Phosphatable Part Starting from a Steel Sheet Coated with a Metallic Coating Based on Aluminum

Abstract

A hardened part coated with a phosphatable coating is provided. The part is made by a method that includes providing a steel sheet pre-coated with a metallic coating including from 4.0 to 20.0% by weight of zinc, from 1.0 to 3.5% by weight of silicon, optionally from 1.0 to 4.0% by weight of magnesium, and optionally additional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each additional element being less than 0.3% by weight, the balance being aluminum and unavoidable impurities and residuals elements. The steel sheet is cut to obtain a blank, the blank is thermally treated at a temperature between 840 and 950° C. to obtain a fully austenitic microstructure in the steel, the blank is transferred into a press tool and hot-formed to obtain a part. The part is cooled to obtain a martensitic or martensitic-bainitic microstructure or made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

Claims

1. A part comprising: a part having a metallic coating obtained by a method including: A) providing a steel sheet pre-coated with a metallic coating including 4.0 to 20.0% by weight of zinc; 1.0 to 3.5% by weight of silicon; a balance being aluminum, unavoidable impurities and residual elements; and a ratio Zn/Si being between 3.2 and 8.0; B) cutting the coated steel sheet to obtain a blank; C) performing a thermal treatment on the blank at a temperature between 840 and 950° C. to obtain a fully austenitic microstructure in the steel; D) transferring the blank into a press tool; E) hot-forming the blank to obtain a part; F) cooling the part in order to obtain a microstructure in the steel being martensitic or martensitic-bainitic or made of at least 75% equiaxed ferrite, 5 to 20% of martensite and bainite in an amount less than or equal to 10%; and G) a phosphating step; a ZnO layer on the metallic coating of the part; and a phosphate crystals layer on the ZnO layer.

2. The part as recited in claim 1 wherein a coverage rate of phosphate crystals on the part surface is equal or greater than 90%.

3. The part as recited in claim 2 wherein the coverage rate of phosphate crystals on the part surface is equal or greater than 99%.

4. The part as recited in claim 1 further comprising an e-coating layer on the phosphate crystals layer.

5. The part as recited in claim 1 wherein the metallic coating comprises an intermetallic layer Fe.sub.3Al and an interdiffusion layer Fe—Si—Al.

6. The part as recited in claim 1 wherein a microstructure of the metallic coating comprises Zn.sub.2Mg phase or Mg.sub.2Si phase or both.

7. The part as recited in claim 1 wherein a microstructure of the metallic coating does not comprise metallic zinc.

8. The part as recited in claim 1 wherein the part is a press hardened steel part having a variable thickness.

9. The part as recited in claim 1 wherein the variable thickness is produced by a continuous flexible rolling process.

10. The part as recited in claim 1 wherein the part is a tailored rolled blank.

11. The part as recited in claim 1 wherein the metallic coating does not comprise elements selected among In and Sn or combinations thereof

12. The part as recited in claim 1 wherein the part is 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.

13. An automotive vehicle comprising: the part as recited in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0022] To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following FIGURE:

[0023] FIG. 1 illustrates one corrosion cycle corresponding to 168 hours of the norm VDA 233-102.

DETAILED DESCRIPTION

[0024] The following terms will be defined: [0025] “coverage rate of phosphate crystals” is defined by a percentage. 0% means that the surface of the part is not covered at all by phosphate crystals, 100% means that the surface of the part is totally covered by phosphate crystals”.

[0026] The designation “steel” or “steel sheet” means a steel sheet for press hardening process having a composition allowing the part to achieve a higher tensile strength greater than or equal to 500 MPa, preferably greater than or equal to 1000 MPa, advantageously greater than or equal to 1500 MPa. 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.

[0027] For example, the steel sheet is 22MnB5 with the following composition: 0.20%≤C≤0.25%; 0.15%≤Si≤0.35%; 1.10%≤Mn≤1.40%; 0%≤Cr≤0.30%; 0%≤Mo≤0.35%; 0%≤P≤0.025%; 0%≤S≤0.005%; 0.020%≤Ti≤0.060%; 0.020%≤Al≤0.060%; 0.002≤% B≤0.004%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0028] The steel sheet can be Usibor®2000 with the following composition: 0.24%≤C≤0.38%; 0.40%≤Mn≤3%; 0.10%≤Si≤0.70%; 0.015%≤Al≤0.070%; 0%≤Cr≤2%; 0.25%≤Ni≤2%; 0.020%≤Ti≤0.10%; 0%≤Nb≤0.060%; 0.0005%≤B≤0.0040%; 0.003%≤N≤0.010%; 0.0001%≤S≤0.005%; 0.0001%≤P≤0.025%; it being understood that the contents of titanium and nitrogen satisfy Ti/N>3.42; and that the contents of carbon, manganese, chromium and silicon satisfy:

[00001]$2.6C+\frac{\mathrm{Mn}}{5.3}+\frac{\mathrm{Cr}}{13}+\frac{\mathrm{Si}}{15}\ge 1.1\%$

the composition optionally comprising one or more of the following: 0.05%≤Mo≤0.65%; 0.001%≤W≤0.30%; 0.0005%≤Ca≤0.005%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0029] For example, the steel sheet is Ductibor®500 with the following 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%; 0%≤Ca≤0.006%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0030] Steel sheet can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be for example between 0.7 and 3.0 mm.

[0031] The invention relates to a method for the manufacture of a hardened part coated with a phosphatable coating. Firstly, the method comprises the provision of a steel sheet pre-coated with a metallic coating comprising from 4.0 to 20.0% by weight of zinc, from 1.0 to 3.5% by weight of silicon, optionally from 1.0 to 4.0% by weight of magnesium, and optionally additional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each additional element being less than 0.3% by weight, the balance being aluminum and unavoidable impurities and residuals elements, wherein the ratio Zn/Si is between 3.2 and 8.0.

[0032] Without willing to be bound by any theory, it seems that if these conditions are not met, in particular if the amount of silicon is greater than 3.5%, there is a risk that the zinc is localized in aluminum matrix or an intermetallic compound Zn—Al is formed. Thus, zinc cannot rise to the surface of the coated steel sheet. Alumina layer, which is not phosphatable, is formed on the surface of the coated steel sheet.

[0033] In most cases, when coverage rate of phosphate crystals is low, there is a risk of poor paint adhesion. However, in some cases, although the coverage rate of phosphate crystals is low, the paint adhesion is good but the corrosion resistance after painting is poor. Indeed, the microroughness of the coated parts surface coated allows for paint adhesion. But, the paint is not evenly distributed on the part surface. In this case, phosphate crystals cannot play the role of binder between the paint and the coating. Consequently, in a corrosive environment, water infiltrates easily under paint resulting in red rust areas.

[0034] Preferably, the metallic coating does not comprise elements selected among Cr, Mn, Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb or their combinations. In another preferred embodiment, the metallic coating does not comprise any of the following compounds: Cr, Mn, Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb. Indeed, without willing to be bound by any theory, it seems that when these compounds are present in the coating, there is a risk that the properties of the coating, such as electrochemical potential, are altered, because of their possible interactions with the essential elements of the coatings.

[0035] Advantageously, the metallic coating comprises from 1.5 to 3.5% by weight of silicon, preferably from 1.5 to 2.5% by weight of silicon. In another preferred embodiment, the coating comprises from 2.1 to 3.5% by weight of silicon.

[0036] Preferably, the metallic coating comprises from 10.0 to 15.0% by weight of zinc.

[0037] In a preferred embodiment, the ratio Zn/Si in the metallic coating is between 5 and 4 and 8, preferably between 4.5 and 7.5 and advantageously between 5 and 7.5.

[0038] Without willing to be bound by any theory, it has been found that when the ratio Zn/Si is not between 3.2 and 8, there is a risk that the coverage rate of phosphate crystals decreases because of a too high content of Al and Fe at the coating surface.

[0039] Advantageously, the coating comprises from 1.1 to 3.0% by weight of magnesium.

[0040] Advantageously, the coating comprises greater than 76% by weight of aluminum.

[0041] The coating can be deposited by any methods known to the man skilled in the art, for example hot-dip galvanization process, electrogalvanization process, physical vapour deposition such as jet vapor deposition or sputtering magnetron. Preferably, the coating is deposited by hot-dip galvanization process. In this process, the steel sheet obtained by rolling is dipped in a molten metal bath.

[0042] The bath comprises zinc, silicon, aluminum and optionally magnesium. It can comprise additional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each additional element being less than 0.3% by weight. These additional elements can improve among others ductibility, coating adhesion on the steel sheet.

[0043] The bath can also contain unavoidable impurities and residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath. Residual element can be iron with a content up to 3.0% by weight.

[0044] The thickness of the metallic coating is usually between 5 and 50 μm, preferably between 10 and 35 μm, advantageously between 12 and 18 μm or between 26 to 31 μm. The bath temperature is usually between 580 and 660° C.

[0045] After the deposition of the coating, the steel sheet is usually wiped with nozzles ejecting gas on both sides of the coated steel sheet. The coated steel sheet is then cooled. Preferably, the cooling rate is greater than or equal to 15° C..Math.s.sup.−1 between the beginning of the solidification and the end of the solidification. Advantageously, the cooling rate between the beginning and the end of the solidification is superior or equal to 20° C..Math.s.sup.−1.

[0046] Then, a skin-pass can be realized and allows work hardening the coated steel sheet and giving it a roughness facilitating the subsequent shaping. A degreasing and a surface treatment can be applied in order to improve for example adhesive bonding or corrosion resistance.

[0047] Then, the coated steel sheet is cut to obtain a blank. A thermal treatment is applied to the blank in a furnace under non protective atmosphere at an austenitization temperature Tm usually between 840 and 950° C., preferably 880 to 930° C. Advantageously, said blank is maintained during a dwell time tm between 1 to 12 minutes, preferably between 3 to 9 minutes. During the thermal treatment before the hot-forming, the coating forms an alloy layer having a high resistance to corrosion, abrasion, wear and fatigue.

[0048] After the thermal treatment, the blank is then transferred to a hot-forming tool and hot-formed at a temperature between 600 and 830° C. The hot-forming comprises the hot-stamping and the roll-forming. Preferably, the blank is hot-stamped. The part is then cooled in the hot-forming tool or after the transfer to a specific cooling tool.

[0049] The cooling rate is controlled depending on the steel composition, in such a way that the final microstructure after the hot-forming comprises mostly martensite, preferably contains martensite, or martensite and bainite, or is made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

[0050] In a preferred embodiment, the part is a press hardened steel part having a variable thickness, i.e. the press hardened steel part of the invention can have a thickness which is not uniform but which can vary. Indeed, it is possible to achieve the desired mechanical resistance level in the zones which are the most subjected to external stresses, and to save weight in the other zones of the press hardened part, thus contributing to the vehicle weight reduction. In particular, the parts with non-uniform thickness can be produced by continuous flexible rolling, i.e. by a process wherein the sheet thickness obtained after rolling is variable in the rolling direction, in relationship with the load which has been applied through the rollers to the sheet during the rolling process.

[0051] Thus, within the conditions of the invention, it is possible to manufacture advantageously vehicle parts with varying thickness in order to obtain for example a tailored rolled blank. 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.

[0052] A phosphatable hardened part according to the invention is obtained.

[0053] Preferably, the microstructure of the metallic coating of the part comprises an intermetallic layer Fe.sub.3Al, an interdiffusion layer Fe—Si—Al, a low quantity of silicon distributed in the coating and a ZnO layer at the surface of the coating. When magnesium is present in the coating, the microstructure comprises also Zn.sub.2Mg phase and/or Mg.sub.2Si phase. Advantageously, the microstructure does not comprise metallic zinc.

[0054] For automotive application, after phosphating step, the part is degreased and phosphated so as to ensure the adhesion of the cataphoresis. After the phosphating, a high coverage rate of phosphate crystals on the surface of the part is obtained. The coverage rate of phosphate crystals on the surface of the part is greater than or equal to 80%, preferably greater than or equal to 90%, advantageously greater than or equal to 99%.

[0055] Then, the part is dipped in an e-coating bath. Usually, the thickness of the phosphate layer is between 1 and 2 μm and the thickness of the e-coating layer is between 15 and 25 μm, preferably less than or equal to 20 μm. The cataphoresis layer ensures an additional protection against corrosion.

[0056] 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.

[0057] The invention will now be explained in trials carried out for information only. They are not limiting.

EXAMPLES

[0058] For all samples, steel sheets used are 22MnB5. The composition of the steel is as follows: C=0.2252%; Mn=1.1735%; P=0.0126%, S=0.0009%; N=0.0037%; Si=0.2534%; Cu=0.0187%; Ni=0.0197%; Cr=0.180%; Sn=0.004%; Al=0.0371%; Nb=0.008%; Ti=0.0382%; B=0.0028%; Mo=0.0017%; As=0.0023% et V=0.0284%.

[0059] All coatings were deposited by hot-dip galvanization process.

Example 1: Phosphating Test

[0060] Phosphatability test is used to determine the adhesion of phosphate crystals on hardened parts by assessing the coverage rate on the part surface.

[0061] Trials 1 to 10 were prepared and subjected to the phosphating test.

[0062] To this end, coated trials were cut in order to obtain a blank. Blanks were then heated at a temperature of 900° C. during a dwell time varying between 5 and 10 minutes. Blanks were transferred into a press tool and hot-stamped in order to obtain a part. Finally, the part was cooled to obtain a hardening by martensitic transformation.

[0063] A degreasing was then realized. It was followed by a phosphating step realized by dipping into a bath comprising a solution of Gardobond® 24 TA, Gardobond® Add H7141, Gardobond® H7102, Gardobond® Add H7257, Gardobond® Add H7101, Gardobond® Add H7155 during 3 minutes at 50° C. Parts were then wiped with water and dried with hot air. The parts surface were observed by SEM. Results are shown in the following Table 1:

TABLE-US-00001 Covering rate after a thermal treatment at 900° C. (%) Dwell Dwell Coating Thickness time = 5 time = 10 Trials Al Si Zn Mg Zn/Si (μm) minutes minutes 1 91 9 — — — 27 0 0 2 81 9 10 — 1.1 27 <5 <10 3 76 9 15 — 1.7 27 0 20 4 71 9 20 — 2.2 27 <10 <10 5 80 5 15 — 3.0 27 50 70 6 78 5 15 2 3.0 27 50 50  7* 82.5 3.5 12 2 3.4 27 >99 >99  8* 88 2 10 — 5 27 95 95  9* 83 2 15 — 7.5 27 >99 >99 10* 81 2 15 2 7.5 27 ND 90 *examples according to the invention, ND: not done.
The above results show that Trials 7 to 10 have a high coverage rate of phosphate crystals on hardened part.

Example 2: Paint Adhesion Test

[0064] This test is used to determine the paint adhesion of the hardened parts.

[0065] An e-coating layer of 20 μm is deposited on Trials 1 to 5 and 7 to 10 prepared in Example 1. To this end, all trials were dipped into a bath comprising an aqueous solution comprising Pigment Paste® W9712-N6 and Resin Blend® W7911-N6 of PPG Industries during 180 seconds at 30° C. A 200V current was applied. Then, the panel was wiped and cured in the oven at 180° C. during 35 minutes.

[0066] Then, painted parts are dipped into a sealed box comprising demineralized water during 10 days at a temperature of 50° C. After the dipping, a grid is realized with a cutter. The paint is ripped with a scotch.

[0067] The removed paint is assessed by naked eyes: 0 means excellent, in other words, there is a little or no paint removed and 5 means very bad, in other words, there are lots of paint removed. Results are shown in the following Table 2:

TABLE-US-00002 Paint adhesion after a thermal treatment at 900° C. (%) Dwell Dwell Coating time = 5 time = 10 Trials Al Si Zn Mg Zn/Si minutes minutes 10 91 9 — — — 0 0 11 81 9 10 — 1.1 5 5 12 76 9 15 — 1.7 5 5 13 71 9 20 — 2.2 5 5 14 80 5 15 — 3.0 0 0  15* 82.5 3.5 12 2 3.4 0 0  16* 88 2 10 — 5.0 0 0  17* 83 2 15 — 7.5 0 0  18* 81 2 15 2 7.5 2 0 *examples according to the invention.

[0068] Trials 15 to 18 according to the present invention show good paint adhesion, as trials 10 and 14.

Example 3: Delamination Test

[0069] This test is used to determine the corrosion after painting of the hardened parts.

[0070] An e-coating layer of 20 μm is deposited on Trials 1 to 5, 8 and 10 prepared at Example 1. To this end, all trials were dipped into a bath comprising an aqueous solution comprising Pigment Paste® W9712-N6 and Resin Blend® W7911-N6 of PPG Industries during 180 seconds at 30° C. A 200V current was applied. Then, the panel was wiped and cured in the oven at 180° C. during 35 minutes.

[0071] Then, scratches were realized on the e-coating layer with a cutter.

[0072] Finally, a test, consisting in submitting panels to corrosion cycles according to the norm VDA 233-102, was realized. Trials were put in a chamber wherein an aqueous solution of sodium chloride of 1% by weight was vaporized on trials with a rate of flow of 3 mL.Math.h.sup.−1. The temperature varied from 50 to −15° C. and the humidity rate varied from 50 to 100%. FIG. 1 illustrates one cycle corresponding to 168 hours, i.e. one week.

[0073] The presence of delamination was observed by naked eyes: 0 means excellent, in other words, there is no delamination and 5 means very bad, in other words, there are lots of delamination. Results are shown in the following Table 3:

TABLE-US-00003 2 corrosion cycles 5 corrosion cycles thermal treatment at 900° C. Coating Dwell time = Dwell time = Dwell time = Dwell time = Trials Al Si Zn Mg Zn/Si 5 minutes 10 minutes 5 minutes 10 minutes 18 91 9 — — — 0.5 1 4.5 5 19 81 9 10 — 1.1 5 0.5 ND ND 20 76 9 15 — 1.7 5 1 5 5 21 71 9 20 — 2.2 4.5 4.5 ND ND 22 80 5 15 — 3.0 2 2 4.5 4  23* 88 2 10 — 5.0 1 1 2.5 3  24* 81 2 15 2 7.5 0.5 0.5 2 2 *examples according to the invention, ND: not done.

[0074] Trials according to the invention (Trials 23 and 24) lead to a little delamination after 2 and 5 weeks of corrosion cycle, in contrary to Trials 18 to 22.