JP2013022609A - Continuous casting method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for steel, capable of obtaining the steel with excellent internal quality, in which a generation of center segregation, center porosity and internal cracks is suppressed.SOLUTION: In the continuous casting method for steel, respective pieces of data on the component analysis value and the temperature of molten steel inside a tundish, a casting speed, the amount and the temperature or the like of cooling water for a mold, the amount and the temperature of secondary cooling water, the amount and the temperature of cooling water for a roll and an outside air temperature are collected in real time during a continuous casting operation, model calculation for estimating a solidification state inside a cast slab is performed with the respective pieces of data or the like as parameters, the roll force of a rolling device is controlled in real time on the basis of the solidification state inside the cast slab estimated from the result of the model calculation, and data on the relation of a center solid phase rate of the cast slab and the roll force set beforehand, and the casting speed is controlled such that an area where the center solid phase rate is estimated to be 0.30-0.80 is at the position of the rolling device.

Description

  The present invention relates to a method for continuous casting of steel, and in particular, the occurrence of center segregation, center porosity and internal cracks in a slab is suppressed, and steel having good internal quality with few center defects can be produced. The present invention relates to a continuous casting method.

  In general, slabs produced by continuous casting often have center segregation, and the cause of the center segregation is the flow of molten steel due to slab bulging or solidification shrinkage at the end of solidification of the slab. It is considered. It is known that light reduction of the slab at the end of solidification is effective for directly preventing the molten steel flow and suppressing the generation of center segregation.

  In Patent Document 1, as a technique for suppressing the center segregation of the slab by light pressure, the solid phase ratio at the center part in the thickness direction of the slab exceeds 0 and is measured at an arbitrary position in the range of 0.3 or less. Compare the measured value of the solidified shell thickness with the calculated value of the solidified shell thickness calculated using the initial concentration and casting conditions of the molten steel at the measurement position, and based on the difference between the calculated value and the measured value. The center segregation degree of carbon in the slab is evaluated online, and based on the evaluation result, the secondary cooling rate and the light rolling roll are adjusted so that the center segregation degree of carbon is in the range of 0.98 to 1.10. A method for adjusting one or more of the refinement gradients is disclosed.

  However, in the technique disclosed in Patent Document 1, although it is possible to predict the center segregation degree of the slab, the solid-liquid coexistence region inside the continuous cast slab is It is difficult to grasp the final solidification position in real time. For this reason, it is difficult to optimally control the rolling roll squeezing gradient, and there is a problem that the center segregation of the slab cannot be sufficiently reduced. The casting conditions correspond to, for example, the casting speed, the composition and superheating degree of the molten steel in the tundish, the amount of mold cooling water, temperature and temperature fluctuations, and the amount of secondary cooling water.

  In Patent Document 2, as a technique for rolling down an unsolidified portion in a slab with a rolling roll at an optimal rolling position, a calculation section perpendicular to the casting direction is set at predetermined intervals in a strand being cast, The solidification calculation model using the cooling history information based on the two-dimensional solidification calculation when passing through multiple cooling zones is used to estimate the slab solidification state in the light rolling roll placement area, and light rolling in the optimal light rolling area A method for controlling the amount of roll reduction is disclosed.

  With the technique disclosed in Patent Document 2, it is possible to predict the solidification state of a slab according to changes in casting speed, molten steel temperature, and the amount and temperature of secondary cooling water. There is no description on what to evaluate and control as the slab solidification state to maintain the optimal light reduction zone (down roll position), simply using the corresponding steel properties, different heat in the same strand It merely describes a method capable of maintaining the calculation even when there are a plurality of steel types having specific physical properties. For this reason, the technique disclosed in this document cannot sufficiently reduce the center segregation of the slab.

  Further, in a generally used method for controlling the amount of roll reduction, the roll and the support frame of the roll, that is, the reduction device, expands as the temperature rises due to heat radiation from the high-temperature slab. Even if it performs, a difference with the amount of reduction which the amount of reduction which actually acts on a slab will produce, and the fluctuation | variation of a reduction reaction force will arise. For this reason, this method has a problem that even if the casting conditions are the same, the center segregation of the slab cannot be sufficiently reduced, resulting in overpressure and the occurrence of internal cracks in the slab.

Japanese Patent No. 3478231 JP 2006-175468 A

  The present invention has been made in view of the above-mentioned problems, and its problem is that continuous segregation of steel, in which the occurrence of center segregation, center porosity, and internal cracks is suppressed, and steel having good internal quality can be obtained. It is to provide a method.

  As a result of studying the above problems, the present inventors have focused on estimating the solidification form inside the slab during casting in real time and controlling the reduction of the slab based on this.

  In general, under unsolidified pressure, it is said that the central solid phase ratio in the continuous cast slab when the reduction is applied is appropriately in the range of 0.3 to 0.8. Solidification form inside the slab, for example, the central solid fraction distribution and final solidification position inside the slab are influenced by casting conditions, specifically the casting speed, the degree of superheat of the molten steel in the tundish, the amount of mold cooling water, It is affected by temperature and temperature fluctuations, and the amount of secondary cooling water, and is most strongly affected by casting speed. As a method for more accurately grasping the change in the solidification form inside the slab during continuous casting operation due to the influence of casting conditions, the present inventors have used solidification simulation using the thermodynamic characteristics of the molten steel and the casting conditions. It was considered that estimating the solidification form inside the slab by model calculation was the most effective for the variation of various molten steel composition and casting conditions.

  In order to reduce the influence of thermal expansion of the slab reduction device, obtain reproducibility of the reduction control, and suppress the occurrence of internal cracks in the slab, the reduction control of the slab is not reduced by the reduction amount. I thought it was effective to do it by force.

  As a result of the above study, the inventors calculated the central solid phase ratio distribution in the slab in real time by solidification simulation model calculation during continuous casting operation, and the result and a preset slab The data of the optimal rolling force corresponding to the central solid fraction at the rolling position was used for rolling control of the slab. As a result of repeated experiments, according to this method, even if the casting conditions fluctuate and the central solid fraction distribution and the final solidification position fluctuate, the rolling force at the time of unsolidified reduction and after complete solidification is appropriately adjusted. It was found that the generation of central defects such as central segregation and central porosity and internal cracks can be controlled.

  The present invention has been made on the basis of the results and knowledge of the above studies, and the gist thereof is the continuous casting method of steel shown in the following (1) and (2).

(1) Molten steel is poured from a tundish into a mold to form a slab, the slab is secondarily cooled at the lower part of the mold, and the secondary cooled slab is reduced by a reduction device having a roll. In the continuous casting method, the component analysis value and temperature of the molten steel in the tundish, the casting speed, the amount of cooling water of the mold, the temperature and temperature change, the amount of secondary cooling water in real time during the continuous casting operation And temperature, the amount and temperature of the cooling water of the roll, and the outside air temperature data, and the liquidus temperature, solidification latent heat amount, specific heat and density of the molten steel calculated from each data and the component analysis value of the molten steel. As a parameter, solidification simulation model calculation for estimating the solidification state inside the slab is performed, and the solidification state inside the slab estimated from the result of the solidification simulation model calculation is calculated in advance. The rolling force of the rolling device is controlled in real time based on the set data on the relationship between the central solid fraction of the slab and the rolling force, and the central solid fraction is estimated to be 0.30 to 0.80. A continuous casting method of steel, wherein a casting speed is controlled so that a region to be moved becomes a position of the reduction device.

(2) The continuous casting method for steel according to (1), wherein the reduction device is a plurality of pinch roll pairs.

  In the following description, “mass%” for the composition of steel is simply expressed as “%”.

  According to the continuous casting method of steel of the present invention, it is possible to obtain a continuous cast slab having good internal quality in which the occurrence of center defects such as center segregation and center porosity and internal cracks is suppressed regardless of the steel type. is there.

It is a figure which shows the outline of the continuous casting apparatus which can apply the continuous casting method of this invention.

1. Basic Configuration of Continuous Casting Apparatus FIG. 1 is a diagram showing an outline of a continuous casting apparatus to which the continuous casting method of the present invention can be applied. The tundish 1 is supplied with molten steel 2 from a ladle (not shown). The molten steel 2 injected from the tundish 1 through the immersion nozzle 3 to form the meniscus 5 in the mold 4 is cooled in the mold 4 and the secondary cooling zone 6 below the mold 4 to form a solidified shell and cast. It becomes piece 7. In the secondary cooling zone 6, the surface of the slab 7 is cooled by water sprayed from a spray nozzle group (not shown).

  The slab 7 is pulled out and reduced by a plurality of reduction roll pairs 9 arranged on the downstream side in the casting direction of the guide roll 8 while being supported by the plurality of guide rolls 8 while holding an unsolidified portion inside. . The area where the slab formed by the reduction roll pair 9 is reduced is hereinafter referred to as “reduction zone”. The slab 7 squeezed by the roll pair 9 is cut by a torch cutting device (not shown) arranged on the downstream side in the casting direction of the roll pair 9.

  The upper roll constituting the reduction roll pair 9 is provided with a slab reduction module 11 that controls the distance between the upper roll and the lower roll and the reduction force by adjusting the position of the roll. Thereby, each reduction roll pair 9 can control reduction force individually. The reduction roll pair 9 has a function of drawing out the slab in addition to an essential function of reducing the slab, and is also referred to as a “pinch roll” hereinafter. The rolls constituting the reduction roll pair 9 are cooled by cooling water.

  As a slab reduction device, a segment in which a plurality of rolls are arranged in one frame may be used instead of the reduction roller pair 9.

  Further, the continuous casting apparatus is provided with a workstation 10 for controlling the reduction force of each reduction roll pair 9.

2. The slab reduction control method The workstation 10 collects the actual casting data in real time during the continuous casting operation by various sensors provided in each part of the continuous casting machine, and uses this data to calculate the solidification simulation model. The solidification form inside the slab during casting is estimated in real time.

  Casting performance data refers to data on thermodynamic characteristics and casting conditions of molten steel. The thermodynamic characteristics of the molten steel used in this solidification simulation model calculation are the component analysis values (concentrations of C, Si, Mn, P, S, etc. in the molten steel) of the molten steel of the heat to be cast and the molten steel according to the component analysis values. Temperature dependence of liquidus temperature, latent heat of solidification, specific heat and density. These are calculated in real time from the component analysis values of the molten steel being cast taken in real time from within the tundish. The heat is a casting unit of molten steel received in the ladle, and the amount of molten steel that can be accommodated in one ladle is 1 heat.

  The casting conditions used in this solidification simulation model calculation are: casting speed, temperature and superheat of molten steel in the tundish, amount of mold cooling water, temperature and temperature fluctuation, amount and temperature of secondary cooling water, roll The item relates to heat removal from the slab, such as the amount and temperature of the cooling water, and the outside air temperature.

  This solidification simulation model calculation is a simulation model that takes into account the temperature dependence of the molten steel specific heat, solidification latent heat amount, and molten steel density according to the composition of molten steel, and takes into account solute redistribution during solidification.

  In the workstation 10, data of the optimal reduction force corresponding to the central solid phase ratio at the reduction position of the slab set in advance is registered. Based on the distribution of the central solid phase ratio calculated by the solidification simulation model calculation and the data on the relation between the central solid phase ratio of the slab and the reduction force, the reduction force of each reduction roll pair 9 is controlled in real time.

  The data of the optimum rolling force registered in the workstation 10 and corresponding to the central solid phase ratio at the slab reduction position is the actual value when a slab having good internal quality can be manufactured, that is, ideal The rolling force is determined for the position of each rolling roll pair 9 based on the casting conditions, and the central solid fraction at each position is calculated by solidification simulation model calculation. This data is set as follows, for example.

  When the center solid phase ratio at the position of the reduction roll pair 9 is as low as 0 to 0.30, in general, there is a high possibility of internal cracks due to the reduction strain at the solidification interface due to the reduction of the slab. The rolling force is set to zero. When the central solid phase ratio is 0.30 to 0.80, in order to suppress the central segregation due to the flow of molten steel concentrated in the solute at the end of solidification, an appropriate reduction force according to the steel type is as described above. Set based on ideal casting conditions. When the central solid fraction is larger than 0.80, depending on the steel type, there is a possibility that the central porosity is generated in the slab by solidification shrinkage. Therefore, the central solid fraction is more than the region where the central solid fraction is 0.30 to 0.80. Set a strong reduction force.

  The central solid phase ratio at the position of each rolling roll pair 9 during operation does not necessarily match the data registered in the workstation 10. Therefore, during casting of a slab of a certain steel type, when the central solid phase ratio at the position of the third reduction roll pair 9 from the upstream side in the casting direction was 0.45, it was registered in the workstation 10. A rolling force obtained by apportioning the rolling force corresponding to the central solid phase ratios of 0.40 and 0.50 is applied.

  Hereinafter, such a control method is also referred to as “dynamic control”. It is also possible to control the rolling gradient and rolling speed of the slab by controlling the rolling force and the rolling amount of each rolling roll pair 9.

  As a result, even if fluctuations occur in the collected casting data, it is possible to control the reduction of the slab according to the solidification form inside the slab at each time point, particularly the central solid phase ratio. Generation of center defects such as center segregation and center porosity of the pieces can be suppressed, and a slab having good internal quality can be stably and continuously cast.

  Further, in this method, since the roll rolling force is controlled, the occurrence of internal cracks due to over-pressing of the slab can be suppressed even if expansion occurs due to the temperature rise of the rolling device.

  The continuous casting method of the present invention can be applied to slabs and round billets as well as bloom as a slab to be cast.

3. Range of casting speed In the continuous casting method of the slab of the present invention, the region where the central solid fraction of the slab estimated by the solidification simulation model calculation by the workstation is 0.30 to 0.80 is The upper limit and the lower limit of the casting speed are determined so as to be located in the reduction zone. Thereby, generation | occurrence | production of the center segregation of a slab can be suppressed. Specific examples of the upper limit and the lower limit of the casting speed will be described later.

  When the casting speed is too slow, the region where the central solid fraction is 0.30 to 0.80 does not reach the reduction zone, so the slab after complete solidification or close to it will be reduced, and the center segregation will occur. It is not possible to suppress the occurrence of Therefore, the lower limit of the casting speed is a speed at which the region where the central solid phase ratio is 0.80 is positioned at least at the end of the rolling zone on the upstream side in the casting direction.

  The upper limit of the casting speed is determined according to the necessity of performing the reduction of the slab after complete solidification in the reduction zone. For steel types that do not require the reduction of the slab after complete solidification in the reduction zone, in order to suppress the occurrence of center segregation, the region where the central solid fraction is 0.80 is at least downstream in the casting direction of the reduction zone To be located at the end of In a steel type in which the reduction of the slab after complete solidification needs to be performed in the reduction band, it is determined so that at least the end of the reduction band on the most downstream side in the casting direction is completely solidified.

4). Rolling device In the continuous casting method of steel of the present invention, the rolling force of the slab can be individually set as a slab rolling device, and a pair of rolling rolls (pinch rolls) to which a driving force is applied is arranged in multiple stages. (Hereinafter also referred to as “multi-stage pinch roll reduction method”). The reason will be described below.

  A method using a segment in which a plurality of rolls are arranged on a frame as a reduction device is referred to as a “segment reduction method”. In order to give a rolling gradient using the segments, it was necessary to provide complicated mechanical equipment such as guide posts and spherical seats. In addition, there is a limit to the rolling gradient that can be set because it is affected by the limit of mechanical contact. The influence of the limit of mechanical contact of the rolling gradient here means that the segment used in the segment rolling method uses a plurality of lifting hydraulic cylinders to tilt the frame. It means that we are subject to general restrictions.

  In addition, a large deformation resistance is generated when a slab of low temperature (600 ° C. or less) is crushed. Therefore, in the segment rolling method, it is necessary to provide a large device that generates a large driving torque in the segment in order to roll down a low-temperature slab.

  On the other hand, as shown in FIG. 1, by arranging the rolling roll pair 9 in multiple stages over the range from the outlet of the secondary cooling zone 6 of the continuous casting machine to the torch cutting device, complicated mechanical equipment is not required, It is possible to set the rolling gradient freely. Further, since the driving force for pulling out the slab is always applied when the slab is being reduced, stable casting of the slab is possible. Since a large driving force can be obtained by using a plurality of reduction roll pairs 9, even when a low-temperature slab is reduced, a large device that generates a large driving torque is not required.

  In the multi-stage arrangement pinch roll reduction method, the reduction force can be adjusted at a roll pitch interval shorter than the length of the segment used in the segment reduction method in the casting direction. Therefore, even if the center solid phase ratio of the slab at the position of each reduction roll pair 9 changes, an appropriate reduction force corresponding to the change can be applied to the slab, so that the casting with a more stable internal quality can be achieved. It is possible to produce pieces.

  In the multi-stage pinch roll reduction method, the unsolidified part of the slab is cast even when casting a wide variety of steel types with different solidification forms, such as low-carbon steel to high-carbon steel, and other steel types. Therefore, it is possible to always select the optimum reduction region in a proper manner. The optimum reduction region under the unsolidified pressure refers to a region having a central solid phase ratio of 0.30 to 0.80. In the multi-stage arrangement pinch roll reduction method, it is possible to easily perform the reduction control of the slab after complete solidification based on the result of the solidification simulation model calculation.

5. An example of casting conditions Next, casting conditions of the continuous casting method of the present invention and calculation conditions of solidification simulation model calculation related thereto will be described with an example.

5-1. Continuous casting apparatus The continuous casting apparatus to be used is of the multi-stage arrangement pinch roll reduction method shown in FIG. In this casting apparatus, a pinch roll having a roll interval control function is disposed at a position of 17 to 30 m from the meniscus in the mold, and constitutes a reduction belt. Each pinch roll has a diameter of 450 mm, a roll pitch of 1200 mm, and a maximum pressure hydraulic pressure of 210 kgf / cm ^2, and 11 pairs are arranged. The cross section of the slab obtained by this casting apparatus is rectangular, and its dimensions are 339 mm × 456 mm in the state before reduction.

5-2. Steel types There are no limitations on the steel types applicable to the continuous casting method of the present invention. For example, carbon steel, alloy steel and bearing steel shown in Table 1 can be used.

5-3. As described above, the range of the casting speed is determined by the solidification simulation model calculation, and the region where the center solid phase ratio of the slab is estimated to be 0.30 to 0.80 is located in the rolling zone of the continuous casting apparatus. To be determined. When continuously casting the steel type slab using the continuous casting apparatus, the lower limit of the casting speed is 0.45 m / min. The upper limit of the casting speed is 1.00 m / min when it is not necessary to perform the reduction of the slab after complete solidification in the reduction zone, and 0.80 m / min when necessary.

  When the casting speed increases, out-of-machine bulging may occur when the slab to be manufactured is a slab. However, for blooms cast with the above size and conditions, the solidified shell has a sufficient thickness even if the part with a central solid fraction of 0.80 exceeds the machine limit position of the continuous casting machine and goes out of the machine. The present inventors have confirmed that bulging does not occur because it is strong and strong.

In order to confirm the effect of the steel continuous casting method of the present invention, the following tests of Examples 1 to 4 were performed, and the results were evaluated.
Example 1: Test when the composition of molten steel changes within the standard range during continuous casting of the same steel type Example 2: When the casting speed is reduced because the molten steel temperature has increased during continuous casting of the same steel type Example 3: Test when temperature rise of mold cooling water, secondary cooling water and roll cooling water occurred during continuous casting of the same steel type Example 4: Variation in casting conditions of Examples 1-3 Test for all occurrences

1. Test conditions and evaluation items (common conditions)
First, test conditions and evaluation items common to each example will be described.

1-1. Continuous casting apparatus The continuous casting apparatus used was a multi-stage pinch roll reduction system shown in FIG. In this casting apparatus, a pinch roll having a roll interval control function is disposed at a position of 17 to 30 m from the meniscus in the mold, and constitutes a reduction belt. Each pinch roll had a diameter of 450 mm, a roll pitch of 1200 mm, and a maximum pressure hydraulic pressure of 210 kgf / cm ^2, and 11 pairs were arranged. The cross section of the slab obtained by this casting apparatus was rectangular, and its dimensions were 339 mm × 456 mm in the state before reduction. Using this continuous casting apparatus, continuous casting was performed by switching the molten steel in the tundish on the way.

1-2. Slab squeezing control method In each example, as a slab reduction control method, tests (test numbers 1-1, 2-1, 3-1 and 4-1) to which static control was applied and the above-described dynamic control were performed. Each of the tests (test numbers 1-2, 2-2, 3-2 and 4-2) applied immediately after the start of casting was performed. Static control refers to the composition and liquidus temperature of molten steel cast in any one heat, casting speed, superheat of molten steel in the tundish, amount of mold cooling water, temperature and temperature rise, This is a conventional method of rolling down a slab based on the central solid phase ratio of a representative slab calculated from the next cooling water amount. When performing the static control, the composition of the molten steel was analyzed for each successive charge of casting, and the distance from the meniscus in the mold at the final solidification position inside the slab of each charge was estimated. In any of the examples, the casting speed range was set so that the region where the central solid fraction was estimated to be 0.30 to 0.80 in the solidification simulation model calculation was located in the rolling zone of the continuous casting apparatus. .

  For the part where the front and rear charge molten steel is mixed (hereinafter also referred to as “mixing part”), the weight of the mixed molten steel is the same as that of the molten steel remaining in the tundish at the time when the rear charge ladle is opened. It was defined as 1.5 times the weight. The component composition of the mixed molten steel was set to an intermediate value of the component composition of each molten steel in the front and rear charge. After the mixing portion defined in this way, the steel was completely replaced with the post-charge molten steel, and the reduction control was performed assuming that the charge switching was completed.

1-3. Evaluation item The evaluation item is an internal quality evaluation index calculated from the segregation degree of the central carbon concentration in the steel slab obtained by rolling the slab as an index related to the internal quality of the slab, and the UT inspection defect rate of this steel slab. did.

  The segregation degree of the central carbon concentration is a value represented by C / Co × 100 [%], where Co is the average carbon concentration in the steel slab and C is the maximum carbon concentration in the steel slab. Also referred to as “C / Co”. The smaller the C / Co, the better the internal quality of the steel slab. In general, as a management index for C / Co, it is preferable that the upper limit threshold is 105%. This almost coincides with a numerical value obtained empirically as a range in which breakage caused by center segregation during product rolling can be stably suppressed.

  The internal quality evaluation index was the standard deviation of C / Co. The larger the standard deviation of C / Co, the greater the variation in C / Co values.

  The UT inspection is an inspection of a central defect by an ultrasonic flaw detection test. The steel piece UT inspection defect rate is a value of the ratio of the number of slabs regarded as defective to the number of slabs to be tested.

2. Conditions and results of each example 2-1. Example 1
2-1-1. Test conditions Example 1 is a test in the case where the composition of molten steel changes within a standard range during continuous casting of the same steel type. Tables 2 to 4 show the molten steel composition, casting conditions, and reduction control method, respectively. Test number 1-1 shown in Table 4 is a conventional example to which static control is applied, and test number 1-2 is an example of the present invention to which dynamic control is applied. Table 4 also shows the standard deviation of C / Co, which is an evaluation item, and the steel piece UT inspection defect rate (the same applies to Examples 2 to 4 below).

  Among the constituent compositions of the molten steel shown in Table 2, since the concentration of C changed within the standard range at the time of continuous casting, the concentration at the start of casting and the concentration at the time when switching of the charge of the molten steel in continuous casting was completed were shown. . Specifically, the concentration of C increased from 0.45% to 0.48% by continuously switching the molten steel.

  The data of each test, such as the casting speed and the specific amount of secondary cooling water shown in Table 3, the standard deviation of the center segregation shown in Table 4 and the steel piece UT inspection failure rate, are data in charge units, The average value of data, that is, a value representing each test charge.

2-1-2. Test results As shown in Tables 2 to 4, in Example 1, although the casting conditions did not change during casting, the C concentration in the molten steel increased. Therefore, it is considered that the final solidification position inside the slab has changed.

  Correspondingly, in test number 1-1 in which static control was performed, as shown in Table 4, the estimated value of the distance from the meniscus in the mold at the final solidification position inside the continuous cast slab is the casting start time. No. 22.5 m, 23.6 m at the time of completion of the molten steel switching for continuous casting, and moved downstream in the casting direction. Along with this, the slab reduction speed in the region where the center solid phase ratio of the slab was estimated to be 0.30 to 0.80 was changed from 0.23 mm / min at the start of casting to the time when the molten steel switching was continuously performed. Then, it was reduced to 0.10 mm / min.

  In test number 1-1, the average value and the maximum value of C / Co (not shown in Table 4) both increase and the central segregation deteriorates. As shown in Table 4, the standard deviation of C / Co Increased from 2.09 at the start of casting to 4.28 at the time of completion of molten steel switching. Moreover, the steel piece UT inspection defect rate has increased from 0.06% to 0.18%, which is caused by insufficient pressure bonding of the center porosity.

  This deterioration of internal quality is optimal at the time of setting because the reduction in the central solid fraction of the slab caused by fluctuations in the casting conditions during continuous casting operation because the slab reduction control was performed by static control. This is considered to be because the slab pressure reduction condition deviated from the optimum condition.

  In test number 1-2, no significant variation was observed in the maximum value of C / Co (not shown in Table 4). In addition, the standard deviations of C / Co at the start of casting and at the time of completion of molten steel switching of continuous casting are 2.14 and 2.08, respectively, and the steel piece UT inspection defect rate is 0.06% and 0.02%, However, a large fluctuation was not observed, and a slab having good internal quality could be manufactured stably. These are considered to be due to the application of dynamic control.

  In addition, the slab reduction speed in the region where the central solid fraction of the slab was estimated to be 0.30 to 0.80 in the solidification simulation model calculation was 0.23 mm / min at the start of casting. At completion, it could be maintained at 0.24 mm / min.

2-2. Example 2
2-2-1. Test conditions Example 2 is a test in the case where the casting rate was reduced for the purpose of avoiding breakout directly under the mold because the composition of the molten steel did not change during continuous casting of the same steel type but the molten steel temperature increased. It is. Tables 2 to 4 show the molten steel composition, casting conditions, and reduction control method, respectively. Test number 2-1 shown in Table 4 is a conventional example to which static control is applied, and test number 2-2 is an example of the present invention to which dynamic control is applied.

  In Example 2, the superheat degree of the molten steel in the tundish increased from 28 ° C. to 47 ° C. due to the continuous change of the molten steel. Correspondingly, the casting speed was reduced from 0.55 m / min to 0.50 m / min in both test numbers 2-1 and 2-2.

2-2-2. Test Results As shown in Tables 2 to 4, in Example 2, the casting speed and the molten steel temperature in the tundish varied among the casting conditions during casting. Due to these changes in casting conditions, the final solidification position inside the slab changes, and the solidification profile (center solid fraction of the slab at each pinch roll position, solidified shell thickness, and slab Temperature) is considered to have changed.

  Therefore, as shown in Table 4, in test number 2-1 where static control is performed, the estimated value of the distance from the meniscus in the mold at the final solidification position inside the continuous cast slab is continuously cast from 23.8 m at the start of casting. When the molten steel switching was completed, it moved to 21.4 m, upstream in the casting direction. Along with this, the slab reduction speed in the region where the center solid phase ratio of the slab is estimated to be 0.30 to 0.80 is changed from 0.37 mm / min at the start of casting to the time when the molten steel switching for continuous casting is completed. Then, it reduced to 0.06 mm / min.

  In Test No. 2-1, the center segregation deteriorated, and as shown in Table 4, the standard deviation of C / Co was from 3.01 at the start of casting, and 4. Increased to 34. Moreover, the billet UT inspection defect rate increased from 0.04% to 0.20%.

  In Test No. 2-2, the standard deviations of C / Co at the start of casting and the completion of continuous cast steel switching are 1.97 and 2.01, respectively, and the slab UT inspection defect rate is 0.03% and 0.8. The slabs with good internal quality could be manufactured stably, with no significant fluctuations at 05%. These are considered to be due to the application of dynamic control.

  In addition, the slab reduction speed in the region where the central solid fraction of the slab is estimated to be 0.30 to 0.80 by solidification simulation model calculation is 0.37 mm / min at the start of casting, and the molten steel is continuously switched. It could be maintained even when completed.

2-3. Example 3
2-3-1. Test conditions In Example 3, when the same steel type was continuously cast, there was no change in the composition of molten steel, casting conditions such as casting speed and molten steel temperature, and the temperature of mold cooling water, secondary cooling water and roll cooling water increased. This is a test for the case where this occurs. Tables 2 to 4 show the molten steel composition, casting conditions, and reduction control method, respectively. Test number 3-1 shown in Table 4 is a conventional example to which static control is applied, and test number 3-2 is an example of the present invention to which dynamic control is applied.

  In Example 3, the mold cooling water, secondary cooling water, and roll cooling water temperatures increased from 32 ° C. to 38 ° C., 29 ° C. to 35 ° C., and 32 ° C. to 36 ° C., respectively, before and after the molten steel was switched to continuous casting. did.

2-3-2. Test Results As shown in Tables 2 to 4, in Example 3, the temperature of various cooling waters of the casting conditions increased during casting, and the cooling strength of the slab decreased. Therefore, it is considered that the final solidification position inside the slab has changed.

  Correspondingly, in test number 3-1 to which static control was applied, the estimated value of the distance from the meniscus in the mold at the final solidification position in the slab was continuously cast from 20.1 m at the start of casting. When the switching was completed, it moved to 21.3 m, downstream in the casting direction. Along with this, the slab reduction speed in the region where the center solid phase ratio of the slab was estimated to be 0.30 to 0.80 was changed from 0.45 mm / min at the start of casting to the time when the molten steel changeover of continuous casting was completed. Then, it reduced to 0.27 mm / min.

  In Test No. 3-1, as in Test Nos. 1-1 and 2-1, deterioration of central segregation was observed, and as shown in Table 4, the standard deviation of C / Co was 2. From 02, it increased to 2.42 at the time of completion of the change of molten steel for continuous casting. Also, the billet UT inspection failure rate increased from 0.03% to 0.15%.

  In test number 3-2 to which dynamic control was applied, the center segregation was stable. In addition, the standard deviations of C / Co at the start of casting and at the time of completion of continuous casting of molten steel are 2.11 and 2.08, respectively, and the steel piece UT inspection defect rate is 0.03% and 0.05%, respectively. However, a large fluctuation was not observed, and a slab having good internal quality could be manufactured stably.

  In addition, the slab reduction speed in the region where the central solid fraction of the slab was estimated to be 0.30 to 0.80 by solidification simulation model calculation was switched from 0.45 mm / min at the start of casting to the molten steel for continuous casting. At the time of completion, it could be maintained at 0.46 mm / min.

2-4. Example 4
2-4-1. Test conditions Example 4 is a test in the case where all the fluctuations in the casting conditions of Examples 1 to 3 occurred. Tables 2 to 4 show the molten steel composition, casting conditions, and reduction control method, respectively. Test number 4-1 shown in Table 4 is a conventional example to which static control is applied, and test number 4-2 is an example of the present invention to which dynamic control is applied.

  In Example 4, the concentrations of C and Mn increased from 0.34% to 0.38% and from 0.69% to 0.73%, respectively, due to the continuous casting of molten steel. Moreover, the superheat degree of the molten steel in the tundish increased from 35 ° C. to 46 ° C. by continuously switching the molten steel. Further, the temperatures of the mold cooling water, the secondary cooling water, and the roll cooling water increased from 33 ° C. to 37 ° C., 28 ° C. to 35 ° C., and 30 ° C. to 35 ° C., respectively, before and after the continuous casting of the molten steel.

2-4-2. Test Results As shown in Tables 2 to 4, in Example 4, various casting conditions were changed, so that the final solidification position inside the slab was considered changed.

  At this time, in test number 4-1 to which static control was applied, the estimated value of the distance from the meniscus in the mold at the final solidification position inside the slab was changed from 22.6 m at the start of casting to the time when the molten steel was continuously switched over. It moved to 24.2 m and the casting direction downstream side. Accordingly, the slab reduction speed in the region where the center solid phase ratio of the slab is estimated to be 0.30 to 0.80 is changed from 0.27 mm / min at the start of casting to the time when the molten steel switching for continuous casting is completed. Then, it reduced to 0.14 mm / min.

  In Test No. 4-1, as in Test Nos. 1-1, 2-1 and 3-1, deterioration of center segregation was observed, and as shown in Table 4, the standard deviation of C / Co was the start of casting. From 1.97 at the time, it increased to 3.13 at the time of completion of molten steel switching of continuous casting. Moreover, the billet UT inspection defect rate increased from 0.04% to 0.16%.

  In test number 4-2 to which dynamic control was applied, the center segregation was stable. In addition, the standard deviation of C / Co at the start of casting and the time of completion of continuous casting of molten steel is 1.84 and 2.21, respectively, and the steel piece UT inspection defect rate is 0.04% and 0.04%. However, a large fluctuation was not observed, and a slab having good internal quality could be manufactured stably.

  In addition, the slab reduction speed in the region where the central solid fraction of the slab was estimated to be 0.30 to 0.80 in the solidification simulation model calculation was 0.27 mm / min at the start of casting, and the molten steel was switched continuously. It could be maintained even when completed.

  According to the steel continuous casting method of the present invention, it is possible to stably obtain a continuous cast slab having good internal quality in which center segregation is suppressed and the center porosity is pressure-bonded.

1: tundish, 2: molten steel, 3: immersion nozzle, 4: mold, 5: meniscus, 6: secondary cooling zone, 7: cast slab, 8: guide roll, 9: rolling roll pair, 10: workstation, 11: Slab reduction module

Claims (2)

A continuous casting method of steel in which molten steel is injected from a tundish into a mold to form a slab, the slab is secondarily cooled at the bottom of the mold, and the secondary cooled slab is reduced by a reduction device having a roll Because
In real time during continuous casting operation, component analysis value and temperature of molten steel in the tundish, casting speed, amount of cooling water of the mold, temperature and temperature change, amount and temperature of secondary cooling water, cooling of the roll Data on the amount and temperature of water, and outside air temperature were collected, and the liquidus temperature, solidification latent heat amount, specific heat and density of the molten steel calculated from each data and component analysis values of the molten steel were used as parameters. Perform solidification simulation model calculation to estimate the solidification state,
Based on the solidification state in the slab estimated from the result of the solidification simulation model calculation and the preset data on the relationship between the central solid fraction of the slab and the reduction force, the reduction force of the reduction device is calculated. While controlling in real time,
A continuous casting method of steel, wherein the casting speed is controlled so that a region where a central solid fraction is estimated to be 0.30 to 0.80 is a position of the reduction device.   2. The steel continuous casting method according to claim 1, wherein the reduction device is a plurality of pinch roll pairs.

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