Continuous casting method of steel

Abstract

This invention provides a continuous casting method for steel that enables high-speed continuous casting even of extremely thick slabs using a vertical unsolidified continuous casting machine, while ensuring the internal quality of the resulting slabs and preventing surface cracking. The continuous casting method of this invention is a method for continuously casting steel slabs using a vertical unsolidified bending continuous casting machine, wherein an alternating current moving magnetic field moving along the width of the mold is applied to the molten steel in the mold using an electromagnetic stirring device within the mold, inducing swirling currents in the molten steel, and continuous casting is performed while stirring the molten steel. The travel speed of the alternating current moving magnetic field, calculated by equation (1) below, is 0.20–1.50 m / s. Here, in equation (1), U is the travel speed of the alternating current moving magnetic field (m / s), τ is the pole spacing of the coil of the electromagnetic stirring device within the mold (m), and f is the frequency (Hz) of the current applied to the coil of the electromagnetic stirring device within the mold. U=2τf………(1).

Description

Technical Field

[0001] This invention relates to a continuous casting method for steel slabs using a vertical unsolidified bending continuous casting machine, and more specifically, to a continuous casting method for steel in which an alternating moving magnetic field is applied to the molten steel in the mold to induce swirling flow in the molten steel while continuous casting is performed. Background Technology

[0002] In boiler steel plates, low-alloy steel plates for pressure vessels, high-strength steel plates for marine structures, and high-strength steel plates for industrial machinery, there are steel plates with a thickness exceeding 100mm used as important components (high-quality ultra-thick steel plates). For these high-quality ultra-thick steel plates, internal quality sometimes becomes an issue from a performance perspective. Therefore, the following manufacturing method has been used in the past to improve the internal quality of high-quality ultra-thick steel plates: manufacturing large ingots using the ingot casting method, and then manufacturing high-quality ultra-thick steel plates by rolling or forging these large ingots with a sufficient reduction ratio.

[0003] On the other hand, due to the low productivity of the aforementioned ingot casting method, a method for manufacturing so-called "ultra-thick slab castings" with thicker slabs using continuous casting was also explored. However, in ultra-thick slab castings obtained using continuous casting, center segregation, known as porosity, easily occurs in the center of the slab thickness. That is, when manufacturing high-quality ultra-thick steel plates from ultra-thick slab castings obtained by continuous casting, because a sufficient reduction ratio cannot be ensured, internal defects remain in the slab, sometimes causing problems with the internal quality of high-quality ultra-thick steel plates. Here, "porosity" refers to the formation of voids between grains, such as bubbles, where the grains are not tightly filled.

[0004] Furthermore, when continuously casting extremely thick slabs using the continuous casting method, extremely low-speed casting is generally performed due to limitations in the length of the continuous casting equipment and the need to prevent slab bulging. In low-speed casting of extremely thick slabs, the amount of molten steel injected into the mold per unit time is small, resulting in a decrease in the temperature of the molten steel at the surface of the molten steel inside the mold (hereinafter also referred to as the "curved surface"), causing the molten steel to solidify and easily forming a skin on the surface of the molten steel inside the mold. When such skin forms, internal defects occur in the extremely thick slabs due to the entrapment of protective slag introduced into the molten steel surface for lubrication and insulation purposes, as well as the introduction of the skin into the interior of the slab.

[0005] Patent Document 1 discloses a method for electromagnetically stirring molten steel in a mold during continuous casting of extremely thick slabs with a thickness of 400 mm or more, thereby imparting a swirling velocity to the molten steel near the meniscus. According to Patent Document 1, by imparting a swirling velocity to the molten steel near the meniscus, it is possible to prevent skin formation at the meniscus, suppress the growth of a solidified shell near the meniscus, and solve the problem caused by the temperature drop of the molten steel at the meniscus in the aforementioned mold.

[0006] Patent Document 2 discloses a method for continuously casting extremely thick slabs with a thickness of 380 mm or more using a vertical continuous casting machine at a casting speed of less than 0.2 m / min. The method involves setting the immersion nozzle at the center relative to the actual thickness of the slab, making the superheat of the molten steel in the tundish relative to the liquidus temperature 10°C to 50°C, and using an electromagnetic stirrer in the mold to stir the molten steel in the mold while continuously casting.

[0007] According to Patent Document 2, the aforementioned continuous casting method generates a large number of equiaxed crystal nuclei in the molten steel, resulting in finer grain size of the equiaxed crystals generated in the center of the extremely thick slab casting, suppressing the formation of porosity, thereby improving the toughness of the steel plate product. Furthermore, Patent Document 2 also discloses that when continuous casting is performed while stirring the molten steel in the mold using an in-mold electromagnetic stirrer, the effect of finer grain size of the equiaxed crystals is enhanced.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 11-277197

[0011] Patent Document 2: Japanese Patent Application Publication No. 2007-229736 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] In recent years, even for the aforementioned extremely thick slab castings, higher continuous casting speeds have been required to improve productivity.

[0014] However, in Patent Document 1, only an example is shown where the drawing speed of the slab is set to 0.25 m / min when the thickness of the extremely thick slab is 400 mm. In addition, regarding the conditions for electromagnetic stirring in the mold, only the method of implementing electromagnetic stirring with the swirling speed of molten steel near the meniscus being 0.2 to 0.4 m / s is described.

[0015] Patent Document 2 uses a vertical continuous casting machine. Due to the length of the casting equipment, the drawing speed of the slab in a vertical continuous casting machine must be slowed down compared to a vertical unsolidified bending continuous casting machine. Therefore, only an example of a drawing speed of 0.15–0.16 m / min is shown when the thickness of the extremely thick slab is 380 mm. Furthermore, the conditions for electromagnetic stirring within the mold in this case are not described.

[0016] Therefore, in the past, when using vertical unsolidified bending continuous casting machines to continuously cast ultra-thick slabs, no conditions were found for applying electromagnetic stirring within the mold to achieve higher casting speeds for ultra-thick slabs. Furthermore, the steel grades used for ultra-thick slabs include peritectic steels, which are prone to surface cracking. Therefore, increasing the casting speed can easily lead to uneven initial solidification within the mold, significantly increasing the risk of surface cracking in ultra-thick slabs.

[0017] In other words, regarding the quality of ultra-thick slab castings, the internal quality was previously the main consideration. However, with the increase in the casting speed of ultra-thick slab castings, it is necessary to set casting conditions that also take into account the prevention of surface cracks.

[0018] The present invention was made in view of the above circumstances, and its object is to provide a continuous casting method for steel that can be continuously cast at higher speeds even for extremely thick slabs using a vertical unsolidified bending type continuous casting machine, and that prevents surface cracking while ensuring the internal quality of the obtained slabs.

[0019] Methods for solving problems

[0020] The main idea of ​​the present invention for solving the above problems is as follows.

[0021] [1] A continuous casting method for steel, which is a continuous casting method for steel slabs cast using a vertical unsolidified bending type continuous casting machine, wherein,

[0022] An alternating magnetic field, moving along the width of the mold, is applied to the molten steel using an electromagnetic stirring device inside the mold. This induces swirling currents in the molten steel, and continuous casting is performed simultaneously with the stirring.

[0023] The speed of the above-mentioned alternating moving magnetic field, calculated by the following formula (1), is 0.20 to 1.50 m / s.

[0024] U=2τf………(1)

[0025] In equation (1), U is the speed of the alternating moving magnetic field (m / s), τ is the pole spacing of the coil of the electromagnetic stirring device in the mold (m), and f is the frequency of the current applied to the coil of the electromagnetic stirring device in the mold (Hz).

[0026] [2] According to the continuous casting method of steel described in [1] above, the frequency of the current applied to the coil of the electromagnetic stirring device in the mold is 0.2 to 1.0 Hz.

[0027] [3] According to the continuous casting method of steel described in [1] or [2] above, in a mold where the height direction of the mold is the center position of the coil of the electromagnetic stirring device in the mold and the thickness direction of the mold is 15 mm away from the inner surface of the long side of the mold, the effective value of the magnetic flux density of the AC moving magnetic field in the thickness direction is 0.008 T or more, calculated as the average value in the width direction of the mold.

[0028] [4] The continuous casting method of steel according to any one of [1] to [3] above, wherein the thickness of the slab casting is 360 mm or more and 540 mm or less.

[0029] [5] The continuous casting method of steel according to any one of [1] to [3] above, wherein the thickness of the slab casting is 400 mm or more and 500 mm or less.

[0030] [6] The continuous casting method of steel according to [4] or [5] above, wherein the drawing speed of the casting sheet is 0.3 to 0.8 m / min.

[0031] [7] The continuous casting method of steel according to any one of [1] to [6] above, wherein the average flow velocity of molten steel at the solidification interface of the slab casting at a position 50 mm below the molten steel surface in the mold in the casting direction is 0.08 to 0.3 m / s.

[0032] Invention Effects

[0033] According to the present invention, when continuously casting slab sheets using a vertical unsolidified bending type continuous casting machine, by appropriately determining the conditions of electromagnetic stirring in the mold, it is possible to continuously cast even extremely thick slab sheets with good internal quality and no surface cracks under casting conditions with higher sheet drawing speed. Attached Figure Description

[0034] Figure 1 The figure shown is an example of the numerical calculation results, which is the result of investigating the effect of the frequency of the current applied to the coil on the temperature distribution of molten steel in the mold. Detailed Implementation

[0035] The embodiments of the present invention will be described in detail below.

[0036] The continuous casting method of the present invention is a method for continuously casting slabs using a vertical unsolidified bending continuous casting machine. In this method, a pair of magnetic poles are arranged on the back side of the pair of long sides of a continuous casting mold, which has a pair of long sides and a pair of short sides, forming a rectangular internal space. These magnetic poles are positioned within a range covering the maximum width of the slab cast using the vertical unsolidified bending continuous casting machine. An alternating current moving magnetic field, generated from these magnetic poles and moving in the direction of the mold width, is applied to the molten steel within the mold, inducing swirling currents in the molten steel and performing continuous casting while simultaneously stirring the molten steel within the mold.

[0037] When an alternating current moving magnetic field is applied to the molten steel in the mold, the molten steel within the range of the alternating current moving magnetic field moves along the solidification interface of the long side of the casting in the direction of the moving magnetic field. By setting the moving directions of the alternating current moving magnetic field applied to a pair of opposing magnetic poles sandwiching a pair of long sides of the mold to opposite directions, the molten steel near the solidification interface of the opposing long sides of the casting moves in opposite directions in the width direction of the mold, thus inducing a swirling flow of molten steel rotating along the width direction of the mold within the mold. As a result, a stirred flow of molten steel with a flow velocity component rotating in the horizontal direction is formed in the molten steel within the mold.

[0038] Regarding the direction of movement of the alternating magnetic field, as long as the directions of movement of the alternating magnetic field applied from a pair of magnetic poles are opposite, the direction of movement of the magnetic field when viewed from directly above the mold can be either clockwise or counterclockwise; either direction is acceptable. The effect is the same regardless of the direction. It should be noted that the alternating magnetic field is applied from the same back side relative to the long side of the mold, with the same direction of movement.

[0039] Here, "vertical unsolidified bending type continuous casting machine" refers to a continuous casting machine in which the mold and the area several meters below the mold are vertical (vertical part), the lower part of the vertical part is bent into an arc shape (bending part), and then the casting sheet is pulled out in the horizontal direction (horizontal part). That is, it is a continuous casting machine that pulls the casting sheet from the vertical part to the bending part while the unsolidified phase exists inside the casting sheet.

[0040] The inventors investigated the flow of molten steel within the mold of extremely thick slabs with a thickness of 400 mm to 500 mm and a width of 1900 mm to 2450 mm in a continuous casting method that utilizes an alternating moving magnetic field as described above for controlling the flow of molten steel within the mold. Here, "extremely thick slab" refers to a slab with a thickness of 360 mm or more. The width of extremely thick slabs is typically about 1000 mm or more; however, when aiming for high-quality extremely thick steel plates, it is preferable to increase the mass per unit length of the extremely thick slab, in which case the width is 1600 mm or more.

[0041] In this investigation, numerical calculations were primarily used to repeatedly determine the velocity distribution of molten steel within the mold by varying the combination of the casting speed and the applied alternating current moving magnetic field. It should be noted that the conditions for the immersion nozzle used to inject molten steel from the tundish into the mold were set as follows: two rectangular discharge holes, each 65mm horizontally and 75mm vertically; the discharge angle of the discharge holes was 15°–25° downwards from the horizontal direction; and the immersion depth was 200mm. Here, "immersion depth of the immersion nozzle" refers to the length (distance) from the meniscus to the top of the discharge hole of the immersion nozzle.

[0042] The results showed that by continuously casting under the condition that the travel speed of the alternating moving magnetic field calculated by the following formula (1) meets the requirement of 0.20 to 1.50 m / s, even under casting conditions where the casting speed is set to 0.3 m / min or higher, high-quality, extremely thick slab castings with fewer defects can be obtained.

[0043] U=2τf………(1)

[0044] In equation (1), U is the speed of the alternating moving magnetic field (m / s), τ is the pole spacing of the coil of the electromagnetic stirring device in the mold (m), and f is the frequency of the current applied to the coil of the electromagnetic stirring device in the mold (Hz).

[0045] The pole spacing (pole pitch) τ of the coil of the electromagnetic stirring device in the mold is usually fixed and cannot be changed. Once the device is installed, it is fixed to a constant value. Therefore, in order to control the travel speed of the AC moving magnetic field calculated by the above equation (1) within the range of 0.20 to 1.50 m / s, the frequency of the current applied to the coil is adjusted in accordance with the pole spacing τ of the coil of the electromagnetic stirring device in the mold. For example, if the pole spacing τ of the coil is 700 mm, the travel speed U of the AC moving magnetic field calculated by equation (1) is 0.20 to 1.50 m / s by making the frequency of the current applied to the coil within the range of 0.143 Hz to 1.071 Hz. That is, when the pole spacing τ of the coil is 700 mm, if the frequency of the current applied to the coil is set within the range of 0.2 to 1.0 Hz, the travel speed U of the AC moving magnetic field calculated by equation (1) is within the range of 0.20 to 1.50 m / s.

[0046] When the traveling speed of the AC moving magnetic field calculated by equation (1) is less than 0.20 m / s, the traveling speed of the AC moving magnetic field is too slow to control the flow of molten steel in the mold. On the other hand, when the traveling speed of the AC moving magnetic field calculated by equation (1) exceeds 1.50 m / s, the horizontal swirling flow induced by the AC moving magnetic field in the molten steel is only near the inner surface of the mold (it is difficult to induce swirling flow in the molten steel near the center of the mold thickness). As a result, the temperature distribution of the molten steel at the surface of the molten steel in the mold becomes significant. That is, compared with the temperature of the molten steel near the center of the mold thickness, the temperature of the molten steel near the inner surface of the mold is lower, and the temperature difference of the molten steel at the surface of the molten steel in the mold becomes larger, which has an adverse effect on the quality of the slab casting. This is because the higher the frequency of the current applied to the coil of the electromagnetic stirring device in the mold, the more difficult it is for the AC moving magnetic field to penetrate towards the center of the mold thickness due to the skin effect.

[0047] Figure 1 An example of numerical calculation results is shown below. Figure 1 This study investigated the effect of the frequency of the current applied to the coil on the temperature distribution of molten steel at a position 2.5 mm from the long side surface of the mold when continuously casting an extremely thick slab with a thickness of 460 mm and a width of 2400 mm at a casting speed of 0.6 m / min. The pole spacing τ of the coil was 700 mm.

[0048] When the frequency of the current applied to the coil is 3.3 Hz, the travel speed of the alternating moving magnetic field calculated by equation (1) is 4.6 m / s, which does not meet the scope of this invention. At this time, as... Figure 1As shown, the difference between the maximum and minimum molten steel temperatures is 2.0℃. Furthermore, a region with lower molten steel temperatures is formed near the short side of the mold. This is believed to indicate that the swirling current caused by the alternating magnetic field does not reach the center of the mold thickness, where the immersion nozzle, which serves as the supply source for the high-temperature molten steel, exists; only the lower-temperature molten steel near the inner surface of the mold rotates due to the alternating magnetic field.

[0049] On the other hand, when the frequency of the current applied to the coil is 0.35 Hz, the travel speed of the moving magnetic field calculated by equation (1) is 0.49 m / s, which meets the scope of the present invention. At this time, as... Figure 1 As shown, the difference between the maximum and minimum molten steel temperatures is 1.6℃. Compared to when a current with a frequency of 3.3Hz is applied to the coil, the temperature difference is smaller, and the temperature distribution of the molten steel within the mold is more uniform. Furthermore, the low-temperature region observed when the current applied to the coil is 3.3Hz is absent; when the current applied to the coil is 0.35Hz, the molten steel temperature is high across most of the mold width. This is believed to be a result of the swirling current induced by the alternating moving magnetic field reaching the center of the mold thickness, allowing the high-temperature molten steel supplied from the immersion nozzle to be supplied throughout the mold. Therefore, in the continuous casting of extremely thick slabs, even with increased casting speed, initial solidification inhomogeneity within the mold is less likely to occur, reducing the risk of surface cracks in extremely thick slab castings.

[0050] It should be noted that, within a mold located at the center of the electromagnetic stirring device coil along its height and 15mm from the long side of the mold along its thickness, the effective value of the flux density of the alternating magnetic field along its thickness, calculated as the average value along its width, is preferably 0.008T or higher. At this location, if a flux density meeting the above conditions can be ensured, appropriate molten steel flow within the mold can be achieved through swirling currents induced by the alternating magnetic field in the molten steel. Furthermore, the stronger the flux density of the alternating magnetic field, the easier it is to induce swirling currents in the molten steel; therefore, an upper limit for the flux density is not required.

[0051] However, in order to increase the magnetic flux density, it is necessary to increase the current density applied to the coil. Considering the equipment cost for forming a device that can withstand high current density and the increase in power cost due to the application of high current, it is practically sufficient as long as the effective value of the magnetic flux density of the alternating moving magnetic field in the mold thickness direction is less than 0.030T, calculated as the average value in the mold width direction.

[0052] Furthermore, it is more preferable that the average flow velocity of molten steel at the solidification interface of the slab casting, located 50 mm below the molten steel surface in the mold in the casting direction, is 0.08 to 0.3 m / s. Here, the average flow velocity refers to the value obtained by spatially averaging the time average of the molten steel flow velocity at a location 50 mm below the molten steel surface in the mold in the casting direction and with a solid fraction fs = 0.5. This value can be obtained through numerical flow analysis that takes into account the solidification of molten steel. For example, it can be obtained by arithmetically averaging the magnitudes (magnitudes of the three-dimensional flow velocity vectors) of the time averages of each flow velocity in the computational grid at a solid fraction fs = 0.5 50 mm below the molten steel surface in the mold in the casting direction.

[0053] When the average flow velocity of molten steel at the solidification interface of the slab casting, located 50 mm below the molten steel surface in the mold, is slower than 0.08 m / s in the casting direction, non-metallic inclusions suspended in the molten steel are more easily captured by the solidified shell, increasing the risk of defects in the slab casting. On the other hand, when the average flow velocity of molten steel at the solidification interface of the slab casting, located 50 mm below the molten steel surface in the mold, exceeds 0.3 m / s in the casting direction, the molten steel flows at high speed, impacting the solidified shell, causing the solidified shell to re-dissolve, and increasing the risk of fracture during continuous casting.

[0054] Based on the above examples, the inventors added conditions to the casting thickness within the range of 360 mm or more and 540 mm or less, and performed numerical calculations, confirming the following trend.

[0055] The continuous casting method of the present invention is more suitable for extremely thick slabs with a thickness of 360 mm or more and 540 mm or less. When the thickness of the slab is less than 360 mm, the slab is thin, so even if the swirling current induced by the alternating moving magnetic field in the molten steel is only near the inner surface of the mold, it still has a stirring effect on the entire molten steel in the mold, and the effect obtained by applying the present invention is small. When the thickness of the slab exceeds 540 mm, in order to make the alternating moving magnetic field penetrate to the vicinity of the center in the thickness direction of the mold, the electromagnetic stirring device in the mold needs to be enlarged, which increases the equipment cost of the electromagnetic stirring device in the mold. It should be noted that the thickness of the slab to be continuously cast is more preferred when it is 400 mm or more and 500 mm or less.

[0056] Furthermore, when continuously casting extremely thick slabs with a thickness of 360 mm or more but less than 540 mm, the effect of this invention is even more significant when applied to continuous casting operations where the casting speed is set to 0.3 m / min to 0.8 m / min, and therefore it is preferred. According to this invention, in the continuous casting of extremely thick slabs, high-speed casting with a casting speed of 0.3 m / min or more, which is difficult to achieve using conventional vertical continuous casting machines, can be realized. It should be noted that in the continuous casting of extremely thick slabs, when the casting speed exceeds 0.8 m / min, it is necessary to extend the length of the continuous casting equipment and enhance the refining capacity of the molten steel supply process; therefore, in practical applications, a casting speed of 0.8 m / min or less is sufficient.

[0057] As explained above, according to the present invention, when continuously casting slab sheets using a vertical unsolidified bending type continuous casting machine, by appropriately determining the conditions of electromagnetic stirring in the mold, it is possible to continuously cast even extremely thick slab sheets with good internal quality and no surface cracks under casting conditions with higher sheet drawing speeds.

[0058] Example 1

[0059] This invention is applied when using a vertical unsolidified bending type continuous casting machine with a vertical section of 4.5m to continuously cast extremely thick slabs of carbon steel with a thickness of 410mm, a width of 1900mm, and a carbon content of 0.12% by mass at a casting speed of 0.8m / min.

[0060] The impregnation nozzle used is a double-hole type impregnation nozzle with rectangular discharge holes of 65mm horizontally and 75mm vertically on the left and right sides of the impregnation nozzle, respectively. The discharge angle (the angle relative to the horizontal direction) of the discharge hole is set to 15° downwards, and the impregnation depth is set to 200mm.

[0061] The magnetic pole spacing τ of the coil of the electromagnetic stirring device used in the mold is 700mm. In this electromagnetic stirring device, the effective value of the magnetic flux density of the alternating moving magnetic field in the mold thickness direction component, which is located at the center of the height direction of the coil of the electromagnetic stirring device and at a position 15mm away from the inner surface of the long side of the mold in the mold thickness direction, is 0.008T, calculated as the average value in the mold width direction.

[0062] In Example 1 of the present invention, the frequency f of the current applied to the coil of the electromagnetic stirring device in the mold is set to 0.4 Hz (the travel speed of the alternating moving magnetic field U = 0.56 m / s) for continuous casting.

[0063] In addition, for comparison, continuous casting was also carried out under the conditions of not applying current to the coil of the electromagnetic stirring device in the mold, i.e., not performing electromagnetic stirring (Comparative Example 1), and under the conditions of setting the frequency f of the current applied to the coil of the electromagnetic stirring device in the mold to 3.3 Hz (the travel speed of the alternating moving magnetic field U = 4.62 m / s) (Comparative Example 2).

[0064] Following continuous casting, the internal and surface quality of the manufactured ultra-thick slabs were investigated. Internal quality was assessed through hydrochloric acid corrosion testing of the ground slab cross-section and sulfur stamping to investigate center segregation, porosity, and internal cracks. Surface quality was determined by shot peening to remove oxide films and other defects from the slab surface, followed by penetrant testing to investigate longitudinal cracks, transverse cracks, and inclusion entrapment.

[0065] In Example 1 of this invention, no defects occurred in the internal or surface quality of the extremely thick slab casting. In contrast, in Comparative Example 1, center segregation and porosity occurred. In Comparative Example 2, the internal quality was reliable, but longitudinal cracks appeared on the surface of the casting.

[0066] Example 2

[0067] This invention is applied when using a vertical unsolidified bending type continuous casting machine with a vertical section of 4.5m to continuously cast extremely thick slabs of carbon steel with a thickness of 460mm, a width of 2200mm, and a carbon content of 0.16% by mass at a casting speed of 0.6m / min.

[0068] The impregnation nozzle used is a double-hole type impregnation nozzle with rectangular discharge holes of 65mm horizontally and 75mm vertically on the left and right sides of the impregnation nozzle, respectively. The discharge angle (the angle relative to the horizontal direction) of the discharge hole is set to 15° downwards, and the impregnation depth is set to 200mm.

[0069] The magnetic pole spacing τ of the coil of the electromagnetic stirring device used in the mold is 700mm. In this electromagnetic stirring device, the effective value of the magnetic flux density of the alternating moving magnetic field in the mold thickness direction component, which is located at the center of the height direction of the coil of the electromagnetic stirring device and at a position 15mm away from the inner surface of the long side of the mold in the mold thickness direction, is 0.008T, calculated as the average value in the mold width direction.

[0070] In Example 2 of the present invention, the frequency f of the current applied to the coil of the electromagnetic stirring device in the mold is set to 0.4 Hz (the travel speed of the alternating moving magnetic field U = 0.56 m / s) for continuous casting.

[0071] In addition, for comparison, continuous casting was also carried out under the condition that the frequency f of the current applied to the coil of the electromagnetic stirring device in the mold was set to 3.3 Hz (the travel speed of the alternating moving magnetic field U = 4.62 m / s) (Comparative Example 3).

[0072] Following continuous casting, the internal and surface qualities of the manufactured ultra-thick slabs were investigated. For internal quality, center segregation, porosity, and internal cracks were investigated through hydrochloric acid corrosion tests and sulfur printing on the ground slab cross-sections. Regarding surface quality, the oxide film and other defects on the slab surface were removed by shot peening, and then longitudinal cracks, transverse cracks, and inclusion entrapment were investigated through penetrant testing.

[0073] In Example 2 of this invention, no defects occurred in either the internal or surface quality of the extremely thick slab casting. In contrast, in Comparative Example 3, the internal quality was reliable, but inclusions were entangled on the surface of the casting.

Claims

1. A continuous casting method for steel, comprising a method for continuously casting steel slabs using a vertical unsolidified bending continuous casting machine, wherein, An alternating magnetic field, moving along the width of the mold, is applied to the molten steel using an electromagnetic stirring device inside the mold. This induces swirling currents in the molten steel, and continuous casting occurs simultaneously with the stirring of the steel. The travel speed of the alternating moving magnetic field, calculated by equation (1) below, is 0.20–1.50 m / s. The frequency of the current applied to the coil of the electromagnetic stirring device inside the mold is 0.2–1.0 Hz. Within a mold located at the center of the electromagnetic stirring device coil along its height and 15 mm from the inner surface of the long side of the mold along its thickness, the effective value of the magnetic flux density of the alternating moving magnetic field along its thickness direction, calculated as the average value along the mold width direction, is 0.008 T or more and 0.030 T or less. U=2τf ………(1) In equation (1), U is the speed of the alternating moving magnetic field (m / s), τ is the pole spacing of the coil of the electromagnetic stirring device in the mold (m), and f is the frequency of the current applied to the coil of the electromagnetic stirring device in the mold (Hz).

2. The continuous casting method for steel according to claim 1, wherein, The thickness of the continuously cast slab is 360 mm or more and 540 mm or less.

3. The continuous casting method for steel according to claim 1, wherein, The thickness of the continuously cast slab is 400mm or more and 500mm or less.

4. The continuous casting method for steel according to claim 2 or claim 3, wherein, The drawing speed of the cast sheet is 0.3 to 0.8 m / min.

5. The continuous casting method for steel according to any one of claims 1 to 3, wherein, In the casting direction, the average flow velocity of molten steel at the solidification interface of the slab casting 50 mm below the molten steel surface in the mold is 0.08–0.3 m / s.

6. The continuous casting method for steel according to claim 4, wherein, In the casting direction, the average flow velocity of molten steel at the solidification interface of the slab casting 50 mm below the molten steel surface in the mold is 0.08–0.3 m / s.

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