Hot-dip plating process

Abstract

The invention relates to a process for hot-dip plating a steel strip comprising: (i) dipping the steel strip in a bath of Zn-based alloy comprising 0.5 to 10.3 wt% aluminium and 0.2 to 6 wt% magnesium, their sum being inferior or equal to 10.5 wt%, (ii) wiping the molten Zn-based alloy dragged out from the bath and adjusting the thickness of the molten Zn-based alloy to a value hm, in m per side of the steel strip, ranging from 0.000032 to 0.000130 m, (iii) passing the steel strip through a confinement zone extending at least from the wiping line and having a height Hbox, in m, measured from the wiping line, the atmosphere in the confinement zone being at pressure Pbox in bar and comprising an oxygen content CO2, in ppm by volume, ranging from 50 to 100,000 ppm, wherein V, Pbox, CO2 and Hbox satisfy specific inequations.

Description

[0001] Hot-dip plating process

[0002]

[0001] The present invention relates to a process for hot-dip plating a steel strip, in particular to a process for hot-dip plating a steel strip with a heavy metallic coating, i.e. a metallic coating having a thickness of at least 32 pm, more particularly to a process for hot-dip plating a steel strip with a heavy ZnAIMg coating.

[0003]

[0002] It is well known to coat a steel strip with a metallic coating by dipping the moving steel strip in a bath of metal liquid. The process is known as hot- dip plating and it is applied on galvanizing lines.

[0004]

[0003] Various types of metallic coatings exist: Zn-based coatings mainly comprise Zn with limited amounts of other elements such as, for example, Al, Si, Mg. Al-based coatings mainly comprise Al with limited amounts of other elements such as, for example, Zn, Si, Mg. ZnAIMg coatings refer to coatings comprising Al (typically up to 12 wt%), Mg (typically up to 6 wt%), the balance being Zn, possible additional elements in limited amount and unavoidable impurities resulting from the processing.

[0005]

[0004] The metallic coating is often applied to protect the steel strip against corrosion. In this context, there is a trend to increase the coating weight of the ZnAIMg coating to increase the corrosion resistance.

[0006]

[0005] It has been observed that there are nevertheless limitations in the capacity of a galvanizing line to increase the thickness of ZnAIMg coatings. Above 32 pm, surface defects, such as wrinkles, tend to appear. Wrinkles are mat oxide folds starting from the strip edge and extending substantially perpendicular to the strip edge. Such surface defects have to be minimized, if not suppressed, to avoid the rejection of the galvanized steel by either the quality control of the galvanizing line or the customers.

[0007]

[0006] The aim of the present invention is therefore to remedy the drawbacks of the hot-dip plating of the prior art by providing a process for hot-dip plating a steel strip with a ZnAIMg coating having a thickness of at least 32pm wherein the occurrence of wrinkles is minimized.

[0008]

[0007] For this purpose, a first object of the present invention consists of a process for hot-dip plating a steel strip moving at a line speed V, in m / s, the process comprising:

[0009] - (i) dipping the steel strip in a bath of molten Zn-based alloy comprising, in percentage by weight, CAI % of aluminium, CM9% of magnesium, the molten Zn-based alloy having a density p in kg / m3and a viscosity p in Pa-s, CAI ranging from 0.5 to 10.3 wt%, CMg ranging from 0.2 to 6 wt% and the sum of CAI and CMg being inferior or equal to 10.5 wt%,

[0010] - (ii) wiping with a wiping nozzle the molten Zn-based alloy dragged out from the bath by the steel strip and adjusting the thickness of the molten Zn-based alloy to a value hm in m per side of the steel strip, hm ranging from 0.000032 to 0.000130 m,

[0011] - (iii) passing the steel strip through a confinement zone extending at least from the wiping line of the wiping nozzle and having a height Hbox, in m, measured from the wiping line, the atmosphere in the confinement zone being at pressure Pbox in bar and comprising an oxygen content C02, in ppm by volume, C02 ranging from 50 to 100,000 ppm, wherein V, Pbox, C02 and Hbox satisfy the following inequations: wherein g is the standard gravity in m / s2and A= C02 when with MAI the molar mass of Al and MMg the molar mass of Mg.

[0012]

[0008] The first object of the invention may also have the optional features listed below, considered individually or in combination:

[0013] - CAI ranges from 1 to 6 wt%,

[0014] - CMg ranges from 0.5 to 4 wt%,

[0015] - the line speed V ranges from 0.3 to 3.3 m / s,

[0016] - hm ranges from 0.000050 to 0.000130 m,

[0017] - Hbox is greater than or equal to 0.25 m,

[0018] - Hbox ranges from 0.5 to 4 m,

[0019] - the confinement zone is delimited: o at the bottom, by the wiping line L of the wiping nozzles and the upper external faces of the wiping nozzles, o at the top, by the upper end of two confinement boxes placed on each side of the strip, just above the nozzles, the upper ends of the confinement boxes being positioned at the height Hbox above the wiping line and, o on the sides, by the lateral parts of the confinement boxes,

[0020] - the oxygen content C02 in the confinement zone ranges from 500 to 30,000 ppm,

[0021] - the oxygen content C02 in the confinement zone ranges from 500 to 5,000 ppm.

[0022]

[0009] A second object of the invention consists of a process for hot-dip plating a steel strip by dipping the steel strip in a bath of molten Zn-based alloy, wiping with a wiping nozzle the molten Zn-based alloy dragged out from the bath by the steel strip and passing the steel strip through a confinement zone extending at least from the wiping line and having a height measured from the wiping nozzle, the process comprising:

[0023] - (i) Acquiring the following preset parameters: o the line speed V, o the percentage by weight of aluminium CAI in the bath of molten Zn-based alloy, CAI ranging from 0.5 to 10.3 wt%, o the percentage by weight of magnesium CM9in the bath of molten Zn-based alloy, CM9ranging from 0.2 to 6 wt% and the sum of CAI and CMg being inferior or equal to 10.5 wt%, o the density p in kg / m3of the molten Zn-based alloy, o the viscosity p in Pa-s of the molten Zn-based alloy, o the thickness hm of the molten Zn-based alloy per side of steel strip in m, hm ranging from 0.000032 to 0.000130 m, o the pressure Pbox in the confinement zone, o the content of oxygen C02 in the confinement zone, in ppm by volume, C02 ranging from 50 to 100,000 ppm, o the height Hbox of the confinement zone, in m,

[0024] - (ii) determining that V, Pbox, C02 and Hbox satisfy the following inequations:

[0025] Wherein g is the standard gravity in m / s2and A= C02 when

[0026] With MAI the molar mass of Al and MMg the molar mass of Mg,

[0027] - (iii) If equation (1 ) is satisfied, hot-dip plating the steel strip based on the acquired preset parameters; otherwise adjusting at least one preset parameter among V, Pbox, C02 and Hbox so that equation (1 ) is satisfied and hot-dip plating the steel strip based on the adjusted preset parameter(s).

[0028]

[0010] The second object of the invention may also have the following optional feature: - the adjustment of at least one preset parameter comprises at least one of following: adjusting V by accelerating or slowing the line speed, adjusting Pbox by modifying the flow rate of an auxiliary supply, adjusting C02 by modifying the composition of the wiping gas and / or the composition of the gas injected in the confinement zone or by modifying the gas flow rate or adjusting Hbox by moving mobile parts of confinement boxes up or down to extend or restrict the confinement zone.

[0029]

[0011] The optional additional features presented above for the first object can also apply to the second object.

[0030]

[0012] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0031]

[0013] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

[0032] - Figure 1 , which is a schematic illustration of a hot-dip galvanizing line,

[0033] - Figures 2 and 3, which are schematic illustrations of wiping nozzles and confinement boxes.

[0034]

[0014] Referring to Figure 1 , in a first step of the process according to a first embodiment of the invention, a steel strip S is provided. The steel strip is not limited, neither in terms of composition nor in terms of grade, microstructure, manufacturing process or dimensions. In terms of grade, the steel strip can notably be an interstitial free (IF) steel, an Aluminium-killed mild steel or a high-strength low-alloy (HSLA) steel.

[0035]

[0015] In one variant, the steel strip S is a hot-rolled steel strip S having a thickness from 1.5 to 6 mm. In another variant, the steel strip S is a cold- rolled steel strip having a thickness from 0.4 to 2.5 mm.

[0036]

[0016] The steel strip S generally undergoes an annealing operation, in an annealing furnace 1 , to recrystallize it after the significant work hardening due to the hot- and / or cold- rolling, and to prepare its surface chemistry in order to promote the chemical reactions necessary for the galvanizing operation. In the annealing furnace, the steel strip is usually brought to a temperature ranging from 500 to 1100°C, preferably 650 to 900°C, and maintained at this temperature from 5 to 400 s, preferably from 5 to 180 s. Then, the steel strip is cooled to a temperature close to that of the temperature of the bath of molten Zn-based alloy.

[0037]

[0017] At the exit of the annealing furnace (if applicable), the steel strip S, moving at a line speed V, passes continuously through a coating bath 2 comprising a molten Zn-based alloy contained in a tank 3.

[0038]

[0018] In the context of the invention, the line speed V is not particularly limited. On current industrial lines, the line speed V is in general ranging from 0.3 m / s to 3.3 m / s, preferably from 0.3 m / s to 2.5 m / s, more preferably from 0.5 to 1 m / s.

[0039]

[0019] The bath is of a molten Zn-based alloy. The latter is defined as an alloy comprising Zn as the majority element, i.e. it contains more than 50 wt% Zn. In the context of the invention, the Zn-based alloy comprises preferably at least 60 wt% Zn, more preferably at least 70 wt% Zn, even more preferably at least 80 wt% Zn, in particular at least 88 wt% Zn. The composition is simply defined by the weight percentage of Aluminium CAI % and the weight percentage of Magnesium CMg %, since it has been determined that Al and Mg are the main elements impacting the appearance of surface defects on heavy coatings.

[0040]

[0020] Overall, the molten metal comprises in percentage by weight CAI % of aluminium, CMg % of magnesium, optional addition elements, the balance being zinc and the unavoidable impurities resulting from the processing.

[0041]

[0021] The weight percentage of Aluminium CAI % ranges from 0.5 to 10.3 wt%, preferably from 1 to 6 wt%, more preferably from 3 to 5.5 wt%. This element allows, on the one hand, to improve the adhesion of the coating to the metal strip and, on the other hand, to protect the strip from corrosion.

[0042]

[0022] The weight percentage of Magnesium CM9% ranges from 0.2 to 6 wt%, preferably from 0.5 to 4 wt%, more preferably from 1 to 3.5 wt%. Magnesium improves the corrosion resistance of the hot-dip steel and in particular its red rust resistance.

[0023] The sum of the weight percentage of Aluminium CAI % and the weight percentage of Magnesium CMg % is inferior or equal to 10.5 wt%. It has been found that, when the sum is higher than 10.5 wt%, wrinkles can still be observed on the coating surface even though inequation (1 ), which is detailed later on, is satisfied.

[0043]

[0024] The composition of the bath may also contain one or more optional addition elements selected from among Be, Ca, Ce, La, Ni, Pb, Sb, Sn, Sr, Y and Zr, in a content of up to 1 wt% for the sum of the addition elements. These various elements may notably improve the corrosion resistance of the coating or its brittleness or its adhesion. A person skilled in the art knowing their effects on the characteristics of the coating will employ them in accordance with the intended complementary purpose.

[0044]

[0025] Finally, the bath may contain unavoidable impurities resulting from the processing, mainly coming from residual elements included in the ingots feeding the bath and / or from the dissolution of the strip surface when passing through the bath. The unavoidable impurities can include As, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mo, N, P, S, Si in a content of up to 0.5 wt% for the sum of the unavoidable impurities.

[0045]

[0026] Preferably, the molten Zn-based alloy comprises, in percentage by weight, from 0.5 to 10.3 wt% Al, from 0.2 to 6 wt% Mg, the sum of the weight percentage of Aluminium and the weight percentage of Magnesium being inferior or equal to 10.5 wt%, possibly one or more addition elements selected from among Be, Ca, Ce, La, Ni, Pb, Sb, Sn, Sr, Y and Zr in a content of up to 1 wt% for the sum of the addition elements, the balance being zinc and possibly one or more of As, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mo, N, P, S or Si in a total content of up to 0.5 wt% as unavoidable impurities resulting from the processing.

[0046]

[0027] The molten Zn-base alloy is also defined by its density p, expressed in kg / m3, at the bath temperature Tbath. It can be calculated from the densities of all elements of the metal bath at the given temperature along with their respective contents. For molten Zn-based alloys comprising, in percentage by weight, from 0.5 to 10.3 wt% Al, from 0.2 to 6 wt% Mg, the sum of the weight percentage of Aluminium and the weight percentage of Magnesium being inferior or equal to 10.5 wt%, at a bath temperature Tbath ranging from 390 to 480°C, the density p is comprised between 5100 and 6500 kg / m3

[0047]

[0028] The molten Zn-base alloy is also defined by its viscosity p, expressed in Pa-s, at the bath temperature Tbath or in a temperature range around Tbath. It is determined using abacuses or through experimentation, notably using an oscillating cup viscometer. For molten Zn-based alloys comprising, in percentage by weight, from 0.5 to 10.3 wt% Al, from 0.2 to 6 wt% Mg, the sum of the weight percentage of Aluminium and the weight percentage of Magnesium being inferior or equal to 10.5 wt%, at a bath temperature Tbath ranging from 390 to 480°C, the viscosity p is comprised between 1.5 x 10’3and 4.0 x 10’3Pa-s.

[0048]

[0029] The bath is maintained at a temperature Tbath. The latter usually ranges from 10°C above the liquidus to 580°C, the temperature of the liquidus varying depending on the bath composition. For the range of coatings used in the present invention, this temperature will therefore preferably range from 360 to 480° C. It is recalled here that the liquidus is the temperature above which an alloy is entirely in the molten state.

[0049]

[0030] After having passed through the bath of molten Zn-base alloy, the steel strip S exits the bath coated on both its faces with the molten Zn-base alloy dragged out from the bath by the steel strip. The molten Zn-base alloy dragged out from the bath is wiped by means of wiping nozzles 5 placed on each side of the strip S and which sprays a wiping gas onto the surfaces of the strip. This conventional operation, well known to those skilled in the art, adjusts the thickness hm of the molten Zn-base alloy on each side of the steel strip. The thickness of the molten Zn-base alloy corresponds to the thickness of the coating obtained once the molten Zn-base alloy has solidified.

[0050]

[0031] In the context of the invention, the thickness hm ranges from 3.2 x10’5to 1.3 x 10’4m, preferably from 5 x 10’5to 8 x 10’5m. The thickness is preferably measured by X-ray fluorescence spectroscopy. It can also be measured by gravimetry or according to the magnetic induction method (ISO 2178:2016).

[0051]

[0032] Wiping is preferably performed with a wiping gas having a low oxidizing power, in particular having an oxidizing power lower than that of an atmosphere consisting of 4% oxygen by volume and 96% nitrogen by volume. It minimizes the presence of oxygen, which would otherwise promote the rapid growth of a surface oxide. In particular, it is possible to use pure nitrogen or pure argon, or else mixtures of nitrogen or argon and oxidizing gases such as, for example, oxygen, CO / CO2 mixtures or H2 / H2O mixtures. It will also be possible to use CO / CO2 mixtures or H2 / H2O mixtures without the addition of an inert gas.

[0052]

[0033] After wiping, the steel strip passes through a confinement zone extending at least from the wiping line L of the wiping nozzles 5 and having a height Hbox, in m, measured from the wiping line.

[0053]

[0034] The height Hbox between the wiping line and the end of the confinement zone is preferably of at least 0.25 m. It favors the relaxation of the wiped surface before surface oxide appears. More preferably, Hbox is comprised between 0.5 and 4m. Limiting the size of the confinement zone can facilitate the control of the atmosphere. The size of the confinement box can also be limited to accommodate some line constraints.

[0054]

[0035] In a preferred variant of the invention illustrated on Figures 2 and 3, the confinement zone is delimited:

[0055] - at the bottom, by the wiping line L of the wiping nozzles and the upper external faces of the wiping nozzles 4,

[0056] - at the top, by the upper end of two confinement boxes 6 placed on each side of the strip, just above the nozzles 4, the upper ends of the confinement boxes being positioned at the height Hbox above the wiping line and,

[0057] - on the sides, by the lateral parts of the confinement boxes.

[0058]

[0036] In the context of the present application, the term "wiping line" is understood to mean the shortest segment connecting the nozzle and the strip, corresponding to the minimum path followed by the wiping gas, as denoted by the letter L in Figure 1 .

[0059]

[0037] The confinement zone can optionally extend below the wiping line. It can extend to an intermediate position between the bath and the wiping line, preferably located at a distance of 10 cm, or even 15 cm, beneath the wiping line. It can also extend to the surface of the bath of molten metal. It limits the appearance of an oxide layer that is removed by the wiping jet and returns to the bath where it accumulates and is picked up by the moving strip. The confinement zone extending below the wiping line can be formed by extensions of the confinement boxes 6 below the wiping line.

[0060]

[0038] The atmosphere in the confinement zone is controlled to limit the oxidation of the surface of molten metal after wiping. The atmosphere is defined by its pressure Pbox in bar and by its oxidizing power expressed in the form of an oxygen content C02, in ppm by volume.

[0061]

[0039] The pressure Pbox preferably ranges from 0.8 to 1.1 bar. Pbox is preferably the atmospheric pressure. It can be measured with a piezoresistive / capacitive transducer, or a Pitot tube, positioned in the confinement zone.

[0062]

[0040] The oxygen content C02 ranges from 50 to 100,000 ppm. Below 50ppm, the confinement has to be very tight which is industrially difficult to manage. Above 100,000, the atmosphere is too oxidizing. C02 ranges preferably from 50 to 60,000 ppm, more preferably from 500 to 30,000 ppm, even more preferably from 500 to 5,000 ppm, to facilitate the line management. C02 can be measured in real time by means of a suitable sensor. The latter is preferably positioned above the wiping nozzles, more preferably from 0.1 to 0.6 m above. The sensor is preferably substantially at the same distance from the steel strip as the wiping nozzles.

[0063]

[0041] The confinement zone, and if applicable the confinement boxes 6, may simply be supplied with the flow of wiping gas escaping from the wiping nozzles. Alternatively, the confinement may also be equipped with an auxiliary supply in inert gas, such as nitrogen or argon, or in a mixture of inert gases. The flow of the auxiliary supply can be adjustable to adjust the oxygen content C02 in the confinement zone.

[0064]

[0042] Although all kinds of wiping nozzles may be used to implement the process according to the invention, it is more particularly preferred to choose nozzles having a blade shaped outlet orifice, the width of which exceeds that of the strip to be coated, since this type of nozzle enables the bottom part of the wiping zone to be properly confined. In particular, nozzles of triangular cross section, as shown schematically in Figure 1 , may advantageously be used. These nozzles are generally located 5 to 50 cm above the surface of the bath.

[0065]

[0043] In this first embodiment of the invention, the process conditions are controlled so that V, Pbox, C02 and Hbox satisfy the following inequations: wherein g is the standard gravity in m / s2and A= C02 when with MAI the molar mass of Al and Mi ig the molar mass of Mg, both expressed in g / mol.

[0066]

[0044] In other words:

[0067]

[0045] The present inventors have found that by hot-plating the steel strip in the conditions of inequation (1 ), the wrinkles, as defined previously, surprisingly disappear from the surface of the heavy ZnAIMg coatings. Without being bound by any theory, it is believed that these conditions favor the relaxation of the surface of the wiped molten metal before the surface topography is disturbed by the appearance of a surface oxide.

[0068]

[0046] During production, at least one of the parameters V, Pbox, C02 and Hbox can be adjusted, in case of deviation of the process conditions, to satisfy the inequations.

[0047] In particular, V can be simply adjusted by accelerating or slowing the line speed. Pbox can be adjusted notably by modifying the flow rate of the auxiliary supply. C02 can be adjusted notably by modifying the composition of the wiping gas and / or the composition of the gas injected in the confinement zone or by modifying the gas flow rate. Hbox can be adjusted by moving mobile parts of the confinement boxes up or down to extend or restrict the confinement zone.

[0069]

[0048] The deviation of the process conditions can be monitored by an operator or a monitoring device. The monitoring device is an electronic device comprising at least a processor and a memory. It can be independent of the line management system or integrated into it.

[0070]

[0049] In one variant of the invention, the monitoring device is configured, for instance programmed, to acquire V, Pbox, C02 and Hbox. By that, it is meant that the parameters are made available to the monitoring device. The latter can acquire them from the line management system. It can also be connected to measurement devices installed on the line to directly collect these parameters. In particular, it can be connected to a line speed measurement device, a pressure sensor positioned in the confinement zone, an oxygen sensor positioned in the confinement zone and / or a sensor measuring the height of the confinement zone. The acquisition can be done dynamically, for example at regular time intervals, preferably continuously. Preferably, the monitoring device is also configured, for instance programmed, to acquire hm, since it can vary during production.

[0071]

[0050] The monitoring device can be configured to determine that V, Pbox, C02 and Hbox satisfy inequation (1 ). The determination can be done dynamically, for example at regular time intervals, preferably continuously.

[0072]

[0051] The monitoring device can also be configured to determine an adjusted parameter for correcting the deviation.

[0073]

[0052] The adjustment can be done manually, for example by an operator. Alternatively, the adjustment is done with a closed-loop controller. Preferably this controller is a proportional integral derivative controller. As, in some cases, only one or two terms of the controller can provide appropriate control, some parameters of the controller can be set to zero to inactivate some terms. In other words, the proportional integral derivative controller can be a P controller, an I controller, a D controller, a PI controller, a PD controller, an ID controller or a PID controller.

[0074]

[0053] In the context of the invention, the conditions of the cooling that follows the wiping are not limited. The average cooling rate between the bath temperature and the liquidus of the molten metal is preferably below 20°C / s, more preferably from 0.5°C / s to 10°C / s.

[0075]

[0054] When the coated strip has completely cooled, it may undergo a skinpass operation to give it a texture, that notably facilitates a possible subsequent forming process or a possible subsequent painting process.

[0076]

[0055] The metallic coating obtained through this process has the same composition as the molten Zn-based alloy. The metallic coating is formed on an inhibition layer that usually appears on the surface of the steel strip when the molten metal of the bath, and in particular the aluminium, reacts with the steel. The inhibition layer usually comprises several phases such as Fe2Al5, FeAh and tau-5C (a AIFeSiZn phase). It can have a thickness ranging from 20 nm to 200 nm. The thickness hm of the molten metal I of the metallic coating is expressed and measured without taking the inhibition layer into account.

[0077]

[0056] A second embodiment of the invention is now described. It mainly differs from the first embodiment in that, before starting the hot-dip plating of the steel strip, it is determined that the parameters V, Pbox, C02 and Hbox satisfy inequation (1 ). At this stage, these parameters are preset parameters since the production has not started yet. Doing so is advantageous to make sure that a specific product can be produced on a given line and / or to dimension the line accordingly.

[0078]

[0057] The specificities of the second embodiment compared to the first embodiment are described below. All other details provided when describing the first embodiment apply to the second embodiment.

[0079]

[0058] In a first step, the preset parameters are acquired. The acquisition can be done by an operator or by a monitoring device, as described in relation to the first embodiment. It can be done any time before starting the hot-dip plating of the steel strip. It can notably be done at an early stage based on the line capabilities. It can be done once an order has been booked. It can also be done before the steel strip is dipped in the bath of molten metal. It can also be done once for all if the parameter in question is not to be modified during the production, which can be the case for example for Pbox. Accordingly, the parameters can be acquired from the line capabilities, from the order book of the line, from the line scheduling system and / or from the line management system.

[0080]

[0059] In particular, the following preset product parameters are acquired:

[0081] - the percentage by weight of aluminium CAI in the bath of molten Zn-based alloy,

[0082] - the percentage by weight of magnesium CMg in the bath of molten Zn- based alloy,

[0083] - the density p in kg / m3of the molten Zn-based alloy,

[0084] - the viscosity p in Pa-s of the molten Zn-based alloy,

[0085] - the thickness hm of the molten Zn-based alloy per side of steel strip in m,

[0060] In particular, the following preset process parameters are acquired:

[0086] - the line speed V, in m / s,

[0087] - the pressure Pbox in the confinement zone, in bar,

[0088] - the content of oxygen C02 in the confinement zone, in ppm by volume,

[0089] - the height Hbox of the confinement zone, in m.

[0090]

[0061] Once the parameters have been acquired, it is determined whether V, Pbox, C02 and Hbox satisfy the inequations. The determination can be done by an operator or by a monitoring device, as described in relation to the first embodiment. The determination can be in the form of a calculation. It can also be done by consulting an abacus that indicates whether the inequations are satisfied as a function of acquired preset parameters. It can be done any time before starting the hot-dip plating of the steel strip, as long as it is done after the acquisition. It can notably be done at an early stage based on the line capabilities. It can be done once an order has been booked. It can also be done before the steel strip is dipped in the bath of molten Zn-based alloy.

[0091]

[0062] Once it has been determined whether V, Pbox, C02 and Hbox satisfy the inequations, the result can be made available to an operator, for example in the form of abacus, or to the line management system, for example in the form of line constraints.

[0092]

[0063] Once it has been determined that V, Pbox, C02 and Hbox satisfy the inequations, the steel strip can be hot-dipped based on the acquired preset parameters. It is meant by “based on” that slight deviations compared to the preset parameters are possible as it is usual on industrial lines. The production can be done any time after the determination.

[0093]

[0064] Otherwise, i.e. in the case where V, Pbox, C02 and Hbox do not satisfy the inequations, at least one preset parameter can be adjusted, as described previously, so that the equations are satisfied. The adjustment can be done by an operator or by a monitoring device, as described in relation to the first embodiment. It can be done any time before starting the hot-dip plating of the steel strip, as long as it is done after the determination. It can notably be done at an early stage based on the line capabilities. It can be done once an order has been booked. It can also be done before the steel strip is dipped in the bath of molten Zn-based alloy.

[0094]

[0065] The first and second embodiments can be combined for an improved control of the process. In particular, the monitoring device can be configured to:

[0095] - Determine, before the hot-dip plating of the steel strip, that the parameters V, Pbox, C02 and Hbox satisfy inequation (1 ),

[0096] - Determine, during hot-dip plating, that the parameters V, Pbox, C02 and Hbox satisfy inequation (1 ),

[0097] - Optionally, determine an adjusted parameter for correcting the deviation.

[0098]

[0066] Examples:

[0099]

[0067] Steel strips were hot-dip coated in a bath of molten Zn-based alloy comprising 3.7wt% Al and 3 wt% Mg, the balance being zinc and the unavoidable impurities resulting from the elaboration and maintained at 430°C. The molten Zn-based alloy had a density p of 5659.426 kg / m3and a viscosity p of 3.4 x 10’3Pa-s. The pressure Pbox in the confinement zone was maintained at atmospheric pressure and the height Hbox of the confinement zone was maintained at 0.9m. The molar mass of Al MAI equals 26.98 u and the molar mass of Mg MMg equals 24.305 u.

[0100]

[0068] The density p was calculated from the composition of the molten Zn- based alloy for a bath temperature of 430°C. The viscosity p was obtained from abacuses for a bath at 430°C. The thickness hm of the metallic coating was evaluated by comparing the weight of hot-dipped samples to the weight of bare samples. The samples were 1 m long (taken in the width of the steel strip) and 0.25m wide.

[0069] The parameters V, C02 and hm were varied as detailed in Table 1 , where SR stands for:

[0101]

[0070] The appearance of defects was evaluated and summarized in Table 2. Evaluations were performed by visual inspection of the surface of the hot- dipped steel strip.

[0102] Table 1 Table 1 (continued) examples according to the invention

[0103] Table 2 Table 2 (continued) examples according to the invention As illustrated by examples 1 to 19, when the parameters are varied so that inequation (1 ) is satisfied, wrinkles are prevented on the surface of the ZnAIMg coating.

[0104] As illustrated by counter-examples 20 to 42, when the parameters do not satisfy inequation (1 ), wrinkles are present on the surface of the ZnAIMg coating. In particular, as illustrated by counter-examples 39 and 42, when the oxygen content

[0105] C02 in the confinement zone is above 100,000 ppm, wrinkles are present on the surface of the ZnAIMg coating, whatever the other parameters.

Claims

1.CLAIMS1 ) Process for hot-dip plating a steel strip moving at a line speed V, in m / s, the process comprising:- (i) dipping the steel strip in a bath of molten Zn-based alloy comprising, in percentage by weight, CAI % of aluminium, CM9% of magnesium, the molten Zn-based alloy having a density p in kg / m3and a viscosity p in Pa-s, CAI ranging from 0.5 to 10.3 wt%, CMg ranging from 0.2 to 6 wt% and the sum of CAI and CMg being inferior or equal to 10.5 wt%,- (ii) wiping with a wiping nozzle the molten Zn-based alloy dragged out from the bath by the steel strip and adjusting the thickness of the molten Zn-based alloy to a value hm in m per side of the steel strip, hm ranging from 0.000032 to 0.000130 m,- (iii) passing the steel strip through a confinement zone extending at least from the wiping line of the wiping nozzle and having a height Hbox, in m, measured from the wiping line, the atmosphere in the confinement zone being at pressure Pbox in bar and comprising an oxygen content C02, in ppm by volume, C02 ranging from 50 to 100,000 ppm, wherein V, Pbox, C02 and Hbox satisfy the following inequations:wherein g is the standard gravity in m / s2and A= C02 whenwith MAI the molar mass of Al and MM9the molar mass of Mg.2) Process according to claim 1 wherein CAI ranges from 1 to 6 wt%.3) Process according to any one of claims 1 or 2 wherein CMg ranges from 0.5 to 4 wt%.4) Process according to any one of claims 1 to 3 wherein the line speed V ranges from 0.3 to 3.3 m / s.5) Process according to any one of claims 1 to 4 wherein hm ranges from 0.000050 to 0.000130 m.6) Process according to any one of claims 1 to 5 wherein Hbox is greater than or equal to 0.25 m.7) Process according to any one of claims 1 to 6 wherein Hbox ranges from 0.5 to 4 m.8) Process according to any one of claims 1 to 7 wherein the confinement zone is delimited:- at the bottom, by the wiping line L of the wiping nozzles and the upper external faces of the wiping nozzles,- at the top, by the upper end of two confinement boxes placed on each side of the strip, just above the nozzles, the upper ends of the confinement boxes being positioned at the height Hbox above the wiping line and,- on the sides, by the lateral parts of the confinement boxes.9) Process according to any one of claims 1 to 8 wherein the oxygen content C02 in the confinement zone ranges from 500 to 30,000 ppm.10)Process according to any one of claims 1 to 8 wherein the oxygen content C02 in the confinement zone ranges from 500 to 5,000 ppm.)Process for hot-dip plating a steel strip by dipping the steel strip in a bath of molten Zn-based alloy, wiping with a wiping nozzle the molten Zn-based alloy dragged out from the bath by the steel strip and passing the steel strip through a confinement zone extending at least from the wiping line and having a height measured from the wiping nozzle, the process comprising:- (i) Acquiring the following preset parameters: o the line speed V, o the percentage by weight of aluminium CAI in the bath of molten Zn-based alloy, CAI ranging from 0.5 to 10.3 wt%, o the percentage by weight of magnesium CMg in the bath of molten Zn-based alloy, CMg ranging from 0.2 to 6 wt% and the sum of CAI and CMg being inferior or equal to 10.5 wt%, o the density p in kg / m3of the molten Zn-based alloy, o the viscosity p in Pa-s of the molten Zn-based alloy, o the thickness hm of the molten Zn-based alloy per side of steel strip in m, hm ranging from 0.000032 to 0.000130 m, o the pressure Pbox in the confinement zone, o the content of oxygen C02 in the confinement zone, in ppm by volume, C02 ranging from 50 to 100,000 ppm, o the height Hbox of the confinement zone, in m,- (ii) determining that V, Pbox, C02 and Hbox satisfy the following inequations:Wherein g is the standard gravity in m / s2and A= C02 whenWith MAI the molar mass of Al and MMg the molar mass of Mg,- (iii) If equation (1 ) is satisfied, hot-dip plating the steel strip based on the acquired preset parameters; otherwise adjusting at least one preset parameter among V, Pbox, C02 and Hbox so that equation (1 ) is satisfied and hot-dip plating the steel strip based on the adjusted preset parameter(s).12) Process according to claim 11 wherein the adjustment of at least one preset parameter comprises at least one of following: adjusting V by accelerating or slowing the line speed, adjusting Pbox by modifying the flow rate of an auxiliary supply, adjusting C02 by modifying the composition of the wiping gas and / or the composition of the gas injected in the confinement zone or by modifying the gas flow rate or adjusting Hbox by moving mobile parts of confinement boxes up or down to extend or restrict the confinement zone.

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