Method of continuous casting of a composite metallic product

Abstract

The invention relates to a method of continuous casting of a composite metallic product having a shell of a defined thickness, comprising the steps of: A. pouring a stream of liquid metal from a tundish into a mold with a flow rate FR using a nozzle comprising: − a dome − two chambers created by an internal wall − means for injecting raw material − means for injecting gas − channels connected to the chambers and opening into the mold at two different immersion depths D1 and D2, B. injecting raw material into one chamber to be mixed with liquid to modify its composition, creating two pools of liquid metal having different compositions, separated by an interface at a depth Li C. injecting gas below said dome D. regulating FR, D1, D2 and the casting speed to regulate Li to obtain a composite metallic product with a defined shell thickness.

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 position of the interface between the pools is one of the main parameters that define the thickness of the shell 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 thus defining the position of the magnetic field in the mold as the position of the interface between the pools.

[0008] However, as the magnetic field is static in the mold, the position of the boundary is also static and only depends on the equipment. Thus, the solution proposed in the prior art provides only very limited control over the thickness of the shell of the composite metallic product.

[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 of a defined thickness and allowing better control over said thickness.

[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 defined thickness, said method comprising the steps of:A. pouring an initial stream of liquid metal from a tundish 2 into a mold 3 with a flow rate FR 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 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,- means for injecting raw material 10 through the dome 6 into at least one of said chambers,- 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 oneoutlet 13, 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,B. injecting raw material into at least one of said chambers 9a, 9b to be mixed with liquid metal flowing into said 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 to facilitate the injection of raw material into said chamber 9a, 9b,D. regulating said flowrate FR, said outlets 13 immersion depths D1 and D2 and the casting speed V to regulate said depth Li of the interface 19 between the two pools in the mold 3 to obtain a composite metallic product with a defined shell thickness.

[0011] The method of continuous casting according to the invention may also have the optional features listed below, considered individually or in combination:- an additional step is performed, said additional step consisting of modifying said flow rate FR, said outlets 13 immersion depths D1 and D2 and the casting speed V to modify the depth Li of the interface 19 between the two pools in the mold, to modify the shell 16 thickness of the composite metallic product during casting,- the liquid metal is steel,- the outlets 13 immersion depths D1 and D2 are further regulated by changing the liquid metal level into the mold 3,- the outlets 13 immersion depths D1 and D2 are further regulated by changing the immersion depth of the nozzle 1 into the mold 3 by modifying the vertical position of the tundish 2.- the immersion depth D1 of the outlets 13 of at least one channel 12b, 12c is set from 100 to 150 mm.- the immersion depth D2 of the outlets 13 of the other channels 12a is set from 250 to 600 mm,- the outlets 13 having an immersion depth D1 and the outlets 13 having an immersion depth D2 are separated by a distance A = | D2-D1 1 ,- the distance A is set from 100 to 500 mm,- the regulation of the flow rate FR is done by a stopper rod 18,- the shape and orientation of the outlets 13 are configured to further adjust the depth Li of the interface 19 between the two pools in the mold 3,

[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 another embodiment of the immersed part of the nozzle used in the invention and the mold

[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, creates at least two chambers 9a, 9b in the upper part 4. In the configuration presented in Fig 1 , two chambers are present 9a, 9b.

[0016] A means for injecting raw material 10 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 means for injecting powder. The means for injecting powder can be an endless screw, for example, linked to a powder tank.

[0017] The lower part 5 of the nozzle 1 is composed, in this embodiment, of three channels 12a, 12b, 12c extending from the chambers 9a, 9b of the upper part 4 and ending into the mold 3, comprising at least one outlet 13 for each channel 12a, 12b, 12c.

[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. As shown on Fig 1 , one channel is longer than the other channels, 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 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 while allowing the thickness of the shell 16 to be defined before and during casting. The method comprises four steps.

[0022] 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 previously described 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.

[0023] An initial stream of steel, with a flow rate FR, flows from the tundish 2 into the upper part 4 of the nozzle 1 .

[0024] 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.

[0025] The separate streams flow then into the different chambers9a, 9b. 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.

[0026] Due to the different lengths of the channels 12a, 12b, 12c, their outlets 13 have different immersion depth 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.

[0027] 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.

[0028] 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...

[0029] With the step of injection, the different chambers 9a, 9b contain two types of steel with different composition.

[0030] The consequence is that due to the channels 12a, 12b, 12c having different lengths, the two types of steel are poured at two different heightsinto the mold 3, thus creating 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.

[0031] The composition of the pools 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 powder 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 powder injected and the composition of the lower pool 15 is the composition of the steel coming from the tundish 2 only.

[0032] The raw material can thus be injected into any of the chambers9a, 9b, depending on the needs of the person skilled in the art.

[0033] The third step consists in injecting gas below the dome. This step allows to facilitate 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.

[0034] The gas is preferably non-oxidizing, Ar for example, to prevent any reaction with the steel during casting.

[0035] The fourth step consists in regulating the different parameters to define the thickness of the shell 16 of the composite metallic product. The main parameters that can be regulated are the flow rate of steel FR, the outlets 13 depths D1 and D2 and the casting speed V.

[0036] 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)V: 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.

[0037] With the presence of two pools, 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 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.

[0038] 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 between the meniscus of the liquid steel and the position of the interface 19 between the upper pool 14 and the lower pool 15. The shell 16 of the solidified product can thus be defined and modified by controlling and modifying the casting speed V and the position of the interface 19 between the two pools. This can be achieved by modifying the casting conditions and the characteristics of the nozzle 1 .

[0039] 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 D2represents the distance from the meniscus of the liquid steel to the bottom of the channel 12a, 12b, 12c like represented for D2 in Fig 4. D1 and D2 are parameters that are defined by the casting conditions and can be modified during casting. 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 |Due to A being a fixed distance, D1 and D2 cannot be modified independently, any modification on D1 implies the same modification on D2.

[0040] 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. The position of the interface 19 is thus located in the gap A.

[0041] To move the position of the interface 19 between the pools,D1 and D2 are the main parameters to modify, together with the casting speed V and the flow rate FR. To modify them, the operator can adjust the level of liquid steel in the mold 3 using a stopper rod 18 to modify the flow rate FR or move up and down the tundish 2 to change the nozzle 1 immersion into the mold 3. Regulating D1 and D2 allows the operator to directly regulate the position of the interface 19 as the position of the outlets 13 into the mold 3 defines where the flows are located into the mold. Increasing the casting speed also increases slightly Li and reversely.

[0042] Other secondary parameters that influence the flows can also change the position of the interface 19 in a moderate way. In the case where the upper flows 20 and the lower flows 21 are similar, the position of the interface 19 is located near the middle of the gap A as illustrated in Fig 3. However, by changing the shape, the angle or the diameter of the outlets 13, the person skilledin the art can influence the flows and thus adjust further the position of the interface 19.

[0043] Fig 4 gives an example of an outlet 13 at the bottom of the longer channel 12a that creates flows directed towards the bottom of the mold 3 thus changing the position of the interface 19 between the pools and thus increasing Li.

[0044] By regulating the flow rate FR, the outlets 13 depths D1 andD2 and the casting speed V, the person skilled in the art can thus obtain a composite metallic product with a defined shell 16 thickness.

[0045] The flow rate FR, the outlets 13 depths D1 and D2 and the casting speed V can also be modified during casting to modify the shell 16 thickness of the composite metallic product during casting.

[0046] 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. Conversely, 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.

[0047] 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.

[0048] Preferably the casting speed V is set from 0.4 m / min to 6 m / min.

[0049] Preferably the flow rate FR is set from 0.3 T / min to 6 T / min.

[0050] The products obtained by this method are preferably billets, blooms, or slabs.

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 defined thickness, said method comprising the steps of:A. pouring an initial stream of liquid metal from a tundish (2) into a mold (3) with a flow rate FR 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 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),- means for injecting raw material (10) through the dome (6) into at least one of said chambers,- 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), 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,B. injecting raw material into at least one of said chambers (9a, 9b) to be mixed with liquid metal flowing into said 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 to facilitate the injection of raw material into said chamber (9a, 9b),D. regulating said flowrate FR, said outlets (13) immersion depths D1 and D2 and the casting speed V to regulate said depth Li of the interface (19)between the two pools in the mold (3) to obtain a composite metallic product with a defined shell thickness.

2. A method of continuous casting according to claim 1 , wherein an additional step is performed, said additional step consisting of modifying said flow rate FR, said outlets (13) immersion depths D1 and D2 and the casting speed V to modify the depth Li of the interface (19) between the two pools in the mold, to modify the shell (16) thickness of the composite metallic product during casting.

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

4. A method of continuous casting according to any of the preceding claims, wherein the outlets (13) immersion depths D1 and D2 are further regulated by changing the liquid metal level into the mold (3).

5. A method of continuous casting according to any of the preceding claims, wherein the outlets (13) immersion depths D1 and D2 are further regulated by changing the immersion depth of the nozzle (1 ) into the mold (3) by modifying the vertical position of the tundish (2).

6. A method of continuous casting according to any of the preceding claims, wherein the immersion depth D1 of the outlets (13) of at least one channel (12b, 12c) is set from 100 to 150 mm.

7. A method of continuous casting according to claim 6, wherein the immersion depth D2 of the outlets (13) of the other channels (12a) is set from 250 to 600 mm.

8. A method of continuous casting according to any of the preceding claims, wherein the outlets (13) having an immersion depth D1 and the outlets (13) having an immersion depth D2 are separated by a distance A = | D2-D1 19. A method of continuous casting according to claim 8, wherein said distance A is set from 100 to 500 mm.

10. A method of continuous casting according to any of the preceding claims, wherein the regulation of the flow rate FR is done by a stopper rod (18).1 1. A method of continuous casting according to any of the preceding claims, wherein the shape and orientation of the outlets (13) are configured tofurther adjust the depth Li of the interface (19) between the two pools in the mold (3).

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