Temper-Passed, Hot-Dipped Steel Sheet

The invention relates to a skin-pass-rolled, hot-dip coated steel sheet.

In skin-pass rolling operations on hot-dip coated flat steel material according to the prior art, in which the forming elements of the skin-pass roll(s) are generally distributed stochastically/randomly on the skin-pass roll surface, there is no observation of any apparent transference of the forming elements distributed uniformly over the entire surface through the coating and into the substrate layer. Confocal reflected light microscopy, which is known from the prior art, can be used for assessment in order to provide evidence as to the extent to which uniformity and/or quality of the embossment has taken place. For this purpose, samples are created, and these are each surveyed with a surface in the coated and uncoated state after chemical decoating, for example. From the visual appearance of the surfaces, it is already possible to identify through-embossing by the die elements of the skin-pass roll in the substrate surface, which can be regarded as an indication of uniformity and/or quality of the embossment. This relationship becomes particularly clear via mathematical correlation evaluations of the topography data of the coated and decoated surfaces. Because of the stochastic/random topography/texture, spatial correspondence of the coated and decoated measurement sites is absolutely necessary. The corresponding correlation images of the coated and decoated surfaces show barely any correlating sheet regions, and so even particularly regularly recurring correlating sheet regions are not apparent.

It is therefore an object of the invention to specify a skin-pass rolled, hot-dip coated steel sheet with which the draw backs from the prior art can be remedied, and in particular the uniformity and/or quality of the embossment can be enhanced.

This object is achieved by the features of claim.

The invention provides a skin-pass rolled, hot-dip coated steel sheet, where at least one of the surfaces of the skin-pass rolled, hot-dip coated steel sheet has embossments in a deterministic arrangement, wherein a cross-correlation function ris used to compare measured height values zon the steel sheet in the hot dip-coated state and measured height values uon the steel sheet in the decoated state in different regions, where the correlation coefficient r that results from the cross-correlation function ris at least 0.40.

The correlation coefficient r is found from the cross-correlation function rwith

where sis the covariance and is determined by

with the averages

and the standard deviations

The measurement or evaluation area should be at least 0.5×0.5 mm. Over and above 1.5×1.5 mm, shape and corrugations can be eliminated by filter measures. The ascertainment of the correlation coefficient and the underlying formulae are prior art.

By contrast with the prior art, the embossing die elements of the skin-pass roll surface are distributed uniformly both in a planar manner in the plane of the sheet and vertically, especially in the layer structure, and can be discovered/are detectable, for example, with the aid of confocal reflected light microscopy. Coated steel sheets that have been subjected to skin-pass rolling in accordance with the invention have been used to create samples that have each been surveyed with a surface in the coated and uncoated state after chemical decoating in different regions. There is thus no overlap between consideration of the coated and decoated region, meaning that a sample examined is viewed in the coated state in a defined measurement area in a first region, and is viewed in the coated state after decoating in a defined measurement area in a second region not corresponding to the first region. The size of the measurement area is preferably the same. In particular, this can indicate uniformity and/or quality of the embossment. If the correlation coefficient is high, at least 0.40, it is concluded that there is an improvement over the prior art, and hence uniformity and/or quality of the embossment.

In particular, the correlation coefficient r is at least 0.450, preferably at least 0.480, more preferably at least 0.50, especially preferably at least 0.520, further preferably at least 0.550, 0.560, 0.570, 0.580, 0.590, 0.60, 0.610, 0.620, 0.630, 0.640, 0.650, 0.660, 0.670, 0.680, 0.690, 0.70, 0.710, 0.720, 0.730, 0.740. A correlation coefficient of 1 would be theoretically possible, but is never likely to occur in practice, and so the correlation coefficient may be up to 1, especially up to 0.90, preferably up to 0.850. It is assumed that the quality and/or uniformity of embossment increases with the correlation coefficient.

The term “steel sheet” generally refers to a flat steel product which may be provided in sheet form (sheet) or else in plate form (plate) or in strip form (steel strip).

The steel sheet has been coated with a hot-dip coating. The skin-pass rolling of a steel sheet having a hot-dip coating thus follows the coating operation, in order to specify improved adhesion in the coating/substrate interface by virtue of the die elements embossed in accordance with the invention.

For example, the steel sheet has been hot-dip coated with a zinc-based coating.

In particular, the coating, in addition to zinc and unavoidable impurities, comprises additional elements such as aluminum with a content of at least 0.1% up to 8.0% by weight and/or magnesium with a content of at least 0.1% up to 8.0% by weight. Steel sheets with a zinc-based coating have very good cathodic corrosion protection and have been used in automotive construction for years. If improved corrosion protection is intended the coating additionally comprises magnesium in a content of at least 0.3% by weight, in particular of at least 0.6% by weight, preferably of at least 0.9% by weight. Aluminum may be present alternatively or additionally to magnesium with a content of at least 0.3% by weight, especially of at least 0.6% by weight, in order for example to improve binding of the coating to the steel sheet and in particular to essentially avoid diffusion of iron out of the steel sheet into the coating during a heat treatment of the coated steel sheet, in order that good suitability for adhesion, for example, can be assured. Unavoidable impurities and optional further constituents are limited to a total of not more than 2.0% by weight.

A thickness of the coating here may be between 1.5 and 15 μm, in particular between 2 and 12 μm, preferably between 3 and 10 μm.

In an alternative variant, the steel sheet has been hot-dip coated with an aluminum-based coating. A thickness of the coating here may be between 1.5 and 15 μm, in particular between 2 and 12 μm, preferably between 3 and 10 μm.

In particular, the coating comprises, in addition to aluminum and unavoidable impurities, additional elements such as silicon with a content of up to 15% by weight, optionally iron up to 4% by weight, optionally alkali metals or alkaline earth metals up to 1.0% by weight, and optional further constituents having a total content limited to not more than 2.0% by weight.

In a preferred variant, the silicon content is either 0.2% to 4.5% by weight or 7% to 13% by weight, especially 8% to 11% by weight. In a preferred variant, the optional iron content is 0.2% to 4.5% by weight, especially 1% to 4% by weight, preferably 1.5% to 3.5% by weight. In a preferred variant, the optional content of alkali metals or alkaline earth metals is 0.01% to 1.0% by weight of magnesium, especially 0.1% to 0.7% by weight of magnesium, preferably 0.1% to 0.5% by weight of magnesium. In addition, the optional content of alkali metals or alkaline earth metals may especially comprise at least 0.0015% by weight of calcium.

For example, the aluminum-based coating, in an alternative variant, in addition to aluminum and unavoidable impurities, comprises additional elements such as zinc 2% to 24% by weight, silicon 1% to 7% by weight, optionally magnesium 1% to 8% by weight when the silicon content should be between 1% and 4% by weight, optionally up to 0.3% by weight in total of Pb, Ni, Zr or Hf.

The skin-pass rolling of a hot-dip coated steel sheet, wherein a steel substrate having a hot-dip coating is provided, is effected in a skin-pass mill and is conducted between two skin-pass rolls. At least one of the skin-pass rolls has a deterministic arrangement of die elements on its surface, where each die element on the surface of the skin-pass roll can have an area Abetween 12.4 and 32 400 μm.

The deterministic arrangement of die elements produced on the skin-pass roll, generally by laser ablation, form a “positive” mold which acts on the surface of a coated steel sheet in the course of skin-pass rolling and embosses a surface structure on the coated steel sheet as “negative” mold. Complete embossment/immersion of the die elements is virtually impossible, and so transference of the form of the geometry of the die elements into or onto the surface of the coated steel sheet is greater than 0%, especially greater than 10%, preferably greater than 20% and less than 100%, especially less than 90%, preferably less than 85%.

Methods and apparatuses for production of laser-textured skin-pass rolls are prior art; cf. EP 2 892 663 B1 inter alia. A deterministic surface topography/surface structure or deterministic arrangement of die elements means recurring surface structures having a defined shape and/or configuration; cf. also EP 2 892 663 B1.

By laser, a deterministic arrangement of die elements is introduced into the surface or on the surface of the skin-pass roll by material removal in that a positive influence is possible via controlled actuation of the energy and pulse duration and choice of a suitable wavelength of a laser beam acting on the surface of the skin-pass roll. With a high or higher pulse duration, there is a rise in the interaction time between the laser beam and the surface of the skin-pass roll and it is possible to remove more material on the surface of the skin-pass roll. A pulse leaves an essentially circular crater on the skin-pass roll surface. A reduction in pulse duration influences crater formation; in particular, it is possible to reduce the diameter of the crater. The die elements are thus the remaining raised regions on the surface of the skin-pass roll that have not been affected by the laser. In particular, the bombardment pattern to be generated by means of the laser on the surface of the skin-pass roll may be computer-assisted.

The skin-pass roll is thus provided with a multitude of raised die elements. The number n of die elements in a (partial) reference area Agives a number-to-area ratio Snr=n/A.

Each die element may have an area A(n) between 12.4 and 32 400 μm. The area Ais in particular at least 50 μm, preferably at least 150 μm, more preferably at least 300 μm, further preferably at least 500 μm, and in particular at most 30 000 μm, preferably at most 25 000 μm, more preferably at most 20 000 μm, further preferably at most 15 000 μm.

The areas of the die elements are measured in a section plane cin which dSmr(c)/dc, the second derivative of the Firestone-Abbott curve Smr(c), has a maximum:

c is the distance of the section plane, i.e. depth, below a reference plane, for example a height value. For suppression of noise, it may be advantageous to smooth the Smr(c) function by means of an average filter with a width of 1 μm. The discretization margin Δc considered may be less than 0.1 μm.

The average area Aof the die elements is apparent from the proportion of material Smr(c) divided by the number of die elements per unit area Snr(c):

For example, each die element on the surface of the skin-pass roll may have an area Abetween 500 and 20 000 μm, especially between 750 and 15 000 μm.

For example, each die element on the surface of the skin-pass roll may have an area Abetween 1001 and 5000 μm.

For example, each die element on the surface of the skin-pass roll may alternatively have an area Abetween 5001 and 10 000 μm.

For example, each of the two skin-pass rolls is formed with a deterministic arrangement of die elements on its surface, where each die element on the surface of the skin-pass roll has an area Abetween 12.4 and 32 400 μm.

The hot-dip coated flat steel sheet is subjected to skin-pass rolling with the skin-pass roll(s) with a skin-pass reduction of at least 0.1% and at most 2.5%, more preferably with establishment of a skin-pass reduction especially of at least 0.3%, preferably of at least 0.5%. In the case of a skin-pass reduction of more than 2.5%, the mechanical properties would be adversely affected. In order to optimize dimensional accuracy and surface characteristics, the skin-pass reduction is especially up to 2.0%, preferably up to 1.7%. The skin-pass rolling operation and the establishment of the skin-pass reduction are prior art.

In a practical test, three coated steel sheets (steel strips) having a zinc-based coating were subjected to skin-pass rolling (EDT1, LT1, LT3). Thickness and composition of the steel strips and thickness and composition of the zinc-based coating were the same for all three. Three further coated steel sheets (steel strips) having an aluminum-based coating were likewise subjected to skin-pass rolling (EDT2, LT2, LT4). Thickness and composition of the steel strips and thickness and composition of the aluminum-based coating were the same for all three. The skin-pass rolling was conducted on two steel strips with two differently stochastically structured EDT skin-pass roll pairs (EDT1, EDT2) in a skin-pass mill, and on the remaining four steel strips with laser-textured skin-pass roll pairs (LT1, LT2, LT3, LT4) having different deterministic arrangements of die elements in a skin-pass mill. The skin pass reduction was the same in all experiments. Further parameters are listed in table 1.

10 samples were taken from each of the six skin-pass rolled coated steel strips, and these were examined in detail. Both the height values of the coated samples and the height values of the chemically decoated samples were measured in different regions by confocal reflected light microscopy with a measurement area of 0.8×0.8 mm. Decoating was effected as follows: rinsing the samples with ethanol and drying: degalvanizing with inhibited hydrochloric acid (essentially complete removal of the coating); rinsing with ethanol and drying. The measured surface areas of the coated and decoated samples were subjected to a mathematical correlation evaluation of the topography data (μsoft analysis premium 6.2.6967) and the correlation coefficient r; see table 2.

In, for execution EDT1, the result of the cross-correlation function is surveyed in a measurement area by reflected light microscopy and mutually compared height values are shown in the coated and decoated state. The light-colored central point symbolizes a local main maximum again which only in the case of congruence, meaning that the area of the measured height values in the coated and decoated state is the same. It is readily apparent that, in the overall diagram, there is no uniformity and/or quality of embossment. Thus, no recurrent pattern is apparent. By contrast, in, for execution LT1, local maxima are in the form of secondary maxima, which especially by shifting of the areas considered, meaning that the first region in the coated state does not correspond to the second region considered in the decoated state, which are recurrently apparent in the diagram, which suggests uniformity and/or quality of the embossment.