Continuous casting method for steel

Abstract

The invention provides a continuous casting method of steel, which can identify a final solidification position with high precision and low cost. A continuous casting method for steel in which a cast piece drawn from a mold is cast by supporting a plurality of pairs of cast piece support rollers facing each other so as to sandwich the cast piece, in which the degree of opening between the facing cast piece support rollers is increased in one section toward the downstream side in the casting direction, and the degree of opening between the facing cast piece support rollers is increased in one section toward the downstream side in the casting direction. In a range from a position where the solid fraction of at least the thickness center portion of the cast piece reaches 0.2 to a position where the solid fraction reaches the flow limit solid fraction, the cast piece is pressed down by a roller section having a plurality of the cast piece support roller pairs, the amount of press-down of which is controlled by hydraulic cylinders, and the pressure difference between the plurality of hydraulic cylinders is measured. The final solidification position of the casting piece is estimated on the basis of the pressure difference, and the amount of reduction to the casting piece is controlled so as to satisfy a predetermined formula in the roller section in which the final solidification position is estimated and the immediately preceding roller section.

Description

Technical Field

[0001] The present disclosure relates to a method for continuous casting of steel. Background Art

[0002] In the continuous casting of steel, in the final solidification process, unsolidified molten steel (called "unsolidified layer") flows in the direction of drawing the cast piece due to solidification shrinkage. In this unsolidified layer, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated. If this concentrated molten steel flows to the center of the cast piece and solidifies, so-called center segregation occurs.

[0003] Center segregation deteriorates the quality of steel products, especially thick steel plates. For example, in pipeline materials for oil and natural gas transportation, hydrogen-induced cracks occur from center segregation due to the action of acidic gas. In addition, the same problem also occurs in marine structures, storage tanks, oil tanks, etc. In recent years, the use environment of steel materials has been required to be used in harsh environments such as lower temperatures or stronger corrosive environments, and the importance of reducing center segregation of cast pieces has increased.

[0004] Therefore, from the continuous casting process to the rolling process, many countermeasures have been proposed to reduce the center segregation of the cast piece or make it harmless. Among the various countermeasures, it is known that the light reduction method at the end of solidification, which is to press down the continuously cast cast piece with an unsolidified layer inside the continuous casting machine, is particularly effective in improving the center segregation. Here, the "light reduction method at the end of solidification" refers to a method in which a reduction roller is arranged near the solidification completion position of the cast piece, and the cast piece in the continuous casting is gradually pressed down by the reduction roller at a reduction speed equivalent to the sum of the solidification shrinkage and the thermal shrinkage. In this way, the generation of voids in the center of the cast piece can be suppressed, the flow of molten steel can be thickened, and the center segregation of the cast piece can be suppressed.

[0005] In order to effectively prevent the occurrence of center segregation of a cast piece by the soft reduction method at the end of solidification, it is important to appropriately set the soft reduction conditions, such as the start and end of the period of soft reduction during the final solidification period of the cast piece, and the amount of soft reduction. Therefore, a method for setting the above conditions and a method for estimating the final solidification position are proposed.

[0006] For example, Patent Document 1 proposes a crater end detection device composed of a pair of work rolls arranged to clamp a cast piece. In the above detection device, a structure is adopted in which the end of a predetermined work roll does not contact the short side corner of the cast piece.

[0007] In addition, in Patent Document 2, a continuous casting method for steel is proposed, which is characterized in that during the continuous casting operation, various data such as the composition analysis value and temperature of the molten steel in the tundish, the casting speed, the amount and temperature of the cooling water of the mold, the amount and temperature of the secondary cooling water, the amount and temperature of the cooling water of the rollers, and the external air temperature are collected, and each data is used as a parameter to perform a model calculation for inferring the solidification state inside the casting, and the pressing force of the pressing device is controlled in real time based on the solidification state inside the casting inferred from the result of the model calculation and the data on the relationship between the central solid phase fraction of the casting and the pressing force set in advance, and the casting speed is controlled in such a way that the area where the central solid phase fraction is inferred to be 0.30 to 0.80 becomes the position of the above-mentioned pressing device.

[0008] Patent Document 1: Japanese Patent Application Publication No. 2018-199137

[0009] Patent Document 2: Japanese Patent Application Publication No. 2013-22609

[0010] However, in the annular pit end detection device described in Patent Document 1, a load measuring device is provided in the bearing box provided at both ends of the support roller. Therefore, a large load is applied to the load measuring device provided in the bearing box under the high temperature environment during casting, and the possibility of failure also increases. In addition, when the load measuring device fails, the section needs to be replaced, which costs a lot of money.

[0011] In the continuous casting method of steel described in Patent Document 2, there is a possibility that an error may occur in the calculation of the center solid phase ratio. Summary of the invention

[0012] The present disclosure has been made in view of the above circumstances, and an object of the present invention is to provide a continuous casting method for steel capable of specifying a final solidification position with high accuracy and at low cost.

[0013] The inventors of the present invention have repeatedly conducted in-depth research to achieve the above-mentioned problem, and as a result, have come up with the idea of ​​using the pressure difference of the hydraulic cylinder to predict the time when the soft reduction should start and the time when the soft reduction should end in the continuous casting of the slab using the soft reduction method at the end of solidification. By measuring the pressure difference of the hydraulic cylinder, the final solidification position can be determined with high accuracy and at low cost.

[0014] The present disclosure is completed based on the above findings. That is, the gist of the present invention is as follows.

[0015] [1] A method for continuous casting of steel, wherein a slab drawn from a mold is supported by a plurality of pairs of slab support rollers for casting, the plurality of pairs of slab support rollers being opposed to each other so as to clamp the slab, wherein:

[0016] The opening between the opposed slab support rolls is increased toward the downstream side in the casting direction in a portion of the section.

[0017] In the casting direction of the cast piece, within the range from a position where the solid phase ratio of at least the center of the thickness of the cast piece becomes 0.2 to a position where the solid phase ratio becomes the flow limit, the cast piece is pressed down by a roll section having a plurality of cast piece supporting roll pairs whose pressing amount is controlled by a hydraulic cylinder.

[0018] measuring the pressure difference between the plurality of hydraulic cylinders and estimating the final solidification position of the cast piece based on the pressure difference;

[0019] In the roll section estimated to be the final solidification position and the roll section immediately before it, the amount of reduction applied to the cast piece is controlled so as to satisfy the following equation (1) and the following equation (2).

[0020] 0.50<V×Z<3.00…(1)

[0021] 0.5R<Db<2R…(2)

[0022] here,

[0023] V: Casting speed (m / min)

[0024] Z: Pressure gradient (mm / m)

[0025] Db: bulge of casting (mm)

[0026] R: total reduction of the cast piece (mm).

[0027] According to the present disclosure, it is possible to provide a continuous casting method for steel capable of specifying a final solidification position with high accuracy and at low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a diagram showing an example of a schematic structure of a slab continuous casting machine.

[0029] Figure 2 This is a schematic diagram showing an example of a roll segment constituting a soft reduction zone of a slab continuous casting machine, and is a schematic diagram viewed from the side of the continuous casting machine.

[0030] Figure 3 It is observed in a cross section perpendicular to the casting direction of the cast piece. Figure 2 A simplified diagram of the roller segments shown.

[0031] Figure 4 Graph showing the pressure difference of the hydraulic cylinders in each roll section versus the casting length. DETAILED DESCRIPTION

[0032] Hereinafter, embodiments of the present disclosure will be described. In addition, the present disclosure is not limited to the following embodiments.

[0033] A method for manufacturing steel according to an embodiment of the present disclosure is a continuous casting method for steel, wherein a slab drawn from a casting mold is supported by a plurality of pairs of slab support rollers for casting, wherein the plurality of pairs of slab support rollers are opposed to each other in a manner of sandwiching the slab, wherein:

[0034] The opening between the opposed slab support rolls is increased toward the downstream side in the casting direction in a portion of the section.

[0035] In the casting direction of the cast piece, within the range from a position where the solid phase ratio of at least the center of the thickness of the cast piece becomes 0.2 to a position where the solid phase ratio becomes the flow limit, the cast piece is pressed down by a roll section having a plurality of cast piece supporting roll pairs whose pressing amount is controlled by a hydraulic cylinder.

[0036] measuring the pressure difference between the plurality of hydraulic cylinders and estimating the final solidification position of the cast piece based on the pressure difference;

[0037] In the roll section estimated to be the final solidification position and the roll section immediately before it, the amount of reduction applied to the cast piece is controlled so as to satisfy the following equation (1) and the following equation (2).

[0038] 0.50<V×Z<3.00…(1)

[0039] 0.5R<Db<2R…(2)

[0040] here,

[0041] V: Casting speed (m / min)

[0042] Z: Pressure gradient (mm / m)

[0043] Db: bulge of casting (mm)

[0044] R: total reduction of the cast piece (mm).

[0045] Figure 1 1 is a diagram showing a schematic structure of a slab continuous casting machine used when implementing a continuous casting method for steel according to an embodiment. Figure 1As shown, in one example, a slab continuous casting machine 1 is provided with a casting mold 5, which is used to inject molten steel 9 and solidify it to form the shell shape of a cast piece 10. A tundish 2 is provided at a predetermined position above the casting mold 5, and the tundish 2 is used to relay the molten steel 9 supplied from a ladle (not shown) to the casting mold 5. A sliding pouring nozzle 3 for adjusting the flow rate of the molten steel 9 is provided at the bottom of the tundish 2, and an immersion pouring nozzle 4 is provided on the lower surface of the sliding pouring nozzle 3. On the other hand, a plurality of pairs of casting piece support rollers 6 composed of support rollers, guide rollers and pinch rollers are arranged below the casting mold 5. A secondary cooling zone is formed in which a spray nozzle (not shown) such as a water spray nozzle or an aerosol spray nozzle is arranged in the gap between the adjacent casting piece support rollers 6 in the casting direction. The casting piece 10 is cooled while being drawn by the cooling water (also called "secondary cooling water") sprayed from the spray nozzle of the secondary cooling zone. In addition, a plurality of conveying rollers 7 are arranged downstream of the slab support rollers 6 at the terminal end in the casting direction, and the plurality of conveying rollers 7 are used to convey the cast slab 10. Moreover, a slab cutter 8 is arranged above the conveying rollers 7, and the slab cutter 8 is used to cut a slab 10a of a predetermined length from the cast slab 10.

[0046] A light reduction zone 14 is provided on the upstream and downstream sides of the casting direction in such a manner as to clamp the solidification completion position 13 of the cast piece 10, and the light reduction zone 14 is composed of a plurality of pairs of cast piece support roller groups. The light reduction zone 14 is set so that the interval between the cast piece support rollers facing each other in such a manner as to clamp the cast piece 10 (this interval is referred to as "roller opening") gradually narrows toward the downstream side in the casting direction. That is, a reduction gradient is set in the light reduction zone 14 (the state of the roll opening being set to gradually narrow toward the downstream side in the casting direction). In the light reduction zone 14, the cast piece 10 can be lightly reduced in its entire area or in a partially selected area. A spray nozzle for cooling the cast piece 10 is also arranged between each cast piece support roller of the light reduction zone 14. The cast piece support roller 6 arranged in the light reduction zone 14 is also called a reduction roller.

[0047] exist Figure 1 In the slab continuous casting machine 1 shown, the light reduction belt 14 is formed by connecting three groups of roller segments, each of which is composed of three pairs of cast sheet support rollers 6, in the casting direction. However, in the present invention, the light reduction belt 14 does not need to be composed of three groups of roller segments, and the roller segments that constitute the light reduction belt 14 may be one group or two groups, or may be four or more groups. In addition, each roller segment is composed of three pairs of cast sheet support rollers 6, but the number of cast sheet support rollers 6 that constitute one roller segment is not particularly limited.

[0048] Figure 2 , Figure 3 An example of the roller segment constituting the light pressure belt 14 is shown. Figure 2 , Figure 31 is a diagram showing an example in which five pairs of slab support rolls 6 serving as reduction rolls are arranged in one roll segment 15. Figure 2 This is a schematic diagram of a continuous casting machine viewed from the side. Figure 3 This is a schematic diagram of a cross section perpendicular to the casting direction of the cast piece.

[0049] like Figure 2 and Figure 3 As shown, in one example, the roll segment 15 is composed of a pair of frames 16 and a frame 16' holding five pairs of cast sheet support rolls 6 via roll bearing seats 21. In addition, a total of four pillars 17 (on both sides of the upstream side and both sides of the downstream side) are arranged through the frames 16 and 16'. The interval between the frames 16 and 16', that is, the reduction gradient of the roll segment 15 can be adjusted by driving the worm jack 19 provided on the pillar 17 by the motor 20. The cast sheet support rolls 6 are held by the frame 16' on the upper surface side via the roll bearing seats 21 connected to the cylinder piston rod (not shown) of the hydraulic cylinder 22 fixedly arranged on the frame 16'.

[0050] In the slab continuous casting machine 1, the molten steel 9 poured from the tundish 2 into the mold 5 via the immersion pouring nozzle 4 is cooled in the mold 5 to form a solidified shell 11. In addition, the molten steel 9 is supported as a cast sheet 10 having an unsolidified layer 12 inside by the cast sheet support roller 6 provided below the mold 5, and is continuously drawn below the mold 5. While passing through the cast sheet support roller 6, the cast sheet 10 is cooled by the secondary cooling water of the secondary cooling belt to increase the thickness of the solidified shell 11, and solidification is completed to the inside at the solidification completion position 13 while being pressed down by the light pressing belt 14. The cast sheet 10 after solidification is cut by the cast sheet cutting machine 8 to become a cast sheet 10a.

[0051] In the steel casting method of the present embodiment, the casting is performed by supporting the casting sheet drawn from the mold 5 by a plurality of pairs of casting sheet support rollers 6, and the plurality of pairs of casting sheet support rollers 6 are opposed to each other in a manner of clamping the casting sheet. The size of the manufactured casting sheet is not particularly limited. The thickness of the casting sheet is preferably 200 mm or more. In addition, the thickness of the casting sheet is preferably 600 mm or less. The width of the casting sheet is preferably 1000 mm or more. In addition, the width of the casting sheet is preferably 2500 mm or less. If the thickness and width of the casting sheet are within the above range, the reduction gradient is more suitable.

[0052] The opening between the opposing slab support rollers 6 increases toward the downstream side in the casting direction in a part of the interval. By increasing the opening between the slab support rollers 6 toward the downstream side in the casting direction, the thickness of the long side of the rectangular slab having an unsolidified layer inside can be bulged. In one example, the thickness of the long side of the slab can be bulged within the range of 0.1 to 10% of the thickness of the slab in the mold. Thus, it is possible to prevent excessive load from being applied to the slab during the subsequent light reduction. If the bulging amount is less than 10%, it is possible to appropriately prevent the strain applied to the solidification interface of the slab from being too large, and the generation of internal cracks can be more appropriately suppressed. The bulging range is preferably arranged between the lower end of the mold 5 and the end position of the liquidus annular pit of the slab 10. This is because, on the upstream side of the liquidus annular pit end position of the slab 10 in the casting direction, the entire center of the slab thickness is the unsolidified layer 12 (liquid phase), the solidified shell 11 of the slab 10 has a high temperature, and the deformation resistance is small, so it can be easily bulged. In addition, when the casting 10 is bulged, if it is bulged at a time when the unsolidified layer 12 present inside the casting 10 is small, the center segregation will be worsened. In contrast, when the casting 10 is bulged on the upstream side of the liquidus ring pit end position in the casting direction, at this time, the molten steel of the initial concentration of the solute element is not thickened and exists abundantly inside the casting, and the molten steel flows easily. Even if the molten steel flows, it will not cause center segregation. Therefore, the bulge on the upstream side of the casting direction than the liquidus ring pit end position will not cause center segregation.

[0053] In the casting direction of the slab, in the range from the position where the solid phase ratio of at least the center of the thickness of the slab becomes 0.2 to the position where the solid phase ratio of the flow limit is reached, the slab is pressed down by a roller section having a plurality of slab support roller pairs whose pressing amount is controlled by a hydraulic cylinder. In addition, as long as the pressing down starts from the position where the solid phase ratio of at least the center of the thickness of the slab becomes 0.2, the pressing down can also be started before the solid phase ratio of the center of the thickness of the slab becomes 0.2. For example, the pressing down can also be started from the position where the solid phase ratio of the center of the thickness of the slab is 0. In addition, the solid phase ratio is defined as setting the solid phase ratio = 0 before the start of solidification and setting the solid phase ratio = 1.0 when the solidification is completed. The solid phase ratio of the center is calculated based on the estimated temperature of the center based on the two-dimensional heat conduction solidification calculation. In addition, the flow limit solid phase ratio is usually 0.7 to 0.8, which can be derived by a known method.

[0054] Here, the part of the casting to which the pressure is applied is not particularly limited, but it is preferred to apply the pressure to the entire width of the casting. For example, as in the method using the load measuring device described in Patent Document 1, when measuring the reaction force in the central part of the width, the measured reaction force is equivalent to the load difference between the ferrostatic pressure of the unsolidified part in the central part of the width and the pressure load of the solidified part. Therefore, if the casting is pressed down under the state of setting the pressure gradient in which the pressure load of the solidified part is balanced with the ferrostatic pressure, there is a possibility that it is impossible to determine whether the casting is solidified. In contrast, by applying pressure to the entire width of the casting, the pressure difference equivalent to the reaction force received by the entire width of the casting is measured, so there is no need to consider the balance between the ferrostatic pressure and the pressure load of the solidified part, and the final solidification position can be inferred with higher accuracy.

[0055] In the steel casting method of the present embodiment, the pressure difference between multiple hydraulic cylinders is measured, and the final solidification position of the above-mentioned cast piece is inferred based on the pressure difference. In the roller segment inferred as the final solidification position and the above-mentioned roller segment immediately before it, the amount of pressure applied to the cast piece is controlled in a manner that satisfies the following formula (1) and the following formula (2).

[0056] 0.50<V×Z<3.00…(1)

[0057] 0.5R<Db<2R…(2)

[0058] here,

[0059] V: Casting speed (m / min)

[0060] Z: Pressure gradient (mm / m)

[0061] Db: bulge of casting (mm)

[0062] R: total reduction of the cast piece (mm).

[0063] By performing the reduction so as to satisfy the equations (1) and (2), internal cracks will not be generated, and center segregation and pores can be effectively prevented.

[0064] Regarding formula (1), V×Z means the rolling speed of the casting (mm / min). When V×Z exceeds 0.50, the flow of the thickened molten steel can be appropriately suppressed, thereby appropriately preventing the generation of V-shaped segregation. When V×Z is less than 3.00, excessive extrusion of the thickened molten steel due to excessive rolling can be appropriately prevented, thereby appropriately preventing the generation of inverted V-shaped segregation. It is preferred that V×Z be set to 0.70 or more. In addition, it is preferred that V×Z be set to 1.20 or less.

[0065] Regarding formula (2), when Db exceeds 0.5R, the pressing reaction force of the casting piece will not become too large. When Db is less than 2R, the pressing reaction force of the casting piece will not become too large, and the internal cracks of the casting piece caused by excessive bulging can be prevented. Preferably, Db is set to 0.7R or more. In addition, it is preferred that Db is set to 1.8R or less.

[0066] Here, the cast piece is pulled by rotating the pinch rolls, and V: cast piece pulling speed (m / min) means the cast piece pulling speed calculated based on the rotation speed of the pinch rolls.

[0067] Z: The reduction gradient (mm / m) means the amount of reduction in the roll opening per 1 m in the casting direction between the roll section estimated as the final solidification position and the roll section immediately preceding it.

[0068] Db: The bulge amount (mm) of the cast piece means the difference between the roll opening at the lower end of the mold and the roll opening in the section before the start of soft reduction.

[0069] R: The total reduction of the cast piece (mm) means the total amount of reduction during casting.

[0070] As described above, the measurement performed by the load measuring device applies a large load in the high temperature environment during casting, and the possibility of failure is high. In addition, when the load measuring device fails, the section needs to be replaced, and the replacement costs a lot of money. In contrast, according to the present invention, the final solidification position of the cast piece is inferred by measuring the pressure difference of multiple hydraulic cylinders, thereby making it easy to specify the final solidification position and apply pressure at an appropriate position. Here, the pressure difference of the hydraulic cylinder is (head pressure on the input side of the roll section) - (head pressure on the output side of the roll section). By measuring the pressure of the hydraulic cylinder 22a on the input side of the roll section and the hydraulic cylinder 22b on the output side of the roll section, the load applied to the support 17a on the input side of the roll section and the support 17b on the output side can be calculated respectively. There are two types of pressure in the hydraulic cylinder: head pressure and rod pressure. The rod pressure is made constant (for example, 20 MPa), and the frame 16' is pressed by the hydraulic cylinder. In contrast, the position of the cast piece support roller 6 is controlled to be constant by balancing the head pressure. When the frame receives a reaction force from the cast sheet, the head pressure is reduced to balance the sum of the reaction force from the cast sheet and the head pressure with the rod pressure. That is, rod pressure = head pressure + reaction force from the cast sheet. The greater the reaction force of the cast sheet, the lower the head pressure. If the solidified cast sheet after the final solidification position is pressed down in the roller section, the reaction force from the roller section becomes larger than that of pressing down the cast sheet with an unsolidified portion. On the other hand, in the roller section where the final solidification position exists, the load on the input side of the section is small, and the load on the output side of the section becomes larger. That is, by using a roller section in which the pressure difference between the hydraulic cylinder provided on the input side of the section and the hydraulic cylinder provided on the output side of the section is greater than a predetermined value or greater than a predetermined value, the final solidification position of the cast sheet can be inferred. The pressure difference between the hydraulic cylinder provided on the input side of the section and the hydraulic cylinder provided on the output side of the section is not specifically limited, and is preferably greater than 2MPa.

[0071] Figure 4 This is a graph showing the pressure difference of the hydraulic cylinder in each roller section relative to the casting length. In the figure, sections 1, 2, and 3 are the data of the roller sections located at the most downstream, the middle, and the most upstream, respectively. It can be seen that the more the roller section is located downstream, the greater the pressure difference between the hydraulic cylinder set at the section input side and the hydraulic cylinder set at the section output side. The pressure difference of the hydraulic cylinder increases from the moment the bottom position of the cast piece enters the roller section, and then the pressure difference decreases when the bottom of the cast piece is separated, and the pressure difference remains large when the final solidification position exists in the roller section. In the example of this figure, the pressure difference drops to less than 2MPa after the roller section 3 and roller section 2 located at the most upstream and the middle pass through the bottom. In contrast, the pressure difference of section 1 located at the most downstream is still more than 2MPa after passing through the bottom, that is, it can be inferred that section 1 is the final solidification position of the cast piece.

[0072] In estimating the final solidification position, when a load measuring device is used, it is necessary to set a load measuring device for each pressing roll. Therefore, if the final solidification position is to be estimated from a wide range in the casting direction, many 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 section. Therefore, even if the thickness of the cast piece, the drawing speed or the steel type changes and the final solidification position varies greatly, it is easy to estimate the final solidification position from a wide range.

[0073] In addition, casting conditions other than the above-mentioned conditions can be based on a conventional method.

[0074] According to the continuous casting method of the steel, the center segregation of the cast piece can be reduced. In the cross section perpendicular to the drawing direction of the cast piece, the carbon concentration is analyzed at equal intervals along the thickness direction of the cast piece, and the maximum value in the thickness direction is defined as C max The carbon concentration analyzed from the molten steel collected from the tundish during casting is set as C0, and C max Therefore, the closer the central segregation is to 1.000, the better the cast piece is with less central segregation. max / C0 is less than 1.100. In addition, the lower limit of the center segregation degree is not particularly limited, and generally, the center segregation degree is greater than 1.000. In addition, according to the continuous casting method of the steel, the generation of pores in the cast piece can be suppressed, and internal cracks can be suppressed.

[0075] Example

[0076] Hereinafter, the present disclosure will be described in further detail based on examples.

[0077] The slab continuous casting machine used for the test Figure 1 The same as the slab continuous casting machine 1 shown. Using this slab continuous casting machine, low-carbon aluminum killed steel was cast. The thickness of the cast piece at the lower end of the mold was set to 250mm, 400mm or 600mm. The width of the cast piece was 2200mm in all tests. Table 1 shows the casting conditions, the central segregation degree of the cast piece, the presence or absence of pores and the presence or absence of internal cracks. Table 1 also shows the casting conditions and the results of the test conducted as a comparative example. In addition, in Table 1, the starting position of the reduction, the end position of the reduction, the position where the solid phase ratio of the thickness center of the cast piece becomes 0.2, and the final solidification position are recorded according to the distance from the curved liquid surface in the mold. In addition, in Table 1, in the test where the reduction range control is displayed as "yes", the reduction start position and the reduction end position deviate from the appropriate range within the initially set light reduction range, so the reduction range is adjusted based on the final solidification position measured during casting.

[0078] [Table 1]

[0079]

[0080] The central segregation of the cast piece used for the test evaluation was measured by the following method. That is, in a cross section perpendicular to the drawing direction of the cast piece, the carbon concentration was analyzed at equal intervals along the thickness direction of the cast piece, and the maximum value in the thickness direction was defined as C. max The carbon concentration analyzed from the molten steel collected from the tundish during casting is set as C0, and C max / C0 is taken as the central segregation degree. Therefore, the closer the central segregation degree is to 1.000, the better the casting is with less central segregation. Here, castings with a central segregation degree of 1.100 or more are judged to have a poor degree of central segregation.

[0081] The presence or absence of pores and internal cracks in the cast piece was determined by microscopic observation of the vicinity of the center of the thickness of the cast piece in a cross section perpendicular to the drawing direction of the cast piece.

[0082] The casting drawing speed at each casting thickness is set at least so that the casting in the range of the solid phase fraction at the center of the casting thickness from 0.2 to the flow limit solid phase fraction is located in the light reduction zone. On this basis, in test numbers 1 to 5, test numbers 9 to 13, and test numbers 17 to 21, the reduction gradient and the bulge amount are set in the roll section estimated as the final solidification position and the roll section immediately before it in a manner that satisfies the above-mentioned equations (1) and (2). As the final solidification position, a roller section is specified in which the pressure difference between the hydraulic cylinder set on the input side of the section and the hydraulic cylinder set on the output side of the section is 2 MPa or more. In addition, in test numbers 6, 14, and 22 conducted as comparative examples, any one of the reduction start position and the reduction end position is set outside the appropriate range. In addition, in test numbers 7, 8, 15, and 23, V×Z deviates from equation (1). In addition, in test numbers 16, 22, and 24, the total reduction amount and the bulge amount were outside the range of formula (2).

[0083] As is clear from the central segregation shown in Table 1, the central segregation in the inventive examples of Test Nos. 1 to 5, 9 to 13, and 17 to 21 was less than 1.100, which was good. In addition, no pores or internal cracks were observed in the cast pieces.

[0084] In test numbers 6, 14, and 22, which were conducted as comparative examples, since either the reduction start position or the reduction end position was outside the appropriate range, the central segregation was 1.100 or more, and pores were also observed inside the cast piece. In test numbers 7 and 15, which were conducted as comparative examples, the reduction gradient was insufficient, the central segregation was 1.100 or more, and pores were also observed inside the cast piece for test number 15. In addition, in test numbers 8 and 23, since the reduction gradient was too large, the central segregation was 1.100 or more, and internal cracks were generated in the cast piece. In test numbers 16, 22, and 24, since the total reduction amount and the bulge amount were outside the range of formula (2), the central segregation was 1.100 or more, and pores were also observed inside the cast piece.

[0085] Description of Reference Numerals

[0086] 1…slab continuous casting machine; 2…tundish; 3…sliding pouring nozzle; 4…immersion pouring nozzle; 5…casting mold; 6…casting sheet support roller; 7…conveying roller; 8…casting sheet cutting machine; 9…molten steel; 10…casting sheet; 11…solidified shell; 12…unsolidified layer; 13…solidification completion position; 14…light pressing belt; 15…roller section; 16…frame; 17…pillar; 19…worm jack; 20…motor; 21…roller bearing seat; 22…hydraulic cylinder.

Claims

1. A method for continuous casting of steel, wherein a cast sheet drawn from a casting mold is supported by a plurality of pairs of cast sheet support rollers, the plurality of pairs of cast sheet support rollers being opposed to each other so as to sandwich the cast sheet, The continuous casting method of steel is characterized in that: The opening between the opposed slab support rolls is increased toward the downstream side in the casting direction in a portion of the section, In the casting direction of the cast piece, within the range from the position where the solid phase ratio of at least the center of the thickness of the cast piece becomes 0.2 to the position where the solid phase ratio becomes the flow limit, the cast piece is pressed down by a roll segment having a plurality of cast piece supporting roll pairs whose pressing amount is controlled by a hydraulic cylinder. measuring the pressure difference between the plurality of hydraulic cylinders and inferring the final solidification position of the cast piece based on the pressure difference, In the roll section estimated as the final solidification position and the roll section immediately before it, the amount of reduction applied to the cast piece 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: Casting speed (m / min) Z: Pressure gradient (mm / m) Db: bulge of casting (mm) R: total reduction of the cast piece (mm).

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