Method of continuous casting of a composite metallic product

Abstract

A method of continuous casting of a composite metallic product having a distinct shell and bulk of different compositions, said shell having a constant thickness, said method comprising the steps of: A. pouring an initial stream of liquid metal from a tundish into a mold with an initial flow rate FRini using a continuous casting nozzle, B. injecting raw material into at least one of said mixing chambers to be mixed with liquid metal flowing into said mixing chamber to modify its composition, C. injecting gas below said dome with a defined gas flowrate GFR, D. regulating the initial flow rate FRini, the gas flowrate GFR, the outlets immersion depths D1 and D2 and the casting speed Vc to obtain a composite metallic product.

Description

METHOD OF CONTINUOUS CASTING OF A COMPOSITE METALLIC PRODUCT

[0001] The invention relates to a continuous casting process. In particular, the invention relates to a continuous casting process made for manufacturing a composite metallic product.

[0002] The continuous casting of steel is a well-known process. It consists in pouring a liquid metal from a ladle into a tundish intended to regulate the stream, then pouring the metal into the upper part of a water-cooled bottomless copper mold undergoing a vertical reciprocating movement. The solidified semifinished product is extracted from the lower part of the mold by rollers. The liquid metal is introduced into the mold by means of a tubular duct called a nozzle placed between the tundish and the mold.

[0003] However, this simple process is not suitable for the casting of a composite metallic product. The nozzle being a simple duct, it can only be used as a pouring tool for the liquid metal between the tundish and the mold. Therefore, the nozzle and the method of casting must be modified to allow the casting of a composite metallic product.

[0004] Japanese Patent Application JP11 197807 describes a continuous casting process for manufacturing a multilayer cast piece, using a nozzle being formed of a vertical duct having multiple discharge ports in the vertical direction, the duct being divided on the inside by a partition wall creating multiple molten steel flow passages and having one or multiple ports for adding raw material.

[0005] The continuous casting process described consists in injecting two types of molten metals, differing in composition, into the mold at different heights thus creating two pools of liquid metal, an upper pool and a lower pool, differing by their respective composition. The metal located in the upper pool solidifies first, creating a shell having the composition of the upper pool. The metal located in the lower pool solidifies then inside the shell, forming the bulk of the material and having the composition of the lower pool, thus creating a composite metallic product.

[0006] When manufacturing a composite metallic product by continuous casting, the stability of the interface between the pools is one of the main parameters that define the quality of the separation between the shell and the bulk of the composite product.

[0007] Japanese Patent Application JP1 1197807 uses a static magnetic field and injects the different streams of liquid metal above and below the magnetic field. The magnetic field acts as a brake towards the liquid metal thus reducing mixing of the two pools and stabilizing the boundary.

[0008] However, this method only relies on the equipment for the stability of the boundary and is thus not adaptable to different casting conditions. Moreover, the magnetic field does not prevent enough remixing for the product to have a sufficient quality of separation between the shell and the bulk.

[0009] The present invention discloses a method of continuous casting for manufacturing a composite metallic product, allowing the casting of a product having a shell and a bulk of different composition with a maximized difference in composition and well defined, thus being of better quality.

[0010] The object of the invention is a method of continuous casting of a composite metallic product having a distinct shell 16 and bulk 17 of different compositions, said shell 16 having a constant thickness, said method comprising the steps of:A. pouring an initial stream of liquid metal from a tundish 2 into a mold 3 with an initial flow rate FRini using a continuous casting nozzle 1 comprising at least:- a dome 6 for splitting said initial stream of liquid metal into a plurality of separate streams,- two mixing chambers 9a, 9b separated by an internal wall 8, to allow said separate streams of liquid metal to flow in each of said chambers 9a, 9b, said internal wall 8 being designed to determine the repartition of the separate streams into each chamber 9a, 9b,- means for injecting raw material 10 through the dome 6 into at least one of said mixing chambers 9a, 9b,- means for injecting gas 1 1 through the dome 6,- two channels 12a, 12b, 12c, each one being connected to at least one chamber 9a, 9b and opening into said mold 3 through at least one outlet 13, allowing the liquid metal from the chambers 9a, 9b to be poured into the mold, wherein at least one of said channels 12a has a length different than the other channels 12b, 12c, thus allowing the outlets 13 of said channels 12a, 12b, 12c to be immersed into the mold 3 at two different immersion depths D1 and D2, thus creating two types of flows of liquid metal 20, 21 into the mold 3, upper flows 20 having a total flow rate FRup = Q*FRini, with Q being a ratio, and lower flows 21 having a total flow rate FRIow = (1 -Q)*FRini, and wherein for each outlet, the ratio between the channel diameter and the outlet diameter is designed to set a speed to the flows of liquid metal flowing out of said outlet.B. injecting raw material into at least one of said mixing chambers 9a, 9b to be mixed with liquid metal flowing into said mixing chamber 9a, 9b to modify its composition, thus creating two pools of liquid metal into the mold 3, an upper pool 14 and a lower pool 15 having different compositions, separated by an interface 19 located at a depth Li into the mold 3,C. injecting gas below said dome 6 with a defined gas flowrate GFR to facilitate the injection of raw material into said chamber 9a, 9b,D. regulating said initial flow rate FRini, said gas flowrate GFR, said outlets 13 immersion depths D1 and D2 and the casting speed Vc to regulate said depth Li of the interface 19 between the two pools 14, 15 in the mold 3 to obtain a composite metallic product.

[0011] The method of continuous casting according to the invention may also have the optional features listed below, considered individually or in combination:- the liquid metal is steel,- the initial flow rate FRini is regulated using a stopper rod 18- the initial flow rate FRini is set from 0.3 T / min to 6 T / min- the gas flow rate GFR is regulated using a valve or a mass flowmeter- the gas flow rate GFR is set from 0.1 Nl / min to 5 Nl / min- the casting speed Vc is set from 0.4 m / min to 6 m / min- the ratio Q is set from 0.2 to 0.8- the injection of raw material is oriented towards the metal stream flowing down the dome- the injection of raw material is done with an angle of 30 to 90° with respect to the horizontal.

[0012] The invention will be described, in a non-limitative way, in reference to the following drawings:- Fig 1 : general view of an embodiment of the installation used in the invention- Fig 2: example of a composite metallic product that can be manufactured using the invention- Fig 3: view of an embodiment of the immersed part of the nozzle used in the invention and the mold- Fig 4: view of the repartition of the flows into an embodiment of the nozzle and the mold- Fig 5: cross-sectional view of an embodiment of the means for injecting raw material into the nozzle- Fig 6: examples of shapes for the bottom of the channels and their outlets

[0013] Fig 1 shows an embodiment of the installation used in the continuous casting process according to the invention. The installation comprises a continuous casting nozzle 1 disposed between a tundish 2 and a mold 3. The nozzle 1 is composed of an upper part 4 and a lower part 5.

[0014] A dome 6 is disposed at the inlet of the upper part 4 and closes a part of it. The top of the dome 6 preferably has a slope of a certain angle, higher than 15° for example. The dome 6 also has a lateral side, preferably forming a sharp edge with the slope. The dome 6 is fixed to the upper part 4 by one or more support arms 7.

[0015] An internal wall 8, located below the dome 6, separates at least two mixing chambers 9a, 9b in the upper part 4. These chambers 9a, 9b,comprise a reduction of their section. In the configuration presented in Fig 1 , two chambers 9a, 9b are present.

[0016] A means for injecting raw material 10 into at least one of the mixing chambers 9a, 9b and a means for injecting gas 1 1 are also comprised in the upper part 4, each one being partly located in one of the support arms 7 and passing through the dome 6. In this configuration, the means for injecting raw material 10 is a powder injector. The powder injector can be an endless screw, for example, linked to a powder tank. The powder injector preferably has a diameter ranging from 10 to 30 mm. The powder injector also preferably has an angle ranging from 30 to 90° with respect to the horizontal in a usage configuration. More preferably, the powder injector has an angle ranging from 30 to 50° with respect to the horizontal.

[0017] The lower part 5 of the nozzle 1 is composed of at least two channels 12a, 12b, 12c, extending from the chambers 9a, 9b of the upper part 4 after the reduction of their section, and opening into the mold 3, through at least one outlet 13 for each channel 12a, 12b, 12c. In this embodiment, three channels 12a, 12b, 12c, are present.

[0018] In the present embodiment, the channels 12a, 12b, 12c are of circular shape. In a preferred embodiment, the channels 12a, 12b, 12c have a round, an elliptic or an oblong section.

[0019] At least one of the channels 12a, 12b, 12c has a length different than the other channels 12a, 12b, 12c. As shown on Fig 1 , one channel 12a is longer than the other channels 12b, 12c, but other configurations can be considered with only two channels or two longer channels.

[0020] The channels 12a, 12b, 12c having different lengths are connected to separate chambers 9a, 9b, but more than one channel 12a, 12b, 12c can be connected to a single chamber 9a, 9b. For example, in this embodiment, two chambers 9a, 9b and three channels 12a, 12b, 12c, are present. The longer channel 12a is connected to one chamber 9a and the two other shorter channels 12b, 12c are connected to the other chamber 9b.

[0021] The channels 12a, 12b, 12c have at least one outlet 13 for each channel 12a, 12b, 12c, but the number of outlets 13, the location of theoutlets 13, the angle of their axis as well as the shape of the bottom of the channel 12a, 12b, 12c can vary depending on the embodiment. The outlets 13 can also be different between the longer channels 12a and the shorter channels 12b, 12c. In the embodiment of Fig 1 , the longer channel 12a has only one outlet 13 located at the bottom of the channel 12a that is vertical, and the shorter channels 12b, 12c have one outlet 13 each, located on their side with no angle with respect to the horizontal, with the bottom of the channel 12b, 12c having a flat shape. In another embodiment, not represented, the longer channel 12a has two outlets 13 located on the side of the channel 12a, have an angle with respect to the horizontal and the bottom of the channel 12a has a dome shape. In a preferred embodiment, in the case where the outlets 13 are located on the side of the channels 12a, 12b, 12c, the bottom of the channel 12a, 12b, 12c has a flat, a recess, a slope or a dome shape.

[0022] Fig 6 shows various shapes for the bottom of the channels12a, 12b, 12c. Fig 6a shows a flat shape for a channel 12a, 12b, 12c with only one outlet 13, Fig 6b shows a recess shape for a channel 12a, 12b, 12c with only one outlet 13, Fig 6c shows a slope shape for a channel 12a, 12b, 12c with only one outlet 13, Fig 6d shows a flat shape for a channel 12a, 12b, 12c with two outlets 13, Fig 6e shows a recess shape for a channel 12a, 12b, 12c with two outlets 13 and Fig 6f shows a dome shape for a channel 12a, 12b, 12c with two outlets 13. The shapes represented in Fig 6 are only given as examples and the bottom of the channels 12a, 12b, 12c can be of any shape.

[0023] The method of continuous casting of the invention consists in manufacturing a composite metallic product having a distinct shell 16 and bulk 17 which are of different composition, maximizing said difference of composition between the shell 16 and the bulk 17, while allowing the thickness of the shell 16 to remain equal to the target value in the final product. The method comprises four steps.

[0024] The first step consists in pouring a liquid metal of a defined composition from a ladle into a tundish 2 then filling the nozzle 1 previouslydescribed to distribute said liquid metal into the mold 3. In a preferred embodiment, the liquid metal is steel, and the invention will be described with it.

[0025] An initial stream of steel, with an initial flow rate FRini, flows from the tundish 2 into the upper part 4 of the nozzle 1 .

[0026] The dome 6, being placed in the trajectory of the steel, forces the initial stream to collide on it. The slope of the dome 6 makes the steel flow towards its edge. The support arms 7 create different areas on the dome 6, splitting the initial stream of steel into a plurality of separate streams. The number of separate streams is determined by the design of the dome 6 and its support arms 7.

[0027] The separate streams flow then into the different mixing chambers 9a, 9b. The design of the internal wall 8 separating the chambers 9a, 9b, determines the repartition of the separate streams into each chamber 9a, 9b by determining their respective volume. The steel accumulates then inside the chambers 9a, 9b due to the reduction of their section. The steel is then distributed into the channels 12a, 12b, 12c and poured into the mold 3 through the outlets 13 of the channels 12a, 12b, 12c.

[0028] Due to the different lengths of the channels 12a, 12b, 12c, their outlets 13 have different immersion depths into the mold 3. The shorter channels 12b, 12c, have outlets 13 at an immersion depth D1 and the longer channels 12a have outlets 13 at an immersion depth D2. This difference of immersion depth allows the liquid steel to be injected at the two different depths D1 and D2 thus creating two types of flows of liquid steel 20, 21 into the mold 3.

[0029] The flows of liquid steel 20 coming from the outlets 13 having the immersion depth D1 are the upper flows 20. The total flow rate of the upper flows FRup is a fraction Q of the initial flow rate FRini injected at the top of the nozzle 1 . FRup can be calculated using the following formula: FRup = Q*FRini.

[0030] The flows of liquid steel 21 coming from the outlets 13 having the immersion depth D2 are the lower flows 21 . The total flow rate of the lower flows FRIow is the other fraction (1 -Q) of the initial flow rate FRini. FRIow can be calculated using the following formula: FRIow = (1 -Q)*FRini.

[0031] Fig 4 shows the repartition of the flows of liquid steel, according to the embodiment of Fig 1 , using the Q ratio. In this embodiment, FRup is partitioned between the two upper outlets 13, with each upper outlet 13 having a flow rate coming out of it of (Q / 2)*FRini. In general, the flow rates are partitioned between the outlets 13 having the same immersion depth. The flow rate FRup is partitioned between all the outlets 13 having an immersion depth D1 and the flow rate FRIow is partitioned between all the outlets 13 having an immersion depth D2.

[0032] The diameter of the outlets respectively to the diameter of the channels is defined to regulate the speed of the flows of liquid metal flowing out of the outlets.

[0033] The second step consists in injecting raw material into at least one of the chambers 9a, 9b to modify the composition of the steel flowing into said chamber 9a, 9b. The design of the chambers 9a, 9b, having a large section at the top allows the steel to flow down the dome 6 like a waterfall and allows the raw material to be injected into the flow without the steel coming into contact with the means for injecting raw material 10. The design of the chambers 9a, 9b allows the steel to be slowed down and to accumulate in the chambers 9a, 9b. The reduction of the section of the chambers allows the steel to be agitated inside the chamber. In consequence, the raw material injected can be mixed efficiently with the steel into said chamber 9a, 9b to modify its composition, and starts melting.

[0034] In a preferred embodiment, the raw material injected into the steel is in the form of powder. It can be of various composition, for example, it can be FeSi, Ni, FeAl, FeTi, FeCr, FeNb, FeB, FeCe, FeMo, etc...

[0035] Fig 5 shows a cross-sectional view of the dome 6 and the top of the mixing chambers 9a, 9b as well as an embodiment of the means for injecting raw material 10. The ending part of the means for injecting raw material 10 is located inside the dome 6 and one of its support arms 7 and opens under the dome 6. It injects raw material into one of the mixing chambers 9a, 9b. In a preferred embodiment, the injection of raw material is oriented towards the metal steel streams flowing down the dome 6. It allows a better mixing of the raw material into the mixing chambers 9a, 9b. For this purpose, the means forinjecting raw material 10 has an angle with respect to the horizontal which is preferably from 30° to 90°. More preferably, the angle is from from 30 to 50°.

[0036] With the step of injection, the different chambers 9a, 9b contain two types of steel with different composition.

[0037] Due to the channels 12a, 12b, 12c having different lengths, the two types of steel are poured at the two different heights into the mold 3. The upper flows 20 and the lower flows 21 create two pools of steel, an upper pool 14 and a lower pool 15, different in composition and separated by an interface 19 at a depth Li.

[0038] The composition of the pools 14, 15 varies depending on the chamber 9a, 9b where the raw material is injected. If the raw material is injected into a chamber 9a, 9b connected to longer channels 12a, the composition of the upper pool 14 is the composition of the steel coming from the tundish 2 only and the composition of the lower pool 15 is the combination of the composition of the steel coming from the tundish 2 and the composition of the raw material injected. On the other hand, if the raw material is injected into a chamber 9a, 9b connected to shorter channels 12b, 12c, the composition of the upper pool 14 is the combination of the composition of the steel coming from the tundish 2 and the composition of the raw material injected and the composition of the lower pool 15 is the composition of the steel coming from the tundish 2 only.

[0039] The raw material can thus be injected into any of the chambers9a, 9b, depending on the needs of the person skilled in the art.

[0040] The third step consists in injecting gas below the dome 6 with a defined gas flowrate GFR. This step facilitates the injection of raw material into the chambers 9a, 9b by creating a gas flow which maintains the steel flowing down the dome 6 towards the exterior of the upper part 4, thus creating a zone below the dome 6 without steel. This hollow zone prevents any contact between the steel and the raw material injection 10 thus avoiding potential clogging of the injection.

[0041] The gas is preferably non-oxidizing, Ar for example, to prevent any reaction with the steel during casting. In a preferred embodiment, the gasflow is regulated using a valve or a mass flowmeter. In a preferred embodiment, the gas flow rate GFR is set from 0.1 Nl / min to 5 Nl / min.

[0042] The fourth step consists in regulating the different parameters to define the depth Li of the interface 19 between the two pools 14, 15 in the mold 3 and to stabilize it, in order to maximize the difference of composition between the shell 16 and the bulk 17 of the composite metallic product as well as keeping constant the thickness of the shell 16 of said product. The main parameters that can be regulated are the initial flow rate of steel FRini, the gas flow rate GFR, the outlets 13 immersion depths D1 and D2 and the casting speed Vc.

[0043] In the mold, the thickness of the solidified part can be determined by a formula. With e being the solidified thickness:e: solidified thickness (mm)L: casting length (m)Vc: casting speed (m.min-1 ) k: constant depending on the cooling speed of the casterThe casting length L is a variable representing the distance from the meniscus of the liquid steel to a variable depth in the mold 3.

[0044] With the presence of two pools 14, 15, the steel of the upper pool 14 solidifies first, creating a solidified part with one composition. The solidified thickness grows until it reaches the interface 19 between the two pools 14, 15 where the second composition in the lower pool 15 starts to solidify until total solidification of the product. The first solidified composition coming from the upper pool 14 represents the shell 16 of the product and the second composition coming from the lower pool 15 represents the bulk 17 of the product. After full solidification, the material obtained is a composite metallic product with a composition different in its shell 16 than in its bulk 17 as illustrated by Fig 2.

[0045] With the formula presented above, the thickness of the shell16 of the composite metallic product can be calculated. With a defined casting speed, the thickness of the shell 16 is calculated using the distance Li betweenthe meniscus of the liquid steel and the position of the interface 19 between the upper pool 14 and the lower pool 15. By defining and regulating the casting speed Vc and the position of the interface between the pools, the thickness of the shell 16 of the solidified product can thus be made constant.

[0046] Fig 3 represents an embodiment of the lower part 5 of the nozzle 1 used in the continuous casting process and the mold 3 where it is immersed. The outlets 13 of the shorter channels 12b, 12c are immersed into the mold 3 at an immersion depth D1 and the outlets 13 of the longer channels 12a are immersed into the mold 3 at an immersion depth D2 > D1 . For a lateral outlet 13, the immersion depth D1 or D2 represents the distance from the meniscus of the liquid steel to the top of the outlet 13, like represented in Fig 3. For an outlet 13 located at the bottom of a channel 12a, 12b, 12c, the distance D1 or D2 represents the distance from the meniscus of the liquid steel to the bottom of the channel 12a, 12b, 12c. D1 and D2 are parameters that are defined by the casting conditions. The gap A between the outlets 13 of the shorter channels 12b, 12c and the outlets 13 of the longer channels 12a, contrary to D1 and D2, is a fixed distance only determined by the design of the nozzle 1. Its value can be calculated using the following formula:A = | D2-D1 |A is a fixed distance, determined by the design of the nozzle.

[0047] Considering the process, the steel for the upper pool 14 is poured into the mold 3 through the outlets 13 of the shorter channels 12b, 12c creating upper flows 20 at the depth D1 , and the steel for the lower pool 15 is poured into the mold 3 through the outlets 13 of the longer channels 12a creating lower flows 21 at the depth D2. The interface 19 between the two pools is created where the upper flows 20 and the lower flows 21 meet.

[0048] D1 and D2 are defined by the immersion of the nozzle 1 into the liquid steel. The two parameters affecting the immersion are the elevation of the tundish 2 to which the nozzle 1 is attached compared to the mold 3 and the height of the meniscus of the liquid steel in the mold 3. Those parameters should be defined before casting and kept constant during casting to avoid thefluctuations of the interface 19 between the pools 14, 15 to obtain a constant thickness of the shell 16 of the solidified product. To modify them, the operator can adjust the level of liquid steel in the mold 3 using a stopper rod 18 to modify the initial flow rate FRini or move up and down the tundish 2 to change the nozzle 1 immersion into the mold 3.

[0049] To ensure a good quality of casting at all times, it may be preferable to set limits for the outlets 13 depths D1 and D2. With a too low immersion depth D1 , the upper flows 20 are close to the meniscus of the liquid steel and thus might drag mold powder into the liquid steel. On the other hand, with a too high immersion depth D1 , the upper flows 20 might not bring enough heat at the top of the mold 3 for the casting operation to perform properly. Concerning D2, it is preferable to have a minimal immersion depth so that the gap A is acceptable so that the upper flows 20 and the lower flows 21 do not mix with each other. A maximum immersion depth is also preferable so that the bottom of the nozzle 1 does not come into contact with the solidified thickness of the product to avoid a potential breakage.

[0048] Given these elements, in a preferred embodiment, D1 is set from 100 to 150 mm, D2 is set from 250 to 500 mm and A is set from 100 to 500 mm.

[0049] During casting, due to the upper flows 20 and the lower flows21 meeting each other as well as the gas flows injected into the nozzle, remixing between the two pools 14, 15 of liquid metal is inevitable. However, to obtain a good quality of separation between the shell 16 and the bulk 17 of the final product, it is important that the elements added with the raw material do not mix with the pool 14, 15 where the addition is not desirable. In the case where a bulk 17 addition is desired, it is important that the added elements do not mix with the upper pool 14. However, the liquid steel coming from the upper pool 14 can go down into the lower pool 15 without consequences on the separation between the shell 16 and the bulk 17. In the other case where a shell 16 addition is desired, it is important that the added elements do not mix with the lower pool 15. However, in the same manner as the other case, the liquid steel coming from thelower pool 15 can go up into the upper pool 14. Remixing is thus acceptable in only one way depending on the type of addition.

[0050] The invention takes into account this fact to maximize the difference of composition between the shell 16 and the bulk 17 and to maximize the separation, by forcing this one way remixing thus limiting the diffusion of the added element into the undesired pool. This is achieved by regulating the flow rates FRup and FRIow of the liquid steel coming from the outlets 13 of the nozzle 1 . These parameters are not directly controllable but they both depend on the initial flow rate FRini which is controllable and on the repartition of this initial flow rate FRini into the two flow rates FRup and FRIow coming out of the nozzle 1 . The ratio Q represents this repartition and is determined by the design of the nozzle 1 .

[0051] To regulate the ratio Q, there are various parameters of the nozzle 1 that have to be taken into account. The first is the initial splitting of the liquid metal of the initial stream into a plurality of separate streams done by the dome 6. The design of the dome 6 determines the number of separate streams. For example, a dome 6 with three support arms 7 will split the initial stream into three separate streams. The second parameter is the chambers 9a, 9b that collect the separate streams. The internal wall 8 that separates the different chambers 9a, 9b is designed so that the different chambers 9a, 9b have defined volumes to collect a defined number of separate streams. In the embodiment of Fig 1 , two chambers 9a, 9b are present, and a first chamber 9a collects one separate stream while the other chamber 9b collects the other two separate streams. The last parameter is the channels 12a, 12b, 12c connected to each chamber 9a, 9b as their length determine the pool 14, 15 where the liquid steel is poured. By adapting these parameters, the operator can thus adjust Q depending on the type of addition needed.

[0052] For an addition in the bulk 17, Q is preferably from 0.5 to 0.7.

[0053] For an addition in the shell 16, Q is preferably from 0.3 to 0.5.

[0054] With a Q value defined, the initial flow rate FRini can be regulated to define the flow rates FRup and FRdown in the mold. In a preferredembodiment, the initial flow rate FRini is regulated using a stopper rod. In a preferred embodiment, the initial flow rate FRini is set from 0.3 T / min to 6 T / min.

[0055] However, even if remixing in one way is forced, it is limited to a minimum so that the final product has a maximized difference of composition between its shell 16 and its bulk 17. The final product also must be of sufficient quality. For this, the composite metallic product must have a clear and constant separation between the bulk 16 and the shell 17. Thus, the thickness of the shell 16 has to remain to the target value throughout the product. To achieve this, the interface 19 between the pools 14, 15 must be as stable as possible during casting and so other parameters must be taken into account, namely the flows of liquid metal 20, 21 themselves and the flows of gas.

[0056] The flows of liquid metal 20, 21 are characterized by their speed and their direction which are defined by their angle and their speed at the exit of the nozzle 1 . The angle of the flows is defined by the angle of the axis of the outlets 13. Concerning the speed of the flows at the exit of the nozzle 1 , it is determined by the diameter of the outlets 13 and the flow rate in the channel 12a, 12b, 12c thus depending on the initial flow rate FRini and the ratio Q.

[0057] To obtain a good stability of the interface 19 between the pools14, 15 and to obtain a good quality of the desired product, the flows of liquid metal 20, 21 are balanced with the gas flow, while allowing sufficient remixing in one way so that the added element do not mix with the wrong pool 14, 15. As the mold 3 geometry and the casting parameters change depending on the desired product, the nozzle 1 and the process parameters have to be modified accordingly. The angle and the speed of the flows have to be adapted to reach the edges of the mold 3 and to not mix with each other. The speed also has an influence on the gas flows. With not enough speed, the gas tends to flow upward directly at the exit of the outlets 13 which can be detrimental to the stability. However, with a sufficient speed, the gas flows are entrained with the metal flows 20, 21 , thus achieving better balancing of the flows.

[0058] Preferred values of the different parameters considering different embodiments are presented in Tab 1.

[0059] In a preferred embodiment, Vc is set from 0.4 m / min to 6 m / min.

[0060] The products obtained by this method are preferably billets, blooms, or slabs.

[0061] The continuous casting process meets the expectations in terms of quality of the semi-finished product. It allows a stable casting of a composite metallic product having a distinct shell 16 and bulk 17, having a maximized difference in composition between said shell 16 and bulk 17 and the thickness of said shell 16 is constant in the final product.

Claims

CLAIMS1 . A method of continuous casting of a composite metallic product having a distinct shell (16) and bulk (17) of different compositions, said shell (16) having a constant thickness, said method comprising the steps of:A. pouring an initial stream of liquid metal from a tundish (2) into a mold (3) with an initial flow rate FRini using a continuous casting nozzle (1 ) comprising at least:- a dome (6) for splitting said initial stream of liquid metal into a plurality of separate streams,- two mixing chambers (9a, 9b) separated by an internal wall (8), to allow said separate streams of liquid metal to flow in each of said chambers (9a, 9b), said internal wall (8) being designed to determine the repartition of the separate streams into each chamber (9a, 9b),- means for injecting raw material (10) through the dome (6) into at least one of said mixing chambers (9a, 9b),- means for injecting gas (1 1 ) through the dome (6),- two channels (12a, 12b, 12c), each one being connected to at least one chamber (9a, 9b) and opening into said mold (3) through at least one outlet (13), allowing the liquid metal from the chambers (9a, 9b) to be poured into the mold (3), wherein at least one of said channels (12a) has a length different than the other channels (12b, 12c), thus allowing the outlets (13) of said channels (12a, 12b, 12c) to be immersed into the mold (3) at two different immersion depths D1 and D2, thus creating two types of flows of liquid metal (20, 21 ) into the mold (3), upper flows (20) having a total flow rate FRup = Q*FRini, , and lower flows (21 ) having a total flow rate FRIow = (1 -Q)*FRini, and wherein for each outlet, the ratio between the channel diameter and the outlet diameter is designed to set a speed to the flows of liquid metal flowing out of said outlet.B. injecting raw material into at least one of said mixing chambers (9a, 9b) to be mixed with liquid metal flowing into said mixing chamber (9a, 9b) tomodify its composition, thus creating two pools (14, 15) of liquid metal into the mold (3), an upper pool (14) and a lower pool (15) having different compositions, separated by an interface (19) located at a depth Li into the mold (3),C. injecting gas below said dome (6) with a defined gas flowrate GFR to facilitate the injection of raw material into said chamber (9a, 9b),D. regulating said initial flow rate FRini, said gas flowrate GFR, said outlets immersion depths D1 and D2 and the casting speed Vc to regulate said depth Li of the interface (19) between the two pools (14, 15) in the mold (3) to obtain a composite metallic product.

2. A method of continuous casting according to claim 1 , wherein the liquid metal is steel.

3. A method of continuous casting according to any of the preceding claims, wherein the initial flow rate FRini is regulated using a stopper rod (18).

4. A method of continuous casting according to claim 3, wherein the initial flow rate FRini is set from 0.3 T / min to 6 T / min.

5. A method of continuous casting according to any of the preceding claims, wherein the gas flow rate GFR is regulated using a valve or a mass flowmeter.

6. A method of continuous casting according to claim 5, wherein the gas flow rate GFR is set from 0.1 Nl / min to 5 Nl / min.

7. A method of continuous casting according to any of the preceding claims, wherein the casting speed Vc is set from 0.4 m / min to 6 m / min.

8. A method of continuous casting according to any of the preceding claims, wherein the ratio Q is set from 0.2 to 0.8.

9. A method of continuous casting according to any of the preceding claims, wherein the injection of raw material is oriented towards the metal stream flowing down the dome.

10. A method of continuous casting according to claim 9, wherein the injection of raw material is done with an angle of 30 to 90° with respect to the horizontal.

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