Steel sheet having excellent corrosion properties after press hardening and method for manufacturing the same

Abstract

A steel sheet, coated with a metallic coating including, by weight percent, from 7.5 to 9.0% of zinc, from 1.1 to 4.0% of silicon, from 1.1 to 8.0% of magnesium, up to 3.0% of iron, optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium and unavoidable impurities up to 0.02%, a balance being aluminum, and wherein the coating weight of the metallic coating is from 50 to 500 g / m2 for the sum of both sides of the-steel sheet.

Description

[0001] The present invention relates to a method for the manufacture of hardened parts starting from a steel sheet coated with a metallic coating. The part has good characteristics with respect to cosmetic corrosion resistance after painting. The present invention is particularly well suited for the manufacture of automotive vehicles.BACKGROUND

[0002] In recent years the use of coated steels in hot-stamping processes for the shaping of parts has become important, especially in the automotive industry. Fabrication of such parts may include the following main steps:

[0003] Coating of a steel sheets, by hot dipping.

[0004] Trimming or cutting for obtaining blanks.

[0005] Heating the blanks in order to obtain alloying of the steel substrate with the coating, as well as the austenitizing of the steel.

[0006] Press hardening of the part in order to obtain a predominantly martensitic structure.SUMMARY OF THE INVENTION

[0007] Thanks to an alloying of the coating with the steel substrate, which 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.

[0008] Hardened parts can be coated with zinc-based coating or aluminum-based coating.

[0009] Zinc-based coatings are generally used because they allow 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 than steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel.

[0010] However, when a press hardening process is performed on such zinc coated steel sheets, for example by hot-stamping, microcracks are observed in steel which spread from the coating. Additionally, 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.

[0011] Aluminum-based coatings have a good aptitude for painting. They allow for a protection by barrier effect and can be welded. However, they do not allow for a cathodic protection or they have a very low cathodic protection.

[0012] It is an object of the present invention to provide a coated steel sheet providing cathodic protection and a suitable method for manufacturing a press hardened part with a good corrosion performance after phosphatizing without prior sanding operations.

[0013] The present invention provides a steel sheet, coated with a metallic coating comprising, by weight percent, from 7.5 to 9.0% of zinc, from 1.1 to 4.0% of silicon, from 1.1 to 8.0% of magnesium, up to 3.0% of iron, optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium and unavoidable impurities up to 0.02%, the balance being aluminum, and wherein the coating weight of said metallic coating is from 50 to 500 g / m2 for the sum of both sides of said steel sheet.

[0014] The present invention also provides a method for the manufacture of a hardened part coated with an anti-corrosion coating comprising the following steps:

[0015] A) the provision of a coated steel sheet as described above,

[0016] B) the cutting of the coated steel sheet to obtain a blank,

[0017] C) the thermal treatment of the blank at a temperature from 840 to 950° C. to obtain a fully austenitic microstructure in the steel,

[0018] D) the transfer of the blank into a press tool,

[0019] E) the press hardening of the blank to obtain a part,

[0020] F) the cooling of the part obtained at step E) in order to obtain a press-hardened part.

[0021] The present invention also provides a press-hardened coated steel part obtained by press-hardening of a coated steel sheet as described above, said coating being topped by a superficial oxide layer on its outer surface, such oxide layer comprising from 20 to 60% weight of zinc on a thickness of 1.5 μm from the outer surface of the coated part.

[0022] A final object of the invention is the use of such a coated part for the manufacture of an automotive vehicle.

[0023] Other characteristics and advantages of the present invention will become apparent from the following detailed description of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] To illustrate the present invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following figures obtained by scanning electron microscopy with a magnification of ×500:

[0025] FIG. 1 illustrates the uniform structure observed by cross-section of the metallic coating after heat-treatment on a 1.5 mm thick steel sheet with a coating comprising 8% by weight of zinc (trial 8), according to the present invention.

[0026] FIG. 2 illustrates the non-uniform structure observed by cross-section of the metallic coating after heat-treatment on a 1.5 mm thick steel sheet, with a coating comprising 15% by weight of zinc (trial 12), not according to the present invention.DETAILED DESCRIPTION

[0027] The present invention provides for a steel sheet coated with a metallic coating comprising, by weight, from 7.5 to 9.0% of zinc, from 1.1 to 7.0% of silicon, from 1.1 to 8.0% of magnesium, up to 3.0% of iron, and unavoidable impurities up to 0.02%, the balance being aluminum.

[0028] Preferably, the coating comprises, in weight percent, from 2.0 to 4.0% of silicon and from 1.1 to 4.0% of magnesium, advantageously from 1.5 to 2.5% of magnesium.

[0029] Optionally, the coating comprises additional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the content by weight of each additional element being inferior to 0.3 wt. %. In a preferred embodiment, up to 100 ppm of calcium is added.

[0030] Finally, the coating may contain unavoidable impurities up to 0.02%, preferably up to 0.01%.

[0031] The steel sheet according to the present invention can be manufactured by hot dip galvanizing in a bath, the temperature of which is set from 600 to 700° C., preferably from 620 to 650° C.

[0032] The coating weight is set during the wiping process by gas knives in a range from 50 to 500 g / m2, possibly from 80 to 150 g / m2 and preferably from 100 and 120 g / m2 for the sum of both sides of the steel sheet.

[0033] Before being coated, the steel sheet according to the present 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.

[0034] The steel substrate to be coated can have any appropriate composition, depending on the final properties required. When the steel is used for press-hardening, its composition is preferably as described below.

[0035] The coated steel sheet according to the present invention can notably be used in a press hardening method. In particular, it can be used in the frame of a method of manufacturing of a press hardened part according to the invention.

[0036] This method according to the present invention comprises the following steps:

[0037] A) the provision of a coated steel sheet according to the invention,

[0038] B) the cutting of the coated steel sheet to obtain a blank,

[0039] C) the thermal treatment of the blank at a temperature between 84° and 950° C. to obtain a fully austenitic microstructure in the steel,

[0040] D) the transfer of the blank into a press tool,

[0041] E) the press-hardening of the blank to obtain a part, and

[0042] F) the cooling of the part obtained at step E) in order to obtain a press-hardened part.

[0043] In step A, any steel can be advantageously used in the frame of the present invention. However, in case steel having high mechanical strength is needed, in particular for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500 MPa, advantageously between 500 and 2000 MPa before or after heat-treatment, can be used. 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] In another embodiment, the steel sheet 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 Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

[0049] 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.

[0050] 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.

[0051] The steel sheet is cut into a blank in step B. Said coated steel blank may have a thickness which is not uniform. This is the case of the so-called “tailored rolled blanks” which are obtained from cutting a sheet obtained by a process of rolling with an effort which is variable along the direction of the length of the sheet. Or this may be also the case of the so-called “tailored welded blanks” obtained by the welding of at least two sub-blanks of different thicknesses.

[0052] In step C, a heat treatment of the blank is performed at a temperature from 800 to 970° C., preferably from 840 to 950° C. Said blank is maintained during a dwell time from 1 to 15 minutes. During the heat treatment before the press hardening, the coating forms an alloy layer having a high resistance to corrosion, abrasion, wear and fatigue.

[0053] In step D, after the heat treatment, the blank is then transferred to a press-hardening tool.

[0054] In step E, the press-hardening takes place at a temperature from 600 to 830° C.

[0055] In step F, the part is cooled in the hot-forming tool or after the transfer to a specific cooling tool. The cooling rate is controlled depending on the steel composition, in such a way that the final microstructure after press hardening is consistent with the targeted mechanical properties. After press hardening, the part can be tempered to reach the targeted microstructure and mechanical properties.

[0056] In a preferred embodiment, the steel microstructure comprises, in terms of volume fraction, at least 95% of martensite.

[0057] In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, at least 50% of martensite and less than 40% of bainite.

[0058] In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, from 5 to 20% of martensite, up to 10% of bainite and at least 75% of equiaxed ferrite.

[0059] A coated part according to the present invention is thus obtained by press hardening but is also achievable by any suitable combination of cold-stamping and press hardening.

[0060] The part obtained in step F is topped by a superficial oxide layer on its outer surface. This oxide layer comprises aluminum, zinc and magnesium from the coating and iron from the steel substrate. Iron has diffused through the coating during heat treatment. The thickness of said oxide layer can vary from 0.2 up to 3 μm, preferably from 0.3 to 1.5 μm. Oxidizable elements have their highest concentration at the vicinity of the surface. The proportion of each element can be obtained by Energy X-ray dispersive spectroscopy. It gives thus the composition of a layer having a thickness of 1.5 μm from the outer surface.

[0061] According to the present invention, the superficial oxide layer comprises zinc from 20 to 60% by weight, preferably 25 to 50%.

[0062] According to the present invention, the superficial oxide layer comprises optionally aluminum from 10 to 27% by weight, preferably 17 to 24%.

[0063] According to the present invention, the superficial oxide layer comprises optionally magnesium from 5 to 10% by weight.

[0064] According to the present invention, the superficial oxide layer comprises optionally iron from 10 to 28% by weight, preferably 14 to 25%.

[0065] Without willing to be bound by any theory, it is believed that the corrosion performance after phosphating step is related to the zinc content in the superficial oxide layer having a depth of 1.5 μm or less from the outer surface of the coating.

[0066] If there is less than 20% in weight of zinc in this superficial oxide layer, the surface is mainly composed of aluminum oxide, which is not phosphatable. It is believed that zinc oxides are not covering enough the upper surface to ensure a proper layer of phosphate crystals after phosphatizing, resulting in a poor corrosion performance.

[0067] If there is more than 60% in weight of zinc in this superficial layer, the surface becomes rough and the layer structure becomes non-uniform, as can be seen on FIG. 2. It is believed that this leads then to poor corrosion performance.

[0068] This present invention is notably relevant to manufacture any parts relevant for crash which are in wet areas. Specifically, the part can be 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.

[0069] For automotive applications, the part is previously degreased and phosphated to ensure the adhesion of the other layers. Then, the part is dipped in an e-coating bath forming a layer by cataphoresis on the part. After the e-coating step, other paint layers can be deposited, for example, a primer coat of paint, a basecoat layer and a top-coat layer.

[0070] Usually, the thickness of the phosphate layer is from 1 to 2 μm and the thickness of the e-coating layer is between 15 and 25 μm, preferably inferior or equal to 20 μm. The cataphoresis layer ensures an additional protection against corrosion

[0071] The present invention will now be explained in trials carried out for information only. They are not limiting.EXAMPLES

[0072] For all samples, steel sheets 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%.

[0073] All coatings were deposited by hot-dip galvanization process. The hot dip bath temperature was set at 620 or 650° C.Example 1: Surface Analysis

[0074] Trials 1 to 13 were therefore prepared as follows: coated samples were cut into blanks. These blanks were then heated at a temperature of 900° C. during a dwell time varying from 5 to 6 minutes. Blanks were transferred into a press tool and hot-stamped to obtain a part. Finally, the part was cooled to obtain a hardening by martensitic transformation. After press hardening and when observed with a microscope, trial 8 has a smoother surface and a uniform layer structure, as can be observed on FIG. 1. The same goes for the other trials according to the present invention. Trials 11 to 13, on the contrary, show a rough surface with non-uniform layer structure as can be seen on FIG. 2.

[0075] After heat treatment, trials were subjected to the EDX test to deliver their superficial composition of the outer layer having a thickness of 1.5 μm: surface analysis test is used to determine the weight percentage of elements on the surface. After heat treatment, the samples were analyzed using Energy X-ray dispersive spectroscopy (EDX) at 15 kV to determine the superficial atomic composition of the oxide layer. Results are gathered in table 1.TABLE 1Samples testedSum ofcoatingHeat TreatmentZn content in theweight onParametersoxide layer ofSheetbothDwellpress hardenedThicknessCoating compositionsidesFurnaceTimepartTrial Nr(mm)ZnMgSiAl(g / m2)T (° C.)(min)(wt %)1 1.205.02.03.2Balance1209005192*1.207.52.03.2Balance1209005313*1.208.22.03.0Balance1209005324*1.208.22.03.0Balance1209006485*1.358.22.03.0Balance1209005336*1.358.22.03.0Balance1209006447*1.508.22.03.0Balance1209005428*1.508.22.03.0Balance1209006299*1.658.22.03.0Balance12090053610* 1.658.22.03.0Balance12090063811 1.2015.0 1.53.5Balance8090056312 1.5015.0 1.53.5Balance12090056513 1.2015.0 1.53.5Balance150900570*Trials according to the present invention, underlined values are not according to the present invention.Example 2: Corrosion Test

[0076] A degreasing of the samples was then 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 concentrations are disclosed in table 2.TABLE 2Component concentrations in phosphating bathGardobond ® product typeR24TAH7101H7141H7257H7102H7141H7255H7004Concentration58.07.05.04.54.02.42.40.4(g / L)

[0077] The samples were then wiped with water and dried with hot air and finally stored in corrosion chambers for 12 cycles according to VDA 233-102 standard.

[0078] All the samples according to the present invention had less than 20% red rust in terms of surface percentage after these cycles. Results are gathered in table 3.TABLE 3Corrosion resultsRed Rust on PO4 after 12VDA cyclesTrial Nr(Surface %) 126.9 2* 7.7 3* 1.1 4* 3.8 5*13.1 6* 8.4 7* 1.7 8* 7.4 9* 7.710* 5.31121.41227.91370.6*Trials according to the present invention, underlined values are not according to the present invention.

[0079] Trials 1, and 11 to 13, the steel sheet coating of which contains respectively 5 and 15 weight % of zinc, have also less than 20% or more than 60 weight % of zinc in the oxide layer after heat treatment. They show more than 20% of red rust in terms of area portion.

[0080] The trials according to the present invention show less than 20% of red rust in terms of area portion.

Examples

example 1

Surface Analysis

[0074]Trials 1 to 13 were therefore prepared as follows: coated samples were cut into blanks. These blanks were then heated at a temperature of 900° C. during a dwell time varying from 5 to 6 minutes. Blanks were transferred into a press tool and hot-stamped to obtain a part. Finally, the part was cooled to obtain a hardening by martensitic transformation. After press hardening and when observed with a microscope, trial 8 has a smoother surface and a uniform layer structure, as can be observed on FIG. 1. The same goes for the other trials according to the present invention. Trials 11 to 13, on the contrary, show a rough surface with non-uniform layer structure as can be seen on FIG. 2.

[0075]After heat treatment, trials were subjected to the EDX test to deliver their superficial composition of the outer layer having a thickness of 1.5 μm: surface analysis test is used to determine the weight percentage of elements on the surface. After heat treatment, the samples were...

example 2

Corrosion Test

[0076]A degreasing of the samples was then 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 concentrations are disclosed in table 2.

TABLE 2Component concentrations in phosphating bathGardobond ® product typeR24TAH7101H7141H7257H7102H7141H7255H7004Concentration58.07.05.04.54.02.42.40.4(g / L)

[0077]The samples were then wiped with water and dried with hot air and finally stored in corrosion chambers for 12 cycles according to VDA 233-102 standard.

[0078]All the samples according to the present invention had less than 20% red rust in terms of surface percentage after these cycles. Results are gathered in table 3.

TABLE 3Corrosion resultsRed Rust on PO4 after 12VDA cyclesTrial Nr(Surface %) 126.9 2* 7.7 3* 1.1 4* 3.8 5*13.1 6* 8.4 7* 1.7 8* 7.4 9* 7.710* 5.31121.41227.91370.6*Trials according to the pre...

[0001] The present invention relates to a method for the manufacture of hardened parts starting from a steel sheet coated with a metallic coating. The part has good characteristics with respect to cosmetic corrosion resistance after painting. The present invention is particularly well suited for the manufacture of automotive vehicles.BACKGROUND

[0002] In recent years the use of coated steels in hot-stamping processes for the shaping of parts has become important, especially in the automotive industry. Fabrication of such parts may include the following main steps:

[0003] Coating of a steel sheets, by hot dipping.

[0004] Trimming or cutting for obtaining blanks.

[0005] Heating the blanks in order to obtain alloying of the steel substrate with the coating, as well as the austenitizing of the steel.

[0006] Press hardening of the part in order to obtain a predominantly martensitic structure.SUMMARY OF THE INVENTION

[0007] Thanks to an alloying of the coating with the steel substrate, which 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.

[0008] Hardened parts can be coated with zinc-based coating or aluminum-based coating.

[0009] Zinc-based coatings are generally used because they allow 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 than steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel.

[0010] However, when a press hardening process is performed on such zinc coated steel sheets, for example by hot-stamping, microcracks are observed in steel which spread from the coating. Additionally, 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.

[0011] Aluminum-based coatings have a good aptitude for painting. They allow for a protection by barrier effect and can be welded. However, they do not allow for a cathodic protection or they have a very low cathodic protection.

[0012] It is an object of the present invention to provide a coated steel sheet providing cathodic protection and a suitable method for manufacturing a press hardened part with a good corrosion performance after phosphatizing without prior sanding operations.

[0013] The present invention provides a steel sheet, coated with a metallic coating comprising, by weight percent, from 7.5 to 9.0% of zinc, from 1.1 to 4.0% of silicon, from 1.1 to 8.0% of magnesium, up to 3.0% of iron, optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium and unavoidable impurities up to 0.02%, the balance being aluminum, and wherein the coating weight of said metallic coating is from 50 to 500 g / m2 for the sum of both sides of said steel sheet.

[0014] The present invention also provides a method for the manufacture of a hardened part coated with an anti-corrosion coating comprising the following steps:

[0015] A) the provision of a coated steel sheet as described above,

[0016] B) the cutting of the coated steel sheet to obtain a blank,

[0017] C) the thermal treatment of the blank at a temperature from 840 to 950° C. to obtain a fully austenitic microstructure in the steel,

[0018] D) the transfer of the blank into a press tool,

[0019] E) the press hardening of the blank to obtain a part,

[0020] F) the cooling of the part obtained at step E) in order to obtain a press-hardened part.

[0021] The present invention also provides a press-hardened coated steel part obtained by press-hardening of a coated steel sheet as described above, said coating being topped by a superficial oxide layer on its outer surface, such oxide layer comprising from 20 to 60% weight of zinc on a thickness of 1.5 μm from the outer surface of the coated part.

[0022] A final object of the invention is the use of such a coated part for the manufacture of an automotive vehicle.

[0023] Other characteristics and advantages of the present invention will become apparent from the following detailed description of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] To illustrate the present invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following figures obtained by scanning electron microscopy with a magnification of ×500:

[0025] FIG. 1 illustrates the uniform structure observed by cross-section of the metallic coating after heat-treatment on a 1.5 mm thick steel sheet with a coating comprising 8% by weight of zinc (trial 8), according to the present invention.

[0026] FIG. 2 illustrates the non-uniform structure observed by cross-section of the metallic coating after heat-treatment on a 1.5 mm thick steel sheet, with a coating comprising 15% by weight of zinc (trial 12), not according to the present invention.DETAILED DESCRIPTION

[0027] The present invention provides for a steel sheet coated with a metallic coating comprising, by weight, from 7.5 to 9.0% of zinc, from 1.1 to 7.0% of silicon, from 1.1 to 8.0% of magnesium, up to 3.0% of iron, and unavoidable impurities up to 0.02%, the balance being aluminum.

[0028] Preferably, the coating comprises, in weight percent, from 2.0 to 4.0% of silicon and from 1.1 to 4.0% of magnesium, advantageously from 1.5 to 2.5% of magnesium.

[0029] Optionally, the coating comprises additional elements chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the content by weight of each additional element being inferior to 0.3 wt. %. In a preferred embodiment, up to 100 ppm of calcium is added.

[0030] Finally, the coating may contain unavoidable impurities up to 0.02%, preferably up to 0.01%.

[0031] The steel sheet according to the present invention can be manufactured by hot dip galvanizing in a bath, the temperature of which is set from 600 to 700° C., preferably from 620 to 650° C.

[0032] The coating weight is set during the wiping process by gas knives in a range from 50 to 500 g / m2, possibly from 80 to 150 g / m2 and preferably from 100 and 120 g / m2 for the sum of both sides of the steel sheet.

[0033] Before being coated, the steel sheet according to the present 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.

[0034] The steel substrate to be coated can have any appropriate composition, depending on the final properties required. When the steel is used for press-hardening, its composition is preferably as described below.

[0035] The coated steel sheet according to the present invention can notably be used in a press hardening method. In particular, it can be used in the frame of a method of manufacturing of a press hardened part according to the invention.

[0036] This method according to the present invention comprises the following steps:

[0037] A) the provision of a coated steel sheet according to the invention,

[0038] B) the cutting of the coated steel sheet to obtain a blank,

[0039] C) the thermal treatment of the blank at a temperature between 84° and 950° C. to obtain a fully austenitic microstructure in the steel,

[0040] D) the transfer of the blank into a press tool,

[0041] E) the press-hardening of the blank to obtain a part, and

[0042] F) the cooling of the part obtained at step E) in order to obtain a press-hardened part.

[0043] In step A, any steel can be advantageously used in the frame of the present invention. However, in case steel having high mechanical strength is needed, in particular for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500 MPa, advantageously between 500 and 2000 MPa before or after heat-treatment, can be used. 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] In another embodiment, the steel sheet 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 Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

[0049] 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.

[0050] 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.

[0051] The steel sheet is cut into a blank in step B. Said coated steel blank may have a thickness which is not uniform. This is the case of the so-called “tailored rolled blanks” which are obtained from cutting a sheet obtained by a process of rolling with an effort which is variable along the direction of the length of the sheet. Or this may be also the case of the so-called “tailored welded blanks” obtained by the welding of at least two sub-blanks of different thicknesses.

[0052] In step C, a heat treatment of the blank is performed at a temperature from 800 to 970° C., preferably from 840 to 950° C. Said blank is maintained during a dwell time from 1 to 15 minutes. During the heat treatment before the press hardening, the coating forms an alloy layer having a high resistance to corrosion, abrasion, wear and fatigue.

[0053] In step D, after the heat treatment, the blank is then transferred to a press-hardening tool.

[0054] In step E, the press-hardening takes place at a temperature from 600 to 830° C.

[0055] In step F, the part is cooled in the hot-forming tool or after the transfer to a specific cooling tool. The cooling rate is controlled depending on the steel composition, in such a way that the final microstructure after press hardening is consistent with the targeted mechanical properties. After press hardening, the part can be tempered to reach the targeted microstructure and mechanical properties.

[0056] In a preferred embodiment, the steel microstructure comprises, in terms of volume fraction, at least 95% of martensite.

[0057] In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, at least 50% of martensite and less than 40% of bainite.

[0058] In another embodiment, the steel microstructure comprises after press hardening, in terms of volume fraction, from 5 to 20% of martensite, up to 10% of bainite and at least 75% of equiaxed ferrite.

[0059] A coated part according to the present invention is thus obtained by press hardening but is also achievable by any suitable combination of cold-stamping and press hardening.

[0060] The part obtained in step F is topped by a superficial oxide layer on its outer surface. This oxide layer comprises aluminum, zinc and magnesium from the coating and iron from the steel substrate. Iron has diffused through the coating during heat treatment. The thickness of said oxide layer can vary from 0.2 up to 3 μm, preferably from 0.3 to 1.5 μm. Oxidizable elements have their highest concentration at the vicinity of the surface. The proportion of each element can be obtained by Energy X-ray dispersive spectroscopy. It gives thus the composition of a layer having a thickness of 1.5 μm from the outer surface.

[0061] According to the present invention, the superficial oxide layer comprises zinc from 20 to 60% by weight, preferably 25 to 50%.

[0062] According to the present invention, the superficial oxide layer comprises optionally aluminum from 10 to 27% by weight, preferably 17 to 24%.

[0063] According to the present invention, the superficial oxide layer comprises optionally magnesium from 5 to 10% by weight.

[0064] According to the present invention, the superficial oxide layer comprises optionally iron from 10 to 28% by weight, preferably 14 to 25%.

[0065] Without willing to be bound by any theory, it is believed that the corrosion performance after phosphating step is related to the zinc content in the superficial oxide layer having a depth of 1.5 μm or less from the outer surface of the coating.

[0066] If there is less than 20% in weight of zinc in this superficial oxide layer, the surface is mainly composed of aluminum oxide, which is not phosphatable. It is believed that zinc oxides are not covering enough the upper surface to ensure a proper layer of phosphate crystals after phosphatizing, resulting in a poor corrosion performance.

[0067] If there is more than 60% in weight of zinc in this superficial layer, the surface becomes rough and the layer structure becomes non-uniform, as can be seen on FIG. 2. It is believed that this leads then to poor corrosion performance.

[0068] This present invention is notably relevant to manufacture any parts relevant for crash which are in wet areas. Specifically, the part can be 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.

[0069] For automotive applications, the part is previously degreased and phosphated to ensure the adhesion of the other layers. Then, the part is dipped in an e-coating bath forming a layer by cataphoresis on the part. After the e-coating step, other paint layers can be deposited, for example, a primer coat of paint, a basecoat layer and a top-coat layer.

[0070] Usually, the thickness of the phosphate layer is from 1 to 2 μm and the thickness of the e-coating layer is between 15 and 25 μm, preferably inferior or equal to 20 μm. The cataphoresis layer ensures an additional protection against corrosion

[0071] The present invention will now be explained in trials carried out for information only. They are not limiting.EXAMPLES

[0072] For all samples, steel sheets 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%.

[0073] All coatings were deposited by hot-dip galvanization process. The hot dip bath temperature was set at 620 or 650° C.Example 1: Surface Analysis

[0074] Trials 1 to 13 were therefore prepared as follows: coated samples were cut into blanks. These blanks were then heated at a temperature of 900° C. during a dwell time varying from 5 to 6 minutes. Blanks were transferred into a press tool and hot-stamped to obtain a part. Finally, the part was cooled to obtain a hardening by martensitic transformation. After press hardening and when observed with a microscope, trial 8 has a smoother surface and a uniform layer structure, as can be observed on FIG. 1. The same goes for the other trials according to the present invention. Trials 11 to 13, on the contrary, show a rough surface with non-uniform layer structure as can be seen on FIG. 2.

[0075] After heat treatment, trials were subjected to the EDX test to deliver their superficial composition of the outer layer having a thickness of 1.5 μm: surface analysis test is used to determine the weight percentage of elements on the surface. After heat treatment, the samples were analyzed using Energy X-ray dispersive spectroscopy (EDX) at 15 kV to determine the superficial atomic composition of the oxide layer. Results are gathered in table 1.TABLE 1Samples testedSum ofcoatingHeat TreatmentZn content in theweight onParametersoxide layer ofSheetbothDwellpress hardenedThicknessCoating compositionsidesFurnaceTimepartTrial Nr(mm)ZnMgSiAl(g / m2)T (° C.)(min)(wt %)1 1.205.02.03.2Balance1209005192*1.207.52.03.2Balance1209005313*1.208.22.03.0Balance1209005324*1.208.22.03.0Balance1209006485*1.358.22.03.0Balance1209005336*1.358.22.03.0Balance1209006447*1.508.22.03.0Balance1209005428*1.508.22.03.0Balance1209006299*1.658.22.03.0Balance12090053610* 1.658.22.03.0Balance12090063811 1.2015.0 1.53.5Balance8090056312 1.5015.0 1.53.5Balance12090056513 1.2015.0 1.53.5Balance150900570*Trials according to the present invention, underlined values are not according to the present invention.Example 2: Corrosion Test

[0076] A degreasing of the samples was then 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 concentrations are disclosed in table 2.TABLE 2Component concentrations in phosphating bathGardobond ® product typeR24TAH7101H7141H7257H7102H7141H7255H7004Concentration58.07.05.04.54.02.42.40.4(g / L)

[0077] The samples were then wiped with water and dried with hot air and finally stored in corrosion chambers for 12 cycles according to VDA 233-102 standard.

[0078] All the samples according to the present invention had less than 20% red rust in terms of surface percentage after these cycles. Results are gathered in table 3.TABLE 3Corrosion resultsRed Rust on PO4 after 12VDA cyclesTrial Nr(Surface %) 126.9 2* 7.7 3* 1.1 4* 3.8 5*13.1 6* 8.4 7* 1.7 8* 7.4 9* 7.710* 5.31121.41227.91370.6*Trials according to the present invention, underlined values are not according to the present invention.

[0079] Trials 1, and 11 to 13, the steel sheet coating of which contains respectively 5 and 15 weight % of zinc, have also less than 20% or more than 60 weight % of zinc in the oxide layer after heat treatment. They show more than 20% of red rust in terms of area portion.

[0080] The trials according to the present invention show less than 20% of red rust in terms of area portion.

Examples

example 1

Surface Analysis

[0074]Trials 1 to 13 were therefore prepared as follows: coated samples were cut into blanks. These blanks were then heated at a temperature of 900° C. during a dwell time varying from 5 to 6 minutes. Blanks were transferred into a press tool and hot-stamped to obtain a part. Finally, the part was cooled to obtain a hardening by martensitic transformation. After press hardening and when observed with a microscope, trial 8 has a smoother surface and a uniform layer structure, as can be observed on FIG. 1. The same goes for the other trials according to the present invention. Trials 11 to 13, on the contrary, show a rough surface with non-uniform layer structure as can be seen on FIG. 2.

[0075]After heat treatment, trials were subjected to the EDX test to deliver their superficial composition of the outer layer having a thickness of 1.5 μm: surface analysis test is used to determine the weight percentage of elements on the surface. After heat treatment, the samples were...

example 2

Corrosion Test

[0076]A degreasing of the samples was then 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 concentrations are disclosed in table 2.

TABLE 2Component concentrations in phosphating bathGardobond ® product typeR24TAH7101H7141H7257H7102H7141H7255H7004Concentration58.07.05.04.54.02.42.40.4(g / L)

[0077]The samples were then wiped with water and dried with hot air and finally stored in corrosion chambers for 12 cycles according to VDA 233-102 standard.

[0078]All the samples according to the present invention had less than 20% red rust in terms of surface percentage after these cycles. Results are gathered in table 3.

TABLE 3Corrosion resultsRed Rust on PO4 after 12VDA cyclesTrial Nr(Surface %) 126.9 2* 7.7 3* 1.1 4* 3.8 5*13.1 6* 8.4 7* 1.7 8* 7.4 9* 7.710* 5.31121.41227.91370.6*Trials according to the pre...

Claims

1-12. (canceled)13. A coated steel sheet comprising:a metallic coating and a steel sheet coated with the metallic coating, the metallic coating comprising, by weight percent:from 7.5 to 9.0% of zinc;from 1.1 to 4.0% of silicon;from 1.1 to 8.0% of magnesium;up to 3.0% of iron;optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%; andoptionally up to 100 ppm of Calcium and unavoidable impurities up to 0.02%, a balance being aluminum; andwherein a coating weight of the metallic coating is from 50 to 500 g / m2 for the sum of both sides of the steel sheet.

14. The coated steel sheet according to claim 13 wherein the metallic coating includes, by weight percent,from 7.5 to 9.0% of zinc;2.0 to 4.0% of silicon;1.1 to 4.0% of magnesium;up to 3.0% of iron,and optional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each element being less than 0.3%, optionally up to 100 ppm of Calcium, and unavoidable impurities up to 0.01%, the balance being aluminum.

14. The coated steel sheet according to claim 13 wherein the coating weight of the coating is from 80 to 150 g / m2 for the sum of both sides of the steel sheet15. A method for manufacturing a hardened part coated with an anti-corrosion coating comprising the following steps:A) providing the coated steel sheet as recited in claim 13;B) cutting the coated steel sheet to obtain a blank;C) thermally treating the blank at a temperature from 840 to 950° C. to obtain a fully austenitic microstructure in the steel sheet;D) transferring the blank into a press tool;E) press hardening of the blank to obtain a part; andF) cooling of the part to obtain a press-hardened part.

16. A press-hardened coated steel part obtained by press-hardening of the coated steel sheet as recited in claim 13, the coating being topped by a superficial oxide layer on an outer surface, the oxide layer including 20 to 60% weight of zinc on a thickness of 1.5 μm from the outer surface of the coated part.

17. The press-hardened coated steel part according to claim 16, wherein the oxide layer includes 25 to 50% weight of zinc.

18. The press-hardened coated steel part according to claim 16 wherein the microstructure of the press-hardened part includes, in terms of volume fraction, at least 95% of martensite.

19. The press-hardened coated steel part according to claim 16 wherein the microstructure of the press-hardened part includes, in terms of volume fraction, at least 50% of martensite and less than 40% of bainite.

20. The press-hardened coated steel part according to claim 16 wherein the microstructure of the press-hardened part includes from 5 to 20% of martensite, up to 10% of bainite and at least 75% of equiaxed ferrite.

21. A method for manufacturing a hardened part of an automotive vehicle comprising employing the coated steel sheet as recited in claim 16.

22. The method as recited in claim 21 wherein the hardened part is a wet area part of the automotive vehicle.

23. The method as recited in claim 22 wherein the hardened part is chosen from at least one of the following group: 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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