Hot-dip plating process
By determining an initial strip width and controlling the hot-dip plating process to accommodate edge under-coating removal, the process achieves higher ZnAIMg coating weights on steel strips without defects, improving production yield and quality.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- ARCELORMITTAL SA
- Filing Date
- 2025-12-08
- Publication Date
- 2026-06-18
Smart Images
Figure IB2025062550_18062026_PF_FP_ABST
Abstract
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 coating weight of at least 400 g / m2for both sides, 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 coating weight of ZnAIMg coatings. Above 400 g / m2, a defect tends to appear along the edges of the steel strip in the form of a thinning of the metallic coating, referred to as edge under-coating. It impacts the corrosion resistance of the galvanized strip and is not accepted by either the quality control of the galvanizing line or the customers. The edge under-coating can also be adjacent to an overcoating of the metallic coating. The latter complicates the coiling of the galvanized strip and may require the use of oscillating coiling. Such coils may not be accepted by customers.
[0006] The only way to remove this defect before sending the coil to the customer is to trim the edges. Accordingly, to make sure that all the edge under-coating, and possibly all the overcoating, is trimmed while the strip width requested by the customer is obtained after trimming, the galvanizing line has to order strips that are significantly wider than the width requested by the customer. This extra width significantly impacts the production yield.
[0007]
[0007] 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 coating weight of at least 400 g / m2with an improved production yield.
[0008]
[0008] For this purpose, a first object of the present invention consists in a process for hot-dip plating a steel strip S on a galvanizing line, the process comprising:
[0009] - (i) Acquiring WP, which is the preset strip width of steel strip S at the exit of the galvanizing line, and CWP, which is the preset coating weight of molten Zn-based alloy for the sum of both sides of the steel strip S, CWPbeing greater than or equal to 400 g / m2,
[0010] - (ii) providing the steel strip S with an initial strip width Wo determined based on WPand CWPso that the difference between Wo and WPis at least two times bigger than an inferred faulty edging E satisfying Equation (1 ): wherein A is a non-zero coefficient, B and C are coefficients, and wherein, when (2),
[0011] - (iii) dipping the steel strip S in a bath of a molten Zn-based alloy comprising, in percentage by weight, from 0.5 to 12 wt% of aluminium and from 0.2 to 6 wt% of magnesium, - (iv) wiping the molten Zn-based alloy dragged out from the bath by the steel strip S and adjusting the coating weight of the molten Zn-based alloy according to CWP,
[0012] - (v) cooling the molten Zn-based alloy,
[0013] - (vi) trimming both edges of the steel strip to obtain the preset strip width WP.
[0014]
[0009] The process according to the invention may also have the optional features listed below, considered individually or in combination:
[0015] - the inferred faulty edging E satisfies Equation (4): wherein As is a non-zero coefficient, Bsand Cs are coefficients, wherein, when > 1.4 (5), wherein T(t) is an expected thermal path of steel strip S at least from after wiping and ts is the time in seconds taken for the molten Zn-based alloy to reach its solidus Tsat least from after wiping,
[0016] - the expected thermal path T(t) and the time ts to reach Ts are estimated from preset parameters acquired in step (i),
[0017] - the expected thermal path T(t) and the time ts to reach Ts are estimated from the preset temperature Tbath of the bath, the preset average cooling rates in at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the length of each of the at least first two cooling boxes, the position of each of the at least first two cooling boxes and the preset line speed V,
[0018] - the expected thermal path T(t) and the time ts to reach Ts are estimated from a cooling model of the galvanizing line as a function of the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the position of each of the at least first two cooling boxes, the preset line speed V, the convective heat transfer coefficient in each of the at least first two cooling boxes, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip, the preset thickness Th of the steel strip and the solidus Ts,
[0019] - the inferred faulty edging E satisfies Equation (7): wherein Anq is a non-zero coefficient, Biiq and Ciiq are coefficients, wherein, when BUq0 and / or CUq0 : wherein T(t) is an expected thermal path of steel strip S at least from after wiping and tisqis the time in seconds taken for the molten Zn-based alloy to reach its liquidus Tnq at least from after wiping,
[0020] - the expected thermal path T(t) and the time tisq to reach Tiiq are estimated from preset parameters acquired in step (i),
[0021] - the expected thermal path T(t) and the time tisq to reach Tiiq are estimated from the preset temperature Tbath of the bath, the preset average cooling rates in at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the length of each of the at least first two cooling boxes, the position of each of the at least first two cooling boxes and the preset line speed V,
[0022] - the expected thermal path T(t) and the time tisq to reach Tiiq are estimated from a cooling model of the galvanizing line as a function of the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the position of each of the at least first two cooling boxes, the preset line speed V, the convective heat transfer coefficient in each of the at least first two cooling boxes, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip, the preset thickness Th of the steel strip and the liquidus Tiiq,
[0023] - the inferred faulty edging E is obtained from an inferred faulty edge table as a function of a plurality of preset parameters acquired in step (i),
[0024] - the plurality of preset parameters is selected among the preset temperature Tbath of the bath, the length of each cooling box of the cooling system, the position of each cooling box, the preset line speed V, the convective heat transfer coefficient in each cooling box, the preset power of the cooling box, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip, the preset thickness Th of the steel strip, the solidus Ts, the liquidus Tnq or the preset coating weight CWP,
[0025] - both edges are equally trimmed,
[0026] - the difference between Wo and WPis from two to four times bigger than the inferred faulty edging E,
[0027] - the difference between Wo and WPis from four to six times bigger than the inferred faulty edging E.
[0028]
[0010] A second object of the invention consists in a determination device comprising at least a processor and a memory, configured to execute the following steps in relation to a galvanizing line comprising a bath of molten Zn-based alloy for hot-dip plating a steel strip S:
[0029] - Acquiring WP, which is the preset strip width of the steel strip S at the exit of a galvanizing line, and CWP, which is the preset coating weight of molten Zn-based alloy for the sum of both sides of the steel strip S, CWPbeing greater than or equal to 400 g / m2,
[0030] - Determining the initial strip width Wo of the steel strip to be provided at the entry of the galvanizing line based on WPand CWPso that the difference between Wo and WPis at least two times bigger than an inferred faulty edging E satisfying Equation (1 ): wherein A is a non-zero coefficient, B and C are coefficients, and wherein, when > 1.4
[0031]
[0011] The determination device according to the invention may also have the optional features listed below, considered individually or in combination: - the determination device is configured to acquire the preset temperature Tbath of the bath, preset average cooling rates in at least two cooling boxes of a plurality of cooling boxes composing the cooling system, the length of each of the at least two cooling boxes, the position of each of the at least two cooling boxes and the preset line speed V,
[0032] - the determination device is configured to acquire the preset temperature Tbath of the bath, the length of each cooling box of the cooling system, the position of each cooling box, the preset line speed V, the convective heat transfer coefficient in each cooling box, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip and the preset thickness Th of the steel strip,
[0033] - the determination device according is configured to determine the initial strip width Wo of the steel strip to be provided at the entry of the galvanizing line with the inferred faulty edging E satisfying Equation (4): wherein As is a non-zero coefficient, Bsand Cs are coefficients, wherein, when
[0034] Bs0 and / or Cs0 , > 1.4 wherein T(t) is an expected thermal path of steel strip S at least from after wiping, ts is the time in seconds taken for the molten Zn-based alloy to reach its solidus Tsat least from after wiping and both T(t) and ts are estimated from the preset parameters acquired in step (i),
[0035] - the determination device is configured to determine the initial strip width Wo of the steel strip to be provided at the entry of the galvanizing line with the inferred faulty edging E satisfying Equation (7): wherein Anq is a non-zero coefficient, Biiq and Ciiq are coefficients, wherein, when BUq0 and / or CUq0 : wherein T(t) is an expected thermal path of steel strip S at least from after wiping and tisqis the time in seconds taken for the molten Zn-based alloy to reach its liquidus Tnq at least from after wiping and both T(t) and ts are estimated from the preset parameters acquired in step (i),
[0036]
[0012] The optional additional features presented above for the first object can also apply to the second object.
[0037]
[0013] A third object of the invention consists in a galvanizing line comprising a bath of molten Zn-based alloy for hot-dip plating a steel strip S and a determination device according to the invention.
[0038]
[0014] The optional additional features presented above for the first object can also apply to the third object.
[0039]
[0015] Other characteristics and advantages of the invention will be described in greater detail in the following description.
[0040]
[0016] 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 Figure 1 , which compares the inferred faulty edging E, as determined in the second step of the process according to the invention, to the maximum edge under-coating measured on the edges of the examples.
[0041]
[0017] In a first step of the process according to the invention, preset parameters are acquired. In particular, some preset parameters of the galvanized steel strip to be produced are acquired. More particularly, the preset strip width WPof the steel strip S at the exit of the galvanizing line and the preset coating weight CWPof the molten Zn-based alloy for the sum of both sides of the steel strip S are acquired.
[0042]
[0018] According to a preferred variant of the first step, other preset parameters of the galvanized steel strip to be produced are acquired and / or preset parameters of the galvanizing line for producing the galvanized steel strip are acquired. Examples of the other preset parameters of the galvanized steel strip to be produced are the preset thickness Th of the steel strip, the emissivity of the molten Zn-based alloy, the solidus Tsof the molten Zn-based alloy, the liquidus Tnq of the molten Zn-based alloy, the density p of the steel strip. Examples of preset parameters of the galvanizing line are the preset temperature Tbath of the bath, the preset line speed V, the preset power(s) Pi, P2... Pn of the cooling box(es), the preset convective heat transfer coefficient(s) Hnin the cooling box(es), the length of the cooling boxes, their position along the strip path, the preset average cooling rate(s) in the cooling box(es), the temperature Tgasof the cooling gas.
[0043]
[0019] The acquisition can be done manually by an operator or automatically by a determination device, in particular the acquisition module of the determination device. The latter is an electronic device comprising at least a processor and a memory. It can be independent of the line scheduling system or line management system or integrated into it. The acquisition can be done any time before providing the steel strip to be galvanized. It can be done once an order has been booked. It can be done as an initial step when the steel strip to be galvanized is ordered to the hot-rolling mill or cold-rolling mill. The acquisition can be done at different times for the different preset parameters. Some preset parameters can be acquired once for all.
[0044]
[0020] The parameters can be acquired from the order book of the line, from the line scheduling system and / or from the line management system, from the line capabilities, from reference charts of the line, from scientific literature.
[0045]
[0021] Once the preset parameters have been acquired or simultaneously, the initial strip width Wo of the steel strip to be provided on the galvanizing line is determined.
[0046]
[0022] The determination of the initial strip width Wo can be done manually by an operator or automatically by the determination device, in particular by the prediction module of the determination device. The determination can be done with the help of preset table(s), as detailed later on. The determination can be done any time before providing the steel strip to be galvanized. It can be done once an order has been booked. It can be done as an initial step when the steel strip to be galvanized is ordered to the hot-rolling mill or coldrolling mill.
[0047]
[0023] The initial strip width Wo is determined based on the preset strip width WPand the preset coating weight CWPso that the difference between Wo and WPis at least two times bigger than an inferred faulty edging E. The latter is the estimation of the maximum width of the edge under-coating for one edge of the steel strip S when hot-dip plated on the galvanizing line. By selecting a difference between Wo and WPat least two times bigger than the inferred faulty edging E, the actual edge under-coating will be completely removed on both edges. Preferably, the difference between Wo and WPis at most six times bigger than the inferred faulty edging E, more preferably at most five times, even more preferably at most four times, in particular at most three times.
[0048]
[0024] In a variant of the invention, the difference between Wo and WPis at least three times bigger than the inferred faulty edging E. Consequently, possible overcoatings adjacent to the under-coating are trimmed in addition to the trimming of the edge under-coating. It has indeed been observed that the size of the overcoating is of the same order of magnitude as the size of the edge under-coating. Preferably, the difference between Wo and WPis from three to six times bigger than the inferred faulty edging E. More preferably, the difference between Wo and WPis from four to five times bigger than the inferred faulty edging E.
[0049]
[0025] In particular, the inferred faulty edging E satisfies Equation (1 ): wherein A is a non-zero coefficient, B and C are coefficients, and wherein, when > 1.4 (2).
[0050]
[0026] It is the inventor’s finding that the size of the edge under-coating is substantially proportional to (CWP)3 / 2and that an offset, which is a linear function of CWP, can be added, in some circumstances, to fine-tune the estimation. The circumstances depend on the specificities of the galvanizing line implementing the invention. In particular, the offset is added when some variability in the size of the edge under-coating is observed as the result of unavoidable process variations during production on the galvanizing line. Equation (2) expresses the fact that, when an offset is added, the offset is small compared to the first term of Equation (1 ). If there is no offset added, Equation (2) does not apply. The ratio of Equation (2) can be as high as 100, 50, 30 or 20.
[0051]
[0027] The coefficients A, B and C can be set empirically based on past productions on the galvanizing line or on other galvanizing lines.
[0052]
[0028] The coefficient A can also be determined based on the expected cooling conditions of the steel strip S during cooling, at least from after wiping until the point where the molten Zn-based alloy reaches its liquidus. Preferably, A is determined based on the expected cooling conditions of the steel strip S during cooling, at least from after wiping until the point where the molten Zn-based alloy reaches its solidus. In particular, the A is determined based on the expected thermal path of the steel strip S during cooling, at least from after wiping until the point where the molten Zn-based alloy reaches its liquidus. More particularly, A is determined based on the expected thermal path of the steel strip S during cooling, at least from after wiping until the point where the molten Zn-based alloy reaches its solidus. The expected thermal path T(t) is the expected temperature variation of the steel strip during cooling.
[0053]
[0029] The expected cooling conditions, and in particular the expected thermal path, can be set empirically based on past productions on the galvanizing line or on other galvanizing lines.
[0054]
[0030] Alternatively, the coefficient A is determined based on preset parameters of the galvanized steel strip to be produced and preset parameters of the galvanizing line for producing the galvanized steel strip.
[0055]
[0031] In a first variant, A satisfies Equation (3):
[0056]
[0032] Accordingly, in that case, the inferred faulty edging E satisfies Equation (4): wherein As is a non-zero coefficient, Bs and Cs are coefficients, and wherein, when Bs0 and / or Cs0 , > 1.4
[0057]
[0033] In these equations, T(t) is the expected thermal path of steel strip S at least from after wiping and ts is the time in seconds taken for the molten Zn- based alloy to reach its solidus Tsat least from after wiping. T(t) and Tsare preferably expressed in °C or K. As, Bsand Cs can be obtained by calibration, i.e. by comparing, for different parameters of the galvanizing line and for different parameters of the galvanized steel strip, the inferred faulty edging E to the actual edge under-coating observed during production.
[0058]
[0034] It is recalled here that the solidus Ts is the temperature below which an alloy is in the completely solidified state. In other words, the solidus is the end of solidification. It is obtained from phase diagrams as a function of the Al and Mg contents in the molten Zn-based alloy.
[0059]
[0035] In a variant, T(t) is the expected thermal path of steel strip S after wiping and ts is the time in seconds taken for the molten Zn-based alloy to reach its solidus Ts from after wiping. In another variant, T(t) is the expected thermal path of steel strip S after it exits the bath and ts is the time in seconds taken for the molten Zn-based alloy to reach its solidus Ts from the bath exit.
[0060]
[0036] The expected thermal path T(t) and the time ts to reach Ts depend on preset parameters and can be estimated from these parameters, notably by numerical modelling.
[0061]
[0037] According to a first option, the expected thermal path T(t) is estimated from the preset temperature Tbath of the bath, the preset average cooling rates in at least the first two cooling boxes of the plurality of cooling boxes composing the cooling system, the length of each cooling box considered, the position of each cooling box considered and the preset line speed V. In that case, the time ts to reach Ts is deducted from the expected thermal path T(t).
[0062]
[0038] The preset average cooling rates are very dependent on the design of the cooling system of the galvanizing line and on the preset parameters. The person skilled in the art knows how to estimate the preset average cooling rates in each cooling box considered depending on that design and on the preset parameters. The preset average cooling rates can notably be estimated from a cooling model of the galvanizing line as a function of the preset temperature Tbath of the bath, the length of each cooling box considered, the position of each cooling box considered, the preset line speed V, the convective heat transfer coefficient Hnin each cooling box considered (which can be obtained from the power of the cooling box), the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip and the preset thickness Th of the steel strip.
[0063]
[0039] According to a second option, the expected thermal path T(t) and the time tsto reach Tsfrom wiping can be estimated from a cooling model of the galvanizing line as a function of the preset temperature Tbath of the bath, the length of at least the first two cooling box of the cooling system, the position of each of the at least first two cooling boxes, the preset line speed V, the convective heat transfer coefficient Hnin each of the at least first two cooling boxes (which can be obtained from the power of the cooling box), the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip and the preset thickness Th of the steel strip.
[0064]
[0040] In a second variant, A satisfies Equation (6):
[0065]
[0041] Accordingly, in that case, the inferred faulty edging E satisfies Equation (7): wherein Aiiq is a non-zero coefficient, Bnq and Ciiq are coefficients, and wherein, when BUq0 and / or CUq0 :
[0066]
[0042] In these equations, T(t) is an expected thermal path of steel strip S at least from after wiping and tiiq is the time in seconds taken for the molten Zn- based alloy to reach its liquidus Tiiq at least from after wiping. Anq, Biiq and Ciiq can be obtained by calibration, as described above for the first variant.
[0067]
[0043] It is recalled here that the liquidus Tiiq is the temperature above which an alloy is entirely in the molten state. In other words, the liquidus is the start of solidification. It is obtained from phase diagrams as a function of the Al and Mg contents in the molten Zn-based alloy.
[0068]
[0044] In a variant, T(t) is the expected thermal path of steel strip S after wiping and tiiq is the time in seconds taken for the molten Zn-based alloy to reach its liquidus Tiiq from after wiping. In another variant, T(t) is the expected thermal path of steel strip S after it exits the bath and tiiq is the time in seconds taken for the molten Zn-based alloy to reach its liquidus Tnq from the bath exit.
[0069]
[0045] The expected thermal path T(t) and the time tiiq to reach Tiiq depend on preset parameters and can be estimated from these parameters, notably by numerical modelling.
[0070]
[0046] In particular, the expected thermal path T(t) and the time tiiq to reach Tnq can be estimated as described above for the first variant.
[0071]
[0047] For both variants, the expected thermal path T(t) can be obtained from a cooling model of the galvanizing line, preferably integrated in the determination device, in particular in the prediction module of the determination device. It can also be made available to the operator or to the determination device in the form of an expected thermal path table. In the table, the expected thermal path, can be given as a function of some preset parameters. These parameters can notably be selected among the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of the plurality of cooling boxes composing the cooling system, the position of each cooling box considered, the preset line speed V, the convective heat transfer coefficient Hnin each cooling box considered, the preset power Pnof each cooling box considered, the preset average cooling rate in each cooling box considered, the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip or the preset thickness Th of the steel strip. This table can be built at any time before implementing the process. It can be built, once for all, for all possible formats of the galvanizing line. It can be built with the help of a cooling model of the galvanizing line.
[0072]
[0048] Similarly, the time ts, respectively the time tiiq, can be obtained from a cooling model of the galvanizing line integrated in the determination device, in particular in the prediction module of the determination device. The time ts, respectively the time tiiq, can also be made available to the operator or to the determination device in the form of a time table. In the table, the time ts, respectively the time tiiq, can be given as a function of some preset parameters. These parameters can notably be selected among the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of the plurality of cooling boxes composing the cooling system, the position of each cooling box considered, the preset line speed V, the convective heat transfer coefficient Hnin each cooling box considered, the preset power Pnof each cooling box considered, the preset average cooling rate in each cooling box considered, the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip, the preset thickness Th of the steel strip, the solidus Tsor the liquidus Tiiq. This table can be built at any time before implementing the process. It can be built, once for all, for all possible formats of the galvanizing line. It can be built with the help of a cooling model of the galvanizing line.
[0073]
[0049] In all variants and options described above, the cooling model of the galvanizing line preferably takes into account the thermal convection and the thermal radiative exchange in at least two cooling boxes of the plurality of cooling boxes composing the cooling system.
[0074]
[0050] Once the expected thermal path T(t) and the time ts, respectively the time tiiq, have been obtained, the inferred faulty edging E can be estimated and thus the initial strip width Wo of the steel strip to be provided on the galvanizing line can be determined. The estimation and determination can be done by the operator or by the determination device.
[0075]
[0051] As an alternative to the expected thermal path table and the time table, the coefficient A can be made available to the operator or to the determination device in the form of a proportionality constant table. In the table, A can be given as a function of some preset parameters. These parameters can notably be selected among the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of the plurality of cooling boxes composing the cooling system, the position of each cooling box considered, the preset line speed V, the convective heat transfer coefficient Hnin each cooling box considered, the preset power Pnof each cooling box considered, the preset average cooling rate in each cooling box considered, the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip, the preset thickness Th of the steel strip, the solidus Tsor the liquidus Tiiq. This table can be built at any time before implementing the process. It can be built, once for all, for all possible formats of the galvanizing line. It can be built with the help of a cooling model of the galvanizing line.
[0076]
[0052] Once the coefficient A has been obtained, the inferred faulty edging E can be estimated and thus the initial strip width Wo of the steel strip to be provided on the galvanizing line can be determined. The estimation and determination can be done by the operator or by the determination device.
[0077]
[0053] In another alternative, the inferred faulty edge E can be made available to the operator or to the determination device in the form of an inferred faulty edge table. In the table, E can be given as a function of some preset parameters. These parameters can notably be selected among the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of the plurality of cooling boxes composing the cooling system, the position of each cooling box considered, the preset line speed V, the convective heat transfer coefficient Hnin each cooling box considered, the preset power Pnof each cooling box considered, the preset average cooling rate in each cooling box considered, the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip, the preset thickness Th of the steel strip, the solidus Ts, the liquidus Tiiq or the preset coating weight CWP. This table can be built at any time before implementing the process. It can be built, once for all, for all possible formats of the galvanizing line. It can be built with the help of a cooling model of the galvanizing line.
[0054] Once the inferred faulty edging E has been obtained, the initial strip width Wo of the steel strip to be provided on the galvanizing line can be determined. The determination can be done by the operator or by the determination device.
[0078]
[0055] In another alternative, the initial strip width Wo can be made available to the operator or to the determination device in the form of an initial strip width table. In the table, Wo can be given as a function of some preset parameters. These parameters can notably be selected among the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of the plurality of cooling boxes composing the cooling system, the position of each cooling box considered, the preset line speed V, the convective heat transfer coefficient Hnin each cooling box considered, the preset power Pnof each cooling box considered, the preset average cooling rate in each cooling box considered, the temperature Tgasof the cooling gas, the emissivity e of the molten Zn-based alloy, the density p of the steel strip, the preset thickness Th of the steel strip, the solidus Ts, the liquidus Tnq, the preset coating weight CWPor the preset strip width WP. This table can be built at any time before implementing the process. It can be built, once for all, for all possible formats of the galvanizing line. It can be built with the help of a cooling model of the galvanizing line.
[0079]
[0056] Once the initial strip width Wo of the steel strip to be provided on the galvanizing line has been determined, the steel strip to be galvanized can be ordered to the hot-rolling mill or cold-rolling mill.
[0080]
[0057] In a second step of the process according to the invention, the steel strip S is provided with an initial strip width Wo as determined at a previous step. 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.
[0081]
[0058] In one variant, the steel strip S is a hot-rolled steel strip 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.
[0059] Once the steel strip S has been provided, it generally undergoes an annealing operation, in an annealing furnace, 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.
[0082]
[0060] In a third step of the process according to the invention, 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 comprising a molten Zn-based alloy contained in a tank.
[0083]
[0061] 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 20 m / min to 200 m / min, preferably ranging from 30 m / min to 160 m / min.
[0084]
[0062] 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 88.5 wt% Zn.
[0085]
[0063] Overall, the bath is of a molten Zn-based alloy comprising, in percentage by weight, at least of 0.5 wt% Al and at least of 0.2 wt% Mg.
[0086]
[0064] The weight percentage of Aluminium preferably ranges from 0.5 to 12 wt%, more preferably from 1 to 6 wt%, even 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.
[0087]
[0065] The weight percentage of Magnesium preferably ranges from 0.2 to 6 wt%, more preferably from 0.5 to 4 wt%, even 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.
[0066] 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.
[0088]
[0067] 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.
[0089]
[0068] Preferably, the molten Zn-based alloy comprises, in percentage by weight, from 0.5 to 12 wt% Al, from 0.2 to 6 wt% Mg, 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.
[0090]
[0069] The bath is usually maintained at a temperature ranging 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 550° C.
[0091]
[0070] In a fourth step of the process according to the invention, after having passed through the bath of molten Zn-based alloy, the steel strip S exits the bath coated on both its faces with the molten Zn-based alloy dragged out from the bath by the steel strip. The molten Zn-based alloy dragged out from the bath is wiped by means of wiping nozzles placed on each side of the strip S and spraying a wiping gas onto the surfaces of the strip. This conventional operation, well known to those skilled in the art, adjusts the coating weight of the molten Zn-based alloy dragged out from the bath by the steel strip.
[0071] In the context of the invention, the coating weight is adjusted according to the preset coating weight CWP. By these terms, it is meant that the wiping conditions are adjusted with the aim of obtaining the preset coating weight CWP. Nevertheless, as variations are inherent to such an industrial process, the invention is not limited to the case where the preset coating weight CWPis actually reached. Preferably, the variations are limited to ± 15% around the preset coating weight CWP.
[0092]
[0072] The preset coating weight CWP, and similarly the actual coating weight, is greater than or equal to 400 g / m2. It ranges preferably from 400 to 1600 g / m2, more preferably from 600 to 1200 g / m2.
[0093]
[0073] In the context of the invention, the wiping conditions are not limited. Preferably, the distance between the steel strip and the wiping nozzle ranges from 10 to 50 mm and the overpressure of wiping gas in the nozzle ranges from 20 to 150 mbar.
[0094]
[0074] In a fifth step of the process according to the invention, after wiping, the molten Zn-based alloy is cooled. Preferably, it is cooled according to the preset parameters, in particular the preset power(s) Pn of the cooling box(es), acquired at a previous step of the process. By these terms, it is meant that process parameters are adjusted with the aim of obtaining the expected cooling conditions, in particular with the aim of obtaining the expected thermal path T(t) and the time tsor tisq. Nevertheless, as variations are inherent to such an industrial process, the invention is not limited to the case where the expected cooling conditions are actually reached. Preferably, the variations are limited to ± 5% around the cooling rate given by the expected thermal path T(t).
[0095]
[0075] In the context of the invention, the cooling system is not limited. It preferably comprises at least one cooling box, more preferably a plurality of cooling boxes, through which the steel strip runs. In each cooling box n, a gas or a mixture of gases is blown on the surfaces of the steel strip. The cooling box(es) can fill only a portion of the cooling tower. In that case, outside of the cooling box(es), the molten Zn-based alloy is cooled by natural convection. The cooling system can be defined notably by the length of each cooling box, the position of each cooling box, the convective heat transfer coefficient Hn in each cooling box, the power Pn of each cooling box. The position of each cooling box can notably be the distance from wiping to each cooling box or the distance from the bath exit to each cooling box, along the strip path.
[0096]
[0076] 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.
[0097]
[0077] In a sixth step of the process according to the invention, after cooling, and after skin-pass if applicable, both edges of the steel strip are trimmed to obtain the preset strip width WP. Preferably, both edges are equally trimmed. The trimming can be done at the exit of the galvanizing line or on a separate line.
[0098]
[0078] 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 Zn-based alloy 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 coating weight of the molten Zn-based alloy I of the metallic coating is expressed and measured without taking the inhibition layer into account.
[0099]
[0079] Examples:
[0100]
[0080] Steel strips, of various preset thicknesses Th, were hot-dip coated in a molten bath comprising 3.7wt% Al and 3 wt% Mg, the balance being zinc and the unavoidable impurities resulting from the processing. The molten bath was maintained at various preset temperatures Tbath. The steel strips were moving at various line speeds V. The steel strips were then wiped to obtain various preset coating weights CWPof molten Zn-based alloy. The steel strips were then cooled in a cooling system comprising five cooling boxes positioned and dimensioned as follows:
[0101] - Cooling box 1 : entry point at 2.4m from the bath exit - length: 4m
[0102] - Cooling box 2: entry point at 6.9m from the bath exit - length: 6m - Cooling box 3: entry point at 13.2m from the bath exit - length: 6m
[0103] - Cooling box 4: entry point at 19.5m from the bath exit - length: 6m
[0104] - Cooling box 5: entry point at 25.85m from the bath exit - length: 6m
[0105]
[0081] The powers Pi, P2, P3, P4 and P5 of the cooling boxes were varied to obtain various cooling conditions. 0% means that there is no cooling medium blown in the cooling box. 100% means that the cooling box is working at its maximum capacity.
[0106]
[0082] The cooling medium was air at ambient temperature (25°C).
[0107]
[0083] After cooling, the steel strips were trimmed to obtain the preset strip width WP.
[0108]
[0084] All above-described parameters are detailed in Table 1 .
[0109]
[0085] A cooling model of the galvanizing line, which takes into account the thermal convection and the thermal radiative exchange in the five cooling boxes, was used to estimate the expected thermal path T(t) of each steel strip after it exists the bath and the time tisqfor the molten Zn-based alloy to reach its liquidus Tnqfrom the bath. For the cooling model, the inputs were: o The line speed V, in m / min, o The thickness Th of the steel strip, in mm, o The bath temperature Tbath, o The preset coating weight CWP, o The positions and length of the cooling boxes, o The preset powers Pnof the cooling boxes, o The temperature Tgasof the cooling medium (air): 298°K, o The density p of the steel strip: 7800 kg / m3, o The emissivity e of the molten Zn-based alloy: 0.08, o The liquidus of the molten Zn-based alloy: 347°C.
[0110]
[0086] To estimate the thermal path T(t = z / K), the height z£from the bath exit was discretized in Nz+ 1 cells of size dz such that z£= dz x i. Thus, T(z£= t£x 7) = T£.
[0111]
[0087] For each position i in the height, the convective heat transfer coefficient Hi was obtained as follows:
[0112] - If z£was positioned in a cooling box n, then Hi was equal to the preset convective heat transfer coefficient Hnin the cooling box n: / p \ 0.75
[0113] Hi 1 = Hnn = Cnn x 150 x H in W m-2K-1, \ioo7 with Cn a calibrated coefficient which depends on the maximum blowing power of the cooling box n,
[0114] - Else Hi = 12 W m-2K"1.
[0115] For these examples, Ci = 1 .66, C2=1 , Cs=1 , C4=1 .2 and Cs=1 .3.
[0116]
[0088] For each position i in the height, the expected thermal path T(t) was estimated with the following equations:
[0117] Csteel = -793.82 + Ti X (5.325 + TtX (-0.007331 + 0.0000037916
[0118] T =o = Tbath
[0119] Wherein:
[0120] - Ti is the temperature at position i in the height, expressed in K,
[0121] - Convi is the thermal convection at position i in the height,
[0122] - Radi is the thermal radiative exchange at position i in the height,
[0123] - o is the Stefan-Boltzmann constant with a value of 5.67 x 10"8W / (m2- K4),
[0124] - Csteei is the specific heat capacity of steel.
[0125]
[0089] The time tiiq was selected as the time where Ti reaches Tnq.
[0126]
[0090] The inferred faulty edging E was estimated with Equation (7) where Anq was 0.0723, Bnq was 0.027 and Cnq was 12.168, these constants having been obtained by calibration. For all examples, Equation (8) was satisfied.
[0127]
[0091] The initial strip width Wo was selected as WP+ 2xE rounded up to the nearest whole number.
[0128]
[0092] The inferred faulty edging E was also compared to the maximum edge under-coating (EUC) observed on the edges of the steel strips. The comparison is expressed as a deviation of E from EUC.
[0129]
[0093] These results are detailed in Table 2 and Figure 1 .
[0130]
[0094] As it is apparent from the results, Equation (7) accurately predicts the appearance of the edge under-coating. Consequently, the initial strip width Wo can be precisely selected to minimize the amount of material to be trimmed. As a result, a manufacturing line can reduce the amount of material trimmed at the end of the manufacturing by 20 to 80%, which improves its production yield accordingly.
[0131]
[0095] Alternatively, the initial strip width Wo of the above examples may have been selected as WP+ 4xE rounded up to the nearest whole number to remove both the edge under-coating and the overcoating adjacent to the under-coating.
[0132] Table 1 Table 2
Claims
CLAIMS1 ) Process for hot-dip plating a steel strip S on a galvanizing line, the process comprising:- (i) Acquiring WP, which is the preset strip width of steel strip S at the exit of the galvanizing line, and CWP, which is the preset coating weight of molten Zn-based alloy for the sum of both sides of the steel strip S, CWPbeing greater than or equal to 400 g / m2,- (ii) providing the steel strip S with an initial strip width Wo determined based on WPand CWPso that the difference between Wo and WPis at least two times bigger than an inferred faulty edging E satisfying Equation (1 ):wherein A is a non-zero coefficient, B and C are coefficients, and wherein, when > 1.4 (2),- (iii) dipping the steel strip S in a bath of a molten Zn-based alloy comprising, in percentage by weight, from 0.5 to 12 wt% of aluminium and from 0.2 to 6 wt% of magnesium,- (iv) wiping the molten Zn-based alloy dragged out from the bath by the steel strip S and adjusting the coating weight of the molten Zn-based alloy according to CWP,- (v) cooling the molten Zn-based alloy,- (vi) trimming both edges of the steel strip to obtain the preset strip width WP.2) Process according to claim 1 wherein the inferred faulty edging E satisfiesEquationwherein As is a non-zero coefficient, Bs and Cs are coefficients, wherein, when > 1.4 (5),wherein T(t) is an expected thermal path of steel strip S at least from after wiping and ts is the time in seconds taken for the molten Zn-based alloy to reach its solidus Tsat least from after wiping.3) Process according to claim 2 wherein the expected thermal path T(t) and the time ts to reach Ts are estimated from preset parameters acquired in step (i).4) Process according to claim 3 wherein the expected thermal path T(t) and the time ts to reach Ts are estimated from the preset temperature Tbath of the bath, the preset average cooling rates in at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the length of each of the at least first two cooling boxes, the position of each of the at least first two cooling boxes and the preset line speed V.5) Process according to claim 3 wherein the expected thermal path T(t) and the time ts to reach Ts are estimated from a cooling model of the galvanizing line as a function of the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the position of each of the at least first two cooling boxes, the preset line speed V, the convective heat transfer coefficient in each of the at least first two cooling boxes, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip, the preset thickness Th of the steel strip and the solidus Ts.6) Process according to claim 1 wherein the inferred faulty edging E satisfies Equation (7):wherein Anq is a non-zero coefficient, Bnq and Ciiq are coefficients, wherein, when BUq0 and / or CUq0 :> 1.4 (8)wherein T(t) is an expected thermal path of steel strip S at least from after wiping and tisqis the time in seconds taken for the molten Zn-based alloy to reach its liquidus Tnqat least from after wiping.7) Process according to claim 6 wherein the expected thermal path T(t) and the time tiiq to reach Tnqare estimated from preset parameters acquired in step (i).8) Process according to claim 7 wherein the expected thermal path T(t) and the time tiiq to reach Tiiq are estimated from the preset temperature Tbath of the bath, the preset average cooling rates in at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the length of each of the at least first two cooling boxes, the position of each of the at least first two cooling boxes and the preset line speed V.9) Process according to claim 7 wherein the expected thermal path T(t) and the time tiiq to reach Tiiq are estimated from a cooling model of the galvanizing line as a function of the preset temperature Tbath of the bath, the length of at least the first two cooling boxes of a plurality of cooling boxes composing the cooling system, the position of each of the at least first two cooling boxes, the preset line speed V, the convective heat transfer coefficient in each of the at least first two cooling boxes, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip, the preset thickness Th of the steel strip and the liquidus Tiiq.10)Process according to claim 1 wherein the inferred faulty edging E is obtained from an inferred faulty edge table as a function of a plurality of preset parameters acquired in step (i).11 )Process according to claim 9 wherein the plurality of preset parameters is selected among the preset temperature Tbath of the bath, the length of each cooling box of the cooling system, the position of each cooling box, the preset line speed V, the convective heat transfer coefficient in each cooling box, thepreset power of the cooling box, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip, the preset thickness Th of the steel strip, the solidus Ts, the liquidus Tnq or the preset coating weight CWP.12) Process according to any one of the preceding claims wherein both edges are equally trimmed.13)Process according to any one of the preceding claims wherein the difference between Wo and WPis from two to four times bigger than the inferred faulty edging E.14) Process according to any one of claims 1 to 12 wherein the difference between Wo and WPis from four to six times bigger than the inferred faulty edging E.15) Determination device comprising at least a processor and a memory, configured to execute the following steps in relation to a galvanizing line comprising a bath of molten Zn-based alloy for hot-dip plating a steel strip S:- Acquiring WP, which is the preset strip width of the steel strip S at the exit of a galvanizing line, and CWP, which is the preset coating weight of molten Zn-based alloy for the sum of both sides of the steel strip S, CWPbeing greater than or equal to 400 g / m2,- Determining the initial strip width Wo of the steel strip to be provided at the entry of the galvanizing line based on WPand CWPso that the difference between Wo and WPis at least two times bigger than an inferred faulty edging E satisfying Equation (1 ):wherein A is a non-zero coefficient, B and C are coefficients, and wherein, when > 1.4 (2).16) Determination device according to claim 15 further configured to acquire the preset temperature Tbath of the bath, preset average cooling rates in at least two cooling boxes of a plurality of cooling boxes composing the cooling system, the length of each of the at least two cooling boxes, the position of each of the at least two cooling boxes and the preset line speed V.17) Determination device according to claim 16 further configured to acquire the preset temperature Tbath of the bath, the length of each cooling box of the cooling system, the position of each cooling box, the preset line speed V, the convective heat transfer coefficient in each cooling box, the temperature of the cooling gas, the emissivity of the molten Zn-based alloy, the density of the steel strip and the preset thickness Th of the steel strip.18) Determination device according to any one of claims 14 to 17 further configured to determine the initial strip width Wo of the steel strip to be provided at the entry of the galvanizing line with the inferred faulty edging E satisfying Equation (4):wherein As is a non-zero coefficient, Bsand Cs are coefficients, wherein, whenBs0 and / or Cs0 , > 1.4 (5),wherein T(t) is an expected thermal path of steel strip S at least from after wiping, ts is the time in seconds taken for the molten Zn-based alloy to reach its solidus Tsat least from after wiping and both T(t) and ts are estimated from the preset parameters acquired in step (i).19) Determination device according to any one of claims 14 to 17 further configured to determine the initial strip width Wo of the steel strip to be provided at the entry of the galvanizing line with the inferred faulty edging E satisfying Equation (7):wherein Anq is a non-zero coefficient, Bnq and Ciiq are coefficients, wherein, when BUq0 and / or CUq0 :wherein T(t) is an expected thermal path of steel strip S at least from after wiping and tisqis the time in seconds taken for the molten Zn-based alloy to reach its liquidus Tnq at least from after wiping and both T(t) and tsare estimated from the preset parameters acquired in step (i). )Galvanizing line comprising a bath of molten Zn-based alloy for hot-dip plating a steel strip S and a determination device according to any one of claims 15 and 19.