STEEL SHEET FOR TOP COVER OF BATTERY PACK AND ITS MANUFACTURING METHOD

Abstract

A top cover of battery pack including a metallic coated steel sheet wherein the metallic coating is based on zinc and includes aluminum, magnesium and unavoidable impurities.

Claims

1-6 (canceled).

7. A top cover of a battery pack comprising: a press stamped metallic coated steel sheet including a steel sheet with a metallic coating, the metallic coating being based on zinc and comprising aluminium, magnesium and unavoidable impurities.

8. The top cover as recited in claim 7 wherein the metallic coating is topped by a passivation metallic coating wherein a surface weight of a passivating element is from 5 to 50 mg/m.sup.2.

9. The top cover as recited in claim 7 wherein the metallic coating comprises by weight from 1.5 to 10% of aluminium, from 1.5 to 10% of magnesium, a balance being zinc and the unavoidable impurities.

10. The top cover as recited in claim 7 wherein the metallic coating has a thickness of 10 to 40 ?m on an inner side of the battery pack.

11. The top cover as recited in claim 7 wherein the metallic coated steel sheet has a metallic coating weight of 50 to 450 g/m.sup.2 in total on both sides.

12. A battery pack comprising the top cover as recited in claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following figures:

[0017] FIG. 1 illustrates a battery pack and its top cover in an electric battery vehicle,

[0018] FIG. 2 illustrates a top cover according to the invention after fire exposure during 130 seconds at a temperature of 1300? C.

[0019] FIG. 3 illustrates a top cover not according to the invention after fire exposure during 130 s at 1000? C.

[0020] FIG. 4 illustrates a top cover according to the invention after fire exposure during 130 seconds at a temperature of 1000? C.

DETAILED DESCRIPTION

[0021] The invention relates to a top cover for battery pack comprising a metallic coated steel sheet wherein said metallic coating is based on zinc and comprises aluminum, magnesium and unavoidable impurities.

[0022] For this purpose, any steel can be used in the frame of the invention. Preferably, steels having a good formability are well suited. For example, the top cover can be made of mild steel for deep drawing such as Interstitial Free steel having the following weight composition: C?0.01%; Si?0.3%; Mn?1.0%; P?0.1%; S?0.025; Al?0.01%; Ti?0.12%; Nb?0.08%; Cu?0.2%.

[0023] For example, the top cover can be made of High Strength Low Alloy (HSLA) steel having the following weight composition: C?0.1%; Si?0.5%; Mn?1.4%; P?0.04%; S?0.025%; Al?0.01%; Ti?0.15%; Nb?0.09%; Cu?0.2%.

[0024] The steel sheet can be obtained by hot rolling of a steel slab and subsequent cold rolling of the obtained steel coil, depending on the desired thickness, which can be for example from 0.6 to 1.0 mm.

[0025] The steel sheet is then coated with a metallic coating by any coating process. For examples, the steel sheet is hot-dip coated in a molten bath based on zinc and comprising aluminum, magnesium and unavoidable impurities.

[0026] The steel sheet can then be cut into a blank. It can be shaped by stamping to form the specific shape of the top cover.

[0027] The metallic coating used in the invention is based on aluminum and optionally comprises silicon and unavoidable impurities coming from the production process.

[0028] Such a coating is fireproof and does not release any gas when submitted to flame temperatures. In case of fire or high temperatures, it won't increase the pressure inside the battery pack.

[0029] In a preferred embodiment, the metallic coating comprises by weight from 1.5 to 10% of aluminum, from 1.5 to 10% of magnesium, the balance being zinc and unavoidable impurities. Such a metallic coating provides good resistance to corrosion.

[0030] For example, the metallic coating is Zagnelis? Protect with the following weight composition: contains 3.7% of aluminum and 3% of magnesium, the remainder being aluminum.

[0031] The metallic coating weight can be of 50 to 450 g/m.sup.2 in total on both sides or less. For example, the metallic coating weight can be 120, 310 or 430 g/m.sup.2.

[0032] For example, the metallic coating thickness on the inner side of the battery pack is 10 to 40 ?m.

[0033] In a preferred embodiment, the surface of the metallic coating is topped by a passivation coating.

[0034] Passivation coatings can be applied on-line after the hot dip coating step with a roll-coat. It can also be applied on steel part by dipping. For both applications, an aqueous solution containing specific metallic passivating elements is deposited on the surface in form of a humid film. After drying, the passivation coating creates a conversion layer on the surface to provide enhanced corrosion protection.

[0035] For example, the passivation coating contains chromium and the chromium surface weight is from 15 to 45 mg/m.sup.2. It can alternatively contain zirconium in the same amount.

[0036] The steel sheet can then be cut into a blank. The blank can be formed by press stamping to the specific shape of the top cover. This specific shape is design related. The top cover being a large horizontal part, it may be subject for vibrations. To reduce these vibrations and subsequent noise, stiffeners are generally punched into the top cover during the stamping operation. Finally, the top cover is attached to the pack by any removable or non-detachable means, for example by screwing, welding or gluing.

EXAMPLES

[0037] In order to determine the resistance to fire of the top covers, several tests were performed. All tests were performed on the same test device.

[0038] The test device was adapted from the test device described in the Standard ISO 2685:1998. Both following adaptations were done: Firstly, the sample was thermally isolated from the structure of the test device by a 10 mm thick plate of calcium silicate. Secondly, the gas burner generating the flame has been calibrated to achieve the targeted temperature on the face of the sample that is exposed to the flame.

[0039] For all tests, the samples have the same dimension of 150?150 mm.sup.2. Each sample is positioned in front of the gas burner to get hit by the flame. The plate between the sample and the burner has an opening area with the dimension of 90?90 mm.sup.2.

[0040] Three materials were tested:

[0041] material 1 is a 0.7 mm thick steel sheet. It is coated with Zagnelis? Protect. This hot-dip coating contains by weight 3.7% of aluminum and 3% of magnesium, the remainder being aluminum. The coating weight is 310 g/m.sup.2. After hot dip coating, the surface has been passivated by roll-coat application of Bonderite? MPA 6010 from supplier HENKEL which contains Chromium (III) ions. It has been then dried by blowing of hot air. The surface weight of chromium on the dried surface is from 25 to 35 mg/m.sup.2.

[0042] material 2 is a 1.0 mm thick aluminum sheet of 6016 series.

[0043] material 3 is a 0.8 mm galvanized steel sheet coated with epoxy-based e-coat. The hot-dip coating contains up to 0.2 of aluminum by weight, the remainder being zinc. The metallic coating weight is 140 g/m.sup.2. After a phosphating step, the sample was dipped in a e-coating bath. The e-coat tested is Powercron? 6200 HE from supplier PPG. The dry thickness of paint after baking is 25 ?m on each face.

[0044] material 4 is 0.8 mm thick steel sheet having the same metallic coating and passivation as material 1

[0045] In the following, sample 1 is made of material 1, sample 2 is made of material 2 and sample 3 is made of material 3, sample 4 of material 4.

[0046] Two scenarios of fire exposure have been tested. In scenario A, the flame temperature is 1300? C. and the exposure time is 130 s. In scenario B, which is less severe, the flame temperature is 1000? C., and the time exposure is first 20 s directly followed by a 60 s time of flame extinction with a final exposure of 10 s.

[0047] Several criteria are considered for analysis of the tests. The integrity of the sheet, i. e. whether the flame has pierced the sheet or not, the temperature of the face unexposed to the flame (back-face) at the end of the test and the presence of bubbles in the coating after the test. The presence of a bubble shows the release of gas.

TABLE-US-00001 TABLE 1 Scenarios of flame exposure Scenario Time of flame exposure Temperature of flame A 130 s 1300? C. B 130 s 1000? C.

TABLE-US-00002 TABLE 2 Scenario A: 130 s at 1300? C. Scenario Temperature on back- Presence A Sample Flame Piercing face after 130 s of bubbles 1300? C. 1* No 615? C. No 130 s 2 Yes after 45 s above 910? C.

[0048] After an exposure of 130 s at 1300? C., the back-face of sample 1 made of steel remains at a temperature of less than 700? C. and doesn't show any signs of melting. On the contrary, the flame has pierced material 2 made of thicker aluminum.

[0049] Moreover, sample 1 doesn't show any bubbles as can be seen on FIG. 2. Its coating didn't release gas.

TABLE-US-00003 TABLE 3 Scenario B: 130 s at 1000? C. Scenario Temparature on back- Presence B Sample Flame Piercing face after 130 s of bubbles 1000? C. 3 No 486? C. Yes 130 s 4* No 590? C. No

[0050] After an exposure of 130 s at 1000? C., the back-face of sample 3 clearly shows bubbles as can be seen on FIG. 3. These open bubbles have released combustion products of the paint in form of gas.

[0051] Sample 4 doesn't show any bubbles as can be seen on FIG. 4