JP2001300704A - Manufacturing method and cast slab of continuous cast slab - Google Patents

Abstract

(57) [Problem] To provide a multi-layer continuous cast slab in which the concentration of an alloy element in the surface layer portion of the slab is higher than the concentration of the alloy element in the inner layer portion of the slab. SOLUTION: A continuous casting powder containing a predetermined alloy element is used, and an electromagnetic stirring device is installed at an upper portion in a continuous casting mold, and a direction of a stirring flow formed in a horizontal section in a molten steel pool in the mold and Switching the flow rate periodically, and
By providing the discharge hole of the immersion nozzle for supplying molten steel in the mold below the stirring area, a region in the mold where the alloy element is uniformly dissolved and mixed by the stirring flow is formed in the upper part, and A region where the concentration of the alloying element is low is formed underneath.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing steel directly from molten steel.
In a continuous casting mold, a method of producing a multilayer continuous cast slab in which the concentration of the alloy element in the surface layer portion of the slab is higher than the concentration of the alloy element in the inner layer portion of the slab, and produced by the method. Slabs.

[0002]

2. Description of the Related Art As shown in FIG. 1, the inventors of the present invention disclosed in Japanese Patent Application Laid-Open No. Hei 8-290235, using a continuous casting powder containing a predetermined alloy element, By installing a stirrer, forming a swirling flow in the horizontal section in the molten steel pool in the mold, and by providing the discharge hole of the immersion nozzle for supplying molten steel in the mold below the stirring area, the inside of the mold In the upper part, a region where the alloy element is uniformly dissolved and mixed by the stirring flow is formed,
Forming a region in which the concentration of the alloying element is low in the lower part, the surface concentration of the alloying element is disclosed as a method for producing a continuous cast slab characterized by producing a multi-layered slab compared to the inner layer, In Japanese Patent Application Laid-Open No. Hei 8-290236, a swirling agitated flow is formed in a molten steel pool at the upper and lower portions of a mold, and an electromagnetic force capable of forming a DC magnetic field is applied under the swirling flow. A method for enriching alloy elements added only to the surface layer of a slab by adding particles or powder of alloy elements to be added to molten steel together with powder has been disclosed.

[0003]

However, if the molten steel pool in the upper part of the mold is vigorously stirred in order to make the concentration of the surface layer of the slab more uniform, irregularities are generated at the molten metal level, and the There was a problem that the addition was not stabilized. In addition, JP-A-8-29023
When a direct current magnetic field is not used below the electromagnetic stirrer as in the method disclosed in No. 5, a large amount of alloying elements are added to secure a predetermined concentration at the surface layer of the slab because the stirring area is wide. There is a problem that must be done.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems, stabilizes the addition of alloy from powder, and improves the uniformity of the concentration of alloy elements in the surface layer of the cast slab. In addition to providing a manufacturing method, a continuous cast slab having different concentrations of alloying elements in a surface layer portion and an inner layer portion of the cast slab is provided, and the gist thereof is as follows.

(1) A continuous casting powder containing a predetermined alloy element is used, and an electromagnetic stirrer is installed at an upper part in a continuous casting mold to form a stirring flow formed in a horizontal section in a molten steel pool in the mold. By periodically switching the direction and the flow rate, and by providing a discharge port of the immersion nozzle for supplying molten steel into the mold below the stirring area, in the mold,
A region where the alloying element is uniformly dissolved and mixed by the agitated flow is formed in the upper portion, and a region in which the concentration of the alloying element is low is formed in the lower portion, and the concentration of the alloying element in the surface layer portion of the slab is reduced. A method for producing a continuous cast slab, comprising producing a multilayer cast slab having a higher concentration of alloying elements in an inner layer portion of the slab.

(2) A continuous casting powder containing a predetermined alloy element is used, and an electromagnetic stirrer is installed at an upper part in a continuous casting mold to form a stirring flow formed in a horizontal section in a molten steel pool in the mold. The direction and the flow rate are periodically switched, and the discharge hole of the immersion nozzle for supplying molten steel into the mold is provided below the stirring area, and further below the electromagnetic stirrer, substantially one in the width direction of the mold. By applying a DC magnetic field having a similar magnetic flux density distribution in the thickness direction of the mold, a region is formed in the upper portion of the mold where the alloy element is uniformly melted and mixed by the agitated flow. Forming a region where the element concentration is low, producing a multilayer slab in which the concentration of the alloying element in the surface layer portion of the slab is higher than the concentration of the alloying element in the inner layer portion of the slab. Manufacture of continuous cast slabs Law.

(3) In the method according to the above (1) or (2), the immersion nozzle for supplying molten steel into a mold is
A method for producing a continuous cast slab, comprising using a submerged nozzle having a discharge hole shaped to form a flow in which molten steel flows out in a short side direction of a mold and a flow flowing out in a casting direction. (4) A continuous cast slab cast using the method according to (1), (2) or (3), wherein the alloy element concentration in the surface layer portion of the slab is in the inner layer portion of the slab. It is higher than the concentration of the alloy element, and the surface layer has a thickness defined by the following formula (1) and has a negative segregation zone having a multilayer structure with a pitch defined by the following formula (2). A continuous cast slab characterized by the above.

D = k (L / V) ⁿ (1) D: thickness L: depth of vibration stir zone V: casting speed k: solidification coefficient n: constant P = U × t / 2 (2) P: Pitch of negative segregation zone U: solidification speed t: oscillation period

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to FIG. First, a grain or powder 1 of a predetermined alloy element
3 is added continuously or intermittently to the entire surface of the molten metal from above the mold 1 using a continuous casting powder 10 containing 3. Then, in the process of melting the continuous casting powder 10 on the meniscus in the mold 1, the grains or powders 13 of the alloy elements come into contact with the molten steel at the meniscus, and are mixed and diffused from the meniscus into the upper molten steel pool 3 in the mold 1. I do. For example, nickel, copper,
If the standard free energy of formation of oxides such as carbon, manganese, phosphorus, sulfur, and molybdenum is silicon or an alloy powder or metal powder of an element larger than boron, all are added in the same form from powder to molten steel. be able to.

Next, it is necessary to make the concentration distribution of the upper molten steel pool 3 in the mold 1 uniform in the horizontal cross section. For this reason, in the present invention, it is necessary to form a unidirectional stirring flow in the horizontal cross section. By periodically changing the stirring direction and the stirring flow rate in the horizontal section over time, the addition of the alloy from the powder to the molten steel is stabilized without generating irregularities at the molten metal level, and the concentration in the horizontal section is reduced. Uniform distribution.

According to the conventional method, the more the vigorous stirring is performed to uniformly mix the concentration in the stirring area, the more the shape disorder at the molten metal level becomes large. By periodically changing the direction over time, it is possible to stably add the alloy from the powder without causing such disturbance. Specifically, an electromagnetic stirrer 30 is provided in the upper part of the continuous casting mold 1 and, as shown in FIG. The vibrating stirring flow 32 can be applied in a horizontal section in the upper molten steel pool 3. Since the stirring direction and the stirring flow rate are periodically switched in the horizontal cross section, the molten metal surface is not disturbed, and in addition to this, the formation of the downward flow of molten steel that collides with the short side and penetrates downward is also suppressed. The penetration depth becomes shallower, and the volume of molten steel in the vibration stirring zone 19 becomes smaller than in the conventional method. Therefore, the concentration distribution of the alloy element can be more efficiently uniformized.

With respect to the flow rate of the vibrating stirring flow (stirring flow rate), the time average flow rate of the square of the flow rate in one cycle is 0.1 m.
If it is not less than / sec, the concentration distribution of the alloy element in the vibration stirring area 19 can be made substantially uniform. Further, by positioning the discharge hole of the immersion nozzle 2 below the vibration stirring area 19, the concentration of the alloy element in the molten steel discharged from the discharge hole is made lower than the concentration of the alloy element in the vibration stirring area 19. Therefore, molten steel having a low alloying element concentration is continuously supplied to the molten steel pool below the discharge hole. As a result, in the surface layer 6 of the slab, it is possible to manufacture a slab that is uniform in the width direction and that has a higher concentration of the alloying element in the surface layer than the concentration of the alloying element in the inner layer 7.

At this time, if the particles or powder 13 sized according to the alloying element is contained in the continuous casting powder 10 by the concentration of the component desired to be added to the surface layer portion 6 of the slab, the vibration stirring region 19
The molten steel component in the steel can be adjusted to a predetermined concentration. that time,
When alloy particles or powder are contained in the powder grains, alloy addition can be made more uniform and stable. In addition, as shown in FIGS. 5A and 5B, the shape of the discharge hole of the immersion nozzle 2 positioned below the vibration stirring area 19 formed in the molten steel pool is generally used.
At the lower end of the hole nozzle, a downward discharge hole is added to
The molten steel flows out from the lower end of the nozzle in the casting direction, or the shape of the discharge hole is changed to a slit-like discharge hole as shown in FIG. And the molten steel was allowed to flow radially in the mold width direction and the casting direction. Further, as shown in FIG. 5D, a slit orthogonal to the slit was added to the shape of the discharge hole. Since the molten steel flows radially in the shape direction and in the width direction, thickness direction and casting direction of the slab, it is possible to reliably supply the molten steel having the required composition in the tundish below the molten steel pool. This is effective in lowering the concentration of the alloy element in the inner layer portion 7 of the piece, that is, in enriching only the surface layer portion of the cast piece with the alloy element added into the mold via powder.

Further, as shown in FIG. 6, a DC magnetic field generator 20 is installed below the electromagnetic stirrer 30, and a substantially uniform magnetic flux is applied to the molten steel located below the vibration stir zone 19 in the width direction of the mold. By applying a DC magnetic field 21 having a density distribution in the thickness direction of the mold to form the damping zone 29, the vibration stirring zone 19 and the lower molten steel pool can be reliably separated, so that the DC magnetic field 21 was added to the mold. The alloy element can be more efficiently enriched only in the surface layer portion of the slab.

The slab thus cast has the following features. That is, regarding the thickness of the surface layer portion of the slab, the thickness of the solidified shell formed in the vibration stirring zone 19 is the thickness of the surface layer portion. Therefore, the thickness of the surface layer portion of the slab is defined by the following equation (1). You. In addition, in the method of the present invention, the stirring flow rate and the stirring direction of the molten steel are intentionally changed with time. For example, as shown in FIG. 4, the stirring flow rate on the front surface of the solidified shell is changed with time. Then, in the solidified shell portion formed in the time zone in which the stirring flow rate is high, the molten steel concentrated between the dendrite trees is washed, so that the concentration of the alloying element is in the solidified shell portion formed in the time of the stirring flow rate of 0. Lower than the concentration of the alloying element, and becomes a negative segregation zone.

In addition, since the stirring flow rate similarly changes with time in the circumferential direction of the cast slab, as shown in FIG.
Are uniformly formed in the circumferential direction of the slab. Further, since the repetitive flow changes with time until the slab passes through the vibration stirring zone 19, several negative segregation zones 8 (see FIG. 3) are defined by the following formula (2) in the thickness direction of the slab. Are formed in layers on the surface layer 6 of the slab at the given pitch.

In the slab, even if the layered negative segregation zone 8 exists, the concentration of the alloying element in the surface layer of the slab is higher than the concentration of the alloying element in the inner layer of the slab. Therefore, the properties and functions of the surface layer portion of the slab do not deteriorate. D = k (L / V) ⁿ (1) D: Thickness L: Depth of vibration stir zone V: Casting speed k: Solidification coefficient n: Constant P = U × t / 2 (2) P: Negative segregation zone Pitch U: solidification speed t: oscillation cycle

[0018]

EXAMPLE The center of the electromagnetic stirrer in the height direction was positioned 150 mm below the level of the molten metal in the mold, and a vibratory stirring flow could be applied in a horizontal cross section to the upper molten steel pool in the mold. In the casting process (see FIG. 2), casting was performed under the conditions shown in Table 1.

[0019]

[Table 1]

The position of the discharge hole of the immersion nozzle that supplies molten steel into the mold is 300 mm from the level of the molten metal to the top of the discharge hole.
Position. In addition, nickel powder is contained in the powder in an amount of 20% by mass, and this powder is cast from above the mold during casting.
It was continuously supplied over the entire surface of the molten metal. For comparison, an experiment was performed also in the case where a unidirectional stirring flow was applied without changing the stirring direction.

In addition, a DC magnetic field generator capable of applying a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction in the thickness direction is provided below the electromagnetic stirrer, and a discharge hole is provided in the braking region to perform casting. Also in the continuous casting process to be performed (see FIG. 6), casting was performed under the conditions shown in Table 2 when the vibrating stirring flow was applied and when the unidirectional stirring flow was applied, and the results were compared. FIG. 7 and FIG. 8 show the results of investigating the nickel concentration distribution in the slab after casting in the double casting process.

[0022]

[Table 2]

FIG. 7 shows a vibration stirring or an electromagnetic stirring device.
It is a nickel concentration distribution in a slab when only unidirectional stirring is applied, (a) is a nickel concentration distribution in the thickness direction at the center of the slab in the width direction, and (b) is a nickel concentration distribution.
It is a nickel concentration distribution in the width direction at 10 mm from the surface of the slab. 7 (a) and 7 (b), according to the method of the present invention, the nickel concentration in the surface layer portion of the slab is higher than that of the conventional method over the entire width direction of the slab. It can be seen that the concentration of nickel is higher than the nickel concentration in the portion, and that the nonuniformity of the alloy element concentration in the width direction observed during strong stirring is eliminated, and the concentration distribution is more uniform.

Further, from FIGS. 7A and 7B, the nickel concentration in the inner layer portion of the slab is lower, and the nickel concentration in the surface layer portion of the slab is higher. However, it can be understood that the surface portion of the slab is efficiently enriched. In addition, when an immersion nozzle having a shape with an increased number of discharge holes is used, it can be seen that the nickel concentration in the inner layer portion of the slab is further reduced.

As shown in FIG. 8, a DC magnetic field generator is provided below the electromagnetic stirrer, and below the vibration stirrer,
In the case where the braking zone was formed, the nickel concentration in the inner layer portion of the slab was further reduced, and it can be seen that nickel added in the mold was more efficiently enriched only in the surface layer portion of the slab. Similar experiments were also performed for carbon, manganese, phosphorus, sulfur, copper, chromium, molybdenum, and niobium. For all elements, the concentration of alloying elements in the surface layer of the slab was higher than that in the inner layer of the slab. A slab having a higher concentration than the element and having a uniform alloy element concentration distribution in the width direction of the slab could be obtained.

[0026]

According to the present invention, a continuous casting powder containing an alloying element is used to melt and dissolve an alloying element in an upper molten steel pool in a mold while applying a vibrating stirring flow in which the direction of flow periodically changes. By forming a region to be mixed and providing a discharge hole of the immersion nozzle below the vibration stirring region to form a region where the concentration of the alloy element is low, or by increasing the concentration of the alloy element in the surface layer portion of the slab. A DC magnetic field generator (electromagnet) capable of applying a DC magnetic field having a uniform magnetic flux density distribution substantially in the width direction of the mold in the thickness direction of the mold below the electromagnetic stirrer, and / or Or
By using an immersion nozzle having a discharge hole shaped such that the molten steel flows downward in the casting direction in addition to the component directed in the short side direction of the mold, the slab is used as an immersion nozzle for supplying molten steel into the mold. The alloy element concentration in the surface layer portion of the slab is higher than the alloy element concentration in the inner layer portion of the slab, and it is possible to produce a multilayer slab in which the alloy element concentration in the surface layer portion of the slab is uniform. it can.

That is, according to the present invention, for example, in the continuous casting of steel containing only copper and tin or copper, a nickel or nickel alloy-containing continuous powder is used to reduce the nickel concentration in the surface layer of the slab. By increasing the value, surface flaws caused by copper specific to this steel type can be suppressed. In addition, the concentration of alloying elements in the surface layer of the slab can be intentionally increased to a level that does not cause various defects such as scale flaws and cracks, plating defects, etc. Defects generated during the process can be prevented.

In addition, it is possible to easily produce a slab having new properties and functions imparted to the surface layer of the slab. Further, the present invention is applicable regardless of the slab size, bloom, casting of small cross-sectional size such as billet, or,
It is also applicable to thin slab casting with a thickness of 100 mm or less.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional continuous casting process for forming a unidirectional stirring flow using an electromagnetic stirring device. (A) shows the mode of the unidirectional stirring flow in the mold, (b) shows the mode of the molten steel pool in the mold, and (c) shows the concentration distribution of the alloy element in the horizontal cross section of the slab. Show.

FIG. 2 is a view showing an embodiment of continuous casting according to the present invention, in which a vibrating stirring flow is applied and a two-hole type immersion nozzle is used.
(A) shows the mode of the vibrating stirring flow in the mold,
(B) shows the mode of the molten steel pool in the mold, and (c)
Shows the concentration distribution of alloy elements in the horizontal section of the slab.

FIG. 3 is a view showing a form of a surface layer portion in a continuously cast slab of the present invention.

FIG. 4 is a diagram showing a time change of the current applied to the electromagnetic stirring device (a) and a time change of the flow speed of the stirring flow at the front of the solidified shell at the height of the coil center (b).

FIG. 5 is a view exemplifying a shape of a discharge hole of a dipping nozzle used in the present invention. (A) shows the shape of a three-hole type discharge hole, (b) shows the shape of a four-hole type discharge hole,
(C) shows the shape of a discharge hole provided with a slit on the lower end and side surface of the nozzle so as to connect two holes on the side surface of the nozzle,
Further, (d) shows the shape of the discharge hole in which a slit is further provided at the center of the width in the shape shown in (c), and the shape of the discharge hole at the lower end is a cross shape.

FIG. 6 shows a two-hole immersion nozzle for applying a vibrating stirring flow and applying a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction of the mold in the thickness direction of the mold below the electromagnetic stirring device. It is a figure which shows the aspect of the continuous casting of this invention using.
(A) shows the mode of the vibrating stirring flow in the mold,
(B) shows the mode of the molten steel pool in the mold, and (c)
Shows the concentration distribution of alloy elements in the horizontal section of the slab.

FIG. 7 shows a case where a vibrating stirring flow is applied and a two-hole type immersion nozzle is used, and a case where a two-hole type side wall is connected to a slit hole at the lower end of the nozzle is used. It is a figure which shows the nickel concentration distribution in a cast piece at the time of giving a unidirectional stirring flow, and using a 2-hole type immersion nozzle. (A) shows the nickel concentration distribution in the thickness direction of the slab, and (b) shows the nickel concentration distribution in the width direction of the slab.

FIG. 8 is a flow diagram showing a method of applying a vibrating stirring flow and applying a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction of the mold in the thickness direction of the mold below the electromagnetic stirrer. FIG. 3 is a diagram showing a nickel concentration distribution in a slab when a two-hole type immersion nozzle is used and only a unidirectional stirring flow is applied for comparison, when used.
(A) shows the nickel concentration distribution in the thickness direction of the slab, and (b) shows the nickel concentration distribution in the width direction of the slab.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Mold 2 ... Immersion nozzle 3 ... Upper molten steel pool 4 ... Lower molten steel pool 6 ... Surface layer part of slab 7 ... Inner layer part of slab 8 ... Negative segregation zone in surface layer part of slab 9 ... Stir zone 10 ... Continuous casting Powder for use 13 ... Granules or powders of alloying elements 19 ... Vibration stir zone 20 ... DC magnetic field generator 21 ... DC magnetic field 29 ... Brake zone 30 ... Electromagnetic stirrer 31 ... Stirring flow 32 ... Vibrating stir flow

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) B22D 27/02 B22D 27/02 W (72) Inventor Takahiro Isono 1-Fuji-cho, Hirohata-ku, Himeji-shi, Hyogo Nippon Steel Corporation Hirohata Works F-term (reference) 4E004 AA09 FB06 MB12 MB14

Claims (4)

[Claims] 1. A continuous casting powder containing a predetermined alloy element is used, and an electromagnetic stirring device is installed at an upper portion in a continuous casting mold, and a direction of a stirring flow formed in a horizontal section in a molten steel pool in the mold. And periodically switch the flow rate, and
By providing the discharge hole of the immersion nozzle for supplying molten steel in the mold below the stirring area, a region in the mold where the alloy element is uniformly dissolved and mixed by the stirring flow is formed in the upper part, and Forming a region where the concentration of alloying elements is low in the lower part, and manufacturing a multilayer slab in which the concentration of alloying elements in the surface layer portion of the slab is higher than the concentration of alloying elements in the inner layer portion of the slab. A method for producing a continuous cast slab. 2. A continuous casting powder containing a predetermined alloy element is used, and an electromagnetic stirring device is installed at an upper part in a continuous casting mold, and a direction of a stirring flow formed in a horizontal section in a molten steel pool in the mold. And periodically switch the flow rate, and
A discharge hole of an immersion nozzle for supplying molten steel into the mold is provided below the stirring area, and a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction of the mold is provided below the electromagnetic stirrer. By applying in the thickness direction of the above, in the mold, to form a region in which the alloy element is uniformly dissolved and mixed by the agitated flow in the upper portion, and to form a region in which the concentration of the alloy element is lower in the lower portion, A method for producing a continuous cast slab, comprising producing a multi-layer cast slab in which the concentration of an alloying element in the surface layer of the slab is higher than the concentration of the alloying element in the inner layer of the slab. 3. The method according to claim 1, wherein
As the immersion nozzle for supplying molten steel into the mold, a flow of molten steel flowing in the direction of the short side of the mold and a immersion nozzle having a discharge hole shaped to form a flow flowing in the casting direction are used. Manufacturing method of continuous cast slab. 4. A continuous cast slab cast by the method according to claim 1, 2 or 3, wherein the concentration of the alloy element in the surface layer portion of the cast slab is the same as that of the alloy element in the inner layer portion of the cast slab. The surface layer has a thickness defined by the following formula (1) and a negative segregation zone having a multilayer structure with a pitch defined by the following formula (2). Continuous cast slab. D = k (L / V) ⁿ (1) D: Thickness L: Depth of vibration stir zone V: Casting speed k: Solidification coefficient n: Constant P = U × t / 2 (2) P: Negative segregation zone Pitch U: solidification speed t: oscillation cycle