WO2024105433A1

WO2024105433A1 - Adhesive bonding assembly of phs coated steel part coating and method to manufacture thereof

Info

Publication number: WO2024105433A1
Application number: PCT/IB2022/061018
Authority: WO
Inventors: Frida Gilbert; Maxime BROSSARD; Tiago MACHADO AMORIM
Original assignee: Arcelormittal
Priority date: 2022-11-16
Filing date: 2022-11-16
Publication date: 2024-05-23

Abstract

An assembly of several parts by crash adhesive bonding, wherein at least one of said parts is a press hardened steel part obtained by press hardening of a steel sheet provided on at least one of its surfaces with a coating, said coating comprising by weight percent, from 7.0 to 9.0 % of zinc, from 1.0 to 10.0 % of silicon, from 1.0 to 10.0 % of magnesium, up to 3.0% of iron as residual element, and optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, the balance being aluminum.

Description

Adhesive bonding Assembly of PHS coated steel part coating and method to manufacture thereof

The present invention deals with the assembly of press hardened parts by adhesive bonding.

In recent years the use of coated steels in hot-stamping processes for the shaping of parts has become important, especially in the automotive industry. Fabrication of such parts may include the following main steps:

- Coating of a steel sheets, by hot dipping

- Trimming or cutting for obtaining blanks

- Heating the blanks in order to obtain alloying of the steel substrate with the coating by diffusion, as well as the austenitizing of the steel

- Press hardening of the part in order to obtain a predominantly martensitic structure.

Thanks to the diffusion of iron from the steel substrate through the coating, intermetallic alloys with high melting temperature are generated. The blanks having such coating may be heated in a temperature range where austenitizing of the metallic substrate takes place, allowing further hardening by quenching.

Steel press hardened parts intended for the manufacture of automobiles can be deep drawn at high temperatures and are quenched in the forming tools to reach the targeted microstructure. In terms of material properties, tensile strength from 500 to 2000 MPa and tensile elongation from 5 to 15 % can be achieved. Press hardening allows to produce parts for energy absorption and crash management in automotive structures.

Adhesive bonding in automotive structure allows to joint automotive parts together. Crash adhesives are specifically designed to make the loads paths predictable in automotive structures in the event of a crash.

Therefore, press hardened parts need a surface that makes them adherent. As a crash may occur many years after vehicle production, mechanical properties of the assembly must be ensured also after ageing. Hardened parts can be coated with zinc-based coating or aluminum-based coating.

Zinc-based coatings are generally used because they allow a protection against corrosion thanks to barrier protection and cathodic protection. However, when heating steps are performed on such zinc coated steel sheets, for example press hardening or welding, cracks are observed in steel which spread from the coating. Indeed, occasionally, there is a reduction of metal mechanical properties due to the presence of cracks in coated steel sheet after heating steps. These cracks appear with the following conditions: high temperature; contact with a liquid metal having a low melting point (such as zinc) in addition to stress; heterogeneous diffusion of molten metal with substrate grain bulk and boundary. The designation for such phenomenon is liquid metal embrittlement (LME).

Aluminum-based coatings are not subject to LME. However, in real vehicle life, the crash may happen after years of usage. This the reason why the bonded assembly are tested after ageing. The mechanical performance of a bonded assembly with standard silicon containing Aluminum-based coating deteriorates with ageing.

The aim of the present invention is to provide a coating for press hardened parts, such coating ensuring good mechanical properties of assemblies bonded with crash adhesives, even after ageing.

The present invention is well suited for the manufacture modules or submodules of an automotive structure which play a role during crash. Crash modules can be for example front module, front end module, rear module, rear end module, side doors, under body or upper body.

According to the invention, the failure pattern of the assembly after ageing has up to 35% adhesive failure, more than 50% cohesive failure and up to 20% red rust in terms of area portion, and a loss in maximal shear stress value of less than 50%.

This is achieved by the assembly of claims 1 to 3. Another object of the invention is to provide a manufacturing method for such an assembly according to claim 4.

Eventually, the last object of this invention is the use of such an assembly for the manufacturing of an automobile according to claims 5 to 7.

To illustrate the invention, figure 1 illustrates the mechanical shear test performed on the bonded specimen to assess the maximal shear stress value of the assembly.

The invention relates to an assembly of several parts by crash adhesive bonding, wherein at least one of said parts is a press hardened steel part 11 obtained by press hardening of a steel sheet provided on at least one of its surfaces with a coating, said coating comprising by weight percent, from 7.0 to 9.0 % of zinc, up to 3.0 % of iron as residual element, and unavoidable impurities up to 0.02 %, the balance being aluminum.

Preferably, the coating comprises, in weight percent, from 7.5 to 8.5 % of zinc.

The coating comprises from 1 .0 to 10.0 % of silicon, and from 1 .0 to 10.0 % of magnesium.

Preferably, the coating comprises, in weight percent, from 1.0 to 4.0 % of silicon and from 1.0 to 4.0 % of magnesium, advantageously from 2.5 to 3.5 % of silicon and from 1 .5 to 3.0 % of magnesium.

Optionally, the coating comprises up to 3.0 % weight iron, when the coating is applied by hot dip coating. Iron comes from the dissolution of the steel sheet in the hot dip coating bath and can vary during production.

Optionally, the coating comprises additional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the content by weight of each additional element being inferior to 0.3 wt.%.

In a preferred embodiment, up to 100 ppm in weight of calcium is added.

Finally, the coating may contain unavoidable impurities up to 0.02 wt.%, preferably up to 0.01 wt.%. At least one of the assembled parts has surface oxides coming from the process hardening process. It is believed that these oxides, depending on both the coating composition and the heat treatment are of major importance for the crash adhesive assembly performance. The surface of the part assembled according to the invention is covered by an oxide layer comprising iron coming from the steel substrate by diffusion, aluminum and zinc oxides and silicon and magnesium oxides from the coating.

The steel sheet used to manufacture the press hardened part can be manufactured by hot dip galvanizing in a bath, the temperature of which is set from 600 to 700°C, preferably from 620 to 650°C.

The coating weight is set during the wiping process by gas knives in a range from 50 to 500 g/m², possibly from 80 to 150 g/m² and preferably from 90 to 120 g/m² for the sum of both sides of the steel sheet.

Before being coated, the steel sheet according to the invention can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be for example between 0.5 and 3.0 mm, preferably from 1.0 to 2.0 mm.

The method according to the invention comprises the following steps:

A) the provision of a steel sheet 11 coated with a metallic coating comprising, by weight percent, from 7.0 to 9.0 % of zinc, from 1.0 to 10.0 % of silicon, from 1.0 to 10.0 % of magnesium, and optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, up to 3.0% of iron as residual element, and unavoidable impurities up to 0.02 %, the balance being aluminum,

B) the cutting of the coated steel sheet to obtain a blank,

C) the thermal treatment of the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel,

D) the transfer of the blank into a press tool,

E) the press-hardening of the blank to obtain a part, including a cooling step in order to obtain a part, F) the application of a crash adhesive bonding film 13 to said press-hardened steel part and at least one other part 12 to obtain an assembly,

G) the curing of the crash adhesive bonding.

In step A), any steel can be advantageously used in the frame of the invention. However, in case steel having high mechanical strength is needed, in particular for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heat-treatment, can be used. The weight composition of steel sheet is preferably as follows: 0.03% < C < 0.50% ; 0.3% < Mn < 3.0% ; 0.05% < Si < 0.8% ; 0.015% < Ti < 0.2% ; 0.005% < Al < 0.1 % ; 0% < Or < 2.50% ; 0% < S < 0.05% ; 0% < P< 0.1 % ; 0% < B < 0.010% ; 0% < Ni < 2.5% ; 0% < Mo < 0.7% ; 0% < Nb < 0.15% ; 0% < N < 0.015% ; 0% < Cu < 0.15% ; 0% < Ca < 0.01 % ; 0% < W < 0.35%, the balance being iron and unavoidable impurities from the manufacture of steel.

For example, the steel sheet is 22MnB5 with the following weight composition: 0.20% < C < 0.25%; 0.15% < Si < 0.35%; 1.10% < Mn < 1.40%; 0% < Or < 0.30%; 0.020% < Ti < 0.060%; 0.020% < Al < 0.060%; 0.002% < B < 0.004%, the remainder being iron and unavoidable impurities from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.24% < C < 0.38%; 0.40% < Mn < 3%; 0.10% < Si < 0.70%; 0.015% < Al < 0.070%; Or < 2%; 0.25% < Ni < 2%; 0.015% < Ti < 0.10%; Nb < 0.060%; 0.0005% < B < 0.0040%; the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

Alternatively, the steel sheet can have the following weight composition: 0.30% < C < 0.40%; 0.5% < Mn < 1.0%; 0.40% < Si < 0.80%; 0.1 % < Or < 0.4%; 0.1 % < Mo < 0.5%; 0.01 % < Nb < 0.1 %; 0.01 % < Al < 0.1 %; 0.008% < Ti < 0.003%; 0.0005% < B < 0.003%; 0.0% < P < 0.02%; 0.0% < Ca < 0.001 %; 0.0% < S < 0.004 %; 0.0% < N < 0.005 %, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.040% < C < 0.100%; 0.80% < Mn < 2.00%; 0% < Si < 0.30%; 0% < S < 0.005%; 0% < P < 0.030%; 0.010% < Al < 0.070%; 0.015% < Nb < 0.100%; 0.030% < Ti < 0.080%; 0% < N < 0.009%; 0% < Cu < 0.100%; 0% < Ni < 0.100%; 0% < Cr < 0.100%; 0% < Mo < 0.100%, the balance being iron and unavoidable impurities from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.06% < C < 0.1 %, 1 % < Mn < 2%, Si < 0.5%, Al <0.1 %, 0.02% < Cr < 0.1 %, 0.02%

< Nb < 0.1 %, 0.0003% < B < 0.01 %, N < 0.01 %, S < 0.003%, P < 0.020% less than 0,1 % of Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.015% < C < 0.25%; 0.5% < Mn < 1.8%; 0.1 % < Si < 1.25%; 0.01 % < Al < 0.1 %; 0.1 % < Cr < 1 .0%; 0.01 % < Ti < 0.1 %; 0% < S < 0.01 %; 0.001 % < B < 0.004%; 0%

< P < 0.020%; 0% < N < 0.01 %; the balance being iron and unavoidable impurities from the manufacture of steel.

Alternatively, the steel sheet has the following weight composition: 0.2% < C

< 0.34%; 0.5% < Mn < 1 .24%; 0.5% < Si < 2.0%; 0% < S < 0.01 %; 0% < P < 0.020%; 0% < N < 0.01 %, the balance being iron and unavoidable impurities from the manufacture of steel.

The steel sheet is cut into a blank in step B. Said coated steel blank may have a thickness which is not uniform. This is the case of the so-called “tailored rolled blanks” which are obtained from cutting a sheet obtained by a process of rolling with an effort which is variable along the direction of the length of the sheet. Or this may be also the case of the so-called “tailored welded blanks” obtained by the welding of at least two sub-blanks of different thicknesses.

In step C, a heat treatment of the blank is performed at a temperature from 840 to 950°C. Said blank is maintained during a dwell time from 3 to 10 minutes to have a full austenitic structure. During the heat treatment, the coating forms an alloy layer having a high resistance to corrosion and abrasion.

In step E, the part is cooled in the press hardening tool or after the transfer to a specific cooling tool. The cooling rate is controlled depending on the steel composition, in such a way that the final microstructure after press hardening is consistent with the targeted mechanical properties. After press hardening, the part can be tempered to reach the targeted microstructure and mechanical properties.

In a preferred embodiment, the steel microstructure comprises, in terms of volume fraction, at least 95% of martensite.

In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, at least 50% of martensite and less than 40 % of bainite.

In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, from 5 to 20 % of martensite, up to 10 % of bainite and at least 75 % of equiaxed ferrite.

The crash adhesive in the assembly of step F has preferably a film thickness from 0.05 to 0.5 mm, more preferably from 0.1 to 0.3 mm.

In step G, the crash adhesive is preferably cured in an oven, for example from 10 to 35 minutes at a temperature from 80 to 200°C, depending on the composition of the adhesive.

Crash adhesive must have a predictable behavior in case of failure of the assembly. The predictability is assessed by the failure pattern: crash adhesive must remain on the bonded parts after failure. In other words, the fracture pattern should be cohesive failure, meaning that the failure occurred inside the adhesive layer.

If the failure is not cohesive, either the tear appears at the interface between the adhesive and the surface of the coated steel in the case of an adhesive failure, or the tear appears at the steel/coating interface, meaning that the Al-based coating is peeled off from the steel sheet by the adhesive in the case of a delamination failure. In both cases, the way the assembly will behave in case of crash is not as predictable as in the case of a cohesive failure.

In real conditions, the crash may happen on a vehicle after years of usage. This the reason why the bonded assembly are tested after ageing.

After ageing, the predictability in case of failure relies on the cohesive failure pattern, but also on the loss in maximal shear stress value compared to said maximal shear stress before ageing. If located in wet area, the corrosion of the bonded part during ageing must be limited as well. The inventors have found that, when there is less than 7.0 % by weight of zinc in the coating, the assembly after ageing exhibits the following poor performances, taken alone or in combination: the crash adhesive failure can be of 35% or more, the cohesive failure can be lower than 50%, the red rust can be of more than 20 %, the maximal shear stress loss can be of 50% or more. This makes the failure of the assembly not predictable enough during the vehicle life.

If the zinc content is above 9.0 wt % by weight in the coating, the ability to be painted or other in use-properties, such as corrosion, will not be sufficient.

The invention will now be illustrated by tests as an illustration and not as a limitation.

Example 1 : Crash Adhesive bonding test after ageing

To assess the compatibility of crash adhesives with press-hardened parts coated with an Al-based coating, several samples were produced from 22MnB5 steel sheets. The composition of the steel is as follows: C = 0.23 %; Mn = 1.2%; Si = 0.25%; %; Cr = 0.2%; Al = 0.04%; Ti = 0.04%; B = 0.003 %.

All coatings were deposited by hot-dip galvanization process. For the samples according to the invention, the bath temperature was set from 620 to 650 °C. The alloying elements in weight percent of coating composition is given in table 1 , the balance being aluminum.

Then test pieces 100 mm long and 25 mm wide were cut to perform an evaluation of the crash adhesive coating performance by a single lap shear test. After cutting, the samples were press-hardened. Two test samples of the same material were then bonded with crash adhesive by overlapping them on a width of 10 mm using beads (diameter: 200pm) to ensure adhesive thickness film of 0.2 mm. Four different crash adhesives were tested in the same modalities: BETAMATE® 1440G (BM 1440), and BETAMATE®1480V203G (BM 1480) from Dupont.

All assemblies were cured in the oven for 20 minutes at 180°C. The samples were then conditioned for 24h before ageing.

Assemblies from trials were submitted to ageing in climatic chamber according to standard VDA 621 -415. The adhesion has been assessed according to DIN EN 1465 standard. In this test, each bonded assembly is fixed in the clamping jaws (gripping 50mm of each test piece in each clamp and leaving 50mm of each test piece free) of a tensile machine using cell force of 50KN. The samples are pulled at a rate of 10 mm/min, at room temperature. The maximal shear stress values are recorded in MPa. The principle of the shear test of a bonded assembly is shown on figure 1 .

The failure pattern is visually classified as:

- AF for Adhesive Failure where the failure occurred at the interface between the adhesive and the surface of the coated steel,

- CF for cohesive failure where the failure occurred inside the adhesive layer,

- DF for delamination failure where the Al-based coating is peeled off by the adhesive from the substrate,

- RR for red rust.

Each failure classification corresponds to a specific appearance of the bonded surface after mechanical test. They are then quantified in proportion of the overall failure pattern area.

The maximal shear stress value is compared with the same value obtained by mechanical before ageing. The loss after ageing is expressed in percent.

For each test condition, 5 assemblies were tested. The result is the mean values of the 5 assemblies.

Results are gathered in table 1 .

Table 1 : Crash adhesive bonding test after ageing:

trials according to the invention, underlined values are not according to the invention.

Claims

1 . An assembly of several parts by crash adhesive bonding, wherein at least one of said parts is a press hardened steel part obtained by press hardening of a steel sheet provided on at least one of its surfaces with a coating, said coating comprising by weight percent, from 7.0 to 9.0 % of zinc, from 1 .0 to 10.0 % of silicon, from 1 .0 to 10.0 % of magnesium, up to 3.0% of iron as residual element, and optional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi , the content by weight of each element being less than 0.3%, and unavoidable impurities up to 0.02 %, the balance being aluminum.

2. An assembly according to claim 1 , wherein said press hardened part is covered by an oxide layer comprising iron coming from the steel substrate by diffusion, aluminum, and zinc oxides, and silicon and magnesium oxides from the metallic coating.

3. An assembly according to claims 1 or 2, wherein said coating comprises, by weight percent, from 7.5 to 8.5 % of zinc, 1 .0 to 4.0 % of silicon, 1 .0 to 4.0 % of magnesium, up to 3.0% of iron, and optional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the content by weight of each element being less than 0.3%, and unavoidable impurities up to 0.02%, the balance being aluminum.

4. A method for the manufacture of an assembly of several parts by crash adhesive bonding, according to the following steps:

A) the provision of a steel sheet coated with a coating comprising, by weight percent, from 7.0 to 9.0 % of zinc, from 1 .0 to 10.0 % of silicon, from 1 .0 to 10.0 % of magnesium, and optional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi , the content by weight of each element being less than 0.3%, up to 3.0% of iron as residual element, and unavoidable impurities up to 0.02 %, the balance being aluminum,

B) the cutting of the coated steel sheet to obtain a blank, C) the thermal treatment of the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel,

D) the transfer of the blank into a press tool,

E) the press-hardening of the blank to obtain a part (11 ), including a cooling step in order to obtain a part,

F) the application of a crash adhesive bonding film (13) to said press-hardened steel part and at least one other part (12) to obtain an assembly,

G) the curing of the crash adhesive bonding.

5. Use of an assembly according to anyone of claims 1 to 3 or obtained according to claim 4, for the manufacture of automotive vehicle.

6. The use of an assembly according to claim 5 to manufacture at least one of the crash modules of an automotive structure chosen from among: front module, front end module, rear module, rear end module, side doors, under body or upper body.

7. The use of an assembly according to claim 6 to manufacture at least one of the crash modules of an automotive structure located in wet areas of an automotive structure, said modules being chosen from among: front module, front end module, rear module, rear end module, side doors, under body or upper body.