Adhesively bonded assemblies of PHS coated steel parts and their manufacturing methods.

Abstract

Assembly of several parts by collision adhesive bonding, at least one of the parts being a press-hardened steel part obtained by press-hardening a steel sheet provided with a coating on at least one of its surfaces, the coating containing, in percentages by weight, 7.0 to 9.0% zinc, 1.0 to 10.0% silicon, 1.0 to 10.0% magnesium, with up to 3.0% iron as a residual element and optional elements selected from Pb, Ni, Zr or Hf, each of which has a weight content of less than 0.3%, the remainder being aluminum.

Description

[Technical Field]

[0001] The present invention deals with the assembly of press-hardened parts with adhesive bonding. [Background technology]

[0002] In recent years, the use of coated steel in hot stamping processes for forming parts has become important, especially in the automotive industry. The manufacture of such parts can include the following main steps: - Hot-dipping coating of steel sheets; - a trimming or cutting step to obtain blanks; - heating the blank to obtain alloying of the steel substrate with the coating by diffusion and austenitization of the steel; - Press hardening process of parts to obtain a primarily martensitic structure.

[0003] Diffusion of iron from the steel substrate through the coating produces an intermetallic alloy with a high melting temperature. Blanks bearing such coatings can be heated in the temperature range where austenitization of the metal substrate occurs, allowing for further hardening by quenching.

[0004] Steel press-hardened parts intended for automotive manufacturing can be deep-drawn at high temperatures and quenched in the forming tool to achieve the desired microstructure. Regarding material properties, tensile strengths of 500 to 2000 MPa and tensile elongations of 5 to 15% can be achieved. Press-hardening makes it possible to manufacture components for energy absorption and crash management in automotive structures.

[0005] Adhesive bonds in automotive structures allow automotive parts to be joined together. Crash adhesives are specifically designed to provide predictable load paths in automotive structures in the event of a crash.

[0006] Press-hardened parts therefore require a surface that makes them sticky. As collisions can occur many years after the vehicle is manufactured, the mechanical properties of the assembly must be guaranteed even after aging.

[0007] The hardened parts can be coated with a zinc-based or aluminum-based coating.

[0008] Zinc-based coatings are commonly used because they provide protection against corrosion thanks to their barrier and cathodic protection. However, when such zinc-coated steel sheets are subjected to a heating process, such as press hardening or welding, cracks propagate from the coating into the steel. In fact, the presence of cracks in coated steel sheets after a heating process sometimes leads to a decrease in the mechanical properties of the metal. These cracks appear under the following conditions: high temperature, stress, and contact with a liquid metal (such as zinc) with a low melting point, and inhomogeneous diffusion of the molten metal with the substrate particle bulk and boundaries. The name for this phenomenon is liquid metal embrittlement (LME).

[0009] Aluminum-based coatings are not susceptible to LME. However, in the actual life of a vehicle, a crash may occur after several years of use. This is why bonded assemblies are tested after aging. The mechanical performance of bonded assemblies with standard silicon-containing aluminum-based coatings deteriorates with aging. Summary of the Invention [Problem to be solved by the invention]

[0010] It is an object of the present invention to provide a coating for press-hardened parts, which coating ensures good mechanical properties of assemblies bonded with impact adhesives, even after aging.

[0011] The present invention is well suited to manufacturing modules or sub-modules of a vehicle structure that play a role in a crash. The crash module can be, for example, a front module, a front-end module, a rear module, a rear-end module, a side door, an underbody or an upper body.

[0012] According to the present invention, the failure pattern of the assembly after aging includes adhesive failure of up to 35% by area, cohesive failure of more than 50%, red rust of up to 20%, and loss of maximum shear stress value of less than 50%. [Means for solving the problem]

[0013] This is achieved by the assemblies of claims 1-3.

[0014] Another object of the present invention is to provide a method for manufacturing an assembly as claimed in claim 4.

[0015] Finally, a final object of the invention is the use of such an assembly for the manufacture of a motor vehicle according to claims 5-7. [Brief explanation of the drawings]

[0016] [Figure 1] To illustrate the invention, FIG. 1 shows mechanical shear tests performed on bonded samples to evaluate the maximum shear stress value of the assembly. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention relates to an assembly of several parts by collision adhesive bonding, at least one of which is a press-hardened steel part 11 obtained by press-hardening a steel sheet provided with a coating on at least one of its surfaces, the coating containing, in weight percent, 7.0 to 9.0% zinc, with up to 3.0% iron as residual elements and up to 0.02% unavoidable impurities, the remainder being aluminum.

[0018] Preferably, the coating contains 7.5 to 8.5% zinc by weight.

[0019] The coating contains 1.0 to 10.0% silicon and 1.0 to 10.0% magnesium.

[0020] Preferably, the coating contains, in percentages by weight, 1.0 to 4.0% silicon and 1.0 to 4.0% magnesium, advantageously 2.5 to 3.5% silicon and 1.5 to 3.0% magnesium.

[0021] Optionally, if the coating is applied by hot-dip coating, the coating contains up to 3.0 wt. % iron, which comes from dissolution of the steel sheet in the hot-dip coating bath and can vary during manufacturing.

[0022] Optionally, the coating comprises an additional element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the weight content of each additional element is less than 0.3 wt.%.

[0023] In a preferred embodiment, up to 100 ppm by weight of calcium is added.

[0024] Finally, the coating may contain up to 0.02% by weight, preferably up to 0.01% by weight, of unavoidable impurities.

[0025] At least one of the assembled components has surface oxides resulting from the process curing process. These oxides are believed to be critical to the performance of the crash adhesive assembly, depending on both the coating composition and heat treatment. The surface of the assembled component according to the present invention is covered by an oxide layer containing oxides of iron, aluminum, and zinc derived from the steel substrate by diffusion, and oxides of silicon and magnesium derived from the coating.

[0026] The steel sheets used to manufacture press-hardened parts can be produced by hot-dip galvanizing in a bath, the temperature of which is set at 600-700°C, preferably 620-650°C.

[0027] The coating weight is 50-500g / m², applied by gas knife during the wiping process, for both sides of the steel sheet. 2 , 80-150g / m 2 , preferably 90 to 120 g / m 2 It is set in the range.

[0028] Before coating, 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 between 1.0 and 2.0 mm.

[0029] The method according to the present invention comprises the following steps: A) providing a steel sheet 11 coated with a metallic coating containing, in weight percent, 7.0-9.0% zinc, 1.0-10.0% silicon, 1.0-10.0% magnesium, optional elements selected from Pb, Ni, Zr, and Hf, each of which has a weight content of less than 0.3%, the remainder being up to 3.0% iron, and unavoidable impurities up to 0.02%, the remainder being aluminum; B) cutting the coated steel sheet to obtain blanks; C) heat treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel; D) Transferring the blank to a press tool; E) press hardening the blank to obtain a part, including a cooling step to obtain the part; F) applying an impact adhesive bonding film 13 to the press hardened steel part and at least one other part 12 to obtain an assembly; G) Curing process of impact adhesive bond.

[0030] In step A), any steel can be advantageously used for the frame of the invention. However, when steel with high mechanical strength is required, in particular for structural parts of automobiles, steels with a tensile strength of better than 500 MPa, advantageously between 500 and 2000 MPa before or after heat treatment, can be used. The steel plate has the following composition by weight: 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%≦Cr≦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%, and the remainder being unavoidable impurities from the production of iron and steel.

[0031] For example, the steel sheet may have the following composition by weight: 0.20%≦C≦0.25%, 0.15%≦Si≦0.35%, 1.10%≦Mn≦1.40%, 0%≦Cr≦0.30%, 0.020%≦Ti≦0.060%, 0.020%≦Al≦0.060%, 0.002%≦B≦0.004%, with the remainder being 22MnB5, which are unavoidable impurities from the production of iron and steel.

[0032] In another embodiment the steel sheet has the following composition by weight: 0.24%≦C≦0.38%, 0.40%≦Mn≦3%, 0.10%≦Si≦0.70%, 0.015%≦Al≦0.070%, Cr≦2%, 0.25%≦Ni≦2%, 0.015%≦Ti≦0.10%, Nb≦0.060%, 0.0005%≦B≦0.0040%, the remainder being unavoidable impurities from the production of iron and steel.

[0033] Alternatively, the steel plate may have the following composition by weight: 0.30%≦C≦0.40%, 0.5%≦Mn≦1.0%, 0.40%≦Si≦0.80%, 0.1%≦Cr≦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 unavoidable impurities from the production of iron and steel.

[0034] In another embodiment, the steel sheet has the following composition by weight: 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%, with the remainder being unavoidable impurities from the production of iron and steel.

[0035] In another embodiment the steel sheet has the following composition by weight: 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% Cu, Ni and Mo, the balance being unavoidable impurities from the production of iron and steel.

[0036] In another embodiment the steel sheet has the following composition by weight: 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 remainder being unavoidable impurities from the production of iron and steel.

[0037] Alternatively, the steel plate has the following composition by weight: 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 remainder being unavoidable impurities from the production of iron and steel.

[0038] In step B, the steel sheet is cut into blanks. The coated steel blanks may have a non-uniform thickness. This is the case for so-called "tailored rolled blanks", which are obtained by cutting a sheet obtained by a process of rolling with a variable force along the length of the sheet. Alternatively, this may be the case for so-called "tailored welded blanks", which are obtained by welding at least two sub-blanks of different thickness.

[0039] In step C, the blank is heat treated at a temperature of 840-950°C. The blank is maintained for a dwell time of 3-10 minutes to obtain a fully austenitic structure. During the heat treatment, the coating forms a highly corrosion- and wear-resistant alloy layer.

[0040] In step E, the part is cooled in the press hardening tool or after transfer to a specific cooling tool. The cooling rate is controlled depending on the steel composition so that the final microstructure after press hardening matches the target mechanical properties. After press hardening, the part can be tempered to reach the target microstructure and mechanical properties.

[0041] In a preferred embodiment, the steel microstructure comprises at least 95% martensite, in terms of volume fraction.

[0042] In another embodiment, the steel microstructure, after press hardening, comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

[0043] In another embodiment, the steel microstructure, after press hardening, comprises, by volume fraction, 5-20% martensite, up to 10% bainite, and at least 75% equiaxed ferrite.

[0044] The collision adhesive in the assembly of step F preferably has a film thickness of 0.05 to 0.5 mm, more preferably 0.1 to 0.3 mm.

[0045] In step G, the collision adhesive is preferably cured in an oven at a temperature of, for example, 80-200° C. for 10-35 minutes depending on the composition of the adhesive.

[0046] Crash adhesives must have predictable behavior when an assembly fails. Predictability is measured by the failure pattern; after failure, the crash adhesive must remain on the joined parts. In other words, the failure pattern should be cohesive, meaning that the failure occurs inside the adhesive layer.

[0047] If the failure is not cohesive, then in the case of adhesive failure, the crack appears either at the interface between the adhesive and the coated steel surface, or at the steel / coating interface, which means that in the case of delamination failure, the Al-based coating is peeled off from the steel sheet by the adhesive. In both cases, the way the assembly behaves in the event of a crash is not as predictable as in the case of cohesive failure.

[0048] In real-life conditions, crashes can occur in vehicles after several years of use, which is why bonded assemblies are tested after aging.

[0049] After aging, the predictability of failure cases depends not only on the cohesive failure pattern, but also on the loss of maximum shear stress values ​​compared to said maximum shear stress before aging. Corrosion of joined parts during aging must also be limited if they are located in wetted areas.

[0050] The inventors have found that when less than 7.0 wt. % zinc is present in the coating, the aged assembly exhibits the following poor performance, either alone or in combination: impact adhesive failure may be 35% or greater, cohesive failure may be less than 50%, red rust may be greater than 20%, and maximum shear stress loss may be 50% or greater, which makes the assembly failure unpredictable over the life of the vehicle.

[0051] If the zinc content exceeds 9.0% by weight in the coating, the paint or other performance in use properties such as corrosion may not be sufficient. [Example]

[0052] The present invention will now be described by way of example and not by way of limitation, with reference to the following tests.

[0053] Example 1: Impact adhesive bond testing after aging To evaluate the compatibility of the crash adhesive with press-hardened parts coated with an Al-based coating, several specimens were prepared from 22MnB5 steel plate with the following composition: C = 0.23%, Mn = 1.2%, Si = 0.25%, Cr = 0.2%, Al = 0.04%, Ti = 0.04%, B = 0.003%.

[0054] All coatings were deposited by a hot dip galvanizing process. For the samples according to the invention, the bath temperature was set at 620-650°C. The alloying elements in weight percent of the coating composition are shown in Table 1, with the remainder being aluminum.

[0055] Next, test pieces 100 mm long and 25 mm wide were cut and subjected to a single lap shear test to evaluate the coating performance of the impact adhesive.

[0056] After cutting, the samples were press-cured. Two test samples of the same material were then joined with collision adhesive by overlapping them to a width of 10 mm using beads (diameter: 200 μm), ensuring a film thickness of 0.2 mm.

[0057] Four different impact adhesives, namely BETAMATE® 1440G (BM1440), and BETAMATE® 1480V203G (BM1480) (manufactured by DuPont), were tested in the same manner.

[0058] The entire assembly was cured in an oven for 20 minutes at 180° C. The samples were then conditioned for 24 hours before aging.

[0059] The assemblies from the trials were subjected to ageing in a climatic chamber according to standard VDA 621-415.

[0060] Adhesion was evaluated according to the DIN EN 1465 standard. In this test, each bonded assembly is fixed in the clamping jaws of a tensile machine with a cell force of 50 KN (each clamp grips 50 mm of each specimen, leaving 50 mm of each specimen free). The specimens are pulled at room temperature at a rate of 10 mm / min. The maximum shear stress value is recorded in MPa. The principle of the shear test of bonded assemblies is shown in Figure 1.

[0061] The failure patterns are visually classified as follows: - AF for adhesive failure when failure occurs at the interface between the adhesive and the coated steel surface; - CF for cohesive failure when failure occurs inside the adhesive layer; - DF for peeling failure when the Al-based coating is peeled off from the substrate by the adhesive, - RR for red rust.

[0062] Each failure classification corresponds to a particular appearance of the bond surface after mechanical testing, which are then quantified in proportion to the total failure pattern area.

[0063] The maximum shear stress values ​​are compared with the same values ​​obtained mechanically before aging. The loss after aging is expressed as a percentage.

[0064] For each test condition, five assemblies were tested, and the results are the average of the five assemblies.

[0065] The results are summarized in Table 1.

[0066] [Table 1]

Claims

1. Assembly of several parts by collision adhesive bonding, at least one of said parts being a press-hardened steel part obtained by press-hardening a steel plate provided with a coating on at least one of its surfaces, said coating containing, in percentages by weight, 7.0 to 9.0% zinc, 1.0 to 10.0% silicon, 1.0 to 10.0% magnesium, with up to 3.0% of iron, optional elements selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi as residual elements, with a weight content of each element being less than 0.3%, and unavoidable impurities up to 0.02%, the remainder being aluminium.

2. 10. The assembly of claim 1, wherein the press-hardened part is covered with an oxide layer comprising oxides of iron, aluminum and zinc coming from the steel substrate by diffusion, and oxides of silicon and magnesium from the metal coating.

3. 3. An assembly according to claim 1 or 2, wherein the coating comprises, in weight percent, 7.5 to 8.5 wt.% zinc, 1.0 to 4.0 wt.% silicon, 1.0 to 4.0 wt.% magnesium, up to 3.0 wt.% optional elements selected from iron, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each element having a weight content of less than 0.3%, and up to 0.02% unavoidable impurities, the balance being aluminium.

4. A method for manufacturing an assembly of several parts by collision adhesive bonding, comprising the following steps: A) providing a steel sheet coated with a metallic coating containing, in weight percent, 7.0-9.0% zinc, 1.0-10.0% silicon, 1.0-10.0% magnesium, optional elements selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, each of which has a weight content of less than 0.3%, the remainder being iron up to 3.0%, and unavoidable impurities up to 0.02%, the remainder being aluminum; B) cutting the coated steel sheet to obtain blanks; C) heat treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel; D) transferring the blank to a press tool; E) press hardening the blank to obtain the part (11), including a cooling step to obtain the part; F) applying an impact adhesive bonding film (13) to said press hardened steel part and at least one other part (12) to obtain an assembly; G) Curing of the impact adhesive bond.

5. Use of an assembly according to any one of claims 1 to 3 or obtained according to claim 4 for the manufacture of a motor vehicle.

6. 6. Use of the assembly according to claim 5 for manufacturing at least one crash module of a motor vehicle structure selected from the group consisting of a front module, a front end module, a rear module, a rear end module, a side door, an underbody or an upper body.

7. 10. Use of the assembly of claim 6 for manufacturing at least one crash module of a vehicle structure located in a wetted area of ​​the vehicle structure, said module being selected from the group consisting of a front module, a front end module, a rear module, a rear end module, a side door, an underbody, or an upper body.

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