JP2012115898A - Method of continuously casting slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of reducing porosity and porosity in a slab center and a shrinkage cavity and porosity in a slab upper part at low equipment cost, without reducing the surface quality, regardless of the size of the sectional area in continuous casting.SOLUTION: In a method of continuously casting slab, after drawing out of the slab is completed using a movable roll rolling device having a roll rolling the slab and vertically movable from a mold while going along the slab, the stopped slab is rolled while vertically moving it. A direction in which the roll is moved while rolling the slab may be oriented upward vertically. The cross section of the slab may be circular.

Description

  The present invention relates to a method for continuously casting a slab that reduces the occurrence of internal defects such as zaku, porosity and shrinkage by reducing the slab using a pair of rolls, and in particular, continuous casting using a movable roll. Regarding the method.

  Currently, in the production of steel, it is common to cast a slab by a continuous casting method and subject the slab to partial rolling, rolling and the like for the final product. However, as a final product, a large material having a large cross section, such as a boiler tank or a large mold material, is a small lot and requires a large cross-section slab. Instead of continuous casting, the piece is casted as a large ingot by pouring molten steel into a mold and solidifying it (hereinafter referred to as “ingot method”).

  Casting large ingots by the ingot method, even in small lots, is much less efficient than the continuous casting method, and there is a need for hot water at the top of the ingot, or for runners and hot water pipes. Considering the remaining of molten steel, the yield is very bad. The hot water referred to herein is to supply molten steel for solidification shrinkage in order to prevent the formation of shrinkage cavities and shrinkage cracks due to solidification shrinkage of the molten steel when casting an ingot.

  Further, when a slab having a large cross section is cast by a continuous casting method, zack generated at the center of the slab, porosity as a bubble defect, and segregation tend to increase. Zaku here refers to a cavity defect that occurs in the center of the slab when casting an alloy slab. At the end of casting, after the supply of molten steel to the mold is stopped, a large shrinkage cavities such as those found in the ordinary ingot method are formed due to solidification shrinkage from the meniscus of the slab to the downstream side in the casting direction. appear. These internal defects and the like not only deteriorate the yield of the product but also remain in the final product in some cases and cause product defects.

  As a method of manufacturing large-section slabs with good internal quality, Patent Document 1 manufactures large steel ingots that are difficult to cast with conventional continuous casting machines, such as extremely thick flat ingots. In semi-continuous casting for this purpose, it has been proposed to use a mold having an upper taper. Further, this document describes that the quality of the steel ingot can be further improved by heating the meniscus at the top (upper part) of the steel ingot by an electric method.

  Patent Document 2 describes that, in continuous casting of a slab, it is possible to reduce the occurrence of internal defects such as zaku and porosity by making the shape of the slab into a tapered shape in which both side surfaces gradually expand upward. Has been.

  On the other hand, in order to reduce internal defects such as porosity and segregation in continuous casting of a slab, a method of reducing the surface of the slab at the end of solidification is generally known. For example, Patent Document 3 describes a method of unsolidifying and reducing a slab.

  By using a tapered mold as in the techniques described in Patent Documents 1 and 2 or by making the shape of the slab into a tapered shape, the role of the conventional hot water supply can be supplemented to some extent. However, these methods have a complicated casting method and a high equipment cost, but the effect of suppressing zaku and porosity is limited, and the effect becomes smaller as the cross section of the slab becomes larger. Also, the method of heating the meniscus at the top of the slab is not effective in improving the internal quality up to the center of the slab if the slab length is long, is expensive in terms of equipment, and is uneconomical in terms of energy. For this reason, it is not a very effective method.

  On the other hand, the method (in-line reduction method) that crushes the slab from the surface with a roll or the like and crushes the internal porosity at the generation stage, as in continuous casting in a normal continuous casting machine, is decisive and difficult. It is an effective method. However, when this in-line reduction method is adopted for continuous casting of a large-section slab, there are the following two problems.

  The first problem is that, in the in-line reduction method, the porosity generated in the slab may not be crimped at any stage of casting, and there is an optimum reduction time. For example, if the slab is squeezed at the porosity generation stage, the central solid phase ratio is said to be good at the end of solidification period from about 0.5 to complete solidification, and after complete solidification, It is said that the temperature immediately after solidification is still sufficiently high in the center of the slab. Therefore, in normal continuous casting, it is common to install a reduction device such as a reduction roll at a specific position such as near the outlet of the continuous casting machine.

  However, when casting a slab having a large cross section, the slab is completely solidified in order to reduce the slab under the optimum conditions for pressure bonding of zaku and porosity by a reduction device installed near the outlet of the continuous casting machine. In order to secure the time until this, it is necessary to increase the length of the continuous casting machine. Here, the length from the meniscus in the mold to the final solidification position of the slab is considered to be proportional to the square of the thickness of the slab. For this reason, for example, if a slab having a thickness of 300 mm is used as a reference, a slab having a thickness of 900 mm requires a continuous casting machine that is nine times as long, and a great construction cost is required.

  On the other hand, if the length of the continuous casting machine cannot be increased, a method of reducing the casting speed can be considered in order to secure time until the slab is completely solidified. It is generally considered that the casting speed at the final solidification position (slab speed) is inversely proportional to the square of the slab thickness. For this reason, for example, if the casting speed of a 300 mm thick slab is 1 m / min, the casting of 900 mm thick must be 0.11 m / min. Such ultra-low speed casting results in insufficient heat supply at the meniscus in the mold, greatly increasing the surface quality of the slab, such as the solidification of the meniscus and the occurrence of rippled cast skin due to shrinkage of the solidified shell at the meniscus. Cause a significant decline. In order to prevent the deterioration of the surface quality, the combined use of plasma heating and meniscus heating by Joule heat is also conceivable, but the equipment cost is high as described above, which is uneconomical in terms of energy.

  The second problem is that when the cross section of the slab is large, the penetration into the slab under reduction is insufficient, and there is a concern that zaku and porosity cannot be sufficiently crimped.

JP 62-161445 A JP 2004-243352 A JP 2000-288705 A

  As described above, in the conventional continuous casting, the method for reducing the crust and porosity at the center of a slab having a large cross section and the shrinkage cavity and zaku at the top of the slab, the cost of equipment, energy, and surface quality There was a problem in terms.

  The present invention has been made in view of such problems in the prior art, and the problem is that, in continuous casting, the equipment cost is low and the surface quality is not reduced, regardless of the size of the cross section. It is an object of the present invention to provide a method for reducing crinkles and porosity at the center of a slab and shrinkage cavities and crusts at the top of the slab.

  In order to solve the above-described problems, the present inventors have studied a slab reduction method in continuous casting. As a result, by using a movable roll to reduce the slab, regardless of the cross-sectional size of the slab, it is possible to reduce the slab at the optimum position for crimping the zaku, porosity, and shrinkage cavity. I found out. In this case, it is not necessary to adjust the length and casting speed of the continuous casting machine as in the case of using a roll fixed at a specific position, and the equipment cost is very low.

  This invention is made | formed based on the said knowledge, The summary exists in the continuous casting method of the slab shown to following (1)-(4).

(1) A method of continuously casting a slab using a pair of rolls that can be switched between guide support and reduction of the slab and that can be moved vertically along the slab below the mold, While the slab is being drawn, the slab is guided and supported while the movement is stopped, and after the slab has been pulled out, the stopped slab is moved down in the vertical direction while being reduced. Continuous casting method.

(2) The continuous casting method of a slab according to (1), wherein the moving direction of the roll while rolling down the slab is upward in the vertical direction.

(3) The continuous casting method of a slab according to (1) or (2), wherein the slab has a circular cross section.

  According to the continuous casting method of the slab of the present invention, the continuous casting machine having a low equipment cost does not cause a reduction in surface quality, and a large zaku, porosity, and shrinkage cavity regardless of the size of the cross section of the slab. The slab can be cast with a high yield.

It is a block diagram of the continuous casting machine which can apply the method of this invention, The figure (a) is a front view, The figure (b) is a side view. It is a figure explaining the casting process by the continuous casting method of this invention, the figure (a) is a state at the time of a casting start, the figure (b) is the state in the middle of drawing of a slab, and the figure (c) is drawing completion. The state in which the movable roll is moved to the lower end of the movable range later, (d) in the figure shows the state in which the movable roll is raised while the slab is being crushed, and (e) shows the state in which the reduction has been completed. It is a figure which shows the relationship between the ratio of the rolling amount with respect to the unsolidified diameter of a slab (rolling amount / unsolidified diameter), and a defect area rate, The figure (a) shows the results in a stationary part, (b) ) Shows the results at the top of the slab.

  FIG. 1 is a configuration diagram of a continuous casting machine to which the method of the present invention can be applied. FIG. 1 (a) is a front view and FIG. 1 (b) is a side view. The continuous casting machine shown in the figure is a vertical type, and the casting direction of the slab is vertically downward. This continuous casting machine includes a ladle 1 for containing molten steel, a mold 2 to which molten steel is supplied from a ladle 1 via an immersion nozzle (not shown), and a movable roll for rolling down a slab 3 drawn downward from the mold 2. A reduction device 4. The mold 2 is a combination of half molds. The movable roll reduction device 4 includes a pair of rolls 5 and a frame 6 that supports the rolls 5. The frame 6 is integrated with the rolls 5 and can be moved vertically up and down along the slab 2 below the mold 2. It is.

  As shown in FIG. 2 to be described later, a support roll group 7 is arranged immediately below the mold 2 (not shown in FIG. 1), and forms a support area for the solidified shell 3a of the slab 3. In the continuous casting machine, it is preferable to support the solidified shell 3a in a region that is about 1/4 to the equivalent of the length of the mold 2 at least immediately below the mold 2. In FIG. 2 described later, a mode in which the length of the support area is equivalent to the length of the mold 2 is shown.

  The roll 5 is configured to be able to switch both roles so as to serve both as a pinch roll for guiding and supporting the slab 3 and as a reduction roll for reducing the slab 3, and from the back so as to contact the slab 3. It is pressed toward the slab by hydraulic pressure. The roll 5 is connected to the large speed reducer 9 via the universal joint 8 and operates as a drive roll.

  The frame 6 is supported by four jack shafts 10 formed of vertically arranged ball screws so as to be movable in the vertical direction, and is given a driving force that can be moved vertically by the jack mechanism of the jack shaft 10.

  Since the roll 5 is integral with the frame 6, the roll 5 can be moved vertically up and down along the slab 2, and the slab 3 can be moved while being reduced, and the slab 3 can be moved while being reduced. The roll 5 can be moved by rotating the roll 5 itself with the slab 3 interposed therebetween, and the moving direction can be changed by changing the rotating direction of the roll 5. Further, when the roll 5 is not in contact with the slab 3, it can be moved by the jack mechanism of the jack shaft 10.

  2A and 2B are diagrams for explaining a casting process according to the continuous casting method of the present invention, in which FIG. 2A is a state at the start of casting, FIG. 2B is a state during drawing of a slab, and FIG. ) Shows a state in which the movable roll is moved to the lower end of the movable range after completion of drawing, FIG. 6D shows a state in which the movable roll is raised while squeezing the slab, and FIG. .

  The continuous casting method of the present invention will be described with reference to FIG. First, casting of the slab 3 is started as shown in FIG. 11A, and the slab 3 is continuously pulled out as shown in FIG. At this time, the roll 5 is arranged directly under the mold 2, actually directly under the support roll group 7, and used as a pinch roll. When the slab 3 is drawn out to the limit of the continuous casting machine, the slab 3 is stopped and the drawing is completed. Thereafter, the roll 5 is moved to the lowest end of the movable range as shown in FIG. Then, it waits until the temperature of the center part of the slab 3 and the thickness of the solidification shell 3a become the optimal conditions for reduction.

  After the state of the slab 3 becomes the optimum condition for the reduction, the roll 5 is pressed against the slab 3 until the reduction amount of the slab 3 reaches a predetermined amount, and the roll 5 is rotated in the direction opposite to the drawing time. Then, as shown in FIG. 2 (d), the slab 3 is pressed down while raising the roll 5 along the axis of the slab 3. When the solidified shell 3a has the unsolidified molten steel 3b, the unsolidified molten steel 3b is placed on the upper meniscus by raising the roll 5 while reducing the slab 3 as shown in FIG. Discharged. The amount of molten steel discharged depends on the size of the unsolidified part when the slab is pressed, but when the slab has a circular cross section, it is not so much as compared with other shapes. 2 can be accommodated. On the other hand, when the roll 5 is raised while being reduced after the slab 3 has completely solidified, there is no discharge of unsolidified molten steel.

  Thus, by rolling down the slab 3 using the movable roll reduction device 4, the entire slab 3 can be efficiently squeezed regardless of the cross-sectional size of the slab 3, and zaku and porosity can be pressure-bonded. Can do. The reduction of the slab 3 may be performed continuously or only necessary portions may be intermittently performed.

  The rolling condition of the slab 3 can be changed by changing the rising speed of the roll 5. For example, by making the rising speed of the roll 5 the same as the drawing speed of the slab 3, the entire slab 3 can be rolled down under the same conditions. This is because the solidification of the unsolidified molten steel in the slab 3 progresses even during the elapse of time after the roll 5 starts rolling down, and the unsolidified portion shrinks. This is because the time from the casting to the reduction is made constant at the reduction position by making it the same as the drawing speed, and the size of the unsolidified portion at the reduction position is kept substantially constant. However, the rising speed of the roll 5 may not be the same as the drawing speed of the slab 3.

  When only the shrinkage nest and the occurrence of zaku under the meniscus are to be suppressed, the roll 5 is raised to a predetermined position in the vicinity of the lower part of the mold 2 without lowering the slab 3, and the upper part from that position is raised. What is necessary is just to roll down the slab 3 while raising the roll 5 to a predetermined position. On the contrary, the slab 3 is moved up to a predetermined position above the predetermined position near the lower part of the mold 2 without being reduced, and the slab is lowered from the position to a predetermined position near the lower part of the mold 2 while lowering the roll 5. 3 may be reduced.

  With the above steps, the steps from one slab drawing to the reduction in rolling are completed. Therefore, after unloading the slab, the next casting may be performed again by repeating the step shown in FIG.

  In this way, by using a movable roll, the same continuous casting machine can be used to produce a slab having a good internal quality at a low equipment cost and without causing a reduction in surface quality, regardless of the size of the cross section. Can be cast. Moreover, since it is continuous casting, a slab can be cast with a higher yield than the ingot method.

  In the above description, the case of using a vertical continuous casting machine as a continuous casting method has been described. However, the continuous casting machine to which the present invention can be applied is not limited to the vertical type, and there is a portion that casts vertically from directly below the mold. If it exists, a vertical bending type, a curved arc type, or the like can be applied.

  It is preferable that the slab to be cast has a circular cross section. When the slab with a circular cross section is squeezed with a pair of flat rolls, the solidified shell around the roll contact part is not greatly deformed against zaku and porosity generated at the center of the slab, This is because deformation only between the pair of roll contact portions is sufficient, and zaku and porosity can be efficiently crimped with a small reduction reaction force.

  Further, when the movable roll reduction device is arranged, the support roll group of slabs provided in the conventional continuous casting machine and the roller apron that holds the slab geometrically interfere with the movable roll reduction device. It is extremely difficult to install. If the support roll group is not installed, there is a concern about occurrence of slab bulging by pressing the solidified shell against the static pressure of the unsolidified molten steel inside the slab. However, when the cross section of the slab is circular, even if the solidified shell is subjected to the molten steel static pressure without the support roll group being installed to some extent, bulging can be made difficult to occur.

  The slab may be reduced in a state where an unsolidified portion remains in the slab or in a state where the slab is completely solidified. Depending on the type of steel to be cast, if the slab is reduced in a state in which an unsolidified portion remains, an internal crack may occur in the slab. In this case, the reduction may be performed after the slab is completely solidified. In addition, since the generated zaku and porosity are not comparatively large depending on the steel type, in this case, zaku and porosity can be sufficiently pressed under pressure after completely solidifying.

  In order to confirm the effect of the continuous casting method of the slab of the present invention, the following casting tests (preliminary test and main test) were performed.

1. Preliminary test 1-1. Test conditions The slab to be cast was a small slab having a diameter of 300 mm and a length of 1800 mm, and the steel type was 13% Cr steel in which zaku and porosity were easily increased. The continuous casting machine shown in FIG. 1 was used. However, a support roll group that supports the solidified shell of the slab was not provided. The movable roll reduction device had a roll diameter of 450 mm, a maximum reduction force of 100 t, and a maximum reduction torque of 50 t · m. The reduction rate of the movable roll reduction device was 0.8 m / min, and after the casting of the entire length of the slab was completed, the rolling was reduced over the entire length of the slab. The reduction amount of the slab was 20 to 70 mm in terms of the reduction amount of the diameter of the slab in the reduction direction. However, the cross-sectional shape of the slab became flat due to the reduction.

  The diameter of the unsolidified portion at the reduced position (hereinafter referred to as “unsolidified diameter”) was 70 mm or 110 mm. This is a value when an isotherm corresponding to a solid phase ratio of 0.8 is defined as a solid-liquid interface. The position of the interface at which the solid fraction becomes 0.8 was determined by unsteady primary heat transfer solidification analysis of a cylindrical cross section. The accuracy of the analysis should be sufficient by comparing the results of measurement of the surface of the slab surface, measurement of the temperature inside the slab with a thermocouple, and measurement of the unsolidified diameter with the addition of a tracer such as S. confirmed.

1-2. Test results After completion of the test, each slab was cut so that the longitudinal cross section passing through the center of the slab was exposed, and the cut surface was cut and polished, and then the occurrence of zaku, porosity, and shrinkage was investigated. Each of these defects exhibited voids in the cross section of the slab, and the degree was calculated by the ratio (void ratio) of the area of the voids to the total area of the cross section. The porosity is divided by the porosity of a slab that is cast separately from the slab that has been reduced, and that is not squeezed by a roll (hereinafter referred to as “non-reduced slab”), and the value obtained by the removal is determined as a defect It was defined as the area ratio and used as an index of defect occurrence. The area of the void was measured using general-purpose image photograph analysis software, but may be measured by other methods.

  FIG. 3 is a diagram showing the relationship between the ratio of the rolling amount to the unsolidified diameter of the slab (rolling amount / unsolidified diameter) and the defect area ratio. FIG. The figure (b) shows the results in the upper part of the slab respectively. The upper part of the slab refers to a region where a crush and a shrinkage cavity are generated in the unsqueezed cast slab, and a region corresponding to a region where a crush and a shrinkage cavity are generated in the unsqueezed slab. A stationary part means the area | region of slabs other than slab upper part.

  As shown in FIG. 3 (a), it has been found that when the value of the amount of reduction / unsolidified diameter increases, zaku and porosity can be significantly reduced. Further, from FIG. 5B, it was confirmed that the upper part of the slab has a further significant defect reduction effect as compared with the steady part.

2. Main test 2-1. Examination of casting conditions Based on the results of preliminary tests, the casting conditions were examined for the case where the scale of molten steel was increased as the main test. The cast slab was 800 mm in diameter and 10 m in length, and the steel type was 13% Cr steel. The amount of molten steel used for casting this slab was about 40 t. This corresponds to four ingot castings (molten steel amount 10 t) by a normal ingot method. Usually, ingots are cast using hot water in order to prevent shrinkage cavities at the top of the slab and the occurrence of zack. Since the amount of molten steel required for the hot water is 10% of the mass per ingot, an extra 4t of molten steel is required. After casting the ingot, it is necessary to cut off the hot metal portion, so that a loss occurs accordingly, but this loss does not occur in the continuous casting method.

  The continuous casting machine shown in FIG. 1 was used. The mold was a water-cooled copper type having a diameter of 800 mm and a length of 800 mm. A support roll group was provided immediately below the mold, and the length of the support area was 800 mm. In the movable roll reduction device, the diameter of the roll provided was 650 mm. The slab was cooled by spray cooling with a specific water amount of 0.2 L / kg-steel. The slab was drawn out at a casting speed of 0.25 m / min, and the drawing was stopped when the length of the slab reached 10 m. The other conditions were the same as in the preliminary test.

  According to the solidification heat transfer analysis performed for continuous casting under the above conditions, the surface temperature of the slab when drawing is stopped is about 1220 ° C. at a location of 4 m from the meniscus in the mold in the casting direction, and at a location of 10 m. Estimated at about 980 ° C. The unsolidified diameter at this point was estimated to be about 620 mm at a position 4 m from the meniscus and 500 mm at a position 10 m from the solid phase ratio of 0.8. Based on the analysis results, the amount of slab reduction by the movable roll reduction device was 225 mm, and the ascent rate of the movable roll reduction device was 0.25 m / min. Since this rising speed is the same as the drawing speed of the slab, the rolling conditions (the unsolidified diameter of the slab at the lower part and the surface temperature of the slab) are the same over the entire area of the slab.

In this case, the unsolidified diameter in the lower part at the start of reduction is about 500 mm, and the surface temperature is 980 ° C. When the reduction amount is 225 mm with respect to the unsolidified diameter of 500 mm, the value of reduction amount / unsolidified diameter is 0.45. Therefore, from FIG. In the upper half, it is estimated that 4.8%, both of which are greatly reduced. Since the diameter of the roll provided in the movable roll reduction device is 650 mm, and the deformation resistance of 13% Cr steel to be cast is 6 kgf / mm ² , the contact angle between the roll and the slab is 32 °. The necessary rolling force is 650 t.

2-2. Test Results The slab cast under the above conditions had less zaku, porosity and shrinkage, and better internal quality and surface quality than when no movable roll reduction device was provided. Moreover, a high yield was obtained as compared with an ingot of the same size cast by the ingot method.

  According to the continuous casting method of the slab of the present invention, the continuous casting machine having a low equipment cost does not cause a reduction in surface quality, and a large zaku, porosity, and shrinkage cavity regardless of the size of the cross section of the slab. The slab can be cast with a high yield.

1: ladle, 2: mold, 3: slab, 3a: solidified shell, 3b: unsolidified molten steel,
4: movable roll reduction device, 5: roll pair, 6: frame, 7: support roll group,
8: Universal joint, 9: Large speed reducer, 10: Jack shaft

Claims (3)

A method for continuously casting a slab,
Using a pair of rolls that can switch the guide support and reduction of the slab, and can move vertically along the slab below the mold,
While the slab is being pulled out, the slab is guided and supported while the movement is stopped.
A method for continuously casting a slab, comprising: after the drawing of the slab has been completed, the stopped slab is moved down in a vertical direction.   The continuous casting method for a slab according to claim 1, wherein the moving direction of the roll while pressing the slab is vertically upward.   The continuous casting method for a slab according to claim 1 or 2, wherein the slab has a circular cross section.

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