WO2023041954A1

WO2023041954A1 - High strength high slenderness part having excellent energy absorption

Info

Publication number: WO2023041954A1
Application number: PCT/IB2021/058364
Authority: WO
Inventors: Alice DUMONT; Arnaud Cocu
Original assignee: Arcelormittal
Priority date: 2021-09-14
Filing date: 2021-09-14
Publication date: 2023-03-23
Also published as: WO2023042031A1; MX2024003166A; CA3229411A1; EP4402298A1; KR20240049308A; CN117916400A

Abstract

The present invention relates to a high strength, high slenderness structural part having excellent energy absorption properties in the case of an impact. In particular, the present invention relates to a structural part for use in an automotive vehicle. The structural part has an ultimate tensile strength higher than 1000MPa, a yield strength to ultimate tensile strength ratio higher than 0,85, a bending angle normalized to 1,5mm thickness higher than 55° and a slenderness ratio higher than 10.

Description

High strength high slenderness part having excellent energy absorption

The present invention relates to a high strength structural part having excellent energy absorption properties in the case of an impact. In particular, the present invention relates to a structural part for use in an automotive vehicle.

High strength high slenderness structural parts play an important role in the crash resistance of a vehicle. They are long and narrow assemblies comprising a hollow cavity. During an impact they act to absorb energy by buckling and thus forming folds which absorb part of the crash energy. They also act as important relays on the load path of the vehicle architecture and contribute to transmitting and diffusing the crash energy from one end of the vehicle to the other, thus ensuring that a maximum amount of the vehicle’s architecture is involved in absorbing the crash energy.

As such, high strength high slenderness structural parts play a fundamental role in promoting the safety of the vehicle’s occupants.

When a crash occurs, the high slenderness parts are subjected to a compressive force, which is not necessarily strictly parallel to the length direction of the part. In order to absorb the maximum amount of energy it is important that the high slenderness part bottles onto itself as much as possible. When there is an angle between the compressive force and the length direction of the part, there is a risk that the part will bend before fully bottling. Once the part is bent it is no more available for bottling and therefore will not have absorbed the maximum amount of energy possible.

The purpose of the current invention is to address this issue by providing a high slenderness part having a robust buckling behavior even in the case of an angled compressive load. This is particularly critical in the case of current vehicles which are submitted to both stringent safety requirements and weight lightning requirements for energy consumption.

The object of the present invention is achieved by providing a high slenderness part according to claim 1 , optionally comprising the features of claims 2 to 5. The invention will now be described in detail and illustrated by examples without introducing limitations, with reference to the appended figures:

-Figure 1 is a schematic of a high slenderness part according to an embodiment of the invention, with Figure 1 a being an insert detailing the definition of the different angles defined in the description,

-Figure 2 is a top view of the compared simulated behavior during a crash test of a high slenderness part according to the invention (11 ) and a reference example (R2) with an angle between the impactor and the longitudinal direction of the part,

-Figure 3 is a side view of the compared simulated behavior during a crash test of a high slenderness part according to the invention (11 ) and a reference example (R2) with an angle between the impactor and the length direction of the part,

The slenderness ratio, commonly used in Leonhard Euler’s buckling theory, is defined by the following formula, where L is the length of the part, S is the area of its straight section, and Imin is the minimum quadratic moment of area in the section being considered.

ln general, the minimum quadratic moment of area Imin over a cross section A in a set of cartesian coordinates (x,y) is defined by the following formula:

For example, the minimum quadratic moment of area Imin for a hollow rectangular section having outer dimension b and h and inner dimensions b1 and hi is calculated using the following formula:

For example, the minimum quadratic moment of area Imin for a hollow annular section having outer radius R and inner radius R1 is calculated using the following formula:

A part can be considered to have a high slenderness when its slenderness ratio is above 10.

The bending angle is measured according to the VDA-238-100 bending standard. In the current invention, the bending angles are measured after springback. For the same material, the bending angle depends on the thickness. For the sake of simplicity, the bending angle values of the current invention refer to a thickness of 1.5mm. If the thickness is different than 1.5mm, the bending angle value needs to be normalized to 1.5mm by the following calculation where a1.5 is the bending angle normalized at 1.5mm, t is the thickness, and at is the bending angle for thickness t: a1.5 = (at x t) I 1.5

The bending angle of a part is representative of the ability of the part to resist deformation without the formation of cracks.

The ultimate tensile strength, the yield strength and the elongation are measured according to ISO standard ISO 6892-1 , published in October 2009. The tensile test specimens are cut-out from flat areas. If necessary, small size tensile test samples are taken to accommodate for the total available flat area on the part.

The term fracture strain refers to the fracture strain criterion defined by Pascal Dietsch et al. in “Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration”, in Metallurgical Research Technology Volume 114, Number 6, 2017. The fracture strain is the equivalent strain within the material at the deformation point when the critical bending angle has been reached. The critical bending angle defines the angle at which the first cracks are detected on the extrados of a sample which has been deformed according to the standardized VDA- 238-100 Standard.

The term “bottling” refers to the mode of deformation of a part subjected to a compressive load, typically a high slenderness part, where the part progressively absorbs the mechanical energy of the compressive load by forming a series of successive waves resulting from successive local buckling deformations of the part. As a result, the length of the part as measured in the direction of the compressive load is smaller after the deformation than the initial length of the part in said direction. In other words, when a part reacts to a compressive load by controlled buckling, it folds onto itself in the same way as a plastic bottle on which a compressive load is applied between the top and the bottom of the bottle.

Hot stamping is a forming technology for steel which involves heating a blank up to a temperature at which the microstructure of the steel has at least partially transformed to austenite, forming the blank at high temperature by stamping it and quenching the formed part to obtain a microstructure having a very high strength, possibly with an additional partitioning or tempering step in the heat treatment. Hot stamping allows to obtain very high strength parts with complex shapes and presents many technical advantages. It should be understood that the thermal treatment to which a part is submitted includes not only the above described thermal cycle of the hot stamping process itself, but also possibly other subsequent heat treatment cycles such as for example the paint baking step, performed after the part has been painted in order to bake the paint. The mechanical properties of hot stamped parts below are those measured after the full thermal cycle, including optionally for example a paint baking step, in case paint baking has indeed been performed.

A blank refers to a flat sheet, which has been cut to any shape suitable for its use. A blank has a top and bottom face, which are also referred to as a top and bottom side or as a top and bottom surface. The distance between said faces is designated as the thickness of the blank. The thickness can be measured for example using a micrometer, the spindle and anvil of which are placed on the top and bottom faces. In a similar way, the thickness can also be measured on a formed part.

Hardness is a measure of the resistance to localized plastic deformation induced by mechanical indentation. It is well correlated to the mechanical properties of a material and is a useful local measurement method which does not require to cut out a sample for tensile testing. In the current invention, the hardness measurements are made using a Vickers indenter according to standard ISO 6507- 1 . The Vickers hardness is expressed using the unit Hv.

The heat affected zone is the area of material surrounding a weld which has been heated up during the welding operation. In the case of high strength materials, for example high strength steels, it is well known that the heat affected zone can have weaker mechanical properties. Indeed, the heat affected zone undergoes a thermal treatment akin to tempering, which can lead to softening.

The hardness drop in the heat affected zone of a material is measured using the following protocol:

1/ A hardness profile of the weld is taken by measuring the Vickers hardness under a 0,5kg load every 0,2mm along a staggered line crossing through the heat affected zones of both welded plates and through the spot weld itself,

2/ The weakest point of said hardness profile, usually present in the heat affected zone, is determined and noted HVmin,

3/ The hardness of the base metal away from the weld is measured by taking the average measurement of several points and is noted HVBM,

4/ The hardness drop is computed as the difference HVBM - HVmin, expressed in Hv.

Referring to figure 1 , a high slenderness part 1 extends in a main longitudinal direction Ldir between two ends E1 and E2 and in a transverse direction Tdir. It comprises a hollow volume 4 encased between a top part 3 and a bottom part 2.

In a particular embodiment, as depicted on figure 1 , the high slenderness part 1 is made by forming separately and then joining together the top part 3, which is a generally omega shaped part, and the bottom part 2, which is a flat closing plate. For example, the top part 3 and the bottom part 2 are joined together by welding, for example by spot welding on flanges 6, which produces spot welds 5.

In another embodiment, the high slenderness part is made in one piece comprising the top part and the bottom part. For example, the high slenderness part is made by extrusion. For example, the high slenderness part is formed by roll forming. For example, the high slenderness part is made from a formed metallic tube.

High slenderness parts abound in vehicle architectures, some examples are the front parts joining the front crash boxes to the rocker assembly, the rear parts joining the rear crash boxes to the rocker assembly, cross parts extending transversally in the vehicle, the rocker panels themselves etc. The high slenderness part is generally attached to the rest of the vehicle structure at each of its ends E1 and E2. High slenderness parts are designed to absorb energy under the compressive stress resulting from a crash. Referring to figure 1 , the high slenderness part is subjected to the joint effort of F1 exerted on E1 and the opposite reaction force exerted on E2. These forces are exerted by the other parts to which the high slenderness part is attached in E1 and E2. Referring to figure 1 a, F1 forms an angle beta with the longitudinal direction Ldir. This angle results in a bending moment on the high slenderness part in addition to the compressive load exerted by F1 and its reaction.

As a result, the high slenderness part is submitted both to a compressive stress, which it can accommodate through a bottling deformation (which keeps the part in the Ldir direction) or a bending deformation away from the Ldir direction, and a bending moment, which it can accommodate through a bending direction away from the Ldir direction. The bottling deformation, which keeps the part in the Ldir direction, and the bending deformation, which bends the part away from the Ldir direction, compete with one another. Once bending starts to occur, the angle between the normal vectors to E1 and E2 will increase, which will further promote the bending deformation mode and discourage the bottling deformation mode. At this point, the high slenderness part will not anymore deform by bottling and instead will deform by bending onto itself only.

Bottling forms multiple folds in the material whereas bending only forms one fold in the material. Therefore, bottling absorbs a much higher amount of energy and it is interesting to promote the bottling deformation mode over the bending mode to increase the energy absorption effectiveness of the part. Furthermore, bottling maintains the general direction of the part during the crash, whereas bending will make it deform in rather unpredictable directions and in a catastrophic way. Bottling therefore makes the behaviour of the part during crash much more predictable then bending and makes it available to collaborate predictably and correctly with the rest of the vehicle’s structure as the crash scenario unfolds, which is a further important advantage of bottling over bending.

It is also interesting to manufacture the high slenderness part with high strength materials in order to absorb as much energy as possible. For example, the material used to manufacture at least a portion or all of the high slenderness part has a tensile strength after forming above 1000MPa. For example, the material used to manufacture at least a portion or all of the high slenderness part has a tensile strength after forming above 1300MPa. For example, the material used to manufacture at least a portion or all of the high slenderness part has a tensile strength after forming above 1500MPa. For example, the material used to manufacture at least a portion or all of the high slenderness part has a tensile strength after forming above 1800MPa.

Surprisingly, the inventors have found that in the case of an angle beta strictly greater than 0, the buckling deformation mode is promoted when using materials having a high Yield Strength over Ultimate Tensile Strength (YS/UTS) ratio. In particular, the inventors have found that high slenderness parts having a YS/UTS ratio above 0,85, even more preferably a YS/UTS ratio above 0,9, exhibit very good bottling behaviour and low bending reactions.

Furthermore, the inventors have found that the energy absorption amount is increased when using material having a higher bending angle. Indeed, this means that the material can form folds without cracks occurring in the highest deformation areas of the folds. Such cracks lower the energy absorption because it takes much less energy to deform a cracked area. Cracks can also lead to crack propagation and catastrophic failure of the part, which is to be prevented to ensure energy absorption and energy transmission through the load path and to stick to a predictable overall vehicle crash scenario.

Even though it is important to have sufficient bending ability of the material in order to avoid cracks occurring, the inventors found that it was not necessary to have a very high bending angle. For example, a bending angle normalized to 1 ,5mm of 55° is sufficient for good energy absorption. More preferably, a material having a bending angle normalized to 1 ,5mm of 70° can be used for good energy absorption.

The inventors have also found that a minimum level of fracture strain can be beneficial for increased energy absorption. For example, it is interesting to have a minimum fracture strain of 0,5 to promote high energy absorption and to avoid catastrophic failure of the part.

In the case of a high slenderness part made of at least two different parts joined together by welding, such as the one illustrated on figure 1 , the mechanical resistance of the spot welds 5 and their surrounding area will have an impact on the total energy absorption.

In the case of high strength materials, for example high strength steels, it is well known that the heat affected zone can have weaker mechanical properties. Indeed, the heat affected zone undergoes a thermal treatment akin to tempering, which can lead to softening.

The inventors have found that in this type of configuration, it is advantageous to use materials exhibiting a low hardness drop in the heat affected zone. More specifically, it is advantageous to use materials exhibiting less than 100Hv hardness drop compared to the base metal in the heat affected zone. Preferably, it is advantageous to have a hardness drop below 80Hv, even more preferably below 50Hv.

In a particular embodiment, the material used to manufacture at least a portion of the high slenderness part or the entire high slenderness part is steel comprising the following elements expressed in weight% :

C: 0.1 - 0.25 %

Mn: 3.00 - 5.00 %

Si: 0.80 - 1.60 %

B: 0.0003 - 0.004 %

S < 0.010 %

P < 0.020 %

N < 0.008 % and comprising optionally one or more of the following elements, in weight percentage:

Ti < 0.04 %

Nb < 0.05 %

Mo < 0.3 % 15

Al < 0.90 %

Cr < 0.80 %

The remainder of the composition being iron and unavoidable impurities resulting from the smelting This material is worked using hot stamping and the resulting hot stamped part has for example a UTS above 10OOMPa, an elongation above 10%, a YS / UTS ratio above 0,9, a bending angle above 55° and a hardness drop in the heat affected zone below 80Hv.

C: 0.03 - 0.18 %

Mn: 6.0 - 11.0 %

Mo: 0.05 - 0.5 %

B: 0.0005 - 0.005%

S < 0.010 % P < 0.020 % N < 0.008 % and comprising optionally one or more of the following elements, in weight percentage:

Al < 3%

Si < 1 .20 %

Ti < 0.050 %

Nb < 0.050 %

Cr < 0.5%

This material is worked using hot stamping and the resulting hot stamped part has for example a UTS above 10OOMPa, an elongation above 10%, a YS / UTS ratio above 0,9, a bending angle above 55° and a hardness drop in the heat affected zone below 80Hv.

In a particular embodiment, the material used to manufacture at least part of the high slenderness part or the entire high slenderness part is steel comprising the following elements expressed in weight%:

C : 0.2 - 0.34 %

Mn: 0.50 - 2.20 %

Si: 0.5 - 2 % P < 0.020 %

S < 0.010 %

N < 0.010 % and comprising optionally one or more of the following elements, by weight percent:

Al: <0.2 %

Cr < 0.8 %

B < 0.005%

Ti < 0.06 %

Nb < 0.06 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting.

For example, the portion of said high slenderness part being made of said material has a microstructure comprising, in surface fraction:

95% or more of tempered martensite, and less than 5 % of bainite.

For example, the portion of said high slenderness part being made of said material has the following mechanical properties: an ultimate tensile strength TS higher than 1000 MPa, a fracture strain higher than 0.5, a bending angle higher than 55 ° and a hardness drop in the heat affected zone below 80Hv.

0.20% < C < 0.25%

1.1 % < Mn < 1.4%

0.15% < Si < 0.35%

Cr < 0.30%

0.020% < Ti < 0.060%

0.020% < Al < 0.060%

S < 0.005%

P < 0.025%

0.002% < B < 0.004% the remainder being iron and unavoidable impurities resulting from the elaboration.

The invention will now be illustrated by the following examples, which are by no way limitative.

The reaction to an impact having an angle on a high slenderness part made with different materials was simulated using LS-DYNA R11 .1 .0. The mesh size used is 3mm.

Referring to figure 1 , the simulated high slenderness part 1 is made by forming separately and then joining together the top part 3, which is a generally omega shaped part, and the bottom part 2, which is a flat closing plate by spot welding on flanges 6, which produces spot welds 5. The joining is performed by 20 spot welds on each side every 30mm along each flange. Each spot weld 5 has a 6mm diameter nugget and the heat affected zone is simulated by a 3mm ring around each nugget.

The high slenderness part 1 has the following dimensions:

-sheet metal thickness of 1 ,5mm before forming,

-length L of 600mm

-closing plate 2 having a total width in the transverse direction of 130mm, comprising two flanges 6 of 25mm each. Hence the width of the closing plate enclosing the hollow volume 4 is 130 - 2*25 = 80mm.

-height of the hollow volume 4 of 60mm

For simplicity sage, the slenderness factor below was calculated for a perfectly rectangular part having the same hollow volume 4 and the same sheet metal thickness. That is to say, the slenderness factor is calculated without taking into account the contribution of the flanges, which will be very minimal.

In the formula below, the factors b1 and b correspond respectively to the inner width (i.e. 60mm) and outer width (i.e. b=b1 +2*thickness = b1 +3mm) of the rectangular part, the factors hi and h correspond respectively to the inner height (i.e. 60mm) and outer height (i.e. h=h1 +2*thickness = h1 +3mm) of the rectangular part. The minimum quadratic moment is given by the formula:

Which computes as:

Imin = min(441881,75; 289491,75 )

Imin = 289491,75

The slenderness ratio is given by the following formula:

ln which the area of the straight section S = h*b - hi *b1

Which computes as

600

Slenderness ratio = -

I 289491,75 63 * 83 — 60 * 80

Slenderness ratio = 23.1

The described shape therefore results in a slenderness ratio of 23.1 .

The part 1 is fixed at one end E2 and impacted at its other end E1 by a flat impactor 7 travelling at an angle beta of 10° with the longitudinal direction Ldir and an initial impact velocity of 16m/s and having a mass of 417kg.

The results are expressed in terms of energy absorption, as provided directly by the software and in terms of deleted elements to represent the level of fracture resulting from the crash.

The behaviour of the spot welds under load was simulated applying the method developed in the Fosta 806 project: “P 806 - Characterization and simplified modeling of the fracture behavior of spot welds from ultra-high strength steels for crash simulation with consideration of the effects of the joints on component behaviour” (Fosta stands for “Forschungsvereinigung Stahlanwendung”, i.e. The Research Association for Steel Application). In order to dissociate the effect of the spot weld behaviour from the rest of the material behaviour, the simulations were done with and without taking into account the presence of spot welds (in table 1 the columns indicating “no weld” for line “Hardness drop in Heat affected zone” correspond to the simulation in which the weld behaviour was not taken into account). The failure behavior and associated deleted elements calculation is simulated using the material cards MAT123 and MAT_ADD_EROSION. Further explanation on the methodology can be found for example in “Simulation of Spot Welds and Weld Seams of Press-Hardened Steel (PHS) Assemblies”, Stanislaw Klimek, International Automotive Body Congress 2008.

The number of deleted elements is an evaluation of the amount of fracture that occurs during the crash. Because the failure modelling does not take into account the propagation of cracks, it can be said that the effect of fracture on the overall results is probably underestimated in the simulations and that in actual physical crash tests the energy absorption levels would probably be lower when the number of deleted elements are high because of failure propagation and eventual total failure of the part (such as for example the part being cut in two). It should be noted that such catastrophic failure is an issue for energy absorption but also for the overall behavior of the part in the predicted crash scenario of the vehicle. Indeed, it disrupts the anticipated load path and means that the different parts of the vehicle will travel in uncontrolled directions because they are not anymore joined together. This lack of control leads to unpredictable and catastrophic behavior of the vehicle during a crash.

Referring to table 1 , the evolution of the amount of absorbed energy and the number of deleted elements is represented throughout the crash scenario, taking sample points at 1/4 of the crash time, 1/2 of the crash time, 3/4 of the crash time and the end crash time. The inventive examples bear references 11 and I1w, corresponding respectively to the case in which the spot weld behavior was not taken into account and in which it was taken into account. The reference examples, corresponding to cases outside the invention, are termed R1 , R1w, R2, R2w, R3 and R3w.

The inventive examples show the highest amount of energy absorption and the lowest number of deleted elements, i.e. the most favorable response.

R1 , R1w have lower YS/LITS ratio and lower bending angle than the invention. This results in a combination of lower energy absorption and higher amount of fracture (number of deleted elements).

R2, R2w have lower YS/LITS ratio than the invention. This results in a lower energy absorption. Referring to figures 2 and 3, which are respectively top and side views of the crash simulation at 1/4, 1/2, 3/4 and the full crash time of 11 and R2, the angle of the impactor 7 results in eventual bending of the part 1 starting at 3/4 crash time. This bending translates in a lower amount of energy absorption for R2. Actually, 11 and R2 have very similar energy absorption levels at 1/4 time and 1/2 time - they start to deviate from one another at 3/4 time, which corresponds to the onset of bending in R2. On the other hand the high slenderness part of 11 continues to deform by bottling until the end of the crash.

R3, R3w have lower bending angles than the invention, which results in a significantly higher number of deleted elements. When comparing the examples with and without the effect of the weld spots, it is apparent that an important hardness drop in the heat affected zone has a detrimental effect on the energy absorption. This is consistently the case for R1 vs R1w, R2 vs R2w and R3 vs R3w, which all have an estimated hardness drop in the heat affected zone of 200Hv. The resulting decrease in energy absorption ranges from 0,4kJ for R1 vs R1w to 2,4kJ for R2 vs R2w.

Table 1: crash simulation results

Claims

1. A part made from materials having an ultimate tensile strength after forming higher than 1000MPa, wherein:

-the ratio between the Yield Strength YS and the ultimate tensile strength UTS of the materials is higher than 0,85

-the bending angle normalized to 1 ,5mm thickness of the materials is higher than 55°

-the slenderness ratio of the part is higher than 10.

2. Part according to claim 1 , wherein the bending angle normalized to 1 ,5mm thickness of the materials used to manufacture said part is higher than 70°.

3. Part according to claim 1 or 2, wherein the part is made of several individual parts assembled together by welding and wherein the hardness drop in the heat affected zones associated to said welding operation is less than 100Hv.

4. Part according to any one of claims 1 to 3, wherein the material to manufacture said part is steel.

5. Part according to claim 4 comprising at least one portion made by hot stamping of steel.