THICK STEEL PLATE MANUFACTURING METHOD

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a steel plate in which hot rolling, shape correction, and accelerated cooling are performed.

Background Art

[0002] In recent years, controlled cooling has been increasingly applied as a manufacturing process of steel plate. However, in general, hot-rolled steel plates do not necessarily have uniformity in shapes, surface properties and the like. Therefore, temperature non-uniformity tends to occur in the steel plates during cooling, as a result of which, for example, deformation, residual stress, and non-uniformity in material quality occur in the steel plates after the cooling, thereby resulting in poor quality and operational problems.

[0003] Therefore, Patent Literature 1 discloses a method in which descaling immediately before and/or immediately after a last pass of finish rolling, hot correction, descaling forced cooling are performed in this order. In addition, Patent Literature 2 discloses a method in which descaling is performed after finish rolling and hot shape correction, and forced cooling is performed thereafter. Further, Patent Literature 3 discloses a method in which, descaling is performed immediately before controlled cooling with controlling impact pressure of cooling water. Further, Patent Literature 4 discloses a steel plate manufacturing facility comprising a hot rolling mill, a hot leveler, a descaler and cooling equipment, wherein a pressure at the point of impact of cooling water sprayed from the descaler to each surface of the steel plate is greater than or equal to 1.5 MPa. However, Patent Literature 4 does not disclose the energy density of the descaling water.

Citation List

Patent Literature

[0004] 

PTL 1: Japanese Unexamined Patent Application Publication No. 9-57327

PTL 2: Japanese Patent No. 3796133

PTL 3: Japanese Unexamined Patent Application Publication No. 2010-247228

PTL 4: EP 2 412 455 A1

Summary of Invention

Technical Problem

[0005] However, when a steel plate is actually manufactured by the aforementioned methods of Patent Literatures 1 and 2, scales are not completely peeled off in the descaling. Rather, scale non-uniformity occurs in which the scales are partly peeled off by the descaling. Therefore, uniform cooling cannot be performed in controlled cooling. In the method in Patent Literature 3, high impact pressure is required to prevent scale non-uniformity. Therefore, low impact pressure causes scale non-uniformity, as a result of which uniform cooling cannot be performed in controlled cooling.

[0006] In particular, in recent years, steel plates are required to have strict levels of uniformity in material quality. Therefore, adverse effects of the non-uniformity of cooling speed in controlled cooling, which is caused by the above-described scale non-uniformity, in particular, on uniformity in material quality in a width direction of steel plate are no longer negligible.

[0007] The present invention is made in view of the aforementioned problems that are not solved by the prior art. It is an object of the present invention to provide a method for manufacturing steel plate having excellent shapes and excellent mechanical properties, by performing uniform cooling in a cooling step by uniformizing scales that are generated on surfaces of the steel plate uniform in a descaling step.

Solution to Problem

[0008] The inventors carried out assiduous studies regarding forces that cause scales to be peeled off by using descaling water, and found out that, when descaling is performed after hot shape correction, if two or more rows of jetting nozzles of descaling apparatus are set in a longitudinal direction of the steel plate, and if the energy density of the descaling water that is jetted to the steel plate from the two or more rows of jetting nozzles is greater than or equal to 0.08 J/mm² in total, the thicknesses of scales that are generated on product surfaces become uniform. As a result, when the steel plate passes through an accelerated cooling apparatus, the steel plate can be uniformly cooled almost without variations in surface temperatures at locations on the steel plate in a width direction thereof, to have excellent shapes.

[0009] The invention provides a method for manufacturing a steel plate according to the appended claims.

Advantageous Effects of Invention

[0010] According to the present invention, it is possible to manufacture steel plates having excellent shapes and excellent mechanical properties by performing uniform cooling in the cooling step as a result of uniformizing scales that are generated on surfaces of the thick steel sheets in the descaling step.

Brief Description of Drawings

[0011] 

[Fig. 1] Fig. 1 is a schematic view of a facility for manufacturing a steel plate according to an embodiment of the method of the present invention.

[Fig. 2] Fig. 2 illustrates temperature distribution of a steel plate in a width direction thereof in a prior art.

[Fig. 3] Fig. 3 is a graph showing, in descaling apparatus, the relationship between energy density of descaling water that is jetted and scale thickness at surfaces of a steel plate product.

[Fig. 4] Fig. 4 shows the relationship between jetting-nozzle jetting distance and fluid velocity in the descaling apparatus.

[Fig. 5] Fig. 5 shows a surface temperature distribution of locations on a steel plate according to the present invention in a width direction thereof.

[Fig. 6] Fig. 6 is a schematic view showing the arrangement relationship of jetting nozzles of the descaling apparatus, with Fig. 6(a) being a schematic view showing the relationship between the positions of jetting nozzles and Fig. 6(b) being a schematic view of a spray pattern.

[Fig. 7] Fig. 7 is a side view of an accelerated cooling apparatus.

[Fig. 8] Fig. 8 is a side view of another accelerated cooling apparatus.

[Fig. 9] Fig. 9 illustrates an exemplary nozzle arrangement at a partition wall.

[Fig. 10] Fig. 10 illustrates flow of drainage cooling water along an upper side of the partition wall.

[Fig. 11] Fig. 11 illustrates another flow of drainage cooling water along the upper side of the partition wall.

[Fig. 12] Fig. 12 illustrates flow of cooling water in the accelerated cooling apparatus.

[Fig. 13] Fig. 13 illustrates flow of cooling water in the accelerated cooling apparatus.

[Fig. 14] Fig. 14 illustrates non-interference with respect to drainage cooling water along the upper side of the partition wall in the accelerated cooling apparatus. Description of Embodiment

[0012] An embodiment according to the method of the present invention is described below with reference to the drawings.

[0013] Fig. 1 is a schematic view of a facility for manufacturing a steel plate according to an embodiment of the method the present invention. In Fig. 1, the direction of an arrow corresponds to a conveyance direction of the steel plate. From an upstream side in the conveyance direction of the steel plate, a heating furnace 1, a descaling apparatus 2, a rolling apparatus 3, a shape correcting apparatus 4, a descaling apparatus 6, a descaling apparatus 7, and an accelerated cooling apparatus 5 are set in this order. In Fig. 1, after re-heating a slab (not shown), which is a rolling material, in the heating furnace 1, the slab is descaled for primary scale removal in the descaling apparatus 2. Then, the rolling apparatus 3 performs rough rolling and finish rolling on the slab, so that the slab is rolled to form a steel plate having a predetermined plate thickness (not shown). Only one rolling apparatus 3, which is illustrated, is used. The rolling apparatus 3 may include a rough rolling apparatus and a finish rolling apparatus. After the shape correcting apparatus 4 has corrected the shape of the steel plate, the descaling apparatus 6 and the descaling apparatus 7 perform descaling for completely removing scale. Thereafter, the accelerated cooling apparatus 5 performs controlled cooling by water cooling or air cooling. Here, with regard to the shape of the steel plate after the cooling, it is suitable to perform accelerated cooling after adjusting the shape of the steel plate via the shape correcting apparatus 4. The shape correcting apparatus 4 corrects distortion of the steel plate that occurs during hot rolling. Fig. 1 shows the shape correcting apparatus of a roller leveler type, which compresses the steel plate by using shape correcting rollers disposed in a staggered arrangement in a vertical direction. The shape correcting apparatus is not limited to the roller leveler type. The shape correcting apparatus may be a skin pass type or a press type. When the rolling apparatus 3 includes a rough rolling machine and a finish rolling apparatus, skin pass correction may be performed by using the finish rolling apparatus.

[0014] In the accelerated cooling apparatus 5, the steel plate is cooled to a predetermined temperature by using cooling water that is jetted from an upper surface cooling facility and a lower surface cooling facility. Thereafter, if necessary, the shape of the steel plate is further corrected by using a shape correcting apparatus (not shown) provided on-line or off-line at a downstream side. This shape correcting apparatus corrects distortion of the steel plate that occurs during the cooling by the accelerated cooling apparatus 5. In the present invention, this shape correcting apparatus need not be used. This shape correcting apparatus may be a skin pass type or a press type in addition to a roller leveler type.

[0015] In the present embodiment, two sets of descaling apparatus, that is, the descaling apparatus 6 and the descaling apparatus 7 are set between the shape correcting apparatus 4 and the accelerated cooling apparatus 5. Energy density E of descaling water that is jetted to surfaces of the steel plate from the descaling apparatus 6 and the descaling apparatus 7 is greater than or equal to 0.08 J/mm² in total for the two rows of jetting nozzles. The descaling machine 6 and the descaling apparatus 7 remove scale generated on the surfaces of the steel plate, and then the accelerated cooling apparatus 5 cools the steel plate to make it possible to improve the shape and the mechanical properties of the steel plate. The descaling apparatus shown in Fig. 1 is formed in only two rows. Descaling apparatus may be formed in three or more rows. When descaling apparatus are formed in three or more rows, the energy density E of the descaling water that is jetted to the surfaces of the steel plate is greater than or equal to 0.08 J/mm² in total for the number of rows.

[0016] The reasons are as follows. In an existing rolling facility, when scales are removed by a descaling apparatus after shape correction, the scales may be partly removed. In this case, since the scales are not uniformly peeled off, variations in scale thickness of approximately 10 to 50 µm occurs. In this case, it is difficult to, thereafter, uniformly cool the steel plate by using the accelerated cooling apparatus. That is, when the steel plate having variations in a scale thickness distribution is subjected to accelerated cooling in an existing rolling facility, variations in surface temperature at locations in a width direction become large as shown in Fig. 2, thereby preventing uniform cooling. As a result, the shape of the steel plate is affected.

[0017] In relation to this, the inventors found out that, depending upon descaling conditions, scales are not sufficiently peeled off and scale non-uniformity is increased instead. In addition, the inventors carried out assiduous studies regarding the conditions that enable scales to be sufficiently peeled off. The result of the studies makes it clear that, when descaling is performed after shape correction, if the descaling apparatus is such as to be formed in two or more rows set in a longitudinal direction of the steel plate between the shape correcting apparatus and the accelerated cooling apparatus, and if the energy density E of the descaling water that is jetted to the surfaces of the steel plate from the two or more rows of jetting nozzles of the descaling apparatus is greater than or equal to 0.08 J/mm² in total for the two or more rows of jetting nozzles, the thickness of scale that is regenerated thereafter becomes uniform at 5 µm or less.

[0018] At the time of descaling, by cooling the scale surface with descaling water, thermal stress is generated in the scale, and impact force acts due to the descaling water. As a result, the scale is removed by peeling or destruction. The inventors carried out assiduous studies and found out that, by performing descaling two or more times after hot shape correction, the effects of thermal stress that is generated at the time of descaling can be provided two or more times. In addition, the inventors found out that, as shown in Fig. 3, the scale can be removed more efficiently when the descaling is performed two times than when the descaling is performed only one time. Further, the inventors found out that, if the energy density E of the descaling water that is jetted to the steel plate from the two rows of jetting nozzles of the descaling apparatus is greater than equal to 0.08 J/mm² in total, the scale thickness of the product is reduced and becomes uniform. The number of jettings shown in Fig. 3 is two. The inventors confirmed that even if the number of jettings is three or more, the same effects are obtained. This is because, by the descaling, scale is completely and uniformly peeled off once, and then, scale is uniformly and thinly regenerated. Therefore, according to the present invention, since the scale thickness of the steel plate before the steel plate passes through the accelerated cooling apparatus is small and uniform, when the steel plate passes through the accelerated cooling apparatus, the steel plate can be uniformly cooled almost without variations in surface temperatures of locations on the steel plate in the width direction thereof. Therefore, the steel plate has an excellent shape and excellent mechanical properties.

[0019] Here, the energy density E (J/mm²) of the descaling water that is jetted to the steel plate indicates the capability of removing scale by descaling, and is defined by the following Expression (1):

where Q: jetting flow rate [m³/s] of descaling water,

d: spray jet thickness [mm] of flat nozzle,

W: spray jet width [mm] of flat nozzle,

fluid density is denoted by ρ [kg/m³],

fluid velocity at the time of collision with steel plate is denoted by v [m/s],

collision time is denoted by t [s] (t = d/1000 V; conveyance velocity is denoted by V [m/s].

[0020] However, it is not necessarily easy to measure the fluid velocity v at the time of collision with the steel plate. Therefore, strictly determining the energy density E defined by Expression (1) is very difficult.

[0021] Accordingly, the inventors carried out further studies and found out that, as a simple definition of energy density E (J/mm²) of the descaling water that is jetted to the steel plate, the expression "(water amount density) × (jetting pressure) × (collision time)" may be used. Here, the water amount density (m³/(mm² · min)) is a value that is calculated by using "jetting flow rate of descaling water ÷ collision area of descaling water". The jetting pressure (N/m² (= Pa)) is defined by ejection pressure of descaling water. The collision time (s) is a value that is calculated by using "collision thickness of descaling water ÷ conveyance velocity of steel plate". In the present invention, the energy density E has no upper limit as descaling capability. When the energy density E becomes greater than or equal to 0.80 J/mm² in total for two or more rows of jetting nozzles, for example, the pump discharge pressure becomes extraordinarily high. Therefore, this is not desirable.

[0022] Next, the inventors carried out studies regarding the fluid velocity v of descaling water that is jetted from the jetting nozzles of the descaling apparatus 6 and the descaling apparatus 7. The inventors found out that the relationship between the fluid velocity v and the jetting distance is as shown in Fig. 4. The fluid velocity, which is indicated along the vertical axis, is determined by solving an equation of motion considering buoyancy and air resistance. The fluid velocity v of descaling water is reduced as the descaling water moves and reaches the steel plate during jetting. Therefore, the smaller the jetting distance, the higher the fluid velocity v at the time of collision with the steel plate, so that a large energy density can be provided. From Fig. 4, in particular, since attenuation becomes large as a jetting distance H exceeds 200 mm, it is desirable that the jetting distance H be less than or equal to 200 mm.

[0023] The shorter the jetting distance, the smaller the jetting pressure, the jetting flow rate, etc., for providing a predetermined energy density can be made, so that it is possible to reduce the pumping power of the descaling apparatus 6 and the descaling apparatus 7. In the embodiment according to the present invention shown in Fig. 1, the steel plate whose shape has been corrected by the shape correcting apparatus 4 moves into the descaling apparatus 6 and the descaling apparatus 7. Therefore, the jetting nozzles of the descaling apparatus 6 and the descaling apparatus 7 can be brought close to the surfaces of the steel plate. However, considering contact between the jetting nozzles and the steel plate, it is desirable that the jetting distance be greater than or equal to 40 mm. From the above, in the present invention, it is desirable that the jetting distance H be greater than or equal to 40 mm and less than or equal to 200 mm.

[0024]  The pump discharge power of the ordinary descaling apparatus 6 and descaling apparatus 7 is greater than or equal to 14.7 MPa. Therefore, it is desirable that the jetting pressure of descaling water be greater than or equal to 14.7 MPa. The upper limit of the jetting pressure is not particularly determined. However, when the jetting pressure becomes large, the pumps that supply descaling water consume an extraordinarily large amount of energy. Therefore, it is desirable that the jetting pressure be less than or equal to 50 MPa.

[0025] In this way, according to the embodiment, the descaling apparatus 6 and the descaling apparatus 7 in which the energy density E of the descaling water that is jetted from two or more jetting nozzles is set greater than or equal to 0.08 J/mm² remove the scale that is generated on the surfaces of the steel plate. As a result, variations in scale thickness are eliminated. Therefore, when the steel plate is cooled by the accelerated cooling apparatus 5, as shown in Fig. 5, the steel plate can be uniformly cooled almost without variations in surface temperatures of locations in the width direction, and have excellent shape and mechanical properties.

[0026] In the descaling apparatus 6 and the descaling apparatus 7, for example, as shown in Fig. 6(a), a descale header 6-1 of the descaling apparatus 6 and a descale header 7-1 of the descaling apparatus 7 are formed in two rows in the longitudinal direction of the steel plate. The descale headers shown in Fig. 6(a) are configured in two rows. Descale headers may be configured in three or more rows. Here, since the above-described effect is no longer increased when the number of rows exceeds three, it is desirable that the upper limit be three rows. Descaling water is jetted from a plurality of jetting nozzles 6-2 and 7-2 of the descale headers to the steel plate, and a spray pattern 22 as shown in Fig. 6(b) is formed.

[0027] Regarding the arrangement relationship of the jetting nozzles 6-2 of the descaling apparatus 6 and the jetting nozzles 7-2 of the descaling apparatus 7, in order to prevent splashed descaling water from the second row from interfering with descaling water from the first row, it is desirable that the jetting nozzles 6-2 be separated from the jetting nozzles 7-2 by 500 mm or more in the longitudinal direction. Further, as shown in Fig. 6(b), it is desirable that jetting patterns in the width direction be such that the first row and the second row are in a staggered arrangement. The energy density of the descaling water jetted from two jetting nozzles, the jetting nozzle 6-2 and the jetting nozzle 7-2, is such that, after a crack has been formed in scale by the thermal stress effect produced by the descaling by the first row, the scale is removed at a high energy density by the descaling by the second row to allow the scale to be efficiently removed. Accordingly, in order to form a crack in the scale by the thermal stress effect produced by the descaling by the first row, it is essential that the energy density of the descaling water from the first row be greater than or equal to 0.01 J/mm², and that the energy density of the descaling water from the second row be greater than that of the descaling water from the first row by 0.04 J/mm² or greater. Even if descaling apparatus is configured in three or more rows, it is desirable that the nozzle rows be separated by 500 mm or more in the longitudinal direction and be in a staggered arrangement. When the descaling apparatus is configured in three or more rows, due to the same reason as when the descaling apparatus is configured in two rows, it is essential that the energy density of the descaling water jetted from the jetting nozzles of the descaling apparatus in a row just before the final row be greater than or equal to 0.01 J/mm² and that the energy density of the descaling water jetted from the jetting nozzles of the descaling apparatus in the final row be greater than the energy density of the descaling water jetted from the jetting nozzles of the descaling apparatus in the row just before the final row by 0.04 J/mm² or greater.

[0028] Since it is after the correction of the shape of the steel plate by the shape correcting apparatus 4, it is possible to bring the jetting nozzles of the descaling apparatus 6 and the jetting nozzles of the descaling apparatus 7 close to the surfaces of the steel plate whose shape has been corrected. As a result, descaling capability is increased.

[0029] The scale on the surfaces of the steel plate that affects the stability of cooling of the steel plate by the accelerated cooling apparatus 5 is such that, in general, the growth of the scale on the steel plate can be determined by diffusion control, and is known to be represented by the next Expression (2):

where ξ: scale thickness, a: constant, Q: activation energy, R: constant, T: temperature [K] of steel plate before cooling, and t: time.

[0030] Therefore, considering the growth of the scale after removing the scale by the descaling apparatus 6 and the descaling apparatus 7, a simulation experiment for the scale growth was conducted for various temperatures and times, the constant in Expression (2) above was experimentally derived, and, further, assiduous tests were carried out regarding the scale thickness and cooling stability. The result is that the cooling becomes stable when the scale thickness is less than or equal to 15 µm, becomes more stable when the scale thickness is less than or equal to 10 µm, and becomes very stable when the scale thickness is less than or equal to 5 µm.

[0031] When the scale thickness is less than or equal to 15 µm, the following Expression (3) can be derived based on Expression (2) above. That is, when the time t [s] from the completion of the removal of the scale on the steel plate by the descaling apparatus 7, which is the downstream-side one of the descaling apparatus 6 and the descaling apparatus 7, to the start of the cooling of the steel plate by the accelerated cooling apparatus 5 satisfies the following Expression (3), the cooling by the accelerated cooling apparatus 5 becomes stable:

where T [K]: temperature of steel plate before cooling.

[0032] When the scale thickness is less than or equal to 10 µm, the following Expression (4) can be derived based on Expression (2) above. That is, when the time t [s] from the completion of the removal of the scale on the steel plate by the descaling apparatus 7 to the start of the cooling of the steel plate by the accelerated cooling apparatus 5 satisfies the following Expression (4), the cooling by the accelerated cooling apparatus 5 becomes more stable:

[0033] Further, when the scale thickness is less than or equal to 5 µm, the following Expression (5) can be derived based on Expression (2) above. That is, when the time t [s] from the completion of the removal of the scale on the steel plate by the descaling apparatus 7 to the start of the cooling of the steel plate by the accelerated cooling apparatus 5 satisfies the following Expression (5), the cooling by the accelerated cooling apparatus 5 becomes very stable:

[0034] A distance L from an exit side of the descaling apparatus 7 to an entrance side of the accelerated cooling apparatus 5 is set so as to satisfy the following Expression (6) in relation to the conveyance velocity of the steel plate V and the time t (time from the completion of a descaling step by the descaling apparatus 7 to the start of a step by the accelerated cooling apparatus 5):

where L: distance (m) from the descaling apparatus 7 to the accelerated cooling apparatus 5, V: conveyance velocity of the steel plate (m/s), and t: time (s).

[0035] The following Expression (7) can be derived from Expression (6) and Expression (3) above. In the present invention, it is desirable that Expression (7) be satisfied:

[0036] The following Expression (8) can be derived from Expression (6) and Expression (4) above. In the present invention, it is desirable that Expression (8) be satisfied:

[0037] Further, the following Expression (9) can be derived from Expression (6) and Expression (5) above. In the present invention, it is desirable that Expression (9) be satisfied:

[0038] On the basis of Expressions (7) to (9) above, for example, in the case where the temperature of the steel plate before the cooling by the accelerated cooling apparatus 5 is 820°C and the conveyance velocity of the steel plate is from 0.28 to 2.50 m/s, the cooling becomes stable when the distance L from the descaling apparatus 7 to the accelerated cooling apparatus 5 is greater than or equal to 12 m and less than or equal to 107 m; the cooling becomes more stable when the distance L is greater than or equal to 5 m and less than or equal to 47 m, and the cooling becomes very stable when the distance L is greater than or equal to 1.3 m and less than or equal to 12 m.

[0039] Therefore, when the distance L from the descaling apparatus 7 to the accelerated cooling apparatus 5 is less than or equal to 12 m, even if the conveyance velocity V of the steel plate is low (for example, V = 0.28 m/s), the cooling becomes stable, whereas when the conveyance velocity of the steel plate V is high (for example, V = 2.50 m/s), the cooling becomes very stable. Therefore, this is desirable. It is more desirable that the distance L from the descaling apparatus 7 to the accelerated cooling apparatus 5 is less than or equal to 5 m.

[0040] Further, in general, considering that most of the steel plates that require controlled cooling are transported at the conveyance velocity V of greater than or equal to 0.5 m/s, it is desirable that the distance L, which is a condition in which the cooling becomes very stable at this conveyance velocity V, be less than or equal to 2.5 m.

[0041] Here, the case in which the temperature of the steel plate before the cooling by the accelerated cooling apparatus 5 is 820°C is described. Even in the case in which the temperature of the steel plate before the cooling by the accelerated cooling apparatus 5 is other than 820°C, the cooling can be similarly made stable when the distance L from the descaling apparatus 7 to the accelerated cooling apparatus 5 is desirably less than or equal to 12 m, is more desirably less than or equal to 5 m, and even more desirably less than or equal to 2.5 m. This is due to the following reason. That is, when the temperature of the steel plate before the cooling by the accelerated cooling apparatus 5 is lower than 820°C, the values on the right side in Expression (7), Expression (8), and Expression (9) above are greater than that when T = 820°C, so that as long as, for T = 820°C, the distance L from the descaling apparatus 7 to the accelerated cooling apparatus 5 is a properly set value, Expression (7), Expression (8), and Expression (9) above are necessarily satisfied. On the other hand, when the temperature of the steel plate before the cooling by the accelerated cooling apparatus 5 is higher than 820°C, Expression (7), Expression (8), and Expression (9) above are also satisfied by adjusting the conveyance velocity V of the steel plate to a low value as appropriate.

[0042] Next, the accelerated cooling apparatus 5 is described. As shown in Fig. 7, the upper surface cooling facility of the accelerated cooling apparatus 5 includes an upper header 11 that supplies cooling water to an upper surface of a steel plate 10, cooling water injection nozzles 13 that are suspended from the upper header 11 and that are used for jetting rod-like cooling water, and a partition wall 15 that is set between the steel plate 10 and the upper header 11. It is desirable that the partition wall 15 have a plurality of water supply ports 16 in which lower end portions of the cooling water injection nozzles 13 are inserted, and a plurality of water drainage ports 17 for draining away the cooling water, supplied to the upper surface of the steel plate 10, to an upper side of the partition wall 15.

[0043] More specifically, the upper surface cooling facility includes the upper header 11 that supplies cooling water to the upper surface of the steel plate 10, the cooling water injection nozzles 13 that are suspended from the upper header 11, and the partition wall 15 that is set horizontally along the width direction of the steel plate and between the upper header 11 and the steel plate 10, and that has a plurality of through holes (the water supply ports 16 and the water drainage ports 17). The cooling water injection nozzles 13 are circular tube nozzles for jetting rod-like cooling water. Ends of the cooling water injection nozzles 13 are inserted into the through holes (the water supply ports 16) in the partition wall 15, and are situated above a lower end portion of the partition wall 15. In order to prevent the cooling water injection nozzles 13 from being clogged by sucking in foreign matter at a bottom portion in the upper header 11, it is desirable that the cooling water injection nozzles 13 penetrate the upper header 11 such that upper ends of the cooling water injection nozzles 13 protrude into the upper header 11.

[0044] Here, the term "rod-like cooling water" according to the present invention refers to cooling water which is jetted in a state in which the cooling water is compressed to a certain extent from circular nozzle jetting ports (including elliptical and polygonal nozzle jetting ports), and which is a continuous and straight stream, the jetting speed of the cooling water from the nozzle jetting ports being 6 m/s or higher and, desirably, 8 m/s or higher, and, the cross section of the stream jetted from the nozzle jetting ports being maintained in a substantially circular shape. That is, the cooling water differs from that which flows so as to fall freely from round tube laminar nozzles, and that which is jetted in liquid drops like a spray.

[0045]  The ends of the cooling water injection nozzles 13 are inserted into the through holes so as to be set above the lower end portion of the partition wall 15, so that, even if a steel plate whose end is warped upward moves in, the cooling water injection nozzles 13 are prevented from becoming damaged by the partition wall 15. This makes it possible to perform cooling for a long time with the cooling water injection nozzles 13 in a good state. Therefore, it is possible to prevent the occurrence of temperature unevenness in the steel plate without, for example, repairing the facility.

[0046] Since the ends of the circular tube nozzles 13 are inserted in the through holes, as shown in Fig. 14, the ends of the circular tube nozzles 13 do not interfere with the flow in the width direction of drainage water that flows along an upper surface of the partition wall 15 and that is indicated by a dotted arrow. Therefore, the cooling water jetted from the cooling water injection nozzles 13 can evenly reach the upper surface of the steel plate regardless of locations in the width direction, so that uniform cooling can be performed in the width direction.

[0047] In an example of the partition wall 15, as shown in Fig. 9, the partition wall 15 has a plurality of through holes, each having a diameter of 10 mm, in a grid pattern and at a pitch of 80 mm in the width direction of the steel plate and at a pitch of 80 mm in the conveyance direction. The cooling water injection nozzles 13, each having an outside diameter of 8 mm, an inside diameter of 3 mm, and a length of 140 mm, are inserted in the water supply ports 16. The cooling water injection nozzles 13 are set in a hound's-tooth check-like form. The through holes in which the cooling water injection nozzles 13 are not inserted correspond to the water drainage ports 17 for the cooling water. Accordingly, the plurality of through holes in the partition wall 15 of the accelerated cooling apparatus include the water supply ports 16 and the water drainage ports 17 that are substantially the same in number, with their roles and functions being divided among the water supply ports 16 and the water drainage ports 17.

[0048] At this time, the total sectional area of the water drainage ports 17 is sufficiently larger than the total sectional area of the inside of the circular tube nozzles 13, which are the cooling water injection nozzles 13, and is approximately 11 times the total sectional area of the inside of the circular tube nozzles 13. As shown in Fig. 7, the cooling water supplied to the upper surface of the steel plate fills a space between a surface of the steel plate and the partition wall 15, flows through the water drainage ports 17, is guided to a location above the partition wall 15, and is quickly discharged. Fig. 10 is a front view illustrating flow of drainage cooling water near an end portion at the upper side of the partition wall in the width direction of the steel plate. A draining direction of the water drainage ports 17 is upward in a direction that is opposite to a cooling water jetting direction. The drainage cooling water that has flown out to a location above the partition wall 15 changes direction towards an outer side in the width direction of the steel plate, flows to a water drain flow path between the upper header 11 and the partition wall 15, and is drained off.

[0049] In an example shown in Fig. 11, the water drainage ports 17 are inclined in the width direction of the steel plate to cause the draining direction to be in an oblique direction towards the outer side in the width direction of the steel plate. This allows drainage water 19 at the upper side of the partition wall 15 to flow smoothly in the width direction of the steel plate, and the water drainage is accelerated. Therefore, this is desirable.

[0050] Here, when, as shown in Fig. 12, the water drainage ports and the corresponding water supply ports are provided in the same through holes, it becomes difficult for the cooling water that has collided with the steel plate to flow out to a location above the partition wall 15, as a result of which the cooling water flows between the steel plate 10 and the partition wall 15 and towards end portions in the width direction of the steel plate. This causes the flow rate of the drainage cooling water between the steel plate 10 and the partition wall 15 to be larger towards the end portions in the width direction of the plate. Therefore, the force for causing jetted cooling water 18 to penetrate a stagnant water film and to reach the steel plate is interfered with to a greater extent towards the end portions in the width direction of the plate.

[0051] In the case of a thin steel sheet, since the sheet width is approximately 2 m at most, the effects thereof are limited. However, in the case of, in particular, a steel plate having a plate width of 3 m or greater, the effects thereof cannot be ignored. Therefore, cooling at the end portions in the width direction of the steel plate is weakened. The temperature distribution in the steel plate in the width direction thereof in this case is an uneven temperature distribution.

[0052]  In contrast, as shown in Fig. 13, the accelerated cooling apparatus includes the water supply ports 16 and the water drainage ports 17 that are separately provided. Since the roles of supplying water and draining off water are divided among the water supply ports 16 and the water drainage ports 17, the drainage cooling water flows through the water drainage ports 17 in the partition wall 15 and smoothly flows to a location above the partition wall 15. Therefore, the drain water after the cooling is quickly drained off from the upper surface of the steel plate, so that cooling water that is subsequently supplied can easily penetrate the stagnant water film, and, thus, a sufficient cooling capacity can be provided. The temperature distribution in the steel plate in the width direction thereof in this case is a uniform temperature distribution, so that a uniform temperature distribution can be provided in the width direction.

[0053] Incidentally, when the total sectional area of the water drainage ports 17 is greater than or equal to 1.5 times the total sectional area of the inside of the circular tube nozzles 13, the cooling water is quickly discharged. This can be realized, for example, when ports having a size that is greater than the outside diameter of the circular tube nozzles 13 are formed in the partition wall 15, and the number of water drainage ports is greater than or equal to the number of water supply ports.

[0054] When the total sectional area of the water drainage ports 17 is less than 1.5 times the total sectional area of the inside of the circular tube nozzles 13, the flow resistance at the water drainage ports is increased and, thus, it becomes difficult to drain off stagnant water. As a result, the amount of cooling water that can penetrate the stagnant water film and reach the surface of the steel plate is considerably reduced, thereby reducing the cooling capacity. Therefore, this is not desirable. It is more desirable that the total sectional area of the water drainage ports 17 be greater than or equal to 4 times the total sectional area of the inside of the circular tube nozzles 13. On the other hand, when there are too many water drainage ports or the sectional diameter of the water drainage ports is too large, the rigidity of the partition wall 15 is reduced, as a result of which the partition wall 15 tends to be damaged when the thick wall sheet collides with the partition wall 15. Therefore, it is desirable that the ratio between the total sectional area of the water drainage ports and the total sectional area of the inside of the circular tube nozzles 13 be in the range of 1.5 to 20.

[0055] It is desirable that gaps between outer peripheral surfaces of the circular tube nozzles 13, which are inserted in the water supply ports 16 in the partition wall 15, and inner surfaces defining the water supply ports 16 be less than or equal to 3 mm in size. When the gaps are large, due to the effects of accompanied flow of the cooling water that is jetted from the circular tube nozzles 13, the drainage cooling water discharged to the upper surface of the partition wall 15 is sucked into the gaps between the water supply ports 16 and the outer peripheral surfaces of the circular tube nozzles 13, and is re-supplied to the steel plate. Therefore, the cooling efficiency is reduced. In order to prevent this, it is more desirable that the outside diameter of the circular tube nozzles 13 be substantially the same as the size of the water supply ports 16. However, considering working accuracy and mounting errors, gaps of up to 3 mm, at which the effects are substantially small, are allowed. It is more desirable that the gaps be less than or equal to 2 mm in size.

[0056] Further, in order to allow the cooling water to penetrate the stagnant water film and to reach the steel plate, the inside diameter and length of the circular tube nozzles 13, the jetting speed of the cooling water, and nozzle distance also need to be optimal values.

[0057] That is, it is desirable that the nozzle inside diameter be 3 to 8 mm. When the nozzle inside diameter is less than 3 mm, a flux of water that is jetted from the nozzles becomes thinner, and, thus, water strength is reduced. On the other hand, when the nozzle diameter exceeds 8 mm, the flow speed is reduced, as a result of which the force for causing the cooling water to penetrate the stagnant water film is reduced.

[0058] It is desirable that the length of each circular tube nozzle 13 be 120 to 240 mm. Here, the length of each circular tube nozzle 13 refers to the length from an inlet at the upper end of each nozzle that penetrates the header by a certain amount to a lower end of each nozzle inserted in the corresponding water supply port in the partition wall. When each circular tube nozzle 13 is shorter than 120 mm, the distance between a lower surface of the header and the upper surface of the partition wall becomes too small (for example, when the header thickness is 20 mm, a protruding amount of the upper end of each nozzle into the header is 20 mm, and an insertion amount of the lower end of each nozzle into the partition wall is 10 mm, the distance becomes less than 70 mm). Therefore, a drain space above the partition wall becomes small, as a result of which the drainage cooling water cannot be smoothly discharged. On the other hand, when the length is greater than 240 mm, pressure loss at each circular tube nozzle 13 becomes large, and, thus, the force for causing the cooling water to penetrate the stagnant water film is reduced.

[0059] The jetting speed of the cooling water from the nozzles needs to be greater than or equal to 6 m/s, and, desirably, greater than or equal to 8 m/s. This is because, when the jetting speed is less than 6 m/s, the force for causing the cooling water to penetrate the stagnant water film becomes extremely weak. When the jetting speed is greater than or equal to 8 m/s, a higher cooling capacity can be provided. Therefore, this is desirable. The distance from the lower end of each cooling water injection nozzle 13, used for cooling the upper surface of the steel plate, to the surface of the steel plate 10 may be 30 to 120 mm. When this distance is less than 30 mm, the frequency with which the steel plate 10 collides with the partition wall 15 is extremely high. Therefore, it becomes difficult to maintain the facility. When this distance exceeds 120 mm, the force for causing the cooling water to penetrate the stagnant water film becomes extremely weak.

[0060]  In cooling the upper surface of the steel plate, draining rollers 20 may be set in front of and behind the upper header 11 so as to prevent the cooling water from spreading in the longitudinal direction of the steel plate. This causes a cooling zone length to be constant, and facilitates temperature control. Here, since the draining rollers 20 intercept the flow of the cooling water in the conveyance direction of the steel plate, the drainage cooling water flows to the outer side in the width direction of the steel plate. However, the cooling water tends to stagnate near the draining rollers 20.

[0061] Accordingly, as shown in Fig. 8, it is desirable that, of the rows of circular tube nozzles 13 that are set side by side in the width direction of the steel plate, the cooling water injection nozzles in an uppermost-stream-side row in the conveyance direction of the steel plate be tilted towards an upstream side in the conveyance direction of the steel plate by 15 to 60 degrees, and the cooling water injection nozzles in a lowermost-stream-side row in the conveyance direction of the thick steel sheet be tilted towards a downstream side in the conveyance direction of the steel plate by 15 to 60 degrees. This makes it possible to also supply the cooling water to locations close to the draining rollers 20, and, thus, increase the cooling efficiency without stagnation of the cooling water near the draining rollers 20.

[0062] The distance between the lower surface of the upper header 11 and the upper surface of the partition wall 15 is such that the sectional area of a flow path in the width direction of the steel plate in a space surrounded by the lower surface of the header and the upper surface of the partition wall is greater than or equal to 1.5 times the total sectional area of the inside of the cooling water injection nozzles, and is, for example, greater than or equal to approximately 100 mm. When the sectional area of the flow path in the width direction of the steel plate is not greater than or equal to 1.5 times the total sectional area of the inside of the cooling water injection nozzles, the drainage cooling water discharged to the upper surface of the partition wall 15 from the water drainage ports 17 in the partition wall cannot be smoothly discharged in the width direction of the steel plate.

[0063] In the accelerated cooling apparatus, the range of the water amount density that is most effective is greater than or equal to 1.5 m³/(m² · min). When the water amount density is lower than this value, the stagnant water film does not become so thick, and, even if a publicly known technology of cooling the steel plate by causing the rod-like cooling water to fall freely is applied, there are cases in which the degree of temperature unevenness in the width direction does not become so large. On the other hand, when the water amount density is greater than 4.0 m³/(m² · min), the use of the technology according to the present invention is effective. However, since there are problems in terms of practical use, such as an increase in facility costs, the range of 1.5 to 4.0 m³/(m² · min) is the most practical water amount density.

[0064] In applying the cooling technology according to the present invention, the case of disposing the draining rollers in front of and behind the cooling header is particularly effective. However, the cooling technology according to the present invention is applicable to the case in which the draining rollers are not provided. For example, it is possible to apply the cooling technology according to the present invention to a cooling facility that prevents water leakage to a non-water-cooling zone by spraying purging water in front of and behind a header that is relatively long in the longitudinal direction (approximately 2 to 4 m).

[0065] In the present invention, a cooling apparatus at a side of a lower surface of the steel plate is not particularly limited. In the embodiment shown in Figs. 7 and 8, an example in which a cooling lower header 12 provided with circular tube nozzles 14 as in the cooling apparatus at the side of the upper surface of the steel plate is given. In cooling the lower surface of the steel plate, since the jetted cooling water falls freely after colliding with the thick steel sheet, a partition wall 15 for evacuating drainage cooling water in the width direction of the steel plate, like the one used in cooling the upper surface of the steel plate, need not be used. A publicly known technology of supplying, for example, membranous cooling water or cooling water in the form of a spray may be used.

[0066] As described above, in the facility for manufacturing a steel plate according to the method of the present invention, when two or more rows of jetting nozzles for descaling water are set as the descaling apparatus 6 and the descaling apparatus 7, and the energy density E that is jetted towards the surfaces of the steel plate 10 from the two or more rows of jetting nozzles is set greater than or equal to 0.08 J/mm² in total, the scale on the steel plate 10 can be made uniform, and uniform cooling can be performed by the accelerated cooling apparatus 5. As a result, the steel plate 10 can be manufactured as one having excellent shape.

[0067] By correcting the shape of the steel plate 10 by the shape correcting apparatus 4, the jetting nozzles of the descaling apparatus 6 and the jetting nozzles of the descaling apparatus 7 can be brought close to the surfaces of the steel plate 10.

[0068] When the jetting distance H (distance from the jetting nozzles of the descaling apparatus 6 and the jetting nozzles of the descaling apparatus 7 to the surfaces of the steel plate 10) is greater than or equal to 40 mm and less than or equal to 200 mm, descaling capacity is increased. The jetting pressure, the jetting flow rate, etc., for obtaining the predetermined energy density E are low. Therefore, it is possible to reduce the pumping power of the descaling apparatus 6 and the descaling apparatus 7.

[0069] When the distance L from the descaling apparatus 7, which, of the descaling apparatus 6 and the descaling apparatus 7, is the descaling apparatus at the downstream side, to the accelerated cooling apparatus 5 satisfies L ≤ V × 5 × 10^-9 × exp(25000/T), it is possible to stabilize the cooling of the steel plate 10 by the accelerated cooling apparatus 5.

[0070]  Further, as shown in Fig. 7, in the accelerated cooling apparatus 5, the cooling water supplied from the upper cooling water injection nozzles 13 through the water supply ports 16 cools the upper surface of the steel plate 10 and becomes hot drain water, and flows in the width direction of the steel plate 10 from above the partition wall 15 with the water drainage ports 17, in which the upper cooling water injection nozzles 13 are not inserted, being drain water flow paths. The drain water after the cooling is quickly removed from the steel plate 10, so that, by successively bringing the cooling water that flows from the upper cooling water injection nozzles 13 through the water supply ports 16 into contact with the steel plate 10, a sufficient cooling capacity can be provided uniformly in the width direction.

[0071] The inventors carried out studies and found out that the degree of temperature unevenness in the width direction of the steel plate subjected to accelerated cooling without being subjected to descaling such as that in the present invention is approximately 40°C. On the other hand, the inventors found out that the degree of temperature unevenness in the width direction of the steel plate cooled by the accelerated cooling apparatus 5 after the descaling by the descaling apparatus 6 and the descaling apparatus 7 above is reduced to approximately 10°C. Further, the inventors found out that the degree of temperature unevenness in the width direction of the steel plate subjected to accelerated cooling by using the accelerated cooling apparatus 5 shown in Fig. 7 after the descaling by the descaling apparatus 6 and the descaling apparatus 7 is reduced to approximately 4°C. Regarding the temperature unevenness of the steel plate, the distribution of the surface temperature in the steel plate after the accelerated cooling is measured by using a scanning type thermometer and the degree of temperature unevenness in the width direction is calculated on the basis of the results of the measurement.

[0072] As in the present invention, any distortion that has occurred during rolling is corrected by the shape correcting apparatus 4 and the steel plate 10 is descaled by the descaling apparatus 6 and the descaling apparatus 7 to stabilize the controllability of cooling. Therefore, the steel plate 10 to be subjected to correction by a shape correcting apparatus that is provided on-line or off-line at a downstream side of the facility for manufacturing the steel plate also has high flatness and uniform temperature by its nature. Therefore, the correcting capability of the shape correcting apparatus that is provided at the downstream side need not be very high. The distance between the accelerated cooling apparatus 5 and the shape correcting apparatus that is provided at the downstream side may be larger than the maximum length of the steel plate 10 that is manufactured in a rolling line. Therefore, since, for example, reverse correction by the shape correcting apparatus, which is provided at the downstream side, is performed often, it is possible to expect the effect of preventing problems, such as the steel plate 10 that is been transported in the opposite direction jumping at the upper side of a conveyance roller and colliding with the accelerated cooling apparatus 5, and the effect of equalizing slight temperature deviations occurring during the cooling at the accelerated cooling apparatus 5 and preventing the occurrence of warping caused by the temperature deviations after the correction.

Example 1

[0073] Controlled cooling was performed from 820°C to 420°C after passing a steel plate having a sheet thickness of 30 mm and a width of 3500 mm and rolled by the rolling apparatus 3 through the shape correcting apparatus 4, the descaling machine 6, and the descaling apparatus 7. Here, when the conditions for stabilizing the cooling are calculated on the basis of Expressions (3), (4), and (5) above, the time t from the completion of the removal of scale on the steel plate by the descaling apparatus 7 to the start of the cooling of the steel plate by the accelerated cooling apparatus 5 is desirably less than or equal to 42 s, more desirably, less than or equal to 19 s, and even more desirably, less than or equal to 5 s.

[0074] The descaling apparatus 6 and the descaling apparatus 7 were such that the nozzle jetting pressure was 17.7 MPa, the jetting flow rate per nozzle was 45 L/min (= 7.5 × 10^-4 m³/s), the jetting distance (the distance from the jetting nozzles of the descaling apparatus 6 and the jetting nozzles of the descaling apparatus 7 to surfaces of the steel plate) was 130 mm, the nozzle jetting angle was 66 degrees, and the attack angle was 15 degrees. The descaling apparatus 6 and the descaling apparatus 7 were such that two rows of nozzles were set in a longitudinal direction such that jetting areas of adjacent nozzles were arranged side by side in the width direction so as to overlap each other to a certain extent, with the spray jet thickness being 3 mm and the spray jet width being 175 mm. The nozzles were flat spray nozzles. Here, the energy density of the descaling water is a value defined by the aforementioned expression "water amount density × jetting pressure × collision time". The collision time (s) is a time when descaling water is jetted to the surfaces of the steel plate, and is determined by dividing the spray jet thickness by the conveyance velocity.

[0075] The accelerated cooling apparatus 5 is a facility including flow paths that allow cooling water supplied to the upper surface of the steel plate to flow to a location above the partition wall as shown in Fig. 7, and further allow the water to be drained off from a side of the steel plate in the width direction as shown in Fig. 10. The partition wall had holes having a diameter of 12 mm and arranged in a grid pattern, and were such that, as shown in Fig. 9, the upper cooling water injection nozzles was inserted into the water supply ports set in a hound's-tooth check-like arrangement, and the remaining ports were water drainage ports. The distance between the lower surface of the upper header and the upper surface of the partition wall was 100 mm.

[0076] The upper cooling water injection nozzles of the accelerated cooling apparatus 5 had an inside diameter of 5 mm, an outside diameter of 9 mm, and a length of 170 mm, and their upper ends protruded into the header. The jetting speed of the rod-like cooling water was 8.9 m/s. The nozzle pitch in the width direction of the steel plate was 50 mm, 10 rows of nozzles were set side by side in the longitudinal direction in a zone of a distance of 1 m between table rollers. The water amount density at the upper surface was 2.1 m³/(m² · min). The lower ends of the upper-surface cooling nozzles were set at intermediate positions between the upper surface and the lower surface of the partition wall having a sheet thickness of 25 mm, and the distance to the surface of the steel plate was 80 mm.

[0077] Regarding the lower surface cooling facility, as shown in Fig. 7, a cooling facility that is the same as the upper surface cooling facility was used except that the cooling facility did not include a partition wall. The water amount density and the jetting speed of the rod-like cooling water were 1.5 times those of the upper surface cooling facility.

[0078] As shown in Table 1, the distance L from the descaling apparatus 7 to the accelerated cooling apparatus 5, the conveyance velocity V of the steel plate, and the time t from the descaling apparatus 7 to the accelerated cooling apparatus 5 were variously changed. T in Table 1 denotes the temperature (K) of each steel plate before cooling.

[0079] Regarding the shape of each steel plate, the re-correction percentage (%) was evaluated. More specifically, if warping of the entire length of any steel plate and/or warping of the entire width of any steel plate was/were within a standard value prescribed by a product standard corresponding to the steel plate, it was determined that the result was acceptable, whereas, if not, it was determined that the steel plate was one to be subjected to shape correction again, with the re-correction percentage being calculated by using the expression "(number of sheets to be subjected to shape correction again)/(total number of sheets) × 100".

[Table 1]

[0080] 

Table 1

Item	Descaling Before Controlled Cooling	Number of Descalings	Energy Density (J/mm2)	Distance from Descaling Apparatus to Accelerated Cooling Apparatus(m)	Conveyance Velocity (m/s)	Time from Completion of Descaling to Start of Accelerated Cooling (s)	Jetting Distance (mm)	Water Amount per Nozzle (m3/s)	Water Amount (m3/(mm2 · s))	Collision Time (s)	Collision Pressure (MPa)	Jetting Pressure (MPa)	T (K)	Re-correction Percentage (%)
Inventive Example 1	Performed	2	0.54	5	0.28	18	130	7.5 ×10-4	1.4 × 10-6	1.1 × 10-2	0.63	17.7	1093	5
Inventive Example 2	Performed	2	0.25	5	0.6	8	130	7.5 × 10-4	1.4 × 10-6	5.0 × 10-3	0.63	17.7	1103	4
Inventive Example 3	Performed	2	0.08	5	1.7	3	130	7.3 × 10-4	1.4 × 10-6	1.8 × 10-3	0.58	16.5	1110	2
Inventive Example 4	Performed	3	0.28	5	0.8	6	130	7.5 × 10-4	1.4 × 10-6	3.8 × 10-3	0.63	17.7	1105	3
Inventive Example 5	Performed	2	0.54	13	0.28	46	130	7.5 × 10-4	1.4 × 10-6	1.1 × 10-2	0.63	17.7	1083	12
Comparative Example 1	Not Performed	-	-	-	-	-	-	-	-	-	-	-	1093	40
Comparative Example 2	Performed	2	0.06	5	1	5	130	6.5 × 10-4	1.1 × 10-6	3.0 × 10-3	0.35	10	1105	70
Comparative Example 3	Performed	1	0.09	5	0.8	6	130	7.5 × 10-4	1.4 × 10-6	3.8 × 10-3	0.63	17.7	1110	72
Comparative Example 4	Performed	3	0.06	5	1.7	3	130	5.6 × 10-4	1.1 × 10-6	1.8 × 10-3	0.35	10	1105	69

[0081] In Inventive Examples 1 to 5 in Table 1, the energy densities were greater than or equal to 0.08 J/mm², so that re-correction percentages based on shape defects were low, and good results were obtained. This is thought to be because, when the cooling is performed by the accelerated cooling apparatus 5, the steel plates are uniformly cooled almost without surface temperature variations at locations in the width direction, mechanical properties are better than those in the prior art, and the flatness thought to result from the temperature distributions of the steel plates are excellent, as a result of which the re-correction percentage based on shape defects is reduced. In addition, in Inventive Examples 1 to 5, scales were removed, and surface properties were good. The surface properties were evaluated by using images of the surfaces of the steel plates cooled to room temperature to determine the presence of scales on the basis of image processing making use of color tone differences between portions where scales remained and portions where scales were peeled off.

[0082] In particular, in Inventive Examples 1 to 4 in which the distance from the descaling apparatus 7, which is at the lowermost stream side with respect to the conveyance direction, to the accelerated cooling apparatus 5 was 5 m, the time t from the completion of the removal of the scale on each steel plate by the descaling apparatus 7 to the start of the cooling of each steel plate by the accelerated cooling apparatus 5 was less than or equal to 19 s, which is the condition at which the cooling by the accelerated cooling apparatus 5 becomes more stable, regardless of the conveyance velocity V of the steel plates. Therefore, the re-correction percentage was good at 5% or less. In Inventive Example 5, the re-correction percentage was 12%, which is a passing percentage, and was not as good as those in Inventive Examples 1 to 4. This is thought to be because, since the time from the completion of the removal of the scale to the start of the cooling by the accelerated cooling apparatus 5 is long at 46 s, the scale becomes thicker, thereby making the cooling unstable.

[0083] On the other hand, in Comparative Example 1 in which the cooling was performed by the accelerated cooling apparatus 5 without scale removal by the descaling apparatus 6 and the descaling apparatus 7, the cooling by the accelerated cooling apparatus 5 was performed without uniformizing the scale on the surfaces of the steel plate. Therefore, the re-correction percentage was 40% due to deteriorated flatness that would be caused by the temperature distribution of the steel plate, and there were also variations in the mechanical properties.

[0084] In Comparative Example 2 in which the setting conditions based on the descaling apparatus 6 and the descaling apparatus 7 were water pressure = 10 MPa, the jetting flow rate per nozzle = 39 L/min (= 6.5 × 10^-4m³/s), the jetting distance = 130 mm, the nozzle jetting angle = 66 degrees, and the nozzle attack angle = 15 degrees; and in which the energy density was 0.06 J/mm², the energy density of the descaling water was not sufficiently high, as a result of which the scale was partly peeled off and the temperature distribution of the steel plate in the width direction thereof deteriorated. Therefore, the re-correction percentage was 70%, and there were also variations in the mechanical properties.

[0085] In Comparative Example 3 in which the number of descalings was one, the nozzle jetting pressure was 17.7 MPa, the jetting flow rate per nozzle was 45 L/min (= 7.5 × 10^-4m³/s), the jetting distance was 130 mm, the nozzle jetting angle was 66 degrees, and the attack angle was 15 degrees; and in which the energy density was 0.09 J/mm², thermal stress occurring during the descaling was effective only once because the number of descalings was one. Therefore, the scale was partly peeled off and the temperature distribution of the steel plate in the width direction thereof deteriorated. Therefore, the re-correction percentage was 72%, and there were also variations in the mechanical properties.

[0086] In Comparative Example 4 in which the number of descalings was three, the nozzle jetting pressure was 10 MPa, the jetting flow rate per nozzle was 34 L/min (= 5.6 × 10^-4m³/s), the jetting distance was 130 mm, the nozzle jetting angle was 66 degrees, and the attack angle was 15 degrees; and in which the energy density was 0.06 J/mm² in total for three descalings, the energy density of the descaling water was not sufficiently high, as a result of which the scale was partly peeled off and the temperature distribution of the steel plate in the width direction thereof deteriorated. Therefore, the re-correction percentage was 69%, and there were also variations in the mechanical properties.

Reference Signs List

[0087] 

1 heating furnace

2 descaling apparatus

3 rolling apparatus

4 shape correcting apparatus

5 accelerated cooling apparatus

6 descaling apparatus

6-1 descaling header

6-2 jetting nozzle

7 descaling apparatus

7-1 descaling 0header

7-2 jetting nozzle

10 steel plate

11 upper header

12 lower header

13 upper cooling water injection nozzle (circular tube nozzle)

14 lower cooling water injection nozzle (circular tube nozzle)

15 partition wall

16 water supply port

17 water drainage port

18 jetting cooling water

19 drainage water

20 draining roller

21 draining roller

22 spray pattern

Claims

1. A method for manufacturing a steel plate comprising a hot rolling step, a hot correction step, and an accelerated cooling step in this order to manufacture the steel plate, the method further comprising:

a descaling step in which descaling is performed on a surface of the steel plate two or more times between the hot correction step and the accelerated cooling step such that energy density E of descaling water is greater than or equal to 0.08 J/mm² in total wherein jetting nozzles of a descaling apparatus are set in two or more rows with respect to a longitudinal direction of the steel plate,

wherein the energy density of the descaling water that is jetted from the jetting nozzles of the descaling apparatus in a row just before the final row is greater than or equal to 0.01 J/mm², and

wherein the energy density of the descaling water that is jetted from the jetting nozzles of the descaling apparatus in the final row is greater than the energy density of the descaling water that is jetted from the jetting nozzles of the descaling apparatus in the row just before the final row by 0.04 J/mm² or greater.

2. The method for manufacturing a steel plate according to claim 1 in which descaling is performed on a surface of the steel plate two times between the hot correction step and the accelerated cooling step.

3. The method for manufacturing a steel plate according to Claim 1 or 2,
wherein a time t [s] from completion of the descaling step to start of the accelerated cooling step satisfies the expression t ≤ 5 × 10 ^-9 × exp(25000/T),
where T (K): temperature of steel plate before cooling.

Ansprüche

1. Verfahren zum Herstellen eines Stahlblechs, das einen Warmwalz-Schritt, einen Warmkorrektur-Schritt und einen Schritt zum beschleunigten Abkühlen zum Herstellen des Stahlblechs in dieser Reihenfolge umfasst, wobei das Verfahren des Weiteren umfasst:

einen Entzunderungs-Schritt, in dem Entzundern an einer Oberfläche des Stahlblechs zweimal oder häufiger zwischen dem Warmkorrektur-Schritt und dem Schritt zum beschleunigten Abkühlen durchgeführt wird, so dass Energiedichte E von Entzunderungs-Wasser insgesamt 0,08 J/mm² oder mehr beträgt, wobei Strahldüsen einer Entzunderungs-Vorrichtung in zwei oder mehr Reihen in Bezug auf eine Längsrichtung des Stahlblechs angeordnet sind,

die Energiedichte des Entzunderungs-Wassers, das über die Strahldüsen der Entzunderungs-Vorrichtung in einer Reihe unmittelbar vor der letzten Reihe ausgestrahlt wird, 0,01 J/mm² oder mehr beträgt, und

die Energiedichte des Entzunderungs-Wassers, das über die Strahldüsen der Entzunderungs-Vorrichtung in der letzten Reihe ausgestoßen wird, um 0,04 J/mm² oder mehr größer ist als die Energiedichte des Entzunderungs-Wassers, das über die Strahldüsen der Entzunderungs-Vorrichtung in der Reihe unmittelbar vor der letzten Reihe ausgestoßen wird.

2. Verfahren zum Herstellen einer Stahlplatte nach Anspruch 1, bei dem zwischen dem Warmkorrektur-Schritt und dem Schritt zum beschleunigten Abkühlen Entzundern an einer Oberfläche der Stahlplatte zweimal durchgeführt wird.

3. Verfahren zum Herstellen eines Stahlblechs nach Anspruch 1 oder 2,
wobei für eine Zeit t [s] von Abschluss des Entzunderungs-Schritts bis zum Beginn des Schritts zum beschleunigten Abkühlen Ausdruck t ≤ 5 × 10 ^-9 × exp(25000/T) gilt,
wobei T (K): Temperatur des Stahlblechs vor Abkühlen.

Revendications

1. Procédé de fabrication d'une plaque d'acier comprenant une étape de laminage à chaud, une étape de correction à chaud et une étape de refroidissement accéléré dans cet ordre pour fabriquer la plaque d'acier, le procédé comprenant en outre :

une étape de décalaminage dans laquelle un décalaminage est effectué sur une surface de la plaque d'acier deux fois ou plus entre l'étape de correction à chaud et l'étape de refroidissement accéléré de sorte que la densité d'énergie E de l'eau de décalaminage soit supérieure ou égale à 0,08 J/mm² au total, où des buses d'éjection d'un appareil de décalaminage sont placées dans deux rangées ou plus par rapport à une direction longitudinale de la plaque d'acier,

dans lequel la densité d'énergie de l'eau de décalaminage qui est éjectée à partir des buses d'éjection de l'appareil de décalaminage dans une rangée juste avant la rangée finale est supérieure ou égale à 0,01 J/mm², et

dans lequel la densité d'énergie de l'eau de décalaminage qui est éjectée à partir des buses d'éjection de l'appareil de décalaminage dans la rangée finale est supérieure à la densité d'énergie de l'eau de décalaminage qui est éjectée à partir des buses d'éjection de l'appareil de décalaminage dans la rangée juste avant la rangée finale de 0,04 J/mm² ou plus.

2. Procédé de fabrication d'une plaque d'acier selon la revendication 1 dans lequel un décalaminage est effectué sur une surface de la plaque d'acier deux fois entre l'étape de correction à chaud et l'étape de refroidissement accéléré.

3. Procédé de fabrication d'une plaque d'acier selon la revendication 1 ou 2,
dans lequel un temps t [s] entre l'achèvement de l'étape de décalaminage et le début de l'étape de refroidissement accéléré satisfait l'expression t ≤ 5 <10 ^-9 × exp (25000/T),
où T (K) : température de la plaque d'acier avant le refroidissement.

Drawing