EP0111985A2 - Process for cooling thin metal strips - Google Patents

Abstract

1. Process for cooling a moving thin steel strip by spraying a refrigerant onto the surface of the strip, in which the said steel strip is subjected to a low intensity cooling phase followed by a high intensity cooling phase, characterized in that the said low intensity phase is carried out with a heat exchange coefficient less than or equal to 3 kW/m**2 degrees C and in the said high intensity phase is carried out with a heat exchange coefficient greater than 3 kW/m**2 degrees C, and in that the said low intensity phase is interrupted when the strip has reached a temperature of between 600 degrees C and 350 degrees C.

Description

The present invention relates to the forced cooling of thin metal strips, in particular steel. It relates in particular to the cooling operation carried out in the continuous heat treatment lines for thin strips.

It is known that if the cooling applied is not homogeneous, the strips can, in the final state, present serious defects. In this regard, one can cite the lack of flatness, which manifests itself when the tensions locally exceed the elastic limit of the material during cooling. The strips may also have vermiculations, that is to say marks of plastic deformation, which appear when the elastic limit of the material is reached for a certain temperature of the product and when the tensile characteristic of the material has a plateau. at this temperature.

In an attempt to prevent the appearance of these two types of defect, various methods have already been proposed using methods for relatively gentle cooling.

A first known method consists in applying cooling by air blowing, for which the heat exchange coefficient is approximately 0.15 kW / m ² ° C. This is a fairly slow cooling, which requires a long period of application and therefore the use of long cooling installations. The consumption of compressed air and the cost of the operation are consequently high.

Another cooling method is known which consists in immersing the thin strip in water maintained at its boiling temperature. In this case, the heat exchange coefficient is approximately 0.28 kW / m ² ° C in the film boiling range, that is to say when the surface temperature of the product is above 300 ° C. Below this temperature, the heat exchange coefficient increases very quickly and cooling by immersion in boiling water does not always make it possible to obtain a flat strip free of vermiculations. In addition, this cooling is also quite slow and has in this respect the same drawbacks as cooling by air blowing.

The subject of the present invention is a method making it possible to remedy the drawbacks which have just been mentioned. To this end, the method of the invention ensures intense cooling of the strip, while making it possible to obtain a quality product, having excellent flatness and free from vermiculations.

The process which is the subject of the present invention, in which a thin metal strip is subjected to cooling sement by spraying a refrigerant, is essentially characterized in that said cooling comprises a phase, called low intensity, during which the heat exchange coefficient defined at 6 ₀₀ ° C is less than ₃ k _{W /} m ² ° _C , and a phase, called high intensity, during which the heat exchange coefficient defined at 6 ₀₀ ° _C is greater than 3 kw / m2 _{° C.}

According to an advantageous implementation of the method of the invention, the cooling phase is carried out first at low intensity, until the strip has reached a predetermined temperature, and then the cooling phase at high intensity. .

_E also according to the invention, it has been found advantageous to interrupt the first stage of cooling when the surface of the strip has reached a value between 350 ° C and 600 ° C.

Still according to the invention, it may be advantageous to provide an additional cooling phase, called an intermediate intensity phase, which is preferably situated between the low intensity phase and the high intensity phase.

In the context of the invention, it has been found to be particularly advantageous to cool the strip by means of jets, for example of water, arranged so as to cover the entire surface of the strip. The water may possibly be hot and / or sprayed in the form of a mist.

In this case, it is advantageous to use the same refrigerant in the various cooling phases, the difference in intensity being obtained by adjusting the total flow of refrigerant in each phase. This adjustment can be performed by modifying either the number of coolant jets, or the flow rate of the various jets by any means known per se.

By way of example, an application of the method of the invention will now be described for the cooling in two phases of a thin strip by means of water jets, with reference to the appended figures.

In this example, a strip of mild steel 0.7 mm thick and moving at a speed of 109 m / min., Is cooled from 750 ° C to a temperature below ₁₇₀ ° _C.

In FIG. 1, the strip A moving in the direction of the arrow B, is subjected to the action of water jets arranged in staggered rows, of which only the three jets of axis perpendicular to the strip at points C, D and E are shown. We also indicated 5 points, numbered O, 1, 2, 3, and 4 located at an increasing distance from point C, as well as a point 5 located at equal distance from points C, D and E.

Figures 2 to 5 reflect the results obtained by a cooling operation, either only at high intensity (fig. 2 and 3), or only at low intensity (fig. 4 and 5).

FIG. 2 shows the curves O to 5 giving the evolution of the heat flux density as a function of the surface temperature of the strip, corresponding respectively to the points O to 5 indicated in FIG. 1. These curves reflect high intensity cooling for which the average heat exchange coefficient is 3.8 kW / m ² ° C to 600 ° C.

These curves are separated from each other, which reveals a significant heterogeneity of the cooling applied.

FIG. 3 shows the evolution of the stresses of thermal origin, superimposed on the traction in the strip (2 _{0 N /} mm ² ), as a function of the surface temperature of the strip, still in the case of cooling. at high intensity of FIG. 2. These curves show that it develops in the band, compressive stresses (line passing through point O) and tensile stresses (line passing through point 4). It also appears that the maximum difference between the tensile and compressive stresses is manifested for a temperature of the strip of the order of 300 ° _C. This FIG. 3 also shows the evolution of the elastic limit Re of the strip, as a function of its temperature, as well as the point M of the appearance of the bearing in the traction curve.

Examination of these curves shows that in the vicinity of the point of appearance of the bearing, the tensile stress is greater than the elastic limit. As a result, the strip treated in this way has a flatness defect as well as worms. It is calculated that under these conditions, the cooling length necessary to bring the strip to a temperature below 170 ° C is equal to 0.73 m.

An identical strip, also moving at a speed of 109 m / min., Was subjected to low intensity cooling, with an average heat exchange coefficient of 1.9 kW / m ² ° C at 600 "C .

Figure 4 shows the curves reflecting the evolution of the heat flux density as a function of the surface temperature of the strip, respectively at points O to 5 of Figure 1. The difference between the curves also reveals a fairly heterogeneous cooling sensitive, but weaker however than in the case of FIG. 2.

Figure 5 shows that the maximum difference between the tensile and compressive stresses is significantly less than in Figure 3. In addition, these stresses at no point exceed the elastic limit of the material. The strips cooled at low intensity therefore do not exhibit any defect in flatness or vermiculations. On the other hand, the cooling length necessary to reach a temperature below 170 ° C is equal to 1.61 m. It is therefore 2.2 times higher than in the case of high-intensity cooling, which leads to an increased total consumption of cooling agent.

The same strip was finally subjected to the cooling process according to the invention, comprising a phase I at low intensity followed by a phase II at high intensity. FIG. 6 shows the evolution of the maximum difference Δσ between the maximum tensile and compression stresses in the strip, as a function of the interruption temperature of phase I.

In a first case, the points of breakthrough of the axes of the jets of phase II are aligned with those of phase I according to straight lines parallel to the longitudinal axis of the strip. This case is illustrated by curve 1 in FIG. 6. The point P of this curve corresponds to an interruption of phase I at 750 ° C; the cooling takes place entirely in phase II and leads to Δσ _max = 172 N / mm ² . If, on the other hand, the interruption temperature is less than 300 ° C., it is brought back to the case where the cooling is entirely carried out in phase I; below this temperature of 300 ° C, the value of Δσ _max remains constant and equal to 78 N / mm ² . An intermediate interruption temperature of 450 ° _C leads to a _maximum Δσ of 8 ₇ N / mm ² .

In a second arrangement, the jets of phase _I I are offset transversely by half a step with respect to those of phase I. This case corresponds to curve 2 of FIG. 6. The points corresponding to temperatures of 750 ° C on the one hand and less than 300 ° C on the other hand, are identical to those of curve 1. It is noted however that when the interruption temperature is between 580 ° C and 300 ° C, the value of Δσ _max is less than the lowest achievable value in the first case (curve 1). For an interruption temperature of 450 ° C, Δσ _max is 56 _{N /} mm ² .

To achieve an interruption temperature of 450 ° C. in the case of this second arrangement of the jets, the phases I and _I I must have respective lengths of 0.88 m and 0.30 m, ie a total length of 1, 18 m.

The method of the invention therefore makes it possible to reduce the duration of cooling, in this case by 27%, and consequently the consumption of refrigerant, compared to low intensity cooling, while avoiding the appearance of flatness defects. and worms in the strips.

It is possible, by the method of the invention, to further reduce the length, therefore the duration of the cooling, without altering the quality of the strip. Curve 2 gives, for an interruption temperature of 580 ° C, a Δσ _max equal to 78 _{N /} m _m ² , but only requires a length of 0.53 m (phase I) + 0.47 m ( phase II) = 1 m, to reach a final temperature below 170 ° C. This value of 78 N / mm ² corresponds to the lowest value achievable by low intensity cooling (curve 1), - but with a length of 1 m instead of 1.61 m, a reduction of 38%. ''

Claims (6)

1. A method of cooling a thin metal strip in motion, by spraying a cooling agent onto the surface of the strip, characterized in that said cooling comprises a phase, called a low intensity phase, during which the coefficient heat exchange defined at 600 ° C is less than or equal to 3 kW / m ² ° C, and a phase, called high intensity, during which the heat exchange coefficient is greater than 3 k _{W /} m ² ° C. 2. Method according to claim 1, characterized in that the said phase is firstly executed at low intensity, until the strip has reached a predetermined temperature, called the interruption temperature, and then the said phase at high intensity from this interruption temperature. 3. Method according to either of Claims 1 or 2, characterized in that the said interruption temperature is chosen between approximately 350 ° C and approximately 600 ° C. 4. Method according to either of claims-1 or 2, characterized in that one performs a cooling phase, called intermediate intensity, between said low intensity phase and said high intensity phase . 5. Method according to either of claims 1 to 4, characterized in that water, possibly hot, is used as the cooling agent, preferably in the form of full conical jets. 6. Method according to either of Claims 1 to ₄ , characterized in that a mist of water and air is used as the cooling agent.

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