Continuous casting method for steel

Abstract

Provided is a continuous casting method for steel, which is capable of suppressing fluctuation in rolling reduction speed in a region where a center solid phase ratio of a slab is 0.7 to 1.0, inclusive.　The continuous casting method for steel includes casting a slab while subjecting the slab to light rolling reduction utilizing a light rolling reduction zone. In the longitudinal direction of the slab, a region in which the center solid phase ratio of the slab is 0.7 to 1.0 is continuously predicted using operating conditions for a continuous casting machine. The roll opening degree of the light rolling reduction zone is dynamically changed so that the fluctuation in the light rolling reduction speed in the region is no more than a predetermined value.

Description

Continuous casting method for steel

[0001] The present invention relates to a continuous casting method for steel that can cast a slab with reduced center segregation.

[0002] In the final stage of continuous casting of steel, solidification shrinkage causes suction flow of unsolidified molten steel (hereinafter referred to as the "unsolidified layer") in the direction of drawing of the slab. Solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated in this unsolidified layer. This solute-element-enriched molten steel flows to the center of the slab and solidifies, resulting in center segregation. Factors that cause the solute-element-enriched 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 static pressure of the molten steel and misalignment of the slab support rolls.

[0003] This center segregation deteriorates the quality of steel products, especially thick steel plates. For example, in line pipe materials for transporting oil and natural gas, hydrogen-induced cracking occurs starting from the center segregation due to the action of sour gas. Similar problems occur in offshore structures, storage tanks, oil tanks, etc. In recent years, steel products are often required to be used in lower temperatures or in more corrosive environments, making it increasingly important to reduce center segregation in cast slabs.

[0004] Therefore, many measures have been proposed to reduce or neutralize center segregation in slabs from the continuous casting process to the rolling process. Among these, a soft reduction method at the end of solidification, in which a continuously cast slab having an internal unsolidified layer is reduced in a continuous casting machine, is known to be particularly effective in improving center segregation in slabs. Here, the "soft reduction method at the end of solidification" refers to a method in which reduction rolls are placed near the end of solidification of the slab, and the slab is gradually reduced during continuous casting using these reduction rolls at a reduction rate equivalent to the amount of solidification shrinkage. In this way, reducing the slab near the end of solidification using reduction rolls suppresses the generation of voids in the center of the slab and the flow of concentrated solute elements in the molten steel, thereby reducing center segregation in the slab.

[0005] In order to effectively suppress centerline segregation in a slab by this soft reduction method in the final solidification stage, it is important to appropriately set the start and end times of the soft reduction period during the final solidification period of the slab, as well as the amount of reduction at each time, and various methods for setting these times 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 solidification stage of the continuous casting, the amount of reduction per unit time of the slab in the section in which soft reduction is applied is determined by the surface temperature of the slab 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 rolling down a region from a point where the temperature reaches a point where the center solid fraction is 0.1 to 0.3 to a point where the temperature reaches a point where the temperature reaches the flow limit solid fraction, using a plurality of roll pairs. Patent Documents 2 and 3 state that it is preferable to increase the rolling down 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 for 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 calculations.

[0009] JP 8-132203, JP 3-90263, JP 3-90259, JP 2012-187636

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

[0011] Here, the rolling rate (mm / min) is a value calculated by multiplying the rolling gradient (mm / m) by the slab withdrawal speed (m / min). The rolling gradient is a state of roll gap that is set so that the distance between opposing rolls (hereinafter sometimes 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 a 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 does not disclose anything about suppressing fluctuations in the reduction rate in accordance with the center solid fraction of the slab.

[0014] The present invention has been made in view of the above problems of the prior art, 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 a range in which the center solid fraction of a slab is 0.7 or more and 1.0 or less.

[0015] The means for solving the above problems are as follows. [1] A method for continuously casting steel in which a slab is cast while soft reduction is performed 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 operating conditions of a continuous casting machine; and dynamically changing a roll gap in the soft reduction zone so that fluctuations in the soft reduction rate in the region are equal to or less than a predetermined value. [2] The method for continuously casting steel according to [1], in which the roll gap in the soft reduction zone is dynamically changed so that fluctuations in the soft reduction rate in the region are equal to or less than 0.08 mm / min. [3] The operating conditions are secondary cooling conditions for 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. 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 measured value of the temperature at the thickness center. δT is calculated using the following equations (1) to (3). ｓｕｒｆ , δT ｃｅｎｔｅｒThe method for continuous casting steel according to [1] or [2], further comprising: correcting a heat transfer correction coefficient of the heat transfer model so that δT is equal to or less than a predetermined value; and predicting a final solidification position of the slab using the heat transfer model including the corrected heat transfer correction coefficient. 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 of the center of the thickness of the slab measured by the electromagnetic ultrasonic solidification state estimation sensor. [4] ｓｕｒｆ , the δT ｃｅｎｔｅｒ and correcting the heat transfer correction coefficient so that the δFS satisfies the following formulas (4) to (6): 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 [6] The method for continuous casting steel according to [3], further comprising: specifying a heat transfer correction coefficient that minimizes δFS and the δFS; and correcting the heat transfer correction coefficient to the specified heat transfer correction coefficient. ａThe 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 The method for continuous casting 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) 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] The time it takes for the unsteady state portion of the slab at the start of casting to become the steady state portion is t ｃ Then, the t ｃ The method for continuous casting 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 in the condition t=t ｃ When , 0.0 < Z ｃ ≦3.0 (10) In the above formula (10), Z ｃ is ｃ [8] The method for continuous casting of steel according to any one of [1] to [7], wherein the roll gap on the upstream side of the soft reduction zone in the casting direction is larger than the thickness of the slab at the outlet side of the mold.

[0016] According to the present invention, the center solid fraction of a slab is continuously predicted, and therefore the reduction rate can be adjusted to follow the solidification position even if it changes due to fluctuations in operating conditions, etc. This makes it possible to suppress fluctuations in the reduction rate in the region where the center solid fraction of the slab is 0.7 to 1.0 more effectively than before, thereby realizing the casting of a slab with reduced center segregation.

[0017] Fig. 1 is a cross-sectional schematic diagram showing a continuous steel casting facility in which the continuous steel casting method according to the present embodiment can be implemented. Fig. 2 is a flow chart illustrating a process for determining a heat transfer correction coefficient α. Fig. 3 is a side view of a roll segment constituting a soft reduction zone. Fig. 4 is a front view of a roll segment. Fig. 5 is a diagram illustrating a time t at the end of casting. ａ , t ０ and t ｂ 1 is a graph showing

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

[0019] 1 is a cross-sectional schematic diagram 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 includes a continuous casting machine 90 and a control device 48 that controls the continuous casting machine 90. The continuous casting machine 90 mainly includes a mold 10, a tundish 12, and multiple pairs of strand support rolls 32. The tundish 12 is installed above the mold 10. A sliding nozzle 14 is installed at the bottom of the tundish 12 to adjust the flow rate of molten steel 18 poured into the mold 10, and a submerged nozzle 16 is installed below 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 is solidified as heat is removed from the inner surface of the mold 10, forming a solidified shell 20. This results in the formation of a cast 24 having the solidified shell 20 as its outer shell and an unsolidified layer 22 made of the molten steel 18 inside.

[0021] Below the mold 10, multiple pairs of strand support rolls 32, including support rolls 26, guide rolls 28, and drive rolls 30, are provided. Of these, the drive rolls 30 support the strand 24 while simultaneously withdrawing it. Spray nozzles (not shown), such as water spray nozzles or air mist spray nozzles, are provided in the gaps between adjacent strand support rolls 32 in the casting direction, thereby forming a secondary cooling zone. The strand 24 is cooled by the cooling water sprayed from the spray nozzles in the secondary cooling zone while being withdrawn, reducing the internal unsolidified layer 22 and growing the solidified shell 20. The strand 24 is then appropriately cooled, solidifying the unsolidified layer 22, and the strand 24 is completely solidified. Downstream of the strand support rolls 32, multiple transport rolls 34 are provided to transport the cast strand 24. Above the transport rolls 34, a strand cutter 36 is provided to cut the cast strand 24 into slabs 38 of a predetermined length. The cast piece 24 is cut by the cast piece cutter 36 to produce slabs 38 of a predetermined length.

[0022] Soft reduction zones 41, which allow for changing the roll gap between the opposing guide rolls 28, 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 of the slab at the thickness center in the soft reduction zone 41. The solidification state estimation sensor 46 is a device that measures the temperature of the slab at the thickness center by utilizing the temperature dependency of ultrasonic propagation 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 thickness center to the control device 48.

[0025] In the above example, the surface thermometer 44 and the solidification state estimation sensor 46 are provided in the soft reduction zone 41, but this is not limiting. The surface thermometer 44 may be installed anywhere downstream of the mold 10 in the casting direction, but is preferably installed in a position close to the final solidification position 40 of the slab 24. The solidification state estimation sensor 46 is preferably installed in a position close to the final solidification position 40, and more preferably installed 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 personal computer that has 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 steel casting process. The control device 48 further acquires 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 installed using a heat transfer model that uses 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 of the thickness center of the slab 24 at the position where the solidification state estimation sensor 46 is installed, 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 soft reduction zone 41, the temperature of the thickness center of the slab 24, and the final solidification position 40 using a heat transfer model that uses a heat flux based on secondary cooling conditions will be described.

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

[0029] At this time, by continuously generating and calculating sliced ​​cross sections of unit length, it is possible to calculate not only the temperature in the steady state portion but also the temperature in the unsteady state portion at the beginning and end of casting. Operating conditions such as actual water cooling data, casting speed, and the temperature of the 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 heat transfer equation is solved as a numerical calculation by differentiating the heat conduction equation. Here, in a typical secondary cooling calculation for continuous casting, for example, a cross section of a slab sliced ​​at unit length along the slab's longitudinal direction (casting direction) is considered. Then, the heat flux Q is calculated based on the following equation (11), which indicates the boundary conditions on the surface of the slab 24 under secondary cooling conditions, such as water cooling, air cooling, mist cooling, and roll cooling, depending on the location within the strand during casting. This heat flux Q can be used to solve the two-dimensional heat transfer equation shown in the following equation (13).

[0031] However, φ, which is a value related to temperature in the above formula (11), is expressed by the following formula (12). Therefore, when applying the above formula (11) to the later-described formula (13), the temperature is replaced by the following formula (12).

[0032] In the above equations (11) and (12), Q is the heat flux (W / m ２ ) k is the thermal conductivity (W / (m×K)). kd is the thermal conductivity at the reference temperature (W / (m×K)). h is the heat transfer coefficient (m ２ .K). T is the model surface temperature (K). Ta is the ambient temperature (K).

[0033] In the above formula (13), c is the specific heat (J / (kg×K)), and ρ is the density (kg / m ３ ) k is the thermal conductivity (W / (m×K)). T is the temperature (K). t is the time (sec), and x and y are the coordinates (m).

[0034] The heat transfer coefficient h in equation (11) is determined by the cooling method (e.g., water cooling, air cooling, mist cooling), the cooling operation amount, the amount of heat removed from the roll, and other secondary cooling conditions. 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 the resulting value is used as h in the calculation. In this embodiment, the following equation (14) is used instead of equation (11) as the equation 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 property values ​​of specific heat, density, and thermal conductivity change with changes in the temperature of the slab 24, so it is necessary to change the physical property values ​​as a function of temperature to solve the equations. If the physical property values ​​are temperature-dependent, the equations cannot be expanded as differential equations as they are. Therefore, in actual calculations, the temperature and heat content are linearized by substituting them into the following equations (15) and (16) using the well-known "temperature-transformed temperature method."

[0037]

[0038] In the above equations (15) and (16), φ is the conversion temperature (K), and H is the heat content (J). Td is the reference temperature (K), and kd is the thermal conductivity (W / (m×K)) at the reference temperature. By substituting these equations (15) and (16) into the above equation (13), the following equation (17) is derived.

[0039]

[0040] By differentiating this equation (17), it becomes possible to perform numerical analysis of heat transfer calculations for each slice. Different differential 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]

[0042]

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

[0044] In the above equation (21), Q is the heat flux (W / m ２ ) In the above equations (20) and (21), l represents the calculation time step, and the value of (l+1) for the next calculation step (time) is calculated from each value of l. Heat transfer is calculated by the difference method using the difference equations (20) and (21).

[0045] In the actual calculation process, heat transfer calculations are performed in steps 1 to 9 below. 1. As the analysis begins, one two-dimensional sheet enters the mold and moves forward. 2. This sheet is calculated using only the external boundary conditions and internal two-dimensional heat conduction. (Heat conduction in the direction of travel is not considered.) 3. Along the way, the speed changes at each time depending on the speed data. 4. Along the way, the spray pattern switches depending on the external cooling pattern data. 5. This one sheet is calculated until the end of the analysis time. 6. When moving to the next sheet, the physical properties and initial temperature are changed to match the input. 7. Once the calculation for one sheet is complete, the calculation for the next sheet begins the same time step later, and calculations are continued until the end of the analysis time. 8. The above calculations are performed for each sheet until the end of the extraction time. 9. Files are output as needed along the way.

[0046] The heat transfer calculation is performed by using a finite difference method to analyze the heat conduction in the slab 24, and the analysis target is half the thickness due to structural symmetry. For example, if the short and long sides are divided into m and n parts, the mesh will be the half part divided into m and n parts.

[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 changes depending on the cooling method (water only, water and air, air only, and the flow rate of each). The heat removal used in practice is the larger value than that of radiative cooling.

[0048] The center solid fraction of the slab 24 is calculated as follows: if the temperature of the center of the slab 24 is lower than the liquidus temperature, the center solid fraction = 1; if the temperature of the center of the slab 24 is higher than the solidus temperature, the center solid fraction = 0. If the temperature of the center of the slab 24 is between the liquidus temperature and the solidus temperature, the center solid fraction is calculated using the following equation (22):

[0049] Central solid fraction = (C ０ -C Ｌ ) / (C Ｓ -C Ｌ ) (22) In the above formula (22), C ０ is the carbon concentration (mass%). Ｓ is the carbon concentration (mass%) at a certain temperature equal to the solidus temperature. Ｌ is the carbon concentration (mass%) at which a certain temperature is equal to the liquidus temperature.

[0050] Inside the mold, the amount of heat removed from the surface is determined by the time the slice passes through the mold. The amount of heat removed 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: 0.02 sec ・Casting speed: 1.4 mpm ・Analysis thickness: 125 mm (half thickness, full thickness 250 mm) ・Analysis width: 1000 mm (half width, full width 2000 m) ・Atmospheric temperature: 30°C ・Secondary cooling water temperature: 28°C ・Molten steel temperature: 1555°C ・Thermal conductivity at reference temperature: Determined based on the composition of the material in question. ・Liquid phase temperature and solid phase temperature calculated from the composition of the molten steel: Determined through experiments or other means. ・Relationship between transformation temperature φ and temperature: Determined through experiments or other means. ・Relationship between heat content H and temperature: Determined through experiments or other means. ・Relationship between density ρ and temperature: Determined through experiments or other means. ・Example of number of divisions in the mesh width direction: Width (n) = 66, thickness (n) = 25

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

[0053] For this reason, the control device 48 acquires the actual measured values ​​of the surface temperature and 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. In addition, 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 using the following equations (1) and (2): ｓｕｒｆ , δT ｃｅｎｔｅｒ becomes equal to or less than a predetermined value, the control device 48 determines 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 equation (3) becomes equal to or less than a predetermined value, the control device 48 determines 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, the control device 48 determines that the predicted value of the final solidification position 40 of the slab 24 matches the estimated value of the final solidification position 40 when δT calculated using the following equations (1) to (3) ｓｕｒｆ , δT ｃｅｎｔｅｒ If δFS exceeds a predetermined value, it is determined that the predicted value does not match the actually 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，ｓｕｒｆis the surface temperature (°C) of the slab 24 predicted by the heat transfer model. ｓｕｒｆ is the actual measured value (°C) of the surface temperature 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. ｃｅｎｔｅｒ is the actual temperature (°C) at the center 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 δFS 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 equations (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 δFS and δFS vary depending on the required level of center segregation required for the steel sheet, which is the final 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 (the above formulas (4) to (6)). 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 flowchart illustrating the process for determining the heat transfer correction coefficient α. This flow is started, for example, when the control device 48 is started and the surface thermometer 44 and the solidification state estimation sensor 46 are ready for measurement.

[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 measured value T of the temperature at the center of the thickness from the solidification state estimation sensor 46 (step S101). ｃｅｎｔｅｒ The control device 48 acquires the actual measured value T of the temperature at the center of the thickness acquired from the solidification state estimation sensor 46 (step S102). ｃｅｎｔｅｒ and the heat transfer model, the final solidification position FS is estimated (step S103).

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

[0062] The control device 48 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). ｓｕｒｆ , δT ｃｅｎｔｅｒ It is determined whether or not the calculated δT satisfies the above formulas (4) to (6) (step S106). ｓｕｒｆ , δT ｃｅｎｔｅｒ If δFS satisfies the above formulas (4) to (6) (step S106: Yes), the control device 48 ｓｉｍ，ｓｕｒｆ , T ｓｉｍ，ｃｅｎｔｅｒ , F.S. ｓｉｍThe heat transfer correction coefficient α used to calculate δT is identified as the heat transfer correction coefficient used to identify the region where the center solid fraction is 0.7 to 1.0 (step S108), and the flow ends. ｓｕｒｆ , δT ｃｅｎｔｅｒ If 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). Thereafter, 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 the δT ｓｕｒｆ , δT ｃｅｎｔｅｒ The processes of steps S104 to S107 are repeated until the calculated δT ｓｕｒｆ , δT ｃｅｎｔｅｒ and δFS can be specified as a 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 48.

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

[0065] In this embodiment, "continuously" means predicting the region in which the center 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 center solid fraction is 0.7 to 1.0, it 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 fluctuations in the reduction rate so that the absolute value of the fluctuations in the reduction rate is 0.08 mm / min or less. 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. By suppressing fluctuations in the reduction rate in the region where the center solid fraction of the slab 24 is 0.7 to 1.0 in this way, it is possible to cast a slab with reduced centerline 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 upstream support columns 60 and downstream support columns 62. Because the upper frame 56 and the lower frame 58 are supported by the upstream support columns 60 and the downstream support columns 62, the upstream support columns 60 and the downstream support columns 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 each fixed to the upper frame 56 or the lower frame 58 via bearings 54. Therefore, the distance between the upper and lower guide rolls can be changed by extending or retracting the support columns using hydraulic cylinders. The amount of bulging can be controlled by setting the roll spacing wider than that of the immediately preceding segment, and the soft reduction gradient can be controlled by setting the distance 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 cylinders that extend or retract the support columns of the guide rolls 28. The control device 48 controls the drive unit to dynamically change the roll gap in the soft reduction zone 41. While this embodiment has been described with reference to an example in which the roll gap in the light reduction zone 41 is dynamically changed to minimize fluctuations in the reduction rate, this is not limiting. For example, the center solid fraction of the slab 24 may be controlled by dynamically changing the roll gap in the light reduction zone 41 and changing the operating conditions of the continuous casting machine 90 to minimize fluctuations in the reduction rate.

[0068] In continuous casting of steel, casting becomes unstable at the beginning and end of casting. Therefore, the beginning and end of casting are considered to be unsteady periods, in contrast to the steady period during which continuous casting operation is stable. In the unsteady period, the slab 24 shrinks, and unless the reduction amount is increased in accordance with the amount of shrinkage, fluctuations in the reduction rate will increase, and center segregation of the slab 24 will worsen.

[0069] FIG. 5 shows the time t ａ, t ０ and t ｂ 5 is a graph showing the time (min) from the end of casting, and the vertical axis is the casting speed (m / min). ａ is the time it takes for the withdrawal speed of the slab 24 to change from the withdrawal speed before the end of casting to the withdrawal speed at the time of head solidification at the end of casting. ０ is the time for fixing the head. ｂ is the time from the end of head fixation to the re-pulling speed.

[0070] After casting is completed, as shown in Figure 5, the casting speed is reduced from the time when casting is completed (when 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). If the top portion of the slab 24 is not yet solidified when it is removed from the mold 10 after casting is completed, molten steel 18 will leak 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 "heading."

[0071] During head hardening, the casting speed is slowed down to firmly harden the top portion. 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 elapsed time t is greater than or equal to 0 and less than t. １ for less than (time t ａ At this time, the casting speed is reduced by the first casting speed V ａ Second casting speed V when head is fixed ０ Slow down and perform head lock.

[0072] After this, the elapsed time t becomes t ２ More than t ３ During the following period (time t ｂ At this time, the casting speed of the slab 24 is increased to the second casting speed V ０ The third casting speed V is the casting speed at the time of re-withdrawing after the end of casting. ｂ The re-pulling is performed by increasing the pulling speed after the head hardening is completed and pulling out the slab 24. When the elapsed time t reaches t３ When the drawing speed exceeds the predetermined value, the slab 24 is cast at the third casting speed V ｂ The period after the end of pouring is called the time t ａ The period is the deceleration process, and the time t ０ The period is the head hardening process, time t ｂ The period is the acceleration step, and the time t ｂ A period later than (t>t ３ ) is also called the re-drawing process.

[0073] The control device 48 determines the time t ａ , t ０ and t ｂ In the above, it is preferable to change the roll gap so that an additional reduction amount that satisfies the following formulas (7) to (9) is added to the soft reduction zone. This allows an appropriate soft reduction to be applied 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) 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.

[0075] Time t a , t ０ , 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 using 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 being withdrawn from the casting machine from the start of casting until it enters the steady state portion within a certain range. ｃ Then, the control device 48 calculates t ｃ In this case, it is preferable to add an additional reduction amount to the soft reduction zone that satisfies 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) at

[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 using 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 light reduction zone 41 compared to the thickness of the slab 24 at the delivery 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 directly below the mold 10 by a maximum of 15 mm compared to the thickness of the slab 24 at the delivery side of the mold 10. This allows the light reduction zone 41 to soften the narrow sides, where the reaction force from the slab 24 to the rolls increases, without excessively reducing them.

[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 concurrently calculate δT for a plurality of heat transfer correction coefficients within a predetermined range. ｓｕｒｆ , δT ｃｅｎｔｅｒand δFS are calculated, and δT ｓｕｒｆ , δT ｃｅｎｔｅｒ The range of the heat transfer correction coefficient may be determined based on the actual values ​​of the heat transfer correction coefficients in 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 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 identified in two steps. In the first step, δT is calculated 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 δFS are smallest is identified. The heat transfer correction coefficient is changed by 5 to 10 levels in increments of 0.01 from the baseline, and δT ｓｕｒｆ , δT ｃｅｎｔｅｒ and the heat transfer correction coefficient that minimizes δFS may be specified.

[0083] The units by which the heat transfer coefficient is changed at each level on the increasing and decreasing sides in the first step do not have to be the same, and may be changed by 0.1 to 0.9 units based on past performance. Similarly, the units by which the heat transfer coefficient is changed at each level on the increasing and decreasing sides in the second step do not have to be the same, and may be changed by 0.01 to 0.09 units 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 multiple 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.

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

[0086] In Examples 1 to 3 of Example 1, a heat transfer model was used to identify the region where the center solid fraction of the slab was 0.7 to 1.0 every second. The roll opening of the reduction rolls in the soft reduction zone was dynamically changed so that the target reduction rate in that region would be 0.28 mm / min, and the absolute value (time average) of the difference from the target reduction rate was controlled to be 0.10 mm / min or less. In Comparative Examples 1 and 2, on the other hand, casting was performed without controlling the fluctuations in the reduction rate. Therefore, the absolute value of the difference from the target reduction rate in Comparative Examples 1 and 2 was larger than in Examples 1 to 3.

[0087] Continuous casting of steel was carried out under these operating conditions, and the center segregation of the cast slabs was evaluated. The center segregation of the slabs was evaluated by the number of center segregated grains. The number of center segregated grains was measured using the following procedures (1) to (5). (1) A 15 mm wide slab sample was taken from a cross section of the slab perpendicular to the casting direction, including a center segregation region at the center, and measuring from the center of the width to one side of the triple point (the point where the solidified shells on the short side and long side meet after growing). (2) The cross section of the taken slab sample perpendicular to the casting direction was polished, and the surface was corroded with, for example, a saturated aqueous solution of picric acid to reveal a segregation zone. The center segregation region was determined to be a region extending from the center of the segregation zone to ±7.5 mm of the slab thickness. (3) The central segregation portion of the slab sample was divided into smaller portions in the slab width direction, and the Mn concentration of the slab sample was analyzed over the entire surface using an electron probe microanalyzer (EPMA) with an electron beam diameter of 100 μm. (4) The distribution of Mn segregation was determined, and connected regions with a Mn segregation degree of 1.33 or more were defined as one segregated grain. The Mn segregation degree was calculated by dividing the Mn concentration of the segregated portion by the Mn concentration at a position 10 mm away from the thickness center. (5) The number of segregated grains was counted, and the number of segregated grains was calculated by dividing the number of segregated grains by the length of the sample in the slab width direction.

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

[0089]

[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 these results, it was confirmed that the region in which the center solid fraction of the slab is 0.7 or more and 1.0 or less is continuously identified, and the opening of the reduction rolls in the light reduction zone is dynamically controlled so that the reduction rate in that region becomes the target reduction rate. This makes it possible to suppress fluctuations in the reduction rate in the region in which the center solid fraction of the slab is 0.7 or more and 1.0 or less, and to cast a slab with reduced center segregation.

[0091] [Example 2] In Example 2, as in 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. The opening of the reduction rolls in the soft reduction zone was then dynamically adjusted to control fluctuations in the reduction rate so that the target reduction rate in that region would be 0.28 mm / min. In Examples 6 to 15 of Example 2, actual measurements of the surface temperature of the slab and the temperature at the center of the thickness were also obtained from the surface thermometer 44 and the solidification state estimation sensor 46. Using the obtained measurements, δT calculated using the above formulas (1) to (3) was calculated. ｓｕｒｆ , δT ｃｅｎｔｅｒ The heat transfer correction coefficient of the heat transfer model was corrected so that δFS and δFS satisfied the above equations (4) to (6). ｓｕｒｆ , δT ｃｅｎｔｅｒ The heat transfer correction coefficient of the heat transfer model was corrected so that δFS and δFS were 0.05, 0.03, and 0.05 or less. Inventive Example 15, the steel required for center segregation was a normal steel type, so δT ｓｕｒｆ , δT ｃｅｎｔｅｒ The heat transfer correction coefficient of the heat transfer model was modified so that δFS was 0.07, 0.05, and 0.07 or less.

[0092] On the other hand, in Example 4, the heat transfer correction coefficient of the heat transfer model was not corrected. ｓｕｒｆ δT ｓｕｒｆ The heat transfer correction coefficient of the heat transfer model was simply modified so that δT ｃｅｎｔｅｒ , δFS was used to correct the heat transfer correction factor.

[0093] In Examples 6 to 15, a heat transfer model including a modified heat transfer correction coefficient was used to identify the region where the center solid fraction of the slab was 0.7 to 1.0 every second. The roll gap in the soft reduction zone was then dynamically changed so that the absolute value (time average) of the difference from the target reduction rate in this 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 beyond the thickness of the slab at the mold exit.

[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 or more and 1.0 or less every second. ｓｕｒｆ The region in which the center 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 modified so that the difference between the reduction rate and the target reduction rate in this region (averaged over time) was 0.08 mm / min or less.

[0095] Continuous casting of steel was carried out under these operating conditions, and the centerline segregation of the cast slabs was evaluated. The centerline segregation of the slabs was evaluated based on the number of centerline 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]

[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 a 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 these results, it is believed that the use of a heat transfer model including the modified heat transfer correction coefficient enabled the region where the center solid fraction of the slab is 0.7 to 1.0 to be identified with high accuracy, thereby suppressing fluctuations in the reduction rate in that region. It is believed that suppressing fluctuations in the reduction rate in the region where the center solid fraction is 0.7 to 1.0 resulted in cast slabs with fewer center segregated grains 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 fluctuations in the reduction rate was 0.08 mm / min or less, as in the examples, but the accuracy of identifying the region where the center solid fraction is 0.7 to 1.0 was low. Therefore, it is believed that fluctuations in the reduction rate in that region could not be suppressed, resulting in an increase in center segregated grains. These results confirmed that the region where the center solid fraction is 0.7 or more and 1.0 or less can be identified with high accuracy by using a heat transfer model including the modified heat transfer correction coefficient. It was also confirmed that by dynamically changing the roll gap of the reduction rolls so as to reduce fluctuations in the reduction rate in the identified region, it is possible to cast a slab with even less center segregation.

[0098] Inventive Example 7, in which the roll gap upstream of the light reduction zone and downstream 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, the number of center segregation grains was smaller than in Inventive Example 6, in which the roll gap was not wider. This result confirmed that center segregation can be reduced by widening the roll gap upstream 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 opening of the reduction rolls was also changed in the first unsteady state section and the second unsteady state section to change the reduction amount in the region of the center solid fraction of 0.7 to 1.0. ａ , t ０ and t ｂ is the t shown in Figure 5 ａ , t ０and t ｂ The same as t in the first half of the unsteady state ｃ It takes about 40 minutes.

[0100] In Example 3, the number of center segregated grains was measured not only for the slab in the steady portion but also for the slab in the latter unsteady portion and the slab in the former unsteady portion. The number of center segregated grains was measured in the same manner as in Example 1. The results of Example 3 are shown in Table 3 below.

[0101]

[0102] As shown in Table 3, in the latter half of the unsteady state, 0.0 < Z ａ ≦3.0 and 0.0<Z ｂ Example 18 does not satisfy the condition 0.0<Z≦3.0. ０ Inventive Example 20, which does not satisfy the condition ≦1.5, the number of center segregations in the latter half of the unsteady state was greater than that of the other inventive examples. ａ , t ０ and t ｂ It was confirmed that by changing the roll gap so that the additional reduction amount in the latter half of the rolling process satisfies the above formulas (7) to (9), it is possible to reduce fluctuations in the rolling rate in the latter half of the unsteady state and thereby reduce centerline segregation in the slab.

[0103] In the first half of the unsteady state, 0.0 < Z ｃ Inventive Example 22, which does not satisfy the condition ≦3.0, the number of center segregations in the first half unsteady state was greater than that of the other inventive examples. ｃ It was confirmed that by changing the roll gap so that the additional reduction amount in the first half portion satisfies the above formula (10), it is possible to reduce fluctuations in the reduction rate in the first half unsteady portion and thereby reduce center segregation of the slab.

[0104] [Example 4] In Example 4, as in Example 1, a heat transfer model was used to identify the region where the center solid fraction of the slab was 0.7 to 1.0 every second. The opening of the reduction rolls in the soft reduction zone was dynamically changed so that the target reduction rate in that region would be 0.28 mm / min, thereby suppressing fluctuations in the reduction rate. In 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 also obtained from the surface thermometer 44 and the solidification state estimation sensor 46. The δT of heat transfer correction coefficients 2 to 12 selected from within a predetermined range of heat transfer correction coefficients was then calculated. ｓｕｒｆ , δT ｃｅｎｔｅｒ and δFS were calculated in parallel using the GPU and CUDA programs, and these values ​​were corrected to the heat transfer correction coefficient that minimized them. Multiple heat transfer correction coefficients were selected so that the heat transfer correction coefficients would be distributed within a range predetermined by the number of selected heat transfer correction coefficients.

[0105] In Examples 23 to 29, a heat transfer model including a modified heat transfer correction coefficient was used to identify the region where the center solid fraction of the slab was 0.7 to 1.0 every second. The roll opening in the soft reduction zone was then 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. 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 measured using the same method as in Example 1. The results of Examples 23 to 29 are shown in Table 4 below.

[0106]

[0107] As shown in Table 4, increasing the number of calculated heat transfer correction coefficients reduces δT ｓｕｒｆ , δT ｃｅｎｔｅｒ It was confirmed that the values ​​of δFS and δFS can be reduced. This makes it possible to identify with high accuracy the region in which the center solid fraction of the heat transfer slab is 0.7 or more and 1.0 or less, and to cast a slab with few center 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.

[0109] REFERENCE SIGNS LIST 10 Mold 12 Tundish 14 Sliding nozzle 16 Submerged entry nozzle 18 Molten steel 20 Solidified shell 22 Unsolidified layer 24 Strand 26 Support roll 28 Guide roll 30 Drive roll 32 Strand support roll 34 Transport roll 36 Strand cutter 38 Slab 40 Final solidification position 41 Soft reduction zone 42 Roll segment 44 Surface thermometer 46 Solidification state estimation sensor 48 Control device 54 Bearing 56 Upper frame 58 Lower frame 60 Upstream support 62 Downstream support 90 Continuous casting machine 100 Steel continuous casting equipment

Claims

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 the continuous casting machine; and dynamically changing the roll gap in the soft reduction zone so that the fluctuation in the soft reduction rate in the region is below a predetermined value.

2. A method for continuous casting of steel as set forth in 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 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; and δT is calculated using the following formulas (1) to (3). ｓｕｒｆ , δT ｃｅｎｔｅｒ 3. The method for continuous casting of steel according to claim 1, further comprising the steps of: correcting a heat transfer correction coefficient of the heat transfer model so that δFS is equal to or less than a predetermined value; and predicting a final solidification position of the slab using the heat transfer model including the corrected heat transfer correction coefficient. 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), 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 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 δT ｓｕｒｆ , the δT ｃｅｎｔｅｒ The method for continuous casting steel according to claim 3, wherein the heat transfer correction coefficient is corrected so that the δFS satisfies the following formulas (4) to (6). 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 the steps of: 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 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 called t ａ The time for head fixation 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 The method for continuous casting steel according to any one of claims 1 to 5, wherein an additional reduction amount satisfying the following formulas (7) to (9) is added to the soft reduction zone in the range of 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. The time from the start of casting until the unsteady part of the slab becomes the steady part is called t ｃ Then, the above t ｃ The method for continuous casting steel according to any one of claims 1 to 5, wherein an additional reduction amount satisfying the following formula (10) is added to the soft reduction zone in the range of 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 8. A method for continuous casting of steel according to any one of claims 1 to 7, wherein the roll gap on the upstream side of the soft reduction zone in the casting direction is set to be larger than the thickness of the slab at the outlet side of the mold.

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