RU2463379C2 - Method of galvanisation by submersion of steel strip - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: in the method a strip is submerged into a galvanising tank with liquid mixture of metals, such as zinc and aluminium, continuously circulating between the specified galvanising tank and a preparation device, the first capacity is determined as provided by a metal strip entering at the first temperature (T₁) into a bath of liquid mixture of the galvanising tank, and the specified bath is stabilised at the second predetermined temperature (T₂), lower than the first temperature (T₁), the second capacity is identified, required to maintain the liquid mixture at the second predetermined temperature (T₂), and this second capacity is compared with the first capacity provided by the strip. If the first capacity is higher than the second capacity, for the first temperature of the strip the specified reduction value is applied, and if the first capacity is below or equal to the second capacity, energy is identified, which is required for continuous melting of an ingot blank in the device in the amount required to compensate the liquid mixture spent when applied onto the strip. In the method the liquid mixture circulation flow between the galvanising tank and the preparation device is corrected, thus maintaining the liquid mixture temperature in the preparation device at the value of the predetermined third temperature (T₃), lower than the predetermined second temperature (T₂), and the fourth temperature (T₄) of the liquid mixture is corrected at the outlet of the preparation device, in order to ensure additional capacity required for thermal balance between the specified outlet and inlet of the galvanising tank supply, at the same time the specified inlet is supplied from the outlet.

EFFECT: method improvement.

27 cl, 10 dwg

Description

The present invention relates to a method of galvanizing by immersion of a steel strip, characterized in the restrictive part of paragraph 1 of the claims.

Immersion galvanizing of continuously moving steel strip steel is a known technology, which basically contains two options, in one of which the strip exiting the galvanizing furnace is lowered at an angle into a molten metal bath containing at least one galvanized metal, such as zinc, aluminum, and then deflected vertically upwards by means of a roller immersed in said molten metal bath. In another embodiment, the strip is deflected vertically upstream of the furnace and then moved in a vertical channel containing molten zinc held by magnetic levitation. A liquid metal bath is a zinc alloy with varying amounts of aluminum, magnesium, or manganese. To simplify the patent, the description only applies to the use of an alloy of zinc and aluminum.

In both cases, the operation is designed to perform on the surface of the steel strip a continuous and tight-fitting coating of a liquid mixture of zinc and aluminum, in which the said strip is moved. The kinetics of the formation of this coating is well known to those skilled in the art and has been disclosed in many publications, including “Modeling of galvanizing reactions” by Georgi et al., “La Revue de Métallurgie-CIT”, October 2004. According to this source, contact with a liquid mixture occurs the dissolution of iron from a steel strip, which, on the one hand, is involved in the formation of a combined layer of a Fe ₂ Al ₃ Zn _x compound about 0.1 μm thick on the strip surface and, on the other hand, diffuses in the direction of the molten metal bath until it forms a solid layer Fe ₂ Al ₃ Zn _x . The Fe ₂ Al ₃ Zn _x layer serves as a substrate for the final zinc protective layer, while dissolved iron contributes to the formation of precipitates in the liquid mixture consisting of Fe iron, aluminum Al and zinc Zn and called “mattes” or “drosses”. These depositions in the form of particles of several microns can lead to the appearance of surface defects on the coated strip (galvanized strip), which can be irreparable, in particular, if we are talking about steel strips designed to perform external car body elements. Metallurgists made a lot of efforts to limit or eliminate the formation of dross in galvanizing baths. The phenomenon of the formation of drosses is known to specialists, for example, from the publication “Numerical simulation of the rate of dross formation in continuous galvanizing baths” Ajersch et al. Depending on the temperature of the liquid zinc bath and on the aluminum content in it, the amount of iron that can dissolve varies widely. When the iron content exceeds the solubility limit, nucleation and growth of certain Fe-Al-Zn compounds become possible. In conventional continuous galvanizing processes, a galvanizing bath containing a liquid mixture intended to be applied to the strip remains continuously saturated with iron, as a result of which all the iron dissolved from the strip and diffusing into the liquid mixture immediately participates in the local formation of drosses.

Among the means provided for controlling the formation of drosses, or at least to reduce their number in a zinc bath, they have long resorted to the manual removal of hardened parts of the melt from the surface of the liquid mixture. This method is rightly considered dangerous for operators, and it was proposed to mechanize and then robotize this hardened part removal operation, as described in JP 2001-064760.

Other methods using overflow, pumping, or emissions have been proposed to remove the dross generated in the zinc bath. So, in document EP 1070765 describes a number of options galvanizing installation, containing, in addition to the galvanizing tank, which forms dross, an auxiliary tank, which remove dross.

In EP 0429351, the inventors went even further and proposed a method and apparatus for circulating a liquid metal mixture between a coating zone on a metal strip and a cleaning zone of a zinc bath containing liquid zinc, to ensure separation of the dross in the cleaning zone and for delivery to the galvanizing zone liquid mixture, "the iron content of which is close to or below the solubility limit." However, having described the current physical principles, this document does not give any indication allowing a specialist to apply these principles, in particular, how to simultaneously control cooling by means of a heat exchanger and induction heating of the same cleaning zone. The document also does not contain any indication of a means for determining the flow rate of liquid zinc.

The present invention is intended to provide a galvanizing method by immersion of a steel strip in a liquid mixture, in which the circulation circuit of the liquid mixture is thermally optimized.

This method can be applied using the methodology proposed in paragraph 1 of the claims.

To more clearly illustrate aspects of the proposed method in accordance with the present invention in FIG. 1 and 2 show the installation for galvanizing by immersion of a steel strip in a liquid mixture and one of its variants, allowing to apply the proposed method.

FIG. 1 is a schematic diagram of an installation in which the proposed method is used.

FIG. 2 is a schematic diagram of an installation option in which the proposed method is used.

In FIG. 1 shows a schematic diagram of an apparatus for applying the method in accordance with the present invention. The steel strip (1) is fed into the installation, ideally, continuously, introducing it at an angle into the galvanizing tank (2) through the channel (3) of the connection to the galvanizing furnace (not shown in the figure and is located in front of the galvanizing tank). The strip is deflected vertically by means of a roller (4) and passed through the liquid mixture (5) of the coating contained in said zinc tank. The deviation of the strip can be carried out using a horizontal roller (4), accompanying the movement of the strip. Channel (6) allows the excess liquid mixture to flow into the preparation device (7) containing two zones: the first zone (71), in which at least one ingot of Zn-Al alloy (8) is melted in an amount sufficient to make up liquid mixture consumed during deposition onto a strip in a zinc tank and inevitable losses (material), and a second zone (72) located immediately after the first zone in the direction of flow of the liquid mixture (zinc tank, then the first zone, then the second zone). These two zones can be in the same tank, as shown in FIG. 1, and in this case they are separated by a separation device (73), such as a wall open in its central part, or can be two separate tanks installed next to each other. Between these two separate and adjacent tanks, the liquid mixture can be moved by pumping or through the connecting channel. Preferably, the level of the start of pumping in the first zone (71) or the input level of the connecting channel is between the upper zone (81) of separating the surface dams and the lower zone (82) of sedimentation of bottom drosses, i.e. in the central third of the height of the zone (71). Indeed, according to the method in accordance with the present invention, at this central height of the preparation device, it is possible to allocate an intermediate space free of drosses between two zones, lower and upper, for accumulating (gradually increasing in the direction of flow (FL)) of said drosses (81, 82).

The liquid mixture emerging from the zinc tank has a sufficiently high temperature to melt the ingot. The energy consumption for melting the ingot leads to cooling of the liquid mixture, which results in the formation of surface (81) and bottom (82) drosses, delayed by the sealed parts at the outlet of the preparation device (73). Between the zinc tank and the preparation device, for example, on their connecting channel (6), it is possible to place means (62) for additional cooling with the cooling effect due to the consumption of ingots. A purified liquid mixture passes into the second zone (72) of the preparation device, which can be heated using heating means (75), preferably induction heating. The nozzle (9) draws the liquid mixture in the second zone (72) and in the case shown in FIG. 1, by means of a pumping device (10) and a return flow pipe (11), it is possible to re-supply the zinc tank (2) through the chute (12) depending on the flow rate of the purified liquid mixture. Devices, for example, such as systems for removing hardened parts or pumping out, allow the dross to be removed from the preparation device (first zone (71)). Preferably, the first zone (71) of the preparation device may comprise partitions separating portions of the liquid mixture and located between several ingots (8) sequentially located in the flow direction. These partitions can be made in the form of a wall open in its central part, which allows one to concentrate bottom (82) and surface (81) drosses for each ingot depending on the aluminum content in them.

Regarding the process of melting the ingots, the first zone (71) of the preparation device preferably contains several ingots (8 ₁ , 8 ₂ , ..., 8 _n ), at least two of which have different aluminum contents, and at least one of the ingot has a content higher than the required content in the liquid mixture in the preparation device. In addition, the first zone (71) of the preparation device comprises means for controlling the melting rate of at least two ingots, preferably by selectively dipping or removing at least one ingot in the first zone (71). Finally, the first compartment of the preparation device may comprise means (6, 62) for regulating a predetermined lowering of the temperature (T2, T3) of the liquid mixture in which the ingots are melted, preferably also by selective immersion or extraction of at least one ingot in the first zone ( 71).

In light of this solution, continuous melting of the ingots (8) in the preparation device is ensured at a total melting rate of at least two ingots. In this case, a set of n ingots is preferably simultaneously immersed in the bath of the liquid mixture, each of them having a different aluminum content, with at least one of them having an aluminum content higher than the required content in the preparation device in order to establish the profile content (or melting rate) that varies over time. This required content itself is determined on the basis of the measured or estimated consumption of aluminum in the zinc tank in the combined Fe ₂ Al ₃ Zn _x layer formed on the strip surface and in the drosses formed in the preparation device. Preferably, the melting rate of each of the n ingots can also be individually controlled in such a way as to bring the aluminum content in the preparation device to the desired value, while maintaining the overall required melting rate.

Continuous melting of the ingots in the preparation device leads to local cooling of the liquid mixture from the second temperature (at the outlet of the galvanizing tank) to a predetermined temperature in the first zone (71) in order to reduce the solubility threshold of iron and ensure local formation of dross in the said preparation device not higher than the solubility threshold at a predetermined temperature. The so-called “surface” drosses with a high aluminum content mainly form near submerged ingots with a high aluminum content, then float to the surface, and the so-called “bottom” drosses with a high aluminum content predominantly form near submerged ingots with a high aluminum content, then settle to the bottom .

After the formation of drosses, the recirculation flow of the liquid mixture entering the zinc tank, when the iron content is equal to the solubility threshold of iron at a predetermined temperature, allows you to limit the content of dissolved iron to a value below the solubility threshold at the second temperature.

Thus, the preparation device (7) can contain only one tank, consisting of two zones (71, 72), separated by a dividing wall (73), while the first zone provides melting of the ingots and localizes the formation of dross, and the purified liquid enters the second zone mixture. In this case, the second zone is equipped with a single and simple means (75) of induction heating, which ensures heating of the purified liquid mixture before it is again fed to the zinc tank to close the thermal cycle of the reverse flow at the end of the flow path to the beginning of a new flow. Two zones (71) and (72) can also be located in two separate tanks connected by a connecting channel.

In FIG. 2 shows a variant of the circuit diagram of the installation shown in FIG. 1, in which the initial galvanizing tank is divided into a strip deflection tank (15) (without a liquid mixture) and a zinc tank (13) containing a liquid mixture bath (5) held by magnetic levitation. Basically, this installation uses a variant of the method in which the bath of the liquid mixture (5) is held due to magnetic levitation in a zinc tank (13) connected to the preparation device shown in figure 1. As is known, the effect of levitation is provided by electromagnetic devices (14). The compartment (15) provides connection to the furnace and the deflection of the strip (1) using the roller (4).

For reasons of clarity and according to the example shown in FIG. 1, the main parameters of the method are presented in FIG. 3.

FIG. 3 - distribution of temperatures, values of aluminum and dissolved iron in the installation circuit.

At the top of FIG. 3 shows a simplified example of the installation shown in FIG. 1 and containing the already mentioned basic elements (zinc tank 2 and its inlet 12 for the reverse flow of the liquid mixture, ingots 8, preparation device 7, the tank for melting ingots in the first zone 71, the cleaning tank in the second zone 72 and its outlet 11, heating means 75), which allows you to best imagine the implementation of the method in accordance with the present invention.

Three distribution profiles are also shown under the setup diagram — temperature T, aluminum Al% and dissolved iron Fe% associated with the solubility threshold of iron SFe — which are obtained by implementing the method in accordance with the present invention. The profiles shown also vary depending on the location in the direction of the flow path from the inlet 12 of the galvanizing tank 2 to the outlet 11 of the cleaning tank 72. It should be noted that outlet 11 is connected to the inlet 12 of the reverse flow channel of the liquid mixture, a separate and opposite flow path. Thus, the invention allows you to align the profile values between the inlet and outlet, as well as between different tanks on the flow path, to ensure the closure of the thermal cycle, as well as the exact maintenance of the required values of aluminum and iron (below the solubility threshold corresponding to a given temperature).

The liquid mixture in the zinc tank (2) near the immersion strip is maintained at the so-called second temperature (T ₂ ). At the inlet (12) of the galvanizing tank (2), which is distant from the immersion zone, the temperature may be lower than the second temperature (T ₂ ), since it is connected with the outlet 11 of the cleaning tank (72) and with the return flow path, where heat losses are unavoidable, although they do not affect the process. Indeed, when the strip is immersed in the liquid mixture of the galvanizing tank, it is envisaged that the strip is at a so-called first temperature higher than the desired second temperature (T ₂ ), therefore, the mentioned second temperature (T ₂ ) can be reached without difficulty, since the strip affects bath liquid mixture due to heat transfer. In addition, the second required temperature (T ₂ ) of the liquid mixture at the outlet of the galvanizing tank and, therefore, at the entrance to the first zone (71) is chosen high enough to ensure the melting of the ingots (8).

The energy consumption necessary for melting the ingots (8) in the first zone (71) of the preparation device (7) leads to a decrease in the second temperature (T ₂ ) of the liquid mixture coming from the zinc tank to the required value, called the third temperature (T ₃ ) . In the second zone (72) of the preparation device (7), the heating means (75) gives, if necessary, power (ΔP = PZ-PB), which raises the temperature of the liquid mixture from the third temperature (T ₃ ) to the fourth temperature (T ₄ < T ₂ ), which must be chosen high enough to compensate for losses in the return flow path and to comply with the temperature requirements at the inlet (12) of the galvanizing tank. Thus, the closure of the thermal cycle is quite simple. The thermal process is regulated only by the strip and, if necessary, by means of heating (75) by adding energy. If at the outlet of the cleaning tank (72) there is no need to add energy, the heating means (75) are turned off.

Between the inlet (12) and the outlet of the galvanizing tank (2) in the direction of the first zone (71), the aluminum content (Al%) in the liquid mixture decreases (Al _c ) depending on the intensity of the loss in the combination layer and passes from the first content value (Al _t ) (the aluminum content in the liquid mixture after the ingots are melted in the preparation device and then by cleaning (second zone 72) and the return flow, i.e., the aluminum content in the liquid mixture sent to the inlet (12) of the zinc tank) to the second content value (Al _v ) at the outlet of the galvanizing tank (2). After passing the outlet of the zinc tank (2), controlled melting of the ingots provides an increase (Al _l ) in the content (or consumption per unit time) of aluminum to the content (Al _m ) in the liquid mixture at the outlet of the first zone (71). At the same time, this last content (Al _m ) should be considered as virtual, since simultaneously with the addition of aluminum from ingots, a part of aluminum is inevitably consumed when drosses appear, which leads to a real decrease in (Al _d ) aluminum content depending on the flow rate to the content value aluminum (Al _t ) in the cleaning tank (second zone 72) corresponding to (and equal to) the aluminum content at the inlet 12 of the return flow to the zinc tank.

In the zinc tank (2), under the influence of temperature and aluminum fluctuations, the solubility threshold of iron (SFe) in the liquid mixture is almost stable in the value (SFe T ₂ ) at the second temperature (T ₂ ), then it significantly decreases to the value (SFe T ₃ ) at the third temperature (T ₃ ) in the melting zone of the ingots and again undergoes an increase to the value (SFe T ₄ ) at the fourth temperature (T ₄ ) in the zone of the heating agent (75) until it returns to the zinc tank (2).

As for the iron content (Fe%) in the liquid mixture, it rises in the zinc tank (2) to a level that remains below the solubility threshold of iron (SFe T ₂ ) in the liquid mixture at the second temperature (T ₂ ) and remains at that level before the deposition of drosses in the first ingot melting zone (71), then reaching a value equal to the saturation threshold with iron (SFe T ₃ ) of the liquid mixture at the third temperature in this first zone. The shaded area (Dross) of the diagram between the curves of changes in the iron content (Fe%) and the solubility threshold of iron (SFe) in the liquid mixture allows us to identify the area of deposition of drosses. Ultimately, in the second cleaning zone (72), the solubility threshold of iron (SFe) of the liquid mixture rises to a higher value (SFe T ₄ ) at the fourth temperature (T ₄ ) (higher than in the first zone 71). In this case, the deposition of drosses is locally avoided so that the liquid mixture in the cleaning tank remains clean and can be returned to the inlet of the galvanizing tank (2), which does not contain any dross.

The description is also accompanied by figures that complement the previous figures and allow you to better imagine and understand the method in accordance with the present invention.

FIG. 4 is a diagram of the solubility of iron (Fe%) in a liquid mixture as a function of temperature (T) and aluminum content (Al%).

FIG. 5 is a part of the solubility diagram of iron (Fe%) in a liquid mixture depending on temperature (T) at a given aluminum content (Al% = 0.19%).

FIG. 6 is a diagram of changes in power (RV) added to the liquid mixture by a moving steel strip and the required power (PZ) to ensure the liquid mixture is melted in the zinc tank (2).

In FIG. Figure 4 shows that at a given temperature (in this case, from T = 440 ° C to T = 480 ° C), the solubility limit of iron (Fe%) in the Zn-Al liquid mixture increases when the aluminum content (Al%) decreases, and at given aluminum content, it rises with temperature. Thus, there are two means to control the solubility limit of iron: change the aluminum content or the temperature of the liquid mixture.

In FIG. Figure 5 shows the change in solubility limit (Fe%) as a function of temperature (T) with an aluminum content (Al%) of 0.19%. At a temperature of T = 470 ° C (point A) of the zinc bath (2), the solubility limit of iron (Fe%) is about 0.015%. At a temperature of T = 440 ° C (point B) below normal, the solubility limit of iron (Fe%) is about 0.07%. Thus, in a liquid mixture saturated or close to the saturation limit at an operating temperature of 470 ° C, the solubility limit is halved at 440 ° C. Provided that it is possible to extract all the drosses formed from the iron released from the solution at this temperature 440 ° C, the content of the remaining dissolved iron is reduced to 0.07%. Heating from this state to 470 ° C allows, thus, without precipitating drosses, to additionally dissolve 0.08% of the iron from the strip intended for coating.

In FIG. Figure 6 shows the changes in power (RV) added to the liquid mixture by a moving steel strip and in the power (PZ) necessary to ensure melting of the mixture consumed in the zinc tank (2). These capacities (PB, PZ) are limited by two parameters characteristic of continuous galvanizing plants: the heating power of the furnace (not shown in FIG. 1, but located in front of the galvanizing tank), on the one hand, and the maximum speed at which the strip treatment remains effective. For example, these limits are approximately 100 tons of treated strip per hour for one furnace (after the strip enters the galvanizing tank) and a little more than 200 m / min of strip speed during processing (at the outlet of the strip from the galvanizing tank). In the presented example, for a strip with a width (L) of 1200 mm and at a strip temperature of 485 ° С, the curve (shown by a dotted line) of the power, also called the power of the “strip” (PB), continuously rises, depending on the thickness (E) of the strip, to horizontal area corresponding to the heating limits of the furnace. The curve (solid line) of the required power (PZ) is first limited by the maximum speed of the strip, which, in turn, is limited by the maximum processing speed, then gradually decreases. For a thickness (E) of 1.2 mm strip and a coating thickness of 15 μm, the power (RV) added by the strip is lower than the power (PZ) required for zinc melting (PZ> PB), and therefore it is necessary to add a power difference (ΔР) by heating the liquid strip during circulation, in particular, until it returns to the zinc tank (2). This power difference in this case is considered as a necessary increase in power (ΔP> 0). Of course, it is also possible to provide for a decrease in power (ΔP <0), in which case it is necessary to change at least one of the parameters affecting the power (furnace temperature, strip speed, etc.) in order to reduce the power added to liquid mixture, while ensuring the melting of the mixture consumed in the zinc tank (2). If necessary, a cooling system can be connected to the galvanizing tank.

Based on the previous figures, it is possible to propose a method in accordance with the present invention, that is, a method of galvanizing by immersion of a continuously moving strip (1) of rolled steel, according to which the strip is immersed in a zinc tank (2) containing a bath (5) of a liquid mixture applied to the strip of metals, such as zinc (Zn) and aluminum (Al), continuously circulating between said zinc tank and preparation device (7), in which the temperature of the liquid mixture is intentionally lowered to lower the solubility threshold of iron, but is maintained residually high to activate in said training device melting, at least one ingot Zn-Al (8) in an amount sufficient to compensate for the liquid mixture, when applied to a consumable strip, and unavoidable loss (about 5%).

Said method comprises the following steps:

- determine the first power (RV) provided by the steel strip entering at the first temperature (T ₁ ) into the bath of the liquid mixture of the galvanizing tank, while said bath is stabilized at a second predetermined temperature (T ₂ ) less _{than the} first temperature (T ₁ ),

- determine the second power (PZ) necessary to maintain the liquid mixture at a second predetermined temperature (T ₂ ), and this second power is compared with the first power (PB) provided by the strip,

- if the first power (RV) is higher than the second power (PZ), for the first temperature (T ₁ ) of the strip, the set lowering value is applied,

- if the first power (RV) is lower than or equal to the second power (PZ), determine the energy required for continuous melting of the ingot (8) in the preparation device in the amount necessary to compensate for the liquid mixture consumed when applied to the strip, as well as any associated losses ,

- adjust the flow rate (Q ₂ ) of the liquid mixture between the zinc tank and the preparation device to provide the energy necessary for continuous melting of the ingot (8), while maintaining the temperature of the liquid mixture in the preparation device in the value of the third temperature (T ₃ ), lower in advance a certain second temperature (T ₂ ),

- adjust the fourth temperature (T ₄ ) of the liquid mixture at the outlet (9) of the preparation device to provide additional power (ΔP = PZ-PB) necessary for thermal equilibrium between the said output and the input (12) of the galvanizing tank power supply, while the said input powered by output (9).

Thus, the method provides the flow rate of the liquid mixture in a continuous and sequential mode on the flow path between the inlet of the galvanizing tank and the output of the preparation device, then on the identical return flow path, different and opposite flow paths. This circulation flow rate is also thermally optimized, as it cyclically closes (flow, return flow) for each necessary heat transfer, that is, it is controlled with sufficient accuracy.

By controlling the second temperature (T ₂ ) and the desired aluminum content (Al _v ), it is possible to control the solubility threshold (SFe T ₂ ) of iron at the second temperature (T ₂ ) in the bath (zinc tank) at such a level that, taking into account the expected rate of dissolution of iron ( QFe) in the zinc tank, maintain the total iron (Fe ₂ ) content below the solubility threshold (SFe T ₂ ) at the second temperature (T ₂ ). Thus, since the galvanizing tank remains free of drafts, the coating is of impeccable quality. To this end, by controlling the second temperature (T ₂ ) and the desired aluminum content (Al _v ), the solubility threshold (SFe T ₂ ) of iron at the second temperature (T ₂ ) in the liquid mixture of the zinc tank is controlled at such a level that, taking into account the expected rate of dissolution of iron (QFe) in the zinc tank, maintain the total iron (Fe ₂ ) content below the solubility threshold (SFe T ₂ ) at the second temperature (T ₂ ).

Preferably, the continuous melting of the ingots is provided at a total melting rate (Vm) of at least two ingots.

With regard to melting, as shown in FIG. 1 (or 2), preferably a variable number (n) of ingots is selectively and simultaneously immersed in the bath of the liquid mixture. Preferably, each of the ingots has a different aluminum content (Al ₁ , Al ₂ , ..., Al _n ), and at least one of the ingots has an aluminum content higher than the required content (Al _t ) in the preparation device (in particular, in the second zone 72 containing purified mixture). Thus, saving or obtaining the desired value of the aluminum content in the zones of the preparation device can be carried out more flexibly and more accurately.

For this set of (n) ingots, it is also possible to individually control the rate of immersion (V ₁ , V ₂ , ..., V _n ) of each of (n) ingots in order to dynamically adjust the aluminum content in the preparation device to the desired content value (Al _t ), while maintaining the required total melting rate (= flow rate) (Vm).

If necessary, you can activate the means of cooling the liquid mixture from the second temperature (T ₂ ) to the third temperature (T ₃ ) as an auxiliary system in the overall cooling achieved by melting ingots. Such additional cooling means allows for better control flexibility in accordance with the present invention.

Advantageously, it is possible to divide into compartments between ingots and according to their respective aluminum content in order to separate different types of gross, the so-called “surface” gross high-aluminum grosses are preferably formed near submerged high-aluminum ingots, and the so-called “bottom” ones »Low-aluminum grasses are preferably formed near submerged low-aluminum ingots. This division into compartments is realized by simply adding partitions between ingots on the surface and at the bottom of the first zone (71).

The method in accordance with the present invention provides for the regulation of the required flow rate of liquid zinc, that is, the recovery rate of the liquid mixture entering the zinc tank, with an iron content equal to the solubility threshold (SFe T ₃ ) of iron at a third temperature (T ₃ ) to limit the increase in the content dissolved iron is much lower than the solubility threshold at a second temperature (T ₂ ) in a zinc tank. This allows you to maintain the amount of dissolved iron coming from the strip in the interval between the solubility threshold (SFe T ₃ ) of iron at the third temperature (T ₃ ) and the solubility threshold (SFe T ₂ ) of iron at the second temperature (T ₂ ).

The closed loop for controlling the first power (RV) provided by the strip controls the addition or decrease of power (ΔP), which leads to an equilibrium in which the first power (RV) is equal to the sum of the second power (PZ) and the addition or decrease of power (ΔP), then there is at which PB = PZ + ΔP. This is achieved by issuing a command for a given decrease (or increase) in the temperature of the strip (T ₁ ) at the entrance to the galvanizing tank.

The method involves the equipment of the preparation device with additional adjustable calorie removal and removal means associated with an adjustable induction heating means, configured to modulate a third temperature (T ₃ ) in the melting zone of the ingots in the temperature range +/- 10 ° C, values close to the temperature defined by external means of regulation or control.

From the point of view of the thermal process, the method assumes that the first temperature (T ₁ ) of the steel strip at the entrance to the zinc tank is ideally between 450 and 550 ° C. Similarly, the second temperature (T ₂ ) of the liquid mixture in the zinc tank should ideally be between 450 and 520 ° C. For maximum efficiency of the method, the temperature difference (ΔT ₁ ) between the steel strip and the liquid mixture in the zinc tank is maintained in the range from 0 to 50 ° C. Thus, ideally, the second temperature (T ₂ ) of the liquid mixture in the zinc tank is maintained with an accuracy of +/- 1 ° C to +/- 3 ° C in the value (T ₁ -ΔT ₁ ) equal to the first temperature (T ₁ ) minus the temperature difference (ΔT ₁ ) between the steel strip and the liquid mixture. Finally, a decrease in temperature (ΔT ₂ = T ₂ -T ₃ ) between the second and third temperature of the liquid mixture in the preparation device is maintained at a value not lower than 10 ° C. With values of zinc, aluminum and iron, these values provide an optimal closed thermal cycle in the circuit (flow / return) of the circulation used for the galvanizing method in accordance with the present invention.

The method provides that the flow rate (Q ₂ ) of the liquid mixture coming from the zinc tank is constantly from 10 to 30 times the amount of the mixture applied to the strip for the same unit of time.

The method in accordance with the present invention also provides for the implementation of the steps of measurement and control, providing regulation / maintenance of a closed thermal cycle, the circulation circuit and the desired values of the content of aluminum, zinc and iron.

In particular, the temperature and the concentration of aluminum in the liquid mixture are measured, preferably continuously, at least in the flow path from the feed inlet (12) in the zinc tank to the outlet (11) of the preparation device. These values are decisive for inclusion in the diagrams of the content of aluminum or iron, depending on the location of the liquid mixture in the closed loop.

Preferably, the level of the liquid mixture is continuously measured in the preparation device and even, if necessary, in the zinc tank. This allows you to adjust the consumption of melting ingots and control the amount of metal deposited on the strip.

In practice, the flow rate (for example, the aluminum content per unit time) and the temperature of the liquid mixture are maintained in pairs of predetermined values by simple regulation. This allows, for example, to simply derive from a diagram (such as the diagrams shown in FIGS. 1 and 2) and quickly obtain the ideal solubility threshold (iron) for a pair of values.

The method includes a function in which the temperature of the strip at the outlet of the galvanizing furnace, connected to the inlet of the galvanizing tank, is maintained in the range of adjustable values. Similarly, the speed of the strip is maintained in the range of adjustable values. Ideally, the method involves measuring or evaluating the width and thickness of the strip at the entrance to the galvanizing tank, if they were not defined as primary input parameters (Primary Data Input PDI) in the control system of the galvanizing plant. These parameters are necessary to determine the entry conditions, in particular in connection with the power provided by the strip in the circulation loop, controlled by the method in accordance with the present invention.

In order to be able to modulate the melting rate of each of the ingots, the introduction and retention of the ingots in the melting zone of the preparation device is carried out dynamically and selectively.

Thus, the method in accordance with the present invention is carried out depending on the dynamic measurement and regulation parameters associated with the strip, with the galvanizing tank and with the preparation device. Ideally, these parameters are controlled centrally and autonomously according to the analytical model of predicted control in real time, which in the optional version is automatically updated. In this regard, you can also apply the external control mode (for example, by simply entering external commands into the analytical model that controls the process), so that, for example, the operator can adjust the aluminum content, strip temperature, etc. The analytical model of process control is also updated in accordance with such external management.

As in the case of the parameters obtained from the galvanizing furnace at the inlet of the galvanizing tank, to control the method in accordance with the present invention, it is possible to obtain measurement and control parameters as a result of the drying process of the strip exiting the galvanizing tank. This makes it possible to better calibrate the preliminary adjustment values associated with the coating thickness and with the required values of the applied metal content.

In this sense, the advantages of the invention are disclosed in the dependent claims.

Examples of the implementation and application of the method are illustrated in the previous figures and in the following figures.

FIG. 7 is a logic diagram for determining power values.

FIG. 8 is a logic diagram for determining the flow rate of a liquid mixture.

FIG. 9 is a logic diagram for determining aluminum content.

FIG. 10 is a logical diagram for determining the melting rate of ingots.

FIG. 11 is a flowchart for verifying the theoretical content of dissolved iron in a liquid mixture.

In FIG. 7 shows a logic diagram for determining values of band power (RV) and required power (PZ) for implementing the method in accordance with the present invention. Based on the data regarding the product (DAT_BAND) and the control conditions (DAT_DRIV) of the installation (see Figs. 1, 2 and 3), that is:

- the width (L) and thickness (E) of the continuously moving strip,

- the thickness of the zinc (EZ) applied on both sides of the strip, and the required speed (V) of the strip,

calculate mass flow rate (QBm) and flow rate per unit surface area (QBs) of the strip, as well as the total consumption of zinc consumed (Q ₁ ), including unavoidable losses.

Based on these flow values, the first temperature (T ₁ ) of the strip at the outlet of the galvanizing furnace in front of the galvanizing tank and the second temperature (T ₂ ) in the galvanizing tank, the strip power (PB) and the required power (PZ) are calculated.

If, as in the case shown in FIG. 6, the required power exceeds the power of the strip (PZ> PB, case “Y”), a number of calculations are performed (see Fig. 8) in the form:

ΔP = PZ-PB (step "1").

On the other hand, the required power may be less than the band power (PZ <PB, case “N”). In this case, the method in accordance with the present invention provides a predetermined cooling value (ORD1) (ΔT) for the first strip temperature (T ₁ ) by lowering the outlet temperature of the galvanizing furnace. After this stage, the temperature of the liquid mixture in the galvanizing tank should return to its value (T ₂ ), taking into account the fact that the temperature of the strip (T ₁ ) at the entrance to the galvanizing tank is equal to the second temperature (T ₂ ), increased by a certain value, in this case cooling (ΔТ) in absolute value, i.e.

T ₁ = T ₂ + ΔT.

In FIG. 8 is a flowchart for determining a flow rate of a liquid mixture associated with the result of step “1” shown in FIG. 1 and also represented as the logical starting point of the present scheme. Based on the desired third temperature (T ₃ ) in the melting zone (71) of the ingots of the preparation device, the initial temperature (T _L ) of the ingots, these ingots can, if necessary, be heated before being introduced into the liquid mixture, and the flow rate (Q ₁ ) consumed zinc determine the energy (W = W _{fus_Zn} ) of the melting of said zinc ingots. This energy is also the energy (W _{inc_Zn} ) provided by liquid zinc coming from the zinc tank.

Taking into account the second temperature (T ₂ ) of the liquid mixture coming from the zinc tank and the pre-calculated energy (W), the flow rate (Q ₂ ) of the liquid mixture coming from the zinc tank and necessary to ensure continuous melting of the ingots is determined. This flow rate (Q ₂ ) also indicates the flow rate of the liquid mixture between the zinc tank and the preparation device.

In FIG. 9 shows a logic diagram for determining the aluminum content (Al _t ) of a liquid mixture as a result of melting ingots in a preparation device (treatment tank 72). Indeed, the formation of certain Fe-Al compounds, which, on the one hand, form a combined layer deposited on the strip, and which, on the other hand, are present in grosses, leads to the consumption of aluminum, respectively (QAl _c ) and (QAl _d ), which is added to the amount usually applied with zinc to the strip. This additional flow rate must be compensated by the aluminum content (Al _t ) in the cleaning tank (72), slightly exceeding the desired aluminum content (Al _v ) in the zinc tank. The flow rates (QAl _c ) and (QAl _d ) are calculated based on the mass flow rate (QBm) of the strip. They are also included in the calculation scheme of the fourth temperature (T ₄ ) of the liquid mixture returning to the zinc tank, depending on the third temperature (T ₃ ) obtained after the ingots are melted, and the additional power (ΔР) necessary to bring the temperature of the liquid mixture to the second temperature (T ₂ ) in the galvanizing tank. After that, the value of the aluminum content (Al _t ) is taken into account from the point of view of flow in order to proceed to step "2", presented in the following figure.

In FIG. 10 is a logical diagram for determining the rate (= flow rate) of melting ingots in a preparation device. Depending on the amount of aluminum loss (QAl _c ) in the combined layer and aluminum loss (QAl _d ) in the gross, which vary, in particular, depending on the width of the processed strip, it is necessary to be able to adapt the aluminum content (Al _t ) resulting from melting ingots in order to maintain the required value of aluminum (Al _v ) in the zinc tank. For this, it is preferable that at least two ingots with different aluminum contents are simultaneously and selectively immersed in the liquid mixture of the preparation device, at least one of which has an aluminum content higher than the aluminum content (Al _t ) in the second zone (72) of the device training. A set of (n) ingots is immersed in a liquid metal at a total melting rate (= flow rate) (V _m ) corresponding to the calculated total zinc consumption (Q ₁ ). Each of (n) ingots with aluminum content (Al ₁ , Al ₂ , ..., Al _n ) is immersed selectively and correspondingly to dynamics (immersion time), which is variable for each ingot taking into account the calculated melting rate (V ₁ , V ₂ , ..., V _n ) in order to ensure the resulting aluminum content (Al _t ) associated with the total melting rate (V _m ), and that the required aluminum content (Al _t ) associated with the intended consumption of aluminum from the value obtained in step "2", presented in the previous FIG. 9 was provided by the aluminum content (Al _t ) obtained by melting the ingots.

In FIG. 11 is a logic diagram for checking the theoretical content of iron (SFe) dissolved in a liquid mixture based on the above described step “1” (see FIGS. 6, 7, 8). The iron content (Fe ₁ ) in the liquid mixture entering the zinc tank is fixed by the solubility threshold (SFe T ₃ ) of iron at the third temperature (T ₃ ) of deposition of gross (Fe ₁ = SFe T ₃ ) (see also FIG. 1). Depending on the data, such as the first temperature (T ₁ ) of the strip at the inlet of the zinc tank, the second temperature (T ₂ ) of the liquid mixture in said zinc tank, the flow rate per unit surface of the strip (QBs) and the aluminum content (Al _v ) in the liquid mixture at the entrance to the preparation device, the method includes calculating, on the one hand, the rate of dissolution of iron (QFe) obtained from both sides of the moving strip, and, on the other hand, the solubility threshold (SFe T ₂ ) of iron in the liquid mixture at the second temperature ( T ₂ ). This dissolution rate added to the iron (Fe ₁ ) content at the inlet of the galvanizing tank allows the iron content in the liquid mixture (Fe ₂ ) to be calculated.

Fe ₂ = (QFe · SFe) + Fe _1,

where the safety factor (S _Fe ) is introduced. A large gradient of iron concentration is established on the surface of the strip, which contributes to the development of the combined Fe ₂ Al ₅ Zn _x layer. The iron content (Fe ₂ ) in the liquid mixture in the zinc tank is in this case the iron content at the end of the gradient and can be considered the total iron content in the bath of the liquid mixture. If the solubility threshold (SFe T ₂ ) of iron in the liquid mixture at the second temperature (T ₂ ) exceeds the actual iron content of the liquid mixture (Fe ₂ ) in the zinc tank (see the case "SFe T ₂ > Fe ₂ "), then various obtained method control parameters (see case “VAL_PA"). Otherwise, these parameters must be changed (see the “MOD_PA” case) to increase (the case “UP (SFe T ₂ )”) the solubility threshold (SFe T ₂ ) of iron in the liquid mixture at the second temperature (T ₂ ) and / or reduce (the case of "DOWN (QFe)") the rate of dissolution of iron (QFe). An increase in said solubility threshold (SFe T ₂ ) is obtained by increasing the second temperature (T ₂ ) and / or lowering the aluminum content (Al _v ) in the zinc tank. A decrease in the dissolution rate of iron (QFe) is obtained by lowering the first temperature (T ₁ ) and / or the second temperature (T ₂ ) and / or the flow rate per unit surface of the strip (QBs) and / or by increasing the aluminum content (Al _v ) in the zinc tank . It is almost preferable to act on the first temperature (T ₁ ) of the strip and / or on its speed (V).

List of basic symbols

1 continuously moving strip

2, 13 galvanizing tank

7 preparation device

71, 72 first and second zones of the preparation device

8 ingot (ingots)

And the limit point of solubility of iron at 470 ° C with an aluminum content of 0.19%

Al aluminum

Al ₁ , ..., Al _n the aluminum content of the ingots 1-n

Al _c consumption of aluminum in the combined layer

Al _d gross aluminum consumption

Al ₁ increase in the aluminum content of the liquid mixture required in the preparation device

Al _m maximum (virtual) aluminum content of the liquid mixture in the preparation device (first zone 71)

Al _t the aluminum content in the liquid mixture obtained from ingots melted in the preparation device (i.e. in the second zone 72)

Al _{v the} desired aluminum content in the liquid mixture at the outlet of the galvanizing tank

At the limit point of solubility of iron at 440 ° C with an aluminum content of 0.19%

DAT_BAND strip data

DAT_DRIV control data

DOWN (x) reduce the variable x

Dross matte, dross

ΔР adding (ΔР> 0) or decreasing (ΔР <0) power

ΔT positive (ΔT> 0) or negative (ΔT <0) temperature change corresponding to the addition or decrease of energy

E strip thickness

EZ Zinc Thickness

Fe iron

Fe ₁ iron content of the liquid mixture at the inlet of the galvanizing tank

Fe ₂ maximum iron content of the liquid mixture in the zinc tank

L strip width

MOD_PA change selected parameters

N no

ORD1 setpoint

PZ power required to maintain zinc at T ₂

PB power provided by the strip

Q ₁ = Q _{1_fus_Zn the} consumption of melting zinc ingots

= Q _{1_cons_Zn} total consumption of zinc-aluminum consumed

Q _{2 the} required flow rate of liquid zinc at the outlet of the galvanizing tank

QAl _c Al loss rate in the composite layer

QAl _d Al loss rate in gross

QBm mass flow strip

QBs flow rate per unit strip surface

QFe flow rate of dissolution of iron in a liquid mixture

SFe solubility / saturation threshold of iron in a liquid mixture

SFe T ₂ SFe for liquid mixture at temperature T ₂

SFe T ₃ SFe for liquid mixture at temperature T ₃

SFe T ₄ SFe for liquid mixture at temperature T ₄

T ₁ 1st strip temperature at the inlet of the galvanizing tank

T _{1_mes} measured T ₁

T ₂ 2nd temperature of the liquid mixture in the zinc tank

T ₃ 3rd temperature of the preparation device (bath)

T ₄ 4th temperature at the outlet of the cleaning tank

T _L initial temperature of the ingots before immersion in the melting zone

UP (x) increment variable (x)

V lane speed

Vm total melting rate of submerged ingots

V _max maximum lane speed

V ₁ , ..., V _{n the} values of the flow rate of melting ingots 1-n

VAL_PA confirmation of selected parameters

W = W _{fus_Zn} melting energy of zinc ingots

= W _{inc_Zn is the} energy added by liquid zinc coming from a galvanizing tank

Y yes

Zn Zinc

Claims (27)

1. A method of galvanizing by immersion of a continuously moving strip (1) of rolled steel, comprising immersing the strip in a zinc tank (2) containing a bath (5) of a liquid mixture of metals deposited onto the strip, such as zinc and aluminum, continuously circulating between the said zinc tank and the device (7) a preparation in which the temperature of the liquid metal mixture is intentionally lowered to lower the solubility threshold of iron, and kept high enough to activate at least one Zn-Al ingot (8) in an amount sufficient to compensate for the liquid metal mixture consumed when applied to the strip, while
determine the first power (RV) provided by the steel strip entering at the first temperature (T ₁ ) into the bath (5) of the liquid mixture of the zinc tank, while said bath is stabilized at a second predetermined temperature (T ₂ ) less _{than the} first temperature (T ₁ ),
determine the second power (PZ) necessary to maintain the liquid metal mixture at a second predetermined temperature (T ₂ ), and this second power is compared with the first power (PB) provided by the strip,
if the first power (RV) is higher than the second power (PZ), a set lowering value is applied to the first temperature (T ₁ ) of the strip,
if the first power (RV) is lower than or equal to the second power (PZ), determine the energy required for continuous melting in the preparation device of the ingot (8) in the amount necessary to compensate for the liquid metal mixture consumed when applied to the strip,
adjust the flow rate (Q ₂ ) of the liquid metal mixture between the zinc tank and the preparation device to provide the energy necessary for continuous melting of the ingot (8), while maintaining the temperature of the liquid metal mixture in the preparation device at a predetermined third temperature (T ₃ ) less than a predetermined second temperature (T ₂ ),
adjust the fourth temperature (T ₄ ) of the liquid metal mixture at the outlet (9) of the preparation device to provide the additional power (ΔP = PZ-PB) necessary for thermal equilibrium between the said output and the input (12) of the galvanizing tank supply, while the said input powered by output (9). 2. The method according to claim 1, in which by adjusting the second temperature (T ₂ ) and the desired aluminum content (Al _v ), the solubility threshold (SFe T ₂ ) of iron at the second temperature (T ₂ ) in the liquid metal mixture of the zinc tank is controlled level so that, taking into account the expected dissolution rate of iron (QFe) in the zinc tank, to maintain the total iron content (Fe ₂ ) below the solubility threshold (SFe T ₂ ) at the second temperature (T ₂ ). 3. The method according to claim 1 or 2, in which they provide continuous melting of the ingots at a total melting rate (Vm) of at least two ingots. 4. The method according to claim 3, in which a variable number (n) of ingots is selectively and simultaneously immersed in the bath (5) of the liquid mixture, each of the ingots having a different aluminum content (Al ₁ , Al ₂ , ..., Al _n ), and at least one of the ingots has an aluminum content higher than the required content (Al _t ) in the preparation device. 5. The method according to claim 4, in which individually control the speed of immersion (V ₁ , V ₂ , ..., V _n ) of each of (n) ingots to adjust the aluminum content in the preparation device according to the desired value of the content (Al _t ), while maintaining the required total melting rate (Vm). 6. The method according to claim 1, in which the preparation device activates the cooling of the liquid metal mixture from the second temperature (T ₂ ) to the third temperature (T ₃ ) in order to lower the solubility threshold of iron and localize the formation of drosses in said preparation device. 7. The method according to claim 1, in which the division into compartments between the ingots and their corresponding aluminum content is carried out in order to separate different types of dross among themselves, while surface drosses with a high aluminum content are mainly formed near submerged ingots with a high aluminum content, and low aluminum bottom drosses are mainly formed near submerged low aluminum ingots. 8. The method according to claim 1, in which the recovery rate (Q ₂ ) of the liquid metal mixture included in the zinc tank is adjusted below the iron content equal to the solubility threshold at the third temperature (T ₃ ), in order to limit the increase in the content of dissolved iron to a value below the threshold solubility at a second temperature (T ₂ ) in a zinc tank. 9. The method according to claim 1, in which the closed loop regulation of the first power (RV) provided by the strip, control the addition or decrease of power (ΔP), which leads to equilibrium, in which the first power (RV) is equal to the sum of the second power (PZ) and adding or decreasing power (ΔP), that is, at which PB = PZ + ΔP, at a given strip temperature. 10. The method according to claim 1, in which the preparation device is equipped with adjustable means for selecting and removing calories associated with an adjustable means of induction heating, configured to control a third temperature (T ₃ ) in the melting zone of the ingots in the temperature range of ± 10 ° C values, close to the value of the set temperature. 11. The method according to claim 1, in which the first temperature (T ₁ ) of the steel strip at the entrance to the galvanizing tank is from 450 to 550 ° C. 12. The method according to claim 1, in which the second temperature (T ₂ ) of the liquid metal mixture in the zinc tank is from 450 to 520 ° C. 13. The method according to one of claims 11 or 12, in which the temperature difference (ΔT ₁ ) between the steel strip and the liquid mixture of metals in the zinc tank is maintained in the range from 0 to 50 ° C. 14. The method according to item 13, in which the second temperature (T ₂ ) of the liquid metal mixture in the zinc tank is preferably supported with an accuracy of ± 1 ° C to ± 3 ° C in the value (T ₁ -ΔT ₁ ) equal to the first temperature ( T ₁ ) minus the temperature difference (ΔT ₁ ) between the steel strip and the liquid metal mixture. 15. The method according to one of claims 11 or 12, in which lowering the temperature (ΔT ₂ = T ₂ -T ₃ ) between the second and third temperatures of the liquid metal mixture in the preparation device is maintained at a value not lower than 10 ° C. 16. The method according to claim 1, in which the flow rate (Q ₂ ) of the liquid metal mixture from the zinc tank is constantly maintained from 10 to 30 times the amount of the metal mixture applied to the strip for the same unit of time. 17. The method according to claim 1, in which the temperature and concentration of aluminum in the liquid metal mixture are measured, preferably continuously, at least in the flow path from the power inlet in the zinc tank to the output of the preparation device. 18. The method according to claim 1, in which preferably continuously measure the level of the liquid mixture of metals in the preparation device. 19. The method according to claim 1, in which the flow rate and temperature of the liquid metal mixture is maintained in the form of pairs of predetermined values by regulation. 20. The method according to claim 1, in which the temperature of the strip at the outlet of the galvanizing furnace, connected to the inlet of the strip into the zinc tank, is maintained in the range of adjustable values. 21. The method according to claim 1, in which the speed of the strip is maintained in the range of adjustable values. 22. The method according to claim 1, in which measure the width and thickness of the strip at the entrance to the galvanizing tank. 23. The method according to claim 1, in which the introduction and retention of the ingots in the melting zone of the preparation device is carried out dynamically. 24. The method according to claim 1, in which the dynamic measurement and regulation parameters associated with the strip, with the galvanizing tank and with the preparation device, are controlled centrally. 25. The method according to claim 1, in which the control parameters are set by entering external commands into the analytical model that controls the above method. 26. The method of claim 25, wherein the analytical process control model is updated automatically. 27. The method according to claim 1, in which to control the aforementioned method, measurement and control parameters are obtained as a result of the drying process of the strip exiting the zinc tank.

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