KR840001144B1 - Process for continuous casting of a slightly deoxidized steel slab - Google Patents

Abstract

Continuous casting of slightly deoxidized steel is described using continuous casting powder and an immersion nozzle. The mold, having two long and two short sides, is subjected to electromagnetic forces to induce a horizontal flow speed of 0.1-1.0m/sec. in the solid-liquid interfacde within the mold. This method allows the amount of deoxidant used to be reduced without rimming or surface defects becoming a problem.

Description

Continuous casting process of slightly deoxidized steel slab

1 is a graph showing a mechanism for generating bubbles;

2 is a graph of the distribution of concentrations of the components contained in the steel,

3 is an explanatory view of a strand to be cast, representing the principles of the invention,

4 is a graph similar to FIG.

5 is a flow rate distribution graph,

6 is a plan view of a continuous casting mold,

7 is a cross-sectional view taken along the line VII-VII of FIG.

8 is a plan view of a four corner slab mold showing the rotational flow of molten steel;

9 is a plan view of a slab mold having a short side of a curve,

10 is a graph of the distribution of flow velocity according to the distance X from one of the short sides of the mold,

11a, b, c show partial plan views of molds that can be used in the present invention,

12a, b are a plan view and a vertical sectional view of the mold, respectively showing the flow of molten steel moving to the center of the mold,

13A and 13B are views similar to those of FIGS. 12A and 12B, respectively, and are similar to FIGS.

14 is a graph of the solidification layer thickness,

15 is a partial cross-sectional view of the mold,

16 and 17 are partial cross-sectional views of immersion nozzles and molds with horizontal outlets,

18 is a partial cross-sectional view of an immersion nozzle and mold with a downward outlet,

19 is a plan view of the mold;

20 is similar to 19,

21 is a schematic explanatory diagram of the macroscopic structure of the strands.

The present invention relates to a process for producing so-called slightly deoxidized steel which is very similar to the limbed or semi-kilted steel by continuous casting.

For many years, continuous casting has attempted to produce steels corresponding to the Limd and Semikilled rivers. However, due to problems in continuous casting and steel materials, particularly defects such as pores on the slab surface, these continuous casting steels have not been produced so far. Phenomena such as the rimming action occurring in the conventional ingot casting have production difficulties such as melt leakage in powder casting, which is adopted in most modern continuous casting processes.

Therefore, deoxidation is controlled so that reaming does not occur before continuous casting.

However, when the amount of free oxygen in the deoxidation-controlled molten steel is more than approximately 50 to 70 ppm at the solidification temperature from 1,520 ° C to 1,550 ° C, pores are formed on the surface of the strand. These pores are exposed to ambient air before rolling, leaving surface scratches on the rolled products as the inner surfaces of the pores are oxidized by the ambient air. The oxygen concentration mentioned above uses zirconium dioxide (ZrO ₂ ) stabilized by calcium oxide (CaO) as a solid electrolyte, and a mixture of chromium and chromium oxide (Cr ₂ O ₃ ) as a standard electrode and relative to iron (Fe). It can be measured by the oxygen concentration battery used as an electrode.

In the current continuous casting technology of steel, which is equivalent to the rim steel or the semidkill steel, the steel is excessively deoxidized by deoxidizer or vacuum degassing to prevent reaming. As a result, the high production rate of continuous casting is fully utilized.

Many reports, on the other hand, draw attention to the fact that surface defects, such as the pores of a strand, are caused by insufficient reaming to remove the pores.

In these reports, processes are proposed to assist in the reaming of slightly deoxidized or undecided steels. In the proposed processes the molten steel is subjected to electromagnetic stirring forces in the mold.

These processes are circulated by a horizontal or vertical process in which the molten steel in the mold is circulated by an electromagnetic stirring device installed in the mold, and by an electromagnetic stirring device in which the molten steel is installed under the mold. The process is divided into agitating processes to generate it. The prior art of the process of installing an electromagnetic whisk in a mold is disclosed in Japanese Patent Laid-Open Publication No. 51-2621 and Japanese Patent Publication No. 53-34164 The prior art of the process of installing an electromagnetic device under a mold is shown in JA-OS number 49-126523 and JA-OS number 50-68915.

When molten steel in the mold is subjected to electromagnetic stirring to help ream, the following inconveniences occur in actual work. The flow rate of the molten steel required to float the bubbles is relatively high since the bubbles generated are moved upwards and are removed by the effect of reaming in the process mentioned above.

It should be noted here that the flow rate depends on the oxygen concentration in the molten steel.

In practice, a flow rate of approximately 3.0 m / sec is required because the oxygen level of the castable deoxidation is not finished or is slightly lower than the oxygen level required for significant reaming action. However, when bubbles move up or are removed by high flow rates, disturbances occur on the molten steel surface due to the strong swiveling motion of the molten steel in the mold.

Continuous casting powder, which should be on the surface of molten steel, has the purpose of lubricating between the mold and strand, preventing the temperature of molten steel, reoxidation of molten steel and absorbing the inclusions in molten steel. The disturbance of the molten steel surface this time causes disturbance of the continuous casting powder on the molten steel surface, and as a result, the essential function of the powder does not appear, and problems such as powder immersion and melt leakage occur.

Since continuous casting powder on the molten steel surface in the mold is indispensable in the continuous casting currently performed, it is essential to prevent disturbance of the molten steel surface.

Therefore, since the turbulence of the molten steel surface is unavoidable, the method of removing air bubbles by reaming is impractical in current continuous casting based on powder casting.

As for the flotation method, considerable disturbance on the molten steel surface does not occur in the case of horizontal rotational flow, but the molten steel's stirring motion to remove the floating and growing bubbles should be performed at extremely high flow rate, and the stirring flow causes the powder to be melted on the molten steel surface. Note that it rotates at.

As a result, the continuous casting powder gradually collects in the center of the mold and finally there is no continuous casting powder on the surface between the molten steel and the brick of the mold.

As a result, melt leakage finally occurs because the powder cannot flow between the mold and the shell of the solidified steel to act as the necessary lubricant.

The concept involved in the process described in JA-OS number 51-2621, mentioned above, imposes a high flow of powder immersion on the entire molten steel in the mold.

Since the current continuous casting is based on the powder casting described above, the casting operation itself is difficult because it must give the same stirring force as the reaming action of the molten steel in the mold. Thus, such a stirring process cannot be practically adopted. In the process disclosed in Belgian Patent No. 864218, a linear motor is installed on both long sides of the slab mold so that the driving force of the linear motor is directed in opposite directions to generate horizontal rotational flow in the center of the mold. . The purpose of this published process is to separate and remove the contents from molten steel by centrifugal force and not to prevent liquid surface disturbances from occurring.

In the process disclosed in this Belgian patent, the molten steel flows in the center of the mold, which also adversely disturbs the continuous casting powder on the molten steel surface.

As a result, normal powder casting on which current continuous casting is based is not performed in this published process.

Conventional techniques for the continuous casting of steels with high oxygen content using electromagnetic stirring forces are shown in JA-OS number 51-122625 in addition to the previous techniques mentioned above. However, no means of preventing powder disturbances are mentioned in JA-OS number 51-122625.

The prior art of electromagnetic stirring apparatus installed in various types of molds

Is shown.

The prior art mentioned above basically represents a means of preventing inclusions or slags from immersing into the solidified shell of the strands and does not represent a specific means of preventing the disturbance of the continuous casting powder.

It is therefore an object of the present invention to eliminate the disadvantages of known processes for producing deoxidized or slightly deoxidized steel by continuous casting and to ensure that the steel does not have reaming action on the molten steel in the mold and the surface is free of surface defects. It is possible to make a continuous casting box to obtain the advantages of producing the above-mentioned steel by continuous casting while at the same time reducing the amount of deoxidizer used to produce a unit of steel.

Another object of the present invention is to make it possible to continuously cast steel which does not end deoxidation or is slightly deoxidized without removing the advantages usually gained by powder casting which are now commonly done.

It is an object of the present invention to prevent disturbances on the solidified surface by a stream ejected from the outlet of an appropriately adopted immersion nozzle as described below.

According to the present invention, there is provided a process of continuously casting a slab of slightly deoxidized steel, in which a continuous casting powder and a nozzle submerged in a molten steel in a mold are used, and this process molds molten steel having a free oxygen concentration in the range of 50 to 20 ppm. Casting, and the inner surface of both short sides of the mold is concave when viewed from the horizontal section of the short side, and the device is installed to generate electromagnetic force on both long sides of the mold and the outlet of the immersion nozzle. The propulsive forces of the device generating the force are directed along the long side in opposite directions, having essentially constant flow rates and rotating horizontally around the solidification surface and its vicinity, from the position of the molten steel surface in the mold to the predetermined vertical position of the solidification surface. Energizes a device that generates a flow that forms near it, from 0.1 to 1.0 It is characterized by providing horizontal flow with a flow rate in the m / sec range.

The invention is described in detail with reference to the drawings.

The inventors first examined in detail the factors related to the generation of bubbles on the strand surface during the solidification of low deoxidation molten steel.

Referring to FIG. 1, the process of forming bubbles during solidification is divided into a stage in which a nucleation of bubbles is generated and a growth phase in which nuclei grow into bubbles.

)의 분압 P_CO이 응고면에서의 핵생성에 필요하지만 이미 발생한 기포내에서는 대략 1atm이나 그 이상(

)의 분압 P_CO이면 기포가 성장하기에 충분하다.As shown in Figure 1, approximately 2-3 atm (

), Partial pressure P _CO is required for nucleation on the solidified surface, but in bubbles already generated, approximately 1 atm or more (

A partial pressure P _CO of) is sufficient for bubbles to grow.

This means that nuclei of bubbles are difficult to generate, but nuclei that have already occurred easily grow into bubbles. Bubble growth ends when the ferrostatic pressure P _Fe of the molten steel applied to the bubble exceeds the partial pressure P _CO . The nuclei of bubbles which will later grow into bubbles are mainly influenced by the concentration of carbon and oxygen in the molten steel.

As can be appreciated in FIG. 2, the component elements in the molten steel are concentrated between the solidification surface, that is, between the solid and liquid, during the solidification process.

In FIG. 2, C _i represents the component concentration at the solidified surface, C _s represents the component concentration in the solid state, and C _e represents the component concentration in the liquid state.

The critical concentration required for the generation of bubble nuclei is indicated by C _x in FIG. 2.

Even if the component concentration C _e of the liquid does not reach the concentration C _x , the concentration C _i can exceed C _x by the above-mentioned enrichment phenomenon, and the nucleus thus generated is subsequently bubbled and the surface portion of the strand 4 Are exposed from.

This fact follows that when the formation of bubble-grown nuclei begins to solidify the molten steel, that is to say, on the molten steel surface 1 in the mold 7, and to suppress the generation of bubble nuclei, The component concentration at the coagulation surface should be adjusted to be below the nucleation critical concentration C _x of the bubble.

The research results by the present inventors are as follows.

A. Bubble nuclei are less likely to occur than their growth, and bubble nuclei require more than a certain concentration of components.

B. The nucleus of the bubble already occurs at the point at which solidification begins, i.

C. The concentrations of components such as carbon and oxygen are relatively high in terms of coagulation.

In the present invention, A and B provide a method of reducing the concentration of components at the solidified surface near the molten steel surface in traveling to a level lower than the critical concentration of bubble nucleation without disturbing the molten steel surface 1 in the present invention. Notice the facts of C.

In the present invention, the rotational flow of the thin film-like molten steel is formed as shown by the hatched side of the solidified surface shown by a thick line like frame in FIG.

The rotational flow, hereafter indicated by reference numeral 3, is essentially formed around the entire periphery of the solidification surface 2 and is limited to only the periphery of the periphery of the solidification surface 2, ie does not extend into the interior of the molten steel, It is formed near the molten steel surface 1.

Due to the thin film-like rotary flow (3) of molten steel, the produced strand (4) has a flawless solidification layer (5) at its surface portion, and the concentration of components in the solidification layer (5) is higher than the threshold for bubble nucleation. small. Because molten steel around the solidification surface 2 near the molten steel surface 1 rotates while casting, the concentration phenomenon of the components in the molten steel can be suppressed, and a given flow of molten steel solidifies in the form of a thin film. Since it is formed only around the surface 2, the disturbance of the molten steel surface 1 and the continuous casting powder on the molten steel surface does not occur due to the flow of the molten steel.

4 also high, but the more the molten steel level C _i is generated in the cell nuclei critical concentration C _x of when the reference stop state (broken line) the concentration of the solidification surface a thin film receiving the same flow as the value of the smaller C _i C _x Decreases.

According to the present invention, the deoxidation incomplete or slightly deoxidized steel having the defect-free solidified layer 5 having no pores on the strand surface is a steel in the range of deoxidation degree as follows.

Below the minimum point of oxygen concentration, pores containing pin holes are generated on the surface of the cast strand 4 without giving any flow to the molten steel. This minimum point depends on operating conditions such as non-oxygen components, molten steel temperature and casting speed, and in most operating conditions it corresponds to an oxygen concentration of approximately 50 to 60 ppm at solidification temperatures ranging from 1,520 ° C to 1,530 ° C. At the peak of the oxygen concentration, casting can be inconveniently adjusted. When the oxygen concentration is too high, the reaming action occurs in the mold, so that the significant disturbance of the molten steel surface 1 thus generated not only prevents normal powder casting but also severely disables the casting operation itself.

The minimum oxygen concentration for reaming is approximately 200 ppm. The deoxidation which is not finished or slightly deoxidized here refers to a steel having an oxygen concentration not less than 50 ppm and not higher than 200 ppm, expressed in free oxygen concentration. Oxygen concentration mentioned above uses zirconium dioxide (ZrO ₂ ) stabilized by calcium oxide (CaO) as a solid electrolyte, chromium (Cr) and chromium dioxide (CrO ₂ ) as standard electrodes, and iron (Fe) as counter electrodes It is measured by an oxygen concentration battery.

When the steel to be cast has an oxygen content higher than the maximum value of 200 ppm, the steel must be subjected to deoxidation by deoxidizers such as Al, Si, and Ca or carbon deoxidation in vacuum degassing and the oxygen concentration is lower than the maximum point. After the adjustment, the casting process of the present invention is received.

In the present invention, the entire periphery of the solidification surface near the molten steel surface 1 in the mold 7 is subjected to the rotational flow 3 of the molten steel having a flow rate as described later.

In the present invention, the entire periphery of the solidification surface near the molten steel surface 1 in the mold 7 is subjected to the rotational flow 3 of the molten steel having a flow rate as described later.

The flow rate of molten steel necessary for the suppression of bubble nuclei only causes the concentration of components at the coagulation surface to decrease to less than the concentration of components necessary for nucleation.

Therefore, the flow rate in the present invention is considerably slower than the flow rate for removing bubbles in the conventional process.

The maximum flow rate is approximately 1.0 m / sec.

In other words, even at speeds slower than approximately 1.0 m / sec, the concentration of components at the surface of coagulation can be reduced to levels lower than those required for nucleation.

If the flow rate exceeds approximately 1.0 m / sec, the flow movement, which is a rotational movement around the solidification surface in the mold 7, causes disturbance of the continuous casting powder on the molten steel surface 1 or the molten steel surface 1. Therefore, the maximum flow rate is adjusted so as not to disturb the molten steel surface 1 and the continuous casting powder 14.

The minimum flow rate is in the range of 0.1 to 0.4 m / sec.

Below the minimum flow rate, the desired effect of concentration reduction cannot be achieved.

The flow rate according to the invention is slow and in the range of 0.1 to 1.0 m / sec, in particular in the range of 0.5 to 0.8 m / sec.

The flow rate within this range should be constant over the entire coagulation surface. Although the flow rate of the present invention may partially overlap with the conventional flow rates for removing bubbles, the present invention is directed to a slow flow rate and there is a difference between the present invention and the conventional process of flowing molten steel in the mechanism for suppressing bubbles. Because. The concept of the present invention lies in the fact that the generation of nuclei is inhibited in the pre-growth stage of bubble nuclei, while in conventional processes the bubbles already grown are moved up and then removed.

The depth of the rotary flow 3 is now described. This depth is related to the thickness of the defect free surface coagulation layer of the strand without pores.

Theoretically, when a minimal, flawless solidification layer 5 is in the surface layer of the strand 4, the pores in the surface layer are pressed firmly during the subsequent rolling step, which does not pose a problem for the practical use of the rolled product.

In practice, however, a significant amount of scale occurs, for example, during casting or in a furnace until rolling, so that if scale is not taken into consideration, the pores will be exposed to the strand surface.

Since the amount of scale is approximately 0.7 to 5 mm, the rotational flow 3 realized around the entire solidification surface extends from the molten steel surface at which solidification begins to the length at which the solidification surface equal to the amount of scale is formed. The rotational flow thus given is formed around the entire solidification surface above the liquid in the mold, and this rotational flow 3 has the form of a strip whose width is perpendicular to the liquid. The location of the solidification layer, having a thickness in the range of 0.7 to 5 mm, depends on the casting speed but is approximately 50 to 200 mm below the molten steel surface when casting conditions are normally employed.

As for the thickness of the rotating flow mentioned above, the smaller the thickness is, the better it is possible to save energy and to minimize the effect of the rotating flow 3 on the molten steel surface 1 to a minimum. The thickness of the rotary flow 3 is now described.

Referring to FIG. 5, the flow in the mold has a velocity distribution with distance from the mold wall.

This distribution depends, for example, on the propulsion force given by the flowimparting device described below and the copper plate thickness of the mold.

When these conditions are properly adjusted, the flow velocity as high as 1.0 m / sec at the surface of the mold wall, as illustrated in FIG. 5, is less than half of 1.0 m / sec at the position in the mold 10 to 20 mm away from the surface of the mold wall. Can be reduced.

Thus the flow adjacent to the mold wall with a thickness of 10 to 20 mm is an important part of the flow that participates in the suppression of bubble nuclei and the flow more than 20 mm from the mold wall has little effect on the behavior of the molten steel surface. In view of the suppression function of the bubble nucleus, only the flow of molten steel having a thickness of 10 to 20 mm participates in this function.

In the present invention, there is provided a specific device for applying rotational flow to the molten steel in the entire periphery of the solidification surface in the upper part of the liquid in the mold.

Such a device is preferable for generating an electromagnetic force, and in particular, a linear motor 8, 8 'that forms a thin film-like rotary flow 3 from the viewpoint of economy and stability.

In the embodiment of the device for applying rotational flow shown in FIGS. 6 and 7, the linear motors 8, 8 ′ are located in the cooling box of both long sides 9 of the mold 7.

The driving forces of the linear motors 8, 8 ′ are directed in opposite directions a, b to provide the rotational flow 3.

The installation position in the vertical direction of the linear motors 8, 8 'is shown in FIG.

The linear motors 8, 8 ′ installed in the position shown in FIG. 7 cause the solidified surface 2 to receive the rotary flow 3 in the form of a strip.

The area of the solidification surface 2 which receives the rotating flow 3 in the form of a strip is the solidification surface whose thickness of the solidification layer is larger than the scale amount of, for example, about 0.7 to 2.0 mm from the point where solidification starts at the molten steel surface 1 It extends to the position of (2).

The thickness of the rotary flow 3 with the predetermined flow rate described above is approximately 10-20 mm.

Theoretically, the entire area of the solidification surface can be subjected to electromagnetic flow as described above, but this is particularly difficult when the product to be cast is a slab with a rectangular cross section. In the case of blooms or billets having a circular or rectangular cross-sectional shape, a smooth electromagnetic flow is obtained relatively easily due to the constant distance from the wall of the mold to the center. In the case of slabs, on the other hand, there is a significant difference between the length of the long side and the short side of the mold, and the electromagnetic flow is not uniform because the distance from the wall to the center of the mold is not constant. Moreover, because the flow rate itself given to the molten steel is slow according to the present invention, the flow of molten steel may be disturbed at the four corners of the slab mold due to the stagnation of the electromagnetic flow and consequently as intended later on with respect to FIG. Goals cannot be achieved.

In FIG. 8, the linear motors 8, 8 'are installed along the direction of the long sides 9 of the slab mold 7 to generate propulsion forces having opposite directions a and b.

Congestion may occur at the four corners C ₁ to C ₄ of the slab mold 7.

In the present invention, the predetermined position of the solidified layer is stably subjected to electromagnetic flow having a predetermined velocity without causing disturbance of the molten steel surface 1 and congestion in the slab mold.

These conditions of electromagnetic flow were investigated and established as follows.

First, the inventors have conducted a detailed study of the shape of the short side of the slab mold having a form in which electromagnetic flow is made without stagnation.

In FIG. 9 the short side 10 of the mold 7 is concave outwardly and inwardly with a radius of curvature R corresponding to half of the effective length curvature radius R of the short side 10 of the mold 7. It has a convex form.

In the case of using the mold 7 shown in FIG. 9, the flow of molten steel in the mold 7 has a shape 5 as shown schematically in FIG. 9. When the short side 10 has a concave shape as shown in FIG. 9, the flow generated on one of the long sides 9 has a short side 10 having a radius of curvature R as shown in FIG. 9. It can be satisfactorily transmitted to one of the) and also to the opposite long side 9 so that a continuous horizontal rotating flow 3 with no congestion can be formed in the mold 7.

The concave formation of the inner surfaces of the short sides 10 of the mold 7 is known by itself from JA-OS number 52-117234 in the continuous casting of the steel.

However, JA-OS number 52-117234 has an arch structure in the solidified shell of the short side 10 of the slab to prevent the short side 10 of the slab from swelling during the casting process. have.

Thus JA-OS number 52-117234 does not propose a continuous horizontal rotating flow of molten steel according to the invention.

Although related to nonferrous metals, the concave shape of the inner surface of the short side 10 of the continuous casting mold is also known from US Pat. No. 2781562.

However, the purpose of this patent is to reduce the coagulation gap between the ingot and the mold and also does not propose the above-mentioned rotary flow.

Secondly, the inventors studied the type of flow obtained by using the short sides of the mold with various radius of curvature R.

As a result of this study, it was proved that a flow form equivalent to the shape shown in FIG. 9 can be obtained at a radius of curvature R ranging from half to twice the effective thickness d of the short sides 10 of the strand.

FIG. 10 shows the relationship between flow rate and distance X in the case of using various types of short sides.

The distance X represents the distance from one of the short sides to the center of the long sides in the direction of the long side at the center of the mold thickness or half the thickness d of the short side of the strand or mold.

As shown in FIG. 10, when the radius of curvature R of the mold is double from half the strand thickness (solid and dashed lines), the relatively slow flow is formed on the short side of the mold and the flow rate suddenly slows down when the flow leaves the short side. .

In contrast, when the radius of curvature R is three or more times the strand thickness d (chain line), the flow rate is relatively less than half the speed of the two curves mentioned above, and furthermore, the long side of the mold There is almost no change in velocity distribution in the central direction.

This fact indicates that electromagnetic flow is efficiently obtained at the solidification plane at the two curvature radii of the former, while in the latter curvature radius, the horizontal rotational flow disperses into the mold without affecting the solidification plane at the short side.

In the latter case, the flow dispersion can cause disturbances or stagnations on the molten steel surface, so that the entire periphery of the solidified surface cannot receive the desired electromagnetic flow.

3d인 경우, 수평회전 유동의 이러한 경향은 직선의 짧은 측면들을 가진 종래의 주형들에서는 일반적인 것이다.R

In 3d, this tendency of horizontal rotational flow is common in conventional molds with short sides of a straight line.

The disturbance of the molten steel surface mentioned above in turn leads to an inhomogeneous distribution of the continuous casting powder on the molten steel surface and the immersion of this powder into the molten steel, while the disruption of electromagnetic flow due to stagnation leads to the formation of bubbles.

As can be understood from the above description, in order for the predetermined area of the solidification surface to receive electromagnetic flow, the shape or shape of the short side is such that the radius of curvature R of the short side is doubled from half the strand thickness d. It should be chosen to be in the range up to and more preferably in the range of half to one time.

The forms of the short sides of the mold as shown in Figures 11a, b, c are embodiments of continuous casting molds.

This concave polygonal shape can be practically employed to provide a relatively effective electromagnetic flow but is not as ideal as short sides with a radius of curvature ranging from half to twice the strand thickness.

In summary, as can be understood in FIG. 8, stagnation is caused by flow disturbances caused by the flow advancing from the long side to the short side and hitting the short side. As a result, when the short side of the mold has a concave shape, including a concave polygonal shape, the entire solidification surface can receive electromagnetic flow, so that the flow is smoothly induced from the long side to the short side.

Third, the inventors conducted experiments on the installation positions of the linear motors 8, 8 'provided on both long sides of the mold in order to stably obtain the thickness of the defect free solidification layer 5 larger than the predetermined thickness. It was.

The location of the linear motor is important because it is difficult to obtain the above-mentioned solidified layer thickness for the slab by means of the above mentioned means, which makes it possible to provide an electromagnetic flow without any congestion.

In an experiment conducted by the inventors, the linear motor was installed at various depths from a position corresponding to the molten steel surface 1 in the mold.

In one experiment, the linear motors 8, 8 ′ are installed substantially below the molten steel surface 1 so that the predetermined position of the molten steel in the mold 7, ie the molten steel surface 1, is shown in FIGS. 12a, b. Subject to this electromagnetic flow.

Moreover, the portion of the coagulation surface which extends from the liquid surface down to the position where a coagulation layer having a thickness of scale is formed is also subjected to electromagnetic flow.

Moreover, the short side of the mold had a radius of curvature R ranging from half to twice the strand thickness d, the optimum radius for receiving electromagnetic flow without stagnation.

The flow rate was slow from 0.1 to 1.0 m / sec.

Although the form of the short side 10 of the mold 7 was ideal and the flow velocity was slow, the obtained force of the electromagnetic flow 13 is not directed in the horizontal direction of rotation but on the sides of the mold at both short sides 10 of the mold. Was oriented in the direction of.

Due to the direction of electromagnetic flow mentioned above, the continuous casting powder 14 on the molten steel surface 1 gathers toward the center of the molten steel surface 1 and consequently any powder on the molten steel surface 1 on both short sides 10. Also does not appear.

Therefore, normal powder casting cannot be performed and accidents such as melt leakage are caused by electromagnetic flow as described above.

Concentration of the continuous casting powder 14 is caused by the component 15 facing upward, as illustrated in FIG. 12B, which shows a vertical cross-sectional view of the mold.

In FIG. 12B the linear motors 8, 8 ′ are constructed such that the components of the electromagnetic flow in the form of lamination along the long depth side from the molten steel surface 1 are not satisfactorily transmitted to the molten steel surface 1. Installed in depth.

When these components of this electromagnetic flow impinge on the short side 10, they are divided into an upward facing component 15 and a downward facing component 16.

The above-mentioned phenomena appear when the upward facing component 15 is stronger than the horizontal electromagnetic flow 3.

As can be understood from this description, when the linear motor 8,8 'is installed below a certain position, the horizontal electromagnetic flow 3 tends to be weak, while later from the short side 10 to the center of the mold. The component 15 facing upwards of the advancing flow tends to be strong and is most influential. Therefore, in the present invention, the linear motors 8 and 8 'are installed as close as possible to the molten steel surface 1, and the mounting position of the linear motors 8 and 8' corresponds to the thickness of the scale mentioned above. The defect-free solidified layer 5 which has a is provided in the range provided.

According to a study conducted by the present inventors, the phenomenon shown in Figs. 12A and 12B is noticeable when the installation of the linear motors 8 and 8 'is 150 mm or more from the molten steel surface 1, and the molten steel surface 1 Especially when it is 200 mm or more. Therefore, the linear motors 8, 8 'are preferably installed at a distance of less than 200 mm, in particular less than 150 mm, from the molten steel surface 1. The value of this distance can be applied to the installation of the linear motors 8, 8 ′ in almost all molds 7 and does not depend on the dimensions of the molds.

In the arrangement shown in FIG. 13a the short side 10 of the mold 7 has a radius of curvature R of half to twice the thickness d of the strand 4 as mentioned above and the arrangement shown in FIG. 13b. In this case, linear motors 8, 8 'were installed below the molten steel surface 1 in the range of 150 to 200 mm, ie 150 mm.

The flow pattern obtained by the mold 7 and the linear motors 8 and 8 'shown in Figs. 13A and 13B is shown in Fig. 13A. As can be seen in FIG. 13a, stagnant electromagnetic flow that does not cause disturbance at a predetermined position on the molten steel surface 1 is ideal for shorter side shapes and optimal installation positions for the linear motors 8, 8 '. If neither is equipped, it will not be obtained. The horizontal electromagnetic flow 3 which will later hit the short side 10 is sufficiently strong to obtain an electromagnetic flow 3 which is continuously distributed only at the solidification surface without disturbing the powder of the molten steel surface 1. The form of electromagnetic flow in the figure does not cause disturbance of the continuous casting powder 14 of the molten steel surface 1.

Moreover, the downward facing portion 16 of the electromagnetic flow is at a position below the solidification surface portion, which is the portion to receive the electromagnetic flow. Thus, this downward component 16 does not have an undesirable effect on the formation of the defect free solidification layer 5.

When the linear motors 8, 8 ′ are installed at approximately 100 mm below the molten steel surface 1, this installation position does not always cause a serious accident such as melt leakage in the case of a short side of a conventional flat form. .

However, it can not be said that the dangers of these accidents clearly do not exist, because when the electromagnetic flow hits the flat short side, the flat shape causes the liquid portion to expand slightly on the short side 10, resulting in the continuous casting powder 14. This is because the amount supplied is too small in the expanded portion of the liquid.

As described in detail above, the shape and linear morphology of the short side 10 of the mold 7 are such that disturbances or stagnation of the molten steel surface 1 do not occur and the predetermined part of the solidification surface is subjected to continuous electromagnetic flow. Both installation positions of the rotor 8, 8 'must satisfy the conditions according to the present invention.

In an ideal embodiment of the invention the linear motors 8, 8 ′ are installed in a cooling box in the mold in such a way that the center of the core is located at the level of the molten steel surface 1. In addition, the liquid portion affected by the laminar flow caused by the linear motors 8, 8 'extends from the molten steel surface 1 to a depth of 200 mm, and the flow rate of the electromagnetic flow 3 in this region. The continuous casting operation is performed in a manner in the range of 0.1 mm / sec to 1.0 mm / sec.

In fact, the linear motors 8,8 'in the ideal positions mentioned above can present installation difficulties.

There is also a disadvantage in the flow rate in the range of 0.1 to 1.0 m / sec at a depth of 200 mm from the molten steel surface 1.

The center of the core of the linear motor 8, 8 ′ is thus located approximately 200 mm below the molten steel surface 1 so that the linear motor 8, 8 ') is practically suitable to install. In such an installation, the surface between the liquid portion and the solidification layer with a thickness of up to 5 mm produces a continuous electromagnetic flow with a flow rate in the range of 0.1 to 1.0 m / sec without causing disturbance or stagnation of the molten steel surface 1. Receive effectively.

The number of linear motors 8,8 'arranged in the vertical direction is one because the main purpose of the present invention is to obtain a solidified solidified layer 5 in the required minimum area of the solidified surface 2 as described above. I can be more than that.

One important aspect of the present invention is now described.

Referring to FIG. 14, the solidification process at the solidification surface 2 which is subjected to electromagnetic flow of molten steel is delayed compared to the solidification process at the conventional process (solid line) which is not subjected to electromagnetic flow (indicated by a dashed line). ). In order to obtain the defect free solidification layer 5 with the required thickness, it is necessary to deepen the position of the electromagnetic flow compared with the position in the conventional method.

In this regard, the relationship between the electromagnetic flow and the flow of molten steel ejected from the immersion nozzle 11 used to inject molten steel into the mold is important in practical terms.

Since the process of the present invention is based on powder casting, the exposed casting flow cannot be adopted for molten steel injection in the present invention. When so-called open pouring is used, disturbance of the continuous casting powder 14 occurs during the injection of molten steel into the mold 7.

Therefore, the immersion nozzle 11 submerged in the molten steel in the mold 7, that is, the injection nozzle of the immersion type, is indispensable in the process of the present invention.

Referring to FIG. 9, linear motors 8, 8 'are installed in the cooling boxes of the slab mold 7 along the long side. The solidification surface whose thickness of the solidification layer is equal to or smaller than the thickness of the scale is subjected to electromagnetic flow generated by the driving force of the linear motors 8 and 8 'in the directions a and b opposite to each other.

According to the present invention electromagnetic flow is generated in such a way that the flow ejected from the immersion nozzle 11 to inject molten steel into the mold 7 does not interfere with the electromagnetic flow that the solidified surface 2 mentioned above receives. do.

Considering conventional immersion nozzles, the exit of such nozzles is in most cases consistent with the location that forms the electromagnetic flow. If such a conventional immersion nozzle is used in the present invention, slow electromagnetic flow is hindered by the flow ejected from the immersion nozzle 11 so as not to disturb the molten steel surface 1 in the mold 7. In other words, electromagnetic flow is only partially formed due to the influence of flow. In this regard, in the region of the solidification surface where the thickness of the solidification layer is less than or equal to the thickness of the scale, the solidification process is delayed due to the electromagnetic flow that the solidification surface 2 receives, as shown in FIG. It should be noted that are shown castings with no electromagnetic flow and castings with electromagnetic flow, respectively.

Thus, as mentioned above, if there is an influence of the erupted flow, the suppression of bubble nuclei and the formation of pseudo-rimmed layers are only partially achieved.

This is the result of the stagnant part of the electromagnetic flow. Because of this stagnation, it is impossible to form a thin film-like electromagnetic flow that must be continuous along the solidification surface 2 in order to form a flawless solidification layer 5 around the entire surface of the strand 4.

Although the degree of discontinuity varies with the direction of flow, adversely discontinuous electromagnetic flows are formed during casting, whether the flow of molten steel is directed toward the short side or the long side of the mold 7.

However, the high velocity of electromagnetic flow may seem to eliminate the above mentioned disadvantages.

However, when the flow rate of the electromagnetic flow is adjusted to exclude the effects of the above-mentioned jetted flow, stagnation disappears but disturbance of the molten steel surface 1 is caused.

This is because the impact force of the electromagnetic flow hitting the wall is large because the flow velocity of the entire flow is fast.

As a result, the components 15 facing upwards of the flow are strong and stronger than the horizontal electromagnetic flow.

Due to the component 15 facing upwards, when disturbances are caused to the molten steel surface 1, the advantages of the powder casting method are lost.

According to the present invention, the immersion nozzle 11 may be provided by having an immersion nozzle 11 having a relatively long length, directing the ejection direction of the immersion nozzle 11 downward, or by combining the ejection direction of the relatively long nozzle and the lower side. The ejection position of is placed deep in the liquid part. As a result of this adjustment method, the above-mentioned electromagnetic flow is formed between the molten steel surface 1 and the flow ejected from the immersion nozzle 11, so that the flow ejected from the immersion nozzle 11 has a predetermined thickness. It does not interfere with the electromagnetic flow required to form the defect free solidification layer 5 having.

A continuous casting apparatus according to an embodiment of the present invention is shown in FIG. 15 in the form of section B-B in FIG.

As shown in FIG. 15, the linear motors 8, 8 ′ are installed in a portion of the mold 7 that corresponds to the height between the molten steel surface 1 in the mold 7 and the ejected stream 17. .

The output of the linear motor 8, 8 ′ allows the region of the solidification surface to undergo continuous electromagnetic flow, to form a solidified layer free of pores, and the region of the solidification surface mentioned above solidifies from the molten steel surface 1. The thickness of the layer is adjusted to extend to the location of the solidification surface, such as the thickness of the scale.

The position of the flow 17 ejected from the immersion nozzle 11 is below the electromagnetic flow.

In addition to the mounting positions of the linear motors 8, 8 'as shown in FIG. 15, the linear motors 8, 8' can be installed at positions as shown in FIG. 16 and FIG. .

In FIG. 16, the linear motors 8, 8 'are installed at the height of the molten steel surface 1, and in FIG. 17, the linear motors 8, 8' are immersion nozzles 11 from the molten steel surface 1; It is installed on the mold area extending to the position of the flow (17) sprayed from the).

In FIG. 16 and FIG. 17, the installation positions of the linear motors 8, 8 'extend from the molten steel surface 1 to the flow 17 ejected to obtain a solid free layer of a predetermined thickness in a predetermined area of the solidified surface. Can be. When the position of the ejected flow 17 is 300 mm below the molten steel surface 1, the installation position of the linear motors 8 and 8 'as shown in FIG. 15 is 10 to 20 below the molten steel surface 1. May be mm.

In this installation of the linear motors 8, 8 ′ a 2-4 mm thick defect free layer 5 having a thickness larger than the thickness of the scale is protected and ejected by the effect of this electromagnetic flow. ) Does not affect the growth of the coagulation layer.

In carrying out the process of the invention, it is important that it is in the direction of the flow 17 ejected from the immersion nozzle 11.

In this regard, it is possible to use the following methods adopted for the technique of continuous casting.

According to these methods the upward flow occurs as low as possible from the flow 17 ejected from the immersion nozzle 11, after which it hits the wall of the mold 7 and the outlet of the immersion nozzle 11 The horizontal rotational flow formed on the ejected flow 17 is provided at an angle that does not disturb.

As a result, the flow 17 ejected from the immersion nozzle 11 can be brought close to the molten steel surface.

The above-mentioned jetted stream 17 may be horizontal as shown in FIGS. 15, 16, 17 degrees or may be directed downward as shown in FIG.

Particularly preferred is the ejected stream 17 of FIG. 18.

As mentioned above, although the concentration of free oxygen is in the range of 50 to 200 ppm, it is advisable to adjust the concentration of free oxygen in the molten steel to be in the range of 50 to 150 ppm.

When the concentration of free oxygen is in the range of 50 to 150 ppm, the generation of bubble nuclei can be reliably suppressed by the electromagnetic flow of molten steel having a flow rate of 0.1 to 1.0 m / sec, in particular 0.5 to 0.8 m / sec.

The solidification surface which is to receive the horizontal electromagnetic flow of the molten steel extends from the molten steel surface 1 to the part of the solidification surface whose thickness of the solidification layer formed in molten steel is at least 5 mm.

In order for this solidified surface to be subjected to horizontal electromagnetic flow with a substantially constant flow rate in the range from 0.1 to 1.0 m / sec, in particular from 0.5 to 0.8 m / sec, the following operating conditions of the linear motor 8,8 'are used. Can be.

The frequency of the current through the coils of the linear motors (8,8 ') is 1 to 10 kHz, and the value of the current in amper turns is multiplied by the number of turns of the coil to 1,500. At 7,000AT.

In determining this condition, the copper sheet thickness of the long side of the mold 7 was considered to be as small as possible, for example, 5 to 12 mm.

At the required position in the mold 7, ie at the solidification surface, the flow rate falls within the range of the above mentioned flow rates, while the flow rate at the inner position within the required position provides a flow of molten steel with an extremely low flow rate distribution.

This distribution of flow rates is obtained by adjusting the frequency of the current passing through the linear motors 8,8 ', which is in the range of 1 to 10 Hz, for example 3 to 10 Hz. Because of this high frequency, the output of the linear motors 8, 8 'is relatively low, making it difficult to obtain flow rates of 0.1 to 1.0 m / sec, in particular 0.5 to 1.0 m / sec.

Therefore, the current must be regulated in such a way that the high magnetomotive force in the above mentioned range, eg 4,000 to 7,000 amps, makes up for the lack of output.

As can be understood from the above description, in the continuous casting process of steel slab based on the adoption of nozzles submerged in molten steel in the continuous casting powder 14 and the mold 7, an improvement according to the invention is the concentration of free oxygen Casts molten steel in the range of 50 to 200 ppm, in particular 50 to 150 ppm, prevents the flow of molten steel ejected from the outlet of the immersion nozzle 11 from adversely affecting the electromagnetic flow of the molten steel, The predetermined area consists of receiving electromagnetic flows. Since there is no undesirable effect on the electromagnetic flow, the powder continuous casting performed by using the immersion nozzle 11 is not disturbed at all and the slab having no pores on its surface can be stably cast.

In order to form a defect free solidification layer on the surface of the strand and not to disturb the continuous casting powder 14 on the molten steel surface 11, a thin film-like rotational flow of molten steel is carried out in the liquid part of the upper part of the mold. It is formed around a solidification surface and has a strip shape having short sides in the vertical direction of the liquid portion.

That is, a thin film-like rotary motion of the molten steel with the required flow rate is formed in the limited portion of the liquid portion adjacent to the mold wall, while the molten steel flows slowly or hardly moves at the other portion, ie the center portion of the liquid portion. Therefore, it is possible to industrially perform continuous casting of deoxidized steel which is not finished or slightly deoxidized, corresponding to the rim and semi-killed steels.

In carrying out the process of the invention, attention should be paid to the distance L (Fig. 19) between the immersion nozzle 11 for injecting molten steel into the mold 7 and the walls of the long sides 9 of the mold.

When the distance L is smaller than 20 mm, the resistance to flow having a predetermined flow rate in one part of the molten steel surface 1 adjacent to the mold wall is very high, so that smooth flow is not always obtained. Therefore, the distance L should be 20mm or more.

The maximum distance L is usually determined according to the dimensions of the mold 7, the diameter of the immersion nozzle 11, and the like.

Since the flow 17 ejected from the immersion nozzle 11 affects the thin filmy rotational flow, the ejected flow 17 is directed down the thin filmy rotational flow to reduce this effect.

The remaining influence of the jet stream 17 directed down the thin membrane-like rotary flow can be effectively eliminated by jetting the stream at the outlet 13 as shown in FIG.

The flow 17 ejected from the immersion nozzle 11 is directed in a direction substantially the same as that of the rotary flow.

The invention is now described in more detail by way of examples and comparative examples.

In these examples, heat 1,2 corresponding to the rim steel and heat 3 and 4 corresponding to the semi-kilted steel were cast continuously as shown in the table below.

As shown in the table, the oxygen concentration was obtained by using deoxidizer in heat 1,2 and vacuum degassing in heat 3 and 4.

[table]

[excuse]

Steel of each heat number was cast under the following conditions.

Form of the mold: The short side is of an arch-shape with a radius of curvature R equal to half the thickness of the strand 4.

Mold dimensions: thickness 250mm, maximum length 2,100mm,

Casting Speed: 0.7m / min

Position of the linear motor 8, 8 ': The center of the linear motor 8, 8' is 200 mm below the molten steel surface.

Immersion nozzle 11: The immersion nozzle 11 has an outer diameter of 100 mm and is located at the center of the mold 7. The ejection position is 250 mm below the molten steel surface 1 and the ejection direction is shorter than the side 10. Headed.

Frequency of Current: 5㎐

Magnetic power: 380 amps x 18 turns

Rotational flow state of molten steel: The effective thickness of the flow was 10 to 20 mm, and the effective depth of the flow was from the molten steel surface 1 to the 200 mm position below the molten steel surface 1, and the 200 mm position below the molten steel surface 1 The coagulation layer thickness was 0 to 3 mm and the flow rate was 0.5 to 0.8 m / sec.

Continuous Casting Powder (14):

(1) CaO / SiO ₂ = 1.0 K ⁺ = 2.5%

_{Al 2 O 3 = 10% F} - = 4%

Na ⁺ = 3.5% C = 4.5%

(2) viscosity of 2.3 poise at 1,500 ° C

(3) 1,150 ° C melting point

In casting 1 to 4 hits, the strands 4 with an intact solidifying layer 5 cause immersion or disturbance of the continuous casting powder 14 on the molten steel surface 1 in the mold 7. Could be obtained without.

The cross-sectional macrostructure of the produced strands 4 of the heat from 1 to 4 was investigated.

It has been demonstrated that a 3 mm thick defect free solidification layer 5 is formed uniformly around the entire surface of the strands 4 of all hits from 1 to 4.

Pores were formed in the defect-free solidified layer 5.

The slab strands 4 produced from 1 to 4 hits were reheated and hot rolled by conventional processes. Some of the strands 4 were then cold rolled by conventional processes.

No surface defects were found in any of the rolled end products.

Comparative Example 1

Molten steels with the same components as those from 1 to 4 were cast under the same conditions as the example except that the area of the horizontal rotating flow extends to the center of the mold 7 and the flow rate at the mold wall is approximately 3.0 m / sec. .

The flow rate of the horizontal rotary flow was 1.0 m / sec.

The molten steel surface 1 was actively disturbed and the continuous casting powder 14 finally gathered at the center of the mold 7.

The migratory tanks had to be stopped because of the increased risk of melt leakage. Regarding the cast strand 4, its structure was observed after solidification. It has been demonstrated that the continuous casting powder 14 has been immersed in a significant number of strands 4.

In this comparative example, the continuous casting powder 14 was disturbed due to the rotational flow extending to the center of the mold.

Comparative Example 2

Molten steels with the same constituents as those from 1 to 4 hits, except that the rotational flow had a flow rate of 0.1 to 1.0 m / sec and no thin film-like rotating flow was given to the molten steel section adjacent to the molten steel surface (1). It was cast under the same conditions as in the example. Pores or pin-holes were found on the surface of all strands 4.

After casting, the strands 4 were placed in a furnace and then rolled to obtain a final product.

Many surface defects occurred in the final product and therefore the recovery of the final product was very low.

In this comparative example in which a thin film-like rotational flow that suppresses the generation of bubble nuclei was not given to the molten steel surface 1, the bubble generation was not suppressed in the outer surface adjacent to the molten steel surface 1 and thus bubbles were generated. It is believed.

Comparative Example 3

Molten steel with components such as those from 1 to 4 were cast under the same conditions as in the example except that the short side of the mold 7 was flat as in conventional molds.

The continuous casting powder 14 finally gathered in the center of the mold 7 and eventually a risk of melt leakage occurred.

Thereafter, casting was performed due to the lack of propulsion of the linear motors 8 and 8 ', and the linear motors 8 and 8' were stopped during casting. As a result, many pin holes occurred on the surface of the strands 4, and the recovery was extremely low.

[Comparative Example 4]

Molten steels having the same components as those from 1 to 4 were cast under the same conditions as in the example except that the installation positions of the linear motors 8 and 8 'were 250 mm and 300 mm from the molten steel surface 1. .

The molten steel surface 1 was actively disturbed and the state of the molten steel surface 1 was similar to that of the comparative example 3.

[Comparative Example 5]

Except that the molten steel having the same component as that of the heat from 1 to 4 changed in the length of the immersion nozzle 11 as follows and the flow rate was 1.0 m / sec or more for the molten steel having the component of the heat 2,4. It was cast under the same conditions as in the example.

The length of the immersion nozzle was varied in the example length such that the upper surface of the jetted stream 17 was 100 mm below the molten steel surface 1. As a result of the casting, many pin-hole defects have occurred in the portion indicated by P on the strands 4 as shown in FIG. 21 which schematically shows the strands 4.

It is believed that this defect arises from the stagnation and discontinuity of the electromagnetic flow due to the influence of the flow 17 ejected from the immersion nozzle 11.

Therefore, the solidified surface could not receive continuous electromagnetic flow. Regarding the molten steel having the components of the heats 2 and 4, the disturbance of the molten steel surface 1 was so significant that a risk of melt leakage occurred. Casting was then carried out without the driving force of the linear motors 8, 8 ', which stopped during the casting.

As a result, a large number of pin-holes and the like occurred on the strands 4, so the recovery was extremely low.

Comparative Example 6

The short side of the mold (7) using molten steel with the same components as those of the heats from 1 to 4 was flat as in conventional molds, and the flow rate of electromagnetic flow was 0.2 m / sec for molten steel with the components of the heats 2,4. It was cast under the same conditions as the example, except that it was and 1.3m / sec.

When molten steel having components of heat 1,2 were cast at a casting speed of 1.3 m / sec, the molten steel surface 1 was severely disturbed and the continuous casting powder 14 gathered at the center of the mold 7.

Therefore, the risk of melt leakage occurred.

Thereafter, casting was performed without the driving force of the linear motors 8 and 8 ', and the linear motors 8 and 8' were stopped during casting. As a result, many pin-holes occurred in the strands 4 and the recovery was extremely low.

As for the molten steel having the components of the heats 2 and 4 cast at a speed of 0.2 m / sec, many pin-holes occurred on the diagonal opposite sides of the short side of the strand 4.

It is believed that the generation of pin-holes results from the flat form of the short sides and from low flow rates that cause congestion and thereby discontinuous electromagnetic flow along the solidification surface.

Comparative Example 7

Molten steels having the same components as those of 1 to 4 hits were cast under the same conditions as in the example except that the flow rate of horizontal rotational flow at the molten steel surface 1 in the mold was 0.1 m / sec or less.

Large pores were also generated on the surface of the slab obtained from molten steel having components of heat 1,2.

Therefore, these slabs could not go through the secondary process. On the other hand, pin-holes were found on all surfaces of the slab obtained from molten steel with components of heats 3 and 4.

The recovery was extremely low because surface defects occurred in a myriad of end products produced from these slabs.

In this comparative example, the flow rate of the horizontal rotational flow in the mold 7 was not high enough to prevent the generation of bubble nuclei.

As a result, surface pores were generated.

Comparative Example 8

The same conditions as in the example except that the molten steel having the same composition as that of the heat from 1 to 4 was changed to 1.0 m / sec or more in the horizontal rotational flow at the molten steel surface 1 in the mold 7. Casting under

Longitudinal balance occurred sporadically on the slab surface obtained from molten steel with components of heat from 1 to 4. No matter how small the dimensions of the longitudinal cracks, the slabs were subjected to Scarfing operations, which were incidentally subjected to conventional processes.

Surface scratches occurred sporadically on the surface of the final products of this process, resulting in less recovery.

It is believed that the occurrence of surface scratches results from secondary scratches formed due to the scuffing of the large longitudinal cracks in the slab thickness.

In the casting of this comparative example, it was observed that the continuous casting powder 14 tended to gather at the center of the mold 7 because of the strong rotational flow.

As a result, the lubrication effect of the continuous casting powder 14 was not provided and longitudinal cracking occurred.

Comparative Example 9

Molten steels with the same constituents as those from 1 to 4 were cast under the same conditions as in the example, except that the solidified surface, whose solidification thickness did not exceed 1.0 mm, was subjected to electromagnetic flow with a flow rate of 0.1 to 0.4 m / sec. .

The outputs of the linear motors 8, 8 ′ have been adjusted to provide this electromagnetic flow.

In this comparative example, in all molten steels having components of heat from 1 to 4, the continuous casting powder 14 was not disturbed and the thickness of the obtained solidified solidified layer 5 was 0.5 mm.

The strands 4 obtained by casting have undergone conventional processes to produce the final product.

The pores were exposed before placing these strands 4 in the furnace.

The recovery was extremely low due to the conditioning of the strands 4.

This is because the defect-free solidified layer 5 thicker than the scale amount was not obtained in the post-casting process.

Claims (1)

In the continuous casting process of the slightly deoxidized steel slab in which the immersion nozzle 11 and the continuous casting powder 14 submerged in the molten steel are used, molten steel having a free oxygen concentration in the molten steel in the range of 50 to 200 ppm is used as a mold. It is cast and has a concave shape when viewed from the horizontal cross section of both short sides of the mold 7, and an apparatus for generating electromagnetic force on both long sides of the mold and the outlet of the immersion nozzle 11 is installed. The propulsion forces of the devices generating the normal force are directed along the long side in opposite directions, essentially at a constant flow rate and completely horizontally rotated around and near the solidification surface and the position of the molten steel surface 1 in the mold 7. To energize the device to generate a flow that forms near the predetermined vertical position of the solidification surface and to provide a horizontal flow with a flow rate in the range of 0.1 to 1.0 Continuous casting process of slightly deoxidized steel slab, characterized in that consisting of ones.

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