WO2008108483A1 - Thin steel sheet excelling in strength and toughness uniformity, process for producing the same, and apparatus therefor - Google Patents

Abstract

A process for producing a steel sheet of given thickness excelling in the uniformity of material properties along the length of the steel sheet with the use of a reversing rolling mill and a through cooling unit. In particular, there is disclosed a rolling-cooling process using a rolling-cooling apparatus having a first-step through cooling unit and a second-step through cooling unit disposed downstream of a reversing hot rolling mill, the process including the steps of: 1. at the rolling just prior to final rolling in the finish rolling, cooling the steel sheet by the first-step through cooling unit so that the rolling finishing temperature of the final rolling at the front end area of the steel sheet is identical with that at the rear end area of the steel sheet; 2. at the cooling of the steel sheet after the finish rolling by means of the second-step through cooling unit, cooling the steel sheet by means of the first-step through cooling unit so that the cooling starting temperature at the front end area of the steel sheet is identical with that at the rear end area of the steel sheet; and 3. cooling the steel sheet by passage thereof through the second-step through cooling unit at a constant rate.

Description

 Description Thin-walled steel plate with excellent strength and toughness uniformity, manufacturing method thereof, and manufacturing equipment thereof

 The present invention relates to a method of manufacturing a steel plate using a reversing rolling mill and a transfer-type cooling device, and more particularly, a direct quenching and tempering process after hot rolling ( Thin wall with excellent homogeneity of material properties in the longitudinal direction of steel sheets subjected to direct quenching and temper treatment or accelerated cooling treatment The present invention relates to a thick steel plate, a rolling-cooling method suitable for its production, and its production equipment. Background art

 As a method for producing high-tensile steel with a tensile strength of 570 MPa or more, production by accelerated cooling after hot rolling is generally used. Conventionally, the steel sheet that has been rolled to a predetermined thickness by hot rolling and then reheated to a normal temperature, and has been subjected to quenching and tempering treatment.In recent years, direct quenching that immediately quenches after hot rolling. After the tempering process or hot rolling, accelerated cooling is performed, the subsequent heat treatment is not performed, and the rapid cooling process is used.

 The material of the thick copper plate depends on the microstructure, and in order to make the microstructure uniform, in the accelerated cooling process, the rolling finishing temperature—cooling start temperature (cooling start temperature) It is important to keep all cooling stop temperatures constant, and in the direct quenching-tempering process, it is important to keep the rolling finish temperature and the cooling start temperature constant.

When the rolling finishing temperature changes, the size and shape of the crystal grain, and the number of crystal nucleation sites of ferrite formed by the rolling strain (number of crystal nucleation) Because the site) is greatly different, the final form of the structure changes, and the strength and toughness change greatly. The cooling start temperature also has a significant effect on the tissue. When cooling with an austenite range force above the Ar ₃ transformation temperature to ensure strength, the temperature decreases before the tail end of steel plate enters the cooling facility. Below the Ar transformation point, ferrite begins to form and ferritic strength decreases, making it difficult to secure a stable material.

 In addition, when cooling from the dual phase region temperature of ferrite and austenite, that is, by dual quenching with accelerated formation of ferrite, or accelerated cooling, a dual phase structure is formed. Even when the steel plate is obtained, the temperature difference at the top and tail end of steel plate changes the volume fraction of the two phases of ferrite and austenite at the start of cooling, thereby increasing the strength and toughness of the steel plate. A big influence.

 Furthermore, the cooling stop temperature is specified in particular in the accelerated cooling process, and is extremely important in order to make the transformation ratio of the resulting structure constant, and the rolling finish temperature-cooling start temperature-cooling stop temperature are all It is important to maintain the uniformity of the material.

 However, in direct quenching and temper after rolling and accelerated cooling, the cooling start temperature differs between the tip and tail of the steel sheet. In the passage cooling process, the cooling stop temperature varied in the longitudinal direction of the plate, making it difficult to create a uniform material.

 However, the accelerated cooling process is an on-line heat treatment using a pass-through cooling device, but it takes time to roll the entire length of the steel plate when the steel plate is thin or the plate length is large. A large cooling start temperature difference occurs between the tip and the tail of the steel plate.

In other words, if the rolling speed is slow in the accelerated cooling process or it is difficult to start cooling all the steel sheets with a plate length of several tens of meters at the same time, the leading edge and tail edge of the steel sheet depend on the conveying speed and length of the steel sheet. As a result, a time difference between the start of cooling and the stop temperature occurs. In the accelerated cooling process, the force to lower the temperature by several hundred ° C from the start to the end of cooling '; the cooling speed is reduced to the desired temperature with a limited cooling equipment length, so the traveling speed is the rolling speed. ^ If it is slower than AmZs), it will cause a temperature difference in the longitudinal direction of the steel sheet. .

For example, if the length of the steel plate product is L == 30m, and the steel plate transport speed (or the speed of entering the cooling facility) is S _V = lm se _C , the cooling start at the tip and tail ends of the steel plate is LZv == Since the difference is 30 seconds, the tail end is air-cooled for 30 seconds more than the tip.

 Because the cooling start temperature and cooling stop temperature during accelerated cooling affect the microstructure, these temperatures are not constant in the longitudinal direction of the steel plate! / In some cases, a homogeneous material is obtained in the longitudinal direction of the copper plate. Is difficult. Limiting the plate length of the steel plate according to the thickness of the copper plate so that the fluctuation range of the material falls within the specified range, or manufacturing by re-heating and quenching and tempering process decreases the production efficiency, Increases manufacturing costs and is undesirable. For this reason, various methods have been proposed to reduce the variation in the steel sheet material.

 In addition, the direct quenching and tempering process is an on-line heat treatment using a through-type cooling device, and when the steel plate is thin or the plate length is large, there is a large difference in the quenching start temperature between the tip and tail of the steel plate. Arise.

 In other words, it is difficult to start cooling all steel plates with a slow rolling speed in quenching cooling or when the steel plate length is several + m at the same time, depending on the copper plate conveyance speed and the copper plate length. As a result, there is a time difference between the start of cooling and the start of cooling.

The cooling start temperature during quenching affects the microstructure and it is difficult to obtain a homogeneous material in the longitudinal direction of the steel sheet. Limiting the plate length of the steel sheet according to the thickness of the steel sheet so that the fluctuation range of the material falls within a predetermined range, or reheating and manufacturing by a quenching and tempering process may reduce manufacturing efficiency, Desirable, which increases manufacturing costs. For this reason, various methods for reducing the variation in the material of the copper plate have been proposed. For example, Japanese Laid-Open Patent Publication No. 9-310117 proposes to produce a copper material with little material change by controlling the cooling rate, and describes that the uniformity of the steel sheet in the thickness direction and between steel materials is improved. Has been. However, in the invention described in Japanese Patent Application Laid-Open No. 9-310117, the material change cannot be suppressed when the cooling start temperature or the rolling finishing temperature changes.

In addition, JP-A-3-173716 describes that the temperature drop at the four circumferences of the steel material is suppressed by a prior cooling method, and the entire material is rolled at a uniform temperature. although it constant, problems force ^s can not be enhanced application homogeneity in quenching material and accelerated coolant to exit the difference in the previous tail of Itacho subsequent cooling start temperature Nirre Te, .

 Furthermore, the technique disclosed in Japanese Patent No. 3784265 is effective in suppressing variations between steel materials, and is not sufficient to make the materials in the plate length direction of the steel plates within the same steel plate uniform. As described above, in the technique disclosed heretofore, since the temperature change in the longitudinal direction of the steel sheet cannot be controlled, the cooling start temperature at the time of cooling cannot be kept constant, resulting in large variations in materials.

 Therefore, in JP-A 61-21332.7, a thermometer is installed on the inlet side of the cooling device, the temperature in the longitudinal direction of the steel sheet is measured, and the approach speed to the cooling device is increased stepwise to reduce the cooling. A method for stabilizing the material by making the stop temperature uniform is disclosed.

 In the method described in Japanese Patent Laid-Open No. 61-213327, as described above, the stabilization of the material of the steel sheet is significantly affected not only by the cooling start temperature but also by the cooling start temperature. I can't. ,

JP-A-8-90042 discloses a method for keeping the cooling start temperature constant in the longitudinal direction of the steel sheet. Inserting a steel plate into the cooling device during hot rolling and keeping a temperature distribution in the longitudinal direction of the steel plate causes a temperature gradient at the leading edge of the steel plate at the end of rolling, and then inserts it into the cooling facility Sometimes the cooling start temperature is kept constant. Therefore, in the method described in JP-A-8-90042, the most important rolling finishing temperature that affects the toughness of the copper plate differs greatly in the longitudinal direction of the copper plate, and a certain toughness cannot be obtained in the longitudinal direction of the steel plate. .

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

 Therefore, the present invention provides a manufacturing method in which, when the length of the steel plate is long, a thin-walled steel plate having a uniform cooling start temperature and cooling stop temperature over the entire length of the copper plate and excellent strength and toughness uniformity is obtained. An object of the present invention is to provide a manufacturing facility and a thin-walled steel plate.

 As described above, in the direct quenching and tempering process using the pass-through quenching device, it is difficult to eliminate the cooling start temperature difference in the longitudinal direction of the steel sheet, and the strength and toughness vary greatly. The ability to limit the length, the reheating and quenching and tempering process had to be costly manufacturing.

 Therefore, the present invention is a direct quenching and tempering process or an accelerated cooling process, where the strength (TS) is 25MPa and the ductile-brittle fracture surface transition temperature (vTrs) is 10 ° C. An object of the present invention is to provide a thin-walled steel sheet having excellent strength and toughness uniformity in the longitudinal direction of the steel sheet, a manufacturing method thereof, and a manufacturing facility thereof. Disclosure of the invention

The inventors of the present invention studied various rolling-cooling processes in which the hot rolling rolling finishing temperature and the subsequent cooling start temperature or the cooling stop temperature are made constant, respectively, The most stable effect in actual machine operation can be obtained by placing a cooling device with a short cooling zone near the rolling mill and applying an appropriate temperature gradient in the longitudinal direction of the steel plate before hot rolling, quenching or accelerated cooling. It was found effective. The present invention has been made based on further studies based on the obtained knowledge. That is, the present invention 1.When water-cooling a steel sheet after finish rolling using a pass-through cooling device, in the first step, the steel sheet is water-cooled in advance so as to give a temperature gradient in the longitudinal direction of the steel sheet. A method for producing a thin-walled steel sheet, characterized in that water cooling is performed at a constant passing speed in the process.

 2.If the first-stage pass-through cooling device is placed upstream of the reversible hot rolling mill, the second-step pass cooling device is placed downstream of the reversible hot rolling mill. In the case where the first-stage passing type cooling device is arranged downstream of the reversible hot rolling mill, the second-step passing type cooling device is arranged downstream of the first-step passing type cooling device. The steel plate finished and rolled by the reversible hot rolling mill is cooled by the passage-type cooling device in the first step and is elongated in length. A method for producing a thin-walled steel sheet, characterized in that a temperature gradient is applied in the direction, and the steel sheet is water-cooled while passing through the passage-type cooling device in the second step at a constant speed.

3.When cooling with the passing type cooling device in the second step, the first start point and the tail end portion of the steel plate are cooled to the Ar ₃ transformation point or the two-phase region temperature so that the cooling start temperature is equal to or higher than the Ar ₃ transformation point. 3. The method for producing a thin-walled steel plate according to 2, wherein a temperature gradient is imparted by a passing type cooling device.

4. When cooling with the passing-type cooling device in the second step, the passing-type cooling in the first step so that the cooling start temperature difference between the tip and tail ends of the steel sheet is within 50 ° C. 4. The method for producing a thin-walled steel plate according to 2 or 3, wherein a temperature gradient is imparted by an apparatus.

 5. A method for producing a thick steel plate using a rolling-cooling device in which a passing-type cooling device is disposed downstream or upstream of the reversible hot rolling mill, which is finish-rolled by the reversible hot rolling mill. In the first step, the steel sheet is first cooled by the through-type cooling device and then cooled in the next second step while being reversely fed. In the first first step, the steel plate is cooled and longitudinally cooled. A method for producing a thin-walled steel sheet, characterized in that a temperature gradient is applied in the direction, and the reverse feed speed of the copper plate in cooling is a constant speed in the next second step.

6. When cooling with the above-mentioned passing type cooling device, in the next second step, the cooling start temperature at the tip and tail ends of the steel plate in cooling should be the Ar ₃ transformation point or higher or the two-phase region temperature. The above 6. The method for producing a thin-walled steel sheet according to 5, wherein a temperature gradient is applied by cooling in the first first step.

 7. When cooling with the passing type cooling device, in the next second step, the first step is performed so that the cooling start temperature difference at the tip and tail ends of the steel plate during cooling is within 50 ° C. 7. The method for producing a thin steel plate according to 5 or 6, wherein a temperature gradient is applied by cooling in the step 1.

8. When cooling with the first-stage pass-through cooling system, the temperature drop difference Δ 注 between the steel plate tip and tail ends satisfies equation (1) by changing the transport speed and / or the amount of water injected. The method for producing a thin-walled thick copper plate according to any one of 2 to 7, wherein cooling is performed.

15L / (hv) ≤ AT≤55L (hv) (l)

Where h: plate thickness (mm), L: length (m), V: entry speed (mZ s) to the passing type cooling device in the second step.

 9.When quenching or accelerating cooling with the pass-through cooling device in the second step, the first cooling step is performed so that the difference in cooling start temperature at the tip and tail ends of the steel sheet is within 30 ° C. 9. The method for producing a thin-walled steel plate according to 8, wherein the temperature of the front end portion and the temperature of the tail end portion in the longitudinal direction of the steel plate is adjusted in the process.

 The length in the steel sheet conveyance direction of the cooling region of the through-type cooling device in the first step described in 10.2 or the cooling region of the through-type cooling device arranged on the downstream side or the upstream side of the reversible rolling mill in 5 is 0.4 m to The method for producing a thin-walled steel plate according to any one of 2 to 9, wherein the thickness is 4 m.

11. The compositional strength of the steel sheet is as follows: C O.01 to 0.20%, Si.O.01—0.80%, Mn: 0.50—2.50%, P: 0.020% or less, S: 0.0070% or less, sol. The method for producing a thin-walled steel plate according to any one of 1 to 10, wherein A1: 0..004-0.1 00% is contained, and the balance is Fe and inevitable impurities.

12. In addition, the steel composition is Ti: 0.005-0.20%, Cu: 0.01—2.0 ^ο / ο, Ν i: 0.01—4.0%, Cr: 0.01—2.0%, Μο: 0.01—2.0%, Nb: 0.003—0.1%, V: 0.003-0.5%, W: 0.003-0.7%, Β: 0.0005—0.0040%, Ca: 0.0001—0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001—0.0200% 1 type or 2 11. The method for producing a thin steel plate according to 11, characterized by containing at least a seed.

 13.If the first-stage pass-through cooling device is placed upstream of the reversible hot rolling mill, the second-pass pass-through cooling device is placed downstream of the reversible hot rolling mill. In the case where the first-stage passing type cooling device is arranged downstream of the reversible hot rolling mill, the second-step passing type cooling device is arranged downstream of the first-step passing type cooling device. Manufacturing equipment for thin thick copper plates.

 14. A production facility for thin-walled steel sheets, which is equipped with a through-type cooling device that can cool the steel sheet after finishing rolling in the downstream or upstream side of the reversible hot rolling mill.

 The length of the cooling region of the through-type cooling device in the first step described in 15.13 or the cooling region of the through-type cooling device arranged on the downstream side or the upstream side of the reversible rolling mill in 14 is 0.4π. ! The manufacturing equipment for thin-walled thick steel plate according to 13 or 14, characterized by being ~ 4 m.

 16. Thickness of 6mn manufactured by hot rolling / direct quenching / tempering process or hot rolling / accelerated cooling process! Plate length 20π at 25mm! 50m thick steel sheet with a difference in tensile strength between the tip and tail of the steel sheet of 25MPa and ductile-brittle fracture surface transition temperature ®tt (vTrs) within 10 ° C. steel sheet.

 17. Ingredient compositional power mass%, C: 0.01—0.20%, Si: 0.01-0.80%, Mn: 0.50—2.50%, P: 0.020% or less, S: 0.0070% or less, sol.A1: 0.004—0.100% 17. The thin-walled thick copper plate according to 16, wherein the balance is Fe and the composition having inevitable impurity power.

 18. In addition, as component composition, Ti: 0.005-0.20%, Cu: 0.01—2.0%, Ni: 0.01—4.0%, Cr: 0.01—2.0%, Mo: 0.01—2.0%, Nb: 0.003—0.1 %, V: 0.003—0.5%, W: 0.003-0.7%, B: 0.0005—0.0040%, Ca: 0.0001—0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200% 18. The thin-walled thick steel plate according to 17, containing two or more kinds.

The steel plate 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 plate surface with a radiation thermometer unless otherwise specified. According to the present invention, the plate thickness is 6mn! Plate length 20π at 25mm! Direct quenching of thin-walled steel sheets up to 50m In one tempering process, the rolling finish temperature and cooling start temperature are all uniform over the entire length of the steel sheet, and in the accelerated cooling process, the rolling finish temperature, cooling start temperature, and cooling stop All of the temperature is uniform over the entire length of the steel sheet, and a thin and thick copper sheet with excellent strength and toughness uniformity is obtained, which is extremely useful in industry. Brief Description of Drawings

 Figure 1: Schematic diagram showing an example of rolling-cooling equipment.

 Fig. 2 is a schematic diagram illustrating the rolling-cooling method in the copper plate manufacturing method according to the present invention. Fig. 2A shows Casel, Fig. 2B shows Case 2, and Fig. 2C shows Case 3.

 Fig. 3 is a schematic diagram for explaining the temperature difference in the longitudinal direction of the steel sheet in the rolling-cooling method according to the present invention. Fig. 3A shows Casel, Fig. 3B shows Case 2, and Fig. 3C shows Case 3.

 Fig. 4: Cooling device with excellent drainage performance suitable as the first-stage pass-through cooling device in the present invention. Fig. 4A does not use a draining roll, and the cooling water is retained on the steel plate by the cooling water injection nozzle. Figure 4B shows a form in which cooling water jet nozzles and draining rolls are used together to retain the cooling water on the copper plate. Figure 4C shows a form in which cooling water is retained on the steel sheet with the draining rolls without using the cooling water jet nozzle.

 Fig. 5: Diagram showing the cooling rate of hot-rolled copper sheet during air cooling.

 Fig. 6: An example of a schematic diagram of the passing-type cooling device in the second step.

(Explanation of symbols)

 1: Cooling tank, 2: Retained cooling water, 3: Upper cooling water injection nozzle,

 4: Steel plate, 5: Transfer roll, 6: Lower cooling water nozzle, 7: Cooling area

 8: draining roller, 9: rolling mill, 10: first-stage cooling unit, 1

1: Passing type cooling device in the second step, 14: Heating furnace, 15: Rod-like water flow, 21: Slit jet nozzle, 22: Circular tube nos, Nore BEST MODE FOR CARRYING OUT THE INVENTION

 In the present invention, a cooling facility capable of adjusting the temperature in the longitudinal direction of the steel sheet is arranged in the vicinity of the upstream side or the downstream side of the finish rolling mill, and further, quenching or accelerated cooling is possible on the downstream side of the cooling facility. Using a hot rolling and cooling facility for steel sheets equipped with various water cooling facilities, the rolling finish temperature, cooling start temperature, and further cooling stop temperature are all made uniform over the entire length of the copper plate. And

 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 Fig. 1, 14 is a reheating fiirnace, 9 is a rolling mill, 10 is a rolling mill 9 (10) is the first-stage cooling device arranged upstream of the rolling mill 9, and 11 is the second-type flow-type cooling device arranged on the downstream side. A cooling device is shown. The heating furnace 14 side of the rolling mill 9 is referred to as the upstream side of the rolling mill 9, and the passing cooling device 11 side of the second step is referred to as the downstream side.

 A thick steel plate (not shown) is heated for rolling in a heating furnace 14, and after finish rolling, an appropriate temperature gradient is imparted in the longitudinal direction of the steel plate in the first-stage passing cooling device 10.

 The steel plate after finish rolling is transferred to the passing water cooling device 11 in the second step for quenching or accelerated cooling, but the cooling start temperature is substantially the same in the longitudinal direction of the steel plate. An appropriate temperature gradient is given in advance in the longitudinal direction of the steel sheet when passing through the passing type cooling device 10 in the first step.

2A, 2B, and 2C are schematic diagrams for explaining the rolling-cooling method in the copper plate manufacturing method according to the present invention. P1 to P4 indicate a pass, P3 indicates a final (finishing) pass, P2 is a pass immediately before the final pass P3, P1 is a pass immediately before P2, and P4 is an empty pass through which the steel sheet after finish rolling or finish rolling and water cooling passes through the rolling mill 9. The empty pass means that the thick copper plate is passed through the rolling mill without rolling. a represents a rolling operation by the rolling mill 9, bl, b2 represents a water cooling operation by the first-stage passage type cooling device, and c represents a water cooling operation by the second step passage-type cooling device. In Figures 2A, 2B, and 2C, the steel plate is not shown.

 Fig. 2A shows the case where the first-stage through-type cooling device 10 and the second-step through-type cooling device 11 are arranged in this order on the downstream side of the rolling mill 9, and the steel sheets after finish rolling are sequentially passed through and cooled. (Case 1), Fig. 2B shows the first-stage passing-type cooling device 10 upstream of the rolling mill 9 and the second-step passing-type cooling device 11 downstream of the rolling mill 9, and finish rolling ( The steel plate after P3) is cooled by the pass-through cooling device 10 in the first step in the empty pass (P4), and then passed through the rolling mill 9 in the empty pass (P4) and passed through the rolling mill 9 in the second step 11 In Fig. 2C, a pass-through cooling device (in this description, the pass-through cooling device 10 in the first step) is placed downstream of the rolling mill 9, and after cooling, A case (Case 3) is shown in which cooling is performed by the above-described cooling device in the second step while back feeding. In (Casel) and (Case3), the force P2 described with P3 as the final (finishing) pass can be used as the final (finishing) pass, and the rolling mill 9 can be empty in the P3 pass.

 FIG. 2C shows the case where the first-stage passing type cooling device 10 is provided downstream of the rolling mill 9! However, it is also possible to provide the pass-type cooling device 10 of the first step on the upstream side of the rolling mill 9 and make P2 the final (finishing) pass.

 3A, 3B, and 3C schematically show the temperature distribution in the longitudinal direction of the steel sheet in Cases 1 to 3 shown in FIGS. 2A, 2B, and 2C, respectively. 2A, 2B, 2C, 3A, 3B, and 3C, the direction of the arrow indicates the traveling direction of the copper plate.In the explanation, the traveling direction side of the copper plate is the tip of the steel plate, and the opposite direction is the direction of the copper plate. It is called the tail end of a steel plate. Hereinafter, the present invention will be described with reference to FIG. 2A, FIG. 2B, FIG. 2C, FIG. 3A, FIG. 3B, and FIG.

In Case 1 shown in Fig. 2A and Fig. 3A, the steel plate is made to have a predetermined plate thickness in the final pass P3, then water-cooled by the first-stage passing cooling device 10 and then passed through the second step. Water cooling is performed in the mold cooling device 11 to give the desired performance. When the water cooling bl is water cooled by the passage type cooling device 11 in the second step, the tail end of the steel plate in the longitudinal direction of the steel plate is ΔΔΤ from the tip so that the cooling start temperature is substantially the same in the steel plate longitudinal direction. High temperature. Water in the second-stage through-type cooling device 11 When the cold c is applied, the steel sheet is allowed to enter the second-stage cooling apparatus 11 in the second step 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 shown in Fig. 2B and Fig. 3B, the steel sheet is set to a predetermined plate thickness by the rolling mill 9 in the final pass P3, and then water-cooled by the first-stage pass-through cooling device 10 and then empty. It passes through the rolling mill 9 in pass P4 and is water-cooled in the second-stage passing cooling device 11 to give the desired performance.

 When the water cooling bl is cooled by the passage type cooling device 11 in the second step c, the tail end portion in the longitudinal direction of the steel sheet is more than Δ 先端 from the front end portion so that the cooling start temperature is substantially the same in the longitudinal direction of the steel sheet. High temperature. When water-cooling c is performed in the second-stage cooling device 11 in the second step, the steel sheet enters the second-step cooling device 11 in the second step at a constant speed, and the cooling stop temperature is substantially the same in the longitudinal direction of the copper plate. .

 In Case 1 and 2, it is possible to make the tail end portion higher in temperature in the longitudinal direction of the copper plate by ΔΤ in the longitudinal direction of the copper plate by passing the steel plate while accelerating the passage type cooling device 10 in the first step.

 In the case of Cse3 shown in Fig. 2C and Fig. 3C, after the copper plate is set to a predetermined plate thickness in the final pass P3, it is cooled with water in the first-stage pass-through cooling device 10 and then again in the first step. Water-cooling b2 is performed while the pass-through cooling device 10 is fed back. In the water-cooled bl, the tip of the steel plate is heated by 厶 T from the tail end in the longitudinal direction of the steel sheet. In the case of performing water cooling bl, the copper plate can be heated by Δ 高温 from the tail end portion by passing the copper plate while decelerating the passage type cooling device 10 in the first step.

When water cooling b2 is performed while feeding back, the tail end portion becomes the tip end portion, and the tip end portion becomes the tail end portion, so that the cooling start temperature becomes substantially the same in the longitudinal direction of the copper plate. When water cooling b2 is performed in the first-stage pass-through cooling device 10 while back feeding, the steel plate enters the first-step pass-type cooling device 10 at a constant speed, and the cooling stop temperature is set in the longitudinal direction of the steel plate. And substantially the same. Reverse feed refers to the case where the steel sheet is conveyed from the downstream side of the rolling mill 9 to the upstream side. In Cases 1 to 3, the temperature difference ΔΤ applied to the tail end and the tip in the longitudinal direction of the steel sheet is the second-stage through-type cooling device 11 or the first-step through-type cooling device 10 that is reversely fed. In the subsequent cooling, the cooling start temperature at the tail end of the steel sheet is applied so as to be the same as that at the tip.

 As shown in Fig. 3 図, Fig. 3Β, and Fig. 3C, it is preferable to change the temperature gradient linearly in the longitudinal direction of the steel sheet. However, the temperature gradient may be changed stepwise in the longitudinal direction of the steel sheet. . In addition, the power S, the cooling start temperature, and the cooling stop temperature are made substantially the same, making the cooling equipment capacity constant after applying a temperature gradient in the longitudinal direction of the steel sheet and making the conveyance speed during cooling constant. If the cooling capacity such as the amount of cooling water can be controlled, the conveying speed in the longitudinal direction of the steel sheet does not have to be constant.For example, the amount of cooling water is increased in proportion to the increase in the conveying speed. You may let them.

 That is, the technical feature of controlling the cooling start temperature and the cooling stop temperature in the second-stage through-type cooling device, which are the characteristics of the invention of the present application, substantially in the same direction with respect to the longitudinal direction of the steel sheet can be achieved. If there are, i.e., increasing the cooling water volume while increasing the conveying speed to increase the cooling capacity, or conversely decreasing the cooling water volume while decreasing the conveying speed to decrease the cooling capacity. By adopting it, if the cooling start temperature and cooling stop temperature can be made substantially the same while changing the conveying speed in the longitudinal direction of the steel sheet, the steel sheet longitudinal strength, which is the target of the present invention, is homogeneity of toughness. It is possible to produce a steel plate that is excellent in the quality.

 Here, the solid line in Fig. 5 shows the relationship between the thickness of the hot-rolled copper sheet at 700 ° C, 850 ° C, and 1000 ° C and the cooling rate when the copper sheet is cooled by air cooling. An example is shown. 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 dissipation, so the cooling rate increases.

The cooling rate is slightly different depending on the transport mode and atmosphere. For example, when the plate thickness is 30 mm and the steel plate surface temperature is 800 ° C, the cooling rate is about 0.8 ° CZs. In general, after cooling with the first-stage through-type cooling device, the second-step through-type cooling device or again with the first-step through-type cooling equipment, bow I will be used for accelerated cooling and quenching when cooling continuously. Since the cooling start temperature is 700 ° C or higher, the cooling rate is higher than the broken line of 15Zh (° CZmm) in Fig. 5.

 On the other hand, since the cooling start temperature for accelerated cooling and quenching hardly exceeds 1000 ° C, the cooling rate is similarly below the broken line of δδΖΐιΓΌΖπιπι).

 Therefore, when the cooling start temperature for accelerated cooling or quenching is 700 ° C or higher and 1000 ° C or lower, the temperature difference Δ T at the tip of the temperature gradient applied in the longitudinal direction of the steel sheet is

15L / (hv) ≤ Α Ύ≤55 ^ (Ιιν) (1)

It is preferable to satisfy this relationship.

However, h (mm): Plate thickness of steel plate, L (m): Length of steel plate, v (mZs): After applying a temperature gradient in the longitudinal direction of the steel plate with the passing-type cooling equipment in the first step, Steel sheet transport speed when entering the cooling system

30s違うので、銅板の尾端部は先端部 より も 30s間分余計に空冷される。 As an example, if the length of the steel plate product is L = 30 m and the copper plate transport speed is v = lmZs,

Since the difference is 30 s, the tail end of the copper plate is air-cooled for 30 s more than the tip.

 When the plate thickness is h = 30mm, the cooling rate when the surface temperature of the steel plate is 800 ° C is 0.8 °. Therefore, the tail end of the steel sheet is 0.8 X L / v = 24 ° C lower than the tip. Therefore, if the tip of the copper plate is cooled at 34 and the tail end of the copper plate is cooled by 10 ° C, the temperature difference at the tail end is 24 ° C higher than the tip, so the cooling start temperature is almost constant.

 In addition, it is preferable to use a cooling device excellent in water-blocking property so that the cooling water does not flow out in the direction of conveying the steel sheet outside the cooling region as the passing-type cooling device in the first or second step. .

Figures 4A, 4B, and 4C show an example of a cooling device with excellent drainage performance. In the figure, 1 is a cooling bath, 2 is cooling water retained on the steel plate 4, and 3 is upward cooling water. Injection nozzle, 4 is steel A plate, 5 is a transport roll, 6 is a lower cooling water nozzle, 7 is a cooling region, 8 is a draining ronor, and 15 is a rod-shaped cooling water.

Fig. 4A shows a cooling device that ejects rod-shaped cooling water 7 from the upper cooling water injection nozzle 3 attached to the cooling tank 1 so as to face each other and retains the cooling water 2 on the copper plate 4, and Fig. 4B shows the conveyance with the draining roll 8 A cooling device in which roll 5 is opposed across steel plate 4 and bar cooling water 15 is jetted from one direction and dammed by draining roll 8, Figure 4C shows two pairs of draining roll 8 and transport roll 5, and between the roll pairs, A cooling device for retaining cooling water 2 on steel plate 4 is shown. In order to obtain a damming effect, it is preferable that the rod-shaped cooling water has a water density of 4 m ³ Zm ² min or more. That is, the amount of water density force m ³ // !!! ² !! ^! The amount of stagnant cooling water 2 that can be dammed up increases, and the cooling water discharged from the plate width end and the cooling supplied The amount of water is balanced and the retained cooling water 2 is kept constant. In the case of thick steel plates, the general plate width is 2 to 5 m. If cooling is performed with a water density of 401 ³ cm 1 ^ 1 ^! 1 or more, the stagnant cooling water 2 can be kept constant at these plate widths. The desired temperature drop can be obtained while passing the steel plate being rolled.

The higher the water density is 4m ³ Zm ² min or more, the more controlled rolled material that eliminates the waiting for cooling. For example, if the water density is small, the waiting for cooling can be solved only with a rolled material with a thin plate thickness, but if the water density is increased, the waiting for cooling can be solved even for a rolled material with a certain thickness. However, the effect of shortening the cooling waiting time for increasing the water volume gradually decreases as the water density increases, so the water density takes into account the effect of shortening the cooling waiting time and the equipment cost. Therefore, it is preferable to determine. A more preferable water density is 4 to: ί0πΤΖι ² ιηίη.

The distance in the transport direction of the cooling region 7 is 0.4π in the first-stage passing cooling system (including the case 3 passing cooling system)! It is preferably set to 4 m. If it is less than 4 m, it is necessary to take a long residence time in the cooling zone in order to cool the copper plate, and it takes too much time to pass the entire steel plate, making it difficult to create a sufficient temperature gradient. On the other hand, if it exceeds 4 m, it is difficult to provide uniform cooling in the cooling region, and it is difficult to provide a sufficient temperature gradient with a steel plate with a short plate length. The

 In addition, the cooling area | region in FIG. 4A, FIG. 4B, and FIG. 4C is shown by the black coating part of a steel plate.

In addition, the cooling area 7 is appropriately set by increasing or decreasing the number of cooling baths and nozzles, or by increasing or decreasing the number of cooling unit units shown in FIGS. 4A, 4B, and 4C. It is possible.

 In order to provide a temperature gradient with the first-stage cooling system, it is preferable to control the passing speed of the steel sheet in the cooling zone because of excellent control responsiveness. Both the passing speed and the cooling capacity may be used so that the cooling facility capacity such as the water injection amount of the first-stage passing cooling system can be controlled.

 The second-stage pass-through cooling system is not particularly limited as long as it has the required cooling capacity and can perform uniform cooling. For example, a through-type cooling device 11 (which can also be used for direct quenching) as shown in FIG. 6 is used. In FIG. 6, 4 is a steel plate, 8 is a draining roll and a draining drain, 21 is a slit jet nozzle, and 22 is a circular pipe nozzle. In this cooling device 11, the steel plate 4 cooled by the first cooling device is conveyed through a plurality of cooling zones between 20 sets of water draining rolls and sewage draining rollers 8, while the upper side is a slit nozzle 21. The lower side is cooled on-line by the cooling water from the circular tube nozzle 22 by the cooling water from. In addition, thermometers are attached to the inlet side and the outlet side of the cooling device 11, respectively, so that the temperature of the thick copper plate can be measured before and after cooling. Each cooling zone is partitioned by upper and lower draining rolls 8, and the amount of cooling water can be adjusted individually.

The cooling start temperature is appropriately selected from the Ar ₃ transformation point or the two-phase region temperature depending on the desired characteristics. This is because, in order to secure the desired strength by generating transformation phases such as martensite and bainite by quenching or accelerated cooling from the temperature range including the austenite phase, the cooling start temperature of quenching and accelerated cooling is the Ar ₃ transformation point. The above-mentioned temperature must be in the two-phase temperature range and the temperature range in which the austenite phase is present. What is necessary is just to select suitably according to the intensity | strength desired.

In addition, it is preferable to temper the hardened steel plate. Tempering can be carried out in a conventional manner.For example, an off-line atmosphere furnace or an on-line induction heating device can be used, and the tempering temperature is below the _ACl transformation point, which is the temperature range where no austenite phase is generated. Preferable to be temperature.

 In the present invention, after a temperature gradient is applied in the longitudinal direction of the steel sheet in the first-stage pass-type cooling device, either the cooling start temperature in the second-step pass-type cooling device or further the cooling stop temperature. It is preferable that the difference between the maximum value and the minimum value in the longitudinal direction of the copper plate be 50 ° C or less. .

 When the temperature difference exceeds 50 ° C, the difference in strength between the tip and tail ends of the steel sheet increases the difference in toughness. More preferably, it is 30 ° C or less.

 In Case 3, quenching or accelerated cooling is performed by cooling b2 using the pass-through cooling device 10 in the first step. The cooling method according to the present invention can be applied to a steel sheet having a composition suitable for direct quenching and tempering or an accelerated cooling process. The direct quenching and tempering process described below or an accelerated cooling process is assumed. The component composition is preferred. % In the component composition is mass%.

 C: 0. 01 -0. 20%

 C must be at least 0.01% to ensure the strength of the steel sheet, and if added over 0.20%, the weldability will be significantly reduced, so 0.01% or more and 0.20% or less (hereinafter, 0. 01 -0. 2 0%).

 Si: 0. 01 -0. 80%

 Si is an element necessary for deoxidation, but if it is less than 0.01%, the effect is small.If it exceeds 0.80%, weldability and base metal toughness are significantly reduced. 80%.

Mn: 0.5— 2. 50% Mn is necessary to ensure the strength of the steel sheet as in C, and if added in excess, the weldability is impaired, so 0.5-2.50%.

 P: 0.020% or less, S: 0.0070% or less

P and S are elements that are unavoidably contained in steel as impurities, and deteriorate the toughness of the steel base material and weld heat affected zone, so it is preferable to reduce it as much as possible in consideration of economy. Mashigu P: 0.020 mass% or less, S: 0.0070 mass% or less.

 A1: 0.004-0.10% or less

A1 is a deoxidizing element, and if it is less than 0.004%, its effect is not sufficient, and adding too much will cause toughness deterioration, so it should be 0.004-0.10% or less.

 The preferred basic component composition of the present invention is the above force S, and in the case of further improving desired characteristics, Ti, Cu, Ni, Cr, Mo, Nb, V, W, B, Ca, Mg, REM or Add two or more kinds as selective elements.

 Ti.O.005-0.20%

 Ti has the strength to ensure the toughness of the base metal and the toughness in the heat affected zone. The force with the specified range is good. If added over 0.20%, the toughness will decrease significantly. 0.20%.

 Cu: 0.01-2.0%

 Cu is an element to increase the strength and exerts its effect at 0.01% or more, and if added over 2.0%, the steel sheet surface properties deteriorate due to hot brittleness. To do.

 Ni: 0.01-4.0%

 Ni can improve the toughness while increasing the strength of the base metal, and is effective at 0.01% or more, and the effect is saturated and economically disadvantageous at 4.0% or more. -4.0%.

 Cr: 0.01-2.0%, Μο: 0.01—2.0%

Cr and Mo are both effective in increasing the strength, and the effect is exhibited at 0.01% or more. If added over 2.0%, the toughness will deteriorate significantly. If added, the content should be 0.01-2.0%.

 Nb: 0.003-0.1%, V: 0.003-0.5%

Nb and V are elements that improve the strength and toughness of the base metal. Addition of 0.003% or more produces an effect. Also, if it exceeds 0.1% and 0.5%, respectively, the toughness may be lowered. Therefore, when adding, Nb: 0.003-0.1% and V: 0.003-0.5%.

 W: 0.003-0.7%

 W is an element that improves strength and corrosion resistance. If it is less than 0.003%, the effect is not good. If it exceeds 0.7%, the weld heat affected zone toughness may be deteriorated.

 B: 0.0005-0.0040%

B can increase the strength by improving the hardenability. This effect becomes prominent at 0.0005% or more, and the effect is saturated even if added over 0.0040%. Therefore, when added, the content should be 0.0005-0.0040%.

 Ca: 0.0001-0.0060%, Mg: 0.0001— 0.0060%, REM: 0.0001-0.020 0%

Ca, Mg. REM works to fix S in steel and improve the toughness of the copper sheet, and has an effect S with an applied force of 0.0001% or more. And then force, respectively 0.0060%, 0.0060%, in order to rather degrade the intervening amount increases and toughness of the copper is added Te Etsumen the 0.0200 ^o / o, the case of adding the, Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.020 0%.

 The balance other than the above components is composed of Fe and inevitable impurities. In the case of a steel plate, the molten copper having the above composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and a slab or the like is prepared by a conventional method such as a continuous forging method or an ingot-bundling method. It is preferable to use a steel material. The melting method and the forging method are not limited to the methods described above.

The dimensions of the thin-walled steel plate targeted by the present invention are the plate thickness of 6-25mm and the plate length of 20π! ~ 50m steel plate. When the plate thickness is less than 6mm, the air cooling rate is high, so the rolling temperature and cooling start temperature- Not all stop temperatures can be made uniform. In addition, when the plate thickness is 25 mm or more, the cooling rate during air cooling is small, so that the temperature difference between the tip and tail ends of the steel plate is small without using the temperature gradient control method of the present invention. If the length of the steel sheet is less than 20 m, the required product length may not be obtained, or the productivity is inferior, and even if the temperature gradient control method of the present invention is not used, The temperature difference between the tip and tail is small. In addition, when the length of the steel sheet exceeds 50 m, the temperature gradient control according to the present invention has a large temperature difference between the leading edge and the trailing edge of the steel sheet, which takes time for rolling and cooling, and the temperature drop at the tail edge of the steel sheet is large. Even with this method, it becomes difficult to reduce the temperature difference between the tip and tail of the copper plate.

 In addition, the mechanical performance of the thin steel plate targeted by the present invention is that the difference in tensile strength is ± 25 MPa between the tip and tail of the steel plate, and the difference in ductile-brittle fracture surface transition temperature (toughness) (vTrs). Is a thin-walled steel plate within the range of ± 10 ° C.

 If the difference in tensile strength between the tip and tail ends of the steel sheet is less than ± 25MPa, if the difference in toughness is out of soil 10 ° C, the tensile strength and toughness of the entire steel sheet will be within the target range (material standard). The probability of control within the range is reduced, the acceptance rate of the steel sheet is reduced, and the workability in the steel sheet is not uniform. In addition, the production conditions such as the rolling finishing temperature and cooling stop temperature are restricted more than necessary, which hinders productivity. Example 1

 A thick steel plate was produced by the rolling-cooling method according to Casel 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-stage pass-through cooling system uses a cooling facility (Fig. 4C) with a cooling area lm between two draining rolls (Fig. 4C). The accelerated cooling equipment shown in Fig. 6 was used as the equipment.

In this example, a recrystallization zone rolling with a cumulative rolling reduction of 50% at a temperature of 980 ° C or higher after rolling at 1150 ° C in a heating furnace using a slab with a plate thickness of 215mm. (rolling in recrystallization zone), controlled rolling with a cumulative rolling reduction of 50% at 900 to 800 ° C., and a steel plate having a plate thickness of 30 mm and a plate length of 30 m was manufactured. In this embodiment, the tip and tail edges of the steel plate having the cooling start temperature and the cooling stop temperature in the second-stage through-type cooling device are provided by Casel, while the temperature gradient is given by the first-type through-type cooling device. The steel plate was manufactured under the manufacturing conditions in which the difference was 50 ° C. or less and the manufacturing conditions in which cooling was performed only with the passing type cooling device in the second step without using the passing type cooling device in the first step. In all the manufacturing conditions, the cooling conveyance speed in the second-stage cooling device in the second step was kept constant at lmZs.

 For the obtained thick copper plate, a tensile test piece having a full thickness was collected and subjected to a tensile test in accordance with the standard of JIS Z 2241 (1998) to obtain the tensile strength TS. In addition, a Charpy impact test specimen with a V-notch standard dimension was taken from the position in the plate thickness direction 1Z2 in accordance with JIS Z 2202 (1998), and impact in accordance with JIS Z 2242 (1998). Conducted tests to determine ductile- brittle fracture transition temperature and vTrs.

 Table 1 shows the composition of the test steel, and Table 2 shows the strength and toughness of the steel sheet obtained. Nol, 2, 6, 7, 11, 12 with a temperature difference ΔΤ between the tip and tail in the first-stage pass-through cooling system are YS (yield strength) and TS ( The difference in tensile strength was uniform within ± 25 MPa, and the toughness was good.

 On the other hand, due to low speed rolling in one direction, the toughness at the tip of the steel sheet decreased for Nos. 3, 8 and 13 where the temperature difference between the tip and tail ends of the steel sheet with a finishing temperature of 50 ° C or more was large. In addition, No4, 9, and 14 are YS at the tip and tail of the steel plate, where the temperature difference between the tip and tail of the steel plate at the cooling start temperature is large at 50 ° C or more. Also, the TS difference was large and the toughness of the tail end was lowered, and a uniform steel plate could not be obtained.

In addition, the difference between the steel plate tip and tail end at the cooling stop temperature in the second-stage cooling system without applying a temperature gradient is as large as 50 ° C or more. The difference in YS and TS at the edges was large, and the toughness at the tip or tail edge of the steel sheet was reduced. Example 2 Thick steel plates were produced by the cooling method according to 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-stage pass-through cooling system uses a cooling facility (Fig. 4C) with a cooling area of lm between two draining rolls (Fig. 4C). The direct quenching equipment shown in Fig. 6 was used as the cooling device. The tempering conditions after direct quenching were the tempering temperatures shown in Tables 4 and 5.

 In this example, a steel plate having a thickness of 6 to 25 mm was produced from a slab having a thickness of 250 mm using various hot rolling conditions, and the steel plates obtained after direct quenching and tempering shown in Tables 4 and 5 were used. For this, tensile test specimens of full thickness were collected and subjected to a tensile test in accordance with JIS Z 2241 (1998) to determine the tensile strength TS. In addition, a Charpy impact test specimen with a V-notch standard dimension was taken from the position in the plate thickness direction 12 in accordance with the provisions of JIS Z 2202 (199 8), and conformed to the provisions of JIS Z 2 242 (1998). An impact test was conducted to determine the ductile-brittle fracture surface transition temperature vTrs. However, for sheet thicknesses of llmmt or less, vTrs was determined using a half-size Charpy specimen.

 Table 3 shows the composition of the test steel, and Tables 4 and 5 show the strength and toughness of the copper plates obtained. In the inventive examples (Nos. 1 to 26), the TS difference between the tip and tail of the copper plate was uniform within ± 25 MPa, and the toughness difference (vTrs difference) was good within ± 10 ° C. On the other hand, in the comparative examples (Nos. 27 to 39), uniform steel sheets with large TS difference between the tip and tail and Z or toughness difference (vTrs) could not be obtained. Example 3

 Thick steel plates were produced by the cooling method according to 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-stage pass-through cooling system uses a cooling facility (Fig. 4C) with a cooling area of lm between two draining rolls (Fig. 4C). As the cooling device, the accelerated cooling equipment shown in Fig. 6 was used.

In this example, a steel plate having a thickness of 6 to 25 mm was produced using various hot rolling conditions such as a 250 mm cross-sectional slab force, and a full thickness tensile test piece was collected from the obtained thick steel plate. Tensile tests were conducted in accordance with JIS Z 2241 (1998) to determine the tensile strength TS. Board In accordance with JIS Z 2202 (1998), a V-notch standard size Charpy impact test piece is taken from the position in the thickness direction 1 2 and subjected to an impact test according to JIS Z 2242 (1998). The ductile one brittle fracture surface transition temperature vTrs was obtained. However, for sheet thicknesses of llmmt or less, vTrs was determined using half-size Charpy specimens.

 Table 3 shows the composition of the test steel, and Tables 6 and 7 show the strength and toughness of the copper plates obtained. In the present invention example (Nos. 1 to 26), the TS difference between the tip and tail ends of the steel sheet was uniform within ± 25 MPa, and the toughness difference (vTrs difference) was good within ± 10 ° C. On the other hand, the comparative examples (Nos. 27 to 39) were strong enough to obtain a uniform steel sheet with a large TS difference and Z or toughness difference (vTrs) between the tip and tail of the copper plate. Industrial applicability

According to the present invention, in the rolling and cooling process of a thick steel plate, the rolling finishing temperature and the cooling start temperature, or the cooling stop temperature are all uniform over the entire length of the steel plate, and the strength and toughness are uniform over the entire length of the steel plate. A thin thick copper plate with excellent resistance is obtained, which is extremely useful industrially.

Note 2: * mark Outside the scope of the present invention

Note 3: ○: Yes, X: No

Note 4: The end that enters the rolling mill or cooling equipment first is the “tip”.

: * Indicates temperature control items that are out of the range. 0: X: Not applicable Note 2: The end that enters the rolling mill or cooling equipment first is the “tip”.

Table 7

2: Enter the _yard

Claims

The scope of the claims 1.When water-cooling a steel sheet after finish rolling using a pass-through cooling device, in the first step, the steel sheet is water-cooled in advance so as to give a temperature gradient in the longitudinal direction of the steel sheet. A method for producing a thin-walled steel sheet that is water-cooled at a constant passing speed in the process. 2.If the first-stage pass-through cooling device is placed upstream of the reversible hot rolling mill, the second-step pass cooling device is placed downstream of the reversible hot rolling mill. In the case where the first-stage passing type cooling device is arranged downstream of the reversible hot rolling mill, the second-step passing type cooling device is arranged downstream of the first-step passing type cooling device. A copper sheet finished and rolled with the reversible hot rolling mill is cooled by the passage-type cooling device in the first step and is elongated in length. A method for producing a thin-walled steel plate, in which a temperature gradient is applied in a direction, and the steel plate is water-cooled while passing through the passage-type cooling device in the second step at a constant speed. 3.When cooling with the passing type cooling device in the second step, the first start point and the tail end portion of the steel plate are cooled to the Ar ₃ transformation point or the two-phase region temperature so that the cooling start temperature is equal to or higher than the Ar ₃ transformation point. 3. The method for producing a thin-walled thick copper plate according to claim 2, wherein a temperature gradient is imparted by a passing type cooling device. 4. When cooling with the passing-type cooling device in the second step, the passing-type cooling in the first step so that the cooling start temperature difference at the tip and tail ends of the copper plate is within 50 ° C. The method for producing a thin-walled steel sheet according to claim 2 or 3, wherein a temperature gradient is imparted by the apparatus. 5. A method for producing a thick copper plate using a rolling-cooling device in which a passing-type cooling device is disposed downstream or upstream of the reversible hot rolling mill, which is finish-rolled by the reversible hot rolling mill. In the first step, the steel plate is first cooled by the passing type cooling device and then fed back to the next. In the second step, cooling is performed, in the first first step, a temperature gradient is applied in the longitudinal direction by cooling, and in the next second step, the reverse feed speed of the steel plate in cooling is A method for producing thin-walled steel plates at a constant speed. 6. When cooling with the above-mentioned passing type cooling device, in the next second step, the cooling start temperature at the tip and tail ends of the steel plate in cooling should be the Ar ₃ transformation point or higher or the two-phase region temperature. 6. The method for producing a thin-walled thick copper plate according to claim 5, wherein a temperature gradient is applied by cooling in the first first step. 7. When cooling with the passing type cooling device, in the next second step, the first step is performed so that the cooling start temperature difference at the tip and tail ends of the steel plate during cooling is within 50 ° C. The method for producing a thin-walled steel sheet according to claim 5 or 6, wherein in step 1, a temperature gradient is applied by cooling. 8. When cooling with the first-stage cooling system, change the transport speed and / or the amount of water injected so that the difference in temperature drop Δ T between the steel plate tip and tail end satisfies equation (1). The method for producing a thin-walled steel sheet according to claim 2, wherein the cooling is performed as follows.  15L / (hv) ≤ AT≤55L / (hv) '... (1)  However, h: Plate thickness (mm), L: Length (m), V: Speed of approach to the passing type cooling device in the second step (m). 9.When quenching or accelerating cooling with the pass-through cooling device in the second step, the first cooling step is performed so that the difference in cooling start temperature at the tip and tail ends of the steel sheet is within 30 ° C. 9. The method for manufacturing a thin-walled steel plate according to claim 8, wherein the temperature of the front end portion and the temperature of the tail end portion in the longitudinal direction of the steel plate is adjusted in the process. The length in the steel sheet conveyance direction of the cooling region of the through-type cooling device in the first step described in 10.2 or the cooling region of the through-type cooling device arranged on the downstream side or the upstream side of the reversible rolling mill in 5 is 0.4 m to The method for producing a thin thick copper plate according to any one of claims 2 to 9, wherein the thickness is 4m. 11. The compositional power of the steel sheet is as follows: C: 0.01-0.20%, Si: 0.01-0.80%, Mn: 0.50-2.50%, P: 0.020% or less, S: 0.0070% or less, sol.A1: 0.004 —0.1 The method for producing a thin-walled steel sheet according to any one of claims 1 to 10, wherein the composition contains 00% and has a remaining force SFe and an inevitable impurity force. 12. In addition, the steel composition is Ti: 0.005-0.20%, Cu.O.01—2.0% _s N i: 0.01-4.0%, Cr: 0.01—2.0%, Mo: 0.01-2.0%, Nb: 0.003—0.1% V: 0.003-0.5%, W: 0.003-0.7%, B: 0.0005—0.0040%, Ca: 0.0001—0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001—0.0200% 12. The method for producing a thin-walled thick steel sheet according to claim 11, comprising one or more kinds. 13.If the first-stage pass-through cooling device is placed upstream of the reversible hot rolling mill, the second-pass pass-through cooling device is placed downstream of the reversible hot rolling mill. In the case where the first-stage passing type cooling device is arranged downstream of the reversible hot rolling mill, the second-step passing type cooling device is arranged downstream of the first-step passing type cooling device. Manufacturing equipment for thin-walled thick steel plates. 14. A manufacturing facility for thin-walled thick copper plates that has a through-type cooling device that can cool the steel sheet after finishing rolling in the downstream or upstream side of the reversible hot rolling mill. 15. A method of conveying steel plates in a cooling region of the first-stage through-type cooling apparatus according to claim 13 or the first-stage reversible rolling mill according to claim 14 disposed downstream or upstream of the reversible rolling mill. The direction length is 0.4π! The equipment for producing a thin-walled steel sheet according to claim 13 or 14, wherein the production facility is ~ 4m. 16. Thickness of 6nm manufactured by hot rolling and direct quenching and tempering process or hot rolling-accelerated cooling process! Plate length 20π at 25mm! A thin, thick copper plate with a tensile strength difference of 25MPa at the tip and tail ends of the steel plate and a difference in ductile-brittle fracture surface transition temperature (vTrs) within ± 10 ° C. 17. Ingredient compositional power mass%, C: 0.01—0.20%, Si: 0.01-0.80%, Mn: 0.50-2.50%, P: 0.020% or less, S: 0.0070% or less, sol.A1: 0.004-0.100% The thin-walled steel plate according to claim 16, wherein the balance is a composition in which the balance is Fe and inevitable impurity power. 18. In addition, the component composition is Ti: 0.005-0.20%, Cu: 0.01-2.0%, Ni: 0.01—4.0%, Cr: 0.01-2.0%, Mo: 0.01-2.0 ^o / _o , Nb: 0.003 — 0.1%, V: 0.003—0.5%, W: 0.003-0.7%, B: 0.0005—0.0040%, Ca: 0.0001—0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001—0.0200% 1 The thin-walled thick steel plate according to claim 17, comprising seeds or two or more kinds.