Continuous casting method for steel

Abstract

A method for continuous casting of steel is provided that can suppress fluctuations in reduction rate in the region where the central solid fraction of a slab is 0.7 or more and 1.0 or less. A method for continuous casting of steel in which a slab is cast while being soft reduced using a soft reduction zone, the method continuously predicts a region in the longitudinal direction of the slab where the central solid fraction of the slab is 0.7 to 1.0 using the operating conditions of the continuous casting machine, and dynamically changes the roll gap in the soft reduction zone so that fluctuations in the soft reduction rate in that region are kept below a predetermined value.

Description

[Technical field]

[0001] The present invention relates to a continuous casting method for steel capable of casting a slab having reduced center segregation. [Background technology]

[0002] In the final solidification process of continuous casting of steel, solidification shrinkage causes a suction flow of unsolidified molten steel (hereinafter referred to as the "unsolidified layer") in the direction of drawing of the slab. In this unsolidified layer, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated. When this solute-element-concentrated molten steel flows to the center of the slab and solidifies, central segregation occurs. Factors that cause solute-element-concentrated molten steel to flow to the center of the slab in the final stage of solidification include not only solidification shrinkage, but also slab bulging between rolls due to molten steel static pressure and misalignment of the rolls supporting the slab.

[0003] This central segregation deteriorates the quality of steel products, especially thick steel plates. For example, in line pipe materials for transporting oil or natural gas, hydrogen-induced cracking occurs starting from the central segregation due to the action of sour gas. The same problem occurs in marine structures, storage tanks, oil tanks, etc. In recent years, steel materials are often required to be used in lower temperatures or in more corrosive environments, and the importance of reducing the central segregation of cast slabs is increasing.

[0004] Therefore, many measures have been proposed to reduce or neutralize the central segregation of the slab from the continuous casting process to the rolling process. Among them, the soft reduction method at the end of solidification, in which a continuously cast slab having an unsolidified layer inside is reduced in a continuous casting machine, is known to be particularly effective in improving the central segregation of the slab. Here, the "soft reduction method at the end of solidification" refers to a method in which a reduction roll is disposed near the solidification completion position of the slab, and the slab during continuous casting is gradually reduced by this reduction roll at a reduction speed equivalent to the amount of solidification shrinkage. In this way, by reducing the slab near the solidification completion position with the reduction roll, the generation of voids in the center of the slab and the flow of concentrated molten steel with solute elements are suppressed, thereby reducing the central segregation of the slab.

[0005] In order to effectively suppress centerline segregation of a slab by this method of soft reduction at the final solidification stage, it is important to appropriately set the start and end times of the period during which soft reduction is applied during the final solidification period of the slab, and the amount of reduction at those times, and various methods for setting these have been proposed.

[0006] Patent Document 1 discloses that in a continuous casting method in which soft reduction is applied to a slab in a final solidified portion of the continuously cast slab, the amount of reduction of the slab per unit time in the section in which soft reduction is applied is determined by the slab surface temperature at the start of reduction and the thickness of the unsolidified layer of the slab at the reduction position.

[0007] Patent Documents 2 and 3 disclose a method of using a plurality of roll pairs to reduce a region from a point where the temperature at the center solid fraction reaches a temperature corresponding to 0.1 to 0.3 to a point where the temperature reaches a temperature corresponding to the flow limit solid fraction. Patent Documents 2 and 3 state that it is preferable to increase the reduction rate of the slab toward the downstream side in the casting direction, where the center solid fraction at the center of the thickness of the slab becomes larger.

[0008] Patent Document 4 discloses a method of correcting a calculation model by referring to the surface temperature of a surface thermometer installed inside the machine in order to improve the accuracy of the solidification state estimated using heat transfer calculation. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 8-132203 [Patent Document 2] Japanese Patent Application Publication No. 3-90263 [Patent Document 3] Japanese Patent Application Publication No. 3-90259 [Patent Document 4] JP 2012-187636 A Summary of the Invention [Problem to be solved by the invention]

[0010] The present inventors have discovered that in continuous casting of a slab using a soft reduction method at the final stage of solidification, in order to effectively suppress center segregation of the slab, it is important to reduce fluctuations in the reduction rate in the region where the center solid fraction of the slab is 0.7 or more and 1.0 or less.

[0011] Here, the rolling speed (mm / min) is a value calculated by multiplying the rolling gradient (mm / m) and the slab withdrawal speed (m / min). The rolling gradient is a state of roll gap set so that the distance between opposing rolls (hereinafter, may be referred to as "roll gap") gradually narrows toward the downstream side in the casting direction, and is usually expressed as the amount of narrowing of the roll gap per meter (mm / m).

[0012] The methods disclosed in Patent Documents 1 to 3 cannot continuously obtain information on the central solid fraction of the slab. Therefore, when the solidification position of the slab changes due to unexpected fluctuations in the operating conditions, air temperature, water temperature, or other environmental factors, the method is unable to follow the change in solidification position, resulting in large fluctuations in the reduction rate.

[0013] The method disclosed in Patent Document 4 is a technique for estimating the solidification state of a slab, and there is no disclosure about suppressing fluctuations in the reduction rate in accordance with the central solid fraction of the slab.

[0014] The present invention has been made in consideration of the problems with the conventional technology, and an object of the present invention is to provide a continuous casting method for steel that can suppress fluctuations in the reduction rate in the range where the central solid fraction of the slab is 0.7 to 1.0. [Means for solving the problem]

[0015] The means for solving the above problems are as follows. [1] A method for continuous casting of steel in which a slab is cast while being soft reduced using a soft reduction zone, the method comprising: continuously predicting a region in the longitudinal direction of the slab in which the central solid fraction of the slab is 0.7 to 1.0 using the operating conditions of a continuous casting machine; and dynamically changing the roll gap of the soft reduction zone so that the fluctuation in the soft reduction rate in the region is kept below a predetermined value. [2] The method for continuous casting of steel according to [1], further comprising dynamically changing a roll gap in the soft reduction zone so that the fluctuation of the soft reduction rate in the zone is 0.08 mm / min or less. [3] The operating conditions are secondary cooling conditions of the slab, and the surface temperature of the slab in the soft reduction zone, the temperature at the thickness center of the slab, and the final solidification position are predicted by a heat transfer model using a heat flux based on the secondary cooling conditions. The actual measured values ​​of the surface temperature of the slab and the temperature at the thickness center of the slab are obtained by a surface thermometer and an electromagnetic ultrasonic solidification state estimation sensor provided in the continuous casting machine, and the final solidification position is estimated using the obtained actual measured value of the temperature at the thickness center. ΔT surf , δT center and predicting a final solidification position of the slab using a heat transfer model including the modified heat transfer correction coefficient, so that ΔFS becomes equal to or less than a predetermined value. δT surf =(T sim,surf- T surf ) / T surf (1) δT center =(T sim,center -T center ) / T center (2) δFS = (FS sim -FS) / FS···(3) T sim,surf is the surface temperature of the slab predicted by the heat transfer model (℃), and T surf is the actual surface temperature of the slab measured by the surface thermometer (°C), and T sim,centeris the temperature at the center of the thickness of the slab predicted by the heat transfer model (°C), and T center is the actual temperature (°C) at the center of the thickness of the slab measured by the electromagnetic ultrasonic solidification state estimation sensor, and F sim is the final solidification position (m) of the slab predicted by the heat transfer model, and FS is the final solidification position (m) of the slab estimated using the actual temperature value at the center of the thickness of the slab measured by the electromagnetic ultrasonic solidification state estimation sensor. [4] The above δT surf , the δT center and correcting the heat transfer correction coefficient so that the ΔFS satisfies the following formulas (4) to (6). δT surf ≦0.1 (4) δT center ≦0.06 (5) δFS≦0.1 (6) [5] The δT is selected from a plurality of heat transfer correction coefficients within a predetermined range of the heat transfer correction coefficients. surf , the δT center and identifying a heat transfer correction coefficient that minimizes the ΔFS, and correcting the heat transfer correction coefficient to the identified heat transfer correction coefficient. [6] The time required for the withdrawal speed to change from the withdrawal speed before the end of casting to the withdrawal speed at the head set at the end of casting is t a The time when the head is fixed is t0, and the time from the end of the head fixing to the re-pulling speed is t b Then, the above t a , t0 and t b The method for continuous casting of steel according to any one of [1] to [5], wherein an additional reduction amount satisfying the following formulas (7) to (9) is added to the soft reduction zone: t=t a When , 0.0 <Z a ≦3.0 (7) When t=t0, it is 0.0 <Z0≦1.5···(8) t=t b When , 0.0 <Z b ≦3.0 (9) In the above formulas (7) to (9), Z a , Z0 and Z b is the additional reduction amount (mm) at each time. [7] The time it takes for the unsteady part of the slab to become a steady part at the start of casting is called t c Then, the above t c The method for continuous casting of steel according to any one of [1] to [5], wherein an additional reduction amount satisfying the following formula (10) is added to the soft reduction zone. t=t c When , 0.0 <Z c ≦3.0 (10) In the above formula (10), Z c is c is the additional reduction (mm) in [8] A method for continuous casting of steel according to any one of [1] to [7], wherein the roll gap upstream of the soft reduction zone in the casting direction is made larger than the thickness of the slab at the outlet side of the mold. Effect of the Invention

[0016] According to the present invention, the central solid fraction of a slab is continuously predicted, so that even if the solidification position changes due to a change in the operating conditions, etc., the reduction rate can be adjusted to follow the change in the position. This makes it possible to suppress the fluctuation of the reduction rate in the region where the central solid fraction of the slab is 0.7 to 1.0 more than conventionally, and as a result, it is possible to cast a slab with reduced central segregation. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a continuous steel casting facility in which the continuous steel casting method according to this embodiment can be carried out. [Diagram 2] FIG. 2 is a flow diagram illustrating a process for determining the heat transfer correction coefficient α. [Diagram 3] FIG. 3 is a side view of a roll segment constituting the soft reduction zone. [Figure 4] FIG. 4 is a front view of a roll segment. [Diagram 5]FIG. 5 is a graph showing the times ta, t0 and tb at the end of pouring. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention will be described below through the embodiments of the present invention. However, the following embodiments are merely preferred examples of the present invention, and the present invention is not limited to these embodiments.

[0019] FIG. 1 is a schematic cross-sectional view showing a continuous steel casting facility 100 in which the continuous steel casting method according to this embodiment can be implemented. The continuous steel casting facility 100 has a continuous casting machine 90 and a control device 48 for controlling the continuous casting machine 90. The continuous casting machine 90 mainly has a mold 10, a tundish 12, and a plurality of pairs of cast piece support rolls 32. The tundish 12 is installed above the mold 10. A sliding nozzle 14 for adjusting the injection flow rate of molten steel 18 into the mold 10 is installed at the bottom of the tundish 12, and a submerged nozzle 16 is installed on the lower surface of the sliding nozzle 14.

[0020] Molten steel 18 is poured into the mold 10 through the submerged nozzle 16. The molten steel 18 poured into the mold 10 solidifies as heat is removed from the inner surface of the mold 10, forming a solidified shell 20. As a result, a cast slab 24 is formed having the solidified shell 20 as an outer shell and an unsolidified layer 22 made of the molten steel 18 inside.

[0021] Below the mold 10, a plurality of pairs of slab support rolls 32 including a support roll 26, a guide roll 28, and a driving roll 30 are provided. Among these, the driving roll 30 is a roll for supporting the slab 24 and simultaneously drawing out the slab 24. A spray nozzle (not shown), such as a water spray nozzle or an air mist spray nozzle, is provided in the gap between the slab support rolls 32 adjacent to each other in the casting direction, thereby forming a secondary cooling zone. The slab 24 is cooled while being drawn out by the cooling water sprayed from the spray nozzle in the secondary cooling zone, so that the internal unsolidified layer 22 is reduced, and the solidified shell 20 grows. Thereafter, the slab 24 is appropriately cooled, so that the unsolidified layer 22 solidifies, and the slab 24 is completely solidified. Downstream of the slab support rolls 32, a plurality of transport rolls 34 for transporting the cast slab 24 are provided, and above the transport rolls 34, a slab cutter 36 for cutting the cast slab 24 into slabs 38 of a predetermined length is provided. The cast piece 24 is cut by a cast piece cutter 36 to produce slabs 38 of a predetermined length.

[0022] Soft reduction zones 41, in which the roll gap between the opposing guide rolls 28 can be changed, are provided before and after the final solidification position 40 of the slab 24 in the casting direction. A surface thermometer 44 and a solidification state estimation sensor 46 are provided in the soft reduction zone 41.

[0023] The surface thermometer 44 measures the temperature distribution on the surface of the slab in the soft reduction zone 41. The surface thermometer 44 may be a radiation thermometer that can directly measure the temperature distribution, or may be a device that measures the temperature distribution by measuring the temperatures at multiple positions and aggregating the temperatures. The surface thermometer 44 outputs information indicating the surface temperature distribution to the control device 48.

[0024] The solidification state estimation sensor 46 measures the temperature at the center of the thickness of the slab in the soft reduction zone 41. The solidification state estimation sensor 46 is a device that measures the temperature at the center of the thickness of the slab by utilizing the temperature dependency of ultrasonic transmission time using, for example, electromagnetic ultrasonic waves (longitudinal ultrasonic waves / transverse ultrasonic waves). The solidification state estimation sensor 46 outputs information indicating the temperature at the center of the thickness to the control device 48.

[0025] In the above, an example has been shown in which the surface thermometer 44 and the solidification state estimating sensor 46 are provided in the soft reduction zone 41, but this is not limiting. The surface thermometer 44 may be provided anywhere downstream of the mold 10 in the casting direction, but is preferably provided in a position close to the final solidification position 40 of the slab 24. The solidification state estimating sensor 46 is preferably provided in a position close to the final solidification position 40, and is more preferably provided between the soft reduction zone 41 and the final solidification position 40 of the slab 24.

[0026] The control device 48 is, for example, a general-purpose computer such as a workstation or a personal computer having a control unit (such as a CPU), a storage unit, an input unit, and an output unit. The control device 48 is connected to each device of the continuous casting machine 90 by wire or wirelessly, and controls the continuous casting process of steel. The control device 48 further obtains operating conditions, including secondary cooling conditions, of the continuous casting machine 90 from a higher-level computer, and controls the operation of the continuous casting machine 90 in accordance with the operating conditions.

[0027] The control device 48 predicts the surface temperature of the slab 24 at the position where the surface thermometer 44 is provided, using a heat transfer model using a heat flux based on secondary cooling conditions, which are operating conditions of the continuous casting machine 90. The control device 48 predicts the temperature at the center of the thickness of the slab 24 at the position where the solidification state estimation sensor 46 is provided, and the final solidification position 40, using a similar heat transfer model. First, a method for predicting the surface temperature of the slab 24 in the light reduction zone 41, the temperature at the center of the thickness of the slab 24, and the final solidification position 40 using a heat transfer model using a heat flux based on secondary cooling conditions will be described.

[0028] The secondary cooling calculation is performed by, for example, considering the cross section of the slab sliced ​​into unit lengths in the casting direction, and giving boundary condition heat fluxes under various conditions (water cooling, air cooling, mist cooling, roll cooling, etc.) depending on the location within the strand during casting, and solving a two-dimensional heat transfer equation.

[0029] At this time, by continuously generating and calculating sliced ​​cross sections of unit length one after another, it is possible to calculate not only the temperature in the steady state, but also the temperature in the unsteady state in the early and final stages of casting. Operational conditions such as actual water cooling data, casting speed, and the temperature of molten steel 18 in the tundish 12 are input online, and secondary cooling calculations are performed in real time. This calculation also makes it possible to calculate the location of the final solidification position 40 of the slab 24 using the solidus temperature.

[0030] The solution of the heat transfer equation is carried out as a numerical calculation by making the heat conduction equation differencing. Here, in the secondary cooling calculation of normal continuous casting, for example, a cross section of a slab sliced ​​at a unit length along the slab's longitudinal direction (casting direction) is considered. Then, depending on the location in the strand during casting, the heat flux Q is calculated based on the following equation (11) which shows the boundary conditions on the surface of the slab 24 under secondary cooling conditions consisting of water cooling, air cooling, mist cooling, roll heat extraction, etc. The calculation can be carried out by using this heat flux Q to solve the two-dimensional heat transfer equation of the following equation (13).

[0031]

number

[0032]

number

[0033]

number

[0034] The heat transfer coefficient h in formula (11) is determined by the secondary cooling conditions such as the cooling method (water cooling, air cooling, mist cooling, etc.), the amount of cooling operation, and the amount of heat removed by the roll. Furthermore, the heat transfer coefficient h is multiplied by a real number α as a correction coefficient for heat transfer calculation (hereinafter referred to as the "heat transfer correction coefficient") and used as the h in the calculation. In this embodiment, the following formula (14) is used instead of formula (11) as the formula for the heat flux under the boundary conditions under the secondary cooling conditions.

[0035] Q ij =αh(T-Ta) (14) In the above formula (14), α is a heat transfer correction coefficient (initial value is "1") (-). i is the correction position in the width direction of the slab 24. j is the correction position in the longitudinal direction of the slab 24. (-) means that the formula is dimensionless.

[0036] Generally, the physical properties of specific heat, density, and thermal conductivity change with the temperature change of the slab 24, so it is necessary to change the physical properties as a function of temperature to solve the equations. If the physical properties are temperature-dependent, the equations cannot be expanded as they are into differential equations. Therefore, in actual calculations, the temperature and heat content are linearized by replacing them as shown in the following equations (15) and (16) using the well-known "temperature-transformed temperature method."

[0037]

number

[0038]

number

[0039]

number

[0040] By making this equation (17) different, it becomes possible to perform a numerical analysis of heat transfer calculations for each slice. Different difference equations are used for internal points and surface points of the slice. On the surface of the slab 24, the above equation (17) can be expressed by the following equation (18), and if the velocity of the slab 24 in the casting direction is v (z direction), the above equation (17) becomes the following equation (19).

[0041]

number

[0042]

number

[0043] Based on the above equations (18) and (19), when the above equation (17) is differentiated (discretized) for the internal points and surface points, the following equations (20) and (21) are obtained.

[0044]

number

[0045] In the actual calculation process, the heat transfer calculation is performed in the following steps 1 to 9. 1. As the analysis begins, a two-dimensional sheet enters the mold and moves forward. 2. This sheet is calculated based only on the external boundary conditions and the two-dimensional internal heat conduction. (The heat conduction in the direction of travel is not considered.) 3. Along the way, the speed changes at each point in time based on the speed data. 4.During the process, the spray pattern is switched based on external cooling pattern data. 5. This single sheet will be calculated up to the end of the analysis time. 6. When moving to the next sheet, change the physical properties and initial temperature according to the input. 7. Once the calculation for one sheet is completed, the calculation for the next sheet is started with a time step delay and continues until the end of the analysis time. 8. Repeat the above calculations for each sheet until the end of withdrawal. 9. Output files as necessary along the way.

[0046] The above heat transfer calculation uses a finite difference method to analyze the heat conduction in the slab 24, and the analysis target is 1 / 2 of the thickness direction due to structural symmetry. For example, if the short side and long side are divided into m and n, the mesh will be the half-part divided into m and n.

[0047] The heat transfer coefficient h in equation (18) is determined by the cooling method (water cooling, air cooling, mist cooling, etc.), the amount of cooling operation, the amount of heat removed from the roll, and other secondary cooling conditions. The calculation formula for the heat transfer coefficient h is changed according to the cooling method (water only, water and air, air only, and the flow rate of each). The heat removal actually used is the larger value compared to these in radiative cooling.

[0048] The central solid fraction of the slab 24 is calculated as follows: when the temperature at the center of the slab 24 is lower than the liquidus temperature, the central solid fraction = 1; when the temperature at the center of the slab 24 is higher than the solidus temperature, the central solid fraction = 0. When the temperature at the center of the slab 24 is between the liquidus temperature and the solidus temperature, the central solid fraction is calculated by the following formula (22).

[0049] Central solid fraction = (C0-C L ) / (C S -C L )···(twenty two) In the above formula (22), C0 is the carbon concentration (mass%). S is the carbon concentration (mass%) at which a certain temperature is equal to the solidus temperature. L is the carbon concentration (mass%) at which a certain temperature is equal to the liquidus temperature.

[0050] In the mold, the amount of heat transfer from the surface is determined by the time the slice passes through the mold. The amount of heat transfer from the surface is determined assuming that both the long and short sides are uniform. The calculation conditions are the operating conditions of the target continuous casting. An example of the operating conditions is shown below.

[0051] Simulation time step: 0.02sec Casting speed: 1.4mpm ·Analysis thickness: 125mm (half thickness, full thickness 250mm) ·Analysis width: 1000mm (half width, full width 2000m) Ambient temperature: 30℃ ·Secondary cooling water temperature: 28℃ ·Molten steel temperature: 1555℃ Thermal conductivity at reference temperature: Determined based on the composition of the material in question. Liquidus temperature and solidus temperature obtained from molten steel composition: Determined by experiments or other means. Relationship between conversion temperature φ and temperature: Determined by experiment or other means. Heat content H-temperature relationship: To be determined by experiment or other means. Density ρ-temperature relationship: To be determined through experiments or other means. Example of mesh width divisions: Width (n) = 66, Thickness (n) = 25

[0052] In accordance with the above, the control device 48 predicts the surface temperature, the temperature at the thickness center, and the final solidification position of the slab 24 in the soft reduction zone 41. However, the surface temperature, the temperature at the thickness center, and the final solidification position predicted using the above method may not match the actual surface temperature, the temperature at the thickness center, and the final solidification position.

[0053] For this reason, the control device 48 acquires the actual measured value of the surface temperature and the actual measured value of the temperature at the thickness center from the surface thermometer 44, and checks whether the predicted surface temperature and temperature at the thickness center of the slab 24 match the actual measured values ​​of the surface temperature and the temperature at the thickness center. Additionally, the control device 48 estimates the final solidification position using the actual measured value of the temperature at the thickness center, and checks whether the estimated value matches the predicted value of the final solidification position.

[0054] Specifically, the control device 48 calculates ΔT surf , δT center When ΔFS calculated using the following formula (3) becomes equal to or less than a predetermined value, the control device 48 judges that the predicted values ​​of the surface temperature and the temperature at the thickness center of the slab 24 match the actually measured values ​​of these temperatures. Similarly, when ΔFS calculated using the following formula (3) becomes equal to or less than a predetermined value, the control device 48 judges that the predicted value of the final solidification position 40 of the slab 24 matches the estimated value of the final solidification position 40. On the other hand, when ΔT surf , δT center If ΔFS exceeds a predetermined value, it is determined that the predicted value does not match the actual measured value and the estimated value, and the heat transfer correction coefficient α of the heat transfer model is corrected.

[0055] δT surf =(T sim,surf -T surf ) / T surf (1) δT center =(T sim,center -T center ) / T center (2) δFS = (FS sim -FS) / FS···(3) T sim,surf is the surface temperature of the slab 24 predicted by the heat transfer model (℃). T surf is the actual surface temperature (°C) of the slab 24 measured by the surface thermometer 44. sim,center is the temperature (°C) at the center of the thickness of the slab 24 predicted by the heat transfer model. T centeris the actual temperature (°C) of the central part of the thickness of the slab 24 measured by the solidification state estimation sensor 46. sim is the final solidification position (m) of the slab 24 predicted by the heat transfer model. FS is the final solidification position (m) of the slab 24 estimated from the actual temperature value of the center of the thickness of the slab 24 measured by the solidification state estimation sensor 46.

[0056] In this embodiment, ΔT surf , δT center The predetermined values ​​of and ΔFS are, for example, 0.1, 0.06 and 0.1, and the control device 48 corrects the heat transfer correction coefficient α so as to satisfy the following expressions (4) to (6).

[0057] δT surf ≦0.1 (4) δT center ≦0.06 (5) δFS≦0.1 (6)

[0058] δT surf , δT center The predetermined values ​​of and δFS vary depending on the required level of center segregation for the final steel product. surf , δT center and δFS are 0.05, 0.03, and 0.05, respectively. For steels with looser requirements for center segregation, δT surf , δT center and δFS are 0.1, 0.06, and 0.1, respectively (see formulas (4) to (6) above). For steels with normal center segregation requirements, δT surf , δT center and δFS are 0.07, 0.05, and 0.07, respectively.

[0059] Next, a method for correcting the heat transfer correction coefficient α by the control device 48 will be described. Fig. 2 is a flow diagram illustrating a process for determining the heat transfer correction coefficient α. This flow is started, for example, on the condition that the control device 48 is started up and the surface thermometer 44 and the solidification state estimation sensor 46 are in a state where they can perform measurements.

[0060] The control device 48 receives the actual measured value T of the surface temperature from the surface thermometer 44. surf The control device 48 acquires the actual temperature measurement value T center The control device 48 acquires the actual temperature measurement value T center and the above heat transfer model to estimate the final solidification position FS (step S103).

[0061] The control device 48 performs temperature calculations for each two-dimensional cross-sectional slice of the unit length in the casting direction from the meniscus to the end of the machine. As a result, the surface temperature T sim,surf and the temperature T sim,center and the final solidification position FS sim (Step S104).

[0062] The control device 48 is sim,surf , T sim,center , F.S. sim , T surf , T center Using FS and the above equations (1) to (3), ΔT surf , δT center and ΔFS (step S105). The control device 48 calculates the calculated ΔT surf , δT center It is determined whether or not ΔFS satisfies the above formulas (4) to (6) (step S106). surf , δT center If ΔFS satisfies the above expressions (4) to (6) (step S106: Yes), the control device 48 sim,surf , T sim,center , F.S. sim The heat transfer correction coefficient α used to calculate ΔT is determined as the heat transfer correction coefficient used to specify the region in which the center solid fraction is 0.7 to 1.0 (step S108), and the flow ends. surf , δT centerIf ΔFS does not satisfy the above formulas (4) to (6) (step S106: No), the control device 48 corrects the heat transfer correction coefficient α (step S107). After that, the control device 48 returns the process to step S104 and executes the processes of steps S104 to S106 again.

[0063] The control device 48 calculates ΔT surf , δT center The processes in steps S104 to S107 are repeated until the calculated ΔT surf , δT center and ΔFS can specify the heat transfer correction coefficient that satisfies the above equations (4) to (6). surf , δT center If ΔFS does not satisfy the above formulas (4) to (6), an error message may be displayed on the output section of the control device .

[0064] When the control device 48 identifies the heat transfer correction coefficient satisfying the above formulas (4) to (6), it uses a heat transfer model including the heat transfer correction coefficient to continuously predict the range in which the central solid fraction of the slab 24 is 0.7 to 1.0. In this way, by predicting the region in which the central solid fraction is 0.7 to 1.0 using a heat transfer model including the heat transfer correction coefficient satisfying the above formulas (4) to (6), it becomes possible to predict the region with high accuracy.

[0065] In this embodiment, "continuously" means predicting a region in which the central solid fraction of the slab 24 is 0.7 to 1.0 at intervals of 1 second or less. However, "continuously" is preferably at intervals of 0.5 seconds or less, and more preferably at intervals of 0.1 seconds or less.

[0066] When the control device 48 predicts a region where the central solid fraction is 0.7 to 1.0, the control device 48 dynamically changes the roll gap in the soft reduction zone 41 so as to reduce fluctuations in the reduction rate within that range. In this embodiment, it is preferable to reduce the fluctuations in the reduction rate so that the absolute value of the fluctuations in the reduction rate is 0.08 mm / min or less, for example. The absolute value of the fluctuations in the reduction rate is the absolute value of the difference between the maximum reduction rate and the minimum reduction rate. In this way, by suppressing the fluctuations in the reduction rate in the region where the central solid fraction of the slab 24 is 0.7 to 1.0, it is possible to cast a slab with reduced central segregation.

[0067] Next, the roll gap of the reduction rolls will be described. Fig. 3 is a side view of the roll segment 42 constituting the soft reduction zone 41, and Fig. 4 is a front view of the roll segment 42. The roll segment 42 has a drive roll 30 that applies a pressing force to the slab 24 from above, and a guide roll 28. The guide rolls are fixed to an upper frame 56 and a lower frame 58 via bearings 54. The upper frame 56 and the lower frame 58 are supported by an upstream support 60 and a downstream support 62. Since the upper frame 56 and the lower frame 58 are supported by the upstream support 60 and the downstream support 62, the upstream support 60 and the downstream support 62 determine the amount of soft reduction applied to the slab 24 by the roll segment 42 as a whole. As described above, the guide rolls 28 are fixed to the upper frame 56 or the lower frame 58 via the bearings 54, and the interval between the upper guide roll and the lower guide roll can be changed by extending or retracting the length of the support column with a hydraulic cylinder. The amount of bulging can be set by setting the roll interval wider than that of the immediately preceding segment, and the soft reduction gradient can be set by setting the interval between the guide rolls on the upstream process side wider than that of the guide rolls on the downstream process side. In this embodiment, a drive unit is provided that drives the hydraulic cylinder that extends or retracts the length of the support columns of the guide rolls 28, and the control device 48 controls the drive unit to dynamically change the roll gap of the light reduction zone 41. In this embodiment, an example has been described in which the roll gap of the light reduction zone 41 is dynamically changed to reduce the fluctuation in the reduction speed, but this is not limited to the above. For example, the center solid fraction of the slab 24 may be controlled by dynamically changing the roll gap of the light reduction zone 41 and changing the operating conditions of the continuous casting machine 90 so as to reduce the fluctuation in the reduction speed.

[0068] In continuous casting of steel, the casting becomes unstable at the beginning and end of casting. Therefore, the beginning and end of casting are considered to be unsteady parts, in contrast to the steady part where the operation of the continuous casting is stable. In the unsteady part, the slab 24 shrinks, so unless the reduction amount is added in accordance with the amount of shrinkage, the reduction speed fluctuates greatly, and the center segregation of the slab 24 worsens.

[0069] Figure 5 shows the time t a , t0 and t b In FIG. 5, the horizontal axis represents the time (min) from the end of casting, and the vertical axis represents the casting speed (m / min). As shown in FIG. a is the time from the withdrawal speed before the end of casting to the withdrawal speed of the slab 24 at the time of head setting at the end of casting. t0 is the time during head setting. t b is the time from the end of head fixation to the re-pulling speed.

[0070] After casting is completed, as shown in Fig. 5, the casting speed is slowed down after casting is completed (the point at which the sliding nozzle 14 is closed), and this low casting speed is maintained for a certain period of time to cool and solidify the top portion of the slab 24 (the rearmost end of the slab 24), thereby performing head hardening. If the top portion of the slab 24 that is pulled out of the mold 10 after casting is completed is not yet solidified, the molten steel 18 will leak out from the top portion. For this reason, the process of pouring cold material toward the molten steel 18 remaining in the mold 10 to solidify the top portion of the slab 24 is called "head hardening."

[0071] During head hardening, the casting speed is slowed down in order to firmly harden the top part. After head hardening, the casting speed is increased and the slab 24 is withdrawn from the continuous casting machine 90. In other words, when the elapsed time from the end of casting is t, the period during which the elapsed time t is equal to or greater than 0 and less than t1 (time t a ) to reduce the casting speed. At this time, the first casting speed V a Then, the casting speed is decelerated to the second casting speed V0 for head hardening, and head hardening is performed.

[0072] After this, while the elapsed time t is between t2 and t3 (time t b At this time, the casting speed of the slab 24 is increased from the second casting speed V0 to the third casting speed V1, which is the casting speed at the time of re-withdrawing after the end of casting. b The speed is increased until the slab 24 is drawn at a higher drawing speed after the head is fixed. When the elapsed time t exceeds t3, the slab 24 is drawn at the third casting speed Vb The period after the end of casting is called the time t a The period is the deceleration process, the period at time t0 is the head-stabilizing process, and the period at time t b The period of acceleration is t b The period after (t>t3) is also called the re-drawing process.

[0073] The control device 48 determines the time t a , t0 and t b In the above, it is preferable to change the roll opening so that an additional reduction amount satisfying the following formulas (7) to (9) is added to the soft reduction zone. This makes it possible to impart an appropriate soft reduction to the unsteady portion of the slab 24 remaining in the continuous casting machine 90 after casting is completed, thereby realizing the casting of a slab 24 with further reduced centerline segregation.

[0074] t=t a When , 0.0 <Z a ≦3.0 (7) When t=t0, it is 0.0 <Z0≦1.5···(8) t=t b When , 0.0 <Z b ≦3.0 (9) In the above formulas (7) to (9), Z a , Z0 and Z b is the additional reduction amount (mm) at each time.

[0075] Time t a , t0, t b The amount of additional rolling reduction is determined by comparing the amount of shrinkage of the slab 24 in the steady state with the amount of shrinkage of the slab 24 at the end of casting. The amount of shrinkage of the slab 24 may be measured with a water column ultrasonic meter or a laser distance meter, or may be estimated from heat transfer calculations and density changes.

[0076] Furthermore, even at the start of casting, it is preferable to control the soft reduction speed of the slab 24 withdrawn from the machine from the start of casting until the casting enters the steady state within a certain range. c Then, the control device 48 calculates t cIn the soft reduction zone, it is preferable to add an additional reduction amount satisfying the following formula (10). This allows the slab 24 in the unsteady state at the start of casting to be appropriately reduced, thereby realizing the casting of a slab 24 with reduced centerline segregation.

[0077] t=t c When , 0.0 <Z c ≦3.0 (10) In the above formula (10), Z c is c is the additional reduction (mm) in

[0078] Time t c and additional reduction amount Z c can be determined by comparing the amount of shrinkage of the slab 24 in the steady state with the amount of shrinkage of the slab 24 at the start of casting. The amount of shrinkage of the slab 24 may be measured with a water column ultrasonic meter or a laser distance meter, or may be estimated from heat transfer calculations and density changes.

[0079] Furthermore, it is preferable to widen the roll gap upstream of the soft reduction zone 41 more than the thickness of the slab 24 at the outlet side of the mold 10. For example, it is preferable to widen the roll gap upstream of the light reduction zone 41 and downstream of the support roll 26 provided immediately below the mold 10 by a maximum of 15 mm more than the thickness of the slab 24 at the outlet side of the mold 10. This makes it possible to soft reduce the narrow side in the light reduction zone 41, where the reaction force from the slab 24 to the rolls increases, without excessively reducing it.

[0080] In the above embodiment, the control device 48 specifies a heat transfer correction coefficient that satisfies the above formulas (4) to (6) and continuously predicts the range in which the center solid fraction of the slab 24 is 0.7 to 1.0 using a heat transfer model including the heat transfer correction coefficient, but the present invention is not limited to this. The control device 48 may calculate ΔT surf , δT center and δFS are calculated, and then δT surf , δT centerThe range of the heat transfer correction coefficient may be determined based on the actual values ​​of the heat transfer correction coefficient in the continuous casting carried out in the past.

[0081] The plurality of heat transfer correction coefficients may be selected such that the predetermined range is equally divided by the number of selected heat transfer correction coefficients. surf , δT center and δFS are calculated, and then δT surf , δT center and the heat transfer correction coefficient at which ΔFS is smallest may be specified.

[0082] δT surf , δT center The heat transfer correction coefficient that minimizes ΔFS may be specified in two steps. In the first step, ΔT is determined for heat transfer correction coefficients that are changed by 5 to 10 levels in increments of 0.1 around a heat transfer correction coefficient of 1.0. surf , δT center In the second step, the baseline of the heat transfer correction coefficient at which ΔFS and ΔT are smallest is identified. surf , δT center and the heat transfer correction coefficient for which ΔFS is smallest may be specified.

[0083] The unit of change at each level of the heat transfer coefficient on the increasing side and the decreasing side in the first step does not have to be the same, and may be changed in units of 0.1 to 0.9 based on past performance. Similarly, the unit of change at each level of the heat transfer coefficient on the increasing side and the decreasing side in the second step does not have to be the same, and may be changed in units of 0.01 to 0.09 based on past performance.

[0084] In order to increase the calculation speed when specifying the heat transfer correction coefficient, the control device 48 preferably executes the process of specifying the heat transfer correction coefficient using a GPU and a CUDA program. By calculating a plurality of heat transfer correction coefficients in parallel in this manner, the calculation cycle can be shortened, and the range in which the center solid fraction of the slab 24 is 0.7 to 1.0 can be predicted with high accuracy. EXAMPLES

[0085] [Example 1] Hereinafter, a description will be given of Example 1 in which low-carbon aluminum-killed steel was continuously cast using the continuous steel casting equipment 100 shown in Fig. 1. The composition of the low-carbon aluminum-killed steel used was C: 0.065 mass%, Si: 0.35 mass%, Mn: 1.55 mass%, Cu: 0.12 mass%, Ni: 0.07 mass%, Nb: 0.015 mass%, and Ti: 0.014 mass%. The cast slab had a thickness of 250 mm and a width of 2000 mm.

[0086] In Examples 1 to 3 of the embodiment 1, a region where the central solid fraction of the slab is 0.7 to 1.0 was identified every second using a heat transfer model. Then, the opening of the rolls in the soft reduction zone was dynamically changed so that the target reduction speed in that region was 0.28 mm / min, and the absolute value (time average) of the difference from the target reduction speed was controlled to be 0.10 mm / min or less. On the other hand, in Comparative Examples 1 and 2, casting was performed without controlling the fluctuation of the reduction speed. Therefore, the absolute value of the difference from the target reduction speed in Comparative Examples 1 and 2 was larger than that in Examples 1 to 3 of the invention.

[0087] Continuous casting of steel was carried out under these operating conditions, and the center segregation of the cast slab was evaluated. The center segregation of the slab was evaluated by the number of center segregated grains. The number of center segregated grains was measured by the following steps (1) to (5). (1) In the cross section of the slab perpendicular to the casting direction, a slab sample was taken that was 15 mm wide, included the central segregation area in the center, and had a length from the center of the width to the triple point on one side (the point where the solidified shells on the short side and long side grew and met). (2) The cross section perpendicular to the casting direction of the collected cast specimen was polished, and the surface was corroded with, for example, a saturated aqueous solution of picric acid to reveal the segregation zone. The area ranging from the center of the segregation zone to ±7.5 mm in the cast specimen thickness was determined as the central segregation area. (3) The central segregation part in the slab sample was divided into small parts in the slab width direction, and the Mn concentration of the slab sample was analyzed over the entire surface using an electron probe micro analyzer (EPMA) with an electron beam diameter of 100 μm. (4) The distribution of Mn segregation was determined, and a region with a Mn segregation degree of 1.33 or more was regarded as one segregated grain. The Mn segregation degree was calculated by dividing the Mn concentration in the segregated area by the Mn concentration at a position 10 mm away from the center of the thickness. (5) The number of segregated grains was counted, and the number of segregated grains was divided by the length of the sample in the slab width direction to obtain the number of center segregated grains.

[0088] The results of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0089] [Table 1]

[0090] As shown in Table 1, the number of center segregated grains in Examples 1 to 3 was smaller than the number of center segregated grains in Comparative Examples 1 and 2. From this result, the region in which the center solid fraction of the slab is 0.7 to 1.0 is continuously identified, and the opening of the reduction rolls in the light reduction zone is dynamically controlled so that the reduction speed in that region becomes the target reduction speed. This makes it possible to suppress fluctuations in the reduction speed in the region in which the center solid fraction of the slab is 0.7 to 1.0, and it has been confirmed that it is possible to cast a slab with reduced center segregation.

[0091] [Example 2] In Example 2, as in the invention example of Example 1, a heat transfer model was used to identify the region where the central solid fraction of the slab was 0.7 to 1.0 every second. Then, the opening of the reduction rolls in the light reduction zone was dynamically adjusted so that the target reduction rate in that region would be 0.28 mm / min, thereby controlling the fluctuation of the reduction rate. In invention examples 6 to 15 of Example 2, furthermore, actual measured values ​​of the surface temperature of the slab and the temperature at the center of the thickness were obtained from the surface thermometer 44 and the solidification state estimation sensor 46. Using the obtained measured values, ΔT calculated using the above formulas (1) to (3) was surf , δT center The heat transfer correction coefficient of the heat transfer model was corrected so that ΔFS and ΔFS satisfied the above formulas (4) to (6). surf , δT center The heat transfer correction coefficient of the heat transfer model was corrected so that δFS was 0.05, 0.03, and 0.05 or less. In Example 15, the center segregation requirement was a normal steel type, so δT surf , δT center The heat transfer correction coefficients of the heat transfer model were modified so that δFS was less than 0.07, 0.05, and 0.07.

[0092] On the other hand, in Example 4 of the invention, the heat transfer correction coefficient of the heat transfer model was not corrected. In Example 5 of the invention, the surface thermometer 44 was used to measure ΔT surf Calculate δT surf The heat transfer correction coefficient of the heat transfer model was modified so that δT center , the heat transfer correction factor was not modified using δFS.

[0093] In Examples 6 to 15, a region where the central solid fraction of the slab is 0.7 to 1.0 was identified every second using a heat transfer model including a modified heat transfer correction coefficient. The roll gap in the soft reduction zone was dynamically changed so that the absolute value (time average) of the difference from the target reduction rate in that region was 0.08 mm / min or less. In Example 7, the roll gap upstream of the soft reduction zone and downstream of the support roll 26 in the casting direction was further increased by 15 mm from the thickness of the slab at the mold exit side to cast the slab.

[0094] In Example 4, a heat transfer model including an unmodified heat transfer correction factor was used to identify the region where the center solid fraction of the slab was 0.7 to 1.0 every second. In Example 5, δT surf The region in which the central solid fraction of the slab was 0.7 to 1.0 was identified every second using a heat transfer model including a heat transfer correction coefficient corrected so that the center solid fraction of the slab was 0.7 to 1.0. In both Examples 4 and 5, the roll opening in the soft reduction zone was dynamically changed so that the absolute value (time average) of the difference between the reduction speed in this region and the target reduction speed became 0.08 mm / min or less.

[0095] Continuous casting of steel was carried out under these operating conditions, and the center segregation of the cast slab was evaluated. The center segregation of the slab was evaluated based on the number of center segregated grains measured in the same manner as in Example 1. The results of Invention Examples 4 to 15 are shown in Table 2 below.

[0096] [Table 2]

[0097] As shown in Table 2, the number of center segregated grains in Examples 6 to 15, in which the region where the center solid fraction of the slab is 0.7 to 1.0 was identified using the heat transfer model including the modified heat transfer correction coefficient, was smaller than the number of center segregated grains in Examples 4 and 5. From this result, it is considered that the region where the center solid fraction of the slab is 0.7 to 1.0 can be identified with high accuracy by using the heat transfer model including the modified heat transfer correction coefficient, and thus the fluctuation of the reduction speed in the relevant region can be suppressed. It is considered that, as a result of suppressing the fluctuation of the reduction speed in the region where the center solid fraction is 0.7 to 1.0, the slab with fewer center segregated grains was cast in Examples 6 to 15. On the other hand, in Examples 4 and 5, the roll gap was changed so that the absolute value of the fluctuation of the reduction speed was 0.08 mm / min or less as in the examples of the invention, but the accuracy of identifying the region where the center solid fraction is 0.7 to 1.0 was low. For this reason, it is considered that the fluctuation of the reduction speed in the relevant region could not be suppressed, and as a result, the center segregated grains increased. These results confirmed that the region where the central solid fraction is 0.7 to 1.0 can be identified with high accuracy by using a heat transfer model including a modified heat transfer correction coefficient. It was also confirmed that the casting of a slab with reduced central segregation can be realized by dynamically changing the roll gap of the reduction rolls so as to reduce the fluctuation of the reduction rate in the identified region.

[0098] Example 7, in which the roll gap on the upstream side of the light reduction zone and downstream side of the support rolls 26 in the casting direction was wider by 15 mm than the thickness of the slab at the mold exit side, had fewer center segregation grains than Example 6, in which the roll gap was not wider. This result confirmed that center segregation can be reduced by widening the roll gap on the upstream side of the light reduction zone in the casting direction.

[0099] [Example 3] In Example 3, a slab was cast and evaluated in the same manner as in Example 6 of Example 2. In Example 3, the roll gap of the reduction rolls was also changed in the first half unsteady state and the second half unsteady state to change the reduction amount in the region where the center solid fraction is 0.7 to 1.0. a , t0 and t b is shown in Figure 5.a and t0 and t b is the same as that. t in the first half unsteady part c is about 40 minutes.

[0100] In Example 3, in addition to the slab in the steady part, the number of central segregation grains was also measured for the slab in the latter half unsteady part and the slab in the first half unsteady part. The number of central segregation grains was measured in the same manner as in Example 1. The results of Example 3 are shown in Table 3 below.

[0101]

Table 3

[0102] As shown in Table 3, in the latter half unsteady part, Invention Example 18 that does not satisfy 0.0 < Z a ≤ 3.0 and 0.0 < Z b ≤ 3.0, and Invention Example 20 that does not satisfy 0.0 < Z0 ≤ 1.5 had a larger number of central segregation in the latter half unsteady part than other invention examples. From this result, in the latter half unsteady part, by changing the roll opening so that the additional reduction amount in t a , t0 and t b satisfies the above formulas (7) to (9), it was confirmed that the variation in the reduction rate in the latter half unsteady part can be reduced and the central segregation of the slab can be reduced.

[0103] In Invention Example 22 that does not satisfy 0.0 < Z c ≤ 3.0 in the first half unsteady part, the number of central segregation in the first half unsteady part was larger than that of other invention examples. From this result, in the first half unsteady part, by changing the roll opening so that the additional reduction amount in t c satisfies the above formula (10), it was confirmed that the variation in the reduction rate in the first half unsteady part can be reduced and the central segregation of the slab can be reduced.

[0104] [Example 4] In Example 4, as in the invention example of Example 1, a heat transfer model was used to identify the region where the central solid fraction of the slab was 0.7 to 1.0 every second. Then, the opening of the reduction rolls in the soft reduction zone was dynamically changed so that the target reduction speed in that region would be 0.28 mm / min, thereby suppressing fluctuations in the reduction speed. In invention examples 23 to 29 of Example 4, the actual measured values ​​of the surface temperature of the slab and the temperature at the center of the thickness were further obtained from the surface thermometer 44 and the solidification state estimation sensor 46. Then, the ΔT surf , δT center and δFS were calculated in parallel using GPU and CUDA programs, and these values ​​were corrected to the heat transfer correction coefficient that was the smallest. Multiple heat transfer correction coefficients were selected so that the heat transfer correction coefficients were distributed within a range predetermined by the number of selected heat transfer correction coefficients.

[0105] In Examples 23 to 29, a region where the central solid fraction of the slab is 0.7 to 1.0 was identified every second using a heat transfer model including a modified heat transfer correction coefficient. The opening of the reduction rolls in the soft reduction zone was dynamically changed so that the absolute value (time average) of the difference from the target reduction rate in the region was 0.08 mm / min or less. Continuous casting of steel was performed under these operating conditions, and the central segregation of the cast slab was evaluated. The central segregation of the slab was evaluated by the number of central segregation grains measured in the same manner as in Example 1. The results of Examples 23 to 29 are shown in Table 4 below.

[0106] [Table 4]

[0107] As shown in Table 4, increasing the number of heat transfer correction coefficients to be calculated reduces δT surf , δT center It was confirmed that the value of δFS can be reduced. This makes it possible to precisely identify the region in which the central solid fraction of the heat transfer slab is 0.7 to 1.0, and to cast a slab with fewer central segregation grains.

[0108] As shown in Table 4, the number of center segregated grains in the slab was significantly reduced by increasing the number of calculated heat transfer correction coefficients from 2 to 4, but the number of center segregated grains in the slab was hardly reduced even when the number of calculated heat transfer correction coefficients was increased to 4 or more. From these results, it was confirmed that it is preferable to set the number of calculated heat transfer correction coefficients to 4 or more. [Explanation of symbols]

[0109] 10 Mold 12 Tundish 14 Sliding Nozzle 16 Submerged nozzle 18 Molten Steel 20 Solidified shell 22 Unsolidified layer 24 Castings 26 Support Roll 28 Guide Roll 30 Driving Roll 32 Cast piece support roll 34 Transport roll 36 Slab cutting machine 38 Slab 40 Final solidification position 41 Lightly Pressed Zone 42 Roll Segments 44 Surface thermometer 46 Coagulation state estimation sensor 48 Control Device 54 Bearings 56 Upper Frame 58 Lower Frame 60 Upstream support 62 Downstream support 90 Continuous casting machine 100 Steel continuous casting equipment

Claims

1. A continuous steel casting method for casting a slab while soft reduction is performed using a soft reduction zone, comprising the steps of: continuously predicting a region in the longitudinal direction of the slab where the center solid fraction of the slab is 0.7 or more and 1.0 or less using the operating conditions of a continuous casting machine; a roll gap in the soft reduction zone that is dynamically changed so that fluctuations in the soft reduction rate in the soft reduction zone are kept below a predetermined value;

2. 2. The method for continuous casting steel according to claim 1, wherein the roll gap in the soft reduction zone is dynamically changed so that the fluctuation in the soft reduction rate in the zone is 0.08 mm / min or less.

3. the operating conditions are secondary cooling conditions for the slab, and a surface temperature of the slab in the soft reduction zone, a temperature at a thickness center of the slab, and a final solidification position are predicted using a heat transfer model that uses a heat flux based on the secondary cooling conditions; a surface thermometer and an electromagnetic ultrasonic solidification state estimation sensor provided in the continuous casting machine, which measure the surface temperature of the slab and the temperature at the thickness center of the slab, and estimate a final solidification position using the measured temperature at the thickness center; δT calculated using the following formulas (1) to (3) ｓｕｒｆ , δT ｃｅｎｔｅｒ modifying the heat transfer correction coefficient of the heat transfer model so that δFS is equal to or less than a predetermined value; 3. The method for continuous casting steel according to claim 1, wherein a final solidification position of the slab is predicted using a heat transfer model including the modified heat transfer correction coefficient. δT ｓｕｒｆ =(T ｓｉｍ，ｓｕｒｆ -T ｓｕｒｆ ) / T ｓｕｒｆ ・・・(1) δT ｃｅｎｔｅｒ =(T ｓｉｍ，ｃｅｎｔｅｒ -T ｃｅｎｔｅｒ ) / T ｃｅｎｔｅｒ ・・・(2) δFS=(FS ｓｉｍ -FS) / FS・・・(3) T ｓｉｍ，ｓｕｒｆ is the surface temperature of the slab predicted by the heat transfer model (°C), and T ｓｕｒｆ is the actual surface temperature (°C) of the slab measured by the surface thermometer, and T ｓｉｍ，ｃｅｎｔｅｒ is the temperature (°C) at the center of the thickness of the slab predicted by the heat transfer model, and T ｃｅｎｔｅｒ is the actual temperature (°C) at the center of the thickness of the slab measured by the electromagnetic ultrasonic solidification state estimation sensor, and FS ｓｉｍ is the final solidification position (m) of the slab predicted by the heat transfer model, and FS is the final solidification position (m) of the slab estimated using the actual temperature value at the center of the thickness of the slab measured by the electromagnetic ultrasonic solidification state estimation sensor.

4. The δT ｓｕｒｆ , the δT ｃｅｎｔｅｒ and correcting the heat transfer correction coefficient so that the δFS satisfies the following equations (4) to (6): δT ｓｕｒｆ ≦0.1・・・(4) δT ｃｅｎｔｅｒ ≦0.06・・・(5) δFS≦0.1 (6)

5. The δT is selected from a plurality of heat transfer correction coefficients within a predetermined range of the heat transfer correction coefficients. ｓｕｒｆ , the δT ｃｅｎｔｅｒ 4. The method for continuous casting steel according to claim 3, further comprising identifying a heat transfer correction coefficient that minimizes δFS and correcting the heat transfer correction coefficient to the identified heat transfer correction coefficient.

6. The time from the withdrawal speed before the end of pouring to the withdrawal speed at the time of head solidification at the end of pouring is t ａ The time for fixing the head is t ０ The time from the end of head fixation to the re-pulling speed is t ｂ Then, the t ａ , t ０ and t ｂ 3. The method for continuous casting steel according to claim 1, wherein an additional reduction amount satisfying the following formulas (7) to (9) is added to the soft reduction zone: t=t ａ When , 0.0 < Z ａ ≦3.0 (7) t=t ０ When , 0.0 < Z ０ ≦1.5 (8) t=t ｂ When , 0.0 < Z ｂ ≦3.0 (9) In the above formulas (7) to (9), Z ａ , Z ０ and Z ｂ is the additional reduction amount (mm) at each time.

7. A method for continuous casting of steel as set forth in claim 3, wherein an additional reduction amount that satisfies the following equations (7) to (9) is added to the light reduction zone at ta, t0, and tb, where ta is the time required for the withdrawal speed to reach the slab withdrawal speed when the head is solidified at the end of casting, t0 is the time required for the head to be solidified, and tb is the time required for the re-withdrawal speed to be reached when the head is solidified at the end of casting. When t = t a , 0.0 < Z a ≦ 3.0 (7) When t=t 0 , 0.0<Z 0 ≦1.5 (8) When t = t b , 0.0 < Z b ≦ 3.0 (9) In the above formulas (7) to (9), Z a , Z 0 and Z b are the additional reduction amounts (mm) at each time.

8. A method for continuous casting of steel as set forth in claim 4, wherein an additional reduction amount that satisfies the following equations (7) to (9) is added to the light reduction zone at ta, t0, and tb, where ta is the time required for the withdrawal speed to reach the slab withdrawal speed when the head is solidified at the end of casting, t0 is the time required for the head to be solidified, and tb is the time required for the re-withdrawal speed to be reached when the head is solidified at the end of casting. When t = t a , 0.0 < Z a ≦ 3.0 (7) When t=t 0 , 0.0<Z 0 ≦1.5 (8) When t = t b , 0.0 < Z b ≦ 3.0 (9) In the above formulas (7) to (9), Z a , Z 0 and Z b are the additional reduction amounts (mm) at each time.

9. A method for continuous casting of steel as set forth in claim 5, wherein an additional reduction amount that satisfies the following equations (7) to (9) is added to the light reduction zone at ta, t0, and tb, where ta is the time required for the withdrawal speed to reach the slab withdrawal speed when the head is solidified at the end of casting, t0 is the time required for the head to be solidified, and tb is the time required for the re-withdrawal speed to be reached when the head is solidified at the end of casting. When t = t a , 0.0 < Z a ≦ 3.0 (7) When t=t 0 , 0.0<Z 0 ≦1.5 (8) When t = t b , 0.0 < Z b ≦ 3.0 (9) In the above formulas (7) to (9), Z a , Z 0 and Z b are the additional reduction amounts (mm) at each time.

10. The time it takes for the unsteady portion of the slab to become a steady portion at the start of casting is called t ｃ Then, Said t ｃ 3. The method for continuous casting steel according to claim 1, wherein an additional reduction amount satisfying the following formula (10) is added to the soft reduction zone: t=t ｃ When , 0.0 < Z ｃ ≦3.0 (10) In the above formula (10), Z ｃ is ｃ is the additional reduction (mm) at

11. When the time from the start of casting until the non-steady-state portion of the slab becomes the steady-state portion is defined as tc, The method for continuous casting steel according to claim 3 , wherein an additional reduction amount satisfying the following formula (10) is added to the soft reduction zone at the time t c : When t = tc, 0.0 < Zc ≦ 3.0 (10) In the above formula (10), Z c is the additional reduction (mm) at t c .

12. When the time from the start of casting until the non-steady-state portion of the slab becomes the steady-state portion is defined as t c , The method for continuous casting steel according to claim 4 , wherein an additional reduction amount satisfying the following formula (10) is added to the soft reduction zone at the time t c : When t = tc, 0.0 < Zc ≦ 3.0 (10) In the above formula (10), Z c is the additional reduction (mm) at t c .

13. When the time from the start of casting until the non-steady-state portion of the slab becomes the steady-state portion is defined as t c , The method for continuous casting steel according to claim 5, wherein an additional reduction amount satisfying the following formula (10) is added to the soft reduction zone at the time t c : When t = tc, 0.0 < Zc ≦ 3.0 (10) In the above formula (10), Z c is the additional reduction (mm) at t c .

14. 3. The method for continuous casting of steel according to claim 1, wherein the roll gap on the upstream side of the soft reduction zone in the casting direction is set larger than the thickness of the slab at the outlet side of the mold.

15. A method for continuous casting of steel as described in claim 3, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

16. A method for continuous casting of steel as described in claim 4, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

17. A method for continuous casting of steel as described in claim 5, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

18. A method for continuous casting of steel as described in claim 6, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

19. A method for continuous casting of steel as described in claim 7, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

20. A method for continuous casting of steel as described in claim 8, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

21. A method for continuous casting of steel as described in claim 9, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

22. A method for continuous casting of steel as described in claim 10, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

23. A method for continuous casting of steel as described in claim 11, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

24. A method for continuous casting of steel as described in claim 12, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

25. A method for continuous casting of steel as described in claim 13, wherein the roll opening upstream of the light reduction zone in the casting direction is made larger than the thickness of the cast piece at the mold exit side.

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