The invention relates to a vehicle underbody structure, comprising a floor panel and an underbody reinforcement element, said underbody reinforcement element comprising at least a front transversal beam and a rear transversal beam, said transversal beams extending parallel to each other and being attached to the floor panel, characterized in that the front transversal beam and the rear transversal beam are made of a press hardening steel having a tensile strength higher than or equal to 1300 MPa.

Conventionally, vehicles comprise reinforcement elements destined to protect the vehicle occupants in case of an impact, in particular a side impact, a front impact or a roof crush during a rollover accident, by limiting intrusions of any kind in the vehicle passenger compartment.

The reinforcement elements comprise, on each side of the vehicle, a body side structure forming a door frame, which is conventionally made of steel, and an underbody reinforcement element forming with a floor panel the vehicle underbody structure.

Reducing the overall weight of the vehicle is very desirable in order to reduce the energy consumption, so as to meet the future environmental requirements. It is thus desirable to reduce the thickness of the parts forming the vehicle, in particular the reinforcement elements, including the underbody reinforcement element.

However, simply reducing the thickness of the reinforcement elements generally leads to a degraded protection of the vehicle occupants against intrusions.

Steels having better mechanical properties such as yield strength or tensile strength may be used. Indeed, such steels provide an enhanced protection that lower grade steels for a given thickness. However, such steels generally have a lower formability.

The object of the invention is to solve the above-mentioned problems, and in particular to provide a vehicle underbody structure comprising an underbody reinforcement element having reduced thickness and weights together with a satisfactory protection to the vehicle occupants.

To that end, the invention relates to a vehicle underbody structure of the aforementioned type, characterized in that the front transversal beam and the rear transversal beam are made of a press hardening steel having a tensile strength higher than or equal to 1300 MPa.

The use of a press hardening steel having a tensile strength higher than or equal to 1300 MPa provides an improved resistance to impacts, in particular side impacts, without requiring thickening the parts forming the underbody reinforcement element, and thus without increasing the weight of the vehicle underbody structure.

According to other advantageous aspects of the invention, the vehicle underbody structure comprises one or more of the following features, considered alone or according to any technically possible combination:

- the underbody reinforcement element further comprises an intermediate transversal beam extending between and parallel to the front transversal beam and the rear transversal beam, said intermediate transversal beam being made of a press hardening steel having a tensile strength higher than or equal to 1300 MPa;

- the underbody reinforcement element comprises at least one longitudinal beam extending from the front transversal beam to the rear transversal beam, said longitudinal beam being made of a press hardening steel having a tensile strength higher than or equal to 1300 MPa;

- the floor panel comprises a longitudinal floor tunnel forming a recess in the floor panel, the underbody reinforcement element comprising two front transversal beams extending transversally on either side of the floor tunnel;

- the underbody reinforcement element comprises two longitudinal beams extending on either side of the floor tunnel and each joining one of the front transversal beam to the rear transversal beam;

- the underbody reinforcement element comprises a tunnel transversal beam extending from one longitudinal beam to the other longitudinal beam and across the floor tunnel, said tunnel transversal beam being made of a press hardening steel having a tensile strength higher than or equal to 1300 MPa;

- the underbody reinforcement element comprises a single rear transversal beam extending transversally from a transversal end of the floor panel to the other transversal end of said floor panel at a longitudinal end of the floor tunnel;

- the press hardening steel is Usibor®;

- the underbody reinforcement element forms an opened profile, said opened profile being closed by the floor panel;

- the material of the floor panel is different than the material of the underbody reinforcement element;

- the vehicle underbody structure further comprises at least one connection element of the vehicle underbody structure to a pick-up chassis, said connection element being attached to one of the transversal beam of the underbody reinforcement element.

The invention also relates to a vehicle body comprising at least a vehicle door frame and a vehicle underbody structure according to the invention, wherein the vehicle door frame comprises at least a front pillar, a center pillar and a side rail joining the front pillar and the center pillar, the front pillar being attached to the front transversal beam of the underbody reinforcement element and the center pillar being attached to the rear transversal beam of the underbody reinforcement element.

According to other advantageous aspects of the invention, the vehicle body comprises one or more of the following features, considered alone or according to any technically possible combination:

- the door frame further comprises a lowerside sill joining the front pillar to the center pillar, the transversal beams of the underbody reinforcement element being welded to said lowerside sill;

- the intermediate transversal beam is attached to the lowerside sill of the door frame;

- the lowerside sill comprises an outer lowerside sill part and an inner lowerside sill part, said transversal beams of the underbody reinforcement element being welded to said inner lowerside sill part;

- said inner lowerside sill part is made of a steel having a fully martensitic microstructure, said steel having a tensile strength higher than or equal to 1700 MPa.

Other features and advantages of the invention will be better understood from a reading of the following description, given with reference to the appended drawings, in which:

- Figure 1 is a perspective view of a vehicle body according to a particular embodiment;

- Figure 2 shows an exploded perspective view of a vehicle door frame of the vehicle body of Figure 1 ;

- Figure 3 is a bottom view of the underbody structure of the vehicle body of Figure

1 ;

Figure 4 shows a cross-section along line IV-IV of Figure 3 of an assembly of the vehicle door frame and the underbody structure.

In the following description, the terms inner, outer, front, rear, transversal, longitudinal, vertical and horizontal are construed with reference to the usual orientation of the illustrated elements, parts or structures when assembled on a vehicle structure.

A vehicle body 10 according to an embodiment of the invention is illustrated on Figure 1 . The vehicle body 10 is a body of a vehicle of the body-on-frame type, for example a pick-up truck. A vehicle of this type comprises a chassis which is separate from the body of the vehicle. The vehicle body 10 comprises a vehicle underbody structure 20, and, on either side of the vehicle underbody structure 20, a vehicle door frame 22.

The vehicle underbody structure 20 includes a floor panel 24 and an underbody reinforcement element 26, shown on Figure 3. The vehicle underbody structure 20 is destined to be connected to the chassis of the vehicle, as described in further details herebelow.

The vehicle door frame 22 and each underbody reinforcement element 26 together form at least part of a vehicle reinforcement structure 30.

The vehicle door frame 22 comprises a side rail 34, a center pillar 36 and a front pillar 38. In the illustrated embodiment, the vehicle door frame 22 further comprises a lowerside sill 40.

The side rail 34 extends substantially horizontally along a longitudinal direction between a front end 34a and a rear end 34b.

The front pillar 38 extends downward from the front end 34a of the side rail 34 along an overall substantially vertical direction. The front pillar 38 comprises an upper front pillar section 42 and a lower front pillar section 44.

The upper front pillar section 42 extends frontward and downward from the front end 34a of the side rail 34 in an oblique direction. The upper front pillar section 42 thus extends between an upper end 42a contiguous to the front end 34a of the side rail 34, and a lower end 42b.

The lower front pillar section 44 extends substantially vertically from the lower end 42b of the upper front pillar section 42. The lower front pillar section 44 extends between an upper end 44a, contiguous to the lower end 42b of the upper front pillar section 42, and a lower end 44b.

The center pillar 36 extends downward from the side rail 34 along a substantially vertical direction. In the illustrated example, the center pillar 36 extends from an intermediate section of the side rail 34 comprised between the front end 34a and the rear end 34b of the side rail 34. The center pillar 36 thus extends between an upper end 36a, contiguous to the intermediate section of the side rail 34, and a lower end 36b.

Thus, the side rail 34 joins the upper ends of the front pillar 38 and the center pillar

36.

The lowerside sill 40 extends substantially along a longitudinal direction between a front end 40a and a rear end 40b.

The lower end of the front pillar 38 is contiguous to the front end 40a of the lowerside sill 40, and the lower end 36b of the center pillar 36 is contiguous to an intermediate section of the lowerside sill 40 comprised between the front end 40a and the rear end 40b of the lowerside sill 40. Thus, the lowerside sill 40 joins the lower ends of the front pillar 38 and the center pillar 36.

Thus, the front pillar 38, the center pillar 36, the side rail 34 and the lowerside sill 40 define a door opening. Indeed, the vehicle door frame 22 is destined to receive a vehicle door, which, when closed, fills the door opening.

As illustrated on Figure 2, the vehicle door frame 22 is formed by the assembly of a vehicle door frame outer reinforcement element 50 and of a vehicle door frame inner reinforcement element 52.

The vehicle door frame inner reinforcement element 52 and the vehicle door frame outer reinforcement element 50 each are three-dimensional elements.

The vehicle door frame inner reinforcement element 52 comprises inner reinforcement parts including an inner side rail part 64, an inner center pillar part 66 and an inner front pillar part 68. The inner reinforcement parts further include a roof reinforcement part 70.

Similarly to the side rail 34, the inner side rail part 64 extends substantially horizontally along a longitudinal direction between a front end 64a and a rear end 64b.

Similarly to the center pillar 36, the inner center pillar part 66 extends downward from the inner side rail part 64 along a substantially vertical direction. In the illustrated example, the inner center pillar part 66 extends from an intermediate section of the inner side rail part 64 comprised between the front end 64a and the rear end 64b of the inner side rail part 64. The inner center pillar part 66 thus extends between an upper end 66a contiguous to the intermediate section of the inner side rail part 64, and a lower end 66b.

The inner front pillar part 68 extends frontward and downward from the front end 64a of the inner side rail part 64 in an oblique direction. The inner front pillar part 68 thus extends between an upper end 68a contiguous to the front end of the side rail 34, and a lower end 68b. Preferably, the lower end 68b is comprised in a horizontal plane higher than the horizontal plane comprising the lower end of the front pillar 38, i.e. the inner front pillar part 68 does not extend along the whole length of the front pillar 38, and for example only extends along the length of the upper front pillar section 42.

Thus, the inner side rail part 64 joins the inner front pillar part 68 and the inner center pillar part 66.

The roof reinforcement part 70 extends in a substantially horizontal plane inwardly from the front end 64a of the inner side rail part 64, in a direction substantially perpendicular to the inner front pillar part 68 and to the inner side rail part 64.

The roof reinforcement part 70 is destined to support a roof front rail element. The roof reinforcement part 70 is for example made integral with the inner side rail part 64.

At least part of the inner reinforcement parts are made of a press hardened steel. For example, all the inner reinforcement parts are made of a press hardened steel. The inner reinforcement parts may be made of different press hardened steels.

Preferably, the press hardened steel has a tensile strength greater than or equal to 1300 MPa.

For example, the press-hardened steel has a composition comprising, in % weight, 0.10%≤ C≤ 0.5%, 0.5%≤ Mn≤ 3%, 0.1 %≤ Si≤ 1 %, 0.01 %≤ Cr≤ 1 %, Ti≤ 0.2%, Al≤ 0.1 %, S≤ 0.05%, P≤ 0.1 %, 0.0005%≤ B≤ 0.010%, the remainder consisting of iron and unavoidable impurities resulting from the production.

The press hardened steel is for example Usibor®, in particular Usibor®1500.

The steel may be coated or uncoated, for example galvannealed or galvanized by any suitable process such as hot dip coating, electrodeposition, vacuum coating.

In particular, the press hardened steel has preferably a structure consisting essentially of ferrite and perlite before the steel is hot stamped, and a structure consisting essentially of martensite after hot stamping.

The vehicle door frame inner reinforcement element 52 has an overall thickness, defined as the smallest dimension of the vehicle door frame inner reinforcement element 52, for example comprised between 0.7 mm and 1 .3 mm.

Preferably, at least one of the inner reinforcement parts has a thickness different from the thickness of the other inner reinforcement parts. For example, the thicknesses of the inner side rail part 64, the inner center pillar part 66 and the inner front pillar part 68 differ one from each other.

Each inner reinforcement part has a thickness adapted to the final position of the part in the vehicle and to the resistance to be achieved.

For example, the inner side rail part 64, together with the roof reinforcement part 70, has a thickness substantially equal to 1 mm. The inner front pillar part 68 has a thickness substantially equal to 0.9 mm. The inner center pillar part 66 has a thickness substantially equal to 1 .1 mm.

The vehicle door frame outer reinforcement element 50 comprises outer reinforcement parts including an outer side rail part 74, an outer center pillar part 76 and an outer front pillar part 78. The outer reinforcement parts further include an outer lowerside sill part 80.

Similarly to the side rail 34, the outer side rail part 74 extends substantially horizontally along a longitudinal direction between a front end 74a and a rear end 74b. Similarly to the center pillar 36, the outer center pillar part 76 extends downward from the outer side rail part 74 along a substantially vertical direction. In the illustrated example, the outer center pillar part 76 extends from an intermediate section of the outer side rail part 74 comprised between the front end 74a and the rear end 74b of the outer side rail part 74. The outer center pillar part 76 thus extends between an upper end 76a, contiguous to the intermediate section of the outer side rail part 74, and a lower end 76b.

Similarly to the front pillar 38, the outer front pillar part 78 extends downward from the front end 74a of the outer side rail part 74 along an overall substantially vertical direction. The outer front pillar part 78 comprises an outer upper front pillar part 82 and an outer lower front pillar part 84.

The outer upper front pillar part 82 extends frontward and downward from the front end 74a of the outer side rail part 74 in an oblique direction. The outer upper front pillar part 82 thus extends between an upper end 82a contiguous to the front end 74a of the outer side rail part 74, which is the upper end of the outer front pillar part 78, and a lower end 82b.

The outer lower front pillar part 84 extends substantially vertically from the lower end of the outer upper front pillar part 82. The outer lower front pillar part 84 extends between an upper end 84a, contiguous to the lower end 82b of the outer upper front pillar part 82, and a lower end 84b, which is the lower end of the outer front pillar part 78.

Thus, the outer side rail part 74 joins the outer front pillar part 78 and the outer center pillar part 76.

Similarly to the lowerside sill 40, the outer lowerside sill part 80 extends substantially along a longitudinal direction between a front end 80a and a rear end 80b.

The lower end 84b of the outer front pillar part 78 is contiguous to the front end 76a of the outer lowerside sill part 80, and the lower end 76b of the outer center pillar part 76 is contiguous to an intermediate section of the outer lowerside sill part 80 comprised between the front end 80a and the rear end 80b of the outer lowerside sill part 80. Thus, the outer lowerside sill part 80 joins the outer lower front pillar part 84 to the outer center pillar part 76.

At least part of the outer reinforcement parts are made of a press hardened steel.

For example, all the outer reinforcement parts are made of a press hardened steel.

Preferably, the press hardened steel has a tensile strength higher than or equal to 1300 MPa.

For example, the press-hardened steel has a composition comprising, in % weight, 0.10%≤ C≤ 0.5%, 0.5%≤ Mn≤ 3%, 0.1 %≤ Si≤ 1 %, 0.01 %≤ Cr≤ 1 %, Ti≤ 0.2%, Al≤ 0.1 %, S≤ 0.05%, P≤ 0.1 %, 0.0005%≤ B≤ 0.010%, the remainder consisting of iron and unavoidable impurities resulting from the production.

The press hardened steel is for example Usibor®, in particular Usibor®1500.

The steel may be coated or uncoated, for example galvannealed or galvanized by any suitable process such as hot dip coating, electrodeposition, vacuum coating.

The outer reinforcement parts may be made of different press hardened steels. For example, the outer lowerside sill part 80 may be made of a press hardened steel different from the other outer parts. In particular, the outer lowerside sill part 80 may be made of Ductibor®, and the other outer reinforcement parts are made of Usibor®.

However, the outer reinforcement parts are preferably all made of the same press hardened steel, so that when subjected to an external stress, the distribution of the deformation is homogeneous within the outer reinforcement element 50.

In particular, the press hardened steel has preferably a structure consisting essentially of ferrite and perlite before the steel is hot stamped, and a structure consisting essentially of martensite after hot stamping.

The door frame outer reinforcement element 50 has an overall thickness, defined as the smallest dimension of the door frame outer reinforcement element 50, for example comprised between 0.8 mm and 2.5 mm.

Preferably, at least one of the outer reinforcement parts has a thickness different from the thickness of the other outer reinforcement parts. For example, the thicknesses of the outer side rail part 74, the outer center pillar part 76 and the outer front pillar part 78 differ one from each other.

Each outer part has a thickness adapted to the final position of the part in the vehicle and to the resistance to be achieved.

For example, the outer side rail part 74 has a thickness substantially equal to 1 .1 mm, the outer upper front pillar part 82 has a thickness substantially equal to 0.9 mm, and the outer lower front pillar part 84 has a thickness substantially equal to 1 .1 mm. According to this example, the outer center pillar part 76 has a thickness substantially equal to 2 mm and the outer lowerside sill part 80 has a thickness substantially equal to 1 .2 mm.

The door frame outer reinforcement element 50 and the door frame inner reinforcement element 52 have complementary shapes such that, once assembled, the door frame outer reinforcement element 50 and the door frame inner reinforcement element 52 form the vehicle door frame 22. In particular, the side rail 34 is formed by the assembly of the inner side rail part 64 and the outer side rail part 74, and the center pillar 36 is formed by the assembly of the inner center pillar part 66 and the outer center pillar part 76.

Furthermore, the front pillar 38 is formed by the assembly of the inner front pillar part 68 and the outer front pillar part 78. More specifically, the upper front pillar section 42 is formed by the assembly of the inner front pillar part 68 and the outer upper front pillar part 82.

In the illustrated example, the lower front pillar section 44 is formed by the outer lower front pillar part 84.

The vehicle door frame outer reinforcement element 50 and the vehicle door frame inner reinforcement element 52 are shaped as complementary opened profiles such that a part of the front pillar 38, the center pillar 36 and the side rail 34 of the vehicle door frame 22 each have a hollow closed section.

In particular, the outer reinforcement parts have an open cross-section. The open cross-section comprises at least a bottom segment and two wall segments extending from either ends of the bottom segment.

A method for producing the vehicle door frame 22 will now be described.

The production of the vehicle door frame 22 comprises producing the door frame inner reinforcement element 52, producing the door frame outer reinforcement element 50, and assembling the door frame inner reinforcement element 52 to the door frame outer reinforcement element 50.

The vehicle door frame inner reinforcement element 52 is produced by hot stamping an inner reinforcement blank, which is itself formed by assembling several blanks.

The production of the vehicle door frame inner reinforcement element 52 thus comprises a step of formation of a substantially planar door frame inner reinforcement blank. The shape of the door frame inner reinforcement blank is adapted so that it may be hot stamped to form the inner reinforcement element having the desired shape.

The inner reinforcement blank is preferably a tailor welded blank.

The formation of the inner reinforcement blank comprises providing an inner center pillar blank, an inner front pillar blank and an inner side rail blank, said inner blanks being substantially planar. Preferably, the formation of the inner reinforcement blank further comprises providing a roof reinforcement blank made integral with the inner side rail blank.

The shapes and thicknesses of the inner blanks are adapted so that, once hot stamped, the inner center pillar blank, the inner front pillar blank, the inner side rail blank and the roof reinforcement blank may form the inner center pillar part 66, the inner front pillar part 68, the inner side rail part 64 and the roof reinforcement part 70 respectively.

The inner blanks are for example obtained by cutting steel sheets, for example sheets made of a press hardening steel such as Usibor®, to the desired shapes.

The inner center pillar blank and the inner front pillar blank are then assembled to the inner side rail blank to form the inner reinforcement blank.

In particular, an upper end of the inner front pillar blank is assembled to a front end of the inner side rail blank and an upper end of the inner center pillar blank is assembled to an intermediate section of the inner side rail blank.

Preferably, the inner blanks are assembled by means of welding, more preferably by laser welding, such that the inner blanks are joined together by continuous weld lines.

In particular, the inner center pillar blank and the inner front pillar blank are joined to the inner side rail blank by continuous weld lines.

The production of the vehicle door frame inner reinforcement element 52 then comprises a step of hot stamping the door frame inner reinforcement blank to shape the three-dimensional vehicle door frame inner reinforcement element 52.

If the inner blanks are made of a press hardening steel, the hot stamping results in a hardening of the steel.

In particular, as stated above, the press hardened steel has preferably a structure consisting essentially of ferrite and perlite before the steel is hot stamped, and a structure consisting essentially of martensite after hot stamping and quenching.

Similarly, the door frame outer reinforcement element 50 is produced by hot stamping an outer reinforcement blank, which is itself formed by assembling several blanks.

The production of the door frame outer reinforcement element 50 thus comprises a step of formation of a substantially planar door frame outer reinforcement blank. The shape of the door frame outer reinforcement blank is adapted so that it may be hot stamped to form the outer reinforcement element having the desired shape.

The outer reinforcement blank is preferably a tailor welded blank.

The formation of the outer reinforcement blank comprises providing an outer center pillar blank, an outer upper front pillar blank, an outer lower front pillar blank, an outer side rail blank, and an outer lowerside sill blank, said outer blanks being substantially planar.

The shapes and thicknesses of the outer blanks are adapted so that, once hot stamped, the outer center pillar blank, the outer upper front pillar blank, the outer lower front pillar blank, the outer side rail blank and the outer lowerside sill blank may form the outer center pillar part 76, the outer upper front pillar part 82, the outer lower front pillar part 84, the outer side rail part 74 and the outer lowerside sill part 80 respectively.

The outer blanks are for example obtained by cutting steel sheets, for example sheets made of a press hardening steel such as Usibor®, to the desired shapes.

The outer center pillar blank and the outer upper front pillar blank are then assembled to the outer side rail blank, the outer lower front pillar blank is assembled to the outer upper front pillar blank, and the outer lowerside sill blank is assembled to the outer lower front pillar blank and to the outer center pillar blank, so as to form the outer reinforcement blank.

In particular, an upper end of the outer upper front pillar blank is assembled to a front end of the outer side rail blank and an upper end of the outer center pillar blank is assembled to an intermediate section of the outer side rail blank. Furthermore, a lower end of the outer upper front pillar blank is assembled to an upper end of the outer lower front pillar blank, a lower end of the outer lower front pillar blank is assembled to a front end of the outer lowerside sill blank, and a lower end of the outer center pillar blank is assembled to an intermediate section of the outer lowerside sill blank.

Preferably, the outer blanks are assembled by means of welding, more preferably by laser welding, such that the outer blanks are joined together by continuous weld lines.

The production of the door frame outer reinforcement element 50 then comprises a step of hot stamping the door frame outer reinforcement blank to shape the three- dimensional vehicle door frame outer reinforcement element 50.

If the outer blanks are made of a press hardening steel, the hot stamping results in a hardening of the steel.

The door frame inner reinforcement element 52 and the door frame outer reinforcement element 50 are then assembled, for example by welding.

The use of a press hardening steel for the inner and outer blanks thus provides both a good formability for the blanks so that the door frame inner and outer reinforcement blanks may be hot stamped to shape the vehicle door frame inner and outer reinforcement elements without obtaining necking or thickening of the steel, and a ultra high strength for the vehicle door frame inner and outer reinforcement elements once hot stamped.

Assembling the inner blanks (respectively the outer blanks) to form the door frame inner reinforcement blank (respectively the door frame outer reinforcement blank) before hot stamping the door frame inner reinforcement blank (respectively the door frame outer reinforcement blank) allows the use of a single press for shaping the whole vehicle door frame inner reinforcement element 52 (respectively the whole door frame outer reinforcement element 50), which reduces the cost of the production of the vehicle door frame inner 52 and outer 50 reinforcement elements.

Moreover, the formation of the door frame inner reinforcement blank (respectively the door frame outer reinforcement blank) by assembling several inner blanks (respectively several outer blanks) allows having a varying thickness between the different parts of the vehicle door frame inner reinforcement element 52 (respectively the door frame outer reinforcement element 50), and further allows the reduction of material use thanks to improved nesting.

Furthermore, assembling the inner blanks (respectively the outer blanks) to form the door frame inner reinforcement blank (respectively the door frame outer reinforcement blank) before hot stamping the door frame inner reinforcement blank (respectively the door frame outer reinforcement blank) allows using laser welding to assemble the inner blanks (respectively the outer blanks), instead of spot welding. Laser welding provides a continuous weld line between the blanks, and consequently provides a better resistance, and thus a better crash resistance, than spot welding.

Referring now to Figure 3, the vehicle underbody structure 20 includes a floor panel 24 and an underbody reinforcement element 26.

The floor panel 24 generally extends along a horizontal plane. The floor panel 24 extends longitudinally between a front side 24a and a rear side 24b, and transversally between a right side 24c and a left side 24d.

The floor panel 24 comprises a longitudinal floor tunnel 100 forming a recess in the floor panel 24, opening downwardly. The floor tunnel 100 extends from the front side 24a of the floor panel 24 towards the rear side 24b, between two side floor parts.

The floor tunnel 100 comprises a substantially horizontal upper wall 100a and two substantially longitudinal vertical side walls 100b, 100c. Each side wall 100b, 100c extends between an upper end, contiguous to the upper wall, and a lower end, contiguous to one of the side floor parts.

The floor panel 24 is for example made of a steel.

The underbody reinforcement element 26 is attached to the lower side floor panel 24 and is destined to provide resistance to the vehicle underbody structure 20.

The underbody reinforcement element 26 comprises a mesh of beams destined to absorb stress when the vehicle is subjected to an impact. In particular, the underbody reinforcement element 26 is destined to absorb stress received by a door ring, for example during a side impact.

The underbody reinforcement element 26 thus comprises several reinforcement beams attached to the floor panel 24. In particular, the reinforcement beams comprise at least a front transversal beam and a rear transversal beam, the front and rear transversal beams extending parallel to each other.

In the illustrated example, the reinforcement beams comprise two front transversal beams 1 12a, 1 12b and a single rear transversal beam 1 14.

The reinforcement beams further comprise two intermediate transversal beams 1 16a, 1 16b, two longitudinal beams 1 18a, 1 18b and a tunnel transversal beam 120.

The two front transversal beams 1 12a, 1 12b extend transversally in the front part of the floor panel 24, on either side of the floor tunnel 100. Indeed, each front transversal beam 1 12a, 1 12b extends between an inner end contiguous to a lower end of a side wall 100b, 100c of the floor tunnel 100 and an outer end contiguous to a side 24c, 24d of the floor panel 24.

The rear transversal beam 1 14 extends in a central part of the floor panel 24, parallel to the front transversal beams 1 12a, 1 12b. The rear front transversal beam 1 14 thus extends between the right 24c and left 24d sides of the floor panel 24.

More specifically, the front transversal beams 1 12a, 1 12b and the rear transversal beam 1 14 are configured so that, when the vehicle underbody structure 20 is assembled to the door frame 22, the outer ends of each front transversal beam 1 12a, 1 12b may join the front pillar 38 of a door frame 22, and each end of the rear transversal panel may join the center pillar 36 of a door frame 22.

The two intermediate transversal beams 1 16a, 1 16b extend between and parallel to the front transversal beams 1 12a, 1 12b and the rear transversal beam 1 14, on either side of the floor tunnel 100. Each intermediate transversal beam 1 16a, 1 16b extends between an inner end contiguous to a lower end of a side wall of the floor tunnel 100 and an outer end contiguous to a side of the floor panel 24.

For example, the intermediate transversal beams 1 16a, 1 16b extend substantially halfway between the front transversal beams 1 12a, 1 12b and the rear transversal beam 1 14.

Thus, the intermediate transversal beams 1 16a, 1 16b are configured so that, when the vehicle underbody structure 20 is assembled to the door frame 22, the outer end of each intermediate transversal beam 1 16a, 1 16b may join the lowerside sill 40 of a door frame 22.

The two longitudinal beams 1 18a, 1 18b extend longitudinally on either side of the floor tunnel 100. Each longitudinal beam 1 18a, 1 18b extends between a front end contiguous to a front transversal beam 1 12a, 1 12b and a rear end contiguous to the rear transversal beam 1 14. Thus, each longitudinal beam 1 18a, 1 18b joins one of the front transversal beams 1 12a, 1 12b to the rear transversal beam 1 14.

Furthermore, each longitudinal beam 1 18a, 1 18b comprises an intermediate section which is contiguous to an inner end of an intermediate transversal beam 1 16a, 1 16b. Thus, each longitudinal beam 1 18a, 1 18b joins one of the front transversal beams 1 12a, 1 12b to an intermediate transversal beam 1 16a, 1 16b and to the rear transversal beam 1 14.

The tunnel transversal beam 120 extends from one longitudinal beam 1 18a to the other longitudinal beam 1 18b and across the floor tunnel 100. The tunnel transversal beam 120 thus comprises a center region 122 crossing the floor tunnel 100 and comprised between two ends 124a, 124b joining the longitudinal beams 1 18a. The center region 120a of the tunnel transversal beam 120 comprises a substantially horizontal upper wall, attached to the floor tunnel 100 upper wall, and two substantially longitudinal vertical side walls, each attached to a floor tunnel 100 side wall.

Preferably, the tunnel transversal beam 120 is not transversally aligned with the intermediate transversal beams 1 16a, 1 16b, so that a longitudinal offset exists between the tunnel transversal beam 120 and the intermediate transversal beams 1 16a, 1 16b.

Owing to this offset, stress received by the intermediate transversal beams 1 16a, 1 16b is not directly transmitted to the tunnel transversal beam 120, but is transmitted through the longitudinal beams 1 18a, 1 18b. Indeed, since the tunnel transversal beam 120 is not straight, it does not work in compression but in flexion when submitted to a transversal stress, and thus bends more easily than a straight beam. The offset between the tunnel transversal beam 120 and the intermediate transversal beams 1 16a, 1 16b thus allows reducing the risks of bending the tunnel transversal beam 120.

Each front transversal beam 1 12a, 1 12b and the rear transversal beam 1 14 are provided with slots 130 for receiving a connection element, also called body mount, destined to connect the vehicle underbody structure 20 to a chassis.

Preferably, the underbody reinforcement element 26 is made of a steel (or several different steels) different from the steel forming the floor tunnel 100, and having a tensile strength higher than the steel forming the floor tunnel 100.

Preferably, at least some of the reinforcement beams are made of a steel having a tensile strength higher than or equal to 1300 MPa. For example, the steel is a press hardening steel, which, once pressed, has a tensile strength higher than or equal to 1300 MPa.

For example, the press-hardened steel has a composition comprising, in % weight,

0.10%≤ C≤ 0.5%, 0.5%≤ Mn≤ 3%, 0.1 %≤ Si≤ 1 %, 0.01 %≤ Cr≤ 1 %, Ti≤ 0.2%, Al≤ 0.1 %, S≤ 0.05%, P≤ 0.1 %, 0.0005%≤ B≤ 0.010%, the remainder consisting of iron and unavoidable impurities resulting from the production.

The press hardened steel is for example Usibor®, in particular Usibor®1500 or Usibor®2000.

The steel may be coated or uncoated, for example galvannealed or galvanized by any suitable process such as hot dip coating, electrodeposition, vacuum coating.

In particular, the press hardened steel has preferably a structure consisting essentially of ferrite and perlite before the steel is pressed, and a structure consisting essentially of martensite after pressing.

The underbody reinforcement element 26 has an overall thickness, defined as the smallest dimension of the underbody reinforcement element 26, for example comprised between 0.7 mm and 1 .5 mm.

Preferably, at least one of the reinforcement beams has a thickness different from the thickness of the other beams. For example, the thicknesses of the front transversal beams 1 12a, 1 12b, the rear transversal beam 1 14, the intermediate transversal beams 1 16a, 1 16b, the longitudinal beams 1 18a, 1 18b and the tunnel transversal beam 120 differ one from each other.

Each reinforcement beam has a thickness adapted to the position of the beam and to the resistance to be achieved.

Preferably, the tunnel transversal beam 120 has a thickness higher than the other beams, due to the non straight geometry of this beam, in particular in order to avoid bending of the tunnel transversal beam 120.

For example, the front transversal beams 1 12a, 1 12b have a thickness substantially equal to 1 .2 mm, and the rear transversal beam 1 14 has a thickness substantially equal to 1 .2 mm. The intermediate transversal beams 1 16a, 1 16b have for example a thickness substantially equal to 1 .3 mm, the longitudinal beams 1 18a, 1 18b have for example a thickness substantially equal to 1 .2 mm, and the tunnel transversal beam 120 has for example a thickness substantially equal to 1 .5 mm.

The reinforcement beams have an open cross section, so that the underbody reinforcement element 26 forms an opened profile. The open cross-section of each reinforcement beam comprises at least a bottom segment and two wall segments extending from either ends of the bottom segment.

The underbody reinforcement element 26 is attached to the floor panel 24 such that the floor panel 24 closes the opened profile.

Preferably, the underbody reinforcement element 26 and the floor panel 24 are attached by welding, for example spot welding. The production of the vehicle underbody structure 20 comprises the production of the floor panel 24, the production of the underbody reinforcement element 26, and the assembling of the underbody reinforcement element 26 to the floor panel 24 so as to form the vehicle underbody structure 20.

The floor panel 24 is for example produced by stamping of a substantially rectangular blank.

The underbody reinforcement element 26 is produced by forming and shaping each reinforcement beam and by assembling the reinforcement beams so as to form the underbody reinforcement element 26.

The production of the underbody reinforcement element 26 thus comprises a step of formation of substantially planar reinforcement beam blanks.

The formation of the reinforcement beam blanks comprises the formation of two front transversal beam blanks, a rear transversal beam blank, two intermediate transversal beam blanks, two longitudinal beam blanks and a tunnel transversal beam blank.

The shapes and thicknesses of the reinforcement beam blanks are adapted so that, once hot stamped, the front transversal beam blanks, the rear transversal beam blank, the intermediate transversal beam blanks, the longitudinal beam blanks and the tunnel transversal beam blank may form the front transversal beams 1 12a, 1 12b, the rear transversal beam 1 14, the intermediate transversal beams 1 16a, 1 16b, the longitudinal beams 1 18a, 1 18b and the tunnel transversal beam 120 respectively.

The reinforcement beam blanks are for example obtained by cutting steel sheets, for example sheets made of a press hardening steel such as Usibor®, to the desired shapes.

The reinforcement beam blanks are then hot stamped so as to shape the reinforcement beams, and subsequently quenched.

The reinforcement beams are then assembled to form the underbody reinforcement element 26.

In particular, the front end of each longitudinal beam 1 18a, 1 18b is assembled to the inner end of a front transversal beam 1 12a, 1 12b, an intermediate section of each longitudinal beam 1 18a, 1 18b is assembled to the inner end of an intermediate transversal beam 1 16a, 1 16b, and the rear end of each longitudinal beam 1 18a, 1 18b is assembled to the rear transversal beam 1 14.

Furthermore, each end of the tunnel transversal beam 120 is assembled to a longitudinal beam 1 18a, 1 18b. Preferably, the reinforcement beams are assembled by means of welding, for example by spot welding or laser welding.

The use of a press hardening steel for at least part of the underbody reinforcement element 26 provides both a good formability for the reinforcement beam blanks so that the reinforcement beam blanks may be hot stamped to shape the reinforcement beams without obtaining compression or thickening of the steel, and a ultra high strength for the reinforcement beams once hot stamped and quenched.

The use of a press hardening steel having a tensile strength higher than or equal to 1300 MPa provides an improved resistance to impacts, in particular side impacts, without requiring thickening the beams, and thus without increasing the weight of the vehicle underbody structure.

The vehicle body 10 is formed by the assembly of the vehicle underbody structure 20 and at least one vehicle door frame 22 on one side of the vehicle underbody structure 20, preferably two vehicle door frames 22 on either side of the vehicle underbody structure 20. The assembling of the vehicle body 10 will now be described with a reference to one vehicle door frame 22, on one side of the vehicle underbody structure 20, but it will be understood that a second vehicle door frame 22 may be assembled in the same way to the other side of the vehicle underbody structure 20

The vehicle door frame 22 is attached to the vehicle underbody structure 20 on a side of the vehicle underbody structure 20, preferably such that the front pillar 38 is attached to a front transversal beam 1 12a of the underbody reinforcement element 26 and the center pillar 36 is attached to the rear transversal beam 1 14 of the underbody reinforcement element 26. Furthermore, an intermediate transversal beam 1 16a is attached to the lowerside sill 40.

Thus, the energy of an impact received by the vehicle door frame 22, in particular by the front and center pillars, can be efficiently transmitted to the vehicle underbody structure 20.

Preferably, the transversal beams 1 12a, 1 14 and 1 16a are attached to the vehicle door frame 22 by means of an inner lowerside sill part 140 (Figure 4), forming a joining steel sheet, which extends between the front transversal beam 1 12a and the rear transversal beam 1 14. For example, the transversal beams 1 12a, 1 14 and 1 16a are attached to the vehicle door frame 22 by welding.

The inner lowerside sill part 140 extends extends substantially along a longitudinal direction. The inner lowerside sill part 140 is assembled, for example by welding, to the outer lowerside sill part 80 to form the lowerside sill 40. The inner lowerside sill part 140 has a shape complementary to that of the outer lowerside sill part 80 such that, when the joining steel sheet 140 and the outer lowerside sill part 80 are assembled, they form the lowerside sill 40, and such that the lowerside sill 40 has a hollow closed section.

The inner lowerside sill part 140 is for example made of a steel having a fully martensitic microstructure, and having a tensile strength preferably higher than or equal to 1700 MPa. The steel forming the inner lowerside sill part 140 is for example MS1700®.

The outer lowerside sill part 80 is itself covered by an outer ornament panel 142.

Thus formed, the vehicle underbody structure 20 improves the resistance of the vehicle in case of an impact, in particular a side impact. In particular, the position of the front and rear transversal beams, facing the front pillar 38 and the center pillar 36 respectively, allows distributing the stress received by the vehicle during the impact, and provides an efficient support to the front and center pillars. Furthermore, the intermediate transversal beam 1 16a, facing the lowerside sill 40 may absorb energy in case of a side impact occurring between the front and center pillars, and thus limits the risks of intrusion in the vehicle compartment in case of a side impact.

It must be understood that the example embodiments presented above are not limiting.