ES2920053T3 - Continuous casting method - Google Patents

Abstract

Trapping of a foreign matter in a solidified shell is effectively suppressed. A continuous casting method is a method that is carried out with the use of a continuous casting device and includes: a discharge passage for discharging molten steel through a discharge port (41a); and a stirring step of stirring the molten steel so that a stirring region includes an entire line segment through which the discharge port (41a) and a reached location (P) are connected with each other, the reached location (P ) is a fired location A surface(s) of the molten steel in a mold at which the molten steel is reached in a case where the molten steel discharged in the discharge passage continues straight. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Continuous casting method

field of invention

The present invention relates to a method for continuously casting steel while electromagnetic stirring is carried out.

Background of the invention

In a continuous casting of steel, there is a problem that foreign matter, such as an air bubble or a rust, which is inevitably incorporated into the molten steel in a mold is captured in a solidified shell, and consequently, a crust is formed. defect (imperfection) in a surface of a steel sheet (plate) obtained through a hot rolling step or a cold rolling step. As a solution to the problem, electromagnetic stirring is widely used. By electromagnetic stirring, a flow of the molten steel in the mold is controlled so that foreign matter is floated in the molten steel and is then captured by a casting powder applied to a surface of the molten steel. Patent Literature 1 discloses an example of such a method.

In the technique disclosed in Patent Literature 1, a submerged nozzle is used having two discharge holes through which molten metal is discharged upward at an angle of 5° to 30°. Through the two discharge holes, the molten metal is discharged to the short sides of a mold. Then, by electromagnetic stirring, a driving force acting in a direction perpendicular to a casting direction is applied to the molten metal in the vicinity of a meniscus near each of the two long side surfaces of the mold. With this configuration, the temperature of the molten metal in the vicinity of its surface is kept high, and a flow of the molten metal is formed, which is uniform and perpendicular to the casting direction.

dating list

[Patent Literature]

[Patent Literature 1] Japanese Patent Application Publication No. 10-166120 (published June 23, 1998).

[Patent Literature 2] European Patent Application EP 2092 998 A1 of NIPPON STEEL CORP (published on August 26, 2009).

[Patent Literature 3] JP 3817209 B2 of NIPPON STEEL CORP (published on September 6, 2006).

[Patent Literature 4] US 4042007 A to ZAVARAS ALEXANDER A ET AL (published August 16, 1977).

[Patent Literature 5] NIPPON STEEL CORP JP 2007319923 A (published Dec 13, 2007) [Patent Literature 6] ABB AB EP 2038081 A1 (published Mar 25, 2009)

[Patent Literature 7] JP Document 2016 022493 A of NIPPON STEEL & SUMITOMO METAL CORP (published on Feb 8, 2016)

[Patent Literature 8] JFE STEEL CORP JP 5791 234 B2 (published Oct 7, 2015 [Patent Literature 9] ABB AB and KOLLBERG STEN ET AL WO2004/050277 A1 (published Jun 17, 2004 )

[Patent Literature 10] Document EP 2151 291 A1 of SUMITOMO METAL IND dated February 10, 2010.

[Patent Literature 11] JFE STEEL CORP JP 2010029936 A dated Feb. 12, 2010.

Brief description of the invention

technical problem

However, Patent Literature 1 does not clearly disclose a method for forming an agitation flow to increase the cleaning of foreign matter from the molten metal. Thus, the technique disclosed in Patent Literature 1 has a problem that the effect of eliminating trapping of foreign matter in a solidified shell is not sufficient. An object of one aspect of the present invention is to effectively eliminate trapping of foreign matter in a solidified shell.

Solution to the problem

The scope of the present invention is defined by independent claim 1, and further embodiments of the invention are specified in dependent claims 2-4.

Favorable effects of the invention

According to one aspect of the present invention, it is possible to effectively suppress trapping of foreign matter in a solidified shell.

Brief description of the drawings

Fig. 1 is a schematic view illustrating a configuration of a continuous caster which is not part of the present invention.

Fig. 2 is a cross-sectional view of the continuous casting device, obtained by cutting the continuous casting device along a horizontal plane at a height of a surface of steel cast in a mold included in the continuous casting device. .

Fig. 3 is a cross-sectional view of a vicinity of the surface of the molten steel in the continuous caster, obtained by cutting the continuous caster along a plane passing through a center of the mold and which is parallel to the long side molds included in the mold.

Fig. 4 is a cross-sectional view of a vicinity of the surface of the molten steel in the continuous caster, obtained by cutting the continuous caster along a plane passing through the center of the mold and is parallel to the short side molds included in the mold.

Fig. 5 is a cross-sectional view of a continuous caster not part of the present invention, and is a cross-sectional view of a vicinity of a cast steel surface in the continuous caster, obtained by cutting the continuous caster along a plane that passes through a center of a mold included in the continuous caster and that is parallel to the long side molds included in the mold. Fig. 6 shows the numbers of macro scratch imperfections per square millimeter in each of the example lines of the present invention and a comparative example line. (a) of Fig. 6 is a graph showing the number of macro scratch imperfections present at a position of 2 mm below a surface layer. (b) of Fig. 6 is a graph showing the number of macro scratch imperfections present at a position of 3 mm below the surface layer.

Description of achievements

The following description will discuss a continuous casting method according to Embodiment 1 of the present invention with reference to Figs. 1 to 4. As used herein, an expression "A to B" means "not less than A and not more than B".

Fig. 1 is a schematic view illustrating a configuration of the continuous caster 1A which is not part of the invention. As illustrated in Fig. 1, the continuous casting device 1A includes a ladle 2, a tundish 3, a mold 10, a submerged nozzle (discharge nozzle) 40A, and electromagnetic stirring devices (stirring devices) 50A and 50B. Ladle 2 receives the supplied molten steel from a steel converter.

The tundish 3 is a member for storing molten steel transferred from ladle 2 and extracting foreign matter such as rust. The molten steel stored in the tundish 3 is transferred to the mold 10 through the submerged nozzle 40A (described later).

The mold 10 is a mold for cooling molten steel, thereby transferring it so as to form a solidified shell C against an inner surface of the mold 10. The solidified shell C is sent out of a bottom of the mold 10. Fig 2 is a cross-sectional view of the continuous caster 1A, obtained by cutting the continuous caster 1A along a horizontal plane at a height of a surface S of molten steel in the mold 10. As illustrated in Fig. 2, a contour shape of the inner surface of the mold 10 that cuts in the flat horizontal plane is a rectangular shape. The mold 10 includes (i) long side molds 11A and 11B that face each other and (ii) short side molds 12A and 12B that face each other. Long side molds 11A and 11B have long side (first surface) surfaces 11Aa and 11Ba, respectively, which form the interior surface of mold 10. Short side molds 12A and 12B have short side (second surface) surfaces 12Aa and 12Ba, respectively, which form the inner surface of the mold 10. In other words, the long side surfaces 11Aa and 11Ba and the short side surfaces 12Aa and 12Ba intersecting the long side surfaces 11Aa and 11Ba form a surrounding structure. Hereinafter, a horizontal direction that is parallel to the long side molds 11A and 11B will be referred to as a "long side LD direction", and the horizontal direction that is parallel to the short side molds 12A and 12B will be referred to as a "long side LD direction". as a "short side SD address" (see Fig. 2).

The submerged nozzle 40A is a member for transferring molten steel stored in the tundish 3 to the mold 10. The submerged nozzle 40A is provided such that an upper end of the submerged nozzle 40A is connected to the tundish 3 and a lower end of the submerged nozzle 40A is located substantially at a center of the mold 10 (that is, in Fig. 2, substantially at a center of the rectangular shape formed by the long side surfaces 11Aa and 11Ba and the surfaces of the short side 12Aa and 12Ba).

Fig. 3 is a cross-sectional view of a neighborhood of the surface S in the continuous caster 1A, obtained by cutting the continuous caster 1A along a plane passing through a center of the mold 10 and that it is parallel to the long side molds 11A and 11B of the mold 10. It should be noted that since the continuous caster 1A has a symmetrical structure around the submerged nozzle 40A, Fig. 3 illustrates a region including the submerged nozzle 40A and the short side mold 12B, in a state where the region is enlarged. mouthpiece The submerged 40A has two discharge ports 41A, as illustrated in Fig. 3. The two discharge ports 41A are each located within the mold 10, and each is a port for discharging molten steel that has been supplied from the mold. tundish 3 and which has passed through an interior of the submerged nozzle 40A. The two discharge holes 41A are formed on the respective sides of the submerged nozzle 40A whose sides are located in the long side LD direction. Molten steel is discharged through the two discharge holes 41A in directions each parallel to the long side surfaces 11Aa and 11Ba. Each of the two discharge ports 41A is formed so that a discharge direction 60 of a discharge flow is directed upward with respect to the horizontal plane. Hereinafter, an angle formed by the discharge direction 60 of each of the two discharge holes 41A and the horizontal plane will be referred to as a discharge angle 9.

According to the continuous casting device 1A, molten steel is continuously supplied to the mold 10 through the two discharge ports 41A of the submerged nozzle 40A, and the surface S (also referred to as a meniscus) of the molten steel is formed. at a given height within the mold 10. It should be noted that, although the surface S sways to a certain extent during continuous casting, an average height of the surface S is considered as a position of the surface S in the present specification. It should also be noted that a casting powder (not illustrated) to capture foreign matter such as an air bubble and rust is applied to the surface S.

The electromagnetic stirring devices 50A and 50B are devices for causing, by electromagnetic forces, a stirring flow (eddy flow) with respect to the molten steel in the vicinity of the surface S in the mold 10. Fig. 4 is a cross-sectional view of a neighborhood of the surface S in the continuous caster 1A, obtained by cutting the continuous caster 1A along a plane passing through the center of the mold 10 and parallel to the short-side molds 12A and 12B of mold 10. Electromagnetic stirring devices 50A and 50B are provided on the rear surfaces of long-side molds 11A and 11B, respectively, as illustrated in Fig. 4. The stirring devices electromagnetic coil 50A and 50B include stirrer coil cores 51A and 51B, respectively. Each of the stirring coil cores 51A and 51B applies an electromagnetic force to the molten steel in the mold 10, at a height where each of the stirring coil cores 51A and 51B is provided. Specifically, the stirring coil core 51A of the stirring device 50A applies, to the molten steel in the vicinity of the long side mold 11A, an electromagnetic force parallel to the long side direction LD. Similarly, the stirring loop core 51B of the electromagnetic stirring device 50B applies an electromagnetic force parallel to the long side direction LD to the molten steel in the vicinity of the long side mold 11B. However, it should be noted that the continuous casting device 1A is configured such that a direction of the electromagnetic force applied to the molten steel by the stirring loop core 51A and a direction of the electromagnetic force applied to the molten steel by the core of stirring loop 51B are opposite each other. This causes an agitated flow to form in a horizontal direction in the immediate vicinity of the surface S in the mold 10 (see the black arrows in Fig. 2). In Embodiment 1, the electromagnetic stirring devices 50A and 50B are provided such that the respective upper ends of the stirring coil cores 51A and 51B are each located at a given distance below the surface S.

In Fig. 4, a region A1 indicates a region between an upper end and a lower end of each of the two holes 41A, which upper end and lower end are located in a vertical direction. As illustrated in Fig. 4, the stirring coil cores 51A and 51B are provided so as to include, in the vertical direction, all of the discharge holes 41A. With this configuration, the electromagnetic forces are applied, through the stirring loop cores 51A and 51B, to the molten steel discharged through the two discharge ports 41A from a point in time when the molten steel is discharged.

The electromagnetic stirring devices 50A and 50B give rise to a stirring flow with respect to the molten steel in the mold 10. It should be noted here that the stirring flow is not formed only in a region located at the height where each of stirring coil cores 51A and 51B is provided. That is, in case a stirring flow is caused in a region A2 in which the stirring coil cores 51A and 51B are provided, the molten steel, present in the regions extending up or down from region A2 for a certain distance, it is also caused to swirl in the mold 10, so that an agitated flow is caused. In the present specification, a region including region A2 and "regions each extending above and below region A2 by a certain distance" will be referred to as a stirring region A3 which is formed by the agitation devices electromagnetic stirrer 50A and 50B (see Fig. 4). Specifically, the stirring region A3 in the present specification means a region where a flow rate of the molten steel falls within a range of 0.20 m/s to 0.40 m/s. It is known that, in a case where a flow rate of molten steel in the vicinity of a surface S falls within a range of 0.20 m/s to 0.40 m/s, it is possible to reduce both a surface defect as an internal defect of the steel to be produced. According to the continuous caster 1A according to Embodiment 1, the stirring coil cores 51A and 51B are provided so that the stirring region A3 includes the surface S.

It should be noted that the continuous casting device 1A is configured such that the electromagnetic stirring devices 50A and 50B are provided such that the respective upper ends of the stirring coil cores 51A and 51B are each located at the given distance below the surface S. However, the continuous caster which is not part of the present invention is not limited to such a configuration, and may alternatively be configured so that the respective upper ends of the stirring coil cores 51A and 51B are each located at a height corresponding to the height of the surface S or each located above the surface S. Also, in such cases, it is possible to configure the continuous caster so that the stirring region includes the surface S.

The continuous casting device 1A according to Embodiment 1 is configured as follows. That is, a substantially total amount of the molten steel discharged through each of the two discharge ports 41A of the submerged nozzle 40A is caused to reach the surface S, by properly setting the discharge angle 9, a distance L between (i) a center of each of the two discharge holes 41A and (ii) the surface S, and a distance W between the short side molds 12A and 12B (that is, a length of each of the long side surfaces 11Aa and 11Ba in the horizontal direction).

Here, the following description will discuss, with reference to Fig. 3, an "attained position" on the surface S which position is reached by the discharge flow caused by molten steel discharged through a discharge port 41A. As illustrated in Fig. 3, a center of a discharge port opening 41A is considered as a starting point, and a point where the surface S intersects a center line extending in the discharge direction 60 from the point starting point is regarded as a point P. In other words, the point P is a point on the surface S whose point is reached by the molten steel, discharged through the discharge port 41A, in a case where the steel discharged through from the discharge port 41A proceeds in a straight line. The discharge flow from the discharge port 41A proceeds through the molten steel in the mold 10 while being dispersed to some extent. By setting a discharge speed, in which the molten steel is discharged through the discharge port 41A, to be greater than a given speed, it is possible to cause substantially the entire amount of the molten steel to discharge through the discharge port 41A. of the discharge port 41A to directly reach the point P and the surface S in a vicinity of the point P. In Embodiment 1, the point P and the vicinity of the point P will be referred to as an "attained position". In Embodiment 1, the stirring region A3 includes a whole line segment through which the center of the discharge port opening 41A and the point P are connected to each other.

It should be noted that, in a case where the discharge angle 9 is excessively wide or in a case where the distance L is excessively short, the surface S is excessively shaken because the discharge flow from the discharge port 41A reaches directly the surface S (position reached), so that the casting powder present on the surface S is highly likely to be trapped in the solidified shell C as foreign matter. Therefore, the discharge angle 9 is not more than 30°. In the present invention, a discharge flow rate is 300 mm/s to 1150 mm/s, and the distance L is not less than 180 mm.

In a case where the distance L is excessively long, the time it takes for the discharge flow to reach the surface S (attained position) is longer. As a result, the time it takes for a high-temperature discharge stream to reach the surface S in a vicinity of each of the short-side molds 12A and 12B becomes longer, and a temperature of the surface S of the molten steel at the vicinity of each of the short side molds 12A and 12B becomes smaller. A decrease in a temperature of the molten steel results in the generation of a uniform initial solidified shell having a hook-like cross-section. This ultimately results in an increase in entrapment of foreign matter in the solidified shell C. Therefore, in the present invention, the discharge flow rate is 300 mm/s to 1150 mm/s, and the distance L is not greater than 230mm.

As an example of the configuration of the continuous casting device 1A, in a case where the distance W is longer than 520mm, it is possible to cause the substantially total amount of the molten steel discharged through the discharge port 41A of the submerged nozzle 40A reaches the surface S (position reached), by setting the discharge angle 9 to 30° and setting the distance L to 150mm (for example, setting a width, in an up and down direction, of the discharge hole 41A to 58mm, and set a distance between the surface S and an upper end of the discharge port 41A to 121mm).

As described, the object of the invention continuous casting method carried out with the use of the continuous casting device 1A includes: an unloading step consisting of unloading, through the two unloading holes 41A located in the mold 10, the steel melted upward with respect to the horizontal direction and in the directions parallel to the long side molds 11A and 11B; and a stirring step of stirring the molten steel so that the stirring region A3 includes a full line segment through which a center of an opening of each of the two discharge ports 41A and the point P are connected to each other.

With this configuration, it is possible to cause most of the high-temperature molten steel discharged through each of the two discharge holes 41A to reach the surface S, and consequently, there is a possible delay in the solidification of the molten steel. in the vicinity of the surface S. This makes it possible to increase the stirring effect of each of the electromagnetic stirring devices 50A and 50B whose stirring effect is carried around the vicinity of the surface S, and makes it possible to effectively suppress capturing foreign matter, which is present in molten steel, in a solidified shell C.

Further, since a distance between each of the two discharge ports 41A and the surface S is short, the discharge flow reaches the surface S before the discharge flow velocity decreases. This makes it easier for foreign matter, such as an air bubble or an inclusion in the molten steel, to float through the discharge flow whose velocity is higher, and makes it easier to cause casting dust to capture the foreign matter. On the other hand, given Since the distance between (i) each of the two holes 41A and (ii) the surface S is short, it is possible to suppress diffusion of the discharge flow in a flow path through which the discharge flow reaches the surface S, and consequently it is possible to avoid blocking the flow of stirring.

The discharge flow which has reached the surface S is divided into (i) a flow towards the short side mold 12A or 12B and (ii) a flow towards the submerged nozzle 40A (i.e. the center of the mold 10), according to is illustrated in Fig. 3. As a result, it is possible to make the temperature of the molten steel in the immediate vicinity of the surface S uniform.

Realization 2

The following description will set forth another embodiment which is not part of the present invention with reference to Fig. 5. It should be noted that, for convenience, an identical reference sign will be given to a member having an identical function to that of a member. described in the preceding embodiment.

A continuous caster 1B according to Embodiment 2 includes a submerged nozzle 40B having a structure different from that of the submerged nozzle 40A in Embodiment 1.

Fig. 5 is a cross-sectional view of a neighborhood of a surface S in the continuous caster 1B, obtained by cutting the continuous caster 1B along a plane passing through a center of the mold 10 and which is parallel to the long side molds 11A and 11B of the mold 10.

As illustrated in Fig. 5, the continuous caster 1B according to Embodiment 2 includes the submerged nozzle 40B instead of the submerged nozzle 40A in Embodiment 1.

The submerged nozzle 40B has two discharge holes 41B (see Fig. 5). Each of the two discharge holes 41B is formed so that a discharge direction 70 of molten steel is directed upward with respect to the horizontal plane. Hereinafter, an angle formed by the discharge direction 70 of each of the two discharge holes 41B and the horizontal plane will be referred to as a discharge angle 9.

The continuous casting device 1B according to Embodiment 2 is configured as follows. That is, a substantially total amount of the molten steel discharged through each of the two discharge ports 41B is caused to reach one of the corresponding one of the short side molds 12A and 12B (more specifically, a solidified shell C formed against the corresponding one of the short side molds 12A and 12B), by properly setting the discharge angle 9, a distance L between (i) a center of each of the two discharge holes 41B and (ii) the surface S, and a distance W, on the surface S, between the short side molds 12A and 12B.

Here, the following description will set forth, with reference to Fig. 5, an "attained position" in each of the short side molds 12A and 12B which position is reached by a discharge flow caused by molten steel discharged through the corresponding one of the two discharge holes 41B. As illustrated in Fig. 5, a center of a discharge port opening 41B is considered as a starting point, and a point where the short side mold 12B intersects a center line extending in the discharge direction 70 from the starting point is considered as a Q point. In other words, the Q point is a point on the short side mold 12B whose point is reached by the molten steel, discharged through the discharge hole 41B, in a case where the steel discharged through the discharge port 41B proceeds in a straight line. The discharge flow caused by the molten steel discharged through the discharge port ^41b proceeds through the molten steel in the mold 10 while being dispersed to some extent. By setting a discharge rate, in which the molten steel is discharged through the discharge port 41A, so as to be greater than a given rate, it is possible to cause substantially the entire amount of the molten steel to discharge through the discharge port 41A. discharge hole 41B to directly reach the short side mold 12B. In Embodiment 2, the Q-point and a vicinity of the Q-point will be referred to as an "attained position". The point Q is located in the immediate vicinity of the surface S.

In Embodiment 2, the continuous caster 1B is configured such that a stirring region, formed by electromagnetic stirring devices 50A and 50B, at least includes, in a vertical direction, a region ranging from the "position reached" (the Q point and the vicinity of the Q point) to the lower end of the discharge port 41B. This causes the stirring region, formed by the electromagnetic stirring devices 50A and 50B, to include a whole line segment through which the center of the discharge port opening 41B and the point Q are connected to each other.

As an example of the configuration of the continuous caster 1B, in a case where the distance W is from 1430mm to 1650mm, and a discharge flow rate is from 300mm/s to 1150mm/s, it is possible to give rise substantially all of the molten steel discharged through the discharge port 41A of the submerged nozzle 40B to reach the "reached position", by setting the discharge angle 9 to 5° and setting the distance L to 125mm (for example , setting a width, in an up and down direction, of the discharge port 41B to 50mm, and setting a distance between the surface S and the discharge port 41A to 100mm).

As described, a continuous casting method carried out using the continuous casting device 1B includes: an unloading step consisting of unloading, through the two unloading holes 41B located in the mold 10, the steel fused upward with respect to the horizontal direction and in directions parallel to the long side molds 11A and 11B; and a stirring step of stirring the molten steel such that the stirring region, formed by the electromagnetic stirring devices 50A and 50B, includes a full line segment through which a center of an opening of each of the two discharge ports 41B and point Q are connected with each other.

With this configuration, it is possible to cause most of the high-temperature molten steel discharged through each of the two discharge ports 41A to reach the vicinity of the surface S, and consequently there is a possible delay in the discharge. solidification of the molten steel in the vicinity of the surface S. This makes it possible to increase the stirring effect of each of the electromagnetic stirring devices 50A and 50B whose stirring effect is carried around the vicinity of the surface S, and makes it possible effectively suppress the trapping of foreign matter, which is present in molten steel, in a solidified shell C.

Each of the continuous casting devices 1A and 1B has a structure that the solidified shell C and part of the molten steel are taken out from a lower end portion of the mold 10. Therefore, the discharge flow of each of the two discharge ports 41B receives a downward force. Thus, in a case where the continuous caster 1B has a narrow discharge angle 9, the discharge flow from each of the two discharge ports 41B is likely to proceed outside the stirring region. Consequently, the molten steel is preferably discharged at an angle of not less than 5°, such as the discharge angle 9. This allows the stirring region to absolutely include the discharge flow from each of the discharge ports 41B.

Realization 3

In Embodiment 3, the preferred configuration of an electromagnetic stirring device in continuous casting will be described.

Parameters in continuous casting in Embodiment 3 are shown below. Note that a term in brackets indicates a unit.

a: a discharge angle of a discharge port 41A or a discharge port 41B [°]

A: A discharge area of a discharge port 41A or the discharge port 41B [m2]

W: a casting width [m] (horizontal distance between the short side surfaces 12Aa and 12Ba)

T: a casting thickness [m] (horizontal distance between long side surfaces 11Aa and 11Ba)

V: a discharge speed at which molten steel is discharged through discharge port 41A or discharge port 41B [m/s]

Vc: a casting speed [m/s]

L: a submerging depth that a submerged nozzle 40A or a submerged nozzle 40B is submerged [m] (distance between a center of the discharge port 41A or the discharge port 41B and a surface S)

B: a magnetic flux density at a position in the molten steel, which position is 15 mm apart, in a horizontal direction, from each of the long side surfaces 11 Aa and 11 Bb [G]

f: a frequency of an electromagnetic stirring device 50A or an electromagnetic stirring device 50B [Hz] a: an electrical conductivity of a secondary conductor (the molten steel at 1500°C) [1/|iQ-m]

First, since a volume of the molten steel discharged through the discharge port 41A or 41B is identical to a volume of the casting steel, the following Expression 1 is established.

(A x 2) x V = W x T x Vc ■• ■(Expression 1}

A discharge rate V is determined according to the following Expression 2 based on Expression 1.

Therefore, a velocity Vy, in a vertical direction, of the molten steel discharged through the discharge port 41A or 41B is obtained according to the following Expression 3.

Vy = V x sin a = W x T x Vc x sin a/2A ... (Expression 3)

Thus, the time t that it takes for the molten steel to reach the surface S of the discharge port 41A or 41B (time t to reach the surface) is obtained according to the following Expression 4.

^{t = L / Vy =L x} w ^{x T x Vc x sin a/2A ...(Expression 4)}

The hydraulic stirring power H applied to the molten steel is obtained according to the following Expression 5.

H = B 2 x f x a

Therefore, an impulse I received by the molten steel, discharged through the discharge port 41A or 41B, at the time when the molten steel reaches the surface S is obtained according to the following Expression 6.

l = H x t = B 2 x f x a x L x W x T « V c x S¡na/2A - (Expression 6) According to a continuous casting method according to Embodiment 3, it is preferable to set the magnetic flux density B , the frequency f, and the casting speed Vc so that the impulse I is 0.4 x 107 G2/pQ-m to 2.5 x 107 G2/pQ-m, in a case where the discharge angle a , the discharge area A, the casting width, the casting thickness T, and the submerging depth L have the respective given values. This makes it possible to form a stirring flow which is highly effective in cleaning foreign matter from molten steel, even in a case where a type of molten steel is changed.

The present invention is not limited to the embodiments, but can be altered by a person skilled in the art within the scope of the claims. The present invention also covers, in its technical scope, any embodiment derived from combining technical means disclosed in different embodiments.

Example 1

The following description will set forth an Example of the present invention.

In Example 1, continuous casting of SUH409L and SUS439, each of which is a type of ferrite-based steel, was carried out under the following conditions.

(Continuous casting conditions)

A discharge angle 9 of each discharge port 41A: 30°

A width, in a vertical direction, of each discharge port 41A: 58 mm

A distance L, in the vertical direction, between a surface S and a center of each discharge hole 41A: 180 mm A distance W between short side molds 12A and 12B: 1042 mm

A casting speed: 1.30 m/min

A discharge speed: 865mm/s

A thickness of a line: 200mm

A magnetic flux density at a position 15 mm apart, in a thickness direction, from each of the long side surfaces 11Aa and 11Ba: 1150 G

The aforementioned conditions are conditions under which, in a case where stirring is carried out with the use of electromagnetic stirring devices 50A and 50B, (i) a substantially total amount of molten steel discharged through each of the discharge holes 41A directly reaches a surface S and (ii) the molten steel is included in a stirring region, formed by the electromagnetic stirring devices 50A and 50B, until the molten steel reaches the surface S when the molten steel is discharged through each discharge port 41A. In Example 1, evaluation was carried out with respect to (i) lines prepared while stirring was carried out with the use of electromagnetic stirring devices 50A and 50B (a line of SUH409L will be referred to as a line of SUH409L). example 1, and a line of SUS439 will be referred to as a line of example 2) and (ii) the lines prepared without performing stirring with the use of electromagnetic stirring devices 50A and 50B (a line of SUH409L will be referred to as a line of comparative example 1, and a line of SUS439 will be referred to as a line of comparative example 2).

An X-ray transmission method was carried out with respect to each of example lines 1 and 2 and comparative example lines 1 and 2, in order to count the number of surface defects (defects formed by the capture of air bubbles or inclusions in a solidified cell) present within 10 mm of a surface layer. In this evaluation, the number of surface defects having a diameter of not less than 0.4 mm was counted. The following Table 1 shows the results of the count. It should be noted that Table 1 shows the number of defects per cubic centimeter.

[Table 1]

Defect density type of steel (number/cm3)

SUH409L Comparative Example Line 10.0029

SUH409L Example line 10,0000

SUS439 Comparative Example Line 20,0024

SUS439 Example line 20,0007

As shown in Table 1, in example lines 1 and 2, it was possible to considerably suppress the formation of a surface defect, compared with comparative example lines 1 and 2. This seems to be because, since each of the example lines 1 and 2 were cast in a state where (i) substantially the entire amount of the molten steel discharged through each of the discharge holes 41A directly reached the surface S and (ii) the molten steel was included in the stirring region, formed by the electromagnetic stirring devices 50A and 50B, until the molten steel reached the surface S from which the molten steel was discharged through each discharge hole 41A, it was possible to carry out more effectively carry out the agitation of the molten steel.

Each of Example Lines 1 and 2 and Comparative Example Lines 1 and 2 thus prepared were subjected to steps (hot rolling, annealing, acid pickling, cold rolling, annealing and acid pickling) to produce a typical ferrite-based stainless steel sheet, so that a plurality of cold-rolled annealed steel sheets, each with a thickness of 1 mm, were produced. A surface of each steel sheet thus produced was inspected, and it was determined whether each steel sheet had product quality. As a result, even though the plate surfaces were ground, a low percentage (3.9% in the case of SUH409L, and 2.2% in the case of SUS439) in the steel sheets prepared with the use of comparative example lines 1 and 2 did not have quality as a product. In contrast, all of the steel sheets prepared using example lines 1 and 2 had product quality, even though the surfaces of the sheets were not ground.

Example 2

The following description will set forth another Example of the present invention.

In Example 2, continuous casting of SUS304 was carried out under the following conditions.

(Continuous casting conditions)

A discharge angle 9 of each discharge port 41B: 5°

A width, in a vertical direction, of each discharge port 41B: 50 mm

A distance L, in the vertical direction, between a surface S and a center of each discharge hole 41B: 220 mm A distance W between short side molds 12A and 12B: 1038 mm

A casting speed: 1.40 m/min

A download speed: 932mm/s

A thickness of a line: 200mm

A magnetic flux density at a position 15 mm apart, in a thickness direction, from each of the long side surfaces 11Aa and 11Ba: 1150 G

The aforementioned conditions are conditions under which, in a case where stirring is carried out with the use of electromagnetic stirring devices 50A and 50B, (i) a substantially complete amount of molten steel discharged through each of the discharge holes 41B directly reaches a surface of the short side 12Aa or 12Ba of a mold 10 and (ii) the molten steel is included in a stirring region, formed by the electromagnetic stirring devices 50A and 50B, until the molten steel reaches the surface of the short side 12Aa or 12Bb of the mold 10 when the molten steel is discharged through each discharge hole 41B.

In Example 2, the evaluation was carried out with respect to (i) an example line 3 prepared at the same time as stirring was carried out with the use of the electromagnetic stirring devices 50a and 50B and (ii) a line of comparative example 3 prepared without carrying out stirring with the use of the electromagnetic stirring devices 50A and ^50b . It should be noted that example line 3 was prepared by carrying out stirring with the use of electromagnetic stirring devices 50A and 50B from which 900 mm of mold 10 was removed.

Radiographic inspections were carried out with respect to each of the example lines 3 and the comparative example line 3 in order to count the number of macro scratch imperfections (cracks caused in a case where a casting powder is incorporated in a solidified shell) present at positions 2 mm and 3 mm below a surface layer. Counting was carried out with respect to a middle part of an upper surface of each line, in positions 800 mm, 1,000 mm, 1,200 mm, 1,500 mm, 2,000 mm, 2,500 mm, and 3,000 mm apart from a casting start position where casting was started. In this evaluation, the number of macro scratch imperfections having a diameter of not less than 0.15 mm was counted. Fig. 6 shows the counting results. Fig. 6 shows the numbers of macro scratch imperfections per square millimeter in each of example lines 3 and comparative example line 3. (a) of Fig. 6 is a graph showing the number of imperfections of Macro stripes present at the position of 2 mm below the surface layer. (b) of Fig. 6 is a graph showing the number of macro scratch imperfections present at the position of 3 mm below a surface layer.

As shown in (a) and (b) of Fig. 6, the numbers of macro scratch imperfections present at the positions of 2 mm and 3 mm below the surface layer of example line 3 was less than those in comparative example line 3. This appears to be because, since example line 3 was cast in a state where (i) the substantially total amount of molten steel discharged through each of the discharge ports 41B directly reached the surface of the short side 12Aa or 12Ba of the mold 10 and (ii) the molten steel was included in the stirring region, formed by the electromagnetic stirring devices 50A and 50B, until the molten steel reached the surface of the short side short 12Aa or 12Ba from where the molten steel was discharged through each discharge port 41B, it was possible to carry out stirring of the molten steel more effectively. In particular, the numbers of macro scratch imperfections were conventionally large at positions 1000 mm to 2000 mm apart from the casting starting position, as in comparative example line 3. In contrast, in example line 3, it was possible to reduce the number of macro scratch imperfections also at positions 1000mm to 2000mm apart from the casting starting position. From these results, it was determined that it was possible to increase the grinding yield of a prepared line from 96.8% to 97.5%.

Example 3

The following description will set forth yet another Example of the present invention.

In Example 3, continuous casting of SUS304 was carried out under the following conditions.

A discharge angle a of each discharge port 41: 5°

A discharge area A of each discharge port 41: 0.0026 m2

A casting width W: 1260 mm

A casting thickness T: 200 mm

A discharge speed V: 0.70 m/min

A casting speed Vc: 0.7 m/min to 1.2 m/min

A submerged depth L: 0.25 m

An electrical conductivity a of a secondary conductor: 1p = 1/1.3 (p represents a specific electrical resistance of SUS304, see "Sutenresuko Binran" (Handbook for Stainless Steel), unit: pQ-m)

In Example 3, a magnetic flux density B was set at 1150 G and a frequency was set at 2.7 Hz, and then a pulse I was calculated according to the aforementioned Expressions 1 to 6. As a result, the following Expression 7 was obtained.

0.4 ^X 107 (G2/|jQ-m) < I < 2.5 ^X 107 (G2/|jQ-m) ^{■■■(Expression7)} In Example 3 where the impulse that fell within an interval represented by Expression 7 was applied to molten steel, it was possible to form a stirring flow which had an excellent effect of cleaning foreign matter from molten steel, and to make it possible to effectively suppress trapping of foreign matter, present in molten steel, in a shell. solidified.

List of reference signs

1A, 1B Continuous casting device

10 mold

11Aa, 11 Ba Long side surface (first surface)

12Aa, 12Ba Short side surface (second surface)

40A, 40B Submerged nozzle

41A, 41B Discharge port

50A, 50B Electromagnetic stirring device (stirring device)

A3 Region of agitation

S Surface

Claims (4)

1. A continuous casting method that is carried out with the use of a continuous casting device (1A, 1B), the continuous casting device (1A, 1B) includes: a mold (10) including a first surface and a second surface, intersecting the first surface, in order to have a surrounding structure; a submerged nozzle (40A, 40B) having a discharge port (41A, 41B) through which molten steel is discharged; Y a stirring device (50A, 50B) that forms a stirring region (A3) by stirring the molten steel in the mold (10), the method comprising: a discharge step of discharging, through the discharge hole (41A, 41B) located in the mold (10), the molten steel upward with respect to a horizontal direction and in a direction parallel to the first surface; and a stirring step of stirring the molten steel such that the stirring region (A3) includes a full line segment through which the discharge port (41A, 41B) and a reached position are connected to each other, the reached position is a position on a surface (S) of the molten steel in the mold (10) or on the second surface whose position is reached by the molten steel in a case where the molten steel discharged in the discharge stage proceeds online straight; wherein, an angle formed by the discharge direction (60, 70) of each of the two discharge ports (41A, 41B) and the horizontal plane is referred to as a discharge angle (©, 0) and is not greater than 30°, and a distance between a center of each of the two discharge holes (41A, 41B) and the surface (S) of the molten steel is not less than 180mm and not more than 230mm, characterized in that , a speed of a discharge flow is 300-1150 mm/s. . 2. The method as set forth in claim 1, wherein a flow rate of the molten steel in the stirring region (A3) falls within the range of 0.20 m/s to 0.40 m/s. 3. The method as set forth in claim 1 or 2, wherein the position reached is the position on the surface(S). 4. The method as set forth in any of claims 1 to 3, wherein an impulse received by the molten steel, discharged from the submerged nozzle (40A, 40B), receives for a time when the molten steel reaches the surface (S ) is 0.4 x 107 G2/pQ-m to 2.5 x 107 G2/pQ-m, the impulse is calculated according to the following expressions 1 to 6: (A ^x 2) ^{x V = W x} T ^{x V e (Expression1)} a discharge rate V is determined according to the following Expression ^V _w ^{= W} _vv ^{x T} ₁ ^x Vc/2A ₍ ^,r _E - _xpresi ^■ _o ^■ _{n 2)} a velocity Vy, in a vertical direction, of the molten steel discharged through the discharge port (41A, 41B) is obtained according to the following Expression ^{V y} = ^{V x} sina = ^{W x} T ^{x V ex} sina/2A ^{(Expression 3)} the time t that it takes for the molten steel to reach the surface S of the discharge hole (41A, 41B) (time t to reach the surface) is obtained according to the following Expression ^{t = L / V y = L x W x T x V ex} sina/2A ^{(Expression 4)} the hydraulic stirring power H applied to the molten steel is obtained according to the following expression H - B x ^f xs ^{(Expression 5)} an impulse I received by the molten steel, discharged through the discharge hole (41A, 41B), at the time when the molten steel reaches the surface S is obtained according to the following Expression ^{l = H xt = B 2 xfxsx | _ x W x T x V cx} Sina/2A ^{(Expression 6)} where a: a discharge angle of a discharge port (41A, 41B) [°]; A: a discharge area of the discharge port (41A, 41B) [m2]; W: a casting width [m] (horizontal distance between short side surfaces (12Aa, 12Ba)); T: a casting thickness [m] (horizontal distance between long side surfaces (11Aa, 11Ba)); V: a discharge speed at which the molten steel is discharged through the discharge port (41A, 41B) [m/s]; Vc: a casting speed [m/s]; L: a submerged depth to which a submerged nozzle (40A, 40B) is submerged [m]; B: a magnetic flux density at a position in the molten steel, which position is 15 mm apart, in a horizontal direction, from each of the long side surfaces (11Aa, 11Bb) [G]; f: a frequency of an electromagnetic stirring device (50A, 50B) [Hz]; s: an electrical conductivity of a secondary conductor (molten steel at 1500°C) [1/mW-m]. Vy: a discharge velocity in a vertical direction that the molten steel is discharged through the discharge hole (41A, 41B) [m/s] t: time it takes for the molten steel to reach the surface S of the discharge hole (41A, 41B) [s] H: hydraulic stirring power I: one pulse [G2/|iQ-m]