SEMI-FINISHED PRODUCT MADE OF ELECTRICAL COMPOSITE STEEL

Abstract

A composite steel semi-product having a shell and bulk, said shell having a chemical composition containing, in weight%, Si: from 2.0% to 4.0%, said bulk having a chemical composition containing, in weight%, Si: from 3.0% to 8.0%, said shell and said bulk compositions comprising also the following elements expressed in weight%: C: 0.0001% - 0.05%, Mn: 0.05% - 2.0%, Al: 0.05% - 1.3%, and can contain one or more optional elements, the remainder composition being composed of iron and unavoidable impurities caused by processing, wherein the amount of Si in the bulk is higher than in the shell and wherein said shell is provided on the four longitudinal sides of said semi-product.

Description

ELECTRICAL COMPOSITE STEEL SEMI-PRODUCT

[0001] The invention relates to an electrical composite steel semi-product and a manufacturing method thereof. In particular, the invention relates to an electrical composite steel semi-product with high silicon content that is able to be hot rolled.

[0002] Electrical steels are important materials for electrical machines such as motors, generators or transformers. To obtain the best properties in terms of high frequency losses, a silicon content near 6.5 wt% is necessary.

[0003] However, as the silicon content increases, the product becomes more brittle and is thus impossible to hot roll due to cracks appearing on the product during the process. Therefore, manufacturing of high-silicon electrical steels is difficult and very limited to some processes.

[0004] CN103014613 describes a method for continuously manufacturing a high-silicon steel sheet using two low-silicon steel sheets uncoiled and sewn together to form a hollow plate then adding silicon-based powder inside the hollow and rolling the low-silicon sheets together with the powder to form a sandwich composite plate.

[0005] The method proposed in prior art allows the manufacturing of a high-silicon electrical steel without causing cracks on the product. However, the method proposed in the prior art is very complex, can lead to bonding issues on the final product and may not be adapted to a simple plant.

[0006] The present invention discloses an electrical composite steel semiproduct with high silicon content able to be hot rolled without forming cracks on the product and a method for manufacturing the electrical composite steel semiproduct with a simple installation.

[0007] The object of the invention is a composite steel semi-product having a shell and bulk, said shell having a chemical composition containing, in weight%, Si: from 2.0% to 4.0%, said bulk having a chemical composition containing, inweight%, Si: from 3.0% to 8.0%, said shell and said bulk compositions comprising also the following elements expressed in weight%:C: 0.0001% -0.05%,Mn: 0.05% - 2.0%, Al: 0.05% - 1.3%, and can contain one or more of the following optional elements,P: <0.15%, S: <0.015%, N: <0.015%, Nb: <0.1%, Ti: <0.1%, V: <0.1%, Cr: < 5%,Mo: < 0.5%,W: <0.1%, Co: < 8%,As: < 0.05%,Cu: < 5%,Ni: < 5%,B: <0.1%, Ca: 0.001% -0.01%, Pb: < 0.2%, Sn: < 0.2%, Sb: < 0.2%, the remainder composition being composed of iron and unavoidable impurities caused by processing, wherein the amount of Si in the bulk is higher than in the shell and wherein said shell is provided on the four longitudinal sides of said semiproduct.

[0008] The composite steel semi-product according to the invention may also have the optional features listed below, considered individually or in combination:- the shell contains in weight%: Si: from 2.0% to 3.5% and the bulk contains in weight%, Si: from 4.0% to 7.5%,- the shell contains in weight%: Si: from 2.0% to 3.5%, and the bulk contains in weight%: Si: from 5.0% to 7.0%,- the shell has a thickness of 1 -25% of the composite semi-product thickness,- the shell has a thickness of from 3 to 40 mm.

[0009] The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention.

[0010] The invention will be described, in a non-limitative way, in reference to the following drawings:- Fig 1 : general view of a composite steel slab according to the invention,- Fig 2: general view of a first embodiment of an equipment used for manufacturing the invention,- Fig 3: general view of a second embodiment of an equipment used for manufacturing the invention,

[0011] The chemical composition of the electrical composite steel semi-product comprises of the following elements:

[0012] Carbon is present in the steel of the present invention from0.0001 % to 0.05%. Carbon is a precipitate forming element that is detrimental for the magnetic properties of the present steel.

[0013] Manganese content of the steel of the present invention is from 0.05% to 2.0%. Manganese provides solid solution strengthening and reduce the iron loss by increasing specific resistance. However, it may considerably reduce the magnetic flux density, and recrystallization of the steel will be hindered during annealing.

[0014] The content of Aluminum is from 0.05% to 1.3%. Aluminum increases the electrical resistivity of the material and can effectively reduce theiron loss of steel. When the content of Aluminum is present more than 1 .3%, the magnetic induction of the steel will be significantly reduced, the magnetostriction increased and it is also detrimental to the rollability of the steel of the present invention.

[0015] Phosphorus content in the Steel of the present invention is from 0% to 0.15%, Phosphorus reduces the hot and cold ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with Manganese. For these reasons, its content is limited to 0.15%.

[0016] Sulfur is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the Sulfur content is preferably as low as possible but 0.015% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides which are detrimental for the magnetic properties of the present invention.

[0017] Nitrogen is limited to 0.015% to minimize the precipitation ofAluminum nitrides during solidification which are detrimental for magnetic properties of the steel.

[0018] Niobium is present in the Steel of the present invention from0% to 0.1 % and suitable for forming carbo-nitrides to increase the strength of the Steel of the present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides.

[0019] Titanium is an optional element and when added to the Steel of the present invention is from 0% to 0.1 %. It forms Titanium-nitrides appearing during solidification of the cast product. The amount of Titanium is limited to 0.1 % to avoid the formation of Titanium-nitrides detrimental for magnetic properties of the steel of the present invention. The Titanium content below 0.015% does not impart any effect on the steel of the present invention.

[0020] Vanadium is present in the Steel of the present invention from0% to 0.1 % and is effective in enhancing the strength of the steel by forming carbides or carbo-nitrides and the upper limit is 0.1 % from economic points of view.

[0021] Chromium is an optional element for the steel of the present invention and is from 0% to 5%. Chromium provide strength to the steel by solid solution strengthening but when used above 1 % impairs the magnetic properties of the steel.

[0022] Molybdenum is an optional element that constitutes 0% to0.5% of the Steel of the present invention. Mo has an effect of coarsening carbide and thus reducing the iron loss. When it exceeds 0.5%, the effect of improving the iron loss is saturated.

[0023] Tungsten is an optional element that constitutes 0% to 0.1 % of the Steel of the present invention. Tungsten has an effect of coarsening carbide and reducing the iron loss, like Mo. However, when the addition amount is less than 0.001 mass%, the above effect cannot be obtained sufficiently, while when it exceeds 0.1 wt.%, the effect of improving the iron loss is saturated.

[0024] Cobalt is an optional element that constitutes 0% to 8% of theSteel of the present invention. Cobalt is an element increasing the magnetic moment of Fe alloy and has an effect of increasing the magnetic flux density and reducing the iron loss. However, when the addition amount is less than 0.01 wt.%, the above effects cannot be obtained sufficiently.

[0025] Arsenic is an optional element that constitutes 0% to 0.05% of the Steel of the present invention. As is a grain boundary segregation element and has an effect of improving the texture and thus reducing the iron loss. The above effect is obtained by the addition of not less than 0.001 wt.%. However, As is an element causing grain boundary embrittlement, and this adverse effect becomes particularly remarkable when it is added by more than 0.05 wt.%. Therefore, As is preferable to be added within the range of 0.001 to 0.05 wt.%.

[0026] Copper may be added as an optional element in an amount of0% to 5% to increase the strength and elongation of the steel of the present invention.

[0027] Nickel may be added as an optional element in an amount of0% to 5% to increase the strength of the steel of the present invention and to improve its strength and elongation.

[0028] Boron is an optional element for the steel of the present invention and may be present between 0% and 0.01 %. Boron forms boro-nitirides and impart additional strength to steel of present invention when added in an amount of at least 0.0001 %.

[0029] Calcium may be present in the steel of the present invention optionally and may be from 0.001 % to 0.01 %. Calcium contributes towards the refining of the Steel by binding the detrimental Sulfur content in globular form thereby retarding the harmful effect of Sulfur.

[0030] Other elements such as Sn, Pb or Sb can be added individually or in combination in the following proportions: Sn 1=0.2%, Pb 1=0.2% and Sb 1=0.2%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. Sn and Sb also have texture improvement potential.

[0031] The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.

[0032] The main feature of the electrical composite steel semiproduct according to the invention is that it is separated into two parts, a bulk and a shell, having a different amount of Si.

[0033] The composition of the bulk, having from 3.0% to 8.0%, preferably from 4.0% to 7.5%, more preferably from 5.0% to 7.0% Si in weight% allows the electrical steel products to have the properties of a high-silicon electrical steel, with enhanced eddy current loss performance thanks to its high electrical resistivity as well as enhanced hysteresis loss performance thanks to its low magnetostriction close to 6.5% Si.

[0034] The shell, having a composition in Si lower than in the bulk from 2.0% to 4.0%, preferably from 2.0% to 3.5% or from 2.5% to 4.0%, more preferably from 2.5% to 3.5%, in combination with the fact that the shell is located on the four longitudinal sides of the steel semi-product allows avoiding cracks during subsequent operations (semi-product transportation, rolling, etc... ).

[0035] The shell is located on the four longitudinal sides of the semi product with a continuity. Consequently, the shell is also located on the comers of the semi product.

[0036] The thickness of the shell is preferably from 1 % to 25% of the thickness of the steel semi-product or from 3 to 40 mm depending on the semiproduct. The thickness of the shell is not necessarily uniform on each of the four longitudinal sides of the steel semi-product.

[0037] For the shell and the bulk, the microstructure is made of ferrite.

[0038] The steel according to the invention can be manufactured by any suitable method. It is however preferable to use the method according to the invention that will be detailed, as a non-limitative example.

[0039] Such preferred method consists in providing a semi-finished casting of steel with a chemical composition of the steel according to the invention.

[0040] The preferred method consists in using an installation comprising at least a nozzle, a mold, and a means for injecting powder, the method wherein:- a liquid steel of a composition according to the invention with Si from 2.0% to 4.0% is poured into said nozzle to be poured into the mold,- powder including additional Si is injected into part of the steel inside the nozzle thus creating a liquid steel with a composition comprising from 3.0% to 8.0% of Si,- the liquid steel of the initial composition solidifies first creating a solidified shell,- the liquid steel with the second composition solidifies then inside the shell creating the bulk,- creating after full solidification a composite steel semi-product.

[0041] Preferably, the composite metallic semi-products are slabs, billets, blooms or ingots. An electrical composite steel slab according to theinvention is represented in Fig 1 . A slab has two narrow faces and two long faces. As represented in Fig 1 , a slab according to the invention has its shell 16 located on its four longitudinal sides, in other words, in the two narrow and long faces. A head and a tail can also be defined for the slab as represented in Fig 1 .

[0042] A first embodiment of the preferred method consists in using the continuous casting equipment and associated method described in Patent application PCT / IB2022 / 062382.

[0043] The equipment described comprises a nozzle disposed between a tundish and a mold, the nozzle comprising at least a dome, mixing chambers, channels of different lengths and a means for injecting powder. The equipment is represented in Fig 2.

[0044] The method of manufacturing the electrical composite steel semi-product using this equipment consists in pouring steel with a defined composition from the tundish into the nozzle. The steel flows into the upper part 4 of the nozzle 1 , thus creating an initial stream. A stopper rod 18 allows the control of the initial flow rate.

[0045] 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. Support arms 7 create different areas on the dome 6, dividing the 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.

[0046] The separate streams flow then into the different mixing chambers 9a, 9b. Si-based powder is injected at the same time into one of the mixing chambers 9a, 9b. The design of the chambers, having a large section at the top, allows the steel to flow down the dome like a waterfall and allows the powder to be injected into the flow without the steel coming into contact with the means for injecting powder. The reduction of the section of the chambers allows the steel to be slowed down and to accumulate in the chambers 9a, 9b. In consequence, the powder can be mixed efficiently with the steel into said chamber 9a, 9b to modify its composition, and starts melting. This step allows the liquid steel into which powder is injected to be homogeneous. The reduction ofthe section of the chambers can be done through various configurations of the walls. For example, the reduction can be done with a regular slope or stepwise or any means for reducing the section.

[0047] The injection of powder is facilitated by a gas injection 11 that creates 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 powder injection 10 thus avoiding potential clogging of the powder injection 10.

[0048] The gas is preferably non-oxidizing, Ar for example, to prevent any reaction with the steel during casting.

[0049] After the injection, the two mixing chambers 9a, 9b contain two types of steel with different composition.

[0050] The steels flow then into the channels 12a, 12b, 12c of the lower part 5 of the nozzle 1 which are connected to the chambers after the reduction of their section. The steel in the chamber where Si powder was injected flows into the central channel 12a and the steel in the other chamber flows into the side channels 12b, 12c. The different steels are then poured into the mold 3 through the outlets 13 of the channels 12a, 12b, 12c.

[0051] The steel from the central channel 12a is poured deeper into the mold 3 due to the central channel 12a being longer than the side channels 12b, 12c. This configuration allows the two types of steels to be poured at different heights into the mold 3, thus creating two pools of steel, an upper pool 14 and a lower pool 15, different in composition. The upper pool 14 is formed by the steel coming from the side channels 12b, 12c and the lower pool 15 is formed by the steel coming from the central channel 12a.

[0052] The outlets 13 of the nozzle 1 are submerged into the different pools of steel during usage. The outlets 13 of the side channels 12b, 12c are submerged into the upper pool 14 and the outlets 13 of the central channel 12a are submerged into the lower pool 15.

[0053] The composition of the upper pool 14 is the composition of the steel injected and having low Si content. The composition of the lower pool 15 isthe composition of the steel injected and having high Si content due to the injection of Si-based powder.

[0054] In the mold 3, the steel of the upper pool 14 solidifies first thus creating a shell 16 with low Si content. The steel of the lower pool 15 solidifies then inside the shell 16 thus creating the bulk 17 of the material with high Si content. After full solidification, the material obtained is a composite metallic semiproduct with a Si content lower in its shell than in its bulk.

[0055] A second embodiment of the preferred method consists in using a casting equipment according to Fig 3.

[0056] A steel having an initial composition is injected into a nozzle 1 having a funnel-shaped upper part 4 that is located into a protection chamber 19 cooled by air. The bottom of the funnel-shaped upper part 4 is closed by a plug 20 having holes that goes through it to let the liquid steel pass through. The steel goes into the lower part 5 of the nozzle 1 and is injected into the bottom of an ingot mold 21 until the mold 21 is almost filled with steel. The steel starts to solidify on the periphery of the mold 21 .

[0057] A powder container 22 containing Si-containing powder is connected to the equipment with a tube 23 that pass through a lateral hole in the plug 20 and ends into the plug 20. The Si-containing powder is injected into the nozzle 1 via an endless screw located inside the tube 23 and controlled by a motor 24 and goes into the lower part 5 of the nozzle through a hole on the bottom of the plug 20. The bottom of the upper part 4 of the nozzle, the plug 20, the top of the bottom part 5 of the nozzle 1 and the end of the tube 23 for injecting powder are located into a preheating furnace 25. The injection of powder is performed once the steel in the mold has already started to solidify.

[0058] The powder is mixed with the steel inside the nozzle 1 to modify the composition of the steel. The steel with the modified composition is injected into the ingot mold 21 and the excess of steel of the initial composition flows out of the mold 21 into a container 26 and the mold 21 is filled with the steel with powder injected. Once the mold 21 is filled, the nozzle 1 is removed, and thesteel is solidified. After full solidification, the material obtained is a composite metallic semi-product with a Si content lower in its shell than in its bulk.

[0059] For every embodiment, an intermediate zone between the shell and the bulk having an intermediate content of Si can be formed during the process but is not detrimental to the technical effect of the steel of the present invention. The depth of the intermediate zone depends on the exact chemistries of the shell and the bulk.

[0060] The powder injected in the step of injection is a Si based powder and can be Fe-Si powder, Fe-AI-Si mixing of powders, Si metal powder or any of their combinations.

[0061] The particle size of the powders is preferably 2.0 mm or less.The Fe-Si powder preferably contains 50% or more Si, the remainder being Fe and unavoidable impurities. The Si metal powder preferably contain 90% or more Si, the remainder being unavoidable impurities. The impurities can consist in one or more of the following elements: Al, Ca, B, S, Ti, Mn, Cr, V, P.

[0062] The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention.

[0063] Steel semi-products made of steels with different compositions are gathered in Table 1 , where the steel semi-products are produced according to process parameters as stipulated in Table 2, respectively. Thereafter, Table 3 gathers the results of the hot rolling tests.Table 1

[0064] The steel semi-products represented in Table 1 are manufactured using the process and the equipment represented in Fig 3, according to the second embodiment.

[0065] The product obtained is a high-silicon electrical steel semiproduct with a shell having less Si content than in the bulk and with a composition according to Table 1 .Table 2

[0066] The results of the hot rolling tests are gathered in Table 3. Two semi-products casted were used for the hot rolling tests (E and F). For the hot rolling tests, the semi-product casted was first cut to obtain several slabs. For tests R1 , R2 and R3, the shell was removed on two opposite sides for each slab. The slabs are then hot rolled according to the parameters of Table 3. The rolling is done from the head of the semi-product to its tail.Table 3

Claims

1. A semi-finished product made of composite steel, having a shell and a main part, wherein the shell has a chemical composition containing, by weight %: Si: 2-4.0, and the main part has a chemical composition containing, by weight %: Si: 3-8.0, wherein the shell and the main part also include the following elements, by weight %: C: 0.0001-0.05, Mn: 0.05-2.0, Al: 0.05-1.3, and may contain one or more of the following optional elements: P: ≤0.15, S: ≤0.015, N: ≤0.015, Nb: ≤0.1, Ti: ≤0.1, V: ≤0.1, Cr: ≤5, Mo: ≤0.5, W: ≤0.1, Co: ≤8, As: ≤0.05, Cu: ≤5, Ni: ≤5%, B: ≤0.1, Ca: 0.001-0.01, Pb: ≤0.2, Sn: ≤0.2, Sb: ≤0.2, The rest of the composition consists of iron and inevitable impurities formed during processing, while the amount of Si in the main part is higher than in the shell, and the shell is located on four longitudinal sides of the semi-finished product.

2. A semi-finished product made of composite steel according to claim 1, wherein said shell contains, by weight: Si: 2-3.5, and the main part contains, by weight: Si: 4-7.

5.

3. A semi-finished product made of composite steel according to claim 2, wherein said shell contains, by weight: Si: 2-3.5, and said main part contains, by weight: Si: 5-7.

0.

4. A semi-finished product made of composite steel according to paragraphs 1-3, in which the said shell has a thickness of 1-25% of the thickness of the composite semi-finished product.

5. A semi-finished product made of composite steel according to paragraphs 1-3, in which the said shell has a thickness of 3-40 mm.