EP0609190A1 - Method of manufacturing a hot rolled steel sheet with high magnetic properties - Google Patents

Abstract

A slab of steel is heated up to a temperature less than or equal to 1100@C, this slab is subjected to a roughing rolling operation in the austenitic region down to an intermediate thickness of between 8 mm and 40 mm, and this slab is cooled down to a temperature below Ac3 in order to carry out the austenite-to-ferrite phase transformation. Next, the slab is subjected to a finishing rolling operation in the ferritic range by means of hot-rolling rolls lubricated with a lubricant able to withstand high temperature, in order to form a hot-rolled sheet having a thickness of between 0.4 mm and 10 mm, this hot-rolled sheet is coiled at a temperature below 850@C, and the coil is then cooled down to ambient temperature.

Description

The present invention relates to a method of manufacturing a hot-rolled steel sheet, having high magnetic properties, in particular very low watt losses.

It is known that 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. It is known in particular that the optimal grain size is approximately 100 μm, because the losses by hysteresis increase and that the losses by eddy currents decrease when the size of the grains decreases. Likewise, 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.

Under current conditions, a steel placed in a magnetic environment must have properties adapted to each particular application.

To ensure a shielding effect, preferably a steel having a high magnetic permeability is used. On the other hand, in a rotating machine, we will seek to minimize the total watt losses, so as to reduce the installed power.

To achieve these objectives, 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.

According to the present invention, 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.

For the implementation of this process, a slab of continuously cast steel is generally used.

After pouring the slab, recent techniques known as hot charging or direct rolling can be applied. However, it has proved preferable to put the slab at low temperature in the reheating oven, in order to minimize the dispersion of the precipitates.

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. For the aforementioned range of sheet thicknesses, the intermediate thickness can be between 8 mm and 40 mm.

The 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 μm and 120 μm and on the other hand a majority crystalline texture (100).

However, it has been found that the optimization of the conditions of hot rolling in the ferritic field was not sufficient to obtain a hot sheet having a grain size and a crystalline texture homogeneous depending on the thickness. It has in fact been observed that the hardening of steel in the ferritic region causes heterogeneity of the microstructure and of the crystal texture according to the thickness of the rolled sheet, according to a mechanism which is not established with certainty. It seems that in contact with the cylinders, the surfaces of the rolled product in the ferritic domain are deformed essentially by shearing, and that this shearing is all the more important as the friction contact with the cylinders is higher. The importance of this shearing of the laminated surfaces increases when the work hardening in the ferrite increases; the heterogeneities of the microstructure and of the crystalline texture which result therefrom then tend to invade the core of the laminated product. The shear also increases when the rolling temperature in the ferritic region decreases. Under the effect of this shearing, there appears in the surface and immediately below the surface a concentration of deformations, which induces a dynamic recrystallization in the regions considered. As a result, in the surface zone of the sheet, a stabilization of a layer of very fine grains, very different in size from the grains produced in the core of the sheet, which is deformed essentially by compression. Shearing has a second effect, namely the production on the surface of a shear texture (110). This texture is less favorable, as regards the magnetic properties, than the texture (100) of the core deformed in compression.

Rolling in the ferritic field with lubricated rolling rolls, recommended by the invention, 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.

Preferably, 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.

According to an advantageous variant of the process of the invention, 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.

In the context of this variant, the aforementioned annealing of the sheet can be carried out either continuously, optionally combining a pickling of the sheets with hydrogen, or in coils.

According to another variant of the process of the invention, 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.

Carbone :

< 0,03 %, de préférence < 0,005 %

Azote :

< 0,004 %, de préférence < 0,002 %

Soufre :

< 0,010 %, de préférence < 0,003 %

Phosphore :

< 0,010 %

Manganèse :

< 0,2 %

Silicium :

< 0,5 %, de préférence < 0,3 %

Aluminium total :

< 0,05 %, de préférence < 0,003 %

Fer :

< reste.

As indicated above, it is essential that the steels have as few sources of heterogeneity as possible, such as inclusions or precipitates. For this purpose, it is preferable that their chemical composition meets the following conditions, by weight:

Carbon:

<0.03%, preferably <0.005%

Nitrogen:

<0.004%, preferably <0.002%

Sulfur:

<0.010%, preferably <0.003%

Phosphorus:

<0.010%

Manganese:

<0.2%

Silicon:

<0.5%, preferably <0.3%

Total aluminum:

<0.05%, preferably <0.003%

Iron :

<stay.

These contents are as low as possible; they can be zero, with the exception of the silicon content.

The improvement of the magnetic properties of hot-rolled steel sheets obtained by the process of the invention is illustrated by comparative examples collected in Table 1.

These examples correspond to the following conditions, listed in the various columns of the table: Column 1: steel type: ELC - ULC 2: sheet thickness, in mm 3: obtained according to the invention: YES (+) - NO (-) 4: type of hot rolling: austenite (A) - ferrite (F) 5: lubricated rolling cylinders: YES (+) - NO (-) 6: winding temperature; in ° C 7: possible cold rolling; reduction rate in% 8: possible annealing; temperature in ° C 9: grain size, in µm 10: main orientation (110) in the rolling plane 11: main orientation (100) in the rolling plane 12: main orientation (111) in the rolling plane 13: maximum magnetic induction B50, in Tesla 14: magnetic losses W15 / 50, in W / kg.

For ULC steels, 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.

Examination of this table indicates that the sheets produced by the process of the invention (sign + in column 3) exhibit increased magnetic induction (B50 - column 13) and reduced magnetic losses (W15 / 50 - column 14) compared to sheets produced by other methods.

Claims (10)

Method for manufacturing a hot-rolled steel sheet, having high magnetic properties, which comprises a finishing rolling operation in the ferritic field, characterized in that a steel slab is heated 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 to an intermediate thickness, in that said slab is cooled to a temperature lower than 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 hot rolling cylinders lubricated with a lubricant resistant to high temperature, to form a hot sheet having a thickness of between 0.4 mm and 10 mm, in that the sheet is hot rolled at a temperature below 850 ° C., and in that it is then cooled to room temperature. Method according to claim 1, characterized in that said intermediate thickness is between 8 mm and 40 mm. Method according to either of claims 1 and 2, characterized in that said finishing rolling is carried out at a temperature between Ac₃ and 650 ° C. Process according to either of Claims 1 to 3, characterized in that the said finishing rolling is carried out at a temperature between 850 ° C and 750 ° C. Process according to either of Claims 1 to 4, characterized in that the said finishing rolling is carried out with a reduction rate at least equal to 90%. Process according to either of Claims 1 to 5, characterized in that the sheet is hot-rolled at a temperature between 800 ° C and 600 ° C. Process according to either of Claims 1 to 5, characterized in that the sheet is hot-rolled at a temperature between 650 ° C and 20 ° C, in that it is annealed in the ferritic domain at a temperature between 600 ° C and 800 ° C and in that it is cooled to room temperature. Process according to either of Claims 1 to 5, characterized in that the hot sheet, after winding, is subjected to work hardening, the rate of which is between 1% and 35%, in that 'It is annealed in the ferritic area at a temperature between 600 ° C and 800 ° C and in that it is cooled to room temperature. Process according to either of Claims 7 and 8, characterized in that the said annealing is carried out continuously under a hydrogen atmosphere. Process according to either of Claims 1 to 9, characterized in that the said sheet is wound immediately after the hot finish rolling.