Method for coating steel sheets or steel strips and method for producing press-hardened components therefrom

Abstract

The invention relates to a method for coating a steel sheet or steel strip to which an aluminium-based coating is applied in a dip-coating process and the surface of the coating is freed of a naturally occurring aluminium oxide layer. In order to provide a low-cost method for coating steel sheets or steel strips that makes the steel sheets or steel strips outstandingly suitable for the production of components by means of press hardening and for the further processing thereof, it is proposed that transition metals or transition metal compounds are subsequently deposited on the freed surface of the coating to form a top layer. The invention also relates to a method for producing press-hardened components from the aforementioned steel sheets or steel strips with an aluminium-based coating.

Claims

1. A method for coating a steel sheet or steel strip, comprising: applying an aluminium-based coat on the steel sheet or steel strip in a hot-dipping process; freeing a surface of the aluminium-based coat of a naturally occurring aluminium oxide layer; depositing transition metals or transition metal compounds, at least one of which includes the chemical element iron, on the freed surface of the coat to thereby form a top layer as a planar deposit with a layer weight, based on said iron, in a range of 7 to 25 mg/m.sup.2.

2. The method of claim 1, wherein the layer weight, based on said iron, is 10 to 15 mg/m.sup.2.

3. The method of claim 1, wherein the transition metals or transition metal compounds comprise beside the chemical element iron at least one further chemical element selected from the group consisting of titanium, vanadium, chromium, and manganese.

4. The method of claim 1, wherein the chemical element iron is present in a predominant part of the transition metals or the transition metal compounds.

5. The method of claim 1, wherein the transition metals or the transition metal compounds are deposited in the presence of at least one further chemical element selected from the group consisting of cobalt, molybdenum, tungsten, and compounds thereof.

6. The method of claim 1, wherein the transition metals or the transition metal compounds are deposited by chemical deposition.

7. The method of claim 6, wherein the chemical deposition includes spraying, dipping or rolling application.

8. The method of claim 6, further comprising removing atmospherically occurring, natural oxide layer and the chemical deposition in a single process step.

9. The method of claim 8, wherein the removal of the atmospherically occurring, natural oxide layer and the chemical deposition are performed in a continuously operating coating installation which is located downstream of a hot-dip coating installation or is separate from the hot-dip coating installation.

10. The method of claim 1, wherein the transition metals or the transition metal compounds are deposited electrolytically.

11. The method of claim 10, wherein the transition metals or transition metal compounds are applied electrolytically in an aqueous medium as an electrolyte at an electrolyte temperature of 25° C. to 85° C., at current densities between 0.05 and 150 A/dm.sup.2.

12. The method of clairn 22, wherein an aluminium oxide layer with mixed oxides from the top layer is formed on the coat with the top layer when exposed to an oxygen atmosphere or when exposed to steam.

13. The method of claim 12, wherein the aluminium oxide layer is formed with the mixed oxides in a furnace at a temperature >750° C. and a furnace dwell time >90 s.

14. The method of claim 12, wherein self-limitation of a layer growth of the aluminium oxide is avoided by formation of the mixed oxides.

15. The method of claim 12, wherein corundum, eskdaite, haematite, karelianite, tistarite, ilmenite, perowskite and/or spinets are formed as the mixed oxides.

16. The method of claim 1, wherein the aluminium-based coat includes aluminium, aluminium-silicon (AS) or aluminium-zinc-silicon (AZ) with optional incorporation of an additional element selected from the group consisting of. magnesium, manganese, titanium, and rare earth.

17. A method for producing a press-hardened component from a steel sheet or steel strip, comprising: applying an aluminium-based coat on the steel sheet or steel strip in a hot-dipping process, with a surface of the aluminium-based coat being freed of a naturally occurring aluminium oxide layer and transition metals or transition metal compounds, least one of which includes the chemical element iron, being deposited on the freed surface of the coat in order to form a top layer as a planar deposit with a layer weight, based on said iron, in a range of 7 to 25 mg/m.sup.2; heating at least a region of the steel sheet or steel strip to a temperature above Ac3; forming the steel sheet or steel strip at saki temperature; cooling the steel sheet or steel strip such as to harden at least a region of the steel sheet or steel strip at a rate which is above a critical cooling rate.

18. The method of claim 17, wherein the steel sheet or steel strip is made of a steel which is hardenable by heat treatment.

19. The method of claim 18, wherein the steel is alloyed with manganese and boron.

20. The method of claim 19, wherein the steel is a 22MnB5 steel.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other features and advantages of the present invention will become apparent upon reading the following description with reference to the accompanying drawing, in which:

(2) FIG. 1 shows a depth profile for the elements Al, Fe and O after the press-hardening of untreated sheets with an AS coat with a treatment in accordance with the invention;

(3) FIG. 2 shows a depth profile for the elements Al, Fe and O after the press-hardening of sheets with an AS coat with a treatment using an iron-containing solution in accordance with the invention; and

(4) FIG. 3 shows by way of example a cross-section polish on a sheet portion with an AS coating and treatment in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) FIGS. 1 and 2 show the depth profile for the elements Al, Fe and O after the press-hardening of sheets with an AS coat with a treatment in accordance with the invention using an iron-containing solution (FIG. 2) in comparison with an untreated sheet (FIG. 1) with a furnace dwell time of 6 min and a furnace temperature of 950° C. in an air atmosphere. FIG. 2 clearly shows the deeper oxygen input in the sample treated in accordance with the invention, which is indicative of a considerably thicker oxide layer in comparison with the untreated sample. Moreover, the enrichment of iron in the oxide layer can be clearly seen.

(6) The inventive treatment of the surface of the coated steel strip can be effected advantageously in a treatment part located downstream of the process part of a continuously producing hot-dip coating installation or a separate installation e.g. via spray bars with nozzles, in a dipping process and by means of electrolytic deposition or spray electrolysis, in each case also in combination. The separate installation can be e.g. a strip coating or electrolytic strip finishing installation. Alkaline cleaning upstream of the treatment in accordance with the invention and final rinsing of the steel sheet or steel strip provided with an aluminium-based coating advantageously eliminates the (natural) oxide layer which occurs by virtue of atmospheric oxidation and thereby provides a defined starting state for the inventive deposition of metallic species.

(7) The treatment of the surface can be effected in accordance with the invention over the entire strip surface or even only partially or on one or both sides. In the case of the external current-free treatment, it is possible to modify the molar quantity of the deposited metal species by concentrating the charge solution, the temperature thereof, the spray pressure, the shear of the sprayed-on solution relative to the surface of the metal strip to be treated and the volume brought into contact with the surface. In the case of electrolytic deposition, the deposited molar quantity of the metal species is determined by electrolyte composition, flow ratios, temperature, current density and treatment time.

(8) Exemplified Embodiments:

(9) Inventive pre-treatments of the samples are e.g. as follows:

(10) The AS-coated sheet is subjected to a dipping treatment in a metal cation-containing alkaline solution at a temperature of 50° C. for a few seconds. The naturally occurring oxide layer is removed and the iron-containing layer is applied.

(11) Alternatively, the AS-coated sheet is subjected to a dipping treatment in a 20% sodium hydroxide solution for 30 s at room temperature in order to remove the naturally occurring oxide layer. Subsequently, rinsing is effected using completely desalinated water. This is followed by the electrolytic deposition of an iron-containing layer at an electrolyte temperature of 50° C. The deposition is effected for in each case 1 and 10 s respectively at a current density of 23 A/dm.sup.2.

(12) Press-Hardening Test Parameters Furnace temperature for the heat treatment: 950° C. Atmosphere: ambient air Furnace dwell time (sheet thickness up to 1.5 mm): 2, 3, 4, 6 min Then cooling in the cooled flat die to <200° C.

(13) Table 1 shows for the purely wet-chemical pre-treatment of the samples that the thickness of the aluminium oxide layers increases significantly as the coverage of active substance (Fe) and the dwell time in the furnace increase. Without the treatment in accordance with the invention, the layer thickness of the oxide layer is less than 10 nm. In the case of an iron top layer of ca. 7 mg/m.sup.2 and dwell time of 2, 3 or 4 min, a significant layer formation is still not achieved. This also applies to an iron top layer of ca. 11 mg/m.sup.2 and a dwell time of 2 min,

(14) TABLE-US-00001 TABLE 1 Layer formation on the sample surface in dependence upon the iron top layer and furnace dwell time Furnace dwell time/min Top layer of 2 3 4 6 iron/mg/m.sup.2 Layer thickness of topmost layer/nm ca. 7 No significant layer formation 170 ca. 11 140 200 230 ca. 15 150 220 230 250

(15) Table 2 shows that the pre-treated AS samples which are press-hardened in an air atmosphere and have an iron-containing coating already have a distinct welding area even after short annealing times. Without the treatment hi accordance with the invention, there is no measurable welding area in the case of short annealing times.

(16) TABLE-US-00002 TABLE 2 Welding area according to SEP1220-2 in dependence upon the top layer and annealing time Furnace dwell time/min. Top layer of 2 3 4 6 iron/mg/m.sup.2 Welding area/kA ca. 7 2.2 2.1 2.1 1.2 ca. 11 2.2 2 1.7 1.7 ca. 15 2.5 2.1 1.7 1.6

(17) The disbanding at the crack after 12 weeks subjected to the Volkswagen PV1210 corrosion test is less on samples undergoing the treatment in accordance with the invention than on untreated samples, as illustrated in Table 3.

(18) TABLE-US-00003 TABLE 3 Disbonding on CD-coated samples after 12 weeks subjected to the Volkswagen PV1210 test in dependence upon the iron top layer and annealing time Disbonding (UW) at the Furnace dwell Top layer of crack/mm after 12 weeks time/min iron/mg/m.sup.2 subjected to the VW PV1210 test 2 ca. 11 UW < 1 ca. 15 UW < 1 3 ca. 7  UW < 1 ca. 11 UW < 1 ca. 15 UW < 1 4 ca. 7  UW < 1 ca. 11 UW < 1 ca. 15 UW < 1 6 ca. 7  UW < 1 ca. 11 UW < 1.5 ca. 15 UW < 1.5 Without the treatment in accordance with the invention 2.5 0 UW > 2 or extensive filiform corrosion 6 0 1.5 < UW < 2

(19) FIG. 3 shows by way of example a cross-section polish on a sheet portion with an AS coating and inventive treatment deposited without external current with an iron top layer of ca. 15 mg/m.sup.2 after press-hardening. The furnace dwell time was 3 min at a furnace temperature of 950° C. in an air atmosphere.

(20) In this case, the letter A designates the base material; B designates the diffusion zone consisting of a matrix of the base material, into which Al and Si are diffused from the coat; C designates a layer which is rich in Fe—Al phases; D designates the alloying zone, consisting of different Al—Fe, Al—Fe—Si phases; E designates the oxide layer of aluminium oxide and iron oxide; F designates the embedding compound.