JP5673530B2 - Hot rolled steel sheet cooling apparatus, cooling method, manufacturing apparatus, and manufacturing method - Google Patents

Description

  The present invention relates to a cooling apparatus, a cooling method, a manufacturing apparatus, and a manufacturing method for a hot-rolled steel sheet. The present invention particularly relates to a hot-rolled steel sheet cooling apparatus, a cooling method and a manufacturing apparatus that are suitably used when manufacturing hot-rolled steel sheets having ultrafine crystal grains, and manufacture of hot-rolled steel sheets having ultrafine crystal grains. Regarding the method.

  Steel materials used for automobiles, structural materials, etc. are required to have excellent mechanical properties such as strength, workability, and toughness. It is effective to reduce the size. Therefore, many manufacturing methods for obtaining a hot rolled steel sheet having fine crystal grains have been sought. Further, if the crystal grains are refined, it is possible to produce a high-strength hot-rolled steel sheet having excellent mechanical properties even if the addition amount of the alloy element is reduced.

  As a method of refining crystal grains of hot-rolled steel sheet, particularly in the latter stage of hot finish rolling, high-pressure rolling is performed to refine austenite grains and accumulate rolling strain in the grains, after cooling (or after transformation) And the like, and the like. And, from the viewpoint of suppressing recrystallization and recovery of austenite grains and promoting ferrite transformation, it is effective to cool the steel sheet to a predetermined temperature or lower (for example, 720 ° C. or lower) in a short time after rolling. . In other words, in order to manufacture hot rolled steel sheets with fine crystal grains, it is effective to install a cooling device that can cool faster than before, followed by hot finish rolling, and rapidly cool the rolled steel sheet. It is.

There have been disclosed several techniques that enable the production of hot-rolled steel sheets having fine crystal grains, or techniques that can be applied to the production of hot-rolled steel sheets having fine crystal grains. For example, Patent Document 1 discloses a method of manufacturing a hot-rolled steel sheet by multi-pass hot rolling a steel sheet or slab made of carbon steel or low alloy steel containing C: 0.01 to 0.3% by mass. A method for producing an ultrafine-grained hot-rolled steel sheet is disclosed, wherein the final rolling pass is terminated at a temperature of ₃ or more points of Ar and then cooled to 720 ° C. or less within 0.4 seconds. Further, in Patent Document 2, a final stand, a first cooling device, a second cooling device, and a winding device in a hot rolling finish rolling mill row are sequentially arranged in the steel plate conveyance direction, In a hot-rolled steel sheet manufacturing facility in which a non-cooled region is provided between the first cooling device and the second cooling device, the first cooling device has a strip or oval shape on the surface to be cooled of the steel plate. A nozzle that should form a jet collision area and a dam roll that dams cooling water sprayed from the nozzle, and a pool of cooling water is formed in the area between the roll of the final stand and the dam roll. In addition, a technique is disclosed in which a weir roll is disposed so that a steel sheet conveyed in the first cooling device is immersed in the cooling water of the pool. Patent Document 3 discloses a cooling facility for supplying cooling water to the upper surface of a steel sheet while passing the steel sheet at a position close to the entry-side and / or exit-side rolling mill of a rolling mill that hot-rolls the steel sheet. The cooling equipment is arranged such that the cooling water after supplying the steel plate with a header having a nozzle that injects rod-shaped cooling water toward the rolling mill side with a depression angle of 30 ° to 60 ° with respect to the upper surface of the steel plate is the rolling mill. A hot-rolling equipment for a steel sheet is disclosed, which is provided at a position where it can be dammed by a work roll. In Patent Document 3, it is preferable that the distance between the tip of the upper nozzle and the pass line be 500 mm to 1800 mm in order to avoid a situation in which the cooling water is dispersed and is not rod-shaped and has no function of blocking the cooling water. It is written.

Japanese Patent Laid-Open No. 2005-213595 Japanese Patent No. 4029865 JP 2007-61838 A

According to the technique disclosed in Patent Document 1, a steel plate having a temperature of Ar ₃ or higher is cooled to 720 ° C. within 0.4 seconds after the end of the final rolling pass. It is considered that a hot-rolled steel sheet having crystal grains having an average grain diameter of 2 μm or less (the same applies hereinafter) can be produced. However, Patent Document 1 does not disclose a detailed configuration of a cooling device that can cool a steel plate to 720 ° C. within 0.4 seconds from the end of the final rolling pass. Moreover, according to the technique disclosed in Patent Document 2, in order to immerse the steel sheet in the pool of cooling water formed in the region between the roll of the final stand of the hot rolling mill row and the weir roll, It is considered that the cooling efficiency of the steel sheet can be improved. Here, the rapid cooling required when producing a hot-rolled steel sheet having ultrafine crystal grains has a cooling rate of at least 400 ° C./s or more as shown in Patent Document 1, for example. Is required to be rapidly cooled by nucleate boiling cooling. However, as disclosed in Patent Document 2, when the cooling water pool is positively formed to cool the steel sheet, the collision pressure of the cooling water that collides with the steel sheet surface is reduced to the extent that nucleate boiling cooling is possible. In order to manufacture a hot-rolled steel sheet that is difficult to increase and has ultrafine crystal grains, there has been a problem that further technical improvement is required. Moreover, in the rapid cooling required when manufacturing a hot-rolled steel sheet having ultrafine crystal grains, the collision pressure of cooling water that collides with the steel sheet surface needs to be a predetermined value or more. In the technique disclosed in No. 3, only the injection angle of the rod-shaped cooling water supplied to the steel sheet is mainly defined. Further, in Patent Document 3, the cooling water sprayed onto the steel sheet flows to the part where the steel sheet and the work roll contact, and therefore cooling from immediately after the part is possible. However, sufficient rapid cooling cannot be achieved, and cooling in this portion hardly contributes to the formation of ultrafine crystal grains. Therefore, there is a problem that it is difficult to manufacture a hot-rolled steel sheet having ultrafine crystal grains even if this technique is simply used.

  Therefore, the present invention can manufacture a hot-rolled steel sheet having ultrafine crystal grains and increase the use efficiency of cooling water, a hot-rolled steel sheet cooling device, a hot-rolled steel sheet cooling method, and a hot-rolled steel sheet It is an object to provide a manufacturing apparatus and a method for manufacturing a hot-rolled steel sheet.

The inventors of the present invention conducted research on the production of hot-rolled steel sheets having ultrafine crystal grains (hereinafter sometimes referred to as “ultrafine grain steel”), and obtained the following knowledge.
(1) As shown in FIG. 11, after rolling in a temperature range of ₃ or more points of Ar, when cooling to 720 ° C. is completed within 0.2 seconds, the crystal grains can be further refined Become.
(2) Ar cooling at 100 ° C. from 820 ° C. to 720 ° C. at ₃ points or more, for example, is completed within 0.2 seconds after rolling, for example, rapid cooling is performed at an average cooling rate of 500 ° C./s or more. It is necessary to perform rapid cooling at a cooling rate of 600 ° C./s or more. Here, the reduction point in the final stand of the hot finish rolling mill (the bottom dead center of the work roll contacting the upper surface of the steel sheet to be rolled and the top dead center of the work roll contacting the lower surface of the steel sheet to be rolled) The same applies hereinafter) to the end of the final stand housing post (hereinafter referred to as “in-stand area”) in the steel plate conveyance direction L1, and rapid cooling in the in-stand area. The length in the steel plate conveyance direction of the section in which the steel sheet can be conveyed is L2, the cooling rate in the section is Z1, the length in the steel plate conveyance direction in the section where rapid cooling is difficult in the stand internal region is L3, and the cooling speed in the section is Z2. When this is done, the cooling rate represented by {L2 × Z1 + L3 × Z2} / L1 is the average cooling rate. When the steel plate is cooled at a cooling rate of 600 ° C./s, the time required for lowering the temperature of the steel plate by 100 ° C. is 0.167 seconds. Therefore, in order to finish the cooling within 0.2 seconds, it is necessary to start the cooling within 0.033 seconds after rolling. For example, when the steel plate is moved at a speed of 10 m / s, the moving distance in 0.033 seconds is 0.33 m. Accordingly, it is preferable that the rapid cooling after rolling starts within a position corresponding to the radius of the work roll of the final stand in the hot rolling mill row, and at least in the final stand in the hot rolling mill row is quenched almost continuously.
(3) For example, when the rolling speed of the steel plate is 10 m / s, the distance that the steel plate moves in 0.2 seconds is 2 m. Further, the distance from the reduction point in the final stand of a general hot finish rolling mill to the housing post exit side of the final stand is also about 2 m. Therefore, the required rapid cooling must be done almost in the final stand. However, as represented by the vicinity of the reduction point pole, there is a portion where it is difficult to perform rapid cooling from the reduction point to the housing post exit side of the final stand. Therefore, in consideration of the existence of a portion where it is difficult to perform rapid cooling, it refers to a rapid cooling possible range (a region obtained by excluding a portion where it is difficult to perform rapid cooling from the region from the reduction point to the stand exit side. In the same manner, the average cooling rate in the region from the final stand reduction point to the housing post exit side of the final stand must be secured.
(4) There is a correlation between the pressure (surface pressure) at which the cooling water sprayed onto the steel plate collides with the steel plate and the cooling rate of the steel plate (see FIG. 6), and the pressure at which the cooling water collides with the steel plate is increased. By making it, it becomes possible to increase the cooling rate of a steel plate. In order to produce ultrafine-grained steel, it is necessary to inject high-pressure jet water toward the steel sheet, and the steel sheet needs to be nucleate-boiling cooled.

  Furthermore, from a metallurgical point of view, it is desirable to cool from within an earlier stage within 0.2 seconds after rolling, and it is desirable to start cooling from a position closer to the reduction point of the final stand. Similarly, it is desirable to cool more strongly at a position near the reduction point of the final stand. The inventors investigated the influence of particularly strong cooling on the refinement of the crystal grains in a portion close to the reduction point of the final stand, that is, within a position corresponding to the radius of the work roll. Specifically, rolling and cooling tests were performed under different cooling conditions between the position corresponding to the radius of the work roll and the housing post exit side thereafter, and the crystal grain size of the ferrite structure of the obtained steel sheet was investigated. . In this investigation, the distance from the reduction point to the housing post exit side is 1.8 m, the work roll radius is 0.35 m, and the point on the most upstream side of the portion where the injected cooling water directly collides with the steel plate (hereinafter, “ The “cooling start point” was 0.15 m from the reduction point, the plate speed was 10 m / s, and the plate thickness of the steel plate was 3 mm. Further, the cooling water supply pressure was 1.5 MPa at the cooling header. Only the conditions under which the crystal grain size target of 2 μm or less was achieved are extracted and the results are shown in Table 1.

  In Table 1, the subscript indicating a region within the work roll radius equivalent position (hereinafter sometimes referred to as “region 1”) is “1”, and the region up to the housing post exit side (hereinafter referred to as “region 1”). The subscript indicating “2” may be referred to as “region 2”. The cooling speeds in the respective regions are V1 and V2, the collision pressure of the cooling water on the steel plate is P1 and P2, and the flow rate of the cooling water. Densities are W1 and W2. Also, the subscript indicating the area from the cooling start point (0.15 m) to the housing post exit side (1.8 m) (hereinafter sometimes referred to as “total cooling area”) is “m”, The average values (Vm, Pm, Wm) of the cooling zone are also noted. As shown in FIG. 6, there is a correlation between the cooling rate V and the collision pressure P, and a high collision pressure was required to obtain a high cooling rate. Further, under the condition that the cooling water pressure (1.5 MPa) in the header is constant, a high cooling water injection speed is required to obtain a high collision pressure. A numerical value instead of the injection speed is a flow rate per unit area, that is, a flow rate density W. If the length of the injected cooling water in the width direction of the steel plate is constant, the cooling area is also constant, so the flow density W is also an indicator of the amount of cooling water to be used, and the required energy of the pump that supplies the cooling water is compared. It becomes an index to

As shown in Table 1, test No. 1 was performed so that V1 = V2. 1, the average cooling rate Vm in the entire cooling region is expressed as Test No. Test No. 1 in which V1 was made larger than V2 while being kept near 615 ° C./s, the same as in FIG. 2 and No. 3 increased the effect of crystal grain refinement. A ferrite grain size finer than 1 was obtained. In addition, Test No. 3 for test no. 4 and test no. 5, when V2 was lowered while keeping V1 constant at 1600 ° C./s, the effect of refining the crystal grains was somewhat reduced, but the average cooling rate was 404 ° C./s. It was found that the target particle size of 2 μm or less can still be obtained even under the condition of 5. Test No. 1 and test no. 4 and test no. 5 is almost the same as the grain refinement effect, while the test no. 4 and test no. Wm of 5 is the test No. It was smaller than 1 Wm, and as a whole, fine granulation could be achieved efficiently with a small amount of cooling water (the use efficiency of cooling water could be increased). It is possible to further increase V1 to increase the effect of refining crystal grains and increase the efficiency of use of cooling water. However, if the local flow density in region 1 is excessively increased, the cooling water in the rolling mill is increased. There is a possibility that the impinging force of the cooling water jet on the steel plate is scraped off by the staying water, and as a result, V1 cannot be increased. Therefore, in consideration of drainage in the rolling mill, the upper limit value of the flow density W1 is preferably about 20 m ³ / m ² · min, and the upper limit of the cooling rate V1 corresponding to the plate thickness of 3 mm is 1600 ° C. / S.

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

  The present invention will be described below. In order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are appended in parentheses, but the present invention is not limited to the illustrated embodiments.

1st aspect of this invention is arrange | positioned in the downstream in the last stand (11g) in a hot finish rolling mill row | line | column (11), and it is high pressure jet water toward the surface of the steel plate (1) conveyed by a pass line. a plurality of nozzles provided to be inject (21a, 21a, ..., 22a , 22a, ...) a cooling device hot-rolled steel sheet having a header (21, 22) comprising a (20), at least the steel plate The nozzle arranged on the most upstream side in the conveyance direction is arranged so that the axial direction intersects the vertical plane of the steel sheet so that high-pressure jet water can be injected obliquely toward the upstream side in the conveyance direction of the steel sheet by an average cooling rate of the steel sheet is cooled by high-pressure jet water collides with the surface of the steel sheet that exist within a radius corresponding position of the work rolls of the final stand V1, the radial position corresponding to the work rolls of the final stand top When the average cooling rate of the steel sheet is cooled by high-pressure jet water collides with the surface of the steel sheet present between the housing post delivery side of the stand V2, and a V1 ≧ V2, and, in the final stand The average cooling rate Vm of the steel sheet surface cooled by the high-pressure jet water colliding with the surface of the steel sheet existing between the cooling start point and the housing post exit side of the final stand is 400 ° C./s or more. This is a cooling device for hot-rolled steel sheets.

  Here, the “downstream side” refers to the downstream side in the conveying direction of the steel plate (1). The “high pressure jet water” refers to jet water having a pressure capable of nucleate boiling cooling the steel plate (1). Further, “the position corresponding to the radius of the work roll of the final stand” means, as shown in FIG. 4, a portion where the rolled steel plate (1) and the work rolls of the final stand (11 gw, 11 gw) are in contact (more specifically) Are the bottom dead center of the work roll (11 gwu) in contact with the upper surface of the steel plate (1) and the top dead center of the work roll (11 gwd) in contact with the lower surface of the steel plate (1). It is sometimes referred to as “point”.) A position that is separated from the steel plate (1) by the radius of the work roll (11gw, 11gw) of the final stand toward the downstream side in the conveying direction of the steel plate (1). In addition, “the surface of the steel sheet existing within the position corresponding to the radius of the work roll of the final stand” means between the position corresponding to the radius of the work roll of the final stand and the reduction point (more than the position corresponding to the radius of the work roll of the final stand. It refers to the surface (upper surface and lower surface) of the steel plate (1) existing on the reduction point side. In addition, the “average cooling rate of the steel plate” is calculated for each of a plurality of parallelogram regions that are taken into account when calculating the vertical component of the steel plate conveyance direction average value of the steel plate surface collision pressure of high-pressure jet water described later, for example. Means the average cooling rate. “V1” is a cooling rate calculated for each parallelogram region by dividing the upper surface (or the lower surface) of the steel sheet existing within the position corresponding to the radius of the work roll of the final stand into a plurality of parallelogram regions. The average value of At that time, the boundary on the upstream side of the region closest to the work roll is the most upstream side of the portion where the high-pressure jet water directly collides with the steel plate, that is, the point close to the reduction point (cooling start point). When the high-pressure jet spray is installed closest to the roll, this corresponds to the point where the tangent drawn from the center of the nozzle injection hole to the roll circumference reaches the steel plate. The “housing post exit side of the final stand” refers to the outer surface of the housing post (11gh) of the final stand (the outer surface on the downstream side in the steel plate conveyance direction). Further, “V2” divides the upper surface (or the lower surface) of the steel plate existing between the position corresponding to the radius of the work roll of the final stand and the housing post exit side of the final stand into a plurality of parallelogram regions, The average value of the cooling rate calculated about each parallelogram area | region is said. In addition, “Vm” is defined by dividing the upper surface (or the lower surface) of the steel plate existing between the cooling start point in the final stand and the housing post exit side of the final stand into a plurality of parallel quadrangular regions. It means the average value of the cooling rates calculated for the quadrilateral region.

The second aspect of the present invention is a high-pressure jet water which is arranged on the downstream side in the final stand (11g) in the hot finish rolling mill row (11) and is directed toward the surface of the steel plate (1) conveyed through the pass line. A hot-rolled steel sheet cooling device (20) comprising a header (21, 22) comprising a plurality of nozzles (21a, 21a,..., 22a, 22a,. The nozzle arranged on the most upstream side in the conveyance direction is arranged so that the axial direction intersects the vertical plane of the steel sheet so that high-pressure jet water can be injected obliquely toward the upstream side in the conveyance direction of the steel sheet is, high-pressure jet water collides with the surface of the steel sheet that exist within a radius corresponding position of the final stand the work rolls, the vertical component of the steel sheet conveyance direction average value of the steel sheet surface collision pressure P1, the final stand of the work roll radius When the vertical component of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water colliding with the surface of the steel plate existing between the position and the housing post exit side of the final stand is P1, P1 ≧ P2 and the high-pressure jet water colliding with the surface of the steel plate existing between the cooling start point in the final stand and the housing post exit side of the final stand, the average value of the steel plate surface collision pressure in the steel plate conveyance direction The cooling device for hot-rolled steel sheet, wherein the vertical component Pm is 2.7 kPa or more.

  Here, "the vertical component of the average value of the steel plate surface collision pressure of the high-pressure jet water" in the steel plate conveyance direction is an arbitrary position in the steel plate width direction, for example, at the center in the width direction, along the line segment in the steel plate conveyance direction. A collision component of high-pressure jet water received on the surface of the steel sheet is measured or calculated, and averaged over a predetermined region, which means a vertical component (hereinafter sometimes referred to as “average collision pressure” or “average collision pressure”). ). In order to uniformly cool the steel plate in the plate width direction, it is desirable to make the vertical component of the average value in the steel plate conveyance direction equal in all regions in the steel plate width direction. Even when considering a plane having a width corresponding to at least the nozzle pitch, it should be equal to the vertical component of the steel plate surface collision pressure obtained on the line segment. Therefore, in obtaining the vertical component of the average value in the steel plate conveyance direction, the average collision pressure on the steel plate surface that is handled by one nozzle is obtained for each nozzle row arranged in the steel plate conveyance direction, and this is averaged in the steel plate conveyance direction. (See FIGS. 4 and 9). When the nozzle is a flat spray nozzle, in the present invention, for example, as shown in FIG. 7, when the nozzle pitch in the steel plate width direction is A and the nozzle pitch in the steel plate transport direction is B, that is, the header interval is B, one nozzle is The average collision pressure on the steel sheet surface is calculated by dividing the cooling water force (collision force) that collides with the parallelogram area whose area is represented by A × B by the area A × B of the parallelogram. Can be calculated. On the other hand, when the nozzle is a columnar nozzle, similarly, when the nozzle pitch in the plate width direction of the steel plate is A and the nozzle pitch in the conveyance direction of the steel plate is B, the average collision pressure on the steel plate surface that one nozzle takes is: It can be calculated by dividing the force (collision force) of the high-pressure jet water colliding with the parallelogram area whose area is represented by A × B by the area A × B of the parallelogram area. In addition, “Pm” is defined by dividing the upper surface (or lower surface) of the steel plate existing between the cooling start point in the final stand and the housing post exit side of the final stand into a plurality of parallel quadrilateral regions. The average value of the average collision pressure calculated for the quadrilateral region.

  Further, in the first aspect of the present invention and the second aspect of the present invention, per unit area of the high-pressure jet water sprayed onto the surface of the steel plate existing within the position corresponding to the radius of the work roll of the final stand. When the amount of water is W1, and the amount of water per unit area of high-pressure jet water sprayed onto the surface of the steel plate existing between the position of the work roll radius of the final stand and the housing post exit side of the final stand is W2. , W1 ≧ W2.

  Here, the “unit area” is not particularly limited as long as the area when the water amount W1 is derived and the area when the water amount W2 is derived are the same. As the “unit area”, for example, an area of a parallelogram used for deriving a vertical component of an average value in the steel plate conveyance direction of the steel plate surface collision pressure of high-pressure jet water can be used.

  In the first aspect of the present invention, the distance between the high pressure jet water injection port of the nozzle disposed at the position closest to the work roll of the final stand and the steel plate is D1, and is closest to the housing post exit side of the final stand. When the distance between the high-pressure jet water injection port of the nozzle disposed at the position and the steel plate is D2, it is preferable that D1 ≦ D2.

  Further, in the first aspect of the present invention, in the section from the position corresponding to the radius of the work roll of the final stand to the housing post exit side of the final stand, high-pressure jet water is supplied from the nozzle toward the upper surface and the lower surface of the steel plate. It is preferable to be configured so that it can be continuously ejected in the conveying direction of the steel sheet.

  Here, “From within the position corresponding to the radius of the work roll of the final stand” means between the position corresponding to the radius of the work roll of the final stand and the reduction point (from the position corresponding to the radius of the work roll of the final stand). The high pressure jet water sprayed from the nozzles (21a, 21a,..., 22a, 22a,...) Is supplied to the upper surface and the lower surface of the steel plate (1) present in FIG. The strict start point of the section in which the high-pressure jet water is continuously injected is the most upstream side of the portion where the high-pressure jet water directly collides with the steel plate within the position corresponding to the radius of the work roll, that is, the point close to the reduction point. When the nozzle for injecting high-pressure jet water is installed closest to the work roll of the final stand, the point where the tangent line drawn from the center of the nozzle injection hole to the surface of the work roll reaches the surface of the steel plate This corresponds to the strict start point of the section where jet water is continuously injected. The “housing post exit side of the final stand” refers to the outer surface of the housing post (11gh) of the final stand (the outer surface on the downstream side in the steel plate conveyance direction). Moreover, "it is comprised so that high pressure jet water can be continuously sprayed in the conveyance direction of a steel plate from a nozzle" means a plurality of nozzles (21a, 21a arranged at predetermined intervals in the conveyance direction of a steel plate (1). , ..., 22a, 22a, ...) means that the high-pressure jet water can be continuously jetted from the upper surface and the lower surface of the steel plate (1).

Further, in the first aspect of the present invention, the average value of the steel plate surface collision pressure of the steel plate surface collision pressure of the high pressure jet water in the section from the position corresponding to the radius of the work roll of the final stand to the housing post exit side of the final stand . The vertical component is preferably 3.5 kPa or more on the upper surface and the lower surface.

  In the first aspect of the present invention, the nozzles (21a, 21a, ..., 22a, 22a, ...) are preferably flat spray nozzles.

  Further, in the first aspect of the present invention, a space capable of discharging cooling water is secured between both end faces in the steel plate width direction of the cooling device (20) and both end faces in the steel plate width direction of the final stand (11g). It is preferable.

  Here, the “steel plate width direction both end surfaces of the cooling device (20)” refers to the outer surfaces of the cooling device (20) on both ends in the width direction of the steel plate (1). Moreover, the “steel plate width direction both end surfaces of the final stand (11g)” refers to the inner surfaces of the housing posts (11gh) of the final stand on both ends in the width direction of the steel plate (1).

  In the first aspect of the present invention, the header (21) and the nozzles (21a, 21a,...) Provided on the upper surface side of the steel plate (1) are provided between the nozzle and the pass line. It is preferable that the upper surface guide (23) is integrally formed.

  Here, the “upper surface guide (23)” means that the steel plate (1) rolled in the final stand (11g) is the work roll (11gwu) of the final stand or the nozzles (21a, 21a,...) Of the cooling device (20). ) Refers to a member of the cooling device (20) installed on the upper surface side of the steel plate (1) for the purpose of preventing collision with the steel plate.

  In the first aspect of the present invention, the header (22) and the nozzles (22a, 22a,...) Provided on the lower surface side of the steel plate (1) are provided between the nozzle and the pass line. It is preferable that the lower surface guide (24) which is provided is integrally formed.

  Here, the “lower surface guide (24)” means that the steel plate (1) rolled by the final stand (11g) is the work roll (11gwd) of the final stand or the nozzles (22a, 22a,...) Of the cooling device (20). ) Refers to a member of the cooling device (20) installed on the lower surface side of the steel plate (1) for the purpose of preventing collision with the steel plate.

  Further, in the first aspect of the present invention, a plurality of headers (21, 31, 22, 32) are provided, and at least a part of the headers are in the conveying direction of the steel plate (1) and the width direction of the steel plate (1). It is preferable that the cooling water can be collectively supplied to the nozzles (31a, 31a, ..., 32a, 32a, ...) arranged in a plurality of rows.

  In the first aspect of the present invention, at least a part of the header is configured to be able to collectively supply cooling water to nozzles arranged in a plurality of rows in each of the conveying direction of the steel sheet and the width direction of the steel sheet. A plurality of headers (21, 31) are arranged on the upper surface side of the steel plate, and among the headers provided on the upper surface side of the steel plate, at least the header (31) arranged on the most upstream side in the conveying direction of the steel plate is It is preferable that the header is configured so that the cooling water can be collectively supplied to the nozzles (31a, 31a,...) Arranged in a plurality of rows in each of the conveying direction and the width direction of the steel plate.

  In the first aspect of the present invention, at least a part of the header is configured to be able to collectively supply cooling water to nozzles arranged in a plurality of rows in each of the conveying direction of the steel sheet and the width direction of the steel sheet. A plurality of headers (22, 32) are disposed on the lower surface side of the steel sheet, and among the headers provided on the lower surface side of the steel sheet, at least the header (32) disposed on the most upstream side in the conveying direction of the steel sheet is It is preferable that the header is configured so that the cooling water can be collectively supplied to the nozzles (32a, 32a,...) Arranged in a plurality of rows in each of the conveying direction and the width direction of the steel plate.

  A third aspect of the present invention is a hot-rolled steel sheet, wherein the steel sheet is cooled using the hot-rolled steel sheet cooling device according to the first aspect of the present invention or the second aspect of the present invention. This is a cooling method.

  The fourth aspect of the present invention is the final stand (11g) in the hot finish rolling mill row (11), and cooling of the hot rolled steel sheet according to the first aspect of the present invention or the second aspect of the present invention. It is a manufacturing apparatus (10) of a hot-rolled steel sheet characterized by comprising an apparatus (20, 20 ') in order in the conveying direction of the steel sheet (1).

  The fifth aspect of the present invention is rolled at the final stand (11g) in the hot finish rolling mill (11) using the hot-rolled steel sheet manufacturing apparatus (10) according to the fourth aspect of the present invention. It is a manufacturing method of a hot-rolled steel plate characterized by including the process of processing the obtained steel plate (1).

  In the present invention, the average cooling rate V1 cooled by the high-pressure jet water sprayed onto the surface of the steel sheet existing on the side of the reduction point from the position corresponding to the work roll radius of the final stand is the position corresponding to the work roll radius corresponding to the final stand. The average cooling rate is V2 or higher, which is cooled by the high-pressure jet water sprayed onto the surface of the steel plate existing between the housing post exit side of the final stand, and Vm ≧ 400 ° C./s. In the present invention, the vertical component P1 of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water that has collided with the surface of the steel plate existing on the side of the reduction point from the position corresponding to the radius of the work roll of the final stand is The vertical component P2 of the average value in the steel plate conveying direction of the steel plate surface collision pressure of the high-pressure jet water colliding with the surface of the steel plate existing between the position corresponding to the radius of the work roll of the final stand and the housing post exit side of the final stand It is set as above and it is set as Pm> = 2.7kPa. Therefore, according to the present invention, the steel sheet can be rapidly cooled immediately after the rolling of the final stand is finished, and the use efficiency of the cooling water used when manufacturing the ultrafine grain steel can be enhanced. By rapidly cooling the steel sheet immediately after the end of rolling, recovery of the austenite structure and the like can be suppressed. Therefore, according to the present invention, a hot-rolled steel sheet cooling device, a hot-rolled steel sheet cooling method, a hot-rolled steel sheet, which can produce a hot-rolled steel sheet having ultrafine crystal grains and can improve the use efficiency of cooling water. It becomes possible to provide the manufacturing apparatus of a steel plate, and the manufacturing method of a hot-rolled steel plate.

It is a figure which shows typically a part of manufacturing apparatus of the hot-rolled steel plate concerning this invention. It is a figure which expands and shows the part by which the cooling device of the hot-rolled steel plate of this invention is arrange | positioned from FIG. It is a figure which shows the example of the form of the cooling device of the hot-rolled steel plate of this invention. It is a figure explaining the concept of the perpendicular | vertical component of the steel roll conveyance direction average value of the position equivalent to the radius of the work roll of the last stand, the exit side of the housing post of the last stand, and the steel plate surface collision pressure of high-pressure jet water. It is a conceptual diagram which shows the pressure distribution of the high pressure jet water injected on the upper surface of the steel plate. It is a figure which shows the relationship between the steel plate conveyance direction average value of the steel plate surface collision pressure of high pressure jet water, and the average cooling rate of a steel plate. It is a figure explaining the average value per nozzle of the steel plate surface collision pressure of high pressure jet water. The shape of the part where the high-pressure jet water jetted from the nozzle collides with the steel plate surface and the parallelogram region considered when deriving the vertical component of the average value of the steel plate surface collision pressure of the high-pressure jet water on the steel plate surface are shown. FIG. FIG. 8A shows a case where the nozzle is a flat spray nozzle, and FIG. 8B shows a case where the nozzle is a columnar nozzle. In the hot rolled steel sheet cooling apparatus according to another embodiment of the present invention, the position corresponding to the radius of the work roll of the final stand, the housing post exit side of the final stand, and the perpendicular value of the average value in the steel sheet conveying direction of the steel plate surface collision pressure of high-pressure jet water. It is a figure explaining the concept of a component. It is a figure which extracts and expands and shows the part by which the cooling device of the hot-rolled steel plate of this invention concerning another form is arrange | positioned. It is a figure which shows the relationship between the required time for cooling to 720 degreeC, and the ferrite particle size obtained.

DESCRIPTION OF SYMBOLS 1 ... Steel plate 10 ... Hot-rolled steel plate manufacturing apparatus 11 ... Hot finishing rolling mill row 11g ... Final stand 11gh ... Final stand housing post 11gw ... Final stand work roll 11gwu ... Final stand work roll 11gwd ... Final stand work Roll 12 ... Transport roll 13 ... Pinch roll 20, 20 '... Cooling device for hot-rolled steel plate 21 ... Header 21a ... Nozzle 22 ... Header 22a ... Nozzle 23 ... Upper surface guide 24 ... Lower surface guide 30 ... Cooling device for hot-rolled steel plate 31 ... Collective header 31a ... Nozzle 32 ... Collective header 32a ... Nozzle

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram schematically showing a part of a hot-rolled steel sheet cooling apparatus 20 according to the present invention and a hot-rolled steel sheet manufacturing apparatus 10 according to the present invention provided with the cooling apparatus 20. In FIG. 1, the steel sheet 1 is conveyed from the left (upstream side) to the right (downstream side) of the drawing, and the vertical direction of the drawing is the vertical direction. Hereinafter, the upstream side / downstream side direction may be described as the conveying direction, and the direction of the plate width of the steel plate to be passed through may be described as the steel plate width direction. For ease of viewing, repeated reference numerals may be omitted in the drawings.

  As shown in FIG. 1, a hot-rolled steel sheet manufacturing apparatus 10 of the present invention (hereinafter sometimes simply referred to as “manufacturing apparatus 10”) includes a hot finish rolling mill row 11 and a hot-rolled steel sheet of the present invention. A cooling device 20 (hereinafter, simply referred to as “cooling device 20”), a transport roll 12 and a pinch roll 13 are provided. Although illustration and explanation are omitted, a heating furnace, a rough rolling mill row, and the like are arranged on the upstream side of the hot finish rolling mill row 11, and the conditions of the steel sheet rolled by the hot finish rolling mill row 11 are adjusted. ing. On the other hand, on the downstream side of the pinch roll 13, other cooling devices, winders and the like are arranged, and various facilities for shipping the steel plate as a coil are arranged.

  A hot-rolled steel sheet is generally manufactured as follows. That is, the rough bar extracted from the heating furnace and rolled to a predetermined thickness by a roughing mill is continuously rolled to a predetermined thickness by the hot finish rolling mill row 11 while the temperature is controlled. Thereafter, it is rapidly cooled by the cooling device 20. Here, the cooling device 20 applies the work rolls 11gw and 11gw of the final stand from the inside of the housing post 11gh of the final stand of the hot finish rolling mill row 11 (hereinafter referred to as the work roll 11gw in contact with the upper surface of the steel plate 1) The roll 11gwu ”and the work roll 11gw in contact with the lower surface of the steel plate 1 may be referred to as“ work roll 11gwd ”) as close as possible. And the steel plate which passed the pinch roll 13 is cooled to predetermined winding temperature by another cooling device after that, and is wound up in a coil form with the winder.

  As described above, the manufacturing apparatus 10 includes the hot finish rolling mill row 11. In this embodiment, seven rolling mills (11a, 11b, 11c,..., 11g) are arranged in parallel along the transport direction. Each of the rolling mills 11a, 11b,..., 11g is a rolling mill that constitutes a so-called stand, and can satisfy conditions such as thickness, mechanical properties, and surface quality required for the final product. Thus, the rolling reduction is set.

  2 and 3 are enlarged views showing a portion where the cooling device 20 is arranged. FIG. 2 shows a state of the cooling device 20 that rapidly cools the upper surface and the lower surface of the steel plate immediately after passing through the reduction point of the final stand 11g, and the dotted line in FIG. 2 represents high-pressure jet water. On the other hand, FIG. 3 shows a state of the cooling device 20 when the work rolls 11gw and 11gw of the final stand 11g are exchanged. Also, FIG. 4 shows the vertical component of the average value in the steel plate conveying direction of the position of the work roll of the final stand, the exit side of the housing post 11gh of the final stand, and the steel plate surface collision pressure of the high pressure jet water. , “Sometimes referred to as“ cooling water collision pressure average value ”). 4 is the upstream side in the steel plate conveyance direction, and the right side in FIG. 4 is the downstream side in the steel plate conveyance direction. FIG. 5 is a conceptual diagram showing the pressure distribution of high-pressure jet water jetted onto the upper surface of the steel plate 1. The vertical axis in FIG. 5 is a vertical component [kPa] of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water jetted onto the upper surface of the steel plate 1, and the horizontal axis in FIG. Is the distance. In FIG. 5, X1 is the position corresponding to the radius of the work roll of the final stand, and X2 is the position of the final stand on the housing post exit side. Hereinafter, the cooling device 20 will be specifically described with reference to FIGS.

As shown in FIGS. 2 and 3, the cooling device 20 is disposed on the downstream side of the final stand 11 g in the hot finish rolling mill row 11. The cooling device 20 includes a header 21 to which a plurality of flat spray nozzles 21a, 21a,... (Hereinafter, simply referred to as “nozzle 21a” or the like) that inject high-pressure jet water toward the upper surface of the steel plate 1 are connected. 21 and headers 22, 22 to which a plurality of flat spray nozzles 22 a, 22 a, (hereinafter simply referred to as “nozzle 22 a”) for injecting high-pressure jet water toward the lower surface of the steel plate 1 are connected. It is equipped with. A plurality of nozzles 21 a, 21 a,... Arranged at a predetermined pitch in the steel plate width direction are connected to the header 21, and the plurality of headers 21, 21,. ing. Similarly, a plurality of nozzles 22a, 22a,... Arranged at a predetermined pitch in the steel plate width direction are connected to the header 22, and the plurality of headers 22, 22,. Is arranged in. The headers 21, 21,... Are configured to be able to supply cooling water to a plurality of nozzles 21 a, 21 a,... Arranged at a predetermined pitch in the width direction of the steel plate 1, and the headers 22, 22,. The cooling water can be collectively supplied to a plurality of nozzles 22a, 22a,... Arranged at a predetermined pitch in the width direction. The two rows of nozzles 21a and 21a on the upper surface side of the steel plate 1 and the two rows of nozzles 22a and 22a on the lower surface side of the steel plate 1 are arranged on the most upstream side in the conveyance direction of the steel plate 1. Each axial direction is arranged so as to intersect with the vertical plane of the steel plate so that the high-pressure jet water can be injected obliquely toward the upstream side of the steel plate . In the cooling device 20, an angle formed by the axial direction of the nozzles 21 a and 22 a arranged on the most upstream side in the conveying direction of the steel plate 1 with respect to the vertical surface of the steel plate (hereinafter referred to as “vertical in-plane inclination angle”). The nozzle 21a, 22a and the steel plate 1 are set to have a vertical in-plane inclination angle equal to or greater than the nozzles 21a, 22a adjacent to the downstream side in the conveyance direction of the steel plate 1. The nozzles 21a, 21a,... And the nozzles 22a, 22a,... Arranged in the vicinity of the work rolls 11gw, 11gw are arranged so as to approach the steel plate 1 as the distance to the work rolls 11gw, 11gw approaches. In addition, the nozzles 21a, 21a,... In the vicinity of the work roll 11gwu are arranged so that the angle (injection angle) with respect to the upper surface of the steel plate 1 of the injected high-pressure jet water becomes smaller as the work roll 11gwu is approached. . Similarly, the nozzles 22a, 22a,... In the vicinity of the work roll 11gwd are arranged such that the angle (injection angle) with respect to the lower surface of the steel plate 1 of the injected high-pressure jet water becomes smaller as the work roll 11gwd gets closer. .

  In the cooling device 20, between the nozzles 21a, 21a,... And the upper surface of the steel plate 1, there are provided upper surface guides 23, 23 for preventing the nozzles 21a, 21a,. .. And the lower surface of the steel plate 1 are provided with lower surface guides 24 and 24 for preventing a collision between the nozzles 22a, 22a,. The cooling device 20 includes a header 21 and an upper surface guide 23 that are provided in proximity to the work roll 11gwu of the final stand 11g, and a header 22 and a lower surface that are provided in proximity to the work roll 11gwd of the final stand 11g. The guide 24 is integrally formed. Therefore, for example, when replacing the work rolls 11gw and 11gw of the final stand, as shown in FIG. 3, the header 21 is moved together with the upper surface guide 23 provided close to the work roll 11gwu of the final stand, and The header 22 can be moved together with the lower surface guide 24 provided in the vicinity of the work roll 11gwd of the final stand, so that the chock (not shown) on the driving side (the back side in FIG. 3) comes out to the operation side. Space is available and roll replacement work is possible.

As shown in FIGS. 2 and 4, when the steel plate 1 is rapidly cooled using the cooling device 20, the collision area of the high-pressure jet injected from the nozzle 21a is reduced below the position corresponding to the work roll radius of the final stand 11g. It reaches the point side region, and the collision area of the high-pressure jet injected from the nozzle 22a reaches the region on the reduction point side from the position corresponding to the work roll radius of the final stand 11g. Further, as shown in FIG. 2 and FIG. 3, the cooling device 20 includes headers 21, 21,..., And a steel plate to which a plurality of nozzles 21a, 21a,. .., To which a plurality of nozzles 22a, 22a,... Arranged at a predetermined pitch in the width direction are connected, are arranged at a predetermined pitch in the conveying direction of the steel sheet. Therefore, by using the cooling device 20, high-pressure jet water is continuously applied to the upper surface and the lower surface of the steel plate 1 in the section from the position corresponding to the work roll radius of the final stand 11g to the exit side of the housing post 11gh of the final stand. Can be injected. 2 and 4, the average cooling rate V1 of the steel sheet 1 located on the side of the reduction point relative to the work roll radius equivalent position of the final stand 11g is equal to the work roll radius equivalent position of the final stand 11g. High-pressure jet water is sprayed toward the steel plate 1 so as to be equal to the average cooling rate V2 of the steel plate 1 located between the housing stand 11gh of the final stand 11g.

  On the other hand, as shown in FIG. 5, the collision pressure average value P1 of the cooling water sprayed from the nozzle 21a to the region closer to the reduction point than the work roll radius equivalent position X1 of the final stand 11g is from the nozzle 21a to the final stand 11g. This is equal to the collision pressure average value P2 of the cooling water injected between the work roll radius equivalent position X1 and the housing post exit side X2 of the final stand 11g. As will be described later, there is a correlation between the average collision pressure value of the cooling water and the average cooling rate of the steel sheet, and by increasing the average collision pressure value of the cooling water, the average cooling speed of the steel sheet can be increased. It becomes possible. Therefore, according to the cooling device 20, not only the upper and lower surfaces of the steel plate existing between the position corresponding to the work roll radius of the final stand 11g and the outer surface of the housing post 11gh of the final stand 11g, but also the work roll of the final stand 11g. The upper surface and the lower surface of the steel plate 1 existing within the position corresponding to the radius can also be rapidly cooled using high-pressure jet water. In addition, as described above, in the cooling device 20, the nozzles 21a, 21a,... In the vicinity of the work roll 11gwu have an angle (injection angle) with respect to the upper surface of the steel plate 1 that is injected as the nozzle 21a, 21a,. ) Is reduced (that is, the inclination angle in the vertical plane is increased), and the nozzles 22a, 22a,... In the vicinity of the work roll 11gwd are ejected as high pressure jet water is closer to the work roll 11gwd. Are arranged so that the angle (injection angle) with respect to the lower surface of the steel plate 1 becomes smaller (that is, the vertical in-plane inclination angle becomes larger). Therefore, the high-pressure jet water that has collided with the upper surface and the lower surface of the steel sheet 1 existing within the position corresponding to the work roll radius of the final stand 11g proceeds to the work rolls 11gwu and 11gwd, and collides with the work rolls 11gwu and 11gwd. Thus, a jet is generated in the vicinity of the work rolls 11gwu and 11gwd (within the position corresponding to the work roll radius of the final stand). Here, when jets are generated on the upper and lower surfaces of the steel plate 1, the pressure of the jet water that collides with the upper and lower surfaces of the steel plate 1 can be increased. Therefore, according to the cooling device 20, high-pressure jet water is sprayed on the upper and lower surfaces of the steel plate 1 existing within the position corresponding to the work roll radius of the final stand 11g, and the upper and lower surfaces of the steel plate 1 collide. Since the subsequent high-pressure jet water generates a jet, the steel plate 1 immediately after passing through the reduction point can be rapidly cooled. Of course, P1 can also be increased by increasing the feed water pressure of the high-pressure spray that collides with the work roll radius, or by changing the nozzle type. That is, by setting the cooling device 20 in such a form, it is possible to cool the upper surface and the lower surface of the steel plate 1 that has passed the reduction point faster, stronger, and continuously. Therefore, according to the present invention, it is possible to provide a cooling device 20 capable of producing ultrafine-grained steel. In addition, even if stagnant water exists on the surface of the steel plate 1, the high pressure jet water can penetrate the boiling film on the steel plate surface. It becomes possible to boil cool (rapidly cool).

  FIG. 6 is a diagram showing the relationship between the average value of the steel plate surface collision pressure of the high-pressure jet water and the average cooling rate of the steel plate. The vertical axis in FIG. 6 is the average cooling rate [° C./s] when cooling the temperature of a steel plate having a thickness of 3 mm with no cooling water on the surface from both sides (upper surface and lower surface) from 750 ° C. to 600 ° C. Yes, the horizontal axis of FIG. 6 is the average value [kPa] in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water. As shown in FIG. 6, there is a correlation between the average value of the steel plate surface collision pressure of the high-pressure jet water and the average cooling rate of the steel plate, and the average value of the high-pressure water jet surface collision pressure of the steel plate surface When the is increased, the average cooling rate of the steel sheet can be increased. In addition, as shown in FIG. 7, the average value of the high-pressure jet water collision surface pressure in the steel plate direction is A, the nozzle pitch in the steel plate width direction of the cooling water reaching the steel plate surface, and the nozzle pitch in the steel plate conveyance direction. , Where B is a nozzle derived by dividing the cooling water force (collision force) that collides with the quadrilateral area represented by A × B by the area A × B of the quadrilateral area. The average collision pressure per piece is averaged in the section in the transport direction.

  In the present invention, the vertical component of the steel plate conveying direction average value of the steel plate surface collision pressure of the high-pressure jet water sprayed from the cooling device 20 to the steel plate 1 is 2.7 kPa or more. From the standpoint of making the steel plate 1 easy to cool while suppressing the recovery of austenite grains, etc., it is preferably 3.5 kPa or more. Moreover, in this invention, it is preferable to quench the steel plate 1 rapidly with an average cooling rate of 1000 degrees C / s or more from a viewpoint of making a crystal grain into the form which can be refined | miniaturized. In the present invention, it is more preferable to set the average collision pressure value of the cooling water to 8 kPa or more from the viewpoint of making the steel sheet 1 capable of being rapidly cooled at an average cooling rate of 1000 ° C./s or more. The cooling rate varies depending on the plate thickness, and is approximately inversely proportional to the plate thickness. If the hot-rolled steel sheet cooling device of the present invention has the ability to rapidly cool a steel sheet having a thickness of 3 mm at an average cooling rate of 1000 ° C./s, an average cooling of a steel sheet having a thickness of 5 mm is 600 ° C./s. It becomes possible to rapidly cool at a speed.

As described above, the average collision pressure per nozzle is equal to the collision force of the high-pressure jet water ejected from the nozzle divided by the cooling area of the nozzle. Therefore, even if the collision force is measured instead of measuring the pressure, the collision pressure average value of the cooling water can be calculated, and the collision force of the high-pressure jet water can be obtained from its flow rate and flow velocity. Since the flow velocity depends on the water supply pressure to the nozzle, the average value of the steel plate surface collision pressure can be estimated from the water supply pressure to the nozzle if a predetermined pressure loss is expected. An example of a method of calculating the steel plate surface collision pressure average value is described below.
Steel plate surface collision pressure average value Ps = F / (A · B) [Pa]
Here, A is the steel plate width direction nozzle pitch [m], B is the transport direction nozzle pitch [m], and F is the impinging force [N] of the high-pressure jet water against the steel plate surface. The collision force F can be obtained by the following equation.
Collision force F = 44.7 · C · q · P ^0.5 [N]
Here, 44.7 is a constant [N ^0.5 s / m ² ] including the 0.5th power of water density, C is a loss factor (about 0.8 to 1.0), and q is a flat spray nozzle. The flow rate [m ³ / s], P is the feed water pressure [Pa]. The flow rate of the flat spray nozzle is determined in relation to the feed water pressure according to the nozzle type (characteristic).

  In the present invention, if the stagnant water exists on the surface of the steel plate, the pressure of the high-pressure jet water jetted from the nozzle 21a is reduced by the stagnant water, and the collision pressure of the high-pressure jet water when reaching the surface of the steel plate 1 is reduced. Easy to reduce. Therefore, it is preferable to reduce the accumulated water on the surface of the steel plate 1 from the viewpoint of making the steel plate 1 easily cooled. From such a viewpoint, in the present invention, it is preferable that a space capable of discharging cooling water is secured between both end faces in the steel plate width direction of the cooling device 20 and both end faces in the steel plate width direction of the final stand 11g.

  In the above description regarding the cooling device 20 of the present invention, the nozzles 21a, 21a,... And the nozzles 22a, 22a,. The form arrange | positioned so that it may approach the steel plate 1 (namely, it becomes D1 <D2) was illustrated. By setting it as this form, in the cooling device 20, the angle with respect to the upper surface of the steel plate 1 of the high pressure jet water injected from the nozzles 21a, 21a,... Even if the angle of the high-pressure jet water jetted from the nozzles 22a, 22a,... With respect to the lower surface of the steel plate 1 is reduced, V1 = V2 and P1 = P2. The cooling device is not limited to this form. In the apparatus for cooling a hot-rolled steel sheet of the present invention, D1 = D2 can be set, and V1> V2 or P1> P2 can be set. In the cooling apparatus for hot-rolled steel sheets of the present invention, when high-pressure jet water is jetted from a nozzle in the vicinity of the work roll so that the angle (spray angle) with respect to the steel sheet surface becomes smaller as it approaches the work roll of the final stand. For example, the amount of water W1 per unit area of high-pressure jet water sprayed to the surface of the steel sheet existing within the position corresponding to the radius of the work roll of the final stand, and the position corresponding to the radius of the work roll of the final stand and the final By injecting the high-pressure jet water so that the amount of water W2 per unit area of the high-pressure jet water injected to the surface of the steel plate existing between the housing post exit side of the stand is W1 ≧ W2, V1 ≧ It is also possible to satisfy V2 or P1 ≧ P2.

  Moreover, in the said description regarding the cooling device 20 of this invention, the angle (injection angle) with respect to the upper surface of the steel plate 1 of the high pressure jet water injected from the nozzles 21a, 21a, ... near the work roll 11gwu as it approaches the work roll 11gwu. , And the closer to the work roll 11gwd, the smaller the angle (injection angle) of the high-pressure jet water jetted from the nozzles 22a, 22a,... However, the hot-rolled steel sheet cooling device of the present invention is not limited to this form. However, the radius of the work roll of the final stand is created by actively colliding the high pressure jet water sprayed within the position corresponding to the radius of the work roll of the final stand with the work roll to generate a jet near the reduction point. From the viewpoint of making the surface of the steel sheet existing within a corresponding position easy to cool rapidly, it is preferable to inject the high-pressure jet water so that the injection angle of the high-pressure jet water becomes smaller as it approaches the work roll.

  In the above description relating to the cooling device 20 of the present invention, the form in which the flat spray nozzles 21a, 21a, ..., 22a, 22a, ... are provided is illustrated, but the hot-rolled steel sheet cooling apparatus of the present invention is limited to this form. Instead of this, a so-called columnar nozzle may be provided. However, cooling in a form that reduces nozzle clogging and easily increases the vertical component of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of high-pressure jet water even when stagnant water is present on the surface. From the viewpoint of providing an apparatus and the like, it is preferable that a flat spray nozzle is provided. In addition, the flat spray nozzle can be devised in the form of arrangement so that directivity can be generated in the drainage of the cooling water present on the surface of the steel sheet, and thus the drainage can be improved.

FIG. 8 shows an example of the shape of a portion where the high-pressure jet water jetted from the nozzle collides with the steel plate surface, and a quadrilateral considered when deriving the vertical component of the average value of the steel plate surface collision pressure of the high-pressure jet water on the steel plate surface. It is a figure which shows the example of a shape of an area | region. FIG. 8A shows a case where the nozzle is a flat spray nozzle, and FIG. 8B shows a case where the nozzle is a columnar nozzle. In FIG. 8, the back / front direction of the paper is the thickness direction of the steel sheet. Moreover, the site | part colored in FIG. 8 represents the site | part which the high pressure jet water collided with the steel plate surface.
As shown to Fig.8 (a), when a nozzle is a flat spray nozzle, the site | part where a high pressure jet water collides with the steel plate surface becomes elliptical shape or oval shape. In this case, the area of the quadrilateral region (parallelogram region) considered when deriving the vertical component of the steel plate conveyance direction average value of the steel plate surface collision pressure of the high-pressure jet water is the nozzle pitch A in the plate width direction of the steel plate. And the nozzle pitch B in the conveying direction of the steel plate can be calculated. Moreover, as shown in FIG.8 (b), when a nozzle is a columnar nozzle, the site | part where high pressure jet water collides with the steel plate surface becomes circular shape. In this case, the area of the quadrilateral region (rectangular region) considered when deriving the vertical component of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water is the nozzle pitch A and the steel plate in the plate width direction of the steel plate. It can be calculated by multiplying by the nozzle pitch B in the transport direction.

  Moreover, in the said description regarding the cooling device 20 of this invention, not only the area to the housing post exit side of the last stand of a hot rolling mill row | line | column but a flat spray nozzle is arrange | positioned also in the area | region downstream from the said area. However, the present invention is not limited to this form. However, it may be assumed that the steel sheet is required to be rapidly cooled to a temperature lower than 720 ° C. within a short time after the end of rolling. Therefore, from the viewpoint of making it possible to provide a cooling device that can rapidly cool the steel sheet to a temperature lower than 720 ° C., the section to the housing post exit side of the final stand of the hot rolling mill row, and the section It is preferable that the flat spray nozzle is continuously arranged in the downstream region.

  Further, in the above description regarding the cooling device 20 of the present invention, the header 21 and the upper surface guide 23 disposed on the upper surface side of the steel plate 1 are integrally configured, and the header 22 and the lower surface disposed on the lower surface side of the steel plate 1. Although the form comprised integrally with the guide 24 was illustrated, the cooling apparatus of the hot-rolled steel plate of this invention is not limited to the said form. In the cooling apparatus for hot-rolled steel sheets of the present invention, the header and the lower surface guide disposed on the lower surface side of the steel sheet are not integrally configured, and the header and the upper surface guide disposed on the upper surface side of the steel sheet are not integrally configured. It is also possible to adopt a form. In order to make it possible to replace the roll provided in the final stand of the hot rolling mill row, a header 21 arranged close to the work roll 11gwu, an upper surface guide 23, a header 22 arranged close to the work roll 11gwd, and The lower surface guide 24 only needs to be configured to be movable, and these can be moved using a known means such as a hydraulic cylinder. However, it is preferable that the header and the upper surface guide disposed on the upper surface side of the steel plate are simultaneously retracted or returned from the viewpoint of making the form easy to improve the work efficiency of the roll exchange, and are configured integrally for that purpose. It is preferable. Similarly, it is preferable that the header disposed on the lower surface side of the steel plate and the lower surface guide are configured integrally.

  Moreover, in the said description regarding the cooling device 20 of this invention, the some header 21,21, ... to which the some nozzle 21a, 21a, ... arrange | positioned by the predetermined pitch to the width direction of the steel plate 1 was connected is the steel plate 1. A plurality of headers 22, 22,... Connected to a plurality of nozzles 22 a, 22 a,... Arranged at a predetermined pitch in the conveyance direction and arranged at a predetermined pitch in the width direction of the steel plate 1 are the conveyance direction of the steel plate 1. However, the hot-rolled steel sheet cooling device of the present invention is not limited to this form. The cooling device of the present invention is a header configured to be able to collectively supply cooling water to a plurality of nozzles arranged at a predetermined pitch in the width direction of the steel plate and the conveying direction of the steel plate (hereinafter referred to as “collective header”). Can be arranged on the upper surface side and / or the lower surface side of the steel plate. FIG. 9 shows an example of a form of the hot-rolled steel sheet cooling device of the present invention provided with a collective header. FIG. 9 is a diagram for explaining an example of a cooling apparatus for hot-rolled steel sheets provided with a collective header. FIG. 9 shows the position corresponding to the radius of the work roll of the final stand and the housing post exit side of the final stand. In addition, the concept of the vertical component of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water is also shown. 9, components similar to those of the manufacturing apparatus 10 and the cooling apparatus 20 are denoted by the same reference numerals as those used in FIG. 4, and description thereof is omitted as appropriate.

  As shown in FIG. 9, the hot-rolled steel sheet cooling device 30 of the present invention (hereinafter, sometimes simply referred to as “cooling device 30”) is provided on the upper surface side of the steel plate 1 on the most upstream side of the steel plate conveying direction. A collective header 31 configured to be able to supply cooling water to the flat spray nozzles 31a, 31a,... (Hereinafter, simply referred to as “nozzle 31a”, etc.) constituting the flat spray nozzle row of the row. The flat spray nozzles 32a, 32a,... (Hereinafter, simply referred to as “nozzle 32a” or the like constituting the three flat spray nozzle rows on the uppermost stream side in the steel plate conveyance direction are also provided on the lower surface side of the steel plate 1. It is configured in the same manner as the cooling device 20 except that a collective header 32 configured to be able to collectively supply cooling water is provided. Two rows of nozzles 31a, 31a connected to the collective header 31 from the most upstream side in the conveying direction of the steel plate 1 are connected to be able to inject high-pressure jet water obliquely toward the upstream side in the conveying direction of the steel plate 1. The two rows of nozzles 32a, 32a connected to the collective header 32 from the most upstream side in the conveying direction of the steel plate 1 are connected so that high-pressure jet water can be injected obliquely toward the upstream side in the conveying direction of the steel plate 1. Has been. In the cooling device 30, the vertical in-plane inclination angles of the nozzles 31 a and 32 a arranged on the most upstream side in the conveyance direction of the steel plate 1 are the same as the nozzles 31 a and 32 a adjacent to the nozzles 31 a and 32 a and the downstream side in the conveyance direction of the steel plate 1. It is set to be equal to or greater than the given vertical in-plane tilt angle. Moreover, the high-pressure jet water sprayed from the nozzles 31a and 32a arranged on the most upstream side in the transport direction of the steel plate 1 reaches a region closer to the reduction point than the position corresponding to the radius of the work roll of the final stand. Therefore, even with such a cooling device 30, it is possible to produce ultrafine-grained steel as with the cooling device 20.

  Thus, by using the cooling devices 20 and 30 of the present invention, it becomes possible to produce ultrafine-grained steel. Therefore, it becomes possible to manufacture ultrafine-grained steel by using a manufacturing apparatus 10 including the cooling device 20 and a manufacturing apparatus for hot-rolled steel sheets including the cooling device 30. Furthermore, by setting it as the form which has the process of processing the steel plate rolled by the last stand in a hot finishing rolling mill row | line | column using the manufacturing apparatus and manufacturing apparatus 10 of a hot-rolled steel plate provided with the cooling device 30, ultra-fine grain steel It becomes possible to provide the manufacturing method of a hot-rolled steel plate which can be manufactured.

  In the present invention, the distance between the nozzle disposed on the upper surface side of the steel plate and the upper surface of the steel plate is not particularly limited, but the collision pressure average value of the cooling water is increased by bringing the nozzle closer to the steel plate surface. This makes it easy to cool the steel sheet. Therefore, in the present invention, it is preferable that the distance between the nozzle surface (jetting surface of high-pressure jet water) facing the steel plate and the steel plate surface is less than 500 mm from the viewpoint of making the steel plate easily cooled. More preferably, it is 350 mm or less.

  Moreover, in the said description, although the form in which the vertical in-plane inclination angle was provided to the nozzle arrange | positioned in the steel plate conveyance direction upstream was illustrated, this invention is not limited to the said form. However, by giving a vertical in-plane inclination angle to one or two or more nozzle rows including the nozzle row arranged at the position closest to the work roll of the final stand, in particular in the steel sheet conveying direction, Within the position corresponding to the radius of the work roll of the stand, it becomes easy to make the high-pressure jet water collide with the upper surface and the lower surface of the steel plate positioned closer to the roll bite, and it becomes easy to rapidly cool the steel plate after rolling. Therefore, from the viewpoint of making the steel sheet easy to cool rapidly, etc., one or more rows including nozzle rows arranged at the position closest to the work roll of the final stand (the uppermost stream side in the steel plate conveyance direction) It is preferable to give a vertical in-plane inclination angle to the nozzle rows (nozzle rows arranged on the upper surface side and the lower surface side of the steel plate), and the more the nozzle arranged on the upstream side in the steel plate conveyance direction, the more the vertical in-plane inclination angle. Is preferably increased. Furthermore, from the viewpoint of making the steel plate easier to cool, a vertical in-plane inclination angle is given to the nozzle row arranged on the most upstream side in the steel plate conveyance direction, and the steel plate is arranged on the most upstream side in the steel plate conveyance direction. More preferably, the distance between the surface of the nozzle row (jetting surface of high-pressure jet water) and the surface of the steel sheet is minimized.

  In the above description, the high-pressure jet water is continuously collided with the steel plate at least in the region from the position corresponding to the radius of the work roll of the final stand in the hot finish rolling mill row to the housing post exit side of the final stand. Thus, the present invention in a form in which the steel sheet is rapidly cooled immediately after passing through the reduction point is mentioned, but the present invention is not limited to this form. In the present invention, if it is possible to cool the steel sheet to 720 ° C. or less within 0.2 seconds after passing through the reduction point, there is a section in the stand area where the high-pressure jet water does not continuously collide with the steel sheet. May be. If there is a part in the stand area where rapid cooling is difficult (the section where high-pressure jet water does not continuously collide with the steel sheet), the cooling rate in the stand area excluding that part is increased, and the stand area By securing the required average cooling rate in step, the steel sheet may be cooled to 720 ° C. or less within 0.2 seconds after passing the reduction point. Examples of the portion in the stand region where it is difficult to perform rapid cooling can include a section between the roll bite position shown in FIG. 4 and the upstream end of the continuous cooling range in the steel sheet conveyance direction. . In addition, for example, as in the case of a hot-rolled steel sheet cooling device 20 ′ shown in FIG. 10, the transport roll 12 is also disposed on the steel sheet lower surface side between the reduction point and the housing post exit side of the final stand. In addition, the portion on the lower surface side of the steel plate where the high pressure jet water is prevented from colliding by the transport roll 12 is also a portion where it is difficult to perform rapid cooling. Even if the cooling device 20 ′ is used, it is possible to form ultrafine grains by cooling the steel sheet to 720 ° C. or less within 0.2 seconds after passing through the reduction point. Therefore, it becomes possible to manufacture ultrafine-grained steel by using a hot-rolled steel sheet manufacturing apparatus provided with a cooling device 20 'and through a cooling process by the cooling device 20'. Furthermore, an ultra fine grain steel is manufactured by having the process which has the process of processing the steel plate rolled by the last stand in a hot finishing rolling mill row | line | column using the manufacturing apparatus of a hot rolled steel plate provided with cooling device 20 '. It is possible to provide a method for manufacturing a hot-rolled steel sheet.

  Using a rolling mill with a roll diameter of 700 mm (radius 350 mm) and a distance from the reduction point to the housing post exit side of 1800 mm, a steel plate containing 0.1% by mass C and 1% by mass Mn was A test was conducted in which the sheet was rolled at a delivery speed of 600 mpm so that the thickness was 3 mm, and then rapidly cooled. The rolling end temperature was 820 ° C., and the rapid cooling start position was 100 mm downstream from the reduction point. The average collision pressure P1 of the cooling spray from the rapid cooling start position to the roll radius equivalent position 350 mm and the average collision pressure P2 from the cooling post to the side of the housing post 1800 mm are changed, and the finally obtained ferrite particle diameter is compared and investigated. did. In addition, when it was not able to cool to 720 degreeC in the area to the housing post exit side, it cooled using the cooling device which followed a housing post exit side. The cooling water supply pressure was 1.5 MPa at the cooling header. The results are shown in Table 2. Condition No. Examples 1 to 6 are examples (examples of the present invention), and cooling speed V1 ≧ V2, average collision pressure P1 ≧ P2, and flow rate W1 ≧ W2 per unit area. Condition No. 7 to 8 are comparative examples, and V1 <V2, P1 <P2, and W1 <W2. In Table 2, the collision pressure average value of the cooling water is described as “collision pressure”, and the ferrite particle diameter is described as “particle diameter”. In Table 2, “height D1” is the distance between the high-pressure jet water injection port of the nozzle disposed at the position closest to the roll bite position and the steel plate, and “height D2” is the housing post exit side This is the distance between the high-pressure jet water injection port of the nozzle arranged at the position closest to and the steel plate. In Table 1, “X to Ymm section” refers to a section whose distance from the reduction point is Xmm to Ymm.

As shown in Table 2, condition no. In 1-6, it was V1> = V2 and the cooling rate was 400 degrees C / s or more as an average value in the whole cooling region whose distance from a reduction point is 100 mm-1800 mm. Therefore, Condition No. In 1 to 6, an ultrafine grain structure having a ferrite grain size of 2 μm or less was obtained. In addition, Condition No. 1 to 6 were P1 ≧ P2, and the collision pressure (average collision pressure) was 2.7 kPa or more as an average value in the entire cooling region where the distance from the reduction point was 100 mm to 1800 mm. Furthermore, condition no. In 1-6, a W1 ≧ W2, the distance from the pressure point as an average over the full cooling zone of 100Mm～1800mm, flow density was ^{2.8m 3 / (m 2 · min} ) or more. On the other hand, Condition No. In No. 7, since V1 <V2, P1 <P2, and W1 <W2, the ferrite grain size was larger than 2 μm. In addition, Condition No. 8, V1 ≧ V2, and P1 ≧ P2 and W1 ≧ W2, but the average value (Vm) in the entire cooling region of the cooling rate does not satisfy the lower limit (400 ° C./s), The average value (Pm) of the collision pressure in the entire cooling region did not satisfy the lower limit (2.7 kPa), and the ferrite particle size was larger than 2 μm. In addition, the condition No. 1 and condition no. No. 4, the same ferrite grain size was obtained, but the condition No. 4 where the difference between V1 and V2 was large and the difference between P1 and P2 was large. In the case of No. 4, the average value of the flow density was small and the amount of cooling water was small. That is, the condition No. 4 is condition No. The use efficiency of cooling water was higher than 1. Similarly, condition no. 3 and condition no. 6 had the same ferrite grain size, but the difference between V1 and V2 was large and the difference between P1 and P2 was large. In the case of No. 6, the average value of the flow density was small and the amount of cooling water was small. That is, the condition No. 6 is condition No. The use efficiency of cooling water was higher than 3. From the above, it has been confirmed that the grain refinement is promoted by setting V1 ≧ V2 and P1 ≧ P2, and that the cooling water use efficiency is improved by increasing the difference between V1 and V2 and the difference between P1 and P2. It was done.

  Although the present invention has been described in connection with embodiments that are presently practical and preferred, the present invention is not limited to the embodiments disclosed herein, but is claimed. Can be appropriately changed without departing from the scope or spirit of the invention that can be read from the entire specification and the specification, and a hot-rolled steel sheet cooling apparatus, a hot-rolled steel sheet cooling method, and a hot-rolled steel sheet manufacturing apparatus with such changes And the manufacturing method of a hot-rolled steel sheet should also be understood as being included in the technical scope of the present invention.

  The apparatus for cooling a hot-rolled steel sheet, the method for cooling a hot-rolled steel sheet, the apparatus for manufacturing a hot-rolled steel sheet, and the method for manufacturing a hot-rolled steel sheet according to the present invention can be used for manufacturing a hot-rolled steel sheet having ultrafine crystal grains. . Moreover, the hot-rolled steel sheet having ultrafine crystal grains can be used as a material used for applications such as automobiles, household appliances, machine structures, and buildings.

Claims (16)

Heat provided with a header having a plurality of nozzles arranged on the downstream side in the final stand in the hot finish rolling mill, and provided with a plurality of nozzles capable of injecting high-pressure jet water toward the surface of the steel sheet conveyed through the pass line. A cooling device for rolled steel sheet,
The nozzle disposed at least on the most upstream side in the conveying direction of the steel sheet is perpendicular to the vertical surface of the steel sheet so that the high-pressure jet water can be injected obliquely toward the upstream side in the conveying direction of the steel sheet. Arranged so as to intersect
The average cooling rate of the steel sheet cooled by the high-pressure jet water colliding with the surface of the steel sheet existing within the position corresponding to the radius of the work roll of the final stand is V1, the position corresponding to the radius of the work roll of the final stand and the position When the average cooling rate of the steel sheet cooled by the high-pressure jet water colliding with the surface of the steel sheet existing between the housing post exit side of the final stand is V2, V1 ≧ V2, and the The average cooling rate Vm of the surface of the steel sheet cooled by the high-pressure jet water colliding with the surface of the steel sheet existing between the cooling start point in the final stand and the housing post exit side of the final stand is 400 ° C. / S or more, The cooling apparatus of the hot-rolled steel plate characterized by the above-mentioned. Heat provided with a header having a plurality of nozzles arranged on the downstream side in the final stand in the hot finish rolling mill, and provided with a plurality of nozzles capable of injecting high-pressure jet water toward the surface of the steel sheet conveyed through the pass line. A cooling device for rolled steel sheet,
The nozzle disposed at least on the most upstream side in the conveying direction of the steel sheet is perpendicular to the vertical surface of the steel sheet so that the high-pressure jet water can be injected obliquely toward the upstream side in the conveying direction of the steel sheet. Arranged so as to intersect
The vertical component of the average value of the steel plate surface impinging pressure of the high-pressure jet water colliding with the surface of the steel sheet existing within the position corresponding to the radius of the work roll of the final stand is P1, and the work roll of the final stand P2 represents a vertical component of an average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water that has collided with the surface of the steel plate existing between the position corresponding to the radius of the steel plate and the housing post exit side of the final stand. When P1 ≧ P2, and the steel plate surface of the high-pressure jet water colliding with the surface of the steel plate existing between the cooling start point in the final stand and the housing post exit side of the final stand The apparatus for cooling a hot-rolled steel sheet, wherein the vertical component Pm of the average value of the impinging pressure in the steel sheet conveying direction is 2.7 kPa or more. The amount of water per unit area of the high-pressure jet water sprayed onto the surface of the steel sheet existing within the position corresponding to the radius of the work roll of the final stand is W1, the position corresponding to the radius of the work roll of the final stand and the final stand when the amount of water per unit area of the high pressure jet water jetted to the surface of the steel sheet present between the housing post exit side W2, and characterized in that it is a W1 ≧ W2, claim The cooling apparatus for hot-rolled steel sheets according to 1 or 2. The distance between the high-pressure jet water injection port of the nozzle disposed at the position closest to the work roll of the final stand and the steel plate is D1, and the nozzle of the nozzle disposed at the position closest to the housing post exit side of the final stand. when the distance between the high-pressure jet water injection port and the steel sheet D2, and characterized in that it is a D1 ≦ D2, the cooling device for a hot-rolled steel sheet according to any one of claims 1 to 3. In the section from the position corresponding to the radius of the work roll of the final stand to the housing post exit side of the final stand, continuous high-pressure jet water from the nozzle in the conveying direction of the steel plate toward the upper and lower surfaces of the steel plate It jettable be characterized by being composed, the cooling device for a hot-rolled steel sheet according to any one of claims 1 to 4. In the section from the position corresponding to the radius of the work roll of the final stand to the housing post exit side of the final stand, the vertical component of the average value in the steel plate conveyance direction of the steel plate surface collision pressure of the high-pressure jet water is the upper surface and the lower surface. The apparatus for cooling a hot-rolled steel sheet according to any one of claims 1 to 5, wherein the temperature is 3.5 kPa or more.
The nozzle, characterized in that it is a flat spray nozzle, the cooling device for a hot-rolled steel sheet according to any one of claims 1 to 6. Between the steel sheet width direction both end faces of the final stand and the steel plate width direction both end surfaces of said cooling apparatus, characterized in that the space capable of discharging the cooling water is ensured, any one of claims 1 to 7 The cooling apparatus for hot-rolled steel sheets according to item 1. Said header and said nozzle is provided on the upper surface of the steel sheet, and the upper surface guide is provided between the between the nozzle pass line, characterized in that it is integrally constructed, claim 1 cooling device for a hot-rolled steel sheet according to any one of 1-8. Said header and said nozzle is provided on the lower surface side of the steel plate, and a lower surface guide is provided between the between the nozzle pass line, characterized in that it is integrally constructed, claim 1 cooling device for a hot-rolled steel sheet according to any one of 1-9. A plurality of the headers are provided, and at least a part of the headers is configured to be able to collectively supply cooling water to the nozzles arranged in a plurality of rows in each of the conveying direction of the steel plate and the width direction of the steel plate. wherein the cooling device for a hot-rolled steel sheet according to any one of claims 1 to 10. A plurality of the headers are arranged on the upper surface side of the steel plate,
Among the headers provided on the upper surface side of the steel plate, at least the headers arranged on the most upstream side in the conveying direction of the steel plate are arranged in a plurality of rows in each of the conveying direction of the steel plate and the width direction of the steel plate. It has been characterized by a cooling water to the nozzle which is the header is configured to be bulk supply, cooling device for a hot-rolled steel sheet according to claim 11. A plurality of the headers are arranged on the lower surface side of the steel plate,
Among the headers provided on the lower surface side of the steel plate, at least the headers arranged on the most upstream side in the conveying direction of the steel plate are arranged in a plurality of rows in each of the conveying direction of the steel plate and the width direction of the steel plate. It has been characterized by a cooling water to the nozzle which is the header is configured to be bulk supply, cooling device for a hot-rolled steel sheet according to claim 11 or 12. Characterized by cooling the steel plate using a cooling apparatus of hot-rolled steel sheet according to any one of claims 1 to 13, the cooling method of hot rolled steel sheet. A final stand in the finish hot rolling mill train, a cooling device for a hot-rolled steel sheet according to any one of claims 1 to 13, characterized in that it comprises in order in the conveyance direction of the steel sheet, the hot rolled steel sheet manufacturing device. By using the manufacturing apparatus of hot-rolled steel sheet according to claim 15, characterized in that it comprises the step of treating the rolled steel sheet in the final stand in the finish hot rolling mill train, a manufacturing method of hot-rolled steel sheet.

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