Temper-passed, hot-dipped steel sheet

Abstract

The invention relates to a hot-dipped, temper-passed steel sheet.

Description

[0001] Tempered, hot-dip coated steel sheet

[0002] The invention relates to a temper-rolled, hot-dip coated steel sheet.

[0003] In state-of-the-art skin-pass processes of hot-dip coated steel flat material, in which the shaped elements of the skin-pass roll(s) are generally stochastically / randomly distributed across the skin-pass roll surface, no discernible transfer of the shaped elements through the coating into the substrate layer can be observed, evenly distributed across the entire surface. Confocal reflected-light microscopy, known from the state of the art, can be used for assessment to provide evidence of the uniformity and / or quality of the embossing. For this purpose, samples are created, each of which is measured with a coated and an uncoated surface, for example, after chemical decoating.Based on the visual appearance of the surfaces, the stamping elements of the skin-passing roll can already be identified in the substrate surface, which can be interpreted as an indication of the uniformity and / or quality of the stamping. This fact becomes particularly clear through mathematical correlation analyses of the topography data of the coated and stripped surfaces. Due to the stochastic / random topography / texture, spatial agreement between the coated and stripped measurement points is essential. The associated correlation images of the coated and stripped surfaces show hardly any correlating sheet metal areas, so that even particularly regularly recurring correlating sheet metal areas are not recognizable.

[0004] The object of the invention is therefore to provide a tempered, hot-dip coated steel sheet with which the disadvantages of the prior art can be eliminated, in particular the uniformity and / or quality of the embossing can be increased.

[0005] The problem is solved with the features of claim 1.

[0006] The invention provides a temper-rolled, hot-dip coated steel sheet, wherein at least one of the surfaces of the temper-rolled, hot-dip coated steel sheet has deterministically arranged impressions, wherein a cross-correlation function r z>u measured height values ​​z, on the steel sheet in the hot-dip coated state and measured height values ​​Uj on the steel sheet in the decoated state are compared in different areas, whereby the resulting from the cross-correlation function r z>u resulting correlation coefficient r is at least 0.40.

[0007] Sz u

[0008] The correlation coefficient r results from the cross-correlation function r z>u with r z u = s

[0009] , where s z , u the covariance is and is determined by s z u = -^=1^1 — z) * (u t — u) with the mean values ​​z = -S”=iz L and it = -£” = 1 u t as well as the standard deviations and s u = .

[0010] The measuring or evaluation area should be at least 0.5 x 0.5 mm 2 From 1.5 x 1.5 mm 2 Shape and waviness can be eliminated through filtering. The determination of the correlation coefficient and the underlying formulas are state-of-the-art.

[0011] In contrast to the prior art, the embossing stamp elements of the skin-pass roll surface are evenly distributed both planar in the sheet plane and vertically, particularly in the layer structure, and can be located / detected, for example using confocal reflected light microscopy. Samples were created from coated steel sheets skin-passed according to the invention, and each of these samples was measured in different areas with a surface in a coated and an uncoated state after chemical decoating. The observation of the coated and decoated areas therefore do not overlap, which means that an examined sample is observed in a defined measuring area in the coated state in a first area and, after decoating, is observed in a defined measuring area in the coated state in a second area, which does not correspond to the first area, in the coated state. The size of the measuring area is preferably identical.In particular, this can indicate the uniformity and / or quality of the embossing. If the correlation coefficient is high, at least 0.40, it is concluded that there is an improvement over the prior art, thus indicating uniformity and / or quality of the embossing.

[0012] In particular, the correlation coefficient r is at least 0.450, preferably at least 0.480, more preferably at least 0.50, particularly 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 with high probability never applies in practice, so that the correlation coefficient can be up to 1, in particular up to 0.90, preferably up to 0.850. It is assumed that the higher the correlation coefficient, the better the uniformity and / or quality of the embossing.

[0013] Steel sheet is generally understood to be a flat steel product which can be provided in sheet form (sheet), in blank form (blank) or in strip form (steel strip).

[0014] The steel sheet is coated with a hot-dip coated coating. The skin-passing of a steel sheet with a hot-dip coated coating is thus carried out after coating in order to provide improved adhesion at the coating / substrate interface through the stamping elements designed according to the invention.

[0015] For example, the steel sheet is hot-dip coated with a zinc-based coating.

[0016] In particular, in addition to zinc and unavoidable impurities, the coating contains additional elements such as aluminum with a content of at least 0.1 up to 8.0 wt.% and / or magnesium with a content of at least 0.1 up to 8.0 wt.%. 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 required, the coating additionally contains magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt. Aluminum can be used alternatively or in addition to magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.-% may be present, for example, to improve the bonding of the coating to the steel sheet and, in particular, to substantially prevent the diffusion of iron from the steel sheet into the coating during heat treatment of the coated steel sheet, thus ensuring, for example, good adhesive properties. Unavoidable impurities and optional additional components are limited to a maximum of 2.0 wt.% in total.

[0017] The thickness of the coating can be between 1.5 and 15 pm, in particular between 2 and 12 pm, preferably between 3 and 10 pm.

[0018] According to an alternative variant, the steel sheet is hot-dip coated with an aluminum-based coating. The coating thickness can be between 1.5 and 15 μm, in particular between 2 and 12 μm, preferably between 3 and 10 μm. In particular, in addition to aluminum and unavoidable impurities, the coating contains additional elements such as silicon in a content of up to 15 wt.%, optionally iron in a content of up to 4 wt.%, optionally alkali or alkaline earth metals in a content of up to 1.0 wt.%, and optionally further components whose total contents are limited to a maximum of 2.0 wt.%.

[0019] In a preferred variant, the silicon content is either 0.2 to 4.5 wt.% or 7 to 13 wt.%, in particular 8 to 11 wt.%. In a preferred variant, the optional iron content comprises 0.2 to 4.5 wt.%, in particular 1 to 4 wt.%, preferably 1.5 to 3.5 wt.%. In a preferred variant, the optional content of alkali or alkaline earth metals comprises 0.01 to 1.0 wt.% magnesium, in particular 0.1 to 0.7 wt.% magnesium, preferably 0.1 to 0.5 wt.% magnesium. Furthermore, the optional content of alkali or alkaline earth metals can in particular comprise at least 0.0015 wt.% calcium.

[0020] For example, in an alternative variant, the aluminum-based coating contains, in addition to aluminum and unavoidable impurities, additional elements such as zinc 2 to 24 wt.%, silicon 1 to 7 wt.%, optionally magnesium 1 to 8 wt.% if the silicon content should be between 1 and 4 wt.%, optionally up to 0.3 wt.% in total Pb, Ni, Zr or Hf.

[0021] The skin-passing of a hot-dip coated steel sheet, wherein a steel substrate with a hot-dip coated coating is provided, takes place in a skin-pass mill and is passed between two skin-pass rolls. At least one of the skin-pass rolls has deterministically arranged punch elements on its surface, wherein each punch element on the surface of the skin-pass roll has an area A P between 12.4 and 32400 pm 2 can have.

[0022] The deterministically arranged stamp elements, which are usually produced by laser ablation on the skin-pass roll, form a “positive” shape which, during skin-passing, acts on the surface of a coated steel sheet and imprints a surface structure on the coated steel sheet as a “negative” shape. Complete imprinting / immersion of the stamp elements is practically impossible, so that a transfer of the shape of the geometry of the stamp elements into or onto the surface of the coated steel sheet is greater than 0%, in particular greater than 10%, preferably greater than 20%, and less than 100%, in particular less than 90%, preferably less than 85%. Methods and devices for producing laser-textured skin-pass rolls are state of the art, cf., among others, EP 2 892 663 B1. Deterministic surface topography / surface structure orDeterministically arranged stamp elements are to be understood as recurring surface structures which have a defined shape and / or design, see also EP 2 892 663 Bl .

[0023] Using a laser, deterministically arranged stamp elements are introduced into or onto the surface of the skin-passing roll by means of material removal. This can be positively influenced by specifically controlling the energy, the pulse duration and selecting a suitable wavelength of a laser beam acting on the surface of the skin-passing roll. With a high or longer pulse duration, the interaction time between the laser beam and the skin-passing roll surface increases and more material can be removed from the surface of the skin-passing roll. A pulse leaves behind an essentially circular crater on the skin-passing roll surface. Reducing the pulse duration influences the formation of a crater; in particular, the diameter of the crater can be reduced. The stamp elements are therefore the remaining raised areas on the surface of the skin-passing roll not detected by the laser.In particular, the firing pattern to be generated using the laser on the surface of the skin-pass roller can be computer-assisted.

[0024] The skin-pass roll is thus provided with a large number of raised stamping elements. The number n of stamping elements in a reference (partial) area A ref results in a number to area ratio Snr = n / A ref .

[0025] Each stamp element can have an area A P (n) between 12.4 and 32400 pm 2 The area A P is in particular at least 50 pm 2 , preferably at least 150 pm 2 , preferably at least 300 pm 2 , more preferably at least 500 pm 2 and in particular a maximum of 30000 pm 2 , preferably a maximum of 25000 pm 2 , preferably maximum 20000 pm 2 , more preferably maximum 15000 pm 2 .

[0026] The surface areas of the stamp elements are in a section plane c p measured in the d 2 Smr(c0) / dc 2 , the second derivative of the Firestone-Abbott curve Smr(c), has a maximum: d 2 Smr(c0) / dc 2 = (Smr(c0+ Ac) - 2 Smr(c0) + Smr(c0- Ac) ) / (Ac) 2 d 2 Smr(c P ) / dc 2 = Maximum c is the distance of the cutting plane, i.e., depth, below a reference plane, such as a height value. For noise reduction, it may be advantageous to smooth the function Smr(c) using a mean filter with a width of 1 pm. The discretization distance Ac can be considered smaller than 0.1 pm.

[0027] The average area A P , m of the stamp elements results from the material proportion Smr(c P ) divided by the number of stamp elements per unit area Snr(c P ) with

[0028] A P.m = Smr(cP ) / Snr(c P ),

[0029] For example, each stamp element on the surface of the skin-pass roll can have an area A P between 500 and 20000 pm 2 , especially between 750 and 15000 pm 2 have.

[0030] For example, each stamp element on the surface of the skin-pass roll can have an area A P between 1001 and 5000 pm 2 have.

[0031] For example, each stamp element can alternatively have an area A on the surface of the skin-pass roll. P between 5001 and 10000 pm 2 have.

[0032] For example, both skin-passing rolls are designed with stamping elements deterministically arranged on their surfaces, each stamping element having an area A on the surface of the skin-passing roll. P between 12.4 and 32400 pm 2 has.

[0033] The hot-dip coated flat steel sheet is skin-passed using the skin-pass roll(s) with a skin-pass degree of at least 0.1% and a maximum of 2.5%, with a skin-pass degree of at least 0.3%, preferably at least 0.5%, being particularly preferred. A skin-pass degree of more than 2.5% would negatively impact the mechanical properties. To optimize dimensional accuracy and surface quality, the skin-pass degree is in particular up to 2.0%, preferably up to 1.7%. The skin-pass process and the adjustment of the skin-pass degree are state of the art. In a practical test, three coated steel sheets (steel strips) with a zinc-based coating were skin-passed (EDT1, LT1, LT3). The thickness and composition of the steel strips as well as the thickness and composition of the zinc-based coating were the same for all three.Three additional coated steel sheets (steel strips) with an aluminum-based coating were also skin-passed (EDT2, LT2, LT4). The thickness and composition of the steel strips, as well as the thickness and composition of the aluminum-based coating, were the same for all three. Skin-passing was carried out on two steel strips using two differently stochastically structured EDT skin-passing roll pairs (EDT1, EDT2) in a skin-passing stand, and on the remaining four steel strips using laser-textured skin-passing roll pairs with differently deterministically arranged punch elements (LT1, LT2, LT3, LT4) in a skin-passing stand. The degree of skin-passing was the same in all tests. Further parameters are listed in Table 1.

[0034] Table 1

[0035] Ten samples were taken from each of the six tempered, coated steel strips and examined in more detail. The height values ​​of both the coated and the chemically stripped samples were measured using confocal reflected-light microscopy with a measuring area of ​​0.8 x 0.8 mm. 2 in different areas. Decoating was performed as follows: rinsing the samples with ethanol and drying; dezincification with inhibited hydrochloric acid (essentially complete removal of the coating); rinsing with ethanol and drying. The measured surfaces of the coated and decoated samples were subjected to a mathematical correlation analysis of the topography data (psoft analysis premium 6.2.6967), and the correlation coefficient r was determined; see Table 2.

[0036] Table 2

[0037] Figure 1 for the EDT1 version shows the result of the cross-correlation function in a measurement area measured using reflected light microscopy and compared with each other. The bright central point symbolizes a local main maximum, which only occurs when the height values ​​are congruent, i.e. the range of the measured height values ​​in the coated and uncoated states is the same. It is clearly visible that there is no uniformity and / or quality of the embossing in the entire representation. Thus, no recurring pattern can be recognized. Figure 2 for the LT1 version is different, with local maxima in the form of secondary maxima, which are particularly caused by shifting the observed areas, i.e.that the first area in the coated state does not correspond to the second area in the decoated state, which can be seen repeatedly in the representation, which suggests a uniformity and / or quality of the embossing.

Claims

Patent claims 1. A tempered, hot-dip coated steel sheet, wherein at least one of the surfaces of the tempered, hot-dip coated steel sheet has deterministically arranged indentations, characterized in that a cross-correlation function r z , u Measured height values ​​Zj on the steel sheet in the hot-dip coated state and measured height values ​​Uj on the steel sheet in the uncoated state are compared in different areas, whereby the cross-correlation function r is derived from the z>u The resulting correlation coefficient r is at least 0.

40.

2. Steel sheet according to claim 1, wherein the steel sheet is hot-dip coated with a zinc-based coating.

3. Steel sheet according to one of the preceding claims, wherein the coating may contain, in addition to zinc and unavoidable impurities, additional elements such as aluminium with a content of at least 0.1 to 8.0 wt.% and / or magnesium with a content of at least 0.1 to 8.0 wt.% in the coating.

4. Steel sheet according to claim 1, wherein the steel sheet is hot-dip coated with an aluminium-based coating.

5. Steel sheet according to claim 4, wherein the coating may contain, in addition to aluminium and unavoidable impurities, additional elements such as silicon with a content of up to 15 wt.%, optionally iron up to 4 wt.%, optionally alkali or alkaline earth metals up to 1.0 wt.% and optional further components, the total content of which is limited to a maximum of 2.0 wt.%.

6. Steel sheet according to claim 4, wherein the coating may contain, in addition to aluminium and unavoidable impurities, additional elements such as zinc 2 to 24 wt.%, silicon 1 to 7 wt.%, optionally magnesium 1 to 8 wt.% if the silicon content should be between 1 and 4 wt.%, optionally up to 0.3 wt.% in total Pb, Ni, Zr or Hf.

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