STEEL PRODUCT WITH AN ANTICORROSIVE COATING OF ALUMINUM ALLOY AND METHOD FOR THE PRODUCTION THEREOF

Abstract

A steel product may include a metallic anticorrosion coating of an aluminum alloy. So that such a steel product possesses high media resistance, more particularly high acid resistance and corrosion resistance, the steel product may be immersed in a liquid-melt coating bath that includes an aluminum alloy. In addition to Aluminum and unavoidable impurities, the aluminum alloy may include at least one of 0.2-2% by weight Mn or 0.2-7% by weight Mg, 0.5-5% by weight Fe, and at least one of 0.05-0.4% by weight Ti or 0.05-0.4% by weight Zr. A method for producing such steel products may involve providing a steel product in a hot-rolled or cold-rolled state, activating a surface of the steel product to remove passive oxides, and coating the surface-activated steel product by immersion in a liquid-melt coating bath that comprises an aluminum alloy having the aforementioned composition.

Claims

1.-18. (canceled)

19. A steel product having a metallic anticorrosion coating of an aluminum alloy, wherein in addition to aluminum and unavoidable impurities the aluminum alloy comprises: at least one of 0.2-2% by weight Mn or 0.2-7% by weight Mg; 0.5-5% by weight Fe; and at least one of 0.05-0.4% by weight Ti or 0.05-0.4% by weight Zr.

20. The steel product of claim 19 wherein the aluminum alloy further comprises at least one of 0.1-15% by weight Si; 0.05-2% by weight Ni; 0.05-0.4% by weight Sb; 0.05-0.4% by weight Cr; at most 0.4% by weight Co; at most 0.1% by weight Cu; or at most 0.1% by weight Zn.

21. The steel product of claim 19 wherein the aluminum alloy includes 0.2-1.5% by weight Mn.

22. The steel product of claim 21 wherein the aluminum alloy includes 3-5% by weight Fe.

23. The steel product of claim 19 wherein the aluminum alloy includes 1.5-2% by weight Mn.

24. The steel product of claim 23 wherein the aluminum alloy includes 1.5-3% by weight Fe.

25. The steel product of claim 19 wherein the aluminum alloy includes 1.5-5% by weight Fe.

26. The steel product of claim 19 wherein the aluminum alloy includes both Mn and Mg, with the aluminum alloy including 0.2-0.6% by weight Mg.

27. A method for producing a steel product with a metallic anticorrosion coating of an aluminum alloy, the method comprising: providing a steel product in a hot-rolled state or a cold-rolled state; activating a surface of the steel product to remove passive oxides from the surface; and coating the surface-activated steel product by immersion in a liquid-melt coating bath that contains an aluminum alloy, wherein in addition to aluminum and unavoidable impurities the aluminum alloy comprises: at least one of 0.2-2% by weight Mn or 0.2-7% by weight Mg, 0.5-5% by weight Fe, and at least one of 0.05-0.4% by weight Ti or 0.05-0.4% by weight Zr.

28. The method of claim 27 wherein the liquid-melt coating bath is operated at a coating bath temperature in a range of 650-750 C.

29. The method of claim 27 wherein activating the surface of the steel product comprises pickling, rinsing, fluxing, and drying, wherein the method further comprises heating the steel product to a temperature that corresponds to a temperature of the liquid-melt coating bath or is at most 50 C. above the temperature of the liquid-melt coating bath.

30. The method of claim 27 wherein activating the surface of the steel product comprises pickling, rinsing, and annealing, with the annealing being performed in a hydrogen-nitrogen atmosphere at a holding temperature in a range of 500-900 C., wherein the steel product is heated or cooled to a temperature that corresponds to a temperature of the liquid-melt coating bath or is at most 50 C. above the temperature of the liquid-melt coating bath.

31. The method of claim 30 wherein the annealed steel product is introduced into the liquid-melt coating bath in a protected form through a snout in which there is a neutral or reductive inert-gas atmosphere.

32. The method of claim 30 wherein annealing of the steel product is performed such that preoxidizing of the steel product, surface nitriding of the steel product, controlled dew point working of the steel product, or combinations thereof are performed.

33. The method of claim 30 wherein annealing of the steel product is performed such that the steel product is decarburized.

34. The method of claim 27 wherein the surface of the hot dip-coated steel product is dressed.

35. The method of claim 27 wherein the surface of the hot dip-coated steel product is anodized.

36. The method of claim 27 wherein the surface of the steel product is activated by annealing, with the steel product being heated in a hydrogen-nitrogen atmosphere to a holding temperature in a range of 600-1100 C., wherein the steel product is heated or cooled to a temperature that corresponds to a temperature of the liquid-melt coating bath or is at most 50 C. above the temperature of the liquid-melt coating bath.

37. The method of claim 36 wherein the annealed steel product is introduced into the liquid-melt coating bath in a protected form through a snout in which there is a neutral or reductive inert-gas atmosphere.

38. The method of claim 36 wherein annealing of the steel product is performed such that preoxidizing of the steel product, surface nitriding of the steel product, controlled dew point working of the steel product, or combinations thereof are performed.

Description

[0039] The invention is illustrated in more detail below with working examples.

[0040] A steel product, typically a flat steel product, is coated with an aluminum coating of the invention by brief immersion of the steel product into a liquid-melt coating bathhence the term hot dip aluminizing which can also be used here. In order to achieve effective wetting and adhesion of the coating material on the steel substrate, a pretreatment is carried out in order to activate the steel surface. This pretreatment and the hot dip aluminization take place preferably in series and also in a continuous operating sequence. This operating sequence comprises the following steps: [0041] a) providing the steel product, preferably flat steel product, in a hot-rolled or cold-rolled state [0042] b) cleaning the surface of the steel product (optional) [0043] c) activating the surface of the steel product [0044] d) hot dip aluminizing, i.e., hot dip coating in a coating bath composition of the invention based on aluminum [0045] e) thermal, chemical, or mechanical aftertreatment (optional in each case).

[0046] The surface activation removes passive oxides from the surface of the steel product, causing this surface after activation to consist very largely of metallic iron. This can be achieved in a variety of ways, specifically as follows:

[0047] c1) Chemical surface activation, comprising pickling, rinsing, fluxing, and drying of the steel surface and conditioning of the steel product at bath immersion temperature before it is immersed into the coating bath. This surface activation or operating sequence is advisable particularly for hot-rolled steel starting material.

[0048] c2) Combined chemical/annealing gas activation, comprising pickling, rinsing, annealing in a hydrogen-nitrogen atmosphere (having a dew point of 60 C. to 0 C.) with a holding temperature in the range from 500 C. to 900 C., and cooling to bath immersion temperature. Optionally here it is possible for an overaging operation to be carried out before the surface-activated steel product is immersed into the coating bath. This surface activation or operating sequence is also advisable for hot-rolled steel starting material.

[0049] c3) Annealing gas activation, comprising heating to a holding temperature in the range from 600 C. to 1100 C. in hydrogen-nitrogen atmosphere (with a dew point of 60 C. to 0 C.) and cooling to bath immersion temperature. Here likewise it is possible optionally to carry out an overaging operation before the surface-activated steel product is immersed into the coating bath. This variant of the surface activation or operating sequence is advisable particularly with cold-rolled steel starting material, since the microstructure of this material can recrystallize at the holding temperature.

[0050] Independently of the choice of operating step sequence c1), c2), or c3), the liquid-melt coating bath is operated with a coating bath temperature in the range from 650 C. to 750 C., preferably in the range from 680 C. to 750 C. The surface-activated steel product to be coated is cooled preferably to a bath immersion temperature in the range from 650 C. to 800 C.

[0051] If a steel product made of alloyed steel is to undergo hot dip aluminization in accordance with the invention, using one of operating step sequences c2) or c3), it may be necessary to operate the annealing gas-metal reaction during the anneal in such a way as to prevent external oxidation of the alloying elements with affinity for oxygen (such as Mn, Al, Cr, B, Si, etc.). For this purpose the annealing gas-metal reaction would additionally comprise, for example, a preoxidation, a surface nitriding, a controlled dew point working, or combinations of these additional measures. It is likewise within the scope of the invention for edge decarburization to be carried out in parallel with the surface activation, in order to improve the formability of the steel product.

[0052] In order to prevent reverse passivation of the steel surface after annealing in the case of operating step sequence c2) or c3), the transfer of the steel product, cooled to bath immersion temperature, into the coating bath is accomplished by way of a snout, in which there is a controlled nitrogen or nitrogen/hydrogen inert gas atmosphere that is neutral or reductive relative to the steel substrate.

[0053] Through optional dressing of the surface of the steel product coated in accordance with the invention it is possible to apply a desired roughness or surface structure. In an optional downstream anodizing operation, the wear resistance of the coating of the invention on the steel product can be boosted. Anodizing brings about an increase in the abrasion resistance, which is greater by a factor of about 3 than that of a conventional stainless steel (1.4301).

[0054] The coating of a steel product subjected in accordance with the invention to hot dip aluminization by means of a coating bath and one of the operating step sequences as described above comprises, further to aluminum and unavoidable impurities, the following elements:

TABLE-US-00004 Mn 0.2-2 wt % and/or Mg 0.2-7 wt % Fe 0.5-5 wt % as obligatory element Ti 0.05-0.4 wt % and/or Zr 0.05-0.4 wt %.

[0055] Table 1 reports a number of working examples for the chemical composition of a metallic coating bath (melt bath) of the invention. The V1 and V3 melts reported in table 1 feature particularly high corrosion resistance even with respect to acids and alkaline media. The melt of type V2 possesses increased resistance toward seawater and alkalis.

[0056] A flat steel product provided with an anticorrosion coating of the invention is suitable for all common joining techniques such as soldering, welding, adhesive bonding, etc., and can be subjected to single-stage or multistage processing by either cold forming or hot forming to give a component. A flat steel product or component of this kind is suitable for general mechanical engineering, aircraft, automotive, and marine construction, construction of household appliances, building construction, especially for exterior facings, decorative elements of everyday use, such as casings for cell phones and notebook computers, for example, and also mining equipment. The use of flat steel products with the anticorrosion coating of the invention, or of components produced from such flat steel products, is particularly advantageous in the sector of the chemical industry and also the food industry, examples being statically loaded plant components and articles such as silo containers, insulators, beverage cans, etc. In the case of the latter application, the aluminum-based anticorrosion coating of the invention allows replacement by low-alloy steels of the stainless steels prescribed in the food industry according to EU Regulation 1935/2004 and also the EU Guidelines on meals, alloys used as food contact materials (Mar. 9, 2001).

TABLE-US-00005 TABLE 1 Elements [wt %] Test melt Mn Mg Si Fe Al V1 (AlMnSi) 0.98 3.23 0.96 bal. V2 (AlMg) 5 3.5 bal. V3 (AlMgMn) 2 7 3.5 bal.