EP0363232B1 - Method for producing a structured element having a high mechanichal resistance - Google Patents

Description

The subject of the invention is a process for producing metal parts made up of a thin wall and having a three-dimensional shape, and more particularly structural parts having a high mechanical strength allowing them to withstand high stresses. The invention applies in particular to the production of elongated structural parts and of large dimensions such as the spars of engine-carrying masts in the aeronautical industry.

To fix the reactors on the fuselage or on the wings of an airplane, masts are used which must be large enough to keep the reactor at the desired distance from its support, while remaining light enough not to weigh down excessively. 'apparatus. However, these parts must withstand very large and variable mechanical stresses and this, with perfect safety, given that they are parts of vital importance. These parts are therefore given a three-dimensional shape determined so as to obtain the desired dimensions, weight and resistance and to reduce the welds as much as possible. In the case of a mast reactor carrier, for example, we often use parts in the form of longitudinal members each consisting of a thin wall of elongated shape and stiffened at its edges by raised wings which give it a U-shaped cross section, but other shapes are conceivable.

In view of the stresses borne and the security requirements, the specifications that such parts must meet are very strict, in particular as regards the composition of the metal, its metallurgical structure and the dimensions to be observed.

We know, in particular, that a very fine-grained metallurgical structure increases resistance (K.J. PASCOE - "An introduction to the Properties of Engineering Materials 3rd edition, 1979, Van Nostrand Reinhold Company, London, page 173).

The composition of the metal is determined to best resist various mechanical or thermal stresses or corrosion with as low a weight as possible and generally use highly alloyed steels of Fe-Ni-Cr or Fe-Ni-Cr-Mo base. , other compositions being possible however. In addition, to resist stresses under good conditions and over a long period, the metal must have a specific metallurgical structure and in particular a very fine-grained structure, for example finer than index 6 of the AFNOR standard (or ASTM). ).

Up to now, in order to meet all of these requirements, it has seemed normal to manufacture such parts by stamping from solid semi-finished products or from a metal bar of the desired shade by means of a press. very great power.

Die-stamping, in fact, is the technique generally used for the manufacture of solid parts meeting severe requirements, but the power of the press obviously depends on the dimensions of the part to be produced. To decrease the necessary power, the part is subjected to a preheating making it possible to reduce the resistance to deformation of the metal. This necessarily results in a modification of the metallurgical structure and in particular a magnification of the grain which is then attenuated by the stamping and, possibly, a heat treatment.

In such a process, therefore, the metallurgical structure first deteriorated by the necessary reheating is restored by the successive effects of stamping and heat treatment. It is not always easy to adjust the stamping so as to obtain both the desired shape from the blank and a determined metallurgical structure.

In addition, too great a rise in temperature before stamping causes such a magnification of the grain that it is very difficult to restore a metallurgical structure with a fine and homogeneous grain throughout the part, the heat treatment not always making it possible to sufficiently refine the grain size.

In the case where a very fine structure must be kept, it is therefore necessary to reduce the preheating temperature which increases the resistance to deformation of the metal and it is thus necessary to use extremely high press powers, which can range up to 65,000 tonnes (64 x 10⁷N). Obviously, such presses are rare and the manufacturing cost extremely expensive. In addition, given the dimensions of the parts that it is desired to manufacture, a power of 65,000 tonnes may still be insufficient.

It was therefore useful to seek a new process if possible less expensive and allowing further extend the possibilities of realization so as to best respond to the evolution of the technique which leads in particular to an increase in the size of the parts and to more stringent requirements.

To make metal parts made up of a thin wall having a three-dimensional shape, possibly of large size, the most banal method obviously consists in cutting from a laminated sheet having the desired thickness, blanks having a profile determined according to of the shape to be obtained and to subject each blank, optionally after reheating, to a press-type deformation of the stamping type to obtain a part having the desired three-dimensional shape. Such a process is obviously very economical but, until now, it has been used essentially for parts produced in very large numbers such as for example in automobile bodywork. Such parts do not have to meet very severe resistance requirements and, moreover, are made of very thin sheet metal.

It had therefore never been envisaged, until now, to use such a process for the production of fairly thick structural parts and having to meet very severe structural requirements to withstand high stresses, the die-casting being considered as the normal process for making such parts.

The inventor nevertheless considered that, in view of the very significant economic advantages brought by being able to produce such parts on moderate power presses, it was justified to continue studies in that Sens.

The subject of the invention is therefore a new method of manufacturing structural parts of large dimensions and intended to withstand high stresses using medium power presses, for example not exceeding 15 tonnes (15 x 10⁷N) and in guaranteeing however the obtaining of the specific metallurgical structure necessary to obtain the required performances.

According to the invention, the shape of the part, its thickness and its specific metallurgical structure are obtained in several separate operations according to claim 1.

Preferably, the blank is made of high-alloy steel in which at least the highest content of additives exceeds 5%, or of a basic austenitic alloy Fe-Ni-Cr or Fe-Ni-Cr-Mo and the final heat treatment is a treatment for dissolving the metal under conditions determined so as not to cause changes in the fineness of the grain of the part obtained.

In a particularly advantageous manner, the blank is made of highly alloyed steel of the Maraging type, that is to say of martensitic structure, hardened by structural hardening, this steel possibly being a Fe-Ni-Co-Mo- base steel. Ti hardened by precipitation of phases rich in titanium and / or molybdenum, in Fe-Cr-Ni-Mo-Al base steel hardened by precipitation in phases rich in aluminum and / or titanium or Fe-Cr- base steel Ni-Cu- or Fe-Cr-Mo-Ni-Cu hardened by the precipitation of copper-rich phases.

In a first particular embodiment, the rolling conditions are adjusted so as to give the rolled product, from this step, the desired metallurgical structure and the temperature of the metal is adjusted, during all of the following operations, to a level sufficiently low to that said specific structure is not modified.

In a second embodiment, the rolling conditions are controlled so as to give the product a determined metallurgical structure and the reheating preceding the drawing is carried out at a temperature adjusted so as to obtain the specific structure desired, the temperature of the metal being adjusted, during all these operations, to a level at most equal to the temperature of final heat treatment, that is to say the solution temperature.

In particular, the desired metallurgical structure can be obtained, during rolling, by controlling the temperature during successive rolling passes, these preferably comprising a large portion, for example greater than 25% of the reduction in thickness. performed, at a relatively low temperature, for example less than 950 ° C., these two parameters depending on the composition of the alloy.

On the other hand, the deformation carried out with the press to obtain the desired shape will be carried out without appreciable reduction in the thickness obtained by rolling and may be, depending on the case, either a true stamping, or even a simple folding with the hurry.

The invention therefore differs from the stamping used hitherto for the production of such parts by the way in which one obtains not only the shape but also the metallurgical structure required.

Previously, in fact, the same stamping operation had, on the one hand to give the blank the desired shape in three dimensions and, at the same time, to restore as much as possible, the metallurgical structure modified by the necessary preheating, precisely for make mastering possible.

In the present invention, on the contrary, the different requirements are met by separate operations which can therefore be better controlled.

To this end, we take advantage of the fact that the very old rolling technique has benefited in recent years from very significant progress which have made it possible not only to achieve the desired thickness reductions, but even to control the metallurgical structure of the laminated sheet (WL ROBERTS - "Hot Rolling of Steel").

In addition, while initially the rolling mills thus perfected were used for the rolling of fairly soft and thin metals, the latest developments in the technique make it possible to obtain the same advantages for the hot rolling of steel sheets, the l thickness can even exceed 10 mm.

In addition, it has been discovered that, in the particular case of highly alloyed steels, in particular of the Maraging type, which are often used for the production of structural parts, the very fine grain size that one can obtain is directly lamination, or, after this, during reheating before stamping, could not be altered by the stamping operation and the final heat treatment.

This is what made it possible, very surprisingly, to produce, by simple stamping, structural parts with high resistance meeting all the requirements imposed.

However, the invention will be better understood from the detailed description of certain examples with reference to the accompanying drawings.

FIG. 1 schematically represents all of the operations for manufacturing a structural part according to an embodiment of the invention.

Figure 2 gives by way of example the shape of a part produced by the method of the invention.

First of all, a blank with the desired composition is produced.

This draft can be obtained by example by forging a metal ingot of suitable composition or by direct casting of a flat product.

In a first step of the process shown diagrammatically at A in FIG. 1, the blank is rolled to a thickness equal to or slightly greater than the thickness desired for the final part. This rolling is carried out by successive or alternating passes in one or more rolling mill stands 10 provided with the improvements necessary to precisely adjust all of the rolling parameters on each pass. The rolling cycle, in particular the number of passes, the reduction in thickness and the temperature, can thus be determined so that the product has a specific metallurgical structure which corresponds to the structure required for the part or else makes it possible to obtain this structure during reheating preceding the subsequent deformation with the press. In particular, part of the reduction is carried out at a relatively low temperature. These parameters obviously depend on the composition of the metal.

For example, for the production of a part such as that which is represented in FIG. 2 and which can constitute a spar of a reactor-carrying mast, one will start from a highly alloyed ingot, of martensitic stainless steel, the composition of which can be for example:
0.03% C - 13% Cr - 8% Ni - 2% Mo - 1% Al.

This ingot, the thickness of which, after rough forging, can be between 200 and 250 mm, is rolled into a sheet of thickness approximately 30 mm, therefore with a reduction in thickness of around 87%. The rolling conditions, and in particular the number of passes, the temperature of the product and the reduction rate at each pass, are adjusted so that the sheet obtained has a metallurgical structure such that, following the reheating preceding the deformation subsequent to the press, the grain size is finer than the AFNOR index 6. In particular, part of the reduction of at least 25% is carried out at a temperature below 950 ° C.

The sheet thus laminated is then cut into one or more blanks having the desired shape.

Finally, the final part is produced by heating it and then deforming it into the third dimension without any significant change in thickness and therefore with an average press power.

The reheating temperature must, in particular, remain lower or, at most equal to the heat treatment temperature of the alloy, that is to say its dissolution temperature.

For example, in the case of the example indicated above and represented in FIG. 2, each blank is preheated to a temperature of 900 ° C., which constitutes a dissolution of the alloy considered and determines a grain size thinner than AFNOR index 6.

This temperature remains, however, sufficient to allow stamping or folding of the blank by means of a moderate power press, for example 12,000 T (12 x 10⁷ N).

Each of these parts then undergoes a heat treatment for dissolution at 920 ° C. and aging according to the specifications. of employment.

It is observed that the size of the grain obtained in the sheet is finer than the index 6 of AFNOR and that it is uniform over its entire extent.

Of course, the invention does not make it possible to produce all the shapes that can be obtained by stamping, but experience has shown that a large number of parts thus produced up to now and in particular the spars of masts reactors have shapes which allow them to be manufactured by the process according to the invention.

In particular, it will be possible to produce structural parts with a thickness greater than 10 mm and whose length can be several meters using a press whose power does not exceed 15,000 tonnes (15 x 10⁷N).

However, the invention is obviously not limited to the sole characteristics and manufacturing methods of the example which has just been described, and can be applied to other alloys and to the manufacture of parts of other shapes.

For example, the stamping operation can be replaced by any forming operation by deformation of a dish requiring only medium power, such as, for example, folding with a press.

In addition, it is possible also to produce by stamping, parts of very diverse shapes without significant reduction in thickness and without requiring exceptional power and the invention is therefore not limited to the manufacture of elongated parts, even if these these constitute preferential application.

Thus it was possible to produce a hemisphere from a ingot of highly alloyed martensitic steel, preforged to a thickness of 120 mm and having the composition: 0.01% C; 18% Ni; 8% CO; 5% Mo; 0.4% Ti. This blank was rolled into a 60 mm thick sheet, therefore with a reduction in thickness slightly less than 73% by carrying out part of this reduction of at least 25% at a temperature below 950 ° C. After cutting this sheet into individual blanks, each of these was preheated to 900 ° C, temperature for which the grain size was finer than the AFNOR index 6. Each blank was then stamped so as to produce a hemisphere whose outside diameter was of the order of 1 m and the thickness of 50mm. Each of these hemispheres was then subjected to a heat treatment for dissolving at 820 ° C and aging according to the specifications for use.

It can therefore be seen that, for the production of structural parts subjected to very high stresses, the invention makes it possible to replace the forging of massive semi-finished products by means of a very powerful press by rolling, cutting and stamping or folding. on a medium power press, the fineness of the grain can be perfectly controlled during the rolling operation by limiting the preheating temperature for stamping, which improves the reliability of the parts thus produced.

Claims (11)

1. Process for producing structural components (20) provided to support high strains and having a metallurgical structure with very fine grain size from a blank consisting of a highly alloyed steel, said blank being prepared after reheating to obtain a three dimentional component the thickness of which may be upper than 10 mm and the length of which may be several meters characterized by firstly rolling the blank to obtain a flat product (1) having the desired thickness, then cutting said flat product (1) in at least a blank (2) and making the deformation by drawing or single bending of said blank (2) to obtain a component (20) having the suitable shape, the rolling conditions being regulated so as to give to said product (1) a determinated metallurgical structure and the metal temperature being adjusted during the whole operation and particularly during rolling and reheating preceding drawing such as to obtain finally the very fine grain size desired specific metallurgical structure, the component (20) being subjected after drawing to a final heat treatment comprising a solution treatment under determinated conditions so as not to cause modification to the specific metallurgical structure thus obtained.
2. Process for producing a structural component according to Claim 1, characterized in that the blank consists of a highly alloyed steel in which at least the highest content of addition elements exceeds 5%, or of an austenitic alloy based on Fe-Ni-Cr or Fe-Ni-Cr-Mo, and in that the final heat treatment is a solution treatment of the metal under specific conditions so as not to cause modification of the fineness of the grain obtained by the rolling operation.
3. Process for producing a structural component according to Claim 2, characterized in that the blank consists of a highly alloyed steel of the Maraging type, that is to say of martensitic structure hardened by structural hardening, it being possible for this steel to be a steel based on Fe-Ni-Co-Mo-Ti hardened by the precipitation of phases rich in titanium and/or molybdenum, a steel based on Fe-Cr-Ni-Mo-Al hardened by the precipitation of phases rich in aluminium and/or titanium or a steel based on Fe-Cr-Ni-Cu or Fe-Cr-Mo-Ni-Cu hardened by the precipitation of phases rich in copper.
4. Process for producing a structural component according to one of Claims 1 to 3, characterized in that the rolling conditions are regulated so as to give the rolled product (1), from this stage on, the desired specific metallurgical structure, and in that the temperature of the metal is adjusted throughout all the subsequent operations to a level which is sufficiently low so that the said specific structure is not modified.
5. Process for producing a structural component according to one of Claims 1 to 3, characterized in that the rolling conditions are controlled so as to give the product (1) a determinated metallurgical structure, and in that reheating is carried out before drawing to a temperature which is regulated so as to obtain the desired specific structure, the temperature of the metal being limited throughout all the operations to a level which is at most equal to the temperature of the final heat treatment.
6. Process for producing a structural component according to one of the preceding claims, characterized in that the specific metallurgical structure is a structure with a very fine grain size, preferably below AFNOR Index 6.
7. Process for producing a structural component according to one of the preceding claims, characterized in that the desired metallurgical structure is obtained in particular by control of the temperature during the operations of rolling the flat product from which the blank or blanks to be stamped are then cut out.
8. Process for producing a structural component according to Claim 7, characterized in that, during rolling, a major part of the reduction in thickness is carried out at a relatively low temperature, this reduction and this temperature depending on the composition of the alloy.
9. Process for producing a structural component according to Claim 8, characterized in that, during rolling, at least 25% of the reduction in thickness is carried out at a temperature below 950°C.
10. Process for producing a structural component according to Claim 10, characterized in that the deformation may be performed on a press (3) whose power does not exceed 15000 t.
11. Process for producing a structural component according to one of the preceding claims, characterized in that the deformation operation is a simple press bending operation.

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