Method for dressing a hot dip coated steel strip and coated steel flat product with slipped areas

Abstract

The invention relates to a method for skin passing a hot-dip coated steel strip (1) with a Zn-Mg-Al coating, which comprises aluminum between 0.5 and 8.0 wt.%, magnesium between 0.5 and 8.0 wt.% and the remainder zinc and unavoidable impurities with an oxide layer formed on the surface of the coating, wherein the steel strip (1) is passed through a roll gap having two skin passing rolls (10) arranged one above the other at a strip speed (v), wherein the skin passing rolls (10) rotate at a roll circumferential speed (u), wherein either the strip speed (v) is set slower or faster in comparison to the roll circumferential speed (u).

Description

[0001] The invention relates to a method for skin-passing a flat steel product hot-dip coated with a Zn-Mg-Al coating and to a slip-passed flat steel product.

[0002] To ensure successful paint bonding to hot-dip coated steel sheets, chemical treatment and modification of the coating's surface is required. In the automotive industry, considerable effort is put into a phosphating process to ensure widespread growth of phosphate crystals on a typically hot-dip coated coating, thus achieving sufficient adhesion and a homogeneous appearance of the paint. Before crystal formation occurs, the surface of the hot-dip coated steel sheet / strip is "pickled" by the phosphoric acid present in the phosphating solution in order to at least partially remove / dissolve the non-reactive oxide layer on the surface of the coating that inevitably forms during the hot-dip coating process.Only after this reaction barrier (oxide layer) is removed can a successful conversion chemistry be developed, see for example DE 10 2019 204 224 and EP 2 474 649A1 A1.

[0003] To counteract this disadvantage, there is a need for a hot-dip coated steel strip which, in addition to excellent corrosion properties, also has improved post-treatment properties compared to the state of the art.

[0004] The object of the invention is to provide a method for skin-passing a steel strip coated with a Zn-Mg-Al coating (ZM coating), which can positively influence the surface of the coating. The aim is to comply with applicable health, safety, and environment (HSE) standards, and in particular, to avoid the use of previously unused toxic substances. Ideally, no additional substances or process steps should be used in the manufacturing process of the final product compared to the state of the art. This supports a safe and sustainable design of operational processes in a cost-effective manner.

[0005] Furthermore, it is an object of the invention to provide a hot-dip coated flat steel product with improved properties for post-treatment compared to the prior art without reduced corrosion properties.

[0006] The problem is solved by the features of patent claims 1 and 6. Further embodiments are described in the subordinate claims.

[0007] The invention relates to a method for skin passing a hot-dip coated steel strip with a Zn-Mg-Al coating, which has aluminum between 0.5 and 8.0 wt.%, magnesium between 0.5 and 8.0 wt.%, the remainder zinc and unavoidable impurities with an oxide layer formed on the surface of the coating, wherein the steel strip is passed through a roll gap having two superimposed skin passing rolls at a strip speed, wherein the roll gap is preferably set to a maximum of the thickness of the hot-dip coated steel strip to be skin passed and the skin passing rolls rotate at a roll peripheral speed, wherein either the strip speed in the roll gap is set slower or faster compared to the roll peripheral speed or the roll peripheral speed is set slower or faster compared to the strip speed in the roll gap.

[0008] In conventional skin-passing, care is taken to ensure that the peripheral speed (= rolling motion) of the skin-passing roll(s) is matched to the strip speed of the coated steel strip being skin-passed, ensuring essentially slippage-free skin-passing. The facing skin-passing rolls are adjusted so that there is always contact and thus a predefined skin-passing pressure on the steel strip being skin-passed, and the skin-passing rolls are always flush with the steel strip. If the skin-passing roll and the steel strip run at the same speed in the same direction, the skin-passing force of the skin-passing roll acts essentially perpendicularly on the steel strip, preventing slippage.

[0009] If slippage occurs unintentionally, the steel strip is pulled away from the exit side of the skin-pass mill with the skin-pass rolls faster than the skin-pass rolls are rotating, resulting in a pressure loss in the skin-pass mill, which is displayed in the system as an operating / process error. Alternatively or additionally, speed monitoring of the skin-pass rolls can be provided, which also displays an operating / process error in the system if the speed changes. If a pressure loss is identified or slippage occurs, the pressure between the skin-pass rolls is automatically increased to a predefined normal range. Excessive contact pressure leads to severe wear on the steel strip and the skin-pass rolls and is therefore undesirable. Excessive contact pressure, on the other hand, has a negative impact on the skin-passing result, and the mechanical characteristics of the steel strip cannot be adjusted according to the specifications.

[0010] With the slippage desired according to the invention, the rolling motion (= peripheral speed) of the skin-passing roll(s) is slowed by friction on the steel strip being skin-passed, while the steel strip being skin-passed continues to run at a (constant) strip speed. Thus, the rolling circumference of the skin-passing roll(s) is smaller than the distance of the passing steel strip, causing the skin-passing roll to slide over the hot-dip coated steel strip being skin-passed instead of rotating. The roll thus temporarily partially pushes the contact areas on the steel strip being skin-passed together instead of (merely) pressing them in.The skin-passing roll thus partially imprints the texture contained on the surface of the skin-passing roll as a negative in the form of a structure into the coating of the hot-dip coated steel strip to be skin-passed. The steel strip is meanwhile fed further, runs faster due to the desired slip, and thus the hot-dip coated steel strip to be skin-passed is pulled against the skin-passing roll surface. As a result, the skin-passing roll not only comes into contact with the embossed depressions (troughs) in the coating (skin-pass valley of the structure), but also with the raised areas (hills) of the coating (plateau of the structure), thus influencing these too. This effect with regard to the raised areas is undesirable in conventional skin-passing, but in combination with a Zn-Mg-Al coating it has a surprisingly positive effect, since in particular part of the (near-surface) oxide layer is removed during skin-passing.The remaining raised areas are hills in the shape of table mountains. Due to the slippage, at least some of the embossed depressions (depressions) have edges that are arranged essentially parallel to the strip travel direction, which is identical to the skin-pass direction.

[0011] This allows the belt speed to be set either slower or faster than the roller peripheral speed. The roller peripheral speed can also be set slower or faster than the belt speed.

[0012] If the strip speed is higher than the roll peripheral speed, an additional force is obtained in the strip running direction, which means that the skin pass force no longer acts completely perpendicularly on the steel strip, resulting in slippage.

[0013] If, however, the strip speed is lower than the peripheral speed of the roll, the contact surface between the steel strip and the skin-pass roll is shifted and the direction of the force is changed, which also leads to slippage.

[0014] Due to the "slip-like" skin-passing, the surface has improved properties with regard to possible post-treatments, such as activation and / or phosphating, coil coating, etc.

[0015] The steel strip can be a hot-rolled steel strip or a cold-rolled steel strip or can be made from a hot-rolled steel strip or a cold-rolled steel strip.

[0016] The thickness of the steel strip is, for example, 0.5 to 4.0 mm, in particular 0.6 to 3.0 mm, preferably 0.7 to 2.5 mm.

[0017] The zinc coating, applied using the hot-dip coating process, comprises a zinc alloy containing, in addition to zinc (balance) and unavoidable impurities, additional elements such as aluminum with a content between 0.1 and 8.0 wt.% and magnesium with a content between 0.1 and 8.0 wt.%. Impurities in the molten bath may include elements from the group consisting of Si, Sb, Bi, Zr, Ni, Cr, Pb, Ti, Ca, Mn, Sn, La, Ce, Fe, and Cr, individually or cumulatively in amounts of up to 0.5 wt.%, in particular up to 0.4 wt.%, preferably up to 0.5 wt.%. Elements from the group Si, Sb, Bi, Zr, Ni, Cr, Pb, Ti, Ca, Mn, Sn, La, Ce, Fe, and Cr may be present as impurities in the coating, either individually or cumulatively, in amounts of up to 0.5 wt.%, in particular up to 0.4 wt.%, preferably up to 0.5 wt.%. Alternatively, the concentration of Fe may be higher due to the diffusion described above. The remainder is zinc.Steel sheets cut from steel strips or steel sheet components produced therefrom with a zinc-based corrosion protection coating have very good cathodic corrosion protection and have been used in automotive engineering for years. If improved corrosion protection is required, the coating has a magnesium content of at least 0.8 wt.%, in particular at least 1.0 wt.%, preferably at least 1.1 wt.% and an aluminum content of at least 0.8 wt.%, in particular at least 1.0 wt.%. The coating has a magnesium content of at most 8.0 wt.%, preferably at most 7 wt.%, particularly preferably 5.0 wt.%, in particular at most 4.0 wt.% and an aluminum content of at most 8.0 wt.%, preferably at most 7 wt.%, particularly preferably 5.0 wt.%, in particular at most 4.0 wt.%.

[0018] In particular, to set a predetermined thickness of the cement coating, which in the solid state can be between 1.0 µm and 60.0 µm per side, the melt applied to the steel strip while still in its liquid state is stripped off. After leaving the molten bath, the steel strip coated with the liquid melt is passed through a stripping device which has means, for example nozzles, in particular slot nozzles, which act on both sides of the steel strip with a gaseous stripping medium to strip off the liquid melt. This makes it possible to achieve an asymmetric coating, i.e. different coating layers on both sides. The thickness of the cement coating can be set, in particular, independently of one another, for each side, between at least 4.0 µm, preferably at least 5.0 µm and a maximum of 58.0 µm, preferably between 5.0 and a maximum of 55.0 µm.

[0019] According to an alternative embodiment, the coating has a thickness between 26 and 75 µm, in particular between 28 and 70 µm, preferably between 30 and 65 µm. Such thicknesses can be used in industrial construction, for example, in the field of solar stud frames, and provide sufficient corrosion protection.

[0020] In a particular embodiment, the thickness of the coating is at least 1.0 µm, preferably at least 2.0 µm, particularly preferably at least 3.0 µm, in particular at least 5.0 µm and at most 25.0 µm, preferably at most 20.0 µm and particularly preferably at most 15.0 µm, in particular at most 10.0 µm, independently of one another, per side.

[0021] Below the minimum limits, sufficient cathodic corrosion protection cannot be guaranteed and above the maximum limit, joining problems may occur when connecting the steel sheet according to the invention or a component made from it to another component.

[0022] Depending on the requirements and intended use, the composition of the coating can vary. If improved corrosion protection is required, the coating contains aluminum and magnesium, each in particular at least 1.0, preferably at least 1.2 wt.%, in order to provide improved cathodic protection. Preferably, aluminum and magnesium in the coating are limited to a maximum of 5.0 wt.% each, preferably a maximum of 4.5 wt.%.

[0023] Further advantageous embodiments and developments will become apparent from the following description. One or more features from the claims, the description, and the drawings can be combined with one or more other features therein to form further embodiments of the invention. One or more features from the independent claims can also be combined with one or more other features.

[0024] According to one embodiment, the strip speed is at least 2% higher or lower than the roll peripheral speed. The strip speed can be at least 3, 4, 5%, preferably at least 6, 7, 8%, preferably at least 9, 10, 11, 12, 13, 14, 15% higher or lower than the roll peripheral speed. The difference can be limited to a maximum of 50%, since effective skin-passing is no longer possible if the speed exceeds or falls below this limit.

[0025] According to one embodiment, a skin-pass ratio is set between 0.2 and 3.0%. The skin-pass ratio corresponds to the difference in the thickness of the hot-dip coated steel strip before and after the roll gap between the skin-pass rolls divided by the thickness of the skin-passed hot-dip coated steel strip after the roll gap. The skin-pass ratio can be set in particular between 0.5 and 2.5%, preferably between 0.8 and 2.0%.

[0026] According to one embodiment, at least one or preferably both skin-pass rolls are each provided with a stochastic surface texture. A stochastic surface texture is known to be introduced into the surface of the skin-pass roll using an EDT process. A stochastic surface texture exhibits a random configuration of peaks and valleys (texture).

[0027] According to an alternative embodiment, at least one or preferably both skin-pass rolls are each provided with a deterministic surface texture. A deterministic surface texture is known to be introduced into the surface of the skin-pass roll by means of a laser. A deterministic surface texture has regularly recurring peaks and / or valleys with a particularly defined shape and / or configuration or dimension. In particular, this also includes surface textures with a (quasi-)stochastic appearance, which are composed of stochastic form elements with a recurring texture.

[0028] The invention further relates to a Zn-Mg-Al hot-dip coated, skin-rolled flat steel product with a surface structure in some areas which has hills and depressions, wherein the base area of ​​at least one depression has an edge with a length of at least 10 µm parallel, with a deviation of + / - 25°, to the strip running direction.

[0029] For the purposes of the invention, deviation is understood as a deviation of the edge from parallelism and / or from a straight line. In the case of a deviation from a straight line, the curved edge spans an arc of a maximum of 25°, 22°, or 20°, preferably 18° or 16°, particularly preferably 15°, in particular 14°, 15°, 12°, 11°, or 10°. The straight line through the two end points of the edge can also exhibit a deviation from parallelism of a maximum of 25°, 22°, 20°, 18°, or 16°, particularly preferably 15°, in particular 14°, 15°, 12°, 11°, or 10°.

[0030] Flat steel products are defined here as rolled products whose length and width are significantly greater than their thickness. These include, in particular, steel strips, steel sheets, and cut pieces obtained from them, such as blanks and the like, as well as formed components.

[0031] In an alternative, the flat steel product is a steel strip produced according to the method according to the invention described above. Preferably, the flat steel product is a steel strip with a slip-rolled finish and the features described above, or steel sheets, blanks, plates, and / or components formed therefrom.

[0032] In a preferred embodiment, the flat steel product is a steel strip as described above, provided with a stochastic surface texture, or a product made therefrom.

[0033] According to the invention, the surface structure, hills, depressions and edges are detected using a scanning electron microscope.

[0034] In a further embodiment, the flat steel product is characterized in that the sheet has at least a 0.025 mm² area with at least 5 edges, preferably at least 6 or 7 edges, particularly preferably at least 8 or 9 edges, in particular at least 10 or more edges, a maximum of 200 edges, preferably a maximum of 100 edges, particularly preferably 80 edges, in particular a maximum of 50 edges with a length of at least 10 µm parallel to the strip travel direction, with a deviation of + / - 25°. For deviation, see above.

[0035] One embodiment relates to the flat steel product described above, characterized in that the sheet has at least one region with a width of 120 µm, preferably 115 µm, particularly preferably 110 µm, in particular 100 µm perpendicular to the strip running direction over at least 80%; preferably at least 85%, particularly preferably at least 90%, in particular at least 95% and a maximum of 100% of the total sheet width with edges with a length of at least 10 µm parallel, with a deviation of + / - 25°, to the strip running direction in an average distribution (density) of at least one edge, preferably at least 1.2 or 1.4 edges, particularly preferably at least 1.6 or 1.8 edges, in particular at least 2 or more edges, a maximum of 40 edges, preferably a maximum of 20 edges, particularly preferably 15 edges, in particular a maximum of 10 edges per 0.005 mm 2<.

[0036] This means, for example, that a flat steel product with a width of one meter has a region perpendicular to the strip travel direction of 120 µm (0.12 mm) wide and 80 cm (800 mm) long (80% of the strip width, length perpendicular to the strip travel direction), i.e., with an area of ​​96 mm², in which a total of at least 192,000 of the edges described above are arranged. This results in an average distribution (density) of at least one edge (96 mm² ÷ 0.005 mm²) over an area of ​​0.005 mm².

[0037] Preferably it is a contiguous area.

[0038] In an alternative, a flat steel product according to the invention has several, repeated regions as described above with the average edge distribution.

[0039] In a further alternative, the flat steel product is characterized by having material accumulations arranged before or after depressions with edges at least 10 µm long, parallel to the strip travel direction, with a deviation of + / - 25°. These accumulations are preferably made of ZM coating, i.e., hills without peaks visible at 1000x magnification, i.e., hills in the shape of table mountains.

[0040] In the following, specific embodiments of the invention are explained in more detail with reference to the drawings. The drawings and the accompanying description of the resulting features are not to be interpreted as limiting the respective embodiments, but serve to illustrate exemplary embodiments. Furthermore, the respective features can be used with each other and with features of the above description for possible further developments and improvements of the invention, especially in additional embodiments not shown.

[0041] The drawing shows Figure 1) a schematic representation of a hot-dip coated steel strip passed between two skin-pass rolls having a roll gap; Figure 2) two SEM images of a partial area of ​​a surface of a conventionally skin-finished steel strip and a steel strip skin-finished according to the invention; Figure 3) a)identical to Figure 2 right, A is the section that is in Figure 3b is shown; b) Surface structure (2) with hills (2.1, 2.11), depressions (2.2) and edges (2.3), the latter with a length of at least 10 µm parallel, with a deviation of + / - 25° to the strip running direction.

[0042] The Figure 1 The schematic principle of skin-passing a hot-dip coated steel strip (1) shown is state of the art. The steel strip (1) is passed through a roll gap comprising two superimposed skin-pass rolls (10) at a strip speed (v). The roll gap is set to a maximum thickness of the hot-dip coated steel strip (1) to be skin-passed, and the skin-pass rolls (10) rotate at a roll peripheral speed (u).

[0043] Recently, increasingly corrosion-resistant zinc-based coatings, comprising aluminum between 0.5 and 8.0 wt.%, magnesium between 0.5 and 8.0 wt.%, the remainder zinc and unavoidable impurities with an oxide layer formed on the surface of the coating, have been put into practice, so-called Zn-Mg-Al coatings, on which the focus is in this invention.

[0044] The Figure 2shows on the left an SEM image of a partial area of ​​a surface of a conventionally skin-passed steel strip and on the right an SEM image of a steel strip skin-passed according to the invention, in which a Zn-Mg-Al coating was used. In the steel strip skin-passed according to the invention, the strip speed (v) was set faster than the roll circumferential speed (u), so that slippage occurred between the skin-pass roll (10) and the steel strip (1). This explains the artifacts clearly visible on the right in comparison to the image on the left, which correspond to local partial removal of the oxide layer on the Zn-Mg-Al coating. This can provide improved properties with regard to possible post-treatments through partial removal of the oxide layer.

[0045] The Figure 3a ) serves only to mark section A, which is Figure 3b ) is shown. In Figure 3b) the surface structure (2) with the depression (2.2) between the hills (2.1) and (2.11) can be seen. The depression (2.2) has two edges (2.3) that run essentially parallel to the belt travel direction, indicated by the arrow on the right. Hill (2.11) exhibits the characteristic table mountain shape caused by the slip.

Claims

1. A method for temper-passing a hot-dip coated steel strip (1) with a Zn-Mg-Al coating, which comprises aluminum between 0.5 and 8.0 wt.%, magnesium between 0.5 and 8.0 wt.%, the remainder zinc and unavoidable impurities with an oxide layer formed on the surface of the coating, wherein the steel strip (1) is passed through a roll gap having two superimposed temper rolls (10) at a strip speed (v), wherein the temper rolls (10) rotate at a roll peripheral speed (u), characterized in that either the belt speed (v) is set slower or faster compared to the roller peripheral speed (u) or the roller peripheral speed (u) is set slower or faster compared to the belt speed (v).

2. Method according to claim 1, wherein the belt speed (v) is at least 2% higher or lower compared to the roller peripheral speed (u).

3. Method according to one of the preceding claims, wherein a tempering degree of between 0.2 and 3.0% is set 4. Method according to one of the preceding claims, wherein at least one of the skin-pass rolls is provided with a stochastic surface texture.

5. Method according to one of claims 1 to 3, wherein at least one of the skin-pass rolls is provided with a deterministic surface texture.

6. A Zn-Mg-Al hot-dip coated, skin-passed flat steel product (1) having a surface structure (2) in some areas, which has hills (2.1, 2.11) and depressions (2.2), wherein the base area of ​​at least one depression has an edge (2.3) with a length of at least 10 µm parallel to, with a deviation of + / - 25°, the strip running direction.

7. Flat steel product according to claim 6, characterized in that the sheet at least an area of ​​0.025 mm 2with at least 5 edges with a length of at least 10 µm parallel, with a deviation of + / - 25°, to the strip running direction.

8. Flat steel product according to claim 6 or 7, characterized in that the sheet has at least one area with a width of 120 µm, perpendicular to the strip running direction over at least 80% of the sheet width with edges with a length of at least 10 µm parallel, with a deviation of + / - 25°, to the strip running direction in an average distribution of at least one edge to 0.005 mm 2 has.

9. Flat steel product according to one of claims 6 to 8 characterized in that before or after sinks with edges of at least 10 µm length, material accumulations are arranged parallel to the strip running direction with a deviation of + / - 25°.

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