JPS6247102B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the refining of metals, particularly steel. In particular, it relates to the field of continuous casting methods, which involve rotating molten metal in a mold about a casting axis. Continuous casting methods, which involve rotating the molten metal in a mold around the casting axis, offer advantages over traditional continuous casting methods in terms of the quality of the semi-finished products obtained (improved surface and subsurface cleanliness). , higher internal health,
This is said to be advantageous in terms of the progress of the casting process itself (such as a significant reduction in segregation and the formation of cavities), as well as in the progress of the casting process itself. That is, it is possible to cast at higher speeds, or the control of superheating of the metal in the distributor may not be as precise. There are actually two known industrial methods for achieving rotation of metal in a mold. The first one is
From the order of things, "centrifugal continuous casting method (CCC)"
In this method, the rotation is effected by mechanical means and is carried out by a combination of rotation of the mold and extractor and tangential eccentric feeding of the molten metal by means of a shoe or curved nozzle. The second is ``magnetic rotary continuous casting'', better known today as ``magnetosil''. The method essentially consists of subjecting the molten metal to a rotating magnetic field induced by a stationary multiphase induction device immersed in a water-cooled chamber surrounding the mold. (French patent no.
No. 2315344) These two methods yield similar results in terms of metallurgy. However, the magnetosyl method
It is substantially superior to the CCC method in that, on the one hand, there is no need to rotate parts of the casting machine (such as the mold), and on the other hand, there are no restrictions on the performance of the mold with respect to molten metal. In fact, the electromagnetic force required to rotate the metal can be easily adjusted and selected to a level sufficient to drive the metal. In principle, the Magnetosil process differs from the CCC process in that the molten metal jet is fed by a long tube that protects it in the presence of a coating powder that forms a slag. This slag protects the free surfaces of the metal in the mold, as in the traditional continuous casting technique, better known as "casting with immersed nozzles and coated slugs." The advantages of such a configuration, as is well understood, are that it rotates the metal yet protects it from re-oxidation, and that it allows the metal to be rotated while protecting it from re-oxidation, and for delicate or difficult-to-cast materials such as alloyed steels or steels with high additions of aluminium, for example. It is possible to cast metals in good condition. However, experience has shown that, on a general scale, the reality of submerged nozzle casting is not very consistent with the rotation of the metal being cast. In fact, as a result of the rotational movement, the free surface of the metal has a concave shape, in the center of which the coating powder tends to collect. This results in inconvenience in two ways. First, the periphery of the free surface is exposed, which undermines the desired result. Another problem is that there is a risk of getting powder or scum into the center of the cast product, which is contrary to the desired result. The present invention aims to define operating conditions in the magnetosil process such that this process is compatible with casting processes using immersed nozzles and coated slugs. To this end, the present invention provides a process for continuous casting of metals by conventional immersion nozzle and coated slug techniques, in which the liquid metal is rotated inside the mold by means of a magnetic field rotating about the casting axis;
The linear velocity around the rotating liquid metal is 0.3 to 0.8
m/s range, align the nozzle with the axis of the mold, and reduce the immersion depth of the nozzle to at least approximately 3 cm.
and use a coating powder having the following properties: ●Melting temperature around 1000-1200℃. ●0.3 at the temperature of molten metal or 1500℃
A viscosity of the order of ~3 poise, in particular a viscosity of about 0.3 to 1 poise or a viscosity of about 1 to 3 poise, depending on whether the withdrawal speed of the product to be cast is higher or lower than 1 m/min. According to a preferred embodiment, the free surface of the molten metal in the mold is coated with a coating powder before the metal is rotated. For the purpose of understanding the invention, the characteristic operating conditions of this method will be explained in more detail. 1. The nozzle must be on the axis of the mold, and this axis, as understood, coincides with the axis of rotation and the axis of the casting. This is ensured by keeping the system symmetrical about the axis and thus not disturbing the rotational motion. in this regard,
In addition to the problem that eccentricity of the jet leads to damage to the nozzle end and the well-known difficulties inherent in small-scale products, the rotational movement of the free surface of the metal, at least to the extent that it can be visually determined, Experience has shown that this significantly impairs the quality of 2. The immersion depth to be ensured is approximately 3 cm. Temporarily tested depths of less than this value, which vary between approximately 1 and 2 cm, indicate that the coating slag and the deposits accumulated in the central part of the free surface of the metal (i.e. the concave side of the liquid). but,
This resulted in the product being engulfed in the axial direction to the inside of the cast product. In principle, the invention places no restrictions on the maximum immersion depth of the nozzle. However, too deep an immersion, for example in the vicinity of about 10 cm, is unfavorable in terms of renewing the liquid at the surface, which solidifies, especially at the lowest temperatures, and sticks to the walls of the mold around the nozzle ('beads'). It is known that there is a risk of the formation of It has been confirmed that the magnetosil method can reduce the above-mentioned concerns thanks to the convection force generated in the metal. In other words, the depth of immersion can be increased compared to conventional casting methods using immersion nozzles. This can be another advantage of the magnetosil method, since the maximum allowable depth does not have to be controlled very precisely. 3 Regarding coated powders, two properties are decisive. melting temperature, and viscosity as a liquid. In all respects it is preferable that the slug be as close to the liquid surface as possible, like a tablecloth. To do this, the slug must rotate at a speed as close as possible to that of the metal on the surface. That is, friction at the metal-slag interface and within the slag layer should be minimized. This is satisfied by the following two conditions. The coating powder shall form a homogeneous liquid phase immediately upon contact with the molten metal. For this, the range of values regarding melting temperature is 1000-1200
℃, and preferably decreases as the withdrawal speed increases. In this respect, the present invention does not differ from the prior art. The liquid slag must be sufficiently fluid. If we also take into account its role as a lubricant between the walls, it follows that the slag must become more fluid as the speed of withdrawal increases. This requirement dictates that the temperature of the molten steel, i.e. approximately
At 1500°C, the range of permissible values of viscosity is divided into two regions according to slow or fast draw. The boundary is approximately 1m/
It's around the minute. At higher speeds, the preferred viscosity of the slag is between 1 and 0.3 poise, and below about 1 to 3 poise. The relationship between the drawing speed of such a cast product and the melt viscosity of the coated powder is as shown in the graph of FIG.
It is represented by a band sandwiching a straight line descending to the right, centering on the poise and the drawing speed of 1 m/min, and the higher the drawing speed, the lower the melt viscosity, and the lower the drawing speed, the higher the melt viscosity. According to this graph, it can be seen that the width of this zone is approximately three times higher and lower than the melt viscosity. Outside this range, the surface of the cast product becomes rough. Furthermore, it is preferable that the melting temperature and viscosity vary within this range depending on the size of the cast product. 4. The area around the rotation, that is, the area allowed for the linear velocity of the rotation of the metal in contact with the wall of the mold, is:
The value ranges from 0.3 to 0.8 m/sec. The lower limit of this region is the fact that at least a certain amount of rotation must be applied in order to ensure the desired metallurgical results, i.e. good homogeneity of the cast product, i.e. 0.3 m/s. determined by. The upper limit of this range is defined by reasons directly related to the manner in which the metal is supplied in the mold. That is, at values above 0.8 m/s, the liquid surface is no longer completely covered with powder, and the powder accumulates in the center of the liquid surface and gets caught inside the product due to a phenomenon called swirling. It can be done. The best value of the velocity within the region determined in this way will not be chosen independently of the range of coating powder properties (melting point, viscosity) indicated in this regard. The selection of each of the above values is made according to the size of the product to be cast. If there is no established relationship, it is not a problem to give optimal values for all possible scales here. The problem, as will be appreciated, is really only in the area of casting operations, which in the course of conventional operations determines the value of each factor in the range most suitable for that size of casting. To facilitate this problem, some examples are presented in a general order. For a given linear velocity of rotation, the angular velocity is used when the scale of the casting is small. In this connection, the free surface of the metal in the mold assumes the concave shape described above. Therefore, in order to properly coat the surface of the slag, it is necessary to use a coating powder that melts more easily and has a lower viscosity. On the contrary, if the coating powder is given (melting point and viscosity fixed within a range of predetermined values), the linear speed of rotation will, within permissible limits, depend on the scale of the casting. The larger the value, the lower the value. Considering the relationship between the linear speed of rotation and the properties of the coating powder, only a mutual combination of the previously described configurations will give the desired result. However, the pursuit of the best possible choice regarding the linear speed of rotation and the properties of the powder, while ensuring optimal results, is not an essential condition for the implementation of the invention. In fact, the first result, that is, the ability to proceed with casting using a submerged nozzle and coating powder, and with the metal rotating in the mold, is guaranteed as long as values lying within the limits mentioned above are adopted. In some cases, of course, the practitioner has to endure the immediate imposition of some constraints, such as increased consumption of powder or lower withdrawal speeds than optimal standards allow. Maybe. Thus, if the above provisions are followed, on the one hand the casting operation proceeds in good conditions and, despite the concave shape resulting from the rotation, the coating powder covers the free surface of the metal uniformly and on the other hand In terms of metallurgical quality, the finished semifinished product exhibits all the characteristics of rotation, in addition to the effects inherent in casting with an immersion nozzle and coated powder. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood, and other aspects and advantages will become more apparent, from the following description, given by way of example, and reference to the accompanying drawings. FIG. 2 shows, in longitudinal section, the upper part of an apparatus for continuously casting circular steel bars. 1 indicates a distributor, and 2 indicates a continuous casting mold. The distributor contains molten steel 3 and has an opening 4 at its bottom through which the liquid steel flows out. The discharge of steel is controlled by a stopper 5, the tip 6 of which cooperates with the apertured valve seat 7. The distributor thus continuously supplies the molten metal in the form of a jet to the mold, and at the same time continuously withdraws the partially solidified steel rod from the bottom of the mold. In accordance with well-known technology, the mold consists of a double-walled box whose inner wall 10 of copper or copper alloy defines the passage for the cast product. The box has a tube-shaped inner enclosure 11 which, together with walls 10, defines an annular space 12 for the circulation of cooling water. A lateral watertight bulkhead 13 provides two superimposed chambers 14 and 1 for cooling water to enter and exit as shown in the figure.
The inside of the mold is divided into 5. According to the magnetozil process, the upper chamber 15 is occupied by a stationary annular electromagnetic induction device 16 surrounding the rod to be cast. This induction device consists of a number of internal electromagnetic coil bobbins distributed regularly around the wall 10. The induction device is connected to a three-phase (or more generally polyphase) current supply, not shown. The wire carries a magnetic field perpendicular to and rotating around the axis of the casting, indicated at 17, with an angular velocity adjusted according to the frequency and strength of the current supplied to the coil. Do so in a suitable (and well-known) manner, such as by creating The induction device 16 thus serves as the stator of a motor with synchronous induction rotation, and the still liquid portion 18 of the rod being cast becomes the rotor. Due to the action of the rotating magnetic field, the liquid metal 18 contained in the mold is caused to undergo a rotational movement about the casting axis 17 in the direction of the magnetic field (as indicated by the arrow). As a result, a concave liquid surface 19 is formed on the free surface of the liquid metal, as shown somewhat exaggerated in the drawing. As mentioned above, this itself is already part of the prior art. In particular, regarding the production of the guiding device and the mold with it, if desired, reference may be made, for example, to French Patent No. 2,315,344 (IRSID) for more details. As shown in the drawing, the jet 8 of molten metal flows through the distributor 1
into a refractory tube or nozzle that runs to the bottom of the mold and reaches into the liquid metal in the mold. This nozzle penetrates into the liquid metal through the center of the liquid level 19 according to one of the features of the invention, with one end in the aperture 4 of the distributor and the other end on the axis 17 of the mold. As in, it is centered. At the same time, according to the invention, the immersion depth is around 5 cm. The liquid surface 19 is covered with a layer 21 of coating powder. When it comes into contact with molten steel, it quickly forms a film 22 of liquid slag. Needless to say, this powder, in addition to having suitable physicochemical properties (melting temperature, viscosity) for the purposes of the present invention, has well-known properties that generally comply with those required for traditional continuous casting. It also has many functions, including thermally and chemically protecting the metal from the surrounding atmosphere, acting as a lubricant for molds, and trapping surface inclusions. have to work. The invention will now be illustrated by a quantitative example relating to the casting of aluminum-loaded steel billets. The cast billet has a circular cross section with a diameter of 120 mm. The submerged nozzle 20 is made of fused silica, better known in the market as "glassrock". Its outer diameter is 70mm and inner diameter is 40mm
It is. The immersion depth was kept at an average of about 5 cm, with temporary fluctuations limited to 1 to 2 cm. Taking into account the billet size, the casting speed (i.e. withdrawal speed) was always adjusted to 2.2-2.4 m/min. The induction device was supplied with a three-phase AC current of 180 A and a frequency of 3 Hz between the phases and the neutral point. Under these conditions, the angular velocity of the steel rotation is 80 revolutions/min, which is the peripheral linear velocity of 0.5 m/sec. The attached table shows the properties of the coating powder used. The average melting temperature is around 1050°C, and the viscosity at 1500°C is about 0.4 poise. The consumption of this powder during the casting process was 0.5 Kg per ton of steel. In accordance with a preferred embodiment of the present invention, casting was started without the induction device in effect, allowing the coating powder to contact the steel and form a film 22 of liquid slag. This design provided effective protection from the beginning of the casting, and a good product was obtained even from the first meter. Of course, this is not an essential feature for the invention. In fact, it is also possible to introduce powder onto the liquid surface after the liquid steel has been rotated. In that case, however, the powder tends to collect in the center and be exposed to a certain temperature (corresponding to the formation of a liquid film) before it covers the entire surface of the liquid.
During that time, the peripheral part of the liquid surface remains exposed,
If the immersion depth of the nozzle is at its limit, some of the powder will become entangled in the shaft of the product. The first meter of the product obtained, as well as some of the starting billets, will be of poorer quality than the rest of the product in terms of internal cleanliness. Several other castings were made in different sizes. One of them concerns the casting of a circular bloom with a diameter of 240 mm. All other conditions were the same as before, but the withdrawal speed was kept around 0.8 to 1 m/min, and the angular velocity of the steel rotation was 50 revolutions/min. This corresponds to a peripheral linear velocity of 0.6 m/sec. The properties of the powders used are shown in the attached table. Average melting temperature is 1170℃, viscosity at 1500℃ is 1.6
It's poise. Needless to say, the method of the present invention is not limited to the embodiments shown here. This applies in particular not only to the casting of circular rods;
Products that do not have a rotating body cross section, such as products with a rectangular cross section such as billets with a rectangular cross section or blooms with a square cross section, or other types of guiding devices whose external dimensions are predetermined. Applicable to the extent that it is compatible with the use of In addition, the invention applies not only to various steels, but also to all metals that can be continuously cast.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a graph showing the preferred range of the melt viscosity of the coated powder versus the withdrawal speed of the cast product, and FIG. 2 is a longitudinal sectional view of the upper part of the apparatus for continuously casting circular steel bars. 1... Distributor, 2... Mold, 3... Molten steel, 8...
... Jet stream, 9 ... Partially solidified cast product, 16 ... Electromagnetic induction device, 17 ... Casting axis, 19 ... Liquid level,
21...Layer of coating powder, 22...Slag film.

Claims (1)

[Scope of Claims] 1. In a continuous steel casting method using an immersion nozzle and a coated powder in which the molten metal rotates in a mold around the casting axis under the action of a rotating magnetic field, the nozzle is The rotational speed of the molten steel in the mold is maintained at a peripheral linear velocity of 0.3
The melting temperature of the coated powder is within the range of 1000 to 1200℃, and the melt viscosity at 1500℃ is maintained at a casting speed of 1m/sec.
A continuous casting method for steel, characterized in that when the casting speed is faster than 1 m/min, the casting speed is 1 poise or less and up to 0.3 poise, and when the casting speed is slower than 1 m/min, the casting speed is 1 poise or more and up to 3 poise. 2. The sequence according to claim 1, wherein coating powder is introduced onto the molten steel in the mold before the molten steel is rotated to form a liquid slag film between the powder layer and the surface of the molten steel. Casting method.