Casting condition setting device, continuous casting device, casting condition setting method, continuous casting method, and high-tensile steel slab manufacturing method

Abstract

To provide a casting condition setter, continuous caster, casting condition setting method, continuous casting method and high-tensile steel slab producing method allowing for inhibiting a steel slab from cracking by preventing a segregation of constituent elements of molten steel occurring at a portion nearby a corner in a longer-sided wall at an inside of a mold.SOLUTION: A casting condition setter according to one embodiment of the present invention comprises a mold, an immersion nozzle, an electromagnetic stirring device and an electromagnetic brake device, so as to set up a casting condition of a continuous caster to continuously cast a steel slab, and further comprises a casting condition calculating section that, in the case of producing a high-tensile steel slab, calculates a casting condition for a hit velocity, at which an ejection flow hits against the longer-sided wall of the mold, to become smaller than a predetermined reference value, and a setting section that sets, for the continuous caster, a setting value allowing for casting under the casting condition calculated by the casting condition calculating section.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a casting condition setting device, a continuous casting device, a casting condition setting method, a continuous casting method, and a method for producing a high-tensile steel slab. [Background technology]

[0002] Steel slabs are sometimes continuously cast using continuous casting equipment. In continuous casting equipment, electromagnetic stirring devices and electromagnetic braking devices are widely used in the mold to improve the surface quality of the steel slabs.

[0003] For example, Patent Document 1 discloses a continuous casting method in which the discharge outlet of the submerged entry nozzle is positioned so that the magnetic flux density at the discharge outlet is 50% or less of the maximum magnetic flux density of an electromagnetic stirrer, and a discharge flow is discharged from the submerged entry nozzle, and this discharge flow is introduced into a static magnetic field formed by an electromagnetic brake, in order to ensure a stable state regarding fluctuations in the molten metal surface during casting and to easily obtain slabs with excellent surface and internal quality.

[0004] Furthermore, Patent Document 2 discloses a continuous steel casting method in which, in order to reduce Ar gas bubbles and inclusions contained in a continuously cast slab and improve the quality of the slab, the molten steel discharged from the immersion nozzle is controlled in a continuous steel casting method in which, while Ar gas is being blown into the immersion nozzle, an electromagnetic stirring device is used to stir the molten steel in the upper part of the mold to form a swirling flow of molten steel in the horizontal cross section of the mold, and an electromagnetic brake device is used to apply a DC magnetic field to the molten steel discharged from the discharge hole of the immersion nozzle to cast the molten steel. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-47195 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-66618 [Non-patent literature]

[0006] [Non-Patent Document 1] T. Takahashi, K. Ichikawa, M. Kudo, and K. Shimabara, "Effect of Molten Metal Flow on Solidification Segregation of Steel Ingots," Iron and Steel, 61 (1975), No. 9, pp. 2198-2213. [Non-patent document 2] Nagata, S., Matsumiya, T., Ozawa, K., and Ohashi, T., "Estimation of the critical strain for internal cracking in continuously cast slabs," Tetsu-to-Hagané, 76 (1990), No. 2, pp. 214-221. [Non-patent document 3] A. Jablonka, K. Harste and K. Schwerdtfeger, “Thermomechanical properties of iron and iron-carbon alloys: density and thermal contraction, Steel Research, 62 (1991), 24-33. [Non-patent document 4] J. Ni and C. Beckermann, “A Volume-Averaged Two-Phase model for Transport Phenomena during Solidification”, Metallurgical Transactions B, 22B, June, 1991, 349-361 Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, the increasing demand for high-value-added products has led to the use of higher alloy steel billets. However, in the continuous casting of high-alloy steel, segregation of elements due to electromagnetic stirring can become a problem. For example, in high-tensile steels containing large amounts of Si and Mn, localized segregation occurs near the corners of the mold due to electromagnetic stirring and the flow caused by the discharge flow. This results in a non-uniform structure in the high-alloy steel billet, making the billet more susceptible to cracking.

[0008] The present invention has been made in view of the above-mentioned circumstances, and has as its object to provide a casting condition setting device, a continuous casting device, a casting condition setting method, a continuous casting method, and a method for producing high-tensile steel slabs, which are capable of preventing segregation of constituent elements of molten steel that occurs near the corners of the long side walls inside the mold, thereby suppressing cracking of the steel slab. [Means for solving the problem]

[0009] To solve the above problems, the inventors conducted extensive research and found that during solidification of an alloy, alloy elements such as C, Si, and Mn are discharged from the solid phase into the molten steel. However, when the molten steel is flowing, these discharged alloy elements are easily washed away, resulting in microsegregation, or variations in element concentration on the order of several millimeters, after solidification. Furthermore, the inventors found that when a continuous casting system equipped with an electromagnetic stirrer is used to produce steel slabs, this macrosegregation is likely to occur near the corners of the long side walls of the mold. Further research led the inventors to find that by using an electromagnetic brake device to brake the discharge flow and suppress macrosegregation near the corners of the long side walls of the mold, spatially uneven peritectic solidification can be suppressed, thereby preventing cracking of the steel slab. This led to the present invention.

[0010] The gist of the present invention is as follows. [1] According to one aspect of the present invention, there is provided a casting condition setting device for setting casting conditions for a continuous casting machine for continuously casting high-tensile steel slabs, the casting condition setting device comprising: a mold having a rectangular cross section formed by a pair of long side walls and a pair of short side walls; an immersion nozzle having two discharge holes disposed opposite each other on the pair of short side walls of the mold and supplying molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field and applies an electromagnetic force to the molten steel inside the mold to stir the molten steel; and an electromagnetic braking device that generates a magnetic field and applies an electromagnetic force to the molten steel constituting a discharge flow that is the main flow of molten steel discharged from the discharge holes to brake the discharge flow, the casting condition setting device setting casting conditions for a continuous casting machine for continuously casting high-tensile steel slabs, the casting conditions being set by the setting device including a casting speed Vc, a distance Hb from the surface of the molten steel to an upper end of a core of the electromagnetic braking device, mass concentrations of alloy components, an average magnetic flux density Bb of the magnetic field generated by the electromagnetic braking device, an average magnetic flux density Bs of the magnetic field generated by the electromagnetic stirring device, and a speed at which the molten steel is supplied to the electromagnetic stirring device. application of the AC current Phase Wavelength Δx, the electromagnetic stirring device application the frequency f of the AC current supplied to the casting machine, the cross-sectional area Sp of the discharge hole, the downward angle θ of the discharge hole, the depth position D of the discharge hole, the outer diameter D1 of the submerged entry nozzle, the cross-sectional area Ss of the horizontal cross section of the internal space of the mold, and the distance W between the long side walls in the internal space. The casting condition setting device includes: a casting condition calculation unit that calculates the casting conditions to make the collision speed when the discharge stream collides with the long side wall of the mold under the action of electromagnetic forces from the electromagnetic stirring device and the electromagnetic brake device less than a predetermined reference value; and a setting unit that sets, for the continuous casting machine, setting values ​​that enable casting under the casting conditions calculated by the casting condition calculation unit, wherein the predetermined reference value is a value that is determined based on the degree of influence of changes in the concentrations of Si and Mn contained in the molten steel due to the discharge stream on peritectic solidification. [2] In the casting condition setting device described in [1] above, the casting conditions may include at least the strength of the magnetic field generated by the electromagnetic stirring device. [3] In the casting condition setting device described in [1] or [2] above, the casting conditions may include the strength of the magnetic field generated by the electromagnetic brake device. [4] In the casting condition setting device according to any one of the above [1] to [3], the setting unit may set the set value so that the collision velocity satisfies the following formula (1). U2<0.015×7500×V / {(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|)-0.015} ···(1) formula where: V=6.45×10 -5 ×K×(Vc / Hb) 0.5 however, %X: Mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) U2: Collision speed (m / s) Vc: Casting speed (m / s) Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) K: Coagulation coefficient (mm / min 0.5 ) [5] In the casting condition setting device according to any one of [1] to [4] above, the setting unit may set the set value so that ΔCE expressed by the following formula (2) is less than 0.015. ΔCE=(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|) / (7500×V / U2+1) ···(2) formula where: V=6.45×10 -5 ×K×(Vc / Hb) 0.5 U2=U0-(σ×Lb×Bb 2 ) (where U0>σ×Lb×Bb 2 in the case of) U2=0 (where U0≦σ×Lb×Bb 2 in the case of) U0=(Vp 2 +4×Cs×Ls) 0.5 Ls = (Hb-D) / sinθ Lb = 0.5 × (W - D1) / cosθ - Ls Cs=0.5×σ×f×Δx×Bs 2 Vp = Vc × (Ss / Sp) however, %X: Mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) σ: Electrical conductivity per unit mass of the molten steel (Sm 2 / kg) U2: Collision speed (m / s) Bb: Average magnetic flux density (T) of the magnetic field generated by the electromagnetic brake device Bs: Average magnetic flux density (T) of the magnetic field generated by the electromagnetic stirrer Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) Vc: Casting speed (m / s) Vp: flow velocity (m / s) of the discharge flow at the discharge hole of the submerged nozzle Sp: Cross-sectional area of ​​the discharge hole (m 2 ) Ss: Cross-sectional area of ​​the horizontal section of the internal space of the mold (m 2 ) W: Distance between the long side walls in the internal space of the mold (m) Δx: AC current applied to the electromagnetic stirrer Phase Wavelength (m) f: frequency of the AC current applied to the electromagnetic stirrer (1 / s) D: Depth position of the discharge hole (m) D1: Outer diameter of the above submerged nozzle (m) θ: downward angle of the discharge hole (rad) K: Coagulation coefficient (mm / min 0.5 ) [6] In the casting condition setting device described in [5] above, U0, Bb, and Lb are set to satisfy the following condition: U0≦σ×Lb×Bb 2 The casting may be performed so as to satisfy the following. [7] In the casting condition setting device according to any one of [1] to [6] above, the molten steel may contain C: 0.1 to 0.5 mass %, and at least one of Si: 0.8 mass % or more and Mn: 4.0 mass % or more.

[0011] [ 8 According to another aspect of the present invention, the above [1] to [7

[0033] The casting conditions are set by a setting device for casting conditions according to any one of high tension A continuous casting apparatus is provided for casting steel billets.

[0012] [9] According to yet another aspect of the present invention, there is provided a method for setting casting conditions for a continuous casting machine for continuously casting high-tensile steel slabs, the continuous casting machine comprising: a mold having a rectangular cross section formed by a pair of long side walls and a pair of short side walls; an immersion nozzle having two discharge holes disposed opposite each other on the pair of short side walls of the mold and supplying molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field and applies an electromagnetic force to the molten steel in the mold to stir the molten steel; and an electromagnetic braking device that generates a magnetic field and applies an electromagnetic force to the molten steel constituting a discharge flow that is the main flow of molten steel discharged from the discharge holes to brake the discharge flow, the method comprising the steps of: setting the casting conditions for a casting speed Vc; a distance Hb from the surface of the molten steel to an upper end of a core of the electromagnetic braking device; a mass concentration of alloy components; an average magnetic flux density Bb of the magnetic field generated by the electromagnetic braking device; an average magnetic flux density Bs of the magnetic field generated by the electromagnetic stirring device; application of the AC current Phase Wavelength Δx, the electromagnetic stirring device application the frequency f of the AC current supplied to the mold, the cross-sectional area Sp of the discharge hole, the downward angle θ of the discharge hole, the depth position D of the discharge hole, the outer diameter D1 of the submerged entry nozzle, the cross-sectional area Ss of the horizontal cross section of the internal space of the mold, and the distance W between the long side walls in the internal space can be changed, and the method includes: a casting condition calculation step of calculating the casting conditions under which the collision speed when the discharge stream collides with the long side wall of the mold under the action of electromagnetic forces from the electromagnetic stirring device and the electromagnetic brake device is less than a predetermined reference value; and a setting step of setting, for the continuous casting device, setting values ​​that enable cast under the casting conditions calculated in the casting condition calculation step, wherein the predetermined reference value is a value determined based on the degree of influence of changes in the concentrations of Si and Mn contained in the molten steel due to the discharge flow on peritectic solidification.

[0013] [ 10 According to yet another aspect of the present invention, 9 The casting conditions were set by the casting condition setting method described in high tension A continuous casting method for casting a steel billet is provided.

[0014]

[11] According to yet another aspect of the present invention, there is provided a continuous casting apparatus for continuously casting high-tensile steel slabs using a continuous casting machine comprising: a mold having a rectangular cross section formed by a pair of long side walls and a pair of short side walls; an immersion nozzle having two discharge holes disposed opposite each other on the pair of short side walls of the mold, for supplying molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field and applies an electromagnetic force to the molten steel in the mold to stir the molten steel; and an electromagnetic braking device that generates a magnetic field and applies an electromagnetic force to the molten steel constituting a discharge flow, which is the main flow of molten steel discharged from the discharge holes, to brake the discharge flow. a magnetic field generated by the electromagnetic stirring device that applies an electromagnetic force to the molten steel in the mold, thereby generating a swirling flow in which the molten steel swirls horizontally; an electromagnetic force acting on the molten steel that constitutes the discharge flow by the magnetic field generated by the electromagnetic brake device that brakes the discharge flow, thereby reducing the collision speed of the discharge flow, which is curved by the swirling flow and collides with a long side wall of the mold, to less than a predetermined reference value; and the predetermined reference value is a value determined based on the degree of influence of changes in the concentrations of Si and Mn contained in the molten steel, caused by the discharge flow, on peritectic solidification.

[12] In the method for producing a high-tensile steel slab described in

[11] above, the collision velocity may satisfy the following formula (1): U2<0.015×7500×V / {(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|)-0.015} ···(1) formula where: V=6.45×10 -5 ×K×(Vc / Hb) 0.5 however, %X: Mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) U2: Collision speed (m / s) Vc: Casting speed (m / s) Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) K: Coagulation coefficient (mm / min 0.5 )

[13] In the method for producing a high-tensile steel slab described in

[11] or

[12] above, the reference value may be ΔCE expressed by the following formula (2), and the setting values ​​of the continuous casting device may be determined so that ΔCE is less than 0.015. ΔCE=(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|) / (7500×V / U2+1) ···(2) formula where: V=6.45×10 -5 ×K×(Vc / Hb) 0.5 U2=U0-(σ×Lb×Bb 2 ) (where U0>σ×Lb×Bb 2 in the case of) U2=0 (where U0≦σ×Lb×Bb 2 in the case of) U0=(Vp 2 +4×Cs×Ls) 0.5 Ls = (Hb-D) / sinθ Lb = 0.5 × (W - D1) / cosθ - Ls Cs=0.5×σ×f×Δx×Bs 2 Vp = Vc × (Ss / Sp) however, %X: Mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) σ: Electrical conductivity per unit mass of the molten steel (Sm 2 / kg) U2: Collision speed (m / s) Bb: Average magnetic flux density (T) of the magnetic field generated by the electromagnetic brake device Bs: Average magnetic flux density (T) of the magnetic field generated by the electromagnetic stirrer Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) Vc: Casting speed (m / s) Vp: flow velocity (m / s) of the discharge flow at the discharge hole of the submerged nozzle Sp: Cross-sectional area of ​​the discharge hole (m 2 ) Ss: Cross-sectional area of ​​the horizontal section of the internal space of the mold (m 2 ) W: Distance between the long side walls in the internal space of the mold (m) Δx: AC current applied to the electromagnetic stirrer Phase Wavelength (m) f: frequency of the AC current applied to the electromagnetic stirrer (1 / s) D: Depth position of the discharge hole (m) D1: Outer diameter of the above submerged nozzle (m) θ: downward angle of the discharge hole (rad) K: Coagulation coefficient (mm / min 0.5 )

[14] The method for producing a high-tensile steel slab according to

[13] above is characterized in that U0, Bb, and Lb satisfy the following conditions: U0≦σ×Lb×Bb 2 The casting may be performed so as to satisfy the following.

[15] In the method for producing a high-tensile steel slab according to any one of

[11] to

[14] above, the molten steel may contain C: 0.1 to 0.5 mass %, and at least one of Si: 0.8 mass % or more and Mn: 4.0 mass % or more.

[0015] According to the present invention, the collision speed of the discharge flow when it collides with the long side wall of the mold is less than a reference value, so that segregation of the constituent elements of the molten steel occurring near the corners of the long side wall inside the mold can be prevented, thereby making it possible to suppress cracking of the steel slab. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic explanatory diagram showing an example of a continuous casting production apparatus. [Figure 2] 2 is a partial cross-sectional view in the YZ plane showing the positional relationship between the mold, the electromagnetic stirring device, and the electromagnetic brake device, as well as the configuration of the electromagnetic stirring device and the electromagnetic brake device. FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along the line AA shown in FIG. 2. [Figure 4] FIG. 4 is a cross-sectional view taken along the line BB shown in FIG. [Figure 5] FIG. 4 is a cross-sectional view taken along the CC cross section shown in FIG. [Figure 6] 4 is a diagram for explaining the direction of an electromagnetic force acting on molten steel by an electromagnetic brake device. FIG. [Figure 7] 1 is a block diagram showing the configuration of a casting condition setting device according to an embodiment of the present invention. [Figure 8] 10A and 10B are conceptual diagrams for explaining the flow of the discharge flow when the electromagnetic brake is not applied and when the electromagnetic brake is fully applied. [Figure 9] FIG. 10 is a conceptual diagram for explaining the flow of a discharge flow. [Figure 10] 1 is a graph showing the relationship between EMBr intensity and segregation ratio Δ%Si / %Si when the EMS intensity is changed. [Figure 11] 1 is a graph showing the relationship between EMBr intensity and segregation ratio Δ%Si / %Si when the casting speed Vc is changed. [Figure 12] 1 is a graph showing the relationship between EMBr intensity and ΔCE′ when the EMS intensity is changed. [Figure 13] 1 is a graph showing the relationship between EMBr intensity and ΔCE′ when the casting speed Vc is changed. [Figure 14] 1 is a graph showing the relationship between the EMS strength and the EMBr electromagnetic braking strength when ΔCE=0.015 when the steel type is 0.13%C-0.1%Si-0.1%Mn steel. [Figure 15] 1 is a graph showing the relationship between the EMS strength and the EMBr electromagnetic braking strength when ΔCE=0.015 when the steel type is 0.13%C-1.0%Si-0.1%Mn steel. [Figure 16] 1 is a graph showing the relationship between the EMS strength and the EMBr electromagnetic braking strength when ΔCE=0.015 when the steel type is 0.13%C-0.1%Si-6.0%Mn steel. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, a steel cast slab manufacturing system according to an embodiment of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiment. In addition, in the drawings shown in this specification, the size of some of the components may be exaggerated for the purpose of explanation. The relative sizes of the components shown in the drawings do not necessarily accurately represent the actual size relationships between the components.

[0018] <<Configuration of continuous casting equipment>> First, the overall configuration of a continuous casting apparatus to which a casting condition setting device according to one embodiment of the present invention can be applied will be described with reference to Fig. 1. Fig. 1 is a schematic explanatory view showing an example of a continuous casting apparatus.

[0019] As shown in Fig. 1, a continuous casting apparatus 1 according to this embodiment is an apparatus for continuously casting molten steel 2 using a continuous casting mold 110 to produce a cast piece 3 such as a slab. The continuous casting apparatus 1 includes the mold 110, a ladle 4, a tundish 5, an immersion nozzle 6, a secondary cooling device 7, and a cast piece cutting machine 8.

[0020] The ladle 4 is a movable container for transporting the molten steel 2 from the outside to the tundish 5. The ladle 4 is disposed above the tundish 5, and the molten steel 2 in the ladle 4 is supplied to the tundish 5. The tundish 5 is disposed above the mold 110, and stores the molten steel 2 therein, removing inclusions from the molten steel 2. The submerged entry nozzle 6 extends downward from the lower end of the tundish 5 toward the mold 110, and its tip is immersed in the molten steel 2 in the mold 110. The submerged entry nozzle 6 continuously supplies the molten steel 2, from which inclusions have been removed in the tundish 5, into the mold 110.

[0021] The mold 110 has a rectangular cylindrical shape corresponding to the width and thickness of the slab 3. For example, the mold 110 is assembled so that a pair of long side mold plates (corresponding to the long side mold plate 111 shown in FIG. 2 and other figures, which will be described later) sandwich a pair of narrow side mold plates (corresponding to the narrow side mold plate 112 shown in FIG. 4 and other figures, which will be described later) from both sides. The long side mold plates and narrow side mold plates (hereinafter sometimes collectively referred to as mold plates) are, for example, water-cooled copper plates provided with water channels through which cooling water flows. The mold 110 cools the molten steel 2 that comes into contact with the mold plates to produce the slab 3. As the slab 3 moves downward in the mold 110, solidification of the internal liquid portion 3b progresses, and the thickness of the outer solidified shell 3a gradually increases. The slab 3 including the solidified shell 3a and liquid portion 3b is pulled out from the bottom end of the mold 110.

[0022] In the following description, the up-down direction (i.e., the direction in which the slab 3 is withdrawn from the mold 110) is also referred to as the Z-axis direction. The Z-axis direction is also referred to as the vertical direction. Two mutually perpendicular directions in a plane (horizontal plane) perpendicular to the Z-axis direction are also referred to as the X-axis direction and the Y-axis direction, respectively. The X-axis direction is defined as the direction parallel to the long sides of the mold 110 in the horizontal plane (i.e., the mold width direction), and the Y-axis direction is defined as the direction parallel to the short sides of the mold 110 in the horizontal plane (i.e., the mold thickness direction). The direction parallel to the XY plane is also referred to as the horizontal direction. In the following description, when expressing the size of each component, the length of the component in the Z-axis direction may be referred to as the height, and the length of the component in the X-axis or Y-axis direction may be referred to as the width.

[0023] Although not shown in FIG. 1 to avoid cluttering the drawing, in this embodiment, an electromagnetic force generator 170 is installed on the outer surface of the long side mold plate of the mold 110. Continuous casting is performed while driving the electromagnetic force generator 170. The electromagnetic force generator 170 includes an electromagnetic stirring device 150 and an electromagnetic brake device 160. In this embodiment, continuous casting is performed while driving the electromagnetic force generator 170, thereby enabling higher-speed casting while ensuring the quality of the cast slab. The configuration of the electromagnetic force generator 170 and its installation position relative to the mold 110 will be described later with reference to FIGS. 2 to 5.

[0024] The secondary cooling device 7 is provided in the secondary cooling zone 9 below the mold 110, and cools the slab 3 withdrawn from the lower end of the mold 110 while supporting and transporting it. The secondary cooling device 7 has multiple pairs of support rolls (e.g., support rolls 11, pinch rolls 12, and segment rolls 13) arranged on both sides of the slab 3 in the thickness direction, and multiple spray nozzles (not shown) that spray cooling water onto the slab 3.

[0025] The support rolls provided in the secondary cooling device 7 are arranged in pairs on both sides in the thickness direction of the slab 3 and function as a support and transport means that supports and transports the slab 3. By supporting the slab 3 from both sides in the thickness direction with the support rolls, breakout and bulging of the slab 3 during solidification in the secondary cooling zone 9 can be prevented.

[0026] The support rolls 11, pinch rolls 12, and segment rolls 13 form a transport path (pass line) for the slab 3 in the secondary cooling zone 9. As shown in FIG. 1 , this pass line is vertical just below the mold 110, then curves, and finally becomes horizontal. In the secondary cooling zone 9, the vertical portion of the pass line is referred to as a vertical section 9A, the curved portion as a curved section 9B, and the horizontal portion as a horizontal section 9C. A continuous casting apparatus 1 having such a pass line is referred to as a vertical bending type continuous casting apparatus 1. Note that the present invention is not limited to the vertical bending type continuous casting apparatus 1 shown in FIG. 1 , but can also be applied to various other continuous casting apparatuses, such as a curved type or vertical type.

[0027] The support rolls 11 are non-driven rolls provided in the vertical section 9A directly below the mold 110, and support the slab 3 immediately after it has been withdrawn from the mold 110. Because the solidified shell 3a of the slab 3 immediately after it has been withdrawn from the mold 110 is thin, it needs to be supported at a relatively short interval (roll pitch) to prevent breakout and bulging. Therefore, it is desirable to use small-diameter rolls as the support rolls 11, which allow the roll pitch to be shortened. In the example shown in FIG. 1, three pairs of support rolls 11, each consisting of a small diameter roll, are provided on both sides of the slab 3 in the vertical section 9A, with a relatively narrow roll pitch.

[0028] The pinch rolls 12 are driven rolls that are rotated by a driving means such as a motor, and have the function of pulling the slab 3 out of the mold 110. The pinch rolls 12 are disposed at appropriate positions in the vertical section 9A, the curved section 9B, and the horizontal section 9C. The slab 3 is pulled out of the mold 110 by the force transmitted from the pinch rolls 12 and transported along the pass line. The arrangement of the pinch rolls 12 is not limited to the example shown in FIG. 1 , and the arrangement positions may be set arbitrarily.

[0029] The segment rolls 13 (also referred to as guide rolls) are non-driven rolls provided in the curved portion 9B and the horizontal portion 9C, and support and guide the slab 3 along the pass line. The segment rolls 13 may be arranged with different roll diameters and roll pitches depending on their positions on the pass line and whether they are provided on the F face (fixed face, the face on the lower left in FIG. 1) or the L face (loose face, the face on the upper right in FIG. 1) of the slab 3.

[0030] The slab cutter 8 is disposed at the end of the horizontal section 9C of the pass line and cuts the slab 3 transported along the pass line to a predetermined length. The cut slab 14 in the shape of a thick plate is transported by table rolls 15 to the equipment for the next process.

[0031] The overall configuration of the continuous casting apparatus 1 according to this embodiment has been described above with reference to Fig. 1. In this embodiment, it is sufficient that an electromagnetic force generating device 170 having a configuration described below is installed in the mold 110 and continuous casting is performed using this electromagnetic force generating device 170, and the configuration of the continuous casting apparatus 1 other than the electromagnetic force generating device 170 may be the same as that of a general conventional continuous casting apparatus. Therefore, the configuration of the continuous casting apparatus 1 is not limited to that shown in the figure, and any configuration may be used as the continuous casting apparatus 1.

[0032] <<Electromagnetic force generator 170>> <Configuration of electromagnetic force generating device 170> The configuration of the electromagnetic force generator 170 installed relative to the above-described mold 110 will be described in detail with reference to FIGS.

[0033] FIG. 2 is a cross-sectional view of the mold equipment 10 according to this embodiment taken along the YZ plane. FIG. 3 is a cross-sectional view of the mold equipment 10 taken along the AA cross section shown in FIG. 2. FIG. 4 is a cross-sectional view of the mold equipment 10 taken along the BB cross section shown in FIG. 3. FIG. 5 is a cross-sectional view of the mold equipment 10 taken along the CC cross section shown in FIG. 3. Since the mold equipment 10 is symmetrical about the center of the mold 110 in the Y-axis direction, only the portion corresponding to one of the long side mold plates 111 is shown in FIGS. 2, 4, and 5. For ease of understanding, molten steel 2 in the mold 110 is also shown in FIGS. 2, 4, and 5.

[0034] Referring to Figures 2 to 5, the mold equipment 10 of this embodiment is configured by installing two water tanks 130, 140 and an electromagnetic force generating device 170 on the outer surface of the long side mold plate 111 of the mold 110 via a backup plate 121.

[0035] As described above, the mold 110 is assembled by sandwiching a pair of narrow side mold plates 112 between a pair of long side mold plates 111. The mold plates 111 and 112 are made of copper plates. However, this embodiment is not limited to this example, and the mold plates 111 and 112 may be made of various materials generally used for molds in continuous casting machines.

[0036] Here, this embodiment is directed to continuous casting of steel slabs, and the size of the slab is approximately 800 to 2300 mm in width (i.e., length in the X-axis direction) and 200 to 300 mm in thickness (i.e., length in the Y-axis direction). That is, the mold plates 111 and 112 also have sizes corresponding to the slab size. That is, the long side mold plate 111 has a width in the X-axis direction that is at least longer than the width of the slab 3, 800 to 2300 mm, and the narrow side mold plate 112 has a width in the Y-axis direction that is approximately the same as the thickness of the slab 3, 200 to 300 mm.

[0037] It is generally known that as solidification of the molten steel 2 progresses within the mold 110, the slab 3 separates from the inner wall of the mold 110 due to solidification shrinkage, which can result in insufficient cooling of the slab 3. For this reason, the length of the mold 110 in the Z-axis direction is limited to approximately 1000 mm at most from the surface of the molten steel.

[0038] The backup plates 121, 122 are made of, for example, stainless steel, and are provided to cover the outer surfaces of the mold plates 111, 112 in order to reinforce the mold plates 111, 112 of the mold 110. Hereinafter, for the sake of distinction, the backup plate 121 provided on the outer surface of the long side mold plate 111 will also be referred to as the long side backup plate 121, and the backup plate 122 provided on the outer surface of the narrow side mold plate 112 will also be referred to as the narrow side backup plate 122.

[0039] Since the electromagnetic force generator 170 applies an electromagnetic force to the molten steel 2 in the mold 110 via the long side backup plate 121, at least the long side backup plate 121 can be made of a non-magnetic material (e.g., non-magnetic stainless steel, etc.).

[0040] The long-side backup plate 121 is further provided with a pair of backup plates 123 extending in a direction perpendicular to the long-side backup plate 121 (i.e., in the Y-axis direction). As shown in FIGS. 3 to 5 , an electromagnetic force generator 170 is installed between the pair of backup plates 123. In this manner, the backup plate 123 can determine the width (i.e., the length in the X-axis direction) and the installation position in the X-axis direction of the electromagnetic force generator 170. In other words, the installation position of the backup plate 123 is determined so that the electromagnetic force generator 170 can apply electromagnetic force to a desired range of the molten steel 2 in the mold 110. Hereinafter, for the sake of distinction, the backup plate 123 will also be referred to as a width-direction backup plate 123. Like the backup plates 121 and 122, the width-direction backup plate 123 is also formed of, for example, stainless steel.

[0041] The water boxes 130, 140 store cooling water for cooling the mold 110. In this embodiment, as shown in the figure, one water box 130 is installed in an area a predetermined distance from the upper end of the long side mold plate 111, and the other water box 140 is installed in an area a predetermined distance from the lower end of the long side mold plate 111. By providing the water boxes 130, 140 at the upper and lower parts of the mold 110, respectively, it is possible to ensure space for installing an electromagnetic force generator 170 between the water boxes 130, 140. Hereinafter, for the sake of distinction, the water box 130 installed above the long side mold plate 111 will be referred to as the upper water box 130, and the water box 140 installed below the long side mold plate 111 will be referred to as the lower water box 140.

[0042] A water channel (not shown) through which cooling water passes is formed inside the long side mold plate 111 or between the long side mold plate 111 and the long side backup plate 121. The water channel extends to the water boxes 130, 140. A pump (not shown) causes cooling water to flow through the water channel from one water box 130, 140 to the other water box 130, 140 (for example, from the lower water box 140 to the upper water box 130). This cools the long side mold plate 111, and the molten steel 2 inside the mold 110 is cooled via the long side mold plate 111. Although not shown, a water box and a water channel are also provided for the narrow side mold plate 112, and cooling water flows through the water channel to cool the narrow side mold plate 112.

[0043] The electromagnetic force generator 170 includes an electromagnetic stirring device 150 and an electromagnetic braking device 160. As shown in the figure, the electromagnetic stirring device 150 and the electromagnetic braking device 160 are installed in the space between the water boxes 130 and 140. In this space, the electromagnetic stirring device 150 is installed at the top and the electromagnetic braking device 160 is installed at the bottom.

[0044] The electromagnetic stirrers 150 apply a dynamic magnetic field to the molten steel 2 in the mold 110, thereby applying an electromagnetic force to the molten steel 2. The electromagnetic stirrers 150 are driven so that an electromagnetic force acts on the molten steel 2 in the width direction (i.e., the X-axis direction) of the long side mold plate 111 on which they are installed. In FIG. 4, the direction of the electromagnetic force acting on the molten steel 2 by the electromagnetic stirrers 150 is shown simulated by thick arrows. Here, the electromagnetic stirrers 150 provided on the long side mold plate 111 (not shown in the figure) (i.e., the long side mold plate 111 facing the long side mold plate 111 shown in the figure) are driven so that an electromagnetic force acts in the width direction of the long side mold plate 111 on which they are installed, in the opposite direction to the direction shown in the figure. In this way, a pair of electromagnetic stirrers 150 are driven to generate a swirling flow in a horizontal plane. The electromagnetic stirring device 150 generates such a swirling flow, which causes the molten steel 2 at the solidified shell interface to flow, thereby achieving a cleaning effect that suppresses the capture of bubbles and inclusions in the solidified shell 3a, thereby improving the surface quality of the cast slab 3.

[0045] The following describes the detailed configuration of the electromagnetic stirring device 150. The electromagnetic stirring device 150 is composed of a case 151, an iron core 152 (hereinafter also referred to as the electromagnetic stirring core 152) stored in the case 151, and a plurality of coils 153 formed by winding a conducting wire around the electromagnetic stirring core 152.

[0046] The case 151 is a hollow member having a substantially rectangular parallelepiped shape. The size of the case 151 can be determined appropriately so that the electromagnetic stirring device 150 can apply an electromagnetic force to a desired range of the molten steel 2, that is, so that the coil 153 provided inside can be disposed at an appropriate position with respect to the molten steel 2. In the electromagnetic stirring device 150, the electromagnetic force is applied to the molten steel 2 from the coil 153 through the side wall of the case 151, and therefore the case 151 is made of a non-magnetic material that has sufficient strength, such as non-magnetic stainless steel or FRP (Fiber Reinforced Plastics).

[0047] The electromagnetic stirring core 152 is a solid member having a substantially rectangular parallelepiped main body and a plurality of teeth 154 protruding from the main body, and is installed in the case 151 so that its longitudinal direction is substantially parallel to the width direction (i.e., the X-axis direction) of the long side mold plate 111. The electromagnetic stirring core 152 is formed, for example, by laminating electromagnetic steel plates.

[0048] Coil 153 is formed by winding a conductor around electromagnetic stirring core 152 with the X-axis direction as the winding axis (i.e., coil 153 is formed so as to magnetize electromagnetic stirring core 152 in the X-axis direction). For example, a copper conductor having a cross section of 10 mm x 10 mm and an internal cooling water channel with a diameter of approximately 5 mm is used as the conductor. When a current is applied, the conductor is cooled using the cooling water channel. The surface of the conductor is insulated with insulating paper or the like, and can be wound in layers. For example, one coil 153 is formed by winding the conductor in approximately two to four layers. Coils 153 having similar configurations are arranged in parallel at predetermined intervals in the X-axis direction.

[0049] Specifically, the electromagnetic stirrer core 152 has a substantially rectangular parallelepiped main body extending in the X-axis direction, and multiple teeth 154 are provided to protrude horizontally from the main body toward the mold 110. These multiple teeth 154 are arranged in the X-axis direction at predetermined intervals. Conductive wires are wound around the regions between the multiple teeth 154, with the X-axis direction as the winding axis, to form multiple coils 153. The width Wt of the teeth 154 (i.e., the length in the X-axis direction) and the width Wc of the coil 153 (i.e., the length of the coil 153 in the X-axis direction, which corresponds to the distance between adjacent teeth 154 in the X-axis direction) are appropriately set in consideration of the width W1 of the electromagnetic stirrer core 152 and the desired stirring force that can improve the quality of the slab 3. For example, the width Wt of the teeth 154 is approximately 30 mm to 120 mm, and the width Wc of the coil 153 is approximately 20 mm to 120 mm.

[0050] A power supply device (not shown) is connected to each of the plurality of coils 153. The power supply device applies an alternating current to the plurality of coils 153 such that the phase of the current is appropriately shifted in accordance with the arrangement order of the plurality of coils 153, thereby applying an electromagnetic force that causes a swirling flow to the molten steel 2. The driving of the power supply device can be appropriately controlled by a control device (not shown) including a processor or the like operating in accordance with a predetermined program. The control device appropriately controls the amount of current applied to each of the coils 153, the phase of the alternating current applied to each of the coils 153, etc., and can thereby control the strength of the electromagnetic force applied to the molten steel 2.

[0051] The width W1 of the electromagnetic stirring core 152 in the X-axis direction can be determined appropriately so that the electromagnetic stirring device 150 can apply electromagnetic force to a desired range of the molten steel 2, that is, so that the coil 153 can be disposed at an appropriate position with respect to the molten steel 2. For example, W1 is about 1800 mm.

[0052] In the illustrated configuration example, electromagnetic stirrer core 152 has teeth 154, but the first embodiment is not limited to this example. In the first embodiment, electromagnetic stirrer core 152 does not have to be provided with teeth 154. In this case, electromagnetic stirrer core 152 is configured in a substantially rectangular parallelepiped shape, and conductive wires are wound around electromagnetic stirrer core 152 at predetermined intervals in the X-axis direction, thereby forming a plurality of coils 153 arranged at predetermined intervals in the X-axis direction.

[0053] The electromagnetic brake device 160 applies a static magnetic field to the molten steel 2 in the mold 110, thereby causing an electromagnetic force to act on the molten steel 2. Here, FIG. 6 is a diagram for explaining the direction of the electromagnetic force acting on the molten steel 2 by the electromagnetic brake device 160. FIG. 6 schematically illustrates a cross section in the XZ plane of the configuration in the vicinity of the mold 110. In addition, FIG. 6 shows the positions of the electromagnetic stirring core 152 and an electromagnetic brake core 162, which will be described later, in a simulated manner using dashed lines.

[0054] As shown in FIG. 6 , the submerged entry nozzle 6 may be provided with a pair of discharge holes 61 positioned opposite the narrow side mold plate 112. The electromagnetic brake device 160 is driven to apply an electromagnetic force to the molten steel 2 in a direction that suppresses the flow (discharge flow) of the molten steel 2 from the discharge holes 61 of the submerged entry nozzle 6. In FIG. 6 , the directions of the discharge flows from the two discharge holes 61 are shown schematically by thin arrows, and the directions of the electromagnetic forces acting on the molten steel 2 from the discharge holes 61 by the electromagnetic brake device 160 are shown schematically by thick arrows. The electromagnetic brake device 160 generates an electromagnetic force in a direction that suppresses the discharge flow, thereby streamlining the flow in the region below the electromagnetic brake. Note that the discharge flow refers to the main flow of the molten steel 2 discharged from the discharge holes 61. For example, if the discharge holes 61 are tilted downward by 30 degrees from the horizontal plane, the flow direction will be tilted approximately 30 degrees downward. Note that the discharge flow is not necessarily linear. In the above example, if the flow is tilted downward at 30 degrees and then hits the mold wall and changes direction, for example, to a vertical direction, the discharge flow may also include the flow after it has changed direction.

[0055] The following describes the detailed configuration of the electromagnetic brake device 160. As shown in Fig. 5, the electromagnetic brake device 160 is composed of a case 161, an electromagnetic brake core 162 housed in the case 161, and a coil 163 formed by winding a conducting wire around the electromagnetic brake core 162.

[0056] The case 161 is a hollow member having a substantially rectangular parallelepiped shape. The size of the case 161 can be determined appropriately so that the electromagnetic force can be applied to a desired range of the molten steel 2 by the electromagnetic brake device 160, that is, so that the coil 163 provided inside can be positioned appropriately with respect to the molten steel 2. However, the width of the electromagnetic stirring device 150 and the width of the electromagnetic brake device 160 may be different.

[0057] Furthermore, in the electromagnetic brake device 160, electromagnetic force is applied to the molten steel 2 from the coil 163 through the side wall of the case 161, so the case 161, like the case 151, is made of a non-magnetic material that can ensure strength, such as non-magnetic stainless steel or FRP.

[0058] A coil 163 is formed by winding a conducting wire around the end of the electromagnetic brake core 162 with the Y-axis direction as the winding axis direction (i.e., the coil 163 is formed so as to magnetize the electromagnetic brake core 162 in the Y-axis direction). The structure of the coil 163 is similar to that of the coil 153 of the electromagnetic stirring device 150 described above.

[0059] A power supply device (not shown) is connected to each of the coils 163. When a direct current is applied to each of the coils 163 by the power supply device, an electromagnetic force that weakens the momentum of the discharge flow can be applied to the molten steel 2. The driving of the power supply device can be appropriately controlled by a control device (not shown) including a processor or the like operating in accordance with a predetermined program. The control device appropriately controls the amount of current applied to each of the coils 163, and the strength of the electromagnetic force applied to the molten steel 2 can be controlled.

[0060] The width W0 of the electromagnetic brake core 162 in the X-axis direction can be determined appropriately so that the electromagnetic stirring device 150 can apply electromagnetic force to a desired range of the molten steel 2, that is, so that the coil 163 can be disposed at an appropriate position with respect to the molten steel 2. For example, W0 is about 1600 mm. So far, the electromagnetic force generator 170 has been described.

[0061] (Molten Steel 2) Molten steel 2 is not particularly limited to the following as long as the steel slab to be cast is one used in a process for producing high-tensile steel plate, but if the molten steel has a composition involving peritectic solidification and contains 0.1 to 0.5 mass% C and relatively large amounts of Si and Mn, cracks are more likely to occur during the production of the steel slab. Therefore, the present invention is particularly preferably applied to cases where molten steel 2 contains 0.1 to 0.5 mass% C and at least one of 0.8 mass% or more Si and 4.0 mass% or more Mn.

[0062] So far, we have described the steel slab manufacturing apparatus (continuous casting apparatus 1). The continuous casting apparatus 1 can produce high-tensile steel slabs under casting conditions set by a casting condition setting device, which will be described later.

[0063] <<Casting condition setting device>> Next, a casting condition setting device according to one embodiment of the present invention will be described with reference to FIG. 7. FIG. 7 is a block diagram showing the configuration of the casting condition setting device according to one embodiment of the present invention. The casting condition setting device 50 shown in FIG. 7 is a casting condition setting device according to one embodiment of the present invention, and includes a casting condition calculation unit 51 that calculates casting conditions for making the collision speed of the discharge flow when it collides with the long side wall of the mold 110 less than a predetermined reference value when producing a high-tensile steel slab, and a setting unit 52 that sets, for the continuous casting apparatus 1, setting values ​​that enable casting under the casting conditions calculated by the casting condition calculation unit 51. The high-tensile steel slab referred to here refers to a steel slab cast in a process for producing a high-tensile steel plate that contains relatively large amounts of Si and Mn and has a tensile strength of, for example, about 780 MPa.

[0064] <Reference value of collision velocity when discharge flow collides with long side wall of mold 110> With reference to Fig. 8, the reference value of the collision speed when the discharge flow collides with the long side wall of the mold 110 will be described. Fig. 8 is a conceptual diagram for explaining the flow of the discharge flow when the electromagnetic brake is not applied and when it is fully applied. Fig. 8 shows the flow of the discharge flow in a cross section (XY plane) of molten steel 2 in the mold 110 perpendicular to the Z axis.

[0065] The molten steel 2 discharged from the discharge port 61 of the submerged entry nozzle 6 is accelerated by an electromagnetic force acting from the magnetic field generated by the electromagnetic stirrer 150 and curves in the swirling direction of the molten steel 2 in the mold 110. If the electromagnetic brake does not act on the discharge flow, as shown in the left diagram of Figure 8, the discharge flow, which is accelerated and curved in the swirling direction of the molten steel 2, collides with the short side walls of the mold 110 and then strongly collides with the long side walls of the corners of the mold 110, which are located in the swirling direction (hereinafter, the velocity of the discharge flow when colliding with the short side walls is considered to be approximately the same as the velocity of the discharge flow when colliding with the long side walls). Solidification of the molten steel 2 also progresses on the long side walls of the corners, but this solidification is affected by the strong flow of the molten steel 2, coupled with the collision of the discharge flow with the long side walls of the corners. On the other hand, when the electromagnetic brake acts sufficiently on the discharge flow, the velocity of the discharge flow is reduced by the electromagnetic force acting on the molten steel 2 constituting the discharge flow, and the discharge flow does not reach the portions near the corners of the long side wall, or even if it does reach them, the degree of collision is weak.

[0066] Normally, when molten steel solidifies without a strong flow, variations in the concentration of components on the scale of several tens of μm, known as microsegregation, occur. However, because the scale is small, it tends to become uniform through diffusion at high temperatures, and microsegregation rarely has a direct adverse effect on the casting process. On the other hand, when there is a strong flow of molten steel, the flow of elements such as C, Si, and Mn, which are discharged into the molten steel during solidification, can easily cause macrosegregation. Si and Mn have a slower diffusion rate in the solid phase than C, and their segregation can adversely affect the steel slab (causing the slab to have a non-uniform structure and prone to cracking). When casting steel types with low Si and Mn contents, such as ordinary steel, the low Si and Mn contents, which have slow diffusion rates in the solid phase, make macrosegregation near the inner wall surface of the mold 110 less likely to have adverse effects. However, in steel types with peritectic solidification, such as high-tensile steel, which contain a large amount of Si or Mn and also contain approximately 0.1 to 0.5 mass% C, macrosegregation can adversely affect the quality of the steel slab. In these steel types, if there are local variations in the concentration distribution of Si, a ferrite-stabilizing element, or Mn, an austenite-stabilizing element, during solidification, the resulting peritectic structure will be spatially non-uniformly distributed. For example, in areas with high Si concentrations, the δ phase is locally abundant and unevenly distributed. In areas with high Si concentrations, the volumetric shrinkage due to the δ-γ transformation (δ-to-γ phase) is greater than that in areas with low δ-phase formation, which can cause distortion in the solidified shell and lead to cracking of the steel slab. Therefore, by braking the discharge flow with the electromagnetic brake device 160 and suppressing macrosegregation near the corners, which would otherwise flow strongly if the electromagnetic brake device 160 were not installed, and by making the peritectic solidification spatially uniform, it is possible to suppress cracking of the steel slab.

[0067] From the above, it is preferable that the reference value of the collision speed when the discharge flow collides with the portion near the corner of the long side wall of the mold 110 is determined based on the degree of influence on peritectic solidification of the change in concentration of Si and Mn contained in the molten steel 2 caused by the discharge flow.

[0068] Next, an example of a reference value for the collision speed when the discharge flow collides with the long side wall of the mold 110 will be described in detail with reference to Fig. 9. One example of the reference value is a calculation formula for determining the influence ΔCE of local Si concentration changes and Mn concentration changes due to segregation on the peritectic structure. Below, we will explain why the influence ΔCE determined by this calculation formula can be used as the reference value.

[0069] (Flow velocity U0 of the discharge flow when it flows into the upper end of the electromagnetic brake core 162) The discharge flow velocity Vp of the molten steel 2 discharged from the discharge hole 61 can be expressed by the following equation (1) using the cross-sectional area Sp of a cross section perpendicular to the flow direction of the discharge hole 61 and including the four sides of the discharge hole 61, the cross-sectional area Ss of a horizontal cross section along the XY plane of the internal space of the mold 110, and the casting velocity Vc. Vp=Vc×(Ss / Sp) (1) formula

[0070] The discharge flow accelerates from the lower end of the discharge hole 61 to the upper end of the electromagnetic brake core 162 due to electromagnetic force F1 generated by the magnetic field generated by the electromagnetic stirring device 150. If the distance from the upper end of the electromagnetic stirring core 152 to the lower end of the discharge hole 61 is D, the distance from the upper end of the electromagnetic stirring core 152 to the upper end of the electromagnetic brake core 162 is Hb, and the angle between the horizontal plane and the discharge flow is θ (rad.), the distance Ls over which the discharge flow accelerates is expressed by the following equation (2). The angle between the horizontal plane and the discharge flow is the angle between the horizontal plane and the direction along the long side wall of the mold 110 at the lower end of the discharge hole 61. Ls=(Hb-D) / sinθ...Equation (2)

[0071] The above Ls is the distance in a region located above the electromagnetic brake core 162, and this region is a region where forced convection is dominant, as the molten steel 2 is strongly affected by the magnetic field generated by the electromagnetic stirring device 150 and the diffusion of momentum due to turbulence generated by the magnetic field and the discharge flow. The electromagnetic force F1 acting on the discharge flow by the electromagnetic stirring device 150 within the range of the above Ls can be approximated by the following equation (3). F1=σ×Es×Bs...Equation (3) Here, σ is the conductivity per unit mass of the molten steel 2 (discharge flow), Es is the electric field generated by the time change of the magnetic field generated by the electromagnetic stirring device 150, and Bs is the spatial average magnetic flux density of the magnetic field generated by the electromagnetic stirring device 150.

[0072] The electric field Es can be approximated using the frequency f of the alternating current applied to the coil 153 of the electromagnetic stirrer 150 and the wavelength Δx of the phase of the alternating current, and is expressed by the following equation (4). Es=f×Δx×Bs (4)

[0073] Therefore, from the above equation (4), the above equation (3) can be expressed as the following equation (5). F1=σ×f×Δx×Bs 2 ...Equation (5)

[0074] Therefore, the equation of motion of the molten steel 2 on which the electromagnetic force F1, which is generated by the magnetic field generated by the electromagnetic stirrer 150, acts is expressed by the following equation (6). du / dt=σ×f×Δx×Bs 2 ...Equation (6)

[0075] Then, when the time when the molten steel 2 is discharged from the discharge port 61 is set to t=0 and the above equation of motion (equation (6)) is solved, the velocity u1 of the molten steel 2 at time t is expressed by the following equation (7). u1=Vp+2×Cs×t...Equation (7) where Cs=0.5×σ×f×Δx×Bs 2 is.

[0076] When equation (7) above is integrated over the range from time t=0 to time t=T1 when the discharge flow reaches the upper end of the electromagnetic brake core 162, the following equation (8) is obtained. When equation (8) is solved for T1, the time T1 when the discharge flow reaches the upper end of the electromagnetic brake core 162 is expressed by equation (9) below. Vp×T1+Cs×T1 2 =Ls...(8) formula T1=0.5 / Cs×(-Vp+(Vp 2 +4×Cs×Ls) 0.5 )...Equation (9)

[0077] Here, by substituting the above equation (9) into equation (7), the flow velocity of the discharge flow at time t=T1, i.e., the flow velocity U0 of the discharge flow when it flows into the upper end P of the electromagnetic brake core 162, is expressed by the following equation (10). U0=(Vp 2 +4×Cs×Ls) 0.5 ...Equation (10)

[0078] (Flow velocity U2 of the discharge flow when it collides with the inner surface of the mold 110 (collision velocity U2)) The discharge flow that has flowed in at the height where the electromagnetic brake core 162 is disposed is decelerated by the magnetic field generated by the electromagnetic brake device 160 until it reaches the short side walls of the mold 110. The distance Lb that the discharge flow that has flowed in at the height where the electromagnetic brake core 162 is disposed travels while being decelerated by the electromagnetic brake device 160 is expressed by the following equation (11) using the width W of the mold 110 (the distance between the short side walls) and the outer diameter D1 of the submerged entry nozzle 6. Lb=0.5×(W-D1) / cosθ-Ls (11) formula

[0079] Here, unless the device is configured such that Lb>0, the electromagnetic brake device 160 cannot be expected to have a direct braking effect on the discharge flow.

[0080] During the distance Lb traveled by the discharge flow while being decelerated by the electromagnetic brake device 160, the electromagnetic force F2 generated on the molten steel 2 by the magnetic field generated by the electromagnetic brake device 160 is expressed by the following equation (12). F2=σ×(u×Bb)×Bb (12) formula Here, u is the flow velocity of the discharge flow, and Bb is the spatial average magnetic flux density of the magnetic field generated by the electromagnetic brake device 160.

[0081] Since this electromagnetic force F2 acts in the direction opposite to the flow velocity u of the molten steel 2, the equation of motion of the molten steel 2 on which the electromagnetic force F2 generated by the magnetic field generated by the electromagnetic brake device 160 acts is expressed by the following equation (13). du / dt=-σ×u×Bb 2 ...Equation (13)

[0082] Then, when the time when the discharge flow flows into the upper end of the electromagnetic brake core 162 is set to t=0, and the above equation of motion (equation (13)) is solved, the velocity u2 of the molten steel 2 at time t is expressed by the following equation (14). u2=U0×exp(-σ×Bb 2 ×t)...Equation (14)

[0083] When the above equation (14) is integrated over the range from time t=0 when the discharge flow flows into the upper end of the electromagnetic brake core 162 to time t=T when the discharge flow has traveled a distance L, the following equation (15) is obtained. (U0 / (σ×Bb 2 ))×(1-exp(-σ×Bb 2 ×T))=L ···(15) formula

[0084] Therefore, from the above formula (15), when the maximum value Lmax of L (the distance the discharge flow travels until the flow velocity becomes 0) satisfies the following formula (16), the discharge flow is sufficiently decelerated by the electromagnetic force acting due to the magnetic field generated by the electromagnetic brake device 160, in other words, by the electromagnetic brake by the electromagnetic brake device 160, and macrosegregation does not occur. Therefore, preferably, U0 / (σ×Bb 2 By operating under conditions that satisfy the relationship )≦Lb, the flow of the molten steel 2 in the surface layer of the mold 110 is almost completely braked, and macrosegregation can be further prevented. Lmax = U0 / (σ×Bb 2 )≦Lb...Equation (16)

[0085] On the other hand, if the maximum value Lmax of L does not satisfy the above equation (16), in other words, U0 / (σ×Bb 2 )>Lb, the flow velocity U2 of the discharge flow at time t=T2 when L=Lb can be approximated as shown in the following equation (17). U2=U0-σ×Lb×Bb 2 ...Equation (17)

[0086] (Effect of flow velocity of molten steel 2 when it solidifies on segregation of molten steel components) Next, we will explain the effect that the flow velocity of molten steel has on the segregation of components in molten steel when it solidifies. According to Non-Patent Document 1, the aforementioned macrosegregation is affected by the flow velocity U of the molten steel and the solidification rate V of the molten steel, and the degree of this can be approximated by the following equation (19): Cm / C0=1-(1-k) / ((7500×V / U)+1) (19) formula Here, Cm is the alloying element concentration in the solid phase after solidification, Co is the alloying element concentration in the molten steel before solidification, and k is the equilibrium distribution coefficient of the alloying element.

[0087] Here, regarding the solidification rate V, the following empirical formula (formula (20)) is often used in relation to the solidified shell thickness d (mm) in continuous casting. By time differentiation of formula (20) and unit conversion, the solidification rate V of the molten steel 2 at the upper end position Hb of the electromagnetic brake core 162 can be approximated as shown in the following formula (21). d=K√t ···(20) formula V=6.45×10 -5 ×K×(Vc / Hb) 0.5 ...Equation (21) In the above formula (20), K is a constant of about 15 to 30, t is time (min.), and in the above formula (21), Vc is the casting speed.

[0088] (The influence of local Si concentration changes and Mn concentration changes due to segregation on the peritectic structure, ΔCE) Next, we will explain the effect of localized changes in Si concentration and Mn concentration due to segregation on the peritectic structure. The Si concentration change ΔC_Si = C0_Si - Cm_Si, which is the difference between the Si concentration C0_Si in the molten steel before solidification and the Si concentration Cm_Si in the solid phase after solidification, is expressed by the following equation (22) based on the above equation (19). ΔC_Si=(C0_Si)×(1-k_Si) / ((7500×V / U)+1) ···(22) Formula In the above equation (22), k_Si is the equilibrium distribution coefficient of Si.

[0089] Similarly, for Mn, the Mn concentration change ΔC_Mn=C0_Mn−Cm_Mn, which is the difference between the Mn concentration C0_Mn in the molten steel before solidification and the Mn concentration Cm_Mn in the solid phase after solidification, is expressed by the following equation (23). ΔC_Mn=(C0_Mn)×(1-k_Mn) / ((7500×V / U)+1) ···(23) Formula In the above equation (23), k_Mn is the equilibrium distribution coefficient of Mn.

[0090] The influence of a change in Si concentration, ΔC_Si, on peritectic solidification can be estimated as 0.1 × ΔC_Si, assuming that the influence of a change in C (carbon) concentration on peritectic solidification is 1.0. Similarly, the influence of Mn can be estimated as 0.02 × ΔC_Mn. The Si concentration and Mn concentration each affect the ease of forming δ and γ phases, but their interaction is small. Therefore, the influence ΔCE of a change in Si concentration, ΔC_Si, and a change in Mn concentration, ΔC_Mn, on peritectic solidification can be expressed by the following equation (24) based on the above equations (22) and (23). Hereinafter, the influence ΔCE of a change in Si concentration, ΔC_Si, and a change in Mn concentration, ΔC_Mn, on peritectic solidification will sometimes be simply referred to as the influence ΔCE. ΔCE=(|0.1×(1-k_Si)×C0_Si|+|0.02×(1-k_Mn)×C0_Mn|) / (7500×V / U+1) ···(24) Formula So far, the reasons why the influence degree ΔCE can be set as the reference value have been explained.

[0091] (Influence degree ΔCE<0.015) The present inventors have found that by casting so that the degree of influence ΔCE is less than 0.015, it is possible to suppress cracking of slabs even in continuous casting of high-tensile steel. By optimizing the operating conditions according to the Si and Mn contents of molten steel 2 and making the degree of influence ΔCE less than 0.015, it is possible to reduce defects in slabs.

[0092] The reason for setting the threshold value of the impact degree ΔCE at 0.015 will be explained. There are various forms of cracks in steel slabs during continuous casting, but particularly with regard to cracks in the high-temperature embrittlement region where cracks occur with slight strain, cracks often occur in steel slabs when the steel slab is distorted beyond the critical strain εcr. For example, according to Non-Patent Document 2, the critical strain εcr in the solidified shell when cracks occur in steel slabs is 0.32 × 10 -2 ~3.8×10 -2 Therefore, the strain generated in the solidified shell during casting is about 0.32 × 10 -2 By making the thickness less than 1000 nm, cracking of the steel slab can be reduced.

[0093] The strain ε generated by the δγ transformation of hypoperitectic steel is ε=|1-(ργ / ρδ) when the strain is isotropic. 1 / 3 When strain occurs in one direction, it is expressed as ε = |1-(ργ / ρδ)|. Here, ργ is the density of the γ phase, and ρδ is the density of the δ phase. The strain that occurs during solidification in the mold is not necessarily isotropic, and strain is greater when concentrated in one direction than when it occurs isotropically. Therefore, the conditions for sufficiently reducing cracking in steel slabs can be expressed by the following equation (25). ε=|1-(ργ / ρδ)|<εcr ···(25) formula

[0094] According to Non-Patent Document 3, the density ργ of the γ phase is expressed by the following formula (26), and the density ρδ of the δ phase is expressed by the following formula (27). ργ=8099.8-0.5×T (26) formula ρδ=7876.0-0.3×T (27) formula

[0095] Assuming that the approximate temperature during casting is T = 1750 K, the strain due to the δ-γ transformation can be calculated as ε ≈ 0.017 from the above formula ε = |1 - (ργ / ρδ)|, the above formulas (26) and (27). Therefore, if the volume fraction of the δ-ferrite phase generated during peritectic solidification is fδ, and the volume fraction of the δ-ferrite phase varies by |Δfδ| due to non-uniform concentration, the strain affecting the solidified shell due to the variation can be estimated as 0.017 × |Δfδ|.

[0096] Here, when the volume fraction of the δ-ferrite phase immediately after complete solidification was calculated using the thermodynamic calculation software Thermo-Calc, the results shown in Table 1 were obtained.

[0097] [Table 1]

[0098] From these results, the change in the volume fraction of the δ-ferrite phase due to the change in C concentration ΔC is C can be estimated as shown in equation (28) below. Δfδ C =-12.7×ΔC (28) formula

[0099] When the influence ΔCE obtained by converting the influence of the Si concentration change ΔC_Si and the Mn concentration change ΔC_Mn into the C concentration is used, the above formula (28) becomes the following formula (29). |Δfδ|=-12.7×ΔCE(ΔC_Si, ΔC_Mn) ···(29) Formula

[0100] From the above, in order to reduce cracks in steel slabs, the above formulas (25), (29), and εcr=0.32×10 -2 Therefore, it is sufficient to satisfy the following equation (30). 0.017×12.7×ΔCE(ΔC_Si, ΔC_Mn)<0.32×10 -2 ···(30) formula

[0101] The following equation (31) is obtained from the above equation (30). ΔCE(ΔC_Si, ΔC_Mn)<1.5×10 -2 ...Equation (31)

[0102] Therefore, according to the above formula (31), by setting the casting conditions so that ΔCE is less than 0.015, it is possible to reduce cracks in the steel slab. So far, we have explained the basis for setting the threshold value of the impact ΔCE at 0.015.

[0103] When the threshold value of ΔCE is set to 0.015, it is preferable that the flow velocity U2 of the discharged flow when it collides with the inner surface of the mold 110 be set so as to satisfy the following formula (32). U2<0.015×7500×V / {(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|)-0.015}...Equation (32)

[0104] <Casting condition calculation unit 51> As described above, the casting condition calculation unit 51 calculates the casting conditions for producing a high-tensile steel slab so that the collision speed of the discharge flow when it collides with the long side wall of the mold 110 is less than a predetermined reference value. As described above, the predetermined reference value is preferably determined based on the influence of the discharge flow on the peritectic solidification of elements contained in the molten steel 2 that have a low diffusion rate in the solid phase, and more preferably, the influence ΔCE = 0.015 is used as the reference value. In conventional steel slab production, cracks often occur on the long sides of the slab. Therefore, the casting condition calculation unit 51 calculates the casting conditions so that the collision speed of the discharge flow when it collides with the long side wall of the mold 110 is less than the predetermined reference value.

[0105] Furthermore, as described above, when the maximum value Lmax of L satisfies the above formula (16), the electromagnetic brake of the electromagnetic brake device 160 sufficiently decelerates the rotor, so macrosegregation is unlikely to occur. 2 It is preferable to operate under conditions that satisfy the following relationship: )≦Lb.

[0106] As described above, the speed of the discharge flow changes depending on the strength of the magnetic field generated by the electromagnetic stirring device 150. The speed of the swirling flow of the molten steel 2 also changes depending on the strength of the magnetic field generated by the electromagnetic stirring device 150, and the behavior of the discharge flow also changes depending on the speed of the swirling flow. Therefore, it is preferable that the casting conditions calculated by the casting condition calculation unit 51 include at least the strength of the magnetic field generated by the electromagnetic stirring device 150.

[0107] As described above, the velocity of the discharge flow changes depending on the strength of the magnetic field generated by the electromagnetic brake device 160. Therefore, it is preferable that the casting conditions calculated by the casting condition calculation unit 51 include the strength of the magnetic field generated by the electromagnetic brake device.

[0108] The casting conditions calculated by the casting condition calculation unit 51 may include, in addition to the above, the casting speed, the shape of the submerged entry nozzle 6, the mold size, etc. The casting condition calculation unit 51 may be configured to input, for example, casting conditions other than the casting conditions to be adjusted (adjustment conditions), and output a range of adjustment conditions such that ΔCE calculated by Equation (24) satisfies Equation (31). For example, if the casting speed is the adjustment condition, other casting conditions such as the mold width, the position of the electromagnetic stirrer 150, the position of the electromagnetic brake device 160, and the submerged entry depth may be input, and the range of casting speeds in which ΔCE is less than 0.015 may be calculated and output. Furthermore, when there are multiple adjustment conditions, the casting condition calculation unit 51 may be configured to output a graph showing the relationship between the adjustment conditions when ΔCE reaches a threshold value that satisfies Equation (31), i.e., when ΔCE = 0.015. In this case, the adjustment conditions that satisfy the ΔCE condition may be determined by visually inspecting the graph. For example, if the three adjustment conditions are the casting speed, the magnetic flux density of the electromagnetic stirring device 150, and the magnetic flux density of the electromagnetic brake device 160, the other casting conditions can be input, and a graph can be created and output with the three adjustment conditions as the x, y, and z axes, respectively, when ΔCE is 0.015. If there are two adjustment conditions, the graph will be two-dimensional, and if there are four or more adjustment conditions, for example, the fourth and subsequent adjustment conditions can be fixed at temporary values, and the process of creating a graph can be performed with multiple temporary values, to create multiple graphs.

[0109] <Settings section 52> As described above, the setting unit 52 sets, for the continuous casting apparatus 1, setting values ​​that enable casting under the casting conditions calculated by the casting condition calculation unit 51. The setting unit 52 receives information on the casting conditions from the casting condition calculation unit 51 and outputs the information to the continuous casting apparatus 1. The continuous casting apparatus 1 casts a steel slab based on the received information.

[0110] Although one embodiment of the present invention has been described above, the present invention is not limited to this example. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0111] For example, Patent Document 2 describes a method for suppressing cellular defects on the surface and inside of a slab when an electromagnetic stirring device and an electromagnetic brake device are used in combination. It states that if the electromagnetic brake is too strong, the flow caused by electromagnetic stirring is hindered, making surface cellular defects more likely to occur. Therefore, if it is desired to suppress cellular defects in addition to suppressing cracking of a steel slab, it is sufficient to use the electromagnetic brake to suppress macrosegregation and slab cracking using the above-mentioned method, and then set an upper limit on the strength of the electromagnetic brake.

[0112] <<How to set casting conditions>> To set the casting conditions, as described above, the casting condition calculation unit 51 calculates the casting conditions, and the setting unit 52 sets, for the continuous casting apparatus 1, setting values ​​that enable casting under the casting conditions calculated by the casting condition calculation unit 51. Therefore, the method for setting casting conditions is a method for setting casting conditions for a continuous casting device that continuously casts steel slabs, and that includes: a mold having a rectangular cross section formed by a pair of long side walls and a pair of short side walls; an immersion nozzle having two discharge holes arranged opposite each of the pair of short side walls of the mold and supplying molten steel from each of the discharge holes into the mold; an electromagnetic stirring device that generates a magnetic field and applies an electromagnetic force to the molten steel inside the mold to stir the molten steel; and an electromagnetic braking device that generates a magnetic field and applies an electromagnetic force to the molten steel that constitutes the discharge flow, which is the flow of molten steel discharged from the discharge holes, to brake the discharge flow.When producing high-tensile steel slabs, the method includes a casting condition calculation step of calculating casting conditions such that the collision speed when the discharge flow collides with the long side walls of the mold is less than a predetermined reference value; and a setting step of setting, for the continuous casting device, setting values ​​that enable casting under the casting conditions calculated in the casting condition calculation step.

[0113] <<Manufacturing Method for High-Tensile Steel Slabs>> By using the continuous casting apparatus 1 and the casting condition setting apparatus 50, high-tensile steel slabs can be produced. Specifically, the method for producing high-tensile steel slabs involves continuously casting high-strength steel slabs using a continuous casting apparatus that includes: a mold having a rectangular cross section formed by a pair of long side walls and a pair of short side walls; an immersion nozzle having two discharge holes arranged opposite each of the pair of short side walls of the mold, which supply molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field and applies an electromagnetic force to the molten steel inside the mold to stir the molten steel; and an electromagnetic braking device that generates a magnetic field and applies an electromagnetic force to the molten steel that constitutes the discharge stream, which is the flow of molten steel discharged from the discharge holes, to brake the discharge stream.The magnetic field generated by the electromagnetic stirring device applies an electromagnetic force to the molten steel in the mold, causing a swirling flow in which the molten steel swirls horizontally, and the magnetic field generated by the electromagnetic braking device acts on the molten steel that constitutes the discharge stream, braking the discharge stream so that the collision speed of the discharge stream, which is curved by the swirling flow and collides with the long side walls of the mold, is less than a predetermined reference value. It goes without saying that the configuration of each device used in the above-described method for producing a high-tensile steel slab is not limited to the configuration provided in the continuous casting device 1. [Example]

[0114] Example 1 A thermo-fluid simulation of the in-mold steel, taking into account solidification and element segregation, was performed for a steel grade expressed as 0.13%C-1.0%Si-0.1%Mn, and the concentration segregation of Si and Mn was calculated. The above in-mold thermo-fluid simulation was performed using the numerical simulation model described in Non-Patent Document 4. Furthermore, from the maximum values ​​of the concentration segregation of Si and Mn obtained in the in-mold thermo-fluid simulation, the influence ΔCE' of changes in the Si and Mn concentrations on peritectic solidification was calculated using the following formula. ΔCE'=|0.1×Δ%Si|+|0.02×Δ%Mn|

[0115] Table 2 shows the ΔCE' values ​​obtained from the results of the above-mentioned in-mold thermal fluid numerical simulation for a steel type with a composition of 0.13%C-1.0%Si-0.1%Mn. Table 2 also shows the influence ΔCE of changes in Si concentration and Mn concentration on peritectic solidification, calculated based on the above-mentioned equation (24). Figure 10 shows a graph of the relationship between EMBr strength and segregation ratio Δ%Si / %Si when the EMS strength is changed. Figure 11 shows a graph of the relationship between EMBr strength and segregation ratio Δ%Si / %Si when the casting speed Vc is changed. The segregation ratio Δ%Si / %Si was calculated using the following equation. Δ%Si / %Si=(C0_Si-Cm_Si) / Cm_Si=ΔC_Si / Cm_Si Figure 12 shows a graph of the relationship between EMBr strength and ΔCE' when the EMS strength is changed. Figure 13 shows a graph of the relationship between EMBr strength and ΔCE' when the casting speed Vc is changed. Figures 10 and 12 were obtained from the results of a thermo-fluid simulation inside the mold when the mold width w was 1.6 m and the casting speed was 1.6 m / min. Figures 11 and 13 were obtained from the results of a thermo-fluid simulation inside the mold when the mold width w was 1.6 m and the AC current applied to the electromagnetic stirrer was 300 A.

[0116] [Table 2]

[0117] As shown in Figures 10 and 11, when the electromagnetic brake was not applied, the segregation ratio Δ%Si / %Si was large. As the strength of the electromagnetic brake (the strength of the magnetic field generated by the electromagnetic stirrer) increased, the segregation ratio Δ%Si / %Si decreased. Furthermore, as shown in Figures 12 and 13, when the electromagnetic brake was not applied, Si segregation was large. The Si content was 1.0 mass%, which is higher than that of conventional steel, resulting in a large ΔCE' value. Applying the electromagnetic brake was effective in suppressing ΔCE'. When the AC current applied to the electromagnetic stirrer was 200 A or 300 A, ΔCE' could be reduced to less than 0.015, even when the average magnetic flux density of the magnetic field generated by the electromagnetic brake was 0.1 T. When the AC current applied to the electromagnetic stirrer was 400 A, ΔCE' could be reduced to less than 0.015 when the average magnetic flux density of the magnetic field generated by the electromagnetic brake was 0.2 T or higher. It was also found that ΔCE has a high correlation with ΔCE', and that by using ΔCE as an indicator to determine the set value, it is possible to prevent segregation of the constituent elements of the molten steel that occurs near the corners of the long side wall inside the mold, thereby suppressing cracks in the steel slab. In addition, in the numerical calculations using the in-mold thermal fluid simulation, even when the average magnetic flux density of the magnetic field generated by the electromagnetic brake device is set to 0.2 T or more, a slight unsteady flow occurs due to the inherent instability of the fluid calculation, so the segregation ratio does not strictly become 0, but it was found that the segregation ratio is sufficiently small and converges to an almost constant value.

[0118] Example 2 Table 3 shows the effect ΔCE of changes in Si and Mn concentration on peritectic solidification, calculated based on the above equation (24), for steels with compositions of 0.13%C-0.1%Si-0.1%Mn (A in Table 3), 0.13%C-1.0%Si-0.1%Mn (B in Table 3), and 0.13%C-0.1%Si-6.0%Mn (C in Table 3). Figure 14 shows the relationship between the EMS strength (strength of the magnetic field generated by the electromagnetic stirrer) and the EMBr electromagnetic brake strength (strength of the magnetic field generated by the electromagnetic brake device) when ΔCE = 0.015 for a 0.13%C-0.1%Si-0.1%Mn steel. Figure 15 shows the relationship between the EMS strength and the EMBr electromagnetic brake strength when ΔCE = 0.015 for a 0.13%C-1.0%Si-0.1%Mn steel. Figure 16 shows the relationship between the EMS strength and the EMBr electromagnetic brake strength when ΔCE = 0.015 for a 0.13%C-0.1%Si-6.0%Mn steel. The mold width w was 1.6 m and the casting speed was 1.6 m / min.

[0119] [Table 3]

[0120] As shown in Figure 14, for 0.13%C-0.1%Si-0.1%Mn steel, which does not contain large amounts of Si and Mn like conventional steel, the absolute change in element concentration due to segregation is small, and ΔCE is less than 0.015 even without the application of the electromagnetic brake. However, for 0.13%C-1.0%Si-0.1%Mn and 0.13%C-0.1%Si-6.0%Mn steels, which contain large amounts of Si and Mn, ΔCE increases without the application of the electromagnetic brake, potentially resulting in quality problems. The curves shown in Figures 15 and 16, and the range of high EMBr strength from these curves, represent the range where ΔCE is greater than 0.015. It was found that ΔCE can be controlled by adjusting the EMS strength and EMBr strength, thereby suppressing cracking in steel slabs. [Explanation of symbols]

[0121] 1. Continuous casting equipment 2. Molten steel 3, 14 Castings 4 Ladle 5 Tundish 6 Submerged Entry Nozzle 7 Secondary cooling device 8 Slab cutting machine 11 Support Roll 12 Pinch Roll 13 Segment Roll 15 Table Roll 16 Secondary cooling zone 50 Casting condition setting device 51 Casting condition calculation section 52 Setting section 61 Discharge hole 110 Mold 111 Long side mold plate 112 Short side mold plate 121, 122 Backup plates 123 Backup plate (width direction backup plate) 130, 140 water box 150 Electromagnetic Stirring Device 151 cases 152 Electromagnetic Stirring Core 153 Coil 154 Teeth 160 Electromagnetic brake device 161 cases 162 Electromagnetic brake core 163 Coil 170 Electromagnetic Force Generator

Claims

1. a submerged nozzle having two discharge holes disposed opposite each other on each of the pair of short side walls of the mold, the submerged nozzle supplying molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field to apply an electromagnetic force to the molten steel inside the mold, thereby stirring the molten steel; and an electromagnetic braking device that generates a magnetic field to apply an electromagnetic force to the molten steel constituting a discharge flow, which is the main flow of the molten steel discharged from the discharge holes, thereby braking the discharge flow, and the casting condition setting device sets the casting conditions of a continuous casting machine that continuously casts high-tensile steel slabs, the casting condition setting device comprising: a mold having a rectangular cross section formed by a pair of long side walls and a pair of short side walls; The setting device can change at least one of the following casting conditions: casting speed Vc, distance Hb from the surface of the molten steel to the upper end of the core of the electromagnetic brake device, mass concentration of alloy components, average magnetic flux density Bb of the magnetic field generated by the electromagnetic brake device, average magnetic flux density Bs of the magnetic field generated by the electromagnetic stirrer, wavelength Δx of the phase of the AC current applied to the electromagnetic stirrer, frequency f of the AC current applied to the electromagnetic stirrer, cross-sectional area Sp of the discharge hole, downward angle θ of the discharge hole, depth position D of the discharge hole, outer diameter D1 of the submerged nozzle, cross-sectional area Ss of the horizontal cross section of the internal space of the mold, and distance W between the long side walls in the internal space, a casting condition calculation unit that calculates the casting conditions for making the collision speed when the discharge flow collides with the long side wall of the mold under the action of electromagnetic forces from the electromagnetic stirring device and the electromagnetic brake device less than a predetermined reference value; a setting unit that sets, for the continuous casting device, setting values ​​that enable casting under the casting conditions calculated by the casting condition calculation unit, The casting condition setting device, wherein the predetermined reference value is a value determined based on the degree of influence that a change in concentration of Si and Mn contained in the molten steel due to the discharge flow has on peritectic solidification.

2. The casting condition setting device according to claim 1 , wherein the casting conditions include at least the strength of the magnetic field generated by the electromagnetic stirring device.

3. 3. The casting condition setting device according to claim 1, wherein the casting conditions include a strength of a magnetic field generated by the electromagnetic brake device.

4. 4. The casting condition setting device according to claim 1, wherein the setting unit sets the set value so that the collision velocity satisfies the following formula (1): U2<0.015×7500×V / {(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|)-0.015}...Equation (1) where: V=6.45×10 －５ ×K×(Vc / Hb) ０．５ however, %X: mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) U2: Collision speed (m / s) Vc: Casting speed (m / s) Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) K: Coagulation coefficient (mm / min ０．５ )

5. The casting condition setting device according to any one of claims 1 to 4, wherein the setting unit sets the set value so that ΔCE expressed by the following formula (2) is less than 0.

015. ΔCE=(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|) / (7500×V / U2+1)...Equation (2) where: V=6.45×10 －５ ×K×(Vc / Hb) ０．５ U2=U0-(σ×Lb×Bb ２ ) (However, U0>σ×Lb×Bb ２ in the case of) U2 = 0 (where U0 ≤ σ × Lb × Bb ２ in the case of) U0=(Vp ２ +4×Cs×Ls) ０．５ Ls=(Hb-D) / sinθ Lb=0.5×(W-D1) / cosθ-Ls Cs=0.5×σ×f×Δx×Bs ２ Vp=Vc×(Ss / Sp) however, %X: mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) σ: Electrical conductivity per unit mass of the molten steel (Sm ２ / kg) U2: Collision speed (m / s) Bb: average magnetic flux density (T) of the magnetic field generated by the electromagnetic brake device Bs: average magnetic flux density (T) of the magnetic field generated by the electromagnetic stirrer Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) Vc: Casting speed (m / s) Vp: flow velocity (m / s) of the discharge flow at the discharge hole of the submerged nozzle Sp: Cross-sectional area of ​​the discharge hole (m ２ ) Ss: cross-sectional area (m ２ ) W: the distance between the long side walls in the internal space of the mold (m) Δx: wavelength (m) of the phase of the AC current applied to the electromagnetic stirrer f: frequency of the alternating current applied to the electromagnetic stirrer (1 / s) D: Depth position of the discharge hole (m) D1: outer diameter of the immersion nozzle (m) θ: downward angle of the discharge hole (rad) K: Coagulation coefficient (mm / min ０．５ )

6. The U0, the Bb, and the Lb are U0≦σ×Lb×Bb ２ 6. The casting condition setting device according to claim 5, wherein casting is performed so as to satisfy the following:

7. The casting condition setting device according to any one of claims 1 to 6, wherein the molten steel contains C: 0.1 to 0.5 mass%, and at least one of Si: 0.8 mass% or more and Mn: 4.0 mass% or more.

8. A continuous casting apparatus for casting a high-tensile steel slab under the casting conditions set by the casting condition setting device according to any one of claims 1 to 7.

9. a submerged entry nozzle having two discharge holes disposed opposite each other on each of the pair of short side walls of the mold, for supplying molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field to apply an electromagnetic force to the molten steel inside the mold, thereby stirring the molten steel; and an electromagnetic braking device that generates a magnetic field to apply an electromagnetic force to the molten steel constituting a discharge flow, which is the main flow of the molten steel discharged from the discharge holes, thereby braking the discharge flow, said method comprising: setting casting conditions for a continuous casting machine that continuously casts high-tensile steel slabs, In the setting method, at least one of the following casting conditions can be changed: casting speed Vc, distance Hb from the surface of the molten steel to the upper end of the core of the electromagnetic brake device, mass concentration of alloy components, average magnetic flux density Bb of the magnetic field generated by the electromagnetic brake device, average magnetic flux density Bs of the magnetic field generated by the electromagnetic stirrer, wavelength Δx of the phase of the AC current applied to the electromagnetic stirrer, frequency f of the AC current applied to the electromagnetic stirrer, cross-sectional area Sp of the discharge hole, downward angle θ of the discharge hole, depth position D of the discharge hole, outer diameter D1 of the submerged entry nozzle, cross-sectional area Ss of the horizontal cross section of the internal space of the mold, and distance W between the long side walls in the internal space, a casting condition calculation step of calculating the casting conditions such that a collision speed when the discharge flow collides with the long side wall of the mold under the action of electromagnetic forces from the electromagnetic stirring device and the electromagnetic brake device is less than a predetermined reference value; a setting step of setting, for the continuous casting device, setting values ​​that allow casting under the casting conditions calculated in the casting condition calculation step, The method for setting casting conditions, wherein the predetermined reference value is a value determined based on the degree of influence of a change in concentration of Si and Mn contained in the molten steel due to the discharge flow on peritectic solidification.

10. A continuous casting method for casting a high-tensile steel slab under the casting conditions set by the method for setting casting conditions according to claim 9.

11. a submerged nozzle having two discharge holes disposed opposite each other on each of the pair of short side walls of the mold, the submerged nozzle supplying molten steel into the mold from each of the discharge holes; an electromagnetic stirring device that generates a magnetic field and applies an electromagnetic force to the molten steel inside the mold to stir the molten steel; and an electromagnetic braking device that generates a magnetic field and applies an electromagnetic force to the molten steel that constitutes a discharge flow, which is the main flow of the molten steel discharged from the discharge holes, to brake the discharge flow, an electromagnetic force is applied to the molten steel in the mold by the magnetic field generated by the electromagnetic stirring device, thereby generating a swirling flow in which the molten steel swirls in a horizontal direction; an electromagnetic force acting on the molten steel constituting the discharge flow by a magnetic field generated by the electromagnetic brake device brakes the discharge flow; The collision speed of the discharge flow that is curved by the swirling flow and collides with the long side wall of the mold is set to be less than a predetermined reference value, A method for producing a high-tensile steel slab, wherein the predetermined reference value is a value determined based on the degree of influence of a change in concentration of Si and Mn contained in the molten steel due to the discharge flow on peritectic solidification.

12. The method for producing a high-tensile steel slab according to claim 11, wherein the collision velocity satisfies the following formula (1): U2<0.015×7500×V / {(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|)-0.015}...Equation (1) where: V=6.45×10 －５ ×K×(Vc / Hb) ０．５ however, %X: mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) U2: Collision speed (m / s) Vc: Casting speed (m / s) Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) K: Coagulation coefficient (mm / min ０．５ )

13. 13. The method for producing a high-tensile steel slab according to claim 11 or 12, wherein the reference value is ΔCE expressed by the following formula (2), and the setting values ​​of the continuous casting device are determined so that ΔCE is less than 0.015: ΔCE=(|0.1×(1-k_Si)×%Si|+|0.02×(1-k_Mn)×%Mn|) / (7500×V / U2+1)...Equation (2) where: V=6.45×10 －５ ×K×(Vc / Hb) ０．５ U2=U0-(σ×Lb×Bb ２ ) (However, U0>σ×Lb×Bb ２ in the case of) U2 = 0 (where U0 ≤ σ × Lb × Bb ２ in the case of) U0=(Vp ２ +4×Cs×Ls) ０．５ Ls=(Hb-D) / sinθ Lb=0.5×(W-D1) / cosθ-Ls Cs=0.5×σ×f×Δx×Bs ２ Vp=Vc×(Ss / Sp) however, %X: mass concentration of component X (mass%) k_X: Equilibrium distribution coefficient of component X (-) σ: Electrical conductivity per unit mass of the molten steel (Sm ２ / kg) U2: Collision speed (m / s) Bb: average magnetic flux density (T) of the magnetic field generated by the electromagnetic brake device Bs: average magnetic flux density (T) of the magnetic field generated by the electromagnetic stirrer Hb: Distance from the surface of the molten steel to the top of the electromagnetic brake core (m) Vc: Casting speed (m / s) Vp: flow velocity (m / s) of the discharge flow at the discharge hole of the submerged nozzle Sp: Cross-sectional area of ​​the discharge hole (m ２ ) Ss: cross-sectional area (m ２ ) W: the distance between the long side walls in the internal space of the mold (m) Δx: wavelength (m) of the phase of the AC current applied to the electromagnetic stirrer f: frequency of the alternating current applied to the electromagnetic stirrer (1 / s) D: Depth position of the discharge hole (m) D1: outer diameter of the immersion nozzle (m) θ: downward angle of the discharge hole (rad) K: Coagulation coefficient (mm / min ０．５ )

14. The U0, the Bb, and the Lb are U0≦σ×Lb×Bb ２ The method for producing a high-tensile steel slab according to claim 13, wherein the casting is carried out so as to satisfy the above condition.

15. The method for producing a high-tensile steel slab according to any one of claims 11 to 14, wherein the molten steel contains 0.1 to 0.5% by mass of C and at least one of 0.8% by mass or more of Si and 4.0% by mass or more of Mn.

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