WO2013068296A1

WO2013068296A1 - Method for continuous casting of steel and submersible nozzle for the same

Info

Publication number: WO2013068296A1
Application number: PCT/EP2012/071670
Authority: WO
Inventors: Edgar Schumacher; Renata Franzky; Konstantin DORN; Sergei PUSTAHANOV
Original assignee: Techcom Gmbh
Priority date: 2011-11-09
Filing date: 2012-11-01
Publication date: 2013-05-16
Also published as: EA016943B1; EA201101485A1

Abstract

The invention relates to ferrous metallurgy, in particular, to production of steel slabs by continuous casting. In the claimed method for continuous casting of steel, superheated metal is fed from an intermediate ladle to a mold having a rectangular cross-section by means of a submersible nozzle, which has a sprue channel in its upper part adjacent to the intermediate ladle; the sprue channel branches in the lower part of the nozzle to at least two closed- bottom outlet channels. The closed-bottom outlet channels are located within the nozzle limited by a skirt that stretches toward the narrow edges of the mold and joins with the nozzle above the level where the closed-bottom outlet channels branch, and by the lower edge of the skirt located below the level of the bottom of the closed-bottom outlet channels. Each closed-bottom outlet channel has two diametrically positioned and horizontally directed outlets, wherein the axes of the outlets form an angle that is not 90º with the wide edge of the mold in the horizontal plane. Another object of the invention is a submersible nozzle for implementation of said steel casting method. The claimed method for continuous casting of steel and submersible nozzle used therein result in an increased rate of casting, providing more uniform volumetric metal temperature and reduced wearing of the submersible nozzle, thus increasing the efficiency of slab continuous casting devices.

Description

METHOD FOR CONTINUOUS CASTING OF STEEL AND SUBMERSIBLE NOZZLE

FOR THE SAME

The invention relates to the field of ferrous metallurgy, in particular, to production of steel slabs by continuous casting of steel.

When casting steel continuously, especially when casting a slab up to a level, it is important to mix a metal, as entering a mold, in the horizontal plane without disturbing the meniscus surface. If this condition is met, a liquid metal temperature in the mold volume and in the surface layers of the meniscus is averaged, and conditions are created for forming a dense structure of an ingot to be cast without defects.

The use of a closed-bottom submersible nozzle for continuous casting of steel, which has outlet channels in its lower part, is well known in the art (see, for example, Japanese patent applications Nos. 11291026, 11320046, 2001087843, 2004283848, 2006150434, 2008200705, etc.).

A method for continuous casting of a slab through a closed-bottom submersible nozzle having horizontal outlet channels directed toward the mold short facets. A disadvantage of this method is formation of intensive flows in surface layers of a liquid metal, which act on the mold walls and cause meniscus swirling, thus leading to blurring of the forming slab skin. Furthermore, a possibility arises that large defects may appear on the surface of a cast slab and that the skin may be broken when the slab comes out of the mold.

RU Patent No. 2137570 (publ. 20.09.1999, IPC B22D11/00) describes a device and a method for producing continuously cast deformed slabs with the use of three submersible dead-bottom nozzles spaced apart from each other. A metal with a lower crystallization temperature is fed through the end nozzles having one opening (channel), and a metal with a higher crystallization temperature is fed through the central nozzle having two openings (channels), the melts being fed to the mold in such a way that they form a mixture after being mixed. This construction is directed to creation of a highly efficient process of producing deformed slabs from a mixture comprising melts of two metals. The technical effect is increased productivity of a method for producing deformed slabs as well as production of solid deformed slabs of random form and thickness with a defined distribution of two metals in the slab section.

The closest to the claimed method as to the technical essence and the achieved result is a continuous casting method described in RU Patent No. 2359782 (publ. 27.06.2009, IPC B22D41/50), which comprises entering of a metal into a mold from an intermediate ladle through a dead-bottom submersible nozzle having a skirt in its lower part, which skirt is attached to the nozzle outer surface above the level of the outlet channels. The horizontal outlet channels are located above the level of the skirt lower edge and have a geometry imparting rotational movement to a metal entering the mold in a plane perpendicular to the direction of the slab pulling. Due to its form extended in the direction of the mold short facets, the skirt excludes direct action of metal flows to the water-cooled walls and forms their horizontal rotation in the whole volume of the mold. This structure of the submersible nozzle ensures production of good-quality cast slabs having a round or circular section. The possibility of arranging two individual submersible nozzles in a slab mold is also provided for (FIG. 4).

However, is a slab has a rectangular section, the nozzle field of application according to RU 2359782 is limited to the size of a long facet that does not exceed 800 mm. When wide slabs having the long facet size more than 800 mm are cast, the intensity of mixing in the outlet channels area increases greatly due to an increase in the mass rate of casting, but with approaching the mold short facets and moving off from the meniscus in the direction of slab pulling the mixing intensity sharply decreases. So, one of the main advantages of this structure, namely, averaging of a metal temperature in the slab cross- section by horizontally rotating metal flows in the whole volume of the mold, is lost. Conditions for melting a slag-forming mixture and forming a slab skin normally become poorer, a possibility of arising defects of the slab structure and surface becomes greater, and this negatively affects the service properties of a ready rolled stock.

In the result of high dirtiness of the slab axial zone due to nonmetal inclusions, sulfur and phosphor and poor axial soundness important properties of ready rolled stock, such as strength and plasticity, become worse, the defect appears that is classified as an axial crack, which may cause delamination of ready rolled stock.

The proposed casting method and device are aimed at eliminating the said disadvantages and enlarge the application field for continuous casting wide slabs. Distribution of a metal coming from a mold into several flows ensures achievement of the following technical effect in comparison with the method disclosed in RU Patent No. 2359782. First, kinetic energy of a vertically directed flow transforms most fully into rotational movement of a metal in the mold volume without disturbing the meniscus upper layers, which enables to increase casting mass rates. Second, more full temperature averaging of the metal volume may be achieved due to formation of a unidirectional flow rotating in a horizontal plane in the whole slab width. Furthermore, conditions are created for increasing casting rates by 3 to 5 per cent due to better heat removal and formation of slag skull, and nozzle wear may be reduced due to decreased velocity of metal flows in the outlet channels.

The claimed technical effect can be achieved due to the fact that during continuous casting of steel an overheated metal is fed from an intermediate ladle into a mold of rectangular section through a submersible nozzle having, in its upper part adjoining the intermediate ladle, a charging channel that is divided at least into two dead-bottom outlet channels in the lower part of the nozzle. The dead-bottom outlet channels are located in the nozzle volume that is limited by a skirt lengthened in the direction of the mold short facets and conjugated to the nozzle above the level, at which the dead-bottom channels are divided, and the skirt lower edge located below the level of the bottom of the dead-bottom outlet channels. Each dead-bottom outlet channel includes at least two diametrically disposed and horizontally directed outlets which axes form an angle different from 90° with a mold long facet in a horizontal plane.

For casting slabs having a long facet size from 800 mm to 1,200 mm the nozzle may comprise two dead-bottom outlet channels, and for casting slabs with a long facet size more than 1,200 mm the submersible nozzle design with three dead-bottom outlet channels is preferable. In the latter case, the diameter of each of the end dead-bottom outlet channels should be made 5.0 to 25% greater than the diameter of the central dead-bottom outlet channel for the purpose of more efficient averaging of a temperature in the mold horizontal section.

In a preferable embodiment the outlet axes of the dead-bottom outlet channels are arranged relative to the mold long facet in a horizontal plane at an angle from 15° to 65°, the greater being these angles, the lesser is the long facet size and the greater is the short facet size of the mold. If the submersible nozzle is made with two dead-bottom outlet channels, the angles between the outlet axes and the mold long facet may be equal. If the submersible nozzle is provided with three dead-bottom outlet channels, the outlet axes of the end dead-bottom outlet channels are arranged at equal angles to the mold long facet, but different from the angle of the axis of the central dead-bottom outlet channel.

The outlets of the dead-bottom outlet channels may be made in such a way that they impart to a metal, as entering the mold, rotational movement of equal direction in a plane perpendicular to the slab pulling direction with equal intensity over its width.

The nozzle should be preferably made force-feed. Further, a total area of all the outlets of the dead-bottom outlet channels should be less than the cross-sectional area of the charging channel.

It is preferable that a molten metal that is overheated to a temperature higher than the steel liquidus temperature is charged to the mold.

The invention is described hereinafter in more detail with reference to the accompanying drawings and the invention embodiments.

Fig. 1 shows a general view and a section of the lower part of the submersible nozzle made with two dead-bottom outlet channels.

Fig. 2 shows a general view and a section of the lower part of the submersible nozzle made with three dead-bottom outlet channels.

A melt is fed from an intermediate ladle (not shown) to the charging channel 1 of the submersible nozzle, and the melt comes out to a mold of rectangular section (not shown) in the lower part 2 of the submersible nozzle through the outlets 3 surrounded by the skirt 4.

A melt flow in its way in the submersible nozzle is divided by the dead-bottom outlet channels 5 into two (Fig. 1) or three (Fig. 2) flows. The bottom level 6 of the dead- bottom outlet channels 5, at which the outlets 3 are made, is located above the edge level 7 of the skirt 4.

The outlets 3 are made in such a way that the axis 8 of each outlet forms an angle different from 90° with the mold long facet, or, equivalently, the wide side of the skirt 4.

It is preferable to charge a molten metal overheated to a temperature above the steel liquidus temperature through the submersible nozzle to the mold. If the wide side size of casted slabs is in the range from 800 mm to 1,200 mm, then it is preferable to use the submersible nozzle with two dead-bottom outlet channels 5, as shown in Fig. 1.

In the case where the wide side size of casted slabs is greater than 1,200 mm, it is preferable to use the submersible nozzle with three dead-bottom outlet channels 5, as shown in Fig. 2. Further, it is preferable that the diameter of each of the end dead-bottom outlet channels 5 is 5.0-25% greater than the diameter of the central dead-bottom outlet channel 5. Due to a difference in the diameters of the central and end dead-bottom outlet channels 5 it is possible to redistribute a greatest mass of a "hot" metal, as coming from the intermediate ladle, to the slab short facets, i.e., to the areas of the most intensive heat removal, which also facilitate quicker formation of the slab body.

Moreover, it is preferable that the axes 8 of the outlets 3 of the end dead-bottom outlet channels 5 are arranged at equal angles to the mold long facet, but different from the angle between the axis 8 of the central dead-bottom outlet channel 5 and the mold long facet, in order to optimize molten metal flows in the mold.

In general, it is preferable that the axes 8 of the outlets 3 are arranged relative to the mold long facet in a horizontal plane at angles from 15° to 65°, the greater being these angles, the lesser is the long facet size and the greater is the short facet size of the mold.

The outlets 3 of the dead-bottom outlet channels 5 may be made with the ppossibility of imparting equally directed rotational movement to a melt, as entering the mold, in a plane perpendicular to the slab pulling direction with equal intensity along its width.

The submersible nozzle may be made may be made force-feed. Further, a total area of all the outlets 3 of the dead-bottom outlet channels 5 should be less than the cross- sectional area of the charging channel 1.

Example 1.

The method for continuous casting of steel is applied, comprising charging of an overheated metal from an intermediate ladle into a mold of rectangular section through a submersible nozzle having a channel in its upper part adjoining the intermediate ladle. The size of the mold long facet is more than 800 mm. The nozzle includes a branching into two dead-bottom outlet channels located in the volume limited by a skirt lengthened in the direction of the mold short facets and conjugated to the nozzle above the level, at which the dead-bottom channels are divided, and the skirt lower edge located below the level of the bottom of the dead-bottom outlet channels having each two diametrically positioned and horizontally directed outlets which axes form an angle different from 90° with a mold long facet in a horizontal plane.

When a slab is cast into a mold with the long facet having a size less than 800 mm, all the advantages can be achieved through using a single-channel dead bottom nozzle with a skirt lengthened along the long facets. Kinetic energy of flows coming out of the outlet channels is sufficient for imparting uniformly directed rotational movement to a metal in a plane perpendicular to the direction of slab pulling, the movement intensity being equal along the slab width. A stable flow is formed that runs in the slab pulling direction and ensures mixing of the metal in the mold below the level of the skirt lower edge.

However, if a single-channel nozzle is used for casting a slab with a long facet more than 800 mm, a significant advantage is lost, namely: it is not possible to ensure efficient mixing of a metal by rotational movement in a plane perpendicular to the slab pulling direction with equal intensity along the slab width. Two differently directed metal flows are formed in the areas of the mold short facets in the volume limited by the skirt, the flows being rotated in a horizontal plane and decreasing as they approach to the short facets and as they move in the slab pulling direction. Rotation of a metal in a horizontal plane decays fully at the level of the skirt lower edge, since:

- the kinetic energy of the metal flows coming out of the horizontally disposed outlet channels is insufficient for imparting uniformly directed rotational movement to the metal along the mold width;

- interaction of differently directed flows at the level of the skirt lower edge results in their sharp decay;

- "cold" zones are formed near the mold short facets, which result in poorer conditions for work of a slag-forming mixture and in occurrence of conditions leading to defects at the surface of a slab to be cast;

- in the result of insufficient mixing effect in the horizontal plane the main portion of a metal coming from the mold with a higher temperature is distributed in the axial zone, which causes a poorer slab structure.

As studies show, when casting a slab with a long facet size more than 800 mm, but less than 1,200 mm, all the above-mentioned disadvantages may be eliminated if a nozzle is used that has in its lower part a branching into two dead-bottom channels. In particular, in comparison with an embodiment of the casting nozzle comprising only one dead-bottom outlet channel, the following effects are achieved:

- more efficient averaging of a metal temperature in the slab horizontal section. Accordingly, the conditions for heat removal and formation of slag skull at the slab surface;

- a more stable flow of a rotating metal appears that precludes an increase of concentration of harmful impurities and nonmetal inclusions in the slab axial zone during the crystallization process, thus positively influencing formation of a denser structure;

- more efficient damping of the vertical component of the metal flow velocity in the mold is achieved. The vertical component fully transforms into the metal rotational movement in the horizontal plane. A more hot metal coming from the mold penetrates to a lesser depth of the slab liquid core, which also have positive influence on the crystallization processes and formation of a denser slab structure in the axial zone;

- formation of vertical vortex flows is precluded in the result of damping the verticval component of the metal flow in the mold.

As a consequence, conditions are created for removal of nonmetal inclusions (uniform floating), more uniform melting of a slag-forming mixture is observed (a temperature gradient along the slab vertical is increased, the upper layer is more hot), entrapment of a slag-forming mixture by vertical vortex flows is precluded (reduction in exogenous nonmetal inclusions). At other equal conditions prerequisites are created for increasing a casting rate.

Example 2.

The method for continuous casting of steel is applied, comprising charging of an overheated metal from an intermediate ladle into a mold of rectangular section through a submersible nozzle having a channel in its upper part adjoining the intermediate ladle. The size of the mold long facet is more than 1,200 mm. The nozzle, in its lower part, includes a branching into three dead-bottom outlet channels located in the volume limited by a skirt lengthened in the direction of the mold short facets and conjugated to the nozzle above the level, at which the dead-bottom channels are divided, and the skirt lower edge located below the level of the bottom of the dead-bottom outlet channels having each two diametrically positioned and horizontally directed outlets which axes form an angle different from 90° with a mold long facet in a horizontal plane. As studies show, if a nozzle having a branching into two dead-bottom outlet channels is used for casting a slab into a mold with the long facet size more than 1,200 mm, this ensures metal mixing due to rotational movement in a plane perpendicular to the slab pulling direction only in the volume limited by the skirt. The two dead-bottom outlet channels are insufficient for forming a stable metal flow rotating in the horizontal plane along the mold width, such a flow is fully damped at the level of the skirt lower edge. As the size of the long facet increases, the negative effects, as described in Example 1, develop:

- in the result of insufficient mixing of a metal in the horizontal plane the main portion of a metal coming from the mold with a higher temperature is distributed in the axial zone, which causes a poorer slab structure.

All the above-mentioned disadvantages relating to casting a slab with a long facet size more than 1,200 mm may be eliminated if a nozzle is used that has in its lower part a branching into three dead-bottom channels.

Example 3.

In order to achieve more efficient averaging of a temperature in the horizontal section of a mold with a long facet size more than 1,200 mm, the diameter of each dead- bottom outlet end channel is increased by 5.0 to 25% relative to the diameter of the dead- bottom outlet central channel. A difference in the diameters of the central and peripheral channels is selected so as to be greater with an increase in the size of the mold long facet.

It is shown that at equal diameters the main portion of a metal enters a mold through the central dead-bottom outlet channel in the direction of a least resistance, which reduces efficiency of the channel work significantly, namely:

- conditions for forming a stable, horizontal, rotating metal flow become worse, since kinetic energy of a small portion of a metal, as exiting the peripheral channels, is insufficient for this;

- a main mass of a "hot" metal coming from the mold is distributed in the central zone of a forming slab, and, accordingly, its structure becomes worse;

- "cold" zones are formed near the short facets; correspondingly, conditions become worse for work of a slag-forming mixture, surface defects are formed more frequently; A reduction in the diameter of the central channel relative to the peripheral ones enables to redistribute metal flows optimally and increase the mixing effect in the areas of the mold short facets, which is of special importance when casting a wide slab. Since a main portion of a "hot" metal is fed to the short facets, i.e., to the area of the most intensive heat removal, the slab-forming rate is increased.

Example 4.

The method for continuous casting of steel is applied, comprising charging of an overheated metal from an intermediate ladle into a mold of rectangular section through a submersible nozzle having a channel in its upper part adjoining the intermediate ladle. The size of the mold long facet is more than 1,200 mm. The nozzle, in its lower part, includes a branching into three dead-bottom outlet channels located in the volume limited by a skirt lengthened in the direction of the mold short facets and conjugated to the nozzle above the branching level of the dead-bottom channels and the skirt lower edge located below the level of the bottom of the dead-bottom outlet channels which axes form an angle less than 15° with the mold long facet.

It is discovered that in this case metal flows from the central dead-bottom outlet channel act on the external walls of the peripheral dead-bottom outlet channels, and, vice versa, metal flows from the horizontal holes located most closely to the mold short facets act on the skirt radial generatrix. As a result, strict orientation of flows is broken, they lose their kinetic energy and take different directions. The conditions for forming a stable metal flow rotating in a horizontal plane along the whole width of the mold are lost completely.

The same takes place when a slab with the long facet size less than 1,200 mm is cast. And in this case exiting metal flows act on the walls of the neighboring dead-bottom outlet channel and also lose their kinetic energy.

Example 5.

The method for continuous casting of steel is applied, comprising charging of an overheated metal from an intermediate ladle into a mold of rectangular section through a submersible nozzle having a channel in its upper part adjoining the intermediate ladle. The size of the mold long facet is more than 1,200 mm. The nozzle, in its lower part, includes a branching into three dead-bottom outlet channels located in the volume limited by a skirt lengthened in the direction of the short facets and conjugated to the nozzle above the branching level of the dead-bottom channels and the skirt lower edge located below the level of the bottom of the channels having each two diametrically located and horizontally directed outlets which axes form an angle more than 65° in the horizontal plane with the mold long facet.

It should be noted that a velocity of a metal horizontal flow sharply decreases at an angle more than 65°. As the angle between the outlets axes and the mold long facet approaches the value of 90°, action of the exiting metal flows on the skirt inner surface increases, and the rotational velocity of the horizontal metal flow verges towards zero. Flows having different directions appear, a part of which acts on the mold walls and on the forming slab skull, which causes disturbance of the meniscus, and the rate of defect formation on the slab surface increases.

The same can be observed when casting a slab having a long facet size less than 1,200 mm.

The technical effect of the claimed method for continuous casting of steel and the charging submersible nozzle used therein is an increase in the casting rate, more full volumetric averaging of a metal temperature, less wear of the submersible nozzle. All this results in increased productivity of slab plants for continuous casting of slabs.

Claims

1. A method for continuous casting of steel, including charging of an overheated metal from an intermediate ladle to a mold of rectangular section through a submersible nozzle having, in its upper part adjoining the intermediate ladle, a charging channel, characterized in that the overheated metal, as passing through the charging channel in the lower part of the submersible nozzle, is divided into at least two flows by directing it to at least two dead-bottom outlet channels located in the nozzle volume limited by a skirt lengthened toward the mold short facets and conjugated to the nozzle above the level of the branching dead-bottom outlet channels and the skirt lower edge being below the bottom level of the dead-bottom outlet channels, and, further, the said at least two metal flows are fed to the mold through two diametrically disposed and horizontally directed outlets made in each dead-bottom outlet channel, the outlet axes forming an angle different from 90° in the horizontal plane with the mold long facet.

2. The method for continuous casting of steel according to Claim 1, characterized in that, when casting slabs having a long facet size from 800 to 1,200 mm, the metal flow is divided into two flows passing through the two dead-bottom outlet channels.

3. The method for continuous casting of steel according to Claim 1, characterized in that, when slabs having a long facet size more than 1,200 mm, the metal flow is divided into three flows passing through three dead-bottom outlet channels.

4. The method for continuous casting of steel according to Claim 2 HJIH Claim 3, characterized in that the metal flows are directed at the mold entrance through the outlets along the axes of the outlets dead-bottom outlet channels directed relative to the mold long facet in the horizontal plane at angles from 15° to 65°, the greater being these angles, the lesser is the long facet size and the greater is the short facet size of the mold.

5. The method for continuous casting of steel according to Claim 1, characterized in that the metal flows exiting the outlets dead-bottom outlet channels and coming to the mold are given uniformly directed rotational movement in a plane perpendicular to the slab pulling direction with equal intensity along the slab width.

6. The method for continuous casting of steel according to Claim 1, characterized in that metal pressure in the submersible nozzle is ensured by making the total area of the outlets dead-bottom outlet channels less than a cross-sectional area of the charging channel.

7. The method for continuous casting of steel according to Claim 3, characterized in that averaging of a metal temperature is ensured in the mold horizontal section by making the diameter of each peripheral dead-bottom outlet channel by 5.0 to 25% greater than the diameter of the central dead-bottom outlet channel.

8. The method for continuous casting of steel according to Claim 1, characterized in that a molten metal, which is overheated to a temperature greater than the steel liquidus temperature, is charged into the mold.

9. A submersible nozzle for continuous casting of steel, comprising: a charging channel in its upper part adjoining an intermediate ladle; the nozzle bottom, the outlets and the skirt, which are located in the lower part of the nozzle, the said skirt being lengthened toward the mold short facets and attached above the outlets in such a way that the nozzle lower part is located in the center of the skirt; characterized in that the charging channel in the nozzle lower part is made branched to at least two dead-bottom outlet channels located in the nozzle volume limited by the skirt conjugated to the nozzle above the branching level of the dead-bottom outlet channels, each dead-bottom outlet channel being provided, above the bottom level, with two outlets disposed above the level of the skirt edge; the said outlets being disposed diametrically, and the outlet axes forming an angle different from 90° in the horizontal plane with the mold long facet.

10. The submersible nozzle according to Claim 9, characterized in that, in order to cast slabs with a long facet size from 800 to 1,200 mm, the nozzle is made with branching into two dead-bottom outlet channels.

11. The submersible nozzle according to Claim 9, characterized in that, in order to cast slabs with a long facet size more than 1,200 mm, the nozzle is made with branching into three dead-bottom outlet channels.

12. The submersible nozzle according to Claim 10 HJIH Claim 11, characterized in that the outlet axes are located relative to the mold long facet in the horizontal plane at angles from 15° to 65°, the greater being these angles, the lesser is the long facet size and the greater is the short facet size of the mold.

13. The submersible nozzle according to Claim 9, characterized in that the outlets of the dead-bottom outlet channels are made so as to impart to a metal, as entering into the mold, uniformly directed rotational movement in a plane perpendicular to the slab pulling direction with equal intensity along the slab width.

14. The submersible nozzle according to Claim 9, characterized in that it is made force- feed, the total area of all the outlets of the dead-bottom outlet channels being lesser than the cross-sectional area of the charging channel.

15. The submersible nozzle according to Claim 11, characterized in that the diameter of each peripheral dead-bottom outlet channel is made 5.0 to 25% greater than the diameter of the central dead-bottom outlet channel.