JP2018069278A - Production method of thick-walled high strength hot-rolled steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a thick-walled high strength hot-rolled steel plate which allows for suppression of loose winding of the steel plate by reducing springback and dispenses with a device for applying a large load to the steel plate.SOLUTION: The production method of a thick-walled high strength hot-rolled steel plate is provided that comprises: a hot-rolling step of applying hot-rolling to a steel raw material; a cooling step of cooling a thick-walled high strength hot-rolled steel plate obtained by the hot-rolling step; and a winding step of winding the hot-rolled steel plate cooled by the cooling step into a coil shape, wherein the finish rolling completion temperature in the hot-rolling step is 790°C or more, and the hot-rolled steel plate after subjected to the finish rolling is cooled by the cooling step, and is wound at 400°C or more in the winding step within 5 sec. after the temperature of the hot-rolled steel plate becomes 600°C or less.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a thick, high-strength hot-rolled steel sheet.

  In recent years, demand for large-diameter, thick-walled, high-strength pipe materials having an API standard of X60 to 100 is increasing mainly for the purpose of improving the transportation efficiency of resources such as crude oil and natural gas in pipelines. In order to efficiently transport these resources, high internal pressure is applied to the inside of the pipe, and characteristics such as high toughness and high strength are also considered for pipe materials in consideration of crustal deformation due to use in cold regions and earthquakes. It has become very important. Pipes used in these pipelines have a large diameter of 10 to 30 mm and an outer diameter of 20 inches or more. Conventionally, as a high-strength pipe, the short side of a thick steel plate that is vertically long in the longitudinal direction is used. After the side is formed into a circular shape, an electric resistance welded tube is often used in which the butt portion is welded in the longitudinal direction to form a pipe. Usually, a thick steel plate is manufactured in a substantially rectangular shape by multi-pass rolling in a thick plate mill having one hot slab or two rolling mills, and its product length is at most 30 m.

  In contrast, in recent years, a thick steel plate is rolled in a hot rolling (hot strip mill) line for thin plate rolling and wound into a coil shape to form a hot rolled steel plate coil (coiled hot rolled steel plate). A spiral steel pipe that is unrolled and cut into a predetermined length and then formed into an ERW pipe, and a hot-rolled steel sheet is formed in a spiral shape in the longitudinal direction, and at the same time, the butt portion at the end of the plate width is welded to the pipe. Demand is increasing. The hot-rolled steel sheet coil can be manufactured up to 45 tons. For example, if the thickness is 20 mm and the plate width is 1900 mm, the length of the hot-rolled steel sheet is 151 m, and the pipe length when formed into a 28-inch diameter spiral steel pipe Is about 128 m. In this way, by manufacturing the base material before pipe forming with a hot strip mill and making it a hot-rolled steel sheet coil, it is possible to manufacture a continuously long pipe as compared with the case of manufacturing from a thick steel sheet manufactured with a thick plate mill. Can be expected to improve productivity.

  By the way, a hot-rolled steel sheet (sometimes referred to as a hot strip or simply a strip) after passing through a finish rolling mill of a hot strip mill in a hot rolling line is usually wound by a winding device (coiler). That is, the tip of the hot-rolled steel sheet is moved downward by a pinch roll and guided between the apron and the throat guide, and further, a plurality of wrapper rolls, arc-shaped strip guides and mandrels arranged around the mandrel. It is sequentially bent according to the three-point support state and wound around a rotating mandrel. Next, the winding proceeds, and finally the hot rolled steel sheet is wound into a coil shape to form a hot rolled steel sheet coil. When the winding is completed, the mandrel is reduced in diameter so that the coil can be pulled out.

  And normally, at the end of winding of the hot-rolled coil, the rotation of the mandrel is stopped so that the tail end portion of the hot-rolled steel sheet is located below the coil. Next, during winding, the coil car waiting under the coil is raised and the cradle roll attached to the coil car is brought into contact with the tail end of the coil so that the looseness of the tail end is suppressed by the weight of the coil. Then, the diameter of the mandrel is reduced, the wrapper roll is opened, and the coil car is lowered while mounted on the cradle roll of the coil car, and the coil is extracted from the coiler. And the hot-rolled steel sheet coil extracted from the coiler is conveyed to a banding device installed adjacent to the coiler to prevent winding looseness during conveyance, and one or a plurality of thin steel sheets The band of the belt is annularly attached to the outermost peripheral portion thereof and bound.

  However, in a thick and high strength hot-rolled steel sheet, since the sheet thickness is large, the cross-sectional area of the steel sheet is large and the strength is very high, so it is very difficult to apply tension to wind tightly during winding. It becomes difficult. In general, high strength materials have a very large spring back after bending, and after coil winding is completed, the cradle roll waiting under the coil is raised and pressed against the coil, and then the coil is extracted. When the wrapper roll is retracted (opened) and the mandrel, which was the center of coil rotation, is reduced in diameter, the elastic strain fixed by the coil mandrel and the wrapper roll is released, causing the spring back to the coil and loosening the winding. Becomes prominent.

  If the coil is loose enough, the coil will not lift up and cannot be pulled out (pulled out) from the coiler. There are various problems such as a problem that the band is cut during transportation after the band is applied. Even if the coil is pulled out of the coiler without any problems, transported by a coil car and cooled to room temperature, the tail end is springbacked at the moment of cutting the binding band when the coil is discharged in the next process, for example, a pipe forming line. The phenomenon of repelling occurred, and there were problems such as flaws in the steel sheet and, in severe cases, damage to the uncoil (dispensing) equipment, and it was necessary to establish some countermeasure technology against springback.

  To solve the above problem, for example, a method of increasing the winding of the outer periphery from the winding temperature of the inner periphery and reducing the springback using the difference in heat shrinkage (Patent Document 1), cooling the runout table before winding and winding Proposal of a method to cool the outer peripheral side only during the removal and put a pre-strain (Patent Document 2) or a method to put a restraining roll between the pinch roll and the coiler and put the pre-strain by bending back (Patent Document 3) Has been.

JP 2002-263734 A JP 2010-162595 A JP 2010-269321 A

  However, in the method described in Patent Document 1, since a temperature difference is provided between the inner periphery and the outer periphery within a range in which quality can be ensured, it is difficult to provide a large temperature difference. For this reason, the amount of reduction in springback is small, and it is difficult to obtain an effect that suppresses loosening of winding.

  In the method described in Patent Document 2, a large thermal stress must be caused in order to strain a thick high-strength steel sheet, and bending and unbending are necessary, and a large temperature difference is required between the upper and lower surfaces. There is. Since the temperature difference is set in a range that does not affect the material as in Patent Document 1, it is difficult to set a large temperature difference. For this reason, the amount of reduction in springback is small, and it is difficult to obtain an effect that suppresses loosening of winding.

  In the method described in Patent Document 3, in order to strain a thick high-strength steel plate, a large bending / bending is necessary, and a device capable of applying a large load needs to be installed. Moreover, since this apparatus must be installed in the way of a steel plate, there exists a problem that a material passing is very difficult.

  The present invention has been made in view of such circumstances, and provides a method for producing a thick-walled high-strength hot-rolled steel sheet that can suppress spring looseness by suppressing spring back and does not require a device that applies a large load. With the goal.

  As a result of studies conducted by the present inventors to solve the above problems, the relationship between finish rolling, cooling and winding is controlled. Specifically, the finish rolling end temperature is set to 790 ° C or higher, and the steel plate temperature is set to 600 ° C or lower. It has been found that by winding in a coil shape at 400 ° C. or more within 5 seconds after becoming, a spring back can be reduced and winding looseness can be suppressed and a device for applying a large load is unnecessary, and the present invention is Completed.

  The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] A hot rolling process in which hot rolling is performed on a steel material, a cooling process for cooling the thick high-strength hot-rolled steel sheet obtained in the hot rolling process, and the hot rolling cooled in the cooling process A winding step of winding the steel plate into a coil shape,
The finish rolling end temperature in the hot rolling step is 790 ° C. or higher,
The hot-rolled steel sheet after the finish rolling is cooled in the cooling step, and the hot-rolled steel sheet is wound at 400 ° C. or higher in the winding step within 5 seconds after the temperature of the hot-rolled steel plate becomes 600 ° C. or lower. A method for producing a thick-walled high-strength hot-rolled steel sheet.

[2] A hot rolling process in which hot rolling is performed on a steel material, a cooling process for cooling the thick high-strength hot-rolled steel sheet obtained in the hot rolling process, and the hot rolling cooled in the cooling process A winding step of winding the steel plate into a coil shape,
The finish rolling end temperature in the hot rolling step is 790 ° C. or higher,
The hot-rolled steel sheet after the finish rolling is cooled in the cooling step, and the hot-rolled steel sheet is wound at the bainite transformation end temperature or more in the winding step within 5 seconds after the temperature of the hot-rolled steel plate becomes equal to or lower than the bainite transformation start temperature. A method for producing a thick-walled high-strength hot-rolled steel sheet, characterized in that it is taken.

  [3] In the winding process described in [1] or [2], the thick high-strength hot-rolled steel sheet at the time of winding has a bainite phase ratio of 80% or less by volume%. A method for producing thick, high-strength hot-rolled steel sheets.

  [4] The method for producing a thick high-strength hot-rolled steel sheet according to any one of [1] to [3], wherein the thick-walled high-strength hot-rolled steel sheet has a thickness of 10 mm or more. .

[5] In the cooling step, water is directed to both sides of the thick-walled high-strength hot-rolled steel sheet from 10 seconds before winding to 2 m ³ / (m ² · min per side. ) The method for producing a thick, high-strength hot-rolled steel sheet according to any one of [1] to [4], wherein the injection is performed at the above flow rate.

  According to the method for producing a thick, high-tensile hot-rolled steel sheet of the present invention, the finish rolling finish temperature is set to 790 ° C. or higher, and the steel sheet is wound in a coil shape at 400 ° C. or higher within 5 seconds after the steel plate temperature reaches 600 ° C. or lower. As a result, the spring back can be reduced and the loosening of the winding can be suppressed. Further, a device for applying a large load is not necessary.

It is the schematic which shows the manufacturing equipment of a thick-walled high intensity | strength hot-rolled steel plate. It is the schematic which shows a winding apparatus.

  The method for producing a thick-walled high-strength hot-rolled steel sheet according to the present invention includes a hot-rolling process for hot-rolling a steel material, a cooling process for cooling the thick-walled high-strength hot-rolled steel sheet obtained in the hot-rolling process, A hot-rolled steel sheet cooled in the cooling process is wound into a coil, and the finish rolling end temperature in the hot rolling process is 790 ° C or higher, and the hot-rolled steel sheet after finish rolling is finished It cools by a cooling process and winds up at 400 degreeC or more by a winding process within 5 second after the temperature of a hot-rolled steel plate becomes 600 degrees C or less. A method for producing such a thick and high-strength hot-rolled steel sheet according to the present invention will be described in detail below with reference to FIGS. The temperature is the surface temperature of a steel material or a steel plate unless otherwise specified. The thermometer that measures the surface temperature is a non-contact type that measures the temperature of a certain point in the width direction of the steel sheet, that measures a plurality of points in the width direction of the steel sheet, What measures the temperature of a full width is preferably used, for example, it can measure with a radiation thermometer etc. The high strength is, for example, X60 or more (tensile strength: 520 MPa or more) in the API standard, and the thick wall is 10 mm or more in plate thickness.

  FIG. 1 is a schematic view showing a manufacturing facility for thick-walled high-strength hot-rolled steel sheet, which is an example of manufacturing equipment to which the method for manufacturing a thick-walled high-strength hot-rolled steel sheet according to the present invention can be applied. FIG. It is the schematic which shows the winding apparatus which equipment has. As shown in FIGS. 1 and 2, a thick-walled high-strength hot-rolled steel sheet manufacturing facility (hereinafter also simply referred to as “manufacturing facility”) 1 has a final stand 2 of a finish rolling mill and hot-rolls a steel material. A cooling machine 4 that cools a rolling mill (hot strip mill; not shown), and a thick, high-strength hot-rolled steel sheet (hereinafter also simply referred to as “hot-rolled steel sheet”) 3 that has passed through the final stand 2 of the finishing mill. And a coiler (winding device) 5 for winding the thick high strength hot rolled steel sheet 3 that has passed through the cooling device 4 into a coil shape. A hot rolling process is performed by the rolling mill, a cooling process is performed by the cooling device 4, and a winding process is performed by the coiler 5.

  The rolling mill hot-rolls the steel material to the product sheet thickness, and the hot-rolled hot-rolled steel sheet 3 is discharged from the final stand 2 of the finish rolling mill.

  In the present invention, the hot rolled steel sheet 3 discharged from the final stand 2 of the finish rolling mill is a thick and high strength steel sheet. Here, in the coil of thick and high strength hot rolled steel sheet with high thickness and high strength, the springback cannot be suppressed by the bending moment due to its own weight, and the winding is loosened especially at the outer periphery of the coil, and the peripheral equipment is damaged. Sometimes. Even if the coil cools down to room temperature without any problems, the phenomenon that the tail end part of the back-end will spring off at the moment of cutting the binding band in the next process, for example, the pipe forming line. In some cases, the uncoil (dispensing) equipment will be damaged. However, in the production method of the present invention, as will be described in detail later, the coil is wound in a coil shape at 400 ° C. or more within 5 seconds after the temperature of the thick high-strength hot-rolled steel sheet 3 becomes 600 ° C. or less. The back can be reduced, and damage to peripheral equipment and wrinkles due to loose winding can be prevented.

  The hot-rolled steel sheet 3 discharged from the final stand 2 has, for example, a thickness of 10 mm or more and has an API standard of X60 or more. In a steel plate having a plate thickness of less than 10 mm, winding looseness due to springback is unlikely to occur. Therefore, it is preferable to apply the present invention to a hot-rolled steel plate having a plate thickness of 10 mm or more. Similarly, since it is difficult for loosening to occur in a hot-rolled steel sheet not having high strength, it is preferable to apply the present invention to a hot-rolled steel sheet having high strength, specifically, an API standard of X60 or higher. The hot-rolled steel sheet 3 preferably has a thickness of 30 mm or less, and the strength is preferably X100 or less in accordance with API standards. However, the effect of the present invention is expected when a problem due to the similar springback occurs regardless of the plate thickness and strength.

  The material (component composition) of the hot-rolled steel sheet 3 is not particularly limited as long as it can be the above-described thick and high-strength steel sheet. For example, C: 0.01% to 0.20%, Si, : 0.10% to 0.55%, Mn: 1.20% to 2.00%, P: 0.030% or less, S: 0.01% or less, Al: 0.08% or less, N : 100 ppm or less, Ti: 0.005% or more and 0.05% or less, Nb: 0.03% or more and 0.10% or less, Mo: 0.05% or less, and the balance is Fe and inevitable impurities.

  The finish rolling finish temperature of hot rolling is 790 ° C. or higher. Thus, when the finish rolling finish temperature is 790 ° C. or higher, the metal structure of the steel sheet discharged from the final stand 2 of the finish rolling mill is austenite. In addition, since recrystallization after finish rolling is faster at higher temperatures, the finish rolling end temperature is more preferably 800 ° C. or higher, and is preferably 980 ° C. or lower from the viewpoint of cooling ability in the cooling step. Further, in order to obtain a finish rolling end temperature of 790 ° C. or higher, the finish rolling start temperature is preferably 810 ° C. or higher.

  The cooling device 4 cools the thick high-strength hot-rolled steel sheet 3 discharged from the final hot rolling stand 2. For example, as shown in FIG. 1, the cooling device 4 is on a runout table, has a plurality of coolers 6, and faces the upper and lower surfaces of the thick high-strength hot-rolled steel sheet 3 that passes between the coolers 6. Water (cooling water) is sprayed from.

The cooler 6 arranges nozzles (for example, a circular pipe nozzle, a slit nozzle, and a spray nozzle in which a plurality of holes are arranged in a header pipe) on the upper side and the lower side of the thick high-strength hot-rolled steel sheet 3 to spray water. , A cooling machine that performs water cooling such as water column cooling, spray cooling, and mist cooling. The jet of water onto the thick high-strength hot-rolled steel sheet 3 may be strong cooling in which water is sprayed on both sides of the steel sheet at a flow rate of ² m ³ / (m ² · min) or more per side.

  The cooling device 4 may perform one type of cooling method or may combine a plurality of types of cooling methods. For example, the cooling device 4 included in the cooling device 4 may be plural or single, and in the case of plural cooling devices, the cooling methods of the respective cooling devices 6 may be the same or different. When a plurality of types of cooling methods are used in combination, cooling is preferably performed by dividing the zone. In any cooling method, the flow rate of water sprayed onto the hot-rolled steel sheet 3 can be adjusted by the opening degree of the valve by using, for example, a cooler provided with a valve upstream of the nozzle as the cooler 6. In order to wind the thick high-strength hot-rolled steel sheet 3 to 600 ° C. or less in the winding process within 5 seconds, it is preferable to cool it with the cooler 6 at a position close to the coiler 5.

  The cooling water sprayed to the hot-rolled steel sheet 3 is, for example, installed a thermometer between one of the final stand 2 of the finish rolling mill, the cooling device 4, the pinch roll 11 or in the cooling device 4, It is preferable to compare the temperature of the thick-walled high-strength hot-rolled steel sheet 3 predicted by the difference calculation with the temperature at each measurement position and sequentially change the flow rate or each cooling zone by feedback control. Further, the coefficient for adjusting the flow rate of the cooling water may be changed by learning and reflected in the cooling pattern of the next material.

  The coiler 5 is a device for winding the thick high strength hot rolled steel sheet 3 discharged from the cooling device 4 into a coil shape, and is composed of a set of rolls for pressing the thick high strength hot rolled steel sheet 3 from above and below. 3, a pinch roll 11 that regulates the conveyance direction 3, an apron 12 and a throat guide 13 that guide the thick high-strength hot-rolled steel sheet 3 whose conveyance direction is regulated by the pinch roll 11, and the thick-thick high-strength hot-rolled steel sheet 3 A mandrel 14, a plurality of wrapper rolls 15 and an arc-shaped strip guide 16 which are arranged around the mandrel 14 and sequentially bend the thick and high strength hot-rolled steel sheet 3 by supporting the thick and high-strength hot-rolled steel sheet 3 together with the mandrel 14. Have Moreover, in the coiler 5 shown in FIG. 1 and FIG. 2, the coil outer periphery cooling device provided in the circumferential direction outer side of the mandrel 14 which cools the thick high strength hot-rolled steel sheet 3 wound in a coil shape. 17.

  The transport direction of the thick high-strength hot-rolled steel sheet 3 that has passed through the cooling device 4 is regulated by the pinch roll 11 (in FIGS. 1 and 2, the transport direction of the hot-rolled steel sheet 3 is changed from the horizontal direction downward), Proceeding downward, guided between the apron 61 and the throat guide 62, and further bent sequentially by the three-point support state of the plurality of wrapper rolls 15, the arc-shaped strip guide 16 and the mandrel 14 arranged around the mandrel And wound around a rotating mandrel 14. The wound thick high-strength hot-rolled steel sheet 3 is cooled by water sprayed from the coil outer periphery cooling device 17 as necessary, and finally, the thick-walled high-strength hot-rolled steel sheet substantially only of bainite. Manufacturing.

  In the present invention, the coiler 5 is wound at a coiling temperature of 400 ° C. or more within 5 seconds after the temperature of the thick high-strength hot-rolled steel sheet 3 becomes 600 ° C. or less. “Winding” means to transform into a coil shape (that is, to wind around a mandrel). The winding temperature is measured with a radiation thermometer or the like at the location indicated by the arrow in FIG. 2 (winding location A) or at the entrance side or the exit side of the pinch roll 11.

  The thick-walled high-strength hot-rolled steel sheet 3 is subjected to a cooling step after the hot rolling step, that is, after being discharged from the final stand 2 of the finish rolling mill, and then taken up by the winding device 5. A process is performed. In the present invention, the thick high-strength hot-rolled steel sheet 3 is wound at a winding temperature of 400 ° C. or more within 5 seconds after the temperature of the thick-walled high-strength hot-rolled steel sheet 3 becomes 600 ° C. or less. That is, the thick high-strength hot rolled steel sheet 3 is positioned at the winding position A within 5 seconds after the temperature of the thick high strength hot rolled steel sheet 3 becomes 600 ° C. or lower.

  The temperature of the thick-walled high-strength hot-rolled steel sheet 3 may be the upper surface temperature or the lower surface temperature of the steel sheet, but is preferably the lower of the upper surface temperature and the lower surface temperature of the steel sheet. Temperature. The temperature of the thick high-strength hot-rolled steel sheet 3 can be measured by a thermometer provided in the middle or front and rear of the cooling device 4 and confirmed to be 600 ° C. or lower.

  Here, as described above, the metal structure of the thick high-strength hot-rolled steel sheet 3 discharged from the final stand 2 of the finish rolling mill is austenite. And when the temperature of this thick high-strength hot-rolled steel sheet 3 falls to 600 ° C. or less, that is, the bainite transformation start temperature or less, transformation from austenite to bainite starts. In the present invention, the thick-walled high-strength hot-rolled steel sheet 3 is wound at a winding temperature of 400 ° C. or more within 5 seconds after the temperature of the hot-rolled steel sheet 3 becomes 600 ° C. or less. The temperature of the high-strength hot-rolled steel sheet 3 is, for example, in the range of 600 ° C. or lower and 400 ° C. or higher (for example, in the range of the bainite transformation start temperature or lower and the bainite transformation end temperature or higher). This transformation is not completed, and the bainite phase ratio (bainite volume fraction) of the thick high-strength hot-rolled steel sheet 3 is, for example, 80% or less, preferably 60% or less in volume%. In other words, in the present invention, at the time of winding, austenite is present at 20% or more by volume. In the present invention,% of the metal structure represents volume%. Then, 20% or more of the austenite is added to the coil outer periphery cooling device 17 or the built-in cooling device (illustrated) built in the mandrel 14 as necessary after the thick high strength hot-rolled steel sheet 3 is wound in a coil shape. None) and the like, and the transformation to bainite is completed. That is, finally, the whole becomes bainite.

  Thus, in the present invention, 20% or more of austenite is transformed into bainite in a state of being wound in a coil shape. When it transforms from austenite to bainite, it expands, but when it transforms from austenite to bainite in a coiled state, it becomes difficult to expand, and strain (transformation strain) occurs. In this way, since the transformation state is generated, the coiled state (coiled state) becomes a stable state. Therefore, in the present invention, the spring back can be reduced. In addition, when the steel sheet is bent with the same curvature, austenite is more easily deformed than bainite (that is, less stress is generated that causes springback), so in the present invention, springback is reduced. Can do. In addition, after the transformation to bainite is completed, the temperature is lowered to, for example, room temperature, but the tightening occurs due to the difference between the inner side and the outer side in the thickness direction of the compressive stress generated by the thermal shrinkage that occurs at that time. Can be suppressed.

  On the other hand, in conventional thick and high strength hot rolled steel sheets such as Patent Documents 1 to 3, the steel sheet is cooled to 600 ° C. or lower and the winding temperature is not higher than 400 ° C. within 5 seconds. The effect of reducing the spring back due to transformation strain could not be exhibited remarkably because it was not wound in a state of 20% or more, and the spring back could not be suppressed as much as in the present invention.

  In the present invention, the transformation of the thick high strength hot-rolled steel sheet 3 into bainite may be started during winding (winding location A), before winding, or after winding.

The method of winding within 5 seconds after the temperature of the thick high-strength hot-rolled steel sheet 3 becomes 600 ° C. or lower (and thus making the transformation rate to bainite at the time of winding 80% or lower) is, for example, a cooling device Adjusting and controlling the combination of the cooling machine 6 and the amount of water, the position of the final stand 2 of the finishing mill 2, the cooling device 4 and the coiler 5, the transport speed of the thick-walled high-strength hot-rolled steel sheet 3, the finishing rolling temperature, etc. The method of doing is mentioned. Specifically, for example, keeping the high temperature in the first half of the run-out table in the small water-cooled or the like having to or water so as not to inject cooling water into the steel plate, to suppress the progress of the transformation to bainite, 2m ³ water in the second half / (M ² · min) The method of performing the strong cooling which injects with the flow rate above is mentioned. For example, water is injected at a flow rate of ² m ³ / (m ² · min) or more per one side toward each side of the thick-walled high-strength hot-rolled steel sheet 3 from 10 seconds before winding to winding. The method of doing is mentioned.

  Thus, as shown in the Example and comparative example which are mentioned later by winding at the coiling temperature: 400 degreeC or more within 5 second after the temperature of the thick high-strength hot-rolled steel sheet 3 becomes 600 degreeC or less. Springback can be significantly reduced.

  When winding the tip of the hot-rolled steel sheet 3 with the coiler 5, the peripheral speed of each roll such as the pinch roll 11, the mandrel 14, and the wrapper roll 15 depends on the thickness and hardness (strength) of the hot-rolled steel sheet 3. ) And the like, the value is controlled so as to have a lead rate of 10 to 30% with respect to the conveying speed of the hot-rolled steel sheet 3, that is, 10 to 30% faster. At this time, the read rate of the mandrel 14 and the wrapper roll 15 is set to be equal to or higher than the read rate of the pinch roll 11.

  Then, the lead rate is cleared at the time when the winding of the tip of the hot-rolled steel sheet is completed, and thereafter the conveyance speed of the hot-rolled steel sheet 3 and the substantial (meaning excluding the influence of heat shrinkage due to cooling and width shrinkage due to tension, etc.) The convergence is controlled in synchronization with Although the number of wrapper rolls 15 is four in the drawing, there are three, and the number of wrapper rolls 15 is not limited. As the coiler 5 of a hot-rolled steel sheet, there are generally 3 to 4 wrapper rolls 15.

  Here, in the coiler 5, when the leading end of the hot-rolled steel sheet 3 is loosened and is wound in a wavy manner, the subsequent winding cannot be continued, or even if it is fortunately wound, When the winding of the winding of the coil is rolled up and the outside of the winding is overlapped, the winding becomes thick, it becomes a size that protrudes from the housing of the coiler 5, becomes clogged and cannot be extracted, and once such a situation occurs, Damage to each part of the coiler 5 is great, and it takes a long time to recover. Therefore, it is necessary to wind up without causing winding looseness.

  After the winding has progressed and the tail end portion of the hot-rolled steel sheet 3 has passed the pinch roll 11, no tension acts on the hot-rolled steel sheet 3. At least one or more of the wrapper rolls 13 out of the wrapper rolls 15 that have been pressed is pressed against the outer periphery of the coil to suppress loosening of the coil tail end.

  In addition, when the tail end part of the hot-rolled steel sheet 3 comes to 20 m before the coiler 5, it is preferable to reduce the conveyance speed (winding speed) of the hot-rolled steel sheet toward the end of winding. And normally, at the end of coil winding, when the tail end portion of the hot-rolled steel sheet 3 is located on the lower side of the coil, the rotation of the mandrel 14 is stopped. Thereafter, a coil car (not shown) having two cradle rolls is raised, brought into contact with the cradle roll at the coil tail end, and the spring back is suppressed by the bending moment generated by the weight of the coil. Is reduced in diameter and mounted on a cradle roll of a coil car, and the coil is extracted from the coiler 5. Thereby, a coil-shaped thick high-strength hot-rolled steel sheet is obtained.

  Hereinafter, the present invention will be described by way of examples for further understanding of the present invention. However, the examples do not limit the present invention.

  The slabs (steel materials) used in the following examples and comparative examples are API standard X70 grades, are 250 mm thick, 1800 mm wide, and 8500 mm long, have a composition of C: 0.06% by mass, Si: 0.28% by mass, Mn: 1.60% by mass, P: 0.010%, S: 0.001% by mass, Al: 0.03% by mass, N: 30ppm, Ti: 0.01% by mass, Nb : 0.07% by mass, Mo: 0.02% by mass, the balance being Fe and inevitable impurities.

  Moreover, the apparatus used by the following Example and the comparative example is the manufacturing equipment 1 and coiler (winding apparatus) 5 which are shown in FIG.1 and FIG.2, the diameter of the wrapper roll 15 is (PHI) 440mm, and the outer diameter of the mandrel 14 is (PHI) 750mm The outer diameter of the coil is Φ1800 mm. Further, the mandrel 14 and the wrapper roll 15 have a tip end portion of the final stand 2 of the finish rolling mill so that winding is started at a predetermined lead rate with respect to the speed of the thick high-strength hot-rolled steel plate 3. I started accelerating when I exited. The lead rate of the mandrel 14 at the start of winding of the tip portion was 15%, and the lead rate of the wrapper roll 15 was 25%.

  Further, the tensile strength is obtained by preparing a JIS No. 5 tensile test piece in a direction parallel to the rolling direction from the obtained coiled steel plate and conducting a tensile test in accordance with the provisions of JIS Z 2241: 2011. Tensile strength.

Moreover, the bainite transformation start temperature was calculated | required by the following (1) formula. The bainite transformation end temperature is determined by the following equation (2).
Bainite transformation start temperature (° C.) = 756-270 × [C] −90 × [Mn] −37 × [Ni] −70 × [Cr] −83 × [Mo] (1)
(In the formula (1), [% M] (M = C, Mn, Ni, Cr, Mo) represents the content of each element in mass%.)
Bainitic transformation end temperature (° C.) = 517−300 × [C] −11 × [Si] −33 × [Mn] −22 × [Cr] −17 × [Ni] −11 × [Mo] (2 )
(In the formula (2), [% M] (M = C, Si, Mn, Ni, Cr, Mo) represents the content of each element in mass%.)

(Examples 1-11 and Comparative Examples 1-4)
The slab was finished to a plate thickness of 20 mm through a rough rolling process and a finish rolling process of a hot rolling line. After finish rolling, both surfaces of the steel plate were cooled by the cooling device 4 on the run-out table. The cooled hot-rolled steel sheet 3 was wound with a coiler 5 to obtain a thick-walled high-strength hot-rolled steel sheet. Table 1 shows the temperature at the end of finish rolling and the speed of the hot-rolled steel sheet 3 on the run-out table (run-out sheet passing speed). The resulting thick-walled high-strength hot-rolled steel sheet had a tensile strength in the range of 625 to 640 MPa and a bainite transformation start temperature of 594 ° C.

  The tail end portion of the thick high-strength hot-rolled steel sheet 3 started decelerating from a position 20 m before the coiler, and the rotation was stopped at a position where the tail end portion was below the coil. At this time, in Examples 1 to 10 and Comparative Examples 1 to 4, the coil outer periphery cooling device 17 performed water cooling from the outer periphery of the coil, and in Example 11, water cooling from the outer periphery was stopped. Then, after the coil winding is completed, the cradle roll that has been waiting under the coil is raised, pressed against the lower surface of the coil to fix the vertical position, and then secured with a band, the wrapper roll is retracted, and the mandrel is shrunk. Diameter was extracted.

  In order to evaluate the loosening of the obtained thick-walled high-strength hot-rolled steel sheet, the band that had been lashed was removed, and the presence or absence of the occurrence of loosening and the amount of the soot were investigated.

  Cooling method on the run-out table, time after finishing rolling to 600 ° C. or lower to winding (time from reaching the bainite transformation start temperature 594 ° C. until reaching the winding point A), winding Table 1 shows the coiling temperature, the volume ratio of austenite and the volume ratio of bainite at the winding point A, the presence or absence of winding looseness, and the amount of the soot.

“La” is the laminar cooling at 0.7 m ³ / (min · m ² ) of water per side (one side), “Sky” is air cooling, "Is spray cooling at 0.5 m ³ / (min · m ² ) of water per side," Mi "is mist cooling at 0.05 m ³ / (min · m ² ) of water per side, and" strong "is The cooling is strong cooling at a water amount of 2.5 m ³ / (min · m ² ) per side, and water is used except for air cooling. The runout table is divided into 10 zones 1 to 10 from the upstream side, and the cooling method for each zone is shown in Table 1. The division here does not divide into equal lengths, but means that a plurality of zones are divided into 10 groups, and the length of each cooling facility is different. In the runout table, the valve is opened and closed and the flow rate is controlled so that the time and winding temperature after the temperature reaches 600 ° C. or less are sequentially controlled by temperature control. The given cooling method is shown.

  The temperature of the hot-rolled steel sheet 3 was measured by measuring the temperature of the upper surface of the hot-rolled steel sheet 3 using a radiation thermometer.

  The time from the temperature lower than 600 ° C. to the winding is such that a plurality of radiation thermometers are provided between the final stand 1 of the finish rolling mill and the pinch roll 11, and the temperature of the upper surface of the hot rolled steel sheet 3 is measured with each thermometer. It was determined from the conveying speed of the hot-rolled steel sheet 3 and the distance to the winding location A.

  The volume fraction of austenite and the volume fraction of bainite at the winding start point A are measured by a magnetic transformation rate meter (a magnetic flux generator and a measuring device are arranged with a steel plate in between, and the magnetic flux generated from the magnetic flux generator penetrates the steel plate. The magnetic flux density was measured with the opposed measuring device, and was measured with a device that reversely calculated the transformation rate from the passed magnetic flux density.

Moreover, when winding looseness generate | occur | produced, it was set to x, and when it did not generate | occur | produce, it was set as (circle). The amount of the soot was judged by the number per 1 m ^2, the number of 20 or more was evaluated as x, the number of 5 or more and less than 20 was evaluated as ◯, and the case where no wrinkle was generated was evaluated as ◎.

  As shown in Table 1, in Examples 1 to 11 in which the time from the temperature of 600 ° C. or less to the start of winding is within 5 seconds, there was no loosening of the winding, and there was little fretting, and the spring back was reduced. Was confirmed. In particular, in Examples 2, 4, 5, 7, 9, and 11 in which the volume fraction of bainite was less than 70%, no loosening of the winding and no scratch occurred. On the other hand, in the comparative example, winding looseness occurred and there were many scratches.

DESCRIPTION OF SYMBOLS 1 Production equipment 2 Final stand 3 Thick high strength hot-rolled steel sheet 4 Cooling device 5 Coiler (winding device)
6 Cooling Machine 11 Pinch Roll 12 Apron 13 Throat Guide 14 Mandrel 15 Wrapper Roll 16 Strip Guide 17 Coil Perimeter Cooling Device A Winding Location

Claims (5)

A hot rolling process for hot rolling the steel material, a cooling process for cooling the thick high strength hot rolled steel sheet obtained in the hot rolling process, and the hot rolled steel sheet cooled in the cooling process is coiled A winding process of winding in a shape,
The finish rolling end temperature in the hot rolling step is 790 ° C. or higher,
The hot-rolled steel sheet after the finish rolling is cooled in the cooling step, and the hot-rolled steel sheet is wound at 400 ° C. or higher in the winding step within 5 seconds after the temperature of the hot-rolled steel plate becomes 600 ° C. or lower. A method for producing a thick-walled high-strength hot-rolled steel sheet. A hot rolling process for hot rolling the steel material, a cooling process for cooling the thick high strength hot rolled steel sheet obtained in the hot rolling process, and the hot rolled steel sheet cooled in the cooling process is coiled A winding process of winding in a shape,
The finish rolling end temperature in the hot rolling step is 790 ° C. or higher,
The hot-rolled steel sheet after the finish rolling is cooled in the cooling step, and the hot-rolled steel sheet is wound at the bainite transformation end temperature or more in the winding step within 5 seconds after the temperature of the hot-rolled steel plate becomes equal to or lower than the bainite transformation start temperature. A method for producing a thick-walled high-strength hot-rolled steel sheet, characterized in that it is taken.   The thick and high strength hot rolled steel sheet according to claim 1 or 2, wherein the thick and high strength hot rolled steel sheet at the time of winding has a bainite phase ratio of 80% or less by volume%. A method for producing rolled steel sheets.   The thickness of the thick high-strength hot-rolled steel sheet is 10 mm or more, The method for producing a thick-walled high-strength hot-rolled steel sheet according to any one of claims 1 to 3. In the cooling step, water is directed to both sides of the thick high-strength hot-rolled steel sheet at least ² m ³ / (m ² · min) per side from 10 seconds before winding to the winding time. The method for producing a thick, high-strength hot-rolled steel sheet according to any one of claims 1 to 4, wherein the injection is performed at a flow rate.

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