JP2000061591A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To establish a continuous casting method of a steel, with which the abnormality of quality caused by cleaning of a bottom cast slab can be prevented from occurring. SOLUTION: In the continuous casting method of the steel in which pouring into a mold 11 is started after storing the molten steel 12 in a tundish 1, the pickup rate of total oxygen concn. in the cast slab at the time of starting the casting and the separating ratio of oxide base inclusion, are pre-measured in the casting condition decided with the combination of three casting factors of whether or not molten steel in the tundish is heated, whether or not molten steel in the tundish is stirred and the molten steel stuck quantity in the tundish. Based on these measured results, the total oxygen concn. in the cast slab at the time of starting the casting and the separating ratio of the oxide base inclusion in the individual casting condition, are estimated and based on these estimated values, the surface cleaning method on the cast slab and the operating method at the time of starting the casting are decided for each product.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention estimates the quality level of a slab at the start of casting from casting conditions, and determines a surface care method and an operating method for the slab based on the estimated value to improve cleanliness. The present invention relates to a continuous casting method for steel capable of preventing the resulting quality abnormality.

[0002]

2. Description of the Related Art In recent years, the demands on the material characteristics of steel plates for automobiles, steel plates for food cans, steel pipes for oil transport, steel pipes for gas transport, and high-strength steel wire rods have become increasingly severe, and homogeneous and clean steel strips have been required. It has been demanded. On the other hand, the diversification of customer needs is due to the continuous casting method suitable for large-lot production, but the smaller lots make it possible to use unsteady parts with poor quality levels at the start of casting, at the end of casting, and when casting different steel types one after another. It causes a problem of increase of slabs.

Of these unsteady-section cast pieces, the one that is particularly problematic in terms of quality is the cast piece at the start of casting (hereinafter referred to as "bottom cast piece"). At the start of casting, molten steel is oxidized by the metal and slag remaining in the air and tundish,
Oxide inclusions (hereinafter referred to as “inclusions”) increase in the molten steel, and the temperature of the molten steel decreases due to conduction heat to the tundish refractory and radiant heat to the air. This is because the floating and separation of the inclusions inside is hindered, and many inclusions remain, resulting in poor cleanability. for that reason,
Many techniques have been disclosed for improving the cleanability of bottom slabs.

[0004] For example, in Japanese Patent Laid-Open No. 53-115617, before the start of injection into a mold, the tundish is pooled until the height of molten steel reaches 400 mm or more, and the inclusions are floated to improve cleanliness. Is disclosed, and
JP-A-1-107948 discloses that the metal and slag adhering to the inside of the tundish are dissolved and removed by a burner, and the metal and the slag flow into the outflow hole from the tundish to the mold during preheating of the tundish. A method of preventing the above and reusing the tundish to improve the cleanability of the bottom slab is disclosed.

[0005]

However, in the above-mentioned prior art, there is a problem that the cleanliness of the bottom slab is not sufficiently improved, rather deteriorates, and sometimes the casting operation cannot be continued. For example, according to the method disclosed in Japanese Patent Laid-Open No. 53-115617, the cleanliness of the slab is improved as the height of the molten steel stored is increased, but the temperature of the molten steel in the tundish is lowered, so that the molten steel flows out due to solidification. The holes are closed to make casting impossible, and the temperature of molten steel in the mold is lowered, so that the slag of the mold powder is impaired, resulting in frequent occurrence of surface defects on the slab. The effect of the lowering of the molten steel temperature lasts longer than in the case of the usual casting start method, and as a result, the range of poor quality at the start of casting is lengthened. In addition, JP-A-1-10
In the method according to Japanese Patent Publication No. 7948, the influence of the amount of adhered metal and the amount of adhered slag is taken into consideration, and if it is not made harmless, the metal or slag adhered in the tundish becomes a pollution source,
Contamination is continued for a long time to increase the range of poor quality at the start of casting.

As a result of investigating and examining the causes of the above problems in the prior art, the inventors of the present invention have found that in the prior art, the size and amount of inclusions in question, and reduction of inclusions by various measures are taken. It was confirmed that the effects were not sufficiently quantified and that the above problems were caused by these.

The present invention has been made in view of the above circumstances,
An object of the object is to provide a continuous casting method for steel capable of preventing a quality abnormality due to cleanliness of a bottom slab in advance.

[0008]

According to a first aspect of the present invention, there is provided a continuous casting method for steel in which a molten steel is stored in a tundish and then injection into a mold is started. The total oxygen in the slab at the start of casting under the casting conditions determined by the combination of the three casting factors, namely, whether the molten steel is heated in the tundish, whether the molten steel is stirred in the tundish, and the amount of metal deposit in the tundish. The concentration pick-up amount and the separation rate of oxide inclusions were measured in advance, and based on the measurement results, the total oxygen concentration of the slab and the separation rate of oxide inclusions at the start of casting under individual casting conditions. It is characterized in that the surface maintenance method and the operating method of the slab at the start of casting are determined for each product based on this estimated value.

The continuous casting method for steel according to the second aspect of the invention is a continuous casting method for steel in which molten steel is stored in a tundish and then poured into a mold to start casting. Temperature change in advance, based on this measurement result to determine the pooling time in the tundish from the measured value of the molten steel temperature immediately after injection into the tundish, and whether or not the molten steel is heated in the tundish. Under the casting conditions that are determined by the combination of the three casting factors of the presence or absence of molten steel stirring in the dish and the amount of metal adhesion in the tundish, the total oxygen concentration pick-up amount and oxide in the slab at the start of casting The separation rate of the system inclusions is measured in advance, and the total oxygen concentration of the slab and the separation rate of the oxide inclusions at the start of casting under the individual casting conditions are estimated based on the measurement result, and It is characterized in that determining the surface UPKEEP and operational methods of the casting starting slab based on value by product.

The inventors of the present invention have started investigations and studies in order to prevent abnormal quality due to cleanliness of bottom slabs. First, the size of inclusions that would be defective for each product was investigated, and the allowable size of inclusions for each product was determined as follows. That is, in a DI can manufactured by deep-drawing a thin steel plate, a flange crack is generated by an Al ₂ O ₃ -based inclusion having a diameter of more than 50 μm, and the allowable size of the inclusion is set to prevent the flange crack. The diameter is 50μ
In the electric resistance welded pipe manufactured by bending a thin steel plate and welding the abutted portions, the allowable size of the inclusion is 1 mm or less in order to prevent ultrasonic flaws caused by the inclusion.
The diameter was 200 μm or less, the diameter was 200 μm or less for the general-purpose thin steel plate, and the diameter was 300 μm or less for the general-purpose thick steel plate.

Further, although the total content of inclusions in the cast slab does not necessarily correspond to the defect in the product, from the experience of the present inventors, those with a large total content of inclusions are inclusions of an allowable size or more. Therefore, the total content of inclusions is expressed by the total oxygen concentration of the slab (also referred to as “T. [O]”), and the allowable total oxygen concentration is 15 ppm for DI cans.
Hereinafter, it was set to 25 ppm or less for the electric resistance welded pipe, 25 ppm or less for the general-purpose thin steel plate, and 35 ppm or less for the general-purpose thick steel plate. Table 1 shows the allowable size of inclusions and the allowable value of total oxygen concentration determined for each product.

[0012]

[Table 1]

The present inventors have found that the amount of total oxygen concentration picked up at each site of bottom slab due to air oxidation or the like depends on whether or not the molten steel is heated by a plasma torch or induction heating in the tundish, and whether it is in the tundish. If the three casting factors, that is, the presence or absence of molten steel agitation with an inert gas or electromagnetic agitation, and the amount of metal in the tundish are the same, the molten steel storage time in the tundish and the molten steel in the mold The total time of adding three times of the pouring time and the elapsed time from the start of slab drawing of each part of the bottom slab (hereinafter,
It was found that they are organized by "total elapsed time").

That is, the pickup amount of the total oxygen concentration in each part of the bottom cast piece can be arranged by the total elapsed time regardless of the cast factors such as the cast piece size and the cast piece drawing speed if the above three casting factors are determined. In the present invention, the pickup amount of the total oxygen concentration in the bottom slab is measured in advance for each casting condition determined by the combination of the above three casting factors, and based on the result, each part of the bottom slab under the individual casting conditions is measured. The total oxygen concentration of is estimated from the total elapsed time.

Further, according to the present invention, the separation rate by size of inclusions in the molten steel in the tundish is measured in advance for each casting condition determined by the combination of the above three casting factors, and the individual casting is performed based on the result. The distribution of inclusions in the bottom slab under the conditions is estimated.

In this way, it becomes possible to estimate the total oxygen concentration and the distribution of inclusions in the bottom slab, and these estimated values and the permissible values and the inclusions of total oxygen concentration for each product shown in Table 1. For the bottom slabs in which the estimated value of the total oxygen concentration exceeds the permissible value by comparing the allowable size of the product, the surface care method of the slab can be changed, or the operation can be changed to other than the planned product. The quality abnormality caused by the cleanliness of the bottom slab is prevented in advance.

Further, in the present invention, the change in molten steel temperature during storage is measured in advance, and based on this measurement result, the storage time in the tundish is determined from the measured value of the molten steel temperature immediately after injection into the tundish. Since it is determined, it is possible to prevent the solidification of the molten steel and to secure the longest storage time and promote the separation of inclusions. Incidentally, the storage time of the molten steel in the tundish in the present invention is the time from the start of pouring the molten steel into the tundish until the start of pouring into the mold, and the pouring of the molten steel into the mold The raising time is the time from the start of pouring molten steel into the mold to the start of withdrawing the slab.

[0018]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below with reference to the drawings. FIG. 1 shows a slab having a rectangular cross section according to the present invention.
It is an outline figure of the front section of the continuous casting equipment of a strand.

In the figure, a tundish 1 having a refractory material inside is mounted on a tundish car (not shown) and is arranged at a predetermined position above a mold 11.
At the bottom of the tundish 1, an upper nozzle 17 fitted to the tundish refractory, a sliding nozzle 3 connected to the lower surface side of the upper nozzle 17 and composed of a fixed plate 18 and a sliding plate 19, and a sliding nozzle 3 An immersion nozzle 2 connected to the lower surface side is provided, and an outflow hole 20 from the tundish 1 to the mold 11 is formed. A pair of weirs 4 having discharge holes at the bottom is installed in the center of the tundish 1, and a long nozzle 16 provided at the bottom of the ladle 15 is surrounded by the pair of weirs 4. A ladle 15 for accommodating the molten steel 12 therein is installed above the tundish 1 so as to be located at.

A plasma heating device 5 is installed above the tundish 1 as a means for heating the molten steel 12 in the tundish 1. The plasma torch 6 of the plasma heating device 5 can move up and down in the tundish 1 surrounded by the pair of weirs 4, and plasma is generated by the plasma torch 6 and the molten steel 12 in the tundish 1 to generate molten steel 1.
2 can be heated. Also, at the bottom of the tundish 1 between the weir 4 and the upper nozzle 17, the tundish 1
As a means for stirring the molten steel 12 therein, a porous brick 7 for blowing an inert gas is installed. The porous brick 7 is connected to an inert gas pipe (not shown), blows an inert gas such as Ar, and melts the molten steel 1 in the tundish 1.
2 can be stirred. The heating means for molten steel and the stirring means for molten steel are not always necessary for the present invention.

Above the tundish 1, a consumable thermocouple 9 is installed in a molten steel 12 in the tundish 1, and a temperature measuring device 8 for measuring the molten steel temperature is installed. The tundish 1 is a load cell 10. Supported, Tundish 1
The weight of the molten steel 12 therein and, in the case of reusing the tundish 1, the amount of metal adhered in the tundish 1 are measured by the load cell 10 based on the tare weight of the tundish 1.

The operation in such a continuous casting facility is as follows.
First, the sliding nozzle 3 is closed and the molten steel 1 is put into the tundish 1 from the ladle 15 through the long nozzle 16.
2 is injected, and molten steel 1 is heated while the molten steel is heated by the plasma heating device 5 and the molten steel is stirred by the porous brick 7 as needed.
2 is stored in the tundish 1 for a predetermined time, then the sliding nozzle 3 is opened, and the immersion nozzle 2
The molten steel 12 is poured into the mold 11 via the. The molten steel 12 poured into the mold 11 is cooled to form a solidified shell 14. Then, the molten steel surface poured into the mold 11 is poured up until it reaches a predetermined position in the mold 11, and when the molten steel surface reaches a predetermined position, the solidification shell 14 is pulled downward together with a dummy bar (not shown). , Start drawing the slab. Mold powder 13 is added into the mold 11 as a heat retaining agent for the molten steel 12 and a lubricant for the mold 11 and the solidified shell 14. Solidified shell 1 pulled out below the mold 11
No. 4 solidifies to the inside to form a cast piece, which is cut by a cast piece cutting machine (not shown) installed below the drawing direction, and continuous casting is performed.

A method of applying the present invention to a continuous casting facility having such a structure and operation will be described below.

First, under various casting conditions, the separation rate due to the floating of inclusions in the molten steel 12 in the tundish 1 is understood. Specifically, the casting conditions determined by the combination of three casting factors, namely, whether or not the molten steel is heated in the tundish 1, whether or not the molten steel is stirred in the tundish 1, and the amount of metal adhered in the tundish 1. Separately, the separation rate of inclusions depending on the storage time in the tundish 1 is grasped in advance. The following is an example of the separation rate of inclusions as understood by the present inventors.

In FIG. 2, the tundish 1 shown in FIG. 1 having a capacity of 40 tons and not used is compared, and the casting conditions are compared by comparing the presence or absence of molten steel agitation by blowing Ar gas from the porous brick 7. It is a figure which shows the result of having investigated the isolation | separation rate of the inclusion by making the storage time of the molten steel 12 in 2 minutes, 5 minutes, and 10 minutes in. The amount of Ar gas blown in is 50 Nl / min per strand. In the figure, the solid line shows the case where molten steel stirring is not performed, and the broken line shows the case where molten steel stirring with Ar gas is performed. As shown in FIG. 2, the greater the diameter of the inclusions and the longer the storage time, the greater the separation rate of the inclusions.
It can be seen that at least 90% of the inclusions of 00 μm float and separate in a storage time of 5 minutes. Further, it is understood that the stirring of molten steel with Ar gas promotes the separation of inclusions.

In FIG. 3, using the same tundish 1 as described above, while the molten steel 12 is being heated by the plasma heating apparatus 5, the storage time is set to 2 minutes, 5 minutes, and 10 minutes to separate the inclusions. It is a figure which shows the result of investigation. As shown in FIG. 3, heating the molten steel 12 in the tundish 1 improves the separation rate of inclusions, and in particular, small inclusions with a diameter of 50 μm are efficiently separated in a short storage time. I understand that. In this way, the separation rate of inclusions is grasped in advance for each casting condition determined by the above three casting factors.

Next, under various casting conditions, the pickup amount of the total oxygen concentration of the bottom slab is grasped. Specifically, the casting conditions determined by the combination of three casting factors, namely, whether or not the molten steel is heated in the tundish 1, whether or not the molten steel is stirred in the tundish 1, and the amount of metal adhered in the tundish 1. Separately, the pickup amount of the total oxygen concentration at each part of the bottom slab is grasped. The following is an example of the pickup amount of the total oxygen concentration as understood by the present inventors.

FIG. 4 shows that the tundish 1 shown in FIG. 1 having a capacity of 40 tons, which is unused, is used to stir molten steel by pouring 50 Nl / min of Ar gas per strand from the porous brick 7 and the pool in the tundish 1 is agitated. Place time 2
It is a figure which shows the result of having investigated the pick-up amount of the total oxygen concentration for every slab length which made 5 seconds, 90 seconds, 4 minutes, and 4 levels, and the cutting position with a bottom crop the starting point.
As shown in FIG. 4, it can be seen that the pickup amount of the total oxygen concentration in the bottom slab decreases as the storage time increases. In addition, the total oxygen concentration pick-up amount is the molten steel 12 in the ladle 15 before the start of injection into the tundish 1.
It was obtained from the difference from the total oxygen concentration in the cast slab, with the analytical value of the total oxygen concentration of the sample (hereinafter referred to as "initial total oxygen concentration") as a reference.

Further, the casting method in which the storage time is 25 seconds is
5 tonne of molten steel 12 was injected into the tundish 1 in 25 seconds, and the injection into the mold 11 was started immediately. Similarly, the casting method in which the pooling time was 90 seconds was 20 tonne in 90 seconds in the tundish 1. The molten steel 12 was poured into the mold 11 immediately, and the storage time was 2
In the casting method of 30 minutes, 30 tons of molten steel 12 was poured into the tundish 1 in 2 minutes and the casting was immediately started into the mold 11. On the other hand, in the casting method in which the storage time is 10 minutes, 9 tons of molten steel 12 is added to the tundish 1.
The injection was performed in 0 seconds, and the injection was performed so as to be 40 tons after 10 minutes.

The slab length on the horizontal axis in FIG. 4 is defined as the storage time under each casting condition, the pouring time into the mold 11, and
Based on the elapsed time from the start of slab drawing, it is converted into the total elapsed time described above, and the total elapsed time is shown on the horizontal axis and the total oxygen concentration pickup amount is shown on the vertical axis. is there. As shown in FIG. 5, the pickup amount of the total oxygen concentration of the bottom slab shows a good correlation with the total elapsed time.

Thus, whether or not the molten steel is heated in the tundish 1, whether or not the molten steel is stirred in the tundish 1,
Also, if the three casting factors of the amount of metal adhered in the tundish 1 are set under the same condition, the pickup amount of the total oxygen concentration of the bottom slab can be easily estimated. 2 and 3 when the molten steel is not stirred or the molten steel is heated.
Based on the above, the relationship between the total elapsed time and the pickup amount of the total oxygen concentration of the bottom slab will be separately obtained.

Further, FIG. 5 shows the tundish 1 in a hot state.
The influence of the amount of metal adhered in the tundish 1 when reusing is indicated by a broken line and a dashed line. When there is a large amount of metal deposit, the metal is oxidized by the next casting and becomes a pollution source at the time of the next casting. The effect is to measure the amount of metal deposit remaining in the tundish 1 at the end of the previous casting. By doing so, it is possible to quantify as shown in FIG.

Further, under various casting conditions, the change with time of the molten steel temperature during storage is grasped. According to the experience of the present inventors, when the molten steel superheat degree in the tundish 1 (the superheat degree is the difference between the molten steel temperature and the solidification temperature) becomes 15 ° C. or less, the molten steel 12 in the immersion nozzle 2 and the sliding nozzle 3 is It has been confirmed that solidification causes nozzle blockage, making casting impossible. If the storage time of the molten steel 12 in the tundish 1 is lengthened, the separation of inclusions is promoted and the cleanability is improved, but on the other hand, the molten steel temperature is lowered and the superheating degree of molten steel cannot be ensured. Therefore, in order to secure the superheated degree of molten steel exceeding 15 ° C., the relationship between the storage time and the change in molten steel temperature is grasped in advance. If there is a heating device such as the plasma heating device 5, the storage time can be arbitrarily selected. The following is an example of the temperature drop during storage that the present inventors have grasped.

FIG. 6 shows a molten steel stirred by the plasma heating device 5 using the tundish 1 shown in FIG. 1 having a capacity of 40 tons and stirring the molten steel by blowing Ar gas at 50 Nl / min per strand from the porous brick 7. It is a figure which shows the result of having investigated the change of the molten steel temperature in the tundish 1 with the temperature measuring device 8 with or without heating. As shown in FIG. 6, when the molten steel is not heated, the molten steel temperature decreases by 9 ° C. in the pooling time of 5 minutes and by 14 ° C. in the pooling time of 10 minutes. You need to decide the time. The data of 0 minutes of storage time in FIG. 6 is an analogy from the data of 30 seconds, 1 minute, and 2 minutes of storage time.

In this way, by combining the three casting factors of whether or not the molten steel is heated in the tundish 1, whether or not the molten steel is stirred in the tundish 1, and the amount of metal adhered in the tundish 1, Under the casting conditions that are determined, the change in molten steel temperature during storage, the total oxygen concentration pickup amount in the bottom slab, and the separation rate of oxide inclusions are known in advance, and when individual casting is performed. First, immediately after starting the injection into the tundish 1, the temperature of the molten steel is measured by the temperature measuring device 8, and the measured value and the drop in the molten steel temperature during the pooling according to the casting conditions that is grasped in advance (for example, FIG. 6). Contrast with, and determine the storage time in Tundish 1. still,
In the equipment provided with the plasma heating device 5, the storage time can be arbitrarily determined.

Next, the total oxygen concentration of the bottom slab in the individual casting is determined by the pickup amount (for example, FIG. 5) of the total oxygen concentration of the bottom slab for each casting condition and the measured initial total oxygen concentration. The distribution is estimated and compared with the allowable value of the total oxygen concentration for each product defined in Table 1, and the total elapsed time corresponding to the position where the total oxygen concentration exceeds the allowable value is obtained. Then, this total elapsed time is converted into a slab position based on the pooling time in individual casting, the pouring time into the mold 11, and the elapsed time from the start of drawing determined from the slab drawing speed, and the bottom crop Taking the length into consideration, the length position in the casting direction of the slab in which the total oxygen concentration exceeds the allowable value is determined. Then, in the cast slab where the total oxygen concentration exceeds the allowable value, the residual amount and distribution of the inclusions having a size corresponding to the allowable inclusion size for each product defined in Table 1 are preliminarily grasped for each casting condition. It is estimated from the separation rate of the product (for example, FIG. 2), and the surface maintenance method of the slab and the operating method of the slab are determined from the estimated residual amount and distribution of inclusions. Since the bottom slab has particularly poor cleanliness in the surface layer of the slab that solidifies first, the cleanability can be improved by fusing the surface layer of the slab with a cold scarf or the like.

By carrying out continuous casting in this way, it becomes possible to perform casting with an optimum storage time, and
It is possible to estimate the total oxygen concentration and residual amount of inclusions for each part of the bottom slab in the lengthwise direction, and it is possible to determine from which part of the bottom slab the applicable product can be applied for each required product. Therefore, it is possible to prevent a quality abnormality due to the cleanliness of the bottom slab.

In the above description, a pair of weirs 4 are provided in the tundish 1 in order to make the molten steel temperature during heating uniform, but the weirs 4 are not always necessary for the present invention and need not be provided. May be. Further, the number of strands is not limited to two, and the present invention can be carried out according to the above even if the number of strands is one or three or more. Further, it goes without saying that the molten steel heating means, the molten steel stirring means, and the temperature measuring means are not limited to the above.

[0039]

[Examples] [Example 1] Examples of steel types for electric resistance welded pipes in the two-strand continuous casting equipment shown in Fig. 1 will be described.
The tundish was unused and had a capacity of 40 tons and was blown with 50 Nl / min of Ar gas per strand from the porous brick. Slab drawing speed v (m / min)
Was cast as in equations (1) and (2). However, in the equations (1) and (2), t is the elapsed time (minutes) from the start of drawing the cast slab. v = 0.3 + 0.1 × t where t <5.5 minutes …… (1) v = 0.85 where t ≧ 5.5 minutes …… (2)

The initial total oxygen concentration in the sample collected from the ladle is 20 ppm, and the allowable amount of pickup of the total oxygen concentration in the electric resistance welded steel grade is 5 ppm from the allowable value in Table 1. In addition, the molten steel superheat degree in the tundish immediately after the start of injection into the tundish was 25 ° C, and it was found from Fig. 6 that the storage time could be set to 5 minutes. After the solution was stored in the mold, injection into the mold was started. The pouring time of the molten steel in the mold was 1 minute, and a height of 0.3 m was cast as the length of the cast piece in this 1 minute, and then the pulling of the cast piece was started at the above-mentioned cast piece drawing speed. In cutting the slab, the bottom crop length was set to 0.6 m. No plasma heating device was used.

Here, T is the total elapsed time, t is the elapsed time from the start of the drawing of the slab, L is the drawing length of the slab, and X is the slab length from the cutting position with the bottom crop in the bottom slab. Then, the total elapsed time (T) at each part of the slab is
It is the sum of the storage time, the pouring time of molten steel in the mold, and the elapsed time (t) from the start of drawing the slab, and
The slab length (X) is determined by the slab withdrawal length (L) obtained by integrating the formulas (1) and (2), the pouring height in the mold, and the bottom crop length. Under the condition of No. 1, the total elapsed time (T) and the slab length (X)
Are expressed as equations (3) and (4), respectively.
The denominator 1.015 in the equation (4) is a hot correction coefficient, which is a coefficient for converting a hot slab length into a cold slab length. T = 5 + 1 + t (3) X = (L + 0.3-0.6) /1.015 (4)

The relationship between the elapsed time (t) from the start of the ingot extraction in Example 1, the total elapsed time (T) calculated by the equations (3) and (4), and the ingot length (X) is shown. 2 shows.

[0043]

[Table 2]

Then, when the total elapsed time (T) when the pickup amount is 5 ppm is obtained based on FIG. 5, the total elapsed time (T) is at the position of 10 minutes. When the slab length (X) corresponding to the total elapsed time (T) of 10 minutes is calculated from Table 2,
The slab length (X) was 1.7 m. Therefore, it was determined that slabs with a slab length (X) of up to 1.7 m were inaccurate in total oxygen concentration as a steel type for ERW pipes. However, according to FIG. 2, it is determined that 70% or more of the inclusions having a diameter of 100 μm or more, which is a problem in the electric resistance welded pipe, are separated even in a cast piece within 1.7 m from the cutting position with the bottom crop. The cast slab was melted and cut by a 2 mm cold scarfer only in a range of 1.7 m from the cutting position with the bottom crop and used for an electric resistance sewing can. As a result of ultrasonic inspection of the product, the cast product had no problem. Although these operations were performed by the operator collecting each management item, all the data were automatically collected and these data were transmitted to the process computer.
You may judge with a computer.

Example 2 Using the same continuous casting equipment as in Example 1, a steel for DI can was cast under the same casting conditions.
The initial total oxygen concentration in the sample collected from the ladle is 13 ppm
The allowable amount of total oxygen concentration is shown in Table 1.
It becomes 2 ppm from the allowable value of. Immediately after the start of injection into the tundish, the degree of superheat of molten steel in the tundish is 25.
Since it was found that it was possible to set the storage time to 5 minutes from Fig. 6, it was stored in the tundish for 5 minutes, then injection into the mold was started, and the above formula (1) and Casting was started at the slab drawing speed of the formula (2). At this time, the plasma heating device could not be used due to equipment failure.

Then, when the total elapsed time (T) at which the pickup amount becomes 2 ppm is obtained based on FIG. 5, the total elapsed time (T) is at the position of 13 minutes. When the slab length (X) corresponding to the total elapsed time (T) of 13 minutes is calculated from Table 2,
The slab length (X) was 4.1 m. Therefore, it was determined that slabs with a slab length (X) of up to 4.1 m were inaccurate as the total steel concentration for DI can steel type. Further, according to FIG. 2, it is judged that only 50% of the inclusions having a diameter of 50 μm or more, which is a problem in the DI can, are float-separated in the slab up to 4.1 m from the cutting position with the bottom crop. , The relevant slab is 4.1 from the cutting position with the bottom crop.
After cutting with m and fusing 2 mm, it was removed from the DI can and diverted to a general-purpose thin steel plate. There was no problem as a result of the inspection of the corresponding slab with a thin steel sheet product, and other slabs excluding the bottom slab were shipped as DI cans without any problem as a result of the inspection, and there was no occurrence of flange cracks. .

[0047]

According to the present invention, it is possible to estimate the total oxygen concentration and the residual amount of inclusions for each length from the start of casting of the bottom slab, and which part of the slab is required for each product. Since it can be determined from the above, it is possible to prevent the quality abnormality due to the cleanliness of the bottom slab.

[Brief description of drawings]

FIG. 1 is a schematic view of a front cross-section of a continuous casting equipment for two strands having a rectangular slab to which the present invention is applied.

FIG. 2 is a diagram showing a comparison result of the inclusion separation ratio in the tundish with and without molten steel agitation.

FIG. 3 is a diagram showing a result of investigation of a separation rate of inclusions in a tundish at the time of plasma heating.

FIG. 4 is a diagram showing a result of investigation on a pickup amount of total oxygen concentration for each length from the start of casting of a bottom slab.

FIG. 5 is a diagram showing the relationship between the total oxygen concentration pickup amount of the bottom slab and the total elapsed time.

FIG. 6 is a diagram showing a result of investigating a change in molten steel temperature in a tundish with and without plasma heating.

[Explanation of symbols]

1 tundish 2 immersion nozzle 3 sliding nozzles 4 weirs 5 Plasma heating device 6 plasma torch 7 Porous brick 8 temperature measuring device 9 Consumable thermocouple 10 load cell 11 molds 12 Molten steel 13 Mold powder 14 solidification shell 15 ladle 16 long nozzles

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Noriko Kubo             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4E004 MB00 MC00

Claims (2)

[Claims] 1. In a continuous casting method of steel, in which molten steel is stored in a tundish and then poured into a mold, in the tundish, whether molten steel is heated, whether molten steel is stirred in the tundish, In addition, under the casting conditions determined by the combination of three casting factors of the amount of metal adhered in the tundish, the pickup amount of the total oxygen concentration in the cast piece at the start of casting and the separation rate of oxide-based inclusions are set in advance. Measure, estimate the total oxygen concentration of the slab and the separation rate of oxide inclusions at the start of casting under individual casting conditions based on this measurement result, and cast at the start of casting based on this estimated value. A method for continuous casting of steel, characterized in that the surface care method and operation method of the piece are determined for each product. 2. In a continuous casting method of steel in which molten steel is stored in a tundish and then poured into a mold, a temperature change of molten steel in the tundish during storage is measured in advance, Based on the measured value of the molten steel temperature immediately after injection into the tundish based on which the time for holding in the tundish is determined, whether the molten steel is heated in the tundish, whether the molten steel is stirred in the tundish,
In addition, under the casting conditions determined by the combination of three casting factors of the amount of metal adhered in the tundish, the pickup amount of the total oxygen concentration in the cast piece at the start of casting and the separation rate of oxide-based inclusions are set in advance. Measure, estimate the total oxygen concentration of the slab and the separation rate of oxide inclusions at the start of casting under individual casting conditions based on this measurement result, and cast at the start of casting based on this estimated value. A method for continuous casting of steel, characterized in that the surface care method and operation method of the piece are determined for each product.