JPS61195702A - Rolling method for hot rolled steel sheet - Google Patents

Abstract

PURPOSE:To suppress scale formation by cooling in the atm. without requiring a large-scale installation in hot rolling in which the temp. of a steel sheet after final hot finish rolling is high by cooling quickly the surface of the steel sheet to a prescribed temp. or below to a prescribed temp. or below right after the rolling and cooling the surface without allowing the temp. to rise to the prescribed temp. or above. CONSTITUTION:The section of about 10m between a final finishing mill 1 and a hot run table cooler 3 is utilized and the steel sheet 5 of >=800 deg.C surface temp. emitted from the mill 1 is quickly cooled to <=650 deg.C surface temp. within several seconds by a quick cooler 2. The sheet 5 is thereafter coiled while the sheet is cooled so as not to be heated to >=650 deg.C owing to the recuperation of heat by the cooler 3. The formation of the surface scale on the steel sheet after the finish rolling in a hot rolling line is thereby considerably suppressed and therefore the need for a pickling treatment prior to the cold rolling is eliminated and the scale loss is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method for hot rolling a steel plate, and particularly to a method for rolling a hot rolled steel plate that can be rolled up while preventing the occurrence of surface scale on the hot rolled steel plate. Regarding the method.

Conventional technology and its problems Surface scale generated during hot rolling is generally removed by pickling, but recently the pickling process has been simplified by preventing scale from occurring during the hot rolling process. Various hot rolling methods have been proposed.

For example, JP-A No. 53-43661 discloses a method of covering the section from the final finishing stand to the winding machine with a tunnel-shaped compartment containing non-oxidizing gas (reducing gas) to prevent the formation and growth of scale. Disclosed. However, this method requires very high equipment costs and poses a safety problem because it is not possible to completely seal the inlet and outlet of the material to be rolled.

Furthermore, it is unclear how to deal with misrolls in one compartment, making it difficult to implement.

In addition, in JP-A-54-56060, a cooling tank is provided in the same section as above, and the cooling is done by immersing in the cooling tank, and the entire section including the cooling tank is made non-oxidizing by the same means as above. A method of preventing scale formation by creating a gas atmosphere has been disclosed, but this method also requires large-scale equipment and poses safety problems, and 5j!
It is difficult to administer.

As described above, conventional techniques that have been proposed as methods for preventing scale generation in the hot rolling process and simplifying or omitting pickling treatment have problems with equipment costs, safety, etc., and are difficult to implement. It was something.

Purpose of the Invention The present invention was made in view of the above-mentioned conventional problems, and it suppresses the generation of surface scale by cooling treatment in the atmosphere without requiring large-scale equipment, and eliminates the need for pickling treatment. The purpose of this paper is to propose a method for hot rolling and rolling steel sheets that can be simplified or omitted.

Structure of the Invention The method for rolling a hot-rolled steel sheet according to the present invention is such that, in hot rolling where the surface temperature of the steel sheet is 800°C or higher after hot-rolling finish rolling, the surface of the steel sheet is heated to 650°C within a few seconds immediately after the rolling. The steel sheet is rapidly cooled to a temperature below 650° C., and thereafter cooled so that the surface temperature of the steel sheet does not exceed 650° C. to prevent scale formation before winding.

Here, the above-mentioned rapid cooling is preferably carried out using rapid cooling equipment that starts cooling the upper and lower surfaces of the steel plate after rolling within 1 second and can cool the steel plate to 650° C. or less within 2 seconds. This is based on the recognition that the amount of scale generated at a steel plate surface temperature of 80°C or higher is significant, and that if the cooling time during this period is shortened as much as possible, the amount of scale generated can be naturally suppressed. It is something that

The method of this invention will be explained in detail below.

As shown in Fig. 5, which shows the outline of the conventional cooling equipment, in the current hot rolling process, a hot feed table roller (6) is installed over a length of about 10,011 mm from the final finishing mill.
A water cooling facility with an upper spray nozzle /L/(7) and a lower spray nozzle /I/(8) is provided in this part. Final finishing mill (1) From the exit side to the water cooling equipment is 1
It is about 0jl away. With this water cooling equipment, the cold rolling direction is generally cooled from the final finishing rolling temperature to around 600°C. For example, for a steel plate with a thickness of 3.2 layers, the ION from the final finishing mill to the water cooling equipment is 1.
5 seconds and 12.7 thick steel plates are air cooled for 5 seconds each,
After that, it takes 10 seconds and 30 seconds to complete the cooling, respectively.

As described above, with the current cooling method, it takes a long time to reach 650° C. or lower, at which point the generation of surface scale is extremely slow, and scale of 10 μm or more is generated.

Therefore, in this invention, by using the 10W1 section between the final finishing rolling mill and the cooling equipment of the hot run table,
In this section, the steel plate leaving the final finishing mill is cooled to 65% by rapid cooling equipment within a few seconds, preferably within 3 seconds.
The method was to rapidly cool the material to below 0°C, and then use a hot-fun table cooling system to cool it so that the temperature did not exceed 650°C due to reheating before winding it up. .

Here, the reason why the steel plate leaving the final finishing mill is rapidly cooled to 650° C. or less within several seconds, preferably within 3 seconds will be explained in detail.

In the case of low alloy steel, it is known that scale formation follows the parabolic law at temperatures above 200°C. That is, Z''=Kp-tz: [μ] Acid oxide film thickness Kp: Proportionality constant (depends on temperature) Dioxidation time (sec) Therefore, in order to reduce the oxide film thickness, the oxidation start time is Immediate post-processing is important.

FIG. 2 shows an example of scale thickness immediately after cooling at a cooling rate of 15'c/sec (air cooling). The temperature in the figure is the cooling start temperature. From this figure, it can be seen that a large amount of scale is generated immediately after oxidation. In the hot rolling process, about 80% of the scale during winding is generated in the first 5 to 6 seconds after final finish rolling.

Moreover, FIG. 3 is a plot of the amount of scale generated when cooling from each of the above-mentioned cooling start temperatures and keeping the cooling rate constant to 200° C. or less.

From FIG. 3, the thickness of the generated scale can be reduced by increasing the cooling rate.

Furthermore, as shown in FIGS. 2 and 3, it can be confirmed that scale formation during the cooling process is suppressed at temperatures below 650°C.

The pickling equipment, waste acid treatment equipment, etc. used in the hot rolling process can be expected to be significantly simplified if the thickness of the generated scale is halved to 5μ, and furthermore, pickling can be omitted if the thickness is reduced to 2μ or less.

Confirming this condition by referring to Figures 2 and 3, if the final finishing rolling temperature is 900°C, the 5μ
It is not possible to set the scale thickness below. Next, the cooling rate of rapid cooling is 100°C/second or more, as shown in Figure 3.
In other words, in the case of rapid cooling from 900°C to 650°C, 2.5
It must be rapidly cooled in seconds or less.

From these results, in order to suppress the formation of scale during hot rolling, it is effective to rapidly cool the steel sheet surface within a few seconds immediately after the start of oxidation (1) and within one second after leaving the final stand for finishing rolling. It can be seen that if cooling is started and the temperature is rapidly cooled to 650° C. or lower within 2 to 3 seconds, scale formation can be suppressed to approximately 5 μm or less.

FIG. 1 shows an example of an apparatus configuration for carrying out the method of the present invention, in which (1) shows a final finishing mill of a hot rolling line;
(2) is a rapid cooling device, (3) is a hot run table cooling device, (4) is a daven coif, and (5) is a steel plate. Since it has to be started, it is installed at a position approximately 1 m away from the rolling mill (1). This cooling device is capable of heating up to 650°C from a high temperature of 800°C or higher within 2 to 3 seconds after starting cooling.
Because it must be rapidly cooled down to below 10°C,
A mist jet cooling device having a cooling capacity of k-1"·hr·° C. is used. The length of this cooling device varies depending on the speed of the rolling mill, but is preferably 10 #1.

In addition, after exiting the final finishing mill (1), a rapid cooling device (
In the air-cooled part (approximately 1 m) during cooling in step 2), the surface of the steel plate may be covered with a water stream or the like to isolate it from the atmosphere, if necessary.

Implementation example: 1m from the exit of the final stand of the finishing mill for hot rolling fins
A mist jet cooling system was installed at a separate location over approximately 10 beaks, and an existing hot run table cooling system was installed over approximately 100'll on the exit side of this cooling system, and a steel plate with a thickness of 3.2 m + at the exit of the final stand was installed as described above. The scale thickness when winding is performed while cooling with a mist jet cooling device and a hot-fun table cooling device is compared with that when winding is performed while cooling with only a hot-fun table cooling device without using a mist-jet cooling device. are shown in Table 1.

In this example, the finish rolling temperature was 900°C, the coiling temperature was 400 to 480°C, the cooling rate of the mist jet cooling device was 340''Q/sec, and the cooling rate of the hot funnel table cooling device was 45°C/sec. .

As is clear from the results in Table 1 @ Table 1, when only the hot run table cooling device is used without using the mist jet cooling device, a large amount of scale is generated, and the amount of scale at the time of winding is 11.
On the other hand, in the case of the method of the present invention, there was little scale formation, and the amount of scale hardly changed during winding, and could be suppressed to 2.5 μm. Note that FIG. 4 shows the results of a computer simulation of changes in temperature and scale thickness after finish rolling with respect to the results in Table 1.

Effects of the Invention As is clear from the above examples, according to the method of the present invention, the formation of surface scale on the steel sheet after finish rolling on the hot rolled fins can be significantly suppressed, so that the pickling treatment before cold rolling can be omitted and Scale loss can be reduced. Furthermore, the method of the present invention can be implemented by installing a rapid cooling device capable of rapid cooling in the atmosphere immediately after the final stand of finish rolling, such as a mist jet cooling device, in combination with an existing sump-out table cooling device. This method has the effect of lowering equipment costs compared to conventional cleaning methods in which the surface of the steel plate is isolated from the atmosphere using non-oxidizing gas, etc., and there is no problem in terms of safety, and it is fully practicable. Ru.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an example of the configuration of an apparatus for carrying out the method of this invention, Fig. 2 is a diagram showing an example of the scale thickness of a deferred steel plate immediately after cooling, and Fig. 3 is a diagram showing an example of the cooling rate of a hot rolled steel plate. FIG. 4 is a diagram showing the relationship between scale thickness; FIG. 4 is a diagram showing the change in scale amount and material temperature in an embodiment of the invention;
The figure is a cross-sectional view schematically showing a conventional cooling facility. 1... Final finishing rolling mill, 2... Rapid cooling device,
3.Hot-fun table cooling device, 4.Download, 5.Steel plate. Applicant Sumitomo Metal Industries Co., Ltd. Agent 1) Yoshihisa, '771J
:Go: Figure 4 Figure 5 Figure 1 Figure 2 Cooling time (see) Figure 3 Cooling rate (°tsu'set)

Claims (1)

[Claims] In hot rolling in which the surface temperature of the steel plate after final finish rolling is 800°C or higher, the surface of the steel plate is rapidly cooled to 650°C or lower within a few seconds immediately after the rolling, and thereafter the surface temperature of the steel plate is 650°C or lower.
A method for rolling a hot-rolled steel sheet, which comprises cooling to a temperature not higher than 50°C to prevent the formation of scale, and then winding the sheet.