Method of manufacturing a continuous hot dip coated steel strip and a hot dip coated steel sheet

Abstract

The invention relates to a method for manufacturing hot dip coated steel strip, in which the coating is carried out by guiding the strip through a molten metal bath, the molten metal bath comprising Al, the balance of the metal being Zn, unavoidable impurities and optionally one or more additional elements in total up to 0.3%, characterized in that the composition of the bath is controlled so as to have an aluminium content greater than 0.50%.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201880034348.9 (corresponding international application PCT / EP2018 / 063850), entitled "Method for Manufacturing Continuously Hot-Dip Coated Steel Strip and Hot-Dip Coated Steel Sheet", dated May 25, 2017. Technical Field

[0002] This invention relates to a method for manufacturing continuously hot-dip coated steel strips and hot-dip coated steel sheets. Background Technology

[0003] The method of manufacturing hot-dip coated steel strips and hot-dip coated steel sheets in the form of galvanized steel sheets is well known in the steel industry.

[0004] Continuous hot-dip coating processes for producing steel sheet products are widely used and utilized worldwide. Originally developed for galvanizing (zinc plating), hot-dip coating is now also used to apply other metals to steel sheets. This method has become a very complex operation.

[0005] Initially, this product was used in applications that did not require high-quality surface finish or high formability. However, in recent years, hot-dip coated steel sheets have been increasingly used in more demanding applications such as automotive hoods, fenders, and doors. These applications have higher requirements for formability and final surface quality. The produced hot-dip coated sheets range in thickness from 0.25 to 4.50 mm.

[0006] In continuous hot-dip coating, steel strip passes continuously through a molten metal bath at speeds up to 200 m / min. In the molten metal bath, the steel strip reacts with the molten metal, and the coating bonds to the strip surface. The strip passes through one or more immersion rollers and exits the bath vertically. Directly above the exit point, a set of air knives wipes away excess molten metal, thus controlling the coating thickness, which is typically expressed as the weight of the coating per unit area on the strip surface. After cooling, the strip is fed into the exit end, which usually includes a leveling machine (finishing machine). Air or nitrogen can be used as the wiping gas. Nitrogen is typically used to produce higher quality coated products.

[0007] For steel and zinc, it is important to form a proper bonding zone, which is usually achieved by adding a controlled amount of aluminum of about 0.15% to 0.20% (all percentages in this article are by weight%) to the bath and controlling the temperature of the steel sheet as it enters the bath and the temperature of the bonding bath.

[0008] The resulting coating is essentially a zinc coating containing, typically, between 0.20% and 0.30% aluminum. This aluminum content is higher than the aluminum content in the bath because aluminum has a greater affinity for iron than zinc. Upon entering the zinc bath, an aluminum-iron layer, a so-called inhibition layer, immediately forms, causing aluminum to accumulate at the steel-zinc interface.

[0009] As mentioned above, current applications of galvanized steel sheets, in addition to formability, also require meeting surface quality requirements. GB-A-2517622 discloses a conventional method for improving the surface quality of hot-dip galvanized steel substrates. The disclosure mentions that the outer surface of galvanized metal sheets has a so-called waviness, which previously could only be compensated for by applying thick layers of paint, at the cost of a so-called "orange peel" appearance, which is unacceptable for, for example, automotive body parts.

[0010] According to GB-A-2517622, ​​the bath composition is based on zinc and contains 0.1-0.5% by weight of aluminum, preferably 0.1-0.4%, and even more preferably 0.1-0.3%. Furthermore, if certain parameters are set according to requirements, conditions, and equations, it is possible to obtain a waviness Wa 0.8 of less than or equal to 0.55 μm, or in one embodiment less than or equal to 0.35 μm, after the coating has cured and before possible finishing. Summary of the Invention

[0011] One object of the present invention is to provide an improved hot-dip coating method.

[0012] Another object of the present invention is to provide an improved hot-dip coated steel.

[0013] These objectives are achieved according to the independent claims. Preferred embodiments are defined in the corresponding dependent claims. It should be noted that the features listed in the claims can be combined in any technically meaningful manner to describe other embodiments of the invention. The following description explains the features of the invention and may suggest other embodiments of the invention.

[0014] According to the present invention, the method is characterized in that the composition of the bath is controlled so that its aluminum content is greater than 0.50%.

[0015] Surprisingly, when using the method according to the invention, a more stable hot-dip coating operation than conventional hot-dip galvanizing is achieved on the one hand, and excellent hot-dip coated steel with superior properties can be obtained on the other hand, especially in terms of the final surface finish.

[0016] As described above, a small amount of aluminum is typically present in the zinc bath of a conventional continuous galvanizing production line to form a (primarily) iron-aluminum-based interface layer between the steel substrate and the molten zinc. This layer, referred to as the inner layer in this patent application, is also commonly known as the inhibition layer. When studying the formation of this layer and its dependence on aluminum, the inventors found that at conventional aluminum concentrations in the zinc bath, the aluminum content in the inhibition layer is highly dependent on the temperature at which the strip enters the bath (strip entry temperature, SET) and the aluminum concentration in the zinc bath. Conversely, at higher aluminum concentrations in the zinc bath, particularly at >0.50% Al, the aluminum content in the inhibition layer is surprisingly completely independent of the SET.

[0017] As the inventors further investigated the problem, they discovered that when the aluminum content was greater than 0.5%, especially about 0.55% or 0.6%, the aluminum content in the inhibition layer became not only independent of SET, but also largely independent of changes in the Al concentration in the zinc bath.

[0018] Therefore, it was found that, if operation is performed according to the present invention, rather than conventional galvanizing, a very stable thin inhibitory layer is formed without adverse effects on product quality, such as spot weldability, zinc adhesion, and wear behavior of hot-dip coated products. The inventors also recognized that in conventional galvanizing, the inhibitory layer is quite sensitive to the process settings of the galvanizing production line; for example, because the relatively low aluminum content in the zinc bath prevents the inhibitory layer from being completely sealed at all times, it will not completely prevent iron from dissolving (further) from the steel substrate into the zinc bath at all locations. Conversely, if the method according to the present invention is implemented, less iron will dissolve in the zinc, which in turn leads to less dross formation, less contamination, fewer surface defects, and fewer high points.

[0019] The inventors have discovered that due to the relatively low aluminum concentration (0.15-0.20%) in the bath during conventional hot-dip galvanizing, aluminum in the thin layer near the steel strip may be depleted. Sealing the barrier layer provided by the Fe2Al5 inhibitor layer takes time. Possibly for this reason, iron can still dissolve near the inhibitor crystals in conventional galvanizing, and the inhibitor layer becomes thicker. In conventional galvanizing baths with a low aluminum concentration of 0.15-0.20%, the iron dissolution rate, iron solubility, and the amount of metal dross particles formed are very high. These metal dross particles are partially removed by a top deslagging operation, another portion deposits on the bath equipment (e.g., rollers), and yet another portion becomes embedded in the zinc coating.

[0020] The deposition of metallic dross particles on bath fixtures causes bearing wear and degrades the surface quality of galvanized strip by causing repeated roller imprinting and poor control of strip vibration. Furthermore, metallic dross particles are also present in the zinc coating. These particles are sometimes visible on the strip surface because they cause abnormal wiping patterns (surface defects). Some metallic dross particles remain invisible and therefore cannot be recorded through surface inspection. These particles then become a problem because they can appear as tiny bright spots (so-called high points) in the stamping workshop and become a cause of scrap. Therefore, it is clear that the presence of metallic dross particles in the bath should be avoided as much as possible.

[0021] According to one aspect of the invention, a method for manufacturing hot-dip coated steel strip is provided, wherein the strip is coated by guiding it through a molten metal bath containing Al, with the balance being Zn, unavoidable impurities, and up to 0.3% of one or more additional elements.

[0022] The one or more additional elements may be selected from Si, Sb, Pb, Ti, Ca, Mn, Mg, Sn, La, Ce, Cr, Zr, or Bi. Small amounts of such elements may be added to the bath.

[0023] The composition of the bath can be controlled to ensure that the aluminum content is 0.55% or higher, or 0.60% or higher. As mentioned above, this is advantageous for stable operation and for the manufactured products.

[0024] The composition of the bath can be controlled to achieve an aluminum content of 0.9% or less, or 0.8% or less. As mentioned above, this is advantageous for production and the product.

[0025] To understand the final performance of the product developed in this invention, we need to consider what happens during curing. For this purpose, we consider the aluminum-zinc phase diagram. As can be seen from this phase diagram, if the percentage of aluminum is less than 1.1%, zinc segregates first during cooling after the liquid zinc coating leaves the bath, followed by aluminum and the remaining zinc. At higher aluminum contents, the ZnAl phase (the β phase in the phase diagram) may form, which is undesirable as it degrades surface quality. Even in non-equilibrium conditions, such as the typically extremely high cooling rates of the coating during wiping, the ZnAl phase is present when the Al percentage is less than 1.1%.

[0026] Depending on production line conditions, especially at low production line speeds and during later cooling, ZnAl phase can be generated at lower Al% contents due to potential diffusion. Therefore, it is preferable to keep the aluminum percentage at 0.9% or less, and even better at 0.8% or less, to ensure that a large amount of ZnAl phase is not generated.

[0027] Since iron may dissolve during the immersion of the strip in the bath, the Fe content in the bath can be less than 70 ppm, preferably less than 50 ppm, more preferably less than 30 ppm, and most preferably less than 20 ppm. The inventors have realized that iron dissolution during immersion of steel strip in a zinc bath can now be reduced, i.e., by having a higher aluminum content in the zinc bath. This avoids critical aluminum depletion near the strip surface, and the Fe2Al5 inhibition layer begins to act as a true barrier layer at the earliest possible moment, thereby reducing iron dissolution into the bath. Furthermore, it has been found, surprisingly, that the formed inhibition layer remains relatively thin, meaning its growth ceases, because any defects in the coating that would allow any iron dissolution are rapidly repaired.

[0028] All of this contradicts the misconception that a higher aluminum concentration in a zinc bath leads to more scum particles because adding aluminum would shift the equilibrium: 2Fe + 5Al <=> Fe2Al5 to the right. Instead, the inventors conceived that, in fact, adding more aluminum results in less iron dissolving from the steel strip into the liquid zinc, thus limiting the amount of metallic scum particles. Furthermore, this is a very stable situation: as long as the amount of dissolved aluminum remains "excessive," the iron content remains low and continues to be low.

[0029] The typical SET temperature in this process is 420℃-490℃, preferably 450℃-470℃.

[0030] The method may include the following steps: guiding the strip to the outlet of the bath by rollers below the level of the bath surface, and wiping the strip leaving the bath by spraying wiping gas onto the coated steel strip through at least one air knife via at least one discharge port, characterized in that parameters D, HS, and US are selected such that D ln(HS US)≤130, where D is the shortest distance (in mm) from the air knife outlet to the strip surface, HS is the height (in mm) of the air knife above the bath surface, and US is the distance (in mm) between the upper edge of the roller and the bath surface. Setting the parameters in this way achieves excellent hot-dip coating, particularly in terms of surface properties, especially the waviness of the hot-dip coated steel. It should be noted that the air knife typically has a generally rectangular outlet with a slit shape, with an average slit width of 0.8–1.3 mm.

[0031] D ln(HS The value of (US) can be 120 or less, preferably 110 or less, more preferably 100 or less, even more preferably 90 or less, even more preferably 80 or less, and most preferably 70 or less. This is a further measure, thereby adjusting the hot-dip coating to achieve excellent hot-dip coated steel.

[0032] The method may include the following steps: wiping the coated steel strip after it leaves the bath by spraying wiping gas onto it through at least one air knife via at least one discharge port, wherein the hot-dip coating and wiping are performed in the following manner:

[0033] [(P-24)÷(5.95D ∧ 0.96 V ∧ 1.39)] ∧ -0.81 ≤ 150,

[0034] Where P is the wiping pressure in the air knife in millibars, D is the shortest distance in mm from the air knife outlet to the surface of the steel strip, and V is the strip speed in m / min.

[0035] On one hand, this method has the following characteristics:

[0036] [(Ρ-24)÷(5.95D ∧ 0.96 V ∧ 1.39)] ∧ -0.81 ≤ 100,

[0037] [(Ρ-24)÷(5.95D ∧ 0.96 V ∧ 1.39)] ∧ -0.81 ≤ 70,

[0038] The preferred option is [(P-24)÷(5.95D)]. ∧ 0.96 V ∧ 1.39)] ∧ -0.81 ≤ 60.

[0039] If these conditions are met, a thinner hot-dip coating can be applied while achieving excellent surface properties. Preferably, the coating thickness is less than 70 g / m². 2 The coating weight, and more preferably less than 60 g / m 2 The resulting hot-dip coated product requires less zinc and is lighter (e.g., up to about 1%). Furthermore, the lower coating weight leads to better solderability and waviness.

[0040] D can be equal to or less than 9 mm, preferably equal to or less than 8 mm, and more preferably equal to or less than 7 mm. A smaller D value can improve the hot-dip coating process and the manufactured product; however, for practical reasons, it is obvious that a certain minimum distance will always be required between the strip and the wiping device. It has been found that the smaller this distance, the lower the pressure in the manifold of the wiping device, which is thought to reduce interference with the "wet" coated surface, which in turn is better for the resulting (lower) waviness.

[0041] The HS value can be 550 mm or less, preferably 400 mm or less, more preferably 300 mm or less, more preferably 250 mm or less, and even more preferably 200 mm or less. It has been found that lower HS values ​​produce better results for the waviness of hot-dip coated products. A minimum of 115 mm can be mentioned; below this value, complex and undesirable flow patterns may occur within and near the bath surface.

[0042] Prior to hot-dip coating, the steel strip can be cold-rolled in a cold rolling mill with a pair of work rolls having a predetermined surface roughness Ra. This method is characterized by Ra being 4.5 μm or less, preferably 1.5 μm or less, and more preferably 0.6 μm or less. This surface roughness, and indeed the surface of the strip, plays a role in the quality of the final product prior to hot-dip coating. This is necessarily related to the sagging behavior of the molten coating material as it travels upwards after passing through a wiping device.

[0043] According to another aspect, a hot-dip coated steel is proposed, comprising a steel substrate and a Zn-Al coating layer, the Zn-Al coating layer comprising an inner layer immediately adjacent to the steel substrate and an outer layer extending between the surface layer of the coated steel and the inner layer, characterized in that the aluminum content in the outer layer is in the range of 0.4-1.0%, and the aluminum content of the entire Zn-Al coating layer is 0.71-0.95%. Such a product possesses all the advantages mentioned above and below, and is a product of surprisingly superior quality to conventional hot-dip galvanized steel, especially in automotive applications. As described in the introduction, the inner layer corresponds to an inhibitory layer formed upon initial contact between the steel and the molten metal, and the outer layer extends between the surface layer and the inner layer of the coated steel, as further explained below with reference to the accompanying drawings.

[0044] It should be noted that the features described in conjunction with the proposed method for manufacturing hot-dip coated steel strip can be used to further explain the features of the proposed hot-dip coated steel, and vice versa.

[0045] The proposed hot-dip coated steel can be manufactured by the proposed manufacturing method and / or is a product of the proposed manufacturing method, and is particularly suitable for automotive applications and especially for full-finish (exposed) applications, such as automotive body panels with excellent paint appearance.

[0046] In one respect, this hot-dip coated steel has the following characteristics: the coating contains Al, with the balance being Zn, unavoidable impurities, and optional <0.3% of one or more additional elements. Although the coating may contain other elements, this product has a zinc-based coating containing some aluminum, with an aluminum content significantly higher than that of conventional hot-dip galvanized steel.

[0047] The one or more additional elements may be selected from Si, Sb, Pb, Ti, Ca, Mn, Mg, Sn, La, Ce, Cr, Zr, or Bi. This defines the additional elements in more detail.

[0048] The Fe content in the outer layer can be less than 70 ppm, preferably less than 50 ppm, more preferably less than 30 ppm, and most preferably less than 20 ppm. The lower the Fe content, the fewer fine dross particles are incorporated into the zinc alloy coating layer, which can affect the appearance of the coated steel. This is particularly important for steel used in the pressing of external automotive parts. Inclusions of hard particles can cause uneven deformation, resulting in surface defects and irregularities, such as tiny protrusions and bumps, even when present on the unexposed side of the automotive part. Surface unevenness leads to undesirable reflections (“bright spots”), thus unacceptably affecting the appearance. This phenomenon is also referred to as a “high-spot” defect.

[0049] The aluminum content of the inner layer can be less than 150 mg / m 2 [mg / m²], and preferably less than 120 mg / m² 2 These values ​​indicate that the inner layer immediately adjacent to the steel substrate is thin, resulting in improved weldability. Additionally, a thinner inner layer is more stable. Finally, the coating is less prone to cracking. Regarding elsewhere and Figure 2 The experiment described herein should be noted that the aluminum content values ​​are slightly higher in real-world scenarios. This means less than 165 mg / m³. 2 The value is preferred.

[0050] The average thickness of the inner layer can be 500 nm or less, preferably 300 nm or less, more preferably 200 nm or less, and most preferably 100 nm or less.

[0051] The aluminum content of the entire coating can be in the range of 0.71-1.1%, which will minimize the presence of Fe2Al5 particles in the bath and in the coating.

[0052] The aluminum content of the entire coating can be 0.75% or higher. This will further reduce the particle size.

[0053] The aluminum content of the entire coating can be 0.90% or less, and can be 0.86% or less. This reduces the Zn-Al phase in the entire hot-dip coating.

[0054] All of these steels according to the present invention have been found to perform very well.

[0055] The waviness characteristic value Wsa of the outer surface of a 5% biaxially deformed cup-shaped article made of coated steel, measured in the rolling direction, can be 0.35 μm or less. This low waviness characteristic is important in automotive applications where aesthetics are required, such as on the outer side of body panels. The waviness Wsa, as described herein, is defined in the following standard: SEP 1941:2012, 'Measurement of the waviness characteristic value Wsa on cold-rolled flat metal products (1-5)'.

[0056] The Wsa of the outer surface of a 5% biaxially deformed cup-shaped article made of coated steel, measured in the rolling direction, can be 0.30 μm or less, preferably 0.25 μm or less. These are preferred values ​​for critical applications, such as automotive exterior panels. Attached Figure Description

[0057] The invention will now be further illustrated by the following non-limiting drawings and examples. The descriptions and features disclosed in or in conjunction with the drawings may be extracted individually and combined with any other features not expressly excluded herein.

[0058] In the diagram:

[0059] - Figure 1 The invention shows the inventors' findings regarding the variation of total Al content in the zinc layer with the aluminum content in the SET and zinc bath;

[0060] - Figure 2 This shows the inventors' discovery regarding the variation of Al content in the inhibition layer with the aluminum content in the SET and zinc bath;

[0061] - Figure 2A A cross-sectional schematic diagram of the structure of a hot-dip coated steel substrate is shown, revealing the identifiable layers;

[0062] - Figure 2B The Al content at different locations within different layers of the hot-dip coating and at the boundaries between the layers is shown.

[0063] - Figure 2C The Fe content at different locations within the hot-dip coating is shown.

[0064] - Figure 2D The Al weight in the inner layer of the hot-dip coating on one side of materials 1-3 according to the present invention and conventional materials 4-6 is shown;

[0065] - Figure 3 A hot-dip coating apparatus is shown;

[0066] - Figure 4 The variation of the total Al content in the hot-dip coating of a sample of hot-dip coated steel manufactured according to the present invention with coating weight (per side) is shown;

[0067] - Figure 5 The Fe content in the outer layer of the hot-dip coated layer of a hot-dip coated steel sample manufactured according to the present invention is shown.

[0068] - Figure 5A The Fe content in the outer layer of the hot-dip coating of the hot-dip coated steel material manufactured according to the present invention is shown;

[0069] - Figure 6 The test specimens that passed the most stringent OEM tests are shown for adhesion testing of hot-dip coatings to steel substrates;

[0070] - Figure 7 A cross-section of a steel sheet coated by hot-dip coating is shown, comprising a steel substrate (partial), an inner layer, and an outer layer;

[0071] - Figure 8 The wear performance of the hot-dip coated steel according to the present invention is shown compared with that of conventional hot-dip galvanized steel;

[0072] - Figure 9 This shows the development of waviness just after the air knife passes through until solidification;

[0073] - Figure 10 The final waviness of the coating before the leveling machine is schematically shown;

[0074] - Figure 11 The flowability of the zinc-aluminum alloy is shown, and

[0075] - Figure 12 The phase diagram of Zn and Al is shown. Detailed Implementation

[0076] Al in Zn

[0077] The inventors have conducted in-depth research on the correlation between the performance of hot-dip coated products and the variables involved in hot-dip coating. Figure 1 A set of results shows that, in the experimental setup, the total Al content in the overall hot-dip coated layer is displayed as a function of the SET (Al content in the zinc bath) and the aluminum content in the zinc bath. These results clearly indicate that when the Al content in the Zn bath is high, the total Al content in the resulting hot-dip coated steel coating becomes independent of the SET. Typical test SET temperatures are in the range of 300-600°C.

[0078] Al in the inner layer

[0079] The results, presented in another manner, indicate that the Al content in the inner layer is a function of SET and the aluminum content in the bath; see [reference needed]. Figure 2As shown, starting with an Al content of 0.50%, the inner layer is stable, indicating that it is completely sealed and Fe diffusion from the substrate into the bath no longer occurs. This, in turn, reduces scum, minimizes surface problems in the product, and reduces high points in the product.

[0080] Determination of Al and Fe in the outer and inner layers

[0081] exist Figure 2A The text describes how the surface layer SL, the outer layer OL (sometimes called the capping layer), and the inner layer IL (sometimes called the inhibition layer) together constitute the overall hot-dip zinc coating on the steel substrate StS. The surface layer can be distinguished from the outer layer by its different Al and Fe contents.

[0082] To determine the Al and Fe content in these layers, the following steps were performed:

[0083] Apply a masking tape (typically 60 × 120 mm) to the center of the material sample to be analyzed (typically 90 × 130 mm). Then apply two coats of protective paint to the top, cut edges, and bottom of the sample. After the paint dries, remove the masking tape to obtain a fully protected sample with only unprotected areas the same size as the masking tape.

[0084] Place the sample horizontally in a petri dish with the unprotected area facing up, and then pour hydrochloric acid solution over the unprotected area, ensuring that the unprotected area is completely covered by the acid solution, but not entirely immersed in the solution. This acid solution is prepared by immersing 270 ml of concentrated (37%) HCl in 2 liters of deionized water. An inhibitor is also added to prevent dissolution of the steel substrate.

[0085] After 30 seconds, remove the acid from the sample and rinse the sample with deionized water. Collect the acid solution and water in a flask.

[0086] Repeat this step several times, collecting the acid solution and rinsing water in a separate flask each time, until the dissolution reaction has completely stopped, indicating that the Zn coating has been completely removed from the surface.

[0087] The Zn, Al, and Fe contents in each flask were determined using ICP-OES (Inductively Coupled Plasma Atomic Emission Spectroscopy). From the results, the weight of the Zn coating, the total Al, and the total Fe can be easily calculated, and the depth distributions of Al and Fe can be prepared, as shown below. Figure 2B and 2C As shown.

[0088] According to the following reference Figure 2BThe steps for determining the Al content in different layers SL, OL, and IL are as follows: The average Al content at locations between 40% and 60% of the total coating thickness is taken and designated as the Al content of the outer layer. Subsequently, the boundary between the surface layer and the outer layer is determined as follows: Starting from the 40% position and moving left, look for locations where the Al content deviates from the aforementioned average by more than 5%. This location is considered the boundary B between the surface layer and the outer layer. With the boundary B between the surface layer and the outer layer known, the amount of Al in the surface layer can then be calculated.

[0089] Assuming that the Al content in the remaining Zn layers, excluding the surface layer, is the same as that determined for the outer layer through the above steps, the Al content in the inner layer can now be calculated. Figure 2D Examples of conventional galvanized materials (materials 4, 5 and 6) and hot-dip coated materials according to the invention (materials 1, 2 and 3) are given.

[0090] By determining boundary B according to the steps described above, from... Figure 2C The measurements shown can be used to calculate the Fe content in the layer in a similar manner.

[0091] Hot dip coating parameters

[0092] Whenever the same reference numerals are used in the accompanying drawings, these reference numerals denote the same parts.

[0093] A typical process leading to the manufacturing steps of coated steel sheets is known, along with some sequential steps that transform the coated steel sheets of this invention into pressed and painted automotive body parts. After casting the steel billet, it is hot-rolled in a hot-roll mill to provide steel strip, processed in a pickling line, cold-rolled in a cold-rolling mill, annealed in a continuous annealing line, hot-dip coated on the annealed strip in a hot-dip coating line, leveled in a leveling mill (also known as finishing in a finishing mill), transported to the customer, pressed, and painted. Some of these operations can be omitted, while others can be added to this sequence of events.

[0094] Events relating to this invention, particularly those occurring during hot-dip coating, certainly play a crucial role in the final performance of the final product, although the surface quality of the steel arriving at the cold rolling mill also has a decisive influence. Therefore, it is explicitly stated that while the disclosure of this invention focuses on the hot-dip coating process, other process steps are not unimportant and should be maintained at their normal high-quality standards.

[0095] Generally, unless otherwise stated in a table or text, the hot-dip coating process according to the present invention is set as follows:

[0096] Aluminum content in the molten metal bath: approximately 0.6%

[0097] D (shortest distance from air knife discharge port to strip surface): 6-12 mm

[0098] HS (height of air knife above bath surface): 200-550 mm

[0099] US (distance between the upper edge of the roller and the bath surface): 110-160 mm

[0100] P (wiping pressure): 100-600 millibars

[0101] V (Speed): 60-160 mpm

[0102] Ra: 0.6-6.5μm

[0103] Now go to Figure 3 The image shows a portion of a hot-dip galvanizing apparatus 11 for applying a zinc-based coating to surfaces 12, 13 of a moving steel strip 14, including a tank 15 containing a liquid bath of a molten coating material 16 to be applied to the moving steel strip 14.

[0104] Figure 3 The diagram illustrates the application of a stabilizing roller 17 and a submerged roller 21 to guide a moving steel strip 14 through a bath, wherein the stabilizing roller 17 provides a predetermined depth US, which is represented in the figure as a "depth" below the surface level 18 of the liquid bath. This US is defined as the distance between the surface level 18 of the liquid bath and the upper edge of the stabilizing roller 17. It should be noted that multiple stabilizing rollers or no stabilizing rollers may also be used, in which case the US parameter relates to the roller that contacts the topmost part of the strip before it leaves the bath.

[0105] At least one air knife 19 is provided above the liquid bath, the air knife having an exhaust port 20 for spraying wiping gas onto a coating provided on the surfaces 12, 13 of the steel strip 14 along the path of the air knife 19. Figure 3 The figure shows the height HS of one or more air knives 19 above the liquid bath relative to their surface level 18, indicated as “height H” in the figure, and the distance D of the two air knives 19 to the surface of the coated steel strip 14.

[0106] Choose the values ​​of at least one air knife 19 relative to the passing steel strip 14, the distance D, the height HS, and US of at least one air knife 19 above the liquid bath 16, to satisfy Equation D. ln(HS US)≤130, where US represents the depth of the last roller 17 below the surface level 18 of the liquid bath in contact with the strip.

[0107] Table 1 shows the effect of HS and D on the waviness Wsa, which is determined by measuring the outer surface of a 5% biaxially deformed cup-shaped article made of coated steel strip 14 in the rolling direction.

[0108] The aluminum content in the baths used in the experiment, as shown in Table 1-3, was 0.6%.

[0109] Table 1

[0110]

[0111] It should be pointed out that D was discovered ln(HS) The lower the US value, the lower the Wsa value will be. Therefore, D is preferred. ln(HS) The value of US) is equal to or lower than 120, 110, 100, 90, 80, 70.

[0112] Table 2 shows the effect of the height HS of the air knife 19 above the liquid bath and the depth US of at least one stabilizing roller 17 below the surface level 18 of the liquid bath.

[0113] Table 2

[0114]

[0115] As can be seen from Tables 1 and 2, preferably, the distance D from at least one air knife 19 to the steel sheet 14 is 9 mm or less, preferably 8 mm or less, and more preferably 7 mm or less. Then, according to the invention, the US value and the height HS of at least one air knife above the liquid bath of the zinc-based coating are selected in order to achieve the best results with respect to the Wsa of the resulting hot-dip coated steel.

[0116] Alternatively, as can be seen from Tables 1 and 2, the height HS of at least one air knife 19 above the liquid bath of zinc-based coating 16 is 550 mm or less, preferably 400 mm or less, preferably 300 mm or less, more preferably 250 mm or less, even more preferably 200 mm or less, and the distance D is selected according to the invention.

[0117] Table 3 shows the repeatability of Wsa values ​​that can be obtained if the present invention is implemented.

[0118] Table 3

[0119]

[0120] In all the above cases in Table 3, D ln(HS The figure for the US is 89.80.

[0121] The method for coating steel sheets according to the present invention produces a batch of coated steel sheets with an average waviness value Wsa of 0.25 μm. It should be mentioned here that values ​​as low as 0.22 μm were even found in some samples. In all these cases, Wsa represents the waviness present on the outer surface of a 5% biaxially deformed cup-shaped article of hot-dip coated steel parts (manufactured) according to the present invention, measured in the rolling direction (RD).

[0122] exist Figure 4 The image shows the results of manufacturing hot-dip coated steel according to the present invention. Samples were manufactured using steel substrates such as DX52, DX54, and BH180, and process settings were selected according to the present invention to achieve different coating weights, ranging from 35-75 g / m². 2 (On each side) unequal. For example... Figure 4 As shown, the Al content in the overall hot-dip coating ranges from 0.71% to 0.86%. Figure 2B The diagram provides the Al content in the outer layer (OL) of three sample materials manufactured according to the present invention. It can be seen that the Al content of the outer layer is approximately 0.6%.

[0123] Here, the inventors wish to explain their views on the waviness of hot-dip coated products. The surface waviness of the final formed article is a result of the surface waviness of the undeformed, i.e., flat steel sheet and the waviness variation caused by the forming process. The difference between the waviness of the formed article and the waviness of the undeformed steel sheet is called the delta waviness ΔWsa. Due to the special nature of the steel strip manufacturing process, the formed surface exhibits a linear pattern, where the lines are perpendicular to the rolling direction. This observation means that the delta waviness in the rolling direction (RD) is higher than in other directions, the direction perpendicular to RD being denoted as the transverse direction (TD). This directional effect is also strongly present in paint appearance values, therefore minimizing the delta waviness in the rolling direction is important.

[0124] These patterns are caused by small localized hardness differences in the substrate, leading to uneven deformation during the forming process, resulting in height differences and consequently increased waviness. These hardness differences occur when the grains in the substrate are not small enough or when the waviness is too high before the leveling mill. In the latter case, the higher areas are pushed into the substrate by the leveling rolling operation, resulting in localized hardness differences.

[0125] According to the invention, reducing the waviness before the leveling mill ensures that the increase in waviness during forming is limited or completely eliminated. Due to the interaction between surface tension and gravity, the waviness level of the hot-dip coating before leveling rolling is related to the leveling of the coating surface. For a better understanding, refer to... Figure 9 A schematic diagram illustrating the formation of ripples from the air knife to solidification.

[0126] First, the hot-dip coating is reduced to its desired thickness at the air knife. Due to the highly turbulent nature of the airflow ejected by the wiping blade, the waviness of the coating surface will be higher than that of the initial substrate (see [link to original text]). Figure 9 (1) In the initial period after the steel strip passes through the air knife, the surface tension will level the surface, resulting in a decrease in waviness (see section 1). Figure 9 (See point 2). It can be noted that longer wavelengths attenuate more slowly than shorter wavelengths, making it crucial to ensure that high-wavelength waves are not generated at the air knife. After this initial leveling, the coating begins to move downwards. Although the descent rate is very low, it is believed that as the coating begins to adapt itself to the initial substrate surface (see point 2), the descent will accelerate. Figure 9 (3) This has a significant impact on waviness. The final waviness of the coating is determined as soon as the coating solidifies. The difference between the coating waviness and the substrate waviness is determined by... Figure 9 The 4 in the text represents...

[0127] Figure 10 The diagram schematically illustrates the final waviness of the hot-dip coating before the leveling machine. The difference between the substrate and coating waviness (see...) Figure 10 2) is determined by the balance between surface tension and gravity. Reducing substrate waviness has a very positive impact on coating waviness, but it tends to plateau for extremely low substrate waviness. The final level is determined by... Figure 10 The 1 in the value represents and is mainly determined by the influence of the air knife.

[0128] According to different aspects of the invention, several detrimental effects are successfully minimized, the combination of which results in excellent surface quality, particularly in terms of waviness. The explanation for the effect of increased aluminum in the zinc bath may be that the coating of the present invention better handles the turbulence of gases from the air knife, due to the change in fluidity of the coating as it passes over the air knife. The fluidity of molten metal is inversely proportional to the freezing range; in other words, the longer the freezing range, the lower the fluidity. For zinc-aluminum alloys, the fluidity is as follows: Figure 11As shown. Clearly, adding aluminum reduces flowability. For higher aluminum contents in the bath, such as above 0.5%, the flowability may be low enough to ensure that zinc segregation has already occurred in the impact zone of the gas from the air knife due to high heat transfer. This significantly increases the viscosity in this zone, which in turn ensures that the effects of air knife pressure fluctuations are minimized, and thus reduces the increase in waviness due to the wiping process. After the air knife, the segregated zinc remelts and reduces the viscosity sufficiently to ensure good leveling.

[0129] Further improvements for achieving excellent waviness are described above and in the claims.

[0130] Table 4. Wsa values ​​(μm) at different stages

[0131]

[0132] Table 4 provides examples of the waviness Wsa values ​​for the outer surface of the cold-rolled steel substrate before hot-dip coating, the hot-dip coated surface before leveling rolling (unleveled NTR), the flat, undeformed hot-dip coated steel according to the invention, and the outer surface of the 5% biaxially deformed cup-shaped article made from the coated steel of the invention, all measured in the rolling direction.

[0133] According to the present invention, the waviness Wsa (WsaNTR) of the hot-dip coating prior to leveling rolling is closely related to the waviness Wsa (WsaSUBSTR) of the substrate. Specifically, when the waviness difference between WsaNTR and WsaSUBSTR is defined as ΔWsaKNIVES = WsaNTR - WsaSUBSTR, the following relationship is found to hold:

[0134] ΔWsaKNIVES <MAX(0.06; 0.16-0.6 WsaSUBSTR),

[0135] Please see the examples in Table 5.

[0136] Table 5

[0137]

[0138] exist Figure 5 and Figure 5AThe figure shows the measurement results of the Fe content of the outer layer of hot-dip coated steel manufactured according to the present invention. Based on the inventors' findings and considerations, the Fe content in the outer layer of the product is very low, below 20 ppm in the tested samples. However, it should be mentioned that satisfactory products can also be obtained if the Fe content is below 30 ppm, 50 ppm, and 70 ppm. DX52 indicates DX52D+Z, DX54 indicates DX54D+Z, and BH 180 indicates HX180BD+Z (see EN10346: 2015).

[0139] To understand the final properties of the hot-dip coated steel products according to the present invention, the inventors have considered the conditions that occur during coating solidification. For this purpose, the aluminum-zinc phase diagram has been considered; see [link to relevant documentation]. Figure 12 If the aluminum percentage is less than 1.1%, zinc will segregate first during the cooling process of the liquid zinc coating after passing through an air knife, followed by aluminum and the remaining zinc.

[0140] High aluminum content leads to the formation of the ZnAl phase, which is undesirable as it degrades surface quality. Under non-equilibrium conditions (such as the typically extremely high cooling rates of the coating during wiping), the ZnAl phase is present at an Al percentage of 1.0%. For this reason, it is best to keep the aluminum percentage well below 1.0%, i.e., below 0.9%, and even preferably below 0.8%, to ensure that only negligible ZnAl phase is produced.

[0141] To test the wear behavior, an experiment was conducted in which coated strips were pulled together between two copolymers by applying a certain force.

[0142] To best simulate the pressing operation, both flat and cylindrical tools with a predetermined roughness (Ra) of 0.4 μm were used. The operation was repeated six times, with no lubrication between subsequent passes, to obtain a similar texture to that typically found in pressing workshops. Results are as follows: Figure 8 As shown in the diagram. Among the three lines, the line with triangular data points represents the wear behavior of conventionally galvanized DX54, the line with diamond-shaped data points represents the wear behavior of BH 180 hot-dip coated in a bath with an aluminum content of 0.66%, and the line with square data points represents the wear behavior of DX54 hot-dip coated in a bath with an aluminum content of 0.64%.

[0143] The inventors discovered that for materials with a high Al percentage, the coefficient of friction did not increase after two passes, while the control group did. This result is excellent and represents the best finding from this testing effort. Similarly, after all subsequent passes, the increase in the coefficient of friction was not as pronounced as with the control material, demonstrating the superiority of the product of this invention.

[0144] The spot welding results of the steel product of this invention are excellent. This may be related to the relatively small thickness of the inhibition layer. Figure 7 The structure of the coating layer can be seen in the figure. The gray area at the bottom of the figure represents the steel substrate, and the white to light gray area at the top of the figure represents the outer layer. Between the very thin inhibition layer (dark gray layer), thicknesses of 50.24 nm, 66.99 nm, 55.82 nm, 94.90 nm, and 128.4 nm can be seen at different locations.

[0145] The study employed a comprehensive adhesion test procedure according to SEP1220-6. As a result, the product according to the invention performed as well as or better than conventional galvanized products in terms of shear strength, shear outcome, and peel outcome.

[0146] Figure 6 The test sample according to the invention (as all test samples) successfully passed the most stringent OEM test regarding the adhesion of the coating to the steel substrate.

[0147] In summary, during the inventive work, which included in-depth research and development, the inventors have discovered, unexpectedly, in the field of this invention, that increasing the aluminum content in the zinc bath does not result in a thicker but rather a thinner inhibitory layer. This is advantageous because a thinner layer has better ductility and provides better electrode life. It is also advantageous because it makes the hot-dip coating operation independent of SET (Self-Touch). The thin inhibitory layer provides optimal coating adhesion and good electrode life in spot welding. The higher aluminum content in the outer layer provides better wear behavior, likely due to the layer's greater hardness. Finally, the higher aluminum content imparts different viscosity and solidification properties to the coating, which proves advantageous for manufacturing, and especially for the resulting coated product, particularly in terms of waviness.

[0148] Preferably, the invention is carried out in combination with a steel substrate having the following composition: all by weight%, C up to 0.007, Mn up to 1.2, Si up to 0.5, Al up to 0.1, P up to 0.15, S 0.003-0.045, N up to 0.01, Ti, Nb, Mo: if Ti ≥ 0.005 and Nb ≥ 0.005: 0.06 ≤ 4Ti + 4Nb + 2Mo ≤ 0.60, otherwise 0.06 ≤ Ti + 2Nb + 2Mo ≤ 0.60, and one or more optional elements: Cu up to 0.10, Cr up to 0.06, Ni up to 0.08, B up to 0.0015, V up to 0.01, Ca up to 0.01, Co up to 0.01, Sn up to 0.01, with the balance being iron and unavoidable impurities.

[0149] Finally, it should be pointed out that in the formula... To represent multiplication, ∧÷ represents exponentiation, and ÷ represents division.

Claims

1. A method for manufacturing a hot-dip coated steel strip with high surface quality for automotive applications, the hot-dip coated steel strip comprising a steel substrate and a Zn-Al coating layer, wherein coating is performed by guiding the steel substrate through a molten metal bath containing Al, wherein the composition of the bath is controlled to have an aluminum content greater than 0.50 wt% and 1.1 wt% or less, wherein the coating layer comprises an inner layer immediately adjacent to the steel substrate and an outer layer extending from a surface layer of the coated steel toward the inner layer, wherein the aluminum content of the inner layer is less than 165 mg / m². 2 The aluminum content in the outer layer is in the range of 0.4-1.0% by weight, and the aluminum content of the entire Zn-Al coating is 0.71-0.95%, the balance of the coating being Zn, unavoidable impurities, and optional total of up to 0.3% by weight of one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Mg, Sn, La, Ce, Cr, Zr, or Bi, and the outer surface Wsa of the 5% biaxially deformed cup-shaped article made from the coated steel strip, measured in the rolling direction, is 0.35 μm or less, as defined in standard SEP 1941:2012.

2. The method according to claim 1, characterized in that... The composition of the bath is controlled to have an aluminum content of 0.55% by weight or more.

3. The method according to claim 1 or 2, characterized in that... The composition of the bath is controlled to have an aluminum content of 0.60% by weight or more.

4. The method according to claim 1 or 2, characterized in that... The composition of the bath is controlled to have an aluminum content of 1.0% by weight or less.

5. The method according to claim 1 or 2, characterized in that... The composition of the bath is controlled to have an aluminum content of 0.9% by weight or less.

6. The method according to claim 1 or 2, characterized in that... The composition of the bath is controlled to have an aluminum content of 0.8% by weight or less.

7. The method according to claim 1 or 2, characterized in that... The Fe content in the bath is less than 90 ppm, preferably less than 70 ppm, more preferably less than 50 ppm, more preferably less than 30 ppm, and most preferably less than 20 ppm.

8. The method of claim 1, the method comprising guiding the strip to the outlet of the bath via rollers below the surface level of the bath, and wiping the strip leaving the bath by spraying wiping gas onto the coated steel strip through at least one air knife via at least one discharge port, characterized in that... Choose parameters D, HS, and US such that D ln(HS US)≤130, Where D is the shortest distance in mm from the air knife outlet to the strip surface, HS is the height in mm above the bath surface of the air knife, and US is the distance in mm between the upper edge of the roller and the bath surface.

9. The method according to claim 8, characterized in that... D ln(HS) The value of (US) is 120 or less, preferably 110 or less, more preferably 100 or less, more preferably 90 or less, more preferably 80 or less, and most preferably 70 or less.

10. The method according to claim 1, wherein the method comprises wiping the strip leaving the bath by spraying wiping gas onto the coated steel strip through at least one air knife via at least one discharge port, characterized in that... Perform heat soaking and wiping as follows: [(P-24)÷(5.95D ∧ 0.96 V ∧ 1.39)] ∧ -0.81 ≤ 150, Where P is the wiping pressure in the air knife in millibars, D is the shortest distance from the air knife outlet to the strip surface in mm, and V is the strip speed in m / min.

11. The method of claim 10, wherein [(P-24)÷(5.95D ∧ 0.96V ∧ 1.39)] ∧ -0.81 ≤ 100。 12. The method according to claim 10, characterized in that, [(P-24)÷(5.95D ∧ 0.96V ∧ 1.39)] ∧ -0.81 ≤ 70, The preferred option is [(P-24)÷(5.95D)]. ∧ 0.96 V ∧ 1.39)] ∧ -0.81 ≤ 60.

13. The method according to any one of claims 8-12, characterized in that... D is 9 mm or less, preferably 8 mm or less, and more preferably 7 mm or less.

14. The method according to any one of claims 8-12, characterized in that... HS is 550 mm or less, preferably 400 mm or less, preferably 300 mm or less, more preferably 250 mm or less, and even more preferably 200 mm or less.

15. The method according to any one of claims 8-12, wherein the steel strip is cold-rolled in a cold rolling mill having a mill stand before hot-dip coating, the mill stand having a pair of work rolls having a predetermined roughness Ra, characterized in that... Ra is 4.5 μm or less, preferably 1.5 μm or less, and more preferably 0.6 μm or less.

16. A hot-dip coated steel for automotive applications, the hot-dip coated steel comprising a steel substrate and a Zn-Al coating layer, the coating layer comprising an inner layer adjacent to the steel substrate and an outer layer extending from a surface layer of the coated steel toward the inner layer, wherein the aluminum content of the inner layer is less than 165 mg / m³. 2 The aluminum content in the outer layer is in the range of 0.4-1.0% by weight, and the aluminum content of the entire Zn-Al coating is 0.71-0.95% by weight, with the balance of the coating being Zn, unavoidable impurities, and optional total of up to 0.3% by weight of one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Mg, Sn, La, Ce, Cr, Zr, or Bi, and wherein the outer surface Wsa of the 5% biaxially deformed cup-shaped article made from the coated steel, measured in the rolling direction, is 0.35 μm or less, as defined in standard SEP 1941:2012.

17. The hot-dip coated steel according to claim 16, for full finish (exposed) applications.

18. The hot-dip coated steel according to claim 16 or 17, characterized in that... The Fe content in the outer layer is less than 70 ppm, preferably less than 50 ppm, more preferably less than 30 ppm, and most preferably less than 20 ppm.

19. The hot-dip coated steel according to claim 16 or 17, characterized in that... The aluminum content of the inner layer is less than 150 mg / m³. 2 And preferably less than 120 mg / m 2 .

20. The hot-dip coated steel according to claim 16 or 17, characterized in that... The aluminum content of the Zn-Al coating is 0.72% by weight or more, preferably 0.75% or more.

21. The hot-dip coated steel according to claim 16 or 17, characterized in that... The aluminum content of the Zn-Al coating is 0.90% or less.

22. The hot-dip coated steel according to claim 16 or 17, characterized in that... The outer surface Wsa of the 5% biaxially deformed cup-shaped article made of the coated steel, measured in the rolling direction, is 0.32 μm or less, preferably 0.29 μm or less, more preferably 0.26 μm or less, as defined in standard SEP 1941:2012.

23. The hot-dip coated steel according to claim 16 or 17, characterized in that... The steel substrate has the following composition, all by weight %: C up to 0.007, Mn up to 1.2, Si up to 0.5, Al up to 0.1, P up to 0.15, S 0.003-0.045, N up to 0.01, Ti, Nb, Mo: if Ti ≥ 0.005 and Nb ≥ 0.005: 0.06 ≤ 4Ti + 4Nb + 2Mo ≤ 0.60, otherwise 0.06 ≤ Ti + 2Nb + 2Mo ≤ 0.60, and one or more optional elements: Cu up to 0.10, Cr up to 0.06, Ni up to 0.08, B up to 0.0015, V up to 0.01, Ca up to 0.01, Co up to 0.01, Sn up to 0.01, with the balance being iron and unavoidable impurities.

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