WO2021165088A1

WO2021165088A1 - Method for producing a surface-finished steel sheet, and surface-finished steel sheet

Info

Publication number: WO2021165088A1
Application number: PCT/EP2021/053004
Authority: WO
Inventors: Fabian JUNGE; Jennifer Schulz; Burak William Cetinkaya
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2020-02-20
Filing date: 2021-02-09
Publication date: 2021-08-26
Also published as: EP4107297A1; DE102020202171A1

Abstract

The invention relates to a method for improving the wettability of a surface-finished steel sheet (10), in which a protective coating (2) based on Zn-Al-Mg is applied, wherein a native oxide layer comprising at least ZnO, Al₂O₃ and MgO is formed after the application of the protective coating (2) and the steel sheet comprising the native oxide layer is subjected to at least one further treatment, to modify the native oxide layer, wherein the at least one further treatment comprises at least one heat treatment at a temperature for a period in a non-reducing atmosphere, wherein the non-reducing atmosphere, the temperature and the duration of the heat treatment are harmonized with one another in such a manner that the native oxide layer post-oxidizes, wherein in particular the resulting post-oxidized oxide layer (3) arising is greater than the native oxide layer.

Description

METHOD FOR MANUFACTURING A SURFACE-FINISHED STEEL SHEET AND SURFACE-FINISHED STEEL SHEET

The invention relates to a method for improving the wettability on a surface-finished steel sheet, in which a protective coating based on Zn-Al-Mg is applied, with a native _{oxide layer comprising at least ZnO, Al 2} O ₃ and MgO forming after application of the protective coating and that the steel sheet having native oxide layer is subjected to at least one further treatment in order to change the native oxide layer. The invention also relates to a correspondingly produced, surface-finished steel sheet.

An addition of different alloying elements leads to the formation of an oxide layer on the surface-finished steel sheet with different chemical compositions and properties, especially when hot-dip coating or when hot-dip finishing of a steel sheet. Immediately after hot-dip processing, a temperature-related "native" oxide layer forms on the coating. Depending on the alloying elements, this oxide layer can have a negative effect on the further process steps in the automotive process, such as suitability for bonding, cleaning or phosphating, in the case of Zn-Al-Mg basic protective coatings. A Zn-Al-Mg base surface must be very easy to wet with aqueous media in order to be able to degrease, activate and phosphate it well. Due to the poorer wettability of Zn-Al-Mg base surfaces, it is known from the prior art that the steel sheet having a native oxide layer must be subjected to at least one further treatment in order to change the native oxide layer, in particular to cover the native oxide layer or at least partially to be removed, see for example WO 2015/004284 A1, WO 2013/160871 A1.

The measures for covering or removing the native oxide layer are associated in particular with a high use of chemicals. In relation to the prior art, there is therefore a need for optimization, without the need for the expensive use of chemicals, nevertheless, an improvement in wettability to be achieved.

The object of the invention is to provide a method for producing a surface-finished steel sheet, with which improved wettability can be achieved without the costly use of chemicals, and to provide a correspondingly surface-finished steel sheet. The object is achieved in relation to the method with the features of claim 1 and in relation to the surface-finished steel sheet with the features of claim 11.

The inventors have surprisingly found that a positive influence can be exerted and thus an improvement in wettability can be achieved if the at least one further treatment comprises at least one heat treatment at a temperature for a duration in a non-reducing atmosphere, the non-reducing atmosphere containing the The temperature and the duration of the heat treatment are coordinated with one another in such a way that the native oxide layer is post-oxidized, in particular the post-oxidized oxide layer formed therefrom being larger than the native oxide layer. The post-oxidized oxide layer is, for example, at least 20%, in particular by at least 50%, preferably by at least 80%, preferably by 150%, particularly preferably by 200% larger than the native oxide layer. The "targeted" post-oxidation of the native oxide layer on the Zn-Al-Mg base protective coating of the surface-refined steel sheet results in a change in the chemical composition on the surface, which has a positive effect on the chemical reactivity on the entire surface of the sheet . The areas close to the surface are oxidized, which manifests itself in the formation of rounded oxide structures, initially as bubble-like structures, then as spherical structures. This increases the surface area, which in turn leads to improved wettability and / or deformability.

Sheet steel is to be understood as meaning a flat steel product in the form of a strip or sheet metal / plate. It has a longitudinal extension (length), a transverse extension (width) and a vertical extension (thickness). The steel sheet can be a hot strip (hot-rolled steel strip) or cold strip (cold-rolled steel strip) or be made from a hot strip or from a cold strip.

The thickness of the steel sheet is, for example, 0.5 to 4.0 mm, in particular 0.6 to 3.0 mm, preferably 0.7 to 2.5 mm.

Further advantageous refinements and developments can be found in the description below. One or more features from the claims, the description as well as the drawing can be combined with one or more other features Embodiments of the invention are linked. One or more features from the independent claims can also be linked by one or more other features.

According to the invention, the heat treatment is carried out at a temperature between at least 400 ° C. and at most Acl. In order to effectively avoid a disadvantageous structural transformation / change in the steel sheet, the temperature is limited to a maximum of Acl, which can be determined depending on the chemical composition of the steel sheet or derived from ZTU / ZTA diagrams. The temperature is in particular limited to a maximum of 700.degree. C., preferably to a maximum of 650.degree. C., preferably to a maximum of 600.degree. C., particularly preferably to a maximum of 560.degree. The temperature is at least 400 ° C., in particular at least 420 ° C., preferably at least 440 ° C., in order to be able to achieve a sufficiently strong oxidation in the shortest possible time, and / or that the phases of the protective coating are predominantly in one are in a liquid state and can thus preferentially oxidize.

According to one embodiment of the method according to the invention, the heat treatment is carried out for a duration between at least 3 s and a maximum of 24 hours. Depending on the heat treatment to be carried out, whether continuous (in a continuous process) or discontinuous, the duration or the dwell time of the surface-refined steel sheet can vary. In the continuous process, where the surface-finished steel sheet is continuously passed through a continuous furnace, the duration according to the invention can be between 3 s and 60 s, in particular between 4 s and 30 s, preferably between 5 s and 15 s. If the heat treatment is discontinuous or stationary , for example in a hood furnace, according to the invention, the duration can alternatively be between 60 s and 24 h, in particular between 2 min and 12 h, preferably between 10 min and 2 h.

According to one embodiment of the method according to the invention, an 0 ₂ -containing atmosphere is used as the non-reducing atmosphere. In particular, _{O 2} -containing gases can support post-oxidation of the native oxide layer. Air, as a cost-neutral or free and freely available gas, can be used with particular preference.

According to the invention, the protective coating based on Zn-Al-Mg has the following chemical composition in% by weight:

Al: 0.1 to 5.0, Mg: 0.1 to 5.0, Balance Zn and unavoidable impurities. In addition to zinc and unavoidable impurities, the protective coating contains both aluminum with a content of at least 0.1% by weight up to 5.0% by weight and magnesium with a content of at least 0.1% by weight up to Contain 5.0% by weight. Steel sheets with a zinc-based protective coating have very good cathodic corrosion protection, which has been used in automotive engineering for years. If improved corrosion protection is provided, the protective coating preferably has aluminum and magnesium, each with at least 0.5% by weight. Due to the presence of the proportions of alloy elements Al and Mg with an affinity for oxygen in the zinc-based protective coating, zinc, aluminum and magnesium oxides are formed on the surface of the protective coating or near the surface. Oxide layers have a negative effect on the wetting behavior, with magnesium oxides exhibiting poorer wetting behavior compared to zinc and aluminum oxides. The “targeted” post-oxidation on the surface of the protective coating dissolves the metal compound and electrons are released because the metal is brought into an energetically unfavorable state, in particular to a correspondingly high energy level due to the temperature. 0 ₂ from the preferred 0 ₂ -containing atmosphere (gaseous or as water vapor) absorbs the electrons of the metal and can thus form a stable “valence electron shell”, which oxidizes the metal.

According to one embodiment of the method according to the invention, the protective coating has a thickness between 2 and 20 μm, in particular between 4 and 15 μm, preferably between 5 and 12 μm.

According to one embodiment of the method according to the invention, the native oxide layer has a thickness between 1 and 100 nm, in particular between 2 and 60 nm, preferably between 3 and 50 nm.

According to one embodiment of the method according to the invention, the surface-finished steel sheet is dressed. Passing can be carried out before or after the post-oxidation heat treatment. Skin-passaging is preferably carried out after heat treatment, since after skin-passaging the surface-refined steel sheet does not undergo any further heat treatment and therefore no longer adversely affects the properties set in or on the surface-refined steel sheet, such as mechanical parameters and / or surface roughness, that are set by skin-passaging will. The skin pass properties are specifically set, these from the chemical composition of the steel sheet and the skin pass, which is at least 0.2% and for example up to 5%, in particular up to up to 4%, preferably up to 3%, preferably up to 2.5%, particularly preferably up to 2%, be dependent. The skin pass degree expresses the ratio of the decrease in thickness (input thickness minus output thickness in the rolling / skin pass stand) of the rolled or passaged steel sheet to the input thickness, in particular taking the reduction in thickness into account. A deterministic surface structure can be introduced into the surface-refined steel sheet through the treatment. A deterministic surface structure is to be understood in particular as regularly recurring surface structures which have a defined shape and / or configuration or dimensioning. In particular, this also includes surface structures with a (guasi-) stochastic appearance, which are composed of stochastic form elements with a recurring structure. Alternatively, the introduction of a stochastic surface structure into the surface-finished steel sheet is also conceivable.

According to one embodiment of the method according to the invention, the surface-finished steel sheet is pickled. In particular, after the heat treatment, inorganic impurities on the surface of the protective coating can be removed with the aid of a liquid by pickling if necessary, whereby chemical dissolution and / or detachment, in particular of all oxidic layers, can be effected from the surface of the protective coating. Pickling is usually done by flooding, immersion or spraying. Electrolytic methods can also be used to speed up the process. The pickling liquids are usually dilute mineral acids, which can contain additives, for example to achieve a uniform pickling attack on the surface.

The surface-finished steel sheet can preferably be phosphated. As a result of the change in the surface and the improved wettability, improved phosphatability can thus also be achieved. In the case of zinc phosphating, the zinc ions can get better into the phosphating bath and form a conversion chemistry, so that the phosphate layer can be formed essentially homogeneously, which can meet the high requirements of the automobile manufacturer.

In particular, the surface-finished steel sheet can be better wetted with aqueous solutions (cleaner, activating solution, phosphating solution). The second teaching of the invention relates to a surface-finished steel sheet with a protective coating based on Zn-Al-Mg and an _{oxide layer comprising at least ZnO, Al 2} 0 ₃ and MgO, which has preferably been produced by the method according to the invention, the surface-finished steel sheet has a surface energy of at least 40 mN / m and a polar component of the surface energy of at least 20%.

Due to the native oxide layer changed to a post-oxidized oxide layer as a result of the heat treatment, the wettability can be improved, which can be determined according to a test method for determining the surface energy by means of static contact angle measurement according to DIN 55660-2: 2011-12. The measurements can be carried out, for example, with the static contact angle measuring device DSA 100 from Kruess.

Using the contact angle determined according to DIN 55660-2, in particular with various test liquids such as ethylene glycol, diiodomethane and / or water, the surface energy can be determined, preferably using the Young's equation, and the polar and disperse portion of the surface energy can be calculated. The contact angle for ethylene glycol, diiodomethane and water is particularly preferably determined.

The surface energy can in particular be at least 45 mN / m, preferably at least 48 mN / m, preferably at least 51 mN / m. The polar component of the surface energy can in particular be at least 30%, preferably at least 40%, preferably at least 50%.

The near-surface chemical composition is determined, for example, by means of X-ray photoelectron spectroscopy (XPS), the procedure for determining the individual chemical compositions being familiar from the prior art. The measurement can be carried out, for example, with the Phi Quantera II SXM Scanning XPS Microprobe from Physical Electronics GmbH. The element concentrations measured by means of the XPS can be taken from overview spectra, which can be recorded for example with a transmission energy of 280 eV in the course of at least 7 cycles and can relate, for example, to a measuring area of 100 x 100 pm ² .

According to the invention, the determined proportions of Zn, Mg and Al are normalized to 100% by weight in the oxide layer and the normalized Mg proportion in the oxide layer is at least 30% by weight. The relative concentration of zinc, magnesium and aluminum is determined by determining the absolute concentration of these elements and then normalizing to 100%. The sum of the concentration of zinc, magnesium and aluminum is set equal to 100 and the proportion of the respective element in this 100% is evaluated or weighted as a relative concentration, i.e. based on 100%. The relative concentration of an element (Al, Mg, Zn) therefore relates to the sum of the concentrations of the elements Mg, Zn and Al, in that this sum represents 100%. Since the absolute concentration of the elements Zn, Mg and Al can vary from protective coating to protective coating, the information for the general method to be used is given as a relative concentration and in percentage points in order to precisely define changes. For the purposes of the invention, the occurrence of the elements zinc, magnesium and aluminum is recorded regardless of the form in which they are present. It is therefore irrelevant whether these elements are present as neutral atoms or as ions, in a compound such as an alloy or intermetallic phases, or in a compound such as complexes, oxides, salts, hydroxides or the like. Thus, the terms “zinc”, “aluminum” and “magnesium” in the context of the invention can encompass not only the elements in pure form, but also oxidic and / or hydroxidic or any form of compounds that contain these elements.

Surprisingly, it was found that the heat treatment can bring about a (further) Magnesiu moxidanricherung with a normalized Mg content in the oxide layer or on the surface of the oxide layer, which despite the known poor wetting behavior of magnesium oxide still achieved an improvement in wettability can be. This is due to the fact that in the course of post-oxidation, wetting and reactivity in general improve by increasing the roughness.

The normalized Mg content in the oxide layer is in particular at least 33% by weight, preferably at least 36% by weight, preferably at least 40% by weight. The specification of the normalized portion corresponds in particular to the determined mean value, with fluctuations within the scope of measurement tolerances (standard deviation).

According to one embodiment, the steel sheet consists of a steel material with the following chemical composition in% by weight:

C up to 0.1, in particular between 0.0002 and 0.1,

Mn up to 2.0, in particular between 0.01 and 2.0, Si up to 0.3, in particular between 0.0002 and 0.3,

P up to 0.1, in particular up to 0.05,

S up to 0.1, in particular up to 0.05,

N up to 0.1, in particular up to 0.01, and optionally one or more alloy elements from the group (Al, Cr, Cu, Nb, Mo, Ti, V, Ni, B, Sn, Ca):

AI up to 0.2, in particular between 0.001 and 0.1,

Cr up to 1.0, in particular up to 0.8,

Cu up to 0.2, in particular up to 0.18,

Nb up to 0.1, in particular up to 0.05,

Mo up to 0.2, in particular up to 0.1,

Ti up to 0.2, in particular up to 0.15,

V up to 0.2, in particular up to 0.1,

Ni up to 0.2, in particular up to 0.18,

B up to 0.005, in particular up to 0.004,

Sn up to 0.1, in particular up to 0.05,

Ca up to 0.1, in particular up to 0.01,

Remainder Fe and unavoidable impurities.

In the following, specific embodiments of the invention are explained in more detail with reference to the drawing. The drawing and accompanying description of the resulting features are not to be read in a limiting manner to the respective configurations, but serve to illustrate exemplary configurations. Furthermore, the respective features can be used with one another as well as with features of the above description for possible further developments and improvements of the invention, especially in the case of additional configurations which are not shown.

The drawing shows in the single FIG. 1 a cross-sectional view in the form of a light microscope recording of a surface-finished steel sheet (10) produced according to the invention. The surface-finished steel sheet (10) comprises a steel sheet (1) with a protective coating (2) based on Zn-Al-Mg and an oxide layer (3) containing at least ZnO, Al2O3 and MgO. In addition to zinc and unavoidable impurities, the protective coating (2) contains Al between 0.1 and 5.0% by weight and Mg between 0.1 and 5.0% by weight. The thickness of the steel sheet (1) is, for example, 0.5 to 4.0 mm. After the protective coating (2) has been applied, which is preferably applied by means of hot-dip coating, a natural ve oxide layer on the protective coating (2). The surface-finished steel sheet with the native oxide layer is subjected to at least one further treatment in order to change the native oxide layer. The method according to the invention therefore provides that the at least one further treatment comprises at least one heat treatment at a temperature for a duration in a non-reducing atmosphere, the non-reducing atom sphere, the temperature and the duration of the heat treatment being coordinated with one another in such a way that that the native oxide layer is post-oxidized, in particular the post-oxidized oxide layer (3) resulting therefrom being larger than the native oxide layer. The heat treatment is carried out at a temperature between at least 400 ° C. and a maximum of Acl, for a duration between at least 3 s and a maximum of 24 h, in an O ₂ -containing atmosphere, preferably in air. After the heat treatment for post-oxidation, the surface-finished steel sheet (10) has a surface energy of at least 40 mN / m and a polar component of the surface energy of at least 20%. The proportions of Zn, Mg and Al are normalized to 100% in the oxide layer (3) and the normalized Mg proportion in the oxide layer (3) is at least 30%.

A steel strip (cold strip) of the quality DP500 was provided with a thickness of 0.7 mm and coated with a protective coating based on Zn-Al-Mg in a hot-dip coating system. The melt contained 1.6% by weight of Al, 1.1% by weight of Mg, the remainder being Zn and unavoidable impurities. The protective coating was set with a thickness of approx. 7 μm. A native oxide layer comprising at least ZnO, Al ₂ O _{3 and MgO formed on the protective coating.} A total of 37 samples were separated from the surface-refined steel strip or sheet steel, which were then fed into further processing steps. The individual processing steps and the results obtained from them are summarized in Table 1. Sample 0 reflects the reference sample, which was not subjected to a heat treatment, but was skinned after coating with a skin-pass degree of 0.8%. Samples 1 to 36 were each subjected to a heat treatment which was carried out at different temperatures and durations. In the case of samples 1 to 36, the heat treatment was also carried out in different ovens, with air being used as the atmosphere in the oven in all cases. Passed samples were passaged with a skin pass degree of 0.8%. In the case of samples that were pickled, the pickling took place before the heat treatment in an acidic immersion bath with phosphoric acid, 5 ml / l, for approx. 30 s and an immersion bath temperature of approx. 25 ° C.

Table 1 The heat treatment increases the thickness of the post-oxidized oxide layer and is at least 20% larger than the native oxide layer. The normalized Mg proportions also change in the oxide layer or on the surface and increase with increasing duration, in particular at the expense of or through a reduction in the normalized Al proportions. The near-surface chemical composition is determined by means of X-ray photoelectron spectroscopy (XPS). The Phi Quantera II SXM Scanning XPS Mini was used for the measurement. croprobe from Physical Electronics GmbH was used, whereby the element concentrations measured by means of the XPS were taken from overview spectra, which were recorded at a permeability of 280 eV in the course of at least 7 cycles and were based on a measuring area of 100xlOOpm ² .

Two further samples 0 were removed and similarly to the samples 1 to 3 of a Wärmebe action-fed, however, the first sample 1 ¹ only for a period min of 5 and the second sample 2 'for a period of 60 minutes under otherwise identical conditions as samples 1 to 3 were heat-treated. The contact angle according to DIN 55660-2 was determined from the six samples, three test liquids (ethylene glycol, diiodomethane and water) being used. The polar and disperse fractions as well as the surface energy were determined from this. The polar component is a measure of the wettability of aqueous solutions, such as those used in pre- and post-treatment. The higher the polar proportion, the better the wetting properties. The measurements were carried out with the static contact angle measuring device DSA 100 from Kruess. The results are summarized in Table 2.

Table 2

The surface energy essentially corresponds to the sum of the polar and disper sen components of the surface energy. If the polar portion corresponds to at least 20% of the surface energy, an improved wetting behavior can already result. With a corresponding surface energy, which is at least 40 mN / m, in particular at least 45 mN / m, preferably at least 48 mN / m, preferably at least 51 mN / m and where the polar portion of the surface energy is in particular at least 30%, preferably wise is at least 40%, preferably at least 50%, surface-refined sheet steel with improved wettability can be provided.

As far as technically possible, the features can all be combined with one another.

Claims

1. A method for improving the wettability on a surface-finished steel sheet (10), in which a protective coating (2) based on Zn-Al-Mg is applied, with a native oxide layer _{comprising at least ZnO, Al 2} O _{3 and MgO} after the protective coating (2) has been applied and the steel sheet facing the native oxide layer is subjected to at least one further treatment in order to change the native oxide layer, characterized in that the protective coating (2) based on Zn-Al-Mg has the following chemical Composition in wt .-% has: Al: 0.1 to 5.0, Mg: 0.1 to 5.0, remainder Zn and unavoidable impurities; and the at least one further treatment comprises at least one heat treatment at a temperature for a duration in a non-reducing atmosphere, the non-reducing atmosphere, the temperature and the duration of the heat treatment being coordinated in such a way that the native oxide layer is post-oxidized, the resulting oxide layer post-oxidized oxide layer (3) is larger than the native oxide layer, the heat treatment being carried out at a temperature between at least 400 ° C and a maximum of Acl, the heat treatment being carried out continuously in a continuous process for a duration between 3 s and 60 s or discontinuously between 60 s and is carried out for 24 hours.

2. The method according to claim 1, wherein the heat treatment is carried out at a temperature between at least 440 ° C and a maximum of 560 ° C.

3. The method according to any one of the preceding claims, wherein the heat treatment is carried out continuously in a continuous process for a duration between 5 s and 15 s or discontinuously between 10 min and 2 h.

4. The method according to any one of the preceding claims, wherein a 0 ₂ -containing atmosphere is used as the non-reducing atmosphere, in particular air.

5. The method according to claim 1, wherein the protective coating (2) based on Zn-Al-Mg has Al and Mg each with at least 0.5% by weight.

6. The method according to any one of the preceding claims, wherein the protective coating (2) has a thickness between 2 and 20 pm.

7. The method according to any one of the preceding claims, wherein the native oxide layer has a thickness between 1 and 100 nm.

8. The method according to any one of the preceding claims, wherein the post-oxidized oxide layer (3) is at least 20% larger than the native oxide layer.

9. The method according to any one of the preceding claims, wherein the oberflächenveredel te steel sheet (10) is dressed.

10. The method according to any one of the preceding claims, wherein the oberflächenveredel te steel sheet (10) is pickled.

11. Surface-refined steel sheet (10) with a protective coating (2) based on Zn-Al-Mg and an _{oxide layer (3) comprising at least ZnO, Al 2} 0 ₃ and MgO, characterized in that the surface-finished steel sheet (10) has a surface energy of at least 40 mN / m and a polar component of the surface energy of at least 20%, whereby in particular the surface energy and the polar component of the surface energy can be determined via the contact angle determined according to DIN 55660-2, the components of Zn, Mg and Al are normalized to 100% by weight in the oxide layer (3) and the normalized Mg content in the oxide layer (3) is at least 30% by weight.

12. Surface-refined steel sheet according to claim 11, wherein the steel sheet (1) consists of egg nem steel material with the following chemical composition in wt .-%:

C to 0.1,

Mn up to 2.0,

Si up to 0.3,

P to 0.1,

S to 0.1,

N to 0.1, and optionally one or more alloy elements from the group (Al, Cr, Cu, Nb, Mo, Ti, V, Ni, B, Sn, Ca):

AI up to 0.2,

Cr up to 1.0, Cu up to 0.2, Nb up to 0.1, Mo up to 0.2, Ti up to 0.2,

V to 0.2,

Ni up to 0.2,

B to 0.005, Sn to 0.1, Ca to 0.1,

Remainder Fe and unavoidable impurities.