JPS5932201B2 - Hot rolling method for steel materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of preventing a decrease in the depth of a steel material during hot rolling of the steel material and rolling a steel material with fewer surface defects.

In recent years, in the hot rolling of steel materials, for the purpose of energy saving, the so-called direct rolling method, in which the billet is directly transferred to the rolling process after blooming and rolling, and the direct rolling method following continuous casting, that is, the CC-DR method, have been adopted. being developed.

In either rolling method, the steel billet must be kept as hot as possible before being transferred to the rolling process. Heat loss during transfer is large, especially at the top, bottom, and widthwise edges of the steel. Heat loss is significant, and the hot water temperature tends to drop to a level unsuitable for rolling.

Now, the scale that is generated in the hot rolling process of steel materials, that is, the primary scale that is generated during the heating of the steel material, and the secondary scale that is generated in the hot rolling process after the primary scale has been removed, both cause surface defects after rolling. Cause.

Therefore, as is well known, it is essential to remove these scales before rolling the steel material.

In the conventional descaling method, the descaling equipment has a high water pressure and a high water flow rate, and high pressure water with a water pressure of more than 100 kg/d and an injection water amount of more than 501/m1 nt is injected onto the steel surface in order to completely remove scale. This led to the use of descaling methods.

Therefore, the cooling of the steel material is large, and the amount of temperature reduction of the steel material per section of the high-pressure water descaling device is 100 to 12
The current temperature is reaching 0℃.

As mentioned above, in the direct rolling method and CC-DFm, there is heat loss during the transfer of the steel billet, and cooling of the steel material when the scale generated in the hot rolling process is descaled with a high-pressure water descaling device, which causes damage to the top part of the steel material, especially in the top part of the steel material. The problem is that the bottom part and widthwise edge part have a noticeable drop in hot metal temperature during rolling, and the material quality deteriorates due to the drop in hot rolling inlet hot rolling temperature, and red scale defects are likely to occur in these parts. be.

In order to avoid such problems, in addition to paying attention to the slurry sampling during transfer, it is desirable to completely remove the scale with a smaller amount of water during descaling, and to minimize the drop in hot water temperature of the steel material. .

Attempts to achieve the above-mentioned objective have been published in the well-known methods, and one of the means is to reduce the secondary scale generated on the surface of the steel material during hot rolling by using a group of low-pressure water jets of less than 10 mm. This is a descaling method using a 3O-50ky/CIIL water jet injection device installed immediately after passing the spray zone and low pressure spray zone, and by using a full-surface water spray in the low pressure spray zone, the steel surface is This is a descaling method in which the scale formed on the surface is exposed to a low-pressure water jet for 1 to 2 seconds and becomes flaked off, and then a high-pressure water jet blows away the flaked scale pieces and eliminates them. .

However, this method has the disadvantage that the temperature of the steel material is considerably lowered by the low-pressure spray zone and the water jet injection device installed immediately after the zone.

Therefore, the present inventors researched a method for completely descaling with an even smaller amount of water, and as a result, developed the method of the present invention, which has the following characteristics: a. It utilizes two-fluid spraying using air and water. (b) Furthermore, the purpose is to reduce the amount of water as much as possible to achieve the objective; (C) Furthermore, the purpose is to further reduce the temperature drop of the steel material to completely descale it.

The gist of the present invention is to provide a hot rolling method for steel materials having a rough rolling and a finish rolling process, in which an air/water spray zone is provided between the rough rolling and finish rolling processes, and the zone is heated within a period of 1 second to 30 seconds. By passing the steel material through the steel material, minute water droplets are attached to the surface of the steel material at a water density in the range of 0.2 to 75 cc 7 m2・sec, and then at the entrance side of the finishing mill, the water pressure is 55 kg/ff1 or more and 130 kg/i or less. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a method for hot rolling a steel material, characterized in that surface scale of the steel material is removed by high-pressure water descaling and finish rolling is performed, and the present invention will be described in detail.

In the present invention, the air/water spray zone is a zone in which minute water droplets with a water density of 0.2 to 75 CC/m2・sec adhere to the surface of the steel material (rolled material) while the steel material passes for 1 to 30 seconds. In order to prevent the temperature of the steel material from decreasing, the water flow rate is kept as low as possible using an air-water spray device, and particles with a particle size of approximately 10 to 200 μ are sprayed to remove scale on the surface of the steel material. Good results are obtained by rapidly cooling only the layer.

In addition to well-known spray devices such as the simple air-water mixer used in salt water spray testers, etc.

The purpose can be achieved by appropriately selecting a commercially available nozzle dedicated to mixing air and water that has a small water flow rate and can spray an appropriate water droplet size.

FIG. 1 is a schematic diagram showing an embodiment of an apparatus for carrying out the method of the present invention, in which 1 is a rough rolling mill, 2 is a material to be rolled, that is, a steel material,
3a to 3d are air and water spray nozzles, 4a and 4b are air pipes,
5a and 5b are the pressure control valves, 6a and 6b are water pipes, and 7
8a and 8b are high-pressure water descaling nozzles, and 9 is a finishing mill.

In FIG. 1, the steel material 2 passes through an air/water spray zone formed by air/water spray nozzles 3a to 3d between a rough rolling mill 1 and a finishing rolling mill 9 at a speed of 0.2 to 75 CC/m''·sec. Microscopic water droplets can be attached to the surface with a water density of .

According to research by the present inventors, when the spray water flow rate and water droplet particle size are large, such as with a well-known spray nozzle, the water droplets become coarse due to spraying, and water vapor forms between the water droplets and the steel surface. It is recognized that because a film is formed, the cooling effect of the scale layer is reduced and the descaling property is not improved much.

In contrast, in the method of the present invention, only the scale layer is rapidly cooled, which increases the cooling thermal stress at the interface between the scale and the steel base, causing the scale to rise from the base steel and become separated, resulting in extremely descaling. It becomes easier to do.

Furthermore, since the absolute amount of water is small, a good result is obtained in that the temperature drop in the steel material is also very small.

In addition, in the present invention, if two-fluid spray nozzles are installed at appropriate intervals or by intermittently spraying, the cycle of water droplet adhesion → evaporation is repeated over time on the steel plate surface, and an even more excellent descaling effect can be obtained. can be obtained.

In the present invention, the reason why the water density is set to 0.2 to 75 CC7m2・sec for the water to be attached to the steel plate surface is as follows.
This is because when using the 75 cc, 'm''・se mountain d method, a water vapor film is formed on the surface of the steel material, especially the edges of the steel material, which are too cooled. As a result, descaling performance is not improved and surface scratches are more likely to occur, so 0.2 CC/
m2·sec or more and 75 cc/m”·sec or less.

Furthermore, the reason why the time for the steel material to pass through the air-water spray zone in the present invention is set to 1 second to 30 seconds was determined through numerous experiments. There is no big difference in descaling performance from the water descaling method, but if it exceeds 30 seconds, the speed of the steel material will slow down and heat dissipation on the transfer table will be significant, and oxidation will become intense, descaling performance will decrease, and heat retention etc. This is because, in addition to increasing equipment costs during the process, problems also arise in terms of operation and productivity.

The existence of this appropriate value for the transit time is presumed to be due to the as-yet unknown reason for the peeling off of the scale by adhering water droplets.

Now, the air-water spray nozzle for forming the air-water spray zone may be installed not only on the upper and lower surfaces of the steel material but also on the side surface, and the air-water spray nozzle for forming the air-water spray zone may be installed in the vertical direction. The air/water spray zone can be installed vertically, or it can be installed so that the length of the air/water spray zone can be changed by changing the orientation angle of the air/water spray nozzle.

Further, the arrangement of the air/water spray nozzles may be arbitrarily selected in the width direction and the longitudinal direction depending on the width of the steel material and the sheet passing speed.

Furthermore, if necessary, a cover may be attached to cover the air/water spray nozzle on the rolling table.

By spraying air and water as described above, the secondary scale generated on the surface of the steel material forms speckled bulges and floats from the surface of the steel material, making it easy to peel off.

Furthermore, since the secondary scale layer generated on the surface of the steel material is mainly rapidly cooled by the air/water spray rather than the main body of the steel material, the temperature of the steel material is significantly less likely to be lowered.

As mentioned above, the secondary scale that has become easily peeled off by air/water spraying is then exposed to a pressure of 55 to 130 k.
It is removed by high-pressure water descaling at g/'ffl, but below 55 kg/i, the secondary scale removal effect decreases, and above 130 kg/i, the scale removal effect becomes saturated, lowering the temperature of the steel material. The reason for limiting the pressure as described above is that it is not economical.

In the present invention, since the water volume density is small, it has been confirmed that a high pressure of 55kj9/d or more is preferable for high-pressure water descaling as described above.
The principle behind this is not clear.

The inventors estimate that there are differences in phenomena such as scale removability, form of bulges, and cracking of secondary scale depending on the particle size, amount, and collision speed of attached water droplets.
This seems to determine the appropriate pressure value for high-pressure water descaling.

The method of the present invention involves not only the so-called reheating method in which the steel ingot is soaked and bloomed, then treated to remove defects, then charged into a heating furnace, and subjected to rough rolling and finish rolling, as well as direct rolling of the steel billet immediately after blooming. In addition to plate steel materials that are rolled in the direct rolling method where the product is transferred to the process and rolled, the method that involves continuous casting followed by heating and rolling, and the direct rolling method that follows continuous casting, it can also be applied to steel sections, steel bars, and wire rods. It also has the remarkable effect of reducing the temperature drop in steel materials, reducing the occurrence of scale defects, and saving the amount of water during descaling.

Example-1 After heating a low carbon rimmed steel ingot in an atmospheric furnace at 1250°C,
After descaling with high pressure water of 0 kg/crit to remove primary scale, hot rough rolling was carried out immediately, and then (1
) air cooling (conventional method-1), (2) cooling with a low-pressure water jet of 10 kg/ffl or less using a commercially available full-surface water spray nozzle (conventional method-2), and (3) after spray cooling with an air-water spray nozzle (invention method), high pressure water descaling water pressure to O
FIG. 2 shows the results of investigating the amount of temperature drop on the surface of the steel material when finish hot rolling was performed after descaling at ~110 kg/ffl, and FIG. 3 shows the results of investigating scale flaws.

Note that the time from rough rolling to descaling and finish rolling was about 30 seconds.

As shown in FIG. 2, in the conventional air cooling method (0) for cooling after rough rolling, when the descaling water pressure is varied from 0 to 11 ok/=, the surface temperature of the steel material decreases by 80 to 105°C.

In addition, when cooling with a low-pressure water jet of 10 kg/i or less using a commercially available one-fluid (water) all-over spray nozzle in conventional method (2), the surface temperature of the steel material decreases by 90 to 115 degrees Celsius during cooling, compared to air cooling. The temperature decreases by about 10°C.

On the other hand, with the spraying using the air-water spray nozzle of the method of the present invention, the surface hot water temperature of the steel material decreases by 80 to 105°C,
The amount of temperature decrease is the same as that of air cooling, and it can be said that there is almost no decrease in the humidity of the steel material due to spraying.

Furthermore, as shown in Fig. 3, conventional method 1 with natural cooling, that is, air cooling, and conventional method 2 with a commercially available all-surface water spray nozzle using a low pressure water jet of 10 kg/i or less,
According to research by the present inventors, scaling defects do not occur when the high pressure water descaling water pressure is 110 kg/i;
It was found that scaling defects occurred when the descaling water pressure was lowered below that level.

On the other hand, in the spraying using the air-water spray nozzle of the method of the present invention, scale defects did not occur even when the high-pressure water descaling pressure was lowered not only to 110 kg/criY but also to 55 kg/cwt.

Example-2 Low carbon rimmed steel ingot (or slab) heated to 1000°C to 1250°C
After heating in an atmospheric furnace at ℃, descaling with high pressure water of 110 kg/CTt, and immediately rough rolling, followed by (1) air cooling, (
2) Cooling with a low-pressure water jet of 10 kg/i or less with a commercially available full-surface water spray nozzle, and (3) When spraying with an air-water spray nozzle, change the spray time from less than 1 second to 30 seconds or less. After that, reduce the high pressure water descaling water pressure to 1
Table 1 shows the results of examining scale flaws in steel materials when descaling was performed by changing to 9/Crl from 0 to 110, and then finishing rolling was performed.

Note that the time from rough rolling to descaling and finish rolling was about 30 seconds.

As shown in Table 1, in the conventional method-1 air cooling after rough rolling, scale defects are likely to occur when the steel material temperature during high-pressure water descaling is on the high and low temperature sides.

In addition, 10 k using conventional method-2, a commercially available full-surface water spray nozzle.
When spraying with a low-pressure water jet of g/crlL or less, scale defects are likely to occur when the steel material temperature during high-pressure water descaling is from a high temperature side to a low temperature side.

On the other hand, in the case of spraying using the air-water spray nozzle of the method of the present invention, if the spraying time is less than 1 second, it is not much different from that in which high-pressure water descaling is performed after air cooling. 110kg after spray cooling for seconds
/cr! When high-pressure water descaling is performed, scale defects will not occur in all areas where the steel material temperature during descaling ranges from high temperature to low temperature.

Furthermore, even if high-pressure water descaling of 110 kg/cwt is performed after spray cooling for 6 to 30 seconds during the process from rough rolling to finish rolling, the descaling temperature remains in all regions from the high temperature side to the low temperature side. No scale defects occurred, and the same effect as spraying for 2 to 5 seconds was observed.

Furthermore, after spraying for 2 to 5 seconds or 6 to 30 seconds during the process from rough rolling to finish rolling, the descaling temperature changes from the high temperature side to the low temperature side even if the high pressure water descaling-IJ song pressure is lowered to 551 g/ffl. No scale defects occurred in all areas, and the result was the same as when descaling with high pressure water of 110 kg/crit was performed after spraying.

However, if the descaling water pressure after the separation of secondary scale by spraying is lowered to 10 vd, scale defects are likely to occur in all regions where the descaling temperature ranges from the high temperature side to the low temperature side.

Therefore, in the present invention, preferably 2 seconds for 1 to 30 seconds.
After ~5 seconds of spray cooling, even if the high-pressure water descaling water pressure was lowered to 55 kg/CIIt, scale defects did not occur in all areas where the descaling temperature ranged from the high temperature side to the low temperature side, and the amount of water was very small. The purpose can be achieved with less temperature drop of the steel material.

As described in detail above, the method of the present invention provides a method for economically producing steel products with few scale defects in hot rolling of steel products.

[Brief explanation of the drawing]

Fig. 1 is a schematic explanatory diagram of an embodiment of an apparatus for carrying out the method of the present invention, and Fig. 2 shows a low carbon rimmed steel ingot heated to 1250°C.
After heating in an atmospheric furnace of
After spraying with a low-pressure water jet of 10 kg/ffl or less using a commercially available full-surface water spray nozzle, and (3) spraying with an air-water spray nozzle, high-pressure water descaling water pressure is set to 0 to 1.
A diagram showing the amount of temperature drop in the steel material when finishing rolling was performed after descaling by changing to 10 U/CrIL, and FIG. 3 is a diagram showing the results of examining scale defects using the above method. 1... Rough rolling mill, 2... Rolled material, 3
... Air water spray nozzle, 4 ... Air piping, 5 ... Pressure control valve, 6 ... Water pipe, 7
...Flow rate control valve, 8...High pressure water descaling nozzle, 9...Finishing rolling mill.

Claims (1)

[Claims] 1. In a hot rolling method for steel materials having rough rolling and finishing rolling steps, an air/water spray zone is provided between the rough rolling and finishing rolling steps, and the steel material is passed through the zone for a period of 1 second or more and 30 seconds or less. By this, the water density is 0 on the steel surface.
．． Allow minute water droplets to adhere in the range of 2 to 75 cc/rn”・sec, and then apply a water pressure of 55 ky on the entry side of the finishing mill.
1. A method for hot rolling a steel material, comprising removing scale on the surface of the steel material by high-pressure water descaling at a pressure of not less than /i and not more than 130 ky/ffl, and then performing finish rolling.