Continuous Casting Method

Abstract

To provide a continuous casting method which can suppress longitudinal cracks, bleeds and breakouts in the vicinity of corners.SOLUTION: Provided is a continuous casting method in which, while molten steel is poured into a mold for continuous casting, a solidified shell obtained by solidifying the molten steel is pulled out to produce a slab with a local heat flux ratio between a local heat flux qn on a short side mold side at positions in which casting direction distances from a meniscus in the mold are the same and which are separated by a prescribed distance from one corner and a local heat flux qw on a long side mold side at positions separated by a prescribed distance from the one corner R=qn / qw being controlled to a range of 0.7 or more to 1.4 or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a continuous casting method for preventing cracking of a cast piece and breakouts caused by the cracking by regulating the local heat flux of a copper plate of a mold, and more particularly to a continuous casting method for medium carbon steel. [Background technology]

[0002] In continuous casting of steel, molten steel poured into a mold is cooled by a water-cooled mold, and the molten steel solidifies at the contact surface with the mold to form a solidified layer (called a "solidified shell"). This solidified shell is continuously drawn down the mold together with the unsolidified layer inside while being cooled by water sprays or air-water sprays installed downstream of the mold. Then, the steel solidifies to the center by cooling with the water sprays or air-water sprays to produce a cast piece.

[0003] When cooling in the mold becomes uneven, the thickness of the solidified shell becomes uneven in the casting direction, the width direction of the slab, and the thickness direction of the slab. Stress due to the shrinkage and deformation of the solidified shell acts on the solidified shell, and in the early stage of solidification, this stress is concentrated in the thin wall part of the solidified shell. This stress causes cracks to form on the surface of the solidified shell. These cracks expand due to external forces such as thermal stress and bending and straightening stress caused by the rolls of the continuous casting machine, and become large surface cracks. When the unevenness of the solidified shell thickness is large, vertical cracks may form in the mold, and breakouts may occur, in which molten steel flows out from the vertical cracks. These vertical cracks may also form near the mold corners, and in such cases, it is known that the possibility of not only deteriorating the slab quality but also causing operational troubles such as bleeding and breakouts is further increased.

[0004] It is known that vertical cracks caused by this non-uniform solidification are likely to occur in the sub-peritectic region of steels with a carbon content in the range of 0.08 to 0.17 mass% that undergo a peritectic reaction. In order to prevent surface cracks in the above-mentioned steel types that undergo a peritectic reaction (referred to as "medium carbon steel"), attempts have been made to slowly cool the solidified shell by using mold powder with a composition that is easily crystallized or a mold that promotes slow cooling, as proposed in Patent Documents 1, 2, and 3, for example.

[0005] Patent Document 4 discloses a continuous casting method for medium carbon steel that prevents cracking of a cast piece or breakouts caused by cracking by monitoring the heat flux of a mold copper plate. The taper rate of the mold copper plate on the short side is increased or decreased by using the time change in the heat flux ratio between the short side heat flux and the long side heat flux to make the heat flux ratio appropriate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2005-297001 A [Patent Document 2] Japanese Patent Application Publication No. 06-304719 [Patent Document 3] Japanese Patent Application Publication No. 09-276994 [Patent Document 4] JP 2009-090309 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques have the following problems. That is, in the techniques disclosed in Patent Documents 1 to 3, although the slow cooling of the solidified shell is effective in suppressing vertical cracks other than near the corners, the effect on vertical cracks near the corners is small. Therefore, there is a problem in that vertical cracks, bleed, and breakout cannot be sufficiently suppressed.

[0008] Furthermore, in the technology disclosed in Patent Document 4, the heat flux measured is a surface-averaged heat flux obtained from the heat extraction of the cooling water, and therefore the local heat flux near the corners is not directly measured, which poses a problem that it is not possible to accurately suppress vertical cracks near the corners.

[0009] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to propose a continuous casting method capable of suppressing vertical cracks, bleeding, and breakouts in the vicinity of corners during continuous casting. [Means for solving the problem]

[0010] The continuous casting method of the present invention, which advantageously solves the above-mentioned problems, is characterized in that when injecting molten steel into a continuous casting mold and withdrawing a solidified shell in which the molten steel has solidified to produce a cast piece, the local heat flux ratio R=qn / qw between a local heat flux qn on a short side mold side at a position a predetermined distance away from one corner, at the same casting direction distance from the meniscus in the mold, and a local heat flux qw on a long side mold side at a position a predetermined distance away from one corner, is in the range of 0.7 or more and 1.4 or less.

[0011] The continuous casting method according to the present invention is as follows: (a) the positions at which the local heat fluxes on the narrow side mold side and the wide side mold side are measured are away from the corners by a distance of 5 mm to 20 mm; (b) when casting a mold powder on the surface of the molten steel poured into the mold, the mold powder contains CaO, SiO2, Al2O3, Na2O and Li2O, and has a basicity represented by a ratio of a CaO concentration to a SiO2 concentration in the mold powder (mass%CaO / mass%SiO2) of 1.0 or more and 2.5 or less; (c) Continuously casting medium carbon steel containing 0.05 to 0.22 mass% C; This is thought to be a more preferable solution. Effect of the Invention

[0012] INDUSTRIAL APPLICABILITY According to the continuous casting method of the present invention, since it is possible to suppress longitudinal cracks, bleeding and breakouts near the corners, it is possible to prevent quality problems of the cast slab and operational problems, and it is industrially useful. [Brief description of the drawings]

[0013] [Figure 1] 1 is a graph showing the effect of the distance in the casting direction from the meniscus of a heat flux measurement point on the relationship between the local heat flux ratio between the short side and long side near a corner of a continuous casting mold and the number of vertical cracks at the corresponding slab corner. [Diagram 2] This is a graph showing the influence of the distance from the corner of a continuous casting mold to the short side and long side of the heat flux measurement point on the relationship between the local heat flux ratio between the short side and long side near the corner of the mold and the number of vertical cracks at the corresponding slab corner. [Diagram 3] 1 is a graph showing the relationship between the number of vertical cracks at a slab corner and the basicity of a mold powder. [Figure 4] 1 is a graph showing the number of vertical cracks at slab corners under each test condition in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The following is a detailed description of the embodiments of the present invention. The following embodiments are merely examples of equipment and methods for embodying the technical idea of ​​the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0015] The inventors conducted extensive research, believing that unevenness in the solidified shell near the corners could be eliminated by keeping the ratio of local heat fluxes between the short side and long side of the mold corner within a specified range.

[0016] First, in the mold copper plate during continuous casting, the local heat flux qw(5) at a position 5 mm away from the corner on the long side and the local heat flux qn(5) at a position 5 mm away from the corner on the short side were measured at positions L = 50 mm, 250 mm, and 500 mm (downward) from the meniscus in the casting direction. The local heat flux ratio R(5, 5) = qn(5) / qw(5) and the number density (m -2 ) are shown in Figures 1(a) to 1(c), respectively. Figure 1(a) shows the case where L = 50 mm in the casting direction from the meniscus, Figure 1(b) shows the case where L = 250 mm in the casting direction from the meniscus, and Figure 1(c) shows the case where L = 500 mm in the casting direction from the meniscus.

[0017] The local heat flow rate was calculated using the temperature measurement value of a thermocouple embedded in a copper plate. When the local heat flux ratio R(5, 5) is smaller than 0.7, the local heat flux on the short side becomes too small compared to the long side, and the shell thickness on the long side becomes thicker than that on the short side. Therefore, the solidified shell on the short side near the corner is pulled toward the long side, and the gap between the solidified shell and the mold on the short side near the corner becomes large. As a result, solidification delay occurs in the solidified shell on the short side near the corner, and it is considered that vertical cracks are likely to occur. On the other hand, when the local heat flux ratio R(5, 5) is larger than 1.4, the local heat flux on the short side becomes too large compared to the long side, and the solidified shell thickness on the short side becomes larger than that on the long side. Therefore, the solidified shell on the long side near the corner is pulled toward the short side, and the gap between the solidified shell and the mold on the long side near the corner becomes large. As a result, solidification delay occurs in the solidified shell on the long side near the corner, and it is considered that vertical cracks are likely to occur. Therefore, by setting the local heat flux ratio R(5, 5) in the range of 0.7 to 1.4, continuous casting can be performed without increasing the difference in solidified shell thickness between the long side and short side near the corner, and the occurrence of vertical cracks can be suppressed.

[0018] Next, the heat flux ratio R(n,w) = qn(n) / qw(w) was measured at positions with the same corner, n, w = 5 mm, 20 mm, 50 mm, and 100 mm away from the corner on the short side and long side, respectively, at a position L = 50 mm (downward) from the meniscus in the casting direction. n and w represent the distances (mm) from the corner on the short side and long side, qn(n) represents the local heat flux at a position n away on the short side, and qw(w) represents the local heat flux at a position w away on the long side. The local heat flux ratio R(n,w) and the vertical crack number density (m -2 ) and the results are shown in Figure 2.

[0019] The local heat flux ratio R(n, w) at positions n, w = 50 mm and 100 mm away from the corner on the short side and long side, respectively, was in the range of 0.95 to 1.05, and it was found that there was a weak correlation with the vertical crack density at the slab corner. In other words, when the vertical crack density at the slab corner was 0.2 m -2 In some cases, it may be as small as 1.0m -2 In some cases, the cracks may occur more frequently than 0.2m. It was also found that the local heat flux ratio R(n, w) at positions w, n = 5mm and 20mm away from the corner on the short side and long side, respectively, shows a strong correlation with the number density of vertical cracks at the slab corner. In other words, when the local heat flux ratio R = qn / qw is in the range of 0.7 to 1.4, the number density of vertical cracks at the slab corner is 0.2m -2 On the other hand, outside that range, the density of vertical cracks in the slab corners is 0.8m -2In addition, when w and n are less than 5 mm from the corner to the short side and long side, respectively, the measurement is performed near the corner, and it is impossible to insert a thermocouple into the copper plate of the mold, so the measurement cannot be performed. Therefore, the position for measuring the local heat flux ratio R(n, w) is set to the short side mold side at a predetermined distance from one corner and the long side mold side at a predetermined distance from the corner, which are at the same casting direction distance from the meniscus in the mold, and the local heat flux ratio R=qn / qw is set in the range of 0.7 to 1.4, so that the number density of vertical cracks in the slab corner can be reduced. Preferably, the position for measuring the local heat flux is set to be 5 mm to 20 mm away from the corner on the short side mold side and the long side mold side. The local heat flux ratio R(n, w) can be appropriately selected by changing the cooling conditions on the short side mold side and the long side mold side, for example, by changing the taper of the short side of the mold or changing the balance of the amount of cooling water between the long side and the short side of the mold. Furthermore, by changing the waveform and frequency of the vibration of the mold that is vibrated in the casting direction, the consumption of mold powder can be changed, and the cooling conditions of the slab in the mold can be changed.

[0020] Furthermore, the chemical composition of the mold powder that influences the initial solidification in the mold was examined. The mold powder used in this embodiment contains CaO, SiO2, Al2O3, Na2O, and Li2O, and has a basicity CaO / SiO2 in the range of 0.7 to 2.9, which is expressed as the ratio of the CaO concentration to the SiO2 concentration in the mold powder (mass%CaO / mass%SiO2). The number density (m -2) and the results are shown in Figure 3. A mold powder basicity of 1.0 or more is preferable because it can suppress the difference in solidified shell thickness near the corners by slow cooling in the mold. If the mold powder basicity CaO / SiO2 exceeds 2.5, the corner vertical crack count density is low, but slow cooling in the mold is promoted too much, resulting in insufficient solidified shell thickness at the mold exit side. This may increase the occurrence rate of breakouts directly below the mold. Therefore, it is preferable to set the basicity, expressed as the ratio of the CaO concentration to the SiO2 concentration in the mold powder (mass%CaO / mass%SiO2), to 1.0 or more and 2.5 or less.

[0021] The continuous casting method of the present embodiment is preferably applied to continuous casting of medium carbon steel with a C content of 0.05 to 0.22% by mass, which is prone to vertical cracks due to solidification shrinkage, etc., to reduce vertical cracks at slab corners, and is particularly preferably used for continuous casting of medium carbon steel with a C content of about 0.08 to 0.17% by mass, which is in the sub-peritectic region accompanied by a peritectic reaction. EXAMPLES

[0022] 300 tons of molten steel in a ladle, which had been subjected to oxygen blowing in a converter and RH vacuum degassing treatment, was continuously cast under the conditions shown in Table 1. The local heat flux R(n, w) was obtained from the temperature measured by a thermocouple at a position w (mm) away on the long side mold side and at a position n (mm) away on the short side mold side from the same corner, as shown in Table 1. The casting direction position from the meniscus where the local heat flux was measured was L = 50 mm. Casting was performed under the conditions of mold powder basicity CaO / SiO2 shown in Table 1. The C content of the molten steel was 0.20 mass%. In addition, as a conventional example (Test No. 1), continuous casting was performed under the conditions of R(5, 5) = 0.6 and mold powder basicity CaO / SiO2 = 0.90. Continuous casting was performed under the conditions of the invention examples (Test Nos. 2 to 13), the comparative examples (Test Nos. 14 to 17), and the conventional example (Test No. 1) shown in Table 1, and the number density of vertical cracks (m -2) are shown in Figure 4. As is clear from Figure 4, it was found that the invention example was able to significantly reduce the number and density of cracks on the slab surface.

[0023] [Table 1] [Industrial Applicability]

[0024] According to the continuous casting method of the present invention, it is possible to suppress longitudinal cracks, bleeding and breakouts near the corners, and therefore it is possible to prevent quality problems and operational problems in the cast slab, which is industrially useful.

Claims

1. When molten steel is poured into a mold for continuous casting and a solidified shell in which the molten steel is solidified is extracted to produce a cast piece, A continuous casting method, wherein a local heat flux ratio R = qn / qw between a local heat flux qn on a short side mold side at a position a predetermined distance away from one corner, the local heat flux qw on a long side mold side at a position a predetermined distance away from one corner, the local heat flux being at the same position in the casting direction from the meniscus in the mold, is in the range of 0.7 to 1.

4.

2. 2. The continuous casting method according to claim 1, wherein the positions at which the local heat fluxes on the narrow side mold side and the wide side mold side are measured are separated from the corners by a distance of 5 mm to 20 mm.

3. When pouring mold powder onto the surface of the molten steel poured into the mold, the mold powder contains CaO, SiO ２ , Al ２ O ３ , Na ２ O and Li ２ O, and the concentration of CaO and SiO ２ Ratio of concentration (mass% CaO / mass% SiO ２ 2. The continuous casting method according to claim 1, wherein the basicity represented by the formula (1) is 1.0 or more and 2.5 or less.

4. The continuous casting method according to claim 1, wherein medium carbon steel containing C: 0.05 to 0.22 mass % is continuously cast.

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