JP5597989B2 - Bottom surface cooling device for hot-rolled steel strip - Google Patents

Description

  When cooling the hot-rolled steel strip that has been hot-rolled, the present invention uses a spray nozzle (nozzle in which cooling water spreads from the nozzle injection port) for cooling the lower surface side of the hot-rolled steel strip. The cooling water from the spray nozzle falls to the upper surface of the hot-rolled steel strip and prevents the end of the width of the hot-rolled steel strip from being overcooled. The present invention relates to a lower surface cooling device for a hot-rolled steel strip that can be cooled.

  In the production line for hot-rolled steel strip, the slab heated at high temperature is rolled into a steel strip of the desired size, and then the steel strip (hot-rolled steel strip) is cooled on the runout table from the standpoint of material adjustment. Is done. The purpose of cooling here is mainly to adjust the material such as the desired strength and elongation by controlling the precipitates and transformation structure of the steel strip (hereinafter, the steel strip simply means hot-rolled steel strip). Has been done. As the cooling medium for the cooling, water with low cost is often used. Here, if the temperature distribution in the width direction after cooling the steel strip is not uniform, mechanical test values such as strength and elongation change in the width direction of the steel strip, and a predetermined material can be obtained locally. Disappear.

  In hot-rolled steel strips, the upper surface is often cooled by laminar flow and the lower surface is often spray-cooled, but in general, the temperature is not uniform in the width direction of the steel strip, especially due to cooling by the laminar flow on the upper surface. Distribution is said to occur.

  That is, when cooling the upper surface of the steel strip by laminar flow, a plurality of nozzles are attached in the width direction to the header provided perpendicular to the traveling direction of the steel strip, and cooling water is jetted from each nozzle at the same time. Since the cooling water that has reached the upper surface of the steel strip forms a water flow in the width direction of the steel strip, the amount of water passing through increases toward the edge portion (width end portion) of the steel strip, resulting in more cooling. For this reason, the portion near the edge in the width direction has a higher cooling capacity than the center portion, and often has a temperature distribution in which both edge portions of the steel strip have a low temperature.

  On the other hand, non-uniform temperature distribution may occur in the width direction of the steel strip due to cooling by spraying the lower surface.

  That is, since the thickness of the steel strip through which the runout table is passed after finish rolling is as thin as about 2 to 4 mm and has low rigidity, the table rolls are densely arranged in order to pass the runout table stably. For example, in many run-out tables, table rolls having a diameter of about 250 to 300φ are arranged at a pitch of 300 to 400 mm to narrow the space between the table rolls. Therefore, when cooling the lower surface of a steel strip, there exists a problem that it is difficult to arrange | position a nozzle between table rolls. Therefore, in the cooling of the bottom surface of the hot-rolled steel strip at the run-out table, the spray nozzle (the nozzle through which the cooling water spreads from the nozzle injection port) can be installed in the width direction in order to install in a narrow space and increase the cooling area. In many cases, a plurality are arranged. In the cooling using this spray nozzle, the cooling water sprayed from the spray nozzle arranged at the position in the width direction (width direction end) where the steel strip does not pass above blown up several hundred mm to several m from the pass line. Although it falls later, some cooling water falls on the upper surface of the steel strip. This falling water also causes overcooling particularly at the end of the steel strip.

  Various proposals have been made so far to prevent overcooling of the width end of the hot-rolled steel strip.

  For example, Patent Document 1 describes a method of providing a hook for adjusting the amount of cooling water falling at the end of the steel strip width to be less than that of the central portion of the steel strip with respect to the upper surface nozzle. A plurality of this technique is disclosed in addition to this Patent Document 1, and a technique of providing a shielding plate so that the cooling water does not fall at the end of the steel strip width is also proposed as an application. In addition, there is an example in which the present technique is applied not only to the upper surface of the steel strip but also to the lower surface of the steel strip.

  In Patent Document 2, in laminar flow cooling on the upper surface of the steel strip, the function is divided into a header that selectively cools the steel strip width end portion and a header that selectively cools the steel strip width center portion, A technique for controlling the temperature distribution in the width direction of the steel strip by providing a flow distribution in the width direction of the steel strip by ON-OFF control of water injection from the header is disclosed. As a similar technique, there is a method of adjusting the flow rate of the cooling water in the width direction by sequentially changing the diameter of the nozzles attached in the width direction in the width direction. In addition, there is an example in which the present technique is applied not only to the upper surface of the steel strip but also to the lower surface of the steel strip.

JP 2005-238283 A JP-A-1-284419

  However, the methods described in Patent Documents 1 and 2 are effective means for the upper surface of the hot-rolled steel strip, but are not practically sufficient for cooling the lower surface of the hot-rolled steel strip. Absent.

  First, the method of adjusting the amount of cooling water at the end of the steel strip by means of scissors or shielding plates, as described in Patent Document 1, etc., is mainly intended for cooling the top side of the steel strip, and is a table roll. And nozzle arrangement when the interval between draining rolls is wide. Therefore, when the table roll interval is only about 50 to 100 mm as in the cooling on the lower surface side of the steel strip in the run-out table of the hot rolled steel strip, there is only a space for arranging the nozzles side by side in the width direction. There is virtually no space for installing the shielding plate as described.

  Moreover, the method of dividing the nozzle in the width direction as described in Patent Document 2 can only deal with a specific steel strip width. Since the width of the hot-rolled steel strip changes in units of several tens of mm, it is necessary to finely divide the header in the width direction in order to cope with this. In addition, the method of adjusting the flow rate distribution of the cooling water in the width direction can cope with a predetermined steel strip width, but when the width changes from about 800 mm to about 2400 mm like a hot-rolled steel strip. Cannot be handled with one type of cooling water flow rate distribution in the width direction.

  Therefore, Patent Documents 1 and 2 disclose that cooling water sprayed from a nozzle disposed at the end in the width direction where there is no steel strip above the nozzle, which is a problem that occurs when spray cooling is employed for cooling the lower surface of the steel strip. It can be said that nothing is described as a means for preventing the problem that the steel sheet is blown up from below and falls to the upper surface of the steel strip and the end of the steel strip width is overcooled.

  The present invention has been made in view of the circumstances as described above, and in a hot-rolled steel strip production line, when the hot-rolled steel strip is cooled at a place where a distance between transport table rolls is narrow like a run-out table. When a spray nozzle is used for cooling the lower surface side of the hot-rolled steel strip, the cooling water from the spray nozzle falls on the upper surface of the hot-rolled steel strip and the width end of the hot-rolled steel strip is supercooled. It is an object of the present invention to provide a lower surface cooling device for a hot-rolled steel strip that can accurately prevent this and cool the hot-rolled steel strip at a uniform temperature in the width direction.

  In order to solve the above problems, the present invention has the following features.

  [1] Heat in which a plurality of spray nozzles are arranged in the width direction of the steel strip as nozzles for injecting cooling water to the lower surface of the hot-rolled steel strip between the transport table rolls. In the lower surface cooling device of the steel strip, the angle from the vertical direction to the central portion of the steel strip width is 10 ° or less at the projection angle in the steel strip width direction as the spread of the cooling water from the nozzle injection port. A lower surface cooling device for a hot-rolled steel strip.

  [2] The spray nozzle has a central axis in a vertical direction, and the cooling water from the nozzle injection port spreads from the central axis more than the spread angle from the central axis toward the central portion of the steel strip width. The lower surface cooling device for a hot-rolled steel strip according to the above [1], wherein the spreading angle toward the end of the steel strip width is larger.

  [3] The spray nozzle has a symmetric spreading angle with respect to the central axis, and the central axis is inclined at an angle of 30 ° or less from the vertical direction toward the end of the steel strip width. The lower surface cooling device for a hot-rolled steel strip according to [1].

  [4] The lower surface cooling device for a hot-rolled steel strip according to any one of [1] to [3], wherein the spray nozzle is a flat spray nozzle.

  In the present invention, when the hot-rolled steel strip is cooled at a place where the distance between the conveying table rolls is narrow like a run-out table, when the spray nozzle is used for cooling the lower surface side of the hot-rolled steel strip, from the spray nozzle The cooling water is blown up and dropped onto the upper surface of the hot-rolled steel strip, and the width end portion of the hot-rolled steel strip is accurately prevented from being overcooled. As a result, the hot-rolled steel strip can be cooled at a uniform temperature in the width direction, and a high-strength steel strip can be produced without variations in mechanical properties.

It is the front view which looked at the general arrangement | positioning of the spray nozzle in the lower surface cooling device of a runout table from the steel strip advancing direction. It is the top view which looked at general arrangement | positioning of the spray nozzle in the lower surface cooling device of a runout table from upper direction. It is a side view with respect to the steel strip advancing direction of the general arrangement | positioning of the spray nozzle in the lower surface cooling device of a runout table. It is the front view which looked at arrangement | positioning of the spray nozzle in Embodiment 1 of this invention from the steel strip advancing direction. It is a figure which shows the spray nozzle in Embodiment 1 of this invention. In a spray nozzle, it is a graph which shows the relationship between the spreading angle (theta) from a perpendicular direction to the steel strip width center part, and the ratio of the cooling water which falls on a steel strip. It is the front view which looked at arrangement | positioning of the spray nozzle in Embodiment 2 of this invention from the steel strip advancing direction. It is a figure which shows the spray nozzle in Embodiment 2 of this invention. It is a figure which shows the spray nozzle in Embodiment 2 of this invention. It is a figure which shows the spray nozzle in Embodiment 2 of this invention. It is a figure which shows the hot-rolled steel strip manufacturing line in the Example of this invention.

  Embodiments of the present invention will be described with reference to the drawings.

  The present invention uses a spray nozzle to cool the lower surface side of a hot-rolled steel strip when the hot-rolled steel strip is cooled in a place where the distance between the conveying table rolls is narrow like a run-out table in a hot-rolled steel strip production line. In this case, the cooling water from the spray nozzle falls on the upper surface of the hot-rolled steel strip, preventing the width end of the hot-rolled steel strip from being overcooled, making the hot-rolled steel strip uniform in the width direction. It is intended to cool at a reasonable temperature.

  First, the general arrangement of spray nozzles in the bottom surface cooling device installed on the run-out table of the hot-rolled steel strip production line will be described. FIG. 1 is a front view seen from the steel strip traveling direction, FIG. 2 is a plan view seen from above, and FIG. 3 is a side view relative to the steel strip traveling direction.

  As shown in FIGS. 1 to 3, the lower surface cooling device installed in the runout table is composed of a header 1 and a flat spray nozzle 2 in which cooling water 3 spreads from the nozzle injection port and is sprayed in a fan shape. ing. Flat spray nozzles (hereinafter, spray nozzles) 2 are installed between the table rolls 4, and a plurality of flat spray nozzles are arranged at a predetermined pitch (for example, 100 mm) in the steel strip width direction. With the cooling water 3 sprayed from the spray nozzles 2 in the width direction, the steel strip is cooled not only immediately above the spray nozzles 2 but also between the adjacent spray nozzles 2. In the case of a run-out table of a hot-rolled steel strip, the table rolls 4 having a diameter of about 250 to 300φ are arranged at a pitch of 300 to 400 mm, so that the gap between the table rolls 4 is about 50 to 100 mm. Narrow, between the table rolls 4, the spray nozzles 2 can be installed only in one row in the longitudinal direction (steel strip traveling direction).

  And when the cooling water 3 is sprayed from the spray nozzle 2 in this state, as shown in FIG. 1, the cooling water 3 sprayed from directly below the steel strip 10 collides with the steel strip 10 and falls downward. Since the cooling water 3 sprayed from the outside of the steel strip 10 spreads and is sprayed, it falls on the upper surface of the steel strip 10. The falling water 9 falling on the upper surface of the steel strip 10 induces supercooling at the width end of the steel strip 10.

  Therefore, in the embodiment of the present invention, the spread angle of the cooling water 3 sprayed from the spray nozzle 2 is adjusted to reduce the spread angle of the cooling water 3 from the vertical direction to the steel strip width central side, and the vertical direction. The spread angle from the steel strip width end to the side is increased. Specifically, the projection angle in the steel strip width direction is set so that the spread angle from the vertical direction toward the steel strip width central portion is 10 ° or less.

[Embodiment 1]
In Embodiment 1 of the present invention, as shown in FIG. 4, with respect to the spray nozzle 2, the mounting direction of the spray nozzle 2 is set to the vertical direction, and the spread angle of the cooling water 3 to be injected is changed from the vertical direction to the steel strip width. The size is reduced toward the center and increased from the vertical direction toward the end of the steel strip. That is, as shown in FIG. 5, when the central axis 2a of the spray nozzle 2 is set in the vertical direction, the spread angle θ1 toward the central part of the steel strip width with respect to the central axis 2a is closer to the steel strip width end side. A spray nozzle having a larger spread angle θ2 is used.

  At that time, the spray nozzles 2 are arranged so that the cooling water 3 spreads symmetrically with respect to the central portion in the width direction of the cooling header 1, and the nozzle pitch P1 between the spray nozzles 2 in the central portion in the width direction is the other spray nozzles. It arrange | positions so that it may narrow with respect to the nozzle pitch P2 between 2 and cooling between the spray nozzles 2 is made.

  As for the spread angle from the vertical direction to the steel strip width center side at this time, as shown in FIG. Then, with respect to the direction of the central axis 2a of the spray nozzle 2), the cooling water sprayed from one spray nozzle 2 is defined as θ (here, θ = θ1) as the spread angle in the steel band width central direction. FIG. 6 shows the result of investigating what percentage falls on the steel strip 10 with respect to the total amount of water 3.

  From this, it can be seen that the cooling water 9 falling on the steel strip 10 decreases as the spread angle θ toward the central portion of the steel strip decreases. In practice, when cooling water is sprayed simultaneously on the upper surface and the lower surface of the steel strip 10, the cooling water 9 falling on the steel strip 10 is 10% of the cooling water 3 sprayed from one spray nozzle 2. If it is as follows, supercooling of the steel strip width end portion does not occur. This is because cooling is also performed from the upper surface of the steel strip 10, so that a liquid film is formed on the steel strip 10, thereby reducing the influence of the cooling water 9 falling on the end of the steel strip width. Therefore, the spread angle θ from the vertical direction to the central portion of the steel strip width may be set to 10 ° or less based on FIG.

  However, when the cooling water is not jetted onto the upper surface of the steel strip 10, the amount of the cooling water 2 that falls on the lower surface of the steel strip 10 needs to be almost zero. The spread angle θ is preferably 0 ° or less. Needless to say, it is more preferable that the spreading angle θ in the central direction of the steel strip width is negative and the cooling water 3 can always be sprayed outward (in the direction of the steel strip width end).

  The reason why the lower surface of the steel strip 10 is used as the spray nozzle 2 is to perform the cooling between the spray nozzles 2, so that one spray nozzle 2 has a certain length in the width direction (at least the nozzle pitch). It is necessary to cool it for the same degree. Therefore, the spread angle θ2 outward in the steel strip width direction (in the steel strip width end direction) needs to be increased to some extent. In practice, a spray nozzle having a structure in which cooling water collides wider than the nozzle pitch may be employed.

[Embodiment 2]
In Embodiment 2 of the present invention, as shown in FIG. 7, a spray nozzle having a symmetrical spread angle with respect to the center axis of the spray nozzle is used, and the center axis of the spray nozzle 2 is directed outward in the steel strip width direction. It is installed with an inclination in the direction of the steel strip width end. That is, as shown in FIG. 8, the central axis 2a is formed by using a spray nozzle in which the spread angle θ1 toward the central part of the steel strip width with respect to the central axis 2a of the spray nozzle 2 is equal to the spread angle θ2 toward the end of the steel strip. Is inclined at an angle φ from the vertical direction toward the end of the steel strip width.

  Also in the second embodiment, as in the first embodiment, the projection of the cooling water 3 sprayed from the spray nozzle 2 as viewed from the traveling direction of the steel strip expands in the central direction of the steel strip width with respect to the vertical direction. The angle may be arranged so that θ (here, θ = φ−θ1) is 10 ° or less.

  At that time, the spray nozzle 2 is arranged so as to be symmetrically inclined with respect to the center portion in the width direction of the cooling header 1, and the spray nozzle 2 at the center portion in the width direction is cooled in the vertical direction without being inclined in the width direction. Water is sprayed so that cooling is performed between the spray nozzles 2 even at the center of the width.

  To incline the spray nozzle 2, there are methods as shown in FIGS. As shown in FIG. 9, the pipe 20 is taken out from the header 1, and the spray nozzle 2 is attached to the tip of the pipe 20 via the inclined joint 21, or the spray nozzle 2 is connected from the header 1 as shown in FIG. The pipe may be inclined and attached to the header 1.

  Here, the inclination angle φ of the spray nozzle 2 is preferably 30 ° or less. If the inclination is larger than 30 °, for example, when the inclination joint 21 is inclined as shown in FIG. 9, a drift occurs at the location of the inclination joint 21, and the cooling water 3 cannot be injected evenly from the spray nozzle 2. It is.

  In the above, in the first embodiment, the nozzle pitch P1 of the spray nozzle arranged in the center in the width direction is arranged narrower than the other nozzle pitch P2, but this is implemented in the second embodiment. It doesn't matter. In the second embodiment, the spray nozzle arranged in the center in the width direction is arranged to spray in the vertical direction, but this may be implemented in the first embodiment.

  Moreover, as a spray nozzle, it is preferable to use the flat spray nozzle from which cooling water is injected in fan shape like Embodiment 1,2. A full cone spray nozzle in which cooling water is spread and injected in a circle or a square blow spray nozzle in which cooling water is spread and injected in a rectangle may be used, in which case, as in Embodiments 1 and 2, If it is attempted to inject the cooling water so that the spread of the cooling water is asymmetric with respect to the vertical direction, the steel strip collision surface cannot maintain a circular shape or a rectangular shape, and becomes an elliptical shape or a rhombus shape, which is difficult to arrange appropriately. In addition, since the cooling water sprayed from the adjacent spray nozzles interferes with each other before colliding with the steel strip, the speed of the cooling water is reduced, and there is a risk that temperature unevenness occurs between the spray nozzles. On the other hand, as shown in FIG. 2, by arranging the flat spray nozzle with twist in the traveling direction of the steel strip, the cooling water sprayed from each adjacent spray nozzle interferes before the steel strip collision. Cooling is possible without causing temperature unevenness between spray nozzles.

  Examples of the present invention will be described.

  FIG. 11 shows the layout of the hot-rolled steel strip production line in the embodiment of the present invention. A 250 mm thick slap is heated to about 1200 ° C. by the heating furnace 60, then rolled to a thickness of 40 mm by the rough rolling mill group 61, and rolled to 2.6 mm by the finish rolling mill group 62. After rolling, the sheet is cooled to a predetermined temperature by the runout table 63 and then wound by the coiler 64.

  In the runout table 63, the upper surface of the steel strip is cooled by the upper surface cooling device 71 (upper surface cooling device group 72) of the hairpin laminar, and the lower surface of the steel strip is cooled by the lower surface cooling device 75 (lower surface cooling device group 76). Do by. Here, the upper surface cooling device 71 is installed in a pair with the lower surface cooling device 75 so that the cooling water falls on each table roll. The temperature distribution of the steel strip before and after cooling on the runout table 63 can be measured by a radiation thermometer 65.

  Here, the geometric relationship between the table roll and the spray nozzle (flat spray nozzle) in the lower surface cooling device 75 was as follows.

Table roll diameter: 140 mm, table roll pitch: 350 mm, distance between spray nozzle and steel strip: 150 mm, spread angle from nozzle central axis: 75 °, spray nozzle width direction mounting pitch: 150 mm, per spray nozzle The flow rate is 20 L / min, the water density between the table rolls is 700 L / min · mm ² , and the cooling water pressure is 0.02 MPa.

  As an example of the present invention, the lower surface cooling device 75 was cooled on the lower surface of the steel strip using the lower surface cooling device (FIGS. 7, 8, and 9) based on the second embodiment of the present invention. That is, the center axis of the spray nozzle is installed to be inclined by an angle φ in the direction of the steel strip width end so that the spread angle in the central direction of the steel strip width with respect to the vertical direction is 10 ° or less. At that time, as shown in Table 1, in Example 1 of the present invention, the inclination angle φ is set to 15 °, and the spread angle θ in the central direction of the steel strip width is set to 8 °. In Example 2 of the present invention, the inclination angle φ is set to 20 °, the steel strip width central direction spreading angle θ was 6 °, and in Example 3 of the present invention, the inclination angle φ was 30 °, and the steel strip width central direction spreading angle θ was −7 °. Further, in Example 4 of the present invention, the inclination angle φ was set to 40 °, and the spread angle θ in the central direction of the steel strip width was set to −17 °.

  On the other hand, as a comparative example, the lower surface cooling device 75 is used to lower the steel strip lower surface by using a lower surface cooling device in which a spray nozzle is arranged so that the spread angle θ in the central portion of the steel strip width with respect to the vertical direction exceeds 10 °. Cooling was performed. That is, as shown in Table 1, in Comparative Example 1, the inclination angle φ is set to 0 °, the steel band width central direction spreading angle θ is set to 23 °, and in Comparative Example 2, the inclination angle φ is set to 10 °. The spread angle θ in the central direction of the steel strip width was set to 13 °.

  Then, the cooling of the steel strip bottom surface was evaluated based on the temperature distribution of the steel strip after cooling measured with the radiation thermometer 65. It should be noted that the temperature deviation in the width direction of the steel strip needs to be 20 ° C. or less, preferably 10 ° C. or less if possible.

  Table 1 shows the evaluation of the cooling of the lower surface of the steel strip in the comparative example and the inventive example. In Table 1, regarding the temperature distribution of the steel strip after cooling, the temperature deviation ΔT1 between the steel strip width center and the steel strip width end and the temperature deviation ΔT2 between the spray nozzles are shown. When ΔT1 and ΔT2 are both 10 ° C. or less, they are evaluated as very good (◎), and both the temperature deviations ΔT1 and ΔT2 are 20 ° C. or less, but at least one of them exceeds 10 ° C. evaluated. On the other hand, a case where at least one of the temperature deviations ΔT1 and ΔT2 exceeded 20 ° C. was evaluated as defective (×).

  As a result, as shown in Table 1, in Comparative Examples 1 and 2, the temperature deviation ΔT2 between the spray nozzles was as small as 1 ° C. and there was no particular problem. Deviation ΔT1 was 50 ° C. in Comparative Example 1 and 45 ° C. in Comparative Example 2, and was evaluated as defective (×). And the predetermined material was not able to be obtained by this supercooling of the steel strip width end.

  On the other hand, as shown in Table 1, in Examples 1 to 3 of the present invention, the temperature deviation ΔT1 between the steel strip width center portion and the steel strip width end portion is within 10 ° C., and the temperature deviation ΔT2 between the spray nozzles is 3 Cooling was carried out very well (）) within the temperature range. As a result, the desired material could be obtained very well in all the steel strip width directions.

  Moreover, in Invention Example 4, the temperature deviation ΔT1 between the steel strip width center portion and the steel strip width end portion was within 7 ° C. and 10 ° C. However, since the inclination angle φ of the spray nozzle was made larger than 30 °, a drift occurred in the cooling water, and the temperature deviation ΔT2 between the spray nozzles became as large as 15 ° C., which was evaluated as good (◯). .

1 Header 2 Spray nozzle (Flat spray nozzle)
2a Center axis of spray nozzle 3 Cooling water 4 Table roll 9 Falling water 10 Steel strip (hot rolled steel strip)
20 piping 21 inclined joint 60 heating furnace 61 rough rolling mill group 62 finishing rolling mill group 63 runout table 64 coiler 65 radiation thermometer 71 upper surface cooling device 72 upper surface cooling device group 75 lower surface cooling device 76 lower surface cooling device group

Claims (4)

As a nozzle for injecting cooling water to the lower surface of the hot-rolled steel strip, a plurality of flat spray nozzles in which the cooling water spreads from the nozzle injection port and is injected between the transport table rolls are arranged in the steel strip width direction . In the lower surface cooling device for hot-rolled steel strips, where flat nozzles are arranged with a twist angle in the direction of steel strip travel, as the cooling water spreads from the nozzle injection port, the projection angle in the steel strip width direction, from the vertical direction An apparatus for cooling the lower surface of a hot-rolled steel strip, characterized in that the angle toward the central portion of the steel strip width is 0 ° or less. The flat spray nozzle has a central axis in a vertical direction, and the cooling water from the nozzle injection port spreads from the central axis to the steel strip rather than the spread angle from the central axis to the central portion of the steel strip width. The lower surface cooling device for a hot-rolled steel strip according to claim 1, wherein a spread angle in a width end direction is larger. The flat spray nozzle has a symmetric spreading angle with respect to the central axis, and the central axis is inclined at an angle of 30 ° or less from the vertical direction toward the end of the steel strip width. The lower surface cooling apparatus of the hot-rolled steel strip according to claim 1. The lower surface cooling device for a hot-rolled steel strip according to any one of claims 1 to 3, wherein an upper-surface cooling device for injecting cooling water onto the upper surface of the hot-rolled steel strip is not provided.

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