Steel sheet provided with a coating offering sacrificial cathodic protection, method for the production of a part using such a sheet, and resulting part

Abstract

This invention relates to a steel sheet provided with a sacrificial cathodic protection layer comprising from 5 to 50% zinc by weight, from 0.1 to 15% silicon by weight and optionally up to 10% magnesium by weight and up to 0.3% by weight, in cumulative content, of additional elements, and also comprising a protection elements to be selected from among tin in a percentage by weight between 0.1 and 5%, indium in a percentage by weight between 0.01 and 0.5% and combinations thereof, the balance consisting of aluminum and residual elements or unavoidable impurities. The invention further relates to a method for the fabrication of parts by hot or cold stamping and the parts that can be thereby obtained.

Claims

1. A steel sheet provided with a sacrificial cathodic protection coating, the coating comprising: zinc, in a percentage by weight from 5 to 50%; silicon, in a percentage by weight from 0.1 to 15%, iron, in a percentage by weight from 2 to 5%; up to 0.3% by weight, in cumulative content, of additional elements, and a protection element selected from among tin in a percentage by weight from 0.1% to 5%, indium in a percentage by weight from 0.01 to 0.5% and combinations thereof, a balance of the coating consisting of aluminum, residual elements or unavoidable impurities, wherein the additional elements include Sb, Pb, Ti, Ca, Mn, La, Ce, Cr, Ni, Zr or Bi.

2. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, the coating further comprising up to 10% magnesium by weight and up to 0.3% by weight, in cumulative content, of additional elements.

3. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, wherein the protection element is tin in a percentage by weight from 1% to 3%.

4. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, wherein the protection element is indium in a percentage by weight from 0.02% to 0.1%.

5. A steel sheet provided with a sacrificial cathodic protection coating, the coating comprising: from 20 to 40% zinc by weight; from 0.1 to 15% silicon by weight; a protection element selected from among tin in a percentage by weight from 0.1% to 5%, indium in a percentage by weight from 0.01 to 0.5% and combinations thereof, the balance of the coating including aluminum, residual elements or unavoidable impurities.

6. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 5, further comprising magnesium in a percentage by weight of 1 to 10%.

7. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 5, wherein the coating comprises from 20 to 30% zinc by weight.

8. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 7, further comprising magnesium in a percentage by weight of 3 to 6%.

9. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, wherein the coating comprises from 8% to 12% silicon by weight.

10. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, wherein the steel comprises, in percentage by weight: 0.15%<C<0.5%, 0.5%<Mn<3%, 0.1%<Si<0.5%, Cr<1%, Ni<0.1%, Cu<0.1%, Ti<0.2%, Al<0.1%, P<0.1%, S<0.05%, and 0.0005%<B<0.08%, the balance of the steel including iron and unavoidable impurities due to the processing of the steel.

11. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, wherein the coating has a thickness from 10 to 50 m.

12. The steel sheet provided with a sacrificial cathodic protection coating as recited in claim 1, wherein the coating is obtained by hot dipping the steel sheet.

13. A method for the fabrication of a steel part provided with a sacrificial cathodic protection coating comprising the following steps, carried out in this order: procuring a coated steel sheet comprising: zinc, in a percentage by weight from 5 to 50%; silicon, in a percentage by weight from 0.1 to 15%, and a protection element selected from among tin in a percentage by weight from 0.1% to 5%, indium in a percentage by weight from 0.01 to 0.5% and combinations thereof, the balance of the coating including aluminum, residual elements or unavoidable impurities; cutting said sheet to obtain a blank; heating the blank in a non-protective atmosphere to an austenitization temperature Tm from 840 to 950 C.; holding the blank at the austenitization temperature Tm for a time period tm from 1 to 8 minutes; hot stamping the blank to obtain a coated steel part which is cooled at a rate so a microstructure of the steel comprises at least one constituent selected from martensite and bainite; the temperature Tm, the time tm, a thickness of the coating and percentages of protection elements and zinc and being selected so a final average content of iron in an upper part of the coating of the part is less than 75% by weight.

14. The method as recited in claim 13, further comprising magnesium in the coating.

15. The method as recited in claim 13, wherein a thickness of the coating is greater than or equal to 27 m and wherein the coating includes tin in a percentage by weight of greater than or equal to 1% and zinc in a percentage by weight of greater than or equal to 20%.

16. A steel part provided with a sacrificial cathodic protection coating obtained by the method as recited in claim 13.

17. A steel part provided with a sacrificial cathodic protection coating obtained by cold stamping of the steel sheet recited in claim 1.

18. A steel sheet provided with a sacrificial cathodic protection coating, the coating comprising: zinc, in a percentage by weight from 5 to 50%; silicon, in a percentage by weight from 8 to 15%, up to 1% magnesium by weight; up to 0.3% by weight, in cumulative content, of additional elements; a protection element selected from among tin in a percentage by weight from 0.1% to 5%, indium in a percentage by weight from 0.01 to 0.5% and combinations thereof; and a balance of the coating including aluminum, iron and unavoidable impurities, wherein the additional elements include Sb, Pb, Ti, Ca, Mn, La, Ce, Cr, Ni, Zr or Bi.

19. A steel sheet provided with a sacrificial cathodic protection coating, the coating comprising: zinc, in a percentage by weight from 5 to 50%; silicon, in a percentage by weight from 0.1 to 15%, a protection element selected from among tin in a percentage by weight from 0.1% to 5%, indium in a percentage by weight from 0.01 to 0.5% and combinations thereof, and a balance of the coating including aluminum, residual elements or unavoidable impurities; a final average content of iron in an upper part of the coating of the part being less than 75% by weight.

Description

TESTS

Example 1AlSiZnInFe Coating

(1) Tests have been conducted with 22MnB5 cold rolled sheets 1.5 mm thick provided with hot dip coatings comprising, in percent by weight, 20% zinc, 10% silicon, 3% iron, 0.1% indium, the remainder consisting of aluminum and unavoidable impurities, and the thicknesses of which are approximately 15 m.

(2) These sheets were subjected to conventional electrochemical measurements in a 5% NaCl environment, with reference to a saturated calomel electrode.

(3) It was noted that the electrochemical potential of the coated sheet is 0.95 V/SCE. The sheet claimed by the invention therefore does have sacrificial cathodic protection. Under the same measurement conditions, it was verified that a sheet that was identical but was provided with a coating that contained neither zinc nor indium had an electrochemical potential of 0.70 V/SCE, which does not provide cathodic protection to the steel.

(4) To evaluate the residual protection after hot stamping, additional tests consisted of heating the sheets claimed by the invention, which were identical to those previously used, to a temperature of 900 C. for variable lengths of time. It was observed that the electrochemical potential of the sheet treated for 3 minutes is still 0.95 V/SCE, thereby demonstrating the preservation of the sacrificial cathodic protection. Above this processing temperature, the average iron content of the upper part of the coating over a thickness of 5 m is greater than 75% by weight and the electrochemical potential falls to 0.70 V/SCE.

(5) With regard to the propagation of micro-cracks from the coating to the sheet, the formation of a thick intermetallic layer was observed at the steel-coating interface, an intermetallic layer that is still present upon completion of the austenitization.

Example 2AlSiZnMgSnFe Coating

(6) Tests have been conducted with cold-rolled 22MnB5 sheet 1.5 mm thick provided with hot dip coatings comprising, in percent by weight, 10% silicon, 10% zinc, 6% magnesium, 3% iron and 0.1% tin, the remainder consisting of aluminum and unavoidable impurities, and the average thicknesses of which are 17 m.

(7) These sheets were subjected to conventional electrochemical measurements in a 5% NaCl environment, with reference to a saturated calomel electrode.

(8) It was noted that the electrochemical potential of the coated sheet is 0.95 V/SCE, while the electrochemical potential of an identical sheet provided with a coating containing 10% silicon, and the rest consisting of aluminum and unavoidable impurities, is 0.70 V/SCE. The sheet claimed by the invention therefore does have sacrificial cathodic protection.

(9) To evaluate the residual protection after hot stamping, additional tests consisted of heating the sheets claimed by the invention, which were identical to those previously used, to a temperature of 900 C. for variable lengths of time. It was observed that the electrochemical potential of the sheet treated for 2 minutes is still 0.95 V/SCE, thereby demonstrating the preservation of the sacrificial cathodic protection. Above this processing temperature, the average iron content of the upper portion of the coating over a thickness of 5 m is greater than 75% by weight and the electrochemical potential falls to 0.70 V/SCE.

(10) It was then verified that the use of a coating with an average thickness of 27 m makes it possible to increase the duration of austenitization Tm to 5 minutes at 900 C. with preservation of this cathodic protection.

(11) With regard to the propagation of microcracks from the coating to the sheet, the formation of a thick intermetallic layer was observed at the steel-coating interface, an intermetallic layer that is still present at the conclusion of the austenitization.

Example 3AlZnSiSnFe Coatings with or without in

(12) Similar additional tests were performed with cold-rolled 22MnB5 sheets 1.5 mm thick provided with hot dip coatings, the characteristics of which are presented in the following table, and the thicknesses of which are approximately 32 m.

(13) TABLE-US-00001 Ref. % Al % Zn % Si % Sn % Fe % ln A 76 10 10 1 3 B 66 20 10 1 3 C 56 30 10 1 3 D 46 40 10 1 3 E 45.9 40 10 1 3 0.1

(14) The results of these tests will confirm that the properties sought by the invention have indeed been achieved.