Method for Manufacturing a Sheet Metal Component from a Flat Steel Product Provided With a Corrosion Protection Coating

Abstract

A method for manufacturing a sheet metal component including: annealing a flat steel product comprising 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, ≤0.06% P, ≤0.01% S, ≤1.0% Al, ≤0.15% Ti, ≤0.6% Nb, ≤0.01% B, ≤1.0% Cr, ≤1.0% Mo, ≤1.0% Cr+Mo, ≤0.2% Ca, ≤0.1% V, remainder iron and impurities in a continuous furnace under an atmosphere consisting of 0.1-15% hydrogen and remainder nitrogen with a specific dew point and temperature profile; applying a coating consisting of ≤15% Si, ≤5% Fe, in total 0.1-5% of at least one alkaline earth or transition metal and a remainder Al and unavoidable impurities; heating the fat steel product to >Ac3 and ≤1000° C. for a time sufficient to introduce a heat energy quantity>100,000-800,000 kJs; hot-forming the flat steel product to form the component; and cooling at least one section of the component at a cooling rate sufficient to generate hardening structures.

Claims

1. A method for manufacturing a sheet metal component from a flat steel product provided with a corrosion protection coating, the method comprising: a) providing a flat steel product produced from a steel which (in wt. %) comprises 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, wherein the total of the contents of Cr and Mo is at most 1.0%, up to 0.2% Ca, up to 0.1% V, and as the remainder of iron and unavoidable impurities; b) annealing the flat steel product in a continuous furnace having four zones A, B, C, D, which are passed through successively by the flat steel product and in which the flat steel product is annealed under an annealing atmosphere consisting in each case of 0.1-15 vol. % hydrogen and as the remainder of nitrogen as well as technically unavoidable impurities with a dew point temperature TP.sub.A, TP.sub.B, TP.sub.C, TP.sub.D at an annealing temperature GT.sub.A, GT.sub.B, GT.sub.C, GT.sub.D, the following specifications apply: TABLE-US-00008 one Dew point temperature TP Annealing temperature GT −10° C. ≤ TP.sub.A ≤ −25° C. 800° C. ≤ GT.sub.A ≤ 950° C. −27° C. ≤ TP.sub.B ≤ −41° C. 800° C. ≤ GT.sub.B ≤ 930° C. −30° C. ≤ TP.sub.C ≤ −80° C. 800° C. ≤ GT.sub.C ≤ 950° C. −30° C. ≤ TP.sub.D ≤ −20° C. 750° C. ≤ GT.sub.D ≤ 950° C. c) applying a corrosion protection coating to the flat steel product obtained in work step b), wherein the corrosion protection coating consists of (in wt. %) up to 15% Si, up to 5% Fe, in total 0.1-5% of at least one alkaline earth or transition metal and as the remainder of Al and unavoidable impurities; d) optionally: dress rolling the flat steel product provided with the corrosion protection coating; e) optionally: separating a board from the flat steel product; f) heating the flat steel product or the board to a hot forming temperature which is higher than the Ac3 temperature of the steel of the flat steel product and does not exceed 1000° C. for a holding time sufficient to introduce a heat energy quantity Js of more than 100,000 kJs and at most 800,000 kJs into the flat steel product or the board; g) hot forming the flat steel product heated to the hot forming temperature or the board heated to the hot forming temperature into the sheet metal component; h) cooling at least one section of the component at a cooling rate sufficient to generate a hardening structure in the at least one section of the sheet metal component.

2. The method according to claim 1, wherein a thickness of the flat steel product provided in work step a) is 0.6-7 mm.

3. The method according to claim 1, wherein the annealing temperature GT.sub.A is 810-940° C. and the dew point temperature TP.sub.A is −15° C. to −25° C. in zone A of the continuous furnace in the annealing completed in work step b).

4. The method according to claim 1, wherein the annealing temperature GT.sub.B is 800-900° C. in zone B of the continuous furnace in the annealing completed in work step b).

5. The method according to claim 1, wherein the annealing temperature GT.sub.C is 800-920° C. and the dew point temperature TP.sub.C is −30° C. to −50° C. in zone C of the continuous furnace in the annealing completed in work step b).

6. The method according to claim 1, wherein the annealing temperature GT.sub.D is 780-930° C. in zone D of the continuous furnace in the annealing completed in work step b).

7. The method according to claim 1, wherein a lambda value λ of the annealing atmosphere maintained in zones A-D is 0.95-1.1 in the annealing completed in work step b).

8. The method according to claim 1, wherein the Si content of the corrosion protection coating applied to the flat steel product in work step c) is at least 3 wt. %.

9. The method according to claim 1, wherein the Fe content of the corrosion protection coating applied to the flat steel product in work step c) is at least 1 wt. %.

10. The method according to claim 1, wherein the corrosion protection coating applied to the flat steel product in work step c) contains in total at least 0.11 wt. % of alkaline earth or transition metals.

11. The method according to claim 1, wherein the content of alkaline earth or transition metals in the corrosion protection coating applied to the flat steel product in work step c) is in total at most 0.6 wt. %.

12. The method according to claim 1, wherein the corrosion protection coating applied to the flat steel product in work step c) contains magnesium as the at least one alkaline earth or transition metal.

13. The method according to claim 1, wherein an amount of the corrosion protection coating applied to the flat steel product in work step c) is 30-100 g/m.sup.2 per coated side of the flat steel product.

14. The method according to claim 1, wherein the application of the corrosion protection coating in work step c) takes place by hot-dip coating.

15. The method according to claim 1, wherein the heating of the flat steel product or the board in work step f) takes place in a continuous furnace by radiant heat and the holding time is 100-900 s.

16. A sheet metal component manufactured from a flat steel product, the steel substrate of which consists of a steel, which (in wt. %) comprises 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, wherein the total of the contents of Cr and Mo is at most 1.0%, up to 0.2% Ca, up to 0.1% V, and as the remainder of iron and unavoidable impurities, and which is coated with a corrosion protection consisting of (in wt. %) up to 15% Si, up to 5% Fe, in total 0.1-5 wt. % of at least one alkaline earth or transition metal and as the remainder of Al and unavoidable impurities, wherein the layer of the corrosion protection coating adjoining the steel substrate is an interdiffusion layer (D) consisting of ferrite with an Al content of up to 50 wt. %, wherein in a cross-section of the interdiffusion layer (D), the proportion of the surface covered by pores with a diameter ≥0.1 μm is less than 10% and wherein the surface covered with pores in the interdiffusion layer (D) is <300 μm.sup.2 over a measurement length of 500 μm.

17. The sheet metal component according to claim 16, wherein the interdiffusion layer (D) has a thickness of 1-30 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] FIG. 1 shows a cross-section of a steel sheet of a sheet metal component manufactured according to the invention by hot forming in 500× magnification. The cross-section was prepared in a conventional manner by etching with 3% Nital in order to clarify the layer structure present on the steel sheet.

[0077] FIG. 2 shows a schematic representation of the cross-section according to FIG. 1.

DESCRIPTION OF THE INVENTION

[0078] The corrosion protection coating K formed on the steel substrate S comprises an interdiffusion layer D directly connected to the steel substrate S, which substantially consists of alpha mixed crystal (i.e. ferrite) with increased Al content. Fe2Al5 is still present here in phases. The interdiffusion layer D is characterised in that it is homogeneously and uniformly formed and that it is virtually pore-free.

[0079] In the direction of the free surface O of the corrosion protection coating K, a first Si-rich layer S.sub.1 has formed on the diffusion layer D. At the boundary between the diffusion layer D and the Si-rich layer S.sub.1, pores P1 are present in the diffusion layer D in small numbers and far apart from one another.

[0080] In the direction of the free surface O on the Si-rich layer S.sub.1, a first intermediate layer Z.sub.1 has formed, which consists of aluminium iron, wherein the majority lies in the aluminium. Traces of Si, alkaline earth and/or transition metals as well as unavoidable impurities may also be present in the layer Si. The intermediate layer Z1 is pore-free.

[0081] In the direction of the free surface O on the intermediate layer Z.sub.1 there is a second Si-rich layer S.sub.2.

[0082] A second intermediate layer Z.sub.2 is formed in the direction of the free surface O on the Si-rich layer S.sub.2. The layer Z2 also consists of aluminium iron, with the majority being aluminium and alkaline earth and/or transition metals may also be present. Traces of Si as well as unavoidable impurities may also be present. The intermediate layer Z2 is also pore-free.

[0083] The second intermediate layer Z2 is covered on its side facing the free surface O with an oxide layer OX, which substantially consists of aluminium, silicon and alkaline earth and/or transition metal oxides. Oxide layer thicknesses of up to 1.5 μm can be present on average on a hot-formed component. Crater-shaped pores P.sub.2, which are open to the environment, have formed in a small number and at large distance from one another on the surface of the oxide layer OX forming the free surface O of the corrosion protection coating K.

[0084] For comparison, a component was formed from a flat steel product which was covered with an AlSi coating according to the sample of the prior art described in EP 2 086 755. Its coating consisted of (in wt. %) 9.5% Si, 3.5% Fe and, as the remainder of aluminium and unavoidable impurities, was therefore free of alkaline earth or transition metals of the type added according to the invention.

[0085] The steel substrate of the flat steel product consisted of (in wt. %) 0.224% C, 0.25% Si, 1.16% Mn, 0.014% P, 0.002% S, 0.039% Al, 0.0034% N, 0.2% Cr, 0.03% Ti and 0.0026% B.

[0086] Before applying the metallic coating and forming into the flat steel product, the flat steel product processed for comparison has undergone an annealing treatment in a continuous furnace with four zones in which the dew point temperatures TP and annealing temperatures GT indicated in Table 6 have been set. The air ratio A in the continuous furnace was 0.98.

[0087] A five-layered layer structure of the corrosion protection coating has also been created for the component produced conventionally for comparison. However, compared to the number of pores in the coat of the component produced conventionally for comparison, in the component produced according to the invention, the number of pores P2 in the oxide layer OX was reduced by at least 25% and the number of pores P1 in the diffusion layer D by at least 40% compared with the pores present in the corresponding layers of the corrosion protection coating of the component produced conventionally for comparison. The area covered with pores P1 was 300 μm.sup.2 after a dwell time in the furnace of 600 s with a measurement length of 500 μm in the layer D.

[0088] The reduction of pores in P2 leads to a reduction of paint craters and improves adhesion and weldability. The pores in P2 have openings in the direction of the atmosphere of a few nm. If a component is now processed further after hot forming as is typical for cars, it will undergo cathodic dip painting in addition to a larger number of cleaning steps. Contact with water-based solutions is unavoidable here. During cleaning, water can penetrate into the pores P2 of the layer, since the surfactants added to the cleaning water improve wetting and significantly reduce the surface tension of the water. Water can also penetrate the opened pores P2 in the cathodic dip painting process. In this particular case, the cleaning water also leads to a separation of the paint particles, which cannot penetrate into the pores P2 due to the size of the opening. Water, which is then present in the pores P2, reaches the boiling point when the paint layers are baked in, which leads to vapour phases which, in a kind of boiling delay, escapes explosively through the paint to the environment. As a result of this reaction, so-called paint craters form, which, in addition to visual influence, also significantly reduce the effect of the paint in terms of corrosion protection. In the case of aluminium-based coats in particular, corrosion and paint infiltration can occur at such points. The red rust that occurs, which is formed due to the high iron contents of the coating and stands out visually, is particularly problematic for the further processor.

[0089] Also, on a surface where many open pores P2 are present, adhesives cannot penetrate into the pores P2 due to their higher viscosity. This may result in incomplete coverage of the surface with adhesive. Cavities also form in the area of the pores, as a result of which adhesion is also impaired.

[0090] The pores P2 present in the layer OX also lead to changed current paths in the material during resistance spot welding, which negatively influence the weldability.

[0091] In the case of a high pore count, there is also an enlarged surface on which water can split during oxidation in the hot forming process. In this way, diffuse hydrogen can penetrate into the material, which is known to increase the risk of hydrogen-induced cracking.

[0092] By minimising the frequency at which the pores P2 occur during the manufacture of a sheet metal component according to the invention, the risks associated with pore formation in conventionally produced components can be effectively reduced.

[0093] The reduction of the number of pores P1 in diffusion layer D also leads to an increase in the transferable force of adhesive bonds and to an improvement in the weldability.

[0094] The pores in P2 represent cavities within the corrosion protection coating K. If the number of pores is too high, there is a risk that the corrosion protection coating K will break up at the boundary region between the diffusion layer D and the first Si-rich layer S1, with the result that the adhesive seam also fails at an early stage. With the reduction of the number of pores P1 achieved according to the invention, the area over which the forces of the adhesive bond are transferred is increased by over 60% and thus the risk of delamination fracture is correspondingly reduced.

[0095] In order to prove the effect of the invention, steel sheets each with a thickness of 1.5 mm and cold-rolled in a conventional manner have been produced from six steels ST1-ST6, the compositions of which are indicated in Table 1 (work step a) of the method according to the invention).

[0096] The steel sheets provided in this way were subjected in nine tests V1-V9 in each case to a continuous annealing G1, G2 or G3 in a continuous furnace, which had four consecutive zones A, B, C, D. Table 2 shows the dew point temperatures TP.sub.A-TP.sub.D set in zones A-D for variants G1-G3 of the annealing, the annealing temperatures GT.sub.A-GT.sub.D as well as the hydrogen content H2 and the nitrogen content N2 of the respective annealing atmosphere, the remainder of which consisted of technically unavoidable impurities (work step b) the method according to the invention).

[0097] The samples annealed in this way are each coated in a conventional manner with a Al-based corrosion protection coating Z1-Z5 with a load AG. The compositions of the corrosion protection coatings Z1-Z5 are indicated in Table 3 (work step c) of the method according to the invention).

[0098] The samples each provided with one of the corrosion protection coatings Z1-Z5 were heated in each case in the tests V1-V9 in the continuous furnace to a hot forming temperature T.sub.WU at which they were held for a holding time t.sub.WU (work step f) of the method according to the invention.

[0099] The steel ST1-ST6, of which the samples each used in the tests V1-V9 consisted, the variants G1-G3 of the annealing each used in the tests V1-V9, the compositions Z1-Z5 of the corrosion protection coatings each produced in the tests V1-V9 and their respective loads AG as well as the hot forming temperatures T.sub.WU and holding times t.sub.WU each selected in the tests V1-V9 are indicated in Table 4.

[0100] The samples heated in this way were taken from the continuous furnace in a transfer time of 3-7 s in each case and placed in a conventional hot forming tool in which they were hot-formed into a component. Subsequently, cooling took place at 270 K/s in each case to room temperature (work steps g) and h) the method according to the invention.

[0101] Of the components obtained in the tests V1-V9, three cross-sections were produced in a manner known per se, which were etched with 3% Nital to clarify the layer structure. Illustrations of the cross-sections were generated in 500× magnification, as shown by way of example in FIG. 1. In the respective illustration, the pores P1, P2 present in the layers OX and D were counted over a section with a length of 550 μm. The arithmetic mean was formed from the counter results determined for the three cross-sections of a sample in each case. This arithmetic mean of the numbers determined for the pores P1 and P2 has been compared with the comparative values determined in the same way for a comparative sample.

[0102] The relative reduction in pore counts P1 and P2 resulting from this comparison and achieved by the invention is indicated in Table 5. Table 5 also shows the proportion of paint craters in the total area of the respective sample, the decrease in the delamination area and the welding region determined in accordance with the steel-iron test sheet SEP 1220-2. Welding regions greater than 1 kA have been classified as “OK”.

TABLE-US-00002 TABLE 1 Steel C Si Mn P S Al Nb Ti B A 0.08 0.33 0.95 0.025 0.02 0.013 0.09 0.01 0.005 B 0.23 0.38 1.3 0.02 0.007 0.013 — 0.03 0.004 C 0.38 0.37 1.38 0.02 0.008 0.013 — 0.1 0.005 D 0.2 0.35 1.35 0.02 0.008 0.012 — 0.02 0.004 E 0.14 0.25 1.07 0.1 0.001 0.08 0.025 0.01 0.002 F 0.24 0.3 1.3 0.022 0.008 0.012 — 0.02 0.004 Information in wt. %, the remainder Fe and unavoidable impurities

TABLE-US-00003 TABLE 2 Annealing Dew point Annealing atmosphere temperature TP temperature GT GA Lambda [° C.] [° C.] [vol %] Annealing value A B C D A B C D H2 N2 G1 1.05 −25 −40 −40 −20 880 880 850 800 7 91 G2 1.1 −20 −40 −45 −25 890 890 830 800 10 87 G3 0.95 −12 −30 −47 −22 890 900 900 820 5 92

TABLE-US-00004 TABLE 3 Corrosion protection coating Mg Si Fe Z1 0.3 9.5 3 Z2 0.5 8 3.5 Z3 0.1 10 3 Z4 2 8 2 Z5 0.8 8 3 Information in wt. %, remainder Al and unavoidable impurities

TABLE-US-00005 TABLE 4 Corrosion protection AG Twu twu Test Steel Annealing coating [g/m.sup.2] [° C.] [s] V1 A G1 Z3 69 920 300 V2 B G2 Z2 70 920 180 V3 C G1 Z3 75 925 360 V4 D G3 Z5 65 920 420 V5 E G1 Z1 70 900 300 V6 F G3 Z4 71 920 360 V7 H G2 Z1 65 925 360 V8 B G1 Z3 72 920 300 V9 D G3 Z2 71 925 300

TABLE-US-00006 TABLE 5 Reduction Reduction Decrease in of of Paint delamination Welding pores P1 pores P2 craters fracture area region Test [%] [kA] V1 25 50 8.4 67 1.1 V2 30 75 10.5 70 1 V3 28 43 9.5 62 1.2 V4 35 52 10.9 60 1.1 V5 33 60 11.6 65 1.2 V6 25 58 7.8 63 1 V7 37 57 12.5 60 1.1 V8 33 70 11.6 70 1 V9 28 65 9.5 69 1.2

TABLE-US-00007 TABLE 6 T.sub.P T.sub.G Zone [° C.] [° C.] A −30 750 B −23 780 C −25 780 D −35 740