JP7371821B1 - Continuous steel casting method - Google Patents

Abstract

To provide a continuous casting method for steel that can accurately and inexpensively specify the final solidification position. A continuous casting method for steel, in which a slab pulled from a mold is cast while being supported by a plurality of pairs of slab support rolls facing each other with the slab in between, and in which the opening between the opposing slab support rolls is The degree of solid phase is increased in some sections toward the downstream side in the casting direction, and at least in the range from the position where the solid phase ratio at the center of the thickness of the slab is 0.2 to the position where the solid phase rate is the flow limit, A reduction is applied to the slab by a roll segment having a plurality of slab supporting roll pairs whose reduction amount is controlled by a hydraulic cylinder, a pressure difference between the plurality of hydraulic cylinders is measured, and a pressure difference is measured based on the pressure difference. estimating the final solidification position of the slab, and controlling the amount of reduction to the slab in the roll segment estimated to be the final solidification position and the roll segment immediately before it so as to satisfy a predetermined formula; Continuous casting method for steel.

Description

The present disclosure relates to a method for continuous casting of steel.

In continuous casting of steel, in the final process of solidification, a suction flow of unsolidified molten steel (referred to as "unsolidified layer") occurs in the direction of withdrawal of the slab due to solidification contraction. In this unsolidified layer, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated. When this concentrated molten steel flows to the center of the slab and solidifies, so-called center segregation occurs.

Center segregation deteriorates the quality of steel products, especially thick steel plates. For example, in line pipe materials for oil transportation and natural gas transportation, hydrogen-induced cracking occurs starting from center segregation due to the action of sour gas. Similar problems also occur in offshore structures, storage tanks, oil tanks, etc. Moreover, in recent years, steel materials are often required to be used in harsh environments such as lower temperatures or more strongly corrosive environments, making it increasingly important to reduce central segregation of slabs. .

Therefore, many measures have been proposed to reduce or render harmless the center segregation of slabs from the continuous casting process to the rolling process. Among various countermeasures, it is known that a light reduction method at the final stage of solidification, in which a continuously cast slab having an internal unsolidified layer is reduced in a continuous casting machine, is particularly effective in improving center segregation. Here, the "light reduction method at the end of solidification" means that a reduction roll is arranged near the solidification completion position of the slab, and this reduction roll reduces the amount of solidification shrinkage and heat shrinkage of the slab during continuous casting. This is a method of gradually rolling down at a corresponding rolling speed. Thereby, it is possible to suppress the generation of voids in the center of the slab and the flow of concentrated molten steel, and to suppress the center segregation of the slab.

In order to effectively prevent the occurrence of center segregation of slabs using the light reduction method at the final stage of solidification, it is necessary to adjust the conditions of light reduction, for example, the beginning and end of the period during which light reduction is applied during the final solidification period of the slab; It is important to appropriately set the amount of reduction under light pressure. Therefore, methods for setting the conditions and methods for estimating the final solidification position have been proposed.

For example, Patent Document 1 proposes a crater end detection device that includes a pair of work rolls arranged to sandwich a slab. The detection device employs a structure in which the end of a predetermined work roll does not come into contact with the corner of the short side of the slab.

Furthermore, in Patent Document 2, during continuous casting operation, component analysis values and temperature of molten steel in the tundish, casting speed, amount and temperature of cooling water for the mold, amount and temperature of secondary cooling water, cooling of the rolls, etc. Collect data on the amount and temperature of water as well as the outside temperature, perform model calculations to estimate the solidification state inside the slab using each data as parameters, and calculate the solidification state inside the slab estimated from the results of the model calculation. The rolling force of the rolling device is controlled in real time based on the condition and preset data on the relationship between the central solid fraction of the slab and the rolling force, and the central solid fraction is 0.30 to 0.80. A continuous casting method for steel has been proposed, which is characterized in that the casting speed is controlled so that the area estimated to be at the position of the reduction device is the position of the reduction device.

Japanese Patent Application Publication No. 2018-199137 Japanese Patent Application Publication No. 2013-22609

However, in the crater end detection device described in Patent Document 1, a load measuring device is installed in a bearing box installed at both ends of the backup roll. Therefore, in the high temperature environment during casting, a large load is placed on the load measuring device installed in the bearing box, increasing the possibility of failure. Furthermore, when the load measuring device breaks down, the segments must be replaced, which incurs a great deal of cost.

In the continuous steel casting method described in Patent Document 2, an error may occur in the calculation of the central solid fraction.

The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a method for continuous casting of steel that can accurately and inexpensively specify the final solidification position.

In order to achieve the above-mentioned problems, the present inventors have conducted intensive studies and have discovered the point at which light reduction should start and the point at which light reduction should end in continuous casting of slab slabs using the late solidification light reduction method. The idea was to use the pressure difference in hydraulic cylinders to predict the desired timing. By measuring the pressure difference in the hydraulic cylinder, it is possible to specify the final solidification position with high accuracy and at low cost.

The present disclosure has been made based on the above findings. That is, the gist of the present invention is as follows.

[1] A continuous steel casting method in which a slab pulled from a mold is cast while being supported by a plurality of pairs of slab support rolls facing each other with the slab in between,
increasing the opening degree between the opposing slab support rolls toward the downstream side in the casting direction in some sections;
In the casting direction of the slab, the reduction amount was controlled by a hydraulic cylinder at least in the range from the position where the solid fraction at the center of the thickness of the slab is 0.2 to the position where the solid fraction is the flow limit. Applying a reduction to the slab by a roll segment having a plurality of pairs of slab supporting rolls,
measuring the pressure difference between the plurality of hydraulic cylinders, estimating the final solidification position of the slab based on the pressure difference,
In the roll segment estimated to be the final solidification position and the roll segment immediately before the roll segment, the amount of reduction to the slab is controlled so as to satisfy the following equation (1) and the following equation (2). Continuous casting method.
0.50<V×Z<3.00...(1)
0.5R<Db<2R...(2)
here,
V: Drawing speed of slab (m/min)
Z: Rolling gradient (mm/m)
Db: Bulging amount of slab (mm)
R: Total reduction amount of slab (mm)
It is.

According to the present disclosure, it is possible to provide a continuous steel casting method that can accurately and inexpensively specify the final solidification position.

1 is a diagram showing an example of a schematic configuration of a continuous slab casting machine. It is a schematic diagram showing an example of a roll segment which constitutes a light reduction zone of a slab continuous casting machine, and is a schematic diagram seen from the side of a continuous casting machine. FIG. 3 is a schematic view of the roll segment shown in FIG. 2 in a cross section perpendicular to the slab casting direction. It is a graph which shows the pressure difference of the hydraulic cylinder with respect to casting length in each roll segment.

Embodiments of the present disclosure will be described below. Note that the present disclosure is not limited to the following embodiments.

A steel manufacturing method according to an embodiment of the present disclosure includes continuous steel casting in which a slab pulled from a mold is cast while being supported by a plurality of pairs of slab support rolls facing each other with the slab in between. A method,
increasing the opening degree between the opposing slab support rolls toward the downstream side in the casting direction in some sections;
In the casting direction of the slab, the reduction amount was controlled by a hydraulic cylinder at least in the range from the position where the solid fraction at the center of the thickness of the slab is 0.2 to the position where the solid fraction is the flow limit. Applying a reduction to the slab by a roll segment having a plurality of pairs of slab supporting rolls,
measuring the pressure difference between the plurality of hydraulic cylinders, estimating the final solidification position of the slab based on the pressure difference,
In the roll segment estimated to be the final solidification position and the roll segment immediately before the roll segment, the amount of reduction to the slab is controlled so as to satisfy the following equation (1) and the following equation (2). This is a continuous casting method.
0.50<V×Z<3.00...(1)
0.5R<Db<2R...(2)
here,
V: Drawing speed of slab (m/min)
Z: Rolling gradient (mm/m)
Db: Bulging amount of slab (mm)
R: Total reduction amount of slab (mm)
It is.

FIG. 1 is a diagram showing a schematic configuration of a continuous slab casting machine used when carrying out a continuous steel casting method according to an embodiment. As shown in FIG. 1, in one example, a continuous slab casting machine 1 is equipped with a mold 5 for injecting and solidifying molten steel 9 to form an outer shell shape of a slab 10. A tundish 2 is installed at a predetermined position above the mold 5 for relaying molten steel 9 supplied from a ladle (not shown) to the mold 5. A sliding nozzle 3 for adjusting the flow rate of molten steel 9 is installed at the bottom of the tundish 2, and an immersion nozzle 4 is installed at the lower surface of this sliding nozzle 3. On the other hand, below the mold 5, a plurality of pairs of slab support rolls 6 consisting of a support roll, a guide roll, and a pinch roll are arranged. A secondary cooling zone in which spray nozzles (not shown) such as water spray nozzles or air mist spray nozzles are arranged is formed in the gap between the slab support rolls 6 adjacent to each other in the casting direction. The slab 10 is cooled while being drawn out by cooling water (also referred to as "secondary cooling water") sprayed from a spray nozzle in this secondary cooling zone. Furthermore, a plurality of transport rolls 7 for transporting the cast slab 10 are installed downstream of the slab support roll 6 at the end in the casting direction. A slab cutting machine 8 for cutting slabs 10a of a predetermined length from the slab 10 to be cast is arranged above the conveyance roll 7.

On the upstream and downstream sides of the solidification completion position 13 of the slab 10 in the casting direction, a light reduction band 14 composed of a plurality of pairs of slab support roll groups is installed. The light reduction zone 14 is set so that the interval between the slab support rolls facing each other with the slab 10 in between (this interval is referred to as the "roll opening degree") gradually narrows toward the downstream side in the casting direction. There is. That is, the light reduction zone 14 has a reduction gradient (a state in which the roll opening degree is set to gradually become narrower toward the downstream in the casting direction). In the light reduction zone 14, it is possible to perform light reduction on the slab 10 over the entire area or in a selected region. A spray nozzle for cooling the slab 10 is also arranged between each slab support roll of the light reduction zone 14. The slab support roll 6 arranged in the light reduction zone 14 is also called a reduction roll.

In the continuous slab casting machine 1 shown in FIG. 1, the light reduction zone 14 is configured by three roll segments each having three pairs of slab support rolls 6 connected in the casting direction. However, in the present invention, the light rolling band 14 does not need to be composed of three roll segments, and the light rolling band 14 may be composed of one or two roll segments. Furthermore, the number may be four or more. Further, each roll segment is composed of three pairs of slab support rolls 6, but the number of slab support rolls 6 that constitute one roll segment is not particularly limited.

FIGS. 2 and 3 show an example of the roll segments that constitute the light compression zone 14. As shown in FIG. 2 and 3 are diagrams showing an example in which five pairs of slab support rolls 6 as reduction rolls are arranged in one roll segment 15, and FIG. 2 is a schematic diagram as seen from the side of the continuous casting machine. , FIG. 3 is a schematic view taken in a cross section perpendicular to the slab casting direction.

As shown in FIGS. 2 and 3, in one example, the roll segment 15 includes a pair of frames 16 and 16' that hold five pairs of slab support rolls 6 via roll chocks 21. A total of four pillars 17 (on both upstream sides and on both downstream sides) are arranged to penetrate the frame 16 and the frame 16'. By driving the worm jack 19 installed on the support column 17 with the motor 20, the distance between the frames 16 and 16' can be adjusted, that is, the rolling slope of the roll segment 15 can be adjusted. The slab support roll 6 is held on the upper frame 16' via a roll chock 21 connected to a cylinder rod (not shown) of a hydraulic cylinder 22 fixedly disposed on the frame 16'.

In the continuous slab casting machine 1, molten steel 9 is injected from the tundish 2 into the mold 5 through the immersion nozzle 4, and is cooled in the mold 5 to form a solidified shell 11. The molten steel 9 is continuously drawn below the mold 5 as a slab 10 having an unsolidified layer 12 therein, while being supported by a slab support roll 6 provided below the mold 5. While the slab 10 passes through the slab support rolls 6, it is cooled by secondary cooling water in the secondary cooling zone, increasing the thickness of the solidified shell 11, and is rolled down in the light reduction zone 14 until it reaches the solidification completion position. At step 13, solidification to the inside is completed. After completion of solidification, the slab 10 is cut by a slab cutting machine 8 to become a slab 10a.

In the steel casting method according to the present embodiment, a slab drawn from a mold 5 is cast while being supported by a plurality of pairs of slab support rolls 6 facing each other with the slab interposed therebetween. The size of the manufactured slab is not particularly limited. The thickness of the slab is preferably 200 mm or more. Further, the thickness of the slab is preferably 600 mm or less. The width of the slab is preferably 1000 mm or more. Further, the width of the slab is preferably 2500 mm or less. If the thickness and width of the slab are within the above range, the rolling gradient is more suitable.

The opening degree between the facing slab support rolls 6 is increased toward the downstream side in the casting direction in some sections. By increasing the opening between the slab support rolls 6 toward the downstream side in the casting direction, it is possible to bulge the thickness of the long side of the rectangular slab that has an unsolidified layer inside. In one example, the thickness of the long side of the slab can be bulged within a range of 0.1 to 10% of the thickness of the slab in the mold. This can prevent an excessive load from being applied to the slab during subsequent light reduction. If the amount of bulging is 10% or less, strain applied to the solidification interface of the slab can be suitably prevented from becoming excessive, and the occurrence of internal cracks can be more suitably suppressed. The range in which bulging is performed is preferably arranged between the lower end of the mold 5 and the liquidus crater end position of the slab 10. Upstream in the casting direction from the liquidus crater end position of the slab 10, the center of the thickness of the slab is entirely an unsolidified layer 12 (liquid phase), and the solidified shell 11 of the slab 10 has a high temperature and has a high deformation resistance. This is because it is small and can be easily bulged. Moreover, when bulging the slab 10, if the bulging is performed at a time when the unsolidified layer 12 existing inside the slab 10 is small, center segregation will worsen. On the other hand, when bulging is performed upstream of the liquidus crater end position of the slab 3 in the casting direction, at this point, molten steel with an initial concentration of solute elements that is not concentrated is abundantly inside the slab. , and this molten steel flows easily. Even if this molten steel flows, center segregation does not occur. Therefore, bulging on the upstream side of the liquidus crater end position in the casting direction does not cause center segregation.

In the casting direction of the slab, at least in the range from the position where the solid fraction at the center of the thickness of the slab is 0.2 to the position where the solid fraction reaches the flow limit, a plurality of A rolling reduction is applied to the slab by a roll segment having a pair of slab supporting rolls. Incidentally, it is sufficient to start rolling at least from the position where the solid fraction at the center of the thickness of the slab reaches 0.2, and the rolling should be started before the solid fraction at the center of the thickness of the slab reaches 0.2. It's okay. For example, rolling may be started from a position where the solid fraction at the center of the thickness of the slab is 0. Note that the solid phase rate is defined as solid phase rate = 0 before the start of solidification and solid phase rate = 1.0 at the time of completion of solidification. The solid fraction in the center is calculated from the estimated temperature in the center by two-dimensional heat transfer solidification calculation. Further, the flow limit solid fraction is generally 0.7 to 0.8, and can be derived by a known method.

Here, the part of the slab to which the reduction is applied is not particularly limited, but it is preferable to apply the reduction to the entire width of the slab. For example, when measuring the reaction force at the center of the width, as in the method using the load measuring device described in Patent Document 1, the reaction force to be measured is the ferrostatic pressure (ferrostatic pressure) in the unsolidified part at the center of the width. ) and the rolling load at the solidification section. Therefore, if the slab is rolled down with a rolling gradient set such that the rolling load in the solidification section is balanced with the static iron pressure, it may not be possible to determine whether the slab is solidified or not. On the other hand, by applying a reduction across the entire width of the slab, the pressure difference corresponding to the reaction force received across the entire width of the slab is measured, so it is necessary to consider the balance between the static iron pressure and the rolling load of the solidified part. Therefore, it becomes possible to estimate the final solidification position with higher accuracy.

In the steel casting method according to the present embodiment, the pressure difference between a plurality of hydraulic cylinders is measured, the final solidification position of the slab is estimated based on the pressure difference, and the roll segment that is estimated to be at the final solidification position is And in the roll segment just before that, the amount of reduction to the slab is controlled so as to satisfy the following equation (1) and the following equation (2).
0.50<V×Z<3.00...(1)
0.5R<Db<2R...(2)
here,
V: Slab drawing speed (m/min)
Z: Rolling gradient (mm/m)
Db: Bulging amount of slab (mm)
R: Total reduction amount (mm) of the slab.

By rolling down so as to satisfy equations (1) and (2), it is possible to effectively prevent center segregation and porosity without generating internal cracks.

In equation (1), V×Z means the rolling speed (mm/min) of the slab. When V×Z is more than 0.50, the flow of the concentrated molten steel can be suitably suppressed, and the occurrence of V segregation can be suitably prevented. When V×Z is less than 3.00, it is possible to suitably prevent the concentrated molten steel from being pushed out too much due to excessive rolling reduction, and suitably prevent the occurrence of reverse V segregation. Preferably, V×Z is 0.70 or more. Further, preferably, V×Z is 1.20 or less.

Regarding formula (2), when Db is more than 0.5R, the effect that the rolling reaction force of the slab does not become excessive can be obtained. When Db is less than 2R, it is possible to prevent the rolling reaction force of the slab from becoming excessive and to prevent internal cracking of the slab due to excessive bulging. Preferably, Db is 0.7R or more. Moreover, Db is preferably 1.8R or less.

Here, in order to pull out the slab, the pinch rolls are rotated to pull out the slab, and V: slab withdrawal speed (m/min) means the slab withdrawal speed calculated based on the rotational speed of the pinch roll. do.

Z: Reduction gradient (mm/m) means the reduction amount of the roll opening degree per 1 m in the casting direction between the roll segment estimated to be the final solidification position and the roll segment immediately before it.

Db: Bulging amount (mm) of a slab means the difference between the roll opening at the lower end of the mold and the roll opening at the segment before light reduction is started.

R: Total reduction amount (mm) of slab means the total amount of reduction in casting.

As mentioned above, measurement using a load measuring device imposes a large load in the high-temperature environment during casting, and there is a high possibility of failure. Furthermore, when the load measuring device breaks down, the segments must be replaced, which incurs a great deal of cost. In contrast, according to the present invention, by estimating the final solidification position of the slab by measuring the pressure difference between multiple hydraulic cylinders, it is possible to easily identify the final solidification position and apply rolling at the appropriate position. It is. Here, the pressure difference in the hydraulic cylinder is (head pressure on the inlet side of the roll segment) - (head pressure on the outlet side of the roll segment). By measuring the pressure of the hydraulic cylinder 22a on the entrance side of the roll segment and the pressure of the hydraulic cylinder 22b on the exit side of the roll segment, the load applied to the pillar 17a on the entrance side of the roll segment and the pillar 17b on the exit side of the roll segment is calculated, respectively. I can do it. There are two types of hydraulic cylinder pressure: head pressure and rod pressure. The frame 16' is pressed down with a hydraulic cylinder while keeping the rod pressure constant (for example, 20 MPa). On the other hand, the position of the slab support roll 6 is controlled to be constant by balancing the head pressure. When the frame receives a reaction force from the slab, the head pressure is reduced to balance the rod pressure with the sum of the reaction force from the slab and the head pressure. That is, rod pressure=head pressure+reaction force from the slab, and the greater the slab reaction force, the lower the head pressure. When a slab that has been solidified after the final solidification position is rolled down into a roll segment, the reaction force from the roll segment becomes larger than when a slab that has an unsolidified portion is rolled down. On the other hand, in the roll segment where the final solidification position exists, the load is small on the segment entry side, and the load is large on the segment exit side. That is, by identifying the roll segment in which the pressure difference between the hydraulic cylinder provided on the segment entry side and the hydraulic cylinder provided on the segment exit side is greater than or equal to a predetermined value, It is possible to estimate the final solidification position of the slab. The pressure difference between the hydraulic cylinder provided on the segment entry side and the hydraulic cylinder provided on the segment exit side is not particularly limited, but is preferably 2 MPa or more.

FIG. 4 is a graph showing the hydraulic cylinder pressure difference with respect to the casting length in each roll segment. In the figure, segments 1, 2, and 3 are data of roll segments located at the most downstream, middle, and most upstream positions, respectively. It can be seen that the further downstream the roll segment is located, the greater the pressure difference between the hydraulic cylinder provided on the segment entry side and the hydraulic cylinder provided on the segment exit side. The pressure difference in the hydraulic cylinder increases from the time the bottom of the slab enters the roll segment, and then decreases when the bottom of the slab comes out, and the final solidification position is within the roll segment. When present, the pressure difference remains large. In the example shown in the figure, the pressure difference between the roll segment 3 and the roll segment 2 located at the most upstream and middle positions decreases to less than 2 MPa after the bottom portion has passed. On the other hand, segment 1 located at the most downstream side has a pressure difference of 2 MPa or more even after passing through the bottom portion, and it can be estimated that segment 1 is the final solidification position of the slab.

When using a load measuring device to estimate the final solidification position, it is necessary to install a load measuring device for each roll-down roll. Therefore, in order to estimate the final solidification position within a wide range in the casting direction, a large number of sensors are required. In contrast, in the present invention, the final solidification position is estimated by calculating the pressure difference of the hydraulic cylinder for each roll segment. Therefore, even if the final solidification position varies greatly due to changes in the thickness, drawing speed, or steel type of the slab, it is easy to estimate the final solidification position from a wide range.

Note that casting conditions other than those described above can be determined by conventional methods.

According to the continuous casting method of this steel, center segregation of the slab can be reduced. In the cross section perpendicular to the drawing direction of the slab, the carbon concentration was analyzed at equal intervals along the thickness direction of the slab, and the maximum value in the thickness direction was taken as C _max . Let C ₀ be the carbon concentration analyzed in , and let C _max /C ₀ be the center segregation degree. Therefore, the closer the degree of center segregation is to 1.000, the better the slab with less center segregation. According to the continuous casting method of this steel, a slab having a C _max /C ₀ of less than 1.100 can be manufactured. Note that the lower limit of the center segregation degree is not particularly limited, but the center segregation degree is usually 1.000 or more. Moreover, according to the continuous casting method of this steel, it is possible to suppress the occurrence of porosity in the slab and also to suppress internal cracks.

Hereinafter, the present disclosure will be described in more detail based on examples.
The continuous slab casting machine used in the test is the same as the continuous slab casting machine 1 shown in FIG. Using this continuous slab casting machine, low carbon aluminum killed steel was cast. The thickness of the slab at the lower end of the mold was 250 mm, 400 mm, or 600 mm. The width of the slab is 2200 mm in all tests. Table 1 shows the results of the investigation on the casting conditions, center segregation degree, presence of porosity, and presence of internal cracks in the cast slab. Table 1 also shows the casting conditions and investigation results of tests conducted as comparative examples. In Table 1, the rolling start position, the rolling finishing position, the position where the solid phase ratio at the center of the thickness of the slab is 0.2, and the final solidification position are indicated by the distance from the meniscus in the mold. In addition, in Table 1, in the tests where the reduction range control is displayed as "with", the reduction start position and reduction end position were outside the appropriate range in the initially set light reduction range, so the measurement during casting was The reduction range was adjusted based on the final solidification position.

The degree of center segregation of the slabs used for test evaluation was measured by the following method. That is, in a cross section perpendicular to the drawing direction of the slab, the carbon concentration is analyzed at equal intervals along the thickness direction of the slab, the maximum value in the thickness direction is taken as C _max , and the carbon concentration is collected from inside the tundish during casting. The carbon concentration analyzed in the molten steel was taken as C ₀ , and C _max /C ₀ was taken as the central segregation degree. Therefore, the closer the degree of center segregation is to 1.000, the better the slab with less center segregation. Here, it was determined that slabs with a degree of center segregation of 1.100 or more had a poor degree of center segregation.

The presence or absence of porosity and internal cracks in the slab was determined by microscopic observation near the center of the thickness of the slab in a cross section perpendicular to the drawing direction of the slab.

The slab withdrawal speed for each slab thickness should be set so that the slab in the area where the solid fraction at the center of the thickness of the slab is at least 0.2 to the flow limit solid fraction is located in the light reduction zone. It was set to In addition, in test numbers 1 to 5, test numbers 9 to 13, and test numbers 17 to 21, the above equations (1) and (2) are The rolling gradient and the amount of bulging were set to satisfy the formula. As the final solidification position, a roll segment where the pressure difference between the hydraulic cylinder provided on the segment entry side and the hydraulic cylinder provided on the segment exit side was 2 MPa or more was identified. In addition, in test numbers 6, 14, and 22 conducted as comparative examples, either the rolling start position or the rolling end position was outside the appropriate range. Further, in test numbers 7, 8, 15, and 23, V×Z deviated from equation (1). Furthermore, in test numbers 16, 22, and 24, the total reduction amount and bulging amount were outside the range of equation (2).

As is clear from the degree of center segregation shown in Table 1, the degree of center segregation in test numbers 1 to 5, test numbers 9 to 13, and test numbers 17 to 21 of the present invention examples were all less than 1.100, which was good. there were. Furthermore, no porosity or internal cracks were observed in the slab.

In test numbers 6, 14, and 22 conducted as comparative examples, either the rolling start position or the rolling end position was outside the appropriate range, so the center segregation degree was 1.100 or more, and there was no porosity inside the slab. observed. In test numbers 7 and 15 conducted as comparative examples, the rolling gradient was insufficient and the center segregation degree was 1.100 or more, and in test number 15, porosity was also observed inside the slab. In addition, in Test Nos. 8 and 23, because the reduction gradient was set too high, the center segregation degree was 1.100 or more, and internal cracks occurred in the slab. In test numbers 16, 22, and 24, the total reduction amount and bulging amount were outside the range of equation (2), so the center segregation degree was 1.100 or more, and porosity was also observed inside the slab.

1 Continuous slab casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Slab supporting roll 7 Conveying roll 8 Slab cutting machine 9 Molten steel 10 Slab 11 Solidified shell 12 Unsolidified layer 13 Solidification completion position 14 Light reduction zone 15 Roll segment 16 Frame 17 Support 19 Worm jack 20 Motor 21 Roll chock 22 Hydraulic cylinder

Claims (1)

A continuous steel casting method in which a slab pulled from a mold is cast while being supported by a plurality of pairs of slab support rolls facing each other with the slab in between, the method comprising:
increasing the opening degree between the opposing slab support rolls toward the downstream side in the casting direction in some sections;
In the casting direction of the slab, the reduction amount was controlled by a hydraulic cylinder at least in the range from the position where the solid fraction at the center of the thickness of the slab is 0.2 to the position where the solid fraction is the flow limit. Applying a reduction to the slab by a roll segment having a plurality of pairs of slab supporting rolls,
measuring the pressure difference between the plurality of hydraulic cylinders, estimating the final solidification position of the slab based on the pressure difference,
In the roll segment estimated to be the final solidification position and the roll segment immediately before the roll segment, the amount of reduction to the slab is controlled so as to satisfy the following equation (1) and the following equation (2). Continuous casting method.
0.50<V×Z<3.00...(1)
0.5R<Db<2R...(2)
here,
V: Drawing speed of slab (m/min)
Z: Rolling gradient (mm/m)
Db: Bulging amount of slab (mm)
R: Total reduction amount of slab (mm)
It is.

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