Continuous casting method for slabs

Abstract

To provide a continuous casting method capable of suppressing surface cracks in a slab by making the slab less susceptible to restrictions on equipment and steel types.SOLUTION: The continuous casting method uses a vertical bent type or curved type continuous casting machine to cast a slab having an Ar3 transformation point of 950°C or less. The continuous casting method includes a quenching step, a reheating step, and a correcting step. In the quenching step, water is sprayed onto the slab (4) in a quenching zone (8) with a specific water content Q of 0.4 to 0.95 (L / kg-steel), thereby lowering a surface temperature of the slab (4) down to the Ar3 transformation point or lower. In the reheating step, water is sprayed onto the slab (4) in a reheating zone (9) at a specific water content Q of 0.1 to 0.2 (L / kg-steel), thereby raising again the surface temperature of the slab (4) to the Ar3 transformation point or higher. In the correcting step, the slab having the surface temperature of 950°C or higher is corrected in the correcting section (7).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for continuous casting of a slab, and more particularly to a method for continuous casting of a slab for producing a thick steel plate. [Background technology]

[0002] Thick steel plates are manufactured from slab cast pieces (hereinafter simply referred to as "slabs") obtained by continuous casting. During this process, cracks (hereinafter referred to as "surface cracks") may occur on the surface of the slab. When slabs for thick steel plates are manufactured using a vertical bending or curved type continuous casting machine, surface cracks tend to occur along a direction approximately perpendicular to the casting direction. These cracks are called "transverse cracks" or "transverse cracks." Transverse cracks occur when the slab passes through the straightening section, i.e., when the curved portion of the slab is stretched into a flat plate shape. This is because the stress applied to the vicinity of the surface of the slab in the straightening section exceeds the limit stress specific to the steel material that constitutes the slab.

[0003] The hot ductility of the material constituting the slab is low in a certain temperature range. Hereinafter, the temperature range in which the hot ductility decreases is referred to as the "brittle temperature range". The brittle temperature range is, for example, in the range of 600 to 850°C based on the temperature of the surface of the slab. If the temperature of the slab in the straightening section is in the brittle temperature range, transverse cracks are likely to occur. For this reason, in conventional continuous casting methods, the temperature of the slab in the straightening section is sometimes controlled to avoid the brittle temperature range, that is, to be lower or higher than the brittle temperature range. Such control can be performed by appropriately setting the conditions for secondary cooling (water cooling) of the slab.

[0004] However, the surface temperature of the slab changes due to variations in the operating conditions of continuous casting, such as the casting speed, secondary cooling conditions, etc. Therefore, when continuously casting a slab of a steel type that has a wide brittle temperature range and is prone to transverse cracks, the above-mentioned method of increasing or decreasing the temperature of the slab in the straightening section may not be able to suppress the occurrence of transverse cracks.

[0005] Patent Document 1 proposes a continuous casting method that can suppress transverse cracks without controlling the temperature of the slab in the straightening section to avoid the embrittlement temperature range. Specifically, in the continuous casting method of Patent Document 1, the surface temperature of the slab is temporarily lowered to the Ar3 transformation point or lower by secondary cooling of the slab immediately below the mold, and a fine structure is formed near the surface of the slab. The fine structure suppresses the occurrence of transverse cracks. This technology is called a surface structure control cooling (SSC). The surface structure control cooling immediately below the mold makes it possible to form a fine structure in a region from the surface to a depth of about several mm, and suppresses the occurrence of cracks in this region.

[0006] Patent Document 2 discloses a continuous cast slab (cast piece) for thick steel plate. In this slab, the gamma grain size (diameter of crystal grains of the austenite phase) is less than 2 mm in a range of 20 to 30 mm in the depth direction from the long side surface (surface that contacts the roll during continuous casting). That is, in the slab of Patent Document 2, a fine structure is formed in a deeper region compared to a general slab produced by SSC. As a result, the slab of Patent Document 2 is less susceptible to transverse cracks. A slab in which a fine structure is formed from the surface to a deep region in this way can be obtained by strengthening secondary cooling of the slab immediately below the mold. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2002-086252 A [Patent Document 2] JP 2015-182110 A Summary of the Invention [Problem to be solved by the invention]

[0008] In the continuous casting method of Patent Document 1, a fine structure can be formed in a range of up to 7 mm from the surface of the slab (hereinafter referred to as the "surface layer"). However, a fine structure cannot necessarily be formed in a deeper region (hereinafter referred to as the "subsurface layer"). In this case, cracks may occur in the subsurface layer at the straightening portion of the slab. The cracks that occur in the subsurface layer are called "subsurface cracks". Depending on the location where the subsurface cracks occur, the density where they occur, etc., the subsurface cracks may reach (propagate) to the surface of the slab during rolling after the casting process, open up, and become surface cracks. In recent years, there has been a demand for higher quality steel. In order to meet this demand, it is necessary to suppress the occurrence of surface cracks caused by subsurface cracks.

[0009] The continuous cast slab of Patent Document 2 may not be produced depending on the type of steel of the slab. In addition, the conditions under which the continuous cast slab of Patent Document 2 can be produced may not be realized depending on the equipment used.

[0010] Neither Patent Document 1 nor Patent Document 2 describes subsurface cracks, and no consideration is given to surface cracks resulting from subsurface cracks. The present invention has been made in view of such problems, and an object of the present invention is to provide a continuous casting method that is less subject to restrictions on equipment and steel type and can suppress surface cracks in a slab cast strip. [Means for solving the problem]

[0011] The continuous casting method according to an embodiment of the present invention is a method for continuously casting a slab having an Ar3 transformation point of 950°C or less using a vertical bending or curved type continuous casting machine having a quenching zone, a retentivity zone, and a straightening zone in the casting direction from the outlet of the mold, the quenching zone being within a range of 2.5 m from the outlet of the mold, a quenching step of spraying water onto the slab in the quenching zone under a condition that a specific water amount Q defined by the following formula (A) is 0.4 to 0.95 (L / kg-steel) to reduce a surface temperature of the slab to an Ar3 transformation point or lower; a recuperation process in which the surface temperature of the slab is restored to the Ar3 transformation point or higher while spraying water onto the slab in the recuperation zone under conditions in which the specific water amount Q is 0.1 to 0.2 (L / kg-steel); and a straightening step of straightening the slab strand having a surface temperature of 950°C or higher in the straightening section. Q = W / (H × D × Vc × ρ) (A) Where: W: Amount of water sprayed onto the slab (L / min) H: Width of the slab (m) D: Thickness of the slab (m) Vc: Casting speed (m / min) ρ: Density of molten steel (kg-steel / m 3 ) It is. Effect of the Invention

[0012] According to the continuous casting method of the embodiment of the present invention, surface cracks in a slab cast piece can be suppressed while being less subject to restrictions on equipment and steel type. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a cross-sectional view of a continuous casting machine that can be used to carry out a continuous casting method according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a graph showing the relationship between the specific water content in the recovery zone and the average number of subsurface cracks. [Diagram 3] FIG. 3 is a graph showing the relationship between the average number of subsurface cracks and the demotion rate. [Figure 4] FIG. 4 is a graph showing the relationship between the specific water content in the quenching zone and the thickness of the fine part. [Diagram 5] FIG. 5 is a diagram showing the relationship between the fine part thickness and the drop rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In this specification, the term "slab" refers not only to a portion where the molten steel is completely solidified, but also to a portion where the molten steel and the shell obtained by solidifying the molten steel coexist in the casting direction. The "surface layer" of a slab refers to a region from the surface of the slab to a depth of 7 mm. The "subsurface layer" of a slab refers to a region deeper than 7 mm from the surface of the slab. In both the "surface layer" and the "subsurface layer," the surface of the slab in an unprocessed state after casting is used as a reference. For example, when the surface of the slab is treated (scraped or ground) after continuous casting, the "surface layer" and "subsurface layer" are defined based on the surface before treatment after continuous casting.

[0015] The representative position below the surface is a position 10 mm deep from the surface of the slab. Based on this, for example, "temperature below the surface" refers to the temperature at a position 10 mm deep from the surface of the slab. Surface cracks include cracks that occur on the surface and subsurface cracks that have reached the surface of the slab.

[0016] As a result of testing and investigation into subsurface cracks, the present inventors have come to the following findings. (i) In the conventional continuous casting method, the temperature of the subsurface layer in the straightening section is low, which often causes subsurface cracks. If the temperature of the subsurface layer in the straightening section is sufficiently high, the occurrence of subsurface cracks can be suppressed even if a microstructure is not necessarily formed deep from the surface. (ii) The temperature below the surface in the straightening section can be controlled by the amount of water (specific water amount) supplied to the quenching zone, i.e., the portion of the slab downstream of the quenching zone. (iii) The deeper the microstructure is formed from the surface, the more effectively it is possible to prevent subsurface cracks from reaching the surface during rolling in the casting process and thereafter.

[0017] The present invention has been completed based on the above findings. A continuous casting method according to an embodiment of the present invention is a method for continuously casting a slab slab having an Ar3 transformation point of 950°C or less, using a vertical bending or curved type continuous casting machine having a quenching zone, a recuperation zone, and a straightening zone in the casting direction from the outlet of the mold, the quenching zone being within a range of 2.5 m from the outlet of the mold. This continuous casting method includes a quenching step, a recuperation step, and a straightening step. In the quenching step, the surface temperature of the slab slab is lowered to the Ar3 transformation point or less by spraying water onto the slab slab in the quenching zone under the condition that the specific water amount Q defined by the following formula (A) is 0.4 to 0.95 (L / kg-steel). In the recuperation step, the surface temperature of the slab slab is restored to the Ar3 transformation point or more while spraying water onto the slab slab in the recuperation zone under the condition that the specific water amount Q is 0.1 to 0.2 (L / kg-steel). In the straightening process, a slab cast piece having a surface temperature of 950° C. or more is straightened in a straightening section. Q = W / (H × D × Vc × ρ) (A) Where: W: Amount of water sprayed onto the slab (L / min) H: Width of slab (m) D: Thickness of slab (m) Vc: Casting speed (m / min) ρ: Density of molten steel (kg-steel / m 3 ) It is.

[0018] According to this continuous casting method, the specific water amount in the retrieval zone is reduced compared to the specific water amount in the retrieval zone of the conventional continuous casting method. As a result, in the straightening process, the surface temperature of the slab cast piece can be set to 950°C or higher, and the subsurface temperature can be made higher than 950°C. As a result, the occurrence of subsurface cracks and surface cracks resulting from these cracks can be suppressed. Such effects are not subject to restrictions on equipment and steel type.

[0019] The specific water amount Q in the quenching step is preferably 0.85 to 0.95 (L / kg-steel). In this case, a thick microstructure can be formed near the surface of the slab cast piece. This can prevent cracks generated under the surface layer from reaching the surface of the slab cast piece during rolling in the casting step and thereafter.

[0020] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following, "%" in relation to a chemical composition means % by mass.

[0021] [Continuous casting machine] In the continuous casting method of this embodiment, a vertical bending type or curved type continuous casting machine is used. These continuous casting machines have a straightening unit that straightens a curved portion of a slab cast piece (hereinafter simply referred to as a "slab") into a horizontal flat plate shape. The continuous casting method of this embodiment does not apply to a continuous casting machine that does not straighten a slab, for example, a continuous casting method using a vertical type continuous casting machine.

[0022] FIG. 1 is a cross-sectional view of a vertical bending type continuous casting machine that can be used to carry out a continuous casting method according to an embodiment of the present invention. The continuous casting machine 1 includes a mold 2 and a plurality of pairs of support rolls 5. Molten steel M is supplied into the mold 2 through a submerged nozzle 3 provided on a tundish (not shown). A portion of the molten steel M adjacent to the mold 2 solidifies to become a shell 4s. A slab 4 is drawn out from an outlet (opening) at the lower end of the mold 2 by a pinch roll (not shown). In the vicinity of the mold 2, the slab 4 includes the shell 4s and the molten steel M therein. The proportion of the molten steel M in the slab 4 decreases the farther away from the mold 2 in the casting direction, and becomes zero at a position some distance away from the mold 2.

[0023] The continuous casting machine 1 has, in order from the outlet of the mold 2 along the casting direction, a quenching zone 8, a retrieval zone 9, and a straightening section 7. The quenching zone 8 is within a range of 2.5 m from the outlet of the mold 2. The continuous casting machine 1 further includes a group of secondary cooling spray nozzles (a plurality of secondary cooling spray nozzles) not shown. In the quenching zone 8 and the retrieval zone 9, water (cooling water) can be sprayed onto the slab 4 by a group of secondary cooling spray nozzles (hereinafter simply referred to as "spray nozzles"). The retrieval zone 9 is a range between the spray nozzles arranged between the quenching zone 8 and the straightening section 7, which is closest to the spray nozzle arranged at the downstream end of the quenching zone 8, and the spray nozzle closest to the roll arranged at the upstream end of the straightening section 7.

[0024] The spray nozzles are provided near one surface and the other surface of the slab 4, i.e., the surfaces facing one roll and the other roll constituting each pair of support rolls 5. In this embodiment, the number of spray nozzles provided on one surface side of the slab 4 and the number of spray nozzles provided on the other surface side of the slab 4 are approximately the same in each of the quenching zone 8 and the refrigeration zone 9. As for the amount of water sprayed onto the slab 4 by the spray nozzles, the specific water amount for one surface of the slab 4 and the specific water amount for the other surface are approximately the same in each of the quenching zone 8 and the refrigeration zone 9.

[0025] The support rolls 5 are arranged vertically in the region directly below the mold 2. Then, as it becomes farther from the mold 2, the arrangement direction of the support rolls 5 gradually changes, and finally becomes horizontal. The slab 4 moves between each pair of support rolls 5 while being supported by the support rolls 5. Therefore, the shape of the slab 4 is flat (straight) in the portion where the support rolls 5 are arranged vertically (vertical portion) and in the portion where the support rolls 5 are arranged horizontally (horizontal portion). In the portion between the vertical portion and the horizontal portion (curved portion), the slab 4 is curved.

[0026] In the continuous casting machine 1, in the bending section 6 which transitions from the vertical section to the curved section, a force acts on the slab 4 so as to curve the flat slab 4. Also, in the straightening section 7 which transitions from the curved section to the horizontal section, a force acts on the slab 4 so as to straighten the curved slab 4 into a flat shape. When the continuous casting machine 1 is of the vertical bending type as in this embodiment, the quenching zone 8 is included in the vertical section, and the bending section 6 is included in the retrieval zone 9. When the continuous casting machine is of the curved type, the quenching zone and the retrieval zone are included in the curved section.

[0027] [Continuous casting method] The continuous casting method of the present embodiment includes a quenching step, a reheating step, and a straightening step. In this continuous casting method, a slab 4 of a steel type having an Ar3 transformation point of 950° C. or less is cast. The steel type of the slab 4 is not particularly limited as long as it satisfies the requirements of the Ar3 transformation point, but for example, it has a composition, strength, etc. specified as JIS SM490 (SM490A, SM490B, or SM490C) or SM570.

[0028] The SM490A is C: 0.20 or less, Si: 0.55% or less, Mn: 1.65% or less, P: 0.035% or less, and S: 0.035% or less and the balance being Fe and impurities.

[0029] SM490 meets the strength requirements stipulated in JIS G3106 (for example, SM490A has a tensile strength of 490 to 610 N / mm 2 As long as the above condition is satisfied, any type and amount of additive elements other than those mentioned above may be contained.

[0030] The SM570 is C: 0.18% or less, Si: 0.55% or less, Mn: 1.70% or less, P: 0.035% or less, and S: 0.035% or less and the balance being Fe and impurities.

[0031] SM570 meets the strength requirements stipulated in JIS G3106 (for example, tensile strength of 570 to 720 N / mm 2 As long as the above condition is satisfied, any type and amount of additive elements other than those mentioned above may be contained.

[0032] As described below, the continuous casting method of this embodiment has the effect of suppressing surface cracking of slabs even for steel types with high cracking sensitivity, such as steel types with Ni content of 0.6% or more or N content of 0.0040% or more. The steel types to be continuously cast by this method, including SM490 and SM570, may contain 1.0% or less Ni and 0.0080% or less N as optional added elements or impurities.

[0033] <Quenching process> In the quenching process, water is sprayed onto the slab 4 in the quenching zone 8 under conditions where the specific water amount Q defined by the following formula (A) is 0.4 to 0.95 (L / kg-steel). This reduces the surface temperature of the slab 4 to below the Ar3 transformation point of the slab 4. Q = W / (H × D × Vc × ρ) (A) Where: W: Water volume sprayed onto slab 4 (L / min) H: Width of slab 4 (length perpendicular to the cross section in Figure 1) (m) D: Thickness of slab 4 (length perpendicular to the longitudinal direction of slab 4 in the cross section of Figure 1) (m) Vc: Casting speed of slab 4 (speed at which slab 4 is drawn out) (m / min) ρ: Density of molten steel M in slab 4 (kg-steel / m 3 ) It is.

[0034] The quenching zone 8 includes a plurality of spray nozzles. When the continuous casting machine 1 has a bending section 6 as in the present embodiment, the continuous casting machine 1 has a quenching zone 8 upstream of the bending section 6 (closer to the mold 2).

[0035] The degree of superheat of the molten steel is, for example, 15 to 50°C. The casting speed Vc is, for example, 0.5 to 0.8 (m / min). The specific water amount Q is the amount of water (L (liters)) supplied to the slab (steel) 4 per unit mass (kg). The amount of water W supplied to the slab 4 in the quenching process is the amount of water sprayed to the slab 4 per unit time (L / min) in the quenching zone 8. When multiple spray nozzles are provided in the quenching zone 8, the amount of water W is the total amount of water supplied by these spray nozzles, and includes the amount of water for one surface and the other surface of the slab 4 (the same applies to the reheating process described later). The amount of water W supplied to the slab 4 in the quenching process is, for example, 2800 to 3200 (L / min). In the quenching zone, the surface of the slab 4 is cooled in a temperature range of 700 to 1000°C at a cooling rate of, for example, 8 to 12°C / sec.

[0036] Surface cracks of the slab 4 are likely to occur when the thickness D of the slab 4 is 250 mm or more, particularly when the thickness D is 300 mm or more. In the continuous casting method of this embodiment, the thickness D of the slab 4 is not particularly limited. However, the continuous casting method of this embodiment can effectively suppress surface cracks when the thickness D is 250 mm or more, particularly when the thickness D is 300 mm or more.

[0037] By spraying water in the quenching zone 8, the surface temperature of the slab 4 is lowered to a temperature lower than the Ar3 transformation point of the steel constituting the slab 4, at least at the downstream end of the quenching zone 8 (a position upstream of the quenching zone 9 and in the vicinity of the spray nozzle closest to the quenching zone 9). As a result, a fine structure is formed in at least the surface layer of the slab 4, i.e., in the entire region from the surface to a depth of 7 mm. The term "fine structure" refers to a solidification structure that is a mixture of ferrite and pearlite with unclear austenite grain boundaries (γ grain boundaries).

[0038] If the specific water amount Q in the quenching process is less than 0.4 (L / kg-steel), a fine structure will not be sufficiently formed on the surface layer of the slab 4. If the specific water amount Q in the quenching process exceeds 0.95 (L / kg-steel), cracks due to supercooling may occur near the surface of the slab 4, and the surface temperature of the slab 4 may not recover to a predetermined temperature in the reheating process and straightening process described below.

[0039] The specific water amount Q in the quenching step is preferably 0.85 to 0.95 (L / kg-steel), which allows the microstructure to be formed in a region extending from the surface of the slab 4 to a depth of more than 7 mm, i.e., in a region extending from the surface layer to a part of the subsurface layer.

[0040] <Recuperation process> In the recuperating process, water is sprayed onto the slab 4 in the recuperating zone 9 under conditions of a specific water amount Q of 0.1 to 0.2 (L / kg-steel). This allows the surface temperature of the slab 4 to be restored to the Ar3 transformation point of the slab 4 or higher. It is sufficient that the surface temperature of the slab 4 is restored to a temperature higher than the Ar3 transformation point of the slab 4 at least at the downstream end of the recuperating zone 9 (a position upstream of the straightening zone 7 and in the vicinity of the spray nozzle closest to the straightening zone 7). The amount of water W supplied to the slab 4 in the recuperating process is, for example, 500 to 600 (L / min). The recuperating zone 9 includes at least one spray nozzle. It is preferable that the recuperating zone 9 includes multiple spray nozzles.

[0041] If the specific water amount Q in the reheating process is less than 0.1 (L / kg-steel), the temperature of the slab 4 becomes too high due to reheating. This increases the load on the equipment and shortens the life of the equipment, particularly the various rolls including the support rolls 5. If the specific water amount Q in the reheating process exceeds 0.2 (L / kg-steel), there is a risk that the surface temperature of the slab 4 will not recover to a temperature higher than the Ar3 transformation point of the steel constituting the slab 4, even at the downstream end of the reheating zone 9.

[0042] <Correcting process> In the straightening process, the slab 4 having a surface temperature of 950°C or more is straightened in the straightening section 7. The surface temperature of the slab 4 is preferably 950°C or more across the entire area between the upstream end and the downstream end of the straightening section 7. However, the surface temperature of the slab 4 may be less than 950°C in a portion between the upstream end and the downstream end of the straightening section 7.

[0043] As described above, the downstream end of the refrigeration zone 9 is the position where the spray nozzles closest to the straightening section 7 are arranged, and therefore the downstream end of the refrigeration zone 9 and the upstream end of the straightening section 7 may be separated from each other. In this case, the surface temperature of the slab 4 usually rises due to recuperation between the downstream end of the refrigeration zone 9 and the upstream end of the straightening section 7.

[0044] The surface temperature of the slab 4 may be 950°C or higher at the downstream end of the refrigeration zone 9. In this case, the surface temperature of the slab 4 is usually maintained at 950°C or higher until the slab 4 reaches the straightening section 7. Furthermore, the surface temperature of the slab 4 may be equal to or higher than the Ar3 transformation point and lower than 950°C at the downstream end of the refrigeration zone 9. In this case, it is sufficient that the surface temperature of the slab 4 is restored to 950°C or higher until the slab 4 reaches the straightening section 7.

[0045] [effect] The microstructure formed in the surface layer of the slab 4 in the quenching process is maintained even after the quenching process. Therefore, a microstructure is formed in the surface layer of the slab 4 to be straightened in the straightening unit 7. This microstructure can suppress the occurrence of cracks in the surface layer during the straightening process. The temperature of the slab 4 is higher the deeper from the surface. Since the slab 4 is made of a steel type whose Ar3 transformation point is 950°C or less, the temperatures of the surface layer and subsurface layer of the slab 4 to be straightened are both equal to or higher than the Ar3 transformation point. Such temperatures are usually higher than the embrittlement temperature range. Therefore, it is possible to suppress the occurrence of cracks (subsurface cracks) even at a depth where no microstructure is formed subsurface.

[0046] Even if subsurface cracks do occur, since a fine structure is formed in the surface layer, the subsurface cracks are unlikely to reach the surface of the slab 4 even when reduced in the casting process and thereafter. In particular, when the specific water content Q in the quenching zone 8 is 0.85 to 0.95 (L / kg-steel), a fine structure is formed in an area extending from the surface of the slab 4 to a depth of more than 7 mm, so that it is possible to suppress cracks from occurring in this area and to sufficiently suppress the subsurface cracks from reaching the surface. This effect can be sufficiently obtained when the surface temperature of the slab 4 is 950°C or higher across the area between the upstream end and the downstream end of the straightening section 7.

[0047] The above-mentioned effects can also be obtained for steel types having high cracking sensitivity, for example, steel types that satisfy at least one of the following conditions (a) and (b). (a) The Ni content is 0.6% or more. (b) The N content is 0.0040% or more.

[0048] In this embodiment, the manufacturing conditions (casting conditions) such as the specific water amount Q (0.4 to 0.95 (L / kg-steel)) in the quenching process can be realized by a general continuous casting machine. Therefore, this continuous casting method is less subject to restrictions on equipment and steel types, and can suppress surface cracks in the slab. EXAMPLES

[0049] In order to confirm the effect of this embodiment, a test was conducted in which a slab was continuously cast from molten steel, and the occurrence of surface cracks in the obtained slab was investigated. In this investigation, the surface temperature of the slab during each test was calculated by computer simulation. At that time, various conditions during each test were input into the computer and the simulation was performed. The various conditions during each test were as follows.

[0050] In this test, a vertical bending type continuous casting machine having the configuration shown in Fig. 1 was used. For this continuous casting machine, the length of the vertical section was 2.5 m, and the strand radius (the radius of curvature of the curved section) was 9.4 m. The distance from the outlet of the mold to the straightening section along the slab was 15.4 m. Along the slab, the quenching zone was in the range of 0.0 to 1.0 m from the outlet of the mold, and the retentivity zone was in the range of 1.0 to 15.4 m from the outlet of the mold.

[0051] Table 1 shows the chemical composition of the slab. Table 2 shows the specific water amounts in the quenching zone and the refrigeration zone, and the evaluation results of the slab. The specific water amounts in the refrigeration zone in Comparative Examples 1 to 3 were typical of those used in conventional continuous casting methods. The Ar3 transformation point of the steel constituting the slab was 850 to 865°C. The embrittlement temperature range of this steel was 600 to 1000°C. The upper limit of this embrittlement temperature range is higher than the upper limit of the embrittlement temperature range of the average steel type. In calculating the specific water amount, the density ρ of the molten steel (molten steel) was set to 7.8 (kg-steel / m 3 ) was decided.

[0052] [Table 1]

[0053] [Table 2]

[0054] The degree of superheat of the molten steel was 30 to 50°C. The casting speed Vc was 0.7 m / min. The dimensions of the cast slabs were a thickness D of 298 to 304 mm and a width H of 2300 mm. For each of the inventive examples and comparative examples, a plurality of slabs (for example, about 10 slabs) were obtained.

[0055] The column "Surface temperature of straightening section" in Table 2 shows the average temperature of the slab surface in the straightening section. In all of the examples of the present invention, the surface temperature of the slab was lower than the Ar3 transformation point at the downstream end of the quenching zone, higher than the Ar3 transformation point at the downstream end of the refrigeration zone, and 950°C or higher in the straightening section. On the other hand, in the comparative example, the surface temperature of the slab was lower than the Ar3 transformation point at the downstream end of the quenching zone, lower than the Ar3 transformation point at the downstream end of the refrigeration zone, and less than 950°C in the straightening section.

[0056] The thickness of the part having a fine structure (hereinafter referred to as "fine part thickness") and the average number of subsurface cracks were determined by the following procedure. First, the slab was cut perpendicular to the length direction, and the cut surface was mirror-polished. The structure was observed on this mirror surface, and the fine part thickness (the length in the direction perpendicular to the original surface of the slab) was measured at 100 mm intervals in the width direction of the slab. Since the diameter of the crystal grains changes abruptly between the part having a fine structure and the other parts, the part having a fine structure can be easily identified.

[0057] Penetrant testing was also performed on the mirror surface. More specifically, a straight line was drawn on the mirror surface at a depth of 15 mm from the initial surface of the slab, i.e., at a depth of 8 mm from the underside of the surface layer. The number of cracks crossing the line was then measured across the entire width of the slab (2300 mm). The number of cracks per 100 mm is shown in the "Subsurface Cracks" column in Table 2.

[0058] The demotion rate was calculated by the following procedure. For each of the casting methods of the present invention and the working examples, the slabs produced were rolled to obtain a plurality of plates (thick steel plates). The number of plates was designated as N0, and the number of plates with surface cracks was designated as N1, and the demotion rate was calculated as N1 / N0×100(%).

[0059] In both the slabs of the invention and the comparative example, a fine structure was formed in the surface layer, that is, over the entire area from the surface of the slab to 7 mm. The number of subsurface cracks was 5.9 / 100 mm on average for the comparative example, whereas the number of subsurface cracks was 2.4 / 100 mm on average for the invention example. In other words, the number of subsurface cracks in the invention example was less than half that of the comparative example.

[0060] Fig. 2 is a diagram showing the relationship between the specific water content in the refracting zone and the average number of subsurface cracks. The smaller the specific water content in the refracting zone, the smaller the average number of subsurface cracks. More specifically, when the specific water content in the refracting zone is 0.2 L / kg-steel or less (examples of the present invention), the average number of subsurface cracks is 5 cracks / 100 mm or less, and in most cases it is 4 cracks / 100 mm or less. In contrast, when the specific water content in the refracting zone exceeds 0.2 L / kg-steel (comparative examples), the average number of subsurface cracks is 3 cracks / 100 mm or more in all cases.

[0061] Figure 3 shows the relationship between the average number of subsurface cracks and the demotion rate. Generally, the smaller the average number of subsurface cracks, the lower the demotion rate. In other words, the smaller the average number of subsurface cracks, the fewer the number of subsurface cracks that reach the surface due to reduction by the reduction rolls. From the results of Figures 2 and 3, it can be seen that the smaller the specific water content in the retrieval zone, the fewer the number of subsurface cracks that reach the surface.

[0062] FIG. 4 is a diagram showing the relationship between the specific water amount in the quenching zone and the thickness of the fine part. The larger the specific water amount in the quenching zone, the larger the thickness of the fine part. FIG. 5 is a diagram showing the relationship between the thickness of the fine part and the downgrade rate for the examples of the present invention. In all of the examples of the present invention, the number of subsurface cracks is equal at 5 / 100 mm or less, but the thicker the fine part thickness, the fewer the surface cracks. From the results of FIG. 4 and FIG. 5, the following can be seen. That is, when the specific water amount in the refrigeration zone is set to 0.1 to 0.2 L / kg-steel, the larger the specific water amount in the quenching zone and the larger the thickness of the fine part, the more difficult it is for the subsurface cracks to reach the surface. [Industrial Applicability]

[0063] This continuous casting method is widely applicable to the continuous casting of steel. [Explanation of symbols]

[0064] 1: Continuous casting machine 2: Mold 3: Submerged nozzle 4: Slab 4s:Shell 5: Support roll 6: Bending section 7: Orthodontic department 8: Quenching zone 9: Tropical reversion M: Molten steel

Claims

1. A vertical bending or curved type continuous casting machine having a quenching zone, a retentivity zone and a straightening zone in the casting direction from the outlet of the mold, the quenching zone being within a range of 2.5 m from the outlet of the mold, is used to cast a sintered steel having a Ni content of 0.6% or more and / or a N content of 0.0040% or more, and a sintered steel having an Ar content of 950° C. or less. ３ A method for continuously casting a slab having a transformation point, comprising the steps of: In the quenching zone, water is sprayed onto the slab under conditions where the specific water amount Q defined by the following formula (A) is 0.4 to 0.95 (L / kg-steel), thereby increasing the surface temperature of the slab to Ar ３ A rapid cooling process to lower the temperature below the transformation point; In the recovery zone, water is sprayed onto the slab under the condition that the specific water amount Q is 0.1 to 0.2 (L / kg-steel), and the surface temperature of the slab is increased by Ar. ３ A reheating process to restore the temperature above the transformation point; and a straightening step of straightening the slab strand having a surface temperature of 950°C or higher in the straightening section. Q=W / (H×D×Vc×ρ)...(A) Where: W: Amount of water sprayed onto the slab (L / min) H: width of the slab (m) D: Thickness of the slab (m) Vc: Casting speed (m / min) ρ: density of molten steel (kg-steel / m ３ ) It is.

2. The continuous casting method according to claim 1, The continuous casting method, wherein the specific water amount Q in the quenching step is 0.85 to 0.95 (L / kg-steel).

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