EP0686209B1 - Process and system for the continuous treatment of a galvanized steel strip - Google Patents

Abstract

A method and apparatus for continuously treating a strip of hot dip-galvanized steel. The method comprises the steps of rapidly reheating the strip to a predetermined temperature between 460 DEG C. and 600 DEG C., with a heating flux greater than 180 kWatts per meter squared based for each face of the strip; maintaining the strip at a substantially constant temperature for a predetermined period of time lasting between 10 and 30 seconds; and subsequently cooling the strip rapidly to a temperature below 420 DEG C., using a cooling flux of a magnitude greater than 100 kWatts per meter squared for each face of the strip. The step of rapidly reheating is performed following a drying operation conducted on the strip as it exits from a zinc bath. The apparatus comprises various elements for performing the method, as well as a vessel containing the zinc bath, redirecting rolls which define an upward and substantially vertical trajectory for the strip and a drying mechanism disposed along the trajectory downstream of the vessel's exit. Preferably, the rapidly heating of the strip is accomplished using an induction furnace operating at a frequency between 100 and 500 kHz, and the rapid cooling is provided by nozzles which, in turn, deliver a mist or fine spray of water and air.

Description

The present invention relates to a method for the continuous treatment of a galvanized steel strip, in particular by galvannealing. She wears also on an installation.

We know that the heat treatment known as galvannealing consists in subjecting a strip of galvanized steel successively to a heating, holding at temperature then cooling. This treatment must ensure the diffusion of the iron from the strip through the zinc of the coating, until a content of between 7% and 13% is reached in this coating. These values define the optimal range of composition of the alloy, out of which, if the iron content is too high, it dusting occurs when stamping, or if the iron content is too high low, the weldability of the product is not sufficient.

Currently, the galvannealing operation is performed in an instalation where the strip describes at least two vertical passes, namely a ascending pass and descending pass. Typically, this installation can also be used to manufacture a galvanized strip conventional.

In a classic galvannealing operation, we have a heating and maintaining temperature above the galvanizing bath, immediately after the wringing device consisting of air knives. This oven is generally retractable, because it is not not used in the production of conventional galvanized strips. Above this oven is located a first cooling device, usually a group of supply air fans, so that do not damage the coating on the deflection rollers. All of the oven and the cooling device determine the height of the ascending pass, which generally does not exceed 50 meters due to the vibrations produced in the air knives. A second device cooling system, for example a second group of fans, is usually placed at the start of the following downward pass.

In such an installation, the coated strip leaves the zinc bath at a temperature of about 450 ° C to 480 ° C then, after spinning with the knives of air, it undergoes the galvannealing operation by heating and keeping in the above-mentioned oven at a temperature between 460 ° C. and 600 ° C depending on the steel grade considered. It is then cooled first by a first group of fans at the end of the pass rising, then at the start of the falling pass, to a temperature suitable for further processing of the coated strip.

As for conventional galvanized strips, they only undergo cooling from about 450 ° C to 480 ° C, which is their temperature after spinning, to a temperature below 330 ° C at the top of the ascending pass, in order to avoid sticking of the strip on the rollers of reference. The cooling then continues in the second device cooling, located at the start of the downdraft, up to the temperature required for further processing of the strip galvanized.

In current practice, said means for heating the strip include a direct fire oven or an induction oven. These means of heaters are capable of increasing from 50 ° C to 100 ° C the temperature of the strip, with a speed which is however moderate and for example, for a strip 0.7 mm thick, 6 ° C / s in a oven equipped with burners and 30 ° C / s in a frequency induction oven 10 kHz. In addition, heating by gas burners has a yield low energy, around 30%, while with conventional induction with multispire coils, with longitudinal flow or transverse, we may have to correct the transverse distribution irregular temperatures.

For their part, the means for maintaining the temperature are generally consisting of an insulated tunnel, possibly equipped with means of heating, for example with electricity or gas; they occupy approximately a quarter of the height of the ascending vertical strand. All ovens heating and holding should be long enough to ensure residence time greater than 10 seconds, and preferably greater at 15 seconds, at a temperature above 450 ° C.

Designed in this way, the installations do not allow the galvannealing operation under optimal conditions: speed low heating of the galvanized strip requires a significant length of this section and therefore limits the length of the temperature maintenance zone where the diffusion of iron from the strip in the zinc, which makes it necessary to resort to higher temperatures high. It is also well known that the risk of dusting during the drawing decreases when the holding temperature is reduced and that the holding time is extended.

The present invention aims to remedy this situation by offering a thermal cycle which ensures excellent conditions of execution of the galvannealing treatment: instead of the cycles that one currently practiced, the method of the invention performs a "square" cycle with a long holding time at a temperature which can be relatively low given the long duration of treatment.

The process of continuous treatment of a steel strip galvanized with hardened, object of the present invention, is characterized in that, after the strip wringing operation at the outlet of the zinc bath, we quickly warms up the strip to a temperature between 460 ° C and 600 ° C with a heating power density greater than 180 kW / m2 per side of product, keeping the strip at substantially constant temperature for a period between 10 seconds and 30 seconds, in that we then quickly cool the tape up to a temperature below 420 ° C with a density of cooling power greater than 100 kW / m2 per product side.

The temperature at which the strip is worn depends on the shade of treated steel, like what happens in the galvannealing operation traditional; in practice, however, it can be a little lower and be for example between 460 and 560 ° C.

The heating power density - respectively cooling - which is expressed in kW / m2 is a concept well known to practitioners, and in particular practitioners of heat treatment of steel sheets. We can easily convert power density to rate of change temperature, depending on the thickness of the product.

For example, a heating power density of 180 kW / m2 per side of product means a heating rate of 100 ° C / s for a sheet of 0.7 mm thick, and 60 ° C / s for a sheet 1.25 mm thick. On the other hand, a cooling power density of 100 kW / m2 per product side means a cooling rate of 54 ° C / s for a thickness of 0.7 mm and 30 ° C / s for a sheet of 1.25 mm.

According to an advantageous method of the process of the invention, the rapid heating of the strip by means of a very high induction furnace frequency, for example between 100 kHz and 500 kHz; this modality achieves very high power densities and therefore to achieve very high heating rates, for example already 150 ° C / s for a 0.7 mm thick strip in an oven at 100 kHz.

According to a preferred variant of this modality, we combine the use of a very high frequency induction furnace with that of a single coil inductor for example made of copper foil surrounding the band. This advantageous variant improves the distribution transverse temperatures in the strip: the problem of variations periodicals of the temperature according to the width of the strip disappears and the edges are heated to the same temperature as the central part.

Still according to the invention, the temperature holding zone is constituted by an enclosure possibly provided with means of reheating such as gas burners to provide calories intended to compensate for local heat losses.

According to the invention, on leaving the temperature-maintaining zone, the strip is cooled to a temperature below 350 ° C. In an advantageous mode of the process, the operating parameters of the cooling system are adjusted to ensure a cooling density greater than 125 kW / m ² per product face.

In a practical implementation of the invention, the cooling is provided rapid strip by means of water / air mist nozzles.

It should be noted that the rise in temperature by heating to induction allows, as we already know, better control of the operation of galvannealing treatment, provided that the distribution of temperatures in the strip be as homogeneous as possible at the outlet of the oven, which depends on the conditions after leaving the zinc bath. In this perspective, it may prove useful, still according to the invention, insert between the rapid temperature rise section and the temperature maintenance zone, a homogenization section of temperatures, for example equipped with burners arranged across the band and provided with individual means for adjusting the feed.

Fig. 1

montre une installation typique de l'état actuel de la technique de galvannealing; la

Fig. 2

représente une modalité de réalisation de l'invention, et la

Fig. 3

représente les diagrammes température/temps du cycle thermique objet de l'invention et d'un cycle thermique conventionnel.

We will now describe in more detail one embodiment of an installation in accordance with the invention, with reference to the appended drawings in which the:

Fig. 1

shows a typical installation of the current state of the art of galvannealing; the

Fig. 2

represents a method of carrying out the invention, and the

Fig. 3

represents the temperature / time diagrams of the thermal cycle which is the subject of the invention and of a conventional thermal cycle.

These figures are of course schematic representations, in which only the necessary elements have been deliberately reproduced understanding the invention. For the sake of clarity, identical or analogous elements are designated by the same references numerical in all figures.

We first refer to FIG. 1, which shows an installation of treatment of a galvanized steel strip, typical of the technique current.

The steel strip 1 coming from an annealing furnace, plunges into a bath of molten zinc 2 contained in a galvanizing tank 3. The strip 1 is deflected by a first deflection roller 4, and it leaves the zinc bath 2 in vertical direction, guided by rollers 5. At the exit of the bath of zinc 2, it passes through a wringing device 6, constituted by air knives, which adjusts the thickness of the zinc layer on the strip steel 1.

Thus coated, the steel strip 1 successively describes a trajectory upward vertical to a second deflection roller 7, a trajectory horizontal to a third deflection roller 8, then a trajectory downward vertical towards a later operation.

In the lower part of the upward vertical path, i.e. shortly after the wringing device 6, an oven is placed comprising a heating section composed of zones 9 and 10 followed a temperature maintenance section 11. This oven makes it possible to operate the heating the strip and keeping it at the temperature chosen for cause the migration of iron into zinc which characterizes the treatment of galvannealing. All or part of this oven is retractable in order to ability to produce conventionally galvanized steel strips, without galvannealing treatment; in particular, this oven can make room for other equipment, such as for example a minimum flowering, often used in conventional galvanization. The strip temperature is around 450 ° C after spinning; by the galvannealing operation in the oven, it is raised up to 460 ° C at 600 ° C depending on the grade of steel treated.

Above the oven, i.e. at the top of the path vertical upward, the conventional installation includes a device 12 for cooling the galvanized steel strip, which usually consists of a group of supply air fans. These devices ensure the cooling of the galvanized strip, possibly after galvannealing, to a sufficiently low temperature to prevent it from sticking to the idler rollers 7.

In such an installation, the thermal cycle deviates significantly from the square cycle, which makes the galvannealing operation poorly controllable. This results in particular in difficulties concerning on the one hand the regulation of the treatment and on the other hand the control of the composition of the iron-zinc alloy, in particular on thick steel strips with a small thickness of zinc.

These problems are solved by the installation which is the subject of the invention, one embodiment of which is illustrated in FIG. 2 and described below.

This modality does not differ from the prior art as far as the actual galvanization of the strip, as well as the spin of the zinc layer.

The essential difference relates to the galvannealing oven which, according to the invention has a short heating section 10 rapid galvanized steel strip, followed by a short zone 13 temperature equalization and a long holding zone 11 at substantially constant temperature; at the outlet of the oven is arranged a rapid cooling section 14 fitted with mist nozzles water / air.

As mentioned above, the length of the vertical path upward (4.7) cannot currently exceed fifty meters, in particular due to transverse vibrations of the strip and difficulty adjusting the thickness of the coating. In the facilities conventional as shown in Figure 1, given the presence of a long cooling device 12 before the first idler roller 7 and the length of the heating oven 10 space available for temperature maintenance zone 11 is limited.

By significantly shortening the heating and cooling of the strip, the invention makes it possible to greatly lengthen the duration of temperature maintenance, which on the one hand facilitates adjustment temperatures and the conduct of treatment and secondly allows to apply a long hold at a lower temperature than in the conventional operation, which is favorable for the properties of the coating.

This difference in the thermal cycles of the conventional treatment of galvannealing and treatment according to the invention is illustrated in Figure 3 which gives the temperature / time diagram respectively of the two operations.

On this diagram where the temperatures are on the ordinate and the time in abscissa, section AB represents the slight cooling which affects the strip at the outlet of the zinc bath at A and the section EF illustrates the temperature decrease following forced cooling before contact with deflection roller 7.

The classic BCDE thermal cycle highlights the slow evolution of product temperature, difficult to control. For its part, the cycle of the invention with its BG rapid heating and HE sections of accelerated cooling allows long GH temperature maintenance constant.

By way of example of implementation of the method of the invention, mention will be made the galvannealing treatment of a steel strip at 0.005% C, 0.110% Mn, 0.009% Ti and 0.015% Nb, 1500 mm wide and 0.7 mm wide strip thick, traveling at a speed of 120 m / min.

At the exit of the zinc bath, the strip cools slightly, passing from 460 ° C to 445 ° C at its entry into an induction furnace whose heating density is 190 kW / m ² and which raises the temperature to 490 ° C; the strip is maintained at this temperature for 15.5 seconds, after which it is subjected to intense cooling in an enclosure 3 m in length, the first part of which is equipped with a battery of water / air jets, the cooling power of which is 180 kW / m ² per product face; the temperature of the strip leaving the enclosure is 330 ° C.

Claims (8)

1. Process for the continuous treatment of a hot-dip galvanized steel strip, characterized in that, after the operation of wiping the strip as it leaves the zinc bath, the strip is rapidly reheated to a temperature of between 460°C and 600°C with a heating power density of greater than 180 kW/m² per side of the product, in that the strip is maintained at an approximately constant temperature for a period of between 10 seconds and 30 seconds and in that the strip is then rapidly cooled to a temperature below 420°C with a cooling power density of greater than 100 kW/m² per side of the product.
2. Treatment process according to Claim 1, characterized in that the strip is rapidly heated by means of an induction furnace operating at a frequency of between 100 kHz and 500 kHz.
3. Treatment process according to Claim 2, characterized in that the heating is provided by combining the use of the induction furnace with that of a one-turn inductor, consisting, for example, of a copper sheet surrounding the strip.
4. Treatment process according to any one of Claims 1 to 3, characterized in that the temperature is maintained in a chamber optionally provided with heating means, such as gas burners.
5. Treatment process according to any one of Claims 1 to 4, characterized in that the strip is rapidly cooled down to a temperature below 350°C.
6. Treatment process according to any one of Claims 1 to 5, characterized in that the strip is rapidly cooled with a cooling power density of greater than 125 kW/m² per side of the product.
7. Treatment process according to any one of Claims 1 to 6, characterized in that the strip is rapidly cooled by means of water/air mist jets.
8. Plant for the continuous treatment of a galvanized steel strip, which includes a tank containing a zinc bath, guide rollers defining an ascending vertical path for the strip, and wiping means placed after the exit of the said tank on the said ascending vertical path for the strip, characterized in that it includes, in succession, means for rapidly heating the strip, consisting of an induction furnace operating at a frequency of between 100 kHz and 500 kHz, means for maintaining the strip at an approximately constant temperature, and rapid cooling means consisting of water/air mist jets, all the means lying along the ascending vertical path for the strip.

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