WO2016079272A1

WO2016079272A1 - Beam for producing a metal framework

Info

Publication number: WO2016079272A1
Application number: PCT/EP2015/077167
Authority: WO
Inventors: Emmanuel LEROY; Grégory GATARD; Christophe Cazes
Original assignee: Autotech Engineering A.I.E.
Priority date: 2014-11-20
Filing date: 2015-11-19
Publication date: 2016-05-26
Also published as: FR3028830B1; FR3028830A1

Abstract

Vehicle beams (1) are disclosed, the beams comprising a main elongation direction oriented along a central axis (a) and that is formed by a first material such that it comprises a higher strength zone (2) formed by the first material, the beam (1) comprising an enlarged portion (7) on which at least one dimension perpendicular to the axis (a) is increased relative to the rest of the beam. The beams include a lower strength zone (3) formed by a second material and that is located on said enlarged portion (7), the second material having a lower strength against intrinsic mechanical stresses than that of the first material; and the zones being located successively along the axis (a). Methods of manufacturing are also disclosed.

Description

BEAM FOR PRODUCING A METAL FRAMEWORK

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of metal parts involved in the manufacture of a metal framework, in particular a frame or bodywork of vehicles.

STATE OF THE ART

Many metal beams for bodyworks of vehicles have already been made. Particularly the production of such beams, comprising at least one zone of lower mechanical strength than that of the rest of the beam has been proposed. The creation of this zone having a lower strength ("soft zone") which may have a ductility higher than that of the rest of the beam, enables the kinematics of the beam deformation to be predicted when the beam is under mechanical loads, for instance during a collision involving a motor vehicle. Effectively, when the beam is under mechanical loads, the beam firstly becomes deformed at the level of the lower strength zone ("soft zone") while absorbing energy. In this way, by changing the arrangement and shape of these lower strength zones arranged along a beam, it is possible to create several deformation kinematics for the same mechanical loads, which are accompanied by different energy absorptions. It is therefore easy to understand that in this way it is possible to choose a specific kinematic enabling maximum protection to be provided to a vehicle user.

Several solutions that enable this lower strength zone to be created are known.

A first solution is to realise a defect (such as a notch), which serves as a trigger for the deformation of the part. Such a solution is described for example in documents US4702515 and US5431445.

Another already known solution is to locally reduce at least one cross- sectional dimension of a region of the beam (for example the thickness of the section) so that this region is more easily deformed than the rest of the beam. Such a solution is for example described in document US3831997.

It is also possible to combine the principle of the defect or the reduction of a cross-sectional dimension with a heat treatment, enabling the mechanical characteristics of the material to be increased locally at the level of the zones comprising the defects or having a lower dimension, thus compensating the local drop in strength. Such a solution enables the deformation to be controlled since the beam is going to bend at the heat-treated zones as they have a lower ductility Such a solution is described in document US2003/0075951 , which teaches a combination of cross-sectional width reduction in a region of a beam and the heat treatment of that region to harden it and ensure a compressive strength similar to that of the rest of the beam. The region thus treated, enables a sort of fusible to be created since it possesses a lower ductility than the rest of the beam while having the same compressive strength.

Finally, a further known solution is to make a beam from different materials, of which one material has a lower strength against mechanical loads than that of the rest of the materials employed, such that the beam first deforms at the level of the region made of that lower strength material. Such a beam may be made from one single material, on which a heat treatment is carried out, enabling the characteristics of the material in that zone to be reduced, and to additionally modify the strength against mechanical loads. A well-known example is described in document US6820924.

In response to problems relative to reducing fuel consumption in vehicles, to mass compensation in new kinematic chains and equipment and to increase safety requirement regarding passive security, motor vehicles manufacturers promote making structures lighter by using high grade materials, which make it possible to reduce the thickness of the parts.

Nevertheless, the use of high grade materials and the creation of such lower strength zones along a beam combined with the thickness reduction, decreases the energy absorbed by the beam during the deformation thereof. Yet, the energy absorbed by the beam during the deformation thereof is a significant factor with regard to the protection of the vehicle user, for example during a car accident. SUMMARY

The aim of the invention is to propose a new geometry for a beam to control the kinematics of the deformation of said beam when it is subjected to mechanical loads and to obtain a good compromise between weight reduction of the beam, strength of the beam against mechanical loads, and energy absorption during the deformation of said beam. The aforementioned aim is attained, according to one aspect of the invention, thanks to a vehicle beam comprising a main elongation direction oriented along a central axis and that is formed by a first material such that it comprises a higher strength zone (or "higher strength region") formed by the first material, the beam comprising an enlarged portion for which at least one dimension perpendicular to the axis is increased relative to the rest of the beam, characterised in that said beam comprises:

- a lower strength zone (or "lower strength region", or "soft zone") formed by a second material and

which is located on said enlarged portion, the second material having a lower intrinsic strength than that of the first material;

the zones (or "regions") being located successively along the axis.

Herein strength is to be understood as strength or resistance against mechanical loads or under mechanical stresses. Strength is used herein as a material property referring both to "ultimate tensile strength" and "yield strength".

"Enlarged" portion is herein to be understood as a portion having an increased dimension in at least one direction with respect to another portion of the beam. Herein "spatially enlarged" and "enlarged" can be used interchangeably. The increased dimension may be one or more of the following: increased thickness, increased width, and increased height. As will be explained with reference to some of the examples, the enlargement or increase may be local or substantially around an entire perimeter of a cross-section.

According to an additional feature, the enlarged portion may extend along the entire perimeter or circumference of the beam.

According to an additional feature, the enlarged portion may be a homothety of the higher strength zone. This means that the enlarged portion is obtained by a uniform scaling of both width and height or by a uniform scaling of the thickness.

According to another feature, the enlarged portion may extend along a fraction of the beam perimeter or circumference.

According to an additional feature, the enlarged portion may have a cross-section perpendicular to the axis which is larger than the cross-section perpendicular to the higher strength zone. According to an additional feature, the enlarged portion may have a greater material thickness than that of the highest strength zone.

According to another feature, the lower strength zone may be centred (centrally located) on the enlarged portion.

According to another aspect, the invention relates to a manufacturing process for a vehicle beam comprising a main elongation direction directed along a central axis, characterised in that it comprises:

- an operation to supply at least one intermediate beam from which the beam is going to be manufactured.

- an operation to create an enlarged portion on a specific portion of the intermediate beam;

- an operation to create a lower strength zone on a specific zone of the intermediate beam;

the operation to create an enlarged portion and the operation to create a lower strength zone being carried out such that the lower strength zone is located on the enlarged portion.

According to an additional feature, the operation to create an enlarged portion may be carried out by stamping.

According to an additional feature, the operation to create an enlarged portion may be carried out by adding a patch, a reinforcement, or by lamination.

According to another feature, the operation to create a lower strength zone may be carried out by heating and then hardening or "quenching" the intermediate beam, and by controlling the cooling of the specific zone of the intermediate beam such that it forms the lower strength zone.

BRIEF DESCRIPTION OF THE FIGURES

Other features, aims and advantages of the invention will become apparent upon reading the following detailed description, with reference to the attached drawings, provided by way of non-limiting examples, wherein:

- figure 1 a represents a perspective view of a beam according to a first embodiment;

- figure 1 b represents a side view along the longitudinal axis of the first embodiment illustrated on figure 1 a;

- figure 1 c represents a cross-sectional view of the first embodiment in plane A-A, and a cross-sectional view of the first embodiment in plane B-B; both planes being perpendicular to the longitudinal axis of the beam;

- figure 2a represents a perspective view of a beam according to a second embodiment;

- figure 2b represents a side view of the beam according to the second embodiment illustrated in figure 2a;

- figure 3a represents a perspective view of a beam according to a third embodiment;

- figure 3b represents a cross-sectional view of the beam according to a third embodiment along a plane D;

- figure 3c represents a cross-sectional view of the third embodiment along in plane A'-A', and the cross-sectional view of the third embodiment in plane B'-B';

- figure 4 represents a cross-sectional view of a beam according to a fourth embodiment;

- figure 5a represents a perspective view of a beam according to a fifth embodiment;

- figure 5b represents a cross-sectional view along a plane D' of the fifth embodiment illustrated in figure 5a;

- figure 5c represents a cross-sectional view of the fifth embodiment along an axis E-E, and a sectional view of the fifth embodiment along an axis F-F;

- figure 6a represents a cross-sectional view of a beam according to a sixth embodiment;

- figure 6b represents a cross-sectional view along an axis G-G of the sixth embodiment illustrated in figure 6a;

- figure 7 represents a perspective view of a beam according to a seventh embodiment;

- figure 8 represents a perspective view of a beam according to an eighth embodiment;

- figure 9 represents a cross-sectional view of a beam according to a ninth embodiment;

- figure 10a represents a perspective view of a beam according to a tenth embodiment;

- figure 10b represents a cross-sectional view of the beam according to the tenth embodiment illustrated in figure 10a;

- figure 10c represents a cross-sectional view of the tenth embodiment along an axis H-H, and a sectional view of said tenth embodiment along an axis - figure 10d represents a deformation of the beam according to the tenth embodiment when it is subjected to mechanical stress;

- figure 1 1 represents a perspective view of a beam according to an eleventh embodiment.

DETAILED DESCRIPTION OF EXAMPLES OF EMBODIMENTS OF THE INVENTION

As represented in figures 1 to 1 1 , a beam 1 according to examples of the invention comprises a main elongation direction oriented along an axis a. The axis a is the central axis of the beam 1 . Therefore, the beam 1 comprises a length oriented along a direction X, the axis a having the same direction.

In the first embodiment, as illustrated in figures 1 a and 1 b, the beam 1 has a cross-section with a square profile, and includes a width oriented along a direction Y and a height oriented along a direction Z. Nevertheless, the invention is not limited to beams having a square section and other sectional profiles may be used, such as for example a circular profile. In addition, the invention may be applied to beams 1 made up in one single part, or beams 1 having multiple parts, said beams 1 may include flanges and assembly interfaces.

The term cross-section as used herein, means a cross-section along a plane perpendicular to the axis a.

In all the illustrated embodiments, the cross-section of the beams is closed along substantially their entire length.

First embodiment:

The beam 1 is formed in a first material and comprises a first higher strength zone 2, which is formed in the first material.

The beam 1 also comprises an enlarged portion 7 which, in the first embodiment, is located on an end of the beam 1 . The enlarged portion 7 extends along a fraction of the beam's length, and in the first embodiment, the enlarged portion 7 extends along the entire (local) perimeter or circumference of the beam 1 .

The beam 1 also comprises a lower strength zone ("soft zone") 3 formed by a second material and that is located on the enlarged portion 7. The intrinsic strength of the second material is lower than those of the first material, which forms the higher strength zone 2. In some embodiments, the lower strength zone 3 may be centred on the enlarged portion 7. According to the first variant shown here, the lower strength zone 3 is off-centred ("eccentric") on the enlarged portion 7, such that said lower strength zone is located closer to one end of the enlarged portion 7 than the other.

In addition, the beam 1 comprises a geometric transition zone 4 located on the enlarged portion 7, between the lower strength zone ("soft zone") 3 and the higher strength zone 2. The geometric transition zone 4 is formed in the first material. In this way, the higher strength zone 2 and the geometric transition zone 4 are formed in the same material.

The beam 1 further comprises a metallurgical transition zone 5 which is located on said enlarged portion 7, between the lower strength zone 3 and the geometric transition zone 4. The metallurgical transition zone 5 is such that it comprises a first end formed by the first material, and a second end formed by the second material.

Finally, the beam 1 comprises an intermediate zone 6 which is located on said enlarged portion 7, between the geometric transition zone 4 and the metallurgical transition zone 5. The intermediate zone 6 is formed by the first material.

The fact that the second material has lower intrinsic strength than the first material, which for two elements having the same dimensions, of which a first element is formed in the first material and a second element is formed in the second material, means that the first element has a higher strength than the second element, for example the energy needed for flexing (or "bending") the first element is higher than the energy needed to flex (or bend) the second element.

As shown in figure 1 c, at least one dimension perpendicular to the axis a is enlarged on the enlarged portion 7 relative to the higher strength zone 2. More generally for all examples, at least one dimension perpendicular to the axis a is enlarged on the enlarged portion 7 relative to the rest of the beam 1 .

The lower strength zone ("soft zone") 3 is located on the enlarged portion 7, and comprises at least one dimension perpendicular to the axis a, which is larger than the corresponding dimension of the higher strength zone 2. In the first embodiment, the enlarged portion 7 has a cross-section that is larger than the cross-section of the higher strength zone 2, or, according to a similar definition, the enlarged portion 7 has a longer length and a higher height than the length and the height of the higher strength zone 2.

In the first embodiment, the cross-section of the lower strength zone 3 is a homothety of the higher strength zone 2.

In the first embodiment, the beam has a constant material thickness along the entire length thereof, and therefore the enlarged portion 7 has a material thickness equal to that of the higher strength zone 2, as may be seen in figure 3.

The section of the metallurgical transition zone 5 and the intermediate zone 6 are equal to the cross-section of the lower strength zone 3.

The geometric transition zone 4 is a zone whose cross-section increases along the direction X. More specifically, at one end of the geometric transition zone the cross-section is equal to the section of the higher strength zone 2, and at the other end of the geometric transition zone the section is equal to the sections of the intermediate zone 6 and the lower strength zone 3. In the first embodiment, the geometric transition zone 5 increases linearly along the direction X, however, according to a variant, the geometric transition zone may be a curve comprising an inflexion point at the centre thereof.

The enlargement of at least one dimension perpendicular to the axis a of the lower strength zone ("soft zone") 3 enables the strength of the lower strength zone ("soft zone") 3 to be increased, so that there is maximum energy absorption when the lower strength zone ("soft zone") 3 becomes deformed, for example during a car accident. Such a solution makes it possible to obtain a good compromise between strength of the beam 1 , mass of the beam 1 , and energy absorbed by the beam 1 during the deformation thereof.

More specifically, the enlargement of at least one dimension perpendicular to the axis a on a zone of the beam 1 , so as to create an enlarged portion 7, makes it possible to increase the moment of inertia on that zone of the beam 1 . The creation of the lower strength zone ("soft zone") 3 on the beam decreases the plastic flexural modulus of the beam 1 on that lower strength zone ("soft zone") 3. By making the beam 1 such that the lower strength zone 3 is located on the enlarged portion 7, the moment of inertia of the lower strength zone 3 is increased, said zone having a low plastic flexural modulus.

In the first embodiment, the cross-sections of the lower strength zone (soft zone) 3 and higher strength zone 2 are centred on the axis a. This centering makes it possible to increase the strength of the lower strength zone ("soft zone") 3 against mechanical loads applied along any direction.

The cross-sectional enlargement of the enlarged portion 7 may be carried out by stamping.

In the first embodiment, the beam 1 is made of steel. Advantageously, the first material forming the higher strength zone 2 is substantially a martensitic steel, for example, 22MnB5 or USIBOR 1500 ® steel, and the second material forming the lower strength zone ("soft zone") 3 is a steel that may contain one or several phases: ferrite and/or bainite. The second material may also contain a limited amount of martensite.

The metallurgical transition zone 5 is thus a zone formed by a steel whose phase varies between the martensite forming the higher strength zone 2 and the steel grade (ferrite and/or bainite) forming the lower strength zone 3.

An example of steel used in the automotive industry is 22MnB5 steel. In order to avoid the decarburization and the scale formation during the forming process, 22MnB5 may be presented with an aluminum-silicon coating. Usibor ® 1500P may be supplied in ferritic-perlitic phase. It is a fine grain structure distributed in a homogenous pattern. The mechanical properties are related to this structure. After heating, a hot stamping process, and subsequent quenching, a martensite microstructure may be created. As a result, maximal strength and yield strength increase noticeably. The composition of Usibor 1500 ® is summarized below in weight percentages (rest is iron (Fe) and impurities):

If the same material is subjected to a different temperature treatment, a microstructure may be obtained that leads to a lower ultimate tensile strength and to higher ductility. The microstructure may include ferrite and/or bainite, rather than martensite.

The manufacturing process of a beam 1 according to the first embodiment comprises:

- An operation to supply at least one intermediate beam (or "blank", which may be a semi-finished product) from which the beam 1 is going to be manufactured.

- An operation to create the lower strength zone ("soft zone") 3 on a portion of the intermediate beam.

- An operation to create the enlarged portion 7 on a portion of the intermediate beam. The operation to create the enlarged portion 7 and the operation to create the lower strength zone ("soft zone") 3 are adapted such that the lower strength zone ("soft zone") is located on the enlarged portion 7.

The operation to create the lower strength zone ("soft zone") 3 may be carried out prior, after, or simultaneously to the operation to create the enlarged portion 7. The operation to create the lower strength zone ("soft zone") 3 may be carried out for example, on an intermediate beam having a shape identical to the shape of the beam 1 , finished and ready to be assembled to form for example, the bodywork of a vehicle.

The operation to create the lower strength zone ("soft zone") 3 may be carried out using an intermediate beam formed in the first material, and carrying out a local heat treatment (an annealing process, for example) on a zone of the intermediate beam (by heating for example with a laser and then allowing to air- cool at room temperature), so as to transform the first material (martensite, for example) into the second material (ferrite, for example). According to another variant, the operation to create the lower strength zone 3 may be carried out using an intermediate beam formed in the second material, and carrying out a local heat treatment (a hardening process, for example), so as to transform the second material (bainite, for example) into the first material (martensite, for example).

The use of a heat treatment to create the lower strength zone ("soft zone") 3 entails the creation of the metallurgical transition zone 5.

The intermediate zone 6 is created by the fact that the zone affected by the heat treatment does not extend along the entire enlarged portion 7. According to a variant, the beam 1 does not comprise an intermediate zone 6, in such a way that the geometric transition zones 4 and metallurgical transition zones 5 are united.

The operation to create the enlarged portion 7 may be carried out by stamping a portion of the intermediate beam using a stamping die. The use of a stamping process entails the creation of the geometric transition zone 4.

According to a preferred manufacturing process, which on the one hand makes it possible to obtain a beam 1 able to absorb a large amount of energy upon deformation, and on the second hand limit the manufacturing stages, the beam 1 is obtained by means of a hot stamping process in which the operation to create the lower strength zone is carried out into the stamping die. More specifically, the manufacturing process is as follows:

- an intermediate beam (a "blank" or a semi-finished product) made of steel is heated in an oven at a temperature above the austenisation temperature thereof;

- the intermediate beam or "blank" is placed on the stamping die and is hot stamped, such that the enlarged portion 7 is formed;

- the intermediate beam ("blank") is cooled inside the stamping die so as to carry out the hardening except for a zone of the beam located on the enlarged portion 7 for which the stamping die is heated in order to slow down the cooling of the intermediate beam and thus preventing the hardening phenomenon from happening.

In this way, a beam 1 is obtained at the exit of the stamping die, said beam comprising a lower strength zone 3 located on the enlarged portion 7.

The cross-section of the lower strength zone 3 should preferably not be enlarged too much so that the strength of said lower strength zone 3 remains below the strength of the higher strength zone 2. The difference in cross-section between the higher strength zone 2 and the lower strength zone 3 may depend on the difference in intrinsic strength of the first material with respect to the second material, and therefore depends on the base material used to manufacture the beam 1 , as well as onthe manufacturing conditions (heat treatment, for example).

Advantageously, for a first material made of USIBOR 1500 ® in its martensitic phase, and a second material made of Soft Zone HT400 ®, the cross-section of the lower strength zone 3 is β times larger than the cross- section of the higher strength zones 2, β being comprised between 1 and 2, and preferentially β is equal to 1 .6.

HT400 is a commercial name for the "soft zone" obtainable using the same or very similar basic steel composition as Usibor, namely 22MnB5 steel. HT400 herein refers to a material having an ultimate tensile strength of between 550 - 650 Mpa. The yield strength is in the range of 350 - 450 Mpa (hence the name HT400).

Second embodiment:

According to a second embodiment shown in figures 2a and 2b, the enlarged portion 7 is not provided on one end of the beam 1 , but on a central portion.

The beam 1 comprises two different higher strength zones 2 which are both formed in the first material. The two higher strength zones 2 are located on both sides of the enlarged portion 7.

Similarly to the first embodiment, the beam 1 comprises a lower strength zone 3 which is located on the enlarged portion 7.

The beam 1 comprises two geometric transition zones 4 which are each located on one end of the enlarged portion 7, on both sides of the lower strength zone ("soft zone") 3, between one higher strength zone 2 and the lower strength zone 3.

The beam 1 also comprises two metallurgical transition zones 5 which are located on the enlarged portion 7, on both sides of the lower strength zone 3, between one geometric transition zone 4 and the lower strength zone 3.

The beam 1 also comprises two intermediate zones 6 which are located on the enlarged portion 7, each intermediate zone 6 being located between one geometric transition zone 4 and one metallurgical transition zone 5.

The manufacturing process of the beam 1 according to the second embodiment may be similar to the manufacturing process of the beam according to the first embodiment. The main difference between these two processes lies in where the operation to create the enlarged zone 7 and operation to create the lower strength zone ("soft zone") 3 on the intermediate beam are carried out. Effectively, they may be carried out on the central portion of the beam 1 , and not on one end of the beam 1 , in contrast with the first embodiment. Third embodiment

According to a third embodiment, which is represented in figures 3a, 3b and 3c, the enlarged portion 7 is obtained by increasing the material thickness on a zone of the beam 1 .

Such a solution may be obtained using a beam of variable thickness comprising an over-thickness (or increased thickness) at the level of the selected zone. According to a variant, it is also possible to increase the material thickness on the enlarged portion 7 using patches, or reinforcements, or even using different assembled blanks.

As illustrated in figures 3a, 3b and 3c, similarly to the second embodiment, the enlarged portion 7 is not arranged on one end of the beam 1 , but on a central portion of said beam 1 . In this way, the arrangement of the higher strength zones 2, lower strength zone 3, geometric transition zones 4, metallurgical transition zones 5, and intermediate zones 6 along the axis a is identical to that of the second embodiment.

Similarly to the first embodiment, the enlarged portion 7 is a homothety of the higher strength zones 2, as may be seen in figure 3c. In addition, the material thickness of the metallurgical transition zone 5 is identical to the thickness of the lower strength zone 3.

In the third embodiment, the geometric transition zones 4 are zones of the beam 1 having an increasing material thickness along the direction X (or decreasing according to the position of the geometric transition zone 4 on the beam 1 ). More specifically, one end of the geometric transition zones 4 has a material thickness equal to the material thickness of the higher strength zone 2, and the other end of the geometric transition zones 4 has a thickness equal to the thickness of the intermediate zones 6, metallurgical transition zones 5 and lower strength zone 3.

In the third embodiment, the increase in thickness of the beam 1 is oriented towards the inside of said beam 1 , such that the enlarged portion 7 projects inside a cavity formed by the beam 1 . Nevertheless, such an arrangement is just a possible example. Effectively, according to another variant, the increase in thickness may be oriented towards the outside of the beam 1 , such that the enlarged portion 7 projects outside the beam 1 . In this way, it should be understood that the enlarged portion 7 may project either inside the cavity formed by the beam 1 , or towards the outside of the beam 1 .

The beam 1 according to the third embodiment may be manufactured by assembling blanks, each comprising a thicker portion of material obtained by lamination, or said beam 1 may be obtained by successively folding a piece of sheet metal, which comprises a thicker portion also obtained by lamination.

The operation to create a lower strength zone 3 may be carried out after the lamination process, on the thicker portion that forms the enlarged portion. In the case where the beam 1 is obtained by assembling several blanks, the operation to create a lower strength zone 3 may be carried out either prior to or after assembling said blanks. In the case where the beam 1 is obtained by folding a single sheet metal, the operation to create a lower strength zone 3 may be carried out either prior to or after folding said sheet metal.

The manufacturing process of a beam according to the third embodiment comprises:

- An operation to supply at least one intermediate beam from which the beam 1 is going to be manufactured.

- An operation to create the enlarged portion 7 on a portion of the intermediate beam by lamination so as to locally increase the material thickness of the intermediate beam.

- An operation to create the lower strength zone (soft zone) 3 on the enlarged portion 7.

As indicated above for the first embodiment, the operation to create the lower strength zone 3 may be carried out by performing a heating process followed by a hardening process, in order to create the higher strength zones 2, and thereafter by locally performing an annealing process in order to create the lower strength zone 3. According to a possible variant, similarly to the first embodiment, the lower strength zone 3 may be obtained by controlling the cooling of one zone of the intermediate beam during the hardening process. Fourth embodiment

According to a fourth embodiment, as featured in figure 4, the beam 1 comprises an enlarged portion 7 which is obtained by the application of a patch. The patch is welded along the entire perimeter of the beam 1 , in order to form a material "over-thickness" (or "increased thickness") oriented towards the inside of the beam 1 . Similarly to the third embodiment, the enlarged portion 7 is located on a central zone of the beam 1 , and said beam 1 comprises a lower strength zone 3, which is located on the enlarged zone; two higher strength zones 2, which are located on both sides of the enlarged portion 7; two metallurgical transition zones 5, which are located on the enlarged portion 7 on both sides of the lower strength zone 3; as well as two intermediate zones 6 located on the enlarged portion 7 such that each of the intermediate zones 6 are surrounded by one higher strength zone 2 and by one metallurgical transition zone 5.

Nevertheless, in contrast with the third embodiment, the beam 1 does not comprise geometric transition zones 4, the beam 1 thickness varies in a discontinuous manner between the higher strength zones 2 and the enlarged portion 7. According to a possible variant, a beam 1 whose enlarged portion 7 is obtained by assembling a patch with bevelled ends, may comprise geometric transition zones 4.

The term patch as used herein, means a piece of sheet metal which is assembled to the beam 1 , in order to locally increase the material thickness of the beam 1 , said piece of sheet metal being assembled prior to the operation to create a lower strength zone 3.

The manufacturing process of the beam 1 , according to the fourth embodiment comprises:

- an operation to assemble a patch on an intermediate beam (or "semifinished" or "blank"), such that the extending zone 7 is formed, the patch being made of the same material as the intermediate beam;

- an operation to create the lower strength zone ("soft zone") 3 by performing on the one hand, a heating process followed by a hardening process of the intermediate beam in order to create the higher strength zones 2, and on the other hand a local heat treatment on the enlarged zone 7 in order to create the lower strength zone 3, metallurgical transition zones 5, and intermediate zones 6.

The local heat treatment which enables the lower strength zone 3 to be created is either an annealing process, or a limitation of the cooling rate in a zone of the enlarged portion 7 during the hardening of the intermediate beam, or a limitation of the maximum heating temperature in a zone of the enlarged portion 7 during the heating of the intermediate beam and prior to the hardening process. Fifth embodiment

According to a fifth embodiment illustrated in figures 5a, 5b and 5c, the enlarged portion 7 extends only over a fraction of the beam 1 perimeter, such that said enlarged portion 7 does not extend over the entire perimeter of the beam 1 .

In the fifth embodiment, the enlarged portion 7 extends only over half of the perimeter of the cross-section of the beam 1 . Nevertheless, according to other variants, the enlarged portion 7 may extend over different fractions.

Similarly to the second and third embodiments, the enlarged portion 7 is realised on a central portion of the beam 1 , and not on one end of said beam 1 . Nevertheless, according to a variant, the enlarged portion 7 may be provided on one end of the beam 1 .

The beam 1 comprises:

- A first and a second higher strength zones 21 and 22, which are located on both sides of the enlarged portion 7.

- A third higher strength zone 23, which is located facing the enlarged portion 7, and between the first and the second higher strength zones 21 and 22, in order to join them together.

- A lower strength zone 3, which is located on the enlarged portion 7.

- Two geometric transition zones 4 which are each located on one end of the enlarged portion 7, between the higher strength zones 21 and 22, and the lower strength zone 3.

- Two metallurgical transition zones 5, which are located on the enlarged portion 7, on both sides of the lower strength zone 3, between the geometric transition zones 4 and the lower strength zone 3.

- Two intermediate zones 6, which are located on the enlarged portion 7, each of the two intermediate zones 6 being located between one geometric transition zone 4 and one metallurgical transition zone 5.

The fact that the enlarged portion 7 extends only over a fraction of the beam 1 perimeter, and therefore the lower strength zone 3 extends only over a fraction of the beam 1 perimeter, makes it possible to influence the beam 1 strength against mechanical stresses, and to provide it with a particular deformation kinematics when it is subjected to a load by creating a preferred deformation direction. It is therefore possible to create several enlarged portions 7 along the beam 1 , a lower strength zone 3 being realised on each of said enlarged portions 7, and thus providing the beam 1 with the desired deformation kinematics according to the distribution of the lower strength zones 3. Sixth embodiment

According to a sixth embodiment illustrated in figures 6a and 6b, the enlarged portion 7 is provided by increasing the material thickness along a fraction of the beam perimeter instead of on the entire beam perimeter. This increase in the material thickness is obtained by adding a patch. According to a variant, this increase in thickness may be obtained by assembling several blanks, of which only certain blanks comprise a local over-thickness obtained by lamination.

Just as in the fourth embodiment, this variant enables the beam 1 to be provided with a preferred deformation direction.

Seventh embodiment

According to a seventh embodiment illustrated in figure 7, the beam 1 comprises several enlarged portions 71 and 72. Each enlarged portion 71 and 72 covers only a fraction of the beam 1 perimeter, and are located successively on the beam 1 along the axis a and spaced apart by a higher strength zone 24.

The enlarged portion 71 is located on a central portion of the beam 1 , such that is located between a higher strength zone 21 and the higher strength zone 24 along the axis a. The enlarged portion 71 extends only along a portion of the beam perimeter, said beam 1 comprises a higher strength zone 25 which is located facing the enlarged portion 71 , and which is located between the higher strength zones 21 and 24, in order to join them together.

The beam 1 comprises:

- A lower strength zone 31 , which is located on the enlarged portion 71 .

- Two geometric transition zones 41 , which are each located on one end of the enlarged portion 71 , each of the zones 41 being located between the lower strength zone 31 and either the higher strength zone 21 , or the higher strength zone 24. In figure 7, only one zone 41 has been represented.

- Two metallurgical transition zones 51 , which are located on both sides of the lower strength zone 31 . Each metallurgical transition zone 51 is located between one geometric transition zone 41 and the lower strength zone 31 .

- Two intermediate zones 61 , which are located on the enlarged portion

71 , each of the intermediate zones 61 being located between one geometric transition zone 41 and one metallurgical transition zone 51 .

The enlarged portion 72 is located on a central portion of the beam 1 , such that is located between the higher strength zone 24 and a higher strength zone 22 along the axis a. The enlarged portion 72 extends only on a portion of the beam 1 perimeter, said beam 1 comprises a higher strength zone 26 which is located facing the enlarged portion 72, and which is located between the higher strength zones 22 and 24, in order to join them together.

The beam 1 further comprises:

- A lower strength zone 32 which is located on the enlarged portion 72.

- Two geometric transition zones 42, which are each located on one end of the enlarged portion 72, each of the zones 42 being located between the lower strength zone 32 and either the higher strength zone 22, or the higher strength zone 24. In figure 7, only one zone 42 has been represented.

- Two metallurgical transition zones 52, which are located on both sides of the lower strength zone 32. Each metallurgical transition zone 52 is located between one geometric transition zone 42 and the lower strength zone 32.

- Two intermediate zones 62, which are located on the enlarged portion

72, each of the intermediate zones 62 being located between one geometric transition zone 42 and one metallurgical transition zone 52.

Such an embodiment enables the flexural deformation kinematics of the beam 1 to be controlled, by forcing said beam 1 to bend first at the lower strength zones 31 and 32, and thus adopt a Z-shape upon deformation.

According to a possible variant, at least one enlarged portion 71 , 72 may be realised on one end of the beam 1 .

Eighth embodiment

According to an eighth embodiment illustrated in figure 8, the beam 1 is made up of two blanks which have been assembled. Each of the blanks is hat- shaped and comprises two flanges 8. The two blanks are assembled through the flanges thereof, by welding, for example, the two blanks having their concavity facing each other.

The beam 1 , similarly to the first embodiment, comprises an enlarged portion 7 on one end of said beam 1 on which the section of the beam 1 is enlarged. In this way, the beam 1 comprises one lower strength zone ("soft zone") 3, one metallurgical transition zone 5, one intermediate zone 6, one metallurgical transition zone 4 and one higher strength zone 2.

Ninth embodiment

According to a ninth embodiment illustrated in figure 9, the enlarged portion 7 is obtained by means of a local increase in the material thickness on a central portion of the beam 1 , which is carried out by adding a patch and a reinforcement. The increase in thickness of the enlarged portion 7 is formed on the first half of the perimeter thereof by the patch and on the second half of the perimeter thereof by the reinforcement. The patch is arranged on the inside of the beam 1 , whereas the reinforcement is arranged on the outside of the beam 1 , however, according to another variant the patch may be arranged on the outside of the beam 1 and the reinforcement on the inside of said beam 1 .

The term reinforcement as used herein, means a piece of sheet metal assembled onto the beam 1 after the operation to create the lower strength zone 3. The reinforcement may be integrally made up of the second material, the same material as the lower strength zone 3.

The beam 1 comprises a lower strength zone ("soft zone") 3, which is located on the enlarged zone 7; two higher strength zones 2, which surround the enlarged portion 7; two metallurgical transition zones 5, which surround the lower strength zone 3; and two intermediate zones 6, which surround the metallurgical transition zones 5.

The manufacturing process of the beam 1 , according to the ninth embodiment comprises:

- An operation to supply at least one intermediate beam ("blank" or "semifinished" beam) from which the beam 1 is going to be manufactured.

- An operation to add a patch on a zone of the intermediate beam.

- An operation to create the lower strength zone 3 by performing a heating process followed by a hardening process on the intermediate beam, and controlling the cooling of the zone of the intermediate beam. - An operation to add a reinforcement on the zone of the intermediate beam, the reinforcement being made up of the second material.

Tenth embodiment

According to a tenth embodiment illustrated in figures 10a, 10b, 10c and

10d, the beam 1 comprises two enlarged portions 73 and 74, the enlarged portion 73 being located on a central portion of the beam 1 , and the enlarged portion 74 being located on one end of the beam 1 . The enlarged portion 73 extends along half of the beam 1 perimeter, and the enlarged portion 74 extends along the entire beam 1 perimeter. The enlarged portions 73 and 74 are made by means of a local enlargement of the cross-section of the beam 1 .

The beam 1 comprises a higher strength zone 27 which is located on the other end of the beam 1 , and a higher strength zone 28 which is surrounded by the enlarged portions 73 and 74, and one higher strength zone 29 located between the higher strength zones 27 and 28, perpendicular to the enlarged portion 73. The higher strength zones 27, 28 and 29 are made from the first material.

The beam 1 is made by assembling two hat-shaped blanks, each comprising two flanges 8, said blanks being attached through their flanges 8.

The beam 1 comprises one lower strength zone 33 located on the enlarged portion 73, and one lower strength zone 34 located on the enlarged portion 74. The two lower strength zones 33 and 34 are made up in the second material.

The beam 1 comprises two geometric transition zones 43 which are located on the enlarged portion 73, such that the lower strength zone ("soft zone") 33 is surrounded. The geometric transition zones 43 are made up in the first material.

The beam 1 further comprises two metallurgical transition zones 53 which are located on the enlarged portion 73, each of the metallurgical transition zones 53 being located between the lower strength zone 33 and one geometric transition zone 43. The metallurgical transition zones 53 are such that each comprises a first end made up of the first material, and a second end formed by the second material.

The beam 1 also comprises two intermediate zones 63 which are located on said enlarged portion 7, each of the intermediate zones 63 being located between one geometric transition zone 43 and one metallurgical transition zone 53. The intermediate zones 6 are formed by the first material.

In addition, the beam 1 comprises a geometric transition zone 44 located on the enlarged portion 74, between the higher strength zone 28 and the lower strength zone 34. The geometric transition zone 44 is formed in the first material.

The beam 1 comprises one metallurgical transition zone 54 which is located on an enlarged portion 74, between the lower strength zone 34 and the geometric transition zone 44. The metallurgical transition zone 54 is such that it comprises a first end formed by the first material, and a second end formed by the second material.

Finally, the beam 1 comprises an intermediate zone 64 which is located on said enlarged portion 7, between the geometric transition zone 44 and the metallurgical transition zone 54. The intermediate zone 64 is formed by the first material.

As represented in figure 10d, such an embodiment makes it possible to obtain flexural or "bending" deformation kinematics, said bending being created by the application of two forces F along the same direction but in opposite directions on two different sites of the Z-shaped beam 1 , which one segment is very small in size when compared to the rest of the segments making up the Z.

Eleventh embodiment

As illustrated in figure 1 1 , the lower strength zone 3 may only extend on a portion of the perimeter of the enlarged portion 7.

The intermediate beam described in the different embodiments described before may be either a piece of sheet metal or a blank.

The different embodiments presented above are just illustrative embodiments of the invention, and of course it is possible to combine them in order to obtain a beam possessing desirable deformation kinematics, strength against mechanical loads as well as energy absorption during deformation.

The beams as disclosed herein may in particular be used as front (side) rails or rear (side) rails in a vehicle bodywork.

Claims

1 . A vehicle beam (1 ) extending mainly along a direction oriented along a central axis (a) and that is formed by a first material such that it comprises a higher strength zone (2, 21 , 22, 23, 24, 25, 26, 27, 28, 29) formed by the first material, the beam (1 ) comprising an enlarged portion (7, 71 , 72, 73, 74) on which at least one dimension perpendicular to the axis (a) is increased relative to the rest of the beam (1 ), characterised in that said beam (1 ) comprises:

- a lower strength zone (3, 31 , 32, 33, 34) formed by a second material and which is located on said enlarged portion (7, 71 , 72, 73, 74), the second material having a lower intrinsic strength against mechanical stresses than that of the first material;

said zones being located successively along the axis (a).

2. The vehicle beam according to claim 1 , characterised in that the enlarged portion (7, 74) extends along the entire perimeter of the beam (1 ).

3. The vehicle beam according to claim 2, characterised in that the enlarged portion (7, 74) is a homothety of the higher strength zone (2, 21 , 22, 23, 24, 25, 26, 27, 28, 29).

4. The vehicle beam according to claim 1 , characterised in that the enlarged portion (7, 71 , 72, 73) extends along a fraction of the beam perimeter (1 ).

5. The vehicle beam according to any of the preceding claims, characterised in that the enlarged portion (7, 71 , 72, 73, 74) has a cross-section perpendicular to the axis (a) which is larger than the cross-section perpendicular to the axis (a) of the higher strength zone (2, 21 , 22, 23, 24, 25, 26, 27, 28, 29).

6. Vehicle beam according to any of the preceding claims characterised in that the enlarged portion (7) has a greater material thickness than that of the highest strength zone (2, 21 , 22, 23, 24, 25, 26, 27, 28, 29).

7. The vehicle beam according to any of the preceding claims, characterised in that the lower strength zone (3, 31 , 32, 33, 34) is centred on the enlarged portion (7, 71 , 72, 73, 74).

8. A manufacturing process for a vehicle beam (1 ) extending mainly along a direction of a central axis (a), characterised in that it comprises:

- an operation to supply at least one intermediate beam from which the beam (1 ) is going to be manufactured.

- an operation to create an enlarged portion (7, 71 , 72, 73, 74) on a specific portion of the intermediate beam;

- an operation to create a lower strength zone (3, 31 , 32, 33, 34) on a specific zone of the intermediate beam;

the operation to create an enlarged portion (7, 71 , 72, 73, 74) and the operation to create a lower strength zone (3, 31 , 32, 33, 34) are done such that the lower strength zone (3, 31 , 32, 33, 34) is located on the enlarged portion (7, 71 , 72, 73, 74).

9. The manufacturing process according to claim 8, characterised in that the operation to create an enlarged portion (7, 71 , 72, 73, 74) is carried out by stamping.

10. The manufacturing process according to claim 8, characterised in that the operation to create an enlarged portion (7, 71 , 72, 73, 74) is carried out either by adding a patch.

1 1 . The manufacturing process according to claim 8, characterised in that the operation to create an enlarged portion (7, 71 , 72, 73, 74) is carried out by adding a reinforcement.

12. The manufacturing process according to claim 8, wherein the operation to create an enlarged portion comprises using a taylor rolled blank.

13. The manufacturing process according to any of the claims 9 - 12, characterised in that the operation to create a lower strength zone (3, 31 , 32, 33, 34) is carried out by heating and then hardening the intermediate beam, and by controlling the cooling of the specific zone of the intermediate beam such that it forms the lower strength zone (3, 31 , 32, 33, 34).