JP2010099697A - Continuous casting method for molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for a molten steel which provides a product steel of high quality. <P>SOLUTION: A molten steel is continuously poured into a water-cooled die, the molten steel is covered with separately charged mold powder, the molten steel is stirred with electromagnetic force, and while pulling out a produced solidified shell to the lower part from the die, cooling is continued so as to produce a perfectly solidified long-length steel slab. In such a case, the thickness of the molten layer in the mold powder is always measured, the measured value is compared with the relationship between the predetermined thickness of the molten layer in the mold powder and the rate of occurrence of defect in the steel slab or a product steel obtained by rolling the steel slab, the casting conditions of the molten steel in the water-cooled die are changed in such a manner that the rate of occurrence of the defect in the steel slab or the product steel becomes a desired value or below, and the thickness of the molten layer in the mold powder is adjusted. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for continuously casting molten steel. Specifically, when continuously casting molten steel, the thickness of a molten layer formed by a powder called mold powder added to the upper part of the molten steel injected into a mold (mold) ( (Hereinafter referred to as “powder melt layer thickness”) is continuously measured online, and the powder melt layer thickness and / or the unmelted layer thickness is obtained by rolling the steel slab or the steel slab. The present invention relates to a technique for obtaining a high-quality product steel by adjusting the occurrence rate to a value that falls within a desired range.

  In order to perform continuous casting of molten steel, as shown in FIG. 4, the molten steel is cooled by pouring into a mold 1 (called a mold) having a water-cooled structure in which the molten steel is vibrated up and down. A solid steel shell 3 (referred to as a shell) is produced, and the solid steel shell 3 is drawn continuously below the mold 1 at a constant speed, and further cooled, and finally a long steel slab that has been completely solidified (see FIG. Not shown). The material of the water-cooled mold 1 is made of copper as a base material, and the surface thereof is plated with chromium, nickel, or an alloy of these metals, if necessary, and the generated solidified shell 3 is smoothly drawn downward. For this, it is necessary to make the frictional resistance between the solidified shell 3 and the inner surface of the mold 1 as small as possible. Therefore, a powder called mold / powder 5 is continuously added to the upper surface of the molten steel 2 at a constant supply amount so as to cover the entire molten steel 2 and enter the gap between the solidified shell 3 and the inner surface of the mold 1. .

This mold powder 5 is based on an oxide such as CaO, SiO ₂ , Al ₂ O ₃ , MgO, MnO, Na ₂ O, K ₂ O, CaF ₂ , MgF ₂ , Li ₂ CO ₃ , ice crystals. Alkali metal or alkaline earth metal oxides such as stone, fluorides or carbonates, CaO as the main component of the base material, calcium carbonate or diatomaceous earth as a component adjusting material for SiO ₂ , carbon to adjust the melting rate As shown in FIG. 4, a part of the molten steel 2 is melted and flows into a gap (not shown) between the mold 1 and the solidified shell 3 as shown in FIG. It demonstrates its function. The layer of the added mold / powder 5 is a lubricant, a heat insulating agent and an antioxidant for molten steel, an absorbent for removing non-metallic inclusions existing in the molten steel, and a meniscus portion (solidified shell and molten steel). Various functions such as a coolant for molten steel in the vicinity of the boundary part) are also exhibited. In particular, the powder melt layer 4 formed by melting the mold powder 5 plays an important role in the function as the lubricant and the cooling effect of the slab near the meniscus portion.

  Accordingly, in an actual operation, it is important how to properly control the thickness of the powder melt layer 4, but as a premise, it is necessary to first accurately measure the powder melt layer thickness. In particular, when the powder melt layer thickness is thin, the unmelted mold powder 5 is caught in the molten steel 2 to become non-metallic inclusions, which may lead to a deterioration in the quality of the manufactured steel slab. In view of this, it is necessary to control the powder melt layer thickness.

  As a means for measuring the powder melt layer thickness, there has been conventionally used a contact type or non-contact type apparatus. As the contact type, a measuring rod with copper plating or aluminum plating on an iron rod is immersed in the molten powder layer until it reaches the molten steel, and the powder molten layer thickness is measured from the difference in the melted state of the measuring rod. There exists a method (for example, refer patent document 1). Also, as a non-contact type, a rod-like γ-ray source along the vertical direction is arranged on one side of the opposing wall of the mold, and a detector that can move up and down on the other side, and the transmittance of the projected γ-ray is molten steel , A method using the difference between the powder melt layer, the powder particle layer and the air layer (see Patent Document 2), a method combining a γ-ray and an eddy current level meter (see Patent Document 3), a resonant eddy current level There is a method (Non-patent Document 1) for obtaining a powder melt layer thickness from the height of the molten steel surface and the upper surface of the powder melt layer using a meter (see Patent Document 4) at two frequencies.

  However, in the contact-type measuring method, the sensor comes into contact with the powder molten layer, the molten steel, and the like, so that the powder is caught in the molten steel from the contact portion. In actual operation, the casting speed will tend to increase in the future, but in that case, the flow speed of the molten steel surface will increase, and it may be possible to increase the entrainment of the powder, and also adhere to the immersion part of the sensor There is a problem in quality such as foreign matter peeling off in the molten steel and becoming non-metallic inclusions. In addition, since the measurement cannot be performed continuously and the so-called “batch” measurement is performed with a time interval, the operation action performed based on the measurement result is delayed, which is not preferable for stabilizing the quality of the steel slab.

  On the other hand, in the method using the non-contact type γ-ray, since the apparatus is enlarged, it is difficult to install on the side surface of the mold, and it is difficult to accurately measure the powder melt layer thickness. In addition, the two-frequency vortex method has a problem in that it is affected by the thickness of the unmelted powder particle layer, and sufficient accuracy cannot be obtained, and it is easily affected by changes in the ambient temperature.

  Therefore, the present applicant has further improved the method for obtaining the powder melt layer thickness from the height of the molten steel surface and the upper surface of the powder melt layer by using the resonance type eddy current level meter (see Patent Document 4) at two frequencies. In a layer thickness measurement method for measuring the thickness of one or a plurality of layers having different electrical conductivities, a step of applying a voltage having one or a plurality of frequencies to the excitation coil, and the generation of each of the layers by the excitation coil A step of detecting a voltage caused by an eddy current with a detection coil that is the same as or different from the excitation coil, a phase difference of the voltage detected by the detection coil with respect to a voltage applied by the excitation coil, and the detection coil And a step of outputting an absolute value of the amplitude of the voltage detected in (2), which has been proposed (see Patent Document 5). According to this measuring method, the melt layer thickness of the mold / powder can be measured very easily and continuously. However, this multi-frequency eddy current mold / powder melt thickness measurement method will confirm the absolute value of the melt thickness unless the output of the measuring instrument is calibrated based on the actual measurement value of the molten steel surface and the actual measurement value of the mold / powder melt thickness. There was also a problem that it was not possible. Therefore, the applicant calibrated using a molten steel simulated material that is electrically close to the molten steel and a solid, and a molten mold / powder simulated material that is electrically close to the molten layer of the mold and powder and is a solid. Also provided is a technique for performing (see Patent Document 6).

  However, when using the techniques of Patent Documents 5 and 6, the measurement accuracy may deteriorate depending on conditions. For example, if the measurement error of the molten powder thickness increases due to the type of mold / powder and the surface temperature of the molten steel, or the thickness of the steel slab to be manufactured decreases, the measurement accuracy of the molten steel deteriorates. The measurement data varies depending on the vibration pattern.

  Among these, the latter two problems, “In other words, when the thickness of the steel slab to be manufactured becomes narrower, the measurement accuracy of the molten steel deteriorates. Depending on the oscillation pattern of oscillation (move the mold up and down), the measurement data may be `` Variation '' could be solved relatively easily, but the problem when the mold / powder type and molten steel surface temperature changed was not simple, as the magnitude of the measurement error varied depending on the mold / powder type and temperature. . Therefore, recently, the present applicant has also proposed a measurement method of a multi-frequency eddy current mold / powder melt layer thickness that can be measured with high accuracy by correcting the error due to the effect of the mold / powder type and molten steel temperature ( (See Patent Document 7).

  As described above, techniques for measuring the thickness of the mold / powder melt layer during continuous casting of molten steel have been improved, and the measurement accuracy of the powder melt layer thickness has been considerably increased. However, to use these measurement techniques for operation, the surface temperature of the mold / powder layer is measured in a non-contact manner, the thickness distribution is estimated, and the mold / powder has a constant thickness based on the estimated value. A mold / powder supply apparatus that automatically supplies a layer to form a layer has only been developed, and it cannot be said that it is sufficient (see Patent Document 8).

However, the method of estimating the molten layer thickness from the surface temperature distribution of such mold powder powder and supplying the mold powder is to insert the mold powder based on the estimation, and how it is measured and how. It was unclear whether there was a divergence and how it affected quality. In fact, the quality of the steel slabs obtained and the quality of the product steel obtained by rolling the steel slabs vary widely, and there is still room for improvement as a continuous casting technology. ing.
Japanese Patent Laid-Open No. 9-79805 JP-A-61-46361 JP-A-62-166065 JP 59-180402 A JP 2005-221282 A JP 2006-205227 A JP 2007-021529 A Yukio Nakamori et al. “Development of Powder Film Thickness Gauge and Molten Powder Pool Thickness Gauge” Steel Research, No. 324, 1987, p. JP 60-49846 A

In view of such circumstances, the present invention constantly measures the melt layer thickness of the mold and powder, judges the quality status of the steel slab online from the measurement information, and determines the powder melt layer thickness and / or the unmelted layer thickness. Method for continuous casting of molten steel capable of obtaining a high-quality product steel by adjusting the value so that the defect occurrence rate of the steel slab and the product steel obtained by rolling the steel slab falls within a desired range The purpose is to provide.

    The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention. That is, the present invention continuously injects molten steel into a water-cooled mold that is vibrated up and down, covers the molten steel with a separately supplied mold and powder, agitates the molten steel with electromagnetic force, and generates the solidified shell as the mold. When cooling down and drawing into a fully solidified long steel slab, the thickness of the molten layer of the mold / powder is constantly measured, and the measured value of the mold / powder determined in advance is measured. In light of the relationship between the thickness of the molten layer and the defect occurrence rate of the steel slab or product steel obtained by rolling the steel slab, the defect occurrence rate of the steel slab or product steel is less than the desired value. Further, the molten steel continuous casting method is characterized in that the thickness of the molten layer of the mold / powder is adjusted by changing the casting condition of the molten steel in the water-cooled mold.

  In this case, the thickness of the unmelted layer of the mold / powder is further determined, and the thickness of the unmelted layer of the mold / powder and the thickness of the molten layer and the steel slab or the steel slab are rolled. In light of the relationship with the defect occurrence rate of the product steel obtained in this way, the casting condition of the molten steel in the water-cooled mold is changed so that the defect occurrence rate of the steel slab or product steel falls within the desired range. It is preferable to adjust the thickness of the molten layer of the powder and the thickness of the unmelted layer. The casting condition of the molten steel in the water-cooled mold to be changed is preferably the amount of mold / powder input and / or the magnitude of electromagnetic stirring force, and the measurement of the mold / powder molten layer is a multi-frequency eddy current thickness meter. It is even better to do it.

  In the present invention, when the molten steel is continuously cast, the thickness of the molten layer (hereinafter referred to as the powder molten layer thickness) formed by the powder called mold powder added to the upper part of the molten steel injected into the mold (mold) is online. While continuously measuring continuously, the powder melt layer thickness and / or the unmelted layer thickness is such that the defect occurrence rate of the steel slab and the product steel obtained by rolling the steel slab falls within a desired range. The value was adjusted by changing the casting conditions, so that high-quality product steel could be obtained stably.

  Hereinafter, the best embodiment of the present invention will be described based on the background of the invention.

  First, in the present invention, molten steel is continuously poured into a water-cooled mold that is vibrated up and down, and the molten steel is covered with a mold powder that exhibits the function of the lubricant that has been separately added. The property (for example, particle size distribution) is not particularly limited. This is because the necessary prior information (data) at the time of continuous casting may be collected according to the type of mold and powder to be employed.

  In the present invention, the thickness of the molten layer of the mold / powder is always measured, but the measuring method is not particularly limited. This is because all of the conventional contact and non-contact measurement methods described above can be used. However, in the present invention, in order to utilize these measured values, the thickness of the molten layer of the pre-molded powder and the defect occurrence rate of the steel slab or product steel obtained by rolling the steel slab, It is necessary to establish the relationship.

Therefore, the present inventor used a vertical bending type continuous casting machine to inject molten steel with a steel type of ultra-low carbon molten steel into a water-cooled mold having a flat cross-sectional area of 260 mm × 1200 mm, and perform normal vibration and electromagnetic stirring of the molten steel. Charged many test operations for continuous casting while performing under certain conditions (1 charge is 280 tons of molten steel received in a molten steel pan, placed above the tundish, and the entire amount of molten steel is lost to the mold via the nozzle. The relationship between the thickness of the molten layer of the mold / powder and the defect occurrence rate of the product steel obtained by rolling the steel slab was determined. Here, mold powder used was the _{(composition, CaO: 36.1mass%, SiO 2} : 35.6mass%, Al 2 O 3: 6.4mass%, MgO: 1.8mass%, F: 2 0.9 mass%, TC: 3.3 mass%, freezing point temperature: 1100 ° C., viscosity at 1300 ° C .: 0.4 Pa · s), and the thickness of the molten layer is a conventional measuring method described in Patent Document 7 Measured with At that time, the slab (slab) obtained by continuous casting was hot-charged and rolled into a hot-rolled steel strip without taking care. The hot charge rolling conditions were a heating furnace temperature: 1100 to 1150 ° C. and a furnace time: 30 to 60 min. Subsequently, the obtained hot-rolled steel strip was cold-rolled to obtain a cold-rolled coil (steel strip) having a thickness of 0.8 mm. About the obtained cold-rolled coil, the number of surface defects generated was visually examined to determine the surface defect generation rate (%) (= (number of defects / measured coil length) × 100). The surface defects measured were bubbles, inclusions, powdered bulges, slivers, and blisters (blisters), and cracked defects were excluded. An example of the obtained relationship is shown in FIG.

  In the present invention, the thickness of the molten layer of the mold / powder obtained by constant measurement during the continuous casting operation is obtained by rolling the steel slab at a predetermined time interval in light of the relationship shown in FIG. The value of the casting conditions of the molten steel is changed and adjusted so that the defect occurrence rate of the product steel material falls within a desired range (0.1% or less). This is because if the defect occurrence rate is 0.1% or less, it is satisfactory as a steel plate material for automobiles.

  In the present invention, the casting conditions are mainly the input amount of mold / powder (kg / min) and the electromagnetic stirring force of molten steel (evaluated by current value). Since the molten layer of the mold powder enters the gap between the mold and the solidified shell during continuous casting, it greatly affects the amount of mold powder consumed, but the molten layer of the mold powder during operation for one charge. This is because the thickness can be adjusted almost satisfactorily by manipulating the amount of mold and powder introduced and the electromagnetic stirring force of the molten steel.

  Thus, the present invention measured the thickness of the molten layer of the mold powder online during operation, and obtained the value by rolling the thickness of the molten layer of the mold powder in FIG. 1 and the steel slab. By using the relationship with the defect occurrence rate of the product steel material, the casting conditions of the molten steel may be changed and adjusted. As casting conditions, in addition to the above, (1) switching to a mold / powder (kind) with a high melting rate, or if it is determined that the molten layer thickness is thick, (2) using an electromagnetic stirrer or the like There are methods such as lowering the flow of molten steel in the mold and lowering the heat supply to the powder, and (3) switching to a powder with a lower melting rate. However, these operations are not in operation within one charge. It is used as a countermeasure when changing the charge or changing the steel type of molten steel.

  Here, it is desirable to measure the mold / powder melt layer thickness over the entire width in the steel slab width direction, but the quality information that combines the mold / powder melt layer thickness information with the quality information of the rolled steel. If a database where the product defect is likely to occur in the width direction can be specified, measurement may be performed with the position fixed in the width direction.

  Next, the inventor examined further improvements of the present invention. The reason for this is that the relationship of FIG. 1 is based on data from past operations or test operations, which may possibly contain errors, and that there is still room for improving accuracy. Then, the inventor has conceived that the purpose is better achieved by considering the influence of the unmelted layer of the mold powder, and has also obtained the thickness of the unmelted layer of the mold powder. Since the upper end of the powder melt layer is known from the measurement of the powder melt layer thickness, if the position of the powder unmelt layer is known, the thickness of the powder melt layer can be easily obtained by calculation. Specifically, an ultrasonic distance meter or the like may be disposed above the mold 1 and the distance to the surface of the mold / powder 5 may be measured (see FIG. 4).

  When the thickness of the powder unmelted layer thus obtained is considered in accordance with the relationship between the thickness of the powder melted layer in FIG. 1 and the defect occurrence rate of the product steel obtained by rolling the steel slab, FIG. The relationship is as shown in. Here, the circles indicate data in which the defect occurrence rate satisfies a desired value (0.05% or less), and the x marks indicate data that does not reach the desired value (over 0.05%). In other words, it has been found from FIG. 2 that the occurrence of defects can be further reduced to 0.05% or less in consideration of the thickness of the powder unmelted layer as compared with the case of using FIG. 1 alone. I made it. That is, “the thickness of the unmelted layer of the mold / powder is obtained and the calculated value is determined in advance, and the relationship between the thickness of the unmelted layer / the melted layer of the mold / powder and the defect occurrence rate of the product steel (FIG. 2) In light of the above, it is also added to the present invention to adjust the thickness of the unmelted layer and the molten layer of the mowed powder 5 by changing the casting conditions.

  In carrying out the above two present inventions, measurement of the mold / powder melt layer is essential, but it is more preferable to perform the measurement with the multi-frequency eddy current thickness meter described in Patent Document 7. According to it, when the thickness of the steel slab to be manufactured is reduced, the measurement accuracy of the molten steel level becomes worse, and depending on the vibration pattern of oscillation (the mold is vibrated up and down), problems such as variation in measurement data can be solved. In addition, the influence of the type of mold and powder and the temperature distribution of the molten steel can be corrected, and the unmelted thickness of the mold and powder can be obtained at the same time.

  In addition, although the relationship with the defect incidence rate of the product steel obtained by rolling this steel slab of FIG.1 and FIG.2 was the steel plate to which the product steel was given CGL, in the present invention, It is not limited to steel plates, and surface defects and internal inclusion defects of all steel products can be used. Since the quality degradation position can be identified from the estimated quality results, surface degradation, surface grinding, and internal defects, if necessary, are degraded to lower grades and cut and removed with a coil sheet to generate quality degraded products. This is because it can be suppressed.

  Using a vertical bend type continuous casting machine (hereinafter referred to as a continuous casting machine), ultra low carbon molten steel for use as a steel sheet for automobiles was cast. The plane cross-sectional area at the upper end of the water-cooled mold 1 is 260 mm × 1200 mm. The water-cooled mold 1 is vibrated up and down at a constant period called “oscillation”, and the molten steel in the mold is subjected to electromagnetic stirring.

First, molten steel decarburized to a very low carbon region by secondary refining using a vacuum degassing apparatus (not shown) after the steel from the converter (main composition is C: 0.001 mass%, Si: 0 .01 mass%, Mn: 0.15 mass%, P: 0.010 mass%, S: 0.010 mass%) in a ladle (not shown) having a capacity of 280 tons, After setting on a tundish (not shown), it was continuously injected into the mold 1 via an immersion nozzle or the like, and operation for one charge was started. At that time, a mold and powder stored in a bunker (not shown) separately provided above the mold 1 was continuously put on the molten steel in the mold 1 through a screw feeder to cover the molten steel. The current value of the electromagnetic stirring device at the beginning is set to 300 amperes. Moreover, mold powder used was composition _{CaO: 36.1mass%, SiO 2:} 35.6mass%, Al 2 O 3: 6.4mass%, MgO: 1.8mass%, F: 2.9mass%, T.A. C: 3.3 mass%, viscosity at freezing point: 1100 ° C., 1300 ° C .: 0.4 Pa · s.

  During operation, the melt layer thickness of the mold / powder was constantly measured by a mold / powder melt layer thickness meter (not shown) also disposed above the mold 1. Since the mold / powder melt layer thickness meter employs the multi-frequency eddy current thickness meter described in the cited document 7 as the prior art, not only can the melt layer thickness be measured, but the unmelted layer thickness can also be calculated. I made it. These measured values are collated with the defect occurrence rate of the product steel shown in FIG. 2 every minute using a process computer, and the defect occurrence rate deviates from 0.05% of the desired value. Can increase or decrease the stirring current of the electromagnetic stirrer or change the supply amount of the mold / powder so as to return to 0.05% or less of the desired value, so that the unmelted layer and the molten layer of the mold / powder are changed. The thickness of was adjusted. The state of the operation is shown in FIG.

  As a result, it was possible to adjust the thickness of the molten layer of the mold / powder to a range of 10 to 15 mm and the thickness of the unmelted layer of the mold / powder to a range of 5 to 10 mm. And the slab (slab) obtained by continuous casting was hot-charge-rolled without maintenance, and it was set as the hot-rolled steel strip. The hot charge rolling conditions were a heating furnace temperature: 1100 to 1150 ° C. and a furnace time: 30 to 60 min. Subsequently, the obtained hot-rolled steel strip was cold-rolled to obtain a cold-rolled coil (steel strip) having a thickness of 0.8 mm. About the obtained cold-rolled coil, the number of surface defects generated was visually examined to determine the surface defect generation rate (%) (= (number of defects / measured coil length) × 100). The surface defects measured were bubbles, inclusions, powdered bulges, slivers, and blisters (blisters), and cracked defects were excluded. The defect occurrence rate in the examples of the present invention was a very low value of 0.05% or less on the average of the total number of charges produced.

  On the other hand, in the continuous casting of the same steel grade, no special action is taken with respect to the mold / powder melt layer thickness etc. A conventional casting method was also carried out, in which the operation was carried out while doing as much as possible.

  As a result, the thickness of the molten layer of the mold powder varied from 0 to 25 mm, and the thickness of the non-molten layer of the mold powder varied greatly from 10 to 30 mm. The average defect rate was as large as 0.25% of the entire product. In other words, by implementing the present invention, it has become possible to obtain a product steel material of higher quality than before.

It is a figure which shows the relationship between the molten layer thickness of a mold powder and the defect incidence rate of a product steel plate. It is a figure which shows the relationship between the thickness of the molten layer and unmelted layer of a mold powder, and the defect incidence rate of a product steel plate. It is a figure which shows the change condition of the molten layer and unmelted layer of a mold / powder at the time of implementing this invention, and the provision condition of action. It is the schematic explaining the condition in the casting_mold | template in the continuous casting of molten steel.

Explanation of symbols

1 Water-cooled mold 2 Molten steel 3 Solidified shell 4 Molten layer of mold / powder 5 Mold / powder (or its unmelted layer)

Claims (4)

The molten steel is continuously poured into a water-cooled mold that is vibrated up and down, and the molten steel is covered with a separately supplied mold and powder, and the molten steel is stirred by electromagnetic force, and the generated solidified shell is cooled while being drawn downward from the mold. To make a fully solidified long steel slab,
The thickness of the molten layer of the mold / powder is constantly measured, the thickness of the molten layer of the mold / powder in which the measured value is predetermined, and the steel slab or the product steel obtained by rolling the steel slab In light of the relationship with the defect rate, the thickness of the molten layer of the mold / powder is changed by changing the casting conditions of the molten steel in the water-cooled mold so that the defect rate of the steel slab or product steel is less than the desired value. A method for continuously casting molten steel, characterized by adjusting the temperature.   Further, the thickness of the unmelted layer of the mold / powder was obtained and obtained by rolling the thickness of the unmelted layer of the mold / powder and the thickness of the molten layer and the steel slab or the steel slab. In light of the relationship with the defect occurrence rate of the product steel material, the casting conditions of the molten steel in the water-cooled mold are changed so that the defect occurrence rate of the steel slab or the product steel material falls within the desired range. The method for continuously casting molten steel according to claim 1, wherein the thickness of the layer and the thickness of the unmelted layer are adjusted.   3. The molten steel continuous casting method according to claim 1, wherein the casting condition of the molten steel in the water-cooled mold to be changed is the amount of mold and powder and / or the magnitude of electromagnetic stirring force.   The method for continuously casting molten steel according to any one of claims 1 to 3, wherein the measurement of the molten mold / powder layer is performed with a multi-frequency eddy current thickness meter.

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