US3888294A - Method of continuously casting steel - Google Patents

Abstract

A continuous steel casting method for a casting capacity of more than 1.5 metric tons of steel per minute, wherein a stream of steel is supplied to below the casting level in the mold through a casting tube having lateral openings and at least one additional bottom opening directed vertically downward and having a cross section smaller than the cross section of the casting tube. The angle of inclination of the axes of the lateral openings, in relation to the horizontal, lie within a range of 70* downward to 90* upward. The overall cross section of all of the casting tube openings corresponds to at least the clear cross section of the casting tube. At the place of origin of the stream of steel above the mold inert gas is supplied in a certain amount to the stream of steel over its entire cross section.

Description

United States Patent 1 Fastner et al.

[ June 10, 1975 METHOD OF CONTINUOUSLY CASTING STEEL [73] Assigneez Vereinigte Osterreichische Eisenund Stahlwerke Alpine Montan Aktiengesellschaft, Linz, Austria 22 Filed: May 24,1974

211 App1.No.:473,212

3,578,064 5/1971 Mills et al. 164/281 3,608,621 9/1971 Bollig et al 164/281 3,738,419 6/1973 Hartmann et a1... l64/28l 3,833,047 9/1974 Tovini 164/66 FOREIGN PATENTS OR APPLICATIONS 228.418 7/1963 Austria .1 164/259 808,711 ll/l936 France ZZZ/DIG. 11

443,705 l/l949 Italy 164/57 445,034 2/1968 Switzerland 164/57 Primary Examiner-R. Spencer Annear Attorney, Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond [57] ABSTRACT A continuous steel casting method for a casting capacity of more than 1.5 metric tons of steel per minute, wherein a stream of steel is supplied to below the casting level in the mold through a casting tube having lateral openings and at least one additional bottom opening directed vertically downward and having a cross section smaller than the cross section of the casting tube. The angle of inclination of the axes of the lateral openings, in relation to the horizontal, lie within a range of 70 downward to 90 upward. The overall cross section of all of the casting tube openings corresponds to at least the clear cross section of the casting tube. At the place of origin of the stream of steel above the mold inert gas is supplied in a certain amount to the stream of steel over its entire cross section.

14 Claims, 6 Drawing Figures METHOD OF CONTINUOUSLY CASTING STEEL The invention relates to a continuous steel casting method for a casting capacity of more than L5 metric tons of steel per minute using a vertically arranged, substantially rectangular mold, into which at least one steel stream containing inert gas is passed via a casting tube below the casting level in the mold, the casting tube being provided with lateral openings, whose axes lie in a vertical plane running through the longer trans verse axis of the mold.

It is known to provide in continuous casting below the bottom opening of the tundish a gas-permeable, annular disk of refractory material, through which an inert gas, preferably argon is blown in radially in direction of the axis of the casting stream. Gas bubbles are carried downward together with the steel stream, then rise from the liquid phase in the continuous casting mold together with nonmetallic particles contained in the steel and wash these particles into a slag layer floating on the casting level in the mold (CONCAST NEWS, Vol. 10, l/l97l, page 4 and FIG. ll). The gas bubbles also cause a reduction of the penetration depth of the metal stream in the mold, which diminishes the danger of crack formation. The use of refractory, gaspermeable disks or of other intermediate pieces has however the disadvantage that the gas has to be supplied under pressure. The regulation of the gas supply is also problematic and operationally not reliable because ofthe danger that the gas channels might become blocked. Moreover, a gas veil forms between the casting tube and the steel stream. it is not possible to supply the gas in such a way that its amount is distributed evenly over the entire stream cross section. It has therefore been proposed to provide in the tundish an outlet comprising two parts, the outer part consisting of a strongly gas permeable and the inner part of a less gaspermeable material, so that the inert gas may be supplied in vertical direction upward as well as radially and perpendicularly towards the steel stream (German Utility Model Pat. No. 7,l49,261); coarse non-metallic inclusions are to be passed upward into the slag layer of the tundish before they enter the outlet, while the remaining gas passes downwardly through the casting tube for braking the casting stream. The construction of this known outlet is complicated, its production is expensive and its operational reliability is not sufficient either; it cannot be guaranteed that the supplied gas is being distributed evenly over the cross section of the metal stream. The same applies analogously for spout stones having a lateral bore for introducing a gas (German Utility Model Pat. No. 6,9l8,019).

In continuous steel casting procedures it is important that the strand should be free from surface-and inner cracks, because defects of this kind lead to material losses, as cracked strands have to be scarfed or even rejected. As modern continuous casting plants are to work fully continuously, a further demand consists in increasing the safety against breakthroughs of liquid steel through the solidified strand skin; the breakthroughs hitherto occurring repeatedly in continuous casting plants may cause considerable damages to the plant and the production loss may be substantial. Practice has shown that these problems are relatively insignificant up to a casting capacity of 1.5 metric tons of steel per minute. If, however, the casting capacity is raised above 1.5 metric tons of steel per minute, it becomes obvious that the present technology is no longer sufficient; in rapid casting plants for slabs having a thickness of lSO up to 250 mm and a width of 800 up to 2500 mm and more the crack formation and the liability to breakthroughs increases strongly as the casting velocity rises, because the steel flow causes cavitations in the range of the strand skin, which cavitations are all the more critical, the thinner the strand skin. As is known the strand skin thickness diminishes as the casting capacity increases. The steel flow in the liquid phase of the strand is determined substantially by the direction of the axes of the lateral openings of the casting tube. Normally one uses refractory casting tubes with a closed bottom and two lateral openings downwardly inclined and directed towards the narrow sides of the mold. When the casting velocity is high mainly edge cracks and breakthroughs in the range of the edges of the strand occur. If one casts with a casting stream directed exclusively vertically downward, longitudinal cracks at the broad side of the slab in the range of the strongest flow may occur. Moreover, nonmetallic inclusions are carried into a great depth of the liquid phase; these have no opportunity to rise and to get into the slag floating on the surface of the liquid steel (casting level). If in a casting tube with closed bottom the lateral outlets are directed steeply or perpendicularly upward, the casting level is moved strongly in the mold at a high casting speed, whereby the slag practice in the mold is made difficult.

It is the object of the invention to eliminate these difficulties in the casting of steel slabs by means of rapidor high capacitycasting machines and to provide a method, in which slabs wider than 800 mm can be cast with a capacity of 1.5 metric tons of steel per minute without crack formation and by using simple and well proven means, wherein the slabs should be poor in nonmetallic inclusions; furthermore the danger of breakthroughs beneath the mold is to be diminished.

According to the invention this object is achieved in a continuous steel casting method of the above described type in that a casting tube is used, which, apart from the lateral openings, is provided with a least one additional bottom opening directed vertically downward, whose cross section is smaller than the clear cross section of the casting tube (steel stream), wherein the inclination angle a of the axes of the lateral openings in relation to the horizontal lies within a range of downward up to upward, preferably 10 to 90 upward, and the total cross section of the lateral openings and the bottom openings directed vertically downward of the casting tube corresponds at least to the clear cross section of the casting tube and that the inert gas is supplied to the steel stream over its entire cross section at the place of its origin above the mold in a quantity of l to 15 Ncm/kg steel, preferably of 3 to 8 Ncm /kg steel. Ncm means normal volume, i.e., volume at 0 and 760 mm H In order to achieve an optimum distribution of the inert gas over the entire cross section of the steel stream the inert gas is, according to a further feature of the invention, supplied coaxially to the steel stream.

It is advantageous that for the gas supply a refractory tube is used, whose outlet is adjustable at a distance above the bottom of a tundish, in which the casting tube is fastened, wherein this distance is preferably equal or a little smaller than the diameter of the casting tube.

For the gas supply one can also use a refractory tube. whose outer diameter is smaller than the inner diameter of the casting tube and whose outlet is adjustable at a distance below the bottom of the tundish, in which the casting tube is fastened, wherein this distance is preferably equal or smaller than the diameter of the casting tube.

It is a feature of the invention that in rapid casting procedures part of the steel quantity is allowed to flow out ofthe casting tube vertically downward, on account of which a further feature of the invention consists in that the cross section of the downwardly directed bottom opening(s) of the casting tube expressed in 7: of the casting tube (steel stream-l cross section in dependence upon the casting capacity expressed in metric tons of steel per minute is adjusted in such a way that at 1.5 metric tons per minute the cross section lies in the range of to 60% and rising proportionately up to a casting capacity of 60 metric tons per minute increases to 50 to 90%.

The invention also comprises the use of a suitable slag or slag powder, respectively, which is applied onto the casting level in the mold. This slag serves for receiv ing non-metallic particles from the steel, in particular from aluminum-containing steel, it is easily melted, is produced preferably synthetically and has the following composition:

20 to 50% fluxing agents of the group Cal- Na O,

K20 and B203 20 to 40% Si0 to 40% CaO less than 59% A1 0 and 0 to 107: oxides of iron, manganese and magnesium. Particularly advantageous is the use of a slag having the following reference analysis:

about 8% Nat O K 0 about B203 about SiO about 30% CaO less than about 2% Al O about 4% FeO about 4% C.

Further features and details of the invention are illustrated by way of example with reference to the accompanying drawings.

FIG. 1 is a schematic illustration of a vertical section through the upper part of a continuous casting plant with a tundish.

FIG. 2 is a longitudinal section through a casting tube to be used in the method according to the invention, wherein various possibilities for the inclination of the axes of the lateral openings in the wall of the casting tube are drawn.

FIG. 3 shows the head of a casting tube, where the lateral openings are directed vertically upward.

FIG. 4 is a horizontal section through a slab mold and a casting tube with oval cross section, which possesses two bottom openings directed perpendicularly down' ward.

FIG. 5 is a graph showing the most favorable cross section for the bottom opening(s) of the casting tube, i.e., on the abscissa the casting capacity is plotted in metric tons per minute and on the ordinate the portion of the cross section of the bottom opening(s) in per cent of the casting tube- (steel stream-) cross section is plotted.

FIG. 6 illustrates a modified embodiment of the supply of inert gas to the steel stream.

In FIG. 1, 1 denotes a refractory tundish with a bottom opening 2, which is capable of being closed by a liftable and lowerable refractory stopper 3. The stopper 3 has an axial bore 4, through which, in the direction of the arrow 5, an inert gas at a quantity of l to 15 Ncm /kg steel is supplied; it has been found that ifa gas quantity of less than lNcm lkg steel is supplied, cracks occur in the slabs, whereas with gas quantities higher than 15 Ncm /kg steel the casting level 12 in the mold bubbles too strongly, so that a perfect control of the slag 13 is no longer possible. The outlet of the bore 4 is adjusted above the upper edge ofa casting tube 6, respectively the bottom 7 of the tundish l at a distance +a in such a way that the supplied gas is sucked in practically without pressure by the steel flow indicated by the arrows 8 in the range of the bottom opening 2; this is the case when a is equal to or smaller than the diameter d of the bottom opening 2 of the casting tube 6. One can, however, also adjust the stopper 3 to a higher distance b, i.e., a distance bigger than d, when the inert gas is blown in under pressure. In each case, by the coaxial gas supply, a uniform distribution of the gas over the entire steel stream- (casting tube-) cross section occurs, so that an optimum braking effect is achieved, in particular because the gas is supplied at the place of the origin of the steel stream at the largest distance possible from the mold. The steel-gas mixture emerges into the liquid phase 15 of the steel through two horizontal openings 9 lying opposite each other, in the wall of the casting tube 6, as well as through a bottom opening 10', the flow direction is marked by arrows 11. As can be seen, the steel-gas mixture is deflected below the casting level 12 without damage occurring in the strand skin 16 forming in the water-cooled mold l4 and without breakthrough of the liquid phase l5. l7 denotes support and guiding rollers for the strand. Below the oscillating mold 14 spray nozzles (not illustrated) are provided for cooling the strand in the secondary cooling zone till the complete solidification. l8 denotes the quantity of steel present in the tundish 1 and 19 is a slag layer protecting the steel against the influence of the atmosphere.

The axes 20 of the lateral casting tube openings 9 lie horizontally in a plane laid through the bigger transverse axis of the mold, which plane corresponds to the plane of the drawing. Non-metallic particles in the steel are washed upward by the gas bubbles into an easily melting slag layer 13 and are taken up by it. So as to avoid that the slag 13 becomes crumbly and viscous and in order to avoid deposits on the casting tube, the slag is, according to the invention, structured on the basis of C210 and CO with a high content of fluxing agents and a low content of A1 0 the high content of fluxing agents guarantees the rapid melting of the casting powder added under continuous casting conditions; the low Al O -content causes that by the alumina particles washed from the steel into the slag no undesired alteration of the melting behaviour occurs. The slag 19 in the tundish can, but need not have the same chemical composition as the slag 12.

In FIG. 2 it is shown, which inclinations of the axes 20 of the lateral openings are possible according to the invention: for achieving a good separation effect for the non-metallic particles the axes may be inclined to the horizontal by an angle +0: of ID to i.e. the range lying between 20' and 20". characterized by the angle 0 of 60. is very favorable; if. however. the segregation of the non-metallic particles is only less important. the axis up to the axis 20" may be inclined downward by an angle a of 70. When the. lateral openings 9 are downwardly inclined without gassupply an .increased sedimentation of non-metallic inclusions on the strand skin 16 would occur; by supplying gas and by providing the bottom opening 10, as well as by adjusting an angle of -70 to +70 cach in relation to the horizontal this disadvantage of the hitherto practised continuous casting technology is eliminated.

According to FIG. 3 it is also possible to use a casting tube head 6 with openings 9' directed vertically upward; in this case the axes 20"" enclose with the horizontal an angle of 90". In all embodiments it is essential that, on the one hand, the cross'sectiongiven by the bottom opening with the diameter d; is smallertha n the cross section of the casting tube 6. respectively the steel stream. given by the diameter d and that, on the other hand. the sum of the cross sections of the bottom opening and the lateral openings 9 with the diameter :1 is either equal to the cross section of the casting tube or bigger than said cross section. The cross section of the casting tube can. as is shown in FIG. 4, have any shape, e.g.. it may be oval, and two bottom openings l0 and two slit-formed lateral openings 9" in the casting tube 6" can be also combined; moreover it is essential that the axes of the opening 9" coincide with the bigger transverse axis 22 of the mold, i.e., that they are directed towards the narrow side of the mold 14.

FIG. 5 illustrates the correlation according to the invention between the cross section of the bottom opening(s) l0, l0, l0" and the casting tube (steel stream-l cross section in dependence upon a casting capacity lying in the range between 1.5 and 6.0 metric tons/min. in rapid continuous steel casting plants; the cross section of the bottom openings must lie in field A, so that no cracks occur in the strand.

In FIG. 6, 23 denotes a refractory tube for the supply with gas, whose cross section is smaller than the cross section of the casting tube 6. This tube 23 is, like the stopper 3, liftable and lowerable. The outlet may also be adjusted at a distance -a below the bottom 7, whereby the inert gas is also sucked in and as a consequence of a strong current in the bottom opening 2 a uniform distribution of the gas bubbles over the entire steel stream cross section occurs; the distance -a is not to be bigger than the tube diameter (1. Such a tube 23 is preferably used, when. below the tundish. a sliding closure is provided for the casting tube arranged below.

In high-capacity casting plants the casting tube diameter d usually measures about 60 mm. This diameter d should possibly not exceed 40% of the narrow side of the mold (slab thickness); with substantially bigger diameters there is the danger of erosion of the strand skin. With very wide slabs two casting tubes may be arranged side by side, the bottom openings of which have to be dimensioned according to the quantity of steel in metric tons per minute flowing through the casting tube, as is shown in FIG. 5.

The invention may be applied in continuous casting procedures for all types of steel, it is however particularly advantageous to use it for wide slabs of aluminumcontaining steel; in particular casting procedure of the invention can be applied for aluminum-killed deep drawing steels containing C 0.03 to 0.0771

Si traces V Mn 0.30 to 0.45%

p max. 0.020%

S max. 0.020%

A1 0.020 to 0.060% which steels are destined for the production of cold rolled sheets with maximum surface quality.

What we claim is: v

1. In a continuous steel casting method for a casting capacity of more than 1.5 metric tons of steel per minute, wherein at least one stream of molten steel is formed and cast into a vertically arranged. substantially rectangular mold, having a larger transverse axis and a smaller transverse axis, through a casting tube having a certain clear cross section and extending to below the level ofmolten steel in the mold, said casting tube hav ingclateral openings whose axes lie in a vertical plane running through the larger transverse axis of the mold. and wherein said at least one stream of molten steel contains inert gas, the improvement comprising casting the at least one stream of molten steel. having a certain cross section, through a casting tube provided, in addition to the said lateral openings, with at least one bottom opening directed vertically downward. said at least one bottom opening having a cross section smaller than the clear cross section of the casting tube and the cross section of the stream of molten steel. the axes of the said lateral openings being inclined by between downward and upward in relation to the horizontal and the total cross section of the lateral openings and of the at least one bottom opening being at least equal to the clear cross section ofthe casting tube, and supplying the said inert gas to the said at least one stream of molten steel in an amount of l to 15 Ncm lkg steel over the entire cross section of the stream of molten steel at the place of origin of said at least one stream of molten steel above the mold.

2. A method according to claim 1, wherein the axes of the lateral openings are inclined by between 10 and 90 upward in relation to the horizontal.

3. A method according to claim I, wherein the inert gas is supplied to the at least one stream of molten steel in an amount of 3 to 8 Ncm /kg steel.

4. A method according to claim 1, wherein the inert gas is supplied to the at least one stream of molten steel coaxially.

5. A method according to claim 1, wherein the at least one stream of molten steel is cast into the mold from a tundish. in whose bottom the casting tube is secured. and wherein the inert gas is supplied coaxially to the at least one stream of molten steel through a refractory tube. said refractory tube having an outlet adjust able at a certain distance above the bottom of the tundish.

6. A method according to claim 5, wherein said distance is substantially equal to the width of the casting tube.

7. A method according to claim 5, wherein said distance is somewhat smaller than the width of the casting tube.

8. A method according to claim I, wherein the at least one stream of molten steel is cast into the mold from a tundish. in whose bottom the casting tube is secured, and wherein the inert gas is supplied coaxially to the at least one stream of molten steel through a refractory tube. said refractory tube having an outer cross section smaller than the inner cross section of the casting tube and an outlet adjustable at a certain distance below the bottom of the tundish.

9. A method according to claim 8, wherein said distance is substantially equal to the width of the casting tube.

10. A method according to claim 8. wherein said distance is smaller than the width of the casting tube.

11. A method according to claim 1, wherein the cross section of the at least one bottom opening of the casting tube expressed in of the cross section of the casting tube in dependence upon the casting capacity expressed in metric tons of steel per minute is adjusted in such a way that at L metric tons per minute the cross section lies in the range of to 60% and rising proportionately up to a casting capacity of 6.0 metric tons per minute increases to 50 to 90%.

12. A method according to claim 1, wherein, for receiving non-metallic particles from the steel. in particular aluminum-containing steel. an easily melting slag having the following composition is used in the mold: 20 to 50% fluxing agents of the group Cal- Nil- O K 0 and 8 0 20 to 40% S0,. 25 to 40% C110, less than 5% M 0 and 0 to 10% oxides of iron, manganese and magneslum.

13. A method according to claim 12, wherein the slag is a synthetically produced slag.

14. A method according to claim 12, wherein the slag has the following reference analysis:

about 12% CaF about 8% Na o K 0,

about 10% 8 0;,

about 30% SiO about 30% C210.

less than about 2% ALO,

about 4% FeO and about 4% C.

Claims (14)

1. IN A CONTINUOUS STEEL CASTING METHOD FOR A CASTING CAPACITY OF MORE THAN 1.5 METRIC TONS OF STEEL PER MINUTE, WHEREIN AT LEAST ONE STREAM OF MOLTEN STEEL IS FORMED AND CAST INTO A VERTICALLY ARRANGED, SUBSTANTIALLY RECTANGULAR MOLD, HAVING A LARGER TRANSVERSE AXIS AND A SMALLER TRANSVERSE AXIS, THROUGH A CASTING TUBE HAVING A CERTAIN CLEAR CROSS SECTION AND EXTENDING TO BELOW THE LEVEL OF MOLTEN STEEL IN THE MOLD, SAID CASTING TUBE HAVING LATERAL OPENINGS WHOSE AXES LIE IN A VERTICAL PLANE RUNNING THROUGH THE LARGER TRANSVERSE AXIS OF THE MOLD, AND WHEREIN SAID AT LEAST ONE STREAM OF MOLTEN STEEL CONTAINS INERT GAS, THE IMPROVEMENT COMPRISING CASTING THE AT LEAST ONE STREAM OF MOLTEN STEEL, HAVING A CERTAIN CROSS SECTION, THROUGH A CASTING TUBE PROVIDED, IN ADDITION TO THE SAID LATERAL OPENINGS, WITH AT LEAST ONE BOTTOM OPENING DIRECTED VERTICALLY DOWNWARD, SAID AT LEAST ONE BOTTOM OPENING HAVING A CROSS SECTION SMALLER THAN THE CLEAR CROSS SECTION OF THE CASTING TUBE AND THE CROSS SECTION OF THE STREAM OF MOLTEN STEEL, THE AXES OF THE SAID LATERAL OPENINGS BEING INCLINED BY BETWEEN 70* DOWNWARD AND 90* UPWARD IN RELATION TO THE HORIZONTAL AND THE TOTAL CROSS SECTION OF THE LATERAL OPENINGS AND OF THE AT LEAST ONE BOTTOM OPENING BEING AT LEAST EQUAL TO THE CLEAR CROSS SECTION OF THE CASTING TUBE, AND SUPPLYING THE SAID INERT GAS TO THE SAID AT LEAST ONE STREAM OF MOLTEN STEEL IN AN AMOUNT OF 1 TO 15 NCM3/KG STEEL OVER THE ENTIRE CROSS SECTION OF THE STREAM OF MOLTEN STEEL AT THE PLACE OF ORIGIN OF SAID AT LEAST ONE STREAM OF MOLTEN STEEL ABOVE THE MOLD.

2. A method according to claim 1, wherein the axes of the lateral openings are inclined by between 10* and 90* upward in relation to the horizontal.

3. A method according to claim 1, wherein the inert gas is supplied to the at least one stream of molten steel in an amount of 3 to 8 Ncm3/kg steel.

4. A method according to claim 1, wherein the inert gas is supplied to the at least one stream of molten steel coaxially.

5. A method according to claim 1, wherein the at least one stream of molten steel is cast into the mold from a tundish, in whose bottom the casting tube is secured, and wherein the inert gas is supplied coaxially to the at least one stream of molten steel through a refractory tube, said refractory tube having an outlet adjustable at a certain distance above the bottom of the tundish.

6. A method according to claim 5, wherein said distance is substantially equal to the width of the casting tube.

7. A method according to claim 5, wherein said distance is somewhat smaller than the width of the casting tube.

8. A method according to claim 1, wherein the at least one stream of molten steel is cast into the mold from a tundish, in whose bottom the casting tube is secured, and wherein the inert gas is supplied coaxially to the at least one stream of molten steel through a refractory tube, said refractory tube having an outer cross section smaller than the inner cross section of the casting tube and an outlet adjustable at a certain distance below the bottom of the tundish.

9. A method according to claim 8, wherein said distance is substantially equal to the width of the casting tube.

10. A method according to claim 8, wherein said distance is smaller than the width of the casting tube.

11. A method according to claim 1, wherein the cross section of the at least one bottom opening of the casting tube - expressed in % of the cross section of the casting tube - in dependence upon the casting capacity - expressed in metric tons of steel per minute - is adjusted in such a way that at 1.5 metric tons per minute the cross section lies in the range of 20 to 60% and -rising proportionately - up to a casting capacity of 6.0 metric tons per minute increases to 50 to 90%.

12. A method according to claim 1, wherein, for receiving non-metallic particles from the steel, in particular aluminum-containing steel, an easily melting slag having the following composition is used in the mold: 20 to 50% fluxing agents of the group CaF2, Na2O, K2O and B2O3, 20 to 40% SiO2, 25 to 40% CaO, less than 5% Al2O3 and 0 to 10% oxides of iron, manganese and magnesium.

13. A method according to claim 12, wherein the slag is a synthetically produced slag.

14. A method according to claim 12, wherein the slag has the following reference analysis: about 12% CaF2, about 8% Na2O + K2O, about 10% B2O3, about 30% SiO2, about 30% CaO, less than about 2% Al2O3, about 4% FeO and about 4% C.

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