Steel continuous casting method

Abstract

To provide a steel continuous casting method capable of reducing center segregation and porosity with a simple structure.SOLUTION: A steel continuous casting method comprises: a cooling step (#5); and a depression step (#10). In the cooling step (#5), within a range until the thickness of a solidifying shell (S) reaches 60 mm, secondary cooling water is injected from a secondary cooling nozzle (6) so that the surface cooling speed of a cast piece (10) until the surface speed of the cast piece (10) from the exist side of a cast piece (4) to a width center becomes 1000°C can be 5°C to 20°C. In the depression step (#10), until a center solid phase rate on the width center of the cast piece (10) reaches 1.0 after reaching 0.3, the cast piece (10) is lightly depressed at the depressing speed of 0.5 mm / min., to 2.0 mm / min. At the start of the depression step (#10), the surface temperature of the cast piece (10) is 300°C to 1000°C.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to a method for continuous casting of steel. [Background technology]

[0002] In the continuous casting of steel, defects such as center segregation and porosity occur in the slab. The slab obtained by continuous casting is rolled to become a product. In recent years, products have become thicker and stronger, and as a result, requirements for the internal quality of the slab have increased significantly. For this reason, it is necessary to further reduce center segregation and porosity in the center of the slab's thickness direction. To improve the internal quality caused by these defects, the slab is usually lightly reduced in the thickness direction within the continuous casting machine. By lightly reducing the unsolidified slab by an amount commensurate with the amount of solidification shrinkage, the positive segregation in the thickness direction is alleviated.

[0003] Patent Documents 1 to 3, for example, are known as techniques for reducing center segregation. Patent Document 1 discloses a technique in which the solidification profile (progression in the thickness of the solidified shell) in the width direction of a slab is determined in advance, and the amount of secondary cooling water in the width direction of the slab is individually controlled to uniformize this profile, thereby increasing negative segregation in the thickness center. Patent Document 2 discloses a technique in which a static magnetic field of 0.15 T or more is applied to a slab in a continuous casting machine, causing the solidification structure in the thickness center of the slab to become columnar. Patent Document 3 describes a technique in which the grain size of center segregation is reduced by adjusting the water flow density in secondary cooling within a range from a start point where the average solid fraction along the thickness direction of the slab is 0.8 or less to an end point where the average solid fraction along the thickness direction of the slab is 1.0 or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6561822 [Patent Document 2] Patent No. 6264524 [Patent Document 3] Patent No. 7355285 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 is based on the premise that negative segregation is formed in the thickness center of a slab. To form negative segregation in the thickness center, it is necessary to apply a large reduction to the unsolidified slab that is greater than the amount of solidification shrinkage and forcibly discharge concentrated molten steel. Therefore, a reduction device that performs large reduction is required.

[0006] In Patent Document 2, a device for applying a static magnetic field to the slab is required. Furthermore, in order to suppress molten steel convection after the solidified shell has developed to a certain extent, the strength of the static magnetic field must be increased. This increases the costs of equipment development, installation, and maintenance, making this method uneconomical in terms of cost-effectiveness.

[0007] In Patent Document 3, the slab is subjected to intense cooling in the area where the solidified shell is thick. Therefore, it is considered necessary to extremely low the slab temperature in order to control the temperature gradient at the solidification interface. In this case, there is a concern that the slab may crack due to thermal stress. Furthermore, in Patent Document 3, since soft reduction is performed on a slab with a low surface temperature and high strength, an extremely large thrust force is required to achieve sufficient soft reduction. As a result, the equipment costs are high, which is disadvantageous in terms of cost-effectiveness.

[0008] An object of the present disclosure is to provide a method for continuous casting of steel that can reduce center segregation and porosity with a simple configuration. [Means for solving the problem]

[0009] The continuous casting method for steel according to the present disclosure uses a continuous casting machine. The continuous casting machine includes a mold, support rolls, secondary cooling nozzles, and soft reduction rolls. The support rolls are arranged along the casting direction of the slab. The secondary cooling nozzles spray secondary cooling water onto the slab. The soft reduction rolls are arranged downstream of the support rolls in the casting direction. The continuous casting method includes a cooling process and a reduction process. In the cooling process, secondary cooling water is sprayed from the secondary cooling nozzles so that the slab's surface cooling rate is 5°C / s or more and 20°C / s or less until the surface temperature of the slab at the width center reaches 1000°C from the outlet side of the mold until the thickness of the solidified shell reaches 60 mm. In the reduction process, the slab is soft reduced in the thickness direction of the slab by the soft reduction rolls at a reduction rate of 0.5 mm / min or more and 2.0 mm / min or less from the time when the central solid fraction at the width center of the slab reaches 0.3 until the central solid fraction reaches 1.0. At the start of the reduction process, the surface temperature of the slab is 300°C or higher and 1000°C or lower. [Effects of the Invention]

[0010] According to the method for continuous casting steel according to the present disclosure, both center segregation and porosity can be reduced with a simple configuration. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing an example of the measurement results of dendrite angles. [Figure 2] FIG. 2 is a schematic diagram showing the cross section of a cast slab. [Figure 3] FIG. 3 is a schematic diagram showing the cross section of a cast slab. [Figure 4] FIG. 4 is a schematic diagram showing the relationship between the index A and the number of V-shaped segregations in the width direction. [Figure 5] FIG. 5 is a schematic diagram showing the relationship between the surface cooling rate of a slab and the number of V-shaped segregations in the width direction. [Figure 6] FIG. 6 is a schematic diagram of a continuous casting machine used in the continuous casting method according to the embodiment. [Figure 7]FIG. 7 is a flow diagram showing a method for continuously casting steel according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] In order to solve the above-mentioned problems, the inventors focused on the growth direction of the solidification structure growing from the surface of the slab during solidification. The growth direction of the solidification structure may vary at an angle with respect to the thickness direction of the slab (the direction perpendicular to the surface) due to the flow of molten steel in the width direction and non-uniformity of temperature distribution during solidification. Furthermore, even if the flow of molten steel in the width direction and non-uniformity of temperature distribution are reduced as much as possible, the growth direction of the solidification structure may fluctuate due to unavoidable minute thermal fluctuations.

[0013] The angle of the growth direction of the solidification structure relative to the thickness direction (hereinafter also referred to as "dendrite angle") can be measured by the following procedure. A sample of a cast slab is cut perpendicular to the casting direction, and the cross section is etched with picric acid or the like to visualize the solidification structure. The dendrite angle is measured by observing this solidification structure. The cross section may be observed in the direction along the casting direction or in the direction opposite to the casting direction.

[0014] An example of the dendrite angle measurement results is shown in Figure 1. Figure 1 plots the dendrite angle of the solidification structure at a position 20 mm from the center in the thickness direction in the cross section of a slab. The position 20 mm from the center in the thickness direction refers to both a position 20 mm from the center in the thickness direction toward one surface of the slab and a position 20 mm from the center in the thickness direction toward the other surface of the slab. Figure 1 shows the dendrite angle when the cross section of the slab is observed in the direction opposite to the casting direction, and the counterclockwise direction with respect to the thickness direction is defined as positive. The dendrite angle is measured along the width direction of the slab. However, because the dendrite angle near the ends in the width direction is affected by the solidification structure toward the inside in the width direction, it is preferable not to measure the dendrite angle within 200 mm from the ends of the slab.

[0015] Next, the effect of the dendrite angle on center segregation and porosity will be explained. FIG. 2 is a schematic diagram showing the cross section of a slab 10. In FIG. 2, the growth direction of the solidified structure 20 is indicated by an arrow. In the example shown in FIG. 2, the dendrite angle of the solidified structure 20 is 0°, and the solidified structure 20 grows in the thickness direction. The solidified structures 20 that grow inward from the surfaces 10a on both sides of the slab collide at the center in the thickness direction and are completely solidified. From the perspective of reducing center segregation and porosity, it is ideal for the solidified structures 20 to collide perpendicularly with the surface 10a of the slab 10, as shown in FIG. 2.

[0016] FIG. 3 is a schematic diagram showing the cross section of a slab 10. In FIG. 3, the growth direction of a solidification structure 20 is indicated by an arrow. In the example shown in FIG. 3, the dendrite angle of the solidification structure 20 varies, and the growth direction of the solidification structure 20 is at an angle with respect to the thickness direction. In this case, the solidification structures 20 growing inward from the surfaces 10a on both sides of the slab collide in a direction deviated from the thickness direction. Therefore, as shown in FIG. 3, localized areas of delayed solidification (dotted line areas in FIG. 3) occur. Center segregation and porosity are likely to occur in these delayed solidification areas.

[0017] From the above, if the variation in dendrite angle can be suppressed, the solidification delay of the slab is less likely to occur, and center segregation and porosity are more likely to be reduced. Therefore, the inventors introduced the index A (%) = (N / N0) × 100 and conducted extensive research to determine a preferable range for index A. Specifically, the inventors counted the number of V-shaped segregations in the width direction in the cross section of the slab and investigated the relationship with index A. As with the dendrite angle, the number of V-shaped segregations was measured by visualizing the solidification structure in the cross section of the slab and observing the solidification structure.

[0018] Index A means the proportion of solidification structures whose absolute value of the angle relative to the thickness direction is 10° or more. In Index A, N means the number of solidification structures whose dendrite angle in the cross section of the slab is outside the range of -10° to +10° (solidification structures whose absolute value of the dendrite angle is 10° or more). NO means the total number of solidification structures in the cross section of the slab.

[0019] Generally, widthwise V-segregation is more likely to occur when there is significant variation in the solidification of the slab in the width direction, and it forms toward the location of the negative pressure area created locally by the variation in solidification. The fewer the number of widthwise V-segregations, the better the center segregation and porosity of the slab. It has been found that if there are 30 or fewer widthwise V-segregations per meter, the center segregation and porosity are good, and the quality of the product (steel plate) after rolling can be ensured.

[0020] FIG. 4 is a schematic diagram showing the relationship between index A and the number of V-segregations in the width direction. FIG. 4 plots the results of calculating index A for multiple cross sections with different numbers of V-segregations in the width direction per meter. The results shown in FIG. 4 show that there is a correlation between index A and the number of V-segregations in the width direction, and that index A needs to be approximately 35% or less in order to keep the number of V-segregations in the width direction per meter at 30 or less. In particular, if index A is 30% or less, the number of V-segregations in the width direction per meter can be more reliably kept at 30 or less.

[0021] The inventors further focused on the secondary cooling conditions within the range until the solidified shell thickness reaches 60 mm. This is because surface cooling of the slab has a significant effect on the internal solidification structure until the solidified shell thickness reaches 60 mm. Specifically, the inventors investigated the relationship between the surface cooling rate (°C / s) of the slab from the mold outlet until the surface temperature of the slab at the width center reaches 1000°C and the number of V-shaped segregations in the width direction. The surface cooling rate refers to the average cooling rate of the slab surface, and is calculated by dividing the difference between the surface temperature of the slab at the mold outlet and 1000°C by the time required for the slab to move from the mold outlet to the width center until the surface temperature of the slab at the width center reaches 1000°C. Hereinafter, the surface cooling rate of the slab from the mold outlet until the surface temperature of the slab at the width center reaches 1000°C may be simply referred to as the "surface cooling rate."

[0022] Figure 5 is a schematic diagram showing the relationship between the surface cooling rate of a slab and the number of V-shaped segregations in the width direction. Figure 5 plots the number of V-shaped segregations in the width direction per meter when the surface cooling rate of the slab is changed. Referring to Figure 5, the number of V-shaped segregations in the width direction decreases with increasing surface cooling rate. This is thought to be because relatively strong cooling of the slab surface increases the temperature gradient in the thickness direction, promoting the growth of the solidification structure along the thickness direction and reducing the variation in dendrite angle. It can also be said that the greater the difference between the temperature at the center of the slab's thickness direction and the surface temperature, the smaller the variation in dendrite angle. From the results shown in Figure 5, it can be seen that a surface cooling rate of 4°C / s or higher is sufficient to reduce the number of V-shaped segregations in the width direction per meter to 30 or less. In particular, a surface cooling rate of 5°C / s or higher can more reliably reduce the number of V-shaped segregations in the width direction per meter to 30 or less. This reduces centerline segregation and porosity.

[0023] The continuous casting method for steel according to the embodiment of the present disclosure has been completed based on the above findings.

[0024] A continuous casting method for steel according to an embodiment uses a continuous casting machine. The continuous casting machine includes a mold, support rolls, secondary cooling nozzles, and soft reduction rolls. The support rolls are arranged along the casting direction of the slab. The secondary cooling nozzles spray secondary cooling water onto the slab. The soft reduction rolls are arranged downstream of the support rolls in the casting direction. The continuous casting method includes a cooling process and a reduction process. In the cooling process, secondary cooling water is sprayed from the secondary cooling nozzles so that the slab's surface cooling rate is 5°C / s or more and 20°C / s or less until the surface temperature of the slab at the widthwise center reaches 1000°C, from the outlet side of the mold, until the thickness of the solidified shell reaches 60 mm. In the reduction process, the slab is soft reduced in the thickness direction of the slab by the soft reduction rolls at a reduction rate of 0.5 mm / min or more and 2.0 mm / min or less, from when the central solid fraction at the widthwise center of the slab reaches 0.3 until the central solid fraction reaches 1.0. At the start of the reduction step, the surface temperature of the slab is 300°C or higher and 1000°C or lower (first configuration).

[0025] In the continuous casting method of the first configuration, the cooling step involves cooling the slab until the thickness of the solidified shell reaches 60 mm. In particular, the slab is intensively cooled so that the surface cooling rate is 5°C / s or more and 20°C / s or less until the surface temperature of the slab at the center in the width direction reaches 1000°C from the outlet of the mold. As described above, a surface cooling rate of 5°C / s or more can reduce V-segregation in the width direction of the slab. Furthermore, intensive cooling of the slab can be achieved by simply adjusting the amount of secondary cooling water sprayed from the secondary cooling nozzles, and no large-scale equipment is required. Therefore, the first configuration allows for a simple reduction in centerline segregation and porosity.

[0026] In the above continuous casting method, the proportion of solidification structures at a position 20 mm from the center in the thickness direction of the slab after the reduction step, in which the absolute value of the angle with respect to the thickness direction is 10° or more, is 30% or less (second configuration). The proportion of solidification structures in which the absolute value of the angle with respect to the thickness direction is 10° or more corresponds to the above-mentioned index A. By setting index A to 30% or less, V-segregation in the width direction of the slab can be reduced. Therefore, according to the second configuration, centerline segregation and porosity can be further reduced.

[0027] Hereinafter, a continuous steel casting method according to an embodiment of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0028] [Continuous casting machine] FIG. 6 is a schematic diagram of a continuous casting machine 1 used in the continuous casting method according to this embodiment. The continuous casting machine 1 produces a slab 10. The slab 10 has a thickness of, for example, 250 mm or more and 300 mm or less. In this embodiment, the continuous casting machine 1 is a vertical bending type. In other words, the continuous casting machine 1 includes a vertical band, a curved portion, and a horizontal band. However, the continuous casting machine 1 may be, for example, a vertical type consisting of only a vertical band, or a curved type consisting of a curved portion and a horizontal band. The continuous casting machine 1 includes a tundish 2, a mold 4, a plurality of support rolls 5, a plurality of secondary cooling nozzles 6, and a plurality of soft reduction rolls 7.

[0029] Molten steel M is supplied to the tundish 2 from a ladle (not shown). The molten steel M in the tundish 2 is supplied to the mold 4 through an immersion nozzle 3. The molten steel M in the mold 4 is cooled by the mold 4. Electromagnetic brakes (not shown) may be disposed on both outer sides of the mold 4 in the thickness direction of the slab 10. The thickness direction is a direction perpendicular to the width direction of the slab 10 and the casting direction.

[0030] A plurality of support rolls 5 and a plurality of secondary cooling nozzles 6 are arranged downstream in the casting direction of the mold 4. The plurality of support rolls 5 are arranged along the casting direction of the slab 10. The secondary cooling nozzles 6 spray secondary cooling water onto the slab 10. The secondary cooling nozzles 6 are arranged, for example, alternately with the plurality of support rolls 5 in the casting direction. However, only some of the secondary cooling nozzles 6 are shown in FIG. 6, and the other secondary cooling nozzles 6 are not shown.

[0031] The molten steel M is cooled in the mold 4 and then further cooled by secondary cooling water sprayed from the secondary cooling nozzles 6. This forms a solidified shell S. The slab 10 in the process of solidifying contains the solidified shell S (solid fraction 1.0) and unsolidified molten steel M (solid fraction less than 1.0). The slab 10 in the process of solidifying and containing the unsolidified molten steel M is guided downstream in the casting direction by a plurality of support rolls 5. During this process, the unsolidified molten steel M gradually decreases, and a completely solidified slab 10 is formed.

[0032] A plurality of light reduction rolls 7 are arranged downstream in the casting direction of the plurality of support rolls 5. Each light reduction roll 7 forms a pair and softly reduces the slab 10. Specifically, a series of light reduction rolls 7 form a light reduction zone 71. The light reduction zone 71 is divided into a plurality of segments, each of which is provided with a plurality of light reduction rolls 7. The reduction amount of the light reduction rolls 7 is controlled for each segment. The plurality of light reduction rolls 7 are arranged at approximately equal intervals between the entrance 71i and exit 71o of the light reduction zone 71. Here, in the light reduction zone 71, the position of the entrance 71i coincides with the position of the light reduction roll 7 arranged furthest upstream in the casting direction, and the position of the exit 71o coincides with the position of the light reduction roll 7 arranged furthest downstream in the casting direction.

[0033] [Continuous casting method] Fig. 7 is a flow diagram showing a continuous casting method for steel according to this embodiment. As shown in Fig. 7, the continuous casting method for steel according to this embodiment includes a cooling step (#5) and a rolling step (#10). The slab 10 obtained by this continuous casting method becomes the raw material for products such as steel plate. Each step shown in Fig. 7 will be described in detail below.

[0034] [Cooling process (#5)] In the cooling step (#5), secondary cooling water is sprayed onto the slab 10 from the secondary cooling nozzles 6. Specifically, in the cooling step (#5), the slab 10 is cooled until the thickness of the solidified shell S reaches 60 mm. A thickness of 60 mm for the solidified shell S means that the thickness of the solidified shell S on one surface 10a of the slab 10 is 60 mm. In particular, the slab 10 is subjected to intensive cooling from the mold outlet until the surface temperature of the slab at the center in the width direction reaches 1000°C. In this specification, intensive cooling refers to cooling performed so that the surface cooling rate of the slab 10 is 5°C / s or more. Intensive cooling of the slab 10 within the above range increases the temperature gradient at the solidification interface and reduces the variation in dendrite angle.

[0035] However, increasing the surface cooling rate of the slab 10 does not necessarily mean that the variation in dendrite angle can be suppressed. The effect of suppressing the variation in dendrite angle saturates at a surface cooling rate of about 20°C / s. Furthermore, the higher the surface cooling rate, the more secondary cooling water is required, which increases the equipment cost and makes the cost-effectiveness unrealistic. Therefore, in the cooling process (#5), the surface cooling rate of the slab 10 is set to 5°C / s or more and 20°C / s or less.

[0036] A slab 10 having a solidified shell S with a thickness of 60 mm or more may also be cooled by spraying secondary cooling water from the secondary cooling nozzle 6. However, as the solidified shell S becomes thicker, the thermal resistance of the solidified shell S also increases. Therefore, even if intensive cooling is performed on a slab 10 having a thickness in this range, the temperature gradient at the solidification interface does not become that large due to the thermal resistance of the solidified shell S. Furthermore, in this case, the surface temperature of the slab 10 may become excessively low, which may increase the strength of the slab 10 to the extent that soft reduction is difficult, or may make the slab 10 more susceptible to cracking. Therefore, it is preferable to cool a slab 10 having a solidified shell S with a thickness of 60 mm or more at a surface cooling rate that is slower than that of a slab 10 having a solidified shell S with a thickness up to 60 mm.

[0037] The solidification state of the slab 10 is highly dependent on the casting speed. Therefore, a position where the thickness of the solidified shell S is 60 mm may be identified in advance by performing a solidification analysis or the like that takes the casting speed into consideration. This allows the range where the slab 10 is to be subjected to intensive cooling to be appropriately set.

[0038] [Reduction process (#10)] In the reduction step (#10), the slab 10 is soft reduced in the thickness direction using a plurality of soft reduction rolls 7 provided in a soft reduction zone 71. In the reduction step (#10), soft reduction is performed on the slab 10 after the cooling step (#5), i.e., the slab 10 with small variation in dendrite angle.

[0039] Soft reduction is performed from when the center solid fraction at the width center of the slab 10 reaches 0.3 until it reaches 1.0. That is, the center solid fraction at the width center of the slab 10 at the entrance 71i of the soft reduction zone 71 is 0.3, and the center solid fraction at the width center of the slab 10 at the exit 71o of the soft reduction zone 71 is 1.0. Hereinafter, the center solid fraction at the width center may be simply referred to as the "center solid fraction." The center solid fraction of the slab 10 is the solid fraction at the thickness center. When the temperature at the thickness center is at the liquidus temperature (the temperature at which solidification begins), the center solid fraction is 0. When the temperature at the thickness center is at the solidus temperature (the temperature at which solidification is completed), the center solid fraction is 1.0. If soft reduction is started after the center solid fraction of the slab 10 reaches 0.3, center segregation and porosity cannot be sufficiently reduced. Similarly, if the soft reduction is terminated before the center solid fraction of the slab 10 reaches 1.0, center segregation and porosity cannot be sufficiently reduced. Furthermore, even if soft reduction is performed on the region where the center solid fraction of the slab 10 is 1.0, i.e., the region where the center of the thickness of the slab 10 is completely solidified, it has almost no effect on reducing center segregation and porosity, so there is no need to soft reduce this region.

[0040] Soft reduction of the region of the slab 10 where the center solid fraction is less than 0.3 does not have a significant effect on reducing the center segregation and porosity formed at the end of solidification, so it is not necessarily required to soft reduce this region. However, soft reduction may be performed on the region of the slab 10 where the center solid fraction is less than 0.3.

[0041] In the reduction step (#10), if the reduction rate in the soft reduction zone 71 is less than 0.5 mm / min, the reduction amount will be insufficient relative to the solidification shrinkage of the slab 10, and center segregation and porosity cannot be sufficiently reduced. Furthermore, if the reduction rate is greater than 2.0 mm / min, the reduction amount will be excessive relative to the solidification shrinkage of the slab 10, and there is a risk of molten steel M flowing back from downstream to upstream in the casting direction. In this case, concentrated molten steel will flow, worsening center segregation. Therefore, the reduction rate of the slab 10 in the reduction step (#10) is 0.5 mm / min or more and 2.0 mm / min or less.

[0042] If the surface temperature of the slab 10 at the start of the reduction step (#10) is lower than 300°C, the strength of the slab 10 will be high, and excessive load capacity will be required to perform the soft reduction using the soft reduction rolls 7. In this case, the cost required for soft reduction will increase. Furthermore, if the surface temperature of the slab 10 at the start of the reduction step (#10) is higher than 1000°C, the strength of the slab 10 will be low, and bulging between the rolls will increase, making it impossible to sufficiently reduce center segregation and porosity. Therefore, the surface temperature of the slab 10 at the start of the reduction step (#10) is 300°C or higher and 1000°C or lower. Preferably, the surface temperature of the slab 10 at the start of the reduction step (#10) is 600°C or higher and 900°C or lower.

[0043] By cutting the slab 10 after the reduction step (#10) perpendicular to the casting direction and observing the solidification structure 20 of the cross section, it can be confirmed that the variation in dendrite angle has been reduced. In this embodiment, the proportion of the solidification structure 20 at a position 20 mm from the center in the thickness direction of the slab 10 after the reduction step (#10) that has an angle of 10° or more with respect to the thickness direction is 30% or less. In other words, the index A of the slab 10 after the reduction step (#10) is 30% or less. Therefore, centerline segregation and porosity can be effectively reduced.

[0044] [effect] In the continuous casting method of this embodiment, in the cooling step (#5), the slab 10 is cooled until the thickness of the solidified shell S reaches 60 mm. In particular, the slab 10 is intensively cooled so that the surface cooling rate of the slab 10 is 5°C / s or more and 20°C / s or less until the surface temperature of the slab 10 at the center in the width direction reaches 1000°C from the outlet side of the mold 4. A surface cooling rate of 5°C / s or more can reduce V-segregation in the width direction of the slab 10. Furthermore, intensive cooling of the slab 10 can be achieved by simply adjusting the amount of secondary cooling water sprayed from the secondary cooling nozzles 6, and no large-scale equipment is required. Therefore, the continuous casting method of this embodiment can reduce centerline segregation and porosity with a simple configuration. [Example]

[0045] In order to confirm the effects of the continuous casting method according to the embodiment, the following tests were carried out and the results were evaluated. Specifically, the Mn segregation degree of the cast slab obtained by continuous casting was evaluated. The Mn segregation degree will be described later.

[0046] In this test, slabs were produced using the continuous casting machine shown in Figure 6. The mold was a water-cooled copper mold. The mold length was 800 mm. The mold cross section was rectangular. The soft reduction zone was located 16 m or more from the meniscus (the molten metal surface in the mold). The slab was soft reduced in the thickness direction using a pair of soft reduction rolls at a set reduction rate of 0.8 mm / min from the time the central solid fraction of the slab reached 0.3 until it reached 1.0. No soft reduction was performed in the region after the central solid fraction of the slab reached 1.0. The surface temperature, shell thickness, central temperature, and solid fraction of the slab were calculated using two-dimensional solidification analysis in the thickness and width directions of the slab.

[0047] The main chemical composition of the slab used in this test was C: 0.05%, Si: 0.2%, Mn: 1.4%, P: 0.011%, and S: 0.003%.

[0048] The degree of Mn segregation in the slab was investigated using the following procedure. The cast slab was cut at the center in the width direction, and a sample was taken from the center of the thickness direction of the cut surface, covering an area 40 mm in the casting direction and 40 mm in the thickness direction. This sample was subjected to area analysis of the Mn concentration using an EPMA (Electron Probe Micro Analyzer). The Mn concentration was integrated over a 2 mm width along the casting direction, centered on the position with the highest Mn concentration, and the average value was taken as the maximum Mn concentration (Cmax). The value (Cmax / C0) obtained by dividing the maximum Mn concentration (Cmax) by the Mn concentration (C0) in the bulk composition of the slab was taken as the Mn segregation degree. The Mn concentration (C0) in the bulk composition was determined by chemical analysis of an analytical sample taken from the slab.

[0049] The test conditions and test results of this example are shown in Table 1. In Table 1, the test conditions include the thickness, width, casting speed, surface cooling rate, index A, the number of V-segregations per meter in the width direction, and the surface temperature at the start of soft reduction of the slab produced by continuous casting. In Table 1, slabs produced under test conditions that satisfy the following condition 1 are designated as invention examples, and slabs produced under test conditions that do not satisfy condition 1 are designated as comparative examples. Since none of comparative examples 1 to 3 shown in Table 1 satisfy condition 1, a mark "*" is added to the surface cooling rate. Furthermore, among invention examples 1 to 5 and comparative examples 1 to 3 shown in Table 1, those that do not satisfy the following condition 2 are designated as index A with a mark "#". Condition 1: The surface cooling rate is 5°C / s or more and 20°C / s or less. Condition 2: Index A is 30% or less.

[0050] [Table 1]

[0051] It is known that if the Mn segregation ratio of the resulting slab is 1.40 or less, toughness can be ensured in the product (steel plate) after rolling the slab. Therefore, in Table 1, the test results are shown as "excellent" if the Mn segregation ratio is 1.40 or less, and "poor" if not.

[0052] As shown in Table 1, all of Examples 1 to 4 satisfied Conditions 1 and 2. Therefore, Examples 1 to 4 produced cast slabs with a good degree of Mn segregation. In other words, centerline segregation was reduced.

[0053] Although Example 5 satisfied Condition 1, it did not satisfy Condition 2 because Index A was greater than 30%. The Mn segregation degree of Example 5 was somewhat higher than that of Examples 1 to 4, but was 1.40 or less, and a cast slab with a good Mn segregation degree was obtained. In other words, centerline segregation was also reduced in Example 5.

[0054] In Comparative Examples 1 to 3, the surface cooling rate was less than 5°C / s, and condition 1 was not satisfied. In addition, in Comparative Examples 1 to 3, index A was less than 30%, and condition 2 was not satisfied. As a result, the Mn segregation degree was worse than that of Invention Examples 1 to 4, and the evaluation was "fail."

[0055] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure. [Explanation of symbols]

[0056] 1: Continuous casting machine 5: Support roll 6: Secondary cooling nozzle 7: Light reduction roll 10: Casting 20: Coagulation tissue S: Solidified shell

Claims

1. A method for continuously casting steel using a continuous casting machine including a mold, support rolls arranged along the casting direction of a slab, secondary cooling nozzles that spray secondary cooling water onto the slab, and soft reduction rolls arranged downstream of the support rolls in the casting direction, a cooling step in which secondary cooling water is sprayed from the secondary cooling nozzles so that the surface cooling rate of the slab is 5°C / s or more and 20°C / s or less until the surface temperature of the slab reaches 1000°C from the outlet side of the mold to the center in the width direction, within a range until the thickness of the solidified shell reaches 60 mm; a reduction step of softly reducing the slab in the thickness direction of the slab with the soft reduction rolls at a reduction rate of 0.5 mm / min or more and 2.0 mm / min or less from the time when the central solid fraction at the width direction central portion of the slab reaches 0.3 until the central solid fraction reaches 1.0, The continuous casting method, wherein the surface temperature of the slab is 300°C or higher and 1000°C or lower at the start of the reduction step.

2. The continuous casting method according to claim 1, a ratio of solidification structures at a position 20 mm from the center in the thickness direction of the slab after the reduction step, the solidification structures having an absolute value of an angle with respect to the thickness direction of 10° or more, is 30% or less.

Images