JP6809237B2 - Continuous steel casting method and continuous casting machine - Google Patents

Description

The present invention relates to a continuous steel casting method and a continuous casting machine.

In continuous steel casting, molten steel is supplied from a tundish, which is a molten steel container, into a mold via a dipping nozzle, and the molten steel is solidified by cooling from the outside to form a slab (also referred to as a slab or a steel piece). To manufacture.

In the process of continuous casting of steel, the powder line of the immersion nozzle (the part of the immersion nozzle that comes into contact with the molten powder) reacts with the molten powder of the meniscus (the molten steel surface in the mold) to melt the immersion nozzle. In order to prevent loss, in the case of casting for a long time, change the immersion nozzle depth, which is the length of the immersion nozzle immersed in the meniscus, and change the position where the immersion nozzle and the molten powder of the meniscus contact. By not concentrating the powder line portion of the immersion nozzle at a specific position, the operation is performed while preventing the immersion nozzle from being melted.

However, when the depth of the immersion nozzle is changed, the height direction of the discharge hole of the immersion nozzle (which is an opening for supplying molten steel from the immersion nozzle into the mold, and the flow of molten steel from this is referred to as a discharge flow). Since the position is inevitably changed, the state of the reversal flow generated by the collision between the discharge flow from the immersion nozzle and the short side of the mold changes, which changes the temperature condition of the meniscus part and causes the inside of the slab to change. It affects the generation status of inclusions such as Al ₂ O ₃ . As a result, it leads to quality defects of the product due to inclusions in the lower process.

In addition, an inert gas such as Ar is blown from a sliding nozzle plate for adjusting the supply amount of molten steel installed in the tundish to prevent blockage of the immersion nozzle (hereinafter referred to as "porous Ar"). It has been. The blown Ar moves inside the immersion nozzle and flows out to the meniscus of molten steel.

Furthermore, in order to control the flow of the discharge flow from the discharge hole of the immersion nozzle, the flow suppression electromagnetic force by the DC magnetic field is applied to the molten steel in the mold to promote the floating of inclusions and bubbles in the molten steel and the discharge flow. Suppression is being done. This is called an electromagnetic brake, and is also called an electromagnetic brake or EMBr.

The decrease in the temperature of the meniscus portion is one of the factors that the inclusions and air bubbles remain in the slab without floating, and in the lower process, these residues cause quality defects of the product. If the temperature is further lowered, it becomes difficult to perform a smooth casting operation.

Although the high temperature of the meniscus part is favorable for the quality of the product described above, it not only requires energy for raising the temperature of the molten steel, but is also high in order to avoid breakout in the casting process for solidifying the molten steel. Cooling capacity is required, which leads to a decrease in casting speed (production volume) and other factors, which in turn leads to a decrease in productivity (cost).

Therefore, in continuous steel casting, it is desirable to lower the molten steel temperature within a range that does not adversely affect smooth work and product quality. It is aimed to set the target value of the molten steel temperature low and control it as close to the target value as possible within the range not below the target value.

On the other hand, the temperature control of the molten steel is usually performed by substituting the temperature measured in the tundish instead of the meniscus closer to the position where the solidification actually starts. Since the temperature drop from the tundish to the meniscus can be grasped empirically (for example, about 15K), if the temperature is set in consideration of a slight margin in the tundish, there is no major hindrance to the continuous casting operation. This is the usual way of thinking about "molten steel heating degree".

In recent years, high-precision temperature control has been required for the purpose of preventing operational troubles such as breakouts and quality troubles, and if temperature setting and temperature measurement are performed with a tundish, the accuracy of temperature control will be improved. The problem of inadequacy has arisen.

In order to solve this problem, for example, Patent Document 1 discloses a mold for continuous casting, which is provided with a mold copper plate on the inner surface and casts a slab having a rectangular cross section. This continuous casting mold is provided with a plurality of heating means capable of independently controlling the temperature at the portion corresponding to the molten steel meniscus of the rectangular corner corresponding portion in the mold copper plate, and the rectangular long side center corresponding portion in the mold copper plate and the central corresponding portion. A temperature measuring means (thermoelectric pair) is provided in each meniscus portion of the corner corresponding portion.

JP-A-11-047888

In the invention disclosed in Patent Document 1, the thermocouple is arranged at a position corresponding to each meniscus portion of the long side center corresponding portion and the corner corresponding portion of the mold copper plate. However, the thickness of the mold copper plate decreases as its use is repeated. Therefore, it is considered that the temperature measurement value of the meniscus portion according to the invention disclosed in Patent Document 1 is easily affected by the thickness of the mold copper plate and other disturbance factors, and an error is likely to occur.

Further, in order to directly measure the temperature of the meniscus portion by the method disclosed in Patent Document 1, considerable equipment modification and its cost are required, and there is a big problem in terms of cost.

Therefore, although it is necessary to control the temperature of the meniscus portion as a quality measure, the temperature of the meniscus portion can be measured and controlled at a lower cost, easily, and with higher accuracy than the invention disclosed in Patent Document 1. It is necessary to improve the quality.

The present invention suppresses the generation of inclusions such as Al ₂ O ₃ when the heat supply to the meniscus during casting is insufficient, and suppresses the occurrence of product quality defects caused by inclusions in the lower process. It is an object of the present invention to provide a continuous casting method and a continuous casting machine for steel capable of producing steel. Specifically, an object of the present invention is to realize the temperature control of the meniscus portion by a highly accurate and simpler means according to the temperature condition of the meniscus portion without increasing the cost such as equipment modification.

As a result of diligent studies to solve the above problems, the present inventor has obtained the following findings and completed the present invention.

FIG. 1 is an explanatory diagram schematically showing a method for measuring the temperature of the meniscus portion in the present invention.

The temperature of the meniscus portion in the mold 3 is directly measured from the thermocouple 2 attached to the measuring instrument 1 shown in FIG. For example, the thermocouple 2 is attached to a probe 7 attached to a measuring instrument 1 having a lead wire 6 for transmitting the generated thermoelectromotive force to the data logger 4. The thermocouple 2 can directly measure the temperature of the meniscus portion by suspending and contacting the molten steel 8 inside the mold 3 with the measuring instrument 1.

The measuring instrument 1 in FIG. 1 has an L-shaped outer shape, and when measuring the temperature of molten steel, the temperature measuring place may change at any time, or the temperature measuring place cannot be fixedly arranged. Even in some cases, it is an instrument that can be carried by the operator as appropriate and has the function of measuring the temperature directly or through a simple support (indicated by a small Y-shape in FIG. 1). is there. Further, the ring portion of the measuring instrument 1 indicates a gripping portion for the operator to grip the measuring instrument 1. Therefore, as long as it has the above functions, the measuring instrument 1 is not limited to a specific shape / structure.

Then, the measured value by the thermocouple 2 is transmitted to the data logger 4 via the lead wire 6, and at least one of the depth of the immersion nozzle 5, the EMBr current value and the porous flow rate is controlled to measure the temperature of the meniscus portion. By setting the molten steel heating degree required by using the above to 15K or more, the thermocouple 2 is brought into direct contact with the molten steel 8 without adding special equipment, so that the temperature is higher than that of the mold copper plate. The temperature of the meniscus part can be measured with.

As a result, it is possible to manufacture a slab that can manufacture a product having few quality defects originating from inclusions in the lower process, and it is possible to suppress the occurrence of product defects in the lower process. Reference numeral 13 in FIG. 1 is a vortex sensor.

The present invention is as listed below.

(1) A flow-suppressing electromagnetic force is applied to the molten steel supplied from the tundish to the mold via the immersion nozzle by an electromagnetic braking device, and an inert gas is blown from the sliding nozzle plate installed in the tundish. In a method of continuous casting of steel to produce slabs while doing one or both of the things
A thermocouple is used to set at least one of the immersion nozzle depth, which is the length of the immersion nozzle immersed in the meniscus, the EMBr current value that adjusts the flow suppression electromagnetic force, and the porous flow rate, which is the flow rate of the inert gas. The meniscus temperature TM (K), which is the molten steel temperature of the meniscus in the mold, and the liquidus temperature TL (K) of the molten steel, which are measured using the above, are controlled so as to satisfy the relationship of TM-TL ≧ 15. , Continuous steel casting method.

(2) The thermocouple is attached to a probe attached to a measuring instrument having a built-in lead wire for transmitting the generated thermoelectromotive force to the data logger, and by contacting with the molten steel inside the mold, the meniscus temperature TM ( The method for continuously casting steel according to item 1, wherein K) is measured.

The probe may be a ready-made probe for measuring the temperature of molten steel, and has an elongated shape with a built-in thermocouple. The outer shell is a paper tube for attaching to the measuring instrument, and a glass tube for the part immersed in molten steel. A certain structure is common, and for example, a manufacturer: Kawaso Electric Industrial Co., Ltd. model number: STP-T (MOL) is exemplified. As illustrated in FIG. 1, the measuring instrument is exemplified as an L-shaped bent pipe.

(3) The meniscus temperature is separated from the center of the immersion nozzle by 50 mm or more in the long side direction of the mold, and is separated from the inner surface of the mold by 50 mm or more in both the long side direction and the short side direction of the mold. The method for continuously casting steel according to item 1 or 2, wherein the measurement is performed in a region where the depth from the meniscus portion is 10 to 50 mm in the vertical direction of the mold.

(4) Item 1 to 3, wherein the immersion nozzle depth is controlled to 200 to 300 mm, the EMBr current value is controlled to 0 to 750 A, and the porous flow rate is controlled to 2.0 to 8.0 L / min. The method for continuously casting steel according to any one.

(5) A tundish having a sliding nozzle plate, a mold, a dipping nozzle for supplying molten steel from the tundish to the mold, and an electromagnetic braking device for applying a flow suppression electromagnetic force to the molten steel supplied to the mold. In a steel continuous casting machine for producing slabs, one or both of the above-mentioned flow-suppressing electromagnetic force is applied and the inert gas is blown from the sliding nozzle plate.
A thermocouple is used to set at least one of the immersion nozzle depth, which is the length of the immersion nozzle immersed in the meniscus, the EMBr current value that adjusts the flow suppression electromagnetic force, and the porous flow rate, which is the flow rate of the inert gas. The meniscus temperature TM (K), which is the molten steel temperature of the meniscus in the mold, and the liquidus temperature TL (K) of the molten steel, which are measured using the above, are controlled so as to satisfy the relationship of TM-TL ≧ 15. A continuous steel casting machine equipped with a control device.

(6) The thermocouple is attached to a probe attached to a measuring instrument having a built-in lead wire for transmitting the generated thermoelectromotive force to the data logger, and by contacting with the molten steel inside the mold, the meniscus temperature TM ( Item 5. The steel continuous casting machine according to Item 5, which measures K).

(7) The meniscus temperature is separated from the center of the immersion nozzle by 50 mm or more in the long side direction of the mold, and is separated from the inner surface of the mold by 50 mm or more in both the long side direction and the short side direction of the mold. The continuous steel casting machine according to item 5 or 6, measured in a region where the depth from the meniscus portion is 10 to 50 mm in the vertical direction of the mold.

(8) Item 5 to 7, wherein the immersion nozzle depth is controlled to 200 to 300 mm, the EMBr current value is controlled to 0 to 750 A, and the porous flow rate is controlled to 2.0 to 8.0 L / min. The steel continuous casting machine described in either.

According to the present invention, the temperature of the meniscus portion at a plurality of positions in the mold is measured with high accuracy by a thermocouple without increasing the cost such as equipment modification, and the meniscus temperature-liquidus temperature ( _TL ) is measured. By controlling at least one of the immersion nozzle depth, the EMBr current value, and the porous Ar flow rate so that ≥15K, the temperature drop of the meniscus portion can be suppressed, and thereby Al ₂ O, which causes product defects. It is possible to suppress the generation of inclusions such as ₃ .

FIG. 1 is an explanatory diagram schematically showing a method for measuring the meniscus temperature. FIG. 2 is a graph showing the relationship between the immersion nozzle depth (immersion depth) and (meniscus temperature-liquidus temperature). FIG. 3 is a graph showing the relationship between the EMBr current value (EMBr applied current value) and (meniscus temperature-liquidus temperature). FIG. 4 is a graph showing the relationship between the porous Ar flow rate and (meniscus temperature-liquidus temperature). 5A and 5B are explanatory views showing a conventional reverse flow near the meniscus, FIG. 5A is a cross-sectional view, and FIG. 5B is a side view (long side side). 6A and 6B are explanatory views showing a reverse flow in the vicinity of the meniscus according to the present invention, FIG. 6A is a cross-sectional view, and FIG. 6B is a side view (long side side). FIG. 7 is a two-view view showing the meniscus temperature measurement range in the embodiment. FIG. 8 is a graph showing the relationship between (meniscus temperature-liquidus temperature) and the UT defect rate.

1. 1. Explanation of terms The terms used herein will be described.
(1-1) Meniscus means the molten steel surface in the mold in a continuous casting machine.
(1-2) Powder Also called continuous casting powder, mold powder, etc. It is mainly composed of oxides and has multiple functions such as a lubricant for molds and molten steel, blocking molten steel from the atmosphere, and taking in inclusions floating on the meniscus. The layer that becomes a molten state on the meniscus is referred to as a "powder molten layer", and the unmelted state above it is referred to as a "powder layer".
(1-3) Meniscus portion This means a region having a depth of about 50 mm from the meniscus. Since the meniscus means the molten steel surface in the mold, the "meniscus portion" does not include the above-mentioned powder molten layer or powder layer.
(1-4) Molten steel superheat degree This means a temperature difference obtained by subtracting the liquidus temperature obtained from an equilibrium state diagram or the like from the actually measured molten steel temperature. In the case of continuous casting, the "molten steel superheat degree" means the difference between the temperature measurement value in the tundish and the liquidus temperature, and is generally used as an operation index. However, in the present invention, the meniscus portion is used. It means the difference between the measured temperature value and the liquidus temperature, and is used as a characteristic index.
(1-5) Liquidus temperature This is the temperature at which the material transforms from a solid to a liquid phase, and the liquidus temperature differs depending on the chemical composition of the steel. The liquidus temperature of steel can be obtained from well-known equilibrium diagrams and thermodynamic data.
(1-6) Breakout This means that the solidified shell is broken and molten steel leaks out (leakage steel).
(1-7) Hook crack When both ends of a steel plate are crimped during welding of a welded steel pipe, a nearby metal flow rises sharply, and along with this, segregation inclusions rise and appear as cracks on the surface. The shape of the cracked fracture surface is a hook shape.
(1-8) UT (Ultrasonic Testing)
It means to evaluate the internal quality using ultrasonic waves (ultrasonic flaw detection test) in order to investigate the presence or absence of the above-mentioned hook cracks in a small-diameter pipe after pipe production.
(1-9) Immersion nozzle depth This means the length of the immersion nozzle immersed in the meniscus (the surface of molten steel in the mold).
(1-10) Meniscus temperature It means the molten steel temperature of the meniscus part. In the present invention, it is preferable to measure the meniscus temperature by actually measuring the molten steel existing in the region where the depth is 10 to 50 mm from the meniscus (the surface of the molten steel in the mold).
(1-11) EMBr current value By applying an electromagnetic force that suppresses flow by a DC magnetic field to the molten steel in the mold, the electromagnetic force of EMBr that promotes the floating of inclusions and bubbles in the molten steel and suppresses the discharge flow is adjusted. Means a parameter.
(1-12) Porous Ar flow rate This is the flow rate of Ar that is blown from the sliding nozzle plate installed in the tundish to prevent blockage of the immersion nozzle, moves in the immersion nozzle, and flows out to the meniscus. It is supplied by the control method.
(1-13) Short side of the mold This means the short side of the mold forming two short sides of the four sides constituting the cross section of the mold having a rectangular outer shape, and the direction in which the short side of the mold exists is the "short side direction". Say.
(1-14) Long side of the mold It means the long side of the mold forming two long sides of the four sides constituting the cross section of the mold having a rectangular outer shape, and the direction in which the long side of the mold exists is the "long side direction". Say.

2. 2. Description of the Invention The present invention basically applies a flow suppression electromagnetic force by a DC magnetic field to the molten steel supplied from the tundish to the mold via the immersion nozzle by an electromagnetic braking device, and installs the tundish. It is a continuous casting method and a continuous casting machine for steel that manufactures a slab while blowing an inert gas from a sliding nozzle plate for adjusting the supply amount of molten steel.

The chemical composition of the steel to which the present invention is applied and the dimensions of the slab are not particularly limited.

In the present invention, as shown in FIG. 1, the temperature of the meniscus portion in the mold 3 is separated from a specific region, preferably the center of the immersion nozzle 5, by 50 mm or more in the long side direction, and the short side of the mold and the mold. The meniscus temperature, which is the temperature of the molten steel 8 existing in the region where the depth is 10 to 50 mm from the meniscus (the surface of the molten steel in the mold), is measured and the discharge flow is separated from the surface of each of the long sides by 50 mm or more. And, depending on the situation of the reverse flow, at least one of the immersion nozzle 5 depth, the EMBr current value and the porous Ar flow rate is changed during casting or during the next casting.

When the measurement region of the meniscus temperature is less than 50 mm in the long side direction from the center of the immersion nozzle 5, the diameter (inside) of the immersion nozzle 5 is, for example, 98 mm, so that the measurement is performed immediately beside the discharge hole of the immersion nozzle 5. This is likely to occur, and the discharge flow and reverse flow directly affect the measured value of the meniscus temperature, making it impossible to obtain accurate data for the entire meniscus portion. Therefore, the measurement region of the meniscus temperature is preferably a region separated from the center of the immersion nozzle 5 in the long side direction by 50 mm or more.

The reason why a region separated from the surface of each of the short side and the long side of the mold by 50 mm or more is preferable is that the inserted thermocouple 2 is captured by the solidified shell at the meniscus portion as shown in FIG. 1 to prevent the solidified shell from growing. This is to prevent the mold 3 from being extruded by the static pressure of the molten steel at the lower end of the mold 3 and leading to operational troubles such as breakout.

Further, in a region less than 10 mm deep from the meniscus, the temperature of the entire meniscus portion may not be accurately measured due to the influence of the surface portion such as the powder molten layer. On the other hand, in the region where the depth from the meniscus exceeds 50 mm, the appropriate meniscus temperature may not be measured because it may be affected by the discharge flow from the immersion nozzle depending on the operating conditions. Therefore, the depth from the meniscus of the temperature measuring unit is preferably 10 mm or more and 50 mm or less.

Needless to say, even within the measurement range of the meniscus temperature specified in the present invention, measurement at a position competing with other equipment, for example, the vortex sensor 13, is not preferable.

As shown in FIG. 1, the temperature of the meniscus portion in the mold 3 is directly measured from the thermocouple 2 attached to the measuring instrument 1. For example, the thermocouple 2 is attached to a probe 7 attached to a measuring instrument 1 having a lead wire 6 for transmitting the generated thermoelectromotive force to the data logger 4. The thermocouple 2 can directly measure the temperature of the meniscus portion by contacting the molten steel 8 inside the mold 3.

Then, the measured value by the thermocouple 2 is transmitted to the data logger 4 via the lead wire 6 to control at least one of the depth of the immersion nozzle 5, the EMBr current value, and the porous flow rate.

FIG. 2 is a graph showing the relationship between the immersion nozzle depth (immersion depth) and (meniscus temperature-liquidus temperature), and FIG. 3 shows the EMBr current value (EMBr applied current value) and (meniscus). It is a graph which shows the relationship with (temperature-liquidus line temperature), and further, FIG. 4 is a graph which shows the relationship between (meniscus temperature-liquidus line temperature) and porous Ar flow rate. The graphs of FIGS. 2 to 4 show the influence of each operating condition on the meniscus temperature-liquidus temperature ( _TL ).

As shown in the graphs of FIGS. 2 to 4, the meniscus temperature tends to increase as the immersion nozzle depth decreases, the EMBr current value decreases, or the porous Ar flow rate decreases. Therefore, the meniscus temperature- At least one of the immersion nozzle depth, the EMBr current value, and the porous Ar flow rate is controlled so as to ensure the liquidus temperature ( _TL ) ≧ 15K.

5A and 5B are explanatory views showing a conventional reversal flow 10 in the vicinity of the meniscus, FIG. 5A is a cross-sectional view, and FIG. 5B is a side view (long side side). 6A and 6B are explanatory views showing a reverse flow 10 in the vicinity of the meniscus according to the present invention, FIG. 6A is a cross-sectional view, and FIG. 6B is a side view (long side side).

As shown in FIGS. 5A and 5B, the high-temperature discharge flow 9 from the immersion nozzle 5 collides with the short side of the mold, and the upward flow (reverse flow) 10 and the downward flow (downward flow). ) 11, but in this state, it is assumed that the temperature near the meniscus is low and bubbles are trapped in the steel.

In this case, as shown in FIGS. 6A and 6B, the depth of the immersion nozzle 5 is decreased, that is, the tundish height is increased, so that the reverse flow 10 is supplied to the meniscus portion. As shown in the graph of FIG. 2, the temperature of the meniscus increases, the temperature in the vicinity of the meniscus can be secured, and the capture of inclusions can be suppressed.

Similarly, since the electromagnetic braking device (EMBr) is attached to suppress the discharge flow 9 from the immersion nozzle 5, when the EMBr current value is reduced, the discharge flow 9 is not suppressed, so that the reverse flow 10 is not suppressed. Also becomes stronger, and as shown in the graph of FIG. 3, the meniscus temperature rises.

Further, since the temperature of the molten steel 8 tends to be deprived by flowing the porous Ar, if the flow rate thereof is suppressed, the decrease in the molten steel temperature can be suppressed, and as shown in the graph of FIG. 4, the meniscus temperature rises. However, since the purpose of the porous Ar flow rate is to suppress clogging of the immersion nozzle 5, if the porous Ar flow rate is lowered too much, it is advantageous in suppressing inclusion defects, but clogging of the immersion nozzle 5 may occur. It will be disadvantageous.

There is no priority for changing the operating conditions of the immersion nozzle 5 depth (immersion depth), EMBr current value, and porous Ar flow rate, and the procedure is appropriately performed in consideration of the operation of the continuous casting machine to which the present invention is applied. Just do it.

The reason why the meniscus temperature-liquidus temperature ( _TL ), which is the standard molten steel heating degree of the meniscus temperature, is set to 15K or more is that if it is less than 15K, the viscosity of the molten steel in the meniscus portion increases and bubbles are generated. And inclusions are less likely to surface and solidify to slabs while still present in the molten steel, which causes product defects. Therefore, in the present invention, the minimum degree of superheat of molten steel required to promote the floating of air bubbles and inclusions is set to 15 K or more.

The steel used for casting was C: 0.28 to 0.32%, Si: 0.16 to 0.21%, Mn: 1.35 to 1.45%, P: 0.012% in mass%. Hereinafter, this was performed with a steel grade containing S: 0.0030% or less, having a chemical composition consisting of the balance Fe and impurities, and being used as a small-diameter tube. For this type of steel, the liquidus temperature range is 1776K to 1781K.

Casting conditions are slab thickness: 250 mm, slab width: 1240 mm width, molten steel superheat degree: 29 to 45 K, casting speed: 1.00 m / min, immersion nozzle depth: 260 mm, EMBr current value: 340 A, porous Ar flow rate. : 5.0 L / min. The immersion nozzle depth, EMBr current value, and porous Ar flow rate value at this time are set as "initial setting values".

FIG. 7 is a two-view view showing the temperature measurement position in the embodiment.

As shown in FIG. 7, the measurement position 12 of the meniscus temperature was set to a position 200 mm from the center of the immersion nozzle 5 (in the direction opposite to that of the vortex sensor 13, 1/2 thickness portion) and a depth of 30 to 40 mm. A glass tube is arranged at the tip of the thermocouple 2 immersed in the molten steel 8. Since the powder layer and the powder molten layer are about 70 mm under the casting conditions according to the present embodiment, the length of the glass tube of the thermocouple 2 Was 105 mm.

The range of various casting conditions (immersion nozzle 5 depth, EMBr current value and porous Ar flow rate) to be changed according to the meniscus temperature is as follows: immersion nozzle depth 240 to 260 mm, EMBr current value 150 to 340 A and porous Ar flow rate. It was set to 0 to 5.0 L / min. The evaluation of the effect is whether or not the meniscus temperature-liquidus temperature _TL ≥ 15K can be secured, and the occurrence rate of hook cracks caused by the presence of inclusions in the welded portion of the small diameter welded steel pipe made of this slab. UT defect rate).

In the example of the present invention, for the purpose of promoting the heat supply of the meniscus, the immersion nozzle depth is changed from 260 mm to 240 mm and the EMBr current value is changed from 340 A to 150 A for the same slab size and the same steel type casting at the same or different timings. The porous Ar flow rate was changed from 5.0 L / min to 3.0 L / min, respectively. The casting conditions (casting speed, ΔT, etc.) and manufacturing conditions (slab width, slab thickness) other than the above were the same as those at the time of measuring the meniscus temperature (in the example of the present invention, the same chemical composition system was used).

Small diameter pipe manufactured from cast slabs (standard: AT30, (i) outer diameter: 34.0 mm, wall thickness: 3.8 mm, (ii) outer diameter: 38.1 mm, wall thickness: 3.6 mm) Was subjected to ultrasonic flaw detection inspection, and the occurrence of inclusions leading to quality defects was evaluated based on the defect rate.

The defect rate referred to here is a value obtained by dividing "the number of steel pipes judged to be abnormal in the ultrasonic flaw detection test" by "the total number of steel pipes subjected to the ultrasonic flaw detection test (number of inspections)". In the ultrasonic flaw detection test, not only foreign matter defects such as hook cracks and foreign matter defects such as air bubble defects but also noise may be detected, so if the UT defect rate is 3.0% or less, it passes. And said.

The present invention has an effect of being improved by applying the conditions of the present invention when the meniscus temperature-liquidus temperature ( _TL ) is less than 15K in the "initial setting value", and thus is applicable or not. The result of comparing the effects of the above is shown in a graph in FIG. FIG. 8 is a graph showing the relationship between (meniscus temperature-liquidus temperature) and the UT defect rate.

In a comparative example in which the operating conditions are not changed according to the meniscus temperature, the meniscus temperature-liquidus temperature _TL <15K (mean value is 10.0K), and as a result of inspection with the number of inspections 14340, the UT defect rate Was 5.9%, which was a failure.

On the other hand, in the example of the present invention in which the operating conditions are changed according to the meniscus temperature, the heat supply state to the meniscus is improved by controlling at least one of the immersion nozzle depth, the EMBr current value, and the porous Ar flow rate value. , Meniscus temperature-liquidus temperature _TL ≥ 15K (mean value 17.2K), and the generation of inclusions such as Al ₂ O ₃ was suppressed, so the UT defect rate decreased to 1.6%. However, the UT defect rate ≤3.0% was achieved, and the result was passed.

1 Measuring instrument 2 Thermocouple 3 Mold 4 Data logger 5 Immersion nozzle 6 Lead wire 7 Probe 8 Molten steel 9 Discharge flow 10 Upward flow (reversal flow)
11 Downward flow (downward flow)
12 Meniscus temperature measurement position 13 Vortex sensor

Claims (6)

One is to apply a flow suppression electromagnetic force to the molten steel supplied from the tundish to the mold via the immersion nozzle by an electromagnetic braking device, and the other is to blow an inert gas from the sliding nozzle plate installed in the tundish. Or in the continuous casting method of steel to manufacture slabs while doing both.
A thermocouple is used to set at least one of the immersion nozzle depth, which is the length of the immersion nozzle immersed in the meniscus, the EMBr current value that adjusts the flow suppression electromagnetic force, and the porous flow rate, which is the flow rate of the inert gas. It was measured using, as the meniscus temperature TM and (K) a meniscus of the molten steel temperature inside the mold and the molten steel liquidus temperature TL (K) satisfies the relationship of TM-TL ≧ 15, the control and ,
The thermocouple is attached to a probe attached to a measuring instrument having a built-in lead wire for transmitting the generated thermoelectromotive force to the data logger, and contacts the molten steel inside the mold to raise the meniscus temperature TM (K). A method of continuous steel casting to measure. The meniscus temperature is separated from the center of the immersion nozzle by 50 mm or more in the long side direction of the mold, and is separated from the inner surface of the mold by 50 mm or more in both the long side direction and the short side direction of the mold, and the mold is separated from the center. in region depth is 10~50mm from the meniscus in the vertical direction, is measured, the continuous casting method of steel according to claim 1. According to claim 1 or 2 , the depth of the immersion nozzle is controlled to 200 to 300 mm, the EMBr current value is controlled to 0 to 750 A, and the porous flow rate is controlled to 2.0 to 8.0 L / min. The method for continuous casting of steel according to the description. A tundish having a sliding nozzle plate, a mold, a dipping nozzle for supplying molten steel from the tundish to the mold, and an electromagnetic braking device for applying a flow suppression electromagnetic force to the molten steel supplied to the mold are provided. In a continuous steel casting machine for producing slabs, one or both of applying an electromagnetic force for suppressing flow and blowing an inert gas from the sliding nozzle plate.
A thermocouple is used to set at least one of the immersion nozzle depth, which is the length of the immersion nozzle immersed in the meniscus, the EMBr current value that adjusts the flow suppression electromagnetic force, and the porous flow rate, which is the flow rate of the inert gas. The meniscus temperature TM (K), which is the molten steel temperature of the meniscus in the mold, and the liquidus temperature TL (K) of the molten steel, which are measured using the above, are controlled so as to satisfy the relationship of TM-TL ≧ 15. Equipped with a control device
The thermocouple is attached to a probe attached to a measuring instrument having a built-in lead wire for transmitting the generated thermoelectromotive force to the data logger, and contacts the molten steel inside the mold to raise the meniscus temperature TM (K). Continuous steel casting machine to measure . The meniscus temperature is separated from the center of the immersion nozzle by 50 mm or more in the long side direction of the mold, and is separated from the inner surface of the mold by 50 mm or more in both the long side direction and the short side direction of the mold, and the mold is separated from the center. The continuous steel casting machine according to claim 4 , wherein the steel continuous casting machine is measured in a region where the depth from the meniscus portion is 10 to 50 mm in the vertical direction. According to claim 4 or 5 , the depth of the immersion nozzle is controlled to 200 to 300 mm, the EMBr current value is controlled to 0 to 750 A, and the porous flow rate is controlled to 2.0 to 8.0 L / min. The steel continuous casting machine described.