JPH0140897B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous heat treatment method for steel plates. This method is particularly advantageous for producing steel sheets for embossing or high-strength steel sheets which exhibit an excellent surface condition and very uniform properties over their width and length. Such qualities are particularly necessary for high tensile strength steel plates used in the automotive industry.

When it is desired to obtain good ductility, embossing and elongation properties from cold rolled steel sheets, the steel sheets are generally subjected to recrystallization annealing while being wound into rolls in a bell furnace.

However, it is clear that such a process is expensive due to its long duration and low productivity. Moreover, not only the physical properties obtained but also the surface chemical properties show considerable variation.

In order to correct these disadvantages, various continuous processing methods have been proposed. And, instead of the conventional batch annealing, the applicant himself also brought the steel plate to a temperature higher than the recrystallization temperature of the steel, and then placed the steel plate in a water bath maintained at a temperature of 75°C or higher, preferably at boiling temperature. It has already been recommended to carry out a continuous heat treatment essentially consisting of immersion. This method, which is the subject of Belgian Patent No. 837458, is used in particular for the production of steel plates for embossing, high tensile strength steel plates and steel plates with high breaking strength and high elongation.

The principle of operation of such a method is now well known, and the role played by the vapor film formed on the surface of the steel sheet at the beginning of the immersion step in the bath is also known.

In the course of research to perfect the mode of implementing this method, the inventors investigated the effect of the disappearance of this vapor film at the moment boiling begins. This extinction occurs when the surface of the immersed steel plate reaches a certain transition temperature, generally between 300 and 350°C. In fact, it can be seen that in the bath there is clearly a "transition front" on the steel plate surface which marks the boundary between the vapor zone and the boiling zone.

The present invention requires heating at a temperature higher than the recrystallization temperature and 75
The subject matter is a method of continuous treatment of steel plates by immersion in a water bath that reaches a temperature of ℃ or higher.
The method is then based on the discovery of the unexpected relationship that exists between the quality of the product obtained, in particular its flatness, and the position of the transition front defined above.

Note that if the bath temperature is lower than 75°C, the desired cooling effect will not be achieved, and the excellent planar properties and mechanical properties (yield strength, tensile strength, etc.) of the steel plate will not be uniform throughout the steel plate. The upper limit of the bath temperature is not critical and varies depending on the composition of the bath and the presence or absence of pressurization, so it cannot be set uniformly, but in general it is around 100 degrees above the boiling point of the bath.
It is ℃. Of course, the boiling point itself differs slightly depending on the depth of the bath and between the surface and bottom.

The excellent flatness of a steel plate is a property that indicates that the surface of the steel plate forms a flat surface as completely as possible. Typical planar defects appear as folds in the steel plate or longitudinal corrugations along the edge of the steel plate.
The importance of sheet flatness in the automotive industry has long been recognized, and many mechanical methods are widely used to correct its defects.

The method of the invention has a very good flat surface,
From a technical and economical point of view the invention provides a very important advance, i.e. making it possible to produce steel plates free of the above-mentioned defects, so that it is no longer necessary to apply the above-mentioned conventional mechanical methods. do.

"Water bath" in the sense of the present invention means any bath based on water, irrespective of the nature or purity of this water. The bath may therefore contain substances in the form of solutions or dispersions.

A feature of this method, which is the object of the invention, is that the operating conditions of the dipping step, in particular the speed of advance of the steel plate and/or the temperature of the steel plate when entering the bath, and/or the length of the journey of the steel plate in the bath, are adjusted. By doing so, the annihilation zone, or "transition front", of the vapor film adhering to the surface of the steel sheet is placed outside the rising portion of the steel sheet in the bath.

According to a first embodiment of the invention, a "metastatic front"
The conditions of the immersion process are adjusted so that no water is present anywhere in the bath. This means in practice that the processing conditions are adjusted so that the steel plate leaves the bath at a temperature above the transition temperature at which the vapor film disappears.

In one preferred variant of this embodiment, this position of the transition front is achieved by imparting a sufficient velocity to the steel plate. In the second variant,
The temperature of the steel plate at the inlet of the bath is adjusted to a sufficiently high value. In a third variant, the length of the steel plate being immersed is adjusted such that the temperature of the steel plate when it leaves the bath is such that it is possible to carry out the next treatment step continuously.

The following example is given as a non-limiting illustration of this first embodiment of the method of the invention.

Example 1 A steel plate with a thickness of 0.5 mm and a width of 1 m was immersed in a bath of industrial water brought to boiling under the following conditions.

Temperature of the steel plate at the inlet of the bath: 700℃ Temperature of the steel plate at the outlet of the bath: 320℃ Cooling rate: 110℃/sec Advance speed of the steel plate: 170m/min The flatness of this steel plate has been proven to be excellent. It was done.

Example 2 Similarly, a steel plate with a thickness of 0.24 mm was heated to a water bath inlet temperature of 700.
Similar good results were obtained when processing at an exit temperature of 350°C and a speed of 330 m/min.

According to a second embodiment of the invention, the conditions of the dipping step are adjusted so that the transition front appears in the descending part of the steel plate in the bath. In addition, care is preferably taken that the transfer front is not located in the area where the steel plate contacts the rollers that direct it towards the surface of the bath.

In one variant of the second embodiment of the method of the invention, the advancing speed of the steel plate is kept low so that the position of the transition front is obtained in the descending part of the steel plate. According to the invention, the temperature at the inlet of the steel sheet bath can be adjusted below a limit value that depends on the working conditions. Alternatively, the height of the bath can be kept above a predetermined minimum value which also depends on the working conditions.

Two examples are given below to illustrate this second aspect of the invention.

Example 3 A steel plate with a thickness of 0.9 mm and a width of 1 m was treated according to the present method under the following conditions.

Speed: 15 m/min Temperature at bath inlet: 700°C Temperature at bath outlet: 100°C Example 4 Thickness of steel plate: 0.5 mm Speed: 150 m/min Temperature at bath inlet: 430°C Temperature at bath outlet: The flatness of the steel sheets obtained at 100°C in both cases proved to be excellent.

The method according to the invention makes it possible to reliably and simply produce steel plates exhibiting excellent flatness.
In fact, in the process of operation, if it is found that the transition front happens to be located in the rising part of the steel plate in the bath, then the process of ensuring a new vapor film on the surface of the steel plate in the entire bath , or to bring the transition front back to the descending part of the steel plate.
It is sufficient to change parameters such as the speed of the steel plate, the temperature at the inlet of the bath, the height of the bath (the length of the steel plate immersed in the bath), etc., according to the circumstances corresponding to the chosen implementation method. .

In fact, it was quite unexpectedly established that when the transition front lies on the rising part of the steel plate in the bath, the results of the treatment are unacceptable, especially from the point of view of the flatness of the product.

Moreover, in some cases metallurgical reasons force the temperature at which the steel leaves the bath to be between 100 and 200°C. Under these conditions it may be necessary to have a vessel containing a bath of nearly impossible height in order to locate the transition front within the descending portion of the steel plate in the bath. In order to avoid such inconveniences, it is recommended that other aspects of the invention be implemented.

In this third embodiment of the invention, 75°C
Use at least two consecutive water baths maintained at a temperature above the above temperature. The operating conditions are then adjusted so that the temperature of the steel plate at the outlet of the penultimate tank is between 300 and 500°C. Under these conditions, the cooling process of the steel plate in this tank takes place only in the steam film area, but in the next tank the transition front can only be located in the descending part of the steel.

To further explain this, when more than one bath, i.e. at least two baths, are used sequentially for cooling the steel plate, the steel plate cools down to the penultimate bath.
It should come out at a temperature of 300-550℃. Therefore, the outlet temperature is higher than 350℃ or 300-350℃, respectively.
℃, the transition front either does not occur in this bath or occurs immediately before the exit of the steel plate. The steel plate then enters the final tank at a temperature of 300-550℃.
In the first case (350°C to 550°C), the transition front is
Since the steel plate leaves this last tank at approximately bath temperature,
This always occurs during the descending part of the steel plate's journey in the final tank. Also in the second case (300-350°C) no transition front occurs at all in the last bath.

A device with two successive vessels, as recommended for the above-mentioned special treatment, makes it possible to process any product at any desired rate in conditions corresponding to the method of the invention. If necessary, one of the two vessels may be taken out of service for further processing.

The present invention is also directed to a method of controlling the continuous heat treatment described above. This confirms a certain relationship between, on the one hand, the position of the transition front and, on the other hand, the sound emitted by the cooling operation of the steel plate in a bath of boiling water when the steel plate no longer exhibits an acceptable planarity ( It is also entirely unexpected).

The feature of the control method of the operation which is the object of the present invention is that it records the sound emitted during the passage of the steel plate into the bath and records the sound generated when the transfer front is not in the rising part of the steel plate in the bath. When it is found that the recorded sound has increased beyond the above range, the operating conditions are changed to return the sound to within the range.

The subject embodiments of the present invention represent a significant advance in the management and control of continuous heat treatment processes for steel sheets by immersion in "boiling" water. The present invention
It can be applied whatever the product is to be made and whatever the metallurgical conditions to be observed.
Because the process is easy to adjust, products of desired quality can be manufactured without difficulty.

Furthermore, the expressions "descending part" and "rising part" of the steel plate in the bath in the sense of the present invention refer to
By a steel plate that enters the bath and exits again without detours after simply circling one return cylinder placed at the bottom of the bath to force the steel plate to follow a prearranged circuit. It should be noted that it is necessary to interpret it as a normal part of the route to be traveled.

It is of course beyond the scope of the invention to subject the steel plate to a reorientation or secondary deviation of the circuit during the journey between the surface and the greatest depth reached, or between this lower level and the surface. It's not a thing.

The effects of the present invention can be seen, for example, in Belgian patent No. 837458.
The purpose of this invention is to provide a steel sheet with even better smoothness than in known methods such as those described in No.

Such effects can be obtained by comparing the already described Example 1 with the attached 1st example.
It will become clearer from the following more detailed description with reference to the drawings.

This example illustrates a preferred embodiment of the invention in which there is no transition front in the bath. That is, the length of the immersed portion of the steel plate is adjusted such that the temperature of the steel plate upon exiting the bath is higher than the temperature at which the transition front occurs.

In FIG. 1, a cold rolled steel strip 1 is heated in a continuous furnace 2 to a temperature higher than its recrystallization temperature. The thus heated strip is lowered by the first reversing roll 3 and enters the hot water bath 4 of the tank 5. The steel is then guided around an immersed return roll 6 before rising, leaving the bath and being directed by a second diverting roll 7 to the next step (for example pickling).

While the steel strip is passing through the aqueous bath, from the bath inlet temperature T ₁ ,
Through the temperature T ₂ at the lowest part of the strip steel in the bath,
It is cooled down to the bath outlet temperature _T3 . The cooling rate is V _r
(°C/s), and the passing velocity of the bath is V _b (m/s).
s).

If all the immersion conditions of the strip remain unchanged, i.e. inlet temperature T ₁ , speed in the bath, temperature and composition of the bath, length of the strip in the bath, intermediate temperature T ₂ and outlet temperature T _{3 .} If is maintained substantially constant, the strip should continue to follow a constant temperature profile as it passes through the bath.

In this type of process, the inlet temperature and passage speed are generally set at constant values, so the parameter that can most easily be varied is the length of the strip section immersed in the bath.

In the steel strip used in this example, the transition front was approximately 300°C.
It occurred in Therefore, the depth of the return roll 6 was adjusted so that the immersion length of the steel strip could be obtained so that the exit temperature of the steel strip would be 320°C, which is higher than 300°C.

The immersion length L=L ₁ +L ₂ is obtained by the following formula.

L=T ₁ −T ₃ /V _r・V _bWhere , T ₁ = Inlet temperature = 700℃ T ₃ = Outlet temperature = 320℃ V _r = Cooling rate = 110℃/sec V _b = Strip speed = 170m /min = 2.8m/sec Therefore, L = 9.80m, and the return roll 6 is L ₁ =
It must be installed so that L ₂ = 4.90m.

The above conditions were compatible with the method of the present invention, whereby the steel strip was ideally cooled and excellent planar properties were obtained.

On the other hand, if we set the long L to a high value, i.e. 11 m (L ₁ =
L ₂ = 5.5 m), the outlet temperature T ₃ was 268°C, and the average temperature T ₂ was 484°C. For this reason, a transition front was generated in the ascending portion of the steel strip, and the planar properties of the obtained steel strip were poor. That is, a waveform was generated at the edge.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing one embodiment of the present invention. In the figure, 1 is the steel strip, 2 is the furnace, 4 is the water bath, 6 is the return roll, T ₁ is the inlet temperature of the steel strip, T ₃ is the outlet temperature of the steel strip, L ₁ is the length of the descending part of the steel strip, L ₂ is the length of the rising section of the steel strip.

Claims (1)

[Claims] 1. A continuous heat treatment method for a steel plate, which comprises heating the steel plate to a temperature higher than its recrystallization temperature, and then immersing the steel plate in a water bath maintained at a temperature of 75°C or higher,
Adjusting at least one of the implementation conditions of the immersion process, such as the advancing speed of the steel plate, the temperature of the steel plate at the inlet of the bath, and the length of the journey of the steel plate in the bath, allows the disappearance zone of the vapor film attached to the surface of the steel plate, i.e. A continuous heat treatment method for a steel plate, characterized in that a "transition front" is prevented from occurring in the ascending portion of the steel plate in the bath. 2. By imparting a sufficient velocity to the steel plate and/or by adjusting the temperature of the steel plate at the entrance to the bath to a sufficiently high value and/or by adjusting the temperature of the steel plate at the entrance of the bath to a sufficiently high value so that no transition front appears anywhere in the bath. 2. The heat treatment method according to claim 1, wherein the conditions for performing the dipping step of the steel sheet are adjusted by limiting the length of the steel sheet at the portion where the steel sheet is soaked. 3. By maintaining the advancing speed of the steel plate so that the transition front appears in the descending part of the steel plate in the bath,
and/or carrying out the immersion process by regulating the temperature at the inlet of the steel plate to the bath below a certain limit value and/or by keeping the height of the bath above a predetermined minimum value depending on the operating conditions. The heat treatment method according to claim 1, characterized in that conditions are adjusted. 4. Record the sound emitted during the passage of the steel plate through the bath, compare this sound with the sound in the range corresponding to the implementation of the method in which the transition front is not present in the rising part of the steel plate, and Claims 1 to 3 are characterized in that when it is recognized that the sound has increased beyond the above range, the conditions for implementing the operation are changed so as to return the sound to within the above range. The heat treatment method described in any one of the above.