JPH06142752A - Manufacture of thin scale hot rolled steel plate - Google Patents

Abstract

PURPOSE:To manufacture a hot-rolled steel plate having thin scale without generating flaw on a product. CONSTITUTION:In interval from the outlet side of a finish rolling mill to a coiling in a hot-rolled steel plate manufacturing line, for instance, a water cooling header 4 in an underwater cooling device 2 is arranged so as to position a nozzle at the bottom surface of it within 20 mm from the surface of the steel plate 1. The water is injected at >=0.1kgf/cm<2> discharging pressure toward the steel plate surface from this nozzle, and the steel plate surface is cooled in the condition of almost perfectly covering with the water to remove the scale. The cooling in the condition of almost perfectly covering the steel plate surface with the water 5, is desirable to execute until the steel plate surface becomes <=650 deg.C in order to prevent reoxidation after descaling. By this method, the thin scale hot-rolled steel plate having <=1mum thickness of the scale can be manufactured by a simple device and low driving motive power.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet which has a thin scale attached to the surface thereof and can reduce the time for descaling due to pickling or the like.

[0002]

2. Description of the Related Art In an ordinary hot rolling line for steel sheets, descaling of steel sheets is performed after the completion of rough rolling and before the start of finish rolling. There are few examples of scaling. This conventional descaling of steel sheets is performed in the atmosphere. In addition, since the steel sheet is exposed to the atmosphere at high temperature from the end of finishing rolling to the winding, the scale of the rolled hot-rolled steel sheet is usually 5 to 15 μm.
Have a thickness of.

If the hot-rolled steel sheet remains attached to scale,
The commercial value of the hot-rolled steel sheet is lowered, and when it is used as a material for the cold-rolled steel sheet, the scale is rolled in and rolled in the next cold rolling step, which deteriorates the surface quality of the product. Further, there is a problem that scales are mixed in the circulating rolling oil to contaminate it.

The hot rolled steel sheet once wound may be descaled by mechanical means such as shot blasting. However, mechanical descaling has low productivity and is not suitable for a large-scale production line, and complete descaling is difficult to perform. Therefore, the hot-rolled steel sheet used as a material for cold rolling is usually subjected to a pickling step with hydrochloric acid or sulfuric acid for descaling before cold rolling. This pickling process is carried out by continuously passing the steel sheet through a long pickling tank.The thicker the scale, the longer the processing time, the longer the equipment, the cost of the acid used and waste acid. The processing cost will also increase.

From the above circumstances, it is desired that the hot rolled steel sheet has a scale as thin as possible. Therefore, JP-A-58-55528, 61-56722, and
Japanese Patent Laid-Open No. 61-195702 proposes a technique in which a cooling device is installed on the exit side of a hot final finish rolling mill to rapidly cool a steel sheet to thin the scale. The idea is to reduce the steel sheet temperature between the rolling mill exit side and the winder by quenching to suppress scale formation, but this cannot remove the scale produced up to the rolling mill exit side. However, sufficient thinning cannot be achieved.

Further, in Japanese Patent Laid-Open No. 53-23827, after finishing rolling and passing through a water cooling section, before entering a winding device, a polishing cleaning material is sprayed onto this hot rolled material by a high pressure liquid. It has been proposed to wind the rolled material while preventing the scale from being generated by descaling by means of descaling and then winding in an inert gas atmosphere. However, in this method, since the abrasive cleaning material is sprayed on the steel plate still soft at high temperature, the abrasive cleaning material bites into the surface of the steel sheet, causing flaws in the product. Further, it is not a practical method due to problems such as the need for equipment for treating abrasives and equipment for shielding the winding device.

[0007]

SUMMARY OF THE INVENTION The object of the present invention is to eliminate the problems in the methods proposed so far, to carry out with a relatively small power and simple equipment, and to produce defects in the product. Another object of the present invention is to provide a method for producing a hot-rolled steel sheet having a small scale. The thin scale targeted in the present invention is a scale of about 3 μm or less which significantly improves the descaling efficiency in the subsequent pickling step.

[0008]

DISCLOSURE OF THE INVENTION The gist of the present invention is the following method for producing a hot rolled steel sheet. From the exit side of the final rolling mill of the hot-rolled steel sheet production line to the winding, from the nozzle installed so that the tip is located within 20 mm from the surface of the steel sheet, 0.1 kgf / cm ² or more toward the steel sheet surface. A method for producing a thin-scale hot-rolled steel sheet, comprising injecting water at a discharge pressure, cooling the steel sheet surface while being almost completely covered with water, and removing scale.

It is desirable that the cooling in the state where the surface of the steel sheet is almost completely covered with water is performed until the surface of the steel sheet reaches a temperature of 650 ° C. or lower in order to prevent reoxidation after descaling.

FIG. 1 is a schematic side view of a part of the hot rolling line for carrying out the method of the present invention (from the final stand of the finishing mill to the winding device), and FIG. 2 is a sectional view taken along the line AA. is there. As shown, the final stand 6 of the finishing mill
Between the machine and the winder 3 there is a device 2 for carrying out the method of the invention. Here, since the steel plate is cooled while being substantially covered with water, this device will be referred to as an "underwater cooling device" for convenience. This device comprises a plurality of sets of pinch rollers 8 and a water cooling header 4 arranged between them. Reference numeral 1 is a hot-rolled steel sheet to be descaled, and 7 is a table roller. Furthermore, the side guide 11 shown in FIG.
There is.

FIG. 3 is a partially cutaway perspective view of the water cooling header 4. The header has a hollow tube 41 for supplying water therein, and a bottom plate 42 is provided with a large number of nozzles 43. The size of the bottom plate 42 is the vertical direction (rolling direction, b in the figure).
Direction) can be arranged almost in contact with the pinch roll 8, and the lateral direction (direction perpendicular to the rolling direction, direction a in the figure) is preferably larger than the width of the hot-rolled steel sheet to be treated. Side guides 11 are placed at the ends in the width direction to prevent water from flowing out.

If the steel sheet surface is almost completely covered with water and the apparatus is cooled by the apparatus shown in FIGS. 1 to 3, the scale can be removed very efficiently.

[0013]

In order to produce a thin-scale hot-rolled steel sheet having a scale thickness of 3 μm or less, it is necessary to remove the scale generated upstream before finishing hot-rolling. There is. Further, it is more effective for thinning the scale to prevent reoxidation by cooling the steel sheet from the atmosphere to a temperature range where it is not further oxidized after removing the scale. However, the above descaling should be as simple as possible and at a low operating cost.

The inventor of the present invention firstly investigated in detail the conditions for peeling off the scale when descaling was performed with high-pressure water. As a result, it was found that the following three forces were applied to the peeling. That is, the impact force of the water flow of the descaling on the scale surface, the thermal stress generated inside the scale due to the temperature drop of the scale due to the water injection, and the volume expansion coefficient of the scale are the base materials. The internal stress generated when the scale grows by being larger than that of (steel).

Of these, the thermal stress particularly has the greatest effect on the descaling property, and if the thermal stress is large, descaling proceeds rapidly. This means that it is not always necessary to use high-pressure water for the descaling, and it is sufficient if cooling can be achieved so as to give a large thermal stress to the scale.

Normally, the surface temperature of the steel sheet after the hot finish rolling is finished (immediately after leaving the final stand 6 in FIG. 1) is 8
It is from 00 to 1000 ° C. After this, cooling water is sprayed from the top and bottom of the steel sheet to cool the steel sheet to a specified temperature (winding temperature) .When such a hot steel sheet is water-cooled, water will contact the steel sheet and evaporate at the same time. The steam film covers the surface of the steel sheet.

FIG. 4 is a diagram schematically showing a general state when cooling water is sprayed on a high temperature steel plate. Nozzle 9
The water jetted from the water instantly becomes steam when it comes into contact with the surface of the hot steel plate 1, and the film 10 covers the steel plate surface.
In such a state of being covered with the water vapor film, even if water is supplied later, the steel sheet cannot be directly contacted, so that the cooling of the steel sheet does not proceed. Therefore, it is not possible to apply thermal stress to the scale due to cooling.

In order to enhance the effect of cooling by jetting water and increase the thermal stress inside the scale, it is essential to quickly remove the water vapor film formed on the scale during cooling. One of the means is to increase the pressure of the injected water and increase the flow velocity of the water to increase the collision force. However, there is a problem that equipment using high-pressure water becomes expensive and power cost for operation increases.

According to the method of the present invention, the steam film on the steel sheet is broken without using special high-pressure water such as 100 to 200 kgf / cm ² which is sometimes used in the conventional descaling method, and the steel sheet and its surface are broken. The idea is to quickly cool the scale. The means is to "inject water toward the steel sheet surface at a discharge pressure of 0.1 kgf / cm ² or more so that the steel sheet surface is almost completely covered with water".

FIG. 5 is a diagram schematically showing a state where the steel plate is cooled by the apparatus of FIG. 1 by enlarging the bottom surface of one water cooling header and the steel plate surface portion. The water film blown out from the nozzle 43 easily breaks the water vapor film, and the water comes into direct contact with the steel plate. When spraying water, the steel plate surface in the area where this header is installed is
It becomes almost completely covered with water. Although only the upper surface of the steel plate is shown in FIG. 5, the same state can be obtained on the lower surface.

In the state as shown in FIG. 4 above, even if high-pressure water is jetted, if there is a distance between the tip of the nozzle and the steel plate, the water flow becomes a water droplet, and the momentum possessed by each water droplet is small,
The water vapor film is hard to break. However, in the state shown in FIG. 5, the water becomes a continuous flow and hits the steel plate. Since the kinetic energy of this water flow is large, the water vapor film can be broken even at a low discharge pressure.

The shape and arrangement of the nozzle 43 of the water-cooling header 4, the supply amount of cooling water, and the discharge pressure are determined so that the above-described state shown in FIG. The conditions will be described below.

The distance between the surface of the steel plate and the bottom plate of the water-cooling header 4, that is, the tip of the nozzle is 20 mm or less. Water is sprayed from the nozzle of the bottom plate onto the steel plate, and the amount of water is 250 liters per second or more per m ^{2 of the} bottom plate from the upper and lower water cooling headers. With this amount of water, the side guide 11 prevents the water from flowing out in the width direction of the steel sheet, so that the space between the steel sheet and the header bottom plate can be filled with water. The amount of water can be reduced by reducing the distance between the pinch roller 8 and the water-cooling header 4 and allowing the pinch roller 8 to act as a weir.

It is desirable that the nozzles provided on the bottom plate of the water-cooled header have a pitch in the width direction (direction a in FIG. 2) smaller than that in the longitudinal direction (direction b in the same drawing) and are arranged in a staggered pattern.

Tip of nozzle (outer surface of bottom plate of water cooling header)
Shall be within a distance of 20 mm from the surface of the steel plate.
If it is separated by 20 mm or more, the flow velocity of water is attenuated, the force of water is weakened, the water vapor film is not broken, the cooling rate becomes slower, and descaling becomes difficult.

The water discharge pressure from the nozzle is set to 0.1 kgf / cm ² or more. If the water pressure is lower than this, the flow velocity of water at the nozzle outlet will be 4 m / sec or less, the pressure of water colliding with the steel plate will be low, the water vapor film on the steel plate cannot be broken, and the cooling rate of the scale will slow down, Scaling efficiency decreases.

In the method of the present invention, water is always present on the surface of the steel sheet during descaling, and water is further jetted from there to the nozzle. Therefore, it is the same as jetting water in water. If you spray water in the atmosphere,
The water that hits the steel plate is only the water that comes out of the nozzle, but if you inject the water in water, the water that comes out of the nozzle will engulf the surrounding water and several times the water that comes out of the nozzle will accompany the violent convection phenomenon. Since the water comes into contact with the steel plate, the cooling efficiency is improved.

Also in the method of the present invention, it is desirable that the discharge pressure at the time of exiting from the nozzle 43 is high. However, if the condition setting, such as, the discharge pressure is 0.1 kgf / cm ² or more, is sufficient up to about about 10 kgf / cm ^2, device (water cooling header dealing with water pressure of this order and its Peripheral equipment) is simple and requires little driving power.

When water is brought into direct contact with the surface of the steel plate as described above, the cooling rate becomes 10 times or more as compared with the case where the water vapor film is separated. Therefore, the scale is rapidly cooled, a large thermal stress is generated inside the scale, and the scale quickly separates from the steel sheet.

After the scale is removed by the method of the present invention, the hot-rolled steel sheet is sent to a winding machine and wound up as usual. However, if the temperature of the steel sheet after the descaling is high, it is reoxidized by the subsequent contact between the atmosphere and the steel sheet, and the steel sheet surface is again covered with scale. Therefore, until the temperature becomes lower than 650 ℃, which slows down the oxidation rate, it is possible to keep the steel sheet surface covered with water and shield it from the atmosphere to prevent reoxidation. Desirable for manufacturing. Therefore, it is preferable to install as many water cooling headers 4 and pinch rollers 8 as shown in FIGS. 1 to 3 as much as possible to lower the surface temperature of the steel plate on the delivery side.

[0031]

Example 1 The method of the present invention was carried out in the rolling line outlined above in FIG. That is, using a low carbon steel sheet as the test material, the temperature (rolling finishing temperature) of the steel sheet at the exit of the final stand 6 of the finishing rolling mill is 900 ° C, the finishing sheet thickness is 5 mm, and the sheet width is
It was kept constant at 300 mm, air-cooled to 850 ° C. on the table roller 7, and then passed through the underwater cooling device 2 to examine the scale peeling condition. Line speed is 100mpm, cooling water temperature is 30 ℃
Is.

The submersible cooling system 2 used is one in which six water cooling headers are arranged on each of the upper and lower sides (12 in total) between 200 mm diameter pinch rollers arranged at intervals of 750 mm. The bottom plate of the water-cooled header has a width (b in Fig. 3) of 500 mm and a length (same as
a) is 650 mm. Nozzles with a diameter of 1.6 mm were arranged in a zigzag pattern with a pitch of 30 mm in the width direction and a pitch of 50 mm in the longitudinal direction, for a total of 186 nozzles.

FIG. 6 shows the results of examining the peelability of the scale by changing the distance between the surface of the steel plate and the tip of the nozzle (bottom surface of the header) (called the nozzle distance) and the discharge pressure of the nozzle.
The scale adhesion state was examined by visually observing the steel sheet after winding.

In FIG. 6, the mark ○ indicates that the scale was completely peeled off, the mark Δ indicates that the scale remained, and the mark X indicates that the scale was not peeled off. As a result of the test, when the nozzle distance exceeds 20 mm, it becomes difficult to peel off and the water pressure is
It was found that peeling becomes difficult even at less than 0.1 kgf / cm ² .

[0035]

Example 2 The apparatus used in Example 1 was used to descale a rolled steel sheet under the same conditions as in Example 1. However, the nozzle distance of the underwater cooling device is 10 mm, and the water pressure is 1 kgf / cm ²
That is, the conditions were set so that the scale was completely peeled off. The number of headers used in the underwater cooling device was changed, the temperature of the steel plate on the outlet side was changed, the steel plate after passing was left in the air as it was, cooled to room temperature, and the thickness of the scale was measured.

The above measurement results are shown in FIG. As can be seen, when the temperature taken out from the submersible cooling device is high, the scale becomes thicker because of reoxidation after coming out of the device. But with the submersible cooler 650
When cooled below ℃, the thickness of the scale is extremely thin, 1 μm or less.

[0037]

According to the method of the present invention, it is possible to produce a thin-scale hot-rolled steel sheet having a scale thickness of 1 μm or less without using high-pressure water. This method can be implemented only by attaching a relatively simple cooling device to the existing hot rolling line, and the increase in operating cost is slight. The hot-rolled steel sheet obtained by this method has a high commercial value due to its thin scale, and the pickling time before using it as a material for a cold-rolled steel sheet or the like is shortened.

[Brief description of drawings]

FIG. 1 is a schematic view of a hot rolling line for carrying out the method of the present invention.

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 is a partially cutaway perspective view showing an example of a cooling water header installed in the line of FIG.

FIG. 4 is a diagram schematically showing a generation state of a water vapor film when a high temperature steel plate is cooled by a high pressure water jet.

FIG. 5 is a diagram schematically showing a water-cooled state of the steel sheet surface in the method of the present invention.

FIG. 6 is a diagram showing a relationship between a nozzle discharge distance and water discharge pressure from a nozzle at which scale peeling occurs.

FIG. 7 is a diagram showing the relationship between the temperature of the steel plate surface and the thickness of the scale when underwater cooling is completed.

[Explanation of symbols]

1: Steel plate, 2: Underwater cooling device, 3: Winding machine,
4: Water cooling header, 41: Water supply pipe for water cooling header, 4
2: Same bottom plate, 43: Same nozzle, 5: Water, 6:
Final stand of finishing rolling mill, 7: table roller,
8: Pinch roller, 9: Nozzle, 10: Water vapor film,
11: Side guide

Claims (2)

[Claims] 1. From a nozzle installed so that the tip is located within 20 mm from the surface of the steel sheet from the exit side of the final rolling mill of the hot-rolled steel sheet production line to the winding, 0.1 kgf toward the steel sheet surface. A method for producing a thin-scale hot-rolled steel sheet, comprising the steps of: injecting water at a discharge pressure of / cm ² or more, cooling the steel sheet surface almost completely with water, and removing scale. 2. The method for producing a thin-scale hot-rolled steel sheet according to claim 1, wherein cooling is performed while the steel sheet surface is almost completely covered with water until the surface temperature of the steel sheet becomes 650 ° C. or lower.