Steel component provided with a metallic coating giving protection against corrosion

Abstract

A steel component having a steel substrate containing 0.3-3 wt.-% manganese, and an anti-corrosion coating applied to the steel substrate including a coating layer having at least 70 mass-% -Fe(Zn,Ni) mixed crystal, the remainder being intermetallic compounds of Zn, Ni and Fe, and which has at its free surface a Mn-containing layer in which the Mn is present in metallic or oxidic form.

Claims

1. A steel component comprising a steel substrate containing 0.3-3 wt.-% manganese, and having an anti-corrosion coating applied to the steel substrate comprising a coating layer at least 70 mass-% of which is composed of -Fe(Zn,Ni) mixed crystal, the remainder of intermetallic compounds of Zn, Ni and Fe, and which has at its free surface a Mn-containing layer in which the Mn is present in metallic or oxidic form.

2. The steel component according to claim 1, wherein the intermetallic compounds are dispersed in the -Fe(Zn,Ni) mixed crystal.

3. The steel component according to claim 1, wherein the coating layer is more than 2 m thick.

4. The steel component according to claim 1, wherein the coating layer contains 1-15 wt.-% Ni.

5. The steel component according to claim 1, wherein the Mn content of the Mn-containing layer is 1-18 wt.-%.

6. The steel component according to claim 1, wherein the thickness of the Mn-containing layer is 0.1-5 m.

7. The steel component according to claim 1, wherein the anti-corrosion coating comprises a zinc-rich layer lying on the coating layer.

8. The steel component according to claim 1, wherein an organic coating is applied to the Mn-containing layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described in what follows by reference to embodiments. In the drawings:

(2) FIG. 1 shows the results of a GDOS measurement of a coating according to the invention after the hot forming, for the elements O, Mn, Zn, Ni and Fe;

(3) FIG. 2 shows the measured result which is shown in FIG. 1 for the element Mn, in isolation;

(4) FIG. 3 is a schematic illustration of the structure of a coating at various times of production;

(5) FIGS. 4, 5 are micrographs of a coating present on a component produced according to the invention.

DESCRIPTION OF THE INVENTION

(6) Specimens A-Z of cold-rolled, recrystallisation annealed and skin-pass-rolled strip materialreferred to below for simplicity's sake simply as specimens A-V2were made available, which had been provided with a layer of zinc-nickel alloy on an electrolytic galvanising line though which they travelled in a continuous pass. A specimen. Z had also been melt dip coated for comparison.

(7) The Mn contents are of significance in the present case and are given in the Mn content column in Table 2 for each of the specimens A-Z, which were composed of a hardenable steel. The Table shows that specimens A-Q and Z each had Mn contents of more than 0.3 wt.-% whereas the Mn contents of specimens V1, V2 were below the limiting level of 0.3 wt.-%.

(8) Each of the specimens A-V2 in strip form first progressed through a cleaning treatment in which it passed through the following operating steps one after the other:

(9) The given specimen A-V2 was first subjected to spray cleaning, with the use of brushes, in an alkaline bath of cleaner at a temperature of 60 C. for a dwell time of 6 s.

(10) Electrolytic degreasing at a current density of 15 A/dm.sup.2 then took place for 3 s.

(11) This was followed by flushing twice with clean water, with the use of brushes. The duration of each of these flushing treatments was 0.3 s.

(12) After this, pickling with hydrochloric acid at a concentration of 150 g/l was carried out at ambient temperature for 8 s.

(13) In conclusion, three-stage cascade flushing with water took place.

(14) The specimens A-V2 which had been pre-treated in this way were subjected to electrolytic coating in an electrolysis cell. The following operating parameters, as respectively set for the specimens A-V2, are given in Table 1: Zn=Zn content of the electrolyte in g/l, Ni=Ni content of the electrolyte in g/l, Na2SO4=Na2SO4 content of the electrolyte in g/l, pH-value=pH-value of the electrolyte, T=temperature of the electrolyte in C., Cell type=orientation of the incident flow on the strip produced by the electrolyte, Speed of flow=speed of flow of the electrolyte in m/s, and Current density=current density in A/dm.sup.2.

(15) Specimen Z was hot galvanised in the conventional way as a comparison.

(16) Shown in Table 2 are not only the Mn contents of the respective specimens A-V2 but also the properties of the ZnNi coatings which were electrolytically deposited under the above conditions. It can be seen that a single-phase -ZnNi coating according to the invention was obtained in the case of variants A-H and N-P, whereas in the case of variants I-K -Zn, i.e. elemental zinc, and -ZnNi were present next to one another.

(17) In the case of variants L and M, before the layer of ZnNi was applied, a thin layer of pure nickel (a so-called nickel flash) was applied to the steel substrate. What this latter layer involved was deposits of pure nickel which were situated below the coating of single-phase -ZnNi. A multi-layered structure of this kind does not have any positive effect on the properties which are to be achieved and because of this these variants have been designated not according to the invention in the same way as the specimens obtained under variants I-K.

(18) The Ni content of specimen Q was too high, and this specimen too was therefore considered to be not according to the invention.

(19) Specimens V1 and V2 were produced from a steel which had a too low Mn content. These specimens too were therefore designated not according to the invention even though they had a -ZnNi coating according to the invention.

(20) In view of the single-phase structure of their coating of ZnNi alloy, the electrolytically coated specimens A-H and N-P could be considered according to the invention and blanks 1 to 23 were taken from them.

(21) In addition to this, blanks 31-35 were taken from the specimens L and M which had a two-layer ZnNi coating with a nickel flash, a blank 36 was taken from specimen Q, which could likewise not be considered according to the invention because of the excessively high Ni content of its coating, and blanks 37 to 40 were taken from the specimens V1 and V2 which were produced for comparison and a blank 41 was taken from the comparison specimen Z.

(22) Blanks 1 to 41 were then heated to the blank temperature T oven which is given in Table 3 for an annealing time t anneal and were each formed into a steel component in a single stage in a conventional die for hot press hardening and were cooled sufficiently quickly for a hardened microstructure to form in the steel substrate.

(23) For each of the steel components produced from blanks 1 to 41, the behaviour when hot formed which was found in the course of the hot press forming was assessed and checked by seeing whether there had been any cracking in the given steel substrate in the course of the hot press forming. The results of this assessment and checking process are also shown in Table 3.

(24) The steel components formed from blanks 1 to 36 and 41 were then subjected to a salt spray test under DIN EN ISO 9227. Where, in this test, any corrosion of the substrate metal was found after 72 h or 144 h, this is noted in the columns headed Substrate metal corrosion 72 h and Substrate metal corrosion 144 h in Table 3.

(25) It was found that the steel components which were produced from blanks 9 to 23 which had Ni contents of 9-13 wt.-% in their originally applied coating of ZnNi alloy not only showed optimum behaviour when formed but also had superior resistances to corrosion.

(26) It is true that good behaviour when hot formed was found for the steel component which was formed from the conventionally coated blank 41 obtained from specimen Z. It did not however meet the requirements laid down for avoidance of cracking of its steel substrate.

(27) Peeling of the coating and an inadequate resistance to corrosion on its part were found for the steel components which were produced from the blanks 37-40 taken from comparison specimens V1 and V2. Because this constituted a criterion for exclusion, no further checks were made on these steel components.

(28) The GDOS measurement process (GDOS=glow discharge optical emission spectrometry) is a standard process for the fast detection of a profile of concentrations for coatings. It is described in, for example, the VDI-Lexikon Werkstofftechnik [VDI Lexicon of Materials Science], edited by Hubert Grfen, VDI-Verlag GmbH, Dsseldorf 1993.

(29) Shown in FIG. 1 is a typical result of the GDOS measurement of the anti-corrosion coating of a steel component produced and obtained in a manner according to the invention. In it, the contents of Mn (line of short dashes), O (dotted line), Zn (line of long dashes), Fe (dotted and dashed line) and Ni (solid line) are plotted against the thickness of the coating layer. It can be seen that at the surface of the coating there is a high concentration of Mn which has diffused from the steel substrate through the coating to the surface of the latter and has there oxidised with the ambient oxygen. In the ZnNi-containing layer of the coating on the other hand the Mn content is considerably lower and only rises again when the steel substrate is reached. This can be seen particularly clearly in FIG. 2. The Ni content of the coating on the other hand is substantially constant over its entire thickness.

(30) In a further test, a recrystallised cold-rolled strip was first coated electrolytically with a single-phase coating of ZnNi alloy composed of the -ZnNi phase, in the same way as specimens according to the invention which were explained above. The thickness of the layer of -ZnNi alloy coating was 7 m with a Ni content of 10%. A 5 m thick Zn layer composed of pure zinc was then applied to this coating of ZnNi alloy, likewise electrolytically.

(31) Blanks were taken from the cold-rolled strip provided with a two-layer anti-corrosion coating which was obtained in this way and were heated to a blank temperature of 880 C. within a length of time of 5 minutes. After the hot forming and hardening, an anti-corrosion layer was present on the steel component obtained. There was also a pronounced layer of Mn oxide present at the surface of this layer, below which there was a Zn-rich layer below which in turn was a layer of ZnNi resting on the steel substrate.

(32) In order to check how the coating applied to the respective blank develops during the heating to the blank temperature and in what way the coating on the finished component obtained is constituted, using specimens provided with a coating of ZnNi alloy in accordance with the inventive method, firstly the structure of the coating is examined after the electrolytic coating, after heating to 750 C. with subsequent cooling and finally on the component which is finish formed and hardened after through-heating to 880 C. The states of the coating at the three moments in time concerned may be described as follows:

(33) a) After coating (FIG. 3, image 1):

(34) The coating is single-phase, intermetallic, composed of gamma-zinc-nickel (Ni5Zn21). At the best, a very thin and native oxide film of negligible effect, which is free from Mn, is present on the surface.

(35) b) Heating to approx. 750 C. (FIG. 3, image 2)

(36) A Zn/Mn oxide layer has formed on the coating. The coating seen metallographically is two-phase. Both gamma phases are shown, wherein in each case Fe is partially replaced by Ni and vice versa. The phases are isomorphous as regards their crystal structure.

(37) It is characteristic that the Ni-content in the coating decreases towards the base material and similarly the Fe-content decreases towards the free surface. This form of the coating structure is present up to approx. 750 C., but can still be demonstrated in the case of very short times, less than those for through-heating of the respective blank. Typical examples for the composition of the -ZnNi(Fe) and the -FeZn(Ni) phase of the coating are indicated in the following table:

(38) TABLE-US-00001 Fe Ni Zn Phase [mass-%] [mass-%] [mass-%] -ZnNi(Fe) 3 14 83 -FeZn(Ni) 16 6 78

(39) c) Result of the annealing process (FIG. 3, images 3, 4):

(40) With further continued heating firstly the coating is as far as possible intermetallic, in some cases both gamma phases -ZnNi and -ZnFe are present next to each other. However, in the course of the annealing process (above approx. 750 C.) an -Fe mixed crystal, in which Zn and Ni are present in solution, forms in the coating.

(41) With further continued heating, the Zn/Mn oxide layer continues to be present. The coating seen metallographically and radiographically is two-phase. A mixed gamma phase (/-ZnNi(Fe)) forms. It is characteristic that this phase is quite rich in Ni. A new phase forms at the steel-coating boundary phase. An -Fe mixed crystal, in which Zn and Ni are in solution, is present. The forced solution takes place due to the swift cooling rate. Typical examples of the composition of the coating layers are indicated in the following table:

(42) TABLE-US-00002 Fe Ni Zn Phase [mass-%] [mass-%] [mass-%] /-ZnNi(Fe) 7 13 80 -Fe(Zn,Ni) 70 3 27 mixed crystal

(43) The finished component always has a two-phase coating, consisting of an -Fe mixed crystal, in which Zn and Ni are present in forced solution, and a mixed gamma phase Zn.sub.xNi(Fe).sub.y in which Ni-atoms are replaced by Fe-atoms and vice versa.

(44) Dependent on the point in time at which the annealing treatment is completed and on the annealing temperature, the mixed gamma phase /-ZnNi(Fe) diffuses in the -Fe(Zn,Ni)-MK -Fe mixed crystal area, which now reaches to below the ZnMn oxide layer. This type of phase structure is promoted by: high temperatures long oven dwell times minimum layer thicknesses

(45) Typical examples of the composition of the coating layers are indicated in the following table:

(46) TABLE-US-00003 Fe Ni Zn Phase [mass-%] [mass-%] [mass-%] /-ZnNi(Fe) 14 13 73 -Fe(Zn,Ni) 71 3 26 mixed crystal

(47) Two states of the coatings reached after completion of the annealing treatment are illustrated by way of example in FIG. 3, images 3 and 4.

(48) FIG. 3, image 3 in this case shows the state of the coating which comes into being if comparably low annealing temperatures, short oven dwell times or large layer thicknesses of the coating are maintained. In FIG. 4 a microscopic flash-assisted photograph of a cross section of a coating produced in the inventive way is shown in this state.

(49) FIG. 3, image 4, however, shows a structure of the coating, which comes into being with high annealing temperatures, comparably long annealing time or minimum layer thickness of the coating. In this case the state shown in FIG. 3, image 3 as well as FIG. 4, illustrates an interim stage, which is undergone on the way to the state illustrated in FIG. 3, image 4. In FIG. 5 a microscopic flash-assisted photograph of a cross section of a coating produced in the inventive way is shown in this state.

(50) It can be confirmed that in phase c) elucidated above (FIG. 3, images 3 and 4) the -Fe(Zn,Ni) mixed crystal contains <30 wt.-% Zn and the mixed gamma phase /-ZnNi(Fe) comprises >65 wt.-% Zn. Due to the high Zn content of the mixed gamma phase /-ZnNi(Fe) an elevated anti-corrosion effect is achieved compared with pure Zn/Fe systems.

(51) With the invention, a method by which a component provided with a well-adhering and particularly effective metallic anti-corrosion coating can be produced in a simple manner is therefore available. For this purpose, a flat steel product produced from steel containing 0.3-3% manganese and having a yield point of 150-1100 MPa as well as tensile strength of 300-1200 MPa is coated with an anti-corrosion coating, which comprises a coating of ZnNi alloy which is electrolytically deposited on the flat steel product which coating is composed in a single phase of -ZnNi phase and which contains, as well as zinc and unavoidable impurities 7-15 wt.-% nickel. A blank is then obtained from the flat steel product and is directly heated to at least 800 C. and is then formed into the steel component or is first formed into the steel component, which is then heated to at least 800 C. The steel component obtained in the respective cases is finally hardened by being cooled sufficiently fast for hardened microstructures to form, from a temperature at which the steel component is in a suitable state for hardened or tempered microstructures to form.

(52) TABLE-US-00004 TABLE 1 Zn Ni Na2SO4 Temp. Speed of flow Current density Specimen [g/l] [g/l] [g/l] pH-value [ C.] Type of cell [m/s] [A/dm.sup.2] A 42 126 28 1.6 65 Horizontal 0.3 10 B 42 126 28 1.6 65 Horizontal 0.3 10 C 42 126 28 1.6 65 Horizontal 0.3 10 D 75 70 23 1.4 60 Vertical 4 40 E 75 79 23 1.4 60 Vertical 4 40 F 75 75 23 1.4 60 Vertical 4 40 G 75 85 23 1.4 60 Vertical 4 40 H 75 90 25 1.4 63 Vertical 4 40 I 75 79 23 1.4 60 Horizontal 3.5 40 J 105 75 23 1.4 60 Horizontal 4.4 40 K 75 79 23 1.4 60 Horizontal 3.5 40 L 42 126 28 1.6 65 Vertical 3.5 40 M 42 126 28 1.6 65 Vertical 3.5 40 N 62 75 27 1.6 65 Horizontal 0.5 20 O 62 75 27 1.6 65 Horizontal 0.5 20 P 62 75 27 1.6 65 Horizontal 0.5 20 Q 36 144 25 1.5 69 Horizontal 0.3 10 V1 75 70 23 1.4 60 Vertical 4 40 V2 75 79 23 1.4 60 Vertical 4 40 Z Melt dip coating - hot-dip galvanised in the conventional way

(53) TABLE-US-00005 TABLE 2 Coating Mn content in Thickness of Ni Thickness of ZnNi Ni content of ZnNi Crystallographic substrate metal flash layer coating coating structure of ZnNi According to the Specimen [% by mass] [m] [m] [% by mass] coating invention? A 1.3 6 14 Yes B 1.3 8 Yes C 1.3 10 Yes D 1 10 9 Yes E 2 10 12 Yes F 1 15 11 Yes G 1.4 8 12 Yes H 1.4 7 13 Yes I 1.5 5 10 + No J 1.5 8 9 + No K 1.5 10 11 + No L 1.5 1 8 14 No M 1.25 2 7 No N 1.25 6 13 Yes O 1.25 8 Yes P 2.2 9 Yes Q 1.3 8 16 No V1 0.1 10 9 No V2 0.2 10 12 No Z 1.2 No

(54) TABLE-US-00006 TABLE 3 Coating T Behaviour Corrosion of Corrosion of According Thickness Ni content oven t anneal when hot substrate metal substrate metal to the Specimen Blank [m] [% by weight] [ C.] [min] formed Cracking 72 h.sup.2) 144 h.sup.2) invention A 1 6 14 880 5 Good No No Yes Yes B 2 8 880 4 Good No No Yes Yes B 3 8 880 5 Good No No Yes Yes C 4 10 880 6 Good No No Yes Yes C 5 10 880 4 Good No No Yes Yes C 6 10 880 5 Good No No Yes Yes C 7 10 860 7 Good No No Yes Yes C 8 10 860 5 Good No No Yes Yes D 9 10 9 880 5 Good No No No Yes D 10 10 880 8 Good No No No Yes E 11 10 12 880 5 Good No No No Yes E 12 10 860 8 Good No No No Yes F 13 15 10.5 880 5 Good No No No Yes F 14 15 880 5 Good No No No Yes H 15 7 13 880 5 Good No No No Yes N 16 6 860 7 Good No No No Yes N 17 6 880 6 Good No No No Yes O 18 8 860 10 Good No No No Yes O 19 8 880 8 Good No No No Yes O 20 8 900 6 Good No No No Yes P 21 9 860 12 Good No No No Yes P 22 9 880 10 Good No No No Yes P 23 9 900 8 Good No No No Yes L 31 (1)8.sup.1) 14 880 3 Good No Yes Yes No L 32 (1)8.sup.1) 880 4 Good No Yes Yes No L 33 (1)8.sup.1) 880 5 Good No Yes Yes No M 34 (2)7.sup.1) 860 4 Good No Yes Yes No M 35 (2)7.sup.1) 860 5 Good No Yes Yes No Q 36 8 16 880 7 Good No Yes Yes No V1 37 10 9 860 8 Poor No further assessment due to poor No V1 38 10 880 5 Poor behaviour when hot formed (local peeling) No V2 39 10 12 880 5 Poor No V2 40 10 860 8 Poor No Z 41 10 880 5 Good Yes No No No .sup.1)Values in ( ) = Thickness of Ni flash .sup.2)Salt spray test under DIN EN ISO 9227