JP2014065077A - Method of manufacturing hot rolled steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a hot rolled steel plate whereby productivity of the hot rolled steel plate can be enhanced by fully suppressing collapse of a coil.SOLUTION: A method for manufacturing a hot rolled steel plate comprises the steps of hot-rolling a steel plate to be hot-rolled with use of a hot rolling mill, then cooling the hot-rolled steel plate on a hot run table and taking up the plate by a coiler thereby manufacturing the hot rolled steel plate of coil form. According to this method, after the hot rolled steel plate is so cooled during hot-rolling that the temperature of the plate immediately after delivery thereof from the hot rolling mill becomes constant along the longitudinal direction of the steel plate, the hot rolled steel plate is delivered onto the hot run table from the hot rolling mill at a constant velocity, and the hot rolled steel plate is so cooled on the hot run table that the cooling velocity and the cooling time become constant along the longitudinal direction of the steel plate during the period from the delivery of the plate from the hot rolling mill to taking up thereof by the coiler.

Description

  The present invention, after hot-rolling a hot-rolled steel sheet with a hot rolling mill, cooling it on a hot run table and winding it with a coiler to produce a coiled hot-rolled (hot-rolled) steel sheet, The present invention relates to a method for producing a hot-rolled steel sheet that suppresses the coiled hot-rolled steel sheet from being crushed by its own weight in a coiler.

  As shown in FIG. 1, in order to manufacture a hot-rolled steel sheet, a slab is heated to a predetermined temperature in a heating furnace 1, and the heated slab is rolled by a rough rolling machine 2 as a hot rolling mill to form a rough bar. Then, this rough bar is rolled by a continuous hot finish rolling mill 3 composed of a plurality of rolling stands, which is also a hot rolling mill, to obtain a hot rolled steel sheet 4 having a predetermined thickness. The hot-rolled steel sheet 4 is cooled by cooling water supplied from above and below by a cooling device 5 installed on a hot run table (HRT), and then wound by a coiler 6 while applying tension in the longitudinal direction, and coiled heat A rolled steel plate (hereinafter sometimes simply referred to as “coil”) 7 is used.

  When the hot-rolled steel sheet 4 is wound by the coiler 6, the tension applied to the hot-rolled steel sheet 4 in the longitudinal direction acts as a radial surface pressure in the coiled hot-rolled steel sheet (coil) 7 after winding. A frictional force is generated between the steel plate layers. Thereby, the slip of a steel plate is suppressed and the rigidity of the coil 7 is improved.

  By the way, in the coiler 6, a phenomenon called so-called “coil collapse” in which the coil 7 in the down-end state (the winding shaft hole faces sideways) is crushed by its own weight may occur. In this case, a process for rewinding the coil 7 is added, and the production efficiency is lowered.

  It has been found that the coil collapse in the coiler 6 occurs when the phase transformation is not completed on the HRT. That is, when the phase transformation is not completed on the HRT, the steel sheet after winding in the coiler 6 undergoes volume expansion due to the phase transformation. When this volume expansion is larger at the outer periphery of the coil than at the inner periphery of the coil, the surface pressure in the radial direction is reduced, and the frictional force between the steel sheet layers is also reduced. Thereby, a steel plate becomes easy to slide and the rigidity of the coil 7 falls. When the rigidity of the coil 7 is significantly reduced, coil collapse occurs, and this coil collapse is in particular C: 0.25% by mass or less (not including 0% by mass), Mn: 1.5% by mass to 2.7% by mass. And Si: It tends to occur easily in the case of a hot-rolled steel sheet to which 1.5% by mass or less (excluding 0% by mass) is added.

  For this reason, conventionally, coil collapse is prevented by extracting the coil 7 after cooling on the winding shaft in the coiler 6 for several tens of seconds. In this way, the coil is held and cooled in the coiler. This method has a disadvantage that the next hot-rolled steel sheet cannot be wound with a coiler while the coil is cooled, and the production efficiency is lowered. Therefore, as described in Patent Document 1, for example, a method of gradually cooling a coil taken out from a coiler to a room temperature on a table has been tried.

JP 2006-281306 A

  In the method described in Patent Document 1, a coil is placed on a pedestal in which a support surface having an angle of 45 to 55 degrees with respect to a horizontal plane is arranged in a V shape. For example, the coil can be prevented from collapsing by receiving the downward force due to the coil's own weight and the lateral force due to the reaction force against its own weight on the V-shaped support surface. The problem of coil extraction failure that makes it difficult or impossible for the coil to be extracted from the winder winding shaft due to the occurrence of the coil cannot be solved.

  For this reason, when the present inventor further researched on the coil collapse, the coil collapse in the coiler is related to the fact that the phase transformation rate in the longitudinal direction of the hot-rolled steel sheet is not only completed on the HRT. Therefore, it has been found that if the change in the phase transformation rate in the longitudinal direction of the hot-rolled steel sheet is suppressed, coil collapse can be suppressed even if the phase transformation is not completed on the HRT.

  In view of the above-mentioned knowledge of the present inventor, the present invention suppresses the change in the phase transformation rate in the longitudinal direction of the hot-rolled steel sheet by controlling the finish rolling speed, cooling control on the HRT, etc. It aims at providing the manufacturing method of the hot-rolled steel plate which suppresses generation | occurrence | production of a coil collapse and improves productivity.

  FIG. 2 shows an example of a TTT diagram (Time-Temperature-Transformation Diagram) of medium carbon steel and medium carbon / high Mn steel. In FIG. 2, curve A is the relationship between ferrite transformation start temperature and time of medium carbon steel, curve B is the relationship between ferrite transformation start temperature and time of medium carbon / high Mn steel, and curve C is the bainite transformation of medium carbon steel. The relationship between the start temperature and time, curve D shows the relationship between the bainite transformation start temperature and time of medium carbon / high Mn steel. In general, the phase transformation includes ferrite transformation, bainite transformation, pearlite transformation, and martensitic transformation. In the steel types targeted by the present invention, the time from finish rolling to winding is 10 to 20 sec. (Seconds), the coiling temperature is 500 ° C. to 650 ° C., and the phase transformation that occurs under these conditions is mainly the ferrite transformation (γ → α transformation), so the present invention focuses on the ferrite transformation.

  In FIG. 2, for medium carbon steel, the start of ferrite transformation is 1 sec. Therefore, the phase transformation is almost completed from finish rolling to winding. On the other hand, for medium carbon / high Mn steel, the start of ferrite transformation is 10 sec. If the temperature is not the temperature at which the phase transformation starts the earliest (ferrite nose temperature), the phase transformation proceeds further slowly, and the phase transformation does not progress so much from finish rolling to winding. That is, the amount of cooling temperature before winding (how many times it has been cooled) and the cooling time, and thus the cooling rate, greatly affect the progress of the phase transformation.

  FIG. 3 shows an example of a TTT diagram of a 590 MPa class high strength steel as a low carbon steel. As shown in the figure, in the low carbon steel, when the temperature is cooled from about 820 ° C. to about 640 ° C., 4-5 sec. When a ferrite nose with a phase transformation rate of 30% is reached and further cooled to about 600 ° C., 60 sec. A ferrite nose with a phase transformation rate of 70% is reached. Here, t is the elapsed time after finishing rolling (sec.), ΔT is the cooling temperature after finishing rolling, and Vc is the cooling rate after finishing rolling (ΔT / t). That is, the phase transformation rate is determined by the cooling time and the cooling temperature (in combination with the cooling rate).

  By the way, in order to suppress the coil crushing in the coiler, the volume expansion amount due to the phase transformation after winding may be made similar between the coil outer peripheral portion and the coil inner peripheral portion. Further, the phase transformation rate of the portion that becomes the outer peripheral portion of the coil may be set to the same level as the phase transformation rate of the portion that becomes the inner peripheral portion of the coil. This is because the phase transformation rate of the portion that becomes the outer peripheral portion of the coil before winding is approximately the same as the phase transformation rate of the portion that becomes the inner peripheral portion of the coil, and the volume expansion after winding is set to the outer peripheral side and inner peripheral side of the coil. The volume expansion amount is used for increasing the circumferential length on the outer peripheral side of the coil than on the inner peripheral side, so that the increase in thickness is less than that on the inner peripheral side and the coil is loosened. This is because it is possible to suppress the coil collapse effectively.

The first aspect of the method for producing a hot-rolled steel sheet according to the present invention, which solves the above problems based on such knowledge,
After hot rolling the hot rolled steel sheet with a hot rolling machine, cooling it on a hot run table and winding it with a coiler to produce a coiled hot rolled steel sheet,
The hot-rolled steel sheet is cooled during hot rolling so that the temperature of the hot-rolled steel sheet immediately after being sent out from the hot rolling mill is constant along the longitudinal direction of the hot-rolled steel sheet. From a hot rolling mill, it is sent out on a hot run table at a constant speed.
The hot-rolled steel sheet is placed on the hot run table so that the cooling rate and cooling time from the delivery from the hot rolling mill to the winding by the coiler are constant along the longitudinal direction of the hot-rolled steel sheet. It is characterized by cooling with.

Moreover, the second aspect of the method for producing a hot-rolled steel sheet of the present invention is as follows:
After hot rolling the hot rolled steel sheet with a hot rolling machine, cooling it on a hot run table and winding it with a coiler to produce a coiled hot rolled steel sheet,
Sending out the hot rolled steel sheet while accelerating from a hot rolling mill onto a hot run table,
The hot-rolled steel sheet has a cooling temperature amount per unit length that increases along the longitudinal direction of the hot-rolled steel sheet in proportion to the increase in the feed rate, and the cooling rate over the entire length of the hot-rolled steel sheet. And is cooled on a hot run table so that the phase transformation is completed over the entire length of the hot-rolled steel sheet at the time of winding or just before the end.

Furthermore, the 3rd aspect of the manufacturing method of the hot rolled steel plate of this invention is as follows.
After hot rolling the hot rolled steel sheet with a hot rolling machine, cooling it on a hot run table and winding it with a coiler to produce a coiled hot rolled steel sheet,
Sending out the hot rolled steel sheet while accelerating from a hot rolling mill onto a hot run table,
The intermediate temperature target on the hot run table and / or the coiler so that the phase transformation rate of the hot-rolled steel sheet is 10% or less at the time of winding with the coiler with respect to the minimum speed and the maximum speed of the feeding speed, respectively. The hot rolling steel sheet is cooled on a hot run table in accordance with the feed speed based on the temperature target.

  In the first aspect of the method for producing a hot-rolled steel sheet of the present invention (a method of constant rolling speed), the temperature of the hot-rolled steel sheet immediately after the hot-rolled steel sheet is fed from the hot rolling mill is After cooling during hot rolling so as to be constant along the longitudinal direction of the hot-rolled steel sheet, the hot-rolled steel sheet is sent out on the hot run table at a constant speed, and the hot-rolled steel sheet is removed from the hot rolling mill. Since the cooling is performed on the hot run table so that the cooling rate and the cooling time from the feeding of the coiler to the coiler are constant along the longitudinal direction of the hot-rolled steel sheet, when winding with the coiler, The tip side of the hot-rolled steel sheet that becomes the inner circumference of the coil undergoes a slight phase transformation until the end of the tail end of the hot-rolled steel sheet that becomes the outer circumference of the coil has been wound. Great difference between the outer periphery and coil inner periphery No.

  Therefore, according to this first aspect, after winding in the coiler, the phase transformation proceeds in the coil outer peripheral portion and the coil inner peripheral portion of the coiled hot-rolled steel sheet in the coiler. Volume expansion occurs. At that time, the outer peripheral side of the coil has a longer peripheral length than the inner peripheral side, and the volume expansion amount is used to increase the circumferential length, so that the increase in thickness is reduced. As a result, the increase in the relative thickness of the coil inner periphery with respect to the coil outer periphery suppresses the loosening of the coil due to the reduction of the radial surface pressure, thereby suppressing the occurrence of coil collapse in the coiler. Productivity can be improved.

  Here, after the hot-rolled steel sheet is cooled during hot rolling so that the temperature of the hot-rolled steel sheet immediately after being sent out from the hot rolling mill is constant along the longitudinal direction of the hot-rolled steel sheet In order to feed the hot rolling table at a constant speed from the hot rolling mill, for example, the base speed (rolling speed at the tip of the steel plate) in rolling at a plurality of rolling stands constituting the finishing rolling mill of the hot rolling mill While the rolling acceleration rate is set to zero and the feed rate is constant, the cooling of the hot-rolled steel sheet between the rolling stands is controlled and the rolling of the hot rolling mill is being rolled. By controlling the cooling of the coarse bar, the temperature of the hot rolled steel sheet immediately after being fed from the hot rolling mill is reduced by reducing the amount of cooling temperature during rolling toward the tail end of the hot rolled steel sheet. heat It may be kept constant in the longitudinal direction of the rolled steel sheet. In this way, even if the amount of cooling water and the cooling position on the hot run table are fixed, a constant cooling rate and cooling over the entire length of the hot rolled steel sheet. As a result, the winding temperature can be secured.

  Further, in the second aspect of the method for producing a hot-rolled steel sheet of the present invention (a method of constant cooling rate), the hot-rolled steel sheet is fed from a hot rolling mill while being accelerated on a hot run table, and the heat Without using an intermediate temperature target, the hot-rolled steel sheet increases in the length of the hot-rolled steel sheet in the longitudinal direction of the hot-rolled steel sheet in proportion to the increase in the feed rate. Cooling on the hot run table so that the cooling rate is constant over the entire length of the steel sheet and the phase transformation is completed over the entire length of the hot-rolled steel sheet at the time of winding or immediately before the completion, for example, preferably the phase transformation rate is 90% or more. Therefore, as the feed rate increases toward the tail end side, the cooling temperature increases in proportion to the increase in the feed rate as the cooling time becomes shorter. Over the entire length The temperature of the tip of the hot-rolled steel sheet is the TTT diagram and the phase transformation start time is the shortest before reaching the coiler due to the relationship between the cooling speed and the length of the hot run table. By setting so as to decrease to the vicinity, at the time of winding with a coiler, the phase transformation is completed over the entire length of the hot-rolled steel plate or just before the end.

  Therefore, according to this second aspect, since the phase transformation is almost completed in the coil outer peripheral portion and the coil inner peripheral portion of the coiled hot-rolled steel sheet in the coiler at the time of winding with the coiler, Later, almost no volume expansion occurs. As a result, since the loosening of the coil due to the reduction of the surface pressure in the radial direction is suppressed, the occurrence of coil crushing in the coiler can be suppressed and the productivity can be improved.

  Here, the hot-rolled steel sheet is cooled over the entire length of the hot-rolled steel sheet by increasing the cooling temperature per unit length in the longitudinal direction of the hot-rolled steel sheet in proportion to the increase in feed rate. To cool on a hot run table so that the speed is constant and the phase transformation is completed or close to the end of the hot rolled steel sheet during winding, for example, the hot rolled steel sheet is laminar cooling water. The laminar (laminar) cooling equipment that cools directly without steam is installed in the hot run table, and the maximum cooling rate for the maximum cooling rate is obtained from the maximum cooling rate of the laminar cooling facility at the maximum rolling speed. The cooling rate of the hot-rolled steel sheet is set so that the passing speed of the laminar cooling equipment is obtained from the exit side speed of the finish rolling mill, that is, the feeding speed. The cooling temperature amount is determined so that the speed is reached, and the cooling water amount is determined from the cooling capacity of the laminar cooling equipment so that the determined cooling temperature amount can be achieved. The speed may be constant.

  Furthermore, in the third aspect (transformation rate lowering method) of the method for producing a hot rolled steel sheet according to the present invention, the hot rolled steel sheet is fed out while being accelerated from a hot rolling mill onto a hot run table, and the fed out. With respect to the minimum speed and the maximum speed, the intermediate temperature target on the hot run table and / or the coiler so that the phase transformation rate of the hot-rolled steel sheet is 10% or less at the time of winding with the coiler. Since the winding temperature target is set and the hot-rolled steel sheet is cooled on the hot run table according to the feed speed based on the temperature target, the phase transformation rate of the hot-rolled steel sheet at the time of winding with a coiler Becomes 10% or less over the entire length, and the hot-rolled steel sheet undergoes phase transformation over the entire length after winding.

  Therefore, according to the third aspect, after coiling by the coiler, the phase transformation proceeds in the coil outer peripheral portion and the coil inner peripheral portion of the coiled hot-rolled steel sheet in the coiler, respectively, so that the volume expansion of substantially the same amount. In this case, since the outer peripheral side of the coil has a longer peripheral length than the inner peripheral side, the volume expansion amount is used to increase the circumferential length, so that the increase in thickness is reduced. As a result, the increase in the relative thickness of the coil inner periphery with respect to the coil outer periphery suppresses the loosening of the coil due to the reduction of the radial surface pressure, thereby suppressing the occurrence of coil collapse in the coiler. Productivity can be improved.

  Here, in order to make the phase transformation rate of the hot-rolled steel sheet 10% or less at the time of winding with a coiler, for example, an intermediate temperature target on a hot run table and / or a winding temperature target with a coiler. Is set as high as possible from the ferrite nose temperature based on the TTT diagram, and the start of the phase transformation may be delayed until the winding time.

  In any of the first to third aspects of the method for producing a hot-rolled steel sheet of the present invention, C: 0.25% by mass or less (excluding 0% by mass), Mn: 1.5% by mass to 2 It is effective when manufacturing a hot-rolled steel sheet to which 0.7% by mass and Si: 1.5% by mass or less (not including 0% by mass) are added.

It is a block diagram which shows the outline of a normal hot rolling line. It is a relationship diagram which illustrates the phase transformation of medium carbon steel and medium carbon and high Mn steel. It is a TTT diagram which illustrates the relationship between the elapsed time after finish rolling of low carbon steel, temperature, and a phase transformation rate (%). It is a block diagram which shows the hot run cooling equipment of the hot rolling line shown in FIG. 1 to which the Example of the manufacturing method of the hot rolled steel plate of this invention is applied. It is a relationship diagram which shows the phase transformation rate of the front-end | tip part (TOP) and tail end part (BOT) of a hot-rolled steel plate in three aspects of the manufacturing method of the hot-rolled steel plate of this invention, and those differences. It is a conceptual diagram which shows the aspect with a fixed rolling speed among the said three aspects, (a) is a state with a constant rolling speed, (b) is a state with a constant cooling rate, (c) is a state with a constant cooling temperature, respectively. It shows from the tip (TOP) to the tail (BOT) of the cold rolled steel sheet. It is a conceptual diagram which shows the aspect with constant cooling rate among the said three aspects, (a) is the state of acceleration of rolling speed, (b) is the state of constant cooling speed, (c) is the state of cooling temperature amount increase, respectively. It shows from the front end (TOP) to the tail end (BOT) of the hot rolled steel sheet.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 4 is a configuration diagram showing an outline of a normal hot rolling line to which one embodiment of each of the above three aspects of the method for producing a hot rolled steel sheet according to the present invention is applied. Also in the rolling line, the slab heated to a predetermined temperature in the heating furnace 1 is rolled by a roughing mill 2 to form a rough bar, and then this rough bar is predetermined by a continuous hot finish rolling mill 3 composed of a plurality of rolling stands. A hot-rolled (hot rolled) steel plate 4 having a thickness is used. And after cooling this hot-rolled steel plate 4 with the cooling water supplied from the upper direction and the downward direction with the cooling device 5 installed in the hot run table (HRT), it winds up with the coiler 6, providing a tension | tensile_strength, and coil-shaped hot-rolled steel plate (Coil) 7.

  When the hot-rolled steel sheet 4 is wound by the coiler 6, the tension applied to the hot-rolled steel sheet 4 in the longitudinal direction acts as a radial surface pressure in the coiled hot-rolled steel sheet (coil) 7 after winding. A frictional force is generated between the steel plate layers. Thereby, the slip of a steel plate is suppressed and the rigidity of the coil 7 is improved.

  By the way, the additive element of steel in which the occurrence of coil crushing has been confirmed is: C: 0.25% by mass or less (excluding 0% by mass), Mn: 1.5% by mass to 2.7% by mass, Si : 1.5% by mass or less (excluding 0% by mass), and in particular, Mn is added in a large amount of 1.5% by mass or more. These are the conditions in which the progress of phase transformation is slow compared to other steel types and coil collapse is likely to occur. For this reason, three aspects P1, P2, P3 of the manufacturing method of the hot-rolled steel sheet of this invention are applied to said steel type. As illustrated in FIGS. 2 and 3, the phase transformation temperature and the target processing temperature differ depending on the material, so it is preferable to use these three modes properly depending on the material.

  In order to sufficiently suppress coil collapse, the phase transformation rate of the outer peripheral portion of the coil 7 in the coiler 6 is set to be the same as the phase transformation rate of the inner peripheral portion by the phase transformation control before winding as described above. The phase transformation rate is, for example, a transformation rate meter using the change in permeability of a hot-rolled steel sheet previously disclosed by the applicant of the present application in Japanese Patent Laid-Open Nos. 08-0621181 and 09-049017. It is possible to confirm by online measurement using the transformation rate meter.

  FIG. 5 shows three modes P1 and P2 of the method for producing a hot-rolled steel sheet according to the present invention for making the phase transformation rate of the outer peripheral portion of the coil 7 in the coiler 6 the same as the phase transformation rate of the inner peripheral portion. , P3 is a relationship diagram showing the phase transformation rates of the front end portion (TOP) and the tail end portion (BOT) of the hot-rolled steel sheet and the difference between them, and Table 1 below shows the three modes P1, The contents of P2 and P3 are shown respectively.

  Here, TOP is an abbreviation for the tip of the hot-rolled steel sheet, and BOT is an abbreviation for the tail end of the hot-rolled steel sheet. In addition, the hot-rolled steel sheet tip here means a part from the most advanced part of the hot-rolled steel sheet to a length range of about 10 m, and the hot-rolled steel sheet tail end part is about 10 m from the most extreme end of the hot-rolled steel sheet. It means a part up to the length of.

  As shown in FIG. 5 and Table 1, the first aspect P1 of the present invention reduces the phase transformation rate difference between TOP and BOT, thereby reducing the phase transformation rate of the outer peripheral portion of the coil 7 in the coiler 6. Is the same as the phase transformation rate of the inner peripheral portion, and for this purpose, the finish rolling from the finishing mill 3 is performed at a constant speed by finish rolling at a constant speed.

  Drawing 6 is a key map showing one embodiment of the 1st mode P1, and it is in a state where rolling speed is constant in (a) about a tip part (TOP) to a tail end part (BOT) of a hot rolled steel plate. (B) shows a state where the cooling rate is constant, and (c) shows a state where the cooling temperature is constant. As is apparent from these figures, in this embodiment of the first aspect P1, the hot rolled steel sheet 4 immediately after being sent out from the continuous hot finishing mill 3 as a hot rolling mill. Is cooled during hot rolling in the continuous hot finish rolling mill 3 so that the temperature of the hot rolled steel sheet 4 becomes constant along the longitudinal direction of the hot rolled steel sheet 4 and then hot run (not shown) from the continuous hot finish rolling mill 3. The steel sheet 4 is fed on a table (HRT) at a constant speed V equal to the rolling speed, and the hot-rolled steel sheet 4 is cooled from the feed from the continuous hot finishing mill 3 to the winding by the coiler 6 and cooling. Cooling is performed on the HRT so that the time t is constant along the longitudinal direction of the hot-rolled steel sheet 4.

  Here, the hot-rolled steel sheet 4 is hot so that the temperature of the hot-rolled steel sheet 4 immediately after the hot-rolled steel sheet 4 is sent out from the continuous hot finish rolling mill 3 is constant along the longitudinal direction of the hot-rolled steel sheet 4. In order to send out at a constant speed from the continuous hot finish rolling mill 3 to the HRT after cooling during rolling, in this embodiment, a base in rolling in a plurality of rolling stands constituting the continuous hot finish rolling mill 3 is used. While increasing the speed (rolling speed at the tip of the steel sheet) and making the feed speed constant with the rolling acceleration rate being zero, by controlling the cooling of the hot-rolled steel sheet 4 between the rolling stands, The temperature of the hot-rolled steel sheet 4 immediately after being sent out from the continuous hot finishing mill 3 is reduced in the longitudinal direction of the hot-rolled steel sheet 4 by decreasing the cooling temperature amount during rolling as it goes to the tail end side of the hot-rolled steel sheet 4. To be constant Keep. Thereby, even if the amount of cooling water and the cooling position in the HRT are fixed, it is possible to ensure a constant cooling rate, cooling time, and coiling temperature over the entire length of the hot-rolled steel sheet 4.

  Therefore, according to the first aspect P1, the hot-rolled steel sheet 4 that becomes the outer peripheral part of the coil 7 is located at the front end side of the hot-rolled steel sheet 4 that becomes the inner peripheral part of the coil 7 during winding by the coiler 6. Although the phase transformation proceeds somewhat before winding the tail end of the coil, there is not much difference between the outer peripheral portion and the inner peripheral portion of the coil 7 in the phase transformation rate after winding. Later, in the coiler 6, the phase transformation proceeds in the coil outer peripheral portion and the coil inner peripheral portion of the coiled hot-rolled steel sheet 7, respectively, and a volume expansion with a little difference occurs. Since the circumferential length is longer than that of the inner circumferential side, the volume expansion amount is used to increase the circumferential length, so that the increase in thickness is reduced. As a result, the increase in the relative thickness of the coil inner periphery with respect to the coil outer periphery suppresses the loosening of the coil 7 due to the reduction of the radial surface pressure, thereby suppressing the occurrence of coil collapse in the coiler 6. And productivity can be improved.

  Moreover, as shown in FIG. 5 and Table 1, the second aspect P2 of the present invention increases the overall phase transformation rate of the hot-rolled steel sheet 4 by cooling at a constant cooling rate, thereby allowing the inside of the coiler 6 to The phase transformation rate of the outer peripheral portion of the coil 7 is set to be the same as the phase transformation rate of the inner peripheral portion. For this purpose, cooling equipment with high cooling capacity is utilized to lower the cooling temperature.

  FIG. 7 is a conceptual diagram showing an embodiment of the second aspect P2, in which the rolling speed acceleration state is shown in (a) from the front end (TOP) to the tail end (BOT) of the hot rolled steel sheet. (B) shows a state where the cooling rate is constant, and (c) shows a state where the cooling temperature is increased. As is apparent from these figures, in this embodiment of the second mode P2, the hot rolled steel sheet 4 is fed from the continuous hot finishing mill 3 as a hot rolling mill while being accelerated on the HRT, and the heat The hot-rolled steel sheet 4 is hot-rolled by increasing the cooling temperature per unit length along the longitudinal direction of the hot-rolled steel sheet 4 in proportion to the increase in feed rate without using the intermediate temperature target. On the HRT, the cooling rate is constant over the entire length of the steel plate 4 and the phase transformation is completed over the entire length of the hot-rolled steel plate 4 at the time of winding or immediately before the completion, that is, here the phase transformation rate is 90% or more. Cool with.

  Here, the hot-rolled steel sheet 4 has a cooling temperature amount per unit length that increases along the longitudinal direction of the hot-rolled steel sheet 4 in proportion to the increase in feed rate, and the total length of the hot-rolled steel sheet 4 increases. In order to cool on the HRT so that the cooling rate becomes constant over the entire length of the hot rolled steel sheet 4 during winding or just before the end of the phase transformation, that is, the phase transformation rate is 90% or more. In this embodiment, a laminar (laminar flow) cooling facility that directly cools the hot-rolled steel plate 4 with laminar cooling water without using water vapor and has a high cooling capacity is provided in the HRT. For each length, obtain the maximum value of the cooling rate by the maximum cooling amount from the maximum cooling amount of the laminar cooling facility at the maximum rolling speed, and set the cooling rate of the hot-rolled steel sheet 4 to be equal to or less than the maximum value, The exit side speed of the finishing mill 3, that is, the feed The passage speed of the laminar cooling equipment is obtained from the delivery speed, the cooling temperature amount is obtained so that the cooling speed is set at the passage speed, and cooling is performed from the cooling capacity of the laminar cooling equipment so that the obtained cooling temperature amount can be achieved. The process of determining the amount of water is performed for the entire length of the hot-rolled steel sheet 4.

  Therefore, according to the second aspect P2, the feed rate increases as it goes to the tail end side of the hot-rolled steel sheet 4, and therefore the cooling temperature is proportional to the increase in the feed rate as the cooling time is shortened. Since the amount increases, the cooling rate becomes constant over the entire length of the hot-rolled steel plate 4, and this cooling rate is related to the hot-rolled steel plate 4 before reaching the coiler 6 due to the relationship between the feed rate and the HRT length. By setting so that the temperature of the tip of the steel plate decreases to near the ferrite nose temperature with the shortest phase transformation start time in the TTT diagram, the phase transformation occurs over the entire length of the hot-rolled steel plate 4 during winding with the coiler 6. Thus, the phase transformation is almost completed in the coil outer peripheral portion and the coil inner peripheral portion of the coiled hot-rolled steel sheet 7 in the coiler 6 at the time of winding with the coiler 6. doing Because, most of the volume expansion after the take-up does not occur. As a result, the loosening of the coil 7 due to the reduction of the surface pressure in the radial direction is suppressed, so that the occurrence of coil collapse in the coiler 6 can be suppressed and the productivity can be improved.

  As shown in FIG. 5 and Table 1, the third aspect P3 of the present invention reduces the overall phase transformation rate of the hot-rolled steel sheet 4, thereby reducing the phase of the outer peripheral portion of the coil 7 in the coiler 6. The transformation rate is set to the same level as the phase transformation rate in the inner periphery, and for this purpose, the cooling temperature is increased.

  For this reason, in this embodiment of the third aspect P3, the hot rolled steel sheet 4 is fed from the continuous hot finish rolling mill 3 as a hot rolling mill while accelerating on the HRT, and the minimum speed and the maximum of the feeding speed are sent. The intermediate temperature target on the HRT and / or the coiling temperature target on the coiler 6 are set so that the phase transformation rate of the hot-rolled steel sheet 6 becomes 10% or less at the time of winding with the coiler 6 with respect to the speed. Since the hot-rolled steel sheet 4 is cooled on the HRT according to the feed speed based on the temperature target, the phase transformation rate of the hot-rolled steel sheet 4 reaches its full length at the time of winding with the coiler 6. Therefore, the hot rolled steel sheet 4 undergoes phase transformation over the entire length after winding.

  Here, in order for the phase transformation rate of the hot-rolled steel sheet 4 to be 10% or less at the time of winding with the coiler 6, in this embodiment, the intermediate temperature target on the HRT and / or the coiler 6 By setting the winding temperature target as high as possible above the ferrite nose temperature based on the TTT diagram, the start of the phase transformation is delayed until the winding time.

  Therefore, according to the third aspect, after winding in the coiler 6, the phase transformation proceeds in the coiler 6 at the coil outer peripheral portion and the coil inner peripheral portion of the coiled hot-rolled steel sheet 7, respectively. In this case, since the outer peripheral side of the coil has a longer peripheral length than the inner peripheral side, the volume expansion amount is used for increasing the circumferential length, so that the increase in thickness is reduced. As a result, the increase in the relative thickness of the coil inner periphery with respect to the coil outer periphery suppresses the loosening of the coil 7 due to the reduction of the radial surface pressure, thereby suppressing the occurrence of coil collapse in the coiler 6. And productivity can be improved.

  Next, the example based on this embodiment demonstrates the case where the hot-rolled steel plate 4 is cooled in the hot run cooling equipment shown in FIG. 4 of the hot rolled steel plate production line shown in FIG. In this hot run cooling facility, the finishing thermometer 8 is located in the vicinity of the exit of the finishing mill 3, and the intermediate thermometer 9 is located at a position 1/5 to 1/2 of the rolling mill side of the distance from the finishing mill 3 to the coiler 6. Among them, the HRT table length provided with the cooling device 5 is installed at a position 1/3 on the finishing mill 3 side, and the winding thermometer 10 is installed in the vicinity of the coiler 6, and the transformation rate for further confirmation. The total 11 is installed in the same place as the winding thermometer 10. As described above, the transformation rate meter 11 can use, for example, a change in magnetic permeability of the hot-rolled steel plate 4.

  By using this hot run cooling equipment and controlling the operating conditions of the cooling device 5 (temperature and amount of cooling water supplied to the hot-rolled steel sheet 4), the hot-rolling is performed in the production methods of Examples 1 to 3 below. A steel plate was produced. The manufacturing method of Example 1 implements said 1st aspect P1, and this Example 1 is C: 0.15 mass%, Mn: 2.0 mass%, and other elements. Is a high-Mn medium carbon steel grade with a thickness of 2.9 mm, which is a minute amount that does not affect the phase transformation behavior, and the applied equipment / application process / application operations are constant at the finish rolling speed and accelerated finish rolling. Rate 0.0 mpm / sec. The cooling control of the cooling device 5 on the HRT is also constant, and the operating / processing conditions are a finishing side temperature of 870 ° C. and a winding temperature of 600 ° C.

  Moreover, the manufacturing method of Example 2 implements said 2nd aspect P2, and this Example 2 differs from Example 1 only in operation conditions, and finish rolling acceleration rate 15 mpm / sec. It is said. And the manufacturing method of Example 3 implements the said 3rd aspect P3, and this Example 3 is C: 0.005 mass% or less, Mn: 2.0 mass%, and others. These elements are high-Mn low-carbon steel grades with a minute amount that does not affect the phase transformation behavior, and hot rolled steel sheets with a thickness of 3.2 mm. 0 mpm / sec. And the operation / processing conditions are a finishing delivery temperature of 890 ° C. and a winding temperature of 640 ° C. The coil inner diameter was 750 mm, and the phase transformation rate was measured by the transformation rate meter 11. As a result, in Example 1, the entire length of the hot-rolled steel plate 4 was within 60% ± 10%. It was within 100% -10% over the range, and in Example 3, it was within 0% + 10% over the entire length of the hot-rolled steel sheet 4.

  In the coil 7 manufactured by the conventional method, the water cooling time in the coiler 6 for suppressing the occurrence of coil crushing was 10 seconds, whereas in the coil 7 manufactured in each of the first to third embodiments, the coil 7 The water cooling time in the coiler 6 for suppressing the occurrence of crushing was 0 second. That is, no coil collapse occurred even without water cooling in the coiler 6.

  As is clear from the results described above, according to the method for manufacturing a hot-rolled steel sheet according to the above three types of aspects of the present invention, any coil loosening due to a reduction in radial surface pressure after winding is suppressed. Productivity can be improved by suppressing the occurrence of coil collapse in the coiler.

  The present invention has been described based on the illustrated examples. However, the present invention is not limited to the above-described examples, and can be appropriately changed within the scope of the claims. For example, in the first aspect P1 described above, In order to keep the temperature of the hot rolled steel sheet 4 immediately after being sent out from the continuous hot finish rolling mill 3 so as to be constant in the longitudinal direction of the hot rolled steel sheet 4, a rough rolling mill of the hot rolling mills. By controlling the cooling of the coarse bar during rolling at 2, the amount of cooling temperature during rolling may be reduced toward the tail end side of the hot rolled steel plate 4.

  Moreover, in the said 2nd aspect P2, if the cooling capacity is sufficient with the normal laminar cooling equipment in order to make the cooling rate of the hot-rolled steel sheet 4 on HRT constant, especially the laminar cooling equipment with a high cooling capacity is used. Ordinary laminar cooling equipment may be used instead.

  Thus, according to the method for producing a hot-rolled steel sheet of the present invention, after coiling with a coiler, coil loosening due to reduction of the radial surface pressure is suppressed, so that occurrence of coil crushing in the coiler is suppressed. Productivity.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Rough rolling mill 3 Finish rolling mill 4 Hot-rolled steel plate 5 Cooling device 6 Coiler 7 Coiled hot-rolled steel plate (coil)
8 Finishing thermometer 9 Intermediate thermometer 10 Winding thermometer 11 Phase transformation rate meter

Claims (6)

After hot rolling the hot rolled steel sheet with a hot rolling machine, cooling it on a hot run table and winding it with a coiler to produce a coiled hot rolled steel sheet,
The hot-rolled steel sheet is cooled during hot rolling so that the temperature of the hot-rolled steel sheet immediately after being sent out from the hot rolling mill is constant along the longitudinal direction of the hot-rolled steel sheet. From a hot rolling mill, it is sent out on a hot run table at a constant speed.
The hot-rolled steel sheet is placed on the hot run table so that the cooling rate and cooling time from the delivery from the hot rolling mill to the winding by the coiler are constant along the longitudinal direction of the hot-rolled steel sheet. The manufacturing method of the hot-rolled steel plate characterized by cooling with.   The cooling during the hot rolling so that the temperature of the hot rolled steel sheet immediately after the hot rolled steel sheet is sent out from the hot rolling mill is constant along the longitudinal direction of the hot rolled steel sheet, The method for producing a hot-rolled steel sheet according to claim 1, wherein cooling is performed between a plurality of rolling stands constituting the finish rolling mill. After hot rolling the hot rolled steel sheet with a hot rolling machine, cooling it on a hot run table and winding it with a coiler to produce a coiled hot rolled steel sheet,
Sending out the hot rolled steel sheet while accelerating from a hot rolling mill onto a hot run table,
The hot-rolled steel sheet has a cooling temperature amount per unit length that increases along the longitudinal direction of the hot-rolled steel sheet in proportion to the increase in the feed rate, and the cooling rate over the entire length of the hot-rolled steel sheet. A method for producing a hot-rolled steel sheet, characterized in that the cooling is performed on the hot run table so that the phase transformation is completed over the entire length of the hot-rolled steel sheet during winding or just before the end.   The method for producing a hot-rolled steel sheet according to claim 3, wherein a phase transformation rate of the hot-rolled steel sheet at the time of winding is 90% or more over its entire length. After hot rolling the hot rolled steel sheet with a hot rolling machine, cooling it on a hot run table and winding it with a coiler to produce a coiled hot rolled steel sheet,
Sending out the hot rolled steel sheet while accelerating from a hot rolling mill onto a hot run table,
The intermediate temperature target on the hot run table and / or the coiler so that the phase transformation rate of the hot-rolled steel sheet is 10% or less at the time of winding with the coiler with respect to the minimum speed and the maximum speed of the feeding speed, respectively. A method for producing a hot rolled steel sheet, comprising setting a coiling temperature target at a temperature and cooling the hot rolled steel sheet on a hot run table according to the feed rate based on the temperature target.   The hot-rolled steel sheet has C: 0.25% by mass or less (excluding 0% by mass), Mn: 1.5% by mass to 2.7% by mass, and Si: 1.5% by mass or less (0% by mass). The method for producing a hot-rolled steel sheet according to any one of claims 1 to 5, wherein a hot-rolled steel sheet according to any one of claims 1 to 5 is added.

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