facing each other with the slab 10 sandwiched therebetween are arranged. A spray nozzle for cooling the slab 10 is also arranged between the slab support rolls 6 of the reduction zone 14. The slab support roll 6 arranged in the reduction band 14 is also called a reduction roll. In FIG. 1, the roll reduction zone 14 is configured by arranging two roll segments each including three pairs of slab support rolls 6 in the casting direction. The radix is not particularly limited, and the logarithm of the slab support roll 6 is not particularly limited.

At the solidification completed position 13, porosity may be formed in the center of the slab, and hydrogen is likely to accumulate in this porosity, and hydrogen remains as a solid solution in the steel sheet obtained by rolling the slab. In some cases, hydrogen-induced cracks may occur in the steel sheet. Therefore, it is desirable to reduce porosity during the continuous casting process. It is possible to reduce the porosity by performing reduction on the slab 10 in the entire area of the reduction zone 14 or in a partially selected area.

As shown in FIG. 1, a plurality of transport rolls 7 for continuously transporting the slab 10 are installed downstream of the reduction zone 14 in the casting direction. A slab cutting machine 8 for cutting the slab 10 is arranged above the transport roll 7. The slab 10 after completion of solidification is cut into a slab 10a having a predetermined length by the slab cutting machine 8.

Next, the roll segments forming the rolling band 14 are shown in FIGS. 2 and 3. 2 and 3 show an example in which five pairs of slab support rolls 6 are arranged in one roll segment 15 as reduction rolls, and FIG. 2 is a side view of the continuous casting machine. 3 is a view showing a cross section orthogonal to the casting direction. The roll segment 15 is composed of a pair of frame 16 and frame 16′ holding five pairs of slab support rolls 6 via the roll chock 21, and a total of four (the upstream side of the frame 16 and frame 16′ are penetrated. Tie rods 17 on both sides and both sides on the downstream side are arranged. The worm jack 19 installed on the tie rod 17 may be driven by the motor 20 to adjust the distance between the frame 16 and the frame 16 ′, that is, the roll down gradient of the roll segment 15.

On the tie rod 17, a disc spring 18 is installed between the frame 16 ′ and the worm jack 19. The disc spring 18 is not configured by one disc spring but is configured by stacking a plurality of disc springs (the rigidity increases as a number of disc springs are stacked). The disc spring 18 has a constant thickness without contracting when a load greater than a predetermined load is not applied to the disc spring 18, but contracts when a predetermined load is applied. Then, after the load exceeds a predetermined load, the load shrinks in proportion to the load.

Since the rolling band 14 has such a roll segment structure, the roll opening degrees of the plurality of pairs of slab support rolls 6 arranged in each roll segment are collectively adjusted. In this case, the amount of movement of the upper frame (corresponding to the frame 16') by the worm jack is measured and controlled by the number of rotations of the worm jack, so that the rolling gradient of each roll segment can be known.

Although it is possible to reduce the porosity in the slab by the reduction in the reduction zone 14, a method for further effectively reducing the porosity is desired. The apparatus shown in FIG. 2 and FIG. 3 is capable of exerting a load of several hundred tons, but as described in Patent Document 2, the rolling reduction is 10% or more and a larger load. If the pressure-reducing device that exerts the (reduction force) configures the pressure-reducing zone 14, the porosity can be reduced more effectively. As such an apparatus, for example, there is a rolling apparatus having a backup roll that supports the slab support roll 6 and a hydraulic rolling-down device that produces a rolling force of several thousand tons. However, since the existing continuous casting machine 1 is generally provided with a rolling down device having the configuration shown in FIG. 2 and FIG. 3, when the rolling down device is to be replaced with the rolling down device, equipment is required. Expenses become excessive.

Therefore, the present inventors diligently studied to establish a method for effectively reducing porosity even in the existing continuous casting machine 1. First, the present inventors have conducted investigations, experiments and consideration to evaluate the effect of the slab structure at the completion of solidification on the porosity after reduction. After the slab 10 is pulled out from the mold 5, and when the slab 10 is subjected to electromagnetic stirring at a position upstream of the solidification completion position 13 in the casting direction, the slab 10 after the completion of the solidification is finely divided. It is generally known that when equiaxed crystals are formed and electromagnetic stirring is not performed, equiaxed crystals having a nonuniform size and a large size are likely to be formed. From this phenomenon, the present inventors consider that the porosity generated due to the coarse equiaxed crystal has a large size, and avoids the formation of the coarse equiaxed crystal in the slab 10 at the completion of solidification. We speculate that it is important to reduce the size of porosity.

Next, the present inventors examined the conditions for controlling the solidification structure of the slab center to columnar crystals and/or branched columnar crystals that are not equiaxed crystals themselves. Generally, it is considered that the solidification structure is determined by the alloy composition of the molten steel, the cooling rate of the slab, the temperature gradient from the slab surface to the center, and the like. For steel with a concentration of 0.05 to 1.00 mass%, the cooling conditions for changing the solidification structure at the center of the slab to columnar crystals and/or branched columnar crystals are calculated to determine the cooling rate of the slab. It was speculated that the solidification structure of the slab at the solidification completion position may be a columnar crystal and/or a branched columnar crystal structure by strengthening and increasing the temperature gradient in the slab. Specifically, the present inventors set the surface temperature of the slab at 600° C. or lower and the temperature difference between the center of the slab and the surface at 400° C. or more at the position where the reduction is started, so that the solidification completion position is reached. It was inferred that the solidified structure of the cast product in (1) was obtained as a structure of columnar crystals and/or branched columnar crystals.

To confirm this conjecture, the present inventors have adjusted the alloying components of the molten steel and the cooling conditions to make the solidification structure of the slab after completion of solidification into equiaxed crystals, columnar crystals or branched columnar crystals. Continuous casting was performed, and in the plurality of continuous castings, an equal rolling force was applied to the slab, and an experiment for comparative evaluation of the porosity size of the slab center was conducted (Experiment 1).

<Experiment 1>
Using the continuous casting machine 1 shown in FIG. 1, a plurality of kinds of molten steel 9 having different carbon concentrations in the range of 0.05 to 1.00 mass% but different components were prepared, and a plurality of cast flakes 10 were cast. (Invention examples 101-116 and comparative examples 101-105). In all the examples of the present invention, the cast piece 10 was pressed down by 5 mm by the roll segment 15 shown in FIGS. 2 and 3. In the comparative example as well, it was decided to carry out the reduction by 5 mm as in the case of the present invention, but one example in which no reduction was performed was provided for comparison with the present invention (Comparative Example 101). The conditions and results of Experiment 1 are shown in Table 1.

In the present invention examples 101 to 116, the slab surface temperature at the position where the reduction is started was 600° C. or less and the temperature difference between the slab center temperature and the surface was 400° C. or more, but in Comparative examples 102 to 105. , The surface temperature of the slab and/or the temperature difference between the center of the slab and the surface is not within this range. The slab surface temperature at the position where the reduction is started is determined by the radiation thermometer at the position of the slab support roll 6 which is arranged most upstream in the casting direction among the plurality of slab support rolls 6 forming the reduction zone 14. The surface temperature of the piece was measured. By carrying out the heat transfer solidification calculation using the measured surface temperature and the casting conditions, it is possible to accurately calculate the temperature distribution of the surface and the center of the slab in the continuous casting machine. Incidentally, the reduction roll is arranged on the downstream side of the continuous casting machine, based on the above heat transfer solidification calculation result, by controlling the casting conditions including secondary cooling, the requirements of the present invention at the reduction start position. It is possible to meet.

The central portion of the slab is a portion within 10 mm from the center in the thickness direction of the slab, and the thickness direction of the slab means the normal direction of the surface of the slab corresponding to the long side of the mold. The width direction of the slab means the normal direction of the surface of the slab corresponding to the mold short piece. Regarding the "solidification structure of the slab center part" in Table 1, with the center in the slab thickness and width direction as the center line, in the vicinity of the solidification completion position 13, 10 mm from the center line in the vertical direction (the slab thickness) and left and right ( A sample of 70 mm in the width direction of the slab and 15 mm in the casting direction was cut out from the slab, and after polishing the sample, the solidification structure of the sample was confirmed by etching and the solidification structure of the center part of the slab was identified.

Also, from the cast slab 10a after reduction, a sample of 10 mm in the vertical direction (cast slab thickness) from the center line, 70 mm in the left and right (cast slab width) direction about the center line, and 15 mm in the casting direction was cut and ultrasonic flaw detection was performed. The sample was measured using an apparatus to calculate the maximum porosity diameter. Although it was confirmed from the ultrasonic flaw detector that the sample had multiple porosities. The shape of the confirmed porosity is not necessarily a true sphere but includes an ellipsoid. Therefore, as the evaluation of the porosity size, the length of the maximum diameter of each porosity was measured and defined as “maximum porosity diameter” in Table 1. It is unknown whether the shape is spherical. However, the maximum length of each porosity along the casting direction was measured, and the maximum length was defined as the "maximum porosity diameter" in Table 1.

As can be seen from Table 1, the maximum porosity diameters of Examples 101 to 116 of the present invention in which the solidification structure in the central portion of the slab is columnar crystals and/or branched columnar crystals and which was pressed after the solidification completion position 13 was less than 1 mm. On the other hand, the maximum porosity diameter is 1 mm or more in Comparative Example 101, which is not rolled, and Comparative Examples 102 to 105, in which the solidified structure of the center of the slab is equiaxed even if it is rolled. From these results, it is understood that the porosity size tends to be smaller when the columnar crystal structure and/or the branched columnar crystal structure is present than when the solidification structure at the center of the slab is the equiaxed crystal structure.

In Experiment 1, the slab 10 was pressed down by 5 mm with the roll segment 15 shown in FIGS. 2 and 3, but the present inventors found that the solidified structure of the slab center was columnar crystals and/or branched columnar crystals. Even in the case, it was inferred that the size of porosity also changed depending on the amount of reduction after the solidification completion position 13. Therefore, as in the case of Experiment 1, the inventors of the present invention adjust the alloying components of the molten steel and the cooling conditions to make the solidification structure of the slab after completion of solidification into equiaxed crystals, columnar crystals or branched columnar crystals. Was continuously cast. However, in the plurality of continuous castings, an experiment was conducted in which the rolling force applied to the slab was changed and the porosity size at the center of the slab was comparatively evaluated (Experiment 2).

<Experiment 2>
In Experiment 2, as in Experiment 1, the continuous casting machine 1 shown in FIG. 1 was used to prepare a plurality of types of molten steel 9 having different carbon concentrations in the range of 0.05 to 1.00 mass% but different components. A plurality of cast pieces 10 were cast (invention examples 201 to 235 and comparative examples 201 to 210). In all of the present invention examples and comparative examples, the casting surface temperature at the position where the reduction is started is set to 600° C. or less, and the temperature difference between the temperature of the casting piece center and the surface is set to 400° C. or more. The solidification structure at the center of one side was columnar crystals and/or branched columnar crystals. In the example of the present invention and the comparative example, the amount of reduction of the cast slab 10 was changed by using the roll segment 15 shown in FIGS. Other than that, the steel was continuously cast in the same manner as in Experiment 1. The conditions and results of Experiment 2 are shown in Table 2.

In Experiment 2, the slab temperature, the solidification structure at the center of the slab, and the maximum porosity diameter mm were determined in the same manner as in Experiment 1. The relationship between the amount of reduction (mm) and the maximum porosity diameter (mm) is shown in FIG. From Table 2 and FIG. 4, the maximum porosity diameter is remarkably reduced by setting the solidification structure in the central portion of the slab at the solidification completion position to be columnar crystals and/or branched columnar crystals and setting the reduction amount to 5 mm or more. I understand.

Further, it is preferable that the reduction amount is 10 mm or more. This increases the possibility that the maximum porosity diameter can be 0.5 mm or less. It should be noted that the maximum porosity diameter is not significantly reduced even if the rolling reduction is 20 mm or more, so it is understood that the rolling reduction amount may be 20 mm at the maximum from the viewpoint of equipment load.

By the way, in Experiment 1, the slab 10 was rolled down by 5 mm by the roll segment 15 shown in FIGS. 2 and 3, and in Experiment 2, in all the examples, the slab surface temperature at the position where the rolling was started was measured. When the temperature difference between the temperature of the slab and the temperature of the surface of the slab is 600° C. or less and 400° C. or more, the solidification structure of the central portion of the slab at the solidification completion position is a columnar crystal and/or a branched columnar crystal. The present inventors have further experimented to evaluate the change in porosity size of the slab center by changing the slab surface temperature and the temperature of the slab center and the temperature difference between the surfaces within the above range. Was performed (Experiment 3).

<Experiment 3>
In Experiment 3, as in Experiment 1, the continuous casting machine 1 shown in FIG. 1 was used to prepare a plurality of types of molten steel 9 having different carbon concentrations in the range of 0.05 to 1.00% by mass. A plurality of cast pieces 10 were cast (invention examples 301 to 308). In all the examples of the present invention, the slab surface temperature at the position where the reduction is started is 600° C. or less, and the temperature difference between the slab center and the surface is 400° C. or more, and the slab center portion at the solidification completion position is Although the solidification structure was columnar crystals, in Experiment 3, the temperatures of the surface of the cast piece and the center of the cast piece were changed. Other than that, the steel was continuously cast in the same manner as in Experiment 1. Table 3 shows the conditions and results of Experiment 3.

Also in Experiment 3, the slab temperature, the solidification structure of the slab center and the maximum porosity diameter were determined in the same manner as in Experiment 1, and the temperature difference (°C) between the center and the surface of the slab and the maximum porosity diameter (mm) were obtained. 5 shows the relationship. From Table 3 and FIG. 5, it can be seen that the larger the temperature difference between the center and the surface of the slab, the smaller the maximum porosity diameter tends to be. It is presumed that the larger the temperature difference, the larger the difference in deformation resistance and the easier it was to destroy the porosity.

When the surface of the slab is cooled to adjust the temperature at the center of the slab, the thickness of the slab has a great influence. When the slab is cooled under the above-mentioned conditions regarding the temperature of the slab surface and the temperature difference between the slab surface and the center of the slab to obtain a target solidified structure, the thickness is preferably 220 mm or more and 500 mm or less. When the thickness is more than 500 mm, even if the cooling of the slab by the secondary cooling is strengthened, it is difficult to increase the cooling rate at the center of the slab and strengthen the temperature gradient. It is difficult to make columnar crystals. On the other hand, if the thickness of the cast piece is less than 220 mm, the reduction ratio after the next step for incorporating the characteristics of the steel sheet obtained from the cast piece may be insufficient, which is not preferable. Here, the reduction ratio is a ratio in which the thickness of the cast slab is the numerator and the thickness of the product is the denominator.

The continuous casting machine may be a vertical bending type or a bending type in consideration of installation space and productivity. In these types of continuous casters, there is a slab straightening band. Since a large force is applied to the slab by the straightening band, if the surface temperature of the slab is lowered too much before the slab passes through the straightening band and solidification proceeds too much, cracks may occur on the surface of the slab. Sexuality increases. Therefore, in the case of lowering the surface temperature of the slab by relatively strong cooling as in the present invention, it is preferable to cool the slab so as to reach the target temperature in the horizontal strip after passing through the straightening strip.

The reduction of the present invention is preferably performed by a pair of slab support rolls 6 (rolling rolls) extending in the width direction of the slab 10 as shown in FIG. In the continuous casting machine, in addition to the configuration shown in FIGS. 2 and 3 as the reduction roll in the roll segment, the roll is divided into a plurality of rolls in the width direction of the slab 10 in order to reduce the reaction force from the slab 10 to the reduction roll. There are rolling rolls in the form described above. However, in a portion between a plurality of reduction rolls, the reduction roll may not be sufficient in a portion where the reduction roll is not in contact with the slab, so that it exists in the slab center portion of that portion. It may be difficult to reduce the porosity. Therefore, it is preferable to perform the reduction of the cast slab 10 by using a reduction roll extending in the width direction of the cast slab 10.

<Experiment 4>
Similar to Experiment 1, the inventors of the present invention used the continuous casting machine 1 shown in FIG. 1, and used a plurality of types of molten steel 9 having different carbon concentrations in the range of 0.05 to 1.00 mass% but different components. Was prepared, and the slab 10 was cast 20 times (invention examples 400 and 401). In Example 400 of the present invention, the slab 10 was pressed by 5 mm by the roll segment 15 shown in FIGS. 2 and 3 in all continuous casting. On the other hand, in Inventive Example 401, the slab support roll 6 of the strip 14 of FIG. 1 is not the form shown in FIGS. 2 and 3, but is a form in which the roll is divided into two in the slab width direction. The slab 10 was pressed down by 5 mm by continuous casting. Then, also in Inventive Examples 400 and 401, the maximum porosity diameter was calculated as in Experiment 1. Table 4 shows the average (mm) of the maximum porosity diameters, the maximum value, the minimum value, and the difference between the maximum value and the minimum value in Examples 400 and 401 of the present invention.

As can be seen from Table 4, in Example 400 of the present invention in which reduction was performed by a pair of reduction rolls extending in the slab width direction, the average of the maximum porosity diameters of 20 times was 0.75 mm, and the minimum value was 0. It became 0.6 mm. On the other hand, in Inventive Example 401 in which the slab was reduced by a reduction roll having a form in which the roll was divided into two in the width direction of the slab, the average of the maximum porosity diameter of 20 times was 0.83 mm, and the minimum value was It became 0.7 mm. The present invention example 400 using the reduction roll of the form extending in the slab width direction has an average of the maximum porosity diameter is larger than that of the present invention example 401 using the reduction roll of the form divided in the slab width direction. In addition to falling below the minimum value of the maximum porosity diameter. That is, it was found that the porosity can be reduced more reliably by using the reduction roll extending in the width direction of the slab than by using the reduction roll divided in the width direction of the slab.

As described above, by continuously casting the steel under the conditions found by the present inventors, it is not necessary to perform reduction with a large force of several thousand tons, and the inside of the slab is reduced by a general force of several hundreds of tons. It is possible to cast a slab with suppressed porosity. By extension, it is possible to improve the internal quality of the steel material obtained from the cast slab.

DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Cast strip support roll 7 Conveying roll 8 Cast strip cutting machine 9 Molten steel 10 Cast strip 10a Cast strip (after cutting)
11 Solidified Shell 12 Unsolidified Layer 13 Solidification Completed Position 14 Rolling Zone 15 Roll Segment 16 Frame 16' Frame 17 Tie Rod 18 Disc Spring 19 Worm Jack 20 Motor 21 Roll Chock 30 Secondary Cooling Zone

Claims (3)

Forming a cast piece by solidifying the molten steel while injecting the molten steel into a continuous casting mold, and pulling out the cast piece from the mold,
A method for continuous casting of steel, comprising cooling the slab after solidifying molten steel in the slab by spraying cooling water toward the slab,
At the position where the reduction is started, the surface temperature of the slab is 600° C. or less and the difference between the center temperature of the slab and the surface temperature is 400° C. or more, and solidification of the molten steel in the slab is completed. At the completion position, the solidified structure of the central portion in the thickness direction of the cast piece is columnar crystals and/or branched columnar crystals,
A continuous casting method for steel, wherein the slab is rolled down by 5 mm or more. The continuous casting method for steel according to claim 1, wherein the slab is reduced by 10 mm or more. The continuous casting method for steel according to claim 1 or 2, wherein the rolling is performed by a pair of rolling rolls extending in the width direction of the slab.