JP5217509B2 - Manufacturing method and equipment for thick steel plate - Google Patents

Description

  The present invention relates to a method for producing a thick steel plate using a reversible rolling mill and a passing type cooling device, and in particular, a production method for directly quenching or accelerating cooling after hot rolling, and particularly excellent in material property uniformity in the longitudinal direction of the steel plate. The present invention relates to a rolling-cooling method suitable for manufacturing a thick steel plate.

  The material of the thick steel plate depends on the microstructure, and in order to make the microstructure uniform, for example, when performing accelerated cooling, all of the rolling finishing temperature, the cooling start temperature, and the cooling stop temperature must be made constant. In addition, when direct quenching is performed, it is important to keep the rolling finishing temperature and the cooling start temperature constant.

  When the rolling finishing temperature changes, the size and shape of the crystal grains, or the number of ferrite nucleation sites formed by rolling strain, etc., vary greatly, resulting in a change in the final form of the structure, resulting in significant changes in strength and toughness. .

The cooling start temperature also has a significant effect on the tissue. When cooling from an austenite region of Ar ₃ points or more in order to ensure strength, the temperature decreases before the tail end of the steel sheet enters the cooling facility, and when the temperature falls below the Ar ₃ point, ferrite starts to form and the strength decreases. It is difficult to secure a stable material.

  Even when a two-phase structure is obtained by quenching from a region where ferrite is partially formed, the temperature difference at the leading end of the steel sheet changes the two-phase fraction of ferrite and austenite and greatly affects the strength and toughness.

  Further, the cooling stop temperature is specified particularly in the case of the accelerated cooling process, and is extremely important for making the transformation rate of the obtained structure constant, and the rolling finishing temperature, the cooling start temperature, and the cooling stop temperature are all kept constant. It is important to maintain the uniformity of the material.

  However, in the direct quenching and accelerated cooling after rolling, the cooling start temperature differs between the tip and tail of the steel sheet, and in the case of accelerated cooling, the cooling stop temperature varies further in the longitudinal direction of the plate in the passing cooling process in the cooling facility. It was difficult to make the material uniform.

  In accelerated cooling, the temperature is lowered by several hundred degrees C from the start to the end of cooling, but in order to cool to a desired temperature with a limited cooling facility length, the conveyance speed may be slower than the rolling speed (2 to 4 m / s). In many cases, this causes a temperature difference in the longitudinal direction of the steel sheet.

  For example, if the length of the steel plate product is L = 30 m and the conveyance speed is v = 1 m / sec, the cooling start is different by L / v = 30 sec at the tip and tail, so the tail end is 30 sec than the tip. It is air-cooled excessively.

  Therefore, in Patent Document 1, a thermometer is installed on the cooling device entrance side, the temperature in the longitudinal direction of the steel sheet is measured, and the approach speed to the cooling device is increased stepwise to make the cooling stop temperature uniform. Thus, a method of stabilizing the material is disclosed.

  In the method described in Patent Document 1, as described above, the stabilization of the material is significantly affected not only by the cooling stop temperature but also by the cooling start temperature, so that a sufficient material uniformity effect cannot be obtained.

  Patent Document 2 discloses a method of making the cooling start temperature constant in the longitudinal direction of the steel sheet. By inserting a steel plate into the cooling device during hot rolling and keeping a temperature distribution in the longitudinal direction of the steel plate, there is a temperature gradient at the leading edge of the steel plate at the end of rolling, and cooling starts when it is inserted into the subsequent cooling equipment Keep temperature constant.

Therefore, in the method described in Patent Document 2, the most important rolling finishing temperature that affects the toughness of the steel sheet differs greatly in the longitudinal direction of the steel sheet, and a constant toughness cannot be obtained in the longitudinal direction of the steel sheet.
Japanese Patent Laid-Open No. 61-213327 JP-A-8-90042

  By the way, as described above, when the length of the product is as long as about L = 30 m, the steel plate conveyance speed at the time of cooling is slower than the rolling speed, so the effect on the material uniformity of the steel sheet is the tip of the steel sheet, The cooling start temperature difference and stop temperature difference are larger than the effect of the finishing temperature difference at the tail end.

  Therefore, the present invention provides a manufacturing method in which, when the steel plate length is long, a steel plate with a cooling start temperature and a cooling stop temperature that are uniform over the entire length of the steel plate and excellent in strength and toughness uniformity can be obtained. For the purpose.

  The present inventors have studied various rolling-cooling processes in which the rolling finishing temperature of hot rolling and the subsequent cooling start temperature and cooling stop temperature are made constant, and a cooling device with a short cooling region is arranged near the hot rolling mill. Thus, it has been found that applying an appropriate temperature gradient in the longitudinal direction of the steel sheet before hot rolling, quenching or accelerated cooling is most effective and effective in actual machine operation.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. When water-cooling the steel sheet after the finish rolling using a pass-type cooling device, the first water cooling is performed so as to preliminarily impart a temperature gradient in the longitudinal direction of the steel sheet, and the second water cooling is kept constant after the first water cooling. The manufacturing method of the thick steel plate characterized by performing by the passage speed of.
2. When the first passage type cooling device is arranged upstream of the reversible hot rolling mill, the second passage type cooling device is arranged downstream of the reversible hot rolling mill, and reversible hot rolling When the first passage type cooling device is arranged on the downstream side of the machine, the thick steel plate using the rolling-cooling device in which the second passage type cooling device is arranged on the downstream side of the first passage type cooling device. In the manufacturing method, the steel plate finish-rolled by the reversible hot rolling mill is cooled by the first passage type cooling device, and a temperature gradient is given in the longitudinal direction. A method for producing a thick steel plate, characterized by passing through a passing-type cooling device at a constant speed.
3. When cooling with the second passage-type cooling device, the first passage-type cooling device is configured such that the cooling start temperature at the tip and tail ends of the steel plate is Ar ₃ point or higher or a two-phase region temperature. 3. The method for producing a thick steel plate according to 2, wherein a temperature gradient is applied.
4). When cooling with the second passage-type cooling device, a temperature gradient is applied by the first passage-type cooling device so that the cooling start temperature difference between the front end portion and the tail end portion of the steel sheet is within 50 ° C. A method for producing a thick steel plate according to 2 or 3, wherein:
5. A method of manufacturing a thick steel plate using a rolling-cooling device in which a passage type cooling device is disposed downstream or upstream of a reversible hot rolling mill, and the steel plate finish-rolled by the reversible hot rolling mill is After the first cooling by the passing type cooling device, the next cooling is performed while being reversely fed, and a temperature gradient is given in the longitudinal direction by the first cooling, and the reverse feeding speed of the steel plate in the next cooling Is a method of manufacturing a thick steel plate, characterized by a constant speed.
6). When cooling with the passage type cooling device, the temperature gradient is set in the first cooling so that the cooling start temperature at the tip and tail ends of the steel plate in the next cooling becomes Ar ₃ points or more or the two-phase region temperature. 6. The method for producing a thick steel plate according to 5, wherein:
7). When cooling with the passage type cooling device, a temperature gradient is imparted in the first cooling so that a cooling start temperature difference between the leading end and the tail end of the steel plate in the next cooling is within 50 ° C. The manufacturing method of the thick steel plate of 5 or 6.
The length of the cooling region of the first passage type cooling device described in 8.2 or the reversible rolling mill described in 5 on the downstream side or upstream side of the passing type cooling device is 0.4 m in the steel sheet conveyance direction. The method for producing a thick steel plate according to any one of 2 to 7, wherein the thickness is 4 m.

  The steel sheet temperatures such as the rolling finishing temperature, the cooling start temperature, and the cooling stop temperature referred to in the present invention are temperatures obtained by measuring the temperature of the steel sheet surface with a radiation thermometer unless otherwise specified.

  Moreover, in this invention, a front-end | tip part and a front-end | tip shall refer to the area | region containing the front-end | tip of the stationary part used for a product, all near the front-end | tip of the advancing direction of a steel plate, without distinguishing each other. Similarly, the tail end and the tail end are both in the vicinity of the rear end in the traveling direction of the steel sheet and are not distinguished from each other, and indicate a region including the rear end of the steady portion used in the product.

  According to the present invention, in the rolling-cooling process of a thick steel plate, a rolling steel plate having excellent uniformity of strength and toughness is obtained in which the rolling finishing temperature, the cooling start temperature, and the cooling stop temperature are all uniform over the entire length of the steel plate. It is extremely useful in industry.

  In the present invention, a cooling facility capable of adjusting the temperature in the longitudinal direction of the steel sheet is disposed in the vicinity of the upstream side and / or the downstream side of the finish rolling mill, and further, quenching or accelerated cooling is performed on the downstream side of the cooling facility. It is characterized in that all of the rolling finishing temperature, the cooling start temperature, and the cooling stop temperature are made uniform over the entire length of the steel sheet by using a hot rolling-cooling equipment for the steel sheet provided with possible water cooling equipment. Hereinafter, the present invention will be described in detail with reference to the drawings.

  FIG. 1 schematically shows the arrangement of rolling mills and cooling devices in a rolling-cooling facility to which the present invention is applied. In the figure, 12 is a heating furnace, 9 is a rolling mill, and 10 is upstream or downstream of the rolling mill 9. When the first passage type cooling device is arranged, the inside of (10) is arranged on the upstream side of the rolling mill 9, and 11 denotes a second passage type cooling device. The heating furnace 12 side of the rolling mill 9 is referred to as the upstream side of the rolling mill 9, and the second passage type cooling device 11 side is referred to as the downstream side.

  A thick steel plate (not shown) is heated for rolling in the heating furnace 12, and after finish rolling, an appropriate temperature gradient is applied in the longitudinal direction of the steel plate by the first passage type cooling device 10.

  The steel plate after finish rolling is conveyed to the second passage type water cooling device 11 for quenching, accelerated cooling, etc., but the first passage type is previously set so that the cooling start temperature is substantially the same in the longitudinal direction of the steel plate. When passing through the cooling device 10, an appropriate temperature gradient is applied in the longitudinal direction of the steel sheet.

  FIG. 2 is a schematic diagram for explaining a rolling-cooling method in the steel sheet manufacturing method according to the present invention, P1 to P4 indicate paths, P3 is a final (finishing) path, P2 is a path immediately before the final path P3, P1 is a pass immediately before P2, and P4 is an empty pass for passing the steel sheet after finish rolling or finish rolling and water cooling through the rolling mill 9.

  a is a rolling operation by the rolling mill 9, b1 and b2 are water cooling operations by the first passage type cooling device, and c is a water cooling operation by the second passage type cooling device. Also in the figure, the steel plate is not shown.

  FIG. 2A shows a case where the first passage type cooling device 10 and the second passage type cooling device 11 are arranged in this order on the downstream side of the rolling mill 9, and the steel plates after finish rolling are sequentially passed and cooled (Case 1). In FIG. 2 (b), the first passage type cooling device 10 is arranged upstream of the rolling mill 9, the second passage type cooling device 11 is arranged downstream of the rolling mill 9, and the steel plate after finish rolling (P3) is empty. When cooled by the first passage type cooling device 10 in the pass (P4) and then passed through the rolling mill 9 by the empty pass (P4) and cooled by the second passage type cooling device 11 (Case 2), FIG. 2 (c) Is a case where a passage-type cooling device (first passage-type cooling device 10 in the present description) is arranged downstream of the rolling mill 9 and cooled at one end, and then cooled by the passage-type cooling device while being fed back ( Case 3) is shown.

  In the above (Case 1) and (Case 3), P3 is described as the final (finishing) pass. However, it is also possible to pass P2 as the final (finishing) pass and to pass the rolling mill 9 in the P3 pass.

  2C shows a case where the first passage type cooling device 10 is provided on the downstream side of the rolling mill 9, but the first passage type cooling device 10 is provided on the upstream side of the rolling mill 9, and P2 Can be the final (finishing) pass.

  FIG. 3 schematically shows the temperature distribution in the longitudinal direction of the steel sheet in Cases 1 to 3 shown in FIG. In the drawing, it is assumed that the steel plate proceeds in the direction of the arrow, and in the description, the traveling direction side of the steel plate is referred to as the front end portion of the steel plate, and the opposite direction is referred to as the tail end portion of the steel plate. Hereinafter, the present invention will be described with reference to FIGS.

  In Case 1, the steel sheet is subjected to a final pass P3, and after having a predetermined thickness, is water-cooled b1 by the first passage type cooling device 10 and then water-cooled c by the second passage type cooling device 11, and the desired performance is obtained. Is granted. When the water-cooling b1 is water-cooled by the second passage type cooling device 11, the tail end is set to a temperature higher than the tip by ΔT in the longitudinal direction of the steel plate so that the cooling start temperature is substantially the same in the longitudinal direction of the steel plate. When water cooling c is performed by the second passage type cooling device 11, the steel sheet is allowed to enter the second passage type cooling device 11 at a constant speed, and the cooling stop temperature is made substantially the same in the longitudinal direction of the steel sheet.

  In Case 2, the steel sheet is made to have a predetermined thickness by the rolling mill 9 in the final pass P3, then water-cooled b1 by the first passage type cooling device 10, and then passes through the rolling mill 9 by the empty pass P4. Water cooling is performed by the second passage type cooling device 11, and desired performance is imparted.

  When the water-cooling b1 is water-cooled by the second passage type cooling device 11, the tail end is set to a temperature higher than the tip by ΔT in the longitudinal direction of the steel plate so that the cooling start temperature is substantially the same in the longitudinal direction of the steel plate. When water cooling c is performed by the second passage type cooling device 11, the steel sheet is allowed to enter the second passage type cooling device 11 at a constant speed, and the cooling stop temperature is made substantially the same in the longitudinal direction of the steel sheet.

  In Cases 1 and 2, by allowing the steel sheet to pass through the first passage type cooling device 10 while accelerating, the tail end portion can be heated at a temperature higher by ΔT than the front end portion in the longitudinal direction of the steel plate.

  In Case 3, the steel plate is subjected to a final pass P3, and after having a predetermined plate thickness, the water cooling b1 is performed by the first passage cooling device 10 and then the water cooling b2 is performed again while the first passage cooling device 10 is fed back. . In the water cooling b1, the tip is heated at a higher temperature by ΔT than the tail end in the longitudinal direction of the steel sheet. When water cooling b1 is performed, the steel sheet can be heated at a temperature higher than the tail end by ΔT by passing the first passing cooling device 10 while decelerating.

  When water cooling b2 is performed while feeding back, the tail end portion becomes the tip portion, and the tip end portion becomes the tail end portion, so that the cooling start temperature is substantially the same in the longitudinal direction of the steel sheet. When water cooling b2 is performed by the first passage type cooling device 10 while being reversely fed, the steel plate enters the first passage type cooling device 10 at a constant speed, and the cooling stop temperature is made substantially the same in the longitudinal direction of the steel plate. Reverse feed refers to the case where the steel sheet is transported from the downstream side of the rolling mill 9 to the upstream side.

  In Cases 1 to 3, the temperature difference ΔT applied to the tail end and the tip end in the longitudinal direction of the steel plate is the steel plate in the subsequent cooling by the second passage type cooling device 11 or the first passage type cooling device 10 performed in the reverse direction. The cooling start temperature at the tail end is applied so as to be the same temperature as the tip.

  As shown in FIG. 3, it is preferable to change the temperature gradient linearly in the longitudinal direction of the steel sheet, but the temperature gradient may be changed stepwise in the longitudinal direction of the steel sheet.

  In addition, it is suitable to make the cooling start temperature and the cooling stop temperature substantially the same by making the cooling equipment capacity constant after making the temperature gradient in the longitudinal direction of the steel plate and making the conveyance speed at the time of cooling constant. Although it is a method, if the cooling capacity can be controlled, the conveying speed in the longitudinal direction of the steel sheet may not be constant.

  For example, if it is possible to achieve the technical feature of controlling the cooling start temperature and the cooling stop temperature in the second passage type cooling device substantially the same with respect to the longitudinal direction of the steel sheet, which is a feature of the present invention, that is, By adopting methods such as increasing the cooling water amount while increasing the conveying speed and increasing the cooling capacity, or conversely decreasing the cooling water amount while decreasing the conveying speed and reducing the cooling capacity, etc. If the cooling start temperature and the cooling stop temperature can be made substantially the same while changing the conveyance speed in the longitudinal direction, a steel sheet excellent in steel sheet longitudinal strength and toughness uniformity, which is the target of the present invention, is manufactured. be able to.

  Here, the solid line in FIG. 5 shows an example in which the relationship between the thickness of the hot-rolled steel plate at 700 ° C., 850 ° C., and 1000 ° C. and the cooling rate when the steel plate is cooled by air cooling is obtained by heat transfer calculation. . The solid line is the calculation result. The smaller the plate thickness, the smaller the heat capacity, and the higher the steel plate temperature, the more radiation and heat radiation, so the cooling rate increases.

  Although the cooling rate differs somewhat depending on the form of transport and atmosphere, for example, the cooling rate when the plate thickness is 30 mm and the steel plate surface temperature is 800 ° C. is about 0.8 ° C./s.

  Generally, after cooling with the first passage type cooling device, the start temperature of accelerated cooling or quenching when the second passage type cooling device or the first cooling facility again cools is 700 ° C. or higher, The cooling rate is not less than the broken line of 15 / h (° C./mm) in FIG.

  On the other hand, since the start temperature of accelerated cooling or quenching hardly exceeds 1000 ° C., the cooling rate is similarly below the broken line of 55 / h (° C./mm).

Accordingly, when the accelerated cooling or quenching start temperature is 700 ° C. or higher and 1000 ° C. or lower, the temperature difference ΔT at the leading end of the temperature gradient applied in the longitudinal direction of the steel sheet is:
15L / (hv) ≦ ΔT ≦ 55L / (hv) (1)
It is preferable to satisfy this relationship.
However, h (mm): the thickness of the steel plate, L (m): the length of the steel plate, v (m / s): a temperature gradient in the longitudinal direction of the steel plate by the first passage type cooling equipment, and then performed. The steel sheet transport speed when entering the cooling facility.

  As an example, when the length of the steel sheet product is L = 30 m and the conveyance speed is v = 1 m / s, the cooling start is different at the tip and the tail, so the tail end is different from the tip. Air-cooled for an additional 30 s.

  When the plate thickness is h = 30 mm, the cooling rate when the steel plate surface temperature is 800 ° C. is about 0.8 ° C./s, so that the tail end portion is 0.8 × L / v = 24 ° C. than the tip end portion. Lower. Therefore, if the tip is cooled by 34 ° C. and the tail end is cooled by 10 ° C., the temperature difference at the tail end is 24 ° C. higher than the tip, so that the cooling start temperature is substantially constant.

  In addition, it is preferable to use the cooling device excellent in the draining property which does not flow out the cooling water in the steel plate conveyance direction other than the cooling region as the first or second passing type cooling device.

  4 (a), 4 (b), and 4 (c) show an example of a cooling device excellent in draining performance, in which 1 is a cooling tank, 2 is cooling water staying on the steel plate 4, and 3 is upper cooling water. The injection nozzle, 4 is a steel plate, 5 is a transport roll, 6 is a lower cooling water nozzle, 7 is a cooling region, 8 is a draining roll, and 13 is rod-shaped cooling water.

(A) is a cooling device which ejects the rod-shaped cooling water 15 from the upper cooling water injection nozzle 3 attached to the cooling tank 1 so as to oppose, and retains the cooling water 2 on the steel plate 4, and (b) is the draining roll 8 And the transport roll 5 are opposed to each other with the steel plate 4 interposed therebetween, and a cooling device that ejects the rod-shaped cooling water 13 from one direction and stops it with the draining roll 8, (c) is a pair of the draining roll 8 and the transport roll 5, The cooling device which makes the cooling water 2 stay on the steel plate 4 is shown. In order to obtain a damming effect, it is preferable that the rod-shaped cooling water 13 has a water density of 4 m ³ / m ² min or more.

  It is preferable that the distance in the transport direction of the cooling region 7 is 0.4 m to 4 m in the first passing type cooling device (including the case 3 passing type cooling device). If it is less than 0.4 m, it is necessary to take a long residence time in the cooling region in order to cool the steel sheet, and it takes too much time to pass through the entire steel sheet, making it difficult to provide a sufficient temperature gradient.

  On the other hand, if it exceeds 4 m, it is difficult to provide uniform cooling in the cooling region 7, and it is difficult to provide a sufficient temperature gradient with a steel plate having a short plate length. To do. In addition, the cooling area | region in FIG. 4 (a), (b), (c) is shown by the black coating part of a steel plate.

  The cooling region 7 is appropriately set by increasing / decreasing the number of cooling tanks and nozzles, or by increasing / decreasing the number of cooling units as shown in FIGS. 4 (a), (b), and (c). Is possible.

  In order to provide a temperature gradient with the first passing type cooling device, it is preferable to control the passing speed of the steel sheet in the cooling region because of excellent control responsiveness. The cooling facility capacity such as the water injection amount of the first passage type cooling device may be controlled, and both the passage speed and the cooling capacity may be used.

  The second passage type cooling device is not particularly limited as long as it has a required cooling capacity and can be uniformly cooled.

As the cooling start temperature, an Ar ₃ point or more and a two-phase region temperature are appropriately selected according to desired characteristics.

The cooled steel sheet is appropriately tempered according to desired characteristics. Tempering may be carried out by a conventional method. For example, an off-line atmosphere furnace or an on-line induction heating device can be used, and the tempering temperature is below the Ac ₁ transformation point, which is a temperature range in which an austenite phase is not generated. It is preferable that it is the temperature of.

  In the present invention, after giving a temperature gradient in the longitudinal direction of the steel sheet with the first passage type cooling device, both the cooling start temperature and the cooling stop temperature in the second passage type cooling device are the maximum value in the longitudinal direction of the steel plate. The difference between the minimum values is preferably 50 ° C. or less.

  When the temperature difference exceeds 50 ° C., the difference in strength and toughness between the tip and tail ends becomes large. More preferably, it shall be 30 degrees C or less.

  In Case 3, quenching or accelerated cooling is performed by cooling b2 by the first passage type cooling device 10.

  A thick steel plate was manufactured by the rolling-cooling method according to Case 1 of the present invention, and the mechanical properties (strength and toughness) of the entire length of the steel plate were investigated. In the rolling-cooling facility shown in FIG. 1, the first passage type cooling device uses a cooling facility (FIG. 4C) having a cooling region of 1 m between two draining rolls, and the second passage type cooling device is The conventional accelerated cooling equipment was used.

  In this example, using a slab having a plate thickness of 215 mm, heating at 1150 ° C. and then performing recrystallization zone rolling at a temperature of 980 ° C. or higher with a cumulative reduction ratio of 50%, controlled rolling with a cumulative reduction ratio of 50% at 900 ° C. to 800 ° C. A thick steel plate having a plate thickness of 30 mm and a plate length of 30 m was manufactured.

  In this embodiment, Case 1 provided a temperature gradient in the first passage type cooling device, and the difference between the cooling start temperature and the cooling stop temperature in the second passage type cooling device was set to 50 ° C. or less. A thick steel plate was manufactured under the manufacturing conditions and the manufacturing conditions in which the first passing type cooling device was not used and the second passing type cooling device was used for cooling. Under any manufacturing condition, the cooling conveyance speed in the second passage type cooling device was fixed at 1 m / s.

  Tensile test specimens of full thickness are sampled at positions near 500 mm from the foremost and rearmost ends of the obtained thick steel plates, and subjected to a tensile test in accordance with the provisions of JIS Z 2241 (1998). We asked for TS. In addition, a Charpy impact test piece having a V-notch standard dimension was taken from a position in the plate thickness direction 1/2 in accordance with the provisions of JIS Z 2202 (1998), and in accordance with the provisions of JIS Z 2242 (1998). An impact test was performed to determine a ductile-brittle fracture surface transition temperature vTrs.

  Table 1 shows the component composition of the test steel, and Table 2 shows the manufacturing conditions and the strength and toughness of the obtained steel sheet. For Nos. 1, 2, 6, 7, 11, and 12, which gave the temperature difference ΔT between the tip and tail in the first-pass type cooling device, the YS and TS differences between the tip and tail are uniform within 25 MPa, and the toughness is also good. Met.

  On the other hand, No. 3, 8, and 13 having a large temperature difference between the tip end and the tail end due to low-speed rolling in one direction had reduced tip toughness. No4,9,14 with large temperature difference between the tip and tail of cooling start temperature and large YS and TS difference between tip and tail, and toughness of tail A uniform steel sheet could not be obtained.

  Nos. 5, 10, and 15 have a large difference between the tip and tail of the cooling stop temperature in the second passage type cooling device without giving a temperature gradient, and the difference between YS and TS between the tip and tail is large. The toughness of the steel decreased.

The schematic diagram which shows an example of rolling-cooling equipment. The schematic diagram explaining the rolling-cooling method in the manufacturing method of the steel plate which concerns on this invention, (a) is Case1, (b) is Case2, (c) is a figure which shows the case of Case3. The schematic diagram explaining the temperature difference of the steel plate longitudinal direction in the rolling-cooling method which concerns on this invention, (a) is Case1, (b) is Case2, (c) is a figure which shows the case of Case3. In the present invention, a cooling device excellent in draining performance suitable as a first-pass type cooling device (a) does not use a draining roll, and a cooling water jet nozzle is used to retain cooling water on a steel plate, (b) is cooling. A form in which the cooling water is retained on the steel sheet by using the water injection nozzle and the draining roll together, and (c) is a form in which the cooling water is retained on the steel sheet by the draining roll without using the cooling water injection nozzle. The figure which shows the cooling rate at the time of air cooling of a hot-rolled steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cooling tank 2 Reserved cooling water 3 Upper cooling water injection nozzle 4 Steel plate 5 Transport roll 6 Lower cooling water nozzle 7 Cooling area 8 Draining roll 9 Rolling mill 10 First passage type cooling device 11 Second passage type cooling device 12 Heating furnace 13 Rod cooling water

Claims (8)

  When water-cooling the steel sheet after the finish rolling using a pass-type cooling device, the first water cooling is performed so as to preliminarily impart a temperature gradient in the longitudinal direction of the steel sheet, and the second water cooling is kept constant after the first water cooling. The manufacturing method of the thick steel plate characterized by performing by the passage speed of.   When the first passage type cooling device is arranged upstream of the reversible hot rolling mill, the second passage type cooling device is arranged downstream of the reversible hot rolling mill, and reversible hot rolling When the first passage type cooling device is arranged on the downstream side of the machine, the thick steel plate using the rolling-cooling device in which the second passage type cooling device is arranged on the downstream side of the first passage type cooling device. In the manufacturing method, the steel plate finish-rolled by the reversible hot rolling mill is cooled by the first passage type cooling device, and a temperature gradient is given in the longitudinal direction. A method for producing a thick steel plate, characterized by passing through a passing-type cooling device at a constant speed. When cooling with the second passage-type cooling device, the first passage-type cooling device is configured such that the cooling start temperature at the tip and tail ends of the steel plate is Ar ₃ point or higher or a two-phase region temperature. A method for producing a thick steel plate according to claim 2, wherein a temperature gradient is applied.   When cooling with the second passage-type cooling device, a temperature gradient is applied by the first passage-type cooling device so that the cooling start temperature difference between the front end portion and the tail end portion of the steel sheet is within 50 ° C. The method for producing a thick steel plate according to claim 2 or 3.   A method of manufacturing a thick steel plate using a rolling-cooling device in which a passage type cooling device is disposed downstream or upstream of a reversible hot rolling mill, and the steel plate finish-rolled by the reversible hot rolling mill is After the first cooling by the passing type cooling device, the next cooling is performed while being reversely fed, and a temperature gradient is given in the longitudinal direction by the first cooling, and the reverse feeding speed of the steel plate in the next cooling Is a method of manufacturing a thick steel plate, characterized by a constant speed. When cooling with the passage type cooling device, the temperature gradient is set in the first cooling so that the cooling start temperature at the tip and tail ends of the steel plate in the next cooling becomes Ar ₃ points or more or the two-phase region temperature. The method for producing a thick steel plate according to claim 5, wherein the steel plate is given.   When cooling with the passage type cooling device, a temperature gradient is imparted in the first cooling so that a cooling start temperature difference between the leading end and the tail end of the steel plate in the next cooling is within 50 ° C. The manufacturing method of the thick steel plate of Claim 5 or 6.   The length of the cooling region of the first passage type cooling device according to claim 2 or the passage type cooling device arranged on the downstream side or the upstream side of the reversible rolling mill according to claim 5 is 0 in the steel sheet conveyance direction. The method for producing a thick steel plate according to any one of claims 2 to 7, wherein the thickness is 4m to 4m.

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