EP0487443B1 - Steel sheet for electrical applications - Google Patents

Abstract

Steel sheet for electrical application, of the type containing less than 0.005 % by weight of carbon, 0.1 to 0.7 % by weight of manganese and at least 0.2 % by weight of silicon, characterised in that it is manufactured as follows: - a steel whose carbon content is already lower than 0.005 % by weight and in which the sum of the contents of phosphorus and silicon is between 0.420 and 0.570 % by weight is produced, cast and rolled when hot and then when cold into the form of sheets; - the said sheets are then subjected to a continuous tempering treatment under neutral atmosphere at a temperature taken from the range approximately from 800 to 900 DEG C, and so as to remain below the alpha -> gamma transformation point of the steel type treated.

Description

The invention relates to steel sheets for electrical use, used in particular for the construction of transformers, motors and alternators. The products usually produced for these uses are traditionally divided into three categories.

Oriented grain "Fully Process" sheets have the particularity of containing high silicon contents (of the order of 3%), and of having a very marked texture orientation of the type (110) 〈001〉, the direction 100 being oriented along the rolling direction. It is also in this direction that the ambient magnetic field is applied. The silicon increases the resistivity of the sheet and thus limits the losses by eddy currents. These sheets are applied in particular to the manufacture of power transformers. Non-oriented grain "Fully Process" sheets contain silicon in proportions which may be lower than for the previous ones (0.5 to 3.2%), and also 0.3 to 0.5% of aluminum. This latter element has the same role as silicon in limiting losses by eddy currents, and its partial substitution for silicon makes it possible not to increase the brittleness of the metal too much. As these sheets are intended for rotating machines (motors, alternators) of medium or high power, it is not desirable that they have a directional texture. They can be applied a surface insulation by varnishing.

These two categories of sheets can be used by customers directly in the delivery state of the producer, without any subsequent heat treatment being necessary to obtain the targeted magnetic properties. This is not the case of sheets called "Semi Process", used in devices from low to medium power: motors of small and medium household appliances, transformers, automobile alternators, etc. These steels contain 0.030 to 0.050% of carbon, 0.2 to 0.7% of manganese, 0.020 to 0.0160% phosphorus and possibly up to 1.2% silicon. They undergo hot and cold rolling, then continuous annealing followed by a pass to the skin pass to harden the metal by a deformation of 4 to 10% in order to increase their elastic limit. It is in this form that the sheets are sent to the customer. But at this stage, their magnetic properties are poor, because they have been degraded by hardening. This work hardening is however necessary so that the metal can be easily cut by the customer. The latter must then impose on the product a heat treatment (annealing at approximately 800 ° C.) aimed at decarburizing up to a carbon content of less than 0.005%, at increasing its grain size (operation facilitated by work hardening which has been carried out by the skin pass), and possibly to isolate it, which can be done by surface oxidation.

This operating mode makes it necessary to multiply the stages of treatment, which is long and costly. In addition, the last steps must be carried out on cut products, which results in a lower productivity of the installation than if they were carried out by the producer during the sheet production cycle.

It has already been proposed (see documents US 3867211 and FR 1437673) to shorten this treatment by directly obtaining the very low carbon content desired in the still liquid metal, by vacuum decarburization thereof. Annealing therefore no longer needs to be a decarburizing annealing, and can be conducted in an atmosphere. neutral or reducing. However, there remains the problem of the cuttability of the products obtained, which must be optimized to meet the requirements of the end user.

The object of the invention is to provide sheets for electrical devices of low to medium power having excellent cuttability, and manufactured according to a short and economical die.

• on élabore, coule et lamine sous forme de tôle un acier dont la teneur en carbone est inférieure à 0,005 % en poids, et dont la somme des teneurs en phosphore et en silicium est comprise entre 0,420 et 0,570 % en poids ;
• puis on fait subir auxdites tôles un traitement de recuit continu sous atmosphère neutre à une température prise dans la gamme 800 à 900°C environ et de manière à rester en dessous du point du point de transformation α → γ de la nuance d'acier traité. Comme on l'aura compris, le procédé consiste d'une part à obtenir directement à l'aciérie la teneur en carbone finale désirée pour la tôle, et d'autre part à faire subir à celle-ci, après son laminage à froid, un recuit permettant de lui donner des propriétés magnétiques satisfaisantes, sans que des traitements thermomécaniques ultérieurs soient forcément nécessaires, et enfin à donner au produit une composition telle que ses propriétés mécaniques le rendent aisément découpable.

To this end, the subject of the invention is a steel sheet for electrical use of the type comprising less than 0.005% by weight of carbon, 0.1 to 0.7% by weight of manganese and at least 0.2% by weight. of silicon, characterized in that it is manufactured as follows:

• a steel whose carbon content is less than 0.005% by weight and whose sum of phosphorus and silicon contents is between 0.420 and 0.570% by weight is produced, cast and rolled in the form of sheet metal;
• then said sheets are subjected to a continuous annealing treatment under a neutral atmosphere at a temperature taken in the range 800 to 900 ° C. approximately and so as to remain below the point of the transformation point α → γ of the grade of treated steel . As will have been understood, the method consists on the one hand in obtaining directly at the steelworks the desired final carbon content for the sheet, and on the other hand in subjecting it, after its cold rolling, an annealing which makes it possible to give it satisfactory magnetic properties, without any subsequent thermomechanical treatments being necessarily necessary, and finally to give the product a composition such that its mechanical properties make it easily cuttable.

The invention will be better understood on reading the description which follows.

The development of steelmaking techniques has made it possible, in recent years, to obtain extremely low carbon contents in liquid steel, including in mass production on installations which can simultaneously process more than 300 tonnes of metal. The different types of processing channels that make it possible to guarantee contents of less than 0.005%, or even often less than 0.003%, and to keep these contents until the metal solidifies, are detailed in the article "Steel development with ultra-low carbon and nitrogen contents "(Revue de Métallurgie-CIT, January 1990 pp. 63-77 and February 1990 pp. 145-155). These channels require the use of an apparatus for degassing the liquid steel under vacuum, preferably a RH or DH type reactor.

According to the invention, such a die is used to develop and cast in the form of ingots or slabs a steel whose carbon content does not exceed 0.005%. Its silicon content is greater than 0.2%. It must be less than 0.6% to avoid surface oxidation of the product which would damage the rolling tools. Its manganese content is between 0.1 and 0.7%. Preferably, this steel is free of aluminum so that the nitrogen is not trapped in the form of nitrides and participates in hardening.

This steel must also be given a phosphorus content which gives it excellent behavior when it is cut. The inventors have determined that a condition to be observed in order to obtain such behavior is to comply with the following condition: the sum of the silicon and phosphorus contents must be between 0.420 and 0.560%. Within these limits, a ratio is obtained for the sheet after heat treatment between the elastic limit Re and the tensile strength Rm with a value equal to at least 0.71, which is very favorable for good de-leaching of the material. All these values are expressed in percentages by weight.

By proceeding in this way, a semi-finished product is therefore obtained directly, the composition of which, in particular the carbon content, is similar to that of the desired final product, without it being subsequently necessary to carry out a decarburizing annealing on the cut parts.

• la phase de maintien s'effectue sous une atmosphère neutre et à une température de 800 à 900°C pendant 30 s à 1 min (pour une épaisseur de tôle de 0,5 à 1 mm), contre une atmosphère décarburante et une température de 800°C environ ;
• la phase de survieillissement à 350-450°C, qui a normalement pour but de précipiter le carbone, est désormais facultative puisque la teneur résiduelle en carbone est très faible (de l'ordre de 10 fois moindre que la teneur habituelle).

Once the slabs have been obtained, they undergo a rolling scheme identical to those usually practiced, so as to obtain sheets whose thickness is generally 0.5 to 1 mm. Thicknesses 0.5 and 0.65 mm are the most common. These sheets then undergo a continuous annealing operation, preferably on a line with vertical strands for thin sheets, which differs from the operation usually practiced after the skin-pass on the following points:

• the holding phase is carried out under a neutral atmosphere and at a temperature of 800 to 900 ° C for 30 s to 1 min (for a sheet thickness of 0.5 to 1 mm), against a decarburizing atmosphere and a temperature of About 800 ° C;
• the overaging phase at 350-450 ° C, which normally aims to precipitate carbon, is now optional since the residual carbon content is very low (of the order of 10 times less than the usual content).

In conventional production of semi-process sheets, the choice of usual annealing temperatures of 700 to 750 ° C for the annealing operation preceding the skin-pass is guided by obtaining recrystallization after cold rolling. For the sheets according to the invention, the annealing is carried out under conditions allowing the growth of grains recrystallized. A high temperature is therefore desirable, but it must not, however, cause the sheet to exceed the transformation point α → γ typical of the grade treated. The sheet structure must therefore remain entirely ferritic. For the most common grades, a temperature of 850 ° C is well suited, but the most decarburized grades and those most loaded with silicon and phosphorus can withstand temperatures of 900 ° C without undergoing austenitization. As for the duration of the treatment during this annealing, it must simply be sufficient for the magnification of the grain to affect the entire product, and is therefore a function of the thickness of the sheet. For conventional thicknesses of 0.5 to 1 mm, annealing lasting 30 s to 1 min is well suited.

Traditionally, semi-process sheets in their final state are classified according to the level of their magnetic performance into two categories defined by standards AFNOR NFC 28295 (for sheets without silicon) and NFC 28926 (for sheets alloyed with silicon). The latter correspond to the best performances in terms of specific total losses. The fact that a semi-process sheet belongs to one or other of these categories depends in particular on its composition: the sheets most loaded with alloying elements (in particular silicon which makes it possible to reduce losses by current of Foucault) are generally found in the 2nd category. On the other hand the magnetic permeability decreases when one passes from non-alloy steels to alloy steels, which can constitute a drawback.

By way of example, it can be said that sheets according to the invention, of composition (in% by weight) C <0.005%, Mn = 0.3-045%, Si = 0.2-0.38%, P = 0.15-0.19%, Al <0.005%, S <0.015%, N <0.009%, thickness 0.50 mm, have maximum specific total losses of the order of 7W / kg at 1.5 T. The sheets manufactured according to the invention therefore have in their raw state magnetic performances intermediate between those of conventional sheets of the non-alloyed category called FeV 890-50HD and FeV 660-50HD which respectively have maximum specific total losses of 8.9 and 6.6 W / kg at 1.5 T. These sheets can therefore be used directly by customers for a large number of applications, in a similar way to "Fully Process" sheets. If the customer considers the performance of the product still insufficient for the applications he envisages, he also has the possibility of carrying out an improvement heat treatment on the sheet which will have been previously slightly hardened so as not to degrade the magnetic properties too much. The sheets thus treated are then found with performances at least equal to the conventional sheets FeV-50HE of the category of alloy steels from the point of view of the specific total losses (that is to say equal to or lower than 4.5 W / kg at 1.5 T for a thickness of 0.50 mm), and even offer better magnetic permeability than these, and this for lower silicon contents.

From the point of view of cuttability, these sheets have a Re / Rm ratio of at least 0.71. A silicon content of 0.32% accompanied by a phosphorus content of 0.18% provides, from this point of view, excellent results, namely a Re / Rm ratio of 0.76.

The sheets described make it possible to lower the production costs of sheets for electrical use for low to medium power devices by shortening their manufacturing process, and by reducing the quantity of alloying elements to be introduced into the liquid metal, to performance at least equal for the final product. In addition, they exhibit excellent behavior during their cutting, and are therefore easy to implement by their end use.

Claims (2)

1. Steel sheet for electrical applications, of the type including less than 0.005% by weight of carbon, 0.1 to 0.7% by weight of manganese and at least 0.2% by weight of silicon, characterized in that it is manufactured as follows:

   - a steel, the carbon content of which is less than 0.005% by weight and the sum of the phosphorus and silicon contents of which is between 0.420 and 0.570% by weight, is smelted, cast and rolled into the form of a sheet;

   - the said sheet is then subjected to a continuous annealing treatment under a neutral atmosphere at a temperature taken within the range of approximately 800 to 900°C and so as to remain below the α→γ transformation point of the treated grade of steel.
2. Sheet according to Claim 1, characterized in that the said sheet includes from 0.27 to 0.38% by weight of silicon and from 0.15 to 0.19% by weight of phosphorus.