METHOD FOR PRODUCING HARDENED STEEL COMPONENTS WITH A CONDITIONED ZINC ANTI-CORROSIVE LAYER

Abstract

A method for producing hardened steel components is provided. A sheet bar is cut from a galvanized strip made of a hardenable steel alloy. The sheet bar is cold-formed into a component blank and heated to a temperature that produces a structural change to austenite. The austenitized component blank is conveyed to a form hardening tool and is held in a form-fitting manner by an upper tool and lower tool, which have a shape essentially corresponding to that of the component blank. Due to the contact of the material of the component blank with the tools, the heat is removed from the steel material quickly enough that a martensitic hardening occurs. After the galvanization of the metal strip and before the temperature increase for achieving the austenitization, tin is applied to the surface of the strip, sheet blank, or component blank.

Claims

1-17. (canceled)

18. A method for producing hardened steel components, comprising the steps of: cutting a steel sheet bar from a galvanized steel strip including a hardenable steel alloy; cold-forming the steel sheet bar into a component blank; heating the component blank to a temperature that produces a structural change in the steel alloy to austenite, yielding an austenitized component blank; conveying the austenitized component blank to a form hardening tool including an upper tool and a lower tool that define a shape substantially corresponding to a shape of the austenitized component blank; holding the austenitized component blank in a form-fitting manner contacting the upper tool and the lower tool, wherein due to the contact of the austenitized component blank with the upper tool and the lower tool, heat is removed from the steel alloy quickly enough to cause martensitic hardening of the steel alloy; and before heating the component blank to the temperature that produces the structural change, applying tin to a surface of at least one of the galvanized steel strip, the steel sheet bar, and the component blank.

19. The method according to claim 18, wherein the tin is applied in an ionic form from a salt solution.

20. The method according to claim 18, wherein the tin is applied using a chemical vapor deposition (CVD) or a physical vapor deposition (PVD) process.

21. The method according to claim 18, wherein the tin is applied from an alkaline or acidic solution.

22. The method according to claim 18, wherein the tin is applied using an aqueous stannate solution, which is adjusted to be alkaline or acidic.

23. The method according to claim 18, wherein the tin is complexed with citric acid and is applied from a solution.

24. The method according to claim 18, wherein the tin is applied from a solution in a layer having a wet thickness of about 1 to about 5 microns and a dry thickness of about 50 to about 150 nanometers.

25. The method according to claim 18, wherein the tin is applied in an amount of about 30 to about 90 mg tin per square meter of the surface.

26. The method according to claim 18, wherein the tin is applied from a solution comprising K.sub.2SnO.sub.3*3H2O, present in a concentration of about 150 to about 250 grams/liter.

27. The method according to claim 26, wherein the solution further comprises KOH in a concentration of about 15 to about 25 grams per liter.

28. The method according to claim 18, wherein the tin is applied from a solution having a pH value of about 12.5 to about 13.5.

29. The method according to claim 18, wherein the tin is complexed with citric acid and is applied from a solution having a pH value of about 4 to about 5.5.

30. The method according to claim 29, wherein the solution comprises the citric acid in a concentration of about 35 to about 40 g/l.

31. The method according to claim 18, wherein the tin is applied from a solution comprising about 200 g/l K.sub.2SnO.sub.3*3H.sub.2O and about 20 g/l KOH.

32. A galvanized cold-formed steel strip coated with about 40 to about 80 mg tin/m.sup.2.

33. The galvanized steel strip according to claim 32, wherein the tin is deposited metallically or in ionic form.

34. The galvanized steel strip according to claim 32, wherein the tin is deposited from a stannate solution.

35. The galvanized steel strip according to claim 32, wherein the tin is deposited using a physical vapor deposition (PVD) or chemical vapor deposition (CVD) process.

36. A method of using a galvanized steel strip formed from a hardenable steel alloy, comprising the steps of: cutting the galvanized steel strip to form a steel sheet bar; cold-forming the steel sheet bar into a component blank; coating at least one of the galvanized steel strip, the sheet bar, and the component blank with tin, resulting in a tin-coated component blank; heating the tin-coated component blank to a temperature that produces austenitization of the steel alloy, yielding an austenitized tin-coated component blank; conveying the austenitized component blank to a form hardening tool including an upper tool and a lower tool; and holding the austenitized component blank in a form-fitting manner contacting the upper tool and the lower tool, thereby removing heat from the steel alloy quickly enough to cause martensitic hardening of the steel alloy; wherein the method is performed without cleaning the galvanized steel strip.

37. The method of claim 36, wherein the tin is provided from a stannate solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0090] The invention will be explained by way of example based on the drawings. In the drawings:

[0091] FIG. 1 shows the production path in conventional form hardening;

[0092] FIG. 2 shows the production path in conventional press hardening;

[0093] FIG. 3 shows the production path in a variant of the multi-step hot forming process, multi-step press-hardening, or Phs-Multiform® process according to the prior art.

[0094] FIG. 3b shows a sheet steel after the annealing without conditioning and a sheet steel after annealing with an annealing coating according to the invention;

[0095] FIG. 4 shows an electron microscope image of the surface that has been conditioned according to the invention after the annealing;

[0096] FIG. 5 shows the element distribution at four different measuring points;

[0097] FIG. 6 shows the surface of a galvanized sheet steel after the annealing with an annealing time of 45 seconds and 200 seconds;

[0098] FIG. 7 shows the surface of the sheet steel after the annealing with a surface conditioning according to the invention after 45 seconds and 200 seconds;

[0099] FIG. 8 shows the electrical resistance of the sheet surface in untreated and treated surfaces;

[0100] FIG. 9 shows the paint infiltration in surfaces not conditioned according to the invention and surfaces conditioned according to the invention after six weeks according to the VDA test.

DETAILED DESCRIPTION OF THE INVENTION

[0101] According to the invention, the surface of a galvanized sheet metal, in particular sheet steel, which is first cold formed in several steps in a form hardening process and then heated as a component blank, transferred to a forming tool, and hardened therein, is conditioned with tin or stannates; the conditioning with stannates will be discussed below.

[0102] The stannates that can be used have already been listed above; a potassium stannate solution is particularly suitable, wherein basically, one approach is to apply stannate or tin to the surface in ionic form.

[0103] In this connection, both alkaline and acidic solutions can be used and in particular, solutions in which the tin is complexed can be used.

[0104] In particular, the aim is to produce an aqueous layer thickness of 1-5 μm, with a dry layer thickness of 50-150 nm, and a tin coating of 30-90 mg tin/m.sup.2 in the form of K.sub.2[SnO.sub.3].

[0105] FIGS. 1 and 2 show conventional methods in which a galvanized sheet steel whose zinc layer contains an element with an affinity for oxygen, for example aluminum, either is austenitized before the forming or is austenitized after the forming and is respectively quench-hardened in a press. After the hardening, the surfaces of the two sheets have a glass-like, hard layer particularly composed of aluminum oxide, which is preferably cleaned.

[0106] According to the invention, it has been discovered that the conditioning of the surface with very small quantities of tin clearly has such a powerful influence on the formation of the glassy or hard layer that it either does not occur in this form or is conditioned to such a degree that it does not have to be cleaned.

[0107] A conventionally produced hardened steel sheet bar has a greenish-beige appearance on the surface, which is caused by oxides.

[0108] In a conditioning with a stannate solution, the sheet exhibits a silvery surface (FIG. 3).

[0109] Whereas with conventional methods, silvery surfaces indicate the lack of a complete reaction of the zinc layers with the underlying steel, this is not the case with the invention. Measurements have shown that the zinc layer has completely reacted in the same way. However, small amounts of oxides have formed on the surface, wherein the surface resistance as a measure for the spot-weldability and the paint infiltration is very low.

[0110] FIG. 2 shows a surface that is embodied and conditioned according to the invention in an electron microscope image, wherein an alkaline solution of potassium stannate with potassium hydroxide was applied with a roll coater before the heat treatment.

[0111] At different measuring points, element measurements were performed (FIG. 5), which indicate the presence of a tin coating.

[0112] The concentration of the solution that is used for the conditioning by means of roll coating is selected so that with a wet film of 1 μm, from 50-60 mg tin/m.sup.2 are deposited. During the annealing, a layer applied to this produces a modification of the oxide layer that forms so that a mechanical cleaning by means of a centrifugal wheel or other mechanical methods is no longer necessary.

[0113] A solution that produces a conditioning according to the invention has a solution concentration of 180-220 g/l K.sub.2SnO.sub.3*3H.sub.2O.

[0114] In order to increase the base capacity, the solution can have 15-25 g/l KOH added to it so that a pH value of approx. 13, i.e. 12.5-13.5 is produced.

[0115] Since in practical operation, acidic solutions are usually used readily, and since stannate solutions often tend to form precipitates during acidification, the tin can be suitably complexed to such an extent that a clear precipitate-free solution is obtained by adding citric acid in a quantity of 30-50 g/l, which results in a pH value of approx. 4.8.

[0116] FIG. 6 once again shows the surface of a conventional sheet that is not conditioned according to the invention after 45 seconds and 200 seconds of annealing time at 870° C. Both sheets exhibit the above-mentioned beige-green color.

[0117] FIG. 7 shows the surfaces of two sheets, which were conditioned according to the invention, after 45 seconds and 200 seconds of annealing time at 870° C. The differences in the surface color are clearly visible.

[0118] FIG. 8 shows the corresponding resistance results, which demonstrate that with the surface conditioning according to the invention, a very low surface resistance is achieved, which gives rise to the expectation of a very good weldability.

[0119] Also with regard to corrosion, the surface conditioning according to the invention achieves an advantage when it comes to paint infiltration because, as the results in FIG. 13 demonstrate, the paint infiltration results are so good that a cathodic immersion paint applied to the sheets without mechanical cleaning has infiltrated only slightly and not to a greater degree than in other sheets.

[0120] The conditioning according to the invention has been presented particularly in conjunction with stannates. But titanates, oxalates, and zirconates also chemically react in essentially the same way. One can therefore assume that they are effective in the same way, particularly the corresponding tin compounds

[0121] Tin appears to be particularly effective, which is why the surface conditioning is also successful if the tin is in metallic form. But the deposition of the tin onto the surface with the aid of stannates, i.e. in ionic form, has the advantage that the application can be carried out in a comparatively simple way using a roll coating method.

[0122] Naturally, all other methods with which liquid ionic solutions can be applied to a surface are also suitable.

[0123] The deposition of metallic tin is nevertheless conceivable and is possible, for example, by means of a CVD or PVD process.

[0124] The application can take place inline on the strip before it is cut into individual sheet bars. The sheet bars cut out from the strip can also be coated in a corresponding way.

[0125] The sheet bars are then formed by means of an in particular multi-step process into a component blank. It is also conceivable to first coat the component blank with the tin compound or the tin. It has turned out, however, that the tin or tin salt coating also tolerates the forming processes well.

[0126] Then a component blank that is obtained in this way is heated to a temperature that produces a structural change to austenite. The austenitized component blank is then conveyed to a form hardening tool in which the component blank is hardened in a single stroke by means of the contact with an upper tool and lower tool, which essentially have the shape of the blank or correspond to it. Due to the placement of the material of the component blank against the—in particular cooled—tools, the heat is removed from the steel material so quickly that a martensitic hardening occurs.

[0127] The invention has the advantage that by means of it, the surface of a sheet steel provided for form hardening or press hardening is successfully conditioned so that it is possible to dispense with a mechanical final cleaning for removing oxidic surface layers so that sheets of this kind can be processed in the same way as hot-dip aluminized sheets, for example, but with the advantage that a very high cathodic corrosion protection effect is achieved in comparison to hot-dip aluminized sheets.