Continuous casting method for steel

Abstract

To provide a steel continuous casting method capable of securely preventing the transversal cracks on the surface of a slab produced by continuous casting and producing a high quality slab also achieving the prevention of the inner cracks of a mold.SOLUTION: In a steel continuous casting method, a mold satisfying both of the following inequalities (1) and (2) is used in which the side length of a chamfered part in the long side of the mold is defined as a (mm), and the side length of the chamfered part in the short side of the mold is defined as b (mm), and the average secondary cooling water volume density applied to a slab corner part from directly below the mold to a bending end in the case of a vertical bending continuous casting machine, and the average secondary cooling water volume density applied to a slab corner part from directly below the mold to a point of 5 m in the case of a curved type continuous casting machine are controlled to a range of 20 to 280 (L / min / m2): 10≤a≤35(1) and 1≤b / a≤3(2).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a continuous casting method for steel, and more particularly to a method for forming chamfered (beveled) portions at the corners of the cast slabs to prevent the occurrence of transverse cracks on the surface of the cast slab corners. [Background technology]

[0002] In the continuous casting process of steel, when casting is performed using a vertical bending die or a curved die continuous casting machine, transverse cracks may occur on the surface of the cast slab. These transverse cracks become defects in subsequent processes and must be removed through finishing, but this reduces yield and causes heat loss, so it is desirable to suppress transverse cracks.

[0003] Generally, transverse cracks on the surface of cast slabs are caused by tensile strain inevitably applied at the bending or straightening points of a continuous casting machine, and are more likely to occur when the surface temperature of the cast slab passing through the bending or straightening point is within the temperature range where ductility decreases (the so-called third embrittlement temperature range). In particular, the corners of the cast slab are cooled from both the long and short sides, so they inevitably tend to be colder than other parts of the cast slab. For this reason, transverse cracks are known to occur easily at the corners of cast slabs.

[0004] A commonly used method for preventing transverse cracking is controlling the slab cooling conditions during casting. This method prevents transverse cracking by controlling the cooling conditions to keep the slab surface temperature at bending and straightening points outside the embrittlement temperature range.

[0005] In recent years, many types of steel have become highly susceptible to cracking. For example, in steel types containing high concentrations of alloying elements such as Al and Nb added for mechanical properties, these elements tend to precipitate as carbides or nitrides at grain boundaries, and the concentration of strain in these precipitates increases crack susceptibility. In such steel types with a wide brittleness temperature range, even with controlled cooling conditions, the surface temperature of the cast slab at the corners, bends, or straightens falls within the brittleness temperature range, leading to transverse surface cracking at the corners and posing a problem.

[0006] According to Patent Document 1, by first lowering the surface temperature of the cast slab to a temperature below the Ar3 transformation point, and then restoring it to a temperature above the Ar3 transformation point, the solidification structure of the cast slab becomes a mixed structure of ferrite and pearlite with indistinct γ grain boundaries, thereby reducing the crack susceptibility of the cast slab and preventing the occurrence of cracks.

[0007] In continuous casting, a method is known for forming chamfered edges on the corners of the cast slabs within the mold. Protruding sections are provided in the continuous casting mold to correspond to the chamfered shape of the cast slab.

[0008] One effective measure to prevent the surface temperature of the cast slab around the corners from rising outside the embrittlement temperature range in steel types with a wide embrittlement temperature range is to form a chamfer on the corner of the cast slab. This method mitigates two-sided cooling by chamfering the corners of conventional rectangular cast slabs, increases the surface temperature of the cast slab in the bent and straightened sections, and prevents lateral cracking at the corners.

[0009] Patent Document 2 describes the chamfer shape of the mold corner, where C is the corner chamfer amount, L is the length of the short side of the cast slab, the C / L ratio is 9-20%, and the average secondary cooling water density from directly below the mold to the lower straightening is 20-60 (L / min / m³). 2 This method is said to prevent surface cracking of the cast slab. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2002-307149 [Patent Document 2] Patent No. 6954514 [Non-patent literature]

[0011] [Non-Patent Document 1] Iron and Steel, vol. 93 (2007), No. 9, p. 566 [Non-Patent Document 2] "Improving the Surface Quality of Cast Slabs by Electromagnetic Stirring in Molds," Junji Nakajima et al., Nippon Steel Technical Report No. 376. [Overview of the project] [Problems that the invention aims to solve]

[0012] However, when continuously casting steel types with high susceptibility to transverse cracking, it has been found that even when using the proposed method in Patent Document 2 to chamfer the cast slab, it is difficult to avoid transverse cracking. For example, in a vertical bending type continuous casting machine, the shape of the chamfered portion is C / L = 0.125 and the average secondary cooling water density from directly below the mold to the lower straightening is 54 (L / min / m³). 2 When test casting was performed using the casting conditions described in Patent Document 2 (Level 16 in Table 3 below), numerous transverse cracks occurred on the lower surface of the cast slab. This is presumed to be because the strong cooling at the top of the continuous casting machine lowers the surface temperature of the cast slab at the bending point, causing the surface temperature of the cast slab near the corner to be within the embrittlement temperature range. Thus, it was found that the method of forming a chamfer at the corner of the cast slab and defining the shape of the chamfer and the average amount of water from directly below the mold to the straightening, as described in Patent Document 2, is insufficient.

[0013] Internal cracking is known as one of the internal defects in continuously cast slabs. Internal cracking caused by bulging below the mold is well known. In addition, internal cracking can occur near the surface of the slab solidifying within the mold, particularly near the corners of the slab. Here, such internal cracking is referred to as "in-mold internal cracking."

[0014] The present invention aims to provide a continuous casting method for steel that can reliably prevent transverse cracking on the surface of the cast slab produced by continuous casting, while also preventing internal cracking within the mold, thereby producing high-quality cast slabs. [Means for solving the problem]

[0015] That is, the gist of the present invention is as follows. [1] In the continuous casting of steel using a vertical bending continuous caster, The cross-section at the lower end of the mold in the casting space through which the slab passes through the mold is called the casting cross-section. The shape of the casting cross-section is a rectangular shape with chamfered portions at the four corners of the rectangle. Let the side length of the chamfered portion on the long side of the rectangular shape of the mold be a (mm), and the side length of the chamfered portion on the short side of the rectangular shape of the mold be b (mm). Use a mold whose chamfered portion size satisfies both of the following formulas (1) and (2). From directly below the mold to the completion of bending of the vertical bending continuous caster, the , the average secondary cooling water density from the long-side cooling device and the short-side cooling device Average secondary cooling water density applied to the slab corner Total is 、100 ~280 (L / min / m 2 ) within the range, From the completion of bending to the completion of straightening of the vertical bending continuous caster Secondary cooling is performed using only the secondary cooling device on the long side. , the average secondary cooling water density applied to the slab corner by the long side cooling device is 50 (L / min / m 2 ) or less. A method for continuous casting of steel, characterized in that. 10 ≦ a ≦ 35 ·····(1) 1 ≦ b / a ≦ 3 ·····(2) [2] A method for continuous casting of steel according to [1], characterized in that the horizontal flow velocity of molten steel in the mold at the solidification interface is controlled to be in the range of 5 to 40 (cm / s) by an electromagnetic stirring device.

Effect of the Invention

[0016] It is possible to surely prevent transverse cracks on the surface of the slab in the slab produced by continuous casting and to create a high-quality slab without internal cracks in the mold of the slab.

Brief Description of the Drawings

[0017] [Figure 1] It is a figure showing the relationship between the shape of the mold cross-section and the shape of the slab cross-section. (A) is a side cross-sectional view of the mold as viewed from the A-A arrow, (B) is a plan cross-sectional view of the mold as viewed from the B-B arrow, (C) is the casting cross-section, and (D) is a figure showing the slab cross-section. [Figure 2]This is a partial cross-sectional view showing the secondary cooling process near the corner of the cast slab. [Figure 3] This is a partial cross-sectional view of a cast slab illustrating a method for evaluating the degree of non-uniform solidification within the mold. [Modes for carrying out the invention]

[0018] In continuous casting, the mold 1 is typically formed by combining one pair of long sides 5 and one pair of short sides 6, as shown in Figures 1(A) and 1(B). The space enclosed by the long sides 5 and short sides 6 becomes the casting space 3. In the present invention, as shown in Figure 1, the cross section at the lower end 9 of the mold in the casting space 3 (Figures 1(A) and 1(B)) through which the cast slab 10 passes is called the casting cross section 4 (Figure 1(C)). The shape of the casting cross section 4 is rectangular, and chamfered portions 7 are provided at the four corners of the rectangle 11. As a result of providing chamfered portions 7 in the casting cross section 4, the cast slab 10 cast using the mold 1 has chamfered portions 7 formed at the four corners of the rectangle in the cast slab cross section 8 perpendicular to the casting direction (Figure 1(D)).

[0019] When continuously casting steel types with high susceptibility to transverse cracking, it was found that in order to prevent transverse cracking by chamfering, not only is it important to specify the size and shape of the chamfered portion 7, but the cooling conditions in the secondary cooling zone below the bottom of the mold are also crucial. In particular, regarding the cooling conditions, it was found that it is necessary to precisely set the amount of cooling water at each point up to the bending point and straightening point, rather than using the average amount of cooling water in the continuous casting machine.

[0020] In the following, as shown in Figure 2, the two new corner sections created by the chamfered section 7 for each corner section will be referred to as the first corner section (long side chamfered end 16) on the longer side and the second corner section (short side chamfered end 17) on the shorter side. Let a be the distance from the vertex 15 of the corner section 14 to the long side chamfered end 16 (hereinafter also referred to as the "long side length"), and let b be the distance from the vertex 15 of the corner section 14 to the short side chamfered end 17 (hereinafter also referred to as the "short side length").

[0021] In the present invention, in which the cast slab 10 has a chamfered portion 7 at the corner (see Figure 1(D)), the definition of the secondary cooling water density near the corner of the slab will be explained. Figure 2 shows the secondary cooling near the corner of the slab 10. On the long side 12, cooling sprays 20 are arranged as long side cooling devices 22, and cooling water jets 21 are formed from each cooling spray 20 toward the long side 12. On the short side 13, cooling sprays 20 are arranged as short side cooling devices 23, and cooling water jets 21 are formed from each cooling spray 20 toward the short side 13.

[0022] Cooling water volume density (L / min / m³) near the slab corner by the long-side cooling device 22 2 ) is the average water density applied to the area within 30 mm from the first corner (end of the long side chamfered section 16) on the long side surface (long side water density evaluation area 18). Cooling water density near the slab corner by the short side cooling device 23 (L / min / m³ 2 ) is the average water density applied to the area within 30 mm from the second corner (short-side chamfered end 17) on the short side surface (short-side water density evaluation area 19). When the cooling water density is constant in the width direction of the cast slab on the long side surface, and when the cooling water density is constant in the thickness direction of the cast slab on the short side surface, the values ​​are the same as the water density according to the usual definition. Note that the increase in spray injection area area due to chamfering is not taken into consideration. The sum of the cooling water density near the cast slab corner by the long-side cooling device 22 and the cooling water density by the short-side cooling device 23, calculated as described above, corresponds to the secondary cooling water density near the cast slab corner.

[0023] To explore suitable mold shapes and secondary cooling conditions for the present invention, tests were conducted using steels of the three components shown in Table 1 in continuous casting.

[0024] [Table 1]

[0025] Continuous casting was performed using a vertical bending type continuous casting machine. In the secondary cooling zone, the area from directly below the mold to the completion of bending was defined as the "upper" region, and the area from the completion of bending to the completion of straightening was defined as the "lower" region. The upper region from directly below the mold to the completion of bending had a casting length of 3.3m, and the lower region from the completion of bending to the completion of straightening had a casting length of 15m. The secondary cooling zone was divided into many cooling zones in the casting direction, and a water density was defined for each cooling zone. The average water density in the upper region as defined above was calculated by considering the water density and zone length of each zone located in the upper region, and using the water density obtained by weighting the water density of each zone by the zone length. The same method was used for the average water density in the lower region.

[0026] Electromagnetic stirring (EMS) in the mold is well known and is used in many continuous casting machines. This is because EMS equalizes the temperature of the molten steel near the meniscus, preventing delayed solidification (Non-Patent Literature 2). However, if the flow due to EMS is too fast, the molten steel collides with the long side, causing remelting and resulting in uneven solidification. The inventors of this invention have learned through experiments that internal cracking in the mold can be improved by equalizing solidification within the mold through electromagnetic stirring in the mold. Therefore, electromagnetic stirring in the mold was implemented during the continuous casting described below.

[0027] (Example 1) First, steel having the composition of steel No. 1 in Table 1 was manufactured under the conditions in Table 3, with a casting speed of 1.1 m / min, so that the slab size was 2200 mm in width and 240 mm in thickness.

[0028] The cooling water density in Table 3 is the sum of the cooling water densities applied to the slab corner by the long-side cooling device 22 and the short-side cooling device 23. However, the lower cooling water density is essentially the amount of cooling water from the long-side cooling device, as in this embodiment, cooling from the short side was not performed after bending was completed.

[0029] After casting, the cast slabs underwent a quality evaluation for surface cracking. Surface crack evaluation was performed on approximately 10m long cast slabs cut from a steady-state section. A 1mm thick section of the surface within a 100mm radius from the corner was scraped, and the presence or absence of cracks was determined by color check (dye penetrant testing). For cast slabs found to have transverse cracks, surface grinding and color check were repeated until no cracks were detected, and the crack depth was evaluated. The measured crack depth was evaluated using a three-stage index. Surface crack index -0 represents cases where no cracks were detected after scraping 1mm of the slab surface, i.e., the crack depth was less than 1mm. Surface crack index -1 represents cases where cracks were detected after scraping 1mm of the slab surface, but not after scraping 2mm, i.e., the crack depth was between 1mm and 2mm and could be easily removed by slab maintenance. Surface crack index -2 represents cases where the crack depth was less than 5mm from the surface and required heavy maintenance. Surface crack index -3 represents cases where the crack depth was 5mm or more, requiring heavy maintenance for removal and resulting in a significant decrease in yield.

[0030] The degree of electromagnetic stirring within the mold was evaluated by the average value of the molten steel flow velocity in the 1 / 4 width and 3 / 4 width of the mold. The molten steel flow velocity in Table 3 was estimated by measuring the dendrite inclination angle in the etch print. The following equation (3) (see Non-Patent Literature 1) was used for this estimation. In Table 3, a level where the molten steel flow velocity is indicated as "<5cm / s" means that the electromagnetic stirring intensity was set to the minimum, and the molten steel flow could be confirmed visually within the mold, but the inclination of the dendrite inclination angle could not be confirmed. This stirring is performed to evaluate the degree of solidification uniformity by creating a white band. lnV=(θ+9.73×lnf+33.7) / (1.45×lnf+12.5) ···(3) Here, V represents the molten steel flow rate (cm / sec), θ represents the dendrite inclination angle (degrees), and f represents the solidification rate (cm / sec).

[0031] Regarding the uniformity of solidification within the mold, a quality evaluation was conducted on the solidification delay of the cast slab after casting. The evaluation of solidification delay involved etching a cross-sectional sample of the cast slab and evaluating it using a three-stage index based on the degree of solidification uniformity calculated from the white band. Here, the white band 24 is a negative segregation zone as shown in Figure 3, and is generated when the solidification shell surface within the mold is washed by the molten steel flow due to electromagnetic stirring. That is, the white band 24 represents the solidification interface at a certain moment (the time when the molten steel flow due to electromagnetic stirring occurred), i.e., the solidification shell shape at a certain moment. The solidification uniformity was defined as the ratio (c / d) of the minimum value (c in Figure 4) to the slab surface from the white band 24 to the cast slab surface. A lower solidification uniformity indicates greater solidification non-uniformity, and it is known that a lower solidification uniformity increases the risk of internal cracking within the mold. Three levels of uniformity index were defined from this solidification uniformity as shown in Table 2. Table 2 shows the relationship between the uniformity index, solidification uniformity, and the risk of internal cracking within the mold.

[0032] [Table 2]

[0033] Table 3 summarizes the slab quality under each casting condition. Values ​​outside the scope of the present invention are underlined.

[0034] [Table 3]

[0035] Based on the experimental results in Table 3, we investigated the preferred conditions for the present invention. First, it was confirmed that satisfying the following equations (1) and (2) is effective for the chamfer shape of the slab corner. 10 ≤ a ≤ 35 ·····(1) 1 ≤ b / a ≤ 3 ·····(2) Furthermore, in the case of a vertical bending continuous casting machine, the average secondary cooling water volume density applied to the slab corner from directly below the mold to the completion of bending (the "upper" region of the secondary cooling zone) should be 20-280 (L / min / m³). 2 By keeping the range within the specified limits, combined with the optimization of the chamfer shape of the slab corner, it became clear that the occurrence of lateral cracks on the lower surface of the slab could be prevented. This is referred to as "the first invention."

[0036] Levels -1 to -5 in Table 3 correspond to the first invention. Levels -1 to -4 indicate that the molten steel flow rate in the mold is less than 5 cm / s, meaning that electromagnetic stirring in the mold is not functioning effectively. Level -5 indicates that the molten steel flow rate in the mold is 45 cm / s, exceeding the preferred range of the third invention described later. By setting the chamfer shape and the upper cooling water density within an appropriate range, the surface temperature of the cast slab at the bend can be kept outside the embrittlement temperature range (first invention), and no transverse cracks occurred on the lower surface of the cast slab. Since the cooling water density in the lower part of the secondary cooling zone was not optimized, minor transverse cracks occurred on the upper surface at index -1. Because electromagnetic stirring in the mold was not functioning or the flow rate was faster than the preferred range, a slight solidification delay occurred with a uniformity index of 1, but no serious internal cracks occurred in the mold. The cause of the minor upper surface cracking is thought to be that the lower cooling water density was greater than the preferred range of the second invention described below, and the surface temperature of the cast slab at the straightening point could not completely avoid the embrittlement temperature range. Furthermore, the cause of the minor solidification delay is thought to be due to non-uniform molten steel temperature in the meniscus at levels -1 to -4, and remelting of the solidified shell due to the collision of the long side of the molten steel at level -5.

[0037] In addition to the requirements of the first invention described above, the long-side cooling device 22 ensures that the average secondary cooling water density applied to the slab corner is 50 (L / min / m³) in the case of a vertical bending continuous casting machine, from the completion of bending to the completion of straightening (the "lower" region of the secondary cooling zone), and in the case of a curved continuous casting machine, from 5 m directly below the mold to the completion of straightening. 2 It has become clear that by doing the following, it is also possible to prevent the occurrence of transverse cracks on the upper surface side of the cast slab. This is referred to as "the second invention." In the lower region of the secondary cooling zone, secondary cooling is not performed on the short side, and only cooling is performed on the long side.

[0038] Levels -5 to 10 in Table 3 correspond to the present second invention. By setting the chamfer shape and the upper and lower cooling water density within an appropriate range, the surface temperature of the cast slab in both the bent and straightened sections could be kept outside the embrittlement temperature range. Although a slight solidification delay occurred, with a uniformity index of 1, no internal cracking occurred within the mold. The cause of the slight solidification delay is thought to be the same as above, due to non-uniformity of the molten steel temperature in the meniscus.

[0039] In addition to the requirements of the first invention described above, and the requirements of the second invention, it has been found that by controlling the horizontal flow velocity of molten steel in the mold at the solidification interface to a range of 5 to 40 cm / s using an electromagnetic stirring device, uniform solidification within the mold can be promoted. This is referred to as the "third invention."

[0040] Levels -11 to -15 in Table 3 correspond to the third invention. Of these, levels -11 to -13 show that the chamfer shape, upper cooling water density, lower cooling water density, and molten steel flow within the mold are all within appropriate ranges, resulting in no cracking on the lower surface, cracking on the upper surface, or delayed solidification. This is thought to be because the surface temperature of the cast slab in both the bent and straightened sections was outside the embrittlement temperature range, and uniform heating occurred in the meniscus.

[0041] Levels 16 to 23 in Table 3 are comparative examples that fall outside the scope of the First Invention. In levels 16 and 17, the upper cooling water density exceeded the upper limit of the First Invention, resulting in significant cracking on the lower surface of the cast slab. This is thought to be because the surface temperature of the cast slab at the bending point decreased, entering the embrittlement temperature range, and resulting in cracking. In levels 18 to 20, the shape of the chamfered portion was outside the scope of the First Invention, and the uniformity index was 2 in all cases, indicating a very high risk of internal cracking within the mold. This is thought to be due to non-uniform solidification occurring in the structure around the chamfered portion of the cast slab. In level 18, the side length a of the chamfered portion was outside the scope of the Invention, in level 19, b / a was outside the lower limit, and in level 20, b / a was outside the upper limit. In level 21, the size of the side length a of the chamfered portion was below the scope of the Invention, and in level 22, no chamfered portion was formed at the corner, and in all cases, significant cracking occurred on the upper and lower surfaces of the cast slab. This is thought to be because the chamfered area was too small or absent, causing the slab to overcool and enter the brittleness temperature range, resulting in cracking.

[0042] (Example 2) Steel with the composition of steel No. 2 shown in Table 1 was cast using a vertical bending type continuous casting machine, with a casting speed of 1.1 m / min, under the conditions shown in Table 4, so that the slab size was 2200 mm in width and 240 mm in thickness. After casting, the slabs were evaluated for surface transverse cracks and internal cracks within the mold. Here, the cooling water density in Table 4 was calculated using the same method as in Example 1, and the evaluation of surface transverse cracks was performed using the same method as in Example 1. For evaluation of internal cracks within the mold, sulfur printing was performed on a cross-sectional sample of the slab, and the presence or absence of internal cracks within the mold was evaluated visually.

[0043] Table 4 summarizes the slab quality under each casting condition. Values ​​outside the scope of the present invention are underlined.

[0044] [Table 4]

[0045] Levels -1 to 3 in Table 4 correspond to the first invention of the present application, and levels -7 to 9 correspond to the first and third inventions of the present application. By setting the chamfer shape and the density of the upper cooling water volume within an appropriate range, the surface temperature of the slab at the bending section can be made to be outside the embrittlement temperature range. No transverse surface cracks occurred on the lower surface, and although minor transverse surface cracks occurred on the upper surface, no serious cracks occurred. The reason for the occurrence of minor surface cracks on the upper surface is considered to be that the amount of lower cooling water was large and the surface temperature of the slab at the correction point could not completely avoid the embrittlement temperature range.

[0046] Levels -4 to 6 in Table 4 correspond to the second invention of the present application, and levels -10 to 12 correspond to the second and third inventions of the present application. By setting both the chamfer shape, the density of the upper cooling water volume, and the density of the lower cooling water volume within an appropriate range, the surface temperature of the slab can be made to be outside the embrittlement temperature range at both the bending section and the correction section. No surface cracks occurred on either the upper surface or the lower surface, nor did any internal cracks occur within the mold.

[0047] Levels -13 and 14 in Table 4 are comparative examples outside the scope of the first invention described above. The density of the upper cooling water volume exceeded the upper limit of the present invention, and serious cracks occurred on the lower surface of the slab. This is considered to be because the surface temperature of the slab at the bending point decreased and entered the embrittlement temperature range, resulting in cracks.

[0048] Also, in this example, the reason why no internal cracks occurred within the mold even under the condition of no electromagnetic stirring is considered to be that this steel grade has low sensitivity to internal cracks within the mold.

[0049] The above describes the case where the continuous casting apparatus is a vertical bending type. The case where the continuous casting apparatus is a curved type will be described below.

[0050] In the first invention, in the case of a curved continuous casting apparatus, as described above, the section from directly below the mold to the 5 m point is defined as the "upper part", and the average secondary cooling water volume density is set within the range of 20 to 280 (L / min / m 2 ). Also, in the second invention, as described above, the section from 5 m directly below the mold to the completion of correction is defined as the "lower part", and the average secondary cooling water volume density is set to 50 (L / min / m 2 ) or less.

[0051] (Example 3) Steel with the composition of steel No. 3 shown in Table 1 was cast using a curved continuous casting machine, with a casting speed of 1.0 m / min, under the conditions shown in Table 5, so that the slab size was 2200 mm in width and 240 mm in thickness. After casting, the slabs were evaluated for surface transverse cracks and internal cracks within the mold. Here, the cooling water density in Table 5 was calculated using the same method as in Example 1, and the evaluation of surface transverse cracks and internal cracks within the mold was performed using the same methods as in Example 1.

[0052] [Table 5]

[0053] Levels -1 to -2 in Table 5 correspond to the second and third inventions of this invention. By setting the chamfered shape and the upper and lower cooling water density within an appropriate range, the surface temperature of the cast slab in the straightened section could be kept outside the embrittlement temperature range, and no transverse cracks occurred on the upper and lower surfaces.

[0054] As described above, Patent Document 1 states that by first lowering the surface temperature of the cast slab to a temperature below the Ar3 transformation point, and then restoring it to a temperature above the Ar3 transformation point, the solidification structure of the cast slab becomes a mixed structure of ferrite and pearlite with indistinct γ grain boundaries, thereby reducing the crack susceptibility of the cast slab and preventing cracking. In contrast to the invention described in Patent Document 1, the present invention does not employ a temperature pattern in which the surface temperature of the cast slab is first lowered to a temperature below the Ar3 transformation point and then restoring it to a temperature above the Ar3 transformation point. In fact, it has been confirmed that none of the examples described in Tables 3 and 4 employ a temperature pattern in which the surface temperature of the cast slab is first lowered to a temperature below the Ar3 transformation point and then restoring it to a temperature above the Ar3 transformation point. In the present invention, by chamfering the corners of the cast slab and optimizing the chamfer shape, it can be estimated that the occurrence of lateral cracks in the cast slab can be prevented without employing the temperature pattern described in Patent Document 1. [Explanation of Symbols]

[0055] 1. Mold 3 Casting space 4. Cast cross-section 5 Long side 6 Short side 7. Chamfered section 8 Slab cross section 9. Lower end of the mold 10 cast slabs 11 rectangle 12 Long side 13 Short side 14 Corner section 15 vertices 16 End of the chamfered section on the long side 17. End of the short-side chamfered section 18 Long-side water volume density evaluation area 19. Short-side water volume density evaluation area 20 Cooling spray 21 Cooling water jet 22 Long side cooling device 23 Short-side cooling device 24 White Band a. Length of the long side and side b. Short side length

Claims

1. In continuous casting of steel using a vertical bending continuous casting machine, The cross-section at the lower end of the mold in the casting space through which the cast slab passes is called the casting cross-section, and the shape of the casting cross-section is rectangular with chamfered edges at all four corners of the rectangle, and the length of the chamfered edge on the long side of the rectangular shape of the mold is a (mm), and the length of the chamfered edge on the short side of the rectangular shape of the mold is b (mm), and the size of the chamfered edge satisfies both of the following equations (1) and (2), The sum of the average secondary cooling water density from the long-side cooling device and the average secondary cooling water density from the short-side cooling device applied to the slab corner from directly below the mold to the completion of bending in a vertical bending continuous casting machine is 100 to 280 (L / min / m). ２ ) within the range, From the completion of bending to the completion of straightening in the vertical bending continuous casting machine, secondary cooling is performed only by the secondary cooling device on the long side, and the average secondary cooling water density applied to the slab corner by the secondary cooling device on the long side is 50 (L / min / m³). ２ A method for continuous casting of steel, characterized by the following: 10 ≤ a ≤ 35 .....(1) 1 ≤ b / a ≤ 3 .....(2)

2. The continuous casting method for steel according to claim 1, characterized in that an electromagnetic stirring device is used to control the horizontal flow velocity of molten steel in the mold at the solidification interface to be in the range of 5 to 40 cm / s.

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