Steel plate having excellent corrosion resistance after press hardening and method for manufacturing the same

Abstract

The steel sheet is coated with a metal coating containing, by weight percentage, 7.5 to 9.0% zinc, 1.1 to 4.0% silicon, 1.1 to 8.0% magnesium, maximum 3.0% iron, an optional element selected from Pb, Ni, Zr or Hf, each of which contains less than 0.3% by weight of the element, optionally a maximum of 100 ppm calcium and a maximum of 0.02% unavoidable impurities, with the balance being aluminum, and the coating weight of the metal coating is 50 to 500 g / m2 based on the total of both sides of the steel sheet. 2 That is, steel plate.

Description

[Technical Field]

[0001] The present invention relates to a method for producing hardened parts starting from steel sheets coated with a metallic coating. The parts have good properties in terms of surface corrosion resistance after painting. The invention is particularly well suited for the production of motor vehicles. [Background technology]

[0002] In recent years, the use of coated steels in hot stamping processes for forming parts has become increasingly important, especially in the automotive industry. The production of such parts involves the following main steps: -Coating steel sheets by hot dip galvanizing - Trimming or cutting to obtain blanks Alloying of the steel substrate with the coating and heating of the blank to obtain austenitization of the steel. -Press hardening of parts, primarily to obtain a martensitic structure may include:

[0003] Thanks to the alloying of the coating with the steel substrate, which has the effect of producing an intermetallic alloy with a high melting temperature, blanks with such a coating can be heated in the temperature range where austenitization of the metal substrate occurs, allowing further hardening by quenching.

[0004] The part to be hardened can be coated with a zinc-based coating or an aluminum-based coating.

[0005] Zinc-based coatings are commonly used because they provide protection against corrosion thanks to their barrier protection and cathodic protection. Sacrificial cathodic protection is based on the fact that zinc is a less noble metal than steel, so when corrosion occurs, zinc is consumed preferentially over steel. Summary of the Invention [Problem to be solved by the invention]

[0006] However, when such zinc-coated steel sheets are press-hardened, for example by hot stamping, microcracks are observed in the steel extending from the coating. Furthermore, painting a zinc-coated hardened part requires a sanding operation before phosphating due to the presence of a weak layer of oxide on the surface of the part.

[0007] Aluminum-based coatings have good suitability for painting. They provide effective barrier protection and can be welded. However, they do not provide cathodic protection or have very low cathodic protection. [Means for solving the problem]

[0008] The object of the present invention is to provide a coated steel sheet that provides cathodic protection and a suitable method for producing press-hardened parts that have good corrosion performance after phosphating without prior sanding operations.

[0009] This is achieved by the steel sheets according to claims 1-3.

[0010] Another subject of the invention is a method for production according to claim 4.

[0011] The invention also encompasses press-hardened parts coated with a metallic coating having an oxide layer on top of the coating according to claims 5-9.

[0012] A final subject of the invention is the use of such coated parts for the manufacture of motor vehicles according to claims 10-12.

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0014] To illustrate the invention, various embodiments and test articles of non-limiting examples will be described with particular reference to the following figures obtained by scanning electron microscopy at 500x magnification. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows the homogeneous structure observed by cross-section of a metal coating after heat treatment on a 1.5 mm thick steel sheet (specimen 8) with a coating containing 8% by weight of zinc according to the invention. [Figure 2] 1 shows the inhomogeneous structure observed by cross-section of a metal coating after heat treatment on a 1.5 mm thick steel sheet (specimen 12) with a coating containing 15% by weight of zinc, not according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention relates to a steel sheet coated with a metallic coating containing, by weight, 7.5 to 9.0% zinc, 1.1 to 7.0% silicon, 1.1 to 8.0% magnesium, maximum 3.0% iron, and maximum 0.02% unavoidable impurities, the balance being aluminum.

[0017] Preferably, the coating contains, by weight percentage, 2.0 to 4.0% silicon and 1.1 to 4.0% magnesium, advantageously 1.5 to 2.5% magnesium.

[0018] Optionally, the coating comprises an additional element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the weight content of each additional element is less than 0.3 wt%. In a preferred embodiment, up to 100 ppm of calcium is added.

[0019] Finally, the coating may contain up to 0.02%, preferably up to 0.01%, of unavoidable impurities.

[0020] The steel sheet according to the present invention can be produced by hot dip galvanizing in a bath, the temperature of which is set at 600 to 700°C, preferably 620 to 650°C.

[0021] The coating weight is 50 to 500 g / m² for both sides of the steel sheet. 2 , in some cases 80-150g / m 2 , preferably 100 to 120 g / m 2 The range is set during the gas knife wiping process.

[0022] Before being coated, the steel sheet according to the invention can be obtained by hot rolling and optionally cold rolling, depending on the desired thickness, which can be for example between 0.5 and 3.0 mm.

[0023] The steel substrate to be coated can have any suitable composition depending on the final properties required. If the steel is to be press hardened, it is preferred that its composition is as follows:

[0024] The coated steel sheets according to the invention can be used in particular in press hardening processes, in particular in the frames of the method according to the invention for producing press hardened parts.

[0025] The method according to the invention comprises the following steps: A) providing a coated steel sheet according to the present invention; B) cutting the coated steel sheet to obtain blanks; C) heat treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel; D) transferring the blank to a press tool; E) press hardening the blank to obtain a part; F) cooling the part obtained in step E) to obtain a press-hardened part.

[0026] Any steel can be advantageously used in the frame of the invention in step A. However, if steel with high mechanical strength is required, especially for components of the structure of a motor vehicle, steel with a tensile strength of more than 500 MPa, advantageously between 500 and 2000 MPa before or after heat treatment, can be used. The weight composition of the steel plate 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 remainder being unavoidable impurities from the production of iron and steel.

[0027] For example, the steel sheet is 22MnB5 having the following composition by weight: 0.20%≦C≦0.25%; 0.15%≦Si≦0.35%; 1.10%≦Mn≦1.40%; 0%≦Cr≦0.30%; 0.020%≦Ti≦0.060%; 0.020%≦Al≦0.060%; 0.002%≦B≦0.004%, the balance being unavoidable impurities from the production of iron and steel.

[0028] In another embodiment, the steel sheet has the following composition by weight: 0.24%≦C≦0.38%; 0.40%≦Mn≦3%; 0.10%≦Si≦0.70%; 0.015%≦Al≦0.070%; Cr≦2%; 0.25%≦Ni≦2%; 0.015%≦Ti≦0.10%; Nb≦0.060%; 0.0005%≦B≦0.0040%, the balance being unavoidable impurities resulting from the production of iron and steel.

[0029] Alternatively, the steel plate may have the following composition by weight: 0.30%≦C≦0.40%; 0.5%≦Mn≦1.0%; 0.40%≦Si≦0.80%; 0.1%≦Cr≦0.4%; 0.1%≦Mo≦0.5%; 0.01%≦Nb≦0.1%; 0.01%≦Al≦0.1%; 0.008%≦Ti≦0.003%; 0.0005%≦B≦0.003%; 0.0%≦P≦0.02%; 0.0%≦Ca≦0.001%; 0.0%≦S≦0.004%; 0.0%≦N≦0.005%, the remainder being unavoidable impurities resulting from the production of iron and steel.

[0030] In another embodiment, the steel sheet has the following composition by weight: 0.040%≦C≦0.100%; 0.80%≦Mn≦2.00%; 0%≦Si≦0.30%; 0%≦S≦0.005%; 0%≦P≦0.030%; 0.010%≦Al≦0.070%; 0.015%≦Nb≦0.100%; 0.030%≦Ti≦0.080%; 0%≦N≦0.009%; 0%≦Cu≦0.100%; 0%≦Ni≦0.100%; 0%≦Cr≦0.100%; 0%≦Mo≦0.100%, the balance being unavoidable impurities from the production of iron and steel.

[0031] In another embodiment, the steel sheet has the following composition by weight: 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% Cu, Ni and Mo, the balance being from the production of iron and steel. It contains unavoidable impurities that may cause

[0032] In another embodiment, the steel sheet has the following composition by weight: 0.015%≦C≦0.25%; 0.5%≦Mn≦1.8%; 0.1%≦Si≦1.25%; 0.01%≦Al≦0.1%; 0.1%≦Cr≦1.0%; 0.01%≦Ti≦0.1%; 0%≦S≦0.01%; 0.001%≦B≦0.004%; 0%≦P≦0.020%; 0%≦N≦0.01%, the balance being unavoidable impurities from the production of iron and steel.

[0033] Alternatively, the steel plate has the following composition by weight: 0.2%≦C≦0.34%; 0.5%≦Mn≦1.24%; 0.5%≦Si≦2.0%; 0%≦S≦0.01%; 0%≦P≦0.020%; 0%≦N≦0.01%, the balance being unavoidable impurities from the production of iron and steel.

[0034] The steel sheet is cut into blanks in step B. The coated steel blanks may have a non-uniform thickness. This is the case of so-called "tailor rolled blanks", which result from cutting the sheet obtained by a rolling process with a variable force along the length of the sheet. Or it may be the case of so-called "tailor welded blanks", which result from welding at least two sub-blanks of different thicknesses.

[0035] In step C, the blank is heat treated at a temperature of 800-970°C, preferably 840-950°C. The blank is maintained for a rest time of 1-15 minutes. During the heat treatment before press hardening, the coating forms an alloy layer that is highly resistant to corrosion, abrasion, wear and fatigue.

[0036] In step D, after heat treatment, the blank is then transferred to a press hardening tool.

[0037] In step E, press hardening is carried out at a temperature of 600-830°C.

[0038] In step F, the part is cooled either in the hot forming tool or after transfer to a specific cooling tool. The cooling rate is controlled depending on the steel composition so that the final microstructure after press hardening matches the target mechanical properties. After press hardening, the part can be tempered to reach the target microstructure and mechanical properties.

[0039] In a preferred embodiment, the steel microstructure comprises at least 95% martensite, in terms of volume fraction.

[0040] In another embodiment, the steel microstructure comprises, by volume fraction, at least 50% martensite and less than 40% bainite after compression hardening.

[0041] In another embodiment, the steel microstructure comprises, by volume fraction, 5-20% martensite, up to 10% bainite and at least 75% equiaxed ferrite after compression hardening.

[0042] Thus, the coated parts according to the invention are obtained by press hardening, but can also be achieved by any suitable combination of cold stamping and press hardening.

[0043] The component obtained in step F has a surface oxide layer covering its outer surface. This oxide layer contains aluminum, zinc, and magnesium from the coating and iron from the steel substrate. The iron diffused through the coating during the heat treatment. The thickness of the oxide layer can vary from 0.2 to 3 μm, preferably from 0.3 to 1.5 μm. The oxidizable elements have the highest concentration near the surface. The proportions of each element can be determined by high-energy X-ray spectroscopy. This therefore provides the composition of the layer 1.5 μm thick from the outer surface.

[0044] According to the present invention, the surface oxide layer contains 20 to 60% by weight of zinc, preferably 25 to 50%.

[0045] According to the present invention, the surface oxide layer optionally comprises 10 to 27% by weight of aluminum, preferably 17 to 24%.

[0046] According to the present invention, the surface oxide layer optionally comprises 5 to 10% by weight of magnesium.

[0047] According to the invention, the surface oxide layer optionally comprises 10 to 28% by weight of iron, preferably 14 to 25%.

[0048] Without being bound by any theory, it is believed that corrosion performance after the phosphating step is related to the zinc content of the surface oxide layer having a depth of 1.5 μm or less from the outer surface of the coating.

[0049] If less than 20% zinc by weight is present in this surface oxide layer, the surface will consist primarily of aluminum oxide and will not be phosphating-capable. It is believed that the zinc oxide does not cover enough of the top surface to ensure an adequate layer of phosphate crystals after phosphating, resulting in poor corrosion performance.

[0050] If there is more than 60% zinc by weight in this surface layer, the surface becomes rough and the structure of the layer becomes non-uniform, as can be seen in Figure 2. This is then thought to lead to poor corrosion performance.

[0051] The present invention is particularly relevant to the manufacture of any crash-related part in a wet area, such as a front rail, a seat cross member, a side sill member, a dash panel cross member, a front floor reinforcement, a rear floor cross member, a rear rail, a B-pillar, a door ring, or a shotgun.

[0052] In automotive applications, the part is first degreased and phosphated to ensure adhesion of other layers. The part is then immersed in an e-coating bath, which deposits a layer on the part by electrophoresis. After the e-coating step, other paint layers can be deposited, such as a primer coat, a base coat, and a top coat of paint.

[0053] Typically, the thickness of the phosphate layer is 1-2 μm, and the thickness of the electrolytic coating layer is between 15-25 μm, preferably less than 20 μm. The electrophoretic layer ensures additional protection against corrosion.

[0054] The present invention will now be described in terms of tests that have been carried out for informational purposes only and are not limiting. [Example]

[0055] For all samples, the steel plate used is 22MnB5. The steel composition is as follows: C = 0.23%; Mn = 1.2%; Si = 0.25%; Cr = 0.2%; Al = 0.04%; Ti = 0.04%; B = 0.003%.

[0056] All coatings were deposited by hot dip galvanizing, with the bath temperature set at 620 or 650°C.

[0057] Example 1: Surface Analysis Thus, specimens 1 to 13 were prepared as follows: the coated samples were cut into blanks. These blanks were then heated at 900°C for varying dwell times of 5 to 6 minutes. The blanks were transferred to a press tool and hot stamped to obtain the parts. Finally, the parts were cooled to obtain hardening through martensitic transformation. After press hardening, specimen 8, when observed under a microscope, has a smoother surface and a uniform layer structure, as can be seen in Figure 1. This is also true for the other specimens according to the invention. In contrast, specimens 11 to 13 exhibit a rough surface with an inhomogeneous layer structure, as can be seen in Figure 2.

[0058] After heat treatment, the specimens were subjected to EDX testing to determine the surface composition of their outer layer, which had a thickness of 1.5 μm: a surface analysis test was used to determine the weight percentage of elements on the surface. After heat treatment, the samples were analyzed using energy dispersive X-ray spectroscopy (EDX) at 15 kV to determine the surface atomic composition of the oxide layer. The results are summarized in Table 1.

[0059] [Table 1]

[0060] Example 2: Corrosion test The samples were then degreased, followed by a phosphating step achieved by immersing them in a bath solution containing phosphate for 3 minutes at 50°C. The components of the phosphating bath are products called Gardobond® obtained from the supplier Chemetall. Their concentrations are disclosed in Table 2.

[0061] [Table 2]

[0062] The samples were then wiped with water, dried with hot air and finally stored in a corrosion chamber for 12 cycles according to the standard VDA 233-102.

[0063] All samples according to the invention had less than 20% red rust on the surface after these cycles. The results are summarized in Table 3.

[0064] [Table 3]

[0065] Furthermore, specimens 1 and 11-13, whose steel sheet coatings contained 5 and 15 wt. % zinc, respectively, had less than 20 wt. % or more than 60 wt. % zinc in the oxide layer after heat treatment. In terms of areas, they showed more than 20% red rust.

[0066] The specimens according to the invention show less than 20% red rust in the area.

Claims

1. A steel sheet coated with a metal coating containing, in weight percentages, 7.5 to 9.0% zinc, 1.1 to 4.0% silicon, 1.1 to 8.0% magnesium, maximum 3.0% iron, optional elements selected from Pb, Ni, Zr or Hf, each of which contains less than 0.3% by weight of the element, optionally maximum 100 ppm calcium and maximum 0.02% unavoidable impurities, the balance being aluminum, and the coating weight of the metal coating is 50 to 500 g / m based on the total of both sides of the steel sheet. ２ That is, steel plate.

2. 2. The steel sheet according to claim 1, wherein the metallic coating comprises, in percentages by weight, 7.5 to 9.0% zinc, 2.0 to 4.0% silicon, 1.1 to 4.0% magnesium, maximum 3.0% iron, and optional elements selected from Pb, Ni, Zr or Hf with a content by weight of each element of less than 0.3%, optionally maximum 100 ppm calcium, and maximum 0.01% unavoidable impurities, the balance being aluminum.

3. The coating weight of the coating is 80 to 150 g / m2 in total on both sides of the steel plate. ２ The steel sheet according to claim 1 or 2,

4. 1. A method of manufacturing a hardened part coated with a corrosion protection coating, comprising the steps of: A) providing a coated steel sheet according to any one of claims 1 to 3, B) cutting the coated steel sheet to obtain blanks; C) heat treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure of the steel; D) transferring the blank to a press tool; E) press hardening said blank to obtain a part; F) cooling the part obtained in step E) to obtain a press-hardened part. A method comprising:

5. 4. A press-hardened coated steel part obtained by press-hardening the coated steel sheet according to any one of claims 1 to 3, wherein the coating is covered on its outer surface by a surface oxide layer, and such oxide layer contains 20 to 60 wt. % zinc at a thickness of 1.5 μm from the outer surface of the coated part.

6. A press-hardened coated steel part according to claim 5, wherein such oxide layer comprises 25 to 50% by weight of zinc.

7. 7. A press-hardened, coated steel part according to claim 5 or 6, wherein the microstructure of the press-hardened part comprises at least 95% martensite by volume fraction.

8. 7. A press-hardened and coated steel part according to claim 5 or 6, wherein the microstructure of the press-hardened part comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

9. 7. A press-hardened, coated steel part according to claim 5 or 6, wherein the microstructure of the press-hardened part comprises 5-20% martensite, a maximum of 10% bainite and at least 75% equiaxed ferrite.

10. Use of a component according to any one of claims 5 to 9 for the manufacture of a motor vehicle.

11. Use of a component according to claim 10 for the manufacture of a component located in a wet area of a motor vehicle.

12. Use of the part according to claim 11 for manufacturing at least one part selected from a front rail, a seat cross member, a side sill member, a dash panel cross member, a front floor reinforcement, a rear floor cross member, a rear rail, a B-pillar, a door ring, and a shotgun.

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