JPH0351482B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a hot rolling method and a hot rolling apparatus for carrying out the method. More specifically, as the rolling progresses, the temperature of the rolled material decreases to below the Ar ₃ transformation point, and then the part of the material to be rolled is heated and rolled at an intermediate stage in the rolling line, thereby increasing the uniformity of the material. The present invention relates to a rolling method and a hot rolling device. (Prior art) In hot rolling, for example, when rolling hot strips, the material to be rolled is generally charged into a heating furnace and heated, and then subjected to rough hot rolling with multiple stands and finishing tandem rolling with multiple stands. Roll to specified dimensions. At this time, due to retention on the line and high-pressure water descaling, the temperature at the edge of the rough hot-rolled intermediate material (hereinafter referred to as bar), which has a large heat dissipation area, decreases, and the hot strip of the finishing hot-rolled bag contains mixed grains. There are defects that cause tissue formation and abnormal profiles. Figure 3 shows a schematic cross-sectional view of the mixed grain structure in the width direction.
101 is a mixed grain or coarse grain region, 102 is a fine grain region,
a is the thickness of the mixed grains on the front side, b is the thickness of the mixed grains on the back side, and t is the plate thickness. Such mixed grains are of poor quality and must be discarded, which causes a decrease in yield. As countermeasures against this problem, conventional methods include performing overcompensated high-temperature heating in anticipation of the temperature drop in the heating furnace, partial heating compensation for the bar edge part or the heating furnace skid mark part during rough rolling or after rough rolling and before finishing hot rolling, and special measures. The edge heating compensation during finish hot rolling proposed in JP-A-57-160502 is known. However, the above method is not a good idea because it involves heating up to the center of the board width where no heating compensation is required, resulting in a large energy loss. It is known that the proposed technology reduces energy loss in this order, but in both cases, the amount of temperature increase due to intermediate heating is such that the temperature at the edge or skid mark where the temperature has decreased is approximately the same as the center of the sheet width. It is proposed that the finishing hot rolling be completed at a temperature higher than the Ar ₃ transformation temperature. However, when the present inventors conducted an actual test and investigation based on these techniques on a line having seven finishing hot rolling stands F ₁ to F ₇ with the contents shown in Table 1 below, it was found that During hot rolling, the edge part, which once dropped below the Ar ₃ transformation point, is heated by electric induction heating to bring it to the same temperature as the center part of the sheet width.
A test was carried out in which the temperature was raised to a temperature above the Ar ₃ transformation point, followed by continuous rolling to ensure the final hot rolling completion temperature was above the Ar ₃ transformation point, a sample was taken from this material, and microscopic structure observation revealed that the edges had a mixed grain structure. There was a problem in which the mixed grain structure could not be resolved.

[Table] (Problems to be Solved by the Invention) The purpose of the present invention is to advantageously solve the problems of the prior art, and to provide a hot-rolled material having a uniform material without a mixed grain structure over the entire length and width. It is to obtain. Another object of the invention is to obtain hot rolled material of uniform quality with minimum energy. Still another object of the present invention is to reduce the local wear of the roll profile due to rolling of the edge temperature lowering material, which occurs in the prior art, to extend the life of the roll and to prevent the occurrence of material with abnormal product profile. (Means and effects for solving the problems) In order to achieve these objectives, the present inventors believe that the portions that have become ferrite grains due to a temperature drop below the Ar ₃ transformation point during hot rolling. At least before final hot rolling, the structure of the steel material is intermediately heated to a temperature exceeding the Ac ₃ transformation point to transform the ferrite structure into an austenite structure, and this austenite structure is subjected to at least one rolling process. ,
It was discovered that it is important to finish the final hot rolling at a temperature above the Ar ₃ transformation point. That is, the present invention provides a hot rolling method having a rough hot rolling stage for rough hot rolling of a steel material and a finishing hot rolling stage for finishing hot rolling of the steel material. During the finishing hot rolling, at least the part of the steel material that has become below the Ar ₃ transformation point is intermediately heated to bring the temperature of the steel material within the range of the intermediate heating target temperature of the Ac ₃ transformation point or above, thereby improving the structure of the entire rolled steel material. austenite, followed by at least one pass of reduction.
completing the finish hot rolling at a temperature within the range of the final hot finish exit target temperature of Ar ₃ transformation point or higher;
The degree of heating in intermediate heating based on at least the former of the difference between the actual measured temperature of the steel material immediately after intermediate heating and the intermediate heating target temperature and the difference between the actual measured temperature of the steel material immediately after final hot finishing and the final hot finishing outlet target temperature. a plurality of rough hot rolling stands, a plurality of finishing hot rolling stands provided successively to the rough hot rolling stands, and an adjacent finishing hot rolling method. An intermediate heating device that is installed between rolling stands or immediately before the most upstream finishing hot rolling stand in the rolling direction and that intermediately heats the rolled steel material and the Ac ₃ transformation point and Ar ₃ transformation point of the rolled steel material based on the composition of the rolled steel material. and this
A target temperature calculation device that calculates a heating target temperature of a steel material in an intermediate heating device _and a target temperature at the completion of finish rolling of a steel material in a final hot rolling stand based mainly on the temperatures of the Ac ₃ transformation point and the Ar 3 transformation point. The target temperature calculation device is operatively connected to the intermediate heating device to determine the heating output value of the intermediate heating device, and is further provided immediately after the intermediate heating device to detect the temperature of the rolled steel material immediately after the intermediate heating. a thermometer device installed immediately after the final finishing rolling stand to measure the temperature of the steel material immediately after finishing hot rolling, and a thermometer device for detecting the temperature immediately after intermediate heating. Determine the difference between the heating target temperature and the actual temperature of the steel material determined by the temperature detection thermometer device immediately after the completion of finish hot rolling and the target temperature at the time of completion of finish hot rolling, and at least based on the difference in the former. This hot rolling apparatus includes a control amount calculation device that changes the amount of heating in an intermediate heating device. In the hot rolling method of the present invention, the heating of the rolled steel material immediately before or during the final hot rolling is performed by:
This is done by finding the difference between the actual measured temperature of the rolled steel material immediately after heating during hot rolling and the heating target temperature, and changing the degree of heating so that this difference becomes virtually zero or within an allowable range. preferable. Furthermore, in the hot rolling method of the present invention, the heating of the rolled steel material immediately after high-pressure water descaling or during finishing hot rolling is performed to reduce the difference between the measured temperature of the rolled steel material immediately after heating during hot rolling and the heating target temperature. Also, find the difference between the actual measured temperature of the rolled steel material immediately after finishing hot rolling during hot rolling and the target temperature at the completion of hot finishing rolling, and make sure that both differences are substantially zero or within an allowable range. More preferably, this is carried out by varying the degree of heating. In the present invention, first, the temperatures of the Ac ₃ transformation point and the Ar ₃ transformation point of the rolled material are determined from the components of the rolled material by, for example, the following calculations. T (Ac ₃ ): Ac ₃ transformation point temperature T (Ar ₃ ): Ar ₃ transformation point temperature, T (Ac ₃ ) = aC + bSi + cMn + dAl + e T (Ar ₃ ) = a′C + b′Si + c′Mn + d′Al + e′ where The numerical values of each coefficient are as shown in Table 2,
Each component is a wt% value.

[First example]

A low carbon steel slab (thickness 245 mm x width 1500 mm x length 9000 mm) containing 0.04% C and 0.21% Mn was first heated to a heating temperature of 1180°C and roughly rolled to a thickness of 35 mm.
× Bar 1 was 1450 mm wide. The bar 1 high-pressure water is descaled using a descaling device 31, and then the bar 1 is descaled using _{a descaling device 31, and then the bar 1 is} heated to a maximum of 660kw/max. An edge heating device 4 consisting of an electromagnetic induction heating device with an output on one side was used to perform local intermediate heating on a 100 mm wide portion in the width direction from the outermost edge with an effective heating power of 600 kW per side of the rolled steel material. . As shown in FIGS. 6a and 6b, the heating device 4 is vertically spaced 40 mm from the upper and lower edges,
Moreover, the length was extended to 710 mm in the rolling direction. This bar 1 was finally hot finish rolled to a size of 2.5 mm thick x 1450 mm wide. FIG. 1 is a schematic diagram of the apparatus used in this first embodiment. In Fig. 1, 31 is a high-pressure water descaling device, 5 and 6 are width-scanning radiation thermometers placed before and after the edge heating device, 7 is a width-scanning radiation thermometer placed on the rear surface of the finished product, and 8 is a radiation thermometer placed in the width direction. A pulse generator counts the number of rotations of the rolling rolls, 9 is a control device for the edge heating device 4, and 10 is a calculator for setting various conditions. The control device 9 monitors the actual temperature from the thermometers 5 and 6.
Input T ₁ and T ₂ , and obtain the rolling speed V _R from the pulse generator 8 and the finish rolling temperature from the rear thermometer 7.
T ₇ , input the Ac ₃ temperature calculated from the plate thickness information t and material components from the host computer 10 and the target temperature △T taking into account the subsequent temperature drop, and calculate the heating output amount of the edge heating device 4. Bar 1 was heated with a calculation of 600kw. The temperature transition at this time is shown in Figure 4△
As shown in the figure, the temperature of the edge where the temperature is the lowest below the Ar ₃ transformation point is increased by the high-pressure water descaling device 31 on the entry side of the finishing hot rolling mill to a temperature above the Ac ₃ transformation point.
The temperature of the steel material is raised by intermediate heating to 910℃,
Thereafter, normal rolling was carried out, and rolling was finally completed at a finishing temperature of 837°C. Here, the Ar ₃ transformation temperature of the steel is
At 824℃, the Ac ₃ transformation temperature is 907℃. Then take a sample from the obtained coil,
Figure 5 shows an investigation of the presence or absence of a mixed grain structure. On the other hand, the comparative example was manufactured under exactly the same conditions as the above example up to the rough hot rolling, and normal rolling was performed without any intermediate heating during the finishing hot rolling. Finish-rolled to dimensions of 2.5mm x width 1450mm x coil. The temperature transition at this time is shown by ● and ▲ in Fig. 4. Samples were taken from the coils obtained in the same manner as in the example of the present invention, and the presence or absence of a mixed grain structure was investigated, which is also shown in FIG. Incidentally, the mixed grain ratio on the vertical axis in FIG. 5 is determined from a+b/t×100% in FIG. 3. As is clear from FIG. 5, in the first embodiment of the present invention, generation of mixed grains was prevented and it was possible to manufacture hot-rolled products with uniform material quality. On the other hand, in the comparative example, a mixed grain structure was observed up to approximately 45 mm inside the edge.
The material was insufficient. [Second Embodiment] FIG. 7 shows a second embodiment of the hot rolling method and apparatus of the present invention. Reference numeral 19 denotes a rolled material specification setting device for setting the plate thickness, plate speed, component specifications, etc. of the rolled material. Based on the component specifications of the rolled material given from the rolled material specification setting device 19, the target temperature calculation device 18 determines the temperatures of the Ac ₃ transformation point and Ar ₃ transformation point of the rolled material, as well as this Ac ₃ transformation point,
Heating target temperature T based on the temperature of Ar ₃ transformation point
(HDA) and finish hot rolling end target temperature T
(FDA) was calculated. Next, the heating target temperature T
(HDA) and finish hot rolling end target temperature T
(FDA) were given as target values to the control amount calculators 16 and 17, respectively. 13 is the same electromagnetic induction edge heating device as in the first embodiment, which has an output of 660 kW/side, and is connected to the first finishing hot rolling mill (F ₁ ) and the second finishing hot rolling mill (F ₂ ). set in between. The specific arrangement of the heating device 13 with respect to the edge of the rolled steel material is the same as in the first embodiment.
Reference numeral 14 designates a widthwise scanning radiation thermometer provided on the exit side of the edge heating device, 15 represents a widthwise scanning radiation thermometer provided on the exiting side of the final finishing hot rolling mill, and 20 represents a widthwise scanning radiation thermometer provided on the inlet side of the heating device. The control amount calculator 16 uses the thermometer 1 to control the actual temperature immediately after heating to the heating target temperature T (HDA).
The control amount M(H) was calculated from the deviation between the target value and the actual measurement value using the actual temperature measured at step 4 as the feedback amount. In addition, the control amount calculator 17 converts the actual measured temperature immediately after finishing hot rolling to finishing hot rolling finishing target temperature T.
(FDA), the control amount M(F) was calculated from the deviation between the target value and the actual measurement value using the actual temperature measured by the thermometer 15 as the feedback amount. These control amounts M
The output of the heating device 13 was changed according to the sum of (H) and M(F). Since actual temperature feedback cannot be applied until the bar 1 reaches the thermometer 14 or 15, the initial value was set using the heating initial value setting device 21 in the same manner as in the first embodiment. The results of the second example are shown in Tables 3 and 4.
The thickness of the rolled material in Examples 1, 2, and 3 at the start of finish hot rolling was 35 mm, the same as in the first example, and the widths were 1250 mm, 1091 mm, and 1112 mm, respectively.

【table】

[Table] Examples 1a, 2a, and 3a shown in Tables 3 and 4 show the prior art. In Example 1a, the finished exit temperature is Ar ₃ or higher, but the heating device exit temperature is higher than Ac ₃ . The material has a mixed grain ratio of 39% because of its lower part. In addition, in Example 2a, the temperature at the exit side of the heating device was significantly lower than Ac ₃ , so it was a poor material with a mixed particle ratio of 43%. On the other hand, in the second embodiment of the present invention in which heating was controlled in Examples 1c, _2c , and _3c , the temperature at the exit side of the heating device and the finished product were Since the side temperature is ensured to be Ac ₃ and Ar ₃ and power saving is achieved, the obtained steel plate has no mixed grain structure and is of good quality. In Tables 3 and 4, 100% heating means that the output of the edge heating device 13 is constantly controlled by manual operation.
Shown is 660w/one side. In the second embodiment described above, the difference between the target temperature and the actual temperature of the steel material is constantly determined for both the exit side of the intermediate heating device and the exit side of the final hot rolling device, and the output of the heating device is controlled based on this difference. did. However, the difference between the measured temperature of the steel material at the exit side of the final hot rolling device and the target temperature may be detected only at the beginning of hot rolling, or may be detected at predetermined intervals. Note that FIG. 8 is an explanatory diagram of the edge heating devices 4 and 13 used in the example. (Effects of the Invention) As described in detail above, the present invention is capable of descaling a part of a rolled material whose temperature has become below the Ar ₃ transformation point during the hot rolling process or after descaling with high pressure water immediately before finishing hot rolling or after finishing hot rolling. In the middle of rolling, intermediate heating was performed to the Ac ₃ transformation point or higher, at least one pass of rolling was applied, and the final hot rolling was completed at the Ar ₃ transformation point or higher.
It is possible to easily obtain a hot-rolled product having uniform material properties in the width direction over the entire length without generation of mixed grain structure. In addition, low-temperature heating is becoming increasingly important in promoting energy conservation, and according to the present invention, low-temperature heating can be advantageously carried out without deteriorating quality. Furthermore, since the temperature at the edges where the temperature drop is particularly large is intermediately heated to above the Ac ₃ transformation point during rolling, local wear of the rolling rolls is significantly reduced.
Not only can the life of the roll be extended, but the occurrence of products with abnormal profiles can be reduced. In addition, heating the leading and trailing ends, which experience a large temperature drop like the edge portions, has excellent industrial effects, such as softening the shock caused by roll biting and preventing roll flaws.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the hot rolling equipment used in the first embodiment of the present invention, FIG. 2 is a diagram showing the temperature rise due to heating of the rolled steel material and the rolling end temperature level in the first embodiment of the present invention, FIG. 3 is a schematic cross-sectional view of the mixed grain structure, FIG. 4 is a diagram showing the temperature history of the center part and edge part in the width direction in the first example of the present invention and a comparative example, and FIG. Figure 6 is a diagram showing the incidence of mixed grain structures in the first example and comparative example, and Figure 6 is a diagram explaining an electromagnetic induction edge heating device used as an intermediate heating device. FIG. 7 is a cross-sectional view taken along line a in FIG. Figure 8 showing the hot rolling equipment
The figure is an explanatory diagram of an edge heating device used in an embodiment of the present invention. 1... Bar, 4, 13... Edge heating device,
5, 6, 7, 14, 15, 20... Width direction scanning radiation thermometer, 8... Pulse generator, 9...
Control device, 10... Calculator, 16, 17... Controlled amount calculator, 18... Target temperature calculator, 19...
Rolled material specification setting device, 21... Heating initial value setting device,
101...Mixed grain or coarse grain region, 102...Fine grain region.

Claims (1)

[Scope of Claims] 1. A hot rolling method comprising a rough hot rolling stage of rough hot rolling of steel material and a finishing hot rolling stage of finishing hot rolling of steel material, including: Immediately before or during finishing hot rolling, the portion of the steel material that has reached at least the Ar ₃ transformation point or lower is intermediately heated to bring the temperature of the steel material within the range of the intermediate heating target temperature of the Ac ₃ transformation point or higher, thereby increasing the temperature of the entire rolled steel material. a stage in which the structure becomes austenite, a stage in which at least one pass of rolling is subsequently applied and finish hot rolling is completed at a temperature within the range of the final hot finishing exit target temperature above the Ar ₃ transformation point, and a stage immediately after intermediate heating. A step of changing the heating degree in intermediate heating based on at least the former of the difference between the actual measured temperature of the steel material and the intermediate heating target temperature, and the difference between the actual measured temperature of the steel material immediately after final hot finishing and the final hot finishing exit side target temperature. A hot rolling method comprising: 2 A plurality of rough hot rolling stands, a plurality of finishing hot rolling stands provided following the rough hot rolling stands, and finishing hot rolling stands most upstream in the rolling direction or between adjacent finishing hot rolling stands. An intermediate heating device is provided immediately before an inter-rolling stand and intermediately heats the rolled steel material, and an intermediate heating device is used to determine the Ac ₃ transformation point and Ar ₃ transformation point of the rolled steel material based on the composition of the rolled steel material, and to calculate the Ac ₃ transformation point and Ar ₃ transformation point of the rolled steel material based on the composition of the rolled steel material. The target temperature calculation device includes a target temperature calculation device that calculates a heating target temperature of the steel material in the intermediate heating device and a target temperature at the time of completion of finish rolling of the steel material in the final hot rolling stand based mainly on the temperature at the point. is operatively connected to the intermediate heating device to determine the heating output value of the intermediate heating device, and further includes a thermometer device provided immediately after the intermediate heating device to detect the temperature of the rolled steel material immediately after the intermediate heating, and a final finishing rolling stand. The difference between the actual temperature of the steel material and the intermediate heating target temperature determined by the thermometer device installed immediately after finishing hot rolling to measure the temperature of the steel material and the thermometer device for detecting the temperature immediately after intermediate heating, and the finish. The difference between the actual temperature of the steel material determined by the temperature detection thermometer device immediately after the completion of hot rolling and the target temperature at the completion of finishing hot rolling is determined, and the amount of heating in the intermediate heating device is calculated based on at least the former difference. A hot rolling apparatus having a control amount calculation device for changing.