EP1857566B1 - Flat steel product provided with a corrosion protection coating and method of its manufacture - Google Patents

Description

The invention relates to a method for producing a flat steel product formed from a steel substrate, such as steel strip or sheet, and a zinc-based anticorrosive coating applied to at least one of the sides of the steel substrate.

To improve their resistance to corrosion, metallic coatings are applied, in particular on steel sheets or strips, which are based on zinc or zinc alloys in the majority of applications. Such zinc or zinc alloy coatings protect due to their barrier and cathodic protection effect the corresponding coated steel sheet in practical use against corrosion.

(t = Einzelblechdicke) aufweisen und spritzerfrei schweißbar sein.However, the thickness of the coating required in the prior art for sufficient corrosion resistance poses problems in processing, ie in forming and welding. This applies z. B. when in high-wear highly corrosive flanges to be welded by spot welding. This requirement exists in particular in the field of construction of automobile bodies, in general construction applications or in the construction of housings for building services. The connection produced in such a weld should, with sufficient welding current, have a minimum point diameter of $4\times \sqrt{t}$

(t = single sheet thickness) and can be welded spatter-free.

Against the background of the problems in the processing of conventional coated with a Zn layer of large thickness sheets highly corrosion resistant Zn-Mg and Zn-Mg-Al layer systems have been developed, the one with a significantly reduced layer thickness with a conventional, 7.5 offer μ m thick zinc coating comparable corrosion protection, but have a significantly better processability.

One way to produce such hot-dip galvanized steel sheets with increased corrosion resistance at the same time reduced coating weight, is in the EP 0 038 904 B1 described. According to this prior art, a zinc coating containing 0.2% by weight of Al and 0.5% by weight of Mg is applied to a steel substrate by hot dip coating. Even if the sheet coated in this way should have an improved resistance to rust formation, in practice it does not meet the requirements imposed today on the corrosion resistance of such sheets, in particular in the area of the connecting flanges of an automobile body.

Another provided with a metallic protective coating sheet with improved corrosion resistance is from the EP 1 621 645 Al known. The one described there Steel sheet is provided with a protective coating by conventional hot-dip galvanizing, containing (in% by weight) 0.3-2.3% Mg, 0.6-2.3% Al, optionally <0.2% other active ingredients and balance Zn and unavoidable impurities. To produce the Zn coating, the steel sheet is passed through a melt bath containing 0.3-2.3% by weight of Mg, 0.5-2.3% by weight of Al, optionally less than 0.2% by weight of others Alloy components and passed as rest Zn and unavoidable impurities. As a result of the high Al and Mg content in its coating, such a sheet has a particularly good corrosion resistance. However, practical experiments have shown that also according to the EP 1 621 645 A1 provided sheets do not meet the requirements imposed by the processing industry on the weldability of such sheets. It also shows that the sheets in question have a suitability for phosphating insufficient by today's standards.

When out of the WO 89/09844 A1 known prior art is the influence of Pb and Al z. T. been examined in the presence of Si on a Zn-Al hot-dip coating or coating alloy. The experiments were usually carried out without Mg. If the examples considered contained Mg at all, Mg was present in such small amounts that it had no discernible influence on the corrosion resistance. The positive influence of Mg on the corrosion protection effect is not mentioned in D2.

The object of the invention was therefore to provide a method for producing a flat steel product, which has an optimum combination of high corrosion resistance and optimized processability and which is particularly suitable for use as a material for automotive body construction, for general construction or for home appliance.

This object has been achieved by the method specified in claim 1.

The invention is based on the recognition that general properties such. As adhesion and also weldability of provided with a corrosion-protective Zn-Mg-Al coating steel sheet or strip decisively depends on the distribution of the aluminum in the coating layer. Thus, it has surprisingly been found that when, as predetermined by the invention, in a near-surface intermediate layer of sufficient thickness according to the invention low Al contents are present, compared to conventionally formed sheets improved weldability is given, although the Al content of the coating on the whole a level is guaranteed by the high corrosion protection. The correspondingly produced according to the invention sheets in the boundary layer at the transition to the steel substrate high Al concentration thereby causes the positive effect of aluminum on the corrosion protection is maintained despite the low proportion of Al in the intermediate layer.

As a result of the low contents of Al on their surface and in the intermediate layer, flat steel products produced according to the invention likewise show a particularly good suitability for phosphating, so that they can be provided with an organic lacquer coating, for example, without any special additional measures.

Elements from the group Pb, Bi, Cd, Ti, B, Si, Cu, Ni, Co, Cr, Mn, Sn and rare earths can be present up to a total of their contents of 0.8% by weight in the coating produced according to the invention be. Pb, Bi and Cd can be used to form a larger crystal structure (zinc flower), Ti, B, Si to improve the formability, Cu, Ni, Co, Cr, Mn to influence the boundary layer reactions, Sn influencing the surface oxidation and rare earths, especially lanthanum and cerium, to improve the flow behavior of the melt. The impurities which may be present in a corrosion protection coating according to the invention also include the constituents which, as a result of the hot-dip coating, enter the coating from the steel substrate in quantities which do not affect the properties of the coating.

It has been found that, given the relatively low Al contents of a melt bath used for carrying out the method according to the invention, the shape of the layer structure desired according to the invention can be directly influenced by a suitable adjustment of the strip inlet and / or bath temperature. By the process control according to the invention is achieved in that adjacent to the steel substrate Bound layer high Al and Mg contents accumulate, while in the intermediate layer in particular low Al contents are present. In this case, the difference between the temperature of the strip during immersion and the temperature of the melt bath is of particular importance. By varying this difference in the range from -10 ° C. to 70 ° C., the inventively minimized presence of Al in the intermediate layer can be set in a safe and targeted manner.

Particularly favorable welding properties occur when the aluminum content of the intermediate layer is reduced as much as possible. Therefore, an advantageous embodiment of the invention provides that the Al content of the intermediate layer is limited to 0.25 wt .-%.

In addition, the layer structure used by the invention then has a particularly positive effect on the weldability and the phosphatability with at the same time still good corrosion protection of the coating, if the thickness of the intermediate layer according to the invention is at least 25% of the total thickness of the anticorrosive coating. The information on the structure of the corrosion-coating layer and its individual layers contained herein and in the claims relates to a layer profile determined by a GDOS measurement (glow discharge optical emission spectrometry). For example, in the VDI Lexicon Materials, ed. The GDOS measuring method described by Hubert Gräfen, VDI-Verlag GmbH, Dusseldorf, 1993 is a standard method for rapidly detecting a concentration profile of coatings.

In the case of flat steel profiles produced according to the invention, such a GDOS measurement shows that in the surface layer immediately adjacent to the surface of the coating, as a result of oxidation, an increased Al content inevitably occurs as a result of oxidation. However, since the thickness of this surface layer is very small compared with the total thickness of the coating, the surface layer is easily broken when welding a flat steel product according to the invention and affects the welding result only insignificantly. In order to exclude a possibly negative influence of the higher Al-containing surface layer, the thickness of the surface layer should be limited to less than 10%, in particular less than 1% of the total thickness of the anticorrosive coating. Practical investigations have confirmed that, in the case of flat steel products produced according to the invention, the surface layer is in each case not more than 0.2 .mu.m thick, so that with conventional coating thicknesses of 6 .mu.m and more, the fraction of the surface boundary layer in the total thickness of the coating overlay is approximately 3.5% and wide below it lies.

In the case of flat steel products produced according to the invention, the coating has contents of Fe which are more than 0.3% by weight, in particular more than 0.4% by weight or even more than 0.5% by weight. The relatively high Fe contents are present in particular in the region of the boundary layer adjacent to the steel substrate. In this case, it is preferable to form an alloy by which an optimized adhesion of the coating to the steel substrate is ensured. In this way, a flat steel product obtained according to the invention has Performance characteristics, which are superior to the conventional flat steel products, even if they have high Mg and Al contents in their protective coating.

In order to further optimize the weldability and phosphatability of a flat steel product produced according to the invention in addition to the layer structure of the anticorrosive coating according to the invention, the Al content of the anticorrosive coating can be limited to less than 0.6% by weight, in particular less than 0.5% by weight.

To ensure its effectiveness, the total thickness of the anticorrosive coating should be at least 5 μm , in particular at least 7 μm . In this case, pad weight distribution of the anticorrosive coating of at least 100 g / m ² prove to be particularly favorable in terms of its protective effect. Despite higher print run thickness and thickness of the anti-corrosion coating, the weldability is not adversely affected due to the invention prescribed distribution of its Al content.

Particularly good product results occur when the bath temperature of the melt bath is 440-480 ° C.

Surprisingly, it has been found that the speed with which the steel substrate passes through the melt bath has only a minor influence on the coating result. Therefore, it can be varied in the range of 50 - 200 m / min, for example, in order to achieve the optimum work result at maximum productivity.

The annealing of the steel strip, which precedes the melt bath, should be carried out under a protective gas atmosphere in order to avoid oxidation of the sheet surface. For this purpose, the protective gas atmosphere in a conventional manner contains more than 3.5 vol .-% H ₂ and in each case as the radical N ₂ . The annealing temperature is also in a known manner in the range of 700 - 900 ° C.

By deviating the strip entrance temperature of the steel substrate in the range of -10 ° C to +70 ° C from the temperature of the melt bath, it is also achieved that the melt bath maintains its optimum temperature evenly despite the entry of the steel substrate.

The melt bath itself preferably contains at best traces of iron, since according to the invention the Fe content of the corrosion-coating layer is to be established by alloying in iron originating from the steel substrate. Accordingly, the Fe content of the melt bath is preferably limited to at most 0.1 wt%, more preferably at most 0.07 wt%.

The good processability, the simultaneously good corrosion protection and the good phosphatability are given regardless of the nature and condition of the steel substrate. Thus, practical experiments have shown that there are no significant differences in the properties of flat steel products produced according to the invention when the steel substrate is an IF steel, for example a conventional micro-alloyed steel, or a normal one alloyed steel, like a conventional quality steel.

Diag. 1

die bildliche Darstellung der durch eine GDOS-Messung ermittelten Verteilung der Gehalte an Zn, Mg, Al und Fe über die Dicke eines auf einem Stahlsubstrat aufgebrachten ersten Korrosionsschutzüberzugs;

Diag. 2

die bildliche Darstellung der Verteilung der durch eine GDOS-Messung ermittelten Gehalte an Zn, Mg, Al und Fe über die Dicke eines auf einem Stahlsubstrat aufgebrachten zweiten Korrosionsschutzüberzugs.

The invention will be explained below with reference to exemplary embodiments. Show it:

Diag. 1

the pictorial representation of the distribution of the contents of Zn, Mg, Al and Fe over the thickness of a first anticorrosive coating applied to a steel substrate, as determined by a GDOS measurement;

Diag. 2

the representation of the distribution of determined by a GDOS measurement levels of Zn, Mg, Al and Fe over the thickness of a deposited on a steel substrate second corrosion protection coating.

For the production of good spot-weldable and phosphatizable samples of high corrosion resistant steel flat products, a steel atmosphere steel strip under a nitrogen atmosphere containing 5% H ₂ and having a dew point of -30 ° C ± 2 ° C is annealed for a holding time of 60 s each Service. The annealing temperature was 800 ° C at a heating rate of 10 ° C / s.

After annealing, the steel strip was quenched at a cooling rate of 5 to 30 ° C / s to a temperature of 470 ° C ± 5 ° C where it was held for 30 seconds. Then the steel strip is with a belt immersion speed of 100 m / min was passed into a melt bath whose bath temperature was 460 ° C ± 5 ° C. The strip inlet temperature of the steel strip was in each case 5 ° C. above the bath temperature of the melt bath.

The respective composition of the melt bath and the analyzes of the hot dip galvanizing in the melt bath on the top and bottom of the anti-corrosion coating are summarized in Table 1 for twelve coated in the manner described above E1 - E12 - as far as determined. It turns out that the coatings formed on the steel substrate each have high levels of Fe. The alloying with Fe which occurs during the production of the coating ensures a particularly high adhesion of the coating to the steel substrate.

In addition, analysis of the distribution of contents of Zn, Al, Mg and Fe over the thickness of the anticorrosive coating formed on the steel substrate revealed that the Al content of the coating in a near-surface intermediate layer each exceeding 25% of the overlay thickness (total thickness ) of the coating is less than 0.2%. The corresponding distribution across the thickness D (surface D = 0 μ m) is shown pictorially for the samples E1 and E2 in the diagrams 1 and 2.

It can be seen in the diagrams that a surface boundary layer has formed on the surface of the respective coating whose Al content is high as a result of oxidation. However, the thickness of this surface boundary layer is at most 0.2 μ m and is therefore easily broken in the spot or laser welding, without causing a deterioration in the quality of the welding result.

The about 2.5 μ m thick intermediate layer to the surface boundary layer includes, whose Al content is below 0.2%, respectively. The thickness of the intermediate layer is therefore approximately 36% of the total overlay thickness of the respective anti-corrosion coating of 7 μm .

The intermediate layer merges into a boundary layer adjacent to the steel substrate, in which the contents of Al, Mg and Fe have increased significantly compared with the corresponding contents of the intermediate layer.

In order to check the dependence of the layer structure and the composition of a corrosion coating produced according to the invention from the respectively processed steel substrate and from the strip inlet or bath temperature, further samples E13-E22 are in laboratory experiment based on a conventional micro-alloyed steel IF and a likewise conventional quality steel QS produced with a corrosion protection coating. The composition of the steels IF and QS are given in Table 3.

The operating parameters set in the laboratory tests and an analysis of the correspondingly produced coating layer are summarized in Table 2. It was found that the result of the coating, in particular with regard to the incorporation of high Fe contents originating from the steel substrate, and the formation of the near-surface intermediate layer having Al contents below 0.25% by weight, irrespective of the composition of the steel substrate.

Overall, investigations carried out on samples E1-E22 have confirmed that in the case of a corrosion protection coating produced according to the invention in the surface boundary layer immediately adjacent to the surface of the coating, the elements Mg and Al are enriched in the form of oxide. In addition, Zn oxide is present on the surface.

In addition, operational tests B1 - B19 were carried out, in which existing steel strips were used as steel substrate made of quality steel QS. The operating parameters set, the respective Schmelzenbadzusammensetzung and an analysis of each obtained on the steel substrate corrosion protection layer are given in Table 4.

The operational tests have fully confirmed the result of the previous laboratory tests. The thickness of the superficial oxidation surface boundary layer is max. 0.2 μ m and is based on the determined in a GDOS measurement layer profile in each case in the range of up to 2.7% of the total overlay thicknesses. The amount of Al enrichment on the immediate surface is at most about 1 wt .-%. This is followed up to a thickness of at least 25% of the total overlay of the coating, the intermediate layer with a low Al content of not more than 0.25 wt .-% of. In the boundary layer, the Al content rises to 4.5% at the Border to the steel substrate. The Mg enrichment on the immediate surface of the coating is significantly greater than the Al enrichment. Mg contents of up to 20% are achieved here. Thereafter, the amount of Mg decreases over the intermediate layer and is 0.5 to 2% at a depth of about 25% of the total overlay thickness of the overlay. An increase of the Mg content in the direction of the steel substrate then takes place via the boundary layer. At the border to the steel substrate, the Mg content is up to 3.5%. Table 1 sample According to the invention? melt bath Layer analysis top Shift analysis bottom al Fe mg al Fe mg Editions weight Thickness al Fe mg Editions weight Thickness % *) % *) g / m ² μ m % *) g / m ² μ m E1 YES 0.201 0.011 1,589 1.16 1.06 1.52 41.5 7.0 ne ne ne n. e 9.0 E2 YES 0,205 0,090 2,024 1.18 1.07 1.90 40.5 7.0 ne ne ne n. e 8.5 E3 YES 0,189 0,021 0.733 0.47 0.37 0.75 75.9 10.6 ne ne ne n. e 7.7 E4 NO 0,189 0,021 0.733 0.66 0.58 0.75 50.0 6.7 1.61 1.69 0.77 17.6 2.1 E5 NO 0.202 0,013 0,790 1.38 1.37 0.76 20.7 4.0 ne ne ne n. e 2.9 E6 YES 0.209 ne 0.825 0.63 0.55 0.81 47.8 ne 0.71 0.61 0.82 43.5 ne E7 YES 0.218 ne 0.498 0.87 0.8 0.48 37.4 ne 1.22 1.25 0.48 24.4 ne E8 YES 0.218 ne 0.498 0.69 0.57 0.47 57.3 ne 1.19 1.11 0.48 30.1 ne E9 YES 0.231 ne 1,265 1.16 1.13 1.29 35.1 ne 1.96 2.15 1.29 20.0 ne E10 YES 0.231 ne 1,265 1.12 1.11 1.24 28.7 ne 1.35 1.42 1.24 21.4 ne E11 YES 0.196 ne 0,288 1.65 1.94 ne 27.3 ne 2.96 3.88 0.27 14.6 ne E12 YES 0,200 0.011 0.297 1.02 1.09 ne 43.2 ne 0.59 0.62 0.27 83.8 ne *) Balance Zn and unavoidable impurities; ne = not determined sample stole Glühtemp. Strip inlet temperature bath temperature bearing weight al Fe mg al Fe mg [° C] [° C] [° C] [g / m ² ] [%] [g / m ² ] E13 IF 800 445 440 51.6 0.52 0.36 1.21 0.27 0.19 0.62 E14 QS 800 445 440 55.9 0.56 0.40 1.16 0.31 0.22 0.65 E15 IF 800 465 460 64.3 0.81 0.75 1.15 0.52 0.48 0.74 E16 QS 750 465 460 54.1 0.98 0.84 1.21 0.53 0.45 0.65 E17 IF 800 485 460 49.4 1.08 0.97 1.18 0.53 0.48 0.58 E18 QS 750 485 460 55.1 0.97 0.84 1.19 0.53 0.46 0.66 E19 IF 800 500 460 54.3 1.14 1.08 1.20 0.62 0.59 0.65 E20 QS 750 500 460 36.7 1.50 1.41 1.19 0.55 0.52 0.44 E21 IF 800 485 480 62.4 1.15 1.26 1.15 0.72 0.79 0.72 E22 QS 750 485 480 43.6 1.57 1.68 1.16 0.68 0.73 0.51 stole C Si Mn P S Ti al [Wt .-%] IF 0,003 0.02 0.13 0,010 0,012 0.07 0.03 QS 0.07 0.04 0.40 0,012 0.005 0.005 0.04 Remaining iron and unavoidable impurities attempt Inventions according to? Strip inlet temperature BET Bath temperature BT Difference BET-BT coating thickness bearing weight al Fe mg al Fe [° C] [.Mu.m] [G / m2] [% By weight] *) [G / m2] B1 NO 516 466 50 4.9 34.7 1.61 1.46 0.81 0.56 0.51 B2 YES 536 478 58 7.8 55.1 1.00 0.88 0.82 0.55 0.48 B3 YES 500 472 28 11.4 80.6 0, 65 0.51 0.82 0, 52 0.41 B4 YES 522 472 50 10.2 72.1 0, 94 0.82 0.81 0.68 0.59 B5 YES 493 467 26 5.7 40.2 0.66 0.47 0.81 0.27 0.19 B6 NO 457 456 1 11.2 79.2 0.43 0.20 0.81 0.34 0.15 B7 NO 483 464 19 4.8 34.4 0, 97 0.92 0.83 0, 33 0.32 B8 YES 509 466 43 9.2 65.5 0.72 0.61 0.81 0.47 0.40 B9 YES 509 466 43 9.5 67.7 0.84 0.74 0.81 0.57 0.50 B10 YES 506 471 35 7.0 49.6 1.14 1.05 0.81 0.56 0.52 B11 YES 506 471 35 5.2 37.1 1.13 1.05 0.81 0.42 0.39 B12 YES 521 457 64 5.5 39.1 1.32 1.22 0.81 0.51 0.48 B13 YES 521 457 64 8.1 57.6 1.01 0, 94 0.81 0.58 0.54 B14 YES 479 460 19 7.3 51.8 0.55 0.41 1.11 0.28 0.21 B15 YES 479 460 19 10.7 75.8 0.46 0.29 1.10 0.35 0.22 B16 NO 460 471 -11 4.3 30.7 0.66 0.56 1.11 0.20 0.17 B17 NO 460 471 -11 7.1 50.5 0.47 0.32 1.11 0.24 0.16 B18 YES 460 460 0 7.2 50.9 0.48 0.32 1.11 0.24 0.16 B19 NO 460 460 0 4, 6 32.6 0.79 0.65 1.11 0.26 0.21 medium medium 494 466 28 7.4 52.9 0.83 0, 42 0.70 0.35 0.91 Max Max 536 478 64 11.4 80.6 1.61 0.68 1.46 0.59 1.11 min min 457 456 -11 4.3 30.7 0.43 0.20 0.20 0.15 0.81 *) Balance Zn and unavoidable impurities;

Claims (4)

1. Method for the production of a flat steel product, in which on the steel substrate, such as a steel belt or a steel sheet, a corrosion protective coating with a total thickness of at least 5 µm is generated, while the steel substrate is annealed under a protective gas atmosphere containing more than 3.5 volume percent H₂ and as a residue N₂ at an annealing temperature of 700 - 900 °C and starting from the annealing temperature is cooled to a belt entry temperature, with which the steel substrate enters into a molten bath (in weight percent) 0.1 - 0.4 % Al, 0.25 - 2.5 % Mg. up to 0.2 % Fe, containing residue zinc as well as unavoidable impurities and heated to a bath temperature of 420 - 500 °C, wherein the difference "BET-BT" between the belt entry temperature "BET" and the bath temperature "BT" is -10 °C to + 70 °C and is varied such that a corrosion protective coating is formed on the steel substrate, which in (in weight percent)

   Mg: 0.25 - 2.5 %,

   Al: 0.2 - 3.0 %,

   Fe: more than 0.3 - 4.0%,

   as well as optionally in total up to 0.8% of one or several elements from the group "Pb, Bi, Cd, Ti, B, Si, Cu, Ni, Co, Cr, Mn, Sn as well as rare earths,"
   contains residue zinc and unavoidable impurities and which in an intermediate layer, which extends between a surface layer directly abutting the surface of the flat steel product and a boundary layer abutting the steel substrate and the thickness of which is at least 20 % of the total thickness of the corrosion protective layer, has an Al content of at most 0.5 weight percent.
2. Method according to Claim 1, characterised in that the bath temperature is 440 - 480 °C.
3. Method according to any one of the preceding claims, characterised in that the belt entry temperature is 410 - 510 °C.
4. Method according to any one of the preceding claims, characterised in that the Fe-content of the molten bath is ≤ 0.1 weight percent.

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