Aluminium-based coating for press-hardened steel part and method to produce the same

Abstract

A press hardened coated steel part consisting of: a hardened steel substrate (1) for hot stamping comprising, a coating layer having a thickness from 25 to 55 µm and comprising successively: an interdiffusion layer 21 in contact with the steel substrate and having a thickness of from 3 to 45 µm, said interdiffusion layer comprising more than 60 wt. % Fe, Si from 0.1 to 11.0 wt. %, Cu from 150 to 3000 wt. ppm, unavoidable impurities up to 1.0 wt.%, the remainder being Al, an alloyed layer (22) comprising Al, Fe, Si, Cu, wherein said alloyed coating layer consists of a binary phase (221) and a ternary phase (222), wherein said binary phase comprises from 35 to 60 wt. % Fe, from 0.1 to 5.0 wt. % Si, unavoidable impurities up to 1.0 wt. %, at least 190 wt. ppm of Cu, the balance being aluminium, and wherein said ternary phase comprises from 20 to 50 wt. % Al, from 6.0 to 20.0 weight % Si, unavoidable impurities up to 1.0 wt. %, at least 100 ppm of Cu, the balance being Fe, and wherein, in the alloyed layer, Cu content in the binary phase is at least 90 ppm more than in the ternary phase.

Description

[0001] Aluminium-based coating for press-hardened steel part and method to produce the same

[0002]

[0001] The present invention relates to a method to produce a press hardened coated steel part. The present invention also relates to said coated steel part. Press hardening is a press forming process, which occurs after prior austenitization of a steel sheet. This way said sheet is much more formable and becomes quenched into press forming tools. PHS stands for press hardening steel.

[0003]

[0002] The process manufacturing PHS parts may include the following main steps:

[0004] - Precoating a steel substrate by dipping into a molten metal bath to from a precoating layer,

[0005] - Heating the precoated steel substrate to obtain alloying by interdiffusion between the base steel and the precoating, and to austenize the said steel,

[0006] - Hot forming followed by rapid cooling to obtain predominantly hard martensite microstructures, covered by an alloyed coating layer.

[0007]

[0003] 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 usually used, such steel sheets being formed by austenitization and subsequent press-hardening to achieve said aimed microstructure.

[0008]

[0004] Austenitization heating also performs an alloying of the precoating by diffusion of the steel substrate in the precoating. This has the effect of creating intermetallic alloys with high melting temperature, the blanks having such coating may be heated in a temperature range where austenitizing of the metallic substrate takes place, allowing further hardening by quenching.

[0005] Hardened parts can be coated with zinc-based pre-coating or aluminum-based pre-coating. Zinc-based coatings are generally used because they allow a protection against corrosion thanks to barrier protection and cathodic protection. Sacrificial cathodic protection is based on the fact that zinc is a metal less noble that steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel. However, the step of painting of hardened parts coated with zinc necessitates sanding operations before phosphatizing due to the presence of a weak layer of oxides at the part surface.

[0009]

[0006] Aluminum-based coatings have a good aptitude for painting. They allow a protection by barrier effect and can be welded. However, they do not bring any cathodic protection, or they have a very low cathodic protection.

[0010]

[0007] So, there is a need to develop a coating based on aluminium with increased corrosion performance after painting.

[0011]

[0008] The aim the present invention is to provide a coated press hardened steel part having an improved corrosion performance, the coating of which is based on aluminium.

[0012]

[0009] The present invention also aims at providing a method to produce said coated press hardened steel part.

[0013]

[0010] Another aim of the present invention is to provide a coated steel sheet suitable to be used in the production of said coated press hardened steel part.

[0014]

[0011] This is achieved by providing a press hardened part according to claim 1. The part can also comprise any characteristics of claims 2 to 7, taken alone or in combination.

[0015]

[0012] The invention also covers a press hardening process according to claims 8 to 12.

[0016]

[0013] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

[0014] Figure 1, which is a cross-sectional scheme of a PHS part according to the invention,

[0017]

[0015] Figure 2, which is a cross-sectional scheme of a steel sheet suitable for press hardening according to the invention,

[0018]

[0016] Figure 3, which shows the content of elements in a coating layer according to the invention, obtained by EPMA (Electron Probe MicroAnalyser). The X axis corresponds to the depth from the coating surface in a direction perpendicular to the surface towards the steel substrate.

[0019]

[0017] Said figures are provided with the following labels:

[0020] 1 : Steel substrate

[0021] 2: Coating layer

[0022] 21: Interdiffusion layer

[0023] 22: Alloyed layer

[0024] 221 : binary phase

[0025] 222: ternary phase

[0026] 3: Intermetallic layer

[0027] 31 : Fist sublayer

[0028] 32: Second sublayer

[0029] 4: Precoating layer

[0030]

[0018] Steel part composition.

[0031]

[0019] A steel substrate 1 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 heattreatment.

[0032]

[0020] The weight composition of the steel part is 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% < Ca < 0.01 % ; 0% < V < 0.35% ; 0% < W < 0.35%, from 200 to 3000 wt. ppm of Cu, the remainder being iron and unavoidable impurities from the manufacturing of steel.

[0021] The remainder of the composition of the steel is iron and impurities resulting from the elaboration process. The level of impurities resulting from the elaboration process will depend on the production route used and the level of scrap used in the steel melt. For example, when using a Basic oxygen furnace route with a low level of steel scrap (recycled steel), the level of impurities will remain very low. It is however also possible to add a high amount of scrap in the converter to the pig iron produced in the basic oxygen furnace, which will increase the level of impurities. Furthermore, when elaborating the steel using an electric furnace for example, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. When using a high level of scrap, the level of Sn can reach 0.05%, As can reach 0.03%, Sb can reach 0.03% and Pb can reach 0.03%. Other impurities can be for example Li, Zr, Ag, P, Bi and Mg in a lesser content. The total amount of unavoidable impurities is up to 1.0 wt. %. Advantageously, the total amount of unavoidable impurities in up to 0.5 wt. %, or even less than 0.2 wt. %.

[0033]

[0022] For example, the steel part 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%, unavoidable impurities from the manufacturing of steel, from 200 to 3000 wt. ppm of Cu, the remainder being iron.

[0034]

[0023] In another embodiment, the steel part 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%; from 200 to 3000 wt. ppm of Cu, unavoidable impurities from the manufacturing of steel, the remainder being iron.

[0035]

[0024] Alternatively, the steel part can have the following weight composition:

[0036] 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 %, unavoidable impurities from the manufacturing of steel, from 200 to 3000 wt. ppm of Cu, the remainder being iron.

[0037]

[0025] In another embodiment, the steel part 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% < Ni < 0.100%; 0% < Cr < 0.100%; 0% < Mo < 0.100%, from 200 to 3000 wt. ppm of Cu, unavoidable impurities from the manufacturing of steel, the balance being iron.

[0038]

[0026] In another embodiment, the steel part has the following weight composition: 0.06% < C < 0.1%, 1% < Mn < 2%, Si < 0.5%, Al <0.1%, 0.02% < Cr < 0.1%, 0.02% < Nb < 0.1%, 0.0003% < B < 0.01%, N < 0.01%, S < 0.003%, P < 0.020% less than 0,1% of Ni and Mo, unavoidable impurities from the manufacturing of steel, from 200 to 3000 wt. ppm of Cu, the remainder being iron and unavoidable impurities.

[0039]

[0027] In another embodiment, the steel part 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%; unavoidable impurities from the manufacturing of steel, from 200 to 3000 wt. ppm of Cu, the balance being iron.

[0040]

[0028] Alternatively, the steel part 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%, unavoidable impurities from the manufacturing of steel, from 200 to 3000 wt. ppm of Cu, the balance being iron.

[0041]

[0029] The steel sheet used to manufacture the press hardened steel part 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.

[0042]

[0030] Coating layer of the steel part.

[0043]

[0031] The coating layer 2 of the press hardened coated steel part results from the alloying by interdiffusion of a steel sheet 1 and a coating 4. Said diffusion occurs at high temperature during the austenitization heat treatment. During said treatment, the elements comprised in the steel substrate, like Fe and Cu will migrate through the coating. Fe will form intermetallic compounds with the elements comprised in the precoating like Al and Si. The coating layer of the part is thicker than the coating layer of the steel sheet. The coating layer has a thickness from 15 to 55 pm, preferably from 25 to 55 pm.

[0044]

[0032] The coating layer 2 comprises an interdiffusion layer 21 and an alloyed layer 22.

[0045]

[0033] The interdiffusion layer 21 grows in contact with the steel substrate 1.

[0046] This layer has a lower level of alloying with the coating of the steel sheet compared to the alloyed layer 22. Said interdiffusion layer 21 comprises more than 60 wt. % Fe, from 0.1 to 11.0 wt. % Si, from 150 to 3000 wt. ppm Cu, unavoidable impurities up to 1.0 wt. %, the reminder being Al. The interdiffusion layer may comprise slightly less Cu than the substrate. The thickness of the interdiffusion thickness depends on the duration and temperature of the austenitization heat treatment. If the interdiffusion layer is thinner than 3 pm, the steel sheet won’t be fully austenitic. This will result in a press hardened part with improper microstructure. If the interdiffusion layer is thicker than 45 pm, the alloyed layer will be too thick, leading to poor corrosion performances. Preferably, the interdiffusion layer comprises from 170 to 3000 ppm Cu.

[0047]

[0034] An alloyed layer 22 forms on top of the intermetallic layer 21. The alloyed layer according to the invention consists of a binary phase 221 and a ternary phase 222.

[0048]

[0035] The binary phase 221 comprises mainly Fe and Al, meaning that Fe and Al together account for more than 50 wt. % in the binary phase. The binary phase comprises from 35 to 60 wt. % Fe, from 0.1 to 5.0 wt. % Si, unavoidable impurities up to 1.0 wt. %, at least 190 wt. ppm of Cu, the balance being Al. Preferably, the binary phase comprises 50 to 60 wt.%.

[0049]

[0036] Preferably, the binary phase 221 comprises AIFe compounds. In a preferred embodiment, the binary phase comprises at least one of Fe2AI5 or FeAI3. The binary phase may also comprise Si in solid solution.

[0037] Preferably, the Cu content in the binary phase is from 250 to 2500 wt. ppm.

[0050]

[0038] The ternary phase 222 comprises mainly Fe, Al, and Si, meaning that Fe, Al, and Si together account for more than 50 wt. % in the ternary phase. The ternary phase 222 comprises from 20 to 50 wt. % Al, from 6.0 to 20.0 wt. % Si, unavoidable impurities up to 1.0 wt. %, at least 100 ppm of Cu, the balance being Fe. Preferably, the ternary phase comprises from 6.0 to 12.0 wt.% Si.

[0051]

[0039] Preferably, the ternary phase comprises AIFeSi compounds. It can comprise ternary elements or FeAl binary compounds with Si in solid solution. In preferred embodiment, the ternary phase comprises at least one of AI3Fe3Si2, AI12Fe3Si2, AI20Fe5Si2 or FeAI(Si). It seems that the proportion between these compounds depends on the heat treatment.

[0052]

[0040] Preferably, the Cu content in the ternary phase is from 150 to 1500 wt.

[0053] ppm.

[0054]

[0041] The distribution of the binary and ternary phases in the alloyed layer depends on the heat treatment. It is not particularly limited. In a preferred embodiment, the ternary phase forms one or two layers within the binary phase. Preferably, the ternary phase is in contact with the interdiffusion layer. The phase distribution may be also discontinuous and have for example the shape of nodules.

[0055]

[0042] According to the invention, the Cu content in the binary phase is at least 90 ppm more than the Cu content in the ternary phase. Without to be bound by theory, the inventors have showed that this minimal difference ensures an improved corrosion performance.

[0056]

[0043] It seems that the Cu content difference between both phases promotes the formation of compact Cu-containing corrosion product reaching the surface of steel where the coating is cracked. This compact corrosion products block the diffusion of oxygen towards steel surface and the corrosion of steel is slowed.

[0057]

[0044] Preferably, the weight ratio Al / Cu in the binary phase and / or the ternary phase is from 100 to 3000. If it is less than 100, the alloying kinetics could be too quick and the interdiffusion layer may grow excessively. If it is more than 3000, the corrosion performance could be decreased.

[0058]

[0045] Advantageously, the weight ratio Al / Cu in the binary phase and / or the ternary phase is from 250 to 2500, or even from 300 to 1500.

[0059]

[0046] The coating is completely through alloyed, meaning that Fe from the steel substrate is present at the surface of the coating. Preferably, Fe is present in form of Fe2AI5 on the surface of the alloyed coating.

[0060]

[0047] Manufacturing process of the coated press hardened part

[0061]

[0048] A steel sheet is coated by dipping into a hot metallic bath comprising from 7.0 to 12.0 wt. % of Si, up to 3.0 wt. % of Fe coming from the steel substrate by dilution, from 80 to 500 wt. ppm of Cu, unavoidable impurities up to 0.4 wt. %, the balance being Al.

[0062]

[0049] Preferably, the bath comprises from 9.0 to 11.0 wt. % of Si.

[0063]

[0050] Preferably, the bath comprises from 100 to 400 wt. ppm of Cu, and advantageously from 150 to 300 wt. ppm of Cu.

[0064]

[0051] Unavoidable impurities present in the metal bath may come from the steel sheet by dilution or from the ingots used to feed the bath. Impurity elements are for example Sn, Sb, Sn, As, Pb, Mg, Ca, La, Ce, Y, Ti, Ni, Mo, Cr, Co, V, Nb, Mn, Sr, B, Li, Zr, Mo, W, Ag, Bi and Zn. The total amount of all impurities in the bath doesn’t exceed 0.4 wt. %, preferably 0.2 wt. %.

[0065]

[0052] When the steel sheet is immersed in the hot dip bath, iron from the steel substrate is diluted in the hot bath and an intermetallic layer 3 comprising Al, Si, Cu, and Iron (Fe) coming from the steel sheet by diffusion is formed on the surface of the steel sheet.

[0066]

[0053] The intermetallic layer may comprise a first sublayer 31 in contact with the steel substrate, and a second sublayer 32 on top of the first.

[0067]

[0054] The thickness of the liquid metal film remaining on the steel sheet can be set by any mean. For example, the liquid metal is wiped on the steel sheet by gas knives. Electromagnetic means may be also used. The liquid metal deposited is then solidified by cooling to form a precoating layer 4. The thickness of the coating layer is from 15 to 35 pm, preferably from 20 to 30 pm. The coating layer has the same composition as the coating bath.

[0055] A coated steel sheet is obtained, which is comprising a steel sheet 1 covered by an intermetallic layer, said intermetallic layer 3 being covered by a coating layer 4, wherein the steel sheet comprises, 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% < Ca < 0.01 % ; 0% < V < 0.35% ; 0% < W < 0.35%, from 200 to 3000 wt. ppm of Cu, unavoidable impurities from the manufacturing process of steel, the balance being iron, wherein the intermetallic layer 3 comprises a first sublayer 31 in contact with the steel substrate and having a thickness from 0.5 to 2.0 pm, and a second sublayer 32 on top of the first sublayer and having a thickness from 3 to 7 pm, wherein said first sublayer comprises from 30 to 60 wt. % Fe, up to 4.5 wt. % Si, from 200 to 3000 wt. ppm Cu and unavoidable impurities, the remainder being Al, and said second sublayer comprises from 20 to 50 wt. % Fe, from 6.0 to 15.0 wt. % Si, from 10 to 1000 wt. ppm Cu and unavoidable impurities, the remainder being Al, and wherein said coating layer on top of said intermetallic layer has a thickness from 10 to 35 pm, said coating layer comprising from 8.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, unavoidable impurities, from 80 to 500 ppm of Cu, the balance being aluminium.

[0068]

[0056] Optionally, a temper rolling step, also called skin pass, is performed on the coated steel sheet. This operation allows to erase some punctual surface defects and to adjust the final mechanical properties of the steel sheet. The temper rolling elongation is from 0.1 to 2.0 %, preferably from 0.1 to 1.0 %, advantageously from 0.2 to 0.5 %.

[0069]

[0057] After coating, the steel sheet is cut and heated in a furnace at a temperature from 850 to 980°C for 3 to 15 minutes, preferably 3 to 10 minutes, to achieve a full austenitic microstructure and a proper alloying of the coating.

[0070]

[0058] The present invention relies on the fact the copper comprised in the alloyed layer comes from the steel substrate by diffusion, from the coating and from the intermetallic layer. Indeed, the alloyed coating according to the invention is achieved by a manufacturing method including Cu not only from the steel substrate, nor only from the coating bath but from both.

[0071]

[0059] If the steel sheet comprises less than 200 wt. ppm of Cu, the alloyed coating layer won’t contain enough Cu. If the steel sheet comprises more than 3000 wt. ppm of Cu, the steel may become difficult to cast.

[0072]

[0060] Preferably, the steel sheet comprises up to 2000 wt. ppm of Cu.

[0073]

[0061] If the coating comprises less than 80 wt. ppm of Cu, the alloyed coating layer won’t contain enough Cu. This will decrease the corrosion performance. If the coating comprises more than 500 wt. ppm of Cu, the kinetics of cooling the liquid metal film may be affected. The resulting intermetallic layer may not be according to the invention. This may prevent diffusion of elements like Cu from the substrate to the alloyed coating during austenitization heat treatment.

[0074]

[0062] Preferably, the coating comprises from 100 to 400 wt. ppm of Cu, and advantageously from 150 to 300 wt. ppm of Cu.

[0075]

[0063] It is then formed by press tools at a temperature from 600 to 830°. If the forming starts at a temperature higher than 830°C, the quenching speed down to the end of martensitic transformation may not be sufficient because of the thermal inertia of the press tools. If the forming starts at a temperature below 600°C microstructure won’t be sufficiently austenitic.

[0076]

[0064] Preferably, the quenching speed in the press tool is from 27 to 50°C / s.

[0077]

[0065] The invention will now be illustrated through examples which are not a limitation.

[0078] EXAMPLES

[0079]

[0066] Manufacturing of samples.

[0080]

[0067] Steel grades SA to SD with a composition according to table 1 where coated according to table 2.

[0081]

[0068] Trials 1 to 4 were temper rolled with an elongation from 0.8 to 1.5 % and then cut into blanks with dimension of 300x250 mm.

[0082]

[0069] Steel sheets of trials 1 to 3 according to the invention have an intermetallic comprising a first sublayer in contact with the steel substrate and having a thickness from 0.5 to 2.0 pm, and a second sublayer on top of the first sublayer and having a thickness from 3 to 7 pm. The first sublayer comprises from 30 to 60 wt. % Fe, up to 4.5 wt. % Si, from 200 to 3000 wt. ppm Cu and unavoidable impurities up to 0,3 wt. %, the remainder being Al. The second sublayer comprises less Cu than the steel substrate. It from 20 to 50 wt. % Fe, from 6.0 to 12.0 wt. %Si, from 10 to 1000 wt. ppm Cu and unavoidable impurities up to 0,3 wt. %, the remainder being Al.

[0083]

[0070] The trials were then heat treated at 900°C for 6 minutes and 30 s under air atmosphere. They were transferred in less than 5 s to ensure a temperature from 600° to 830°C to press tool and were eventually hot formed.

[0084]

[0071] Trials 1 to 4 show a coating layer from 40 to 55 pm, and the interdiffusion layer is from 3 to 35 pm. For the trials 1 to 3, the interdiffusion layer comprises more than 60 wt. % Fe, Si from 0.1 to 11.0 wt. %, Cu from 150 to 3000 wt. ppm, unavoidable impurities up to 1.0 wt. %, the remainder being Al.

[0085]

[0072] Samples were cut in cross-section. The cross-section was embedded in resin and then polished. They are analyzed by mean of EPMA (Electron Probe MicroAnalyzer) on WDS (Wavelength Dispersive Energy) spectrometers of model JXA 8530F Plus form equipment manufacturer JEOL with the following parameters:

[0086] Analysis pitch 0.5 pm,

[0087] Acceleration voltage 17KV,

[0088] Quantitative analysis by peak and background measurement at each point.

[0089]

[0073] Thanks to EPMA analysis, the content of each element can be determined along the thickness, as shown on figure 3. Coming from the coating surface, the limit between the coating and the steel is defined as the first the point where the local Al content is lower than 3 wt. %. This is also the limit between the steel and the interdiffusion layer.

[0090]

[0074] Corrosion Test.

[0091]

[0075] Samples were prepared as follows:

[0092] Cutting of modalities after heat treatment in size 100 x 100 mm The corrosion test is performed on 3 samples 100 mm x 100 mm per modality for a good reproducibility. Edges are protected to avoid the corrosion initiation.

[0093] Samples are stored for 6 weeks in a corrosion chamber according to standard ISO11997-3.

[0094]

[0076] Then the corrosion products are removed by chemical dissolution (HCI+HMTA inhibitor). The corroded volume was determined by use of a confocal bench with the following parameters:

[0095] X = 80mm

[0096] Y = 80mm

[0097] Step = 15pm

[0098] Frequency = 1000 Hz

[0099] Using Mountain Software (Version 7.4.8164) for the acquisition.

[0100]

[0077] Results are shown in table 3.

[0101]

[0078] Table 1: Steel composition

[0102]

[0103]

[0079] Table 2: Manufacturing process

[0104]

[0105] *trials according to the invention

[0106] Underlined values are not according to the invention

[0107]

[0080] Table 3: Alloyed coating analysis and corrosion results.

[0108]

[0109] *trials according to the invention

[0110] Underlined values are not according to the invention

[0111] rem. stands for remainder.

[0112] IDL stands for interdiffusion layer.

[0113]

[0081] Trials 1 to 3 according to the invention show a reduced corroded volume compared to trial 4.

Claims

1. CLAIMS1. A press hardened coated steel part consisting of:3.a hardened steel substrate (1) comprising, 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% < Ca < 0.01 % ; 0% < V < 0.35%, 0% < W < 0.35%, from 200 to 3000 wt. ppm of Cu, unavoidable impurities from the manufacturing of steel up to 1.0 wt. %, the balance being iron, and4.a coating layer (2) having a thickness from 25 to 55 pm and comprising successively:5.an interdiffusion layer (21) in contact with the steel substrate and having a thickness of from 3 to 45 pm, said interdiffusion layer comprising more than 60 wt. % Fe, Si from 0.1 to 11.0 wt. %, Cu from 150 to 3000 wt. ppm, unavoidable impurities up to 1.0 wt. %, the remainder being Al,6.an alloyed layer (22) comprising Al, Fe, Si, Cu, wherein said alloyed coating layer consists of a binary phase (221) and a ternary phase (222), wherein said binary phase comprises from 35 to 60 wt. % Fe, from 0.1 to 5.0 wt. % Si, unavoidable impurities up to 1.0 wt. %, at least 190 wt. ppm of Cu, the balance being aluminium, and wherein said ternary phase comprises from 20 to 50 wt. % Al, from 6.0 to 20.0 weight % Si, unavoidable impurities up to 1.0 wt. %, at least 100 ppm of Cu, the balance being Fe, and7.wherein, in the alloyed layer, Cu content in the binary phase is at least 90 ppm more than in the ternary phase.

2. A press hardened steel part according to claim 1, wherein the binary phase (221 ) comprises at least one of Fe2AI5 or FeAI3.

3. A press hardened steel part according to claims 1 or 2, wherein the ternary phase comprises at least one of AI3Fe3Si2, AI12Fe3Si2, AI20Fe5Si2 or FeAI(Si).

4. A press hardened steel part according to anyone of claims 1 to 3, wherein the Cu content in the ternary phase is from 150 to 1500 wt. ppm.

5. A press hardened steel part according to anyone of claims 1 or 4, wherein the Cu content in the binary phase is from 250 to 2500 wt. ppm.

6. A press hardened steel part according to anyone of claims 1 to 5, wherein the Al / Cu content ratio in the binary and / or ternary phases of the coating layer is from 100 to 3000.

7. A press hardened steel part according to anyone of claims 1 to 6, wherein Fe2AI5 compounds are present at the surface of the alloyed coating.

8. A method to produce a press hardened coated steel sheet comprising the following steps:14.A) Providing a steel sheet comprising, 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% < Ca < 0.01% ; 0% < V < 0.35% ; 0% < W < 0.35%, from 200 to 3000 wt. ppm of Cu, unavoidable impurities from the manufacturing of steel, the balance being iron,15.B) Dipping said steel sheet in a metal bath comprising from 8.0 to 12.0 wt% of silicon, up to 3.0 wt% of iron, unavoidable impurities up to 0.4 wt%, from 80 to 500 ppm of Cu, the balance being aluminium,16.C) Setting the thickness of the liquid metal film from 15 to 35 pm, D) Cooling said liquid metal film to from a solid precoating layer, E) Feeding the metal bath to compensate the liquid metal staying on the steel sheet while maintain the bath composition according to step B),17.F) Cutting the steel sheet into a blank,18.G) Heating the said blank at a temperature from 850 to 980°c for 3 to 15 minutes,19.H) Transferring said coated steel to a press forming tool, I) Hot forming said coated steel substrate at a temperature from 600 to 830°C,20.J) Quenching said formed coated steel substrate in the forming tool to obtain a press hardened steel part.

9. A method according to claim 8, wherein in step E), is performed by the metal bath to compensate the liquid metal staying on the steel sheet with ingots comprising from 7.0 to 12.0 wt% of silicon, unavoidable impurities up to 0.4 wt. %, from 10 to 200 wt ppm of copper, the balance being aluminium.

10. A method according to claims 8 or 9, wherein after D and before step F, the steel sheet is temper rolled.

11. A method according to claim 10, wherein the temper roll elongation is from 0.1 to 2.0 %.

12. A method according to claim 8 to 11 , wherein step J occurs at a cooling speed from 27 to 50°C / s.

13. A coated steel sheet suitable for press hardening comprising a steel sheet 1 covered by an intermetallic layer, said intermetallic layer 3 being covered by a coating layer 4, wherein the steel sheet comprises, 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% < Ca < 0.01 % ; 0% < V < 0.35% ; 0% < W < 0.35%, from 200 to 3000 wt. ppm of Cu, unavoidable impurities from the manufacturing process of steel, the balance being iron, wherein the intermetallic layer 3 comprises a first sublayer 31 in contact with the steel substrate and having a thickness from 0.5 to 2.0 pm, and a second sublayer 32 on top of the first sublayer and having a thickness from 3 to 7 pm, wherein said first sublayer comprises from 30 to 60 wt. % Fe, up to 4.5 wt. % Si, from 200 to 3000 wt. ppm Cu and unavoidable impurities, the remainder being Al, and said second sublayer comprises from 20 to 50 wt. % Fe, from 6.0 to 15.0 wt. % Si, from 10 to 1000 wt. ppm Cu and unavoidable impurities, the remainder being Al, and wherein said coating layer on top of said intermetallic layer has a thickness from 10 to 35 pm, said coating layer comprising from 8.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, unavoidable impurities, from 80 to 500 ppm of Cu, the balance being aluminium.

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