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

Abstract

A method for producing hardened steel components is provided. Sheet bars are cut out from an alloy-galvanized strip made of a hardenable steel alloy and the sheet bars are heated to a temperature that produces a structural change to austenite, preferably to a temperature above the respective Ac3 point. The austenitized sheet bars are then conveyed to a press hardening tool in which the sheet bars are hot formed in a single stroke or multiple strokes by means of an upper and lower tool, wherein the formed sheet bar is cooled against the tools at a speed above the critical cooling rate so that a martensitic hardening occurs.After the galvanization, which can be hot-dip galvanization of the steel strip and before the temperature increase for achieving the austenitization, tin is applied to the surface of the strip or sheet bar.

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; heating the steel sheet bar to a temperature that produces a structural change in the steel alloy to austenite, to produce an austenitized steel sheet bar; conveying the austenitized steel sheet bar to a press hardening tool that includes an upper tool and a lower tool; hot forming the austenitized steel sheet bar in a single stroke or multiple strokes using the upper tool and the lower tool; cooling the austenitized steel sheet bar at a speed above a critical cooling rate of the steel alloy to cause a martensitic hardening of the steel sheet bar; and before heating the steel sheet bar to the temperature that produces the structural change, applying tin to a surface of at least one of the galvanized steel strip and the steel sheet bar.

19. The method of claim 18, wherein the step of heating the steel sheet bar to the temperature that produces the structural change comprises heating the steel sheet bar to a temperature that is above an Ac3 temperature of the steel alloy.

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

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

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

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

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

25. 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.

26. 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.

27. 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.

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

29. 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.

30. 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.

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

32. 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.

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

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

35. The galvanized steel strip according to claim 33, wherein the tin is deposited from a stannate solution.

36. The galvanized steel strip according to claim 33, wherein the tin is deposited using a physical vapor deposition (PVD) or chemical vapor deposition (CVD) process.

37. 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; coating at least one of the galvanized steel strip and the sheet bar with tin, resulting in a tin-coated steel sheet bar; heating the tin-coated steel sheet bar to a temperature that produces austenitization of the steel alloy, yielding an austenitized tin-coated steel sheet bar; conveying the austenitized steel sheet bar to a press hardening tool that includes an upper tool and a lower tool; hot forming the austenitized steel sheet bar in a single stroke or multiple strokes using the upper tool and the lower tool; and cooling the austenitized steel sheet bar at a speed above a critical cooling rate of the steel alloy to cause a martensitic hardening of the steel sheet bar; wherein the method is performed without cleaning the galvanized steel strip.

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 the form hardening process or phs-ultraform® process according to the prior art;

[0092] FIG. 2 shows the production path in the hot-forming process, press hardening, or phs-directform® process according to the prior art;

[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. 4 shows a line layout of a hot-dip galvanization line according to the prior art;

[0095] FIG. 5 shows a line layout of an electrolytic galvanization line according to the prior art;

[0096] FIG. 6 shows an electron microscope image of the surface after the annealing without conditioning (prior art);

[0097] FIG. 7 shows an electron microscope image of the surface that has been conditioned according to the invention after the annealing;

[0098] FIG. 8 is the comparison of two steel sheets after the annealing, shown without conditioning on the left and shown with conditioning according to the invention on the right;

[0099] FIG. 9 shows a polished micrograph of the steel sheet that has been conditioned according to the invention, with the element distribution at four different measuring points;

[0100] FIG. 10 shows the surface of a galvanized sheet steel after the annealing with an annealing time of 45 seconds and 200 seconds;

[0101] FIG. 11 shows the surface of the sheet steel after the annealing with a surface conditioning according to the invention after 45 seconds and 200 seconds;

[0102] FIG. 12 shows the electrical resistance of the sheet surface in surfaces treated according to the invention;

[0103] FIG. 13 shows the paint infiltration in surfaces conditioned according to the invention after six weeks according to the VDA test.

DETAILED DESCRIPTION OF THE INVENTION

[0104] According to the invention, the surface of a galvanized sheet metal, in particular sheet steel, which is formed and hardened in one step in a press hardening process, is conditioned with tin or stannates; the conditioning with stannates will be discussed below.

[0105] 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.

[0106] In this connection, both alkaline and acidic solutions can be used and in particular, solutions in which the tin is complexed can be used.

[0107] In particular, the aim is to produce an aqueous layer thickness of 1-5 μm, with a dry layer thickness of 50-250 nm, preferably 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].

[0108] FIGS. 4 and 5 show a hot-dip galvanization line or electrolytic galvanization line. In this case, the application of the stannate can preferably be carried out in the vicinity of the chemical passivation (in FIG. 4) or “passivation” station (in FIG. 5).

[0109] FIGS. 1 to 3 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. This corresponds to the phs-ultraform® process (FIG. 1), wherein after a cold forming, the formed part is then austenitized above Ac3 and then form-hardened. FIG. 2 shows the phs-directform® process in which first, the sheet bar is austenitized, then formed in the hot state, and then the trimming is performed. FIG. 3 shows a variant of this, the so-called phs-multiform® process, in which after the austenitization and an optional pre-cooling, particularly to a temperature of 450° C. to 650° C., a multi-step process is carried out with several forming steps and/or cutting and stamping procedures that are subsumed under the term “hot forming steps”. After the hardening, the surface of the sheets that have been heat treated in this way has a layer particularly composed of aluminum oxide and zinc oxide, which is preferably cleaned.

[0110] 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 oxide 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.

[0111] A conventionally produced hardened steel sheet bar has a greenish-beige appearance on the surface, which is caused by an increased formation of zinc oxides and manganese oxides. This is shown in FIG. 6.

[0112] In a conditioning with a stannate solution, the sheet exhibits a silvery surface (FIG. 7) chiefly consisting of zinc oxides or tin oxides.

[0113] Whereas with conventional methods, silvery surfaces indicate the lack of a complete reaction of the zinc layer 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 aluminum oxides have formed on the surface, wherein the surface resistance as a measure for the spot-weldability and the paint infiltration is very low.

[0114] FIG. 8 once again shows a comparison of a hardened galvanized steel sheet according to the prior art to one that has been treated according to the invention. Both sheets, each of the 22MnB5 grade with a zinc layer coating of 140 g/m.sup.2 (on both sides), were annealed for 45 seconds at a temperature above Ac3. The appearance of the sheet according to the prior art is significantly darker.

[0115] FIG. 9 shows a surface that is embodied and conditioned according to the invention in a sectional electron microscope image, wherein an alkaline solution of potassium stannate with potassium hydroxide was applied with a roll coater before the heat treatment. In this case, the steel grade 340LAD with a zinc layer of 180 g/m.sup.2 was annealed at 870° C. for 200 seconds. The layers above measuring point 7 (MP7) are preparation-related CSP redeposits and are therefore of no significance. It is clear that the lighter-colored layer at the level of MP7 represents the Sn/Zn oxide; this is also substantiated by the constituents of MP7, which exhibit significantly high values of Sn. The layer is very thin and is present over virtually the entire surface of the strip. Under this is a darker layer composed of Al oxide (MP6), which is likewise present over virtually the entire surface of the strip. Under this in turn is the reacted Zn/Fe layer, some of which can have slightly oxidized regions (at MP4, which is not shown in FIG. 9, however).

[0116] At different measuring points, element measurements were performed, which indicate the presence of the above-described tin coating.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] Since in practical operation, acidic solutions are usually used readily, and since stannate solutions often tend to form precipitates during acidification, as an alternative to KOH, 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.

[0121] FIG. 10 once again shows the surface of a conventional sheet that is not conditioned according to the invention for a different steel grade (22MnB5 with a zinc layer Z140-140 g/m.sup.2) after 45 seconds and 200 seconds of annealing time at 870° C. Both sheets exhibit the above-mentioned beige-green color.

[0122] FIG. 11 shows the surfaces of two sheets (once again 22MnB5 with a zinc layer Z140-140 g/m.sup.2), which were conditioned according to the invention, after 45 seconds and 200 seconds of annealing time at 870° C. (i.e. above Ac3). The differences in the surface color are clearly visible.

[0123] FIG. 12 shows the corresponding resistance results for different steel grades and annealing times respectively at 870° C., 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. The third grade 20MnB8 was coated with a zinc-iron layer, i.e. a so-called galvannealed layer of 180 g/m.sup.2.

[0124] 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. To demonstrate this, the VDA 233-102 climate change test was conducted and in this case, the paint infiltration in mm and also the respective cross-hatch adhesion value in a cross-cut according to DIN EN ISO 16276-2 were determined before and after the above-mentioned corrosion test according to VDA 233-102. The scale here is from 0 (very good) to 5 (total delamination). In this case, it is clear that the value before and after the test was usually 0, in other words outstanding. Sometimes, small regions flaked off, which resulted in values of 1 and sometimes 2.

[0125] The conditioning according to the invention has been presented particularly in conjunction with stannates, but titanates, oxalates, and zirconates also react in essentially the same way. One can therefore assume that they are effective in the same way, particularly the corresponding tin compounds.

[0126] But 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 or dip-squeeze method.

[0127] Naturally, all other methods with which liquid ionic solutions can be applied to a surface are also suitable.

[0128] The deposition of metallic tin is nevertheless conceivable and is possible, for example, by means of a CVD or PVD process.

[0129] 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.

[0130] The sheet bars are then heated to a temperature that produces a structural change to austenite. The austenitized sheet bars are then conveyed to a press hardening tool in which the sheet bar is formed in a single stroke or multiple strokes by means of an upper and lower tool. This can be carried out in the above-mentioned phs-directform® or phs-multiform® process with multiple stamping and/or trimming operations and with or without pre-cooling. Due to the placement of the material of the formed sheet bar against the—in particular cooled—tools, the heat is removed from the steel material so quickly that a martensitic hardening occurs.

[0131] 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.