JPH08300040A - Straightening method of thick steel plate - Google Patents

Abstract

PURPOSE: To obtain a hot straightening method to minimize restraightening rate in a desirable straightening method of hot rolled steel plate. CONSTITUTION: In executing hot straightening of the thick steel plate after hot rolling, straightening is started at the timing where an average temp. B in steel plate surface is in the range from A3 transformation start temp. to A1 transformation completion temp of the steel. Further, straightening can be started at the timing where an average temp. G in steel plate surface is turned roughly equal to A3 transformation start temp. or A1 transformation completion temp. of the steel. At this time, an average temp. in steel surface is can be either of an arithmetic average temp. of max temp. and min temp., a linear average temp. or an area average temp. in steel plate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for straightening hot-rolled steel sheet online.

[0002]

2. Description of the Related Art When a thick steel sheet subjected to finish rolling is conveyed on a conveyor table and hot-corrected, if there is temperature nonuniformity in the steel sheet, residual stress occurs in the steel sheet after air cooling, resulting in defective shape. It is known to occur. Possible causes of temperature nonuniformity during hot straightening include heating nonuniformity during slab heating, temperature nonuniformity during rolling, and nonuniform cooling during descaling.

Conventionally, in order to solve this problem, at the time of hot straightening, the temperature distribution is made uniform, the central portion of the steel sheet is weakly cooled, and the width end portion of the steel sheet which is usually at a low temperature is heated. Has been considered.

In Japanese Patent Laid-Open No. 62-81217, the temperature distribution in the plate width direction of the steel plate on the entrance side of the roll straightening machine is measured, and the temperature of the measured temperature distribution in the plate width direction between the high temperature part and the low temperature part is measured. If the temperature difference is a temperature difference that does not cause cooling distortion after straightening cooling, it is corrected as it is.If the temperature difference is a temperature difference that causes cooling distortion after straightening cooling, the temperature difference is cooling distortion after straightening cooling. There is disclosed a method of controlling the temperature of the steel sheet in the plate width direction so as to be equal to or less than a temperature difference that does not cause the temperature difference and then correcting the temperature. The temperature control method is to heat the low temperature part of the steel plate,
Alternatively, it is performed by cooling the high temperature portion.

The straightening method described in Japanese Patent Application Laid-Open No. 6-7847 is a method of finish rolling a thick steel plate and then straightening it with a hot straightening machine. Air cooling, thickened steel plate central part with cooling water amount of 0.1 m ³
/ M ² / min to 0.4 m ³ / m ² / min. This is because by making the temperature of the steel sheet edge part after hot straightening higher than the temperature inside the steel sheet, the residual stress after air cooling is made a tensile stress at the steel sheet edge part, suppressing the occurrence of seismic waves, and making a flat thick steel plate. Let's manufacture.

In the method described in Japanese Patent Laid-Open No. 6-285530, the width end of a steel plate having a thickness of 10 mm or less after rolling is at least 50 mm from the width end and 200 mm at maximum.
In the range from the end of rolling to the end of hot straightening over the entire length of the steel sheet, and the temperature difference between the center temperature of the steel sheet and the average temperature of the heating range satisfies the specified conditional expression. is there. This is to reduce or distribute the compressive stress generated at the steel plate width end by heating the steel plate width end during the process from the end of rolling to the completion of hot straightening.
The stress distribution does not cause buckling.

[0007]

However, in such a method, although the flatness of the steel sheet can be improved to some extent, partial cooling or heating must be performed,
None of them are economical, and the incidence of re-correction has not been significantly reduced. An object of the present invention is to provide a method for reducing the residual stress of a steel sheet after straightening by a simple means so that the occurrence rate of re-straightening can be significantly reduced.

[0008]

[Means for Solving the Problems] The above-mentioned problems are as follows. When the average temperature obtained from the maximum temperature and the minimum temperature in the plane of the steel plate on the inlet side of the straightening machine is between the transformation start temperature and the transformation end temperature of the steel sheet, Alternatively, the invention described below has been achieved based on the finding that it can be achieved by air-cooling or water-cooling so that the temperature becomes almost equal to the transformation start temperature or the transformation end temperature and then the correction is started.

According to the first aspect of the present invention, when the thick steel sheet after hot rolling is hot-corrected, the average temperature in the plane of the steel sheet is in the range from the A ₃ transformation start temperature to the A ₁ transformation end temperature of the steel type. There is provided a method for straightening a thick steel plate, characterized by starting straightening at a certain point.

According to the second aspect of the invention, when the thick steel sheet after hot rolling is hot-corrected, the average temperature in the plane of the steel sheet is almost equal to the A ₃ transformation start temperature or A ₁ transformation end temperature of the steel type. Provided is a method for straightening thick steel plates, which is characterized by starting straightening when they are equal.

According to the invention of claim 3, the average temperature is
The arithmetic mean temperature obtained from the maximum temperature and the minimum temperature, the line average temperature obtained by dividing the measurement result of the line temperature distribution in the steel plate plane by the line length, or the surface temperature distribution in the steel plate plane The area average temperature obtained by integrating the measurement result and dividing the result by the total area is used as one of the areas.
There is provided a method for straightening a thick steel plate described in any one of 1.

[0012]

The principle of the present invention can be explained as follows. FIG. 1 schematically shows a thermal expansion curve when carbon steel is heated to an austenite state and cooled.
The vertical axis of FIG. 1 represents the change in length, the upper part represents expansion and the lower part represents contraction. The horizontal axis represents temperature. The A ₃ transformation starts at point A and the A ₁ transformation ends at point B in the figure.

Further, when the temperature of the steel sheet after rolling is near the A ₃ transformation start point A point, if the maximum temperature in the plane of the steel sheet is H point and the minimum temperature is F point, the maximum temperature and the minimum temperature are If the average temperature G point near point A comes close to point A, the thermal strains of the steel plate portions corresponding to points H and F can be made almost the same, so if hot correction is performed from this point, the entire plate surface will be at room temperature. It is considered that the strain, which is generated when it becomes, that is, the residual stress can be minimized.

Similarly, when the temperature of the steel sheet after rolling is near the A ₁ transformation end point B, the maximum temperature in the plane of the steel sheet is point E, and the minimum temperature is point C. If the average temperature of point D is near the point B, the thermal strains of the steel plate portions corresponding to points E and C can be made almost the same, so if hot correction is performed from this point, the entire plate surface will be at room temperature. It is considered that the strain, which is generated when it becomes, that is, the residual stress can be minimized.

When the steel sheet is straightened in the cooling process after rolling, point A is the Ar ₃ transformation point and point B is A
This is the end point of the r ₁ transformation. In the case where the present invention corrects the steel sheet by reheating it, the point A is the Ac ₃ transformation point, and the point B is the Ac ₃ transformation point.
_{It is one} transformation point.

From the viewpoint of thermal strain only, the average temperature is A
It is subjected to hot straightening at any ₁ transformation finish point or A ₃ transformation start point time, although the residual stress after hot straightening should no difference, the average temperature reaches the A ₁ transformation end point It is preferable to carry out hot straightening at that point because the possibility of plastic deformation is reduced because the average temperature of the steel sheet is lowered.

As the above-mentioned average temperature, the arithmetic average temperature of the maximum temperature and the minimum temperature ((maximum temperature + minimum temperature) / 2) may be used, but the measurement result of the linear temperature distribution in the steel sheet plane is linearly integrated. Then, the line average temperature obtained by dividing by the length of the line or the temperature in the steel plate surface was measured with a surface scanning type temperature distribution meter, the area of the measurement result was integrated, and the area obtained by dividing by the total area was obtained. The average temperature is desirable because the thermal strain of the entire steel sheet can be made smaller.

Although the above is the basic idea of the present invention, a simulation calculation was carried out under predetermined conditions for more specific examination. Plate thickness 2 where there is a temperature difference on the steel plate surface
A 5 mm steel plate was hot-corrected, and the relationship between the residual stress generated after air cooling and the temperature distribution on the hot-straightening machine entrance side was examined by simulation using the thermoelastic-plastic finite element method.

Here, the temperature difference between the maximum temperature and the minimum temperature is 40 ° C., and the cooling condition is that the steel sheet is air-cooled. In addition,
The steel sheet is supposed to be cooled symmetrically in the vertical plane. Also, the Ar ₃ transformation start temperature is 750 ° C and the Ar ₁ transformation end temperature is 700 ° C.
I assumed. Table 1 shows the calculation results of seven cases in which the temperature conditions of the steel plate surface are different.

The residual stress ratio in the table is the residual stress value which is the difference between the maximum residual tensile stress and the minimum residual compressive stress after air cooling, and the residual stress values of Cases 1 to 7 are normalized by the residual stress value of Case 1. It is the value.

In Table 1, Cases 2 and 3 are cases in which the arithmetic average temperature as the average temperature of the maximum temperature and the minimum temperature of the plate surface is approximately equal to the A ₃ transformation start temperature of 750 ° C. _No. 6 is an example in which the arithmetic average temperature of the maximum temperature and the minimum temperature of the plate surface is substantially equal to the A ₁ transformation end temperature of 700 ° C. It can be seen that if the average temperature of the maximum temperature and the minimum temperature of the steel sheet surface becomes substantially equal to the A ₃ transformation start temperature or the A ₁ transformation end temperature, if the straightening is started, the residual stress is significantly reduced.

As described above, by making the average temperature obtained from the maximum temperature and the minimum temperature of the steel sheet surface approximately equal to the A ₃ transformation start temperature or the A ₁ transformation end temperature, the residual stress is reduced as described in FIG. It can be proved by calculation that it is minimized.

[0023]

EXAMPLE FIG. 2 shows a schematic configuration of a rolling line for carrying out the present invention. 1 is a finish rolling mill, 2 is an accelerated cooling device, 3 is a shower device, and 4 is a hot straightening device. This hot straightening machine has a roll diameter of 280 mmφ and is composed of four rolls on the upper side and five rolls on the lower side.

The starting temperature of the A ₃ transformation during slow cooling is 814 ° C.
The A ₁ transformation end temperature is 720 ° C. Using the as-rolled general material having strength of 40 kgf and 50 kgf, the temperature condition of the steel sheet on the inlet side of the hot-rolling machine 4 was changed to perform hot-rolling. . In Cases 1 to 4, the weak cooling using the accelerated cooling device 2 was not performed, but the cases 1 to 4 were transported to the hot straightening machine 4 by air cooling and were hot straightened at a predetermined temperature.

In case 5, the rolling finish temperature, the cooling in the accelerated cooling device, and the strip running speed were set so that the straightening start temperature was almost equal to the A ₁ transformation end temperature (720 ° C.), and the material test value was affected. No weak cooling was done. At the time of weak cooling by the accelerated cooling device, edge masking was performed to prevent overcooling of the widthwise end of the steel sheet. And hot straightener 4
The error from the target value was finely adjusted by shower water cooling with a thermometer (not shown) installed on the inlet side of the. The shower may be cooled by air instead of water.

With a thermometer installed on the inlet side of the hot straightening machine 4, the temperature distribution in the width direction of the central portion of the steel sheet is obtained and subjected to line integration,
The value obtained by dividing by the length in the width direction was taken as the average temperature. In each case, the quality of the plate shape was judged by a flatness meter using a laser range finder after the steel plate reached room temperature.

The results are shown in Table 2. Table 2 shows the dimensions of the steel sheet, the rolling finish temperature, the maximum temperature of the steel sheet, the minimum temperature, and the inlet temperature of the hot straightening machine. Case 2 is the average temperature of the steel sheet surface, the hot straightening machine entry side temperature A ₃ transformation starting temperature 814
When the temperature was set to 822 ° C., which is close to 0 ° C., the plate shape after cooling was good.

In case 5, the average temperature of the steel sheet surface is set to 715 ° C., which is near the A ₁ transformation end temperature of 720 ° C. as the inlet temperature of the hot straightener, and the plate shape after cooling is case 2 Was even better than. In Cases 1, 3, and 4 carried out as comparative examples, the plate shape was poor, and re-correction was necessary.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

As is apparent from the above description, according to the straightening method of the present invention, the residual stress of the steel sheet after straightening can be measured at a cost of installing a temperature measuring device on the inlet side of the hot straightening machine. It can be greatly reduced. Further, as a result, the re-correction occurrence rate can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an expansion curve when carbon steel is cooled from an austenite phase.

FIG. 2 is a schematic diagram of rolling equipment for carrying out the present invention.

[Explanation of symbols]

 1 Finishing rolling mill 2 Acceleration cooling device 3 Shower device 4 Hot straightening machine

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Miyazato 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inoue Munehiro Ishioka 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (3)

[Claims] 1. A hot rolled steel plate after hot rolling is hot-corrected, when the average temperature in the steel plate surface is within the range from the A ₃ transformation start temperature to the A ₁ transformation end temperature of the steel type. A method for straightening thick steel plates, which comprises starting straightening. 2. At the time of hot straightening a thick steel plate after hot rolling, when the average temperature in the steel plate surface becomes substantially equal to the A ₃ transformation start temperature or A ₁ transformation end temperature of the steel type. A method for straightening thick steel plates, which comprises starting straightening. 3. The average temperature is obtained by dividing the arithmetic mean temperature of the maximum temperature and the minimum temperature in the steel sheet surface and the measurement result of the line temperature distribution in the steel sheet surface by line integration and dividing by the length of the line. 3. The line average temperature or the area average temperature obtained by dividing the measurement result of the surface temperature distribution of the steel plate surface by the area and dividing the result by the total area. 3. For straightening thick steel plates.