Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
  • C21D8/1216—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the working steps
  • C21D8/1222—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
  • C21D8/1216—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the working steps
  • C21D8/1233—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
  • C21D8/1244—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment

Definitions

• the present invention relates to a method of manufacturing a hot-rolled steel sheet, having high magnetic properties, in particular very low watt losses.
• the watt losses, or magnetic losses, of a steel are constituted by the energy dissipated in the form of heat in the steel when the latter is subjected to a variable magnetic field. These losses occur in electrical machines such as motors and transformers, where the magnetic field varies in an alternating cyclic fashion. Watts losses are broken down into hysteresis losses, which are proportional to the area of the hysteresis curve, and eddy current losses, which are proportional to the square of the induction.
• the magnetic properties of a steel depend on its chemical as well as physical characteristics.
• the chemical composition has a significant influence on the magnetic permeability and the coercive field.
• the effect of alloying elements is well known in the art. For information, we will briefly recall here the action of the alloying elements most commonly encountered in steels for magnetic sheets. Carbon has a predominant effect, which is particularly marked for contents lower than 0.006%; it decreases the magnetic permeability and it increases the coercive field, and therefore the losses in watts. Carbon is also harmful not only in the form of interstitial atoms, but also in the form of iron carbides of reduced size and finely dispersed. Nitrogen plays essentially the same role as carbon, both if it is in solution and if it is present in the form of fine precipitates.
• the elements present in solid solution have various effects. Phosphorus increases magnetic losses; its action is very marked for contents less than 0.02%. At contents between 0.5% and 5%, silicon reduces magnetic losses thanks to its double action of reducing losses by hysteresis and eddy currents by increasing the resistivity of steel. Overall, manganese reduces total magnetic losses by reducing losses by eddy currents by an increase in resistivity, despite an increase in losses by hysteresis. In its frequent form of MnS sulfide, sulfur increases magnetic losses; on the other hand, if it is in solution, the sulfur contributes to reducing the losses by eddy currents. Aluminum, partially present in the form of oxide (Al2O3) or nitride (AlN), has the same effect as sulfur. Chromium reduces total magnetic losses, as long as it does not form carbides.
• the magnetic properties also depend to a large extent on the heterogeneities present in the steel.
• the presence of inclusions or precipitates, as well as a finer grain, have the effect of increasing the losses in watts and the coercive field and of decreasing the relative magnetic permeability.
• the optimal grain size is approximately 100 ⁇ m, because the losses by hysteresis increase and that the losses by eddy currents decrease when the size of the grains decreases.
• internal stresses, crystal boundaries and crystal orientation of the grains play an important role, due to the anisotropy of the phenomena. In this regard, the crystal texture (100) of the steel is best suited to minimize the watt losses.
• a steel having a high magnetic permeability is used.
• steels having magnetic properties adapted to each type of application are currently used, by varying the chemical composition of the steel, by modifying its crystalline texture and by seeking a high grain size.
• the object of the present invention is to propose a method for manufacturing a hot-rolled steel sheet having magnetic properties, the values of which, but also the uniformity, are clearly improved compared with the magnetic steels of the prior art.
• a method of manufacturing a hot-rolled steel sheet, having high magnetic properties which comprises a finishing rolling operation in the ferritic field, is characterized in that a steel slab up to a temperature less than or equal to 1100 ° C., in that this slab is subjected to a roughing rolling operation in the austenitic domain up to an intermediate thickness, in that one cools said slab to a temperature below Ac3 to effect the phase transformation of austenite into ferrite, in that said slab is subjected to a finishing rolling operation in the ferritic field by means of rolling cylinders hot lubricated by a lubricant resistant to high temperature, to form a hot sheet having a thickness between 0.4 mm and 10 mm, in that one coils this sheet to c haud at a temperature below 850 ° C, and in that said coil is then cooled to room temperature.
• a slab of continuously cast steel is generally used.
• the reheating temperature limited to 1100 ° C. makes it possible to limit the re-solution of the nitrides AlN in the steels calmed with aluminum, the dissolution of the sulphide MnS in the steels with low manganese content as well as its reprecipitation in fine and dispersed form. , which would be penalizing for the subject of the invention.
• the roughing rolling in the austenitic field has the effect of giving the product an intermediate thickness, which depends in particular on the thickness of the initial slab and the thickness of the final sheet.
• the intermediate thickness can be between 8 mm and 40 mm.
• phase transformation of austenite into ferrite takes place during the transfer of the product from the roughing rolling train to the finishing rolling train. In general, this transformation takes place completely for ultra-low carbon steels, without the need for additional cooling. In some cases, it may not be complete, in particular in steels with a higher carbon content; the cooling of the product can then be accelerated by any suitable means, in order to complete the phase transformation before the product enters the hot finish rolling train.
• the finishing hot rolling must be mainly carried out in the ferritic field.
• the optimal temperature range is between Ac3 and 650 ° C and preferably between 850 ° C and 750 ° C.
• the reduction rate is at least 90%
• the range of rolling temperatures in the ferritic region as well as the work hardenings carried out in the successive rolling passes are optimally chosen so that, on the coiled sheet, a microstructure with large ferritic grains, the size of which develops is preferably between 50 ⁇ m and 120 ⁇ m and on the other hand a majority crystalline texture (100).
• Rolling in the ferritic field with lubricated rolling rolls makes it possible to very significantly reduce the shearing of the sheet metal surfaces in contact with the rolls. Therefore, dynamic recrystallization under the surface of the laminated sheet is largely avoided and the texture (110) is no longer created in this area.
• the conditions of application of the lubricating agent as well as the quantities used can be optimized as a function of the temperature of the product, in order to eliminate the aforementioned shearing and its consequences, and thus to produce a sheet having a microstructure and a homogeneous crystalline texture on the surface. 'thickness.
• the sheet is rolled in the ferritic region to a thickness of between 0.4 mm and 1.5 mm.
• the winding temperature of the hot-rolled sheet is advantageously between 800 ° C and 600 ° C, and more preferably between 750 ° C and 700 ° C.
• the winding in this temperature range allows, after recrystallization of the deformed ferrite, to achieve the growth of the ferrite grains to an optimal size.
• This winding also makes it possible to ensure the precipitation of carbides and nitrides - for the fractions which may be dissolved - in a coarse form, which is not detrimental to the magnetic properties of the sheet.
• the winding is advantageously carried out immediately after the finishing rolling, to avoid the risk of uncontrolled deformation of the sheet, in particular for sheets of very small thickness.
• the sheet can be hot-rolled at temperatures which can be markedly lower, that is to say between 650 ° C. and 20 ° C., to produce a steel not recrystallized. This is then, without intermediate work hardening, annealed at a high temperature in the ferritic region, that is to say between 600 ° C and 800 ° C, and preferably between 700 ° C and 750 ° C, then the sheet metal is cooled to room temperature. It is thus possible to develop the optimal microstructure for the magnetic properties.
• the aforementioned annealing of the sheet can be carried out either continuously, optionally combining a pickling of the sheets with hydrogen, or in coils.
• the hot-rolled sheet which is in a coil in the recrystallized or work-hardened state, can be subjected to a limited work hardening, the rate of which is between 1% and 35% and preferably between 5% and 20%, then a final annealing according to the process described in the previous variant.
• the process of the invention is preferably applied to steels of the ELC type, that is to say with a carbon content of less than 0.03%, and of the ULC type, that is to say with a carbon content of less than 0.005%.
• Such low carbon contents are generally obtained by a vacuum decarburization operation at the steelworks.
• columns 9 to 12 have two values, which reflect the situation respectively in the skin and in the heart of a sheet having a heterogeneous structure.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Crystallography & Structural Chemistry (AREA)
• Thermal Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Electromagnetism (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Soft Magnetic Materials (AREA)
• Manufacturing Of Steel Electrode Plates (AREA)

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Cited By (5)

Citations (6)

• 1993
  • 1993-01-29 BE BE9300093A patent/BE1006599A6/fr not_active IP Right Cessation
• 1994
  • 1994-01-27 EP EP94870013A patent/EP0609190A1/de not_active Withdrawn

Patent Citations (6)

Non-Patent Citations (1)

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