A method for manufacturing press-hardened a steel part provided with an al-based coating and corresponding steel sheet

Abstract

A method to manufacture a press hardening part comprising the following steps: A) Providing a coated steel sheet (4), comprising a base steel for heat treatment (1), coated with an aluminium based metallic coating (2), B) Applying a surface treatment (3) on the coated steel sheet (4) with an aqueous solution to form a wet film having a thickness from 0.5 to 6.0 µm, said aqueous solution having a mass concentration in chromium from 1.0 to 30.0 g / L and a mass concentration in phosphorous from 0.8 to 26.0 g / L, C) Drying said wet film to obtain a treated coated steel sheet (5), D) Trimming or cutting said treated steel sheet (5) into a blank, E) Heating said blank in a furnace at a temperature set from 850 to 950°C, F) Transferring said hot blank into a press tool, G) Forming said hot blank into a part and press hardening said part into a press-hardened part.

Description

[0001] A method for manufacturing press-hardened a steel part provided with an Al-based coating and corresponding steel sheet The present invention deals with the press hardening of steel parts coated with an aluminium based metallic coating. It is known that certain applications, especially in the automotive field, require metal structures to be further lightened and strengthened in the event of an impact. To this end, steel sheets having improved mechanical properties are implemented, such steel sheets being hot formed and subsequently hardened in the forming tools. In recent years the use of metallic coated steels in hot stamping and press- hardening processes to manufacture parts has become important, especially in the automotive industry. Fabrication of such parts may include the following main steps: - Coating of a steel sheet by hot dipping in a metallic bath, - Trimming or cutting said sheet into a blank, - Heating said blank to obtain the complete transformation of the steel microstructure into austenite, - Transferring said hot blank into a press tool, - Forming and press hardening of the part. For corrosion resistance purposes, most of the metallic coatings designed for steel comprise zinc, because zinc is a less noble material than iron. This way, when iron from steel meets oxygen from the ambient air, steel remains intact while zinc is oxidized. This is sacrificial protection. However, coatings based on zinc have a lower melting point than the austenitization temperature of steel. Therefore, zinc coatings are not well adapted to press hardening processes. Aluminium based coatings have the advantage of having a higher melting point than zinc. When the coated steel blank is heated at an austenitization temperature, the molten metal coating is easier to manage with an aluminium based coating than with a zinc based coating. An aluminium based coating constitutes a barrier between the steel sheet and the oxygen from the ambient air, but it does not provide any sacrificial corrosion. The sensitivity to delayed cracking increases with the mechanical strength. Especially after press-hardening, high residual stresses are liable to remain after deformation. In combination with atomic hydrogen possibly present in the steel sheet, these stresses are liable to result in delayed cracking, meaning that cracking occurs a certain time after the deformation itself. Hydrogen may progressively build up by diffusion into the crystal lattice defects, such as the matrix / inclusion interfaces, twin boundaries and grain boundaries. It is in the latter defects that hydrogen may become harmful when it reaches a critical concentration after a certain time. This delay results from the residual stress distribution field and from the kinetics of hydrogen diffusion, the hydrogen diffusion coefficient at room temperature being low. In addition, hydrogen localized at the grain boundaries weakens their cohesion and favors the appearance of delayed intergranular cracks. Press hardening is also known as critical for hydrogen absorption, increasing the sensitivity to delayed fracture. Absorption may occur at the austenitization heat treatment, which is the heating step prior to the hot press forming itself. At high temperature the water in the furnace dissociates at the surface of the steel sheet into hydrogen and oxygen. Said water dissociation induces potential hydrogen absorption and delayed fracture risk in press hardened parts. The aim of the present invention is to produce press hardened steel parts which have an increased corrosion performance and a lower risk of delayed fracture, compared to the aluminium based coatings from the prior art. The invention will now be exposed in detail with reference to figure 1 which is a cross-section of a steel sheet according to the invention for illustration purpose. A first object of the invention is achieved by a method to manufacture a press hardened part and comprising the following steps: A) Providing a coated steel sheet comprising a base steel for heat treatment, coated with an aluminium based metallic coating, B) Applying a surface treatment on the coated steel sheet with an aqueous solution to form a wet film having a thickness from 0.5 to 6.0 µm, said aqueous solution having a mass concentration in chromium (Cr) from 1.0 to 30.0 g / L and a mass concentration in phosphorous (P) from 0.8 to 26.0 g / L, C) Drying said wet film to obtain a treated coated steel sheet, D) Trimming or cutting said treated steel sheet into a blank, E) Heating said blank in a furnace at a temperature set from 850 to 950°C, F) Transferring said hot blank into a press tool, G) Forming said hot blank into a part and press hardening said part into a press-hardened part. In step A), the steel sheet used is made of steel for heat treatment as described in the European Standard EN 10083. It can have a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heat-treatment. 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% ≤ 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 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% ≤ Cr ≤ 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%; Cr ≤ 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% ≤ 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 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%, AI ≤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 can be obtained by hot rolling and optionally cold rolling depending on the desired thickness. The steel sheet according to the invention can be from 0.5 to 3.0 mm thick After rolling to the desired thickness, the steel sheet is coated with an aluminium based coating. The coating can be deposited by any mean. For example, it is hot dipped in a liquid bath of molten metal and subsequently wiped by air knifes to adjust the coating thickness. The bath may contain iron from the dissolution of the steel sheet in the liquid metal. Advantageously, the aluminium based coating also comprises silicon. In a preferred embodiment, the aluminum based coating comprises, by weight, 8 to 11 % silicon, up to 3 % iron as residual elements, impurities form the manufacturing process up to 0.2%, the balance being aluminium. The thickness of the aluminium based coating is not limited. Preferably, said thickness it is from 10 to 40 µm per side, advantageously from 20 to 30 µm per side. After metallic coating, a temper rolling can be performed. The temper rolling operation occurs on a single stand temper rolling mill, wherein the steel strip is rolled between the two working rolls of said mill. A pressure force is applied on the steel strip by the work rolls, which in turn exert a lineic pressure along the generatrix in contact with the strip. Temper rolling ensures the mechanical characteristics of the coated steel sheet and erases small surface defects. In step B), the surface treatment with an aqueous solution can be carried out by any mean. For example, the steel sheet can be continuously treated at the end of the hot dip coating line before coiling. The surface treatment in form of an aqueous solution can be applied by any mean. For example, it can be applied by immersion, with a roll coater or by spraying droplets of the aqueous solution. A spin coater can also be used to achieve a uniform application. However, application by spin coater is not adapted for a high speed on-line application. As well the humid thickness applied with a spin coater is higher than humid thickness achieved with a more productive roll coater. The wet film thickness is managed, depending on the process. The wet film thickness can be controlled online by infra-red measurement, or with a rolling wheel. The amount of chromium and phosphorous applied by surface treatment is determined by both the concentration in Cr and in P in the aqueous solution and the by the wet film thickness. The wet film according to the invention has a thickness from 0.5 to 6.0 µm. Preferably, the wet film has a thickness from 0.5 to 4.0 µm. In another advantageous embodiment, the wet film thickness is from 0.5 to 2.0 µm. In a preferred embodiment, the aqueous solution is applied at a temperature below 90°C, advantageously below 80°C. In another preferred embodiment, the aqueous solution is applied at ambient temperature. The aqueous solution comprises chromium and phosphorous. Chromium and phosphorous can be present in the aqueous solution under various chemical species. The molecules carrying the chromium and phosphorous atoms are not limited. For example, the aqueous solution comprises phosphates, phosphonates, acid of any kind, silicates, nitrates, alone or in combination with other chemical elements. Other elements comprised in the solution are not limited. For examples, other chemical elements are oxides, bromides, chlorides, sulfides, fluorides or hydroxides, carbonates, zirconates. Preferably, the aqueous solution is an acidic or an alkaline solution where the concentration in Cr an in P evolves proportionally to the pH of the diluted solution. In this embodiment, the concentration in Cr and P can be controlled by measuring on-line the pH of the applied solution with a pH-meter. The concentration in Cr and in P in the aqueous solution can be adjusted by dissolution. The man skilled in the art will be able to adjust the concentration depending on the wet film thickness and on the application technology of the aqueous solution. The mass concentration in Cr and P in the aqueous solution for surface treatment can be controlled by inductively coupled plasma mass spectrometry (ICP). It is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. ICP is known and used for its ability to detect metals and several non-metals in liquid samples at very low concentrations. The aqueous solution for surface treatment according to the invention has a mass concentration in Cr from 1.0 to 30.0 g / L and a mass concentration in P from 0.8 to 26.0 g / L. In a preferred embodiment where the humid film thickness is from 0.5 to 4.0 µm, the aqueous solution has a mass concentration in Cr from 1.0 to 20.0 g / L and a mass concentration in P from 0.8 to 17.0 g / L of P. In an advantageous embodiment where the humid film thickness is from 0,5 to 2.0 µm, the aqueous solution has a mass concentration in Cr from 1.0 to 10.0 g / L and a mass concentration in P from 0.8 to 8.5 g / L. In step C) the wet film can be dried by any mean. For example, the steel sheet can be heated by induction or infra-red radiation. Preferably, the wet film is dried by blowing hot air on the steel sheet The weight of Cr and P on the surface of the treated steel sheet 5 can be controlled offline by any mean. For example, the coating weights can be measured by Xray fluorescence (XRF). According to the invention, the weight of chromium and phosphorous on the surface of the treated steel sheet 5 is from 20 to 150 mg / m² for Cr and from 8 to 80 mg / m² for P. Preferably, the weight of chromium and phosphorous on the surface of the treated steel sheet 5 is from 40 to 100 mg / m² for Cr and from 15 to 60 mg / m² for P. Advantageously, the weight of chromium and phosphorous on the surface of the treated steel sheet 5 is from 50 to 80 mg / m² for Cr and from 30 to 50 mg / m² for P. In step E), the steel sheet is heated in a furnace at a temperature set from 850 to 950°C, preferably from 900 to 930°C for a time during from 3 to 12 minutes. In step G), the treated steel sheet is transferred in the press forming tools and hot formed at a temperature from 600 to 900°C. After forming, the steel part is quenched into the press forming tools or transferred to a specific quenching tool. Quenching is performed at a cooling speed being faster than the critical cooling rate. The microstructure obtained is predominantly martensitic. The amounts of ferrite, bainite or retained austenite are limited to a volume fraction depending on the steel grade. The surface of the press-hardened part obtained in step E) comprises oxides resulting from the heating and subsequent hot forming process. Said oxides comprise iron coming from the steel substrate by diffusion, aluminium, and optionally silicon from the coating, and chromium and phosphorous from the surface treatment. The invention will now be exposed by examples as an illustration and not a limitation. Examples To assess the performance of the invention several tests were carried out, all on the same materials. For all samples, carbon steel coils used are 22MnB5. 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 steel coils were continuously rolled to desired thickness. After rolling, they were annealed and continuously coated with a coating deposited by hot dipping in a metallic bath. This coating comprises 9% by weight of Silicon, up to 3% by weight of iron, the balance being aluminum. The coating thickness is adjusted by wiping to 23 to 28 µm per side. After hot-dip aluminizing, the steel sheet is cut into samples. A surface treatment is applied on the samples, except the reference. The following treatment is tested: Bonderite® MPA 6010 from supplier Henkel, which contains Chromium comprised in Chromium (III) nitrates. It also comprises phosphorous in phosphoric acid and phosphates. The concentration of the surface treatment in aqueous solution is adjusted by dissolution and implemented according to the supplier’s instructions. In a first preparation, the mass concentration in Chromium was set to 20 g / L and the mass concentration in Phosphorous was set to 17,0 g / L. In a second, more diluted preparation, the mass concentration in Chromium was set to 8 g / L and the mass concentration in Phosphorous was set to 6,8 g / L. The chemical concentration in Cr and P of the aqueous solution was measured by inductively coupled plasma mass spectrometry (ICP). Accordingly, the surface treatment was tested with two levels of concentration. It was applied at ambient temperature by spin coater to ensure a unform wet film thickness of 4 µm. After application, they were dried for 30 seconds in a drying oven at 85°C. After drying, the weight of chromium and silicon on the surface of the treated steel sheet is measured by Xray fluorescence (XRF), with a portable device X- MET8000 from manufacturer Hitachi. This device was previously calibrated for each measured element (Cr or P) on AluSi® samples. Corrosion test To assess the corrosion performance, the steel samples used were 1.0 mm thick, and the surface treatment was applied. Then the samples were heat treated in an oven at 900°C for 5 minutes, and subsequently quenched in pressing tools. The cosmetic corrosion test was carried out on samples which were previously phosphatized. A degreasing of the samples was first realized. It was followed by a phosphating step realized by dipping them into a bath solution comprising during 3 minutes at 50°C. The components of the phosphating bath are Gardobond® products from supplier Chemetall. Their mass concentrations are disclosed in table 1. The phosphatized samples were then stored in corrosion chambers for 4 cycles according to VDA 233-102 standard. Examples according to the invention have less than 99% corroded surface after these cycles. They also exhibit less corrosion than the reference without surface treatment. The pitting corrosion test was performed on samples which were previously painted by e-coat: a degreasing of the samples was first realized. It was followed by a phosphating step realized by dipping them into a bath solution comprising during 3 minutes at 50°C. The components of the phosphating bath are Gardobond® products from supplier Chemetall. Their mass concentrations are disclosed in table 1. Table 1: Component concentrations in phosphating bath for pitting corrosion test Gardobond® product typeR24TA H7101 H7141 H7257 H7102 H7141 H7255 H7004Concentration (g / L)58.0 7.0 5.0 4.5 4.0 2.4 2.4 0.4Finally, an e-coat layer was applied on the samples. The e-coat is DOTO Generation 6 from supplier PPG. It was deposited by immersion in an electrolytic bath while submitted to a voltage of 220 to 240 V for 1 to 2 minutes. It was then cured in an oven at 180°C for 30 minutes. The dry film thickness is from 18 to 22 µm, measured with a Deltascope. The samples were then stored in corrosion chambers for 12 cycles according to VDA 233-102 standard. Examples according to the invention have less than 90% corroded surface after these cycles. They also exhibit less corrosion than the reference without surface treatment. Diffusive hydrogen test To assess the resistance to delayed fracture, the steel samples used were 1.6 mm thick, and the surface treatment was applied on both sides. Then the samples were heat treated in an oven at 900°C during 5 and 10 minutes and subsequently quenched in pressing tools. The dew point in the ambient air of the heating furnace and the pressing tools was 8.3°C. The hydrogen content absorbed by the steel sheet during the heat treatment was measured by thermic desorption using a Thermal Desorption Analyzer or TDA. To this end, each sample was placed in a quartz room and heated slowly in an infra- red furnace under a nitrogen flow. The released mixture hydrogen / nitrogen was picked up by a leak detector and the hydrogen concentration was measured by a mass spectrometer. The trials provided with a surface treatment according to the invention show a limited diffusive hydrogen content and a better corrosion performance compared to reference without surface treatment.

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Claims

CLAIMS1. A method to manufacture a press hardening part comprising the followingsteps: A) Providing a coated steel sheet (4), comprising a base steel for heattreatment (1), coated with an aluminium based metallic coating (2), B) Applying a surface treatment (3) on the coated steel sheet (4) withan aqueous solution to form a wet film having a thickness from 0.5 to 6.0 µm, said aqueous solution having a mass concentration in chromium from 1.0 to 30.0 g / L and a mass concentration in phosphorous from 0.8 to 26.0 g / L, C) Drying said wet film to obtain a treated coated steel sheet (5),D) Trimming or cutting said treated steel sheet (5) into a blank,E) Heating said blank in a furnace at a temperature set from 850 to950°C, F) Transferring said hot blank into a press tool,G) Forming said hot blank into a part and press hardening said partinto a press-hardened part.

2. The method of claim 1, wherein in step B) the wet film has a thickness from0.5 to 4.0 µm, and the mass concentration of the aqueous solution in chromium is from 1.0 to 20.0 g / L and the mass concentration in phosphorous is from 0.8 to 17.0 g / L.

3. The method of claim 2, wherein in step B) the wet film has a thickness from0.5 to 2.0 µm, and the mass concentration of the aqueous solution in chromium is from 1.0 to 10.0 g / L and the mass concentration in phosphorous is from 0.8 to 8.5 g / L.

4. The method according to anyone of claims 1 to 3, wherein in step B), theaqueous solution is an acidic or an alkaline solution where the concentration in Cr an in P is in proportion to the pH.

5. The method according to claim 4, wherein in step B), the concentration in Cr and P of the aqueous solution is controlled by measuring on-line the pH with a pH-meter.

6. The method according to anyone of claims 1 to 3, wherein in step B), the aqueous solution is applied at a temperature below 90°C.

7. The method according to anyone of claims 1 to 3, wherein in step B), the aqueous solution is applied by a roll coater.

8. The method according to anyone of claims 1 to 3, wherein in step B), the humid thickness is controlled online by infra-red measurement.

9. The method of anyone of claims 1 to 3, wherein in step C), the wet film is dried by blowing hot air on the steel sheet.

10. A treated steel sheet for press hardening (5) comprising: - a base steel for heat treatment (1) - an aluminium based metallic coating (2), - a surface treated surface (3) comprising from 20 to 150 mg / m² of Cr and from 8 to 80 mg / m² of P.

11. The treated steel sheet according to claim 10, wherein the treated surface comprises from 40 to 100 mg / m² of Cr and from 15 to 60 mg / m² of P.

12. The treated steel sheet according to claim 11, wherein the aluminium based coating (2) comprises, by weight, 8 to 11 % silicon, up to 3 % iron as residual element, impurities form the manufacturing process up to 0.2%, the balance being aluminium.

13. The treated steel sheet according to claim 12, wherein the thickness of the aluminium based coating (2) is from 10 to 40 µm per side.

14. A press hardened steel part provided with an aluminium based coating having on its surface oxides comprising iron, aluminium, silicon, chromium and phosphorous.

15. Utilization of the part of claim 14 in the structure of an automobile.

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