JP5741402B2 - Continuous casting method for circular section slabs - Google Patents

Description

  The present invention relates to a continuous casting method of a slab having a circular cross section, and more particularly to a vertical continuous casting method capable of producing a slab excellent in internal quality.

  At present, a steel product is generally made of a slab produced by continuous casting, and this material is divided and rolled into a final product by processing such as hot rolling.

  However, when the final product has a large cross section, for example, when the final product is a boiler tank or a large forged steel roll, it is a small lot and the material must have a large cross section. For this reason, in such a case, not a slab produced by continuous casting, but a large ingot cast by the ingot-making method is used as a raw material. The current state is the final product. Here, the large cross section of the slab or ingot means a cross section having a diameter of 600 mm or more.

  Since the ingot-making method is disadvantageous in terms of energy consumption and cost compared to the continuous casting method, in recent years, attempts have been made to apply continuous casting to large-section slabs that have been considered difficult in the past.

  However, when a large-section slab is continuously cast, the center porosity and zaku generated at the center of the slab tend to increase, and the cast slab tends to have a problem that the internal quality is low. In addition, after the hot water supply to the mold is stopped, there is a problem that the yield is low because shrinkage cavities and zack are generated in the top portion of the slab along with the solidification shrinkage.

  In Patent Document 1, as a method of casting special steel including high alloy steel with few internal defects such as center porosity and center segregation by the vertical continuous casting method, while expanding the mold in the width direction and thickness direction of the slab, A method of casting and imparting a taper to a slab has been proposed.

  However, as a result of the investigation by the present inventors, the method proposed in Patent Document 1 plays a role of controlling porosity by a conventional feeder by giving a taper to the slab, but the equipment cost is expensive. It was found that the effect of suppressing the occurrence of internal defects was limited compared to.

  In Patent Document 2, as a method for producing a slab having excellent inner quality without center segregation or porosity, a method in which an electromagnetic stirrer is disposed at the end of solidification in a continuous casting machine, and unsolidified molten steel is stirred with a predetermined electromagnetic force. Has been proposed.

  In the method proposed in Patent Document 2, the quality of the slab can be improved by electromagnetic stirring. However, when this method is applied to large-section slabs, the slab is thick, so the attenuation of the magnetic field is large, and it is necessary to increase the magnetic field strength to sufficiently stir the molten steel. A stirrer is required, increasing the equipment cost.

  In Patent Document 3, as a method for suppressing a decrease in yield due to shrinkage of the top portion of a continuous cast slab, the slab top portion is heated or heated by a burner or an arc after the slab top portion passes through the mold, A method of reducing the contraction surface of the part has been proposed.

  Similar to the method proposed in Patent Document 1, the method proposed in Patent Document 3 is a method aiming at the same effect as the porosity suppression by the hot water of the slab top, but when the slab is long However, it is difficult to obtain this effect up to the center of the slab, and the equipment is expensive and uneconomical in terms of energy.

  In order to suppress the generation of center porosity and zaku and improve the quality of the slab, it is effective to reduce the slab by an in-line reduction method, that is, by a roll or other reduction device installed in a normal continuous casting machine. is there. However, when this method is applied to a large-section slab, there are the following problems.

  First, in order to improve the quality of the slab by the in-line reduction method, it is important to perform the reduction of the slab when the solidified state of the slab is optimum. Therefore, it is necessary to draw out the slab of the optimal solidification state to a fixed reduction device. In the case of a large-section slab, it is difficult to secure the length of the continuous casting machine necessary for adjusting the fixing position of the reduction device to the optimum solidification time of the slab.

  On the other hand, if the casting speed is extremely low, the optimum solidification time of the slab can be matched with the installation position of the reduction device even if the length of the continuous casting machine is short. However, in this case, hot water skinning, ripple marks (water bath), entrainment of mold powder, etc. occur, and the surface quality and internal quality are significantly impaired. The molten metal skinning occurs due to insufficient heat supply on the molten metal surface in the mold, and the ripple mark occurs due to the molten metal overflowing between the mold and the solidified shell in the mold due to the shrinkage of the solidified shell.

  When the problem when the casting speed is extremely low is solved by the method proposed in Patent Document 3, the facilities become complicated, which is uneconomical.

JP 2002-361374 A JP 58-97470 A Japanese Patent No. 1963157

  The present invention has been made in view of the above-described problems, and the inner quality of the slab is suppressed in the generation of the center porosity and zaku in the steady casting portion of the slab, and the formation of the shrinkage nest and zaku in the slab top portion. An object of the present invention is to provide a method capable of stably casting a good slab at a low equipment cost even if it is a slab having a circular large cross section. Here, the slab top portion refers to the final portion of drawing with a continuous casting machine.

  As a result of studying the above problems, the present inventors have found that even with a slab having a large circular cross section, after the casting is completed, solidification of the unsolidified portion proceeds while the slab is stationary, and the solidified state is determined as a slab. It was found that the slab can be squeezed in the optimum solidified state by reducing it with a movable squeezing device after making it optimal for improving the quality of the steel. In this case, it is not necessary to draw the slab in the optimally solidified state to a fixed reduction device, and the length of the continuous casting machine can be as long as the casting length, so use a continuous casting machine with a long length as described above. There is no need to do. Moreover, it is not necessary to make the casting speed very low.

  In addition, as a result of repeated various tests and studies, the present inventors have determined that when the solid-liquid interface defined by the isotherm with a solid phase ratio of 0.8 is the solidification interface, Shrinkage at the top of the slab (final part of drawing in the continuous casting machine) by setting the ratio r1 / d1 of the ratio of the reduction amount r1 and the diameter d1 of the unsolidified part at the start of reduction to 0.4 or more And found that it is possible to suppress the generation of zaku. In addition, by setting r1 / d1 in the steady casting part excluding the slab top part to 0.8 or more, it is possible to suppress the generation of center porosity and zaku in the center part of the slab in the steady casting part. I found out that there was.

This invention is made | formed based on this knowledge, The summary exists in the continuous casting method of the circular cross-section slab shown to following (1) and (2) .

(1) A method of continuously casting a slab having a cross section with a diameter of 300 mm or more using a vertical continuous casting machine, and after casting is completed, a slab having an unsolidified portion inside is vertically When rolling down from the bottom of the slab to the top by a rolling device that can move in the direction, the ratio d1 / D1 of the diameter d1 of the unsolidified portion and the diameter D1 of the slab at the start of rolling is 0.367 or less and then, in a state where no pressure at constant casting portion on the downstream side except for the lower than reduction completion position of the slab top portion shrinkage cavities are formed, reduction rate r1 and pressure at the start of the ratio of the unsolidified portion of diameter d1 A continuous casting method of a circular cross-section slab, wherein the value r1 / d1 is set to 0.8 or more and 1.5 or less.

(2) The circular cross-section casting according to (1) above , wherein r1 / d1 is set to 0.4 or more and 1.5 or less from a portion downstream of the reduction completion position in the slab top portion. Method for continuous casting of pieces.

  According to the continuous casting method of a circular cross-section slab of the present invention, even a slab having a large circular cross section can be obtained by adding a reduction device that can move in the vertical direction to a conventional vertical continuous casting machine. It is possible to reduce the slab when it is in a solidified state optimal for quality improvement. For this reason, a circular continuous cross-section with a reasonable continuous casting method compared to the ingot-making method, which suppresses the formation of center porosity and zaku at the center of the slab and the formation of shrinkage nest and zaku at the top of the slab. It is possible to manufacture the slab having a low equipment cost.

It is a block diagram which shows an example of a vertical type continuous casting machine provided with the reduction | decrease apparatus which can move to a perpendicular direction, The figure (a) is a figure which shows the middle of casting immediately after the start of casting, and the figure (b). It is a block diagram of the continuous casting machine at the time of rolling down a slab, The figure (a) is a figure just before a rolling start, the figure (b) is in the middle of rolling, and the figure (c) is a figure which shows the time of rolling completion. It is a figure which shows the relationship between the amount of rolling reduction, and the amount of internal defects, The figure (a) is a relationship between the amount of rolling reduction and the area ratio of the shrinkage | contraction nest of a top part, The figure (b) is a center of rolling reduction amount and a steady casting part. It is a figure which shows the relationship with a porosity.

1. 1. Configuration of continuous casting machine and casting method of slab 1-1. FIG. 1 is a block diagram showing an example of a vertical continuous casting machine equipped with a reduction device that is movable in the vertical direction. FIG. It is a figure which shows the middle of casting. As shown in the figure, molten steel is poured from a ladle 1 through an immersion nozzle 2 into a bottomless mold 3 installed on a cast floor. The casting mold 3 is subjected to vertical movement using an oscillation device (not shown). Injection of molten steel into the mold 3 may be performed through a tundish provided between the ladle 1 and the immersion nozzle 2.

  At the time of casting, molten steel is poured into the mold 3 whose bottom is closed by the dummy bar 4, the dummy bar 4 is lowered while the mold 3 is moved up and down, and the cast piece is pulled out below the mold 3 while being supported by the foot roll 5. Cast to a predetermined length and complete the casting. At this time, the casting speed is set in a range in which bulging is not generated in the slab by the molten steel static pressure. The slab when drawn from the mold 3 is in a state in which the solidified shell 6 is formed on the surface and the unsolidified molten steel 7 exists inside.

  Subsequently, when the slab reduction is completed, the slab is removed from the continuous casting machine to complete the continuous casting operation. The slab reduction will be described next.

1-2. Casting reduction method 1-2-1. As shown in FIG. 1, the continuous casting machine is provided with a reduction device 8 that is movable in the vertical direction. If the reduction device 8 is within a movable range, the reduction is performed on the slab. Can be applied. The reduction device 8 is provided with a frame 8a and a pair of drive-type reduction rollers 8b disposed in the frame 8a.

  The diameter of the reduction roll 8b is preferably 450 mm or more so that the internal permeability under reduction is a sufficient value and the angle of engagement with the slab is a value that enables stable reduction. Moreover, 1000 mm or less is preferable in order to ensure a sufficient amount of reduction.

The internal permeability under rolling is a value represented by the following formula (1).
η = (d1−d2) / (D1−D2) × 100 (1)
Here, η: internal permeability (%) under reduction, D1: diameter of slab before reduction (mm), D2: diameter of slab after reduction (mm), d1: diameter of unsolidified part before reduction (Mm), d2: thickness (mm) of the unsolidified portion after the reduction.

  The shape of the reduction roll 8b may be a crown shape or a caliber shape, but a flat roll is preferable in consideration of ensuring the reduction force and formability.

  The driving force for rotation of the rolling-down roll 8b is applied by an electric motor and a speed reducer (not shown) provided in the rolling-down device 8. The reduction force of the reduction roll 8 b is applied by a hydraulic cylinder (not shown) provided in the reduction device 8. The frame 8a of the reduction device 8 has rigidity that can withstand this reduction force.

  The vertical movement of the reduction device 8 in the vertical direction is performed by a jack (not shown) connected to an electric motor. You may carry out using the mechanism which suspends and rolls down the rolling-down apparatus 8 on a wire.

  The above-described vertical movement of the dummy bar 4 can be performed using an independent driving device. However, as shown in FIG. 1, using the reduction device 8 is preferable because it is not necessary to provide new equipment.

  The slab support is preferably as small as possible in consideration of the interference of the reduction device 8. FIG. 1 shows a case where only the foot roll 5 is disposed directly under the mold as a member for supporting the cast slab.

1-2-2. FIG. 2 is a configuration diagram of a continuous casting machine when the slab is crushed. FIG. 2 (a) shows the state immediately before the tumbling starts, FIG. 2 (b) shows the state of the crushed, and FIG. FIG.

  When the casting of the slab is completed, as shown in FIG. 5A, a molten steel outflow prevention pipe 9 having a circular or polygonal cross section is disposed as an outlet for the discharged molten steel at the upper part of the slab. . The reduction device 8 lifts the dummy bar 4 and then raises it to the reduction start position. When the slab is in a stationary state and solidification of the unsolidified molten steel 7 proceeds to a predetermined solidified state, the rolling roll 8b is rotated and the rolling device 8 is raised to reduce the slab by a predetermined rolling amount. To start. Control of the ascending speed of the rolling-down device 8 is performed in synchronization with the peripheral speed of rotation of the rolling-down roll 8b. While the reduction device 8 is raised and the slab is being reduced, the unsolidified molten steel 7 inside the slab is squeezed upward from the slab, and the discharged molten steel 10 is accommodated in the molten steel outflow prevention pipe 9.

  The solidification state of the slab at the start of reduction is to prevent the occurrence of internal cracks due to reduction, so that the solid-liquid interface defined by the isotherm with a solid phase ratio of 0.8 is the solidification interface, and the solidification state at the start of reduction The ratio d1 / D1 of the ratio of the diameter d1 of the part and the diameter D1 of the slab is preferably 0.3 or less.

  The reduction amount r1 of the slab is related to the diameter d1 of the unsolidified portion at the start of reduction, and the ratio r1 / d1 of the ratio between r1 and d1 at the slab top portion (final portion of drawing in the continuous casting machine) By setting so that it may become 0.4 or more, the shrinkage cavity and zaku in a slab top part can be suppressed. Moreover, in the steady casting part excluding the slab top part where the shrinkage cavity is formed when the slab is not squeezed, by setting r1 / d1 to 0.8 or more, the center part of the slab in the steady casting part The generation of center porosity and zaku can be suppressed.

  Thus, the appropriate value of r1 / d1 in the case where the shrinkage cavity or zaku is suppressed at the slab top portion and the case where the center porosity or zaku generation is suppressed in the steady casting portion excluding the slab top portion. The reason for the difference is presumed as follows. In other words, there are some equiaxed crystals that are squeezed from the bottom of the slab as it progresses under unsolidified pressure, and at the top of the slab where the accumulated distance of squeezing is long, voids are caused by the squeezed equiaxed crystals. It is presumed that because the filling is small, the generation of center porosity and zaku can be suppressed even if the amount of reduction is small and the value of r1 / d1 is small.

  For improving the quality of the slab, it is preferable that the amount of slab reduction is as large as possible. However, in order to increase r1 / d1 beyond 1.5, a very large reduction force is required, which increases equipment costs. R1 / d1 is 1.5 or less.

  The slab reduction is not necessarily performed from the lower end to the upper end of the slab, and the reduction target may be limited to the vicinity of the slab top portion as necessary.

2. Examination for scale-up In the examples described later, in order to confirm the effect of the present invention, the diameter D1 of the circular cross-section slab is 300 mm, the diameter d1 of the unsolidified portion at the start of rolling is 70 mm, and the steel type is A test was conducted for the case of 13% Cr steel shown in Table 1 described later. As a result, by setting r1 / d1 at the slab top part to 0.4 or more, generation of shrinkage cavities at the slab top part can be suppressed, and in the steady casting part excluding the slab top part. By setting r1 / d1 to be 0.8 or more, it was confirmed that the center porosity generation at the center portion of the slab in the steady casting portion was suppressed, and a slab having a good internal quality could be obtained.

  Here, the effect of suppressing internal defects according to the present invention when the diameter of the slab was larger than 300 mm (when scaled up) was examined by simulation using a three-dimensional finite element method.

2-1. First Simulation Deformation analysis was performed when a slab cast using the vertical continuous casting machine shown in FIG. 1 was rolled down with an unsolidified portion inside. Deformation analysis was performed when the diameter D1 of the slab was 300 mm, 600 mm, and 800 mm, and the behavior of the solidification interface during reduction was compared. At this time, in each case, the diameter d1 of the unsolidified portion at the start of the reduction was set so that the value d1 / D1 of the ratio between d1 and D1 was 70 / 300≈0.233.

  As a result, the internal permeability η under the reduction expressed by the above formula (1) depends on the diameter d1 of the unsolidified portion at the start of the reduction and the thickness (D1−d1) / 2 of the solidified shell, and It was found that if the value of d1 / D1 is constant, η is constant regardless of the diameter D1 of the slab. That is, it has been found that increasing the diameter of the slab does not affect the internal deformation efficiency of the slab if the value of d1 / D1 is constant.

  Therefore, if d1 / D1 is set to a predetermined value, the continuous casting method of the present invention can be applied to the case of D1> 300 mm including D1> 600 mm, as in the case of D1 = 300 mm. Thus, it was found that the production of internal defects can be suppressed and a slab having a good internal quality can be obtained.

2-2. Second Simulation A simulation similar to the first simulation was performed assuming a reduction of different steel types. Here, only the strength characteristics (yield strength and work hardening coefficient) of the material were set to various values different from those in the first simulation. As a result, it was found that η was constant regardless of the strength characteristics of the material. That is, it is considered that the relationship between the reduction amount and the amount of internal defects shown in FIG.

2-3. Third simulation A simulation similar to the first simulation was performed under the following conditions to estimate the rolling force.

  The slab was made to have a diameter of 800 mm, a length of a predetermined length, and a steel type of 13% Cr steel. The casting speed was 0.50 m / min, and the reduction roll included in the reduction device was 650 mm in diameter. The diameter d1 of the unsolidified portion at the start of slab reduction was 190 mm, the reduction amount r1 was 190 mm, and the surface temperature of the slab was 720 ° C. In this case, d1 / D1 = 0.238, which is almost the same value as 70/300. As a result of this simulation, the estimated necessary rolling force was 750 t.

  At this time, since r1 / d1 is 1.0, from the relationship between the amount of reduction shown in FIG. 3 described later and the amount of internal defects, by rolling down the steady cast part and the slab top part under these conditions, It can be predicted that the generation of shrinkage cavities in the top part and the generation of center porosity and zaku in the center part of the slab in the steady casting part can be suppressed almost completely.

  In order to confirm the effect of the continuous casting method of the circular cross-section slab of the present invention, the following tests were conducted and the results were evaluated.

1. Test Conditions A slab having a circular cross section with a diameter D1 of 300 mm and a length of 1800 mm was cast using the vertical continuous casting machine shown in FIG. The cast slab was brought into a stationary state after completion of casting, and when solidification progressed to a predetermined solidified state, the reduction device was raised to lower the slab from its lower end to 1300 mm above. The ascending speed of the reduction device is 0.8 m / min, and when the slab is reduced, the diameter d1 of the unsolidified portion becomes 70 mm (d1 / D1 = 0.233) and 110 mm (d1 / D1 = 0.367). Various reductions r1 were performed.

  The diameter d1 of the unsolidified portion was evaluated by an isotherm corresponding to a solid phase ratio of 0.8 calculated by unsteady one-dimensional heat transfer analysis. The accuracy of the heat transfer analysis was confirmed in advance by measuring the surface temperature of the slab, measuring the internal temperature with a thermocouple, and measuring the diameter of the unsolidified part by adding a tracer.

  The reduction roll used was a flat roll having a diameter of 450 mm, and the reduction force was a maximum of 100 t. The steel type of the slab was a 13% Cr steel having the components shown in Table 1 that is likely to generate zaku and porosity.

  The unsolidified molten steel inside the slab rises from the bottom of the slab and is discharged from the upper part due to the reduction of the part including the unsolidified part, but the discharged molten steel has been reduced without overflowing by the molten steel outflow prevention pipe. (See FIG. 2 above).

  The obtained slab was cut along a cross section passing through the center of the shaft, the cut surface was polished, and the state of the voids of the center porosity in the shrinkage nest of the slab top part and the steady casting part was investigated. In order to grasp the situation of the void, a photograph of the cut surface was taken, and image analysis was applied to this photograph to calculate the void area. Moreover, the area ratio of shrinkage nest and center porosity was calculated using this void area.

The test piece at the top of the slab was taken from the range from 300 mm to the downstream side of the slab from the rolling completion part of the slab. Test specimens for investigation of the center porosity in the steady casting part were collected from a range of 300 mm from the center in the longitudinal direction of the slab to the downstream side .

As the diameter of the slab increases, the portion affected by the shrinkage cavity at the top of the slab tends to be larger. Therefore, when the diameter of the slab is larger than 300 mm, the slab reaches the downstream of the slab it is preferred to collect the specimens in a range wider than the range of up to 300mm to the side.

  Here, the area ratio of the shrinkage cavity and the center porosity is a test piece in which the ratio r1 / d1 of the ratio of the reduction amount r1 of the slab to the diameter d1 of the unsolidified portion at the start of the reduction is 0, that is, reduction. It is the ratio of each area when the area of the shrinkage cavity or the center porosity of the slab test piece (non-rolled material) is 100%.

2. Test Results FIG. 3 is a diagram showing the relationship between the amount of reduction and the amount of internal defects. FIG. 3A shows the relationship between the amount of reduction and the area ratio of the shrinkage nest of the top portion, and FIG. 3B shows the amount of reduction. It is a figure which shows the relationship between the center porosity of a stationary casting part. In the figure, the horizontal axis is r1 / d1, and the vertical axis is the shrinkage nest area ratio in the figure (a), and the center porosity area ratio in the figure (b).

  From FIG. 3, the area ratio of the shrinkage nest and the area ratio of the center porosity hardly change when the amount of reduction is small, starts to decrease as the amount of reduction increases, and does not decrease further when reaching a certain amount of reduction. The result was obtained. The reason why there is almost no change when the amount of reduction is small is considered that there is almost no reduction effect.

  Further, from FIG. 3, when the reduction amount r1 is set so that r1 / d1 is 0.4 or more, the shrinkage nest of the slab top portion is about 5% of the non-reduction material. Recognize. Further, it can be seen that when the reduction amount r1 is set so that r1 / d1 is 0.8 or more and the reduction is performed, the center porosity of the steady cast part is 10 to 30% of the non-reduced material.

2. Product Production Test As an example of the present invention, reduction was performed with an unsolidified part under the condition that r1 / d1 was 0.8, and a slab having an area ratio of the center porosity of the steady cast part of about 30% was produced. Moreover, the slab which does not give reduction as a comparative example was produced. The diameter of each slab was 300 mm. Each slab was subjected to forging with a forging ratio of 2, and a test for producing a hollow tube by piercing and rolling was performed.

  In the comparative example, wrinkles due to porosity occurred on the inner surface of the hollow tube, whereas in the present invention example, there were no such inner surface wrinkles. Thus, by using the slab manufactured by the method of the present invention as a raw material, a product having good inner surface properties could be manufactured.

  According to the continuous casting method for a circular cross-section slab of the present invention, an in-line reduction method is applied to a conventional vertical continuous casting machine by adding a reduction device that can move in the vertical direction even for a slab having a large circular cross section. As a result, it is possible to reduce the slab when it is in a solidified state optimal for improving the quality of the slab. For this reason, a circular continuous cross-section with a reasonable continuous casting method compared to the ingot-making method, which suppresses the formation of center porosity and zaku at the center of the slab and the formation of shrinkage nest and zaku at the top of the slab. It is possible to manufacture the slab having a low equipment cost.

1: ladle, 2: immersion nozzle, 3: mold, 4: dummy bar, 5: foot roll,
6: Solidified shell, 7: Unsolidified molten steel, 8: Reduction device, 8a: Frame,
8b: Rolling roll, 9: Molten steel outflow prevention pipe, 10: Discharged molten steel

Claims (2)

A method of continuously casting a slab having a cross section with a diameter of 300 mm or more using a vertical continuous casting machine,
After the casting is completed, when the slab having an unsolidified portion inside is squeezed downward from the bottom of the slab by a reduction device that is movable in the vertical direction,
The value d1 / D1 of the ratio of the diameter d1 of the unsolidified portion at the start of rolling and the diameter D1 of the slab is set to 0.367 or less,
Of the slab top portion where the shrinkage cavity is formed without being reduced, the ratio r1 between the reduction amount r1 and the diameter d1 of the unsolidified portion at the start of reduction in the downstream steady casting portion excluding the lower portion from the reduction completion position. / D1 is 0.8 or more and 1.5 or less, The continuous casting method of the circular section slab characterized by the above-mentioned. 2. The continuous casting of a circular cross-section slab according to claim 1, wherein r1 / d1 is set to be 0.4 or more and 1.5 or less from a portion of the slab top portion downstream from the reduction completion position. Method.

Images