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    • 专利摘要:
    steel sheet, method for preparing a steel sheet, method for preparing a press-hardened part, hardened part and use of a press-hardened part. the invention relates to a steel sheet coated with an aluminum-based coating, which additionally comprises a second coating of zinc, the thickness of which is less than or equal to 1.1 µm; a method for producing the coated steel sheet; a method for producing a press-hardened part from the steel sheet; a press-hardened part, and the use of the press-hardened part.c
    公开号:BR112017016132B1
    申请号:R112017016132-0
    申请日:2016-01-22
    公开日:2021-06-29
    发明作者:Christian Allely；Eric Jacqueson；Daniel Chaleix
    申请人:Arcelormittal；
    IPC主号:
    专利说明:

    FIELD OF THE INVENTION
    [001] The invention relates to a method for preparing press-hardened steel parts manufactured from sheets coated with an aluminum-based coating and a zinc coating. The part has satisfactory characteristics with respect to phosphating and therefore exhibits satisfactory paint adhesion. In particular, it is intended for the manufacture of motor vehicle parts. BACKGROUND OF THE INVENTION
    [002] Press-hardened parts can be coated with an aluminum-based alloy that has satisfactory anti-corrosion and thermal properties. In general, the method for preparing these parts comprises acquiring a steel sheet, cutting the sheet to obtain a raw block, heating the raw block, press hardening followed by cooling to obtain hardening by martensitic or martensitic-bainitic transformation, so that the steel structure comprises at least 75% of exaxed iron, an amount of martensite greater than or equal to 5% and less than or equal to 20%, an amount of bainite less than or equal to 10%. The hardened parts thus obtained have very satisfactory mechanical characteristics.
    [003] In general, a paint film is added to the hardened parts by pressing, in particular, a cataphoresis layer. Phosphating is often carried out beforehand. In this way, phosphate crystals form on the surface of the hardened part to be coated, increasing paint adhesion and, in particular, adhesion of the cataphoresis layer.
    [004] The parts coated with an aluminum-based alloy are not amenable to phosphating, that is, no phosphate crystal is formed on the surface of the coating. The addition of a paint film is thus carried out directly without prior phosphating. The microroughness of the surface of parts coated with an aluminum-based alloy allows paint adhesion. However, under some conditions, the paint is not evenly distributed on the surface of the part, resulting in areas of red rust. Red rust appears in areas where the paint is deeply embedded in the aluminum-based coating.
    [005] Patent application no EP2270257 is known. It discloses a press hardening steel sheet comprising a first coating based on aluminium, and a second coating having a wurtzite structure, the compound being preferably zinc oxide (ZnO). A ZnO coating allows phosphating to be carried out after the press hardening method. However, in practice, the implementation of this method results in a relatively low degree of phosphate crystal coverage on the surface of the part, on the order of 20 to 70%. This degree of coverage is unacceptable due to the fact that it does not allow satisfactory paint adhesion to the surface of the part. DESCRIPTION OF THE INVENTION
    [006] The purpose of the present invention is to remedy the disadvantages of the prior art by providing a press-hardened part capable of phosphating, which therefore exhibits satisfactory ink adhesion, from a coated steel sheet. It aims to provide, in particular, a press-hardened part capable of being phosphated in order to obtain a high degree of phosphate crystal coverage on the surface of the part, in particular a degree of coverage greater than or equal to 80%.
    [007] It also refers to a method for preparing said steel sheet.
    [008] It also refers to a method for manufacturing a press-hardened part capable of phosphating, under advantageous productivity conditions.
    [009] It also refers to the use of a press-hardened part to manufacture motor vehicle parts.
    [010] For this purpose, the object of the invention is a steel sheet, according to claim 1. The steel sheet can also comprise functions according to claims 2 to 9.
    [011] An object of the invention is also a method for preparing a coated steel sheet according to claim 10. This method of preparation may also comprise functions of claims 11 to 13.
    [012] The object of the invention is also a method for preparing a press-hardened part according to claim 14. This method of preparation can also comprise functions according to claims 15 to 16.
    [013] The purpose of the invention is also a press-hardened part according to claim 17. This part can also comprise functions according to claims 18 to 20.
    [014] Finally, the purpose of the invention is the use of a press-hardened part according to claim 21.
    [015] Additional functions and advantages of the invention become evident upon reading the following description. BRIEF DESCRIPTION OF THE DRAWINGS
    [016] In order to illustrate the invention, various embodiments and tests will be described by way of non-limiting examples, in particular, with reference to the Figures they represent.
    [017] Figure 1 is a schematic representation of a coated steel sheet according to an embodiment of the invention.
    [018] Figure 2 is a schematic representation of a coated steel sheet according to another embodiment of the invention.
    [019] For Figures 1 and 2, the thickness of the layers shown is provided for illustration purposes only and cannot be considered a scale representation of the various layers.
    [020] Figures 3 and 4 show the actual size of two sides of hardened parts of steel sheets provided with a first aluminum-based coating having a thickness of 25 µm and on one side a second coating of zinc deposited by galvanization which has a thickness of 1 µm, 2 µm, and 3 µm.
    [021] Figure 5 shows a corrosion cycle for the VDA 231-102 standard. DESCRIPTION OF ACHIEVEMENTS OF THE INVENTION
    [022] The following terms are defined: - "zinc coating" means a coating that comprises primarily zinc and possibly impurities. Preferably, the coating contains zinc and possibly impurities, - "aluminium-based coating" means a coating comprising in composition by weight an amount of aluminum greater than 50%, preferably greater than or equal to 70%, more preferably greater or equal to 85%, advantageously greater than or equal to 88%. This coating can be alloyed or unalloyed, - "phosphate crystal coverage degree" is defined by a percentage. 0% means the surface of the part is not covered with phosphate crystals at all, 100% means the surface of the part is entirely covered.
    [023] For all Figures 1 to 5, and subsequent to the invention, the designation "steel" or "steel sheet" refers to the press hardening steel sheet that has a composition that allows the part to achieve a resistance to traction greater than or equal to 500 MPa, preferably greater than or equal to 1,000 MPa, advantageously greater than or equal to 1,500 MPa. The composition by weight of the 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% < 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%, the balance of the composition comprising iron and unavoidable impurities resulting from the process.
    [024] For example, the steel sheet is a 22MnB5 sheet that has 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% < Mo < 0.35% 0% < P < 0.025% 0% < S < 0.005% 0.020% < Ti < 0.060% 0.020% < Al < 0.060% 0.002% < B < 0.004%, with the balance of the composition being iron and unavoidable impurities resulting from the process.
    [025] The steel sheet can also be a Usibor®2000 sheet which 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% 0% < Cr < 2% 0.25% < Ni < 2% 0.020% < Ti < 0.10% 0% < Nb < 0.060% 0.0005% < B < 0.0040% 0.003% < N < 0.010% 0.0001% < S < 0.005% 0.0001% < P < 0.025% It is understood that the titanium and nitrogen content satisfies: Ti/N > 3.42, and that the carbon, manganese, chromium and silicon content satisfies: Mn Cr Si 2.6C +53 +13 +15 - 1.1% the composition optionally includes one or more of the following elements: 0.05% < Mo < 0 .65% 0.001% < W < 0.30% 0.0005% < Ca < 0.005%, with the balance of the composition being iron and unavoidable impurities resulting from the process.
    [026] In another example, the steel sheet is a Ductibor® 500 sheet which 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% 0% < Ca < 0.006%, the balance of the composition being iron and unavoidable impurities resulting from the process.
    [027] The steel sheets are produced by hot rolling and can optionally be cold rolled according to the desired thickness.
    [028] It is understood that, in the first place, the invention relates to a coated steel sheet. Figure 1 shows an embodiment of the sheet according to the invention. It is provided with an aluminum-based coating 2 which additionally comprises a second zinc coating 3 having a thickness less than or equal to 1.1 µm; preferably less than or equal to 1.0 µm; more preferably less than or equal to 0.7 µm and preferably less than or equal to 0.5 µm. Advantageously, the zinc coating has a thickness greater than or equal to 0.1 µm.
    [029] Without being limited to a theory, apparently, if these thickness conditions are not met, in particular, if the zinc thickness is too high, not all zinc is oxidized to zinc oxide (ZnO) during heat treatment . In fact, the oxygen present in the furnace will take a long time to diffuse into the zinc coating to completely oxidize it. Therefore, some of the zinc will remain in the liquid state for a long time, and the zinc still in the liquid state will diffuse into the aluminum-based coating and then into steel. Zinc that has diffused deep into the substrate can cause fragility and loss of adhesion of the upper layers, ie, the aluminum-based coating and the cataphoresis layer.
    [030] Furthermore, if the thickness conditions are not met, it is also possible that the microroughness of the surface will increase as the thickness of the zinc coating increases, thereby promoting areas of red rust after deposition of the cataphoresis layer.
    [031] Therefore, in most cases, if the degree of phosphate crystal coverage is low, there is a risk of poor adhesion of the cataphoresis layer. However, in some cases, although the degree of phosphate crystal coverage is high, there is a risk of loss of adhesion of the cataphoresis layer and poor red rust resistance under this cataphoresis layer.
    [032] Finally, there is a risk that zinc, which remains in a liquid state for a long time, will pollute the furnace in which the heat treatment takes place.
    [033] Aluminum-based coating is preferably performed by hot immersion in a bath of molten metal. In general, the bath comprises up to 3% iron and 9% to 12% silicon, with the balance being aluminum. The thickness of the layer is, for example, between 5 μm and 50 μm, preferably between 10 μm and 35 μm. During heat treatment prior to hot deformation, the coating forms an alloyed layer that exhibits high resistance to corrosion, abrasion, wear and fatigue.
    [034] Preferably, the product of the thickness of the aluminum-based coating and the thickness of the zinc coating is comprised between 2 and 25, preferably between 4 and 25, more preferably between 4 and 16, and advantageously between 6 and 13; the thickness of the zinc coating being less than or equal to 1.1 μm, preferably less than or equal to 1.0 μm, more preferably less than or equal to 0.7 μm and preferably less than or equal to 0.5 μm .
    [035] Without being limited to a theory, it is possible that if these conditions are not met, in particular, if the thickness of the aluminum-based coating is too high, the aluminum-based coating will remain for a long time in the liquid phase . Therefore, the ZnO layer formed at the beginning of the heat treatment and the aluminum-based coating will interact for a long time. This interaction can result in the reduction by aluminum of the layer from ZnO to Zn, and this process is governed by slow kinetics.
    [036] The zinc coating 3 can be deposited by any suitable method, for example by cementation, by galvanizing, or by sonic jet vapor deposition (JVD) under vacuum.
    [037] According to one embodiment, the zinc coating is deposited by galvanizing, for example, by immersion in a bath of zinc sulfate (ZnSO4). Advantageously, the temperature of bath T3 is less than 70 °C, T3 preferably ranges from 50 °C to 60 °C.
    [038] According to another embodiment, the zinc coating is deposited by cementation. When the zinc coating is deposited by cementation, it is done either by spraying or dipping. In this way, the steel sheet coated with an aluminum-based metal coating is immersed in a bath or sprinkled with a solution of sodium hydroxide (NaOH) and zinc oxide (ZnO). Advantageously, the immersion or spraying is carried out for a time t4 ranging from 1 to 20 seconds, t4 preferably ranging from 5 to 10 seconds. It is carried out at a T4 temperature ranging from 40 to 60 °C, T4 being preferably 60 °C.
    [039] Without being limited to a particular theory, the mixture of NaOH and ZnO allows the formation of sodium zincate (Na2ZnO2), according to the following reaction: 2NaOH + ZnO ^ Na2ZnO2 + H2O. Then, Na2ZnO2 will react with the aluminum present in the aluminum-based coating to form a zinc coating on the steel sheet coated with the aluminum-based coating according to the following reactions: 2Al + 3Na2ZnO2 + 2H2O ^ 3Zn + 2NaAlO2 + 4NaOH.
    [040] According to another embodiment, a zinc coating is deposited by sonic jet vapor deposition under vacuum. According to this embodiment, zinc metal vapor is generated by induction heating of a crucible containing a bath of zinc heated to a temperature T5 of at least 600 °C, preferably T5 is 700 °C, in a chamber of vacuum at a pressure P5 preferably between 6 Pa and 20 Pa (6 10-2e 2 10-1 mbar). The steam escapes from the crucible through a conduit that leads to an exit orifice, preferably calibrated, in order to form a sonic velocity jet directed to the surface of the substrate to be coated.
    [041] Advantageously, prior to the zinc deposition carried out by the JVD, a steel sheet provided with an aluminum-based coating is coated with a thin layer of stainless steel comprising by weight composition at least 10% of chromium, the balance is iron, additional elements such as nickel, carbon, molybdenum, silicon, manganese, phosphorus or sulfur and impurities associated with the manufacturing process. Preferably, the stainless steel layer comprises at least 10% chromium, at least 8% nickel, the balance being iron, additional elements as described above and impurities associated with the manufacturing process. This layer is preferably stainless steel 316 which comprises, by weight, the following: 16 < Cr < 18% and 10 < Ni < 14%. For example, this layer is 316L stainless steel whose composition by weight is as follows: C = 0.02%; 16 < Cr < 18%; 10.5 < Ni < 13%; 2 < Mo < 2.5%, Si = 1%; Mn = 2%; P = 0.04% and S = 0.03%. Its thickness can be, for example, greater than or equal to 2 nm, preferably between 2 and 15 nm. Figure 2 depicts that embodiment in which sheet 21 is coated with an aluminum-based coating 22, a thin layer of stainless steel 23, and a third coating of zinc 24. The stainless steel layer can be deposited by any known method to the elements versed in the technique. It is, for example, deposited by galvanizing or magnetron sputtering.
    [042] Regardless of the deposition method used to deposit the zinc coating, alkaline degreasing can be performed to increase the adhesion of a zinc coating layer. Advantageously, degreasing is carried out using an alkaline bath comprising, for example, NaOH and/or potassium hydroxide (KOH), and surfactants. Degreasing is advantageously carried out by immersion or spraying for a time t6 ranging from 1 to 120 seconds, preferably from 2 to 20 seconds. The degreasing temperature T6 ranges from 30°C to 90°C, T6 being preferably 60°C. The pH of the bath ranges from 10 to 14.
    [043] Acid degreasing can also be considered. In this case, the chemical bath comprises, for example, phosphoric acid (H3PO4) and surfactants. The pH of the bath ranges from 0 to 2.
    [044] Regardless of the type of degreasing implanted, it is usually followed by rinsing.
    [045] The steel sheets according to the invention are then hardened by pressing. This method involves providing a previously coated steel sheet in accordance with the invention, then cutting the sheet to obtain a raw block. This raw block is then heated in a non-protective atmosphere to the T1 austenizing temperature of the steel ranging from 840 °C to 950 °C, preferably from 840 °C to 930 °C. Preferably, the raw block is heated for a time t1 ranging from 3 to 12 minutes, preferably 4 to 10 minutes, in an inert atmosphere or an atmosphere comprising air. During this heat treatment, the zinc coating is oxidized to ZnO.
    [046] After heat treatment, the raw block is transferred to a hot stamping tool, then hot stamped at a temperature T2 ranging from 600 to 830 °C. The obtained part is then cooled both in the tool itself and after transfer to a specific cooling tool.
    [047] The cooling rate is controlled as a function of the steel composition, so that its final microstructure at the end of the press hardening process comprises primarily martensite, preferably containing martensite, or martensite and bainite or comprising at least 75% of equiaxed ferrite, an amount of martensite greater than or equal to 5% and less than or equal to 20%, and an amount of bainite less than or equal to 10%.
    [048] A phosphatable press-hardened part having a layer of ZnO is thereby obtained according to the invention.
    [049] For automotive applications, after phosphating, each part is immersed in a cataphoresis bath. It is also possible to consider sequentially applying a primary paint layer, a base coat paint layer, and optionally a top coat layer.
    [050] Before applying the cataphoresis layer separately, it is degreased beforehand then phosphated in order to ensure the adhesion of the cataphoresis layer. After phosphating, a hardened part according to the invention, a press hardened part is obtained which comprises a ZnO layer and also a phosphate crystal layer on the ZnO layer. There is a high degree of phosphate crystal coating on the surface of the hardened part. Preferably, the degree of phosphate crystal coverage on the surface part is greater than or equal to 80%, more preferably greater than or equal to 90% and advantageously greater than or equal to 99%.
    [051] The cataphoresis layer provides additional protection to the part against corrosion. The primary paint layer, usually applied by spray gun, prepares the final appearance of the part and protects it against stone chips and UV. The basecoat paint layer provides the part with its final color and appearance. The topcoat layer provides the surface of the part with satisfactory mechanical strength, resistance to aggressive chemical agents, and a satisfactory surface appearance.
    [052] In general, the thickness of the phosphating layer is between 1 μm and 2 μm. The paint films used to protect and ensure an optimal surface appearance for the parts comprise, for example, a cataphoresis layer from 8 μm to 25 μm thick, a primary paint layer from 35 μm to 45 μm thick and a base paint layer from 40 µm to 50 µm thick.
    [053] In cases where paint films additionally comprise a topcoat layer, the thicknesses of the various paint layers are generally as follows: - cataphoresis layer: between 8 μm and 25 μm, preferably less than 20 μm. - primary ink layer: less than 45 µm. - base paint layer: less than 20 μm, and top coat layer: less than 55 μm.
    [054] Preferably, the total thickness of the paint films is less than 120 µm, or even less than 100 µm.
    [055] The invention will now be explained through tests that are indicative, but not limiting. EXAMPLES
    [056] For all tests, the following steels were used: Usibor®1500 or 22MnB5. They were coated with an aluminum-based coating comprising by weight composition 9% silicon, 3% iron, and 88% aluminum, which has a thickness of 14 or 25 µm.
    [057] The composition by weight of the Usibor®1500 steel sheet used in the examples is as follows: C = 0.2252%; Mn = 1.1735%; P = 0.0126%, S = 0.0009%; N=0.0037%; Si = 0.2534%; Cu = 0.0187%; Ni = 0.0197%; Cr = 0.180%; Sn = 0.004%; Al = 0.0371%; Nb = 0.008%; Ti = 0.0382%; B = 0.0028%; Mo = 0.0017%; As = 0.0023% and V = 0.0284%.
    [058] For example, "Usibor® AluSi® 14 μm" indicates a Usibor®1500 steel as defined above that has an aluminum-based coating comprising, by weight composition, 9% silicon, 3% iron, and 88% aluminum, having a thickness of 14 µm.
    [059] For example, "22MnB5 AlSi 14 μm" indicates a 22MnB5 steel of an aluminum-based coating comprising, by weight composition, 9% silicon, 3% iron, and 88% aluminum having a thickness of 14 µm. EXAMPLE 1: PHOSPHATE CAPACITY TEST AND ZINC BEHAVIOR TEST
    [060] The phosphatizing capacity test is used to determine the distribution of phosphate crystals in a press-hardened part, in particular, by evaluating the degree of coverage on the surface of the part. EXAMPLE 1A
    [061] First, a series of 3 samples is created, designated 1, 2, and 3.
    [062] Samples 1 and 2 are obtained from a 22MnB5 steel sheet provided with a first aluminum-based coating and a second ZnO coating which has a thickness of 0.2 μm according to the method described in the document in EP2270257.
    [063] Sample 3 is obtained from a 25 μm Usibor® AluSi® steel sheet.
    [064] Sample 4 is obtained from a 25 μm Usibor® AluSi® steel sheet additionally provided with a zinc coating deposited by cementation having a thickness of 0.17 μm. Zinc deposition is carried out by immersion in a bath comprising NaOH at a concentration of 150g L-1 and ZnO at a concentration of 15g L-1 for 5 seconds. The bath temperature is 60 °C.
    [065] For each of the samples, the coated steel sheet is cut to obtain a rough block. The raw block is then heated to a temperature of 900 °C for a period ranging from 5 minutes to 5 minutes and 30 seconds. The raw block is then transferred to a press, then hot stamped to obtain a part. Finally, said part is cooled to obtain martensitic transformation hardening.
    [066] Degreasing is then carried out using a solution of Gardoclean® 5176 and Gardobond® H 7352 at a temperature of 55 °C. the same is followed by rinsing with water. The surface of the part is then activated by immersion using a solution of Gardolene® V6513 at room temperature. Finally, phosphating is carried out by immersion for 3 minutes in a bath comprising a solution of Gardobond® 24 TA at a temperature of 50 °C. The part is rinsed with water then dried with hot air. The surface of these phosphate samples is observed using SEM. The results are shown in the Table below.


    [067] It is observed that only sample 4 exhibits a high degree of phosphate crystal coverage on the hardened part. EXAMPLE 1B
    [068] Another series of samples is created, designated 4, 5, and 6.
    [069] Sample 4 is obtained from a 25 μm Usibor® AluSi® steel sheet, additionally provided on one side of the sheet with a second galvanized deposited zinc coating which has a thickness of 1 μm.
    [070] Sample 5 is obtained from a 25 μm Usibor® AluSi® steel sheet additionally provided on one side of the sheet with a second galvanized deposited zinc coating which has a thickness of 2 μm.
    [071] Sample 6 is obtained from a 25 μm Usibor® AluSi® steel sheet provided additionally on one side of the sheet with a second galvanized deposited zinc coating which has a thickness of 3 μm.
    [072] For zinc plating deposition, first, degreasing of the sheet is carried out by immersion for 3 seconds in a Novaclean® 301 solution at a temperature of 80 °C. Degreasing is followed by rinsing with water. Then, brightening is carried out by immersion for 4 seconds in a sulfuric acid solution at room temperature. Brightening is followed by rinsing with water. The sheet is then immersed in an electrolytic bath comprising sulfuric acid (H2SO4) and zinc sulfate (ZnSO4) at a temperature of 50°C. The current density applied to the steel sheet is 80 A/dm2. The pH of the bath is 0.8.
    [073] The press hardening method described in Example 1a is then applied to these samples. In this case, the raw block is heated to a T1 temperature of 900 °C for a time t1 of 6 minutes of 30 seconds. The surface of the phosphated samples is then observed using SEM. The results are shown in the Table below.

    [074] It is observed that only sample 4 exhibits a high degree of phosphate crystal coverage on the surface of the hardened part.
    [075] The behavior of zinc during heat treatment at 900 °C was also observed for each of samples 4 to 6. The behavior of zinc on the side where zinc deposition occurred and on the opposite side that was not coated was studied with zinc. Figures 9 and 10* show samples 4, 5 and 6 arranged in that order. The results are shown in the Table below.

    [076] It is observed that the zinc behavior was satisfactory during the heat treatment, that is, there is nothing notable to report for sample 4. However, when the zinc layer thickness is 2 μm or 3 μm ( samples 5 and 6), zinc diffuses on the opposite side. Zinc diffusion is detrimental on an industrial scale in the furnace in which heat treatment for austenization and press hardening takes place. EXAMPLE 1C
    [077] Another series of samples is created, designated 7, 8, and 9.
    [078] Samples are obtained from 14μm Usibor® AluSi® steel sheet additionally provided on one side of the sheet with a second zinc coating deposited by galvanizing using a zinc sulfate bath. Depending on the samples, the thickness of the zinc coating varies from 0.5 µm to 1.5 µm.
    [079] For zinc plating deposition, first, degreasing of the sheet is carried out by immersion for 3 seconds in a Novaclean® 301 solution at a temperature of 80 °C. Degreasing is followed by rinsing with water. Then, brightening is carried out by soaking for 4 seconds in a sulfuric acid solution at room temperature. Brightening is followed by rinsing with water. The sheet is then immersed in an electrolytic bath comprising H2SO4 and ZnSo4 at a temperature of 50°C. The current density applied to the steel sheet is 15 A/dm2. The pH of the bath is 3.
    [080] For each sample, the coated sheet is cut to obtain a raw block. The raw block is then heated to a temperature of 900 °C for a period of 5 minutes 30 seconds. The raw block is then transferred to a press, then hot stamped to obtain a part. Finally, said part is cooled to obtain martensitic transformation hardening.
    [081] The degreasing and phosphating as defined in Example 1a are then carried out on the hardened parts obtained. The surface of these phosphate samples is observed with SEM. The results are shown in the Table below.

    [082] It is observed that all samples exhibit a high degree of phosphate crystal coverage on the surface of the hardened part. EXAMPLE 2: CATAPHORESIS LAYER ADHESION TEST
    [083] This method is used to determine the adhesion of the cataphoresis layer deposited on the press-hardened part.
    [084] A 20 μm cataphoresis layer is deposited on samples 7 to 9 obtained in Example 1c. For this purpose, samples 7 to 9 are immersed in a bath comprising an aqueous solution comprising Pigment paste® W9712 and Resin blend® W7911 from PPG Industries. A 180V voltage is applied for 180 seconds with a voltage gradient from 0 to 180V for the first 30 seconds. After the cataphoresis deposition is carried out, the part is rinsed in water and then cured in an oven at a temperature of 178 °C for 30 minutes. Hardened parts painted by pressing are obtained in this way. EXAMPLE 2A: DRY E-COAT ADHESION TEST
    [085] A cutter is used to create a grid on the painted parts. Then, the paint that has peeled off the part is evaluated with the naked eye: 0 indicates excellent, ie no paint peeled off, and 5 indicates very weak, ie more than 65% of the paint has peeled off. The results are shown in the Table below.

    [086] It is observed that the paint adhesion is excellent when the hardened part is prepared from a steel sheet provided with a zinc coating that has a thickness of 0.5 μm or 1.0 μm. However, when the hardened part is prepared from a steel sheet which has a zinc layer which has a thickness of 1.5 µm, the adhesion of the cataphoresis layer is very poor. EXAMPLE 2B: WET E-COAT ADHESION TEST
    [087] Firstly, painted parts are immersed in demineralized water in a closed chamber for 10 days at a temperature of 50 °C. then, the grid described in Example 2a is created. The following results were obtained.

    [088] It is observed that paint adhesion is very satisfactory when the hardened part is prepared from a steel sheet provided with a zinc coating that has a thickness of 0.5 μm or 1.0 μm. However, when the hardened part is prepared from a steel sheet provided with a zinc layer which has a thickness of 1.5 µm, the adhesion of the cataphoresis layer is poor. EXAMPLE 3: CORROSION TEST
    [089] This method is used to determine corrosion resistance, particularly the presence of red rust on a painted part.
    [090] Samples 16 to 18 were prepared by applying the method described in Example 2. In this example, the thickness of the cataphoresis layer is 8 µm deposited. For this purpose, the total time for cataphoresis layer deposition is reduced to 30 seconds.
    [091] This test consists of submitting the painted parts to 6 corrosion cycles according to the new standard VDA 231-102. One cycle consists of placing the samples in a closed chamber in which a 1% by weight aqueous solution of sodium chloride is sprayed onto the samples at a rate of 3 ml/h while the temperature is varied from 50 °C to -15 ° C and the humidity level is varied from 50% to 100%. This cycle is shown in Figure 5.
    [092] The presence of red rust is assessed with the naked eye: 3 indicates excellent, in other words there is little or no red rust and 0 indicates very weak, in other words there is a lot of red rust.

    [093] It is observed that, when the painted part is prepared from a steel sheet provided with a zinc coating which has a thickness of 0.5 μm or 1.0 μm, the resistance to red rust is very satisfactory. However, when the painted part is prepared from a steel sheet with a zinc coating which has a thickness of 1.5 µm, the resistance to red rust is poor.
    [094] The hardened parts according to the invention thereby allow satisfactory paint adhesion and satisfactory resistance to red rust. EXAMPLE 4: ADHESION TEST
    [095] This method is used to determine the force required to loosen two steels bonded together to form a sandwich. These steels are obtained from Usibor® AluSi® steel sheets which are either uncoated or coated with zinc according to the invention.
    [096] For this purpose, samples 19 and 20 were obtained from a 14 μm Usibor® AluSi® steel sheet additionally provided with a second zinc coating deposited by galvanizing which has a thickness of 0.6 μm.
    [097] Samples 21 and 22 were obtained from a 25 μm Usibor® AluSi® steel sheet additionally provided with a second coating of zinc deposited by galvanizing which has a thickness of 0.6 μm.
    [098] Samples 23 and 24 is obtained from a 25 μm Usibor® AluSi® steel sheet.
    [099] For zinc deposition by galvanizing, first of all, a first degreasing of the sheet is carried out by spraying a solution of Garboclean® S5170-S5093 at a temperature of 80 °C. Then, a second degreasing is carried out by immersion in the same solution at a temperature of 40 °C. Degreasing is followed by rinsing with water. Then, brightening is carried out with a solution of sulfuric acid. Brightening is followed by rinsing with water. The sheet is then immersed in an electrolytic bath comprising H2SO4 and ZnSO4.
    [0100] Then, in each case, an uncoated or zinc-coated Usibor® AluSi® steel sheet is cut to obtain a raw block. The raw block is then heated to a temperature of 900 °C for a period ranging from 5 to 10 minutes. The raw block is then transferred to a press, then it is hot pressed to obtain a part. Finally, said part is cooled to obtain martensitic transformation hardening.
    [0101] For each of the samples, two equivalent steel sheets were bonded using DOW company Betamate 1480V203G collision adhesives. These samples were then heated for 30 minutes at 180 °C to cure the bump adhesive. After cooling, a mechanical stress is applied to both sides of each sheet in order to loosen the two sheets of steel. Mean shear stress is then measured in Megapascal (MPa). The results are shown in the Table below.

    [0102] It is observed that a high shear stress is obtained for each sample. Thus, for samples 19 to 22, this means the bonded interfaces coated with zinc, that is, the interface between the aluminum-based coating and the ZnO layer obtained during the heat treatment and the interface between the same layer of ZnO and collision adhesive are not brittle. EXAMPLE 5: DIFFUSED HYDROGEN TEST
    [0103] This method is used to determine the amount of hydrogen absorbed during austenitic heat treatment.
    [0104] Sample 25 was obtained from a 25 μm Usibor® AluSi® steel sheet additionally provided with a second zinc coating deposited by galvanization having a thickness of 0.6 μm.
    [0105] Sample 26 is obtained from a 25 μm Usibor® AluSi® steel sheet.
    [0106] For zinc plating deposition, the method described in Example 4 was repeated.
    [0107] Then, for each sample, the uncoated or zinc coated Usibor® AluSi® steel sheet is cut to obtain a raw block. Then, the raw block is heated to a temperature of 900 °C for a time of 6 minutes and 30 seconds. The raw block is then transferred to a press, and then it is hot pressed to obtain a part. Finally, said part is cooled to obtain martensitic transformation hardening.
    [0108] Finally, measurement of the amount of hydrogen absorbed by samples during heat treatment was performed by thermal desorption with the use of a TDA or Thermal Desorption Analyzer. For this purpose, each sample, placed in a quartz chamber, is slowly heated in an infrared furnace under a stream of nitrogen. The hydrogen/nitrogen mixture released is captured by a leak detector and the hydrogen concentration is measured with a mass spectrometer. The results are shown in the Table below.

    [0109] It is observed that the amount of desorbed diffused oxygen is similar if the sample is an uncoated steel sheet or a zinc coated steel sheet according to the invention. EXAMPLE 6: WELDING TEST EXAMPLE 6A: WELDING RANGE
    [0110] This method is used to determine the valid welding intensity range for a sample, that is, the intensity range over which spot welding can be performed without disadvantages such as expelling metal from the metal coating. The welding range should generally be greater than 1kA as per car manufacturer specifications.
    [0111] Sample 27 was obtained from a 14 μm Usibor® AluSi® steel sheet, additionally provided with a second zinc coating deposited by galvanization having a thickness of 0.6 μm.
    [0112] For plating deposition, the method described in Example 4 was repeated.
    [0113] Then, the sample was cut to obtain a raw block. The raw block was heated to a temperature of 900 °C for a period of 5 minutes 30 seconds. The raw block was then transferred to a press, then hot stamped to obtain a part. Finally, said part was cooled to obtain martensitic transformation hardening.
    [0114] Sample 28 was prepared by applying the method described in Example 5.
    [0115] In this test, for each of the samples, two equivalent steel sheets were spot welded. For this purpose, electrodes are placed on both sides of each coated steel sheet according to the invention. A compressive force of 450 daN is applied between the electrodes. Alternating current of frequency 50Hz applied to the electrodes passes through the sample, thereby allowing the two sheets of steel to be welded together. Furthermore, an intensity of 3 kA is set at the beginning of the test and is gradually increased in order to determine the maximum intensity (Imax) at which molten metal is expelled. Then, the intensity is gradually decreased in order to determine the minimum intensity (Imin) at which the weld diameter per spot is smaller than the permissible threshold value. The welding range limits are Imax and Imin. The results are shown in the Table below.


    [0116] It is observed that the welding range of each sample is greater than 1 kA. EXAMPLE 6B: MECHANICAL PROPERTIES
    [0117] This method is used to determine the mechanical properties of a spot weld. For this purpose, mechanical stress is applied to a spot weld until it breaks.
    [0118] Samples 29 and 31 are, respectively, samples 27 and 28 of Example 6a.
    [0119] Samples 30 and 32 are obtained respectively from Usibor® AluSi® 14 μm and Usibor® AluSi® 25 μm steel sheets.
    [0120] In this test, samples comprise two equivalent sheets of steel welded together in the shape of a cross. Force is applied so as to break the spot weld. This force, known as the crossing tensile strength (CTS), is expressed in daN. It is a function of the diameter of the spot weld and the thickness of the metal, that is, the thickness of the steel and metal coating. This makes it possible to calculate the coefficient α which is the ratio of the CTS value to the product of the weld diameter per spot multiplied by the substrate thickness. This coefficient is expressed in daN/mm2. The results are shown in the Table below:


    [0121] It is observed that the α coefficients for samples 29 and 30 and for samples 31 and 32 are similar. EXAMPLE 6C: USEFUL LIFE OF ELECTRODE
    [0122] This method is used to determine electrode service life, that is, the number of spot welds that can be performed with a pair of electrodes before the electrodes are too deteriorated and the quality of spot welds is no longer acceptable.
    [0123] Samples 33 and 34 are, respectively, samples 27 and 28 of Example 6a.
    [0124] This test must be performed with the same pair of electrodes used to perform spot welds and their diameters must be measured every 100 welds. In fact, the more a pair of electrodes is used for welding, the more the diameter of spot welds will decrease. Therefore, there is a critical diameter for spot welds in automaker specifications. This critical diameter is generally described as 4xt where t is the thickness of the metal, ie the thickness of the sheet in mm. below this diameter, the electrode is considered to be depleted.
    [0125] The diameter of welds per spot was measured as a function of the number of welds performed. As the metal thickness of samples 33 and 34 is 1.5 mm, the critical diameter of the spot weld is 4.89 mm. Testing was discontinued after 1000 weld points. The results are shown in the Table below:

    [0126] It is observed that the diameter of welds per spot is well above the limiting value of 4.89 mm for all welds performed. Furthermore, it is observed that the diameter of the welds decreases very little from 200 to 1,000 welds.
    权利要求:
    Claims (21)
    [0001]
    1. STEEL SHEET (1) characterized in that it is for press hardening coated with an aluminum-based coating (2) which additionally comprises a second zinc coating (3) having a thickness less than or equal to 1.1 μm .
    [0002]
    2. SHEET according to claim 1, characterized in that the thickness of the zinc coating (3) is less than or equal to 1.0 μm.
    [0003]
    3. SHEET according to claim 2, characterized in that the thickness of the zinc coating (3) is less than or equal to 0.7 μm.
    [0004]
    4. SHEET, according to any one of claims 1 to 3, characterized in that the aluminum-based coating (2) comprises up to 3% of iron and from 9% to 12% of silicon, the balance being aluminum.
    [0005]
    5. SHEET according to any one of claims 1 to 4, characterized in that it is coated on at least one side with an aluminum-based coating (2) having a thickness of between 5 μm and 50 μm.
    [0006]
    6. SHEET, according to any one of claims 1 to 5, characterized in that the aluminum-based coating (2) is directly in contact with the zinc coating.
    [0007]
    7. SHEET according to any one of claims 1 to 6, characterized in that the product of the thickness of the aluminum-based coating (2) and the thickness of the zinc coating (3) is between 2 and 25.
    [0008]
    8. SHEET according to any one of claims 1 to 7, characterized in that it has a composition by weight as follows: 0.20% < C < 0.25% 0.15% < Si < 0.35% 1, 10% < Mn < 1.40% 0% < Cr < 0.30% 0% < Mo < 0.35% 0% < P < 0.025% 0% < S < 0.005% 0.020% < Ti < 0.060% 0.020% < Al < 0.060% 0.002% < B < 0.004%, with the balance of the composition being iron and unavoidable impurities resulting from the process.
    [0009]
    9. SHEET according to any one of claims 1 to 7, characterized in that its composition by weight is as follows: 0.24% < C < 0.38% 0.40% < Mn < 3% 0, 10% < Si < 0.70% 0.015% < Al < 0.070% 0% < Cr < 2% 0.25% < Ni < 2% 0.020% < Ti < 0.10% 0% < Nb < 0.060% 0, 0005% < B < 0.0040% 0.003% < N < 0.010% 0.0001% < S < 0.005% 0.0001% < P < 0.025% it is understood that the amounts of titanium and nitrogen satisfy:
    [0010]
    10. METHOD FOR PREPARING A STEEL SHEET (1), as defined in any one of claims 1 to 9, characterized in that the zinc coating (3) is carried out by cementation, galvanization, or sonic jet steam deposition under vacuum .
    [0011]
    11. METHOD, according to claim 10, characterized in that when the zinc coating (3) is deposited by cementation or galvanization, said coating is directly deposited on the aluminum-based coating (2).
    [0012]
    12. METHOD, according to claim 11, characterized in that the zinc coating (3) deposited by cementation is carried out by immersion or by spraying.
    [0013]
    13. METHOD according to claim 11, characterized in that when the coating is deposited under vacuum by sonic jet steam, a thin layer of stainless steel is deposited between the aluminum-based coating (2) and the zinc coating (3).
    [0014]
    14. METHOD FOR PREPARING A PRESS HARDENED PART coated with a coating capable of phosphating characterized by comprising the following sequential steps in which: A) a steel sheet is provided, for press hardening, as defined in any one of claims 1 to 9, or manufactured by a method as defined in any one of claims 10 to 13, B) the sheet obtained in step A) is cut to obtain a raw block, then C) the raw block is heated to a temperature T1 in a range from 840 °C to 950 °C to obtain a completely austenitic structure in the steel, D) the raw block is then transferred to a press, E) the raw block is hot pressed to obtain a part, F) the part obtained in the step E) is cooled to obtain a martensitic or martensitic-bainitic structure in the steel composed of at least 75% iron equiaxed, a quantity of martensite greater than or equal to 5% and less than or equal to 20%, a quantity of bainite less or i equal to 10%.
    [0015]
    15. METHOD according to claim 14, characterized in that step C) is deployed for a time t1 of 3 to 12 minutes in an inert atmosphere or in an atmosphere comprising air.
    [0016]
    16. METHOD according to any one of claims 14 to 15, characterized in that, during step E), the hot pressing is carried out at a temperature T2 ranging from 600 °C to 830 °C.
    [0017]
    17. HARDENED PART capable of being obtained by a method as defined in any one of claims 14 to 16, characterized in that it comprises a layer of ZnO and which additionally comprises a phosphate crystal layer on the layer of ZnO obtained after a step G ) of additional phosphating.
    [0018]
    18. HARDENED PART, according to claim 17, characterized in that the degree of phosphate crystal coverage on the surface of the part is greater than or equal to 90%.
    [0019]
    19. HARDENED PART according to claim 18, characterized in that the degree of phosphate crystal coverage on the surface of the part is greater than or equal to 99%.
    [0020]
    The hardened part according to any one of claims 17 to 19, characterized in that it additionally comprises a cataphoresis layer on the phosphate crystal layer.
    [0021]
    USE OF A PRESS HARDENED PART as defined in any one of claims 17 to 20 manufactured by a method as defined in any one of claims 14 to 16, characterized in that it is for manufacturing motor vehicle parts.
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    同族专利:
    公开号 | 公开日
    ZA201704918B|2019-05-29|
    CN107250414A|2017-10-13|
    CA2977173A1|2016-08-25|
    UA117994C2|2018-10-25|
    RU2017132500A|2019-03-19|
    RU2686164C2|2019-04-24|
    ES2707368T3|2019-04-03|
    BR112017016132A2|2018-04-17|
    EP3259380B1|2018-10-24|
    US20180044774A1|2018-02-15|
    CN107250414B|2019-04-02|
    TR201900880T4|2019-02-21|
    EP3259380A1|2017-12-27|
    RU2017132500A3|2019-03-19|
    MX2017010574A|2017-11-16|
    CA2977173C|2019-07-02|
    KR102203043B1|2021-01-14|
    WO2016132165A1|2016-08-25|
    PL3259380T3|2019-04-30|
    JP6511155B2|2019-05-15|
    KR20200027578A|2020-03-12|
    KR20170116102A|2017-10-18|
    WO2016132194A1|2016-08-25|
    HUE041821T2|2019-05-28|
    JP2018513909A|2018-05-31|
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    法律状态:
    2020-02-18| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|
    2021-04-20| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-06-29| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 22/01/2016, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    PCT/IB2015/000194|WO2016132165A1|2015-02-19|2015-02-19|Method of producing a phosphatable part from a sheet coated with an aluminium-based coating and a zinc coating|
    IBPCT/IB2015/000194|2015-02-19|
    PCT/IB2016/000037|WO2016132194A1|2015-02-19|2016-01-22|Method of producing a phosphatable part from a sheet coated with an aluminium-based coating and a zinc coating|
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